NOVEL TENASCIN VARIANTS WITH A DISTINCTIVE PATTERN OF EXPRESSION IN THE AVIAN EMBRYO

Previous studies have shown that several forms of the glycoprotein ten ascin are present in the embryonic extracellular matrix. These forms a re the result of alternative splicing, which generates tenascin varian ts with different numbers of fibronectin type III repeats. We have use d degenerate primers and PCR to isolate a novel tenascin exon from an avian genomic library. Genomic clones contained a sequence encoding a fibronectin type III repeat that corresponds to repeat 'C' from the va riable domain of human tenascin. To demonstrate that tenascin containi ng repeat 'C' is actually synthesized by avian cells, a monospecific a ntiserum was raised against a repeat 'C' fusion protein. This antiseru m recognized a novel high-molecular-weight variant on immunoblots of t enascin isolated from chicken embryo fibroblast-conditioned medium, an d stained tendons on frozen sections of chicken embryos. A cDNA probe specific for mRNA encoding repeat 'C' was used for in situ hybridizati on, This probe hybridized in a subset of the embryonic tissues labelle d with a universal tenascin probe, including tendons, ligaments and me senchyme at sites of epithelial-mesenchymal interactions. Finally, we provide evidence that additional fibronectin type ITC repeats, one cor responding to a recently discovered human repeat as well as one entire ly novel sequence, also exists in chicken tenascin mRNA. These data in dicate that tenascin is present in the embryonic matrix in a multitude of forms and that these forms have distinctive distributions that may reflect more than one function for tenascin in development.