Expression of the orphan GPR50 protein in rodent and human dorsomedial hypothalamus, tanycytes and median eminence

Thirty-seven years of MT1 and MT2 melatonin receptor localization in the brain: Past and future challenges

Identifying the target cells of a hormone is a key step in understanding its function. Once the molecular nature of the receptors for a hormone has been established, researchers can use several techniques to detect these receptors. Here I will review the different tools used over the years to localize melatonin receptors and the problems associated with each of these techniques. The radioligand 2-[125I] iodomelatonin was the first tool to allow localization of melatonin receptors on tissue sections. Once the MT1 and MT2 receptors were cloned, in situ hybridization could be used to detect the messenger RNA for these receptors. The deduced amino acid sequences for MT1 and MT2 receptors allowed the production of peptide immunogens to generate antibodies against the MT1 and MT2 receptors. Finally, transgenic reporters driven by the promoter elements of the MT1 and MT2 genes have been used to map the expression of MT1 and MT2 in the brain and the retina. Several issues have complicated the localization of melatonin receptors and the characterization of melatonin target cells over the last three decades. Melatonin receptors are expressed at low levels, leading to sensitivity issues for their detection. The second problem are specificity issues with antibodies directed against the MT1 and MT2 melatonin receptors. These receptors are G protein-coupled receptors and many antibodies directed against such receptors have been shown to present similar problems concerning their specificity. Despite these specificity problems which start to be seriously addressed by recent studies, antibodies will be important tools in the future to identify and phenotype melatonin target cells. However, we will have to be more stringent than previously when establishing their specificity. The results obtained by these antibodies will have to be confronted and be coherent with results obtained by other techniques.

Implications of Hypothalamic Neural Stem Cells on Aging and Obesity-Associated Cardiovascular Diseases

The hypothalamus, one of the major regulatory centers in the brain, controls various homeostatic processes, and hypothalamic neural stem cells (htNSCs) have been observed to interfere with hypothalamic mechanisms regulating aging. NSCs play a pivotal role in the repair and regeneration of brain cells during neurodegenerative diseases and rejuvenate the brain tissue microenvironment. The hypothalamus was recently observed to be involved in neuroinflammation mediated by cellular senescence. Cellular senescence, or systemic aging, is characterized by a progressive irreversible state of cell cycle arrest that causes physiological dysregulation in the body and it is evident in many neuroinflammatory conditions, including obesity. Upregulation of neuroinflammation and oxidative stress due to senescence has the potential to alter the functioning of NSCs. Various studies have substantiated the chances of obesity inducing accelerated aging. Therefore, it is essential to explore the potential effects of htNSC dysregulation in obesity and underlying pathways to develop strategies to address obesity-induced comorbidities associated with brain aging. This review will summarize hypothalamic neurogenesis associated with obesity and prospective NSC-based regenerative therapy for the treatment of obesity-induced cardiovascular conditions.

Singular Adult Neural Stem Cells Do Not Exist

Adult neural stem cells (aNSCs) are the source for the continuous production of new neurons throughout life. This so-called adult neurogenesis has been extensively studied; the intermediate cellular stages are well documented. Recent discoveries have raised new controversies in the field, such as the notion that progenitor cells hold similar self-renewal potential as stem cells, or whether different types of aNSCs exist. Here, we discuss evidence for heterogeneity of aNSCs, including short-term and long-term self-renewing aNSCs, regional and temporal differences in aNSC function, and single cell transcriptomics. Reviewing various genetic mouse models used for targeting aNSCs and lineage tracing, we consider potential lineage relationships between Ascl1-, Gli1-, and Nestin-targeted aNSCs. We present a multidimensional model of adult neurogenesis that incorporates recent findings and conclude that stemness is a phenotype, a state of properties that can change with time, rather than a cell property, which is static and immutable. We argue that singular aNSCs do not exist.

Modeling the Heterodimer Interfaces of Melatonin Receptors

Melatonin receptors are Class A G protein-coupled receptors (GPCRs) that regulate a plethora of physiological activities in response to the rhythmic secretion of melatonin from the pineal gland. Melatonin is a key regulator in the control of circadian rhythm and has multiple functional roles in retinal physiology, memory, immunomodulation and tumorigenesis. The two subtypes of human melatonin receptors, termed MT₁ and MT₂, utilize overlapping signaling pathways although biased signaling properties have been reported in some cellular systems. With the emerging concept of GPCR dimerization, melatonin receptor heterodimers have been proposed to participate in system-biased signaling. Here, we used computational approaches to map the dimerization interfaces of known heterodimers of melatonin receptors, including MT₁/MT₂, MT₁/GPR50, MT₂/GPR50, and MT₂/5-HT_2C. By homology modeling and membrane protein docking analyses, we have identified putative preferred interface interactions within the different pairs of melatonin receptor dimers and provided plausible structural explanations for some of the unique pharmacological features of specific heterodimers previously reported. A thorough understanding of the molecular basis of melatonin receptor heterodimers may enable the development of new therapeutic approaches against aliments involving these heterodimeric receptors.

What is the physiological role of hypothalamic tanycytes in metabolism?

In vertebrates, the energy balance process is tightly controlled by complex neural circuits that sense metabolic signals and adjust food intake and energy expenditure in line with the physiological requirements of optimal conditions. Within neural networks controlling energy balance, tanycytes are peculiar ependymoglial cells that are nowadays recognized as multifunctional players in the metabolic hypothalamus. However, the physiological function of hypothalamic tanycytes remains unclear, creating a number of ambiguities in the field. Here, we review data accumulated over the years that demonstrate the physiological function of tanycytes in the maintenance of metabolic homeostasis, opening up new research avenues. The presumed involvement of tanycytes in the pathophysiology of metabolic disorders and age-related neurodegenerative diseases will be finally discussed.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

GPR50-Ctail cleavage and nuclear translocation: a new signal transduction mode for G protein-coupled receptors

Transmission of extracellular signals by G protein-coupled receptors typically relies on a cascade of intracellular events initiated by the activation of heterotrimeric G proteins or β-arrestins followed by effector activation/inhibition. Here, we report an alternative signal transduction mode used by the orphan GPR50 that relies on the nuclear translocation of its carboxyl-terminal domain (CTD). Activation of the calcium-dependent calpain protease cleaves off the CTD from the transmembrane-bound GPR50 core domain between Phe-408 and Ser-409 as determined by MALDI-TOF-mass spectrometry. The cytosolic CTD then translocates into the nucleus assisted by its 'DPD' motif, where it interacts with the general transcription factor TFII-I to regulate c-fos gene transcription. RNA-Seq analysis indicates a broad role of the CTD in modulating gene transcription with ~ 8000 differentially expressed genes. Our study describes a non-canonical, direct signaling mode of GPCRs to the nucleus with similarities to other receptor families such as the NOTCH receptor.

Tetramethylpyrazine Attenuated Sevoflurane-Induced Neurotoxicity by Enhancing Autophagy through GPR50/CREB Pathway in SH-SY5Y Cells

Tetramethylpyrazine has shown neuroprotective and axonal outgrowth-promoting effects and can improve cognitive deficit in a rat model of chronic hypoperfusion. However, the role of tetramethylpyrazine in sevoflurane-induced neurotoxicity is still vague. Therefore, this study was designed to investigate the effects and mechanisms of tetramethylpyrazine on sevoflurane-induced autophagy, apoptosis, and the expression of BACE1 and A[Formula: see text] in SH-SY5Y cells. We measured the expression levels of the apoptosis protein markers Bax and Bcl-2, autophagy protein markers Atg5 and LC3-II, BACE1, and A[Formula: see text] in SH-SY5Y cells after sevoflurane treatment and determined the effects of tetramethylpyrazine on sevoflurane-induced expression of these proteins after silencing GPR50 or Atg5 with siRNA in vitro. We found that exposure to 3.4% sevoflurane for 6 h decreased the expression of autophagy protein markers and increased the expression of the apoptosis protein markers, BACE1, and A[Formula: see text] in SH-SY5Y cells. The number of red puncta (autolysosomes) and yellow puncta (autophagosomes) in each SH-SY5Y cell decreased after transient transfection with the mRFP-GFP-LC3 expression plasmid. Silencing of GPR50 decreased the expression of pCREB, Atg5, and LC3-II, while silencing of Atg5 increased the expression of BACE1 and A[Formula: see text] in SH-SY5Y cells. Our results demonstrate that tetramethylpyrazine attenuated sevoflurane-induced neurotoxicity by enhancing autophagy through the GPR50/CREB pathway in SH-SY5Y cells.

Association of the intronic rs2072621 polymorphism of the X-linked GPR50 gene with affective disorder with seasonal pattern

This case-control study found an association between Seasonal Affective Disorder (SAD) and a single nucleotide polymorphism (intronic rs2072621) of the gene encoding GPR50 (an orphan member of the G protein-coupled melatonin receptor subfamily) in females. This may represent a gender-specific risk factor and a molecular link between melatonin and SAD.

The role of orphan G protein-coupled receptors in the pathophysiology of multiple sclerosis: A review

G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that are expressed in many organs and serve as important drug targets. A new subgroup, namely orphan GPCRs, comprising many of these receptors has been discovered. These receptors exhibit diverse physiological functions and have been considered in many neurological disorders including Alzheimer's disease, Parkinson's disease, and multiple sclerosis (MS). GPR17, GPR30, GPR37, GPR40, GPR50, GPR54, GPR56, GPR65, GPR68, GPR75, GPR84, GPR97, GPR109, GPR124, and GPR126 are orphan GPCRs that have been reported with considerable effects in the prevention and/or treatment of MS in preclinical studies. In the present article, we reviewed the most recent findings regarding the role of orphan GPCRs in the treatment of MS.

Detection of recombinant and endogenous mouse melatonin receptors by monoclonal antibodies targeting the C-terminal domain

Melatonin receptors play important roles in the regulation of circadian and seasonal rhythms, sleep, retinal functions, the immune system, depression, and type 2 diabetes development. Melatonin receptors are approved drug targets for insomnia, non-24-hour sleep-wake disorders, and major depressive disorders. In mammals, two melatonin receptors (MTRs) exist, MT₁ and MT₂ , belonging to the G protein-coupled receptor (GPCR) superfamily. Similar to most other GPCRs, reliable antibodies recognizing melatonin receptors proved to be difficult to obtain. Here, we describe the development of the first monoclonal antibodies (mABs) for mouse MT₁ and MT₂ . Purified antibodies were extensively characterized for specific reactivity with mouse, rat, and human MT₁ and MT₂ by Western blot, immunoprecipitation, immunofluorescence, and proximity ligation assay. Several mABs were specific for either mouse MT₁ or MT₂ . None of the mABs cross-reacted with rat MTRs, and some were able to react with human MTRs. The specificity of the selected mABs was validated by immunofluorescence microscopy in three established locations (retina, suprachiasmatic nuclei, pituitary gland) for MTR expression in mice using MTR-KO mice as control. MT₂ expression was not detected in mouse insulinoma MIN6 cells or pancreatic beta-cells. Collectively, we report the first monoclonal antibodies recognizing recombinant and native mouse melatonin receptors that will be valuable tools for future studies.

Tanycyte Gene Expression Dynamics in the Regulation of Energy Homeostasis

Animal survival relies on a constant balance between energy supply and energy expenditure, which is controlled by several neuroendocrine functions that integrate metabolic information and adapt the response of the organism to physiological demands. Polarized ependymoglial cells lining the floor of the third ventricle and sending a single process within metabolic hypothalamic parenchyma, tanycytes are henceforth described as key components of the hypothalamic neural network controlling energy balance. Their strategic position and peculiar properties convey them diverse physiological functions ranging from blood/brain traffic controllers, metabolic modulators, and neural stem/progenitor cells. At the molecular level, these functions rely on an accurate regulation of gene expression. Indeed, tanycytes are characterized by their own molecular signature which is mostly associated to their diverse physiological functions, and the detection of variations in nutrient/hormone levels leads to an adequate modulation of genetic profile in order to ensure energy homeostasis. The aim of this review is to summarize recent knowledge on the nutritional control of tanycyte gene expression.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

A comparative study of the neural stem cell niche in the adult hypothalamus of human, mouse, rat and gray mouse lemur (Microcebus murinus)

The adult brain contains niches of neural stem cells that continuously add new neurons to selected circuits throughout life. Two niches have been extensively studied in various mammalian species including humans, the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. Recently, studies conducted mainly in rodents have identified a third neurogenic niche in the adult hypothalamus. In order to evaluate whether a neural stem cell niche also exists in the adult hypothalamus in humans, we performed multiple immunofluorescence labeling to assess the expression of a panel of neural stem/progenitor cell (NPC) markers (Sox2, nestin, vimentin, GLAST, GFAP) in the human hypothalamus and compared them with the mouse, rat and a non-human primate species, the gray mouse lemur (Microcebus murinus). Our results show that the adult human hypothalamus contains four distinct populations of cells that express the five NPC markers: (a) a ribbon of small stellate cells that lines the third ventricular wall behind a hypocellular gap, similar to that found along the lateral ventricles, (b) ependymal cells, (c) tanycytes, which line the floor of the third ventricle in the tuberal region, and (d) a population of small stellate cells in the suprachiasmatic nucleus. In the mouse, rat and mouse lemur hypothalamus, co-expression of NPC markers is primarily restricted to tanycytes, and these species lack a ventricular ribbon. Our work thus identifies four cell populations with the antigenic profile of NPCs in the adult human hypothalamus, of which three appear specific to humans.

The orphan GPR50 receptor promotes constitutive TGFβ receptor signaling and protects against cancer development

Transforming growth factor-β (TGFβ) signaling is initiated by the type I, II TGFβ receptor (TβRI/TβRII) complex. Here we report the formation of an alternative complex between TβRI and the orphan GPR50, belonging to the G protein-coupled receptor super-family. The interaction of GPR50 with TβRI induces spontaneous TβRI-dependent Smad and non-Smad signaling by stabilizing the active TβRI conformation and competing for the binding of the negative regulator FKBP12 to TβRI. GPR50 overexpression in MDA-MB-231 cells mimics the anti-proliferative effect of TβRI and decreases tumor growth in a xenograft mouse model. Inversely, targeted deletion of GPR50 in the MMTV/Neu spontaneous mammary cancer model shows decreased survival after tumor onset and increased tumor growth. Low GPR50 expression is associated with poor survival prognosis in human breast cancer irrespective of the breast cancer subtype. This describes a previously unappreciated spontaneous TGFβ-independent activation mode of TβRI and identifies GPR50 as a TβRI co-receptor with potential impact on cancer development.

Transforming growth factor-β (TGFβ) regulates many cellular processes. Here the authors show that the orphan G-protein coupled receptor GPR50 can activate the TGFβ receptor I, in the absence of TGFβ, by stabilizing its active conformation and show antitumor activity in a mouse model of breast cancer.

Characterization of the Mel1c melatoninergic receptor in platypus (Ornithorhynchus anatinus)

Melatonin is a neurohormone produced in both animals and plants. It binds at least three G-protein-coupled receptors: MT1 and MT2, and Mel1cGPR. Mammalian GPR50 evolved from the reptilian/avian Mel1c and lost its capacity to bind melatonin in all the therian mammal species that have been tested. In order to determine if binding is lost in the oldest surviving mammalian lineage of monotremes we investigated whether the melatonin receptor has the ability to bind melatonin in the platypus (Ornithorhynchus anatinus), and evaluated its pharmacological profile. Sequence and phylogenetic analysis showed that platypus has in fact retained the ancestral Mel1c and has the capacity to bind melatonin similar to other mammalian melatonin receptors (MT1 and MT2), with an affinity in the 1 nM range. We also investigated the binding of a set of melatoninergic ligands used previously to characterize the molecular pharmacology of the melatonin receptors from sheep, rats, mice, and humans and found that the general profiles of these compounds make Mel1c resemble human MT1 more than MT2. This work shows that the loss of GPR50 binding evolved after the divergence of monotremes less than 190MYA in therian mammals.

Orphan G protein-coupled receptors: The role in CNS disorders

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

Orphan GPR61, GPR62 and GPR135 receptors and the melatonin MT2 receptor reciprocally modulate their signaling functions

Understanding the function of orphan G protein-coupled receptors (GPCRs), whose cognate ligand is unknown, is of major importance as GPCRs are privileged drug targets for many diseases. Recent phylogenetic studies classified three orphan receptors, GPR61, GPR62 and GPR135 among the melatonin receptor subfamily, but their capacity to bind melatonin and their biochemical functions are not well characterized yet. We show here that GPR61, GPR62 and GPR135 do not bind [³H]-melatonin nor 2-[¹²⁵I]iodomelatonin and do not respond to melatonin in several signaling assays. In contrast, the three receptors show extensive spontaneous ligand-independent activities on the cAMP, inositol phosphate and ß-arrestin pathways with distinct pathway-specific profiles. Spontaneous ß-arrestin recruitment internalizes all three GPRs in the endosomal compartment. Co-expression of the melatonin binding MT₂ receptor with GPR61, GPR62 or GPR135 has several consequences such as (i) the formation of receptor heteromers, (ii) the inhibition of melatonin-induced ß-arrestin2 recruitment to MT₂ and (iii) the decrease of elevated cAMP levels upon melatonin stimulation in cells expressing spontaneously active GPR61 and GPR62. Collectively, these data show that GPR61, GPR62 and GPR135 are unable to bind melatonin, but show a reciprocal regulatory interaction with MT₂ receptors.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

Tanycytes As Regulators of Seasonal Cycles in Neuroendocrine Function

Annual cycles of physiology and behavior are highly prevalent in organisms inhabiting temperate and polar regions. Examples in mammals include changes in appetite and body fat composition, hibernation and torpor, growth of antlers, pelage and horns, and seasonal reproduction. The timing of these seasonal cycles reflects an interaction of changing environmental signals, such as daylength, and intrinsic rhythmic processes: circannual clocks. As neuroendocrine signals underlie these rhythmic processes, the focus of most mechanistic studies has been on neuronal systems in the hypothalamus. Recent studies also implicate the pituitary stalk (pars tuberalis) and hypothalamic tanycytes as key pathways in seasonal timing. The pars tuberalis expresses a high density of melatonin receptors, so is highly responsive to changes in the nocturnal secretion of melatonin from the pineal gland as photoperiod changes across the year. The pars tuberalis in turn regulates tanycyte function in the adjacent hypothalamus via paracrine signals. Tanycytes are radial glial cells that persist into adulthood and function as a stem cell niche. Their cell soma are embedded in the ependymal lining of the third ventricle, and they also send elaborate projections through the arcuate nucleus, many of which terminate on capillaries in the median eminence. This anatomy underlies their function as sensors of nutrients in the circulation, and as regulators of transport of hormones and metabolites into the hypothalamus. In situ hybridization studies reveal robust seasonal changes in gene expression in tanycytes, for example, those controlling transport and metabolism of thyroid hormone and retinoic acid. These hormonal signals play a key role in the initial development of the brain, and experimental manipulation of thyroid hormone availability in the adult hypothalamus can accelerate or block seasonal cyclicity in sheep and Siberian hamsters. We hypothesize that seasonal rhythms depends upon reuse of developmental mechanisms in the adult hypothalamus and that tanycytes are key orchestrators of these processes.

Hypothalamic Wnt Signalling and its Role in Energy Balance Regulation

Wnt signalling and its downstream effectors are well known for their roles in embryogenesis and tumourigenesis, including the regulation of cell proliferation, survival and differentiation. In the nervous system, Wnt signalling has been described mainly during embryonic development, although accumulating evidence suggests that it also plays a major role in adult brain morphogenesis and function. Studies have predominantly concentrated on memory formation in the hippocampus, although recent data indicate that Wnt signalling is also critical for neuroendocrine control of the developed hypothalamus, a brain centre that is key in energy balance regulation and whose dysfunction is implicated in metabolic disorders such as type 2 diabetes and obesity. Based on scattered findings that report the presence of Wnt molecules in the tanycytes and ependymal cells lining the third ventricle and arcuate nucleus neurones of the hypothalamus, their potential importance in key regions of food intake and body weight regulation has been investigated in recent studies. The present review brings together current knowledge on Wnt signalling in the hypothalamus of adult animals and discusses the evidence suggesting a key role for members of the Wnt signalling family in glucose and energy balance regulation in the hypothalamus in diet-induced and genetically obese (leptin deficient) mice. Aspects of Wnt signalling in seasonal (photoperiod sensitive) rodents are also highlighted, given the recent evidence indicating that the Wnt pathway in the hypothalamus is not only regulated by diet and leptin, but also by photoperiod in seasonal animals, which is connected to natural adaptive changes in food intake and body weight. Thus, Wnt signalling appears to be critical as a modulator for normal functioning of the physiological state in the healthy adult brain, and is also crucial for normal glucose and energy homeostasis where its dysregulation can lead to a range of metabolic disorders.

The emerging role of GPR50 receptor in brain

GPR50 receptor one of the member of G protein-coupled receptors (GPCRs) is extensively expressed in the pituitary, hypothalamus,cortex, midbrain, pons, amygdala, and in several brainstem nuclei. The exact function of this receptor in brain is remains unclear. This review presents current knowledge regarding the function of GPR50 receptor in brain, with a focus on role of this receptor in the hypothalamus-pituitary-adrenal (HPA) axis and the glucocorticoid receptor (GR) signaling, leptin signaling, adaptive thermogenesis, torpor, neurite outgrowth, and self-renewal and neuronal differentiation of neural progenitor cells NPCs. Although the results are encouraging, further research is needed to clarify GPR50 role in neurobiology of mood disorders, adaptive thermogenesis, torpor, and in the pathophysiology of neurological disorders.

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice

Background

We recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.

Methods

LPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.

Results

In both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.

Conclusions

The results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Involvement of GPR50 polymorphisms in depression: independent replication in a prospective elderly cohort

INTRODUCTION

Despite the explosion in genetic association studies over the last decade, clearly identified genetic risk factors for depression remain scarce and replication studies are becoming increasingly important. G-protein-coupled receptor 50 (GPR50) has been implicated in psychiatric disorders in a small number of studies, although not consistently.

METHODS

Data were obtained from 1010 elderly men and women from the prospective population-based ESPRIT study. Logistic regression and survival models were used to determine whether three common GPR50 polymorphisms were associated with depression prevalence or the incidence of depression over 12-years. The analyses were adjusted for a range of covariates such as comorbidity and cholesterol levels, to determine independent associations.

RESULTS

All three variants showed some evidence of an association with late-life depression in women, although these were not consistent across outcomes, the overall effect sizes were relatively small, and most would not remain significant after correction for multiple testing. Women heterozygous for rs13440581, had a 1.6-fold increased risk of baseline depression, while the odds of depression comorbid with anxiety were increased fourfold for women homozygous for the minor allele of rs2072621. When depressed women at baseline were excluded from the analysis, however, neither variant was associated with the 12-year incidence of depression. In contrast, rs561077 was associated with a 1.8-fold increased risk of incident depression specifically. No significant associations were observed in men.

DISCUSSION

Our results thus provide only weak support for the involvement of GPR50 variants in late-life depression, which appear specific to certain subgroups of depressed individuals (i.e., women and those with more severe forms of depression).

DCX-expressing cells in the vicinity of the hypothalamic neurogenic niche: a comparative study between mouse, sheep, and human tissues

Neural stem and precursor cells persist postnatally throughout adulthood and are capable of responding to numerous endogenous and exogenous signals by modifying their proliferation and differentiation. Whereas adult neurogenesis has been extensively studied in the dentate gyrus of the hippocampal formation and in the subventricular zone adjacent to the wall of the lateral ventricles, we and others have recently reported constitutive adult neurogenesis in other brain structures, including the hypothalamus. In this study, we used immunohistochemistry to study the expression of the neuroblast marker doublecortin (DCX), and compared its expression pattern in adult ovine, mouse, and human hypothalamic tissues. Our results indicate that DCX-positive cells resembling immature and developing neurons occur in a wide range of hypothalamic nuclei in all three species, although with different distribution patterns. In addition, the morphology of DCX-positive cells varied depending on their location. DCX-positive cells near the third ventricle had the morphology of very immature neuroblasts, a round shape with no processes, whereas those located deeper in the parenchyma such as in the ventromedial nucleus were fusiform and showed a bipolar morphology. Extending this observation, we showed that among the cohort of immature neurons entering the ventromedial nucleus, some appeared to undergo maturation, as revealed by the partial colocalization of DCX with markers of more mature neurons, e.g., human neuronal protein C and D (HuC/D). This study provides further confirmation of the existence of an adult hypothalamic neurogenic niche and argues for the potential existence of a migratory path within the hypothalamus.

Research resource: Gene profiling of G protein-coupled receptors in the arcuate nucleus of the female

The hypothalamic arcuate nucleus controls many critical homeostatic functions including energy homeostasis, reproduction, and motivated behavior. Although G protein-coupled receptors (GPCRs) are involved in the regulation of these functions, relatively few of the GPCRs have been identified specifically within the arcuate nucleus. Here, using TaqMan low-density arrays we quantified the mRNA expression of nonolfactory GPCRs in mouse arcuate nucleus. An unprecedented number of GPCRs (total of 292) were found to be expressed, of which 183 were known and 109 were orphan GPCRs. The known GPCR genes expressed were classified into several functional clusters including hormone/neurotransmitter, growth factor, angiogenesis and vasoactivity, inflammation and immune system, and lipid messenger receptors. The plethora of orphan genes expressed in the arcuate nucleus were classified into 5 structure-related classes including class A (rhodopsin-like), class B (adhesion), class C (other GPCRs), nonsignaling 7-transmembrane chemokine-binding proteins, and other 7-transmembrane proteins. Therefore, for the first time, we provide a quantitative estimate of the numerous GPCRs expressed in the hypothalamic arcuate nucleus. Finally, as proof of principle, we documented the expression and function of one of these receptor genes, the glucagon-like peptide 1 receptor (Glp1r), which was highly expressed in the arcuate nucleus. Single-cell RT-PCR revealed that Glp1r mRNA was localized in proopiomelanocortin neurons, and using whole-cell recording we found that the glucagon-like peptide 1-selective agonist exendin-4 robustly excited proopiomelanocortin neurons. Thus, the quantitative GPCR data emphasize the complexity of the hypothalamic arcuate nucleus and furthermore provide a valuable resource for future neuroendocrine/endocrine-related experiments.

Hypothalamic subependymal niche: a novel site of the adult neurogenesis

The discovery of undifferentiated, actively proliferating neural stem cells (NSCs) in the mature brain opened a brand new chapter in the contemporary neuroscience. Adult neurogenesis appears to occur in specific brain regions (including hypothalamus) throughout vertebrates’ life, being considered an important player in the processes of memory, learning, and neural plasticity. In the adult mammalian brain, NSCs are located mainly in the subgranular zone (SGZ) of the hippocampal dentate gyrus and in the subventricular zone (SVZ) of the lateral ventricle ependymal wall. Besides these classical regions, hypothalamic neurogenesis occurring mainly along and beneath the third ventricle wall seems to be especially well documented. Neurogenic zones in SGZ, SVZ, and in the hypothalamus share some particular common features like similar cellular cytoarchitecture, vascularization pattern, and extracellular matrix properties. Hypothalamic neurogenic niche is formed mainly by four special types of radial glia-like tanycytes. They are characterized by distinct expression of some neural progenitor and stem cell markers. Moreover, there are numerous suggestions that newborn hypothalamic neurons have a significant ability to integrate into the local neural pathways and to play important physiological roles, especially in the energy balance regulation. Newly formed neurons in the hypothalamus can synthesize and release food intake regulating neuropeptides and they are sensitive to the leptin. On the other hand, high-fat diet positively influences hypothalamic neurogenesis in rodents. The nature of this intriguing new site of adult neurogenesis is still so far poorly studied and requires further investigations.

Action of melatonin on squamous cell carcinoma and other tumors of the oral cavity (Review)

Melatonin (MLT; N-acetyl-5-metoxy-tryptamine) is a hormone that is principally synthesized in the pineal gland. MLT has been shown to exhibit a variety of functions. The hormone, which is a free radical scavenger, plays an immunomodulatory role, stimulates the proliferation and synthesis of type I collagen and promotes bone formation. Moreover, MLT exerts oncostatic activity through several biological mechanisms, including antiproliferative functions, stimulation of anticancer immunity, modulation of oncogene expression and anti-inflammatory, antioxidant and antiangiogenic effects. In addition, MLT inhibits human cancer cell growth in culture, and previous clinical studies have also confirmed its anticancer properties in vivo. With regard to the underlying mechanisms of MLT in tumor processes, including oral cavity tumors such as epidermoid carcinoma, knowledge of the role played by the MT1 and 2 membrane receptors, MT3 and the calmodulin cytosolic binding sites, as well as the nuclear receptors of the RZR/ROR family, is increasing. It has been hypothesized that exogenous restoration of MT1 (MTNR1A) expression inhibits the growth of oral squamous cell carcinoma cells lacking the expression of the receptor. The tumor suppressing functions of MLT and the presence of the MT1 receptor in various tumors indicate that the receptor may play a pivotal role in oral carcinogenesis. The current review discusses the clinical significance of MLT in oral cancer.

Distribution of MT1 melatonin receptor promoter-driven RFP expression in the brains of BAC C3H/HeN transgenic mice

The pineal hormone melatonin activates two G-protein coupled receptors (MT1 and MT2) to regulate in part biological functions. The MT1 and MT2 melatonin receptors are heterogeneously distributed in the mammalian brain including humans. In the mouse, only a few reports have assessed the expression of the MT1 melatonin receptor expression using 2-iodomelatonin binding, in situ hybridization and/or polymerase chain reaction (PCR). Here, we described a transgenic mouse in which red fluorescence protein (RFP) is expressed under the control of the endogenous MT1 promoter, by inserting RFP cDNA at the start codon of MTNR1a gene within a bacterial artificial chromosome (BAC) and expressing this construct as a transgene. The expression of RFP in the brain of this mouse was examined either directly under a fluorescent microscope or immunohistochemically using an antibody against RFP (RFP-MT1). RFP-MT1 expression was observed in many brain regions including the subcommissural organ, parts of the ependyma lining the lateral and third ventricles, the aqueduct, the hippocampus, the cerebellum, the pars tuberalis, the habenula and the habenula commissure. This RFP-MT1 transgenic model provides a unique tool for studying the distribution of the MT1 receptor in the brain of mice, its cell-specific expression and its function in vivo.

Laser-capture microdissection and transcriptional profiling of the dorsomedial nucleus of the hypothalamus

Identifying neuronal molecular markers with restricted patterns of expression is a crucial step in dissecting the numerous pathways and functions of the brain. While the dorsomedial nucleus of the hypothalamus (DMH) has been implicated in a host of physiological processes, current functional studies have been limited by the lack of molecular markers specific for DMH. Identification of such markers would facilitate the development of mouse models with DMH-specific genetic manipulations. Here we used a combination of laser-capture microdissection (LCM) and gene expression profiling to identify genes that are highly expressed within the DMH relative to adjacent hypothalamic regions. Six of the most highly expressed of these genes, Gpr50, 4930511J11Rik, Pcsk5, Grp, Sulf1, and Rorβ, were further characterized by real-time polymerase chain reaction (PCR) analysis and in situ hybridization histochemistry. The genes identified in this article will provide the basis for future gene-targeted approaches for studying DMH function.

Hypothalamic tanycytes: potential roles in the control of feeding and energy balance

Tanycytes, glial-like cells that line the third ventricle, are emerging as components of the hypothalamic networks that control body weight and energy balance. They contact the cerebrospinal fluid (CSF) and send processes that come into close contact with neurons in the arcuate and ventromedial hypothalamic nuclei. Tanycytes are glucosensitive and are able to respond to transmitters associated with arousal and the drive to feed. At least some tanycytes are stem cells and, in the median eminence, may be stimulated by diet to generate new neurons. The quest is on to understand how tanycytes detect and respond to changes in energy balance and how they communicate with the rest of the nervous system to effect their functions.

Molecules affecting hypothalamic control of core body temperature in response to calorie intake

Core body temperature (CBT) and calorie intake are main components of energy homeostasis and two important regulators of health, longevity, and aging. In homeotherms, CBT can be influenced by calorie intake as food deprivation or calorie restriction (CR) lowers CBT whereas feeding has hyperthermic effects. The finding that in mice CBT prolonged lifespan independently of CR, suggested that the mechanisms modulating CBT may represent important regulators of aging. Here we summarize the current knowledge on the signaling molecules and their receptors that participate in the regulation of CBT responses to calorie intake. These include hypothalamic neuropeptides regulating feeding but also energy expenditure via modulation of thermogenesis. We also report studies indicating that nutrient signals can contribute to regulation of CBT by direct action on hypothalamic preoptic warm-sensitive neurons that in turn regulate adaptive thermogenesis and hence CBT. Finally, we show the role played by two orphans G protein-coupled receptor: GPR50 and GPR83, that were recently demonstrated to regulate temperature-dependent energy expenditure.

Developmental expression of orphan G protein-coupled receptor 50 in the mouse brain

Mental disorders have a complex etiology resulting from interactions between multiple genetic risk factors and stressful life events. Orphan G protein-coupled receptor 50 (GPR50) has been identified as a genetic risk factor for bipolar disorder and major depression in women, and there is additional genetic and functional evidence linking GPR50 to neurite outgrowth, lipid metabolism, and adaptive thermogenesis and torpor. However, in the absence of a ligand, a specific function has not been identified. Adult GPR50 expression has previously been reported in brain regions controlling the HPA axis, but its developmental expression is unknown. In this study, we performed extensive expression analysis of GPR50 and three protein interactors using rt-PCR and immunohistochemistry in the developing and adult mouse brain. Gpr50 is expressed at embryonic day 13 (E13), peaks at E18, and is predominantly expressed by neurons. Additionally we identified novel regions of Gpr50 expression, including brain stem nuclei involved in neurotransmitter signaling: the locus coeruleus, substantia nigra, and raphe nuclei, as well as nuclei involved in metabolic homeostasis. Gpr50 colocalizes with yeast-two-hybrid interactors Nogo-A, Abca2, and Cdh8 in the hypothalamus, amygdala, cortex, and selected brain stem nuclei at E18 and in the adult. With this study, we identify a link between GPR50 and neurotransmitter signaling and strengthen a likely role in stress response and energy homeostasis.

Neuroanatomical distribution of the orphan GPR50 receptor in adult sheep and rodent brains

GPR50, formerly known as melatonin-related receptor, is one of three subtypes of the melatonin receptor subfamily, together with the MT(1) and MT(2) receptors. By contrast to these two high-affinity receptor subtypes and despite its high identity with the melatonin receptor family, GPR50 does not bind melatonin or any other known ligand. Specific and reliable immunological tools are therefore needed to be able to elucidate the physiological functions of this orphan receptor that are still largely unknown. We have generated and validated a new specific GPR50 antibody against the ovine GPR50 and used it to analyse the neuroanatomical distribution of the GPR50 in sheep, rat and mouse whole brain. We demonstrated that GPR50-positive cells are widely distributed in various regions, including the hypothalamus and the pars tuberalis of the pituitary, in all the three species studied. GPR50 expressing cells are abundant in the dorsomedial nucleus of the hypothalamus, the periventricular nucleus and the median eminence. In rodents, immunohistochemical studies revealed a broader distribution pattern for the GPR50 protein. GPR50 immunoreactivity is found in the medial preoptic area (MPA), the lateral septum, the lateral hypothalamic area, the bed nucleus of the stria terminalis, the vascular organ of the laminae terminalis and several regions of the amygdala, including the medial nuclei of amygdala. Additionally, in the rat brain, GPR50 protein was localised in the CA1 pyramidal cell layer of the dorsal hippocampus. In mice, moderate to high numbers of GPR50-positive cells were also found in the subfornical organ. Taken together, these results provide an enlarged distribution of GPR50 protein, give further insight into the organisation of the melatoninergic system, and may lay the framework for future studies on the role of the GPR50 in the brain.

A role for the melatonin-related receptor GPR50 in leptin signaling, adaptive thermogenesis, and torpor

The ability of mammals to maintain a constant body temperature has proven to be a profound evolutionary advantage, allowing members of this class to thrive in most environments on earth. Intriguingly, some mammals employ bouts of deep hypothermia (torpor) to cope with reduced food supply and harsh climates [1, 2]. During torpor, physiological processes such as respiration, cardiac function, and metabolic rate are severely depressed, yet the neural mechanisms that regulate torpor remain unclear [3]. Hypothalamic responses to energy signals, such as leptin, influence the expression of torpor [4-7]. We show that the orphan receptor GPR50 plays an important role in adaptive thermogenesis and torpor. Unlike wild-type mice, Gpr50(-/-) mice readily enter torpor in response to fasting and 2-deoxyglucose administration. Decreased thermogenesis in Gpr50(-/-) mice is not due to a deficit in brown adipose tissue, the principal site of nonshivering thermogenesis in mice [8]. GPR50 is highly expressed in the hypothalamus of several species, including man [9, 10]. In line with this, altered thermoregulation in Gpr50(-/-) mice is associated with attenuated responses to leptin and a suppression of thyrotropin-releasing hormone. Thus, our findings identify hypothalamic circuits involved in torpor and reveal GPR50 to be a novel component of adaptive thermogenesis in mammals.