Zebrafish bashful/laminin-alpha 1 mutants exhibit multiple axon guidance defects

Laminin is known to provide a highly permissive substratum and in some cases directional information for axon outgrowth in vitro. However, there is still little known about laminin function in guiding axons in vivo. We investigated the axon guidance role of laminin-alpha 1 in the developing zebrafish nervous system. Analysis of zebrafish bashful (bal)/laminin-a1 mutants revealed multiple functions for laminin-alpha 1 in the outgrowth and guidance of central nervous system (CNS) axons. Most CNS axon pathways are defective in bal embryos. Some axon types, including retinal ganglion cell axons, early forebrain axons, and hindbrain reticulospinal axons, make specific pathfinding errors, suggesting laminin-alpha 1 is required for directional decisions. Other axon tracts are defasciculated or not fully extended in bal embryos, suggesting a function for laminin-alpha 1 in regulating adhesion or providing a permissive substratum for growth. In addition, some neurons have excessively branched axons in bal, indicating a potential role for laminin-alpha 1 in branching. In contrast to CNS axons, most peripheral axons appear normal in bal mutants. Our results, thus, reveal important and diverse functions for laminin-alpha 1 in guiding developing axons in vivo.