Anti-integrin monoclonal antibodies

Luminescent sensing of conformational integrin activation in living cells

Integrins are major receptors for secreted extracellular matrix, playing crucial roles in physiological and pathological contexts, such as angiogenesis and cancer. Regulation of the transition between inactive and active conformation is key for integrins to fulfill their functions, and pharmacological control of those dynamics may have therapeutic applications. We create and validate a prototypic luminescent β1 integrin activation sensor (β1IAS) by introducing a split luciferase into an activation reporting site between the βI and the hybrid domains. As a recombinant protein in both solution and living cells, β1IAS accurately reports β1 integrin activation in response to (bio)chemical and physical stimuli. A short interfering RNA (siRNA) high-throughput screening on live β1IAS knockin endothelial cells unveils hitherto unknown regulators of β1 integrin activation, such as β1 integrin inhibitors E3 ligase Pja2 and vascular endothelial growth factor B (VEGF-B). This split-luciferase-based strategy provides an in situ label-free measurement of integrin activation and may be applicable to other β integrins and receptors.

A Review of Talin- and Integrin-Dependent Molecular Mechanisms in Cancer Invasion and Metastasis

Cancer is the second most common cause of death in the world, representing one of the main economic burdens in health care and research. The effort of research has mainly focused on limiting the growth of a localized tumor, but most recently, there has been more attention focused on restricting the spreading of the cancer via invasion and metastasis. The signaling pathways behind these two processes share many molecules with physiological pathways regulating cell adhesion and migration, and, moreover, adhesion and migration processes themselves underlie tumor potential for invasion. In this work, we reviewed the latest literature about cancer development and invasion and their regulation by cell migration- and adhesion-related proteins, with a specific focus on talins and integrins. We also summarized the most recent developments and approaches to anti-cancer therapies, concentrating on cell migration-related therapies.

The tumor microenvironment: shaping cancer progression and treatment response

The tumor microenvironment (TME) plays a crucial role in cancer progression and treatment response. It comprises a complex network of stromal cells, immune cells, extracellular matrix, and blood vessels, all of which interact with cancer cells and influence tumor behaviour. This review article provides an in-depth examination of the TME, focusing on stromal cells, blood vessels, signaling molecules, and ECM, along with commonly available therapeutic compounds that target these components. Moreover, we explore the TME as a novel strategy for discovering new anti-tumor drugs. The dynamic and adaptive nature of the TME offers opportunities for targeting specific cellular interactions and signaling pathways. We discuss emerging approaches, such as combination therapies that simultaneously target cancer cells and modulate the TME. Finally, we address the challenges and future prospects in targeting the TME. Overcoming drug resistance, improving drug delivery, and identifying new therapeutic targets within the TME are among the challenges discussed. We also highlight the potential of personalized medicine and the integration of emerging technologies, such as immunotherapy and nanotechnology, in TME-targeted therapies. This comprehensive review provides insights into the TME and its therapeutic implications. Understanding the TME's complexity and targeting its components offer promising avenues for the development of novel anti-tumor therapies and improved patient outcomes.

Apical integrins as a switchable target to regulate the epithelial barrier

Tight junctions regulate epithelial barrier function and have been shown to be influenced by multiple classes of proteins. Apical integrins have been identified as potential regulators of epithelial barrier function; however, only indirect approaches have been used to measure integrin regulation of the epithelial barrier. Here, we used polymeric nanowires conjugated with anti-integrin β1 antibodies to specifically target apically localized integrins in either their closed or open conformation. Barrier regulation by apical integrins was found to be conformation specific. Nanowires targeting integrins in the closed conformation increased epithelial permeability and caused zonula occludens-1 (ZO-1, also known as TJP1) to change from a linear to a ruffled morphology. Claudin-2 and claudin-4 colocalized with ZO-1 and were also ruffled; however, claudin-1 and claudin-7 remained linear. Ruffling was dependent on myosin light chain kinases (MLCKs) and Rho kinases (ROCKs). Conversely, targeting integrins in the open conformation decreased epithelial permeability and made junctions more linearized. Anti-integrin β1 nanowires differentially affected actin and talin (analyzed using pan-talin antibodies), depending on whether they contained activating or inhibitory antibodies. Thus, apical integrins can act as a conformation-sensitive switch that regulates epithelial barrier function.

Monocyte uptake of polymeric peptidoglycan is bimodal and governed by complement C3 and C4 opsonins

Peptidoglycans (PGNs) are structural polymers of the bacterial cell wall and a common microbial molecular pattern encountered by the immune system daily. Low levels of PGNs are constitutively present in the systemic circulation in humans and rise during inflammatory pathologies. Since all known PGN sensors are intracellular, PGN internalization is a prerequisite for the initiation of cellular immune responses. Here, we report the mechanisms controlling the recognition and uptake of polymeric PGNs by circulating human mononuclear phagocytes. We found that complement C3 and C4 opsonins govern PGN recognition and internalization, but no single opsonin is indispensable because of multiple uptake redundancies. We observed a bimodal internalization of polymeric PGNs with distinct requirements for complement C4. At low PGN concentrations, C3 mediated PGN recognition by surface receptors while the efficient internalization of PGN polymers critically required C4. Supraphysiologic PGN concentrations triggered a secondary uptake modality that was insensitive to C4 and mediated instead by C3 engagement of complement receptors 1 and 3. To our knowledge, this is the first description of nonoverlapping C3 and C4 opsonophagocytoses working in parallel. Controlling these uptake mechanisms has the potential to modulate PGN clearance or the dysregulated immune responses during bacterial infections.

Strengthening E-cadherin adhesion via antibody-mediated binding stabilization

E-cadherins (Ecads) are a crucial cell-cell adhesion protein with tumor suppression properties. Ecad adhesion can be enhanced by the monoclonal antibody 66E8, which has potential applications in inhibiting cancer metastasis. However, the biophysical mechanisms underlying 66E8-mediated adhesion strengthening are unknown. Here, we use molecular dynamics simulations, site-directed mutagenesis, and single-molecule atomic force microscopy experiments to demonstrate that 66E8 strengthens Ecad binding by stabilizing the primary Ecad adhesive conformation: the strand-swap dimer. By forming electrostatic interactions with Ecad, 66E8 stabilizes the swapped β-strand and its hydrophobic pocket and impedes Ecad conformational changes, which are necessary for rupture of the strand-swap dimer. Our findings identify fundamental mechanistic principles for strengthening of Ecad binding using monoclonal antibodies.

Stem cell heterogeneity, plasticity, and regulation

As a population of homogeneous cells with both self-renewal and differentiation potential, stem cell pools are highly compartmentalized and contain distinct subsets that exhibit stable but limited heterogeneity during homeostasis. However, their striking plasticity is showcased under natural or artificial stress, such as injury, transplantation, cancer, and aging, leading to changes in their phenotype, constitution, metabolism, and function. The complex and diverse network of cell-extrinsic niches and signaling pathways, together with cell-intrinsic genetic and epigenetic regulators, tightly regulate both the heterogeneity during homeostasis and the plasticity under perturbation. Manipulating these factors offers better control of stem cell behavior and a potential revolution in the current state of regenerative medicine. However, disruptions of normal regulation by genetic mutation or excessive plasticity acquisition may contribute to the formation of tumors. By harnessing innovative techniques that enhance our understanding of stem cell heterogeneity and employing novel approaches to maximize the utilization of stem cell plasticity, stem cell therapy holds immense promise for revolutionizing the future of medicine.

Defined extracellular matrix compositions support stiffness-insensitive cell spreading and adhesion signaling

Integrin-dependent adhesion to the extracellular matrix (ECM) mediates mechanosensing and signaling in response to altered microenvironmental conditions. In order to provide tissue- and organ-specific cues, the ECM is composed of many different proteins that temper the mechanical properties and provide the necessary structural diversity. Despite most human tissues being soft, the prevailing view from predominantly in vitro studies is that increased stiffness triggers effective cell spreading and activation of mechanosensitive signaling pathways. To address the functional coupling of ECM composition and matrix rigidity on compliant substrates, we developed a matrix spot array system to screen cell phenotypes against different ECM mixtures on defined substrate stiffnesses at high resolution. We applied this system to both cancer and normal cells and surprisingly identified ECM mixtures that support stiffness-insensitive cell spreading on soft substrates. Employing the motor-clutch model to simulate cell adhesion on biochemically distinct soft substrates, with varying numbers of available ECM-integrin-cytoskeleton (clutch) connections, we identified conditions in which spreading would be supported on soft matrices. Combining simulations and experiments, we show that cell spreading on soft is supported by increased clutch engagement on specific ECM mixtures and even augmented by the partial inhibition of actomyosin contractility. Thus, "stiff-like" spreading on soft is determined by a balance of a cell's contractile and adhesive machinery. This provides a fundamental perspective for in vitro mechanobiology studies, identifying a mechanism through which cells spread, function, and signal effectively on soft substrates.

Imaging and quantitative analysis of integrin-dependent cell-matrix adhesions

Integrin-dependent cell-extracellular matrix adhesion is essential for wound healing, embryonic development, immunity, and tissue organization. Here, we present a protocol for the imaging and quantitative analysis of integrin-dependent cell-matrix adhesions. We describe steps for cell culture; virus preparation; lentiviral transduction; imaging with widefield, confocal, and total internal reflection fluorescence microscopy; and using a script for their quantitative analysis. We then detail procedures for analyzing adhesion dynamics by live-cell imaging and fluorescence recovery after photobleaching (FRAP). For complete details on the use and execution of this protocol, please refer to Margadant et al. (2012),1 van der Bijl et al. (2020),2 Amado-Azevedo et al. (2021).3.

Strengthening E-cadherin adhesion via antibody mediated binding stabilization

E-cadherins (Ecads) are a crucial cell-cell adhesion protein with tumor suppression properties. Ecad adhesion can be enhanced by the monoclonal antibody 66E8, which has potential applications in inhibiting cancer metastasis. However, the biophysical mechanisms underlying 66E8 mediated adhesion strengthening are unknown. Here, we use molecular dynamics simulations, site directed mutagenesis and single molecule atomic force microscopy experiments to demonstrate that 66E8 strengthens Ecad binding by stabilizing the primary Ecad adhesive conformation: the strand-swap dimer. By forming electrostatic interactions with Ecad, 66E8 stabilizes the swapped β-strand and its hydrophobic pocket and impedes Ecad conformational changes, which are necessary for rupture of the strand-swap dimer. Our findings identify fundamental mechanistic principles for strengthening of Ecad binding using monoclonal antibodies.

Identification and Characterization of the Wilms Tumor Cancer Stem Cell

A nephrogenic progenitor cell (NP) with cancer stem cell characteristics driving Wilms tumor (WT) using spatial transcriptomics, bulk and single cell RNA sequencing, and complementary in vitro and transplantation experiments is identified and characterized. NP from WT samples with NP from the developing human kidney is compared. Cells expressing SIX2 and CITED1 fulfill cancer stem cell criteria by reliably recapitulating WT in transplantation studies. It is shown that self-renewal versus differentiation in SIX2+CITED1+ cells is regulated by the interplay between integrins ITGβ1 and ITGβ4. The spatial transcriptomic analysis defines gene expression maps of SIX2+CITED1+ cells in WT samples and identifies the interactive gene networks involved in WT development. These studies define SIX2+CITED1+ cells as the nephrogenic-like cancer stem cells of WT and points to the renal developmental transcriptome changes as a possible driver in regulating WT formation and progression.

Application of a Magnetic Platform in α6 Integrin-Positive iPSC-TM Purification

The emergence of induced pluripotent stem cell (iPSC) technology has provided a new approach to regenerating decellularized trabecular meshwork (TM) in glaucoma. We have previously generated iPSC-derived TM (iPSC-TM) using a medium conditioned by TM cells and verified its function in tissue regeneration. Because of the heterogeneity of iPSCs and the isolated TM cells, iPSC-TM cells appear to be heterogeneous, which impedes our understanding of how the decellularized TM may be regenerated. Herein, we developed a protocol based on a magnetic-activated cell sorting (MACS) system or an immunopanning (IP) method for sorting integrin subunit alpha 6 (ITGA6)-positive iPSC-TM, an example of the iPSC-TM subpopulation. We first analyzed the purification efficiency of these two approaches by flow cytometry. In addition, we also determined cell viability by analyzing the morphologies of the purified cells. To conclude, the MACS-based purification could yield a higher ratio of ITGA6-positive iPSC-TM and maintain a relatively higher cell viability than the IP-based method, allowing for the preparation of any iPSC-TM subpopulation of interest and facilitating a better understanding of the regenerative mechanism of iPSC-based therapy.

The βI domain promotes active β1 integrin clustering into mature adhesion sites

Modulation of integrin function is required in many physiological and pathological settings, such as angiogenesis and cancer. Integrin allosteric changes, clustering, and trafficking cooperate to regulate cell adhesion and motility on extracellular matrix proteins via mechanisms that are partly defined. By exploiting four monoclonal antibodies recognizing distinct conformational epitopes, we show that in endothelial cells (ECs), the extracellular βI domain, but not the hybrid or I-EGF2 domain of active β1 integrins, promotes their FAK-regulated clustering into tensin 1-containing fibrillar adhesions and impairs their endocytosis. In this regard, the βI domain-dependent clustering of active β1 integrins is necessary to favor fibronectin-elicited directional EC motility, which cannot be effectively promoted by β1 integrin conformational activation alone.

Targeting integrin pathways: mechanisms and advances in therapy

Integrins are considered the main cell-adhesion transmembrane receptors that play multifaceted roles as extracellular matrix (ECM)-cytoskeletal linkers and transducers in biochemical and mechanical signals between cells and their environment in a wide range of states in health and diseases. Integrin functions are dependable on a delicate balance between active and inactive status via multiple mechanisms, including protein-protein interactions, conformational changes, and trafficking. Due to their exposure on the cell surface and sensitivity to the molecular blockade, integrins have been investigated as pharmacological targets for nearly 40 years, but given the complexity of integrins and sometimes opposite characteristics, targeting integrin therapeutics has been a challenge. To date, only seven drugs targeting integrins have been successfully marketed, including abciximab, eptifibatide, tirofiban, natalizumab, vedolizumab, lifitegrast, and carotegrast. Currently, there are approximately 90 kinds of integrin-based therapeutic drugs or imaging agents in clinical studies, including small molecules, antibodies, synthetic mimic peptides, antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T-cell therapy, imaging agents, etc. A serious lesson from past integrin drug discovery and research efforts is that successes rely on both a deep understanding of integrin-regulatory mechanisms and unmet clinical needs. Herein, we provide a systematic and complete review of all integrin family members and integrin-mediated downstream signal transduction to highlight ongoing efforts to develop new therapies/diagnoses from bench to clinic. In addition, we further discuss the trend of drug development, how to improve the success rate of clinical trials targeting integrin therapies, and the key points for clinical research, basic research, and translational research.

Fetal and neonatal alloimmune thrombocytopenia: Current pathophysiological insights and perspectives for future diagnostics and treatment

FNAIT is a pregnancy-associated condition caused by maternal alloantibodies against paternally-inherited platelet antigens, most frequently HPA-1a on integrin β3. The clinical effects range from no symptoms to fatal intracranial hemorrhage, but underlying pathophysiological determinants are poorly understood. Accumulating evidence suggests that differential antibody-Fc-glycosylation, activation of complement/effector cells, and integrin function-blocking effects contribute to clinical outcome. Furthermore, some antibodies preferentially bind platelet integrin αIIbβ3, but others bind αvβ3 on endothelial cells and trophoblasts. Defects in endothelial cells and angiogenesis may therefore contribute to severe anti-HPA-1a associated FNAIT. Moreover, anti-HPA-1a antibodies may cause placental damage, leading to intrauterine growth restriction. We discuss current insights into diversity and actions of HPA-1a antibodies, gathered from clinical studies, in vitro studies, and mouse models. Assessment of all factors determining severity and progression of anti-HPA-1a-associated FNAIT may importantly improve risk stratification and potentially reveal novel treatment strategies, both for FNAIT and other immunohematological disorders.

Activation and expansion of presynaptic signaling foci drives presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) adaptively regulates synaptic transmission in health and disease. Despite identification of numerous genes that are essential for PHP, we lack a dynamic framework to explain how PHP is initiated, potentiated, and limited to achieve precise control of vesicle fusion. Here, utilizing both mice and Drosophila, we demonstrate that PHP progresses through the assembly and physical expansion of presynaptic signaling foci where activated integrins biochemically converge with trans-synaptic Semaphorin2b/PlexinB signaling. Each component of the identified signaling complexes, including alpha/beta-integrin, Semaphorin2b, PlexinB, talin, and focal adhesion kinase (FAK), and their biochemical interactions, are essential for PHP. Complex integrity requires the Sema2b ligand and complex expansion includes a ∼2.5-fold expansion of active-zone associated puncta composed of the actin-binding protein talin. Finally, complex pre-expansion is sufficient to accelerate the rate and extent of PHP. A working model is proposed incorporating signal convergence with dynamic molecular assemblies that instruct PHP.

An innovative strategy to identify new targets for delivering antibodies to the brain has led to the exploration of the integrin family

Background

Increasing brain exposure of biotherapeutics is key to success in central nervous system disease drug discovery. Accessing the brain parenchyma is especially difficult for large polar molecules such as biotherapeutics and antibodies because of the blood-brain barrier. We investigated a new immunization strategy to identify novel receptors mediating transcytosis across the blood-brain barrier.

Method

We immunized mice with primary non-human primate brain microvascular endothelial cells to obtain antibodies. These antibodies were screened for their capacity to bind and to be internalized by primary non-human primate brain microvascular endothelial cells and Human Cerebral Microvascular Endothelial Cell clone D3. They were further evaluated for their transcytosis capabilities in three in vitro blood-brain barrier models. In parallel, their targets were identified by two different methods and their pattern of binding to human tissue was investigated using immunohistochemistry.

Results

12 antibodies with unique sequence and internalization capacities were selected amongst more than six hundred. Aside from one antibody targeting Activated Leukocyte Cell Adhesion Molecule and one targeting Striatin3, most of the other antibodies recognized β1 integrin and its heterodimers. The antibody with the best transcytosis capabilities in all blood-brain barrier in vitro models and with the best binding capacity was an anti-αnβ1 integrin. In comparison, commercial anti-integrin antibodies performed poorly in transcytosis assays, emphasizing the originality of the antibodies derived here. Immunohistochemistry studies showed specific vascular staining on human and non-human primate tissues.

Conclusions

This transcytotic behavior has not previously been reported for anti-integrin antibodies. Further studies should be undertaken to validate this new mechanism in vivo and to evaluate its potential in brain delivery.

A general chemical principle for creating closure-stabilizing integrin inhibitors

Integrins are validated drug targets with six approved therapeutics. However, small-molecule inhibitors to three integrins failed in late-stage clinical trials for chronic indications. Such unfavorable outcomes may in part be caused by partial agonism, i.e., the stabilization of the high-affinity, extended-open integrin conformation. Here, we show that the failed, small-molecule inhibitors of integrins αIIbβ3 and α4β1 stabilize the high-affinity conformation. Furthermore, we discovered a simple chemical feature present in multiple αIIbβ3 antagonists that stabilizes integrins in their bent-closed conformation. Closing inhibitors contain a polar nitrogen atom that stabilizes, via hydrogen bonds, a water molecule that intervenes between a serine residue and the metal in the metal-ion-dependent adhesion site (MIDAS). Expulsion of this water is a requisite for transition to the open conformation. This change in metal coordination is general to integrins, suggesting broad applicability of the drug-design principle to the integrin family, as validated with a distantly related integrin, α4β1.

Cryo-electron Microscopic Analysis of Single-Pass Transmembrane Receptors

Single-pass transmembrane receptors (SPTMRs) represent a diverse group of integral membrane proteins that are involved in many essential cellular processes, including signal transduction, cell adhesion, and transmembrane transport of materials. Dysregulation of the SPTMRs is linked with many human diseases. Despite extensive efforts in past decades, the mechanisms of action of the SPTMRs remain incompletely understood. One major hurdle is the lack of structures of the full-length SPTMRs in different functional states. Such structural information is difficult to obtain by traditional structural biology methods such as X-ray crystallography and nuclear magnetic resonance (NMR). The recent rapid development of single-particle cryo-electron microscopy (cryo-EM) has led to an exponential surge in the number of high-resolution structures of integral membrane proteins, including SPTMRs. Cryo-EM structures of SPTMRs solved in the past few years have tremendously improved our understanding of how SPTMRs function. In this review, we will highlight these progresses in the structural studies of SPTMRs by single-particle cryo-EM, analyze important structural details of each protein involved, and discuss their implications on the underlying mechanisms. Finally, we also briefly discuss remaining challenges and exciting opportunities in the field.

Molecular mechanism for strengthening E-cadherin adhesion using a monoclonal antibody

E-cadherin (Ecad) is an essential cell-cell adhesion protein with tumor suppression properties. The adhesive state of Ecad can be modified by the monoclonal antibody 19A11, which has potential applications in reducing cancer metastasis. Using X-ray crystallography, we determine the structure of 19A11 Fab bound to Ecad and show that the antibody binds to the first extracellular domain of Ecad near its primary adhesive motif: the strand-swap dimer interface. Molecular dynamics simulations and single-molecule atomic force microscopy demonstrate that 19A11 interacts with Ecad in two distinct modes: one that strengthens the strand-swap dimer and one that does not alter adhesion. We show that adhesion is strengthened by the formation of a salt bridge between 19A11 and Ecad, which in turn stabilizes the swapped β-strand and its complementary binding pocket. Our results identify mechanistic principles for engineering antibodies to enhance Ecad adhesion.

The Effects of Mechanical Stretch on Integrins and Filopodial-Associated Proteins in Normal and Glaucomatous Trabecular Meshwork Cells

The trabecular meshwork (TM) is the tissue responsible for regulating aqueous humor fluid egress from the anterior eye. If drainage is impaired, intraocular pressure (IOP) becomes elevated, which is a primary risk factor for primary open angle glaucoma. TM cells sense elevated IOP via changes in their biomechanical environment. Filopodia cellular protrusions and integrin transmembrane proteins may play roles in detecting IOP elevation, yet this has not been studied in detail in the TM. Here, we investigate integrins and filopodial proteins, such as myosin-X (Myo10), in response to mechanical stretch, an in vitro technique that produces mechanical alterations mimicking elevated IOP. Pull-down assays showed Myo10 binding to α5 but not the β1 subunit, αvβ3, and αvβ5 integrins. Several of these integrins colocalized in nascent adhesions in the filopodial tip and shaft. Using conformation-specific antibodies, we found that β1 integrin, but not α5 or αvβ3 integrins, were activated following 1-h mechanical stretch. Cadherin -11 (CDH11), a cell adhesion molecule, did not bind to Myo10, but was associated with filopodia. Interestingly, CDH11 was downregulated on the TM cell surface following 1-h mechanical stretch. In glaucoma cells, CDH11 protein levels were increased. Finally, mechanical stretch caused a small, yet significant increase in Myo10 protein levels in glaucoma cells, but did not affect cellular communication of fluorescent vesicles via filopodia-like tunneling nanotubes. Together, these data suggest that TM cell adhesion proteins, β1 integrin and CDH11, have relatively rapid responses to mechanical stretch, which suggests a central role in sensing changes in IOP elevation in situ.

New Therapeutic Targets for Hepatic Fibrosis in the Integrin Family, α8β1 and α11β1, Induced Specifically on Activated Stellate Cells

A huge effort has been devoted to developing drugs targeting integrins over 30 years, because of the primary roles of integrins in the cell-matrix milieu. Five αv-containing integrins, in the 24 family members, have been a central target of fibrosis. Currently, a small molecule against αvβ1 is undergoing a clinical trial for NASH-associated fibrosis as a rare agent aiming at fibrogenesis. Latent TGFβ activation, a distinct talent of αv-integrins, has been intriguing as a therapeutic target. None of the αv-integrin inhibitors, however, has been in the clinical market. αv-integrins commonly recognize an Arg-Gly-Asp (RGD) sequence, and thus the pharmacophore of inhibitors for the 5-integrins is based on the same RGD structure. The RGD preference of the integrins, at the same time, dilutes ligand specificity, as the 5-integrins share ligands containing RGD sequence such as fibronectin. With the inherent little specificity in both drugs and targets, "disease specificity" has become less important for the inhibitors than blocking as many αv-integrins. In fact, an almighty inhibitor for αv-integrins, pan-αv, was in a clinical trial. On the contrary, approved integrin inhibitors are all specific to target integrins, which are expressed in a cell-type specific manner: αIIbβ3 on platelets, α4β1, α4β7 and αLβ2 on leukocytes. Herein, "disease specific" integrins would serve as attractive targets. α8β1 and α11β1 are selectively expressed in hepatic stellate cells (HSCs) and distinctively induced upon culture activation. The exceptional specificity to activated HSCs reflects a rather "pathology specific" nature of these new integrins. The monoclonal antibodies against α8β1 and α11β1 in preclinical examinations may illuminate the road to the first medical agents.

Plating human iPSC lines on micropatterned substrates reveals role for ITGB1 nsSNV in endoderm formation

Quantitative analysis of human induced pluripotent stem cell (iPSC) lines from healthy donors is a powerful tool for uncovering the relationship between genetic variants and cellular behavior. We previously identified rare, deleterious non-synonymous single nucleotide variants (nsSNVs) in cell adhesion genes that are associated with outlier iPSC phenotypes in the pluripotent state. Here, we generated micropatterned colonies of iPSCs to test whether nsSNVs influence patterning of radially ordered germ layers. Using a custom-built image analysis pipeline, we quantified the differentiation phenotypes of 13 iPSC lines that harbor nsSNVs in genes related to cell adhesion or germ layer development. All iPSC lines differentiated into the three germ layers; however, there was donor-specific variation in germ layer patterning. We identified one line that presented an outlier phenotype of expanded endodermal differentiation, which was associated with a nsSNV in ITGB1. Our study establishes a platform for investigating the impact of nsSNVs on differentiation.

Efficient fabrication of monodisperse hepatocyte spheroids and encapsulation in hybrid hydrogel with controllable extracellular matrix effect

Three-dimensional (3D) culture techniques, such as spheroid and organoid cultures, have gained increasing interest in biomedical research. However, the understanding and control of extracellular matrix (ECM) effect in spheroid and organoid culture has been limited. Here, we report a biofabrication approach to efficiently form uniform-sized 3D hepatocyte spheroids and encapsulate them in a hybrid hydrogel composed of alginate and various ECM molecules. Cells were seeded in a microwell platform to form spheroid before being encapsulated directly in a hybrid hydrogel containing various ECM molecules, including collagen type I (COL1), collagen type IV (COL4), fibronectin (FN), and laminin (LM). A systematic analysis of the effect of ECM molecules on the primary mouse hepatocyte phenotype was then performed. Our results showed that hydrogel encapsulation of hepatocyte spheroid promoted hepatic marker expression and secretory functions. In addition, different ECM molecules elicited distinct effects on hepatic functions in 3D encapsulated hepatocyte spheroids, but not in 2D hepatocyte and 3D non-encapsulated spheroids. When encapsulated in hybrid hydrogel containing LM alone or COL1 alone, hepatocyte spheroids exhibited improved hepatic functions overall. Analysis of gene and protein expression showed an upregulation of integrinα1 and integrinα6 when LM was introduced in the hybrid hydrogel, suggesting a possible role of integrin signaling involved in the ECM effect. Finally, a combinatorial screening was performed to demonstrate the potential to screen a multitude of 3D microenvironments of varying ECM combinations that exhibited synergistic influence, indicating a strong positive effect of COL1 and a negative interaction effect of COL1·LM on both albumin and urea secretion. These findings illustrate the broad application potential of this biofabrication approach in identifying optimal ECM composition(s) for engineering 3D tissue, and elucidating defined ECM cues for tissue engineering and regenerative medicine.

Myosin-X and talin modulate integrin activity at filopodia tips

Filopodia assemble unique integrin-adhesion complexes to sense the extracellular matrix. However, the mechanisms of integrin regulation in filopodia are poorly defined. Here, we report that active integrins accumulate at the tip of myosin-X (MYO10)-positive filopodia, while inactive integrins are uniformly distributed. We identify talin and MYO10 as the principal integrin activators in filopodia. In addition, deletion of MYO10's FERM domain, or mutation of its β1-integrin-binding residues, reveals MYO10 as facilitating integrin activation, but not transport, in filopodia. However, MYO10's isolated FERM domain alone cannot activate integrins, potentially because of binding to both integrin tails. Finally, because a chimera construct generated by swapping MYO10-FERM by talin-FERM enables integrin activation in filopodia, our data indicate that an integrin-binding FERM domain coupled to a myosin motor is a core requirement for integrin activation in filopodia. Therefore, we propose a two-step integrin activation model in filopodia: receptor tethering by MYO10 followed by talin-mediated integrin activation.

Above the Matrix: Functional Roles for Apically Localized Integrins

Integrins are transmembrane proteins that are most typically thought of as integrating adhesion to the extracellular matrix with intracellular signaling and cell regulation. Traditionally, integrins are found at basolateral and lateral cell surfaces where they facilitate binding to the ECM and intercellular adhesion through cytosolic binding partners that regulate organization of actin microfilaments. However, evidence is accumulating that integrins also are apically localized, either endogenously or due to an exogenous stimulus. Apically localized integrins have been shown to regulate several processes by interacting with proteins such as connexins, tight junction proteins, and polarity complex proteins. Integrins can also act as receptors to mediate endocytosis. Here we review these newly appreciated roles for integrins localized to the apical cell surface.

Structural insights into integrin α5β1 opening by fibronectin ligand

Integrin α₅β₁ is a major fibronectin receptor critical for cell migration. Upon complex formation, fibronectin and α₅β₁ undergo conformational changes. While this is key for cell-tissue connections, its mechanism is unknown. Here, we report cryo-electron microscopy structures of native human α₅β₁ with fibronectin to 3.1-angstrom resolution, and in its resting state to 4.6-angstrom resolution. The α₅β₁-fibronectin complex revealed simultaneous interactions at the arginine-glycine-aspartate loop, the synergy site, and a newly identified binding site proximal to adjacent to metal ion-dependent adhesion site, inducing the translocation of helix α1 to secure integrin opening. Resting α₅β₁ adopts an incompletely bent conformation, challenging the model of integrin sharp bending inhibiting ligand binding. Our biochemical and structural analyses showed that affinity of α₅β₁ for fibronectin is increased with manganese ions (Mn²⁺) while adopting the half-bent conformation, indicating that ligand-binding affinity does not depend on conformation, and α₅β₁ opening is induced by ligand-binding.

Toggle switch residues control allosteric transitions in bacterial adhesins by participating in a concerted repacking of the protein core

Critical molecular events that control conformational transitions in most allosteric proteins are ill-defined. The mannose-specific FimH protein of Escherichia coli is a prototypic bacterial adhesin that switches from an 'inactive' low-affinity state (LAS) to an 'active' high-affinity state (HAS) conformation allosterically upon mannose binding and mediates shear-dependent catch bond adhesion. Here we identify a novel type of antibody that acts as a kinetic trap and prevents the transition between conformations in both directions. Disruption of the allosteric transitions significantly slows FimH's ability to associate with mannose and blocks bacterial adhesion under dynamic conditions. FimH residues critical for antibody binding form a compact epitope that is located away from the mannose-binding pocket and is structurally conserved in both states. A larger antibody-FimH contact area is identified by NMR and contains residues Leu-34 and Val-35 that move between core-buried and surface-exposed orientations in opposing directions during the transition. Replacement of Leu-34 with a charged glutamic acid stabilizes FimH in the LAS conformation and replacement of Val-35 with glutamic acid traps FimH in the HAS conformation. The antibody is unable to trap the conformations if Leu-34 and Val-35 are replaced with a less bulky alanine. We propose that these residues act as molecular toggle switches and that the bound antibody imposes a steric block to their reorientation in either direction, thereby restricting concerted repacking of side chains that must occur to enable the conformational transition. Residues homologous to the FimH toggle switches are highly conserved across a diverse family of fimbrial adhesins. Replacement of predicted switch residues reveals that another E. coli adhesin, galactose-specific FmlH, is allosteric and can shift from an inactive to an active state. Our study shows that allosteric transitions in bacterial adhesins depend on toggle switch residues and that an antibody that blocks the switch effectively disables adhesive protein function.

A DARPin targeting activated Mac-1 is a novel diagnostic tool and potential anti-inflammatory agent in myocarditis, sepsis and myocardial infarction

The monocyte β₂-integrin Mac-1 is crucial for leukocyte–endothelium interaction, rendering it an attractive therapeutic target for acute and chronic inflammation. Using phage display, a Designed-Ankyrin-Repeat-Protein (DARPin) was selected as a novel binding protein targeting and blocking the α_M I-domain, an activation-specific epitope of Mac-1. This DARPin, named F7, specifically binds to activated Mac-1 on mouse and human monocytes as determined by flow cytometry. Homology modelling and docking studies defined distinct interaction sites which were verified by mutagenesis. Intravital microscopy showed reduced leukocyte–endothelium adhesion in mice treated with this DARPin. Using mouse models of sepsis, myocarditis and ischaemia/reperfusion injury, we demonstrate therapeutic anti-inflammatory effects. Finally, the activated Mac-1-specific DARPin is established as a tool to detect monocyte activation in patients receiving extra-corporeal membrane oxygenation, as well as suffering from sepsis and ST-elevation myocardial infarction. The activated Mac-1-specific DARPin F7 binds preferentially to activated monocytes, detects inflammation in critically ill patients, and inhibits monocyte and neutrophil function as an efficient new anti-inflammatory agent.

Natural Killer Cell Integrins and Their Functions in Tissue Residency

Integrins are transmembrane receptors associated with adhesion and migration and are often highly differentially expressed receptors amongst natural killer cell subsets in microenvironments. Tissue resident natural killer cells are frequently defined by their differential integrin expression compared to other NK cell subsets, and integrins can further localize tissue resident NK cells to tissue microenvironments. As such, integrins play important roles in both the phenotypic and functional identity of NK cell subsets. Here we review the expression of integrin subtypes on NK cells and NK cell subsets with the goal of better understanding how integrin selection can dictate tissue residency and mediate function from the nanoscale to the tissue environment.

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.

Discovery of a new class of integrin antibodies for fibrosis

Lung fibrosis, or the scarring of the lung, is a devastating disease with huge unmet medical need. There are limited treatment options and its prognosis is worse than most types of cancer. We previously discovered that MK-0429 is an equipotent pan-inhibitor of αv integrins that reduces proteinuria and kidney fibrosis in a preclinical model. In the present study, we further demonstrated that MK-0429 significantly inhibits fibrosis progression in a bleomycin-induced lung injury model. In search of newer integrin inhibitors for fibrosis, we characterized monoclonal antibodies discovered using Adimab's yeast display platform. We identified several potent neutralizing integrin antibodies with unique human and mouse cross-reactivity. Among these, Ab-31 blocked the binding of multiple αv integrins to their ligands with IC50s comparable to those of MK-0429. Furthermore, both MK-0429 and Ab-31 suppressed integrin-mediated cell adhesion and latent TGFβ activation. In IPF patient lung fibroblasts, TGFβ treatment induced profound αSMA expression in phenotypic imaging assays and Ab-31 demonstrated potent in vitro activity at inhibiting αSMA expression, suggesting that the integrin antibody is able to modulate TGFβ action though mechanisms beyond the inhibition of latent TGFβ activation. Together, our results highlight the potential to develop newer integrin therapeutics for the treatment of fibrotic lung diseases.

RAB5A promotes the formation of filopodia in pancreatic cancer cells via the activation of cdc42 and β1-integrin

Filopodia are slender actin-rich plasma membrane protrusions that function to drive cell migration and invasion. Despite the observation of defective filopodia formation in many malignant tumors, the regulation mechanism remained unknown to date. In the present study, for the first time, we demonstrate that RAB5A, a Rab GTPase family protein, is a potent regulator of filopodia formation in pancreatic cancer cells. High expression of RAB5A was associated with filopodia formation and migration in pancreatic cancer cells. Overexpression of RAB5A promoted filopodia formation and migration in CF Pac-1 cells. In contrast, down-regulation of RAB5A expression in SW1990 cells with a high endogenous RAB5A expression level impeded the formation of filopodia. Further analysis indicated that RAB5A was required for cdc42 activation in CF Pac-1 and SW1990 cells. Moreover, to investigate the underlying mechanism by which the activation of cdc42 mediates RAB5A-induced filopodia formation, the active state of β1-integrin was examined in cells with different expression levels of RAB5A. We observed that RAB5A regulated the accumulation of the active β1-integrin. We demonstrated that down-regulation of the expression of β1-integrin strongly suppressed filopodia formation and cdc42 activation mediated by RAB5A. These results indicate the important role of RAB5A in the regulation of filopodia formation in pancreatic cancer cells, which is dependent on the activation of cdc42 and β1-integrin.

Mapping the Endothelial Cell S-Sulfhydrome Highlights the Crucial Role of Integrin Sulfhydration in Vascular Function

BACKGROUND

In vascular endothelial cells, cysteine metabolism by the cystathionine γ lyase (CSE), generates hydrogen sulfide-related sulfane sulfur compounds (H₂S_n), that exert their biological actions via cysteine S-sulfhydration of target proteins. This study set out to map the "S-sulfhydrome" (ie, the spectrum of proteins targeted by H₂S_n) in human endothelial cells.

METHODS

Liquid chromatography with tandem mass spectrometry was used to identify S-sulfhydrated cysteines in endothelial cell proteins and β3 integrin intraprotein disulfide bond rearrangement. Functional studies included endothelial cell adhesion, shear stress-induced cell alignment, blood pressure measurements, and flow-induced vasodilatation in endothelial cell-specific CSE knockout mice and in a small collective of patients with endothelial dysfunction.

RESULTS

Three paired sample sets were compared: (1) native human endothelial cells isolated from plaque-free mesenteric arteries (CSE activity high) and plaque-containing carotid arteries (CSE activity low); (2) cultured human endothelial cells kept under static conditions or exposed to fluid shear stress to decrease CSE expression; and (3) cultured endothelial cells exposed to shear stress to decrease CSE expression and treated with solvent or the slow-releasing H₂S_n donor, SG1002. The endothelial cell "S-sulfhydrome" consisted of 3446 individual cysteine residues in 1591 proteins. The most altered family of proteins were the integrins and focusing on β3 integrin in detail we found that S-sulfhydration affected intraprotein disulfide bond formation and was required for the maintenance of an extended-open conformation of the β leg. β3 integrin S-sulfhydration was required for endothelial cell mechanotransduction in vitro as well as flow-induced dilatation in murine mesenteric arteries. In cultured cells, the loss of S-sulfhydration impaired interactions between β3 integrin and Gα13 (guanine nucleotide-binding protein subunit α 13), resulting in the constitutive activation of RhoA (ras homolog family member A) and impaired flow-induced endothelial cell realignment. In humans with atherosclerosis, endothelial function correlated with low H₂S_n generation, impaired flow-induced dilatation, and failure to detect β3 integrin S-sulfhydration, all of which were rescued after the administration of an H₂S_n supplement.

CONCLUSIONS

Vascular disease is associated with marked changes in the S-sulfhydration of endothelial cell proteins involved in mediating responses to flow. Short-term H₂S_n supplementation improved vascular reactivity in humans highlighting the potential of interfering with this pathway to treat vascular disease.

Biological Relevance of RGD-Integrin Subtype-Specific Ligands in Cancer

Integrins are heterodimeric transmembrane proteins able to connect cells with the micro-environment. They represent a family of receptors involved in almost all the hallmarks of cancer. Integrins recognizing the Arg-Gly-Asp (RGD) peptide in their natural extracellular matrix ligands have been particularly investigated as tumoral therapeutic targets. In the last 30 years, intense research has been dedicated to designing specific RGD-like ligands able to discriminate selectively the different RGD-recognizing integrins. Chemists' efforts have led to the proposition of modified peptide or peptidomimetic libraries to be used for tumor targeting and/or tumor imaging. Here we review, from the biological point of view, the rationale underlying the need to clearly delineate each RGD-integrin subtype by selective tools. We describe the complex roles of RGD-integrins (mainly the most studied αvβ3 and α5β1 integrins) in tumors, the steps towards selective ligands and the current usefulness of such ligands. Although the impact of integrins in cancer is well acknowledged, the biological characteristics of each integrin subtype in a specific tumor are far from being completely resolved. Selective ligands might help us to reconsider integrins as therapeutic targets in specific clinical settings.

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.

Thiol switches in membrane proteins - Extracellular redox regulation in cell biology

Redox-mediated signal transduction depends on the enzymatic production of second messengers such as hydrogen peroxide, nitric oxide and hydrogen sulfite, as well as specific, reversible redox modifications of cysteine-residues in proteins. So-called thiol switches induce for instance conformational changes in specific proteins that regulate cellular pathways e.g., cell metabolism, proliferation, migration, gene expression and inflammation. Reduction, oxidation and disulfide isomerization are controlled by oxidoreductases of the thioredoxin family, including thioredoxins, glutaredoxins, peroxiredoxins and protein dsisulfide isomerases. These proteins are located in different cellular compartments, interact with substrates and catalyze specific reactions. Interestingly, some of these proteins are released by cells. Their extracellular functions and generally extracellular redox control have been widely underestimated. Here, we give an insight into extracellular redox signaling, extracellular thiol switches and their regulation by secreted oxidoreductases and thiol-isomerases, a topic whose importance has been scarcely studied so far, likely due to methodological limitations. We focus on the secreted redox proteins and characterized thiol switches in the ectodomains of membrane proteins, such as integrins and the metalloprotease ADAM17, which are among the best-characterized proteins and discuss their underlying mechanisms and biological implications.

Essential Functions of Glycans in Human Epithelia Dissected by a CRISPR-Cas9-Engineered Human Organotypic Skin Model

The glycome undergoes characteristic changes during histogenesis and organogenesis, but our understanding of the importance of select glycan structures for tissue formation and homeostasis is incomplete. Here, we present a human organotypic platform that allows genetic dissection of cellular glycosylation capacities and systematic interrogation of the roles of distinct glycan types in tissue formation. We used CRISPR-Cas9 gene targeting to generate a library of 3D organotypic skin tissues that selectively differ in their capacity to produce glycan structures on the main types of N- and O-linked glycoproteins and glycolipids. This tissue library revealed distinct changes in skin formation associated with a loss of features for all tested glycoconjugates. The organotypic skin model provides phenotypic cues for the distinct functions of glycoconjugates and serves as a unique resource for further genetic dissection and identification of the specific structural features involved. The strategy is also applicable to other organotypic tissue models.

Dissection of α4β7 integrin regulation by Rap1 using novel conformation-specific monoclonal anti-β7 antibodies

Integrin activation is associated with conformational regulation. In this study, we developed a system to evaluate conformational changes in α₄β₇ integrin. We first inserted the PA tag into the plexin-semaphorin-integrin (PSI) domain of β₇ chain, which reacted with an anti-PA tag antibody (NZ-1) in an Mn²⁺-dependent manner. The small GTPase Rap1 deficiency, as well as chemokine stimulation and the introduction of the active form of Rap1, Rap1V12, enhanced the binding of NZ-1 to the PA-tagged mutant integrin, and increased the binding affinity to mucosal addressing cell adhesion molecule-1 (MAdCAM-1). Furthermore, we generated two kinds of hybridomas producing monoclonal antibodies (mAbs) that recognized Mn²⁺-dependent epitopes of β₇. Both epitopes were exposed to bind to mAbs on the cells by the introduction of Rap1V12. Although one epitope in the PSI domain of β₇ was exposed on Rap1-deficienct cells, the other epitope in the hybrid domain of β₇ was not. These data indicate that the conversion of Rap1-GDP to GTP exerts two distinct effects stepwise on the conformation of α₄β₇. The induction of colitis by Rap1-deficient CD4⁺ effector/memory T cells suggests that the removal of constraining effect by Rap1-GDP on α₄β₇ is sufficient for homing of these pathogenic T cells into colon lamina propria (LP).

A functionally neutral single chain antibody to measure beta-1 integrin uptake and recycling

Integrin‐mediated cell adhesion and signaling are critical for many physiological processes. The dynamic turnover of integrins and their associated adhesion complexes through endocytic and recycling pathways has emerged as an important mechanism for controlling cell migration and invasion in cancer. Thus, the regulation of integrin trafficking and how this may be altered by disease‐specific molecular mechanisms has generated considerable interest. However, current tools available to study integrin trafficking may cause artifacts and/or do not provide adequate kinetic information. Here, we report the generation of a functionally neutral and monovalent single chain antibody to quantitatively and qualitatively measure β1 integrin trafficking in cells. Our novel probe can be used in a variety of assays and allows for the biochemical characterization of rapid recycling of endogenous integrins. We also demonstrate its potential utility in live cell imaging, providing proof of principle to guide future integrin probe design.

The dynamic turnover of integrins through endocytic trafficking pathways has emerged as a key mechanism for cell migration and invasion. Lakoduk et al. report the generation of a functionally neutral and monovalent antibody‐based probe to track and measure endogenous beta‐1 integrin uptake and fast recycling in multiple cell types. Their tool, scFv^K20, serves as proof of principle inspiration for future integrin probe design.

Cell matrix adhesion in cell migration

The ability of cells to migrate is a fundamental physiological process involved in embryonic development, tissue homeostasis, immune surveillance and wound healing. In order for cells to migrate, they must interact with their environment using adhesion receptors, such as integrins, and form specialized adhesion complexes that mediate responses to different extracellular cues. In this review, we discuss the role of integrin adhesion complexes (IACs) in cell migration, highlighting the layers of regulation that are involved, including intracellular signalling cascades, mechanosensing and reciprocal feedback to the extracellular environment. We also discuss the role of IACs in extracellular matrix remodeling and how they impact upon cell migration.

Cell-Surface Proteomics Identifies Differences in Signaling and Adhesion Protein Expression between Naive and Primed Human Pluripotent Stem Cells

Naive and primed human pluripotent stem cells (hPSC) provide valuable models to study cellular and molecular developmental processes. The lack of detailed information about cell-surface protein expression in these two pluripotent cell types prevents an understanding of how the cells communicate and interact with their microenvironments. Here, we used plasma membrane profiling to directly measure cell-surface protein expression in naive and primed hPSC. This unbiased approach quantified over 1,700 plasma membrane proteins, including those involved in cell adhesion, signaling, and cell interactions. Notably, multiple cytokine receptors upstream of JAK-STAT signaling were more abundant in naive hPSC. In addition, functional experiments showed that FOLR1 and SUSD2 proteins are highly expressed at the cell surface in naive hPSC but are not required to establish human naive pluripotency. This study provides a comprehensive stem cell proteomic resource that uncovers differences in signaling pathway activity and has identified new markers to define human pluripotent states.

Conformationally active integrin endocytosis and traffic: why, where, when and how?

Spatiotemporal control of integrin-mediated cell adhesion to the extracellular matrix (ECM) is critical for physiological and pathological events in multicellular organisms, such as embryonic development, angiogenesis, platelet aggregation, leukocytes extravasation, and cancer cell metastatic dissemination. Regulation of integrin adhesive function and signaling relies on the modulation of both conformation and traffic. Indeed, integrins exist in a dynamic equilibrium between a bent/closed (inactive) and an extended/open (active) conformation, respectively endowed with low and high affinity for ECM ligands. Increasing evidence proves that, differently to what hypothesized in the past, detachment from the ECM and conformational inactivation are not mandatory for integrin to get endocytosed and trafficked. Specific transmembrane and cytosolic proteins involved in the control of ECM proteolytic fragment-bound active integrin internalization and recycling exist. In the complex masterplan that governs cell behavior, active integrin traffic is key to the turnover of ECM polymers and adhesion sites, the polarized secretion of endogenous ECM proteins and modifying enzymes, the propagation of motility and survival endosomal signals, and the control of cell metabolism.

Mitotic cell responses to substrate topological cues are independent of the molecular nature of adhesion

Correct selection of the cell division axis is important for cell differentiation, tissue and organ morphogenesis, and homeostasis. Both integrins, which mediate interactions with extracellular matrix (ECM) components such as fibronectin, and cadherins, which mediate interactions between cells, are implicated in the determination of spindle orientation. We found that both cadherin- and integrin-based adhesion resulted in cell divisions parallel to the attachment plane and elicited identical spindle responses to spatial adhesive cues. This suggests that adhesion topology provides purely mechanical spatial cues that are independent of the molecular nature of the interaction or signaling from adhesion complexes. We also demonstrated that cortical integrin activation was indispensable for correct spindle orientation on both cadherin and fibronectin substrates. These data suggest that spindle orientation responses to adhesion topology are primarily a result of force anisotropy on the cell cortex and show that integrins play a central role in this process that is distinct from their role in cell-ECM interactions.

In situ antibody phage display yields optimal inhibitors of integrin α11/β1

Integrins are transmembrane multi-conformation receptors that mediate interactions with the extracellular matrix. In cancer, integrins influence metastasis, proliferation, and survival. Collagen-binding integrin-α11/β1, a marker of aggressive tumors that is involved in stroma-tumor crosstalk, may be an attractive target for anti-cancer therapeutic antibodies. We performed selections with phage-displayed synthetic antibody libraries for binding to either purified integrin-α11/β1 or in situ on live cells. The in-situ strategy yielded many diverse antibodies, and strikingly, most of these antibodies did not recognize purified integrin-α11/β1. Conversely, none of the antibodies selected for binding to purified integrin-α11/β1 were able to efficiently recognize native cell-surface antigen. Most importantly, only the in-situ selection yielded functional antibodies that were able to compete with collagen-I for binding to cell-surface integrin-α11/β1, and thus inhibited cell adhesion. In-depth characterization of a subset of in situ-derived clones as full-length immunoglobulins revealed high affinity cellular binding and inhibitory activities in the single-digit nanomolar range. Moreover, the antibodies showed high selectivity for integrin-α11/β1 with minimal cross-reactivity for close homologs. Taken together, our findings highlight the advantages of in-situ selections for generation of anti-integrin antibodies optimized for recognition and inhibition of native cell-surface proteins, and our work establishes general methods that could be extended to many other membrane proteins.

A Novel αIIbβ3 Antagonist from Snake Venom Prevents Thrombosis without Causing Bleeding

Life-threatening thrombocytopenia and bleeding, common side effects of clinically available α_IIbβ₃ antagonists, are associated with the induction of ligand-induced integrin conformational changes and exposure of ligand-induced binding sites (LIBSs). To address this issue, we examined intrinsic mechanisms and structure–activity relationships of purified disintegrins, from Protobothrops flavoviridis venom (i.e., Trimeresurus flavoviridis), TFV-1 and TFV-3 with distinctly different pro-hemorrhagic tendencies. TFV-1 with a different α_IIbβ₃ binding epitope from that of TFV-3 and chimeric 7E3 Fab, i.e., Abciximab, decelerates α_IIbβ₃ ligation without causing a conformational change in α_IIbβ₃, as determined with the LIBS antibody, AP5, and the mimetic, drug-dependent antibody (DDAb), AP2, an inhibitory monoclonal antibody raised against α_IIbβ₃. Consistent with their different binding epitopes, a combination of TFV-1 and AP2 did not induce FcγRIIa-mediated activation of the ITAM–Syk–PLCγ2 pathway and platelet aggregation, in contrast to the clinical antithrombotics, abciximab, eptifibatide, and disintegrin TFV-3. Furthermore, TFV-1 selectively inhibits Gα₁₃-mediated platelet aggregation without affecting talin-driven clot firmness, which is responsible for physiological hemostatic processes. At equally efficacious antithrombotic dosages, TFV-1 caused neither severe thrombocytopenia nor bleeding in FcγRIIa-transgenic mice. Likewise, it did not induce hypocoagulation in human whole blood in the rotational thromboelastometry (ROTEM) assay used in perioperative situations. In contrast, TFV-3 and eptifibatide exhibited all of these hemostatic effects. Thus, the α_IIbβ₃ antagonist, TFV-1, efficaciously prevents arterial thrombosis without adversely affecting hemostasis.

Stem Cells and Plastic Surgery

Pluripotent stem cells can help recreate a variety of different tissues. Stem cells are already in use in a variety of ways in the medical field but plastic surgeons have particular interest because of the constant need to produce additional tissue or mold existing tissue. More and more commercial products are being marketed with far-reaching goals and some with proven and promising results. In this article, the authors discuss the basic science behind stem cells and the theories on how they work. They then discuss some active uses of stem cells that should be understood by all plastic surgeons. The reader should then have an understanding and basis to evaluate new technologies and commercial products as they develop.

Changes in Cholesterol Level Alter Integrin Sequestration in Raft-Mimicking Lipid Mixtures

The influence of cholesterol (CHOL) level on integrin sequestration in raft-mimicking lipid mixtures forming coexisting liquid-ordered (l_o) and liquid-disordered (l_d) lipid domains is investigated using complementary, single-molecule-sensitive, confocal detection methods. Systematic analysis of membrane protein distribution in such a model membrane environment demonstrates that variation of CHOL level has a profound influence on l_o-l_d sequestration of integrins, thereby exhibiting overall l_d preference in the absence of ligands and l_o affinity upon vitronectin addition. Accompanying photon-counting histogram analysis of integrins in the different model membrane mixtures shows that the observed changes of integrin sequestration in response to variations of membrane CHOL level are not associated with altering integrin oligomerization states. Instead, our experiments suggest that the strong CHOL dependence of integrin sequestration can be attributed to CHOL-mediated changes of lipid packing and bilayer thickness in coexisting l_o and l_d domains, highlighting the significance of a biophysical mechanism of CHOL-mediated regulation of integrin sequestration. We envision that this model membrane study may help clarify the influence of CHOL in integrin functionality in plasma membranes, thus providing further insight into the role of lipid heterogeneities in membrane protein distribution and function in a cellular membrane environment.

β1 integrin is a sensor of blood flow direction

Endothelial cell (EC) sensing of fluid shear stress direction is a critical determinant of vascular health and disease. Unidirectional flow induces EC alignment and vascular homeostasis, whereas bidirectional flow has pathophysiological effects. ECs express several mechanoreceptors that respond to flow, but the mechanism for sensing shear stress direction is poorly understood. We determined, by using in vitro flow systems and magnetic tweezers, that β1 integrin is a key sensor of force direction because it is activated by unidirectional, but not bidirectional, shearing forces. β1 integrin activation by unidirectional force was amplified in ECs that were pre-sheared in the same direction, indicating that alignment and β1 integrin activity has a feedforward interaction, which is a hallmark of system stability. En face staining and EC-specific genetic deletion studies in the murine aorta revealed that β1 integrin is activated and is essential for EC alignment at sites of unidirectional flow but is not activated at sites of bidirectional flow. In summary, β1 integrin sensing of unidirectional force is a key mechanism for decoding blood flow mechanics to promote vascular homeostasis.This article has an associated First Person interview with the first author of the paper.