Agenda

Brain development is a complex process involving several coordinated mechanisms to fine-tune gene expression across thousands of neurons. In the Drosophila central nervous system, maturation of neurons and circuit assembly occur in a stepwise manner starting from metamorphosis, and are likely governed by intricate gene regulatory networks. Since microRNAs represent an attractive mechanism to fine-tune gene expression at a post-transcriptional level, we investigated how they regulate the maturation of neurons. MicroRNAs have been found to mediate various roles in nervous system development, but previous studies did not evaluate the effect of their promiscuous interactions with mRNAs, making it difficult to understand their precise contribution and mode of action. So, instead of investigating individual miRNA-mRNA interactions, we used a recently established technique, in vivo AGO-APP, along with miRNA sequencing to generate a comprehensive dataset of active neuronal miRNAs across development from late larval to adult stages and validated this data using different transgenics. We show that the complete loss of mature miRNAs during late larval stages results in precocious maturation of neurons. From the sequencing dataset, we identified a module of miRNAs enriched in late larvae that are predicted to target the axon-dendrite morphogenesis pathway. Overexpression of individual miRNAs from the module caused severe axon guidance defects in mushroom body neurons, and the knockdown of the miRNA module resulted in early neuron maturation. Therefore, using AGO-APP, we have profiled active miRNAs in neuronal maturation and identified a miRNA module with roles in axon guidance.