New insights into form and function of fibronectin splice variants

Involvement of O-glycosylation defining oncofetal fibronectin in epithelial-mesenchymal transition process

The process termed "epithelial-mesenchymal transition" (EMT) was originally discovered in ontogenic development, and has been shown to be one of the key steps in tumor cell progression and metastasis. Recently, we showed that the expression of some glycosphingolipids (GSLs) is down-regulated during EMT in human and mouse cell lines. Here, we demonstrate the involvement of GalNAc-type (or mucin-type) O-glycosylation in EMT process, induced with transforming growth factor β (TGF-β) in human prostate epithelial cell lines. We found that: (i) TGF-β treatment caused up-regulation of oncofetal fibronectin (onfFN), which is defined by mAb FDC6, and expressed in cancer or fetal cells/tissues, but not in normal adult cells/tissues. The reactivity of mAb FDC6 requires the addition of an O-glycan at a specific threonine, inside the type III homology connective segment (IIICS) domain of FN. (ii) This change is associated with typical EMT characteristics; i.e., change from epithelial to fibroblastic morphology, enhanced cell motility, decreased expression of a typical epithelial cell marker, E-cadherin, and enhanced expression of mesenchymal markers. (iii) TGF-β treatment up-regulated mRNA level of FN containing the IIICS domain and GalNAc-T activity for the IIICS domain peptide substrate containing the FDC6 onfFN epitope. (iv) Knockdown of GalNAc-T6 and T3 inhibited TGF-β-induced up-regulation of onfFN and EMT process. (v) Involvement of GSLs was not detectable with the EMT process in these cell lines. These findings indicate the important functional role of expression of onfFN, defined by site-specific O-glycosylation at IIICS domain, in the EMT process.

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes.

A molecular insight into algal-oomycete warfare: cDNA analysis of Ectocarpus siliculosus infected with the basal oomycete Eurychasma dicksonii

Brown algae are the predominant primary producers in coastal habitats, and like land plants are subject to disease and parasitism. Eurychasma dicksonii is an abundant, and probably cosmopolitan, obligate biotrophic oomycete pathogen of marine brown algae. Oomycetes (or water moulds) are pathogenic or saprophytic non-photosynthetic Stramenopiles, mostly known for causing devastating agricultural and aquacultural diseases. Whilst molecular knowledge is restricted to crop pathogens, pathogenic oomycetes actually infect hosts from most eukaryotic lineages. Molecular evidence indicates that Eu. dicksonii belongs to the most early-branching oomycete clade known so far. Therefore Eu. dicksonii is of considerable interest due to its presumed environmental impact and phylogenetic position. Here we report the first large scale functional molecular data acquired on the most basal oomycete to date. 9873 unigenes, totalling over 3.5 Mb of sequence data, were produced from Sanger-sequenced and pyrosequenced EST libraries of infected Ectocarpus siliculosus. 6787 unigenes (70%) were of algal origin, and 3086 (30%) oomycete origin. 57% of Eu. dicksonii sequences had no similarity to published sequence data, indicating that this dataset is largely unique. We were unable to positively identify sequences belonging to the RXLR and CRN groups of oomycete effectors identified in higher oomycetes, however we uncovered other unique pathogenicity factors. These included putative algal cell wall degrading enzymes, cell surface proteins, and cyclophilin-like proteins. A first look at the host response to infection has also revealed movement of the host nucleus to the site of infection as well as expression of genes responsible for strengthening the cell wall, and secretion of proteins such as protease inhibitors. We also found evidence of transcriptional reprogramming of E. siliculosus transposable elements and of a viral gene inserted in the host genome.

Extra domain A of fibronectin primes leukotriene biosynthesis and stimulates neutrophil migration through activation of Toll-like receptor 4

OBJECTIVE

There is increasing evidence of a role for Toll-like receptors (TLRs) in inflammatory arthritis. The extra domain A (ED-A)-containing isoform of fibronectin is generated under pathologic conditions such as rheumatoid arthritis, and ED-A has been identified as an endogenous TLR-4 ligand. Leukotriene B4 (LTB4) and polymorphonuclear neutrophils (PMNs) play a critical role in murine models of inflammatory arthritis. The aim of this study was therefore to investigate the putative effects of ED-A on leukotriene biosynthesis and PMN migration through TLR signaling.

METHODS

The effect of recombinant human ED-A (rhED-A) on leukotriene biosynthesis was evaluated in isolated human blood PMNs and monocytes by high-performance liquid chromatography. The capacity of rhED-A to stimulate PMN migration was evaluated using a transendothelial/matrix migration assay in vitro and the mouse air-pouch model in vivo.

RESULTS

Recombinant human ED-A efficiently primed the biosynthesis of LTB4 in PMN and monocyte suspensions. This priming effect was dependent on TLR-4 activation, since the TLR-4-signaling inhibitor CLI-095 completely blocked the effect of rhED-A but not that of other TLR ligands (R-848, Pam2 CSK4) or cytokines. Moreover, rhED-A stimulated transendothelial migration of PMNs in vitro, which was inhibited by 50-60% with the LTB4 receptor 1 (BLT1) antagonist CP105,696 or the cytosolic phospholipase A2 α inhibitor pyrrophenone. In vivo, rhED-A induced a significant PMN recruitment into the air pouch of C3H/HeOuJ mice (expressing functional TLR-4), but not in C3H/HeJ mice (expressing nonsignaling TLR-4).

CONCLUSION

These results demonstrate the ability of rhED-A to promote LTB4 biosynthesis and PMN migration through TLR-4 activation, thus providing new insights on TLR-dependent mechanisms of regulation of LTB4 biosynthesis and PMN infiltration in inflammatory joint diseases.

Extra domain A-positive fibronectin-positive feedback loops and their association with cutaneous inflammatory disease

Cutaneous inflammation can show Th1 or Th2 predominance, but the precise mechanisms by which such selectivity is determined are unknown. A recent study has demonstrated that Th1 cells, but not Th2 cells, produce an endogenous ligand for Toll-like receptor (TLR) 4, namely extradomain A+ fibronectin containing extra type III domain A (FnEDA+). As TLR4 stimulation leads to production of proinflammatory cytokines that recruit (via altered endothelial adhesion molecule expression and chemokine production) more Th1/Th17 cells, a positive feedback mechanism for Th1/Th17 inflammation exists. We propose that FnEDA+ positive feedback loops are a potential driver of Th1/Th17 inflammation. Conversely, the inflammatory EDA+ fibronectin loop is negatively regulated in atopic dermatitis, Th2 cytokines actively suppress TLR4 expression of Th1 cytokines, and recruited Th2 cells do not produce FnEDA+. In psoriasis, there are multiple FnEDA+ loops, comprising inflammatory, keratinocyte, and autoimmune loops. In allergic contact dermatitis, a single inflammatory loop operates. In atopic dermatitis, the FnEDA+ loop is actively suppressed by Th2 cytokines, and recruited Th2 cells do not "feedback" FnEDA+. We review endogenous ligands for TLR in relation to inflammatory disease, FnEDA+ function, and the potential role for FnEDA+ in psoriasis, allergic contact dermatitis, and atopic dermatitis.

Fibronectin and beta-catenin act in a regulatory loop in dermal fibroblasts to modulate cutaneous healing

β-Catenin is an important regulator of dermal fibroblasts during cutaneous wound repair. However, the factors that modulate β-catenin activity in this process are not completely understood. We investigated the role of the extracellular matrix in regulating β-catenin and found an increase in β-catenin-mediated Tcf-dependent transcriptional activity in fibroblasts exposed to various extracellular matrix components. This occurs through an integrin-mediated GSK3β-dependent pathway. The physiologic role of this mechanism was demonstrated during wound repair in extra domain A-fibronectin-deficient mice, which exhibited decreased β-catenin-mediated signaling during the proliferative phase of healing. Extra domain A-fibronectin-deficient mice have wounds that fail at a lower tensile strength and contain fewer fibroblasts compared with wild type mice. This phenotype was rescued by genetic or pharmacologic activation of β-catenin signaling. Because fibronectin is a transcriptional target of β-catenin, this suggests the existence of a feedback loop between these two molecules that regulates dermal fibroblast cell behavior during wound repair.

Different patterns of fibronectin and tenascin-C splice variants expression in primary and metastatic melanoma lesions

We have investigated the staining patterns of primary and metastatic melanoma lesions using F8, L19 and F16. These three clinical-stage antibodies are currently being studied in clinical trials for the pharmacodelivery of cytokines or therapeutic radionuclides to neoplastic sites in patients with cancer. Frozen sections of 24 primary and 29 metastatic melanoma lesions were stained, using immunofluorescence procedures, with biotinylated preparations of the F8, L19 and F16 antibodies, which are specific to the alternatively spliced extra domain A and extra domain B domains of fibronectin and A1 domain of tenascin-C, respectively. Blood vessels were costained using von Willebrand factor-specific antibodies. In primary cutaneous melanoma lesions, F16 and F8 (but not L19) strongly stained the basal lamina at the interface between epidermis and dermis, with a strikingly complementary pattern. By contrast, metastatic melanoma lesions displayed a strong and diffuse pattern of immunoreactivity with all three antibodies. It was found that the extracellular matrix in melanoma undergoes extensive remodelling during the transition from primary to metastatic lesions. The intense staining of metastatic melanoma lesions by the F8, L19 and F16 antibodies provides a strong rationale for the use of these antibodies and their derivatives for the treatment of melanoma patients and possibly for the personalized choice of the best performing antibody in individual patients.

Evolutionary connections between coding and splicing regulatory regions in the fibronectin EDA exon

Research on exonic coding sequences has demonstrated that many substitutions at the amino acid level may also reflect profound changes at the level of splicing regulatory regions. These results have revealed that, for many alternatively spliced exons, there is considerable pressure to strike a balance between two different and sometimes conflicting forces: the drive to improve the quality and production efficiency of proteins and the maintenance of proper exon recognition by the splicing machinery. Up to now, the systems used to investigate these connections have mostly focused on short alternatively spliced exons that contain a high density of splicing regulatory elements. Although this is obviously a desirable feature in order to maximize the chances of spotting connections, it also complicates the process of drawing straightforward evolutionary pathways between different species (because of the numerous alternative pathways through which the same end point can be achieved). The alternatively spliced fibronectin extra domain A exon (also referred to as EDI or EIIIA) does not have these limitations, as its inclusion is already known to depend on a single exonic splicing enhancer element within its sequence. In this study, we have compared the rat and human fibronectin EDA exons with regard to RNA structure, exonic splicing enhancer strengths, and SR protein occupancy. The results gained from these analyses have then been used to perform an accurate evaluation of EDA sequences observed in a wide range of animal species. This comparison strongly suggests the existence of an evolutionary connection between changes at the nucleotide levels and the need to maintain efficient EDA recognition in different species.

Oncostatin M is expressed in atherosclerotic lesions: a role for Oncostatin M in the pathogenesis of atherosclerosis

OBJECTIVE

Chronic inflammation plays a pivotal role in the development and progression of atherosclerosis. The inflammatory response is mediated by cytokines. The aim of this study was to determine if Oncostatin M (OSM), a monocyte and T-lymphocyte specific cytokine is present in atherosclerotic lesions. We also investigated the roles of signal transducer and activator of transcription (STAT)-1 and STAT-3 in regulating OSM-induced smooth muscle cell (SMC) proliferation, migration and cellular fibronectin (cFN) synthesis.

METHODS AND RESULTS

Immunostaining of atherosclerotic lesions from human carotid plaques demonstrated the expression of OSM antigen in both macrophages and SMCs. Explanted SMCs from human carotid plaques expressed OSM mRNA and protein as determined by RT-PCR and Western blotting. Using the chow-fed ApoE(-/-) mouse model of atherosclerosis, we observed that OSM was initially expressed in the intima at 20 weeks of age. By 30 weeks, OSM was expressed in both the intima and media. In vitro studies show that OSM promotes SMC proliferation, migration and cFN synthesis. Lentivirus mediated-inhibition of STAT-1 and STAT-3 prevented OSM-induced SMC proliferation, migration and cellular fibronectin synthesis.

CONCLUSIONS

These findings demonstrate that OSM is expressed in atherosclerotic lesions and may contribute to the progression of atherosclerosis by promoting SMC proliferation, migration and extracellular matrix protein synthesis through the STAT pathway.

Histone deacetylase activity modulates alternative splicing

There is increasing evidence to suggest that splicing decisions are largely made when the nascent RNA is still associated with chromatin. Here we demonstrate that activity of histone deacetylases (HDACs) influences splice site selection. Using splicing-sensitive microarrays, we identified ∼700 genes whose splicing was altered after HDAC inhibition. We provided evidence that HDAC inhibition induced histone H4 acetylation and increased RNA Polymerase II (Pol II) processivity along an alternatively spliced element. In addition, HDAC inhibition reduced co-transcriptional association of the splicing regulator SRp40 with the target fibronectin exon. We further showed that the depletion of HDAC1 had similar effect on fibronectin alternative splicing as global HDAC inhibition. Importantly, this effect was reversed upon expression of mouse HDAC1 but not a catalytically inactive mutant. These results provide a molecular insight into a complex modulation of splicing by HDACs and chromatin modifications.

Impaired splicing of fibronectin is associated with thoracic aortic aneurysm formation in patients with bicuspid aortic valve

OBJECTIVE

Thoracic aortic aneurysm is a common complication in patients with bicuspid aortic valve (BAV). Alternatively spliced extra domain A (EDA) of fibronectin (FN) has an essential role in tissue repair. Here we analyze the expression of FN spliceforms in dilated and nondilated ascending aorta of tricuspid aortic valve (TAV) and BAV patients.

METHODS AND RESULTS

The mRNA expression was analyzed in the ascending aorta by Affymetrix Exon arrays in patients with TAV (n=40) and BAV (n=69). EDA and extra domain B (EDB) expression was increased in dilated aorta from TAV patients compared with nondilated aorta (P<0.001 and P<0.05, respectively). In contrast, EDA expression was not increased in dilated aorta from BAV patients (P=0.25), whereas EDB expression was upregulated (P<0.01). The expression of EDA correlated with maximum aortic diameter in TAV (ρ=0.58) but not in BAV (ρ=0.15) patients. Protein analyses of EDA-FN showed concordant results. Transforming growth factor-β treatment influenced the splicing of FN and enhanced the formation of EDA-containing FN in cultured medial cells from TAV patients but not in cells derived from BAV patients. Gene set enrichment analysis together with multivariate and univariate data analyses of mRNA expression suggested that differences in the transforming growth factor-β signaling pathway may explain the impaired EDA inclusion in BAV patients.

CONCLUSIONS

Decreased EDA expression may contribute to increased aneurysm susceptibility of BAV patients.

Comparative in vivo analysis of the atherosclerotic plaque targeting properties of eight human monoclonal antibodies

OBJECTIVE

The selective in vivo localization of antibody derivatives in atherosclerotic plaques may open novel diagnostic and therapeutic applications. Here, we present a comparative in vivo localization analysis of eight radioiodinated human monoclonal antibodies in apolipoprotein E-deficient (ApoE(-/-)) mice.

METHODS

Animals were fed with a cholesterol-rich diet, followed by harvesting of the aorta 24h after intravenous antibody injection and investigated by autoradiographic analysis. Localization of F8 antibody on atherosclerotic plaque structures was further studied in three-color fluorescence microscopy.

RESULTS

The study revealed that the F8 antibody, specific to the alternatively spliced EDA domain of fibronectin, exhibited the highest plaque-targeting potential among the antibodies analyzed in this study, with an ability to preferentially localize to all plaques within the aorta. Targeting results were confirmed by injection of fluorescein-labeled F8 antibody, followed by three-color fluorescence microscopy analysis.

CONCLUSION

These findings open novel biomolecular avenues for the in vivo imaging of atherosclerotic plaques and for pharmacodelivery applications, since F8 had previously been reported by our group to strongly stain atherosclerotic plaques in human carotid arteries.

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.

Chronic OVA allergen challenged Siglec-F deficient mice have increased mucus, remodeling, and epithelial Siglec-F ligands which are up-regulated by IL-4 and IL-13

BACKGROUND

In this study we examined the role of Siglec-F, a receptor highly expressed on eosinophils, in contributing to mucus expression, airway remodeling, and Siglec-F ligand expression utilizing Siglec-F deficient mice exposed to chronic allergen challenge.

METHODS

Wild type (WT) and Siglec-F deficient mice were sensitized and challenged chronically with OVA for one month. Levels of airway inflammation (eosinophils), Siglec-F ligand expresion and remodeling (mucus, fibrosis, smooth muscle thickness, extracellular matrix protein deposition) were assessed in lung sections by image analysis and immunohistology. Airway hyperreactivity to methacholine was assessed in intubated and ventilated mice.

RESULTS

Siglec-F deficient mice challenged with OVA for one month had significantly increased numbers of BAL and peribronchial eosinophils compared to WT mice which was associated with a significant increase in mucus expression as assessed by the number of periodic acid Schiff positive airway epithelial cells. In addition, OVA challenged Siglec-F deficient mice had significantly increased levels of peribronchial fibrosis (total lung collagen, area of peribronchial trichrome staining), as well as increased numbers of peribronchial TGF-β1+ cells, and increased levels of expression of the extracellular matrix protein fibronectin compared to OVA challenged WT mice. Lung sections immunostained with a Siglec-Fc to detect Siglec-F ligand expression demonstrated higher levels of expression of the Siglec-F ligand in the peribronchial region in OVA challenged Siglec-F deficient mice compared to WT mice. WT and Siglec-F deficient mice challenged intranasally with IL-4 or IL-13 had significantly increased levels of airway epithelial Siglec-F ligand expression, whereas this was not observed in WT or Siglec-F deficient mice challenged with TNF-α. There was a significant increase in the thickness of the peribronchial smooth muscle layer in OVA challenged Siglec-F deficient mice, but this was not associated with significant increased airway hyperreactivity compared to WT mice.

CONCLUSIONS

Overall, this study demonstrates an important role for Siglec-F in modulating levels of chronic eosinophilic airway inflammation, peribronchial fibrosis, thickness of the smooth muscle layer, mucus expression, fibronectin, and levels of peribronchial Siglec-F ligands suggesting that Siglec-F may normally function to limit levels of chronic eosinophilic inflammation and remodeling. In addition, IL-4 and IL-13 are important regulators of Siglec-F ligand expression by airway epithelium.

Jekyll and Hyde: the role of the microenvironment on the progression of cancer

It is now recognized that the host microenvironment undergoes extensive change during the evolution and progression of cancer. This involves the generation of cancer-associated fibroblasts (CAFs), which, through release of growth factors and cytokines, lead to enhanced angiogenesis, increased tumour growth and invasion. It has also been demonstrated that CAFs may modulate the cancer stem cell (CSC) phenotype, which has therapeutic implications. The altered fibroblast phenotype also contributes to the development of an altered extracellular matrix (ECM), with synthesis of ECM isoforms rarely found in normal tissues, including tenascin-C isoforms and the fibronectin EDA isoform. There is also emerging evidence of how the tensile strength of the tumour-associated ECM may be modified and lead to altered signalling in tumour cells. The hypoxic environment of the tumour stimulates angiogenesis and also impacts on other aspects of cell signalling, including the c-met pathway and lysyl oxidase-mediated signalling, which can directly promote tumour cell invasion. The inflammatory infiltrate associated with many solid tumours also modulates tumour function, having both anti- and pro-tumour effects. All of these components of the microenvironment provide potential targets for therapeutic attack, with a number of molecules already in clinical trials. It is also becoming evident that characterizing the tumour microenvironment can provide important prognostic and predictive information about tumours, independent of the tumour cell phenotype.

Fragmentation of fibronectin by inherent autolytic and matrix metalloproteinase activities

Fibronectin (FN) purified by gelatin affinity chromatography is unstable and undergoes fragmentation. The cleavage has been ascribed to inherent autolytic protease activities as well as co-purified matrix metalloproteinases (MMP). Understanding the mechanism by which the proteolysis of FN occurs is important, because the FN fragments have biological activities that differ from those of intact FN. Having excluded contributions of other plasma-derived proteases, the present experiments demonstrated that cleavage of FN by MMP-2 to distinct fragments occurred in synergy with inherent FN activities. Limited heat treatment of FN at 56°C for 30 min inactivated the inherent protease activities sharply reducing autolysis of FN in a manner similar to that seen in the presence of serine proteinase inhibitors. Heat treatment did not alter cell attachment to FN, but significantly increased the susceptibility of FN to enzymatic cleavage by MMP-2. The carboxyl-terminal hemopexin-like domain (PEX) of MMP-2 was shown to possess critical exodomain properties required for the interactions of MMP-2 with FN, and FN was cleaved at a significantly reduced rate by an MMP-2 variant with deletion of PEX. Verifying the specificity of interactions, isolated PEX competed FN cleavage by MMP-2 in a concentration-dependent manner. These results have further elucidated the synergistic contributions of inherent autolytic serine protease-like activities and MMP-2 to fragmentation of FN and provide the rationale and basis for modified preparation and handling of FN used in biological research.

The response of annulus fibrosus cell to fibronectin-coated nanofibrous polyurethane-anionic dihydroxyoligomer scaffolds

Tissue engineering of the annulus fibrosus(AF), a component of the intervertebral disc, has proven to be challenging due to its complex oriented lamellar structure. Previously it was demonstrated that polyurethane (PU) scaffolds containing an anionic dihydroxy oligomers (ADO) may be suitable to use in this application. The current study examines whether matrix protein(s) coatings (collagen type I, collagen type I and fibronectin, fibronectin, or vitronectin) would promote cell and collagen orientation that more closely mimics native AF. The greatest cell attachment occurred when scaffolds were pre-coated with Fn. Cells on Fn-coated scaffolds were aligned parallel to scaffold fibers, a process that involved α5β1 integrin, as determined by integrin-specific blocking antibodies, which in turn reduced AF cell spreading and alignment. Cell shape was regulated by the actin cytoskeleton as cells grown in the presence of cytochalasin D did not spread. Cells on Fn-coated PU scaffolds formed fibrillar Fn, synthesized significantly more collagen, and showed linear alignment of the secreted type I collagen when compared to cells grown on the other protein-coated scaffolds and the non-coated control. Thus Fn-coating of PU-ADO scaffolds appears to promote properly oriented AF cells and collagen, which should facilitate developing AF tissue that more closely mimics the native tissue.

Extracellular matrix roles during cardiac repair

The cardiac extracellular matrix (ECM) provides a platform for cells to maintain structure and function, which in turn maintains tissue function. In response to injury, the ECM undergoes remodeling that involves synthesis, incorporation, and degradation of matrix proteins, with the net outcome determined by the balance of these processes. The major goals of this review are a) to serve as an initial resource for students and investigators new to the cardiac ECM remodeling field, and b) to highlight a few of the key exciting avenues and methodologies that have recently been explored. While we focus on cardiac injury and responses of the left ventricle (LV), the mechanisms reviewed here have pathways in common with other wound healing models.

Cross-talk between the insulin-like growth factor (IGF) axis and membrane integrins to regulate cell physiology

The biology of cross-talk between activated growth factor receptors and cell-surface integrins is an area which has attracted much interest in recent years (Schwartz and Ginsberg, 2002). This review discusses the relationship between the insulin-like growth factor (IGF) axis and cell-surface integrin receptors in the regulation of various aspects of cell physiology. Key to these interactions are signals transmitted between integrins and the IGF-I receptor (IGF-IR) when either or both are bound to their cognate ligands and we will review the current state of knowledge in this area. The IGF axis comprises many molecular components and we will also discuss the potential role of these species in cross-talk with the integrin receptor. With respect to integrin ligands, we will mainly focus on the well-characterized interactions of the two extracellular matrix (ECM) glycoproteins fibronectin (FN) and vitronectin (VN) with cell-surface ligands, and, how this affects activity through the IGF axis. However, we will also highlight the importance of other integrin activation mechanisms and their impact on IGF activity.

Fibronectin splicing variants in human intervertebral disc and association with disc degeneration

STUDY DESIGN

In this study, normal intervertebral disc (IVD) tissues and degenerative human IVD tissues were compared for presence of fibronectin (FN) mRNA splice variants and for FN fragments (FN-f).

OBJECTIVE

To further understand FN RNA splice forms and protein fragments in disc degeneration.

SUMMARY OF BACKGROUND DATA

FN splice variants play important roles in regulating cell-matrix and matrix-matrix interactions in skeletogenesis and skeletal function in limbs and other sites. However, presence and possible roles of FN splice variants and fragments in human IVD have not been determined.

METHODS

Normal infant and adult IVD tissues were obtained from organ donors, and degenerative human IVD tissues were obtained from patients undergoing spinal surgeries. FN splice patterns were assessed by reverse transcriptase polymerase chain reaction. Relative expression levels were semiquantified by densitometry. FN and its fragments were studied by Western blot analysis.

RESULTS

Both the EDB and EDB splice variants were present in normal and degenerative IVD tissues. The EDB to EDB ratio was highest in moderately degenerative tissue. The EDA domain was only expressed in infant but not adult tissue. Variable-region (V) splice forms were present in all tissues studied. A splice form with the entire V-region, the 15th type III domain, and 10th type I domain adjacent to the 3'end of V region omitted (referred to as [V+III-15+I-10], also known as [V+C] splice form) was present at higher levels in adult than in infant samples. FN-f were also detected in degenerating tissue, but not in normal IVD tissue samples.

CONCLUSION

The data indicate that higher levels of EDB isoform and FN-f are associated with IVD degeneration. This shift in alternative splicing may reflect an attempt of tissue repair and remodeling. Novel information gathered in this study will lead to a better understanding of pathologic processes associated with disc malfunction and degeneration.

EDA-containing cellular fibronectin induces fibroblast differentiation through binding to alpha4beta7 integrin receptor and MAPK/Erk 1/2-dependent signaling

Fibroblast differentiation is an essential step during wound healing and fibrosis. Fibronectin (FN) is a major component of the extracellular matrix and occurs in two main forms: plasma and cellular FN. The latter includes the alternatively spliced domain A (EDA). Although EDA-containing cellular fibronectin (EDA-FN) is associated with fibroblast differentiation, how EDA-FN promotes differentiation is incompletely understood. In this study, we investigate the mechanism by which EDA-FN contributes to fibroblast differentiation with emphasis on the characterization of the EDA-FN receptor. We show that EDA-FN increases α-SMA expression (immunofluorescence), collagen deposition, cell contractility, and focal adhesion kinase (FAK) activation (immunoblotting); whereas plasma FN, a form lacking EDA, shows no effect. Primary lung fibroblasts constitutively express α(4)β(7) integrin receptor (FACS and RT-PCR). Blocking of α(4)β(7) reduces fibroblast adhesion to EDA-FN and inhibits α-SMA expression, collagen deposition, and FAK activation induced by EDA-FN. Using recombinant EDA-containing peptides, we demonstrate that the EDA segment is sufficient to induce fibroblast differentiation via binding to α(4)β(7). EDA-FN induces MAPK-Erk1/2 activation and inhibition of MEK1/2 attenuates EDA-FN-induced α-SMA expression. Our findings demonstrate that EDA-FN induces fibroblast differentiation by a mechanism that involves binding of EDA to α(4)β(7) integrin followed by activation of FAK and MAPK-associated signaling pathways.

Control of fibroblast fibronectin expression and alternative splicing via the PI3K/Akt/mTOR pathway

Fibronectin (FN), a ubiquitous glycoprotein that plays critical roles in physiologic and pathologic conditions, undergoes alternative splicing which distinguishes plasma FN (pFN) from cellular FN (cFN). Although both pFN and cFN can be incorporated into the extracellular matrix, a distinguishing feature of cFN is the inclusion of an alternatively spliced exon termed EDA (for extra type III domain A). The molecular steps involved in EDA splicing are well-characterized, but pathways influencing EDA splicing are less clear. We have previously found an obligate role for inhibition of the tumor suppressor phosphatase and tensin homologue on chromosome 10 (PTEN), the primary regulator of the PI3K/Akt pathway, in fibroblast activation. Here we show TGF-beta, a potent inducer of both EDA splicing and fibroblast activation, inhibits PTEN expression and activity in mesenchymal cells, corresponding with enhanced PI3K/Akt signaling. In pten(-/-) fibroblasts, which resemble activated fibroblasts, inhibition of Akt attenuated FN production and decreased EDA alternative splicing. Moreover, inhibition of mammalian target of rapamycin (mTOR) in pten(-/-) cells also blocked FN production and EDA splicing. This effect was due to inhibition of Akt-mediated phosphorylation of the primary EDA splicing regulatory protein SF2/ASF. Importantly, FN silencing in pten(-/-) cells resulted in attenuated proliferation and migration. Thus, our results demonstrate that the PI3K/Akt/mTOR axis is instrumental in FN transcription and alternative splicing, which regulates cell behavior.

Impairment of pre-mRNA splicing in liver disease: Mechanisms and consequences

Pre-mRNA splicing is an essential step in the process of gene expression in eukaryotes and consists of the removal of introns and the linking of exons to generate mature mRNAs. This is a highly regulated mechanism that allows the alternative usage of exons, the retention of intronic sequences and the generation of exonic sequences of variable length. Most human genes undergo splicing events, and disruptions of this process have been associated with a variety of diseases, including cancer. Hepatocellular carcinoma (HCC) is a molecularly heterogeneous type of tumor that usually develops in a cirrhotic liver. Alterations in pre-mRNA splicing of some genes have been observed in liver cancer, and although still scarce, the available data suggest that splicing defects may have a role in hepatocarcinogenesis. Here we briefly review the general mechanisms that regulate pre-mRNA splicing, and discuss some examples that illustrate how this process is impaired in liver tumorigenesis, and may contribute to HCC development. We believe that a more thorough examination of pre-mRNA splicing is still needed to accurately draw the molecular portrait of liver cancer. This will surely contribute to a better understanding of the disease and to the development of new effective therapies.

Nrk2b-mediated NAD+ production regulates cell adhesion and is required for muscle morphogenesis in vivo: Nrk2b and NAD+ in muscle morphogenesis

Cell-matrix adhesion complexes (CMACs) play fundamental roles during morphogenesis. Given the ubiquitous nature of CMACs and their roles in many cellular processes, one question is how specificity of CMAC function is modulated. The clearly defined cell behaviors that generate segmentally reiterated axial skeletal muscle during zebrafish development comprise an ideal system with which to investigate CMAC function during morphogenesis. We found that Nicotinamide riboside kinase 2b (Nrk2b) cell autonomously modulates the molecular composition of CMACs in vivo. Nrk2b is required for normal Laminin polymerization at the myotendinous junction (MTJ). In Nrk2b-deficient embryos, at MTJ loci where Laminin is not properly polymerized, muscle fibers elongate into adjacent myotomes and are abnormally long. In yeast and human cells, Nrk2 phosphorylates Nicotinamide Riboside and generates NAD+ through an alternative salvage pathway. Exogenous NAD+ treatment rescues MTJ development in Nrk2b-deficient embryos, but not in laminin mutant embryos. Both Nrk2b and Laminin are required for localization of Paxillin, but not beta-Dystroglycan, to CMACs at the MTJ. Overexpression of Paxillin in Nrk2b-deficient embryos is sufficient to rescue MTJ integrity. Taken together, these data show that Nrk2b plays a specific role in modulating subcellular localization of discrete CMAC components that in turn plays roles in musculoskeletal development. Furthermore, these data suggest that Nrk2b-mediated synthesis of NAD+ is functionally upstream of Laminin adhesion and Paxillin subcellular localization during MTJ development. These results indicate a previously unrecognized complexity to CMAC assembly in vivo and also elucidate a novel role for NAD+ during morphogenesis.

Pref-1 interacts with fibronectin to inhibit adipocyte differentiation

Pref-1/Dlk1 is made as an epidermal growth factor (EGF) repeat-containing transmembrane protein but is cleaved by tumor necrosis factor alpha converting enzyme (TACE) to generate a biologically active soluble form. Soluble Pref-1 inhibits adipocyte differentiation through the activation of extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) and the subsequent upregulation of Sox9 expression. However, others have implicated Notch in Pref-1 signaling and function. Here, we show that Pref-1 does not interact with, or require, Notch for its function. Instead, we show a direct interaction of Pref-1 and fibronectin via the Pref-1 juxtamembrane domain and fibronectin C-terminal domain. We also show that fibronectin is required for the Pref-1-mediated inhibition of adipocyte differentiation, the activation of ERK/MAPK, and the upregulation of Sox9. Furthermore, disrupting fibronectin binding to integrin by the addition of RGD peptides or by the knockdown of alpha 5 integrin prevents the Pref-1 inhibition of adipocyte differentiation. Pref-1 activates the integrin downstream signaling molecules, FAK and Rac, and ERK activation by Pref-1 is blunted by the knockdown of Rac or by the forced expression of dominant-negative Rac. We conclude that, by interacting with fibronectin, Pref-1 activates integrin downstream signaling to activate MEK/ERK and to inhibit adipocyte differentiation.

Enhanced fibrillin-2 expression is a general feature of wound healing and sclerosis: potential alteration of cell attachment and storage of TGF-beta

Wound healing and sclerosis are characterized by an increase of extracellular matrix proteins, which are characteristically expressed in the embryo-fetal period. We analyzed the expression of fibrillin-2, which is typically found in embryonic tissues, but only scarcely in adult skin. In wound healing and sclerotic skin diseases such as lipodermatosclerosis and scleroderma, a marked increase of fibrillin-2 expression was found by immunohistology. Double labelling of fibrillin-2 and tenascin-C, which is also expressed in wound healing and sclerosis, showed co-localization of both proteins. Solid-phase and slot blot-overlay assays showed a dose-dependent binding of the recombinant N-terminal half of fibrillin-2 (rFBN2-N) to tenascin-C. Real-time PCR showed an increase of the fibrillin-2 gene expression in cell culture triggered by typical mediators for fibroblast activation such as serum, IL-4, and TGF-beta. By contrast, prolonged hypoxia is not associated with changes in fibrillin-2 expression. Tenascin-C is an anti-adhesive substrate for fibroblasts, whereas fibrillin-2 stimulates cell attachment. Attachment assays using mixed substrates showed decreased cell attachment when tenascin-C and rFBN2-N were coated together, compared with the attachment to rFBN2-N alone. Fibrillins are involved in storage and activation of TGF-beta. Immunohistology with an antibody against the latency-associated peptide (LAP (TGF-beta1)) showed a marked increase of inactive LAP-bound TGF-beta1 in wound healing and sclerotic skin whereas normal skin showed only a weak expression. Double immunofluorescence confirmed a partial colocalization of both proteins. In conclusion, we show that a stimulation of the fibrillin-2 expression is a characteristic feature of fibroblasts present in wound healing and sclerosis, which may be involved in the alteration of cell attachment and storage of inactive TGF-beta in the matrix.

Emerging roles of fibronectin in thrombosis

Fibronectin (FN) is a glycoprotein recognized originally in the 1940's as a contaminant of fibrinogen in Cohn fraction I of plasma. Decades of research demonstrated FN synthesis by a variety of cells and defined FN as an essential component of the extracellular matrix with roles in embryogenesis, development, and wound healing. More recently, FN has emerged as player in platelet thrombus formation and diseases associated with thrombosis including vascular remodeling, atherosclerosis, and cardiac repair following a myocardial infarct. We discuss the mechanisms by which this might occur and conclude that FN may have a unique role in thrombosis without affecting normal hemostasis and therefore may be a reasonable therapeutic target for the prevention of thrombotic diseases.

Erythropoietin promotes Schwann cell migration and assembly of the provisional extracellular matrix by recruiting beta1 integrin to the cell surface

In peripheral nerve injury, Schwann cells undergo profound phenotypic modulation, adopting a migratory phenotype and remodeling the extracellular matrix so that it is permissive for axonal regrowth. Erythropoietin (Epo) and its receptor (EpoR) are expressed by Schwann cells after nerve injury, regulating inflammatory cytokine expression and minimizing the duration of neuropathic pain. The mechanism of Epo activity in the injured peripheral nerve remains incompletely understood. Herein, we demonstrate that Epo promotes Schwann cell migration in vitro on fibronectin (FN)-coated surfaces. Epo also rapidly recruits beta1 integrin subunit to the Schwann cell surface by a JAK-2-dependent pathway. Although beta1 integrin subunit-containing integrins were not principally responsible for Schwann cell adhesion or migration on FN under basal conditions, beta1 gene-silencing blocked the ability of Epo to promote cell migration. Epo also induced Schwann cell FN expression in vitro and in vivo. The FN was organized into insoluble fibrils by Epo-treated Schwann cells in vitro and into an extensive matrix surrounding Schwann cells in vivo. Our results support a model in which Epo promotes Schwann cell migration and assembly of the provisional extracellular matrix in the injured peripheral nerve by its effects on integrin recruitment to the cell surface and local FN production.

Psoriasis and extra domain A fibronectin loops

We have previously postulated that as well as T-helper (Th) 1 and Th17 cells, the transforming growth factor (TGF)-beta/fibronectin (FN)/alpha5beta1 pathway is central to psoriasis pathogenesis. EDA+ FN refers to an alternatively spliced isoform of FN with an additional domain known as extra domain A. EDA+ FN has two important properties pertinent to psoriasis lesions: it stimulates keratinocyte hyperproliferation, and, through stimulation of Toll-like receptor (TLR) 4, stimulates production of proinflammatory cytokines. EDA+ FN production induced by TGF-beta stimulation can be maintained in psoriasis lesions via two main feedback loops. Firstly, EDA+ FN stimulates proliferation of keratinocytes, which, in an autocrine fashion, will release more EDA+ FN. Secondly, EDA+ FN stimulates TLR4 expressed by antigen-presenting cells resulting in the production of proinflammatory cytokines such as tumour necrosis factor-alpha, interleukin (IL)-1, IL-6 and IL-12. The resultant promotion of cutaneous inflammation results in the recruitment of Th1 cells, which also produce EDA+ FN. We propose that these 'FN loops' contribute to the maintenance and progression of psoriatic lesions. Finally, although the association between psoriasis and heart/thrombotic disease remains unclear one plausible link may be the promotion of atherosclerosis and thrombotic heart disease by EDA+ FN.

Identification of a novel family of laminin N-terminal alternate splice isoforms: structural and functional characterization

The laminins are a family of heterotrimeric basement membrane proteins that play roles in cellular adhesion, migration, and tissue morphogenesis. Through in silico analysis of the laminin-encoding genes, we identified a novel family of alternate splice isoforms derived from the 5'-end of the LAMA3 and LAMA5 genes. These isoforms resemble the netrins in that they contain a laminin N-terminal domain followed by a short stretch of laminin-type epidermal growth factor-like repeats. We suggest the terms LaNt (laminin N terminus) alpha3 and LaNt alpha5, for the predicted protein products of these mRNAs. RT-PCR confirmed the presence of these transcripts at the mRNA level. Moreover, they exhibit differential, tissue-specific, expression profiles. To confirm the existence of LaNt alpha3 protein, we generated an antibody to a unique domain within the putative polypeptide. This antibody recognizes a protein at the predicted molecular mass of 64 kDa by immunoblotting. Furthermore, immunofluorescence analyses revealed a basement membrane staining in epithelial tissue for LaNt alpha3 and LaNt alpha3 localized along the substratum-associated surface of cultured keratinocytes. We have also tested the functionality LaNt alpha3 through RNAi-mediated knockdown. Keratinocytes exhibiting specific knockdown of LaNt alpha3 displayed impaired adhesion, stress resistance, and reduced ability to close scratch wounds in vitro.

Bidirectional extracellular matrix signaling during tissue morphogenesis

Normal tissue development and function are regulated by the interplay between cells and their surrounding extracellular matrix (ECM). The ECM provides biochemical and mechanical contextual information that is conveyed from the cell membrane through the cytoskeleton to the nucleus to direct cell phenotype. Cells, in turn, remodel the ECM and thereby sculpt their local microenvironment. Here we review the mechanisms by which cells interact with, respond to, and influence the ECM, with particular emphasis placed on the role of this bidirectional communication during tissue morphogenesis. We also discuss the implications for successful engineering of functional tissues ex vivo.

Comparison of the results obtained by ELISA and surface plasmon resonance for the determination of antibody affinity

The aim of this study was to compare the affinity values obtained for a monoclonal antibody/antigen complex using two different techniques, surface plasmon resonance (SPR) and an enzyme linked immunosorbent assay (ELISA) approach recently described by Bobrovnik S.A. and by Stevens F.J. These two techniques can be used in particular to determine the equilibrium dissociation constant, K(D), of the complex in solution or on a surface. Bobrovnik's method gives two K(D) values that differ by a factor of 100, demonstrating that two populations of complexes are present in solution. In an initial step, one protein binds relatively weakly to the other (high K(D)) and this is followed by a conformational change in the most flexible portion of the antigen, which increases the affinity (low K(D)). Only the higher of the two K(D) values can be detected when complex formation in solution is investigated using SPR, because the interaction measured concerns the fibronectin/antibody complexes of lowest affinity. In contrast, when measuring association at the sensor surface, SPR gives an average result between the two K(D) values because complexes corresponding to both affinities can form in this situation. The constants that characterise the kinetics of the fibronectin-antibody interaction obtained by SPR and ELISA are therefore different, because the methods do not allow the same phenomena to be observed. However they are consistent and complementary.

Fibronectin maintains survival of mouse natural killer (NK) cells via CD11b/Src/beta-catenin pathway

Tissue microenvironment and stroma-derived extracellular matrix (ECM) molecules play important roles in the survival and differentiation of cells. Mouse natural killer (NK) cells usually die within 24 hours once isolated ex vivo. Exogenous cytokines such as interleukin-12 (IL-12) and IL-15 are required to maintain the survival and activity of mouse NK cells cultured in vitro. Whether and how ECM molecules such as fibronectin can support the survival of NK cells remain unknown. We demonstrate that fibronectin, just like IL-15, can maintain survival of mouse NK cells in vitro. Furthermore, we show that fibronectin binds to the CD11b on NK cells, and then CD11b recruits and activates Src. Src can directly interact with beta-catenin and trigger nuclear translocation of beta-catenin. The activation of beta-catenin promotes extracellular signal-related kinase (ERK) phosphorylation, resulting in the increased expression of antiapoptotic protein B-cell leukemia 2 (Bcl-2), which may contribute to the maintenance of NK-cell survival. Consistently, fibronectin cannot maintain the survival of CD11b(-) NK cells and beta-catenin-deficient NK cells in vitro, and the number of NK cells is dramatically decreased in the beta-catenin-deficient mice. Therefore, fibronectin can maintain survival of mouse NK cells by activating ERK and up-regulating Bcl-2 expression via CD11b/Src/beta-catenin pathway.

Running GAGs: myxoid matrix in tumor pathology revisited: what's in it for the pathologist?

Ever since Virchow introduced the entity myxoma, abundant myxoid extracellular matrix (ECM) has been recognized in various reactive and neoplastic lesions. Nowadays, the term "myxoid" is commonly used in daily pathological practice. But what do today's pathologists mean by it, and what does the myxoid ECM tell the pathologist? What is known about the exact composition and function of the myxoid ECM 150 years after Virchow? Here, we give an overview of the composition and constituents of the myxoid ECM as known so far and demonstrate the heterogeneity of the myxoid ECM among different tumors. We discuss the possible role of the predominant constituents of the myxoid ECM and attempt to relate them to differences in clinical behavior. Finally, we will speculate on the potential relevance of this knowledge in daily pathological practice.

Immunotherapeutic strategies in kidney cancer--when TKIs are not enough

FDA approval of the multitargeted, antiangiogenic tyrosine kinase inhibitors (TKIs) sunitinib and sorafenib, and the serine and threonine mammalian target of rapamycin inhibitor, temsirolimus, has revolutionized the management of metastatic clear-cell renal-cell carcinoma (CC-RCC). The inability of these targeted therapies to provide durable complete responses, however, is a serious limiting factor to their clinical usefulness. Although immunotherapeutic approaches in advanced disease are increasingly regarded as a historical treatment paradigm, we propose that a fundamental understanding of immunobiology in CC-RCC can improve the selection of patients for high-dose intravenous interleukin 2 and facilitate the development of novel immunotherapeutic strategies. In our opinion, immunotherapeutic strategies have an important place in the management of advanced CC-RCC in the era of biological targeted therapy.

Extracellular matrix molecules: potential targets in pharmacotherapy

The extracellular matrix (ECM) consists of numerous macromolecules classified traditionally into collagens, elastin, and microfibrillar proteins, proteoglycans including hyaluronan, and noncollagenous glycoproteins. In addition to being necessary structural components, ECM molecules exhibit important functional roles in the control of key cellular events such as adhesion, migration, proliferation, differentiation, and survival. Any structural inherited or acquired defect and/or metabolic disturbance in the ECM may cause cellular and tissue alterations that can lead to the development or progression of disease. Consequently, ECM molecules are important targets for pharmacotherapy. Specific agents that prevent the excess accumulation of ECM molecules in the vascular system, liver, kidney, skin, and lung; alternatively, agents that inhibit the degradation of the ECM in degenerative diseases such as osteoarthritis would be clinically beneficial. Unfortunately, until recently, the ECM in drug discovery has been largely ignored. However, several of today's drugs that act on various primary targets affect the ECM as a byproduct of the drugs' actions, and this activity may in part be beneficial to the drugs' disease-modifying properties. In the future, agents and compounds targeting directly the ECM will significantly advance the treatment of various human diseases, even those for which efficient therapies are not yet available.

Fibronectins in vascular morphogenesis

Fibronectin is an extracellular matrix protein found only in vertebrate organisms containing endothelium-lined vasculature and is required for cardiovascular development in fish and mice. Fibronectin and its splice variants containing EIIIA and EIIIB domains are highly upregulated around newly developing vasculature during embryogenesis and in pathological conditions including atherosclerosis, cardiac hypertrophy, and tumorigenesis. However, their molecular roles in these processes are not entirely understood. We review genetic studies examining functions of fibronectin and its splice variants during embryonic cardiovascular development, and discuss potential roles of fibronectin in vascular disease and tumor angiogenesis.

Identification of alternative splicing markers for breast cancer

Breast cancer is the most common cause of cancer death among women under age 50 years, so it is imperative to identify molecular markers to improve diagnosis and prognosis of this disease. Here, we present a new approach for the identification of breast cancer markers that does not measure gene expression but instead uses the ratio of alternatively spliced mRNAs as its indicator. Using a high-throughput reverse transcription-PCR-based system for splicing annotation, we monitored the alternative splicing profiles of 600 cancer-associated genes in a panel of 21 normal and 26 cancerous breast tissues. We validated 41 alternative splicing events that significantly differed in breast tumors relative to normal breast tissues. Most cancer-specific changes in splicing that disrupt known protein domains support an increase in cell proliferation or survival consistent with a functional role for alternative splicing in cancer. In a blind screen, a classifier based on the 12 best cancer-associated splicing events correctly identified cancer tissues with 96% accuracy. Moreover, a subset of these alternative splicing events could order tissues according to histopathologic grade, and 5 markers were validated in a further blind set of 19 grade 1 and 19 grade 3 tumor samples. These results provide a simple alternative for the classification of normal and cancerous breast tumor tissues and underscore the putative role of alternative splicing in the biology of cancer.