Tenascins are a family of 4 oligomeric extracellular glycoproteins, tenascin (TN) C, R, X, and N (also called 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 N 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 to be discovered and is the best characterised. Its subunits vary greatly in size 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 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 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). TNN, first identified in zebrafish (Weber et al. 1998), is the least well characterized member of the tenascin family. It forms hexamers (Degen et al. 2007) and is expressed in developing skeletal tissue and neural crest cells, a pattern that partially overlaps with TNC.
TNC and TNR bind with high affinity to fibronectin (FN) (Chiquet-Ehrismann et al. 1991, Chung et al. 1995, Chung & Erickson 1997, Hauzenberger et al. 1999, Ingham et al. 2004, To & Midwood 2011, Pesheva et al. 1994), modulating the cell adhesion function of FN either by binding or restricting access of FN to integrin binding sites (Lightner & Erickson 1990) or by binding to cell receptors and altering their responsiveness to FN (Prieto et al. 1992, Fischer et al. 1997). The interaction of Tenascin and FN impacts tissue structure by controlling the assembly, maintenance, and turnover of the ECM at the cell surface (To & Midwood 2010).