SPECIFICATION OF MOTOR-NEURON IDENTITY BY THE MNR2 HOMEODOMAIN PROTEIN

Sonic hedgehog signaling controls the differentiation of motor neurons in the ventral neural tube, but the intervening steps are poorly unde rstood. A differential screen of a cDNA library derived from a single Shh-induced motor neuron has identified a novel homeobox gene, MNR2, e xpressed by motor neuron progenitors and transiently by postmitotic mo tor neurons. The ectopic expression of MNR2 in neural cells initiates a program of somatic motor neuron differentiation characterized by the expression of homeodomain proteins, by neurotransmitter phenotype, an d by axonal trajectory. Our results suggest that the Shh-mediated indu ction of a single transcription factor, MNR2, is sufficient to direct somatic motor neuron differentiation.