Kindlin-1 and -2 directly bind the C-terminal region of beta integrin cytoplasmic tails and exert integrin-specific activation effects

The role of FERMT2 in the tumor microenvironment and immunotherapy in pan-cancer using comprehensive single-cell and bulk sequencing

FERMT2 has been identified as a participant in integrin-linked kinase signaling pathways, influencing epithelial-mesenchymal transition and thereby affecting tumor initiation, progression, and invasion. While the character of FERMT2 in the tumor microenvironment (TME) as well as its implications for immunotherapy remain unclear. Thus, we conducted a comprehensive analysis to assess the prognostic significance of FERMT2 using Kaplan-Meier analysis. In addition, we employed enrichment analysis to uncover potential underlying molecular mechanisms. Using "Immunedeconv" package, we evaluated the immune characteristics of FERMT2 within TME. Furthermore, we determined the expression levels of FERMT2 in various cell types within TME, based on single-cell sequencing data. To confirm the co-expression of FERMT2 and markers of cancer-associated fibroblasts (CAFs), we performed multiplex immunofluorescence staining on tissue paraffin sections across various cancer types. Our analysis disclosed a significant correlation between elevated FERMT2 expression and unfavorable prognosis in specific cancer types. Furthermore, we identified a strong correlation between FERMT2 expression and diverse immune-related factors, including immune checkpoint molecules, immune cell infiltration, microsatellite instability (MSI), and tumor mutational burden (TMB). Additionally, there was a significant correlation between FERMT2 expression and immune-related pathways, particularly those associated with activating, migrating, and promoting the growth of fibroblasts in diverse cancer types. Interestingly, we observed consistent co-expression of FERMT2 in both malignant tumor cells and stromal cells, particularly within CAFs. Notably, our findings also indicated that FERMT2, in particular, exhibited elevated expression levels within tumor tissues and co-expressed with α-SMA in CAFs based on the multiplex immunofluorescence staining results.

Paxillin interactome identified by SILAC and label-free approaches coupled to TurboID sheds light on the compositions of focal adhesions in mouse embryonic stem cells

Self-renewal and differentiation of mouse embryonic stem cells (mESCs) are greatly affected by the extracellular matrix (ECM) environment; the composition and stiffness of which are sensed by the cells via integrin-associated focal adhesions (FAs) which link the cells to the ECM. Although FAs have been studied extensively in differentiated cells, their composition and function in mESCs are not as well elucidated. To gain more detailed knowledge of the molecular compositions of FAs in mESCs, we adopted the proximity-dependent biotinylation (BioID) proteomics approach. Paxillin, a known FA protein (FAP), is fused to the promiscuous biotin ligase TurboID as bait. We employed both SILAC- and label-free (LF)-based quantitative proteomics to strengthen as well as complement individual approach. The mass spectrometry data derived from SILAC and LF identified 38 and 443 proteins, respectively, with 35 overlapping candidates. Fifteen of these shared proteins are known FAPs based on literature-curated adhesome and 7 others are among the reported "meta-adhesome", suggesting the components of FAs are largely conserved between mESCs and differentiated cells. Furthermore, the LF data set contained an additional 18 literature-curated FAPs. Notably, the overlapped proteomics data failed to detect LIM-domain proteins such as zyxin family proteins, which suggests that FAs in mESCs are less mature than differentiated cells. Using the LF approach, we are able to identify PDLIM7, a LIM-domain protein, as a FAP in mESCs. This study illustrates the effectiveness of TurboID in mESCs. Importantly, we found that application of both SILAC and LF methods in combination allowed us to analyze the TurboID proteomics data in an unbiased, stringent and yet comprehensive manner.

Extracellular matrix-induced signaling pathways in mesenchymal stem/stromal cells

The extracellular matrix (ECM) is a crucial component of the stem cell microenvironment, or stem-cell niches, and contributes to the regulation of cell behavior and fate. Accumulating evidence indicates that different types of stem cells possess a large variety of molecules responsible for interactions with the ECM, mediating specific epigenetic rearrangements and corresponding changes in transcriptome profile. Signals from the ECM are crucial at all stages of ontogenesis, including embryonic and postnatal development, as well as tissue renewal and repair. The ECM could regulate stem cell transition from a quiescent state to readiness to perceive the signals of differentiation induction (competence) and the transition between different stages of differentiation (commitment). Currently, to unveil the complex networks of cellular signaling from the ECM, multiple approaches including screening methods, the analysis of the cell matrixome, and the creation of predictive networks of protein-protein interactions based on experimental data are used. In this review, we consider the existing evidence regarded the contribution of ECM-induced intracellular signaling pathways into the regulation of stem cell differentiation focusing on mesenchymal stem/stromal cells (MSCs) as well-studied type of postnatal stem cells totally depended on signals from ECM. Furthermore, we propose a system biology-based approach for the prediction of ECM-mediated signal transduction pathways in target cells. Video Abstract.

A specialized integrin-binding motif enables proTGF-β2 activation by integrin αVβ6 but not αVβ8

Activation of latent transforming growth factor (TGF)-β2 is incompletely understood. Unlike TGF-β1 and β3, the TGF-β2 prodomain lacks a seven-residue RGDLXX (L/I) integrin-recognition motif and is thought not to be activated by integrins. Here, we report the surprising finding that TGF-β2 contains a related but divergent 13-residue integrin-recognition motif (YTSGDQKTIKSTR) that specializes it for activation by integrin αVβ6 but not αVβ8. Both classes of motifs compete for the same binding site in αVβ6. Multiple changes in the longer motif underlie its specificity. ProTGF-β2 structures define interesting differences from proTGF-β1 and the structural context for activation by αVβ6. Some integrin-independent activation is also seen for proTGF-β2 and even more so for proTGF-β3. Our findings have important implications for therapeutics to αVβ6 in clinical trials for fibrosis, in which inhibition of TGF-β2 activation has not been anticipated.

Kindlin2 enables EphB/ephrinB bi-directional signaling to support vascular development

Direct contact between cells expressing either ephrin ligands or Eph receptor tyrosine kinase produces diverse developmental responses. Transmembrane ephrinB ligands play active roles in transducing bi-directional signals downstream of EphB/ephrinB interaction. However, it has not been well understood how ephrinB relays transcellular signals to neighboring cells and what intracellular effectors are involved. Here, we report that kindlin2 can mediate bi-directional ephrinB signaling through binding to a highly conserved NIYY motif in the ephrinB2 cytoplasmic tail. We show this interaction is important for EphB/ephrinB-mediated integrin activation in mammalian cells and for blood vessel morphogenesis during zebrafish development. A mixed two-cell population study revealed that kindlin2 (in ephrinB2-expressing cells) modulates transcellular EphB4 activation by promoting ephrinB2 clustering. This mechanism is also operative for EphB2/ephrinB1, suggesting that kindlin2-mediated regulation is conserved for EphB/ephrinB signaling pathways. Together, these findings show that kindlin2 enables EphB4/ephrinB2 bi-directional signal transmission.

Tension Enhances the Binding Affinity of β1 Integrin by Clamping Talin Tightly: An Insight from Steered Molecular Dynamics Simulations

Integrin activation is a predominant step for cell-cell and cell-ECM interactions. Talin and Kindlin are mechanosensitive adaptor proteins that bind to the integrin cytoplasmic tail and mediate integrin activation, cytoskeleton rearrangement, and focal adhesion assembly. However, knowledge about how Talin and Kindlin synergistically assist integrin activation remains unclear. Here, we performed so-called "ramp-clamp" SMD simulations, which modeled the mechanosignaling from Kindlin, to investigate the effect of tension on the interaction of the β1 integrin cytoplasmic tail with the Talin-F3 domain. The present results showed that mild but not excessive stretching enhanced the binding of integrin with Talin. This mechanical regulation on integrin affinity to Talin referred to an event cascade, in which under stretching, the integrin cytoplasmic tail adopted allostery in response to the mechanical stimulus, remodeling of integrin in favor of Talin-association ensued, and finally, a stable, close-knit complex was formed. In the cascade, the torsion angle transition of integrin was the cue for the stable interaction of the complex under tensile force. The present work suggested a model for Talin and Kindlin to synergistically activate integrin. It should help understand integrin activation and its mechanochemical regulation mechanism, integrin-related innate cellular immune responses, cell adhesion, cell-cell interaction, and integrin-related drug development.

Molecular basis for integrin adhesion receptor binding to p21-activated kinase 4 (PAK4)

Integrin adhesion receptors provide links between extracellular ligands and cytoplasmic signaling. Multiple kinases have been found to directly engage with integrin β tails, but the molecular basis for these interactions remain unknown. Here, we assess the interaction between the kinase domain of p21-activated kinase 4 (PAK4) and the cytoplasmic tail of integrin β5. We determine three crystal structures of PAK4-β5 integrin complexes and identify the PAK-binding site. This is a region in the membrane-proximal half of the β5 tail and confirmed by site-directed mutagenesis. The β5 tail engages the kinase substrate-binding groove and positions the non-phosphorylatable integrin residue Glu767 at the phosphoacceptor site. Consistent with this, integrin β5 is poorly phosphorylated by PAK4, and in keeping with its ability to occlude the substrate-binding site, weakly inhibits kinase activity. These findings demonstrate the molecular basis for β5 integrin-PAK4 interactions but suggest modifications in understanding the potential cellular role of this interaction.

Integrins in cardiac fibrosis

Cells sense mechanical stress and changes in their matrix environment through the integrins, a family of heterodimeric surface receptors that bind to extracellular matrix ligands and trigger cytoskeletal remodeling, while transducing a wide range of intracellular signals. Integrins have been extensively implicated in regulation of inflammation, repair and fibrosis in many different tissues. This review manuscript discusses the role of integrin-mediated cascades in myocardial fibrosis. In vitro studies have demonstrated that β1 and αv integrins play an important role in fibrogenic conversion of cardiac fibroblast, acting through direct stimulation of FAK/Src cascades, or via accentuation of growth factor signaling. Fibrogenic actions of αv integrins may be mediated, at least in part, through pericellular activation of latent TGF-β stores. In vivo evidence supporting the role of integrin heterodimers in fibrotic cardiac remodeling is limited to associative evidence, and to experiments using pharmacologic inhibitors, or global loss-of-function approaches. Studies documenting in vivo actions of integrins on fibroblasts using cell-specific strategies are lacking. Integrin effects on leukocytes may also contribute to the pathogenesis of fibrotic myocardial responses by mediating recruitment and activation of fibrogenic macrophages. The profile and role of integrins in cardiac fibrosis may be dependent on the underlying pathologic condition. Considering their cell surface localization and the availability of small molecule inhibitors, integrins may be attractive therapeutic targets for patients with heart failure associated with prominent fibrotic remodeling.

A gain-of-function filamin A mutation in mouse platelets induces thrombus instability

BACKGROUND

Filaminopathies A are rare disorders affecting the brain, intestine, or skeleton, characterized by dominant X-linked filamin A (FLNA) gene mutations. Macrothrombocytopenia with functionally defective platelets is frequent. We have described a filaminopathy A male patient, exhibiting a C-terminal frame-shift FLNa mutation (Berrou et al., Arterioscler Thromb Vasc Biol. 2017;37:1087-1097). Contrasting with female patients, this male patient exhibited gain of platelet functions, including increased platelet aggregation, integrin αIIbβ3 activation, and secretion at low agonist concentration, raising the issue of thrombosis risk.

OBJECTIVES

Our goal is to assess the thrombotic potential of the patient FLNa mutation in an in vivo model.

METHODS

We have established a mutant FlnA knock-in mouse model.

RESULTS

The mutant FlnA mouse platelets phenocopied patient platelets, showing normal platelet count, lower expression level of mutant FlnA, and gain of platelet functions: increased platelet aggregation, secretion, and αIIbβ3 activation, as well as increased spreading and clot retraction. Surprisingly, mutant FlnA mice exhibited a normal bleeding time, but with increased re-bleeding (77%) compared to wild type (WT) FlnA mice (27%), reflecting hemostatic plug instability. Again, in an in vivo thrombosis model, the occlusion time was not altered by the FlnA mutation, but arteriolar embolies were increased (7-fold more frequent in mutant FlnA mice versus WT mice), confirming thrombus instability.

CONCLUSIONS

This study shows that the FlnA mutation found in the male patient induced gain of platelet functions in vitro, but thrombus instability in vivo. Implications for the role of FLNa in physiology of thrombus formation are discussed.

The structural basis of β2 integrin intra-cellular multi-protein complexes

In multicellular organisms, cell adhesion is a pivotal physiological process which is essential for cell-cell communications, cell migration, and interactions with extracellular matrix. Integrins, a family of large hetero-dimeric type I membrane proteins, are known for driving cell adhesion functions. Among 24 different integrins, four β2 integrins, αL β2, αM β2, αX β2 and αD β2, are specific for cell adhesion and migration of leukocytes. Many cytosolic proteins interact with short cytosolic tails (CTs) of β2 and other integrins which are essential in bi-directional signaling processes. Further, phosphorylation of CTs of integrins regulates binding of intra-cellular proteins and signaling systems. In this review, recent advances in structures and interactions of multi-protein complexes of integrin tails, with a focus on β2 integrin, and cytosolic proteins are discussed along with a proposed future direction.

The combination of danhong injection plus tissue plasminogen activator ameliorates mouse tail thrombosis-induced by κ-carrageenan

BACKGROUND

After thrombosis, t-PA thrombolysis is the first choice, but the use of t-PA can easily lead to hemorrhagic injury and neurotoxicity. The combination of Danhong injection (DHI) and tissue plasminogen activator (t-PA) therapy may be a new strategy to find high-efficiency anti-thrombosis and low bleeding risk. However, nothing is about the effect of DHI plus t-PA on platelet activation.

PURPOSE

The present research was to explore the optimal dose of DHI and t-PA in vivo and mechanisms involved with the treatment of combining DHI and t-PA for thrombotic disease and determined whether DHI plus t-PA affects thrombotic processes related to platelet activation.

METHODS

Mice were induced by administering κ-carrageenan intraperitoneally, the ratio of different doses of DHI and t-PA in vivo, and the optimal dose effects on platelet aggregation, platelet adhesion, thrombosis formation, and platelet activation were determined. The effects of the αIIbβ3 signaling pathway were analyzed in mice.

RESULTS

In vitro, DHI (62% v/v), t-PA (1 mg/ml), and DHI + t-PA (62% v/v + 1 mg/ml) decreased rat platelet aggregation and adhesion, with a stronger effect from the combination as compared to t-PA monotherapy. In vivo, injections of κ-carrageenan were used to induce BALB/c mice. The optimal dose of DHI, t-PA, and DHI + t-PA is 12 ml/kg, 10 mg/kg, and 12 ml/kg + 7.5 mg/kg. The administration of DHI (12 ml/kg), t-PA (10 mg/kg), and DHI + t-PA (12 ml/kg + 7.5 mg/kg) decreased thrombi in mouse tissue vessels. Furthermore, the reduction of thrombosis formation by DHI, t-PA, and DHI + t-PA was related to lower collagen deposition, and lowered expressions of collagen I, matrix metalloproteinase 2 (MMP-2), and metalloproteinase 9 (MMP-9) in mouse tails, with increased efficacy in combination as compared to t-PA alone. The anti-thrombosis actions of DHI, t-PA, and their combination regulated the expression of CD41, purinergic receptor (P2Y₁₂), guanine nucleotide-binding protein G (q) subunit alpha (GNAQ), phosphatidylinositol phospholipase c beta (PLCβ), Ras-related protein 1 (Rap1), RIAM, talin1, fibrinogen alpha chain (FG), kindlin-3, and RAS guany1-releasing protein 1 (RasGRP1).

CONCLUSIONS

Based on expression, the mechanism responsible for thrombosis may be attributed to platelet activation via the αIIbβ3 signaling pathway. Combination therapy with DHI and t-PA exerted potent thrombolytic effects. Thus, our data can be used as a foundation for further clinical studies examining the efficacy of traditional Chinese medicines for the treatment of thrombosis.

Integrin Regulators in Neutrophils

Neutrophils are the most abundant leukocytes in humans and are critical for innate immunity and inflammation. Integrins are critical for neutrophil functions, especially for their recruitment to sites of inflammation or infections. Integrin conformational changes during activation have been heavily investigated but are still not fully understood. Many regulators, such as talin, Rap1-interacting adaptor molecule (RIAM), Rap1, and kindlin, are critical for integrin activation and might be potential targets for integrin-regulating drugs in treating inflammatory diseases. In this review, we outline integrin activation regulators in neutrophils with a focus on the above critical regulators, as well as newly discovered modulators that are involved in integrin activation.

Paxillin: A Hub for Mechano-Transduction from the β3 Integrin-Talin-Kindlin Axis

Focal adhesions are specialized integrin-dependent adhesion complexes, which ensure cell anchoring to the extracellular matrix. Focal adhesions also function as mechano-signaling platforms by perceiving and integrating diverse physical and (bio)chemical cues of their microenvironment, and by transducing them into intracellular signaling for the control of cell behavior. The fundamental biological mechanism of creating intracellular signaling in response to changes in tensional forces appears to be tightly linked to paxillin recruitment and binding to focal adhesions. Interestingly, the tension-dependent nature of the paxillin binding to adhesions, combined with its scaffolding function, suggests a major role of this protein in integrating multiple signals from the microenvironment, and accordingly activating diverse molecular responses. This minireview offers an overview of the molecular bases of the mechano-sensitivity and mechano-signaling capacity of core focal adhesion proteins, and highlights the role of paxillin as a key component of the mechano-transducing machinery based on the interaction of cells to substrates activating the β3 integrin-talin1-kindlin.

New insights into regulation of αIIbβ3 integrin signaling by filamin A

Background

Filamin (FLN) regulates many cell functions through its scaffolding activity cross‐linking cytoskeleton and integrins. FLN was shown to inhibit integrin activity, but the exact mechanism remains unclear.

Objectives

The aim of this study was to evaluate the role of filamin A (FLNa) subdomains on the regulation of integrin αIIbβ3 signaling.

Methods

Three FLNa deletion mutants were overexpressed in the erythro‐megakaryocytic leukemic cell line HEL: Del1, which lacks the N‐terminal CH1‐CH2 domains mediating the FLNa‐actin interaction; Del2, lacking the Ig‐like repeat 21, which mediates the FLNa‐β3 interaction; and Del3, lacking the C‐terminal Ig repeat 24, responsible for FLNa dimerization and interaction with the small Rho guanosine triphosphatase involved in actin cytoskeleton reorganisation. Fibrinogen binding to HEL cells in suspension and talin‐β3 proximity in cells adherent to immobilized fibrinogen were assessed before and after αIIbβ3 activation by the protein kinase C agonist phorbol 12‐myristate 13‐acetate.

Results

Our results show that FLNa‐actin and FLNa‐β3 interactions negatively regulate αIIbβ3 activation. Moreover, FLNa‐actin interaction represses Rac activation, contributing to the negative regulation of αIIbβ3 activation. In contrast, the FLNa dimerization domain, which maintains Rho inactive, was found to negatively regulate αIIbβ3 outside‐in signaling.

Conclusion

We conclude that FLNa negatively controls αIIbβ3 activation by regulating actin polymerization and restraining activation of Rac, as well as outside‐in signaling by repressing Rho.

ATP allosterically stabilizes integrin-linked kinase for efficient force generation

The pseudokinase integrin-linked kinase (ILK) is a central component of focal adhesions, cytoplasmic multiprotein complexes that integrate and transduce biochemical and mechanical signals from the extracellular environment into the cell and vice versa. However, the precise molecular functions, particularly the mechanosensory properties of ILK and the significance of retained adenosine triphosphate (ATP) binding, are still unclear. Combining molecular-dynamics simulations with cell biology, we establish a role for ATP binding to pseudokinases. We find that ATP promotes the structural stability of ILK, allosterically influences the interaction between ILK and its binding partner parvin at adhesions, and enhances the mechanoresistance of this complex. On the cellular level, ATP binding facilitates efficient traction force buildup, focal adhesion stabilization, and efficient cell migration.

Focal adhesions link the actomyosin cytoskeleton to the extracellular matrix regulating cell adhesion, shape, and migration. Adhesions are dynamically assembled and disassembled in response to extrinsic and intrinsic forces, but how the essential adhesion component integrin-linked kinase (ILK) dynamically responds to mechanical force and what role adenosine triphosphate (ATP) bound to this pseudokinase plays remain elusive. Here, we apply force–probe molecular-dynamics simulations of human ILK:α-parvin coupled to traction force microscopy to explore ILK mechanotransducing functions. We identify two key salt-bridge–forming arginines within the allosteric, ATP-dependent force-propagation network of ILK. Disrupting this network by mutation impedes parvin binding, focal adhesion stabilization, force generation, and thus migration. Under tension, ATP shifts the balance from rupture of the complex to protein unfolding, indicating that ATP increases the force threshold required for focal adhesion disassembly. Our study proposes a role of ATP as an obligatory binding partner for structural and mechanical integrity of the pseudokinase ILK, ensuring efficient cellular force generation and migration.

Examining the Effect of Kindlin-3 Binding Site Mutation on LFA-1-ICAM-1 Bonds by Force Measuring Optical Tweezers

Integrins in effector T cells are crucial for cell adhesion and play a central role in cell-mediated immunity. Leukocyte adhesion deficiency (LAD) type III, a genetic condition that can cause death in early childhood, highlights the importance of integrin/kindlin interactions for immune system function. A TTT/AAA mutation in the cytoplasmic domain of the β2 integrin significantly reduces kindlin-3 binding to the β2 tail, abolishes leukocyte adhesion to intercellular adhesion molecule 1 (ICAM-1), and decreases T cell trafficking in vivo. However, how kindlin-3 affects integrin function in T cells remains incompletely understood. We present an examination of LFA-1/ICAM-1 bonds in both wild-type effector T cells and those with a kindlin-3 binding site mutation. Adhesion assays show that effector T cells carrying the kindlin-3 binding site mutation display significantly reduced adhesion to the integrin ligand ICAM-1. Using optical trapping, combined with back focal plane interferometry, we measured a bond rupture force of 17.85 ±0.63 pN at a force loading rate of 30.21 ± 4.35 pN/s, for single integrins expressed on wild-type cells. Interestingly, a significant drop in rupture force of bonds was found for TTT/AAA-mutant cells, with a measured rupture force of 10.08 ± 0.88pN at the same pulling rate. Therefore, kindlin-3 binding to the cytoplasmic tail of the β2-tail directly affects catch bond formation and bond strength of integrin–ligand bonds. As a consequence of this reduced binding, CD8+ T cell activation in vitro is also significantly reduced.

Characterization of amino acid substitutions and deletions in the kindlin-1 FERM domain: relevance for precision medicine

Kindler epidermolysis bullosa is a genodermatosis that manifests with cutaneous and mucosal fragility, and with photosensitivity. No cure is available to date. Kindlin-1, the deficient protein binds to β intergrin and is required for its activation. Using a previously established experimental workflow we addressed the consequences of three naturally occurring mutations leading either to single amino acid substitutions, p.Y293D and p.W559R, or to a single amino acid deletion p.I623del in kindlin-1. We show that p.Y293D disrupts kindlin-1 localization to focal adhesions and cell spreading. Although, treatment with a chemical chaperone increases the amount of mutant protein, spreading does not improve and cellular stress increases. In contrast, the mutations p.W559R and p.I623del do not interfere with kindlin-1 localization to focal adhesions and support cell adhesion and survival. These mutants are also responsive to the treatment with chemical chaperone, and the increased mutant proteins improve cell spreading. These findings suggest that low levels of mutant kindlins, p.W559R and p.I623del are able to rescue some important cellular functions. Patients carrying these mutations could benefit from treatment with promotors of proteostasis. Our results show that each mutation must be individually tested on genetic, molecular and cellular level to tailor personalized treatments for patients.

Integrin-Mediated Tumorigenesis and Its Therapeutic Applications

Integrins, a family of adhesion molecules generally exist on the cell surface, are essential for regulating cell growth and its function. As a bi-directional signaling molecule, they mediate cell-cell and cell-extracellular matrix interaction. The recognitions of their key roles in many human pathologies, including autoimmunity, thrombosis and neoplasia, have revealed their great potential as a therapeutic target. This paper focuses on the activation of integrins, the role of integrins in tumorigenesis and progression, and advances of integrin-dependent tumor therapeutics in recent years. It is expected that understanding function and signaling transmission will fully exploit potentialities of integrin as a novel target for tumors.

Integrin β1 Promotes Pancreatic Tumor Growth by Upregulating Kindlin-2 and TGF-β Receptor-2

The tumor microenvironment plays a critical role in defining the growth and malignancy of solid tumors. Extracellular matrix (ECM) proteins such as collagen, vitronectin, and fibronectin are major components of the tumor microenvironment. Tumor growth-promoting reciprocal interaction between ECM and cytoplasmic proteins is regulated by the cell surface receptors called integrins. This study investigated the mechanism by which integrin β1 promotes pancreatic tumor growth. In MIA PaCa-2 pancreatic cancer cell line, the loss of integrin β1 protein reduced the ability of cells to proliferate in a 3D matrix and compromised the ability to form a focal adhesion complex. Decreased expression of integrin α5 was observed in KO cells, which resulted in impaired cell spreading and adhesion on vitronectin and fibronectin. Reduced expression of the integrin-associated protein, kindlin-2 was also recorded. The downregulation of kindlin-2 decreased the phosphorylation of Smad2/3 by reducing the expression of TGF-β receptor 2. These results unravel a new mechanism of integrin β1 in tumor growth by modifying the expression of kindlin-2 and TGF-β receptor 2 signaling.

Molecular motion and tridimensional nanoscale localization of kindlin control integrin activation in focal adhesions

Focal adhesions (FAs) initiate chemical and mechanical signals involved in cell polarity, migration, proliferation and differentiation. Super-resolution microscopy revealed that FAs are organized at the nanoscale into functional layers from the lower plasma membrane to the upper actin cytoskeleton. Yet, how FAs proteins are guided into specific nano-layers to promote interaction with given targets is unknown. Using single protein tracking, super-resolution microscopy and functional assays, we link the molecular behavior and 3D nanoscale localization of kindlin with its function in integrin activation inside FAs. We show that immobilization of integrins in FAs depends on interaction with kindlin. Unlike talin, kindlin displays free diffusion along the plasma membrane outside and inside FAs. We demonstrate that the kindlin Pleckstrin Homology domain promotes membrane diffusion and localization to the membrane-proximal integrin nano-layer, necessary for kindlin enrichment and function in FAs. Using kindlin-deficient cells, we show that kindlin membrane localization and diffusion are crucial for integrin activation, cell spreading and FAs formation. Thus, kindlin uses a different route than talin to reach and activate integrins, providing a possible molecular basis for their complementarity during integrin activation.

Kindlin2 regulates neural crest specification via integrin-independent regulation of the FGF signaling pathway

The focal adhesion protein Kindlin2 is essential for integrin activation, a process that is fundamental to cell-extracellular matrix adhesion. Kindlin 2 (Fermt2) is widely expressed in mouse embryos, and its absence causes lethality at the peri-implantation stage due to the failure to trigger integrin activation. The function of kindlin2 during embryogenesis has not yet been fully elucidated as a result of this early embryonic lethality. Here, we showed that kindlin2 is essential for neural crest (NC) formation in Xenopus embryos. Loss-of-function assays performed with kindlin2-specific morpholino antisense oligos (MOs) or with CRISPR/Cas9 techniques in Xenopus embryos severely inhibit the specification of the NC. Moreover, integrin-binding-deficient mutants of Kindlin2 rescued the phenotype caused by loss of kindlin2, suggesting that the function of kindlin2 during NC specification is independent of integrins. Mechanistically, we found that Kindlin2 regulates the fibroblast growth factor (FGF) pathway, and promotes the stability of FGF receptor 1. Our study reveals a novel function of Kindlin2 in regulating the FGF signaling pathway and provides mechanistic insights into the function of Kindlin2 during NC specification.

Emerging evidence for kindlin oligomerization and its role in regulating kindlin function

Integrin-mediated cell-extracellular matrix (ECM) interactions play crucial roles in a broad range of physiological and pathological processes. Kindlins are important positive regulators of integrin activation. The FERM-domain-containing kindlin family comprises three members, kindlin-1, kindlin-2 and kindlin-3 (also known as FERMT1, FERMT2 and FERMT3), which share high sequence similarity (identity >50%), as well as domain organization, but exhibit diverse tissue-specific expression patterns and cellular functions. Given the significance of kindlins, analysis of their atomic structures has been an attractive field for decades. Recently, the structures of kindlin and its β-integrin-bound form have been obtained, which greatly advance our understanding of the molecular functions that involve kindlins. In particular, emerging evidence indicates that oligomerization of kindlins might affect their integrin binding and focal adhesion localization, positively or negatively. In this Review, we presented an update on the recent progress of obtaining kindlin structures, and discuss the implication for integrin activation based on kindlin oligomerization, as well as the possible regulation of this process.

Phosphorylation of Kindlins and the Control of Integrin Function

Integrins serve as conduits for the transmission of information between cells and their extracellular environment. Signaling across integrins is bidirectional, transducing both inside-out and outside-signaling. Integrin activation, a transition from a low affinity/avidity state to a high affinity/avidity state for cognate ligands, is an outcome of inside-signaling. Such activation is particularly important for the recognition of soluble ligands by blood cells but also influences cell-cell and cell-matrix interactions. Integrin activation depends on a complex series of interactions, which both accelerate and inhibit their interconversion from the low to the high affinity/avidity state. There are three components regarded as being most proximately involved in integrin activation: the integrin cytoplasmic tails, talins and kindlins. The participation of each of these molecules in integrin activation is highly regulated by post-translation modifications. The importance of targeted phosphorylation of integrin cytoplasmic tails and talins in integrin activation is well-established, but much less is known about the role of post-translational modification of kindlins. The kindlins, a three-member family of 4.1-ezrin-radixin-moesin (FERM)-domain proteins in mammals, bind directly to the cytoplasmic tails of integrin beta subunits. This commentary provides a synopsis of the emerging evidence for the role of kindlin phosphorylation in integrin regulation.

Cardiac Fibrosis: Key Role of Integrins in Cardiac Homeostasis and Remodeling

Cardiac fibrosis is a common finding that is associated with the progression of heart failure (HF) and impacts all chambers of the heart. Despite intense research, the treatment of HF has primarily focused upon strategies to prevent cardiomyocyte remodeling, and there are no targeted antifibrotic strategies available to reverse cardiac fibrosis. Cardiac fibrosis is defined as an accumulation of extracellular matrix (ECM) proteins which stiffen the myocardium resulting in the deterioration cardiac function. This occurs in response to a wide range of mechanical and biochemical signals. Integrins are transmembrane cell adhesion receptors, that integrate signaling between cardiac fibroblasts and cardiomyocytes with the ECM by the communication of mechanical stress signals. Integrins play an important role in the development of pathological ECM deposition. This review will discuss the role of integrins in mechano-transduced cardiac fibrosis in response to disease throughout the myocardium. This review will also demonstrate the important role of integrins as both initiators of the fibrotic response, and modulators of fibrosis through their effect on cardiac fibroblast physiology across the various heart chambers.

Exogenous Integrin αIIbβ3 Inhibitors Revisited: Past, Present and Future Applications

The integrin αIIbβ3 is the most abundant integrin on platelets. Upon platelet activation, the integrin changes its conformation (inside-out signalling) and outside-in signalling takes place leading to platelet spreading, platelet aggregation and thrombus formation. Bloodsucking parasites such as mosquitoes, leeches and ticks express anticoagulant and antiplatelet proteins, which represent major sources of lead compounds for the development of useful therapeutic agents for the treatment of haemostatic disorders or cardiovascular diseases. In addition to hematophagous parasites, snakes also possess anticoagulant and antiplatelet proteins in their salivary glands. Two snake venom proteins have been developed into two antiplatelet drugs that are currently used in the clinic. The group of proteins discussed in this review are disintegrins, low molecular weight integrin-binding cysteine-rich proteins, found in snakes, ticks, leeches, worms and horseflies. Finally, we highlight various oral antagonists, which have been tested in clinical trials but were discontinued due to an increase in mortality. No new αIIbβ3 inhibitors are developed since the approval of current platelet antagonists, and structure-function analysis of exogenous disintegrins could help find platelet antagonists with fewer adverse side effects.

Quantitative single-protein imaging reveals molecular complex formation of integrin, talin, and kindlin during cell adhesion

Single-molecule localization microscopy (SMLM) enabling the investigation of individual proteins on molecular scales has revolutionized how biological processes are analysed in cells. However, a major limitation of imaging techniques reaching single-protein resolution is the incomplete and often unknown labeling and detection efficiency of the utilized molecular probes. As a result, fundamental processes such as complex formation of distinct molecular species cannot be reliably quantified. Here, we establish a super-resolution microscopy framework, called quantitative single-molecule colocalization analysis (qSMCL), which permits the identification of absolute molecular quantities and thus the investigation of molecular-scale processes inside cells. The method combines multiplexed single-protein resolution imaging, automated cluster detection, in silico data simulation procedures, and widely applicable experimental controls to determine absolute fractions and spatial coordinates of interacting species on a true molecular level, even in highly crowded subcellular structures. The first application of this framework allowed the identification of a long-sought ternary adhesion complex—consisting of talin, kindlin and active β1-integrin—that specifically forms in cell-matrix adhesion sites. Together, the experiments demonstrate that qSMCL allows an absolute quantification of multiplexed SMLM data and thus should be useful for investigating molecular mechanisms underlying numerous processes in cells.

Single-molecule localisation microscopy is limited by low labeling and detection efficiencies of the molecular probes. Here the authors report a framework to obtain absolute molecular quantities on a true molecular scale; the data reveal a ternary adhesion complex underlying cell-matrix adhesion.

Kindlin-2 promotes rear focal adhesion disassembly and directional persistence during cell migration

Cell migration involves front-to-rear asymmetric focal adhesion (FA) dynamics, which facilitates trailing edge detachment and directional persistence. Here, we show that kindlin-2 is crucial for FA sliding and disassembly in migrating cells. Loss of kindlin-2 markedly reduced FA number and selectively impaired rear FA sliding and disassembly, resulting in defective rear retraction and reduced directional persistence during cell migration. Kindlin-2-deficient cells failed to develop serum-induced actomyosin-dependent tension at FAs. At the molecular level, kindlin-2 directly interacted with myosin light chain kinase (MYLK, hereafter referred to as MLCK), which was enhanced in response to serum stimulation. Serum deprivation inhibited rear FA disassembly, which was released in response to serum stimulation. Overexpression of the MLCK-binding kindlin-2 F0F1 fragment (amino acid residues 1-167), which inhibits the interaction of endogenous kindlin-2 with MLCK, phenocopied kindlin-2 deficiency-induced migration defects. Inhibition of MLCK, like loss of kindlin-2, also impaired trailing-edge detachment, rear FA disassembly and directional persistence. These results suggest a role of kindlin-2 in promoting actomyosin contractility at FAs, leading to increased rear FA sliding and disassembly, and directional persistence during cell migration.

Active integrins regulate white adipose tissue insulin sensitivity and brown fat thermogenesis

Objective

Reorganization of the extracellular matrix is a prerequisite for healthy adipose tissue expansion, whereas fibrosis is a key feature of adipose dysfunction and inflammation. However, very little is known about the direct effects of impaired cell–matrix interaction in adipocyte function and insulin sensitivity. The objective of this study was to determine whether integrin activity can regulate insulin sensitivity in adipocytes and thereby systemic metabolism.

Methods

We characterized integrin activity in adipose tissue and its consequences on whole-body metabolism using adipose-selective deletion of β1 integrin (Itgb1^adipo-cre) and Kindlin-2 (Kind2^adipo-cre) in mice.

Results

We demonstrate that integrin signaling regulates white adipocyte insulin action and systemic metabolism. Consequently, loss of adipose integrin activity, similar to loss of adipose insulin receptors, results in a lipodystrophy-like phenotype and systemic insulin resistance. However, brown adipose tissue of Kind2^adipo-cre and Itgb1^adipo-cre mice is chronically hyperactivated and has increased substrate delivery, reduced endothelial basement membrane thickness, and increased endothelial vesicular transport.

Conclusions

Thus, we establish integrin-extracellular matrix interactions as key regulators of white and brown adipose tissue function and whole-body metabolism.

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β1 and β3 integrins interact with insulin signaling to regulate white adipocyte insulin sensitivity and systemic metabolism.

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Impaired integrin activity results in lipodystrophy in the absence of hepatosteatosis.

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β1 integrin activity regulates energy expenditure and vascular permeability in brown adipose tissue.

Crystal structure of the FERM-folded talin head reveals the determinants for integrin binding

Binding of the intracellular adapter proteins talin and its cofactor, kindlin, to the integrin receptors induces integrin activation and clustering. These processes are essential for cell adhesion, migration, and organ development. Although the talin head, the integrin-binding segment in talin, possesses a typical FERM-domain sequence, a truncated form has been crystallized in an unexpected, elongated form. This form, however, lacks a C-terminal fragment and possesses reduced β3-integrin binding. Here, we present a crystal structure of a full-length talin head in complex with the β3-integrin tail. The structure reveals a compact FERM-like conformation and a tightly associated N-P-L-Y motif of β3-integrin. A critical C-terminal poly-lysine motif mediates FERM interdomain contacts and assures the tight association with the β3-integrin cytoplasmic segment. Removal of the poly-lysine motif or disrupting the FERM-folded configuration of the talin head significantly impairs integrin activation and clustering. Therefore, structural characterization of the FERM-folded active talin head provides fundamental understanding of the regulatory mechanism of integrin function.

Kindlin3-Dependent CD11b/CD18-Integrin Activation Is Required for Potentiation of Neutrophil Cytotoxicity by CD47-SIRPα Checkpoint Disruption

The CD47-signal regulatory protein-alpha (SIRPα) immune checkpoint constitutes a therapeutic target in cancer, and initial clinical studies using inhibitors of CD47-SIRPα interactions in combination with tumor-targeting antibodies show promising results. Blockade of CD47-SIRPα interaction can promote neutrophil antibody-dependent cellular cytotoxicity (ADCC) toward antibody-opsonized targets. Neutrophils induce killing of antibody-opsonized tumor cells by a process identified as trogoptosis, a necrotic/lytic type of cancer cell death that involves trogocytosis, the antibody-mediated endocytic acquisition of cancer membrane fragments by neutrophils. Both trogocytosis and killing strictly depend on CD11b/CD18-(Mac-1)-mediated neutrophil-cancer cell conjugate formation, but the mechanism by which CD47-SIRPα checkpoint disruption promotes cytotoxicity has remained elusive. Here, by using neutrophils from patients with leukocyte adhesion deficiency type III carrying FERMT3 gene mutations, hence lacking the integrin-associated protein kindlin3, we demonstrated that CD47-SIRPα signaling controlled the inside-out activation of the neutrophil CD11b/CD18-integrin and cytotoxic synapse formation in a kindlin3-dependent fashion. Our findings also revealed a role for kindlin3 in trogocytosis and an absolute requirement in the killing process, which involved direct interactions between kindlin3 and CD18 integrin. Collectively, these results identified a dual role for kindlin3 in neutrophil ADCC and provide mechanistic insights into the way neutrophil cytotoxicity is governed by CD47-SIRPα interactions.

Recent Advances and Prospects in the Research of Nascent Adhesions

Nascent adhesions are submicron transient structures promoting the early adhesion of cells to the extracellular matrix. Nascent adhesions typically consist of several tens of integrins, and serve as platforms for the recruitment and activation of proteins to build mature focal adhesions. They are also associated with early stage signaling and the mechanoresponse. Despite their crucial role in sampling the local extracellular matrix, very little is known about the mechanism of their formation. Consequently, there is a strong scientific activity focused on elucidating the physical and biochemical foundation of their development and function. Precisely the results of this effort will be summarized in this article.

PPM1F controls integrin activity via a conserved phospho-switch

Control of integrin activity is vital during development and tissue homeostasis, while derailment of integrin function contributes to pathophysiological processes. Phosphorylation of a conserved threonine motif (T788/T789) in the integrin β cytoplasmic domain increases integrin activity. Here, we report that T788/T789 functions as a phospho-switch, which determines the association with either talin and kindlin-2, the major integrin activators, or filaminA, an integrin activity suppressor. A genetic screen identifies the phosphatase PPM1F as the critical enzyme, which selectively and directly dephosphorylates the T788/T789 motif. PPM1F-deficient cell lines show constitutive integrin phosphorylation, exaggerated talin binding, increased integrin activity, and enhanced cell adhesion. These gain-of-function phenotypes are reverted by reexpression of active PPM1F, but not a phosphatase-dead mutant. Disruption of the ppm1f gene in mice results in early embryonic death at day E10.5. Together, PPM1F controls the T788/T789 phospho-switch in the integrin β1 cytoplasmic tail and constitutes a novel target to modulate integrin activity.

The F1 loop of the talin head domain acts as a gatekeeper in integrin activation and clustering

Integrin activation and clustering by talin are early steps of cell adhesion. Membrane-bound talin head domain and kindlin bind to the β integrin cytoplasmic tail, cooperating to activate the heterodimeric integrin, and the talin head domain induces integrin clustering in the presence of Mn2+ Here we show that kindlin-1 can replace Mn2+ to mediate β3 integrin clustering induced by the talin head, but not that induced by the F2-F3 fragment of talin. Integrin clustering mediated by kindlin-1 and the talin head was lost upon deletion of the flexible loop within the talin head F1 subdomain. Further mutagenesis identified hydrophobic and acidic motifs in the F1 loop responsible for β3 integrin clustering. Modeling, computational and cysteine crosslinking studies showed direct and catalytic interactions of the acidic F1 loop motif with the juxtamembrane domains of α- and β3-integrins, in order to activate the β3 integrin heterodimer, further detailing the mechanism by which the talin-kindlin complex activates and clusters integrins. Moreover, the F1 loop interaction with the β3 integrin tail required the newly identified compact FERM fold of the talin head, which positions the F1 loop next to the inner membrane clasp of the talin-bound integrin heterodimer.This article has an associated First Person interview with the first author of the paper.

Topological features of integrin adhesion complexes revealed by multiplexed proximity biotinylation

Integrin adhesion complexes (IACs) bridge the extracellular matrix to the actin cytoskeleton and transduce signals in response to both chemical and mechanical cues. The composition, interactions, stoichiometry, and topological organization of proteins within IACs are not fully understood. To address this gap, we used multiplexed proximity biotinylation (BioID) to generate an in situ, proximity-dependent adhesome in mouse pancreatic fibroblasts. Integration of the interactomes of 16 IAC-associated baits revealed a network of 147 proteins with 361 proximity interactions. Candidates with underappreciated roles in adhesion were identified, in addition to established IAC components. Bioinformatic analysis revealed five clusters of IAC baits that link to common groups of prey, and which therefore may represent functional modules. The five clusters, and their spatial associations, are consistent with current models of IAC interaction networks and stratification. This study provides a resource to examine proximal relationships within IACs at a global level.

The intercalated disc: a mechanosensing signalling node in cardiomyopathy

Cardiomyocytes, the cells generating contractile force in the heart, are connected to each other through a highly specialised structure, the intercalated disc (ID), which ensures force transmission and transduction between neighbouring cells and allows the myocardium to function in synchrony. In addition, cardiomyocytes possess an intrinsic ability to sense mechanical changes and to regulate their own contractile output accordingly. To achieve this, some of the components responsible for force transmission have evolved to sense changes in tension and to trigger a biochemical response that results in molecular and cellular changes in cardiomyocytes. This becomes of particular importance in cardiomyopathies, where the heart is exposed to increased mechanical load and needs to adapt to sustain its contractile function. In this review, we will discuss key mechanosensing elements present at the intercalated disc and provide an overview of the signalling molecules involved in mediating the responses to changes in mechanical force.

Differences in self-association between kindlin-2 and kindlin-3 are associated with differential integrin binding

The integrin family of transmembrane adhesion receptors coordinates complex signaling networks that control the ability of cells to sense and communicate with the extracellular environment. Kindlin proteins are a central cytoplasmic component of these networks, directly binding integrin cytoplasmic domains and mediating interactions with cytoskeletal and signaling proteins. The physiological importance of kindlins is well established, but how the scaffolding functions of kindlins are regulated at the molecular level is still unclear. Here, using a combination of GFP nanotrap association assays, pulldown and integrin-binding assays, and live-cell imaging, we demonstrate that full-length kindlins can oligomerize (self-associate) in mammalian cells, and we propose that this self-association inhibits integrin binding and kindlin localization to focal adhesions. We show that both kindlin-2 and kindlin-3 can self-associate and that kindlin-3 self-association is more robust. Using chimeric mapping, we demonstrate that the F2PH and F3 subdomains are important for kindlin self-association. Through comparative sequence analysis of kindlin-2 and kindlin-3, we identify kindlin-3 point mutations that decrease self-association and enhance integrin binding, affording mutant kindlin-3 the ability to localize to focal adhesions. Our results support the notion that kindlin self-association negatively regulates integrin binding.

αvβ8 integrin adhesion and signaling pathways in development, physiology and disease

Cells must interpret a complex milieu of extracellular cues to modulate intracellular signaling events linked to proliferation, differentiation, migration and other cellular processes. Integrins are heterodimeric transmembrane proteins that link the extracellular matrix (ECM) to the cytoskeleton and control intracellular signaling events. A great deal is known about the structural and functional properties for most integrins; however, the adhesion and signaling pathways controlled by αvβ8 integrin, which was discovered nearly 30 years ago, have only recently been characterized. αvβ8 integrin is a receptor for ECM-bound forms of latent transforming growth factor β (TGFβ) proteins and promotes the activation of TGFβ signaling pathways. Studies of the brain, lung and immune system reveal that the αvβ8 integrin-TGFβ axis mediates cell-cell contact and communication within complex multicellular structures. Perturbing components of this axis results in aberrant cell-cell adhesion and signaling leading to the initiation of various pathologies, including neurodegeneration, fibrosis and cancer. As discussed in this Review, understanding the functions for αvβ8 integrin, its ECM ligands and intracellular effector proteins is not only an important topic in cell biology, but may lead to new therapeutic strategies to treat human pathologies related to integrin dysfunction.

GIV•Kindlin Interaction Is Required for Kindlin-Mediated Integrin Recognition and Activation

Cells perceive and respond to the extracellular matrix via integrin receptors; their dysregulation has been implicated in inflammation and cancer metastasis. Here we show that a guanine nucleotide-exchange modulator of trimeric-GTPase Gαi, GIV (a.k.a Girdin), directly binds the integrin adaptor Kindlin-2. A non-canonical short linear motif within the C terminus of GIV binds Kindlin-2-FERM3 domain at a site that is distinct from the binding site for the canonical NPxY motif on the -integrin tail. Binding of GIV to Kindlin-2 allosterically enhances Kindlin-2's affinity for β1-integrin. Consequently, integrin activation and clustering are maximized, which augments cell adhesion, spreading, and invasion. Findings elucidate how the GIV•Kindlin-2 complex has a 2-fold impact: it allosterically synergizes integrin activation and enables β1-integrins to indirectly access and modulate trimeric GTPases via the complex. Furthermore, Cox proportional-hazard models on tumor transcriptomics provide trans-scale evidence of synergistic interactions between GIV•Kindlin-2•β1-integrin on time to progression to metastasis.

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GIV and Kindlin (K2), two integrin adaptors that promote metastasis, bind each other

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Binding of GIV or integrin to K2 allosterically enhances GIV•K2•integrin complexes

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Binding is required for the maximal recruitment of GIV and K2 to active integrins

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Binding facilitates integrin clustering, activation, tumor cell adhesion, invasion

Chapter 22: Structural and signaling functions of integrins

The integrin family of transmembrane adhesion receptors is essential for sensing and adhering to the extracellular environment. Integrins are heterodimers composed of non-covalently associated α and β subunits that engage extracellular matrix proteins and couple to intracellular signaling and cytoskeletal complexes. Humans have 24 different integrin heterodimers with differing ligand binding specificities and non-redundant functions. Complex structural rearrangements control the ability of integrins to engage ligands and to activate diverse downstream signaling networks, modulating cell adhesion and dynamics, processes which are crucial for metazoan life and development. Here we review the structural and signaling functions of integrins focusing on recent advances which have enhanced our understanding of how integrins are activated and regulated, and the cytoplasmic signaling networks downstream of integrins.

The focal adhesion protein kindlin-2 controls mitotic spindle assembly by inhibiting histone deacetylase 6 and maintaining α-tubulin acetylation

Kindlins are focal adhesion proteins that regulate integrin activation and outside-in signaling. The kindlin family consists of three members, kindlin-1, -2, and -3. Kindlin-2 is widely expressed in multiple cell types, except those from the hematopoietic lineage. A previous study has reported that the Drosophila Fit1 protein (an ortholog of kindlin-2) prevents abnormal spindle assembly; however, the mechanism remains unknown. Here, we show that kindlin-2 maintains spindle integrity in mitotic human cells. The human neuroblastoma SH-SY5Y cell line expresses only kindlin-2, and we found that when SH-SY5Y cells are depleted of kindlin-2, they exhibit pronounced spindle abnormalities and delayed mitosis. Of note, acetylation of α-tubulin, which maintains microtubule flexibility and stability, was diminished in the kindlin-2-depleted cells. Mechanistically, we found that kindlin-2 maintains α-tubulin acetylation by inhibiting the microtubule-associated deacetylase histone deacetylase 6 (HDAC6) via a signaling pathway involving AKT Ser/Thr kinase (AKT)/glycogen synthase kinase 3β (GSK3β) or paxillin. We also provide evidence that prolonged hypoxia down-regulates kindlin-2 expression, leading to spindle abnormalities not only in the SH-SY5Y cell line, but also cell lines derived from colon and breast tissues. The findings of our study highlight that kindlin-2 regulates mitotic spindle assembly and that this process is perturbed in cancer cells in a hypoxic environment.

Stimulation of Fibronectin Matrix Assembly by Lysine Acetylation

Diabetic nephropathy, a devastating consequence of diabetes mellitus, is characterized by the accumulation of extracellular matrix (ECM) that disrupts the kidney's filtration apparatus. Elevated glucose levels increase the deposition of a fibronectin (FN) matrix by mesangial cells, the primary matrix-producing cells of the kidney, and also increase acetyl-CoA leading to higher levels of lysine acetylation. Here, we investigated the connection between acetylation and the ECM and show that treatment of mesangial cells with deacetylase inhibitors increases both acetylation and FN matrix assembly compared to untreated cells. The matrix effects were linked to lysine 794 (K794) in the β1 integrin cytoplasmic domain based on studies of cells expressing acetylated (K794Q) and non-acetylated (K794R) mimetics. β1(K794Q) cells assembled significantly more FN matrix than wildtype β1 cells, while the non-acetylated β1(K794R) form was inactive. We show that mutation of K794 affects FN assembly by stimulating integrin-FN binding activity and cell contractility. Wildtype and β1(K794Q) cells but not β1(K794R) cells further increased their FN matrix when stimulated with deacetylase inhibitors indicating that increased acetylation on other proteins is required for maximum FN assembly. Thus, lysine acetylation provides a mechanism for glucose-induced fibrosis by up-regulation of FN matrix assembly.

Filamin A: key actor in platelet biology

Filamins (FLNs) are large dimeric actin-binding proteins that regulate actin cytoskeleton remodeling. In addition, FLNs serve as scaffolds for signaling proteins, such as tyrosine kinases, GTPases, or phosphatases, as well as for adhesive receptors, such as integrins. Thus, they connect adhesive receptors to signaling pathways and to cytoskeleton. There are 3 isoforms of FLN (filamin a [FLNa], FLNb, FLNc) that originate from 3 homologous genes. FLNa has been the recent focus of attention because its mutations are responsible for a wide spectrum of defects called filaminopathies A, affecting brain (peri-ventricular nodular heterotopia), heart (valve defect), skeleton, gastrointestinal tract, and, more recently, the megakaryocytic lineage. This review will focus on the physiological and pathological roles of FLNa in platelets. Indeed, FLNa mutations alter platelet production from their bone marrow precursors, the megakaryocytes, yielding giant platelets in reduced numbers (macrothrombocytopenia). In platelets per se, FLNa mutations may lead to impaired αIIbβ3 integrin activation or in contrast, increased αIIbβ3 activation, potentially enhancing the risk of thrombosis. Experimental work delineating the interaction of FLNa with its platelet partners, including αIIbβ3, the von Willebrand factor receptor GPIb-IX-V, the tyrosine kinase Syk, and the signaling pathway of the collagen receptor GPVI, will also be reviewed.

Kindlin-2 modulates MafA and β-catenin expression to regulate β-cell function and mass in mice

β-Cell dysfunction and reduction in β-cell mass are hallmark events of diabetes mellitus. Here we show that β-cells express abundant Kindlin-2 and deleting its expression causes severe diabetes-like phenotypes without markedly causing peripheral insulin resistance. Kindlin-2, through its C-terminal region, binds to and stabilizes MafA, which activates insulin expression. Kindlin-2 loss impairs insulin secretion in primary human and mouse islets in vitro and in mice by reducing, at least in part, Ca²⁺ release in β-cells. Kindlin-2 loss activates GSK-3β and downregulates β-catenin, leading to reduced β-cell proliferation and mass. Kindlin-2 loss reduces the percentage of β-cells and concomitantly increases that of α-cells during early pancreatic development. Genetic activation of β-catenin in β-cells restores the diabetes-like phenotypes induced by Kindlin-2 loss. Finally, the inducible deletion of β-cell Kindlin-2 causes diabetic phenotypes in adult mice. Collectively, our results establish an important function of Kindlin-2 and provide a potential therapeutic target for diabetes.

Beta cell dysfunction and reduction in beta cell mass are hallmark events in the pathogenesis of diabetes mellitus. We identify focal adhesion protein Kindlin-2 as a key factor that controls insulin synthesis and secretion and beta cell mass by modulating MafA and beta-catenin proteins in pancreatic beta cells.

Cell Adhesion by Integrins

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

Sharpin suppresses β1-integrin activation by complexing with the β1 tail and kindlin-1

Background

Previously sharpin has been identified as an endogenous inhibitor of β1-integrin activation by directly binding to a conserved region in the cytoplasmic tails (CTs) of the integrin β1-associated α subunits.

Methods

Here we employed biochemical approaches and cellular analyses to evaluate the function and molecular mechanism of the sharpin-kindlin-1 complex in regulating β1-integrin activation.

Results

In this study, we found that although the inhibition of sharpin on β1-integrin activation could be confirmed, sharpin had no apparent effect on integrin αIIbβ3 activation in CHO cell system. Notably, a direct interaction between sharpin and the integrin β1 CT was detected, while the interaction of sharpin with the integrin αIIb and the β3 CTs were substantially weaker. Importantly, sharpin was able to inhibit the talin head domain binding to the integrin β1 CT, which can mechanistically contribute to inhibiting β1-integrin activation. Interestingly, we also found that sharpin interacted with kindlin-1, and the interaction between sharpin and the integrin β1 CT was significantly enhanced when kindlin-1 was present. Consistently, we observed that instead of acting as an activator, kindlin-1 actually suppressed the talin head domain mediated β1-integrin activation, indicating that kindlin-1 may facilitate recruitment of sharpin to the integrin β1 CT.

Conclusion

Taken together, our findings suggest that sharpin may complex with both kindlin-1 and the integrin β1 CT to restrict the talin head domain binding, thus inhibiting β1-integrin activation.

Inhibition of cyclin-dependent kinase activity exacerbates H2 O2 -induced DNA damage in Kindler syndrome keratinocytes

Kindler syndrome (KS) is an autosomal recessive skin disorder characterized by skin blistering and photosensitivity. KS is caused by loss of function mutations in FERMT1, which encodes Kindlin-1. Kindlin-1 is a FERM domain containing adaptor protein that is found predominantly at cell-extracellular matrix adhesions where it binds to integrin β subunits and is required for efficient integrin activation. Using keratinocytes derived from a patient with KS, into which wild-type Kindlin-1 (Kin1WT) has been expressed, we show that Kindlin-1 binds to cyclin-dependent kinase (CDK)1 and CDK2. CDK1 and CDK2 are key regulators of cell cycle progression, however, cell cycle analysis showed only small differences between the KS and KS-Kin1WT keratinocytes. In contrast, G2/M cell cycle arrest in response to oxidative stress induced by hydrogen peroxide (H₂ O₂ ) was enhanced in KS keratinocytes but not KS-Kin1WT cells, following inhibition of CDK activity. Furthermore, KS keratinocytes were more sensitive to DNA damage in response to H₂ O₂ and this was exacerbated by treatment with the CDK inhibitor roscovitine. Thus, in Kindlin-1 deficient keratinocytes, CDK activity can further regulate oxidative damage induced cell cycle arrest and DNA damage. This provides further insight into the key pathways that control sensitivity to oxidative stress in KS patients.

Integrins, CAFs and Mechanical Forces in the Progression of Cancer

Cells respond to both chemical and mechanical cues present within their microenvironment. Various mechanical signals are detected by and transmitted to the cells through mechanoreceptors. These receptors often contact with the extracellular matrix (ECM), where the external signals are converted into a physiological response. Integrins are well-defined mechanoreceptors that physically connect the actomyosin cytoskeleton to the surrounding matrix and transduce signals. Families of α and β subunits can form a variety of heterodimers that have been implicated in cancer progression and differ among types of cancer. These heterodimers serve as the nexus of communication between the cells and the tumor microenvironment (TME). The TME is dynamic and composed of stromal cells, ECM and associated soluble factors. The most abundant stromal cells within the TME are cancer-associated fibroblasts (CAFs). Accumulating studies implicate CAFs in cancer development and metastasis through their remodeling of the ECM and release of large amounts of ECM proteins and soluble factors. Considering that the communication between cancer cells and CAFs, in large part, takes place through the ECM, the involvement of integrins in the crosstalk is significant. This review discusses the role of integrins, as the primary cell-ECM mechanoreceptors, in cancer progression, highlighting integrin-mediated mechanical communication between cancer cells and CAFs.

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.

Prognostic value of Kindlin-2 expression in patients with solid tumors: a meta-analysis

Background

Kindlin-2 is one of the Kindlin family members which are evolutionarily conserved focal adhesion proteins with integrin β-binding affinity. Recently, accumulative studies have suggested that Kindlin-2 plays important roles in tumor biology. However, the prognostic significance of Kindlin-2 in patients with solid tumors remains controversial. Therefore, this study aimed to clarify the prognostic value of Kindlin-2 in solid tumors via meta-analysis.

Methods

A comprehensive search was performed in PubMed, Embase, Web of Science and EBSCO for all relevant studies reporting the prognostic significance of Kindlin-2 expression in solid cancer patients. The summary hazard ratio (HR) and corresponding 95% confidence interval (CI) were calculated to estimate the association between Kindlin-2 expression with survival of solid cancer patients.

Results

We included 14 eligible studies containing 1869 patients in our meta-analysis. The pooled results indicated that high Kindlin-2 expression was significantly associated with poor overall survival (OS) (pooled HR 1.66, 95% CI 1.44–1.92, P < 0.0001), disease-free survival (DFS)/recurrence-free survival (RFS)/progression-free survival (PFS) (pooled HR 1.73, 95% CI 1.16–2.57, P = 0.0067). For certain tumor types, high Kindlin-2 expression was significantly correlated with a poor outcome in patients with solid tumors, including pancreatic ductal adenocarcinoma (DFS/RFS/PFS), esophageal squamous cell carcinoma (OS, DFS/RFS/PFS), hepatocellular carcinoma (OS), clear cell renal cell carcinoma (OS), bladder cancer (OS, DFS/RFS/PFS), chondrosarcoma (OS), osteosarcoma (OS), gastric cancer (DFS/RFS/PFS), and glioma (OS).

Conclusions

Our meta-analysis demonstrated that high Kindlin-2 expression might indicate poor outcome in patients with solid tumors and could serve as a prognostic biomarker for solid cancer patients.

Kindlin-2 interacts with a highly conserved surface of ILK to regulate focal adhesion localization and cell spreading

The integrin-associated adaptor proteins integrin-linked kinase (ILK) and kindlin-2 play central roles in integrin signaling and control of cell morphology. A direct ILK-kindlin-2 interaction is conserved across species and involves the F2PH subdomain of kindlin-2 and the pseudokinase domain (pKD) of ILK. However, complete understanding of the ILK-kindlin-2 interaction and its role in integrin-mediated signaling has been impeded by difficulties identifying the binding site for kindlin-2 on ILK. We used conservation-guided mapping to dissect the interaction between ILK and kindlin-2 and identified a previously unknown binding site for kindlin-2 on the C-lobe of the pKD of ILK. Mutations at this site inhibit binding to kindlin-2 while maintaining structural integrity of the pKD. Importantly, kindlin-binding-defective ILK mutants exhibit impaired focal adhesion localization and fail to fully rescue the spreading defects seen in ILK knockdown cells. Furthermore, kindlin-2 mutants with impaired ILK binding are also unable to fully support cell spreading. Thus, the interaction between ILK and kindlin-2 is critical for cell spreading and focal adhesion localization, representing a key signaling axis downstream of integrins.This article has an associated First Person interview with the first author of the paper.