Transcriptional regulation of mouse hypoglossal motor neuron somatotopic map formation.

Somatic motor neurons in the hypoglossal nucleus innervate tongue muscles controlling vital functions such as chewing, swallowing and respiration. Formation of functional hypoglossal nerve circuits depends on the establishment of precise hypoglossal motor neuron maps correlating with specific tongue muscle innervations. Little is known about the molecular mechanisms controlling mammalian hypoglossal motor neuron topographic map formation. Here we show that combinatorial expression of transcription factors Runx1, SCIP and FoxP1 defines separate mouse hypoglossal motor neuron groups with different topological, neurotransmitter and calcium-buffering phenotypes. Runx1 and SCIP are coexpressed in ventromedial hypoglossal motor neurons involved in control of tongue protrusion whereas FoxP1 is expressed in dorsomedial motor neurons associated with tongue retraction. Establishment of separate hypoglossal motor neuron maps depends in part on Runx1-mediated suppression of ventrolateral and dorsomedial motor neuron phenotypes and regulation of FoxP1 expression pattern. These findings suggest that combinatorial actions of Runx1, SCIP and FoxP1 are important for mouse hypoglossal nucleus somatotopic map formation.