Michael Wilcox

The function of PS integrins during Drosophila embryogenesis

The Drosophila position-specific (PS) antigens are homologous to the vertebrate fibronectin receptor family, or integrins. A Drosophila gene required for embryonic morphogenesis, l(1)myospheroid, codes for a product homologous to the beta subunit of the vertebrate integrins. l(1)myospheroid mutants die during embryogenesis. We show here that they lack the beta subunit of the PS antigens. In the absence of the beta subunit in mutant embryos, the PS alpha subunits are not expressed on the cell surface. We conclude that the l(1)myospheroid phenotype represents the lack-of-function phenotype for these Drosophila integrins. In wild-type embryos, PS antigens are found at the interface between mesoderm and ectoderm, and later mainly at the attachment sites of muscles to the epidermis and gut. Together these results indicate that during embryogenesis, Drosophila integrins are used to attach mesoderm to ectoderm, and are required for the proper assembly of the extracellular matrix and for muscle attachment.

The Drosophila PS2 antigen is an invertebrate integrin that, like the fibronectin receptor, becomes localized to muscle attachments

We establish that the position-specific antigen 2 (PS2), a Drosophila cell surface glycoprotein complex, is an invertebrate member of the vertebrate fibronectin receptor (integrin) family. New monoclonal antibodies show that in Drosophila embryos and larvae PS2 alpha subunits have a size of ca. 140 kd. Analysis of cDNA and genomic clones revealed that the canonical PS2 alpha subunit contains 1394 amino acids and has extensive homology to the heavy and light chains of integrin alpha subunits. The distribution of the PS2 antigen is regulated at the level of PS2 alpha subunit mRNA. In early Drosophila development the protein is restricted to mesoderm and appears to be involved in muscle attachment. We suggest that PS2, like vertebrate fibronectin receptors, mediates changes in cell shape and cell-extracellular matrix adhesion by binding to a basement membrane protein.

Developmentally regulated alternative splicing of Drosophila integrin PS2 α transcripts

Abstract We report the characterization of a chromosomal integrin gene that encodes the Drosophila PS2 α subunit. The gene is composed of 12 exons spanning 31 kb. By employing a novel method for directed cDNA cloning, we have analyzed over 300 independent cDNA clones for the existence of alternate RNA products. Two forms of PS2 α mRNA are frequently observed: a canonical (C) form and a form lacking the 75 nucleotide exon 8 (m8). The relative ratio of these two forms varies widely during development. Although region A, derived from exon 8 and the adjacent 25 amino acids, shows weak conservation among the sequences of α subunits that bind to different ligands, it is highly conserved in the homologous PS2 α gene of the distantly related Mediterranean fruitfly. We suggest that the variable region A may be important in determining the specificity and affinity of integrin receptors for their ligands.

Drosophila position-specific antigens resemble the vertebrate fibronectin-receptor family.

The Drosophila position‐specific (PS) antigens are a family of cell surface glycoprotein complexes thought to be involved in morphogenesis. Their overall structures and biochemical properties are similar to those of a group of vertebrate receptors, including those for fibronectin, fibrinogen and vitronectin, and also the leukocyte antigens Mac‐1, LFA‐1 and p150,95 and the VLA family of cell surface antigens. The N‐terminal sequences of the alpha subunits of some of these molecules are homologous to the N‐terminus of a PS antigen component. The Drosophila PS antigens thus appear to be homologous to these vertebrate receptors.

A position-specific cell surface antigen in the drosophila wing imaginal disc

The antibody produced by the hybrid cell line DK. 1A4 recognizes an antigen present initially on all the epithelial cells of the D. melanogaster wing imaginal disc. This antigen becomes progressively restricted to cells in the dorsal region of the disc during the final larval instar. The presence of the antigen does not correlate with the specific adult structures to which the cells will eventually contribute, but rather with the position of the cells in the disc. In late discs, the line bounding the region in which the antigen persists corresponds to the boundary between the dorsal and ventral compartments are revealed by a clonal analysis of the undifferentiated disc. Together, these data suggest that the antigens disappearance may be specific to the cells of the ventral compartment of the wing disc.

The Drosophila position-specific antigens are a family of cell surface glycoprotein complexes.

Position‐specific (PS)1 and PS2 monoclonal antibodies bind non‐uniformly to the mature wing imaginal disc of Drosophila with respect to the boundary separating the dorsal and ventral developmental compartments. PS1 antibodies preferentially recognize dorsal cells, PS2 antibodies ventral cells. Antibodies of the two classes extract distinct sets of glycoproteins from an imaginal disc lysate. PS3 antibodies bind to both dorsal and ventral disc cells and extract both PS1 and PS2 glycoprotein sets together with an additional component. We show that the PS antigens are related multimeric glycoprotein complexes on the cell surface. PS3 antibodies recognize a glycoprotein present in all complexes, while PS1 and PS2 antibodies recognize unique components of their own complexes. Spatial and temporal correlations suggest the molecules may have a function in development.

Cloning and characterization of αPS1, a novel Drosophila melanogaster integrin

Abstract The Drosophila position-specific integrins (PS integrins or PS antigens) comprise two heterodimeric complexes, αPS1βPS and αPS2βPS. With the cloning of αPS1 described here, we complete the characterization of the primary structure of the three PS integrin subunits. We have purified the αPS1 subunit, obtained peptide sequence and isolated genomic and cDNA clones. The encoded αPS1 protein contains pattern of the cleaved alpha integrins, three putative metal binding domains and shows the other characteristic features of alpha integrins. Regions of sequence variation indicate that αPS1 is distinct from all other alpha chains. The transcript analysis shows that the patterns of both αPS1 mRNA and protein expression are the same, suggesting that the gene is controlled transcriptionally. We compare the gene structures of the DrosophilaαPS1, αPS2, the human αPS1and αPS2 (p150,95) and the C. elegans F54G8.3 integrins. We find several positions and phases of introns conserved which, supported by conservation also in the amino acid sequence, indicates that they all derive from a common ancestral gene.

The Developmental Biology of Heterocyst and Akinete Formation in Cyanobacteria

We will be concerned with the two major differentiated cell types of filamentous cyanobacteria--the heterocyst and the akinete. The former is generally accepted to be the site of aerobic nitrogen fixation in heterocystous cyanobacteria. The latter is a spore-like cell capable of withstanding certain environmental extremes and of germination. A short general introduction to cyanobacteria and their cell types will be included. The remainder of the review will fall into four main sections. The first will deal with the metabolism of the heterocyst and akinete, with particular reference to nitrogen fixation in the former. The next will be concerned with a description of the metabolic and ultrastructural changes associated with heterocyst and a kinete development. A third section will describe the special arrangement of the heterocyst and akinete (one of the features which makes this group of prokaryotes unique), the relationship between the cell types, and methods of altering this normal regular spatial pattern. The final section will describe in detail present theories of pattern control in cyanobacteria and the mechanisms by which the process of differentiation itself is regulated.

A Minute encoding a ribosomal protein enhances wing morphogenesis mutants

E(Dl)KP135 has been isolated previously as a recessive lethal Drosophila P element insertion line with a dominant enhancing effect on the phenotype of Delta, a gene encoding a surface membrane protein. We show here that this P insertion also enhances the wing phenotype of nd1, an allele of Notch encoding another transmembrane protein, the putative receptor of Delta, as well as that of if3, an allele of the integrin gene PS2 alpha. Moreover, we noticed that this P insertion causes a severe Minute phenotype. Molecular characterisation revealed that the P element disrupts the putative mRNA leader sequence of the ribosomal protein L19 gene. We tested further Minute genes and found that two of them, similarly to E(Dl)KP135, strongly enhance the nd1 wing phenotype. Our results suggest that the pleiotropic Minute syndrome can affect, probably indirectly, one or more steps of wing morphogenesis that involve surface adhesion of epithelial cells.

Clonal analysis of the undifferentiated wing disk of Drosophila

Using a new cell marker, we have examined the early clonal restrictions in wing imaginal disks from late third instar larvae of Drosophila. Large clones do not significantly alter the gross structure of the disks, allowing us to map the clones relative to morphological landmarks. Clones in the posterior region of the disks behave in a similar way to clones in the adult cuticle; that is, they appear to be restricted to a defined compartment, and the presumptive anterior/posterior compartment border defined by these clones is located in a similar place in every disk. In contrast, clones in the anterior region of the wing disks often cross into the region normally occupied by the posterior compartment and, especially near the margins of the disk, show no common posterior boundary. We believe that the anterior clones are “pushing” the anterior/posterior compartment border, and that this pushing is related to the growth advantage of the marked cells, which are Minute+ in a Minute background. Finally, we find that clones do not cross between the adepithelial cells, which contribute to the adult musculature, and the disk epithelium.