Contrasting roles for integrin beta 1 and beta 5 cytoplasmic domains in subcellular localization, cell proliferation, and cell migration

Integrins: role in cell adhesion and communication

Adhesive interactions are crucial for the integrity and function of all cells and tissues. As one of the major families of cell adhesion receptors, the integrins have been the focus of scientific interest for more than a decade. The resulting studies have tremendously enhanced the understanding of integrin-mediated adhesive interactions and have unveiled novel integrin functions in the cytoskeletal organization of microfilaments and in the activation of diverse signaling pathways. These functions are critically involved in the regulation of multiple processes, such as tissue development, inflammation, tumor cell growth and metastasis, and programmed cell death. The global view of integrin receptor biology has radically changed and has become much more subtle and elaborate. The enormous complexity of integrin function is determined by the heterodimeric formation of more than 20 functional integrin receptors, the cell type-specific distribution, the receptor activation state, the presence of different activation and deactivation signals, and the subsequent employment of distinct cytoskeletal and signaling complexes within a more dimensional network of time and space. This article summarizes the structural and functional properties of the integrin receptors and emphasizes some of the major achievements made in the past to enhance the understanding of integrin biology.

Maspin acts at the cell membrane to inhibit invasion and motility of mammary and prostatic cancer cells

Maspin, a novel serine protease inhibitor (serpin), inhibits tumor invasion and metastasis of mammary carcinoma. We show here that recombinant maspin protein blocks the motility of these carcinoma cells in culture over 12 h, as demonstrated by time-lapse video microscopy. Lamellopodia are withdrawn but ruffling continues. Both exogenous recombinant maspin and maspin expressed by tumor transfectants exhibit inhibitory effects on cell motility and cell invasion as shown in modified Boyden chamber assays. In addition, three prostatic cancer cell lines treated with recombinant maspin exhibited similar inhibition of both invasion and motility, suggesting a similar mode of maspin action in these two glandular epithelial cancers. When mammary carcinoma cells were treated with recombinant maspin, the protein was shown by immunostaining to bind specifically to the cell surface, suggesting that maspin activity is membrane associated. When pretreated with antimaspin antibody, maspin loses its inhibitory effects on both invasion and motility. However, when maspin is added to these cells preceding antibody treatment, the activity of maspin is no longer inhibited by subsequent addition of the antibody. It is concluded therefore that the inhibition of invasion and motility by maspin is initially localized to the cell surface.

Modulation of cell migration by integrin-mediated cytoskeletal linkages and ligand-binding affinity

Integrin cell surface adhesion receptors play a central role in mediating cell migration. We have developed a model system consisting of CHO cells ectopically expressing the alpha IIb beta 3 integrin to study integrin affinity and cytoskeletal interactions during cell migration. The alpha IIb beta 3 integrins are suited for study of integrin receptors during cell migration because they are well characterized with respect to ligand binding, cytoskeletal interactions, and signal transduction, and mutants with altered receptor function are available. The alpha IIb beta 3 receptor specifically mediates migration of alpha IIb beta 3-transfected CHO cells. The migration of transfected CHO cells was studied on a fibrinogen substrate both by time lapse videomicroscopy and by random and haptotactic transwell assays. Haptotactic and random transwell assays measured distinct aspects of migration, with the random transwell assay correlating most closely with time lapse videomicroscopy. Mutations in the cytoplasmic domains that increase ligand affinity or activation of the alpha IIb beta 3 receptor into a high affinity state by the LIBS6 antibody decreased the migration rate. Likewise, mutations that increase cytoskeletal organization without affecting affinity also decreased the migration rate. In contrast, truncation of the beta chain, which alters cytoskeletal associations as assayed by absence of focal adhesions, decreased haptotactic migration while increasing random migration. These effects on the migration rate were partially compensated for by altering substrate concentration, demonstrating optimum substrate concentrations that supported maximal migration. For example, cells expressing integrins locked in the high affinity state showed maximal migration at lower substrate concentrations than cells expressing low affinity receptor. Together, these results implicate the strength of adhesion between cell and substrate, as modulated by receptor affinity, organization of adhesive complexes, and substrate concentration, as important regulators of cell migration rate. Further, we demonstrate a dominant effect of high affinity integrin in inhibiting migration regardless of the organization of adhesive complexes. These observations have potential implications for tumor metastasis and its therapy.

Divalent cations regulate the organization of integrins alpha v beta 3 and alpha v beta 5 on the cell surface

Extracellular divalent cations are important regulators of integrin ligand binding activity. In this study we evaluated how divalent cations affect the organization of integrins into focal adhesion sites. Integrins alpha v beta 3 and alpha v beta 5 were compared because they share a high degree of structural homology and because both integrins mediate cell adhesion to vitronectin. On MG-63 osteosarcoma cells, we found that both the extent and pattern of integrin organization was regulated by the type of extracellular divalent ion. Integrin alpha v beta 3 organized in focal contacts when Mn2+ or Mg2+ was present, but not in Ca2+. In contrast, alpha v beta 5 organized in focal contacts only when Ca2+ or Mg2+ was present. Integrin alpha v beta 5 clustered in a centrally located punctate field on the ventral surface of the cell in the presence of Mn2+. These observations reveal a previously unappreciated role for divalent ions in regulating the organization of integrins into focal adhesion sites.

Protein kinase C regulates alpha v beta 5-dependent cytoskeletal associations and focal adhesion kinase phosphorylation

Integrins alpha v beta 3 and alpha v beta 5 both mediate cell adhesion to vitronectin yet trigger different postligand binding events. Integrin alpha v beta 3 is able to induce cell spreading, migration, angiogenesis, and tumor metastasis without additional stimulators, whereas alpha v beta 5 requires exogenous activation of protein kinase C (PKC) to mediate these processes. To investigate this difference, the ability of beta 3 or beta 5 to induce colocalization of intracellular proteins was assessed by immunofluorescence in hamster CS-1 melanoma cells. We found that alpha v beta 5 induced colocalization of talin, alpha-actinin, tensin, and actin very weakly relative to alpha v beta 3. alpha v beta 5 was able to efficiently induce colocalization of focal adhesion kinase (FAK); however, it was unable to increase phosphorylation of FAK on tyrosine. Activation of PKC by adding phorbol ester to alpha v beta 5-expressing cells induced spreading, increased colocalization of alpha-actinin, tensin, vinculin, p130cas and actin, and triggered tyrosine phosphorylation of FAK. Unexpectedly, talin colocalization remained low even after activation of PKC. Treatment of cells with the PKC inhibitor calphostin C inhibited spreading and the colocalization of talin, alpha-actinin, tensin, and actin for both alpha v beta 3 and alpha v beta 5. We conclude that PKC regulates localization of cytoskeletal proteins and phosphorylation of FAK induced by alpha v beta 5. Our results also show that FAK can be localized independent of its phosphorylation and that cells can spread and induce localization of other focal adhesion proteins in the absence of detectable talin.

Down-regulation of beta 1C integrin, an inhibitor of cell proliferation, in prostate carcinoma

The beta 1C integrin, a member of the cell adhesion receptor superfamily, is an alternatively spliced variant of the beta 1A subunit and, in contrast to its wild-type counterpart, inhibits cell proliferation in vitro. The expression of beta 1C integrin in tumor cell growth was investigated. In benign and neoplastic human prostate tissues, immunohistochemical analysis performed using affinity-purified antibodies specific for beta 1C demonstrated a predominant epithelial expression of beta 1C in benign prostate glands with marked staining of the apical, basal, and lateral surfaces. In the adjacent prostate adenocarcinoma glands, the beta 1C variant was dramatically down-regulated in 27 of 34 (79%) analyzed cases, whereas the expression and distribution of its wild-type counterpart, beta 1A, remained unchanged. Tumors exhibiting different Gleason's patterns showed that beta 1C was down-regulated in comparison with the benign tissue regardless of the histological grade. Immunoblotting analysis, using affinity-purified antibodies specific for beta 1C, was performed, in a quantitative manner, to compare beta 1C expression in benign and tumor prostate tissue. The results showed that beta 1C was expressed in benign prostate tissue whereas it was undetectable in prostate adenocarcinoma. Taken together, these data show that beta 1C integrin down-regulation in prostate tissues correlates with a neoplastic phenotype consistent with its in vitro growth-inhibitory properties. These findings indicate a novel pathophysiological role for this integrin variant in tumorigenesis.

Changes in endothelial cell and smooth muscle cell integrin expression during closure of the ductus arteriosus: an immunohistochemical comparison of the fetal, preterm newborn, and full-term newborn rhesus monkey ductus

Anatomical closure of the ductus arteriosus requires normally quiescent luminal endothelial cells and medial smooth muscle cells to migrate into the subendothelial space forming intimal mounds that eventually coalesce and occlude the vessel's lumen. The migration of endothelial cells and smooth muscle cells requires the presence of integrin receptors that interact with the surrounding matrix. We used immunohistochemical staining to examine the repertoires of integrins expressed by endothelial cells and smooth muscle cells during postnatal closure of the ductus arteriosus in full-term and preterm rhesus monkeys. In the fetal ductus, luminal endothelial cells have a limited repertoire of integrins. During postnatal ductus closure, luminal endothelial cells, of both term and preterm monkeys, change their phenotype and express the full repertoire of integrins found on growing capillary endothelial cells (alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha v beta 1, alpha 6 beta 4, and alpha v beta 5). Similarly, during ductus closure, smooth muscle cells of both term and preterm monkeys expand their integrin repertoire to include the alpha 5 beta 1 and alpha v beta 3 integrins; these two integrins have been shown to be essential for smooth muscle cell migration in vitro. These changes in integrin profile occur at the same time the endothelial and smooth muscle cells invade their neighboring compartments. In contrast, preterm monkeys with a persistently patent ductus lumen fail to develop these changes in integrin expression and fail to develop neointimal mounds. No evidence of intimal thickening occurs in the absence of changes in integrin expression. Therefore, endothelial cells and smooth muscle cells change phenotypes to produce the intimal thickening required for ductus closure.

Alpha L beta 2 integrin/LFA-1 binding to ICAM-1 induced by cytohesin-1, a cytoplasmic regulatory molecule

The avidity of integrin adhesion receptors for extracellular ligands is subject to dynamic regulation by intracellular programs that have yet to be elucidated. We describe here a protein, cytohesin-1, which specifically interacts with the intracellular portion of the integrin beta 2 chain (CD18). The molecule shows homology to the yeast SEC7 gene product and bears a pleckstrin homology (PH) domain. Overexpression of either the full-length cytohesin-1 or the SEC7 domain induces beta 2 integrin-dependent binding of Jurkat cells to ICAM-1, whereas expression of the isolated cytohesin-1 PH domain inhibits T cell receptor-stimulated adhesion. Similar inhibition is not exhibited by PH domains taken from other proteins, showing that the interaction is specific and that individual PH domains are capable of discriminating between alternative targets.

Serine 752 in the cytoplasmic domain of the beta 3 integrin subunit is not required for alpha v beta 3 postreceptor signaling events

A naturally occurring point mutation (Ser752Pro substitution) in the beta subunit cytoplasmic domain of the platelet fibrinogen receptor GPIIb-IIIa (integrin alpha IIb beta 3), causing Glanzmann's thrombasthenia, has been shown to abrogate bidirectional transmembrane signaling of GPIIb-IIIa when expressed in heterologous cells (Chen YP, 1994, Blood 84, 1857-1865). As the vitronectin receptor alpha v beta 3 constitutively mediates cell attachment to RGD containing extracellular matrix proteins, the purpose of this study was to explore the regulatory role of Ser752 in alpha v beta 3 vitronectin receptor function, by cotransfecting recombinant human alpha v cDNA together with human beta 3 mutant cDNA into Chinese hamster ovary (CHO) cells. CHO cells expressing wild type human alpha v beta 3 acquired the ability to attach and spread on fibrinogen and von Willebrand factor, in contrast to non transfected CHO cells that only bound to vitronectin and fibronectin. Overexpression of a truncated recombinant beta 3 subunit (beta 3 delta 744) generated alpha v (hamster) beta 3 (human) chimers that mediated attachment but lost the ability to promote cell spreading on vitronectin, von Willebrand factor and fibrinogen, and to concentrate in focal contact sites, demonstrating a negative effect of beta 3 delta 744 on alpha v beta 3 dependent postreceptor occupancy events. Transfection of beta 3Ser752Pro reproduced the same negative effect as beta 3 delta 744, whereas beta 3Ser752Ala restored normal receptor function by allowing pronounced attachment and spreading on fibrinogen and von Willebrand factor. Our results provide evidence that (1) the C-terminal cytoplasmic domain of beta 3 (amino acids 744-762) is essential for alpha v beta 3 integrin postreceptor occupancy events; (2) within this domain, the Ser752Pro mutation affects alpha v beta 3 postreceptor occupancy events by preventing cell spreading and focal contact localization; (3) the defective receptor function of the vitronectin receptor alpha v beta 3 is due to the presence of Pro752, rather than the absence of Ser752, as a Ser to Ala substitution at position 752 restores normal beta 3 integrin cell spreading and adhesive plaque formation.

Extracellular matrix and lung inflammation

Lung injury triggers an acute inflammatory response characterized by increased expression and deposition of extracellular matrix (ECM) components such as fibronectin and collagen. Although the function of newly deposited matrices in injured lungs is unknown, their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. Interactions between immune and nonimmune cells with the lung ECM are mediated via cell surface receptors of the integrin family which link the ECM with intracellular molecules involved in signal transduction. Activation of integrin-mediated intracellular signals may promote inflammation by facilitating leukocyte recruitment and cytokine expression.

Beta 1D integrin displaces the beta 1A isoform in striated muscles: localization at junctional structures and signaling potential in nonmuscle cells

The cytoplasmic domains of integrins provide attachment of these extracellular matrix receptors to the cytoskeleton and play a critical role in integrin-mediated signal transduction. In this report we describe the identification, expression, localization, and initial functional characterization of a novel form of beta 1 integrin, termed beta 1D. This isoform contains a unique alternatively spliced cytoplasmic domain of 50 amino acids, with the last 24 amino acids encoded by an additional exon. Of these 24 amino acids, 11 are conserved when compared to the beta 1A isoform, but 13 are unique (Zhidkova, N. I., A. M. Belkin, and R. Mayne. 1995. Biochem. Biophys. Res. Commun. 214:279-285; van der Flier, A., I. Kuikman, C. Baudoin, R, van der Neuf, and A. Sonnenberg. 1995. FEBS Lett. 369:340-344). Using an anti-peptide antibody against the beta 1D integrin subunit, we demonstrated that the beta 1D isoform is synthesized only in skeletal and cardiac muscles, while very low amounts of beta 1A were detected by immunoblot in striated muscles. Whereas beta 1A could not be detected in adult skeletal muscle fibers and cardiomyocytes by immunofluorescence, beta 1D was localized to the sarcolemma of both cell types. In skeletal muscle, beta 1D was concentrated in costameres, myotendinous, and neuromuscular junctions. In cardiac muscle this beta 1 isoform was found in costamers and intercalated discs. beta 1D was associated with alpha 7A and alpha 7B in adult skeletal muscle. In cardiomyocytes of adult heart, alpha 7B was the major partner for the beta 1D isoform. beta 1D could not be detected in proliferating C2C12 myoblasts, but it appeared immediately after myoblast fusion and its amount continued to rise during myotube growth and maturation. In contrast, expression of the beta 1A isoform was downregulated during myodifferentiation in culture and it was completely displaced by beta 1D in mature differentiated myotubes. We also analyzed some functional properties of the beta 1D integrin subunit. Expression of human beta 1D in CHO cells led to its localization at focal adhesions. Clustering of this integrin isoform on the cell surface stimulated tyrosine phosphorylation of pp125FAK (focal adhesion kinase) and caused transient activation of mitogen-activated protein (MAP) kinases. These data indicate that beta 1D and beta 1A integrin isoforms are functionally similar with regard to integrin-mediated signaling.

Chick sensory neuronal growth cones distinguish fibronectin from laminin by making substratum contacts that resemble focal contacts

The adhesive interactions of nerve growth cones stabilize elongating nerve fibers and mediate transmembrane signaling to regulate growth cone behaviors. We used interference reflection microscopy and immunocytochemistry to examine the dynamics and composition of substratum contacts that growth cones of chick sensory neurons make with extracellular adhesive glycoproteins, fibronectin and laminin. Interference reflection microscopy indicated that sensory neuronal growth cones on fibronectin-treated substrata, but not on laminin, make contacts that have the appearance and immobility of fibroblastic focal contacts. Interference reflection microscopy and subsequent immunocytochemical staining showed that beta 1 integrin and phosphotyrosine residues were concentrated at growth cone sites that resemble focal contacts. Two other components of focal contacts, paxillin and zyxin, were also co-localized with concentrated phosphotyrosine residues at sites that resemble focal contacts. Such staining patterns were not observed on laminin-treated substrata. Growth cone migration on fibronectin-treated substrata was inhibited by herbimycin A, a tyrosine kinase inhibitor. We conclude that sensory neuronal growth cones distinguish fibronectin from laminin by making contacts with distinct organization and regulation of cytoskeletal components at the adhesive sites. This finding suggests that growth cone interactions with different adhesive molecules lead to distinctive transmembrane organization and signaling to regulate nerve fiber elongation.

The novel structural motif Gln795-Gln802 in the integrin beta 1C cytoplasmic domain regulates cell proliferation

Alternative splicing of the integrin beta 1 subunit mRNA generates a variant form, beta 1C, with a unique cytoplasmic domain that differs from beta 1 for a 48-amino acid COOH-terminal sequence. The potential role of this unique sequence in modulating cellular functions was investigated using Chinese hamster ovary (CHO)1 cells transiently transfected with cDNAs coding for human integrin beta 1C or beta 1 subunits or mutants containing truncated forms of the beta 1C cytoplasmic domain. A differential effect of beta 1C and beta 1 on cell proliferation was observed. Expression of wild type beta 1 was associated with a 6-10-fold increase in cell proliferation in response to serum, as measured by [3H]thymidine incorporation. In contrast, only a 2-fold increase in cell proliferation was observed in transfectants expressing comparable levels of beta 1C. Cells expressing the beta 1C mutant truncated at Leu794 and lacking the last 31 amino acids of the cytoplasmic domain showed a 12-fold proliferation increase in response to serum. However, three beta 1C deletion mutants, lacking the COOH-terminal 23, 13, and 8 amino acids, which all contained residues Gln795-Gln802 of the variant cytoplasmic domain responded to serum stimulation with a 2-fold increase in [3H]thymidine uptake. The effect of beta 1C expression on cell proliferation was not associated with changes in exposure of integrin functional epitopes, as judged by the finding that CHO transfectants expressing beta 1C, full-length or deletion mutants, or beta 1 equally adhered to a functionally inhibitory monoclonal antibody against human beta 1 integrin. Expression of beta 1C inversely correlated with the mitogenic potential of vascular cells. Absent on growing cultured endothelial cells, surface expression of beta 1C was induced in growth-arrested, tumor necrosis factor-stimulated endothelial cells. These findings suggest that integrin alternative splicing may provide an accessory mechanism to modulate cell type-specific growth regulatory pathways during vascular cell injury in vivo.

An anti-alpha v-integrin antibody that blocks integrin function inhibits the development of a human melanoma in nude mice

A series of murine monoclonal antibodies were raised against purified human alpha v beta 3 integrin and against M21 human melanoma cells. Five notable hybridomas were identified by ELISA on purified integrins, and the isolated antibodies bound the alpha v-chain. These antibodies, 17E6, 20A9, 23G5, 14D9.F8 and 10G2, recognised the extracellular domains of the integrin, and were shown to be reactive in FACS, immunoprecipitation, ELISA, and ELISA on fixed cells with M21, M21-L4, and UCLA-P3, but not with the alpha v-deficient M21-L or M21-L-IIb (M21-L transfected with GpIIb integrin). One antibody, 17E6, strongly perturbed cell attachment mediated by alpha v integrins, reacting at least with alpha v beta 3, alpha v beta 5, and alpha v beta 1, and strongly inhibiting cell attachment to alpha v-ligands vitronectin and fibronectin with an IC50 of approximately 0.1 microgram ml-1. Furthermore, 17E6 at this concentration could induce cell retraction from the substrate, while LM609 (anti-alpha v beta 3) and control antibody 14E2 (anti-200 kDa melanoma surface protein) at 1,000-fold higher concentrations had minimal effects on cell morphology. The action of 17E6 was reversible and was not due to toxic effects: in vitro 17E6 at 0.1 mg ml-1 did not affect either cell proliferation or DNA synthesis. In two nude-mouse tumour models, subcutaneous tumour development and a lung colonisation (‘experimental metastasis’) assay, injection of 17E6 strongly inhibited tumour development, while isotype-matched controls had no effect. There was no obvious mechanism of cell or of complement-mediated tumour cytotoxicity; the antibody did not mediate ADCC or AECDC, or complement fixation. The data strongly support previous studies which have indicated the importance of alpha v-integrins, and especially alpha v beta 3, in the tumour progression of human melanoma.

Specific association of CD63 with the VLA-3 and VLA-6 integrins

We screened monoclonal antibodies to cell-surface proteins and selected an antibody, called 6H1, that recognizes a putative integrin-associated protein. The 6H1 monoclonal antibody (mAb) indirectly coprecipitated alpha 3 beta 1 and/or alpha 6 beta 1, but not alpha 2 beta 1, or alpha 5 beta 1 from Brij 96 detergent lysates of multiple cell lines. Large scale purification using the 6H1 mAb yielded a single protein of 45-60 kDa with an amino-terminal sequence that exactly matched CD63. Confirming that the 6H1 mAb recognized the CD63 protein, 6H1 and a known anti-CD63 mAb yielded identical coprecipitation results and identical colocalization into lysosomal granules containing cathepsin D. Furthermore, we used an established anti-CD63 mAb to detect this protein in an alpha 3 beta 1 immunoprecipitate, and also we observed VLA-3 and CD63 colocalization in cellular "footprints." Notably, the cytoplasmic domain of alpha 3 was neither required nor sufficient for CD63 association, suggesting that it occurred elsewhere within the alpha 3 beta 1 complex. Knowledge of these specific CD63-alpha 3 beta 1 and CD63-alpha 6 beta 1 biochemical associations should lead to critical insights into the specialized functions of alpha 3 beta 1, alpha 6 beta 1, and CD63.

Specialized functional properties of the integrin alpha 4 cytoplasmic domain

For functional studies of the integrin alpha 4 cytoplasmic domain, we have expressed the following in K562 and Chinese hamster ovary (CHO) cells: 1) wild-type alpha 4 (called X4C4), 2) two chimeric forms of alpha 4 (called X4C2 and X4C5) that contain the cytoplasmic domains of alpha 2 and alpha 5, respectively, and 3) alpha 4 with no cytoplasmic domain (X4C0). Cytoplasmic domain exchange had no effect on VLA-4-dependent static cell adhesion or tethering to VCAM-1 in conditions of shear flow. However, the presence of the alpha 2 or alpha 5 tails markedly enhanced VLA-4-dependent K562 cells spreading (X4C2 > X4C5 > X4C4 > X4C0), increased localization of VLA-4 into focal adhesion-like complexes in CHO cells (X4C2 > X4C5 > X4C4), and strengthened CHO and K562 cell resistance to detachment from VCAM-1 in conditions of shear flow (X4C2 > X4C5 > X4C4 > X4C0). Conversely, the alpha 4 tail supported greater VLA-4-dependent haptotactic and chemotactic cell migration. In the absence of any alpha tail (i.e., X4C0), robust focal adhesions were observed, even though cell spreading and adhesion strengthening were minimal. Thus, such focal adhesions may have relatively little functional importance, and should not be compared with focal adhesions formed when alpha tails are present. Together, these results indicate that all three alpha-chain tails exert defined positive effects (compared with no tail at all), but suggest that the alpha 4 cytoplasmic domain may be specialized to engage in weaker cytoskeletal interactions, leading to diminished focal adhesion formation, cell spreading, and adhesion strengthening, while augmenting cell migration and facilitating rolling under shear flow. These properties of the alpha 4 tail are consistent with the role of alpha 4 integrins on highly motile lymphocytes, monocytes, and eosinophils.

Beta 8 integrins mediate interactions of chick sensory neurons with laminin-1, collagen IV, and fibronectin

Integrins are major receptors used by cells to interact with extracellular matrices. In this paper, we identify the first ligands for the beta 8 family of integrins, presenting evidence that integrin heterodimers containing the beta 8 subunit mediate interactions of chick sensory neurons with laminin-1, collagen IV, and fibronectin. A polyclonal antibody, anti-beta 8-Ex, was prepared to a bacterial fusion protein expressing an extracellular portion of the chicken beta 8 subunit. In nonreducing conditions, this antibody immunoprecipitated from surface-labeled embryonic dorsal root ganglia neurons a M(r) 100 k protein, the expected M(r) of the beta 8 subunit, and putative alpha subunit(s) of M(r) 120 k. Affinity-purified anti-beta 8-Ex strongly inhibited sensory neurite outgrowth on laminin-1, collagen IV, and fibronectin-coated substrata. Binding sites were identified in a heat-resistant domain in laminin-1 and in the carboxyl terminal, 40-kDa fibronectin fragment. On substrates coated with the carboxyl terminal fragment of fibronectin, antibodies to beta 1 and beta 8 were only partially effective alone, but were completely effective in combination, at inhibiting neurite outgrowth. Results thus indicate that the integrin beta 8 subunit in association with one or more alpha subunits forms an important set of extracellular matrix receptors on sensory neurons.

Integrin alpha 8 beta 1 promotes attachment, cell spreading, and neurite outgrowth on fibronectin

The integrin alpha 8 subunit, isolated by low stringency hybridization, is a novel integrin subunit that associates with beta 1. To identify ligands, we have prepared a function-blocking antiserum to the extracellular domain of alpha 8, and we have established by transfection K562 cell lines that stably express alpha 8 beta 1 heterodimers on the cell surface. We demonstrate here by cell adhesion and neurite outgrowth assays that alpha 8 beta 1 is a fibronectin receptor. Studies on fibronectin fragments using RGD peptides as inhibitors show that alpha 8 beta 1 binds to the RGD site of fibronectin. In contrast to the endogenous alpha 5 beta 1 fibronectin receptor in K562 cells, alpha 8 beta 1 not only promotes cell attachment but also extensive cell spreading, suggesting functional differences between the two receptors. In chick embryo fibroblasts, alpha 8 beta 1 is localized to focal adhesions. We conclude that alpha 8 beta 1 is a receptor for fibronectin and can promote attachment, cell spreading, and neurite outgrowth on fibronectin.

Involvement of alpha v beta 3 integrin in mediating fibrin gel retraction

Platelet integrin alpha IIb beta 3 (GPIIb-IIIa) plays important roles in platelet-mediated clot retraction. However, little is known about the mechanisms of clot retraction mediated by nucleated cells. In this report, we demonstrate that another member of the beta 3 integrin family, alpha v beta 3, is involved in clot retraction mediated by nucleated cells. Retraction of fibrin clots was observed using a human melanoma cell line, C32TG, which contains no alpha IIb beta 3 complex. This retraction was inhibited by RGD-containing peptide, monoclonal anti-beta 3, and anti-alpha v beta 3 antibodies. Immunoelectron microscopic studies revealed a direct interaction between beta 3 integrin and fibrin fibers at an early stage of clot retraction. We found that another human embryonal cell line, 293, which is known to express alpha v beta 1, but no alpha v beta 3, lacks fibrin gel retractile activity. Upon transfection of beta 3 DNA into 293 cells, the beta 3 subunit formed a complex with an endogenous alpha v subunit. The beta 3-bearing transfectants were found to retract fibrin gels, which was specifically inhibited by anti-beta 3 antibody. In addition, a point mutation at Asp119 in the beta 3 ligand binding domain abolished the clot retractile activity of 293 transfectants, indicating the requirement of alpha v beta 3 ligand-binding activity. Our findings suggest that alpha v beta 3 is involved in mediating the interaction between the three-dimensional fibrin network and nucleated cells and in promoting "post-receptor occupancy" events.

A developmental analysis of oligodendroglial integrins in primary cells: changes in alpha v-associated beta subunits during differentiation

We have examined the expression of integrins on primary oligodendroglial cells during the differentiation of the proliferative oligodendrocyte precursor (O-2A progenitor) cell to the postmitotic oligodendrocyte. Cells of the oligodendrocyte lineage expressed a limited repertoire of integrins: alpha 6 beta 1 and alpha v integrins including alpha v beta 1, alpha v beta 3 and alpha v beta 5, as well as a potentially novel integrin alpha v beta 80 kDa. Integrin expression was developmentally regulated; during differentiation alpha v beta 1 was reduced and alpha v beta 5 upregulated. These results suggest that laminin and vitronectin are important extracellular matrix ligands for oligodendrocytes, and provide a rational explanation for previous observations that RGD peptides inhibit the expression of myelin-specific genes. They also suggest a simple model by which switching of integrin beta subunits might regulate differentiation. As chimeric beta 1 integrins with a beta 5 cytoplasmic domain support proliferation less well than normal beta 1 integrins (Pasqualini and Hemler (1994), J. Cell Biol. 125, 447–460) the switch from alpha v beta 1 to alpha v beta 5 might play a key instructive role in the cessation of proliferation and subsequent differentiation.

Differential perturbations in the morphogenesis of anterior structures induced by overexpression of truncated XB- and N-cadherins in Xenopus embryos

Cadherins, a family of Ca-dependent adhesion molecules, have been proposed to act as regulators of morphogenetic processes and to be major effectors in the maintenance of tissue integrity. In this study, we have compared the effects of the expression of two truncated cadherins during early neurogenesis in Xenopus laevis. mRNA encoding deleted forms of XB- and N-cadherin lacking most of the extracellular domain were injected into the four animal dorsal blastomeres of 32-cell stage Xenopus embryos. These truncated cadherins altered the cohesion of cells derived from the injected blastomeres and induced morphogenetic defects in the anterior neural tissue to which they chiefly contributed. Truncated XB-cadherin was more efficient than N-cadherin in inducing these perturbations. Moreover, the coexpression of both truncated cadherins had additive perturbation effects on neural development. The two truncated cadherins can interact with the three known catenins, but with distinct affinities. These results suggest that the adhesive signal mediated by cadherins can be perturbed by overexpressing their cytoplasmic domains by competing with different affinity with catenins and/or a common anchor structure. Therefore, the correct regulation of cadherin function through the cytoplasmic domain appears to be a crucial step in the formation of the neural tissue.

Single subunit chimeric integrins as mimics and inhibitors of endogenous integrin functions in receptor localization, cell spreading and migration, and matrix assembly

The ability of single subunit chimeric receptors containing various integrin beta intracellular domains to mimic and/or inhibit endogenous integrin function was examined. Chimeric receptors consisting of the extracellular and transmembrane domains of the small subunit of the human interleukin-2 receptor connected to either the beta 1, beta 3, beta 3B, or beta 5 intracellular domain were transiently expressed in normal human fibroblasts. When expressed at relatively low levels, the beta 3 and beta 5 chimeras mimicked endogenous ligand-occupied integrins and, like the beta 1 chimera (LaFlamme, S. E., S. K. Akiyama, and K. M. Yamada. 1992. J. Cell Biol. 117:437), concentrated with endogenous integrins in focal adhesions and sites of fibronectin fibril formation. In contrast, the chimeric receptor containing the beta 3B intracellular domain (a beta 3 intracellular domain modified by alternative splicing) was expressed diffusely on the cell surface, indicating that alternative splicing can regulate integrin receptor distribution by an intracellular mechanism. Furthermore, when expressed at higher levels, the beta 1 and beta 3 chimeric receptors functioned as dominant negative mutants and inhibited endogenous integrin function in localization to fibronectin fibrils, fibronectin matrix assembly, cell spreading, and cell migration. The beta 5 chimera was a less effective inhibitor, and the beta 3B chimera and the reporter lacking an intracellular domain did not inhibit endogenous integrin function. Comparison of the relative levels of expression of the transfected beta 1 chimera and the endogenous beta 1 subunit indicated that in 10 to 15 h assays, the beta 1 chimera can inhibit cell spreading when expressed at levels approximately equal to the endogenous beta 1 subunit. Levels of chimeric receptor expression that inhibited cell spreading also inhibited cell migration, whereas lower levels were able to inhibit alpha 5 beta 1 localization to fibrils and matrix assembly. Our results indicate that single subunit chimeric integrins can mimic and/or inhibit endogenous integrin receptor function, presumably by interacting with cytoplasmic components critical for endogenous integrin function. Our results also demonstrate that beta intracellular domains, expressed in this context, display specificity in their abilities to mimic and inhibit endogenous integrin function. Furthermore, the approach that we have used permits the analysis of intracellular domain function in the processes of cell spreading, migration and extracellular matrix assembly independent of effects due to the rest of integrin dimers. This approach should prove valuable in the further analysis of integrin intracellular domain function in these and other integrin-mediated processes requiring the interaction of integrins with cytoplasmic components.

Integrin alpha 2 cytoplasmic domain deletion effects: loss of adhesive activity parallels ligand-independent recruitment into focal adhesions

Chinese hamster ovary (CHO) cells transfected with the integrin alpha 2 subunit formed a stable VLA-2 heterodimer that mediated cell adhesion to collagen. Within CHO cells spread on collagen, but not fibronectin, wild-type alpha 2 subunit localized into focal adhesion complexes (FACs). In contrast, alpha 2 with a deleted cytoplasmic domain was recruited into FACs whether CHO cells were spread on collagen or fibronectin. Thus, as previously seen for other integrins, the alpha 2 cytoplasmic domain acts as a negative regulator, preventing indiscriminate integrin recruitment into FACs. Notably, ligand-independent localization of the VLA-2 alpha 2 subunit into FACs was partially prevented if only one or two amino acids were present in the alpha 2 cytoplasmic domain (beyond the conserved GFFKR motif) and was completely prevented by four to seven amino acids. The addition of two alanine residues (added to GFFKR) also partially prevented ligand-independent localization. In a striking inverse correlation, the same mutants showing increased ligand-independent recruitment into FACs exhibited diminished alpha 2-dependent adhesion to collagen. Thus, control of VLA-2 localization may be closely related to the suppression of cell adhesion to collagen. In contrast to FAC localization and collagen adhesion results, VLA-2-dependent binding and infection by echovirus were unaffected by either alpha 2 cytoplasmic domain deletion or exchange with other cytoplasmic domains.