MYC regulation of the miR-92-Robo1 axis in Slit-mediated commissural axon guidance

In the developing spinal cord, translational repression of Robo1 expression by microRNA-92 (miR-92) in precrossing commissural axons (CAs) inhibits Slit/Robo1-mediated repulsion facilitating commissural axon projection and midline crossing; however, the regulatory mechanisms governing miR-92 expression in the developing commissural neurons are currently lacking. Here, we propose that the transcription factor MYC regulates miR-92 expression in the developing spinal cord (of either sex) to control Robo1 levels in precrossing CAs, modulating Slit/Robo1-mediated repulsion and midline crossing. MYC, miR-92, and Robo1 are differentially expressed in the developing chicken spinal cord. MYC binds to the promoter region upstream of the gga-miR-92 gene in vitro. MYC knockdown dramatically decreases miR-92 expression and increases chicken Robo1 (cRobo1) levels. In contrast, overexpression of MYC significantly induces miR-92 expression and reduces cRobo1 levels. MYC knockdown or overexpression results in significant inhibition or induction of miR-92 activity in the developing chicken spinal cord, respectively. Disruption of the MYC-dependent regulation of the miR-92-cRobo1 axis affects Slit2-mediated CA growth cone collapse in vitro and impairs CA projection and midline crossing in vivo. These results elucidate the role of the MYC-miR-92-cRobo1 axis in Slit2/Robo1-mediated CA repulsion and midline crossing.