Fibronectin isoform distribution in the mouse. I. The alternatively spliced EIIIB, EIIIA, and V segments show widespread codistribution in the developing mouse embryo

Deletion of the alternatively spliced fibronectin EIIIA domain in mice reduces atherosclerosis

The alternatively spliced and highly conserved EIIIA domain of fibronectin (FN) is included in most FN of the extracellular matrix in embryos. In adults, both extracellular matrix and plasma FN essentially lack EIIIA. In diverse inflammatory situations however, EIIIA is specifically included by regulated RNA splicing. In atherosclerotic lesions, FN, including the EIIIA domain (EIIIA-FN), is abundant, whereas FN in the flanking vessel wall lacks EIIIA. Lesional EIIIA-FN is localized with endothelial cells and macrophage foam cells. To directly test the function of EIIIA-FN, we generated EIIIA-null (EIIIA–/–) mice that lack the EIIIA exon and crossed them with apolipoprotein E (ApoE)–null (ApoE–/–) mice that develop arterial wall lesions. Compared with ApoE–/– controls, EIIIA–/–ApoE–/– mice had significantly smaller lesions throughout the aortic tree. EIIIA-FN was increased in ApoE–/– plasma, and total plasma cholesterol was reduced in EIIIA–/–ApoE–/– mice, specifically in large lipoprotein particles, suggesting a functional role for plasma EIIIA-FN. To assess a role for macrophage EIIIA-FN in the vessel wall, we conducted in vitro foam cell assays. EIIIA–/–ApoE–/– macrophages accumulated significantly less intracellular lipid than control ApoE–/– cells. These results provide genetic evidence that suggests roles for EIIIA-FN in plasma lipoprotein metabolism and in foam cell formation.

Assembly of exogenous fibronectin by fibronectin-null cells is dependent on the adhesive substrate

The role of endogenously synthesized fibronectin (FN) in assembly was studied with cells lacking or expressing FN. Cells were cultured as homogeneous or mixed populations on surfaces coated with different matrix proteins. Compared with FN-expressing cells, FN-null cells poorly assembled exogenous plasma FN (pFN) when adhered to vitronectin or the recombinant cell-binding domain (III(7-10)) of FN. Vitronectin had a suppressive effect that was overcome by co-adsorbed pFN or laminin-1 but not by soluble FN. In co-cultures of FN-expressing cells and FN-null cells, endogenous FN was preferentially assembled around FN-expressing cells regardless of the adhesive ligand. If the adhesive ligand was vitronectin, exogenous pFN assembled preferentially around cells expressing cellular FN or recombinant EDa- or EDa+ FN. In co-cultures on vitronectin of FN-null cells and beta(1) integrin subunit-null cells, fibrils of cellular FN and pFN were preferentially deposited by FN-null (beta(1)-expressing) cells immediately adjacent to (FN-secreting) beta(1)-null cells. In co-cultures on vitronectin of FN-null cells and beta(1)-null cells expressing a chimera with the extracellular domain of beta(1) and the cytoplasmic domain of beta(3), preferential assembly was by the chimera-expressing cells. These results indicate that the adhesive ligand is a determinant of FN assembly by cells not secreting endogenous FN (suppressive if vitronectin, non-suppressive but non-supportive if III(7-10), supportive if pFN or laminin-1) and suggest that efficient interaction of freshly secreted cellular FN with a beta(1) integrin, presumably alpha(5)beta(1), substitutes for integrin-mediated adherence to a preformed matrix of laminin-1 or pFN to support assembly of FN.

Mammary epithelial-mesenchymal interaction regulates fibronectin alternative splicing via phosphatidylinositol 3-kinase

The way alternative splicing is regulated within tissues is not understood. A relevant model of this process is provided by fibronectin, an important extracellular matrix protein that plays a key role in cell adhesion and migration and contains three alternatively spliced regions known as EDI, EDII, and IIICS. We used a cell culture system to simulate mammary epithelial-stromal communication, a process that is crucial for patterning and function of the mammary gland, and studied the effects of extracellular signals on the regulation of fibronectin pre-mRNA alternative splicing. We found that soluble factors from a mammary mesenchymal cell-conditioned medium, as well as the growth factors HGF/SF (hepatocyte growth factor/scatter factor), KGF (keratinocyte growth factor), and aFGF (acidic fibroblast growth factor), stimulate EDI and IIICS but not EDII inclusion into fibronectin mRNA in the mammary epithelial cell line SCp2, favoring fibronectin isoforms associated with proliferation, migration, and tissue remodeling. We explored the signaling pathways involved in this regulation and found that the mammary mesenchymal cell-conditioned medium and HGF/SF act through a phosphatidylinositol 3-kinase-dependent cascade to alter fibronectin alternative splicing. This splicing regulation is independent from promoter structure and de novo protein synthesis but does require two exonic elements within EDI. These results shed light on how extracellular stimuli are converted into changes in splicing patterns.

Plasma levels of fibronectin bearing the alternatively spliced EIIIB segment are increased after major trauma

Using Western-blot analysis and enzyme-linked immunosorbent assay (ELISA) of N-deglycosylated samples, we have observed that plasma levels of fibronectin (FN) bearing the alternatively spliced EIIIB segment (EIIIB(+) FN) increase in patients after admission to the intensive-care unit (ICU) for acute major trauma. Although not increased at the first sampling ("0 hour"), taken within 24 hours of ICU admission, levels measured 24, 48, and 72 hours later were significantly increased compared with levels obtained in healthy controls. Furthermore, average concentrations at these latter time points were significantly increased relative to the 0-hour sampling. EIIIB(+) FN levels then decreased in plasma samples taken 1 month after hospital discharge, such that no significant difference was found between ELISA-measured values at this time and 0 hour or control values. On the basis of comparisons with previous measurements in these samples, it is apparent that after acute major trauma, circulating levels of soluble EIIIB(+) FN exhibit temporal changes that are qualitatively similar to those encountered for FN isoforms bearing the alternatively spliced EIIIA segment (EIIIA(+) FN), yet different from those observed for the total pool of circulating FN. This is the first report of measurement of FN in clinical plasma samples with antibodies specific for the highly conserved EIIIB segment. Like EIIIA(+) forms of FN, EIIIB(+) FNs are recognized as soluble bloodborne markers for vascular tissue injury.

Alternative splicing during chondrogenesis: modulation of fibronectin exon EIIIA splicing by SR proteins

The alternative exon EIIIA of the fibronectin gene is included in mRNAs produced in undifferentiated mesenchymal cells but excluded from differentiated chondrocytes. As members of the SR protein family of splicing factors have been demonstrated to be involved in the alternative splicing of other mRNAs, the role of SR proteins in chondrogenesis-associated EIIIA splicing was investigated. SR proteins interacted with chick exon EIIIA sequences that are required for exon inclusion in a gel mobility shift assay. Addition of SR proteins to in vitro splicing reactions increased the rate and extent of exon EIIIA inclusion. Co-transfection studies employing cDNAs encoding individual SR proteins revealed that SRp20 decreased mRNA accumulation in HeLa cells, which make A+ mRNA, apparently by interfering with pre-mRNA splicing. Co-transfection studies also demonstrated that SRp40 increased exon EIIIA inclusion in chondrocytes, but not in HeLa cells, suggesting the importance of cellular context for SR protein activity. Immunoblot analysis did not reveal a relative depletion of SRp40 in chondrocytic cells. Possible mechanisms for regulation of EIIIA splicing in particular, and chondrogenesis associated splicing in general, are discussed.

Spatial expression of the alternatively spliced EIIIB and EIIIA segments of fibronectin in the early chicken embryo

Using domain-specific antibodies, we have analyzed the tissue distribution of fibronectins (FNs) containing the alternatively spliced EIIIB and EIIIA segments relative to total FN in early chicken embryos. The results show a selective loss of EIIIA+ FN staining in the notochordal sheath and in cartilaginous structures between 4.5 and 7.0 days of development. In other regions, EIIIB+ and EIIIA+ FNs are extensively codistributed in and around mesoderm-derived structures (somites, notochord, heart, and blood vessels), in basal laminae of endoderm and ectoderm-derived structures, as well as within the vicinity of neural crest formation and migration. We also noted that EIIIA staining overlaps with spatial patterns of distribution that have previously been described for the alpha4 integrin subunit, a component of the EIIIA receptor alpha4beta1.

The EIIIA segment of fibronectin is a ligand for integrins alpha 9beta 1 and alpha 4beta 1 providing a novel mechanism for regulating cell adhesion by alternative splicing

Alternative splicing of the fibronectin gene transcript gives rise to forms that include the EIIIA (or ED-A) segment. EIIIA-containing fibronectins are prominently expressed during embryogenesis and wound healing and appear to mediate changes in cell adhesion and gene expression. Nonetheless, integrins that bind the EIIIA segment have not been identified. We previously mapped the epitope for two function-blocking monoclonal antibodies to the C-C' loop region of the EIIIA segment (Liao, Y.-F., Wieder, K. G., Classen, J. M., and Van De Water, L. (1999) J. Biol. Chem. 274, 17876-17884). The sequence of this epitope ((39)PEDGIHELFP(48)) resembles the sequence within tenascin-C to which the integrin alpha(9)beta(1) binds. We now report that either integrin alpha(9)beta(1) or alpha(4)beta(1) can mediate cell adhesion to the EIIIA segment. Moreover, this interaction is blocked both by epitope-mapped EIIIA antibodies as well as by the respective anti-integrins. Deletion mutants of the EIIIA segment that include the C-C' loop and flanking sequence bind cells expressing either alpha(9)beta(1) or alpha(4)beta(1). Adhesion of alpha(4)beta(1)-containing MOLT-3 cells to the EIIIA segment stimulates phosphorylation of p44/42 MAP kinase. Our observation that two integrins bind the EIIIA segment establishes a novel mechanism by which cell adhesion to fibronectin is regulated by alternative splicing.

Preferential recognition of a fragment species of osteoarthritic synovial fluid fibronectin by antibodies to the alternatively spliced EIIIA segment

OBJECTIVE

To characterize the species of synovial fluid (SF) fibronectin (FN) bearing the alternatively spliced EIIIA segment.

METHODS

SF from patients with osteoarthritis (OA) and rheumatoid arthritis (RA), as well as corresponding affinity isolation products, were subjected to 1-dimensional and 2-dimensional electrophoresis followed by Western blot analysis.

RESULTS

Regardless of the clinical type of arthritis, a polyclonal antibody that recognizes antigenic determinants throughout the FN molecule produced staining of predominantly approximately 200+ and approximately 170-kd species in reduced 1-dimensional electrophoresis. Despite the overall prevalence of the larger species, 4 monoclonal antibodies (mAb) reactive with sequences lying near the center of the EIIIA segment exhibited a relative failure to recognize the larger of these 2 species in OA, but not RA, SF. The absence of recognition of EIIIA sequences within the approximately 200+ kd forms of OA SF FN was unrelated to their derivation from dimers, since anti-EIIIA mAb recognized the smaller fragment species in preference to both monomeric and dimeric forms. The approximately 170-kd EIIIA+ fragments were observed to have minimal gelatin-binding capacity and appeared on 2-dimensional electrophoresis to extend from the N-terminus of FN through at least the center of the EIIIA segment. Similar results were obtained for samples obtained by needle aspiration or arthroscopic lavage, suggesting a widespread applicability of these findings.

CONCLUSION

The approximately 170-kd EIIIA+ species of FN could potentially constitute a soluble "vehicle" by which chondrocyte-regulating EIIIA sequences, liberated from inhibitory flanking C-terminal sequences, could reach cells in the arthritic joint. Additionally, "FN species-specific" recognition of this segment within OA SF could constitute a marker by which to gauge the activity of the OA disease process.

Expression of fibronectin splice variants in the postischemic rat kidney

Using an in vivo rat model of unilateral renal ischemia, we previously showed that the expression and distribution of fibronectin (FN), a major glycoprotein of plasma and the extracellular matrix, dramatically changes in response to ischemia-reperfusion. In the distal nephron in particular, FN accumulates in tubular lumens, where it may contribute to obstruction. In the present study, we examine whether the tubular FN is the plasma or cellular form, each of which is produced by alternative splicing of a single gene transcript. We demonstrate that FN in tubular lumens does not contain the extra type III A (EIIIA) and/or the extra type III B (EIIIB) region, both of which are unique to cellular FN. It does, however, contain the V95 region, which in the rat is a component of FNs in both plasma and the extracellular matrix. Expression of FN containing EIIIA increases dramatically in the renal interstitium after ischemic injury and continues to be produced at high levels 6 wk later. V95-containing FN also increases in the interstitial space, albeit more slowly and at lower levels than FN containing EIIIA; it also persists 6 wk later. FN containing the EIIIB region is not expressed in the injured kidney. The presence of V95 but not the EIIIA or EIIIB regions of FN in tubular lumens identifies the origin of FN in this location as the plasma; tubular FN is ultimately voided in the urine. The data indicate that both plasma and cellular FNs containing the V95 and/or EIIIA regions may contribute to the pathogenesis of acute renal failure and to the repair of the injured kidney.

Integrin receptors are required for cell survival and proliferation during development of the peripheral glial lineage

Proliferation and survival of Schwann cells are important for nerve development and for disease processes in peripheral nerves. We have analyzed embryos lacking alpha4- or alpha5-integrins and show here that these integrins contribute to the control of glial cell numbers. To overcome early embryonic lethality an explant and grafting system that allows the study of isolated glial progenitor cells both in vitro and in vivo was used. Schwann cells differentiate in the absence of alpha5 but their numbers and the proliferation rate of early progenitor cells are reduced, suggesting that alpha5 is essential for normal proliferation. Survival, rather than proliferation, is compromised in alpha4-deficient explants. Conditional immortalization allowed further characterization and revealed that alpha4 contributes to survival in a cell-density-dependent fashion. In addition, transplants into chicken embryos were used to analyze in vivo cell migration and showed that cell death occurs mainly in highly motile, individually migrating cells. The cell death patterns in vitro and in vivo argue that alpha4-integrins play a role in survival during cell migration. Neural crest migration has been suggested to require these integrins; however, no defects in migration were observed in the absence of alpha4 or alpha5. We conclude that integrins can complement growth factors in the control of glial cell numbers.

Role of the extracellular matrix during neural crest cell migration

Once specified to become neural crest (NC), cells occupying the dorsal portion of the neural tube disrupt their cadherin-mediated cell-cell contacts, acquire motile properties, and embark upon an extensive migration through the embryo to reach their ultimate phenotype-specific sites. The understanding of how this movement is regulated is still rather fragmentary due to the complexity of the cellular and molecular interactions involved. An additional intricate aspect of the regulation of NC cell movement is that the timings, modes and patterns of NC cell migration are intimately associated with the concomitant phenotypic diversification that cells undergo during their migratory phase and the fact that these changes modulate the way that moving cells interact with their microenvironment. To date, two interplaying mechanisms appear central for the guidance of the migrating NC cells through the embryo: one involves secreted signalling molecules acting through their cognate protein kinase/phosphatase-type receptors and the other is contributed by the multivalent interactions of the cells with their surrounding extracellular matrix (ECM). The latter ones seem fundamental in light of the central morphogenetic role played by the intracellular signals transduced through the cytoskeleton upon integrin ligation, and the convergence of these signalling cascades with those triggered by cadherins, survival/growth factor receptors, gap junctional communications, and stretch-activated calcium channels. The elucidation of the importance of the ECM during NC cell movement is presently favoured by the augmenting knowledge about the macromolecular structure of the specific ECM assembled during NC development and the functional assaying of its individual constituents via molecular and genetic manipulations. Collectively, these data propose that NC cell migration may be governed by time- and space-dependent alterations in the expression of inhibitory ECM components; the relative ratio of permissive versus non-permissive ECM components; and the supramolecular assembly of permissive ECM components. Six multidomain ECM constituents encoded by a corresponding number of genes appear to date the master ECM molecules in the control of NC cell movement. These are fibronectin, laminin isoforms 1 and 8, aggrecan, and PG-M/version isoforms V0 and V1. This review revisits a number of original observations in amphibian and avian embryos and discusses them in light of more recent experimental data to explain how the interaction of moving NC cells with these ECM components may be coordinated to guide cells toward their final sites during the process of organogenesis.

Identification of two amino acids within the EIIIA (ED-A) segment of fibronectin constituting the epitope for two function-blocking monoclonal antibodies

Alternative splicing of the fibronectin gene transcript gives rise to a group of adhesive glycoproteins showing restricted spatial and temporal expression during embryonic development, tumor growth, and tissue repair. Alternative splicing occurs in three segments termed EIIIB, EIIIA, and V. The EIIIA (or ED-A) segment of fibronectin is expressed prominently but transiently in healing wounds coincident with fibroblast expression of an activation marker, smooth muscle cell alpha-actin. A monoclonal antibody (IST-9) to the EIIIA segment blocks transforming growth factor-beta-mediated smooth muscle cell alpha-actin expression by fibroblasts in culture. A second monoclonal antibody (DH1) blocks chondrocyte condensation in chicken embryos. We find that IST-9 and DH1 react with human, rat, and chicken but not with mouse or frog EIIIA, suggesting that His44 may be important for antibody binding. A series of deletion mutants of rat EIIIA, constructed as glutathione S-transferase fusion proteins, do not react with either IST-9, DH1, or a third monoclonal antibody (3E2). Mutations of pairs of amino acids to alanine have little effect, except for either (Val34Thr35) or (Tyr36Ser37), which are located in a beta strand upstream from His44. For these double mutants, the binding to all three monoclonal antibodies is markedly reduced. By contrast, single mutants at Thr35, Tyr36, or Ser37 retain full activity, suggesting that the epitope for these antibodies is determined in part by conformation. Alanine-scanning mutagenesis of rat EIIIA demonstrates the importance of Ile43 and His44 for binding. Mutation of frog EIIIA (normally Val43Lys44) to rat (Ile43His44) is sufficient to restore fully IST-9 binding and much of the activity of DH1 and 3E2. Our findings demonstrate that the function-blocking antibodies, IST-9 and DH1, bind to the Ile43 and His44 residues in a conformationally dependent fashion, implicating the loop region encompassing both residues as critical for mediating EIIIA function.

Expression of embryonic fibronectin isoform EIIIA parallels alpha-smooth muscle actin in maturing and diseased kidney

In this study we examined if an association exists between expression of an alternatively spliced "embryonic" fibronectin isoform EIIIA (Fn-EIIIA) and alpha-smooth muscle actin (alpha-SMA) in the maturing and adult rat kidney and in two unrelated models of glomerular disease, passive accelerated anti-glomerular basement membrane (GBM) nephritis and Habu venom (HV)-induced proliferative glomerulonephritis, using immunohistochemistry and in situ hybridization. Fn-EIIIA and alpha-SMA proteins were abundantly expressed in mesangium and in periglomerular and peritubular interstitium of 20-day embryonic and 7-day (D-7) postnatal kidneys in regions of tubule and glomerular development. Staining was markedly reduced in these structures in maturing juvenile (D-14) kidney and was largely lost in adult kidney. Expression of Fn-EIIIA and alpha-SMA was reinitiated in the mesangium and the periglomerular and peritubular interstitium in both models and was also observed in glomerular crescents in anti-GBM nephritis. Increased expression of Fn-EIIIA mRNA by in situ hybridization corresponded to the localization of protein staining. Dual labeling experiments verified co-localization of Fn-EIIIA and alpha-SMA, showing a strong correlation of staining between location and staining intensity during kidney development, maturation, and disease. Expression of EIIIA mRNA corresponded to protein expression in developing and diseased kidneys and was lost in adult kidney. These studies show a recapitulation of the co-expression of Fn-EIIIA and alpha-SMA in anti-GBM disease and suggest a functional link for these two proteins.

Extracellular matrix-related genes in kidney after ischemic injury: potential role for TGF-beta in repair

The renal expression of transforming growth factor-beta1 (TGF-beta1) is enhanced following induction of ischemic injury in rat. In cultured renal cells, TGF-beta stimulates the synthesis of extracellular matrix. To link TGF-beta1 expression with the regulation of extracellular matrix postischemia, we characterized the expression of several genes known to regulate extracellular matrix synthesis at various times during recovery from acute ischemic renal injury in rat. Levels of mRNA for plasminogen activator inhibitor-1 (PAI-1), tissue inhibitor of metalloprotease-1 (TIMP-1), alpha1(IV) collagen, and fibronectin-EIIIA (FN-EIIIA) mRNAs were significantly enhanced in kidneys within 12 h to 3 days after injury and remained elevated at 7-28 days postischemia relative to levels in kidneys of sham-operated controls. PAI-1 mRNA and peptide were localized in regenerating proximal tubules at 3 and 7 days postischemic injury. alpha1(IV) Collagen and FN-EIIIA mRNAs were expressed primarily in regenerating proximal tubule cells. Immunoreactivity for FN-EIIIA was enhanced in the tubular basement membrane (TBM) of regenerating proximal tubules, and alpha1(IV) collagen immunoreactivity was detected in thickened tubulointerstitial spaces. In contrast, TIMP-1 immunoreactivity was enhanced in distal nephron structures postischemia. Immunoneutralization of TGF-beta in vivo attenuated the increases in FN-EIIIA, alpha1(IV) collagen, PAI-1, and TIMP-1 mRNAs by 52%, 73%, 43%, and 27%, respectively. These data are consistent with TGF-beta expression postischemic injury participating in renal regeneration of extracellular matrix homeostasis in the proximal TBM.

Alternative splicing of the fibronectin EIIIB exon depends on specific TGCATG repeats

The fibronectin EIIIB exon is alternatively spliced in a cell-type-specific manner, and TGCATG repeats in the intron downstream of EIIIB have been implicated in this regulation. Analysis of the intron sequence from several vertebrates shows that the pattern of repeats in the 3' half of the intron is evolutionarily conserved. Point mutations in certain highly conserved repeats greatly reduce EIIIB inclusion, suggesting that a multicomponent complex may recognize the repeats. Expression of the SR protein SRp40, SRp20, or ASF/SF2 stimulates EIIIB inclusion. Studies of the interplay between mutations in the repeats and SRp40-stimulated inclusion suggest that the repeats are recognized in many, if not all, cell types, and that EIIIB inclusion may be regulated by quantitative changes in multiple factors.

Fibronectins Are Essential for Heart and Blood Vessel Morphogenesis But Are Dispensable for Initial Specification of Precursor Cells

The underlying mechanisms of lethal cardiovascular defects associated with the fibronectin-null (FN.null) mutation in mouse embryos were investigated by lineage analysis of myocardial, endocardial, and endothelial cells. A wide variation in phenotype was observed on two genetic backgrounds. In the less severe class (C57/BL6 background), FN.null embryos display a defective heart. Myocardial cells express the specific marker MF-20 and are correctly localized in the anterior trunk region, but myocardial organization is disrupted, resulting in a bulbous heart tube. Endocardial cells express the specific marker platelet-endothelial cell adhesion molecule-1 (PECAM-1) and are localized within the myocardium, but the endocardium appears collapsed. Endothelial cells of two vascular beds are specified, but the aortae are distended and lack contact with the surrounding mesenchyme, while no vessels form in the yolk sac. Defects in the more severe class suggest that FNs are essential earlier in development on the 129/Sv background. Myocardial and endocardial cells are specified, but morphogenesis of the myocardium and endocardium does not occur. Aortic endothelial cells are specified and localized normally, but remain scattered. Yolk sac endothelial cells resemble those of the less severe class. We conclude that FNs are essential for organization of heart and blood vessels, but are dispensable for cellular specification in the appropriate regions within the embryo.

The region encoded by the alternatively spliced exon IIIA in mesenchymal fibronectin appears essential for chondrogenesis at the level of cellular condensation

Fibronectin in the extracellular matrix of tissues acts as a substrate for cell adhesion and migration during development. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. Fibronectin mRNA alternative splicing patterns change from B+A+V+ to B+A-V+ during chondrogenesis. In this report, immunohistochemical analysis demonstrates that while fibronectin protein containing the region encoded by exon IIIB is present throughout the limb at all stages of development, fibronectin protein containing the region encoded by exon IIIA disappears from cartilaginous regions just after condensation in vivo and in high-density mesenchymal micromass cultures in vitro. Treatment of mesenchymal micromass cultures prior to condensation with an antibody specific for the region encoded by exon IIIA disrupts the formation of cellular condensations and inhibits subsequent chondrogenesis in a dose- and time-dependent manner. Furthermore, microinjection of the exon IIIA antibody into embryonic chick limb primordia in vivo results in malformations characterized by smaller limbs and loss of limb skeletal elements. These results strongly suggest that the presence of the region encoded by exon IIIA in mesenchymal fibronectin is necessary for the condensation event that occurs during chondrogenesis.

alpha3beta1 Integrin is required for normal development of the epidermal basement membrane

Integrins alpha3beta1 and alpha6beta4 are abundant receptors on keratinocytes for laminin-5, a major component of the basement membrane between the epidermis and the dermis in skin. These integrins are recruited to distinct adhesion structures within keratinocytes; alpha6beta4 is present in hemidesmosomes, while alpha3beta1 is recruited into focal contacts in cultured cells. To determine whether differences in localization reflect distinct functions of these integrins in the epidermis, we studied skin development in alpha3beta1-deficient mice. Examination of extracellular matrix by immunofluorescence microscopy and electron microscopy revealed regions of disorganized basement membrane in alpha3beta1-deficient skin. Disorganized matrix was first detected by day 15.5 of embryonic development and became progressively more extensive as development proceeded. In neonatal skin, matrix disorganization was frequently accompanied by blistering at the dermal-epidermal junction. Laminin-5 and other matrix proteins remained associated with both the dermal and epidermal sides of blisters, suggesting rupture of the basement membrane itself, rather than detachment of the epidermis from the basement membrane as occurs in some blistering disorders such as epidermolysis bullosa. Consistent with this notion, primary keratinocytes from alpha3beta1-deficient skin adhered to laminin-5 through alpha6 integrins. However, alpha3beta1-deficient keratinocytes spread poorly compared with wild-type cells on laminin-5, demonstrating a postattachment requirement for alpha3beta1 and indicating distinct roles for alpha3beta1 and alpha6beta4. Our findings support a novel role for alpha3beta1 in establishment and/or maintenance of basement membrane integrity, while alpha6beta4 is required for stable adhesion of the epidermis to the basement membrane through hemidesmosomes.