Integrin alpha6beta1-laminin interactions regulate early myotome formation in the mouse embryo

We addressed the potential role of cell-laminin interactions during epaxial myotome formation in the mouse embryo. Assembly of the myotomal laminin matrix occurs as epaxial myogenic precursor cells enter the myotome. Most Myf5-positive and myogenin-negative myogenic precursor cells localise near assembled laminin, while myogenin-expressing cells are located either away from this matrix or in areas where it is being assembled. In Myf5(nlacZ/nlacZ) (Myf5-null) embryos, laminin, collagen type IV and perlecan are present extracellularly near myogenic precursor cells, but do not form a basement membrane and cells are not contained in the myotomal compartment. Unlike wild-type myogenic precursor cells, Myf5-null cells do not express the alpha6beta1 integrin, a laminin receptor, suggesting that integrin alpha6beta1-laminin interactions are required for myotomal laminin matrix assembly. Blocking alpha6beta1-laminin binding in cultured wild-type mouse embryo explants resulted in dispersion of Myf5-positive cells, a phenotype also seen in Myf5(nlacZ/nlacZ) embryos. Furthermore, inhibition of alpha6beta1 resulted in an increase in Myf5 protein and ectopic myogenin expression in dermomyotomal cells, suggesting that alpha6beta1-laminin interactions normally repress myogenesis in the dermomyotome. We conclude that Myf5 is required for maintaining alpha6beta1 expression on myogenic precursor cells, and that alpha6beta1 is necessary for myotomal laminin matrix assembly and cell guidance into the myotome. Engagement of laminin by alpha6beta1 also plays a role in maintaining the undifferentiated state of cells in the dermomyotome prior to their entry into the myotome.     

The major laminin receptor of mouse embryonic stem cells is a novel isoform of the alpha 6 beta 1 integrin

Laminin is the first extracellular matrix protein expressed in the developing mouse embryo. It is known to influence morphogenesis and affect cell migration and polarization. Several laminin receptors are included in the integrin family of extracellular matrix receptors. Ligand binding by integrin heterodimers results in signal transduction events controlling cell motility. We report that the major laminin receptor on murine embryonic stem (ES) cells is the integrin heterodimer alpha 6 beta 1, an important receptor for laminin in neurons, lymphocytes, macrophages, fibroblasts, platelets and other cell types. However, the cytoplasmic domain of the ES cell alpha 6 (alpha 6 B) differs totally from the reported cytoplasmic domain amino acid sequence of alpha 6 (alpha 6 A). Comparisons of alpha 6 cDNAs from ES cells and other cells suggest that the alpha 6 A and alpha 6 B cytoplasmic domains derive from alternative mRNA splicing. Anti-peptide antibodies to alpha 6 A are unreactive with ES cells, but react with mouse melanoma cells and embryonic fibroblasts. When ES cells are cultured under conditions that permit their differentiation, they become positive for alpha 6 A, concurrent with the morphologic appearance of differentiated cell types. Thus, expression of the alpha 6 B beta 1 laminin receptor may be favored in undifferentiated, totipotent cells, while the expression of alpha 6 A beta 1 receptor occurs in committed lineages. While the functions of integrin alpha chain cytoplasmic domains are not understood, it is possible that they contribute to transferring signals to the cell interior, e.g., by delivering cytoskeleton organizing signals in response to integrin engagement with extracellular matrix ligands. It is therefore reasonable to propose that the cellular responses to laminin may vary, according to what alpha subunit isoform (alpha 6 A or alpha 6 B) is expressed as part of the alpha 6 beta 1 laminin receptor. The switch from alpha 6 B to alpha 6 A, if confirmed in early embryos, could then be of striking potential relevance to the developmental role of laminin.     

Laminin receptors in the retina: sequence analysis of the chick integrin alpha 6 subunit. Evidence for transcriptional and posttranslational regulation.

The integrin alpha 6 beta 1 is a prominent laminin receptor used by many cell types. In the present work, we isolate clones and determine the primary sequence of the chick integrin alpha 6 subunit. We show that alpha 6 beta 1 is a prominent integrin expressed by cells in the developing chick retina. Between embryonic days 6 and 12, both retinal ganglion cells and other retinal neurons lose selected integrin functions, including the ability to attach and extend neurites on laminin. In retinal ganglion cells, we show that this is correlated with a dramatic decrease in alpha 6 mRNA and protein, suggesting that changes in gene expression account for the developmental regulation of the interactions of these neurons with laminin. In other retinal neurons the expression of alpha 6 mRNA and protein remains high while function is lost, suggesting that the function of the alpha 6 beta 1 heterodimer in these cells is regulated by posttranslational mechanisms.     

Antibodies against domain E3 of laminin-1 and integrin alpha 6 subunit perturb branching epithelial morphogenesis of submandibular gland, but by different modes

Branching epithelial morphogenesis requires interactions between the surrounding mesenchyme and the epithelium, as well as interactions between basement membrane components and the epithelium. Embryonic submandibular gland was used to study the roles of two mesenchymal proteins, epimorphin and tenascin-C, as well as the epithelial protein laminin-1 and one of its integrin receptors on branching morphogenesis. Laminin-1 is a heterotrimer composed of an alpha 1 chain and two smaller chains (beta 1 and gamma 1). Immunofluorescence revealed a transient expression of laminin alpha 1 chain in the epithelial basement membrane during early stages of branching morphogenesis. Other laminin-1 chains and alpha 6, beta 1, and beta 4 integrin subunits seemed to be expressed constitutively. Expression of epimorphin, but not tenascin-C, was seen in the mesenchyme during early developmental stages, but a mAb against epimorphin did not perturb branching morphogenesis of this early epithelium. In contrast, inhibition of branching morphogenesis was seen with a mAb against the carboxy terminus of laminin alpha 1 chain, the E3 domain. An inhibition of branching was also seen with a mAb against the integrin alpha 6 subunit. The antibodies against laminin alpha 1 chain and integrin alpha 6 subunit perturbed development in distinct fashions. Whereas treatment with the anti-E3 resulted in discontinuities of the basement membrane at the tips of the branching epithelium, treatment with the mAb against alpha 6 integrin subunit seemed to leave the basement membrane intact. We suggest that the laminin E3 domain is involved in basement membrane formation, whereas alpha 6 beta 1 integrin binding to laminin-1 may elicit differentiation signals to the epithelial cells.     

Beta1 integrins activate a MAPK signalling pathway in neural stem cells that contributes to their maintenance

The emerging evidence that stem cells develop in specialised niches highlights the potential role of environmental factors in their regulation. Here we examine the role of beta1 integrin/extracellular matrix interactions in neural stem cells. We find high levels of beta1 integrin expression in the stem-cell containing regions of the embryonic CNS, with associated expression of the laminin alpha2 chain. Expression levels of laminin alpha2 are reduced in the postnatal CNS, but a population of cells expressing high levels of beta1 remains. Using neurospheres - aggregate cultures, derived from single stem cells, that have a three-dimensional architecture that results in the localisation of the stem cell population around the edge of the sphere - we show directly that beta1 integrins are expressed at high levels on neural stem cells and can be used for their selection. MAPK, but not PI3K, signalling is required for neural stem cell maintenance, as assessed by neurosphere formation, and inhibition or genetic ablation of beta1 integrin using cre/lox technology reduces the level of MAPK activity. We conclude that integrins are therefore an important part of the signalling mechanisms that control neural stem cell behaviour in specific areas of the CNS.     

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.     

Distribution of integrins and their ligands in the trunk of Xenopus laevis during neural crest cell migration

We have examined the distribution in Xenopus embryos of beta 1 subunits of integrin, as recognized by cross-reactive antibodies against the avian integrin beta 1 subunit. These antibodies recognize a doublet of bands of approximately 120 kD in Xenopus embryos. The distribution pattern of these integrin cell surface receptors was compared with that of two possible ligands, fibronectin and laminin, in the extracellular matrix during the time of neural crest cell migration. Integrin immunoreactivity in the early neurula was observed lightly outlining somite and epidermal cells and the notochord. The integrin immunostaining increased with developmental age and was observed on most cell types in the embryo but was particularly notable in the intersomitic clefts through which motoraxons grow. The immunoreactivity in this region was not, however, wholly on the axon surfaces, since intersomitic integrin remained detectable in embryos in which the neural tube had been ablated. Fibronectin and laminin were more extensively distributed than integrin at all stages examined. Immunoreactivity for both was observed around the neural tube, notochord, somites, epidermis, dorsal mesentery, and lateral plate mesoderm. The distribution of laminin and fibronectin around the somites was particularly interesting since it was non-uniform and similar to that of integrin. Strongest staining was observed in the intersomitic clefts, and weakest staining was observed on the medial surface of the somites, which faces the neural tube and notochord. The major differences in distribution pattern between the fibronectin and laminin immunoreactivities were that only fibronectin was detected in the mesenchyme of the dorsal fin. Our results demonstrate that a molecule homologous to avian integrin is present in Xenopus embryos during neural crest cell migration and motoraxon outgrowth. Its presence in the intersomitic clefts and on the surface of many embryonic cell types together with the abundant distribution of its ligands are consistent with a potentially important developmental function in neurite outgrowth and/or muscle development.     

Developmentally regulated expression and functional role of alpha 7 integrin in the chick embryo

Integrin alpha 7 beta 1 is a specific cellular receptor for laminin. In the present work, we studied the distribution pattern of the alpha 7 subunit by immunofluorescence and immunoprecipitation and the role of the integrin by blocking antibodies in early chick embryos. alpha 7 immunoreactivity was first detectable in the neural plate during neural furrow formation (stage HH5, early neurula, Hamburger & Hamilton 1951) and its expression was upregulated in the neural folds during primary neurulation. The alpha 7 expression domain spanned the entire neural tube by stage HH8 (4 somites), and was then downregulated and confined to the neuroepithelial cells in the germinal region near the lumen and the ventrolateral margins of the neural tube in embryos by the onset of stage HH17 (29 somites). Expression of alpha 7 in the neural tube was transient suggesting that alpha 7 functions during neural tube closure and axon guidance and may not be required for neuronal differentiation or for the maintenance of the differentiated cell types. alpha 7 immunoreactivity was strong in the newly formed epithelial somites, although this expression was restricted only to the myotome in the mature somites. The most intense alpha 7 immunoreactivity was detectable in the paired heart primordia and the endoderm apposing the heart primordia in embryos at stage HH8. In the developing heart, alpha 7 immunoreactivity was: (i) intense in the myocardium; (ii) milder in the endocardial cushions of the ventricle; (iii) intense in the sinus venosus; (iv) distinct in the associated blood vessels; and (v) undetectable in the dorsal mesocardium of embryos at stage HH17. Inhibition of function of alpha 7 by blocking antibodies showed that alpha 7 integrin-laminin signaling may play a critical role in tissue organization of the neural plate and neural tube closure, in tissue morphogenesis of the heart tube but not in the directional migration of pre-cardiac cells, and in somite epithelialization but not in segment formation in presomitic mesoderm. In embryos treated with alpha 7 antibody, the formation of median somites in place of a notochord was intriguing and suggested that alpha 7 integrin-laminin signaling may have played a role in segment re-specification in the mesoderm.     

Neuronal determination without cell division in Xenopus embryos

Cell division in the Xenopus CNS was blocked by incubating embryos in a mixture of the DNA synthesis inhibitors hydroxyurea and aphidicolin. Surprisingly, embryos treated at the beginning of gastrulation proceeded normally through neurulation, neural tube closure, and CNS subdivision. Thus, cell division is not critical for neural induction or early morphogenetic events in the CNS. Neuroblasts in treated embryos differentiated into neurons of many classes, indicating that cellular determination in the CNS can be dissociated from lineage and birth date. Axonal tracts and embryonic reflexes also developed. The remarkable amount of normal CNS development that occurs in these animals may be explained by a series of sequential inductions that are largely independent of cell proliferation.     

Opposing roles of integrin alpha6Abeta1 and dystroglycan in laminin-mediated extracellular signal-regulated kinase activation

Laminin-integrin interactions can in some settings activate the extracellular signal-regulated kinases (ERKs) but the control mechanisms are poorly understood. Herein, we studied ERK activation in response to two laminins isoforms (-1 and -10/11) in two epithelial cell lines. Both cell lines expressed beta1-containing integrins and dystroglycan but lacked integrin alpha6beta4. Antibody perturbation assays showed that both cell lines bound to laminin-10/11 via the alpha3beta1and alpha6beta1 integrins. Although laminin-10/11 was a stronger adhesion complex than laminin-1 for both cell lines, both laminins activated ERK in only one of the two cell lines. The ERK activation was mediated by integrin alpha6beta1 and not by alpha3beta1 or dystroglycan. Instead, we found that dystroglycan-binding domains of both laminin-1 and -10/11 suppressed integrin alpha6beta1-mediated ERK activation. Moreover, the responding cell line expressed the two integrin alpha6 splice variants, alpha6A and alpha6B, whereas the nonresponding cell line expressed only alpha6B. Furthermore, ERK activation was seen in cells transfected with the integrin alpha6A subunit, but not in alpha6B-transfected cells. We conclude that laminin-1 and -10/11 share the ability to induce ERK activation, that this is regulated by integrin alpha6Abeta1, and suggest a novel role for dystroglycan-binding laminin domains as suppressors of this activation.     

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

Integrins are essential for the development of the two genetically tractable invertebrate model organisms, the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Just two integrins are present in C. elegans: one putative RGD binding integrin alphapat-2betapat-3, corresponding to Drosophila alphaPS2betaPS and vertebrate alpha5beta1, alphaVbeta1 and alpha8beta1, and one putative laminin binding integrin alphaina-1betapat-3, corresponding to Drosophila alphaPS1betaPS and vertebrate alpha3beta1, alpha6beta1 and alpha7beta1. In this review, the function of this minimal set of integrins during the development of these two invertebrates is compared. Despite the differences in bodyplan and developmental strategy, integrin adhesion to the extracellular matrix is required for similar processes: the formation of the link that translates muscle contraction into movement of the exoskeleton, cell migration, and morphogenetic interactions between epithelia. Other integrin functions, such as regulation of gene expression, have not yet been experimentally demonstrated in both organisms. Additional proteins have been characterised in each organism that are essential for integrin function, including extracellular matrix ligands and intracellular interacting proteins, but so far different proteins have been found in the two organisms. This in part represents the fact that the characterisation of the full set of interacting proteins is not complete in either system. However, in other cases different proteins appear to be used for similar functions in the two animals. The continued use of genetic approaches to identify proteins required for integrin function in these two model organisms should lead to the identification of the minimal set of conserved components that form integrin adhesive structures.     

Expression of alpha 1 integrin, a laminin-collagen receptor, during myogenesis and neurogenesis in the avian embryo.

In this study, we have examined the spatiotemporal distribution of the alpha 1 integrin subunit, a putative laminin and collagen receptor, in avian embryos, using immunofluorescence microscopy and immunoblotting techniques. We used an antibody raised against a gizzard 175 x 10(3) M(r) membrane protein which was described previously and which we found to be immunologically identical to the chicken alpha 1 integrin subunit. In adult avian tissues, alpha 1 integrin exhibited a very restricted pattern of expression; it was detected only in smooth muscle and in capillary endothelial cells. In the developing embryo, alpha 1 integrin subunit expression was discovered in addition to smooth muscle and capillary endothelial cells, transiently, in both central and peripheral nervous systems and in striated muscles, in association with laminin and collagen IV. alpha 1 integrin was practically absent from most epithelial tissues, including the liver, pancreas and kidney tubules, and was weakly expressed by tissues that were not associated with laminin and collagen IV. In the nervous system, alpha 1 integrin subunit expression occurred predominantly at the time of early neuronal differentiation. During skeletal muscle development, alpha 1 integrin was expressed on myogenic precursors, during myoblast migration, and in differentiating myotubes. alpha 1 integrin disappeared from skeletal muscle cells as they became contractile. In visceral and vascular smooth muscles, alpha 1 integrin appeared specifically during early smooth muscle cell differentiation and, later, was permanently expressed after cell maturation. These results indicate that (i) the expression pattern of alpha 1 integrin is consistent with a function as a laminin/collagen IV receptor; (ii) during avian development, expression of the alpha 1 integrin subunit is spatially and temporally regulated; (iii) during myogenesis and neurogenesis, expression of alpha 1 integrin is transient and correlates with cell migration and differentiation.     

Disruption of transforming growth factor beta-regulated laminin receptor function by expression of antisense laminin, a chain RNA in human colon cancer cells

Transforming growth factor beta1 (TGFbeta) simultaneously induces the expression of fibronectin, fibronectin receptor, laminin, and laminin receptor (alpha6beta1 integrin) in the human colon cancer cell line Moser (Int J Cancer, 57:742, 1994). Induction of fibronectin and induction of fibronectin receptor by TGFB are tightly coupled, and disrupting fibronectin induction disrupts the induction of fibronectin receptor and cellular adhesion to fibronectin (J Cellular Physiol, 170:138, 1997). We recently demonstrated the efficacy of using antisense chain-specific laminin RNA expression vectors to disrupt the induction by TGFP of the multichain laminin molecule (J Cellular Physiol, 178:296, 1999). We now show in this report that Moser cells used alpha6 and beta1 integrins to adhere to laminin, and, as is the fibronectin and fibronectin receptor system, disrupting the induction by TGFbeta of the ligand laminin by the expression of antisense laminin A chain RNA disrupted the induction of 125I-laminin binding and cellular adhesion to laminin. Disrupting laminin induction also blocked the induction of alpha6 and beta1 integrin laminin receptor by TGFbeta. We conclude that disrupting the induction of the ligand laminin by TGFbeta disrupts TGFbeta-regulated laminin receptor function by suppressing the induction of alpha6 and beta1 integrins. Therefore, targeted disruption of the ligand laminin may be an effective means in disrupting the function of both the ligand and its receptor in cells that utilize the laminin and laminin receptor system in malignant cell behavior.     

Implantation-associated changes in bovine uterine expression of integrins and extracellular matrix

Appropriate integrin expression appears to be necessary for successful implantation of human embryos and varies considerably among species. The present study was undertaken to determine the distributions of integrin subunits alpha(1), alpha(3), and alpha(6) as well as the extracellular matrix (ECM) components collagen IV and laminin in implanting bovine trophoblast and endometrium. Immunohistochemical staining of cryostat sections prepared from nonpregnant endometrium, of preattachment through to early villus development pregnant endometrium (Days 18, 21, 24, and 30), and of isolated trophoblast binucleate cells was performed. Trophoblast down-regulated the integrin alpha(1) subunit as attachment proceeded, whereas reactivity scores for alpha(6) antibody tended to increase from Day 18 through 24 and remained high. A subpopulation of trophoblast binucleate cells expressed the alpha(3) integrin subunit. Uterine epithelium constitutively expressed alpha(3) and alpha(6) integrin subunits, but the alpha(1) subunit was down-regulated as the luminal epithelium was modified. Collagen IV and laminin reactivity increased in the basal lamina and underlying subepithelial stroma as pregnancy proceeded. The results suggest that binucleate cell fusion with the maternal epithelium initiates integrin and ECM changes in the subepithelial stroma.     

Retinoid signaling is required to complete the vertebrate cardiac left/right asymmetry pathway

Vitamin A-deficient (VAD) quail embryos have severe abnormalities, including a high incidence of reversed cardiac situs. Using this model we examined in vivo the physiological function of vitamin A in the left/right (L/R) cardiac asymmetry pathway. Molecular analysis reveals the expression of early asymmetry genes activin receptor IIa, sonic hedgehog, Caronte, Lefty-1, and Fgf8 to be unaffected by the lack of retinoids, while expression of the downstream genes nodal-related, snail-related (cSnR), and Pitx2 is altered. In VAD embryos nodal expression in left lateral plate mesoderm (LPM) is severely downregulated and the expression domain altered during neurulation. Similarly, the expression of cSnR in the right LPM and of Pitx2 in the left side posterior heart-forming region (HFR) is downregulated in the VAD embryos. The lack of retinoids does not cause randomization or ectopic expression of nodal, cSnR, or Pitx2. At the six- to eight-somite stage nodal is expressed transiently in the left posterior HFR of normal quail embryos; this expression is missing in VAD embryos and may be linked to the loss of Pitx2 expression in this region of VAD quail embryos. Administration of retinoids to VAD embryos prior to the six-somite stage rescues the expression of nodal, cSnR, and Pitx2 as well as the randomized VAD cardiac phenotype. There is an absolute requirement for retinoids at the four- to five-somite developmental window for cardiogenesis and cardiac L/R specification to proceed normally. We conclude that retinoids do not regulate the left/right-specific sidedness assignments for expression of genes on the vertebrate cardiac asymmetry pathway, but are required during neurulation for the maintenance of adequate levels of their expression and for the development of the posterior heart tube and a loopable heart. Cardiac asymmetry may be but one of several critical events regulated by retinoid signaling in the retinoid-sensitive developmental window.     

Targeted disruption of the LAMA3 gene in mice reveals abnormalities in survival and late stage differentiation of epithelial cells.

Laminin 5 regulates anchorage and motility of epithelial cells through integrins alpha6beta4 and alpha3beta1, respectively. We used targeted disruption of the LAMA3 gene, which encodes the alpha3 subunit of laminin 5 and other isoforms, to examine developmental functions that are regulated by adhesion to the basement membrane (BM). In homozygous null animals, profound epithelial abnormalities were detected that resulted in neonatal lethality, consistent with removal of all alpha3-laminin isoforms from epithelial BMs. Alterations in three different cellular functions were identified. First, using a novel tissue adhesion assay, we found that the mutant BM could not induce stable adhesion by integrin alpha6beta4, consistent with the presence of junctional blisters and abnormal hemidesmosomes. In the absence of laminin 5 function, we were able to detect a new ligand for integrin alpha3beta1 in the epidermal BM, suggesting that basal keratinocytes can utilize integrin alpha3beta1 to interact with an alternative ligand. Second, we identified a survival defect in mutant epithelial cells that could be rescued by exogenous laminin 5, collagen, or an antibody against integrin alpha6beta4, suggesting that signaling through beta1 or beta4 integrins is sufficient for survival. Third, we detected abnormalities in ameloblast differentiation in developing mutant incisors indicating that events downstream of adhesion are affected in mutant animals. These results indicate that laminin 5 has an important role in regulating tissue organization, gene expression, and survival of epithelium.     

Differential distribution and modulation of expression of alpha 1/beta 1 integrin on human endothelial cells

In this paper we report that the integrin complex alpha 1/beta 1, a laminin/collagen receptor, is expressed on cultured foreskin microvascular endothelium, but is absent on endothelial cells from large vessels such as the aorta and umbilical and femoral veins. The restricted expression of integrin alpha 1/beta 1 to microvascular endothelium was also demonstrated in vivo, by immunohistochemical staining of human tissue sections. Alpha 1 specific antibodies reacted strongly with endothelial cells of small blood vessels and capillaries in several tissues, but not with endothelium of vein and arteries of umbilical cord. Expression of integrin alpha 1 can be induced in cultured umbilical vein endothelial cells by treatment with 5 ng/ml tumor necrosis factor alpha (TNF alpha). Induction of alpha 1 subunit expression also occurred after treatment of umbilical vein endothelium with 10(-5) M retinoic acid or with 10 nM PMA; Maximal induction of alpha 1 integrin was reached after 48 h of treatment and costimulation with TNF alpha and PMA resulted in a synergistic effect. The induction of alpha 1 integrin changed the adhesive properties of umbilical vein endothelial cells, by increasing the adhesiveness to collagen, laminin, and laminin fragment P1, while adhesion to fibronectin and laminin fragment E8 remained constant. The alpha 1 integrin is thus a marker of a specific population of endothelial cells and its expression confers distinctive properties of interaction with the underlying basal membrane.