Integrins and morphogenesis.

The Drosophila position specific (PS) integrins consist of two cell surface heterodimers, PS1 (alpha PS1 beta PS) and PS2 (alpha OS2 beta PS), which are expressed on complementary sides of attachments between cell layers and are essential for these attachments. Current evidence suggests that the PS integrins bind to components of the extracellular matrix, similar to the majority of vertebrate integrins, but specific Drosophila ligands have not yet been identified. In the embryo PS1 is found on the surface of the epidermis and endoderm, while PS2 is restricted to the mesoderm. The integrins are concentrated at the sites where the somatic muscles attach to the epidermis and at the interface between the visceral mesoderm and the endoderm. In myospheroid mutant embryos, which lack the beta PS subunit, the adhesion between the mesoderm and the other cell layers fails. The PS integrins are also required for the adhesion of the dorsal to the ventral surface of the wing during metamorphosis. PS1 is expressed on the basal surface of the dorsal cells and PS2 is expressed on the ventral cells. Loss of PS integrin function in the wing results in balloon shaped wings because of the failure of the two surfaces of the wing blade to adhere to each other. These and other aspects of the phenotypes of mutations in the genes encoding the PS integrins indicate that integrins play an important role in the adhesion of different cell layers to each other and thus an essential role in the morphogenesis of the organism. The use of extracellular matrix receptors in this role may aid in keeping the different cell layers distinct.