INTEGRIN-ALPHA SUBUNIT MESSENGER-RNAS ARE DIFFERENTIALLY EXPRESSED INEARLY XENOPUS-EMBRYOS

Adhesion of cells to extracellular matrix proteins is mediated, in lar ge part, by transmembrane receptors of the integrin family. The identi fication of specific integrins expressed in early embryos is an import ant first step to understanding the roles of these receptors in develo pmental processes. We have used polymerase chain reaction methods and degenerate oligodeoxynucleotide primers to identify and clone Xenopus integrin alpha subunits from neurula-stage (stage 17) cDNA. Partial cD NAs encoding integrin subunits alpha2, alpha3, alpha4, alpha5, alpha6 and an alpha(IIb)-related subunit were cloned and used to investigate integrin mRNA expression in early embryos by RNase protection assay an d whole-mount in situ hybridization methods. Considerable integrin div ersity is apparent early in development with integrins alpha2, alpha3, alpha4, alpha5 and alpha6 each expressed by the end of gastrulation. Both alpha3 and alpha5 are expressed as maternal mRNAs. Zygotic expres sion Of alpha2, alpha3, alpha4 and alpha6 transcripts begins during ga strulation. Integrin alpha5 is expressed at relatively high levels dur ing cleavage, blastula and gastrula stages suggesting that it may repr esent the major integrin expressed in the early embryo. We demonstrate d previously that integrin beta1 protein synthesis remains constant fo llowing induction of stage 8 animal cap cells with activin (Smith, J. C., Symes, K., Hynes, R. 0. and DeSimone, D. W. (1990) Development 108 , 289-298.). Here we report that integrin alpha3, alpha4 and alpha6 mR NA levels increase following induction with 10 U/ml activin-A whereaS alpha5, beta1 and beta3 mRNA levels remain unchanged. Whole-mount in s itu hybridization reveals that alpha3 mRNAs are expressed by cells of the involuting mesoderm in the dorsal lip region of early gastrulae. A s gastrulation proceeds, alpha3 expression is localized to a stripe of presumptive notochordal cells along the dorsal midline. In neurulae, alpha3 mRNA is highly expressed in the notochord but becomes progressi vely more restricted to the caudalmost portion of this tissue as devel opment proceeds from tailbud to tadpole stages. In addition, alpha3 is expressed in the forebrain region of later stage embryos. These data suggest that integrin-mediated adhesion may be involved in the process of mesoderm involution at gastrulation and the organization of tissue s during embryogenesis.