Cytoskeletal motors, cargo adaptors, and signaling cascades are required for proper myonuclear positioning and muscle function in Drosophila Melanogaster.

摘要

Nuclei are precisely positioned within all cells, occupying distinct intracellular locations that serve to optimize cell function. Skeletal myofibers are multinucleated cells with peripherally located myonuclei that are evenly distributed throughout the myofiber to maximize internuclear distance. Disruptions to this arrangement of myonuclei are commonly found in tissue biopsies from patients with various muscle diseases. To better understand muscle disease, we study the process of myonuclear positioning in the conserved myofiber using Drosophila melanogaster as a model system. Myonuclear positioning is known to be a microtubule-dependent process, and we identified that the microtubule motor proteins, Kinesin and Dynein, are both required for proper myonuclear positioning. Moreover, a number of regulatory proteins influence the activity of each motor, including Kinesin light chain, Dynein light chain, Rapsynoid, CLIP-190, Lis-1, and Glued/p150. Additionally, Sunday Driver (Syd) is another type of regulator, known as a motor adaptor, which interacts with both Kinesin and Dynein to coordinate their activities and promote proper myonuclear positioning. Together, these proteins collectively work to position myonuclei via multiple mechanisms. Kinesin and Dynein work directly on the nucleus to promote myonuclear translocation. Kinesin also regulates the transport of Dynein to a distant location at the cell cortex, where Dynein activity mediates pulling of microtubules, and the attached myonuclei, into proper position. The latter mechanism requires Syd to mediate interactions between Kinesin and Dynein, but Syd only activates this pathway in response to intracellular cues from the JNK signaling pathway. Moreover, evidence suggests that these distinct mechanisms are temporally controlled, and we propose that Syd dictates a shift in motor protein activity from one pathway to the next. Finally, disruption of either pathway leads to muscle dysfunction, mimicking disease states in humans, but importantly, expression of JIP3, the mammalian ortholog of Syd, rescues both myonuclear position and locomotive ability. These data highlight the high degree of conservation across species and, thus, the utility of model organisms in elucidating precise cellular mechanisms that are directly applicable to mammalian biology. With these studies, we advanced the field of muscle biology and identified a number of proteins that, when disrupted, lead to disease phenotypes.