Spatiotemporal analysis of dystrophin expression during muscle repair

Abstract

Dystrophin mRNA is produced from a very large genetic locus and transcription of a single mRNA requires approximately 16 hours. This prolonged interval between transcriptional initiation and completion results in unusual transcriptional behaviour: in skeletal muscle, myonuclei express dystrophin continuously and robustly, yet degrade mature transcripts shortly after completion, such that most dystrophin mRNA is nascent, not mature. This implies dystrophin expression is principally controlled post-transcriptionally, a mechanism that circumvents transcriptional delay, allowing rapid responses to change in demand. Dystrophin protein is however highly stable, with slow turnover: in healthy muscle, despite constant production of dystrophin mRNA, demand is low and the need for responsive expression is minimal. We reasoned this system instead exists to control dystrophin expression during rare periods of elevated but changing demand, such as during muscle development or repair, when newly formed fibres must establish sarcolemmal dystrophin rapidly. By assessing dystrophin mRNA and protein expression in regenerating skeletal muscle following injury, we reveal a complex program that suggests control at multiple levels: nascent transcription begins even prior to myoblast fusion, effectively paying in advance to minimise subsequent delay. During myotube differentiation and maturation, when sarcolemmal demands are high, initiation increases only modestly while mature transcript stability increases markedly to generate high numbers of mature dystrophin transcripts, a state that persists until repair is complete, when a state of oversupply and degradation resumes. Our data demonstrate that dystrophin mRNA is indeed chiefly controlled by turnover, not initiation: degradation consequently represents a potential therapeutic target for maximising efficacy of even modest dystrophin restoration.