Mechanical tension and spontaneous muscle twitching precede the formation of cross-striated muscle in vivo

Abstract

Muscle forces are produced by repeated stereotypical actomyosin units called sarcomeres. Sarcomeres are chained into linear myofibrils spanning the entire muscle fiber. In mammalian body muscles, myofibrils are aligned laterally, resulting in their typical cross-striated morphology. Despite this detailed textbook knowledge about the adult muscle structure, it is still unclear how cross-striated myofibrils are built in vivo. Here, we investigate the morphogenesis of Drosophila abdominal muscles and establish them as an in vivo model for cross-striated muscle development. By performing live imaging, we find that long immature myofibrils lacking a periodic actomyosin pattern are built simultaneously in the entire muscle fiber and then align laterally to give mature cross-striated myofibrils. Interestingly, laser micro-lesion experiments demonstrate that mechanical tension precedes the formation of the immature myofibrils. Moreover, these immature myofibrils do generate spontaneous Ca2+-dependent contractions in vivo, which, when chemically blocked, result in cross-striation defects. Taken together, these results suggest a myofibrillogenesis model in which mechanical tension and spontaneous muscle twitching synchronize the simultaneous self-organization of different sarcomeric protein complexes to build highly regular cross-striated myofibrils spanning the length of large muscle fibers.

Integrin signaling downregulates filopodia in muscle-tendon attachment

“…The integrin expressed by the muscles is the heterodimer αPSβPS, which attaches the muscle to the tendon cell so that the force of muscle contractions is transmitted to the exoskeleton for movement (Maartens and Brown, 2015;Devenport et al, 2007). We observed, similarly to previous studies in indirect flight muscles development (Weitkunat et al, 2017), that the filopodial downregulation occurred at the same time as the accumulation of βPS integrin at the attachment sites ( Fig. 1D, E).…”

Dissection of <em>Drosophila melanogaster</em> Flight Muscles for Omics Approaches

“…The myofibers then grow to span the entire length of the thorax, with myofibrils undergoing an initial growth phase focused on sarcomere addition until about 48 h APF, and then transitioning to a maturation phase, in which sarcomeres grow in length and width and are remodeled to establish stretch-activation by 72 h APF ( Figure 1A-D) 32,35 . The onset of fiber maturation is at least partially controlled by Salm and E2F 32,36,37 , and multiple IFM-specific sarcomere protein isoforms whose splicing is controlled by Bru1 are incorporated during this phase 16,17 .…”

Systematic transcriptomics reveals a biphasic mode of sarcomere morphogenesis in flight muscles regulated by Spalt

“…It was recently proposed in Drosophila cross-striated abdominal muscles and in the developing cross-striated zebrafish muscles that spontaneous contractions are important for the proper formation of the cross-striated pattern (Mazelet et al, 2016;Weitkunat et al, 2017). A similar role for contractions was found in C2C12 cells by stimulating the contractions optogenetically (Asano et al, 2015).…”

“…Is this a general concept for sarcomere morphogenesis? Reviewing the literature, one finds that in other Drosophila muscle types which display a tubular crossstriated myofibril organisation, such as the fly abdominal muscles, the striated sarcomeres also first assemble and then grow in length (Pérez-Moreno et al, 2014;Weitkunat et al, 2017), suggesting a conserved mechanism. In developing zebrafish skeletal muscles, young myofibers present in younger somites show a short sarcomere length of about 1.2 µm, which increases to about 2.3 µm when somites and muscle fibers mature (Sanger et al, 2017;2009).…”