GEF-H1 controls focal adhesion signaling that regulates mesenchymal stem cell lineage commitment

High-viscosity driven modulation of biomechanical properties of human mesenchymal stem cells promotes osteogenic lineage

Biomechanical cues could effectively govern cell gene expression to direct the differentiation of specific stem cell lineage. Recently, the medium viscosity has emerged as a significant mechanical stimulator that regulates the cellular mechanical properties and various physiological functions. However, whether the medium viscosity can regulate the mechanical properties of human mesenchymal stem cells (hMSCs) to effectively trigger osteogenic differentiation remains uncertain. The mechanism by which cells sense and respond to changes in medium viscosity, and regulate cell mechanical properties to promote osteogenic lineage, remains elusive. In this study, we demonstrated that hMSCs, cultured in a high-viscosity medium, exhibited larger cell spreading area and higher intracellular tension, correlated with elevated formation of actin stress fibers and focal adhesion maturation. Furthermore, these changes observed in hMSCs were associated with activation of TRPV4 (transient receptor potential vanilloid sub-type 4) channels on the cell membrane. This feedback loop among TRPV4 activation, cell spreading and intracellular tension results in calcium influx, which subsequently promotes the nuclear localization of NFATc1 (nuclear factor of activated T cells 1). Concomitantly, the elevated intracellular tension induced nuclear deformation and promoted the nuclear localization of YAP (YES-associated protein). The concurrent activation of NFATc1 and YAP significantly enhanced alkaline phosphatase (ALP) for pre-osteogenic activity. Taken together, these findings provide a more comprehensive view of how viscosity-induced alterations in biomechanical properties of MSCs impact the expression of osteogenesis-related genes, and ultimately promote osteogenic lineage.

S100A11 promotes focal adhesion disassembly via myosin II-driven contractility and Piezo1-mediated Ca2+ entry

S100A11 is a small Ca2+-activatable protein known to localize along stress fibers (SFs). Analyzing S100A11 localization in HeLa and U2OS cells further revealed S100A11 enrichment at focal adhesions (FAs). Strikingly, S100A11 levels at FAs increased sharply, yet transiently, just before FA disassembly. Elevating intracellular Ca2+ levels with ionomycin stimulated both S100A11 recruitment and subsequent FA disassembly. However, pre-incubation with the non-muscle myosin II (NMII) inhibitor blebbistatin or with an inhibitor of the stretch-activatable Ca2+ channel Piezo1 suppressed S100A11 recruitment, implicating S100A11 in an actomyosin-driven FA recruitment mechanism involving Piezo1-dependent Ca2+ influx. Applying external forces on peripheral FAs likewise recruited S100A11 to FAs even if NMII activity was inhibited, corroborating the mechanosensitive recruitment mechanism of S100A11. However, extracellular Ca2+ and Piezo1 function were indispensable, indicating that NMII contraction forces act upstream of Piezo1-mediated Ca2+ influx, in turn leading to S100A11 activation and FA recruitment. S100A11-knockout cells display enlarged FAs and had delayed FA disassembly during cell membrane retraction, consistent with impaired FA turnover in these cells. Our results thus demonstrate a novel function for S100A11 in promoting actomyosin contractility-driven FA disassembly.

Modulation of gene expression by YTH domain family (YTHDF) proteins in human physiology and pathology

The advent of high throughput techniques in the past decade has significantly advanced the field of epitranscriptomics. The internal chemical modification of the target RNA at a specific site is a basic feature of epitranscriptomics and is critical for its structural stability and functional property. More than 170 modifications at the transcriptomic level have been reported so far, among which m6A methylation is one of the more conserved internal RNA modifications, abundantly found in eukaryotic mRNAs and frequently involved in enhancing the target messenger RNA's (mRNA) stability and translation. m6A modification of mRNAs is essential for multiple physiological processes including stem cell differentiation, nervous system development and gametogenesis. Any aberration in the m6A modification can often result in a pathological condition. The deregulation of m6A methylation has already been described in inflammation, viral infection, cardiovascular diseases and cancer. The m6A modification is reversible in nature and is carried out by specialized m6A proteins including writers (m6A methyltransferases) that add methyl groups and erasers (m6A demethylases) that remove methyl groups selectively. The fate of m6A-modified mRNA is heavily reliant on the various m6A-binding proteins ("readers") which recognize and generate a functional signal from m6A-modified mRNA. In this review, we discuss the role of a family of reader proteins, "YT521-B homology domain containing family" (YTHDF) proteins, in human physiology and pathology. In addition, we critically evaluate the potential of YTHDF proteins as therapeutic targets in human diseases.

The effects of locomotion on bone marrow mesenchymal stem cell fate: insight into mechanical regulation and bone formation

Bone marrow mesenchymal stem cells (BMSCs) refer to a heterogeneous population of cells with the capacity for self-renewal. BMSCs have multi-directional differentiation potential and can differentiate into chondrocytes, osteoblasts, and adipocytes under specific microenvironment or mechanical regulation. The activities of BMSCs are closely related to bone quality. Previous studies have shown that BMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. Thus, a goal of this review is to discuss how these ubiquious signals arising from mechanical stimulation are perceived by BMSCs and then how the cells respond to them. Studies in recent years reported a significant effect of locomotion on the migration, proliferation and differentiation of BMSCs, thus, contributing to our bone mass. This regulation is realized by the various intersecting signaling pathways including RhoA/Rock, IFG, BMP and Wnt signalling. The mechanoresponse of BMSCs also provides guidance for maintaining bone health by taking appropriate exercises. This review will summarize the regulatory effects of locomotion/mechanical loading on BMSCs activities. Besides, a number of signalling pathways govern MSC fate towards osteogenic or adipocytic differentiation will be discussed. The understanding of mechanoresponse of BMSCs makes the foundation for translational medicine.

S-1-propenylcysteine improves TNF-α-induced vascular endothelial barrier dysfunction by suppressing the GEF-H1/RhoA/Rac pathway

Background

Vascular endothelial barrier function is maintained by cell-to-cell junctional proteins and contributes to vascular homeostasis. Various risk factors such as inflammation disrupt barrier function through down-regulation of these proteins and promote vascular diseases such as atherosclerosis. Previous studies have demonstrated that aged garlic extract (AGE) and its sulfur-containing constituents exert the protective effects against several vascular diseases such as atherosclerosis. In this study, we examined whether AGE and its sulfur-containing constituents improve the endothelial barrier dysfunction elicited by a pro-inflammatory cytokine, Tumor-necrosis factor-α (TNF-α), and explored their mode of action on TNF-α signaling pathway.

Methods

Human umbilical vein endothelial cells (HUVECs) were treated with test substances in the presence of TNF-α for various time periods. The endothelial permeability was measured by using a transwell permeability assay. The localization of cell-to-cell junctional proteins and actin cytoskeletons were visualized by immunostaining. RhoA and Rac activities were assessed by using GTP-binding protein pulldown assay. Gene and protein expression levels of signaling molecules were analyzed by real-time PCR and western blotting, respectively.

Results

We found that AGE and its major sulfur-containing constituent, S-1-propenylcysteine (S1PC), reduced hyperpermeability elicited by TNF-α in HUVECs. In addition, S1PC inhibited TNF-α-induced production of myosin light chain (MLC) kinase and inactivation of MLC phosphatase through the suppression of the Rac and RhoA signaling pathways, respectively, which resulted in the dephosphorylation of MLC2, a key factor of actin remodeling. Moreover, S1PC inhibited the phosphorylation and activation of guanine nucleotide exchange factor-H1 (GEF-H1), a common upstream key molecule and activator of Rac and RhoA. These effects of S1PC were accompanied by its ability to prevent the disruption of junctional proteins on the cell–cell contact regions and the increase of actin stress fibers induced by TNF-α.

Conclusions

The present study suggested that AGE and its major constituent, S1PC, improve endothelial barrier disruption through the protection of junctional proteins on plasma membrane.

Revealing Alteration in the Hepatic Glucose Metabolism of Genetically Improved Carp, Jayanti Rohu Labeo rohita Fed a High Carbohydrate Diet Using Transcriptome Sequencing

Although feed cost is the greatest concern in aquaculture, the inclusion of carbohydrates in the fish diet, and their assimilation, are still not well understood in aquaculture species. We identified molecular events that occur due to the inclusion of high carbohydrate levels in the diets of genetically improved 'Jayanti rohu' Labeo rohita. To reveal transcriptional changes in the liver of rohu, a feeding experiment was conducted with three doses of gelatinized starch (20% (control), 40%, and 60%). Transcriptome sequencing revealed totals of 15,232 (4464 up- and 4343 down-regulated) and 15,360 (4478 up- and 4171 down-regulated) differentially expressed genes. Up-regulated transcripts associated with glucose metabolisms, such as hexokinase, PHK, glycogen synthase and PGK, were found in fish fed diets with high starch levels. Interestingly, a de novo lipogenesis mechanism was found to be enriched in the livers of treated fish due to up-regulated transcripts such as FAS, ACCα, and PPARγ. The insulin signaling pathways with enriched PPAR and mTOR were identified by Kyoto Encyclopedia of Genes and Genome (KEGG) as a result of high carbohydrates. This work revealed for the first time the atypical regulation transcripts associated with glucose metabolism and lipogenesis in the livers of Jayanti rohu due to the inclusion of high carbohydrate levels in the diet. This study also encourages the exploration of early nutritional programming for enhancing glucose efficiency in carp species, for sustainable and cost-effective aquaculture production.

Regulation and functions of the RhoA regulatory guanine nucleotide exchange factor GEF-H1

Since the discovery by Madaule and Axel in 1985 of the first Ras homologue (Rho) protein in Aplysia and its human orthologue RhoB, membership in the Rho GTPase family has grown to 20 proteins, with representatives in all eukaryotic species. These GTPases are molecular switches that cycle between active (GTP bound) and inactivate (GDP bound) states. The exchange of GDP for GTP on Rho GTPases is facilitated by guanine exchange factors (GEFs). Approximately 80 Rho GEFs have been identified to date, and only a few GEFs associate with microtubules. The guanine nucleotide exchange factor H1, GEF-H1, is a unique GEF that associates with microtubules and is regulated by the polymerization state of microtubule networks. This review summarizes the regulation and functions of GEF-H1 and discusses the roles of GEF-H1 in human diseases.

Cell-Cell Adhesion-Driven Contact Guidance and Its Effect on Human Mesenchymal Stem Cell Differentiation

Contact guidance has been extensively explored using patterned adhesion functionalities that predominantly mimic cell-matrix interactions. Whether contact guidance can also be driven by other types of interactions, such as cell-cell adhesion, still remains a question. Herein, this query is addressed by engineering a set of microstrip patterns of (i) cell-cell adhesion ligands and (ii) segregated cell-cell and cell-matrix ligands as a simple yet versatile set of platforms for the guidance of spreading, adhesion, and differentiation of mesenchymal stem cells. It was unprecedently found that micropatterns of cell-cell adhesion ligands can induce contact guidance. Surprisingly, it was found that patterns of alternating cell-matrix and cell-cell strips also induce contact guidance despite providing a spatial continuum for cell adhesion. This guidance is believed to be due to the difference between the potencies of the two adhesions. Furthermore, patterns that combine the two segregated adhesion functionalities were shown to induce more human mesenchymal stem cell osteogenic differentiation than monofunctional patterns. This work provides new insight into the functional crosstalk between cell-cell and cell-matrix adhesions and, overall, further highlights the ubiquitous impact of the biochemical anisotropy of the extracellular environment on cell function.

Lineage Commitment, Signaling Pathways, and the Cytoskeleton Systems in Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) from adult tissues are promising candidates for personalized cell therapy and tissue engineering. Significant progress was achieved in our understanding of the regulation of MSCs proliferation and differentiation by different cues during the past years. Proliferation and differentiation of MSCs are sensitive to the extracellular matrix (ECM) properties, physical cues, and chemical signaling. Sheath stress, matrix stiffness, surface adhesiveness, and micro- and nanotopography define cell shape and dictate lineage commitment of MSCs even in the absence of specific chemical signals. We discuss mechanotransduction as the major route from ECM through the cytoskeleton toward signaling pathways and gene expression. All components of the cytoskeleton from primary cilium and focal adhesions (FAs) to actin, microtubules (MTs), and intermediate filaments (IFs) are involved in the mechanotransduction. Differentiation of MSCs is regulated via the complex network of interrelated signaling pathways, including RhoA/ROCK, Akt/Erk, and YAP/TAZ effectors of Hippo pathway. These pathways could be regulated both by chemical and mechanical stimuli. Attenuation of these pathways in MSCs results in specific changes in FAs and actin cytoskeleton. Besides, differentiation of MSCs affects MTs and IFs. Recent findings highlight the role of intranuclear actin in the regulation of transcription factors in response to mechanical environmental stimuli. Alterations of cytoskeletal components reflect the MSC senescence state and their migratory capacity. In this review, we discuss the relationships between the molecular interactions in signaling pathways and morphological response of cytoskeletal components and reveal the complex interrelations between cytoskeleton systems and signaling pathways during lineage commitment of MSCs. Impact Statement This review describes the complex network of relationships between mechanical and biochemical stimuli in mesenchymal stem cells (MSC) and their balance which defines the morphological changes of cell shape due to rearrangement of cytoskeletal systems during lineage commitment of MSCs.

Enhanced bone regeneration capability of chitosan sponge coated with TiO2 nanoparticles

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Chitosan hybridized with titanium dioxide nanoparticles improves its bone regeneration capability.

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Nano titanium dioxide addition to the matrix of chitosan sponges was done successfully, as depicted from an even distribution of nano titanium dioxide on the surface of the sponges.

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Chitosan – nanoTiO₂ scaffold results in significantly improved sponge robustness, biomineralization, and bone regeneration capability, as indicated by DMP1 and OCN gene upregulation in chitosan-50% nanoTiO₂ sample.

Chitosan has been a popular option for tissue engineering, however exhibits limited function for bone regeneration due to its low mechanical robustness and non-osteogenic inductivity. Here we hybridized chitosan with TiO₂ nanoparticles to improve its bone regeneration capability. Morphology and crystallographic analysis showed that TiO₂ nanoparticles in anatase-type were distributed evenly on the surface of the chitosan sponges. Degradation test showed a significant effect of TiO₂ nanoparticles addition in retaining its integrity. Biomineralization assay using simulated body fluid showed apatite formation in sponges surface as denoted by PO⁴⁻ band observed in FTIR results. qPCR analysis supported chitosan - TiO₂ sponges in bone regeneration capability as indicated by DMP1 and OCN gene upregulation in TiO₂ treated group. Finally, cytotoxicity analysis supported the fact that TiO₂ nanoparticles added sponges were proved to be biocompatible. Results suggest that chitosan-50% TiO₂ nanoparticles sponges could be a potential novel scaffold for bone tissue engineering.

Integrin signaling and mechanotransduction in regulation of somatic stem cells

Somatic stem cells are characterized by their capacity for self-renewal and differentiation, making them integral for normal tissue homeostasis. Different stem cell functions are strongly affected by the specialized microenvironment surrounding the cells. Consisting of soluble signaling factors, extracellular matrix (ECM) ligands and other cells, but also biomechanical cues such as the viscoelasticity and topography of the ECM, these factors are collectively known as the niche. Cell-ECM interactions are mediated largely by integrins, a class of heterodimeric cell adhesion molecules. Integrins bind their ligands in the extracellular space and associate with the cytoskeleton inside the cell, forming a direct mechanical link between the cells and their surroundings. Indeed, recent findings have highlighted the importance of integrins in translating biophysical cues into changes in cell signaling and function, a multistep process known as mechanotransduction. The mechanical properties of the stem cell niche are important, yet the underlying molecular details of integrin-mediated mechanotransduction in stem cells, especially the roles of the different integrin heterodimers, remain elusive. Here, we introduce the reader to the concept of integrin-mediated mechanotransduction, summarize current knowledge on the role of integrin signaling and mechanotransduction in regulation of somatic stem cell functions, and discuss open questions in the field.

Molecular Organization of Integrin-Based Adhesion Complexes in Mouse Embryonic Stem Cells

The mechanical microenvironment serves as an important factor influencing stem cell differentiation. Mechanobiological responses depend strongly on actomyosin contractility and integrin-based cell-extracellular matrix (ECM) interactions mediated by adhesive structures such as focal adhesions (FAs). While the roles of FAs in mechanobiology have been intensively studied in many mesenchymal and migratory cell types, recently it has been recognized that certain pluripotent stem cells (PSCs) exhibited significantly attenuated FA-mediated mechanobiological responses. FAs in such PSCs are sparsely distributed and much less prominent in comparison to "classical" FAs of typical adherent cells. Despite these differences, insights into how FAs in PSCs are structurally organized to perform their functions are still elusive. Using mouse embryonic stem cells (mESCs) to study PSC-ECM interactions, here we surveyed the molecular composition and nanostructural organization of FAs. We found that, despite being small in size, mESC FAs appeared to be compositionally mature, containing markers such as vinculin, zyxin, and α-actinin, and dependent on myosin II contractility. Using super-resolution microscopy, we revealed that mESC FAs were organized into a conserved multilayer nanoscale architecture. However, the nanodomain organization was compressed in mESCs, with the force transduction layer spanning ∼10 nm, significantly more compact than in FAs of other cell types. Furthermore, we found that the position and orientation of vinculin, a key mechanotransduction protein, were modulated in an ECM-dependent manner. Our analysis also revealed that while most core FA genes were expressed, the expression of LIM domain proteins was comparatively lower in PSCs. Altogether our results suggest that while core structural and mechanosensitive elements are operational in mESC FAs, their structural organization and regulatory aspects may diverge significantly from "classical" FAs, which may account for the attenuated mechanobiological responses of these cell types.

Intra-hydrogel culture prevents transformation of mesenchymal stem cells induced by monolayer expansion

In this study, we report that the intra-hydrogel culture system mitigates the transformation of mesenchymal stem cells (MSCs) induced by two-dimensional (2D) expansion. MSCs expanded in monolayer culture prior to encapsulation in collagen hydrogels (group eMSCs-CH) featured impaired stemness in chondrogenesis, comparing with the freshly isolated bone marrow mononuclear cells seeded directly in collagen hydrogels (group fMSCs-CH). The molecular mechanism of the in vitro expansion-triggered damage to MSCs was detected through genome-wide microarray analysis. Results indicated that pathways such as proteoglycans in cancer and pathways in cancer expansion were highly enriched in eMSCs-CH. And multiple up-regulated oncoma-associated genes were verified in eMSCs-CH compared with fMSCs-CH, indicating that expansion in vitro triggered cellular transformation was associated with signaling pathways related to tumorigenicity. Besides, focal adhesion (FA) and mitogen-activated protein kinase (MAPK) signaling pathways were also involved in in vitro expansion, indicating restructuring of the cell architecture. Thus, monolayer expansion in vitro may contribute to vulnerability of MSCs through the regulation of FA and MAPK. This study indicates that intra-hydrogel culture can mitigate the monolayer expansion induced transformation of MSCs and maintain the uniformity of the stem cells, which is a viable in vitro culture system for stem cell therapy.

Non-muscle myosin IIB (Myh10) is required for epicardial function and coronary vessel formation during mammalian development

The coronary vasculature is an essential vessel network providing the blood supply to the heart. Disruptions in coronary blood flow contribute to cardiac disease, a major cause of premature death worldwide. The generation of treatments for cardiovascular disease will be aided by a deeper understanding of the developmental processes that underpin coronary vessel formation. From an ENU mutagenesis screen, we have isolated a mouse mutant displaying embryonic hydrocephalus and cardiac defects (EHC). Positional cloning and candidate gene analysis revealed that the EHC phenotype results from a point mutation in a splice donor site of the Myh10 gene, which encodes NMHC IIB. Complementation testing confirmed that the Myh10 mutation causes the EHC phenotype. Characterisation of the EHC cardiac defects revealed abnormalities in myocardial development, consistent with observations from previously generated NMHC IIB null mouse lines. Analysis of the EHC mutant hearts also identified defects in the formation of the coronary vasculature. We attribute the coronary vessel abnormalities to defective epicardial cell function, as the EHC epicardium displays an abnormal cell morphology, reduced capacity to undergo epithelial-mesenchymal transition (EMT), and impaired migration of epicardial-derived cells (EPDCs) into the myocardium. Our studies on the EHC mutant demonstrate a requirement for NMHC IIB in epicardial function and coronary vessel formation, highlighting the importance of this protein in cardiac development and ultimately, embryonic survival.

Fibronectin in cell adhesion and migration via N-glycosylation

Directed cell migration is an important step in effective wound healing and requires the dynamic control of the formation of cell-extracellular matrix interactions. Plasma fibronectin is an extracellular matrix glycoprotein present in blood plasma that plays crucial roles in modulating cellular adhesion and migration and thereby helping to mediate all steps of wound healing. In order to seek safe sources of plasma fibronectin for its practical use in wound dressing, we isolated fibronectin from human (homo) and porcine plasma and demonstrated that both have a similar ability as a suitable substrate for the stimulation of cell adhesion and for directing cell migration. In addition, we also defined the N-glycosylation sites and N-glycans present on homo and porcine plasma fibronectin. These N-glycosylation modifications of the plasma fibronectin synergistically support the integrin-mediated signals to bring about mediating cellular adhesion and directed cell migration. This study not only determines the important function of N-glycans in both homo and porcine plasma fibronectin-mediated cell adhesion and directed cell migration, but also reveals the potential applications of porcine plasma fibronectin if it was applied as a material for clinical wound healing and tissue repair.

A proteomics view on integrin-mediated adhesions

Individual cells in multicellular organisms constantly explore their microenvironment, or niche, to obtain spatial information that is used to regulate cell behavior to maintain tissue integrity. The extracellular matrix (ECM) is an important source of such spatial information. Binding of the integrin family receptors to the ECM triggers formation of integrin adhesion complexes (IACs) that link the ECM network to cellular cytoskeleton via remarkably large multiprotein complexes collectively referred to as the integrin adhesome. Recent advances in proteomics have enabled researchers to study the IAC composition in detail. Various biochemical IAC isolation methods and culture conditions have been employed to study the composition and dynamics of integrin-mediated adhesions mainly in fibroblasts and lymphoblasts. These studies have led to identification of daunting lists of potential IAC components. This review focuses on the current status of proteomics-driven research seeking to understand integrin functions by comprehensive analysis of IAC components. These systems level approaches have revealed the complexity of biochemical and biomechanical signals that are processed at IACs and provide a novel insight into how these signals are conveyed to regulate cellular behavior.

The integrin adhesome network at a glance

The adhesion nexus is the site at which integrin receptors bridge intracellular cytoskeletal and extracellular matrix networks. The connection between integrins and the cytoskeleton is mediated by a dynamic integrin adhesion complex (IAC), the components of which transduce chemical and mechanical signals to control a multitude of cellular functions. In this Cell Science at a Glance article and the accompanying poster, we integrate the consensus adhesome, a set of 60 proteins that have been most commonly identified in isolated IAC proteomes, with the literature-curated adhesome, a theoretical network that has been assembled through scholarly analysis of proteins that localise to IACs. The resulting IAC network, which comprises four broad signalling and actin-bridging axes, provides a platform for future studies of the regulation and function of the adhesion nexus in health and disease.

miR-194 is a negative regulator of GEF-H1 pathway in melanoma

The incidence and associated mortality of melanoma continues to increase worldwide. At present, there is no curative therapy for advanced stage of melanoma. It is necessary to find new indicators of prognosis and therapeutic targets. Increasing evidence shows that miRNA can provide potential candidate biomarkers for melanoma and therapeutic targets. GEF-H1, a regulator of RhoA, as oncogenic driver in melanoma, promotes the growth and invasion of melanoma. miR-194 is a tumor-suppressor gene in multiple tumors, such as bladder and non-small cell lung cancer, and clear cell renal cell carcinoma. In the present study, we demonstrated that GEF-H1 serves as target of miR-194. Overexpression of miR-194 downregulates the GEF-H1/RhoA pathway, inhibits melanoma cancer cell proliferation and metastasis. Furthermore, miR-194 expression is negatively associated with tumor-node-metastasis (TNM) stages. Briefly, our findings provided new theoretical basis for melanoma treatment.

Tight junctions: from simple barriers to multifunctional molecular gates

Epithelia and endothelia separate different tissue compartments and protect multicellular organisms from the outside world. This requires the formation of tight junctions, selective gates that control paracellular diffusion of ions and solutes. Tight junctions also form the border between the apical and basolateral plasma-membrane domains and are linked to the machinery that controls apicobasal polarization. Additionally, signalling networks that guide diverse cell behaviours and functions are connected to tight junctions, transmitting information to and from the cytoskeleton, nucleus and different cell adhesion complexes. Recent advances have broadened our understanding of the molecular architecture and cellular functions of tight junctions.

Modulation of FAK and Src adhesion signaling occurs independently of adhesion complex composition

Integrin adhesion complexes (IACs) form mechanochemical connections between the extracellular matrix and actin cytoskeleton and mediate phenotypic responses via posttranslational modifications. Here, we investigate the modularity and robustness of the IAC network to pharmacological perturbation of the key IAC signaling components focal adhesion kinase (FAK) and Src. FAK inhibition using AZ13256675 blocked FAK(Y397) phosphorylation but did not alter IAC composition, as reported by mass spectrometry. IAC composition was also insensitive to Src inhibition using AZD0530 alone or in combination with FAK inhibition. In contrast, kinase inhibition substantially reduced phosphorylation within IACs, cell migration and proliferation. Furthermore using fluorescence recovery after photobleaching, we found that FAK inhibition increased the exchange rate of a phosphotyrosine (pY) reporter (dSH2) at IACs. These data demonstrate that kinase-dependent signal propagation through IACs is independent of gross changes in IAC composition. Together, these findings demonstrate a general separation between the composition of IACs and their ability to relay pY-dependent signals.

Definition of a consensus integrin adhesome and its dynamics during adhesion complex assembly and disassembly

Integrin receptor activation initiates the formation of integrin adhesion complexes (IACs) at the cell membrane that transduce adhesion-dependent signals to control a multitude of cellular functions. Proteomic analyses of isolated IACs have revealed an unanticipated molecular complexity; however, a global view of the consensus composition and dynamics of IACs is lacking. Here, we have integrated several IAC proteomes and generated a 2,412-protein integrin adhesome. Analysis of this data set reveals the functional diversity of proteins in IACs and establishes a consensus adhesome of 60 proteins. The consensus adhesome is likely to represent a core cell adhesion machinery, centred around four axes comprising ILK-PINCH-kindlin, FAK-paxillin, talin-vinculin and α-actinin-zyxin-VASP, and includes underappreciated IAC components such as Rsu-1 and caldesmon. Proteomic quantification of IAC assembly and disassembly detailed the compositional dynamics of the core cell adhesion machinery. The definition of this consensus view of integrin adhesome components provides a resource for the research community.

Mechanosensitivity of integrin adhesion complexes: role of the consensus adhesome

Cell and tissue stiffness have been known to contribute to both developmental and pathological signalling for some time, but the underlying mechanisms remain elusive. Integrins and their associated adhesion signalling complexes (IACs), which form a nexus between the cell cytoskeleton and the extracellular matrix, act as a key force sensing and transducing unit in cells. Accordingly, there has been much interest in obtaining a systems-level understanding of IAC composition. Proteomic approaches have revealed the complexity of IACs and identified a large number of components that are regulated by cytoskeletal force. Here we review the function of the consensus adhesome, an assembly of core IAC proteins that emerged from a meta-analysis of multiple proteomic datasets, in the context of mechanosensing. As IAC components have been linked to a variety of diseases involved with rigidity sensing, the field is now in a position to define the mechanosensing function of individual IAC proteins and elucidate their mechanisms of action.

Proteomic analysis of integrin-associated complexes from mesenchymal stem cells

Purpose

Multipotent mesenchymal stem cells (MSCs) have the capability to differentiate down adipocyte, osteocyte and chondrocyte lineages and as such offer a range of potential therapeutic applications. The composition and stiffness of the extracellular matrix (ECM) environment that surrounds cells dictates their transcriptional programme, thereby affecting stem cell lineage decision‐making. Cells sense force via linkages between themselves and their microenvironment, and this is transmitted by integrin receptors and associated adhesion signalling complexes. To identify regulators of MSC force sensing, we sought to catalogue MSC integrin‐associated adhesion complex composition.

Experimental design

Adhesion complexes formed by MSCs plated on the ECM ligand fibronectin were isolated and characterised by MS. Identified proteins were interrogated by comparison to a literature‐based reference set of cell adhesion‐related components and using ontological and protein–protein interaction network analyses.

Results

Adhesion complex‐specific proteins in MSCs were identified that comprised predominantly cell adhesion‐related adaptors and actin cytoskeleton regulators. Furthermore, LIM domain‐containing proteins in MSC adhesion complexes were highlighted, which may act as force‐sensing components.

Conclusion and clinical relevance

These data provide a valuable resource of information regarding the molecular connections that link integrins and adhesion signalling in MSCs, and as such may present novel opportunities for therapeutic intervention.

Dexamethasone-induced cellular tension requires a SGK1-stimulated Sec5-GEF-H1 interaction

Dexamethasone, a synthetic glucocorticoid, is often used to induce osteoblast commitment of mesenchymal stem cells (MSCs), and this process requires RhoA-dependent cellular tension. The underlying mechanism is unclear. In this study, we show that dexamethasone stimulates expression of fibronectin and integrin α5 (ITGA5), accompanied by an increase in the interaction of GEF-H1 (also known as ARHGEF2) with Sec5 (also known as EXOC2), a microtubule (MT)-regulated RhoA activator and a component of the exocyst, respectively. Disruption of this interaction abolishes dexamethasone-induced cellular tension and GEF-H1 targeting to focal adhesion sites at the cell periphery without affecting dexamethasone-induced levels of ITGA5 and fibronectin, and the extracellular deposition of fibronectin at adhesion sites is specifically inhibited. We demonstrate that dexamethasone stimulates the expression of serum-glucocorticoid-induced protein kinase 1 (SGK1), which is necessary and sufficient for the induction of the Sec5-GEF-H1 interaction. Given the function of SGK1 in suppressing MT growth, our data suggest that the induction of SGK1 through treatment with dexamethasone alters MT dynamics to increase Sec5-GEF-H1 interactions, which promote GEF-H1 targeting to adhesion sites. This mechanism is essential for the formation of fibronectin fibrils and their attachment to integrins at adhesion sites in order to generate cellular tension.