# Sirt1 coordinates the mitochondrial UPR and myocellular proteostasis to preserve muscle integrity during muscle atrophy in zebrafish

## TL;DR

This study shows that Sirt1 helps protect muscle from atrophy by linking mitochondrial health to protein quality control in zebrafish.

## Contribution

The study reveals a novel regulatory mechanism where Sirt1 coordinates mitochondrial UPR and proteostasis to preserve muscle integrity during atrophy.

## Key findings

- Sirt1 inhibition worsens muscle deterioration and proteostasis impairment in zebrafish.
- Sirt1 activates UPRmt, which promotes mFAO gene expression to preserve muscle integrity.
- Pharmacological inhibition of mFAO mimics Sirt1 inhibition effects, linking mitochondrial metabolism to proteostasis.

## Abstract

The decline of mitochondrial homeostasis and proteostasis, the two key cell quality control mechanisms, is the hallmark of aging and age-related diseases. One of the most notable examples is the age-related progressive loss of muscle mass, quality, and strength --a condition known as sarcopenia. In atrophic muscle, mitochondrial dysfunction and proteostasis impairment frequently occur together, indicating a potential association between the decline of mitochondrial homeostasis and proteostasis. However, the mechanism by which these two modes of cell quality control are coordinated remains poorly understood.

We employed dexamethasone-induced muscle atrophy models in both larval and adult zebrafish to investigate the role of cell stress responses in muscle maintenance. Mitochondrial stress was assessed by measuring the mitochondrial unfolded protein response (UPRmt) activity using qRT-PCR and reporter analyses. Proteostasis impairment was evaluated by detecting insoluble polyubiquitinated protein aggregates via Western blotting. Muscle integrity was examined histologically in larval and adult tissues. We performed these assays in sirt1 loss of function conditions (genetic mutation and pharmacological inhibition). Furthermore, to elucidate the mechanism by which Sirt1 regulates proteostasis and muscle preservation, we inhibited the mitochondrial fatty acid oxidation (mFAO) using etomoxir.

Inhibition of Sirt1 markedly exacerbated muscle deterioration and proteostasis impairment under dexamethasone-induced muscle atrophy in zebrafish. Mechanistically, Sirt1 is required for activation of the UPRmt, which in turn promotes expression of the mFAO gene cpt1b. Pharmacological inhibition of Cpt1 using etomoxir phenocopied the defects in muscle integrity and proteotoxic stress observed following Sirt1 inhibition. Importantly, enhancement of proteostasis via hormetic heat shock partially rescued the etomoxir-induced muscle defects.

We have demonstrated that muscle atrophic stress induced by dexamethasone treatment activates the UPRmt in zebrafish. The UPRmt is part of the activity of a cell stress regulator, Sirt1, to promote mitochondrial function and preserve muscle integrity during muscle atrophy. Notably, suppressing the UPRmt via Sirt1 inhibition leads to protein aggregation and the ultimate loss of muscle mass, indicating a link between mitochondrial function and proteostasis. We have further shown that mitochondrial metabolism plays a role in proteostasis regulation, as pharmacological inhibition of the mFAO exacerbates dexamethasone-induced proteotoxicity. Collectively, our findings have uncovered a previously uncharacterized regulatory mechanism linking UPRmt signaling to myocellular proteostasis, and highlight the activity of Sirt1, which coordinates these two key cell quality control mechanisms, in muscle preservation during muscle atrophy.