Mitochondria as a Therapeutic Target for Burn Injury

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the review “Mitochondria as a therapeutic target for burn injury” authors describe studies of mitochondrial changes post-burn, with an emphasis on targeting for therapeutic purposes. The topic is interesting and the authors should be applauded for a clearly articulated and well-written review on the topic. I believe some improvements can be made to the organization of the material, which will help inform when, where, and how the mitochondria can be targeted in the continuum of burns.  Additional figures could also help, and a couple of additional aspects of this broad topic that should be included for the review to be more comprehensive. Specific comments are here:

 

• From a organization standpoint, the continuum of where/how the mitochondria could be targeted is lost. Explicit separation of whether findings implicate targeting during acute resuscitation, wound healing, or post-healing hypermetabolism would help. For example, current section 4 on muscle wasting could be included with the portion of section 2 on skeletal muscle but tell a story on the dynamic ways in which the mitochondria could be targeted (perhaps initial potentiation, but inhibition during the more chronic cachexic response). This chronological presentation could also bring in the discussion on adipose (currently in section 3) to light, wherein hypermetabolism and wound healing could both benefit from targeting adipose mitochondria.
• Similarly, an overarching figure of where in the body different changes in mitochondria occur would help. Something that illustrates which organs have been shown to undergo alterations in mitophagy, fission, fusion, apoptosis, etc secondary to burn would be very appealing. This could also inform a discussion on logistically how to target the mitochondria (e.g., routes of administration, timing, etc).
• Some consideration should be given to mitochondrial function of circulating blood cells. While PBMC respiration has been investigated mor in the context of sepsis, mitochondrial activity of these cells has been analyzed in both animal models and studies of burn sepsis diagnosis. Even as a therapeutic target, changes in PBMC mitochondria should be considered in section 2.
• Sections 6 and 7 could be combined. In general, the listing of findings from literature should always be accentuated at the end of paragraphs/sections by a summary of how the findings implicate the mitochondria as a target. This would help some sections read less of a laundry list and emphasize the point more.
• Intact mitochondria as a therapy should also be considered. For example, section 9 should consider the anti-inflammatory role of mitochondrial transplantation per se. It has been shown in a variety of ischemia reperfusion models (e.g., myocardial infarction, acute kidney injury) that delivery of mitochondria are beneficial and reduce injury. This juxtaposed with their contents being pro-inflammatory is an interesting consideration for their use as a therapy.

 

Minor comments

• Define TLR on first use
• Could Figure 1 legend elaborate on which cells/organs the authors propose to be affected by DAMPS/catecholamines?
• A figure summarizing mitophagy in section10 (similar to Figure 1) would be helpful.

 

Author Response

We are grateful to Reviewer 1 for insightful constructive critiques that helped to improve the manuscript. Below are point-by-point responses.

1.From an organization standpoint, the continuum of where/how the mitochondria could be targeted is lost. Explicit separation of whether findings implicate targeting during acute resuscitation, wound healing, or post-healing hypermetabolism would help. For example, current section 4 on muscle wasting could be included with the portion of section 2 on skeletal muscle but tell a story on the dynamic ways in which the mitochondria could be targeted (perhaps initial potentiation, but inhibition during the more chronic cachexic response). This chronological presentation could also bring in the discussion on adipose (currently in section 3) to light, wherein hypermetabolism and wound healing could both benefit from targeting adipose mitochondria.

Based on the suggestion of Reviewer 1, we included the discussion of muscle wasting and adipose browning in section 2. Thus, this section gives the all-encompassing overlook of burn-induced mitochondrial changes in various organs and tissues. In the concluding part of this section, we stressed the importance of mitochondria as a potential target to achieve the stimulation of all the aspects of burn injury treatment.. Considering the systemic effects of severe burn determined by humoral and nervous connections between different organs, we tend to think that generalized approaches similar to those described in the final section of the review have highest therapeutic potential. The only obvious exception would be the local treatment of burn wound. In any case, earlier the treatment starts better results should be expected.

 

2.Similarly, an overarching figure of where in the body different changes in mitochondria occur would help. Something that illustrates which organs have been shown to undergo alterations in mitophagy, fission, fusion, apoptosis, etc secondary to burn would be very appealing. This could also inform a discussion on logistically how to target the mitochondria (e.g., routes of administration, timing, etc).

This is an exciting idea, however at present there is not enough data about burn effects on mitophagy, mitofusion and mitofission in specific organs. Thus, we had to be limited by the available information about mitochondrial function, which is sufficiently reflected in Table 1.

 

3.Some consideration should be given to mitochondrial function of circulating blood cells. While PBMC respiration has been investigated mor in the context of sepsis, mitochondrial activity of these cells has been analyzed in both animal models and studies of burn sepsis diagnosis. Even as a therapeutic target, changes in PBMC mitochondria should be considered in section 2.

There is not much published data about the effects of burn per se (not complicated by sepsis) on blood cells. However, we mentioned in the final paragraph of chapter 2 the importance of studying leukocyte mitochondria after burn injury.

 

4.Sections 6 and 7 could be combined. In general, the listing of findings from literature should always be accentuated at the end of paragraphs/sections by a summary of how the findings implicate the mitochondria as a target. This would help some sections read less of a laundry list and emphasize the point more.

Thank you, we have followed your advice and combined former sections 6 and 7. Also, all major sections have concluding remarks.

 

5.Intact mitochondria as a therapy should also be considered. For example, section 9 should consider the anti-inflammatory role of mitochondrial transplantation per se. It has been shown in a variety of ischemia reperfusion models (e.g., myocardial infarction, acute kidney injury) that delivery of mitochondria are beneficial and reduce injury. This juxtaposed with their contents being pro-inflammatory is an interesting consideration for their use as a therapy.

 We included in the final part of section 7 a brief discussion of mitochondria transplantation perspectives in burn treatment.

 

6.Define TLR on first use

Done. See second paragraph of section 1.

 

6.Could Figure 1 legend elaborate on which cells/organs the authors propose to be affected by DAMPS/catecholamines?

All organs can be affected due to ubiquitous expression of corresponding receptors. This explanation is now given in the legend to Figure 1.

 

7.A figure summarizing mitophagy in section10 (similar to Figure 1) would be helpful.

Done, see Figure 2

 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present an interesting review on the role of mitochondrial injury and burn injuries. I found the review interesting and complete. I had a couple of "thought" questions that you could address:

1) How do the studies on burns differentiate cell damage from cell death? Clearly burns kill a lot of cells, so a lot of the findings could be related to the release of signals from dead and dying cells, not just the release of signals from live cells.

2) How do you differentiate a lot of the signals of cell injury that come from the mitochondria, cytoplasm or outside the cells? 

3) Finally, many treatments, such as TXA or antioxidants, may augment tissue perfusion, as opposed to directly treating cellular or mitochondrial damage. Any thoughts?

Author Response

We are grateful to the Reviewer 2 for interest in our paper and insightful questions:

• How do the studies on burns differentiate cell damage from cell death? Clearly burns kill a lot of cells, so a lot of the findings could be related to the release of signals from dead and dying cells, not just the release of signals from live cells.
• This is an excellent question. Damage signals can be released both from dying cells and stressed live cells. In the last case, it happens through the pathways known as non-classical secretion. We included this explanation in the revised manuscript: in the second paragraph of section 3.
• How do you differentiate a lot of the signals of cell injury that come from the mitochondria, cytoplasm or outside the cells? 
• The signals can be represented by molecules normally located in mitochondria (e.g. mtDNA, formylated polypeptides) or in cytoplasm and even nucleus (e.g. HMGB1, S100 family proteins). Upon stress, mitochondrial signaling molecules can pass through cytoplasm or be secreted in mitochondria-derived vesicles.
• Finally, many treatments, such as TXA or antioxidants, may augment tissue perfusion, as opposed to directly treating cellular or mitochondrial damage. Any thoughts?
• This is a valid observation. Most of the available treatments are not ideally focused. However, inclusion of TXA in the discussion is based on its ability to both activate mitochondria and decrease inflammation. On the other hand, ROS, targets of antioxidants, are mostly produced by mitochondria.