The mRNA expression of serotonin 2C subtype receptors uncoupled with inositol hydrolysis in NG108-15 cells

Serotonin 2C receptor as a superhero: diversities and talents in the RNA universe for editing, variant, small RNA and other expected functional RNAs

The serotonin 2C receptor subtype (5-HT2C) has a unique profession and continues to provide exciting and critical new information. The 5-HT2C is modulated at the RNA level by several mechanisms, including editing, short variant generation, and small RNAs. Recently, these phenomena, which had been demonstrated individually, were shown to be associated with each other. At present, many reports provide information about the influence of RNA regulation on receptor protein activities and expression, which was thought to be the final functional product. However, complicated behavior at the RNA stage allows us to imagine that the RNA itself has functional roles in the RNA universe. The 5-HT2C RNA may play several roles. This review will outline previous 5-HT2C studies and prospects for future studies.

Serotonin 2C receptor (5-HT2CR) mRNA editing-induced down-regulation of 5-HT2CR function in Xenopus oocytes: the significance of site C editing

Serotonin 2C receptor (5-HT2CR) mRNA receives editing at 5 nucleotide positions (sites A-E) located in the sequence encoding the second intracellular loop of 5-HT2CR. 5-HT2CR mRNA without editing and with editing at sites AB, ABD, ABC, ABCD, and C are translated to 6 isoforms of 5-HT2CR: INI(non-edited), VNI(AB), VNV(ABD), VSI(ABC), VSV(ABCD), and ISI(C), respectively. In this study, we investigated electrophysiologically the ability of these isoforms to couple with the G protein/phospholipase C (PLC) system using Xenopus oocytes injected with edited 5-HT2CR RNAs and muscarinic M(1) receptor (M1R) RNA. The efficacy with which 5-HT stimulated each isoform was calculated by comparing 5-HT-induced current with 100 microM acetylcholine-induced M1R current. Stimulation with 5-HT of INI(non-edited), VNI(AB), VNV(ABD), VSI(ABC), VSV(ABCD), and ISI(C) expressed in Xenopus oocytes showed concentration-dependent responses with EC(50) values of 8.6, 17.2, 76,5, 22.0, 91.2, and 20.3 nM, respectively. No significant difference in the ability of 5-HT to induce currents among the oocytes expressing these isoforms was detected, but in the oocytes expressing VSI(ABC) or VSV(ABCD), 5-HT had a significantly reduced ability to induce currents. These results suggest that editing at site C together with sites A and B and/or D markedly reduces 5-HT2CR function by generating isoforms with reduced ability to activate PLC.

BNIP-3: a novel candidate for an intrinsic depression-related factor found in NG108-15 cells treated with Hochu-ekki-to, a traditional oriental medicine, or typical antidepressants

Wakan-yaku is a type of Japanese and Sino traditional, systematized medical care that has been practiced for hundreds of years. To search for novel intrinsic factors related to the action of antidepressants, we used Hochu-ekki-to (HET), a Wakan-yaku medicine with antidepressive effects. First, we verified the quality of the HET by three-dimensional high-performance liquid chromatography and a cytotoxicity check in NG108-15 cells. We performed a DNA microarray analysis of the gene expression in cells treated with 50 micro/ml HET for more than 20 days. HET enhanced the expression of 125 (2.9%) genes and decreased the expression of 255 (6.0%) genes among the 4277 genes that were tested. The concentration-dependent increase in the expression of BCL2/adenovirus E1B 19-kDa protein-interacting protein 3 (BNIP-3) mRNA was particularly remarkable. A concentration-dependent increase in the expression of BNIP-3 mRNA was also observed when cells were treated with imipramine, mianserin, or milnacipran. These results suggest that BNIP-3 is a candidate for an intrinsic factor related to antidepressive effects and that Wakan-yaku theory may be useful for the identification of other intrinsic functional molecules.

Molecular pathopharmacology of 5-HT2C receptors and the RNA editing in the brain

Among the 14 kinds of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes (5-HTR), 5-HT(2C) receptor (5-HT2CR) has been intensively investigated because of its physiologically and pathophysiologically important role in the brain. 5-HT2CR has been suggested to be involved in depressive disorders based on findings from pharmacological/neurochemical/behavioral studies using autopsy preparations of humans suffering from depression, animal models of depression, and animals treated with antidepressant drugs. Recently the editing of 5-HT2CR mRNA has been reported to participate in the pathogenesis of depressive disease. The RNA editing of 5-HT2CR induced by the presumable alteration of deaminase during a pathological state in depression causes changes of a base to another base (e.g., adenosine to guanosine, cytidine to uracil (thymidine)), followed by changes in amino acids constituting the second intracellular transmembrane loop that couples G proteins. Thus 5-HT2CR receptor-mediated signal transduction is changed. In the present review, the pathopharmacological significance of 5-HT2CR in special reference to RNA editing of receptors is reviewed and discussed from the aspect of development of novel therapeutics for depression.

The mRNA Expression Differences of RNA Editing Enzymes in Differentiated and Undifferentiated NG108-15 Cells

The alteration of the editing pattern of serotonin 2C receptor (5-HT2CR) mRNA has been reported during cell differentiation. Since editing of 5-HT2CR mRNA is regulated by adenosine deaminases acting on RNA (ADARs), it is of interest to investigate if the expression of these enzymes changes during cell differentiation. The level of ADAR1 mRNA in NG108-15 cells was decreased by cell differentiation. These results suggest that the decrease of ADAR1 expression during cell differentiation may play an important role in the differentiation-induced alteration of the 5-HT2CR editing pattern in NG108-15.

RNA Editing and Short Variant of Serotonin 2C Receptor mRNA in Neuronally Differentiated NG108-15 Cells

Two types of serotonin 2C subtype receptor mRNA, receptor-type and short variant, has been reported. The expression of the receptor-type mRNA could be detected as well as the short variant in NG108-15 cells by using a high temperature stable reverse transcriptase and the expression of the receptor-type mRNA was enhanced in drug-induced neuronal differentiated cells. The deleted sequence of the short variant include the RNA editing site by adenosine deaminase. Analysis of the sequence at the editing site revealed that the mRNA of undifferentiated cells was highly edited at sites A and B and that cytosine deaminase activity may also be involved in neuronal differentiation.

Chronic treatment with imipramine inhibits cell growth and enhances serotonin 2C receptor mRNA expression in NG 108-15 cells

We previously reported that NG108-15 cells contain intrinsic serotonin 2C receptor (5-HT2CR). The effects of imipramine, a 5-HT2CR antagonist, on cell growth, cell viability, and the 5-HT2CR mRNA level were investigated in this study. Repeated treatment with imipramine at concentrations of 1 - 10 microM for 5 days inhibited cell growth in a concentration-dependent manner without affecting cell viability. In addition, the level of 5-HT2CR mRNA was elevated. At 30 microM, imipramine significantly reduced cell viability. Our findings suggest that the effect of imipramine on neuronal growth may be related to its effects on 5-HT2CR.