Characterization of the novel human serotonin receptor subunits 5-HT3C,5-HT3D, and 5-HT3E.
Within the family of serotonin receptors, the 5-hydroxytryptamine-3 (5-HT(3)) receptor is the only ligand-gated ion channel. It is composed of five subunits, of which the 5-HT(3A) and 5-HT(3B) subunits are best characterized. Several studies, however, have reported on the functional diversity of native 5-HT(3) receptors, which cannot solely be explained on the basis of the 5-HT(3A) and 5-HT(3B) subunits. After our discovery of further putative 5-HT(3) serotonin receptor-encoding genes, HTR3C, HTR3D, and HTR3E, we investigated whether these novel candidates and the isoform 5-HT(3Ea) are able to form functional 5-HT(3) receptor complexes. Using immunofluorescence and immunoprecipitation studies of heterologously expressed proteins, we found that each of the respective candidates coassembles with 5-HT(3A). To investigate whether the novel subunits modulate 5-HT(3) receptor function, we performed radioligand-binding assays and calcium-influx studies in human embryonic kidney 293 cells. Our experiments revealed that the 5-HT(3C),5-HT(3D), 5-HT(3E), and 5-HT(3Ea) subunits alone cannot form functional receptors. Coexpression with 5-HT(3A), however, results in the formation of functional heteromeric complexes with different serotonin efficacies. Potencies of two agonists and antagonists were nearly identical with respect to homomeric 5-HT(3A) and heteromeric complexes. However, 5-HT showed increased efficacy with respect to 5-HT(3A/D) and 5-HT(3A/E) receptors, which is consistent with the increased surface expression compared with 5-HT(3A) receptors. In contrast, 5-HT(3A/C) and 5-HT(3A/Ea) receptors exhibited decreased 5-HT efficacy. These data show for the first time that the novel 5-HT(3) subunits are able to form heteromeric 5-HT(3) receptors, which exhibit quantitatively different functional properties compared with homomeric 5-HT(3A) receptors.
Niesler
Beate
B
Department of Human Molecular Genetics, University of Heidelberg, Im Neuenheimer Feld 366, 69120 Heidelberg, Germany. beate.niesler@med.uni-heidelberg.de
Walstab
Jutta
J
Combrink
Sandra
S
Möller
Dorothee
D
Kapeller
Johannes
J
Rietdorf
Jens
J
Bönisch
Heinz
H
Göthert
Manfred
M
Rappold
Gudrun
G
Brüss
Michael
M
D000595:Amino Acid Sequence; D019921:Biotinylation; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005455:Fluorescent Antibody Technique; D006801:Humans; D047468:Immunoprecipitation; D008969:Molecular Sequence Data; D021122:Protein Subunits; D044406:Receptors, Serotonin, 5-HT3 / Q000737:chemistry* / Q000235:genetics / Q000502:physiology
D021122:Protein Subunits; D044406:Receptors, Serotonin, 5-HT3; D002118:Calcium