miR-124 regulates Notch and NeuroD1 to mediate transition states of neuronal development

MicroRNAs (miRNAs) regulate gene expression by destabilizing target mRNA and/or inhibiting translation in animal cells. The ability to mechanistically dissect the function of miR-124 during specification, differentiation, and maturation of neurons during development within a single system has not been accomplished. Using the sea urchin (Strongylocentrotus purpuratus) embryo, we take advantage of the manipulability of the embryo and its well-documented gene regulatory networks (GRNs). We incorporated NeuroD1 as part of the sea urchin neuronal GRN and determined that miR-124 inhibition resulted in decreased gut contractions, swimming velocity, and neuronal development. We further integrated post-transcriptional regulation of miR-124 into the neuronal GRN. Inhibition of miR-124 resulted in increased number of cells expressing transcription factors associated with progenitor neurons and a concurrent decrease of mature and functional neurons. Results revealed that miR-124 regulates undefined factors early in neurogenesis during neuronal specification and differentiation in the early blastula and gastrula stages. In the late gastrula and larval stages, miR-124 regulates Notch and NeuroD1. Specifically, miR-124 regulates the transition between neuronal differentiation and maturation, by directly suppressing NeuroD1. Removal of miR-124s suppression of NeuroD1 results in increased mature neurons with decreased Synaptagmin B-positive mature, functional neurons. By removing both miR-124 suppression of NeuroD1 and Notch, we were able to phenocopy miR-124 inhibitor induced defects. Overall, we have improved the neuronal GRN and identified miR-124 to play a prolific role in regulating various transitions of neuronal development.