Sonic hedgehog promotes neuronal differentiation of murine spinal cord precursors and collaborates with neurotrophin 3 to induce islet-1

Sonic hedgehog (Shh) is strongly implicated in the development of ventral s tructures in the nervous system. Addition of Sonic hedgehog protein to chic k spinal cord explants induces floor plate and motoneuron development. Whet her Shh acts directly to induce these cell types or whether their induction is mediated by additional factors is unknown. To further investigate the r ole of Shh in spinal neuron development, we have used low-density cultures of murine spinal cord precursor cells. Shh stimulated neuronal differentiat ion; however, it did not increase the proportion of neurons expressing the first postmitotic motoneuron marker Islet-1. Moreover, Shh did induce Islet -l expression in neural tube explants, suggesting that it acts in combinati on with neural tube factors to induce motoneurons. Another factor implicate d in motoneuron development is neurotrophin 3 (NT3), and when assayed in is olated precursor cultures, it had no effect on Islet-1 expression. However, the combination of N-terminal Shh and NT3 induced Islet-1 expression in th e majority of neurons in low-density cultures of caudal intermediate neural plate. Furthermore, in explant cultures, Shh-mediated Islet-1 expression w as blocked by an anti-NT3 antibody. Previous studies have shown expression of NT3 in the region of motoneuron differentiation and that spinal fusimoto r neurons are lost in NT3 knock-out animals. Taken together, these findings suggest that Shh can act directly on spinal cord precursors to promote neu ronal differentiation, but induction of Islet-1 expression is regulated by factors additional to Shh, including NT3.