Mechanisms underlying the functional consequences of α-actinin-3 deficiency in skeletal muscle

An estimated 1.5 billion people worldwide are deficient in the skeletal muscle protein α-actinin-3 due to homozygosity for the common ACTN3 R577X polymorphism. α-Actinin-3 deficiency influences muscle performance in elite athletes and the general population. The sarcomeric α-actinins were originally characterised as scaffold proteins at the muscle Z-line. Through studying the Actn3 knockout (KO) mouse and α-actinin-3 deficient humans, significant progress has been made in understanding how ACTN3 genotype alters muscle function, leading to an appreciation of the diverse roles that α-actinins play in muscle. The α-actinins interact with a number of partner proteins, which broadly fall into three biological pathways - structural, metabolic and signalling. Differences in functioning of these pathways have been identified in α-actinin-3 deficient muscle that together contributes to altered muscle performance in mice and humans. In this thesis we identify the possible molecular mechanisms underlying the phenotypic differences in these biological pathways. Deficiency in α-actinin-3 is associated with enhanced calcineurin signalling. We have discovered that the α-actinins act cooperatively with calsarcin-2 (an inhibitor of calcineurin activity) to regulate calcineurin signaling. α-Actinin-3 deficient muscles are also structurally different; they generate less force, are more susceptible to contraction-induced damage but have better resistance to contraction-induced fatigue. Our study points to altered protein composition and protein complexes at the Z-disk as the mechanism that underlies the altered contractile properties in these muscles. Finally, the metabolism of α-actinin-3 deficient muscle shows a shift from anaerobic to oxidative pathways. Glycogen metabolism is also altered with our lab previously reporting reduced activity of glycogen phosphorylase (GPh) leading to accumulation of glycogen in Actn3 KO muscle. In this work we present multiple novel post-translational modification sites of GPh in the form of sulfation residues. We also show altered localisation of GPh between WT and Actn3 KO muscle. We hypothesise that vi these changes lead to decreases in GPh activity and that muscle adapts to the changes in glycogen metabolism with a compensatory shift towards oxidative metabolism resulting in consequences for athletic performance. The structural, metabolic and signalling changes seen in the Actn3 KO mouse muscle also begin to provide a mechanistic explanation for the selective advantage of the ACTN3 577X allele during human evolution and the association between ACTN3 genotype and muscle performance in humans today. Continued advances in understanding the interplay between the structural, metabolic and signalling pathways will present a clearer picture on why α-actinin-3 deficiency affects muscle function and how this normal human variation continues to influence skeletal muscle both in health and disease.