microRNAs control temporal patterning of the vertebrate nervous system
Temporal patterning is essential for generation of neuronal diversity in the developing vertebrate nervous system. A shared sequence of temporal transcription factors (tTFs) partitions neurons by birth date across much of the central nervous system. The molecular mechanisms that control the timing of expression of these tTFs are incompletely understood. Here, we provide evidence that this process is under the control of microRNAs (miRNAs). Perturbing miRNA biogenesis by conditional deletion of Dgcr8 from the developing mouse nervous system extends the window during which neurons with early identities are generated, but abolishes late identities in the spinal cord, hindbrain and midbrain, and holds progenitors in an early transcriptional state. We further identify miRNAs of the let-7 and miR-9 families, which target transcriptional regulators of early progenitor and neuronal identities, as candidates for mediating this effect. Throughout the nervous system, levels of these miRNAs rise during the neurogenic period in progenitors, while levels in neurons correlate with their birth-date. Moreover, the sensitivity of tTF-encoding transcripts to miR-9 inversely correlates with their expression sequence, suggesting a model in which miR-9 controls the chronology of neuronal cell fate transitions by targeting specifiers of early progenitor and neuronal temporal identity in a level-dependent manner. Consistently, perturbations of miR-9 expression dynamics by gain and loss-of-function approaches perturb temporal patterning. Together, our results demonstrate a critical, evolutionary conserved role for miRNAs in temporal patterning and suggest that a temporal miR-9 expression gradient controls the timing of tTF expression in large parts of the developing vertebrate nervous system.