Domain-specific interactions between entactin and neutrophil integrins. G2 domain ligation of integrin alpha3beta1 and E domain ligation of the leukocyte response integrin signal for different responses

Neutrophil swarming: Is a good offense the best defense?

The phenomenon of swarming has long been observed in nature as a strategic event that serves as a good offense toward prey and predators. Imaging studies have uncovered that neutrophils employ this swarm-like tactic within infected and inflamed tissues as part of the innate immune response. Much of our understanding of neutrophil swarming builds from observations during sterile inflammation and various bacterial, fungal, and parasitic infections of the skin. However, the architecture and function of the skin differ significantly from vital organs where highly specialized microenvironments carry out critical functions. Therefore, the detrimental extent this perturbation may have on organ function remains unclear. In this review, we examine organ-specific swarming within the skin, liver, and lungs, with a detailed focus on swarming within microvascular environments. In addition, we examine potential "swarmulants" that initiate both transient and persistent swarms that have been implicated in disease.

Looking the cow in the eye: deletion in the NID1 gene is associated with recessive inherited cataract in Romagnola cattle

Cataract is a known condition leading to opacification of the eye lens causing partial or total blindness. Mutations are known to cause autosomal dominant or recessive inherited forms of cataracts in humans, mice, rats, guinea pigs and dogs. The use of large-sized animal models instead of those using mice for the study of this condition has been discussed due to the small size of rodent lenses. Four juvenile-onset cases of bilateral incomplete immature nuclear cataract were recently observed in Romagnola cattle. Pedigree analysis suggested a monogenic autosomal recessive inheritance. In addition to the cataract, one of the cases displayed abnormal head movements. Genome-wide association and homozygosity mapping and subsequent whole genome sequencing of a single case identified two perfectly associated sequence variants in a critical interval of 7.2 Mb on cattle chromosome 28: a missense point mutation located in an uncharacterized locus and an 855 bp deletion across the exon 19/intron 19 border of the bovine nidogen 1 (NID1) gene (c.3579_3604+829del). RT-PCR showed that NID1 is expressed in bovine lenses while the transcript of the second locus was absent. The NID1 deletion leads to the skipping of exon 19 during transcription and is therefore predicted to cause a frameshift and premature stop codon (p.1164fs27X). The truncated protein lacks a C-terminal domain essential for binding with matrix assembly complexes. Nidogen 1 deficient mice show neurological abnormalities and highly irregular crystal lens alterations. This study adds NID1 to the list of candidate genes for inherited cataract in humans and is the first report of a naturally occurring mutation leading to non-syndromic catarct in cattle provides a potential large animal model for human cataract.

Skin basement membrane: the foundation of epidermal integrity--BM functions and diverse roles of bridging molecules nidogen and perlecan

The epidermis functions in skin as first defense line or barrier against environmental impacts, resting on extracellular matrix (ECM) of the dermis underneath. Both compartments are connected by the basement membrane (BM), composed of a set of distinct glycoproteins and proteoglycans. Herein we are reviewing molecular aspects of BM structure, composition, and function regarding not only (i) the dermoepidermal interface but also (ii) the resident microvasculature, primarily focusing on the per se nonscaffold forming components perlecan and nidogen-1 and nidogen-2. Depletion or functional deficiencies of any BM component are lethal at some stage of development or around birth, though BM defects vary between organs and tissues. Lethality problems were overcome by developmental stage- and skin-specific gene targeting or by cell grafting and organotypic (3D) cocultures of normal or defective cells, which allows recapitulating BM formation de novo. Thus, evidence is accumulating that BM assembly and turnover rely on mechanical properties and composition of the adjacent ECM and the dynamics of molecular assembly, including further "minor" local components, nidogens largely functioning as catalysts or molecular adaptors and perlecan as bridging stabilizer. Collectively, orchestration of BM assembly, remodeling, and the role of individual players herein are determined by the developmental, tissue-specific, or functional context.

Extracellular matrix lumican promotes bacterial phagocytosis, and Lum-/- mice show increased Pseudomonas aeruginosa lung infection severity

Phagocytosis is central to bacterial clearance, but the exact mechanism is incompletely understood. Here, we show a novel and critical role for lumican, the connective tissue extracellular matrix small leucine-rich repeat proteoglycan, in CD14-mediated bacterial phagocytosis. In Psuedomonas aeruginosa lung infections, lumican-deficient (Lum(-/-)) mice failed to clear the bacterium from lungs, tissues, and showed a dramatic increase in mortality. In vitro, phagocytosis of nonopsonized gram-negative Escherichia coli and P. aeruginosa was inhibited in Lum(-/-) peritoneal macrophages (MΦs). Lumican co-localized with CD14, CD18, and bacteria on Lum(+/+) MΦ surfaces. Using two different P. aeruginosa strains that require host CD14 (808) or CD18/CR3 (P1) for phagocytosis, we showed that lumican has a larger role in CD14-mediated phagocytosis. Recombinant lumican (rLum) restored phagocytosis in Lum(-/-) MΦs. Surface plasmon resonance showed specific binding of rLum to CD14 (K(A) = 2.15 × 10(6) M(-1)), whereas rLumY20A, and not rLumY21A, where a tyrosine in each was replaced with an alanine, showed 60-fold decreased binding. The rLumY20A variant also failed to restore phagocytosis in Lum(-/-) MΦs, indicating Tyr-20 to be functionally important. Thus, in addition to a structural role in connective tissues, lumican has a major protective role in gram-negative bacterial infections, a novel function for small leucine-rich repeat proteoglycans.

Neutrophil elastase cleaves the murine hemidesmosomal protein BP180/type XVII collagen and generates degradation products that modulate experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease associated with autoantibodies against the hemidesmosomal proteins BP180 and BP230. In the IgG passive transfer model of BP, blister formation is triggered by anti-BP180 IgG and depends on complement activation, mast cell degranulation, and neutrophil recruitment. Mice lacking neutrophil elastase (NE) do not develop experimental BP. Here, we demonstrated that NE degrades recombinant mouse BP180 within the immunodominant extracellular domain at amino acid positions 506 and 561, generating peptide p561 and peptide p506. Peptide p561 is chemotactic for neutrophils both in vitro and in vivo. Local injection of NE into B6 mice recruits neutrophils to the skin, and neutrophil infiltration is completely blocked by co-injection with the NE inhibitor α1-proteinase inhibitor. More importantly, NE directly cleaves BP180 in mouse and human skin, as well as the native human BP180 trimer molecule. These results demonstrate that (i) NE directly damages the extracellular matrix and (ii) NE degradation of mouse BP180 generates neutrophil chemotactic peptides that amplify disease severity at the early stage of the disease.

Extracellular matrix lumican deposited on the surface of neutrophils promotes migration by binding to beta2 integrin

During inflammation, circulating polymorphonuclear neutrophils (PMNs) receive signals to cross the endothelial barrier and migrate through the extracellular matrix (ECM) to reach the injured site. Migration requires complex and poorly understood interactions of chemokines, chemokine receptors, ECM molecules, integrins, and other receptors. Here we show that the ECM protein lumican regulates PMN migration through interactions with specific integrin receptors. Lumican-deficient (Lum(-/-)) mice manifest connective tissue defects, impaired innate immune response, and poor wound healing with reduced PMN infiltration. Lum(-/-) PMNs exhibit poor chemotactic migration that is restored with exogenous recombinant lumican and inhibited by anti-lumican antibody, confirming a role for lumican in PMN migration. Treatment of PMNs with antibodies that block beta(2), beta(1), and alpha(M) integrin subunits inhibits lumican-mediated migration. Furthermore, immunohistochemical and biochemical approaches indicate binding of lumican to beta(2), alpha(M), and alpha(L) integrin subunits. Thus, lumican may regulate PMN migration mediated by MAC-1 (alpha(M)/beta(2)) and LFA-1 (alpha(L)/beta(2)), the two major PMN surface integrins. We detected lumican on the surface of peritoneal PMNs and not bone marrow or peripheral blood PMNs. This suggests that PMNs must acquire lumican during or after crossing the endothelial barrier as they exit circulation. We also found that peritoneal PMNs do not express lumican, whereas endothelial cells do. Taken together these observations suggest a novel endothelial lumican-mediated paracrine regulation of neutrophils early on in their migration path.

MIG-17/ADAMTS controls cell migration by recruiting nidogen to the basement membrane in C. elegans

Mutations in the a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS) family of secreted proteases cause diseases linked to ECM abnormalities. However, the mechanisms by which these enzymes modulate the ECM during development are mostly unexplored. The Caenorhabditis elegans MIG-17/ADAMTS protein is secreted from body wall muscle cells and localizes to the basement membrane (BM) of the developing gonad where it controls directional migration of gonadal leader cells. Here we show that specific amino acid changes in the ECM proteins fibulin-1C (FBL-1C) and type IV collagen (LET-2) result in bypass of the requirement for MIG-17 activity in gonadal leader cell migration in a nidogen (NID-1)-dependent and -independent manner, respectively. The MIG-17, FBL-1C and LET-2 activities are required for proper accumulation of NID-1 at the gonadal BM. However, mutant FBL-1C or LET-2 in the absence of MIG-17 promotes NID-1 localization. Furthermore, overexpression of NID-1 in mig-17 mutants substantially rescues leader cell migration defects. These results suggest that functional interactions among BM molecules are important for MIG-17 control of gonadal leader cell migration. We propose that FBL-1C and LET-2 act downstream of MIG-17-dependent proteolysis to recruit NID-1 and that LET-2 also activates a NID-1-independent pathway, thereby inducing the remodeling of the BM required for directional control of leader cell migration.

Nidogens-Extracellular matrix linker molecules

Nidogens/entactins are a family of highly conserved, sulfated glycoproteins. Biochemical studies have implicated them as having a major structural role in the basement membrane. However despite being ubiquitous components of this specialized extracellular matrix and having a wide spectrum of binding partners, genetic analysis has shown that they are not required for the overall architecture of the basement membrane. Rather in development they play an important role in its stabilization especially in tissues undergoing rapid growth or turnover. Nidogen breakdown has been implicated as a key event in the basement membrane degradation occurring in mammary gland involution. A number of studies, most compellingly those in C. elegans, demonstrated that nidogens may have other nonstructural roles and be involved in axonal pathfinding and synaptic transmission.

Neutrophil elastase cleaves laminin-332 (laminin-5) generating peptides that are chemotactic for neutrophils

Proteolytic processing of laminin-332 by matrix metalloproteinase (MMP)-2 and MMP-14 has been shown to yield fragments that are promigratory for epithelial cells. During acute and chronic inflammation, proteases are elaborated by neutrophils and macrophages that can degrade basement membranes. We investigated the susceptibility of laminin-332 to degradation by the following neutrophil and macrophage proteases: neutrophil elastase (NE), cathepsin G, proteinase-3, and MMPs-2, -8, -9, and -12. Protease-specific differences were seen in the capacity to cleave the individual chains of laminin-332. NE and MMP-12 showed the greatest activity toward the gamma2 chain, generating a fragment similar in size to the gamma2x fragment generated by MMP-2. The digestion pattern of laminin-332 by degranulated neutrophils was nearly identical to that generated with NE alone. Digestion by supernatants of degranulated neutrophils was blocked by an inhibitor of NE, and NE-deficient neutrophils were essentially unable to digest laminin-332, suggesting that NE is the major neutrophil-derived protease that degrades laminin-332. In vivo, laminin gamma2 fragments were found in the bronchoalveolar lavage fluid of wild-type mice treated with lipopolysaccharide, whereas that obtained from NE-deficient mice showed a different cleavage pattern. In addition, NE cleaved a synthetic peptide derived from the region of human laminin gamma2 containing the MMP-2 cleavage site, suggesting that NE may generate laminin-332 fragments that are also promigratory. Both laminin-332 fragments generated by NE digestion and NE-digested laminin gamma2 peptide were found to be chemotactic for neutrophils. Collectively, these data suggest that degradation of laminin-332 by NE generates fragments with important biological activities.

Integrin alphaVbeta3 Binds to the RGD motif of glycoprotein B of Kaposi's sarcoma-associated herpesvirus and functions as an RGD-dependent entry receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) envelope-associated glycoprotein B (gB) is involved in the initial steps of binding to host cells during KSHV infection. gB contains an RGD motif reported to bind the integrin alpha(3)beta(1) during virus entry. Although the ligand specificity of alpha(3)beta(1) has been controversial, current literature indicates that alpha(3)beta(1) ligand recognition is independent of RGD. We compared alpha(3)beta(1) to the RGD-binding integrin, alpha(V)beta(3), for binding to envelope-associated gB and a gB(RGD) peptide. Adhesion assays demonstrated that beta(3)-CHO cells overexpressing alpha(V)beta(3) specifically bound gB(RGD), whereas alpha(3)-CHO cells overexpressing alpha(3)beta(1) did not. Function-blocking antibodies to alpha(V)beta(3) inhibited the adhesion of HT1080 fibrosarcoma cells to gB(RGD), while antibodies to alpha(3)beta(1) did not. Using affinity-purified integrins and confocal microscopy, alpha(V)beta(3) bound to gB(RGD) and KSHV virions, demonstrating direct receptor-ligand interactions. Specific alpha(V)beta(3) antagonists, including cyclic and dicyclic RGD peptides and alpha(V)beta(3) function-blocking antibodies, inhibited KSHV infection by 70 to 80%. Keratinocytes from alpha(3)-null mice lacking alpha(3)beta(1) were fully competent for infection by KSHV, and reconstitution of alpha(3)beta(1) function by transfection with alpha(3) cDNA reduced KSHV infectivity from 74% to 55%. Additional inhibitory effects of alpha(3)beta(1) on the cell surface expression of alpha(V)beta(3) and on alpha(V)beta(3)-mediated adhesion of alpha(3)-CHO cells overexpressing alpha(3)beta(1) were detected, consistent with previous reports of transdominant inhibition of alpha(V)beta(3) function by alpha(3)beta(1). These observations may explain previous reports of an inhibition of KSHV infection by soluble alpha(3)beta(1). Our studies demonstrate that alpha(V)beta(3) is a cellular receptor mediating both the cell adhesion and entry of KSHV into target cells through binding the virion-associated gB(RGD).

Nidogen is a prosurvival and promigratory factor for adult Schwann cells

Schwann cells provide a favorable microenvironment for successful regeneration of the injured peripheral nerve. Even though the roles of extracellular matrix proteins in the Schwann cell physiology have long been studied, the precise function of nidogen, a ubiquitous component of the basal lamina, in Schwann cells is unknown. In this study, we show that the protein and mRNA messages for nidogens are up-regulated in the sciatic nerve after sciatic nerve transection. We demonstrate that recombinant nidogen-1 increased the process formation of Schwann cells cultured from adult rat sciatic nerves and that nidogen-1 prevented Schwann cells from serum-deprivation-induced death. In addition, nidogen-1 promoted spontaneous migration of Schwann cells in two-independent migration assays. The Schwann cell responses to the recombinant nidogen-1 were specific because the nidogen-binding ectodomain of tumor endothelial marker 7 inhibited the nidogen responses without affecting Schwann cell response to laminin. Finally, we found that beta1 subunit-containing integrins play a key role in the nidogen-induced process formation, survival, and migration of Schwann cells. Altogether, these results indicate that nidogen has a prosurvival and promigratory activity on Schwann cells in the peripheral nerve.

The kringle domain of urokinase-type plasminogen activator potentiates LPS-induced neutrophil activation through interaction with {alpha}V{beta}3 integrins

Urokinase plasminogen activator (uPA) is a serine protease that catalyzes the conversion of plasminogen to plasmin. In addition, uPA has been shown to have proinflammatory properties, particularly in potentiating lipopolysaccharide (LPS)-induced neutrophil responses. To explore the mechanisms by which uPA exerts these effects, we examined the ability of specific uPA domains to increase cytokine expression in murine and human neutrophils stimulated with LPS. Whereas the addition of intact uPA to neutrophils cultured with LPS increased mRNA and protein levels of interleukin-1beta, macrophage-inflammatory protein-2, and tumor necrosis factor alpha, deletion of the kringle domain (KD) from uPA resulted in loss of these potentiating effects. Addition of purified uPA KD to LPS-stimulated neutrophils increased cytokine expression to a degree comparable with that produced by single-chain uPA. Inclusion of the arginine-glycine-aspartic but not the arginine-glycine-glutamic peptide to neutrophil cultures blocked uPA kringle-induced potentiation of proinflammatory responses, demonstrating that interactions between the KD and integrins were involved. Antibodies to alpha(V) or beta(3) integrins or to the combination of alpha(V)beta(3) prevented uPA kringle-induced enhancement of expression of proinflammatory cytokines and also of adhesion of neutrophils to the uPA KD. These results demonstrate that the KD of uPA, through interaction with alpha(V)beta(3) integrins, potentiates neutrophil activation.

Morphofunctional studies of the glomerular wall in mice lacking entactin-1

The architecture of the basement membranes is essential for proper function. This architecture is based on interactions among its components, which assemble in a complex network. Entactin-1 appears to be the mastermind of this assembling. In entactin-1-null transgenic mice, immunocytochemistry established the absence of entactin-1 in the glomerular basement membrane, and morphological thickening of this membrane was demonstrated. This prompted us to investigate the organization of other components of the glomerular basement membrane in the transgenic animals. The distribution of type IV collagen and laminin remained unchanged, whereas that of anionic charges was significantly altered. We also evaluated the impact of the absence of entactin-1 on cell relays by studying the alpha(3)- and the alpha(v)-integrins along the endothelial and epithelial glomerular cell plasma membranes. Only the density of alpha(v) was found to be increased. Finally, the filtration properties of the glomerular wall were evaluated by revealing endogenous albumin distribution across the basement membrane. This was altered in transgenic animals, suggesting changes in permselectivity properties. Entactin-1 appears to be an essential component in basement membranes because its absence appears to modify the molecular organization leading to alterations in functional properties.

Neurologic defects and selective disruption of basement membranes in mice lacking entactin-1/nidogen-1

Entactin-1 (nidogen-1) is an ubiquitous component of basement membranes. From in vitro experiments, entactin-1 was assigned a role in maintaining the structural integrity of the basement membrane because of its binding affinity to other components, such as type IV collagen and laminin. Entactin-1 also interacts with integrin receptors on the cell surface to mediate cell adhesion, spreading, and motility. Targeted disruption of the entactin-1 gene in the mouse presented in this study revealed a duplication of the entacin-1 locus. Homozygous mutants for the functional locus lacked entactin-1 mRNA and protein and often displayed seizure-like symptoms and loss of muscle control in the hind legs. The behavior patterns suggested the presence of neurologic deficits in the central nervous system, thus providing genetic evidence linking entactin-1 to proper functions of the neuromuscular system. In homozygous mutants, structural alterations in the basement membranes were found only in selected locations including brain capillaries and the lens capsule. The morphology of the basement membranes in other tissues examined superficially appeared to be normal. These observations suggest that the lost functions of entactin-1 result in pathologic changes that are highly tissue specific.

Binding of mouse nidogen-2 to basement membrane components and cells and its expression in embryonic and adult tissues suggest complementary functions of the two nidogens

Nidogen-1 binds several basement membrane components by well-defined, domain-specific interactions. Organ culture and gene targeting approaches suggest that a high-affinity nidogen-binding site of the laminin gamma1 chain (gamma1III4) is important for kidney development and for nerve guidance. Other proteins may also bind gamma1III4, although human nidogen-2 binds poorly to the mouse laminin gamma1 chain. We therefore characterized recombinant mouse nidogen-2 and its binding to basement membrane proteins and cells. Mouse nidogen-1 and -2 interacted at comparable levels with collagen IV, perlecan, and fibulin-2 and, most notably, also with laminin-1 fragments P1 and gamma1III3-5, which both contain the gamma1III4 module. In embryos, nidogen-2 mRNA was produced by mesenchyme at sites of epithelial-mesenchymal interactions, but the protein was deposited on epithelial basement membranes, as previously shown for nidogen-1. Hence, binding of both nidogens to the epithelial laminin gamma1 chain is dependent on epithelial-mesenchymal interactions. Epidermal growth factor stimulated expression of both nidogens in embryonic submandibular glands. Both nidogens were found in all studied embryonic and adult basement membranes. Nidogen-2 was more adhesive than nidogen-1 for some cell lines and was mainly mediated by alpha3beta1 and alpha6beta1 integrins as shown by antibody inhibition. These findings revealed extensive coregulation of nidogen-1 and -2 expression and much more complementary functions of the two nidogens than previously recognized.

Pharmacodynamics and pharmacogenomics of diverse receptor-mediated effects of methylprednisolone in rats using microarray analysis

Corticosteroids such as methylprednisolone (MPL) produce many of their anti-inflammatory, immunosuppressive, and exaggerated physiological effects by receptor and gene-mediated mechanisms. The temporal pattern of change in four genes in rat tissues was measured by quantitative Northern hybridization and rtPCR after a single dose of MPL. Two profiles were observed: two genes with enhanced expression showed a slow onset and moderate rate of decline within a 24 hr time frame while two genes with reduced expression exhibited a rapid onset and prolonged suppression over a > or = 72 hr time span. These patterns are consistent with and rationalized by pharmacodynamic expectations based on earlier models. cDNA microarrays used to assess the expression levels of 5200 genes at one optimal time-point showed marked variation in baseline values. Of these, 20 genes showed statistically significant enhanced expression with increases ranging from 130 to 1690%, 31 genes exhibited reduced expression ranging from 31 to 72% of control. Many genes could be categorized as affecting acute phase/immune response, energy metabolism, microsomal metabolism, and hepatic function. These studies provide the first simultaneous assessment of the diversity in pharmacogenomic effects of corticosteroids. They also provide some insight into the advantages and limitations of microarray measurements in regard to the pharmacodynamics of drugs having complex, multi-faceted, and integrated mechanisms of action.

Recombinant domains of mouse nidogen-1 and their binding to basement membrane proteins and monoclonal antibodies

The basement membrane protein, nidogen-1, was previously shown to consist of three globular domains, G1 to G3, and two connecting segments. Nidogen-1 is a major mediator in the formation of ternary complexes with laminins, collagen IV, perlecan and fibulins. In the present study, we have produced recombinant proteins of these predicted domains in mammalian cells and used these proteins for crystallographic and binding epitope analyses. These fragments included G1, G2, the rod domain and a slightly larger G3 structure; all were obtained in good yields and were shown to be properly folded using electron microscopy. Surface plasmon resonance assays demonstrated high affinity binding (Kd = 3-9 nM) of domain G2 for collagen IV, perlecan domain IV-1 and fibulin-2, and a more moderate Kd for fibulin-1C. Domain G3 contained high affinity binding sites for the laminin gamma1 chain and collagen IV (Kd = 1 nM) and weaker binding sites for fibulin-1C and fibulin-2. A moderate binding affinity was also observed between domain G1 and fibulin-2, while no activity could be detected for the nidogen rod domain. Together, these data indicate the potential of nidogen-1 for multiple interactions within basement membranes. A similar binding repertoire was also identified for seven rat monoclonal antibodies that bound with Kd = 2-30 nM to either G1, G1-G2, G2, the rod domain or G3. Three of the antibodies showed strongly reduced binding to G2 and G3 after complex formation with either a perlecan domain or laminin-1.

Phosphorylation of a conserved integrin alpha 3 QPSXXE motif regulates signaling, motility, and cytoskeletal engagement

Integrin alpha 3A cytoplasmic tail phosphorylation was mapped to amino acid S1042, as determined by mass spectrometry, and confirmed by mutagenesis. This residue occurs within a "QPSXXE" motif conserved in multiple alpha chains (alpha 3A, alpha 6A, alpha 7A), from multiple species. Phosphorylation of alpha 3A and alpha 6A did not appear to be directly mediated by protein kinase C (PKC) alpha, beta, gamma, delta, epsilon, zeta, or mu, or by any of several other known serine kinases, although PKC has an indirect role in promoting phosphorylation. A S1042A mutation did not affect alpha 3-Chinese hamster ovary (CHO) cell adhesion to laminin-5, but did alter 1) alpha 3-dependent tyrosine phosphorylation of focal adhesion kinase and paxillin (in the presence or absence of phorbol 12-myristate 13 acetate stimulation), and p130(CAS) (in the absence of phorbol 12-myristate 13 acetate stimulation), 2) the shape of cells spread on laminin-5, and 3) alpha 3-dependent random CHO cell migration on laminin-5. In addition, S1042A mutation altered the PKC-dependent, ligand-dependent subcellular distribution of alpha 3 and F-actin in CHO cells. Together, the results demonstrate clearly that alpha 3A phosphorylation is functionally relevant. In addition, the results strongly suggest that alpha 3 phosphorylation may regulate alpha 3 integrin interaction with the cytoskeleton.

Adhesion receptors in health and disease

Cell adhesion molecules have been recognized to play a major role in a variety of physiological and pathological phenomena. They determine the specificity of cell-cell binding and the interactions between cells and extracellular matrix proteins. Some of them may also function as receptors that trigger intracellular pathways and participate in cellular processes like migration, proliferation, differentiation, and cell death. The receptors that mediate adhesion between epithelial cells that are discussed in this review include integrins, selectins, the immunoglobulin superfamily members, and cadherins. The intent of this review is to inform the reader about recent advances in cellular and molecular functions of certain receptors, specifically those that are considered important in cell adhesion. We have deliberately not provided all-inclusive detailed information on every molecule, but instead, have presented a generalized overview in order to give the reader a global perspective. This information will be useful in enhancing the reader's understanding of the molecular pathology of diseases and recognizing the potential role of these receptors and ligands as therapeutic agents.

Nidogen-1 regulates laminin-1-dependent mammary-specific gene expression

Nidogen-1 (entactin) acts as a bridge between the extracellular matrix molecules laminin-1 and type IV collagen, and thus participates in the assembly of basement membranes. To investigate the role of nidogen-1 in regulating cell-type-specific gene expression in mammary epithelium, we designed a culture microecosystem in which each component, including epithelial cells, mesenchymal cells, lactogenic hormones and extracellular matrix, could be controlled. We found that primary and established mesenchymal and myoepithelial cells synthesized and secreted nidogen-1, whereas expression was absent in primary and established epithelial cells. In an epithelial cell line containing mesenchymal cells, nidogen-1 was produced by the mesenchymal cells but deposited between the epithelial cells. In this mixed culture, mammary epithelial cells express (beta)-casein in the presence of lactogenic hormones. Addition of either laminin-1 plus nidogen-1, or laminin-1 alone, to mammary epithelial cells induced (beta)-casein production. We asked whether recombinant nidogen-1 alone could signal directly for (beta)-casein. Nidogen-1 did not induce (beta)-casein synthesis in epithelial cells, but it augmented the inductive capacity of laminin-1. These data suggest that nidogen-1 can cooperate with laminin-1 to regulate (beta)-casein expression. Addition of full-length nidogen-1 to the mixed cultures had no effect on (beta)-casein gene expression; however, a nidogen-1 fragment containing the laminin-1 binding domain, but lacking the type IV collagen-binding domain, had a dominant negative effect on (beta)-casein expression. These data point to a physiological role for nidogen-1 in the basement membrane-induced gene expression by epithelial cells.

Negative regulation of phagocytosis in murine macrophages by the Src kinase family member, Fgr

Ingestion of opsonized pathogens by professional phagocytes results in the generation and release of microbicidal products that are essential for normal host defense. Because these products can result in significant tissue injury, phagocytosis must be regulated to limit damage to the host while allowing for optimal clearance and destruction of opsonized pathogens. To pursue negative regulation of phagocytosis, we assessed the effect of the Src kinase family member, Fgr, on opsonin-dependent phagocytosis by mouse macrophages. We chose Fgr because it is present in high concentrations in circulating phagocytes but is not essential for Fcgamma receptor-mediated ingestion by mouse macrophages. Although expression of Fgr both in a macrophage cell line and in primary macrophages significantly attenuates ingestion mediated by Fcgamma receptors and CR3, it does not affect macropinocytosis or receptor-mediated endocytosis. This selective effect of Fgr is independent of its tyrosine kinase function. After Fcgamma receptor cross-linking, Fgr becomes associated with the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor, SIRPalpha (a member of the signal-regulatory protein family, also known as Src homology 2 domain-containing protein tyrosine phosphatase [SHP] substrate 1 [SHPS-1], brain immunoglobulin-like molecule with tyrosine-based activation motifs [BIT], and P84) and potentiates the association of the phosphatase SHP-1 with SIRPalpha. This association is responsible, at least in part, for decreasing positive signaling essential for optimal phagocytosis. These data demonstrate an important negative regulatory role for this Src kinase family member and suggest that this homeostatic function must be overcome for optimal uptake and clearance of opsonized pathogens.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Identification of an alpha(3)beta(1) integrin recognition sequence in thrombospondin-1

A synthetic peptide containing amino acid residues 190-201 of thrombospondin-1 (TSP1) promoted adhesion of MDA-MB-435 breast carcinoma cells when immobilized and inhibited adhesion of the same cells to TSP1 when added in solution. Adhesion to this peptide was enhanced by a beta(1) integrin-activating antibody, Mn(2+), and insulin-like growth factor I and was inhibited by an alpha(3)beta(1) integrin function-blocking antibody. The soluble peptide inhibited adhesion of cells to the immobilized TSP1 peptide or spreading on intact TSP1 but at the same concentrations did not inhibit attachment or spreading on type IV collagen or fibronectin. Substitution of several residues in the TSP1 peptide with Ala residues abolished or diminished the inhibitory activity of the peptide in solution, but only substitution of Arg-198 completely inactivated the adhesive activity of the immobilized peptide. The essential residues for activity of the peptide as a soluble inhibitor are Asn-196, Val-197, and Arg-198, but flanking residues enhance the inhibitory activity of this core sequence, either by altering the conformation of the active sequence or by interacting with the integrin. This functional sequence is conserved in all known mammalian TSP1 sequences and in TSP1 from Xenopus laevis. The TSP1 peptide also inhibited adhesion of MDA-MB-435 cells to the laminin-1 peptide GD6, which contains a potential integrin-recognition sequence Asn-Leu-Arg and is derived from a similar position in a pentraxin module. Adhesion studies using recombinant TSP1 fragments also localized beta1 integrin-dependent adhesion to residues 175-242 of this region, which contain the active sequence.

Differential regulation of beta1 integrins by chemoattractants regulates neutrophil migration through fibrin

Chemoattractants differ in their capacity to stimulate neutrophils to adhere to and to migrate through matrices containing fibrin. Formyl methionyl leucyl phenylalanine (fMLP) stimulates neutrophils to adhere closely to, but not to migrate into, fibrin gels. Leukotriene B4 (LTB4) stimulates neutrophils to adhere loosely to and to migrate through fibrin gels. We report that alpha5beta1 integrins regulate the different migratory behaviors on fibrin gels of neutrophils in response to these chemoattractants. fMLP, but not LTB4, activated neutrophil beta1 integrins, as measured by binding of mAb 15/7 to an activation epitope on the beta1 integrins. Antibodies or peptides that block alpha5beta1 integrins prevented fMLP-stimulated neutrophils from forming zones of close apposition on fibrin and reversed fMLP's inhibitory effect on neutrophil chemotaxis through fibrin. In contrast, neither peptides nor antibodies that block beta1 integrins affected the capacity of LTB4-stimulated neutrophils to form zones of loose apposition or to migrate through fibrin gels. These results suggest that chemoattractants generate at least two different messages that direct neutrophils, and perhaps other leukocytes, to accumulate at specific anatomic sites: a general message that induces neutrophils to crawl and a specific message that prepares neutrophils to stop when they contact appropriate matrix proteins for activated beta1 integrins.

Renal cell carcinomas and pancreatic adenocarcinomas produce nidogen in vitro and in vivo

The production of nidogen by four renal cell carcinoma (RCC) and three pancreatic adenocarcinoma (PAc) cell lines has been studied in cell culture and in xenografted tumours in nude mice. In RCC cells, immunoreactivity for nidogen was seen only after exposure to monensin to induce cytoplasmic accumulation of secretory proteins. In PAc cells, immunoreaction was also detectable in control cells. Immunoblotting of control and monensin-exposed cells and immunoprecipitation of culture media of radioactively labelled cells demonstrated the production of nidogen polypeptide of Mr ca. 150000 by six of the seven cell lines. Basement membranes (BMs) and stroma of the xenografted tumours derived from these six cell lines demonstrated immunoreactivity for both human and mouse nidogen, as revealed with species-specific antibodies. The ability of the cells to produce nidogen in vitro and deposit in vivo was positively correlated with high histological grade of the xenografted tumours, although the small number of cell lines studied calls for further studies to confirm this. The distribution of nidogen in human RCC and PAc specimens was also studied by immunohistochemistry. There was strong immunoreactivity for nidogen in tumour stroma, BM of carcinoma cell nests, and endothelial basal lamina, but no conclusions could be drawn regarding histological grade and immunostaining patterns, because stromal production could not be ruled out. The results show that nidogen is produced by human carcinoma cells both in vitro and in vivo.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.

Highly stoichiometric, stable, and specific association of integrin alpha3beta1 with CD151 provides a major link to phosphatidylinositol 4-kinase, and may regulate cell migration

Here we describe an association between alpha3beta1 integrin and transmembrane-4 superfamily (TM4SF) protein CD151. This association is maintained in relatively stringent detergents and thus is remarkably stable in comparison with previously reported integrin-TM4SF protein associations. Also, the association is highly specific (i.e., observed in vitro in absence of any other cell surface proteins), and highly stoichiometric (nearly 90% of alpha3beta1 associated with CD151). In addition, alpha3beta1 and CD151 appeared in parallel on many cell lines and showed nearly identical skin staining patterns. Compared with other integrins, alpha3beta1 exhibited a considerably higher level of associated phosphatidylinositol-4-kinase (PtdIns 4-kinase) activity, most of which was removed upon immunodepletion of CD151. Specificity for CD151 and PtdIns 4-kinase association resided in the extracellular domain of alpha3beta1, thus establishing a novel paradigm for the specific recruitment of an intracellular signaling molecule. Finally, antibodies to either CD151 or alpha3beta1 caused a approximately 88-92% reduction in neutrophil motility in response to f-Met-Leu-Phe on fibronectin, suggesting an functionally important role of these complexes in cell migration.

Nidogen-2: a new basement membrane protein with diverse binding properties

Human nidogen-2 was cloned and sequenced (1375 residues) and found to share 46% sequence identity and a similar domain arrangement with the previously characterized basement membrane protein nidogen-1. Recombinant nidogen-2 was purified as a 200 kDa protein from transfected mammalian cell medium, showed a high level of N and O-glycosylation, and could be clearly distinguished from nidogen-1 (150 kDa) by specific antibodies. Electron microscopy demonstrated that the two isoforms have a similar shape, consisting of three globular domains connected by two threads, but differ somewhat in length. Northern blots and immunological assays demonstrated co-expression of the nidogens in various tissues and cultured cells. Immunofluoresence revealed colocalization in vessel walls and other basement membrane zones but some differences in heart and skeletal muscle. Nidogen-2 interacted with collagens I and IV, and perlecan at a comparable level to nidogen-1 but failed to bind to fibulins. Nidogen-2 bound to laminin-1, but only moderately to the epitope on the laminin gamma1 chain, which promotes high-affinity binding of nidogen-1. Both nidogens were cell-adhesive for a restricted number of cell lines, with nidogen-2 having a higher activity. Together, these data suggest that nidogen-2 can compensate for some but not all functional activities ascribed to nidogen-1.

Human corneal epithelial basement membrane and integrin alterations in diabetes and diabetic retinopathy

Corneas of diabetic patients have abnormal healing and epithelial adhesion, which may be due to alterations of the corneal extracellular matrix (ECM) and basement membrane (BM). To identify such alterations, various ECM and BM components and integrin receptors were studied by immunofluorescence on sections of normal and diabetic human corneas. Age-matched corneas from 15 normal subjects, 10 diabetics without diabetic retinopathy (DR), and 12 diabetics with DR were used. In DR corneas, the composition of the central epithelial BM was markedly altered, compared to normal or non-DR diabetic corneas. In most cases the staining for entactin/nidogen and for chains of laminin-1 (alpha1beta1gamma1) and laminin-10 (alpha5beta1gamma1 was very weak, discontinuous, or absent over large areas. Other BM components displayed less frequent changes. The staining for alpha3beta1 (VLA-3) laminin binding integrin was also weak and discontinuous in DR corneal epithelium. Components of stromal ECM remained unchanged even in DR corneas. Therefore, distinct changes were identified in the composition of the epithelial BM in DR corneas. They may be due to increased degradation or decreased synthesis of BM components and related integrins. These alterations may directly contribute to the epithelial adhesion and wound healing abnormalities found in diabetic corneas.