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During embryogenesis motor axons navigate to their target muscles, where individual motor axons develop complex branch morphologies. The mechanisms that control axonal branching morphogenesis have been studied intensively, yet it still remains unclear when branches begin to form or how branch locations are determined. Live cell imaging of individual zebrafish motor axons reveals that the first axonal branches are generated at the ventral extent of the myotome via bifurcation of the growth cone. Subsequent branches are generated by collateral branching restricted to their synaptic target field along the distal portion of the axon. This precisely timed and spatially restricted branching process is disrupted in  turnout  mutants we identified in a forward genetic screen. Molecular genetic mapping positioned the  turnout  mutation within a 300 kb region encompassing eight annotated genes, however sequence analysis of all eight open reading frames failed to unambiguously identify the  turnout  mutation. Chimeric analysis and single cell labeling reveal that  turnout  function is required cell non-autonomously for intraspinal motor axon guidance and peripheral branch formation.  turnout  mutant motor axons form the first branch on time via growth cone bifurcation, but unlike wild-type they form collateral branches precociously, when the growth cone is still navigating towards the ventral myotome. These precocious collateral branches emerge along the proximal region of the axon shaft typically devoid of branches, and they develop into stable, permanent branches. Furthermore, we find that null mutants of the guidance receptor  plexin  A3 display identical motor axon branching defects, and time lapse analysis reveals that precocious branch formation in  turnout  and  plexin  A3 mutants is due to increased stability of otherwise short-lived axonal protrusions. Thus,  plexin A3  dependent intrinsic and  turnout  dependent extrinsic mechanisms suppress collateral branch morphogenesis by destabilizing membrane protrusions before the growth cone completes navigation into the synaptic target field.

Plexin A3 and Turnout Regulate Motor Axonal Branch Morphogenesis in Zebrafish