Fibronectin splice variants are differentially incorporated into the extracellular matrix of tumorigenic and non-tumorigenic hybrids between normal fibroblasts and sarcoma cells

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.

Basement membrane and growth factor gene expression in normal and diabetic human retinas

PURPOSE

Recently, we found abnormal accumulation of several extracellular matrix components in retinal basement membranes in human diabetic retinopathy (DR). Others have described increased levels of various growth factors within the vitreous of DR patients. This study examined mRNA levels of these extracellular matrix components and growth factors within human retinal tissues.

METHODS

Total retinal RNA was analyzed by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR products were identified by Southern blotting. Samples were normalized with respect to beta2-microglobulin cDNA. Twenty-one retinas were analyzed: 6 normal, 7 diabetic without DR and 8 diabetic with DR.

RESULTS

In diabetic retinas without DR, the expression levels of most genes were similar to normal. In DR retinas, tenascin-C mRNA expression increased compared to both normal and diabetics without DR. By RT-PCR and Northern blotting, mainly small tenascin-C mRNA isoforms were expressed, and some of them were elevated in DR retinas. Fibronectin mRNA was elevated in DR compared to normal retinas, possibly due to the overexpression of extradomain A-containing isoform (ED-A+, or cellular fibronectin). In DR retinas, gene expression of vascular endothelial growth factor and placenta growth factor was elevated compared to normal, although mRNA for these growth factors receptors (VEGFR-1/Flt-1 and VEGFR-2/KDR) did not change significantly. Transforming growth factor-beta1 mRNA also increased in DR retinas.

CONCLUSIONS

The data suggest that proliferative DR development may be associated with increased retinal expression of vascular endothelial growth factor, placenta growth factor and transforming growth factor-beta1 that possibly triggers the deposition of small tenascin-C isoforms in the blood vessel walls. Angiogenesis-stimulating tenascin-C may further promote diabetic retinal neovascularization.

Differential expression of fibronectin splice variants, oncofetal glycosylated fibronectin and laminin isoforms in nodular palmar fibromatosis

The tissue formation process in nodular palmar fibromatosis (Morbus Dupuytren) was investigated by the demonstration of fibronectin splice variants (ED-A and ED-B fibronectin), de novo glycosylated fibronectin and laminin isoforms (A, M, B1, B2, s chains) in association to the proliferative activity (Ki-67 antigen) and the occurrence of myofibroblast phenotype (alpha-smooth muscle actin, desmin). The proliferative noduli of the fibromatosis were characterized by a diffuse immunostaining for alpha-smooth muscle actin, and single cells positive for desmin and the Ki-67 antigen. In contrast to the surrounding aponeurosis as extracellular matrix, components of the whole proliferative noduli were defined: ED-A, ED-B and de novo glycosylated fibronectin, B1 and B2 laminin chain, tenascin and collagen type IV. The demonstration of the A and M laminin chain was restricted to a few cells of the proliferative noduli. S laminin could be visualized in the majority of palmar aponeurotic fibroblasts. As revealed by mRNA, in situ hybridization a de novo synthesis of fibronectin could only be detected within proliferative noduli. There is a positive correlation between the myofibroblast phenotype formation, cellular proliferation and the occurrence of ED-A and ED-B containing fibronectin, as well as de novo glycosylated fibronectin in Dupuytren's disease. The ultrastructural irregularities of myofibroblastic basal lamina and the heterogeneity of the myofibroblast phenotype are equivalent to the variability of laminin isoform immunostaining.

Expression of the alternatively spliced EIIIB segment of fibronectin

Previous descriptions of the expression and distribution of the alternatively spliced EIIIB segment of fibronectin (FN) relied upon an antibody which, on subsequent testing, was shown not to recognize this segment directly. This raises concerns regarding the reliability of all such previous descriptions. In order to prepare reagents directly reactive with this segment, we raised polyclonal antibodies to two different bacterial fusion proteins containing intact EIIIB segments, and to a synthetic 36 amino acid peptide from the center of the EIIIB segment. Antibodies raised to each of these three immunogens recognized fusion proteins containing the EIIIB segment, but failed to recognize full length EIIIB+ FNs produced by mammalian cells, suggesting that oligosaccharide linked to Asn1359 within the EIIIB segment, or potentially to other residues in FN, might interfere with antibody recognition of this segment. Consistent with this hypothesis, N-deglycosylation of recombinant full and partial length EIIIB+ FNs permitted their specific recognition by the anti-fusion protein (but not anti-peptide) antibodies. Using anti-fusion protein antibodies coupled with deglycosylation procedures, we provide a series of new results relevant to the functions of the EIIIB segment: 1) Endogenously synthesized EIIIB+ FN is incorporated into the extracellular matrix of cultured fibroblasts, where it appears by immunofluorescence microscopy and radioimmunoprecipitation analyses to have a distribution very similar to both EIIIA+ forms and the total pool of FNs. 2) No reproducible changes can be shown to occur in the extent of synthesis or matrix incorporation of EIIIB+ FNs upon cellular transformation. 3) Low levels of EIIIB+ FN are normally present in blood plasma and consequently also in purified preparations of plasma FN. 4) EIIIB+ FN is present in blood platelets, where it constitutes a minor fraction of total platelet FN, yet is greater than 4-fold enriched relative to plasma FN. 5) EIIIB+ FN is synthesized by first passage cultured endothelial cells, suggesting that the endothelium could constitute a source for this FN isoform in the circulating blood. The antibodies and methods used in this study constitute the first direct assays of EIIIB+ FN protein expression and are applicable to a variety of species.