Neural expression of G protein-coupled receptors GPR3, GPR6, and GPR12 up-regulates cyclic AMP levels and promotes neurite outgrowth

Role of orphan G-protein coupled receptors in tissue ischemia: A comprehensive review

Ischemic events lead to many diseases and deaths worldwide. Ischemia/reperfusion (I/R) occurs due to reduced blood circulation in tissues followed by blood reflow. Reoxygenation of ischemic tissues is characterized by oxidative stress, inflammation, energy distress, and endoplasmic reticulum stress. There are still no adequate clinical protocols or pharmacological approaches to address the consequences of I/R damage. G protein-coupled receptors (GPCRs) are important therapeutic targets. They compose a large family of seven transmembrane-spanning proteins that are involved in many biological functions. Orphan GPCRs are a large subgroup of these receptors expressed in different organs. In the present review, we summarized the literature regarding the role of orphan GPCRs in I/R in different organs. We focused on the effect of these receptors on modulating cellular and molecular processes underlying ischemia including apoptosis, inflammation, and autophagy. The study showed that GPR3, GPR4, GPR17, GPR30, GPR31, GPR35, GPR37, GPR39, GPR55, GPR65, GPR68, GPR75, GPR81, and GPR91 are involved in ischemic events, mainly in the brain and heart. These receptors offer new possibilities for treating I/R injuries in the body.

Molecular insights into orphan G protein-coupled receptors relevant to schizophrenia

Schizophrenia remains a sizable socio-economic burden that continues to be treated with therapeutics based on 70-year old science. All currently approved therapeutics primarily target the dopamine D2 receptor to achieve their efficacy. Whilst dopaminergic dysregulation is a key feature in this disorder, the targeting of dopaminergic machinery has yielded limited efficacy and an appreciable side effect burden. Over the recent decades, numerous drugs that engage non-dopaminergic G protein-coupled receptors (GPCRs) have yielded a promise of efficacy without the deleterious side effect profile, yet none have successfully completed clinical studies and progressed to the market. More recently, there has been increased attention around non-dopaminergic GPCR-targeting drugs, which demonstrated efficacy in some schizophrenia symptom domains. This provides renewed hope that effective schizophrenia treatment may lie outside of the dopaminergic space. Despite the potential for muscarinic receptor- (and other well-characterised GPCR families) targeting drugs to treat schizophrenia, they are often plagued with complications such as lack of receptor subtype selectivity and peripheral on-target side effects. Orphan GPCR studies have opened a new avenue of exploration with many demonstrating schizophrenia-relevant mechanisms and a favourable expression profile, thus offering potential for novel drug development. This review discusses centrally expressed orphan GPCRs: GPR3, GPR6, GPR12, GPR52, GPR85, GPR88 and GPR139 and their relationship to schizophrenia. We review their expression, signalling mechanisms and cellular function, in conjunction with small molecule development and structural insights. We seek to provide a snapshot of the growing evidence and development potential of new classes of schizophrenia therapeutics. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Exploring orphan GPCRs in neurodegenerative diseases

Neurodegenerative disorders represent a significant and growing health burden worldwide. Unfortunately, limited therapeutic options are currently available despite ongoing efforts. Over the past decades, research efforts have increasingly focused on understanding the molecular mechanisms underlying these devastating conditions. Orphan receptors, a class of receptors with no known endogenous ligands, emerge as promising druggable targets for diverse diseases. This review aims to direct attention to a subgroup of orphan GPCRs, in particular class A orphans that have roles in neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Multiple sclerosis. We highlight the diverse roles orphan receptors play in regulating critical cellular processes such as synaptic transmission, neuronal survival and neuro-inflammation. Moreover, we discuss the therapeutic potential of targeting orphan receptors for the treatment of neurodegenerative disorders, emphasizing recent advances in drug discovery and preclinical studies. Finally, we outline future directions and challenges in orphan receptor research.

Identification of oleic acid as an endogenous ligand of GPR3

Although GPR3 plays pivotal roles in both the nervous system and metabolic processes, such as cold-induced thermogenesis, its endogenous ligand remains elusive. Here, by combining structural approach (including cryo-electron microscopy), mass spectrometry analysis, and functional studies, we identify oleic acid (OA) as an endogenous ligand of GPR3. Our study reveals a hydrophobic tunnel within GPR3 that connects the extracellular side of the receptor to the middle of plasma membrane, enabling fatty acids to readily engage the receptor. Functional studies demonstrate that OA triggers downstream Gs signaling, whereas lysophospholipids fail to activate the receptor. Moreover, our research reveals that cold stimulation induces the secretion of OA in mice, subsequently activating Gs/cAMP/PKA signaling in brown adipose tissue. Notably, brown adipose tissues from Gpr3 knockout mice do not respond to OA during cold stimulation, reinforcing the significance of GPR3 in this process. Finally, we propose a "born to be activated and cold to enhance" model for GPR3 activation. Our study provides a starting framework for the understanding of GPR3 signaling in cold-stimulated thermogenesis.

Orphan peptide and G protein-coupled receptor signalling in alcohol use disorder

Neuropeptides and G protein-coupled receptors (GPCRs) have long been, and continue to be, one of the most popular target classes for drug discovery in CNS disorders, including alcohol use disorder (AUD). Yet, orphaned neuropeptide systems and receptors (oGPCR), which have no known cognate receptor or ligand, remain understudied in drug discovery and development. Orphan neuropeptides and oGPCRs are abundantly expressed within the brain and represent an unprecedented opportunity to address brain function and may hold potential as novel treatments for disease. Here, we describe the current literature regarding orphaned neuropeptides and oGPCRs implicated in AUD. Specifically, in this review, we focus on the orphaned neuropeptide cocaine- and amphetamine-regulated transcript (CART), and several oGPCRs that have been directly implicated in AUD (GPR6, GPR26, GPR88, GPR139, GPR158) and discuss their potential and pitfalls as novel treatments, and progress in identifying their cognate receptors or ligands.

Orphan G protein-coupled receptors: the ongoing search for a home

G protein-coupled receptors (GPCRs) make up the largest receptor superfamily, accounting for 4% of protein-coding genes. Despite the prevalence of such transmembrane receptors, a significant number remain orphans, lacking identified endogenous ligands. Since their conception, the reverse pharmacology approach has been used to characterize such receptors. However, the multifaceted and nuanced nature of GPCR signaling poses a great challenge to their pharmacological elucidation. Considering their therapeutic relevance, the search for native orphan GPCR ligands continues. Despite limited structural input in terms of 3D crystallized structures, with advances in machine-learning approaches, there has been great progress with respect to accurate ligand prediction. Though such an approach proves valuable given that ligand scarcity is the greatest hurdle to orphan GPCR deorphanization, the future pairings of the remaining orphan GPCRs may not necessarily take a one-size-fits-all approach but should be more comprehensive in accounting for numerous nuanced possibilities to cover the full spectrum of GPCR signaling.

Assessing Dose- and Sex-Dependent Antinociceptive Effects of Cannabidiol and Amitriptyline, Alone and in Combination, and Exploring Mechanism of Action Involving Serotonin 1A Receptors

Inflammatory pain is caused by tissue hypersensitization and is a component of rheumatic diseases, frequently causing chronic pain. Current guidelines use a multimodal approach to pain and sociocultural changes have renewed interest in cannabinoid use, particularly cannabidiol (CBD), for pain. The tricyclic antidepressant amitriptyline (AT) is approved for use in pain-related syndromes, alone and within a multimodal approach. Therefore, we investigated sex- and dose-dependent effects of CBD and AT antinociception in the 2.5% formalin inflammatory pain model. Male and female C57BL/6J mice were pretreated with either vehicle, CBD (0.3-100 mg/kg), or AT (0.1-30 mg/kg) prior to formalin testing. In the acute phase, CBD induced antinociception after administration of 30-100 mg/kg in males and 100 mg/kg in females and in the inflammatory phase at doses of 2.5-100 mg/kg in males and 10-100 mg/kg in females. In the acute phase, AT induced antinociception at 10 mg/kg for all mice, and at 0.3 mg/kg in males and 3 mg/kg in female mice in the inflammatory phase. Combining the calculated median effective doses of CBD and AT produced additive effects for all mice in the acute phase and for males only in the inflammatory phase. Use of selective serotonin 1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1 piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635) maleate (0.1 mg/kg) before co-administration of CBD and AT reversed antinociception in the acute and partially reversed antinociception in the inflammatory phase. Administration of AT was found to enhance cannabinoid receptor type 1mRNA expression only in female mice. These results suggest a role for serotonin and sex in mediating cannabidiol and amitriptyline-induced antinociception in inflammatory pain. SIGNIFICANCE STATEMENT: Inflammatory pain is an important component of both acute and chronic pain. We have found that cannabidiol (CBD) and amitriptyline (AT) show dose-dependent, and that AT additionally shows sex-dependent, antinociceptive effects in an inflammatory pain model. Additionally, the combination of CBD and AT was found to have enhanced antinociceptive effects that is partially reliant of serotonin 1A receptors and supports the use of CBD within a multimodal approach to pain.

Development of Fluorescent AF64394 Analogues Enables Real-Time Binding Studies for the Orphan Class A GPCR GPR3

The orphan G protein-coupled receptor (oGPCR) GPR3 represents a potential drug target for the treatment of Alzheimer's disease and metabolic disorders. However, the limited toolbox of pharmacological assays hampers the development of advanced ligands. Here, we developed a signaling pathway-independent readout of compound-GPR3 interaction. Starting from computational binding pose predictions of the most potent GPR3 ligand, we designed a series of fluorescent AF64394 analogues and assessed their suitability for BRET-based binding studies. The most potent ligand, 45 (UR-MB-355), bound to GPR3 and closely related receptors, GPR6 and GPR12, with similar submicromolar affinities. Furthermore, we found that 45 engages GPR3 in a distinct mode compared to AF64394, and coincubation studies with the GPR3 agonist diphenyleneiodonium chloride revealed allosteric modulation of 45 binding. These insights provide new cues for the pharmacological manipulation of GPR3 activity. This novel binding assay will foster the development of future drugs acting through these pharmacologically attractive oGPCRs.

Nuclear orphan receptors: A novel therapeutic agent in neuroinflammation

Orphan receptors constitute a historically varied subsection of a superfamily of nuclear receptors. Nuclear receptors regulate gene expression in response to ligand signals and are particularly alluring therapeutic targets for chronic illnesses. Neuroinflammation and neurodegenerative diseases have been linked to these orphan nuclear receptors. Preclinical and clinical evidence suggests that orphan receptors could serve as future targets in neuroinflammation, such as Parkinson's disease (PD), Alzheimer's Disease (AD), Huntington's Disease (HD), Multiple Sclerosis (MS), and Cerebral Ischemia. Given the therapeutic relevance of certain orphan receptors in a variety of disorders, their potential in neuroinflammation remains unproven. There is substantial evidence that ligand-activated transcription factors have great promise for preventing neurodegenerative and neurological disorders, with certain orphan nuclear receptors i.e., PPARγ, NR4As, and orphan GPCRs holding particularly high potential. Based on previous findings, we attempted to determine the contribution of PPAR, NR4As, and orphan GPCRs-regulated neuroinflammation to the pathogenesis of these disorders and their potential to become novel therapeutic targets.

The development of diphenyleneiodonium analogs as GPR3 agonists

G protein-coupled receptor 3 (GPR3) is an orphan receptor potentially involved in many important physiological processes such as drug abuse, neuropathic pain, and anxiety and depression related disorders. Pharmacological studies of GPR3 have been limited due to the restricted number of known agonists and inverse agonists for this constitutively active receptor. In this medicinal chemistry study, we report the discovery of GPR3 agonists based off the diphenyleneiodonium (DPI) scaffold. The most potent full agonist was the 3-trifluoromethoxy analog (32) with an EC50 of 260 nM and 90% efficacy compared to DPI. Investigation of a homology model of GPR3 from multiple sequence alignment resulted in the finding of a binding site rich in potential π-π and π-cation interactions stabilizing DPI-scaffold agonists. MMGBSA free energy analysis showed a good correlation with trends in observed EC50s. DPI analogs retained the same high receptor selectivity for GPR3 over GPR6 and GPR12 as observed with DPI. Collectively, the DPI analog series shows that order of magnitude improvements in potency with the scaffold were attainable; however, attempts to replace the iodonium ion to make the scaffold more druggable failed.

Opportunities and challenges in drug discovery targeting the orphan receptor GPR12

G-protein-coupled receptor 12 (GPR12) is a brain-specific expression orphan G-protein-coupled receptor (oGPCR) that regulates various physiological processes. It is an emerging therapeutic target for central nervous system (CNS) disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), attention deficit hyperactivity disorder (ADHD), and schizophrenia, as well as other human diseases, such as cancer, obesity, and metabolic disorders. GPR12 remains a less extensively investigated oGPCR, particularly in terms of its biological functions, signaling pathways, and ligand discovery. The discovery of drug-like small-molecule modulators to probe the brain functions of GPR12 or to act as a potential drug candidates, as well as the identification of reliable biomarkers, are vital to elucidate the roles of this receptor in various human diseases and develop novel target-based therapeutics.

Structural insight into the constitutive activity of human orphan receptor GPR12

G protein-coupled receptor 12 (GPR12) is an orphan G protein-coupled receptor that is highly expressed in the thalamus of the brain and plays a vital role in driving thalamocortical functions in short-term memory. GPR12 performs high constitutive activity and couples with Gs, increasing the intracellular cyclic adenosine monophosphate (cAMP) level when it is expressed. However, exploitation for drug development is limited since it is unclear how GPR12 initiates self-activation and signal transduction, and whether it can be modulated by endogenous or synthetic ligands. Here, we report the cryo-electron microscopy structure of the GPR12-Gs complex in the absence of agonists. Our structure reveals the key determinants for the intrinsically high basal activity of GPR12, including extracellular loop 2 partially occupying the orthosteric binding pocket, a tight-packed TM1 and TM7, and unique activation-related residues in TM6 and TM7. Together with mutagenesis data, this study will improve our understanding of the function and self-activation of the orphan receptor GPR12, enable the identification of endogenous ligands, and guide drug discovery efforts that target GPR12.

Targeting G Protein-Coupled Receptors in the Treatment of Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized in part by the deterioration of dopaminergic neurons which leads to motor impairment. Although there is no cure for PD, the motor symptoms can be treated using dopamine replacement therapies including the dopamine precursor L-DOPA, which has been in use since the 1960s. However, neurodegeneration in PD is not limited to dopaminergic neurons, and many patients experience non-motor symptoms including cognitive impairment or neuropsychiatric disturbances, for which there are limited treatment options. Moreover, there are currently no treatments able to alter the progression of neurodegeneration. There are many therapeutic strategies being investigated for PD, including alternatives to L-DOPA for the treatment of motor impairment, symptomatic treatments for non-motor symptoms, and neuroprotective or disease-modifying agents. G protein-coupled receptors (GPCRs), which include the dopamine receptors, are highly druggable cell surface proteins which can regulate numerous intracellular signaling pathways and thereby modulate the function of neuronal circuits affected by PD. This review will describe the treatment strategies being investigated for PD that target GPCRs and their downstream signaling mechanisms. First, we discuss new developments in dopaminergic agents for alleviating PD motor impairment, the role of dopamine receptors in L-DOPA induced dyskinesia, as well as agents targeting non-dopamine GPCRs which could augment or replace traditional dopaminergic treatments. We then discuss GPCRs as prospective treatments for neuropsychiatric and cognitive symptoms in PD. Finally, we discuss the evidence pertaining to ghrelin receptors, β-adrenergic receptors, angiotensin receptors and glucagon-like peptide 1 receptors, which have been proposed as disease modifying targets with potential neuroprotective effects in PD.

GPR3 expression in retinal ganglion cells contributes to neuron survival and accelerates axonal regeneration after optic nerve crush in mice

Glaucoma is an optic neuropathy and is currently one of the most common diseases that leads to irreversible blindness. The axonal degeneration that occurs before retinal ganglion neuronal loss is suggested to be involved in the pathogenesis of glaucoma. G protein-coupled receptor 3 (GPR3) belongs to the class A rhodopsin-type GPCR family and is highly expressed in various neurons. GPR3 is unique in its ability to constitutively activate the Gαs protein without a ligand, which elevates the basal intracellular cAMP level. Our earlier reports suggested that GPR3 enhances both neurite outgrowth and neuronal survival. However, the potential role of GPR3 in axonal regeneration after neuronal injury has not been elucidated. Herein, we investigated retinal GPR3 expression and its possible involvement in axonal regeneration after retinal injury in mice. GPR3 was relatively highly expressed in retinal ganglion cells (RGCs). Surprisingly, RGCs in GPR3 knockout mice were vulnerable to neural death during aging without affecting high intraocular pressure (IOP) and under ischemic conditions. Primary cultured neurons from the retina showed that GPR3 expression was correlated with neurite outgrowth and neuronal survival. Evaluation of the effect of GPR3 on axonal regeneration using GPR3 knockout mice revealed that GPR3 in RGCs participates in axonal regeneration after optic nerve crush (ONC) under zymosan stimulation. In addition, regenerating axons were further stimulated when GPR3 was upregulated in RGCs, and the effect was further augmented when combined with zymosan treatment. These results suggest that GPR3 expression in RGCs helps maintain neuronal survival and accelerates axonal regeneration after ONC in mice.

Potential role of inducible GPR3 expression under stimulated T cell conditions

G protein-coupled receptor 3 (GPR3) constitutively activates Gαs proteins without any ligands and is predominantly expressed in neurons. Since the expression and physiological role of GPR3 in immune cells is still unknown, we examined the possible role of GPR3 in T lymphocytes. The expression of GPR3 was upregulated 2 h after phorbol 12-myristate 13-acetate (PMA)/ionomycin stimulation and was sustained in Jurkat cells, a human T lymphocyte cell line. In addition, the expression of nuclear receptor 4 group A member 2 (NR4A2) was highly modulated by GPR3 expression. Additionally, GPR3 expression was linked with the transcriptional promoter activity of NR4A in Jurkat cells. In mouse CD4+ T cells, transient GPR3 expression was induced immediately after the antigen receptor stimulation. However, the expression of NR4A2 was not modulated in CD4+ T cells from GPR3-knockout mice after stimulation, and the population of Treg cells in thymocytes and splenocytes was not affected by GPR3 knockout. By contrast, spontaneous effector activation in both CD4+ T cells and CD8+ T cells was observed in GPR3-knockout mice. In summary, GPR3 is immediately induced by T cell stimulation and play an important role in the suppression of effector T cell activation.

GPR3 accelerates neurite outgrowth and neuronal polarity formation via PI3 kinase-mediating signaling pathway in cultured primary neurons

During neuronal development, immature neurons extend neurites and subsequently polarize to form an axon and dendrites. We have previously reported that G protein-coupled receptor 3 (GPR3) levels increase during neuronal development, and that GPR3 has functions in neurite outgrowth and neuronal differentiation in cerebellar granular neurons. Moreover, GPR3 is transported and concentrated at the tips of neurite, thereby contributing to the local activation of protein kinase A (PKA). However, the signaling pathways for GPR3-mediated neurite outgrowth and its subsequent effects on neuronal polarization have not yet been elucidated. We therefore analyzed the signaling pathways related to GPR3-mediated neurite outgrowth, and also focused on the possible roles of GPR3 in axon polarization. We demonstrated that, in cerebellar granular neurons, GPR3-mediated neurite outgrowth was mediated by multiple signaling pathways, including those of PKA, extracellular signal-regulated kinases (ERKs), and most strongly phosphatidylinositol 3-kinase (PI3K). In addition, the GPR3-mediated activation of neurite outgrowth was associated with G protein-coupled receptor kinase 2 (GRK2)-mediated signaling and phosphorylation of the C-terminus serine/threonine residues of GPR3, which affected downstream protein kinase B (Akt) signaling. We further demonstrated that GPR3 was transiently increased early in the development of rodent hippocampal neurons. It was subsequently concentrated at the tip of the longest neurite, and was thus associated with accelerated polarity formation in a PI3K-dependent manner in rat hippocampal neurons. In addition, GPR3 knockout in mouse hippocampal neurons led to delayed neuronal polarity formation, thereby affecting the dephosphorylation of collapsing response mediator protein 2 (CRMP2), which is downstream of the PI3K signaling pathway. Taken together, these findings suggest that the intrinsic expression of GPR3 in differentiated neurons constitutively activates PI3K-mediated signaling pathway predominantly, thus accelerating neurite outgrowth and further augmenting polarity formation in primary cultured neurons.

Complement as a powerful "influencer" in the brain during development, adulthood and neurological disorders

The complement system was long considered as only a powerful effector arm of the immune system that, while critically protective, could lead to inflammation and cell death if overactivated, even in the central nervous system (CNS). However, in the past decade it has been recognized as playing critical roles in key physiological processes in the CNS, including neurogenesis and synaptic remodeling in the developing and adult brain. Inherent in these processes are the interactions with cells in the brain, and the cascade of interactions and functional consequences that ensue. As a result, investigations of therapeutic approaches for both suppressing excessive complement driven neurotoxicity and aberrant sculpting of neuronal circuits, require broad (and deep) knowledge of the functional activities of multiple components of this highly evolved and regulated system to avoid unintended negative consequences in the clinic. Advances in basic science are beginning to provide a roadmap for translation to therapeutics, with both small molecule and biologics. Here, we present examples of the critical roles of proper complement function in the development and sculpting of the nervous system, and in enabling rapid protection from infection and clearance of dying cells. Microglia are highlighted as important command centers that integrate signals from the complement system and other innate sensors that are programed to provide support and protection, but that direct detrimental responses to aberrant activation and/or regulation of the system. Finally, we present promising research areas that may lead to effective and precision strategies for complement targeted interventions to promote neurological health.

Habenula orphan G-protein coupled receptors in the pathophysiology of fear and anxiety

The phasic emotion, fear, and the tonic emotion, anxiety, have been conventionally inspected in clinical frameworks to epitomize memory acquisition, storage, and retrieval. However, inappropriate expression of learned fear in a safe environment and its resistance to suppression is a cardinal feature of various fear-related disorders. A significant body of literature suggests the involvement of extra-amygdala circuitry in fear disorders. Consistent with this view, the present review underlies incentives for the association between the habenula and fear memory. G protein-coupled receptors (GPCRs) are important to understand the molecular mechanisms central to fear learning due to their neuromodulatory role. The efficacy of a pharmacological strategy aimed at exploiting habenular-GPCR desensitization machinery can serve as a therapeutic target combating the pathophysiology of fear disorders. Originating from this milieu, the conserved nature of orphan GPCRs in the brain, with some having the highest expression in the habenula can lead to recent endeavors in understanding its functionality in fear circuitry.

Cannabinoids: Revealing their complexity and role in central networks of fear and anxiety

The first aim of the present review is to provide an in-depth description of the cannabinoids and their known effects at various neuronal receptors. It reveals that cannabinoids are highly diverse, and recent work has highlighted that their effects on the central nervous system (CNS) are surprisingly more complex than previously recognized. Cannabinoid-sensitive receptors are widely distributed throughout the CNS where they act as primary modulators of neurotransmission. Secondly, we examine the role of cannabinoid receptors at key brain sites in the control of fear and anxiety. While our understanding of how cannabinoids specifically modulate these networks is mired by their complex interactions and diversity, a plausible framework(s) for their effects is proposed. Finally, we highlight some important knowledge gaps in our understanding of the mechanism(s) responsible for their effects on fear and anxiety in animal models and their use as therapeutic targets in humans. This is particularly important for our understanding of the phytocannabinoids used as novel clinical interventions.

Endocannabinoid signaling pathways: beyond CB1R and CB2R

The search for cannabinoid receptors other than CB1R and CB2R has been ongoing for over a decade. A number of orphan receptors have been proposed as potential cannabinoid receptors primarily based on phylogenic arguments and reactivity towards known endocannabinoids and phytocannabinoids. Seven putative cannabinoid receptors are described and discussed, and evidence for and against their inclusion in this category are presented.

Development of CVN424: A Selective and Novel GPR6 Inverse Agonist Effective in Models of Parkinson Disease

GPR6 is an orphan G-protein-coupled receptor that has enriched expression in the striatopallidal, indirect pathway and medium spiny neurons of the striatum. This pathway is greatly impacted by the loss of the nigro-striatal dopaminergic neurons in Parkinson disease, and modulating this neurocircuitry can be therapeutically beneficial. In this study, we describe the in vitro and in vivo pharmacological characterization of (R)-1-(2-(4-(2,4-difluorophenoxy)piperidin-1-yl)-3-((tetrahydrofuran-3-yl)amino)-7,8-dihydropyrido[3,4-b]pyrazin-6(5H)-yl)ethan-1-one (CVN424), a highly potent and selective small-molecule inverse agonist for GPR6 that is currently undergoing clinical evaluation. CVN424 is brain-penetrant and shows dose-dependent receptor occupancy that attained brain 50% of receptor occupancy at plasma concentrations of 6.0 and 7.4 ng/ml in mice and rats, respectively. Oral administration of CVN424 dose-dependently increases locomotor activity and reverses haloperidol-induced catalepsy. Furthermore, CVN424 restored mobility in bilateral 6-hydroxydopamine lesion model of Parkinson disease. The presence and localization of GPR6 in medium spiny neurons of striatum postmortem samples from both nondemented control and patients with Parkinson disease were confirmed at the level of both RNA (using Nuclear Enriched Transcript Sort sequencing) and protein. This body of work demonstrates that CVN424 is a potent, orally active, and brain-penetrant GPR6 inverse agonist that is effective in preclinical models and is a potential therapeutic for improving motor function in patients with Parkinson disease. SIGNIFICANCE STATEMENT: CVN424 represents a nondopaminergic novel drug for potential use in patients with Parkinson disease.

Characterization of Four Orphan Receptors (GPR3, GPR6, GPR12 and GPR12L) in Chickens and Ducks and Regulation of GPR12 Expression in Ovarian Granulosa Cells by Progesterone

The three structurally related orphan G protein-coupled receptors, GRP3, GPR6, and GPR12, are reported to be constitutively active and likely involved in the regulation of many physiological/pathological processes, such as neuronal outgrowth and oocyte meiotic arrest in mammals. However, the information regarding these orphan receptors in nonmammalian vertebrates is extremely limited. Here, we reported the structure, constitutive activity, and tissue expression of these receptors in two representative avian models: chickens and ducks. The cloned duck GPR3 and duck/chicken GPR6 and GPR12 are intron-less and encode receptors that show high amino acid (a.a.) sequence identities (66–88%) with their respective mammalian orthologs. Interestingly, a novel GPR12-like receptor (named GPR12L) sharing 66% a.a. identity to that in vertebrates was reported in the present study. Using dual-luciferase reporter assay and Western blot, we demonstrated that GPR3, GPR6, GPR12, and GPR12L are constitutively active and capable of stimulating the cAMP/PKA signaling pathway without ligand stimulation in birds (and zebrafish), indicating their conserved signaling property across vertebrates. RNA-seq data/qRT-PCR assays revealed that GPR6 and GPR12L expression is mainly restricted to the chicken brain, while GPR12 is highly expressed in chicken ovarian granulosa cells (GCs) and oocytes of 6 mm growing follicles and its expression in cultured GCs is upregulated by progesterone. Taken together, our data reveal the structure, function, and expression of GPR3, GPR6, GPR12, and GPR12L in birds, thus providing the first piece of evidence that GPR12 expression is upregulated by gonadal steroid (i.e., progesterone) in vertebrates.

Rapid separation and purification of two C25 steroids with bicyclic [4.4.1] A/B rings from the marine fungus Aspergillus sp. LS116 by high-speed counter-current chromatography in stepwise elution mode

Two C25 steroids with bicyclic [4.4.1] A/B rings were successfully separated from the marine fungus Aspergillus sp. LS116 by a two-step high-speed counter current chromatography (HSCCC). Petroleum ether/ethyl acetate/methanol/water (5.5:11:5:7, v/v) and petroleum ether/ethyl acetate/methanol/water (5:6:5:7, v/v) were selected as two optimum two-phase systems to purify two C25 steroids, neocyclocitrinol B (1) and threo-23-O-methylneocyclocitrinol (2). The purity of two compounds was over 94%. Their structures were determined by comprehensive spectroscopic techniques. This is the first report about rapid separation and identification of C25 steroids with bicyclic [4.4.1] A/B rings by HSCCC.

Component of nicotine-induced intracellular calcium elevation mediated through α3- and α5-containing nicotinic acetylcholine receptors are regulated by cyclic AMP in SH-SY 5Y cells

The pathway from the medial habenular nucleus to the interpeduncular nucleus, in which nicotinic acetylcholine receptor (nAChR) including the α3 and α5 subunits (α3 * and α5 * nAChRs) are expressed, is implicated in nicotine dependence. We investigated whether α3 * and α5 * nAChRs are regulated by cAMP using SH-SY5Y cells to clarify the significance of these receptors in nicotine dependence. We analyzed the nicotine-induced elevation of intracellular Ca²⁺ ([Ca²⁺]i). Nicotine induces a concentration-dependent increase in [Ca²⁺]i. The elimination of Ca²⁺ from extracellular fluid or intracellular stores demonstrated that the nicotine-induced [Ca²⁺]i elevation was due to extracellular influx and intracellular mobilization. The effects of tubocurarine on nicotine-induced [Ca²⁺]i elevation and current suggest that intracellular mobilization is caused by plasma membrane-permeating nicotine. The inhibition of α3 *, α5 *, α7 nAChR and voltage-gated Ca²⁺ channels by using siRNAs and selective antagonists revealed the involvement of these nAChR subunits and channels in nicotine-induced [Ca²⁺]i elevation. To distinguish and characterize the α3 * and α5 * nAChR-mediated Ca²⁺ influx, we measured the [Ca²⁺]i elevation induced by nonmembrane-permeating acetylcholine when muscarinic receptors, α7nAChR and Ca²⁺ channels were blocked. Under this condition, the [Ca²⁺]i elevation was significantly inhibited with a 48-h treatment of dibutyryl cAMP, which was accompanied by the downregulation of α3 and β4 mRNA. These findings suggest that α3 * and α5 * nAChR-mediated Ca²⁺ influx is possibly regulated by cAMP at the transcriptional level.

Detailed neuronal distribution of GPR3 and its co-expression with EF-hand calcium-binding proteins in the mouse central nervous system

The G-protein coupled receptor 3 (GPR3), a member of the class A rhodopsin-type GPR family, constitutively activates Gαs proteins without any ligands. Although there have been several reports concerning the functions of GPR3 in neurons, the physiological roles of GPR3 have not been fully elucidated. To address this issue, we analyzed GPR3 distribution in detail using fluorescence-based X-gal staining in heterozygous GPR3 knockout/LacZ knock-in mice, and further investigated the types of GPR3-expressing neurons using fluorescent double labeling with various EF-hand Ca²⁺-binding proteins. In addition to the previously reported GPR3-expressing areas, we identified GPR3 expression in the basal ganglia and in many nuclei of the cranial nerves, in regions related to olfactory, auditory, emotional, and motor functions. In addition, GPR3 was not only observed in excitatory neurons in layer V of the cerebral cortex, the CA2 region of the hippocampus, and the lateral nucleus of the thalamus, but also in γ-aminobutyric acid (GABA)-ergic interneurons in the cortex, hippocampus, thalamus, striatum, and cerebellum. GPR3 was frequently co-expressed with neuronal Ca²⁺-binding protein 2 (NECAB2) in neurons in various regions of the central nervous system, especially in the hippocampal CA2, medial habenular nucleus, lateral thalamic nucleus, dorsolateral striatum, brainstem, and spinal cord anterior horn. Furthermore, GPR3 also co-localized with NECAB2 at the tips of neurites in differentiated PC12 cells. These results suggest that GPR3 and NECAB2 are highly co-expressed in specific neurons, and that GPR3 may modulate Ca²⁺ signaling by interacting with NECAB2 in specific areas of the central nervous system.

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.

Advances in Neurobiology and Pharmacology of GPR12

GPR12 is a G protein-coupled orphan receptor genetically related to type 1 and type 2 cannabinoid receptors (CB₁ and CB₂) which are ancient proteins expressed all over the body. Both cannabinoid receptors, but especially CB₁, are involved in neurodevelopment and cognitive processes such as learning, memory, brain reward, coordination, etc. GPR12 shares with CB₁ that both are mainly expressed into the brain. Regrettably, very little is known about physiology of GPR12. Concerning its pharmacology, GPR12 seems to be endogenously activated by the lysophospholipids sphingosine-1-phosphate (S1P) and sphingosyl-phosphorylcholine (SPC). Exogenously, GPR12 is a target for the phytocannabinoid cannabidiol (CBD). Functionally, GPR12 seems to be related to neurogenesis and neural inflammation, but its relationship with cognitive functions remains to be characterized. Although GPR12 was initially suggested to be a cannabinoid receptor, it does not meet the five criteria proposed in 2010 by the International Union of Basic and Clinical Pharmacology (IUPHAR). In this review, we analyze all the direct available information in PubMed database about expression, function, and pharmacology of this receptor in central nervous system (CNS) trying to provide a broad overview of its current and prospective neurophysiology. Moreover, in this mini-review we highlight the need to produce more relevant data about the functions of GPR12 in CNS. Hence, this work should motivate further research in this field.

New Insights in Cannabinoid Receptor Structure and Signaling

BACKGROUND

Cannabinoid has long been used for medicinal purposes. Cannabinoid signaling has been considered the therapeutic target for treating pain, addiction, obesity, inflammation, and other diseases. Recent studies have suggested that in addition to CB1 and CB2, there are non-CB1 and non-CB2 cannabinoid-related orphan GPCRs including GPR18, GPR55, and GPR119. In addition, CB1 and CB2 display allosteric binding and biased signaling, revealing correlations between biased signaling and functional outcomes. Interestingly, new investigations have indicated that CB1 is functionally present within the mitochondria of striated and heart muscles directly regulating intramitochondrial signaling and respiration.

CONCLUSION

In this review, we summarize the recent progress in cannabinoid-related orphan GPCRs, CB1/CB2 structure, Gi/Gs coupling, allosteric ligands and biased signaling, and mitochondria-localized CB1, and discuss the future promise of this research.

Discovery of endogenous inverse agonists for G protein-coupled receptor 6

The orphan G protein-coupled receptor 6 (GPR6) is highly expressed in the striatum and has been linked to multiple striatal pathologies. The identification of endogenous ligands and their mechanisms of action at GPR6 will help to elucidate the physiological and pathological roles of the receptor. In the current study, we tested the concentration-dependent effects of a variety of endocannabinoid-like N-acylamides on GPR6 signaling. Here, we demonstrate for the first time that N-arachidonoyl dopamine, N-docosahexaenoyl dopamine, N-oleoyl dopamine and N-palmitoyl dopamine exert inverse agonism at GPR6. This effect was concentration-dependent, with potencies in the micromolar range, and functionally selective for β-arrestin2 recruitment. Structure-activity relationship studies demonstrate that both the N-acyl side chain and the dopamine head group are important for these ligands to act on GPR6. Our discovery of these N-acyl dopamines as endogenous inverse agonists for GPR6 moves us one step further in understanding the roles GPR6 play in neurodegenerative and neuropsychiatric disorders related to striatal dysfunction.

Role of Sphingosylphosphorylcholine in Tumor and Tumor Microenvironment

Sphingosylphosphorylcholine (SPC) is a unique type of lysosphingolipid found in some diseases, and has been studied in cardiovascular, neurological, and inflammatory phenomena. In particular, SPC’s studies on cancer have been conducted mainly in terms of effects on cancer cells, and relatively little consideration has been given to aspects of tumor microenvironment. This review summarizes the effects of SPC on cancer and tumor microenvironment, and presents the results and prospects of modulators that regulate the various actions of SPC.

Development of a High-Throughput Screening-Compatible Assay for Discovery of GPR3 Inverse Agonists Using a cAMP Biosensor

While G-protein-coupled receptors (GPCRs) represent the largest class of cell surface proteins, there are ≥100 orphan GPCRs whose endogenous ligands are unknown. Accordingly, these could prove to be potential therapeutic targets for the pharmaceutical intervention of various diseases. Constitutively active orphan GPCRs are activated without ligands; thus, inverse agonists may be very useful pharmacological tools for inhibiting constitutive activity. However, in general, inverse agonist screening is considered more difficult to perform with high quality than antagonist screening, particularly due to the narrow assay window. We developed a high-throughput screening (HTS)-compatible assay to identify inverse agonists of GPR3. GPR3 is expressed in the central nervous system (CNS) and is known to be related to Alzheimer's disease and other CNS diseases. The GPR3 inducible cell line was established using T-REx 293 cells that stably expressed the tetracycline repressor protein, and the cAMP biosensor, GloSensor, was stably co-expressed. After optimization of the induction level of GPR3 and assay conditions, the GloSensor assay showed an approximately 20-fold signal-to-background ratio and high sensitivity. Using the HTS method, we successfully screened a library of hundreds of thousands of compounds for the inhibition of constitutive activity with good quality and excellent reproducibility. Finally, 35 compounds were identified as GPR3 selective inverse agonists. This inverse agonist screening approach using GloSensor in combination with the inducible expression of orphan GPCR indicates universal applicability to the search for inverse agonists of constitutively active orphan GPCRs.

GPR3, GPR6, and GPR12 as novel molecular targets: their biological functions and interaction with cannabidiol

The G protein-coupled receptors 3, 6, and 12 (GPR3, GPR6, and GPR12) comprise a family of closely related orphan receptors with no confirmed endogenous ligands. These receptors are constitutively active and capable of signaling through G protein-mediated and non-G protein-mediated mechanisms. These orphan receptors have previously been reported to play important roles in many normal physiological functions and to be involved in a variety of pathological conditions. Although they are orphans, GPR3, GPR6, and GPR12 are phylogenetically most closely related to the cannabinoid receptors. Using β-arrestin2 recruitment and cAMP accumulation assays, we recently found that the nonpsychoactive phytocannabinoid cannabidiol (CBD) is an inverse agonist for GPR3, GPR6, and GPR12. This discovery highlights these orphan receptors as potential new molecular targets for CBD, provides novel mechanisms of action, and suggests new therapeutic uses of CBD for illnesses such as Alzheimer's disease, Parkinson's disease, cancer, and infertility. Furthermore, identification of CBD as a new inverse agonist for GPR3, GPR6, and GPR12 provides the initial chemical scaffolds upon which potent and efficacious agents acting on these receptors can be developed, with the goal of developing chemical tools for studying these orphan receptors and ultimately new therapeutic agents.

Novel inverse agonists for the orphan G protein-coupled receptor 6

The orphan G protein-coupled receptor 6 (GPR6) displays unique promise as a therapeutic target for the treatment of neuropsychiatric disorders due to its high expression in the striatopallidal neurons of the basal ganglia. GPR6, along with closely related orphan receptors GPR3 and GPR12, are phylogenetically related to CB1 and CB2 cannabinoid receptors. In the current study, we performed concentration-response studies on the effects of three different classes of cannabinoids: endogenous, phyto-, and synthetic, on both GPR6-mediated cAMP accumulation and β-arrestin2 recruitment. In addition, structure-activity relationship studies were conducted on cannabidiol (CBD), a recently discovered inverse agonist for GPR6. We have identified four additional cannabinoids, cannabidavarin (CBDV), WIN55212-2, SR141716A and SR144528, that exert inverse agonism on GPR6. Furthermore, we have discovered that these cannabinoids exhibit functional selectivity toward the β-arrestin2 recruitment pathway. These novel, functionally selective inverse agonists for GPR6 can be used as research tools and potentially developed into therapeutic agents.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

Towards a better understanding of the cannabinoid-related orphan receptors GPR3, GPR6, and GPR12

GPR3, GPR6, and GPR12 are three orphan receptors that belong to the Class A family of G-protein-coupled receptors (GPCRs). These GPCRs share over 60% of sequence similarity among them. Because of their close phylogenetic relationship, GPR3, GPR6, and GPR12 share a high percentage of homology with other lipid receptors such as the lysophospholipid and the cannabinoid receptors. On the basis of sequence similarities at key structural motifs, these orphan receptors have been related to the cannabinoid family. However, further experimental data are required to confirm this association. GPR3, GPR6, and GPR12 are predominantly expressed in mammalian brain. Their high constitutive activation of adenylyl cyclase triggers increases in cAMP levels similar in amplitude to fully activated GPCRs. This feature defines their physiological role under certain pathological conditions. In this review, we aim to summarize the knowledge attained so far on the understanding of these receptors. Expression patterns, pharmacology, physiopathological relevance, and molecules targeting GPR3, GPR6, and GPR12 will be analyzed herein. Interestingly, certain cannabinoid ligands have been reported to modulate these orphan receptors. The current debate about sphingolipids as putative endogenous ligands will also be addressed. A special focus will be on their potential role in the brain, particularly under neurological conditions such as Parkinson or Alzheimer's disease. Reported physiological roles outside the central nervous system will also be covered. This critical overview may contribute to a further comprehension of the physiopathological role of these orphan GPCRs, hopefully attracting more research towards a future therapeutic exploitation of these promising targets.

[Signaling and functions of G-protein-coupled receptor 3 in cerebellar granular neurons]

G-protein-coupled receptor 3 (GPR3) is a member of the class A rhodopsin-type GPCR family and is highly expressed in various neurons. A unique feature of GPR3 is its ability to constitutively activate the Gαs protein without the addition of ligands, which results in the elevation of the basal level of intracellular cAMP. During the development of the cerebellum, GPR3 expression is upregulated in cerebellar granular neurons (CGNs) and maintained thereafter. In our previous studies, we showed that the intrinsic expression of GPR3 in CGNs is highly associated with neurite outgrowth, neurite differentiation, and neuronal survival. Recently, we have focused on the possible signaling pathways associated with GPR3-mediated neurite outgrowth in CGNs. Interestingly, GPR3-mediated neurite outgrowth is mediated by not only PKA-dependent signaling pathways but also PI3K-mediated signaling pathways. Moreover, the Gβγ-mediated signaling pathway is involved in GPR3-mediated neurite outgrowth. These results suggested that neural expression of GPR3 stimulates multiple downstream signaling pathways, contributing to the maintenance of homeostasis in neurons. Further precise analyses of constitutively active GPCRs may help in unveiling novel neuronal functions.

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.

An Update on Non-CB1, Non-CB2 Cannabinoid Related G-Protein-Coupled Receptors

The endocannabinoid system (ECS) has been shown to be of great importance in the regulation of numerous physiological and pathological processes. To date, two Class A G-protein-coupled receptors (GPCRs) have been discovered and validated as the main therapeutic targets of this system: the cannabinoid receptor type 1 (CB1), which is the most abundant neuromodulatory receptor in the brain, and the cannabinoid receptor type 2 (CB2), predominantly found in the immune system among other organs and tissues. Endogenous cannabinoid receptor ligands (endocannabinoids) and the enzymes involved in their synthesis, cell uptake, and degradation have also been identified as part of the ECS. However, its complex pharmacology suggests that other GPCRs may also play physiologically relevant roles in this therapeutically promising system. In the last years, GPCRs such as GPR18 and GPR55 have emerged as possible missing members of the cannabinoid family. This categorization still stimulates strong debate due to the lack of pharmacological tools to validate it. Because of their close phylogenetic relationship, the Class A orphan GPCRs, GPR3, GPR6, and GPR12, have also been associated with the cannabinoids. Moreover, certain endo-, phyto-, and synthetic cannabinoid ligands have displayed activity at other well-established GPCRs, including the opioid, adenosine, serotonin, and dopamine receptor families. In addition, the cannabinoid receptors have also been shown to form dimers with other GPCRs triggering cross-talk signaling under specific conditions. In this mini review, we aim to provide insight into the non-CB1, non-CB2 cannabinoid-related GPCRs that have been reported thus far. We consider the physiological relevance of these molecular targets in modulating the ECS.

Cannabidiol, a novel inverse agonist for GPR12

GPR12 is a constitutively active, G_s protein-coupled receptor that currently has no confirmed endogenous ligands. GPR12 may be involved in physiological processes such as maintenance of oocyte meiotic arrest and brain development, as well as pathological conditions such as metastatic cancer. In this study, the potential effects of various classes of cannabinoids on GPR12 were tested using a cAMP accumulation assay. Our data demonstrate that cannabidiol (CBD), a major non-psychoactive phytocannabinoid, acted as an inverse agonist to inhibit cAMP accumulation stimulated by the constitutively active GPR12. Thus, GPR12 is a novel molecular target for CBD. The structure-activity relationship studies of CBD indicate that both the free hydroxyl and the pentyl side chain are crucial for the effects of CBD on GPR12. Furthermore, studies using cholera toxin, which blocks G_s protein and pertussis toxin, which blocks G_i protein, revealed that G_s, but not G_i is involved in the inverse agonism of CBD on GPR12. CBD is a promising novel therapeutic agent for cancer, and GPR12 has been shown to alter viscoelasticity of metastatic cancer cells. Since we have demonstrated that CBD is an inverse agonist for GPR12, this provides novel mechanism of action for CBD, and an initial chemical scaffold upon which highly potent and efficacious agents acting on GPR12 may be developed with the ultimate goal of blocking cancer metastasis.

Methods for the Development of In Silico GPCR Models

The Reggio group has constructed computer models of the inactive and G-protein-activated states of the cannabinoid CB1 and CB2 receptors, as well as, several orphan receptors that recognize a subset of cannabinoid compounds, including GPR55 and GPR18. These models have been used to design ligands, mutations, and covalent labeling studies. The resultant second-generation models have been used to design ligands with improved affinity, efficacy, and subtype selectivity. Herein, we provide a guide for the development of GPCR models using the most recent orphan receptor studied in our lab, GPR3. GPR3 is an orphan receptor that belongs to the Class A family of G-protein-coupled receptors. It shares high sequence similarity with GPR6, GPR12, the lysophospholipid receptors, and the cannabinoid receptors. GPR3 is predominantly expressed in mammalian brain and oocytes and it is known as a Gαs-coupled receptor activated constitutively in cells. GPR3 represents a possible target for the treatment of different pathological conditions such as Alzheimer's disease, oocyte maturation, or neuropathic pain. However, the lack of potent and selective GPR3 ligands is delaying the exploitation of this promising therapeutic target. In this context, we aim to develop a homology model that helps us to elucidate the structural determinants governing ligand-receptor interactions at GPR3. In this chapter, we detail the methods and rationale behind the construction of the GPR3 active-and inactive-state models. These homology models will enable the rational design of novel ligands, which may serve as research tools for further understanding of the biological role of GPR3.

GPR3 and GPR6, novel molecular targets for cannabidiol

GPR3 and GPR6 are members of a family of constitutively active, Gs protein-coupled receptors. Previously, it has been reported that GPR3 is involved in Alzheimer's disease whereas GPR6 plays potential roles in Parkinson's disease. GPR3 and GPR6 are considered orphan receptors because there are no confirmed endogenous agonists for them. However, GPR3 and GPR6 are phylogenetically related to the cannabinoid receptors. In this study, the activities of endocannabinoids and phytocannabinoids were tested on GPR3 and GPR6 using a β-arrestin2 recruitment assay. Among the variety of cannabinoids tested, cannabidiol (CBD), the major non-psychoactive component of marijuana, significantly reduced β-arrestin2 recruitment to both GPR3 and GPR6. In addition, the inhibitory effects of CBD on β-arrestin2 recruitment were concentration-dependent for both GPR3 and GPR6, with a higher potency for GPR6. These data show that CBD acts as an inverse agonist at both GPR3 and GPR6 receptors. These results demonstrate for the first time that both GPR3 and GPR6 are novel molecular targets for CBD. Our discovery that CBD acts as a novel inverse agonist on both GPR3 and GPR6 indicates that some of the potential therapeutic effects of CBD (e.g. treatment of Alzheimer's disease and Parkinson's disease) may be mediated through these important 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.

One-step generation of mice carrying a conditional allele together with an HA-tag insertion for the delta opioid receptor

G protein-coupled receptors (GPCRs) are important modulators of many physiological functions and excellent drug targets for many diseases. However, to study the functions of endogenous GPCRs is still a challenging task, partially due to the low expression level of GPCRs and the lack of highly potent and selective GPCR antibodies. Overexpression or knock-in of tagged GPCRs, or knockout of specific GPCRs in mice, are common strategies used to study the in vivo functions of these receptors. However, generating separate mice carrying tagged GPCRs or conditional alleles for GPCRs is labor intensive, and requires additional breeding costs. Here we report the generation of mice carrying an HA-tagged DOR (delta opioid receptor) flanked by LoxP sequences at the endogenous DOR locus using a single recombination step, aided by the TALEN system. These animals can be used directly to study the expression, localization, protein-protein interaction and signal transduction of endogenous DOR using anti-HA antibodies. By crossing with mice expressing tissue-specific Cre, these mice can also generate offspring with DOR knockout within specific tissues. These mice are powerful tools to study the in vivo functions of DOR. Furthermore, the gene modification strategy could also be used to study the functions of many other GPCRs.

Increased susceptibility of G-protein coupled receptor 6 deficient mice to MPTP neurotoxicity

The G-protein coupled receptor 6 (GPR6) is a constitutive active orphan GPCR which is predominantly expressed in striatopallidal neurons. GPR6 deficiency in mice may alter the susceptibility of the nigrostriatal dopaminergic system relevant for Parkinson's disease (PD). Here, we investigated the effect of GPR6 deficiency in mice on neurotoxicity induced by the dopaminergic neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). GPR6^-/-- and control mice were treated with MPTP (4×12.5mg/kg, i.p., 2h intervals) and analyzed after seven days. Striatal dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA) concentrations were measured by HPLC. The number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra pars compacta (SNpc) was analyzed by immunohistochemistry. In a separate group of mice, MPP⁺ (500μM for 20min) was administered via an intrastriatal microdialysis probe to measure the MPP⁺-induced DA release. MPTP produced a significant reduction in striatal DA, DOPAC, HVA and an increase in dopamine turnover in control and GPR6^-/--mice. The MPTP-induced DA and HVA depletion was significantly more pronounced in GPR6^-/--mice. Consistently, the MPTP-induced reduction of TH-positive neurons in the SPpc was significantly higher in GPR6^-/--mice. Furthermore, the MPP⁺-induced dopamine release was significantly higher in GPR6^-/--mice. In conclusion, we showed that MPTP induces an enhanced dopaminergic neurodegeneration in GPR6^-/--mice indicated by alterations at the striatal and nigral level. We propose that GPR6 signaling is involved in the cascade of neurodegenerative events of the parkinsonian neurotoxin MPTP and suggest that pharmacological modulation of GPR6 might represent an entry point to further investigate GPR6 in PD.

Loss of GPR3 reduces the amyloid plaque burden and improves memory in Alzheimer's disease mouse models

The orphan G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) GPR3 regulates activity of the γ-secretase complex in the absence of an effect on Notch proteolysis, providing a potential therapeutic target for Alzheimer's disease (AD). However, given the vast resources required to develop and evaluate any new therapy for AD and the multiple failures involved in translational research, demonstration of the pathophysiological relevance of research findings in multiple disease-relevant models is necessary before initiating costly drug development programs. We evaluated the physiological consequences of loss of Gpr3 in four AD transgenic mouse models, including two that contain the humanized murine Aβ sequence and express similar amyloid precursor protein (APP) levels as wild-type mice, thereby reducing potential artificial phenotypes. Our findings reveal that genetic deletion of Gpr3 reduced amyloid pathology in all of the AD mouse models and alleviated cognitive deficits in APP/PS1 mice. Additional three-dimensional visualization and analysis of the amyloid plaque burden provided accurate information on the amyloid load, distribution, and volume in the structurally intact adult mouse brain. Analysis of 10 different regions in healthy human postmortem brain tissue indicated that GPR3 expression was stable during aging. However, two cohorts of human AD postmortem brain tissue samples showed a correlation between elevated GPR3 and AD progression. Collectively, these studies provide evidence that GPR3 mediates the amyloidogenic proteolysis of APP in four AD transgenic mouse models as well as the physiological processing of APP in wild-type mice, suggesting that GPR3 may be a potential therapeutic target for AD drug development.

G Protein-Coupled Receptors (GPCRs) in Alzheimer's Disease: A Focus on BACE1 Related GPCRs

The G protein coupled receptors (GPCRs) have been considered as one of the largest families of validated drug targets, which involve in almost overall physiological functions and pathological processes. Meanwhile, Alzheimer’s disease (AD), the most common type of dementia, affects thinking, learning, memory and behavior of elderly people, that has become the hotspot nowadays for its increasing risks and incurability. The above fields have been intensively studied, and the link between the two has been demonstrated, whereas the way how GPCRs perturb AD progress are yet to be further explored given their complexities. In this review, we summarized recent progress regarding the GPCRs interacted with β-site APP cleaving enzyme 1 (BACE1), a key secretase in AD pathogenesis. Then we discussed the current findings on the regulatory roles of GPCRs on BACE1, and the possibility for pharmaceutical treatment of AD patients by the allosteric modulators and biased ligands of GPCRs. We hope this review can provide new insights into the understanding of mechanistic link between GPCRs and BACE1, and highlight the potential of GPCRs as therapeutic target for AD.

The Subcellular Dynamics of the Gs-Linked Receptor GPR3 Contribute to the Local Activation of PKA in Cerebellar Granular Neurons

G-protein-coupled receptor (GPR) 3 is a member of the GPR family that constitutively activates adenylate cyclase. We have reported that the expression of GPR3 in cerebellar granular neurons (CGNs) contributes to neurite outgrowth and modulates neuronal proliferation and survival. To further identify its role, we have analyzed the precise distribution and local functions of GPR3 in neurons. The fluorescently tagged GPR3 protein was distributed in the plasma membrane, the Golgi body, and the endosomes. In addition, we have revealed that the plasma membrane expression of GPR3 functionally up-regulated the levels of PKA, as measured by a PKA FRET indicator. Next, we asked if the PKA activity was modulated by the expression of GPR3 in CGNs. PKA activity was highly modulated at the neurite tips compared to the soma. In addition, the PKA activity at the neurite tips was up-regulated when GPR3 was transfected into the cells. However, local PKA activity was decreased when endogenous GPR3 was suppressed by a GPR3 siRNA. Finally, we determined the local dynamics of GPR3 in CGNs using time-lapse analysis. Surprisingly, the fluorescent GPR3 puncta were transported along the neurite in both directions over time. In addition, the anterograde movements of the GPR3 puncta in the neurite were significantly inhibited by actin or microtubule polymerization inhibitors and were also disturbed by the Myosin II inhibitor blebbistatin. Moreover, the PKA activity at the tips of the neurites was decreased when blebbistatin was administered. These results suggested that GPR3 was transported along the neurite and contributed to the local activation of PKA in CGN development. The local dynamics of GPR3 in CGNs may affect local neuronal functions, including neuronal differentiation and maturation.

G-protein genomic association with normal variation in gray matter density

While detecting genetic variations underlying brain structures helps reveal mechanisms of neural disorders, high data dimensionality poses a major challenge for imaging genomic association studies. In this work, we present the application of a recently proposed approach, parallel independent component analysis with reference (pICA-R), to investigate genomic factors potentially regulating gray matter variation in a healthy population. This approach simultaneously assesses many variables for an aggregate effect and helps to elicit particular features in the data. We applied pICA-R to analyze gray matter density (GMD) images (274,131 voxels) in conjunction with single nucleotide polymorphism (SNP) data (666,019 markers) collected from 1,256 healthy individuals of the Brain Imaging Genetics (BIG) study. Guided by a genetic reference derived from the gene GNA14, pICA-R identified a significant SNP-GMD association (r=-0.16, P=2.34×10(-8)), implying that subjects with specific genotypes have lower localized GMD. The identified components were then projected to an independent dataset from the Mind Clinical Imaging Consortium (MCIC) including 89 healthy individuals, and the obtained loadings again yielded a significant SNP-GMD association (r=-0.25, P=0.02). The imaging component reflected GMD variations in frontal, precuneus, and cingulate regions. The SNP component was enriched in genes with neuronal functions, including synaptic plasticity, axon guidance, molecular signal transduction via PKA and CREB, highlighting the GRM1, PRKCH, GNA12, and CAMK2B genes. Collectively, our findings suggest that GNA12 and GNA14 play a key role in the genetic architecture underlying normal GMD variation in frontal and parietal regions.

Novel Therapeutic GPCRs for Psychiatric Disorders

G protein-coupled receptors (GPCRs) are the most common targets of the neuropharmacological drugs in the central nervous system (CNS). GPCRs are activated by manifold neurotransmitters, and their activation in turn evokes slow synaptic transmission. They are deeply involved in multiple neurological and psychiatric disorders such as Parkinson’s disease and schizophrenia. In the brain, the striatum is strongly innervated by the ventral tegmental area (VTA) and plays a central role in manifestation of psychiatric disorders. Recently, anatomical and comprehensive transcriptome analysis of the non-odorant GPCR superfamily revealed that the orphan GPCRs GPR88, GPR6, and GPR52, as well as dopamine D1 and D2 receptors and the adenosine A2a receptor, are the most highly enriched in the rodent striatum. Genetically engineered animal models and molecular biological studies have suggested that these striatally enriched GPCRs have a potential to be therapeutic psychiatric receptors. This review summarizes the current understanding of the therapeutic GPCR candidates for psychiatric disorders.

Hunting for the function of orphan GPCRs - beyond the search for the endogenous ligand

Seven transmembrane-spanning proteins (7TM), also called GPCRs, are among the most versatile and evolutionary successful protein families. Out of the 400 non-odourant members identified in the human genome, approximately 100 remain orphans that have not been matched with an endogenous ligand. Apart from the classical deorphanization strategies, several alternative strategies provided recent new insights into the function of these proteins, which hold promise for high therapeutic potential. These alternative strategies consist of the phenotypical characterization of organisms silenced or overexpressing orphan 7TM proteins, the search for constitutive receptor activity and formation of protein complexes including 7TM proteins as well as the development of synthetic, surrogate ligands. Taken together, a variety of ligand-independent functions can be attributed to orphan 7TM proteins that range from constitutive activity to complex formation with other proteins and include 'true' orphans for which no ligand exist and 'conditional' orphans that behave like orphans in the absence of ligand and as non-orphans in the presence of ligand.