Targeted deletions of Mel1a and Mel1b melatonin receptors affect pCREB levels in lactotroph and pars intermedia cells of mice

Melatonin-induced ApoE expression in mouse astrocytes protects endothelial cells from OGD-R induced injuries

Stroke is a leading reason of death and long-term disability, and most studies mainly focus on efforts to protect neurons. However, failed clinical trials suggest that therapies against single target in neurons may not be sufficient and the involvement of endothelial cells and glial cells have been underestimated. Astrocytes are the major source of ApoE in the brain and endothelial cells express high level of ApoE receptors. Thus, ApoE may mediate the interaction between astrocytes and endothelial cells. To address whether and how ApoE-mediated astrocytes-endothelial cells interaction contributes to the pathogenesis of stroke, we used oxygen and glucose deprivation-reoxygenation (OGD-R) as a stroke model and investigated the effects of OGD-R on astrocytes-endothelial cell co-cultures in the current study. We find that OGD-R leads to various damages to endothelial cells, including compromised cell viability, increased ROS level, enhanced caspase activity, and higher apoptotic rate. Meanwhile, mouse astrocytes could secrete ApoE to activate PI3K/eNOS signaling in endothelial cells to prevent OGD-R induced injuries. In addition, OGD-R induces down-regulation of ApoE in astrocyte-endothelial cell co-cultures while melatonin restores astrocytic ApoE expression via pCREB pathway and protects endothelial cell in OGD-R treated co-cultures. Our study provides evidence that astrocytes could protect endothelial cells via ApoE in OGD-R condition and Melatonin could induce ApoE expression to protect endothelial cells.

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 prevents adverse myocardial infarction remodeling via Notch1/Mfn2 pathway

Mitochondrial dysfunction is linked with myocardial infarction (MI), a disorder in which Notch1 has attracted increasing attention. However, the involvement of Notch1 in mitochondrial impairment after an MI is poorly understood, as is the role of mitochondrial fusion-associated protein 2 (Mfn2). Moreover, whether melatonin potentiates the Notch1/Mfn2 pathway in post-MI cardiac damage remains unclear. In our study, small interfering RNAs against Notch1 or Mfn2 and Jagged1 peptide were delivered via intramyocardial injection. At 3 days after these treatments, MI was induced by ligation of the anterior descending branch. We found that this ablation of Notch1 or Mfn2 aggravated post-MI injury, including worsened mitochondrial damage and increased generation of reactive oxygen species (ROS). In contrast, Jagged1 improved mitochondrial structure and function, decreased ROS production and attenuated post-MI injury. Interestingly, though Mfn2 expression was mildly regulated by Notch1 signaling in myocardium, Mfn2 deficiency nearly eliminated the cardioprotection by Jagged1, as evidenced by suppressed cardiac function, aggravated myocardial fibrosis, increased cell apoptosis, worsened mitochondrial impairment and enhanced oxidative stress. These observations revealed that Mfn2 plays an indispensable role in protection against MI-induced injury by Notch1. The mechanism might involve disrupting a damaging cycle of mitochondrial damage and ROS generation. Furthermore, melatonin activated Notch1 signaling and increased Mfn2 expression were reversed by luzindole, a nonselective antagonist of the melatonin receptor. Notably, melatonin attenuated post-MI injury in normal mice, but not in mice deficient in Notch1 or Mfn2. These results demonstrate that melatonin attenuates post-MI injury via the Notch1/Mfn2 pathway in a receptor-dependent manner.

Reduced silent information regulator 1 signaling exacerbates myocardial ischemia-reperfusion injury in type 2 diabetic rats and the protective effect of melatonin

Diabetes mellitus (DM) increases myocardial oxidative stress and endoplasmic reticulum (ER) stress. Melatonin confers cardioprotective effect by suppressing oxidative damage. However, the effect and mechanism of melatonin on myocardial ischemia-reperfusion (MI/R) injury in type 2 diabetic state are still unknown. In this study, we developed high-fat diet-fed streptozotocin (HFD-STZ) rat, a well-known type 2 diabetic model, to evaluate the effect of melatonin on MI/R injury with a focus on silent information regulator 1 (SIRT1) signaling, oxidative stress, and PERK/eIF2α/ATF4-mediated ER stress. HFD-STZ treated rats were exposed to melatonin treatment in the presence or the absence of sirtinol (a SIRT1 inhibitor) and subjected to MI/R surgery. Compared with nondiabetic animals, type 2 diabetic rats exhibited significantly decreased myocardial SIRT1 signaling, increased apoptosis, enhanced oxidative stress, and ER stress. Additionally, further reduced SIRT1 signaling, aggravated oxidative damage, and ER stress were found in diabetic animals subjected to MI/R surgery. Melatonin markedly reduced MI/R injury by improving cardiac functional recovery and decreasing myocardial apoptosis in type 2 diabetic animals. Melatonin treatment up-regulated SIRT1 expression, reduced oxidative damage, and suppressed PERK/eIF2α/ATF4 signaling. However, these effects were all attenuated by SIRT1 inhibition. Melatonin also protected high glucose/high fat cultured H9C2 cardiomyocytes against simulated ischemia-reperfusion injury-induced ER stress by activating SIRT1 signaling while SIRT1 siRNA blunted this action. Taken together, our study demonstrates that reduced cardiac SIRT1 signaling in type 2 diabetic state aggravates MI/R injury. Melatonin ameliorates reperfusion-induced oxidative stress and ER stress via activation of SIRT1 signaling, thus reducing MI/R damage and improving cardiac function.

Understanding melatonin receptor pharmacology: latest insights from mouse models, and their relevance to human disease

Melatonin, the neuro-hormone synthesized during the night, has recently seen an unexpected extension of its functional implications toward type 2 diabetes development, visual functions, sleep disturbances, and depression. Transgenic mouse models were instrumental for the establishment of the link between melatonin and these major human diseases. Most of the actions of melatonin are mediated by two types of G protein-coupled receptors, named MT1 and MT2 , which are expressed in many different organs and tissues. Understanding the pharmacology and function of mouse MT1 and MT2 receptors, including MT1 /MT2 heteromers, will be of crucial importance to evaluate the relevance of these mouse models for future therapeutic developments. This review will critically discuss these aspects, and give some perspectives including the generation of new mouse models.

Influence of melatonin receptor signalling on parameters involved in blood glucose regulation

The pineal hormone melatonin is known to influence insulin secretion via the G-protein-coupled receptor isoforms MT1 and MT2. The present study was aimed to further elucide the impact of melatonin on blood glucose regulation. To this end, mouse lines were used, in which one of the two or both melatonin receptors were deleted. In comparison with wild-type mice of the same age (8-12 months old), increased plasma insulin and melatonin levels and decreased blood glucose levels and body weights were detected in the MT1- and double-knockout lines. The elimination of melatonin receptor signalling also altered blood glucose concentrations, body weight and melatonin and insulin levels when comparing wild-type and receptor knockout mice of different ages (6 wk and 8-12 months old); such changes, however, were dependent on the type of receptor deleted. Furthermore, reverse transcription polymerase chain reaction results provided evidence that melatonin receptor deficiency has an impact on transcript levels of pancreatic islet hormones as well as on pancreatic and hepatic glucose transporters (Glut1 and 2). Under stimulated insulin secretion in the presence of melatonin in the rat insulinoma β-cells INS-1, the Glut1 transcript level was decreased. In conclusion, the present findings demonstrate that melatonin receptor knockout types affect blood glucose levels, body weight, plasma levels of melatonin and insulin, as well as pancreatic hormone and Glut1 expression in significantly different manners.

Melatonin influences insulin secretion primarily via MT(1) receptors in rat insulinoma cells (INS-1) and mouse pancreatic islets

Several studies have revealed that melatonin affects the insulin secretion via MT(1) and MT(2) receptor isoforms. Owing to the lack of selective MT(1) receptor antagonists, we used RNA interference technology to generate an MT(1) knockdown in a clonal β-cell line to evaluate whether melatonin modulates insulin secretion specifically via the MT(1) receptor. Incubation experiments were carried out, and the insulin concentration in supernatants was measured using a radioimmunoassay. Furthermore, the intracellular cAMP was determined using an enzyme-linked immunosorbent assay. Real-time RT-PCR indicated that MT(1) knockdown resulted in a significant increase in the rIns1 mRNA and a significantly elevated basal insulin secretion of INS-1 cells. Incubation with melatonin decreased the amount of glucagon-like peptide 1 or inhibited the glucagon-stimulated insulin release of INS-1 cells, while, in MT(1) -knockdown cells, no melatonin-induced reduction in insulin secretion could be found. No decrease in 3-isobutyl-1-methylxanthine-stimulated intracellular cAMP in rMT(1) -knockdown cells was detectable after treatment with melatonin either, and immunocytochemistry proved that MT(1) knockdown abolished phosphorylation of cAMP-response-element-binding protein. In contrast to the INS-1 cells, preincubation with melatonin did not sensitize the insulin secretion of rMT(1) -knockdown cells. We also monitored insulin secretion from isolated islets of wild-type and melatonin-receptor knockout mice ex vivo. In islets of wild-type mice, melatonin treatment resulted in a decrease in insulin release, whereas melatonin treatment of islets from MT(1) knockout and MT(1/2) double-knockout mice did not show a significant effect. The data indicate that melatonin inhibits insulin secretion, primarily via the MT(1) receptor in rat INS-1 cells and isolated mouse islets.