RT Journal Article SR Electronic T1 Oculomotor nerve requires an early interaction with muscle precursors for nerve guidance and branch patterning JF bioRxiv FD Cold Spring Harbor Laboratory SP 483396 DO 10.1101/483396 A1 Brielle Bjorke A1 Katherine G. Weller A1 G. Eric Robinson A1 Michelle Vesser A1 Lisheng Chen A1 Philip J. Gage A1 Thomas W. Gould A1 Grant S. Mastick YR 2018 UL http://biorxiv.org/content/early/2018/11/29/483396.abstract AB Muscle function is dependent on innervation by the correct motor nerves. Motor nerves are composed of motor axons that extend through peripheral tissues as a compact bundle, but then diverge to create nerve branches to specific muscle targets. A transition point typically occurs as motor nerves grow near their targets, where the fasciculated nerve halts further growth, then later initiates branching to muscles. The motor nerve transition point is potentially an intermediate target acting as a guidepost to present specific cellular and molecular signals for navigation. Here we describe the navigation of the oculomotor nerve with respect to eye muscle precursor cells in mouse embryos. We found that the oculomotor nerve initially grew to the eye three days prior to the appearance of any eye muscles. The oculomotor axons spread to form a plexus within a mass of eye muscle precursors, then the nerve growth paused for more than two days. This plexus persisted during primary extraocular myogenesis, with a subsequent phase in which the nerve branched out to specific muscles. To test the functional significance of the nerve-precursor contact in the plexus, we genetically ablated muscle precursors early in nerve development, prior to nerve contact. Ablation of muscle precursors resulted in oculomotor nerve fibers failing to stop to form the plexus, but instead growing past the eye. In contrast, ablating the precursor pool at later stages, after the nerve has contacted the precursor cells, results in ectopic branching restricted near the eye. These results demonstrate that muscle precursors act as an intermediate target for nerve guidance, and are required for the oculomotor nerve to transition between nerve growth and distinct stages of terminal axon branching.Several individuals contributed technical assistance in genotyping, and assistance in optimizing immunohistochemistry including Minkyung Kim, Thomas Kidd, and Hillary Price.Author contributionsBB, GSM, KGW, and TWG formulated ideas for these experiments. Methodology: BB performed nerve tracing, antibody labeling, and analysis; several antibody labels were performed by GER, MV and KGW. LC and PJG contributed Pitx2 mutant embryos and advice on Pitx2 antibody labeling. BB and GSM drafted the manuscript, with editing by KGW and TWG. All authors reviewed and approved the manuscript.