A new alternatively spliced exon between v9 and v10 provides a molecular basis for synthesis of soluble CD44.
The numerous isoforms of murine CD44 contain a common peptide region that is encoded by exons 1-5, 16-18, and 20 and variant regions derived from exons 6-15, usually referred to as v1-v10. We have obtained evidence for expression of an additional exon between v9 (or exon 14) and the exon previously termed v10 (or exon 15). Thus, we now number the variant exons as follows: v1-v9 (exons 6-14), v10 (exon 15), and v11 (exon 16); the remaining 3'-exons become exons 17-21 (newly numbered exons are underlined). The new exon, now termed exon v10, contains 93 base pairs and can be internally spliced; the 5'-region is termed v10a, and the 3'-region, v10b. Stop codons are positioned in v10a such that translated protein would be truncated prior to the transmembrane domain and secreted as a soluble protein. We have also found that the previously described v9 exon (now termed v9a), which is 90 base pairs in length, is actually the 5'-region of a longer exon of 142 base pairs (the 3'-region is termed v9b) and thus arises by internal splicing of the longer exon. Using reverse transcription-polymerase chain reaction, four different cDNAs for CD44 isoforms that use different combinations of the new exonic sequences have been found. The mRNAs containing the new exonic sequences are restricted in their expression; to date, we have demonstrated their presence in murine G8 myoblasts in culture and in embryonic muscle and cartilage tissues in vivo. Of these new isoforms, the predominant, full-length amplified product is encoded by exons 1-5, exon 13 (v8), the 5'-part of exon 14 (v9a), exon 15 (v10), exon 16 (v11), exons 17-19, and exon 21. When COS-7 cells were transfected with v10-containing cDNA constructs, the cells secreted low molecular weight soluble CD44 into their medium. Thus, the stop codons within the new exon v10 provide a molecular basis for de novo synthesis of soluble CD44 isoforms.