Common polymorphisms in MTNR1B, G6PC2 and GCK are associated with increased fasting plasma glucose and impaired beta-cell function in Chinese subjects

Background

Previous studies identified melatonin receptor 1B (MTNR1B), islet-specific glucose 6 phosphatase catalytic subunit-related protein (G6PC2), glucokinase (GCK) and glucokinase regulatory protein (GCKR) as candidate genes for type 2 diabetes (T2D) acting through elevated fasting plasma glucose (FPG). We examined the associations of the reported common variants of these genes with T2D and glucose homeostasis in three independent Chinese cohorts.

Methodology/Principal Findings

Five single nucleotide polymorphisms (SNPs), MTNR1B rs10830963, G6PC2 rs16856187 and rs478333, GCK rs1799884 and GCKR rs780094, were genotyped in 1644 controls (583 adults and 1061 adolescents) and 1342 T2D patients. The G-allele of MTNR1B rs10830963 and the C-alleles of both G6PC2 rs16856187 and rs478333 were associated with higher FPG (0.0034<P<6.6×10⁻⁵) in healthy controls. In addition to our previous report for association with FPG, the A-allele of GCK rs1799884 was also associated with reduced homeostasis model assessment of beta-cell function (HOMA-B) (P = 0.0015). Together with GCKR rs780094, the risk alleles of these SNPs exhibited dosage effect in their associations with increased FPG (P = 2.9×10⁻⁹) and reduced HOMA-B (P = 1.1×10⁻³). Meta-analyses strongly supported additive effects of MTNR1B rs10830963 and G6PC2 rs16856187 on FPG.

Conclusions/Significance

Common variants of MTNR1B, G6PC2 and GCK are associated with elevated FPG and impaired insulin secretion, both individually and jointly, suggesting that these risk alleles may precipitate or perpetuate hyperglycemia in predisposed individuals.

Melatonin receptors in a pleuronectiform species, Solea senegalensis: Cloning, tissue expression, day-night and seasonal variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the synchronization of circadian and circannual rhythms. In this study we have cloned three melatonin receptor subtypes (MT1, MT2 and Mel1c) in the Senegalese sole and analyzed their central and peripheral tissue distribution. The full-length MT1 (1452 nt), MT2 (1728 nt) and Mel1c (1980 nt) cDNAs encode different proteins of 345, 373, 355 amino acids, respectively. They were mainly expressed in retina, brain and pituitary, but MT1 was also expressed in gill, liver, intestine, kidney, spleen, heart and skin. At peripheral level, MT2 expression was only evident in gill, kidney and skin whereas Mel1c expression was restricted to the muscle and skin. This pattern of expression was not markedly different between sexes or among the times of day analyzed. The real-time quantitative PCR analyses showed that MT1 displayed higher expression at night than during the day in the retina and optic tectum. Seasonal MT1 expression was characterized by higher mRNA levels in spring and autumn equinoxes for the retina, and in winter and summer solstices for the optic tectum. An almost similar expression profile was found for MT2, but differences were less conspicuous. No day-night differences in MT1 and MT2 expression were observed in the pituitary but a seasonal variation was detected, being mRNA levels higher in summer for both receptors. Mel1c expression did not exhibit significant day-night variation in retina and optic tectum but showed seasonal variations, with higher transcript levels in summer (optic tectum) and autumn (retina). Our results suggest that day-night and seasonal variations in melatonin receptor expression could also be mediating circadian and circannual rhythms in sole.

Changes in the expression of melatonin receptors induced by melatonin treatment in hepatocarcinoma HepG2 cells

Hepatocellular carcinoma (HCC) is one of the most common cancers and its incidence is increasing worldwide. Melatonin, an indoleamine hormone, exerts anti-oxidant, immunomodulatory, anti-aging, and antitumor effects. Previous studies have shown that melatonin can act through specific receptors, including MT(1), MT(2), MT(3) receptors as well as a nuclear receptor belonging to the orphan nuclear receptor family. Recently, we have described their role in the oncostatic and pro-apoptotic effects of melatonin on HepG2 human HCC cells. However, the potential role of the different melatonin cellular receptors on its antiproliferative effects remains unknown. In the present study, we examined the effect of melatonin treatment on HepG2 human HCC cells, analyzing cell cycle arrest and melatonin receptor expression. Melatonin was administered for 2, 4, and 6 days at 1000 or 2500 microm. Melatonin induced a dose- and time-dependent inhibition on cell proliferation. This treatment caused an alteration in the cell cycle, with an increase in the number of cells in G(2)/M phase at both 1000 and 2500 microm melatonin concentrations, and a significant increase on S phase cell percentage by the highest dose. Furthermore, increases in protein expression of MT(1), MT(3), and retinoic acid-related orphan receptor-alpha were found after melatonin treatments. These increases were coincident with a significant induction in the expression of p21 protein, which negatively regulates cell cycle progression. Our results confirm the antitumor effect of melatonin in HCC cells, suggesting that its oncostatic properties are related, at least in part, to changes on the expression of their different subtypes of receptors.

The melatonin-producing system is fully functional in retinal pigment epithelium (ARPE-19)

Since melatonin production has been documented in extrapineal and extraneuronal tissues, we investigated the expression of molecular elements of the melatoninergic system in human RPE cells (ARPE-19). The expression of key enzymes for melatonin synthesis: tryptophan hydroxylases (TPH1 and TPH2); arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT) was detected in ARPE-19 cells using RT-PCR. TPH1 and AANAT proteins were detected in ARPE by Western blotting, while sequential metabolism of tryptophan, serotonin and N-acetylserotonin to melatonin was shown by RP-HPLC. We also demonstrated, by means of RT-PCR, that ARPE expressed mRNA encoding the melatonin receptors: MT2 (but not MT1), two isoforms of nuclear receptor (RORalpha1 and RORalpha4/RZR1), and quinone oxidoreductase (NQO2). By analogy with other peripheral tissues, for example the skin, the expression of these metabolic elements in RPE cells suggests that the RPE represents an additional source of melatonin in the eye, to regulate local homeostasis and prevent from oxidative damage in intra-, auto- and/or paracrine fashions.

Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene

Melatonin is a circulating hormone that is primarily released from the pineal gland. It is best known as a regulator of seasonal and circadian rhythms; its levels are high during the night and low during the day. Interestingly, insulin levels also exhibit a nocturnal drop, which has previously been suggested to be controlled, at least in part, by melatonin. This regulation can be explained by the proposed inhibitory action of melatonin on insulin release. Indeed, both melatonin receptor 1A (MTNR1A) and MTNR1B are expressed in pancreatic islets. The role of melatonin in the regulation of glucose homeostasis has been highlighted by three independent publications based on genome-wide association studies of traits connected with type 2 diabetes, such as elevated fasting glucose, and, subsequently, of the disease itself. The studies demonstrate a link between variations in the MTNR1B gene, hyperglycaemia, impaired early phase insulin secretion and beta cell function. The risk genotype predicts the future development of type 2 diabetes. Carriers of the risk genotype exhibit increased expression of MTNR1B in islets. This suggests that these individuals may be more sensitive to the actions of melatonin, leading to impaired insulin secretion. Blocking the inhibition of insulin secretion by melatonin may be a novel therapeutic avenue for type 2 diabetes.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Increased melatonin synthesis in pineal glands of rats in streptozotocin induced type 1 diabetes

It is well-documented that melatonin influences insulin secretion. The effects are mediated by specific, high-affinity, pertussis-toxin-sensitive, G protein-coupled membrane receptors (MT(1) as well MT(2)), which are present in both the pancreatic tissue and islets of rats and humans, as well as in rat insulinoma cells (INS1). Via the Gi-protein-adenylatecyclase-3',5'-cyclic adenosine monophosphate (cAMP) and, possibly, the guanylatecyclase-cGMP pathways, melatonin decreases insulin secretion, whereas, by activating the Gq-protein-phospholipase C-IP(3) pathway, it has the opposite effect. For further analysis of the interactions between melatonin and insulin, diabetic rats were investigated with respect to melatonin synthesis in the pineal gland and plasma insulin levels. In this context, recent investigations have proven that type 2 diabetic rats and humans display decreased melatonin levels, whereas type 1 diabetic IDDM rats or those with diabetes induced by streptozotocin (STZ) of the present study show increased plasma melatonin levels and elevated AA-NAT-mRNA. Furthermore, the mRNA of pineal insulin receptors and beta1-adrenoceptors, including the clock genes Per1 and Bmal1 and the clock-controlled output gene Dbp, increases in both young and middle-aged STZ rats. The results therefore indicate that the decreased insulin levels in STZ-induced type 1 diabetes are associated with higher melatonin plasma levels. In good agreement with earlier investigations, it was shown that the elevated insulin levels observed in type 2 diabetes, are associated with decreased melatonin levels. The results thus prove that a melatonin-insulin antagonism exists. Astonishingly, notwithstanding the drastic metabolic disturbances in STZ-diabetic rats, the diurnal rhythms of the parameters investigated are maintained.

Human placental trophoblasts synthesize melatonin and express its receptors

Although the role of melatonin on fetal development has been the subject of a number of studies, little is known about the function of melatonin in the placenta. We previously showed that melatonin receptors are expressed and are functional in JEG-3 and BeWo cell lines, both in vitro models of human trophoblast. Local synthesis of melatonin in placenta has been proposed, but the human placenta's ability to synthesize melatonin de novo has never been studied. The purpose of this study was to investigate the expression [reverse transcription-polymerase chain reaction (RT-PCR) and western blot analysis] and activity (radiometric assay) of melatonin synthesizing enzymes, and characterize the expression of the melatoninergic receptors in human term villous trophoblast. The results show that arylalkylamine N-acetyltransferase and hydroxyindole O-methyltransferase melatonin synthesizing enzymes are expressed and active in villous trophoblast as well as in JEG-3 and BeWo placental choriocarcinoma cells. In addition, immunohistochemical analysis demonstrated the presence of MT1, MT2, and retinoid-related orphan nuclear receptor alpha melatonin receptor proteins in both villous cytotrophoblast and syncytiotrophoblast (STB) as well as in endothelial cells surrounding the fetal capillaries and in the villous mesenchymal core. RT-PCR and western blot analysis in primary cultures of human term trophoblast confirmed the expression of all three melatonin receptors in villous cytotrophoblast and STB cells. This study demonstrates for the first time a local synthesis of melatonin and expression of its receptors in human trophoblasts and strongly suggests a paracrine, autocrine, and/or intracrine role for this indolamine in placental function and development as well as in protection from oxidative stress.

Photoperiodic signalling through the melatonin receptor turns full circle

Photoperiod exerts profound influence on the physiology of mammals through the action of melatonin on the neuroendocrine system. Over the last 20 years, studies have moved away from a melatonin receptor-focused approach to understanding how photoperiod regulates neuroendocrine activity through studies of downstream effects on gene expression. This paper reviews the recent progress made in our understanding of the effects of photoperiod on gene expression in the hypothalamus, and considers how this new information can be reconciled with the species-specific location of melatonin receptors.

Cloning and retinal expression of melatonin receptors in the European sea bass, Dicentrarchus labrax

Melatonin contributes to synchronizing behaviors and physiological functions to daily and seasonal rhythm in fish. However, no coherent vision emerges because the effects vary with the species, sex, age, moment of the year or sexual cycle. And, scarce information is available concerning the melatonin receptors, which is crucial to our understanding of the role melatonin plays. We report here the full length cloning of three different melatonin receptor subtypes in the sea bass Dicentrarchus labrax, belonging, respectively, to the MT1, MT2 and Mel1c subtypes. MT1, the most abundantly expressed, was detected in the central nervous system, retina, and gills. MT2 was detected in the pituitary gland, blood cells and, to a lesser extend, in the optic tectum, diencephalon, liver and retina. Mel1c was mainly expressed in the skin; traces were found in the retina. The cellular sites of MT1 and MT2 expressions were investigated by in situ hybridization in the retina of pigmented and albino fish. The strongest signals were obtained with the MT1 riboprobes. Expression was seen in cells also known to express the enzymes of the melatonin biosynthesis, i.e., in the photoreceptor, inner nuclear and ganglion cell layers. MT1 receptor mRNAs were also abundant in the retinal pigment epithelium. The results are consistent with the idea that melatonin is an autocrine (neural retina) and paracrine (retinal pigment epithelium) regulator of retinal function. The molecular tools provided here will be of valuable interest to further investigate the targets and role of melatonin in nervous and peripheral tissues of fish.

Diurnal differences in melatonin effect on intracellular Ca2+ concentration in chicken spleen leukocytes in vitro

Melatonin plays a pleiotropic role in the immune system of mammals and birds. Endogenous and exogenous melatonin modulates lymphocyte proliferation via specific MT(1), MT(2) and Mel(1c) membrane receptors, although the mechanisms behind this process are poorly understood. The diurnal changes in the expression and function of melatonin membrane receptors within the immune system have so far received little attention. We investigated the day/night differences in melatonin membrane receptor mRNA expression in chicken lymphoid organs and cultured splenocytes and examined the in vitro effect of melatonin and 2-iodomelatonin on the intracellular Ca(2+) concentration ([Ca(2+)](i)) in chicken splenocytes. In whole organs, expression of all subtypes of Mel membrane receptors was observed, and the level did not change significantly with the time of day. Interestingly, we observed a significant increase in the expression of the transcripts of all receptor subtypes in cultured splenocytes isolated at night compared with cells obtained during the day. In chicken spleen leukocytes isolated during the day, melatonin and 2-iodomelatonin increased [Ca(2+)](i), with only 2-iodomelatonin being effective in the 'night' cells. Luzindole modulated the [Ca(2+)](i) increase caused by melatonin receptor agonists: it potentiated the stimulatory effect of melatonin during the day, but counteracted that evoked by 2-iodomelatonin at night. The results of this study demonstrate that melatonin can induce changes in [Ca(2+)](i) in chicken spleen leukocytes that should modulate proliferation. The effect of melatonin on [Ca(2+)](i) is less pronounced at night, possibly caused by receptor desensitization.

Melatonin signaling regulates locomotion behavior and homeostatic states through distinct receptor pathways in Caenorhabditis elegans

Melatonin is a hormone that controls circadian rhythms and seasonal behavioral changes in vertebrates. Recent studies indicate that melatonin participates in diverse physiological functions including the modulation of neural activities. Melatonin is also detected in many other organisms that do not exhibit obvious circadian rhythms, but their precise functions are not known. To understand the role of melatonin and its genetic pathway in vivo, we examined the effects of melatonin and its receptor antagonists on various behaviors in Caenorhabditis elegans. Exogenously applied melatonin specifically decreased locomotion rates in 15-min treatments, suggesting that melatonin directly regulates neural activities for locomotion. This melatonin signaling functions through MT1-like melatonin receptors, because the MT1/2 receptor antagonist luzindole effectively blocked the effect of melatonin on locomotion, while MT2-specific antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) and MT3-selective antagonist prazosin had no effect. Alternatively, long-term treatment with prazosin specifically altered homeostatic states of the worm, suggesting another melatonin-signaling pathway through MT3-like receptors. We also found that two G-protein alpha subunit mutants and newly isolated five mutants exhibited defects in response to melatonin. Our findings imply that melatonin acts as a neuromodulator by regulating locomotion behavior and as a ligand for homeostatic control through distinct receptor pathways in C. elegans.

Expression of the melatonin receptor Mel1c in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.

Asymmetric expression of melatonin receptor mRNA in bilateral paravertebral muscles in adolescent idiopathic scoliosis

STUDY DESIGN

Comparison of melatonin receptor mRNA expression in bilateral paravertebral muscles in adolescent idiopathic scoliosis (AIS). OBJECTIVES.: To investigate the change of melatonin receptor mRNA expression in bilateral paravertebral muscles in AIS, congenital scoliosis (CS), and control in order to analyze its association to the pathogenesis of AIS.

SUMMARY OF BACKGROUND DATA

Muscle imbalance and asymmetry of stretch receptors in the paravertebral muscles of patients with AIS were supposed to have a large role to play in the development and production of the deformity. Melatonin is a focus of studies of the mechanism underlying the development of scoliosis, and there is no research on the expression of melatonin receptors in the paravertebral muscles of patients with AIS.

METHODS

Twenty cases with average age of 15.1 +/- 2.2 years and average Cobb angle of 56.2 degrees +/- 16.1 degrees, including 10 cases with Cobb angle >50 degrees and 10 cases with Cobb angle < or =50 degrees, were included in AIS group. The apical vertebrae were from T6 to T11. Twelve cases with an average age of 11.6 +/- 3.2 years and average Cobb angle of 59.2 degrees +/- 33.3 degrees were included in CS group. The apical vertebrae were from T7 to T12. Ten cases without scoliosis were in the control group. The mRNA expression of melatonin receptor subtype MT1 and MT2 was detected by the RT-PCR method.

RESULTS

The MT2 mRNA expression on the concave side of the paravertebral muscle was higher than that on the convex side in AIS and CS groups (P < 0.05), but the MT1 mRNA expression showed no significant difference (P > 0.05). In the AIS group, the ratio of MT2 mRNA expression on the concave side compared with the convex side in cases with Cobb angle >50 degrees and cases with Cobb angle < or =50 degrees showed no significant difference (P > 0.05). The MT1 and MT2 mRNA expression showed no significant difference in control group (P > 0.05).

CONCLUSION

The melatonin receptor expression in bilateral paravertebral muscles in AIS is asymmetric, which may be a secondary change. The bilateral asymmetry in force exerted on the scoliotic spine may be the cause.

Melatonin effects on the hypothalamo-pituitary axis in fish

Melatonin, a hormonal output signal of vertebrate circadian clocks, contributes to synchronizing behaviors and neuroendocrine regulations with the daily and annual variations of the photoperiod. Conservation and diversity characterize the melatonin system: conservation because its pattern of production and synchronizing properties are a constant among vertebrates; and diversity because regulation of both its synthesis and modes of action have been profoundly modified during vertebrate evolution. Studies of the targets and modes of action of melatonin in fish, and their parallels in mammals, are of interest to our understanding of time-related neuroendocrine regulation and its evolution from fish to mammals, as well as for aquacultural purposes.

Differences in Binding Sites of Two Melatonin Receptors Help to Explain Their Selectivity to Some Melatonin Analogs: A Molecular Modeling Study

Numerous diseases have been linked to the malfunction of G-protein coupled receptors (GPCRs). Their adequate treatment requires rational design of new high-affinity and high-selectivity drugs targeting these receptors. In this work, we report three-dimensional models of the human MT(1) and MT(2) melatonin receptors, members of the GPCR family. The models are based on the X-ray structure of bovine rhodopsin. The computational approach employs an original procedure for optimization of receptor-ligand structures. It includes rotation of one of the transmembrane alpha-helices around its axis with simultaneous assessment of quality of the resulting complexes according to a number of criteria we have developed for this purpose. The optimal geometry of the receptor-ligand binding is selected based on the analysis of complementarity of hydrophobic/hydrophilic properties between the ligand and its protein environment in the binding site. The elaborated "optimized" models are employed to explore the details of protein-ligand interactions for melatonin and a number of its analogs with known affinity to MT(1) and MT(2) receptors. The models permit rationalization of experimental data, including those that were not used in model building. The perspectives opened by the constructed models and by the optimization procedure in the design of new drugs are discussed.

Association between expression of reproductive seasonality and alleles of melatonin receptor 1A in goats

To determine whether a link exists between reproductive seasonality and the structure of the melatonin receptor 1A (MTNR1A) gene, the latter was studied in year-round estrous breeds (Jining Grey and Boer goats) and seasonal estrous breeds (Liaoning Cashmere, Inner Mongolia Cashmere, Wendeng milk and Beijing native goats). A large fragment of exon 2 of MTNR1A gene was amplified by PCR using sheep sense and antisense primers in 260 does of six breeds. The uniform 824 bp PCR product was digested with restriction endonucleases MnII and RsaI, and checked for the presence of restriction sites. No polymorphism at the MnII cleavage sites was detected in all six goat breeds and no relationship could be established between the MnII cleavage sites of MTNR1A gene and reproductive seasonality in goats. For polymorphic RsaI cleavage site at base position 53, only genotype RR (267 bp/267 bp) was detected in Jining Grey goats, both genotype RR and genotype Rr (267 bp/320 bp) were found in all other goat breeds, no genotype rr (320 bp/320 bp) was detected in all six goat breeds. Frequency of genotype RR was obviously higher, and frequency of genotype Rr was obviously lower in year-round estrous goat breeds than in seasonal estrous goat breeds. Sequencing revealed one mutation (G52A) in genotype Rr compared with genotype RR. For polymorphic RsaI cleavage site, the differences of genotype distributions were significant (P<0.05) between year-round estrous goat breeds and seasonal estrous goat breeds. These results preliminarily showed an association between genotype RR and year-round estrus in goats, and an association between genotype Rr and seasonal estrus in goats.

[Effects of MTNR1A gene on litter size in a large white and a landrace herd]

Two pairs of primers were designed based on the known sequence in GenBank for amplification of MTNR1A gene in a Large White and a Landrace herd. Using PCR-SSCP (single strand conformation polymorphism), we found a single nucleotide polymorphism(SNP) within the product amplified from the first pair of primers. PCR products from randomly selected different genotypes were sequenced after were recovered and purified. Results revealed a synonymous single base mutation(G-->A) at +159bp(sequence numbering based on Genbank accession number U73326) for the BB genotype. When analyzed for association with litter size traits, this MTNR1A SNP was found to have no significant effect on litter size traits.

[Comparison of melatonin receptor mRNA expression in bilateral paravertebral muscles in adolescent idiopathic scoliosis]

OBJECTIVE

To study the change of melatonin receptor mRNA expression in paravertebral muscles in adolescent idiopathic scoliosis (AIS), and analyze its relationship with the etiology of AIS.

METHODS

Twenty cases with average age of (15.1 +/- 2.2) years and average Cobb angle of 56 degrees +/- 16 degrees , including 10 cases with Cobb angle > 50 degrees and 10 cases with Cobb angle <or= 50 degrees , were in AIS group. The apical vertebrae were from T(6 - 11). Twelve cases with average age of (11.6 +/- 3.2) years and average Cobb angle of 59 degrees +/- 33 degrees were in congenital scoliosis (CS) group. The apical vertebrae were from T(7 - 12). Ten cases without scoliosis were in control group. The mRNA expression of melatonin receptor subtype MT1 and MT2 were detected by RT-PCR method.

RESULTS

The MT2 mRNA expression of paravertebral muscles on concave side was higher than that on convex side in AIS and CS groups (P < 0.05), but the MT1 mRNA expression showed no significant difference (P > 0.05). In AIS group, the ratio of MT2 mRNA expression on concave side compared with convex side in cases with Cobb angle > 50 degrees and cases with Cobb angle <or= 50 degrees showed no significant difference (P > 0.05). The MT1 and MT2 mRNA expression showed no significant difference in control group (P > 0.05).

CONCLUSIONS

The melatonin receptor expression in bilateral paravertebral muscles in AIS is asymmetric, which may be a secondary change and has no important role in the pathogenesis of AIS.

Melatonin synthesized by Jurkat human leukemic T cell line is implicated in IL-2 production

Human lymphocytes have recently been described as an important physiological source of melatonin (N-acetyl-5-methoxytryptamine), which could be involved in the regulation of the human immune system. On the other hand, stimulation of IL-2 production by exogenous melatonin has been shown in the Jurkat human lymphocytic cell line. Furthermore, both melatonin membrane and nuclear receptors are present in these cells. In this study, we show that the necessary machinery to synthesize melatonin is present and active in resting and stimulated Jurkat cells. Accordingly, we have found that cells synthesize and release melatonin in both conditions. Therefore, we investigated whether endogenous melatonin produced by Jurkat cells was involved in the regulation of IL-2 production. When melatonin membrane and nuclear receptors were blocked using specific antagonists, luzindole and CGP 55644, respectively, we found that IL-2 production decreased, and this drop was reverted by exogenous melatonin. Additionally, PHA activation of Jurkat cells changed the profile of melatonin nuclear receptor mRNA expression. A previous study showed that exogenous melatonin is able to counteract the decrease in IL-2 production caused by prostaglandin E2 (PGE2) in human lymphocytes via its membrane receptor. In our model, when we blocked the melatonin membrane receptor with luzindole, the inhibitory effect of PGE2 on IL-2 production was higher. Therefore, we have demonstrated the physiological role of endogenous melatonin in this cell line. These findings indicate that endogenous melatonin synthesized by human T cells would contribute to regulation of its own IL-2 production, acting as an intracrine, autocrine, and/or paracrine substance.

Melatonin-depleted blood from premenopausal women exposed to light at night stimulates growth of human breast cancer xenografts in nude rats

The increased breast cancer risk in female night shift workers has been postulated to result from the suppression of pineal melatonin production by exposure to light at night. Exposure of rats bearing rat hepatomas or human breast cancer xenografts to increasing intensities of white fluorescent light during each 12-hour dark phase (0-345 microW/cm2) resulted in a dose-dependent suppression of nocturnal melatonin blood levels and a stimulation of tumor growth and linoleic acid uptake/metabolism to the mitogenic molecule 13-hydroxyoctadecadienoic acid. Venous blood samples were collected from healthy, premenopausal female volunteers during either the daytime, nighttime, or nighttime following 90 minutes of ocular bright, white fluorescent light exposure at 580 microW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with daytime-collected melatonin-deficient blood, human breast cancer xenografts and rat hepatomas perfused in situ, with nocturnal, physiologically melatonin-rich blood collected during the night, exhibited markedly suppressed proliferative activity and linoleic acid uptake/metabolism. Tumors perfused with melatonin-deficient blood collected following ocular exposure to light at night exhibited the daytime pattern of high tumor proliferative activity. These results are the first to show that the tumor growth response to exposure to light during darkness is intensity dependent and that the human nocturnal, circadian melatonin signal not only inhibits human breast cancer growth but that this effect is extinguished by short-term ocular exposure to bright, white light at night. These mechanistic studies are the first to provide a rational biological explanation for the increased breast cancer risk in female night shift workers.

Essential hypertension seems to result from melatonin-induced epigenetic modifications in area postrema

Essential hypertension is a complex multifactorial disorder with epigenetic and environmental factors contributing to its prevalence. Epigenetic system is a genetic regulatory mechanism that allows humans to maintain extraordinarily stable patterns of gene expression over many generations. Sympathetic nervous system plays a major role in the maintenance of hypertension and the rostral ventrolateral medulla is the main source of this sympathetic activation. A possible mechanism to explain the sympathetic hyperactivity in the rostral ventrolateral medulla is an action of the area postrema. Area postrema seems to be the region where a shift of the set-point to a higher operating pressure occurs resulting in hypertension. But, how can a shift occur in the area postrema. We propose that melatonin-induced epigenetic modifications in the neurons of area postrema plays a role in this shift. Area postrema is reported to contain high levels of melatonin receptors that play a role in the epigenetic modifications in certain cells. Environmental stressors cause epigenetic modifications in the neurons of area postrema via the pineal hormone melatonin and these changes lead to a shift in the set-point to a higher operating pressure. This signal is then sent via efferent projections to key medullary sympathetic nuclei in rostral ventrolateral medulla resulting in increases in sympathetic nerve activity. This model may explain the long-term alterations in sympathetic activity in essential hypertension.

[Melatoninergic receptor agonists and antagonists: pharmacological aspects and therapeutic perspective]

Melatonin, or N-acetyl 5-methoxytryptamine, a neurohormone produced in the pineal gland during periods of darkness, plays a key role in the regulation of circadian and seasonal biological rhythms. In mammals, specific MT1 and MT2 receptors are located in the central nervous system, mainly in suprachiasmatic nuclei, and also in a number of peripheral sites. Besides its chronobiotic action on light-dependant functions, such as sleep/waking alternance or seasonal depression, melatonin exerts modulatory effects on immune, endocrine and metabolic functions. However, its short half-life and extensive metabolism lead to a poor bioavailability. This prompted to search for metabolically stable analogs displaying new and innovative properties. The S 20098 compound, a melatoninergic agonist, has proven potent antidepressive and anxiolytic actions. The S 20928 compound, a melatonin antagonist, was shown to enhance basal metabolism and reduce weight gain. Thus, both of these melatonin derivatives open perspectives for the development of innovative therapeutic agents in the fields of depression and obesity.

The role of melatonin receptor 1B gene (MTNR1B) in adolescent idiopathic scoliosis--a genetic association study

Many studies have demonstrated the role of melatonin in the etiology of AIS. Previous studies have shown that there is no evidence of mutations in the melatonin receptor 1A gene in AIS patients. In this study, we have examined the role of melatonin receptor 1B in predisposition for AIS. Using haplotype block tagging technique, a set of tagging SNPs were defined for MTNR1B from the Han Chinese data of the International HapMap project. The association between the tagging of single nucleotide polymorphisms (tSNPs) in MTNR1B region and the occurrence of AIS was studied.

METHOD

473 AIS girls and 311 normal controls were recruited. The age range of the patients was between 10 and 18 years old. The maximum Cobb was recorded at latest follow-up in AIS patients. Three of five tSNPs were studied; they were all located within the coding region of the MTNR1B gene.

RESULTS

There was no significant difference in the genotype or allelic frequencies (AF) of the 3 tSNPs between AIS and controls. In a case-only analysis, no difference in curve severity in AIS patients was found among patients with different genotypes (by one-way ANOVA).

DISCUSSION

The 3 tSNPs showed no association with either the occurrence of AIS or the maximum Cobb angle within AIS girls. Further analysis of the remaining tSNPs within the regulatory region of the MTNR1B gene and other related genes in the melatonin signaling pathway may provide further information on the role of the melatonin in AIS girls.

Asymmetric expression of melatonin receptor mRNA in bilateral paravertebral muscles in adolescent idiopathic scoliosis

To investigate the change of melatonin receptor mRNA expression in bilateral paravertebral muscles in AIS, congenital scoliosis (CS) and controls in order to analyze its relationship to the pathogenesis of AIS. 20 cases with average age of 15.1 +/- 2.2 years and average Cobb angle of 56.2 degrees +/- 16.1 degrees were included in AIS group. 12 cases with average age of 11.6 +/- 3.2 years and average Cobb angle of 59.2 degrees +/- 33.3 degrees were included in congenital scoliosis (CS) group. 10 cases without scoliosis comprised a control group. The mRNA expression of melatonin receptor subtype MT1 and MT2 were detected by RT-PCR method. The MT2 mRNA expression on the concave side of paravertebral muscle was higher than that on the convex side in AIS and CS groups (p<0.05), but the MT1 mRNA expression showed no significant difference. In the AIS group, the ratio of MT2 mRNA expression on the concave side compared with the convex side in cases with a Cobb angle less than 50 degrees and cases with a Cobb angle greater than 50 degrees showed no significant difference. The melatonin receptor expression in bilateral paravertebral muscles in AIS is asymmetric, which may be a secondary change.

Melatonin as a local regulator of human placental function

Melatonin plays a critical role in a variety of mammalian reproductive processes not only acting on the central nervous system but also behaving as a peripheral physiologic regulator. To address the relevance of melatonin to the maintenance of pregnancy at the feto-maternal interface, we investigated the expression of two types of membrane melatonin receptors, MT1 and MT2, as well as arylalkylamine N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT), the two enzymes required for the conversion of serotonin to melatonin, in the human placenta and the effect of melatonin on the release of human chorionic gonadotropin (hCG) from cultured human trophoblast cells. RT-PCR analysis and DNA sequencing revealed that transcripts of MT1, MT2, AA-NAT, and HIOMT were present in the first-trimester human placenta. We also found that melatonin significantly potentiated hCG secretion at optimal concentrations. These results suggest that melatonin may regulate human placental function in a paracrine/autocrine manner, providing evidence for a novel role in human reproduction.

Melatonin stimulates inositol-1,4,5-trisphosphate and Ca2+ release from INS1 insulinoma cells

The effects of melatonin in mammalian cells are exerted via specific receptors or are related to its free radical scavenging activity. It has previously been reported that melatonin inhibits insulin secretion in the pancreatic islets of the rat and in rat insulinoma INS1 cells via Gi-protein-coupled MT1 receptors and the cyclic adenosine 3',5'-monophosphate pathway. However, the inositol-1,4,5-trisphosphate (IP3) pathway is involved in the insulin secretory response as well, and the melatonin signal may play a part in its regulation. This paper addresses the involvement of the second messengers IP3 and intracellular Ca2+ ([Ca2+]i) in the signalling cascade of melatonin in the rat insulinoma INS1 cell, a model for the pancreatic beta-cell. For this purpose melatonin at concentrations ranging from 1 to 100 nmol/L, carbachol and the nonselective melatonin receptor antagonist luzindole were used to stimulate INS1 cell batches, followed by an IP3-mass assay and Ca2+ imaging. Molecular biological studies relating to the mRNA of IP3 receptor (IP3R) subtypes and their relative abundance in INS1 cells showed expression of IP3R-1, IP3R-2 and IP3R-3 mRNA. In conclusion, we found that in rat insulinoma INS1 cells there is a dose-dependent stimulation of IP3 release by melatonin, which is accompanied by a likewise transient increase in [Ca2+]i concentrations. The melatonin effect observed mimics carbachol action. It can be abolished by 30 micromol/L luzindole and is sustained in Ca2+-free medium, suggesting a mechanism that includes the depletion of Ca2+ from intracellular stores.

Novel targets for valproic acid: up-regulation of melatonin receptors and neurotrophic factors in C6 glioma cells

Valproic acid (VPA) is a potent anti-epileptic and effective mood stabilizer. It is known that VPA enhances central GABAergic activity and activates the mitogen-activated protein kinase-extracellular signal-regulated kinase (MAPK-ERK) pathway. It can also inhibit various isoforms of the enzyme, histone deacetylase (HDAC), which is associated with modulation of gene transcription. Recent in vivo studies indicate a neuroprotective role for VPA, which has been found to up-regulate the expression of brain-derived neurotrophic factor (BDNF) in the rat brain. Given the interaction between the pineal hormone, melatonin, and GABAergic systems in the central nervous system, the effects of VPA on the expression of the mammalian melatonin receptor subtypes, MT1 and MT2, were examined in rat C6 glioma cells. The effects of VPA on the expression of glial cell line-derived neurotrophic factor (GDNF) and BDNF were also examined. RT-PCR studies revealed a significant induction of melatonin MT1 receptor mRNA in C6 cells following treatment with 3 or 5 mm VPA for 24 h or 5 mm VPA for 48 h. Western analysis and immunocytochemical detection confirmed that the VPA-induced increase in MT1 mRNA results in up-regulation of MT1 protein expression. Blockade of the MAPK-ERK pathway by PD98059 enhanced the effect of VPA on MT1 expression, suggesting a negative role for this pathway in MT1 receptor regulation. In addition, significant increases in BDNF, GDNF and HDAC mRNA expression were observed after treatment with VPA for 24 or 48 h. Taken together, the present findings suggest that the neuroprotective properties of VPA involve modulation of neurotrophic factors and receptors for melatonin, which is also thought to play a role in neuroprotection. Moreover, the foregoing suggests that combinations of VPA and melatonin could provide novel therapeutic strategies in neurological and psychiatric disorders.

Positive relationship between melatonin receptor type 1B polymorphism and rheumatoid factor in rheumatoid arthritis patients in the Korean population

Melatonin is reported to be an anti-inflammatory agent. No genetic study concerning the association between melatonin and inflammatory disease has yet been reported. Here we performed a polymorphism study on the melatonin receptor type 1B (MTNR1B) in Korean rheumatoid arthritis (RA) patients and controls. The polymorphism of MTNR1B located in 3'-untranslated region (rs 1562444) was selected for its higher rate of heterozygosity among other single nucleotide polymorphisms (SNPs) in both MTNR1A and MTNR1B genes and investigated in RA patients (n = 173) and healthy controls (n = 195) by polymerase chain reaction-restriction fragment length polymorphism assay using NlaIII restriction enzyme. No statistically significant difference in either genotype distribution or allele frequency was observed between RA patients and controls. The genotype distributions and allele frequencies of rheumatoid factor negative [RF(-)] patients were similar to those of controls. However, statistical analysis of genotype revealed a significant association (chi2 = 6.42, P = 0.04) is present between RF(+) and MTNR1B SNP (rs 1562444). Although no statistically significant difference in allele frequency between RF(+) and controls was observed (chi2 = 2.75, P = 0.10), the results might suggest that MTNR1B SNP (rs 1562444) is associated with the presence of RF in RA. This is the first study, to our knowledge, to report a positive genetic relationship between melatonin and RA.

Stimulation of Melatonin Receptors Decreases Calcium Levels in Xenopus Tectal Cells by Activating GABAC Receptors

To investigate the physiological effects of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased γ-aminobutyric acid-C (GABAC)–receptor activity, as demonstrated by the ability of the GABAC-receptor antagonists, picrotoxin and TPMPA, to abolish the effects of melatonin. In contrast, neither the GABAA-receptor antagonist bicuculline nor the GABAB-receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the 3 known melatonin receptors, MT1 (Mel1a), MT2 (Mel1b), and Mel1c. Because the effect of melatonin on tectal calcium increases was antagonized by an MT2-selective antagonist, 4-P-PDOT, we performed Western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting by MT2 receptors, modulates GABAC receptor activity in the optic tectum and that this effect is influenced by the light–dark cycle.

Melatonin affects the temporal organization of the song of the zebra finch

In birds and mammals, including humans, melatonin-binding sites are abundant in brain areas that have no known clock function. Although the role of such binding sites is still unclear, it is assumed that these sites link neural functions to circadian or circannual demands of neuroendocrine homeostasis and reproduction. To investigate a possible direct role of melatonin in motor control, we studied the song and neural song system of the zebra finch. Neurons of two sensory-motor areas of the descending song control circuit that are crucial for the organization of the song pattern, the HVC and RA, express the melatonin-1B receptor (Mel1B), while the hypoglossal motor neurons of the song circuit express melatonin-1C receptors (Mel1C). Application of melatonin to brain slices decreases the firing-rate of RA-neurons. Systemic administration of a Mel1B antagonist at the beginning of the night shortens the song and motif length and affects the song syllable lengths produced the next day. The temporal pattern of the song, however, does not undergo daily changes. Thus, melatonin is likely to affect a non-circadian motor pattern by local modulation of song control neurons and in consequence alters a sexual signal, the song of the zebra finch.

Organs from mice deleted for NRH:quinone oxidoreductase 2 are deprived of the melatonin binding site MT3

Two melatonin receptors (MT1 and MT2) have been cloned. A third melatonin binding site, MT3, is known with remarkable and distinct pharmacological properties. We previously reported the purification of MT3 and identified it as the enzyme dihydronicotinamide riboside:quinone reductase 2 (NQO2). To investigate the relationship between NQO2 and MT3, we generated a NQO2-/- mouse strain. These mice no longer present MT3 binding sites as measured with 2-[125I]-iodo, 5-methoxycarbonylamino-N-acetyltryptamine, the specific MT3 radioligand. These data establish NQO2 as part of the MT3 binding sites in vivo and resolve the matter of the nature of the third melatonin binding site.

Melatonin induces the expression of gonadotropin-inhibitory hormone in the avian brain

We recently identified a novel hypothalamic neuropeptide inhibiting gonadotropin release in quail and termed it gonadotropin-inhibitory hormone (GnIH). Cell bodies and terminals containing the dodecapeptide GnIH are localized in the paraventricular nucleus (PVN) and median eminence, respectively. To understand the physiological role of GnIH, we investigated the mechanisms that regulate GnIH expression. In this study, we show that melatonin originating from the pineal gland and eyes induces GnIH expression in the quail brain. Pinealectomy (Px) combined with orbital enucleation (Ex) (Px plus Ex) decreased the expression of GnIH precursor mRNA and content of mature GnIH peptide in the diencephalon, which includes the PVN and median eminence. Melatonin administration to Px plus Ex birds caused a dose-dependent increase in expression of GnIH precursor mRNA and production of mature peptide. The expression of GnIH was photoperiodically controlled and increased under short-day photoperiods, when the duration of melatonin secretion increases. To identify the mode of melatonin action on GnIH induction, we investigated the expression of Mel(1c), a melatonin receptor subtype, in GnIH neurons. In situ hybridization of Mel(1c) mRNA combined with immunocytochemistry for GnIH revealed that Mel(1c) mRNA was expressed in GnIH-immunoreactive neurons in the PVN. Melatonin receptor autoradiography further revealed specific binding of melatonin in the PVN. These results indicate that melatonin is a key factor for GnIH induction. Melatonin appears to act directly on GnIH neurons through its receptor to induce GnIH expression. This is the first demonstration, to our knowledge, of a direct action of melatonin on neuropeptide induction in any vertebrate class.

Transition from embryonic to adult transcription pattern of serotonin N-acetyltransferase gene in avian pineal gland

The study reports the change of transcription pattern of serotonin N-acetyltransferase gene and melatonin receptor genes during ontogenesis of the avian pineal gland. The RT-PCR technique was used to investigate the expression of the arylalkylamine N-acetyltransferase (AA-NAT) and melatonin receptor genes during development of the pineal glands isolated from Japanese quail (Coturnix coturnix japonica) embryos incubated from 3 days on until hatching (17 days), and in some organs (pineal, brain hemisphere, eye, leg, heart) of the 3-day-old quail embryo. It was shown that two phases of AA-NAT expression are observed during pineal gland development. The first, embryonic-type phase, lasts from the beginning until 7-10 days of incubation, and is marked by the presence of two RT-PCR products for AA-NAT: the shorter mature form without intron (238 bp), and the longer form (323 bp) containing an unprocessed intron of 85 bp. The second, adult-type phase is characterized by the presence of a single mature transcript, containing no intron; it starts from 7 to 10 days of incubation and lasts until hatching and in the adult pineal. The duration of this transition time from the embryonic to the adult transcription pattern in the quail pineal gland from 7 to 10 days of incubation we attribute to asynchronic embryo development, because quail chicks usually hatch between the 16th and 19th day of incubation. Analysis of the AA-NAT protein sequences for chick and quail (GeneBank accession no. U 46 502 and AF 007 068, respectively) revealed their perfect homology with the part of protein read from the sequence present in the adult-type phase of the pineal gland (the RT-PCR product of 238 bp). The presence of the intron (in the 323 bp RT-PCR product, accession no. AY 197 460) in the embryonic-phase of the pineal gland changes the reading frame of the mRNA sequence and the hypothetical resulting protein loses its homology with the chick and quail AA-NAT enzyme starting with 105th amino acid of the complete chick AA-NAT protein comprising 205 amino acids (accession no. U 46 502). In the whole embryos at stages 1-8 (according to the Hamburger-Hamilton classification) both RT-PCR products with and without intron were consistently found, and individual tissues from 3-day-old embryos also produced two AA-NAT products, i.e., the expression was of the embryonic-type. At the time of transition from the embryonic to the adult AA-NAT transcription pattern, in 7-11-day-old embryos, all three melatonin receptor transcripts (mel-1a, mel-1b, and mel-1c) were observed in the pineals, without consistent modifications of the band intensity. In the adult pineal, a single mature AA-NAT transcript was present as well as all three melatonin receptor transcripts, usually with preferential expression of the mel-1a band. The transition time from the embryonic to adult AA-NAT expression pattern coincides well with the acquisition of functional activity and the appearance of melatonin synthesis in the embryonic pineal reported for chicken, as related to quail. We suggest that the change in transcription pattern of the AA-NAT gene may reflect another, still unknown mechanism of regulating AA-NAT activity during ontogenesis, at the level of mRNA processing, whose specificity (or not) for embryonic development we wish to establish in the future.

Expression of genes coding melatonin and serotonin receptors in rodent skin

Targeted search for expression of melatonin and serotonin receptors genes in the skin of C57BL/6J mice showed expression of MT1B (but not MT1A). Mouse skin and hamster melanomas also expressed 5HT2B and 5HT7. We identified two novel isoforms of MT1A and 5HT7.

Immortalized cells from the rat suprachiasmatic nucleus express functional melatonin receptors

Immortalized SCN2.2 cells retain most biochemical and biophysical characteristics of the native rat SCN including the expression of clock genes and circadian regulatory proteins, and its distinctive pacemaker function. This study assessed the expression and signaling of MT(1) and MT(2) melatonin receptors in SCN2.2 cells. SCN2.2 cells express MT(1) and MT(2) receptors mRNA as detected by RT-PCR. In situ hybridization with digoxigenin-labeled probes demonstrated that mRNA for MT(1) and MT(2) melatonin receptors is expressed mostly in cells with neuronal-like morphology, representing 10.8+/-2.2% and 9.8+/-0.2%, respectively, of the SCN2.2 cell population. MT(1) and MT(2) melatonin receptor proteins are expressed in both rat SCN2.2 cells and rat SCN tissue as demonstrated by Western blot analysis with specific receptor antiserum. Melatonin (0.1-100 nM) inhibited forskolin (20 microM)-stimulated cAMP formation in a dose-dependent manner and this effect was blocked by the competitive melatonin receptor antagonist luzindole (100-1000 nM). Furthermore, melatonin (1 nM) stimulated protein kinase C (PKC) activity by approximately 2-fold. The selective MT(2) receptor antagonist 4P-PDOT (100 nM) blocked this effect, indicating that the melatonin-mediated increase in PKC activity occurs through activation of MT(2) melatonin receptors. We conclude that SCN2.2 cells express functional melatonin receptors, providing an in vitro model to unveil the melatonin signaling pathway(s) involved in the regulation of circadian rhythms.

Embryonic and post-embryonic expression of arylalkylamine N-acetyltransferase and melatonin receptor genes in the eye and brain of chum salmon (Oncorhynchus keta)

Melatonin and arylalkylamine N-acetyltransferase (AANAT), the rate-limiting enzyme in melatonin synthesis, have taken on special importance in vertebrate circadian biology. Recent identification of genes encoding two AANAT (AANAT(1) and AANAT(2)) and two subtypes of melatonin receptor (Mel-R; Mel(1a) and Mel(1b)) in several fish species has led to rapid advances in characterizing the physiological roles of melatonin. In the present study, partial cDNAs encoding these four genes were cloned from the eye and brain of chum salmon (Oncorhynchus keta). Based on the nucleotide sequences, we developed highly sensitive real-time PCR systems for these four mRNAs. The development of daily rhythmicity in AANAT(1), AANAT(2), Mel(1a), and Mel(1b) transcript levels was examined in the eye and brain of chum salmon during embryonic and post-embryonic stages (from day -9 to day +180). In a parallel experiment, ocular and brain melatonin levels were measured by radioimmunoassay. Parallelism in developmental changes and in circadian rhythms of AANAT mRNAs and melatonin levels in the eye and the brain supports a hypothesis that the developmental increases of nocturnal melatonin levels results partly from the elevated transcription of AANAT genes. Moreover, abundant expression of AANAT and Mel-R mRNAs in the optic tectum, thalamus, hypothalamus, cerebellum, and eye indicates possible roles of melatonin in visual processing and neuroendocrine regulation, through which melatonin might be involved in migratory behavior of chum salmon.

Expression of membrane and nuclear melatonin receptors in mouse peripheral organs

Previous studies have shown that melatonin acts through specific receptors, including MT(1) and MT(2) membrane receptors as well as a nuclear receptor belonging to the orphan nuclear receptor family. Therefore, the goal of this study was to determine whether melatonin receptors mRNA is expressed in mouse peripheral tissues. To study the different receptors subtype expression, we have used a reverse-transcription polymerase chain reaction (RT-PCR) procedure followed by Southern hybridization with specific digoxigenin-labeled probes. RT-PCR studies revealed the presence of both MT(1) membrane receptors and ROR(alpha)1 nuclear receptors in all the peripheral tissues examined (brain, heart, lung, liver and kidney). Moreover, the expression of ROR(alpha)1 nuclear receptors was also demonstrated by Western-blot. In contrast, expression of MT(2) membrane receptors was only observed in brain and lung. These results suggest that melatonin, acting through its different subtypes receptors, plays a role in the neuroendocrine regulation of peripheral tissues function.

Molecular pharmacology of the ovine melatonin receptor: comparison with recombinant human MT1 and MT2 receptors

The variations of the pharmacological properties of melatonin receptors between different mammalian species in transfected cell lines have been poorly investigated. In the present study, melatonin analogues have been used to characterize the pharmacology of the recombinant ovine melatonin receptor (oMT1) expressed in CHO cell lines and the native oMT1 from the pars tuberalis (PT). Studies with selective ligands on native and transfected oMT1 showed similar properties for binding affinities [r2(PT/CHO) = 0.85]. The affinities and the functional activities of these ligands were compared with the human receptors (hMT1 or hMT2) expressed in CHO cells as well. The oMT1 and hMT1 receptors had similar pharmacological profiles (r2=0.82). Nevertheless, some of the selective compounds at the human receptor presented a reduced affinity at the ovine receptor. Furthermore, some compounds showed marked different functional activities at oMT1 vs. hMT1 receptors. Our findings demonstrated differences in the pharmacological properties of melatonin receptors in ovine and human species.

Top-down Approaches to the Study of Natural Variation in Complex Physiological Pathways Using the White-footed Mouse (Peromyscus leucopus) as a Model

Variation in complex physiological pathways has important effects on human function and medical treatment. Complex pathways involve cells at multiple locations, which serve different functions regulated by many genes and include complex neuroendocrine pathways that regulate physiological function. One of two competing hypotheses regarding the effects of selection on complex pathways predicts that variability should be common within complex pathways. If this hypothesis is correct, then we should expect wide variation in neuroendocrine function to be typical within natural populations. To test this hypothesis, a complex neuroendocrine pathway that regulates photoperiod-dependent changes in fertility in a natural population of white-footed mice (Peromyscus leucopus) was used to test for natural genetic variability in multiple components of the pathway. After testing only six elements in the photoperiod pathway in P. leucopus, genetic variation in the following four of these elements was evident: the circadian clock, melatonin receptor abundance or affinity, sensitivity of the reproductive axis to steroid negative feedback, and gonadotropin-releasing hormone neuronal activity. If this result can be extended to humans, the prediction would be that significant variation at multiple loci in complex neuroendocrine pathways is common among humans, and that variation would exist even in human populations from a common genetic background. This finding could only be drawn from an "exotic" animal model derived from a natural source population, confirming the continuing importance of nontraditional models alongside the standard laboratory species.

Tracking the elusive antiestrogenic effect of melatonin: a new methodological approach

OBJECTIVES

Detection of the antiestrogenic effect of melatonin on various breast cancer cell lines and its dependence of the differential expression of estrogen receptors (ERalpha and ERbeta) and melatonin receptors (mt1 and RZRalpha).

SETTING AND DESIGN

Dose-response curves of estradiol were determined in 6 different breast cancer cell lines using a colorimetric proliferation assay in the absence or presence of various melatonin concentrations.

METHODS

In order to detect the minor growth inhibitory effect of melatonin, a simple yet novel approach was employed: instead of incubating cells at single estradiol-concentrations at increasing melatonin levels, breast cancer cells were grown in microwell-plates for 4 days at increasing concentrations of estradiol (10(-12)M - 10(-10)M) in the absence or presence of melatonin (10(-9)M - 10(-8)M). Cell number was determined using Alamar blue and colorimetry. RT-PCR was performed for the expression of ERalpha, ERbeta, RZRalpha and mt1.

RESULTS

Melatonin at concentrations of 10(-9)M and 5 x 10(-9)M shifted the dose-response curves of estradiol to higher concentrations. Responsiveness to melatonin depended on expression of ERalpha but not on ERbeta. mRNA of ERbeta was not detectable in the breast cancer cell lines used. Only small amounts of mt1 transcripts were detectable in MCF-7 cells of one source. In MCF-7 cells transfected with the mt1 gene and in an ovarian cancer cell line mt1 was expressed at significant levels. RZRalpha was expressed in all tested cell lines at different amounts.

CONCLUSION

The growth of all ERalpha-positive breast cancer cell lines can be inhibited by melatonin. The effect in most cell lines is weak yet clearly reproducible. RZRalpha clearly contributes to the growth inhibitory effect of melatonin.

Expression of membrane and nuclear melatonin receptor mRNA and protein in the mouse immune system

The neurohormone melatonin plays a fundamental role in neuroimmunomodulation of several mammalian species, including mice. This effect is supported by the existence of specific melatonin-binding sites in murine immunocompetent organs. Moreover, using melatonin receptor analogues, several effects of the neurohormone on mice physiology through its membrane and nuclear receptors have been described. The expression of these receptors has never been studied, despite indirect evidence showing the presence of melatonin receptor in the murine immune system. At present, the MT1 and MT2 membrane receptors, and nuclear receptors belonging to the RZR/ROR family have been related to the immunomodulator effect of melatonin. Here, we show the presence of membrane and nuclear melatonin-binding sites in mouse thymus and spleen, using the specific melatonin membrane (S 20098) and nuclear (CGP 52608) receptor agonist. To confirm the presence of melatonin receptors, we analyzed the presence of membrane and nuclear receptor mRNA and protein by RT-PCR, Southern blot, and Western blot. Thus, we show that MT1 and RORalpha receptor mRNA and protein are expressed in both thymus and spleen, while MT2 receptor mRNA is only detected in the thymus. This expression of melatonin receptors strongly supports the idea of an immunomodulatory role of melatonin through its receptors.

Allelic variants of human melatonin 1A receptor in patients with familial adolescent idiopathic scoliosis

STUDY DESIGN

A genetic study of patients with familial adolescent idiopathic scoliosis.

OBJECTIVES

The purpose of this study was to evaluate the evidence for linkage on chromosome 4q and determine whether mutations in the gene coding for melatonin receptor are present.

SUMMARY OF BACKGROUND DATA

Adolescent idiopathic scoliosis is the most common spine deformity arising during childhood, but its cause remains unknown. The fact that adolescent idiopathic scoliosis is often seen in several members of the same family strongly suggests a genetic factor. Recent work by Wise et al provides evidence for linkage of adolescent idiopathic scoliosis at several different chromosome sites, including 4q. In addition, there is some evidence that adolescent idiopathic scoliosis may be related to a disturbance in melatonin metabolism, and the human melatonin-1A receptor is known to be located on chromosome 4q.

METHODS

Probands having clinically relevant idiopathic scoliosis (Cobb angle >30 degrees) and their relatives were identified. Radiographic confirmation was required for a positive diagnosis. Linkage analysis was performed with 15 microsatellite markers of chromosome 4q spaced at approximately 10-cM resolution and 5 microsatellite markers surrounding the site for human melatonin receptor. The gene for human melatonin receptor was screened for mutations in the coding region using genomic DNA samples by single-strand conformational polymorphism analysis. Amplimers showing a band shift were reamplified and sequenced bidirectionally.

RESULTS

There was no evidence for linkage at chromosome 4q in this study population. Twenty-nine individuals demonstrated aberrant single-strand conformation polymorphism band patterns, and sequence evaluation demonstrated six genetic polymorphisms for the gene for human melatonin receptor. These genetic variations were found in both affected and nonaffected individuals, and there was no correlation between gene variants and the phenotype for adolescent idiopathic scoliosis.

CONCLUSIONS

The results of this study demonstrated no evidence of linkage to chromosome 4q and no mutations in the coding region of the gene for human melatonin receptor. The identification of variants in the human melatonin receptor could provide a useful tool for testing the gene in the predisposition to various other melatonin-related disorders and for clarifying the role of melatonin in adolescent idiopathic scoliosis.

Gene expression and functional characterization of melatonin receptors in the spinal cord of the rat: implications for pain modulation

Recently, a species-dependent distribution of melatonin binding sites have been found in lamina I-V and lamina X of the spinal cord. In order to learn more about the function of spinal melatonin receptors, we investigated (i) the gene expression for melatonin receptor subtypes in lumbar and thoracal spinal cord tissue by means of the reverse-transcriptase polymerase chain reaction (RT-PCR) technique, and (ii) the electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of spinal cord mRNA by means of the voltage clamp technique. Because ample evidence indicates an antinociceptive effect of melatonin, (iii) the role of spinal melatonin receptors for maintaining mechanical and thermal hyperalgesia was studied in a rat model for postoperative pain. The RT-PCR data revealed that transcripts for MT1 and MT2 melatonin receptors are present in the dorsal and ventral horn of lumbar and thoracal spinal cord tissue. Injection of mRNA from lumbar spinal cord tissue into Xenopus oocytes led to the functional reconstitution of melatonin receptors which activate calcium-dependent chloride inward currents. Melatonin responses were abolished by simultaneous administration of the antagonists, 2-phenylmelatonin and luzindole and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Intrathecal administration of different melatonin doses (10-100 nmol) did not inhibit mechanical or thermal hyperalgesia. However, intrathecal application of a low dose of morphine together with melatonin caused a brief antinociceptive effect suggesting an enhanced morphine analgesia by melatonin. In conclusion, the present study demonstrated for the first time the presence of transcripts of MT1 and MT2 receptors located in the dorsal and ventral horn of the spinal cord. Furthermore, spinal melatonin enhanced the antinociceptive effect of morphine indicating that melatonin acts as a neuromodulator in the spinal cord.

Bidirectional communication between the pineal gland and the immune system

The pineal gland is a vertebrate neuroendocrine organ converting environmental photoperiodic information into a biochemical message (melatonin) that subsequently regulates the activity of numerous target tissues after its release into the bloodstream. A phylogenetically conserved feature is increased melatonin synthesis during darkness, even though there are differences between mammals and birds in the regulation of rhythmic pinealocyte function. Membrane-bound melatonin receptors are found in many peripheral organs, including lymphoid glands and immune cells, from which melatonin receptor genes have been characterized and cloned. The expression of melatonin receptor genes within the immune system shows species and organ specificity. The pineal gland, via the rhythmical synthesis and release of melatonin, influences the development and function of the immune system, although the postreceptor signal transduction system is poorly understood. Circulating messages produced by activated immune cells are reciprocally perceived by the pineal gland and provide feedback for the regulation of pineal function. The pineal gland and the immune system are, therefore, reciprocally linked by bidirectional communication.

A molecular perspective of the genetic relationships of G-protein coupled melatonin receptor subtypes

Successful cloning of melatonin receptors from various target tissues in the past few years has increased our understanding of the molecular signal transduction mechanisms of G-protein coupled melatonin receptors, of which three subtypes (MEL-1A, MEL-1B, and MEL-1C) have been reported in different vertebrates. Based upon melatonin receptor sequences available in the Genbank database, we have performed phylogenetic analyses of the nucleotide and encoded amino acid sequences of G-protein-coupled melatonin receptors, and determined the range of amino acid identities between melatonin receptors of the same and different subtypes. Besides the three well-known subtypes, a potential novel subtype of MEL-1D, as exemplified by unique separation of Xenopus X2.0 sequence (Genbank accession No. U31826) from the others in the protein phylogenetic tree, possibly exists. In addition, one of the chicken brain melatonin receptor sequences has been identified as belonging to the MEL-1B subtype. Our analyses showed that melatonin receptors of the same subtype and different subtypes are likely to share > or = 75% and < 65% amino acid identities, respectively. Phylogenetic analysis based on amino acid comparisons will be needed to determine the subtype status of any pair of melatonin receptor sequences that exhibit > or = 65% to < 75% amino acid identity. Despite the usefulness of genetic relatedness in the subtype classification of G-protein-coupled melatonin receptors, functional correlation of molecular structure may ultimately prove the most comprehensive approach in melatonin receptor classification.