Promotion of embryonic cortico-cerebral neuronogenesis by miR-124

Glutamatergic neurons of the murine cerebral cortex are generated within periventricular proliferative layers of the embryonic pallium, directly from apical precursors or indirectly via their basal progenies. Cortical neuronogenesis is the result of different morphogenetic subroutines, including precursor proliferation and death, changes in histogenetic potencies, and post-mitotic neuronal differentiation. Control of these processes is extremely complex, involving numerous polypeptide-encoding genes. Moreover, many so-called 'non-coding genes' are also expressed in the developing cortex. Currently, their implication in corticogenesis is the subject of intensive functional studies. A subset of them encodes microRNAs (miRNAs), a class of small RNAs with complex biogenesis that regulate gene expression at multiple levels and modulate histogenetic progression and are implicated in refinement of positional information. Among the cortical miRNAs, miR-124 has been consistently shown to promote neuronogenesis progression in a variety of experimental contexts. Some aspects of its activity, however, are still controversial, and some have to be clarified. An in depth in vivo characterization of its function in the embryonic mammalian cortex is still missing.

By integrating locked nucleic acid (LNA)-oligo in situ hybridization, electroporation of stage-specific reporters and immunofluorescence, we reconstructed the cortico-cerebral miR-124 expression pattern during direct neuronogenesis from apical precursors and indirect neuronogenesis via basal progenitors. The miR-124 expression profile in the developing embryonic cortex includes an abrupt upregulation in apical precursors undergoing direct neuronogenesis as well as a two-step upregulation in basal progenitors during indirect neuronogenesis. Differential post-transcriptional processing seems to contribute to this pattern. Moreover, we investigated the role of miR-124 in embryonic corticogenesis by gain-of-function approaches, both in vitro, by lentivirus-based gene transfer, and in vivo, by in utero electroporation. Following overexpression of miR-124, both direct neuronogenesis and progression of neural precursors from the apical to the basal compartment were stimulated.

We show that miR-124 expression is progressively up-regulated in the mouse embryonic neocortex during the apical to basal transition of neural precursor cells and upon their exit from cell cycle, and that miR-124 is involved in the fine regulation of these processes.