The ISL LIM-homeobox 2 transcription factor is negatively regulated by circadian adrenergic signaling to repress the expression of Aanat in pinealocytes of the rat pineal gland

Melatonin is synthesized in the pineal gland during nighttime in response to nocturnal increase in the activity of the enzyme aralkylamine N-acetyltransferase (AANAT), the transcription of which is modulated by several homeodomain transcription factors. Recent work suggests that the homeodomain transcription factor ISL LIM homeobox 2 (ISL2) is expressed in the pineal gland, but its role is currently unknown. With the purpose of identifying the mechanisms that control pineal expression of Isl2 and the possible function of Isl2 in circadian pineal biology, we report that Isl2 is specifically expressed in the pinealocytes of the rat pineal gland. Its expression exhibits a 24 h rhythm with high transcript and protein levels during the day and a trough in the second half of the night. This rhythm persists in darkness, and lesion studies reveal that it requires intact function of the suprachiasmatic nuclei, suggesting intrinsic circadian regulation. In vivo and in vitro experiments show that pineal Isl2 expression is repressed by adrenergic signaling acting via cyclic AMP; further, Isl2 is negatively regulated by the nocturnal transcription factor cone-rod homeobox. During development, pineal Isl2 expression is detectable from embryonic day 19, preceding Aanat by several days. In vitro knockdown of Isl2 is accompanied by an increase in Aanat transcript levels suggesting that ISL2 represses its daytime expression. Thus, rhythmic expression of ISL2 in pinealocytes is under the control of the suprachiasmatic nucleus acting via adrenergic signaling in the gland to repress nocturnal expression, while ISL2 itself negatively regulates daytime pineal expression of Aanat and thereby suggestively enhances the circadian rhythm in melatonin synthesis.

SlTDC1 Overexpression Promoted Photosynthesis in Tomato under Chilling Stress by Improving CO2 Assimilation and Alleviating Photoinhibition

Chilling causes a significant decline in photosynthesis in tomato plants. Tomato tryptophan decarboxylase gene 1 (SlTDC1) is the first rate-limiting gene for melatonin (MT) biosynthesis and is involved in the regulation of photosynthesis under various abiotic stresses. However, it is not clear whether SlTDC1 participates in the photosynthesis of tomato under chilling stress. Here, we obtained SlTDC1 overexpression transgenic tomato seedlings, which showed higher SlTDC1 mRNA abundance and MT content compared with the wild type (WT). The results showed that the overexpression of SlTDC1 obviously alleviated the chilling damage to seedlings in terms of the lower electrolyte leakage rate and hydrogen peroxide content, compared with the WT after 2 d of chilling stress. Moreover, the overexpression of SlTDC1 notably increased photosynthesis under chilling stress, which was related to the higher chlorophyll content, normal chloroplast structure, and higher mRNA abundance and protein level of Rubisco and RCA, as well as the higher carbon metabolic capacity, compared to the WT. In addition, we found that SlTDC1-overexpressing seedlings showed higher W_k (damage degree of OEC on the PSII donor side), φ_Eo (quantum yield for electron transport in the PSII reaction center), and PI_ABS (photosynthetic performance index) than WT seedlings after low-temperature stress, implying that the overexpression of SlTDC1 decreased the damage to the reaction center and donor-side and receptor-side electron transport of PSII and promoted PSI activity, as well as energy absorption and distribution, to relieve the photoinhibition induced by chilling stress. Our results support the notion that SlTDC1 plays a vital role in the regulation of photosynthesis under chilling stress.

Role and neural regulation of clock genes in the rat pineal gland: Clock modulates amplitude of rhythmic expression of Aanat encoding the melatonin-producing enzyme

Circadian clock gene expression in the suprachiasmatic nucleus (SCN) controls 24h rhythms in body functions, but clock genes are also expressed in extra-hypothalamic tissues, including the melatonin-producing pineal gland. The nocturnal increase in pineal melatonin synthesis is a hallmark in circadian biology, but the role of local clock gene oscillations in the mammalian pineal gland is unknown. The aim of this work is to determine the role of clock genes in endocrine function of the pineal gland with focus on the Aanat transcript encoding the rhythm-generating enzyme of melatonin synthesis. Using the rat as a model, we here established 24h expression patterns of clock genes in the pineal gland in vivo. Lesion studies showed that rhythmic clock gene expression in the pineal gland to a large extent depends on the SCN; further, clock gene rhythms could be reestablished in cultured pineal cells synchronized by rhythmic stimulation with norepinephrine in 12 h pulses, suggesting that pineal cells house a slave oscillator controlled by adrenergic signaling in the gland. Histological analyses showed that clock genes are expressed in pinealocytes and co-localize with Aanat transcripts, thus potentially enabling clock gene products to control cellular melatonin production. To test this, cultured pineal cells were transfected using small interfering RNA to knock down clock gene expression. While successful knockdown of Per1 had a minor effect on Aanat, Clock knockdown produced a marked overexpression of Aanat in the pinealocytes. Our study suggests that SCN-dependent rhythmic Clock gene expression in the pinealocytes regulates the daily profile of Aanat expression. This article is protected by copyright. All rights reserved.

Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light

Throughout the history of melatonin research, almost exclusive focus has been on nocturnally-generated pineal melatonin production, which accounts for its circadian rhythm in the blood and cerebrospinal fluid; these light/dark melatonin cycles drive the daily and seasonal photoperiodic alterations in organismal physiology. Because pineal melatonin is produced and secreted primarily at night, it is referred to as the chemical expression of darkness. The importance of the other sources of melatonin has almost been ignored. Based on current evidence, there are at least four sources of melatonin in vertebrates that contribute to the whole-body melatonin pool. These include melatonin produced by (1) the pineal gland; (2) extrapineal cells, tissues, and organs; (3) the microbiota of the skin, mouth, nose, digestive tract, and vagina as well as (4) melatonin present in the diet. These multiple sources of melatonin exhibit differentially regulated mechanisms for its synthesis. Visible light striking the retina or an intense physical stimulus can suppress nocturnal pineal melatonin levels; in contrast, there are examples where extrapineal melatonin levels are increased during heavy exercise in daylight, which contains the whole range of NIR radiation. The cumulative impact of all cells producing augmented extrapineal melatonin is sufficient to elevate sweat concentrations, and potentially, if the exposure is sustained, to also increasing the circulating values. The transient increases in sweat and plasma melatonin support the premise that extrapineal melatonin has a production capacity that exceeds by far what can be produced by the pineal gland, and is used to maintain intercellular homeostasis and responds to rapid changes in ROS density. The potential regulatory mechanisms of near infrared light (NIR) on melatonin synthesis are discussed in detail herein. Combined with the discovery of high levels of melanopsin in most fat cells and their response to light further calls into question pineal centric theories. While the regulatory processes related to microbiota-derived melatonin are currently unknown, there does seem to be crosstalk between melatonin derived from the host and that originating from microbiota.

Geographic clines in Daphnia magna's circadian clock gene expression: Local adaptation to photoperiod

Nearly all organisms show daily and seasonal physiological and behavioural responses that are necessary for their survival. Often these responses are controlled by the rhythmic activity of an endogenous clock that perceives day length. Day length differs not only between seasons but also along latitudes, with different seasonal day lengths between the north and the south. Both seasonal and latitudinal differences in day length are discussed to be perceived/processed by the endogenous clock. Some species are distributed over a wide range of latitudes; it should be highly adaptive for these species to be able to time physiological responses (e.g. migration behaviour and diapause) according to the organisms' respective photoperiod, i.e. their respective seasonal and latitudinal day length. The mediator of day length is the indoleamine hormone melatonin which is synthesized by melatonin-producing enzymes (AANAT and HIOMT). These enzymes are in turn controlled by an endogenous clock. The ubiquitous aquatic keystone organism Daphnia possess clock and melatonin synthesis genes that are rhythmically expressed over 24hours. We were able to show that the 24-h rhythm of D. magna's clock persists in constant darkness and is thus truly circadian. In one particular photoperiod, all D. magna clones produced a similar melatonin concentration due to a fixed AANAT activity. However, we have demonstrated that clones originating from different latitudes are adapted to their respective photoperiod by showing a geographic cline in clock and downstream melatonin synthesis gene expression. These findings hint at the problem locally adapted organisms face when they are forced to leave their respective photoperiod, e.g. because of climate change-driven range-expansion. If such a species is incapable of adjusting its endogenous clock to an unknown photoperiod, it will likely become extinct.

Exogenous Melatonin Application Enhances Rhizophagus irregularis Symbiosis and Induces the Antioxidant Response of Medicago truncatula Under Lead Stress

Melatonin is a new kind of plant growth regulator. The aim of this study was to figure out the effect of melatonin on arbuscular mycorrhizal (AM) symbiosis and heavy metal tolerance. A three-factor experiment was conducted to determine the effect of melatonin application on the growth, AM symbiosis, and stress tolerance of Medicago truncatula. A two-factor (AM inoculation and Pb stress) experiment was conducted to determine the effect of AM fungus on melatonin accumulation under Pb stress. AM plants under Pb stress had a higher melatonin accumulation than non-mycorrhizal (NM) plants under Pb stress. Acetylserotonin methyltransferase (ASMT) is the enzymatic reaction of the last step in melatonin synthesis. The accumulation of melatonin may be related to the expression of MtASMT. Melatonin application increased the relative expression of MtPT4 and AM colonization in AM plants. Melatonin application decreased Pb uptake with and without AM inoculation. Both melatonin application and AM inoculation improved M. truncatula growth and increased antioxidant response with Pb stress. These results indicated that melatonin application has positive effects on AM symbiosis and Pb stress tolerance under Pb stress. AM inoculation improve melatonin synthesis capacity under Pb stress. Melatonin application may improve AM plant growth by enhancing AM symbiosis, stimulating antioxidant response, and inhibiting Pb uptake.

Oral Supplementation of Sodium Butyrate Attenuates the Progression of Non-Alcoholic Steatohepatitis

Sodium butyrate (SoB) supplementation has been suggested to attenuate the development of non-alcoholic fatty liver disease (NAFLD). Here, we determined the therapeutic potential of SoB on NAFLD progression and molecular mechanism involved. Eight-week old C57BL/6J mice were pair-fed a fat-, fructose- and cholesterol-rich diet (FFC) or control diet (C). After 8 weeks, some mice received 0.6g SoB/kg bw in their respective diets (C+SoB; FFC+SoB) or were maintained on C or FFC for the next 5 weeks of feeding. Liver damage, markers of glucose metabolism, inflammation, intestinal barrier function and melatonin metabolism were determined. FFC-fed mice progressed from simple steatosis to early non-alcoholic steatohepatitis, along with significantly higher TNFα and IL-6 protein levels in the liver and impaired glucose tolerance. In FFC+SoB-fed mice, disease was limited to steatosis associated with protection against the induction of Tlr4 mRNA and iNOS protein levels in livers. SoB supplementation had no effect on FFC-induced loss of tight junction proteins in the small intestine but was associated with protection against alterations in melatonin synthesis and receptor expression in the small intestine and livers of FFC-fed animals. Our results suggest that the oral supplementation of SoB may attenuate the progression of simple steatosis to steatohepatitis.

TLR2-Melatonin Feedback Loop Regulates the Activation of NLRP3 Inflammasome in Murine Allergic Airway Inflammation

Toll-like receptor 2 (TLR2) is suggested to initiate the activation of NLRP3 inflammasome, and considered to be involved in asthma. The findings that melatonin modulates TLRs-mediated immune responses, together with the suppressing effect of TLRs on endogenous melatonin synthesis, support the possibility that a feedback loop exists between TLRs system and endogenous melatonin synthesis. To determine whether TLR2-melatonin feedback loop exists in allergic airway disease and regulates NLRP3 inflammasome activity, wild-type (WT) and TLR2^−/− mice were challenged with OVA to establish allergic airway disease model. Following OVA challenge, WT mice exhibited increased-expression of TLR2, activation of NLRP3 inflammasome and marked airway inflammation, which were all effectively inhibited in the TLR2^−/− mice, indicating that TLR2-NLRP3 mediated airway inflammation. Meanwhile, melatonin biosynthesis was reduced in OVA-challenged WT mice, while such reduction was notably rescued by TLR2 deficiency, suggesting that TLR2-NLRP3-mediated allergic airway inflammation was associated with decreased endogenous melatonin biosynthesis. Furthermore, addition of melatonin to OVA-challenged WT mice pronouncedly ameliorated airway inflammation, decreased TLR2 expression and NLRP3 inflammasome activation, further implying that melatonin in turn inhibited airway inflammation via suppressing TLR2-NLRP3 signal. Most interestingly, although melatonin receptor antagonist luzindole significantly reduced the protein expressions of ASMT, AANAT and subsequent level of melatonin in OVA-challenged TLR2^−/− mice, it exhibited null effect on leukocytes infiltration, Th2-cytokines production and NLRP3 activity. These results indicate that a TLR2-melatonin feedback loop regulates NLRP3 inflammasome activity in allergic airway inflammation, and melatonin may be a promising therapeutic medicine for airway inflammatory diseases such as asthma.

Homeobox genes in melatonin-producing pinealocytes: Otx2 and Crx act to promote hormone synthesis in the mature rat pineal gland

Homeobox genes encode transcription factors that regulate developmental processes; however, in the pineal gland, a neuroendocrine organ responsible for nocturnal melatonin synthesis, expression of the homeobox genes Otx2 (orthodenticle homeobox 2) and Crx (cone-rod homeobox) persists postnatally. We here show that OTX2 and CRX are exclusively present in melatonin-producing pinealocytes of the rat pineal gland. To understand the roles of Otx2 and Crx in the mature pineal gland, we used siRNA technology in cultured rat pinealocytes with the nocturnal situation mimicked by adding norepinephrine to the culture media. siRNA-induced knockdown of Otx2 was found to reduce expression levels of the enzymes involved in melatonin synthesis at both transcript and protein levels. Similar results were obtained when knocking down Crx. Knocking down Otx2 and Crx simultaneously produced an even larger reduction in both transcript and protein levels of the melatonin-producing enzymes and also reduced the levels of melatonin released to the culture media. These results suggest that Otx2 and Crx, both alone and in combination, act to control pineal melatonin synthesis.

Mitochondria: Central Organelles for Melatonin's Antioxidant and Anti-Aging Actions

Melatonin, along with its metabolites, have long been known to significantly reduce the oxidative stress burden of aging cells or cells exposed to toxins. Oxidative damage is a result of free radicals produced in cells, especially in mitochondria. When measured, melatonin, a potent antioxidant, was found to be in higher concentrations in mitochondria than in other organelles or subcellular locations. Recent evidence indicates that mitochondrial membranes possess transporters that aid in the rapid uptake of melatonin by these organelles against a gradient. Moreover, we predicted several years ago that, because of their origin from melatonin-producing bacteria, mitochondria likely also synthesize melatonin. Data accumulated within the last year supports this prediction. A high content of melatonin in mitochondria would be fortuitous, since these organelles produce an abundance of free radicals. Thus, melatonin is optimally positioned to scavenge the radicals and reduce the degree of oxidative damage. In light of the "free radical theory of aging", including all of its iterations, high melatonin levels in mitochondria would be expected to protect against age-related organismal decline. Also, there are many age-associated diseases that have, as a contributing factor, free radical damage. These multiple diseases may likely be deferred in their onset or progression if mitochondrial levels of melatonin can be maintained into advanced age.

JNK-TLR9 signal pathway mediates allergic airway inflammation through suppressing melatonin biosynthesis

Toll-like receptors (TLRs) play pivotal role in the pathogenesis of allergic airway diseases such as asthma. TLR9 is one of the most extensively studied TLRs as an approach to treat asthma. In this study, we investigated the role of TLR9 in the allergic airway inflammation and the underlying mechanism. Wild-type (WT) mice and TLR9(-/-) mice were sensitized and challenged with OVA to establish allergic airway disease model. We found that the expression of TLR9 was elevated concomitantly with airway inflammation post-OVA challenge, and TLR9 deficiency effectively inhibited airway inflammation, including serum OVA-specific immunoglobulin E (IgE), pulmonary inflammatory cell recruitment, mucus secretion, and bronchoalveolar lavage fluid (BALF) inflammatory cytokine production. Meanwhile, the protein expression of hydroxyindole-o-methyltransferase (HIOMT) in lung tissues, the level of melatonin in serum, and BALF were reduced in OVA-challenged WT mice, while these reductions were significantly restored by TLR9 deficiency. Additionally, we showed that although TLR9 deficiency had no effect on OVA-induced phosphorylation of JNK, inhibition of JNK by specific inhibitor SP600125 significantly decreased OVA-induced expression of TLR9, suggesting that JNK is the upstream signal molecular of TLR9. Furthermore, SP600125 treatment promoted resolution of allergic airway inflammation in OVA-challenged WT mice, but not further ameliorated allergic airway inflammation in OVA-challenged TLR9(-/-) mice. Similarly, SP600125 significantly restored the protein expression of HIOMT and the level of melatonin in OVA-challenged WT mice, while such effect was not further enhanced by TLR9 deficiency. Collectively, our results indicated that JNK-TLR9 signal pathway mediates allergic airway inflammation through suppressing melatonin biosynthesis.

Microarray analysis of genes differentially expressed in melatonin-rich transgenic rice expressing a sheep serotonin N-acetyltransferase

Transgenic rice plants overexpressing a sheep serotonin N-acetyltransferase led to an enhanced production of melatonin with various physiological effects, including seminal root elongation and resistance against cold and oxidative stress, which raises the possibility that melatonin may alter gene expression profiles in the transgenic rice. Therefore, we performed a microarray analysis to investigate the regulatory role of melatonin using the melatonin-rich transgenic rice. We identified 260 and 204 genes that were up- or downregulated in the melatonin-rich transgenic rice when compared with the wild type. Of these, 20 upregulated genes were identified in the seedlings of melatonin-rich rice at more than twice the levels in the wild type (P < 0.05), while 23 downregulated genes were also detected. The representative upregulated genes included caleosin, a Ca(2+) -binding oil-body surface protein involved in the degradation of lipids stored in oil bodies and various signaling proteins such as a cyclin F-box protein and leucine-rich repeat protein. In contrast, jasmonate-induced protein, senescence-associated protein, and polygalacturonase were included in the downregulated gene group. These results suggest that melatonin has an important role in modulating a wide range of gene expression, reflecting its pleiotropic physiological roles in plant growth and development.

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.

Acetylcholine triggers L-glutamate exocytosis via nicotinic receptors and inhibits melatonin synthesis in rat pinealocytes

Rat pinealocytes, melatonin-secreting endocrine cells, contain peripheral glutaminergic systems. L-Glutamate is a negative regulator of melatonin synthesis through a metabotropic receptor-mediated inhibitory cAMP cascade. Previously, we reported that depolarization of pinealocytes by externally added KCl and activation of L-type Ca2+ channels resulted in secretion of L-glutamate by microvesicle exocytosis. What is unknown is how and what kinds of stimuli trigger glutamate exocytosis under physiological conditions. Here, we report that the nicotinic acetylcholine receptor can trigger glutamate exocytosis from cultured rat pinealocytes. Moreover, acetylcholine or nicotine inhibited norepinephrine-dependent serotonin N-acetyltransferase activity, which results in decreased melatonin synthesis. These activities were blocked by (2S,3S, 4S)-2-methyl-2-(carboxycyclopropyl)glycine, an antagonist of the metabotropic glutamate receptor. These results suggest that cholinergic stimulation initiates the glutaminergic signaling cascade in pineal glands and that parasympathetic neurons innervating the gland exert negative control over melatonin synthesis by way of the glutaminergic systems.