The neural crest is the organ system whose presence defines vertebrates. The onset of migration of neural crest cells is an archetypal epithelium to mesenchyme transition (EMT), and this event identifies the cell lineage. Little is known yet of the establishment of the neural crest, although the zinc finger gene Slug seems to be involved in specifying EMT competence. The details, especially the temporal order of events in neural crest EMT, vary between different species and between different axial levels, but several important features have emerged from observations in situ and experiments in vitro and in vivo. EMT seems to be strongly associated with decrease in cell-cell adhesion, and particularly with loss of N-cadherin on the surface of neural crest cells at the time of onset of migration. The related adhesion molecule T-cadherin is also present, but correlated changes have not yet been described, while the unrelated adhesion molecule N-CAM also declines on neural crest cells, but with a time course unrelated to EMT. The extracellular matrix is also important: EMT-related changes in matrix receptor (i.e. integrin) activity are recorded in avian crest cells, while the nature of the matrix itself changes in urodele amphibians. Changes in cell shape and in cell motility also occur at the time of EMT, consistent with changes in the cytoskeleton. These concerted changes can be triggered by TGF-β family growth factors, of which dorsalin-1 appears particularly important. These may act through pathways involving controlled alterations in phosphorylation to effect the complex of responses that make up EMT. Although much remains to be understood, the spatiotemporal definability of this system makes it a very useful model for studying EMTs in general.

Based on genetic, functional and histological studies, the extracellular matrix molecule fibronectin has been proposed to play a key role in the migration of neural crest cells in the vertebrate embryo. In the present study, we have analyzed in vitro the repertoire and function of integrin receptors involved in the adhesive and locomotory responses of avian truncal neural crest cells to fibronectin. Immunoprecipitation experiments showed that neural crest cells express multiple integrins, namely (alpha)3(beta)1, (alpha)4(beta)1, (alpha)5(beta)1, (alpha)8(beta)1, (alpha)v(beta)1, (alpha)v(beta)3 and a (beta)8 integrin, as potential fibronectin receptors, and flow cytometry analyses revealed no major heterogeneity among the cell population for expression of integrin subunits. In addition, the integrin repertoire expressed by neural crest cells was found not to change dramatically during migration. At the cellular level, only (alpha)v(beta)1 and (alpha)v(beta)3 were concentrated in focal adhesion sites in connection with the actin microfilaments, whereas the other integrins were predominantly diffuse over the cell surface. In inhibition assays with function-perturbing antibodies, it appeared that complete abolition of cell spreading and migration could be achieved only by blocking multiple integrins of the (beta)1 and (beta)3 families, suggesting possible functional compensations between different integrins. In addition, these studies provided evidence for functional partitioning of integrins in cell adhesion and migration. While spreading was essentially mediated by (alpha)v(beta)1 and (alpha)8(beta)1, migration involved primarily (alpha)4(beta)1, (alpha)v(beta)3 and (alpha)8(beta)1 and, more indirectly, (alpha)3(beta)1. (alpha)5(beta)1 and the (beta)8 integrin were not found to play any major role in either adhesion or migration. Finally, consistent with the results of inhibition experiments, recruitment of (alpha)4(beta)1 and (alpha)v(beta)3, individually or in combination using antibodies or recombinant VCAM-1 and PECAM-1 molecules as a substratum, was required for migration but was not sufficient to produce migration of the cell population as efficiently as with fibronectin. In conclusion, our study indicates that neural crest cells express a multiplicity of fibronectin-binding integrins and suggests that dispersion of the cell population requires cooperation between distinct integrins regulating different events of cell adhesion, locomotion and, possibly, proliferation and survival.

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