Melatonin synthesis in human colostrum mononuclear cells enhances dectin-1-mediated phagocytosis by mononuclear cells

IL-10-induced STAT3/NF-κB crosstalk modulates pineal and extra-pineal melatonin synthesis

Immune-pineal axis activation is part of the assembly of immune responses. Proinflammatory cytokines inhibit the pineal synthesis of melatonin while inducing it in macrophages by mechanisms dependent on nuclear factor-κB (NF-κB) activation. Cytokines activating the Janus kinase/signal transducer and activator of transcription (STAT) pathways, such as interferon-gamma (IFN-γ) and interleukin-10 (IL-10), modulate melatonin synthesis in the pineal, bone marrow (BM), and spleen. The stimulatory effect of IFN-γ upon the pineal gland depends on STAT1/NF-κB interaction, but the mechanisms controlling IL-10 effects on melatonin synthesis remain unclear. Here, we evaluated the role of STAT3 and NF-κB activation by IL-10 upon the melatonin synthesis of rats' pineal gland, BM, spleen, and peritoneal cells. The results show that IL-10-induced interaction of (p)STAT3 with specific NF-κB dimmers leads to different cell effects. IL-10 increases the pineal's acetylserotonin O-methyltransferase (ASMT), N-acetylserotonin, and melatonin content via nuclear translocation of NF-κB/STAT3. In BM, the nuclear translocation of STAT3/p65-NF-κB complexes increases ASMT expression and melatonin content. Increased pSTAT3/p65-NF-κB nuclear translocation in the spleen enhances phosphorylated serotonin N-acetyltransferase ((p)SNAT) expression and melatonin content. Conversely, in peritoneal cells, IL-10 leads to NF-κB p50/p50 inhibitory dimmer nuclear translocation, decreasing (p)SNAT expression and melatonin content. In conclusion, IL-10's effects on melatonin production depend on the NF-κB subunits interacting with (p)STAT3. Thus, variations of IL-10 levels and downstream pathways during immune responses might be critical regulatory factors adjusting pineal and extra-pineal synthesis of melatonin.

Development of indolealkylamine derivatives as potential multi-target agents for COVID-19 treatment

COVID-19 is a complex disease with short-term and long-term respiratory, inflammatory and neurological symptoms that are triggered by the infection with SARS-CoV-2. As many drugs targeting single targets showed only limited effectiveness against COVID-19, here, we aimed to explore a multi-target strategy. We synthesized a focused compound library based on C2-substituted indolealkylamines (tryptamines and 5-hydroxytryptamines) with activity for three potential COVID-19-related proteins, namely melatonin receptors, calmodulin and human angiotensin converting enzyme 2 (hACE2). Two molecules from the library, 5e and h, exhibit affinities in the high nanomolar range for melatonin receptors, inhibit the calmodulin-dependent calmodulin kinase II activity and the interaction of the SARS-CoV-2 Spike protein with hACE2 at micromolar concentrations. Both compounds inhibit SARS-CoV-2 entry into host cells and 5h decreases SARS-CoV-2 replication and M^Pro enzyme activity in addition. In conclusion, we provide a proof-of-concept for the successful design of multi-target compounds based on the tryptamine scaffold. Optimization of these preliminary hit compounds could potentially provide drug candidates to treat COVID-19 and other coronavirus diseases.

Melatonin: A Potential Candidate for the Treatment of Experimental and Clinical Perinatal Asphyxia

Perinatal asphyxia is considered to be one of the major causes of brain neurodegeneration in full-term newborns. The worst consequence of perinatal asphyxia is neurodegenerative brain damage, also known as hypoxic-ischemic encephalopathy. Hypoxic-ischemic encephalopathy is the leading cause of mortality in term newborns. To date, due to the complex mechanisms of brain damage, no effective or causal treatment has been developed that would ensure complete neuroprotection. Although hypothermia is the standard of care for hypoxic-ischemic encephalopathy, it does not affect all changes associated with encephalopathy. Therefore, there is a need to develop effective treatment strategies, namely research into new agents and therapies. In recent years, it has been pointed out that natural compounds with neuroprotective properties, such as melatonin, can be used in the treatment of hypoxic-ischemic encephalopathy. This natural substance with anti-inflammatory, antioxidant, anti-apoptotic and neurofunctional properties has been shown to have pleiotropic prophylactic or therapeutic effects, mainly against experimental brain neurodegeneration in hypoxic-ischemic neonates. Melatonin is a natural neuroprotective hormone, which makes it promising for the treatment of neurodegeneration after asphyxia. It is supposed that melatonin alone or in combination with hypothermia may improve neurological outcomes in infants with hypoxic-ischemic encephalopathy. Melatonin has been shown to be effective in the last 20 years of research, mainly in animals with perinatal asphyxia but, so far, no clinical trials have been performed on a sufficient number of newborns. In this review, we summarize the advantages and limitations of melatonin research in the treatment of experimental and clinical perinatal asphyxia.

Immunoendocrinology and Ecoimmunology in Brazilian Anurans

This paper reviews several aspects of immunoendocrinology and ecoimmunology in Brazilian species of anurans under investigation for more than a decade, including (1) patterns of annual covariation of circulating steroids, calling behavior and innate immunity, (2) endocrine and immune correlates of calling performance, (3) behavioral and physiological correlates of parasite load, (4) thermal sensitivity of immune function, and (5) endocrine and immunomodulation by experimental exposure to acute and chronic stressors, as well as to endocrine manipulations and simulated infections. Integrated results have shown an immunoprotective role of increased steroid plasma levels during reproductive activity in calling males. Moreover, a higher helminth parasite load is associated with changes in several behavioral and physiological traits under field conditions. We also found anuran innate immunity is generally characterized by eurythermy, with maximal performance observed in temperatures close to normal and fever thermal preferendum. Moreover, the aerobic scope of innate immune response is decreased at fever thermal preferendum. Experimental exposure to stressors results in increased corticosterone plasma levels and immune redistribution, with an impact on immune function depending on the duration of the stress exposure. Interestingly, the fate of immunomodulation by chronic stressors also depends in part on individual body condition. Acute treatment with corticosterone generally enhances immune function, while prolonged exposure results in immunosuppression. Still, the results of hormonal treatment are complex and depend on the dose, duration of treatment, and the immune variable considered. Finally, simulated infection results in complex modulation of the expression of cytokines, increased immune function, activation of the Hypothalamus-Pituitary-Interrenal axis, and decreased activity of the Hypothalamus-Pituitary-Gonadal axis, as well as reduced melatonin plasma levels, suggesting that anurans have a functional Immune-Pineal axis, homologous to that previously described for mammals. These integrated and complementary approaches have contributed to a better understanding of physiological mechanisms and processes, as well as ecological and evolutionary implications of anuran immunoendocrinology.

Physiological and pharmacological perspectives of melatonin

The pineal gland is a interface between light-dark cycle and shows neuro-endocrine functions. Melatonin is the primary hormone of pineal gland, secreted at night. The night-time melatonin peak regulates the physiological functions at dark. Melatonin has several unique features as it synchronises internal rhythm with daily and seasonal variations, regulates circadian rhythm and sleep-wake cycle. Physiologically melatonin involves in detoxification of free radicals, immune functions, neuro-protection, oncostatic effects, cardiovascular functions, reproduction, and foetal development. The precise functions of melatonin are exhibited by specific receptors. In relation to pathophysiology, impaired melatonin secretion promotes sleep disorder, cancer progression, type-2 diabetes, and neurodegenerative diseases. Several reports have highlighted the therapeutic benefits of melatonin specially related to cancer protection, sleep disorder, psychiatric disorders, and jet lag problems. This review will touch the most of the area of melatonin-oriented health impacts and its therapeutic aspects.

The Immune Response in Adipocytes and Their Susceptibility to Infection: A Possible Relationship with Infectobesity

The current obesity pandemic has been expanding in both developing and developed countries. This suggests that the factors contributing to this condition need to be reconsidered since some new factors are arising as etiological causes of this disease. Moreover, recent clinical and experimental findings have shown an association between the progress of obesity and some infections, and the functions of adipose tissues, which involve cell metabolism and adipokine release, among others. Furthermore, it has recently been reported that adipocytes could either be reservoirs for these pathogens or play an active role in this process. In addition, there is abundant evidence indicating that during obesity, the immune system is exacerbated, suggesting an increased susceptibility of the patient to the development of several forms of illness or death. Thus, there could be a relationship between infection as a trigger for an increase in adipose cells and the impact on the metabolism that contributes to the development of obesity. In this review, we describe the findings concerning the role of adipose tissue as a mediator in the immune response as well as the possible role of adipocytes as infection targets, with both roles constituting a possible cause of obesity.

The Role of Delivery Route on Colostrum Melatonin and Serum Il-6 Levels: a Prospective Controlled Study

INTRODUCTION

the aim of this study was to determine whether maternal serum IL-6 and postnatal melatonin levels change with the mode of delivery.

MATERIALS AND METHODS

a prospective controlled study was performed on pregnant women (17-43 years) over 37 weeks of pregnancy. Patients were divided into three groups according to the route of delivery: Group 1) 30 women delivering by vaginal route; Group 2) 30 delivering by iterative cesarean section (CS); Group 3) delivering by emergency CS. Maternal serum IL-6 levels were measured before and after delivery, and maternal colostrum melatonin levels after delivery, and the results between the 3 groups compared.

RESULTS

pre-delivery and post-delivery maternal serum IL-6 levels were significantly higher in patients who delivered vaginally than in patients who delivered by the abdominal route (p<0.01). Maternal colostrum melatonin levels of patients after delivery were significantly higher in patients who delivered vaginally (32.88±7.16 ng/L) than in patients who delivered by elective and emergent cesarean deliveries (24.86±2.40 ng/L and 23.73±4.03 ng/L, respectively) (p<0.01).

CONCLUSION

These data support, should there ever be a further need, the benefit of vaginal delivery over cesarean section, in which cytokine and melatonin levels are reduced compared to vaginal delivery.

Possible Role of Pineal and Extra-Pineal Melatonin in Surveillance, Immunity, and First-Line Defense

Melatonin is a highly conserved molecule found in prokaryotes and eukaryotes that acts as the darkness hormone, translating environmental lighting to the whole body, and as a moderator of innate and acquired defense, migration, and cell proliferation processes. This review evaluates the importance of pineal activity in monitoring PAMPs and DAMPs and in mounting an inflammatory response or innate immune response. Activation of the immune-pineal axis, which coordinates the pro-and anti-inflammatory phases of an innate immune response, is described. PAMPs and DAMPs promote the immediate suppression of melatonin production by the pineal gland, which allows leukocyte migration. Monocyte-derived macrophages, important phagocytes of microbes, and cellular debris produce melatonin locally and thereby initiate the anti-inflammatory phase of the acute inflammatory response. The role of locally produced melatonin in organs that directly contact the external environment, such as the skin and the gastrointestinal and respiratory tracts, is also discussed. In this context, as resident macrophages are self-renewing cells, we explore evidence indicating that, besides avoiding overreaction of the immune system, extra-pineal melatonin has a fundamental role in the homeostasis of organs and tissues.

Circadian immune circuits

Immune responses are gated to protect the host against specific antigens and microbes, a task that is achieved through antigen- and pattern-specific receptors. Less appreciated is that in order to optimize responses and to avoid collateral damage to the host, immune responses must be additionally gated in intensity and time. An evolutionary solution to this challenge is provided by the circadian clock, an ancient time-keeping mechanism that anticipates environmental changes and represents a fundamental property of immunity. Immune responses, however, are not exclusive to immune cells and demand the coordinated action of nonhematopoietic cells interspersed within the architecture of tissues. Here, we review the circadian features of innate immunity as they encompass effector immune cells as well as structural cells that orchestrate their responses in space and time. We finally propose models in which the central clock, structural elements, and immune cells establish multidirectional circadian circuits that may shape the efficacy and strength of immune responses and other physiological processes.

Melatonin controversies, an update

Melatonin was discovered more than 60 years ago. Since then, several seminal discoveries have allowed us to define its function as a neuroendocrine hormone and its molecular targets in mammals and many other species. However, many fundamental issues have not yet been solved such as the subcellular localization of melatonin synthesis and the full spectrum of its molecular targets. In addition, a considerable number of controversies persist in the field, mainly concerning how many functions melatonin has. Altogether, this illustrates how "immature" the field still is. The intention of this opinion article is to note the controversies and limitations in the field, to initiate a discussion and to make proposals/guidelines to overcome them and move the field forward.

Hormonal daily variation co-varies with immunity in captive male bullfrogs (Lithobates catesbeianus)

Almost all physiological processes within the organism, including immune parameters and hormones, follow a circadian rhythm. These daily fluctuations are often observed in free-living organisms; however, little is known regarding hormonal and immune daily variations in anurans, particularly under laboratory conditions. This study aimed to investigate the hormonal and immune daily variation in captive-bred Bullfrogs (Lithobates catesbeianus) under constant conditions (21 °C and 12:12 LD cycle). Our results showed a daily variation for plasma corticosterone (CORT), testosterone (T), and melatonin (MEL), as well as for blood leukocyte profile, phagocytic activity, and plasma bacterial killing ability (BKA). Hormonal profile and immune activity were higher at the dark when compared with the light phase; however, monocytes and lymphocytes followed the opposite pattern. Moreover, CORT was positively correlated with phagocytosis percentage of blood cells, BKA, and monocytes, while MEL and T showed a positive correlation with PP. Our results demonstrate the daily covariation of different immune variables and immunomodulatory hormones. These 24 h-day variations and covariation certainly have broad implications and need to be considered for better understanding anuran physiology both in the context of laboratory and field studies.

The interplay between mast cells, pineal gland, and circadian rhythm: Links between histamine, melatonin, and inflammatory mediators

Our daily rhythmicity is controlled by a circadian clock with a specific set of genes located in the suprachiasmatic nucleus in the hypothalamus. Mast cells (MCs) are major effector cells that play a protective role against pathogens and inflammation. MC distribution and activation are associated with the circadian rhythm via two major pathways, IgE/FcεRI- and IL-33/ST2-mediated signaling. Furthermore, there is a robust oscillation between clock genes and MC-specific genes. Melatonin is a hormone derived from the amino acid tryptophan and is produced primarily in the pineal gland near the center of the brain, and histamine is a biologically active amine synthesized from the decarboxylation of the amino acid histidine by the L-histidine decarboxylase enzyme. Melatonin and histamine are previously reported to modulate circadian rhythms by pathways incorporating various modulators in which the nuclear factor-binding near the κ light-chain gene in B cells, NF-κB, is the common key factor. NF-κB interacts with the core clock genes and disrupts the production of pro-inflammatory cytokine mediators such as IL-6, IL-13, and TNF-α. Currently, there has been no study evaluating the interdependence between melatonin and histamine with respect to circadian oscillations in MCs. Accumulating evidence suggests that restoring circadian rhythms in MCs by targeting melatonin and histamine via NF-κB may be promising therapeutic strategy for MC-mediated inflammatory diseases. This review summarizes recent findings for circadian-mediated MC functional roles and activation paradigms, as well as the therapeutic potentials of targeting circadian-mediated melatonin and histamine signaling in MC-dependent inflammatory diseases.

Prolonged melatonin treatment promote testicular recovery by enhancing RAC1-mediated apoptotic cell clearance and cell junction-dependent spermatogensis after heat stress

INTRODUCTION

A decline in semen quality caused by global warming and torrid working conditions is a major cause of human male infertility, and heat stress-induced decreases in male reproductive ability results in economic losses in livestock husbandry. Increasing evidence suggests that melatonin exerts protective effects on stress-induced DNA damage and apoptosis in germ cells. However, few studies have assessed the effects of melatonin on testicular recovery during post-heat stress and the underlying mechanisms.

METHODS AND RESULTS

In vivo studies using 8-week-old male CD-1 mice revealed that melatonin pretreatment (50 mg/kg for 5 days) did not alleviate heat stress-induced germ cell loss and disrupted testicular histomorphology, however, long-term melatonin administration after heat stress accelerated germ cell apoptosis, spermatogenic cell regeneration, and testicular weight recovery. In vitro studies demonstrated that melatonin enhanced RAC1 activity, resulting in increased phagocytosis of apoptotic germ cells by Sertoli cells. In addition, melatonin restored gap junctions and tight junctions after heat stress, thereby promoting hollow seminiferous tubule filling.

DISCUSSION

Long-term melatonin administration accelerated testicular recovery after heat stress by enhancing the phagocytotic activity of Sertoli cells and the regeneration of spermatogenic cells. This finding suggests that melatonin is a potential therapeutic for heat stress-induced male infertility.

Immune-pineal axis protects rat lungs exposed to polluted air

Environmental pollution in the form of particulate matter <2.5 μm (PM_2.5 ) is a major risk factor for diseases such as lung cancer, chronic respiratory infections, and major cardiovascular diseases. Our goal was to show that PM_2.5 eliciting a proinflammatory response activates the immune-pineal axis, reducing the pineal synthesis and increasing the extrapineal synthesis of melatonin. Herein, we report that the exposure of rats to polluted air for 6 hours reduced nocturnal plasma melatonin levels and increased lung melatonin levels. Melatonin synthesis in the lung reduced lipid peroxidation and increased PM_2.5 engulfment and cell viability by activating high-affinity melatonin receptors. Diesel exhaust particles (DEPs) promoted the synthesis of melatonin in a cultured cell line (RAW 264.7 cells) and rat alveolar macrophages via the expression of the gene encoding for AANAT through a mechanism dependent on activation of the NFκB pathway. Expression of the genes encoding AANAT, MT1, and MT2 was negatively correlated with cellular necroptosis, as disclosed by analysis of Gene Expression Omnibus (GEO) microarray data from the human alveolar macrophages of nonsmoking subjects. The enrichment score for antioxidant genes obtained from lung gene expression data (GTEx) was significantly correlated with the levels of AANAT and MT1 but not the MT2 melatonin receptor. Collectively, these data provide a systemic and mechanistic rationale for coordination of the pineal and extrapineal synthesis of melatonin by a standard damage-associated stimulus, which activates the immune-pineal axis and provides a new framework for understanding the effects of air pollution on lung diseases.

Rhythmic expression of the melatonergic biosynthetic pathway and its differential modulation in vitro by LPS and IL10 in bone marrow and spleen

Daily oscillation of the immune system follows the central biological clock outputs control such as melatonin produced by the pineal gland. Despite the literature showing that melatonin is also synthesized by macrophages and T lymphocytes, no information is available regarding the temporal profile of the melatonergic system of immune cells and organs in steady-state. Here, the expression of the enzymes arylalkylamine-N-acetyltransferase (AA-NAT), its phosphorylated form (P-AA-NAT) and acetylserotonin-O-methyltransferase (ASMT) were evaluated in phagocytes and T cells of the bone marrow (BM) and spleen. We also determined how the melatonergic system of these cells is modulated by LPS and the cytokine IL-10. The expression of the melatonergic enzymes showed daily rhythms in BM and spleen cells. Melatonin rhythm in the BM, but not in the spleen, follows P-AA-NAT daily variation. In BM cells, LPS and IL10 induced an increase in melatonin levels associated with the increased expressions of P-AA-NAT and ASMT. In spleen cells, LPS induced an increase in the expression of P-AA-NAT but not of melatonin. Conversely, IL10 induced a significant increase in melatonin production associated with increased AA-NAT/P-AA-NAT expressions. In conclusion, BM and spleen cells present different profiles of circadian production of local melatonin and responses to immune signals.

Melatonin mediates mucosal immune cells, microbial metabolism, and rhythm crosstalk: A therapeutic target to reduce intestinal inflammation

Nowadays, melatonin, previously considered only as a pharmaceutical product for rhythm regulation and sleep aiding, has shown its potential as a co-adjuvant treatment in intestinal diseases, however, its mechanism is still not very clear. A firm connection between melatonin at a physiologically relevant concentration and the gut microbiota and inflammation has recently established. Herein, we summarize their crosstalk and focus on four novelties. First, how melatonin is synthesized and degraded in the gut and exerts potentially diverse phenotypic effects through its diverse metabolites. Second, how melatonin mediates the activation and proliferation of intestinal mucosal immune cells with paracrine and autocrine properties. By modulating T/B cells, mast cells, macrophages and dendritic cells, melatonin immunomodulatory involved in regulating T-cell differentiation, intervening T/B cell interaction and attenuating the production of pro-inflammatory factors, achieving its antioxidant action via specific receptors. Third, how melatonin exerts antimicrobial action and modulates microbial components, such as lipopolysaccharide, amyloid-β peptides via nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) or signal transducers and activators of transcription (STAT1) pathway to modulate intestinal immune function in immune-pineal axis. The last, how melatonin mediates the effect of intestinal bacterial activity signals on the body rhythm system through the NF-κB pathway and influences the mucosal epithelium oscillation via clock gene expression. These processes are achieved at mitochondrial and nuclear levels to control the host immune cell development. Considering unclear mechanisms and undiscovered actions of melatonin in gut-microbiome-immune axis, it's time to reveal them and provide new insight for the outlook of melatonin as a potential therapeutic target in the treatment and management of intestinal diseases.

Possible Protective Role of Melatonin in Pediatric Infectious Diseases and Neurodevelopmental Pathologies

Melatonin, produced in every cell that possesses mitochondria, acts as an endogenous free radical scavenger, and improves energetic metabolism and immune function, by complex molecular crosstalk with other intracellular compounds. There is greatly increasing evidence regarding beneficial effects of acute and chronic administration of high melatonin doses, in infectious, developmental, and degenerative pathologies, as an endothelial cell and every cell protectant.

STAT1-NFκB crosstalk triggered by interferon gamma regulates noradrenaline-induced pineal hormonal production

Melatonin production by pineal glands is modulated by several immune signals. The nuclear translocation of nuclear factor kappa-B (NFκB) homodimers, lacking transactivation domains, once induced by lipopolysaccharide (LPS) or tumor necrosis factor (TNF), inhibits the expression of Aanat gene and the synthesis of noradrenaline (NA)-induced melatonin. Interferon gamma (IFN-γ), on the other hand, increases melatonin synthesis. Furthermore, this cytokine activates the signal transducer as well as the activator of transcription 1 (STAT1) pathway, which was never evaluated as a melatonin synthesis modulator before. Reports demonstrated that IFN-γ might also activate NFκB. The present study evaluated the role of STAT1-NFκB crosstalk triggered by IFN-γ regarding the regulation of NA-induced pineal glands' hormonal production. Moreover, IFN-γ treatment increased NA-induced Aanat transcription, in addition to the synthesis of N-acetylserotonin (NAS) and melatonin. These effects were associated with STAT1 nuclear translocation, confirmed by the co-immunoprecipitation of STAT1 and Aanat promoter. Pharmacological STAT1 enhancement augmented NA-induced Aanat transcription as well as NAS and melatonin production. Additionally, IFN-γ induced the nuclear translocation of RelA-NFκB subunits. The blockade of this pathway prevented IFN-γ effects on the pineal function. The present data show that STAT1 and NFκB crosstalk controls melatonin production through a synergistic mechanism, disclosing a new integrative mechanism regarding pineal hormonal activity control.

Melatonin Action on the Activity of Phagocytes from the Colostrum of Obese Women

Background and objectives: Breastfeeding promotion is an important public health strategy for counter-balancing the negative effects of maternal overweight and obesity. Colostrum contains melatonin, which can attenuate the impacts of excessive maternal weight and boost the infant’s immune system. Therefore, the objective of this study was to analyze the effects of melatonin on mononuclear (MN) phagocytes from the colostrum of women with pre-gestational obesity. Materials and Methods: Colostrum samples were collected postpartum from 100 women at a public hospital in São Paulo, Brazil. The donors were divided into two groups: the control group and the high body mass index (BMI) group. Melatonin levels in the colostrum were determined by an ELISA Kit, and the functional activity of MN cells was assessed using the phagocytosis assay by flow cytometry, and reactive oxygen species (ROS), intracellular calcium, and apoptosis were assessed by fluorimetry using a microplate reader. Results: The colostrum of mothers with pre-gestational high BMI exhibited higher melatonin levels (p < 0.05) and lower phagocytosis (p < 0.05) and ROS release (p < 0.05). Superoxide release was similar between the normal and high BMI groups (p > 0.05). Intracellular calcium release and apoptosis were also higher in the high BMI group (p < 0.05). Melatonin levels likely increased the phagocytosis rate and reduced intracellular calcium release and the apoptosis index (p < 0.05). Conclusions: The results suggest that melatonin is a possible mechanism for maternal–infant protection against obesity and restores the functional activity of colostrum phagocytes in obese mothers.

Pineal gland dysfunction in Alzheimer's disease: relationship with the immune-pineal axis, sleep disturbance, and neurogenesis

Alzheimer’s disease (AD) is a globally common neurodegenerative disease, which is accompanied by alterations to various lifestyle patterns, such as sleep disturbance. The pineal gland is the primary endocrine organ that secretes hormones, such as melatonin, and controls the circadian rhythms. The decrease in pineal gland volume and pineal calcification leads to the reduction of melatonin production. Melatonin has been reported to have multiple roles in the central nervous system (CNS), including improving neurogenesis and synaptic plasticity, suppressing neuroinflammation, enhancing memory function, and protecting against oxidative stress. Recently, reduced pineal gland volume and pineal calcification, accompanied by cognitive decline and sleep disturbances have been observed in AD patients. Here, I review current significant evidence of the contribution of pineal dysfunction in AD to the progress of AD neuropathology. I suggest new insights to understanding the relationship between AD pathogenesis and pineal gland function.

Daily variation in macrophage phagocytosis is clock-independent and dispensable for cytokine production

Innate immune responses vary in a circadian manner, and more recent investigations aim to understand the underlying molecular mechanisms. Cytokine production varies significantly over the course of a day depending on the time of stimulation by pathogens or Toll-like receptor ligands, and multiple signaling pathways linked to the cell-autonomous circadian clock modulate innate immunity. Recognition of foreign material, especially by innate immune cells, engages a myriad of receptors, which trigger inflammatory responses, as well as endocytosis and degradation and/or processing for antigen presentation. Because of the close connection between particle engulfment and inflammation, it has been proposed that phagocytic uptake may drive cytokine production in phagocytes. Here we show that bacterial particle ingestion by mouse peritoneal macrophages displays temporal variation, but is independent of the cell-intrinsic circadian clock in an ex vivo setting. Although cytokine production is dependent on phagocytosis, uptake capacity across 12 hr does not translate into 24-hr rhythms in cytokine production. In vivo, time-of-day variations in phagocytic capacity are not found, whereas a time of day and clock-dependent cytokine response is maintained. These data show that efficiency of bacterial phagocytosis and the 24-hr rhythmicity of cytokine production by macrophages are independent of one another and should be studied separately.

Melatonin Target Proteins: Too Many or Not Enough?

The neurohormone N-acetyl-5-methoxytryptamine, better known as melatonin, is a tryptophan derivative with a wide range of biological effects that is present in many organisms. These effects are believed to rely either on the chemical properties of melatonin itself as scavenger of free radicals or on the binding of melatonin to protein targets. More than 15 proteins, including receptors (MT₁, MT₂, Mel1c, CAND2, ROR, VDR), enzymes (QR2, MMP-9, pepsin, PP2A, PR-10 proteins), pores (mtPTP), transporters (PEPT1/2, Glut1), and other proteins (HBS, CaM, tubulin, calreticuline), have been suggested to interact with melatonin at sub-nanomolar to millimolar melatonin concentrations. In this review we assemble for the first time the available information on proposed melatonin targets and discuss them in a comprehensive manner to evaluate the robustness of these findings in terms of methodology, physiological relevance, and independent replication.

Melatonin Stimulates STAR Expression and Progesterone Production via Activation of the PI3K/AKT Pathway in Bovine Theca Cells

Melatonin is present in mammalian follicular fluid and plays an important role in regulating steroidogenesis in follicular cells. In this study, we report the effect of melatonin on steroidogenesis in the theca interna (TI) in small bovine follicles and theca cells (TCs) cultured in vitro. Treatment with melatonin significantly increased the expression of steroidogenic acute regulatory protein (STAR) and the production of progesterone in both TI and in TCs. Melatonin stimulated the phosphorylation of AKT but not ERK1/2, and the addition of luzindole (a nonspecific MT1 and MT2 inhibitor) or 4P-PDOT (specific MT2 inhibitor) reduced melatonin-induced STAR expression, progesterone secretion, and PI3K/AKT pathway activation. The effect of melatonin on the TI in follicles was more obvious than on the TCs in vitro. Results indicate that melatonin stimulates the steroidogenesis of TCs mainly via the activation of the PI3K/AKT pathway by MT1 and MT2.

Melatonin suppresses TLR9-triggered proinflammatory cytokine production in macrophages by inhibiting ERK1/2 and AKT activation

Toll-like receptor (TLR) signaling plays major roles in innate immune response in macrophages. Melatonin regulates TLR3- and TLR4-mediated innate immune responses in macrophages. However, it remains unknown whether melatonin regulates TLR9-mediated innate immune responses in macrophages. Here we demonstrated that melatonin suppressed TLR9 ligand-induced proinflammatory cytokines mRNA and protein production in peritoneal macrophages without interrupting the viability of peritoneal macrophages. Using a melatonin membrane receptors MT1/MT2 antagonist luzindole, we found that MT1 and MT2 were dispensable for melatonin’s inhibitory effects on TLR9-mediated proinflammatory cytokines production, even though melatonin upregulated mRNA expression of MT1 and MT2 in macrophages. Furthermore, melatonin did not affect mRNA expressions of TLR9 and MyD88 but attenuated TLR9 ligand-induced ERK1/2 and AKT phosphorylation without affecting p38 and NF-κB p65 phosphorylation. Also, melatonin inhibited TLR9-mediated proinflammatory cytokines production in vivo. Taken together, our results demonstrate that melatonin suppresses TLR9-triggered proinflammatory cytokines production in macrophages via melatonin membrane receptor-independent manners and probably through inhibiting ERK1/2 and AKT activation, which further elucidates the roles of melatonin in regulating TLR-mediated innate immune responses in macrophages.

β-Adrenoceptors Trigger Melatonin Synthesis in Phagocytes

Melatonin (5-methoxy-N-acetylserotonin), the pineal hormone, is also synthesized by immune-competent cells. The pineal hormone signals darkness, while melatonin synthesized on demand by activated macrophages at any hour of the day acts locally, favoring regulatory/tolerant phenotypes. Activation of β-adrenoceptors in pinealocytes is the main route for triggering melatonin synthesis. However, despite the well-known role of β-adrenoceptors in the resolution macrophage phenotype (M2), and the relevance of macrophage synthesized melatonin in facilitating phagocytic activity, there is no information regarding whether activation of β-adrenoceptors would induce melatonin synthesis by monocytes. Here we show that catecholamines stimulate melatonin synthesis in bone marrow-derived dendritic cells and RAW 264.7 macrophages. Activation of β-adrenoceptors promotes the synthesis of melatonin by stimulating cyclic AMP/protein kinase A (PKA) pathway and by activating the nuclear translocation of NF-κB. Considering the great number of macrophages around sympathetic nerve terminals, and the relevance of this system for maintaining macrophages in stages compatible to low-grade inflammation, our data open the possibility that extra-pineal melatonin acts as an autocrine/paracrine signal in macrophages under resolution or tolerant phenotypes.

Immune-pineal axis - acute inflammatory responses coordinate melatonin synthesis by pinealocytes and phagocytes

Melatonin is well known for its circadian production by the pineal gland, and there is a growing body of data showing that it is also produced by many other cells and organs, including immune cells. The chronobiotic role of pineal melatonin, as well as its protective effects in vitro and in vivo, have been extensively explored. However, the interaction between the chronobiotic and defence functions of endogenous melatonin has been little investigated. This review details the current knowledge regarding the coordinated shift in melatonin synthesis from the pineal gland (circadian and monitoring roles) to the regulation of acute immune responses via immune cell production and autocrine effects, producing systemic interactions termed the immune-pineal axis. An acute inflammatory response drives the transcription factor, NFκB, to switch melatonin synthesis from pinealocytes to macrophages/microglia and, upon acute inflammatory resolution, back to pinealocytes. The potential pathophysiological relevance of immune-pineal axis dysregulation is highlighted, with both research and clinical implications, across several medical conditions, including host/parasite interaction, neurodegenerative diseases and cancer. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Melatonin: Pharmacology, Functions and Therapeutic Benefits

BACKGROUND

Melatonin synchronizes central but also peripheral oscillators (fetal adrenal gland, pancreas, liver, kidney, heart, lung, fat, gut, etc.), allowing temporal organization of biological functions through circadian rhythms (24-hour cycles) in relation to periodic environmental changes and therefore adaptation of the individual to his/her internal and external environment. Measures of melatonin are considered the best peripheral indices of human circadian timing based on an internal 24-hour clock.

METHODS

First, the pharmacology of melatonin (biosynthesis and circadian rhythms, pharmacokinetics and mechanisms of action) is described, allowing a better understanding of the short and long term effects of melatonin following its immediate or prolonged release. Then, research related to the physiological effects of melatonin is reviewed.

RESULTS

The physiological effects of melatonin are various and include detoxification of free radicals and antioxidant actions, bone formation and protection, reproduction, and cardiovascular, immune or body mass regulation. Also, protective and therapeutic effects of melatonin are reported, especially with regard to brain or gastrointestinal protection, psychiatric disorders, cardiovascular diseases and oncostatic effects.

CONCLUSION

This review highlights the high number and diversity of major melatonin effects and opens important perspectives for measuring melatonin as a biomarker (biomarker of early identification of certain disorders and also biomarker of their follow-up) and using melatonin with clinical preventive and therapeutic applications in newborns, children and adults based on its physiological regulatory effects.

Dual Effect of Catecholamines and Corticosterone Crosstalk on Pineal Gland Melatonin Synthesis

BACKGROUND/AIM

The nocturnal production of melatonin by the pineal gland is triggered by sympathetic activation of adrenoceptors and may be modulated by immunological signals. The effect of glucocorticoids on nocturnal melatonin synthesis is controversial; both stimulatory and inhibitory effects have been reported. During pathophysiological processes, an increased sympathetic tonus could result in different patterns of adrenoceptor activation in the pineal gland. Therefore, in this investigation, we evaluated whether the pattern of adrenergic stimulation of the pineal gland drives the direction of the glucocorticoid effect on melatonin production.

METHODS

The corticosterone effect on the pineal hormonal production induced by β-adrenoceptor or β+α1-adrenoceptor activation was evaluated in cultured glands. We also investigated whether the in vivo lipopolysaccharide (LPS)-induced inhibition of melatonin is dependent on the interaction of glucocorticoids and the α1-adrenoceptor in adrenalectomized animals and on the in vivo blockade of glucocorticoid receptors (GRs) or the α1-adrenoceptor.

RESULTS

Corticosterone potentiated β-adrenoceptor-induced pineal melatonin synthesis, whilst corticosterone-dependent inhibition was observed when melatonin production was induced by β+α1-adrenoceptors agonists. The inhibitory effect of corticosterone is mediated by GR, as it was abolished in the presence of a GR antagonist. Moreover, LPS-induced reduction in melatonin nocturnal plasma content was reversed by adrenalectomy and by antagonizing GR or α1-adrenoceptors.

CONCLUSIONS

The dual effect of corticosterone on pineal melatonin synthesis is determined by the activation pattern of adrenoceptors (β or β+α1) in the gland during GR activation, suggesting that increased activation of the sympathetic system and the hypothalamic-pituitary-adrenal axis are necessary for the control of melatonin production during defense responses.

Melatoninergic System in Parkinson's Disease: From Neuroprotection to the Management of Motor and Nonmotor Symptoms

Melatonin is synthesized by several tissues besides the pineal gland, and beyond its regulatory effects in light-dark cycle, melatonin is a hormone with neuroprotective, anti-inflammatory, and antioxidant properties. Melatonin acts as a free-radical scavenger, reducing reactive species and improving mitochondrial homeostasis. Melatonin also regulates the expression of neurotrophins that are involved in the survival of dopaminergic neurons and reduces α-synuclein aggregation, thus protecting the dopaminergic system against damage. The unbalance of pineal melatonin synthesis can predispose the organism to inflammatory and neurodegenerative diseases such as Parkinson's disease (PD). The aim of this review is to summarize the knowledge about the potential role of the melatoninergic system in the pathogenesis and treatment of PD. The literature reviewed here indicates that PD is associated with impaired brain expression of melatonin and its receptors MT₁ and MT₂. Exogenous melatonin treatment presented an outstanding neuroprotective effect in animal models of PD induced by different toxins, such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, paraquat, and maneb. Despite the neuroprotective effects and the improvement of motor impairments, melatonin also presents the potential to improve nonmotor symptoms commonly experienced by PD patients such as sleep and anxiety disorders, depression, and memory dysfunction.

Update on melatonin receptors: IUPHAR Review 20

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.

Melatonin modulates the functions of porcine granulosa cells via its membrane receptor MT2 in vitro

Melatonin (N-acetyl-5-methoxytryptamine) is documented as a hormone involved in the circadian regulation of physiological and neuroendocrine function in mammals. Herein, the effects of melatonin on the functions of porcine granulosa cells in vitro were investigated. Porcine granulosa cells were cultivated with variable concentrations of melatonin (0, 0.001, 0.01, 0.1, 1.0, and 10ng/mL) for 48h. Melatonin receptor agonist (IIK7) and antagonist (Luzindole, 4P-PDOT) were used to further examine the action of melatonin. The results showed optimum cell viability and colony-forming efficiency of porcine granulosa cells at 0.01ng/mL melatonin for 48-h incubation period. The percentage of apoptotic granulosa cells was significantly reduced by 0.01 and 0.1ng/mL melatonin within the 48-h incubation period as compared with the rest of the treatments. Estradiol biosynthesis was significantly stimulated by melatonin supplementation and suppressed for the progesterone secretion; the minimum ratio of progesterone to estradiol was 1.82 in 0.01ng/mL melatonin treatment after 48h of cultivation. Moreover, the expression of BCL-2, CYP17A1, CYP19A1, SOD1, and GPX4 were up-regulated by 0.01ng/mL melatonin or combined with IIK7, but decreased for the mRNA levels of BAX, P53, and CASPASE-3, as compared with control or groups treated with Luzindole or 4P-PDOT in the presence of melatonin. In conclusion, the study demonstrated that melatonin mediated proliferation, apoptosis, and steroidogenesis in porcine granulosa cells predominantly through the activation of melatonin receptor MT2 in vitro, which provided evidence of the beneficial role of melatonin as well as its functional mechanism in porcine granulosa cells in vitro.

The RelA/cRel nuclear factor-κB (NF-κB) dimer, crucial for inflammation resolution, mediates the transcription of the key enzyme in melatonin synthesis in RAW 264.7 macrophages

Lipopolysaccharide (LPS) modulates the transcription of the gene that codifies the enzyme arylalkylamine-N-acetyltransferase (AA-NAT) through nuclear translocation of the transcription factor nuclear factor-κ-light-chain-enhancer of activated B cells (NF-κB). AA-NAT converts serotonin to N-acetylserotonin, the ultimate precursor of melatonin. Activation of kappa B elements (aa-nat-κB), localized in the promoter (nat-κB1 and nat-κB2), leads to Aa-nat transcription in RAW 264.7 macrophages. Competitive electrophoretic mobility shift assay (EMSA) with oligonucleotide probes corresponding to each of the two elements, as well as a NF-κB consensus corresponding probe, revealed different specificities for each κB element. In addition, activator protein-1 (AP-1) as well as signal transducers and activator of transcription-1 and 3 (STAT-1; STAT-3) competed with NF-κB for binding to nat-κB1, while only STAT-3 competed with NF-κB for binding to nat-κB2. According to co-immunoprecipitation (ChiP) assays, these two sites are able to distinguish NF-κB subunits. The sequence nat-κB1 bound dimers containing p52, RelA, and cRel, while nat-κB2 bound preferentially p50, p52, and RelA, and did not bind cRel. The expression of RelA and cRel is essential for the induction of Aa-nat expression and melatonin synthesis. Considering that the expression of cRel is induced by the earlier expressed p50/RelA, the differential effects of NF-κB dimers may be intimately associated with the temporal regulation of inflammatory responses, with the resolution phase being associated with paracrine and autocrine melatonin effects. Such data suggest that the proven effects of exogenous melatonin in the resolution phase of inflammation are paralleled by the effects of locally synthesized melatonin in immune cells.

Possible Involvement of the Inhibition of NF-κB Factor in Anti-Inflammatory Actions That Melatonin Exerts on Mast Cells

Melatonin is a molecule endogenously produced in a wide variety of immune cells, including mast cells (RBL-2H3). It exhibits immunomodulatory, anti-inflammatory and anti-apoptotic properties. The physiologic mechanisms underlying these activities of melatonin have not been clarified in mast cells. This work is designed to determine the anti-inflammatory effect and mechanism of action of melatonin on activated mast cells. RBL-2H3 were pre-treated with exogenous melatonin (MELx) at physiological (100nM) and pharmacological (1 mM) doses for 30 min, washed and activated with PMACI (phorbol 12-myristate 13-acetate plus calcium ionophore A23187) for 2 h and 12 h. The data shows that pre-treatment of MELx in stimulated mast cells, significantly reduced the levels of endogenous melatonin production (MELn), TNF-α and IL-6. These effects are directly related with the MELx concentration used. MELx also inhibited IKK/NF-κB signal transduction pathway in stimulated mast cells. These results indicate a molecular basis for the ability of melatonin to prevent inflammation and for the treatment of allergic inflammatory diseases through the down-regulation of mast cell activation. J. Cell. Biochem. 117: 1926-1933, 2016. © 2016 Wiley Periodicals, Inc.

Melatonin attenuates Leishmania (L.) amazonensis infection by modulating arginine metabolism

Acute inflammatory responses induced by bacteria or fungi block nocturnal melatonin synthesis by rodent pineal glands. Here, we show Leishmania infection does not impair daily melatonin rhythm in hamsters. Remarkably, the attenuated parasite burden and lesion progression in hamsters infected at nighttime was impaired by blockage of melatonin receptors with luzindole, whereas melatonin treatment during the light phase attenuated Leishmania infection. In vitro studies corroborated in vivo observations. Melatonin treatment reduced macrophage expression of Cat-2b, Cat1, and ArgI, genes involved in arginine uptake and polyamine synthesis. Indeed, melatonin reduced macrophage arginine uptake by 40%. Putrescine supplementation reverted the attenuation of infectivity by melatonin indicating that its effect was due to the arrest of parasite replication. This study shows that the Leishmania/host interaction varies in a circadian manner according to nocturnal melatonin pineal synthesis. Our results provide new data regarding Leishmania infectiveness and show new approaches for applying agonists of melatonin receptors in Leishmaniasis therapy.

Selective protection of the cerebellum against intracerebroventricular LPS is mediated by local melatonin synthesis

Although melatonin is mainly produced by the pineal gland, an increasing number of extra-pineal sites of melatonin synthesis have been described. We previously demonstrated the existence of bidirectional communication between the pineal gland and the immune system that drives a switch in melatonin production from the pineal gland to peripheral organs during the mounting of an innate immune response. In the present study, we show that acute neuroinflammation induced by lipopolysaccharide (LPS) injected directly into the lateral ventricles of adult rats reduces the nocturnal peak of melatonin in the plasma and induces its synthesis in the cerebellum, though not in the cortex or hippocampus. This increase in cerebellar melatonin content requires the activation of nuclear factor kappa B (NF-κB), which positively regulates the expression of the key enzyme for melatonin synthesis, arylalkylamine N-acetyltransferase (AA-NAT). Interestingly, LPS treatment led to neuronal death in the hippocampus and cortex, but not in the cerebellum. This privileged protection of cerebellar cells was abrogated when G-protein-coupled melatonin receptors were blocked by the melatonin antagonist luzindole, suggesting that the local production of melatonin protects cerebellar neurons from LPS toxicity. This is the first demonstration of a switch between pineal and extra-pineal melatonin production in the central nervous system following a neuroinflammatory response. These results have direct implications concerning the differential susceptibility of specific brain areas to neuronal death.

Immune-pineal axis: nuclear factor κB (NF-kB) mediates the shift in the melatonin source from pinealocytes to immune competent cells

Pineal gland melatonin is the darkness hormone, while extra-pineal melatonin produced by the gonads, gut, retina, and immune competent cells acts as a paracrine or autocrine mediator. The well-known immunomodulatory effect of melatonin is observed either as an endocrine, a paracrine or an autocrine response. In mammals, nuclear translocation of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) blocks noradrenaline-induced melatonin synthesis in pinealocytes, which induces melatonin synthesis in macrophages. In addition, melatonin reduces NF-κB activation in pinealocytes and immune competent cells. Therefore, pathogen- or danger-associated molecular patterns transiently switch the synthesis of melatonin from pinealocytes to immune competent cells, and as the response progresses melatonin inhibition of NF-κB activity leads these cells to a more quiescent state. The opposite effect of NF-κB in pinealocytes and immune competent cells is due to different NF-κB dimers recruited in each phase of the defense response. This coordinated shift of the source of melatonin driven by NF-κB is called the immune-pineal axis. Finally, we discuss how this concept might be relevant to a better understanding of pathological conditions with impaired melatonin rhythms and hope it opens new horizons for the research of side effects of melatonin-based therapies.