The MicroRNA-17-92 Cluster Enhances Axonal Outgrowth in Embryonic Cortical Neurons

MicroRNAs (miRNAs) regulate dendritogenesis and plasticity. However, the biological function of miRNAs in axons has not been extensively investigated. Here, using rat primary cortical neurons cultured in a microfluidic chamber, we found that the distal axons of the neurons expressed the miR-17-92 cluster, and proteins that regulate production and activity of mature miRNAs, Dicer and Argonaute 2, respectively, were present in the distal axons. Overexpression of the miR-17-92 cluster in cortical neurons substantially increased axonal outgrowth, whereas distal axonal attenuation of endogenous miR-19a, a key miRNA of the miR-17-92 cluster, with the miRNA hairpin inhibitor suppressed axonal outgrowth. Moreover, overexpression of the miR-17-92 cluster reduced phosphatase and tensin homolog (PTEN) proteins and elevated phosphorylated mammalian target of rapamycin (mTOR) in the distal axons. In contrast, distal axonal attenuation of miR-19a increased PTEN proteins and inactivated mTOR in the axons, but did not affect these protein levels in the cell bodies. Overexpression of PTEN and attenuation of endogenous PTEN prevailed over the enhancement and inhibitory effects of the miR-19a on axonal outgrowth, respectively. Axonal application of LY294002, a phosphoinositide3-kinase inhibitor, or rapamycin, an mTOR inhibitor, abolished axonal outgrowth enhanced by overexpression of the miR-17-92 cluster. Collectively, these findings demonstrate that axonal alteration of miR-17-92 cluster expression regulates axonal outgrowth and that local modulation of PTEN protein levels by miR-19a likely contributes to the axonal outgrowth.