Melatonin Receptor Signaling: Impact of Receptor Oligomerization on Receptor Function

Melatonin receptor structure and signaling

Melatonin (5-methoxy-N-acetyltryptamine) binds with high affinity and specificity to membrane receptors. Several receptor subtypes exist in different species, of which the mammalian MT1 and MT2 receptors are the best-characterized. They are members of the G protein-coupled receptor superfamily, preferentially coupling to Gi/o proteins but also to other G proteins in a cell-context-depending manner. In this review, experts on melatonin receptors will summarize the current state of the field. We briefly report on the discovery and classification of melatonin receptors, then focus on the molecular structure of human MT1 and MT2 receptors and highlight the importance of molecular simulations to identify new ligands and to understand the structural dynamics of these receptors. We then describe the state-of-the-art of the intracellular signaling pathways activated by melatonin receptors and their complexes. Brief statements on the molecular toolbox available for melatonin receptor studies and future perspectives will round-up this review.

Comprehensive review of melatonin as a promising nutritional and nutraceutical supplement

Background

Melatonin is an indoleamine hormone secreted by the pineal gland at night and has an essential role in regulating human circadian rhythms (the internal 24-h clock) and sleep-wake patterns. However, it has recently gained considerable attention for its demonstrated ability in disease management. This review discusses the major biological activities of melatonin, its metabolites as nutritional supplements, and its bioavailability in food sources.

Methods

The information acquisition process involved conducting a comprehensive search across academic databases including PubMed, Scopus, Wiley, Embase, and Springer using relevant keywords. Only the most recent, peer-reviewed articles published in the English language were considered for inclusion.

Results

The molecular mechanisms by which melatonin induces its therapeutic effects have been the subject of various studies.

Conclusion

While melatonin was initially understood to only regulate circadian rhythms, recent studies indicate that it has a far-reaching effect on various organs and physiological systems, such as immunity, cardiovascular function, antioxidant defense, and lipid hemostasis. As a potent antioxidant, anti-cancer, anti-inflammatory, and immunoregulatory agent, multiple therapeutic applications have been proposed for melatonin.

Melatonin Offers Dual-Phase Protection to Brain Vessel Endothelial Cells in Prolonged Cerebral Ischemia-Recanalization Through Ameliorating ER Stress and Resolving Refractory Stress Granule

Ischemic-reperfusion injury limits the time window of recanalization therapy in cerebral acute ischemic stroke (AIS). Brain vessel endothelial cells (BVECs) form the first layer of the blood-brain barrier (BBB) and are thus the first sufferer of ischemic-reperfusion disorder. The current study demonstrates that melatonin can reduce infarct volume, alleviate brain edema, ameliorate neurological deficits, and protect BBB integrity in prolonged-stroke mice. Here, we demonstrate that endoplasmic reticulum (ER)-associated injury contributes to BVEC death in the dural phase of reperfusion after prolonged ischemia. When encountering ischemia, ER stress arises, specifically activating PERK-EIF2α signaling and the subsequent programmed cell death. Prolonged ischemia leads stress granules (SGs) to be refractory, which remain unresolved and accumulate in ER during recanalization. During reperfusion, refractory SGs activate PKR-EIF2α and further exacerbate BVEC injury. We report that melatonin treatment downregulates ER stress in the ischemic period and enhances dissociation of the refractory SGs during reperfusion, thus offering dual-phase protection to BVECs in prolonged cerebral stroke. Mechanistically, melatonin enhances autophagy in BVECs, which preserves ER function and resolves refractory SGs. We, therefore, propose that melatonin is a potential treatment to extend the time window of delayed recanalization therapy in AIS.

High expression of GPR50 promotes the proliferation, migration and autophagy of hepatocellular carcinoma cells in vitro

G protein-coupled receptors (GPCRs) play important roles in tumorigenesis and the development of hepatocellular carcinoma (HCC). GPR50 is an orphan GPCR. Previous studies have indicated that GPR50 could protect against breast cancer development and decrease tumor growth in a xenograft mouse model. However, its role in HCC remains indistinct. To detect the role and the regulation mechanism of GPR50 in HCC, GPR50 expression was analyzed in HCC patients (gene expression omnibus database (GEO) (GSE45436)) and detected in HCC cell line CBRH-7919, and the results showed that GPR50 was significantly up-regulated in HCC patients and CBRH-7919 cell line compared to the corresponding normal control. Gpr50 cDNA was transfected into HCC cell line CBRH-7919, and we found that Gpr50 promoted the proliferation, migration, and autophagy of CBRH-7919. The regulation mechanism of GPR50 in HCC was detected by isobaric tags for relative and absolute quantification (iTRAQ) analysis, and we found that GPR50 promoted HCC was closely related to CCT6A and PGK1. Taken together, GPR50 may promote HCC progression via CCT6A-induced proliferation and PGK1-induced migration and autophagy, and GPR50 could be an important target for HCC.

Therapeutic potential of melatonin and its derivatives in aging and neurodegenerative diseases

Aging is associated with increasing impairments in brain homeostasis and represents the main risk factor across most neurodegenerative disorders. Melatonin, a neuroendocrine hormone that regulates mammalian chronobiology and endocrine functions is well known for its antioxidant potential, exhibiting both cytoprotective and chronobiotic abilities. Age-related decline of melatonin disrupting mitochondrial homeostasis and cytosolic DNA-mediated inflammatory reactions in neurons is a major contributory factor in the emergence of neurological abnormalities. There is scattered literature on the possible use of melatonin against neurodegenerative mechanisms in the aging process and its associated diseases. We have searched PUBMED with many combinations of key words for available literature spanning two decades. Based on the vast number of experimental papers, we hereby review recent advancements concerning the potential impact of melatonin on cellular redox balance and mitochondrial dynamics in the context of neurodegeneration. Next, we discuss a broader explanation of the involvement of disrupted redox homeostasis in the pathophysiology of age-related diseases and its connection to circadian mechanisms. Our effort may result in the discovery of novel therapeutic approaches. Finally, we summarize the current knowledge on molecular and circadian regulatory mechanisms of melatonin to overcome neurodegenerative diseases (NDDs) such as Alzheimer's, Parkinson's, Huntington's disease, and amyotrophic lateral sclerosis, however, these findings need to be confirmed by larger, well-designed clinical trials. This review is also expected to uncover the associated molecular alterations in the aging brain and explain how melatonin-mediated circadian restoration of neuronal homeodynamics may increase healthy lifespan in age-related NDDs.

A novel combination of sitagliptin and melatonin ameliorates T2D manifestations: studies on experimental diabetic models

INTRODUCTION

Type 2 diabetes (T2D) is an endocrine disorder characterized by hyperglycemia, insulin resistance, dysregulated glucose and lipid metabolism, reduced pancreatic β-cell function and mass, and a reduced incretin effect. Circadian rhythm disruption is associated with increased T2D risk. We have investigated the therapeutic potential of a combination of melatonin (M) and sitagliptin (S), a dipeptidyl peptidase IV (DPP-IV) inhibitor, in the amelioration of T2D manifestations in high-fat diet (HFD) induced T2D mouse model and also on β-cell proliferation under gluco-lipotoxicity stress in vitro.

METHODS

For in vivo study, mice were fed with HFD for 25 weeks to induce T2D and were treated with monotherapies and S + M for four weeks. For the in vitro study, primary mouse islets were exposed to normal glucose and high glucose + palmitate to induce gluco-lipotoxic stress.

RESULTS

Our results suggest that monotherapies and S + M improve metabolic parameters and glyco-lipid metabolism in the liver and adipose tissue, respectively, and improve mitochondrial function in the skeletal muscle. Moreover, it increases peripheral insulin sensitivity. Our in vitro and in vivo studies suggest that β-cell mass was preserved in all the drug-treated groups.

CONCLUSION

The combination treatment is superior to monotherapies in the management of T2D.

Melatonin functions as a broad-spectrum antifungal by targeting a conserved pathogen protein kinase

Melatonin is a low-cost natural small indole molecule with versatile biological functions. However, melatonin's fungicidal potential has not been fully exploited, and the mechanism remains elusive. Here, we report that melatonin broadly inhibited 13 plant pathogens. In the rice blast fungal pathogen Magnaporthe oryzae, melatonin inhibited fungal growth, formation of infection-specific structures named appressoria, and plant infection, reducing disease severity. Melatonin entered fungal cells efficiently and colocalized with the critical mitogen-activated protein kinase named Mps1, suppressing phosphorylation of Mps1. Melatonin's affinity for Mps1 via two hydrogen bonds was demonstrated using surface plasmon resonance and chemical modifications. To improve melatonin's efficiency, we obtained 20 melatonin derivatives. Tert-butyloxycarbonyl melatonin showed a 25-fold increase in fungicidal activities, demonstrating the feasibility of chemical modifications in melatonin modification. Our study demonstrated the broad-spectrum fungicidal effect of melatonin by suppressing Mps1 as one of the targets. Through further systematic modifications, developing an eco-friendly melatonin derivative of commercial values for agricultural applications appears promising.

Imaging strategies for receptor tyrosine kinase dimers in living cells

Receptor tyrosine kinases (RTKs) are the essential regulators of cell signal transduction pathways and play important roles in biological processes. RTK dimerization is generally considered the first step in receptor activation and cell communication. And the abnormal expression of RTK dimers is closely related to the occurrence and development of many diseases. Therefore, the visualization of RTK dimerization is of great significance for monitoring physiological processes. The genetic and nongenetic imaging strategies have attracted widespread attention due to their high efficiency and high sensitivity. In this review, the RTKs and their dimers as well as the advances in strategies for imaging RTK dimers are introduced. Furthermore, we analyze the limitations of existing imaging strategies and put forward suggestions for the future development of imaging probes. We expect that this review will inspire more in-depth investigation of RTK dimers, which will also broaden the application of strategies of RTK dimers in biomedical areas.

Practical Synthesis of 7-Azaserotonin and 7-Azamelatonin

A practical method for synthesizing 7-azaserotonin and 7-azamelatonin was developed by using 3-bromo-5-methoxy-1-tosyl-2,3-dihydro-1H-pyrrolo[2,3-b]pyridin-2-ol as a starting material. This compound is a useful reactant for the formal C3-electrophilic reaction. The lactone derivative obtained by the reaction with Meldrum’s acid was used as a key intermediate, in which the C2 unit was introduced into the 7-azaindole skeleton.

Hypotensive effects of melatonin in rats: Focus on the model, measurement, application, and main mechanisms

The hypotensive effects of melatonin are based on a negative correlation between melatonin levels and blood pressure in humans. However, there is a positive correlation in nocturnal animals that are often used as experimental models in cardiovascular research, and the hypotensive effects and mechanism of melatonin action are often investigated in rats and mice. In rats, the hypotensive effects of melatonin have been studied in normotensive and spontaneously or experimentally induced hypertensive strains. In experimental animals, blood pressure is often measured indirectly during the light (passive) phase of the day by tail-cuff plethysmography, which has limitations regarding data quality and animal well-being compared to telemetry. Melatonin is administered to rats in drinking water, subcutaneously, intraperitoneally, or microinjected into specific brain areas at different times. Experimental data show that the hypotensive effects of melatonin depend on the experimental animal model, blood pressure measurement technique, and the route, time and duration of melatonin administration. The hypotensive effects of melatonin may be mediated through specific membrane G-coupled receptors located in the heart and arteries. Due to melatonin's lipophilic nature, its potential hypotensive effects can interfere with various regulatory mechanisms, such as nitric oxide and reactive oxygen species production and activation of the autonomic nervous and circadian systems. Based on the research conducted on rats, the cardiovascular effects of melatonin are modulatory, delayed, and indirect.

Bos taurus and Cervus elaphus as Non-Seasonal/Seasonal Models for the Role of Melatonin Receptors in the Spermatozoon

Melatonin is crucial in reproduction due its antioxidant, hormonal, and paracrine action. Melatonin membrane receptors (MT₁/MT₂) have been confirmed on spermatozoa from several species, but functionality studies are scarce. To clarify their role in ruminants as reproductive models, bull (Bos taurus, non-seasonal) and red deer (Cervus elaphus, highly seasonal) spermatozoa were analyzed after 4 h of incubation (38 °C, capacitating media) in 10 nM melatonin, MT₁/MT₂ agonists (phenylmelatonin and 8M-PDOT), and antagonists (luzindole and 4P-PDOT). Motility and functionality (flow cytometry: viability, intracellular calcium, capacitation status, reactive oxygen species (ROS) production, and acrosomal and mitochondrial status) were assessed. In bull, MT₁ was related to sperm viability preservation, whereas MT₂ could modulate cell functionality to prevent excess ROS produced by the mitochondria; this action could have a role in modulating sperm capacitation. Deer spermatozoa showed resistance to melatonin and receptor activation, possibly because the samples were of epididymal origin and collected at the breeding season's peak, with high circulating melatonin. However, receptors could be involved in mitochondrial protection. Therefore, melatonin receptors are functional in the spermatozoa from bull and deer, with different activities. These species offer models differing from traditional laboratory experimental animals on the role of melatonin in sperm biology.

In silico drug discovery of melatonin receptor ligands with therapeutic potential

INTRODUCTION

The neurohormone melatonin (N-acetyl-5-methoxytryptamine) regulates circadian rhythms exerting a variety of effects in the central nervous system and in periphery. These activities are mainly mediated by activation of MT₁ and MT₂ GPCRs. MT₁/MT₂ agonist compounds are used clinically for insomnia, depression, and circadian rhythm disturbances.

AREA COVERED

The following review describes the design strategies that have led to the identification of melatonin receptor ligands, guided by in silico approaches and molecular modeling. Initial ligand-based design, mainly relying on pharmacophore modeling and 3D-QSAR, has been flanked by structure-based virtual screening, given the recent availability of MT₁ and MT₂ crystal structures. Receptor ligands with different activity profiles, agonist/antagonist and subtype-selective compounds, are available.

EXPERT OPINION

An insight on the pharmacological characterization and therapeutic perspectives for relevant ligands is provided. In silico drug discovery has been instrumental in the design of novel ligands targeting melatonin receptors. Ligand-based approaches has led to the construction of a solid framework defining structure-activity relationships to obtain compounds with a tailored pharmacological profile. Structure-based techniques could integrate previous knowledge and provide compounds with novel chemotypes and pharmacological activity as drug candidates for disease conditions in which melatonin receptor ligands are currently being investigated, including cancer and pain.

G protein-coupled receptor-effector macromolecular membrane assemblies (GEMMAs)

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in cellular signaling across the plasma membrane and a major class of drug targets. The canonical model for GPCR signaling involves three components - the GPCR, a heterotrimeric G protein and a proximal plasma membrane effector - that have been generally thought to be freely mobile molecules able to interact by 'collision coupling'. Here, we synthesize evidence that supports the existence of GPCR-effector macromolecular membrane assemblies (GEMMAs) comprised of specific GPCRs, G proteins, plasma membrane effector molecules and other associated transmembrane proteins that are pre-assembled prior to receptor activation by agonists, which then leads to subsequent rearrangement of the GEMMA components. The GEMMA concept offers an alternative and complementary model to the canonical collision-coupling model, allowing more efficient interactions between specific signaling components, as well as the integration of the concept of GPCR oligomerization as well as GPCR interactions with orphan receptors, truncated GPCRs and other membrane-localized GPCR-associated proteins. Collision-coupling and pre-assembled mechanisms are not exclusive and likely both operate in the cell, providing a spectrum of signaling modalities which explains the differential properties of a multitude of GPCRs in their different cellular environments. Here, we explore the unique pharmacological characteristics of individual GEMMAs, which could provide new opportunities to therapeutically modulate GPCR signaling.

Membrane Melatonin Receptors Activated Cell Signaling in Physiology and Disease

The pineal hormone melatonin has attracted great scientific interest since its discovery in 1958. Despite the enormous number of basic and clinical studies the exact role of melatonin in respect to human physiology remains elusive. In humans, two high-affinity receptors for melatonin, MT1 and MT2, belonging to the family of G protein-coupled receptors (GPCRs) have been cloned and identified. The two receptor types activate Gi proteins and MT2 couples additionally to Gq proteins to modulate intracellular events. The individual effects of MT1 and MT2 receptor activation in a variety of cells are complemented by their ability to form homo- and heterodimers, the functional relevance of which is yet to be confirmed. Recently, several melatonin receptor genetic polymorphisms were discovered and implicated in pathology-for instance in type 2 diabetes, autoimmune disease, and cancer. The circadian patterns of melatonin secretion, its pleiotropic effects depending on cell type and condition, and the already demonstrated cross-talks of melatonin receptors with other signal transduction pathways further contribute to the perplexity of research on the role of the pineal hormone in humans. In this review we try to summarize the current knowledge on the membrane melatonin receptor activated cell signaling in physiology and pathology and their relevance to certain disease conditions including cancer.

Free-Energy Simulations Support a Lipophilic Binding Route for Melatonin Receptors

The effects of the neurohormone melatonin are mediated by the activation of the GPCRs MT₁ and MT₂ in a variety of tissues. Crystal structures suggest ligand access to the orthosteric binding site of MT₁ and MT₂ receptors through a lateral channel between transmembrane (TM) helices IV and V. We investigated the feasibility of this lipophilic entry route for 2-iodomelatonin, a nonselective agonist with a slower dissociation rate from the MT₂ receptor, applying enhanced sampling simulations and free-energy calculations. 2-Iodomelatonin unbinding was investigated with steered molecular dynamics simulations which revealed different trajectories passing through the gap between TM helices IV and V for both receptors. For one of these unbinding trajectories from the MT₁ receptor, an umbrella-sampling protocol with path-collective variables provided a calculated energy barrier consistent with the experimental dissociation rate. The side-chain flexibility of Tyr5.38 was significantly different in the two receptor subtypes, as assessed by metadynamics simulations, and during ligand unbinding it frequently assumes an open conformation in the MT₁ but not in the MT₂ receptor, favoring 2-iodomelatonin egress. Taken together, our simulations are consistent with the possibility that the gap between TM IV and V is a way of connecting the orthosteric binding site and the membrane core for lipophilic melatonin receptor ligands. Our simulations also suggest that the open state of Tyr5.38 generates a small pocket on the surface of MT₁ receptor, which could participate in the recognition of MT₁-selective ligands and may be exploited in the design of new selective compounds.

Melatonin and Pathological Cell Interactions: Mitochondrial Glucose Processing in Cancer Cells

Melatonin is synthesized in the pineal gland at night. Since melatonin is produced in the mitochondria of all other cells in a non-circadian manner, the amount synthesized by the pineal gland is less than 5% of the total. Melatonin produced in mitochondria influences glucose metabolism in all cells. Many pathological cells adopt aerobic glycolysis (Warburg effect) in which pyruvate is excluded from the mitochondria and remains in the cytosol where it is metabolized to lactate. The entrance of pyruvate into the mitochondria of healthy cells allows it to be irreversibly decarboxylated by pyruvate dehydrogenase (PDH) to acetyl coenzyme A (acetyl-CoA). The exclusion of pyruvate from the mitochondria in pathological cells prevents the generation of acetyl-CoA from pyruvate. This is relevant to mitochondrial melatonin production, as acetyl-CoA is a required co-substrate/co-factor for melatonin synthesis. When PDH is inhibited during aerobic glycolysis or during intracellular hypoxia, the deficiency of acetyl-CoA likely prevents mitochondrial melatonin synthesis. When cells experiencing aerobic glycolysis or hypoxia with a diminished level of acetyl-CoA are supplemented with melatonin or receive it from another endogenous source (pineal-derived), pathological cells convert to a more normal phenotype and support the transport of pyruvate into the mitochondria, thereby re-establishing a healthier mitochondrial metabolic physiology.

Pan-cancer analyses reveal genomics and clinical characteristics of the melatonergic regulators in cancer

Melatonin, an endogenous hormone, plays protective roles in cancer. In addition to regulating circadian rhythms, sleep, and neuroendocrine activity, melatonin functions in various survival pathways. However, the mechanisms of melatonin regulation in cancer remain unknown. In the present study, we performed a comprehensive characterization of melatonin regulators in 9125 tumor samples across 33 cancer types using multi-omic data from The Cancer Genome Atlas and Cancer Cell Line Encyclopedia. In the genomic landscape, we identified the heterozygous amplification of AANAT and GPR50, and heterozygous deletion of PER3, CYP2C19, and MTNR1A as the dominant alteration events. Expression analysis revealed methylation-mediated downregulation of melatonergic regulator expression. In addition, we found that melatonergic regulator expression could be used to predict patient survival in various cancers. In depth, microRNA (miRNA) analysis revealed an miRNA-mRNA interaction network, and the deregulated miRNAs were involved in melatonin secretion and metabolism by targeting circadian clock genes. Pathway analysis showed that melatonergic regulators were associated with inhibition of apoptosis, the cell cycle, the DNA damage response, and activation of RAS/MAPK and RTK signaling pathways. Importantly, by mining the Genomics of Drug Sensitivity in Cancer database, we discovered a number of potential drugs that might target melatonergic regulators. In summary, this study revealed the genomic alteration and clinical characteristics of melatonergic regulators across 33 cancers, which might clarify the relationship between melatonin and tumorigenesis. Our findings also might provide a novel approach for the clinical treatment of cancers.

Empowering Melatonin Therapeutics with Drosophila Models

Melatonin functions as a central regulator of cell and organismal function as well as a neurohormone involved in several processes, e.g., the regulation of the circadian rhythm, sleep, aging, oxidative response, and more. As such, it holds immense pharmacological potential. Receptor-mediated melatonin function mainly occurs through MT1 and MT2, conserved amongst mammals. Other melatonin-binding proteins exist. Non-receptor-mediated activities involve regulating the mitochondrial function and antioxidant cascade, which are frequently affected by normal aging as well as disease. Several pathologies display diseased or dysfunctional mitochondria, suggesting melatonin may be used therapeutically. Drosophila models have extensively been employed to study disease pathogenesis and discover new drugs. Here, we review the multiple functions of melatonin through the lens of functional conservation and model organism research to empower potential melatonin therapeutics to treat neurodegenerative and renal diseases.

Melatonin reduces inflammation in intestinal cells, organoids and intestinal explants

Inflammatory bowel diseases (IBDs) are chronic and recurrent diseases that often occur in young people and place a heavy burden on public health in both developed and developing countries. Melatonin has been confirmed to be useful in various diseases, including Alzheimer's disease, liver injuries and diseases, and cancers, while its role in IBDs remains unclear. To uncover the function of melatonin in IBDs, three intestinal models, including Caco-2 cells, 3D intestinal organoids and intestinal explants, were used. It was found that different concentrations of melatonin could significantly inhibit the expression levels of NFκB and its downstream cytokines, including IL6 and IL8 in Caco-2 cells (*P < 0.05, **P < 0.01), 3D intestinal organoids (*P < 0.05, **P < 0.01) and intestinal explants (*P < 0.05, **P < 0.01). Melatonin abolished the activation of LPS on the expression levels of NFκB, IL6, and IL8 in three intestinal models (*P < 0.05, **P < 0.01, ***P < 0.001). Importantly, the roles of melatonin in the regulation of inflammation was dependent on its receptor (i.e., MTNR1), since it was found that silencing of the melatonin receptor (MTNR1A) abolished the reduction in inflammation induced by melatonin in Caco-2 cells (***P < 0.001) and 3D intestinal organoids (***P < 0.01, ****P < 0.0001). Herein, the findings in this study might provide useful information for understanding the pathogenesis of IBDs and developing novel drugs to treat the diseases.

The role of endocrine hormones in the pathogenesis of adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a common spinal deformity characterized by changes in the three-dimensional structure of the spine. It usually initiates during puberty, the peak period of human growth when the secretion of numerous hormones is changing, and it is more common in females than in males. Accumulating evidence shows that the abnormal levels of many hormones including estrogen, melatonin, growth hormone, leptin, adiponectin and ghrelin, may be related to the occurrence and development of AIS. The purpose of this review is to provide a summary and critique of the research published on each hormone over the past 20 years, and to highlight areas for future study. It is hoped that the presentation will help provide a better understanding of the role of endocrine hormones in the pathogenesis of AIS.

Melatonin receptors, brain functions, and therapies

In mammals, including humans, the neurohormone melatonin is mainly secreted from the pineal gland at night and acts on two high-affinity G protein-coupled receptors, the melatonin MT₁ and MT₂ receptors. Major functions of melatonin receptors in the brain are the regulation of circadian rhythms and sleep. Correspondingly, the main indications of the currently available drugs for these receptors indicate this as targets. Yet these drugs may not only improve circadian rhythm- and sleep-related disorders but may also be beneficial for complex diseases like major depression, Alzheimer's disease, autism, and attention-deficit/hyperactivity disorders. Here, we will focus on the hypothalamic functions of melatonin receptors by updating our knowledge on their hypothalamic expression pattern at normal, aged, and disease states, by discussing their capacity to regulate circadian rhythms and sleep and by presenting the clinical applications of the melatonin receptor-targeting drugs ramelteon, tasimelteon, and agomelatine or of prolonged-release melatonin formulations. Finally, we speculate about future trends in the field of melatonin receptor drugs.

Melatonin receptor ligands: A pharmaco-chemical perspective

Melatonin MT₁ and MT₂ receptor ligands have been vigorously explored for the last 4 decades. Inspection of approximately 80 publications in the field revealed that most melatonergic ligands were structural analogues of melatonin combining three essential features of the parent compound: an aromatic ring bearing a methoxy group and an amide side chain in a relative arrangement similar to that present in melatonin. While several series of MT₂ -selective agents-agonists, antagonists, or partial agonists-were reported, the field was lacking MT₁ -selective agents. Herein, we describe various approaches toward the development of melatonergic ligands, keeping in mind that most of the molecules/pharmacophores obtained were essentially melatonin copies, even though diverse tri- or tetra-cyclic compounds were explored. In addition to lack of structural diversity, only few studies examined the activity of the reported melatonergic ligands in vivo. Moreover, an extensive pharmacological characterization including biopharmaceutical stability, pharmacokinetic properties, specificity toward other major receptors to name a few remained scarce. For example, many of the antagonists described were not stable in vivo, were not selective for the melatonin receptor subtype of interest, and were not fully characterized from a pharmacological standpoint. Indeed, virtual screening of large compound libraries has led to the recent discovery of potent and selective melatonin receptor agonists and partial agonists of new chemotypes. Having said this, the melatonergic field is still lacking subtype-selective melatonin receptor antagonists "active" in vivo, which are critical to our understanding of melatonin and melatonin receptors' role in basic physiology and disease.

Ionizing Radiation as a Source of Oxidative Stress-The Protective Role of Melatonin and Vitamin D

Ionizing radiation (IR) has found widespread application in modern medicine, including medical imaging and radiotherapy. As a result, both patients and healthcare professionals are exposed to various IR doses. To minimize the negative side effects of radiation associated with oxidative imbalance, antioxidant therapy has been considered. In this review, studies on the effects of melatonin and vitamin D on radiation-induced oxidative stress are discussed. According to the research data, both substances meet the conditions for use as agents that protect humans against IR-induced tissue damage. Numerous studies have confirmed that melatonin, a hydro- and lipophilic hormone with strong antioxidant properties, can potentially be used as a radioprotectant in humans. Less is known about the radioprotective effects of vitamin D, but the results to date have been promising. Deficiencies in melatonin and vitamin D are common in modern societies and may contribute to the severity of adverse side effects of medical IR exposure. Hence, supporting supplementation with both substances seems to be of first importance. Interestingly, both melatonin and vitamin D have been found to selectively radiosensitise cancer cells, which makes them promising adjuvants in radiotherapy. More research is needed in this area, especially in humans.

Orphan G Protein Coupled Receptors in Affective Disorders

G protein coupled receptors (GPCRs) are the main mediators of signal transduction in the central nervous system. Therefore, it is not surprising that many GPCRs have long been investigated for their role in the development of anxiety and mood disorders, as well as in the mechanism of action of antidepressant therapies. Importantly, the endogenous ligands for a large group of GPCRs have not yet been identified and are therefore known as orphan GPCRs (oGPCRs). Nonetheless, growing evidence from animal studies, together with genome wide association studies (GWAS) and post-mortem transcriptomic analysis in patients, pointed at many oGPCRs as potential pharmacological targets. Among these discoveries, we summarize in this review how emotional behaviors are modulated by the following oGPCRs: ADGRB2 (BAI2), ADGRG1 (GPR56), GPR3, GPR26, GPR37, GPR50, GPR52, GPR61, GPR62, GPR88, GPR135, GPR158, and GPRC5B.

Melatonin membrane receptors MT1 and MT2 are expressed in ram spermatozoa from non-seasonal breeds

In mammals, many melatonin biological functions are mediated through its interaction with the membrane receptors MT₁ and MT₂. We have previously reported their presence in ram spermatozoa from males located in temperate climates, but there is no information on their presence in spermatozoa from rams in areas with an equatorial photoperiod (12L:12D). Thus, we have investigated the existence and cellular distribution of melatonin receptors in spermatozoa from three sheep breeds in Colombia (Colombian Creole, Hampshire, and Romney Marsh) during dry and rainy seasons, using indirect immunofluorescence and western blot. Our results indicated the presence of melatonin receptors in spermatozoa from these rams, and that their distribution differs from that previously found in spermatozoa from rams in temperate climates. Moreover, two new immunotypes of MT₂ were identified: type N, with staining only in the neck, and type E with a band of immunofluorescence in the upper part of the post-acrosome and the apical edge. Likewise, differences between breeds and climate seasons were detected for both receptors. However, densitometry analysis of western blot bands only revealed differences between seasons in the Creole rams for MT₁ and the Romney Marsh rams for MT₂, whereas differences between breeds were only detected for MT₂. It could be inferred that melatonin receptors in rams subjected to an equatorial photoperiod might be more closely related to sperm quality than seasonal control. Therefore, the presence of these receptors suggests that melatonin could be a useful tool to increase the fertility of rams located in tropical or equatorial climates.

Melatonin Non-Linearly Modulates Bull Spermatozoa Motility and Physiology in Capacitating and Non-Capacitating Conditions

Bull spermatozoa physiology may be modulated by melatonin. We washed ejaculated spermatozoa free of melatonin and incubated them (4 h, 38 °C) with 0-pM, 1-pM, 100-pM, 10-nM and 1-µM melatonin in TALP-HEPES (non-capacitating) and TALP-HEPES-heparin (capacitating). This range of concentrations encompassed the effects mediated by melatonin receptors (pM), intracellular targets (nM–µM) or antioxidant activity (µM). Treatment effects were assessed as motility changes by computer-assisted sperm analysis (CASA) of motility and physiological changes by flow cytometry. Melatonin effects were more evident in capacitating conditions, with 100 pM reducing motility and velocity (VCL) while increasing a “slow” subpopulation. All concentrations decreased apoptotic spermatozoa and stimulated mitochondrial activity in viable spermatozoa, with 100 pM–1 µM increasing acrosomal damage, 10 nM–1 µM increasing intracellular calcium and 1 pM reducing the response to a calcium-ionophore challenge. In non-capacitating media, 1 µM increased hyperactivation-related variables and decreased apoptotic spermatozoa; 100 pM–1 µM increased membrane disorders (related to capacitation); all concentrations decreased mitochondrial ROS production. Melatonin concentrations had a modal effect on bull spermatozoa, suggesting a capacitation-modulating role and protective effect at physiological concentrations (pM). Some effects may be of practical use, considering artificial reproductive techniques.

Melatonin up-regulates bone marrow mesenchymal stem cells osteogenic action but suppresses their mediated osteoclastogenesis via MT2 -inactivated NF-κB pathway

BACKGROUND AND PURPOSE

Melatonin is a neurohormone involved in bone homeostasis. Melatonin directs bone remodelling and the role of bone marrow mesenchymal stem cells (BMMSCs) in the regulating melatonin-mediated bone formation-resorption balance remains undefined.

EXPERIMENTAL APPROACH

Osteoporosis models were established and bone tissue and serum were collected to test the effects of melatonin on bone homeostasis. Melatonin receptors were knocked down, the NF-κB signalling pathway and receptor activator of NF-κB ligand (RANKL) expression were investigated. Communication between bone marrow mesenchymal stem cells and osteoclasts was detected with direct-contact or indirect-contact system.

KEY RESULTS

Bone loss and microstructure disorder in mice were reversed after melatonin treatment, as a result of anabolic and anti-resorptive effects. In vitro, a physiological (low) concentration of melatonin promoted the bone marrow mesenchymal stem cells, osteogenic lineage commitment and extracellular mineralization but had no impact on extracellular matrix synthesis. After MT knockdown, especially MT₂ , the positive effects of melatonin on osteogenesis were attenuated. The canonical NF-κB signalling pathway was the first discovered downstream signalling pathway after MT receptor activation and was found to be down-regulated by melatonin during osteogenesis. Melatonin suppressed BMMSC-mediated osteoclastogenesis by inhibiting RANKL production in BMMSCs and this effect only occurred when BMMSCs and osteoclast precursors were co-cultured in an indirect-contact manner.

CONCLUSION AND IMPLICATIONS

Our work suggests that melatonin plays a crucial role in bone balance, significantly accelerates the osteogenic differentiation of bone marrow mesenchymal stem cells by suppressing the MT₂ -dependent NF-κB signalling pathway, and down-regulates osteoclastogenesis via RANKL paracrine secretion.

β-Arrestin-2 BRET Biosensors Detect Different β-Arrestin-2 Conformations in Interaction with GPCRs

β-Arrestins are critical regulators of G protein-coupled receptors (GPCRs) that desensitize G protein signaling, promote receptor internalization, and initiate signaling on their own. Recent structural findings indicate that β-arrestins adopt different conformations upon interaction with agonist-activated GPCRs. Here, we established a β-arrestin-2 conformational bioluminescence resonance energy transfer (BRET) sensor composed of the bright Nanoluc BRET donor and the red-shifted CyOFP1 BRET acceptor. The sensor monitors early intramolecular conformational changes of β-arrestin-2 in complex with a wide panel of different class A and class B GPCRs upon agonist activation and with orphan GPCRs known to spontaneously recruit β-arrestin-2. The introduction of the R170E mutant in the β-arrestin-2 sensor allowed the detection of a partially active β-arrestin-2 conformation, which is not dependent on receptor phosphorylation, while the deletion of the β-arrestin-2 finger-loop region detected the "tail-conformation" corresponding to the interaction of β-arrestin with the carboxyl-terminal domain of GPCRs. The new sensors combine the advantages of the BRET technique in terms of sensitivity, robustness, and suitability for real-time measurements with a high responsiveness toward early conformational changes to help to elucidate the different conformational states of β-arrestins associated with GPCR activation in living cells.

Melatonin and the control of intraocular pressure

Melatonin is not only synthesized by the pineal gland but by several ocular structures. This natural indoleamine is of great importance for regulating several eye processes, among which pressure homeostasis is included. Glaucoma, the most prevalent eye disease, also known as the silent thief of vision, is a multifactorial pathology that is associated to age and, often, to intraocular hypertension (IOP). Indeed IOP is the only modifiable risk factor and as such medications are available to control it; however, novel medications are sought to minimize undesirable side effects. Melatonin and analogues decrease IOP in both normotensive and hypertensive eyes. Melatonin activates its cognate membrane receptors, MT₁ and MT₂, which are present in numerous ocular tissues, including the aqueous-humor-producing ciliary processes. Melatonin receptors belong to the superfamily of G-protein-coupled receptors and their activation would lead to different signalling pathways depending on the tissue. This review describes the molecular mechanisms underlying differential functionalities that are attributed to melatonin receptors. Accordingly, the current work highlights the important role of melatonin and its analogues in the healthy and in the glaucomatous eyes, with special attention to the control of intraocular pressure.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

Differential Function of Melatonin MT1 and MT2 Receptors in REM and NREM Sleep

The pathophysiological function of the G-protein coupled melatonin MT1 and MT2 receptors has not yet been well-clarified. Recent advancements using selective MT1/ MT2 receptor ligands and MT1/MT2 receptor knockout mice have suggested that the activation of the MT1 receptors are mainly implicated in the regulation of rapid eye movement (REM) sleep, whereas the MT2 receptors selectively increase non-REM (NREM) sleep. Studies in mutant mice show that MT1 knockout mice have an increase in NREM sleep and a decrease in REM sleep, while MT2 knockout mice a decrease in NREM sleep. The localization of MT1 receptors is also distinct from MT2 receptors; for example, MT2 receptors are located in the reticular thalamus (NREM area), while the MT1 receptors in the Locus Coeruleus and lateral hypothalamus (REM areas). Altogether, these findings suggest that these two receptors not only have a very specialized function in sleep, but that they may also modulate opposing effects. These data also suggest that mixed MT1-MT2 receptors ligands are not clinically recommended given their opposite roles in physiological functions, confirmed by the modest effects of melatonin or MT1/MT2 non-selective agonists when used in both preclinical and clinical studies as hypnotic drugs. In sum, MT1 and MT2 receptors have specific roles in the modulation of sleep, and consequently, selective ligands with agonist, antagonist, or partial agonist properties could have therapeutic potential for sleep; while the MT2 agonists or partial agonists might be indicated for NREM-related sleep and/or anxiety disorders, the MT1 agonists or partial agonists might be so for REM-related sleep disorders. Furthermore, MT1 but not MT2 receptors seem involved in the regulation of the circadian rhythm. Future research will help further develop MT1 and/or MT2 receptors as targets for neuropsychopharmacology drug development.

Melatonin in type 2 diabetes mellitus and obesity

Despite considerable advances in the past few years, obesity and type 2 diabetes mellitus (T2DM) remain two major challenges for public health systems globally. In the past 9 years, genome-wide association studies (GWAS) have established a major role for genetic variation within the MTNR1B locus in regulating fasting plasma levels of glucose and in affecting the risk of T2DM. This discovery generated a major interest in the melatonergic system, in particular the melatonin MT₂ receptor (which is encoded by MTNR1B). In this Review, we discuss the effect of melatonin and its receptors on glucose homeostasis, obesity and T2DM. Preclinical and clinical post-GWAS evidence of frequent and rare variants of the MTNR1B locus confirmed its importance in regulating glucose homeostasis and T2DM risk with minor effects on obesity. However, these studies did not solve the question of whether melatonin is beneficial or detrimental, an issue that will be discussed in the context of the peculiarities of the melatonergic system. Melatonin receptors might have therapeutic potential as they belong to the highly druggable G protein-coupled receptor superfamily. Clarifying the precise role of melatonin and its receptors on glucose homeostasis is urgent, as melatonin is widely used for other indications, either as a prescribed medication or as a supplement without medical prescription, in many countries in Europe and in the USA.

Melatonin Synthesis and Function: Evolutionary History in Animals and Plants

Melatonin is an ancient molecule that can be traced back to the origin of life. Melatonin's initial function was likely that as a free radical scavenger. Melatonin presumably evolved in bacteria; it has been measured in both α-proteobacteria and in photosynthetic cyanobacteria. In early evolution, bacteria were phagocytosed by primitive eukaryotes for their nutrient value. According to the endosymbiotic theory, the ingested bacteria eventually developed a symbiotic association with their host eukaryotes. The ingested α-proteobacteria evolved into mitochondria while cyanobacteria became chloroplasts and both organelles retained their ability to produce melatonin. Since these organelles have persisted to the present day, all species that ever existed or currently exist may have or may continue to synthesize melatonin in their mitochondria (animals and plants) and chloroplasts (plants) where it functions as an antioxidant. Melatonin's other functions, including its multiple receptors, developed later in evolution. In present day animals, via receptor-mediated means, melatonin functions in the regulation of sleep, modulation of circadian rhythms, enhancement of immunity, as a multifunctional oncostatic agent, etc., while retaining its ability to reduce oxidative stress by processes that are, in part, receptor-independent. In plants, melatonin continues to function in reducing oxidative stress as well as in promoting seed germination and growth, improving stress resistance, stimulating the immune system and modulating circadian rhythms; a single melatonin receptor has been identified in land plants where it controls stomatal closure on leaves. The melatonin synthetic pathway varies somewhat between plants and animals. The amino acid, tryptophan, is the necessary precursor of melatonin in all taxa. In animals, tryptophan is initially hydroxylated to 5-hydroxytryptophan which is then decarboxylated with the formation of serotonin. Serotonin is either acetylated to N-acetylserotonin or it is methylated to form 5-methoxytryptamine; these products are either methylated or acetylated, respectively, to produce melatonin. In plants, tryptophan is first decarboxylated to tryptamine which is then hydroxylated to form serotonin.

Melatonin: Clinical Perspectives in Neurodegeneration

Prevention of neurodegenerative diseases is presently a major goal for our Society and melatonin, an unusual phylogenetically conserved molecule present in all aerobic organisms, merits consideration in this respect. Melatonin combines both chronobiotic and cytoprotective properties. As a chronobiotic, melatonin can modify phase and amplitude of biological rhythms. As a cytoprotective molecule, melatonin reverses the low degree inflammatory damage seen in neurodegenerative disorders and aging. Low levels of melatonin in blood characterizes advancing age. In experimental models of Alzheimer's disease (AD) and Parkinson's disease (PD) the neurodegeneration observed is prevented by melatonin. Melatonin also increased removal of toxic proteins by the brain glymphatic system. A limited number of clinical trials endorse melatonin's potentiality in AD and PD, particularly at an early stage of disease. Calculations derived from animal studies indicate cytoprotective melatonin doses in the 40-100 mg/day range. Hence, controlled studies employing melatonin doses in this range are urgently needed. The off-label use of melatonin is discussed.