1 RT ± PCR technique was used to clone the human 5-HT 4(e) receptor (h5-HT 4(e) ) from heart atrium. We showed that this h5-HT 4(e) receptor splice variant is restricted to brain and heart atrium. 2 Recombinant h5-HT 4(e) receptor was stably expressed in CHO and C6-glial cell lines at 347 and 88 fmol mg 71 protein, respectively. Expression of h5-HT 4(e) receptors at the cell membrane was con®rmed by immunoblotting. 3 The receptor binding pro®le, determined by competition with [3 H]-GR113808 of a number of 5-HT 4 ligands, was consistent with that previously reported for other 5-HT 4 receptor isoforms. Surprisingly, we found that the rank order of potencies (EC 50 ) of 5-HT 4 agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of anities (K i ) obtained from binding assays. Furthermore, EC 50 values for 5-HT, renzapride and cisapride were 2 fold lower in C6-glial cells than in CHO cells. 4 ML10302 and renzapride behaved like partial agonists on the h5-HT 4(e) receptor. These results are in agreement with the reported low ecacy of the these two compounds on L-type Ca 2+ currents and myocyte contractility in human atrium. 5 A constitutive activity of the h5-HT 4(e) receptor was observed in CHO cells in the absence of any 5-HT 4 ligand and two 5-HT 4 antagonists, GR113808 and ML10375, behaved as inverse agonists. 6 These data show that the h5-HT 4(e) receptor has a pharmacological pro®le which is close to the native h5-HT 4 receptor in human atrium with a functional potency which is dependent on the cellular context in which the receptor is expressed.

1 Among the ®ve human 5-HT 4 (h5-HT 4 ) receptor isoforms, the h5-HT 4(a) receptor was studied with a particular emphasis on the molecular interactions involved in ligand binding. For this purpose, we used site-directed mutagenesis of the transmembrane domain. Twelve mutants were constructed with a special focus on the residue P4.53 of helix IV which substitutes in h5-HT 4 receptors the highly conserved S residue among the rhodopsin family receptors. The mutated receptors were transiently expressed in COS-7 cells.

The 52‐kDa SSA/Ro (Ro52) ribonucleoprotein is an antigenic target strongly associated with the autoimmune response in mothers whose children develop neonatal lupus and congenital heart block. When sera from patients with systemic lupus erythematosus were used as autoimmune controls in an enzyme immunoassay to screen for antibodies against the human serotoninergic 5‐HT4‐receptor, a high correlation was found between the presence of anti‐Ro52 protein antibodies in such sera and antibodies reacting with a synthetic peptide, corresponding to the second extracellular loop of the human 5‐HT4 receptor (amino acid residues 165–185). Homology scanning between the 5‐HT4 peptide and the sequence of the Ro52 protein indicated two potential common epitopes located between residues 365 and 396 of the Ro52 protein. Cross‐reactivity was found between the peptide derived from the 5‐HT4 receptor, and a peptide corresponding to residues 365–382 of the Ro52 protein. Autoantibodies, affinity‐purified on the 5‐HT4 receptor peptide, specifically recognized both the Ro52 protein and the 5‐HT4 receptor protein in immunoblots. The affinity‐purified antibodies antagonized the serotonin‐induced L‐type Ca channel activation on human atrial cells. This effect could explain the electrophysiological abnormalities in neonatal lupus.