THE CANINE BETAINE GAMMA-AMINO-N-BUTYRIC ACID TRANSPORTER GENE - DIVERSE MESSENGER-RNA ISOFORMS ARE REGULATED BY HYPERTONICITY AND ARE EXPRESSED IN A TISSUE-SPECIFIC MANNER

The Na+- and Cl--coupled betaine transporter, designated BGT1, a membe r of the neurotransmitter transporter gene family, is responsible for accumulation of betaine in hypertonic Madin-Darby canine kidney (MDCK) cells and presumably in the hypertonic renal medulla, The canine gene for the betaine gamma-amino-n-butyric acid transporter has been clone d and analyzed, The gene extends over 28 kb and consists of 18 exons, The 5' end of the gene has three alternative first exons (1A, 1B, and 1C+D), Analysis of BGT1 mRNA revealed that there is considerable diver gence in the 5' untranslated sequence resulting from three different 5 ' end motifs (A, B, and C) followed by an alternative motif (D) as wel l as two internal acceptor sites for splicing, Eight kinds of BGT1 mRN A were classified into three types (A, B, and C) according to the 5' e nd sequence, Northern blot analysis using probes specific for the A, B , or C motif revealed that hypertonicity induces all three types in MD CK cells, Reverse transcription and polymerase chain reaction showed t hat each type was expressed in a tissue-specific manner. Primer extens ion and/or RNase protection assays as well as transfection assays into MDCK cells demonstrated that exons 1A, 1B, and 1C+D have independent transcription initiation sites under control of independent promoters, Diverse mRNA isoforms are regulated by hypertonicity and are expresse d in a tissue-specific manner.