Identification of putative ligand binding sites within I domain of integrin alpha 2 beta 1 (VLA-2, CD49b/CD29)

The Kingella kingae PilC1 MIDAS Motif Is Essential for Type IV Pilus Adhesive Activity and Twitching Motility

Kingella kingae is an emerging pathogen that has recently been identified as a leading cause of osteoarticular infections in young children. Colonization with K. kingae is common, with approximately 10% of young children carrying this organism in the oropharynx at any given time. Adherence to epithelial cells represents the first step in K. kingae colonization of the oropharynx, a prerequisite for invasive disease. Type IV pili and the pilus-associated PilC1 and PilC2 proteins have been shown to mediate K. kingae adherence to epithelial cells, but the molecular mechanism of this adhesion has remained unknown. Metal ion-dependent adhesion site (MIDAS) motifs are commonly found in integrins, where they function to promote an adhesive interaction with a ligand. In this study, we identified a potential MIDAS motif in K. kingae PilC1 which we hypothesized was directly involved in mediating type IV pilus adhesive interactions. We found that the K. kingae PilC1 MIDAS motif was required for bacterial adherence to epithelial cell monolayers and extracellular matrix proteins and for twitching motility. Our results demonstrate that K. kingae has co-opted a eukaryotic adhesive motif for promoting adherence to host structures and facilitating colonization.

Travelling ultrasound promotes vasculogenesis of three-dimensional-monocultured human umbilical vein endothelial cells

To generate three‐dimensional tissue in vitro, promoting vasculogenesis in cell aggregates is an important factor. Here, we found that ultrasound promoted vasculogenesis of human umbilical vein endothelial cells (HUVECs). Promotion of HUVEC network formation and lumen formation were observed using our method. In addition to morphological evaluations, protein expression was quantified by western blot assays. As a result, expression of proteins related to vasculogenesis and the response to mechanical stress on cells was enhanced by exposure to ultrasound. Although several previous studies have shown that ultrasound may promote vasculogenesis, the effect of ultrasound was unclear because of unregulated ultrasound, the complex culture environment, or two‐dimensional‐cultured HUVECs that cannot form a lumen structure. In this study, regulated ultrasound was propagated on three‐dimensional‐monocultured HUVECs, which clarified the effect of ultrasound on vasculogenesis. We believe this finding may be an innovation in the tissue engineering field.

Biomechanical Contributions to Macrophage Activation in the Tumor Microenvironment

Alterations in extracellular matrix composition and organization are known to promote tumor growth and metastatic progression in breast cancer through interactions with tumor cells as well as stromal cell populations. Macrophages display a spectrum of behaviors from tumor-suppressive to tumor-promoting, and their function is spatially and temporally dependent upon integrated signals from the tumor microenvironment including, but not limited to, cytokines, metabolites, and hypoxia. Through years of investigation, the specific biochemical cues that recruit and activate tumor-promoting macrophage functions within the tumor microenvironment are becoming clear. In contrast, the impact of biomechanical stimuli on macrophage activation has been largely underappreciated, however there is a growing body of evidence that physical cues from the extracellular matrix can influence macrophage migration and behavior. While the complex, heterogeneous nature of the extracellular matrix and the transient nature of macrophage activation make studying macrophages in their native tumor microenvironment challenging, this review highlights the importance of investigating how the extracellular matrix directly and indirectly impacts tumor-associated macrophage activation. Additionally, recent advances in investigating macrophages in the tumor microenvironment and future directions regarding mechano-immunomodulation in cancer will also be discussed.

Expression of integrins to control migration direction of electrotaxis

Proper control of cell migration is critically important in many biologic processes, such as wound healing, immune surveillance, and development. Much progress has been made in the initiation of cell migration; however, little is known about termination and sometimes directional reversal. During active cell migration, as in wound healing, development, and immune surveillance, the integrin expression profile undergoes drastic changes. Here, we uncovered the extensive regulatory and even opposing roles of integrins in directional cell migration in electric fields (EFs), a potentially important endogenous guidance mechanism. We established cell lines that stably express specific integrins and determined their responses to applied EFs with a high throughput screen. Expression of specific integrins drove cells to migrate to the cathode or to the anode or to lose migration direction. Cells expressing αMβ2, β1, α2, αIIbβ3, and α5 migrated to the cathode, whereas cells expressing β3, α6, and α9 migrated to the anode. Cells expressing α4, αV, and α6β4 lost directional electrotaxis. Manipulation of α9 molecules, one of the molecular directional switches, suggested that the intracellular domain is critical for the directional reversal. These data revealed an unreported role for integrins in controlling stop, go, and reversal activity of directional migration of mammalian cells in EFs, which might ensure that cells reach their final destination with well-controlled speed and direction.-Zhu, K., Takada, Y., Nakajima, K., Sun, Y., Jiang, J., Zhang, Y., Zeng, Q., Takada, Y., Zhao, M. Expression of integrins to control migration direction of electrotaxis.

Regulation and functions of integrin α2 in cell adhesion and disease

Integrins are cell adhesion molecules that are composed of an alpha (α) subunit and a beta (β) subunit with affinity for different extracellular membrane components. The integrin family includes 24 known members that actively regulate cellular growth, differentiation, and apoptosis. Each integrin heterodimer has a particular function in defined contexts as well as some partially overlapping features with other members in the family. As many reviews have covered the general integrin family in molecular and cellular studies in life science, this review will focus on the specific regulation, function, and signaling of integrin α2 subunit (CD49b, VLA-2; encoded by the gene ITGA2) in partnership with β1 (CD29) subunit in normal and cancer cells. Its roles in cell adhesion, cell motility, angiogenesis, stemness, and immune/blood cell regulations are discussed. The pivotal role of integrin α2 in many diseases such as cancer suggests its potential to be used as a novel therapeutic target.

Salt-bridge modulates differential calcium-mediated ligand binding to integrin α1- and α2-I domains

Integrins are transmembrane cell-extracellular matrix adhesion receptors that impact many cellular functions. A subgroup of integrins contain an inserted (I) domain within the α–subunits (αI) that mediate ligand recognition where function is contingent on binding a divalent cation at the metal ion dependent adhesion site (MIDAS). Ca²⁺ is reported to promote α1I but inhibit α2I ligand binding. We co-crystallized individual I-domains with MIDAS-bound Ca²⁺ and report structures at 1.4 and 2.15 Å resolution, respectively. Both structures are in the “closed” ligand binding conformation where Ca²⁺ induces minimal global structural changes. Comparisons with Mg²⁺-bound structures reveal Mg²⁺ and Ca²⁺ bind α1I in a manner sufficient to promote ligand binding. In contrast, Ca²⁺ is displaced in the α2I domain MIDAS by 1.4 Å relative to Mg²⁺ and unable to directly coordinate all MIDAS residues. We identified an E152-R192 salt bridge hypothesized to limit the flexibility of the α2I MIDAS, thus, reducing Ca²⁺ binding. A α2I E152A construct resulted in a 10,000-fold increase in Mg²⁺ and Ca²⁺ binding affinity while increasing binding to collagen ligands 20%. These data indicate the E152-R192 salt bridge is a key distinction in the molecular mechanism of differential ion binding of these two I domains.

Structural aspects of integrins

Structural studies on integrins have recently made great strides in recent years. Crystal structures of the complete extracellular fragments of three integrins in open and closed conformations, 6 α-I domains in complex with ligands, and at least 20 intracellular proteins in complex with cytosolic tails have been obtained; and several transmembrane and cytosolic complexes have been determined by NMR. High resolution EM studies complement these atomic resolution techniques by studying the integrin in different activation states. Although we still have only a few experimental examples among integrin family members, the high level of sequence homology between integrins means that reliable models can be built for the other members of the integrin family. These structures make sense of a lot of preceding biochemical, biophysical and mutagenesis studies, and generate many new testable hypotheses of integrin function. This chapter emphasizes new structural insights applicable to all integrins, with an emphasis on those integrins that contain an α-I domain. The structural data reinforce the notion of the integrin as a molecule in dynamic equilibrium at the cell surface, regulated by binding both to extracellular and intracellular ligands.

Inhibitors of the interactions between collagen and its receptors on platelets

At sites of vascular injury, collagen-mediated platelet adhesion and activation have long been known as one of the first events in platelet-dependent thrombus formation. Studying patients with bleeding disorders that are caused by defective platelet adhesion to collagen resulted in the identification of several platelet collagen receptors, with glycoprotein VI and integrin α2β1 being the most important ones. Subsequent development of specific collagen receptor knockout mice and various inhibitors of platelet binding to collagen have further proven the role of these receptors in haemostasis and thrombosis. The search for clinically applicable inhibitors for use as antithrombotic drug has led to the identification of inhibitory antibodies, soluble receptor fragments, peptides, collagen-mimetics and proteins from snake venoms or haematophagous animals. In experimental settings, these inhibitors have a good antithrombotic effect, with little prolongation of bleeding times, suggesting a larger therapeutic window than currently available antiplatelet drugs. However, at present, none of the collagen receptor blockers are in clinical development yet.

Predictive role of multilocus genetic polymorphisms in cardiovascular disease and inflammation-related genes on chronic kidney disease in Type 2 diabetes--an 8-year prospective cohort analysis of 1163 patients

BACKGROUND

Chinese diabetic patients are at greater risk of developing chronic kidney disease (CKD) than Caucasian counterparts. In this hypothesis-generating study, we examined the independent and joint effects of multiple genetic variants on CKD in a prospective Chinese cohort of Type 2 diabetic patients.

METHODS

Seventy-seven single-nucleotide polymorphisms (SNPs) of 54 candidate genes for cardiorenal diseases and inflammation were genotyped in 1163 patients with no past history of CKD at baseline. CKD was defined as the first estimated glomerular filtration rate <60 mL/min/1.73 m(2) or the first hospitalization with a diagnosis of renal disease.

RESULTS

In Cox-regression analysis, 15 SNPs of 13 genes were associated with incident CKD. After correction for multiple comparisons, 6 SNPs including PON1 55Met, PON2 311Cys CETP-629C, ITGA2 873A, LTA 26Asn and LTA 252Gly remained independently associated with CKD, with respective hazard ratios (95% confidence interval):2.6 (1.4-4.8, P = 0.002), 1.5 (1.2-1.9, P = 0.003), 1.4 (1.1-1.7, P = 0.001), 2.2 (1.3-3.7, P = 0.002), 1.6 (1.1-2.2, P = 0.008) and 1.5 (1.1-2.1, P = 0.019). Analysis of joint effect of the six SNPs showed stepwise increase in risk of CKD with the accumulation of risk alleles and weighted genetic risk score (P(trend) = 8.9 × 10(-7) and 4.0 × 10(-5), respectively).

CONCLUSIONS

In Type 2 diabetes, there are independent and joint effects of multiple genetic variants on risk of CKD. Risk associations with PON1, PON2, CETP, ITGA2 and LTA genetic polymorphisms underline the importance of lipid metabolism, haemostasis and inflammation in the development of CKD in patients with Type 2 diabetes.

Determinants of the specificity of rotavirus interactions with the alpha2beta1 integrin

The human α2β1 integrin binds collagen and acts as a cellular receptor for rotaviruses and human echovirus 1. These ligands require the inserted (I) domain within the α2 subunit of α2β1 for binding. Previous studies have identified the binding sites for collagen and echovirus 1 in the α2 I domain. We used CHO cells expressing mutated α2β1 to identify amino acids involved in binding to human and animal rotaviruses. Residues where mutation affected rotavirus binding were located in several exposed loops and adjacent regions of the α2 I domain. Binding by all rotaviruses was eliminated by mutations in the activation-responsive αC-α6 and αF helices. This is a novel feature that distinguishes rotavirus from other α2β1 ligands. Mutation of residues that co-ordinate the metal ion (Ser-153, Thr-221, and Glu-256 in α2 and Asp-130 in β1) and nearby amino acids (Ser-154, Gln-215, and Asp-219) also inhibited rotavirus binding. The importance of most of these residues was greatest for binding by human rotaviruses. These mutations inhibit collagen binding to α2β1 (apart from Glu-256) but do not affect echovirus binding. Overall, residues where mutation affected both rotavirus and collagen recognition are located at one side of the metal ion-dependent adhesion site, whereas those important for collagen alone cluster nearby. Mutations eliminating rotavirus and echovirus binding are distinct, consistent with the respective preference of these viruses for activated or inactive α2β1. In contrast, rotavirus and collagen utilize activated α2β1 and show an overlap in α2β1 residues important for binding.

Collagen VI microfibril formation is abolished by an {alpha}2(VI) von Willebrand factor type A domain mutation in a patient with Ullrich congenital muscular dystrophy

Collagen VI is an extracellular protein that most often contains the three genetically distinct polypeptide chains, α1(VI), α2(VI), and α3(VI), although three recently identified chains, α4(VI), α5(VI), and α6(VI), may replace α3(VI) in some situations. Each chain has a triple helix flanked by N- and C-terminal globular domains that share homology with the von Willebrand factor type A (VWA) domains. During biosynthesis, the three chains come together to form triple helical monomers, which then assemble into dimers and tetramers. Tetramers are secreted from the cell and align end-to-end to form microfibrils. The precise molecular mechanisms responsible for assembly are unclear. Mutations in the three collagen VI genes can disrupt collagen VI biosynthesis and matrix organization and are the cause of the inherited disorders Bethlem myopathy and Ullrich congenital muscular dystrophy. We have identified a Ullrich congenital muscular dystrophy patient with compound heterozygous mutations in α2(VI). The first mutation causes skipping of exon 24, and the mRNA is degraded by nonsense-mediated decay. The second mutation is a two-amino acid deletion in the C1 VWA domain. Recombinant C1 domains containing the deletion are insoluble and retained intracellularly, indicating that the mutation has detrimental effects on domain folding and structure. Despite this, mutant α2(VI) chains retain the ability to associate into monomers, dimers, and tetramers. However, we show that secreted mutant tetramers containing structurally abnormal C1 VWA domains are unable to associate further into microfibrils, directly demonstrating the critical importance of a correctly folded α2(VI) C1 domain in microfibril formation.

Negative regulation of activated α2 integrins during thrombopoiesis

Circulating platelets exhibit rapid signaling and adhesive responses to collagen that facilitate hemostasis at sites of vessel injury. Because platelets are anuclear, their collagen receptors must be expressed by megakaryocytes, platelet precursors that arise in the collagen-rich environment of the bone marrow. Whether and how megakaryocytes regulate collagen adhesion during their development in the bone marrow are unknown. We find that surface expression of activated, but not wild-type, α2 integrins in hematopoietic cells in vivo results in the generation of platelets that lack surface α2 receptors. Culture of hematopoietic progenitor cells ex vivo reveals that surface levels of activated, but not wild-type, α2 integrin receptors are rapidly down-regulated during cell growth on collagen but reach wild-type levels when cells are grown in the absence of collagen. Progenitor cells that express activated α2 integrins are normally distributed in the bone marrow in vivo and exhibit normal migration across a collagen-coated membrane ex vivo. This migration is accompanied by rapid down-regulation of activated surface integrins. These studies identify ligand-dependent removal of activated α2 receptors from the cell surface as a mechanism by which integrin function can be negatively regulated in hematopoietic cells during migration between the adhesive environment of the bone marrow and the nonadhesive environment of the circulating blood.

Competitive interactions of collagen and a jararhagin-derived disintegrin peptide with the integrin alpha2-I domain

Integrin alpha2beta1 is a major receptor required for activation and adhesion of platelets, through the specific recognition of collagen by the alpha2-I domain (alpha2-I), which binds fibrillar collagen via Mg(2+)-bridged interactions. The crystal structure of a truncated form of the alpha2-I domain, bound to a triple helical collagen peptide, revealed conformational changes suggestive of a mechanism where the ligand-bound I domain can initiate and propagate conformational change to the full integrin complex. Collagen binding by alpha2-I and fibrinogen-dependent platelet activity can be inhibited by snake venom polypeptides. Here we describe the inhibitory effect of a short cyclic peptide derived from the snake toxin metalloprotease jararhagin, with specific amino acid sequence RKKH, on the ability of alpha2-I to bind triple helical collagen. Isothermal titration calorimetry measurements showed that the interactions of alpha2-I with collagen or RKKH peptide have similar affinities, and NMR chemical shift mapping experiments with (15)N-labeled alpha2-I, and unlabeled RKKH peptide, indicate that the peptide competes for the collagen-binding site of alpha2-I but does not induce a large scale conformational rearrangement of the I domain.

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.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

Combinatorial chemistry identifies high-affinity peptidomimetics against alpha4beta1 integrin for in vivo tumor imaging

Small peptide-based agents have attracted wide interest as cancer-targeting agents for diagnostic imaging and targeted therapy. There is a need to develop new high-affinity and high-specificity peptidomimetic or small-molecule ligands against cancer cell surface receptors. Here we report on the identification of a high-affinity peptidomimetic ligand (LLP2A; IC50 = 2 pM) against alpha4beta1 integrin using both diverse and highly focused one-bead-one-compound combinatorial peptidomimetic libraries in conjunction with high-stringency screening. We further demonstrate that LLP2A can be used to image alpha4beta1-expressing lymphomas with high sensitivity and specificity when conjugated to a near infrared fluorescent dye in a mouse xenograft model. Thus, LLP2A provides an important tool for noninvasive monitoring of alpha4beta1 expression and activity during tumor progression, and it shows great potential as an imaging and therapeutic agent for alpha4beta1-positive tumors.

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.

The highly conserved cardiac glycoside binding site of Na,K-ATPase plays a role in blood pressure regulation

The Na,K-ATPase contains a binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin, which is highly conserved among species ranging from Drosophila to humans. Although advantage has been taken of this site to treat congestive heart failure with drugs such as digoxin, it is unknown whether this site has a natural function in vivo. Here we show that this site plays an important role in the regulation of blood pressure, and it specifically mediates adrenocorticotropic hormone (ACTH)-induced hypertension in mice. We used genetically engineered mice in which the Na,K-ATPase alpha2 isoform, which is normally sensitive to cardiac glycosides, was made resistant to these compounds. Chronic administration of ACTH caused hypertension in WT mice but not in mice with an ouabain-resistant alpha2 isoform of Na,K-ATPase. This finding demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays an important role in blood pressure regulation, most likely by responding to a naturally occurring ligand. Because the alpha1 isoform is sensitive to cardiac glycosides in humans, we developed mice in which the naturally occurring ouabain-resistant alpha1 isoform was made ouabain-sensitive. Mice with the ouabain-sensitive "human-like" alpha1 isoform and an ouabain-resistant alpha2 isoform developed ACTH-induced hypertension to greater extent than WT animals. This result indicates that the cardiac glycoside binding site of the alpha1 isoform can also mediate ACTH-induced hypertension. Taken together these results demonstrate that the cardiac glycoside binding site of the alpha isoforms of the Na,K-ATPase have a physiological function and supports the hypothesis for a role of the endogenous cardiac glycosides.

A duplexed microsphere-based cellular adhesion assay

Interactions of integrin cellular adhesion molecules with matrix proteins play important roles in complex bidirectional signaling pathways. To investigate these interactions, a novel flow-cytometry-based cellular adhesion assay has been developed. Based on the concept of microcarrier cell culture, derivatized polystyrene microspheres (9.6 microm) are used as a substrate for the immobilization of type I collagen to which cells then adhere. Using cytometric detection, the extent of cellular adhesion can be precisely determined by comparison of adhered and nonadhered populations based on the side scatter properties of the microspheres. In combination with immunostaining, the novel format of this assay enables the correlation of adhesive function to other cellular characteristics such as surface expression. In this work, the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) was used to stimulate increased adhesion in Chinese hamster ovary cells stably transfected with the collagen receptor integrin alpha2beta1. Multiple clones of varying expression distributions were analyzed, and correlations of adherent populations versus receptor distributions show a threefold increase in functional cellular adhesion to collagen upon treatment with TPA. Probability binning analysis of duplexed data revealed subtle changes in adhesion versus receptor distribution mediated by TPA which otherwise would not have been detectable.

The platelet glycoprotein Ib-von Willebrand factor interaction activates the collagen receptor alpha2beta1 to bind collagen: activation-dependent conformational change of the alpha2-I domain

Integrin alpha2beta1 (glycoprotein [GP] Ia/IIa) is a major platelet receptor for collagen, containing its collagen-binding site within the alpha2 I domain. alpha2beta1 changes conformation upon platelet activation, increasing its affinity for collagen. We observed that 2 antibodies known to bind within the alpha2I domain, 12F1 and 6F1, bound preferentially to adenosine diphosphate (ADP)-activated platelets. Interestingly, when whole blood was perfused over a surface coated with either 12F1 or 6F1, only 6F1 supported the adhesion of unstimulated platelets. To test whether the interaction of GP Ib with von Willebrand factor (VWF) directly activates alpha2beta1, we used 12F1 as a probe of integrin activation. We perfused blood over a surface coated with a mixture of VWF-A1 domain (a GP Ib ligand) and 12F1 or VWF-A1 and mouse immunoglobulin G (IgG). Platelets rolled and did not attach stably on the A1/IgG surface, but they firmly bound and covered the A1/12F1 surface. We corroborated that 12F1 binds an active conformation of the I domain by showing that it binds with higher affinity to a gain-of-function mutant than to either wild-type I domain or a loss-of-function mutant. These results strongly suggest that the interaction of platelet GP Ib with VWF mediates the activation of alpha2beta1, increasing its affinity for collagen.

Platelet receptor interplay regulates collagen-induced thrombus formation in flowing human blood

The platelet glycoproteins (GPs) Ib, integrin α2β1, and GPVI are considered central to thrombus formation. Recently, their relative importance has been re-evaluated based on data from murine knockout models. To examine their relationship during human thrombus formation on collagen type I fibers at high shear (1000 s–1), we tested a novel antibody against GPVI, an immunoglobulin single-chain variable fragment, 10B12, together with specific antagonists for GPIbα (12G1 Fab2) and α2β1 (6F1 mAb or GFOGER-GPP peptide). GPVI was found to be crucial for aggregate formation, Ca2+ signaling, and phosphatidylserine (PS) exposure, but not for primary adhesion, even with more than 97% receptor blockade. Inhibiting α2β1 revealed its involvement in regulating Ca2+ signaling, PS exposure, and aggregate size. Both GPIbα and α2β1 contributed to primary adhesion, showing overlapping function. The coinhibition of receptors revealed synergism in thrombus formation: the coinhibition of adenosine diphosphate (ADP) receptors with collagen receptors further decreased adhesion and aggregation, and, crucially, the complete eradication of thrombus formation required the coinhibition of GPVI with either GPIbα or α2β1. In summary, human platelet deposition on collagen depends on the concerted interplay of several receptors: GPIb in synergy with α2β1 mediating primary adhesion, reinforced by activation through GPVI, which further regulates the thrombus formation.

Structural and functional analysis of integrin alpha2I domain interaction with echovirus 1

Integrins are cell surface receptors for several microbial pathogens including echovirus 1 (EV1), a picornavirus. Cryo-electron microscopy revealed that the functional domain (alpha(2)I) of human alpha(2)beta(1) integrin binds to a surface depression on the EV1 capsid. This three-dimensional structure of EV1 bound to alpha(2)I domain provides the first structural details of an integrin interacting with a picornavirus. The model indicates that alpha(2)beta(1) integrin cannot simultaneously bind both EV1 and the physiological ligand collagen. Compared with collagen binding to the alpha(2)I domain, the virus binds with a 10-fold higher affinity but in vitro uncoating of EV1 was not observed as a result of attachment of alpha(2)I. A molecular model, constructed on the basis of the EV1-integrin complex, shows that multiple alpha(2)beta(1) heterodimers can bind at adjacent sites around the virus 5-fold symmetry axes without steric hindrance. In agreement with this, virus attachment to alpha(2)beta(1) integrin on the cell surface was found to result in integrin clustering, which can give rise to signaling and facilitate the initiation of the viral entry process that takes place via caveolae-mediated endocytosis.

A consensus tetrapeptide selected by phage display adopts the conformation of a dominant discontinuous epitope of a monoclonal anti-VWF antibody that inhibits the von Willebrand factor-collagen interaction

Monoclonal antibody (mAb) 82D6A3 is an anti-von Willebrand factor (VWF) mAb directed against the A3-domain of VWF that inhibits the VWF binding to fibrillar collagens type I and III in vitro and in vivo. To identify the discontinuous epitope of this mAb, we used phage display, mutant analysis, and peptide modeling. All 82D6A3-binding phages displayed peptides containing the consensus sequence SPWR that could be aligned with P981W982 in the VWF A3-domain. Next, the binding of mAb 82D6A3 to 27 Ala mutants with mutations in the A3-domain of VWF revealed that amino acids Arg963, Pro981, Asp1009, Arg1016, Ser1020, Met1022, and His1023 are part of the epitope of mAb 82D6A3. Inspection of residues Ser1020, Arg1016, Pro981, and Trp982 in the three-dimensional structure of the A3-domain demonstrated that these residues are close together in space, pointing out that the structure of the SPWR consensus sequence might mimic this discontinuous epitope. Modeling of a cyclic 6-mer peptide containing the consensus sequence and superposition of its three-dimensional structure onto the VWF A3-domain demonstrated that the Ser and Arg in the peptide matched the Ser1020 and Arg1016 in the A3-domain. The Pro residue of the peptide served as a spacer, and the side chain of the Trp pointed in the direction of Trp982. In conclusion, to our knowledge, this is the first report where a modeled peptide containing a consensus sequence could be fitted onto the three-dimensional structure of the antigen, indicating that it might adopt the conformation of the discontinuous epitope.

Monkey rotavirus binding to alpha2beta1 integrin requires the alpha2 I domain and is facilitated by the homologous beta1 subunit

Rotaviruses utilize integrins during virus-cell interactions that lead to infection. Cell binding and infection by simian rotavirus SA11 were inhibited by antibodies (Abs) to the inserted (I) domain of the alpha2 integrin subunit. To determine directly which integrins or other proteins bind rotaviruses, cell surface proteins precipitated by rotaviruses were compared with those precipitated by anti-alpha2beta1 Abs. Two proteins precipitated by SA11 and rhesus rotavirus RRV from MA104 and Caco-2 cells migrated indistinguishably from alpha2beta1 integrin, and SA11 precipitated beta1 from alpha2beta1-transfected CHO cells. These viruses specifically precipitated two MA104 cell proteins only, but an additional 160- to 165-kDa protein was precipitated by SA11 from Caco-2 cells. The role of the alpha2 I domain in rotavirus binding, infection, and growth was examined using CHO cell lines expressing wild-type or mutated human alpha2 or alpha2beta1. Infectious SA11 and RRV, but not human rotavirus Wa, specifically bound CHO cell-expressed human alpha2beta1 and, to a lesser extent, human alpha2 combined with hamster beta1. Binding was inhibited by anti-alpha2 I domain monoclonal Abs (MAbs), but not by non-I domain MAbs to alpha2, and required the presence of the alpha2 I domain. Amino acid residues 151, 221, and 254 in the metal ion-dependent adhesion site of the alpha2 I domain that are necessary for type I collagen binding to alpha2beta1 were not essential for rotavirus binding. Rotavirus-alpha2beta1 binding led to increased virus infection and RRV growth. SA11 and RRV require the alpha2 I domain for binding to alpha2beta1, and their binding to this integrin is distinguishable from that of collagen.

Structural basis of type VI collagen dimer formation

We have determined the interactive sites required for dimer formation in type VI collagen. Despite the fact that type VI collagen is a heterotrimer composed of alpha1(VI), alpha2(VI), and alpha3(VI) chains, the formation of dimers is determined principally by interactions of the alpha2(VI) chain. Key components of this interaction are the metal ion-dependent adhesion site (MIDAS) motif of the alpha2C2 A-domain and the GER sequence in the helical domain of another alpha2(VI) chain. Replacement of the alpha2(VI) C2 domain with the alpha3(VI) domain abolished dimer formation, whereas alterations in the alpha2(VI) C1 domain did not disrupt dimer formation. When the helical sequences were investigated, replacement of the alpha2(VI) sequence GSPGERGDQ with the alpha3(VI) sequence GEKGERGDV abolished dimer formation. Mutating the Pro-108 to a Lys-108 in this alpha2(VI) sequence did not influence dimer formation and suggests that, unlike the integrin I-domain/triple-helix interaction, hydroxyproline is not required in collagen VI A-domain/helix interaction. These results demonstrate that the alpha2(VI) chain position in the assembled triple-helical molecule is critical for antiparallel dimer formation and identify the interacting collagenous and MIDAS sequences involved. These interactions underpin the subsequent assembly of type VI collagen.

Polymerization of type I and III collagens is dependent on fibronectin and enhanced by integrins alpha 11beta 1 and alpha 2beta 1

Polymerization of the ECM proteins fibronectin and laminin has been shown to take place in close vicinity to the cell surface and be facilitated by beta(1) integrins (Lohikangas, L., Gullberg, D., and Johansson, S. (2001) Exp. Cell Res. 265, 135-144 and Wennerberg, K., Lohikangas, L., Gullberg, D., Pfaff, M., Johansson, S., and Fassler, R. (1996) J. Cell Biol. 132, 227-238). We have studied the role of collagen receptors, integrins alpha(11)beta(1) and alpha(2)beta(1), and fibronectin in collagen polymerization using fibronectin-deficient mouse embryonic fibroblast cell lines. In contrast to the earlier belief that collagen polymerization occurs via self-assembly of collagen molecules we show that a preformed fibronectin matrix is essential for collagen network formation and that collagen-binding integrins strongly enhance this process. Thus, collagen deposition is regulated by the cells, both indirectly through integrin alpha(5)beta(1)-dependent polymerization of fibronectin and directly through collagen-binding integrins.

The 807T allele in alpha2 integrin is protective against atherosclerotic arterial wall thickening and the occurrence of plaque in patients with type 2 diabetes

Polymorphism of alpha2 integrin (C807T) is shown to be associated with an increased incidence of thrombotic cardiovascular events. However, it is not clear whether this polymorphism is associated with atherosclerotic arterial wall thickening. In this study, we examined the association of C807T polymorphism with arterial wall thickness in 265 control subjects and 272 patients with type 2 diabetes. In all subjects, intima-media thickness of the right carotid artery in the 807TT group (0.649 +/- 0.028 mm [SE]) was significantly (P = 0.0228, Scheffe's F test) less than in the 807CC group (0.767 +/- 0.033). This effect of polymorphism is gene dose dependent (P = 0.0227, ANOVA). The similar association was also observed in patients with diabetes but not in control subjects. Multiple regression analysis in all subjects revealed that the T allele was inversely (beta = -0.095, P = 0.021) associated with intima-media thickness independent of age, HbA(1c), and HDL cholesterol. Finally, an inverse relation between the occurrence of carotid plaque and the T allele was observed in patients with diabetes with an adjusted odds ratio of 0.487 (P = 0.031) in multiple logistic regression analyses. These results suggest that the number of 807T alleles in alpha2 integrin is protective against atherosclerotic arterial wall thickening and the occurrence of plaque in patients with type 2 diabetes.

alpha11beta1 integrin is a receptor for interstitial collagens involved in cell migration and collagen reorganization on mesenchymal nonmuscle cells

alpha11beta1 integrin constitutes a recent addition to the integrin family. Here, we present the first in vivo analysis of alpha11 protein and mRNA distribution during human embryonic development. alpha11 protein and mRNA were present in various mesenchymal cells around the cartilage anlage in the developing skeleton in a pattern similar to that described for the transcription factor scleraxis. alpha11 was also expressed by mesenchymal cells in intervertebral discs and in keratocytes in cornea, two sites with highly organized collagen networks. Neither alpha11 mRNA nor alpha11 protein could be detected in myogenic cells in human embryos. The described expression pattern is compatible with alpha11beta1 functioning as a receptor for interstitial collagens in vivo. To test this hypothesis in vitro, full-length human alpha11 cDNA was stably transfected into the mouse satellite cell line C2C12, lacking endogenous collagen receptors. alpha11beta1 mediated cell adhesion to collagens I and IV (with a preference for collagen I) and formed focal contacts on collagens. In addition, alpha11beta1 mediated contraction of fibrillar collagen gels in a manner similar to alpha2beta1, and supported migration on collagen I in response to chemotactic stimuli. Our data support a role for alpha11beta1 as a receptor for interstitial collagens on mesenchymally derived cells and suggest a multifunctional role of alpha11beta1 in the recognition and organization of interstitial collagen matrices during development.

Structure of von Willebrand factor and its function in platelet adhesion and thrombus formation

The adhesive protein von Willebrand factor mediates the initiation and progression of thrombus formation at sites of vascular injury. von Willebrand factor is synthesized in endothelial cells and megakaryocytes as a very large polymer composed of identical subunits. In the plasma, it appears as a series of multimers of regularly decreasing molecular mass, from several thousand to 500 kDa. The size of circulating von Willebrand factor multimers is controlled by proteolytic cleavage carried out by a specific protease. The biological functions of von Willebrand factor are exerted through specific domains that interact with extracellular matrix components and cell membrane receptors to promote the initial tethering and adhesion of platelets to subendothelial surfaces, as well as platelet aggregation. Moreover, von Willebrand factor binds the procoagulant co-enzyme, factor VIII, contributing to its stability and, indirectly, to its function in the generation of fibrin. This chapter presents a review of current knowledge on the structure, biosynthesis and functions of von Willebrand factor.

Structure-function of the putative I-domain within the integrin beta 2 subunit

The central region (residues 125-385) of the integrin beta(2) subunit is postulated to adopt an I-domain-like fold (the beta(2)I-domain) and to play a critical role in ligand binding and heterodimer formation. To understand structure-function relationships of this region of beta(2), a homolog-scanning mutagenesis approach, which entails substitution of nonconserved hydrophilic sequences within the beta(2)I-domain with their homologous counterparts of the beta(1)I-domain, has been deployed. This approach is based on the premise that beta(1) and beta(2) are highly homologous, yet recognize different ligands. Altogether, 16 segments were switched to cover the predicted outer surface of the beta(2)I-domain. When these mutant beta(2) subunits were transfected together with wild-type alpha(M) in human 293 cells, all 16 beta(2) mutants were expressed on the cell surface as heterodimers, suggesting that these 16 sequences within the beta(2)I-domain are not critically involved in heterodimer formation between the alpha(M) and beta(2) subunits. Using these mutant alpha(M)beta(2) receptors, we have mapped the epitopes of nine beta(2)I-domain specific mAbs, and found that they all recognized at least two noncontiguous segments within this domain. The requisite spatial proximity among these non-linear sequences to form the mAb epitopes supports a model of an I-domain-like fold for this region. In addition, none of the mutations that abolish the epitopes of the nine function-blocking mAbs, including segment Pro(192)-Glu(197), destroyed ligand binding of the alpha(M)beta(2) receptor, suggesting that these function-blocking mAbs inhibit alpha(M)beta(2) function allosterically. Given the recent reports implicating the segment equivalent to Pro(192)-Glu(197) in ligand binding by beta(3) integrins, these data suggest that ligand binding by the beta(2) integrins occurs via a different mechanism than beta(3). Finally, both the conformation of the beta(2)I-domain and C3bi binding activity of alpha(M)beta(2) were dependent on a high affinity Ca(2+) binding site (K(d) = 105 microm), which is most likely located within this region of beta(2).

Identification of the collagen-binding site of the von Willebrand factor A3-domain

Von Willebrand factor (vWF) is a multimeric glycoprotein that mediates platelet adhesion and thrombus formation at sites of vascular injury. vWF functions as a molecular bridge between collagen and platelet receptor glycoprotein Ib. The major collagen-binding site of vWF is contained within the A3 domain, but its precise location is unknown. To localize the collagen-binding site, we determined the crystal structure of A3 in complex with an Fab fragment of antibody RU5 that inhibits collagen binding. The structure shows that RU5 recognizes a nonlinear epitope consisting of residues 962-966, 981-997, and 1022-1026. Alanine mutants were constructed of residues Arg(963), Glu(987), His(990), Arg(1016), and His(1023), located in or close to the epitope. Mutants were expressed as fully processed multimeric vWF. Mutation of His(1023) abolished collagen binding, whereas mutation of Arg(963) and Arg(1016) reduced collagen binding by 25-35%. These residues are part of loops alpha3beta4 and alpha1beta2 and alpha-helix 3, respectively, and lie near the bottom face of the domain. His(1023) and flanking residues display multiple conformations in available A3-crystal structures, suggesting that binding of A3 to collagen involves an induced-fit mechanism. The collagen-binding site of A3 is located distant from the top face of the domain where collagen-binding sites are found in homologous integrin I domains.

Gliding motility and cell invasion by Apicomplexa: insights from the Plasmodium sporozoite

Apicomplexa constitute one of the largest phyla of protozoa. Most Apicomplexa, including those pathogenic to humans, are obligate intracellular parasites. Their extracellular forms, which are highly polarized and elongated cells, share two unique abilities: they glide on solid substrates without changing their shape and reach an intracellular compartment without active participation from the host cell. There is now ample ultrastructural evidence that these processes result from the backward movement of extracellular interactions along the anteroposterior axis of the parasite. Recent work in several Apicomplexa, including genetic studies in the Plasmodium sporozoite, has provided molecular support for this 'capping' model. It appears that the same machinery drives both gliding motility and host cell invasion. The cytoplasmic motor, a transmembrane bridge and surface ligands essential for cell invasion are conserved among the main apicomplexan pathogens.

Multiple binding sites in collagen type I for the integrins alpha1beta1 and alpha2beta1

Integrins alpha(1)beta(1) and alpha(2)beta(1) are two major collagen receptors on the surface of eukaryotic cells. Binding to collagen is primarily due to an A-domain near the N terminus of the alpha chains. Previously, we reported that recombinant A-domain of alpha(1)beta(1) (alpha(1)A) had at least two affinity classes of binding sites in type I collagen (Rich, R. L., et al. (1999) J. Biol. Chem. 274, 24906-24913). Here, we compared the binding of the recombinant A-domain of alpha(2)beta(1) (alpha(2)A) to type I collagen with that of alpha(1)A using surface plasmon resonance and showed that alpha(2)A exhibited only one detectable class of binding sites in type I collagen, with a K(D) of approximately 10 microm at approximately 3 binding sites per collagen molecule. We further demonstrated that alpha(1)A and alpha(2)A competed with each other for binding to type I collagen in enzyme-linked immunosorbent assay (ELISA), suggesting that the binding sites in collagen for the two A-domains overlap or are adjacent to each other. By using rotary shadowing, the complexes of alpha(1)A- and alpha(2)A-procollagen were visualized. Morphometric analyses indicated three major binding regions (near the N terminus, in the central part, and near the C terminus) along the type I procollagen molecule for both A-domains. The positions of the respective binding regions for alpha(1)A and alpha(2)A were overlapping with or adjacent to each other, consistent with the ELISA results. Analysis of the sequences of type I collagen revealed that GER or GER-like motifs are present at each of the binding regions, and notably, the central region contains the GFOGER sequence, which was previously identified as a high affinity site for both alpha(1)A and alpha(2)A (Knight, C. G., et al. (2000) J. Biol. Chem. 275, 35-40). Peptides containing GLOGERGRO (peptide I, near the N terminus), GFOGERGVQ (peptide II, central), and GASGERGPO (peptide III, near the C terminus) were synthesized. Peptides I and II effectively inhibited the binding of alpha(1)A and alpha(2)A to type I collagen, while peptide III did so moderately. The N-terminal site in type I collagen has the sequence GLOGER in all three chains. Thus, it seems that peptide I represents a newly discovered native high affinity site for alpha(1)A and alpha(2)A.

The role of alpha and beta chains in ligand recognition by beta 7 integrins

Integrins alpha(E)beta(7) and alpha(4)beta(7) are involved in localization of leukocytes at mucosal sites. Although both alpha(E)beta(7) and alpha(4)beta(7) utilize the beta(7) chain, they have distinct binding specificities for E-cadherin and mucosal addressin cell adhesion molecule-1 (MAdCAM-1), respectively. We found that mutation of the metal ion-dependent adhesion site (MIDAS) in the alpha(E) A-domain (D190A) abolished E-cadherin binding, as did mutation F298A on the A-domain surface near the MIDAS cleft. A docking model of the A-domain with E-cadherin domain 1 indicates that coordination of the alpha(E) MIDAS metal ion by E-cadherin Glu(31) and a novel projection of Phe(298) into a hydrophobic pocket on E-cadherin provide the basis for the interaction. The location of the binding site on the alpha(E) A-domain resembles that on other integrins, but its structure appears distinctive and particularly adapted to recognize the tip of E-cadherin, a unique integrin ligand. Additionally, mutation of the beta(7) MIDAS motif (D140A) abolished alpha(E)beta(7) binding to E-cadherin and alpha(4)beta(7)-mediated adhesion to MAdCAM-1, and alpha(4) chain mutations that abrogated binding of alpha(4)beta(1) to vascular cell adhesion molecule-1 and fibronectin similarly reduced alpha(4)beta(7) interaction with MAdCAM-1. Thus, although specificity can be determined by the integrin alpha or beta chain, common structural features of both subunits are required for recognition of dissimilar ligands.

Structural basis of collagen recognition by integrin alpha2beta1

We have determined the crystal structure of a complex between the I domain of integrin alpha2beta1 and a triple helical collagen peptide containing a critical GFOGER motif. Three loops on the upper surface of the I domain that coordinate a metal ion also engage the collagen, with a collagen glutamate completing the coordination sphere of the metal. Comparison with the unliganded I domain reveals a change in metal coordination linked to a reorganization of the upper surface that together create a complementary surface for binding collagen. Conformational changes propagate from the upper surface to the opposite pole of the domain, suggesting both a basis for affinity regulation and a pathway for signal transduction. The structural features observed here may represent a general mechanism for integrin-ligand recognition.

Distinct recognition of collagen subtypes by alpha(1)beta(1) and alpha(2)beta(1) integrins. Alpha(1)beta(1) mediates cell adhesion to type XIII collagen

Two integrin-type collagen receptors, alpha(1)beta(1) and alpha(2)beta(1), are structurally very similar. However, cells can concomitantly express the both receptors and they might have independent functions. Here, Chinese hamster ovary (CHO) cells, which lack endogenous collagen receptors, were transfected with either alpha(1) or alpha(2) integrin cDNA. Cells were allowed to adhere to various collagen types and their integrin function was tested by observing the progression of cell spreading. The cells expressing alpha(1)beta(1) integrin could spread on collagen types I, III, IV, and V but not on type II, while alpha(2)beta(1) integrin could mediate cell spreading on collagen types I-V. Type XIII is a transmembrane collagen and its interaction with the integrins has not been previously studied. CHO-alpha1beta1 cells could spread on human recombinant type XIII collagen, unlike CHO-alpha2beta1 cells. Integrins alpha(1)beta(1) and alpha(2)beta(1) recognize collagens with the specific alphaI domains. The alpha(1)I and alpha(2)I domains were produced as recombinant proteins, labeled with europium and used in a sensitive solid-phase binding assay based on time-resolved fluorescence. alpha(1)I domain, unlike the alpha(2)I domain, could attach to type XIII collagen. The results indicate, that alpha(1)beta(1) and alpha(2)beta(1) have different ligand binding specificity. Distinct recognition of different collagen subtypes by the alphaI domains can partially explain the differences seen in cell spreading. However, despite the fact that CHO-alpha1beta1 cells could not spread on type II collagen alpha(1)I domain could bind to this collagen type. Thus, the cell spreading on collagens may also be regulated by factors other than the integrins.

Association between diabetic retinopathy and genetic variations in α2β1 integrin, a platelet receptor for collagen

Platelets might be involved in the pathogenesis of diabetic microangiopathy. Wide interindividual variations in the density of a platelet collagen receptor (2β1 integrin or glycoprotein Ia/IIa) are reportedly associated with polymorphism(s) in the gene encoding the  subunit of the receptor, including a Bgl II polymorphism in intron 7. The aim of the present study was to determine the relationship between the Bgl II polymorphism and the susceptibility to diabetic microangiopathy. A case-control study comparing 227 patients with type II diabetes mellitus (119 with versus 108 without diabetic retinopathy) as well as 169 nondiabetic subjects demonstrated that genotypes with Bgl II (+) allele had a significant increase in the risk for retinopathy. The odds ratio for Bgl II (+/+) to Bgl II (−/−) was 3.41 (95% CI, 1.49-7.78, P = .0036) when analysis was confined to those with a disease duration of diabetes of 10 years or more. The present study suggests that the presence of a Bg II (+) allele is a genetic risk factor for diabetic retinopathy.

Mapping the collagen-binding site in the I domain of the glycoprotein Ia/IIa (integrin alpha(2)beta(1))

The I domain present within the alpha2 chain of the integrin alpha(2)beta(1) (GPIa/IIa) contains the principal collagen-binding site. Based on the crystal structure of the alpha2-I domain, a hypothetical model was proposed in which collagen binds to a groove on the upper surface of the I domain (Emsley, J., King, S. L., Bergelson, J. M., and Liddington, R. C. (1997) J. Biol. Chem. 272, 28512-28517). We have introduced point mutations into 13 residues on the upper surface of the domain. Recombinant mutant proteins were assayed for binding to monoclonal antibodies 6F1 and 12F1, to collagen under static conditions, and for the ability to retain adhesive activity under flow conditions. The mutations to residues surrounding the metal ion-dependent adhesion site that caused the greatest loss of collagen binding under both static and flow conditions are N154S in the betaA-alpha1 turn, N190D in the betaB-betaC turn, D219R in the alpha3-alpha4 turn, and E256V and H258V in the betaD-alpha5 turn. Mutation in one of the residues that coordinate the metal binding, S155A, completely lost the adhesive activity under flow but bound normally under static conditions, whereas the mutation Y285F had the converse effect. We conclude that the upper surface of the domain, including the metal ion-dependent adhesion site motif, defines the collagen recognition site.

Integrin alpha(2)I domain recognizes type I and type IV collagens by different mechanisms

The collagens are recognized by the alphaI domains of the collagen receptor integrins. A common structural feature in the collagen-binding alphaI domains is the presence of an extra helix, named helix alphaC. However, its participation in collagen binding has not been shown. Here, we have deleted the helix alphaC in the alpha(2)I domain and tested the function of the resultant recombinant protein (DeltaalphaCalpha(2)I) by using a real-time biosensor. The DeltaalphaCalpha(2)I domain had reduced affinity for type I collagen (430 +/- 90 nM) when compared with wild-type alpha(2)I domain (90 +/- 30 nM), indicating both the importance of helix alphaC in type I collagen binding and that the collagen binding surface in alpha(2)I domain is located near the metal ion-dependent adhesion site. Previous studies have suggested that the charged amino acid residues, surrounding the metal ion-dependent adhesion site but not interacting with Mg(2+), may play an important role in the recognition of type I collagen. Direct evidence indicating the participation of these residues in collagen recognition has been missing. To test this idea, we produced a set of recombinant alpha(2)I domains with mutations, namely D219A, D219N, D219R, E256Q, D259N, D292N, and E299Q. Mutations in amino acids Asp(219), Asp(259), Asp(292), and Glu(299) resulted in weakened affinity for type I collagen. When alpha(2) D219N and D292N mutations were introduced separately into alpha(2)beta(1) integrin expressed on Chinese hamster ovary cells, no alterations in the cell spreading on type I collagen were detected. However, Chinese hamster ovary cells expressing double mutated alpha(2) D219N/D292N integrin showed remarkably slower spreading on type I collagen, while spreading on type IV collagen was not affected. The data indicate that alpha(2)I domain binds to type I collagen with a different mechanism than to type IV collagen.

The collagen-binding A-domains of integrins alpha(1)beta(1) and alpha(2)beta(1) recognize the same specific amino acid sequence, GFOGER, in native (triple-helical) collagens

We have previously assigned an integrin alpha(2)beta(1)-recognition site in collagen I to the sequence, GFOGERGVEGPOGPA (O = Hyp), corresponding to residues 502-516 of the alpha(1)(I) chain and located in the fragment alpha(1)(I)CB3 (Knight, C. G., Morton, L. F., Onley, D. J., Peachey, A. R., Messent, A. J., Smethurst, P. A., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (1998) J. Biol. Chem. 273, 33287-33294). In this study, we show that recognition is entirely contained within the six-residue sequence GFOGER. This sequence, when in triple-helical conformation, readily supports alpha(2)beta(1)-dependent cell adhesion and exhibits divalent cation-dependent binding of isolated alpha(2)beta(1) and recombinant alpha(2) A-domain, being at least as active as the parent collagen. Replacement of E by D causes loss of recognition. The same sequence binds integrin alpha(1) A-domain and supports integrin alpha(1)beta(1)-mediated cell adhesion. Triple-helical GFOGER completely inhibits alpha(2) A-domain binding to collagens I and IV and alpha(2)beta(1)-dependent adhesion of platelets and HT 1080 cells to these collagens. It also fully inhibits alpha(1) A-domain binding to collagen I and strongly inhibits alpha(1)beta(1)-mediated adhesion of Rugli cells to this collagen but has little effect on either alpha1 A-domain binding or adhesion of Rugli cells to collagen IV. We conclude that the sequence GFOGER represents a high-affinity binding site in collagens I and IV for alpha(2)beta(1) and in collagen I for alpha(1)beta(1). Other high-affinity sites in collagen IV mediate its recognition of alpha(1)beta(1).

Role of the alpha1 and alpha2 integrin cytoplasmic domains in cell morphology, motility and responsiveness to stimulation by the protein kinase C pathway

The alpha1beta1 and alpha2beta1 integrins, extracellular matrix receptors for collagens and/or laminins, have similarities in structure and ligand binding. Recent studies suggest that the two receptors mediate distinct post-ligand binding events and are not simply redundant receptors. To discern the mechanisms by which the two receptors differ, we focused on the roles of the cytoplasmic domains of the alpha subunits. We expressed either full-length alpha1 integrin subunit cDNA (X1C1), full-length alpha2 integrin subunit cDNA (X2C2), chimeric cDNA composed of the extracellular and transmembrane domains of alpha2 subunit and the cytoplasmic domain of alpha1 (X2C1), chimeric cDNA composed of the extracellular and transmembrane domains of alpha1 subunit and the cytoplasmic domain of alpha2 (X1C2), alpha1 cDNA truncated after the GFFKR sequence (X1C0) or alpha2 cDNA truncated after the GFFKR sequence (X2C0) in K562 cells. Although the cytoplasmic domains of the alpha1 and alpha2 subunits were not required for adhesion, the extent of adhesion at low substrate density was enhanced by the presence of either the alpha1 or alpha2 cytoplasmic tail. Spreading was also influenced by the presence of an alpha subunit cytoplasmic tail. Activation of the protein kinase C pathway with phorbol dibutyrate-stimulated motility that was dependent upon the presence of the alpha2 cytoplasmic tail. Both the phosphatidylinosotide-3-OH kinase and the mitogen-activated protein kinase pathways were required for phorbol-activated, alpha2-cytoplasmic tail-dependent migration.

Integrins alpha2beta1 and alpha4beta1 can mediate SA11 rotavirus attachment and entry into cells

Most mammalian rotaviruses contain tripeptide amino acid sequences in outer capsid proteins VP4 and VP7 which have been shown to act as ligands for integrins alpha2beta1 and alpha4beta1. Peptides containing these sequences and monoclonal antibodies directed to these integrins block rotavirus infection of cells. Here we report that SA11 rotavirus binding to and infection of K562 cells expressing alpha2beta1 or alpha4beta1 integrins via transfection is increased over virus binding to and infection of cells transfected with alpha3 integrin or parent cells. The increased binding and growth were specifically blocked by a monoclonal antibody to the transfected integrin subunit but not by irrelevant antibodies. In our experiments, integrin activation with phorbol ester did not affect virus binding to cells. However, phorbol ester treatment of K562 parent and transfected cells induced endogenous gene expression of alpha2beta1 integrin, which was detectable by flow cytometry 16 h after treatment and quantitatively correlated with the increased level of SA11 virus growth observed after this time. Virus binding to K562 cells treated with phorbol ester 24 h previously and expressing alpha2beta1 was elevated over binding to control cells and was specifically blocked by the anti-alpha2 monoclonal antibody AK7. Virus growth in alpha4-transfected K562 cells which had also been induced to express alpha2beta1 integrin with phorbol ester occurred at a level approaching that in the permissive MA104 cell line. We therefore have demonstrated that two integrins, alpha2beta1 and alpha4beta1, are capable of acting as cellular receptors for SA11 rotavirus.

A Point Mutation Thr799Met on the 2 Integrin Leads to the Formation of New Human Platelet Alloantigen Sita and Affects Collagen-Induced Aggregation

A new platelet-specific alloantigen, termed Sita, was identified in a severe case of neonatal alloimmune thrombocytopenia. The Sita alloantigen is of low frequency (1/400) in the German population. Immunochemical studies demonstrated that the Sita epitopes reside on platelet glycoprotein (GP) Ia. Nucleotide sequence analysis of GPIa cDNA derived from Sita-positive platelets showed C2531→T2531 point mutation, resulting in Thr799Met dimorphism. Analysis of genomic DNA from 22 Sita-negative normal individuals showed that the Thr799 is encoded by ACG2532 (90.9%) or ACA2532 (9.1%). To establish a DNA typing technique, we elucidated the organization of the GPIa gene adjacent to the polymorphic bases. The introns (421 bp and 1.2 kb) encompass a 142-bp exon with the 2 polymorphic bases 2531 and 2532. Polymerase chain reaction-restriction fragment length polymorphism analysis on DNA derived from 100 donors using the restriction enzyme MaeIII showed that the Met799 form of GPIa is restricted to Sita (+) phenotype. Analysis of stable Chinese hamster ovary transfectants expressing allele-specific recombinant forms of GPIa showed that anti-Sita exclusively reacted with the Glu505Met799, but not with the Glu505Thr799 and the Lys505Thr799 isoforms. In contrast, anti-Bra (HPA-5b) only recognized the Lys505Thr799 form, whereas anti-Brb(HPA-5a) reacted with both Glu505Thr799 and Glu505Met799 isoforms. These results demonstrated that the Met799 is responsible for formation of the Sita alloantigenic determinants, whereas amino acid 505 (Lys or Glu) specifically controls the expression of Bra and Brb epitopes, respectively. Platelet aggregation responses of Sita (+) individuals were diminished in response to collagen, indicating that the Thr799Met mutation affects the function of the GPIa/IIa complex.

A Point Mutation Thr799Met on the 2 Integrin Leads to the Formation of New Human Platelet Alloantigen Sita and Affects Collagen-Induced Aggregation

A new platelet-specific alloantigen, termed Sita, was identified in a severe case of neonatal alloimmune thrombocytopenia. The Sita alloantigen is of low frequency (1/400) in the German population. Immunochemical studies demonstrated that the Sita epitopes reside on platelet glycoprotein (GP) Ia. Nucleotide sequence analysis of GPIa cDNA derived from Sita-positive platelets showed C2531→T2531 point mutation, resulting in Thr799Met dimorphism. Analysis of genomic DNA from 22 Sita-negative normal individuals showed that the Thr799 is encoded by ACG2532 (90.9%) or ACA2532 (9.1%). To establish a DNA typing technique, we elucidated the organization of the GPIa gene adjacent to the polymorphic bases. The introns (421 bp and 1.2 kb) encompass a 142-bp exon with the 2 polymorphic bases 2531 and 2532. Polymerase chain reaction-restriction fragment length polymorphism analysis on DNA derived from 100 donors using the restriction enzyme MaeIII showed that the Met799 form of GPIa is restricted to Sita (+) phenotype. Analysis of stable Chinese hamster ovary transfectants expressing allele-specific recombinant forms of GPIa showed that anti-Sita exclusively reacted with the Glu505Met799, but not with the Glu505Thr799 and the Lys505Thr799 isoforms. In contrast, anti-Bra (HPA-5b) only recognized the Lys505Thr799 form, whereas anti-Brb(HPA-5a) reacted with both Glu505Thr799 and Glu505Met799 isoforms. These results demonstrated that the Met799 is responsible for formation of the Sita alloantigenic determinants, whereas amino acid 505 (Lys or Glu) specifically controls the expression of Bra and Brb epitopes, respectively. Platelet aggregation responses of Sita (+) individuals were diminished in response to collagen, indicating that the Thr799Met mutation affects the function of the GPIa/IIa complex.

Functional analysis of a recombinant glycoprotein Ia/IIa (Integrin alpha(2)beta(1)) I domain that inhibits platelet adhesion to collagen and endothelial matrix under flow conditions

The interaction of platelets with collagen plays an important role in primary hemostasis. Glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) is a major platelet receptor for collagen. The binding site for collagen has been mapped to the I domain within the alpha(2) subunit (GPIa). In order to assess the role of the alpha(2)-I domain structure in GPIa/IIa binding to collagen, a recombinant I domain (amino acids 126-337) was expressed in Escherichia coli. The alpha(2)-I protein bound human types I and III collagen in a saturable and divalent cation-dependent manner and was blocked by the alpha(2)beta(1) function blocking antibody 6F1. The alpha(2)-I protein inhibited collagen-induced platelet aggregation (IC(50) = 600 nM). Unexpectedly, 6F1, an antibody that fails to inhibit platelet aggregation in platelet-rich plasma, blocked the inhibitory effect of the alpha(2)-I protein. The alpha(2)-I protein was able to prevent platelet adhesion to a collagen surface exposed to flowing blood under low shear stress. Interestingly, it inhibited platelet adhesion to extracellular matrix at high shear stress. These results, taken together, provide firm evidence that GPIa/IIa directly mediates the first contact of platelets with collagen under both stirring and flow conditions.

Review article: platelet-collagen interactions: membrane receptors and intracellular signalling pathways

Platelet adhesion to and activation by exposed subendothelial collagen plays a critical role in normal haemostasis and pathological thrombosis. Recent advances in elucidating the mechanisms underlying platelet-collagen interaction support a 'two-site, two-step' model. Direct platelet binding to integrin alpha2beta1 mainly sustains adhesion and allows recognition of glycoprotein VI. The latter interaction is responsible for characteristic intracellular signalling events leading to p72Syk and PLCgamma2 activation. The present review describes the known collagen receptors on platelets and discusses the current understanding of signal transduction promoted by collagen.