Alternative splicing of chicken fibronectin in embryos and in normal and transformed cells

Adaptation of the group A Streptococcus adhesin Scl1 to bind fibronectin type III repeats within wound-associated extracellular matrix: implications for cancer therapy

The human‐adapted pathogen group A Streptococcus (GAS) utilizes wounds as portals of entry into host tissue, wherein surface adhesins interact with the extracellular matrix, enabling bacterial colonization. The streptococcal collagen‐like protein 1 (Scl1) is a major adhesin of GAS that selectively binds to two fibronectin type III (FnIII) repeats within cellular fibronectin, specifically the alternatively spliced extra domains A and B, and the FnIII repeats within tenascin‐C. Binding to FnIII repeats was mediated through conserved structural determinants present within the Scl1 globular domain and facilitated GAS adherence and biofilm formation. Isoforms of cellular fibronectin that contain extra domains A and B, as well as tenascin‐C, are present for several days in the wound extracellular matrix. Scl1‐FnIII binding is therefore an example of GAS adaptation to the host's wound environment. Similarly, cellular fibronectin isoforms and tenascin‐C are present in the tumor microenvironment. Consistent with this, FnIII repeats mediate GAS attachment to and enhancement of biofilm formation on matrices deposited by cancer‐associated fibroblasts and osteosarcoma cells. These data collectively support the premise for utilization of the Scl1‐FnIII interaction as a novel method of anti‐neoplastic targeting in the tumor microenvironment.

Multifaced Roles of the αvβ3 Integrin in Ehlers-Danlos and Arterial Tortuosity Syndromes' Dermal Fibroblasts

The αvβ3 integrin, an endothelial cells’ receptor-binding fibronectin (FN) in the extracellular matrix (ECM) of blood vessels, regulates ECM remodeling during migration, invasion, angiogenesis, wound healing and inflammation, and is also involved in the epithelial mesenchymal transition. In vitro-grown human control fibroblasts organize a fibrillar network of FN, which is preferentially bound on the entire cell surface to its canonical α5β1 integrin receptor, whereas the αvβ3 integrin is present only in rare patches in focal contacts. We report on the preferential recruitment of the αvβ3 integrin, due to the lack of FN–ECM and its canonical integrin receptor, in dermal fibroblasts from Ehlers–Danlos syndromes (EDS) and arterial tortuosity syndrome (ATS), which are rare multisystem connective tissue disorders. We review our previous findings that unraveled different biological mechanisms elicited by the αvβ3 integrin in fibroblasts derived from patients affected with classical (cEDS), vascular (vEDS), hypermobile EDS (hEDS), hypermobility spectrum disorders (HSD), and ATS. In cEDS and vEDS, respectively, due to defective type V and type III collagens, αvβ3 rescues patients’ fibroblasts from anoikis through a paxillin-p60Src-mediated cross-talk with the EGF receptor. In hEDS and HSD, without a defined molecular basis, the αvβ3 integrin transduces to the ILK-Snail1-axis inducing a fibroblast-to-myofibroblast-transition. In ATS cells, the deficiency of the dehydroascorbic acid transporter GLUT10 leads to redox imbalance, ECM disarray together with the activation of a non-canonical αvβ3 integrin-TGFBRII signaling, involving p125FAK/p60Src/p38MAPK. The characterization of these different biological functions triggered by αvβ3 provides insights into the multifaced nature of this integrin, at least in cultured dermal fibroblasts, offering future perspectives for research in this field.

Interepithelial signaling with nephric duct is required for the formation of overlying coelomic epithelial cell sheet

In most organs of the body, epithelial tissues are supported by their own basement membrane and underlying stroma, the latter being regarded as a complex of amorphous cells, extracellular matrices, and soluble factors. We demonstrate here that an epithelial tube can serve as a component of stroma that supports the formation of epithelial cell sheet derived from a different origin. During development of the mesonephros in chicken embryos, the intermediate mesoderm (IMM), which contains the Wolffian duct (WD) and its associated tubules, is overlain by a sheet of epithelial cells derived from lateral plate (coelomic) mesoderm. We describe that in normal embryos, epitheliogenesis of IMM tubes and the adjacent coelomic cell sheet proceed in a coordinated manner. When the WD was surgically ablated, the overlying coelomic epithelium exhibited aberrant morphology accompanied by a punctated basement membrane. Furthermore, the WD-ablated coelomic epithelium became susceptible to latent external stress; electroporation of Rac1 resulted in epithelial-to-mesenchymal transitions (EMTs) within the coelomic epithelium. The distorted coelomic epithelium was rescued by implanting fibronectin-producing cells in place of the WD, suggesting that fibronectin provided by WD has an important role acting interepithelially. This notion was corroborated further by directly visualizing a translocation of EGFP-tagged fibronectin from fibronectin-producing to -receiving epithelia in vivo. Our findings provide a novel insight into interepithelial signaling that also might occur in adult tissues to protect against EMT and suggest a possible new target for anticancer therapeutic strategy.

EDA-containing fibronectin increases proliferation of embryonic stem cells

Embryonic stem cells (ESC) need a set of specific factors to be propagated. They can also grow in conditioned medium (CM) derived from a bovine granulosa cell line BGC (BGC-CM), a medium that not only preserves their main features but also increases ESC´s proliferation rate. The mitogenic properties of this medium were previously reported, ascribing this effect to an alternative spliced generated fibronectin isoform that contains the extra domain A (FN EDA⁺). Here, we investigated if the FN EDA⁺ isoform increased proliferation of mouse and human ES cells. We analyzed cell proliferation using conditioned media produced by different mouse embryonic fibroblast (MEF) lines genetically engineered to express FN constitutively including or excluding the EDA domain (FN EDA^-), and in media supplemented with recombinant peptides containing or not the EDA. We found that the presence of EDA in the medium increased mouse and human ESC’s proliferation rate. Here we showed for the first time that this FN isoform enhances ESC’s proliferation. These findings suggest a possible conserved behavior for regulation of ES cells proliferation by this FN isoform and could contribute to improve their culturing conditions both for research and cell therapy.

Fibronectins, their fibrillogenesis, and in vivo functions

Fibronectin (FN) is a multidomain protein with the ability to bind simultaneously to cell surface receptors, collagen, proteoglycans, and other FN molecules. Many of these domains and interactions are also involved in the assembly of FN dimers into a multimeric fibrillar matrix. When, where, and how FN binds to its various partners must be controlled and coordinated during fibrillogenesis. Steps in the process of FN fibrillogenesis including FN self-association, receptor activities, and intracellular pathways have been under intense investigation for years. In this review, the domain organization of FN including the extra domains and variable region that are controlled by alternative splicing are described. We discuss how FN-FN and cell-FN interactions play essential roles in the initiation and progression of matrix assembly using complementary results from cell culture and embryonic model systems that have enhanced our understanding of this process.

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.

BMP2 and BMP7 play antagonistic roles in feather induction

Feathers, like hairs, first appear as primordia consisting of an epidermal placode associated with a dermal condensation that is necessary for the continuation of their differentiation. Previously, the BMPs have been proposed to inhibit skin appendage formation. We show that the function of specific BMPs during feather development is more complex. BMP2 and BMP7, which are expressed in both the epidermis and the dermis, are involved in an antagonistic fashion in regulating the formation of dermal condensations, and thus are both necessary for subsequent feather morphogenesis. BMP7 is expressed earlier and functions as a chemoattractant that recruits cells into the condensation, whereas BMP2 is expressed later, and leads to an arrest of cell migration, likely via its modulation of the EIIIA fibronectin domain and alpha4 integrin expression. Based on the observed cell proliferation, chemotaxis and the timing of BMP2 and BMP7 expression, we propose a mathematical model, a reaction-diffusion system, which not only simulates feather patterning, but which also can account for the negative effects of excess BMP2 or BMP7 on feather formation.

The monoclonal antibody SM5-1 recognizes a fibronectin variant which is widely expressed in melanoma

Background

Previously we have generated the monoclonal antibody SM5-1 by using a subtractive immunization protocol of human melanoma. This antibody exhibits a high sensitivity for primary melanomas of 99% (248/250 tested) and for metastatic melanoma of 96% (146/151 tested) in paraffin embedded sections. This reactivity is superior to the one obtained by HMB-45, anti-MelanA or anti-Tyrosinase and is comparable to anti-S100. However, as compared to anti-S100, the antibody SM5-1 is highly specific for melanocytic lesions since 40 different neoplasms were found to be negative for SM5-1 by immunohistochemistry. The antigen recognized by SM5-1 is unknown.

Methods

In order to characterize the antigen recognized by mAb SM5-1, a cDNA library was constructed from the metastatic human melanoma cell line SMMUpos in the Uni-ZAP lambda phage and screened by mAb SM5-1. The cDNA clones identified by this approach were then sequenced and subsequently analyzed.

Results

Sequence analysis of nine independent overlapping clones (length 3100–5600 bp) represent fibronectin cDNA including the ED-A, but not the ED-B region which are produced by alternative splicing. The 89aa splicing variant of the IIICS region was found in 8/9 clones and the 120aa splicing variant in 1/9 clones, both of which are included in the CS1 region of fibronectin being involved in melanoma cell adhesion and spreading.

Conclusion

The molecule recognized by SM5-1 is a melanoma associated FN variant expressed by virtually all primary and metastatic melanomas and may play an important role in melanoma formation and progression. This antibody is therefore not only of value in immunohistochemistry, but potentially also for diagnostic imaging and immunotherapy.

Novel form of fibronectin from zebrafish mediates infectious hematopoietic necrosis virus infection

The presence of a novel form of zebrafish fibronectin (FN2) on the cell surface increased the cell's susceptibility to infection by infectious hematopoietic necrosis virus (IHNV). Unlike other fibronectins, FN2 possesses a truncated structure and accumulates on the cell surface instead of in the extracellular matrix. Fish embryo cells expressing recombinant FN2 were more susceptible to IHNV infection, with a greater percentage of cells exhibiting cytopathic effect (CPE) compared to nontransfected control cells. Incubation of nontransfected cells with soluble recombinant FN2 increased IHNV infection, as measured by plaque assay. The number of plaques increased in correlation with the amount of protein added and the length of time that cells were incubated with the protein. Incubation of IHNV with soluble FN2 before addition to cells also increased infection. FN2 immobilized on the culture surface inhibited IHNV infection. The results indicate that FN2 present on the cell surface is able to mediate IHNV attachment and cell entry.

Transcription factor ATF3 partially transforms chick embryo fibroblasts by promoting growth factor-independent proliferation

Activating Transcription Factor 3 (ATF3) is a member of the bZip family of transcription factors. Previous studies in mammalian cells suggested that like other bZip family members e.g. Jun and Fos, ATF3 might play a role in the control of cell proliferation and participate in oncogenic transformation. To investigate this putative ATF3 function directly, the rat ATF3 protein was compared with v-Jun for its ability to transform primary cultures of chick embryo fibroblasts (CEFs). Like CEFs accumulating v-Jun, CEFs accumulating the ATF3 protein displayed a typical, fusiform morphology, associated with an enhanced capacity to grow in medium with reduced amount of serum. However, in contrast to v-Jun-transformed CEFs, the ATF3 overexpressing cells could not promote colony formation from single cells in agar. Partial transformation induced by ATF3 was found to be associated with repression of multiple cellular genes that are also down-regulated by v-Jun, including those coding for the extracellular components fibronectin, decorin, thrombospondin 2, and the pro-apoptotic protein Par-4. These data demonstrate that, at least in primary avian cells, rat ATF3 possesses an intrinsic oncogenic potential. Moreover, the results suggest that ATF3 might induce growth factor independence by down-regulating a subset of the genes repressed by v-Jun.

Fibronectin regulates assembly of actin filaments and focal contacts in cultured cells via the heparin-binding site in repeat III13

Fibroblasts, when plated on the extracellular matrix protein fibronectin (FN), rapidly spread and form an organized actin cytoskeleton. This process is known to involve both the central alpha5beta1 integrin-binding and the C-terminal heparin-binding regions of FN. We found that within the heparin-binding region, the information necessary for inducing organization of stress fibers and focal contacts was located in a 29-amino acid segment of FN type III module 13 (III13). We did not find a cytoskeleton-organizing role for repeat III14, which had previously been implicated in this process. Within III13, the same five basic amino acids known to be most important for heparin binding were also necessary for actin organization. A substrate of III13 alone was only weakly adhesive but strongly induced formation of filopodia and lamellipodia. Stress fiber formation required a combination of III13 and III7-11 (which contains the integrin alpha5beta1 recognition site), either as a single fusion protein or as separate polypeptides, and the relative amounts of the two binding sites appeared to determine whether stress fibers or filopodia and lamellipodia were the predominant actin structures formed. We propose that a balance of signals from III13 and from integrins regulates the type of actin structures assembled by the cell.

Crystal structure of a heparin- and integrin-binding segment of human fibronectin

The crystal structure of human fibronectin (FN) type III repeats 12-14 reveals the primary heparin-binding site, a clump of positively charged residues in FN13, and a putative minor site approximately 60 A away in FN14. The IDAPS motif implicated in integrin alpha4beta1 binding is at the FN13-14 junction, rendering the critical Asp184 inaccessible to integrin. Asp184 clamps the BC loop of FN14, whose sequence (PRARI) is reminiscent of the synergy sequence (PHSRN) of FN9. Mutagenesis studies prompted by this observation reveal that both arginines of the PRARI sequence are important for alpha4beta1 binding to FN12-14. The PRARI motif may represent a new class of integrin-binding sites. The spatial organization of the binding sites suggests that heparin and integrin may bind in concert.

Modulation of cell proliferation and differentiation through substrate-dependent changes in fibronectin conformation

Integrin-mediated cell adhesion to extracellular matrices provides signals essential for cell cycle progression and differentiation. We demonstrate that substrate-dependent changes in the conformation of adsorbed fibronectin (Fn) modulated integrin binding and controlled switching between proliferation and differentiation. Adsorption of Fn onto bacterial polystyrene (B), tissue culture polystyrene (T), and collagen (C) resulted in differences in Fn conformation as indicated by antibody binding. Using a biochemical method to quantify bound integrins in cultured cells, we found that differences in Fn conformation altered the quantity of bound alpha5 and beta1 integrin subunits but not alphav or beta3. C2C12 myoblasts grown on these Fn-coated substrates proliferated to different levels (B > T > C). Immunostaining for muscle-specific myosin revealed minimal differentiation on B, significant levels on T, and extensive differentiation on C. Differentiation required binding to the RGD cell binding site in Fn and was blocked by antibodies specific for this site. Switching between proliferation and differentiation was controlled by the levels of alpha5beta1 integrin bound to Fn, and differentiation was inhibited by anti-alpha5, but not anti-alphav, antibodies, suggesting distinct integrin-mediated signaling pathways. Control of cell proliferation and differentiation through conformational changes in extracellular matrix proteins represents a versatile mechanism to elicit specific cellular responses for biological and biotechnological applications.

HRS/SRp40-mediated inclusion of the fibronectin EIIIB exon, a possible cause of increased EIIIB expression in proliferating liver

Serine-arginine (SR)-rich proteins are believed to be important in mediating alternative pre-mRNA splicing. HRS/SRp40 expression is elevated in liver cell proliferation during development, regeneration, and oncogenesis. We tested whether HRS expression correlates with the appearance of alternatively spliced fibronectin transcripts during liver growth. HRS was highly expressed during the proliferative phase of liver development, correlating with expression of the fibronectin EIIIB alternative exon. In regenerating liver, HRS protein was induced in a time course consistent with the observed increase in fibronectin transcripts containing the EIIIB exon, particularly in nonparenchymal liver cells. Furthermore, in an in vivo assay, HRS, and not other SR proteins, directly mediated EIIIB exon inclusion in the fibronectin transcript. This alternative splicing was dependent on a purine-rich region within the EIIIB exon to which HRS specifically bound. We have established that HRS has the potential to contribute to the regulation of fibronectin pre-mRNA splicing during liver growth. Changes in fibronectin forms may be important in modifying liver architecture during the proliferative response, thus providing a potential mechanism by which SR proteins may participate in cellular growth control.

The role of cell adhesion in retinoic acid-induced modulation of chondrocyte phenotype

Retinoic acid (RA) treatment of a suspension of quail chondrocytes inhibits the expression of cartilage collagens and induces cell adhesion along with fibronectin expression. We asked whether the RA-induced modulation of the chondrocyte phenotype was dependent on cell adhesion. Prevention of cell adhesion blocks cell growth and many of the effects associated with RA, such as collagen II inhibition, collagen I activation and fibronectin induction. The activity of the bone/tendon promoter of the alpha 2(I) collagen gene was determined by measuring the transient expression of COL1A2-CAT, a chimaeric gene bearing 3500 bp from upstream of the transcription start site of the human alpha 2(I) gene fused to the chloramphenicol acetyltransferase (CAT) gene. This promoter is activated only in permissive conditions for cell adhesion. The attachment activities of chondrocytes on protein substrates was studied by an in vitro cell adhesion assay. Untreated cells or cells maintained in suspension while undergoing RA treatment do not attach when replated on protein substrates. Chondrocytes treated with RA in permissive conditions for cell adhesion rapidly attach and spread instead on collagen-coated wells. Altogether the results suggest that cell adhesion plays a major role in RA-induced modulation of the chondrocyte phenotype.

Tear Fluid Cellular Fibronectin Levels After Photorefractive Keratectomy

BACKGROUND

Fibronectin is supposed to have an important role in wound healing. The extradomain A-containing cellular fibronectin (EDAcFn) refers to fibronectin, which instead of being a hepatocyte derived component of blood plasma or body fluids, is produced locally. The present study was undertaken to clarify the possible changes in excretion of EDAcFn in tears following excimer laser photorefractive keratectomy (PRK).

METHODS

An immunoassay was used to determine EDAcFn concentrations in human tear fluid samples of 11 eyes after PRK. Tear fluids were collected with scaled microcapillaries preoperatively as well as 1, 2, and 7 days after PRK. The time used to collect a known volume of tears was registered. This was done to estimate the dilution effect related to the hypersecretion of tears after PRK.

RESULTS

The mean preoperative tear fluid EDAcFn concentration was 0.28 +/- 0.07 ng/microliter with a wide range (0.05 to 0.63). The tear fluid flow-corrected excretion of EDAcFn was 1.36 +/- 0.35 ng/min (range, 0.145 to 3.50). There was a significant increase in both postoperative tear fluid flow and excretion of EDAcFn on days 1 and 2. The elevation of the mean EDAcFn concentration did not decrease in spite of reflex tearing. The mean excretion of EDAcFn in tears was 28-fold on the first and 17-fold on the second postoperative day. Normal level was reached by day 7.

CONCLUSION

There is a rapid increase in excretion of EDAcFn in tears following PRK. This seems to last only as long as an epithelial defect persists. The epithelium of all eyes healed in 3 to 4 days in spite of wide interindividual variations in both tear fluid flow and EDAcFn excretion.

Changes in the fibronectin-specific integrin expression pattern modify the migratory behavior of sarcoma S180 cells in vitro and in the embryonic environment

The molecules that mediate cell-matrix recognition, such as fibronectins (FN) and integrins, modulate cell behavior. We have previously demonstrated that FN and the beta 1-integrins are used during neural crest cell (NCC) migration in vitro as well as in vivo, and that the FN cell-binding domains I and II exhibit functional specificity in controlling either NCC attachment, spreading, or motility in vitro. In the present study, we have analyzed the effect of changes in the integrin expression patterns on migratory cell behavior in vivo. We have generated, after stable transfection, S180 cells expressing different levels of alpha 4 beta 1 or alpha 5 beta 1 integrins, two integrins that recognize distinct FN cell-binding domains. Murine S180 cells were chosen because they behave similarly to NCC after they are grafted into the NCC embryonic pathways in the chicken embryo. Thus, they provide a model system with which to investigate the mechanisms controlling in vitro and in vivo migratory cell behavior. We have observed that either the overexpression of alpha 5 beta 1 integrin or the induction of alpha 4 beta 1 expression in transfected S180 cells enhances their motility on FN in vitro. These genetically modified S180 cells also exhibit different migratory properties when grafted into the early trunk NCC migratory pathways. We observe that alpha 5 and low alpha 4 expressors migrate in both the ventral and dorsolateral paths simultaneously, in contrast to the parental S180 cells or the host NCC, which are delayed by 24 h in their invasion of the dorsolateral path. Moreover, the alpha 4 expressors exhibit different migratory properties according to their level of alpha 4 expression at the cell surface. Cells of the low alpha 4 expressor line invade both the ventral and dorsolateral pathways. In contrast, the high expressors remain as an aggregate at the graft site, possibly the result of alpha 4 beta 1-dependent homotypic aggregation. Thus, changes in the repertoire of FN-specific integrins enable the S180 cells to exploit different pathways in the embryo and regulate the speed with which they disperse in vivo and in culture. Our studies correlate well with known changes in integrin expression during neural crest morphogenesis and strongly suggest that neural crest cells that migrate into the dorsolateral path, i.e., melanoblasts, do so only after they have upregulated the expression of FN receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of fibronectin expression in rat regenerating liver

Fibronectin (FN) expression displays a complex regulation that results in precisely defined isoform patterns during different developmental stages, ageing and injury. The qualitative and quantitative changes that are observed derive from modulation of the rate of transcription of the single FN pre-mRNA and its specific differential processing in the EIIIA, EIIIB and V regions of rat FN. The liver is the major source of plasma FN which is characterised by the absence of the EIIIA and EIIIB exons. Here we show that in the rat regenerating liver there is a significant reprogramming of the splicing machinery that results in the synthesis by the liver of up to 17% of EIIIA+ FN linked with all the three V forms. On the other hand the EIIIB+ form is totally absent both in normal and regenerating liver. Furthermore there is a variation of the V pattern observed in the regenerating tissue, the V120 form (linked to both EIIIA+ and EIIIA- messengers) increases from 11 to 32%. The quantitative RT-PCR method was used to estimate the FN transcription rate, before and after partial hepatectomy. We have shown a 3-fold increase in FN mRNA in liver that is specifically linked to the regeneration process and not to the surgical stress.

Expression of variant fibronectins in wound healing: cellular source and biological activity of the EIIIA segment in rat hepatic fibrogenesis

We have examined the cell-specific expression of two fibronectin isoforms, EIIIA and EIIIB, during experimental hepatic fibrosis induced by ligation of the biliary duct. AT the mRNA level, EIIIA and EIIIB were undetectable in normal liver but expressed early injury, preceding fibrosis. The cellular sources of these changes were determined by fractionating the liver at various time points after bile duct ligation into its constituent cell populations and extracting RNA from the fresh isolates. EIIIA-containing fibronectin mRNA was undetectable in normal sinusoidal endothelial cells but increased rapidly within 12 h of injury. By contrast, the EIIIB form was restricted to hepatic lipocytes (Ito or fat-storing cells) and appeared only after a lag of 12-24 h: it was minimal in sinusoidal endothelial cells. Both forms were minimal in hepatocytes. At the protein level, EIIIA-containing fibronectin was markedly increased within two days of injury and exhibited a sinusoidal distribution. Secretion of this form by endothelial cells was confirmed in primary culture. Matrices deposited in situ by endothelial cells from injured liver accelerated the conversion ("activation") of normal lipocytes to myofibroblast-like cells, and pretreatment of matrices with monoclonal antibody to the EIIIA segment blocked this response. Finally, recombinant fibronectin peptide containing the EIIIA segment was stimulatory to lipocytes in culture. We conclude that expression of EIIIA fibronectin by sinusoidal endothelial cells is a critical early event in the liver's response to injury and that the EIIIA segment is biologically active, mediating the conversion of lipocytes to myofibroblasts.

Expression and functional significance of alternatively spliced CS1 fibronectin in rheumatoid arthritis microvasculature

Expression of fibronectin (FN) isoforms containing CS1, a 25-amino acid sequence present within the alternatively spliced IIICS region of FN, has been analyzed in rheumatoid arthritis (RA) synovium. Unexpectedly, CS1-containing FN variants were exclusively found on endothelium but not extracellular matrix (ECM) of RA synovium. Lumenal expression of CS1 on RA endothelial cells, as observed by electron microscopy, correlated with inflammation in RA, since normal synovium expressed little CS1 without appreciable decrease in ECM FN. CS1 expression on human endothelial cells was further shown by FN mRNA analyses. In adhesion assays on frozen RA synovial sections, T lymphoblastoid cells expressing functionally activated alpha 4 beta 1 integrin specifically attached to the intravascular surface of RA endothelium. Binding was abrogated by both anti-alpha 4 integrin and CS1 peptides. Our observations suggest direct involvement of CS1-containing FN in recruitment of alpha 4 beta 1-expressing mononuclear leukocytes in synovitis, and provide basis for therapeutic intervention in RA.

Cellular fibronectin and tenascin in an orbital nylon prosthesis removed because of infection caused by Staphylococcus aureus

An orbital nylon prosthesis was removed because of an infection caused by Staphylococcus aureus that was resistant to antimicrobials. It was processed for histopathology and immunohistochemistry. Within 3 weeks the implant had an extensive ingrowth of fibrovascular tissue containing chronic inflammatory cells, foreign body giant cells, and myofibroblasts. By using the indirect immunofluorescent method, this tissue was found to react with monoclonal antibodies (Mabs) against extradomain A of cellular fibronectin (EDA-cFN) and tenascin (TN). The presence of EDA-cFN and TN within the implant are indicative of an active healing process, since both of these proteins, scarce in adult tissues, have been shown to be reexpressed during tissue regeneration. The findings suggest that fibronectin plays a definite role in bacterial adherence and foreign body infections.

A novel tenascin type III repeat is part of a complex of tenascin mRNA alternative splices

Sequence analysis of two human tenascin encoding cDNA clones from a cDNA library of the U251 glioblastoma cell line revealed the presence of a novel 276 bp tenascin type III fibronectin like repeat. This alternatively spliced type III repeat designated AD1 is located between the previously identified repeats 10 and 11 and has sequence homology with human, chicken and mouse tenascin type III repeats. These results show that tenascin has at least 16 consecutive fibronectin like type III repeats. PCR amplification of random primed mRNA with specific type III repeat primers revealed a pattern of multiple alternative splices of AD1 and flanking type III repeats. The alternative splice variants were confirmed by direct sequencing. Differences were observed in the expression of the various alternative splices of tenascin mRNA between tumor and normal cells and may thus indicate differences in tenascin isoform expression and function in normal and tumor cells. PCR and Southern analysis of genomic DNA indicate that AD1 is coded by a single exon present in both human and mouse genome.

Accumulation of fetal fibronectin mRNAs during the development of rat cardiac hypertrophy induced by pressure overload

Cardiac pressure overload induces a shift towards the fetal form of major proteins expressed by the myocytes, and an accumulation of extracellular matrix proteins. One of them, fibronectin (FN), accumulates soon after the imposition of pressure overload. Because FN exists both as cellular FN (c-FN) locally synthesized by nonmuscle cells and as "plasma-FN" (p-FN) synthesized by the hepatocytes, the first issue of this study was to determine whether FN accumulation within the myocardium in response to pressure overload is paralleled by a local increase in mRNA. The expression of c-FN isoforms being developmentally regulated in a tissue-specific manner, the types of FN exons expressed by cardiac cells were analyzed. Pressure overload was induced in 25-d-old rats by stenosis of the thoracic aorta. Using in situ hybridization, we show that the mRNAs encoding the fetal forms of c-FN are accumulated in the interstitial tissue of fetal rat hearts but are absent in adult. 1-3 d after aortic stenosis, the fetal forms of c-FN mRNAs were found in the wall of coronary arteries and in focal areas of the myocardium. Thus nonmuscle cells and smooth muscle cells, like myocytes, do respond to pressure overload by reexpressing fetal gene transcripts.

Adhesion of immature thymocytes to thymic stromal cells through fibronectin molecules and its significance for the induction of thymocyte differentiation

Only 10-15% of unseparated thymocytes adhered to culture plates precoated with fibronectin (FN), but 60-70% of the CD4-8- (double-negative) thymocyte population did. This population bound to FN but not to collagen, laminin, or vitronectin. Its binding to FN was inhibited by anti-FN antibody or a mixture of synthetic peptides corresponding to two different sites of FN, termed the V10 sequence and the RGDS (Arg-Gly-Asp-Ser) sequence, which interact, respectively, with the VLA-4 and VLA-5 FN receptors expressed on T-lineage cells. CD4-8- thymocytes also adhered to a monolayer of a thymic stromal cell clone, MRL104.8a, that induces growth-maintenance and differentiation of such thymocytes. The involvement of FN-FN receptor interaction in this adhesion was demonstrated by the following lines of evidence: (i) the MRL104.8a cells expressed FN molecules on their surface and (ii) the adhesion of CD4-8- thymocytes to MRL104.8a monolayers was almost completely inhibited by simultaneous addition of anti-FN antibody and a mixture of peptides (V10 plus RGDS) capable of binding to anti-FN receptors (VLA-4 and -5). Most important, blocking the adhesion of CD4-8- thymocytes to the thymic stromal cell monolayer resulted in potent inhibition of the differentiation of these thymocytes, which was otherwise induced toward the expression of CD4 and/or CD8 molecules. These results indicate that immature CD4-8- thymocytes adhere to thymic stromal cells preferentially through FN-FN receptor interaction and that such adhesion has a critical role in inducing and/or supporting the differentiation of these thymocytes.

Tissue-specific splicing pattern of fibronectin messenger RNA precursor during development and aging in rat

Fibronectin isoforms are generated by the alternative splicing of a primary transcript derived from a single gene. In rat at least three regions of the molecule are involved: EIIIA, EIIIB, and V. This study investigated the splicing patterns of these regions during development and aging, by means of ribonuclease protection analysis. Between fetal and adult rat, the extent of inclusion of the EIIIA and/or EIIIB region in fibronectin mRNA varied according to the type of tissue analyzed; but the inclusion of the V region, and in particular the V25 alternative variant, was significantly higher in all fetal than in adult tissues. These data suggest a crucial role of the V25 variant, possibly related to its interaction with the alpha 4 beta 1 integrin receptor during development. On the other hand, during aging, the only significant change observed in the splicing pattern was a decrease in the EIIIA variant in brain. The high inclusion levels of the EIIIA and EIIIB regions in young adult brain suggest that these segments may play an important role in differentiated brain tissue. The decreasing levels of inclusion of the EIIIA segment in brain fibronectin mRNA during aging may be an age-related marker with functional consequences.

In vitro splicing of fibronectin pre-mRNAs

We have investigated the alternative splicing of the EIIIB exon of the rat fibronectin gene. Mini-gene constructs containing this exon and portions of adjacent introns and exons, when transfected into HeLa cells, are transcribed and spliced, but omit the EIIIB exon. In vitro, HeLa nuclear extracts similarly splice out (skip) the EIIIB exon from similarly structured transcripts. Therefore, the HeLa splicing apparatus recognizes as atypical the EIIIB exon and its flanking intron sequences, both in vivo and in vitro. We also report that alterations in the ionic conditions of the in vitro splicing reaction can promote the initiation of EIIIB exon inclusion, as reflected by the formation of intermediate and product RNAs related to the removal of the intron upstream of EIIIB. Processing of this intron correlates with the formation of complexes resembling intermediates in spliceosome assembly. The branch sites involved in this alternative processing pathway are rather distant from the EIIIB 3' splice site, and lie within a region which is well conserved in the fibronectin genes of other species. Thus, the intron upstream of EIIIB shows singular structure and behavior which probably have a bearing on the regulated alternative splicing of this exon.

Retroviral expression of alternatively spliced forms of rat fibronectin

We describe the construction in retroviral vectors and the expression of recombinant rat fibronectin (FN) cDNAs corresponding with the various alternatively spliced forms of FN. In NIH 3T3 cells, the exogenous rat FN subunits are efficiently secreted as heterodimers with endogenous mouse subunits. In contrast, in lymphoid WEHI231 cells, there is no endogenous FN synthesis and the recombinant FNs are secreted and can be purified as homogeneous proteins. We show that the purified recombinant FNs are biochemically and biologically functional. In basic assays for adhesion, spreading, cytoskeletal organization, and migration using various established adherent cell lines, different forms of FNs containing the different alternatively spliced segments show no marked differences in activity. We have used these recombinant FNs to investigate three systems in which earlier results had suggested potential differences between different forms of FN. First, all forms tested appear equally active in restoring normal morphology to a transformed cell line. Second, we detect minor differences in their ability to assemble into preexisting extracellular matrices. Finally, we report that only those forms of FN that contain the V segment will promote the spreading of a lymphoid cell line indicating that this segment confers additional biological functions for some cell types, a result that confirms and extends earlier data. These homogeneous, biologically active recombinant FNs will allow further studies of the role of the alternatively spliced segments of FN.

Cell-type-specific expression of alternatively spliced human fibronectin IIICS mRNAs

Fibronectin polypeptide diversity is generated to a large extent by alternative splicing of the fibronectin primary transcript at three sites: two extra domain exons encoding extra structural repeats and a region of nonhomologous sequence termed the type-III connecting segment (IIICS). A novel double primer extension assay was developed to identify and quantify simultaneously each of the five human IIICS mRNA splicing variants. Expression of the five IIICS variants was analyzed in a variety of human normal and tumor cell types as well as in human liver. Differences in IIICS expression patterns were observed among different cell types, among fibroblasts of different tissue origins, and between comparable normal and transformed cells. The most predominant cell-type-specific differences were in the abundance of the one IIICS- mRNA variant relative to the four IIICS+ variants. The percentage of O variant (IIICS-) mRNAs within the total fibronectin mRNA pool varied between 3 and 17% among tumor cells and between 7 and 46% among normal cells. The O variant composed 57% of the fibronectin mRNA in liver tissue, correlating with the previously described increased abundance of IIICS- polypeptide subunits in plasma fibronectin, compared with those in cellular fibronectins. Additional cell-type-specific changes among the expression levels of the four IIICS+ mRNA variants are consistent with a proposed model in which regulation of an alternative selection of a 3'splice site predominates over regulation of the selection of a 5' splice site in generating specific patterns of IIICS mRNA expression.

Cell-type-specific expression of alternatively spliced human fibronectin IIICS mRNAs

Fibronectin polypeptide diversity is generated to a large extent by alternative splicing of the fibronectin primary transcript at three sites: two extra domain exons encoding extra structural repeats and a region of nonhomologous sequence termed the type-III connecting segment (IIICS). A novel double primer extension assay was developed to identify and quantify simultaneously each of the five human IIICS mRNA splicing variants. Expression of the five IIICS variants was analyzed in a variety of human normal and tumor cell types as well as in human liver. Differences in IIICS expression patterns were observed among different cell types, among fibroblasts of different tissue origins, and between comparable normal and transformed cells. The most predominant cell-type-specific differences were in the abundance of the one IIICS- mRNA variant relative to the four IIICS+ variants. The percentage of O variant (IIICS-) mRNAs within the total fibronectin mRNA pool varied between 3 and 17% among tumor cells and between 7 and 46% among normal cells. The O variant composed 57% of the fibronectin mRNA in liver tissue, correlating with the previously described increased abundance of IIICS- polypeptide subunits in plasma fibronectin, compared with those in cellular fibronectins. Additional cell-type-specific changes among the expression levels of the four IIICS+ mRNA variants are consistent with a proposed model in which regulation of an alternative selection of a 3'splice site predominates over regulation of the selection of a 5' splice site in generating specific patterns of IIICS mRNA expression.

Selective secretion of alternatively spliced fibronectin variants

We demonstrate that the alternatively spliced variable (V) region of fibronectin (FN) is required for secretion of FN dimers during biosynthesis. Alternative splicing of the V segment of the rat FN transcript generates three subunit variants (V120, V95, V0) that differ by the inclusion or omission of an additional 120 or 95 amino acids. We are exploring the functions of this segment by expressing variant cDNAs in normal and transformed fibroblasts. Like FN itself, the cDNA-encoded polypeptides (deminectins [DNs]) containing the V120 or V95 segment are efficiently secreted as disulfide-bonded homodimers. However, few homodimers of DNs lacking this region, V0 DNs, are secreted. V0 homodimers do form inside the cell, as demonstrated by biosynthetic analyses of dimer formation and secretion using pulse-chase and time course experiments, but these dimers seldom reach the cell surface and are probably degraded intracellularly. Coexpression of V0 and V120 subunits results in intracellular formation of three types of dimers, V0-V0, V0-V120, and V120-V120, but only the V120-containing dimers are secreted. This selective retention of V0 homodimers indicates that the V region is required for formation and secretion of native FN dimers. In an analogous in vivo situation, we show that plasma FN also lacks V0-V0 dimers and consists of V0-V+ and V+-V+ combinations. Dissection of V region sequences by deletion mapping localizes the major site involved in DN dimer secretion to an 18-amino acid segment within V95. In addition, high levels of dimer secretion can be restored by insertion of V into a heterologous site 10 kD COOH terminal to its normal location. We discuss the potential role of intracellular protein-protein interactions in FN dimer formation.

Nucleotide substitutions within the cardiac troponin T alternative exon disrupt pre-mRNA alternative splicing

The cardiac troponin T (cTNT) pre-mRNA contains a single alternative exon (exon 5) which is either included or excluded from the processed mRNA. Using transient transfection of cTNT minigenes, we have previously localized pre-mRNA cis elements required for exon 5 alternative splicing to three small regions of the pre-mRNA which include exons 4, 5, and 6. In the present study, nucleotide substitutions were introduced into the region containing exon 5 to begin to define specific nucleotides required for exon 5 alternative splicing. A mutation within the 5' splice site flanking the cTNT alternative exon that increases its homology to the consensus sequence improves splicing efficiency and leads to increased levels of mRNAs that include the alternative exon. Surprisingly, substitution of as few as four nucleotides within the alternative exon disrupts cTNT pre-mRNA alternative splicing and prevents recognition of exon 5 as a bona fide exon. These results establish that the cTNT alternative exon contains information in cis that is required for its recognition by the splicing machinery.

Reappearance of an embryonic pattern of fibronectin splicing during wound healing in the adult rat

The adhesive extracellular matrix glycoprotein fibronectin (FN) is thought to play an important role in the cell migration associated with wound healing. Immunolocalization studies show abundant FN in healing wounds; however, these studies cannot define the cellular site(s) of FN synthesis, nor do they distinguish the different and potentially functionally distinct forms of FN that can arise from alternative splicing of the primary gene transcript. To examine these questions of FN synthesis and splicing during wound healing, we have performed in situ hybridization with segment-specific probes on healing wounds in adult rat skin. We find that the FN gene is expressed at increased levels after wounding both in the cells at the base of the wound and in subjacent muscle and dermis lateral to the wound. Interestingly, however, the pattern of splicing of FN mRNA was different in these areas. In adjacent dermis and muscle, the splicing pattern remains identical with that seen in normal adult rat skin, with two of the three spliced segments (EIIIA and EIIIB) excluded from FN mRNA. In contrast, these two segments are included in the FN mRNA present in the cells at the base of the wound. As a result, the mRNA in this region is spliced in a pattern identical with that found during early embryogenesis. The finding that the pattern of FN splicing during wound healing resembles an embryonic pattern suggests that alternative splicing may be used during wound healing as a mechanism to generate forms of FN that may be functionally more appropriate for the cell migration and proliferation associated with tissue repair.

Expression of extra domain A fibronectin sequence in vascular smooth muscle cells is phenotype dependent

Different fibronectin (FN) variants arise from the single gene transcript alternatively spliced in a tissue-specific manner (Hynes, R. O. 1985. Annu. Rev. Cell Biol. 1:67-90; Owens, R. J., A. R. Kornblihtt, and F. E. Baralle. 1986. Oxf. Surv. Eurcaryotic Genes. 3:141-160). We used mAb IST-9, specific for extra domain A (ED-A) FN sequence, and cDNA probe to ED-A exon to determine whether ED-A is present in FN synthesized by vascular smooth muscle cells (SMCs) and, if so, whether expression of ED-A is SMC phenotype dependent. ED-A-containing FN (A-FN) was not revealed in tunica media of human arteries and normal rat aorta by immunofluorescence and immunoblotting techniques. A cDNA probe to ED-A exon did not hybridize with RNA isolated from human aortic media. A positive reaction with IST-9 was observed in (a) diffuse intimal thickening and atherosclerotic plaque from human arteries; (b) experimentally induced intimal thickening in rat aorta; and (c) cultured vascular SMCs. A-FN mRNA was present in the RNA preparation from human aortic intima as judged by hybridization with cDNA probe to ED-A. On the other hand, an mAb interacting with an epitope common for all FN variants revealed FN in both intima and media of human arteries and in the normal rat aorta. A cDNA probe to a sequence shared by all FN variants hybridized with RNA from both intima and media of human aorta, though the level of expression was higher in intima. The data suggest that ED-A exon is omitted during splicing of the FN mRNA precursor in medial SMCs while the expression of A-FN is characteristic of "modulated" SMCs--those of intimal thickenings, of atherosclerotic lesions, and growing in culture.

A tumor-associated fibronectin isoform generated by alternative splicing of messenger RNA precursors

Fibronectin (FN) represents the mixture of a number of structurally different molecules (isoforms) whose make-up varies depending on the FN sources. FN from cultured transformed human cells has a very different isoform composition with respect to its normal counterpart. In fact, SV-40-transformed WI-38VAI3 human fibroblasts produce high levels of a FN isoform (B-FN) which is very poorly expressed in their normal, WI-38, counterpart. We have recently demonstrated that the B-FN isoform derives from a differential splicing pattern of the FN primary transcript which leads, in transformed cells, to a high level expression of the exon ED-B (Zardi, L., B. Carnemolla, A. Siri, T. E. Petersen, G. Paolella, G. Sebastio, and F. E. Baralle. 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:2337-2342). Here we report on the production and characterization of a monoclonal antibody (BC-1) which recognizes an epitope within the protein sequence coded for by the ED-B exon. This monoclonal antibody makes it possible to carry out immunohistochemical analysis of the distribution of the ED-B-containing FN isoform (B-FN) in human tissues. The results show that while in normal, adult, human tissues total FN has a widespread distribution, the B-FN isoform is restricted only to synovial cells, to some vessels and areas of the interstitium of the ovary, and to the myometrium. On the contrary, the B-FN isoform has a much greater expression in fetal and tumor tissues. These results demonstrate that, in vivo, different FN isoforms have a differential distribution and indicate that the B-FN isoform may play a role in ontogenesis and oncogenetic processes.

Neurite extension of chicken peripheral nervous system neurons on fibronectin: relative importance of specific adhesion sites in the central cell-binding domain and the alternatively spliced type III connecting segment

Fibronectin contains at least two domains that support cell adhesion. One is the central cell-binding domain that is recognized by a variety of cell types, including fibroblasts. The second, originally identified by its ability to support melanoma cell adhesion, is located in the alternatively spliced type III connecting segment (IIICS). Using specific adhesive ligands and inhibitory probes, we have examined the role of each of these domains in fibronectin-mediated neurite extension of neurons from chick embryo dorsal root and sympathetic ganglia. In studies using explanted ganglia, both fl3, a 75-kD tryptic fragment of human plasma fibronectin containing the central cell-binding domain, and CS1-IgG, a synthetic peptide-IgG conjugate containing the principal cell adhesion site from the IIICS, supported neurite outgrowth after adsorption onto the substrate. The maximal activities of fl3 and CSl-IgG were 45-55% and 25-30% that of intact fibronectin, respectively. Co-coating of the substrate with f13 and CS1-IgG produced an additive stimulation of neurite outgrowth, the extent of which approached that obtained with fibronectin. Similar results were obtained with purified neuronal cell preparations isolated by tryptic dissociation of dorsal root ganglia. In complementary studies, blockage of the adhesive function of either the central cell-binding domain (with mAb 333, an antiadhesive monoclonal antibody) or the IIICS (with CS1 peptide), resulted in approximately 60 or 30% reduction in fibronectin-mediated neurite outgrowth, respectively. When tested in combination, the inhibitory activities of mAb 333 and CSl were additive. From these results, we conclude that neurons from the peripheral nervous system can extend neurites on both the central cell-binding domain and the IIICS region of fibronectin, and that these cells are therefore the first normal, embryonic cell type shown to adhere to the IIICS. These results suggest that spatiotemporal fluctuations in the alternative mRNA splicing of the IIICS region of fibronectin may be important in regulation of cell adhesive events during development of the peripheral nervous system.