Talin and kindlin: the one-two punch in integrin activation

Polymerizing laminins in development, health, and disease

Polymerizing laminins are multi-domain basement membrane (BM) glycoproteins that self-assemble into cell-anchored planar lattices to establish the initial BM scaffold. Nidogens, collagen-IV and proteoglycans then bind to the scaffold at different domain loci to create a mature BM. The LN domains of adjacent laminins bind to each other to form a polymer node, while the LG domains attach to cytoskeletal-anchoring integrins and dystroglycan, as well as to sulfatides and heparan sulfates. The polymer node, the repeating unit of the polymer scaffold, is organized into a near-symmetrical triskelion. The structure, recently solved by cryo-electron microscopy in combination with AlphaFold2 modeling and biochemical studies, reveals how the LN surface residues interact with each other and how mutations cause failures of self-assembly in an emerging group of diseases, the LN-lamininopathies, that include LAMA2-related dystrophy and Pierson syndrome.

Tensin Regulates Fundamental Biological Processes by Interacting with Integrins of Tonsil-Derived Mesenchymal Stem Cells

Human tonsil-derived mesenchymal stem cells (TMSCs) have a superior proliferation rate and differentiation potential compared to adipose-tissue-derived MSCs (AMSCs) or bone-marrow-derived MSCs (BMSCs). TMSCs exhibit a significantly higher expression of the tensin3 gene (TNS3) than AMSCs or BMSCs. TNS is involved in cell adhesion and migration by binding to integrin beta-1 (ITG β1) in focal adhesion. Here, we investigated the roles of four TNS isoforms, including TNS3 and their relationship with integrin in various biological processes of TMSCs. Suppressing TNS1 and TNS3 significantly decreased the cell count. The knockdown of TNS1 and TNS3 increased the gene and protein expression levels of p16, p19, and p21. TNS1 and TNS3 also have a significant effect on cell migration. Transfecting with siRNA TNS3 significantly reduced Oct4, Nanog, and Sox-2 levels. Conversely, when TNS4 was silenced, Oct4 and Sox-2 levels significantly increase. TNS1 and TNS3 promote osteogenic and adipogenic differentiation, whereas TNS4 inhibits adipogenic differentiation of TMSCs. TNS3 is involved in the control of focal adhesions by regulating integrin. Thus, TNS enables TMSCs to possess a higher proliferative capacity and differentiation potential than other MSCs. Notably, TNS3 plays a vital role in TMSC biology by regulating ITGβ1 activity.

Talin variant P229S compromises integrin activation and associates with multifaceted clinical symptoms

Adhesion of cells to the extracellular matrix (ECM) must be exquisitely coordinated to enable development and tissue homeostasis. Cell-ECM interactions are regulated by multiple signalling pathways that coordinate the activation state of the integrin family of ECM receptors. The protein talin is pivotal in this process, and talin's simultaneous interactions with the cytoplasmic tails of the integrins and the plasma membrane are essential to enable robust, dynamic control of integrin activation and cell-ECM adhesion. Here, we report the identification of a de novo heterozygous c.685C>T (p.Pro229Ser) variant in the TLN1 gene from a patient with a complex phenotype. The mutation is located in the talin head region at the interface between the F2 and F3 domains. The characterization of this novel p.P229S talin variant reveals the disruption of adhesion dynamics that result from disturbance of the F2-F3 domain interface in the talin head. Using biophysical, computational and cell biological techniques, we find that the variant perturbs the synergy between the integrin-binding F3 and the membrane-binding F2 domains, compromising integrin activation, adhesion and cell migration. Whilst this remains a variant of uncertain significance, it is probable that the dysregulation of adhesion dynamics we observe in cells contributes to the multifaceted clinical symptoms of the patient and may provide insight into the multitude of cellular processes dependent on talin-mediated adhesion dynamics.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

Kindlin-2 promotes Src-mediated tyrosine phosphorylation of androgen receptor and contributes to breast cancer progression

Androgen receptor (AR) signaling plays important roles in breast cancer progression. We show here that Kindlin-2, a focal adhesion protein, is critically involved in the promotion of AR signaling and breast cancer progression. Kindlin-2 physically associates with AR and Src through its two neighboring domains, namely F1 and F0 domains, resulting in formation of a Kindlin-2-AR-Src supramolecular complex and consequently facilitating Src-mediated AR Tyr-534 phosphorylation and signaling. Depletion of Kindlin-2 was sufficient to suppress Src-mediated AR Tyr-534 phosphorylation and signaling, resulting in diminished breast cancer cell proliferation and migration. Re-expression of wild-type Kindlin-2, but not AR-binding-defective or Src-binding-defective mutant forms of Kindlin-2, in Kindlin-2-deficient cells restored AR Tyr-534 phosphorylation, signaling, breast cancer cell proliferation and migration. Furthermore, re-introduction of phosphor-mimic mutant AR-Y534D, but not wild-type AR reversed Kindlin-2 deficiency-induced inhibition of AR signaling and breast cancer progression. Finally, using a genetic knockout strategy, we show that ablation of Kindlin-2 from mammary tumors in mouse significantly reduced AR Tyr-534 phosphorylation, breast tumor progression and metastasis in vivo. Our results suggest a critical role of Kindlin-2 in promoting breast cancer progression and shed light on the molecular mechanism through which it functions in this process.

Force-enhanced biophysical connectivity of platelet β3 integrin signaling through Talin is predicted by steered molecular dynamics simulations

Platelet β3-integrin signaling through Talin is crucial in platelet transmembrane signaling, activation, adhesion, spreading and aggregation, and remains unclear in mechano-microenvironments. In order to examine Talin-β3 integrin biophysical connectivity, a series of “ramp-clamp” steered molecular dynamics (SMD) simulations were performed on complex of F3 domain of Talin and cytoplasmic tail of β3 integrin to imitate different force-loads in platelet. Pull-induced allostery of the hydrophobic pocket in F3 domain might markedly enhance complex rupture-force (> 150pN) and slow down breakage of the complex; the complex should mechano-stable for its conformational conservation under loads (≤ 80pN); increasing force below 60pN would decrease the complex dissociation probability, and force-induced extension of β5 strand on Talin and binding site residues, ASP⁷⁴⁰ and ALA⁷⁴² as well as Asn⁷⁴⁴, on β3-integrin were responsible for the force-enhanced linkage of the Talin-β3 integrin. Force might enhance biophysical connectivity of β3-integrin signaling through Talin by a catch bond mechanism, which be mediated by the force-induced allostery of complex at clamped stage. This work provides a novel insight into the force-regulated transmembrane β3-integrin signaling and its molecular basis for platelet activation, and exhibited a potential power of the present computer strategy in predicting mechanical regulation on ligand-receptor interaction under loads.

Platelets drive fibronectin fibrillogenesis using integrin αIIbβ3

Platelets interact with multiple adhesion proteins during thrombogenesis, yet little is known about their ability to assemble fibronectin matrix. In vitro three-dimensional superresolution microscopy complemented by biophysical and biochemical methods revealed fundamental insights into how platelet contractility drives fibronectin fibrillogenesis. Platelets adhering to thrombus proteins (fibronectin and fibrin) versus basement membrane components (laminin and collagen IV) pull fibronectin fibrils along their apical membrane versus underneath their basal membrane, respectively. In contrast to other cell types, platelets assemble fibronectin nanofibrils using αIIbβ3 rather than α5β1 integrins. Apical fibrillogenesis correlated with a stronger activation of integrin-linked kinase, higher platelet traction forces, and a larger tension in fibrillar-like adhesions compared to basal fibrillogenesis. Our findings have potential implications for how mechanical thrombus integrity might be maintained during remodeling and vascular repair.

Targeting the RT loop of Src SH3 in Platelets Prevents Thrombosis without Compromising Hemostasis

Conventional antiplatelet agents indiscriminately inhibit both thrombosis and hemostasis, and the increased bleeding risk thus hampers their use at more aggressive dosages to achieve adequate effect. Blocking integrin αIIbβ3 outside-in signaling by separating the β3/Src interaction, yet to be proven in vivo, may nonetheless resolve this dilemma. Identification of a specific druggable target for this strategy remains a fundamental challenge as Src SH3 is known to be responsible for binding to not only integrin β3 but also the proteins containing the PXXP motif. In vitro and in vivo mutational analyses show that the residues, especially E97, in the RT loop of Src SH3 are critical for interacting with β3. DCDBS84, a small molecule resulting from structure-based virtual screening, is structurally validated to be directed toward the projected target. It specifically disrupts β3/Src interaction without affecting canonical PXXP binding and thus inhibits the outside-in signaling-regulated platelet functions. Treatment of mice with DCDBS84 causes a profound inhibition of thrombosis, equivalent to that induced by extremely high doses of αIIbβ3 antagonist, but does not compromise primary hemostasis. Specific targets are revealed for a preferential inhibition of thrombosis that may lead to new classes of potent antithrombotics without hemorrhagic side effects.

Intrinsically disordered proteins play diverse roles in cell signaling

Abstract

Signaling pathways allow cells to detect and respond to a wide variety of chemical (e.g. Ca²⁺ or chemokine proteins) and physical stimuli (e.g., sheer stress, light). Together, these pathways form an extensive communication network that regulates basic cell activities and coordinates the function of multiple cells or tissues. The process of cell signaling imposes many demands on the proteins that comprise these pathways, including the abilities to form active and inactive states, and to engage in multiple protein interactions. Furthermore, successful signaling often requires amplifying the signal, regulating or tuning the response to the signal, combining information sourced from multiple pathways, all while ensuring fidelity of the process. This sensitivity, adaptability, and tunability are possible, in part, due to the inclusion of intrinsically disordered regions in many proteins involved in cell signaling. The goal of this collection is to highlight the many roles of intrinsic disorder in cell signaling. Following an overview of resources that can be used to study intrinsically disordered proteins, this review highlights the critical role of intrinsically disordered proteins for signaling in widely diverse organisms (animals, plants, bacteria, fungi), in every category of cell signaling pathway (autocrine, juxtacrine, intracrine, paracrine, and endocrine) and at each stage (ligand, receptor, transducer, effector, terminator) in the cell signaling process. Thus, a cell signaling pathway cannot be fully described without understanding how intrinsically disordered protein regions contribute to its function. The ubiquitous presence of intrinsic disorder in different stages of diverse cell signaling pathways suggest that more mechanisms by which disorder modulates intra- and inter-cell signals remain to be discovered.

Graphical abstract

Adhesion and proliferation properties of type I collagen-derived peptide for possible use in skin tissue engineering application

The surface properties of three-dimensional scaffolds are improved by coating or covalently linking certain adhesion-promoting proteins or peptides. In the present study, the effect of type I collagen-derived peptide (GKNGDDGEA) on adhesion and proliferation of HaCaT keratinocytes and NIH3T3 murine fibroblast cell lines was studied to assess its suitability for possible skin tissue engineering applications. Cell adhesion and proliferation of HaCaT and NIH3T3 were found to be enhanced by peptide coating. The optimum peptide coating densities to obtain the best cell adhesion and proliferation were found to be 0.827 µmoles/cm² and 0.62 µmoles/cm² for HaCaT and NIH3T3, respectively. Cell adhesion, in the presence of anti-integrin α1 antibody, inhibited attachment of NIH3T3 cells indicating the involvement of integrin α1 receptor. However, the attachment of HaCaT cells was not affected by anti-integrin treatment. The higher expression of paxillin confirmed the effect of the peptide in mediating focal adhesion kinases (FAKs) in cell adhesion and proliferation. Gene expression analysis was performed on cell migration proteins like Rho, Rac, Cdc42, integrin receptor α1, and β1, and the extracellular matrix modulating proteins like MMP2, TIMP, and COL1A1 to validate their role on the peptide-mediated cell proliferation. Immunofluorescence analysis showed the distribution and localisation of phospho-FAK on cells cultured on the peptide-coated surfaces. Results support the role of peptides in enhancing cell adhesion and proliferation properties.

Megakaryocyte Cytoskeletal Proteins in Platelet Biogenesis and Diseases

Thrombopoiesis governs the formation of blood platelets in bone marrow by converting megakaryocytes into long, branched proplatelets on which individual platelets are assembled. The megakaryocyte cytoskeleton responds to multiple microenvironmental cues, including chemical and mechanical stimuli, sustaining the platelet shedding. During the megakaryocyte's life cycle, cytoskeletal networks organize cell shape and content, connect them physically and biochemically to the bone marrow vascular niche, and enable the release of platelets into the bloodstream. While the basic building blocks of the cytoskeleton have been studied extensively, new sets of cytoskeleton regulators have emerged as critical components of the dynamic protein network that supports platelet production. Understanding how the interaction of individual molecules of the cytoskeleton governs megakaryocyte behavior is essential to improve knowledge of platelet biogenesis and develop new therapeutic strategies for inherited thrombocytopenias caused by alterations in the cytoskeletal genes.

Kindlin-3 disrupts an intersubunit association in the integrin LFA1 to trigger positive feedback activation by Rap1 and talin1

Integrin activation by the intracellular adaptor proteins talin1 and kindlin-3 is essential for lymphocyte adhesion. These adaptors cooperatively control integrin activation through bidirectional (inside-out and outside-in) activation signals. Using single-molecule measurements, we revealed the distinct dynamics of talin1 and kindlin-3 interactions with the integrin LFA1 (αLβ2) and their functions in LFA1 activation and LFA1-mediated adhesion. The kinetics of talin1 binding to the tail of the β2 subunit corresponded to those of LFA1 binding to its ligand ICAM1. ICAM1 binding induced transient interactions between the membrane-proximal cytoplasmic region of the β2 subunit with an N-terminal domain of kindlin-3, leading to disruption of the association between the integrin subunits (the α/β clasp) and unbending of the ectodomains of the α/β heterodimer. These conformational changes promoted high-affinity talin1 binding to the β2 tail that required the talin rod domain and the actomyosin cytoskeleton. Inside-out signaling induced by the GTPase Rap1 did not markedly stabilize the binding of talin1 and kindlin-3 to LFA1. In contrast, ligand-induced outside-in signaling, the stabilization of open LFA1 conformers, or shear force substantially altered the dynamics of talin1 and kindlin-3 association with LFA1 and enhanced both Rap1 and LFA1 activation. In migrating lymphocytes, asymmetrical distribution of talin1 and kindlin-3 correlated with the maturation of LFA1 from a low-affinity conformation at the leading edge to a high-affinity conformation in the adherent mid-body. Our results suggest that kindlin-3 spatiotemporally mediates a positive feedback circuit of LFA1 activation to control dynamic adhesion and migration of lymphocytes.

Endothelial connexin-integrin crosstalk in vascular inflammation

Cardiovascular diseases including blood vessel disorders represent a major cause of death globally. The essential roles played by local and systemic vascular inflammation in the pathogenesis of cardiovascular diseases have been increasingly recognized. Vascular inflammation triggers the aberrant activation of endothelial cells, which leads to the functional and structural abnormalities in vascular vessels. In addition to humoral mediators such as pro-inflammatory cytokines and prostaglandins, the alteration of physical and mechanical microenvironment - including vascular stiffness and shear stress - modify the gene expression profiles and metabolic profiles of endothelial cells via mechano-transduction pathways, thereby contributing to the pathogenesis of vessel disorders. Notably, connexins and integrins crosstalk each other in response to the mechanical stress, and, thereby, play an important role in regulating the mechano-transduction of endothelial cells. Here, we provide an overview on how the inter-play between connexins and integrins in endothelial cells unfold during the mechano-transduction in vascular inflammation.

The Activation and Regulation of β2 Integrins in Phagocytes and Phagocytosis

Phagocytes, which include neutrophils, monocytes, macrophages, and dendritic cells, protect the body by removing foreign particles, bacteria, and dead or dying cells. Phagocytic integrins are greatly involved in the recognition of and adhesion to specific antigens on cells and pathogens during phagocytosis as well as the recruitment of immune cells. β2 integrins, including αLβ2, αMβ2, αXβ2, and αDβ2, are the major integrins presented on the phagocyte surface. The activation of β2 integrins is essential to the recruitment and phagocytic function of these phagocytes and is critical for the regulation of inflammation and immune defense. However, aberrant activation of β2 integrins aggravates auto-immune diseases, such as psoriasis, arthritis, and multiple sclerosis, and facilitates tumor metastasis, making them double-edged swords as candidates for therapeutic intervention. Therefore, precise regulation of phagocyte activities by targeting β2 integrins should promote their host defense functions with minimal side effects on other cells. Here, we reviewed advances in the regulatory mechanisms underlying β2 integrin inside-out signaling, as well as the roles of β2 integrin activation in phagocyte functions.

Immunization and Drug Metabolizing Enzymes: Focus on Hepatic Cytochrome P450 3A

OBJECTIVE

Infectious disease emergencies like the 2013-2016 Ebola epidemic and the 2009 influenza and current SARS-CoV-2 pandemics illustrate that vaccines are now given to diverse populations with preexisting pathologies requiring pharmacological management. Many natural biomolecules (steroid hormones, fatty acids, vitamins) and ~60% of prescribed medications are processed by hepatic cytochrome P450 (CYP) 3A4. The objective of this work was to determine the impact of infection and vaccines on drug metabolism.

METHODS

The impact of an adenovirus-based vaccine expressing Ebola glycoprotein (AdEBO) and H1N1 and H3N2 influenza viruses on hepatic CYP 3A4 and associated nuclear receptors was evaluated in human hepatocytes (HC-04 cells) and in mice.

RESULTS

CYP3A activity was suppressed by 55% in mice 24 h after administration of mouse-adapted H1N1, while ˂10% activity remained in HC-04 cells after infection with H1N1 and H3N2 due to global suppression of cellular translation capacity, indicated by reduction (70%, H1N1, 56%, H3N2) of phosphorylated eukaryotic translation initiation factor 4e (eIF4E). AdEBO suppressed CYP3A activity in vivo (44%) and in vitro (26%) 24 hours after infection.

CONCLUSION

As the clinical evaluation of vaccines for SARS-CoV-2 and other global pathogens rise, studies to evaluate the impact of new vaccines and emerging pathogens on CYP3A4 and other metabolic enzymes are warranted to avoid therapeutic failures that could further compromise the public health during infectious disease emergencies.

Atypical structure and function of integrin αV β8

Integrins are heterodimeric transmembrane proteins that play important roles in various biological processes. Most integrins serve as adhesion molecules and transmit bidirectional signaling across the cell membrane through global conformational changes from the bent closed to the extended open conformation. However, integrin β₈ is distinctive in structure and function. Its cytoplasmic domain lacks the conserved protein-binding sequence, which is important in transmitting inside-out signals, suggesting that integrin β₈ may have a different activation mechanism or lack such signaling. In addition, the ligand-binding or activating metal ion Mn²⁺ does not induce a global conformational change in integrin β₈ . It may have only one conformation, that is, an extended, closed conformation, but with high affinity for ligands under physiological conditions, and is, therefore, considered an atypical integrin member. The extended structure and high ligand-binding affinity of integrin α_v β₈ make it ideal for encountering and binding ligands expressed on an opposing cell or in the extracellular matrix. In this review, we summarize the progress in integrin β₈ research with a focus on its distinctive function and structure among integrin members.

Kindlin-2 Mediates Mechanical Activation of Cardiac Myofibroblasts

We identify the focal adhesion protein kindlin-2 as player in a novel mechanotransduction pathway that controls profibrotic cardiac fibroblast to myofibroblast activation. Kindlin-2 is co-upregulated with the myofibroblast marker α-smooth muscle actin (α-SMA) in fibrotic rat hearts and in human cardiac fibroblasts exposed to fibrosis-stiff culture substrates and pro-fibrotic TGF-β1. Stressing fibroblasts using ferromagnetic microbeads, stretchable silicone membranes, and cell contraction agonists all result in kindlin-2 translocation to the nucleus. Overexpression of full-length kindlin-2 but not of kindlin-2 missing a putative nuclear localization sequence (∆NLS kindlin-2) results in increased α-SMA promoter activity. Downregulating kindlin-2 with siRNA leads to decreased myofibroblast contraction and reduced α-SMA expression, which is dependent on CC(A/T)-rich GG(CArG) box elements in the α-SMA promoter. Lost myofibroblast features under kindlin-2 knockdown are rescued with wild-type but not ∆NLS kindlin-2, indicating that myofibroblast control by kindlin-2 requires its nuclear translocation. Because kindlin-2 can act as a mechanotransducer regulating the transcription of α-SMA, it is a potential target to interfere with myofibroblast activation in tissue fibrosis.

Phosphorylation of RIAM by src promotes integrin activation by unmasking the PH domain of RIAM

Integrin activation controls cell adhesion, migration, invasion, and extracellular matrix remodeling. RIAM (RAP1-GTP-interacting adaptor molecule) is recruited by activated RAP1 to the plasma membrane (PM) to mediate integrin activation via an inside-out signaling pathway. This process requires the association of the pleckstrin homology (PH) domain of RIAM with the membrane PIP2. We identify a conserved intermolecular interface that masks the PIP2-binding site in the PH domains of RIAM. Our data indicate that phosphorylation of RIAM by Src family kinases disrupts this PH-mediated interface, unmasks the membrane PIP2-binding site, and promotes integrin activation. We further demonstrate that this process requires phosphorylation of Tyr267 and Tyr427 in the RIAM PH domain by Src. Our data reveal an unorthodox regulatory mechanism of small GTPase effector proteins by phosphorylation-dependent PM association of the PH domain and provide new insights into the link between Src kinases and integrin signaling.

Comparative Proteomic Analysis Identifies EphA2 as a Specific Cell Surface Marker for Wharton's Jelly-Derived Mesenchymal Stem Cells

Wharton’s jelly-derived mesenchymal stem cells (WJ-MSCs) are a valuable tool in stem cell research due to their high proliferation rate, multi-lineage differentiation potential, and immunotolerance properties. However, fibroblast impurity during WJ-MSCs isolation is unavoidable because of morphological similarities and shared surface markers. Here, a proteomic approach was employed to identify specific proteins differentially expressed by WJ-MSCs in comparison to those by neonatal foreskin and adult skin fibroblasts (NFFs and ASFs, respectively). Mass spectrometry analysis identified 454 proteins with a transmembrane domain. These proteins were then compared across the different cell-lines and categorized based on their cellular localizations, biological processes, and molecular functions. The expression patterns of a selected set of proteins were further confirmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR), Western blotting, and immunofluorescence assays. As anticipated, most of the studied proteins had common expression patterns. However, EphA2, SLC25A4, and SOD2 were predominantly expressed by WJ-MSCs, while CDH2 and Talin2 were specific to NFFs and ASFs, respectively. Here, EphA2 was established as a potential surface-specific marker to distinguish WJ-MSCs from fibroblasts and for prospective use to prepare pure primary cultures of WJ-MSCs. Additionally, CDH2 could be used for a negative-selection isolation/depletion method to remove neonatal fibroblasts contaminating preparations of WJ-MSCs.

Role of Kindlin-2 in cancer progression and metastasis

Cancer metastasis is a complex and multistep process whereby cancer cells escape the confines of the primary site to establish a new residency at distant sites. This multistep process is also known as the invasion-metastasis cascade. The biological and molecular mechanisms that control the invasion-metastasis cascade, which ultimately leads to the spread of cancer cells into distant sites, remain poorly understood. Kindlin-2 (K2) belongs to the 4.1-ezrin-ridixin-moesin (FERM) domain family of proteins, which interact with the cytoplasmic tails of β-integrin subunits, leading to the activation of extensive biological functions. These biological functions include cell migration, differentiation, cancer initiation, development, and invasion. In this review, we will discuss the various molecular signaling pathways that are regulated by K2 during the invasion-metastasis cascade of cancer tumors. These signaling pathways include TGFβ, Wnt/β-Catenin, Hedgehog, p53 and senescence, and cancer stem cell (CSC) maintenance. We will also discuss the molecular signaling pathways that regulate K2 function both at the transcriptional and the posttranslational levels. Finally, we will consider molecular mechanisms to specifically target K2 as novel therapeutic options for cancer treatment.

Integrin β3 Deficiency Results in Hypertriglyceridemia via Disrupting LPL (Lipoprotein Lipase) Secretion

OBJECTIVE

Integrin β3 is implicated in numerous biological processes such as its relevance to blood triglyceride, yet whether β3 deficiency affects this metabolic process remains unknown. Approach and Results: We showed that the Chinese patients with β3-deficient Glanzmann thrombasthenia had a 2-fold higher serum triglyceride level together with a lower serum LPL (lipoprotein lipase) level than those with an αIIb deficiency or healthy subjects. The β3 knockout mice recapitulated these phenotypic features. The elevated plasma triglyceride level was due to impaired LPL-mediated triglyceride clearance caused by a disrupted LPL secretion. Further analysis revealed that β3 directly bound LPL via a juxtamembrane TIH (threonine isoleucine histidine)^720-722 motif in its cytoplasmic domain and functioned as an adaptor protein by interacting with LPL and PKD (protein kinase D) to form the PKD/β3/LPL complex that is required for β3-mediated LPL secretion. Furthermore, the impaired triglyceride clearance in β3 knockout mice could be corrected by adeno-associated virus serotype 9 (AAV9)-mediated delivery of wild-type but not TIH^720-722-mutated β3 genes.

CONCLUSIONS

This study reveals a hypertriglyceridemia in both β3-deficient Chinese patients and mice and provides novel insights into the molecular mechanisms of the significant roles of β3 in LPL secretion and triglyceride metabolism, drawing attention to the metabolic consequences in patients with β3-deficient Glanzmann thrombasthenia.

Kindlin Assists Talin to Promote Integrin Activation

Integrin αIIbβ3 is a predominant type of integrin abundantly expressed on the surface of platelets and its activation regulates the process of thrombosis. Talin and kindlin are cytoplasmic proteins that bind to integrin and modulate its affinity for extracellular ligands. Although the molecular details of talin-mediated integrin activation are known, the mechanism of kindlin involvement in this process remains elusive. Here, we demonstrate that the interplay between talin and kindlin promotes integrin activation. Our all-atomic molecular dynamics simulations on complete transmembrane and cytoplasmic domains of integrin αIIbβ3, talin1 F2/F3 subdomains, and the kindlin2 FERM domain in an explicit lipid-water environment over a microsecond timescale unraveled the role of kindlin as an enhancer of the talin interaction with the membrane proximal region of β-integrin. The cooperation of kindlin with talin results in a complete disruption of salt bridges between R995 on αIIb and D723/E726 on β3. Furthermore, kindlin modifies the molecular mechanisms of inside-out activation by decreasing the crossing angle between transmembrane helices of integrin αIIbβ3, which eventually results in parallelization of integrin dimer. In addition, our control simulation featuring integrin in complex with kindlin reveals that kindlin binding is not sufficient for unclasping the inner-membrane and outer-membrane interactions of integrin dimer, thus ruling out the possibility of solitary action of kindlin in integrin activation.

The Extracellular Matrix: An Accomplice in Gastric Cancer Development and Progression

The extracellular matrix (ECM) is a dynamic and highly organized tissue structure, providing support and maintaining normal epithelial architecture. In the last decade, increasing evidence has emerged demonstrating that alterations in ECM composition and assembly strongly affect cellular function and behavior. Even though the detailed mechanisms underlying cell-ECM crosstalk are yet to unravel, it is well established that ECM deregulation accompanies the development of many pathological conditions, such as gastric cancer. Notably, gastric cancer remains a worldwide concern, representing the third most frequent cause of cancer-associated deaths. Despite increased surveillance protocols, patients are usually diagnosed at advanced disease stages, urging the identification of novel diagnostic biomarkers and efficient therapeutic strategies. In this review, we provide a comprehensive overview regarding expression patterns of ECM components and cognate receptors described in normal gastric epithelium, pre-malignant lesions, and gastric carcinomas. Important insights are also discussed for the use of ECM-associated molecules as predictive biomarkers of the disease or as potential targets in gastric cancer.

Clathrin-containing adhesion complexes

An understanding of the mechanisms whereby cell adhesion complexes (ACs) relay signals bidirectionally across the plasma membrane is necessary to interpret the role of adhesion in regulating migration, differentiation, and growth. A range of AC types has been defined, but to date all have similar compositions and are dependent on a connection to the actin cytoskeleton. Recently, a new class of AC has been reported that normally lacks association with both the cytoskeleton and integrin-associated adhesome components, but is rich in components of the clathrin-mediated endocytosis machinery. The characterization of this new type of adhesion structure, which is emphasized by mitotic cells and cells in long-term culture, identifies a hitherto underappreciated link between the adhesion machinery and clathrin structures at the plasma membrane. While this discovery has implications for how ACs are assembled and disassembled, it raises many other issues. Consequently, to increase awareness within the field, and stimulate research, we explore a number of the most significant questions below.

GGA2 and RAB13 promote activity-dependent β1-integrin recycling

β1-integrins mediate cell-matrix interactions and their trafficking is important in the dynamic regulation of cell adhesion, migration and malignant processes, including cancer cell invasion. Here, we employ an RNAi screen to characterize regulators of integrin traffic and identify the association of Golgi-localized gamma ear-containing Arf-binding protein 2 (GGA2) with β1-integrin, and its role in recycling of active but not inactive β1-integrin receptors. Silencing of GGA2 limits active β1-integrin levels in focal adhesions and decreases cancer cell migration and invasion, which is in agreement with its ability to regulate the dynamics of active integrins. By using the proximity-dependent biotin identification (BioID) method, we identified two RAB family small GTPases, i.e. RAB13 and RAB10, as novel interactors of GGA2. Functionally, RAB13 silencing triggers the intracellular accumulation of active β1-integrin, and reduces integrin activity in focal adhesions and cell migration similarly to GGA2 depletion, indicating that both facilitate active β1-integrin recycling to the plasma membrane. Thus, GGA2 and RAB13 are important specificity determinants for integrin activity-dependent traffic.

Adhesive dynamics simulations quantitatively predict effects of kindlin-3 deficiency on T-cell homing

Leukocyte adhesion is important for the proper functioning of the immune system. While leukocyte homing is mediated by adhesion receptors, the activation of these receptors is modulated by intracellular signaling molecules. In Leukocyte Adhesion Deficiency Type 3, the loss of the kindlin-3 prevents the activation of Leukocyte Function-associated Antigen-1 (LFA-1), which leads to a defect in adhesion, causing recurrent infections and bleeding disorders. Here, we use Integrated Signaling Adhesive Dynamics, a computer model of leukocyte rolling and adhesion combined with a simulated intracellular signaling cascade, to predict the response of T cells to depletion of kindlin-3. Our model predicts that cell adhesion is hypersensitive to the amount of kindlin-3 in the cell, while the rolling velocity is independent of kindlin-3 concentration. In addition, our simulation predicted that the time to stop, an important metric of adhesion, would increase with decreasing kindlin-3 expression. These predictions were confirmed experimentally in experiments using Jurkat cells with reduced expression of kindlin-3. These results suggest that Adhesive Dynamics is a versatile tool for quantifying adhesion in the immune response and predicting the effects of engineering cellular components.

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

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

Integrin Signaling in Cancer: Mechanotransduction, Stemness, Epithelial Plasticity, and Therapeutic Resistance

Integrins mediate cell adhesion and transmit mechanical and chemical signals to the cell interior. Various mechanisms deregulate integrin signaling in cancer, empowering tumor cells with the ability to proliferate without restraint, to invade through tissue boundaries, and to survive in foreign microenvironments. Recent studies have revealed that integrin signaling drives multiple stem cell functions, including tumor initiation, epithelial plasticity, metastatic reactivation, and resistance to oncogene- and immune-targeted therapies. Here, we discuss the mechanisms leading to the deregulation of integrin signaling in cancer and its various consequences. We place emphasis on novel functions, determinants of context dependency, and mechanism-based therapeutic opportunities.

Platelet integrin αIIbβ3: signal transduction, regulation, and its therapeutic targeting

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

Connexins and Integrins in Exosomes

Connexins and integrins, the two structurally and functionally distinct families of transmembrane proteins, have been shown to be inter-connected by various modes of cross-talk in cells, such as direct physical coupling via lateral contact, indirect physical coupling via actin and actin-binding proteins, and functional coupling via signaling cascades. This connexin-integrin cross-talk exemplifies a biologically important collaboration between channels and adhesion receptors in cells. Exosomes are biological lipid-bilayer nanoparticles secreted from virtually all cells via endosomal pathways into the extracellular space, thereby mediating intercellular communications across a broad range of health and diseases, including cancer progression and metastasis, infection and inflammation, and metabolic deregulation. Connexins and integrins are embedded in the exosomal membranes and have emerged as critical regulators of intercellular communication. This concise review article will explain and discuss recent progress in better understanding the roles of connexins, integrins, and their cross-talk in cells and exosomes.

Integrin activity in neuronal connectivity

The formation of correct synaptic structures and neuronal connections is paramount for normal brain development and a functioning adult brain. The integrin family of cell adhesion receptors and their ligands play essential roles in the control of several processes regulating neuronal connectivity - including neurite outgrowth, the formation and maintenance of synapses, and synaptic plasticity - that are affected in neurodevelopmental disorders, such as autism spectrum disorders (ASDs) and schizophrenia. Many ASD- and schizophrenia-associated genes are linked to alterations in the genetic code of integrins and associated signalling pathways. In non-neuronal cells, crosstalk between integrin-mediated adhesions and the actin cytoskeleton, and the regulation of integrin activity (affinity for extracellular ligands) are widely studied in healthy and pathological settings. In contrast, the roles of integrin-linked pathways in the central nervous system remains less well defined. In this Review, we will provide an overview of the known pathways that are regulated by integrin-ECM interaction in developing neurons and in adult brain. We will also describe recent advances in the identification of mechanisms that regulate integrin activity in neurons, and highlight the interesting emerging links between integrins and neurodevelopment.

Differential Binding of Active and Inactive Integrin to Talin

Bi-directional signaling of integrins plays an important role in platelet and leukocyte function. Talin plays a key role in integrin bi-directional signaling and its binding to integrin is highly regulated. The precise regulation of the recruitment and binding of talin to integrin is still being elucidated. In particular, the recruitment of talin to integrin is controlled by the RAP-1 and RIAM/lamellipodin signaling axis and the affinity between talin and integrin is regulated by the conformation or protease cleavage of talin. However, whether the binding between integrin and talin is also regulated by integrin conformation has not been thoroughly explored before. In this work, we used biochemical binding assays to study the potential role of integrin conformational changes in integrin-talin interactions. Constitutively active integrin αIIbb3 binds markedly stronger to talin than inactive αIIbb3. Inactive αIIbb3 markedly increases its binding to talin once activated, regardless of how αIIbb3 is activated. Further, the increased binding to talin is b3 tail dependent. Our results suggest that integrin conformation is another regulatory mechanism for integrin-talin interaction.

Selectins and chemokines use shared and distinct signals to activate β2 integrins in neutrophils

Rolling neutrophils receive signals while engaging P- and E-selectin and chemokines on inflamed endothelium. Selectin signaling activates β2 integrins to slow rolling velocities. Chemokine signaling activates β2 integrins to cause arrest. Despite extensive study, key aspects of these signaling cascades remain unresolved. Using complementary in vitro and in vivo assays, we found that selectin and chemokine signals in neutrophils triggered Rap1a-dependent and phosphatidylinositol-4-phosphate 5-kinase γ (PIP5Kγ90)-dependent pathways that induce integrin-dependent slow rolling and arrest. Interruption of both pathways, but not either pathway alone, blocked talin-1 recruitment to and activation of integrins. An isoform of PIP5Kγ90 lacking the talin-binding domain (PIP5Kγ87) could not activate integrins. Chemokines, but not selectins, used phosphatidylinositol-4,5-bisphosphate 3-kinase γ (PI3Kγ) in cooperation with Rap1a to mediate integrin-dependent slow rolling (at low chemokine concentrations), as well as arrest (at high chemokine concentrations). High levels of chemokines activated β2 integrins without selectin signals. When chemokines were limiting, they synergized with selectins to activate β2 integrins.

Forecast of actin-binding proteins as the oncotarget in osteosarcoma - a review of mechanism, diagnosis and therapy

Osteosarcoma (OS) is the most common bone malignant tumor with a high rate of lung metastasis and principally emerges in children and adolescents. Although neoadjuvant chemotherapy is widely used around the world, a high rate of chemoresistance occurs and frequently generates a poor prognosis. Therefore, finding a new appropriate prognostic marker for OS is a valuable research direction, which will give patients a better chance to receive proper therapy. Actin-binding proteins (ABPs) are a group of proteins that interact with actin cytoskeleton and play a crucial role in the regulation of the cell motility and morphology in eukaryotes. Meanwhile, ABPs also act as a bridge between the cytomembrane and nucleus, which transmit the outside-in and inside-out signals in cytoplasm. Furthermore, ABPs alter the dynamic structure of actin and regulate the invasion and metastasis of cancer. Hence, ABPs have a wide application in predicting the prognosis, and may be new targets, in tumor therapy. This review focuses on a series of ABPs and discusses their modulatory functions. It provides a new insight into the classification of ABPs’ functions in the process of invasion and metastasis in OS and illuminates the potential ability in predicting the prognosis of OS patients.

Nanodiscs: A Controlled Bilayer Surface for the Study of Membrane Proteins

The study of membrane proteins and receptors presents many challenges to researchers wishing to perform biophysical measurements to determine the structure, function, and mechanism of action of such components. In most cases, to be fully functional, proteins and receptors require the presence of a native phospholipid bilayer. In addition, many complex multiprotein assemblies involved in cellular communication require an integral membrane protein as well as a membrane surface for assembly and information transfer to soluble partners in a signaling cascade. Incorporation of membrane proteins into Nanodiscs renders the target soluble and provides a native bilayer environment with precisely controlled composition of lipids, cholesterol, and other components. Likewise, Nanodiscs provide a surface of defined area useful in revealing lipid specificity and affinities for the assembly of signaling complexes. In this review, we highlight several biophysical techniques made possible through the use of Nanodiscs.

Mechanotransduction of matrix stiffness in regulation of focal adhesion size and number: reciprocal regulation of caveolin-1 and β1 integrin

Focal adhesion (FA) assembly, mediated by integrin activation, responds to matrix stiffness; however, the underlying mechanisms are unclear. Here, we showed that β1 integrin and caveolin-1 (Cav1) levels were decreased with declining matrix stiffness. Soft matrix selectively downregulated β1 integrin by endocytosis and subsequent lysosomal degradation. Disruption of lipid rafts with methyl-β-cyclodextrin or nystatin, or knockdown of Cav1 by siRNA decreased cell spreading, FA assembly, and β1 integrin protein levels in cells cultured on stiff matrix. Overexpression of Cav1, particularly the phospho-mimetic mutant Cav1-Y14D, averted soft matrix-induced decreases in β1 integrin protein levels, cell spreading, and FA assembly in NMuMG cells. Interestingly, overexpression of an auto-clustering β1 integrin hindered soft matrix-induced reduction of Cav1 and cell spreading, which suggests a reciprocal regulation between β1 integrin and Cav1. Finally, co-expression of this auto-clustering β1 integrin and Cav1-Y14D synergistically enhanced cell spreading, and FA assembly in HEK293T cells cultured on either stiff ( > G Pa) or soft (0.2 kPa) matrices. Collectively, these results suggest that matrix stiffness governs the expression of β1 integrin and Cav1, which reciprocally control each other, and subsequently determine FA assembly and turnover.

Talin regulates integrin β1-dependent and -independent cell functions in ureteric bud development

Kidney collecting system development requires integrin-dependent cell-extracellular matrix interactions. Integrins are heterodimeric transmembrane receptors consisting of α and β subunits; crucial integrins in the kidney collecting system express the β1 subunit. The β1 cytoplasmic tail has two NPxY motifs that mediate functions by binding to cytoplasmic signaling and scaffolding molecules. Talins, scaffolding proteins that bind to the membrane proximal NPxY motif, are proposed to activate integrins and to link them to the actin cytoskeleton. We have defined the role of talin binding to the β1 proximal NPxY motif in the developing kidney collecting system in mice that selectively express a Y-to-A mutation in this motif. The mice developed a hypoplastic dysplastic collecting system. Collecting duct cells expressing this mutation had moderate abnormalities in cell adhesion, migration, proliferation and growth factor-dependent signaling. In contrast, mice lacking talins in the developing ureteric bud developed kidney agenesis and collecting duct cells had severe cytoskeletal, adhesion and polarity defects. Thus, talins are essential for kidney collecting duct development through mechanisms that extend beyond those requiring binding to the β1 integrin subunit NPxY motif.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

α-Actinin Induces a Kink in the Transmembrane Domain of β3-Integrin and Impairs Activation via Talin

Integrin-mediated signaling is crucial for cell-substrate adhesion and can be triggered from both intra- and extracellular interactions. Although talin binding is sufficient for inside-out activation of integrin, other cytoplasmic proteins such as α-actinin and filamin can directly interfere with talin-mediated integrin activation. Specifically, α-actinin plays distinct roles in regulating αIIbβ3 versus α5β1 integrin. It has been shown that α-actinin competes with talin for binding to the cytoplasmic tail of β3-integrin, whereas it cooperates with talin for activating integrin α5β1. In this study, molecular dynamics simulations were employed to compare and contrast molecular mechanisms of αIIbβ3 and α5β1 activation in the presence and absence of α-actinin. Our results suggest that α-actinin impairs integrin signaling by both undermining talin binding to the β3-integrin cytoplasmic tail and inducing a kink in the transmembrane domain of β3-integrin. Furthermore, we showed that α-actinin promote talin association with β1-integrin by restricting the motion of the cytoplasmic tail and reducing the entropic barrier for talin binding. Taken together, our results showed that the interplay between talin and α-actinin regulates signal transmission via controlling the conformation of the transmembrane domain and altering natural response modes of integrins in a type-specific manner.

Integrin αIIbβ3 outside-in signaling

Integrin α_IIbβ₃ is a highly abundant heterodimeric platelet receptor that can transmit information bidirectionally across the plasma membrane, and plays a critical role in hemostasis and thrombosis. Upon platelet activation, inside-out signaling pathways increase the affinity of α_IIbβ₃ for fibrinogen and other ligands. Ligand binding and integrin clustering subsequently stimulate outside-in signaling, which initiates and amplifies a range of cellular events driving essential platelet processes such as spreading, thrombus consolidation, and clot retraction. Integrin α_IIbβ₃ has served as an excellent model for the study of integrin biology, and it has become clear that integrin outside-in signaling is highly complex and involves a vast array of enzymes, signaling adaptors, and cytoskeletal components. In this review, we provide a concise but comprehensive overview of α_IIbβ₃ outside-in signaling, focusing on the key players involved, and how they cooperate to orchestrate this critical aspect of platelet biology. We also discuss gaps in the current understanding of α_IIbβ₃ outside-in signaling and highlight avenues for future investigation.

Looking "Under the Hood" of Cellular Mechanotransduction with Computational Tools: A Systems Biomechanics Approach across Multiple Scales

Signal modulation has been developed in living cells throughout evolution to promote utilizing the same machinery for multiple cellular functions. Chemical and mechanical modules of signal transmission and transduction are interconnected and necessary for organ development and growth. However, due to the high complexity of the intercommunication of physical intracellular connections with biochemical pathways, there are many missing details in our overall understanding of mechanotransduction processes, i.e., the process by which mechanical signals are converted to biochemical cascades. Cell-matrix adhesions are mechanically coupled to the nucleus through the cytoskeleton. This modulated and tightly integrated network mediates the transmission of mechanochemical signals from the extracellular matrix to the nucleus. Various experimental and computational techniques have been utilized to understand the basic mechanisms of mechanotransduction, yet many aspects have remained elusive. Recently, in silico experiments have made important contributions to the field of mechanobiology. Herein, computational modeling efforts devoted to understanding integrin-mediated mechanotransduction pathways are reviewed, and an outlook is presented for future directions toward using suitable computational approaches and developing novel techniques for addressing important questions in the field of mechanotransduction.

Smurf1 inhibits integrin activation by controlling Kindlin-2 ubiquitination and degradation

Integrin-mediated cellular functions require integrin activation by the proteins Kindlin-2 and Talin. Wei et al. show that the E3 ligase Smurf1 permits precise modulation of integrin-mediated adhesion by interacting with and promoting Kindlin-2 ubiquitination and degradation.

Integrin activation is an indispensable step for various integrin-mediated biological functions. Kindlin-2 is known to coactivate integrins with Talin; however, molecules that restrict integrin activation are elusive. Here, we demonstrate that the E3 ubiquitin ligase Smurf1 controls the amount of Kindlin-2 protein in cells and hinders integrin activation. Smurf1 interacts with and promotes Kindlin-2 ubiquitination and degradation. Smurf1 selectively mediates degradation of Kindlin-2 but not Talin, leading to inhibition of αIIbβ3 integrin activation in Chinese hamster ovary cells and β1 integrin activation in fibroblasts. Enhanced activation of β1 integrin was found in Smurf1-knockout mouse embryonic fibroblasts, which correlates with an increase in Kindlin-2 protein levels. Similarly, a reciprocal relationship between Smurf1 and Kindlin-2 protein levels is found in tissues from colon cancer patients, suggesting that Smurf1 mediates Kindlin-2 degradation in vivo. Collectively, we demonstrate that Smurf1 acts as a brake for integrin activation by controlling Kindlin-2 protein levels, a new mechanism that permits precise modulation of integrin-mediated cellular functions.

Integrin signaling in atherosclerosis

Atherosclerosis, a chronic lipid-driven inflammatory disease affecting large arteries, represents the primary cause of cardiovascular disease in the world. The local remodeling of the vessel intima during atherosclerosis involves the modulation of vascular cell phenotype, alteration of cell migration and proliferation, and propagation of local extracellular matrix remodeling. All of these responses represent targets of the integrin family of cell adhesion receptors. As such, alterations in integrin signaling affect multiple aspects of atherosclerosis, from the earliest induction of inflammation to the development of advanced fibrotic plaques. Integrin signaling has been shown to regulate endothelial phenotype, facilitate leukocyte homing, affect leukocyte function, and drive smooth muscle fibroproliferative remodeling. In addition, integrin signaling in platelets contributes to the thrombotic complications that typically drive the clinical manifestation of cardiovascular disease. In this review, we examine the current literature on integrin regulation of atherosclerotic plaque development and the suitability of integrins as potential therapeutic targets to limit cardiovascular disease and its complications.

Role of the Helix in Talin F3 Domain (F3 Helix) in Talin-Mediated Integrin Activation

Increases in ligand binding to cellular integrins (activation) play an important role in platelet and leukocyte function. Talin is necessary in vivo and sufficient in vitro for integrin αIIbβ3 activation. The precise mechanisms by which talin activates integrin are still being elucidated. In particular, talin undergoes conformational changes (around the F3 helix) and inserts the F3 helix into lipid bilayer; however, the connection between this lipid-inserting mechanism of talin and talin's capacity to activate integrin has never been explored before. In this work, we used rational mutagenesis, modeled cell systems, and structural modeling to study the potential role of membrane-induced talin conformational changes in talin-mediated integrin activation. Mutations of the residues critical for talin F3 helix to insert into membrane completely abolished talin-mediated integrin activation without affecting the binding of talin to integrins. Furthermore, mutations of the lipid-binding sequences in talin F3 helix significantly reduced the capacity of talin to activate integrin. Our results suggest that the F3 helix may contribute to talin-mediated integrin activation.

ICAP-1 monoubiquitination coordinates matrix density and rigidity sensing for cell migration through ROCK2- MRCKα balance

Cell migration is a complex process requiring density and rigidity sensing of the microenvironment to adapt cell migratory speed through focal adhesion and actin cytoskeleton regulation. ICAP-1, a β1 integrin partner, is essential for ensuring integrin activation cycle and focal adhesion formation. We show that ICAP-1 is monoubiquitinated by Smurf1, preventing ICAP-1 binding to β1 integrin. The non-ubiquitinable form of ICAP-1 modifies β1 integrin focal adhesion organization and interferes with fibronectin density sensing. ICAP-1 is also required for adapting cell migration in response to substrate stiffness in a β1 integrin-independent manner. ICAP-1 monoubiquitination regulates rigidity sensing by increasing MRCKα-dependent cell contractility through myosin phosphorylation independently of substrate rigidity. We provide evidence that ICAP-1 monoubiquitination helps in switching from ROCK2-mediated to MRCKα-mediated cell contractility. ICAP-1 monoubiquitination serves as a molecular switch to coordinate extracellular matrix density and rigidity sensing thus acting as a critical modulator of cell migration and mechanosensing.