Skeletal muscle mitochondrial dysfunction in mice is linked to bone loss via the bone marrow immune microenvironment

BACKGROUND

Mitochondrial dysfunction has been extensively associated with loss of muscle bulk and atrophy [1]. Progressive loss of muscle function is, in turn, a critical factor for the development of bone alterations, such as osteoporosis, due to the reduction of bone strength caused by a decrease in the mechanical loading on the skeleton [2]. During bone remodelling carried out by bone cells (osteoclasts and osteoblasts), bone marrow (BM) immune cells undertake specific functions that regulate the function of bone cells. In fact, in the context of autoinflammatory diseases, BM T cells provide a specific pro-inflammatory environment that allows osteoclast differentiation, which results in bone resorption. Thus, bearing in mind that during muscle atrophy, pro-inflammatory cytokines can be secreted to the system, the authors hypothesized that there could be a mechanism linking muscle mitochondrial dysfunction, bone marrow signalling and bone loss.

 

Figure 1: Crosstalk between skeletal muscle and bone. Inflammatory cytokines from the muscle (myokines) can signal to the bone and trigger bone loss (osteopenia). On the other hand, cytokines released by bone cells (osteokines) can also signal to the skeletal muscle and promote muscle wasting (sarcopenia). Edited from Kirk B., et al., 2020.

 

In order to address this hypothesis, the authors used a CRIF1 knockout (KO) mouse model for skeletal muscle mitochondrial dysfunction. CRIF1 is a component of the large subunit of the mitoribosome, which has an essential role in the translation of mitochondrial oxidative phosphorylation (OxPhos) polypeptides. Indeed, alterations in the OxPhos system have been widely related to decreased mitochondrial respiratory capacity, and therefore, mitochondrial dysfunction [3]. Thus, using skeletal muscle-specific CRIF1 KO (MKO) mice, the authors collect extensive transcriptomic data as well as data on the cell and tissue biology of BM and muscle that, along with functional assays, allows them to properly describe the impact of muscle mitochondrial dysfunction on the inflammatory response and bone remodelling.

 

KEY FINDINGS

First, the authors validated the phenotype observed in the MKO mice: OxPhos dysfunction and stress response in the skeletal muscle. This is characterized by decreased expression levels of the OxPhos complex subunits, accumulation of abnormal, swollen mitochondria with disrupted cristae, and increased expression levels of proteins related to stress responses, such as the unfolded protein response (UPR) or ER stress. Besides, evaluation of the physical performance of MKO mice showed loss of body mass, motor coordination and strength, suggesting that muscle mitochondrial dysfunction results in muscle dysfunction and atrophy. Furthermore, the authors describe bone loss in these mice, characterized by alterations in various bone histomorphometric parameters including: bone mass, mineral density, trabecular bone number, thickness and volume, osteoblast and osteoclast activity, and serum levels of the bone formation and reabsorption markers (procollagen I N-terminal propeptide and C-terminal telopeptide of collagen, respectively).

In order to gain insights into the connection between muscle mitochondrial dysfunction and bone loss, the authors performed transcriptomic analyses in the EDL muscle, in which significantly higher levels of the myokine FGF21 were found in MKO mice compared to wild-type (WT). Nevertheless, downregulation of FGF21 in MKO mice did not rescue or restore bone loss, suggesting that the effects observed in the bones of these mice were independent of the role of FGF21.

Considering that muscle mitochondrial dysfunction has been described to trigger the upregulation of inflammatory cytokines [4], the authors performed various analyses in BM to test whether increased inflammation in the BM caused by muscle dysfunction could lead to bone loss. The following were assessed: i) expression levels of pro-inflammatory molecules in BM cells of MKO and WT by transcriptomic analyses, ii) presence of immune cells in the BM and their characterization by flow cytometry, and iii) serum levels of proinflammatory cytokines. The results of these analyses suggested that MKO mice present local (not systemic) BM inflammation and T cell infiltration, which could be driving bone loss. Besides, gene enrichment analyses revealed that genes associated with myopathy, osteoporosis, activation of the immune system and adipogenesis were upregulated in BM cells. Particularly, the authors observed activation of the CXCL12-CXCR4 axis which, besides its implication in angiogenesis and metastasis, is involved in certain inflammatory autoimmune disorders such as rheumatoid arthritis. Indeed, treatment of MKO mice with a CXCR4 antagonist not only resulted in reduced T cell infiltration and pro-inflammatory cytokine levels in the BM, but also in the prevention of bone loss. Thus, these data suggest that mitochondrial muscle dysfunction promotes the presence of a pro-inflammatory environment in the BM, characterized by activation of the CXCL12-CXCR4 axis, which in turn results in bone loss. Last but not least, these data were validated in human subjects with hip fracture and lower body mass index, which correlated with a higher inflammatory response, therefore confirming the role of inflammation in the development of bone complications in the context of muscle dysfunction.

 

WHY I CHOSE THIS PREPRINT

Considering the lack of evidence regarding the molecular pathways that link muscle atrophy and bone remodelling, I found the approaches that the authors of the preprint used to shed light on this subject very interesting. They obtained valuable data on the transcriptomics of muscle and bone marrow in the context of muscle mitochondrial dysfunction, as well as extensively characterized the residing immune cells. In my opinion the authors elegantly provide new insights into the link between muscle atrophy and bone loss, which will be very relevant in the context of systemic effects of muscle diseases.

 

MY QUESTIONS TO THE AUTHORS

1. Considering that you were able to discard the effect of FGF21 in the connection between muscle dysfunction and bone loss, do you speculate that other myokines could be acting as a bridge between muscle dysfunction and the BM? If so, do you have an idea of what molecule or molecules could be playing that role?
2. If the adipogenesis could be reduced or prevented in the BM of MKO mice, would you expect a reduction of the pro-inflammatory environment, and therefore, bone loss prevention?
3. Do you think that other types of mitochondrial dysfunction like alterations in mitochondrial dynamics, mitophagy, or calcium and iron homeostasis, can also result in the effects observed in the BM and bones of MKO mice?

 

REFERENCES

[1] Sartori R. et al., Mechanisms of muscle atrophy and hypertrophy: implications in health and disease. Nat Commun., 2021. doi: 10.1038/s41467-020-20123-1.

[2] Johanssen DL et al., Ectopic lipid accumulation and reduced glucose tolerance in elderly adults are accompanied by altered skeletal muscle mitochondrial activity. J Clin Endocrinol Metab, 2012. 97, 242-250, doi:10.1210/jc.2011-1798

[3] Boenzi S and Diodato D. Biomarkers for mitochondrial energy metabolism diseases. Essays Biochem., 2018. doi: 10.1042/EBC20170111.

[4] Missiroli S., et al., The role of mitochondria in inflammation: from cancer to neurodegenerative disorders. J Clin Med, 2020. doi: 10.3390/jcm9030740

 

doi: https://doi.org/10.1242/prelights.27438

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