Epithelial morphogenesis in the Drosophila egg chamber requires Parvin and ILK

Integrins are the major family of transmembrane proteins that mediate cell-matrix adhesion and have a critical role in epithelial morphogenesis. Integrin function largely depends on the indirect connection of the integrin cytoplasmic tail to the actin cytoskeleton through an intracellular protein network, the integrin adhesome. What is currently unknown is the role of individual integrin adhesome components in epithelia dynamic reorganization. Drosophila egg chamber consists of the oocyte encircled by a monolayer of somatic follicle epithelial cells that undergo specific cell shape changes. Egg chamber morphogenesis depends on a developmental array of cell-cell and cell-matrix signalling events. Recent elegant work on the role of integrins in the Drosophila egg chamber has indicated their essential role in the early stages of oogenesis when the pre-follicle cells assemble into the follicle epithelium. Here, we have focused on the functional requirement of two key integrin adhesome components, Parvin and Integrin-Linked Kinase (ILK). Both proteins are expressed in the developing ovary from pupae to the adult stage and display enriched expression in terminal filament and stalk cells, while their genetic removal from early germaria results in severe disruption of the subsequent oogenesis, leading to female sterility. Combining genetic mosaic analysis of available null alleles for both Parvin and Ilk with conditional rescue utilizing the UAS/Gal4 system, we found that Parvin and ILK are required in pre-follicle cells for germline cyst encapsulation and stalk cell morphogenesis. Collectively, we have uncovered novel developmental functions for both Parvin and ILK, which closely synergize with integrins in epithelia.

Differential Expression of Drosophila Transgelins Throughout Development

Transgelins are a conserved family of actin-binding proteins involved in cytoskeletal remodeling, cell contractility, and cell shape. In both mammals and Drosophila, three genes encode transgelin proteins. Transgelins exhibit a broad and overlapping expression pattern, which has obscured the precise identification of their role in development. Here, we report the first systematic developmental analysis of all Drosophila transgelin proteins, namely, Mp20, CG5023, and Chd64 in the living organism. Drosophila transgelins display overall higher sequence identity with mammalian TAGLN-3 and TAGLN-2 than with TAGLN. Detailed examination in different developmental stages revealed that Mp20 and CG5023 are predominantly expressed in mesodermal tissues with the onset of myogenesis and accumulate in the cytoplasm of all somatic muscles and heart in the late embryo. Notably, at postembryonic developmental stages, Mp20 and CG5023 are detected in the gut's circumferential muscles with distinct subcellular localization: Z-lines for Mp20 and sarcomere and nucleus for CG5023. Only CG5023 is strongly detected in the adult fly in the abdominal, leg, and synchronous thoracic muscles. Chd64 protein is primarily expressed in endodermal and ectodermal tissues and has a dual subcellular localization in the cytoplasm and the nucleus. During the larval-pupae transition, Chd64 is expressed in the brain, eye, legs, halteres, and wings. In contrast, in the adult fly, Chd64 is expressed in epithelia, including the alimentary tract and genitalia. Based on the non-overlapping tissue expression, we predict that Mp20 and CG5023 mostly cooperate to modulate muscle function, whereas Chd64 has distinct roles in epithelial, neuronal, and endodermal tissues.

Integrin intracellular machinery in action

Integrin-mediated adhesion to the extracellular matrix involves a surprisingly large number of intracellular proteins, the integrin-associated proteins (IAPs), which are a fraction of the total integrin adhesome. In this review we discuss how genetic approaches have improved our understanding of how each IAP contributes to integrin function, especially in the context of building a functional organism during development. We then begin the process of assembling IAP roles together into an integrated mechanism.

Transcriptome and proteome analyses reveal complex mechanisms of reproductive diapause in the two-spotted spider mite, Tetranychus urticae

Although a variety of factors underlying diapause have been identified in arthropods and other organisms, the molecular mechanisms regulating diapause are still largely unknown. Here, to better understand this process, we examined diapause-associated genes in the two-spotted spider mite, Tetranychus urticae, by comparing the transcriptomes and proteomes of early diapausing and reproductive adult females. Amongst genes underlying diapause revealed by the transcriptomic and proteomic data sets, we described the noticeable change in Ca²⁺ -associated genes, including 65 Ca²⁺ -binding protein genes and 23 Ca²⁺ transporter genes, indicating that Ca²⁺ signalling has a substantial role in diapause regulation. Other interesting changes in diapause included up-regulation of (1) glutamate receptors that may be involved in synaptic plasticity changes, (2) genes involved in cytoskeletal reorganization including genes encoding each of the components of thick and thin filaments, tubulin and members of integrin signalling and (3) genes involved in anaerobic energy metabolism, which reflects a shift to anaerobic energy metabolism in early diapausing mites.

Rapid IFM Dissection for Visualizing Fluorescently Tagged Sarcomeric Proteins

Sarcomeres, the smallest contractile unit of muscles, are arguably the most impressive actomyosin structure. Yet a complete understanding of sarcomere formation and maintenance is missing. The Drosophila indirect flight muscle (IFM) has proven to be a very valuable model to study sarcomeres. Here, we present a protocol for the rapid dissection of IFM and analysis of sarcomeres using fluorescently tagged proteins.

Drosophila vinculin is more harmful when hyperactive than absent, and can circumvent integrin to form adhesion complexes

Vinculin is a highly conserved protein involved in cell adhesion and mechanotransduction, and both gain and loss of its activity causes defective cell behaviour. Here, we examine how altering vinculin activity perturbs integrin function within the context of Drosophila development. Whereas loss of vinculin produced relatively minor phenotypes, gain of vinculin activity, through a loss of head–tail autoinhibition, caused lethality. The minimal domain capable of inducing lethality is the talin-binding D1 domain, and this appears to require talin-binding activity, as lethality was suppressed by competition with single vinculin-binding sites from talin. Activated Drosophila vinculin triggered the formation of cytoplasmic adhesion complexes through the rod of talin, but independently of integrin. These complexes contain a subset of adhesion proteins but no longer link the membrane to actin. The negative effects of hyperactive vinculin were segregated into morphogenetic defects caused by its whole head domain and lethality caused by its D1 domain. These findings demonstrate the crucial importance of the tight control of the activity of vinculin.

Summary: Development is more sensitive to gain of vinculin activity than its loss, and vinculin can promote cytoplasmic adhesion complexes independently of the usual integrin cue.

Parvin-ILK: An intimate relationship

Integrin-linked kinase (ILK), PINCH and Parvin proteins form the IPP-complex that has been established as a core component of the integrin-actin link. Our recent genetic studies on Drosophila parvin, reveal that loss of function mutant defects phenocopy those observed upon loss of ILK or PINCH in the muscle and the wing, strengthening the notion that these proteins function together in the organism. Our work identified that ILK is necessary and sufficient for parvin subcellular localization, corroborating previous data indicating a direct association between these two proteins. Further genetic epistasis analysis of the IPP-complex assembly at integrin adhesion sites reveals that depending on the cell context each component is required differently. At the muscle attachment sites of the embryo, ILK is placed upstream in the hierarchy of genetic interactions required for the IPP-complex assembly. By contrast, in the wing epithelium the three proteins are mutually interdependent. Finally, we uncovered a novel property for the CH1-domain of parvin: its recruitment at the integrin-containing junctions in an ILK-dependent manner. Apparently, this ability of the CH1-domain is controlled by the inter-CH linker region. Thus, an intramolecular interaction within parvin could serve as a putative regulatory mechanism controlling the ILK-Parvin interaction.

Identification of novel elements of the Drosophila blisterome sheds light on potential pathological mechanisms of several human diseases

Main developmental programs are highly conserved among species of the animal kingdom. Improper execution of these programs often leads to progression of various diseases and disorders. Here we focused on Drosophila wing tissue morphogenesis, a fairly complex developmental program, one of the steps of which – apposition of the dorsal and ventral wing sheets during metamorphosis – is mediated by integrins. Disruption of this apposition leads to wing blistering which serves as an easily screenable phenotype for components regulating this process. By means of RNAi-silencing technique and the blister phenotype as readout, we identify numerous novel proteins potentially involved in wing sheet adhesion. Remarkably, our results reveal not only participants of the integrin-mediated machinery, but also components of other cellular processes, e.g. cell cycle, RNA splicing, and vesicular trafficking. With the use of bioinformatics tools, these data are assembled into a large blisterome network. Analysis of human orthologues of the Drosophila blisterome components shows that many disease-related genes may contribute to cell adhesion implementation, providing hints on possible mechanisms of these human pathologies.

A dual role for integrin-linked kinase and β1-integrin in modulating cardiac aging

Cardiac performance decreases with age, which is a major risk factor for cardiovascular disease and mortality in the aging human population, but the molecular mechanisms underlying cardiac aging are still poorly understood. Investigating the role of integrin-linked kinase (ilk) and β1-integrin (myospheroid, mys) in Drosophila, which colocalize near cardiomyocyte contacts and Z-bands, we find that reduced ilk or mys function prevents the typical changes of cardiac aging seen in wildtype, such as arrhythmias. In particular, the characteristic increase in cardiac arrhythmias with age is prevented in ilk and mys heterozygous flies with nearly identical genetic background, and they live longer, in line with previous findings in Caenorhabditis elegans for ilk and in Drosophila for mys. Consistent with these findings, we observed elevated β1-integrin protein levels in old compared with young wild-type flies, and cardiac-specific overexpression of mys in young flies causes aging-like heart dysfunction. Moreover, moderate cardiac-specific knockdown of integrin-linked kinase (ILK)/integrin pathway-associated genes also prevented the decline in cardiac performance with age. In contrast, strong cardiac knockdown of ilk or ILK-associated genes can severely compromise cardiac integrity, including cardiomyocyte adhesion and overall heart function. These data suggest that ilk/mys function is necessary for establishing and maintaining normal heart structure and function, and appropriate fine-tuning of this pathway can retard the age-dependent decline in cardiac performance and extend lifespan. Thus, ILK/integrin-associated signaling emerges as an important and conserved genetic mechanism in longevity, and as a new means to improve age-dependent cardiac performance, in addition to its vital role in maintaining cardiac integrity.

ILK: a pseudokinase with a unique function in the integrin-actin linkage

ILK (integrin-linked kinase) is a central component of cell-matrix adhesions and an important regulator of integrin function. It forms a ternary complex with two other adaptor proteins, PINCH (particularly interesting cysteine- and histidine-rich protein) and parvin, forming the IPP (ILK-PINCH-parvin) complex that regulates the integrin-actin linkage as well as microtubule dynamics. These functions are essential for processes such as cell migration and matrix remodelling. The present review discusses the recent advances on the structural and functional characterization of ILK and the long-standing debate regarding its reported kinase activity.

Interactions by 2D Gel Electrophoresis Overlap (iGEO): a novel high fidelity approach to identify constituents of protein complexes

Background

Here we describe a novel approach used to identify the constituents of protein complexes with high fidelity, using the integrin-associated scaffolding protein PINCH as a test case. PINCH is comprised of five LIM domains, zinc-finger protein interaction modules. In Drosophila melanogaster, PINCH has two known high-affinity binding partners—Integrin-linked kinase (ILK) that binds to LIM1 and Ras Suppressor 1 (RSU1) that binds to LIM5—but has been postulated to bind additional proteins as well.

Results

To purify PINCH complexes, in parallel we fused different affinity tags (Protein A and Flag) to different locations within the PINCH sequence (N- and C-terminus). We expressed these tagged versions of PINCH both in cell culture (overexpressed in Drosophila S2 cell culture in the presence of endogenous PINCH) and in vivo (at native levels in Drosophila lacking endogenous PINCH). After affinity purification, we analyzed PINCH complexes by a novel 2D-gel electrophoresis analysis, iGEO (interactions by 2D Gel Electrophoresis Overlap), with mass spectrometric identification of individual spots of interest. iGEO allowed the identification of protein partners that associate with PINCH under two independent purification strategies, providing confidence in the significance of the interaction. Proteins identified by iGEO were validated against a highly inclusive list of candidate PINCH interacting proteins identified in previous analyses by MuDPIT mass spectrometry.

Conclusions

The iGEO strategy confirmed a core complex comprised of PINCH, RSU1, ILK, and ILK binding partner Parvin. Our iGEO method also identified five novel protein partners that specifically interacted with PINCH in Drosophila S2 cell culture. Because of the improved reproducibility of 2D-GE methodology and the increasing affordability of the required labeling reagents, iGEO is a method that is accessible to most moderately well-equipped biological laboratories. The biochemical co-purifications inherent in iGEO allow for rapid and unambiguous identification of the constituents of protein complexes, without the need for extensive follow-up experiments.

Dynamic regulation of the structure and functions of integrin adhesions

Integrin-mediated cell adhesions to the extracellular matrix (ECM) contribute to tissue morphogenesis and coherence and provide cells with vital environmental cues. These apparently static structures display remarkable plasticity and dynamic properties: they exist in multiple, interconvertible forms that are constantly remodeled in response to changes in ECM properties, cytoskeletal organization, cell migration, and signaling processes. Thus, integrin-mediated environmental sensing enables cells to adapt to chemical and physical properties of the surrounding matrix by modulating their proliferation, differentiation, and survival. This intriguing interplay between the apparently robust structure of matrix adhesions and their highly dynamic properties is the focus of this article.

Elevated expression of the integrin-associated protein PINCH suppresses the defects of Drosophila melanogaster muscle hypercontraction mutants

A variety of human diseases arise from mutations that alter muscle contraction. Evolutionary conservation allows genetic studies in Drosophila melanogaster to be used to better understand these myopathies and suggest novel therapeutic strategies. Integrin-mediated adhesion is required to support muscle structure and function, and expression of Integrin adhesive complex (IAC) proteins is modulated to adapt to varying levels of mechanical stress within muscle. Mutations in flapwing (flw), a catalytic subunit of myosin phosphatase, result in non-muscle myosin hyperphosphorylation, as well as muscle hypercontraction, defects in size, motility, muscle attachment, and subsequent larval and pupal lethality. We find that moderately elevated expression of the IAC protein PINCH significantly rescues flw phenotypes. Rescue requires PINCH be bound to its partners, Integrin-linked kinase and Ras suppressor 1. Rescue is not achieved through dephosphorylation of non-muscle myosin, suggesting a mechanism in which elevated PINCH expression strengthens integrin adhesion. In support of this, elevated expression of PINCH rescues an independent muscle hypercontraction mutant in muscle myosin heavy chain, Mhc^Samba1. By testing a panel of IAC proteins, we show specificity for PINCH expression in the rescue of hypercontraction mutants. These data are consistent with a model in which PINCH is present in limiting quantities within IACs, with increasing PINCH expression reinforcing existing adhesions or allowing for the de novo assembly of new adhesion complexes. Moreover, in myopathies that exhibit hypercontraction, strategic PINCH expression may have therapeutic potential in preserving muscle structure and function.

A wide variety of diseases of the muscle are caused by mutations that alter either the actin and myosin contractile machinery or its regulation. One class of mutations of interest results in hypercontraction of the muscle—actin and myosin fibers contract, but cannot efficiently relax. We have used the fruit fly as a model to study these mutations because of the striking similarity of fly and human muscle and because of the many genetic techniques that are available in the fly. Using a genetic approach we identified a protein, PINCH, whose increased expression can rescue the defects observed in hypercontraction mutants. PINCH is a component of integrin adhesion complexes, responsible for anchoring cells in their environment. This suggests that strengthening the anchorage of muscles via PINCH may be an effective strategy to prevent or reduce the muscle damage that occurs in diseases of muscle hypercontraction.

Purification and SAXS analysis of the integrin linked kinase, PINCH, parvin (IPP) heterotrimeric complex

The heterotrimeric protein complex containing the integrin linked kinase (ILK), parvin, and PINCH proteins, termed the IPP complex, is an essential component of focal adhesions, where it interacts with many proteins to mediate signaling from integrin adhesion receptors. Here we conduct a biochemical and structural analysis of the minimal IPP complex, comprising full-length human ILK, the LIM1 domain of PINCH1, and the CH2 domain of α-parvin. We provide a detailed purification protocol for IPP and show that the purified IPP complex is stable and monodisperse in solution. Using small-angle X-ray scattering (SAXS), we also conduct the first structural characterization of IPP, which reveals an elongated shape with dimensions 120×60×40 Å. Flexibility analysis using the ensemble optimization method (EOM) is consistent with an IPP complex structure with limited flexibility, raising the possibility that inter-domain interactions exist. However, our studies suggest that the inter-domain linker in ILK is accessible and we detect no inter-domain contacts by gel filtration analysis. This study provides a structural foundation to understand the conformational restraints that govern the IPP complex.

Parvin overexpression uncovers tissue-specific genetic pathways and disrupts F-actin to induce apoptosis in the developing epithelia in Drosophila

Parvin is a putative F-actin binding protein important for integrin-mediated cell adhesion. Here we used overexpression of Drosophila Parvin to uncover its functions in different tissues in vivo. Parvin overexpression caused major defects reminiscent of metastatic cancer cells in developing epithelia, including apoptosis, alterations in cell shape, basal extrusion and invasion. These defects were closely correlated with abnormalities in the organization of F-actin at the basal epithelial surface and of integrin-matrix adhesion sites. In wing epithelium, overexpressed Parvin triggered increased Rho1 protein levels, predominantly at the basal side, whereas in the developing eye it caused a rough eye phenotype and severely disrupted F-actin filaments at the retina floor of pigment cells. We identified genes that suppressed these Parvin-induced dominant effects, depending on the cell type. Co-expression of both ILK and the apoptosis inhibitor DIAP1 blocked Parvin-induced lethality and apoptosis and partially ameliorated cell delamination in epithelia, but did not rescue the elevated Rho1 levels, the abnormal organization of F-actin in the wing and the assembly of integrin-matrix adhesion sites. The rough eye phenotype was suppressed by coexpression of either PTEN or Wech, or by knock-down of Xrp1. Two main conclusions can be drawn from our studies: (1), high levels of cytoplasmic Parvin are toxic in epithelial cells; (2) Parvin in a dose dependent manner affects the organization of actin cytoskeleton in both wing and eye epithelia, independently of its role as a structural component of the ILK-PINCH-Parvin complex that mediates the integrin-actin link. Thus, distinct genetic interactions of Parvin occur in different cell types and second site modifier screens are required to uncover such genetic circuits.

Cell adhesion in Drosophila: versatility of cadherin and integrin complexes during development

We highlight recent progress in understanding cadherin and integrin function in the model organism Drosophila. New functions for these adhesion receptors continue to be discovered in this system, emphasising the importance of cell adhesion within the developing organism and showing that the requirement for cell adhesion changes between cell types. New ways to control adhesion have been discovered, including controlling the expression and recruitment of adhesion components, their posttranslational modification, recycling and turnover. Importantly, even ubiquitous adhesion components can function differently in distinct cellular contexts.

ILK: a pseudokinase in the center stage of cell-matrix adhesion and signaling

Integrin-linked kinase (ILK) is a widely expressed and evolutionally conserved component of cell-extracellular matrix (ECM) adhesions. Although initially named as a kinase, ILK contains an unusual pseudoactive site that is incapable of catalyzing phosphorylation. Instead, ILK acts as a central component of a heterotrimer (the PINCH-ILK-parvin complex) at ECM adhesions mediating interactions with a large number of proteins via multiple sites including its pseudoactive site. Through higher level protein-protein interactions, this scaffold links integrins to the actin cytoskeleton and catalytic proteins and thereby regulates focal adhesion assembly, cytoskeleton organization and signaling. This review summarizes recent advances in our understanding of the structure and functions of the PINCH-ILK-parvin complex, and discusses emerging new features of the molecular mechanisms by which it regulates diverse cellular, physiological and pathological processes.