A complex of platelet glycoproteins Ic and IIa identified by a rat monoclonal antibody

Imaging and quantitative analysis of integrin-dependent cell-matrix adhesions

Integrin-dependent cell-extracellular matrix adhesion is essential for wound healing, embryonic development, immunity, and tissue organization. Here, we present a protocol for the imaging and quantitative analysis of integrin-dependent cell-matrix adhesions. We describe steps for cell culture; virus preparation; lentiviral transduction; imaging with widefield, confocal, and total internal reflection fluorescence microscopy; and using a script for their quantitative analysis. We then detail procedures for analyzing adhesion dynamics by live-cell imaging and fluorescence recovery after photobleaching (FRAP). For complete details on the use and execution of this protocol, please refer to Margadant et al. (2012),1 van der Bijl et al. (2020),2 Amado-Azevedo et al. (2021).3.

FER regulates endosomal recycling and is a predictor for adjuvant taxane benefit in breast cancer

Elevated expression of non-receptor tyrosine kinase FER is an independent prognosticator that correlates with poor survival of high-grade and basal/triple-negative breast cancer (TNBC) patients. Here, we show that high FER levels are also associated with improved outcomes after adjuvant taxane-based combination chemotherapy in high-risk, HER2-negative patients. In TNBC cells, we observe a causal relation between high FER levels and sensitivity to taxanes. Proteomics and mechanistic studies demonstrate that FER regulates endosomal recycling, a microtubule-dependent process that underpins breast cancer cell invasion. Using chemical genetics, we identify DCTN2 as a FER substrate. Our work indicates that the DCTN2 tyrosine 6 is essential for the development of tubular recycling domains in early endosomes and subsequent propagation of TNBC cell invasion in 3D. In conclusion, we show that high FER expression promotes endosomal recycling and represents a candidate predictive marker for the benefit of adjuvant taxane-containing chemotherapy in high-risk patients, including TNBC patients.

ECM-integrin signalling instructs cellular position-sensing to pattern the early mouse embryo

Development entails patterned emergence of diverse cell types within the embryo. In mammals, cells positioned inside the embryo give rise to the inner cell mass (ICM), which eventually forms the embryo itself. Yet, the molecular basis of how these cells recognise their ‘inside’ position to instruct their fate is unknown. Here, we show that provision of extracellular matrix (ECM) to isolated embryonic cells induces ICM specification and alters the subsequent spatial arrangement between epiblast (EPI) and primitive endoderm (PrE) cells that emerge within the ICM. Notably, this effect is dependent on integrin β1 activity and involves apical-to-basal conversion of cell polarity. We demonstrate that ECM-integrin activity is sufficient for ‘inside’ positional signalling and is required for correct EPI/PrE patterning. Thus, our findings highlight the significance of ECM-integrin adhesion in enabling position sensing by cells to achieve tissue patterning.

Summary: The importance of patterned cell-extracellular matrix (ECM) interactions in early mouse development: ECM signals can modulate both cell fate and the relative spatial arrangement between cells.

Absence of integrin α3β1 promotes the progression of HER2-driven breast cancer in vivo

Background

HER2-driven breast cancer is correlated with poor prognosis, especially during its later stages. Numerous studies have shown the importance of the integrin α3β1 during the initiation and progression of breast cancer; however, its role in this disease is complex and often opposite during different stages and in different types of tumors. In this study, we aim to elucidate the role of integrin α3β1 in a genetically engineered mouse model of HER2-driven mammary tumorigenesis.

Methods

To investigate the role of α3β1 in HER2-driven tumorigenesis in vivo, we generated a HER2-driven MMTV-cNeu mouse model of mammary tumorigenesis with targeted deletion of Itga3 (Itga3 KO mice). We have further used several established triple-negative and HER2-overexpressing human mammary carcinoma cell lines and generated ITGA3-knockout cells to investigate the role of α3β1 in vitro. Invasion of cells was assessed using Matrigel- and Matrigel/collagen I-coated Transwell assays under static or interstitial fluid flow conditions. The role of α3β1 in initial adhesion to laminin and collagen was assessed using adhesion assays and immunofluorescence.

Results

Tumor onset in mice was independent of the presence of α3β1. In contrast, the depletion of α3β1 reduced the survival of mice and increased tumor growth and vascularization. Furthermore, Itga3 KO mice were significantly more likely to develop lung metastases and had an increased metastatic burden compared to WT mice. In vitro, the deletion of ITGA3 caused a significant increase in the cellular invasion of HER2-overexpressing SKBR3, AU565, and BT474 cells, but not of triple-negative MDA-MB-231. This invasion suppressing function of α3β1 in HER2-driven cells depended on the composition of the extracellular matrix and the interstitial fluid flow.

Conclusion

Downregulation of α3β1 in a HER2-driven mouse model and in HER2-overexpressing human mammary carcinoma cells promotes progression and invasiveness of tumors. The invasion-suppressive role of α3β1 was not observed in triple-negative mammary carcinoma cells, illustrating the tumor type-specific and complex function of α3β1 in breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1146-8) contains supplementary material, which is available to authorized users.

Adaptive adhesion systems mediate glioma cell invasion in complex environments

Diffuse brain invasion by glioma cells prevents effective surgical or molecular-targeted therapy and underlies a detrimental outcome. Migrating glioma cells are guided by complex anatomical brain structures but the exact mechanisms remain poorly defined. To identify adhesion receptor systems and matrix structures supporting glioma cell invasion into brain-like environments we used 2D and 3D organotypic invasion assays in combination with antibody-, peptide- and RNA-based interference. Combined interference with β1 and αV integrins abolished the migration of U-251 and E-98 glioma cells on reconstituted basement membrane; however, invasion into primary brain slices or 3D astrocyte-based scaffolds and migration on astrocyte-deposited matrix was only partly inhibited. Any residual invasion was supported by vascular structures, as well as laminin 511, a central constituent of basement membrane of brain blood vessels. Multi-targeted interference against β1, αV and α6 integrins expressed by U-251 and E-98 cells proved insufficient to achieve complete migration arrest. These data suggest that mechanocoupling by integrins is relatively resistant to antibody- or peptide-based targeting, and cooperates with additional, as yet unidentified adhesion systems in mediating glioma cell invasion in complex brain stroma.

Fv-clasp: An Artificially Designed Small Antibody Fragment with Improved Production Compatibility, Stability, and Crystallizability

Antibody fragments are frequently used as a "crystallization chaperone" to aid structural analysis of complex macromolecules that are otherwise crystallization resistant, but conventional fragment formats have not been designed for this particular application. By fusing an anti-parallel coiled-coil structure derived from the SARAH domain of human Mst1 kinase to the variable region of an antibody, we succeeded in creating a novel chimeric antibody fragment of ∼37 kDa, termed "Fv-clasp," which exhibits excellent crystallization compatibility while maintaining the binding ability of the original IgG molecule. The "clasp" and the engineered disulfide bond at the bottom of the Fv suppressed the internal mobility of the fragment and shielded hydrophobic residues, likely contributing to the high heat stability and the crystallizability of the Fv-clasp. Finally, Fv-clasp antibodies showed superior "chaperoning" activity over conventional Fab fragments, and facilitated the structure determination of an ectodomain fragment of integrin α6β1.

Endothelial basement membrane laminin 511 is essential for shear stress response

Shear detection and mechanotransduction by arterial endothelium requires junctional complexes containing PECAM-1 and VE-cadherin, as well as firm anchorage to the underlying basement membrane. While considerable information is available for junctional complexes in these processes, gained largely from in vitro studies, little is known about the contribution of the endothelial basement membrane. Using resistance artery explants, we show that the integral endothelial basement membrane component, laminin 511 (laminin α5), is central to shear detection and mechanotransduction and its elimination at this site results in ablation of dilation in response to increased shear stress. Loss of endothelial laminin 511 correlates with reduced cortical stiffness of arterial endothelium in vivo, smaller integrin β1-positive/vinculin-positive focal adhesions, and reduced junctional association of actin-myosin II In vitro assays reveal that β1 integrin-mediated interaction with laminin 511 results in high strengths of adhesion, which promotes p120 catenin association with VE-cadherin, stabilizing it at cell junctions and increasing cell-cell adhesion strength. This highlights the importance of endothelial laminin 511 in shear response in the physiologically relevant context of resistance arteries.

The rod domain is not essential for the function of plectin in maintaining tissue integrity

Plectin is a cytoskeletal linker protein that consists of a central rod domain connecting two globular domains. Rodless plectin is able to functionally compensate for the loss of full-length plectin in mice and, like full-length plectin, is able to form dimers.

Epidermolysis bullosa simplex associated with late-onset muscular dystrophy (EBS-MD) is an autosomal recessive disorder resulting from mutations in the plectin gene. The majority of these mutations occur within the large exon 31 encoding the central rod domain and leave the production of a low-level rodless plectin splice variant unaffected. To investigate the function of the rod domain, we generated rodless plectin mice through conditional deletion of exon 31. Rodless plectin mice develop normally without signs of skin blistering or muscular dystrophy. Plectin localization and hemidesmosome organization are unaffected in rodless plectin mice. However, superresolution microscopy revealed a closer juxtaposition of the C-terminus of plectin to the integrin β4 subunit in rodless plectin keratinocytes. Wound healing occurred slightly faster in rodless plectin mice than in wild-type mice, and keratinocytes migration was increased in the absence of the rod domain. The faster migration of rodless plectin keratinocytes is not due to altered biochemical properties because, like full-length plectin, rodless plectin is a dimeric protein. Our data demonstrate that rodless plectin can functionally compensate for the loss of full-length plectin in mice. Thus the low expression level of plectin rather than the absence of the rod domain dictates the development of EBS-MD.

Isolation of a hemidesmosome-rich fraction from a human squamous cell carcinoma cell line

Hemidesmosomes are cell-to-matrix adhesion complexes anchoring keratinocytes to basement membranes. For the first time, we present a method to prepare a fraction from human cultured cells that are highly enriched in hemidesmosomal proteins. Using DJM-1 cells derived from human squamous cell carcinoma, accumulation of hemidesmosomes was observed when these cells were cultured for more than 10 days in a commercial serum-free medium without supplemental calcium. Electron microscopy demonstrated that numerous electron-dense adhesion structures were present along the basal cell membranes of DJM-1 cells cultured under the aforementioned conditions. After removing cellular materials using an ammonia solution, hemidesmosomal proteins and deposited extracellular matrix were collected and separated by electrophoresis. There were eight major polypeptides, which were determined to be plectin, BP230, BP180, integrin α6 and β4 subunits, and laminin-332 by immunoblotting and mass spectrometry. Therefore, we designated this preparation as a hemidesmosome-rich fraction. This fraction contained laminin-332 exclusively in its unprocessed form, which may account for the promotion of laminin deposition, and minimal amounts of Lutheran blood group protein, a nonhemidesmosomal transmembrane protein. This hemidesmosome-rich fraction would be useful not only for biological research on hemidesmosomes but also for developing a serum test for patients with blistering skin diseases.

Targeting integrin α6 stimulates curative-type bone metastasis lesions in a xenograft model

Laminin-binding integrin receptors are key mediators of epithelial cell migration and tumor metastasis. Recent studies have demonstrated a role for the α6 integrin (ITGA6/CD49f) in maintaining stem cell compartments within normal bone marrow and in residency of tumors metastatic to bone. In this study, we tested a function-blocking antibody specific for ITGA6, called J8H, to determine if preexisting cancer lesions in bone could be slowed and/or animal survival improved. Human prostate tumors were established by intracardiac injection into male SCID mice and treatment with J8H antibody was initiated after 1 week. Tumor progression was monitored by micro-computed tomography (CT) imaging of skeletal lesions. Animals that received weekly injections of the anti-ITGA6 antibody showed radiographic progression in only 40% of osseous tumors (femur or tibia), compared with control animals, where 80% of the lesions (femur or tibia) showed progression at 5 weeks. Kaplan-Meier survival analysis demonstrated a significant survival advantage for J8H-treated animals. Unexpectedly, CT image analysis revealed an increased proportion of bone lesions displaying a sclerotic rim of new bone formation, encapsulating the arrested lytic lesions in animals that received the anti-ITGA6 antibody treatment. Histopathology of the sclerotic lesions demonstrated well-circumscribed tumor within bone, surrounded by fibrosis. These data suggest that systemic targeting of the ITGA6-dependent function of established tumors in bone may offer a noncytotoxic approach to arrest the osteolytic progression of metastatic prostate cancer, thereby providing a new therapeutic strategy for advanced disease.

Nesprin-3 connects plectin and vimentin to the nuclear envelope of Sertoli cells but is not required for Sertoli cell function in spermatogenesis

Nesprin-3 regulates perinuclear localization of plectin and vimentin in Sertoli cells but is dispensable for Sertoli cell function in spermatogenesis. In addition, nuclear positioning and anchorage are not disturbed in nesprin-3–knockout mice.

Nesprin-3 is a nuclear envelope protein that connects the nucleus to intermediate filaments by interacting with plectin. To investigate the role of nesprin-3 in the perinuclear localization of plectin, we generated nesprin-3–knockout mice and examined the effects of nesprin-3 deficiency in different cell types and tissues. Nesprin-3 and plectin are coexpressed in a variety of tissues, including peripheral nerve and muscle. The expression level of nesprin-3 in skeletal muscle is very low and decreases during myoblast differentiation in vitro. Of interest, plectin was concentrated at the nuclear envelope in only a few cell types. This was most prominent in Sertoli cells of the testis, in which nesprin-3 is required for the localization of both plectin and vimentin at the nuclear perimeter. Testicular morphology and the position of the nucleus in Sertoli cells were normal, however, in the nesprin-3–knockout mice and the mice were fertile. Furthermore, nesprin-3 was not required for the polarization and migration of mouse embryonic fibroblasts. Thus, although nesprin-3 is critical for the localization of plectin to the nuclear perimeter of Sertoli cells, the resulting link between the nuclear envelope and the intermediate filament system seems to be dispensable for normal testicular morphology and spermatogenesis.

Blood pressure influences end-stage renal disease of Cd151 knockout mice

Podocytes of the kidney adhere tightly to the underlying glomerular basement membrane (GBM) in order to maintain a functional filtration barrier. The clinical importance of podocyte binding to the GBM via an integrin-laminin-actin axis has been illustrated in models with altered function of α3β1 integrin, integrin-linked kinase, laminin-521, and α-actinin 4. Here we expanded on the podocyte-GBM binding model by showing that the main podocyte adhesion receptor, integrin α3β1, interacts with the tetraspanin CD151 in situ in humans. Deletion of Cd151 in mouse glomerular epithelial cells led to reduced adhesive strength to laminin by redistributing α3β1 at the cell-matrix interface. Moreover, in vivo podocyte-specific deletion of Cd151 led to glomerular nephropathy. Although global Cd151-null B6 mice were not susceptible to renal disease, as has been shown previously, increasing blood and transcapillary filtration pressure induced nephropathy in these mice. Importantly, blocking the angiotensin-converting enzyme in renal disease-susceptible global Cd151-null FVB mice prolonged their median life span. Together, these results establish CD151 as a crucial modifier of integrin-mediated adhesion of podocytes to the GBM and show that blood pressure is an important factor in the initiation and progression of Cd151 knockout-induced nephropathy.

Recreating the perivascular niche ex vivo using a microfluidic approach

Stem cell niches are composed of numerous microenvironmental features, including soluble and insoluble factors, cues from other cells, and the extracellular matrix (ECM), which collectively serve to maintain stem cell quiescence and promote their ability to support tissue homeostasis. A hallmark of many adult stem cell niches is their proximity to the vasculature in vivo, a feature common to neural stem cells, mesenchymal stem cells (MSCs) from bone marrow and adipose tissue, hematopoietic stem cells, and many tumor stem cells. In this study, we describe a novel 3D microfluidic device (MFD) as a model system in which to study the molecular regulation of perivascular stem cell niches. Endothelial cells (ECs) suspended within 3D fibrin gels patterned in the device adjacent to stromal cells (either fibroblasts or bone marrow-derived MSCs) executed a morphogenetic process akin to vasculogenesis, forming a primitive vascular plexus and maturing into a robust capillary network with hollow well-defined lumens. Both MSCs and fibroblasts formed pericytic associations with the ECs but promoted capillary morphogenesis with distinct kinetics. Biochemical assays within the niche revealed that the perivascular association of MSCs required interaction between their α6β1 integrin receptor and EC-deposited laminin. These studies demonstrate the potential of this physiologically relevant ex vivo model system to study how proximity to blood vessels may influence stem cell multipotency.

Endothelial alpha3beta1-integrin represses pathological angiogenesis and sustains endothelial-VEGF

Integrin alpha3beta1 is a major receptor for laminin. The expression levels of laminins-8 and -10 in the basement membrane surrounding blood vessels are known to change during tumor angiogenesis. Although some studies have suggested that certain ligands of alpha3beta1 can affect angiogenesis either positively or negatively, either a direct in vivo role for alpha3beta1 in this process or its mechanism of action in endothelial cells during angiogenesis is still unknown. Because the global genetic ablation of alpha3-integrin results in an early lethal phenotype, we have generated conditional-knockout mice where alpha3 is deleted specifically in endothelial cells (ec-alpha3-/-). Here we show that ec-alpha3-/- mice are viable, fertile, and display enhanced tumor growth, elevated tumor angiogenesis, augmented hypoxia-induced retinal angiogenesis, and increased vascular endothelial growth factor (VEGF)-mediated neovascularization ex vivo and in vivo. Furthermore, our data provide a novel method by which an integrin may regulate angiogenesis. We show that alpha3beta1 is a positive regulator of endothelial-VEGF and that, surprisingly, the VEGF produced by endothelial cells can actually repress VEGF-receptor 2 (Flk-1) expression. These data, therefore, identify directly that endothelial alpha3beta1 negatively regulates pathological angiogenesis and implicate an unexpected role for low levels of endothelial-VEGF as an activator of neovascularization.

PECAM-1 functions as a negative regulator of laminin-induced platelet activation

SUMMARY BACKGROUND

Interaction of resting platelets with exposed components of the subendothelial matrix is an important early activating event that takes place at sites of vascular injury. Platelet responses to collagen are mediated by integrin alpha(2)beta(1) and the glycoprotein (GP)VI-Fc receptor (FcR) gamma-chain complex, whereas platelet activation by laminin is mediated by the related integrin, alpha(6)beta(1), and similarly requires signaling through GPVI-FcR gamma-chain.

OBJECTIVE

Because the cell adhesion and signaling receptor PECAM-1 has previously been shown to dampen collagen-induced platelet activation, we sought to determine whether PECAM-1 might similarly regulate platelet activation by laminin.

METHODS/RESULTS

We found that PECAM-1 became tyrosine phosphorylated on its cytoplasmic immunoreceptor tyrosine-based inhibitory motifs following adhesion of either human or murine platelets to immobilized laminin. Whereas the presence or absence of PECAM-1 had no effect on either the rate or extent of platelet adhesion or spreading on laminin, PECAM-1 inhibited laminin-induced phosphorylation of GPVI-FcR gamma-chain immunoreceptor tyrosine-based activation motifs (ITAMs) and activation of its downstream effector, Syk kinase, and suppressed granule secretion.

CONCLUSIONS

Taken together, these data are consistent with previous findings in platelets and other blood and vascular cells that PECAM-1 functions by modulating ITAM-mediated signaling pathways that amplify cellular activation.

Laminin-421 produced by lymphatic endothelial cells induces chemotaxis for human melanoma cells

Melanoma has a high tendency to metastasize to lymph nodes, which is one of the clinicopathological factors to indicate poor prognosis. Recent investigations have shown the importance of lymphangiogenesis in lymph node metastasis in a variety of human tumors including melanoma. However, molecular mechanism of lymphatic metastasis is still poorly defined. We examined influence of interactions between normal lymphatic endothelial cells (LECs) and melanoma cells on cell migration. Medium conditioned with LEC (LEC-CM) contained chemotactic and chemokinetic activities for human melanoma cell lines. The chemotactic activity was fractionated in more than 100 kDa, and inactivated by heat-treatment. The chemotactic activity of LEC-CM was abolished by immunodepletion with anti-laminin-1 antibody. And immunoprecipitation and Western blot analyses revealed that LEC-CM contained laminin-421. When melanoma C8161 cells were treated with function-blocking antibodies to integrin alpha3 or alpha6, their chemotactic responses to LEC-CM were markedly reduced. Furthermore, the knock-down of tetraspanin CD151 weakened the chemotactic responses of C8161 and MeWo cells to LEC-CM. These data suggest that laminin-421 secreted by LEC possibly facilitates lymphatic metastasis through the induction of chemotaxis of melanoma cells.

Differential expression of the integrins alpha6Abeta4 and alpha6Bbeta4 along the crypt-villus axis in the human small intestine

The integrin alpha6 subunit exists as two different variants, termed alpha6A and alpha6B. These two variants have been shown to harbor potentially distinct biochemical properties but little is known about their cellular function. The aim of this work was to characterize the expression of the integrin alpha6A and B variants in relation to cell proliferation and differentiation in the human small intestinal epithelium. The results showed distinct expression patterns for the two variants along the crypt-villus axis. Indeed, proliferative cells of the crypt were found to predominantly express alpha6A, while differentiated enterocytes and Paneth cells expressed the alpha6B variant. A similar relationship was observed in intestinal cell models by competitive RT-PCR. Further studies in the Caco-2 cell model showed that manipulating the cellular balance of the two alpha6 variants can influence transcriptional activities related to cell proliferation but not differentiation. This suggests that differential expression of the alpha6 subunits is involved in the intestinal epithelial cell renewal process. Further studies will be needed to substantiate this hypothesis.

Development of monoclonal antibodies to integrin receptors

Integrins are heterodimeric cell surface receptors composed of an alpha and a beta subunit. They are involved in homotopic and heterotopic cell adhesion and also function as receptors for extracellular matrix molecules such as collagen, fibronectin and laminin. The family to which an integrin belongs is defined by the presence of a particular beta subunit paired with a unique alpha subunit. In this chapter we describe methods to produce monoclonal antibodies to the family of integrin subunits characterized by beta1 and provide detailed instructions for the development of a monoclonal antibody to the alpha6 integrin receptor expressed by human prostate carcinoma cells (PC3 cells). Data are presented that correlate the functional capabilities of an antibody with its biochemical characterization.

Temporal and spatial regulation of alpha6 integrin expression during the development of the cochlear-vestibular ganglion

The neurons of the cochlear-vestibular ganglion (CVG) that innervate the sensory hair cells of the inner ear are derived from the otic epithelium early in development. Neuroblasts detach from neighboring cells, migrate into the mesenchyme where they coalesce to form the ganglion complex, then send processes back into the epithelium. Cell migration and neuronal process formation involve changes in cellular interactions with other cells and proteins in the extracellular matrix that are orchestrated by cell surface-expressed adhesion molecules, including the integrins. I studied the expression pattern of the alpha6 integrin subunit during the early development of the CVG using immunohistochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) in murine tissue sections, otocyst, and ganglion explants. At embryonic day (E)10.5 alpha6 integrin was expressed in the otic epithelium but not in migrating neuroblasts. Importantly, the loss of alpha6 was associated with exit from the epithelium, not neuronal determination, revealing differentiation cues acutely associated with the cellular environment. Markers of glial and neuronal phenotype showed that alpha6-expressing cells present in the CVG at this stage were glia of neural crest origin. By E12.5 alpha6 expression in the ganglion increased alongside the elaboration of neuronal processes. Immunohistochemistry applied to otocyst cultures in the absence of glia revealed that neuronal processes remained alpha6-negative at this developmental stage and confirmed that alpha6 was expressed by closely apposed glia. The spatiotemporal modulation of alpha6 expression suggests changing roles for this integrin during the early development of inner ear innervation.

Serine phosphorylation of the integrin beta4 subunit is necessary for epidermal growth factor receptor induced hemidesmosome disruption

Hemidesmosomes (HDs) are multiprotein adhesion complexes that promote attachment of epithelial cells to the basement membrane. The binding of alpha6beta4 to plectin plays a central role in their assembly. We have defined three regions on beta4 that together harbor all the serine and threonine phosphorylation sites and show that three serines (S1356, S1360, and S1364), previously implicated in HD regulation, prevent the interaction of beta4 with the plectin actin-binding domain when phosphorylated. We have also established that epidermal growth factor receptor activation, which is known to function upstream of HD disassembly, results in the phosphorylation of only one or more of these three residues and the partial disassembly of HDs in keratinocytes. Additionally, we show that S1360 and S1364 of beta4 are the only residues phosphorylated by PKC and PKA in cells, respectively. Taken together, our studies indicate that multiple kinases act in concert to breakdown the structural integrity of HDs in keratinocytes, which is primarily achieved through the phosphorylation of S1356, S1360, and S1364 on the beta4 subunit.

The regulation of MacMARCKS expression by integrin β3

Integrin-mediated adhesion regulates multiple signaling pathways. Our group previously showed that ectopic expression of different integrin beta-subunits in the neuroepithelial cell line GE11, has distinct effects on cell morphology, actin cytoskeletal organization, and on focal contact distribution. In this report we have investigated changes in gene transcription levels resulting from overexpression of the integrin beta3 subunit. We found that beta3 overexpression leads to the transcriptional downregulation of MARCKS related protein (MRP) resulting in a decreased expression of the MRP protein. Furthermore, we show that the Ras/MAPK pathway controls the basal level of MRP expression but beta3 overexpression bypasses this pathway downstream of ERK to downregulate MRP. Further studies indicate that a region of the cytoplasmic tail of beta3 containing part of the NITY motif is responsible for increased cell spreading and MRP downregulation. However, MRP overexpression failed to inhibit the beta3-induced increase in cell spreading while the knock down of MRP expression in GE11 cells did not increase cell spreading. We suggest that the downregulation of MRP by beta3 is not required for increased cell spreading but instead that MRP downregulation is a secondary effect of increased cell spreading.

Regulated synthesis and functions of laminin 5 in polarized madin-darby canine kidney epithelial cells

Renal tubular epithelial cells synthesize laminin (LN)5 during regeneration of the epithelium after ischemic injury. LN5 is a truncated laminin isoform of particular importance in the epidermis, but it is also constitutively expressed in a number of other epithelia. To investigate the role of LN5 in morphogenesis of a simple renal epithelium, we examined the synthesis and function of LN5 in the spreading, proliferation, wound-edge migration, and apical-basal polarization of Madin-Darby canine kidney (MDCK) cells. MDCK cells synthesize LN5 only when subconfluent, and they degrade the existing LN5 matrix when confluent. Through the use of small-interfering RNA to knockdown the LN5 alpha3 subunit, we were able to demonstrate that LN5 is necessary for cell proliferation and efficient wound-edge migration, but not apical-basal polarization. Surprisingly, suppression of LN5 production caused cells to spread much more extensively than normal on uncoated surfaces, and exogenous keratinocyte LN5 was unable to rescue this phenotype. MDCK cells also synthesized laminin alpha5, a component of LN10, that independent studies suggest may form an assembled basal lamina important for polarization. Overall, our findings indicate that LN5 is likely to play an important role in regulating cell spreading, migration, and proliferation during reconstitution of a continuous epithelium.

Integrin α6β1-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 Myf5nlacZ/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 α6β1 integrin, a laminin receptor, suggesting that integrin α6β1-laminin interactions are required for myotomal laminin matrix assembly. Blockingα6β1-laminin binding in cultured wild-type mouse embryo explants resulted in dispersion of Myf5-positive cells, a phenotype also seen in Myf5nlacZ/nlacZ embryos. Furthermore, inhibition ofα6β1 resulted in an increase in Myf5 protein and ectopic myogenin expression in dermomyotomal cells, suggesting that α6β1-laminin interactions normally repress myogenesis in the dermomyotome. We conclude that Myf5 is required for maintaining α6β1 expression on myogenic precursor cells, and that α6β1 is necessary for myotomal laminin matrix assembly and cell guidance into the myotome. Engagement of laminin byα6β1 also plays a role in maintaining the undifferentiated state of cells in the dermomyotome prior to their entry into the myotome.

Fibronectin promotes brain capillary endothelial cell survival and proliferation through alpha5beta1 and alphavbeta3 integrins via MAP kinase signalling

We showed previously that blood vessel maturation in the CNS is associated with a developmental switch in brain capillary endothelial cells (BCEC), from fibronectin signalling during angiogenesis to laminin signalling in the adult. To investigate the functional significance of this switch, we have examined the response of BCEC to different extracellular matrix (ECM) proteins. This showed that BCEC proliferation was significantly promoted by fibronectin (28.2 +/- 4.0%) and by vitronectin (14.8 +/- 2.1%) compared with uncoated glass (7.2 +/- 0.7%), while BCEC survival was significantly promoted by fibronectin (1130 +/- 131 cells), vitronectin (830 +/- 63 cells), collagen IV (703 +/- 77 cells) and laminin (680 +/- 34 cells) compared with the uncoated glass (367 +/- 48 cells). Biochemical studies showed that BCEC express a limited repertoire of integrins, including the beta1 integrins, alpha3beta1, alpha5beta1 and alpha6beta1, and the alphavbeta3 integrin. Function-blocking studies showed that the response to fibronectin was mediated equally by the alpha5beta1 and alphavbeta3 integrins. Analysis of signalling pathways revealed that fibronectin stimulated activation of the p44/p42 MAP kinase signalling pathway and pharmacological inhibitors of this pathway blocked BCEC proliferation on fibronectin. Taken together, these findings show that fibronectin exerts a strong angiogenic influence on endothelial cells (EC) in the CNS, and that this is mediated through the alpha5beta1 and alphavbeta3 integrins via MAP kinase signalling. In addition to a fundamental role in development, these findings may also have implications in pathological conditions of the CNS where fibronectin is re-expressed.

PECAM-1, alpha6 integrins and neutrophil elastase cooperate in mediating neutrophil transmigration

The heterogeneous nature of the perivascular basement membrane (composed primarily of laminin and collagen type IV) suggests the existence of an elaborate array of adhesive interactions and possibly proteolytic events in leukocyte migration through this barrier. In this context, blockade of alpha6 integrins (laminin receptors), neutrophil elastase (NE) or both inhibited neutrophil migration through interleukin-1beta (IL-1beta)-stimulated mouse cremasteric venules, as observed by intravital microscopy. Furthermore, analysis of tissues by confocal microscopy indicated a synergistic role for alpha6 integrins and NE in mediating neutrophil migration through the perivascular basement membrane. Using a combined in vitro and in vivo experimental approach, the findings of this study also suggest that alpha6 integrins and NE are mobilized from intracellular stores to the cell surface of transmigrating mouse neutrophils, although these events occur via mechanisms dependent on and independent of platelet/endothelial-cell adhesion molecule 1 (PECAM-1, CD31), respectively. Despite different regulatory mechanisms, blockade of alpha6 integrins or NE inhibited migration of murine neutrophils through laminin-coated filters in vitro. Collectively, the findings suggest that, whereas regulation of the expression of alpha6 integrins and NE occur via different adhesive mechanisms, these molecules might act in a cooperative manner in mediating neutrophil migration through venular walls, in particular the perivascular basement membrane.

Blockade of α6integrin inhibits IL-1β- but not TNF-α-induced neutrophil transmigration in vivo

In vitro and in vivo evidence supports a functional role for the integrin alpha6beta1 in neutrophil migration through the perivascular basement membrane, a response that in vivo appears to be associated with platelet/endothelial cell adhesion molecule-1 (PECAM-1)-mediated up-regulation of alpha6beta1 on the cell surface of transmigrating leukocytes. As the involvement of PECAM-1 in leukocyte migration is cytokine-specific, the aim of the present study was to investigate whether alpha6beta1 exhibited a similar profile of stimulus specificity in this context. The cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha) were used to elicit neutrophil migration in two murine models of inflammation, migration through cremasteric venules, as observed by intravital microscopy, and migration into the peritoneal cavity. The role of alpha6beta1 was investigated using an alpha6 integrin-blocking monoclonal antibody GoH3. In both models, GoH3 significantly inhibited neutrophil transmigration induced by IL-1beta but not TNF-alpha. This cytokine-specific role of alpha6 integrin was associated with enhanced cell-surface expression of alpha6beta1 on transmigrated neutrophils (as compared with blood cells) in response to IL-1beta but not TNF-alpha. Using lipopolysaccharide as an inflammatory stimulus in the cremaster muscle model, the study also provides evidence for the involvement of alpha6 integrin in leukocyte transmigration as mediated by endogenously generated IL-1beta. Collectively, the findings demonstrate that alpha6beta1 blockade inhibits neutrophil migration induced by exogenous and endogenous IL-1beta but not TNF-alpha, observations that are associated with increased expression of the integrin on transmigrated leukocytes.

Selective modulation of type 1 insulin-like growth factor receptor signaling and functions by beta1 integrins

We show here that β₁ integrins selectively modulate insulin-like growth factor type I receptor (IGF-IR) signaling in response to IGF stimulation. The β_1A integrin forms a complex with the IGF-IR and insulin receptor substrate-1 (IRS-1); this complex does not promote IGF-I mediated cell adhesion to laminin (LN), although it does support IGF-mediated cell proliferation. In contrast, β_1C, an integrin cytoplasmic variant, increases cell adhesion to LN in response to IGF-I and its down-regulation by a ribozyme prevents IGF-mediated adhesion to LN. Moreover, β_1C completely prevents IGF-mediated cell proliferation and tumor growth by inhibiting IGF-IR auto-phosphorylation in response to IGF-I stimulation. Evidence is provided that the β₁ cytodomain plays an important role in mediating β₁ integrin association with either IRS-1 or Grb2-associated binder1 (Gab1)/SH2-containing protein-tyrosine phosphate 2 (Shp2), downstream effectors of IGF-IR: specifically, β_1A associates with IRS-1 and β_1C with Gab1/Shp2. This study unravels a novel mechanism mediated by the integrin cytoplasmic domain that differentially regulates cell adhesion to LN and cell proliferation in response to IGF.

Integrin clipping: a novel adhesion switch?

During human prostate cancer progression, the majority of normally expressed integrins are suppressed with the exception of the alpha6, alpha3, and beta1 integrins. We have shown that in prostate cancer, the alpha6 integrin is found paired with the beta1 integrin and that a novel form of the alpha6 integrin that lacks a large portion of the extracellular domain (alpha6p) exists. The alpha6pbeta1 integrin is found in human prostate cancer tissue specimens as well as tissue culture cell lines and is formed on the cell surface. This review discusses the mechanism of alpha6pbeta1 production and the potential functions of this integrin variant. Our current working model predicts that the alpha6pbeta1 integrin maintains the intracellular cytoskeletal connections associated with the heterodimer while allowing for an alteration in cell adhesion. The mechanism provides a selective advantage for cancer cell metastasis.

Identification of a Lethal Form of Epidermolysis Bullosa Simplex Associated with a Homozygous Genetic Mutation in Plectin

Genetic mutations in plectin, a cytoskeleton linker protein expressed in a large variety of tissues including skin, muscle, and nerves, cause epidermolysis bullosa simplex with muscular dystrophy, a recessive inherited disease characterized by blistering of the skin and late onset of muscular dystrophy, and Ogna epidermolysis bullosa simplex, a rare dominant inherited form of epidermolysis bullosa simplex with no muscular involvement. Here we report a novel homozygous genetic mutation (2727del14) in the plectin gene (PLEC1) associated with a lethal form of recessive inherited epidermolysis bullosa in a consanguineous family with three affected offspring. This new clinical variant of epidermolysis bullosa is characterized by general skin blistering, aplasia cutis of the limbs, developmental complications, and rapid demise after birth. Mutation 2727del14 is the first genetic defect described in PLEC1 that disrupts the plakin domain of plectin. The severe phenotype of the patients may be linked to the role of the N-terminal domain in the function of plectin and develops the understanding of the genotype-phenotype correlations in the genodermatoses affecting the dermal-epidermal junction.

Rapid Decay of α6 Integrin Caused by a Mis-Sense Mutation in the Propeller Domain Results in Severe Junctional Epidermolysis Bullosa with Pyloric Atresia

Genetic mutations in alpha6beta4 integrin cause junctional epidermolysis bullosa with pyloric atresia, a genodermatosis characterized by blistering of the skin and pyloric occlusion. The lethal form of junctional epidermolysis bullosa with pyloric atresia has been mainly associated with the presence of premature termination codons in the mRNA encoding either the alpha6 or beta4 subunit causing rapid decay of the mutated transcript and absence of alpha6beta4 integrin. In this study, we disclose the genetic background of lethal junctional epidermolysis bullosa with pyloric atresia in a patient presenting absent expression of alpha6 integrin despite normal steady-state level of the alpha6beta4 mRNA. Screening for mutation in the alpha6 gene detected a homozygous base pair substitution (286 C-to-T), which results in the substitution of a serine with a leucine residue (S47L). The amino acid substitution S47L localizes in the first beta-strand of the seven-bladed beta-propeller structure of the extracellular head of alpha6 integrin, and triggers a rapid proteolysis of the aberrant polypeptides involving the lysosomal degradation pathway. This study provides new insight into the pathogenic effect of a mis-sense mutation affecting a functional domain of a protein, and identifies a critical peptide sequence of the beta-propeller domain conserved among the alpha integrin cell receptors.

The alphavbeta5 integrin of hematopoietic and nonhematopoietic cells is a transduction receptor of RGD-4C fiber-modified adenoviruses

Epithelial and endothelial cells expressing the primary Coxsackie virus B adenovirus (Ad) receptor (CAR) and integrin coreceptors are natural targets of human Ad infections. The fiber knob of species A, C, D, E and F Ad serotypes binds CAR by mimicking the CAR-homodimer interface, and the penton base containing arginine-glycine-aspartate (RGD) motifs binds with low affinity to alphav integrins inducing cell activation. Here, we generated seven different genetically modified Ad vectors with RGD sequences inserted into the HI loop of fiber knob. All mutants bound and infected CAR and alphav integrin-positive epithelial cells with equal efficiencies. However, the Ads containing two additional cysteines, both N and C terminals of the RGD sequence (RGD-4C), were uniquely capable of transducing CAR-less hematopoietic and nonhematopoietic human tumor cell lines and primary melanoma cells. Both binding and transduction of RGD-4C Ad were blocked by soluble RGD peptides. Flow cytometry of cell surface integrins and virus binding to CAR-less cells in the presence of function-blocking anti-integrin antibodies indicated that the alphavbeta5 integrin, but not alphavbeta3, alphaIIbbeta3 or beta1,alpha5 or alpha6-containing integrins served as a functional transduction receptor of the RGD-4C Ads. However, in cells with low levels of alphavbeta5 integrin, the function-blocking anti-alphavbeta5 antibodies were not effective, unlike soluble RGD peptides. Collectively, our data demonstrate that the alphavbeta5 integrin is a functional transduction receptor of RGD-4C Ads in the absence of CAR, and that additional RGD receptors are targets of these viruses. The RGD-4C vectors further extend the tropism of Ads towards potential human therapies.

Alpha 4 integrin increases anoikis of human osteosarcoma cells

Cell motility, growth, and proliferation are regulated by adhesion to the extracellular matrix. Detachment of adherent cells from extracellular matrix results in induction of apoptosis ("anoikis"). Transformed cells often show an anchorage-independent growth that enables them to acquire a motile, invasive phenotype. This phenotype has been associated with the altered expression and function of the integrin family of transmembrane proteins that mediate cell adhesion to the extracellular matrix. Although alpha4 integrin is normally expressed on leukocyte subpopulations, a number of metastatic melanomas and sarcomas express it as well. In this study, we demonstrated the expression of alpha4 integrins on the human osteosarcoma cell line SAOS and on metastatic osteosarcoma lesions from the lung and pericardium. We further demonstrated that alpha4 integrin is coupled to the beta1 subunit by biochemical analysis and by using a mAb directed against a combinatorial epitope unique to the alpha4beta1 molecule. SAOS cells undergo anoikis when adherence is denied. Anoikis involved the activation of caspase 3 and the release of cytochrome c from mitochondria. Treatment of non-adherent SAOS with an anti-alpha4 mAb increased anoikis while anti-beta1 integrin mAbs did not alter anoikis, thus indicating a novel function for the alpha4 subunit in the control of cell death. Since integrins can control cell migration, proliferation, and apoptosis these results demonstrate a potential role for alpha4 integrin during multiple aspects of osteosarcoma metastasis.

PECAM-1 (CD31) homophilic interaction up-regulates alpha6beta1 on transmigrated neutrophils in vivo and plays a functional role in the ability of alpha6 integrins to mediate leukocyte migration through the perivascular basement membrane

Platelet-endothelial cell adhesion molecule (PECAM)-1 has been implicated in leukocyte migration through the perivascular basement membrane (PBM) though the mechanisms involved are unclear. The present results demonstrate that the ability of alpha(6) integrins to mediate neutrophil migration through the PBM is PECAM-1 dependent, a response associated with PECAM-1-mediated increased expression of alpha(6)beta(1) on transmigrating neutrophils in vivo. An anti-alpha(6) integrins mAb (GoH3) inhibited (78%, P < 0.001) neutrophil migration through interleukin (IL)-1beta-stimulated cremasteric venules, primarily at the level of the PBM, as analyzed by intravital and electron microscopy. In PECAM-1-deficient mice (KO), a reduced level of neutrophil transmigration elicited by IL-1beta (4-h reaction) was observed in both the cremaster muscle (55% inhibition, P < 0.05) and in the peritoneum (57% inhibition, P < 0.01) but GoH3 had no additional inhibitory effect on these responses. FACS((R)) analysis of neutrophils demonstrated increased expression of alpha(6)beta(1) on transmigrated peritoneal neutrophils, as compared with blood neutrophils, in wild-type but not KO mice even though neutrophils from both strains of mice exhibited comparable levels of intracellular expression of alpha(6) as observed by immunofluorescent staining and confocal microscopy. Furthermore, mice deficient in either leukocyte or endothelial cell PECAM-1, as developed by bone marrow transplantation, demonstrated a similar level of reduced neutrophil transmigration and expression of alpha(6)beta(1) on transmigrated neutrophils as that detected in KO mice. The results demonstrate a role for PECAM-1 homophilic interaction in neutrophil transmigration and increased expression of alpha(6)beta(1) on the cell surface of transmigrated neutrophils in vivo, a response that could contribute to the mechanism of PECAM-1-mediated neutrophil migration through the PBM.

Dynamics of the alpha6beta4 integrin in keratinocytes

The integrin alpha6beta4 has been implicated in two apparently contrasting processes, i.e., the formation of stable adhesions, and cell migration and invasion. To study the dynamic properties of alpha6beta4 in live cells two different beta4-chimeras were stably expressed in beta4-deficient PA-JEB keratinocytes. One chimera consisted of full-length beta4 fused to EGFP at its carboxy terminus (beta4-EGFP). In a second chimera the extracellular part of beta4 was replaced by EGFP (EGFP-beta4), thereby rendering it incapable of associating with alpha6 and thus of binding to laminin-5. Both chimeras induce the formation of hemidesmosome-like structures, which contain plectin and often also BP180 and BP230. During cell migration and division, the beta4-EGFP and EGFP-beta4 hemidesmosomes disappear, and a proportion of the beta4-EGFP, but not of the EGFP-beta4 molecules, become part of retraction fibers, which are occasionally ripped from the cell membrane, thereby leaving "footprints" of the migrating cell. PA-JEB cells expressing beta4-EGFP migrate considerably more slowly than those that express EGFP-beta4. Studies with a beta4-EGFP mutant that is unable to interact with plectin and thus with the cytoskeleton (beta4(R1281W)-EGFP) suggest that the stabilization of the interaction between alpha6beta4 and LN-5, rather than the increased adhesion to LN-5, is responsible for the inhibition of migration. Consistent with this, photobleaching and recovery experiments revealed that the interaction of beta4 with plectin renders the bond between alpha6beta4 and laminin-5 more stable, i.e., beta4-EGFP is less dynamic than beta4(R1281W)-EGFP. On the other hand, when alpha6beta4 is bound to laminin-5, the binding dynamics of beta4 to plectin are increased, i.e., beta4-EGFP is more dynamic than EGFP-beta4. We suggest that the stability of the interaction between alpha6beta4 and laminin-5 is influenced by the clustering of alpha6beta4 through the deposition of laminin-5 underneath the cells. This clustering ultimately determines whether alpha6beta4 will inhibit cell migration or not.

Initial interaction of U2OS cells with noncoated and calcium phosphate coated titanium substrates

From previous studies, we know that calcium phosphate (CaP) coated implants stimulate bone formation compared to uncoated implants. Nevertheless, the mechanism by which substrate surface characteristics affect cell function is unclear. In this study, we examined the initial interaction (30 min to 24 h) of U2OS cells with titanium substrates with or without a CaP coating. The effect of substrate roughness was also studied. When cell attachment was studied, we found that cells attached more readily to rough than to smooth surfaces. Also, more cells attached to the uncoated than to the CaP coated surface. After 24 h, cell numbers were similar for all substrate surfaces. Further, cells spread to a larger area on noncoated titanium than on the CaP coated substrates. At 24 h, the sequence of cell size was smooth titanium > rough titanium > CaP coated titanium. Shape measurements showed differences in cell shape between the cells on the different materials only at 7 h, not at different culture times. Cells expressed alpha2, alpha3, alpha5, alpha6, alphav, and beta1 subunits. Expression of alpha1, alpha4, alphavbeta3, beta3, beta4, and beta7 was extremely low or was not found. The beta1 integrin expression was higher on the coated than on the noncoated titanium at 3 h, but not on the other studied times. Expression of alpha2, alpha5, alpha6, and alphav expression was found to be upregulated at 24 h compared to earlier culture times on coated titanium, but not on uncoated titanium substrates. From this we conclude that the surface characteristics of a material (roughness and composition) can affect the initial interaction of cells with the material.

Animal models for skin blistering conditions: absence of laminin 5 causes hereditary junctional mechanobullous disease in the Belgian horse

Recent achievements in the genetic correction of keratinocytes isolated from patients with junctional epidermolysis bullosa have paved the way to a gene therapy approach for the disease. Because gene therapy protocols require preclinical validation in animals, we have characterized spontaneous animal models of junctional epidermolysis bullosa. In this study we have elucidated the genetic basis of the hereditary junctional mechanobullous disease in the Belgian horse, a condition characterized by blistering of the skin and mouth epithelia, and exungulation (loss of the hoof). Immunofluorescence analysis associated the condition to the absent expression of the gamma2 chain of laminin 5 and designated Lamc2 as the candidate gene. Comparative analysis of the nucleotide sequence of the full-length gamma2 cDNA isolated by reverse transcription polymerase chain reaction amplification of total RNA purified from the epithelium of a junctional epidermolysis bullosa foal and a healthy control disclosed a homozygous basepair insertion (1368insC) in the affected animal. Mutation 1368insC results in a downstream premature termination codon and is predicted to cause absent expression of the laminin gamma2 polypeptide. Our results also show that: (i) the horse junctional epidermolysis bullosa genetically corresponds to the severe Herlitz form of junctional epidermolysis bullosa in man; (ii) the amino acid sequence and structure of the horse laminin gamma2 chain are virtually identical to the human counterpart; (iii) the moderate eruption of skin blisters in the affected animals with respect to the human Herlitz junctional epidermolysis bullosa patients correlates with the protection provided by hair. Our observations suggest that the affected foals are a convenient source of epithelial cells from tissues that cannot be obtained from human junctional epidermolysis bullosa patients, and imply that hairless strains of animals with recessive skin disorders would be the best models for in vivo gene therapy approaches to skin blistering diseases.

Localization of laminin alpha4-chain in developing and adult human tissues

Recent studies suggest important functions for laminin-8 (Ln-8; alpha4beta1gamma1) in vascular and blood cell biology, but its distribution in human tissues has remained elusive. We have raised a monoclonal antibody (MAb) FC10, and by enzyme-linked immunoassay (EIA) and Western blotting techniques we show that it recognizes the human Ln alpha4-chain. Immunoreactivity for the Ln alpha4-chain was localized in tissues of mesodermal origin, such as basement membranes (BMs) of endothelia, adipocytes, and skeletal, smooth, and cardiac muscle cells. In addition, the Ln alpha4-chain was found in regions of some epithelial BMs, including epidermis, salivary glands, pancreas, esophageal and gastric glands, intestinal crypts, and some renal medullary tubules. Developmental differences in the distribution of Ln alpha4-chain were detected in skeletal muscle, walls of vessels, and intestinal crypts. Ln alpha4- and Ln alpha2-chains co-localized in BMs of fetal skeletal muscle cells and in some epithelial BMs, e.g., in gastric glands and acini of pancreas. Cultured human pulmonary artery endothelial (HPAE) cells produced Ln alpha4-chain as M(r) 180,000 and 200,000 doublet and rapidly deposited it to the growth substratum. In cell-free extracellular matrices of human kidney and lung, Ln alpha4-chain was found as M(r) 180,000 protein.

Function of laminins and laminin-binding integrins in gingival epithelial cell adhesion

BACKGROUND

In human gingiva, epithelial cells attach to their adjacent tissues by means of specialized molecular adhesion complexes and a basement membrane. Little is known about the synthesis of adhesion proteins by gingival keratinocytes; we, therefore, studied how cultured immortalized gingival epithelial cells produce laminins and express laminin-binding integrins. We presumed that different laminins and integrins would be involved in the adhesion of gingival epithelial cells.

METHODS

We cultured gingival keratinocytes and studied their production of laminins and expression of integrins using immunofluorescence microscopy, immunoprecipitation, and immunoblotting methods and by quantitative cell adhesion experiments. We also studied how gingival tissue expresses these adhesion proteins in vivo by using immunofluorescence microscopy.

RESULTS

In immunofluorescence microscopy, the cells were seen to organize chains of laminin-5 (alpha3beta03gamma2) to extracellular patches, whereas the alpha5 chain of laminin-10 (alpha5betalgamma1) could only be seen intracellularly. Of the laminin-binding integrin subunits, integrin a6 subunit was organized to dotted arrays, typical of prehemidesmosomal adhesions, whereas integrin alpha3 subunit was located at cell-cell junctions, in prehemidesmosomal structures, and at some locations also in small focal-contact like patches. Integrin beta1 subunit was found at cell-cell junctions and in focal contacts. Immunoprecipitation experiments showed that the cells synthesize and secrete chains of laminin-5 and laminin-10. In quantitative cell adhesion experiments, the cells adhered efficiently to these laminins by using cooperatively integrin alpha3beta1 and alpha6beta1 integrin complexes. None of the other known laminin-binding integrin subunits appeared to be significantly involved in cell adhesion to these laminin isoforms.

CONCLUSIONS

Our results provide new information on gingival epithelial cell adhesion and extracellular matrix production and may thus aid in the understanding of periodontal physiology.

Association of the tetraspanin CD151 with the laminin-binding integrinsα3β1, α6β1, α6β4 and α7β1 in cells in culture and in vivo

CD151 is a cell surface protein that belongs to the tetraspanin superfamily. It forms complexes with the laminin-binding integrinsα3β1, α6β1 and α6β4 and is codistributed with these integrins in many tissues at sites of cell-matrix interactions. In this study we show that CD151 can also form stable complexes with the laminin-binding integrin α7β1. The strength of this interaction is comparable to that between CD151 and α3β1. Complexes ofα3β1, α6β1 and α7β1 with CD151 are equally well formed with all splice variants of the α3, α6 and α7 subunits, and complex formation is not affected by mutations that prevent the cleavage of the integrin α6 subunit. Like the expression ofα3β1 and α6β1, expression of α7β1 in K562 cells results in increased levels of CD151 at its surface. Two non-integrin laminin receptors, dystroglycan and the polypeptide on which the Lutheran blood group antigens are expressed, are also often colocalized with CD151, but no association with CD151-α3β1 complexes was found with biochemical analysis.

The anti-CD151 antibody TS151R detects an epitope at a site at which CD151 interacts with integrins, and therefore it cannot react with CD151 when it is bound to an integrin. Comparison of the straining patterns produced by TS151R with that by of an anti-CD151 antibody recognizing an epitope outside the binding site (P48) revealed that most tissues expressing one or more laminin-binding integrins reacted with P48 but not with TS151R. However,smooth muscle cells that express α7β1 and renal tubular epithelial cells that express α6β1 were stained equally well by TS151R and P48. These results suggest that the interactions between CD151 and laminin-binding integrins are subject to cell-type-specific regulation.

Mesothelial cells in suspension expose an enriched integrin repertoire capable of capturing soluble fibronectin and laminin

Pleural cavities are lined by a polarized monolayer of mesothelial cells (MC). During pleuritis, MC are shed into effusions, and pleural obstruction may occur. Integrins are cell surface receptors mediating interactions with extracellular matrix (ECM) proteins. The distribution of beta 1-, beta 3-, beta 4-integrins and fibronectin and laminin in normal and chronically inflamed pleura and in/on MC from pleural effusions was examined by immunomorphology and flow cytometry. Adhesion assays of MC to fibronectin and laminin were performed. In situ, resting MC expressed beta 1-, beta 3-, and beta 4-, and alpha v-subunits. Activated MC were beta 1- and alpha v-positive and also expressed alpha 3 and alpha 6; beta 4 was confined to the basal surface of MC; beta 3 was absent. Floating MC from effusions neoexpressed alpha 5 and reexpressed beta 3. In vitro, MC surface expressed beta 1, beta 3, alpha 3, alpha 5, alpha 6, alpha v, and also alpha 1 and alpha 2. In normal pleura, fibronectin and laminin were components of the basement membrane. In pleuritis, the basement membrane was desintegrated. Instead, newly formed fibronectin/laminin containing fibrils extended into the submesothelial connective tissue. Floating MC freshly isolated from effusions carried fibronectin and laminin on their surface and showed specific binding to these ECM proteins. Binding was blocked by anti-beta 1 or anti-alpha 5 and anti-alpha 6 antibodies, respectively. MC incubated with fibronectin showed a clear shift to the S phase, while laminin had no effect. In conclusion, activated and detached MC progressively enrich their integrin repertoire. By capturing soluble fibronectin and laminin and by matrix-mediated bridging, readhering MC may contribute to pleural obstruction. Further, soluble fibronectin bound to alpha 5 beta 1 might be life-sustaining for floating MC by driving cells into cell cycle.

Novel mutations in the LAMC2 gene in non-Herlitz junctional epidermolysis bullosa: effects on laminin-5 assembly, secretion, and deposition

Laminin-5 is the major adhesion ligand of epithelial cells. Mutations in the three genes (LAMA3, LAMB3, LAMC2) encoding the laminin-5 chains cause junctional epidermolysis bullosa, a clinically and genetically heterogeneous blistering skin disease. Here, we describe a non-Herlitz junctional epidermolysis bullosa patient, compound heterozygote for two novel mutations affecting the LAMC2 gene. The mutation in the paternal allele is a de novo splice site mutation (522-1G-->A) that results in in-frame skipping of exon 4 and synthesis of a mutated gamma2 polypeptide (gamma2Delta4) carrying a 33 amino acid deletion within the N-terminal domain V. The maternal mutation is a one base pair insertion (3511insA) in the 3' terminal exon of LAMC2 resulting in a frameshift and a premature termination codon. Mutation 3511insA is predicted to lead to the synthesis of a gamma2 polypeptide (gamma2t) disrupted in its alpha-helical C-terminal structure and truncated of the last 25 amino acids. Keratinocytes isolated from the patient's skin showed a markedly decreased level of gamma2 chain mRNA and secreted scant amounts of laminin-5, which undergoes physiologic proteolytic processing. To investigate the biologic function of the laminin-5 molecules synthesized by the patient, mutant gamma2 cDNAs were transiently expressed in gamma2-null keratinocytes. Transfection of the gamma2Delta4 cDNA resulted in restoration of laminin-5 deposition onto the culture substrate, which demonstrates that the gamma2 polypeptides carrying a deletion in domain V, upstream of the gamma2 proteolytic cleavage site, are assembled into native laminin-5 that is secreted and extracellularly processed. In contrast, transfection of a mutant cDNA expressing the gamma2t chain failed to restore laminin-5 immunoreactivity, which indicates that integrity of the gamma2 C-terminal amino acid sequences is required for laminin-5 assembly. These results correlate for the first time a functional alteration in a laminin-5 domain with a mild junctional epidermolysis bullosa phenotype.

Identification of a novel structural variant of the alpha 6 integrin

The alpha(6) integrin is a 140-kDa (nonreduced) laminin receptor. We have identified a novel 70-kDa (nonreduced) form of the alpha(6) integrin called alpha(6)p for the latin word parvus, meaning small. The variant was immunoprecipitated from human cells using four different alpha(6)-specific monoclonal antibodies but not with alpha(3) or alpha(5) antibodies. The alpha(6)p integrin contained identical amino acid sequences within exons 13--25, corresponding to the extracellular "stalk region" and the cytoplasmic tail of the alpha(6) integrin. The light chains of alpha(6) and alpha(6)p were identical as judged by alpha(6)A-specific antibodies and electrophoretic properties. The alpha(6)p variant paired with either beta(1) or beta(4) subunits and was retained on the cell surface three times longer than alpha(6). Reverse transcription/polymerase chain reaction analysis revealed a single polymerase chain reaction product. The alpha(6)p variant was found in human prostate (DU145H, LnCaP, PC3) and colon (SW480) cancer cell lines but not in normal prostate (PrEC), breast cancer (MCF-7), or lung cancer (H69) cell lines or a variant of a prostate carcinoma cell line (PC3-N). Protein levels of alpha(6)p increased 3-fold during calcium-induced terminal differentiation in a normal mouse keratinocyte model system. A novel form of the alpha(6) integrin exists on cell surfaces that contains a dramatically altered extracellular domain.

Reduced expression of the epithelial adhesion ligand laminin 5 in the skin causes intradermal tissue separation

Laminin 5, the major keratinocyte adhesion ligand, is found in the lamina lucida subregion of the epidermal basement membrane of the skin, where it colocalizes with the anchoring filaments. Mutations in the genes encoding laminin 5 cause junctional epidermolysis bullosa, an inherited skin blistering disease characterized by abnormal hemidesmosomes and cleavage of the lamina lucida leading to epidermal detachment. In this work we describe the genetic basis of a new subtype of lethal inherited epidermolysis bullosa associated with reduced skin reactivity to laminin 5, presence of mature hemidesmosomes, and intradermal cleavage of the skin. The epidermolysis bullosa patients were heterozygous for a nonsense mutation (Q896X) and a splice site mutation (764-10T-->G) in the gene (LAMC2) for the gamma2 chain of laminin 5. The nonsense mutation causes accelerated decay of the corresponding mRNA, while the splice site mutation results in maturation of a cryptic wild-type gamma2 mRNA leading to reduced expression of wild-type laminin 5. In vitro studies using the probands' keratinocytes showed that secretion of reduced amounts of functional laminin 5 in the patient, although permitting formation of hemidesmosomes, fail to restore efficient cell adhesion. Our results provide the first evidence that laminin 5 contributes to the firm adhesion of the epithelial basement membrane to the underlying stroma. They also show that a low expression level of laminin 5 induces assembly of mature hemidesmosomes in vivo but fails to assure a stable cohesion of the dermal-epidermal junction.

HEMCAM/CD146 downregulates cell surface expression of (β)1 integrins

HEMCAM/gicerin, an immunoglobulin superfamily protein, is involved in homophilic and heterophilic adhesion. It interacts with NOF (neurite outgrowth factor), a molecule of the laminin family. Alternative splicing leads to mRNAs coding for HEMCAM with a short (HEMCAM-s) or a long cytoplasmic tail (HEMCAM-l). To investigate the cellular function of these two variants, we stably transfected murine fibroblasts with either form of HEMCAM. Expression of each isoform of this protein in L cells delayed proliferation and modified their adhesion properties to purified extracellular matrix proteins. Expression of either HEMCAM-s or HEMCAM-l inhibited integrin-dependent adhesion and spreading of fibroblasts to laminin 1, showing that this phenomenon did not depend on the cytoplasmic region. By contrast, L-cell adhesion and spreading to fibronectin depended on the HEMCAM isoform expressed. Flow cytometry and immunoprecipitation studies revealed that the expression of HEMCAM downregulated expression of the laminin-binding integrins (α)3 (β)1, (α)6 (β)1 and (α)7 (β)1, and fibronectin receptor (α)5 (β)1 from the cell surface. Semi-quantitative PCR and northern blot experiments showed that the expression of (α)6 (β)1 integrin modified by HEMCAM occurred at a translation or maturation level. Thus, our data demonstrate that HEMCAM regulates fibroblast adhesion by controlling (β)1 integrin expression. http://www.biologists.com/JCS/movies/jcs1886.html

Effect of the hepatitis B virus HBx protein on integrin-mediated adhesion to and migration on extracellular matrix

BACKGROUND/AIMS

The hepatitis B virus HBx protein is associated with the development of hepatocellular carcinoma (HCC). However, its possible contribution to tumor spreading has not been explored. The migration of tumor cells through the extracellular matrix (ECM) represents a crucial step in tumor metastasis. Our aim was to study the effect of HBx on the integrin-mediated cell-ECM interaction, and its possible consequences for cell migration.

METHODS

Cell-ECM interaction was evaluated by static adhesion experiments, using blocking and stimulating anti-beta1 integrin mAbs. ECM receptor expression was analyzed by flow cytometry. The cellular distribution of the activated beta1 integrin subunit was determined by immunofluorescence analysis, and cell motility was determined by wound-healing assays.

RESULTS

HBx-bearing cells showed decreased adhesion to fibronectin, which correlated with a decreased expression of the alpha5 integrin subunit. The activated beta1 subunit was redistributed to the tips of pseudopodial protrusions of HBx-bearing cells, whereas it was evenly localized in the control cells. HBx-induced cell migration was abrogated by irreversible stimulation of beta1 integrins.

CONCLUSIONS

These results suggest that HBx might play a role in tumor spreading by modulating the adhesion-deadhesion balance of the cells in the primary tumor site and favoring integrin-mediated cell migration.