RGS2 is a feedback inhibitor of melatonin production in the pineal gland

Influence of Circadian Rhythm in the Eye: Significance of Melatonin in Glaucoma

Circadian rhythm and the molecules involved in it, such as melanopsin and melatonin, play an important role in the eye to regulate the homeostasis and even to treat some ocular conditions. As a result, many ocular pathologies like dry eye, corneal wound healing, cataracts, myopia, retinal diseases, and glaucoma are affected by this cycle. This review will summarize the current scientific literature about the influence of circadian patterns on the eye, focusing on its relationship with increased intraocular pressure (IOP) fluctuations and glaucoma. Regarding treatments, two ways should be studied: the first one, to analyze if some treatments could improve their effect on the ocular disease when their posology is established in function of circadian patterns, and the second one, to evaluate new drugs to treat eye pathologies related to the circadian rhythm, as it has been stated with melatonin or its analogs, that not only could be used as the main treatment but as coadjutant, improving the circadian pattern or its antioxidant and antiangiogenic properties.

Single Cell Sequencing of the Pineal Gland: The Next Chapter

The analysis of pineal cell biology has undergone remarkable development as techniques have become available which allow for sequencing of entire transcriptomes and, most recently, the sequencing of the transcriptome of individual cells. Identification of at least nine distinct cell types in the rat pineal gland has been made possible, allowing identification of the precise cells of origin and expression of transcripts for the first time. Here the history and current state of knowledge generated by these transcriptomic efforts is reviewed, with emphasis on the insights suggested by the findings.

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.

Genome-Wide Screen for Genes Involved in Caenorhabditis elegans Developmentally Timed Sleep

In Caenorhabditis elegans, Notch signaling regulates developmentally timed sleep during the transition from L4 larval stage to adulthood (L4/A) . To identify core sleep pathways and to find genes acting downstream of Notch signaling, we undertook the first genome-wide, classical genetic screen focused on C. elegans developmentally timed sleep. To increase screen efficiency, we first looked for mutations that suppressed inappropriate anachronistic sleep in adult hsp::osm-11 animals overexpressing the Notch coligand OSM-11 after heat shock. We retained suppressor lines that also had defects in L4/A developmentally timed sleep, without heat shock overexpression of the Notch coligand. Sixteen suppressor lines with defects in developmentally timed sleep were identified. One line carried a new allele of goa-1; loss of GOA-1 Gαo decreased C. elegans sleep. Another line carried a new allele of gpb-2, encoding a Gβ5 protein; Gβ5 proteins have not been previously implicated in sleep. In other scenarios, Gβ5 GPB-2 acts with regulators of G protein signaling (RGS proteins) EAT-16 and EGL-10 to terminate either EGL-30 Gαq signaling or GOA-1 Gαo signaling, respectively. We found that loss of Gβ5 GPB-2 or RGS EAT-16 decreased L4/A sleep. By contrast, EGL-10 loss had no impact. Instead, loss of RGS-1 and RGS-2 increased sleep. Combined, our results suggest that, in the context of L4/A sleep, GPB-2 predominantly acts with EAT-16 RGS to inhibit EGL-30 Gαq signaling. These results confirm the importance of G protein signaling in sleep and demonstrate that these core sleep pathways function genetically downstream of the Notch signaling events promoting sleep.

RGS2 expression predicts amyloid-β sensitivity, MCI and Alzheimer's disease: genome-wide transcriptomic profiling and bioinformatics data mining

Alzheimer's disease (AD) is the most frequent cause of dementia. Misfolded protein pathological hallmarks of AD are brain deposits of amyloid-β (Aβ) plaques and phosphorylated tau neurofibrillary tangles. However, doubts about the role of Aβ in AD pathology have been raised as Aβ is a common component of extracellular brain deposits found, also by in vivo imaging, in non-demented aged individuals. It has been suggested that some individuals are more prone to Aβ neurotoxicity and hence more likely to develop AD when aging brains start accumulating Aβ plaques. Here, we applied genome-wide transcriptomic profiling of lymphoblastoid cells lines (LCLs) from healthy individuals and AD patients for identifying genes that predict sensitivity to Aβ. Real-time PCR validation identified 3.78-fold lower expression of RGS2 (regulator of G-protein signaling 2; P=0.0085) in LCLs from healthy individuals exhibiting high vs low Aβ sensitivity. Furthermore, RGS2 showed 3.3-fold lower expression (P=0.0008) in AD LCLs compared with controls. Notably, RGS2 expression in AD LCLs correlated with the patients' cognitive function. Lower RGS2 expression levels were also discovered in published expression data sets from postmortem AD brain tissues as well as in mild cognitive impairment and AD blood samples compared with controls. In conclusion, Aβ sensitivity phenotyping followed by transcriptomic profiling and published patient data mining identified reduced peripheral and brain expression levels of RGS2, a key regulator of G-protein-coupled receptor signaling and neuronal plasticity. RGS2 is suggested as a novel AD biomarker (alongside other genes) toward early AD detection and future disease modifying therapeutics.

Degradation of Serotonin N-Acetyltransferase, a Circadian Regulator, by the N-end Rule Pathway

Serotonin N-acetyltransferase (AANAT) converts serotonin to N-acetylserotonin (NAS), a distinct biological regulator and the immediate precursor of melatonin, a circulating hormone that influences circadian processes, including sleep. N-terminal sequences of AANAT enzymes vary among vertebrates. Mechanisms that regulate the levels of AANAT are incompletely understood. Previous findings were consistent with the possibility that AANAT may be controlled through its degradation by the N-end rule pathway. By expressing the rat and human AANATs and their mutants not only in mammalian cells but also in the yeast Saccharomyces cerevisiae, and by taking advantage of yeast genetics, we show here that two "complementary" forms of rat AANAT are targeted for degradation by two "complementary" branches of the N-end rule pathway. Specifically, the N(α)-terminally acetylated (Nt-acetylated) Ac-AANAT is destroyed through the recognition of its Nt-acetylated N-terminal Met residue by the Ac/N-end rule pathway, whereas the non-Nt-acetylated AANAT is targeted by the Arg/N-end rule pathway, which recognizes the unacetylated N-terminal Met-Leu sequence of rat AANAT. We also show, by constructing lysine-to-arginine mutants of rat AANAT, that its degradation is mediated by polyubiquitylation of its Lys residue(s). Human AANAT, whose N-terminal sequence differs from that of rodent AANATs, is longer-lived than its rat counterpart and appears to be refractory to degradation by the N-end rule pathway. Together, these and related results indicate both a major involvement of the N-end rule pathway in the control of rodent AANATs and substantial differences in the regulation of rodent and human AANATs that stem from differences in their N-terminal sequences.

The world of protein acetylation

Acetylation is one of the major post-translational protein modifications in the cell, with manifold effects on the protein level as well as on the metabolome level. The acetyl group, donated by the metabolite acetyl-coenzyme A, can be co- or post-translationally attached to either the α-amino group of the N-terminus of proteins or to the ε-amino group of lysine residues. These reactions are catalyzed by various N-terminal and lysine acetyltransferases. In case of lysine acetylation, the reaction is enzymatically reversible via tightly regulated and metabolism-dependent mechanisms. The interplay between acetylation and deacetylation is crucial for many important cellular processes. In recent years, our understanding of protein acetylation has increased significantly by global proteomics analyses and in depth functional studies. This review gives a general overview of protein acetylation and the respective acetyltransferases, and focuses on the regulation of metabolic processes and physiological consequences that come along with protein acetylation.

Adenosine triphosphate inhibits melatonin synthesis in the rat pineal gland

Adenosine triphosphate (ATP) is released onto the pinealocyte, along with noradrenaline, from sympathetic neurons and triggers P2Y1 receptors that enhance β-adrenergic-induced N-acetylserotonin (NAS) synthesis. Nevertheless, the biotransformation of NAS into melatonin, which occurs due to the subsequent methylation by acetylserotonin O-methyltransferase (ASMT; EC 2.1.1.4), has not yet been evaluated in the presence of purinergic stimulation. We therefore evaluated the effects of purinergic signaling on melatonin synthesis induced by β-adrenergic stimulation. ATP increased NAS levels, but, surprisingly, inhibited melatonin synthesis in an inverse, concentration-dependent manner. Our results demonstrate that enhanced NAS levels, which depend on phospholipase C (PLC) activity (but not the induction of gene transcription), are a post-translational effect. By contrast, melatonin reduction is related to an ASMT inhibition of expression at both the gene transcription and protein levels. These results were independent of nuclear factor-kappa B (NF-kB) translocation. Neither the P2Y1 receptor activation nor the PLC-mediated pathway was involved in the decrease in melatonin, indicating that ATP regulates pineal metabolism through different mechanisms. Taken together, our data demonstrate that purinergic signaling differentially modulates NAS and melatonin synthesis and point to a regulatory role for ATP as a cotransmitter in the control of ASMT, the rate-limiting enzyme in melatonin synthesis. The endogenous production of melatonin regulates defense responses; therefore, understanding the mechanisms involving ASMT regulation might provide novel insights into the development and progression of neurological disorders since melatonin presents anti-inflammatory, neuroprotective, and neurogenic effects.

Control of mammalian G protein signaling by N-terminal acetylation and the N-end rule pathway

Rgs2, a regulator of G proteins, lowers blood pressure by decreasing signaling through Gαq. Human patients expressing Met-Leu-Rgs2 (ML-Rgs2) or Met-Arg-Rgs2 (MR-Rgs2) are hypertensive relative to people expressing wild-type Met-Gln-Rgs2 (MQ-Rgs2). We found that wild-type MQ-Rgs2 and its mutant, MR-Rgs2, were destroyed by the Ac/N-end rule pathway, which recognizes N(α)-terminally acetylated (Nt-acetylated) proteins. The shortest-lived mutant, ML-Rgs2, was targeted by both the Ac/N-end rule and Arg/N-end rule pathways. The latter pathway recognizes unacetylated N-terminal residues. Thus, the Nt-acetylated Ac-MX-Rgs2 (X = Arg, Gln, Leu) proteins are specific substrates of the mammalian Ac/N-end rule pathway. Furthermore, the Ac/N-degron of Ac-MQ-Rgs2 was conditional, and Teb4, an endoplasmic reticulum (ER) membrane-embedded ubiquitin ligase, was able to regulate G protein signaling by targeting Ac-MX-Rgs2 proteins for degradation through their N(α)-terminal acetyl group.