Tenascin-Y: a protein of novel domain structure is secreted by differentiated fibroblasts of muscle connective tissue

Tenascin-Y was identified in chicken as a novel member of the tenascin (TN) family of ECM proteins. Like TN-C, TN-R, and TN-X, TN-Y is a multidomain protein consisting of heptad repeats, epidermal growth factor-like repeats, fibronectin type III-like (FNIII) domains and a domain homologous to fibrinogen. In contrast to all other known TNs, the series of FNIII domains is interrupted by a novel domain, rich in serines (S) and prolines (P) that occur as repeated S-P-X-motifs, where X stands for any amino acid. Interestingly, the TN-Y-type FNIII domains are 70-100% identical with respect to their DNA sequence. Different TN- Y variants are created by alternative splicing of FNIII domains. Although, based on sequence comparisons TN-Y is most similar to mammalian TN-X, these molecules are not species homologues. TN-Y is predominantly expressed in embryonic and adult chicken heart and skeletal muscle and, to a lower extent, also in several non-muscular tissues. Two major transcripts of approximately 6.5 and 9.5 kb are differentially expressed during heart and skeletal muscle development and are also present in the adult. Anti-TN-Y antibodies recognize a approximately 400-kD double band and a approximately 300-kD form of TN- Y on immunoblots of chicken heart extracts. In situ hybridization and immunofluorescence analysis of aortic smooth muscle, heart, and skeletal muscle revealed that TN-Y is mainly expressed and secreted by cells within muscle-associated connective tissue. Cultured primary muscle fibroblasts released a approximately 220-kD doublet and a approximately 170-kD single TN-Y variant only when cultured in 10% horse serum but not in medium containing 10% fetal calf serum. All TN-Y variants isolated bind to heparin under physiologically relevant conditions that may indicate an important function retained in all tenascins.