Genomic organization and characterization of splice variants of the human histamine H-3 receptor

In the present paper we report the genomic organization of the human histam ine H-3-receptor gene, which consists of four exons spanning 5.5 kb on chro mosome 20. Using PCR, six alternative splice variants of the H-3 receptor w ere cloned from human thalamus. These variants were found to be coexpressed in human brain. but their relative distribution varied in a region-specifi c manner. These isoforms displayed either a deletion in the putative second transmembrane domain (TM). H-3(Delta TM2,H-431aa) or a variable deletion i n the third intracellular loop (i3), H-3(Delta i3415aa), H-3(Delta 13365aa) , H-3(Delta i3,H-329aa) and H3(Delta TM5+Delta i3326aa). In order to determ ine the biological role of the H, receptor variants compared with the 'orig inal' H-3(445aa) receptor. three isoforms. namely H-3(445aa) H-3(Delta TM2, H-431aa) and H-3(Delta i1,H-365aa), were expressedin CHO cells and their ph armacological properties were investigated. Binding studies showed that H,, ,,,,.,,,,,, transiently expressed in CHO cells was unable to bind [I-125]io doproxyfan. whereas both the H-3(445aa) and H-3(Delta i3,H-365aa) receptors displayed a high affinity for [I-125]iodoproxyfan [K-d = 28 +/- 5 pM (n = 4) and 8 +/- 1 pM (n = 5) respectively]. In addition, H-3(Delta i3,H-365aa) possessed the same pharmacological profile as the H,,,,,,,, receptor. Howe ver, in CHO cells expressing H-3(Delta i3,H-365aa), HE agonists did not inh ibit forskolin-induced cAMP production, stimulate [S-35]guanosine 5'-[gamma -thio]triphosphate ([S-35]GTP[S]) binding or stimulate intracellular Ca2mobilization. Therefore the 80-amino-acid sequence located at the C-termina l portion of i3 plays an essential role in H, agonist-mediated signal trans duction. The existence of multiple H, isoforms with different signal transd uction capabilities suggests that H-3-mediated biological functions might b e tightly regulated through alternative splicing mechanisms.