Fibronectin isoform distribution in the mouse. I. The alternatively spliced EIIIB, EIIIA, and V segments show widespread codistribution in the developing mouse embryo

Integrin β5 subunit regulates hyperglycemia-induced vascular endothelial cell apoptosis through FoxO1-mediated macroautophagy

BACKGROUND

Hyperglycemia frequently induces apoptosis in endothelial cells and ultimately contributes to microvascular dysfunction in patients with diabetes mellitus (DM). Previous research reported that the expression of integrins as well as their ligands was elevated in the diseased vessels of DM patients. However, the association between integrins and hyperglycemia-induced cell death is still unclear. This research was designed to investigate the role played by integrin subunit β5 (ITGB5) in hyperglycemia-induced endothelial cell apoptosis.

METHODS

We used leptin receptor knockout (Lepr-KO) ( db / db ) mice as spontaneous diabetes animal model. Selective deletion of ITGB5 in endothelial cell was achieved by injecting vascular targeted adeno-associated virus via tail vein. Besides, we also applied small interfering RNA in vitro to study the mechanism of ITGB5 in regulating high glucose-induced cell apoptosis.

RESULTS

ITGB5 and its ligand, fibronectin, were both upregulated after exposure to high glucose in vivo and in vitro . ITGB5 knockdown alleviated hyperglycemia-induced vascular endothelial cell apoptosis and microvascular rarefaction in vivo.In vitro analysis revealed that knockdown of either ITGB5 or fibronectin ameliorated high glucose-induced apoptosis in human umbilical vascular endothelial cells (HUVECs). In addition, knockdown of ITGB5 inhibited fibronectin-induced HUVEC apoptosis, which indicated that the fibronectin-ITGB5 interaction participated in high glucose-induced endothelial cell apoptosis. By using RNA-sequencing technology and bioinformatic analysis, we identified Forkhead Box Protein O1 (FoxO1) as an important downstream target regulated by ITGB5. Moreover, we demonstrated that the excessive macroautophagy induced by high glucose can contribute to HUVEC apoptosis, which was regulated by the ITGB5-FoxO1 axis.

CONCLUSION

The study revealed that high glucose-induced endothelial cell apoptosis was positively regulated by ITGB5, which suggested that ITGB5 could potentially be used to predict and treat DM-related vascular complications.

Matrix stiffness, endothelial dysfunction and atherosclerosis

Atherosclerosis (AS) is the leading cause of the human cardiovascular diseases (CVDs). Endothelial dysfunction promotes the monocytes infiltration and inflammation that participate fundamentally in atherogenesis. Endothelial cells (EC) have been recognized as mechanosensitive cells and have different responses to distinct mechanical stimuli. Emerging evidence shows matrix stiffness-mediated EC dysfunction plays a vital role in vascular disease, but the underlying mechanisms are not yet completely understood. This article aims to summarize the effect of matrix stiffness on the pro-atherosclerotic characteristics of EC including morphology, rigidity, biological behavior and function as well as the related mechanical signal. The review also discusses and compares the contribution of matrix stiffness-mediated phagocytosis of macrophages and EC to AS progression. These advances in our understanding of the relationship between matrix stiffness and EC dysfunction open the avenues to improve the prevention and treatment of now-ubiquitous atherosclerotic diseases.

Mechanistic insights into the development of severe fetal growth restriction

Fetal growth restriction (FGR), which most commonly results from suboptimal placental function, substantially increases risks for adverse perinatal and long-term outcomes. The only "treatment" that exists is delivery, which averts stillbirth but does not improve outcomes in survivors. Furthermore, the potential long-term consequences of FGR to the fetus, including cardiometabolic disorders, predispose these individuals to developing FGR in their future pregnancies. This creates a multi-generational cascade of adverse effects stemming from a single dysfunctional placenta, and understanding the mechanisms underlying placental-mediated FGR is critically important if we are to improve outcomes and overall health. The mechanisms behind FGR remain unknown. However, placental insufficiency derived from maldevelopment of the placental vascular systems is the most common etiology. To highlight important mechanistic interactions within the placenta, we focus on placental vascular development in the setting of FGR. We delve into fetoplacental angiogenesis, a robust and ongoing process in normal pregnancies that is impaired in severe FGR. We review cellular models of FGR, with special attention to fetoplacental angiogenesis, and we highlight novel integrin-extracellular matrix interactions that regulate placental angiogenesis in severe FGR. In total, this review focuses on key developmental processes, with specific focus on the human placenta, an underexplored area of research.

A new mechanism of fibronectin fibril assembly revealed by live imaging and super-resolution microscopy

Fibronectin (Fn1) fibrils have long been viewed as continuous fibers composed of extended, periodically aligned Fn1 molecules. However, our live-imaging and single-molecule localization microscopy data are inconsistent with this traditional view and show that Fn1 fibrils are composed of roughly spherical nanodomains containing six to eleven Fn1 dimers. As they move toward the cell center, Fn1 nanodomains become organized into linear arrays, in which nanodomains are spaced with an average periodicity of 105±17 nm. Periodical Fn1 nanodomain arrays can be visualized between cells in culture and within tissues; they are resistant to deoxycholate treatment and retain nanodomain periodicity in the absence of cells. The nanodomain periodicity in fibrils remained constant when probed with antibodies recognizing distinct Fn1 epitopes or combinations of antibodies recognizing epitopes spanning the length of Fn1. Treatment with FUD, a peptide that binds the Fn1 N-terminus and disrupts Fn1 fibrillogenesis, blocked the organization of Fn1 nanodomains into periodical arrays. These studies establish a new paradigm of Fn1 fibrillogenesis. This article has an associated First Person interview with the first author of the paper.

Fibronectin regulates anoikis resistance via cell aggregate formation

The loss of cell-matrix interactions induces apoptosis, known as anoikis. For successful distant metastasis, circulating tumor cells (CTCs) that have lost matrix attachment need to acquire anoikis resistance in order to survive. Cell aggregate formation confers anoikis resistance, and CTC clusters are more highly metastatic compared to single cells; however, the molecular mechanisms underlying this aggregation are not well understood. In this study, we demonstrated that cell detachment increased cell aggregation and upregulated fibronectin (FN) levels in lung and breast cancer cells, but not in their normal counterparts. FN knockdown decreased cell aggregation and increased anoikis. In addition, cell detachment induced cell-cell adhesion proteins, including E-cadherin, desmoglein-2, desmocollin-2/3, and plakoglobin. Interestingly, FN knockdown decreased the levels of desmoglein-2, desmocollin-2/3, and plakoglobin, but not E-cadherin, suggesting the involvement of desmosomal junction in cell aggregation. Accordingly, knockdown of desmoglein-2, desmocollin-2, or plakoglobin reduced cell aggregation and increased cell sensitivity to anoikis. Previously, we reported that NADPH oxidase 4 (Nox4) upregulation is important for anoikis resistance. Nox4 inhibition by siRNA or apocynin decreased cell aggregation and increased anoikis with the downregulation of FN, and, consequently, decreased desmoglein-2, desmocollin-2/3, or plakoglobin. The coexpression of Nox4 and FN was found to be significant in lung and breast cancer patients, based on cBioPortal data. In vivo mouse lung metastasis model showed that FN knockdown suppressed lung metastasis and thus enhanced survival. FN staining of micro tissue array revealed that FN expression was positive for human lung cancer (61%) and breast cancer (58%) patients. Furthermore, the expression levels of FN, desmoglein-2, desmocollin-2, and plakoglobin were significantly correlated with the poor survival of lung and breast cancer patients, as per the Kaplan-Meier plotter analysis. Altogether, our data suggest that FN upregulation and enhanced desmosomal interactions are critical for cell aggregation and anoikis resistance upon cell detachment.

FN-EDA mediates angiogenesis of hepatic fibrosis via integrin-VEGFR2 in a CD63 synergetic manner

Pathological angiogenesis is an important component of hepatic fibrosis along with fibrous deposition, but its role is not well understood. Here, we demonstrated that fibronectin containing extra domain A(FN-EDA), a fibronectin splice variant highly expressed in hepatic fibrosis, mediated angiogenesis in disease progression. FN-EDA was positively correlated with pathological angiogenesis in hepatic fibrosis, and a reduction in FN-EDA expression was associated with diminished intrahepatic angiogenesis and fibrosis. FN-EDA mostly colocalized with hepatic stellate cells (HSCs) and interference or blockage of FN-EDA attenuated migration and tube formation in co-cultured endothelial cells. Mechanistic studies indicated that FN-EDA was secreted to promote phosphorylation of VEGFR2 with the assistance of integrin and CD63. Targeting FN-EDA-integrin combination postponed the progression of hepatic angiogenesis and fibrosis in vivo. These results indicated that FN-EDA plays an emerging role in angiogenesis in hepatic fibrosis and could be a potential therapeutic intervention for the disease.

TGF-β in Hepatic Stellate Cell Activation and Liver Fibrogenesis-Updated 2019

Liver fibrosis is an advanced liver disease condition, which could progress to cirrhosis and hepatocellular carcinoma. To date, there is no direct approved antifibrotic therapy, and current treatment is mainly the removal of the causative factor. Transforming growth factor (TGF)-β is a master profibrogenic cytokine and a promising target to treat fibrosis. However, TGF-β has broad biological functions and its inhibition induces non-desirable side effects, which override therapeutic benefits. Therefore, understanding the pleiotropic effects of TGF-β and its upstream and downstream regulatory mechanisms will help to design better TGF-β based therapeutics. Here, we summarize recent discoveries and milestones on the TGF-β signaling pathway related to liver fibrosis and hepatic stellate cell (HSC) activation, emphasizing research of the last five years. This comprises impact of TGF-β on liver fibrogenesis related biological processes, such as senescence, metabolism, reactive oxygen species generation, epigenetics, circadian rhythm, epithelial mesenchymal transition, and endothelial-mesenchymal transition. We also describe the influence of the microenvironment on the response of HSC to TGF-β. Finally, we discuss new approaches to target the TGF-β pathway, name current clinical trials, and explain promises and drawbacks that deserve to be adequately addressed.

The fibronectin ED-A domain enhances recruitment of latent TGF-β-binding protein-1 to the fibroblast matrix

Dysregulated secretion and extracellular activation of TGF-β1 stimulates myofibroblasts to accumulate disordered and stiff extracellular matrix (ECM) leading to fibrosis. Fibronectin immobilizes latent TGF-β-binding protein-1 (LTBP-1) and thus stores TGF-β1 in the ECM. Because the ED-A fibronectin splice variant is prominently expressed during fibrosis and supports myofibroblast activation, we investigated whether ED-A promotes LTBP-1-fibronectin interactions. Using stiffness-tuneable substrates for human dermal fibroblast cultures, we showed that high ECM stiffness promotes expression and colocalization of LTBP-1 and ED-A-containing fibronectin. When rescuing fibronectin-depleted fibroblasts with specific fibronectin splice variants, LTBP-1 bound more efficiently to ED-A-containing fibronectin than to ED-B-containing fibronectin and fibronectin lacking splice domains. Function blocking of the ED-A domain using antibodies and competitive peptides resulted in reduced LTBP-1 binding to ED-A-containing fibronectin, reduced LTBP-1 incorporation into the fibroblast ECM and reduced TGF-β1 activation. Similar results were obtained by blocking the heparin-binding stretch FNIII12-13-14 (HepII), adjacent to the ED-A domain in fibronectin. Collectively, our results suggest that the ED-A domain enhances association of the latent TGF-β1 by promoting weak direct binding to LTBP-1 and by enhancing heparin-mediated protein interactions through HepII in fibronectin.

ED-B fibronectin expression is a marker of epithelial-mesenchymal transition in translational oncology

Fibronectin is a component of the extracellular matrix that links collagen fibers to integrins on the cell's surface. The splicing isoforms, containing the ED-B domain, are not expressed in adult tissues but only in tumor stroma or during embryonic development. Fibroblasts and endothelial cells express ED-B fibronectin during angiogenesis. Also cancer cells can synthetize ED-B fibronectin, but its function in tumor growth needs to be further elucidated.

We evaluated the expression of ED-B fibronectin in prostate cancer cell lines: PC3 and DU145. Using TGF-β, we induced epithelial to mesenchymal transition in culture and observed an increase of ED-B fibronectin expression. Thereafter, we evaluated the expression of ED-B fibronectin in multipotent mesangiogenic progenitor cells, and in mesenchymal stromal cells. The expression of ED-B fibronectin was much higher in mesenchymal than prostate cancer cells even after the epithelial to mesenchymal transition.

Epithelial to mesenchymal transition is a key step for tumor progression contributing to the metastatic spread. Therefore, circulating cancer cells could seed into the metastatic niche taking advantage from the ED-B fibronectin that secrete their own.

Endothelium-derived fibronectin regulates neonatal vascular morphogenesis in an autocrine fashion

Fibronectin containing alternatively spliced EIIIA and EIIIB domains is largely absent from mature quiescent vessels in adults, but is highly expressed around blood vessels during developmental and pathological angiogenesis. The precise functions of fibronectin and its splice variants during developmental angiogenesis however remain unclear due to the presence of cardiac, somitic, mesodermal and neural defects in existing global fibronectin KO mouse models. Using a rare family of surviving EIIIA EIIIB double KO mice, as well as inducible endothelial-specific fibronectin-deficient mutant mice, we show that vascular development in the neonatal retina is regulated in an autocrine manner by endothelium-derived fibronectin, and requires both EIIIA and EIIIB domains and the RGD-binding α5 and αv integrins for its function. Exogenous sources of fibronectin do not fully substitute for the autocrine function of endothelial fibronectin, demonstrating that fibronectins from different sources contribute differentially to specific aspects of angiogenesis.

Integrins in Vascular Development and Pathology

During vascular development, endothelial cells (ECs) and neighboring stromal cells interact and communicate through autocrine and paracrine signaling mechanisms involving extracellular matrix (ECM) proteins and their cell surface integrin adhesion receptors. Integrin-mediated adhesion and signaling pathways are crucial for normal vascular development and physiology, and alterations in integrin expression and/or function drive several vascular-related pathologies including thrombosis, autoimmune disorders, and cancer. The purpose of this chapter is to discuss integrin adhesion and signaling pathways important for EC growth, survival, and migration. Integrin-mediated paracrine links between ECs and surrounding stromal cells in the organ microenvironment will also be discussed. Lastly, we will review roles for integrins in vascular pathologies and discuss possible targets for therapeutic intervention.

Fibronectin-targeted drug delivery in cancer

Fibronectin is an extracellular matrix protein with pivotal physiological and pathological functions in development and adulthood. Alternative splicing of the precursor mRNA, produced from the single copy fibronectin gene, occurs at three sites coding for the EDA, EDB and IIICS domains. Fibronectin isoforms comprising the EDA or EDB domains are known as oncofetal forms due to their developmental importance and their re-expression in tumors, contrasting with restricted presence in normal adult tissues. These isoforms are also recognized as important markers of angiogenesis, a crucial physiological process in development and required by tumor cells in cancer progression. Attributed to this feature, EDA and EDB domains have been extensively used for the targeted delivery of cytokines, cytotoxic agents, chemotherapy drugs and radioisotopes to fibronectin-expressing tumors to exert therapeutic effects on primary cancers and metastatic lesions. In addition to drug delivery, the EDA and EDB domains of fibronectin have also been utilized to develop imaging strategies for tumor tissues. Furthermore, EDA and EDB based vaccines seem to be promising for the treatment and prevention of certain cancer types. In this review, we will summarize recent advances in fibronectin EDA and EDB-based therapeutic strategies developed to treat cancer.

Neural crest cell-autonomous roles of fibronectin in cardiovascular development

The chemical and mechanical properties of extracellular matrices (ECMs) modulate diverse aspects of cellular fates; however, how regional heterogeneity in ECM composition regulates developmental programs is not well understood. We discovered that fibronectin 1 (Fn1) is expressed in strikingly non-uniform patterns during mouse development, suggesting that regionalized synthesis of the ECM plays cell-specific regulatory roles during embryogenesis. To test this hypothesis, we ablated Fn1 in the neural crest (NC), a population of multi-potent progenitors expressing high levels of Fn1. We found that Fn1 synthesized by the NC mediated morphogenesis of the aortic arch artery and differentiation of NC cells into vascular smooth muscle cells (VSMCs) by regulating Notch signaling. We show that NC Fn1 signals in an NC cell-autonomous manner through integrin α5β1 expressed by the NC, leading to activation of Notch and differentiation of VSMCs. Our data demonstrate an essential role of the localized synthesis of Fn1 in cardiovascular development and spatial regulation of Notch signaling.

Blood Vessel Maturation by Disintegrin in Oxygen-Induced Retinopathy

PURPOSE

Although Arg-Gly-Asp (RGD) motif-containing disintegrins are associated with integrin inhibition and the activation of various biological processes, little is known about the role of RGD motif-containing disintegrin in vascular development and remodeling. We therefore investigated the role of RGD-containing disintegrin in vascular remodeling in oxygen-induced retinopathy (OIR) mouse model.

MATERIALS AND METHODS

EGT022, an RGD-containing disintegrin originated from human a disintegrin and metalloproteinase 15 (ADAM15), was used to investigate the role of the disintegrin in vascular development in OIR mouse model. To analyze the functional effects of EGT022 on retinal vascular development, the immunohistochemistry on mouse retinas after fluorescein isothiocyanate (FITC) perfusion was conducted and the vessel integrity was examined using modified Mile's permeability assay.

RESULTS

EGT022 was able to reduce overall retinopathy scores by 75%, indicating its efficacy in retinal microvessel maturation stimulation. Pericyte coverage was greatly stimulated by EGT022 treatment in OIR mouse model. EGT022 was also effective to significantly improve blood vessel integrity.

CONCLUSIONS

RGD-containing disintegrin EGT022 stimulated vascular maturation in OIR mouse model. Experimental results suggest that EGT022 is useful for treatments to improve ischemia in nonproliferative diabetic retinopathy (NPDR), the early stage of diabetic retinopathy.

ADAMTS9-Mediated Extracellular Matrix Dynamics Regulates Umbilical Cord Vascular Smooth Muscle Differentiation and Rotation

Despite the significance for fetal nourishment in mammals, mechanisms of umbilical cord vascular growth remain poorly understood. Here, the secreted metalloprotease ADAMTS9 is shown to be necessary for murine umbilical cord vascular development. Restricting it to the cell surface using a gene trap allele, Adamts9(Gt), impaired umbilical vessel elongation and radial growth via reduced versican proteolysis and accumulation of extracellular matrix (ECM). Both Adamts9(Gt) and conditional Adamts9 deletion revealed that ADAMTS9 produced by mesenchymal cells acted non-autonomously to regulate smooth muscle cell (SMC) proliferation, differentiation, and orthogonal reorientation during growth of the umbilical vasculature. In Adamts9(Gt/Gt), we observed interference with PDGFRβ signaling via the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, which regulates cytoskeletal dynamics during SMC rotation. In addition, we observed disrupted Shh signaling and perturbed orientation of the mesenchymal primary cilium. Thus, ECM dynamics is a major influence on umbilical vascular SMC fate, with ADAMTS9 acting as its principal mediator.

Fibronectin extra domain-A promotes hepatic stellate cell motility but not differentiation into myofibroblasts

BACKGROUND & AIMS

Myofibroblasts are the primary cell type involved in physiologic wound healing and its pathologic counterpart, fibrosis. Cellular fibronectin that contains the alternatively spliced extra domain A (EIIIA) is up-regulated during these processes and is believed to promote myofibroblast differentiation. We sought to determine the requirement for EIIIA in fibrosis and differentiation of myofibroblasts in rodent livers.

METHODS

We used a mechanically tunable hydrogel cell culture system to study differentiation of primary hepatic stellate cells and portal fibroblasts from rats into myofibroblasts. Liver fibrosis was induced in mice by bile duct ligation or administration of thioacetamide.

RESULTS

EIIIA was not required for differentiation of rat hepatic stellate cells or portal fibroblasts into fibrogenic myofibroblasts. Instead, hepatic stellate cells cultured on EIIIA-containing cellular fibronectin formed increased numbers of lamellipodia; their random motility and chemotaxis also increased. These increases required the receptor for EIIIA, the integrin α(9)β(1). In contrast, the motility of portal fibroblasts did not increase on EIIIA, and these cells expressed little α(9)β(1). Male EIIIA(-/-) mice were modestly protected from thioacetamide-induced fibrosis, which requires motile hepatic stellate cells, but not from bile duct ligation-induced fibrosis, in which portal fibroblasts are more important. Notably, myofibroblasts developed during induction of fibrosis with either thioacetamide or bile duct ligation in EIIIA(-/-) mice.

CONCLUSIONS

EIIIA is dispensable for differentiation of hepatic stellate cells and portal fibroblasts to myofibroblasts, both in culture and in mouse models of fibrosis. Our findings, however, indicate a role for EIIIA in promoting stellate cell motility and parenchymal liver fibrosis.

Matrikines and the lungs

The extracellular matrix is a complex network of fibrous and nonfibrous molecules that not only provide structure to the lung but also interact with and regulate the behaviour of the cells which it surrounds. Recently it has been recognised that components of the extracellular matrix proteins are released, often through the action of endogenous proteases, and these fragments are termed matrikines. Matrikines have biological activities, independent of their role within the extracellular matrix structure, which may play important roles in the lung in health and disease pathology. Integrins are the primary cell surface receptors, characterised to date, which are used by the matrikines to exert their effects on cells. However, evidence is emerging for the need for co-factors and other receptors for the matrikines to exert their effects on cells. The potential for matrikines, and peptides derived from these extracellular matrix protein fragments, as therapeutic agents has recently been recognised. The natural role of these matrikines (including inhibitors of angiogenesis and possibly inflammation) make them ideal targets to mimic as therapies. A number of these peptides have been taken forward into clinical trials. The focus of this review will be to summarise our current understanding of the role, and potential for highly relevant actions, of matrikines in lung health and disease.

Fibronectin splice variants: understanding their multiple roles in health and disease using engineered mouse models

The extracellular matrix (ECM) is a highly dynamic network of proteins, glycoproteins, and proteoglycans. Numerous diseases result from mutation in genes coding for ECM proteins, but only recently it has been reported that mutations in the fibronectin (FN) gene were associated with a human disorder. FN is one of the main components of the ECM. It generates protein diversity through alternative splicing of a single pre-mRNA, having at least 20 different isoforms in humans. The precise function of these protein isoforms has remained obscure in most cases. Only in the recent few years, it was possible to shed light on the multiple roles of the alternatively spliced FN isoforms. This substantial progress was achieved basically with the knowledge derived from engineered mouse models bearing subtle mutations in specific FN domains. These data, together with a recent report associating mutations in the FN gene to a form of glomerulopathy, clearly show that mutations in constitutive exons or misregulation of alternatively spliced domains of the FN gene may have nonlethal pathological consequences. In this review, we focus on the pathological consequences of mutations in the FN gene, by connecting the function of alternatively spliced isoforms of fibronectin to human diseases.

Serum response factor is required for cell contact maintenance but dispensable for proliferation in visceral yolk sac endothelium

Background

Endothelial-specific knockout of the transcription factor serum response factor (SRF) results in embryonic lethality by mid-gestation. The associated phenotype exhibits vascular failure in embryos as well as visceral yolk sac (VYS) tissues. Previous data suggest that this vascular failure is caused by alterations in cell-cell and cell-matrix contacts. In the current study, we sought to more carefully address the role of SRF in endothelial function and cell contact interactions in VYS tissues.

Results

Tie2-Cre recombinase-mediated knockout of SRF expression resulted in loss of detectable SRF from VYS mesoderm by E12.5. This loss was accompanied by decreased expression of smooth muscle alpha-actin as well as vascular endothelial cadherin and claudin 5, endothelial-specific components of adherens and tight junctions, respectively. Focal adhesion (FA) integrins alpha5 and beta1 were largely unchanged in contrast to loss of the FA-associated molecule vinculin. The integrin binding partner fibronectin-1 was also profoundly decreased in the extracellular matrix, indicating another aspect of impaired adhesive function and integrin signaling. Additionally, cells in SRF-null VYS mesoderm failed to reduce proliferation, suggesting not only that integrin-mediated contact inhibition is impaired but also that SRF protein is not required for proliferation in these cells.

Conclusions

Our data support a model in which SRF is critical in maintaining functional cell-cell and cell-matrix adhesion in endothelial cells. Furthermore, we provide evidence that supports a model in which loss of SRF protein results in a sustained proliferation defect due in part to failed integrin signaling.

Myofibroblast differentiation during fibrosis: role of NAD(P)H oxidases

Progression of fibrosis involves interstitial hypercellularity, matrix accumulation, and atrophy of epithelial structures, resulting in loss of normal function and ultimately organ failure. There is common agreement that the fibroblast/myofibroblast is the cell type most responsible for interstitial matrix accumulation and consequent structural deformations associated with fibrosis. During wound healing and progressive fibrotic events, fibroblasts transform into myofibroblasts acquiring smooth muscle features, most notably the expression of alpha-smooth muscle actin and synthesis of mesenchymal cell-related matrix proteins. In renal disease, glomerular mesangial cells also acquire a myofibroblast phenotype and synthesize the same matrix proteins. The origin of interstitial myofibroblasts during fibrosis is a matter of debate, where the cells are proposed to derive from resident fibroblasts, pericytes, perivascular adventitial, epithelial, and/or endothelial sources. Regardless of the origin of the cells, transforming growth factor-beta1 (TGF-β1) is the principal growth factor responsible for myofibroblast differentiation to a profibrotic phenotype and exerts its effects via Smad signaling pathways involving mitogen-activated protein kinase and Akt/protein kinase B. Additionally, reactive oxygen species (ROS) have important roles in progression of fibrosis. ROS are derived from a variety of enzyme sources, of which the nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase family has been identified as a major source of superoxide and hydrogen peroxide generation in the cardiovasculature and kidney during health and disease. Recent evidence indicates that the NAD(P)H oxidase homolog Nox4 is most accountable for ROS-induced fibroblast and mesangial cell activation, where it has an essential role in TGF-β1 signaling of fibroblast activation and differentiation into a profibrotic myofibroblast phenotype and matrix production. Information on the role of ROS in mesangial cell and fibroblast signaling is incomplete, and further research on myofibroblast differentiation during fibrosis is warranted.

Autocrine fibronectin directs matrix assembly and crosstalk between cell–matrix and cell–cell adhesion in vascular endothelial cells

Cellular fibronectin (cFN) variants harboring extra FN type 3 repeats, namely extra domains B and A, are major constituents of the extracellular matrix around newly forming blood vessels during development and angiogenesis. Their expression is induced by angiogenic stimuli and their assembly into fibrillar arrays is driven by cell-generated tension at α5β1 integrin-based adhesions. Here, we examined the role and functional redundancy of cFN variants in cultured endothelial cells by isoform-selective RNA interference. We show that FN fibrillogenesis is a cell-autonomous process whereby basally directed secretion and assembly of cellular FN are tightly coupled events that play an important role not only in signaling at cell–matrix adhesions but also at cell–cell contacts. Silencing of cFN variants differentially affects integrin usage, cell spreading, motility and capillary morphogenesis in vitro. cFN-deficient cells undergo a switch from α5β1- to αvβ3-based adhesion, accompanied by a Src-regulated disruption of adherens junctions. These studies identify a crucial role for autocrine FN in subendothelial matrix assembly and junctional integrity that provides spatially and temporally restricted control of endothelial plasticity during angiogenic blood vessel remodeling.

Fibronectin and integrin alpha 5 play essential roles in the development of the cardiac neural crest

Cardiac neural crest (CNC) plays a requisite role during cardiovascular development and defects in the formation of CNC-derived structures underlie several common forms of human congenital birth defects. Migration of the CNC cells to their destinations as well as expansion and maintenance of these cells are important for the normal development of the cardiac outflow tract and aortic arch arteries; however, molecular mechanisms regulating these processes are not well-understood. Fibronectin (FN) protein is present along neural crest migration paths and neural crest cells migrate when plated on FN in vitro; therefore, we tested the role of FN during the development of the CNC in vivo. Our analysis of the fate of the neural crest shows that CNC cells reach their destinations in the branchial arches and the cardiac outflow tract in the absence of FN or its cellular receptor integrin α5β1. However, we found that FN and integrin α5 modulate CNC proliferation and survival, and are required for the presence of normal numbers of CNC cells at their destinations.

Endothelial alpha5 and alphav integrins cooperate in remodeling of the vasculature during development

Integrin cell adhesion receptors and fibronectin, one of their extracellular matrix ligands, have been demonstrated to be important for angiogenesis using functional perturbation studies and complete knockout mouse models. Here, we report on the roles of the alpha5 and alphav integrins, which are the major endothelial fibronectin receptors, in developmental angiogenesis. We generated an integrin alpha5-floxed mouse line and ablated alpha5 integrin in endothelial cells. Unexpectedly, endothelial-specific knockout of integrin alpha5 has no obvious effect on developmental angiogenesis. We provide evidence for genetic interaction between mutations in integrin alpha5 and alphav and for overlapping functions and compensation between these integrins and perhaps others. Nonetheless, in embryos lacking both alpha5 and alphav integrins in their endothelial cells, initial vasculogenesis and angiogenesis proceed normally, at least up to E11.5, including the formation of apparently normal embryonic vasculature and development of the branchial arches. However, in the absence of endothelial alpha5 and alphav integrins, but not of either alone, there are extensive defects in remodeling of the great vessels and heart resulting in death at ~E14.5. We also found that fibronectin assembly is somewhat affected in integrin alpha5 knockout endothelial cells and markedly reduced in integrin alpha5/alphav double-knockout endothelial cell lines. Therefore, neither alpha5 nor alphav integrins are required in endothelial cells for initial vasculogenesis and angiogenesis, although they are required for remodeling of the heart and great vessels. These integrins on other cells, and/or other integrins on endothelial cells, might contribute to fibronectin assembly and vascular development.

Recognition of the alternatively spliced segments of fibronectin by the RCJ 3.1C5.18 chondrocytic rat cell line

OBJECTIVES

To define, for the C5.18 chondrocyte-restricted rat cell line, (1) the capacities for recognition of alternatively spliced segments of the adhesion protein fibronectin (FN), (2) the integrin subunits required for such recognition, and (3) differences in such FN recognition vs the multipotential chondroprogenitor line, RCJ 3.1.

METHODS

C5.18 and RCJ 3.1 cells were tested for their capacities to adhere to recombinant alternatively spliced segments of rat FN, presented on plastic surfaces either in isolation or in partial FNs spanning the 7th through 15th type III repeats (III7-15 FNs). The effects on such adhesion of cations and integrin subunit-specific antibodies were tested.

RESULTS

Despite significant augmentation in chondrocyte-specific gene expression in C5.18 relative to the RCJ 3.1 cells, the two lines exhibited similar recognition of FN spliced segments and partial isoforms. Specifically, both lines adhered to the extra type III repeat A (EIIIA) and V, but not extra type III repeat B (EIIIB), segments. There were different cation and integrin subunit requirements for adhesion to EIIIA vs V segments, and only the V segment was recognized in the context of a III7-15 FN. Such recognition was mediated via a "second" arginine-glycine-aspartic acid (RGD) sequence that is present in the V95 subsegment in rat, but not human, FN.

CONCLUSION

The chondrocyte lineage-committed C5.18 cell line, similar to its multipotential chondroprogenitor, RCJ 3.1, recognizes the "cartilage-restricted" EIIIA and V segments of FN with cation, integrin, and molecular context requirements that are specific to each of these segments.

Quantification of fibronectin 1 (FN1) splice variants, including two novel ones, and analysis of integrins as candidate FN1 receptors in bovine preimplantation embryos

Background

Fibronectin 1 (FN1), a glycoprotein component of the extracellular matrix, exerts different functions during reproductive processes such as fertilisation, gastrulation and implantation. FN1 expression has been described to increase significantly from the morula towards the early blastocyst stage, suggesting that FN1 may also be involved in early blastocyst formation. By alternative splicing at 3 defined regions, different FN1 isoforms are generated, each with a unique biological function. The analysis of the alternative FN1 splicing on the one hand and the search for candidate FN1 receptors on the other hand during early bovine embryo development may reveal more about its function during bovine preimplantation embryo development.

Results

RT-qPCR quantification of the FN1 splice isoforms in oocytes, embryos, cumulus cells and adult tissue samples revealed a large variation in overall FN1 expression and in splice variant expression. Moreover, two new FN1 transcript variants were identified, the first one expressed in bovine preimplantation embryos and the second one expressed in cumulus cells.

In the search for candidate receptors for the new embryo specific FN1 isoform, RNA expression analysis identified 5 α integrin subunits (ITGA2B, ITGA3, ITGA5, ITGA8, ITGAV) and 2 β integrin subunits (ITGB1 and ITGB3) with a similar or overlapping RNA expression pattern as compared to FN1. But double immunofluorescent stainings could not confirm complete co-localisation between FN1 and one out of 3 selected integrins alpha subunits (ITGA3, ITGA5, ITGAV).

Conclusion

The existence of a new FN1 transcript variant, specifically expressed in morulae and blastocysts strengthens the idea that FN1 is involved in the process of compaction and blastocyst formation. Analysis of the integrin expression could not identify the binding partner for the embryo specific FN1 transcript variant making further steps necessary for the identification of the FN1 receptor and the downstream effects of FN1-receptor binding.

New insights into form and function of fibronectin splice variants

The extracellular matrix (ECM) is a highly dynamic structure that not only provides a physical framework for cells within connective tissues, but also imparts instructive signals for development, tissue homeostasis and basic cell functions through its composition and ability to exert mechanical forces. The ECM of tissues is composed of, in addition to proteoglycans and hyaluronic acid, a number of proteins, most of which are generated after alternative splicing of their pre-mRNA. However, the precise function of these protein isoforms is still obscure in most cases. Fibronectin (FN), one of the main components of the ECM, is also one of the best-known examples of a family of proteins generated by alternative splicing, having at least 20 different isoforms in humans. Over the last few years, considerable progress on elucidating the functions of the alternatively spliced FN isoforms has been achieved with the essential development of key engineered mouse strains. Here we summarize the phenotypes of the mouse strains having targeted mutations in the FN gene, which may lead to novel insights linking function of alternatively spliced isoforms of fibronectin to human pathologies.

Ligand-based vascular targeting of disease

This review illustrates the basic principles of ligand-based vascular targeting and presents some of the most advanced results obtained in this field, not only in terms of biopharmaceuticals, which are currently being investigated in clinical and preclinical studies, but also in terms of enabling technologies that facilitate target and ligand discovery. Whereas most of the vascular targeting research activities have so far concentrated on tumoral angiogenesis, the development of non-oncological applications has recently gained momentum and is likely to become an important area of modern pharmaceutical research.

CD98hc (SLC3A2) participates in fibronectin matrix assembly by mediating integrin signaling

Integrin-dependent assembly of the fibronectin (Fn) matrix plays a central role in vertebrate development. We identify CD98hc, a membrane protein, as an important component of the matrix assembly machinery both in vitro and in vivo. CD98hc was not required for biosynthesis of cellular Fn or the maintenance of the repertoire or affinity of cellular Fn binding integrins, which are important contributors to Fn assembly. Instead, CD98hc was involved in the cell's ability to exert force on the matrix and did so by dint of its capacity to interact with integrins to support downstream signals that lead to activation of RhoA small GTPase. Thus, we identify CD98hc as a membrane protein that enables matrix assembly and establish that it functions by interacting with integrins to support RhoA-driven contractility. CD98hc expression can vary widely; our data show that these variations in CD98hc expression can control the capacity of cells to assemble an Fn matrix, a process important in development, wound healing, and tumorigenesis.

Multiple cardiovascular defects caused by the absence of alternatively spliced segments of fibronectin

Alternatively spliced variants of fibronectin (FN) containing exons EIIIA and EIIIB are expressed around newly forming vessels in development and disease but are downregulated in mature vasculature. The sequences and patterns of expression of these splice variants are highly conserved among vertebrates, suggestive of their biological importance; however the functions of EIIIA and EIIIB-containing FNs are unknown. To understand the role(s) of these splice variants, we deleted both EIIIA and EIIIB exons from the FN gene and observed embryonic lethality with incomplete penetrance by embryonic day 10.5. Deletion of both EIIIA and EIIIB exons did not affect synthesis or cell surface deposition of FN, indicating that embryonic lethality was due specifically to the absence of EIIIA and EIIIB exons from FN. EIIIA/EIIIB double-null embryos displayed multiple embryonic cardiovascular defects, including vascular hemorrhage, failure of remodeling embryonic and yolk sac vasculature, defective placental angiogenesis and heart defects. In addition, we observed defects in coverage and association with dorsal aortae of alpha-smooth-muscle-actin-positive cells. Our studies indicate that the presence or absence of EIIIA and EIIIB exons alters the function of FN and demonstrate the requirement for these alternatively spliced exons in cardiovascular development.

Cell-matrix adhesion in vascular development

Vascular development requires correct interactions among endothelial cells, pericytes and surrounding cells. These interactions involve many cell adhesion interactions, including cell-matrix interactions both with basement membranes and with surrounding extracellular matrices. Investigations of the contributions of these various interactions in vascular development and angiogenesis have been rather uneven and incomplete over the past 10-15 years. There has been considerable concentration on a few receptors, matrix proteins and proteolytic fragments with the goal of finding means to control angiogenesis. Many other potential contributors have received much less attention. Even for those molecules that have been subject to intensive investigation, our knowledge is incomplete. This review will survey the spectrum of extracellular matrix (ECM) proteins and cell-matrix adhesion receptors (particularly integrins) that are likely to contribute to angiogenesis and discuss what is known and not known about the roles of each of them.

Neurite outgrowth on a fibronectin isoform expressed during peripheral nerve regeneration is mediated by the interaction of paxillin with alpha4beta1 integrins

BACKGROUND

The regeneration of peripheral nerve is associated with a change in the alternative splicing of the fibronectin primary gene transcript to re-express embryonic isoforms containing a binding site for alpha4beta1 integrins that promote neurite outgrowth. Here we use PC12 cells to examine the role of the interaction between paxillin and the alpha4 integrin cytoplasmic domain in neurite outgrowth.

RESULTS

Expression of alpha4 with mutations in the paxillin-binding domain reduced neurite outgrowth on recombinant embryonic fibronectin fragments relative to wild type alpha4. Over-expression of paxillin promoted neurite outgrowth while a mutant isoform lacking the LD4 domain implicated in the regulation of ARF and Rac GTPases was less effective. Optimal alpha4-mediated migration in leucocytes requires spatial regulation of alpha4 phosphorylation at Ser988, a post-translational modification that blocks paxillin binding to the integrin cytoplasmic domain. In keeping with this alpha4(S988D), which mimics phosphorylated alpha4, did not promote neurite outgrowth. However, alpha4 was not phosphorylated in the PC12 cells, and a non-phosphorylatable alpha4(S988A) mutant promoted neurite outgrowth indistinguishably from the wild type integrin.

CONCLUSION

We establish the importance of the alpha4 integrin-paxillin interaction in a model of axonal regeneration and highlight differing dependence on phosphorylation of alpha4 for extension of neuronal growth cones and migration of non-neural cells.

Preconditioning injury-induced neurite outgrowth of adult rat sensory neurons on fibronectin is mediated by mobilisation of axonal α5 integrin

A preconditioning sciatic nerve crush promotes the capacity of adult sensory neurons to regenerate following a subsequent injury to their axons. The increase in regeneration is detected in cultures of dissociated neurons, as an earlier and enhanced rate of neurite elongation. We compare neurotrophin-stimulated neurite outgrowth from sensory neurons on laminin and fibronectin. There is a poor response of sensory neurons to fibronectin in comparison to laminin, but this is enhanced by a preconditioning lesion to the sciatic nerve 7 days prior to culture. By using specific integrin-binding fibronectin fragments and function-blocking antibodies, we demonstrate that the enhanced preconditioned neurite outgrowth on fibronectin is largely mediated by alpha5beta1 integrin. Preconditioning injury alter the subcellular localisation of alpha5 integrin in preconditioned neurites. We show that alpha5 integrin localises to adhesion complexes in the growth cone and neurites of preconditioned neurons, but not control neurons.

Transforming growth factor-beta1 regulates fibronectin isoform expression and splicing factor SRp40 expression during ATDC5 chondrogenic maturation

Fibronectin (FN) isoform expression is altered during chondrocyte commitment and maturation, with cartilage favoring expression of FN isoforms that includes the type II repeat extra domain B (EDB) but excludes extra domain A (EDA). We and others have hypothesized that the regulated splicing of FN mRNAs is necessary for the progression of chondrogenesis. To test this, we treated the pre-chondrogenic cell line ATDC5 with transforming growth factor-beta1, which has been shown to modulate expression of the EDA and EDB exons, as well as the late markers of chondrocyte maturation; it also slightly accelerates the early acquisition of a sulfated proteoglycan matrix without affecting cell proliferation. When chondrocytes are treated with TGF-beta1, the EDA exon is preferentially excluded at all times whereas the EDB exon is relatively depleted at early times. This regulated alternative splicing of FN correlates with the regulation of alternative splicing of SRp40, a splicing factor facilitating inclusion of the EDA exon. To determine if overexpression of the SRp40 isoforms altered FN and FN EDA organization, cDNAs encoding these isoforms were overexpressed in ATDC5 cells. Overexpression of the long-form of SRp40 yielded an FN organization similar to TGF-beta1 treatment; whereas overexpression of the short form of SRp40 (which facilitates EDA inclusion) increased formation of long-thick FN fibrils. Therefore, we conclude that the effects of TGF-beta1 on FN splicing during chondrogenesis may be largely dependent on its effect on SRp40 isoform expression.

Decreased Plasma Fibronectin Leads to Delayed Thrombus Growth in Injured Arterioles

Objective— Plasma fibronectin (FN) is decreased in several clinical conditions. We were interested to study the thrombotic and hemostatic consequences of the decrease in plasma FN (pFN), the role of FN splice variants in thrombosis, and to examine whether pFN incorporates into thrombi in vivo.

Methods and Results— We compared the thrombotic response to a vessel injury in FN heterozygous ( FN +/−) mice and corresponding FN +/+ mice. Although normal thrombosis in venules was observed, a decrease to half in the pFN concentration in FN +/− mice caused a delay in the appearance of thrombi in arterioles and consequently a delay in their occlusion. We were able to rescue the thrombotic defect in the FN +/− mice by infusion of rat pFN. Additionally, we could show intense incorporation of fluorescent pFN-coated microspheres into the developing thrombi. Moreover, we found that mice expressing FN without the EIIIA or EIIIB domains specific to cellular FN including platelet FN had no thrombotic defect.

Conclusions— Mice heterozygous for FN have a striking defect in thrombus initiation and growth in arterioles attributable to the decrease of pFN. Our study is an example of haploid insufficiency for FN, and it emphasizes the fundamental role of this plasma protein in thrombosis in the arterial system.

Analysis of the proepicardium-epicardium transition during the malformation of the RXRalpha-/- epicardium

The epicardium of the heart originates from a cluster of mesothelial-derived cells that develop beneath the sinus venosus in the embryonic day (E) 9.0-9.5 mouse. The subsequent proepicardium-epicardium transition that forms the epicardial layer of epithelial cells covering the myocardial surface is nearly complete by E10.0-E10.5 and results in a fully covered heart by E11.0. In this study, we show that an established model of congenital heart disease, the retinoid X receptor alpha knockout (RXRalpha-/-) embryo, displays a malformed epicardium. At E10.0-E10.5, the RXRalpha-/- has several large regions of myocardium that remain bare. Furthermore, by E11.5-E12.5, when a complete epithelial layer is formed in the mutant, large regions of the epicardium become distended from the underlying myocardium. Close examination of the E9.5 mutant revealed an elevated apoptosis level within the proepicardial cluster of mesothelial cells. Additionally, among the extracellular matrix proteins analyzed, expression of fibronectin was elevated in the RXRalpha-/- as assessed by immunostaining in paraffin-embedded sections and proepicardial explants. We propose that these events contribute to a developmental delay in the formation of the epicardium, which leads to an abnormal epicardium and ultimately contributes to the cardiac malformations seen in the RXRalpha-/-.

Selective expression of presenilin 1 in neural progenitor cells rescues the cerebral hemorrhages and cortical lamination defects in presenilin 1-null mutant mice

Mice with a null mutation of the presenilin 1 gene (Psen1(-/-)) die during late intrauterine life or shortly after birth and exhibit multiple CNS and non-CNS abnormalities, including cerebral hemorrhages and altered cortical development. The cellular and molecular basis for the developmental effects of Psen1 remain incompletely understood. Psen1 is expressed in neural progenitors in developing brain, as well as in postmitotic neurons. We crossed transgenic mice with either neuron-specific or neural progenitor-specific expression of Psen1 onto the Psen1(-/-) background. We show that neither neuron-specific nor neural progenitor-specific expression of Psen1 can rescue the embryonic lethality of the Psen1(-/-) embryo. Indeed neuron-specific expression rescued none of the abnormalities in Psen1(-/-) mice. However, Psen1 expression in neural progenitors rescued the cortical lamination defects, as well as the cerebral hemorrhages, and restored a normal vascular pattern in Psen1(-/-) embryos. Collectively, these studies demonstrate that Psen1 expression in neural progenitor cells is crucial for cortical development and reveal a novel role for neuroectodermal expression of Psen1 in development of the brain vasculature.

Shared expression of phenotypic markers in systemic sclerosis indicates a convergence of pericytes and fibroblasts to a myofibroblast lineage in fibrosis

The mechanisms by which microvascular damage leads to dermal fibrosis in diffuse cutaneous systemic sclerosis (dcSSc) are unclear. We hypothesized that microvascular pericytes constitute a cellular link between microvascular damage and fibrosis by transdifferentiating into myofibroblasts. We used a combination of immunohistochemistry and double immunofluorescence labelling of frozen skin biopsies taken from normal and dcSSc patients to determine whether a phenotypic link between pericytes and myofibroblasts exists in dcSSc. Using α-smooth muscle actin, the ED-A splice variant of fibronectin (ED-A FN) and Thy-1 to identify myofibroblasts, we demonstrated the presence of myofibroblasts in fibrotic dcSSc skin. Myofibroblasts were totally absent from control skin, atrophic stage dcSSc skin and non-lesional skin. Using double immunofluorescence labelling, both myofibroblasts and pericytes were shown to express ED-A FN and Thy-1 in dcSSc skin but not in control skin. Proliferating cell nuclear antigen was also expressed by myofibroblasts and pericytes in dcSSc skin while being absent in control skin. These observations suggest that the presence of myofibroblasts may represent a transitional phase during the fibrotic stages of dcSSc and that Thy-1^+ve pericytes participate in the fibrogenic development of dcSSc by synthesizing ED-A FN, which may be associated with a proliferation and transition of pericytes and fibroblasts to myofibroblasts, thus linking microvascular damage and fibrosis.

Tumour vascular targeting

It is now accepted that the growth of solid tumours is dependent on their capacity to acquire a blood supply, and much effort has been directed towards the development of agents (known as anti-angiogenics) that disrupt this process. More recently, it has become apparent that targeted destruction of the established tumour vasculature is another avenue for exciting therapeutic opportunities. In this article, we present evidence that vascular targeting is an effective antitumour strategy in animal models, describe strategies for identifying putative tumour vascular targets and discuss future prospects for vascular targeting in the clinic.

The role of fibronectin in fibroblast migration during tissue repair

Fibronectin is known to stimulate cell growth and migration, but research does not provide a complete picture of all the mechanisms involved. Further studies are needed before practitioners can apply in vitro results to in vivo environments.

Dexamethasone-Induced Prenatal Alveolar Wall Thinning Is Associated with a Decrease in EIIIA+ Fibronectin Isoform in the Fetal Rat Lung

BACKGROUND

Glucocorticoid hormones play an important role in architectural and biochemical lung maturation. Although much of the molecular mechanism of their action in the lung is not fully understood, glucocorticoids directly or indirectly regulate lung maturation. Indirect effects of glucocorticoids may involve the modulation of cell-cell or cell-matrix interactions. Fibronectin (FN) is the major constituent of the pulmonary extracellular matrix and exists in multiple isoforms arising from alternative RNA splicing. EIIIA is the major alternatively spliced segment, and its expression is regulated in a spatiotemporal and oncodevelopmental manner.

OBJECTIVES

The present study focuses on the regulation of EIIIA-containing FN isoforms (referred to as EIIIA+ FN) by glucocorticoids in the developing lung.

METHODS

Dexamethasone (DEX) or saline was injected daily into pregnant rats from day 15 of gestation (term = day 22) until 24 h before sacrifice. The expression of EIIIA+ FN and proliferating cell nuclear antigen (PCNA), a biochemical marker for cell proliferation, was investigated in the fetal rat lung.

RESULTS

At day 20 of gestation (the canalicular stage), the DEX-treated lung showed a significant decrease in weight and saccular septal wall thickness, while the messenger RNA expression of the surfactant protein SP-B was increased in the DEX-treated lung, as compared with the control lung. The expression of EIIIA+ FN and PCNA around the distal airspaces was less extensive in the DEX-treated lung than in the control lung at day 20 of gestation.

CONCLUSIONS

Given the finding in vitro that EIIIA+ FN regulated the cell cycle, our results suggest that the change of EIIIA+ FN expression in the DEX-treated lung affected pulmonary cell proliferation.

Identification and characterization of a second fibronectin gene in zebrafish

Fibronectin (FN) is a highly conserved extracellular matrix protein that plays crucial roles in vertebrate embryogenesis. Previously, it was reported that zebrafish possess a single FN gene (fn1a). Here we report the presence of a second zebrafish FN gene (fn1b) that encodes a protein with a predicted primary structure that is similar to FNs identified in other vertebrates possessing 12 type I, 2 type II and 17 type III repeats including two alternative splice sites (EIIIA and EIIIB) and a variable region (V). Zebrafish FN1b exhibits 62.0% amino acid identity with zebrafish FN1a, 54% with human and 55% with Xenopus laevis FNs respectively. Employing RT-PCR analysis, we demonstrate that EIIIB- and V- isoforms are produced by alternative splicing of a single fn1b transcript. The FN1b EIIIA- isoform was not detected in zebrafish embryos or adult tissues nor were EIIIA, EIIIB or V region splice variants of fn1a found. FN1b mRNA was detected by RT-PCR in embryos at the gastrula-stage (8hpf) through 72hpf and in various adult tissues. EIIIB- and V+ are the predominant forms of FN1b present in the zebrafish embryo. Unlike FN1a, which is present at a relatively high amount at the embryonic stages before gastrulation, the FN1b isoforms are present at very low amounts at the early cleavage stage. The presence of multiple isoforms of FN1b along with a different pattern of expression compared to FN1a indicates that the two fn genes have separate roles in zebrafish development.

Transforming growth factor-β1 (TGF-β1) regulates ATDC5 chondrogenic differentiation and fibronectin isoform expression

Regulated splicing of fibronectin (FN) occurs during the mesenchymal to chondrocyte transition and ultimately results in the relative enrichment of an extra domain B (EDB) exon-containing FN isoform with the suggestion that FN isoforms may play a functional role in chondrogenesis. Promotion of chondrogenesis can also be achieved by treatment with transforming growth factor-beta (TGF-beta), which also regulates FN isoform expression. We have examined the effects of TGF-beta treatment on the assumption of the chondrogenic phenotype in the teratoma-derived cell line ATDC5 and tested whether these effects on chondrogenesis are paralleled by appropriate changes in FN isoform expression. ATDC5 cells were maintained in a pre-chondrogenic state and, in this state, treated with 10 ng/ml TGF-beta. The cells started to elaborate a matrix rich in sulfated proteoglycans, such that within the first 12 days of culture, TGF-beta1 treatment appeared to slightly accelerate early acquisition of an Alcian blue-stained matrix, and caused a dose- and time-dependent decrease in collagen type I expression; changes in collagen type II expression were variable. At later times, cells treated with TGF-beta became indistinguishable from those of the controls. Interestingly, TGF-beta treatment caused a significant dose- and time-dependent decrease in the proportion of FN containing the extra domain A (EDA) and the EDB exons. These data suggest that TGF-beta induces the early stages of chondrogenic maturation in this pre-chondrogenic line and that TGF-beta treatment increases expression of FN isoforms that lack the EDA and EDB exons.

The Spatial and Temporal Expression Patterns of Integrin α9β1 and One of Its Ligands, the EIIIA Segment of Fibronectin, in Cutaneous Wound Healing

The fibronectins (FN) comprise a family of adhesive extracellular matrix proteins thought to mediate important functions in cutaneous wounds. Plasma fibronectin (pFN) extravasates for days from intact hyperpermeable vessels following injury whereas mRNAs encoding the cellular fibronectins (cFN) that include two segments, termed EIIIA (EDA) and EIIIB (EDB), are expressed by wound cells. Wounds in mice null for pFN appear to heal normally whereas those in EIIIA null mice exhibit defects, suggesting that cFN may play a role when pFN is missing. Integrin alpha9beta1, a receptor for several extracellular matrix proteins as well as the EIIIA segment, is expressed normally in the basal layer of squamous epithelia. We report results from immunohistochemistry on healing wounds demonstrating that EIIIA-containing cFN are deposited abundantly but transiently from day 4 to 7 whereas EIIIB-containing cFN persist at least through day 14. Elevated expression of alpha9beta1 is seen in basal and suprabasal epidermal keratinocytes in wounds. The spatial expression patterns of cFN and alpha9beta1 are distinct, but overlap in the dermal-epidermal junction, and both are expressed contemporaneously. These observations suggest a role for alpha9beta1-EIIIA interactions in wound keratinocyte function.

Expression of Fibronectin Isoforms Bearing the Alternatively Spliced EIIIA, EIIIB, and V Segments in Corneal Alkali Burn and Keratectomy Wound Models in the Rat

PURPOSE

To better understand the healing process in the wounded cornea, fibronectin (FN) isoforms bearing the alternatively spliced EIIIA, EIIIB, and V segments (EIIIA+, EIIIB+, and V+ FNs) were evaluated in alkali burn and keratectomy wound models in the rat.

METHODS

Alkali burn or keratectomy wounds (both 2 mm) were created, and corneas were harvested at various time points and analyzed by indirect immunofluorescence using antibodies specific for the EIIIA, EIIIB, and V segments as well as for the total pool of FN (total FN).

RESULTS

There was minimal staining for any variety of FN in the epithelium or basement membrane zone (BMZ) in normal cornea, but each antibody produced granular staining in the stroma. Bright staining for V+ and total FNs was evident at the denuded stromal surface 1 day following keratectomy. In contrast, staining for EIIIA+ and EIIIB+ FNs was negligible at 24 hours but appeared on the wound surface under the migrating unstained epithelium by the second day. BMZ staining for FN then gradually subsided, such that there was little or no staining by 6 weeks. In contrast, alkali burn wounds exhibited very little BMZ staining throughout the time course. Although there was preferential staining of the anterior aspect of Descemet membrane by anti-EIIIA and anti-EIIIB antibodies under normal conditions, the staining intensity of the anterior and posterior aspects became similar following corneal wounding.

CONCLUSION

Deposition of EIIIA+ and EIIIB+ FNs in the BMZ of the keratectomy wound occurs more slowly than deposition of V+ and total FNs. EIIIA+ FN is expressed in a distribution that overlaps with that previously described for the alpha 9 integrin subunit following corneal debridement, suggesting that EIIIA-alpha 9 interactions could occur during corneal wound healing. In contrast, the relative lack of FN deposition in alkali burn wounds suggests that proteolytic degradation of FN may occur; and this, along with impairment of new FN synthesis because of cellular damage, could play a role in the high prevalence of recurrent epithelial erosions in alkali-wounded corneas.

Direct test of potential roles of EIIIA and EIIIB alternatively spliced segments of fibronectin in physiological and tumor angiogenesis

Fibronectin splice variants containing the EIIIA and/or EIIIB exons are prominently expressed in the vasculature of a variety of human tumors but not in normal adult tissues. To understand the functions of these splice variants in physiological and tumor angiogenesis, we used EIIIB-null and EIIIA-null strains of mice to examine neovascularization of mouse retinas, pancreatic tumors in Rip-Tag transgenic mice, and transplanted melanomas. Contrary to expectations, physiological and tumor angiogenesis was not significantly affected by the absence of either EIIIA or EIIIB splice variants. Tumor growth was also not affected. In addition, the expression levels of smooth muscle alpha actin, believed to be modulated by EIIIA-containing fibronectins, were not affected either. Our experiments show that despite their tight regulation during angiogenesis, the presence of EIIIA or EIIIB splice variants individually is not essential for neovascularization.

Alternative splicing of fibronectin mRNAs in chondrosarcoma cells: role of far upstream intron sequences

The fibronectin (FN) gene encodes multiple mRNAs through the process of alternative splicing, and production of certain isoforms is characteristic of a given cell type. Chondrocytes produce FNs that completely lack alternative exon EIIIA, and loss of inclusion of the exon is tightly linked to chondrogenic condensation of mesenchymal cells. The inclusion of a second exon, EIIIB, is high in embryonic cartilage, but declines with age. Multiple exons are omitted to produce the (V + C)-form that is highly specific for cartilage and chondrocytes. A rat chondrosarcoma cell line, RCS, was identified that preserves key features of the cartilage-specific splicing phenotype. RCS cells, which exclude exon EIIIA, and HeLa cells, which include exon EIIIA similar to mesenchymal cells, were used to assess the contribution of intron sequences flanking exon EIIIA to splicing regulation. Deletion of most of the intron downstream of the exon had little effect on splicing in either cell type. However, deletions within upstream intron 32-A reduced inclusion of the alternative exon in both cell types. The sequences involved lie more than 200 nucleotides away from the exon, but could not be localized to a single region by deletion mapping. These intronic sequences contribute to the efficiency of exon EIIIA recognition, but not to cell-type specific regulation. The normally inhibitory factor polypyrimidine tract binding protein promotes exon EIIIA inclusion in a manner that is partially dependent on the regulatory sequences within intron 32-A.