Membrane-proximal α/β Stalk Interactions Differentially Regulate Integrin Activation

Nascent adhesions shorten the period of lamellipodium protrusion through the Brownian ratchet mechanism

Directional cell migration is driven by the conversion of oscillating edge motion into lasting periods of leading edge protrusion. Actin polymerization against the membrane and adhesions control edge motion, but the exact mechanisms that determine protrusion period remain elusive. We addressed this by developing a computational model in which polymerization of actin filaments against a deformable membrane and variable adhesion dynamics support edge motion. Consistent with previous reports, our model showed that actin polymerization and adhesion lifetime power protrusion velocity. However, increasing adhesion lifetime decreased the protrusion period. Measurements of adhesion lifetime and edge motion in migrating cells confirmed that adhesion lifetime is associated with and promotes protrusion velocity, but decreased duration. Our model showed that adhesions' control of protrusion persistence originates from the Brownian ratchet mechanism for actin filament polymerization. With longer adhesion lifetime or increased-adhesion density, the proportion of actin filaments tethered to the substrate increased, maintaining filaments against the cell membrane. The reduced filament-membrane distance generated pushing force for high edge velocity, but limited further polymerization needed for protrusion duration. We propose a mechanism for cell edge protrusion in which adhesion strength regulates actin filament polymerization to control the periods of leading edge protrusion.

Integrin αIIbβ3 intermediates: From molecular dynamics to adhesion assembly

The platelet integrin αIIbβ3 undergoes long-range conformational transitions associated with its functional conversion from inactive (low-affinity) to active (high-affinity) during hemostasis. Although new conformations that are intermediate between the well-characterized bent and extended states have been identified, their molecular dynamic properties and functions in the assembly of adhesions remain largely unexplored. In this study, we evaluated the properties of intermediate conformations of integrin αIIbβ3 and characterized their effects on the assembly of adhesions by combining all-atom simulations, principal component analysis, and mesoscale modeling. Our results show that in the low-affinity, bent conformation, the integrin ectodomain tends to pivot around the legs; in intermediate conformations, the headpiece becomes partially extended, away from the lower legs. In the fully open, active state, αIIbβ3 is flexible, and the motions between headpiece and lower legs are accompanied by fluctuations of the transmembrane helices. At the mesoscale, bent integrins form only unstable adhesions, but intermediate or open conformations stabilize the adhesions. These studies reveal a mechanism by which small variations in ligand binding affinity and enhancement of the ligand-bound lifetime in the presence of actin retrograde flow stabilize αIIbβ3 integrin adhesions.

Electron microscopy shows that binding of monoclonal antibody PT25-2 primes integrin αIIbβ3 for ligand binding

The murine monoclonal antibody (mAb) PT25-2 induces αIIbβ3 to bind ligand and initiate platelet aggregation. The underlying mechanism is unclear, because previous mutagenesis studies suggested that PT25-2 binds to the αIIb β propeller, a site distant from the Arg-Gly-Asp-binding pocket. To elucidate the mechanism, we studied the αIIbβ3-PT25-2 Fab complex by negative-stain and cryo-electron microscopy (EM). We found that PT25-2 binding results in αIIbβ3 partially exposing multiple ligand-induced binding site epitopes and adopting extended conformations without swing-out of the β3 hybrid domain. The cryo-EM structure showed PT25-2 binding to the αIIb residues identified by mutagenesis but also to 2 additional regions. Overlay of the cryo-EM structure with the bent αIIbβ3 crystal structure showed that binding of PT25-2 creates clashes with the αIIb calf-1/calf-2 domains, suggesting that PT25-2 selectively binds to partially or fully extended receptor conformations and prevents a return to its bent conformation. Kinetic studies of the binding of PT25-2 compared with mAbs 10E5 and 7E3 support this hypothesis. We conclude that PT25-2 induces αIIbβ3 ligand binding by binding to extended conformations and by preventing the interactions between the αIIb and β3 leg domains and subsequently the βI and β3 leg domains required for the bent-closed conformation.

Insight Into Pathological Integrin αIIbβ3 Activation From Safeguarding The Inactive State

The inhibition of physiological activation pathways of the platelet adhesion receptor integrin αIIbβ3 may fail to prevent fatal thrombosis, suggesting that the receptor is at risk of activation by yet an unidentified pathway. Here, we report the discovery and characterization of a structural motif that safeguards the receptor by selectively destabilizing its inactive state. At the extracellular membrane border, an overpacked αIIb(W968)-β3(I693) contact prevents αIIb(Gly972) from optimally assembling the αIIbβ3 transmembrane complex, which maintains the inactive state. This destabilization of approximately 1.0 kcal/mol could be mitigated by hydrodynamic forces but not physiological agonists, thereby identifying hydrodynamic forces as pathological activation stimulus. As reproductive life spans are not generally limited by cardiovascular disease, it appears that the evolution of the safeguard was driven by fatal, hydrodynamic force-mediated integrin αIIbβ3 activation in the healthy cardiovascular system. The triggering of the safeguard solely by pathological stimuli achieves an effective increase of the free energy barrier between inactive and active receptor states without incurring an increased risk of bleeding. Thus, integrin αIIbβ3 has evolved an effective way to protect receptor functional states that indicates the availability of a mechanical activation pathway when hydrodynamic forces exceed physiological margins.

Modulating Integrin αIIbβ3 Activity through Mutagenesis of Allosterically Regulated Intersubunit Contacts

Integrin αIIbβ3, a transmembrane heterodimer, mediates platelet aggregation when it switches from an inactive to an active ligand-binding conformation following platelet stimulation. Central to regulating αIIbβ3 activity is the interaction between the αIIb and β3 extracellular stalks, which form a tight heterodimer in the inactive state and dissociate in the active state. Here, we demonstrate that alanine replacements of sensitive positions in the heterodimer stalk interface destabilize the inactive conformation sufficiently to cause constitutive αIIbβ3 activation. To determine the structural basis for this effect, we performed a structural bioinformatics analysis and found that perturbing intersubunit contacts with favorable interaction geometry through substitutions to alanine quantitatively accounted for the degree of constitutive αIIbβ3 activation. This mutational study directly assesses the relationship between favorable interaction geometry at mutation-sensitive positions and the functional activity of those mutants, giving rise to a simple model that highlights the importance of interaction geometry in contributing to the stability between protein-protein interactions.

Drug-induced activation of integrin alpha IIb beta 3 leads to minor localized structural changes

Integrins are transmembrane proteins involved in hemostasis, wound healing, immunity and cancer. In response to intracellular signals and ligand binding, integrins adopt different conformations: the bent (resting) form; the intermediate extended form; and the ligand-occupied active form. An integrin undergoing such conformational dynamics is the heterodimeric platelet receptor αIIbβ3. Although the dramatic rearrangement of the overall structure of αIIbβ3 during the activation process is potentially related to changes in the protein secondary structure, this has not been investigated so far in a membrane environment. Here we examine the Mn2+- and drug-induced activation of αIIbβ3 and the impact on the structure of this protein reconstituted into liposomes. By quartz crystal microbalance with dissipation monitoring and activation assays we show that Mn2+ induces binding of the conformation-specific antibody PAC-1, which only recognizes the extended, active integrin. Circular dichroism spectroscopy reveals, however, that Mn2+-treatment does not induce major secondary structural changes of αIIbβ3. Similarly, we found that treatment with clinically relevant drugs (e.g. quinine) led to the activation of αIIbβ3 without significant changes in protein secondary structure. Molecular dynamics simulation studies revealed minor local changes in the beta-sheet probability of several extracellular domains of the integrin. Our experimental setup represents a new approach to study transmembrane proteins, especially integrins, in a membrane environment and opens a new way for testing drug binding to integrins under clinically relevant conditions.

Integrin α4β7 switches its ligand specificity via distinct conformer-specific activation

CCL25, CXCL10, and Mn²⁺ induce three distinct active conformations of integrin α4β7, which have selective high affinity for either MAdCAM-1, VCAM-1, or nonselective high affinity for both ligands. Via this mechanism, integrin α4β7 adopts different active conformations to switch its ligand-binding specificity.

Chemokine (C-C motif) ligand 25 (CCL25) and C-X-C motif chemokine 10 (CXCL10) induce the ligand-specific activation of integrin α4β7 to mediate the selective adhesion of lymphocytes to mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1) or vascular cell adhesion molecule-1 (VCAM-1). However, the mechanism underlying the selective binding of different ligands by α4β7 remains obscure. In this study, we demonstrate that CCL25 and CXCL10 induce distinct active conformers of α4β7 with a high affinity for either MAdCAM-1 or VCAM-1. Single-cell force measurements show that CCL25 increases the affinity of α4β7 for MAdCAM-1 but decreases its affinity for VCAM-1, whereas CXCL10 has the opposite effect. Structurally, CCL25 induces a more extended active conformation of α4β7 compared with CXCL10-activated integrin. These two distinct intermediate open α4β7 conformers selectively bind to MAdCAM-1 or VCAM-1 by distinguishing their immunoglobulin domain 2. Notably, Mn²⁺ fully opens α4β7 with a high affinity for both ligands. Thus, integrin α4β7 adopts different active conformations to switch its ligand-binding specificity.

On the contributing role of the transmembrane domain for subunit-specific sensitivity of integrin activation

Integrins are α/β heterodimeric transmembrane adhesion receptors. Evidence exists that their transmembrane domain (TMD) separates upon activation. Subunit-specific differences in activation sensitivity of integrins were reported. However, whether sequence variations in the TMD lead to differential TMD association has remained elusive. Here, we show by molecular dynamics simulations and association free energy calculations on TMDs of integrin αIIbβ3, αvβ3, and α5β1 that αIIbβ3 TMD is most stably associated; this difference is related to interaction differences across the TMDs. The order of TMD association stability is paralleled by the basal activity of these integrins, which suggests that TMD differences can have a decisive effect on integrin conformational free energies. We also identified a specific order of clasp disintegration upon TMD dissociation, which suggests that the closed state of integrins may comprise several microstates. Our results provide unprecedented insights into a possibly contributing role of TMD towards subunit-specific sensitivity of integrin activation.

Low-affinity binding in cis to P2Y2R mediates force-dependent integrin activation during hantavirus infection

Atomic force microscopy is used to establish that low-affinity integrins bind in cis to P2Y₂R. Integrin activation is initiated by a membrane-normal switchblade motion triggered by integrin priming after the virus binds to the integrin PSI domain. Tensile force between the P2Y₂R and unbending integrin stimulates outside-in signaling.

Pathogenic hantaviruses bind to the plexin-semaphorin-integrin (PSI) domain of inactive, β₃ integrins. Previous studies have implicated a cognate cis interaction between the bent conformation β₅/β₃ integrins and an arginine-glycine-aspartic acid (RGD) sequence in the first extracellular loop of P2Y₂R. With single-molecule atomic force microscopy, we show a specific interaction between an atomic force microscopy tip decorated with recombinant α_IIbβ₃ integrins and (RGD)P2Y₂R expressed on cell membranes. Mutation of the RGD sequence to RGE in the P2Y₂R removes this interaction. Binding of inactivated and fluorescently labeled Sin Nombre virus (SNV) to the integrin PSI domain stimulates higher affinity for (RGD)P2Y₂R on cells, as measured by an increase in the unbinding force. In CHO cells, stably expressing α_IIbβ₃ integrins, virus engagement at the integrin PSI domain, recapitulates physiologic activation of the integrin as indicated by staining with the activation-specific mAB PAC1. The data also show that blocking of the Gα₁₃ protein from binding to the cytoplasmic domain of the β₃ integrin prevents outside-in signaling and infection. We propose that the cis interaction with P2Y₂R provides allosteric resistance to the membrane-normal motion associated with the switchblade model of integrin activation, where the development of tensile force yields physiological integrin activation.

The importance of N-glycosylation on β3 integrin ligand binding and conformational regulation

N-glycosylations can regulate the adhesive function of integrins. Great variations in both the number and distribution of N-glycosylation sites are found in the 18 α and 8 β integrin subunits. Crystal structures of α_IIbβ₃ and α_Vβ₃ have resolved the precise structural location of each N-glycan site, but the structural consequences of individual N-glycan site on integrin activation remain unclear. By site-directed mutagenesis and structure-guided analyses, we dissected the function of individual N-glycan sites in β₃ integrin activation. We found that the N-glycan site, β₃-N320 at the headpiece and leg domain interface positively regulates α_IIbβ₃ but not α_Vβ₃ activation. The β₃-N559 N-glycan at the β₃-I-EGF3 and α_IIb-calf-1 domain interface, and the β₃-N654 N-glycan at the β₃-β-tail and α_IIb-calf-2 domain interface positively regulate the activation of both α_IIbβ₃ and α_Vβ₃ integrins. In contrast, removal of the β₃-N371 N-glycan near the β₃ hybrid and I-EGF3 interface, or the β₃-N452 N-glycan at the I-EGF1 domain rendered β₃ integrin more active than the wild type. We identified one unique N-glycan at the βI domain of β₁ subunit that negatively regulates α₅β₁ activation. Our study suggests that the bulky N-glycans influence the large-scale conformational rearrangement by potentially stabilizing or destabilizing the domain interfaces of integrin.

A Conserved Ectodomain-Transmembrane Domain Linker Motif Tunes the Allosteric Regulation of Cell Surface Receptors

In many families of cell surface receptors, a single transmembrane (TM) α-helix separates ecto- and cytosolic domains. A defined coupling of ecto- and TM domains must be essential to allosteric receptor regulation but remains little understood. Here, we characterize the linker structure, dynamics, and resulting ecto-TM domain coupling of integrin αIIb in model constructs and relate it to other integrin α subunits by mutagenesis. Cellular integrin activation assays subsequently validate the findings in intact receptors. Our results indicate a flexible yet carefully tuned ecto-TM coupling that modulates the signaling threshold of integrin receptors. Interestingly, a proline at the N-terminal TM helix border, termed NBP, is critical to linker flexibility in integrins. NBP is further predicted in 21% of human single-pass TM proteins and validated in cytokine receptors by the TM domain structure of the cytokine receptor common subunit β and its P441A-substituted variant. Thus, NBP is a conserved uncoupling motif of the ecto-TM domain transition and the degree of ecto-TM domain coupling represents an important parameter in the allosteric regulation of diverse cell surface receptors.

The integrin αL leg region controls the Mg/EGTA mediated activation of LFA-1

We have shown that Mg/EGTA (5 mM Mg(2+) and 1.5 mM EGTA) could effectively promote the adhesion of integrin αLβ2 to its ligand ICAM-1 but could not promote that of the αMβ2 to denatured BSA. In order to determine the structural differences between αL and αM that specifically contribute to Mg/EGTA sensitivity, a series of αL/αM chimeras were constructed. Our results showed that αLβ2 with αM calf-1 domain completely lost the response to Mg/EGTA activation. In the reverse experiment, αMβ2 would require the presence of both the αL calf-1 and calf-2 domain to initiate the Mg/EGTA sensitivity.

The α-subunit regulates stability of the metal ion at the ligand-associated metal ion-binding site in β3 integrins

The aspartate in the prototypical integrin-binding motif Arg-Gly-Asp binds the integrin βA domain of the β-subunit through a divalent cation at the metal ion-dependent adhesion site (MIDAS). An auxiliary metal ion at a ligand-associated metal ion-binding site (LIMBS) stabilizes the metal ion at MIDAS. LIMBS contacts distinct residues in the α-subunits of the two β3 integrins αIIbβ3 and αVβ3, but a potential role of this interaction on stability of the metal ion at LIMBS in β3 integrins has not been explored. Equilibrium molecular dynamics simulations of fully hydrated β3 integrin ectodomains revealed strikingly different conformations of LIMBS in unliganded αIIbβ3 versus αVβ3, the result of stronger interactions of LIMBS with αV, which reduce stability of the LIMBS metal ion in αVβ3. Replacing the αIIb-LIMBS interface residue Phe(191) in αIIb (equivalent to Trp(179) in αV) with Trp strengthened this interface and destabilized the metal ion at LIMBS in αIIbβ3; a Trp(179) to Phe mutation in αV produced the opposite but weaker effect. Consistently, an F191/W substitution in cellular αIIbβ3 and a W179/F substitution in αVβ3 reduced and increased, respectively, the apparent affinity of Mn(2+) to the integrin. These findings offer an explanation for the variable occupancy of the metal ion at LIMBS in αVβ3 structures in the absence of ligand and provide new insights into the mechanisms of integrin regulation.

Redox-relevant aspects of the extracellular matrix and its cellular contacts via integrins

SIGNIFICANCE

The extracellular matrix (ECM) fulfills essential functions in multicellular organisms. It provides the mechanical scaffold and environmental cues to cells. Upon cell attachment, the ECM signals into the cells. In this process, reactive oxygen species (ROS) are physiologically used as signalizing molecules.

RECENT ADVANCES

ECM attachment influences the ROS-production of cells. In turn, ROS affect the production, assembly and turnover of the ECM during wound healing and matrix remodeling. Pathological changes of ROS levels lead to excess ECM production and increased tissue contraction in fibrotic disorders and desmoplastic tumors. Integrins are cell adhesion molecules which mediate cell adhesion and force transmission between cells and the ECM. They have been identified as a target of redox-regulation by ROS. Cysteine-based redox-modifications, together with structural data, highlighted particular regions within integrin heterodimers that may be subject to redox-dependent conformational changes along with an alteration of integrin binding activity.

CRITICAL ISSUES

In a molecular model, a long-range disulfide-bridge within the integrin β-subunit and disulfide bridges within the genu and calf-2 domains of the integrin α-subunit may control the transition between the bent/inactive and upright/active conformation of the integrin ectodomain. These thiol-based intramolecular cross-linkages occur in the stalk domain of both integrin subunits, whereas the ligand-binding integrin headpiece is apparently unaffected by redox-regulation.

FUTURE DIRECTIONS

Redox-regulation of the integrin activation state may explain the effect of ROS in physiological processes. A deeper understanding of the underlying mechanism may open new prospects for the treatment of fibrotic disorders.

Ligand capture and activation of human platelets at monolayer modified gold surfaces

The effect of RGD peptides, alkane and PEG in self assembled mixed monolayers on gold on platelet adhesion and activation is explored.

Epitope mapping for monoclonal antibody reveals the activation mechanism for αVβ3 integrin

Epitopes for a panel of anti-αVβ3 monoclonal antibodies (mAbs) were investigated to explore the activation mechanism of αVβ3 integrin. Experiments utilizing αV/αIIb domain-swapping chimeras revealed that among the nine mAbs tested, five recognized the ligand-binding β-propeller domain and four recognized the thigh domain, which is the upper leg of the αV chain. Interestingly, the four mAbs included function-blocking as well as non-functional mAbs, although they bound at a distance from the ligand-binding site. The epitopes for these four mAbs were further determined using human-to-mouse αV chimeras. Among the four, P3G8 recognized an amino acid residue, Ser-528, located on the side of the thigh domain, while AMF-7, M9, and P2W7 all recognized a common epitope, Ser-462, that was located close to the α-genu, where integrin makes a sharp bend in the crystal structure. Fibrinogen binding studies for cells expressing wild-type αVβ3 confirmed that AMF-7, M9, and P2W7 were inhibitory, while P3G8 was non-functional. However, these mAbs were all unable to block binding when αVβ3 was constrained in its extended conformation. These results suggest that AMF-7, M9, and P2W7 block ligand binding allosterically by stabilizing the angle of the bend in the bent conformation. Thus, a switchblade-like movement of the integrin leg is indispensable for the affinity regulation of αVβ3 integrin.

α(V)β(3) integrin crystal structures and their functional implications

Many questions about the significance of structural features of integrin α(V)β(3) with respect to its mechanism of activation remain. We have determined and re-refined crystal structures of the α(V)β(3) ectodomain linked to C-terminal coiled coils (α(V)β(3)-AB) and four transmembrane (TM) residues in each subunit (α(V)β(3)-1TM), respectively. The α(V) and β(3) subunits with four and eight extracellular domains, respectively, are bent at knees between the integrin headpiece and lower legs, and the headpiece has the closed, low-affinity conformation. The structures differ in the occupancy of three metal-binding sites in the βI domain. Occupancy appears to be related to the pH of crystallization, rather than to the physiologic regulation of ligand binding at the central, metal ion-dependent adhesion site. No electron density was observed for TM residues and much of the α(V) linker. α(V)β(3)-AB and α(V)β(3)-1TM demonstrate flexibility in the linker between their extracellular and TM domains, rather than the previously proposed rigid linkage. A previously postulated interface between the α(V) and β(3) subunits at their knees was also not supported, because it lacks high-quality density, required rebuilding in α(V)β(3)-1TM, and differed markedly between α(V)β(3)-1TM and α(V)β(3)-AB. Together with the variation in domain-domain orientation within their bent ectodomains between α(V)β(3)-AB and α(V)β(3)-1TM, these findings are compatible with the requirement for large structural changes, such as extension at the knees and headpiece opening, in conveying activation signals between the extracellular ligand-binding site and the cytoplasm.

Reconstruction of integrin activation

Integrins are integral membrane proteins that mediate cell-matrix and cell-cell adhesion. They are important for vascular development and hematopoiesis, immune and inflammatory responses, and hemostasis. Integrins are also signaling receptors that can transmit information bidirectionally across plasma membranes. Research in the past 2 decades has made progress in unraveling the mechanisms of integrin signaling and brings the field to the moment of attempting synthetic reconstruction of the signaling pathways in vitro. Reconstruction of biologic processes provides stringent tests of our understanding of the process, as evidenced by studies of other biologic machines, such as ATP synthase, lactose permease, and G-protein-coupled receptors. Here, we review recent progress in reconstructing integrin signaling and the insights that we have gained through these experiments.

Identification of immunoreactive regions of homology between soluble epidermal growth factor receptor and α5-integrin

Proteins encoded by the epidermal growth factor receptor (EGFR/HER1/ERBB1) gene are being studied as diagnostic, prognostic, and theragnostic biomarkers for numerous human cancers. The clinical application of these tissue/tumor biomarkers has been limited, in part, by discordant results observed for epidermal growth factor receptor (EGFR) expression using different immunological reagents. Previous studies have used EGFR-directed antibodies that cannot distinguish between full-length and soluble EGFR (sEGFR) expression. We have generated and characterized an anti-sEGFR polyclonal antiserum directed against a 31-mer peptide (residues 604-634) located within the unique 78-amino acid carboxy-terminal sequence of sEGFR. Here, we use this antibody to demonstrate that sEGFR is coexpressed with EGFR in a number of carcinoma-derived cell lines. In addition, we show that a second protein of ~140 kDa (p140) also is detected by this antibody. Rigorous biochemical characterization identifies this second protein to be α5-integrin. We show that a 26-amino acid peptide in the calf domain of α5-integrin (residues 710-735) is 35% identical in sequence with a 31-mer carboxy-terminal sEGFR peptide and exhibits an approximately 5-fold lower affinity for anti-sEGFR than the homologous 31-mer sEGFR peptide does. We conclude that the carboxy terminus of sEGFR and the calf-1 domain of α5-integrin share a region of sequence identity, which results in their mutual immunological reactivity with anti-sEGFR. We also demonstrate that anti-sEGFR promotes three-dimensional tissue cohesion and compaction in vitro, further suggesting a functional link between sEGFR and α5-integrin and a role of the calf-1 domain in cell adhesion. These results have implications for the study of both EGFR and sEGFR as cancer biomarkers and also provide new insight into the mechanisms of interaction between cell surface EGFR isoforms and integrins in complex processes such as cell adhesion and survival signaling.

Structural requirements for activation in alphaIIb beta3 integrin

Integrins are postulated to undergo structural rearrangement from a low affinity bent conformer to a high affinity extended conformer upon activation. However, some reports have shown that a bent conformer is capable of binding a ligand, whereas another report has shown that integrin extension does not absolutely lead to activation. To clarify whether integrin affinity is indeed regulated by the so-called switchblade-like movement, we have engineered a series of mutant αIIbβ3 integrins that are constrained specifically in either a bent or an extended conformation. These mutant αIIbβ3 integrins were expressed in mammalian cells, and fibrinogen binding to these cells was examined. The bent integrins were created through the introduction of artificial disulfide bridges in the β-head/β-tail interface. Cells expressing bent integrins all failed to bind fibrinogen unless pretreated with DTT to disrupt the disulfide bridges. The extended integrins were created by introducing N-glycosylation sites in amino acid residues located close to the α-genu, where the integrin legs fold backward. Among these mutants, activation was maximized in one integrin with an N-glycosylation site located behind the α-genu. This extension-induced activation was completely blocked when the swing-out of the hybrid domain was prevented. These results suggest that the bent and extended conformers represent low affinity and high affinity conformers, respectively, and that extension-induced activation depends on the swing-out of the hybrid domain. Taken together, these results are consistent with the current hypothesis that integrin affinity is regulated by the switchblade-like movement of the integrin legs.

Modulation of integrin activation by an entropic spring in the {beta}-knee

We show that the length of a loop in the β-knee, between the first and second cysteines (C1-C2) in integrin EGF-like (I-EGF) domain 2, modulates integrin activation. Three independent sets of mutants, including swaps among different integrin β-subunits, show that C1-C2 loop lengths of 12 and longer favor the low affinity state and masking of ligand-induced binding site (LIBS) epitopes. Shortening length from 12 to 4 residues progressively increases ligand binding and LIBS epitope exposure. Compared with length, the loop sequence had a smaller effect, which was ascribable to stabilizing loop conformation, and not interactions with the α-subunit. The data together with structural calculations support the concept that the C1-C2 loop is an entropic spring and an emerging theme that disordered regions can regulate allostery. Diversity in the length of this loop may have evolved among integrin β-subunits to adjust the equilibrium between the bent and extended conformations at different set points.

Structural basis of transmembrane domain interactions in integrin signaling

Cell surface receptors of the integrin family are pivotal to cell adhesion and migration. The activation state of heterodimeric alphabeta integrins is correlated to the association state of the single-pass alpha and beta transmembrane domains. The association of integrin alphaIIbbeta3 transmembrane domains, resulting in an inactive receptor, is characterized by the asymmetric arrangement of a straight (alphaIIb) and tilted (beta3) helix relative to the membrane in congruence to the dissociated structures. This allows for a continuous association interface centered on helix-helix glycine-packing and an unusual alphaIIb(GFF) structural motif that packs the conserved Phe-Phe residues against the beta3 transmembrane helix, enabling alphaIIb(D723)beta3(R995) electrostatic interactions. The transmembrane complex is further stabilized by the inactive ectodomain, thereby coupling its association state to the ectodomain conformation. In combination with recently determined structures of an inactive integrin ectodomain and an activating talin/beta complex that overlap with the alphabeta transmembrane complex, a comprehensive picture of integrin bi-directional transmembrane signaling has emerged.

Effects of limiting extension at the alphaIIb genu on ligand binding to integrin alphaIIbbeta3

Structural data of integrin alphaIIbbeta3 have been interpreted as supporting a model in which: 1) the receptor exists primarily in a "bent," low affinity conformation on unactivated platelets and 2) activation induces an extended, high affinity conformation prior to, or following, ligand binding. Previous studies found that "clasping" the alphaIIb head domain to the beta3 tail decreased fibrinogen binding. To study the role of alphaIIb extension about the genu, we introduced a disulfide "clamp" between the alphaIIb thigh and calf-1 domains. Clamped alphaIIbbeta3 had markedly reduced ability to bind the large soluble ligands fibrinogen and PAC-1 when activated with monoclonal antibody (mAb) PT25-2 but not when activated by Mn(2+) or by coexpressing the clamped alphaIIb with a beta3 subunit containing the activating mutation N339S. The clamp had little effect on the binding of the snake venom kistrin (M(r) 7,500) or alphaIIbbeta3-mediated adhesion to immobilized fibrinogen, but it did diminish the enhanced binding of mAb AP5 in the presence of kistrin. Collectively, our studies support a role for alphaIIb extension about the genu in the binding of ligands of 340,000 and 900,000 M(r) with mAb-induced activation but indicate that it is not an absolute requirement. Our data are consistent with alphaIIb extension resulting in increased access to the ligand-binding site and/or facilitating the conformational change(s) in beta3 that affect the intrinsic affinity of the binding pocket for ligand.

Focal adhesions are sites of integrin extension

FRET experiments model integrin conformation changes in adherent cells.

Integrins undergo global conformational changes that specify their activation state. Current models portray the inactive receptor in a bent conformation that upon activation converts to a fully extended form in which the integrin subunit leg regions are separated to enable ligand binding and subsequent signaling. To test the applicability of this model in adherent cells, we used a fluorescent resonance energy transfer (FRET)–based approach, in combination with engineered integrin mutants and monoclonal antibody reporters, to image integrin α5β1 conformation. We find that restricting leg separation causes the integrin to adopt a bent conformation that is unable to respond to agonists and mediate cell spreading. By measuring FRET between labeled α5β1 and the cell membrane, we find extended receptors are enriched in focal adhesions compared with adjacent regions of the plasma membrane. These results demonstrate definitely that major quaternary rearrangements of β1-integrin subunits occur in adherent cells and that conversion from a bent to extended form takes place at focal adhesions.

Foot-and-mouth disease virus assembly: processing of recombinant capsid precursor by exogenous protease induces self-assembly of pentamers in vitro in a myristoylation-dependent manner

The assembly of foot-and-mouth disease virus (FMDV) particles is poorly understood. In addition, there are important differences in the antigenic and receptor binding properties of virus assembly and dissociation intermediates, and these also remain unexplained. We have established an experimental model in which the antigenicity, receptor binding characteristics, and in vitro assembly of capsid precursor can be studied entirely from purified components. Recombinant capsid precursor protein (P1 region) was expressed in Escherichia coli as myristoylated or unmyristoylated protein. The protein sedimented in sucrose gradients at 5S and reacted with monoclonal antibodies which recognize conformational or linear antigen determinants on the virion surface. In addition, it bound the integrin alpha(v)beta(6), a cellular receptor for FMDV, indicating that unprocessed recombinant capsid precursor is both structurally and antigenically similar to native virus capsid. These characteristics were not dependent on the presence of 2A at the C terminus but were altered by N-terminal myristoylation and in mutant precursors which lacked VP4. Proteolytic processing of myristoylated precursor by recombinant FMDV 3C(pro) in vitro induced a shift in sedimentation from 5S to 12S, indicating assembly into pentameric capsid subunits. Nonmyristoylated precursor still assembled into higher-order structures after processing with 3C(pro), but these particles sedimented in sucrose gradients at approximately 17S. In contrast, mutant precursors lacking VP4 were antigenically distinct, were unable to form pentamers, and had reduced capacity for binding integrin receptor. These studies demonstrate the utility of recombinant capsid precursor protein for investigating the initial stages of assembly of FMDV and other picornaviruses.

Crystal structure of the complete integrin alphaVbeta3 ectodomain plus an alpha/beta transmembrane fragment

We determined the crystal structure of 1TM-alphaVbeta3, which represents the complete unconstrained ectodomain plus short C-terminal transmembrane stretches of the alphaV and beta3 subunits. 1TM-alphaVbeta3 is more compact and less active in solution when compared with DeltaTM-alphaVbeta3, which lacks the short C-terminal stretches. The structure reveals a bent conformation and defines the alpha-beta interface between IE2 (EGF-like 2) and the thigh domains. Modifying this interface by site-directed mutagenesis leads to robust integrin activation. Fluorescent lifetime imaging microscopy of inactive full-length alphaVbeta3 on live cells yields a donor-membrane acceptor distance, which is consistent with the bent conformation and does not change in the activated integrin. These data are the first direct demonstration of conformational coupling of the integrin leg and head domains, identify the IE2-thigh interface as a critical steric barrier in integrin activation, and suggest that inside-out activation in intact cells may involve conformational changes other than the postulated switch to a genu-linear state.

Linking integrin conformation to function

Integrins are alphabeta heterodimeric adhesion receptors that relay signals bidirectionally across the plasma membrane between the extracellular matrix and cell-surface ligands, and cytoskeletal and signalling effectors. The physical and chemical signals that are controlled by integrins are essential for intercellular communication and underpin all aspects of metazoan existence. To mediate such diverse functions, integrins exhibit structural diversity, flexibility and dynamism. Conformational changes, as opposed to surface expression or clustering, are central to the regulation of receptor function. In recent years, there has been intense interest in determining the three-dimensional structure of integrins, and analysing the shape changes that underpin the interconversion between functional states. Considering the central importance of the integrin signalling nexus, it is perhaps no surprise that obtaining this information has been difficult, and the answers gained so far have been complicated. In this Commentary, we pose some of the key remaining questions that surround integrin structure-function relationships and review the evidence that supports the current models.

Key interactions in integrin ectodomain responsible for global conformational change detected by elastic network normal-mode analysis

Integrin, a membrane protein with a huge extracellular domain, participates in cell-cell and cell-extracellular-matrix interactions for metazoan. A group of integrins is known to perform a large-scale structural change when the protein is activated, but the activation mechanism and generality of the conformational change remain to be elucidated. We performed normal-mode analysis of the elastic network model on integrin alpha(V)beta(3) ectodomain in the bent form and identified key residues that influenced molecular motions. Iterative normal-mode calculations demonstrated that the specific nonbonded interactions involving the key residues work as a snap to keep integrin in the bent form. The importance of the key residues for the conformational change was further verified by mutation experiments, in which integrin alpha(IIb)beta(3) was used. The conservation pattern of amino acid residues among the integrin family showed that the characteristic pattern of residues seen around these key residues is found in the limited groups of integrin beta-chains. This conservation pattern suggests that the molecular mechanism of the conformational change relying on the interactions found in integrin alpha(V)beta(3) is unique to the limited types of integrins.

Interaction and functional association of protein disulfide isomerase with alphaVbeta3 integrin on endothelial cells

Adhesive properties of endothelial cells are influenced by the thioldisulfide balance. However, the molecular mechanism of this effect is unclear, although recent observations indicate that integrin receptors may be direct targets for redox modulation. The purpose of this study was to examine whether protein disulfide isomerase (PDI) is directly involved in this process. As manganese ions are known to affect the thioldisulfide balance and activate integrins to maximal affinity, we searched for PDI interactions with integrins, particularly with alpha(V)beta(3), in Mn(2+)-treated endothelial cells. By employing confocal microscopy, flow cytometry and coimmunoprecipitation experiments, we showed that exposure of endothelial cells to Mn(2+) resulted in: (a) the appearance of surface protein thiol groups, which can be found in PDI and alpha(V)beta(3), and both proteins colocalizing on the cellular surface; and (b) the formation of the PDI-alpha(V)beta(3) complex, which dissociates upon reduction. In addition, PDI in a complex with alpha(V)beta(3) induces conversion of the integrin to the ligand-competent high-affinity state, as evidenced by increased binding of vitronectin. The membrane-impermeable sulfhydryl blockers 3-N-maleimidylpropionyl biocytin 3-N-maleimidylpropionyl biocytin and p-chloromercuriphenyl sulfonate, as well as the PDI inhibitors bacitracin, MA3 018, and MA3 019, abolished the binding of vitronectin and LM609 to endothelial cells that is activated by Mn(2+). Consistently, LM609 almost completely blocked binding of vitronectin to such cells. The formation of the PDI-alpha(V)beta(3) stoichiometric complex was further demonstrated by surface plasmon resonance analysis, which showed that the initial reversible binding of PDI becomes irreversible in the presence of Mn(2+), probably mediated by disulfide bonds. Thus, we show that Mn(2+) simultaneously modulates the thiol isomerase activity of PDI that is bound to alpha(V)beta(3) and induces its transition to the ligand-competent state, suggesting an alternative mechanism of integrin regulation.

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.

Purification, analysis, and crystal structure of integrins

Integrins are large modular cell-surface receptors that regulate almost every aspect of cellular function through bidirectional signals transmitted across the lipid bilayer. Regulation of integrin activity is accomplished by complex and still incompletely understood biochemical pathways that modify integrin ligand binding, clustering, trafficking, and signaling functions. The dynamic tertiary and quaternary changes required to channel some of these activities have hampered, until recently, the crystal structure determination of these heterodimeric receptors. In this chapter, we review the methods used to purify and characterize these proteins biophysically and functionally, and to derive their three-dimensional structures.

The beta-tail domain (betaTD) regulates physiologic ligand binding to integrin CD11b/CD18

Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (betaTD) and the ligand-binding betaA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alphaMbeta2) with its physiologic ligand iC3b. We replaced the betaTD CD loop residues D658GMD of the CD18 (beta2) subunit with the equivalent D672SSG of the beta3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca(2+) plus Mg(2+) (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the betaTD-modified integrins bound in Ca(2+)Mg(2+) to the betaA-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal betaTD as an allosteric modulator of integrin activation.

Evidence for a differential functional regulation of the two beta(3)-integrins alpha(V)beta(3) and alpha(IIb)beta(3)

The functional regulation of integrins is a major determinant of cell adhesion, migration and tissue maintenance. The binding of cytoskeletal proteins to various sites of integrin cytoplasmic domains is a key mechanism of this functional regulation. Expression of recombinant integrin alpha(IIb)beta(3) and alpha(M)beta(2) lacking the GFFKR-region in CHO cells results in constitutively activated integrins. In contrast, CHO cells stably expressing either a GFFKR-deleted alpha(V(del))beta(3) or a FF to AA-substituted alpha(V(AA))beta(3) do not reveal a constitutively activated integrin. Adhesion to immobilized fibrinogen is strongly impaired in alpha(V(del))beta(3) or alpha(V(AA))beta(3)-expressing cells, whereas it is not impaired in alpha(IIb)beta(3) and alpha(M)beta(2), both lacking the GFFKR-region. In a parallel plate flow chamber assay, alpha(V)beta(3)-expressing cells adhere firmly to fibrinogen and spread even at shear rates of 15 to 20 dyn/cm(2), whereas alpha(V(del))beta(3) or alpha(V(AA))beta(3) cells are detached at 15 dyn/cm(2). Actin stress fiber formation and focal adhesion plaques containing alpha(V)beta(3) are observed in alpha(V)beta(3) cells but not in alpha(V(del))beta(3) or alpha(V(AA))beta(3)-expressing cells. As an additional manifestation of impaired outside-in signaling, phosphorylation of pp125(FAK) was reduced in these cells. In summary, we report that the GFFKR-region of the alpha(V)-cytoplasmic domain and in particular two phenylalanines are essential for integrin alpha(V)beta(3) function, especially for outside-in signaling. Our results suggest that the two beta(3)-integrins alpha(IIb)beta(3) and alpha(V)beta(3) are differentially regulated via their GFFKR-region.

Targeting integrin structure and function in disease

Initially linked to the pathogenesis of inflammatory and hematologic diseases, integrins have become validated drug targets with the approval of five drugs. Moreover, there are several promising drug candidates in preclinical and clinical stages of development for multiple clinical indications. Integrins are attractive drug targets as their antagonism can block several steps in disease progression or maintenance. Integrin inhibitors can block the proliferation, migration, or tissue localization of inflammatory, angiogenic, and tumor cells, as well as signaling and gene expression contributing to disease. There has been a rapid increase in the elucidation of integrin structure, their allosteric mechanisms of bidirectional signaling, and the structure of complexes with drugs. This information brings greater focus to how integrins support various cellular functions and how they have been and may be targeted to develop novel drugs. Here we review conformational switches, including an internal ligand, which allosterically regulate the transition from low- to high-affinity ligand binding. We address some of the successes, disappointments, and challenges in targeting competitive or allosteric sites to develop therapeutics. We also discuss new opportunities, including a structure-based approach to discover novel drugs to treat inflammatory and other diseases. This approach targets structural relatives of the von Willebrand factor A-domain present in integrins and many functionally diverse proteins.