Integrin activation and cytoskeletal interaction are essential for the assembly of a fibronectin matrix

Extracellular matrix-derived materials for tissue engineering and regenerative medicine: A journey from isolation to characterization and application

Biomaterial choice is an essential step during the development tissue engineering and regenerative medicine (TERM) applications. The selected biomaterial must present properties allowing the physiological-like recapitulation of several processes that lead to the reestablishment of homeostatic tissue or organ function. Biomaterials derived from the extracellular matrix (ECM) present many such properties and their use in the field has been steadily increasing. Considering this growing importance, it becomes imperative to provide a comprehensive overview of ECM biomaterials, encompassing their sourcing, processing, and integration into TERM applications. This review compiles the main strategies used to isolate and process ECM-derived biomaterials as well as different techniques used for its characterization, namely biochemical and chemical, physical, morphological, and biological. Lastly, some of their applications in the TERM field are explored and discussed.

Matrix stiffness controls megakaryocyte adhesion, fibronectin fibrillogenesis, and proplatelet formation through Itgβ3

Megakaryocytes (MKs) are the precursor cells of platelets, located in the bone marrow (BM). Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the BM biomechanics. We sought to investigate how MKs perceive and adapt to modifications of the stiffness of their environment. Although the BM is one of the softest tissue of the body, its rigidification results from excess fibronectin (FN), and other matrix protein deposition occur upon myelofibrosis. Here, we have shown that mouse MKs are able to detect the stiffness of a FN-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility, and FN fibrils assembly at the expense of proplatelet formation. Itgb3, but not Itgb1, is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. In contrast, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for MK functions with potential pathophysiological implications during pathologies that deregulate FN deposition and modulate stiffness in the marrow.

Highly stable fibronectin-mimetic-peptide-based supramolecular hydrogel to accelerate corneal wound healing

Without timely treatment, poor wound healing in corneal injuries can seriously impair vision and lead to blindness. Thus, it is vital to develop a therapeutic strategy to accelerate corneal re-epithelialization. The conjugation of self-assembled motifs with a fibronectin-mimetic peptide sequence (PHSRN) drastically improves the chemical stability of PHSRN against protease hydrolysis and minimally affects its biological activity to promote the migration of corneal epithelial cells. The optimized Nap-FFPHSRN self-assembled into bioactive supramolecular hydrogels increases cell motility by remolding F-actin and boosts the tight junction of the corneal epithelium by increasing the expression of zonula occludens-1 (ZO-1). An in vivo experiment showed that a Nap-FFPHSRN hydrogel provided extended precorneal retention with good ocular tolerance after topical instillation. An animal model of corneal scrape showed that a single daily dose of Nap-FFPHSRN hydrogel had a superior therapeutic effect in facilitating corneal re-epithelialization with complete morphological and architectural recovery. With a rational approach to mimic bioactive proteins, this study presents a new strategy to demonstrate the potential of peptide-based supramolecular hydrogels for use in clinical treatment of corneal injury. STATEMENT OF SIGNIFICANCE: Here we systematically investigate the self-assembly behavior and chemical stability of designed peptide amphiphiles (Nap-FPHRSN, Nap-FFPHSRN and Nap-FFFPHSRN). The introduction of self-assembled motifs (Nap-F, Nap-FF and Nap-FFF) drastically enhances the chemical stability of fibronectin-mimetic peptide (PHSRN). Moreover, topical instillation of Nap-FFPHSRN hydrogel once daily, exhibits a better in vivo effect than PHSRN and the same in vivo effect as fibronectin, both of which are instilled three times daily, for promoting full morphological and architectural recovery after corneal re-epithelialization. As a rational design of conjugating bioactive peptides with self-assembled motifs to mimic bioactive proteins, this work may lead to a new approach that improves the in vivo therapeutic effect for treating corneal injury in clinic settings.

Developmental Changes in Patterns of Distribution of Fibronectin and Tenascin-C in the Chicken Cornea: Evidence for Distinct and Independent Functions during Corneal Development and Morphogenesis

The cornea forms the tough and transparent anterior part of the eye and by accurate shaping forms the major refractive element for vision. Its largest component is the stroma, a dense collagenous connective tissue positioned between the epithelium and the endothelium. In chicken embryos, the stroma initially develops as the primary stroma secreted by the epithelium, which is then invaded by migratory neural crest cells. These cells secrete an organised multi-lamellar collagenous extracellular matrix (ECM), becoming keratocytes. Within individual lamellae, collagen fibrils are parallel and orientated approximately orthogonally in adjacent lamellae. In addition to collagens and associated small proteoglycans, the ECM contains the multifunctional adhesive glycoproteins fibronectin and tenascin-C. We show in embryonic chicken corneas that fibronectin is present but is essentially unstructured in the primary stroma before cell migration and develops as strands linking migrating cells as they enter, maintaining their relative positions as they populate the stroma. Fibronectin also becomes prominent in the epithelial basement membrane, from which fibronectin strings penetrate into the stromal lamellar ECM at right angles. These are present throughout embryonic development but are absent in adults. Stromal cells associate with the strings. Since the epithelial basement membrane is the anterior stromal boundary, strings may be used by stromal cells to determine their relative anterior-posterior positions. Tenascin-C is organised differently, initially as an amorphous layer above the endothelium and subsequently extending anteriorly and organising into a 3D mesh when the stromal cells arrive, enclosing them. It continues to shift anteriorly in development, disappearing posteriorly, and finally becoming prominent in Bowman's layer beneath the epithelium. The similarity of tenascin-C and collagen organisation suggests that it may link cells to collagen, allowing cells to control and organise the developing ECM architecture. Fibronectin and tenascin-C have complementary roles in cell migration, with the former being adhesive and the latter being antiadhesive and able to displace cells from their adhesion to fibronectin. Thus, in addition to the potential for associations between cells and the ECM, the two could be involved in controlling migration and adhesion and subsequent keratocyte differentiation. Despite the similarities in structure and binding capabilities of the two glycoproteins and the fact that they occupy similar regions of the developing stroma, there is little colocalisation, demonstrating their distinctive roles.

Anastellin impacts on the processing of extracellular matrix fibronectin and stimulates release of cytokines from coronary artery smooth muscle cells

Anastellin, a recombinant protein fragment from the first type III module of fibronectin, mimics a partially unfolded intermediate implicated in the assembly of fibronectin fibrils. Anastellin influences the structure of fibronectin and initiates in vitro fibrillation, yielding "superfibronectin", a polymer with enhanced cell-adhesive properties. This ability is absent in an anastellin double mutant, L37AY40A. Here we demonstrate that both wild-type and L37AY40A anastellin affect fibronectin processing within the extracellular matrix (ECM) of smooth muscle cells. Fibronectin fibrils are diminished in the ECM from cells treated with anastellin, but are partially rescued by supplementation with plasma fibronectin in cell media. Proteomic analyses reveal that anastellin also impacts on the processing of other ECM proteins, with increased collagen and decreased laminin detected in media from cells exposed to wild-type anastellin. Moreover, both anastellin forms stimulate release of inflammatory cytokines, including interleukin 6. At the molecular level, L37AY40A does not exhibit major perturbations of structural features relative to wild-type anastellin, though the mutant showed differences in heparin binding characteristics. These findings indicate that wild-type and L37AY40A anastellin share similar molecular features but elicit slightly different, but partially overlapping, responses in smooth muscle cells resulting in altered secretion of cytokines and proteins involved in ECM processing.

Force tuning through regulation of clathrin-dependent integrin endocytosis

Integrin endocytosis is essential for many fundamental cellular processes. Whether and how the internalization impacts cellular mechanics remains elusive. Whereas previous studies reported the contribution of the integrin activator, talin, in force development, the involvement of inhibitors is less documented. We identified ICAP-1 as an integrin inhibitor involved in mechanotransduction by co-working with NME2 to control clathrin-mediated endocytosis of integrins at the edge of focal adhesions (FA). Loss of ICAP-1 enables β3-integrin-mediated force generation independently of β1 integrin. β3-integrin-mediated forces were associated with a decrease in β3 integrin dynamics stemming from their reduced diffusion within adhesion sites and slow turnover of FA. The decrease in β3 integrin dynamics correlated with a defect in integrin endocytosis. ICAP-1 acts as an adaptor for clathrin-dependent endocytosis of integrins. ICAP-1 controls integrin endocytosis by interacting with NME2, a key regulator of dynamin-dependent clathrin-coated pits fission. Control of clathrin-mediated integrin endocytosis by an inhibitor is an unprecedented mechanism to tune forces at FA.

The Extracellular Matrix Environment of Clear Cell Renal Cell Carcinoma

Simple Summary

The extracellular matrix (ECM) controls fundamental properties of tumors, including growth, blood vessel investment, and invasion. The ECM defines rigidity of tumor tissue and individual ECM proteins have distinct biological effects on tumor cells. This article reviews the composition and biological functions of the ECM of clear cell renal cell carcinoma (ccRCC). The most frequent initiating genetic mutation in ccRCC inactivates the VHL gene, which plays a direct role in organizing the ECM. This is predicted to result in local ECM modification, which promotes the growth of tumor cells and the invasion of blood vessels. Later in tumor growth, connective tissue cells are recruited, which are predicted to produce large amounts of ECM, affecting the growth and invasive behaviors of tumor cells. Strategies to therapeutically control the ECM are under active investigation and a better understanding of the ccRCC ECM will determine the applicability of ECM-modifying drugs to this type of cancer.

Abstract

The extracellular matrix (ECM) of tumors is a complex mix of components characteristic of the tissue of origin. In the majority of clear cell renal cell carcinomas (ccRCCs), the tumor suppressor VHL is inactivated. VHL controls matrix organization and its loss promotes a loosely organized and angiogenic matrix, predicted to be an early step in tumor formation. During tumor evolution, cancer-associated fibroblasts (CAFs) accumulate, and they are predicted to produce abundant ECM. The ccRCC ECM composition qualitatively resembles that of the healthy kidney cortex in which the tumor arises, but there are important differences. One is the quantitative difference between a healthy cortex ECM and a tumor ECM; a tumor ECM contains a higher proportion of interstitial matrix components and a lower proportion of basement membrane components. Another is the breakdown of tissue compartments in the tumor with mixing of ECM components that are physically separated in healthy kidney cortex. Numerous studies reviewed in this work reveal effects of specific ECM components on the growth and invasive behaviors of ccRCCs, and extrapolation from other work suggests an important role for ECM in controlling ccRCC tumor rigidity, which is predicted to be a key determinant of invasive behavior.

Targeting Fibronectin to Overcome Remyelination Failure in Multiple Sclerosis: The Need for Brain- and Lesion-Targeted Drug Delivery

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease with unknown etiology that can be characterized by the presence of demyelinated lesions. Prevailing treatment protocols in MS rely on the modulation of the inflammatory process but do not impact disease progression. Remyelination is an essential factor for both axonal survival and functional neurological recovery but is often insufficient. The extracellular matrix protein fibronectin contributes to the inhibitory environment created in MS lesions and likely plays a causative role in remyelination failure. The presence of the blood–brain barrier (BBB) hinders the delivery of remyelination therapeutics to lesions. Therefore, therapeutic interventions to normalize the pathogenic MS lesion environment need to be able to cross the BBB. In this review, we outline the multifaceted roles of fibronectin in MS pathogenesis and discuss promising therapeutic targets and agents to overcome fibronectin-mediated inhibition of remyelination. In addition, to pave the way for clinical use, we reflect on opportunities to deliver MS therapeutics to lesions through the utilization of nanomedicine and discuss strategies to deliver fibronectin-directed therapeutics across the BBB. The use of well-designed nanocarriers with appropriate surface functionalization to cross the BBB and target the lesion sites is recommended.

Fibronectin isoforms in skeletal development and associated disorders

The extracellular matrix is an intricate and essential network of proteins and non-proteinaceous components that provide a conducive microenvironment for cells to regulate cell function, differentiation, and survival. Fibronectin is one key component in the extracellular matrix that participates in determining cell fate and function crucial for normal vertebrate development. Fibronectin undergoes time dependent expression patterns during stem cell differentiation, providing a unique stem cell niche. Mutations in fibronectin have been recently identified to cause a rare form of skeletal dysplasia with scoliosis and abnormal growth plates. Even though fibronectin has been extensively analyzed in developmental processes, the functional role and importance of this protein and its various isoforms in skeletal development remains less understood. This review attempts to provide a concise and critical overview of the role of fibronectin isoforms in cartilage and bone physiology and associated pathologies. This will facilitate a better understanding of the possible mechanisms through which fibronectin exerts its regulatory role on cellular differentiation during skeletal development. The review discusses the consequences of mutations in fibronectin leading to corner fracture type spondylometaphyseal dysplasia and presents a new outlook towards matrix-mediated molecular pathways in relation to therapeutic and clinical relevance.

A Layered View on Focal Adhesions

Simple Summary

The cytoskeleton is a network of protein fibres within cells that provide structure and support intracellular transport. Focal adhesions are protein complexes associated with the outer cell membrane that are found at the ends of specialised actin fibres of this cytoskeleton. They mediate cell adhesion by connecting the cytoskeleton to the extracellular matrix, a protein and sugar network that surrounds cells in tissues. Focal adhesions also translate forces on actin fibres into forces contributing to cell migration. Cell adhesion and migration are crucial to diverse biological processes such as embryonic development, proper functioning of the immune system or the metastasis of cancer cells. Advances in fluorescence microscopy and data analysis methods provided a more detailed understanding of the dynamic ways in which proteins bind and dissociate from focal adhesions and how they are organised within these protein complexes. In this review, we provide an overview of the advances in the current scientific understanding of focal adhesions and summarize relevant imaging techniques. One of the key insights is that focal adhesion proteins are organised into three layers parallel to the cell membrane. We discuss the relevance of this layered nature for the functioning of focal adhesion.

Abstract

The cytoskeleton provides structure to cells and supports intracellular transport. Actin fibres are crucial to both functions. Focal Adhesions (FAs) are large macromolecular multiprotein assemblies at the ends of specialised actin fibres linking these to the extracellular matrix. FAs translate forces on actin fibres into forces contributing to cell migration. This review will discuss recent insights into FA protein dynamics and their organisation within FAs, made possible by advances in fluorescence imaging techniques and data analysis methods. Over the last decade, evidence has accumulated that FAs are composed of three layers parallel to the plasma membrane. We focus on some of the most frequently investigated proteins, two from each layer, paxillin and FAK (bottom, integrin signalling layer), vinculin and talin (middle, force transduction layer) and zyxin and VASP (top, actin regulatory layer). Finally, we discuss the potential impact of this layered nature on different aspects of FA behaviour.

Cromolyn platform suppresses fibrosis and inflammation, promotes microglial phagocytosis and neurite outgrowth

Neurodegenerative diseases are characterized by chronic neuroinflammation and may perpetuate ongoing fibrotic reactions within the central nervous system. Unfortunately, there is no therapeutic available that treats neurodegenerative inflammation and its sequelae. Here we utilize cromolyn, a mast cell inhibitor with anti-inflammatory capabilities, and its fluorinated analogue F-cromolyn to study fibrosis-related protein regulation and secretion downstream of neuroinflammation and their ability to promote microglial phagocytosis and neurite outgrowth. In this report, RNA-seq analysis shows that administration of the pro-inflammatory cytokine TNF-α to HMC3 human microglia results in a robust upregulation of fibrosis-associated genes. Subsequent treatment with cromolyn and F-cromolyn resulted in reduced secretion of collagen XVIII, fibronectin, and tenascin-c. Additionally, we show that cromolyn and F-cromolyn reduce pro-inflammatory proteins PLP1, PELP1, HSP90, IL-2, GRO-α, Eotaxin, and VEGF-Α, while promoting secretion of anti-inflammatory IL-4 in HMC3 microglia. Furthermore, cromolyn and F-cromolyn augment neurite outgrowth in PC12 neuronal cells in concert with nerve growth factor. Treatment also differentially altered secretion of neurogenesis-related proteins TTL, PROX1, Rab35, and CSDE1 in HMC3 microglia. Finally, iPSC-derived human microglia more readily phagocytose Aβ42 with cromolyn and F-cromolyn relative to controls. We propose the cromolyn platform targets multiple proteins upstream of PI3K/Akt/mTOR, NF-κB, and GSK-3β signaling pathways to affect cytokine, chemokine, and fibrosis-related protein expression.

Fibronectin as a multiregulatory molecule crucial in tumor matrisome: from structural and functional features to clinical practice in oncology

Deciphering extracellular matrix (ECM) composition and architecture may represent a novel approach to identify diagnostic and therapeutic targets in cancer. Among the ECM components, fibronectin and its fibrillary assembly represent the scaffold to build up the entire ECM structure, deeply affecting its features. Herein we focus on this extraordinary protein starting from its complex structure and defining its role in cancer as prognostic and theranostic marker.

Integrin affinity modulation critically regulates atherogenic endothelial activation in vitro and in vivo

While vital to platelet and leukocyte adhesion, the role of integrin affinity modulation in adherent cells remains controversial. In endothelial cells, atheroprone hemodynamics and oxidized lipoproteins drive an increase in the high affinity conformation of α5β1 integrins in endothelial cells in vitro, and α5β1 integrin inhibitors reduce proinflammatory endothelial activation to these stimuli in vitro and in vivo. However, the importance of α5β1 integrin affinity modulation to endothelial phenotype remains unknown. We now show that endothelial cells (talin1 L325R) unable to induce high affinity integrins initially adhere and spread but show significant defects in nascent adhesion formation. In contrast, overall focal adhesion number, area, and composition in stably adherent cells are similar between talin1 wildtype and talin1 L325R endothelial cells. However, talin1 L325R endothelial cells fail to induce high affinity α5β1 integrins, fibronectin deposition, and proinflammatory responses to atheroprone hemodynamics and oxidized lipoproteins. Inducing the high affinity conformation of α5β1 integrins in talin1 L325R endothelial cells suggest that NF-κB activation and maximal fibronectin deposition require both integrin activation and other integrin-independent signaling. In endothelial-specific talin1 L325R mice, atheroprone hemodynamics fail to promote inflammation and macrophage recruitment, demonstrating a vital role for integrin activation in regulating endothelial phenotype.

Targeting lysyl oxidase (LOX) overcomes chemotherapy resistance in triple negative breast cancer

Chemoresistance is a major obstacle in triple negative breast cancer (TNBC), the most aggressive breast cancer subtype. Here we identify hypoxia-induced ECM re-modeler, lysyl oxidase (LOX) as a key inducer of chemoresistance by developing chemoresistant TNBC tumors in vivo and characterizing their transcriptomes by RNA-sequencing. Inhibiting LOX reduces collagen cross-linking and fibronectin assembly, increases drug penetration, and downregulates ITGA5/FN1 expression, resulting in inhibition of FAK/Src signaling, induction of apoptosis and re-sensitization to chemotherapy. Similarly, inhibiting FAK/Src results in chemosensitization. These effects are observed in 3D-cultured cell lines, tumor organoids, chemoresistant xenografts, syngeneic tumors and PDX models. Re-expressing the hypoxia-repressed miR-142-3p, which targets HIF1A, LOX and ITGA5, causes further suppression of the HIF-1α/LOX/ITGA5/FN1 axis. Notably, higher LOX, ITGA5, or FN1, or lower miR-142-3p levels are associated with shorter survival in chemotherapy-treated TNBC patients. These results provide strong pre-clinical rationale for developing and testing LOX inhibitors to overcome chemoresistance in TNBC patients.

The development of chemoresistance is a major hurdle in triple negative breast cancer (TNBC). Here, the authors show that lysyl oxidase (LOX) is overexpressed in chemoresistant TNBCs, and when inhibited reduces collagen cross-linking, fibronectin fibril assembly, and downstream integrin signalling, overcoming resistance.

Basement Membrane Regulates Fibronectin Organization Using Sliding Focal Adhesions Driven by a Contractile Winch

We have discovered that basement membrane and its major components can induce rapid, strikingly robust fibronectin organization. In this new matrix assembly mechanism, α5β1 integrin-based focal adhesions slide actively on the underlying matrix toward the ventral cell center through the dynamic shortening of myosin IIA-associated actin stress fibers to drive rapid fibronectin fibrillogenesis distal to the adhesion. This mechanism contrasts with classical fibronectin assembly based on stable or fixed-position focal adhesions containing αVβ3 integrins plus α5β1 integrin translocation into proximal fibrillar adhesions. On basement membrane components, these sliding focal adhesions contain standard focal adhesion constituents but completely lack classical αVβ3 integrins. Instead, peripheral α3β1 or α2β1 adhesions mediate initial cell attachment but over time are switched to α5β1 integrin-based sliding focal adhesions to assemble fibronectin matrix. This basement-membrane-triggered mechanism produces rapid fibronectin fibrillogenesis, providing a mechanistic explanation for the well-known widespread accumulation of fibronectin at many organ basement membranes.

TLR3 agonists induce fibronectin aggregation by activated astrocytes: a role of pro-inflammatory cytokines and fibronectin splice variants

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system which eventually results in axonal loss mainly due to failure of remyelination. Previously we have shown that the persistent presence of stable astrocyte-derived fibronectin aggregates in MS lesions impairs OPC differentiation, and thereby remyelination. Here we set out to discern whether and, if so, how inflammatory mediators as present in MS lesions trigger astrocytes to form fibronectin aggregates. Our findings revealed that in slice cultures only upon demyelination, the TLR3 agonist Poly(I:C) evoked astrocytes to form fibronectin aggregates. Consistently, pro-inflammatory cytokine-pretreated astrocytes were more susceptible to Poly(I:C)-induced fibronectin aggregation, indicating that astrocytes form fibronectin aggregates upon a double hit by inflammatory mediators. The underlying mechanism involves disrupted fibronectin fibrillogenesis at the cell surface as a result of a cytokine-induced increase in relative mRNA levels of EIIIA^pos-Fn over EIIIB^pos-Fn and a Poly(I:C)-mediated decrease in integrin affinity. Remarkably, fibronectin aggregation is exacerbated by white matter astrocytes compared to grey matter astrocytes, which may be a reflection of higher expression levels of EIIIA^pos-fibronectin in white matter astrocytes. Hence, interfering with alternative fibronectin splicing and/or TLR3-mediated signaling may prevent fibronectin aggregation and overcome remyelination failure in MS lesions.

Kindlin-2 suppresses transcription factor GATA4 through interaction with SUV39H1 to attenuate hypertrophy

Kindlin-2 plays an important role in the regulation of cardiac structure and function. Depletion of Kindlin-2 contributes to cardiac hypertrophy and progressive heart failure, however, the precise mechanisms involved in this process remain unclear. GATA4 is a critical transcription factor in regulating cardiogenesis. We found that Kindlin-2 suppresses the expression of GATA4 through binding to its promoter and prevents cardiomyocytes from hypertrophy induced by isoproterenol (ISO) treatment. Mechanistically, Kindlin-2 interacts with histone methyltransferase SUV39H1 and recruits it to GATA4 promoter leading to the occupancy of histone H3K9 di- and tri-methylation. Furthermore, to confirm the function of Kindlin-2 in vivo, we generated mice with targeted deletion of cardiac Kindlin-2. We found that 6-month-old Kindlin-2 cKO mice have developed hypertrophic cardiomyopathy and that this pathological process can be accelerated by ISO-treatment. GATA4 expression was markedly activated in cardiac tissues of Kindlin-2 cKO mice compared to wild-type animals. Collectively, our data revealed that Kindlin-2 suppresses GATA4 expression by triggering histone H3K9 methylation in part and protects heart from pathological hypertrophy.

Proteomic Analysis of Porcine Pre-ovulatory Follicle Differentiation Into Corpus Luteum

The luteinization of the follicular cells, following a LH surge, causes extensive molecular and structural changes in preovulatory follicles (POF) that lead to ovulation and ultimate formation of the corpus luteum (CL). The objective of this study was to identify proteins expressed in porcine POF before the LH surge and a new CL formed, 2-3 days after ovulation, and evaluate proteome changes associated with formation of the CL from a follicle. We used 2D-gel electrophoresis-based proteomics and tandem mass spectrometry followed by a functional analysis using Ingenuity Pathway analysis (IPA) to evaluate functional pathways associated with the luteinization process. Protein lysates were prepared from isolated POFs and from the newly formed CL. A total of 422 protein spots were identified in both structures. A total of 15 and 48 proteins or their proteoforms were detected only in the POFs and CL, respectively. An IPA analysis of a POF proteome showed that most of the follicular proteins were involved in cellular infiltration, endoplasmic stress responses, and the protein ubiquitination pathway. Most of the early luteal proteins were associated with steroid metabolism, cell death and survival, free radical scavenging, and the protein ubiquitination pathway. A comparison of a follicular proteome with that of an early luteal proteome revealed that 167 identified proteins or their proteoforms were differentially regulated between POFs and the newly formed CL (p < 0.05 and a fold change of >1.8). Proteins that were significantly more abundant in follicles included cAMP-dependent protein kinase, histone binding protein RBBP4, reticulocalbin, vimentin, and calumenin; more abundant luteal proteins included albumin, farnesyl diphosphate synthase, serine protease inhibitors, elongation factor-1, glutaredoxin, and selenium-binding protein. Proteins that were significantly altered with luteal formation were found to be associated with cholesterol biosynthesis, cell death and survival, and acute phase response. Moreover, upstream regulators of differentially abundant proteins in CL were identified that included insulin growth factor-1, sterol regulatory element-binding transcription factor-1, and nuclear factor erythroid-derived 2. We have identified novel proteins that advance our understanding of (1) processes associated with differentiation of POFs into the CL, (2) possible mechanisms of luteal cell survival, and (3) pathways regulating steroidogenesis in the newly formed CL.

Adult stem cells at work: regenerating skeletal muscle

Skeletal muscle regeneration is a finely tuned process involving the activation of various cellular and molecular processes. Satellite cells, the stem cells of skeletal muscle, are indispensable for skeletal muscle regeneration. Their functionality is critically modulated by intrinsic signaling pathways as well as by interactions with the stem cell niche. Here, we discuss the properties of satellite cells, including heterogeneity regarding gene expression and/or their phenotypic traits and the contribution of satellite cells to skeletal muscle regeneration. We also summarize the process of regeneration with a specific emphasis on signaling pathways, cytoskeletal rearrangements, the importance of miRNAs, and the contribution of non-satellite cells such as immune cells, fibro-adipogenic progenitor cells, and PW1-positive/Pax7-negative interstitial cells.

The Molecular Dance of Fibronectin: Conformational Flexibility Leads to Functional Versatility

Fibronectin, a large multimodular protein and one of the major fibrillar components of the extracellular matrix, has been the subject of study for many decades and plays critical roles in embryonic development and tissue homeostasis. Moreover, fibronectin has been implicated in the pathology of many diseases, including cancer, and abnormal depositions of fibronectin have been identified in a number of amyloid and nonamyloid lesions. The ability of fibronectin to carry all these diverse functionalities depends on interactions with a large number of molecules, including adhesive and signaling cell surface receptors, other components of the extracellular matrix, and growth factors and cytokines. The regulation and integration of such large number of interactions depends on the modular architecture of fibronectin, which allows a large number of conformations, exposing or destroying different binding sites. In this Review, we summarize the current knowledge regarding the conformational flexibility of fibronectin, with an emphasis on how it regulates the ability of fibronectin to interact with various signaling molecules and cell-surface receptors and to form supramolecular assemblies and fibrillar structures.

Shear-dependent fibrillogenesis of fibronectin: Impact of platelet integrins and actin cytoskeleton

Soluble plasma fibronectin (Fn) with its inactive compact structure requires unfolding to assemble into active fibrils, which play a role in hemostasis and thrombosis. Fn fibril assembly involves Fn binding to cell receptors, biomechanical coupling of Fn to the cytoskeleton by integrins, exposure of self-assembly sites via contractile cell forces, and elongation of fibrils by Fn polymerization. In this report, we investigated the effect of platelet integrins and actin cytoskeleton on conformational changes of Fn induced by shear. Plasma Fn, in the presence or absence of washed platelets, was exposed to dynamic shear simulating venous or arterial flow conditions. Platelet integrins (αIIbβ3, αvβ3, and α5β1) were blocked by inhibitory antibodies to determine their contribution to shear-induced Fn fibrillogenesis. To examine the role of platelet cytoskeleton in Fn fibrillogenesis induced by shear, platelets were preincubated with cytoskeleton drugs, i. e jasplakinolide to stabilize actin or cytochalasin D to inhibit actin polymerization. Microscopic analyses demonstrated that flow and resulting shear stress over a broad range of physiological and pathological rates (50-5000 s^-1) could induce conformational changes of plasma Fn. In addition, the formation of Fn fibrils is modulated by platelet integrins. In this respect, β3 integrins play a dominant role in terms of Fn fibrillogenesis induced by shear. Disruption of the actin polymerization markedly diminished Fn unfolding and assembly. These observations lead to the conclusion that Fn-integrin β3-cytoskeleton interaction is crucial for the assembly of plasma Fn matrix under flow conditions.

Inductions of granulosa cell luteinization and cumulus expansion are dependent on the fibronectin-integrin pathway during ovulation process in mice

It has been known that EGF-like factor secreted from LH-stimulated granuloma cells acts on granulosa cells and cumulus cells to induce ovulation process. Granulosa cells are changed the morphology with differentiating cell functions to produce progesterone. Cumulus cells are detached to make a space between the cells to accumulate hyaluronan rich matrix. LH also changes extracellular matrix (ECM) components including fibronectin in the follicular walls and granulosa cell layers. EGF like factor and fibronectin synergistically play important roles in numerous cell functions, especially cancer cell migration, estimating that fibronectin would impact on granulosa cells and cumulus cells. To clear this hypothesis, the localizations of fibronectin and its receptor integrin were observed by immunofluorescence technique. The functions were monitored by the detection of downstream signaling pathway, focal adhesion kinase (FAK). The pharmacological approach in both in vivo and in vitro were used for analyzing the physiological roles of FAK during ovulation process. The immunofluorescence staining revealed that fibronectin and integrin were observed in granulosa cells, cumulus cells and the space between cumulus cells and oocyte at 4 and 8 h after hCG injection. Concomitantly with the changes of fibronectin-integrin localization, FAK was phosphorylated in periovulatory follicles. The injection of FAK inhibitor suppressed not only ovulation but also luteinization of granulosa cells and cumulus expansion. In cultured-granulosa cells, fibronectin-integrin synergistically activated FAK with amphiregulin (AREG). Such cooperative stimulations induced a morphological change in granulosa cells, which resulted in the maximum level of progesterone production via the induction of Hsd3b. When cumulus-oocyte complexes (COCs) were cultured with AREG in the presence of serum, the maximum level of cumulus expansion was observed. The AREG-induced cumulus expansion was also suppressed by FAK inhibitor. Thus, it is concluded that fibronectin and AREG synergistically activate FAK not only in granulosa cells and cumulus cells to induce successful ovulation process.

Basement Membrane Defects in Genetic Kidney Diseases

The glomerular basement membrane (GBM) is a specialized structure with a significant role in maintaining the glomerular filtration barrier. This GBM is formed from the fusion of two basement membranes during development and its function in the filtration barrier is achieved by key extracellular matrix components including type IV collagen, laminins, nidogens, and heparan sulfate proteoglycans. The characteristics of specific matrix isoforms such as laminin-521 (α5β2γ1) and the α3α4α5 chain of type IV collagen are essential for the formation of a mature GBM and the restricted tissue distribution of these isoforms makes the GBM a unique structure. Detailed investigation of the GBM has been driven by the identification of inherited abnormalities in matrix proteins and the need to understand pathogenic mechanisms causing severe glomerular disease. A well-described hereditary GBM disease is Alport syndrome, associated with a progressive glomerular disease, hearing loss, and lens defects due to mutations in the genes COL4A3, COL4A4, or COL4A5. Other proteins associated with inherited diseases of the GBM include laminin β2 in Pierson syndrome and LMX1B in nail patella syndrome. The knowledge of these genetic mutations associated with GBM defects has enhanced our understanding of cell-matrix signaling pathways affected in glomerular disease. This review will address current knowledge of GBM-associated abnormalities and related signaling pathways, as well as discussing the advances toward disease-targeted therapies for patients with glomerular disease.

Heparin-fibronectin interactions in the development of extracellular matrix insolubility

During extracellular matrix (ECM) assembly, fibronectin (FN) fibrils are irreversibly converted into a detergent-insoluble form which, through FN's multi-domain structure, can interact with collagens, matricellular proteins, and growth factors to build a definitive matrix. FN also has heparin/heparan sulfate (HS) binding sites. Using HS-deficient CHO cells, we show that the addition of soluble heparin significantly increased the amount of FN matrix that these cells assemble. Sulfated HS glycosaminoglycan (GAG) mimetics similarly increased FN assembly and demonstrated a dependence on GAG sulfation. The length of the heparin chains also plays a role in assembly. Chains of sufficient length to bind to two FN molecules gave maximal stimulation of assembly whereas shorter heparin had less of an effect. Using a decellularized fibroblast matrix for proteolysis, detergent fractionation, and mass spectrometry, we found that the predominant domain within insoluble fibril fragments is FN's major heparin-binding domain HepII (modules III_12-14). Multiple HepII domains bind simultaneously to a single heparin chain in size exclusion chromatography analyses. We propose a model in which heparin/HS binding to the HepII domain connects multiple FNs together to facilitate the formation of protein interactions for insoluble fibril assembly.

β1-Integrin Signaling is Essential for Lens Fiber Survival

Integrins have been proposed to play a major role in lens morphogenesis. To determine the role of β1-integrin and its down-stream signaling partner, integrin linked kinase (ILK), in lens morphogenesis, eyes of WT mice and mice with a nestin-linked conditional knockout of β1-integrin or ILK were analyzed for defects in lens development. Mice, lacking the genes encoding the p1-integrin subunit ( Itgb1) or ILK ( Ilk), showed a perinatal degeneration of the lens. Early signs of lens degeneration included vacuolization, random distribution of lens cell nuclei, disrupted fiber morphology and attenuation and separation of the lens capsule. The phenotype became progressively more severe during the first postnatal week eventually leading to the complete loss of the lens. A more severe phenotype was observed in ILK mutants at similar stages. Eyes from embryonic day 13 β1-integrin-mutant embryos showed no obvious signs of lens degeneration, indicating that mutant lens develops normally until peri-recombination. Our findings suggest that β1-integrins and ILK cooperate to control lens cell survival and link lens fibers to the surrounding extracellular matrix. The assembly and integrity of the lens capsule also appears to be reliant on integrin signaling within lens fibers. Extrapolation of these results indicates a novel role of integrins in lens cell-cell adhesions as well as a potential role in the pathogenesis of congenital cataracts.

Clustering of integrin α5 at the lateral membrane restores epithelial polarity in invasive colorectal cancer cells

Apicobasolateral polarity is a fundamental property of epithelial cells, and its loss is a hallmark of cancer. Integrin-mediated contact with the extracellular matrix defines the basal surface, setting in motion E-cadherin-mediated cell-cell contact, which establishes apicobasolateral polarity. Role(s) for lateral integrins in this polarization process and the consequences of their disruption are incompletely understood. We show that addition of an integrin β1-activating monoclonal antibody, P4G11, to invasive colorectal cancer cells in three-dimensional type 1 collagen reverts the invasive phenotype and restores apicobasolateral polarity. P4G11 induces clustering of integrin α5β1 at lateral, intercellular surfaces. This leads to deposition and polymerization of fibronectin and recruitment of paxillin to sites of lateral integrin α5β1 clustering and is followed by tight junction formation, as determined by ZO-1 localization. Inducible elimination of integrin α5 abrogates the epithelial-organizing effects of P4G11. In addition, polymerization of fibronectin is required for the effects of P4G11, and addition of polymerized superfibronectin is sufficient to induce tight junction formation and apicobasolateral polarization. In the normal human colon, we show that integrin α5 localizes to the lateral membrane of terminally differentiated colonocytes and that integrin α5 staining may be reduced in colorectal cancer. Thus we propose a novel role for integrin α5β1 in regulating epithelial morphogenesis.

Control of Myofibroblast Differentiation and Function by Cytoskeletal Signaling

The cytoskeleton consists of three distinct types of protein polymer structures - microfilaments, intermediate filaments, and microtubules; each serves distinct roles in controlling cell shape, division, contraction, migration, and other processes. In addition to mechanical functions, the cytoskeleton accepts signals from outside the cell and triggers additional signals to extracellular matrix, thus playing a key role in signal transduction from extracellular stimuli through dynamic recruitment of diverse intermediates of the intracellular signaling machinery. This review summarizes current knowledge about the role of cytoskeleton in the signaling mechanism of fibroblast-to-myofibroblast differentiation - a process characterized by accumulation of contractile proteins and secretion of extracellular matrix proteins, and being critical for normal wound healing in response to tissue injury as well as for aberrant tissue remodeling in fibrotic disorders. Specifically, we discuss control of serum response factor and Hippo signaling pathways by actin and microtubule dynamics as well as regulation of collagen synthesis by intermediate filaments.

Fibronectin binding protein and Ca2+ play an access key role to mediate pathogenesis in Mycobacterium tuberculosis: An overview

The anomalous distribution of adhesive proteins throughout on the cell surface of the Mycobacterium tuberculosis H₃₇ Rv and their contribution in cell surface adhesion and host-pathogen interaction remain elusive. The completion of M. tuberculosis H₃₇ Rv genome sequence analysis gives some interesting information about polymorphic GC-rich repetitive sequence (PGRS) subfamily of M. tuberculosis that encodes fibronectin binding proteins (FnBP), which have been extensively studied, but the function in the pathogenesis of most of these proteins remains unknown and unclear. This review addresses the M. tuberculosis entry mechanism in the host cell. In particular, an effort has been made to focus on several aspects, (a) association of FnBP encodes by PE_PGRS protein family of M. tuberculosis during host-pathogen interactions. (b) Effect of calcium ions in and outside of the host cell is overriding to maintenance of calcium trafficking in phagocytosis. Furthermore, FnBP may be a potential source of antigenic variation that participating in evoking immune response. M. tuberculosis entry mechanism does not have a major influence alone, involvement of calcium ions, perhaps shed light on host-pathogen interaction relationship, and could open up new avenues for development of novel drug by targeting M. tuberculosis FnBP and blockade of selective adhesions could be useful for therapeutics.

Potential of Ca2+ in Mycobacterium tuberculosis H37Rv Pathogenesis and Survival

The host-pathogen interaction and involvement of calcium (Ca²⁺) signaling in tuberculosis infection is crucial and plays a significant role in pathogenesis. Ca²⁺ is known as a ubiquitous second messenger that could control multiple processes and is included in cellular activities like division, motility, stress response, and signaling. However, Ca²⁺ is thought to be a regulative molecule in terms of TB infection but its binding relation with proteins/substrates molecules which are influenced with Ca²⁺ concentrations in host-pathogen interaction requires attention. So, in this review, our primary goal is to focus on some Ca²⁺ substrates/proteins and their imperative involvement in pathogenesis, which is unclear. We have discussed several Ca²⁺-binding substrate and protein that affect intracellular mechanism of infected host cell. The major involvement of these proteins/substrates including calmodulin (CaM), calpain, annexin, surfactant protein A (SP-A), surfactant protein D (SP-D), calprotectin (MRP8/14), and PE_PGRS family protein are considered to be significant; however, their detailed understanding in mycobacterium infection is limited. In this aspect, this study will help in adding up our understanding in TB biology and additionally in the development of new therapeutic approach to reduce TB pandemic worldwide.

Blood Brothers: Hemodynamics and Cell-Matrix Interactions in Endothelial Function

SIGNIFICANCE

Alterations in endothelial function contribute to a variety of vascular diseases. In pathological conditions, the endothelium shows a reduced ability to regulate vasodilation (endothelial dysfunction) and a conversion toward a proinflammatory and leaky phenotype (endothelial activation). At the interface between the vessel wall and blood, the endothelium exists in a complex microenvironment and must translate changes in these environmental signals to alterations in vessel function. Mechanical stimulation and endothelial cell interactions with the vascular matrix, as well as a host of soluble factors, coordinately contribute to this dynamic regulation.

RECENT ADVANCES

Blood hemodynamics play an established role in the regulation of endothelial function. However, a growing body of work suggests that subendothelial matrix composition similarly and coordinately regulates endothelial cell phenotype such that blood flow affects matrix remodeling, which affects the endothelial response to flow.

CRITICAL ISSUES

Hemodynamics and soluble factors likely affect endothelial matrix remodeling through multiple mechanisms, including transforming growth factor β signaling and alterations in cell-matrix receptors, such as the integrins. Likewise, differential integrin signaling following matrix remodeling appears to regulate several key flow-induced responses, including nitric oxide production, regulation of oxidant stress, and activation of proinflammatory signaling and gene expression. Microvascular remodeling responses, such as angiogenesis and arteriogenesis, may also show coordinated regulation by flow and matrix.

FUTURE DIRECTIONS

Identifying the mechanisms regulating the dynamic interplay between hemodynamics and matrix remodeling and their contribution to the pathogenesis of cardiovascular disease remains an important research area with therapeutic implications across a variety of conditions. Antioxid. Redox Signal. 25, 415-434.

Syndecan-1-Induced ECM Fiber Alignment Requires Integrin αvβ3 and Syndecan-1 Ectodomain and Heparan Sulfate Chains

Expression of the cell surface proteoglycan syndecan-1 (Sdc1) is frequently induced in stromal fibroblasts of invasive breast carcinomas. We have recently identified a correlation between stromal Sdc1 expression and extracellular matrix (ECM) fiber alignment, both in vitro and in vivo. ECMs derived from Sdc1-positive human mammary fibroblasts (HMF) showed an aligned fiber architecture, which contrasted markedly with the more random fiber arrangement in the ECM produced by Sdc1-negative HMFs. We further demonstrated that aligned fiber architecture promotes the directional migration and invasion of breast carcinoma cells. To decipher the molecular mechanisms governing the formation of an aligned, invasion-permissive ECM, a series of Sdc1 mutants was introduced into HMF. We found that both the ectodomain and heparan sulfate chains of Sdc1 were required for full activity of Sdc1 in regulating ECM alignment, while transmembrane and cytoplasmic domains were dispensable. Sdc1 regulates the activities of several integrins via its ectodomain. Integrins are key players in the assembly of fibronectin-rich ECM. In addition, integrins are capable of regulating cell morphology and cell shape and orientation may affect ECM architecture. Therefore, we investigated the role of integrins in Sdc1-mediated ECM fiber alignment. Sdc1-overexpressing HMF gained an enhanced spindle-shaped morphology when cultured in an overconfluent state under conditions permissive for ECM production, which was partially reversed by siRNA-mediated silencing of β3 integrin expression. Moreover, suppression of αvβ3 integrin activity by a function-blocking antibody or β3 knockdown largely abolished the aligned ECM fiber architecture and consequently the invasion-permissive properties of the ECM induced by Sdc1. The results suggest that Sdc1 may modulate fibronectin fibrillogenesis and/or alter cell morphology during ECM production through αvβ3 integrin, thereby mediating ECM fiber alignment. Understanding the mechanisms governing ECM organization may lead to the development of novel stroma-targeted therapy for breast cancer, aiming at converting an invasion-permissive to an invasion-restrictive microenvironment.

Fibronectin remodelling: cell-mediated regulation of the microenvironment

The biophysical, mechanical and chemical characteristics of extracellular matrixes influence many cellular functions to control tissue homoeostasis and drive progression of cancer and inflammatory diseases. To maintain normal tissue function, fibronectin-rich matrixes are subject to dynamic cell-mediated structural and chemical modification. In this article, we discuss how localized application of mechanical force, heterodimer-specific integrin engagement and matrix proteolysis regulate fibronectin assembly and turnover. We also speculate that recently identified integrin trafficking, syndecan signalling and adhesion receptor–growth factor receptor cross-talk mechanisms might dynamically control the function, assembly and mechanical properties of a viable, and mechanoresponsive, fibronectin network.

The extracellular matrix: Structure, composition, age-related differences, tools for analysis and applications for tissue engineering

The extracellular matrix is a structural support network made up of diverse proteins, sugars and other components. It influences a wide number of cellular processes including migration, wound healing and differentiation, all of which is of particular interest to researchers in the field of tissue engineering. Understanding the composition and structure of the extracellular matrix will aid in exploring the ways the extracellular matrix can be utilised in tissue engineering applications especially as a scaffold. This review summarises the current knowledge of the composition, structure and functions of the extracellular matrix and introduces the effect of ageing on extracellular matrix remodelling and its contribution to cellular functions. Additionally, the current analytical technologies to study the extracellular matrix and extracellular matrix–related cellular processes are also reviewed.

CD98hc (SLC3A2) loss protects against ras-driven tumorigenesis by modulating integrin-mediated mechanotransduction

CD98hc (SLC3A2) is the heavy chain component of the dimeric transmembrane glycoprotein CD98, which comprises the large neutral amino acid transporter LAT1 (SLC7A5) in cells. Overexpression of CD98hc occurs widely in cancer cells and is associated with poor prognosis clinically, but its exact contributions to tumorigenesis are uncertain. In this study, we showed that genetic deficiency of CD98hc protects against Ras-driven skin carcinogenesis. Deleting CD98hc after tumor induction was also sufficient to cause regression of existing tumors. Investigations into the basis for these effects defined two new functions of CD98hc that contribute to epithelial cancer beyond an intrinsic effect of CD98hc on tumor cell proliferation. First, CD98hc increased the stiffness of the tumor microenvironment. Second, CD98hc amplified the capacity of cells to respond to matrix rigidity, an essential factor in tumor development. Mechanistically, CD98hc mediated this stiffness sensing by increasing Rho kinase (ROCK) activity, resulting in increased transcription mediated by YAP/TAZ, a nuclear relay for mechanical signals. Our results suggest that CD98hc contributes to carcinogenesis by amplifying a positive feedback loop, which increases both extracellular matrix stiffness and resulting cellular responses. This work supports a rationale to explore the use of CD98hc inhibitors as cancer therapeutics.

LIM kinase regulation of cytoskeletal dynamics is required for salivary gland branching morphogenesis

LIMK regulation of actin and microtubule dynamics is required for epithelial regulation of early- and late-stage cleft stabilization and progression. LIMK stimulates focal adhesion assembly and integrin β1 activation in cleft regions, causing fibronectin fibrillogenesis and promoting cleft progression during salivary gland branching morphogenesis.

Coordinated actin microfilament and microtubule dynamics is required for salivary gland development, although the mechanisms by which they contribute to branching morphogenesis are not defined. Because LIM kinase (LIMK) regulates both actin and microtubule organization, we investigated the role of LIMK signaling in mouse embryonic submandibular salivary glands using ex vivo organ cultures. Both LIMK 1 and 2 were necessary for branching morphogenesis and functioned to promote epithelial early- and late-stage cleft progression through regulation of both microfilaments and microtubules. LIMK-dependent regulation of these cytoskeletal systems was required to control focal adhesion protein–dependent fibronectin assembly and integrin β1 activation, involving the LIMK effectors cofilin and TPPP/p25, for assembly of the actin- and tubulin-based cytoskeletal systems, respectively. We demonstrate that LIMK regulates the early stages of cleft formation—cleft initiation, stabilization, and progression—via establishment of actin stability. Further, we reveal a novel role for the microtubule assembly factor p25 in regulating stabilization and elongation of late-stage progressing clefts. This study demonstrates the existence of multiple actin- and microtubule-dependent stabilization steps that are controlled by LIMK and are required in cleft progression during branching morphogenesis.

Recombinant Protein Hydrogels for Cell Injection and Transplantation

As found in nature, full-length proteins consist of a genetically specified sequence of the 20 canonical amino acids, of a defined length. This sequence of chemically diverse functional groups enables the many highly controlled interactions with other molecules found in natural proteins. Recombinant proteins can be engineered to incorporate some of nature's palette of protein functionality into hydrogels for cell delivery. Current work demonstrates how this level of molecular precision can be used to address challenges in cell therapies, such as post-implantation viability, localization, and control, via specified gelation mechanics and tailored bioactive domains. Leveraging recombinant technology, including protein engineering, gene synthesis, expression, and purification, materials scientists have appropriated and modified naturally occurring proteins to achieve hydrogels that combine defined gelation mechanics with specified bioactive protein chemistries. Here, we specifically review recent developments in recombinant protein hydrogels that are either inspired by native extracellular matrix proteins (e.g. elastin, collagen, and resilin) or designed from non-matrix peptides (e.g. mixing-induced two-component hydrogels). In many of these case studies, domain- and sequence-level engineering enables a broad range of biochemical activity and mechanical control via gelation. Despite the remaining challenges of scalability and forward-designed predictability, hydrogels made of recombinant proteins offer exciting possibilities for sophisticated delivery of therapeutic cells, including multifactorial control, native-like mechanics, and sensitivity to signals from delivered cells or host tissues.

Integrin Regulation of Epidermal Functions in Wounds

Significance: Integrins are bidirectional signaling receptors for extracellular matrix that regulate both inside-out signaling that controls keratinocyte-mediated changes to the wound microenvironment and outside-in signaling that controls keratinocyte responses to microenvironmental changes. As such, integrins represent attractive therapeutic targets for treatment of chronic wounds or general promotion of wound healing. Advances in wound management are particularly important as the elderly and diabetic populations within the United States continue to grow. Recent Advances: Although integrins are best known for mediating cell adhesion and migration, integrins in wound epidermis also control cell survival, proliferation, matrix remodeling, and paracrine crosstalk to other cellular compartments of the wound. Importantly, the concept of targeting integrins in the clinic has been established for treatment of certain cancers and other diseases, laying the groundwork for similar exploitation of integrins as targets to treat chronic wounds. Critical Issues: Despite their attractiveness as therapeutic targets, integrins have complex roles in wound healing that are impacted by both their own expression and a highly dynamic wound microenvironment that determines ligand availability. Therefore, identifying relevant integrin ligands in the wound and understanding both distinct and overlapping functions that different integrins play in the epidermis will be critical to determine their precise roles in wound healing. Future Directions: Future research should focus on gaining a thorough understanding of the highly coordinated functions of different integrins in wound epidermis, and on determining which of these functions go awry in pathological wounds. This focus should facilitate development of integrin-targeting therapeutics for treating chronic wounds.

Contribution of distinct platelet integrins to binding, unfolding, and assembly of fibronectin

Fibronectin (FN) fibrillogenesis depends on the binding of FN to cellular receptors and subsequent unfolding of bound FN. Integrins αIIbβ3, αvβ3, and α5β1 are known to assemble FN fibrils on platelets. In our study, we examined the contribution of these integrins to FN binding, unfolding, and assembly on platelets in suspension and adherent platelets in the presence or absence of agonists. Phorbol 12-myristate 13-acetate (PMA), but not adenosine diphosphate (ADP), induced binding of FN to platelets in suspension. In contrast, adherent platelets were able to deposit FN on their surfaces in the absence of agonists. β3 integrins had a major impact on the interaction of FN on platelets. αvβ3 showed a similar contribution to the binding of FN as αIIbβ3 on PMA-stimulated platelets in suspension but had a lesser contribution to unfolding and deposition of FN on adherent platelets. α5β1 also participated in the interaction of FN with platelets by mediating the unfolding and assembly of FN, but to a lesser extent than β3 integrins. None of the distinct antibodies directed against one of the three integrins caused a complete inhibition of binding, unfolding, and assembly of FN by platelets. Thus, it is likely that αIIbβ3, αvβ3, and α5β1 or another still unknown receptor can be substituted.

Invadopodia formation in blood clots: Not so SLUGgish after all

Blood clotting specifically supports the metastatic dissemination of malignant cells to the lung. We have recently demonstrated that 2 tumor types that are prone to form lung metastases, renal cell carcinoma and soft tissue sarcoma, share specific adhesive mechanisms that support the invasion and colonization of blood clots in the pulmonary vasculature.

Involvement of CD137 ligand signaling in neural stem cell death

CD137 is a member of the tumor necrosis factor/nerve growth factor receptor superfamily. Interaction of CD137 with its ligand (CD137L) affects the apoptosis, proliferation and differentiation of immune cells. Interestingly, the CD137 receptor/ligand system involves the bi-directional transduction of signals. The expression of CD137 and its ligand is not restricted to immune organs, but can also be detected in a wide variety of tissues such as the brain, kidney, lung and heart. However, its role in brain is largely unknown. This study was performed to determine the role of CD137L reverse signaling in the apoptosis of neural stem cells. We identified the expression of CD137 and its ligand in C17.2 neural stem cells derived from mouse embryonic cerebellum. We found that the activation of CD137L reverse signaling by CD137 resulted in a decrease in cell adhesion to the fibronectin-coated culture basement, thus causing detachment-induced cell death. Furthermore, we showed that the cell death induced by CD137 was completely ameliorated by integrin activators and caspase inhibitors. Therefore we suggest that CD137L reverse signaling exerts a pro-apoptotic effect by suppressing integrin-mediated survival signals in neural stem cells.

Integrin αvβ3 and fibronectin upregulate Slug in cancer cells to promote clot invasion and metastasis

The blood clotting cascade is selectively involved in lung metastasis, but the reason for this selectivity is unclear. Here, we show that tumor cells that metastasize predominantly to the lung, such as renal cell carcinoma (RCC) and soft tissue sarcoma (STS), have an inherent capacity to generate extensive invadopodia when embedded in a blood clot. Compared with other metastatic cancer cells tested, RCC and STS cells exhibited increased levels of expression of fibronectin and an activated form of the integrin αvβ3, which coordinately supported the generation of an elaborate fibronectin matrix and actin stress fibers in fibrin-embedded tumor cells. Together, fibronectin and αvβ3 induced upregulation of the transcription factor Slug, which mediates epithelial-mesenchymal transition as well as fibrin invasion and lung metastasis. This mechanism is clinically significant, because primary cancer cells from patients with metastatic RCC strongly invaded fibrin and this correlated with fibronectin matrix formation and Slug expression. In contrast, tumor cells from patients with localized RCC were largely noninvasive. Together, our findings establish that activated integrin αvβ3 and fibronectin promote lung metastasis by upregulating Slug, defining a mechanism through which cancer cells can colonize blood clots in the lung vasculature.

Mechanisms of talin-dependent integrin signaling and crosstalk

Cells undergo dynamic remodeling of the cytoskeleton during adhesion and migration on various extracellular matrix (ECM) substrates in response to physiological and pathological cues. The major mediators of such cellular responses are the heterodimeric adhesion receptors, the integrins. Extracellular or intracellular signals emanating from different signaling cascades cause inside-out signaling of integrins via talin, a cystokeletal protein that links integrins to the actin cytoskeleton. Various integrin subfamilies communicate with each other and growth factor receptors under diverse cellular contexts to facilitate or inhibit various integrin-mediated functions. Since talin is an essential mediator of integrin activation, much of the integrin crosstalk would therefore be influenced by talin. However, despite the existence of an extensive body of knowledge on the role of talin in integrin activation and as a stabilizer of ECM-actin linkage, information on its role in regulating inter-integrin communication is limited. This review will focus on the structure of talin, its regulation of integrin activation and discuss its potential role in integrin crosstalk. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Vitronectin alters fibronectin organization at the cell-material interface

Cells assemble fibronectin (FN) into fibrils in a process mediated by integrins. For this process to occur, it is known that the presence of other serum proteins is necessary. However, the individual effect of these proteins on FN fibrillogenesis has not been addressed so far. In this study, the effect of vitronectin (VN), an ECM adhesion protein, on material-driven FN fibrillogenesis and cell-mediated FN reorganization is investigated. Poly(ethyl acrylate), PEA, which has previously shown the ability to induce the organization of FN into well-developed physiological-like networks upon adsorption, was employed as a material substrate. FN adsorption, cell adhesion and cellular FN reorganization in the presence or absence of VN were studied. Both FN surface density, quantified via western blot, and its distribution on PEA surfaces, determined via atomic force microscopy, were altered when FN was adsorbed competitively with VN at certain compositions. Moreover, the presence of VN on the material surfaces enhanced cell-mediated FN reorganization and secretion, in comparison with the process which took place in the presence of serum proteins.

SLLISWD sequence in the 10FNIII domain initiates fibronectin fibrillogenesis

Fibronectin (FN) assembly into extracellular matrix is tightly regulated and essential to embryogenesis and wound healing. FN fibrillogenesis is initiated by cytoskeleton-derived tensional forces transmitted across transmembrane integrins onto RGD binding sequences within the tenth FN type III (10FNIII) domains. These forces unfold 10FNIII to expose cryptic FN assembly sites; however, a specific sequence has not been identified in 10FNIII. Our past steered molecular dynamics simulations modeling 10FNIII unfolding by force at its RGD loop predicted a mechanical intermediate with a solvent-exposed N terminus spanning the A and B β-strands. Here, we experimentally confirm that the predicted 23-residue cryptic peptide 1 (CP1) initiates FN multimerization, which is mediated by interactions with 10FNIII that expose hydrophobic surfaces that support 8-anilino-1-napthalenesulfonic acid binding. Localization of multimerization activity to the C terminus led to the discovery of a minimal 7-amino acid "multimerization sequence" (SLLISWD), which induces polymerization of FN and the clotting protein fibrinogen in addition to enhancing FN fibrillogenesis in fibroblasts. A point mutation at Trp-6 that reduces exposure of hydrophobic sites for 8-anilino-1-napthalenesulfonic acid binding and β-structure formation inhibits FN multimerization and prevents physiological cell-based FN assembly in culture. We propose a model for cell-mediated fibrillogenesis whereby cell traction force initiates a cascade of intermolecular exchange starting with the unfolding of 10FNIII to expose the multimerization sequence, which interacts with strand B of another 10FNIII domain via a Trp-mediated β-strand exchange to stabilize a partially unfolded intermediate that propagates FN self-assembly.