Cell-cell adhesion via the ECM: integrin genetics in fly and worm

Drosophila as Model System to Study Ras-Mediated Oncogenesis: The Case of the Tensin Family of Proteins

Oncogenic mutations in the small GTPase Ras contribute to ~30% of human cancers. However, tissue growth induced by oncogenic Ras is restrained by the induction of cellular senescence, and additional mutations are required to induce tumor progression. Therefore, identifying cooperating cancer genes is of paramount importance. Recently, the tensin family of focal adhesion proteins, TNS1-4, have emerged as regulators of carcinogenesis, yet their role in cancer appears somewhat controversial. Around 90% of human cancers are of epithelial origin. We have used the Drosophila wing imaginal disc epithelium as a model system to gain insight into the roles of two orthologs of human TNS2 and 4, blistery (by) and PVRAP, in epithelial cancer progression. We have generated null mutations in PVRAP and found that, as is the case for by and mammalian tensins, PVRAP mutants are viable. We have also found that elimination of either PVRAP or by potentiates Ras^V12-mediated wing disc hyperplasia. Furthermore, our results have unraveled a mechanism by which tensins may limit Ras oncogenic capacity, the regulation of cell shape and growth. These results demonstrate that Drosophila tensins behave as suppressors of Ras-driven tissue hyperplasia, suggesting that the roles of tensins as modulators of cancer progression might be evolutionarily conserved.

N-glycosylomic analysis provides new insight into the molecular mechanism of firmness of fish fillet

Post-translational protein modification affects muscle physiochemistry. To understand the roles of N-glycosylation in this process, the muscle N-glycoproteomes of crisp grass carp (CGC) and ordinary grass carp (GC) were analyzed and compared. We identified 325 N-glycosylated sites with the NxT motif, classified 177 proteins, and identified 10 upregulated and 19 downregulated differentially glycosylated proteins (DGPs). Gene Ontology and Kyoto Encyclopedia of Genes and Genomes annotations revealed that these DGPs participate in myogenesis, extracellular matrix content formation, and muscle function. The DGPs partially accounted for the molecular mechanisms associated with the relatively smaller fiber diameter and higher collagen content observed in CGC. Though the DGPs diverged from the identified differentially phosphorylated proteins and differentially expressed proteins detected in previous study, they all shared similar metabolic and signaling pathways. Thus, they might independently alter fish muscle texture. Overall, the present study provides novel insights into the mechanisms underlying fillet quality.

Comprehensive analyses reveal the prognosis and biological function roles of chromatin regulators in lung adenocarcinoma

The present study explored the prognosis and biological function roles of chromatin regulators (CRs) in patients with lung adenocarcinoma (LUAD). Using transcriptome profile and clinical follow-up data of LUAD dataset, we explored the molecular classification, developed, and validated a CR prognostic model, built an individual risk scoring system in LUAD, and compared the clinical and molecular characteristics between different subtypes and risk stratifications. We investigated the chemotherapy sensitivity and predicted potential immunotherapy response. Lastly, we collected the clinical samples and validated the prognosis and potential function role of NAPS2. Our study indicated that LUAD patients could be classified into two subtypes that had obviously different clinical background and molecular features. We constructed a prognostic model with eight CR genes, which was well validated in several other population cohort. We built high- and low-risk stratifications for LUAD patients. Patients from high-risk group were totally different from low-risk groups in clinical, biological function, gene mutation, microenvironment, and immune infiltration levels. We idented several potential molecular compounds for high-risk group treatment. We predicted that high-risk group may have poor immunotherapy response. We finally found that Neuronal PAS Domain Protein 2 (NPAS2) involved in the progression of LUAD via regulating cell adhesion. Our study indicated that CR involved in the progression of LUAD and affect their prognosis. Different therapeutic strategies should be developed for different molecular subtypes and risk stratifications. Our comprehensive analyses uncover specific determinants of CRs in LUAD and provides implications for investigating disease-associated CRs.

Integrins control epithelial stem cell proliferation in the Drosophila ovary by modulating the Notch pathway

Cell proliferation and differentiation show a remarkable inverse relationship. The temporal coupling between cell cycle withdrawal and differentiation of stem cells (SCs) is crucial for epithelial tissue growth, homeostasis and regeneration. Proliferation vs. differentiation SC decisions are often controlled by the surrounding microenvironment, of which the basement membrane (BM; a specialized form of extracellular matrix surrounding cells and tissues), is one of its main constituents. Years of research have shown that integrin-mediated SC-BM interactions regulate many aspects of SC biology, including the proliferation-to-differentiation switch. However, these studies have also demonstrated that the SC responses to interactions with the BM are extremely diverse and depend on the cell type and state and on the repertoire of BM components and integrins involved. Here, we show that eliminating integrins from the follicle stem cells (FSCs) of the Drosophila ovary and their undifferentiated progeny increases their proliferation capacity. This results in an excess of various differentiated follicle cell types, demonstrating that cell fate determination can occur in the absence of integrins. Because these phenotypes are similar to those found in ovaries with decreased laminin levels, our results point to a role for the integrin-mediated cell-BM interactions in the control of epithelial cell division and subsequent differentiation. Finally, we show that integrins regulate proliferation by restraining the activity of the Notch/Delta pathway during early oogenesis. Our work increases our knowledge of the effects of cell-BM interactions in different SC types and should help improve our understanding of the biology of SCs and exploit their therapeutic potential.

Bioinspired Collagen/Gelatin Nanopillared Films as a Potential Implant Coating Material

Collagen-based Sharpey's fibers are naturally located between alveolar bone and tooth, and they have critical roles in a well-functioning tooth such as mechanical stability, facile differentiation, and disease protection. The success of Sharpey's fibers in these important roles is due to their unique location, vertical alignment with respect to tooth surface, as well as their micronanofiber architecture. Inspired by these structures, herein, we introduce the use of nanoporous anodic aluminum oxide molds in a drop-casting setup to fabricate biopolymeric films possessing arrays of uniform Collagen:Gelatin (Col:Gel) nanopillars. Obtained structures have diameters of ∼90 nm and heights of ∼300 nm, yielding significantly higher surface roughness values compared to their flat counterparts. More importantly, the nanostructures were parallel to each other but perpendicular to the underlying film surface imitating the natural collagenous structures of Sharpey's fibers regarding nanoscale morphology, geometrical orientation, as well as biochemical content. Viability testing showed that the nanopillared Col:Gel films have high cell viabilities (over 90%), and they display significantly improved attachment (ca. ∼ 2 times) and mineralization for Saos-2 cells when compared to flat Col:Gel films and Tissue Culture Polystyrene (TCPS) controls, plausibly due to their largely increased surface roughness and area. Hence, such Sharpey's fiber-inspired bioactive nanopillared Col:Gel films can be used as a dental implant coating material or tissue engineering platform with enhanced cellular and osteogenic properties.

Tension-driven multi-scale self-organisation in human iPSC-derived muscle fibers

Human muscle is a hierarchically organised tissue with its contractile cells called myofibers packed into large myofiber bundles. Each myofiber contains periodic myofibrils built by hundreds of contractile sarcomeres that generate large mechanical forces. To better understand the mechanisms that coordinate human muscle morphogenesis from tissue to molecular scales, we adopted a simple in vitro system using induced pluripotent stem cell-derived human myogenic precursors. When grown on an unrestricted two-dimensional substrate, developing myofibers spontaneously align and self-organise into higher-order myofiber bundles, which grow and consolidate to stable sizes. Following a transcriptional boost of sarcomeric components, myofibrils assemble into chains of periodic sarcomeres that emerge across the entire myofiber. More efficient myofiber bundling accelerates the speed of sarcomerogenesis suggesting that tension generated by bundling promotes sarcomerogenesis. We tested this hypothesis by directly probing tension and found that tension build-up precedes sarcomere assembly and increases within each assembling myofibril. Furthermore, we found that myofiber ends stably attach to other myofibers using integrin-based attachments and thus myofiber bundling coincides with stable myofiber bundle attachment in vitro. A failure in stable myofiber attachment results in a collapse of the myofibrils. Overall, our results strongly suggest that mechanical tension across sarcomeric components as well as between differentiating myofibers is key to coordinate the multi-scale self-organisation of muscle morphogenesis.

Computational modeling and analysis of the morphogenetic domain signaling networks regulating C. elegans embryogenesis

Caenorhabditis elegans, often referred to as the ‘roundworm’, provides a powerful model for studying cell autonomous and cell–cell interactions through the direct observation of embryonic development in vivo. By leveraging the precisely mapped cell lineage at single cell resolution, we are able to study at a systems level how early embryonic cells communicate across morphogenetic domains for the coordinated processes of gene expressions and collective cellular behaviors that regulate tissue morphogenesis. In this study, we developed a computational framework for the exploration of the morphogenetic domain cell signaling networks that may regulate C. elegans gastrulation and embryonic organogenesis. We demonstrated its utility by producing the following results, i) established a virtual reference model of developing C. elegans embryos through the spatiotemporal alignment of individual embryo cell nuclear imaging samples; ii) integrated the single cell spatiotemporal gene expression profile with the established virtual embryo model by data pooling; iii) trained a Machine Learning model (Random Forest Regression), which predicts accurately the spatial positions of the cells given their gene expression profiles for a given developmental time (e.g. total cell number of the embryo); iv) enabled virtual 4-dimensional tomographic graphical modeling of single cell data; v) inferred the biology signaling pathways that act in each of morphogenetic domains by meta-data analysis. It is intriguing that the morphogenetic domain cell signaling network seems to involve some crosstalk of multiple biology signaling pathways during the formation of tissue boundary pattern. Lastly, we developed the Software tool ‘Embryo aligner version 1.0’ and provided it as an Open Source program to the research community for virtual embryo modeling, and phenotype perturbation analyses (https://github.com/csniuben/embryo_aligner/wiki and https://bioinfo89.github.io/C.elegansEmbryonicOrganogenesisweb/).

The basement membrane controls size and integrity of the Drosophila tracheal tubes

Biological tubes are fundamental units of most metazoan organs. Their defective morphogenesis can cause malformations and pathologies. An integral component of biological tubes is the extracellular matrix, present apically (aECM) and basally (BM). Studies using the Drosophila tracheal system established an essential function for the aECM in tubulogenesis. Here, we demonstrate that the BM also plays a critical role in this process. We find that BM components are deposited in a spatial-temporal manner in the trachea. We show that laminins, core BM components, control size and shape of tracheal tubes and their topology within the embryo. At a cellular level, laminins control cell shape changes and distribution of the cortical cytoskeleton component α-spectrin. Finally, we report that the BM and aECM act independently-yet cooperatively-to control tube elongation and together to guarantee tissue integrity. Our results unravel key roles for the BM in shaping, positioning, and maintaining biological tubes.

Proteomics of purified lamellocytes from Drosophila melanogaster HopTum-l identifies new membrane proteins and networks involved in their functions

In healthy Drosophila melanogaster larvae, plasmatocytes and crystal cells account for 95% and 5% of the hemocytes, respectively. A third type of hemocytes, lamellocytes, are rare, but their number increases after oviposition by parasitoid wasps. The lamellocytes form successive layers around the parasitoid egg, leading to its encapsulation and melanization, and finally the death of this intruder. However, the total number of lamellocytes per larva remains quite low even after parasitoid infestation, making direct biochemical studies difficult. Here, we used the Hop^Tum-l mutant strain that constitutively produces large numbers of lamellocytes to set up a purification method and analyzed their major proteins by 2D gel electrophoresis and their plasma membrane surface proteins by 1D SDS-PAGE after affinity purification. Mass spectrometry identified 430 proteins from 2D spots and 344 affinity-purified proteins from 1D bands, for a total of 639 unique proteins. Known lamellocyte markers such as PPO3 and the myospheroid integrin were among the components identified with specific chaperone proteins. Affinity purification detected other integrins, as well as a wide range of integrin-associated proteins involved in the formation and function of cell-cell junctions. Overall, the newly identified proteins indicate that these cells are highly adapted to the encapsulation process (recognition, motility, adhesion, signaling), but may also have several other physiological functions (such as secretion and internalization of vesicles) under different signaling pathways. These results provide the basis for further in vivo and in vitro studies of lamellocytes, including the development of new markers to identify coexisting populations and their respective origins and functions in Drosophila immunity.

Structural mechanism of laminin recognition by integrin

Recognition of laminin by integrin receptors is central to the epithelial cell adhesion to basement membrane, but the structural background of this molecular interaction remained elusive. Here, we report the structures of the prototypic laminin receptor α6β1 integrin alone and in complex with three-chain laminin-511 fragment determined via crystallography and cryo-electron microscopy, respectively. The laminin-integrin interface is made up of several binding sites located on all five subunits, with the laminin γ1 chain C-terminal portion providing focal interaction using two carboxylate anchor points to bridge metal-ion dependent adhesion site of integrin β1 subunit and Asn189 of integrin α6 subunit. Laminin α5 chain also contributes to the affinity and specificity by making electrostatic interactions with large surface on the β-propeller domain of α6, part of which comprises an alternatively spliced X1 region. The propeller sheet corresponding to this region shows unusually high mobility, suggesting its unique role in ligand capture.

Development and maintenance of tendons and ligaments

Tendons and ligaments are fibrous connective tissues vital to the transmission of force and stabilization of the musculoskeletal system. Arising in precise regions of the embryo, tendons and ligaments share many properties and little is known about the molecular differences that differentiate them. Recent studies have revealed heterogeneity and plasticity within tendon and ligament cells, raising questions regarding the developmental mechanisms regulating tendon and ligament identity. Here, we discuss recent findings that contribute to our understanding of the mechanisms that establish and maintain tendon progenitors and their differentiated progeny in the head, trunk and limb. We also review the extent to which these findings are specific to certain anatomical regions and model organisms, and indicate which findings similarly apply to ligaments. Finally, we address current research regarding the cellular lineages that contribute to tendon and ligament repair, and to what extent their regulation is conserved within tendon and ligament development.

An insight on Drosophila myogenesis and its assessment techniques

Movement assisted by muscles forms the basis of various behavioural traits seen in Drosophila. Myogenesis involves developmental processes like cellular specification, differentiation, migration, fusion, adherence to tendons and neuronal innervation in a series of coordinated event well defined in body space and time. Gene regulatory networks are switched on-off, fine tuning at the right developmental stage to assist each cellular event. Drosophila is a holometabolous organism that undergoes myogenesis waves at two developmental stages, and is ideal for comparative analysis of the role of genes and genetic pathways conserved across phyla. In this review we have summarized myogenic events from the embryo to adult focussing on the somatic muscle development during the early embryonic stage and then on indirect flight muscles (IFM) formation required for adult life, emphasizing on recent trends of analysing muscle mutants and advances in Drosophila muscle biology.

The Members of the Highly Diverse Crassostrea gigas Integrin Family Cooperate for the Generation of Various Immune Responses

Studies on invertebrate immune receptors can provide insights into characteristics specific to innate immune system. Here, eight α and three β integrins are identified from an invertebrate, the Pacific oyster Crassostrea gigas, and their possible immune functions are studied. Oyster α/β integrins exhibit a higher degree of sequence and structural variability than the members from Homo sapiens and Drosophila melanogaster. The analysis reveals that oyster RGD- and laminin-binding receptor homologs are present in the phylogenetic tree of α integrins, but the other six oyster α integrins mainly form a species-specific branch; meanwhile, oyster β integrins are clustered with insect β integrins but distinct from a member from the mollusk Biomphalaria glabrata. Although phylogenetically lacking the important α integrin branches of LDV-binding, PS3-type, and αI-containing integrins, oyster integrins can bind to most ECM ligands, including RGDCP, LDVCP, GFOGERCP, and laminin protein in a distinct binding pattern. Besides, oyster integrins are distributed in different hemocyte subpopulations, while only specific integrins are selectively involved in hemocyte phagocytosis, migration, and encapsulation, and some of them participate in more than one immune response in a sophisticated pattern. Especially, oyster β integrins are arranged in the core to mediate complex immune responses, unlike the counterparts in humans that mainly depend on αI-containing integrins to incite immune reactions. This study represents the first comprehensive attempt to reveal the structural and evolutionary features of the integrin family and their involvement in cellular immune responses in the non-model invertebrate C. gigas and sheds light on the characteristics specific to the innate immune system in the integrin family.

RSU-1 Maintains Integrity of Caenorhabditis elegans Vulval Muscles by Regulating α-Actinin

Egg-laying behavior in Caenorhabditis elegans is a well-known model for investigating fundamental cellular processes. In egg-laying, muscle contraction is the relaxation of the vulval muscle to extrude eggs from the vulva. Unlike skeletal muscle, vulval muscle lacks visible striations of the sarcomere. Therefore, vulval muscle must counteract the mechanical stress, caused by egg extrusion and body movement, from inducing cell-shape distortion by maintaining its cytoskeletal integrity. However, the underlying mechanisms that regulate the cellular integrity in vulval muscles remain unclear. Here, we demonstrate that C. elegans egg-laying requires proper vulval muscle 1 (vm1), in which the actin bundle organization of vm1 muscles is regulated by Ras suppressor protein 1 (RSU-1). In the loss of RSU-1, as well as Ras^LET-60 overactivation, blister-like membrane protrusions and disorganized actin bundles were observed in the vm1 muscles. Moreover, Ras^LET-60 depletion diminished the defected actin-bundles in rsu-1 mutant. These results reveal the genetic interaction of RSU-1 and Ras^LET-60 in vivo In addition, our results further demonstrated that the fifth to seventh leucine-rich region of RSU-1 is required to promote actin-bundling protein, α-actinin, for actin bundle stabilization in the vm1 muscles. This expands our understanding of the molecular mechanisms of actin bundle organization in a specialized smooth muscle.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Transcriptomic analysis of the testicular fusion in Spodoptera litura

Background

Lepidoptera is one group of the largest plant-feeding insects and Spodoptera litura (Lepidoptera: Noctuidae) is one of the most serious agricultural pests in Asia countries. An interesting and unique phenomenon for gonad development of Lepidoptera is the testicular fusion. Two separated testes fused into a single one during the larva-to-pupa metamorphosis, which is believed to contribute to sperm production and the prevalence in field. To study the molecular mechanism of the testicular fusion, RNA sequencing (RNA-seq) experiments of the testes from 4-day-old sixth instar larvae (L6D4) (before fusion), 6-day-old sixth instar larvae (L6D6, prepupae) (on fusing) and 4-day-old pupae (P4D) (after fusion) of S. litura were performed.

Results

RNA-seq data of the testes showed that totally 12,339 transcripts were expressed at L6D4, L6D6 and P4D stages. A large number of differentially expressed genes (DEGs) were up-regulated from L6D4 to L6D6, and then more genes were down-regulated from L6D6 to P4D. The DEGs mainly belongs to the genes related to the 20E signal transduction pathway, transcription factors, chitin metabolism related enzymes, the families of cytoskeleton proteins, extracellular matrix (ECM) components, ECM-related protein, its receptor integrins and ECM-remodeling enzymes. The expression levels of these genes that were up-regulated significantly during the testicular fusion were verified by qRT-PCR. The matrix metalloproteinases (MMPs) were found to be the main enzymes related to the ECM degradation and contribute to the testicular fusion. The testis was not able to fuse if MMPs inhibitor GM6001 was injected into the 5th abdomen region at L6D6 early stage.

Conclusions

The transcriptome and DEGs analysis of the testes at L6D4, L6D6 and P4D stages provided genes expression information related to the testicular fusion in S. litura. These results indicated that cytoskeleton proteins, ECM-integrin interaction genes and ECM-related proteins were involved in cell migration, adhesion and fusion during the testicular fusion. The ECM degradation enzymes MMPs probably play a critical role in the fusion of testis.

α-Integrins dictate distinct modes of type IV collagen recruitment to basement membranes

Basement membranes (BMs) are cell-associated extracellular matrices that support tissue integrity, signaling, and barrier properties. Type IV collagen is critical for BM function, yet how it is directed into BMs in vivo is unclear. Through live-cell imaging of endogenous localization, conditional knockdown, and misexpression experiments, we uncovered distinct mechanisms of integrin-mediated collagen recruitment to Caenorhabditis elegans postembryonic gonadal and pharyngeal BMs. The putative laminin-binding αINA-1/βPAT-3 integrin was selectively activated in the gonad and recruited laminin, which directed moderate collagen incorporation. In contrast, the putative Arg-Gly-Asp (RGD)-binding αPAT-2/βPAT-3 integrin was activated in the pharynx and recruited high levels of collagen in an apparently laminin-independent manner. Through an RNAi screen, we further identified the small GTPase RAP-3 (Rap1) as a pharyngeal-specific PAT-2/PAT-3 activator that modulates collagen levels. Together, these studies demonstrate that tissues can use distinct mechanisms to direct collagen incorporation into BMs to precisely control collagen levels and construct diverse BMs.

Integrins are required for synchronous ommatidial rotation in the Drosophila eye linking planar cell polarity signalling to the extracellular matrix

Integrins mediate the anchorage between cells and their environment, the extracellular matrix (ECM), and form transmembrane links between the ECM and the cytoskeleton, a conserved feature throughout development and morphogenesis of epithelial organs. Here, we demonstrate that integrins and components of the ECM are required during the planar cell polarity (PCP) signalling-regulated cell movement of ommatidial rotation in the Drosophila eye. The loss-of-function mutations of integrins or ECM components cause defects in rotation, with mutant clusters rotating asynchronously compared to wild-type clusters. Initially, mutant clusters tend to rotate faster, and at later stages they fail to be synchronous with their neighbours, leading to aberrant rotation angles and resulting in a disorganized ommatidial arrangement in adult eyes. We further demonstrate that integrin localization changes dynamically during the rotation process. Our data suggest that core Frizzled/PCP factors, acting through RhoA and Rho kinase, regulate the function/activity of integrins and that integrins thus contribute to the complex interaction network of PCP signalling, cell adhesion and cytoskeletal elements required for a precise and synchronous 90° rotation movement.

Integrins Have Cell-Type-Specific Roles in the Development of Motor Neuron Connectivity

Formation of the nervous system requires a complex series of events including proper extension and guidance of neuronal axons and dendrites. Here we investigate the requirement for integrins, a class of transmembrane cell adhesion receptors, in regulating these processes across classes of C. elegans motor neurons. We show α integrin/ina-1 is expressed by both GABAergic and cholinergic motor neurons. Despite this, our analysis of hypomorphic ina-1(gm144) mutants indicates preferential involvement of α integrin/ina-1 in GABAergic commissural development, without obvious involvement in cholinergic commissural development. The defects in GABAergic commissures of ina-1(gm144) mutants included both premature termination and guidance errors and were reversed by expression of wild type ina-1 under control of the native ina-1 promoter. Our results also show that α integrin/ina-1 is important for proper outgrowth and guidance of commissures from both embryonic and post-embryonic born GABAergic motor neurons, indicating an ongoing requirement for integrin through two phases of GABAergic neuron development. Our findings provide insights into neuron-specific roles for integrin that would not be predicted based solely upon expression analysis.

Ordering of myosin II filaments driven by mechanical forces: experiments and theory

Myosin II filaments form ordered superstructures in both cross-striated muscle and non-muscle cells. In cross-striated muscle, myosin II (thick) filaments, actin (thin) filaments and elastic titin filaments comprise the stereotypical contractile units of muscles called sarcomeres. Linear chains of sarcomeres, called myofibrils, are aligned laterally in registry to form cross-striated muscle cells. The experimentally observed dependence of the registered organization of myofibrils on extracellular matrix elasticity has been proposed to arise from the interactions of sarcomeric contractile elements (considered as force dipoles) through the matrix. Non-muscle cells form small bipolar filaments built of less than 30 myosin II molecules. These filaments are associated in registry forming superstructures ('stacks') orthogonal to actin filament bundles. Formation of myosin II filament stacks requires the myosin II ATPase activity and function of the actin filament crosslinking, polymerizing and depolymerizing proteins. We propose that the myosin II filaments embedded into elastic, intervening actin network (IVN) function as force dipoles that interact attractively through the IVN. This is in analogy with the theoretical picture developed for myofibrils where the elastic medium is now the actin cytoskeleton itself. Myosin stack formation in non-muscle cells provides a novel mechanism for the self-organization of the actin cytoskeleton at the level of the entire cell.This article is part of the theme issue 'Self-organization in cell biology'.

Diverse integrin adhesion stoichiometries caused by varied actomyosin activity

Cells in an organism are subjected to numerous sources of external and internal forces, and are able to sense and respond to these forces. Integrin-mediated adhesion links the extracellular matrix outside cells to the cytoskeleton inside, and participates in sensing, transmitting and responding to forces. While integrin adhesion rapidly adapts to changes in forces in isolated migrating cells, it is not known whether similar or more complex responses occur within intact, developing tissues. Here, we studied changes in integrin adhesion composition upon different contractility conditions in Drosophila embryonic muscles. We discovered that all integrin adhesion components tested were still present at muscle attachment sites (MASs) when either cytoplasmic or muscle myosin II was genetically removed, suggesting a primary role of a developmental programme in the initial assembly of integrin adhesions. Contractility does, however, increase the levels of integrin adhesion components, suggesting a mechanism to balance the strength of muscle attachment to the force of muscle contraction. Perturbing contractility in distinct ways, by genetic removal of either cytoplasmic or muscle myosin II or eliminating muscle innervation, each caused unique alterations to the stoichiometry at MASs. This suggests that different integrin-associated proteins are added to counteract different kinds of force increase.

Integrin β3 plays a novel role in innate immunity in silkworm, Bombyx mori

Integrins are transmembrane receptors that play essential roles in many physiological and pathological processes through cell-to-cell and cell-to-extracellular matrix (ECM) interactions. In the current study, a 2653-bp full-length cDNA of a novel integrin β subunit (designated Bmintegrin β3) was obtained from silkworm hemocytes. Bmintegrin β3 has the typical conserved structure of the integrin β family. The qRT-PCR results showed that Bmintegrin β3 was specifically expressed in the hematological system and that its expression was significantly increased after challenge with different types of PAMPs and bacteria. The recombinant Bmintegrin β3 protein displayed increased aggregation with S. aureus, suggesting that Bmintegrin β3 might directly bind to PAMPs. Interestingly, Bmintegrin β3 knockdown promoted PPO1, PPO2, BAEE, SPH78, SPH125, and SPH127 expression and accelerated the melanization process. Unexpectedly, the expression of genes related to phagocytosis, the Toll pathway, and the IMD pathway was also up-regulated after Bmintegrin β3 knockdown. Thus, Bmintegrin β3 might be a pattern recognition protein (PRP) for PAMPs and might directly bind to bacteria and enhance the phagocytosis activity of hemocytes. Moreover, Bmintegrin β3 and its ligand might negatively regulate the expression of immune-related genes through an unknown mechanism. In summary, our studies provide new insights into the immune functions of Bmintegrin β3 from the silkworm, Bombyx mori.

Mechanical Control of Myotendinous Junction Formation and Tendon Differentiation during Development

The development of the musculoskeletal system is a great model to study the interplay between chemical and mechanical inter-tissue signaling in cell adhesion, tissue morphogenesis and differentiation. In both vertebrates and invertebrates (e.g., Drosophila melanogaster) the formation of muscle-tendon interaction generates mechanical forces which are required for myotendinous junction maturation and tissue differentiation. In addition, these forces must be withstood by muscles and tendons in order to prevent detachment from each other, deformation or even losing their integrity. Extracellular matrix remodeling at the myotendinous junction is key to resist mechanical load generated by muscle contraction. Recent evidences in vertebrates indicate that mechanical forces generated during junction formation regulate chemical signaling leading to extracellular matrix remodeling, however, the mechanotransduction mechanisms associated to this response remains elusive. In addition to extracellular matrix remodeling, the ability of Drosophila tendon-cells to bear mechanical load depends on rearrangement of tendon cell cytoskeleton, thus studying the molecular mechanisms involved in this process is critical to understand the contribution of mechanical forces to the development of the musculoskeletal system. Here, we review recent findings regarding the role of chemical and mechanical signaling in myotendinous junction formation and tendon differentiation, and discuss molecular mechanisms of mechanotransduction that may allow tendon cells to withstand mechanical load during development of the musculoskeletal system.

Kon-tiki/Perdido enhances PS2 integrin adhesion and localizes its ligand, Thrombospondin, in the myotendinous junction

Cell-extracellular matrix adhesion is mediated by cell receptors, mainly integrins and transmembrane proteoglycans, which can functionally interact. How these receptors are regulated and coordinated is largely unknown and key to understand cell adhesion in development. We show that the conserved transmembrane proteoglycan Kon-tiki/Perdido (Kon) interacts with αPS2βPS integrin to mediate muscle-tendon adhesion. Double mutant embryos for kon and inflated show a synergistic increase in muscle detachment. Furthermore, Kon modulates αPS2βPS signaling at the muscle attachment, since P-Fak is reduced in kon mutants. This reduction in integrin signaling can be rescued by the expression of a truncated Kon protein containing the transmembrane and extracellular domains, suggesting that these domains are sufficient to mediate this signaling. We show that these domains are sufficient to properly localize the αPS2βPS ligand, Thrombospondin, to the muscle attachment, and to partially rescue Kon dependent muscle-tendon adhesion. We propose that Kon can engage in a protein complex with αPS2βPS and enhance integrin-mediated signaling and adhesion by recruiting its ligand, which would increase integrin-binding affinity to the extracellular matrix, resulting in the consolidation of the myotendinous junction.

Direct interaction of avermectin with epidermal growth factor receptor mediates the penetration resistance in Drosophila larvae

With the widespread use of avermectins (AVMs) for managing parasitic and agricultural pests, the resistance of worms and insects to AVMs has emerged as a serious threat to human health and agriculture worldwide. The reduced penetration of AVMs is one of the main reasons for the development of the resistance to the chemicals. However, the detailed molecular mechanisms remain elusive. Here, we use the larvae of Drosophila melanogaster as the model organism to explore the molecular mechanisms underlying the development of penetration resistance to AVMs. We clearly show that the chitin layer is thickened and the efflux transporter P-glycoprotein (P-gp) is overexpressed in the AVM-resistant larvae epidermis. We reveal that the activation of the transcription factor Relish by the over-activated epidermal growth factor receptor (EGFR)/AKT/ERK pathway induces the overexpression of the chitin synthases DmeCHS1/2 and P-gp in the resistant larvae. Interestingly, we discover for the first time, to the best of our knowledge, that AVM directly interacts with EGFR and leads to the activation of the EGFR/AKT/ERK pathway, which activates the transcription factor Relish and induces the overexpression of DmeCHS1/2 and P-gp. These findings provide new insights into the molecular mechanisms underlying the development of penetration resistance to drugs.

Peak BMP Responses in the Drosophila Embryo Are Dependent on the Activation of Integrin Signaling

Within a 3D tissue, cells need to integrate signals from growth factors, such as BMPs, and the extracellular matrix (ECM) to coordinate growth and differentiation. Here, we use the Drosophila embryo as a model to investigate how BMP responses are influenced by a cell’s local ECM environment. We show that integrins, which are ECM receptors, are absolutely required for peak BMP signaling. This stimulatory effect of integrins requires their intracellular signaling function, which is activated by the ECM protein collagen IV. Mechanistically, integrins interact with the BMP receptor and stimulate phosphorylation of the downstream Mad transcription factor. The BMP-pathway-enhancing function of integrins is independent of focal adhesion kinase, but it requires conserved NPXY motifs in the β-integrin cytoplasmic tail. Furthermore, we show that an α-integrin subunit is a BMP target gene, identifying positive feedback between integrin signaling and BMP pathway activity that may contribute to robust cell fate decisions.

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Drosophila embryos lacking integrin function have disrupted BMP responses

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Collagen IV activates integrin signaling to enhance levels of the pMad transducer

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Integrins bind BMP receptors and promote pMad levels after BMP receptor activation

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BMP activates expression of an α-integrin, representing a positive feedback loop

Mechanical force sensing in tissues

Tissue size, shape, and organization reflect individual cell behaviors such as proliferation, shape change, and movement. Evidence suggests that mechanical signals operate in tandem with biochemical cues to properly coordinate cell behavior and pattern tissues. The objective of this chapter is to present recent evidence demonstrating that forces transmitted between cells act as signals that coordinate cell behavior across tissues. We first briefly summarize molecular and cellular mechanisms by which forces are sensed by cells with an emphasis on forces generated and transmitted by cytoskeletal networks. We then discuss evidence for these mechanisms operating in multicellular contexts to coordinate complex cell and tissue behaviors that occur during embryonic development: specifically growth and morphogenesis.

Morphogenesis of the somatic musculature in Drosophila melanogaster

In Drosophila melanogaster, the somatic muscle system is first formed during embryogenesis, giving rise to the larval musculature. Later during metamorphosis, this system is destroyed and replaced by an entirely new set of muscles in the adult fly. Proper formation of the larval and adult muscles is critical for basic survival functions such as hatching and crawling (in the larva), walking and flying (in the adult), and feeding (at both larval and adult stages). Myogenesis, from mononucleated muscle precursor cells to multinucleated functional muscles, is driven by a number of cellular processes that have begun to be mechanistically defined. Once the mesodermal cells destined for the myogenic lineage have been specified, individual myoblasts fuse together iteratively to form syncytial myofibers. Combining cytoplasmic contents demands a level of intracellular reorganization that, most notably, leads to redistribution of the myonuclei to maximize internuclear distance. Signaling from extending myofibers induces terminal tendon cell differentiation in the ectoderm, which results in secure muscle-tendon attachments that are critical for muscle contraction. Simultaneously, muscles become innervated and undergo sarcomerogenesis to establish the contractile apparatus that will facilitate movement. The cellular mechanisms governing these morphogenetic events share numerous parallels to mammalian development, and the basic unit of all muscle, the myofiber, is conserved from flies to mammals. Thus, studies of Drosophila myogenesis and comparisons to muscle development in other systems highlight conserved regulatory programs of biomedical relevance to general muscle biology and studies of muscle disease.

Changes in organelle position and epithelial architecture associated with loss of CrebA

Drosophila CrebA facilitates high-level secretion by transcriptional upregulation of the protein components of the core secretory machinery. In CrebA mutant embryos, both salivary gland (SG) morphology and epidermal cuticle secretion are abnormal, phenotypes similar to those observed with mutations in core secretory pathway component genes. Here, we examine the cellular defects associated with CrebA loss in the SG epithelium. Apically localized secretory vesicles are smaller and less abundant, consistent with overall reductions in secretion. Unexpectedly, global mislocalization of cellular organelles and excess membrane accumulation in the septate junctions (SJs) are also observed. Whereas mutations in core secretory pathway genes lead to organelle localization defects similar to those of CrebA mutants, they have no effect on SJ-associated membrane. Mutations in tetraspanin genes, which are normally repressed by CrebA, have mild defects in SJ morphology that are rescued by simultaneous CrebA loss. Correspondingly, removal of several tetraspanins gives partial rescue of the CrebA SJ phenotype, supporting a role for tetraspanins in SJ organization.

Anchors and signals: the diverse roles of integrins in development

Integrins mediate cell adhesion by providing a link between the actin cytoskeleton and the extracellular matrix. As well as acting to anchor cells, integrin adhesions provide sensory input via mechanotransduction and synergism with signaling pathways, and provide the cell with the conditions necessary for differentiation in a permissive manner. In this review, we explore how integrins contribute to development, and what this tells us about how they work. From a signaling perspective, the influence of integrins on cell viability and fate is muted in a developmental context as compared to cell culture. Integrin phenotypes tend to arise from a failure of normally specified cells to create tissues properly, due to defective adhesion. The diversity of integrin functions in development shows how cell adhesion is continuously adjusted, both within and between animals, to fit developmental purpose.

Netrins and Frazzled/DCC promote the migration and mesenchymal to epithelial transition of Drosophila midgut cells

Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.

Control of the basement membrane and cell migration by ADAMTS proteinases: Lessons from C. elegans genetics

The members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of secreted proteins, MIG-17 and GON-1, play essential roles in Caenorhabditis elegans gonadogenesis. The genetic and molecular analyses of these proteinases uncovered novel molecular interactions regulating the basement membrane (BM) during the migration of the gonadal leader cells. MIG-17, which is localized to the gonadal BM recruits or activates fibulin-1 and type IV collagen, which then recruits nidogen, thereby inducing the remodeling of the BM that is required for directional control of leader cell migration. GON-1 acts antagonistically with fibulin-1 to regulate the levels of type IV collagen accumulation in the gonadal BM, which facilitates active migration of the leader cells. The cooperative action of MIG-17 and GON-1 represents an excellent model for understanding the mechanisms of organogenesis mediated by ADAMTS proteinases.

A novel granulocyte-specific α integrin is essential for cellular immunity in the silkworm Bombyx mori

Haemocytes play crucial roles in immune responses and survival in insects. Specific cell markers have proven effective in clarifying the function and haematopoiesis of haemocytes. The silkworm Bombyx mori is a good model for studying insect haemocytes; however, little is known about haemocyte-specific markers or their functions in silkworm. In this study, we identified the α subunit of integrin, BmintegrinαPS3, as being specifically and highly expressed in silkworm haemocytes. Immunofluorescence analysis validated the specificity of BmintegrinαPS3 in larval granulocytes. Further analyses indicated that haemocytes dispersed from haematopoietic organs (HPOs) into the circulating haemolymph could differentiate into granulocytes. In addition, the processes of encapsulation and phagocytosis were controlled by larval granulocytes. Our work demonstrated that BmintegrinαPS3 could be used as a specific marker for granulocytes and could be applied to future molecular cell biology studies.

Drosophila perlecan regulates intestinal stem cell activity via cell-matrix attachment

Stem cells require specialized local microenvironments, termed niches, for normal retention, proliferation, and multipotency. Niches are composed of cells together with their associated extracellular matrix (ECM). Currently, the roles of ECM in regulating niche functions are poorly understood. Here, we demonstrate that Perlecan (Pcan), a highly conserved ECM component, controls intestinal stem cell (ISC) activities and ISC-ECM attachment in Drosophila adult posterior midgut. Loss of Pcan from ISCs, but not other surrounding cells, causes ISCs to detach from underlying ECM, lose their identity, and fail to proliferate. These defects are not a result of a loss of epidermal growth factor receptor (EGFR) or Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling activity but partially depend on integrin signaling activity. We propose that Pcan secreted by ISCs confers niche properties to the adjacent ECM that is required for ISC maintenance of stem cell identity, activity, and anchorage to the niche.

Mechanotransduction in C. elegans morphogenesis and tissue function

Mechanobiology is an emerging field that investigates how living cells sense and respond to their physical surroundings. Recent interest in the field has been sparked by the finding that stem cells differentiate along different lineages based on the stiffness of the cell surroundings (Engler et al., 2006), and that metastatic behavior of cancer cells is strongly influenced by the mechanical properties of the surrounding tissue (Kumar and Weaver, 2009). Many questions remain about how cells convert mechanical information, such as viscosity, stiffness of the substrate, or stretch state of the cells, into the biochemical signals that control tissue function. Caenorhabditis elegans researchers are making significant contributions to the understanding of mechanotransduction in vivo. This review summarizes recent insights into the role of mechanical forces in morphogenesis and tissue function. Examples of mechanical regulation across length scales, from the single-celled zygote, to the intercellular coordination that enables cohesive tissue function, to the mechanical influences between tissues, are considered. The power of the C. elegans system as a gene discovery and in vivo quantitative bioimaging platform is enabling an important discoveries in this exciting field.

Drosophila importin-7 functions upstream of the Elmo signaling module to mediate the formation and stability of muscle attachments

Establishment and maintenance of stable muscle attachments is essential for coordinated body movement. Studies in Drosophila have pioneered a molecular understanding of the morphological events in the conserved process of muscle attachment formation, including myofiber migration, muscle-tendon signaling, and stable junctional adhesion between muscle cells and their corresponding target insertion sites. In both Drosophila and vertebrate models, integrin complexes play a key role in the biogenesis and stability of muscle attachments through the interactions of integrins with extracellular matrix (ECM) ligands. We show that Drosophila importin-7 (Dim7) is an upstream regulator of the conserved Elmo-Mbc→Rac signaling pathway in the formation of embryonic muscle attachment sites (MASs). Dim7 is encoded by the moleskin (msk) locus and was identified as an Elmo-interacting protein. Both Dim7 and Elmo localize to the ends of myofibers coincident with the timing of muscle-tendon attachment in late myogenesis. Phenotypic analysis of elmo mutants reveal muscle attachment defects similar to those previously described for integrin mutants. Furthermore, Elmo and Dim7 interact both biochemically and genetically in the developing musculature. The muscle detachment phenotype resulting from mutations in the msk locus can be rescued by components in the Elmo signaling pathway, including the Elmo-Mbc complex, an activated Elmo variant, or a constitutively active form of Rac. In larval muscles, the localization of Dim7 and activated Elmo to the sites of muscle attachment is attenuated upon RNAi knockdown of integrin heterodimer complex components. Our results show that integrins function as upstream signals to mediate Dim7-Elmo enrichment to the MASs.

The conserved ADAMTS-like protein lonely heart mediates matrix formation and cardiac tissue integrity

Here we report on the identification and functional characterization of the ADAMTS-like homolog lonely heart (loh) in Drosophila melanogaster. Loh displays all hallmarks of ADAMTSL proteins including several thrombospondin type 1 repeats (TSR1), and acts in concert with the collagen Pericardin (Prc). Loss of either loh or prc causes progressive cardiac damage peaking in the abolishment of heart function. We show that both proteins are integral components of the cardiac ECM mediating cellular adhesion between the cardiac tube and the pericardial cells. Loss of ECM integrity leads to an altered myo-fibrillar organization in cardiac cells massively influencing heart beat pattern. We show evidence that Loh acts as a secreted receptor for Prc and works as a crucial determinant to allow the formation of a cell and tissue specific ECM, while it does not influence the accumulation of other matrix proteins like Nidogen or Perlecan. Our findings demonstrate that the function of ADAMTS-like proteins is conserved throughout evolution and reveal a previously unknown interaction of these proteins with collagens.

Cellular adhesion and tissue integrity in multicellular organisms strongly depend on the molecular network of the extracellular matrix (ECM). The number, topology and function of ECM molecules are highly diverse in different species, or even in single matrices in one organism. In our study we focus on the protein class of ADAMTS-like proteins. We identified Lonely heart (Loh) a member of this protein family and describe its function using the cardiac system of Drosophila melanogaster as model. Loh constitutes a secreted protein that resides in the ECM of heart cells and mediates the adhesion between different cell types - the pericadial cells and the cardiomyocytes. Lack of Loh function induces the dissociation of these cells and consequently leads to a breakdown of heart function. We found evidence that the major function of Loh is to recruit the collagen Pericardin (Prc) to the ECM of the cells and allow the proper organization of Prc into a reticular matrix. Since the function of Loh homologous proteins in other systems is rather elusive, this work provides new important insights into the biology of cell adhesion, matrix formation and indicates that ADAMTS-like proteins might facilitate an evolutionary conserved function.

A dual role for the βPS integrin myospheroid in mediating Drosophila embryonic macrophage migration

Throughout embryonic development, macrophages not only act as the first line of defence against infection but also help to sculpt organs and tissues of the embryo by removing dead cells and secreting extracellular matrix components. Key to their function is the ability of embryonic macrophages to migrate and disperse throughout the embryo. Despite these important developmental functions, little is known about the molecular mechanisms underlying embryonic macrophage migration in vivo. Integrins are key regulators of many of the adult macrophage responses, but their role in embryonic macrophages remains poorly characterized. Here, we have used Drosophila macrophages (haemocytes) as a model system to address the role of integrins during embryonic macrophage dispersal in vivo. We show that the main βPS integrin, myospheroid, affects haemocyte migration in two ways; by shaping the three-dimensional environment in which haemocytes migrate and by regulating the migration of haemocytes themselves. Live imaging revealed a requirement for myospheroid within haemocytes to coordinate the microtubule and actin dynamics, and to enable haemocyte developmental dispersal, contact repulsion and inflammatory migration towards wounds.

Cbl-associated protein regulates assembly and function of two tension-sensing structures in Drosophila

Cbl-associated protein (CAP) localizes to focal adhesions and associates with numerous cytoskeletal proteins; however, its physiological roles remain unknown. Here, we demonstrate that Drosophila CAP regulates the organization of two actin-rich structures in Drosophila: muscle attachment sites (MASs), which connect somatic muscles to the body wall; and scolopale cells, which form an integral component of the fly chordotonal organs and mediate mechanosensation. Drosophila CAP mutants exhibit aberrant junctional invaginations and perturbation of the cytoskeletal organization at the MAS. CAP depletion also results in collapse of scolopale cells within chordotonal organs, leading to deficits in larval vibration sensation and adult hearing. We investigate the roles of different CAP protein domains in its recruitment to, and function at, various muscle subcellular compartments. Depletion of the CAP-interacting protein Vinculin results in a marked reduction in CAP levels at MASs, and vinculin mutants partially phenocopy Drosophila CAP mutants. These results show that CAP regulates junctional membrane and cytoskeletal organization at the membrane-cytoskeletal interface of stretch-sensitive structures, and they implicate integrin signaling through a CAP/Vinculin protein complex in stretch-sensitive organ assembly and function.

The Par complex and integrins direct asymmetric cell division in adult intestinal stem cells

The adult Drosophila midgut is maintained by intestinal stem cells (ISCs) that generate both self-renewing and differentiating daughter cells. How this asymmetry is generated is currently unclear. Here, we demonstrate that asymmetric ISC division is established by a unique combination of extracellular and intracellular polarity mechanisms. We show that Integrin-dependent adhesion to the basement membrane induces cell-intrinsic polarity and results in the asymmetric segregation of the Par proteins Par-3, Par-6, and aPKC into the apical daughter cell. Cell-specific knockdown and overexpression experiments suggest that increased activity of aPKC enhances Delta/Notch signaling in one of the two daughter cells to induce terminal differentiation. Perturbing this mechanism or altering the orientation of ISC division results in the formation of intestinal tumors. Our data indicate that mechanisms for intrinsically asymmetric cell division can be adapted to allow for the flexibility in lineage decisions that is required in adult stem cells.

Integrin αPS3/βν-mediated phagocytosis of apoptotic cells and bacteria in Drosophila

Integrins exert a variety of cellular functions as heterodimers of two transmembrane subunits named α and β. Integrin βν, a β-subunit of Drosophila integrin, is involved in the phagocytosis of apoptotic cells and bacteria. Here, we searched for an α-subunit that forms a complex and cooperates with βν. Examinations of RNAi-treated animals suggested that αPS3, but not any of four other α-subunits, is required for the effective phagocytosis of apoptotic cells in Drosophila embryos. The mutation of αPS3-encoding scb, deficiency, insertion of P-element, or alteration of nucleotide sequences, brought about a reduction in the level of phagocytosis. The defect in phagocytosis by deficiency was reverted by the forced expression of scb. Furthermore, flies in which the expression of both αPS3 and βν was inhibited by RNAi showed a level of phagocytosis almost equal to that observed in flies with RNAi for either subunit alone. A loss of αPS3 also decreased the activity of larval hemocytes in the phagocytosis of Staphylococcus aureus. Finally, a co-immunoprecipitation analysis using a Drosophila cell line treated with a chemical cross-linker suggested a physical association between αPS3 and βν. These results collectively indicated that integrin αPS3/βν serves as a receptor in the phagocytosis of apoptotic cells and bacteria by Drosophila phagocytes.

"Importin" signaling roles for import proteins: the function of Drosophila importin-7 (DIM-7) in muscle-tendon signaling

The formation of a mature myotendinous junction (MTJ) between a muscle and its site of attachment is a highly regulated process that involves myofiber migration, cell-cell signaling, and culminates with the stable adhesion between the adjacent muscle-tendon cells. Improper establishment or maintenance of muscle-tendon attachment sites results in a decrease in force generation during muscle contraction and progressive muscular dystrophies in vertebrate models. Many studies have demonstrated the important role of the integrins and integrin-associated proteins in the formation and maintenance of the MTJ. We recently demonstrated that moleskin (msk), the gene that encodes for Drosophila importin-7 (DIM-7), is required for the proper formation of muscle-tendon adhesion sites in the developing embryo. Further studies demonstrated an enrichment of DIM-7 to the ends of muscles where the muscles attach to their target tendon cells. Genetic analysis supports a model whereby msk is required in the muscle and signals via the secreted epidermal growth factor receptor (Egfr) ligand Vein to regulate tendon cell maturation. These data demonstrate a novel role for the canonical nuclear import protein DIM-7 in establishment of the MTJ.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

Polydom/SVEP1 is a ligand for integrin α9β1

A variety of proteins, including tenascin-C and osteopontin, have been identified as ligands for integrin α9β1. However, their affinities for integrin α9β1 are apparently much lower than those of other integrins (e.g. α3β1, α5β1, and α8β1) for their specific ligands, leaving the possibility that physiological ligands for integrin α9β1 still remain unidentified. In this study, we found that polydom (also named SVEP1) mediates cell adhesion in an integrin α9β1-dependent manner and binds directly to recombinant integrin α9β1 with an affinity that far exceeds those of the known ligands. Using a series of recombinant polydom proteins with N-terminal deletions, we mapped the integrin-binding site to the 21st complement control protein domain. Alanine-scanning mutagenesis revealed that the EDDMMEVPY sequence (amino acids 2636-2644) in the 21st complement control protein domain was involved in the binding to integrin α9β1 and that Glu(2641) was the critical acidic residue for the integrin binding. The importance of this sequence was further confirmed by integrin binding inhibition assays using synthetic peptides. Immunohistochemical analyses of mouse embryonic tissues showed that polydom colocalized with integrin α9 in the stomach, intestine, and other organs. Furthermore, in situ integrin α9β1 binding assays using frozen mouse tissues showed that polydom accounts for most, but not all, of the integrin α9β1 ligands in tissues. Taken together, the present findings indicate that polydom is a hitherto unknown ligand for integrin α9β1 that functions as a physiological ligand in vivo.

Exoskeleton anchoring to tendon cells and muscles in molting isopod crustaceans

Specialized mechanical connection between exoskeleton and underlying muscles in arthropods is a complex network of interconnected matrix constituents, junctions and associated cytoskeletal elements, which provides prominent mechanical attachment of the epidermis to the cuticle and transmits muscle tensions to the exoskeleton. This linkage involves anchoring of the complex extracellular matrix composing the cuticle to the apical membrane of tendon cells and linking of tendon cells to muscles basally. The ultrastructural arhitecture of these attachment complexes during molting is an important issue in relation to integument integrity maintenance in the course of cuticle replacement and in relation to movement ability. The aim of this work was to determine the ultrastructural organization of exoskeleton – muscles attachment complexes in the molting terrestrial isopod crustaceans, in the stage when integumental epithelium is covered by both, the newly forming cuticle and the old detached cuticle. We show that the old exoskeleton is extensively mechanically connected to the underlying epithelium in the regions of muscle attachment sites by massive arrays of fibers in adult premolt Ligia italica and in prehatching embryos and premolt marsupial mancas of Porcellio scaber. Fibers expand from the tendon cells, traverse the new cuticle and ecdysal space and protrude into the distal layers of the detached cuticle. They likely serve as final anchoring sites before exuviation and may be involved in animal movements in this stage. Tendon cells in the prehatching embryo and in marsupial mancas display a substantial apicobasally oriented transcellular arrays of microtubules, evidently engaged in myotendinous junctions and in apical anchoring of the cuticular matrix. The structural framework of musculoskeletal linkage is basically established in described intramarsupial developmental stages, suggesting its involvement in animal motility within the marsupium.

Lentiviral gene transfer method to study integrin function in T lymphocytes

Integrins play critical roles in adhesion and migration of T cells during an immune response and inflammation. It is of great importance to understand the molecular pathways that regulate integrin function in T cells. Lentiviral vector-based gene transfer method has emerged in the past decade as an efficient means of transferring genes into both resting and activated hard-to-transfect cells, including T cells to knockdown gene expression. Therefore, this technology could be utilized effectively to study different aspects of integrin function or even to perform genome-wide RNAi screens to look globally for regulators of integrin function in T cells. In this chapter, we provide the simplest protocol to infect activated CD4(+) human T cells with high efficiency.

Genetic interaction between Caenorhabditis elegans teneurin ten-1 and prolyl 4-hydroxylase phy-1 and their function in collagen IV-mediated basement membrane integrity during late elongation of the embryo

A whole-genome RNAi screen identified phy-1 as a novel interaction partner of the Caenorhabditis elegans gene ten-1. It is shown that the catalytic subunit of prolyl 4-hydroxylase, which is coded for by phy-1, is important for type IV collagen secretion and that the transmembrane protein TEN-1 links the epidermis to muscle cells through the basement membrane.

Teneurins are a family of phylogenetically conserved proteins implicated in pattern formation and morphogenesis. The sole orthologue in Caenorhabditis elegans, ten-1, is important for hypodermal cell migration, neuronal migration, path finding and fasciculation, gonad development, and basement membrane integrity of some tissues. However, the mechanisms of TEN-1 action remain to be elucidated. Using a genome-wide RNA interference approach, we identified phy-1 as a novel interaction partner of ten-1. phy-1 codes for the catalytic domain of collagen prolyl 4-hydroxylase. Loss of phy-1 significantly enhanced the embryonic lethality of ten-1 null mutants. Double-mutant embryos arrested during late elongation with epidermal defects, disruption of basement membranes, and detachment of body wall muscles. We found that deletion of phy-1 caused aggregation of collagen IV in body wall muscles in elongated embryos and triggered the loss of tissue integrity in ten-1 mutants. In addition, phy-1 and ten-1 each genetically interact with genes encoding collagen IV. These findings support a functional mechanism in which loss of ten-1, together with a reduction of assembled and secreted basement membrane collagen IV protein, leads to detachment of the epidermis from muscle cells during late elongation of the embryo when mechanical stress is generated by muscle contractions.