Migfilin and Mig-2 link focal adhesions to filamin and the actin cytoskeleton and function in cell shape modulation

Calpain and caspase regulate Aβ peptide production via cleavage of KINDLIN2 encoded by the AD-associated gene FERMT2

The adapter protein KINDLIN2, encoded by the Alzheimer's disease (AD) genetic risk factor FERMT2, was identified as a modulator of APP processing. KINDLIN2 directly interacts with APP to modulate its metabolism, and KINDLIN2 underexpression impairs long-term potentiation in an APP-dependent manner. Altogether, these data suggest that loss of KINDLIN2 could have a detrimental effect on synaptic function and promote AD pathophysiological process. In this study, we identified KINDLIN2 as a novel substrate of caspases and calpain I, two well-characterized cysteine proteases involved in the regulation of synaptic plasticity. These cleavages resulted in the dissociation of the F0 and F1 domains of KINDLIN2 that are necessary for it to function as an adapter protein. Furthermore, we demonstrate that these cleavages lead to a decrease in KINDLIN2's ability to control APP processing. Overall, these KINDLIN2 cleavages appear as potential new mechanisms in the regulation of KINDLIN2 functions at the synapse and could be of interest for the pathophysiology of AD.

Molecular basis of β2 integrin activation by talin unveils subunit-specific mechanisms of integrin signaling

Integrins consist of 24 species, each with unique tissue expression profiles and distinct biological functions. The β subunit of integrin interacts with the FERM-folded head domain of talin through an NPxY/F motif, triggering integrin activation. Although this motif is conserved across most integrin-β subunits, the precise molecular mechanism governing talin's selective recognition of different integrin-β subunits remains unclear. We identify two distinct configurations of the talin head when interacting with β2 and β3 integrins, providing critical insights into subunit-specific recognition of integrins. Structural studies reveal that mutations at the subdomain interface of the talin head can shift its β2-bound configuration to a β3-bound configuration. This shift enhances β2-integrin affinity, leading to increased lymphocyte function-associated antigen-1 (LFA-1)-mediated natural killer cell activity. Together, our data elucidate the structural basis of talin's role in mediating integrin activation in a subunit-specific manner and advance our understanding of how talin may regulate diverse functions of various integrin species.

Risk of Alzheimer's disease in Down syndrome: Insights gained by multi-omics

Individuals with Down syndrome (DS) are highly susceptible to Alzheimer's disease (AD). The integration of genomics, transcriptomics, epigenomics, proteomics, and metabolomics enables unprecedented understanding of DS-AD, offering a detailed picture of this complex issue. The vast -omics data also present challenges that reflect the complexity of genetic information flow. These studies nonetheless reveal critical mechanisms behind AD risk, including unique observations in DS that differ from those seen in the general population and familial dominant AD. In addition, the correlations between the AD polygenic risk score and proteins related to female infertility and autoimmune thyroiditis corroborate clinical observations. Metabolomic data reveal disrupted metabolic networks, offering prospects for a dynamic score to create specialized nutritional interventions. By adopting a multidimensional perspective with integrated reductionism, the evolving landscape presents an opportunity to identify promising directions for developing precision strategies to mitigate the impact of AD in the DS population. HIGHLIGHTS: Individuals with Down syndrome (DS) are highly susceptible to Alzheimer's disease (AD). DS-AD is characterized by its polygenic nature, extending beyond chromosome 21 with significant contributions from various chromosomes. DS-AD also presents unique features that differ from those observed in the general population and familial dominant AD. Our review consolidates key findings from genomics, transcriptomics, epigenomics, proteomics, and metabolomics, providing a comprehensive view of the molecular mechanisms underlying DS-AD. We highlight promising research directions to further elucidate the pathogenesis of DS-AD.

Mesenchymal stromal cells promote the formation of lung cancer organoids via Kindlin-2

BACKGROUND

Patient-derived lung cancer organoids (PD-LCOs) demonstrate exceptional potential in preclinical testing and serve as a promising model for the multimodal management of lung cancer. However, certain lung cancer cells derived from patients exhibit limited capacity to generate organoids due to inter-tumor or intra-tumor variability. To overcome this limitation, we have created an in vitro system that employs mesenchymal stromal cells (MSCs) or fibroblasts to serve as a supportive scaffold for lung cancer cells that do not form organoids.

METHODS

We successfully established an MSCs/fibroblast co-culture system to form LCOs. We analyzed the morphological and histological similarities between LCOs co-cultured with fibroblast and primary lung cancer lesions through HE and IF staining. We evaluated whether LCOs co-cultured with fibroblast retained the original genetic mutations of their source tumors based on WES. RNA sequencing was used to analyze the differences in gene expression profiles between LCOs co-cultured with fibroblast and paracancerous organoids (POs). Importantly, we have successfully validated the impact of Kindlin-2 on the regulation of MSCs in organoid formation through lentiviral vector-mediated interference or overexpression of kindlin-2.

RESULTS

Our findings demonstrate that the addition of MSCs/fibroblasts to three tumor samples, initially incapable of forming organoids by traditional methods, successfully facilitated the cultivation of tumor organoids. Importantly, these organoids co-cultured with fibroblast faithfully recapitulate the tissue morphology of original lung tumors and replicate the genetic profile observed in the parental tumors even after prolonged in vitro culture. Moreover, drug responses exhibited by these organoids co-cultured with MSCs/fibroblasts are consistent with those observed in the original tumors. Mechanistically, we have also identified kindlin-2 as a crucial regulator linking extracellular matrix (ECM) and mitochondria that influence MSC/fibroblast-mediated support for tumor organoid formation.

CONCLUSION

The results obtained from our research enhance the understanding of the mechanisms implicated in the formation of tumor organoids and aid in creating stronger patient-specific tumor organoid models. This advancement supports the refinement of personalized drug response assessments for use in clinical settings.

The filamins of Drosophila

The actin cytoskeleton is a dynamic mesh of filaments that provide structural support for cells and respond to external deformation forces. Active sensing of these forces is crucial for the function of the actin cytoskeleton, and some actin crosslinkers accomplish it. One such crosslinker is filamin, a highly conserved actin crosslinker dimeric protein with an elastic region capable of responding to mechanical changes in the actin cytoskeleton. Filamins are required across various cells and tissues. In Drosophila early and recent studies have provided many details about filamin functions. This review centers on the two Drosophila filamins encoded by the cheerio and jitterbu g genes. We examine the structural and evolutionary aspects of filamin genes in flies, contrasting them with those of other model organisms. Then, we synthesize phenotypic data across diverse cell types. Additionally, we outline the genetic tools available for both genes. We also propose to divide filamins into typical and atypical based on the number of actin-binding domains and their relationship with other filamins.

Multi-domain interaction mediated strength-building in human α-actinin dimers unveiled by direct single-molecule quantification

α-Actinins play crucial roles in cytoskeletal mechanobiology by acting as force-bearing structural modules that orchestrate and sustain the cytoskeletal framework, serving as pivotal hubs for diverse mechanosensing proteins. The mechanical stability of α-actinin dimer, a determinant of its functional state, remains largely unexplored. Here, we directly quantify the force-dependent lifetimes of homo- and hetero-dimers of human α-actinins, revealing an ultra-high mechanical stability of the dimers associated with > 100 seconds lifetime within 40 pN forces under shear-stretching geometry. Intriguingly, we uncover that the strong dimer stability is arisen from much weaker sub-domain pair interactions, suggesting the existence of distinct dimerized functional states of the dimer, spanning a spectrum of mechanical stability, with the spectrin repeats (SRs) in folded or unfolded conformation. In essence, our study supports a potent mechanism for building strength in biomolecular dimers through weak, multiple sub-domain interactions, and illuminates multifaceted roles of α-actinin dimers in cytoskeletal mechanics and mechanotransduction.

Perception and response of skeleton to mechanical stress

Mechanical stress stands as a fundamental factor in the intricate processes governing the growth, development, morphological shaping, and maintenance of skeletal mass. The profound influence of stress in shaping the skeletal framework prompts the assertion that stress essentially births the skeleton. Despite this acknowledgment, the mechanisms by which the skeleton perceives and responds to mechanical stress remain enigmatic. In this comprehensive review, our scrutiny focuses on the structural composition and characteristics of sclerotin, leading us to posit that it serves as the primary structure within the skeleton responsible for bearing and perceiving mechanical stress. Furthermore, we propose that osteocytes within the sclerotin emerge as the principal mechanical-sensitive cells, finely attuned to perceive mechanical stress. And a detailed analysis was conducted on the possible transmission pathways of mechanical stress from the extracellular matrix to the nucleus.

Force-Dependent Structural Changes of Filamin C Rod Domains Regulated by Filamin C Dimer

Filamin C (FLNC), a large dimeric actin-binding protein in muscle cells, plays a critical role in transmitting force in the cytoskeleton and that between membrane receptors and the cytoskeleton. It performs crucial mechanosensing and downstream mechanotransduction functions via force-dependent interactions with signaling proteins. Mutations in FLNC have been linked to muscle and heart diseases. The mechanical responses of the force-bearing elements in FLNC have not been determined. This study investigated the mechanical responses of FLNC domains and their dimerization interface using magnetic tweezers. Results showed high stability of the N-terminal domains in the rod-1 segment but significant changes in the rod-2 domains in response to forces of a few piconewtons (pN). The dimerization interface, formed by the R24 domain, has a lifetime of seconds to tens of seconds at pN forces, and it dissociates within 1 s at forces greater than 14 pN. The findings suggest the FLNC dimerization interface provides sufficient mechanical stability that enables force-dependent structural changes in rod-2 domains for signaling protein binding and maintains structural integrity of the rod-1 domains.

Integrative analyses of bulk and single-cell RNA-seq identified cancer-associated fibroblasts-related signature as a prognostic factor for immunotherapy in NSCLC

An emerging view regarding cancer-associated fibroblast (CAF) is that it plays a critical role in tumorigenesis and immunosuppression in the tumor microenvironment (TME), but the clinical significance and biological functions of CAFs in non-small cell lung cancer (NSCLC) are still poorly explored. Here, we aimed to identify the CAF-related signature for NSCLC through integrative analyses of bulk and single-cell genomics, transcriptomics, and proteomics profiling. Using CAF marker genes identified in weighted gene co-expression network analysis (WGCNA), we constructed and validated a CAF-based risk model that stratifies patients into two prognostic groups from four independent NSCLC cohorts. The high-score group exhibits a higher abundance of CAFs, decreased immune cell infiltration, increased epithelial-mesenchymal transition (EMT), activated transforming growth factor beta (TGFβ) signaling, and a limited survival rate compared with the low-score group. Considering the immunosuppressive feature in the high-score group, we speculated an inferior clinical response for immunotherapy in these patients, and this association was successfully verified in two NSCLC cohorts treated with immune checkpoint blockades (ICBs). Furthermore, single-cell RNA sequence datasets were used to clarify the molecular mechanisms underlying the aggressive and immunosuppressive phenotype in the high-score group. We found that one of the genes in the risk model, filamin binding LIM protein 1 (FBLIM1), is mainly expressed in fibroblasts and upregulated in CAFs compared to fibroblasts from normal tissue. FBLIM1-positive CAF subtype was correlated with increased TGFβ expression, higher mesenchymal marker level, and immunosuppressive tumor microenvironment. Finally, we demonstrated that FBLIM1 might serve as a poor prognostic marker for immunotherapy in clinical samples. In conclusion, we identified a novel CAF-based classifier with prognostic value in NSCLC patients and those treated with ICBs. Single-cell transcriptome profiling uncovered FBLIM1-positive CAFs as an aggressive subtype with a high abundance of TGFβ, EMT, and an immunosuppressive phenotype in NSCLC.

Static and Dynamic: Evolving Biomaterial Mechanical Properties to Control Cellular Mechanotransduction

The extracellular matrix (ECM) is a highly dynamic system that constantly offers physical, biological, and chemical signals to embraced cells. Increasing evidence suggests that mechanical signals derived from the dynamic cellular microenvironment are essential controllers of cell behaviors. Conventional cell culture biomaterials, with static mechanical properties such as chemistry, topography, and stiffness, have offered a fundamental understanding of various vital biochemical and biophysical processes, such as cell adhesion, spreading, migration, growth, and differentiation. At present, novel biomaterials that can spatiotemporally impart biophysical cues to manipulate cell fate are emerging. The dynamic properties and adaptive traits of new materials endow them with the ability to adapt to cell requirements and enhance cell functions. In this review, an introductory overview of the key players essential to mechanobiology is provided. A biophysical perspective on the state-of-the-art manipulation techniques and novel materials in designing static and dynamic ECM-mimicking biomaterials is taken. In particular, different static and dynamic mechanical cues in regulating cellular mechanosensing and functions are compared. This review to benefit the development of engineering biomechanical systems regulating cell functions is expected.

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.

The NOGO receptor NgR2, a novel αVβ3 integrin effector, induces neuroendocrine differentiation in prostate cancer

Androgen deprivation therapies aimed to target prostate cancer (PrCa) are only partially successful given the occurrence of neuroendocrine PrCa (NEPrCa), a highly aggressive and highly metastatic form of PrCa, for which there is no effective therapeutic approach. Our group has demonstrated that while absent in prostate adenocarcinoma, the αVβ3 integrin expression is increased during PrCa progression toward NEPrCa. Here, we show a novel pathway activated by αVβ3 that promotes NE differentiation (NED). This novel pathway requires the expression of a GPI-linked surface molecule, NgR2, also known as Nogo-66 receptor homolog 1. We show here that NgR2 is upregulated by αVβ3, to which it associates; we also show that it promotes NED and anchorage-independent growth, as well as a motile phenotype of PrCa cells. Given our observations that high levels of αVβ3 and, as shown here, of NgR2 are detected in human and mouse NEPrCa, our findings appear to be highly relevant to this aggressive and metastatic subtype of PrCa. This study is novel because NgR2 role has only minimally been investigated in cancer and has instead predominantly been analyzed in neurons. These data thus pave new avenues toward a comprehensive mechanistic understanding of integrin-directed signaling during PrCa progression toward a NE phenotype.

Respiratory syncytial virus disrupts the airway epithelial barrier by decreasing cortactin and destabilizing F-actin

Respiratory syncytial virus (RSV) infection is the leading cause of acute lower respiratory tract infection in young children worldwide. Our group recently revealed that RSV infection disrupts the airway epithelial barrier in vitro and in vivo. However, the underlying molecular pathways were still elusive. Here, we report the critical roles of the filamentous actin (F-actin) network and actin-binding protein cortactin in RSV infection. We found that RSV infection causes F-actin depolymerization in 16HBE cells, and that stabilizing the F-actin network in infected cells reverses the epithelial barrier disruption. RSV infection also leads to significantly decreased cortactin in vitro and in vivo. Cortactin-knockout 16HBE cells presented barrier dysfunction, whereas overexpression of cortactin protected the epithelial barrier against RSV. The activity of Rap1 (which has Rap1A and Rap1B forms), one downstream target of cortactin, declined after RSV infection as well as in cortactin-knockout cells. Moreover, activating Rap1 attenuated RSV-induced epithelial barrier disruption. Our study proposes a key mechanism in which RSV disrupts the airway epithelial barrier via attenuating cortactin expression and destabilizing the F-actin network. The identified pathways will provide new targets for therapeutic intervention toward RSV-related disease. This article has an associated First Person interview with the first author of the paper.

The Connection Between Rap1 and Talin1 in the Activation of Integrins in Blood Cells

Integrins regulate the adhesion and migration of blood cells to ensure the proper positioning of these cells in the environment. Integrins detect physical and chemical stimuli in the extracellular matrix and regulate signaling pathways in blood cells that mediate their functions. Integrins are usually in a resting state in blood cells until agonist stimulation results in a high-affinity conformation ("integrin activation"), which is central to integrins' contribution to blood cells' trafficking and functions. In this review, we summarize the mechanisms of integrin activation in blood cells with a focus on recent advances understanding of mechanisms whereby Rap1 regulates talin1-integrin interaction to trigger integrin activation in lymphocytes, platelets, and neutrophils.

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.

Migfilin: Cell Adhesion Effect and Comorbidities

Cell adhesion manifests as cell linkages to neighboring cells and/or the extracellular matrix (ECM). Migfilin is a widely expressed adhesion protein. It comprises three LIM domains in the C-terminal region and one proline-rich sequence in the N-terminal region. Through interplay with its various binding partners, such as Kindlin-2, Filamin, vasodilator-stimulated phosphoprotein (VASP) protein and the transcription factor CSX, Migfilin facilitates the dynamic association of connecting actomyosin fibers, orchestrating cell morphogenetic movement and cell adhesion, proliferation, migration, invasion, differentiation and signal transduction. In this review, to further elucidate the functional contributions of and pathogenesis induced by Migfilin, we focused on the structure of Migfilin and the targets which it directly binds with. We also summarized the role of Migfilin and its binding partners in the progression of different diseases and malignancies. As a possible candidate for coordinating various cellular processes and because of its association with both the pathogenesis and progression of certain tumors, Migfilin likely has utility as a therapeutic target against multiple diseases in the clinic.

YBX1 knockdown induces renal cell carcinoma cell apoptosis via Kindlin-2

Among urological tumors, renal cell carcinoma (RCC) is the third-highest mortality rate tumor, and 20%-30% of RCC patients present with metastases at the time of diagnosis. While the treatment of RCC has been improved over the last few years, its mortality stays high. Y-box binding protein 1 (YBX1) is a well-known oncoprotein that has tumor-promoting functions. YBX1 is widely considered to be an attractive therapeutic target in cancer. To develop novel therapeutics to target YBX1, it is of great importance to understand how YBX1 is finely regulated in cancer. Our previous studies showed that YBX1 in RCC cells significantly promoted cell adhesion, migration, and invasion. However, the role of YBX1 in RCC cells apoptosis has not been reported. In this study, we investigated the effect of YBX1 on cell apoptosis and elucidated the mechanisms involved. Results showed that YBX1 regulated RCC cells apoptosis and reactive oxygen species (ROS) generation via Kindlin-2. These findings indicated that YBX1 inhibited RCC cells apoptosis and may serve as a candidate RCC prognostic marker and a potential therapeutic target. Abbreviations: RCC: Renal cell carcinoma; YBX1: Y-box binding protein 1; ROS: Reactive oxygen species; ccRCC: Clear cell renal cell carcinoma; mccRCC: Metastatic clear cell renal cell carcinoma; G3BP1: Ras-GTPase activating protein SH3 domain-binding proteins 1; SPP1: Secreted phosphoprotein 1; NF-κB: Nuclear factor kappa beta; ECM: Extracellular matrix; EMT: Epithelial-mesenchymal transition; PYCR1: Pyrroline-5-carboxylate reductase 1; MEM: Eagle's Minimum Essential Medium; DMEM: Dulbecco's modified Eagle medium; FBS: Fetal bovine serum; PCR: Polymerase chain reaction; shRNA: Short hairpin RNA; siRNA: Small interfering RNA; BSA: Bovine serum albumin; DCFH-DA: 2,7-Dichlorodihydrofluorescein diacetate; FITC: Fluorescein isothiocyanate; PI: Propidium iodide.

Bipolar disorder with Melnick-Needles syndrome and periventricular nodular heterotopia: two case reports and a review of the literature

Background

Melnick–Needles syndrome and periventricular nodular heterotopia are two usually mutually exclusive phenotypes of F-actin-binding cytoskeletal phosphoprotein Filamin-A mutations. Melnick–Needles syndrome is a rare X-linked condition that is lethal in males and shows great phenotypic variability in affected females. It is caused by mutations in Filamin-A gene, which encodes the protein Filamin A. Defects of the human Filamin-A gene also cause X-linked periventricular nodular heterotopia, a malformation of neuronal migration characterized by nodules of neurons in inappropriate location adjacent to the walls of the lateral ventricles.

Case presentation

We report on two Caucasian adolescent females, sisters, diagnosed with Melnick–Needles syndrome and bilateral periventricular nodular heterotopia, who developed bipolar disorder and somatic symptoms disorder at a young age. We also present a review of the literature about mental disorders associated with periventricular nodular heterotopia. Our report shows that patients presenting with atypical and heterogeneous psychiatric disease may have an underrecognized anatomical brain abnormality on genetic basis.

Conclusions

We found records of psychiatric disorders associated with periventricular nodular heterotopia; nevertheless, this is the first report of bipolar disorder occurring in individuals with periventricular nodular heterotopia, and the first report of any psychiatric disorder in individuals affected by Melnick–Needles syndrome. In conclusion, this case report may contribute to characterizing the phenotype of this very rare syndrome.

Filamin C Cardiomyopathy Variants Cause Protein and Lysosome Accumulation

[Figure: see text].

Fermitin family homolog 2 (Kindlin-2) affects vascularization during the wound healing process by regulating the Wnt/β-catenin signaling pathway in vascular endothelial cells

Kindlin-2 is a member of the FERM-containing cytoskeletal protein family that regulates cell–matrix interactions. Previous studies have shown that Kindlin-2 recruits focal adhesion proteins and regulates integration by binding to the focal adhesion region of the integrin β-segment. Although Kindlin-2 has been reported to be involved in various skin diseases and many kinds of tumors, its role in the skin wound healing process remains unclear. The aim of the present study was to investigate the role of Kindlin-2 in the regulation of wound healing. The effects of Kindlin-2 on wound healing were studied by a wound healing model, kindlin-2 (±) mice. The effects of Kindlin-2 on cell migration, cellular tube formation, and cell adhesion and spreading were evaluated in human umbilical vein endothelial cells (HUVECs) with downregulated Kindlin-2 expression. We found that the expression of kindlin-2 was elevated in wound healing tissues and that interfering with the expression of Kindlin-2 delayed the wound healing process and reduced neovascularization. We found that the wound healing of kindlin-2 (±) mice was delayed, with a decreased number of new blood vessels. Furthermore, depletion of Kindlin-2 impaired HUVEC spreading, migration and tube formation. Intriguingly, we found that kindlin-2 binds to β-catenin in the Wnt/β-catenin signaling pathway and cooperates with β-catenin to enter the nucleus from the cytoplasm, activating the downstream Wnt/β-catenin signaling pathway. Taken together, these results help to elucidate the mechanism of Kindlin-2 in the regulation of the wound healing process and provide a theoretical basis for further study of wound healing and abnormal healing.

PINCH-1 promotes Δ1-pyrroline-5-carboxylate synthase expression and contributes to proline metabolic reprogramming in lung adenocarcinoma

Proline metabolic reprogramming is intimately involved in cancer progression. We recently identified a critical role of PINCH-1, a cell-extracellular matrix (ECM) adhesion protein whose expression is elevated in lung adenocarcinoma, in the promotion of proline biosynthesis, fibrosis and lung adenocarcinoma growth. How PINCH-1 promotes proline biosynthesis, however, was incompletely understood. In this study, we show that PINCH-1 promotes the expression of Δ¹-pyrroline-5-carboxylate synthase (P5CS), a key enzyme that links glutamate metabolism to proline biosynthesis. Depletion of PINCH-1 from lung adenocarcinoma cells reduced the protein but not mRNA level of P5CS, resulting in down-regulation of the cellular level of P5C and cell proliferation. Treatment of the cells with protease inhibitor leupeptin effectively reversed PINCH-1 deficiency-induced reduction of the P5CS level. At the molecular level, PINCH-1, through its LIM2 domain, physically associated with P5CS in lung adenocarcinoma cells. Re-expression of wild type PINCH-1, but not that of the PINCH-1 LIM2 deletion mutant, in PINCH-1 deficient lung adenocarcinoma cells restored P5CS expression, proline biosynthesis and cell proliferation. Finally, P5CS expression, like that of PINCH-1, is elevated in human and mouse lung adenocarcinoma. Using a mouse model of lung adenocarcinoma in which PINCH-1 is conditionally ablated, we show that knockout of PINCH-1 from lung adenocarcinoma effectively reduced the P5CS level in vivo. Our results reveal an important role of PINCH-1 in the promotion of P5CS expression, which likely contributes to proline metabolic reprogramming and consequently lung adenocarcinoma progression.

How signaling pathways link extracellular mechano-environment to proline biosynthesis: A hypothesis: PINCH-1 and kindlin-2 sense mechanical signals from extracellular matrix and link them to proline biosynthesis

We propose a signaling pathway in which cell-extracellular matrix (ECM) adhesion components PINCH-1 and kindlin-2 sense mechanical signals from ECM and link them to proline biosynthesis, a vital metabolic pathway for macromolecule synthesis, redox balance, and ECM remodeling. ECM stiffening promotes PINCH-1 expression via integrin signaling, which suppresses dynamin-related protein 1 (DRP1) expression and mitochondrial fission, resulting in increased kindlin-2 translocation into mitochondria and interaction with Δ¹ -pyrroline-5-carboxylate (P5C) reductase 1 (PYCR1). Kindlin-2 interaction with PYCR1 protects the latter from proteolytic degradation, leading to elevated PYCR1 level. Additionally, PINCH-1 promotes P5C synthase (P5CS) expression and P5C synthesis, which, together with increased PYCR1 level, support augmented proline biosynthesis. This signaling pathway is frequently activated in fibrosis and cancer, resulting in increased proline biosynthesis and excessive collagen matrix production, which in turn further promotes ECM stiffening. Targeting this signaling pathway, therefore, may provide an effective strategy for alleviating fibrosis and cancer progression.

Kindlin-2 Acts as a Key Mediator of Lung Fibroblast Activation and Pulmonary Fibrosis Progression

Pulmonary fibrosis is a progressive and fatal lung disease characterized by activation of lung fibroblasts and excessive deposition of collagen matrix. We show here that the concentrations of kindlin-2 and its binding partner PYCR1, a key enzyme for proline synthesis, are significantly increased in the lung tissues of human patients with pulmonary fibrosis. Treatment of human lung fibroblasts with TGF-β1 markedly increased the expression of kindlin-2 and PYCR1, resulting in increased kindlin-2 mitochondrial translocation, formation of the kindlin-2-PYCR1 complex, and proline synthesis. The concentrations of the kindlin-2-PYCR1 complex and proline synthesis were markedly reduced in response to pirfenidone or nintedanib, two clinically approved therapeutic drugs for pulmonary fibrosis. Furthermore, depletion of kindlin-2 alone was sufficient to suppress TGF-β1-induced increases of PYCR1 expression, proline synthesis, and fibroblast activation. Finally, using a bleomycin mouse model of pulmonary fibrosis, we show that ablation of kindlin-2 effectively reduced the concentrations of PYCR1, proline, and collagen matrix and alleviate the progression of pulmonary fibrosis in vivo. Our results suggest that kindlin-2 is a key promoter of lung fibroblast activation, collagen matrix synthesis, and pulmonary fibrosis, underscoring the therapeutic potential of targeting the kindlin-2 signaling pathway for control of this deadly lung disease.

Structure basis of the FERM domain of kindlin-3 in supporting integrin αIIbβ3 activation in platelets

Kindlin-3, a protein 4.1, ezrin, radixin, and moesin (FERM) domain-containing adaptor in hematopoietic cells, is essentially required for supporting the bidirectional integrin αIIbβ3 signaling in platelets by binding to the integrin β3 cytoplasmic tail. However, the structural details of kindlin-3's FERM domain remain unknown. In this study, we crystalized the kindlin-3's FERM domain protein and successfully solved its 3-dimensional structure. The structure shows that the 3 kindlin-3's FERM subdomains (F1, F2, and F3) compact together and form a cloverleaf-shaped conformation, which is stabilized by the binding interface between the F1 and F3 subdomains. Interestingly, the FERM domain of kindlin-3 exists as a monomer in both crystal and solution, which is different from its counterpart in kindlin-2 that is able to form a F2 subdomain-swapped dimer; nonetheless, dimerization is required for kindlin-3 to support integrin αIIbβ3 activation, indicating that kindlin-3 may use alternative mechanisms for formation of a functional dimer in cells. To evaluate the functional importance of the cloverleaf-like FERM structure in kindlin-3, structure-based mutations were introduced into kindlin-3 to disrupt the F1/F3 interface. The results show that integrin αIIbβ3 activation is significantly suppressed in platelets expressing the kindlin-3 mutant compared with those expressing wild-type kindlin-3. In addition, introduction of equivalent mutations into kindlin-1 and kindlin-2 also significantly compromises their ability to support integrin αIIbβ3 activation in CHO cells. Together, our findings suggest that the cloverleaf-like FERM domain in kindlins is structurally important for supporting integrin αIIbβ3 activation.

Bottom-up reconstitution of focal adhesion complexes

Focal adhesions (FA) are large macromolecular assemblies relevant for various cellular and pathological events such as migration, polarization, and metastatic cancer formation. At FA sites at the migrating periphery of a cell, hundreds of players gather and form a network to respond to extra cellular stimuli transmitted by the integrin receptor, the most upstream component within a cell, initiating the FA signaling pathway. Numerous cellular experiments have been performed to understand the FA architecture and functions; however, their intricate network formation hampers unraveling the precise molecular actions of individual players. Here, in vitro bottom‐up reconstitution presents an advantageous approach for elucidating the FA machinery and the hierarchical crosstalk of involved cellular players.

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.

Kindlin-2 mediates mechanotransduction in bone by regulating expression of Sclerostin in osteocytes

Osteocytes act as mechanosensors in bone; however, the underlying mechanism remains poorly understood. Here we report that deleting Kindlin-2 in osteocytes causes severe osteopenia and mechanical property defects in weight-bearing long bones, but not in non-weight-bearing calvariae. Kindlin-2 loss in osteocytes impairs skeletal responses to mechanical stimulation in long bones. Control and cKO mice display similar bone loss induced by unloading. However, unlike control mice, cKO mice fail to restore lost bone after reloading. Osteocyte Kindlin-2 deletion impairs focal adhesion (FA) formation, cytoskeleton organization and cell orientation in vitro and in bone. Fluid shear stress dose-dependently increases Kindlin-2 expression and decreases that of Sclerostin by downregulating Smad2/3 in osteocytes; this latter response is abolished by Kindlin-2 ablation. Kindlin-2-deficient osteocytes express abundant Sclerostin, contributing to bone loss in cKO mice. Collectively, we demonstrate an indispensable novel role of Kindlin-2 in maintaining skeletal responses to mechanical stimulation by inhibiting Sclerostin expression during osteocyte mechanotransduction.

Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)

Chronic nonbacterial osteomyelitis (CNO) is an inflammatory bone disorder that most frequently affects children and adolescents. Chronic recurrent multifocal osteomyelitis (CRMO) is a severe form of CNO, usually characterized by symmetrical inflammatory bone lesions and its waxing and waning character. Sometimes severe and chronic pain can significantly affect the quality of life and psychosocial development of individuals affected. In the absence of prospectively tested and widely accepted diagnostic criteria or disease biomarkers, CNO remains a diagnosis of exclusion, and infections, malignancy and other differentials require consideration (1). The pathophysiology of CNO is not fully understood, but imbalanced cytokine expression and increased inflammasome activation in monocytes from CNO patients contribute to a pro-inflammatory phenotype that contributes to bone inflammation (2). Currently, no medications are licensed for the use in CNO. Most patients show at least some response to nonsteroidal anti-inflammatory drugs, others require more aggressive treatment that can include corticosteroids, cytokine-blocking agents and/or bisphosphonates (3). While under the care of an experienced team and sufficient treatment, the prognosis is good, but some patients will develop sequalae which can include vertebral compression fractures (1).

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CNO is an autoinflammatory bone disorder mostly affecting children and adolescents.

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Dysregulated cytokine expression and pathological activation of inflammasomes play a central role.

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Treatment is based on experience from case series and expert consensus treatment plans.

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Understanding the exact molecular pathophysiology will allow patient stratification and individualized treatment.

Updates on Genes and Genetic Mechanisms Implicated in Primary Angle-Closure Glaucoma

Primary angle-closure glaucoma (PACG) is estimated to affect over 30 million people worldwide by 2040 and is highly prevalent in the Asian population. PACG is more severe and carries three times the higher risk of blindness than primary open-angle glaucoma, thus representing a significant public health concern. High heritability and ethnic-specific predisposition to PACG suggest the involvement of genetic factors in disease development. In the recent past, genetic studies have led to the successful identification of several genes and loci associated with PACG across different ethnicities. The precise cellular and molecular roles of these multiple loci in the development and progression of PACG remains to be elucidated. Nonetheless, these studies have significantly increased our understanding of the emerging cellular processes and biological pathways that might provide more significant insights into the disease’s genetic etiology and may be valuable for future clinical applications. This review aims to summarize and update the current knowledge of PACG genetics analysis research.

The Role of Z-disc Proteins in Myopathy and Cardiomyopathy

The Z-disc acts as a protein-rich structure to tether thin filament in the contractile units, the sarcomeres, of striated muscle cells. Proteins found in the Z-disc are integral for maintaining the architecture of the sarcomere. They also enable it to function as a (bio-mechanical) signalling hub. Numerous proteins interact in the Z-disc to facilitate force transduction and intracellular signalling in both cardiac and skeletal muscle. This review will focus on six key Z-disc proteins: α-actinin 2, filamin C, myopalladin, myotilin, telethonin and Z-disc alternatively spliced PDZ-motif (ZASP), which have all been linked to myopathies and cardiomyopathies. We will summarise pathogenic variants identified in the six genes coding for these proteins and look at their involvement in myopathy and cardiomyopathy. Listing the Minor Allele Frequency (MAF) of these variants in the Genome Aggregation Database (GnomAD) version 3.1 will help to critically re-evaluate pathogenicity based on variant frequency in normal population cohorts.

Examination of FERMT1 expression in placental chorionic villi and its role in HTR8-SVneo cell invasion

Transmembrane integrin receptors mediate cell-extracellular matrix as well as cell-cell adhesion. As placental trophoblast cells undergo differentiation they display changes in integrin expression or switching, but the mechanism(s) of integrin activation that supports this differentiation is still unknown. The Fermitin family of adapter proteins (FERMT 1-3) are integrin activators that mediate integrin-mediated signaling. In this study, we examined the spatiotemporal pattern of expression of FERMT1 in human chorionic villi throughout gestation and its role in HTR8-SVneo substrate adhesion and invasion. Placental villous tissue was obtained from patients undergoing elective terminations at weeks 8-14, as well as from term deliveries at weeks 37-40 and analyzed by immunofluorescence. Additionally, HTR8-SVneo trophoblast cells were transfected with FERMT1-specific siRNA or non-targeting siRNA (control) and used in cell-substrate adhesion as well as invasion assays. FERMT1 was primarily localized to membrane-associated regions at the base or around the periphery of the villous cytotrophoblast and proximal as well as distal cell column trophoblast. FERMT1 was also localized to endothelial cells of blood vessels in chorionic villi. siRNA-mediated depletion of FERMT1 in HTR8-SVneo cells did not markedly alter HTR8-SVneo cell-substrate adhesion but did significantly decrease invasion (P < 0.05) compared to control cells. These novel findings identify the presence of the integrin activator FERMT1 in trophoblast cells and that FERMT1 can regulate HTR8-SVneo cell invasion. FERMT1 may directly influence integrin activation and the subsequent integrin-mediated signaling and differentiation that underlies the acquisition of the invasive trophoblast phenotype in vivo.

Influence of integrins on thrombus formation: a road leading to the unravelling of DVT

Integrins are a group of transmembrane glycoprotein receptors that are responsible for platelet activation through bidirectional signalling. These receptors have left their footprints in various cellular events and have intrigued many groups of scientists that have led to some significant discoveries. A lot of the recent understanding of haemostasis has been possible due to the integrins filling the gaps in between several cellular mechanism. Apart from this, other important functions carried out by integrins are growth and maturation of cardiomyocytes, mechano-transduction, and interaction with actin cytoskeleton. The signalling cascade for integrin activation involves certain intracellular interacting proteins, which initiates the step-by-step activation procedure through 'inside-out' signalling. The signalling cascade gets activated through 'outside-in' signalling with the involvement of agonists such as ADP, Fibronectin, Vitronectin, and so on. This is a crucial step for the downstream processes of platelet spreading, followed by aggregation, clot progression and finally thrombus formation. Researchers throughout the world have shown direct relation of integrins with CVD and cardiac remodelling. The present review aims to summarize the information available so far on the involvement of integrins in thrombosis and its relationship to DVT. This information could be a bedrock of hidden answers to several questions on pathogenesis of deep vein thrombosis.

A Case of Prenatally Diagnosed Periventricular Nodular Heterotopia in a Surviving Male Patient with FLNA Mutation

FLNA is a gene on the X chromosome that encodes Filamin A, a widely expressed protein crucial for forming the cell cytoskeleton and mediating cell signaling. Loss-of-function mutations have been associated with periventricular nodular heterotopia (PVNH) with associated epilepsy and intellectual deficits, as well as cardiovascular disease, connective tissue disorders, pulmonary disease, bleeding diathesis, and gastrointestinal disease. Alternatively, gain-of-function mutations have been described with otopalatodigital spectrum disorders.

The loss-of-function variants of FLNA associated with PVNH have historically been considered lethal in males, often prenatally or by the first year of life. However, more surviving males with FLNA variants are being described. Most of the surviving males have missense or distal truncating mutations or a degree of mosaicism. Others are thought to have splice site mutations or in-frame exon skipping leading to production of some degree of functional Filamin A as possible mechanisms of survival.

Here, we presented a case of a 20-month-old small but developmentally appropriate and healthy male infant who was prenatally diagnosed with PVNH, and postnatally found to have a nonsense variant of the FLNA gene. This mutation has not been previously clinically described or published to our knowledge.

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.

Molecular Tuning of Filamin A Activities in the Context of Adhesion and Migration

The dynamic organization of actin cytoskeleton meshworks relies on multiple actin-binding proteins endowed with distinct actin-remodeling activities. Filamin A is a large multi-domain scaffolding protein that cross-links actin filaments with orthogonal orientation in response to various stimuli. As such it plays key roles in the modulation of cell shape, cell motility, and differentiation throughout development and adult life. The essentiality and complexity of Filamin A is highlighted by mutations that lead to a variety of severe human disorders affecting multiple organs. One of the most conserved activity of Filamin A is to bridge the actin cytoskeleton to integrins, thereby maintaining the later in an inactive state. We here review the numerous mechanisms cells have developed to adjust Filamin A content and activity and focus on the function of Filamin A as a gatekeeper to integrin activation and associated adhesion and motility.

Migfilin supports hemostasis and thrombosis through regulating platelet αIIbβ3 outside-in signaling

Elucidating the regulation mechanism of integrin αIIbβ3 is key to understand platelet biology and thrombotic diseases. Previous in vitro studies have implicated a role of migfilin in the support of platelet αIIbβ3 activation, however, contribution of migfilin to thrombosis and hemostasis in vivo and a detailed mechanism of migfilin in platelets are not known. In this study, with migfilin deletion (migfilin-/-) mice, we report that migfilin is a pivotal positive regulator of hemostasis and thrombosis. Migfilin-/- mice showed a nearly doubled tail-bleeding time and a prolonged occlusion time in Fecl3-induced mesenteric arteriolar thrombosis. Migfilin deficiency impedes platelet thrombi formation on collagen surface and impairs platelet aggregation and dense-granule secretion. Supported by characteristic functional readings and phosphorylation status of distinctive signaling molecules in the bidirectional signaling processes of αIIbβ3, the functional defects of migfilin-/- platelets appear to be mechanistically associated with a compromised outside-in signaling, rather than inside-out signaling. A synthesized cell-permeable migfilin peptide harboring filamin A binding sequence rescued the defective function and phosphorylation of signaling molecules of migfilin-/- platelets. Finally, migfilin does not influence the binding of filamin A and β3 subunit of αIIbβ3 in resting platelets, but hampers the re-association of filamin A and β3 during the conduct of outside-in signaling, suggesting that migfilin functions through regulating the interaction dynamics of αIIbβ3 and filamin A in platelets. Our study enhances the current understanding of platelet integrin αIIbβ3-mediated outside-in signaling and proves that migfilin is an important regulator for platelet activation, hemostasis and thrombosis.

Genetic aetiology of blood pressure relates to aortic stiffness with bi-directional causality: evidence from heritability, blood pressure polymorphisms, and Mendelian randomization

AIMS

Haemodynamic determinants of blood pressure (BP) include cardiac output (CO), systemic vascular resistance (SVR), and arterial stiffness. We investigated the heritability of these phenotypes, their association with BP-related single-nucleotide polymorphisms (SNPs), and the causal association between BP and arterial stiffness.

METHODS AND RESULTS

We assessed BP, central BP components, and haemodynamic properties (during a single visit) including CO, SVR, and pulse wave velocity (PWV, measure of arterial stiffness) in 3531 (1934 monozygotic, 1586 dizygotic) female TwinsUK participants. Heritability was estimated using structural equation modelling. Association with 984 BP-associated SNP was examined using least absolute shrinkage and selection operator (LASSO) and generalized estimating equation regression. One and two-sample Mendelian randomization (MR) was used to estimate the causal direction between BP and arterial stiffness including data on 436 419 UK Biobank participants. We found high heritability for systolic and pulsatile components of BP (>50%) and PWV (65%) with overlapping genes accounting for >50% of their observed correlation. Environmental factors explained most of the variability of CO and SVR (>80%). Regression identified SNPs (n = 5) known to be associated with BP to also be associated with PWV. One-sample MR showed evidence of bi-directional causal association between BP and PWV in TwinsUK participants. Two-sample MR, confirmed a bi-directional causal effect of PWV on BP (inverse variance weighted (IVW) beta = 0.11, P < 0.02) and BP on arterial stiffness (IVW beta = 0.004, P < 0.0001).

CONCLUSION

The genetic basis of BP is mediated not only by genes regulating BP but also by genes that influence arterial stiffness. Mendelian randomization indicates a bi-directional causal association between BP and arterial stiffness.

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.

Mitochondrial metabolism and cancer metastasis

Metastasis is regarded as the most important cause of cancer-related deaths around the world. During the complicated metastatic cascade, altered mitochondrial metabolism adapts to serve distinct conditions and microenvironments. In this review, we discuss how cells regulate their mitochondria metabolism to adapt to environmental cues during the metastasis, as well as how cancer cells and their tumor micro-environment (TME) are metabolically coupled during the metastatic cascade. We place a strong emphasis on how mitochondrial proline metabolism and extracellular matrix (ECM) are coupled.

Collagen content and extracellular matrix cause cytoskeletal remodelling in pancreatic fibroblasts

In many solid tumours a desmoplastic reaction takes place, which results in tumour tissue stiffening due to the extensive production of extracellular matrix (ECM) proteins, such as collagen, by stromal cells, mainly fibroblasts (FBs) and cancer-associated fibroblasts (CAFs). In this study, we investigated the effect of collagen stiffness on pancreatic FBs and CAFs, particularly on specific cytoskeleton properties and gene expression involved in tumour invasion. We found that cells become stiffer when they are cultured on stiff substrates and express higher levels of alpha-smooth muscle actin (α-SMA). Also, it was confirmed that on stiff substrates, CAFs are softer than FBs, while on soft substrates they have comparable Young's moduli. Furthermore, the number of spread FBs and CAFs was higher in stiffer substrates, which was also confirmed by Ras-related C3 botulinum toxin substrate 1 ( RAC1) mRNA expression, which mediates cell spreading. Although stress fibres in FBs become more oriented on stiff substrates, CAFs have oriented stress fibres regardless of substrate stiffness. Subsequently, we demonstrated that cells' invasion has a differential response to stiffness, which was associated with regulation of Ras homologue family member ( RhoA) and Rho-associated, coiled-coil containing protein kinase 1 ( ROCK-1) mRNA expression. Overall, our results demonstrate that collagen stiffness modulates FBs and CAFs cytoskeleton remodelling and alters their invasion properties.

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.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

Site-specific phosphorylation regulates the functions of kindlin-3 in a variety of cells

Studies of isolated cells, mice, and humans have demonstrated the vital role of the FERM domain protein kindlin-3 in integrin activation in certain hematopoietic and non-hematopoietic cells, consequent to binding to integrin β-subunits. To explore regulatory mechanisms, we developed a monoclonal antibody that selectively recognizes the phosphorylated form of Ser⁴⁸⁴ (pS⁴⁸⁴) in kindlin-3. Activation of platelets, HEL megakaryocytic-like cells and BT549 breast cancer cells led to enhanced expression of pS⁴⁸⁴ as assessed by immunofluorescence or Western blotting. In platelets, pS⁴⁸⁴ rose rapidly and transiently upon stimulation. When a mutant form of kindlin-3, T⁴⁸²S⁴⁸⁴/AA kindlin-3, was transduced into mouse megakaryocytes, it failed to support activation of integrin α_IIbβ₃, whereas wild-type kindlin-3 did. In MDA-MB231 breast cancer cells, expression of T⁴⁸²S⁴⁸⁴/AA kindlin-3 suppressed cell spreading, migration, invasion, and VEGF production. Wild-type kindlin-3 expressing cells markedly increased tumor growth in vivo, whereas T⁴⁸²S⁴⁸⁴/AA kindlin-3 significantly blunted tumor progression. Thus, our data establish that a unique phosphorylation event in kindlin-3 regulates its cellular functions.

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.

IKKε phosphorylates kindlin-2 to induce invadopodia formation and promote colorectal cancer metastasis

Invadopodia formation is a key driver of cancer metastasis. The noncanonical IkB-related kinase IKKε has been implicated in cancer metastasis, but its roles in invadopodia formation and colorectal cancer (CRC) metastasis are unclear. Methods: Immunofluorescence, gelatin-degradation assay, wound healing assay and transwell invasion assay were used to determine the influence of IKKε over-expression, knockdown and pharmacological inhibition on invadopodia formation and the migratory and invasive capacity of CRC cells in vitro. Effects of IKKε knockdown or pharmacological inhibition on CRC metastasis were examined in mice. Immunohistochemistry staining was used to detect expression levels of IKKε in CRC patient tissues, and its association with prognosis in CRC patients was also analyzed. Immunoprecipitation, western blotting and in vitro kinase assay were constructed to investigate the molecular mechanisms. Results: IKKε co-localizes with F-actin and the invadopodia marker Tks5 at the gelatin-degrading sites of CRC cells. Genetic over-expression/knockdown or pharmacological inhibition of IKKε altered invadopodia formation and the migratory and invasive capacity of CRC cells in vitro. In vivo, knockdown or pharmacological inhibition of IKKε significantly suppressed metastasis of CRC cells in mice. IKKε knockdown also inhibited invadopodia formation in vivo. Clinical investigation of tumor specimens from 191 patients with CRC revealed that high IKKε expression correlates with metastasis and poor prognosis of CRC. Mechanistically, IKKε directly binds to and phosphorylates kindlin-2 at serine 159; this effect mediates the IKKε-induced invadopodia formation and promotion of CRC metastasis. Conclusions: We identify IKKε as a novel regulator of invadopodia formation and a unique mechanism by which IKKε promotes the metastasis of CRC. Our study suggests that IKKε is a potential target to suppress CRC metastasis.

Tumor-associated macrophages regulate gastric cancer cell invasion and metastasis through TGFβ2/NF-κB/Kindlin-2 axis

Objective

Recent studies have shown that tumor-associated macrophages (TAMs) play an important role in cancer invasion and metastasis. Our previous studies have reported that TAMs promote the invasion and metastasis of gastric cancer (GC) cells through the Kindlin-2 pathway. However, the mechanism needs to be clarified.

Methods

THP-1 monocytes were induced by PMA/interleukin (IL)-4/IL-13 to establish an efficient TAM model in vitro and M2 macrophages were isolated via flow cytometry. A dual luciferase reporter system and chromatin immunoprecipitation (ChIP) assay were used to investigate the mechanism of transforming growth factor β2 (TGFβ2) regulating Kindlin-2 expression. Immunohistochemistry was used to study the relationships among TAM infiltration in human GC tissues, Kindlin-2 protein expression, clinicopathological parameters and prognosis in human GC tissues. A nude mouse oncogenesis model was used to verify the invasion and metastasis mechanisms in vivo.

Results

We found that Kindlin-2 expression was upregulated at both mRNA and protein levels in GC cells cocultured with TAMs, associated with higher invasion rate. Kindlin-2 knockdown reduced the invasion rate of GC cells under coculture condition. TGFβ2 secreted by TAMs regulated the expression of Kindlin-2 through the transcription factor NF-кB. TAMs thus participated in the progression of GC through the TGFβ2/NF-κB/Kindlin-2 axis. Kindlin-2 expression and TAM infiltration were significantly positively correlated with TNM stage, and patients with high Kindlin-2 expression had significantly poorer overall survival than patients with low Kindlin-2 expression. Furthermore, Kindlin-2 promoted the invasion of GC cells in vivo.

Conclusions

This study elucidates the mechanism of TAMs participating in GC cell invasion and metastasis through the TGFβ2/NF-κB/Kindlin-2 axis, providing a possibility for new treatment options and approaches.

The Integrin Activating Protein Kindlin-3 Is Cleaved in Human Platelets during ST-Elevation Myocardial Infarction

Kindlins are important proteins for integrin signaling and regulation of the cytoskeleton, but we know little about their precise function and regulation in platelets during acute ischemic events. In this work, we investigated kindlin-3 protein levels in platelets isolated from patients with ST-elevation myocardial infarction (STEMI) compared to patients with non-ischemic chest pain. Platelets from twelve patients with STEMI and twelve patients with non-ischemic chest pain were isolated and analyzed for kindlin-3 protein levels and intracellular localization by immunoblotting and two-dimensional gel electrophoresis. Platelet proteome analysis by two-dimensional gel electrophoresis and protein sequencing identified kindlin-3 as a protein that is cleaved in platelets from patients with myocardial infarction. Kindlin-3 full-length protein was significantly decreased in patients with STEMI compared to patients with non-ischemic chest pain (1.0 ± 0.2 versus 0.28 ± 0.2, p < 0.05) by immunoblotting. Kindlin-3 showed a differential distribution and was primarily cleaved in the cytosolic and membrane compartment of platelets in myocardial infarction. Platelet activation with thrombin alone did not affect kindlin-3 protein levels. The present study demonstrates that kindlin-3 protein levels become significantly reduced in platelets of patients with myocardial infarction compared to controls. The results suggest that kindlin-3 cleavage in platelets is associated with the ischemic event of myocardial infarction.