Embryonic expression of tenascin‐X suggests a role in limb, muscle, and heart development

Abstract

Tenascin-X (TN-X) is the newest member of the tenascin family of extracellular matrix proteins and it is highly expressed in muscular tissues during development. To gain insight into the possible functions of TN-X during development, we evaluated its expression in the rat embryo. Using an 800 bp cDNA encoding the fibrinogen-like domain of TN-X, we show that TN-X expression begins in migrating cells of the epicardium in the El2 heart. The epicardium provides progenitors of fibrous and vascular tissue to the developing heart. After the epicardium is complete, TN-X is expressed in the sub-epicardial space in association with developing blood vessels, and later by non-myocytes dispersed through the myocardial wall. A similar pattern of TN-X expression, first in connective tissue surrounding muscle, and then by a subset of cells within muscle, was seen in para-axial, body wall, craniofacial, and appendicular muscle. This pattern suggests a role in connective tissue cell migration and late muscle morphogenesis. TN-X is also highly expressed in the interdigital space at El5 and surrounding developing tendons, suggesting an additional role in cell fate determination. Although the pattern of TN-X expression is distinct from that of tenascin C, they are frequently expressed in close proximity. Indirect genetic evidence in humans suggests an essential function for TN-X, and the pattern of TN-X expression in heart, skeletal muscle, and limb is consistent with this hypothesis.

Joint hypermobility syndromes: The pathophysiologic role of tenascin‐X gene defects

“…In developing rats, tenascin-X is expressed in tendon sheaths, ligaments, synovium, muscle, epicardium, and blood vessel adventitia (48), and a similar distribution has also been shown for pigs (49). In mice, tenascin-C and tenascin-X are coexpressed in muscle, gut, skin, and vasculature (50).…”

The epicardium and epicardially derived cells (EPDCs) as cardiac stem cells

“…This subepicardium is a hydrated ECM, rich in proteins, including collagens I, IV, V, and VI and fibronectin (Tidball, 1992;Hurlé et al, 1994;Ká lmá n et al, 1995;Bouchey et al, 1996;Kim et al, 1999), flectin (Tsuda et al, 1998), fibulin-2 (Tsuda et al, 2001), GP68 (Morita et al, 1998), laminin and proteoglycans (Ká lmá n et al, 1995), vitronectin, fibrillin-2, elastin (Bouchey et al, 1996), and tenascin-X (Burch et al, 1995). It has been shown that both the epicardium and myocardium contribute proteins to this matrix (Bouchey et al, 1996).…”

Tenascins: regulation and putative functions during pathological stress

“…Like tenascin-C and tenascin-R, tenascin-X is weakly active as a cell adhesion protein and cells in general do not spread on a tenascin-X substrate [11]. Its major site of expression is in the connective tissue of heart and skeletal muscle as well as in the dermis of skin [12,13]. In the chicken the closest homologue of tenascin-X is tenascin-Y, and judging from its sequence similarity as well as its analogous expression pattern it can be considered as the avian orthologue of tenascin-X [14,15].…”