The human integrin VLA-2 is a collagen receptor on some cells and a collagen/laminin receptor on others

Integrin α11β1 as a Key Collagen Receptor in Human Skin Dermis: Insight into Fibroblast Function and Skin Dermal Aging

Collagen-binding integrins play a crucial role in facilitating fibroblast-collagen interactions and regulating cellular functions. In this study, we identified that among 4 collagen-binding integrins, integrin α11 was the predominant type in human skin dermal fibroblasts and that loss of integrin α11 expression contributed to skin dermal aging. Integrin α11β1 was critical for regulating fibroblast-collagen interactions, including cell adhesion, spreading, morphology, mechanical tension, and the production of collagenous extracellular matrix. TGF-β was recognized as the primary regulator of integrin α11 expression. Notably, dermal fibroblasts in aged human skin demonstrated impaired TGF-β signaling, which coincided with a loss of integrin α11 expression, whereas the expression of other collagen-binding integrins remained unchanged. Similarly, in senescent dermal fibroblasts in vitro, impaired TGF-β signaling was associated with a significant reduction in integrin α11 expression, whereas other collagen-binding integrins were upregulated or unaffected. Furthermore, collapsed dermal fibroblasts, a key characteristic of dermal fibroblasts in aged human skin, specifically downregulated integrin α11, whereas other collagen-binding integrins were upregulated or remained unchanged. These findings suggest a negative feedback loop in which an impaired TGF-β-integrin α11β1 axis and fibroblast collapse promote dermal aging in human skin. This self-reinforcing cycle reflects the progressive and unidirectional nature of biological aging.

The effects of inhibition and siRNA knockdown of collagen-binding integrins on human umbilical vein endothelial cell migration and tube formation

Blood vessels in the body are lined with endothelial cells which have vital roles in numerous physiological and pathological processes. Collagens are major constituents of the extracellular matrix, and many adherent cells express several collagen-binding adhesion receptors. Here, we study the endothelium-collagen interactions mediated by the collagen-binding integrins, α1β1, α2β1, α10β1 and α11β1 expressed in human umbilical vein endothelial cells (HUVECs). Using qPCR, we found expression of the α10 transcript of the chondrocyte integrin, α10β1, along with the more abundant α2, and low-level expression of α1. The α11 transcript was not detected. Inhibition or siRNA knockdown of the α2-subunit resulted in impaired HUVEC adhesion, spreading and migration on collagen-coated surfaces, whereas inhibition or siRNA knockdown of α1 had no effect on these processes. In tube formation assays, inhibition of either α1 or α2 subunits impaired the network complexity, whereas siRNA knockdown of these integrins had no such effect. Knockdown of α10 had no effect on cell spreading, migration or tube formation in these conditions. Overall, our results indicate that the collagen-binding integrins, α1β1 and α2β1 play a central role in endothelial cell motility and self-organisation.

CLN7/MFSD8 may be an important factor for SARS-CoV-2 cell entry

The SARS-CoV-2 virus has triggered a worldwide pandemic. According to the BioGrid database, CLN7 (MFSD8) is thought to interact with several viral proteins. The aim of this work was to investigate a possible involvement of CLN7 in the infection process. Experiments on a CLN7-deficient HEK293T cell line exhibited a 90% reduced viral load compared to wild-type cells. This observation may be linked to the finding that CLN7 ko cells have a significantly reduced GM1 content in their cell membrane. GM1 is found highly enriched in lipid rafts, which are thought to play an important role in SARS-CoV-2 infection. In contrast, overexpression of CLN7 led to an increase in viral load. This study provides evidence that CLN7 is involved in SARS-CoV-2 infection. This makes it a potential pharmacological target for drug development against COVID-19. Furthermore, it provides insights into the physiological function of CLN7 where still only little is known about.

Functional and Genetic Characterization of Porcine Beige Adipocytes

Beige adipocytes are a distinct type of fat cells with a thermogenic activity that have gained substantial attention as an alternative cellular anti-obesity target in humans. These cells may provide an alternative strategy for the genetic selection of pigs with reduced fat deposition. Despite the presence of beige adipocytes in piglets, the molecular signatures of porcine beige adipocytes remain unclear. Here, white and beige adipocytes from Tibetan piglets were primarily cultured and differentiated. Compared to the white adipocytes, the beige adipocytes exhibited a stronger thermogenic capacity. RNA-sequencing-based genome-wide comparative analyses revealed distinct gene expression profiles for white and beige adipocytes. In addition, two genes, integrin alpha-2 (ITGA2) and calponin 1 (CNN1), which were specifically differentially expressed in porcine beige adipocytes, were further functionally characterized using a loss-of-function approach. Our data showed that both genes were involved in differentiation and thermogenesis of porcine beige adipocytes. Collectively, these data furthered our understanding of gene expression in porcine white and beige adipocytes. Elucidating the genetic basis of beige adipogenesis in pigs will pave the way for molecular design breeding in both pigs and large animal models of human diseases.

Integrin α2β1 as a negative regulator of the laminin receptors α6β1 and α6β4

Integrin α₂β₁ is a widely expressed collagen I receptor which also mediates laminin-111 binding in some cell types, but the functional relevance of collagen versus laminin binding for different cell types is poorly understood. Here we use AFM-based singe-cell force spectroscopy (SCFS) to compare α₂β₁-mediated adhesion strength to collagen and laminin in different cell types. Chinese Hamster Ovary (CHO) cells stably expressing integrin α₂β₁ (CHO-A2) displayed enhanced adhesion to collagen, but weak adhesion to laminin, consistent with a role of α₂β₁ as a receptor only for collagen in these cells. Inversely, the α₂β₁-deficient CHO wildtype cells (CHO-WT) showed weak adhesion to collagen, but strong adhesion to laminin-111, in turn suggesting that integrin α₂β₁ expression suppresses laminin binding. Analogous results were obtained in a pair of SAOS-2 human osteosarcoma cell lines. Again, wildtype cells (SAOS-WT) adhered strongly to laminin and poorly to collagen, while expression of integrin α₂β₁ (SAOS-A2) induced strong adhesion to collagen, but reduced adhesion to laminin. Expression of α₂β₁ also shifted cell spreading preference from laminin to collagen and suppressed laminin-dependent transmigration. In agreement with reduced laminin adhesion, α₂β₁ expression downregulated transcription and expression of integrin subunits α₆ and β₄, components of the main laminin-111 binding receptors integrin α₆β₁ and α₆β₄ in these cells. Integrin α₆ and β₄ expression was also reduced when α₂ expression was chemically induced using tetradecanoyl-phorbol-acetate (TPA). Our results thus show that integrin α₂β₁ expression negatively regulates integrin α₆β₁ and α₆β₄-mediated adhesion, spreading and invasion on laminin in different cancer cell types. In contrast to SAOS-WT, but similar to SAOS-A2 osteosarcoma cells, primary Human osteoblasts (HOB) cells express α₂ but only low levels of β₄ integrin, preferentially adhere to and spread on collagen over laminin and show suppressed laminin-dependent transmigration. By enhancing collagen binding directly and suppressing laminin binding indirectly through laminin receptor downregulation, α₂β₁ expression may thus re-direct migrating cancer cells from laminin-rich to collagenous tissues and partially revert osteosarcoma cells towards an untransformed phenotype.

Collagen Assembly at the Cell Surface: Dogmas Revisited

With the increased awareness about the importance of the composition, organization, and stiffness of the extracellular matrix (ECM) for tissue homeostasis, there is a renewed need to understand the details of how cells recognize, assemble and remodel the ECM during dynamic tissue reorganization events. Fibronectin (FN) and fibrillar collagens are major proteins in the ECM of interstitial matrices. Whereas FN is abundant in cell culture studies, it is often only transiently expressed in the acute phase of wound healing and tissue regeneration, by contrast fibrillar collagens form a persistent robust scaffold in healing and regenerating tissues. Historically fibrillar collagens in interstitial matrices were seen merely as structural building blocks. Cell anchorage to the collagen matrix was thought to be indirect and occurring via proteins like FN and cell surface-mediated collagen fibrillogenesis was believed to require a FN matrix. The isolation of four collagen-binding integrins have challenged this dogma, and we now know that cells anchor directly to monomeric forms of fibrillar collagens via the α1β1, α2β1, α10β1 and α11β1 integrins. The binding of these integrins to the mature fibrous collagen matrices is more controversial and depends on availability of integrin-binding sites. With increased awareness about the importance of characterizing the total integrin repertoire on cells, including the integrin collagen receptors, the idea of an absolute dependence on FN for cell-mediated collagen fibrillogenesis needs to be re-evaluated. We will summarize data suggesting that collagen-binding integrins in vitro and in vivo are perfectly well suited for nucleating and supporting collagen fibrillogenesis, independent of FN.

VLA-1 Binding to Collagen IV Controls Effector T Cell Suppression by Myeloid-Derived Suppressor Cells in the Splenic Red Pulp

Myeloid-derived suppressor cells (MDSCs) represent a major population controlling T cell immune responses. However, little is known about their molecular requirements for homing and T cell interaction to mediate suppression. Here, we investigated the functional role of the homing and collagen IV receptor VLA-1 (α1β1-integrin) on in vitro GM-CSF generated murine MDSCs from wild-type (WT) and CD49a/α1-integrin (Itga1^−/−) gene-deficient mice. Here, we found that effector (Teff) but not naive (Tn) CD4⁺ T cells express VLA-1 and monocytes further up-regulated their expression after culture in GM-CSF when they differentiated into the monocytic subset of resting MDSCs (R-MDSCs). Subsequent activation of R-MDSCs by LPS+IFN-γ (A-MDSCs) showed increased in vitro suppressor potential, which was independent of VLA-1. Surprisingly, VLA-1 deficiency did not influence A-MDSC motility or migration on collagen IV in vitro. However, interaction times of Itga1^−/− A-MDSCs with Teff were shorter than with WT A-MDSCs on collagen IV but not on fibronectin substrate in vitro. After injection, A-MDSCs homed to the splenic red pulp where they co-localized with Teff and showed immediate suppression already after 6 h as shown by inhibition of T cell proliferation and induction of apoptosis. Injection of A-MDSCs from Itga1^−/− mice showed equivalent homing into the spleen but a reduced suppressive effect. Interaction studies of A-MDSCs with Teff in the subcapsular red pulp with intravital two-photon microscopy revealed also here that MDSC motility and migration parameters were not altered by VLA-1 deficiency, but the interaction times with Teff were reduced. Together, our data point to a new role of VLA-1 adhesion to collagen IV as a prerequisite for extended contact times with Teff required for suppression.

Cadherins, Selectins, and Integrins in CAM-DR in Leukemia

The interaction between leukemia cells and the bone microenvironment is known to provide drug resistance in leukemia cells. This phenomenon, called cell adhesion-mediated drug resistance (CAM-DR), has been demonstrated in many subsets of leukemia including B- and T-acute lymphoblastic leukemia (B- and T-ALL) and acute myeloid leukemia (AML). Cell adhesion molecules (CAMs) are surface molecules that allow cell-cell or cell-extracellular matrix (ECM) adhesion. CAMs not only recognize ligands for binding but also initiate the intracellular signaling pathways that are associated with cell proliferation, survival, and drug resistance upon binding to their ligands. Cadherins, selectins, and integrins are well-known cell adhesion molecules that allow binding to neighboring cells, ECM proteins, and soluble factors. The expression of cadherin, selectin, and integrin correlates with the increased drug resistance of leukemia cells. This paper will review the role of cadherins, selectins, and integrins in CAM-DR and the results of clinical trials targeting these molecules.

Exosomes-Mediated Transfer of Itga2 Promotes Migration and Invasion of Prostate Cancer Cells by Inducing Epithelial-Mesenchymal Transition

Although integrin alpha 2 subunit (ITGA2) mediates cancer progression and metastasis, its transfer by exosomes has not been investigated in prostate cancer (PCa). We aimed to determine the role of exosomal ITGA2 derived from castration-resistant PCa (CRPC) cells in promoting aggressive phenotypes in androgen receptor (AR)-positive cells. Exosomes were co-incubated with recipient cells and tested for different cellular assays. ITGA2 was enriched in exosomes derived from CRPC cells. Co-culture of AR-positive cells with CRPC-derived exosomes increased their proliferation, migration, and invasion by promoting epithelial-mesenchymal transition, which was reversed via ITGA2 knockdown or inhibition of exosomal uptake by methyl-β-cyclodextrin (MβCD). Ectopic expression of ITGA2 reproduced the effect of exosomal ITGA2 in PCa cells. ITGA2 transferred by exosomes exerted its effect within a shorter time compared to that triggered by its endogenous expression. The difference of ITGA2 protein expression in localized tumors and those with lymph node metastatic tissues was indistinguishable. Nevertheless, its abundance was higher in circulating exosomes collected from PCa patients when compared with normal subjects. Our findings indicate the possible role of the exosomal-ITGA2 transfer in altering the phenotype of AR-positive cells towards more aggressive phenotype. Thus, interfering with exosomal cargo transfer may inhibit the development of aggressive phenotype in PCa cells.

Serendipity: Reflections on Being Mentored by Dr. Peter Doherty

This is a semiautobiographical and scientific account of my time in the Doherty Laboratory from 1994 to 1999. It includes personal vignettes as well as discussion of how our work has impacted the fields of influenza, respiratory infections and immunity. I also point out the long-term impacts on my career.

α2β1 Integrin Is Required for Optimal NK Cell Proliferation during Viral Infection but Not for Acquisition of Effector Functions or NK Cell-Mediated Virus Control

NK cells play an important role in antiviral resistance. The integrin α2, which dimerizes with integrin β1, distinguishes NK cells from innate lymphoid cells 1 and other leukocytes. Despite its use as an NK cell marker, little is known about the role of α2β1 in NK cell biology. In this study, we show that in mice α2β1 deficiency does not alter the balance of NK cell/ innate lymphoid cell 1 generation and slightly decreases the number of NK cells in the bone marrow and spleen without affecting NK cell maturation. NK cells deficient in α2β1 had no impairment at entering or distributing within the draining lymph node of ectromelia virus (ECTV)-infected mice or at becoming effectors but proliferated poorly in response to ECTV and did not increase in numbers following infection with mouse CMV (MCMV). Still, α2β1-deficient NK cells efficiently protected from lethal mousepox and controlled MCMV titers in the spleen. Thus, α2β1 is required for optimal NK cell proliferation but is dispensable for protection against ECTV and MCMV, two well-established models of viral infection in which NK cells are known to be important.

Distinct Migratory Properties of M1, M2, and Resident Macrophages Are Regulated by αDβ2 and αMβ2 Integrin-Mediated Adhesion

Chronic inflammation is essential mechanism during the development of cardiovascular and metabolic diseases. The outcome of diseases depends on the balance between the migration/accumulation of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages in damaged tissue. The mechanism of macrophage migration and subsequent accumulation is still not fully understood. Currently, the amoeboid adhesion-independent motility is considered essential for leukocyte migration in the three-dimensional environment. We challenge this hypothesis by studying the contribution of leukocyte adhesive receptors, integrins α_Mβ₂, and α_Dβ₂, to three-dimensional migration of M1-polarized, M2-polarized, and resident macrophages. Both integrins have a moderate expression on M2 macrophages, while α_Dβ₂ is upregulated on M1 and α_Mβ₂ demonstrates high expression on resident macrophages. The level of integrin expression determines its contribution to macrophage migration. Namely, intermediate expression supports macrophage migration, while a high integrin density inhibits it. Using in vitro three-dimensional migration and in vivo tracking of adoptively-transferred fluorescently-labeled macrophages during the resolution of inflammation, we found that strong adhesion of M1-activated macrophages translates to weak 3D migration, while moderate adhesion of M2-activated macrophages generates dynamic motility. Reduced migration of M1 macrophages depends on the high expression of α_Dβ₂, since α_D-deficiency decreased M1 macrophage adhesion and improved migration in fibrin matrix and peritoneal tissue. Similarly, the high expression of α_Mβ₂ on resident macrophages prevents their amoeboid migration, which is markedly increased in α_M-deficient macrophages. In contrast, α_D- and α_M-knockouts decrease the migration of M2 macrophages, demonstrating that moderate integrin expression supports cell motility. The results were confirmed in a diet-induced diabetes model. α_D deficiency prevents the retention of inflammatory macrophages in adipose tissue and improves metabolic parameters, while α_M deficiency does not affect macrophage accumulation. Summarizing, β₂ integrin-mediated adhesion may inhibit amoeboid and mesenchymal macrophage migration or support mesenchymal migration in tissue, and, therefore, represents an important target to control inflammation.

Pannexin- and Connexin-Mediated Intercellular Communication in Platelet Function

The three major blood cell types, i.e., platelets, erythrocytes and leukocytes, are all produced in the bone marrow. While red blood cells are the most numerous and white cells are the largest, platelets are small fragments and account for a minor part of blood volume. However, platelets display a crucial function by preventing bleeding. Upon vessel wall injury, platelets adhere to exposed extracellular matrix, become activated, and form a platelet plug preventing hemorrhagic events. However, when platelet activation is exacerbated, as in rupture of an atherosclerotic plaque, the same mechanism may lead to acute thrombosis causing major ischemic events such as myocardial infarction or stroke. In the past few years, major progress has been made in understanding of platelet function modulation. In this respect, membrane channels formed by connexins and/or pannexins are of particular interest. While it is still not completely understood whether connexins function as hemichannels or gap junction channels to inhibit platelet aggregation, there is clear-cut evidence for a specific implication of pannexin1 channels in collagen-induced aggregation. The focus of this review is to summarize current knowledge of the role of connexins and pannexins in platelet aggregation and to discuss possible pharmacological approaches along with their limitations and future perspectives for new potential therapies.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Functional Display of an α2 Integrin-Specific Motif (RKK) on the Surface of Baculovirus Particles

The use of baculovirus vectors shows promise as a tool for gene delivery into mammalian cells. These insect viruses have been shown to transduce a variety of mammalian cell lines, and gene transfer has also been demonstrated in vivo. In this study, we generated two recombinant baculovirus vectors displaying an integrin-specific motif, RKK, as a part of two different loops of the green fluorescent protein (GFP) fused with the major envelope protein gp64 of Autographa californica M nucleopolyhedrovirus. By enzyme linked immunosorbent assays, these viruses were shown to bind a peptide representing the receptor binding site of an α2 integrin, the α2I-domain. However, the interaction was not strong enough to overcome binding of wild type gp64 to the unknown cellular receptor(s) on the surface of α2 integrin-expressing cells (CHO-α2β1) or enhance the viral uptake. After treatment of these cells with phospholipase C, internalization of all viruses was blocked or decreased significantly. However, one of the RKK displaying viruses, AcGFP(K)gp64, was still able to internalize into CHO-α2β1 cells, although at a lower level as compared to non-treated cells. This may indicate the possible utilization of a PLC independent alternative route via, in this case, the α2β1 integrin.

Integrin α3β1 Binding to Fibronectin Is Dependent on the Ninth Type III Repeat

Fibronectin (Fn) is a promiscuous ligand for numerous cell adhesion receptors or integrins. The vast majority of Fn-integrin interactions are mediated through the Fn Arg-Gly-Asp (RGD) motif located within the tenth type III repeat. In the case of integrins αIIbβ3 and α5β1, the integrin binds RGD and the synergy site (PHSRN) located within the adjacent ninth type III repeat. Prior work has shown that these synergy-dependent integrins are exquisitely sensitive to perturbations in the Fn integrin binding domain conformation. Our own prior studies of epithelial cell responses to recombinant fragments of the Fn integrin binding domain led us to hypothesize that integrin α3β1 binding may also be modulated by the synergy site. To explore this hypothesis, we created a variety of recombinant variants of the Fn integrin binding domain: (i) a previously reported (Leu → Pro) stabilizing mutant (FnIII9'10), (ii) an Arg to Ala synergy site mutation (FnIII9(R)→(A)10), (iii) a two-Gly (FnIII9(2G)10) insertion, and (iv) a four-Gly (FNIII9(4G)10) insertion in the interdomain linker region and used surface plasmon resonance to determine binding kinetics of integrin α3β1 to the Fn fragments. Integrin α3β1 had the highest affinity for FnIII9'10 and FnIII9(2G)10. Mutation within the synergy site decreased integrin α3β1 binding 17-fold, and the four-Gly insertion decreased binding 39-fold compared with FnIII9'10. Cell attachment studies demonstrate that α3β1-mediated epithelial cell binding is greater on FnIII9'10 compared with the other fragments. These studies suggest that the presence and spacing of the RGD and synergy sites modulate integrin α3β1 binding to Fn.

Type 1 regulatory T cells: a new mechanism of peripheral immune tolerance

The lack of immune response to an antigen, a process known as immune tolerance, is essential for the preservation of immune homeostasis. To date, two mechanisms that drive immune tolerance have been described extensively: central tolerance and peripheral tolerance. Under the new nomenclature, thymus-derived regulatory T (tT(reg)) cells are the major mediators of central immune tolerance, whereas peripherally derived regulatory T (pT(reg)) cells function to regulate peripheral immune tolerance. A third type of T(reg) cells, termed iT(reg), represents only the in vitro-induced T(reg) cells(1). Depending on whether the cells stably express Foxp3, pT(reg), and iT(reg) cells may be divided into two subsets: the classical CD4(+)Foxp3(+) T(reg) cells and the CD4(+)Foxp3(-) type 1 regulatory T (Tr1) cells(2). This review focuses on the discovery, associated biomarkers, regulatory functions, methods of induction, association with disease, and clinical trials of Tr1 cells.

Influence of extracellular matrix components on the expression of integrins and regeneration of adult retinal ganglion cells

PURPOSE

Retinal ganglion cells (RGCs) are exposed to injury in a variety of optic nerve diseases including glaucoma. However, not all cells respond in the same way to damage and the capacity of individual RGCs to survive or regenerate is variable. In order to elucidate factors that may be important for RGC survival and regeneration we have focussed on the extracellular matrix (ECM) and RGC integrin expression. Our specific questions were: (1) Do adult RGCs express particular sets of integrins in vitro and in vivo? (2) Can the nature of the ECM influence the expression of different integrins? (3) Can the nature of the ECM affect the survival of the cells and the length or branching complexity of their neurites?

METHODS

Primary RGC cultures from adult rat retina were placed on glass coverslips treated with different substrates: Poly-L-Lysine (PL), or PL plus laminin (L), collagen I (CI), collagen IV (CIV) or fibronectin (F). After 10 days in culture, we performed double immunostaining with an antibody against βIII-Tubulin to identify the RGCs, and antibodies against the integrin subunits: αV, α1, α3, α5, β1 or β3. The number of adhering and surviving cells, the number and length of the neurites and the expression of the integrin subunits on the different substrates were analysed.

RESULTS

PL and L were associated with the greatest survival of RGCs while CI provided the least favourable conditions. The type of substrate affected the number and length of neurites. L stimulated the longest growth. We found at least three different types of RGCs in terms of their capacity to regenerate and extend neurites. The different combinations of integrins expressed by the cells growing on different substrata suggest that RGCs expressed predominantly α1β1 or α3β1 on L, α1β1 on CI and CIV, and α5β3 on F. The activity of the integrins was demonstrated by the phosphorylation of focal adhesion kinase (FAK).

CONCLUSIONS

Adult rat RGCs can survive and grow in the presence of different ECM tested. Further studies should be done to elucidate the different molecular characteristics of the RGCs subtypes in order to understand the possible different sensitivity of different RGCs to damage in diseases like glaucoma in which not all RGCs die at the same time.

α2β1 Integrin

The α2β1 integrin, also known as VLA-2, GPIa-IIa, CD49b, was first identified as an extracellular matrix receptor for collagens and/or laminins [55, 56]. It is now recognized that the α2β1 integrin serves as a receptor for many matrix and nonmatrix molecules [35, 79, 128]. Extensive analyses have clearly elucidated the α2 I domain structural motifs required for ligand binding, and also defined distinct conformations that lead to inactive, partially active or highly active ligand binding [3, 37, 66, 123, 136, 137, 140]. The mechanisms by which the α2β1 integrin plays a critical role in platelet function and homeostasis have been carefully defined via in vitro and in vivo experiments [76, 104, 117, 125]. Genetic and epidemiologic studies have confirmed human physiology and disease states mediated by this receptor in immunity, cancer, and development [6, 20, 21, 32, 43, 90]. The role of the α2β1 integrin in these multiple complex biologic processes will be discussed in the chapter.

Echovirus 1 entry into polarized Caco-2 cells depends on dynamin, cholesterol, and cellular factors associated with macropinocytosis

Enteroviruses invade their hosts by crossing the intestinal epithelium. We have examined the mechanism by which echovirus 1 (EV1) enters polarized intestinal epithelial cells (Caco-2). Virus binds to VLA-2 on the apical cell surface and moves rapidly to early endosomes. Using inhibitory drugs, dominant negative mutants, and small interfering RNAs (siRNAs) to block specific endocytic pathways, we found that virus entry requires dynamin GTPase and membrane cholesterol but is independent of both clathrin- and caveolin-mediated endocytosis. Instead, infection requires factors commonly associated with macropinocytosis, including amiloride-sensitive Na(+)/H(+) exchange, protein kinase C, and C-terminal-binding protein-1 (CtBP1); furthermore, EV1 accumulates rapidly in intracellular vesicles with dextran, a fluid-phase marker. These results suggest a role for macropinocytosis in the process by which EV1 enters polarized cells to initiate infection.

Leukocyte integrins αLβ2, αMβ2 and αXβ2 as collagen receptors--receptor activation and recognition of GFOGER motif

Integrins αLβ2, αMβ2 and αXβ2 are expressed on leukocytes. Their primary ligands are counter transmembrane receptors or plasma proteins, such as intercellular cell adhesion molecule-1 (ICAM-1) or components of complement system (iC3b, iC4b), respectively. Function blocking antibodies for these integrins may also reduce cell adhesion to collagens. To make the first systematical comparison of human α(L)β2, α(M)β2 and α(X)β2 as collagen receptors, we produced the corresponding integrin αI domains both in wild-type and activated form and measured their binding to collagens I-VI. In the "closed" (wild-type) conformation, the α(L)I and α(M)I domains bound with low avidity to their primary ligands, and the interaction with collagens was also very weak. Gain-of-function mutations α(L) I306G, α(L) K287C/K294C and α(M) I316G are considered to mimic "open", activated αI domains. The binding of these activated αI domains to the primary ligands was clearly stronger and they also recognized collagens with moderate avidity (K(d)400 nM). After activation, the αLI domain favored collagen I (K(d )≈ 80 nM) when compared to collagen IV. The integrin αXI domain acted in a very different manner since already in native, wild-type form it bound to collagen IV and iC3b (K(d) ≈ 200-400 nM). Antibodies against αXβ2 and αMβ2 blocked promyelocytic leukemia cell adhesion to the collagenous GFOGER motif, a binding site for the β1 integrin containing collagen receptors. In brief, leukocyte β2 integrins may act as collagen receptors in a heterodimer specific manner.

alpha2beta1 integrin controls association of Rac with the membrane and triggers quiescence of endothelial cells

Integrin receptors and their extracellular matrix ligands provide cues to cell proliferation, survival, differentiation and migration. Here, we show that alpha2beta1 integrin, when ligated to the basement membrane component laminin-1, triggers a proliferation arrest in primary endothelial cells. Indeed, in the presence of strong growth signals supplied by growth factors and fibronectin, alpha2beta1 engagement alters assembly of mature focal adhesions by alpha5beta1 and leads to impairment of downstream signaling and cell-cycle arrest in the G1 phase. Although the capacity of alpha5beta1 to signal for GTP loading of Rac is preserved, the joint engagement of alpha2beta1 interferes with membrane anchorage of Rac. Adapting the 'split-ubiquitin' sensor to screen for membrane-proximal alpha2 integrin partners, we identified the CD9 tetraspanin and further establish its requirement for destabilization of focal adhesions, control of Rac subcellular localization and growth arrest induced by alpha2beta1 integrin. Altogether, our data establish that alpha2beta1 integrin controls endothelial cell commitment towards quiescence by triggering a CD9-dependent dominant signaling.

Adhesion of monocytes to type I collagen stimulates an APP-dependent proinflammatory signaling response and release of Abeta1-40

BACKGROUND

Amyloid precursor protein (APP) is a ubiquitously expressed cell surface protein reported to be involved in mediating cell-cell or cell-matrix interactions. Prior work has demonstrated that APP co-localizes with beta1 integrin in different cell types.

METHODS

In an effort to determine the function of APP on monocytic lineage cells, in particular, the human monocyte cell line, THP-1, was used to assess the role of APP during adhesion to the extracelluar matrix component type I collagen.

RESULTS

Pull-down assays demonstrated that THP-1 adhesion to collagen stimulated a tyrosine kinase-associated signaling response which included subsequent phosphorylation of p38 MAP kinase and increased association of APP with alpha2beta1 integrin, specifically. In addition, cell adhesion was dependent upon APP expression since APP siRNA knockdown attenuated THP-1 adhesion to collagen compared to mock transfected controls. One consequence of the tyrosine kinase-dependent signaling response was increased secretion of interleukin-1beta (IL-1beta) and Abeta1-40 but not the Abeta1-42 fragment of APP. Increased secretion of IL-1beta was dependent upon p38 MAP kinase activity while Abeta1-40 secretion required Src family kinase activity since the specific p38 inhibitor, SB202190, and the Src family kinase inhibitor, PP2, attenuated IL-1beta and Abeta1-40 secretion, respectively.

CONCLUSIONS

These data demonstrate that APP is involved in classic integrin-dependent tyrosine kinase-associated adhesion and activation of peripheral monocytic cells. Moreover, divergent APP-dependent signaling is required for increased secretion of both IL-1beta and Abeta1-40 as a component of the adhesion-dependent change in phenotype. This suggests that APP may have a broad role in not only mediating cell-matrix adhesion but also in the function of peripheral immune cells.

Integrins in the endometrium

The endometrium expresses many of the same integrins displayed by other tissues. Endometrial epithelial cells maintain the ‘classic’ epithelial integrins, including α2, α3, α6, and β4, while the stroma expresses the fibronectin receptor, α5β1. During the menstrual cycle, the endometrium undergoes dynamic changes in morphology in preparation for implantation. With these histological changes are concomitant alterations in integrin expression that appear to ‘frame’ the window of implantation, by the co-expression of glandular αvβ3 and α4β1 during days 20 to 24 of the menstrual cycle. The changes in integrin expression shift from epithelial to stroma predominance late in the menstrual cycle, extending into early pregnancy. Decidual integrins that appear upregulated in pregnancy include α1β1, α3β1, α6β1 and αvβ3. Markers of uterine receptivity hold promise for a better understanding of the implantation process and may help to explain many different types of infertility. These markers will be essential for monitoring and improving infertility therapies. The importance of integrins in the human endometrium now seems well established and promises to be an area of great clinical and basic science activity in the future.

Interleukin-11 and leukemia inhibitory factor regulate the adhesion of endometrial epithelial cells: implications in fertility regulation

Embryo implantation requires the closely harmonized processes of apposition, attachment, and adhesion of the conceptus to the maternal endometrial epithelium. IL-11 and leukemia inhibitory factor (LIF), two IL-6 family cytokines, are produced by the endometrium and are absolutely required for implantation in mice. We examined the effect of IL-11 and LIF on human endometrial epithelial cell adhesion. Both cytokines increased adhesion of primary human endometrial epithelial cells to fibronectin and collagen IV. IL-11 stimulated, whereas LIF had no effect on the adhesion of trophoblast to endometrial epithelial cells. Focused oligogene arrays were used to identify extracellular matrix and adhesion molecules mRNAs regulated by endometrial epithelial cells. We demonstrated by real-time RT-PCR and antibody arrays that both cytokines increased integrin-alpha2 mRNA and protein by endometrial epithelial cells. Signal transducers and activators of transcription (STAT)-3 inhibition reduced IL-11- and LIF-mediated epithelial cell adhesion to fibronectin, suggesting both cytokines regulated adhesion via phosphorylation of STAT3. Addition of either IL-11 neutralizing antibody and IL-11 or LIF and LIF antagonist to endometrial epithelial cells abolished cytokine induced phosphorylated STAT3. LIF but not IL-11 induced adhesion to collagen IV was reduced by an integrin-alpha2beta1 neutralizing antibody. This study demonstrated that IL-11 and LIF regulated endometrial epithelial cell adhesion, suggesting that targeting IL-11 and LIF may be useful in regulating fertility by either enhancing or blocking implantation.

Integrins in the normal and healing corneal epithelium

Integrins are heterodimeric plasma membrane glycoproteins involved in cell-matrix and cell-cell interactions. The present communication reviews the distribution of several of the currently known integrin subunits in the corneal epithelium. The corneal epithelium contains the following integrin heterodimers: alpha 2 beta 1, alpha 3 beta 1, and alpha 6 beta 4. The expression of alpha v with an unknown beta subunit is also recognized, whereas the expression of alpha 4 and alpha 5 subunits remains controversial. Some of the changes occurring in the distribution of integrins in response to wounding will also be discussed.

Estradiol and tamoxifen differently affects the inhibitory effects of vitamin A and their metabolites on the proliferation and expression of alpha2beta1 integrins in MCF-7 breast cancer cells

PURPOSE

Retinoids are well known inhibitors of estrogen-dependent breast cancer cell growth and differentiation. alpha2beta1 integrins are involved in the normal growth and differentiation of breast cells, they also take part in many pathological processes including malignancies. The aim of the study was to evaluate the effect of estradiol and tamoxifen on the inhibitory action of retinoids on the proliferation of MCF-7 breast cancer cells and alpha2beta1 integrin expression.

MATERIALS AND METHODS

Evaluation was based on [3H]thymidine incorporation and the proliferative activity of PCNA- and Ki 67-positive cells. Expression of alpha2beta1 was assessed through immunocytochemical analysis.

RESULTS

Treatment of cancer cells with the examined compounds and tamoxifen (10 microM) revealed that only 13-cis retinoic acid (13-cis RA) and all-trans retinoic acid (ATRA) (10(-5) M) decreased cells proliferation compared to the tamoxifen group (30.84%+/-3.32, p<0.01 and 31.05%+/-4.67, p<0.01, respectively). The lowest fraction of PCNA positive cells was also observed after the simultaneous addition ATRA (10(-5) M) and tamoxifen (10 microM) (30.75%+/-0.95, p<0.01, compared to the tamoxifen group). Our results showed that the decrease of alpha2beta1 integrin expression by 13-cis RA (10(-5) M, 49.6+/-3.25%) and ATRA (10-9 M, 15.0%+/-5.0) was augmented by tamoxifen and to a lesser extent by estradiol, particularly in the case of ATRA at 10(-7) or 10(-9) M.

CONCLUSIONS

This data suggest that tamoxifen augments the inhibitory effect of retinoids on proliferation and alpha2beta1 integrin expression in MCF-7 cells.

Induction of a VLA-2 (CD49b)-expressing effector T cell population by a cell-based neuroblastoma vaccine expressing CD137L

In malignancies where no universally expressed dominant Ag exists, the use of tumor cell-based vaccines has been proposed. We have modified a mouse neuroblastoma cell line to express either CD80 (B7.1), CD137L (4-1BBL), or both receptors on the tumor cell surface. Vaccines expressing both induce a strong T cell response that is unique in that among responding CD8 T cells, a T effector memory cell (T(EM)) response arises in which a large number of the T(EM) express the alpha-chain of VLA-2, CD49b. We demonstrate using both in vitro and in vivo assays that the CD49b(+) CD8 T cell population is a far more potent antitumor effector cell population than nonfractionated CD8 or CD49b(-) CD8 T cells and that CD49b on vaccine-induced CD8 T cells mediates invasion of a collagen matrix. In in vivo rechallenge studies, CD49b(+) T cells no longer expanded, indicating that CD49b T(EM) expansion is restricted to the initial response to vaccine. To demonstrate a mechanistic link between the expression of costimulatory molecules on the vaccine and CD49b on responding T cells, we stimulated naive T cells in vitro with artificial APC expressing different combinations of anti-CD3, anti-CD28, and CD137L. Although some mRNA encoding CD49b was induced by combining anti-CD3 with anti-CD28 or CD137L, the highest level was induced when all three signals were present. This indicates that CD49b expression results from additive costimulation and that the level of CD49b message serves as an indicator of the effectiveness of T cell activation by a cell-based vaccine.

Molecular profile of endothelial invasion of three-dimensional collagen matrices: insights into angiogenic sprout induction in wound healing

Sprouting angiogenesis is a multistep process consisting of basement membrane degradation, endothelial cell (EC) activation, proliferation, invasion, lumen formation, and sprout stabilization. Such complexity is consistent with a requirement for orchestration of individual gene expression alongside multiple signaling pathways. To better understand the mechanisms that direct the transformation of adherent ECs on the surface of collagen matrices to develop multicellular invading sprouts, we analyzed differential gene expression with time using a defined in vitro model of EC invasion driven by the combination of sphingosine-1-phosphate, basic FGF, and VEGF. Gene expression changes were confirmed by real-time PCR and Western blot analyses. A cohort of cell adhesion molecule genes involved in adherens junction and cell-extracellular matrix (ECM) interactions were upregulated, whereas a set of genes associated with tight junctions were downregulated. Numerous genes encoding ECM proteins and proteases were induced, indicating that biosynthesis and remodeling of ECM is indispensable for sprouting angiogenesis. Knockdown of a highly upregulated gene, a disintegrin and metalloproteinase with thrombospondin-type repeats-1 (ADAMTS1), decreased invasion responses, confirming a role for ADAMTS1 in mediating EC invasion. Furthermore, differential expression of multiple members of the Wnt and Notch pathways was observed. Functional experiments indicated that inhibition and activation of the Notch signaling pathway stimulated and inhibited EC invasion responses, respectively. This study has enhanced the molecular road map of gene expression changes that occur during endothelial invasion and highlighted the utility of three-dimensional models to study EC morphogenesis.

Autocrine semaphorin3A stimulates alpha2 beta1 integrin expression/function in breast tumor cells

The axon repulsion factor semaphorin3A (SEMA3A) and its receptor neuropilin-1 (NP-1) are expressed in breast tumor cells, and function as suppressors of tumor cell migration. Based on the knowledge that both SEMA3A and the alpha2beta1 integrin suppress breast tumor cell migration, we studied the impact of SEMA3A signaling on alpha2beta1 integrin expression/function. The incubation of breast tumor cells with SEMA3A increased alpha2 and beta1 integrin levels, and stimulated tumor cell adhesion to the alpha2beta1-binding matrix protein collagen I. Conversely, reducing SEMA3A expression in breast tumor cells decreased alpha2beta1 levels and collagen adhesion. The ability of SEMA3A to increase tumor cell adhesion to collagen was dependent on both the SEMA3A receptor NP-1 and the glycogen synthase kinase-3. The incubation of breast tumor cells with SEMA3A disrupted the actin cytoskeleton, and reduced both tumor cell migratory and invasive behavior. Importantly, using an alpha2beta1-neutralizing antibody, we demonstrated that SEMA3A suppression of tumor cell migration is dependent on alpha2beta1. Our studies indicate that expression of the alpha2beta1 integrin, a suppressor of metastatic breast tumor growth, is stimulated in breast tumor cells by an autocrine SEMA3A pathway.

Bedeutung von Polymorphismen für venöse und arterielle Thrombosen

Die Polymorphismen der thrombozytären Glykoproteine Ia-IIa (GP Ia C807T) und HPA-1a/1b (GP IIIa T1565C), des Fibrinolyseinhibitors PAI-1 675 4G/5G sowie der Methylentetrahydrofolatreduktase MTHFR C677T werden immer wieder als Risikofaktoren für arterielle und venöse Thrombembolien diskutiert. Ihr prädiktiver Wert für das Auftreten von thrombembolischen Ereignissen ist jedoch aufgrund einer widersprüchlichen Datenlage unklar. Nach derzeitigem Kenntnisstand gehen die genannten Polymorphismen nicht gesichert mit einem erhöhten Thrombembolierisiko einher. Daher ist ihre Untersuchung zurzeit weder in der arteriellen noch in der venösen Thrombophiliediagnostik indiziert. Aufgrund der hohen Prävalenzen dieser genetischen Merkmale bedarf es weiterer Untersuchungen, um die Grenzen zwischen Confoundereffekt und Risikofaktor zu definieren.

In vivo imaging of the immune response in the eye

The immune system is governed by dynamic events involving in part direct intercellular interactions between an immune cell and other cells or the cell's environment. Owing to its unique optical characteristics, the eye offers remarkable opportunities for the analysis of the immune system by intravital microscopy. In this review, we present a brief overview of the current state of knowledge of leukocyte trafficking in each of three anatomically distinct and medically important regions of the eye (cornea, iris, retina) as determined by the application of intravital microscopy to animal models of disease. Additionally, we discuss the use of ocular imaging in patients and volunteers. Finally, we examine the future prospects for this field in terms of its potential for impacting our understanding of fundamental immunological phenomena.

Cytofluorometric analysis of phenotypes of human bone marrow and umbilical fibroblast-like cells

Comparative analysis of the expression of some surface markers of human bone marrow mesenchymal stem cells, umbilical fibroblast-like cells, and skin fibroblasts was carried out by the flow cytofluorometry method. Mesenchymal stem cells and umbilical fibroblast-like cells were similar by the levels of expression of the main histocompatibility complex antigens, adhesion molecules, and some growth factor receptors. The profile of skin fibroblast surface antigens was characterized by higher expression of the markers typical of differentiated cells. The results prove the possibility of using umbilical fibroblast-like cells as an alternative source of mesenchymal stem cells for cell replacement therapy.

Cellular immune response to an engineered cell-based tumor vaccine at the vaccination site

The engineered expression of the immune co-stimulatory molecules CD80 and CD137L on the surface of a neuroblastoma cell line converts this tumor into a cell-based cancer vaccine. The mechanism by which this vaccine activates the immune system was investigated by capturing and analyzing immune cells responding to the vaccine cell line embedded in a collagen matrix and injected subcutaneously. The vaccine induced a significant increase in the number of activated CD62L(-) CCR7(-) CD49b(+) CD8 effector memory T cells captured in the matrix. Importantly, vaccine responsive cells could be detected in the vaccine matrix within a matter of days as demonstrated by IFN-gamma production. The substitution of unmodified tumor cells for the vaccine during serial vaccination resulted in a significant decrease in activated T cells present in the matrix, indicating that immune responses at the vaccine site are a dynamic process that must be propagated by continued co-stimulation.

A simple cell motility assay demonstrates differential motility of human periodontal ligament fibroblasts, gingival fibroblasts, and pre-osteoblasts

During periodontal regeneration, multiple cell types can invade the wound site, thereby leading to repair. Cell motility requires interactions mediated by integrin receptors for the extracellular matrix (ECM), which might be useful in guiding specific cell populations into the periodontal defect. Our data demonstrate that fibroblasts exhibit differential motility when grown on ECM proteins. Specifically, gingival fibroblasts are twice as motile as periodontal ligament fibroblasts, whereas osteoblasts are essentially non-motile. Collagens promote the greatest motility of gingival fibroblasts in the following order: collagen III>collagen V>collagen I. Differences in motility do not correlate with cell proliferation or integrin expression. Osteoblasts display greater attachment to collagens than does either fibroblast population, but lower motility. Gingival fibroblast motility on collagen I is generally mediated by alpha2 integrins, whereas motility on collagen III involves alpha1 integrins. Other integrins (alpha10 or alpha11) may also contribute to gingival fibroblast motility. Thus, ECM proteins do indeed differentially promote the cell motility of periodontal cells. Because of their greater motility, gingival fibroblasts have more of a potential to invade periodontal wound sites and to contribute to regeneration. This finding may explain the formation of disorganized connective tissue masses rather than the occurrence of the true regeneration of the periodontium.

Biological activity of rainbow trout Ea4-peptide of the pro-insulin-like growth factor (pro-IGF)-I on promoting attachment of breast cancer cells (MDA-MB-231) via alpha2- and beta1-integrin

E-peptide of pro-IGF-I was considered as biologically inactive. We have demonstrated that rainbow trout (rt) Ea4-peptide exerted biological activities in several established tumor cell lines [Chen et al., 2002; Kuo and Chen, 2002]. Here we report the activity of rtEa4-peptide in promoting attachment of human breast cancer cells (MDA-MB-231). While rtEa2-, rtEa3-, and rtEa4-peptides enhanced the attachment of MDA-MB-231 cells in a dose dependent manner, rtEa4-peptide possessed the highest activity. Antibodies specific to alpha2 and beta1 integrins significantly inhibited the attachment of cells to rtEa4-peptide coated-plates by 40%. In addition, rtEa4-peptide induced the expression of fibronectin 1 and laminin receptor genes in MDA-MB-231 cells. Blocking new protein synthesis by cycloheximide significantly reduced the attachment of MDA-MB-231 cells to rtEa4-peptide coated wells by 50%. These results suggest that rtEa4-peptide may promote cell attachment by interacting with alpha2/beta1 integrin receptors at the cell surface and by inducing the expression of fibronectin 1 and laminin receptor genes. Expression of fibronectin 1 gene induced by rtEa4-peptide in MDA-MB-231 cells was abolished by inhibitors of PI3K, PKC, Mek1/2, JNK1/2, and p38 MAPK signaling transduction molecules. These results suggested that induction of fibronectin 1 gene expression in MDA-MB-231 cells by rtEa4-peptide may be mediated via PI3K, PKC, Mek1/2, JNK1/2, and p38 MAPK signal transduction molecules.

The alpha2beta1 integrin: a novel collectin/C1q receptor

Our laboratory focuses on the alpha2beta1 integrin, a receptor for a number of matrix and non-matrix ligands, including collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses. The alpha2beta1 integrin is expressed on numerous different cell types, including epithelial cells, endothelial cells, fibroblasts, and hematopoietic elements, including platelets and specific subsets of leukocytes. Although alpha2beta1 integrin expression is widespread, it is not ubiquitous. Rather, it is expressed in a differentiation-dependent and activation-dependent manner. Interactions between the alpha2beta1 integrin and extracellular matrix ligands have been implicated in important biological processes including inflammation and immunity. Studies from a number of laboratories have demonstrated a role for the alpha2beta1 integrin during the immune response. Our laboratory generated an alpha2beta1 integrin-deficient mouse to define the role of the alpha2beta1 integrin in vivo. Our studies demonstrated that the alpha2-null mice have a profound defect in the innate immune response. We have recently reported the identification of a novel family of ligands for the alpha2beta1 integrin, which include C1q and the collectins. The goal of this article is to review the important role that the interaction between the alpha2beta1 integrin and C1q plays in the innate immune response. The identification of C1q and the collectins as ligands for the alpha2beta1 integrin suggests that the integrin may play important roles in a number of immunological responses.

Haemostasis

When the continuity of the vascular endothelium is disrupted, platelets and fibrin seal off the defect. Haemostatic processes are classified as primary (mainly involving platelets) and secondary (mainly related to fibrin formation or blood coagulation). When the blood clot is no longer required for haemostasis, the fibrinolytic system will dissolve it. The pivotal ligand for initial platelet recruitment to injured vessel wall components is von Willebrand factor (vWF), a multimeric protein present in the subendothelium and in plasma, where it is conformationally activated by shear forces. Adhering activated platelets recruit additional platelets, which are in turn activated and form a platelet aggregate. Coagulation is initiated by a reaction, activating factors IX and X. Once critical amounts of factor Xa are generated, thrombin generation is initiated and soluble fibrinogen is converted into insoluble fibrin. Excessive thrombin generation is prevented via inhibition by antithrombin and also via downregulation of its further generation by activation of the protein C pathway. Activation of the fibrinolytic system results from conversion of the proenzyme plasminogen into the active serine proteinase plasmin by tissue-type or urokinase-type plasminogen activators. Plasmin digests the fibrin component of a blood clot. Inhibition of the fibrinolytic system occurs at the level of the plasminogen activator (by plasminogen activator inhibitors) or at the level of plasmin (by alpha2-antiplasmin). Together, these physiological processes act to maintain normal functioning blood vessels and a non-thrombotic state.

Interactions of primary fibroblasts and keratinocytes with extracellular matrix proteins: contribution of α2β1 integrin

The α2β1 integrin is a collagen-binding protein with very high affinity for collagen I. It also binds several other collagens and laminins and it is expressed by many cells, including keratinocytes and fibroblasts in the skin. In the past, α2β1 integrin was suggested to be responsible for cell attachment, spreading and migration on monomeric collagen I and contraction of three-dimensional collagen lattices. In view of these functions, normal development and fertility in integrin α2-deficient mice, which we generated by targeting the integrin α2 gene, came as a surprise. This suggested the existence of compensatory mechanisms that we investigate here using primary fibroblasts and keratinocytes isolated from wild-type and α2-deficient mice, antibodies blocking integrin function and downregulation of integrin α2 expression. The results show that the α2β1 integrin is absolutely required for keratinocyte adhesion to collagens whereas for fibroblasts other collagen-binding integrins partially back-up the lack of α2β1 in simple adhesion to collagen monomers. A prominent requirement for α2β1 integrins became apparent when fibroblasts executed mechanical tasks of high complexity in three-dimensional surroundings, such as contracting free-floating collagen gels and developing isometric forces in tethered lattices. The deficits observed for α2-deficient fibroblasts appeared to be linked to alterations in the distribution of force-bearing focal adhesions and deregulation of Rho-GTPase activation.

Antigen-specific accumulation of naïve, memory and effector CD4 T cells during anterior uveitis monitored by intravital microscopy

Uveitis is an immune-mediated ocular disease and a leading cause of blindness. We characterized a novel model of uveitis with intravital microscopy. Transfer of ovalbumin-specific T cells from DO11.10 spleen to BALB/c recipients and subsequent challenge with ovalbumin in the anterior chamber of the eye resulted in anterior uveitis. Antigen-specificity was verified by injection of irrelevant antigen and transfer of T cells with a different specificity. Subsets of CD4 T cells, including naive (DO11.10 RAG(-/-)) and in vitro-activated Th2 effector CD4 T cells, infiltrated anterior segment tissues early in the inflammation. Memory-like CD44(high) CD4 T cells from unprimed transgenic mice and in vitro-activated Th1 effector CD4 T cells accumulated to larger numbers than naive or Th2 effector cells at 48 and 72 h. Of these, the alpha(2)-integrin+CD4 unprimed T cells entered the eye more efficiently, and antibody to alpha(2)-integrin markedly inhibited the inflammatory response. Intravital microscopy revealed the early arrival and antigen-specific accumulation of CD4 T cells in inflamed tissue and should be helpful in understanding T cell migration to other organs.

Two Functional Active Conformations of the Integrin α2β1, Depending on Activation Condition and Cell Type

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin alpha2beta1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of alpha2beta1 because it only binds to the activated state of alpha2beta1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of alpha2beta1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of alpha2beta1 are dependent on the cell type where alpha2beta1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and alpha2beta1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of alpha2beta1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells.

Association of polymorphisms of platelet membrane integrins alpha IIb(beta)3 (HPA-1b/Pl) and alpha2(beta)1 (alpha807TT) with premature myocardial infarction

Conflicting results of an association of the human platelet antigen 1b (HPA-1b/PlA2), localized on the beta-subunit of the integrin alpha(IIb)beta3, and the alpha(2)807TT genotype of the integrin alpha2beta1 with coronary atherosclerosis and myocardial infarction have been reported. Both platelet receptor polymorphisms were genotyped in 3261 patients who had undergone coronary angiography, including 1175 survivors of a myocardial infarction, 1211 individuals with coronary artery disease but no history of myocardial infarction, and 571 control patients without angiographic coronary artery disease, and in 793 blood donors. In a case-control design, the prevalence of HPA-1b and alpha(2)807TT genotypes did not differ significantly between the patient groups with coronary artery disease or myocardial infarction and patient controls or blood donors. By contrast, using a multivariate case-only design, it was found that the median age of onset of myocardial infarction was 5.2 years earlier (P = 0.006) in carriers of the HPA-1b allele and 6.3 years earlier (P = 0.006) in carriers of the alpha(2)807TT genotype in the 264 survivors of myocardial infarction of recent onset with one- or two-vessel coronary artery disease. A significant interaction with the conventional risk factors hypercholesterolemia, smoking, diabetes, hypertension, and hyperfibrinogenemia was excluded. Human platelet antigen 1b and alpha(2)807TT are associated with premature myocardial infarction but not with coronary artery disease, suggesting a role of distinct integrin genotypes for increased platelet thrombogenicity. This association requires confirmation in follow-up studies.

Stepwise mechanical stretching inhibits chondrogenesis through cell-matrix adhesion mediated by integrins in embryonic rat limb-bud mesenchymal cells

Biomechanical forces are major epigenetic factors that determine the form and differentiation of skeletal tissues, and may be transduced through cell adhesion to the intracellular biochemical signaling pathway. To test the hypothesis that stepwise stretching is translated to molecular signals during early chondrogenesis, we developed a culture system to study the proliferation and differentiation of chondrocytes. Rat embryonic day-12 limb buds were microdissected and dissociated into cells, which were then micromass cultured on a silicone membrane and maintained for up to 7 days. Stepwise-increased stretching was applied to the silicone membrane, which exerted shearing stress on the cultures on day 4 after the initiation of chondrogenesis. Under stretched conditions, type II collagen expression was significantly inhibited by 44% on day 1 and by 67% on day 2, and this difference in type II collagen reached 80% after 3 days of culture. Accumulation of type II collagen protein and the size of the chondrogenic nodules had decreased by 50% on day 3. On the other hand, expression of the non-chondrogenic marker fibronectin was significantly upregulated by 1.8-fold on day 3, while the up-regulation of type I collagen was minimal, even by day 3. The downregulation in the expression of chondrogenic markers was completely recovered when cell-extracellular matrix attachment was inhibited by Gly-Arg-Gly-Asp-Ser-Pro-Lys peptide or by the application of blocking antibodies for alpha2, alpha5 or beta1 integrins. We conclude that shearing stress generated by stepwise stretching inhibits chondrogenesis through integrins, and propose that signal transduction from biomechanical stimuli may be mediated by cell-extracellular matrix adhesion.