Induction of motor neurons by Sonic hedgehog is independent of floor plate differentiation

BACKGROUND The differentiation of floor plate cells and motor neurons in the vertebrate neural tube appears to be induced by signals from the notochord. The secreted protein encoded by the Sonic hedgehog (Shh) gene is expressed by axial midline cells and can induce floor plate cells in vivo and in vitro. Motor neurons can also be induced in vitro by cells that synthesize Sonic hedgehog protein (Shh). It remains unclear, however, if the motor-neuron-inducing activity of Shh depends on the synthesis of a distinct signaling molecule by floor plate cells. To resolve this issue, we have developed an in vitro assay which uncouples the notochord-mediated induction of motor neurons from floor plate differentiation, and have used this assay to examine whether Shh induces motor neurons in the absence of floor plate differentiation. RESULTS Floor plate cells and motor neurons were induced in neural plate explants grown in contact with the notochord, but only motor neurons were induced when explants were separated from the notochord. COS cells transfected with Shh induced both floor plate cells and motor neurons when grown in contact with neural plate explants, whereas only motor neurons were induced when the explants were grown at a distance from Shh-transfected COS cells. Direct transfection of neural plate cells with an Shh-expression construct induced both floor plate cells and motor neurons, with motor neuron differentiation occurring prior to, or coincidentally with, floor plate differentiation. The induction of motor neurons appears, therefore, not to depend on floor plate differentiation. CONCLUSIONS The induction of motor neurons by Shh does not depend on distinct floor-plate-derived signaling molecules. Shh can, therefore, initiate the differentiation of two cell types that are generated in the ventral region of the neural tube. These results show that the early development of motor neurons involves the inductive action of Shh, whereas the survival of motor neurons at later stages of embryonic development requires neurotrophic factors.