Biogenesis and function of microRNA miR-124

MicroRNAs are 19-25 nucleotides long non-coding RNAs that base-pair with cognate mRNA targets and regulate their translation and/or stability. Different miRNAs have been implicated in the regulation of essential biological processes like differentiation, proliferation and apoptosis. miR-124 is a conserved, abundantly expressed neuron-specific microRNA that has been previously shown to contribute to neurogenesis by targeting several important transcripts. miR-124 is expressed in neurons, but not glial cells, and the levels of miR-124 increase over time in the developing nervous system (NS). Similar to a few other vertebrate microRNAs, mature miR-124 is encoded by three distinct non-allelic genes in human and mouse genome. To understand biological significance of this genetic redundancy, we examined spatio-temporal expression patterns of the three genes in developing mouse embryo. Two miR-124 genes were expressed at detectable levels in both neural stem cells (NSCs) and post-mitotic neurons, whereas the third gene was expressed exclusively in neurons. Notably, knocking out the neuron-specific miR-124 gene in mouse led, in addition to other interesting phenotypes, to over-proliferation of the neuroepithelial layer containing embryonic NSCs which are positive for certain proliferation markers. These data suggest that one of the functions of the multiple miR-124 genes in mouse could be limiting the NSC proliferation potential and ensuring temporal precision of neuronal differentiation. In a separate line of experiments, we have investigated post-transcriptional mechanisms underlying miR-124 biogenesis. Interestingly, processing of miR-124 from the pri-miR-124-2 precursor requires the presence of a functional intron, consistent with the native genetic structure of this precursor. We show that the unusually strong dependence of 124-2 on the splicing reaction is due to the presence of a tunable cis-regulatory element that induces rapid degradation of the pri-miR-124-2 precursor unless this element is positioned within a spliceable intron. These data uncover a novel post-transcriptional mechanism regulating miRNA expression which could also possibly modulate the mature miRNA output of the 124-2 gene in vivo.DOCTOR OF PHILOSOPHY (SBS