Modulating inter-mitochondrial contacts to increase membrane potential for mitigating blue light damage

Reviewer #1 (Public review):

Summary:

Blue light exposure has been shown to induce mitochondrial dysfunction, including reduced mitochondrial membrane potential (MMP). In the present study, the authors present a protein-based optogenetic system capable of inducing mito-contacts upon blue LED illumination, and show that this technical platform attenuated blue-light-induced mitochondrial dysfunction and cytotoxicity via restoring mitochondrial membrane potential.

Strengths:

The overall study design is well organized, and the data appear to support the conclusions. Additionally, demonstrating effects in human retinal cells and C. elegans enhances the perceived robustness and translational potential of the findings.

Weaknesses:

(1) Quantification of MMP at contact sites: The use of Rhodamine 123 (Rh123) for MMP measurement can be problematic, as it is not ratiometric; its signals depend on loading conditions, cell size, mitochondrial mass, and focal thickness, rather than solely on ΔΨm. If mitochondrial content changes (e.g., via biogenesis or mitophagy), Rh123 readings can be misleading. This is particularly relevant here, as the mito-contact-induced MMP changes appear to be localized events. The authors should include controls for at least one experiment using FCCP/CCCP (to collapse ΔΨm) and oligomycin (to induce hyperpolarization in many cell types) to confirm the dynamic range of the assay. Where possible, Rh123 fluorescence intensity should be normalized to mitochondrial mass (e.g., using a mass marker or mitochondrial protein). Moreover, MMP changes should be validated using an alternative indicator, such as JC-1 or a genetically encoded probe, as this is foundational to the study.

(2) Mechanisms of mito-contact-induced MMP hyperpolarization: Building on the above, what is the mechanism by which mito-contacts induce MMP hyperpolarization? Does this involve fusion of the outer or inner mitochondrial membranes? MMP hyperpolarization typically reflects an increase in protons in the intermembrane space relative to the matrix. Where do these protons originate? The kinetics of mito-contact-induced MMP changes should also be investigated in more detail.

(3) Building on the above, what is the ratio of contact area to the overall mitochondrial surface area? If MMP increases only at relatively small contact sites, how does this translate to an overall increase in MMP and energy production?

(4) Blue light causes mitochondrial damage via increased reactive oxygen species (ROS), and MMP hyperpolarization can itself lead to excessive oxidative stress. The authors should measure ROS levels and discuss their potential impact on the observed effects.

(5) Although the main focus is on blue LED-mediated injury, the protective effects of the optogenetic system against other stressors (e.g., ischemia-reperfusion, H₂O₂, or FCCP exposure) should be examined. This would help exclude confounds related to blue light, which is central to both the manipulation and the damage model in the current study, and increase the overall impact of the findings.

Reviewer #2 (Public review):

Summary:

This paper describes a novel tool (CRYO2PHR-MiroTM), which aims to create contact sites between mitochondria. One elegant aspect of the technique is that it is controlled by the exposure of cells to blue-light and reversible when cells are put back in the dark. Through an unknown and unexplored mechanism, the mitochondrial membrane potential is raised at the mitochondrial contact sites. The oligomerization of CRYOPHR-MiroTM is protective against the toxic effect of prolonged blue light exposure in cells and nematodes.

Strengths:

This work might open novel perspectives in the fundamental study of mitochondria.

(1) CRYO2PHR-MiroTM represents an interesting tool to manipulate mitochondria interaction/proximity/distribution without playing with the classical components of the mitochondrial fusion and fission machinery.

(2) This work suggests that, without the need for fusion, the relative proximity of mitochondria might influence their activity, opening novel fields of investigation in mitochondrial biology.

(3) Finally, targeting CRYO2PHR not only to mitochondria but also to their partner organelles (ER, LD, peroxisomes...) could provide a tool to reversibly manipulate the interaction of mitochondria with the rest of the organelle community.

Weaknesses:

As detailed below, the claims made by the author that CRYOPHR induce mitochondrial contact sites are not fully convincing at this stage. The method used to define and analyse contact sites is not clear enough, and the image presented in the present manuscript does not convincingly illustrate contact sites between mitochondria. Finally, the evidence that CRYOPHR does not trigger mitochondrial fusion should be strengthened.

Comments on the results:

(1) The quantification of mitochondrial contacts is a crucial point of this study. At this stage, the data are not sufficient to demonstrate that CRYOPHR-MiroTM oligomerisation tethers mitochondria. CRYOPHR-MiroTM can oligomerise in Trans, leading to mitochondrial tethering, but it can also oligomerise in Cis. In that later case, one could hypothesise that the massive aggregation of CRYOPHR-MiroTM at the mitochondrial outer membrane could locally push lipids away and/or create membrane curvature. The image and quantification provided by the author make it difficult to decide whether CRYOPHR-MiroTM tethers mitochondria or pinches their membranes. Below are detailed comments on these aspects:

a) It is claimed that "the proportion of mitochondria having one or more mito-contacts increased by nearly 50% following optogenetic stimulation". However, it is unclear how the authors have calculated this parameter. In the methods for contact ratio calculation, it is written that "the contacted area of CRY2PHR puncta was calculated", but I do not understand what it means and how it relates to contact ratio calculation. Then the authors have written, "Based on the area or distance (between mitochondria), the mitochondria were classified as either non-contact or contact". It is not clear to which parameter the term " area " refers: the area of mito-contacts based on MitoTracker or the area of CRY2PHR puncta. It is not clear how the authors integrate the two parameters "area" and "distance" to decide whether two mitochondria are in contact or not.

b) The method states that "Contact ratio refers to the number of contact mitochondria by the total number of mitochondria". What does "number of contact mitochondria" mean? The number of contacts between mitochondria? The number of mitochondria in contact? What is the distance range between two mitochondria, taking into account optic resolution, for which the authors consider that two mitochondria are "in contact"?

c) The quantification of the contact ratio made on the TEM picture should be explained.

d) The following data should be added, as contact site formation is a critical point. On cells treated or not with blue light, the author should measure systematically what is the distance of a given mitochondrion to the nearest one. The distribution of these distance values should be shown and analysed to determine whether or not there are more mitochondria at short distances upon blue light induction of CRYOPHR oligomerization. In addition, the author should determine the number of CRYO2PHR puncta that are simply lying on a mitochondrion and the number of CRYO2PHR puncta that are bridging two clear, distinct mitochondria.

e) Based on the images provided in Figure 1, there is no convincing evidence of mitochondrial contacts. In image 1g, the CRYO2PHR puncta seem to be lying on mitochondrial tubules. Sometimes, it looks that CRYO2PHR puncta decorate mitochondrial constriction sites, suggesting that the CRYOPHR might pinch membranes. The authors claim that they "found various types of mitochondrial contacts (Figure 1f, 1g), such as head-to-head, side-by-side, and head-to-side", but it is not clearly visible on the images. One problem is that the authors show the merge of MTDR and CRYOPHR-mCherry staining, in which the mitochondria contact are hidden by very bright CRYOPHR-mCherry aggregates. The authors should provide high magnification images (like in 1g) showing not only the merge of mitochondria and CRYOPHR-mCherry but also the staining of mitochondria by themselves. The authors should mark "head-to-head, side-by-side, and head-to-side contacts" with arrows.

f) Continuing on Figure 1f and 1g, it does not sound optimal to use CRYOPHR-mcCherry in combination with MTDR (MitoTracker Deep Red) to precisely delimitate subtle membrane contact sites between mitochondria because the emission and excitation spectra of these two fluorochromes partially overlap. One better alternative could be to use MTG (MitoTracker Green) as for Figure 1a. However, here we come to the point that MitoTraker stains the mitochondrial matrix that is delimited by the mitochondrial inner membrane, which can be discontinuous in a given mitochondrion. To formally visualise mitochondrial contact sites and demonstrate that CRYOPHR tethers mitochondria, the author should rather mark the mitochondrial outer membrane (with TOM20::GFP and anti-TOM20, for instance).

g) Figure S2 presents snapshots of a movie clearly showing the rapid aggregation of CRYOPHR into distinct puncta upon blue light exposure. The author should perform the same experiment on cells in which mitochondria would be stained with a fluorophore, allowing live imaging (MTG or TOM20::GP, for instance). This would allow for tracking of mitochondria and CRYOPHR puncta at the same time. Hence, high magnification views should allow for capturing events where CRYOPHR puncta formation coincides with mitochondrial tethering if the authors' claims are correct, or with, for instance, membrane pinching if they are wrong.

h) If CRYOPHR-TMMiro bring mitochondrial membrane closer, it would be surprising that it does not increase the probability of Mitofusin-dependent fusion events. The author should conduct analysis of the mitochondrial network in cells exposed to the conditions shown in Figure 1. Rather than relying only on the aspect ratio (as shown in Figure 2 in cells stressed by prolonged blue light exposure), the author should also analyse the mitochondrial total branch length (sum of the length of all branches from a mitochondrion) and the number of branches on each mitochondrion.

i) Ideally, the author should not only rely on the analysis of mitochondrial architecture, which only partially informs on mitochondrial fusion rate. Fragmented mitochondria can indeed fuse efficiently via kiss-and-run events, for instance. To formally demonstrate that there are no permanent nor transcient fusion at the mitochondrial contact sites induced by CRYOPHR, the most powerful method would be to analyse diffusion of matrix fluorescent dyes. This can be conducted using photoconvertible probes (mt-dendra2) (Pham et al., 2012) or a PEG-induced cell fusion assay (Detmer et al., 2007).

(2) Regarding the quantification of local MMP at mitochondrial contact, it would be important to better explain how the authors have set up their microscope to avoid technical issues that could lead to fluorescent artifacts at CRYOPHR puncta. Because the emission of Rhodamine 123 overlaps the excitation of mCherry, it should be explained in the methods how the detection of Rhodamine 123 has been filtered to avoid the detection of the red light coming from the mCherry light coming from CRYOPHR puncta. This is critical as fluorescent protein aggregates can be very bright.

Comments on the introduction and discussion

(1) In the results section, the authors state that they were "Inspired by previous studies indicating that nanoscale proximity of a charged membrane or protein 119 condensate to a membrane amplifies the local membrane potential". It could be useful to the readers to have a bit of background regarding these observations (references 55 and 56) to better understand what supports the rationale of the authors' strategy. Then, the discussion part should address in more detail the possible mechanisms that could explain why bringing the mitochondrial membranes without fusing them influences mitochondrial membrane potential.

(2) I would suggest finding a simple name for the CRYOPHR-MiroTM tool that could evoke more clearly that it is an optogenetic tool designed to tether mitochondria with blue light.