電子研セミナー Trans-synaptic coordination of synaptic development by the conserved microRNA miR-8

The formation of functional neuronal circuits during neural development relies upon the reliable assembly and fine-tuning of synaptic connections between neurons and their precise synaptic partners. Using Drosophila as a model to understand the regulation of synapse morphogenesis, we have discovered that microRNA function plays an important role in a trans-synaptic coordination between motor neurons and muscle target cells during multiple stages of neuromuscular junction (NMJ) development. In particular, we find that the highly conserved microRNA miR-8 controls multiple steps of NMJ formation, from the initial refinement of exuberant nerve-target contacts into precise nascent synapses, to the later expansion of the synapse in response to the ongoing growth of target cells. Analysis of miR-8 function using a novel tool that we created for microRNA inhibition with spatiotemporal specificity revealed that at later stages of NMJ development, miR-8 controls presynaptic morphogenesis largely via post-synaptic regulation of target genes. Comprehensive profiling of mRNA and protein expression at different stages in miR-8 mutants combined with computational tools to predict miR-target interactions, followed by functional assays to confirm functional relationships, allowed us to classify several genes downstream of miR-8 that contribute in different ways to NMJ formation. This dynamic process begins at the late stage of embryonic development with miR-8 regulation of expression or localization of the synaptic cell adhesion molecules (CAMs) Fasciclin II, Fasciclin III and Neuroglian. However, as NMJ development proceeds to maturity, normal NMJ growth control also relies upon direct miR-8 regulation of the actin-associated protein Enabled. In this later phase, miR-8 function becomes restricted in the presynaptic compartment despite co-expression of miR-8 and Ena in motor neurons, suggesting the existence of a mechanism that protects key target genes from down-regulation in mature neurons.