Evidence for chronological diversification of spinal neuron subtypes by a shared sequence of transcription factors.

Uncovering the mechanisms that generate the immense diversity of neuronal cell types remains a fundamental goal of developmental biology. Different "cardinal classes" of spinal neurons that share a common molecular identity are produced from spatially segregated progenitor domains. Within many classes, a stereotyped sequence of neuronal subtypes of related function is generated over time, raising the question of the mechanisms that control this process. Here, we show that the successive expression of mouse transcription factors Onecut2, Pou2f2, and Pou3f1 correlates with the emergence of sequentially generated subpopulations within several cardinal classes. We demonstrate that loss of Pou2f2 impairs the development of two early-born motor neuron columns and respecifies anterolateral system projection neurons into a later-born subset. Similarly, we show that Pou3f1 expression is required for the normal development of later-born subsets of motor neurons and anterolateral system projection neurons. Together, our observations provide functional evidence that horologic diversification of spinal circuits is driven by a conserved sequence of transcription factors.