Poster abstracts

Poster number 79 submitted by Jona Hilario

Collagen XIXa1 functions in zebrafish motor axon guidance.

Jona D. Hilario (Molecular, Cellular and Developmental Biology Program, Center for Molecular Neurobiology), Chunping Wang (Center for Molecular Neurobiology), Christine E. Beattie (Center for Molecular Neurobiology, Department of Neuroscience)

Abstract:
Formation of proper motoneuron-muscle connections is a vital step during development. Axons extend to their targets in a stereotyped manner. Many cues responsible for guiding motor axons have been identified but some aspects of this process remain unclear. stumpy mutants were obtained in a genetic screen designed to find cues that affect pathfinding by the Caudal Primary (CaP) motor axon in the developing zebrafish. In stumpy homozygotes CaP axons stall at the first intermediate target, the horizontal myoseptum at 24 hours post fertilization. At day two of development other motor axons also stall at inappropriate locations along their pathways. We have mapped the stumpy loci to the zebrafish homolog of the atypical collagen collagen XIX. Collagens function as structural proteins in the extracellular matrix. Collagen XIX is a member of the family of fibril associated collagens with interrupted triple helices (FACITs) and has no known role in axon guidance. In one stumpy allele we found two base pair mutations, a lysine to arginine mutation and a stop mutation. The K to R mutation occurs in a conserved collagen domain and the stop mutation produces a truncated protein without two C-terminal domains that were shown to be critical to trimerization of ColXIX molecules in vitro. To verify that ColXIX is involved in motor axon guidance, we knocked down ColXIX in wild-type zebrafish using antisense morpholinos. We saw a phenocopy of the stumpy phenotype in the knockdown. ColXIX mouse RNA rescues this phenotype. Overexpression of mouse ColXIX RNA containing both stumpy mutations into wild type embryos, induces stumpy-like phenotypes. In addition the mutant mouse ColXIX RNA was not able to rescue the morpholino phenotype. These data indicate that mutant ColXIX protein in stumpy mutants is acting in a dominant negative manner to inhibit proper motor axon pathfinding and that ColXIX trimerization is important for guiding zebrafish motor axons past intermediate targets.

Keywords: axon guidance, zebrafish, collagens