Contribution of synaptic release mechanisms to the building of sensory maps

Abstract

Numerous neurotransmitters have been implicated in neurodevelopmental processes. In addition, developing neurons show an abundance of vesicles in the growth cones, and express proteins of the SNARE complex early on. This has led to propose a role for vesicular fusion machinery in axonal growth and synapse formation. However, as the molecular machinery of vesicular fusion started to unveil, and knockouts for the major proteins of this complex were generated, it came as a surprise that none of these proteins was essential for the construction of brain architecture, although they were crucial for vital functions of the organism, leading to early mortality of exocytosis mutants. Because of this early death, conditional ablation of these genes in well-defined neuronal populations was necessary to study their role at later stages of neural circuit development, when activity-dependent mechanisms are best defined. Early studies showed that mutants of Munc18-1, a gene essential for both constitutive and calcium triggered release, were required for target dependent cell survival but not for axon growth or early refinement of topographic targeting, at least in the retinotectal system. Conditional knockout of the Rim1 and Rim2 genes allowed to interrogate more specifically the role of calcium-triggered release. Rims (rab interacting molecules) play a key role in the assembly of calcium channels and their coupling to the SNARE complex alters calcium-triggered release with little effect on constitutive release. When Rim1/Rim2 genes were ablated in the thalamus, layer IV neurons failed to organize into barrel structures, and to form the characteristic asymmetric distribution of their dendrites. More surprisingly, thalamocortical axons still organized in precise topographic maps and formed well differentiated synapses despite considerable reduction of calcium-induced synaptic release. However, this reduction in release probability altered axon targeting in the visual system where axons from both eyes compete for the same target. Thus, genetic tools targeting the exocytosis machinery are allowing to dissect more precisely the contribution of synaptic and non-synaptic mechanisms to activity-dependent circuit wiring.