Dynamics in enzymatic protein complexes offer a novel principle for the regulation of melatonin synthesis in the human pineal gland

Ectopic expression of MmCYP1A1, a mouse cytochrome P450 gene, positively regulates stress tolerance in apple calli and Arabidopsis

Ectopic expression of MmCYP1A1 gene from Mus musculus in apple calli and Arabidopsis increased the levels of melatonin and 6-hydroxymelatonin, and improved their stress resistance. Melatonin occurs widely in organisms, playing a key regulatory role. CYP1A1 is a cytochrome P450 monooxygenase, involved in the melatonin metabolism, and is responsible for the synthesis of 6-hydroxymelatonin from melatonin. Melatonin and 6-hydroxymelatonin have strong antioxidant activities in animals. Here, we cloned MmCYP1A1 from Mus musculus and found that ectopic expression of MmCYP1A1 improved the levels of melatonin and 6-hydroxymelatonin in transgenic apple calli and Arabidopsis. Subsequently, we observed that MmCYP1A1 increased the tolerance of transgenic apple calli and Arabidopsis to osmotic stress simulated by polyethylene glycol 6000 (PEG 6000), as well as resistance of transgenic Arabidopsis to drought stress. Further, the number of lateral roots of MmCYP1A1 transgenic Arabidopsis were enhanced significantly after PEG 6000 treatment. The expression of MmCYP1A1 remarkably reduced malondialdehyde (MDA) content, electrolyte leakage, accumulation of H₂O₂ and O₂^- during stress treatment. Moreover, MmCYP1A1 enhanced stress tolerance in apple calli and Arabidopsis by increasing the expression levels of resistance genes. MmCYP1A1 also promoted stomatal closure in transgenic Arabidopsis to reduce leaf water loss during drought. Our results indicate that MmCYP1A1 plays a key role in plant stress tolerance, which may provide a reference for future plant stress tolerance studies.

Retinoic Acid Signalling in the Pineal Gland Is Conserved across Mammalian Species and Its Transcriptional Activity Is Inhibited by Melatonin

The pineal gland is integral to the circadian timing system due to its role in nightly melatonin production. Retinoic acid (RA) is a potent regulator of gene transcription and has previously been found to exhibit diurnal changes in synthesis and signalling in the rat pineal gland. This study investigated the potential for the interaction of these two systems. PCR was used to study gene expression in mouse and human pineal glands, ex-vivo organotypic cultured rat pineal gland and cell lines. The mouse and human pineal glands were both found to express the necessary components required for RA signalling. RA influences the circadian clock in the brain, therefore the short-term effect of RA on clock gene expression was determined in ex vivo rat pineal glands but was not found to rapidly regulate Per1, Per2, Bmal1, or Cry1. The interaction between RA and melatonin was also investigated and, unexpectedly, melatonin was found to suppress the induction of gene transcription by RA. This study demonstrates that pineal expression of the RA signalling system is conserved across mammalian species. There is no short-term regulation of the circadian clock but an inhibitory effect of melatonin on RA transcriptional activity was demonstrated, suggesting that there may be functional cross-talk between these systems.

Identification of a serotonin N-acetyltransferase from Staphylococcus pseudintermedius ED99

Serotonin N-acetyltransferase (SNAT) catalyzes the biosynthesis of N-acetylserotonin (NAS) and N-acetyltryptamine (NAT), two pleiotropic molecules with neurotransmitter functions. Here, we report the identification of a SNAT protein in the genus Staphylococcus. The SNAT gene identified in Staphylococcus pseudintermedius ED99, namely SPSE_0802, encodes a 140 residues-long cytoplasmic protein. The recombinant protein SPSE_0802 was expressed in E. coli BL21 and found to acetylate serotonin (SER) and tryptamine (TRY) as well as other trace amines in vitro. The production of the neuromodulators NAS and NAT was detected in the cultures of different members of the genus Staphylococcus and the role of SPSE_0802 in this production was confirmed in an ED99 SPSE_0802 deletion mutant. A search for SNAT homologues showed that the enzyme is widely distributed across the genus which correlated with the SNAT activity detected in 22 out of the 40 Staphylococcus strains tested. The N-acetylated products of SNAT are precursors for melatonin synthesis and are known to act as neurotransmitters and activate melatonin receptors, among others, inducing various responses in the human body. The identification of SNAT in staphylococci could contribute to a better understanding of the interaction between those human colonizers and the host peripheral nervous system.

Transcriptomic Changes of Photoperiodic Response in the Hypothalamus Were Identified in Ovariectomized and Estradiol-Treated Sheep

Accurate timing of seasonal changes is an essential ability for an animal’s survival, and the change in the photoperiod is the key factor affecting reproductive seasonality in mammals. Emerging evidence has suggested that multiple hypothalamic genes participate in the photoperiod-induced regulation of reproductive activities in sheep, but the mechanism is still unclear. In this study, we initially examined the plasma level of two major reproductive hormones, namely, follicle-stimulating hormone (FSH) and prolactin (PRL), under different photoperiods in ovariectomized and estradiol-treated (OVX + E2) sheep using radioimmunoassay (RIA). Of the two hormones, the concentration of PRL significantly increased with the extension of the photoperiod, while FSH showed the opposite trend. Subsequently, an examination of the transcriptomic variation between the short photoperiod (SP) and long photoperiod (LP) was conducted. Differential expression analyses and functional annotation showed that several key genes in the insulin secretion (VAMP2, PRKACB, PRKCG, and PLCB1), GnRH (MAPK13, CGA, CDC42, ATF4, and LHB) pathways, and circadian entrainment (KCNJ5, PER1, GNB2, MTNR1A, and RASD1), as well as numerous lncRNAs, including XR_173257.3, XR_173415.3, XR_001435315.1, XR_001024596.2, and XR_001023464.2, were shown potentially vital for the hypothalamic photoperiodic response. Four of the differentially expressed mRNAs and lncRNAs were validated by qPCR. The constructed mRNA–mRNA interaction networks further revealed that transcripts potentially participated in hypothalamic thyroid hormone synthesis, endocrine resistance, and neuroactive ligand–receptor interactions. The interactome analysis of lncRNAs and their targets implied that XR_173257.3 and its target arylalkylamine N-acetyltransferase (AANAT) and XR_173415.3 and its target TH might participate in the regulation of seasonal reproduction. Together, the changes in reproductive hormones and transcriptome will help to determine the important photoperiod-induced lncRNAs and mRNAs and provide a valuable resource for further research on reproductive seasonality in sheep.

Negative regulation of melatonin secretion by melatonin receptors in ovine pinealocytes

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms and reproduction. The photoperiodic information is detected by retinal photoreceptors and transmitted through nerve transmissions to the pineal gland, where MLT is synthesized and secreted at night into the blood. MLT interacts with two G protein-coupled receptors, MT1 and MT2. The aim of our work was to provide evidence for the presence of MLT receptors in the ovine pineal gland and define their involvement on melatonin secretion. For the first time, we identified the expression of MLT receptors with the specific 2-[125I]-MLT agonistic radioligand in ovin pinealocytes. The values of Kd and Bmax are 2.24 ± 1.1 nM and 20 ± 6.8 fmol/mg. MLT receptors are functional and inhibit cAMP production and activate ERK1/2 through pertussis toxin-sensitive Gi/o proteins. The MLT receptor antagonist/ inverse agonist luzindole increased cAMP production (189 ± 30%) and MLT secretion (866 ± 13%). The effect of luzindole on MLT secretion was additive with the effect of well-described activators of this pathway such as the β-adrenergic agonist isoproterenol and the α-adrenergic agonist phenylephrine. Co-incubation of all three compounds increased MLT secretion by 1236 ± 199%. These results suggest that MLT receptors are involved in the negative regulation of the synthesis of its own ligand in pinealocytes. While adrenergic receptors promote MLT secretion, MLT receptors mitigate this effect to limit the quantity of MLT secreted by the pineal gland.

Mass-spectrometry analysis of the human pineal proteome during night and day and in autism

The human pineal gland regulates day-night dynamics of multiple physiological processes, especially through the secretion of melatonin. Using mass-spectrometry-based proteomics and dedicated analysis tools, we identify proteins in the human pineal gland and analyze systematically their variation throughout the day and compare these changes in the pineal proteome between control specimens and donors diagnosed with autism. Results reveal diverse regulated clusters of proteins with, among others, catabolic carbohydrate process and cytoplasmic membrane-bounded vesicle-related proteins differing between day and night and/or control versus autism pineal glands. These data show novel and unexpected processes happening in the human pineal gland during the day/night rhythm as well as specific differences between autism donor pineal glands and those from controls.

Biological Timing and Neurodevelopmental Disorders: A Role for Circadian Dysfunction in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are a spectrum of neurodevelopmental disorders characterized by impaired social interaction and communication, as well as stereotyped and repetitive behaviors. ASDs affect nearly 2% of the United States child population and the worldwide prevalence has dramatically increased in recent years. The etiology is not clear but ASD is thought to be caused by a combination of intrinsic and extrinsic factors. Circadian rhythms are the ∼24 h rhythms driven by the endogenous biological clock, and they are found in a variety of physiological processes. Growing evidence from basic and clinical studies suggest that the dysfunction of the circadian timing system may be associated with ASD and its pathogenesis. Here we review the findings that link circadian dysfunctions to ASD in both experimental and clinical studies. We first introduce the organization of the circadian system and ASD. Next, we review physiological indicators of circadian rhythms that are found disrupted in ASD individuals, including sleep-wake cycles, melatonin, cortisol, and serotonin. Finally, we review evidence in epidemiology, human genetics, and biochemistry that indicates underlying associations between circadian regulation and the pathogenesis of ASD. In conclusion, we propose that understanding the functional importance of the circadian clock in normal and aberrant neurodevelopmental processes may provide a novel perspective to tackle ASD, and clinical treatments for ASD individuals should comprise an integrative approach considering the dynamics of daily rhythms in physical, mental, and social processes.

Involvement of orexin A in nocturnal melatonin secretion into the cerebrospinal fluid and the blood plasma in seasonal sheep

In sheep, differences in orexin A (OXA) gene expression and activity are related to changes in energy demand and seasonal reproduction. However, the mechanism by which and the key place where the OXA signal is integrated with photoperiod, whose main biochemical expression is melatonin (MEL), remain unknown. We examined the effects of cisterna magna injections of OXA (0.3 μg/kg body weight) on nocturnal cerebrospinal fluid (CSF) and plasma MEL concentrations; mRNA and protein expression of two rate-limiting enzymes for MEL biosynthesis, tryptophan 5-hydroxylase-1 (TPH1) and arylalkylamine-N-acetyltransferase (AA-NAT); and OXA receptor (OX1R, OX2R) expression in the pineal gland (PG) obtained from twenty ewes during the short-day (SD) and long-day (LD) seasons. OXA increased (P < 0.001) CSF and plasma MEL concentrations regardless of the season. Plasma MEL was positively correlated (P < 0.001) with CSF MEL in the OXA-treated sheep in both seasons. OXA had no effect (P > 0.05) on TPH1 transcript or protein level but upregulated (P < 0.05) AA-NAT mRNA and protein expression in both seasons. OXA enhanced (P < 0.05) OX1R mRNA level only during the LD season. Our results show that the endocrine activity of the ovine PG is regulated by day length and non-photic signals via hypothalamic OXA. These results are important for understanding the work of the biological clock and recognizing mechanisms responsible for the adaptation of seasonal animals to the changing external environment conditions. OXA and MEL are both involved in the regulation of the sleep-wakefulness system, therefore our results can be used in the study on the circadian rhythm disorders in humans (e.g. jet lag, insomnia, seasonal depression).

MicroRNA-7 inhibits melatonin synthesis by acting as a linking molecule between leptin and norepinephrine signaling pathways in pig pineal gland

MicroRNAs, including microRNA-7 (miR-7), are important modulators of numerous gene expressions and the related biological processes. Melatonin is a key hormone regulating daily and seasonal rhythms, in which a variety of positive and negative regulatory factors, such as norepinephrine (NE) and leptin, are involved. However, the interactions among these factors and the mechanisms remain to be elucidated. The aims of the present study were to identify the functions and the related mechanisms of miR-7 in regulating melatonin synthesis and secretion through in vitro and in vivo experiments in pineal gland of pigs, which is an important animal model for agricultural and biomedical studies. Our results firstly show that miR-7 is specifically expressed in porcine pinealocytes and negatively regulates melatonin synthesis. The further functional studies show that the dynamic expression levels of miR-7 are contrary to the melatonin levels throughout the day, and the forced inhibition of endogenous miR-7 in porcine pinealocytes sharply increases arylalkylamine N-acetyltransferase (AANAT) expression by 80.0% (P = 0.0031) and melatonin levels by 81.0% (P = 0.0421), whereas miR-7 over-expression down-regulates AANAT expression by 38.6% (P = 0.0004) and melatonin levels by 37.6% (P = 0.0212). In addition, the miR-7 expression is up-regulated by leptin through the JAK/STAT3 signaling pathway, and the in vivo intracerebroventricular injection of leptin increases miR-7 expression by 80.0% (P = 0.0044) in porcine pineal glands and reduces melatonin levels by 57.1% (P = 0.0060) compared with the controls. This functional inhibition of melatonin synthesis by miR-7 is accomplished by its binding to the 3'-UTR of Raf1. Further, our results demonstrate that the RAF1/MEK/ERK signaling pathway mediates NE-induced AANAT expression, whereas leptin attenuates NE's function through miR-7. Taken together, the results demonstrated that leptin activates the JAK/STAT3 signaling pathway to increase the expression of miR-7, which acts as a negative regulatory molecule inhibiting NE-activated RAF1/MEK/ERK signaling pathway by targeting Raf1, resulting in decreased AANAT expression and melatonin synthesis. These findings suggest that miR-7 is a novel negative regulator of melatonin synthesis and links leptin- and NE-mediated signaling pathways in porcine pineal glands, which will contribute to our understanding in the establishment of the biological rhythms resulting from melatonin.

Choroid plexus is an additional source of melatonin in the brain

The cerebrospinal fluid melatonin is released from the pineal gland, directly into the third ventricle, or produced de novo in the brain from extrapineal melatonin sources leading to a melatonin concentration gradient in the cerebrospinal fluid. Despite the interest on this topic, the brain areas capable of producing melatonin are not yet clear. Bearing this in mind, we hypothesized that the choroid plexus (CP) could be one of these melatonin sources. We analyzed and confirmed the presence of the four enzymes required for melatonin synthesis in rat CP and demonstrated that arylalkylamine N-acetyltransferase shows a circadian expression in female and male rat CP. Specifically, this enzyme colocalizes with mitochondria in rat CP epithelial cells, an organelle known to be involved in melatonin function and synthesis. Then, we demonstrated that melatonin is synthesized by porcine CP explants, although without a circadian pattern. In conclusion, our data show that the CP is a local source of melatonin to the central nervous system, probably contributing to its high levels in the cerebrospinal fluid. We believe that in the CP, melatonin might be regulated by its endogenous clock machinery and by the hormonal background.

Trehalose Activates CRE-Dependent Transcriptional Signaling in HT22 Mouse Hippocampal Neuronal Cells: A Central Role for PKA Without cAMP Elevation

Cyclic adenosine 3′,5′monophosphate (cAMP) regulated element binding protein (CREB) is a transcription factor involved in many different signaling processes including memory storage and retrieval. The mouse hippocampal neuronal cell line HT22 is widely used as a model system for neuronal cell death and cellular signal pathway investigations. For the present work a variant of HT22 with a stably expressed CRE-luciferase (CRE-luc) reporter (HT22CRE) is introduced, characterized and used to investigate cAMP-dependent and independent CRE-dependent signal processes. Trehalose (Mykose or 1-α-Glucopyranosyl-1-α-glucopyranosid) is a naturally occurring disaccharide consisting of two α,α′,1,1-glycosidic connected glucose molecules in a wide range of organisms but usually not found in mammals. Trehalose has been shown to activate autophagy, a process which regulates the degradation and recycling of proteins and organelles. The exact processes how trehalose application works on mammalian neuronal cells is not yet understood. The present work shows that trehalose application dose-dependently elevates CRE-luc activity in HT22 cells and acts synergistically with cAMP-elevating agents. In this pathway cAMP-dependent protein kinase (PKA) appears to be the most important factor and the stress kinase p38 and protein tyrosine kinases (PTKs) act as modulators.

The Timing of Melatonin Administration Is Crucial for Its Antidepressant-Like Effect in Mice

Melatonin is synthesized by the pineal gland with a circadian rhythm in synchrony with the environmental light/dark cycle. A gradual increase in circulating levels of melatonin occur after lights off, reaching its maximum around the middle of the dark phase. Agonists of melatonin receptors have proved effectiveness as antidepressants in clinical trials. However, there is contradictory evidence about the potential antidepressant effect of melatonin itself. Herein we studied melatonin administration in mice at two zeitgeber times (ZT; ZT = 0 lights on; 12:12 L/D), one hour before the beginning (ZT11) and at the middle (ZT18) of the dark phase after either a single or a three-dose protocol. Behavioral despair was assessed through a forced-swimming test (FST) or a tail suspension test (TST), at ZT18.5. A single dose of 4 mg/kg melatonin at ZT11 was effective to reduce the immobility time in both tests. However, acute administration of melatonin at ZT18 was not effective in mice subjected to FST, and a higher dose (16 mg/kg) was required to reduce immobility time in the TST. A three-dose administration protocol of 16 mg/kg melatonin (ZT18, ZT11, and ZT18) significantly reduced immobility time in FST. Data indicate that the timely administration of melatonin could improve its antidepressant-like effect.

LIM homeobox transcription factor Isl1 is required for melatonin synthesis in the pig pineal gland

Melatonin is a key hormone that regulates circadian rhythms, metabolism, and reproduction. However, the mechanisms of melatonin synthesis and secretion have not been fully defined. The purpose of this study was to investigate the functions of the LIM homeobox transcription factor Isl1 in regulating melatonin synthesis and secretion in porcine pineal gland. We found that Isl1 is highly expressed in the melatonin-producing cells in the porcine pineal gland. Further functional studies demonstrate that Isl1 knockdown in cultured primary porcine pinealocytes results in the decline of melatonin and arylalkylamine N-acetyltransferase (AANAT) mRNA levels by 29.2% and 72.2%, respectively, whereas Isl1 overexpression raised by 1.3-fold and 2.7-fold. In addition, the enhancing effect of norepinephrine (NE) on melatonin synthesis was abolished by Isl1 knockdown. The in vivo intracerebroventricular NE injections upregulate Isl1 mRNA and protein levels by about threefold and 4.5-fold in the porcine pineal gland. We then examined the changes in Isl1 expression in the pineal gland and global melatonin levels throughout the day. The results show that Isl1 protein level at 24:00 is 2.5-fold higher than that at 12:00, which is parallel to melatonin levels. We further found that Isl1 increases the activity of AANAT promoter, and the effect of NE on Isl1 expression was blocked by an ERK inhibitor. Collectively, the results presented here demonstrate that Isl1 positively modulates melatonin synthesis by targeting AANAT, via the ERK signaling pathway of NE. These suggest that Isl1 plays important roles in maintaining the daily circadian rhythm.

14-3-3 proteins in platelet biology and glycoprotein Ib-IX signaling

Members of the 14-3-3 family of proteins function as adapters/modulators that recognize phosphoserine/phosphothreonine-based binding motifs in many intracellular proteins and play fundamental roles in signal transduction pathways of eukaryotic cells. In platelets, 14-3-3 plays a wide range of regulatory roles in phosphorylation-dependent signaling pathways, including G-protein signaling, cAMP signaling, agonist-induced phosphatidylserine exposure, and regulation of mitochondrial function. In particular, 14-3-3 interacts with several phosphoserine-dependent binding sites in the major platelet adhesion receptor, the glycoprotein Ib-IX complex (GPIb-IX), regulating its interaction with von Willebrand factor (VWF) and mediating VWF/GPIb-IX-dependent mechanosignal transduction, leading to platelet activation. The interaction of 14-3-3 with GPIb-IX also plays a critical role in enabling the platelet response to low concentrations of thrombin through cooperative signaling mediated by protease-activated receptors and GPIb-IX. The various functions of 14-3-3 in platelets suggest that it is a possible target for the treatment of thrombosis and inflammation.

Disruption of melatonin synthesis is associated with impaired 14-3-3 and miR-451 levels in patients with autism spectrum disorders

Autism spectrum disorders (ASD) are characterized by a wide genetic and clinical heterogeneity. However, some biochemical impairments, including decreased melatonin (crucial for circadian regulation) and elevated platelet N-acetylserotonin (the precursor of melatonin) have been reported as very frequent features in individuals with ASD. To address the mechanisms of these dysfunctions, we investigated melatonin synthesis in post-mortem pineal glands - the main source of melatonin (9 patients and 22 controls) - and gut samples - the main source of serotonin (11 patients and 13 controls), and in blood platelets from 239 individuals with ASD, their first-degree relatives and 278 controls. Our results elucidate the enzymatic mechanism for melatonin deficit in ASD, involving a reduction of both enzyme activities contributing to melatonin synthesis (AANAT and ASMT), observed in the pineal gland as well as in gut and platelets of patients. Further investigations suggest new, post-translational (reduced levels of 14-3-3 proteins which regulate AANAT and ASMT activities) and post-transcriptional (increased levels of miR-451, targeting 14-3-3ζ) mechanisms to these impairments. This study thus gives insights into the pathophysiological pathways involved in ASD.

Melatonin Synthesis: Acetylserotonin O-Methyltransferase (ASMT) Is Strongly Expressed in a Subpopulation of Pinealocytes in the Male Rat Pineal Gland

The rat pineal gland has been extensively used in studies of melatonin synthesis. However, the cellular localization of melatonin synthesis in this species has not been investigated. Here we focus on the localization of melatonin synthesis using immunohistochemical methods to detect the last enzyme in melatonin synthesis, acetylserotonin O-methyltransferase (ASMT), and in situ hybridization techniques to study transcripts encoding ASMT and two other enzymes in melatonin synthesis, tryptophan hydroxylase (TPH)-1 and aralkylamine N-acetyltransferase. In sections of the rat pineal gland, marked cell-to-cell differences were found in ASMT immunostaining intensity and in the abundance of Tph1, Aanat, and Asmt transcripts. ASMT immunoreactivity was localized to the cytoplasm in pinealocytes in the parenchyma of the superficial pineal gland, and immunopositive pinealocytes were also detected in the pineal stalk and in the deep pineal gland. ASMT was found to inconsistently colocalize with S-antigen, a widely used pinealocyte marker; this colocalization was seen in cells throughout the pineal complex and also in displaced pinealocyte-like cells of the medial habenular nucleus. Inconsistent colocalization between ASMT and TPH protein was also detected in the pineal gland. ASMT protein was not detected in extraepithalamic parts of the central nervous system or in peripheral tissues. The findings in this report are of special interest because they provide reason to suspect that melatonin synthesis varies significantly among individual pinealocytes.

Sulfation of 6-hydroxymelatonin, N-acetylserotonin and 4-hydroxyramelteon by the human cytosolic sulfotransferases (SULTs)

1. This study aimed to investigate the involvement of sulfation in the metabolism of 6-hydroxymelatonin (6-OH-Mel), N-acetylserotonin (NAS) and 4-hydroxyramelteon (4-OH-Ram), and to identify and characterize the human cytosolic sulfotransferases (SULTs) capable of sulfating these drug compounds. 2. A systematic analysis using 13 known human SULTs revealed that SULT1A1 displayed the strongest activity in catalyzing the sulfation of 6-OH-Mel and 4-OH-Ram, whereas SULT1C4 exhibited the strongest sulfating-activity towards NAS. pH-dependence and kinetic parameters of these SULT enzymes in mediating the sulfation of respective drug compounds were determined. A metabolic labeling study showed the generation and release of [³⁵S]sulfated 6-OH-Mel, NAS and 4-OH-Ram by HepG2 human hepatoma cells and Caco-2 human colon adenocarcinoma cells labeled with [³⁵S]sulfate in the presence of these drug compounds. Cytosols of human lung, liver, kidney and small intestine were examined to verify the presence of 6-OH-Mel-, NAS- and 4-OH-Ram-sulfating activity in vivo. Of the four human organ samples tested, small intestine and liver cytosols displayed considerably higher 6-OH-Mel-, NAS- and 4-OH-Ram-sulfating activities than those of lung and kidney. 3. Collectively, these results provided a molecular basis for the metabolism of 6-OH-Mel, NAS and 4-OH-Ram through sulfation.

Heterogeneous ribonucleoprotein R regulates arylalkylamine N-acetyltransferase synthesis via internal ribosomal entry site-mediated translation in a circadian manner

Rhythmic arylalkylamine N-acetyltransferase (AANAT) synthesis is a prominent circadian-controlled response that occurs in most mammals. AANAT is the core enzyme in melatonin production; because melatonin participates in many physiological processes, the regulation of AANAT is an important research topic. In this study, we focused on the role of heterogeneous ribonucleoprotein R (hnRNP R) in the translation of AANAT. A novel RNA-binding protein hnRNP R widely interacted with the 5' untranslated region (UTR) of AANAT mRNA and contributed to translation through an internal ribosomal entry site (IRES). Fine-tuning of AANAT protein synthesis occurred in response to knockdown and overexpression of hnRNP R. Nocturnal elevation of AANAT protein was dependent on the rhythmic changes of hnRNP R, whose levels are elevated in the pineal gland during nighttime. Increases in hnRNP R additionally improved AANAT production in rat pinealocytes under norepinephrine (NE) treatment. These results suggest that cap-independent translation of AANAT mRNA plays a role in the rhythmic synthesis of melatonin through the recruitment of translational machinery to hnRNP R-bound AANAT mRNA.

Placental melatonin system is present throughout pregnancy and regulates villous trophoblast differentiation

Melatonin is highly produced in the placenta where it protects against molecular damage and cellular dysfunction arising from hypoxia/re-oxygenation-induced oxidative stress as observed in primary cultures of syncytiotrophoblast. However, little is known about melatonin and its receptors in the human placenta throughout pregnancy and their role in villous trophoblast development. The purpose of this study was to determine melatonin-synthesizing enzymes, arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole O-methyltransferase (HIOMT), and melatonin receptors (MT1 and MT2) expression throughout pregnancy as well as the role of melatonin and its receptors in villous trophoblast syncytialization. Our data show that the melatonin generating system is expressed throughout pregnancy (from week 7 to term) in placental tissues. AANAT and HIOMT show maximal expression at the 3rd trimester of pregnancy. MT1 receptor expression is maximal at the 1st trimester compared to the 2nd and 3rd trimesters, while MT2 receptor expression does not change significantly during pregnancy. Moreover, during primary villous cytotrophoblast syncytialization, MT1 receptor expression increases, while MT2 receptor expression decreases. Treatment of primary villous cytotrophoblast with an increasing concentration of melatonin (10 pM-1 mM) increases the fusion index (syncytium formation; 21% augmentation at 1 mM melatonin vs. vehicle) and β-hCG secretion (121% augmentation at 1 mM melatonin vs. vehicle). This effect of melatonin appears to be mediated via its MT1 and MT2 receptors. In sum, melatonin machinery (synthetizing enzymes and receptors) is expressed in human placenta throughout pregnancy and promotes syncytium formation, suggesting an essential role of this indolamine in placental function and pregnancy well-being.

Multiple sclerosis: the role of melatonin and N-acetylserotonin

Multiple sclerosis (MS) is an immune mediated disorder that is under intensive investigation in an attempt to improve on available treatments. Many of the changes occurring in MS, including increased mitochondrial dysfunction, pain reporting and depression may be partly mediated by increased indoleamine 2,3-dioxygenase, which drives tryptophan to the production of neuroregulatory tryptophan catabolites and away from serotonin, N-acetylserotonin and melatonin production. The consequences of decreased melatonin have classically been attributed to circadian changes following its release from the pineal gland. However, recent data shows that melatonin may be produced by all mitochondria containing cells to some degree, including astrocytes and immune cells, thereby providing another important MS treatment target. As well as being a powerful antioxidant, anti-inflammatory and antinociceptive, melatonin improves mitochondrial functioning, partly via increased oxidative phosphorylation. Melatonin also inhibits demyelination and increases remyelination, suggesting that its local regulation in white matter astrocytes by serotonin availability and apolipoprotein E4, among other potential factors, will be important in the etiology, course and treatment of MS. Here we review the role of local melatonin and its precursors, N-acetylserotonin and serotonin, in MS.

The serotonin-N-acetylserotonin-melatonin pathway as a biomarker for autism spectrum disorders

Elevated whole-blood serotonin and decreased plasma melatonin (a circadian synchronizer hormone that derives from serotonin) have been reported independently in patients with autism spectrum disorders (ASDs). Here, we explored, in parallel, serotonin, melatonin and the intermediate N-acetylserotonin (NAS) in a large cohort of patients with ASD and their relatives. We then investigated the clinical correlates of these biochemical parameters. Whole-blood serotonin, platelet NAS and plasma melatonin were assessed in 278 patients with ASD, their 506 first-degree relatives (129 unaffected siblings, 199 mothers and 178 fathers) and 416 sex- and age-matched controls. We confirmed the previously reported hyperserotonemia in ASD (40% (35-46%) of patients), as well as the deficit in melatonin (51% (45-57%)), taking as a threshold the 95th or 5th percentile of the control group, respectively. In addition, this study reveals an increase of NAS (47% (41-54%) of patients) in platelets, pointing to a disruption of the serotonin-NAS-melatonin pathway in ASD. Biochemical impairments were also observed in the first-degree relatives of patients. A score combining impairments of serotonin, NAS and melatonin distinguished between patients and controls with a sensitivity of 80% and a specificity of 85%. In patients the melatonin deficit was only significantly associated with insomnia. Impairments of melatonin synthesis in ASD may be linked with decreased 14-3-3 proteins. Although ASDs are highly heterogeneous, disruption of the serotonin-NAS-melatonin pathway is a very frequent trait in patients and may represent a useful biomarker for a large subgroup of individuals with ASD.

Extrapineal melatonin: sources, regulation, and potential functions

Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.

Adrenergic activation of melatonin secretion in ovine pineal explants in short-term superfusion culture occurs via protein synthesis independent and dependent phenomena

The ovine pineal is generally considered as an interesting model for the study on adrenergic regulation of melatonin secretion due to some functional similarities with this gland in the human. The present investigations, performed in the superfusion culture of pineal explants, demonstrated that the norepinephrine-induced elevation of melatonin secretion in ovine pinealocytes comprised of two subsequent periods: a rapid increase phase and a slow increase phase. The first one included the quick rise in release of N-acetylserotonin and melatonin, occurring parallel to elevation of NE concentration in the medium surrounding explants. This rapid increase phase was not affected by inhibition of translation. The second, slow increase phase began after NE level had reached the maximum concentration in the culture medium and lasted about two hours. It was completely abolished by the treatment with translation inhibitors. The obtained results showed for the first time that the regulation of N-acetylserotonin synthesis in pinealocytes of some species like the sheep involves the on/off mechanism, which is completely independent of protein synthesis and works very fast. They provided strong evidence pointing to the need of revision of the current opinion that arylalkylamines N-acetyltransferase activity in pinealocytes is controlled exclusively by changes in enzyme abundance.

Comparison of arylalkylamine N-acetyltransferase and melatonin receptor type 1B immunoreactivity between young adult and aged canine spinal cord

Melatonin affects diverse physiological functions through its receptor and plays an important role in the central nervous system. In the present study, we compared immunoreactivity patterns of arylalkylamine N-acetyltransferase (AANAT), an enzyme essential for melatonin synthesis, and melatonin receptor type 1B (MT2) in the spinal cord of young adult (2~3 years) and aged (10~12 years) beagle dogs using immunohistochemistry and Western blotting. AANAT-specific immunoreactivity was observed in the nuclei of spinal neurons, and was significantly increased in aged dog spinal neurons compared to young adult spinal neurons. MT2-specific immunoreactivity was found in the cytoplasm of spinal neurons, and was predominantly increased in the margin of the neuron cytoplasm in aged spinal cord compared to that in the young adult dogs. These increased levels of AANAT and MT2 immunoreactivity in aged spinal cord might be a feature of normal aging and associated with a feedback mechanism that compensates for decreased production of melatonin during aging.

Molecular cloning of rice serotonin N-acetyltransferase, the penultimate gene in plant melatonin biosynthesis

Because of the absence of an arylalkylamine N-acetyltransferase (AANAT) homolog in the plant genome, the proposal was made that a GCN5-related N-acetyltransferase superfamily gene (GNAT) could be substituted for AANAT. To clone rice serotonin N-acetyltransferase (SNAT), we expressed 31 rice GNAT cDNAs in Escherichia coli and screened SNAT activity by measuring N-acetyltryptamine after application with 1 mm tryptamine. GNAT5 was shown to produce high levels of N-acetyltryptamine in E. coli, suggesting a possible rice SNAT. To confirm SNAT activity, the GNAT5 protein was purified through affinity purification from E. coli culture. The purified recombinant GNAT5 showed high SNAT enzyme activity catalyzing serotonin into N-acetylserotonin. The values for Km and Vmax were 385 μm and 282 pmol/min/mg protein, respectively. An in vitro enzyme assay of purified SNAT showed N-acetylserotonin formation to be proportional to enzyme concentration and time, with peak activity at pH 8.8. High substrate concentrations above 1 mm serotonin inhibited SNAT activity. Finally, the mRNA level of SNAT was higher in shoots than in roots, but it was expressed constitutively, unlike N-acetylserotonin methyltransferase (ASMT), the terminal enzyme in melatonin synthesis. These results suggest that ASMT rather than SNAT is the rate-limiting enzyme of melatonin biosynthesis in plants.

The melatonergic system in mood and anxiety disorders and the role of agomelatine: implications for clinical practice

Melatonin exerts its actions through membrane MT1/MT2 melatonin receptors, which belong to the super family of G-protein-coupled receptors consisting of the typical seven transmembrane domains. MT1 and MT2 receptors are expressed in various tissues of the body either as single ones or together. A growing literature suggests that the melatonergic system may be involved in the pathophysiology of mood and anxiety disorders. In fact, some core symptoms of depression show disturbance of the circadian rhythm in their clinical expression, such as diurnal mood and other symptomatic variation, or are closely linked to circadian system functioning, such as sleep-wake cycle alterations. In addition, alterations have been described in the circadian rhythms of several biological markers in depressed patients. Therefore, there is interest in developing antidepressants that have a chronobiotic effect (i.e., treatment of circadian rhythm disorders). As melatonin produces chronobiotic effects, efforts have been aimed at developing agomelatine, an antidepressant with melatonin agonist activity. The present paper reviews the role of the melatonergic system in the pathophysiology of mood and anxiety disorders and the clinical characteristics of agomelatine. Implications of agomelatine in "real world" clinical practice will be also discussed.

Clock gene expression in the human pituitary gland

Pituitary function relies on strictly timed, yet plastic mechanisms, particularly with respect to the daytime-dependent coordination of hormone synthesis and release. In other systems, clock genes and their protein products are well-described candidates to anticipate the daily demands in neuroendocrine coupling and to manage cellular adaptation on changing internal or external circumstances. To elucidate possible mechanisms of time management, a total of 52 human autoptic pituitary glands were allocated to the 4 time-of-day groups, night, dawn, day, and dusk, according to reported time of death. The observed daytime-dependent dynamics in ACTH content supports a postmortem conservation of the premortem condition, and thus, principally validates the investigation of autoptic pituitary glands. Pituitary extracts were investigated for expression of clock genes Per1, Cry1, Clock, and Bmal1 and corresponding protein products. Only the clock gene Per1 showed daytime-dependent differences in quantitative real-time PCR analyses, with decreased levels observed during dusk. Although the overall amount in clock gene protein products PER1, CRY1, and CLOCK did not fluctuate with time of day in human pituitary, an indication for a temporally parallel intracellular translocation of PER1 and CRY1 was detected by immunofluorescence. Presented data suggest that the observed clock gene expression in human pituitary cells does not provide evidence for a functional intrinsic clockwork. It is suggested that clock genes and their protein products may be directly involved in the daytime-dependent regulation and adaptation of hormone synthesis and release and within homeostatic adaptive plasticity.

Peripheral reproductive organ health and melatonin: ready for prime time

Melatonin has a wide variety of beneficial actions at the level of the gonads and their adnexa. Some actions are mediated via its classic membrane melatonin receptors while others seem to be receptor-independent. This review summarizes many of the published reports which confirm that melatonin, which is produced in the ovary, aids in advancing follicular maturation and preserving the integrity of the ovum prior to and at the time of ovulation. Likewise, when ova are collected for in vitro fertilization-embryo transfer, treating them with melatonin improves implantation and pregnancy rates. Melatonin synthesis as well as its receptors have also been identified in the placenta. In this organ, melatonin seems to be of particular importance for the maintenance of the optimal turnover of cells in the villous trophoblast via its ability to regulate apoptosis. For male gametes, melatonin has also proven useful in protecting them from oxidative damage and preserving their viability. Incubation of ejaculated animal sperm improves their motility and prolongs their viability. For human sperm as well, melatonin is also a valuable agent for protecting them from free radical damage. In general, the direct actions of melatonin on the gonads and adnexa of mammals indicate it is an important agent for maintaining optimal reproductive physiology.

Melatonin potentiates running wheel-induced neurogenesis in the dentate gyrus of adult C3H/HeN mice hippocampus

This study assessed the role of melatonin in modulating running wheel(RW)-induced hippocampal neurogenesis in adult C3H/HeN mice. Chronic melatonin (0.02 mg/mL, oral for 12 days) treatment did not affect cell proliferation or cell survival determined by the number of BrdU-positive cells in dentate gyrus of mice with access to fixed wheel (FW). RW activity significantly increased cell proliferation [RW (n = 8) versus FW (n = 6): dorsal, 199 ± 18 versus 125 ± 12, P < 0.01; ventral, 211 ± 15 versus 123 ± 13, P < 0.01] and newborn cell survival [RW (n = 7) versus FW (n = 8): dorsal, 45 ± 8.5 versus 15 ± 1.8, P < 0.01; ventral, 48 ± 8.1 versus 15 ± 1.4)] in the dorsal and ventral dentate gyrus. Oral melatonin treatment further potentiated RW activity-induced cell survival in both areas of the dentate gyrus [melatonin (n = 10) versus vehicle (n = 7): dorsal, 63 ± 5.4 versus 45 ± 8.5 P < 0.05; ventral, 75 ± 7.9 versus 48 ± 8.1, P < 0.01] and neurogenesis [melatonin (n = 8) versus vehicle (n = 8): dorsal, 46 ± 3.4, versus 34 ± 4.5, P < 0.05; ventral, 41 ± 3.4 versus 25 ± 2.4, P < 0.01]. We conclude that melatonin potentiates RW-induced hippocampal neurogenesis by enhancing neuronal survival suggesting that the combination of physical exercise and melatonin may be an effective treatment for diseases affecting the hippocampus neurogenesis.

Melatonin prevents ultraviolet radiation-induced alterations in plasma membrane potential and intracellular pH in human keratinocytes

Melatonin exhibits protective effects against ultraviolet radiation (UVR) via modulation of proinflammatory mediators and its free radical scavenging capacity. To date, several reports presented protective mechanisms of this agent against UVR-induced alterations in mitochondria and nuclei. This investigation evaluates the potent preventing action of melatonin regarding early-stage UVR-mediated perturbations in plasma membrane potential (mbΔψ) and intracellular (cytosolic) pH (pH i) analyzed by flow cytometry. Experiments were carried out in a dose- and time-dependent manner using human keratinocytes [HaCaT and normal human epidermal keratinocytes (NHEK)]. First investigations, which used viability/cytotoxicity assays, showed the gradual mortality with increasing UVR doses and cultivation time. Pre-incubation with melatonin (10(-3) m) prior to UVR exposure reduced lactate dehydrogenase release by 30% (HaCaT) and 28% (NHEK) at the dose of 50 mJ/cm(2) after 48 hr (P < 0.001). Furthermore, UVR caused hyperpolarization of mbΔψ immediately (0 hr) after irradiation (25 or 50 mJ/cm(2)). At the dose of 50 mJ/cm(2), cells cultivated for 48 hr manifested a marked increase in mbΔψ by 112% (HaCaT) and 123% (NHEK). The presence of melatonin significantly protected the cells by 12% (HaCaT) and 14% (NHEK) (P < 0.001). Simultaneously, 50 mJ/cm(2) induced dramatic acidification reaching after 24 hr the level of 6.40 (without melatonin), 6.56 (with melatonin) for HaCaT and 6.11 (without melatonin), 6.43 (with melatonin) for NHEK. The results presented provide information about the protective mechanisms of melatonin itself on one hand and, combined with data reported so far, confirm the potent antiapoptotic action of melatonin.

A novel enzyme-dependent melatonin metabolite in humans

Exogenous melatonin is widely used in humans for multiple pharmacologic purposes. The metabolic pathways of melatonin reflect the fate and functions of melatonin in vivo. This study was designed to re-profile melatonin metabolism in humans using a metabolomic approach. In the urine of healthy subjects treated with 10 mg melatonin, sulfate- or glucuronide-conjugated metabolites of melatonin were detected, including 6-hydroxymelatonin sulfate, 6-hydroxymelatonin glucuronide, N-acetylserotonin glucuronide, N-acetylserotonin sulfate, and an unknown sulfated metabolite (X). The molecular weight of metabolite X was 14 Da smaller than 6-hydroxymelatonin sulfate, but 16 Da larger than N-acetylserotonin sulfate. Further studies suggest that metabolite X was produced via O-demethylation, 6-hydroxylation, and sulfation. The antioxidant products of melatonin, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine and N(1)-acetyl-5-methoxy-kynuramine, were not detected in human urine. In summary, this study provided a global view of melatonin metabolism in humans and extended our knowledge of enzyme-dependent pathways of melatonin metabolism.

Luminal melatonin stimulates pancreatic enzyme secretion via activation of serotonin-dependent nerves

BACKGROUND

Serotonin (5-HT) is released from enterochromaffin cells in the gastrointestinal tract. 5-HT, via the activation of 5-HT2 and 5-HT3 receptors on vagal fibers, mediates pancreatic secretion through the mechanism independent from cholecystokinin. Melatonin (5-HT derivative) or L-tryptophan (melatonin or 5-HT precursor) given systemically or intraduodenally to the rats stimulate amylase secretion, but the mechanism is not clear. The aim of this study was to investigate the involvement of 5-HT in the pancreatostimulatory effect of melatonin or L-tryptophan, administered intraduodenally.

METHODS

Wistar rats were surgically equipped with silicone catheters; inserted into pancreato-biliary duct and into the duodenum. Melatonin, L-tryptophan or 5-HT were given to the rats as a bolus. Combination of 5-HT2 or 5-HT3 receptor antagonists: ketanserin (100 μg/kg) and MDL72222 (250 μg/kg) was given intraperitoneally to the animals, 15 min. prior to the administration of the examined substances. The role of the vagal nerve, sensory fibers and CCK in the control of pancreatic exocrine function were determined. Blood samples were taken for the determination of 5-HT.

RESULTS

Melatonin, 5-HT or L-tryptophan increased pancreatic amylase secretion. The stimulatory effect of the above substances was decreased by pretreatment of the rats with ketanserin and MDL72222. Bilateral vagotomy completely abolished the increase of amylase output caused by 5-HT, while capsaicin deactivation of sensory nerves or blockade of CCK1 receptor only partially reversed the stimulatory effect of 5-HT on the pancreas. Intraduodenal L-tryptophan, but not melatonin, increased plasma 5-HT concentrations in a dose- and time-dependent manner.

CONCLUSION

Stimulation of pancreatic exocrine function caused by intraluminal administration of melatonin, or L-tryptophan is modified, at least in part, by serotoninergic mechanisms and vagal nerves.

Crystal structure and functional mapping of human ASMT, the last enzyme of the melatonin synthesis pathway

Melatonin is a synchronizer of many physiological processes. Abnormal melatonin signaling is associated with human disorders related to sleep, metabolism, and neurodevelopment. Here, we present the X-ray crystal structure of human N-acetyl serotonin methyltransferase (ASMT), the last enzyme of the melatonin biosynthesis pathway. The polypeptide chain of ASMT consists of a C-terminal domain, which is typical of other SAM-dependent O-methyltransferases, and an N-terminal domain, which intertwines several helices with another monomer to form the physiologically active dimer. Using radioenzymology, we analyzed 20 nonsynonymous variants identified through the 1000 genomes project and in patients with neuropsychiatric disorders. We found that the majority of these mutations reduced or abolished ASMT activity including one relatively frequent polymorphism in the Han Chinese population (N17K, rs17149149). Overall, we estimate that the allelic frequency of ASMT deleterious mutations ranges from 0.66% in Europe to 2.97% in Asia. Mapping of the variants on to the 3-dimensional structure clarifies why some are harmful and provides a structural basis for understanding melatonin deficiency in humans.

Mood disorders, circadian rhythms, melatonin and melatonin agonists

Recent advances in the understanding of circadian rhythms have led to an interest in the treatment of major depressive disorder with chronobiotic agents. Many tissues have autonomous circadian rhythms, which are orchestrated by the master clock, situated in the suprachiasmatic nucleus (SNC). Melatonin (N-acetyl-5-hydroxytryptamine) is secreted from the pineal gland during darkness. Melatonin acts mainly on MT1 and MT2 receptors, which are present in the SNC, regulating physiological and neuroendocrine functions, including circadian entrainment, referred to as the chronobiotic effet. Circadian rhythms has been shown to be either misaligned or phase shifted or decreased in amplitude in both acute episodes and relapse of major depressive disorder (MDD) and bipolar disorder. Manipulation of circadian rhythms either using physical treatments (such as high intensity light) or behavioral therapy has shown promise in improving symptoms. Pharmacotherapy using melatonin and pure melatonin receptor agonists, while improving sleep, has not been shown to improve symptoms of depression. A novel antidepressant, agomelatine, combines 5HT2c antagonist and melatonin agonist action, and has shown promise in both acute treatment of MDD and in preventing relapse.