Slow muscle precursors lay down a collagen XV matrix fingerprint to guide motor axon navigation

Abstract

The extracellular matrix (ECM) provides local positional information to guide motoneuron axons toward their muscle target. Collagen XV is a basement membrane component mainly expressed in skeletal muscle. We have identified two zebrafish paralogs of the humanCOL15A1gene,col15a1aandcol15a1b, which display distinct expression patterns. Here we show thatcol15a1bis expressed and deposited in the motor path ECM by slow muscle precursors also called adaxial cells. We further demonstrate that collagen XV-B deposition is both temporally and spatially regulated before motor axon extension from the spinal cord in such a way that it remains in this region after the adaxial cells have migrated toward the periphery of the myotome. Loss- and gain-of-function experiments in zebrafish embryos demonstrate thatcol15a1bexpression and subsequent collagen XV-B deposition and organization in the motor path ECM depend on a previously undescribed two-step mechanism involving Hedgehog/Gli andunplugged/MuSK signaling pathways.In silicoanalysis predicts a putative Gli binding site in thecol15a1bproximal promoter. Usingcol15a1bpromoter-reporter constructs, we demonstrate thatcol15a1bparticipates in the slow muscle genetic program as a direct target of Hedgehog/Gli signaling. Loss and gain ofcol15a1bfunction provoke pathfinding errors in primary and secondary motoneuron axons both at and beyond the choice point where axon pathway selection takes place. These defects result in muscle atrophy and compromised swimming behavior, a phenotype partially rescued by injection of asmyhc1:col15a1bconstruct. These reveal an unexpected and novel role for collagen XV in motor axon pathfinding and neuromuscular development.