Meet the Tenascins: Multifunctional and Mysterious (2005)

The tenascins are a highly conserved family of large oligomeric glycoproteins found in the extracellular matrix (ECM) 1The abbreviations used are: ECM, extracellular matrix; EGF, epidermal growth factor. 1The abbreviations used are: ECM, extracellular matrix; EGF, epidermal growth factor. of vertebrate organisms. Two decades ago, the molecule now known as tenascin-C was among the first proteins shown to have an adhesion modulatory role antagonizing cell attachment to fibronectin (1Chiquet-Ehrismann R. Kalla P. Pearson C.A. Beck K. Chiquet M. Cell. 1988; 6: 383-390Abstract Full Text PDF Scopus (463) Google Scholar). Cells that normally demonstrate a stationary phenotype on the fibronectin-containing matrix by spreading out and forming cortical actin stress fibers (Fig. 1A) will show morphological changes when tenascin-C is included in the matrix by extending membrane protrusions and exhibiting decreased stress fiber formation (Fig. 1B), characteristics more typical of a migratory phenotype (2Wenk M.B. Midwood K.S. Schwarzbauer J.E. J. Cell Biol. 2000; 150: 913-920Crossref PubMed Scopus (105) Google Scholar). The presence of ECM proteins such as tenascins during developmental and pathological states is now well recognized if still not well understood. Although each type has a distinctive expression pattern, the tenascins share the characteristic of having tightly regulated expression during development and throughout an organism's life. Tenascins also share the characteristic of modulating cell-matrix interactions and mediating a state of matrix attachment that promotes motility while also influencing other cell functions. As such, this protein family has important functions not only during development but also during pathological states in the adult such as tissue injury and tumorigenesis where remodeling processes are prominent. Reviews in recent years have covered pertinent aspects of tenascin biology including domain structure, modulation of cell functions, in vivo null deletion phenotypes, and contributions to human pathology (3Pesheva P. Probstmeier R. Prog. Neurobiol. 2000; 61: 465-493Crossref PubMed Scopus (78) Google Scholar, 4Orend G. Int. J. Biochem. Cell Biol. 2005; 37: 1066-1083Crossref PubMed Scopus (92) Google Scholar, 5Jones F.S. Jones P.L. Dev. Dyn. 2000; 218: 235-259Crossref PubMed Scopus (549) Google Scholar, 6Chiquet-Ehrismann R. Int. J. Biochem. Cell Biol. 2004; 36: 986-990Crossref PubMed Scopus (112) Google Scholar, 7Chiquet-Ehrismann R. Chiquet M. J. Pathol. 2003; 200: 488-499Crossref PubMed Scopus (443) Google Scholar, 8Chiquet-Ehrismann R. Tucker R.P. Int. J. Biochem. Cell Biol. 2004; 36: 1085-1089Crossref PubMed Scopus (119) Google Scholar, 9Joester A. Faissner A. Matrix Biol. 2001; 20: 13-22Crossref PubMed Scopus (150) Google Scholar, 10Mackie E. Tucker R. J. Cell Sci. 1999; 112: 3847-3853Crossref PubMed Google Scholar). This minireview will provide a concise summary of information from these reviews, which the reader is encouraged to refer to for greater detail, and will also discuss a few of the more recent developments in the field. Nomenclature and Structure—Tenascins are considered unique to vertebrates. Although Drosophila proteins such as ten-m/Odz were once thought to be possible orthologues due to shared motifs, the identification of more closely related vertebrate equivalents has left the tenascin family without any known invertebrate orthologue (11Rubin B.P. Tucker R.P. Martin D. Chiquet-Ehrismann R. Dev. Biol. 1999; 216: 195-209Crossref PubMed Scopus (79) Google Scholar, 12Erickson H.P. Curr. Opin. Cell Biol. 1993; 5: 869-876Crossref PubMed Scopus (355) Google Scholar). There are currently four tenascin paralogues that have been identified in mammals, each designated with a letter derived, for the most part, from earlier eponyms: C, R, X, and W. The primary structures of tenascin family proteins have common motifs all ordered in the same consecutive sequence (Fig. 2): amino-terminal heptad repeats, epidermal growth factor (EGF)-like repeats, fibronectin type III domain repeats, and a carboxyl-terminal fibrinogen-like globular domain. The heptad repeats lie in a highly conserved amino-terminal oligomerization region allowing individual subunits to assemble, usually into trimers. In some tenascins, additional cysteine residues allow the assembly of two trimers into a hexamer. Each protein member is associated with typical variations among different species in the number and nature of EGF-like and fibronectin type III repeats. Isoform variants produced through alternative splicing within the fibronectin type III repeats have been described across the family; however to date, only tenascin-C has shown splice variants being expressed in significant numbers and diversity (5Jones F.S. Jones P.L. Dev. Dyn. 2000; 218: 235-259Crossref PubMed Scopus (549) Google Scholar, 7Chiquet-Ehrismann R. Chiquet M. J. Pathol. 2003; 200: 488-499Crossref PubMed Scopus (443) Google Scholar). Tenascin-C—Assembly into hexamers is a classic feature of tenascin-C. Glycosylation differences can cause the sizes of individual subunits to be quite variable, but most are generally around 200 kDa, although some human tenascin-C monomers have been estimated to be over 300 kDa (5Jones F.S. Jones P.L. Dev. Dyn. 2000; 218: 235-259Crossref PubMed Scopus (549) Google Scholar, 9Joester A. Faissner A. Matrix Biol. 2001; 20: 13-22Crossref PubMed Scopus (150) Google Scholar). Mammalian protein subunits typically have 14.5 EGF-like repeats with 8 fibronectin type III repeats that are shared by all tenascin-C isoforms. Alternative splicing of an additional 9 distinctive repeats that can be independently included or excluded in a combinatorial manner allows tenascin-C to show the greatest number and diversity in isoforms with as many as 27 different mRNA variants having been identified in the developing mouse brain (13Joester A. Faissner A. J. Biol. Chem. 1999; 274: 17144-17151Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). Although much data exist demonstrating that splice variants are differentially expressed during tissue morphogenesis and tumorigenesis under the influence of growth factors and cytokines such as transforming growth factor-β and basic fibroblast growth factor, the determining mechanisms underlying tenascin-C alternative splicing are still not known (9Joester A. Faissner A. Matrix Biol. 2001; 20: 13-22Crossref PubMed Scopus (150) Google Scholar). As the first tenascin to be identified, tenascin-C remains the best studied member of the family, accounting for most reports examining the component domains shared by tenascin family proteins. These domains and their functions, however, also remain poorly understood. Although it is believed that the different regions of tenascin-C have distinct actions and functions, it is also probable that the overall effects of tenascin-C on cells and their interactions with the ECM require the concerted action of multiple domains (14Fischer D. Brown-Ludi M. Schulthess T. Chiquet-Ehrismann R. J. Cell Sci. 1997; 110: 1513-1522Crossref PubMed Google Scholar). A multiplicity of binding sites have been identified for integrin cell surface receptors, proteoglycans, and cell adhesion molecules of the immunoglobulin family, as well as annexin II receptor proteins and ECM components such as heparin, fibronectin, and collagen. Almost all known binding sites lie in the fibronectin type III repeats or the fibrinogen globe, but evidence has been reported suggesting that the EGF-like repeats act as low affinity ligands for EGF receptors (15Swindle C.S. Tran K.T. Johnson T.D. Banerjee P. Mayes A.M. Griffith L. Wells A. J. Cell Biol. 2001; 154: 459-468Crossref PubMed Scopus (223) Google Scholar). As a consequence, cell interactions with tenascin-C are quite complex, and several signaling mechanisms have been identified, including recent observations that tenascin-C blocks focal adhesion kinase- and Rho-mediated signaling pathways activated by fibronectin (2Wenk M.B. Midwood K.S. Schwarzbauer J.E. J. Cell Biol. 2000; 150: 913-920Crossref PubMed Scopus (105) Google Scholar, 16Midwood K.S. Schwarzbauer J.E. Mol. Biol. Cell. 2002; 13: 3601-3613Crossref PubMed Scopus (121) Google Scholar, 17Midwood K.S. Valenick L.V. Hsia H.C. Schwarzbauer J.E. Mol. Biol. Cell. 2004; 15: 5670-5677Crossref PubMed Scopus (114) Google Scholar) as well as stimulating Wnt and other growth-promoting pathways (18Ruiz C. Huang W. Hegi M.E. Lange K. Hamou M.F. Fluri E. Oakeley E.J. Chiquet-Ehrismann R. Orend G. Cancer Res. 2004; 64: 7377-7385Crossref PubMed Scopus (84) Google Scholar). For a detailed discussion of these mechanisms, please refer to Gertrude Orend's recent review (4Orend G. Int. J. Biochem. Cell Biol. 2005; 37: 1066-1083Crossref PubMed Scopus (92) Google Scholar). In the developing embryo, tenascin-C is expressed during neural, skeletal, and vascular morphogenesis. It then virtually disappears in the adult organism with continued basal expression detectable only in tendon-associated tissues; however, sharp up-regulation in expression occurs in tissues undergoing remodeling processes seen during wound repair and neovascularization or in pathological states such as inflammation or tumorigenesis. Given this expression pattern and that its structure has been well conserved among vertebrates and appears to be capable of numerous interactions and potential functions, reports during the 1990s demonstrating that mice with complete tenascin-C gene deletions are viable and appear to develop normally were unexpected (19Forsberg E. Hirsch E. Frohlich L. Meyer M. Ekblom P. Aszodi A. Werner S. Fassler R. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 6594-6599Crossref PubMed Scopus (203) Google Scholar, 20Saga Y. Yagi T. Ikawa Y. Sakakura T. Aizawa S. Genes Dev. 1992; 6: 1821-1831Crossref PubMed Scopus (531) Google Scholar). Later reports have since noted more subtle abnormalities, and the issue of tenascin knock-out phenotypes will be discussed further below. Tenascin-R—Having subunits 160–180 kDa in size that can form oligomers of 2 or 3 polypeptide chains, tenascin-R shares a high degree of structural homology with tenascin-C. Its expression seems limited exclusively to the central nervous system, although one report of expression in a cell line originating from the peripheral nervous system has been published (21Probstmeier R. Nellen J. Gloor S. Wernig A. Pesheva P. J. Neurosci. Res. 2001; 64: 70-78Crossref PubMed Scopus (30) Google Scholar). Subunits typically have 4.5 EGF-like repeats and 8–9 fibronectin type III repeats. Alternative splicing at the sixth fibronectin type III repeat produces two isoforms that, even when compared with other tenascins, are very highly conserved across different species (3Pesheva P. Probstmeier R. Prog. Neurobiol. 2000; 61: 465-493Crossref PubMed Scopus (78) Google Scholar, 22Carnemolla B. Leprini A. Borsi L. Querze G. Urbini S. Zardi L. J. Biol. Chem. 1996; 271: 8157-8160Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar). Although the 160-kDa isoform tends to form dimers and the larger 180-kDa isoform trimers, the functional significance of the two isoforms is not understood (3Pesheva P. Probstmeier R. Prog. Neurobiol. 2000; 61: 465-493Crossref PubMed Scopus (78) Google Scholar). Tenascin-R expression has some degree of overlap with tenascin-C expression in the developing nervous system, but unlike tenascin-C, the onset of tenascin-R expression occurs at later time points (23Schachner M. Taylor J. Bartsch U. Pesheva P. Perspect. Dev. Neurobiol. 1994; 2: 33-41PubMed Google Scholar). In vitro studies have demonstrated that the protein influences neural pattern formation through adhesive and anti-adhesive effects on cell-matrix interactions (3Pesheva P. Probstmeier R. Prog. Neurobiol. 2000; 61: 465-493Crossref PubMed Scopus (78) Google Scholar); however, like their tenascin-C counterparts, tenascin-R knock-out mice are viable and fertile and only recently has it been demonstrated that they also show behavioral differences (24Weber P. Bartsch U. Rasband M.N. Czaniera R. Lang Y. Bluethmann H. Margolis R.U. Levinson S.R. Shrager P. Montag D. Schachner M. J. Neurosci. 1999; 19: 4245-4262Crossref PubMed Google Scholar, 25Freitag S. Schachner M. Morellini F. Behav. Brain Res. 2003; 145: 189-207Crossref PubMed Scopus (68) Google Scholar, 26Montag-Sallaz M. Montag D. Genes Brain Behav. 2003; 2: 20-31Crossref PubMed Scopus (52) Google Scholar). Tenascin-X—Over 400 kDa in size, tenascin-X is the largest known member of the family and widely expressed during development. Adult expression, however, is mostly limited to musculoskeletal, cardiac, and dermis tissue. Individual subunits have 18.5 EGF-like repeats and 29 or more fibronectin type III repeats depending on the species (27Bristow J. Tee M.K. Gitelman S.E. Mellon S.H. Miller W.L. J. Cell Biol. 1993; 122: 265-278Crossref PubMed Scopus (254) Google Scholar, 28Ikuta T. Sogawa N. Ariga H. Ikemura T. Matsumoto K. Gene (Amst.). 1998; 217: 1-13Crossref PubMed Scopus (50) Google Scholar, 29Elefteriou F. Exposito J.Y. Garrone R. Lethias C. J. Biol. Chem. 1997; 272: 22866-22874Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). Although tenascin-X appears capable of forming trimers, it differs from other family members in that it lacks the amino-terminal cysteine residues involved in hexamer formation. Although alternative splicing of the fibronectin type III repeats has not been described for tenascin-X in humans, splice variants have been reported for the mouse homologue (28Ikuta T. Sogawa N. Ariga H. Ikemura T. Matsumoto K. Gene (Amst.). 1998; 217: 1-13Crossref PubMed Scopus (50) Google Scholar, 30Matsumoto K. Saga Y. Ikemura T. Sakakura T. Chiquet-Ehrismann R. J. Cell Biol. 1994; 125: 483-493Crossref PubMed Scopus (145) Google Scholar, 31Speek M. Barry F. Miller W.L. Hum. Mol. Genet. 1996; 5: 1749-1758Crossref PubMed Scopus (35) Google Scholar). A close relative sharing similar expression patterns, but smaller in size (170–220 kDa subunits) with significantly fewer EGF-like repeats, was originally identified in chickens as a new tenascin paralogue called tenascin-Y (32Hagios C. Koch M. Spring J. Chiquet M. Chiquet-Ehrismann R. J. Cell Biol. 1996; 134: 1499-1512Crossref PubMed Scopus (82) Google Scholar). It contains among the fibronectin type III repeats a region rich in amino acid triplets of serine and proline, an unusual domain that has been found in some variants of tenascin-X (5Jones F.S. Jones P.L. Dev. Dyn. 2000; 218: 235-259Crossref PubMed Scopus (549) Google Scholar). More recently, although the two proteins differ greatly in their respective numbers of EGF-like repeats, their many other similarities have led to the suggestion that tenascin-Y is an avian orthologue of tenascin-X (6Chiquet-Ehrismann R. Int. J. Biochem. Cell Biol. 2004; 36: 986-990Crossref PubMed Scopus (112) Google Scholar, 7Chiquet-Ehrismann R. Chiquet M. J. Pathol. 2003; 200: 488-499Crossref PubMed Scopus (443) Google Scholar). Tenascin-X is the first tenascin whose deficiency has been clearly associated with a pathological disorder in humans, a variant of a heritable connective tissue disorder known as Ehler-Danlos Syndrome, which is associated with fibrillar collagen defects. The tenascin-X gene overlaps on human chromosome 6 with the gene coding for the steroid 21-hydroxylase whose deficiency results in congenital adrenal hyperplasia. The first instances of human tenascin-X deficiency were found in patients exhibiting clinical signs of both congenital adrenal hyperplasia and Ehler-Danlos Syndrome who were found to have a contiguous deletion encompassing both genes (33Burch G.H. Gong Y. Liu W. Dettman R.W. Curry C.J. Smith L. Miller W.L. Bristow J. Nat. Genet. 1997; 17: 104-108Crossref PubMed Scopus (272) Google Scholar, 34Schalkwijk J. Zweers M.C. Steijlen P.M. Dean W.B. Taylor G. van Vlijmen I.M. van Haren B. Miller W.L. Bristow J. N. Engl. J. Med. 2001; 345: 1167-1175Crossref PubMed Scopus (302) Google Scholar). Tenascin-W—First identified in zebrafish (35Weber P. Montag D. Schachner M. Bernhardt R.R. J. Neurobiol. 1998; 35: 1-16Crossref PubMed Scopus (81) Google Scholar), tenascin-W remains the least well characterized member of the tenascin family. Trimers of 130-kDa subunits have been isolated from zebrafish tissues with each subunit containing 3.5 EGF-like repeats and 5 fibronectin type III repeats. Isoforms from alternative splicing have not been reported for zebrafish tenascin-W. Expression has been found in developing skeletal tissue and neural crest cells with a pattern that partially overlaps tenascin-C expression patterns. Although descriptions of tenascin-W expression have been almost exclusively limited to zebrafish, recent reports have identified possible mammalian orthologues (see below and Fig. 2). Tenascin-C and -R Knock-out Phenotypes—Observations that tenascin-C- and tenascin-R-null mice (including single and double knock-outs) have grossly normal phenotypes belied the presumed essential, albeit unknown, role suggested by their highly regulated expression and well conserved presence in vertebrate genomes. Subsequent studies, however, have revealed subtle abnormalities in behavior and wound healing, suggesting that tenascins be for they have subtle actions that in adult E. Tucker R. J. Cell Sci. 1999; 112: 3847-3853Crossref PubMed Google Scholar). This of a on adult has been by recent reports in the of mice M. Hirsch E. J. Cell Mol. Biol. 2004; PubMed Scopus (50) Google Scholar) and that tenascin-R-null mice show behavioral differences S. Schachner M. Morellini F. Behav. Brain Res. 2003; 145: 189-207Crossref PubMed Scopus (68) Google Scholar, 26Montag-Sallaz M. Montag D. Genes Brain Behav. 2003; 2: 20-31Crossref PubMed Scopus (52) Google Scholar). Although the mechanisms are not it has been suggested that tenascins an important role in as well as with cells on observations that in mice show through the central nervous system extracellular E. 2004; PubMed Scopus Google Scholar). Tenascin-R in and subtle behavioral abnormalities but grossly normal of mice have led to that the protein be more important for and adult central nervous system development and S. Schachner M. Morellini F. Behav. Brain Res. 2003; 145: 189-207Crossref PubMed Scopus (68) Google Scholar, 26Montag-Sallaz M. Montag D. Genes Brain Behav. 2003; 2: 20-31Crossref PubMed Scopus (52) Google Scholar). from mice decreased decreased to when with new and in and suggesting that tenascin-R is important for behavior a possible for its highly conserved of tenascin-R with distinct in during development in suggesting a by which the modulatory functions of tenascin-R during the of an organism A. Pesheva P. D. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). evidence has suggested potential significant for the protein in adult neural In zebrafish, tenascin-R has been shown to act as a molecule for injury in J. J. Schachner M. T. Mol. Cell. Neurosci. 2004; PubMed Scopus Google Scholar) and to of in adult mice A. A. Schachner M. P.M. Nat. Neurosci. 2004; PubMed Scopus Google Scholar). In to effects on tenascin-R a role through its to the of cells that are activated central nervous system injury M. M. Probstmeier R. Pesheva P. J. Neurosci. 1998; PubMed Google Scholar). It has been reported recently that this modulation be the of distinct but actions by the EGF-like and fibronectin type III domains on receptors H. S. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). Mammalian reports have described mammalian proteins with close similarities to zebrafish tenascin-W J. S. M. J. Schachner M. Mol. Cell. Neurosci. 2003; PubMed Scopus Google Scholar, A. Tucker R.P. E. M. Martin D. Chiquet-Ehrismann R. J. 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The form is in with collagen that have subtle such as of and but appear in tenascin-X demonstrate in collagen that Ehler-Danlos Syndrome, that tenascin-X a significant role in collagen Taylor G. Dean W.B. Bristow J. Nat. Genet. 2002; PubMed Scopus Google Scholar). The underlying mechanisms are still not well but recent evidence that tenascin-X can the of collagen formation by binding to collagen or through the of type which is known to formation T. T. Y. G. M. H. T. F. K. Ariga H. Matsumoto K. Cell Res. 2004; PubMed Scopus Google Scholar). in tenascin-X also have fibers M.C. van I.M. van R.P. Steijlen P.M. Bristow J. J. J. 2004; 122: Full Text Full Text PDF PubMed Scopus Google Scholar), suggesting although it is not tenascin-X deficiency fiber recent evidence also that tenascin-X deficiency results in in and the of in mouse in such a as to its K. T. S. Y. H. K. J. Ariga H. Genes 2004; PubMed Scopus Google Scholar). biology seems to be in a of that human biology as a The nature of tenascins seems from the shared similarities of their domains with proteins such as EGF, fibronectin, and highly conserved structures and tightly regulated expression only the that tenascins have some in vertebrate The to any and however, has led to tenascins being over a as proteins in of H.P. Curr. Opin. Cell Biol. 1993; 5: 869-876Crossref PubMed Scopus (355) Google Scholar). Although a of has been these proteins in the including an with a human they remain whose functions are still not well understood. it is that tenascins an important of extracellular proteins that have a to cell behavior and significant in and of a and of human