ACTN3 genotype influences androgen response in skeletal muscle

Androgens are vital for the maintenance of muscle mass and their anabolic effects are primarily exerted through the androgen receptor (AR). Accumulating evidence in humans and mice suggests that circulating androgens, AR and androgen response are influenced by ACTN3 ( α- actinin-3), also known as “the gene for speed”. One in 5 people worldwide are α-actinin-3 deficient due to homozygous inheritance of a common null polymorphism (577X) in ACTN3 . In this study, we show that α-actinin-3 deficiency decreases baseline AR in skeletal muscles of mice and humans, in both males and females, and that AR expression directly correlates with ACTN3 in a dosage dependent manner. We further demonstrate in Actn3 knockout mice that α- actinin-3 deficiency increases muscle wasting induced by androgen deprivation and reduces the muscle hypertrophic response to dihydrotestosterone and this is mediated by differential activation of pathways regulating amino acid metabolism, intracellular transport, MAPK signalling, autophagy, mitochondrial activity and calcineurin signalling. Gene set enrichment and protein analyses indicate that the absence of α-actinin-3 results in a failure to coactivate many of these pathways in response to changes in androgens, and relies on leveraging mitochondrial remodelling and calcineurin signalling to restore muscle homeostasis. We further identified 7 genes that are androgen sensitive and α-actinin-3-dependent in expression, and whose functions correspond to these processes. Our results highlight the pivotal role of α- actinin-3 in various processes associated with the regulation of protein turnover and muscle mass, and suggest that ACTN3 genotype is a genetic modifier of androgen action in skeletal muscle.