THE DISTRIBUTION OF TENASCIN-X IS DISTINCT AND OFTEN RECIPROCAL TO THAT OF TENASCIN-C

We have isolated a cDNA encoding mouse tenascin-X (TN-X), a new member of the family of tenascin genes. The TN-X gene lies in the major hist ocompatibility complex (MHC) class III region, as it is the case for i ts human counterpart. On Northern blots we detected a TN-X mRNA of app roximately 13 kb in most tissues analyzed, whereas in various mouse ce ll lines mRNAs of approximately II and 13 kb were detected, suggesting the possibility of alternative splicing of TN-X transcripts. We raise d antibodies against mouse TN-X fragments expressed in bacteria and us ed these antibodies to identify the TN-X protein in heart cell extract s and in the conditioned medium of a renal carcinoma cell line. The su bunit molecular size of TN-X is approximately 500 kD, suggesting that the protein may contain up to 40 fibronectin type III repeats, making it the largest tenascin family member known yet. TN-X in conditioned m edium, as well as the purified protein bind to heparin, but no binding to tenascin-C (TN-C), fibronectin, laminin or collagens could be dete cted. Thus the heparin-binding activity may be a common feature of the tenascins. The TN-X mRNA as well as the protein are predominantly exp ressed in heart and skeletal muscle, but the mRNA is found in most tis sues at a low level. Immunostaining showed the protein to be associate d with the extracellular matrix of the muscle tissues and with blood v essels in all of the tissues analyzed. Although the TN-X gene lies in the MHC class III locus, it is not expressed in the lymphoid organs an alyzed, except for the staining around blood vessels. In skin and tiss ues of the digestive tract often a reciprocal distribution of TN-X and TN-C was observed.