Separation of neural induction and neurulation in Xenopus

Cellular interactions with laminin are important for numerous morphogenetic events. In Xenopus, the first of these is neurulation. The integrin alpha6 subunit mediates an attachment of the cells of the neural plate to the underlying basal lamina. A disruption of this interaction results in embryos that fail to neurulate (T. E. Lallier et al., 1996, Development 122, 2539-2554). Here we provide evidence supporting the specificity of this phenomenon and characterize developmental events as either disrupted or unaffected by a perturbation of alpha6 integrin expression. First, reduction of alpha6 integrin expression does not halt mitotic division throughout the embryo, indicating that the neural defects observed are not simply a global perturbation of all developmental processes. Second, a gene associated with dorsal mesoderm formation, brachyury, is expressed normally in alpha6 integrin-perturbed embryos. Third, the expression of BMP4, noggin, chordin, and follistatin, all of which are critical for neural induction, are at near normal levels. In addition, several genes expressed shortly after neural induction (N-CAM, nrp1, and Xanf1) are not perturbed in nonneurulating embryos. Interestingly, expression of one neural-specific gene (synaptobrevin), which is normally detectable late in neurulation, is abolished in these alpha6 integrin-perturbed embryos. Furthermore, the spatial expression of several transcripts is expanded in alpha6 integrin-perturbed embryos (orthodenticle and engrailed). Taken together, these data indicate that while alpha6 integrin-mediated interactions with laminin are required for neurulation, they are not required for the initial processes of neural induction. However, these cell-extracellular matrix interactions appear to be important in later inductive events and rostrocaudal patterning of the neural tube.