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.

Characterization of mouse fibronectin alternative mRNAs reveals an unusual isoform present transiently during liver development

Fibronectins are found in many extracellular matrices as well as being abundant plasma proteins. The plasma isoforms of fibronectin, which are synthesized in the adult by liver hepatocytes, differ from those derived from most other cells and tissues due to alternative mRNA splicing. Studies in several vertebrates have indicated that FN alternative splicing is regulated spatially and temporally during development. The mouse represents an attractive organism in which to study the regulation of fibronectin splicing during development, but the patterns of fibronectin alternative splicing were not known for this species. Mouse fibronectin cDNA clones were isolated and sequenced, revealing > 95% identity with rat fibronectin at the amino acid level; all three segments that undergo alternative splicing are well conserved. RNase protection and RT-PCR were used to determine the patterns of alternative splicing that occur in fibroblasts and adult liver, sources of cellular and plasma fibronectins. Only A-B-mRNAs were detected in liver, and three V region variants were observed, corresponding to the protein isoforms V120, V95, and V0. Fibroblasts produced mRNAs that were heterogeneous for A and B splicing, but all RNAs contained V120. These patterns contrast with the embryonic form (B+A+V120). Characterization of fibronectin mRNAs from livers of fetal and newborn mice revealed that a significant level of B+ mRNA was present throughout late gestation, declining at birth. Little A+ mRNA was present, and the adult liver V region pattern was observed at all stages. Thus, fibronectin splicing changes during liver development are noncoordinate. One consequence of this temporal regulation is the transient synthesis of B+ mRNAs, including a novel isoform, B+A-V0.

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.

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.

Type III and V collagens modulate the expression and assembly of EDA(+) fibronectin in the extracellular matrix of defective Ehlers-Danlos syndrome fibroblasts

BACKGROUND

Alternative splicing of EDA fibronectin (FN) region is a cell type- and development-regulated mechanism controlled by pathological processes, growth factors and extracellular matrix (ECM). Classic and vascular Ehlers-Danlos syndrome (cEDS and vEDS) are connective tissue disorders caused by COL5A1/COL5A2 and COL3A1 gene mutations, leading to an in vivo abnormal collagen fibrillogenesis and to an in vitro defective organisation in the ECM of type V (COLLV) and type III collagen (COLLIII). These defects induce the FN-ECM disarray and the decrease of COLLs and FN receptors, the α2β1 and α5β1 integrins. Purified COLLV and COLLIII restore the COLL-FN-ECMs in both EDS cell strains.

METHODS

Real-time PCR, immunofluorescence microscopy, and Western blotting were used to investigate the effects of COLLs on FN1 gene expression, EDA region alternative splicing, EDA(+)-FN-ECM assembly, α5β1 integrin and EDA(+)-FN-specific α9 integrin subunit organisation, α5β1 integrin and FAK co-regulation in EDS fibroblasts.

RESULTS

COLLV-treated cEDS and COLLIII-treated vEDS fibroblasts up-regulate the FN1 gene expression, modulate the EDA(+) mRNA maturation and increase the EDA(+)-FN levels, thus restoring a control-like FN-ECM, which elicits the EDA(+)-FN-specific α9β1 integrin organisation, recruits the α5β1 integrin and switches on the FAK binding and phosphorylation.

CONCLUSION

COLLs regulate the EDA(+)-FN-ECM organisation at transcriptional and post-transcriptional level and activate the α5β1-FAK complexes. COLLs also recruit the α9β1 integrin involved in the assembly of the EDA(+)-FN-ECM in EDS cells.

GENERAL SIGNIFICANCE

The knowledge of the COLLs-ECM role in FN isotype expression and in EDA(+)-FN-ECM-mediated signal transduction adds insights in the ECM remodelling mechanisms in EDS cells.

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.

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.

Redefining the role of ectoderm in somitogenesis: a player in the formation of the fibronectin matrix of presomitic mesoderm

The absence of ectoderm impairs somite formation in cultured presomitic mesoderm (PSM) explants, suggesting that an ectoderm-derived signal is essential for somitogenesis. Here we show in chick that the standard enzymatic treatments used for explant isolation destroy the fibronectin matrix surrounding the anterior PSM, which fails to form somites when cultured for 6 hours. By contrast, explants isolated with collagenase retain their fibronectin matrix and form somites under identical culture conditions. The additional presence of ectoderm enhances somite formation, whereas endoderm has no effect. Furthermore, we show that pancreatin-isolated PSM explants cultured in fibronectin-supplemented medium, form significantly more somites than control explants. Interestingly, ectoderm is the major producer of fibronectin (Fn1) transcripts, whereas all but the anterior-most region of the PSM expresses the fibronectin assembly receptor, integrin alpha5 (Itga5). We thus propose that the ectoderm-derived fibronectin is assembled by mesodermal alpha5beta1 integrin on the surface of the PSM. Finally, we demonstrate that inhibition of fibronectin fibrillogenesis in explants with ectoderm abrogates somitogenesis. We conclude that a fibronectin matrix is essential for morphological somite formation and that a major, previously unrecognised role of ectoderm in somitogenesis is the synthesis of fibronectin.

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.

Identification and characterization of a second fibronectin gene in zebrafish

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

Extra-domain-A fibronectin: a new marker of fibrosis in cutaneous graft-versus-host disease

One of the major complications that limit the success of allogeneic stem cell transplantation is graft-versus-host disease (GVHD). The major target organ in GVHD is the skin. Cutaneous GVHD can eventually lead to fibrosis of the skin. Fibronectin mediates a variety of cellular interactions with the extracellular matrix. The molecular and functional diversity of fibronectin (FN) arises from alternative splicing of pre-mRNA. In normal circumstances endothelial cells and fibroblasts synthesize FN without the ED-A domain. In tissue repair and pathologic circumstances such as fibrosis, the ED-A domain is expressed. We hypothesize that expression of ED-A FN is upregulated in patients with cutaneous GVHD. In frozen skin biopsies the expression of ED-A FN was measured at the protein level by immunohistochemistry and at the mRNA level by quantitative real-time PCR (qPCR). In normal control skin, immunohistochemistry showed slight deposits of ED-A FN just under the basal layer. The expression of ED-A FN significantly increased in acute cutaneous GVHD (p<0.05) and ED-A FN was strongly deposited in chronic cutaneous GVHD (p<0.001). Quantitative PCR also showed increased expression of ED-A FN mRNA in acute and chronic cutaneous GVHD compared with normal control skin (p=0.07 and 0.039, respectively). The expression of ED-A FN is increased in the skin of patients with cutaneous GVHD measured both with immunohistochemistry and qPCR. ED-A FN is a new marker of fibrosis in the skin of patients with cutaneous GVHD.

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

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

A truncated form of fibronectin is expressed in fish and mammals

Fibronectin is a highly conserved and well-characterized extracellular matrix protein that plays a crucial role in many cellular processes. In amphibians, birds and mammals, multiple isoforms of fibronectin are generated by differential RNA splicing at three exons. Previously, we identified a novel form of fibronectin (FN2) in zebrafish that possesses a truncated structure and is generated by a unique RNA splicing pattern. Unlike other fibronectins, FN2 exists as a monomer and localizes on the cell surface. Here we show that this novel fibronectin is evolutionarily conserved, found in multiple species of fish and in mice and humans. The unusual structure, unique cellular distribution and conserved pattern of expression indicate that FN2 may have a novel function in normal cellular processes.

Wound healing-associated proteins Hsp47 and fibronectin are elevated in Dupuytren’s contracture

BACKGROUND

Dupuytren's contracture or disease (DD) affects hand function by causing irreversible contraction of the palmar fascia. Histological analysis has shown that DD and wound granulation tissue share many cellular and biochemical characteristics, suggesting that DD may be an exaggerated wound-healing response. The goal of the present study was to examine the possible involvement of two important wound-healing-associated proteins-heat shock protein 47 (Hsp47), fibronectin (Fn), and its oncofetal isoforms-in DD, using clinical tissue samples and primary cell cultures.

MATERIALS AND METHODS

We examined the expression of Hsp47, Fn, and an oncofetal isoform of fibronectin (IIICS) in both normal and disease-matched surgical specimens and primary cell cultures using Western blot analysis, and immunocytochemistry (ICC).

RESULTS

Our results indicate that Hsp47 and total fibronectin is elevated in DD lesional tissue. In addition, Western and ICC analysis of patient-matched (normal and disease) primary cultures show significantly elevated levels of oncofetal fibronectin (IIICS spliced variant) within disease primary cell cultures.

CONCLUSIONS

The high levels of expression of Hsp47 and adult and oncofetal fibronectin in DD suggests that cell-mediated alterations in the extracellular environment may play an important role in the disease process. Furthermore, the involvement of these wound healing-associated proteins in DD supports the notion that this disease may be an exaggerated form of wound healing.

Functional analysis of fibronectin isoforms in chondrogenesis: Full-length recombinant mesenchymal fibronectin reduces spreading and promotes condensation and chondrogenesis of limb mesenchymal cells

Fibronectin (FN), a large dimeric glycoprotein, functions primarily as a connecting molecule in the extracellular matrices of tissues by mediating both cell-matrix and matrix-matrix interactions. All members of the FN family are products of a single FN gene; heterogeneity arises from the alternative splicing of at least three regions (IIIB, IIIA, and V) during processing of a common primary transcript. During chick embryonic limb chondrogenesis, FN structure changes from B+A+ in precartilage mesenchyme to B+A- in differentiated cartilage, and exon IIIA has been shown to be necessary for the process of mesenchymal cellular condensation, a requisite event that precedes overt expression of chondrocyte phenotype. This study aims to investigate the mechanistic action of the FN isoforms in mesenchymal chondrogenesis and, in particular, to identify the specific cellular function in mesenchymal condensation mediated by the mesenchymal (B+A+) FN isoform. Full-length cDNAs corresponding to four splice variants (B+A+, B+A-, B-A+, B-A-) of FN were constructed, and expressed the corresponding proteins using a baculovirus expression vector system. Cell adhesion assays with purified proteins showed that, although the relative levels of cell attachment were approximately the same, chick limb-bud mesenchymal cells spread up to 40 % less on mesenchymal (B+A+) FN than on cartilage (B+A-) FN, (B-A+) FN, or plasma (B-A-) FN. Cellular condensation and chondrogenic differentiation were also promoted in high-density micromass cultures of limb mesenchymal cells plated onto B+A+ FN. These observations suggest that the process of mesenchymal condensation is mediated at least in part by the enhanced ability of chondrogenic mesenchymal cells to migrate and aggregate as a consequence of residing in and interacting with mesenchymal FN. Our findings are consistent with and provide a mechanistic basis for previous observations that rounding of limb mesenchymal cells precedes the onset of chondrogenesis.

Spatial expression of the alternatively spliced EIIIB and EIIIA segments of fibronectin in the early chicken embryo

Using domain-specific antibodies, we have analyzed the tissue distribution of fibronectins (FNs) containing the alternatively spliced EIIIB and EIIIA segments relative to total FN in early chicken embryos. The results show a selective loss of EIIIA+ FN staining in the notochordal sheath and in cartilaginous structures between 4.5 and 7.0 days of development. In other regions, EIIIB+ and EIIIA+ FNs are extensively codistributed in and around mesoderm-derived structures (somites, notochord, heart, and blood vessels), in basal laminae of endoderm and ectoderm-derived structures, as well as within the vicinity of neural crest formation and migration. We also noted that EIIIA staining overlaps with spatial patterns of distribution that have previously been described for the alpha4 integrin subunit, a component of the EIIIA receptor alpha4beta1.

The EIIIA segment of fibronectin is a ligand for integrins alpha 9beta 1 and alpha 4beta 1 providing a novel mechanism for regulating cell adhesion by alternative splicing

Alternative splicing of the fibronectin gene transcript gives rise to forms that include the EIIIA (or ED-A) segment. EIIIA-containing fibronectins are prominently expressed during embryogenesis and wound healing and appear to mediate changes in cell adhesion and gene expression. Nonetheless, integrins that bind the EIIIA segment have not been identified. We previously mapped the epitope for two function-blocking monoclonal antibodies to the C-C' loop region of the EIIIA segment (Liao, Y.-F., Wieder, K. G., Classen, J. M., and Van De Water, L. (1999) J. Biol. Chem. 274, 17876-17884). The sequence of this epitope ((39)PEDGIHELFP(48)) resembles the sequence within tenascin-C to which the integrin alpha(9)beta(1) binds. We now report that either integrin alpha(9)beta(1) or alpha(4)beta(1) can mediate cell adhesion to the EIIIA segment. Moreover, this interaction is blocked both by epitope-mapped EIIIA antibodies as well as by the respective anti-integrins. Deletion mutants of the EIIIA segment that include the C-C' loop and flanking sequence bind cells expressing either alpha(9)beta(1) or alpha(4)beta(1). Adhesion of alpha(4)beta(1)-containing MOLT-3 cells to the EIIIA segment stimulates phosphorylation of p44/42 MAP kinase. Our observation that two integrins bind the EIIIA segment establishes a novel mechanism by which cell adhesion to fibronectin is regulated by alternative splicing.

Regulation of fibronectin alternative splicing by a basement membrane-like extracellular matrix

Hepatocytes are the source of plasma fibronectin (FN) which lacks the alternatively spliced EDI segment, distinctive of oncofetal FN. When hepatic or other epithelial cells are cultured on plastic, EDI inclusion is triggered. Here we report that EDI inclusion is inhibited when hepatic cells are cultured on a basement membrane-like extracellular matrix (ECM), demonstrating a new role for the ECM in the control of gene expression. The effect is duplicated by collagen IV and laminin but not by collagen I; is not observed with another alternatively spliced FN exon (EDII); and correlates with a decrease in cell proliferation, consistently with high EDI inclusion levels observed in many physiological and pathological proliferative processes.

X-rays modulate extracellular matrix in vivo

X-rays have an antiangiogenic effect in the chicken embryo chorioallantoic membrane (CAM) model of in vivo angiogenesis. Our study demonstrates that X-rays induce an early apoptosis of CAM cells, modulate the synthesis and deposition of extracellular matrix (ECM) proteins involved in regulating angiogenesis and affect angiogenesis induced by tumour cells implanted onto the CAM. Apoptosis was evident within 1-2 hr, but not later than 6 hr after irradiation. Fibronectin, laminin, collagen type I, integrin alpha(v)beta3 and MMP-2 protein amounts were all decreased 6 hr after irradiation. In contrast, collagen type IV, which is restricted to basement membrane, was not affected by irradiation of the CAM. There was a similar decrease of gene expression for fibronectin, laminin, collagen type I and MMP-2, 6 hr after irradiation. The levels of mRNA for integrin alpha(v)beta3 and collagen type IV were unaffected up to 24 hr after irradiation. The decrease in both protein and mRNA levels was reversed at later time points and 48 hr after irradiation, there was a significant increase in the expression of all the genes studied. When C6 glioma tumour cells were implanted on irradiated CAMs, there was a significant increase in the angiogenesis induced by tumour cells, compared to that in non-irradiated CAMs. Therefore, although X-rays have an initial inhibitory effect on angiogenesis, their action on the ECM enhances new vessel formation induced by glioma cells implanted on the tissue.

Identification and characterization of a novel fibronectin in zebrafish

Sequence analysis of zebrafish fibronectin (FN) cDNAs indicates that at least two forms of the protein exist in fish. One form (FN1) is very similar to FNs identified in other vertebrates possessing 12 type I, 2 type II, and 17 type III repeats including two alternative splice sites (EIIIA and EIIIB) and a variable region (V). Zebrafish FN1 contains the RGD cell adhesion site in type III(10) and a second cell-binding site (LDV) in the V region. In addition to this conserved form of FN, a novel truncated form of zebrafish FN (FN2) was identified. The predicted structure of FN2 is identical to FN1 at the N-terminal region possessing 9 type I, 2 type II, and the first 3 type III repeats. Following III(3), FN2 contains a unique 20-amino-acid C-terminal tail that is different from the C-terminus of FN1, lacking the two cysteines that are usually involved in the formation of interchain disulfide bonds. Genomic sequence analysis has revealed that FN2 is generated by an alternative RNA splicing pattern that has not been described for FN in other organisms. Reverse transcription-polymerase chain reaction analysis and RNase protection assays reveal that FN2 mRNA is present in the zebrafish embryo throughout development as well as in cultures of an established liver cell line. Experiments conducted with recombinant FN2 synthesized in insect cells demonstrate that the protein promotes the attachment and spreading of fish embryo cells in culture.

Follicular stage-dependent tumor necrosis factor alpha-induced hen granulosa cell integrin production and survival in the presence of transforming growth factor alpha in vitro

The link between cell adhesion to extracellular matrix and integrin-mediated survival signals has been established in several physiological systems, and roles for the cytokines tumor necrosis factor alpha (TNF alpha) and transforming growth factor alpha (TGF alpha) have been suggested. TGF alpha stimulates fibronectin production in hen granulosa cells and is an important survival factor during follicular maturation. In contrast, the role of TNF alpha and its possible interaction with TGF alpha in the regulation of granulosa cell fate (death versus survival) during ovarian follicular development have not been fully elucidated. The object of the current study was to determine if TNF alpha and TGF alpha interact in the regulation of hen granulosa cell fibronectin and integrin content in the context of cell death and survival during follicular development. TGF alpha (0.1 or 10 ng/ml), but not TNF alpha (0.1 or 10 ng/ml), increased both cellular and secreted fibronectin content in granulosa cell cultures of F5,6 but not F1 follicles. The expression of integrin beta(3) subunit was also stimulated by TGF alpha in a follicular stage-dependent manner, and culture of F5,6 granulosa cells with TNF alpha in the presence of maximal stimulatory concentrations of TGF alpha potentiated this response. TGF alpha increased both F5,6 and F1 granulosa cell [(3)H]thymidine incorporation but not 3-(4,5-dimethylthiazol-2-yl)3,5-diphenyl tetrazolium bromide (MTT) metabolism. Although TNF alpha had no effect on [(3)H]thymidine incorporation irrespective of the presence of the growth factor, MTT metabolism was higher in F5,6 granulosa cells cultured for 24 h with both TNF alpha and TGF alpha than with either cytokine alone. Incubation of F5,6 granulosa cells for 48 and 72 h resulted in a TGF alpha-inhibited loss of cellular adhesion and detachment of granulosa cells from the growth surface. Although TNF alpha alone had no effect on cell morphology, it facilitated the reorganization of the granulosa cells into multicellular follicle-like structures in the presence of the growth factor. DNA degradation significantly increased between 0 and 72 h of culture in the absence of the cytokine but was suppressed by the addition of TGF alpha but not of TNF alpha. However, fluorometric analysis indicated that the primary type of cell death exhibited by F5,6 granulosa cells during extended culture and attenuated by the presence of TNF alpha and TGF alpha was necrosis and not apoptosis. The current study demonstrates that TNF alpha and TGF alpha interact in the regulation of granulosa cell integrin content and cell survival in vitro in a follicular stage-dependent manner. These findings suggest that follicular development is accompanied by a change in the intraovarian role of TNF alpha; it is atretogenic prior to follicular selection but prevents follicular demise during preovulatory growth.

Expression of fibronectin splice variants in the postischemic rat kidney

Using an in vivo rat model of unilateral renal ischemia, we previously showed that the expression and distribution of fibronectin (FN), a major glycoprotein of plasma and the extracellular matrix, dramatically changes in response to ischemia-reperfusion. In the distal nephron in particular, FN accumulates in tubular lumens, where it may contribute to obstruction. In the present study, we examine whether the tubular FN is the plasma or cellular form, each of which is produced by alternative splicing of a single gene transcript. We demonstrate that FN in tubular lumens does not contain the extra type III A (EIIIA) and/or the extra type III B (EIIIB) region, both of which are unique to cellular FN. It does, however, contain the V95 region, which in the rat is a component of FNs in both plasma and the extracellular matrix. Expression of FN containing EIIIA increases dramatically in the renal interstitium after ischemic injury and continues to be produced at high levels 6 wk later. V95-containing FN also increases in the interstitial space, albeit more slowly and at lower levels than FN containing EIIIA; it also persists 6 wk later. FN containing the EIIIB region is not expressed in the injured kidney. The presence of V95 but not the EIIIA or EIIIB regions of FN in tubular lumens identifies the origin of FN in this location as the plasma; tubular FN is ultimately voided in the urine. The data indicate that both plasma and cellular FNs containing the V95 and/or EIIIA regions may contribute to the pathogenesis of acute renal failure and to the repair of the injured kidney.

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.

Increased expression of TGF-beta1 but not of its receptors contributes to human obstructive nephropathy

Increased expression of TGF-beta1 but not of its receptors contributes to human obstructive nephropathy.

BACKGROUND

Previous studies have revealed an increased expression of transforming growth factor-beta1 (TGF-beta1) and deposition of extracellular matrix in the kidney of animals with ureteral obstruction. However, these relationships have not been elucidated in the hydronephrotic kidney of humans.

METHODS

We analyzed the tissue expression of extracellular matrix proteins, TGF-beta1, and its receptors in the human kidney with ureteral obstruction by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). Obstructed kidneys (OBKs) were obtained from patients with ureteral tumors. A kidney specimen from patients with a renal tumor was used as control (CNKs).

RESULTS

The interstitial volume was significantly increased in OBKs in comparison with CNKs. OBKs showed increased deposition of collagen types I and IV and fibronectin in the renal interstitium. RT-PCR revealed overexpression of collagen alpha1(IV) mRNA and fibronectin mRNA in OBKs. OBKs showed a significantly increased mRNA expression of TGF-beta1 in comparison with CNKs. The immunoreactivity for TGF-beta1 increased markedly in the interstitium of OBKs. There was a significant correlation between the TGF-beta1 mRNA level and the interstitial volume. However, there was no significant difference between OBKs and CNKs in the relative mRNA level nor in immunoreactivity for TGF-beta receptors.

CONCLUSIONS

These data suggest that TGF-beta1 may contribute to the interstitial fibrosis found in the human kidney with ureteral obstruction, mainly because of an increase in the expression of this cytokine without significant changes to its receptors.

Identification of two amino acids within the EIIIA (ED-A) segment of fibronectin constituting the epitope for two function-blocking monoclonal antibodies

Alternative splicing of the fibronectin gene transcript gives rise to a group of adhesive glycoproteins showing restricted spatial and temporal expression during embryonic development, tumor growth, and tissue repair. Alternative splicing occurs in three segments termed EIIIB, EIIIA, and V. The EIIIA (or ED-A) segment of fibronectin is expressed prominently but transiently in healing wounds coincident with fibroblast expression of an activation marker, smooth muscle cell alpha-actin. A monoclonal antibody (IST-9) to the EIIIA segment blocks transforming growth factor-beta-mediated smooth muscle cell alpha-actin expression by fibroblasts in culture. A second monoclonal antibody (DH1) blocks chondrocyte condensation in chicken embryos. We find that IST-9 and DH1 react with human, rat, and chicken but not with mouse or frog EIIIA, suggesting that His44 may be important for antibody binding. A series of deletion mutants of rat EIIIA, constructed as glutathione S-transferase fusion proteins, do not react with either IST-9, DH1, or a third monoclonal antibody (3E2). Mutations of pairs of amino acids to alanine have little effect, except for either (Val34Thr35) or (Tyr36Ser37), which are located in a beta strand upstream from His44. For these double mutants, the binding to all three monoclonal antibodies is markedly reduced. By contrast, single mutants at Thr35, Tyr36, or Ser37 retain full activity, suggesting that the epitope for these antibodies is determined in part by conformation. Alanine-scanning mutagenesis of rat EIIIA demonstrates the importance of Ile43 and His44 for binding. Mutation of frog EIIIA (normally Val43Lys44) to rat (Ile43His44) is sufficient to restore fully IST-9 binding and much of the activity of DH1 and 3E2. Our findings demonstrate that the function-blocking antibodies, IST-9 and DH1, bind to the Ile43 and His44 residues in a conformationally dependent fashion, implicating the loop region encompassing both residues as critical for mediating EIIIA function.

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.

Rapid and reversible regulation of collagen XII expression by changes in tensile stress

We studied the expression of the fibril-associated collagen XII by fibroblasts cultured on attached (stretched) or floating (relaxed) collagen I gels. Accumulation of collagen XII in the medium as determined by semiquantitative immunoblotting was 8-16 times higher under stretched compared to relaxed conditions. Northern blot experiments showed that tensile stress controls collagen XII expression at the mRNA level. Tenascin-C mRNA levels were also influenced, whereas relative amounts of fibronectin and matrix metalloproteinase-2 mRNA were barely affected. The response to a change in tensile stress is rapid, since de novo biosynthesis of collagen XII was fully down-regulated 12 h after relaxation of a stretched culture. To demonstrate that the effect is also reversible, we mounted collagen gels with attached cells to movable polyethylene plugs. The cultures were relaxed or stretched at intervals of 24 and 48 h, and media samples were analyzed every 24 h. By ELISA, the amount of collagen XII secreted into the medium was found to increase or decrease in accordance with the tensile stress applied. This is evidence that the mechanical stimulus per se, rather than an indirect secondary effect, was responsible for the observed changes in collagen XII production.

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.

Affinity reagents against tumour-associated extracellular molecules and newforming vessels

Here we report some recent results of tumour targeting using extracellular matrix components of tumour stroma as targets. The possibility of using human recombinant antibodies in tumour targeting is also described. Preliminary results indicate that neovasculature markers can be targeted by recombinant antibodies and that they allow long residence time in tumours, thus exploiting the avidity properties of multivalent recombinant fragments.

Blockade of very late antigen-4 integrin binding to fibronectin in allograft recipients: I. Treatment with connecting segment-1 peptides prevents acute rejection by suppressing intragraft mononuclear cell accumulation, endothelial activation, and cytokine expression

BACKGROUND

Allograft rejection is associated with infiltration of inflammatory cells and local deposition of fibronectin (FN). This study was carried out to examine the hypothesis that peptides known to specifically block adhesive interactions between the connecting segment-1 (CS1)-binding domain of FN and alpha4beta1 integrin on circulating cells may interfere with the immune cascade, which would lead to acute rejection in transplant recipients.

METHODS AND RESULTS

Cardiac allografts from Lewis x Brown Norway F1 hybrids were rejected in 7+/-1 days in Lewis rats. Treatment with bioactive CS1 peptides (4 mg/kg/day i.v. for 7 days) abrogated acute rejection and prolonged cardiac allograft survival to 13+/-1 days (P<0.001). This effect correlated with decreased expression of total fibronectin and cell adhesion molecules, such as alpha4beta1, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, as well as reduced infiltration by CD4+ and CD8+ T cells at the graft site. Treatment with CS1 peptides decreased alloantigen activation, as evidenced by decreased intragraft infiltration by CD25+ cells, and diminished expression of mRNA coding for Th1 (interleukin [IL]-2, interferon-gamma)- and Th2 (IL-4, IL-5, IL-6)-type cytokines. CS1-mediated immunosuppressive effects could be reversed and acute rejection recreated after adjunctive treatment of rats with recombinant IL-2.

CONCLUSION

Our data are consistent with the model in which in vivo interaction between the alpha4beta1 integrin receptor and the cell-associated CS1 motif of FN is critical for rejection cascade. The novel therapeutic approach of selectively blocking the alpha4beta1-FN activation pathway with CS1 peptides prevents acute allograft rejection by inhibiting expansion of antigen-specific T cells and inducing a transient state of cytokine-responsive anergy in the residual T-cell population.

Osteoblast cytoskeletal modulation in response to mechanical strain in vitro

The structural integrity of microfilaments has been shown to be necessary for the signal transduction of mechanical stimuli within osteoblasts. Qualitative and quantitative changes within the cytoskeleton of osteoblasts may therefore be crucial components of the signal transduction processes of these cells in response to mechanical stimulation. Avian osteoblasts were strained with a device that deforms a flexible, cell-laden membrane at a defined frequency and intensity in a uniform biaxial manner. We examined the effects of mechanical strain on the accumulation of protein and the expression of the major cytoskeletal elements and specific integrin-binding (arginine-glycine-aspartic acid) proteins of these cells. Mechanical strain increased the level of total extracellular matrix-accumulated fibronectin by approximately 150% and decreased that of osteopontin by approximately 60% but had no quantifiable effect on the accumulation of beta1 integrin subunit or collagen type I. An examination of the major elements of the cytoskeleton demonstrated that neither the level of actin nor that of the intermediate filament protein vimentin changed; however, the amount of tubulin decreased by approximately 75% and the amount of vinculin, a major protein of focal adhesion complexes, increased by approximately 250%. An analysis of protein synthesis by two-dimensional gel electrophoresis of [35S]methionine-labeled cytoskeletal proteins demonstrated that the changes in the accumulation of vinculin and tubulin resulted from their altered synthesis. Messenger RNA analysis confirmed that the changes in accumulation and protein synthesis observed for vinculin, fibronectin, and osteopontin were controlled at a pretranslational level. Immunofluorescent microscopy demonstrated that mechanical strain led to increased formation and thickening of actin stress fibers, with a commensurate dissociation in microtubules and a clear increase in levels of vinculin at the peripheral edges of the cells. In conclusion, the elevated rate of synthesis and the increased accumulation of vinculin and fibronectin, as well as the increase in the number and size of stress fibers and focal adhesion complexes, suggest that mechanical strain leads to a coordinated change both in the cytoskeleton and in extracellular matrix proteins that will facilitate tighter adhesion of an osteoblast to its extracellular matrix.

A highly conserved region upstream of the fibronectin alternative exon EIIIA 3' splice site interacts with cell-type-specific nuclear proteins

Sequencing of chicken fibronectin genomic DNA and interspecies sequence comparisons reveal a highly conserved region upstream of the alternatively spliced exon EIIIA. UV-crosslinking of RNAs corresponding to this region from the chicken and rat genes with HeLa nuclear extract demonstrates that both RNAs interact with similar proteins. However, both RNAs crosslink to a 70 kDa protein present in nuclear extracts from cells and tissues that include exon EIIIA, but not in extracts from tissues that exclude the exon. This protein represents a candidate cell-type-specific factor involved in exon EIIIA inclusion.

Expression and potential role of the extracellular matrix in hepatic ontogenesis: a review

Studies from a number of laboratories have provided information on the temporal and spatial expression of a variety of extracellular matrix (ECM) components in the developing liver and insight into their potential roles in hepatogenesis. Collagen type IV and laminin are present in the basement membranes of the capsular mesothelium, vascular structures of the portal and hepatic vein branches, and the ductular elements of the developing liver. The mesothelial, vascular, and ductular epithelial cells synthesize laminin and type IV collagen. In contrast, fibronectin and type I collagen are restricted to the adjacent or surrounding interstitium of those ductal and vascular elements, but are not within the basement membrane proper. The hepatic perisinusoidal space (Space of Disse) of the fetal rat develops a delicate extracellular matrix by 12.5 days of gestation, which is characterized by banded collagen fibrils and bundles associated with filamentous and flocculent material. Fibronectin, laminin, and collagen types I, III, and IV are present in the developing perisinusoidal space by this early gestational date, with laminin being the most prevalent component detected. The laminin chains localized to that region in the fetal/neonatal period are alpha 2, beta 1, beta 2, and gamma 1, whereas the alpha 1 chain of laminin is absent from the developing Space of Disse. Similar data have been reported on the laminin phenotype in the perisinusoidal space during hepatic regeneration. Electron microscopy immunohistochemistry studies have demonstrated that the sinusoidal lining cells and hepatocytes synthesize these ECM proteins during hepatogenesis. By 6 to 8 weeks of postnatal life, laminin is not detectable in the perisinusoidal space. Both the transient expression of laminin and the similarity of the laminin chain phenotype expressed in the perisinusoidal space in the developing and regenerating liver suggests a role for this protein in the organization of the hepatic lobule in those forms of hepatic morphogenesis.

The region encoded by the alternatively spliced exon IIIA in mesenchymal fibronectin appears essential for chondrogenesis at the level of cellular condensation

Fibronectin in the extracellular matrix of tissues acts as a substrate for cell adhesion and migration during development. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. Fibronectin mRNA alternative splicing patterns change from B+A+V+ to B+A-V+ during chondrogenesis. In this report, immunohistochemical analysis demonstrates that while fibronectin protein containing the region encoded by exon IIIB is present throughout the limb at all stages of development, fibronectin protein containing the region encoded by exon IIIA disappears from cartilaginous regions just after condensation in vivo and in high-density mesenchymal micromass cultures in vitro. Treatment of mesenchymal micromass cultures prior to condensation with an antibody specific for the region encoded by exon IIIA disrupts the formation of cellular condensations and inhibits subsequent chondrogenesis in a dose- and time-dependent manner. Furthermore, microinjection of the exon IIIA antibody into embryonic chick limb primordia in vivo results in malformations characterized by smaller limbs and loss of limb skeletal elements. These results strongly suggest that the presence of the region encoded by exon IIIA in mesenchymal fibronectin is necessary for the condensation event that occurs during chondrogenesis.

Expression and splicing of the fibronectin gene in healthy and diseased periodontal tissue

Fibronectin is a major component of the extracellular matrix and is considered to have an important role in chronic inflammatory periodontal disease. The fibronectin gene product has been shown to be subject to alternative splicing in 3 regions, each generating different mRNA transcripts associated specifically with normal adult tissue, embryogenesis, tissue regeneration, and wound healing. In the present study, using the reverse-transcribed polymerase chain reaction to examine splicing profiles of the primary transcript, we found that healthy periodontal tissue expressed all alternatively spliced embryonic isoforms, indicative of the extensive and ongoing rebuilding processes which occur in these tissues. In marked contrast, only the exon-skipped transcripts were generated in tissue from chronic inflammatory periodontal disease patients. The loss of the high molecular weight isoforms in lesional tissues may be due to the excess production of inflammatory mediators in this disease, since we observed that high concentrations of the cytokine IL-1beta caused down-regulation of these transcripts in normal periodontal cells in tissue culture. Moreover, we also demonstrated that growth factors likely to be involved in periodontal regeneration and repair, such as PDGF, IGF-1 and TGF-beta, elicited pronounced upregulation of the embryonic isoforms of fibronectin in these cells. Although the functional activities of the antigens corresponding to the alternatively spliced variants of fibronectin are not yet known, our finding that they are selectively expressed suggests that they have highly specific roles in both periodontal breakdown and repair.

RT-PCR investigation of fibronectin mRNA isoforms in malignant, normal and reactive oral mucosa

This study aimed to establish patterns of cellular fibronectin mRNA splice variants in normal oral mucosa, oral squamous cell carcinoma, oral leukoplakias with and without atypia, and focal reactive overgrowths of oral mucosa. Particular emphasis was placed on evaluation of either the EDA or EDB domains as markers of malignancy. Total RNA was extracted from normal oral mucosa, oral squamous cell carcinoma, oral leukoplakias with and without atypia, reactive epulides, fibroepithelial polyps and denture-related hyperplasia. Reverse transcriptase polymerase chain reaction (RT-PCR) was used to identify different fibronectin transcripts at three splice sites (EDA, EDB and IIICS). All the tissues investigated produced EDA+, EDA-, EDB+ and EDB- splice variants, and this study did not support RT-PCR-based detection of either EDA or EDB domains as markers of malignancy in oral tissues. Variations in IIICS splice patterns were observed, although these were not specific to any lesion group. In particular, there were differences in either the inclusion or omission of the domain coding for the CS-5 binding site for alpha 4 beta 1 integrin, whereas the CS-1 binding site for alpha 4 beta 1 integrin was typically present when additional domains were included at the IIICS splice site. In conclusion, complex patterns of fibronectin splice variant transcripts exist in normal and pathological oral mucosa. This may reflect the multiple biological functions identified for fibronectin proteins, although the significance of different specific fibronectin splice variants has yet to be fully elucidated.

Fibronectin lacking the ED-B domain is a major structural component of tracheal cartilage

Fibronectin is a highly conserved dimeric glycoprotein found in high concentrations in plasma and widely distributed in low concentrations in the extracellular matrix of tissues. The protein is the product of a single gene, but multiple splicing variants are expressed that show tissue specificity. Three exons (IIIA, IIIB, and V) can be alternatively spliced to produce different fibronectin isoforms. We report here that fibronectin is a remarkably abundant component of the extracellular matrix of bovine tracheal cartilage, increasing with age to more than 20% of the tissue, dry weight. This matrix form of fibronectin is inextractable by 4 M guanidine HCl, indicating that it is a covalently cross-linked structural component. By protein sequence analysis, the main molecular form of fibronectin in bovine tracheal cartilage was shown to lack the ED-B domain encoded by exon IIIB.

Phage antibodies with pan-species recognition of the oncofoetal angiogenesis marker fibronectin ED-B domain

Fibronectin (FN) exists in several polymorphic forms due to alternative splicing. The B-FN isoform (with ED-B domain inserted by splicing) is present in the stroma of foetal and neoplastic tissues and in adult and neoplastic blood vessels during angiogenesis but is undetectable in mature vessels. This isoform, therefore, represents a promising marker for angiogenesis, as already shown using the mouse monoclonal antibody (MAb) BC-1 directed against an epitope on human B-FN. However, this MAb does not directly recognise the human ED-B domain nor does it recognise B-FN of other species; therefore, it cannot be used as a marker of angiogenesis in animal models. In principle, antibodies directed against the human ED-B domain should provide pan-species markers for angiogenesis as the sequence of this domain is highly conserved in different species (and identical in humans and mice). As it has proved difficult to obtain such antibodies by hybridoma technology, we used phage display technology. Here, we describe the isolation of human antibody fragments against the human ED-B domain that bind to human, mouse and chicken B-FN. As shown by immunohistochemistry, the antibody fragments stain human neoplastic tissues and the human, mouse and chicken neovasculature.

RGD and other recognition sequences for integrins

Proteins that contain the Arg-Gly-Asp (RGD) attachment site, together with the integrins that serve as receptors for them, constitute a major recognition system for cell adhesion. The RGD sequence is the cell attachment site of a large number of adhesive extracellular matrix, blood, and cell surface proteins, and nearly half of the over 20 known integrins recognize this sequence in their adhesion protein ligands. Some other integrins bind to related sequences in their ligands. The integrin-binding activity of adhesion proteins can be reproduced by short synthetic peptides containing the RGD sequence. Such peptides promote cell adhesion when insolubilized onto a surface, and inhibit it when presented to cells in solution. Reagents that bind selectively to only one or a few of the RGD-directed integrins can be designed by cyclizing peptides with selected sequences around the RGD and by synthesizing RGD mimics. As the integrin-mediated cell attachment influences and regulates cell migration, growth, differentiation, and apoptosis, the RGD peptides and mimics can be used to probe integrin functions in various biological systems. Drug design based on the RGD structure may provide new treatments for diseases such as thrombosis, osteoporosis, and cancer.

Mesodermal development in mouse embryos mutant for fibronectin

Three independent mutations were made by homologous recombination in two different regions of the fibronectin (FN) gene; all three appeared to be functional null mutations. The embryonic lethal phenotypes of these mutations were indistinguishable; all three FN mutant strains show mesodermal defects and fail to develop notochord or somites. Nevertheless analysis with lineage markers (Brachyury, sonic hedgehog, Notch-1, and mox-1) showed that both the notochord and the somite lineages were induced at the correct times and places. Furthermore, notochord precursor cells showed extensive cell migration in the absence of FN. However, neither notochord nor somites condensed properly in the absence of FN. These results show that specification of notochordal and somitic mesodermal lineages and significant cell migration are independent of fibronectin but that correct morphogenesis of these structures is FN-dependent.