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Molecular mechanisms of axon guidance in the developing spinal cord


The formation of intricate neural circuits requires that nascent axons navigate the complex environment of the developing embryo and precisely determine the trajectories that would lead them to their final synaptic targets. Although many guidance cues and their receptors have been identified, the intracellular signaling cascades that control and direct axonal growth remained to be fully understood. Developing axons constantly reassess the immediate environment to distinguish between a myriad of guidance molecules that are simultaneously present along their trajectory. Exactly how the axon interpret these signals at a specific time and space are limited, thus understanding how guidance cues work in concert during axonal pathfinding is a major goal in developmental neurobiology and in the regeneration of axonal connections after injury and disease. In this dissertation we studied the pathfinding of spinal cord commissural axons to understand the molecular mechanisms underlying changes in axonal responses from attraction to repulsion during floor plate crossing at the spinal cord ventral midline. Here, we found that chemorepellent signals emanating from floor plate cells of the Slit and class 3 Semaphorin family of proteins synergize in vitro to repel precrossing commissural axons possibly through Neuropilin2 pathways. In addition, we propose that Sonic Hedgehog (Shh) plays a pivotal role in mediating commissural axon repulsive guidance during floor plate crossing. We provide evidence that Shh is a regulator of class 3 Semaphorin signals in precrossing axons and specifically show that knockdown of Shh pathway components, Patched1 (PTC1) and Smoothened (Smo) disrupted proper guidance of commissural axons during floor plate crossing. We also show that both Smo and PTC1 are expressed around the developmental stages we studied and are key determinants of this regulation. The third major finding of this dissertation is that the activation of Semaphorin3 signals by Shh might be induced through the alteration of cAMP/PKA levels via Smo dependent activation of inhibitory G alpha proteins. Together the data presented in this dissertation propose new exciting models in which axon guidance regulatory pathways can induce profound changes in axonal navigational responses by successfully integrating and interpreting signaling inputs from external and internal axon guidance systems

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