Of all the organs of the body, the large arteries, particularly the aorta, are subject to the greatest mechanical stress. They have a well-organized fibrous framework. In the tunica media, multilayered elastin sheets (lamellae) connected by fine elastin fibers form a three-dimensional continuous network that links smooth muscle cells. This elastin network of the arterial wall functions as an elastic reservoir protecting the tissue from destructive stress. The outermost layer, the tunica adventitia, consists of a collagen-rich ECM and helps prevent vascular rupture at extremely high pressures (Wagenseil and Mecham, 2009). In the heart, the major structural component of the ECM is collagen, which also forms a three-dimensional network interconnecting myocytes to each other and to the vasculature (Caulfield and Borg, 1979; Borg and Caulfield, 1981). The fibrous skeleton composed of collagen is continuous with the annulus fibrosus cordis, the support apparatus of the tricuspid, mitral, and aortic valves to the cardiac muscle in a manner analogous to the attachment of tendons to skeletal muscle (Hinton and Yutzey, 2011). This stress-tolerant collagenous network not only contributes to passive elastic properties of the heart but also to the transmission of mechanical forces to and from the cardiomyocytes (reviewed in Sussman et al., 2002; Bowers et al., 2010; Borg and Baudino, 2011).

In addition to the fibrous ECM, a unique functional category of non-structural ECM, matricellular proteins, are receiving increasing attention (Bornstein, 2009). Matricellular proteins constitute a growing family (Table 1) that originally included thrombosondin-1 (TSP1), SPARC (secreted protein, acid and rich in cysteine; osteonectin), and TNC (Sage and Bornstein, 1991), and then TSP2, osteopontin, CCN1, CTGF (CCN2), and tenascin-X were added (Bornstein and Sage, 2002). They have common unique properties: (1) expressed at high levels during development and in response to injury; (2) do not subserve structural roles but function as modulators of cell-matrix interactions; (3) bind to many cell-surface receptors, other ECM molecules, growth factors, cytokines, and proteases; (4) generally induce de-adhesion, in contrast to the positive adhesivity of most matrix proteins (Bornstein and Sage, 2002). The term has become used more widely and new members, such as galectins and periostin, have joined the group (Bornstein, 2009). In cardiovascular development, significant roles of periostin have been reported (Conway and Molkentin, 2008; Inai et al., 2008; Norris et al., 2008, 2009; Ghatak et al., 2014). It is also noteworthy that some members, such as SPARC, osteopontin, and periostin, have been found to be related to developing bone and teeth, which are subjected to strong mechanical stress.

Tenascins are a family of multimeric ECM glycoprotein characterized by an N-terminal globular domain and heptad repeats, which facilitate multimerization; one or more tenascin-type epidermal growth factor (EGF)-like repeats; a series of fibronectin (FN) type III domains, and a C-terminal fibrinogen-related domain. There are six names for the tenascin gene products: tenascin-C, X, R, Y, W, and N (Tucker et al., 2006; Tucker and Chiquet-Ehrismann, 2009). TNC was the first tenascin found to be highly expressed in tendons and embryonic ECM (Chiquet-Ehrismann et al., 1986). It was discovered independently in several laboratories as glioma mesenchymal ECM antigen, myotendinous antigen, cytotactin, and J1 glycoprotein (reviewed in Tucker et al., 2006; Chiquet-Ehrismann and Tucker, 2011). Tenascin-R is the second member and is predominantly expressed in the central and peripheral nervous systems (Rathjen et al., 1991). Tenascin-X is a mammalian tenascin primarily expressed in loose connective tissue such as the dermis, epimysium, and blood vessels (Matsumoto et al., 1992; Bristow et al., 1993) Mutations in tenascin-X can lead to a type of Ehlers–Danlos Syndrome (reviewed in Bristow et al., 2005) Tenascin-Y is an avian tenascin similar to mammalian tenascin-X (Hagios et al., 1996). Tenascin-W (Weber et al., 1998) is found primarily in pre-osteogenic areas, the kidney, smooth muscle, and most prominently also in cancer stroma. Tenascin-N is most recently discovered tenascin and is similar to tenascin-W (Neidhardt et al., 2003).

TNC is the best characterized member of the family (Orend and Chiquet-Ehrismann, 2006; Midwood and Orend, 2009; Chiquet-Ehrismann and Tucker, 2011; Midwood et al., 2011; Udalova et al., 2011; Brellier and Chiquet-Ehrismann, 2012; Chiquet-Ehrismann et al., 2014) and is a typical matricellular protein. It is a huge molecule of approximately 220–400 kDa as an intact monomer and is assembled as a hexamer. TNC is found in many developing organs of embryos, down-regulated after birth to a few tissues bearing high tensile stress and locations of high cell turnover, but highly up-regulated during injury, inflammation, regeneration, and cancer (Chiquet-Ehrismann et al., 2014). A number of in vitro studies suggest that TNC has a wide range of effects on cell adhesion, motility, differentiation, growth control, and ECM organization via multiple cell surface receptors including integrins α9β 1, αvβ 3, and αvβ 6, Toll-like receptor 4 (TLR4) and syndecan-4 (Orend and Chiquet-Ehrismann, 2006; Midwood and Orend, 2009). As in the case of target disruption of several other matricellular protein genes, TNC knockout mice develop normally (Saga et al., 1992; Forsberg et al., 1996). Recent detailed investigations of various disease models using TNC KO have suggested that TNC may promote tissue healing but enhances inflammation and fibrosis (Midwood et al., 2011; Udalova et al., 2011; Brellier and Chiquet-Ehrismann, 2012; Imanaka-Yoshida, 2012; Chiquet-Ehrismann et al., 2014).

During embryogenesis and tissue remodeling, TNC is expressed transiently at specific sites, suggesting that the expression of TNC is tightly regulated dependent on the cell type and tissue microenvironment (Tucker and Chiquet-Ehrismann, 2009). Many different growth factors, such as TGFβ, FGF, PDGF, and proinflammatory cytokines, are able to induce TNC expression (for a review, see Orend and Chiquet-Ehrismann, 2006; Tucker and Chiquet-Ehrismann, 2009).

A variety of signaling pathways and transcription factors are known to stimulate TNC transcription (reviewed in Chiquet-Ehrismann and Tucker, 2011). These include TGF/Smad 3/4 (Jinnin et al., 2004), TLR4/NFkB (Goh et al., 2010), c-Jun/NFkB (Mettouchi et al., 1997), Notch (Sivasankaran et al., 2009), Sox4 (Scharer et al., 2009), PDGF/Ets (Jinnin et al., 2006), and MEF2c with scleraxis (della Gaspera et al., 2009). Conversely, TNC can trigger a variety of signaling pathways via multiple cell surface receptors. Interestingly, it affects some of the same signaling pathways that initially trigger the expression leading to negative or positive feedback loops (Chiquet-Ehrismann and Tucker, 2011). For example, PDGF can induce TNC expression via the phosphoinositide 3-kinase/Akt pathway (Jinnin et al., 2006) and MAPK pathways (Chiquet et al., 2004) and, in turn, TNC enhances PDGF signaling by cross-talk between PDGFR-β and integrin αvβ 3 with activation of focal adhesion kinase and Src tyrosine kinase (Ishigaki et al., 2011). In contrast, a negative feedback loops is created in the case of small GTPase RhoA as discussed in the next section.

Mechanical stress is also a strong inducer of TNC. Just as one of its original names, “myotendinous antigen,” suggests, TNC is highly expressed at the myotendinous and osteotendinous junctions (Jarvinen et al., 1999, 2000, 2003) at sites subjected to mechanical stress. High expression of TNC is often observed at the branching point of arteries (Mackie et al., 1992), although the expression level of TNC is generally low in adult blood vessels. Based on this distribution of the molecule, the close association of mechanical stress and TNC has been proposed. Supporting this possibility, load-induced bone remodeling or muscle overload up-regulates the expression of TNC (Webb et al., 1997; Fluck et al., 2000; Mikic et al., 2000; Mackey et al., 2011), while immobilizing tendons down-regulates the expression. In culture, various mechanical stresses including stretching (Chiquet et al., 2004), compression (Jagodzinski et al., 2008), and shear stress (Tan et al., 2013), up-regulate TNC synthesis by fibroblasts, chondrocytes, smooth muscle cells, and endothelial cells.

Several types of cell-surface proteins, including stretch-sensitive ion channels, are known to sense mechanical forces and translate them into biochemical signals (Kung, 2005). Mechanical inputs can be also detected by mechanosensing apparatus of the focal adhesion complex and transduced to the cytoskeleton (Wang et al., 2009). Chiquet and coworkers have shown a mechanism by which a mechano-signal is transduced at the linkage between the ECM and cytoskeleton, which controls TNC transcription mediated by megakaryoblastic leukemia 1 (MAL or MKL1)/myocardin-related transcription factor A (MRTFA) (Chiquet et al., 2007, 2009; Asparuhova et al., 2009, 2011; Brosig et al., 2010). The cycle stretch of fibroblasts up-regulates TNC transcription, independent of de novo protein synthesis, paracrine factors such as TGFβ, and mitogen-activated protein kinases (MAPKs), but depends on actomyosin contractility controlled by the RhoA/ROCK pathway (Sarasa-Renedo et al., 2006) (Figure 1). Mechanical stimuli activate the signaling pathway involving integrin β 1 (Chiquet et al., 2007) and integrin-linked kinase (ILK) (Maier et al., 2008), which induces actin assembly and stress fiber formation via mDia and ROCK (Ridley and Hall, 1992). MAL/MLK1/MRTFA is a coactivator of serum response factor (SRF) and is predominantly localized in the cytoplasm through an interaction with G-actin (Miralles et al., 2003; Guettler et al., 2008). Therefore, depletion of the cytoplasmic G-actin pool following Rho activation causes translocation of MAL into the nucleus, where it induces TNC transcription, partly dependent on SRF (Asparuhova et al., 2011).

RhoA-dependent mechanotransduction requires pericellular fibronectin (Lutz et al., 2010). TNC binds fibronectin at the binding site to syndecan-4, a coreceptor for integrin α5β 1, and has a negative impact on focal adhesion formation and activation of RhoA (Midwood et al., 2006; Lange et al., 2008; Van Obberghen-Schilling et al., 2011). Therefore, mechanically induced TNC may lead to negative feedback from the mechanotrasduction signal. Moreover, since TNC is an elastic molecule that can be stretched to several times its resting length in vitro (Oberhauser et al., 1998; Marin et al., 2003), it may contribute to tissue elasticity and protect against mechanical stress.

Specific roles of TNC in heart morphogenesis have long been anticipated based on its strictly regulated temporal expression at specific sites closely associated with cell migration and epithelial-mesenchymal/mesenchymal-epithelial transition: (Wagenseil and Mecham, 2009) differentiation of precardiomyocytes, (Caulfield and Borg, 1979) cushion tissue formation, (Borg and Caulfield, 1981) valve formation, and (Hinton and Yutzey, 2011) coronary vessel formation (Imanaka-Yoshida et al., 2003).

During the development of mouse embryos, the initial expression of TNC is detected in mesodermal cells in the first heart field (FHH), which undergo mesenchymal-epithelial transition and differentiate to cardiomyocytes and endocardial cells. Once the cells differentiate to cardiomyocytes, they rapidly stop expressing TN-C, while endocardial cells continue to express TNC. TNC expression is also detected at the recruitment of precardiac cells from the second heart field (SHF) (Imanaka-Yoshida et al., 2003). Interestingly, cardiomyocytes from the SHF in the outflow tract maintain the expression of TN-C during looping and shortening.

Another possibility is that TNC may play a role in blood vessel development. In coronary vessels, most vascular progenitors come from the proepicardial organ (PEO) between the primitive heart and the liver bud. Mesenchymal cells from the PEO migrate to the heart and form the epicardium. Epicardial cells undergo EMT, differentiate into endothelial cells and vascular smooth muscle cells (VSMCs), and form a primitive capillary network, which eventually connects to the aorta (see Nakajima and Imanaka-Yoshida, 2013, for review). During this process, TNC is transiently expressed in PEO before cell migration and at epicardial EMT. It is worthy of note that TNC is highly up-regulated and associated with thickening of the vascular wall after the premature vessels are linked with the aorta (Ando et al., 2011), possibly promoting the recruitment of vascular mural cells by facilitating PDGF-BB/PDGFRβ signaling (Ishigaki et al., 2011).

Similar up-regulation of TNC in the vascular wall associated with hemodynamic change is observed during the development of the aorta (Imanaka-Yoshida et al., in press). In E12-13 mouse embryos, weak expression of TNC is detected in the ascending, arch and descending aorta. After ED14-15, when the systemic circulatory system is established, TNC expression is evidently up-regulated and becomes even stronger after birth. In normal adults, the expression of TNC in the aortic wall is generally reduced, although the infra-renal aorta continues to express TNC.

Despite its intriguing expression pattern during cardiovascular development, targeting deletion of the TNC gene causes a grossly normal phenotype (Saga et al., 1992; Forsberg et al., 1996). Our recent preliminary data suggested that over-expression of TNC in the heart may not cause a distinct phenotype, either (unpublished data). Compensatory mechanisms should be present in tissue morphogenesis of the embryo although it is not identified. However, increasing number of studies indicate that TNC is a “stress protein” whose importance becomes apparent when organ homeostasis is challenged by injury or destructive stress such as mechanical overload (Chiquet-Ehrismann et al., 2014), while it is masked during embryonic development.

In the heart, extracellular and intercellular mechanical loads are linked to the myofibrils in cardiomyocytes via various mechanosensing complexes (McCain and Parker, 2011). Cadherins links with myofibrils of neighboring cells at intercalated disks, while integrins attach Z-discs laterally to the connective tissue at costameres (Pardo et al., 1983a,b). Costameres are structures related to the focal adhesion complex and critical cytoskeletal elements involved in environmental mechanochemical signal transduction into cardiomyocytes (Samarel, 2005; Russell et al., 2010). They are also the sites where contractile forces generated within cardiomyocytes are transmitted to the surrounding interstitial collagen network (Danowski et al., 1992; Imanaka-Yoshida et al., 1996, 1999, 2004). Costameres may correspond to the myotendinous junction in the sense of transmitting contraction forces of muscle to connective tissue.

Although TNC is not detected in the normal myocardium, it transiently appears upon tissue injury and inflammation in various heart disease (Imanaka-Yoshida, 2012; Okamoto and Imanaka-Yoshida, 2012).

In an acute myocardial infarction model animal, TNC is exclusively localized at the border zone between the intact and infarcted lesion, the most active site of tissue remodeling (Imanaka-Yoshida et al., 2001; Nishioka et al., 2010). As a typical matricellular protein, TNC could loosen the strong costameric adhesion (Imanaka-Yoshida et al., 2001). This “de-adhesion” function may be useful to release surviving cardiomyocytes to reorganize their shape and arrangement; on the other hand, it should reduce the efficiency of the transduction of contraction force of cardiomyocytes. Furthermore, the border zone should be sites subjected to strong stress due to the difference in the physical property of the intact myocardium and necrotic tissue. By exploiting its elastic properties (Oberhauser et al., 1998; Marin et al., 2003), as discussed in the previous section TNC may protect surviving cardiomyocytes in the border zone as a shock absorber. However, there is no formal proof of this concept. In fact, deletion of TNC attenuates adverse ventricular remodeling and improves cardiac function after myocardial infarction in model mice (Nishioka et al., 2010). Therefore, the adaptive role of TNC in heart tissue remodeling has remained elusive.

Recently, we found that TNC plays an adaptive role in maintaining the tissue strength of the aorta upon hemodynamic and humoral stress and protects aortic tissue from destructive events (Kimura et al., 2014). In this section we summarize our findings and propose the logic of a maintenance mechanism of tissue strength involving TNC. The aorta must maintain tensile strength to tolerate blood pressure, and must also maintain mechanical flexibility and elasticity to accommodate the stroke volume during the systolic phase and to keep the blood flowing during the diastolic phase. Because the blood pressure and stroke volume fluctuate during the cardiac cycle, circadian rhythm, and depending on physical and mental activities, aortic tissue must have a mechanism that locally optimizes these mechanical properties to meet the changes in hemodynamic demands. The failure of such a mechanism would lead to a mismatch between the mechanical properties and hemodynamic demands, causing central arterial hypertension in the case of excessive aortic stiffness (Agabiti-Rosei et al., 2007) or destructive aortic tissue remodeling including aortic aneurysm and aortic dissection (Cronenwett and Johnston, 2010). Because the mechanical properties of aortic tissue are determined mainly by the composition and architecture of ECM (Cronenwett and Johnston, 2010), the maintenance mechanism of aortic mechanical properties is expected to be tightly coupled with the ECM metabolism. TNC is one of the candidate molecules to maintain the strength of the tissue against mechanical stress.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.