Disruption of Myosin II Increases Axonal Elongation in Drosophila by Accelerating Bulk Advance of the Growth Cone

Abstract

A pivotal question in cell biology is whether axons elongate by the assembly of new material or bulk advance of the growth cone. While classic studies suggest the former, recent work suggests forces drive translocation of the growth cone. Here we ask three questions: Is the mechanism of growth cone advance conserved between vertebrate and invertebrate neurons? How do growth cones advance in vivo? And, what is the role of myosin II force generation ¬in growth cone motility? To address conservation, we analyzed the movement of organelles and microtubules in neurons cultured from Drosophila embryos. We found these moved in bulk as observed in chick sensory and Xenopus spinal cord neurons. To assess transport in vivo, we co-expressed myr-td-tomato and mito-GFP in stage 16 Drosophila embryos using the pan neuronal driver elav. using time-lapse confocal microscopy to track the elongation of the aCC pioneer motor axon in intact embryos, we also found bulk advance of docked mitochondria. To better understand the role of myosin II in axonal elongation, we cultured Drosophila neurons that were null for myosin II (Zipper) and monitored bulk transport and growth cone motility. We found both rates were significantly higher. using force calibrated towing needles, we found disruption of myosin II significantly decreased neuronal tension. Together, this suggests axonal myosin II acts antagonistically against forces generated in the growth cone to modulate translocation of the growth cone. This work has important implications for the development of treatments for stroke, peripheral nerve damage, and spinal cord injury.