Control of spinal motor neuron terminal differentiation through sustained <i>Hoxc8</i> gene activity.

Abstract

Spinal motor neurons (MNs) constitute cellular substrates for several movement disorders. Although their early development has received much attention, how spinal MNs become and remain terminally differentiated is poorly understood. Here, we determined the transcriptome of mouse MNs located at the brachial domain of the spinal cord at embryonic and postnatal stages. We identified novel transcription factors (TFs) and terminal differentiation genes (e.g. ion channels, neurotransmitter receptors, adhesion molecules) with continuous expression in MNs. Interestingly, genes encoding homeodomain TFs (e.g. HOX, LIM), previously implicated in early MN development, continue to be expressed postnatally, suggesting later functions. To test this idea, we inactivated <i>Hoxc8</i> at successive stages of mouse MN development and observed motor deficits. Our in vivo findings suggest that <i>Hoxc8</i> is not only required to establish, but also maintain expression of several MN terminal differentiation markers. Data from in vitro generated MNs indicate <i>Hoxc8</i> acts directly and is sufficient to induce expression of terminal differentiation genes. Our findings dovetail recent observations in <i>Caenorhabditis elegans</i> MNs, pointing toward an evolutionarily conserved role for Hox in neuronal terminal differentiation.