Identification of a novel integrin binding site in fibronectin. Differential utilization by beta 3 integrins

Shaping Oncogenic Microenvironments: Contribution of Fibronectin

The extracellular matrix (ECM) is a complex network of proteins and glycans, dynamically remodeled and specifically tailored to the structure/function of each organ. The malignant transformation of cancer cells is determined by both cell intrinsic properties, such as mutations, and extrinsic variables, such as the mixture of surrounding cells in the tumor microenvironment and the biophysics of the ECM. During cancer progression, the ECM undergoes extensive remodeling, characterized by disruption of the basal lamina, vascular endothelial cell invasion, and development of fibrosis in and around the tumor cells resulting in increased tissue stiffness. This enhanced rigidity leads to aberrant mechanotransduction and further malignant transformation potentiating the de-differentiation, proliferation and invasion of tumor cells. Interestingly, this fibrotic microenvironment is primarily secreted and assembled by non-cancerous cells. Among them, the cancer-associated fibroblasts (CAFs) play a central role. CAFs massively produce fibronectin together with type I collagen. This review delves into the primary interactions and signaling pathways through which fibronectin can support tumorigenesis and metastasis, aiming to provide critical molecular insights for better therapy response prediction.

Cancer Cell Targeting, Magnetic Sorting, and SERS Detection through Cell Surface Receptors

Integrins are cellular surface receptors responsible for the activation of many cellular pathways in cancer. These integrin proteins can be specifically targeted by small peptide sequences that offer the potential for the differentiation of cellular subpopulations by using magnetically assisted cellular sorting techniques. By adding a gold shell to the magnetic nanoparticles, these integrin-peptide interactions can be differentiated by surface-enhanced Raman spectroscopy (SERS), providing a quick and reliable method for on-target binding. In this paper, we demonstrate the ability to differentiate the peptide-protein interactions of the small peptides CDPGYIGSR and cyclic RGDfC functionalized on gold-coated magnetic nanoparticles with the integrins they are known to bind to using their SERS signal. SW480 and SW620 colorectal cancer cells known to have the integrins of interest were then magnetically sorted using these functionalized nanoparticles, suggesting differentiation between the sorted populations and integrin populations among the two cell lines.

Osteocyte β3 integrin promotes bone mass accrual and force-induced bone formation in mice

Background

Cell culture studies demonstrate the importance of β3 integrin in osteocyte mechanotransduction. However, the in vivo roles of osteocyte β3 integrin in the regulation of bone homeostasis and mechanotransduction are poorly defined.

Materials and methods

To study the in vivo role of osteocyte β3 integrin in bone, we utilized the 10-kb Dmp1 (dentin matrix acidic phosphoprotein 1)-Cre to delete β3 integrin expression in osteocyte in mice. Micro-computerized tomography (μCT), bone histomorphometry and in vitro cell culture experiments were performed to determine the effects of osteocyte β3 integrin loss on bone mass accrual and biomechanical properties. In addition, in vivo tibial loading model was applied to study the possible involvement of osteocyte β3 integrin in the mediation of bone mechanotransduction.

Results

Deletion of β3 integrin in osteocytes resulted in a low bone mass and impaired biomechanical properties in load-bearing long bones in adult mice. The loss of β3 integrin led to abnormal cell morphology with reduced number and length of dentritic processes in osteocytes. Furthermore, osteocyte β3 integrin loss did not impact the osteoclast formation, but significantly reduced the osteoblast-mediated bone formation rate and reduced the osteogenic differentiation of the bone marrow stromal cells in the bone microenvironment. In addition, mechanical loading failed to accelerate the anabolic bone formation in mutant mice.

Conclusions

Our studies demonstrate the essential roles of osteocyte β3 integrin in regulating bone mass and mechanotransduction.

The Role of the Fibronectin Synergy Site for Skin Wound Healing

Skin is constantly exposed to injuries that are repaired with different outcomes, either regeneration or scarring. Scars result from fibrotic processes modulated by cellular physical forces transmitted by integrins. Fibronectin (FN) is a major component in the provisional matrix assembled to repair skin wounds. FN enables cell adhesion binding of α5β1/αIIbβ3 and αv-class integrins to an RGD-motif. An additional linkage for α5/αIIb is the synergy site located in close proximity to the RGD motif. The mutation to impair the FN synergy region (Fn1^syn/syn) demonstrated that its absence permits complete development. However, only with the additional engagement to the FN synergy site do cells efficiently resist physical forces. To test how the synergy site-mediated adhesion affects the course of wound healing fibrosis, we used a mouse model of skin injury and in-vitro migration studies with keratinocytes and fibroblasts on FN^syn. The loss of FN synergy site led to normal re-epithelialization caused by two opposing migratory defects of activated keratinocytes and, in the dermis, induced reduced fibrotic responses, with lower contents of myofibroblasts and FN deposition and diminished TGF-β1-mediated cell signalling. We demonstrate that weakened α5β1-mediated traction forces on FN^syn cause reduced TGF-β1 release from its latent complex.

Concentration-Independent Multivalent Targeting of Cancer Cells by Genetically Encoded Core-Crosslinked Elastin/Resilin-like Polypeptide Micelles

Valency is a fundamental principle to control macromolecular interactions and is used to target specific cell types by multivalent ligand-receptor interactions using self-assembled nanoparticle carriers. At the concentrations encountered in solid tumors upon systemic administration, these nanoparticles are, however, likely to show critical micelle concentration (CMC)-dependent disassembly and thus loss of function. To overcome this limitation, core-crosslinkable micelles of genetically encoded resilin-/elastin-like diblock polypeptides were recombinantly synthesized. The amphiphilic constructs were covalently photo-crosslinked through the genetically encoded unnatural amino acid para-azidophenylalanine in their hydrophobic block and they carried different anticancer ligands on their hydrophilic block: the wild-type tenth human fibronectin type III domain, a GRGDSPAS peptide-both targeting α_vβ₃ integrin-and an engineered variant of the third fibronectin type III domain of tenascin C that is a death receptor 5 agonist. Although uncrosslinked micelles lost most of their targeting ability below their CMC, the crosslinked analogues remained active at concentrations up to 1000-fold lower than the CMC, with binding affinities that are comparable to antibodies.

Fibronectin: Molecular Structure, Fibrillar Structure and Mechanochemical Signaling

The extracellular matrix (ECM) plays a key role as both structural scaffold and regulator of cell signal transduction in tissues. In times of ECM assembly and turnover, cells upregulate assembly of the ECM protein, fibronectin (FN). FN is assembled by cells into viscoelastic fibrils that can bind upward of 40 distinct growth factors and cytokines. These fibrils play a key role in assembling a provisional ECM during embryonic development and wound healing. Fibril assembly is also often upregulated during disease states, including cancer and fibrotic diseases. FN fibrils have unique mechanical properties, which allow them to alter mechanotransduction signals sensed and relayed by cells. Binding of soluble growth factors to FN fibrils alters signal transduction from these proteins, while binding of other ECM proteins, including collagens, elastins, and proteoglycans, to FN fibrils facilitates the maturation and tissue specificity of the ECM. In this review, we will discuss the assembly of FN fibrils from individual FN molecules; the composition, structure, and mechanics of FN fibrils; the interaction of FN fibrils with other ECM proteins and growth factors; the role of FN in transmitting mechanobiology signaling events; and approaches for studying the mechanics of FN fibrils.

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.

Expression of NR5A2, NUP153, HNF4A, USP15 and FNDC3B is consistent with their use as novel biomarkers for bovine mammary stem/progenitor cells

Mammary stem cells (MaSC) are essential for growth and maintenance of mammary epithelium. Previous studies have utilized morphological characteristics or retention of bromodeoxyuridine (BrdU) label to identify MaSC and progenitor cells, these approaches may not be feasible or may not identify all resident stem cells. Alternatively, these special cells may be identified by assessing protein and mRNA expression of appropriate markers. The focus of this study was to assess the staining patterns and in situ quantification of novel candidate markers for bovine MaSC/progenitor cells. The candidate markers for MaSC/progenitor cells for immunohistochemical analysis were: NR5A2, NUP153, HNF4A, USP15 and FNDC3B and for in situ transcripts quantification were HNF4A and NUP153. We also evaluated protein expression pattern of presumptive MaSC markers known from the literature namely, ALDH1, MSI1 and Notch3. We found that NR5A2, NUP153, HNF4A and USP15-labeled cells represented 2.5-6% of epithelial cells prepubertally and were distributed in a fashion consistent with the location and abundance of MaSC/progenitor cells. A transient increase (10-37%) in expression of these markers was observed at peak lactation. FNDC3B was localized mainly in the nucleus prepubertally and in the cytoplasm of myoepithelial cells and nuclei of a limited number of alveolar cells during lactation. Abundant expression (~ 48%) and luminal localization of ALDH1 precludes its use as a bovine MaSC marker but may include transamplifying progenitor cells. MSI1 staining was consistent with MaSC localization. Onset of lumen formation in mammary ducts of prepubertal gland was associated with Notch 3 expression in the apical surface of luminal cells. RNAscope analysis of HNF4A and NUP153 transcripts in calf mammary gland showed very low copy numbers in a few epithelial cells, supporting the idea that these markers are expressed by fewer cells of epithelial origin. This study suggests that NR5A2, NUP153, HNF4A, USP15 and FNDC3B are likely markers for bovine MaSC/progenitor cells. Quantification of RNA transcripts of HNF4A and NUP153 in bovine MEC as potential MaSC markers are novel. Further studies to correlate protein expression of these markers with their transcripts level using single cell analysis in larger samples in lactating cow at different physiological stages are warranted.

Usher Syndrome: Genetics and Molecular Links of Hearing Loss and Directions for Therapy

Usher syndrome (USH) is an autosomal recessive (AR) disorder that permanently and severely affects the senses of hearing, vision, and balance. Three clinically distinct types of USH have been identified, decreasing in severity from Type 1 to 3, with symptoms of sensorineural hearing loss (SNHL), retinitis pigmentosa (RP), and vestibular dysfunction. There are currently nine confirmed and two suspected USH-causative genes, and a further three candidate loci have been mapped. The proteins encoded by these genes form complexes that play critical roles in the development and maintenance of cellular structures within the inner ear and retina, which have minimal capacity for repair or regeneration. In the cochlea, stereocilia are located on the apical surface of inner ear hair cells (HC) and are responsible for transducing mechanical stimuli from sound pressure waves into chemical signals. These signals are then detected by the auditory nerve fibers, transmitted to the brain and interpreted as sound. Disease-causing mutations in USH genes can destabilize the tip links that bind the stereocilia to each other, and cause defects in protein trafficking and stereocilia bundle morphology, thereby inhibiting mechanosensory transduction. This review summarizes the current knowledge on Usher syndrome with a particular emphasis on mutations in USH genes, USH protein structures, and functional analyses in animal models. Currently, there is no cure for USH. However, the genetic therapies that are rapidly developing will benefit from this compilation of detailed genetic information to identify the most effective strategies for restoring functional USH proteins.

USH2A variants in Chinese patients with Usher syndrome type II and non-syndromic retinitis pigmentosa

Aims

To reveal the Usher syndrome type IIA (USH2A) gene variant profile in a large cohort of Chinese patients with non-syndromic retinitis pigmentosa (RP) or Usher syndrome type II (USH2) and to explore the genotype–phenotype correlation.

Methods

Targeted exome capture plus next-generation sequencing confirmed that 284 patients from 260 unrelated Chinese families carried USH2A disease-associated variants. Both personal medical history and family histories were reviewed. Ocular examinations were performed and audiograms were recorded if hearing loss was suspected. The genotype–phenotype correlation was evaluated by statistical analyses.

Results

A total of 230 variants in the USH2A gene were identified, of which 90 (39.13%) were novel. The most common variants in the RP and USH2 probands were p.Cys934Trp and p.Tyr2854_2894del, respectively, and 26.42% and 63.64% of the alleles in the RP and USH2 groups were truncating, respectively. Patients harbouring biallelic truncating variants had a younger age at the initial clinical visit and symptom onset than patients with missense variants; furthermore, the patients with USH2 had a younger age at the initial clinical visit and nyctalopia onset compared with the patients with RP (p<0.001). For the patients with USH2, the age of nyctalopia onset was positively correlated with that of hearing loss (p<0.05, r=0.219). In addition, three pseudo-dominant pedigrees were identified carrying biallelic USH2A variants.

Conclusions

This study enrolled the largest cohort of Chinese patients with USH2A and identified the most prevalent USH2A variants in USH2 and RP. We found that the patients with USH2 had more truncating variants and experienced an earlier decline in visual function. The findings enhance the current knowledge of USH2A heterogeneity and provide valuable information for future therapies.

The immobilization of fibronectin- and fibroblast growth factor 2-derived peptides on a culture plate supports the attachment and proliferation of human pluripotent stem cells

Pluripotent stem cells (PSCs) offer a promising tool for regenerative medicine. The clinical application of PSCs inevitably requires a large-scale culture in a highly defined environment. The present study aimed to devise defined coating materials for the efficient adhesion and proliferation of human PSCs (hPSCs). We tested the activity of seven fibronectin-derived peptides and three laminin-derived peptides for the attachment and proliferation of hPSCs through their immobilization on the bottom of culture dishes by creating a fusion protein with the mussel adhesion protein. Among the extracellular matrix (ECM) mimetics tested, one fibronectin-derived peptide, PHSRN-GRGDSP, significantly promoted adhesion, enhanced alkaline phosphatase activity, and increased pluripotency-related gene expression in hPSCs compared to Matrigel. Furthermore, co-immobilization of a particular canofin peptide derived from fibroblast growth factor 2 increased pluripotency marker expression, which may offer the possibility of culture without growth factor supplementation. Our findings afford a novel defined condition for the efficient culture of hPSCs and may be utilized in future clinical applications.

Avidity and Cell Uptake of Integrin-Targeting Polypeptide Micelles is Strongly Shape-Dependent

We describe a genetically encoded micelle for targeted delivery consisting of a diblock polypeptide with segments derived from repetitive protein motifs inspired by Drosophila melanogaster Rec-1 resilin and human tropoelastin with a C-terminal fusion of an integrin-targeting fibronectin type III domain. By systematically varying the weight fraction of the hydrophilic elastin-like polypeptide (ELP) block and molecular weight of the diblock polypeptide, we designed micelles of different morphologies that modulate the binding avidity of the human wild-type 10th fibronectin domain (Fn3) as a function of shape. We show that wormlike micelles that present the Fn3 domain have a 1000-fold greater avidity for the α_vβ₃ receptor compared to the monomer ligand and an avidity that is greater than a clinically relevant antibody that is driven by their multivalency. The amplified avidity of these micelles leads to significantly increased cellular internalization, a feature that may have utility for the intracellular delivery of drugs that are loaded into the core of these micelles.

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.

Knockout of ush2a gene in zebrafish causes hearing impairment and late onset rod-cone dystrophy

Most cases of Usher syndrome type II (USH2) are due to mutations in the USH2A gene. There are no effective treatments or ideal animal models for this disease, and the pathological mechanisms of USH2 caused by USH2A mutations are still unknown. Here, we constructed a ush2a knockout (ush2a^-/-) zebrafish model using TALEN technology to investigate the molecular pathology of USH2. An early onset auditory disorder and abnormal morphology of inner ear stereocilia were identified in the ush2a^-/- zebrafish. Consequently, the disruption of Ush2a in zebrafish led to a hearing impairment, like that in mammals. Electroretinography (ERG) test indicated that deletion of Ush2a affected visual function at an early stage, and histological analysis revealed that the photoreceptors progressively degenerated. Rod degeneration occurred prior to cone degeneration in ush2a^-/- zebrafish, which is consistent with the classical description of the progression of retinitis pigmentosa (RP). Destruction of the outer segments (OSs) of rods led to the down-regulation of phototransduction cascade proteins at late stage. The expression of Ush1b and Ush1c was up-regulated when Ush2a was null. We also found that disruption of fibronectin assembly at the retinal basement membrane weakened cell adhesion in ush2a^-/- mutants. In summary, for the first time, we generated a ush2a knockout zebrafish line with auditory disorder and retinal degeneration which mimicked the symptoms of patients, and revealed that disruption of fibronectin assembly may be one of the factors underlying RP. This model may help us to better understand the pathogenic mechanism and find treatment for USH2 in the future.

Fundamentals of protein and cell interactions in biomaterials

The extracellular matrix (ECM) is an active and complex microenvironment with outstanding biomechanical, biophysical, and biochemical characteristics, which can indirectly or directly controls cell adhesion, migration, proliferation, and differentiation, as well as partaking in regeneration and homeostasis of organs and tissues. The ECM has captivated a great deal of attention with the rapid progress of tissue engineering (TE) in the field of regenerative medicine (RM). Approaches to TE, RM and cancer therapy center on the necessity to deliver cell signals to direct cell proliferation and differentiation. These "external signals" are induced from cell-cell, and cell-ECM, interactions, as well as from physico-chemical, mechanical stimuli and growth factors. With the advent of new biomaterials such as casein, we gave a general insight into cell-ECM protein interactions in biomaterials and their applications in TE, RM and cancer therapy. An account of the main ECM molecules and cellular receptors with emphasis on integrins and its ligands was given, their effect on the induction of particular signal transduction pathways is also elucidated.

The fibronectin synergy site re-enforces cell adhesion and mediates a crosstalk between integrin classes

Fibronectin (FN), a major extracellular matrix component, enables integrin-mediated cell adhesion via binding of α5β1, αIIbβ3 and αv-class integrins to an RGD-motif. An additional linkage for α5 and αIIb is the synergy site located in close proximity to the RGD motif. We report that mice with a dysfunctional FN-synergy motif (Fn1^syn/syn) suffer from surprisingly mild platelet adhesion and bleeding defects due to delayed thrombus formation after vessel injury. Additional loss of β3 integrins dramatically aggravates the bleedings and severely compromises smooth muscle cell coverage of the vasculature leading to embryonic lethality. Cell-based studies revealed that the synergy site is dispensable for the initial contact of α5β1 with the RGD, but essential to re-enforce the binding of α5β1/αIIbβ3 to FN. Our findings demonstrate a critical role for the FN synergy site when external forces exceed a certain threshold or when αvβ3 integrin levels decrease below a critical level.

DOI:http://dx.doi.org/10.7554/eLife.22264.001

Fibronectin maintains the balance between hemostasis and thrombosis

Fibronectin is a dimeric protein widely distributed in solid tissues and blood. This major extracellular matrix protein is indispensable for embryogenesis and plays crucial roles in many physiological and pathological processes. Fibronectin pre-mRNA undergoes alternative splicing to generate over 20 splicing variants, which are categorized as either plasma fibronectin (pFn) or cellular fibronectin (cFn). All fibronectin variants contain integrin binding motifs, as well as N-terminus collagen and fibrin binding motifs. With motifs that can be recognized by platelet integrins and coagulation factors, fibronectin, especially pFn, has long been suspected to be involved in hemostasis and thrombosis, but the exact function of fibronectin in these processes is controversial. The advances made using intravital microscopy models and fibronectin deficient and mutant mice have greatly facilitated the direct investigation of fibronectin function in vivo. Recent studies revealed that pFn is a vital hemostatic factor that is especially crucial for hemostasis in both genetic and anticoagulant-induced deficiencies of fibrin formation. pFn may also be an important self-limiting regulator to prevent hemorrhage as well as excessive thrombus formation and vessel occlusion. In addition to pFn, cFn is found to be prothrombotic and may contribute to thrombotic complications in various diseases. Further investigations of the role of pFn and cFn in thrombotic and hemorrhagic diseases may provide insights into development of novel therapeutic strategies (e.g., pFn transfusion) for the maintenance of the fine balance between hemostasis and thrombosis.

Force regulated conformational change of integrin αVβ3

Integrins mediate cell adhesion to extracellular matrix and transduce signals bidirectionally across the membrane. Integrin α_Vβ₃ has been shown to play an essential role in tumor metastasis, angiogenesis, hemostasis and phagocytosis. Integrins can take several conformations, including the bent and extended conformations of the ectodomain, which regulate integrin functions. Using a biomembrane force probe, we characterized the bending and unbending conformational changes of single α_Vβ₃ integrins on living cell surfaces in real-time. We measured the probabilities of conformational changes, rates and speeds of conformational transitions, and the dynamic equilibrium between the two conformations, which were regulated by tensile force, dependent on the ligand, and altered by point mutations. These findings provide insights into how α_Vβ₃ acts as a molecular machine and how its physiological function and molecular structure are coupled at the single-molecule level.

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.

Extracellular matrix structure

Extracellular matrix (ECM) is a non-cellular three-dimensional macromolecular network composed of collagens, proteoglycans/glycosaminoglycans, elastin, fibronectin, laminins, and several other glycoproteins. Matrix components bind each other as well as cell adhesion receptors forming a complex network into which cells reside in all tissues and organs. Cell surface receptors transduce signals into cells from ECM, which regulate diverse cellular functions, such as survival, growth, migration, and differentiation, and are vital for maintaining normal homeostasis. ECM is a highly dynamic structural network that continuously undergoes remodeling mediated by several matrix-degrading enzymes during normal and pathological conditions. Deregulation of ECM composition and structure is associated with the development and progression of several pathologic conditions. This article emphasizes in the complex ECM structure as to provide a better understanding of its dynamic structural and functional multipotency. Where relevant, the implication of the various families of ECM macromolecules in health and disease is also presented.

In situ modulation of cell behavior via smart dual-ligand surfaces

Due to the highly complex nature of the extracellular matrix (ECM), the design and implementation of dynamic, stimuli-responsive surfaces that present well-defined ligands and serve as model ECM substrates have been of tremendous interest to biomaterials, biosensor, and cell biology communities. Such tools provide strategies for identifying specific ligand-receptor interactions that induce vital biological consequences. Herein, we report a novel dual-ligand-presenting surface methodology that modulates dynamic ECM properties to investigate various cell behaviors. Peptides PHSRN, cRGD, and KKKTTK, which mimic the cell- and heparan sulfate-binding domains of fibronectin, and carbohydrates Gal and Man were combined with cell adhesive RGD to survey possible synergistic or antagonist ligand effects on cell adhesion, spreading, growth, and migration. Soluble molecule and enzymatic inhibition assays were also performed, and the levels of focal adhesion kinase in cells subjected to different ligand combinations were quantified. A redox-responsive trigger was incorporated into this surface strategy to spontaneously release ligands in the presence of adhered cells, and cell spreading, growth, and migration responses were measured and compared. The identity and nature of the dual-ligand combination directly influenced cell behavior.

Nicotinamide phosphoribosyltransferase can affect metastatic activity and cell adhesive functions by regulating integrins in breast cancer

NAD(+) metabolism is an essential regulator of cellular redox reactions, energy pathways, and a substrate provider for NAD(+) consuming enzymes. We recently demonstrated that enhancement of NAD(+)/NADH levels in breast cancer cells with impaired mitochondrial NADH dehydrogenase activity, through augmentation of complex I or by supplementing tumor cell nutrients with NAD(+) precursors, inhibits tumorigenicity and metastasis. To more fully understand how aberrantly low NAD(+) levels promote tumor cell dissemination, we here asked whether inhibition of NAD(+) salvage pathway activity by reduction in nicotinamide phosphoribosyltransferase (NAMPT) expression can impact metastasis and tumor cell adhesive functions. We show that knockdown of NAMPT, the enzyme catalyzing the rate-limiting step of the NAD(+) salvage pathway, enhances metastatic aggressiveness in human breast cancer cells and involves modulation of integrin expression and function. Reduction in NAMPT expression is associated with upregulation of select adhesion receptors, particularly αvβ3 and β1 integrins, and results in increased breast cancer cell attachment to extracellular matrix proteins, a key function in tumor cell dissemination. Interestingly, NAMPT downregulation prompts expression of integrin αvβ3 in a high affinity conformation, known to promote tumor cell adhesive interactions during hematogenous metastasis. NAMPT has been selected as a therapeutic target for cancer therapy based on the essential functions of this enzyme in NAD(+) metabolism, cellular redox, DNA repair and energy pathways. Notably, our results indicate that incomplete inhibition of NAMPT, which impedes NAD(+) metabolism but does not kill a tumor cell can alter its phenotype to be more aggressive and metastatic. This phenomenon could promote cancer recurrence, even if NAMPT inhibition initially reduces tumor growth.

Structural basis for pure antagonism of integrin αVβ3 by a high-affinity form of fibronectin

Integrins are important therapeutic targets. However, current RGD-based anti-integrin drugs are also partial agonists, inducing conformational changes that trigger potentially fatal immune reactions and paradoxical cell adhesion. Here we describe the first crystal structure of αVβ3 bound to a physiologic ligand, the tenth type III RGD domain of wild-type fibronectin (wtFN10), or to a high-affinity mutant (hFN10) shown here to act as a pure antagonist. Comparison of these structures revealed a central π-π interaction between Trp1496 in the RGD-containing loop of hFN10 and Tyr122 of the β3 subunit that blocked conformational changes triggered by wtFN10 and trapped hFN10-bound αVβ3 in an inactive conformation. Removing the Trp1496 or Tyr122 side chains or reorienting Trp1496 away from Tyr122 converted hFN10 into a partial agonist. These findings offer new insights into the mechanism of integrin activation and a basis for the design of RGD-based pure antagonists.

Atomic basis for the species-specific inhibition of αV integrins by monoclonal antibody 17E6 is revealed by the crystal structure of αVβ3 ectodomain-17E6 Fab complex

The function-blocking, non-RGD-containing, and primate-specific mouse monoclonal antibody 17E6 binds the αV subfamily of integrins. 17E6 is currently in phase II clinical trials for treating cancer. To elucidate the structural basis of recognition and the molecular mechanism of inhibition, we crystallized αVβ3 ectodomain in complex with the Fab fragment of 17E6. Protein crystals grew in presence of the activating cation Mn(2+). The integrin in the complex and in solution assumed the genuflected conformation. 17E6 Fab bound exclusively to the Propeller domain of the αV subunit. At the core of αV-Fab interface were interactions involving Propeller residues Lys-203 and Gln-145, with the latter accounting for primate specificity. The Propeller residue Asp-150, which normally coordinates Arg of the ligand Arg-Gly-Asp motif, formed contacts with Arg-54 of the Fab that were expected to reduce soluble FN10 binding to cellular αVβ3 complexed with 17E6. This was confirmed in direct binding studies, suggesting that 17E6 is an allosteric inhibitor of αV integrins.

Molecular docking characterization of a four-domain segment of human fibronectin encompassing the RGD loop with hydroxyapatite

Fibronectin adsorption on biomaterial surfaces plays a key role in the biocompatibility of biomedical implants. In the current study, the adsorption behavior of the 7–10th type III modules of fibronectin (FN-III_7–10) in the presence of hydroxyapatite (HAP) was systematically investigated by using molecular docking approach. It was revealed that the FN-III₁₀ is the most important module among FN-III_7–10 in promoting fibronectin binding to HAP by optimizing the interaction energy; the arginine residues were observed to directly interact with the hydroxyl group of HAP through electrostatic forces and hydrogen bonding. Moreover, it was found that the HAP-binding sites on FN-III₁₀ are mainly located at the RGD loop region, which does not affect the interaction between the fibronectin protein and its cognate receptors on the cell surface.

The mechanism of kindlin-mediated activation of integrin αIIbβ3

Increased ligand binding to cellular integrins ("activation") plays important roles in processes such as development, cell migration, extracellular matrix assembly, tumor metastasis, hemostasis, and thrombosis. Integrin activation encompasses both increased integrin monomer affinity and increased receptor clustering and depends on integrin-talin interactions. Loss of kindlins results in reduced activation of integrins. Kindlins might promote talin binding to integrins through a cooperative mechanism; however, kindlins do not increase talin association with integrins. Here, we report that, unlike talin head domain (THD), kindlin-3 has little effect on the affinity of purified monomeric αIIbβ3, and it does not enhance activation by THD. Furthermore, studies with ligands of varying valency show that kindlins primarily increase cellular αIIbβ3 avidity rather than monomer affinity. In platelets or nucleated cells, loss of kindlins markedly reduces αIIbβ3 binding to multivalent but not monovalent ligands. Finally, silencing of kindlins reduces the clustering of ligand-occupied αIIbβ3 as revealed by total internal reflection fluorescence and electron microscopy. Thus, in contrast to talins, kindlins have little primary effect on integrin αIIbβ3 affinity for monovalent ligands and increase multivalent ligand binding by promoting the clustering of talin-activated integrins.

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.

Induction of albumin expression in HepG2 cells using immobilized simplified recombinant fibronectin protein

Optimization of the extracellular environment is very important for hepatocyte function in vitro. We expressed new chimeric proteins of the collagen-binding domain (CBD) with cell attachment site (CAS) of fibronectin to enhance hepatocyte function, and the CBD-CAS proteins were immobilized on collagen-coated plates. We hypothesized that the high density of CAS would increase activity of the integrin-dependent intracellular signaling pathway, thus inducing hepatocyte function. Expression of albumin in the human hepatocyte cell line HepG2 was assessed on CBD-CAS-immobilized dishes. The results indicated that the CBD-CAS-immobilized plates induced albumin expression. Immobilized CBD-CAS induced activation of focal adhesion kinase and integrin-ligand clustering on the cell membrane. These results suggest that immobilized CBD-CAS improves the function of HepG2 cells. This system could therefore be applied to drug metabolism assay in the development of new drugs.

Preparation of gradient surfaces by using a simple chemical reaction and investigation of cell adhesion on a two-component gradient

This article describes a simple method for the generation of multicomponent gradient surfaces on self-assembled monolayers (SAMs) on gold in a precise and predictable manner, by harnessing a chemical reaction on the monolayer, and their applications. A quinone derivative on a monolayer was converted to an amine through spontaneous intramolecular cyclization following first-order reaction kinetics. An amine gradient on the surface on a scale of centimeters was realized by modulating the exposure time of the quinone-presenting monolayer to the chemical reagent. The resulting amine was used as a chemical handle to attach various molecules to the monolayer with formation of multicomponent gradient surfaces. The effectiveness of this strategy was verified by cyclic voltammetry (CV), matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), MS imaging, and contact-angle measurements. As a practical application, cell adhesion was investigated on RGD/PHSRN peptide/peptide gradient surfaces. Peptide PHSRN was found to synergistically enhance cell adhesion at the position where these two ligands are presented in equal amounts, while these peptide ligands were competitively involved in cell adhesion at other positions. This strategy of generating a gradient may be further expandable to the development of functional gradient surfaces of various molecules and materials, such as DNA, proteins, growth factors, and nanoparticles, and could therefore be useful in many fields of research and practical applications.

Rv2468c, a novel Mycobacterium tuberculosis protein that costimulates human CD4+ T cells through VLA-5

Mtb regulates many aspects of the host immune response, including CD4+ T lymphocyte responses that are essential for protective immunity to Mtb, and Mtb effects on the immune system are paradoxical, having the capacity to inhibit (immune evasion) and to activate (adjuvant effect) immune cells. Mtb regulates CD4+ T cells indirectly (e.g., by manipulation of APC function) and directly, via integrins and TLRs expressed on T cells. We now report that previously uncharacterized Mtb protein Rv2468c/MT2543 can directly regulate human CD4+ T cell activation by delivering costimulatory signals. When combined with TCR stimulation (e.g., anti-CD3), Rv2468c functioned as a direct costimulator for CD4+ T cells, inducing IFN-γ secretion and T cell proliferation. Studies with blocking antibodies and soluble RGD motifs demonstrated that Rv2468c engaged integrin VLA-5 (α5β1) on CD4+ T cells through its FN-like RGD motif. Costimulation by Rv2468c induced phosphorylation of FAKs and Pyk2. These results reveal that by expressing molecules that mimic host protein motifs, Mtb can directly engage receptors on CD4+ T cells and regulate their function. Rv2468c-induced costimulation of CD4+ T cells could have implications for TB immune pathogenesis and Mtb adjuvant effect.

Using self-assembled monolayers to understand α8β1-mediated cell adhesion to RGD and FEI motifs in nephronectin

Nephronectin is an extracellular matrix protein that interacts with the α8β1 integrin receptor and plays a role in tissue and organ development, though the motifs that mediate adhesion to the receptor remain unclear. This paper describes the use of self-assembled monolayers to study the adhesion of α8β1-presenting cells to the RGD and DLFEIFEIER ligands in nephronectin and found that both ligands can independently mediate cell adhesion through nonoverlapping binding sites on the integrin. Peptide truncation experiments showed FEI to be the minimal binding sequence within the DLFEIFEIER sequence, and adhesion experiments with peptides that include both the RGD and FEI sequences demonstrate that the two peptides bind synergistically to the receptor. Finally, a peptide array was used to establish a strict requirement for the glutamate residue of FEI and tolerance of other aromatic and hydrophobic residues in the first and third positions, respectively. This work provides an enhanced understanding of the binding of nephronectin with α8β1 and identifies a peptide ligand that can be used for targeting the α8β1 integrin.

Advantages of RGD peptides for directing cell association with biomaterials

Despite many years of in vitro research confirming the effectiveness of RGD in promoting cell attachment to a wide variety of biomaterials, animal studies evaluating tissue responses to implanted RGD-functionalized substrates have yielded more variable results. The goals of this report are to present some of the reasons why cell culture studies may not always reliably predict in vivo responses, and more importantly, to highlight potential applications that may benefit from the use of RGD peptides.

Probing the folded state of fibronectin type III domains in stretched fibrils by measuring buried cysteine accessibility

Fibronectin (FN) is an extracellular matrix protein that is assembled into fibrils by cells during tissue morphogenesis and wound healing. FN matrix fibrils are highly elastic, but the mechanism of elasticity has been debated: it may be achieved by mechanical unfolding of FN-III domains or by a conformational change of the molecule without domain unfolding. Here, we investigate the folded state of FN-III domains in FN fibrils by measuring the accessibility of buried cysteines. Four of the 15 FN-III domains (III-2, -3, -9, and -11) appear to unfold in both stretched fibrils and in solution, suggesting that these domains spontaneously open and close even in the absence of tension. Two FN-III domains (III-6 and -12) appear to unfold only in fibrils and not in solution. These results suggest that domain unfolding can at best contribute partially to the 4-fold extensibility of fibronectin fibrils.

Neisseria gonorrhoeae pilin glycan contributes to CR3 activation during challenge of primary cervical epithelial cells

Expression of type IV pili by Neisseria gonorrhoeae plays a critical role in mediating adherence to human epithelial cells. Gonococcal pilin is modified with an O-linked glycan, which may be present as a di- or monosaccharide because of phase variation of select pilin glycosylation genes. It is accepted that bacterial proteins may be glycosylated; less clear is how the protein glycan may mediate virulence. Using primary, human, cervical epithelial (i.e. pex) cells, we now provide evidence to indicate that the pilin glycan mediates productive cervical infection. In this regard, pilin glycan-deficient mutant gonococci exhibited an early hyper-adhesive phenotype but were attenuated in their ability to invade pex cells. Our data further indicate that the pilin glycan was required for gonococci to bind to the I-domain region of complement receptor 3, which is naturally expressed by pex cells. Comparative, quantitative, infection assays revealed that mutant gonococci lacking the pilin glycan did not bind to the I-domain when it is in a closed, low-affinity conformation and cannot induce an active conformation to complement receptor 3 during pex cell challenge. To our knowledge, these are the first data to directly demonstrate how a protein-associated bacterial glycan may contribute to pathogenesis.

Talin and signaling through integrins

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton. A wide range of experimental approaches have contributed to the current understanding of the importance of talin in integrin signaling. Here, we describe two methods that have been central to our investigations of talin; a biochemical assay that has allowed characterization of interactions between integrin cytoplasmic tails and talin, and a fluorescent-activated cell-sorting procedure to assess integrin activation in cultured cells expressing talin domains, mutants, dominant negative constructs, or shRNA.

Using self-assembled monolayers to model cell adhesion to the 9th and 10th type III domains of fibronectin

Most mammalian cells must adhere to the extracellular matrix (ECM) to maintain proper growth and development. Fibronectin is a predominant ECM protein that engages integrin cell receptors through its Arg-Gly-Asp (RGD) and Pro-His-Ser-Arg-Asn (PHSRN) peptide binding sites. To study the roles these motifs play in cell adhesion, proteins derived from the 9th (containing PHSRN) and 10th (containing RGD) type III fibronectin domains were engineered to be in frame with cutinase, a serine esterase that forms a site-specific, covalent adduct with phosphonate ligands. Self-assembled monolayers (SAMs) that present phosphonate ligands against an inert background of tri(ethylene glycol) groups were used as model substrates to immobilize the cutinase-fibronectin fusion proteins. Baby hamster kidney cells attached efficiently to all protein surfaces, but only spread efficiently on protein monolayers containing the RGD peptide. Cells on RGD-containing protein surfaces also displayed defined focal adhesions and organized cytoskeletal structures compared to cells on PHSRN-presenting surfaces. Cell attachment and spreading were shown to be unaffected by the presence of PHSRN when compared to RGD alone on SAMs presenting higher densities of protein, but PHSRN supported an increased efficiency in cell attachment when presented at low protein densities with RGD. Treatment of suspended cells with soluble RGD or PHSRN peptides revealed that both peptides were able to inhibit the attachment of FN10 surfaces. These results support a model wherein PHSRN and RGD bind competitively to integrins--rather than a two-point synergistic interaction--and the presence of PHSRN serves to increase the density of ligand on the substrate and therefore enhance the sticking probability of cells during attachment.

Emerging concepts in engineering extracellular matrix variants for directing cell phenotype

Directing specific, complex cell behaviors, such as differentiation, in response to biomaterials for regenerative medicine applications is, at present, a mostly unrealized goal. To date, current technological advances have been inspired by the reductionist point of view, focused on developing simple and merely adequate environments that facilitate simple cellular adhesion. However, even if extracellular matrix (ECM)-derived peptides, such as Arg-Gly-Asp (RGD), have largely demonstrated their utility in supporting cell adhesion, their lack of biological specificity is simply not optimal for controlling more integrated processes, such as cell differentiation. These more complex cellular processes require specific integrin-signaling scaffolds and presumably synergistic integrin and growth factor-receptor signaling. This article will introduce some current efforts to engineer ECM variants that incorporate additional levels of complexity for directing greater integrin specificity and synergistic ECM growth factor signaling toward directing cell phenotype.

Accessibility to the Fibronectin Synergy Site in a 3D Matrix Regulates Engagement of α 5β 1 versus α vβ 3 Integrin Receptors

Cell adhesion and migration on fibronectin (FN) extracellular matrix are mediated by integrin receptors. Integrins alpha5beta1 and alphavbeta3 require the RGD cell-binding sequence in FN, but alpha5beta1 also requires the nearby synergy site for maximal binding. In this study, we investigated how differences in the numbers of RGD or synergy sites within a three-dimensional (3D) FN-rich matrix influence cell adhesion and migration. CHO cell adhesion, spreading, and migration were reduced on 3D chimeric matrix containing FN lacking RGD (FN(RGD-)). Incorporation of FN with mutation of the synergy site (FN(syn-)), however, resulted in selective usage of integrins. CHO cells expressing alpha5beta1 showed decreased interactions with FN(syn-) chimeric matrix. In contrast, the presence of FN(syn-) had no effect on CHOalphavbeta3 cell migration. Interestingly, CHOalpha5/alphavbeta3 cells expressing both integrins selectively used alpha5beta1 for migration on wild type FN matrix but preferred alphavbeta3 for migration on FN(syn-) chimeric matrix. Thus sequestration or exposure of the FN synergy site within a 3D matrix may represent a novel mechanism for regulating cell functions through differential usage of integrin receptors. [Supplementary materials are available for this article. Go to the publisher's online edition of Cell Communication and Adhesion for the following free supplemental resource: a video recording shows migration of HT1080 cells on 3D matrix. HT1080 cells were allowed to attach to the matrix in serum-free DMEM for 2 h. FBS was then added to the medium to a final concentration of 10% and video recording was started. Images were taken every 5 min for 2 h. The video plays at 6 frames/s.].

Integrin switching modulates adhesion dynamics and cell migration

When cells are stimulated to move, for instance during development, wound healing or angiogenesis, they undergo changes in the turnover of their cell-matrix adhesions. This is often accompanied by alterations in the expression profile of integrins-the extracellular matrix receptors that mediate anchorage within these adhesions. Here, we discuss how a shift in expression between two different types of integrins that bind fibronectin can have dramatic consequences for cell-matrix adhesion dynamics and cell motility.

Disruption of the novel gene fad104 causes rapid postnatal death and attenuation of cell proliferation, adhesion, spreading and migration

The molecular mechanisms at the beginning of adipogenesis remain unknown. Previously, we identified a novel gene, fad104 (factor for adipocyte differentiation 104), transiently expressed at the early stage of adipocyte differentiation. Since the knockdown of the expression of fad104 dramatically repressed adipogenesis, it is clear that fad104 plays important roles in adipocyte differentiation. However, the physiological roles of fad104 are still unknown. In this study, we generated fad104-deficient mice by gene targeting. Although the mice were born in the expected Mendelian ratios, all died within 1 day of birth, suggesting fad104 to be crucial for survival after birth. Furthermore, analyses of mouse embryonic fibroblasts (MEFs) prepared from fad104-deficient mice provided new insights into the functions of fad104. Disruption of fad104 inhibited adipocyte differentiation and cell proliferation. In addition, cell adhesion and wound healing assays using fad104-deficient MEFs revealed that loss of fad104 expression caused a reduction in stress fiber formation, and notably delayed cell adhesion, spreading and migration. These results indicate that fad104 is essential for the survival of newborns just after birth and important for cell proliferation, adhesion, spreading and migration.

Dentin matrix protein-1 isoforms promote differential cell attachment and migration

Dentin matrix protein-1 (DMP1), bone sialoprotein (BSP), and osteopontin (OPN) are three SIBLINGs (small integrin-binding ligand, N-linked glycoproteins) co-expressed/secreted by skeletal and active ductal epithelial cells. Although etiological mechanisms remain unclear, DMP1 is the only one of these three genes currently known to have mutations resulting in human disease, and yet it remains the least studied. All three contain the highly conserved integrin-binding tripeptide, RGD, and experiments comparing the cell attachment and haptotactic migration-enhancing properties of DMP1 to BSP and OPN were performed using human skeletal (MG63 and primary dental pulp cells) and salivary gland (HSG) cells. Mutation of any SIBLING's RGD destroyed all attachment and migration activity. Using its alphaVbeta5 integrin, HSG cells attached to BSP but not to DMP1 or OPN. However, HSG cells could not migrate onto BSP in a modified Boyden chamber assay. Expression of alphaVbeta3 integrin enhanced HSG attachment to DMP1 and OPN and promoted haptotactic migration onto all three proteins. Interchanging the first four coding exons or the conserved amino acids adjacent to the RGD of DMP1 with corresponding sequences of BSP did not enhance the ability of DMP1 to bind alphaVbeta5. For alphaVbeta3-expressing cells, intact DMP1, its BMP1-cleaved C-terminal fragment, and exon six lacking all post-translational modifications worked equally well but the proteoglycan isoform of DMP1 had greatly reduced ability for cell attachment and migration. The sequence specificity of the proposed BMP1-cleavage site of DMP1 was verified by mutation analysis. Direct comparison of the three proteins showed that cells discriminate among these SIBLINGs and among DMP1 isoforms.

Fibronectin stimulates TRPV1 translocation in primary sensory neurons

Extracellular matrix (ECM) molecules are highly variable in their composition and receptor recognition. Their ubiquitous expression profile has been linked to roles in cell growth, differentiation, and survival. Recent work has identified certain ECM molecules that serve as dynamic signal modulators, versus the more-recognized role of chronic modulation of signal transduction. In this study, we investigated the role that fibronectin (FN) plays in the dynamic modulation of transient receptor potential family V type 1 receptor (TRPV1) translocation to the plasma membrane in trigeminal ganglia (TG) sensory neurons. Confocal immunofluorescence analyses identify co-expression of the TRPV1 receptor with integrin subunits that bind FN. TG neurons cultured upon or treated with FN experienced a leftward shift in the EC(50) of capsaicin-stimulated neuropeptide release. This FN-induced increase in TRPV1 sensitivity to activation is coupled by an increase in plasma membrane expression of TRPV1, as well as an increase in tyrosine phosphorylation of TRPV1 in TG neurons. Furthermore, TG neurons cultured on FN demonstrated an increase in capsaicin-mediated Ca(2+) accumulation relative to neurons cultured on poly-D-lysine. Data presented from these studies indicate that FN stimulates tyrosine-phosphorylation-dependent translocation of the TRPV1 receptor to the plasma membrane, identifying FN as a critical component of the ECM capable of sensory neuron sensitization.