TENASCIN MESSENGER-RNA ISOFORMS IN THE DEVELOPING MOUSE-BRAIN

The extracellular matrix glycoprotein tenascin is expressed in the dev eloping mouse cerebellum as a group of four protein species of differe nt molecular weights. The difference is most likely due to alternative splicing which is known to occurr in tenascin mRNA within the region of the fibronectin type III repeats. In order to systematically analyz e tenascin mRNA isoforms that would account for this heterogeneity, te nascin splice variants were isolated from mouse brain by the polymeras e chain reaction (PCR). In agreement with Northern blot analysis, ampl ification by PCR revealed a general decrease in tenascin mRNA expressi on during development from embryonic and early postnatal to adult stag es. This decrease was more pronounced for isoforms of high molecular w eight compared to those of low molecular weight. In accord with the ob servations at the protein level, four splice variants were found to be predominantly expressed, containing insertions of either six, five, o r one fibronectin type III repeat, or comprising no insertion. In addi tion, a minor splice variant with an insertion of four fibronectin typ e III repeats was isolated. Three of the isolated mRNA splice variants have not yet been described for mouse tenascin. Among them, an isofor m containing six alternatively spliced repeats was found to include a novel fibronectin type III repeat. The sequence of this repeat display s 96.7% similarity to a corresponding type III repeat in human tenasci n, revealing a strict evolutionary conservation between tenascin molec ules from different species in the region of alternative splicing. Sou thern blot analysis of the amplified mRNA isoforms showed that the nov el mouse type III repeat is confined to splice variants with an insert ion of six fibronectin type III repeats. Furthermore, in situ hybridiz ation on sections from mouse embryos indicated that tenascin-specific mRNAs containing the novel type III repeat are predominantly expressed in the central nervous system. (C) 1994 Wiley-Liss, Inc.