Altered rate of fibronectin matrix assembly by deletion of the first type III repeats

The assembly of fibronectin (FN) into a fibrillar matrix is a complex stepwise process that involves binding to integrin receptors as well as interactions between FN molecules. To follow the progression of matrix formation and determine the stages during which specific domains function, we have developed cell lines that lack an endogenous FN matrix but will form fibrils when provided with exogenous FN. Recombinant FNs (recFN) containing deletions of either the RGD cell-binding sequence (RGD-) or the first type III repeats (FN delta III1-7) including the III1 FN binding site were generated with the baculovirus insect cell expression system. After addition to cells, recFN matrix assembly was monitored by indirect immunofluorescence and by insolubility in the detergent deoxycholate (DOC). In the absence of any native FN, FN delta III1-7 was assembled into fibrils and was converted into DOC-insoluble matrix. This process could be inhibited by the amino-terminal 70 kD fragment of FN, showing that FN delta III1-7 follows an assembly pathway similar to FN. The progression of FN delta III1-7 assembly differed from native FN in that the recFN became DOC-insoluble more quickly. In contrast, RGD- recFNs were not formed into fibrils except when added in combination with native FN. These results show that the RGD sequence is essential for the initiation step but fibrils can form independently of the III1-7 modules. The altered rate of FN delta III1-7 assembly suggests that one function of the missing repeats might be to modulate an early stage of matrix formation.

Changes in cell spreading and cytoskeletal organization are induced by adhesion to a fibronectin-fibrin matrix

Plasma fibronectin (pFN) cross-linked to fibrin during the injury response provides a provisional matrix required for cells to begin tissue repair. Using a synthetic matrix of pFN and fibrin as a substrate for cell adhesion and spreading, we have determined that pFN covalently cross-linked to fibrin into a complex multimer is functionally distinct from pFN immobilized onto a plastic surface. NIH-3T3 cells on a FN-fibrin matrix reach 50% of the maximal cell area of cells spread on FN-coated plastic. They neither attach nor spread on cross-linked fibrin alone. Cells on pFN-fibrin matrices form few prominent stress fibers and exhibit clear differences in membrane ruffling and filopodial extension when stained with rhodamine-labeled phalloidin. Interestingly, these differences are enhanced by upregulation of protein kinase C. These data suggest that cell-FN interactions can be modified by the molecular context of the protein within the extracellular matrix resulting in distinct cell morphology and cytoskeletal organization.

Differential targeting of recombinant fibronectins in AtT-20 cells based on their efficiency of aggregation

In pituitary-derived AtT-20 cells, recombinant fibronectin containing the N-terminal matrix assembly domain and the C-terminal half of fibronectin does not follow the regulated secretory pathway but instead concentrates in distinct organelles prior to secretion. These organelles are larger than the dense-core granules and localize to the cell body at sites that differ from lysosomes, endosomes and endoplasmic reticulum. Unlike the dense-core granules, their discharge is not stimulated by 8-bromo-cyclic-AMP or phorbol esters. The kinetics of intracellular transport and secretion of the recombinant fibronectin suggest that it is present in a post-Golgi pool that turns over more slowly than constitutive vesicles. Indeed, the fibronectin-containing organelles disappear with a half-time of 3 hours after inhibiting protein synthesis. Presence of the organelles correlates with intracellular aggregation of dimeric fibronectin polypeptides. The organelles are absent in cells expressing monomeric recombinant fibronectin (lacking C-terminal dimerization sites) or the C-terminal half of fibronectin (which dimerizes but lacks the N-terminal matrix assembly domain), both of which aggregate less efficiently than dimeric fibronectin. Instead, the latter polypeptides enter the dense-core granules. Thus while the formation of the fibronectin-containing organelles may require efficient aggregation, it may not require a specific structural signal. Moreover, efficient aggregation is not necessarily a prerequisite for following the regulated pathway.

Identification of an enhancer involved in tissue-specific regulation of the rat fibronectin gene

Fibronectin (FN) is a widely distributed extracellular matrix protein that is essential for cell adhesion in a variety of biological processes such as wound healing, tissue development and remodeling and oncogenic transformation. Appropriate FN levels are obtained by induction or repression of the FN gene in response to specific factors or circumstances in vivo. In order to identify regulatory regions involved in tissue-specific expression of FN, we have examined the transcriptional activity of overlapping fragments, within 4 kb upstream of the rat FN gene, following transfection into different cell types. Two regions conferred increases in transcription. The region between -1.08 and -2.6 displayed tissue-specificity and was active in fibroblasts but not hepatoma cells. The second region, between -3.2 and -3.9, was active in both cell types. Further characterization of the -1.08 to -2.6 segment demonstrated that it acts as an enhancer. Exonuclease III deletions of the 3' and 5' ends of the enhancer localized essential sequences between -1.5 and -1.7 and indicate that this fragment acts in concert with other sites between -1.08 and -2.6 to provide maximum enhancer activity. Gel mobility shift assays demonstrated fibroblast-specific binding of nuclear protein(s) to a 65 bp fragment within the essential region and DNase I footprinting localized this binding to a 27 bp sequence. Deletion of the sequence abolished the activity of the 1.5 kb enhancer. These studies show that a novel DNA sequence at -1688 is involved in regulating transcription of the FN gene in fibroblasts.

Rapid intracellular assembly of tenascin hexabrachions suggests a novel cotranslational process

Tenascin, an extracellular matrix protein that modulates cell adhesion, exists as a unique six-armed structure called a hexabrachion. The human hexabrachion is composed of six identical 320 kDa subunits and the structure is stabilized by inter-subunit disulfide bonds between amino-terminal segments. We have examined the biosynthesis of tenascin and its assembly into hexabrachions using pulsechase labeling of U-138 MG human glioma cells. Newly synthesized tenascin hexamers are secreted within 60 minutes of translation initiation. Intracellularly, as early as full length tenascin can be detected in pulse-labeled cell lysates, it is already in hexameric form. No precursors, such as monomers, dimers, or trimers, were identified that could be chased into hexamers. This lack of assembly intermediates suggests that nascent tenascin polypeptides associate prior to completion of translation. In contrast, fibronectin monomers in the same lysates are gradually formed into disulfide-bonded dimers. Although hexamer assembly is rapid, the rate-limiting step in secretion appears to be transport to the medial Golgi as endoglycosidase H-resistance was not detected until after a 30 minute chase. These results provide evidence for a novel co-translational mechanism of tenascin assembly which would be facilitated by its length and by the amino-terminal location of the assembly domain.

Fibronectin self-association is mediated by complementary sites within the amino-terminal one-third of the molecule

The formation of a fibrillar fibronectin (FN) extracellular matrix requires self-association of FN dimers. In this report, we show that the major sites for self-association are the amino-terminal repeats I1-5 and the first type III repeats. Recombinant FNs and fragments were generated by baculovirus expression of cysteine-rich domains and by bacterial expression of type III repeats as fusion proteins with maltose binding protein. When recombinant polypeptides were immobilized on microtiter wells, FN bound to 70-kDa amino-terminal fragment and to fusion proteins containing repeats III1-2 and III1-6 but not to other type III repeats. Similar results were obtained with a gel overlay assay. Binding was concentration-dependent and saturable. The amino-terminal binding site for III1-2 was further localized to repeats I1-5. Therefore, at least two different sites for FN-FN interaction reside near the amino terminus of the molecule. A model for the regulation of FN matrix assembly is proposed based on intramolecular interactions between these amino-terminal sites.

Interactions between fibronectin and chondroitin sulfate are modulated by molecular context

Interactions between fibronectin (FN) and glycosaminoglycans are essential for extracellular matrix morphology and cell adhesion. One of the most abundant glycosaminoglycans is chondroitin sulfate, and here we show that recombinant FNs (deminectins (DN)) containing the carboxyl-terminal cell, heparin, and fibrin domains bind specifically to chondroitin sulfate in affinity chromatography assays. Using a panel of mutant DNs, important determinants for chondroitin sulfate binding have been localized to repeats III13 and III14 within the heparin domain. In particular, mutation of an arginine pair in repeat III13 to neutral residues ablated binding to chondroitin sulfate as we previously reported for heparin (Barkalow, F.J.B., and Schwarzbauer, J.E. (1991) J. Biol. Chem. 266, 7812-7818). These results, in combination with the ability of heparin and chondroitin sulfate to compete for binding to DNs, demonstrate that these two glycosaminoglycans interact with similar or overlapping sites in FN. One important difference between FN interactions with heparin and chondroitin sulfate is that, while FN and DNs bound equally to heparin, FN bound less efficiently than DNs to chondroitin sulfate. Reduced binding to chondroitin sulfate was also observed with a larger recombinant FN lacking internal repeats III1-7 indicating that the amino-terminal region acts to limit binding to the carboxyl-terminal domain. Our results demonstrate that interactions between FN and chondroitin sulfate are modulated by molecular context.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

The Caenorhabditis elegans homologue of the extracellular calcium binding protein SPARC/osteonectin affects nematode body morphology and mobility

The extracellular matrix-associated protein, SPARC (osteonectin [Secreted Protein Acidic and Rich in Cysteine]), modulates cell adhesion and induces a change in cell morphology. SPARC expression in mammals is developmentally regulated and is highest at sites of extracellular matrix assembly and remodeling such as parietal endoderm and bone. We have isolated cDNA and genomic DNA clones encoding the Caenorhabditis elegans homologue of SPARC. The gene organization is highly conserved, and the proteins encoded by mouse, human, and nematode genes are about 38% identical. SPARC consists of four domains (I-IV) based on predicted secondary structure. Using bacterial fusion proteins containing nematode domain I or the domain IV EF-hand motif, we show that, like the mammalian proteins, both domains bind calcium. In transgenic nematodes expressing a SPARC-lacZ fusion gene, beta-galactosidase staining accumulated in a striated pattern in the more heavily stained muscle cells along the body. Comparison of the pattern of transgene expression to unc-54-lacZ animals demonstrated that SPARC is expressed by body wall and sex muscle cells. Appropriate levels of SPARC are essential for normal C. elegans development and muscle function. Transgenic nematodes overexpressing the wild-type SPARC gene were abnormal. Embryos were deformed, and adult hermaphrodites had vulval protrusions and an uncoordinated (Unc) phenotype with reduced mobility and paralysis.

The alternatively spliced V region contributes to the differential incorporation of plasma and cellular fibronectins into fibrin clots

During blood clot formation in vivo, plasma fibronectin (pFN) is cross-linked to fibrin by coagulation factor XIIIa. Cellular FN (cFN), which localizes to connective tissue, is distinguished from pFN by the inclusion of alternatively spliced segments. To determine if these two FNs are functionally equivalent in blood clotting, the cross-linking of rat pFN and cFN to fibrin was compared in an in vitro clotting assay. Fibrinogen and FN were incubated at physiological ratios in the presence of thrombin and factor XIIIa. Cross-linking of FN to fibrin was monitored by SDS-PAGE and immunoblotting. Over 24 h, cFN was incorporated at a significantly slower rate than pFN and was not completely cross-linked to fibrin at a temperature that favors this interaction (0 degrees C). This difference was observed with purified fibrinogens from human, rat, and bovine and with rat plasma and was maintained even after incubation of pFN with rat fibroblasts for several days. Using the same assay, purified recombinant V(+)-V0 and V(+)-V+ FN dimers resembling pFN and cFN, respectively, showed a similar difference in cross-linking kinetics. These results suggest that the asymmetric distribution of the V region among pFN dimers plays a role in regulating its incorporation into blood clots. In fibrin clots, cFN was converted into a set of cross-linked intermediates distinct from those of pFN. For example, while pFN was initially cross-linked into a pFN-fibrin alpha heterodimer, this product was not a major intermediate in clots formed with cFN. This finding, in conjunction with evidence for the formation of factor XIIIa-catalyzed cFN-cFN cross-links, indicated that cFN molecules interact with each other, and with fibrin, differently from pFN. Together, these results show an important functional distinction between pFN and cFN.

Alternative splicing of fibronectin: three variants, three functions

Fibronectin (FN) is a multi-functional extracellular matrix protein required for cell adhesion and migration, blood clotting, wound healing, and oncogenic transformation. The functional complexity is paralleled by structural diversity in that multiple forms of FN are generated by cell type-specific alternative splicing. In the rat, up to 12 different combinations of the three alternatively spliced segments (EIIIA, EIIIB, and the V region) are produced. What effects do these segments have on FN function? Recently, progress has been made in the identification of specific activities for the three variants of the V region, V120, V95, and V0. FN-mediated cell adhesion, FN synthesis and secretion, and incorporation into blood clots are differentially affected by these isoforms. These results suggest that cellular behavior is modulated by environmental cues provided by different types and proportions of alternatively spliced FN variants.

Fibronectin: from gene to protein

Recent advances in several key areas of fibronectin biology are discussed. These include its expression, from transcription to secretion of dimers, the structural requirements for several of the binding activities, potential roles for alternatively spliced segments in cell adhesion, and the assembly of a fibronectin matrix.

Identification of the fibronectin sequences required for assembly of a fibrillar matrix

During extracellular matrix assembly, fibronectin (FN) binds to cell surface receptors and initiates fibrillogenesis. As described in this report, matrix assembly has been dissected using recombinant FN polypeptides (recFNs) expressed in mammalian cells via retroviral vectors. RecFNs were assayed for incorporation into the detergent-insoluble cell matrix fraction and for formation of fibrils at the cell surface as detected by indirect immunofluorescence. Biochemical and immunocytochemical data are presented defining the minimum domain requirements for FN fibrillogenesis. The smallest functional recFN is half the size of native FN and contains intact amino- and carboxy-terminal regions with a large internal deletion spanning the collagen binding domain and the first seven type III repeats. Five type I repeats at the amino terminus are required for assembly and have FN binding activity. The dimer structure mediated by the carboxy-terminal interchain disulfide bonds is also essential. Surprisingly, recFNs lacking the RGDS cell binding site formed a significant fibrillar matrix. Therefore, FN-FN interactions and dimeric structure appear to be the major determinants of fibrillogenesis.

Localization of the major heparin-binding site in fibronectin

We have identified the major site required for the interaction of fibronectin (FN) with heparin. Affinity chromatography was used to test the binding ability of a library of truncated, monomeric forms of fibronectin (deminectins) containing deletions or two point mutations in the heparin-binding domain. This domain consists of type III repeats 12, 13, and 14. Deletions of individual repeats showed that both III13 and III14 are required for complete binding. Small deletions within these repeats localized a major site of heparin interaction to the amino-terminal half of III13. Site-directed mutagenesis of adjacent arginines within this sequence to uncharged residues reduced heparin binding by 98%, identifying these positively charged amino acids as essential for the interaction. A significant role for the flanking alternatively spliced regions and for repeat III12 was not found. We conclude that, while both repeats III13 and III14 participate in heparin binding, there is a major site of interaction in repeat III13 that accounts for nearly all of the activity. The significance of multiple heparin-binding sites within this domain is discussed and a model is proposed to account for how these sites may function in vivo.

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.

Organization of the fibronectin gene provides evidence for exon shuffling during evolution

We report the organization of the two ends of the rat fibronectin gene which encode the type I and II repeating units of the protein. We show that each of these modular structural units is encoded by a separate exon. Homologous type I and II repeats are known to occur in tissue plasminogen activator, factor XII and a bovine seminal plasma protein. Comparison of these sequences and the exon structures of the fibronectin and tissue plasminogen activator genes indicates that exons encoding type I and type II repeats have reassorted during evolution. We also report analyses of the extreme 5' and 3' ends of the fibronectin gene including the promoter region and the exon encoding the prepro sequence of fibronectin and we show that the gene is transcribed from a single initiation site to a single polyadenylation site. These data provide information pertinent to the transcriptional regulation of the gene, the alternative splicing of the primary transcript and the structure of the primary translation product.

Multiple sites of alternative splicing of the rat fibronectin gene transcript

We describe analyses of the structure and expression of the rat fibronectin gene with particular attention to the 40-kb stretch from the center of the gene which encodes 17 type-III repeating units. Each repeat is precisely separated from its neighbors by introns and most are encoded by pairs of exons. Three repeats are encoded precisely by single exons and two of these (EIIIA and EIIIB) are alternatively spliced in a cell type-specific fashion. A third site of alternative splicing (EIIIB) reported here is similar in expression to the previously described EIIIA segment. Both are excluded from mRNA in liver cells and are, therefore, absent from plasma fibronectin. These two alternative splices, plus a third one (V) reported previously, can occur in all possible combinations giving 12 fibronectin mRNAs from a single gene. These splicing variations account for most but not all of the known fibronectin subunit variants. We report investigations designed to detect other regions of alternative splicing. We also show that the pattern of alternative splicing is somewhat altered on oncogenic transformation.

Efficient and stable expression of recombinant fibronectin polypeptides

We describe retroviral expression vectors containing cDNAs encoding part of fibronectin preceded by the signal and "pro" sequences of parathyroid hormone. The recombinant retroviruses were used to generate NIH 3T3 cell lines stably producing functionally active fragments of fibronectin. The recombinant fibronectins (deminectins) are processed and secreted by the cells and form disulfide-bonded dimers with themselves and with endogenous fibronectin subunits. The fibronectin-deminectin heterodimers are incorporated into the extracellular matrix. We describe cell lines producing six variant forms of deminectin corresponding to variant forms of fibronectin produced by alternative splicing. In constructing fibronectin cDNAs encoding the six variant forms, we also made use of the ability of retroviral vectors to generate cDNAs by accurate splicing of cloned genomic segments. These constructs should be valuable in analyses of the structure-function relationships of fibronectins.

Cell-type-specific fibronectin subunits generated by alternative splicing

Multiple fibronectin mRNAs arise by alternative splicing of the primary transcript of a single gene. We describe analyses of the contribution of this alternative splicing to fibronectin subunit heterogeneity in three different cell types using antisera directed against specific segments of fibronectin. beta-galactosidase-fibronectin fusion proteins produced with the lambda gt11 bacterial expression vector were used as immunogens. One region of alternative splicing accounts for differences in subunit size, while a second contributes to differences between the fibronectins present in blood plasma and in fibroblastic cells. We also show, however, that these two regions of alternative splicing do not account for all detectable subunits. We have also used these segment-specific antisera to show that blood platelets contain a spectrum of fibronectin subunits distinct from that found in blood plasma.

Fibronectin: a versatile gene for a versatile protein

We have isolated cDNA and genomic clones for rat fibronectins. A single gene gives rise to three different mRNA species by alternative splicing at a complex intron-exon boundary within the coding region. The fibronectins encoded by these three mRNAs differ by the insertion of different protein segments in the C-terminal heparin-binding domain. The amino acid sequences of the cell-, heparin- and fibrin-binding domains in the C-terminal third of fibronectin were deduced from the DNA sequences. The cell- and heparin-binding regions each consist of several similar repeating sequences known as type III homologies, while the fibrin-binding region comprises three repeats of a different type (type I homologies). The sequences suggest several hypotheses for the structure-function relationships of these domains of fibronectin; these hypotheses are now being tested. Determination of intron-exon boundaries within the fibronectin gene is beginning to reveal the modular structure of the gene and its relation to the repeating structure of fibronectin.

On the origin of species of fibronectin

Multiple different subunits of fibronectin are known to occur and their origin has been unclear. Recent results showing that a single fibronectin gene can give rise to several different mRNAs by alternative splicing suggested an explanation for some of this diversity of fibronectin subunits. Because the alternative splicing events occur within the coding region, the mRNAs differ in coding potential. We have prepared recombinant phage containing a rat fibronectin cDNA segment that is present in some fibronectin mRNAs and not in others. This segment was inserted in the beta-galatosidase gene of lambda gt11, and fusion protein produced by lysogens of the recombinant phage was purified and used as immunogen. The resulting antisera recognized some subunits of rat and hamster fibronectins but not others, indicating that inclusion or removal of this segment gives rise to mRNAs that encode different fibronectin subunits. In particular, presence or absence of a 95 amino acid segment appears to account for differences in size among the subunits of plasma fibronectin, whose origin is therefore explained by alternative patterns of RNA splicing.

A single rat fibronectin gene generates three different mRNAs by alternative splicing of a complex exon

Three fibronectin mRNAs exist in rat liver, differing by the presence or absence of segments of 285 or 360 bases at a point within the coding region. We previously proposed that the three mRNAs are encoded by a single gene and arise via alternative splicing of a common transcript. In order to test this hypothesis, we have isolated clones spanning approximately half of the fibronectin gene from a Fisher rat genomic library; blot hybridization analyses reveal the presence of only one fibronectin gene in the haploid rat genome. We determined the sequence of a portion (1221 nucleotides) of this gene. This sequence shows clearly that the three fibronectin mRNAs encoded by this gene are generated by a pattern of alternative splicing in which one 5' splice site can be paired with any one of three 3' splice sites, one at the beginning of, and two within, a single complex exon.