Characterization of mouse serum tenascin-X.

The interstitial extracellular matrix tenascin-X (iTNX), which has a molecular mass of roughly 450 kDa, is expressed at high levels in muscular tissues and skin. In this study, we identified the serum form of TNX (sTNX) with a molecular mass of 200 kDa in the mouse. Western blot analysis with specific antibodies against fibronectin type III-like (FNIII) repeats of TNX and N-terminal sequence analysis of 200-kDa sTNX revealed that the N-terminus of sTNX is located in the juncture between the 16th FNIII (M16) and 17th FNIII (M17) repeats of iTNX. The 200-kDa sTNX contains 15 FNIII repeats and a fibrinogen domain identical to the Cterminal portion of the iTNX. TNX-deficient mice lacked not only iTNX but also sTNX. Furthermore, 200-kDa sTNX was generated by cleavage of the spleen iTNX by spleen homogenate, and its generation was inhibited by protease inhibitors. These results suggest that sTNX is generated by proteolytic cleavage of iTNX.