Integrin cytoplasmic interactions and bidirectional transmembrane signalling

The Fcγ Receptor–Mediated Respiratory Burst of Rolling Neutrophils to Cytokine-Activated, Immune Complex-Bearing Endothelial Cells Depends on L-Selectin But Not on E-Selectin

Intracellular H2O2 generation, as a measure of the respiratory burst, was determined after stimulation of neutrophils by immune complex (IC)-bearing human umbilical vein endothelial cells. Under static conditions, neutrophils basically responded to the immune deposits on resting endothelial cells. The rotating shear forces of ≈0.7 dynes/cm2, corresponding to the physiological flow in postcapillary venules, completely abolished this basal H2O2 generation. After activation of the IC-bearing endothelial layers with interleukin-1 (IL-1) or tumor necrosis factor (TNF), or both, for 4 hours, rolling adhesion of the neutrophils was induced, accompanied by considerable H2O2 production. The neutrophil respiratory burst was prominently inhibited by anti-FcγRIII MoAb 3G8 (72.4%), and partially by MoAb 2E1 against FcγRII (38.5%). Both MoAbs together inhibited the Fc-mediated H2O2generation by 93.4%. The respiratory burst and rolling adhesion were markedly blocked by MoAb LAM1-3 against L-selectin (91.3%), whereas the nonfunctional anti-L-selectin MoAb LAM1-14 was ineffective. F(ab)2′ fragments of MoAb 7A9 against E-selectin inhibited neutrophil rolling by 98.6%, but not the respiratory burst. Moreover, rolling adhesion of neutrophils and the related oxidative burst were CD11b/CD18- independent. In summary, L-selectin has a unique auxiliary function in triggering the FcγR-mediated respiratory burst of rolling neutrophils to IC-bearing endothelial cells, thereby substituting CD11b/CD18 under conditions of flow.

Apoptosis precedes detachment of germ cells from the seminiferous epithelium after hormone suppression by short-term oestradiol treatment of rats

Spermatogenesis is a highly synchronized process in which FSH and testosterone are considered the major regulators. Nevertheless, the mechanism by which these hormones act on germ cells is unclear. Cell adhesion has been proved to play an essential role in the regulation of programmed cell death in epithelial cells and it is now known that FSH and testosterone withdrawal results in the triggering of apoptosis as well as germ cell detachment from the seminiferous epithelium. Therefore, it seemed important to investigate whether the triggering of apoptosis in germ cells by experimental hormone suppression occurred as a result of their previous detachment from the epithelium. To achieve this goal, adult male rats were injected with 50 micrograms oestradiol benzoate for 1, 2, 3, 4, 5 or 10 days to suppress gonadotrophin secretion and thus intratesticular levels of testosterone. Germ cell apoptosis was assessed in testes from these animals by in situ 3' end-labelling of DNA fragments and quantified in seminiferous tubule sections at stages VII-VIII. Serial sections throughout the epididymides from these animals were analysed to search for immature germ cells detached from the epithelium. These cells were scored and quantified in non-consecutive randomly selected epididymal sections. Our data indicate that the triggering of apoptosis in germ cells precedes germ cell detachment, suggesting that detachment of germ cells from the epithelium, occurring after hormone suppression, is not necessary for germ cell apoptosis.

Effect of substratum on growth, cell morphology and lactoferrin synthesis and secretion in bovine mammary cell culture

The role of extracellular matrix in morphology, growth and lactoferrin synthesis and secretion in bovine mammary cells from a developing gland is poorly defined. In this study, bovine mammary cells from a hormone-primed developing gland were isolated and cultured on plastic, collagen, embedded within collagen, or on EHS-matrix, with the hormones prolactin, insulin, and cortisol in the presence or absence of fetal calf serum. Mammary cells on plastic or collagen spread and formed confluent cells sheets, while those embedded within collagen or on EHS-matrix maintained their acinar-like structure. Histological and ultrastructural analysis of cells showed that cells on plastic and collagen grew in multilayers, while those embedded within collagen or on EHS-matrix lacked any lumen structure. The ultrastructure of cells on different substrata more resembled an undifferentiated phenotype. Mammary cells secreted lactoferrin in increasing concentrations throughout the culture period. The total amount secreted in culture was regulated by extracellular matrix and fetal calf serum. Cells embedded within collagen in serum-free cultures secreted the lowest amounts of lactoferrin (up to 619 ng/ml; day 14), while those on collagen and supplemented with fetal calf serum secreted up to 4920 ng/ml at day 14. Fetal calf serum induced higher lactoferrin secretion within each substratum on which the cells were cultured. No intracellular accumulation of lactoferrin was noted in cells on plastic or collagen or those embedded within collagen, whereas those on EHS-matrix accumulated more than 500 ng/ml of lactoferrin intracellularly/intracinarly. Furthermore, when cultured on a similar substratum, cells from a developing gland secreted higher lactoferrin than cells from a lactating gland.

A key function for alphav containing integrins in mesodermal cell migration during Pleurodeles waltl gastrulation

During cleavage of Pleurodeles waltl amphibian embryos, inner cells of the blastocoel roof (presumptive ectodermal and mesodermal cells) organize a fibrillar extracellular matrix (ECM) containing fibronectin on their basal surface by a beta1-integrin-dependent process. This matrix is used as a migratory substrate by mesodermal cells during gastrulation. While alpha5beta1 integrin is expressed on both ectodermal and mesodermal cell surface, we have shown previously that alphav containing integrins are essentially restricted to the surface of mesodermal cells (Alfandari, D., Whittaker, C. A., DeSimone, D. W., and Darribère, T., Dev. Biol. 170, 249-261, 1995). To investigate the function of alphav integrins during gastrulation, we have generated a function blocking antibody directed against the extracellular domain of the Pleurodeles integrin alphav subunit. The antibody did not prevent fibronectin fibril formation, whereas an antibody against the alpha5beta1 integrin did. When injected into the blastocoel, the antibody against integrin alphav subunit perturbed gastrulation and further development in a stage-dependent manner. Developmental defects were correlated to an abnormal positioning of the mesoderm layer. In vitro, the antibody blocked spreading of mesodermal cell to fibronectin or blastocoel roof ECM but not their attachment. In contrast, the antibody directed against the alpha5beta1 integrin inhibited both cell attachment and spreading to the same substrates. We propose that the alpha5beta1 integrin is required for fibronectin assembly into fibrils and mesodermal cell attachment to the blastocoel roof ECM, while the alphav containing integrins are necessary for cell spreading, and possibly migration, on this complex network.

The calpain-calpastatin system and protein degradation in fusing myoblasts

Calpain (Ca(2+)-activated cysteine protease) induced proteolysis has been suggested to play a role in myoblast fusion. We previously found that calpastatin (the endogenous inhibitor of calpain) diminishes markedly in myoblasts during myoblast differentiation just prior to the start of fusion, allowing Ca(2+)-induced calpain activation at that stage. Here, we show that a limited degradation of some proteins occurs within the myoblasts undergoing fusion, but not in proliferating myoblasts. The protein degradation is observed at the stage when calpastatin is low. Protein degradation within the myoblasts and myoblast fusion are inhibited by EGTA, by the cysteine protease inhibitors calpeptin and E-64d and by calpastatin. The degradation appears to be selective for certain myoblast proteins. Integrin beta 1 subunit, talin and beta-tropomyosin are degraded in the fusing myoblasts, whereas alpha-actinin, beta-tubulin and alpha-tropomyosin are not. A similar pattern of degradation is observed in lysates of proliferating myoblasts when Ca2+ and excess calpain are added, a degradation that is inhibited by calpastatin. The results support the notion that degradation of certain proteins is required for myoblast fusion and that calpain participates in the fusion-associated protein degradation. Participation of calpain is made possible by a change in calpain/calpastatin ratio, i.e., by a diminution in calpastatin level from a high level in the proliferating myoblasts to a low level in the differentiating myoblasts. Degradation of certain proteins, known to be responsible for the stability of the membrane-skeleton organization and for the interaction of the cell with the extracellular matrix, would allow destabilization of the membrane and the creation of membrane fusion-potent regions.

Epidermal differentiation and basement membrane formation by HaCaT cells in surface transplants

The immortal human keratinocyte line HaCaT has been employed in many studies as paradigm for epidermal keratinocytes. In order to demonstrate its potential to form stable epidermal structures in response to connective tissue, this was challenged in surface transplants on nude mice, where normal keratinocytes rebuild a typical epidermis within two weeks. During the initial regeneration phase (day 1-4) multilayered but poorly organized epithelia formed with proliferating cells in all layers in analogy to normal keratinocytes. Similarly, with tissue consolidation (around day 7) proliferation was reduced and restricted to cells in basal position marked by keratin K14 and beta1-integrin immunostaining. The strong suprabasal reaction for K1 and K10, the appearance of the late markers K2e, filaggrin and loricrin as well as the polarized distribution of alpha2beta1 and alpha3beta1 indicated advancing tissue normalization (day 14). Keratinization further improved at around three weeks switching from the initial parakeratotic to the regular orthokeratotic type which was prominent at six weeks. Accordingly, most ultrastructural features typical for epidermis or normal keratinocyte grafts were detectable including a complete basement membrane (BM) with regular attachment structures. Matrix- and BM-components appeared sequentially with marked linear deposition of laminin-5 (day 4) followed by accumulation of collagen-IV and 'classical' BM-laminin between one and two weeks. With the general codistribution of integrin alpha6beta4 and BM-molecules (day 14) collagen-VII lining of BM became prominent, while epithelium and host connective tissue were still separated by the collagen matrix. In accordance with the delayed orthokeratinization, wound-matrix molecules (fibronectin, tenascin) persisted longer than in normal keratinocyte transplants. Finally, grafts of long-term passaged (no. 310) cells demonstrated a remarkable stability in the expression of epidermal markers. Thus, the immortalized HaCaT cells reveal a generally high competence to realize an epidermal phenotype in a natural environment and appear therefore qualified for in vitro studies on structural and regulatory aspects of keratinocyte physiology and pathology.

EGF receptor regulation of cell motility: EGF induces disassembly of focal adhesions independently of the motility-associated PLCgamma signaling pathway

A current model of growth factor-induced cell motility invokes integration of diverse biophysical processes required for cell motility, including dynamic formation and disruption of cell/substratum attachments along with extension of membrane protrusions. To define how these biophysical events are actuated by biochemical signaling pathways, we investigate here whether epidermal growth factor (EGF) induces disruption of focal adhesions in fibroblasts. We find that EGF treatment of NR6 fibroblasts presenting full-length WT EGF receptors (EGFR) reduces the fraction of cells presenting focal adhesions from approximately 60% to approximately 30% within 10 minutes. The dose dependency of focal adhesion disassembly mirrors that for EGF-enhanced cell motility, being noted at 0.1 nM EGF. EGFR kinase activity is required as cells expressing two kinase-defective EGFR constructs retain their focal adhesions in the presence of EGF. The short-term (30 minutes) disassembly of focal adhesions is reflected in decreased adhesiveness of EGF-treated cells to substratum. We further examine here known motility-associated pathways to determine whether these contribute to EGF-induced effects. We have previously demonstrated that phospholipase C(gamma) (PLCgamma) activation and mobilization of gelsolin from a plasma membrane-bound state are required for EGFR-mediated cell motility. In contrast, we find here that short-term focal adhesion disassembly is induced by a signaling-restricted truncated EGFR (c'973) which fails to activate PLCgamma or mobilize gelsolin. The PLC inhibitor U73122 has no effect on this process, nor is the actin severing capacity of gelsolin required as EGF treatment reduces focal adhesions in gelsolin-devoid fibroblasts, further supporting the contention that focal adhesion disassembly is signaled by a pathway distinct from that involving PLCgamma. Because both WT and c'973 EGFR activate the erk MAP kinase pathway, we additionally explore here this signaling pathway, not previously associated with growth factor-induced cell motility. Levels of the MEK inhibitor PD98059 that block EGF-induced mitogenesis and MAP kinase phosphorylation also abrogate EGF-induced focal adhesion disassembly and cell motility. In summary, we characterize for the first time the ability of EGFR kinase activity to directly stimulate focal adhesion disassembly and cell/substratum detachment, in relation to its ability to stimulate migration. Furthermore, we propose a model of EGF-induced motogenic cell responses in which the PLCgamma pathway stimulating cell motility is distinct from the MAP kinase-dependent signaling pathway leading to disassembly and reorganization of cell-substratum adhesion.

Transmembrane-truncated alphavbeta3 integrin retains high affinity for ligand binding: evidence for an 'inside-out' suppressor?

The molecular mechanisms of alphavbeta3 integrin affinity regulation have important biological implications in tumour development, wound repair and angiogenesis. We expressed, purified and characterized recombinant forms of human alphavbeta3 (r-alphavbeta3) and compared the activation state of these with alphavbeta3 in its cellular environment. The ligand specificity and selectivity of recombinant full-length and double transmembrane truncations of r-alphavbeta3 cloned in BacPAK6 vectors and expressed in Sf9 and High Five insect cells were compared with those of native placental alphavbeta3 and the receptor in situ on the cell surface. r-alphavbeta3 integrins were purified by affinity chromatography from detergent extracts of cells (full-length), and from the culture medium of cells expressing double-truncated r-alphavbeta3. r-alphavbeta3 had the same epitopes, ligand-binding specificities, bivalent cation requirements and susceptibility to RGD-containing peptides as native alphavbeta3. On M21-L4 melanoma cells, alphavbeta3 mediated binding to vitronectin, but not to fibrinogen unless activated with Mn2+. Non-activated alphaIIbbeta3 integrin as control in M21-L-IIb cells had the opposite profile, mediating binding to fibrinogen, but not to vitronectin unless activated with Mn2+. Thus these receptors had moderate to low ligand affinity. In marked contrast, purified alphavbeta3 receptors, with or without transmembrane and cytoplasmic domains, were constitutively of high affinity and able to bind strongly to vitronectin, fibronectin and fibrinogen under physiological conditions. Our data suggest that, in contrast with the positive regulation of alphaIIbbeta3 in situ, intracellular controls lower the affinity of alphavbeta3, and the cytoplasmic domains may act as a target for negative regulators of alphavbeta3 activity.

The integrin beta subunit

The integrin family of cell adhesion receptors plays a fundamental role in the processes involved in cell division, differentiation and movement. The extracellular domains of integrin alpha/beta heterodimers mediate cell-matrix and cell-cell contacts while their cytoplasmic tails associate with the cytoskeleton. Integrins are capable of transducing information in a bidirectional manner and the beta subunit is now recognised to play an important role in this process. Recent studies have led to the identification of a ligand-binding region on the beta subunit similar to that already characterised on some alpha subunits, and sequences in the cytoplasmic tails of the beta subunits that interact with cytoskeletal and signalling components. Adhesive events can also play a role in the progression of all four major classes of human disease--neoplastic, inflammatory, traumatic and infectious--and the specific nature of integrin adhesion mechanisms make them an attractive target for therapy.

Rack1, a receptor for activated protein kinase C, interacts with integrin beta subunit

The integrin beta subunit cytoplasmic domains are important for activation-dependent cell adhesion and adhesion-dependent signaling events. We report an interaction between integrin beta subunit cytoplasmic domain and Rack1, a Trp-Asp (WD) repeat protein that has been shown to bind activated protein kinase C. The Rack1-binding site on integrin beta 2 subunit resides within a conserved, membrane-proximal region. In the yeast two-hybrid assay, WD repeats five to seven of Rack1 (Rack1-WD5/7) interact with integrin beta 1, beta 2, and beta 5 cytoplasmic domain. In eukaryotic cells, Rack1 co-immunoprecipitates with at least two different beta integrins, beta 1 integrins in 293T cells and beta 2 integrins in JY lymphoblastoid cells. Whereas Rack1-WD5/7 binds integrins constitutively, the association of full-length Rack1 to integrins in vivo requires a treatment with phorbol esters, which promotes cell spreading and adhesion. These findings suggest that Rack1 may link protein kinase C directly to integrins and participate in the regulation of integrin functions.

Integrin-linked protein kinase regulates fibronectin matrix assembly, E-cadherin expression, and tumorigenicity

Fibronectin (Fn) matrix plays important roles in many biological processes including morphogenesis and tumorigenesis. Recent studies have demonstrated a critical role of integrin cytoplasmic domains in regulating Fn matrix assembly, implying that intracellular integrin-binding proteins may be involved in controlling extracellular Fn matrix assembly. We report here that overexpression of integrin-linked kinase (ILK), a newly identified serine/threonine kinase that binds to the integrin beta1 cytoplasmic domain, dramatically stimulated Fn matrix assembly in epithelial cells. The integrin-linked kinase activity is involved in transducing signals leading to the up-regulation of Fn matrix assembly, as overexpression of a kinase-inactive ILK mutant failed to enhance the matrix assembly. Moreover, the increase in Fn matrix assembly induced by ILK overexpression was accompanied by a substantial reduction in the cellular E-cadherin. Finally, we show that ILK-overexpressing epithelial cells readily formed tumors in nude mice, despite forming an extensive Fn matrix. These results identify ILK as an important regulator of pericellular Fn matrix assembly, and suggest a novel critical role of this integrin-linked kinase in cell growth, cell survival, and tumorigenesis.

Key role of the Cdx2 homeobox gene in extracellular matrix-mediated intestinal cell differentiation

To explore the role of homeobox genes in the intestine, the human colon adenocarcinoma cell line Caco2-TC7 has been stably transfected with plasmids synthesizing Cdx1 and Cdx2 sense and antisense RNAs. Cdx1 overexpression or inhibition by antisense RNA does not markedly modify the cell differentiation markers analyzed in this study. In contrast, Cdx2 overexpression stimulates two typical markers of enterocytic differentiation: sucrase-isomaltase and lactase. Cells in which the endogenous expression of Cdx2 is reduced by antisense RNA attach poorly to the substratum. Conversely, Cdx2 overexpression modifies the expression of molecules involved in cell-cell and cell-substratum interactions and in transduction process: indeed, E-cadherin, integrin-beta4 subunit, laminin-gamma2 chain, hemidesmosomal protein, APC, and alpha-actinin are upregulated. Interestingly, most of these molecules are preferentially expressed in vivo in the differentiated villi enterocytes rather than in crypt cells. Cdx2 overexpression also results in the stimulation of HoxA-9 mRNA expression, an homeobox gene selectively expressed in the colon. In contrast, Cdx2-overexpressing cells display a decline of Cdx1 mRNA, which is mostly found in vivo in crypt cells. When implanted in nude mice, Cdx2-overexpressing cells produce larger tumors than control cells, and form glandular and villus-like structures. Laminin-1 is known to stimulate intestinal cell differentiation in vitro. In the present study, we demonstrate that the differentiating effect of laminin-1 coatings on Caco2-TC7 cells is accompanied by an upregulation of Cdx2. To further document this observation, we analyzed a series of Caco2 clones in which the production of laminin-alpha1 chain is differentially inhibited by antisense RNA. We found a positive correlation between the level of Cdx2 expression, that of endogenous laminin-alpha1 chain mRNA and that of sucrase-isomaltase expression in these cell lines. Taken together, these results suggest (a) that Cdx1 and Cdx2 homeobox genes play distinct roles in the intestinal epithelium, (b) that Cdx2 provokes pleiotropic effects triggering cells towards the phenotype of differentiated villus enterocytes, and (c) that Cdx2 expression is modulated by basement membrane components. Hence, we conclude that Cdx2 plays a key role in the extracellular matrix-mediated intestinal cell differentiation.

The two phenylalanines in the GFFKR motif of the integrin alpha6A subunit are essential for heterodimerization

The membrane-proximal domain of the integrin alpha subunit contains a conserved motif of five amino acid residues, GFFKR. We deleted this motif from the human alpha6A subunit and found that in COS-7 cells this mutant cannot associate with the beta1 subunit and is retained in the endoplasmic reticulum. Point mutations in the GFFKR motif of the glycine residue or the two highly charged amino acids, or deletion of the lysine and arginine residues, had no effect on the ability of alpha6 to interact with beta1 and to be expressed at the cell surface. In contrast, by replacing either of the two phenylalanines with alanine, or by deletion of both of these residues, alpha6 was incapable of associating with beta1. The alpha6 point mutants that associated with beta1 were expressed in K562 cells and their responsiveness to integrin-activating factors was determined. None of these transfectants bound spontaneously to laminin-1, but binding could be induced by either PMA or the stimulating anti-beta1 antibody TS2/16 to the same extent as that of the wild-type transfectant. The ability of these mutants to initiate focal-contact formation in CHO cells plated on laminin-1 substrates also appeared to be unaltered. Thus the behaviour of alpha6 mutants involving the glycine, lysine or arginine residues was indistinguishable from that of wild-type alpha6 both in inside-out and outside-in signalling. In contrast, deletion of the cytoplasmic domain of alpha6 C-terminal of the GFFKR motif resulted in a loss of responsiveness of alpha6beta1 to PMA stimulation and formation of focal contacts on laminin-1. However, this mutant was targeted to focal contacts formed by other integrins, even when they had not bound ligand. Together, these results suggest that the two phenylalanine residues of the GFFKR motif provide a site for interaction of the alpha6A subunit with beta1, whereas the cytoplasmic domain C-terminal of this motif is involved in the regulation of bidirectional signalling via alpha6Abeta1.

Regulation of the pp72syk protein tyrosine kinase by platelet integrin alpha IIb beta 3

pp72syk is essential for development and function of several hematopoietic cells, and it becomes activated through tandem SH2 interaction with ITAM motifs in immune response receptors. Since Syk is also activated through integrins, which do not contain ITAMs, a CHO cell model system was used to study Syk activation by the platelet integrin, alpha IIb beta 3. As in platelets, Syk underwent tyrosine phosphorylation and activation during CHO cell adhesion to alpha IIb beta 3 ligands, including fibrinogen. This involved Syk autophosphorylation and the tyrosine kinase activity of Src, and it exhibited two novel features. Firstly, unlike alpha IIb beta 3-mediated activation of pp125FAK, Syk activation could be triggered by the binding of soluble fibrinogen and abolished by truncation of the alpha IIb or beta 3 cytoplasmic tail, and it was resistant to inhibition by cytochalasin D. Secondly, it did not require phosphorylated ITAMs since it was unaffected by disruption of an ITAM-interaction motif in the SH2(C) domain of Syk or by simultaneous overexpression of the tandem SH2 domains. These studies demonstrate that Syk is a proximal component in alpha IIb beta 3 signaling and is regulated as a consequence of intimate functional relationships with the alpha IIb beta 3 cytoplasmic tails and with Src or a closely related kinase. Furthermore, there are fundamental differences in the activation of Syk by alpha IIb beta 3 and immune response receptors, suggesting a unique role for integrins in Syk function.

Arginine-glycine-aspartic acid-specific binding by foot-and-mouth disease viruses to the purified integrin alpha(v)beta3 in vitro

The integrin alpha(v)beta3 has been shown to act as the receptor for internalization of foot-and-mouth disease virus (FMDV) (A12), with attachment being through a highly conserved RGD motif located on the G-H loop of viral capsid protein VP1. In addition, however, we have recently shown that efficient infection of culture-grown cells by FMDV (O1BFS) requires binding to cell surface heparan sulfate. In this study, we have used a solid-phase receptor binding assay to characterize the binding by FMDV to purified alpha(v)beta3 in the absence of heparan sulfate and other cell surface components. In this assay, FMDV (O1BFS) successfully replicated authentic ligand binding by cellular alpha(v)beta3 in terms of its high affinity, dependence on divalent cations, and activation by manganese ions. Virus binding to this preparation of alpha(v)beta3 was exquisitely sensitive to competition by short RGD-containing peptides (50% inhibition at < 10(-8) M peptide), and this inhibition was highly sequence specific, with the equivalent RGE peptide being at least 10(4) fold less effective as a competitor. Representative viruses of the other six serotypes of FMDV bound to alpha(v)beta3 in a similar RGD-specific manner, although significant differences in sensitivity to RGD peptides suggest that the affinity of the different FMDV serotypes for alpha(v)beta3 is influenced, in part, by the variable amino acid residues in the VP1 G-H loop on either side of the RGD.

A role for the VLA-4 integrin in the activation of human memory B cells

It is generally recognized that activation through membrane effector molecules such as CD40 or the B cell receptor (BCR) is mandatory to allow B cells to proliferate and differentiate into antibody (Ab)-secreting cells in response to cytokines. We show here that purified tonsillar B cells can be stimulated directly by a cytokine combination to proliferate and secrete immunoglobulins when cultures are performed at high cell density. The contact-mediated activation of B cells in this experimental system is strongly inhibited both by anti-very late antigen (VLA)-4 monoclonal Ab and by a peptide containing the LDV sequence specifically recognized by the alpha 4 integrin binding site. These reagents also significantly suppressed the B cell responses elicited by engagement of the BCR or CD40. Our data reveal that memory B cells but not virgin or germinal center B cells are sensitive to the direct stimulatory effect of cytokines in high-density cultures. Finally, we found that the dual expression of the alpha and beta chains of VLA-4 is a distinctive feature of the memory B cell population. Collectively, our findings support the notion that VLA-4-dependent homotypic B cell interactions can mediate a co-stimulatory signal to human memory B cells and might participate in the B cell activation triggered through the BCR and CD40.

Identification and characterization of a mouse protein kinase that is highly homologous to human integrin-linked kinase

Integrin-linked kinase (ILK) is a recently identified human protein kinase that has been implicated in integrin-mediated signal transduction and tumorigenesis. We have identified a mouse molecule that is highly homologous to human ILK. The mouse ILK homologue protein is readily recognized by antibodies raised against the human ILK protein, and the gene encoding the mouse ILK homologue is widely expressed in mouse tissues. The mouse ILK homologue gene has been mapped to chromosome 7E1 band. A second locus in the mouse chromosome 9E1-3 region has also been detected with a mouse ILK homologue cDNA probe by fluorescence in situ hybridization, suggesting the possible existence of an ILK pseudo-gene or a family of ILK genes in the mouse.

Integrins mediate adhesion to agrin and modulate agrin signaling

Agrin, a basal lamina-associated proteoglycan, is a crucial nerve-derived organizer of postsynaptic differentiation at the skeletal neuromuscular junction. Because integrins serve as cellular receptors for many basal lamina components, we asked whether agrin interacts with integrins. Agrin-induced aggregation of acetylcholine receptors on cultured myotubes was completely blocked by antibodies to the beta1 integrin subunit and partially blocked by antibodies to the alpha(v) integrin subunit. Agrin-induced clustering was also inhibited by antisense oligonucleotides to alpha(v) and a peptide that blocks the alpha(v) binding site. Non-muscle cells that expressed alpha(v) and beta1 integrin subunits adhered to immobilized agrin, and this adhesion was blocked by anti-alpha(v) and anti-beta1 antibodies. Integrin alpha(v)-negative cells that did not adhere to agrin were rendered adherent by introduction of alpha(v). Together, these results implicate integrins, including alpha(v)beta1, as components or modulators of agrin's signal transduction pathway.

Integrin cytoplasmic domains as connectors to the cell's signal transduction apparatus

Integrins mediate the bidirectional transfer of signals across the plasma membrane. Integrin cytoplasmic domains provide one pathway linking integrin engagement with the cell's signal transduction apparatus. Recent structure-function studies have defined regions of beta cytoplasmic domains required for integrin function and have identified distinct roles for individual alpha cytoplasmic domains in regulating cell behavior. Newly identified proteins that bind to integrin alpha and beta cytoplasmic domains have provided new insights and new questions into the mechanisms involved in integrin signaling.

Integrins and inside-out signal transduction: converging signals from PKC and PIP3

Recent studies have identified molecules that interact with integrins and appear to participate in the signaling pathways that regulate integrin adhesiveness. Clues provided by studies of these molecules point to the integration by integrins of signal transduction pathways implicated in cell division and activation.

Integrin signaling

Integrins provide dynamic links between cells and extracellular matrix molecules. Although integrins were originally viewed as relatively simple adhesion molecules, it soon became clear that intracellular signal transduction initiated by integrins is centrally involved in many cellular processes. In fact, a remarkable number of classical signaling pathways are now known to be activated or modified by the interactions of cells with matrix proteins via integrins. These integrin signaling responses can also involve many other extracellular and intracellular molecules. The following mini-reviews were solicited from some of the future leaders in the field of integrin signaling. They examine selected important portions of this field, provide conceptual syntheses from a large and confusing literature, and then propose novel testable ideas. These ideas should encourage dialogue and open new avenues of research in this rapidly expanding, exciting field.

RGDN peptide interaction with endothelial alpha5beta1 integrin causes sustained endothelin-dependent vasoconstriction of rat skeletal muscle arterioles

The ability of an integrin-binding Arg-Gly-Asp-Asn (RGDN)- containing peptide to influence vascular tone by interacting with the alpha5beta1 integrin was studied using rat skeletal muscle arterioles. After blockade of beta3 integrin function, isolated arterioles with spontaneous tone showed concentration-dependent vasoconstrictions to topical application of GRGDNP, a peptide that shows a greater ability to interact with alpha5beta1 than with alphavbeta3. The constriction to GRGDNP (2.1 mM) was inhibited by blocking alpha5 integrin function, and was intensified by blocking beta3 integrin function. In contrast, GRGDSP, a peptide that interacts better with alphavbeta3, was unable to induce sustained constrictions. Removal of the endothelium abolished the vasoconstriction in response to GRGDNP, suggesting that the response was due to release of an endothelium-dependent factor. Indeed, blockade of ETA endothelin receptors with BQ-610 (1 microM), similar to removal of the endothelium and alpha5 integrin blockade, inhibited the vasoconstriction. These data indicate that interaction of RGD peptides, and in particular the RGDN sequence with endothelial cell alpha5beta1, causes endothelin-mediated arteriolar vasoconstriction. These results indicate that integrins are novel signaling receptors within the vascular wall that affect vasomotor tone, and may play an important role in vascular control.

Coordinated intercellular calcium waves induced by noradrenaline in rat hepatocytes: dual control by gap junction permeability and agonist

Calcium-mobilizing agonists induce intracellular Ca2+ concentration ([Ca2+]i) changes thought to trigger cellular responses. In connected cells, rises in [Ca2+]i can propagate from cell to cell as intercellular Ca2+ waves, the mechanisms of which are not elucidated. Using fura2-loaded rat hepatocytes, we studied the mechanisms controlling coordination and intercellular propagation of noradrenaline-induced Ca2+ signals. Gap junction blockade with 18 alpha-glycyrrhetinic acid resulted in a loss of coordination between connected cells. We found that second messengers and [Ca2+]i rises in one hepatocyte cannot trigger Ca2+ responses in connected cells, suggesting that diffusion across gap junctions, while required for coordination, is not sufficient by itself for the propagation of intercellular Ca2+ waves. In addition, our experiments revealed functional differences between noradrenaline-induced Ca2+ signals in connected hepatocytes. These results demonstrate that intercellular Ca2+ signals in multicellular systems of rat hepatocytes are propagated and highly organized through complex mechanisms involving at least three factors. First, gap junction coupling ensures coordination of [Ca2+]i oscillations between the different cells; second, the presence of hormone at each hepatocyte is required for cell-cell Ca2+ signal propagation; and third, functional differences between adjacent connected hepatocytes could allow a 'pacemaker-like' intercellular spread of Ca2+ waves.

Cell-matrix interactions induce tyrosine phosphorylation of MAP kinases ERK1 and ERK2 and PLCgamma-1 in two-dimensional and three-dimensional cultures of human fibroblasts

Using immunoprecipitation and phosphotyrosine detection by Western blotting, intracellular signaling intermediates were analyzed in human primary dermal fibroblasts, either seeded as monolayers on collagen I coats (2D) or seeded within three-dimensional collagen I lattices (3D). Previous results demonstrated that integrin activation in these systems resulted in a cascade of protein tyrosine phosphorylation, including focal adhesion kinase (D. Roeckel and T. Krieg, 1994, Exp. Cell Res. 211, 42-48). Further downstream signaling events are now shown to include coordinate activation of ERK1 and ERK2 at 2 h after cell-collagen contact, irrespective of 2D or 3D culture conditions. Applying U-73122, an inhibitor of PLC, inhibits collagen lattice contraction in a dose-dependent fashion. Immunoprecipitation identified the isoform PLCgamma-1 as playing a role as signaling intermediate in fibroblast-collagen interactions. PLCgamma-1 becomes phosphorylated within 10 min after culture initiation and declines after 2 h. So far, no qualitative differences in signaling intermediates between 2D and 3D cultures have been identified.

Molecular evolution of integrins: genes encoding integrin beta subunits from a coral and a sponge

The integrin family of cell surface receptors is strongly conserved in higher animals, but the evolutionary history of integrins is obscure. We have identified and sequenced cDNAs encoding integrin beta subunits from a coral (phylum Cnidaria) and a sponge (Porifera), indicating that these proteins existed in the earliest stages of metazoan evolution. The coral betaCn1 and, especially, the sponge betaPo1 sequences are the most divergent of the "beta1-class" integrins and share a number of features not found in any other vertebrate or invertebrate integrins. Perhaps the greatest difference from other beta subunits is found in the third and fourth repeats of the cysteine-rich stalk, where the generally conserved spacings between cysteines are highly variable, but not similar, in betaCn1 and betaPo1. Alternatively spliced cDNAs, containing a stop codon about midway through the full-length translated sequence, were isolated from the sponge library. These cDNAs appear to define a boundary between functional domains, as they would encode a protein that includes the globular ligand-binding head but would be missing the stalk, transmembrane, and cytoplasmic domains. These and other sequence comparisons with vertebrate integrins are discussed with respect to models of integrin structure and function.

Platelet Adhesion to Collagen Under Flow Causes Dissociation of a Phosphoprotein Complex of Heat-Shock Proteins and Protein Phosphatase 1

Phosphorylation/dephosphorylation events in human blood platelets were investigated during their adhesion to collagen under flow conditions. Using 32P-labeled platelets and one-dimensional gel electrophoresis, we found that adhesion to collagen mediated primarily by the α2β1 integrin resulted in a strong dephosphorylation of several protein bands. Neither adhesion to polylysine nor thrombin-induced aggregation caused similar protein dephosphorylation. In addition, treatment with okadaic acid (OA), an inhibitor of serine/threonine protein phosphatases type 1 (PP1) and 2A (PP2A), caused significant inhibition of adhesion, suggesting that adhesion is regulated by OA-sensitive phosphatases. Recent studies indicate that phosphatases may be associated with the heat-shock proteins. Immunoprecipitations with antibodies against either the heat-shock cognate protein 70 (hsc70) or heat-shock protein 90 (hsp90) showed the presence of a phosphoprotein complex in 32P-labeled, resting human platelets. Antibody probing of this complex detected hsc70, hsp90, two isoforms of the catalytic subunit of PP1, PP1Cα and PP1Cδ, as well as the M regulatory subunit of PP1 (PP1M). OA, at concentrations that markedly blocked platelet adhesion to collagen, caused hyperphosphorylation of the hsc70 complex. In platelets adhering to collagen, hsc70 was completely dephosphorylated and hsp90, PP1α, and PP1M were dissociated from the complex, suggesting involvement of heat-shock proteins and protein phosphatases in platelet adhesion.

Platelet Adhesion to Collagen Under Flow Causes Dissociation of a Phosphoprotein Complex of Heat-Shock Proteins and Protein Phosphatase 1

Phosphorylation/dephosphorylation events in human blood platelets were investigated during their adhesion to collagen under flow conditions. Using 32P-labeled platelets and one-dimensional gel electrophoresis, we found that adhesion to collagen mediated primarily by the α2β1 integrin resulted in a strong dephosphorylation of several protein bands. Neither adhesion to polylysine nor thrombin-induced aggregation caused similar protein dephosphorylation. In addition, treatment with okadaic acid (OA), an inhibitor of serine/threonine protein phosphatases type 1 (PP1) and 2A (PP2A), caused significant inhibition of adhesion, suggesting that adhesion is regulated by OA-sensitive phosphatases. Recent studies indicate that phosphatases may be associated with the heat-shock proteins. Immunoprecipitations with antibodies against either the heat-shock cognate protein 70 (hsc70) or heat-shock protein 90 (hsp90) showed the presence of a phosphoprotein complex in 32P-labeled, resting human platelets. Antibody probing of this complex detected hsc70, hsp90, two isoforms of the catalytic subunit of PP1, PP1Cα and PP1Cδ, as well as the M regulatory subunit of PP1 (PP1M). OA, at concentrations that markedly blocked platelet adhesion to collagen, caused hyperphosphorylation of the hsc70 complex. In platelets adhering to collagen, hsc70 was completely dephosphorylated and hsp90, PP1α, and PP1M were dissociated from the complex, suggesting involvement of heat-shock proteins and protein phosphatases in platelet adhesion.

Affinity modulation of platelet integrin alphaIIbbeta3 by beta3-endonexin, a selective binding partner of the beta3 integrin cytoplasmic tail

Platelet agonists increase the affinity state of integrin alphaIIbbeta3, a prerequisite for fibrinogen binding and platelet aggregation. This process may be triggered by a regulatory molecule(s) that binds to the integrin cytoplasmic tails, causing a structural change in the receptor. beta3-Endonexin is a novel 111-amino acid protein that binds selectively to the beta3 tail. Since beta3-endonexin is present in platelets, we asked whether it can affect alphaIIbbeta3 function. When beta3-endonexin was fused to green fluorescent protein (GFP) and transfected into CHO cells, it was found in both the cytoplasm and the nucleus and could be detected on Western blots of cell lysates. PAC1, a fibrinogen-mimetic mAb, was used to monitor alphaIIbbeta3 affinity state in transfected cells by flow cytometry. Cells transfected with GFP and alphaIIbbeta3 bound little or no PAC1. However, those transfected with GFP/beta3-endonexin and alphaIIbbeta3 bound PAC1 specifically in an energy-dependent fashion, and they underwent fibrinogen-dependent aggregation. GFP/beta3-endonexin did not affect levels of surface expression of alphaIIbbeta3 nor did it modulate the affinity of an alphaIIbbeta3 mutant that is defective in binding to beta3-endonexin. Affinity modulation of alphaIIbbeta3 by GFP/beta3-endonexin was inhibited by coexpression of either a monomeric beta3 cytoplasmic tail chimera or an activated form of H-Ras. These results demonstrate that beta3-endonexin can modulate the affinity state of alphaIIbbeta3 in a manner that is structurally specific and subject to metabolic regulation. By analogy, the adhesive function of platelets may be regulated by such protein-protein interactions at the level of the cytoplasmic tails of alphaIIbbeta3.

A homozygous mutation in the integrin alpha6 gene in junctional epidermolysis bullosa with pyloric atresia

The alpha6 integrin subunit participates in the formation of both alpha6beta1 and alpha6beta4 laminin receptors, which have been reported to play an important role in cell adhesion and migration and in morphogenesis. In squamous epithelia, the alpha6beta4 heterodimer is the crucial component for the assembly and stability of hemidesmosomes. These anchoring structures are ultrastructurally abnormal in patients affected with junctional epidermolysis bullosa with pyloric atresia (PA-JEB), a recessively inherited blistering disease of skin and mucosae characterized by an altered immunoreactivity with antibodies specific to integrin alpha6beta4. In this report, we describe the first mutation in the alpha6 integrin gene in a PA-JEB patient presenting with generalized skin blistering, aplasia cutis, and defective expression of integrin alpha6beta4. The mutation (791delC) is a homozygous deletion of a single base (C) leading to a frameshift and a premature termination codon that results in a complete absence of alpha6 polypeptide. We also describe the DNA-based prenatal exclusion of the disease in this family at risk for recurrence of PA-JEB. Our results demonstrate that, despite the widespread distribution of the alpha6 integrin subunit, lack of expression of the alpha6 integrin chain is compatible with fetal development, and results in a phenotype indistinguishable from that caused by mutations in the beta4 chain, which is expressed in a more limited number of tissues.

Overexpression of the integrin-linked kinase promotes anchorage-independent cell cycle progression

Cell adhesion to substratum has been shown to regulate cyclin A expression as well as cyclin D- and E-dependent kinases, the latter via the up-regulation of cyclin D1 and the down-regulation of cyclin-Cdk inhibitors p21 and p27, respectively. This adhesion-dependent regulation of cell cycle is thought to be mediated by integrins. Here we demonstrate that stable transfection and overexpression of the integrin-linked kinase (ILK), which interacts with the beta1 and beta3 integrin cytoplasmic domains, induces anchorage-independent cell cycle progression but not serum-independent growth of rat intestinal epithelial cells (IEC18). ILK overexpression results in increased expression of cyclin D1, activation of Cdk4 and cyclin E-associated kinases, and hyperphosphorylation of the retinoblastoma protein. In addition, ILK overexpression results in the expression of p21 and p27 Cdk inhibitors with altered electrophoretic mobilities, with the p27 from ILK-overexpressing cells having reduced inhibitory activity. The transfer of serum-exposed IEC18 cells from adherent cultures to suspension cultures results in a rapid down-regulation of expression of cyclin D1 and cyclin A proteins as well as in retinoblastoma protein dephosphorylation. In marked contrast, transfer of ILK-overexpressing cells from adherent to suspension cultures results in continued high levels of expression of cyclin D1 and cyclin A proteins, and a substantial proportion of the retinoblastoma protein remains in a hyperphosphorylated state. These results indicate that, when overexpressed, ILK induces signaling pathways resulting in the stimulation of G1/S cyclin-Cdk activities, which are normally regulated by cell adhesion and integrin engagement.

Multimerization of the cytoplasmic domain of syndecan-4 is required for its ability to activate protein kinase C

The transmembrane proteoglycan syndecan-4, which is a coreceptor with integrins in cytoskeleton-matrix interactions, appears to be multimerized in vivo. Both purified and recombinant core proteins form sodium dodecyl sulfate-resistant oligomers, and we now report that a synthetic peptide corresponding to the central region of syndecan-4 cytoplasmic domain (4V) also oligomerizes. The degree of oligomerization correlates with the previously reported ability to bind protein kinase C (PKC) and regulate its activity. Only multimeric recombinant syndecan-4 core protein, but not the monomeric protein, potentiated the activity of PKCalpha, and only oligomeric syndecan-4 cytoplasmic peptides were active. Changes in peptide sequence caused parallel loss of stable oligomeric status and ability to regulate a mixture of PKCalphabetagamma activity. A synthetic peptide encompassing the whole cytoplasmic domain of syndecan-4 (4L) containing a membrane-proximal basic sequence did not form higher order oligomers and could not regulate the activity of PKCalphabetagamma unless induced to aggregate by phosphatidylinositol 4,5-bisphosphate. Oligomerization and PKC regulatory activity of the 4V peptide were both increased by addition of N-terminal cysteine and reduced by phosphorylation of the cysteine thiol group. Concentration of syndecan-4 at sites of focal adhesion formation may enhance multimerization and both localize PKC and potentiate its activity to induce stable complex formation.

Occludin confers adhesiveness when expressed in fibroblasts

Occludin is an integral membrane protein specifically associated with tight junctions. Previous studies suggest it is likely to function in forming the intercellular seal. In the present study, we expressed occludin under an inducible promotor in occludin-null fibroblasts to determine whether this protein confers intercellular adhesion. When human occludin is stably expressed in NRK and Rat-1 fibroblasts, which lack endogenous occludin and tight junctions but do have well developed ZO-1-containing adherens-like junctions, occludin colocalizes with ZO-1 to points of cell-cell contact. In contrast, L-cell fibroblasts which lack cadherin-based adherens junctions, target neither ZO-1 nor occludin to sites of cell contact. Occludin-induced adhesion was next quantified using a suspended cell assay. In NRK and Rat-1 cells, occludin expression induces adhesion in the absence of calcium, thus independent of cadherin-cadherin contacts. In contrast, L-cells are nonadhesive in this assay and show no increase in adhesion after induction of occludin expression. Binding of an antibody to the first of the putative extracellular loops of occludin confirmed that this sequence was exposed on the cell surface, and synthetic peptides containing the amino acid sequence of this loop inhibit adhesion induced by occludin expression. These results suggest that the extracellular surface of occludin is directly involved in cell-cell adhesion and the ability to confer adhesiveness correlates with the ability to colocalize with its cytoplasmic binding protein, ZO-1.

Calreticulin is essential for integrin-mediated calcium signalling and cell adhesion

Integrins are important mediators of cell adhesion to extracellular ligands and can transduce biochemical signals both into and out of cells. The cytoplasmic domains of integrins interact with several structural and signalling proteins and consequently participate in the regulation of cell shape, motility, growth and differentiation. It has been shown that calreticulin associates with the cytoplasmic domains of integrin alpha-subunits and that this interaction can influence integrin-mediated cell adhesion to extracellular matrix. We have now developed calreticulin-deficient embryonic stem (ES) cells and isolated embryonic fibroblasts from calreticulin mutant mice. We find that in both cell types integrin-mediated adhesion is severely impaired, although integrin expression is unaltered. Expression of recombinant calreticulin in double knockout ES cells by complementary DNA transfection rescued integrin-mediated adhesion. In wild-type cells, engagement of surface integrins induced a transient elevation in cytosolic calcium concentration owing to influx of extracellular calcium. This calcium transient was absent in calreticulin-deficient cells. In contrast, the amount of calcium in endomembrane stores, which is sensitive to both inositol 1,4,5-trisphosphate and thapsigargin, was indistinguishable in the two cell types. Our results indicate that calreticulin is an essential modulator both of integrin adhesive functions and integrin-initiated signalling, but that it may not play a significant role in the storage of luminal calcium.

A conserved sequence motif in the integrin beta3 cytoplasmic domain is required for its specific interaction with beta3-endonexin

Integrin signaling is mediated by interaction of integrin cytoplasmic domains with intracellular signaling molecules. Recently, we identified a novel 111-amino acid polypeptide, termed beta3-endonexin, which interacts selectively with the integrin beta3 cytoplasmic domain. In the present study we conducted a systematic mutational analysis of both the integrin beta3 cytoplasmic domain and beta3-endonexin to map sites required for interaction. The interaction of the full-length beta3 integrin subunit with beta3-endonexin in vitro required the beta3 cytoplasmic domain. In a yeast two-hybrid system, both membrane-proximal and membrane-distal residues of the beta3 cytoplasmic domain were necessary for interaction with beta3-endonexin. In particular, the membrane-distal NITY motif at beta3 756-759 was critical for the interaction. Exchange of beta3 residues 756-759 (NITY) for the corresponding residues in beta1 (NPKY) endowed the beta1 cytoplasmic domain with the ability to interact with beta3-endonexin. Conversely, exchange of the NPKY motif at beta1 772-775 for the NITY motif in beta3 abolished interaction of this chimeric cytoplasmic domain with beta3-endonexin. Because the NITY motif is present in the beta3 but not the beta1 cytoplasmic domain, these results explain the selective interaction of this cytoplasmic domain with beta3-endonexin. In addition, deletional analysis suggested that a core 91-residue sequence of beta3-endonexin is sufficient for specific binding to the beta3 cytoplasmic domain. These studies have identified a cytoplasmic domain sequence motif that specifies an integrin-specific protein-protein interaction.