Translational control of the rat angiotensin type 1a receptor by alternative splicing

The rat angiotensin type 1a receptor (AT1aR) is comprised of three exons. Two transcripts are possible due to alternative splicing of exon 2 (E1,3 and E1,2,3). Both transcripts code for identical AT1aR proteins since they differ only in the length of their 5' leader sequence (5'LS). We investigated the functional differences of these two transcripts in stably transfected Chinese hamster ovary (CHO) cells and also determined the splice variant composition in rat tissues. E1,3 expressing cells exhibited 1.8-fold higher AT1R densities and five-fold higher levels of Ang II-stimulated inositol phosphate production compared to E1,2,3 expressing cells. No differences in E1,3 and E1,2,3 mRNA levels or mRNA stability were seen. In vitro translation assays revealed 1.8-fold higher AT1aR protein levels from E1,3 compared to E1,2,3 transcripts, suggesting exon 2 reduces functional AT1R expression by inhibiting translation. Deletion of 10 nucleotides in exon 2 increased translation of the mutated E1,2,3 transcript to levels which were indistinguishable from E1,3, suggesting that this loop region of a predicted hairpin contributes to the inhibitory RNA cis element within exon 2. Comparison of AT1aR exonic composition and AT1R densities in rat tissues suggests alternative splicing is regulated in a tissue-specific manner and contributes to tissue-specific differences in AT1R density.