Unexpected diversity of cnidarian integrins: expression during coral gastrulation

BACKGROUND: Adhesion mediated through the integrin family of cell surface receptors is central to early development throughout the Metazoa, playing key roles in cell-extra cellular matrix adhesion and modulation of cadherin activity during the convergence and extension movements of gastrulation. It has been suggested that Caenorhabditis elegans, which has a single β and two α integrins, might reflect the ancestral integrin complement. Investigation of the integrin repertoire of anthozoan cnidarians such as the coral Acropora millepora is required to test this hypothesis and may provide insights into the original roles of these molecules. RESULTS: Two novel integrins were identified in Acropora. AmItgα1 shows features characteristic of α integrins lacking an I-domain, but phylogenetic analysis gives no clear indication of its likely binding specificity. AmItgβ2 lacks consensus cysteine residues at positions 8 and 9, but is otherwise a typical β integrin. In situ hybridization revealed that AmItgα1, AmItgβ1, and AmItgβ2 are expressed in the presumptive endoderm during gastrulation. A second anthozoan, the sea anemone Nematostella vectensis, has at least four β integrins, two resembling AmItgβ1 and two like AmItgβ2, and at least three α integrins, based on its genomic sequence. CONCLUSION: In two respects, the cnidarian data do not fit expectations. First, the cnidarian integrin repertoire is more complex than predicted: at least two βs in Acropora, and at least three αs and four βs in Nematostella. Second, whereas the bilaterian αs resolve into well-supported groups corresponding to those specific for RGD-containing or laminin-type ligands, the known cnidarian αs are distinct from these. During early development in Acropora, the expression patterns of the three known integrins parallel those of amphibian and echinoderm integrins.