Sonic hedgehog synergizes with the extracellular matrix protein Vitronectin to induce spinal motor neuron differentiation

Patterning of the vertebrate neural tube depends on intercellular signals e manating from sources such as the notochord and the floor plate. The secret ed protein Sonic hedgehog and the extracellular matrix protein Vitronectin are both expressed in these signalling centres and have both been implicate d in the generation of ventral neurons. The proteolytic processing of Sonic hedgehog is fundamental for its signalling properties. This processing gen erates two secreted peptides with all the inducing activity of Shh residing in the highly conserved 19 kDa amino-terminal peptide (N-Shh), Here we sho w that Vitronectin is also proteolitically processed in the embryonic chick notochord, floor plate and ventral neural tube and that this processing is spatiotemporally correlated with the generation of motor neurons. The proc essing of Vitronectin produces two fragments of 54 kDa and 45 kDa, as previ ously described for Vitronectin isolated from chick yolk. The 45 kDa fragme nt lacks the heparin-binding domain and the integrin-binding domain, RGD, p resent in the non-processed Vitronectin glycoprotein, Here we show that N-S hh binds to the three forms of Vitronectin (70, 54 and 35 kDa) isolated fro m embryonic tissue, although is preferentially associated with the 45 kDa f orm. Furthermore, in cultures of dissociated neuroepithelial cells, the com bined addition of N-Shh and Vitronectin significantly increases the extent of motor neuron differentiation, as compared to the low or absent inducing capabilities of either N-Shh or Vitronectin alone. Thus, we conclude that t he differentiation of motor neurons is enhanced by the synergistic action o f N-Shh and Vitronectin, and that Vitronectin may be necessary for the prop er presentation of the morphogen N-Shh to one of its target cells? the diff erentiating motor neurons.