Myoglobin-derived iron causes wound enlargement and impaired regeneration in pressure injuries of muscle

  1. Nurul Jannah Mohamed Nasir
  2. Hans Heemskerk
  3. Julia Jenkins
  4. Nur Hidayah Hamadee
  5. Ralph Bunte
  6. Lisa Tucker-Kellogg  Is a corresponding author
  1. Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore
  2. Centre for Computational Biology, Duke-NUS Medical School, Singapore
  3. BioSyM and CAMP Interdisciplinary Research Group, Singapore-MIT Alliance for Research and Technology, CREATE, Singapore

Abstract

The reasons for poor healing of pressure injuries are poorly understood. Vascular ulcers are worsened by extracellular release of hemoglobin, so we examined the impact of myoglobin (Mb) iron in murine muscle pressure injuries (mPI). Tests used Mb-knockout or treatment with deferoxamine iron chelator (DFO). Unlike acute injuries from cardiotoxin, mPI regenerated poorly with a lack of viable immune cells, persistence of dead tissue (necro-slough), and abnormal deposition of iron. However, Mb-knockout or DFO-treated mPI displayed a reversal of the pathology: decreased tissue death, decreased iron deposition, decrease in markers of oxidative damage, and higher numbers of intact immune cells. Subsequently, DFO treatment improved myofiber regeneration and morphology. We conclude that myoglobin iron contributes to tissue death in mPI. Remarkably, a large fraction of muscle death in untreated mPI occurred later than, and was preventable by, DFO treatment, even though treatment started 12 hr after pressure was removed. This demonstrates an opportunity for post-pressure prevention to salvage tissue viability.

Editor's evaluation

It is known that muscle pressure injury (mPI), tissue damage caused by sustained pressure, is difficult to be healed although the mechanism underlying has been poorly understood. In this study, the authors demonstrated a convincing evidence that myoglobin (Mb) released at the site of injury plays an important role in the size, severity, oxidative damage, and poor healing of mPI by causing the induction of immune cell (in particular phagocyte) death and delaying the clearance of dead tissues using Mb KO mice and iron chelation by deferoxamine. The authors’ findings are valuable in developing a novel therapeutic option for mPI although further clinical corroboration of these findings would be of even greater value.

https://doi.org/10.7554/eLife.85633.sa0

Introduction

Pressure injuries (also called pressure ulcers, bedsores, decubiti, or pressure sores) are tissue damage caused by sustained pressure. They are extremely painful (Girouard et al., 2008), costly to prevent and treat (Padula and Delarmente, 2019), and increase the risk of patient sepsis and death (Wassel et al., 2020). Tissue death can be caused by mechanical deformation, ischemia, or both. Ischemia is often studied as ischemia-reperfusion injury, but poor clearance of damage factors may have unappreciated importance (Gray et al., 2016).

Pressure injuries often heal poorly (Mervis and Phillips, 2019), especially if they involve deeper layers such as muscle (Bouten et al., 2003; Preston et al., 2017), but the reasons are for poor quality and quantity of healing are not clear. Some cases can be explained by complications or comorbidities (infection, incontinence, poor circulation, hyperglycemia, chronic reinjury, advanced age), but pressure injuries can affect any immobile person (e.g., young adults with spinal cord injury). In this study, we ask whether some aspect of pressure-induced injury is intrinsically inhospitable to regeneration and in need of intervention.

Chronic ulcers of veins or arteries (e.g., venous stasis ulcers, sickle cell ulcers) have high levels of extracellular hemoglobin (Hb) released in the wound. For example, many have deposits of hemosiderin. Extracellular Hb and its breakdown products (e.g., hemin, iron) create oxidative stress (Goldman et al., 1998; Reeder and Wilson, 2005) and other effects that are detrimental to regeneration. For example, Hb decreases nitric oxide for angiogenesis (Kato et al., 2017; Nader et al., 2020) and signals as a damage-associated molecular pattern (DAMP) to increase inflammation (Mendonça et al., 2016; Bozza and Jeney, 2020). Systems exist to detoxify Hb (Cherayil, 2011), but Hb is also an innate immune factor with evolutionarily conserved antimicrobial function. When extracellular Hb is activated by proteolytic cleavage, bacterial binding, or conformational change, it increases production of reactive oxygen species (ROS) Bogdan, 2007; this process has been called ‘oxidative burst without phagocytes’ (Bogdan, 2007; Jiang et al., 2007). Our earlier work used evolutionary conservation to identify ROS-producing fragments of Hb and crosstalk with tissue factor coagulation (Bahl et al., 2014; Bahl et al., 2011). Therefore, globin proteins have multiple functions that may be detrimental to chronic wounds (Tchanque-Fossuo et al., 2017).

Myoglobin release into plasma or urine has been observed after muscle pressure injuries (mPI) in multiple studies including deep tissue injury (DTI) (Traa et al., 2019; Makhsous et al., 2010; Loerakker et al., 2012; Levine, 1993; Traa, 2019), but was studied as a readout of damage rather than a source of damage. We reason by analogy to hemoglobin that extracellular myoglobin might create a hostile wound environment. An extracellular environment oxidizes globins to a ferric (Fe3+) state, which can be further oxidized to ferryl (FeIV=O) globin in the presence of endogenous peroxides such as hydrogen peroxide (Reeder et al., 2008). Hydrogen peroxide is ubiquitous in contexts of cell stress, mitochondrial permeabilization, and cell death (Rojkind et al., 2002). Ferryl-Mb can oxidize macromolecules directly (Goldman et al., 1998; Kapralov et al., 2009; Plotnikov et al., 2009) and can form heme-to-protein crosslinks (Osawa and Williams, 1996). Most importantly, ferryl myoglobin can participate in a catalytic cycle of pseudo-peroxidase activity (redox cycling) (Boutaud and Roberts, 2011). In a tissue context, myoglobin can induce ferroptosis, which is a form of non-apoptotic cell death associated with iron and characterized by lipid peroxidation (Dixon et al., 2012). Dissociation of myoglobin into free heme or iron results in additional forms of toxicity, as described for hemoglobin.

We hypothesize that mPI will have Mb-dependent pathologies, and that introducing Mb-knockout or iron chelation therapy will partially normalize the mPI pathologies. Deferoxamine (DFO), also known as desferrioxamine or desferoxamine, is an FDA-approved small-molecule iron chelator that improves iron overload syndromes (Velasquez and Wray, 2022; Karnon et al., 2012). DFO binds free iron and heme at a 1-to-1 ratio (Velasquez and Wray, 2022), scavenges free radicals (Reeder et al., 2008; Morel et al., 1992), reduces ferryl myoglobin to ferric myoglobin (Plotnikov et al., 2009; Vanek and Kozhli, 2022), inhibits crosslinking of heme to protein (Reeder and Wilson, 2005), and prevents the formation of pro-oxidant globin and heme species. DFO can function as an activator of Hif1α (Tchanque-Fossuo et al., 2017; Xiao et al., 2013), a tool for promoting angiogenesis (Duscher et al., 2015; Holden and Nair, 2019), an antioxidant (Sundin et al., 2000), or can join an anti-ischemic cocktail (Soloniuk et al., 1992). DFO appears in hundreds of studies of ischemic or inflammatory pathologies. In our study, subcutaneous DFO is used for testing the hypothesized role of myoglobin iron and as an anti-DAMP therapy for combating local iron overload.

Assessing the contribution of myoglobin iron to pressure injury pathophysiology provides an opportunity to test several additional hypotheses about pressure injuries. First, our prior work in mathematical modeling (Jagannathan and Tucker-Kellogg, 2016) predicted that oxidative stress from myoglobin and other DAMPs could create secondary progression of pressure ulcers. Secondary progression means that otherwise viable tissue dies later from the environmental consequences of injury, rather than dying directly from the original injury. Pressure injuries are known to have gradual expansion of tissue death (Stadler et al., 2004), consistent with secondary progression, but blocking secondary progression has not been clinically recognized as a goal for intervention (European Pressure Ulcer Advisory Panel et al., 2019). Therefore, our studies are designed to test whether tissue margins can be saved from dying, if we initiate iron chelation therapy 12 hr after pressure has ended. Second, we hypothesize that iron chelation therapy, by improving the early stages of injury response, will lead to better muscle tissue architecture (better morphogenesis) in long-term regeneration, even after treatment has ended. This hypothesis will be tested by breeding inducible fluorescence into satellite cells (muscle stem cells bearing Pax7); this fluorescence causes newly regenerated muscle fibers light up against the dark background of pre-existing muscle. Third, establishing a pressure injury mouse model provides an opportunity to learn how much of the poor healing is independent of comorbidities and complications. To the best of our knowledge, pressure injuries have never been assessed for poor regeneration under aseptic conditions in young, healthy animals. We hypothesize that even under these ideal circumstances mPI will heal slowly and incompletely. Our fourth and final additional hypothesis is inspired by prior studies of blood-related conditions in which high levels of hemoglobin or heme/hemin could impair the survival, chemotaxis, and phagocytosis (Nader et al., 2020; Ferris and Harding, 2019; Sindrilaru et al., 2011; Ballart et al., 1986; Liu et al., 2019; Martins et al., 2016; Chen et al., 2009; Yefimova et al., 2002) of phagocytic cells. Given that pressure ulcers often have slough or eschar, we hypothesize that necrotic tissue will persist in the mPI wound bed, and that sterile mPI will have slough, despite the absence of bacterial biofilm. If correct, this would imply that slough by itself is not sufficient to indicate infection (or bacterial colonization) of a wound.

Results

Magnet-induced pressure injury causes delayed healing and failure of muscle regeneration

To compare wound healing between acute and chronic wounds, we injured the dorsal skinfold of mice using either cardiotoxin (CTX) or pressure (Figure 1—figure supplement 1A–C) in healthy young adult mice under specific pathogen-free conditions. Both groups of mice received sham treatment (injected with 0.9% saline subcutaneously for 16 d or until mouse sacrifice, whichever was sooner). The normal uninjured mouse skinfold contains the following parallel layers: a thin epithelium (epidermis), a thicker layer of dermis, dermal white adipose tissue (dWAT), a muscle layer called the panniculus carnosus (PC), and a wavy layer of loose areolar tissue (Figure 1—figure supplement 1D). Despite comparable diameters of dead muscle between CTX and mPI at day 3, the wound diameters were vastly different at day 10 (Supplementary file 1a; p=0.578 at day 3 and p<0.0001 at day 10). In the CTX injury at day 3, many blood vessels were intact and carrying red blood cells, but in mPI at day 3, intact vasculature was not observed in the compressed region (Figure 1—figure supplement 2). Prior work showed that pressure affects blood and lymphatic vessels in many ways including potentially loss of flow (Gray et al., 2016; Kimura et al., 2020; Karahan et al., 2018). After CTX killed the panniculus muscle, substantial muscle regeneration occurred by 10 d, in which immature muscle fibers displayed central nuclei (Figure 1A). At 40 d after acute injury, muscle was completely regenerated and mature (evidenced by peripherally located nuclei, Figure 1C). In contrast, the pressure-injured wound bed remained filled with dead tissue at day 10 (Figure 1B and E–F; p<0.0001). Our pressure injuries showed no signs of infection and no epibole (Figure 1E). The dead epidermis, dermis, dWAT, and panniculus layers were pushed upward at day 7 ± 2 as slough (necroslough, per the nomenclature of Nasser, 2019) and remained at the surface, eventually becoming a dry eschar (Figure 1E). When the eschar dropped off (by day 15), the size of the epithelial opening was smaller than the eschar, meaning that re-epithelialization had occurred underneath the eschar. Re-epithelialization completed at day 21 ± 2. Despite successful closure of the epithelial layer, pressure injuries at 40 d had only partial regeneration of the panniculus carnosus layer (Figure 1D and G; p<0.0001). At 90 d after pressure injury, the dermis and epidermis had regenerated, but a hole remained in the panniculus muscle layer (Figure 1H), indicating a failure to regenerate (Nasir et al., 2022). Supplementary file 1b summarizes the timelines. We conclude that the mPI healed poorly.

Figure 1 with 2 supplements see all
Poor regeneration of muscle pressure injury (mPI), a model of chronic wound, compared with cardiotoxin (CTX), a model of acute injury.

H&E-stained sections of saline-treated wound tissues (A, B) at day 10 post-injury and (C, D) day 40 post-injury. Red arrows point to the panniculus carnosus layer. ‘+’ indicates where the panniculus layer should be. Scale bars: 50 µm. Uninjured control tissue, stained with H&E, is shown in Supplementary file 1C. (E) Full cross-section of H&E-stained mPI, including uninjured edges, at day 10 post-injury. Note the eschar attached to the wound surface. Scale bar: 500 µm. (F, G) Comparison of the regeneration scores for the panniculus layer between mPI and CTX injuries at days 10 and 40. (H) A multi-channel confocal image of the panniculus layer shows a round hole remains in the muscle, 90 d after mPI. Scale bar: 1000 µm. All injuries are saline-treated to permit comparison against later mPI treatments. All quantitative data are reported as means ± SD. n = 4–7. ****<0.0001 Statistical significance in (F) and (G) was computed by unpaired Student’s t test.

Compressed regions of pressure injury display an absence of viable immune cells

To investigate why dead tissue remains at day 10, we studied tissue sections from day 3 post-injury. Both CTX-induced and pressure-induced injuries had comprehensive death of muscle tissue, as indicated by karyolysis (dissolution of nuclear components), karyorrhexis (fragmentation of the nucleus), and acidification (eosinification) in H&E staining of the panniculus carnosus (Figure 2B). CTX and pressure injuries had a difference in morphology: cells in pressure-injured tissues were flattened, and the thickness of the panniculus muscle layer was half of uninjured (Figure 2A–C; p<0.0001). Even more striking was the difference in immune cell numbers. The muscle layer of CTX wounds had sixfold higher levels of immune cell infiltrate than mPI (p<0.0001; Figure 2A–B and D). The panniculus layer of mPI was nearly devoid of intact immune cells. Some viable immune cells were found at the margins of the wound (at the boundary between injured and uninjured tissue), but not in the compressed region of mPI (Figure 2—figure supplement 1). The absence of immune cell infiltrate is noteworthy because iron-scavenging is performed by macrophages (Kristiansen et al., 2001; Soares and Hamza, 2016), and because free iron, when not adequately scavenged, can overstimulate the innate immune response (Bessman et al., 2020).

Figure 2 with 2 supplements see all
Early-stage pathologies in the injury-response of muscle pressure injury (mPI) (3 d after injury).

(A, B) H&E-stained sections of saline-treated wound tissues, comparing magnet-induced mPI versus cardiotoxin (CTX) injury. Red arrows point to the panniculus carnosus (PC) layer. (C) Thickness of PC layer. (D) Histopathology scoring of immune infiltrate into the injured PC layer. (E, F) Immunostaining for citrullinated histone-3 (CitH3, a marker for extracellular traps, in red) in mPI versus CTX. DNA/nuclei were co-stained blue with DAPI. (G) Quantification of CitH3 staining. (H, I) Perls’ Prussian blue iron staining (dark blue-gray), with nuclear fast red co-stain. (J) Quantification of Perls’ staining. (K, L) Immunostaining for heme oxygenase-1 (HO-1, a marker of heme and iron) in mPI versus CTX. Note that the HO-1-positive signal in CTX injury is localized to the panniculus carnosus and is widespread across all layers in mPI. (M) Quantification of HO-1 staining. Scale bars: 50 µm. All quantitative data are reported as means ± SD. n = 4–7 mice. *<0.05, ****<0.0001. Statistical significance in (C, D), (G), (J), and (M) was computed by unpaired Student’s t test.

Another difference between mPI and CTX was in the level of citrullinated histone-3 (citH3), a marker of extracellular traps (ETs). ETs are formed when phagocytes citrullinate their histones and eject nuclear and/or mitochondrial DNA (and associated factors), which can trap and kill pathogens during infection. The process, ETosis, often kills the host cell. ETs have been observed in sickle cell ulcers (Chen et al., 2014). In day 3 wounds, levels of citH3 were tenfold higher in mPI than in CTX (Figure 2E–G; p=0.0280). Highest levels occurred near the muscle layer, such as the interface between the panniculus layer and the dWAT or dermis. This is consistent with the possibility that immune cells may have been present in the muscle layer of mPI before day 3 and then died of ETosis. Oxidative stress is a well-studied trigger of ETosis, and other stimuli include hemin and heme-activated platelets (Okubo et al., 2018; Ohbuchi et al., 2017).

Free iron remains in wound tissues after pressure injury

Iron deposition was very high in mPI, as measured by Perls’ Prussian blue stain (Figure 2I), but iron was undetectable at the same time point after CTX injury (Figure 2H–J; p=0.0332). Prussian blue detects accumulation of ferric Fe3+, typically in the form of ferritin and hemosiderin, but it only shows high levels of iron, because it is unable ‘to detect iron except in massive deposition’ (Perl and Good, 1992; Liu et al., 2014). The blue speckles in Figure 2I are concentrated iron deposits in the extracellular matrix, and the blue ovals are iron-loaded immune cells (Kristiansen et al., 2001; Soares and Hamza, 2016). Levels of myoglobin and hemoglobin were also elevated in mPI compared to CTX injury (Figure 2—figure supplement 2; p=0.0091 for myoglobin and p=0.0474 for hemoglobin).

Heme oxygenase-1 (HO-1) is an enzyme that performs heme degradation and serves as a marker of high heme or iron. HO-1 was expressed by mPI wound tissues, at similar levels to CTX-injured tissue (Figure 2K–M; p=0.998). However, HO-1 expression was localized to the panniculus layer after CTX injury, but it was widespread across all layers after mPI (Figure 2K–L). Levels of hemopexin and haptoglobin (iron detoxification factors) are shown in Figure 2—figure supplement 2.

Myoglobin knockout mPI have less tissue death and greater immune infiltrate

To measure the contribution of myoglobin iron to mPI pathogenesis, we developed myoglobin knockout mice (Mb−/−) via CRISPR deletion of the entire gene from the germline. Note that prior studies of adult Mb−/− mice found no obvious phenotype (Garry et al., 1998; Gödecke et al., 1999; Meeson et al., 2001). Mb−/− is often lethal to cardiac development during E9.5-E10.5, but some Mb−/− embryos survive to term (Meeson et al., 2001). Among our Mb−/− that were born, all developed with normal feeding, weight gain, grooming, social behavior, and life span. Deletion of Mb was confirmed by western blotting (Figure 3—figure supplement 1A), immunostaining (Figure 3—figure supplement 1B and C), and DNA gel electrophoresis (Figure 3—figure supplement 1D). With H&E staining, we detected no knockout-induced changes to the tissues of our injury model (skin, panniculus carnosus layer, or loose areolar tissue) other than increased capillary density (by 17%, p=0.0455; Figure 3—figure supplement 1E) and increased thickness of the dWAT layer in Mb−/− mice (by 43%, p=0.0232, Figure 3—figure supplement 1E). Total iron was not significantly decreased (p=0.0664, Figure 3—figure supplement 1F).

We compared pressure ulcers in Mb−/− versus Mb+/+ mice (Figure 3—figure supplement 1G) using elderly 20-month-old animals. (The mPI in elderly were similar to mPI in young, except with milder increases in pressure-induced oxidative damage, Figure 3—figure supplement 2.) At day 3 post-injury, Mb+/+ mice had high levels of iron (Figure 3A), which appeared in the muscle, dWAT, and dermis, including both the extracellular space and in the small numbers of infiltrating immune cells. In contrast, Mb−/− mice had no detectable signal from Perls’ stain in any layer of the wound (Figure 3B and C; p=0.0020). HO-1 was also decreased by 57% in Mb−/− mPI compared to Mb+/+ (Figure 3D–F; p=0.0438). Levels of immune cell infiltrate were 233% greater in Mb−/− compared to Mb+/+ (Figure 3G–I; p=0.0250).

Figure 3 with 2 supplements see all
Myoglobin knockout decreased iron deposits and tissue death after muscle pressure injury (mPI).

(A, B) Perls’ Prussian blue iron staining. (A) Note that iron deposits in the extracellular space and in immune cells of Mb+/+ wound tissue, and note that (B) Mb−/− tissues have no iron deposits in extracellular regions. (C) Quantification of Perls’ staining. (D, E) Immunostaining for HO-1 in Mb+/+ versus Mb−/− tissues at day 3 after mPI. Note that HO-1 is elevated in all layers of Mb−/− except epidermis. (F) Quantification of HO-1 immunostaining. (G, H) H&E-stained sections of Mb+/+ versus Mb−/− mPI. Paraffin-embedded wound sections derived from elderly Mb-wildtype mice had poor cohesiveness (compared to elderly Mb-knockout or young) and exhibited greater cracking during identical sample handling. (I) Amount of immune infiltrate, quantified by histopathology scoring on a scale of 0–5, performed on day 3 sections. (J) External wound area in Mb+/+ and Mb−/− mice in the initial days following mPI from 12 mm magnets. Statistical analysis compared four wounds from two age- and sex-matched animals using a Student’s t test for each day. Consistent with these results, Supplementary file 1C shows additional animals treated with different-sized magnets. (K) Tissue death in the PC muscle layer by histopathology scoring (3 indicates pervasive death). All quantitative data are reported as means ± SD. n = 3 mice. *<0.05, **<0.01. Statistical significance in (C), (F), and (I–K) was computed by unpaired Student’s t test.

The wound size was smaller in Mb−/− versus Mb+/+ (p=0.0297 at day 3, measured as external area, Figure 3J and Supplementary file 1c). Histopathology scoring in the center of the wound showed 50% decreased tissue death (p=0.0016, Figure 3G, H and K). Oxidative damage was lower in Mb-knockout wounds: DNA damage (8-hydroxy-2’-deoxyguanosine [8-OG]) was decreased by 87% (p=0.0485; Figure 4A–E), and lipid peroxidation (measured using BODIPY 581/591) was decreased by 61% (p=0.0298; Figure 4F–J). BODIPY 581/591 is a fatty acid analogue with specific sensitivity to oxidation (Drummen et al., 2002). When oxidized, its emission fluorescence shifts from 595 nm (red) to a maximal emission at 520 nm (green). The green fluorescence is what is shown in the figure. Similarly, Mb−/− had roughly 56% decrease in CitH3 (p=0.0127; Figure 4K–Q). These improvements in the wound microenvironment extended beyond the muscle layer because Mb+/+ wounds had high levels of BODIPY in the dWAT and dermis, and high levels of CitH3 throughout the wound. An additional measure of oxidative damage, 3-nitrotyrosine, showed the same pattern (56% decrease, p=0.0328; Figure 4—figure supplement 1). In summary, the wounds had less oxidative stress when myoglobin was absent. We also measured the impact of Mb on a monocyte/macrophage cell line in vitro. After pre-incubation with pro-inflammatory stimuli, RAW264.7 cells showed an increase in ROS and decrease in phagocytosis (measured as impaired efferocytosis or clearance of dead cells) in response to Mb treatment (Figure 4—figure supplement 2). A panel of cytokines, chemokines, and growth factors were measured in muscle homogenates of mPI from Mb−/− versus Mb+/+ at day 3 (Supplementary file 1d). There were no significant differences except the knockout wounds had lower levels of CXCL16 (a cytokine associated with lipid peroxidation Ma et al., 2018, Supplementary file 1d; p=0.0110) and higher levels of PAI-1 (also called Serpin E1, a protease inhibitor associated with TGFβ, Supplementary file 1d; p=0.0080). There were no significant differences in total protein between Mb−/− and Mb+/+.

Figure 4 with 2 supplements see all
Myoglobin knockout caused a more hospitable wound environment in muscle pressure injury (mPI).

(A–D) Immunostaining of 8- oxaguanine (8-OG; in magenta) in Mb+/+ versus Mb−/− mPI. Nuclei were co-stained blue with DAPI. (B) and (D) are brightfield images of (A) and (C), respectively. (E) Quantification of 8-OG staining. (F–I) BODIPY staining (for lipid peroxidation) in Mb+/+ versus Mb−/−. (G) and (I) are brightfield images of (F) and (H), respectively. (J) Quantification of BODIPY staining. (K–P) Immunostaining for CitH3 (in red) in Mb+/+ versus Mb−/−. (L, O) DNA/nuclei were co-stained blue with DAPI. (M) and (P) are brightfield images of (K) and (N), respectively. (Q) Quantification of CitH3 staining. Scale bars: 50 µm. Blue dashed lines refer to mean fluorescence intensities for uninjured dorsal skinfolds. All quantitative data are reported as means ± SD. n = 3 mice. *<0.05. Statistical significance in (E), (J), and (Q) was computed by unpaired Student’s t test.

We next sought an orthogonal intervention to test the causal role of myoglobin iron. The FDA-approved iron chelation drug DFO was administered via injection under the dorsal skinfold of 5-month-old mice, starting the morning after pressure induction finished, and repeated twice daily for up 16 d (Figure 5A). DFO- or saline-treated tissues were analyzed at 3, 7, 10, 40, and 90 d (Supplementary file 1e). The same cohort of saline-treated mPI were compared against saline-treated CTX in Figures 1 and 2 (for days 3, 10, and 40 post-injury) and compared against DFO-treated mPI in Figures 5 and 6.

Figure 5 with 2 supplements see all
Deferoxamine (DFO) iron chelation therapy decreased iron deposits at day 3 after muscle pressure injury (mPI).

(A) Experimental schedule shows confetti tamoxifen induction of fluorescence, mPI induction, treatment with DFO or saline, and tissue harvest. (B) Diameter of the dead region of the panniculus carnosus (PC) muscle in tissue sections from DFO- versus saline-treated mPI, 3 d post-injury. (C) Perls ’ Prussian blue iron staining of DFO-treated wounds at the wound center. (D) Quantification of Perls ’ staining, showing comparison against saline-treated mPI from Figure 2I. (E) Immunostaining of HO-1 in DFO-treated mPI. (F) Quantification of HO-1 staining, showing comparison against saline-treated mPI from Figure 2L. (G) H&E-stained sections of DFO-treated mPI at the wound center. (H) Histopathology scoring of immune infiltrate at all layers of the wound center at day 3, comparing DFO-treated versus saline-treated mPI, which appear in Figure 2B and (D). (I) Confirmation that injuries were properly created, according to death of PC tissue at the center of the wound (histopathology scoring where 3 indicates pervasive tissue death). (J) Skin wound area following mPI in 5-month-old Mb+/+ saline and DFO-treated mice, and in 20-month-old Mb+/+ and Mb-/- saline-treated mice. Scale bars: 50 µm. Blue dashed lines refer to histology scores and mean fluorescence intensities for uninjured dorsal skinfolds. All quantitative data are reported as means ± SD. n = 6–7 mice. *<0.05, **<0.01. Statistical significance in (B), (D), (F), and (H–J) was computed by paired Student’s t test.

Figure 6 with 3 supplements see all
Deferoxamine (DFO) treatment improved the muscle pressure injury (mPI) microenvironment at early time point (day 3).

(A–D) Immunostaining of 8-OG (for DNA damage) in saline-treated versus DFO-treated mPI. Nuclei were stained blue with DAPI. (B) and (D) are brightfield images of (A) and (C), respectively. (E) Quantification of 8-OG. (F–I) BODIPY staining (for lipid peroxidation) and brightfield in saline- versus DFO-treated mPI. (J) Quantification of BODIPY. (K) CitH3 immunostaining (L) with DNA/nuclear co-stain and (M) brightfield in DFO-treated mPI at day 3. (N) Quantification of CitH3 staining in DFO-treated versus the saline-treated mPI, which was analyzed in Figure 2F and G. Scale bars: 50 µm. Blue dashed lines refer to mean fluorescence intensities for uninjured dorsal skinfolds. All quantitative data are reported as means ± SD. n = 6 mice. *<0.05, **<0.01, ***<0.001. Statistical significance in (E), (J), and (N) was computed by paired Student’s t test.

Effects of iron chelation therapy on secondary progression of the wound

Remarkably, DFO treatment caused a decrease in the amount of muscle tissue that died from the initial pressure injury, even though the pressure inductions were identical, and treatments did not begin until 12 hr after the last cycle of pressure. That is, intervention only started after deformation injury and reperfusion injury had already occurred. At day 3 of treatment, DFO-treated wounds had 35% smaller diameter of dead PC tissue (Figure 5B; p=0.0043). This observation of secondary progression confirms a prediction of our prior computational modeling (Jagannathan and Tucker-Kellogg, 2016). We also observed a decrease in apoptotic debris (measured as TUNEL staining, Figure 5—figure supplement 1; p=0.0105), which might reflect a decrease in apoptotic death and/or an increase in phagocytosis/efferocytosis.

Effects of iron chelation therapy on injury response at day 3

Consistent with decreased death, DFO-treated mPI displayed 77% lower levels of iron deposition by Perls’ Prussian blue (Figure 5C and D; p=0.0230) compared to saline control (Figure 2I) at day 3. As in the elderly mPI, the young saline-treated mPI had iron accumulations in the extracellular space and immune cells of multiple layers, not just the panniculus muscle layer (Figure 2I). Similarly, levels of HO-1 were 65% decreased (Figure 5E and F and Figure 2L; p=0.0071). To confirm that wounds were properly induced, Figure 5G–I shows complete tissue death at the centers of the wounds in all animals. Re-epithelialization was not affected by the subcutaneous drug (Figure 5J and Figure 5—figure supplement 2).

DFO-treated wounds displayed less oxidative damage, as indicated by a 41% decrease in 8-OG (p=0.0048; Figure 6A–E) and 50% decrease in BODIPY 581/591 (p=0.0004; Figure 6F–J) at day 3. Change in tyrosine nitration was not significant (p=0.965; Figure 6—figure supplement 1). CitH3 dropped to undetectable levels in DFO-treated sections (p=0.0105; Figure 6K–N and Figure 2F). The high CitH3 found in control wounds was partially co-localized with F4/80, a pan-macrophage marker (Figure 6—figure supplement 2), but these markers may reflect debris or noncellular localizations. Myeloperoxidase, a marker of neutrophil extracellular traps (NETs/NETosis), was significantly higher in saline-treated mPI compared to CTX (p=0.0173), but showed similar levels to DFO-treated mPI (p=0.946; Figure 6—figure supplement 3). Supplementary file 1f shows additional measurements of cytokines, chemokines, and growth factors in DFO- versus saline-treated mPI at day 3, and Supplementary file 1g shows the same analytes for day 10. These analytes showed no significant differences between DFO and saline treatments, except for CXCL16 levels, which decreased by 36% (p=0.0174) at day 3. Other differences between saline- and DFO-treatment are shown in Figure 6—figure supplement 3.

Effects of iron chelation therapy on immune infiltration and regeneration

At 7 d post-injury, DFO-treated tissues displayed 156% greater abundance of immune infiltrate, according to brightfield and DAPI staining (p=0.0273; Figure 7C–D and L). Co-staining with iNOS showed strong co-localization with the infiltrate. iNOS is a marker of pro-inflammatory stimulation in many cell types, and it showed a 43% increase after DFO treatment (p=0.0183; Figure 7A–D and K). CD38, a marker of CD4+, CD8+, B, and natural killer cells, was not significantly different between saline- and DFO-treated mPI (p=0.0941; Figure 6—figure supplement 3). On the other hand, Mer tyrosine kinase (MerTK) exhibited a 170% increase in expression in DFO-treated tissues (p=0.0189; Figure 7—figure supplement 1). MerTK is a marker of monocytes and other specialized cell types. It has been shown to promote macrophage survival (Anwar et al., 2009) and phagocytic function (specifically efferocytosis) (Scott et al., 2001).

Figure 7 with 2 supplements see all
Deferoxamine (DFO) treatment improved immune infiltration (7 and 10 d after muscle pressure injury [mPI]).

(A, B) Immunostaining of inducible nitric oxide synthase (iNOS, associated with pro-inflammatory activation) in saline- versus DFO-treated mPI at day 7 post-injury. (C, D) Nuclei are stained blue with DAPI to show the count of infiltrating cells. (E, F) Immunostaining of Arginase-1 (Arg-1) in saline- versus DFO-treated mPI at day 10 post-injury. (G, H) Nuclei are co-stained blue with DAPI. (I, J) Merged brightfield and Arg-1 immunostaining. (K) Quantification of iNOS staining at day 7 between saline- and DFO-treated. (L) Scoring of immune infiltrate into the injured tissue at day 7. (M) Scoring of immune infiltrate into the injured tissue at day 10. (N) Quantification of Arg-1 staining. (O) Count of DAPI nuclei. (P) Distance of tissue infiltrated by Arg-1+ cells in saline- versus DFO-treated tissues. Scale bars: 50 µm. Blue dashed lines refer to histology scores and mean fluorescence intensities for uninjured dorsal skinfolds. All quantitative data are reported as means ± SD. n = 4–7 mice. *<0.05, **<0.01, ***<0.001. Statistical significance in (K–P) was computed by paired Student’s t test.

Later at day 10, immune infiltrate was still elevated in DFO-treated wounds (Figure 7M, p=0.0014 for histopathology and p=0.0004 for count of DAPI nuclei). Arginase-1 (Arg-1) is a marker of regenerative activity and promotes polyamine biosynthesis for wound healing. Arg-1 levels were twofold higher in the wound bed at day 10 (p=0.0269; Figure 7N) and much of the day 10 infiltrate was positive for Arg-1 (Figure 7E–J and M–O). Arg1-positive cells showed 119% greater distance of infiltration into injured tissue (p=0.0061; Figure 7P) compared to saline-treated control. Granulation was also dramatically improved. Granulation is the regenerative stage of wound healing, characterized by angiogenesis (formation of new capillaries) and proliferation of epithelial, endothelial, and fibroblast cells, along with continued presence of immune infiltrate. Angiogenesis is widely reported to increase after DFO treatment (Duscher et al., 2015; Holden and Nair, 2019), and DFO-treated mPI had a similar trend, evidenced by a twofold increase in small blood vessels (p=0.0038; Figure 7—figure supplement 2A–G).

Quality and quantity of regeneration

DFO treatment improved the extent of muscle regeneration (Figure 8A–F and Figure 7—figure supplement 2H). Much of this improvement was complete by day 40 when treatment improved regeneration at the wound center (p=0.0345; Figure 8E) and wound edge (p=0.0121; Figure 8F and Figure 8—figure supplement 1). In saline-treated wounds, myoblastic cells were observed at 40 d (Figure 8A and C), indicating that muscle regeneration was still underway.

Figure 8 with 1 supplement see all
Deferoxamine (DFO) increased the extent of muscle regeneration at day 40.

(A, B) Confocal fluorescent and (C, D) H&E-stained cross-sections of regenerated muscle fibers from saline- and DFO-treated muscle pressure injury (mPI), 40 d post-injury. Scale bars: 100 µm. (E, F) Histological scoring of panniculus carnosus (PC) muscle regeneration in saline- and DFO-treated mPI versus acute cardiotoxin (CTX)-injured tissues at the center and edge of the wound, 40 d post-injury. Red dashed lines refer to PC regeneration scores at center and intermediate regions after CTX injury at day 40. All quantitative data are reported as means ± SD. n = 7 mice. *<0.05, **<0.01. Statistical significance in (E, F) was computed by paired Student’s t test.

When we extended the study to 90 d, no further increase in panniculus regeneration was detected (from 40 to 90 d) in saline-treated mPI. Compared to healthy regeneration (Figure 9A–C, CTX-injured), the newly regenerated myofibers in saline-treated wounds had pathological morphology, seen from branched or split fibers, thin fibers, wavy instead of straight axes, and disjoint angles between different bundles of fibers (Figure 9D–F). Newly regenerated myofibers were distinguishable from preexisting or never injured myofibers by their expression of fluorescent proteins because our mice expressed the confetti transgene in Pax7-positive satellite cells and their progeny (Nasir et al., 2022; Tierney et al., 2018). DFO-treated tissues displayed a far lower frequency (p=0.0021) of morphological malformations than saline-treated tissues (Figure 9G–J). At the day 90 endpoint, DFO-treated wounds had significantly smaller gaps in the original region of muscle (46% smaller area of unregenerated muscle, p=0.0087; Figure 9K) compared to saline-treated.

Deferoxamine (DFO) improved muscle morphology.

Confocal microscopy of confetti-labeled muscle regeneration in ex vivo tissue blocks. Note that unlabeled tissue types such as blood vessels and hair create black shadows on top of the images. (A–C) Left column shows healthy regeneration in the control condition, toxin-induced injury, at 40 d. Experiment was halted at 40 d for cardiotoxin (CTX) only due to complete regeneration. (D–F) Middle column shows regenerated muscle fibers of saline-treated muscle pressure injury (mPI), 90 d post-injury. Note the presence of non-parallel fibers, bent fibers, and split fibers (i.e., fibers with one or more branches). (G–I) Right column shows regenerated muscle fibers of DFO-treated mPI, 90 d post-injury. Scale bars: 100 μm. (J) Quantification of muscle fiber malformations. (K) Diameter of gap (unregenerated region) in panniculus carnosus (PC) muscle layer at day 90 post-mPI between saline- and DFO-treated wounds. All quantitative data are reported as means ± SD. n = 4–5 mice. **<0.01, ***<0.001, ****<0.0001. Statistical significance in (J, K) was computed by one-way ANOVA with Tukey’s post hoc test.

Discussion

Our study established a mouse model of mPI that exhibited poor healing in the absence of any infection, hyperglycemia, or old age. With blood vessels collapsed or disrupted, the mPI had dead tissue persisting in the wound bed for more than a week, and regeneration was impaired. The debris field also contained high accumulations of iron, which motivated us to study myoglobin-knockout mice and iron chelation therapy. These experiments showed that the iron was myoglobin-dependent, and the myoglobin iron caused oxidative damage. The damage was consistent with lipid peroxidation, ETs, and ferroptosis. Finally, we showed that administering DFO after pressure caused decreased oxidative stress, decreased death of tissue margins, and improved muscle regeneration. The numbers of intact immune cells in mPI were positively correlated with regeneration and negatively correlated with oxidative damage, suggesting that excessive inflammation was not the primary pathology causing poor healing in these wounds.

Induction of mPI used a standard magnet method in mice, but instead of studying longitudinal re-epithelialization, we used tissue section timepoints to study the thin muscle layer of panniculus carnosus, also called the cutaneous muscle or the panniculus layer. This muscle regenerated slowly and incompletely from the magnet injury (Figure 1H), even under ideal conditions of youth, health, and lack of microbial pathogens. A hole remained in the panniculus layer at 40 d (in seven out of seven mice) and 90 d (in five out of five mice). For example, in Figure 1D, note that the adipose layer is immediately adjacent to loose areolar tissue at the wound center, indicating that the panniculus layer is absent. Thus, we obtained a normoglycemic noninfected animal model of impaired healing.

Mb knockout tested the hypothesis that myoglobin iron contributes to the pathologies of mPI. This hypothesis is based on analogy to poor-healing wounds that have hemolysis and poor drainage of hemoglobin (e.g., venous stasis), and also inspired by papers showing myoglobin is detectable in distal fluids after pressure ulcers (Traa et al., 2019; Levine, 1993; Traa, 2019). As expected, the iron deposits seen after wildtype mPI injury were absent from Mb−/−. Similarly, Mb−/− had far less oxidative damage. Because Mb normally serves to supply oxygen in muscle tissue, one might expect Mb−/− mice to experience increased tissue death due to the hypoxia of ischemia, but prior work showed Mb−/− mice are ‘surprisingly well adapted’ to hypoxic conditions (Schlieper et al., 2004), capable of withstanding adrenergic stimulation (Meeson et al., 2001), exercise, and oxygen flux (Gödecke et al., 1999). In the mPI context, carrying wildtype Mb caused roughly twice as much tissue death as Mb−/−. Some of that difference might be explained by developmental compensation (Gödecke et al., 1999) (e.g., 17% increase in capillary density), but we interpret that a large fraction of tissue death from mPI was downstream of Mb. Multiple other DAMPs including hemoglobin might have contributed to the iron accumulation and oxidative damage seen in the Mb+/+ case, but removing Mb was sufficient to alleviate the overload.

The region of myoglobin-related damage extended beyond muscle. Given the potential toxicity of myoglobin during ischemia-reperfusion (Garry et al., 1998; Gödecke et al., 1999; Meeson et al., 2001; Hazarika et al., 2008; Meisner et al., 2013), we were not surprised that Mb knockout caused greater survival of muscle. However, we were surprised that most other layers of the wound improved too. Some types of ROS created by iron are highly reactive (e.g., hydroxyl radical from the Fenton reaction), so they act locally where they are created. Long-distance damage might arise from transport of the globin iron, or from milder ROS species, or both. Given the large effect sizes seen in the Mb−/− and DFO experiments, and given the small amount of muscle in mPI, we suspect that unappreciated mechanisms of amplification or vulnerability might be downstream of myoglobin iron in the mPI context.

Clearance of myoglobin iron can be seen in CTX injuries, which had destruction of muscle but no accumulation of iron, according to Perls’ stain (Figure 2J). CTX and naniproin are snake venoms that cause myolysis (cytolysis) of myofibers (Guardiola et al., 2017; Harvey et al., 1982), and have been reported to spare nerve and vessel viability at the concentrations used (Averin and Utkin, 2021; Wang et al., 2022). One presumes that myolysis would cause myoglobin release, but continued function of the circulatory system may have allowed the myoglobin iron to be transported, diluted, and/or detoxified below the detection limit of Perls’ stain. In contrast, levels of iron in mPI were greater than the detection limit for Perls’ stain (Figure 2I), and many dysfunctions ensued. Pressure has been reported to cause disruption to arterial supply, venous clearance, and lymphatic drainage (Gray et al., 2016; Kimura et al., 2020; Karahan et al., 2018), although not always (Stekelenburg et al., 2006). Lymph vessels are occluded at very low pressure, and impaired lymphatic flow can persist after pressure is removed (Worsley et al., 2020). Impaired circulation and/or impaired drainage could cause waste factors such as myoglobin or iron to accumulate in the mPI, and could impair the influx of iron detoxification factors. Poor drainage also occurs in chronic ulcers of venous insufficiency, lymphedema, and other vascular diseases. Toxicity from globin iron might be a shared pathology among these wounds. mPI research might also benefit from analogy to known globin overload syndromes such as compartment syndrome, rhabdomyolysis, and hemoglobinopathies.

Pressure injury prevention usually refers to pre-injury prevention, which is to intervene with cushioning or turning before prolonged pressure occurs (Sundin et al., 2000). What we studied is post-pressure prevention because we asked whether tissue could be saved from dying by interventions beginning 12 hr after pressure had finished. In healthy young mice, magnets were placed and removed in 12 hr cycles as part of a 2-d process of creating a pressure ulcer. (See the detailed timeline in Figure 5A.) Drug injections began on the third day after pressure began, which we refer to as day 1 post-injury. The remarkable observation (Figure 5B) is that the amount of death on day 3 post-injury was not constant across the tissue sections, even though the induction of the pressure ulcers was identical. mPI that received post-pressure DFO showed significantly smaller diameters of muscle death than saline-treated comparisons. Our quantification used cross-sectional tissue slides, but a 35% decreased diameter of death equates to a 58% smaller circle area of death. We conclude that there is window of opportunity to intervene and save viability of tissue, after the established mechanisms of injury have occurred (such as mechanical, hypoxic, reoxygenation, and nutrient stress), but before the full extent of secondary progression and DAMP-induced stress have propagated. In this mouse model, the opportunity occurred between 12 hr and 72 hr after off-loading.

Secondary progression of wounds is well documented in thermal burns, where post-burn treatment can lessen the progression of partial-thickness burns toward full-thickness. However, secondary progression has not, to the best of our knowledge, been previously targeted for medical intervention in pressure ulcers (European Pressure Ulcer Advisory Panel et al., 2019). Mechanisms of secondary progression may include oxidative stress and ferroptosis (Li et al., 2020; Li et al., 2021), and we cannot rule out reperfusion injury, platelet activation, ETs, and DAMP-induced apoptosis/necroptosis (Jagannathan and Tucker-Kellogg, 2016).

CitH3, a marker of ETs, was elevated in the high-iron conditions (young and elderly mPI) and low in the three conditions that lacked concentrated deposits of iron (CTX, knockout, DFO-treated). This is consistent with the ability of heme, oxidative stress, or heme-activated platelets to trigger ETs (Okubo et al., 2018; Ohbuchi et al., 2017). ETs can aid in antimicrobial defense, but they are detrimental to regeneration and contribute to many disorders of sterile inflammation. Our induction of CitH3 was likely sterile as well: the mouse environment was negative for 39 categories of pathogen (Supplementary file 1h, including common dermal microbiota Pseudomonas aeruginosa and Staphylococcus aureus), and no bacteria could be detected in or near the wound by Gram staining.

The persistence of dead tissue was particularly striking in our noninfected mPI wound. Viable immune cells were essentially absent from the pressure-injured muscles of young healthy mice at day 3. The wounds exhibited a normal spike in immune cell infiltration outside the wound margins, moving toward the wound (Figure 2—figure supplement 1), but intact infiltrate was not seen inside the compressed region, almost as if infiltration had been halted at the boundary. Initially we questioned whether the compressed architecture of the tissue might have occluded chemotaxis, but closer inspection revealed dead fragments and markers such as CitH3 in the compressed region. Our interpretation is that immune cells (tissue-resident and/or infiltrated) may have been present but then died. We do not know whether this death was necrotic or programmed. Possible explanations for the persistence of dead tissue, in addition to poor cell survival, are decreased migration and decreased efferocytosis/phagocytosis. Diseases characterized by the presence of plaque (atherosclerosis) and ectopic tissue (endometriosis) have impaired function of phagocytes (Liu et al., 2019; Schrijvers et al., 2005). Other syndromes with iron overload have poor function, migration, and/or survival of phagocytes (Kao et al., 2016; Van Asbeck et al., 1984; Porto and De Sousa, 2007). For example, in patients with thalassemia major, neutrophils and macrophages exhibit poor chemotaxis, defective lysis, and impaired phagocytosis (Ballart et al., 1986; Martins et al., 2016; Cantinieaux et al., 1987).

Intact immune cells were present at days 7 and 10 of mPI. DFO increased their abundance, especially in the loose areolar tissue, but also in muscle, fat, and skin (Figure 7L and M). One theory of nonhealing wounds is that excessive inflammation causes oxidative damage and blocks progress of granulation. Our mPI observations are not well aligned with that theory because DFO caused an increase in the immune infiltrate and iNOS staining, but it decreased the oxidative damage and the size of the hole. Another theory is that nonhealing wounds lack pro-regenerative M2 macrophages. Nothing observed about mPI in this first study would contradict that theory. Our work is also consistent with prior work that found iron-scavenging macrophages from an iron-loaded tissue displayed both inflammatory (iNOS, IFNγ) and alternately-activated (Arg1, IL10) markers (Ali et al., 2020). Prior studies also found that environmental iron can cause M2 macrophages to decrease (Handa et al., 2019) or increase (Agoro et al., 2018), depending on dose/context. Future studies can use our mPI timeline to design an immunobiology study that isolates mPI immune cells for detailed analysis.

Surface visibility of dead tissue was delayed until competent inflammation and granulation pushed it upward. This delay might give a false impression that slough was created by infection or inflammation. The slough detached spontaneously by day 15, but as it was pushed up, the slough remained attached to the healthy margins and bent the margins upward (Figure 1E and Figure 6—figure supplement 2). Since healthy margins organize the geometry of regeneration, distorting the margins might impair regeneration. Nonhealing wounds are sometimes believed to have excess proteolysis, but mPI raises the question of whether increased proteolytic activity might help prevent the extrusion process from distorting the healthy margins.

The toxicity of slough is a topic of clinical debate. In our mPI, re-epithelialization proceeded beneath the necro-slough and proliferative granulation tissue was adjacent to its center, so we conclude it was not cytotoxic. This is consistent with clinical practice to avoid debridement of an intact eschar unless there are signs of infection (Manna et al., 2021). Interestingly, our slough arose from a mass that was probably toxic when it formed. Toxicity at early timepoints is inferred from the oxidative damage at day 3 and indirectly from the phenomenon of secondary progression. It is possible that toxicity could disappear spontaneously if ROS are driven by an energy source that gets depleted (e.g., hydrogen peroxide, ATP, mitochondrial membrane potentials).

Muscle regeneration improved with DFO treatment, evidenced by smaller holes in the muscle layer at multiple timepoints, and improved muscle morphology at the endpoint. The limited muscle regeneration that did occur in mPI without DFO displayed frequent myofiber defects such as branching and nonparallel alignment. Split fibers are considered defective regeneration (Eriksson et al., 2006) and are especially vulnerable to reinjury (Pichavant et al., 2016). Loss of the panniculus muscle layer causes the skin and adipose to experience harsher stress and strain (Nasir et al., 2022; Soh et al., 2020), even though the panniculus is a thin component of the tissue. Previous studies of muscle pressure injury considered larger muscles (Ahmed et al., 2016; Siu et al., 2009; Wassermann et al., 2009; Salcido et al., 1994; Salcido et al., 2007), and the impact of DFO should be retesting in other geometries. The panniculus layer is highly relevant to human pressure injuries because humans have a panniculus layer at the heel (Nasir et al., 2022; Cichowitz et al., 2009), and the heel is one of the most common locations for high-stage pressure injuries.

Skin regeneration was not significantly affected by subcutaneous DFO (Figure 5J and Figure 5—figure supplement 2). Previous work found DFO administered via a transdermal patch accelerated skin closure (Duscher et al., 2015), but these studies used topical delivery, as well as using a different mouse background. Other work studied the antioxidant and angiogenic benefits of DFO in preclinical models of ischemia-reperfusion injury (González-Montero et al., 2018; Morris et al., 1993; Yang et al., 2020; Peeters-Scholte et al., 2003) and related conditions (Qayumi et al., 1994). We observe similar effects in muscle.

Caveats to this work include the following. Mouse wounds differ from human in several obvious ways, such as having denser hair follicle coverage, faster kinetics of DFO, different timing and ratio of phagocytic cells (Zomer and Trentin, 2018), and higher propensity for wound contraction. Although mice can heal skin wounds by contraction, our multilayer mPI exhibited minimal contraction, as seen from prior work (Nasir et al., 2022) and from the hole in the panniculus muscle layer at day 90. To decrease the impact of hair follicles on results, we placed magnets on regions of skin that were in the resting phase of the hair cycle. Another limitation of our study is the limited number of myoglobin knockout mice (n = 3) compared with n = 7 for key timepoints of Mb+/+ analysis. This was necessary because Mb germline knockout had high rates of embryonic lethality. Despite the smaller sample size, several readouts were statistically significant.

Conclusion

Myoglobin iron contributed to the size, severity, oxidative damage, and poor healing of mPI. DFO injections prevented death of tissue at the margins of the wound, when administered starting 12 hr after pressure was removed. Intervention to block secondary progression is a dramatic new opportunity for pressure injuries, like burns or strokes, to be stabilized shortly after injury, to decrease further loss of salvageable tissue. Iron chelation therapy has side effects when used systemically, so muscle wounds might be safer than vascular wounds for future research on human biodistribution (in conjunction with dressings and polymer scaffolds). Also, DFO led to higher quality of muscle, with straighter and more parallel muscle fibers, which should improve muscle viability. Healthy muscle is a crucial defense against wound recurrence, when a patient eventually puts weight on the same location again.

Methods

Mice

Animal experiments were approved by the institutional animal care and use committee (IACUC SHS/2016/1257) of SingHealth, Singapore. To conditionally label Pax7+ muscle satellite stem cells in C57BL6 mice, the Pax7-Cre-ERT2 mouse (JAX stock #012476, Jackson Laboratory, ME) was crossed with the Brainbow2.1 (confetti; JAX stock #017492, Jackson Laboratory) mouse. Pax7-Cre-ERT2 provides the Cre-ERT2 transgene downstream of the Pax7 stop codon, thereby limiting the Cre-ERT2 expression to Pax7+ cells. The Cre-ERT2 system provides tamoxifen-dependent induction of Cre recombinase, so that affected cells carry heritable rather than transient modification. Upon tamoxifen treatment, the Cre induction causes recombination of the confetti construct at its loxP loci, leading to gene expression of one of the four fluorescent proteins in the construct. The fluorescent proteins are mCerulean (CFPmem), hrGFP II (GFPnuc), mYFP (YFPcyt), and tdimer2 (Mendonça et al., 2016) (RFPcyt). CFPmem contains a localization sequence enabling its transport to the myofiber membrane (sarcolemma) while GFPnuc contains a nuclear localization sequence. YFPcyt and RFPcyt have no localization sequences and are expected to localize to the cytoplasm. Experimental mice include 5-month-old adult mice and 20-month-old elderly mice.

Myoglobin knockout mice (homozygous in the germline) were created by Cyagen (CA) using CRISPR/Cas9 nuclease-mediated genome editing (Qin et al., 2015; Qin et al., 2016). 20-month-old mice (n = 3) were used in the knockout experiments. The myoglobin knockout mice were not injected with tamoxifen.

Mice were euthanized via CO2 inhalation, followed by cervical dislocation, at timepoints of 3, 7, 10, 40, and 90 d following completion of the injury protocol (pressure or toxin). Pressure injury via magnetic compression created two wounds (right and left) on the dorsum of each mouse. Similarly, two CTX injections were performed on each mouse at the right and left dorsal skinfold. Tissue samples were isolated, and one wound tissue (from either pressure or toxin) was fixed in 4% paraformaldehyde at 4°C for 8 hr, and then embedded in paraffin. The remaining wound tissue was snap-frozen with isopentane in a liquid nitrogen bath.

Murine pressure injury model

Mice were shaved and remaining hair was removed by hair removal cream (Veet, Reckitt, Slough, England) prior to injury. Muscle pressure ulcers were created in 4–5-month-old transgenic mice by applying a pair of ceramic magnets (Magnetic Source, Castle Rock, CO, part number: CD14C, grade 8) to the dorsal skinfold. The magnets were 5 mm thick and 12 mm in diameter, with an average weight of 2.7 g and pulling force of 640 g. One of the elderly mice lacked sufficient skinfold for 12 mm magnets, so in that mouse and in its age- and sex-matched control, we used 5 mm magnets (neodymium magnets with a 1 mm spacer on one side; Liftontech Supreme Pte Ltd, Singapore). Because hair follicle (HF) stem cells contribute to wound healing, we attempted to minimize the HF differences by synchronizing the hair growth cycle and applying injury only to skin with HFs in telogen phase (non-pigmented skin). In the case where pigmented skin could not be avoided during magnet placement, that is, one of the two wounds fell on skin with active hair cycle, that half was excluded from further analysis. Pressure ulcer induction was performed in two cycles. Each cycle was made up of a 12 hr period of magnet placement followed by a 12 hr period without magnets (Figure 1—figure supplement 1). This procedure induces two pressure wounds on the back of the mouse, on the left and right side of the dorsal skinfold. The dorsal skinfold includes skin, adipose tissue, panniculus carnosus muscle, and loose areolar tissue. The mice were given an analgesic (buprenorphine at 0.1 mg/kg; Bupredyne, Jurox Animal Health, New Zealand) prior to magnet placement and again prior to magnet removal. Timepoints post-injury are measured from the end of last magnet cycle. No dressings were used, and no debridements were performed.

To treat the pressure injuries, mice were subcutaneously injected with DFO while control mice were injected with 0.9% saline (n = 7 each for day 3, 10, and 40 timepoints, n = 4 for day 7, and n = 5 for day 90; Supplementary file 1e). The study was initially conducted using 3, 10, and 40 d. When the day 7 and 90 timepoints were added to the study, we already knew that DFO treatment would give a large effect size. The updated effect size caused our power calculation to give a smaller sample size for days 7 and 90. The day 7 timepoint was added later because of the dramatic changes observed between days 3 and 10, and the day 90 timepoint was added later because day 40 mPI showed signs of ongoing regeneration (myoblastic cells and immature myofibers), indicating that steady state had not yet been reached at day 40.

DFO was administered twice per day at 30 mg/kg body weight (or 0.9% saline for control animals) for 16 d or until mouse sacrifice, whichever was sooner. Under anesthesia, the dorsal skinfold (cranial to the wound) was pulled away from the spine to make a ‘tent,’ and the needle was inserted into the tent (over the spine, near the left and right wounds). The first DFO treatment was given 12 hr after completion of the second and final cycle of pressure (Figure 5A). The dosing rationale was as follows: the recommended dose of DFO for iron overload in human patients is 20–60 mg/kg per day (maximum 80 mg/kg; Barata et al., 1996). Mice metabolize DFO faster than humans, so we took the 60 mg/kg per day human dosing and divided it into two half-doses per day.

Murine cardiotoxin injury model

To induce an acute injury in the panniculus carnosus muscle in the skinfold, mice were shaved and fur removed by hair removal cream (Veet, Reckitt, Slough, England) prior to injury. The mice were anesthetized and 30 μl of toxin at 10 μM concentration was injected intramuscularly into the panniculus carnosus of the dorsal skinfold of each wound (left and right). Because commercial distribution of CTX from Naja mossambica was discontinued, some mice were injected with naniproin instead, while the term ‘cardiotoxin’ has been used for the entire cohort. Naniproin is a Naja nigricollis homolog of CTX with 95% sequence identity. Naniproin induces myolysis comparable to CTX. All toxin-injected mice were given an analgesic (buprenorphine at 0.1 mg/kg; Bupredyne, Jurox Animal Health) prior to injection. To control for the post-injury injections given to mPI mice, the toxin-injured mice were injected subcutaneously with 0.9% saline (n = 7) twice daily for 16 d or until mouse sacrifice, whichever was sooner (Figure 1—figure supplement 1). Tissues were harvested at day 3, 10, or 40 (n = 4). Uninjured healthy tissues were also collected as controls (n = 4).

Pressure wound assessment

External wounds were assessed daily with a digital calliper (Tactix, CO, USA) positioned at the borders of the wounds to measure the length and width of each wound. In addition, each wound was photographed with an Olympus pocket camera alongside a wound mapping marker (KISS Healthcare Inc, CA). Measurements and photographs were obtained until re-epithelialization (external wound closure). Wound area was computed by ImageJ after manual segmentation of the wound margins.

Histopathology scoring of H&E-stained sections

Paraformaldehyde-fixed paraffin-embedded tissue sections were stained with hematoxylin (Shandon Gill Hematoxylin 2; Thermo Fisher, MA) and eosin (Eosin Y; Sigma-Aldrich, MO) according to the manufacturer’s instructions. H&E-stained slides were blinded and scored on the extent of tissue death, immune infiltration, granulation, and regeneration. The scoring for tissue death was defined as follows: 0, healthy tissue with zero or minimal death (<10% tissue area dead); 1, mild death (11–33%); 2, moderate death (34–67%); and 3, extensive tissue death (>67%). Death was identified by karyolysis, karyorrhexis, and acidification (eosinification). Scoring for tissue regeneration was defined as follows: 0 (minimal, i.e., <10% regenerated), 1 (mild, i.e., 11–33% regenerated), 2 (moderate, i.e., 34–67% regenerated) to 3 (extensive, i.e.,>67% regenerated). Scoring for granulation was defined as follows: 0, normal tissue with neither granulation nor neo-angiogenesis; 1, minimal granulation (less than 10% of tissue area); 2, mild granulation (10–25% of tissue area); 3, moderate granulation (26–50% of tissue area); 4, high granulation (51–75% of tissue area); and 5, extensive granulation (more than 75% of tissue area consisting of granulation tissue). Likewise, the scores for immune infiltration were defined as follows: 0, normal tissue without immune infiltration; 1, minimal immune infiltration (in less than 10% of tissue); 2, mild immune infiltration (in 10–25% of tissue); 3, moderate immune infiltration (in 26–50% of tissue); 4, high immune infiltration (51–75% of tissue area); and 5, extensive immune infiltration (more than 75% of tissue infiltrated by immune cells). Therefore, on the scales for tissue death, regeneration, granulation, and immune infiltration, an uninjured tissue would receive scores of 0, 3, 0, and 0, respectively. Scoring was performed for all treatments and all timepoints. The scoring of death, immune infiltration, and regeneration was performed for the overall wound and each tissue layer.

Perl’s Prussian blue staining

8 μm sections of fixed, paraffin-embedded tissues were deparaffinized and rehydrated prior to staining. The sections were then incubated for 5 min in Perl’s Prussian blue iron stain (mix of potassium ferrocyanide and hydrochloric acid; Abcam, Cambridge, UK, ab150674) rinsed in water, and counterstained in nuclear fast red (Abcam, Singapore, ab150674). Iron deposits were observed as deep blue dots or speckles, and the number and fractional area occupied by deep blue pixels were quantified by thresholding in MATLAB (MathWorks, MA).

Confocal microscopy

Tissue samples were imaged using a Zeiss LSM710 confocal microscope (Carl Zeiss, Oberkochen, Germany) and Olympus FV3000 laser scanning confocal microscope (Olympus, Tokyo, Japan). Excitation and detection wavelengths used for the respective fluorophores were: CFPmem: Ex. 457 nm and Em. 466–495 nm, GFPnuc: Ex. 488 nm and Em. 498–510 nm, YFPcyt: Ex. 515 nm and Em. 521–560 nm, RFPcyt: Ex. 559 nm and Em. 590–650 nm. Images were processed and analyzed using Fiji (ImageJ) software. The number of muscle fiber malformations in each mouse was quantified by counting the malformations in a field of view, and averaging over four fields of view for each mouse.

Immunofluorescence staining

10 μm fixed paraffin-embedded sections or cryosections were blocked with 10% normal serum and permeabilized with 0.2% Tween 20. Staining was performed using antibodies against 8-oxoguanine (8-OG or 8-hydroxy-2’-deoxyguanosine; Abcam, ab206461), F4/80 (Santa Cruz Biotechnology Inc, TX, sc-52664), Arginase-1 (Arg1; Abcam, ab60176, and Proteintech Group Inc, IL, 16001-1-AP), MerTK (Abcam, ab95925), inducible nitric oxide synthase (iNOS; Cell Signalling Technology, MA, #13120), citrullinated histone H3 (citH3; Abcam, ab5103), nitrotyrosine (Abcam, ab7048), and myoglobin (Cell Signaling Technology, 25919S). For detection, we used Alexa Fluor 488-, 594-, and 647-conjugated secondary antibodies (Abcam, ab150129, ab150064, and ab150075, respectively) raised in appropriate species for the experiments. The slides were mounted with Vectashield Hardset with DAPI (Vector Laboratories, CA) and images were acquired on a Leica TCS SP8 confocal microscope (Leica Microsystems, Wetzlar, Germany) and analyzed using LAS X software. For quantification, each wound consists of 512 × 512 image frames, covering all layers of the wound, and the number of frames depends on the size of the wound. Each 512 × 512 image frame was thresholded using Fiji (ImageJ) as outlined by Shihan et al., 2021, and the mean fluorescence intensity was computed. The mean intensity for each wound is computed by taking the mean of the frames. Text description has been added if specific layers were disproportionately responsible for the intensity.

BODIPY 581/591 C11 detection

10 μm tissue cryosections were fixed in 4% paraformaldehyde, blocked with 10% normal serum, and permeabilized with 0.2% Tween 20. Tissue sections were stained with 10 μM BODIPY 581/591 C11 probe (Thermo Fisher, D3861). After 30 min in the dark, the slides were mounted with Vectashield Hardset with DAPI (Vector Laboratories) and viewed under a Leica TCS SP8 confocal microscope (Leica Microsystems). Each wound section was imaged from edge to edge with consecutive frames.

TUNEL staining

Deparaffinized wound sections of CTX-injured or saline-treated mPI and DFO-treated mPI (8 μm sections) were stained with the DeadEnd Fluorometric TUNEL System (G3250; Promega, WI) according to the manufacturer’s protocol. DAPI was used for nuclear staining. Images were visualized and captured with a Leica TCS SP8 confocal microscope (Leica Microsystems).

Protein extraction and detection

Snap-frozen tissues were homogenized using MAGNAlyser beads (Roche Life Science, Penzberg, Germany) in RIPA lysis buffer (Sigma-Aldrich) with protease inhibitors and phosphatase inhibitors (Nacalai Tesque Inc, Kyoto, Japan). The samples were centrifuged for 20 min and the supernatant homogenate collected. Protein concentrations were determined using the Pierce BCA reagent (Thermo Fisher).

Luminex ELISA was carried out for the detection of murine interleukins, chemokines, and growth factors (R&D Systems, MN; LXSAMSM-21). Sample preparation followed the manufacturer’s protocol, and analytes were read using the MAGPIX Platform (Luminex Corporation, TX).

For blotting, 30  μg samples were separated by SDS-PAGE (Bio-Rad Laboratories Inc, CA) and transferred to PVDF membranes (Bio-Rad Laboratories Inc). The PVDF membranes were blocked with 5% blotting grade blocker (Bio-Rad Laboratories Inc) at room temperature (RT) for 1  hr and then incubated with the primary antibodies (anti-myoglobin antibody, 25919S, Cell Signaling Technology) overnight at 4°C. Then, the membranes were washed with TBST at RT and incubated with the secondary antibody for 1  hr. Protein bands were detected with Pierce ECL substrate (Thermo Fisher) according to the manufacturer’s instructions. Data were analyzed with the ChemiDoc Imaging System (Bio-Rad Laboratories Inc).

Iron assay

An iron assay kit (Abcam; ab83366) was used to measure total iron (ferrous [Fe2+] and ferric [Fe3+]) in the uninjured tibialis anterior (calf) muscles of Mb−/− and Mb+/+ mice. Snap-frozen muscle tissues were homogenized as previously described, and the assay was performed according to the manufacturer’s instructions.

RAW264.7 monocyte cell culture and differentiation

We cultured RAW264.7 macrophages (TIB-71, ATCC) in High Glucose DMEM with pyruvate (Gibco, UK) supplemented with 10% fetal bovine serum (FBS; GE Healthcare, USA), 100 units/ml penicillin, 100 µg/ml streptomycin, 0.25 µg/ml Amphotericin B (Fungizone, PSF; GE Healthcare), and 5 µg/ml Plasmocin (GE Healthcare) at 37°C under 5% CO2. RAW264.7 monocytes were pre-incubated for 24 hr with 10 ng/ml recombinant murine IFNγ (Stem Cell Technologies, Vancouver, Canada) with 100 ng/ml lipopolysaccharide (LPS; Sigma-Aldrich) to resemble pro-inflammatory or M1 activation (Cao et al., 2017).

To dose the myoglobin treatment, we first used a mouse myoglobin ELISA kit (Abcam, ab157722) to obtain the myoglobin concentrations in the calf (67.4 µg/ml) and bicep (40.8 µg/ml) muscles of our mice. From this, we chose 15, 50, and 150 µg/ml to be the treatment concentrations of myoglobin (M0630 from horse muscle, Sigma-Aldrich) for the RAW264.7 cells (24 hr after pro-inflammatory treatment). Negative and positive controls were provided by untreated (0 µg/ml myoglobin) and menadione-treated macrophages. The cells were treated for 24 hr in the different conditions prior to performing assays for efferocytosis and oxidative stress.

In vitro assays

After pro-inflammatory pretreatment and myoglobin (or control) treatment, the RAW264.7 cells were characterized as follows. The cells were measured for oxidative stress using the ROS-Glo H2O2 assay according to the manufacturer’s instructions (G8820, Promega). Luminescence was measured using a Tecan Spark M10 microplate reader (Tecan, Männedorf, Switzerland). The cells were also measured using an efferocytosis assay kit (Cayman Chemical, MI) according to the manufacturer’s instructions. Briefly, apoptotic bodies were created from staurosporine-treated C2C12 myoblasts (CRL-1772, ATCC) and labeled with CFSE fluorescence. Apoptotic bodies (or cell-free media control [CFM]) were added to wells for 16 hr prior to aspiration of the unengulfed apoptotic bodies, washing, and measurement. Absolute fluorescence was measured as integrated intensity for each well using a microplate reader (Tecan Spark M10, Männedorf, Switzerland) with excitation at 490 nm and emission at 525 nm.

RAW264.7 monocyte and C2C12 myoblast cell lines were authenticated using a commercial STR profiling service (Mouse Cell Authentication Service, ATCC 137‐XV) and tested negative for mycoplasma using the MycoAlert PLUS mycoplasma detection kit (LT07-705; Lonza, Basel, Switzerland).

Statistical analyses

Sample sizes for the mouse studies were determined using a power analysis for comparing two means (two-sample, two-sided equality).

In the analysis of DFO treatment in adult 5-month-old mice at multiple timepoints, treated mice were paired with age- and-sex-matched controls. Therefore, significance was measured using a paired test (two-tailed Student’s t test for mPI + DFO versus mPI + saline). Mice were not paired for the other comparisons (i.e., adult 5-month-old mPI + saline versus CTX + saline cohorts2, and elderly 20-month-old Mb−/− mPI + saline versus Mb+/+ mPI + saline cohorts), and statistical significance was analyzed using an unpaired two-tailed Student’s t test. For multiple comparisons in Figure 9,a one-way ANOVA was followed by the Tukey’s post hoc test. For multiple comparisons in the Luminex assays (Supplementary file 1df and g), the Student’s t test was performed for each analyte, followed by the Bonferroni–Dunn correction for multiple hypothesis testing. Tests and plots were generated by GraphPad Prism (version 9.0.0 for Windows, GraphPad Software, CA). *p<0.05, **p<0.01, ***p<0.001, and ****p<0.0001. ‘ns’ means not significant.

Appendix 1

Appendix 1—key resources table
Reagent type (species) or resourceDesignationSource or referenceIdentifiersAdditional information
Strain, strain background (Mus musculus, C57BL6, male and female)Pax7-Cre-ERT2Jackson Laboratory#012476Transgenic mice
Strain, strain background (M. musculus, C57BL6, male and female)Brainbow2.1Jackson Laboratory#017492Transgenic mice
Strain, strain background (M. musculus, C57BL6, male and female)Myoglobin-KOCyagenGermline knockout mice
Cell line (M. musculus)RAW264.7ATCCTIB71
Cell line (M. musculus)C2C12ATCCCRL-1772Myoblasts (apoptosed)
AntibodyAnti-heme oxygenase 1 (rabbit polyclonal)Abcamab13243IF (1:150)
AntibodyAnti-8-oxoguanine (mouse monoclonal)Abcamab206461IF (1:150)
AntibodyAnti-mouse F4/80 (rat monoclonal)Santa Cruz Biotechnologysc-52664IF (1:50)
AntibodyAnti-arginase-1 (goat polyclonal)Abcamab60176IF (1:150)
AntibodyAnti-MERTK (rabbit polyclonal)Abcamab95925IF (1:150)
AntibodyAnti-mouse iNOS (rabbit monoclonal)Cell Signaling Technology#13120IF (1:100)
AntibodyAnti-myeloperoxidase (rabbit polyclonal)Abcamab9535IF (1:75)
AntibodyAnti-CD38 (rabbit monoclonal)Cell Signaling Technology#68336IF (1:75)
AntibodyAnti-myoglobin (rabbit monoclonal)Cell Signaling Technology#25919IF (1:100)
WB (1:1000)
AntibodyAnti-GAPDH (mouse monoclonal)MerckMAB374WB (1:5000)
AntibodyAnti-mouse light chain (goat polyclonal)MerckAP200PHRP conjugate
WB (1:10,000)
AntibodyAnti-rabbit IgG (goat polyclonal)MerckAP132PHRP conjugate (1:3000)
AntibodyAnti-mouse citH3 (rabbit polyclonal)Abcamab5103IF (1:150)
AntibodyAnti-nitrotyrosine (mouse monoclonal)Abcamab7048IF (1:100)
AntibodyAnti-hemoglobin (rabbit polyclonal)Proteintech16665-1-APIF (1:100)
AntibodyAnti-hemopexin (rabbit polyclonal)Proteintech15736-1-APIF (1:100)
AntibodyAnti-haptoglobin (rabbit polyclonal)Proteintech14537-1-APIF (1:100)
AntibodyAnti-rabbit IgG (donkey polyclonal)Abcamab150064Conjugated to Alexa Fluor 594
IF (1:1000)
AntibodyAnti-rabbit IgG (donkey polyclonal)Abcamab150075Conjugated to Alexa Fluor 647
IF (1:1000)
AntibodyAnti-goat IgG (donkey polyclonal)Abcamab150129Conjugated to Alexa Fluor 488
IF (1:1000)
AntibodyAnti-mouse IgG (donkey polyclonal)Abcamab150107Conjugated to Alexa Fluor 647 IF (1:1000)
Sequence-based reagentMyoglobin common reverseIDTPCR primersCCTTGCTCTGACTGTGTTAGCCTCAG
Sequence-based reagentMyoglobin wildtype forwardIDTPCR primersGTGTGACAGAGTGGCTGTCATACTTTGT
Sequence-based reagentMyoglobin knockout forwardIDTPCR primersCCGTTGACCCACCTTGTCTCAA
Sequence-based reagentPax7 common forwardIDTPCR primersGCTGCTGTTGATTACCTGGC
Sequence-based reagentPax7 wildtype reverseIDTPCR primersCTGCACTGAGACAGGACCG
Sequence-based reagentPax7 transgene reverseIDTPCR primersCAAAAGACGGCAATATGGTG
Sequence-based reagentBrainbow2.1 common reverseIDTPCR primersCCAGATGACTACCTATCCTC
Sequence-based reagentBrainbow2.1 wildtype forwardIDTPCR primersAAAGTCGCTCTGAGTTGTTAT
Sequence-based reagentBrainbow2.1 transgene forwardIDTPCR primersGAATTAATTCCGGTATAACTTCG
Peptide, recombinant proteinMyoglobin (equine)Sigma-AldrichM0630
Peptide, recombinant proteinInterferon gammaStem Cell Technologies78021.1
Commercial assay or kitGenElute Mammalian Genomic DNA Miniprep KitsSigma-AldrichG1N70
Commercial assay or kitIron assay kitAbcamab83366
Commercial assay or kitPerls’ Prussian blue iron stainAbcamab150674
Commercial assay or kitDeadEnd Fluorometric TUNEL SystemPromegaG3250
Commercial assay or kitMouse Luminex Discovery AssayR&D SystemsLXSAMSM-21
Commercial assay or kitROS-Glo H2O2 AssayPromegaG8821
Commercial assay or kitEfferocytosis Assay KitCayman Chemical601770
Commercial assay or kitMycoAlert PLUS mycoplasma detection kitLonzaLT07-705
Commercial assay or kitFTA Sample Collection Kit for Mouse Cell Authentication ServiceATCC137-XV
Chemical compound, drugCardiotoxinSigma-AldrichC9759N. mossambica venom
Chemical compound, drugNaniproinPMID:27173146N. nigricollis venom
Chemical compound, drugDeferoxamine mesylateSigma-AldrichD9533
Chemical compound, drugTamoxifenSigma-AldrichT5648
Chemical compound, drugBuprenorphine (Bupredyne)Jurox Animal Health
Chemical compound, drugIsofluranePiramal Critical Care
Chemical compound, drugMenadioneSigma-AldrichM5625
Chemical compound, drugLipo-polysaccharideSigma-AldrichL4391
Chemical compound, drugSodium chloride solution, 0.9%Sigma-AldrichS8776
Software, algorithmGraphPad PrismGraphPad Software
Software, algorithmMATLABThis paperThresholding of blue color from Prussian blue iron deposits
OtherCeramic magnetsMagnetic SourceCD14CSee ‘Murine pressure injury model’
OtherNeodymium magnetsLiftontech Supreme Pte LtdSee ‘Murine pressure injury model’
OtherVectashield Hardset with DAPIVector LaboratoriesH-1500-10See ‘Immunofluorescence staining’
OtherBODIPY 581/591C11 probeInvitrogenD3861See ‘BODIPY 581/591C11 detection’
OtherEosin Y solutionSigma-AldrichHT110116See ‘Histopathology scoring of H&E-stained sections’
OtherShandon Gill Hematoxylin 2Thermo Fisher6765007See ‘Histopathology scoring of H&E-stained sections’

Data availability

All data including primary images, quantification spreadsheets, and mouse tables have been uploaded to Zenodo at doi: https://doi.org/10.5281/zenodo.7754554.

The following data sets were generated
    1. Nasir NJM
    2. Heemskerk H
    3. Jenkins J
    4. Hamadee NH
    5. Bunte R
    6. Tucker-Kellogg L
    (2023) Zenodo
    Myoglobin-derived iron causes wound enlargement and impaired regeneration in pressure injuries of muscle.
    https://doi.org/10.5281/zenodo.7754554

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Decision letter

  1. Sakae Tanaka
    Reviewing Editor; University of Tokyo, Japan
  2. Mone Zaidi
    Senior Editor; Icahn School of Medicine at Mount Sinai, United States

In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses.

[Editors' note: this paper was reviewed by Review Commons.]

https://doi.org/10.7554/eLife.85633.sa1

Author response

General Statements

Issue A. Removing the claim of phagocytic dysfunction.

Reviewer #1:

“The study is well written, clear, and the experiments are carefully presented and conducted. Although the text is usually very detailed and nicely referenced, some of the claims should be dampened. Notably the title since the phagocytic dysfunction is not evidenced by the results…

Concerning immune cells, the conclusion cannot be that myoglobin impedes their phagocytic function. All the data concur that in the absence of myoglobin there are more immune cells in the regenerating muscle at day 3. Consequently, more macrophages will lead to a better cleansing of debris. Thus, the difference would not rely on phagocytic properties per se, but more on the number of macrophages that arrive at the site of injury.”

We clarify a major misunderstanding. All instances of the phrase “failure of phagocytosis” or phagocytic dysfunction” should be re-worded as “persistence of dead tissue or simply “necro-slough.” The word “phagocytic” contains an implication of “cells” or micro-scale issues, which was not our thinking. We apologize for the ambiguity. Our claim is strictly about the millimeter-scale tissue outcome, not about cell activities to cause the tissue outcome.

Manuscript revisions:

  • Changed the title to omit the term “phagocytic dysfunction”.

  • Changed the text to emphasize tissue physiology and not evoke concepts of cell biology. Changed terminology so that all “failure of phagocytosis” will be written as “persistence of dead tissue.”

  • Our abstract now says, “Unlike acute injuries (from cardiotoxin), mPI regenerated poorly with a lack of viable immune cells, persistence of dead tissue (necro-slough), and abnormal deposition of iron.” (The old version had said, “mPI regenerated poorly with a lack of viable immune cells, failure of phagocytosis….”)

  • Our hypothesis statement now reads as follows: “Given that pressure ulcers often have slough or eschar, we hypothesize that mPI will have persistence of dead tissue in the wound bed, and that sterile mPI will have slough, despite the absence of bacterial biofilm.” This is a clinically oriented claim about the relationship between bacterial infection and sloughing, not a cell biology claim about the relationship between macrophages and efferocytosis.

Issue B. Macrophage characterisation

Reviewer #1:

“It would be interesting to see if myoglobin prevents monocyte or macrophage migration/chemotaxis.”

This is an interesting question. We performed in vitro testing of RAW264.7 cell migration/chemotaxis using serum differential in a trans-well migration assay. Results are shown in Author response image 1. Because this assay is yielding large error bars without affecting our argument (positively or negatively), we prefer to de-emphasise the issue of monocyte/macrophage migration, and instead emphasise the issue of vasculature (see Issue D below).

Author response image 1
Chemotaxis of macrophages measured using a Transwell migration assay.

RAW 264.7 monocyte cells were incubated with pro-inflammatory stimuli (10ng/ml IFNγ + 100ng/ml LPS), for 24h prior to 24h treatment with varying concentrations of myoglobin. Macrophages (1×104 cells/well in DMEM without FBS) were loaded into the upper portion of a 6.5-mm Transwell with 8.0-µm Pore Polycarbonate Membrane Insert (Corning, NY, USA). DMEM with 10% FBS supplementation was placed in the lower chamber. The cells in the bottom chamber and bottom of the insert (i.e., the migrated cells) were collected and quantified using the Cell Titre-Glo assay, with luminescence measured using a microplate reader. n = 3-4 replicates. CFM = cell-free media. Mean ± standard deviation. “ns” not significant, **<0.01 Statistical significance was computed by one-way analysis of variance (ANOVA) with Tukey post-hoc test.

Issue C. Macrophage phagocytosis / efferocytosis

Reviewer #2:

“7. Macrophage phenotype (inflammatory vs. anti-inflammatory/reparative) can be achieved by RTqPCR, using well-define combination of mRNA encoding proteins associated with inflammatory (e.g. iNos, Cox-2, Cd86) or anti-inflammatory (Ym1, Arg-1, RELMa, Cd206).

8. The in vitro experiments with macrophages could be further supported by in vivo experiments, where types of injury (mPI vs. CTX) and mouse genotypes (Mb-/- vs. WT) could be evaluated for the ability of macrophages to perform efferocytosis: coupling apoptotic cell detection (Tunel staining) to macrophage immunostaining. Quantification of overlapping signal would give some information (albeit indirect) regarding the macrophages' ability to clear the tissue from dead cells. From my perspective, this would be the minimal set of data required to highlight a potential "efferocytic failure" in mPI.”

Reviewer #1 cross-comment:

“Point 8 should be investigated if the authors wish to claim about efferocytosis.”

Reviewer #2 cross-comment:

“I would rather suggest the authors to focus on strengthening the description of their model of injury (mPI) versus CTX, on key points that are known to influence tissue repair/regeneration as well as their main findings: evaluation of the injur’'s surface area, myoglobin deposition/accumulation in the wound, better describing the immune response ensuing the injury. Beyond these points, the authors can then re-assess whether or not to include the role of myoglobin on monocyte/macrophage infiltration on the site of injury and the phagocytic activity of these recruited macrophages as part of this manuscript.”

We did not intend to make a claim about efferocytosis or phagocytosis of monocytes/macrophages. We have amended the title and text to omit “phagocyte dysfunction”, as mentioned in Issue A.

That being said, both reviewers requested additional study of macrophages, and provided various options for how to address the underlying question of what macrophages in mPI are doing. Reviewer #2 recommended functional assays of phagocytic capacity in vitro, so we assessed the impact of myoglobin on M1-differentiated RAW264.7 macrophages using an efferocytosis assay. Results showed that Mb treatment caused decreased efferocytosis in vitro, which is now reported in Figure 4—figure supplement 2A. Note that we maximised cell viability during this Mb treatment assay by using an M1 polarisation protocol [1] with a lower dose of IFNγ (10 ng/ml instead of 20 ng/ml) and shorter duration (see Supplementary Methods). We have placed this data in the Supplement (and do not mention it in the Discussion) because we believe impaired efferocytosis could be a side-effect of cell stress, and not necessarily a specific effect of Mb. Mb treatment induced ROS in a dose-dependent manner (Figure 4—figure supplement 2B).

In tissue sections ex vivo, TUNEL staining (for apoptotic debris) was far higher in saline-treated mPI than in CTX or DFO-treated mPI (Figure 5—figure supplement 1), but we were unable to find co-localization of F4/80 and TUNEL staining in our samples, so we could not measure increased or decreased co-localization (as a readout of macrophage engulfment of debris). The increased TUNEL staining might have arisen from increased production (e.g., secondary progression of the wound) or from decreased uptake (e.g., impaired efferocytosis).

Manuscript revisions:

  • Removed the poor wording that suggested a claim about macrophage phagocytosis

  • New data in Figure 4—figure supplement 2

  • New data in Figure 5—figure supplement 1

Issue D. Vasculature

Reviewer #1:

“Another aspect is how cells reach the injured area. In the Discussion section, there is reference to a SupplFig6, which seems to be not the good one in the document. In the FigS6 described in the text, it is mentioned that cells are kind of "stopped" at the boundaries of the damage. This is very interesting. If the vessels are physically flattened or squashed by the injury, extravasation can not occur properly, in comparison with cardiotoxin. Then the role of myoglobin in extravasation, or in the "reshaping" of the vessels after the removal of the magnet would be interesting to investigate. Of note, in the myoglobin deficient mice, the vascular network is increased, favoring immune cell infiltration.

Given that the study is already quite huge with numerous experiments, the reviewer is reluctant to ask for additional experiments… Also the points mentioned above, about the role of myoglobin in immune cell infiltration and the role of myoglobin in the vessel properties should be at least discussed, if they are not experimentally addressed.

– Discussion end of page 10. Unfortunately, suplFig6 is missing (and is not called in the result section).”

We apologise for the omission of SupplFig6 from the journal version. The previous version of Suppl. Figure 6 (in bioRxiv) showed side-by-side comparisons of young and old saline-treated mPI, and it has been included in the current submission (renumbered as Figure 3—figure supplement 2). We also provide additional data (Figure 2—figure supplement 1) showing cells reaching, or failing to reach, the injured area at day 3 post-injury. Specifically, it shows that the uncompressed periphery of mPI wounds have intact immune cells, while the compressed region lacks viable immune infiltrate (Figure 2—figure supplement 1).

In addition, we provide new data showing damaged blood vessels in Mb-wildtype mPI compared to intact vasculature in CTX at day 3 post-injury (Figure 1—figure supplement 2). Damaged vasculature has a number of important implications, which are now elaborated in the Discussion at lines 371-377, as follows: “Pressure has been reported to cause disruption to arterial supply, venous clearance, and lymphatic drainage (4, 51, 52), although not always (79). Lymph vessels are occluded at very low pressure, and impaired lymphatic flow can persist after pressure is removed (80). Impaired circulation and/or impaired drainage could cause waste factors such as myoglobin or iron to accumulate in the mPI, and could impair the influx of iron detoxification factors. Poor drainage also occurs in chronic ulcers of venous insufficiency, lymphedema, and other vascular diseases. Toxicity from globin iron might be a shared pathology among these wounds.”

Manuscript revisions:

  • New data in Figure 1—figure supplement 2

  • New data in Figure 2—figure supplement 1

  • Next text about damaged vasculature (Results section, lines 120-123, Discussion section, lines 371-377)

  • Figure 3—figure supplement 2 (previously Suppl. Figure 6 – comparison of young and old saline-treated mPI) is now mentioned in the Results section

Issue E. Length of Discussion

Reviewer #1:

“Also the text is very long, notably the discussion, which contains 18 sections (!) and several sections are not very informative and poorly referenced, looking more as a thesis dissertation than as an article discussion… Given that the study is already quite huge with numerous experiments, the reviewer is reluctant to ask for additional experiments. Rather, the reviewer suggests to reshape the text, remove unnecessary details to get straight to the points and to emphasize the important results”

“- Discussion sections about oxidative stress, endogenous iron, prevention studies, antiDAMPs strategies, slough and debridement are poorly informative and poorly referenced and should be either removed or shortened”.

We have shortened the discussion and conclusion by 33%, especially the sections entitled “the context of oxidative stress”, “anti-DAMP strategies”, and “debridement of slough.” (These changes were submitted in our December 2022 proposed revision and have not been marked-up as changes in the current submission.)

Issue F. Comparing mPI and CTX wounds

Reviewer #2:

“The authors hypothesis regarding the role of Mb in the pathophysiology of ulcerative pressure injuries is interesting. However, the work here seems quite preliminary with major points remaining to be clarified before considering reaching the author's conclusion. I find the direct comparison made between the two types of injury, CTX and mPI, difficult to interpret. From my perspective, a more rigorous and systematic comparison between the two models of injury would be key to convincingly convey the findings of this work, especially regarding key features impacting repair. My major comments are listed below.

1. Time for tissue repair not only depends on the type of injury, but also on the extent of the injury. In other words, how the mPI and CTX models compare in terms of surface of injured tissue (and resulting ischemia)”

We interpret the feedback to mean that the surface area of injured muscle should be comparable between CTX and mPI, in order to claim that the tissue repair is different between the two injuries. We now provide measurements showing that both CTX and mPI have comparable size of dead muscle at the initial timepoint (Supplementary File 1a).

However, we caution readers that different types of wounds are not truly comparable. We have provided the contrast between CTX and mPI for hypothesis-generation, not as a controlled experiment for hypothesis-testing. To clarify the limits of using CTX as a reference for understanding mPI, we provide additional data mentioned in Issue D showing dissimilarity between CTX and mPI in vasculature and thickness.

Manuscript revisions:

  • New data in Supplementary File 1a

  • New data in Figure 1—figure supplement 2

  • We delete “normal” from our description of CTX regeneration, and better explain the purpose of comparing CTX and mPI.

Issue G. Accumulation and detoxification of globin proteins in the wounds

Reviewer #2:

“2. Is myoglobin also released in the injured tissue after CTX and how does it compare to mPI (Mb+ surface area, quantity)?

9. What about expression levels of proteins involved in heme/iron detoxifying proteins haptoglobin and hemopexin? Are they present in the injured tissue and are they differentially expressed between types of injury and mouse genotypes? Same goes for their receptors (CD163 and CD91, respectively): are they differentially expressed on macrophages found in the injured tissue.

– Is hemoglobin found mPI and CTX wounds from WT or Mb-/- mice?”

We interpret this feedback to mean that measurements of myoglobin and iron detoxification factors would help explain why mPI has higher iron and what consequences the iron may have. We agree this might be interesting, provided we also bear in mind the destruction of vessels in mPI (described in Figure 1—figure supplement 2 and Issue D above). Damaged vasculature could explain why iron-containing waste accumulates in mPI, while the same waste gets removed in CTX. To assess levels of globin proteins in the wound, we performed IF for myoglobin and hemoglobin in mPI at day 3 post-injury. Results showed higher levels of myoglobin and hemoglobin in mPI than in CTX (Figure 2—figure supplement 2A-H and Figure 2—figure supplement 2Q-R, for n = 3, with all wounds saline-treated).

To assess iron detoxification factors in the wound, we performed IF for haptoglobin and hemopexin (Figure 2—figure supplement 2I-P and Figure 2—figure supplement 2S-T), which were very low, relative to our positive controls (liver and spleen, not shown). We found no significant difference between mPI and CTX at day 3 post-injury. Possible interpretations of this finding are that day 3 might be too late to see the peak of iron detoxification for the CTX injury, and the damaged vasculature of mPI might decrease the influx of iron detoxification factors into high-iron mPI wounds.

Manuscript revisions:

  • New data in Figure 2—figure supplement 2A-H and Figure 2—figure supplement 2Q-R

  • New data in Figure 2—figure supplement 2I-P and Figure 2—figure supplement 2S-T

  • Next text about damaged vasculature (Results section, lines 120-123, Discussion section, lines 371-377)

Issue H. Additional mouse models

Reviewer #2:

“3. Does Mb co-injection with CTX mimics mPI injury in terms of inflammation and repair kinetics?”

Reviewer #2 cross-comment:

“I would rather suggest the authors to focus on strengthening the description of their model of injury (mPI) versus CTX, on key points that are known to influence tissue repair/regeneration as well as their main findings: evaluation of the injury's surface area, myoglobin deposition/accumulation in the wound, better describing the immune response ensuing the injury.”

Reviewers did not prioritize an additional mouse injury model, and our Dec 2022 revision plan offered other experiments instead.

Reviewer #2:

“4. Does in situ Mb supplementation in Mb-/- mice worsens mPI repair to an extent that is comparable to WT mice?

5. A better characterization of the inflammatory between types of injury and mice (Mb-/- vs. WT) before and after 3- and 10-days post-injury would be very informative. Comparing the relative proportions of leukocyte populations would provide valuable information regarding the kinetics of the repair process.”

Reviewers did not prioritize further study of knockout mice (Mb-/-), and our Dec 2022 revision plan offered other experiments instead.

Issue I. Characterising the immune landscape in mPI

Reviewer #2:

“6. Macrophages in particular play a central role in the orchestration of tissue repair, through their immunomodulation abilities. On the same token, characterizing macrophage infiltrates (number per surface area of injured tissue) and phenotype would potentially provide valuable information to link observed differences between types of injuries to Mb. Ideally, assessment of leukocyte and macrophages infiltration and populations would be analyzed by flow cytometry after injured (vs. uninjured) tissue dissociation (enzymatic or mechanical). Otherwise, although less quantitative, this can also be done by cell infiltration using specific immunostaining and quantification (cell number/injured tissue surface area).

To assess leukocytes in mPI wounds, we performed immunostaining for CD38 and myeloperoxidase at day 7 post-injury. CD38 is a marker of CD4+, CD8+, B and Natural Killer cells, and myeloperoxidase is a marker of neutrophil extracellular traps (NETs/NETosis). Immunostaining (Figure 6—figure supplement 3A-G) showed that myeloperoxidase staining was high in mPI+Saline, and lower in DFO-treated. The lower level of myeloperoxidase staining in DFO-treated (day 7) resembled the level in CTX injury at day 3 (n = 3 replicates each). Immunostaining for CD38 (Figure 6—figure supplement 3H-N) showed similar expression levels between mPI+Saline at day 7 and CTX+Saline (CTX) at day 3, and increased expression in mPI+DFO at day 7, compared to CTX (n = 3 replicates each). These findings are now mentioned in the Results section alongside CitH3 and MerTK immuno-staining, but are not mentioned in the discussion. As some of our staining may not localize to cells but to cell debris and extracellular molecules, we chose not to quantify fluorescence based on cell number per tissue area. Instead, we provide quantification using fluorescence thresholding [2] of images of injured sections, and report the average of the injured frames for each wound/mouse.

We also performed immuno-staining with additional antibodies that failed quality-control checks; these were for Ly6G, a marker of neutrophils, and CD86, a marker of dendritic cells, macrophages, B cells and other antigen-presenting cells.

For the broader goal of characterising immune response in mPIs, we did already measure many cytokines and general immune-related factors at different timepoints. Surprisingly few of the analytes had any significance change upon Mb knock-out or DFO treatment (Supplementary Files 1d, 1f and 1g), despite the large fold-changes seen in damage-related epitopes of oxidative stress. We wish to avoid making any claims about the immune system in mPI, other than the absence of intact immune cells in untreated day-3 mPI wounds, and the DFO-induced increase in the presence/influx of immune cells at days 7 & 10. These serendipitous findings about immune cells are not required for any of the five hypotheses listed in our introduction.

Manuscript revisions:

  • New data in Figure 6—figure supplement 3

Issue J. Minor comments

Reviewer #1:

“- Page 5: "The panniculus layer of mPI was nearly devoid of intact immune cells." It is not clear here if the authors refer to the absence of immune cells or to damage of immune cells present in the injured area.”

We refer death and damage of the immune cells in the injured area, rather than complete absence of these cells. This is discussed in lines 384-396.

“- Page 6: "In summary, measures of innate immune response became less abnormal after Mb knockout". Cardiotoxin injury is not the "normal" situation since this kind of injury is not physiological and is highly inflammatory as compared with others (Hardy et al., Plos One 2016).”

Thank you. We have omitted the word “normal” in our description of acute cardiotoxin injury.

Reviewer #2:

“- Figure 2: HO-1 staining seem decreased in mPI compared to CTX and thus doesn't support the quantifications. Please 2x-check quantifications and images to provide consistent quantifications-illustrations pairing.”

Thank you. We have doublechecked the quantifications-illustrations pairing and replaced Figure 2L and Figure 5E with more representative images. Primary images, data and quantification spreadsheets have been uploaded to Zenodo at doi:10.5281/zenodo.7750287. The Data Availability section containing the Zenodo link has been moved from Supplementary Methods to Methods (in the main text).

“– Figure 5 C, E, G: please provide illustrations for control treatment.

– Figure 6 K, L, M: please provide illustrations for control treatment.”

The controls for Figure 5C, 5E, and 5G were already shown in Figure 2 (2I, 2L and 2B), and we hesitate to show them again without permission to duplicate from the editor. Similarly, controls for Figure 6K, 6L and 6M are already shown in Figure 2F-G.

“– Figure 5J: it would have been nice to add Mb-/- mice to the comparison.”

We have added Mb-/- mice to the comparison of external wound area, up to the day 3 timepoint.

“– Figure 8: please maintain consistency in the way you convey data between timepoints: area of regenerated (E, F) or unregenerated (G) tissue.”

For consistency, we have moved Figure 8G to Figure 9 (Figure 9K) such that Figure 8 shows regeneration at the 40-day timepoint and Figure 9 shows regeneration and fiber morphology at the 90-day timepoint.

“– Bodipy is not a probe for lipid peroxidation. Due to its lipophilic nature, this dye can be used as a generic lipid satin to image intracellular lipid depots. Therefor the experiments using bodipy as a proxy for lipid peroxidation is incorrect and derived conclusions erroneous. Modulation in bodipy signals probably reflects modulation of intracellular lipid deposition.”

Clarifying text and citation on the BODIPY 581/591 fluorescent probe that we used (Invitrogen, MA, US) have been added in the Results section.

“– Provide concentrations and treatment times in figure legends (sup Figure 3).”

Thank you, we have added the concentrations and treatment times to the figure legend. Further details are in the Methods.

“– Show all the data mentioned in the manuscript (DNA gel electrophoresis supp Figure 4)”

We have added the gel electrophoresis results to Figure 3—figure supplement 1 (previously Suppl. Figure 4).

“– Indicate the number of experimental repeats and the statistical tests used in the figure legends.”

We have added the n numbers, statistical tests, and definitions of centre to the figure legend. We have edited the main text to report exact p-values instead of the p-value range (e.g., p = 0.0251 instead of p < 0.05).

“– Missing information in supp Figure 5 A-D: which images from WT or Myb-KO?”

Thank you. We have added labels for Figure 4—figure supplement 1 (previously Suppl. Figure 5), Figure 6—figure supplement 1 and Figure 7—figure supplement 1.

Issue K. Significance

Reviewer #1 (Significance):

“This study is of interest because it provides insights on muscle regeneration after an injury that may occur in daily life just as contusion, or crush, to the contrary of the whole muscle necrosis induced by cardiotoxin (the main model used in the field). In that aspect, it is more physiological and of interest for the readers in the fields of in muscle regeneration and tissue trauma. The study is descriptive, but is very well conducted and well discussed. Additional experiments investigating the impact of myoglobin on vessel properties/extravasation of immune cells would raise the impact of the study but the study is publishable as it is (with editing as suggest above). The field of expertise of the reviewer is muscle regeneration and inflammation.”

Reviewer #2 (Significance):

“The field of tissue repair and regeneration is an exciting field and improving our understanding of the molecular mechanisms involved in muscle tissue injury has clear and impactful clinical applications. The pathophysiological mechanism involving Mb that the author address in this work has the potential to interest both basic science and clinical researchers and can potentially benefit not only the field of skeletal muscle regeneration but also the field of cardiac remodeling.”

We are very grateful to the reviewers for their insightful and highly constructive feedback.

Issue L. Minor edits made by the authors

  • The section, “Data availability”, which discloses the link to the Zenodo database containing the mice numbers and primary data has been moved from Suppl. Methods (Suppl. Text) to Methods in the main text. The Zenodo description and DOI have been updated to reflect the changes made in this revision.

  • Suppl. Figure 13A-D have a different image set to show F4/80, CitH3 and DAPI triple-stain in saline-treated mPI (day 3). DAPI staining was not shown previously (only F4/80 and CitH3).

  • Suppl. Figure 15E has a blue dashed line added to the graph for the level of MerTK fluorescence in uninjured skinfold.

  • Exact p values are now reported in the main text, as requested by the editorial office.

  • Statistical tests used, exact values of n, definitions of center, and methods of multiple test correction methods have been included in the figure captions, as requested by the editorial office.

  • The methods for TUNEL staining, cell culture conditions and treatments and in vitro assays have been added in the Supplementary Methods.

  • Cell authentication of the cell lines by STR profiling has been provided, as requested by the editorial office.

  • The mycoplasma status of the cell lines has been included, as requested by the editorial office.

  • A Key Resources Table has been provided, as requested by the editorial office.

  • The Supplementary Figures and Supplementary Tables have been separated for better readability.

  • The acknowledgements section has been updated.

References

[1] Cao Q, Yao J, Li H, Tao B, Cai Y, Xiao P, Cheng H, Ke Y. (2017). Cellular phenotypic analysis of macrophage activation unveils kinetic responses of agents targeting phosphorylation. SLAS Discov, 22(1):51-57.

[2] Shihan MH, Novo SG, Le Marchand SJ, Wang Y, Duncan MK. (2021). A simple method for quantitating confocal fluorescent images. Biochemistry and Biophysics Reports, 25:100916.

https://doi.org/10.7554/eLife.85633.sa2

Article and author information

Author details

  1. Nurul Jannah Mohamed Nasir

    1. Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    2. Centre for Computational Biology, Duke-NUS Medical School, Singapore, Singapore
    Contribution
    Investigation, Data analysis, Writing
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-3700-9036
  2. Hans Heemskerk

    1. Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    2. BioSyM and CAMP Interdisciplinary Research Group, Singapore-MIT Alliance for Research and Technology, CREATE, Singapore, Singapore
    Contribution
    Investigation, Methodology
    Competing interests
    No competing interests declared
  3. Julia Jenkins

    Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    Contribution
    Investigation, Project administration
    Competing interests
    No competing interests declared
  4. Nur Hidayah Hamadee

    Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    Contribution
    Sample preparation
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0009-0002-8993-8419
  5. Ralph Bunte

    Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    Contribution
    Methodology
    Competing interests
    No competing interests declared
  6. Lisa Tucker-Kellogg

    1. Cancer & Stem Cell Biology, Duke-NUS Medical School, Singapore, Singapore
    2. Centre for Computational Biology, Duke-NUS Medical School, Singapore, Singapore
    3. BioSyM and CAMP Interdisciplinary Research Group, Singapore-MIT Alliance for Research and Technology, CREATE, Singapore, Singapore
    Contribution
    Conceptualization, Supervision, Funding acquisition, Writing
    For correspondence
    tuckerNUS@gmail.com
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-1301-7069

Funding

National Medical Research Council (NMRC/OFIRG/0007/2016)

  • Lisa Tucker-Kellogg

Ministry of Education - Singapore (MOE2019-T2-1-138)

  • Lisa Tucker-Kellogg

National Research Foundation Singapore (CAMP IRG)

  • Hans Heemskerk
  • Lisa Tucker-Kellogg

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Acknowledgements

We acknowledge funding from the Singapore Ministry of Health’s National Medical Research Council (NMRC/OFIRG/0007/2016) and also from the Singapore Ministry of Education’s Tier2 Grant (MOE2019-T2-1-138) and the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program, through Singapore MIT Alliance for Research and Technology (SMART): Critical Analytics for Manufacturing Personalised-Medicine (CAMP) Inter-Disciplinary Research Group.

We thank Manjunatha Kini and Koh Cho Yeow for naniproin; Ann-Marie Chacko, Tham Jing Yang, and Sarina Heng for detection methods; N Suhas Jagannathan for image analysis; Aadya S Deshpande, Korn Laongkul, Khadijah Zulkifli, and Colin Nicholas Sng for assistance; Peter TC So, Paul Matsudaira, Jasmine Chin, Mynn Varela, Ruklanthi de Alwis, Keshmarathy Sacadevan, the Mechanobiology Institute (Singapore) Microscopy facility, and the SingHealth Advanced BioImaging facility for advice.

Ethics

Animal experiments were approved by the institutional animal care and use committee (IACUC SHS/2016/1257) of SingHealth, Singapore.

Senior Editor

  1. Mone Zaidi, Icahn School of Medicine at Mount Sinai, United States

Reviewing Editor

  1. Sakae Tanaka, University of Tokyo, Japan

Version history

  1. Preprint posted: March 8, 2022 (view preprint)
  2. Received: January 31, 2023
  3. Accepted: April 25, 2023
  4. Version of Record published: June 2, 2023 (version 1)

Copyright

© 2023, Nasir et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

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  1. Nurul Jannah Mohamed Nasir
  2. Hans Heemskerk
  3. Julia Jenkins
  4. Nur Hidayah Hamadee
  5. Ralph Bunte
  6. Lisa Tucker-Kellogg
(2023)
Myoglobin-derived iron causes wound enlargement and impaired regeneration in pressure injuries of muscle
eLife 12:e85633.
https://doi.org/10.7554/eLife.85633

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https://doi.org/10.7554/eLife.85633

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