Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Measurement of Integrin Activation and Conformational Changes on the Cell Surface by Soluble Ligand and Antibody Binding Assays

Soluble ligand and conformation-dependent antibody binding assay of integrins on the cell surface is an effective approach to evaluate the activation status of integrins in live cells. The ligands or antibodies are usually labeled with biotin or a fluorescent dye and incubated with integrin-expressing cells in suspension. The cell-bound ligands and antibodies are then detected by flow cytometry. Here we describe the detailed protocols of soluble ligand or antibody binding assay for α_IIbβ₃, α_Vβ₃, α₅β₁, and α_Lβ₂ integrins that are transiently or stably expressed in the model cell lines such as HEK293 or CHO-k1 cells.

Mechanically Regulated Outside-In Activation of an I-Domain-Containing Integrin

Integrins are heterodimeric transmembrane proteins that mediate cellular adhesion and bidirectional mechanotransductions through their conformational allostery. The allosteric pathway of an I-domain-containing integrin remains unclear because of its complexity and lack of effective experiments. For a typical I-domain-containing integrin α_Xβ₂, molecular dynamics simulations were employed here to investigate the conformational dynamics in the first two steps of outside-in activation, the bindings of both the external and internal ligands. Results showed that the internal ligand binding is a prerequisite to the allosteric transmission from the α- to β-subunits and the exertion of external force to integrin-ligand complex. The opening state of αI domain with downward movement and lower half unfolding of α₇-helix ensures the stable intersubunit conformational transmission through external ligand binding first and internal ligand binding later. Reverse binding order induces a, to our knowledge, novel but unstable swingout of β-subunit Hybrid domain with the retained close states of both αI and βI domains. Prebinding of external ligand greatly facilitates the following internal ligand binding and vice versa. These simulations furthered the understanding in the outside-in activation of I-domain-containing integrins from the viewpoint of internal allosteric pathways.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

An integrin αIIbβ3 intermediate affinity state mediates biomechanical platelet aggregation

Integrins are membrane receptors that mediate cell adhesion and mechanosensing. The structure-function relationship of integrins remains incompletely understood, despite the extensive studies carried out because of its importance to basic cell biology and translational medicine. Using a fluorescence dual biomembrane force probe, microfluidics and cone-and-plate rheometry, we applied precisely controlled mechanical stimulations to platelets and identified an intermediate state of integrin α_IIbβ₃ that is characterized by an ectodomain conformation, ligand affinity and bond lifetimes that are all intermediate between the well-known inactive and active states. This intermediate state is induced by ligand engagement of glycoprotein (GP) Ibα via a mechanosignalling pathway and potentiates the outside-in mechanosignalling of α_IIbβ₃ for further transition to the active state during integrin mechanical affinity maturation. Our work reveals distinct α_IIbβ₃ state transitions in response to biomechanical and biochemical stimuli, and identifies a role for the α_IIbβ₃ intermediate state in promoting biomechanical platelet aggregation.

Two purified proteins from royal jelly with in vitro dual anti-hepatic damage potency: Major royal jelly protein 2 and its novel isoform X1

Liver diseases are serious life-threating conditions that should be controlled. Here, we identify a protein fraction from royal-jelly (RJ) that represents the most effective composite against CCl₄-induced hepatotoxicity and HepG2 cell growth. Two closely related proteins were purified from this fraction by a new simple method and identified by MALDI-TOF MS as major RJ protein 2 (MRJP2) and its predicted isoform X1. The in silico assessment (3D structures and functions) of these proteins were performed using Iterative Threading ASSEmbly Refinement (I-TASSER) analysis and RAMPAGE program. These two purified proteins were able to relieve the necrotic hepatocytes (by 60.4%) via reducing tumor necrosis factor (TNF)-α, mixed lineage kinase domain-like protein (MLKL) and intracellular reactive species. The latter effects associated with improving hepatocyte functions. Furthermore, they revealed the potent anticancer effect via induction of caspase-dependent apoptosis and controlling the expression of both Bcl-2 and p53 in HepG2 cells. Thus, MRJP2 and its isoform X1 can be a promising dual strategy for fighting hepatic injury and cancer in future animal and human studies.

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.

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.

The membrane-distal regions of integrin α cytoplasmic domains contribute differently to integrin inside-out activation

Functioning as signal receivers and transmitters, the integrin α/β cytoplasmic tails (CT) are pivotal in integrin activation and signaling. 18 α integrin subunits share a conserved membrane-proximal region but have a highly diverse membrane-distal (MD) region at their CTs. Recent studies demonstrated that the presence of α CTMD region is essential for talin-induced integrin inside-out activation. However, it remains unknown whether the non-conserved α CTMD regions differently regulate the inside-out activation of integrin. Using αIIbβ3, αLβ2, and α5β1 as model integrins and by replacing their α CTMD regions with those of α subunits that pair with β3, β2, and β1 subunits, we analyzed the function of CTMD regions of 17 α subunits in talin-mediated integrin activation. We found that the α CTMD regions play two roles on integrin, which are activation-supportive and activation-regulatory. The regulatory but not the supportive function depends on the sequence identity of α CTMD region. A membrane-proximal tyrosine residue present in the CTMD regions of a subset of α integrins was identified to negatively regulate integrin inside-out activation. Our study provides a useful resource for investigating the function of α integrin CTMD regions.

A pivotal role for a conserved bulky residue at the α1-helix of the αI integrin domain in ligand binding

The ligand-binding βI and αI domains of integrin are the best-studied von Willebrand factor A domains undergoing significant conformational changes for affinity regulation. In both βI and αI domains, the α1- and α7-helixes work in concert to shift the metal-ion-dependent adhesion site between the resting and active states. An absolutely conserved Gly in the middle of the α1-helix of βI helps maintain the resting βI conformation, whereas the homologous position in the αI α1-helix contains a conserved Phe. A functional role of this Phe is structurally unpredictable. Using α_Lβ₂ integrin as a model, we found that the residue volume at the Phe position in the α1-helix is critical for α_Lβ₂ activation because trimming the Phe by small amino acid substitutions abolished α_Lβ₂ binding with soluble and immobilized intercellular cell adhesion molecule 1. Similar results were obtained for α_Mβ₂ integrin. Our experimental and molecular dynamics simulation data suggested that the bulky Phe acts as a pawl that stabilizes the downward ratchet-like movement of β6-α7 loop and α7-helix, required for high-affinity ligand binding. This mechanism may apply to other von Willebrand factor A domains undergoing large conformational changes. We further demonstrated that the conformational cross-talk between α_L αI and β₂ βI could be uncoupled because the β₂ extension and headpiece opening could occur independently of the αI activation. Reciprocally, the αI activation does not inevitably lead to the conformational changes of the β₂ subunit. Such loose linkage between the αI and βI is attributed to the αI flexibility and could accommodate the α_Lβ₂-mediated rolling adhesion of leukocytes.

Receptor-mediated cell mechanosensing

Mechanosensing depicts the ability of a cell to sense mechanical cues, which under some circumstances is mediated by the surface receptors. In this review, a four-step model is described for receptor-mediated mechanosensing. Platelet GPIb, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

Mechanosensing describes the ability of a cell to sense mechanical cues of its microenvironment, including not only all components of force, stress, and strain but also substrate rigidity, topology, and adhesiveness. This ability is crucial for the cell to respond to the surrounding mechanical cues and adapt to the changing environment. Examples of responses and adaptation include (de)activation, proliferation/apoptosis, and (de)differentiation. Receptor-mediated cell mechanosensing is a multistep process that is initiated by binding of cell surface receptors to their ligands on the extracellular matrix or the surface of adjacent cells. Mechanical cues are presented by the ligand and received by the receptor at the binding interface; but their transmission over space and time and their conversion into biochemical signals may involve other domains and additional molecules. In this review, a four-step model is described for the receptor-mediated cell mechanosensing process. Platelet glycoprotein Ib, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

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.

CD99-Derived Agonist Ligands Inhibit Fibronectin-Induced Activation of β1 Integrin through the Protein Kinase A/SHP2/Extracellular Signal-Regulated Kinase/PTPN12/Focal Adhesion Kinase Signaling Pathway

The human CD99 protein is a 32-kDa glycosylated transmembrane protein that regulates various cellular responses, including cell adhesion and leukocyte extravasation. We previously reported that CD99 activation suppresses β1 integrin activity through dephosphorylation of focal adhesion kinase (FAK) at Y397. We explored a molecular mechanism underlying the suppression of β1 integrin activity by CD99 agonists and its relevance to tumor growth in vivo CD99-Fc fusion proteins or a series of CD99-derived peptides suppressed β1 integrin activity by specifically interacting with three conserved motifs of the CD99 extracellular domain. CD99CRIII3, a representative CD99-derived 3-mer peptide, facilitated protein kinase A-SHP2 interaction and subsequent activation of the HRAS/RAF1/MEK/ERK signaling pathway. Subsequently, CD99CRIII3 induced FAK phosphorylation at S910, which led to the recruitment of PTPN12 and PIN1 to FAK, followed by FAK dephosphorylation at Y397. Taken together, these results indicate that CD99-derived agonist ligands inhibit fibronectin-mediated β1 integrin activation through the SHP2/ERK/PTPN12/FAK signaling pathway.

Ligand-induced Epitope Masking: DISSOCIATION OF INTEGRIN α5β1-FIBRONECTIN COMPLEXES ONLY BY MONOCLONAL ANTIBODIES WITH AN ALLOSTERIC MODE OF ACTION

We previously demonstrated that Arg-Gly-Asp (RGD)-containing ligand-mimetic inhibitors of integrins are unable to dissociate pre-formed integrin-fibronectin complexes (IFCs). These observations suggested that amino acid residues involved in integrin-fibronectin binding become obscured in the ligand-occupied state. Because the epitopes of some function-blocking anti-integrin monoclonal antibodies (mAbs) lie near the ligand-binding pocket, it follows that the epitopes of these mAbs may become shielded in the ligand-occupied state. Here, we tested whether function-blocking mAbs directed against α5β1 can interact with the integrin after it forms a complex with an RGD-containing fragment of fibronectin. We showed that the anti-α5 subunit mAbs JBS5, SNAKA52, 16, and P1D6 failed to disrupt IFCs and hence appeared unable to bind to the ligand-occupied state. In contrast, the allosteric anti-β1 subunit mAbs 13, 4B4, and AIIB2 could dissociate IFCs and therefore were able to interact with the ligand-bound state. However, another class of function-blocking anti-β1 mAbs, exemplified by Lia1/2, could not disrupt IFCs. This second class of mAbs was also distinguished from 13, 4B4, and AIIB2 by their ability to induce homotypic cell aggregation. Although the epitope of Lia1/2 was closely overlapping with those of 13, 4B4, and AIIB2, it appeared to lie closer to the ligand-binding pocket. A new model of the α5β1-fibronectin complex supports our hypothesis that the epitopes of mAbs that fail to bind to the ligand-occupied state lie within, or very close to, the integrin-fibronectin interface. Importantly, our findings imply that the efficacy of some therapeutic anti-integrin mAbs could be limited by epitope masking.

Function and conformation analyses of an aspartate substitution of the invariant glycine in the integrin βI domain α1-α1′ helix

We showed that the αLβ2 integrin with the non-functional mutation G150D cannot be induced with Mg/EGTA to express the mAb KIM127 epitope, which reports the leg-extended conformation. We extended the study to the αIIbβ3, an integrin without an αI domain. The equivalent mutation, i.e. G161D, also resulted in an expressible, but non-adhesive αIIbβ3 integrin. An NMR study of synthetic peptides spanning the α1-α1′ helix of the β3 I domain shows that both wild-type and mutant peptides are α-helical. However, whereas in the wild-type peptide this helix is continuous, the mutant presents a discontinuity, or kink, precisely at the site of mutation G161D. Our results suggest that the mutation may lock integrin heterodimers in a bent conformation that prevents integrin activation via conformational extension.

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Integrin αLβ2-G150D cannot be activated to an extended conformation.

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Integrin αIIbβ3-G161D can be expressed on transfectants.

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Transfectants expressing the αIIbβ3-G161D integrin cannot adhere to fibrinogen.

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The Asp locks the α1-α1′ helices to into a kinked structure.

Force regulated conformational change of integrin αVβ3

Integrins mediate cell adhesion to extracellular matrix and transduce signals bidirectionally across the membrane. Integrin α_Vβ₃ has been shown to play an essential role in tumor metastasis, angiogenesis, hemostasis and phagocytosis. Integrins can take several conformations, including the bent and extended conformations of the ectodomain, which regulate integrin functions. Using a biomembrane force probe, we characterized the bending and unbending conformational changes of single α_Vβ₃ integrins on living cell surfaces in real-time. We measured the probabilities of conformational changes, rates and speeds of conformational transitions, and the dynamic equilibrium between the two conformations, which were regulated by tensile force, dependent on the ligand, and altered by point mutations. These findings provide insights into how α_Vβ₃ acts as a molecular machine and how its physiological function and molecular structure are coupled at the single-molecule level.

The dual structural roles of the membrane distal region of the α-integrin cytoplasmic tail during integrin inside-out activation

Studies on the mechanism of integrin inside-out activation have been focused on the role of β-integrin cytoplasmic tails, which are relatively conserved and bear binding sites for the intracellular activators including talin and kindlin. Cytoplasmic tails for α-integrins share a conserved GFFKR motif at the membrane-proximal region and this forms a specific interface with the β-integrin membrane-proximal region to keep the integrin inactive. The α-integrin membrane-distal regions, after the GFFKR motif, are diverse both in length and sequence and their roles in integrin activation have not been well-defined. In this study, we report that the α-integrin cytoplasmic membrane-distal region contributes to maintaining integrin in the resting state and to integrin inside-out activation. Complete deletion of the α-integrin membrane-distal region diminished talin- and kindlin-mediated integrin ligand binding and conformational change. A proper length and suitable amino acids in α-integrin membrane-distal region was found to be important for integrin inside-out activation. Our data establish an essential role for the α-integrin cytoplasmic membrane-distal region in integrin activation and provide new insights into how talin and kindlin induce the high-affinity integrin conformation that is required for fully functional integrins.

Disruption of integrin-fibronectin complexes by allosteric but not ligand-mimetic inhibitors

Failure of Arg-Gly-Asp (RGD)-based inhibitors to reverse integrin-ligand binding has been reported, but the prevalence of this phenomenon among integrin heterodimers is currently unknown. In the present study we have investigated the interaction of four different RGD-binding integrins (α5β1, αVβ1, αVβ3 and αVβ6) with fibronectin (FN) using surface plasmon resonance. The ability of inhibitors to reverse ligand binding was assessed by their capacity to increase the dissociation rate of pre-formed integrin-FN complexes. For all four receptors we showed that RGD-based inhibitors (such as cilengitide) were completely unable to increase the dissociation rate. Formation of the non-reversible state occurred very rapidly and did not rely on the time-dependent formation of a high-affinity state of the integrin, or the integrin leg regions. In contrast with RGD-based inhibitors, Ca2+ (but not Mg2+) was able to greatly increase the dissociation rate of integrin-FN complexes, with a half-maximal response at ~0.4 mM Ca2+ for αVβ3-FN. The effect of Ca2+ was overcome by co-addition of Mn2+, but not Mg2+. A stimulatory anti-β1 monoclonal antibody (mAb) abrogated the effect of Ca2+ on α5β1-FN complexes; conversely, a function-blocking mAb mimicked the effect of Ca2+. These results imply that Ca2+ acts allosterically, probably through binding to the adjacent metal-ion-dependent adhesion site (ADMIDAS), and that the α1 helix in the β subunit I domain is the key element affected by allosteric modulators. The data suggest an explanation for the limited clinical efficacy of RGD-based integrin antagonists, and we propose that allosteric antagonists could prove to be of greater therapeutic benefit.