Decreased ryanodine receptor type 1 (RyR1) protein is a hallmark of recessive RYR1-related myopathies (RyR1-RM), which are caused by recessive mutations in the RYR1 gene. It is not clear how the decrease in the RyR1 protein triggers muscular disorders. Furthermore, it is a hot topic whether a decrease in RyR1 protein levels can also occur during non-RYR1-related myopathies. In this study, we first show that reduced RYR1 transcripts are associated with various human myopathies, and that RyR1 protein levels are significantly decreased in muscle samples analysed in inflammatory myopathies (IM) and mitochondrial myopathies (MM), both of which are non-RYR1-RM. Secondly, proteomic data show that exclusive depletion of RyR1 protein in vitro recapitulates the common altered molecular pathways observed during myopathies. RyR1 protein depletion impairs ER-mitochondria tethering and Ca2+ transfer to mitochondria, decreases mitophagy genes and induces an accumulation of dysfunctional mitochondria. This phenomenon is also associated with altered lipid homeostasis with an increase in deleterious sphingolipid species. Finally, decreased RyR1 protein levels lead to an increase in the ER stress markers GRP78-Bip and CHOP in muscle cell in vitro, and in mouse and human muscles. Overall, our results indicate an important role of RyR1 protein depletion and ER stress in the pathogenesis of myopathies.

Adipose tissue regulates whole‐body energy homeostasis. Both lipodystrophy and obesity, the extreme and opposite aspects of adipose tissue dysfunction, result in metabolic disorders: insulin resistance and hepatic steatosis. Cyclin‐dependent kinases (CDKs) have been reported to be involved in adipose tissue development and functions. Using adipose tissue‐specific knockout mice, here we demonstrate that the deletion of CDK7 in adipose tissue results in progressive lipodystrophy, insulin resistance, impaired adipokine secretion and downregulation of fat‐specific genes, which are aggravated on high‐fat diet and during ageing. Our studies suggest that CDK7 is a key regulatory component of adipose tissue maintenance and systemic energy homeostasis.