Regulation of spinal motoneuron differentiation by the combined action of Sonic hedgehog and neurotrophin 3.

1. The development of ventral cell types in the spinal cord, including motor neurons, requires the growth factor Sonic hedgehog (Shh). However, it is still unknown whether Shh acts directly on precursors to induce these cell types and whether additional factors are required for induction. 2. To further investigate the precise role of Shh in spinal motoneuron development, we used low-density cultures of murine spinal cord precursor cells. 3. Although purified Shh stimulated neuronal differentiation, it did not increase the proportion of neurons expressing the motoneuron marker Islet-1 (Isl-1). In contrast, purified Shh induced Isl-1 expression in neural tube explants, suggesting that additional neural tube-derived factors are required to induce motoneuron differentiation. 4. A factor implicated in motoneuron development, neurotrophin 3 (NT3), had no effect on Isl-1 expression; however, in combination with Shh, it induced Isl-1 expression in the majority of neurons in the low-density cultures. Furthermore, in explant cultures, Shh-induced Isl-1 expression was blocked by an anti-NT3 antibody. 5. In addition, previous studies have shown expression of NT3 in the region of motoneuron differentiation and the loss of spinal fusimotor neurons in NT3-knockout animals. 6. Taken together, these findings suggest that Shh can act directly on spinal cord precursors to promote neuronal differentiation, but induction of Isl-1 expression and motor differentiation requires additional factors, including NT3.