Drosophila cell adhesion molecules

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Influence of calcium on Manduca sexta plasmatocyte spreading and network formation

Plasmatocytes are a class of insect hemocytes important in the cellular defense response. In some species, they are phagocytic, protecting the insect from smaller pathogens. In many insects, they work in concert with other hemocytes (particularly other plasmatocytes and granular cells) to form nodules and to encapsulate foreign material. To perform these functions, plasmatocytes attach to, spread on, and surround suitable targets. Because of their importance, because we had previously observed that prolonged incubation of hemocytes in solutions containing the divalent cation chelator ethylenediaminetetraacetic acid (EDTA) inhibited plasmatocyte spreading, and because of the importance of divalent cations in many immune-related functions, we investigated the effect of calcium and magnesium on spreading of plasmatocytes from fifth instar Manduca sexta larvae. On glass slides, plasmatocytes spread more quickly and elongated in Grace's medium containing 5 mM calcium, compared to calcium-free medium. In the presence of calcium, plasmatocyte adhesion, spreading, and network formation were not visibly different in magnesium-free and magnesium-containing Grace's medium. Using immunomicroscopy with a monoclonal antibody specific for plasmatocytes, we measured the length and width of plasmatocytes incubated with several different concentrations of calcium. Plasmatocyte length positively correlated with calcium concentration to 5 mM (maximum concentration tested and approximately the hemolymph concentration). Mean plasmatocyte width was less in 0 and 5 mM calcium than in 0.05 or 0.5 mM calcium. On plastic, hemocytes survived longer than on glass (they survived beyond 24 h) and, in 5 mM calcium, formed an extensive network readily visible by phase-contrast microscopy. This network was never as extensive in the absence of calcium. Network formation in the absence of magnesium, but presence of calcium, resembled network formation in standard Grace's medium.

Monoclonal antibody MS13 identifies a plasmatocyte membrane protein and inhibits encapsulation and spreading reactions of Manduca sexta hemocytes

Lepidopterans generally can successfully defend themselves against a variety of parasites or parasitoids. One mechanism they use is to encapsulate the invader in many layers of hemocytes. For encapsulation to occur, the hemocytes must attach to the foreign material, spread, and adhere to each other. The molecules that mediate these processes are not known. One method to identify proteins potentially necessary for adhesion, spreading, and, thus, encapsulation is to use monoclonal antibodies that interfere with these functions. In this paper, we report that a monoclonal antibody against Manduca sexta plasmatocytes effectively inhibited encapsulation of synthetic beads in vitro and in vivo. Furthermore, it inhibited plasmatocyte spreading in vitro. Other anti-hemocyte antibodies did not have these effects. The plasmatocyte-specific monoclonal antibody, mAb MS13, recognized a protein of approximately 90,000 daltons as indicated by Western blot analysis of hemocyte lysate proteins. The epitope recognized by mAb MS13 was present on the exterior surface of plasmatocytes. Using indirect immunohistochemistry with hemocyte-specific antibodies, we also determined that during encapsulation plasmatocytes were the first cells bound to latex beads and later layers consisted of both plasmatocytes and granular cells. Arch.

Invertebrate integrins: structure, function, and evolution

Integrins are a family of molecules that have fundamental roles in cell-cell and cell-matrix adhesion. It is thought that all metazoan cells have one or more integrin receptors on their surface and that these molecules may have been key in the evolution of multicellularity. Knowledge of the structure, function, and distribution of integrin subunits in invertebrate phyla remains incomplete. However, through the recent use of polymerase chain reaction, integrin subunits have been identified in at least five phyla; sponges, cnidarians, nemadodes, arthropods, and echinoderms. The structure of all of the invertebrate subunits is remarkably similar to that of vertebrate integrin subunits. Some experimental data and patterns of expression indicate that invertebrate integrins have a range of functions similar to those of vertebrate integrins. The ligands are not well characterized but at least two laminin-binding receptors have been identified and two other receptors appear to bind using Arg-Gly-Asp motifs. Invertebrate integrins are present during development, in adults, and on a range of cell types including cells with immunological functions such as hemocytes and coelomocytes. Analysis of the invertebrate beta subunits indicates that the invertebrate integrins have diverged independently within each phylum. The two major clades of vertebrate integrins (beta 1, beta 2, beta 7 and beta 3, beta 5, beta 6, beta 8) appear to have radiated since the divergence of the deuterostomes and there are no distinct orthologous subunits in any of the invertebrate phyla. Since fundamental functions of integrins appear to be conserved, studies of invertebrate integrins have the potential of contributing to our understanding of this important group of receptors.

Replacement of the glycoinositol phospholipid anchor of Drosophila acetylcholinesterase with a transmembrane domain does not alter sorting in neurons and epithelia but results in behavioral defects

Drosophila has a single glycoinositol phospholipid (GPI)-anchored form of acetylcholinesterase (AChE) encoded by the Ace locus. To assess the role that GPI plays in the physiology, of AChE, we have replaced the wild-type GPI-AChE with a chimeric transmembrane form (TM-AChE) in the nervous system of the fly. Ace null alleles provided a genetic background completely lacking in endogenous GPI-AChE, and Ace minigene P transposon constructs were used to express both GPI- and TM-AChE forms in the tissues where AChE is normally expressed. Control experiments with the GPI-AChE minigene demonstrated a threshold between 9 and 12% of normal AChE activity for adult viability. Ace mutant flies were rescued by GPI-AChE minigene lines that expressed 12-40% of normal activity and were essentially unchanged from wild-type flies in behavior. TM-AChE minigene lines were able to rescue Ace null alleles, although with a slightly higher threshold than that for GPI-AChE. Although rescued flies expressing GPI-AChE at a level of 12% of normal activity were viable, flies expressing 13-16% of normal activity from the TM-AChE transgene died shortly after eclosion. Flies expressing TM-AChE at about 30% of normal levels were essentially unchanged from wild-type flies in gross behavior but had a reduced lifespan secondary to subtle coordination defects. These flies also showed reduced locomotor activity and performed poorly in a grooming assay. However, light level and electron microscopic immunocytochemistry showed no differences in the localization of GPI- and TM-AChE. Furthermore, endogenous and ectopic-induced expression of both AChEs in epithelial tissues of the adult and embryo, respectively, showed that they were sorted identically. Most epithelial cells sorted GPI- and TM-AChE to the apical surface, but cuticle-secreting epithelia sorted both proteins basolaterally. Our data suggest that rather than having a primary role in protein sorting, the GPI anchor or AChE plays some other more subtle cellular role in neuronal physiology.

CD9 of mouse brain is implicated in neurite outgrowth and cell migration in vitro and is associated with the alpha 6/beta 1 integrin and the neural adhesion molecule L1

We describe here a novel monoclonal antibody (mab H6) which recognizes CD9, an integral cell surface constituent previously described in cells of the hematopoietic lineage and involved in the aggregation of platelets. Mab H6 was raised against membranes of immature mouse astrocytes and reacted with a protein of 25-27 kD in detergent extracts of adult mouse brain membranes. Sequence analysis of the N-terminal amino acids revealed an identity of 96% with CD9 from mouse kidney. CD9 was localized in the central and peripheral mouse nervous systems: in the spinal cord of 11-day-old mouse embryos, CD9 was strongly expressed in the floor and roof plates. In the adult mouse sciatic nerve, myelin sheaths were highly CD9-immunoreactive. Mab H6 reacted with the cell surfaces of both glial cells and neurons in culture and inhibited migration of neuronal cell bodies, neurite fasciculation and outgrowth of astrocytic processes from cerebellar microexplants. Neurite outgrowth from isolated small cerebellar neurons was increased in the presence of mab H6 on substrate-coated laminin, but not on substrate-coated poly-L-lysine. Addition of mab H6 elicited an increase in intracellular Ca2+ concentration in these cells on substrate-coated laminin. Immunoprecipitates of CD9 from cultured mouse neuroblastoma N2A cells contained the alpha 6/beta 1 integrin. Moreover, preparations of CD9 immunoaffinity-purified from adult mouse brain using a mab H6 column contained the neural adhesion molecule L1, but not other neural adhesion molecules. CD9 bound to L1, but not to NCAM or MAG. Both the alpha 6/beta 1 integrin and L1 could be induced to coredistribute with CD9 on the surface of cultured neuroblastoma N2A cells. The combined observations suggest that CD9 can associate with L1 and alpha 6/beta 1 integrin to influence neural cell interactions in vitro.

Role of the PS integrins in Drosophila development

The PS1 and PS2 integrins of Drosophila are heterodimers of alphaPS1betaPS and alphaPS2betaPS subunits, respectively, with very strong structural similarity to vertebrate integrins. Cell transfection experiments indicate that the PS integrins are receptors for extracellular matrix components and are functionally analogous to vertebrate integrins. Matrix ligands from Drosophila tissues have been identified for PS1 and PS2 integrins, using transformed cells and a cell-spreading assay. Mutations in all three subunit genes have been identified, and the phenotypes of mutants indicate that PS integrins are required for the proper morphogenesis of a number of embryonic tissues. Using methods to produce genetic mosaics and transformation of integrin transgenes into whole animals, integrin functions in adult morphogenesis also have been examined. In the pupal wing, integrins are critically required to maintain the connection between dorsal and ventral epithelia, and recent results suggest that in early pupal development, the integrins are acting as specific receptors, as opposed to a non-specific cell-matrix glue.

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.

Masquerade: a novel secreted serine protease-like molecule is required for somatic muscle attachment in the Drosophila embryo

Diverse developmental processes, such as neuronal growth cone migration and cell shape changes, are mediated by the interactions of cells with the extracellular matrix. We describe here a secreted molecule encoded by the Drosophila masquerade (mas) gene. Total loss of mas function causes defective muscle attachment. This mutant phenotype suggests that mas normally acts to stabilize cell-matrix interaction and represents a novel functional and limiting component in the adhesion process. mas encodes a 1047-amino-acid preproprotein that is further processed by proteolytic cleavage to generate two polypeptides. The carboxy-terminal polypeptide is highly similar to serine proteases and has an extracellular localization; however, it is unlikely to possess proteolytic activity, because the catalytic site serine has been substituted by a glycine residue. During embryonic development, the mas amino- and carboxy-terminal polypeptides are differentially localized. The mas carboxy-terminal polypeptide accumulates at all somatic muscle attachment sites, which corresponds well with the morphological defect seen in the mas mutants. Our findings demonstrate the involvement of an extracellular component in somatic muscle attachment. We propose that mas acts via its modified serine protease motif, either as a novel adhesion molecule and/or as a competitive antagonist of serine proteases, to stabilize muscle attachment.

Drosophila integrins and their ligands

The major advance during the past year was the identification of ligands for two of the previously known position-specific integrins in Drosophila. At the same time, two new Drosophila integrin subunits (one alpha and one beta) were discovered, and significant progress was made on developmental genetic analyses of integrin functions, shedding light on the roles of integrins in Drosophila development.

Tiggrin, a novel Drosophila extracellular matrix protein that functions as a ligand for Drosophila alpha PS2 beta PS integrins

Genetic and other studies of Drosophila integrins have implicated these extracellular matrix receptors in various morphogenetic events, but identification of their endogenous ligands has been elusive. We report the biochemical purification and cloning of tiggrin, a novel extracellular matrix protein from Drosophila. This 255 × 10(3) M(r) polypeptide contains the potential integrin recognition sequence Arg-Gly-Asp (RGD) and 16 repeats of a novel 73–77 amino acid motif. The tiggrin gene is at chromosome locus 26D1-2 and is expressed by embryonic hemocytes and fat body cells. Tiggrin protein is detected in matrices, especially at muscle attachment sites that also strongly express integrins. Tiggrin-coated surfaces support primary embryo cell culture and provide excellent substrates for alpha PS2 beta PS integrin-mediated cell spreading. Soluble RGD-peptides inhibit this cell spreading.

Differentiation, extracellular matrix synthesis, and integrin assembly by Drosophila embryo cells cultured on vitronectin and laminin substrates

Two contrasting substrates, Drosophila laminin and human vitronectin, caused determined primary Drosophila embryo cells to follow alternate intermediate differentiation steps without affecting the final outcome of differentiation. Integrin alpha PS2 beta PS3 was essential for the initial spreading of myocytes on vitronectin: focal contacts rich in beta PS3 integrins formed and were connected by actin- and myosin-containing stress fibers. While alpha PS2 beta PS3 was unnecessary for myotube formation on laminin, it was required for the subsequent change to a sarcomeric cytoarchitecture. The differentiating primary cultures synthesized integrins and assembled them into detergent-insoluble, cytoskeleton-associated complexes. Collagen IV, laminin, glutactin, papilin, and other extracellular matrix proteins were made primarily by hemocytes and were secreted into the medium. Further differentiation within the cultures was influenced by secreted components and by later addition of vitronectin or bovine serum. Comparison of the differentiation of various cell types on the two substrates showed that vitronectin provided a selective advantage for the differentiation of myocytes, with enrichment over epithelia, epidermal cells, and neurites.