The role of the specificity-determining loop of the integrin beta subunit I-like domain in autonomous expression, association with the alpha subunit, and ligand binding

Structural insights into the molecular recognition of integrin αVβ3 by RGD-containing ligands: The role of the specificity-determining loop (SDL)

Integrin αVβ3 is a prominent member of the "RGD-recognizing" integrin family of cell surface receptors. αVβ3 binds to various extracellular matrix (ECM) proteins and oxysterols such as 25-hydroxycholesterol, is implicated in several diseases, including cancer metastasis, lung fibrosis, inflammation, and autoimmune diseases, and is pursued as a valuable therapeutic target. Despite enormous efforts to seek a pure antagonist, to date, no single drug candidate has successfully reached clinics due to associated partial agonism and toxicity issues. Developing effective and safe inhibitors require a thorough understanding of the molecular interactions and structural changes related to the receptor's activation and inhibition mechanisms. This study offers a comprehensive residue-residue contact and network analyses of the ligand-binding β-propeller βI domains (headpiece) based on all available experimental structures of integrin αVβ3 in unliganded, agonist-, antagonist-, and antibody-bound states. The analyses reveal many critical interactions that were not reported before and show that specific orientation and interactions of residues from the specificity-determining loop (SDL) are critical in molecular recognition and regulation. Also, the network analysis reveals that residues from the nearby allosteric site (site II) connect to the primary RGD-binding site via SDL, which likely acts as an interface between the two sites. Our results provide valuable insights into molecular interactions, structural changes, distinct features of the active and inactive headpiece conformations, the role of SDL in ligand recognition, and SDL-mediated allostery. Thus, the insights from this study may facilitate the designing of pure antagonists or site II-mediated allosteric modulators to integrin αVβ3 to treat various diseases.

A novel class of inhibitors that disrupts the stability of integrin heterodimers identified by CRISPR-tiling-instructed genetic screens

The plasma membrane is enriched for receptors and signaling proteins that are accessible from the extracellular space for pharmacological intervention. Here we conducted a series of CRISPR screens using human cell surface proteome and integrin family libraries in multiple cancer models. Our results identified ITGAV (integrin αV) and its heterodimer partner ITGB5 (integrin β5) as the essential integrin α/β pair for cancer cell expansion. High-density CRISPR gene tiling further pinpointed the integral pocket within the β-propeller domain of ITGAV for integrin αVβ5 dimerization. Combined with in silico compound docking, we developed a CRISPR-Tiling-Instructed Computer-Aided (CRISPR-TICA) pipeline for drug discovery and identified Cpd_AV2 as a lead inhibitor targeting the β-propeller central pocket of ITGAV. Cpd_AV2 treatment led to rapid uncoupling of integrin αVβ5 and cellular apoptosis, providing a unique class of therapeutic action that eliminates the integrin signaling via heterodimer dissociation. We also foresee the CRISPR-TICA approach to be an accessible method for future drug discovery studies.

Molecular basis for the recognition of 24-(S)-hydroxycholesterol by integrin αvβ3

A growing body of evidence suggests that oxysterols such as 25-hydroxycholesterol (25HC) are biologically active and involved in many physiological and pathological processes. Our previous study demonstrated that 25HC induces an innate immune response during viral infections by activating the integrin-focal adhesion kinase (FAK) pathway. 25HC produced the proinflammatory response by binding directly to integrins at a novel binding site (site II) and triggering the production of proinflammatory mediators such as tumor necrosis factor-α (TNF) and interleukin-6 (IL-6). 24-(S)-hydroxycholesterol (24HC), a structural isomer of 25HC, plays a critical role in cholesterol homeostasis in the human brain and is implicated in multiple inflammatory conditions, including Alzheimer's disease. However, whether 24HC can induce a proinflammatory response like 25HC in non-neuronal cells has not been studied and remains unknown. The aim of this study was to examine whether 24HC produces such an immune response using in silico and in vitro experiments. Our results indicate that despite being a structural isomer of 25HC, 24HC binds at site II in a distinct binding mode, engages in varied residue interactions, and produces significant conformational changes in the specificity-determining loop (SDL). In addition, our surface plasmon resonance (SPR) study reveals that 24HC could directly bind to integrin αvβ3, with a binding affinity three-fold lower than 25HC. Furthermore, our in vitro studies with macrophages support the involvement of FAK and NFκB signaling pathways in triggering 24HC-mediated production of TNF. Thus, we have identified 24HC as another oxysterol that binds to integrin αvβ3 and promotes a proinflammatory response via the integrin-FAK-NFκB pathway.

The challenges and opportunities of αvβ3-based therapeutics in cancer: From bench to clinical trials

Integrins are main cell adhesion receptors serving as linker attaching cells to extracellular matrix (ECM) and bidirectional hubs transmitting biochemical and mechanical signals between cells and their environment. Integrin αvβ3 is a critical family member of integrins and interacts with ECM proteins containing RGD tripeptide sequence. Accumulating evidence indicated that the abnormal expression of integrin αvβ3 was associated with various tumor progressions, including tumor initiation, sustained tumor growth, distant metastasis, drug resistance development, maintenance of stemness in cancer cells. Therefore, αvβ3 has been explored as a therapeutic target in various types of cancers, but there is no αvβ3 antagonist approved for human therapy. Targeting-integrin αvβ3 therapeutics has been a challenge, but lessons from the past are valuable to the development of innovative targeting approaches. This review systematically summarized the structure, signal transduction, regulatory role in cancer, and drug development history of integrin αvβ3, and also provided new insights into αvβ3-based therapeutics in cancer from bench to clinical trials, which would contribute to developing effective targeting αvβ3 agents for cancer treatment.

Cryo-Electron Microscopy Structure of the αIIbβ3-Abciximab Complex

OBJECTIVE

The αIIbβ3 antagonist antiplatelet drug abciximab is the chimeric antigen-binding fragment comprising the variable regions of murine monoclonal antibody 7E3 and the constant domains of human IgG1 and light chain κ. Previous mutagenesis studies suggested that abciximab binds to the β3 C177-C184 specificity-determining loop (SDL) and Trp129 on the adjacent β1-α1 helix. These studies could not, however, assess whether 7E3 or abciximab prevents fibrinogen binding by steric interference, disruption of either the αIIbβ3-binding pocket for fibrinogen or the β3 SDL (which is not part of the binding pocket but affects fibrinogen binding), or some combination of these effects. To address this gap, we used cryo-electron microscopy to determine the structure of the αIIbβ3-abciximab complex at 2.8 Å resolution. Approach and Results: The interacting surface of abciximab is comprised of residues from all 3 complementarity-determining regions of both the light and heavy chains, with high representation of aromatic residues. Binding is primarily to the β3 SDL and neighboring residues, the β1-α1 helix, and β3 residues Ser211, Val212 and Met335. Unexpectedly, the structure also indicated several interactions with αIIb. As judged by the cryo-electron microscopy model, molecular-dynamics simulations, and mutagenesis, the binding of abciximab does not appear to rely on the interaction with the αIIb residues and does not result in disruption of the fibrinogen-binding pocket; it does, however, compress and reduce the flexibility of the SDL.

CONCLUSIONS

We deduce that abciximab prevents ligand binding by steric interference, with a potential contribution via displacement of the SDL and limitation of the flexibility of the SDL residues.

Laminin is the ECM niche for trophoblast stem cells

Laminin functions as an ECM niche factor for trophoblast stem cells and secures trophoblast stem cell expansion through its interactions with integrin.

The niche is a specialized microenvironment for tissue stem cells in vivo. It has long been emphasized that niche ECM molecules act on tissue stem cells to regulate their behavior, but the molecular entities of these interactions remain to be fully elucidated. Here, we report that laminin forms the in vivo ECM niche for trophoblast stem cells (TSCs), the tissue stem cells of the placenta. TSCs expressed fibronectin-binding, vitronectin-binding, and laminin-binding integrins, whereas the integrin ligands present in the TSC niche were collagen and laminin. Therefore, the only niche integrin ligand available for TSCs in vivo was laminin. Laminin promoted TSC adhesion and proliferation in vitro in an integrin binding–dependent manner. Importantly, when the integrin-binding ability of laminin was genetically ablated in mice, the size of the TSC population was significantly reduced compared with that in control mice. The present findings underscore an ECM niche function of laminin to support tissue stem cell maintenance in vivo.

A Glanzmann thrombasthenia family associated with a TUBB1-related macrothrombocytopenia

BACKGROUND

Macrothrombocytopenia (MTP) is a rare but enigmatic complication of Glanzmann thrombasthenia (GT), an inherited bleeding disorder caused by the absence of platelet aggregation due to deficiencies of the αIIbβ3 integrin.

OBJECTIVES

We report a family with type I GT and a prolonged bleeding time but unusually associated with congenital mild thrombocytopenia and platelet size heterogeneity with giant forms.

METHODS AND RESULTS

Sanger sequencing of DNA from the propositus identified 2 heterozygous ITGB3 gene mutations: p.P189S and p.C210S both of which prevent αIIbβ3 expression and are causative of GT but without explaining the presence of enlarged platelets. High-throughput screening led to the detection of a predicted disease-causing heterozygous mutation in the TUBB1 gene: p.G146R, encoding β1-tubulin, a component of the platelet cytoskeleton and a gene where mutations are a known cause of MTP.

CONCLUSIONS

Family screening confirmed that this rare phenotype results from oligogenic inheritance while suggesting that the GT phenotype dominates clinically.

Integrin activation by the lipid molecule 25-hydroxycholesterol induces a proinflammatory response

Integrins are components of cell-matrix adhesions, and function as scaffolds for various signal transduction pathways. So far no lipid ligand for integrin has been reported. Here we show that a lipid, oxysterol 25-hydroxycholesterol (25HC), directly binds to α5β1 and αvβ3 integrins to activate integrin-focal adhesion kinase (FAK) signaling. Treatment of macrophages and epithelial cells with 25HC results in an increase in activated αvβ3 integrin in podosome and focal adhesion matrix adhesion sites. Moreover, activation of pattern recognition receptor on macrophages induces secretion of 25HC, triggering integrin signaling and the production of proinflammatory cytokines such as TNF and IL-6. Thus, the lipid molecule 25HC is a physiologically relevant activator of integrins and is involved in positively regulating proinflammatory responses. Our data suggest that extracellular 25HC links innate immune inflammatory response with integrin signaling.

Autonomous conformational regulation of β3 integrin and the conformation-dependent property of HPA-1a alloantibodies

Integrin α/β heterodimer adopts a compact bent conformation in the resting state, and upon activation undergoes a large-scale conformational rearrangement. During the inside-out activation, signals impinging on the cytoplasmic tail of β subunit induce the α/β separation at the transmembrane and cytoplasmic domains, leading to the extended conformation of the ectodomain with the separated leg and the opening headpiece that is required for the high-affinity ligand binding. It remains enigmatic which integrin subunit drives the bent-to-extended conformational rearrangement in the inside-out activation. The β₃ integrins, including α_IIbβ₃ and α_Vβ₃, are the prototypes for understanding integrin structural regulation. The Leu33Pro polymorphism located at the β₃ PSI domain defines the human platelet-specific alloantigen (HPA) 1a/b, which provokes the alloimmune response leading to clinically important bleeding disorders. Some, but not all, anti-HPA-1a alloantibodies can distinguish the α_IIbβ₃ from α_Vβ₃ and affect their functions with unknown mechanisms. Here we designed a single-chain β₃ subunit that mimics a separation of α/β heterodimer on inside-out activation. Our crystallographic and functional studies show that the single-chain β₃ integrin folds into a bent conformation in solution but spontaneously extends on the cell surface. This demonstrates that the β₃ subunit autonomously drives the membrane-dependent conformational rearrangement during integrin activation. Using the single-chain β₃ integrin, we identified the conformation-dependent property of anti-HPA-1a alloantibodies, which enables them to differently recognize the β₃ in the bent state vs. the extended state and in the complex with α_IIb vs. α_V This study provides deeper understandings of integrin conformational activation on the cell surface.

Laminin γ1 C-terminal Glu to Gln mutation induces early postimplantation lethality

Mouse embryos with an ablated ability of integrins to bind laminins are still able to form basement membranes, but die just after implantation because of deficient extraembryonic development.

Laminin–integrin interactions regulate various adhesion-dependent cellular processes. γ1C-Glu, the Glu residue in the laminin γ1 chain C-terminal tail, is crucial for the binding of γ1-laminins to several integrin isoforms. Here, we investigated the impact of γ1C Glu to Gln mutation on γ1-laminin binding to all possible integrin partners in vitro, and found that the mutation specifically ablated binding to α3, α6, and α7 integrins. To examine the physiological significance of γ1C-Glu, we generated a knock-in allele, Lamc1^EQ, in which the γ1C Glu to Gln mutation was introduced. Although Lamc1^EQ/EQ homozygotes developed into blastocysts and deposited laminins in their basement membranes, they died just after implantation because of disordered extraembryonic development. Given the impact of the Lamc1^EQ allele on embryonic development, we developed a knock-in mouse strain enabling on-demand introduction of the γ1C Glu to Gln mutation by the Cre-loxP system. The present study has revealed a crucial role of γ1C-Glu–mediated integrin binding in postimplantation development and provides useful animal models for investigating the physiological roles of laminin–integrin interactions in vivo.

Structure of an extended β3 integrin

Cells use adhesion receptor integrins to communicate with their surroundings. Integrin activation and cellular signaling are coupled with change from bent to extended conformation. β3 integrins, including αIIbβ3, which is essential for the function of platelets in hemostasis and thrombosis, and αVβ3, which plays multiple roles in diverse cell types, have been prototypes in understanding integrin structure and function. Despite extensive structural studies, a high-resolution integrin structure in an extended conformation remains to be determined. The human β3 Leu33Pro polymorphism, located at the PSI domain, defines human platelet-specific alloantigens 1a and 1b (HPA-1a/b), immune response to which is a cause of posttransfusion purpura and fetal/neonatal alloimmune thrombocytopenia. Leu33Pro substitution has also been suggested to be a risk factor for thrombosis. Here we report the crystal structure of the β3 headpiece in either Leu33 or Pro33 form, both of which reveal intermediate and fully extended conformations coexisting in 1 crystal. These were used to build high-resolution structures of full-length β3 integrin in the intermediate and fully extended states, agreeing well with the corresponding conformations observed by electron microscopy. Our structures reveal how β3 integrin becomes extended at its β-knee region and how the flexibility of β-leg domains is determined. In addition, our structures reveal conformational changes of the PSI and I-EGF1 domains upon β3 extension, which may affect the binding of conformation-dependent anti-HPA-1a alloantibodies. Our structural and functional data show that Leu33Pro substitution does not directly alter the conformation or ligand binding of β3 integrin.

Coupling integrin dynamics to cellular adhesion behaviors

Visualizing fluorescent proteins is essential for understanding cellular function. While advances in microscopy can now resolve individual molecules, determining whether the labeled molecules report native behaviors and how the measured behaviors can be coupled to cellular outputs remains challenging. Here, we used integrin alpha-beta heterodimers - which connect extracellular matrix (ECM) and the cytoskeleton - to quantify the mobility and conformation of labeled integrins. We found that while unlabeled and labeled integrins all localized to adhesions and support anchorage-dependent cell function, integrin mobility decreased when the beta rather than the alpha subunit was labeled. In contrast to unlabeled and alpha labeled subunits, beta labeled subunits changed cellular behavior; decreasing protrusive activity and increasing adhesion size and the extent of cell spreading. Labeling the beta subunit changed the integrin conformation, extending the molecule and exposing an epitope that is revealed by activation with Mn²⁺ treatment. Our findings indicate labeling induced changes in dynamic integrin behavior alter molecular conformation as well as cellular adhesion-dependent function to demonstrate a coupling between molecular inputs and distinct cellular outputs.This article has an associated First Person interview with the first author of the paper.

Activation mechanisms of αVβ3 integrin by binding to fibronectin: A computational study

Integrin αVβ3 plays an important role in regulating cellular activities and in human diseases. Although the structure of αVβ3 has been studied by crystallography and electron microscopy, the detailed activation mechanism of integrin αVβ3 induced by fibronectin remains unclear. In this study, we investigated the conformational and dynamical motion changes of Mn2+ -bound integrin αVβ3 by binding to fibronectin with molecular dynamics simulations. Results showed that fibronectin binding to integrin αVβ3 caused the changes of the conformational flexibility of αVβ3 domains, the essential mode of motion for the domains of αV subunit and β3 subunit and the degrees of correlated motion of residues between the domains of αV subunit and β3 subunit of integrin αVβ3. The angle of Propeller domain with respect to the Calf-2 domain of αV subunit and the angle of Hybrid domain with respect to βA domain of β3 subunit significantly increased when integrin αVβ3 was bound to fibronectin. These changes could result in the conformational change tendency of αVβ3 from a bend conformation to an extended conformation and lead to the open swing of Hybrid domain relative to βA domain of β3 subunit, which have demonstrated their importance for αVβ3 activation. Fibronectin binding to integrin αVβ3 significantly decreased the relative position of α1 helix to βA domain and that to metal ion-dependent adhesion site, stabilized Mn2+ ions binding in integrin αVβ3 and changed fibronectin conformation, which are important for αVβ3 activation. Results from this study provide important molecular insight into the "outside-in" activation mechanism of integrin αVβ3 by binding to fibronectin.

Epitopes in α8β1 and other RGD-binding integrins delineate classes of integrin-blocking antibodies and major binding loops in α subunits

Identification of epitopes for integrin-blocking monoclonal antibodies (mAbs) has aided our understanding of structure-function relationship of integrins. We mapped epitopes of chicken anti-integrin-α8-subunit-blocking mAbs by mutational analyses, examining regions that harboured all mapped epitopes recognized by mAbs against other α-subunits in the RGD-binding-integrin subfamily. Six mAbs exhibited blocking function, and these mAbs recognized residues on the same W2:41-loop on the top-face of the β-propeller. Loop-tips sufficiently close to W2:41 (<25 Å) contained within a footprint of the mAbs were mutated, and the loop W3:34 on the bottom face was identified as an additional component of the epitope of one antibody, clone YZ5. Binding sequences on the two loops were conserved in virtually all mammals, and that on W3:34 was also conserved in chickens. These indicate 1) YZ5 binds both top and bottom loops, and the binding to W3:34 is by interactions to conserved residues between immunogen and host species, 2) five other blocking mAbs solely bind to W2:41 and 3) the α8 mAbs would cross-react with most mammals. Comparing with the mAbs against the other α-subunits of RGD-integrins, two classes were delineated; those binding to "W3:34 and an top-loop", and "solely W2:41", accounting for 82% of published RGD-integrin-mAbs.

In Situ Detection of Integrin Ligands

Integrins are cell surface receptors for cell adhesion. Integrin-mediated cell adhesion regulates various cellular processes, including cell survival, migration, proliferation, and differentiation. In vivo, ligands for integrins are immobilized within extracellular matrices, insoluble sheet-like or fibrous supramolecular complexes that associate with or surround cells. To better understand the molecular basis of integrin-mediated regulation of cellular behavior in vivo, it is of critical importance to collect information regarding the activities as well as spatial distributions of integrin ligands in situ. This unit describes a protocol for detecting the spatial distribution of the complement of integrin ligands in situ by overlaying soluble recombinant integrins.

In situ detection of integrin ligands

Integrins are cell surface receptors for cell adhesion. Integrin-mediated cell adhesion regulates various cellular processes, including cell survival, migration, proliferation, and differentiation. In vivo, ligands for integrins are immobilized within extracellular matrices, insoluble sheet-like or fibrous supramolecular complexes that associate with or surround cells. To better understand the molecular basis of integrin-mediated regulation of cellular behavior in vivo, it is of critical importance to collect information regarding the activities as well as spatial distributions of integrin ligands in situ. This unit describes a protocol for detecting the spatial distribution of the complement of integrin ligands in situ by overlaying soluble recombinant integrins.

Structural determinants of integrin β-subunit specificity for latent TGF-β

Eight integrin α-β heterodimers recognize ligands with an Arg-Gly-Asp (RGD) motif. However, the structural mechanism by which integrins differentiate among extracellular proteins with RGD motifs is not understood. Here, crystal structures, mutations and peptide-affinity measurements show that αVβ6 binds with high affinity to a RGDLXXL/I motif within the prodomains of TGF-β1 and TGF-β3. The LXXL/I motif forms an amphipathic α-helix that binds in a hydrophobic pocket in the β6 subunit. Elucidation of the basis for ligand binding specificity by the integrin β subunit reveals contributions by three different βI-domain loops, which we designate specificity-determining loops (SDLs) 1, 2 and 3. Variation in a pair of single key residues in SDL1 and SDL3 correlates with the variation of the entire β subunit in integrin evolution, thus suggesting a paradigmatic role in overall β-subunit function.

Stereoselectivity of isoflurane in adhesion molecule leukocyte function-associated antigen-1

BACKGROUND

Isoflurane in clinical use is a racemate of S- and R-isoflurane. Previous studies have demonstrated that the effects of S-isoflurane on relevant anesthetic targets might be modestly stronger (less than 2-fold) than R-isoflurane. The X-ray crystallographic structure of the immunological target, leukocyte function-associated antigen-1 (LFA-1) with racemic isoflurane suggested that only S-isoflurane bound specifically to this protein. If so, the use of specific isoflurane enantiomers may have advantage in the surgical settings where a wide range of inflammatory responses is expected to occur. Here, we have further tested the hypothesis that isoflurane enantioselectivity is apparent in solution binding and functional studies.

METHODS

First, binding of isoflurane enantiomers to LFA-1 was studied using 1-aminoanthracene (1-AMA) displacement assays. The binding site of each enantiomer on LFA-1 was studied using the docking program GLIDE. Functional studies employed the flow-cytometry based ICAM binding assay.

RESULTS

Both enantiomers decreased 1-AMA fluorescence signal (at 520 nm), indicating that both competed with 1-AMA and bound to the αL I domain. The docking simulation demonstrated that both enantiomers bound to the LFA-1 "lovastatin site." ICAM binding assays showed that S-isoflurane inhibited more potently than R-isoflurane, consistent with the result of 1-AMA competition assay.

CONCLUSIONS

In contrast with the x-ray crystallography, both enantiomers bound to and inhibited LFA-1. S-isoflurane showed slight preference over R-isoflurane.

C-type lectin binds to β-integrin to promote hemocytic phagocytosis in an invertebrate

Phagocytosis is a conserved cellular response among metazoans. Opsonins are some molecules that label targets to increase their susceptibility to phagocytosis. Opsonins are usually captured by receptors on the surface of phagocytes. Our previous study found the C-type lectin FcLec4 from Chinese white shrimp Fenneropenaeus chinensis might function as an opsonin to facilitate bacterial clearance. In the present study we purified the native FcLec4 protein and confirmed its opsonic activity in the near relation, kuruma shrimp Marsupenaeus japonicus. The possible receptor of FcLec4 was identified as β-integrin by panning a T7 phage display library of shrimp hemocytes and then confirmed by co-immunoprecipitation assay. We further proved that the interaction between FcLec4 and β-integrin did not rely on the carbohydrate recognition domain but on the N terminus of FcLec4. In addition, inhibition of FcLec4 expression using RNAi delayed bacterial clearance, and β-integrin knockdown suppressed the opsonic activity of FcLec4. This study is the first to show the direct interaction between an opsonin and its receptor in crustaceans. Our study provides new insights into invertebrate phagocytosis and the functions of C-type lectins.

Isoflurane binds and stabilizes a closed conformation of the leukocyte function-associated antigen-1

We previously demonstrated that isoflurane targets lymphocyte function-associated antigen-1 (LFA-1), a critical adhesion molecule for leukocyte arrest. However, it remains to be determined how isoflurane interacts with the full ectodomain LFA-1 and modulates its conformation and function. Isoflurane binding sites on the full ectodomain LFA-1 were probed by photolabeling using photoactivatable isoflurane (azi-isoflurane). The adducted residues were determined by liquid chromatography/mass spectrometry analysis. Separately, docking simulations were performed to predict binding sites. Point mutations were introduced around isoflurane binding sites. The significance of isoflurane's effect was assessed in both intracellular adhesion molecule-1 (ICAM-1) binding assays and epitope mapping of activation-sensitive antibodies using flow cytometry. Two isoflurane binding sites were identified using photolabeling and were further validated by the docking simulation: one at the hydrophobic pocket in the ICAM-1 binding domain (the αI domain); the other at the βI domain. Mutagenesis of the α'1 helix showed that isoflurane binding sites at the βI domain were significantly important in modulating LFA-1 function and conformation. Epitope mapping using activation-sensitive antibodies suggested that isoflurane stabilized LFA-1 in the closed conformation. This study suggested that isoflurane binds to both the αI and βI domains allosteric to the ICAM-1 binding site, and that isoflurane binding stabilizes LFA-1 in the closed conformation.

N-glycosylation of ß4 integrin controls the adhesion and motility of keratinocytes

α6ß4 integrin is an essential component of hemidesmosomes and modulates cell migration in wound healing and cancer invasion. To elucidate the role of N-glycosylation on ß4 integrin, we investigated keratinocyte adhesion and migration through the re-expression of wild-type or N-glycosylation-defective ß4 integrin (ΔNß4) in ß4 integrin null keratinocytes. N-glycosylation of ß4 integrin was not essential for the heterodimer formation of ß4 integrin with α6 integrin and its expression on a cell surface, but N-glycosylation was required for integrin-mediated cell adhesion and migration. Concomitantly with the reduction of ß4 integrin in the membrane microdomain, the intracellular signals of Akt and ERK activation were decreased in cells expressing ΔNß4 integrin. Forced cross-linking of ß4 integrin rescued the decreased ERK activation in ΔNß4 integrin-expressing cells to a similar extent in wild-type ß4 integrin-expressing cells. Surprisingly, compared with cells expressing wild-type ß4 integrin, an alternation in N-glycan structures expressed on epidermal growth factor receptor (EGFR), and the induction of a stronger association between EGFR and ß4 integrin were observed in ΔNß4 integrin-expressing cells. These results clearly demonstrated that N-glycosylation on ß4 integrin plays an essential role in keratinocyte cellular function by allowing the appropriate complex formation on cell surfaces.

Cation-pi interaction regulates ligand-binding affinity and signaling of integrin alpha4beta7

Integrin α(4)β(7) mediates rolling and firm adhesion of leucocytes, two of the critical steps in leukocyte migration and tissue specific homing. Affinity of α(4)β(7) for ligand is dynamically regulated by three interlinked metal ion-binding sites in β(7)-subunit I domain. In this study, we found that Phe185 (F185), a highly conserved aromatic residue in β(7)-subunit, links the specificity-determining loop and the synergistic metal ion-binding site (SyMBS) through cation-π interaction. Mutations of F185 that disrupted the SyMBS cation-F185 interaction led to deficient firm cell adhesion mediated by high affinity α(4)β(7), and only slightly affected rolling adhesion mediated by low affinity α(4)β(7). Disruption of SyMBS cation-F185 interaction induced partial extension of integrin ectodomain and separation of cytoplasmic tails, and impaired α(4)β(7)-mediated bidirectional signaling. In addition, loss of SyMBS cation-F185 interaction increased paxillin expression and promoted paxillin-integrin binding, leading to deficient cell spreading. Furthermore, integrin α(4)β(7)-mediated cell migration was decreased by the abolishment of SyMBS cation-F185 interaction. Thus, these findings reveal a cation-π interaction playing vital roles in the regulation of integrin affinity, signaling, and biological functions.

Role of altered sialylation of the I-like domain of beta1 integrin in the binding of fibronectin to beta1 integrin: thermodynamics and conformational analyses

N-glycosylation of the I-like domain of beta1 integrin plays an essential role in integrin structure and function, and the altered sialylation of beta1 integrin regulates beta1 integrin binding to fibronectin. However, the structural basis underlying the effect of altered sialylation of the beta1 I-like domain on beta1 integrin binding to fibronectin remains largely unknown. In this study, we used a combination of molecular dynamics simulations and binding free energy analyses to investigate changes in binding thermodynamics and in conformation of the glycosylated beta1 I-like domain-FN-III(9-10) complex caused by altered sialylation of the beta1 I-like domain. Binding free energy analyses showed that desialylation of beta1 I-like domain increased beta1 integrin binding to fibronectin, consistent with experimental results. Interaction analyses showed that altered sialylation of the beta1 I-like domain resulted in significant changes in the interaction of the N-glycans of the I-like domain with both the I-like domain and fibronectin, and these changes could directly affect the allosteric regulation of the interaction between the I-like domain and fibronectin. Altered sialylation of the beta1 I-like domain caused significant conformational changes in key functional sites of both the beta1 I-like domain and fibronectin. In addition, altered sialylation of the beta1 I-like domain resulted in changes in the degree of correlated motions between residues in the I-like domain and residues in fibronectin, and in the degree of motion changes in fibronectin, which could affect beta1 integrin binding to fibronectin. We believe results from this study provide thermodynamic and structural evidence for a role of altered sialylation of beta1 integrin in regulating beta1 integrin binding to fibronectin and it's induced cellular activities.

Shedding of large functionally active CD11/CD18 Integrin complexes from leukocyte membranes during synovial inflammation distinguishes three types of arthritis through differential epitope exposure

CD18 integrins are adhesion molecules expressed on the cell surface of leukocytes and play a central role in the molecular mechanisms supporting leukocyte migration to zones of inflammation. Recently, it was discovered that CD11a/CD18 is shed from the leukocyte surface in models of inflammation. In this study, we show that shedding of human CD11/CD18 complexes is a part of synovial inflammation in rheumatoid arthritis and spondyloarthritis but not in osteoarthritis. In vivo and in vitro data suggest that the shedding is driven by TNF-α, which links the process to central events in the inflammatory response. The shed complexes contain multiple heterodimers of CD11/CD18, are variable in size, and differ according to the type of synovial inflammation. Furthermore, the differential structures determine the avidity of binding of the complexes to the ICAM-1. With the estimated concentrations of CD11/CD18 in plasma and synovial fluid a significant coverage of binding sites in ICAM-1 for CD18 integrins is expected. Based on cell adhesion experiments in vitro, we hypothesize that the large soluble complexes of CD11/CD18 act in vivo to buffer leukocyte adhesion by competing with the membrane-bound receptors for ICAM-1 binding sites. As reported here for synovial inflammation changes in the concentration or structure of these complexes should be considered as likely contributors to disease activity.

The reactive tumor microenvironment: MUC1 signaling directly reprograms transcription of CTGF

The MUC1 cytoplasmic tail (MUC1.CT) conducts signals from spatial and extracellular cues (growth factor and cytokine stimulation) to evoke a reprogramming of the cellular transcriptional profile. Specific phosphorylated forms of the MUC1.CT achieve this function by differentially associating with transcription factors and redirecting their transcriptional regulatory capabilities at specific gene regulatory elements. The specificity of interaction between MUC1.CT and several transcription factors is dictated by the phosphorylation pattern of the 18 potential phosphorylation motifs within the MUC1.CT. To better appreciate the scope of differential gene expression triggered by MUC1.CT activation, we performed microarray gene expression analysis and ChIP-chip promoter analysis and identified the genome-wide transcriptional targets of MUC1.CT signaling in pancreatic cancer. On a global scale, MUC1.CT preferentially targets genes relating to invasion, angiogenesis and metastasis, suggesting that MUC1.CT signaling contributes to establishing a reactive tumor microenvironment during tumor progression to metastatic disease. We examined in detail the molecular mechanisms of MUC1.CT signaling that induces expression of connective tissue growth factor (CTGF/CCN2), a potent mediator of ECM remodeling and angiogenesis. We demonstrate a robust induction of CTGF synthesis and secretion in response to serum factors that is enabled only when MUC1 is highly expressed. We demonstrate the requirement of phosphorylation at distinct tyrosine motifs within the MUC1.CT for MUC1-induced CTGF expression and demonstrate a phosphorylation-specific localization of MUC1.CT to the CTGF promoter. We found that MUC1 reorganizes transcription factor occupancy of genomic regions upstream of the CTGF gene, directing β-catenin and mutant p53 to CTGF gene regulatory elements to promote CTGF expression and destabilizing the interaction at these regions of the transcriptional repressor, c-Jun. With this example we illustrate the capacity of MUC1.CT to mediate transcription factor activity in a context-dependent manner to achieve widespread and robust changes in gene expression and facilitate creation of the reactive tumor microenvironment.

N-glycosylation of the I-like domain of beta1 integrin is essential for beta1 integrin expression and biological function: identification of the minimal N-glycosylation requirement for alpha5beta1

N-Glycosylation of integrin alpha5beta1 plays a crucial role in cell spreading, cell migration, ligand binding, and dimer formation, but the detailed mechanisms by which N-glycosylation mediates these functions remain unclear. In a previous study, we showed that three potential N-glycosylation sites (alpha5S3-5) on the beta-propeller of the alpha5 subunit are essential to the functional expression of the subunit. In particular, site 5 (alpha5S5) is the most important for its expression on the cell surface. In this study, the function of the N-glycans on the integrin beta1 subunit was investigated using sequential site-directed mutagenesis to remove the combined putative N-glycosylation sites. Removal of the N-glycosylation sites on the I-like domain of the beta1 subunit (i.e. the Delta4-6 mutant) decreased both the level of expression and heterodimeric formation, resulting in inhibition of cell spreading. Interestingly, cell spreading was observed only when the beta1 subunit possessed these three N-glycosylation sites (i.e. the S4-6 mutant). Furthermore, the S4-6 mutant could form heterodimers with either alpha5S3-5 or alpha5S5 mutant of the alpha5 subunit. Taken together, the results of the present study reveal for the first time that N-glycosylation of the I-like domain of the beta1 subunit is essential to both the heterodimer formation and biological function of the subunit. Moreover, because the alpha5S3-5/beta1S4-6 mutant represents the minimal N-glycosylation required for functional expression of the beta1 subunit, it might also be useful for the study of molecular structures.

Unexpected diversity of cnidarian integrins: expression during coral gastrulation

BACKGROUND

Adhesion mediated through the integrin family of cell surface receptors is central to early development throughout the Metazoa, playing key roles in cell-extra cellular matrix adhesion and modulation of cadherin activity during the convergence and extension movements of gastrulation. It has been suggested that Caenorhabditis elegans, which has a single beta and two alpha integrins, might reflect the ancestral integrin complement. Investigation of the integrin repertoire of anthozoan cnidarians such as the coral Acropora millepora is required to test this hypothesis and may provide insights into the original roles of these molecules.

RESULTS

Two novel integrins were identified in Acropora. AmItgalpha1 shows features characteristic of alpha integrins lacking an I-domain, but phylogenetic analysis gives no clear indication of its likely binding specificity. AmItgbeta2 lacks consensus cysteine residues at positions 8 and 9, but is otherwise a typical beta integrin. In situ hybridization revealed that AmItgalpha1, AmItgbeta1, and AmItgbeta2 are expressed in the presumptive endoderm during gastrulation. A second anthozoan, the sea anemone Nematostella vectensis, has at least four beta integrins, two resembling AmItgbeta1 and two like AmItgbeta2, and at least three alpha integrins, based on its genomic sequence.

CONCLUSION

In two respects, the cnidarian data do not fit expectations. First, the cnidarian integrin repertoire is more complex than predicted: at least two betas in Acropora, and at least three alphas and four betas in Nematostella. Second, whereas the bilaterian alphas resolve into well-supported groups corresponding to those specific for RGD-containing or laminin-type ligands, the known cnidarian alphas are distinct from these. During early development in Acropora, the expression patterns of the three known integrins parallel those of amphibian and echinoderm integrins.

Spatial separation and bidirectional trafficking of proteins using a multi-functional reporter

Background

The ability to specifically label proteins within living cells can provide information about their dynamics and function. To study a membrane protein, we fused a multi-functional reporter protein, HaloTag^®, to the extracellular domain of a truncated integrin.

Results

Using the HaloTag technology, we could study the localization, trafficking and processing of an integrin-HaloTag fusion, which we showed had cellular dynamics consistent with native integrins. By labeling live cells with different fluorescent impermeable and permeable ligands, we showed spatial separation of plasma membrane and internal pools of the integrin-HaloTag fusion, and followed these protein pools over time to study bi-directional trafficking. In addition to combining the HaloTag reporter protein with different fluorophores, we also employed an affinity tag to achieve cell capture.

Conclusion

The HaloTag technology was used successfully to study expression, trafficking, spatial separation and real-time translocation of an integrin-HaloTag fusion, thereby demonstrating that this technology can be a powerful tool to investigate membrane protein biology in live cells.

Three novel beta3 domain-deletion peptides for the sensitive and specific detection of HPA-4 and six low frequency beta3-HPA antibodies

BACKGROUND

Antibodies against human platelet antigens (HPA) are clinically important in fetal-maternal alloimmune thrombocytopenia, refractoriness to platelet transfusions and post-transfusion purpura. Of the 16 HPAs, nine are located on the beta3 subunit of the alphaIIb beta3 integrin. Antibody detection is generally based on platelet-derived alphaIIb beta3 from HPA-genotyped donors. Recombinant allelic beta3 peptides, expressed at high levels would improve consistency in antibody detection, but the expression of soluble and monomeric integrins expressing complex dependent epitopes has previously proved challenging.

OBJECTIVES

We aimed to generate three recombinant beta3 peptides for the detection of antibodies against HPA-4, HPA-8bw and five of the six remaining low frequency beta3 alloantigens.

METHODS

The removal of the specificity-determining loop from the betaA domain and fusion of truncated beta3 to calmodulin was exploited to obtain expression of monomeric protein. Using site-directed mutagenesis, the mutations for HPA-4b and HPA-8bw were introduced in the ITGB3*001 haplotype. A third peptide for the detection of antibodies against HPA coded by non-synonymous single nucleotide polymorphisms of low frequency was generated by the introduction of five mutations forming the basis of HPA-6bw, -7bw, -10bw, -11bw, and -16bw antigens.

RESULTS

Reactivity of the three peptides with beta3-specific murine monoclonal antibodies and human HPA-1a phage antibodies confirmed the structural integrity of the recombinant fragments, and reactivity with a unique panel of polyclonal anti-HPA sera confirmed expression of the relevant HPA epitopes.

CONCLUSIONS

These data demonstrate that beta3 integrin domain-deletion fragments are suitable molecular targets for HPA antibody detection.

Immunologic and structural analysis of eight novel domain-deletion beta3 integrin peptides designed for detection of HPA-1 antibodies

BACKGROUND

The single-nucleotide polymorphism (SNP) rs5918 in the ITGB3 gene defines the human platelet antigen-1 (HPA-1) system encoding a Leu (HPA-1a) or Pro (HPA-1b) at position 33. HPA-1 antibodies are clinically the most relevant in the Caucasoid population, but detection currently requires alpha(IIb)beta3 integrin from the platelets of HPA-genotyped donors.

OBJECTIVES

We set out to define the beta3 integrin domains required for HPA-1a antibody binding and produce recombinant soluble beta3 peptides for HPA-1 antibody detection.

METHODS

We designed two sets (1a and 1b) of four soluble beta3 domain-deletion peptides (deltaSDL, deltabetaA, PSIHybrid, PSI), informed by crystallography studies and computer modeling. The footprints of three human HPA-1a-specific phage antibodies were defined by analyzing binding patterns to the beta3 peptides and canine platelets, and models of antibody-antigen interfaces were derived. Specificity and sensitivity for HPA-1a detection were assessed using sera from 140 cases of fetomaternal alloimmune thrombocytopenia (FMAIT).

RESULTS

Fusion of recombinant proteins to calmodulin resulted in high-level expression in Drosophila S2 cells of all eight beta3 peptides. Testing of FMAIT samples indicated that deltabetaA-Leu33 is the superior peptide for HPA-1a antibody detection, with 96% sensitivity and 95% specificity. The existence of type I and II categories of HPA-1a antibodies was confirmed by the study of HPA-1a phage antibody footprints and the reactivity pattern of clinical samples with the four beta3-Leu33 peptides, but there was no correlation between antibody category and clinical severity of FMAIT.

CONCLUSIONS

Soluble recombinant beta3 peptides can be used for detection of clinical HPA-1a antibodies.

Tyr178 of beta3 is critical for alphaIIb maturation and macromolecular ligand binding to alphaIIbbeta3

To explore the structural basis of ligand binding to alphaIIbbeta3, we conducted a site-directed mutagenesis of Y178, which is located in the ligand-specificity region (C177-C184) of the beta3 subunit. Two mutant beta3 constructs, Y178A and Y178I, were transfected into CHO cells and co-expressed with human alphaIIb subunit on the cell surface. Our results showed that the Y178A mutation affected processing and cell surface exposure of recombinant alphaIIbbeta3 receptor, abrogated the binding of PAC-1, a ligand-mimetic antibody, to alphaIIbbeta3 pre-treated with the known activator DTT. The Y178A mutation also resulted in reduced adhesion of alphaIIbbeta3 on immobilized fibrinogen. In contrast, the interaction of alphaIIbbeta3 with the small molecular ligand RGDS was unaffected by Y178A mutation, as evidenced by the elevated LIBS-1 epitope expression following RGDS addition. Interestingly however, Y178I mutation did not affect the receptor synthesis and function at all. As for post-receptor occupancy, neither Y178A nor Y178I prevented alphaIIbbeta3 translocation to focal adhesion contacts. These results suggest that Y178 is involved in alphaIIb maturation and alphaIIbbeta3 complex expression. This residue is also critical for alphaIIbbeta3 interaction with its macromolecular ligand or ligand-mimetic mAb, possibly due to its involvement in other ligand-binding sites distinct from the RGD-binding pocket. We also propose that a residue with appropriate side-chain size and hydrophobicity at position 178 is spatially required for formation of the correct tertiary structure of the site.

Pactolus I-domain: functional switching of the Rossmann fold

Murine Pactolus is a neutrophil-specific single chain glycoprotein that plays a role as an apoptosis marker for macrophages. The extracellular region of the protein shows strong sequence similarities to integrin beta-subunits. Critical sequence modifications differentiate its function when compared to the integrin family. We show experimentally that Pactolus I-domain does not bind divalent metal ions, indicating that ligand binding is not mediated through a metal ion-dependent adhesion site (MIDAS). NMR data was used to map secondary structure and the strand pairing within the beta-sheet to confirm an overall Rossmann fold topology. Homology modeling enhanced by the NMR data was used to determine the overall structure, with two key loop insertions/deletions (insertion 2 and SDL) that distinguish the Pactolus I-domain from the integrin alpha I-domain and beta I-domains. NMR peak exchange broadening is observed due to dimerization, correlating to the beta I-domain and beta propeller heterodimerization region within the integrin headpiece. Two unique N-linked glycosylation sites (Asn151 and Asn230) within this region disrupt dimerization and may account for why Pactolus is not found to associate with an alpha-subunit. These changes in quaternary structure, ligand binding loops, glycosylation, and metal sites illustrate how evolution has rapidly and effectively altered key aspects of the integrin beta-subunit to derive a protein of novel function on an existing protein scaffold.

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.

Beta-integrin mediates WSSV infection

White Spot Syndrome Virus (WSSV) is a virulent and widespread dsDNA virus with a wide range of hosts. Although remarkable progress has been made on virus characterization, however, its mechanism of infection is poorly understood. In this study, by analyzing the phage display library of the WSSV genome, a WSSV envelope protein VP187 (wsv209) was found to interact with shrimp integrin. VP187 possesses the RGD motif. The interaction between integrin and VP187 was confirmed with coimmunoprecipitation. These results demonstrate for the first time an interaction between the WSSV envelope protein and a cell surface molecule. Soluble integrin, integrin-specific antibody and an RGD-containing peptide were found to block the WSSV infection in vivo and in vitro. Gene silencing using a sequence-specific dsRNA targeting beta-integrin effectively inhibited the virus infection. These findings suggest that beta-integrin may function as a cellular receptor for WSSV infection.

Mutation of a conserved asparagine in the I-like domain promotes constitutively active integrins alphaLbeta2 and alphaIIbbeta3

The leukocyte beta2 integrins are heterodimeric adhesion receptors required for a functional immune system. Many leukocyte adhesion deficiency-1 (LAD-1) mutations disrupt the expression and function of beta2 integrins. Herein, we further characterized the LAD-1 mutation N329S in the beta2 inserted (I)-like domain. This mutation converted alphaLbeta2 from a resting into a high affinity conformer because alphaLbeta2N329S transfectants adhered avidly to ligand intercellular adhesion molecule (ICAM)-3 in the absence of additional activating agent. An extended open conformation is adopted by alphaLbeta2N329S because of its reactivity with the beta2 activation reporter monoclonal antibodies MEM148 and KIM127. A corresponding mutation in beta3 generated constitutively active alphaIIbbeta3 that adhered to fibrinogen. This Asn is conserved in all human beta subunits, and it resides before the last helix of the I-like domain, which is known to be important in activation signal propagation. By mutagenesis studies and review of existing integrin structures, we conjectured that this conserved Asn may have a primary role in shaping the I-like domain by stabilizing the conformation of the alpha7 helix and the beta6-alpha7 loop in the I-like domain.

The COOH-Terminal Globular Domain of Fibrinogen γ Chain Suppresses Angiogenesis and Tumor Growth

Fibrinogen is a major plasma protein (350 kDa) that induces proliferative signals by serving as a scaffold to support the binding of growth factors and to promote the cellular responses of adhesion, proliferation, and migration during wound healing, angiogenesis, and tumor growth. Fibrin(ogen) degradation products generated during fibrinolysis are implicated in tissue injury. The fibrinogen gamma chain has a COOH-terminal globular domain (gamma C, residues 151-411 of the gamma chain, 30 kDa) to which several integrin cell adhesion receptors (e.g., platelet alpha(IIb)beta(3), endothelial alpha(v)beta(3), and leukocyte alpha(M)beta(2)) bind. Integrins play a critical role in signal transduction from fibrin(ogen). We found that gamma C and its truncation mutant (designated gamma C399tr), with a deletion of the COOH-terminal 12 residues, induced apoptosis of endothelial cells and blocked tube formation of endothelial cells. DLD-1 human colon cancer cells that secrete gamma C or gamma C399tr grew at similar levels in vitro but grew much slower in vivo than mock-transfected cells. The recombinant purified gamma C399tr fragment markedly suppressed tumor growth, development of intratumoral vasculature, and tumor metastasis in vivo in the highly metastatic Met-1 breast cancer model. The determinant responsible for binding to endothelial cells is cryptic in native fibrinogen but is exposed in gamma C and gamma C399tr. These results suggest that fibrinogen has a novel cryptic determinant, which can exert apoptosis-inducing activity on endothelial cells when exposed, and polypeptides containing this determinant have therapeutic potential.

Identification of multiple integrin beta1 homologs in zebrafish (Danio rerio)

BACKGROUND

Integrins comprise a large family of alpha,beta heterodimeric, transmembrane cell adhesion receptors that mediate diverse essential biological functions. Higher vertebrates possess a single beta1 gene, and the beta1 subunit associates with a large number of alpha subunits to form the major class of extracellular matrix (ECM) receptors. Despite the fact that the zebrafish (Danio rerio) is a rapidly emerging model organism of choice for developmental biology and for models of human disease, little is currently known about beta1 integrin sequences and functions in this organism.

RESULTS

Using RT-PCR, complete coding sequences of zebrafish beta1 paralogs were obtained from zebrafish embryos or adult tissues. The results show that zebrafish possess two beta1 paralogs (beta1-1 and beta1-2) that have a high degree of identity to other vertebrate beta1 subunits. In addition, a third, more divergent, beta1 paralog is present (beta1-3), which may have altered ligand-binding properties. Zebrafish also have other divergent beta1-like transcripts, which are C-terminally truncated forms lacking the transmembrane and cytoplasmic domains. Together with beta1-3 these truncated forms comprise a novel group of beta1 paralogs, all of which have a mutation in the ADMIDAS cation-binding site. Phylogenetic and genomic analyses indicate that the duplication that gave rise to beta1-1 and beta1-2 occurred after the divergence of the tetrapod and fish lineages, while a subsequent duplication of the ancestor of beta1-2 may have given rise to beta1-3 and an ancestral truncated paralog. A very recent tandem duplication of the truncated beta1 paralogs appears to have taken place. The different zebrafish beta1 paralogs have varied patterns of temporal expression during development. Beta1-1 and beta1-2 are ubiquitously expressed in adult tissues, whereas the other beta1 paralogs generally show more restricted patterns of expression.

CONCLUSION

Zebrafish have a large set of integrin beta1 paralogs. beta1-1 and beta1-2 may share the roles of the solitary beta1 subunit found in other vertebrates, whereas beta1-3 and the truncated beta1 paralogs may have acquired novel functions.

Ligand-binding specificities of laminin-binding integrins: a comprehensive survey of laminin-integrin interactions using recombinant alpha3beta1, alpha6beta1, alpha7beta1 and alpha6beta4 integrins

The interactions of cells with basement membranes are primarily mediated via the engagement of laminins by a group of integrin family proteins, including integrins alpha3beta1, alpha6beta1, alpha7beta1 and alpha6beta4. To explore the ligand-binding specificities of these laminin-binding integrins, we produced these integrins, including two alpha7beta1 splice variants (alpha7X1beta1 and alpha7X2beta1), as soluble recombinant proteins and determined their binding specificities and affinities toward a panel of purified laminin isoforms containing distinct alpha chains. Among the five laminin-binding integrins investigated, alpha3beta1 and alpha6beta4 exhibited a clear specificity for laminin-332 (alpha3beta3gamma2) and laminin-511 (alpha5beta1gamma1)/521 (alpha5beta2gamma1), while integrin alpha6beta1 showed a broad specificity, binding to all laminin isoforms with a preference for laminin-111 (alpha1beta1gamma1), laminin-332 and laminin-511/521. The two alpha7beta1 variants were distinct from alpha3beta1, alpha6beta1 and alpha6beta4 in that they did not bind to laminin-332. alpha7X1beta1 bound to all laminins, except laminin-332, with a preference for laminin-211 (alpha2beta1gamma1)/221 (alpha2beta2gamma1) and laminin-511/521, while alpha7X2beta1 bound preferentially to laminin-111 and laminin-211/221. Laminin-511/521 was the most preferred ligand for all the laminin-binding integrins, except for alpha7X2beta1, whereas laminin-411 was the poorest ligand, capable of binding to alpha6beta1 and alpha7X1beta1 with only modest binding affinities. These comprehensive analyses of the interactions between laminin-binding integrins and a panel of laminins clearly demonstrate that the isoforms of both integrins and laminins differ in their binding specificities and affinities, and provide a molecular basis for better understanding of the adhesive interactions of cells with basement membranes of defined laminin compositions.

Integrin structure, allostery, and bidirectional signaling

Alphabeta heterodimeric integrins mediate dynamic adhesive cell-cell and cell-extracellular matrix (ECM) interactions in metazoa that are critical in growth and development, hemostasis, and host defense. A central feature of these receptors is their capacity to change rapidly and reversibly their adhesive functions by modulating their ligand-binding affinity. This is normally achieved through interactions of the short cytoplasmic integrin tails with intracellular proteins, which trigger restructuring of the ligand-binding site through long-range conformational changes in the ectodomain. Ligand binding in turn elicits conformational changes that are transmitted back to the cell to regulate diverse responses. The publication of the integrin alphaVbeta3 crystal structure has provided the context for interpreting decades-old biochemical studies. Newer NMR, crystallographic, and EM data, reviewed here, are providing a better picture of the dynamic integrin structure and the allosteric changes that guide its diverse functions.

Distinct roles for the alpha and beta subunits in the functions of integrin alphaMbeta2

Integrin alphaMbeta2 (Mac-1, CD11b/CD18) is a noncovalently linked heterodimer of alphaM and beta2 subunits on the surface of leukocytes, where it plays a pivotal role in the adhesion and migration of these cells. Using HEK293 cells expressing alphaMbeta2 or the individual constituent chains on their surface, we analyzed the contributions of the alphaM or beta2 subunits to functional responses mediated by the integrin. In cells expressing only alphaM or beta2, the individual subunits were not associated with the endogenous integrins of the cells, and other partners for the subunits were not detected by surface labeling and immunoprecipitation under a variety of conditions. The alphaM cells mediated adhesion and spreading on a series of alphaMbeta2 ligands (fibrinogen, Factor X, iC3b, ICAM-1 (intercellular adhesion molecule-1), and denatured ovalbumin) but could not support cell migration to any of these. The spreading of the alphaM cells suggested an unanticipated linkage of this subunit to the cytoskeleton. The beta2 cells supported migration and attachment but not spreading on a subset of the alphaMbeta2 ligands. The heterodimeric receptor and its individual subunits were purified from the cells by affinity chromatography and recapitulated the ligand binding properties of the corresponding cell lines. These data indicate that each subunit of alphaMbeta2 contributes distinct properties to alphaMbeta2 and that, in most but not all cases, the response of the integrin is a composite of the functions of its individual subunits.

Human integrin alphavbeta5: homology modeling and ligand binding

The recently reported crystal structures of the extracellular domains of the alphavbeta3 integrin in its unligated state and in complex with the peptide cyclo(-RGDf[NMe]V-) have dramatically increased our understanding of ligand binding to integrins. Nonetheless, ligand selectivity toward different integrin subtypes is still a challenging problem complicated by the fact that 3D structures of most of the integrin subtypes remain unknown. In this study, a three-dimensional model for the human alphavbeta5 integrin was obtained using homology modeling based on the crystal coordinates of alphavbeta3 in its bound conformation as template. The modeled receptor was refined using energy minimization and molecular dynamics simulations in explicit solvent. The refined alphavbeta5 model was used to explore the interactions between this integrin and alphavbeta3/alphavbeta5 dual and alphavbeta3-selective ligands in the attempt to provide a preliminary rationalization, at the molecular level, of ligand selectivity toward the two alphav integrins. It was found that, in the RGD binding site of the alphavbeta5 receptor, a partial "roof" composed mainly of the SDL residues Tyr179 and Lys180 is present and hampers the binding of compounds containing bulky substituents in the proximity of the carboxylate group. This study provides a testable hypothesis for alphav integrins subtype ligand binding selectivity, in line with both mutagenesis data and SARs studies.

Integrin beta3 regions controlling binding of murine mAb 7E3: implications for the mechanism of integrin alphaIIbbeta3 activation

Abciximab, a derivative of the murine mAb 7E3, protects against ischemic complications of percutaneous coronary interventions by inhibiting ligand binding to the alphaIIbbeta3 receptor. In this study we identified regions on integrin beta3 that control 7E3 binding. Murine/human amino acid substitutions were created in two regions of the betaA domain that previous studies found to influence 7E3 binding: the C177-C184 loop and K125-N133. The T182N substitution and a K125Q mutation reduced 7E3 binding to human beta3 in complex with alphaIIb. The introduction of both the human C177-C184 region and human W129 into murine beta3 was necessary and sufficient to permit 7E3 binding to the human alphaIIb/murine beta3 complex. Although we cannot exclude allosteric effects, we propose that 7E3 binds between C177-C184 and W129, which are within 15 A of each other in the crystal structure and close to the beta3 metal ion-dependent adhesion site. We previously demonstrated that 7E3 binds more rapidly to activated than unactivated platelets. Because it has been proposed that alphaIIbbeta3 changes from a bent to an extended conformation upon activation, we hypothesized that 7E3 binds less well to the bent than the extended conformation. In support of this hypothesis we found that 7E3 bound less well to an alphaIIbbeta3 construct locked in a bent conformation, and unlocking the conformation restored 7E3 binding. Thus, our data are consistent with alphaIIbbeta3 existing in variably bent conformations in equilibrium with each other on unactivated platelets, and activation resulting in alphaIIbbeta3 adopting a more extended conformation.

Analysis of the Drosophila betaPS subunit indicates that regulation of integrin activity is a primal function of the C8-C9 loop

Integrin-ligand interactions can be influenced by the sequence in a disulfide-bridged loop between the 8th and 9th beta subunit cysteines. Previous experiments are consistent with C8-C9 loop residues being involved in direct ligand-integrin interactions and/or being important in heterodimer regulation. In betaPS from Drosophila melanogaster and three other dipterans, the C8-C9 loop consists of only two amino acids, and exists in two forms that arise by differential splicing of exon 4. In these species, the betaPS4A isoform has an acidic residue in the first loop position (C8+1), with an alanine or proline in the corresponding position of betaPS4B. Mutations in both isoforms (in combination with alphaPS2) can reduce cell spreading during normal growth, but function is generally restored under conditions that enhance integrin activation. Replacement of the betaPS4A acidic residue with a basic lysine has relatively modest effects on integrin function. Spread cells bearing C8-C9 mutations tend to become less elongated, with reduced frequencies of actin stress fibers. The results indicate that even a minimal, two-residue C8-C9 loop contains structural information that can differentially regulate integrin activity and/or integrin signaling, and that this regulation does not rely on direct molecular interactions involving the variable C8+1 side chains.

Crucial role of the specificity-determining loop of the integrin beta4 subunit in the binding of cells to laminin-5 and outside-in signal transduction

Within each hemidesmosome, alpha6beta4 integrin plays a crucial role in hemidesmosome assembly by binding to laminin-5 in the basement membrane zone of epithelial tissue. Recent analyses have implicated "specificity-determining loops" (SDLs) in the I-like domain of beta integrin in regulating ligand binding. Here, we investigated the function of an SDL-like motif within the extracellular I-like domain of beta4 integrin. We generated point mutations within the SDL of beta4 integrin tagged with green fluorescent protein (GFP-beta4K150A and GFP-beta4Q155L). We also generated a mutation within the I-like domain of the beta4 integrin, lying outside the SDL region (GFP-beta4V284E). We transfected constructs encoding the mutated beta4 integrins and a GFP-conjugated wild type beta4 integrin (GFP-beta4WT) into 804G cells, which assemble hemidesmosomes, and human endothelial cells, which express little endogenous beta4 integrin. In transfected 804G cells, GFP-beta4WT and GFP-beta4V284E colocalize with hemidesmosome proteins, whereas hemidesmosomal components in cells expressing GFP-beta4K150A and GFP-beta4Q155L are aberrantly localized. In endothelial cells, GFP-beta4WT and mutant proteins are co-expressed at the cell surface with alpha6 integrin. When transfected endothelial cells are plated onto laminin-5 matrix, GFP-beta4WT and GFP-beta4V284E localize with laminin-5, whereas GFP-beta4K150A and GFP-beta4Q155L do not. GFP-beta4WT and GFP-beta4V284E expressed in endothelial cells associate with the adaptor protein Shc when the cells are stimulated with laminin-5. However, GFP-beta4K150A and GFP-beta4Q155L fail to associate with Shc even when laminin-5 is present, thus impacting downstream signaling. These results provide evidence that the SDL segment of the beta4 integrin subunit is required for ligand binding and is involved in outside-in signaling.

The interacting binding domains of the beta(4) integrin and calcium-activated chloride channels (CLCAs) in metastasis

CLCA (chloride channel, calcium-activated) proteins are novel pulmonary vascular addresses for blood-borne, lung-metastatic cancer cells. They facilitate vascular arrest of cancer cells via adhesion to beta4 integrin and promote early, intravascular, metastatic growth. Here we identify the interacting binding domains of endothelial CLCA proteins (e.g. hCLCA2, mCLCA5, mCLCA1, and bCLCA2) and beta4 integrin. Endothelial CLCAs share a common beta4-binding motif (beta4BM) in their 90- and 35-kDa subunits of the sequence F(S/N)R(I/L/V)(S/T)S, which is located in the second extracellular domain of the 90-kDa CLCA and near the N terminus of the 35-kDa CLCA, respectively. Using enzyme-linked immunosorbent, pull-down, and adhesion assays, we showed that glutathione S-transferase fusion proteins of beta4BMs from the 90- and 35-kDa CLCA subunits bind to the beta4 integrin in a metal ion-dependent manner. Fusion proteins from fibronectin and the integrins beta1 and beta3 served as negative controls. beta4BM fusion proteins competitively blocked the beta4/CLCA adhesion and prevented lung colonization of MDA-MB-231 breast cancer cells. A disrupted beta4BM in hCLCA1, which is not expressed in endothelia, failed to interact with beta4 integrin. The corresponding CLCA-binding domain of the beta4 integrin is localized to the specific determining loop (SDL). Again enzyme-linked immunosorbent, pull-down, and adhesion assays were used to confirm the interaction with CLCA proteins using a glutathione S-transferase fusion protein representing the C-terminal two-thirds of beta4 SDL (amino acids 184-203). A chimeric beta4 integrin in which the indicated SDL sequence had been replaced with the corresponding sequence from the beta1 integrin failed to bind hCLCA2. The dominance of the CLCA ligand in beta4 activation and outside-in signaling is discussed in reference to our previous report that beta4/CLCA ligation elicits selective signaling via focal adhesion kinase to promote metastatic growth.