Tenascins are a family of 4 oligomeric extracellular glycoproteins, tenascin (TN) C, R, X, and W. In rotary shadowing images TNC is seen as a symmetrical structure called a hexabrachion (Erickson & Iglesias 1984). This hexamer is formed from initial trimers (Kammerer et al. 1988). All members of the family are believed able to form trimers but only C, R and W have the extra cysteine required for form hexamers. All have amino-terminal heptad repeats, epidermal growth factor (EGF)-like repeats, fibronectin type III domain repeats, and a carboxyl-terminal fibrinogen-like globular domain (Hsia & Schwartzbauer 2005). TNC was the first of the family to be discovered and is the best characterised (Midwood et al. 2011). Its subunits vary greatly in size (between 190 and 330 kDa of the tenascin-C monomer) due to glycosylation and splicing isoforms (Joester & Faissner 1999). During embryonic development TNC is expressed in neural, skeletal, and vascular tissues. In adults it is detectable only in tendon and tissues undergoing remodeling processes such as wound repair and neovascularization, or in pathological processes such as inflammation and tumorigenesis. TNR forms dimers and trimers (Norenberg et al. 1992) and is expressed only in the developing and adult central nervous system. TNC and TNR-null mice (single and double knock-outs) have surprisingly normal gross phenotypes, but exhibit behavioural and wound healing abnormalities (Mackie & Tucker 1999, Montag-Sallaz & Montag 2003). TNX (termed tenascin-Y in chicken) is the largest member of the family and is widely expressed during development, but in adults is limited to musculoskeletal, cardiac, and dermal tissue. It can form trimers, though it lacks the amino-terminal cysteine residues involved in hexamer formation. It is clearly associated with a variant of a heritable connective tissue disorder known as Ehler-Danlos Syndrome, which is associated with fibrillar collagen defects (Burch et al. 1997, Mao et al. 2002). TNY is thought to be an avian orthologue of TNX (Chiquet-Ehrismann 2004). TNW, first identified in zebrafish (Weber et al. 1998), is the least well characterized member of the tenascin family. It forms trimers and is expressed in developing skeletal tissue, neural crest cells and kidney, a pattern that partially overlaps with TN-C.
TNC binds syndecan-3 (Salmivirta et al. 1991, Koyama et al. 1996) Syndecans are type I transmembrane proteins, with an N-terminal ectodomain that contains several consensus sequences for glycosaminoglycan attachment and a short C-terminal cytoplasmic domain. Syndecan-1 and -3 glycosaminoglycan attachment sites occur in two distinct clusters, one near the N-terminus and the other near the membrane-attachment site, separated by a proline and threonine-rich 'spacer'. Syndecan ectodomain sequences are poorly conserved in the family and between species, but the transmembrane and cytoplasmic domains are highly conserved. Syndecan-1 and -3 form a subfamily. Syndecan-3 on the cell surface is frequently oligomeric (Asundi & Carey 1995). Syndecans bind extracellular ligands via their attached heparan sulphate chains, playing roles in cell to matrix and cell to cell adhesion.