Accumulation of dysfunctional chondrocytes has detrimental consequences on the cartilagehomeostasis and is thus thought to play a crucial role during the pathogenesis of osteoarthritis(OA). However, the underlying mechanisms of phenotypical alteration in chondrocytes areincompletely understood. Here, we provide evidence that disruption of the intracellularvimentin network and consequent phenotypical alteration in human chondrocytes results in anexternalization of the intermediate filament. The presence of the so-called cell surfacevimentin (CSV) on chondrocytes was associated with the severity of tissue degeneration inclinical OA samples and was enhanced after mechanical injury of cartilage tissue. By meansof a doxorubicine-based in vitro model of stress-induced premature senescence (SIPS), wecould confirm the connection between cellular senescence and amount of CSV. AlthoughsiRNA-mediated silencing of CDKN2A clearly reduced the senescent phenotype as well asCSV levels of human chondrocytes, cellular senescence could not be completely reversed.Interestingly, knockdown of vimentin resulted in a SIPS-like phenotype and consequentlyincreased CSV. Therefore, we concluded that the integrity of the intracellular vimentinnetwork is crucial to maintain cellular function in chondrocytes. This assumption could beconfirmed by chemically- induced collapse of the vimentin network, which resulted in cellularstress and enhanced CSV expression. Regarding its biological function, CSV was found to beassociated with enhanced chondrocyte adhesion and plasticity. While osteogenic capacitiesseemed to be enhanced in chondrocytes expressing high levels of CSV, the chondrogenicpotential was clearly compromised. Overall, our study reinforces the importance of thevimentin network in maintenance of the chondrogenic phenotype and introduces CSV as anovel membrane-bound marker of dysfunctional chondrocytes.
The manuscript provides evidence for an association between cell surface vimentin (CSV) and chondrocyte senescence. The authors have provided solid in vitro evidence about the role of cell surface vimentin (CSV) in chondrocyte senescence and its potential in OA development, although it is lacking in vivo evidence to prove the conclusion. Overall, this is well suited for a broad audience and is important for the OA/chondrocyte biology community.https://doi.org/10.7554/eLife.91453.sa0
Osteoarthritis (OA) is the most common joint disease worldwide and associated with various pathophysiological processes, including oxidative stress, cell death, inflammation, and catabolism. The presence of inflammatory cytokines, reactive oxygen species (ROS), and damage-associated molecular patterns create a harmful pathophysiological environment, which leads to cellular damage and phenotypical alteration of chondrocytes (Riegger and Brenner, 2020). By means of single-cell RNA sequencing, researchers could provide evidence for the accumulation of dysfunctional chondrocyte subtypes in OA cartilage, exhibiting a senescent, hypertrophic, or fibroblast-like phenotype (Ji et al., 2019; Chou et al., 2020). Phenotypical alteration of chondrocytes and subsequent dysfunctional behavior plays a crucial role in progressive cartilage degeneration. In fact, the secretome of OA chondrocytes contains excessive amounts of cytokines and chemokines, including IL-6, IL-8, and CXCL1, as well as catabolic enzymes, such as MMPs and ADAMTS (Riegger and Brenner, 2020). In case of senescent chondrocytes, the secretion of detrimental proteins is also referred to as senescence-associated secretory phenotype (SASP) (Jeon et al., 2018). SASP factors not only drive ongoing destruction of the extracellular matrix (ECM), but might also affect neighboring cells, in a paracrine manner. This leads to the activation of immunocompetent cells and subsequent inflammation, on one hand, and spreading of the senescent phenotype among chondrocytes, on the other hand (Jeon et al., 2018; Vinatier et al., 2018).
Dysfunctional and senescent chondrocytes have emerged as a central target in OA therapy. Accordingly, clearance of senescent chondrocytes has been found to attenuate the risk of injury-induced OA (Jeon et al., 2017). However, identification, characterization, and therapeutic targeting of senescent chondrocytes remains challenging due to the high heterogeneity in cellular senescence (Kirschner et al., 2020). Therefore, further cell type-specific investigation of the senescence-associated (SA) characteristics as well as the underlying mechanisms involved in senescence are needed.
Recently, an oxidized form of membrane-bound vimentin, namely malondialdehyde (MDA)-vimentin, was identified on senescent fibroblasts (Frescas et al., 2017). Initially, CSV has been considered as a marker of cancer stem-like cells, exhibiting epithelial-mesenchymal transition (EMT) characteristics (Mitra et al., 2015). However, current studies imply that CSV might also be expressed on non-cancer cells, such as bone marrow-derived mesenchymal stem cells (bmMSC) (Ise et al., 2019) and apoptotic neutrophils (Moisan and Girard, 2006). Despite increasing interest in the dislocated intermediate filament, the role and biological function of CSV is only poorly understood. Interestingly, disassembly of the intracellular vimentin network impaired chondrogenic characteristics in chondrocytes and thus contributes to an OA phenotype, as proposed by Blain et al. (Tolstonog et al., 2001; Blain et al., 2006). Since there are no reports about a potential link between the intracellular vimentin network and CSV, we wondered whether CSV can be found on human articular chondrocytes (hAC) and if the dislocation results from vimentin network disruption and consequent phenotypical alteration of cartilage cells.
Here, we provide first evidence of enhanced CSV on senescent hAC. CSV was associated with the severity of OA and was triggered by mechanical injury of cartilage tissue or doxorubicine (Doxo) stimulation and subsequent stress-induced premature senescence (SIPS). Interestingly, CSV was enhanced in response to the disruption of the intracellular vimentin network and ROS accumulation in vitro, indicating a link between intracellular integrity of vimentin, cellular stress, and the externalization of the intermediate filament. Furthermore, the results imply that CSV is associated with chondrocyte plasticity and dedifferentiation, underlining the importance of the vimentin network in the stabilization of the chondrogenic phenotype. Overall, this is the first report of CSV as novel membrane-associated indicator of chondrocyte plasticity and senescence.
In order to investigate the incidence of chondrosenescence and CSV-positive cells in OA, we compared hAC isolated from macroscopically intact (OARSI grade ≤1) and highly degenerated human cartilage tissue (OARSI grade ≥3) derived from the same patient. Exemplary characterization of the corresponding tissues using Saf-O staining confirmed clear differences regarding the proteoglycan content, structural surface integrity, and cellularity between macroscopically intact and highly degenerated tissue (Figure 1a and b). mRNA levels of CDKN1A and CDKN2A were significantly enhanced in OARSI grade ≥3 as compared to OARSI grade ≤1 tissue, while that of SIRT1 was decreased (Figure 1c). SIRT1 has been described as an essential negative regulator in cellular senescence and was found to protect chondrocytes against IL-17-induced senescence (Lee et al., 2019; Wang et al., 2021).
Moreover, SA-β-galactosidase (SA-β-gal) and CSV were significantly higher in isolated hAC derived from highly degenerated cartilage as compared to hAC from donor-matched macroscopically intact tissue (Figure 1d–g). Taken together, increased activity of SA-β-gal positively correlated with the percentage of CSV-positive hAC (Figure 1h).
Our ex vivo human cartilage trauma model is suitable to study injury-induced oxidative stress, cell death, and degenerative processes (Riegger and Brenner, 2020; Riegger and Brenner, 2019; Riegger et al., 2016). Since ROS and cellular damage, in general, have been described as crucial triggers in chondrosenescence (Jeon et al., 2018; Jeon et al., 2017), we investigated the expression of SA markers and CSV on hAC after ex vivo traumatization. Seven days after traumatization, we could histologically confirm structural and cellular alteration of the formerly macroscopically intact cartilage, similarly to that of highly degenerated samples (Figure 1i). The traumatized tissue exhibited fissures in the surface, proteoglycan loss, hypocellularity, and cluster formation – altogether typical hallmarks of OA (Riegger et al., 2018b; Riegger et al., 2018a; Lotz et al., 2010). Moreover, the expression pattern of CDKN1A, CDKN2A, and SIRT1 in traumatized cartilage explants was comparable to that in OARSI grade ≥3 tissue (Figure 1j–l). While mRNA levels of CDKN1A remained elevated over 14 days after cartilage trauma, that of CDKN2A and SIRT1 were largely restored to the control levels by time. Moreover, CSV was significantly higher on hAC derived from traumatized cartilage as compared to donor-matched hAC from unimpacted tissue (Figure 1m).
To verify that CSV was associated with a senescent phenotype of hAC, we used our Doxo-based in vitro model to induce SIPS in isolated hAC (Kang et al., 2019; Kirsch et al., 2022). As expected, Doxo treatment resulted in enhanced SA marker expression as well as SA-β-gal activity (Figure 2a and b), which was accompanied by a significant increase in CSV (Figure 2c and d).
For further investigation of the flow cytometric data, we defined four regions. Q1: CSV+/SSC-H <400 (normal granularity). Q2: CSV+/SSC-H >400 (unusually high granularity). Q3: CSV-/SSC-H <400. Q4: CSV-/SSC-H >400. This way, we found significant changes in the distribution to the four regions in hAC after Doxo stimulation as compared to the untreated control, characterized by a shift of the main population from SSC-H <400 to SSC-H >400, indicating an enhanced granularity, but also significantly increased numbers of CSV-positive cells, located in Q1 and Q2 (Figure 2e–g). Moreover, Doxo treatment resulted in morphological alteration of hAC, which was comparable to hAC isolated from highly degenerated cartilage (Supplementary file 1). Development of an abnormal fibroblast-like or flattened cell shape as well as increased granularity in Doxo-treated hAC has been previously described (Kirsch et al., 2022).
To visualize enhanced CSV levels on Doxo-treated hAC and further investigate whether there are similarities in the binding sites between the intracellular VIM antibody [clone EPR3776] and the CSV antibody [clone 84–1], non-permeabilized hAC were co-stained with the corresponding antibodies (Figure 2h). In line with the cytometric analysis, immunofluorescence staining confirmed increased CSV levels on hAC after Doxo treatment. While EPR3776 staining, which binds to the C-terminus of vimentin (aa 425–466), was mainly co-localized with 84-1 on control cells, the CSV-specific staining was predominantly found in the absence of EPR3776, and particularly in Doxo-treated cells, implying the presence of a modified vimentin structure, which could not be detected by EPR3776. Further information about the CSV-specific antibody can be found in the patent (international publication number WO2014138183A1). This preliminary experiment implies that the structure of externalized vimentin is possibly modified. Moreover, we performed a scanning electron microscopy (SEM) analysis, which indicated that CSV does not form a filamentous structure (Figure 2i). That CSV might form oligomers of about 4–12 monomers instead of filaments has been previously been proposed (Hwang and Ise, 2020).
Immunofluorescence staining of intracellular vimentin revealed that Doxo-treated chondrocytes exhibited an abnormal vimentin arrangement, characterized by increased vimentin bundling, or a (nearly) complete loss of vimentin (Figure 2j). Correlation analysis between CSV-positivity and relative mRNA levels of VIM confirmed a negative association between the gene expression and translocation of vimentin (Figure 2k). These findings indicate that the disruption of the vimentin network might be connected to a SIPS-like phenotype and CSV expression.
The CDKN2A gene encodes for p14ARF and p16INK4A, which are crucial regulators in cell cycle arrest and consequent senescence. As described above, both CSV and CDKN2A expression were similarly enhanced in OA as well as after cartilage trauma or Doxo stimulation. Therefore, we assumed a connection between CDKN2A-mediated senescence and CSV-positivity.
As siRNA-mediated silencing of CDKN2A was previously described to result in a recovery of chondrogenic characteristics and reduction of SA-β-gal in OA hAC (Zhou et al., 2004), we considered a knockdown of CDKN2A to further clarify the impact of p14ARF/p16INK4A on chondrosenescence and CSV-positivity. By means of a special transfection regime (Figure 3a), we achieved a knockdown of CDKN2A in hAC over 7 days, which was confirmed by qRT-PCR and immunofluorescence staining (Figure 3b–d). Moreover, knockdown of CDKN2A significantly decreased the gene expression of SA markers CDKN1A, CXCL1, and IL8 (Figure 3e) and enhanced cell proliferation (Figure 3f). However, reduction in CDKN2A could not completely reverse the senescent phenotype and had only minor, though significant, effect on SA-β-gal activity (Figure 3g and h) and CSV (Figure 3i). Subsequent correlation analysis between relative CDKN2A expression and CSV-positivity did not reveal any significant association in case of the knockdown experiment but after Doxo stimulation (Figure 3j). Taken together, knockdown of CDKN2A could not completely reverse CSV presentation on hAC, implying that externalization of vimentin and chondrosenescence do not exclusively depend on CDKN2A.
Overexpression of vimentin was found to induce a senescence-like morphology in fibroblasts, characterized by a flattened and enlarged cell shape (Nishio et al., 2001). However, disruption of the vimentin network has been associated with a decline in chondrogenic characteristics and an OA phenotype in chondrocytes (Blain et al., 2006). As we assumed a connection between alterations in the intracellular vimentin network and CSV, we performed an siRNA-mediated knockdown of VIM in hAC.
By using the transfection regime depicted before (Figure 3a), we applied a knockdown over 7 days, which was confirmed by quantitative real-time PCR (qRT-PCR), flow cytometry, and immunofluorescence staining (Figure 4a–c). Reduction in intracellular vimentin did not result in cell death. However, we observed that the typical cage-like distribution of vimentin around the nuclei was lost and the filaments were condensed in siVIM-transfected hAC, indicating a disruption of the remaining vimentin network (Figure 4c). Furthermore, silencing of VIM resulted in a progressive alteration of the cell morphology, comparable to that of OA hAC (Supplementary file 1) and provoked significant accumulation of ROS and SA-β-gal (Figure 4d–g). In line with this, the gene expression levels of various SA markers were enhanced, which was significant in case of CDKN1A, CDKN2A, and IL6 (Figure 4h). Additionally, we confirmed nuclear accumulation of p53 (Figure 4i) and found indications of cell cycle arrest in siVIM-transfected hAC as demonstrated by lower proliferation and an increased percentage of cells in G0/G1 phase (Figure 4j and k). Moreover, silencing of VIM clearly suppressed COL2 production of hAC (Supplementary file 1). These findings are in agreement with Tolstonog et al., 2001, who observed enhanced oxidative stress and SIPS in embryonic fibroblasts from vimentin knockout mice.
Interestingly, CSV was significantly enhanced after knockdown of VIM (Figure 4l and m). Correlation analysis between CSV-positivity and relative mRNA levels of VIM confirmed a negative association between the gene expression and translocation of vimentin (Figure 4n).
Therefore, we assumed that chondrosenescence and consequent increase of CSV can be attributed to the disturbance of the intracellular vimentin network and subsequent oxidative stress.
To further confirm our assumption that disturbance of the vimentin network results in chondrocyte senescence and vimentin dislocation to the membrane, we investigated if direct disruption of the vimentin network by simvastatin (Sim) might increase CSV and promote SIPS. According to Trogden et al., 2018, who previously described the effects of Sim on the intermediate filament in adrenal carcinoma SW13 cells, we observed a reorganization of intracellular vimentin. Co-staining with phalloidin confirmed that Sim had only minor effect on the actin cytoskeleton but resulted in a strong condensation of vimentin filaments, which were selectively located in close proximity to the nuclei (Figure 5a). Similar to the phenotype observed after siRNA-mediated VIM knockdown, the vimentin ‘cage’ around the nucleus was collapsed, resulting in smaller size and elliptical shape of the nuclei (Figure 5a). Sim-treated hAC further exhibited enhanced levels of CSV (Figure 5b and c), cytoplasmatic and mitochondrial ROS (Figure 5d–f), and SA-β-gal activity (Figure 5g, h). Accumulation of mitochondrial ROS might be explained by the fact that some cytoskeletal components, and in particular vimentin, play a crucial role in mitochondrial morphology and function (Tolstonog et al., 2001; Mado et al., 2019).
Overall, these results support the assumption that CSV on hAC is triggered by breakdown or structural alteration of the intracellular vimentin network and subsequent cellular stress and senescence.
Chondrocytes are known to rapidly lose their chondrogenic phenotype and undergo SA alterations in 2D culture (Lee et al., 2019). Dedifferentiated chondrocytes at passages ≥4 are characterized by enhanced levels of stem cell-associated markers (Fickert et al., 2004; Barbero et al., 2003), augmented cellular plasticity, and tri-linear differentiation capacities (Barbero et al., 2003; Caron et al., 2012; Varela-Eirín et al., 2018). Interestingly, Varela-Eirín et al. described increasing levels of CD105 and CD166 during in vitro expansion of chondrocytes as indicator of an immature phenotype with high cellular plasticity but also SA characteristics (Varela-Eirín et al., 2018).
Consequently, we evaluated CSV levels on hAC during in vitro expansion and its association to cellular plasticity. In fact, CSV increased in a passage-dependent manner (Figure 6a and b) and strongly correlated with the expression of CD105 and CD166 (Figure 6c).
To further investigate the role of CSV as a potential marker of cellular plasticity in hAC, dedifferentiated chondrocytes (passages 4–5) were re-differentiated into osteogenic or chondrogenic lineage for 28 days, which was confirmed by expression analysis of corresponding markers and exemplary staining (Supplementary file 1). CSV levels were significantly diminished after differentiation, while subsequent culturing at basal conditions and sub-confluent density restored CSV (Figure 6d) and decreased osteogenic as well as chondrogenic markers (Supplementary file 1-Figure S3a,b,d,e). Additional mRNA analysis confirmed the dynamics of CD105 and CD166 as well as VIM expression during the differentiation experiment, and lower mRNA levels of VIM in dedifferentiated chondrocytes (Supplementary file 1-Figure S4a-c), reinforcing the assumption that VIM gene expression is inversive to the presentation of CSV.
Since CSV correlated with stem-like markers, we wondered whether CSV might be a progenitor-specific marker. While isolated bm-MSC, indeed, exhibited very high levels of CSV, we could not find any significant difference in CSV levels between passage-matched hAC and cartilage-derived chondrogenic stem progenitor cells (CSPC) (Figure 6e). Due to the fact that CSV was linked to a dysfunctional behavior in hAC, so far, we assumed that CSV might indicate a compromised chondrogenic potential. Indeed, we found a positive correlation between CSV-positivity and matrix calcification after osteogenic differentiation of high-passage hAC (Figure 6f), while in vitro chondrogenesis was declined in a CSV-dependent manner (Figure 6g).
Altogether these findings suggest that an increased level of CSV on hAC is associated with an enhanced cellular plasticity but decreased chondrogenic capacities.
Vimentin is known to contribute to integrin-mediated cell adhesion and binding to ECM components, respectively (Bhattacharya et al., 2009). Interestingly, we observed that the adhesion of hAC on culture plates increased with higher CSV levels. Accordingly, cells detached within 20 min upon EDTA exposure had significantly lower CSV levels as compared to the cells, which remained on the plates (Figure 7a). By means of an adherence assay, it could be confirmed that donors with low CSV-positivity (below 14%) detached significantly faster as compared to that with CSV levels of 16% and higher (Figure 7b). Subsequent correlation analysis revealed a significant association between the CSV-positivity and adherence (Figure 7c). A similar outcome was obtained by using the cancer cell lines HeLa (low CSV) and SaOs-II (high CSV) (Supplementary file 1). Further investigation regarding the adhesion of hAC to different ECM components demonstrated a significant association between CSV levels and binding affinity to fibronectin and vitronectin, while no association was found in case of tenascin (Figure 7d) or Collagen I, II, and IV (Figure 7e) as determined by means of an ECM cell adhesion array. Interestingly, knockdown of vimentin, which induced cellular stress and enhanced CSV levels as described above, significantly increased the gene expression of αV, α5, and β1 integrin subunits in hAC (Figure 7f).
In sum, these results imply that CSV contributes to adhesion strength of hAC on plastic and might be involved in the interaction with certain ECM components.
Senescent chondrocytes are characterized by a dysfunctional behavior, which is thought to contribute to ongoing cartilage degeneration and thus OA progression (Jeon et al., 2018; Coryell et al., 2021). Here, we provide first evidence for the importance of the vimentin network in chondrosenescence. In this context, we introduce CSV as a membrane-associated indicator of vimentin disruption and consequent damage and senescence in hAC. In sum, we could demonstrate that CSV was (1) enhanced in damaged and dysfunctional hAC after traumatic injury or due to OA and (2) was associated with an OA-like phenotype as well as cell plasticity. Moreover, CSV on hAC was (3) strongly enhanced in response to the disruption of the intracellular vimentin network and consequent stress and (4) loss of the chondrogenic capacities (Figure 7g).
Vimentin is an essential component of the cytoskeleton. Besides cell morphology and mechanical integrity, vimentin is involved in different cellular processes, including migration, adhesion, and differentiation (Bobick et al., 2010; Ivaska et al., 2007). However, overexpression of vimentin is also considered as a hallmark of EMT in cancer cells (Ivaska et al., 2007) and was found to induce a senescent cell morphology in fibroblasts (Nishio et al., 2001). In chondrocytes, IL-1β stimulation resulted in reduced gene expression of vimentin via p38-signaling, which is also associated with posttraumatic cartilage degeneration and OA progression (Joos et al., 2008; Ding et al., 2010). Moreover, disassembly and collapse of the cage-like vimentin network, observed after acrylamide treatment or traumatic cartilage loading, has been linked to decreased cell stiffness as found in OA chondrocytes (Henson and Vincent, 2008; Haudenschild et al., 2011). Accordingly, it has been hypothesized that the vimentin network is involved in the maintenance of the chondrogenic phenotype while its disassembly, in turn, can disturb chondrocyte characteristics, contributing to the development of OA (Blain et al., 2006). Beyond that, we provide clear evidence that alterations of the vimentin network and enhanced CSV levels, respectively, are associated with compromised chondrogenic capacities and a SIPS phenotype.
Besides increased CSV levels, Sim-mediated breakdown of the vimentin network remarkably changed the nuclear shape and enhanced mitochondrial ROS levels. In fact, vimentin filaments are involved in the regulation of the nuclear shape, mechanics, and chromatin condensation, as well as mitochondrial structure and function (Trogden et al., 2018; Keeling et al., 2017). The interaction between vimentin and mitochondria depends on the small GTPase Rac1 – a target of Sim (Matveeva et al., 2015). In contrast to other vimentin-binding molecules, such as withaferin A, which also affect microtubules and microfilaments, Sim is considered to bind vimentin with high specificity (Trogden et al., 2018; Grin et al., 2012; Lavenus et al., 2020). Although enhanced oxidative stress and subsequent induction of senescence via the p53/p21 pathway has been previously found in Sim-treated human melanoma cells (Guterres et al., 2013), it should be noted that Sim has also been described as an antioxidative and chondroprotective drug in cartilage (Riegger et al., 2022).
Previously, other surface-associated markers have been reported on senescent chondrocytes, such as urokinase plasminogen activator surface receptor (uPAR) (Kirsch et al., 2022; Schwab et al., 2004), dipeptidyl peptidase 4 (DPP-4) (Bi et al., 2019), and connexin43 (Varela-Eirín et al., 2018). Except for DPP-4, all three surface markers (CSV, connexin43, and uPAR) are connected with EMT, thus indicating alteration in cell properties, in particular migration and adhesion. Moreover, increased expression of different EMT makers, such as Twist-1, N-cadherin, and vimentin, have been reported in OA chondrocytes (Varela-Eirín et al., 2018; Hasei et al., 2017). Nevertheless, the hypothesis that chondrocytes might undergo an EMT-like process remains controversially discussed, because chondrocytes are mesenchymal and not epithelial cells. In a recent review, Gems and Kern, 2022 propose to consider senescent chondrocytes as activated and hyperfunctional remodeling cells occurring during OA progression (Loeser, 2014). Accordingly, chondrosenescence might represent an unsuccessful attempt of tissue repair. They further suppose that the senescent or activated chondrocytes are associated with a hypertrophic, bone-forming phenotype, following the process of bone development rather than hyaline cartilage formation. In line with this, we observed that CSV was associated with enhanced osteogenic capacities and a decline in chondrogenic properties.
Moreover, CSV has also been linked to stemness, due to its strong expression on stem cell-like cancer cells as well as on rat bmMSC (Mitra et al., 2015; Ise et al., 2019). However, neither the underlying mechanism of vimentin externalization nor its biological function could be sufficiently clarified so far. Previously, Frescas et al., 2017 identified an MDA-modified vimentin, on the cell surface of senescent fibroblasts. They hypothesized that the presentation of MDA-vimentin might serve as a ‘eat me’-signal to phagocytes. Although CSV has also been described on apoptotic neutrophils, it could not be clarified whether this really represents an ‘eat me’-signal or not (Moisan and Girard, 2006). In our study, we could not observe any connection between cell death and CSV-positivity of hAC, not even after direct induction of apoptosis by means of cycloheximide and TNF (Supplementary file 1).
In the current study, we observed that CSV on chondrocytes was reduced by siRNA-mediated silencing of CDKN2A and increased after Doxo treatment or cartilage trauma. While we confirmed that mRNA levels of both CDKN1A and CDKN2A were significantly enhanced upon injury but exhibited different expression levels over time, we determined CSV-positive cells only at one time point after ex vivo cartilage trauma. Future studies might also consider earlier and later time points after cartilage injury to identify a potential time-dependent peak or decline in CSV-positive chondrocytes. In this way a potential association between CSV and the expression levels of CDKN1A and CDKN2A, which are thought to play differential roles in initiating and maintenance of senescence, respectively (Riegger et al., 2023), might be clarified.
Our preliminary results concerning the biological function of CSV imply that CSV promotes plastic adhesion, as well as the binding affinity to fibronectin and vitronectin. Both observations might be closely linked as plastic adhesion is mainly mediated by fetal bovine serum (FBS)-derived vitronectin and/or fibronectin. In chondrocytes, interaction with fibronectin and vitronectin mainly involves α5β1 and αVβ3 integrins, which both recognize the Arg-Gly-Asp (RGD) sequence of these ECM components (Loeser, 2014). Interestingly, it has previously been described that β1 and β3 integrins can recruit vimentin to the cell membrane, where the intermediate filaments interact with focal adhesion proteins and increase the adhesive strength on plastic (Bhattacharya et al., 2009). Moreover, intracellular vimentin was found to recruit paxillin to the focal adhesion complex, which enhanced β1 integrin clustering and consequent collagen binding (Ostrowska-Podhorodecka et al., 2021). Although we found clear indications for an association between CSV and cell adhesion, as well as increased mRNA levels of certain integrin subunits after silencing of vimentin, the underlying mechanisms and potential interdependence with integrins clearly deserve further investigation.
The usage of human material should be considered as a strength of this study; however, the OA origin also represents a crucial limitation. Although the material was classified in macroscopically intact (grade ≤1) and highly degenerated (grade ≥3) tissue, it cannot be completely excluded that the intact cartilage has been affected by OA on molecular level. Nevertheless, we still could demonstrate significant differences regarding SA marker expression in highly degenerated or impacted cartilage as compared to macroscopically intact tissue.
In sum, our study provides first evidence of CSV as a novel surface marker of phenotypical alteration and SIPS in hAC. Although we could not clarify the underlying mechanism of vimentin translocation, the negative association between intracellular vimentin levels and CSV-positivity implies that the externalization does not require additional vimentin but rather represents a reallocation of existing filaments in response to cellular stress. Overall, our findings suggest that presentation of CSV most likely results from vimentin network disturbance and is associated with a dysfunctional phenotype in hAC.
hAC were enzymatically isolated from human cartilage with either OARSI grade ≥3 (highly degenerated) or OARSI grade ≤1 (macroscopically intact) (Waldstein et al., 2016). In short, full-thickness cartilage was minced and digested for 45 min with 0.2% pronase (Sigma-Aldrich) and overnight with 0.025% collagenase (Sigma-Aldrich). CSPC were isolated by outgrowth of the migratory active cells from intact human cartilage by as previously described (Riegger et al., 2018b).
Cells were cultured in serum-containing chondrocyte (=basal) medium (1:1 DMEM 1 g/L glucose/Ham’s F12, 10% FBS, 0.5% penicillin/streptomycin, 2 mM L-glutamine, and 10 μg/mL 2-phospho-L-ascorbic acid trisodium salt) and split at a confluence of 80%.
Full-thickness cartilage explants (Ø=6 mm) from macroscopically intact tissue (OARSI score ≤1) were harvested and cultivated in chondrocyte medium for 24 hr in an incubator (37°C, 5% CO2, 95% humidity). Afterward, the explants were subjected to a single impact load of 0.59 J using a drop-tower model as previously described (Riegger and Brenner, 2019; Riegger et al., 2016; Riegger et al., 2018a) and further cultivated under serum-free conditions (DMEM, 1% sodium pyruvate, 0.5% L-glutamine, 1% non-essential amino acids, 0.5% penicillin/streptomycin, 10 μg/mL 2-phospho-L-ascorbic acid trisodium salt, and 0.1% insulin-transferrin-sodium selenite).
SIPS was induced by stimulation with 0.1 µM Doxo (Selleckchem, S1208) for 10 days (Kirsch et al., 2022). Activated Sim was applied for 20 hr at a concentration of 10 µM. Induction of apoptosis was attained by stimulation with cycloheximide (10 µg/mL; Selleckchem, S7418) and TNF (10 ng/mL; Peprotech, 300-01A) as previously reported (Riegger and Brenner, 2019). All stimulations were performed in serum-reduced chondrocyte medium (5% FBS).
Quantitative measurement of cell proliferation/cytotoxicity was attained by means of an alamarBlue assay (Bio-Rad, Munich, Germany). Cells were incubated for 3 hr in a 5% alamarBlue solution (in DMEM) at 37°C. Fluorescence intensities were detected at 550 nm excitation and 590 nm emission by using the multimode microplate reader Infinite M200 Pro (Tecan Austria GmbH Groedig, Austria). Blank values (5% alamarBlue solution in empty well) were subtracted from measured values. Results were calculated relative to unstimulated cells.
To analyze surface markers on freshly isolated hAC, cartilage tissue was directly digested by means of collagenase overnight, without pronase. In case of cultured hAC, cells were detached with PBS-buffered EDTA (5 mM). hAC were stained with anti-CSV (clone 84-1, abnova, #H00007431-MF08) or IgG2bκ isotype control (BD Pharmingen) for 40 min. A minimum of 2×104 cells was analyzed on a Becton Dickinson FACSCalibur flow cytometer (BD Biosciences) with dual-laser technology using the corresponding software CellQuest version 5.2.1. A maximum of 1% isotype control-positive cells was accepted. Cancer cell lines HeLa (low CSV levels; mean: 11.8%±4.2) and SaOs-II (mediate/high CSV levels; mean: 34.4%±8.8) were used to validate the anti-CSV antibody and the staining protocol (Supplementary file 1). While CVS was found to be degraded by trypsin and accutase treatment, no influence was observed in case of collagenase relative to EDTA (Supplementary file 1).
SA-β-gal staining was performed using an SA-β-gal staining kit according to the manufacturer’s protocol (Cell Signaling Technology, Danvers, MA, USA). In short, cells were seeded on chamber slides (5000 cells/cm2). Cells were fixed in a 2% formaldehyde and 0.2% glutaraldehyde solution for 15 min. After washing with PBS, cells were stained overnight in an X-gal staining solution at 37°C (dry incubator; low CO2).
In case of the SA-β-gal activity assay, a fluorometric cellular senescence assay kit was used according to the manufacturer’s protocol (Cell Biolabs Inc, San Diego, CA, USA). In short, cells were seeded on a 96-well plate (5000 cells per well). The next day, cells were lysed using the cell lysis buffer and centrifuged. The supernatants of the cell lysates were mixed with the assay buffer and incubated for 2 hr at 37°C in the dark. After stopping the reaction using the stop solution, fluorescence was measured on a black bottom 96-well plate at 360 nm excitation and 465 nm emission using an infinite M200 PRO TECAN reader.
Adherent hAC were incubated in a digest solution containing 1 mg/mL collagenase, 1 mg/mL protease, and 40 U/mL hyaluronidase to increase siRNA transfection efficacy. Detached cells were counted and seeded at a density of 20,000 cells/cm2. Silencer Select siRNA (Thermo Fisher Scientific) against CDKN2A (ID: s218; targeting both p16INK4a and the related transcript p14ARF), VIM (ID: s14800), or Silencer Negative Control siRNA was diluted in Lipofectamine P3000 (Thermo Fisher Scientific) and added to the cells at a final concentration of 5 pmol/μL in serum-free Opti-MEM (Thermo Fisher Scientific; 31985062) without antibiotics. After 4 hr at 37°C, transfection medium was removed and serum-containing chondrocyte medium was added. After 72 hr, hAC were trypsinized, counted, and seeded as mentioned above. The next day, transfection was repeated in adherence. This protocol allows a consistent silencing over 7 days.
Detection of cytoplasmatic and mitochondrial ROS levels was performed by means of a commercial DCFDA/H2DCFDA – Cellular ROS Assay Kit (abcam, ab113851) and MitoSOX Red Mitochondrial Superoxide Indicator (Invitrogen, Thermo Fisher Scientific, M36008), according to the manufacturer’s instructions.
For total RNA isolation, cryopreserved cartilage explants were pulverized with a microdismembrator S (B. Braun Biotech, Melsungen, Germany). Subsequently, RNA was isolated using a Lipid Tissue Mini Kit (QIAGEN, Hilden, Germany). In case of isolated cells, mRNA of least 50,000 cells was isolated using a RNeasy Mini Kit (QIAGEN). RNA was reverse transcribed with Superscript II (Thermo Fisher) and used for qRT-PCR analysis (StepOne-PlusTM Real-Time PCR System, Applied Biosystems, Darmstadt, Germany). Relative expression levels were calculated using the ∆∆Ct method, except of correlation analysis depicted in Figures 2j–4n—4, in which the ∆Ct method was required. GAPDH, HPRT1, and TMEM199 served as housekeeping genes. TMEM199 has previously been described as appropriate endogenous control to study gene expression in senescent chondrocytes (Kuwahara et al., 2020), while GAPDH or HPRT1 are the most commonly used housekeeping genes in studies on chondrosenescence.
Isolated cells were fixed with formalin, blocked for 1 hr at 37°C, and stained with anti-CSV (clone 84-1, abnova, H00007431-MF08 or H00007431-MB08), anti-VIM (clone EPR3776, abcam, ab185030), and anti-CDKN2A (abcam, ab108349) for 2 hr at RT. In case of intracellular staining, cells were permeabilized with 0.1% PBS-Tween 20. Unconjugated anti-CDKN2A was further incubated with a biotinylated link antibody (DAKO), followed by iFluor568-conjugated avidin (ATT Bioquest), each for 20 min at RT. In case of immunocytochemistry, anti-CSV H00007431-MB08 was incubated as described above, followed by further incubated with strepavidin-bound HRP (DAKO) for 20 min each. After addition of AEC substrate for 15 min, nuclei were counterstained with hematoxylin.
For chondrogenic differentiation, chondrocytes were cultivated for 28 days in CDM (DMEM 4.5 g/L glucose and Ham’s F12 (1:1), 100 U/mL penicillin/streptomycin, 40 ng/mL L-proline, 2 mM L-glutamine, 1 mM Na-pyruvate, 0.1 µM dexamethasone, 50 μg/mL ascorbic acid, 10 μL/mL ITS [Sigma-Aldrich], 10 ng/mL rhTGF-β3 [Peprotech], 10 ng/mL rhBMP6 [Peprotech]) under serum-reduced (in case of 2D culture; 1% FSC) or serum-free (in case of pellet culture) conditions as previously described (Riegger et al., 2018b). For osteogenic differentiation, chondrocytes were cultured for 28 days in ODM (DMEM 1 g/L glucose, 10% FBS, 100 U/mL penicillin/streptomycin, 2 mM L-glutamine, 0.1 µM dexamethasone, 0.2 mM ascorbic acid, and 10 mM β-glycerophosphate).
Experiments were analyzed using GraphPad Prism8 (GraphPad Software, Inc, La Jolla, CA, USA). Data sets with n≥5 were tested for outliers by means of the Grubbs outlier test. Outliers were not included in statistical analyses. Each data point represents an independent biological replicate (donor); technical replicates were not performed. Significance level was set to α=0.05. Information about the statistical analysis applied is provided in the corresponding figure legends.
All data generated or analysed during this study are included in the manuscript and supporting file.
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Di ChenReviewing Editor; Chinese Academy of Sciences, China
Mone ZaidiSenior 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.91453.sa1
Point-by-point description of the revisions
Reviewer #1 (Evidence, reproducibility and clarity (Required)):
The manuscript by Rigger and Brenner details the role of vimentin network, in advancing OA pathogenesis by exacerbating premature senescence. The data is well presented and the study of interest, in that there is little known about vimentin in cartilage biology.
The authors used OA derived cartilage explants and chondrocytes cultures, were graded for severity and compared accordingly. Figure 1 shows that markers of senescence are increased with structural damage, which is well established and consistant with the literature. Using a DOX model the authors induce premature senescence and exhibit a disrupted vimentin network. However, upon KD of CDKN2A, a marker of senescence, but did not observe complete reversal of CSV presentation.
Next the authors show in figure 4 and 5, that the reduction or dismemberment of vimentin structures are linked to senescence and may act as contributing factors.
Figures 6 and 7 then go on to show that upon advanced passage chondrocytes lose their vimentin network, and tend to senesce and mineralize.
Reviewer #1 (Significance (Required)): Strength:
This is a very novel study showing a link between vimentin and senescence in chondrocytes. The data are in line with other data. The work is clearly written structured and well displayed.
We thank reviewer #1 for their interest in our work and their overall positive report.
Suggestions for improvement:
While the study is very thorough ought in describing the markers of senescence and vimentin network, it lacks insight regarding mechanism which isn't completely deciphered. Are there links to key transcription factors?
The transcriptional regulation of vimentin in human cells is very complex. The VIM promoter region comprises multiple elements, such as a NF-κB- binding site, a PEA3-binding site and two AP1-binding sites (Zhang et al., 2003). Moreover, it was recently demonstrated that redox signaling is involved in vimentin expression at the wound margin after tissue injury in zebra fish (LeBert et al., 2018). However, it has also been reported that IL-1ß stimulation results in reduced gene expression of vimentin via p38-signalling in cartilage degeneration and OA progression (see manuscript REF. 36,37).
In our study, we observed that enhanced CSV levels are associated with a decreased vimentin gene expression, indicating a lower stability of the mRNA or decreased transcription of VIM in senescent chondrocytes (maybe due to enhanced p38-signalling as mentioned above). Since the transcriptome in senescent cells is radically changed, this question cannot be answered easily.
In future studies, we will rather try to clarify the underlying mechanism of vimentin externalization. There are still many questions to be answered: is the CSV anchored in the cell membrane (which anchor protein?) and is there still a connection to the intracellular vimentin network? Which proteins are involved in the externalization process: maybe comparable to phosphatidylserine exposure, mediated by flippases, scramblases, and lipid transfer proteins or rather by vesicles?
Literature mentioned above (not included in manuscript):
LeBert et al., 2018: Damage-induced reactive oxygen species regulate vimentin and dynamic collagen-based projections to mediate wound repair. DOI: 10.7554/eLife.30703
Zhang et al., 2003: ZBP-89 represses vimentin gene transcription by interacting with the transcriptional activator, Sp1. DOI: 10.1093/nar/gkg380
It is also unclear if disruption of the network is more detrimental than KD in promoting senescence.
KD of Vimentin led to a gradually decrease of intracellular Vimentin content and consequent stress. The cells were analyzed 7 days after induction of the KD and exhibited a stable senescent phenotype, comparable to Doxorubicin-treated chondrocytes (treated with very low concentrations over several days to produce only mild but ongoing stress). These models might reflect the pathophysiologic situation: We think that cellular stress due to mechanical impact and subsequent oxidative stress/ low-grade inflammation might lead to a gradual disruption or re-organization of the vimentin network, which is accompanied by decreased vimentin gene expression.
In case of the disruption of the vimentin network by Simvastatin, the stress response was very intense and rapid (24 h), and was only conducted as a proof-of-principle experiment. Despite the upregulation of some senescence-associated markers, we don`t think that permanent Simvastatin treatment would be suitable to obtain a stable senescent phenotype, but rather expect the cells to die due to excessive stress.
It would have been good to include models OA murine models to understand these processes better, and make a stronger physiological connection with OA of the joint.
The CSV antibody is only suitable for human cells and cannot be used for immunohistochemistry. Therefore, all previous reports of CSV are based on human (isolated) cells. At the current time point, it would not be possible to stain CSV in joints of mice after induction of PTOA due to the methodological limitations. We actually tested the CSV-antibody in isolated lapine chondrocytes and found a high percentage of CSV-positive cells, even at low passages. Although stress increased the amount of CSV-positive lapine cells, we did not consider the results as reliable due to the high percentage in un-stressed cells, which might result from unspecific antibody binding.
Overall, we think that the usage of clinical OA samples is convincing and reflect the pathophysiologic situation in the human OA joint.
Reviewer #2 (Evidence, reproducibility and clarity (Required)):
The manuscript provides solid evidence for an association between cell surface vimentin (CSV) and chondrocyte senescence. Human cartilage and cultured chondrocytes are used with a wide range of approaches to provoke senescence: natural osteoarthritis, traumatic loading ex vivo, doxorubicin to cells in monolayer, vimentin siRNA, and simvastatin. In contrast, relatively little was done to try and interrupt or reverse the role of CSV in senescence, with CDKN2A siRNA representing one attempted intervention. The manuscript is well written and the data are presented in a logical and clear manner, with a high likelihood of being reproduced in subsequent studies.
We thank reviewer #2 for their interest in our work and their mainly positive report.
Regarding their comment on our attempts to reverse CSV on senescent chondrocytes, we would like to add the following: Reversal of cellular senescence is a very ambitious challenge. But in fact, we are currently preparing a manuscript in which we characterize an appropriate senolytic strategy to “rejuvenate” human chondrocytes and plan to use this approach to reduce the amount of senescent and thus CSV-positive cells in future experiments.
In the doxorubicin experiments, the senescent cells show a spread morphology as expected. Given the importance of vimentin in cell spreading (as the authors own data show), the possibility that spread morphology itself (and not senescence) leads to CSV should probably be examined. This could perhaps be achieved by plating with different concentrations of fibronectin or other matrix proteins that produce a spread morphology to a degree that matches the doxo. If the cells remain spread for ~10 days but don't become senescent and don't have CSV, this would provide further support for a direct relationship.
We agree that cell spreading is associated with various cellular processes (for example by the YAP signaling pathway). Moreover, we would like to thank the reviewer for the proposed experiment.
Seeding of cartilage cells on fibronectin coated plates is a commonly used procedure to isolate chondrogenic stem progenitor cells, due to their higher affinity to fibronectin. The cells are usually cultured for several days on the coated plates and do not exhibit a flattened, senescent-like phenotype (as we observe for Doxorubicin-treated cells), but an elongated, fibroblast-/ stem cell-like shape. Our results (Figure 6E) demonstrate that CSPC have no increased CSV levels, despite their elongated (not flat) morphology.
There are some findings supporting the assumption that CSV leads to enhanced cell adhesion, but not that adhesion or cell spreading promotes CSV: we included experiments with HeLa (low CSV levels) and SaOS-2 (high CSV levels), which demonstrated that high CSV levels are associated with increased plastic adhesion (Figure S5). In line with this, we demonstrated that higher CSV levels on chondrocytes were associated with enhanced fibronectin and vitronectin binding, which might explain increased plastic adhesion. Moreover, Simvastatin stimulation and subsequent cellular stress by Vimentin disruption resulted in enhanced CSV but did not lead to cell spreading (Actin not affected, cells rather elongated, not flattened).
The CSV antibody and staining method appeared to have generated some signal from debris, which makes it challenging to assess the localization of true staining. Presumably the true staining would be present only on the cell surface. While the widefield view is appreciated, perhaps insets with a higher magnification would clarify.
In Figure 2h and Figure 2i, we provide insets of the IF-staining and an exemplary image made by scanning electron microscopy (SEM). CSV is not localized on debris – Figure 2h, actually represents the cell surface. The magnified, Doxo-treated cell is highly senescent and thus flattened. The uneven (rather spotted) staining pattern of CSV and the unusual shape of the cell might suggest that this is debris, not the cell membrane.
For figure 1k, it is a bit surprising that CDKN2A would peak so early after injury and then drop off. Most studies in other systems show a gradual increase in CDKN2A levels with persistent stress as opposed to a rapid increase in response to acute stress. Could the drop-off be due to preferential death of these cells? The CSV % in 1m was taken from 7d after trauma (plus 7 days in monolayer it appears). Further discussion on the timing of traditional senescence markers as compared to the emergence of CSV would be useful.
We would like to thank the reviewer for this comment. That CDKN1A was induced by mechanical trauma without significant decrease at the later time points was in line with the P53 expression, which we detected via immunohistochemistry (IHC; positive staining of chondrocyte nuclei in cartilage). P53 and P21 are regarded as interconnected senescence markers. Interestingly, P53 is not regulated on gene expression level upon cartilage trauma or Doxorubicine stimulation – but there is a significant increase in P53 nuclear translocation.
Although such a discrepancy between gene expression and protein activity has not been reported in case of P16 or P21, we plan to investigate the dynamics of these cell cycle regulators and its connection to CSV after cartilage trauma in more detail in future studies.
We included the following statement in the discussion part:
“In the current study, we observed that CSV on chondrocytes was reduced by siRNA-mediated silencing of CDKN2A and increased after Doxo treatment or cartilage trauma. While we confirmed that mRNA levels of both CDKN1A and CDKN2A were significantly enhanced upon injury but exhibited different expression levels over time, we determined CSV-positive cells only at one time point after ex vivo cartilage trauma. Future studies might also consider earlier and later time points after cartilage injury to identify a potential time-dependent peak or decline in CSV-positive chondrocytes. In this way a potential association between CSV and the expression levels of CDKN1A and CDKN2A, which are thought to play differential roles in initiating and maintenance of senescence, respectively , might be clarified.”
 Stein G, Drullinger L, Soulard A, and Dulić V. Differential Roles for Cyclin-Dependent Kinase Inhibitors p21 and p16 in the Mechanisms of Senescence and Differentiation in Human Fibroblasts. Mol Cell Biol. 1999;19(3): 2109–2117. https://doi.org/10.1128/mcb.19.3.2109.
There is no CSV staining shown for figures 4 and 5. While the quantification of CSV was done by flow cytometry, it would nice confirmation to see the increase in CSV on the surface of cells with either siRNA for vimentin or the simvastatin.
There is no CSV staining shown for figures 4 and 5. While the quantification of CSV was done by flow cytometry, it would nice confirmation to see the increase in CSV on the surface of cells with either siRNA for vimentin or the simvastatin. CSV-IF of simvastatin-treated chondrocytes is provided in Figure 5 (b). We did not perform exemplary staining of CSV after VIM-KD, because the quantification was performed via flow cytometry.
Reviewer #2 (Significance (Required)):
The strengths of the study include a rigorous design and the establishment of a potential new cell surface marker of chondrocyte senescence. The main limitation is that the conclusions are largely descriptive in nature.
If CSV is confirmed as a robust marker of senescence, this would be of value to the field. While this marker has been explored previously in other systems, there is value in this manuscript given the wide range of contexts investigated for a cell type in which senescence likely has an important role.
Reviewer #3 (Evidence, reproducibility and clarity (Required)):
This study presents a sound piece of science in the puzzle about extracellular vimentin in the differentiation/dedifferentiation of human chondrocytes and senescence and osteoarthritis. Eventhough, no mechanism is elucidated, the results clearly point towards a correlation of the amount of extra cellular vimentin and the level of chondrocyte senescence, and therefore signs of osteoarthritic changes in the cultivated chondrocytes. The methods applied are state-of-the art and provide the means to generate meaningful results in this experimental setting. The paper is concise and clearly written, there are only minor remarks.
We thank reviewer #3 for their interest in our work and their overall positive report.
Minor comments: 1. The main clue of the paper is extra cellular vinemtin around chondrites in culture, please provide better pictures (1g) to support this. Why is the extra cellular staining seen so broad and not concentrated on the cells surface? The picture chosen imply a huge amount of vimentin to be externilized in disease states. It also indicates that in diseased chondrocytes no intact or semi-intact vimentin network is found intracellular. Please comment.
In Figure 1g, CSV is located on the cell membrane. The pattern of the staining was surprising to us, as well. CSV was not equally distributed on the membrane, but rather represented an inconsistent pattern. Sometimes the staining was located at the filopodia of the cells, sometimes the whole cell was covered by spots. We also observed this on cancer cells, which was in line with other studies using this antibody. It remains unclear whether the distribution of the CSV has any effect. But we assume that the high abundance in filopodia might be connected with cell adhesion and mobility, which was positively associated with CSV.
Yes, chondrocytes isolated from highly degenerated tissue exhibited higher CSV levels as compared to cells derived from macroscopically intact regions. Although we did not investigate the vimentin network of these cells, our observations in Doxo-treated cells imply, indeed, that intracellular vimentin might be altered in diseased chondrocytes. According to this, Blain et al. (Ref. 13) reported that there is a disassembly of the intracellular vimentin network in OA chondrocytes, which can disturb the chondrocyte phenotype and contributes to the development of OA (see discussion).
2. In the doxo experiment no extracellular vimentin is found? Please explain.
Doxo-treated cells are highly positive for CSV (=extracellular vimentin on membrane). However, the intracellular vimentin is strongly decreased and some cells seem to be negative. We have not clarified the underlying mechanism by now, but it seems that senescence/ disease progression negatively affects the transcription of vimentin and, at the same time, promotes the externalization of the existing intracellular vimentin. Altogether, this might result in a decline in intracellular vimentin.
3. The SEM picture is showing what. IGH? The red dots are colloidal gold particles? In any case the quantity of stain gathered EM level would not correlate to the huge amount seen in LM staining. Please comment.
For the SEM analysis, a gold particle-coated secondary antibody was used. The positive signal usually appears in white and was subsequently colored via a software. In IF and ICC staining, we had a signal amplification due to the biotin-streptavidin system and the magnification makes, of course, a huge difference.
4. Why the ICC in Figure 3c? The siRNA is not detected in the KD? A reduction of Vimentin could be shown via WB.
In Figure 3c, the KD of P16 was confirmed on protein level. In addition to the gene expression analysis, we chose the ICC (IF) to confirm that there is a decline in active (nuclear) CDKN2A. In case of P53, we made the experience that gene expression and the amount of cytoplasmic/ nuclear protein might not be consistent.
In Figure 4, we confirmed the successful KD of vimentin on mRNA and protein level (flow cytometry plus IF). Of course, WB would also be possible, but we decided to use the methods in which the antibody was well established and we wanted to visualize the disturbance of the intracellular vimentin network upon KD.
5. Figure 4c, why are there no remnants of the vimentin networks seen in the chondrocytes? A Knock-down, not a KO is shown.
In fact, most of the intracellular vimentin seems to be gone. However, there are some remnants (condensed fibers/ bundles) of the former vimentin network. We applied the VIM-KD over seven days. Usually, a KD experiment is only conducted for 2-3 days. But since we were not sure how stable the vimentin protein would be, we chose seven days. This long-lasting KD might have resulted in a strong decline of the protein. Moreover, the CSV levels on these cells were very high, indicating that existing vimentin was externalized and additionally decreased the amount of intracellular vimentin.
6. Please comment of the concentration of simvastatin, why not nmolar?
The concentration of Simvastatin was chosen in accordance with Trogden et al. (Ref. 26), who first described the effects of simvastatin on the vimentin network. A lower concentration might have had the advantage, that the effects were less severe, allowing a longer observation time than 24h. However, as a proof-of-principle model to demonstrate the connection between vimentin network collapse ant CSV expression, the concentration worked quite well.
7. CSV+ is misleading in Figure 6g, it's not an over expression.
We would like to thank the reviewer for this comment and removed the “+” to make it less misleading.
8. The concept of EMT is debatable, at least in kidney fibrosis, and chondrocytes are not epithelial cells. Please add a more critical discussion point.
The authors agree with the reviewer’s argument that chondrocytes are no epithelial cells ant that the term EMT doesn’t seem to be appropriate. However, this is one leading hypothesis proposed by the working group of Prof. Mayán, who described CX43 and other EMT-markers on/ in senescent chondrocytes (see reference 31; more recently: Cell Death Dis. 2022;13(8):681. doi: 10.1038/s41419-022-05089-w).
We added the following passage in the discussion part to indicate that this hypothesis is a controversial concept:
“Nevertheless, the hypothesis that chondrocytes might undergo an EMT-like process remains controversially discussed, because chondrocytes are mesenchymal and not epithelial cells. In a recent review, Gems and Kern propose to consider senescent chondrocytes as activated and hyperfunctional remodeling cells occurring during OA progression . Accordingly, chondrosenescence might represent an unsuccessful attempt of tissue repair. They further suppose that the senescent or activated chondrocytes are associated with a hypertrophic, bone-forming phenotype, following the process of bone development rather than hyaline cartilage formation. In line with this, we observed that CSV was associated with enhanced osteogenic capacities and a decline in chondrogenic properties.”
 Gems and Kern, 2022: Geroscience. 2022;44(5):2461-2469. doi: 10.1007/s11357-022-00652-x.
Reviewer #3 (Significance (Required)):
The manuscript provides novel insight in the role of intermediary filaments, i.e. vimentin, on chondrocyte senescence and osteoarthritic changes in vitro. It's strength is a thorough elucidation of the connection with a wealth of experimental data, a weakness is the missing elucidation, or first experiments in the direction, of the cell biological mechanism.
It is well suited for a broad audience, because it deals with fundamental cell biological phenomena, definitely it's important for the OA /chondrocyte biology community.
- Jana Riegger
- Jana Riegger
- Jana Riegger
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
The authors would like to thank Natalie Braun for excellent technical assistance and Hartmut Geiger for his constructive input. This research study was supported by the European Social Fund and by the Ministry of Science, Research and Arts Baden-Württemberg, as well as the University of Ulm (Bausteinprogramm). Jana Riegger Margarete von Wrangell fellowship (European Social Fund and by the Ministry of Science, Research and Arts Baden-Württemberg); start-up funding of the University of Ulm (Bausteinprogramm). The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Human cartilage was obtained from donors undergoing total knee joint replacement due to OA. Informed consent was obtained from all patients according to the terms of the Ethics Committee of the University of Ulm (ethical approval number 353/18).
- Mone Zaidi, Icahn School of Medicine at Mount Sinai, United States
- Di Chen, Chinese Academy of Sciences, China
© 2023, Riegger and Brenner
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.
Cylicins are testis-specific proteins, which are exclusively expressed during spermiogenesis. In mice and humans, two Cylicins, the gonosomal X-linked Cylicin 1 (Cylc1/CYLC1) and the autosomal Cylicin 2 (Cylc2/CYLC2) genes, have been identified. Cylicins are cytoskeletal proteins with an overall positive charge due to lysine-rich repeats. While Cylicins have been localized in the acrosomal region of round spermatids, they resemble a major component of the calyx within the perinuclear theca at the posterior part of mature sperm nuclei. However, the role of Cylicins during spermiogenesis has not yet been investigated. Here, we applied CRISPR/Cas9-mediated gene editing in zygotes to establish Cylc1- and Cylc2-deficient mouse lines as a model to study the function of these proteins. Cylc1 deficiency resulted in male subfertility, whereas Cylc2-/-, Cylc1-/yCylc2+/-, and Cylc1-/yCylc2-/- males were infertile. Phenotypical characterization revealed that loss of Cylicins prevents proper calyx assembly during spermiogenesis. This results in decreased epididymal sperm counts, impaired shedding of excess cytoplasm, and severe structural malformations, ultimately resulting in impaired sperm motility. Furthermore, exome sequencing identified an infertile man with a hemizygous variant in CYLC1 and a heterozygous variant in CYLC2, displaying morphological abnormalities of the sperm including the absence of the acrosome. Thus, our study highlights the relevance and importance of Cylicins for spermiogenic remodeling and male fertility in human and mouse, and provides the basis for further studies on unraveling the complex molecular interactions between perinuclear theca proteins required during spermiogenesis.
Previously we showed that 2D template matching (2DTM) can be used to localize macromolecular complexes in images recorded by cryogenic electron microscopy (cryo-EM) with high precision, even in the presence of noise and cellular background (Lucas et al., 2021; Lucas et al., 2022). Here, we show that once localized, these particles may be averaged together to generate high-resolution 3D reconstructions. However, regions included in the template may suffer from template bias, leading to inflated resolution estimates and making the interpretation of high-resolution features unreliable. We evaluate conditions that minimize template bias while retaining the benefits of high-precision localization, and we show that molecular features not present in the template can be reconstructed at high resolution from targets found by 2DTM, extending prior work at low-resolution. Moreover, we present a quantitative metric for template bias to aid the interpretation of 3D reconstructions calculated with particles localized using high-resolution templates and fine angular sampling.