Utrophin regulates modal gating of mechanosensitive ion channels in dystrophic skeletal muscle

Key points Loss of the cytoskeletal protein dystrophin leads to muscle degeneration in Duchenne muscular dystrophy. The mdx mouse lacks dystrophin but muscle disease is mild, in part because upregulation of utrophin compensates for dystrophin. Evidence suggests that Ca2+ entry through mechanosensitive (MS) ion channels contributes to disease pathogenesis, but the role of utrophin in regulating Ca2+ entry has not been studied. Depletion of utrophin in mdx mice produces a gene dose‐dependent increase in MS channel open probability, single‐channel conductance in some patches, and an increase in the pressure required to MS channels. The overall effect of loss of utrophin is increased Ca2+ entry. The results suggest utrophin organizes local membrane microdomains containing MS channels. Disruption of these domains causes changes in membrane composition, which favour channel open states and subunit aggregation. Utrophin‐replacement therapies may act by correcting MS channel gating, and specific MS channel inhibitors have the potential to prevent disease progression. Dystrophin is a large, submembrane cytoskeletal protein, absence of which causes Duchenne muscular dystrophy. Utrophin is a dystrophin homologue found in both muscle and brain whose physiological function is unknown. Recordings of single‐channel activity were made from membrane patches on skeletal muscle from mdx, mdx/utrn+/– heterozygotes and mdx/utrn–/– double knockout mice to investigate the role of these cytoskeletal proteins in mechanosensitive (MS) channel gating. We find complex, gene dose‐dependent effects of utrophin depletion in dystrophin‐deficient mdx muscle: (1) increased MS channel open probability, (2) a shift of MS channel gating to larger pressures, (3) appearance of modal gating of MS channels and small conductance channels and (4) expression of large conductance MS channels. We suggest a physical model in which utrophin acts as a scaffolding protein that stabilizes lipid microdomains and clusters MS channel subunits. Depletion of utrophin disrupts domain composition in a manner that favours open channel area expansion, as well as allowing diffusion and aggregation of additional MS channel subunits.