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Call For Papers: Comparative Genomics

Alternative splicing and exon duplication generates 10 unique porcine 5-HT₄ receptor splice variants including a functional homofusion variant

¹ Heymans Institute of Pharmacology, Ghent University, Ghent
² Department of Internal Medicine, Johnson & Johnson Pharmaceutical Research and Development, Beerse, Belgium
³ Department of Functional Genomics, Johnson & Johnson Pharmaceutical Research and Development, Beerse, Belgium

5-HT₄ receptors are present in human and porcine atrial myocytes while they are absent from the hearts of small laboratory animals. The pig is therefore the only available nonprimate animal model in which to study cardiac 5-HT₄ receptor function under physiological conditions. While several human splice variants of the 5-HT₄ receptor have been described, the splicing behavior of this receptor in porcine tissue is currently unknown. Here we report on the identification of nine novel COOH-terminal splice variants of the porcine 5-HT₄ receptor, which were named 5-HT_{4(b2, j, k, l, m, o, p, q, r)}. The internal h-variant was found in combination with several COOH-terminal exons. In addition, splice variants were found that comprised duplicated exons fused to the common region of the 5-HT₄ receptor, thereby providing evidence for a duplication of the porcine HTR4 gene. One of these variants putatively encoded a nine transmembrane-spanning domain homofusion receptor, 5-HT_4(9TM); also the other variants with a duplicated region might translate into functional, transcriptionally fused dimeric 5-HT₄ receptor variants. The elucidation of the genomic context confirmed that the variants were not genomic artefacts but originated from alternative splicing. This was further corroborated by a functional analysis of the variants 5-HT_4(a), 5-HT_4(r), and 5-HT_4(9TM). To our knowledge, our data are the first to report on a functional GPCR with more than seven predicted transmembrane domains. These findings urge for caution when interpreting data on 5-HT₄ receptor-related pharmacology obtained in the pig; validation at the molecular level might be needed before extrapolating results to human.

7 transmembrane receptor; 9 transmembrane receptor; pig; G protein-coupled receptor; 5-hydroxytryptamine