School of Medicine, University of Tasmania, Hobart, Tas, Australia, 7001
The precise connectivity that underlies the neural circuitry of the brain begins with axon guidance. The highly specialized sensory organ at the tip of extending axons, the growth cone, responds to extrinsic guidance cues as it navigates the embryonic environment. Defects in axon guidance are thought to underlie a range of neurological disorders such as autism and schizophrenia. It is known that spatially-restricted cytosolic calcium transients determine most growth cone responses to guidance cues. However, the mechanisms that regulate the spatial and temporal nature of growth cone calcium signals are not clear. We hypothesize that the calcium-sensing protein Stromal Interacting Protein 1 (STIM1) is necessary for the spatiotemporal regulation of calcium within the growth cone. STIM1 is located in the membrane of the endoplasmic reticulum (ER), a major internal calcium source within growth cones. We have previously shown that STIM1 is necessary for growth cone navigation in vitro and we have extended this to zebrafish motor axons navigating in vivo. Importantly, our data indicate that STIM1 is not only required for growth cone calcium signaling, but that STIM1 expression was also essential for microtubule assembly and organization during growth cone turning. Reduction of STIM1 expression disrupted the dynamics of microtubule polymerisation (p<0.0005) as well as the assymetric distribution of microtubules in growth cones as they turned towards a source of BDNF (p<0.0005) or away from a source of Sema3a (p<0.05). The data support the hypothesis that STIM1 is critical in regulating the spatial localisation of growth cone calcium signals. Our work suggests that STIM1 regulates asymmetric ER translocation to the motile side of the growth cone, thereby controlling the spatiotemporal release of calcium and subsequent instructive calcium signals. This work significantly builds on our current understanding of how calcium regulates the growth cone navigation that underlies early neuronal connectivity.