Restoring neuronal function after an injury requires the successful migration of the regenerating axon back to its target cells. Cell surface heparan sulfate proteoglycans (HSPGs) mediate important interactions with extracellular signals and have been shown to change expression after neuronal injury (Murakami et al. Neuroscience. 2006), suggesting that they may mediate some aspects of axon regeneration. In order to elucidate a potential role for HSPGs in regeneration, we severed fluorescently labeled GABAergic motorneurons in HSPG mutants using a pulsed dye laser, and assessed subsequent regeneration. A novel regeneration phenotype was observed in mutants for the transmembrane HSPG syndecan (
sdn-1), but not for the GPI-anchored glypicans. Although axons in
sdn-1 mutants initiate regrowth as frequently as wild type, average neurite lengths are decreased and many
sdn-1 mutants axons are characterized by branch-like structures and dysmorphic growth cones. Further, very few regenerating axons in
sdn-1 mutants reach their target on the dorsal nerve cord. These data demonstrate that syndecan affects the migration step of axon regeneration, possibly through an underlying defect in growth cone morphology.
Syndecan is post-translationally modified by the addition of heparan sugar chains, which are themselves modified by sulfation, epimerization, and deacetylation (Bernfield et al. Annu Rev Biochem. 1999). The HSPG modifying enzymes are essential for axon guidance and cell migration during C. elegans development (Bulow and Hobert. Neuron. 2004), and
sdn-1 mutant worms recapitulate many of these neuronal defects (Rhiner et al. Development. 2005). Interestingly, mutants for the modifying enzymes
hse-5,
hst-2,
hst-6,
hst-3.1,
hst-3.2, and
sul-1 showed no obvious regeneration phenotypes, and the HSPG synthesizing enzyme
rib-2 mutant did not phenocopy the
sdn-1 regeneration defects. These results indicate that syndecan may function independently of heparan sulfate sugars to affect growth cone migration and morphology during regeneration, and may represent a mechanistically distinct function from its known role in axon guidance. Our preliminary data suggest that syndecan functions in the hypodermis to affect axon guidance, while we are currently testing the hypothesis that syndecan functions cell-intrinsically in neurons to regulate the cytoskeleton. Thus, our data suggest that syndecan may have two separable functions: a canonical function in axon guidance, and a novel function in growth cone morphology and migration during regeneration.