Microtubules coordinate mitochondria transport with myofibril morphogenesis during muscle development

Muscle cells contain numerous energy-producing mitochondria and contractile myofibrils, whose myosin motors need ATP to generate force. Thus, myofibrils and mitochondria are in intimate contact in mature muscles. However, how their morphogenesis is coordinated during development remains largely unknown. Here, we used in vivo imaging to investigate myofibril and mitochondria network dynamics in developing Drosophila flight muscles. We found that mitochondria intercalate from the surface of actin bundles to their interior, and concomitantly, actin filaments condense to individual myofibrils. This ensures that mitochondria locate in proximity to every myofibril. Notably, antiparallel microtubules bundle with the assembling myofibrils, suggesting a key role in myofibril orientation. Indeed, microtubule severing affects myofibril orientation, whereas kinesin knockdown specifically blocks mitochondria intercalation. Importantly, mitochondria intercalation and their kinesin-dependent microtubule-based transport are conserved in mammalian muscle. Together, these data identify a key role for microtubules in coordinating mitochondria and myofibril morphogenesis to build functional muscles.