A mechanically regulated liquid-liquid phase separation of the transcriptional regulator Tono instructs muscle development

Abstract

Muscle morphogenesis is a multi-step program, starting with myoblast fusion, followed by myotube-tendon attachment and sarcomere assembly, with subsequent sarcomere maturation, mitochondrial amplification and specialisation. The correct chronological order of these steps requires precise control of the transcriptional regulators and their effectors. How this regulation is achieved during muscle development is not well understood. In a genome-wide RNAi screen in Drosophila, we identified the BTB-zinc finger protein Tono (CG32121) as a muscle-specific transcriptional regulator. tono mutant flight muscles display severe deficits in mitochondria and sarcomere maturation, resulting in uncontrolled contractile forces causing muscle atrophy during development. Tono protein is expressed during sarcomere maturation and localises in distinct condensates in flight muscle nuclei. Interestingly, internal pressure exerted by the maturing sarcomeres deforms the muscle nuclei into elongated shapes and changes the Tono condensates, suggesting that Tono senses the mechanical status of the muscle cells. Indeed, external mechanical pressure on the muscles triggers rapid liquid-liquid phase separation of Tono utilising its BTB domain. Thus, we propose that Tono senses high mechanical pressure to in turn adapt muscle transcription specifically at the sarcomere maturation stage. Consistently, tono mutant muscles display specific defects in a transcriptional switch that represses early muscle differentiation genes and boosts late ones. We hypothesise that a similar mechano-responsive regulation mechanism may control the activity of related BTB-zinc finger proteins that, if mutated, can result in uncontrolled force production in human muscle.