| During development neurons form precise synaptic connections with each other, assembling into circuits whose pattern of connectivity sculpts nervous system function. As neuronal growth cones approach their targets, a critical step in formation of these circuits is selection of the appropriate synaptic partner within the target field. The classical cadherins are a diverse family of cell adhesion that have been proposed to mediate interactions between pre- and postsynaptic cells that are necessary for synapse formation. I provide the first direct, genetic evidence in favor of this model by examining the role of N-cadherin in controlling the pattern of synaptic connections made by photoreceptor axons in Drosophila melanogaster. By creating somatic mosaic flues in which individual photoreceptors are mutant in an otherwise heterozygous and phenotypically wild-type organism, I examine N-cadherin's function in different cells with high resolution. N-cadherin is required in both individual photoreceptors and their target neurons for photoreceptor axon extension. Cell-by-cell reconstruction of wild-type photoreceptor axons extending within mosaic patches of mutant target cells shows that N-cadherin mediates attractive interactions between photoreceptors and their targets. This interaction is not limited to those cells that will become the synaptic partners of photoreceptors. Multiple N-cadherin isoforms are produced, but single isoforms can substitute for endogenous N-cadherin activity. This suggests that N-cadherin mediates a homophilic, attractive interaction between photoreceptor growth cones and their targets that stabilizes this interaction and precedes synaptic partner choice. In photoreceptors the scaffolding protein Liprin-alpha and the receptor tyrsoine phosphatase LAR are also required cell-autonomously for target selection. However, unlike N-cadherin, neither Liprin-alpha nor LAR is required postsynaptically for R cells to project to their correct targets. Thus, these two proteins, unlike N-cadherin, are functionally asymmetric between axons and dendrites, suggesting that the adhesive mechanisms that link pre- and postsynaptic cells before synapse formation may be differentially regulated in these two compartments. As N-cadherin, LAR, and Liprin-alpha act at the same step in target selection, genetic interactions between these proteins were evaluated. These studies demonstrate that N-cadherin and LAR interact positively with each other, but also that they act as negative regulators of Liprin-alpha. These observations are corroborated by biochemical studies that demonstrate that LAR physically associates with N-cadherin in Drosophila embryon. Together, these data demonstrate that N-cadherin mediates stabilizing, adhesive interactions between pre-synaptic neurons and their targets that are necessary for selection of the appropriate target. LAR and Liprin-alpha are also necessary for target selection but function asymmetrically, only in the axon, forming a complex with N-cadherin that may regulate cadherin mediated cell adhesion. |
