Sonic hedgehog (Shh) attracts spinal cord commissural axons toward the floorplate. How Shh elicits changes in the growth cone cytoskeleton that drive growth cone turning is unknown. We find that the turning of rat commissural axons up a Shh gradient requires protein synthesis. In particular, Shh stimulation increases β-actin protein at the growth cone, even when the cell bodies have been removed. Thus Shh induces the local translation of β-actin at the growth cone. We hypothesised that this requires zipcode binding protein 1 (ZBP1), an mRNA binding protein that transports β-actin mRNA and releases it for local translation upon phosphorylation. We found that Shh stimulation increases phospho-ZBP1 levels in the growth cone. Disruption of ZBP1 phosphorylation in vitro abolished the turning of commissural axons towards a Shh gradient. Disruption of ZBP1 function in vivo in mouse and chick resulted in commissural axon guidance errors. Therefore, ZBP1 is required for Shh to guide commissural axons. This identifies ZBP1 as a new mediator of non-canonical Shh signaling in axon guidance.
Sonic hedgehog (Shh) guides axons via a non-canonical signaling pathway that is distinct from the canonical Hedgehog signaling pathway that specifies cell fate and morphogenesis. Axon guidance is driven by changes in the growth cone in response to gradients of guidance molecules. Little is known about the molecular mechanism of how Shh orchestrates changes in the growth cone cytoskeleton that are required for growth cone turning. In this paper, we show that the guidance of axons by Shh requires protein synthesis. ZBP1 is an mRNA-binding protein that regulates the local translation of proteins, including actin, in the growth cone. We demonstrate that ZBP1 is required for Shh-mediated axon guidance, identifying a new member of the non-canonical Shh signaling pathway.
The authors declare no competing financial interests.
This work was supported by grants from the Canadian Institutes of Health Research (CIHR), the Fonds de Recherche du Québec-Santé (FRQS) and the Canada Foundation for Innovation (CFI). FC holds the Canada Research Chair in Developmental Neurobiology. We thank Cornelia Zorca for comments on the manuscript, and Shirin Makihara, Julien Ferent, and Cornelia Zorca for technical assistance. The 4D7 antibody was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa.