Transient opening of fibronectin type III (FNIII) domains: the interaction of the third FNIII domain of FN with anastellin

Insights into the implementation of Fibronectin 1 in the cartilage tissue engineering

Recently, cartilage tissue engineering has become a cornerstone to treat cartilage degeneration and osteoarthritis (OA). Fibronectin1 (FN1) is described as multiple functional glycoproteins that play an essential role in chondrogenic and osteogenic differentiation. Few studies reported the potential of FN1 to enhance tissue engineering and reduce the death of chondrocytes in OA. Further, FN1 possesses multiple binding domains including collagen, integrin, and heparin that can interact with heparan sulfate proteoglycans at the surface of chondrocyte leading to promote cell signaling and differentiation. Recent studies suggested that FN1 can promote chondrocyte differentiation by upregulating TGF-β/PI3K/Akt pathways. Further, FN1 can inhibit the apoptosis of chondrocytes by preventing the release of metalloproteinases through lowering the expression of p-PI3K/PI3K and p-AKT/AKT pathways. However, the use of FN1 in cartilage repair studies using animal models or clinical trials was rarely reported. Therefore, this article provides new insights into the importance of FN1 in cartilage tissue engineering to encourage more studies concerning FN1 in cartilage repair studies. Further, we provided new suggestions for advanced applications of FN1 to treat OA and cartilage degeneration.

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.

Peroxynitrous acid (ONOOH) modifies the structure of anastellin and influences its capacity to polymerize fibronectin

Anastellin (AN), a fragment of the first type III module in fibronectin (FN), initiates formation of superfibronectin, a polymer which resembles the native cell-derived fibrillar FN found in the extracellular matrix of many tissues, but which displays remarkably different functional properties. Here we demonstrate that exposure of AN to the biologically-important inflammatory oxidant, peroxynitrous acid (ONOOH), either as a bolus or formed at low levels in a time-dependent manner from SIN-1, impairs the capability of AN to polymerize FN. In contrast, exposure of FN to ONOOH does not seem to affect superfibronectin formation to the same extent. This oxidant-induced loss-of-function in AN occurs in a dose-dependent manner, and correlates with structural perturbations, loss of the amino acid tyrosine and tryptophan, and dose-dependent formation of modified amino acid side-chains (3-nitrotyrosine, di-tyrosine and 6-nitrotryptophan). Reagent ONOOH also induces formation of oligomeric species which decrease in the presence of bicarbonate, whereas SIN-1 mainly generates dimers. Modifications were detected at sub-stoichiometric (0.1-fold), or greater, molar excesses of oxidant compared to AN. These species have been localized to specific sites by peptide mass mapping. With high levels of oxidant (>100 times molar excess), ONOOH also induces unfolding of the beta-sheet structure of AN, thermal destabilization, and formation of high molecular mass aggregates. These results have important implications for the understanding of FN fibrillogenesis in vivo, and indicates that AN is highly sensitive to pathophysiological levels of oxidants such as ONOOH.

Unraveling the Mechanobiology of Extracellular Matrix

Cells need to be anchored to extracellular matrix (ECM) to survive, yet the role of ECM in guiding developmental processes, tissue homeostasis, and aging has long been underestimated. How ECM orchestrates the deterioration of healthy to pathological tissues, including fibrosis and cancer, also remains poorly understood. Inquiring how alterations in ECM fiber tension might drive these processes is timely, as mechanobiology is a rapidly growing field, and many novel mechanisms behind the mechanical forces that can regulate protein, cell, and tissue functions have recently been deciphered. The goal of this article is to review how forces can switch protein functions, and thus cell signaling, and thereby inspire new approaches to exploit the mechanobiology of ECM in regenerative medicine as well as for diagnostic and therapeutic applications. Some of the mechanochemical switching concepts described here for ECM proteins are more general and apply to intracellular proteins as well.

Direct Binding of the Flexible C-Terminal Segment of Periaxin to β4 Integrin Suggests a Molecular Basis for CMT4F

The process of myelination in the nervous system requires a coordinated formation of both transient and stable supramolecular complexes. Myelin-specific proteins play key roles in these assemblies, which may link membranes to each other or connect the myelinating cell cytoskeleton to the extracellular matrix. The myelin protein periaxin is known to play an important role in linking the Schwann cell cytoskeleton to the basal lamina through membrane receptors, such as the dystroglycan complex. Mutations that truncate periaxin from the C terminus cause demyelinating peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease type 4F, indicating a function for the periaxin C-terminal region in myelination. We identified the cytoplasmic domain of β4 integrin as a specific high-affinity binding partner for periaxin. The C-terminal region of periaxin remains unfolded and flexible when bound to the third fibronectin type III domain of β4 integrin. Our data suggest that periaxin is able to link the Schwann cell cytoplasm to the basal lamina through a two-pronged interaction via different membrane protein complexes, which bind close to the N and C terminus of this elongated, flexible molecule.

Fibronectin Produced by Cerebral Endothelial and Vascular Smooth Muscle Cells Contributes to Perivascular Extracellular Matrix in Late-Delayed Radiation-Induced Brain Injury

Late-delayed radiation-induced brain injury (RIBI) is a major adverse effect of fractionated whole-brain irradiation (fWBI). Characterized by progressive cognitive dysfunction, and associated cerebrovascular and white matter injury, RIBI deleteriously affects quality of life for cancer patients. Despite extensive morphological characterization of the injury, the pathogenesis is unclear, thus limiting the development of effective therapeutics. We previously reported that RIBI is associated with increased gene expression of the extracellular matrix (ECM) protein fibronectin (FN1). We hypothesized that fibronectin contributes to perivascular ECM, which may impair diffusion to the dependent parenchyma, thus contributing to the observed cognitive decline. The goal of this study was to determine the localization of fibronectin in RIBI and further characterize the composition of perivascular ECM, as well as identify the cell of origin for FN1 by in situ hybridization. Briefly, fibronectin localized to the vascular basement membrane of morphologically normal blood vessels from control comparators and animals receiving fWBI, and to the perivascular space of edematous and fibrotic vascular phenotypes of animals receiving fWBI. Additional mild diffuse parenchymal staining in areas of vascular injury suggested blood-brain-barrier disruption and plasma fibronectin extravasation. Perivascular ECM lacked amyloid and contained lesser amounts of collagens I and IV, which localized to the basement membrane. These changes occurred in the absence of alterations in microvascular area fraction or microvessel density. Fibronectin transcripts were rarely expressed in control comparators, and were most strongly induced within cerebrovascular endothelial and vascular smooth muscle cells after fWBI. Our results demonstrate that fibronectin is produced by cerebrovascular endothelial and smooth muscle cells in late-delayed RIBI and contributes to perivascular ECM, which we postulate may contribute to diffusion barrier formation. We propose that pathways that antagonize fibronectin deposition and matrix assembly or enhance degradation may serve as potential therapeutic targets in RIBI.

The Interaction between the Third Type III Domain from Fibronectin and Anastellin Involves β-Strand Exchange

Anastellin is a small recombinant fragment derived from the extracellular matrix protein fibronectin; it comprises the first type III (FN3) domain without the two N-terminal β-strands. It inhibits angiogenesis, tumor growth, and metastasis in mouse models and requires endogenous fibronectin for its in vivo anti-angiogenic activity. It binds to fibronectin in vitro and converts the soluble protein to insoluble fibrils that structurally and functionally resemble fibronectin fibrils deposited in the extracellular matrix by cells. Anastellin binds to several FN3 domains in fibronectin, but how it interacts with these domains and why the interactions lead to aggregation of fibronectin are not well understood. In this work, we investigated the interaction between anastellin and the third FN3 domain (3FN3) from fibronectin. We show that anastellin binds with high affinity to a peptide comprising the two N-terminal β-strands from 3FN3, and we present here the structure of the resulting complex. The peptide and anastellin form a composite FN3 domain, with the two N-terminal β-strands from 3FN3 bound in place of the two β-strands that are missing in anastellin. We also demonstrate using disulfide cross-linking that a similar interaction involving the two N-terminal β-strands of 3FN3 occurs when intact 3FN3 binds to anastellin. 3FN3 adopts a compact globular fold in solution, and to interact with anastellin in a manner consistent with our data, it has to open up and expose a β-strand edge that is not accessible in the context of the folded domain.

8-Anilino-1-naphthalenesulfonate/Layered Double Hydroxide Ultrathin Films: Small Anion Assembly and Its Potential Application as a Fluorescent Biosensor

The fluorescent dye 8-anilino-1-naphthalenesulfonate (ANS) is a widely used fluorescent probe molecule for biochemistry analysis. This paper reported the fabrication of ANS/layered double hydroxide nanosheets (ANS/LDH)n ultrathin films (UTFs) via the layer-by-layer small anion assembly technique based on electrostatic interaction and two possible weak interactions: hydrogen-bond and induced electrostatic interactions between ANS and positive-charged LDH nanosheets. The obtained UTFs show a long-range-ordered periodic layered stacking structure and weak fluorescence in dry air or water, but it split into three narrow strong peaks in a weak polarity environment induced by the two-dimensional (2D) confinement effect of the LDH laminate; the fluorescence intensity increases with decreasing the solvent polarity, concomitant with the blue shift of the emission peaks, which show good sensoring reversibility. Meanwhile, the UTFs exhibit selective fluorescence enhancement to the bovine serum albumin (BSA)-like protein biomolecules, and the rate of fluorescence enhancement with the protein concentration is significantly different with the different protein aggregate states. The (ANS/LDH)n UTF has the potential to be a novel type of biological flourescence sensor material.

Dynamic structure of plasma fibronectin

Fibronectin is a large vertebrate glycoprotein that is found in soluble and insoluble forms and involved in diverse processes. Protomeric fibronectin is a dimer of subunits, each of which comprises 29-31 modules - 12 type I, two type II and 15-17 type III. Plasma fibronectin is secreted by hepatocytes and circulates in a compact conformation before it binds to cell surfaces, converts to an extended conformation and is assembled into fibronectin fibrils. Here we review biophysical and structural studies that have shed light on how plasma fibronectin transitions from the compact to the extended conformation. The three types of modules each have a well-organized secondary and tertiary structure as defined by NMR and crystallography and have been likened to "beads on a string". There are flexible sequences in the N-terminal tail, between the fifth and sixth type I modules, between the first two and last two of the type III modules, and at the C-terminus. Several specific module-module interactions have been identified that likely maintain the compact quaternary structure of circulating fibronectin. The quaternary structure is perturbed in response to binding events, including binding of fibronectin to the surface of vertebrate cells for fibril assembly and to bacterial adhesins.

Structure and unfolding of the third type III domain from human fibronectin

Fibronectin is a modular extracellular matrix protein that is essential for vertebrate development. The third type III domain (3FN3) in fibronectin interacts with other parts of fibronectin and with anastellin, a protein fragment that causes fibronectin aggregation. 3FN3 opens readily both as an isolated domain in solution and when part of fibronectin in stretched fibrils, and it was proposed that this opening is important for anastellin binding. We determined the structure of 3FN3 using nuclear magnetic resonance spectroscopy, and we investigated its stability, folding, and unfolding. Similar to most other FN3 domains, 3FN3 contains two antiparallel β-sheets that are composed of three (A, B, and E) and four (C, D, F, and G) β-strands, respectively, and are held together by a conserved hydrophobic interface. cis-trans isomerization of P847 at the end of β-strand C leads to observable conformational heterogeneity in 3FN3, with a cis peptide bond present in almost one-quarter of the molecules. The chemical stability of 3FN3 is relatively low, but the folding rate constant in the absence of denaturant is in the same range as those of other, more stable FN3 domains. Interestingly, the unfolding rate constant in the absence of denaturant is several orders of magnitude higher than the unfolding rate constants of other FN3 domains investigated to date. This unusually fast rate is comparable to the rate of binding of 3FN3 to anastellin at saturating anastellin concentrations, consistent with the model in which 3FN3 has to unfold to interact with anastellin.

Cryptic activity within the Type III1 domain of fibronectin regulates tissue inflammation and angiogenesis

The fibronectin matrix provides mechanical and biochemical information to regulate homeostatic and pathological processes within tissues. Fibronectin consists of independently-folded modules termed Types I, II and III. In response to cellular contractile force, Type III domains unfold to initiate a series of homophilic binding events which result in the assembly of a complex network of intertwining fibrils. The unfolding of Type III modules provides elasticity to the assembled fibronectin matrix allowing it to function as a dynamic scaffold which provides binding sites for cellular receptors, growth factors and other matrix molecules. Access to bioactive sites within the fibronectin matrix is under complex regulation and controlled through a combination of mechanical and proteolytic activity. Mechanical unfolding of Type III modules and limited proteolysis can alter the topographical display of bioactive sites within the fibronectin fibrils by exposing previously cryptic sites and disrupting functional sites. In this review we will discuss cryptic activity found within the first Type III module of fibronectin and its impact on tissue angiogenesis and inflammation.

Mechano-transduction: from molecules to tissues

Biological mechano-transduction and force-dependent changes scale from protein conformation (â„« to nm) to cell organization and multi-cell function (mm to cm) to affect cell organization, fate, and homeostasis.

External forces play complex roles in cell organization, fate, and homeostasis. Changes in these forces, or how cells respond to them, can result in abnormal embryonic development and diseases in adults. How cells sense and respond to these mechanical stimuli requires an understanding of the biophysical principles that underlie changes in protein conformation and result in alterations in the organization and function of cells and tissues. Here, we discuss mechano-transduction as it applies to protein conformation, cellular organization, and multi-cell (tissue) function.

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.

Intrinsic disorder of the extracellular matrix

The extracellular matrix is very well organized at the supramolecular and tissue levels and little is known on the potential role of intrinsic disorder in promoting its organization. We predicted the amount of disorder and identified disordered regions in the human extracellular proteome with established computational tools. The extracellular proteome is significantly enriched in proteins comprising more than 50% of disorder compared to the complete human proteome. The enrichment is mostly due to long disordered regions containing at least 100 consecutive disordered residues. The amount of intrinsic disorder is heterogeneous in the extracellular protein families, with the most disordered being collagens and the small integrin-binding ligand N-linked glycoproteins. Although most domains found in extracellular proteins are structured, the fibronectin III domains contain a variable amount of disordered residues (up to 92%). Binding sites for heparin and integrins are found in disordered sequences of extracellular proteins. Intrinsic disorder is evenly distributed in hubs and ends in the interaction network of extracellular proteins with their extracellular partners. In contrast, extracellular hubs are significantly enriched in disorder in the network of extracellular proteins with their extracellular, membrane and intracellular partners. Disorder could thus provide the structural plasticity required for the hubs to interact with membrane and intracellular proteins. Organization and assembly of the extracellular matrix, development of mineralized tissues and cell-matrix adhesion are the biological processes overrepresented in the most disordered extracellular proteins. Extracellular disorder is associated with binding to growth factors, glycosaminoglycans and integrins at the molecular level.

Fibronectin aggregation and assembly: the unfolding of the second fibronectin type III domain

The mechanism of fibronectin (FN) assembly and the self-association sites are still unclear and contradictory, although the N-terminal 70-kDa region ((I)1-9) is commonly accepted as one of the assembly sites. We previously found that (I)1-9 binds to superfibronectin, which is an artificial FN aggregate induced by anastellin. In the present study, we found that (I)1-9 bound to the aggregate formed by anastellin and a small FN fragment, (III)1-2. An engineered disulfide bond in (III)2, which stabilizes folding, inhibited aggregation, but a disulfide bond in (III)1 did not. A gelatin precipitation assay showed that (I)1-9 did not interact with anastellin, (III)1, (III)2, (III)1-2, or several (III)1-2 mutants including (III)1-2KADA. (In contrast to previous studies, we found that the (III)1-2KADA mutant was identical in conformation to wild-type (III)1-2.) Because (I)1-9 only bound to the aggregate and the unfolding of (III)2 played a role in aggregation, we generated a (III)2 domain that was destabilized by deletion of the G strand. This mutant bound (I)1-9 as shown by the gelatin precipitation assay and fluorescence resonance energy transfer analysis, and it inhibited FN matrix assembly when added to cell culture. Next, we introduced disulfide mutations into full-length FN. Three disulfide locks in (III)2, (III)3, and (III)11 were required to dramatically reduce anastellin-induced aggregation. When we tested the disulfide mutants in cell culture, only the disulfide bond in (III)2 reduced the FN matrix. These results suggest that the unfolding of (III)2 is one of the key factors for FN aggregation and assembly.

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.

Assembly of fibronectin extracellular matrix

In the process of matrix assembly, multivalent extracellular matrix (ECM) proteins are induced to self-associate and to interact with other ECM proteins to form fibrillar networks. Matrix assembly is usually initiated by ECM glycoproteins binding to cell surface receptors, such as fibronectin (FN) dimers binding to α5ß1 integrin. Receptor binding stimulates FN self-association mediated by the N-terminal assembly domain and organizes the actin cytoskeleton to promote cell contractility. FN conformational changes expose additional binding sites that participate in fibril formation and in conversion of fibrils into a stabilized, insoluble form. Once assembled, the FN matrix impacts tissue organization by contributing to the assembly of other ECM proteins. Here, we describe the major steps, molecular interactions, and cellular mechanisms involved in assembling FN dimers into fibrillar matrix while highlighting important issues and major questions that require further investigation.