Requirements for the cytoplasmic domain of the alphaPS1, alphaPS2 and betaPS integrin subunits during Drosophila development.

The integrins are a family of transmembrane heterodimeric proteins that mediate adhesive interactions and participate in signaling across the plasma membrane. In this study we examine the functional significance of the cytoplasmic domains of the alphaPS1, alphaPS2 and betaPS subunits of the Drosophila Position Specific (PS) integrin family by analyzing the relationship between cytoplasmic domain structure and function in the context of a developing organism. By examining the ability of ssPS molecules lacking the cytoplasmic domain to rescue embryonic abnormalities associated with PS integrin loss, we find that although many embryonic events require the betaPS cytoplasmic domain, this portion of the molecule is not required for at least two processes requiring PS integrins: formation of midgut constrictions and maintaining germband integrity. Furthermore, our studies demonstrate that mutant proteins affecting four highly conserved amino acid residues in the cytoplasmic tail function with different efficiencies during embryonic development, suggesting that interaction of PS integrins with cytoplasmic ligands is developmentally modulated during embryogenesis. We have also examined the ability of alphaPS1 and alphaPS2 to function without their cytoplasmic domains. By analyzing the ability of transgenes producing truncated alphaPS molecules to rescue abnormalities associated with integrin loss, we find that the cytoplasmic tail of alphaPS2 is essential for both embryonic and postembryonic processes, while this portion of alphaPS1 is not required for function in the wing and in the retina. Furthermore, temperature-shift experiments suggest roles for the alphaPS2 cytoplasmic domain in signaling events occurring in the developing wing.

A novel alpha integrin subunit associates with betaPS and functions in tissue morphogenesis and movement during Drosophila development.

We have identified a novel alpha integrin subunit in Drosophila, that associates with betaPS integrin. We report the temporal expression of the gene encoding this integrin subunit, which we have called alphaPS3, throughout development and the localization of its expression during embryogenesis. AlphaPS3 RNA was localized to tissues undergoing invagination, tissue movement and morphogenesis such as salivary gland, trachea, midgut, dorsal vessel, midline of the ventral nerve cord, amnioserosa and the amnioproctodeal invagination. AlphaPS3 DNA localized to the chromosomal vicinity of scab (scb), previously identified by a failure of dorsal closure. Embryos homozygous for the 119 allele of scb had no detectable alphaPS3 RNA and the 1035 allele of scb contains a P element inserted just 5' of the coding region for the shorter of the gene's two transcripts. Furthermore, mutations in the scb locus exhibit additional defects corresponding to sites of alphaPS3 transcription, including abnormal salivary glands, mislocalization of the pericardial cells and interrupted trachea. Removal of both maternal and zygotic betaPS produced similar defects, indicating that these two integrin subunits associate in vivo and function in the movement and morphogenesis of tissues during development in Drosophila. Phenotypic similarities suggest that laminin A is a potential ligand for this integrin, at least in some tissues.

Alternatively spliced forms of the Drosophila alphaPS2 subunit of integrin are sufficient for viability and can replace the function of the alphaPS1 subunit of integrin in the retina.

The Drosophila inflated (if) gene encodes the alphaPS2 subunit of the PS family of integrins. The if transcript is spliced such that alphaPS2 is found in two alternative forms, alphaPS2(C) and alphaPS2(m8), which differ by 25 amino acid residues in a region shown to affect cation requirements and ligand specificity. In this study, we examine the functional significance of the protein isoforms of if by analyzing the ability of transgenes producing only one isoform to rescue developmental abnormalities associated with complete loss of PS2 integrin. We find that either form of alphaPS2 is sufficient to rescue if- animals to viability; however, the alphaPS2(C) form promotes higher survival of the organism. Furthermore, these studies suggest distinct roles for alphaPS2(C) and alphaPS2(m8) during development. When expressed in the developing wing, alphaPS2(m8) is more efficient at rescuing the if wing blister phenotype than is alphaPS2(C). Expression of alphaPS2(C) in the eye produces dominant disruption of photoreceptor organization. We have also examined the ability of alphaPS2 and alphaPS1 to maintain photoreceptor organization in the Drosophila retina. Clonal analysis of sectioned eyes suggests a requirement for alphaPS1, but not alphaPS2. However, ectopic expression of if(m8) or if(C) shows that either splice form Of alphaPS2 can functionally replace alphaPS1 and rescue the mew eye phenotype.

Nonequivalent requirements for PS1 and PS2 integrin at cell attachments in Drosophila: genetic analysis of the alpha PS1 integrin subunit.

We report on the generation and phenotype of mutant alleles of multiple edematous wings (mew), the gene encoding the alpha PS1 subunit of the PS1 integrin of Drosophila. None of the six alleles examined makes detectable protein, and one allele results from a chromosome break near the middle of the translated sequence, so we are confident that we have described the null phenotype. In contrast to if (alpha PS2) and mys (beta PS) mutants, most mutant mew embryos hatch, to die as larvae. Mutant mew embryos display abnormal gut morphogenesis but, unlike mys or if embryos, there is no evidence of defects in the somatic muscles. Thus, the complementary distributions of PS1 (alpha PS1 beta PS) and PS2 (alpha PS2 beta PS) integrin on tendon cells and muscle, respectively, do not reflect equivalent requirements at the myotendinous junction. Dorsal herniation, characteristic of the mys lethal phenotype, is not observed in mew or in mew if embryos. Clonal analysis experiments indicate that eye morphogenesis is disrupted in mew clones, but if clones in the eye are relatively normal in morphology. Adult wings display blisters around large dorsal but not ventral mew clones. In contrast to dorsal mys clones, small mew patches do not necessarily display morphogenetic abnormalities. Thus, another integrin in addition to PS1 appears to function on the dorsal wing surface.

Functions for PS integrins in tissue adhesion, migration, and shape changes during early embryonic development in Drosophila.

The position-specific (PS) integrins of Drosophila are a family of cell surface heterodimers that function as receptors for extracellular matrix molecules. In this study we used time-lapse videomicroscopy to examine living embryos lacking both maternal and zygotic expression of the genes that encode the alpha PS1, alpha PS2, and beta PS integrin subunits. We demonstrate roles for these molecules as early as gastrulation. Abnormalities in mutant embryos include: separation and twisting of the embryonic germband, abnormal shape and migration of midgut primordia, irregular visceral mesoderm, detachment of amnioserosa cells, rupture of the cuticle along the dorsal midline, lack of midgut constriction, and detachment of somatic muscles. These observations suggest multiple roles for PS integrins in the adhesion of cells and in the formation, organization, and migration of embryonic tissues. We also show that although alpha PS1 beta PS and alpha PS2 beta PS are often expressed in adjacent embryonic tissues, this distribution does not necessarily reflect equivalent requirements. Furthermore, the complete loss of both alpha subunits does not produce all of the phenotypes observed in embryos lacking beta PS. This suggests that alpha PS1 beta PS and alpha PS2 beta PS are not required in all embryonic processes utilizing PS integrins.