Integrin α subunit mRNAs are differentially expressed in early Xenopus embryos

Adhesion of cells to extracellular matrix proteins is mediated, in large part, by transmembrane receptors of the integrin family. The identification of specific integrins expressed in early embryos is an important first step to understanding the roles of these receptors in developmental processes. We have used polymerase chain reaction methods and degenerate oligodeoxynucleotide primers to identify and clone Xenopus integrin sub-units from neurula-stage (stage 17) cDNA. Partial cDNAs encoding integrin subunits α₂, α₃, α₄, α₅, α₆ and an α_IIb-related subunit were cloned and used to investigate integrin mRNA expression in early embryos by RNase protection assay and whole-mount in situ hybridization methods. Considerable integrin diversity is apparent early in development with integrins α₂, α₃, α₄, α₅ and α₆ each expressed by the end of gastrulation. Both α₃ and α₅ are expressed as maternal mRNAs. Zygotic expression of α₂, α₃, α₄ and α₆ transcripts begins during gastrulation. Integrin α₅ is expressed at relatively high levels during cleavage, blastula and gastrula stages suggesting that it may represent the major integrin expressed in the early embryo. We demonstrated previously that integrin α₁ protein synthesis remains constant following induction of stage 8 animal cap cells with activin (Smith, J. C., Symes, K., Hynes, R. O. and DeSimone, D. W. (1990) Development 108, 289–298.). Here we report that integrin α₃, α₄ and α₆ mRNA levels increase following induction with 10 U/ml activin-A whereas α₅, α₁ and α₃ mRNA levels remain unchanged. Whole-mount in situ hybridization reveals that α₃ mRNAs are expressed by cells of the involuting mesoderm in the dorsal lip region of early gastrulae. As gastrulation proceeds, ₃ expression is localized to a stripe of presumptive notochordal cells along the dorsal midline. In neurulae, α₃ mRNA is highly expressed in the notochord but becomes progressively more restricted to the caudalmost portion of this tissue as development proceeds from tailbud to tadpole stages. In addition, α₃ 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 tissues during embryogenesis.