Cell- and heparin-binding domains of the hexabrachion arm identified by tenascin expression proteins

The role of the ninth and tenth type III domains of human fibronectin in cell adhesion

Fibronectins (FN) contain sites, in addition to the cell recognition site RGD in the tenth type III domain (FIII10), that are required for adhesive activity. The role of FIII10 and the adjacent FIII9 was analysed in functional cell adhesion assays recombinant FIII domains in which the domain boundaries were strictly conserved. FIII9 had no adhesive activity. FIII10, and FIII9 plus FIII10 had less activity than FN, whereas the activity of FIII9-10 was similar to FN. We conclude that FIII9 acts synergistically with FIII10 in cell adhesion, and that this synergy is dependent upon the structural integrity of the FIII9-10 pair of domains.

Tenascin is induced at implantation sites in the mouse uterus and interferes with epithelial cell adhesion

Expression of tenascin, an extracellular matrix protein associated with morphogenetic events and altered states of cellular adhesion, was examined in mouse uterus during the peri-implantation period. A uniform low level expression of tenascin was detected in stromal extracellular matrix during the estrous cycle and days 1 through 4 of early pregnancy. During the period of blastocyst attachment (day 4.5), an intense deposition of tenascin fibrils was located in the extracellular matrix of stroma immediately subjacent to the uterine epithelium surrounding the attaching blastocyst. This localized intensity of tenascin expression was both spatially and temporally restricted. By day 5.5, differentiation of stroma in the immediate area around the embryo to form the primary decidual zone was accompanied by a reduced amount of tenascin expression in the form of fragmented fibrils. Tenascin also could be induced by an artificial stimulus in uterine stroma of mice that had been hormonally prepared for implantation. The ability of artificial stimuli to induce tenascin expression suggested that the tenascin-inducing signals were derived from uterine cells, presumably lumenal epithelium, rather than embryonic cells. Consistent with this, conditioned medium from primary cultures of uterine epithelium was found to induce tenascin expression (2- to 4-fold) in isolated uterine stroma. Artificial stimuli generated a temporal pattern of tenascin expression similar to that observed during early pregnancy; however, in the artificially induced model, tenascin was induced in stroma immediately subjacent to lumenal epithelium along the entire length of the uterus. Purified tenascin and a recombinant tenascin fragment consisting of alternatively spliced fibronectin type III repeats, interfered with maintenance of uterine epithelial cell adhesion to Matrigel. In contrast, other recombinant tenascin fragments or fibronectin had no effect in this regard. Tenascin had no effect on adhesion of uterine stroma. Collectively, these results suggest that stimulation of TN expression in stromal extracellular matrix in vivo occurs via hormonally regulated, epithelial-mesenchymal interactions and serves as an early marker for uterine receptivity and the attachment phase of implantation. Furthermore, tenascin may facilitate embryo penetration by disrupting uterine epithelial cell adhesion to underlying basal lamina.

Tenascin: does it play a role in epidermal morphogenesis and homeostasis?

Tenascin is a large glycoprotein of the extracellular matrix. Its complex multidomain structure, along with its unique distribution during embryogenesis, inflammation, wound healing, and tumorigenesis suggest this protein may play a significant role in regulating cell proliferation, migration, and differentiation. In this review I will summarize the structural features of tenascin and its localization in skin and discuss some of the potential roles of tenascin in the regulation of keratinocyte biology.

Novel tenascin variants with a distinctive pattern of expression in the avian embryo

Previous studies have shown that several forms of the glycoprotein tenascin are present in the embryonic extracellular matrix. These forms are the result of alternative splicing, which generates tenascin variants with different numbers of fibronectin type III repeats. We have used degenerate primers and PCR to isolate a novel tenascin exon from an avian genomic library. Genomic clones contained a sequence encoding a fibronectin type III repeat that corresponds to repeat ‘C’ from the variable domain of human tenascin. To demonstrate that tenascin containing repeat ‘C’ is actually synthesized by avian cells, a monospecific antiserum was raised against a repeat ‘C’ fusion protein. This antiserum recognized a novel high-molecular-weight variant on immunoblots of tenascin isolated from chicken embryo fibroblast-conditioned medium, and stained tendons on frozen sections of chicken embryos. A cDNA probe specific for mRNA encoding repeat ‘C’ was used for in situ hybridization. This probe hybridized in a subset of the embryonic tissues labelled with a universal tenascin probe, including tendons, ligaments and mesenchyme at sites of epithelial-mesenchymal interactions. Finally, we provide evidence that additional fibronectin type III repeats, one corresponding to a recently discovered human repeat as well as one entirely novel sequence, also exists in chicken tenascin mRNA. These data indicate that tenascin is present in the embryonic matrix in a multitude of forms and that these forms have distinctive distributions that may reflect more than one function for tenascin in development.

Tenascin mRNA isoforms in the developing mouse brain

The extracellular matrix glycoprotein tenascin is expressed in the developing mouse cerebellum as a group of four protein species of different molecular weights. The difference is most likely due to alternative splicing which is known to occur in tenascin mRNA within the region of the fibronectin type III repeats. In order to systematically analyze tenascin mRNA isoforms that would account for this heterogeneity, tenascin splice variants were isolated from mouse brain by the polymerase chain reaction (PCR). In agreement with Northern blot analysis, amplification by PCR revealed a general decrease in tenascin mRNA expression during development from embryonic and early postnatal to adult stages. This decrease was more pronounced for isoforms of high molecular weight compared to those of low molecular weight. In accord with the observations at the protein level, four splice variants were found to be predominantly expressed, containing insertions of either six, five, or one fibronectin type III repeat, or comprising no insertion. In addition, a minor splice variant with an insertion of four fibronectin type III repeats was isolated. Three of the isolated mRNA splice variants have not yet been described for mouse tenascin. Among them, an isoform containing six alternatively spliced repeats was found to include a novel fibronectin type III repeat. The sequence of this repeat displays 96.7% similarity to a corresponding type III repeat in human tenascin, revealing a strict evolutionary conservation between tenascin molecules from different species in the region of alternative splicing. Southern blot analysis of the amplified mRNA isoforms showed that the novel mouse type III repeat is confined to splice variants with an insertion of six fibronectin type III repeats. Furthermore, in situ hybridization on sections from mouse embryos indicated that tenascin-specific mRNAs containing the novel type III repeat are predominantly expressed in the central nervous system.

Isoforms of cellular fibronectin and tenascin in amniotic fluid

Amniotic fluid (AF) obtained from second trimester pregnancies presented extradomain (ED) A, B and an oncofetal (onc-f) domain containing isoforms of cellular fibronectin (cFn) in Western blotting of gelatin-bound polypeptides and directly of AF. Western blotting after sequential immunoprecipitation suggested at least three Fn molecules: one containing EDA and the onc-f domain and another minor component distinctly containing all the domains, and a third one only containing EDA. The immunoblotting results for EDA-cFn and onc-f-cFn were closely similar to that for total Fn, whereas in plasma samples of normal and pregnant women only traces of EDA-cFn and onc-f-cFn, but no EDB-cFn, were found. Western blotting of AF also indicated the presence of three isoforms of tenascin (Tn), M(r) 190,000 and 280,000 polypeptides earlier found in many cells, and a M(r) 200,000 polypeptide, novel for AF and not present in plasma. The results suggest a novel extracellular matrix polypeptide composition for AF.

Production and characterization of monoclonal antibodies specific for different epitopes of human tenascin

We have obtained and characterized 11 monoclonal antibodies (mAbs) specific for different domains of human tenascin (TN). Five of these mAbs reacted with epitopes contained in the TN area that undergoes alternative splicing and are thus able to recognize specific TN isoforms. These mAbs are a useful tool to study the expression and distribution of TN and its different isoforms in normal and pathological tissues.

Dual function of tenascin: simultaneous promotion of neurite growth and inhibition of glial migration

The extracellular matrix molecule tenascin is expressed within the developing peripheral nervous system, first by migrating neural crest cells and later by satellite (Schwann precursor) cells at the growing tips of peripheral nerves. Here we found that the neurite promoting activity of tenascin for sensory neurons is developmentally regulated: very young sensory ganglia of stage 23 (4 days old) embryos grew neurites on tenascin as fast as on laminin and fibronectin. The growth response of older (day 7 and 9) ganglia on laminin and fibronectin was similar to that of 4-day-old ganglia, while on tenascin neurite growth occurred only after a lag phase and at a slower rate. Neurite growth on tenascin was inhibited by antibodies to beta 1 integrin and by heparin. While tenascin promotes neurite outgrowth of peripheral neurons, we found that it does not allow satellite cell migration when it is present on the substratum, and it inhibits migration of satellite cells on fibronectin when added in soluble form. In contrast, soluble tenascin did not significantly alter the rate of neurite growth on tenascin, fibronectin or laminin substrata, although neurites were straighter and less attached. When isolated satellite cells were added to neurites grown on tenascin, they preferentially adhered to and elongated along neurite surfaces. Using patterned substrata of tenascin versus fibronectin or laminin confirmed that tenascin borders allow neurites to pass but act as barriers to migrating satellite cells. We postulate that tenascin or related molecules with dual functions in cell adhesion are important for peripheral nerve morphogenesis. Tenascin allows axonal growth, but may restrict random satellite cell migration into the fibronectin-rich mesenchyme, thereby inducing the compaction of nerve fascicles.

Endothelial cells adhere to the RGD domain and the fibrinogen-like terminal knob of tenascin

We have found that endothelial cells adhere much more strongly than fibroblasts to domains of tenascin and fibronectin. Endothelial cells adhered weakly, without spreading, to bacterial expression proteins corresponding to the tenth fibronectin type III (FN-III) domain of fibronectin, which contains the RGD. A larger fibronectin protein, containing this domain and the three amino-terminal ‘synergy’ domains gave strong adhesion and spreading. Two widely separated domains of tenascin gave adhesion. The third FN-III domain, TNfn3, which contains an RGD sequence in human and chicken tenascin, gave very strong adhesion and spreading of endothelial cells when tested as an isolated domain. Larger segments containing TNfn3 and the adjacent TNfn2 gave weaker adhesion, probably because the RGD sequence is partially blocked. Adhesion to this domain required divalent cations, was exquisitely sensitive to soluble GRGDSP peptide, and was blocked by antisera to the integrin alpha v beta 3. The second tenascin adhesion domain was the fibrinogen-like C-terminal knob, TNfbg. Cells adhered to but did not spread on this domain. This adhesion required divalent cations and was also sensitive to GRGDSP peptide, so it may be mediated by an integrin receptor. We have explored a range of conditions for preparing the adhesion substratum, and our results may resolve the controversy over whether tenascin can act as a substratum adhesion molecule. When coated for short times (1-2 hours) on plastic, tenascin had no adhesion activity, in contrast to fibronectin and the expression proteins, which gave strong adhesion under these conditions. When coated for longer times (12-24 hours) on plastic, the tenascin substratum supported good adhesion, but not spreading, of endothelial cells. Tenascin coated on nitrocellulose gave substantially stronger adhesion than on plastic, but still required long coating times for maximal activity. Adhesion of endothelial cells to native TN was inhibited by GRDGSP peptide. The cell adhesion activity demonstrates the presence on endothelial cells of tenascin receptors, which may play a supportive role in angiogenesis, in the structure of blood vessels, or in binding tenascin to the cell surface to elicit or enhance a signalling function.