DEVELOPMENTALLY-REGULATED ALTERNATIVE SPLICING GENERATES A COMPLEX ARRAY OF DROSOPHILA-PARA SODIUM-CHANNEL ISOFORMS

The para locus encodes the predominant class of sodium channels expres sed in Drosophila neurons. Previous sequence analysis of para cDNAs in dicated the occurrence of alternative splicing at several sites within the open reading frame. Here we report a detailed analysis of this al ternative splicing and its regulation during development. We have used a combination of RNA-PCR and sequence analysis to examine a 1.7 kilob ase region of the para mRNA that encompasses the previously reported s ites of alternative splicing. Five sites of alternative splicing were identified; 48 different splice variants could be generated by the dif ferential exon usage observed. The number of splice forms and their re lative frequency in vivo were characterized in RNA samples of both emb ryos and adults. The range of splice types was found to be much more d iverse in adults than in embryos; of a total of 19 different combinati ons of alternative exons, 11 splice types were found in embryos and 18 in adults. Usage of some individual alternative exons changed during development; a newly identified exon, which is found in one of two for ms either 24 or 30 base pairs long, was present in about 85% of para t ranscripts from embryos but only 7% of those in adults. These data sug gest that a wide variety of subtly distinct Na channel isoforms are pr esent in Drosophila, and that these may provide a range of voltage-gat ed sodium channel functions. Although multiple sodium channel genes ha ve already been described in both Drosophila and mammalian systems, th is study provides a clear indication that sodium channel variability m ay be much greater than previously thought.