GPR55 and the regulation of glucose homeostasis

Theaflavin 3'-gallate activates G protein-coupled receptor 55 (GPR55) and enhances GLP-1 secretion via Ca2+/CaMKII/ERK signaling in enteroendocrine STC-1 cells, mitigating postprandial hyperglycemia in mice

The antihyperglycemic effect of black tea is well-known, and theaflavins (TFs) are considered active compounds. It is, however, unclear whether glucagon-like peptide-1 (GLP-1) is involved in the antihyperglycemic effects of TFs. We demonstrate that TFs suppress postprandial hyperglycemia by stimulating GLP-1 secretion in mice. In STC-1 cells, theaflavin 3'-gallate (TF2B), possessing a galloyl group at the 3'-position, showed the strongest effect on GLP-1 secretion among the four TFs. TF2B activated G protein-coupled receptor 55 (GPR55) and was confirmed to bind to the receptor, notably exhibiting the highest binding affinity. Moreover, GPR55 antagonist canceled TF2B-induced GLP-1 secretion. Downstream, TF2B increased intracellular Ca2+ levels and activated the Ca2+/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinases 1/2 (ERK1/2) pathways. Inhibitors of Ca2+ signaling, CaMKII, and ERK pathways abolished TF2B-stimulated GLP-1 secretion. These findings suggest that TF2B from black tea prevents hyperglycemia through GPR55-dependent stimulation of GLP-1 secretion via Ca2+-Ca2+/CaMKII and ERK1/2 pathways.

GPCRs involved in metabolic diseases: pharmacotherapeutic development updates

G protein-coupled receptors (GPCRs) are expressed in a variety of cell types and tissues, and activation of GPCRs is involved in enormous metabolic pathways, including nutrient synthesis, transportation, storage or insulin sensitivity, etc. This review intends to summarize the regulation of metabolic homeostasis and mechanisms by a series of GPCRs, such as GPR91, GPR55, GPR119, GPR109a, GPR142, GPR40, GPR41, GPR43 and GPR120. With deep understanding of GPCR's structure and signaling pathways, it is attempting to uncover the role of GPCRs in major metabolic diseases, including metabolic syndrome, diabetes, dyslipidemia and nonalcoholic steatohepatitis, for which the global prevalence has risen during last two decades. An extensive list of agonists and antagonists with their chemical structures in a nature of small molecular compounds for above-mentioned GPCRs is provided as pharmacologic candidates, and their preliminary data of preclinical studies are discussed. Moreover, their beneficial effects in correcting abnormalities of metabolic syndrome, diabetes and dyslipidemia are summarized when clinical trials have been undertaken. Thus, accumulating data suggest that these agonists or antagonists might become as new pharmacotherapeutic candidates for the treatment of metabolic diseases.

Thienopyrimidine Derivatives as GPR55 Receptor Antagonists: Insight into Structure-Activity Relationship

GPR55 is an orphan G-protein coupled receptor involved in various pathophysiological conditions. However, there are only a few noncannabinoid GPR55 ligands reported so far. The lack of potent and selective GPR55 ligands precludes a deep exploration of this receptor. The studies presented here focused on a thienopyrimidine scaffold based on the GPR55 antagonist ML192, previously discovered by high-throughput screening. The GPR55 activities of the new synthesized compounds were assessed using β-arrestin recruitment assays in Chinese hamster ovary cells overexpressing human GPR55. Some derivatives were identified as GPR55 antagonists with functional efficacy and selectivity versus CB1 and CB2 cannabinoid receptors.

Identification of G protein-coupled receptor 55 (GPR55) as a target of curcumin

The identification of molecular targets of bioactive food components is important to understand the mechanistic aspect of their physiological functions. Here, we have developed a screening system that enables us to determine the activation of G protein-coupled receptors (GPCRs) by food components and have identified GPR55 as a target for curcumin. Curcumin activated GPR55 and induced serum-response element- and serum-response factor-mediated transcription, which were inhibited by Rho kinase and GPR55 antagonists. Both the methoxy group and the heptadienone moiety of curcumin were required for GPR55 activation. The F190^5.47 residue of GPR55 was important for the interaction with curcumin. The curcumin-induced secretion of glucagon-like peptide-1 in GLUTag cells was inhibited by a GPR55 antagonist. These results indicate that expression screening is a useful system to identify GPCRs as targets of food components and strongly suggest that curcumin activates GPR55 as an agonist, which is involved in the physiological function of curcumin.

GPR55 and GPR119 Receptors Contribute to the Processing of Neuropathic Pain in Rats

Orphan G-protein-coupled receptors (GPCR) comprise a large number of receptors which are widely distributed in the nervous system and represent an opportunity to identify new molecular targets in pain medicine. GPR55 and GPR119 are two orphan GPCR receptors whose physiological function is unclear. The aim was to explore the participation of spinal GPR55 and GPR119 in the processing of neuropathic pain in rats. Mechanical allodynia was evaluated using von Frey filaments. Protein localization and modulation were measured by immunohistochemistry and western blotting, respectively. Intrathecal administration of CID16020046 (selective GPR55 antagonist) or AS1269574 (selective GPR119 agonist) produced a dose-dependent antiallodynic effect, whereas O1062 (GPR55 agonist) and G-protein antagonist peptide dose-dependently prevented the antiallodynic effect of CID16020046 and AS1269574, respectively. Both GPR55 and GPR119 receptors were expressed in spinal cord, dorsal root ganglia and sciatic nerve, but only GPR119 was downregulated after 14 days of spinal nerve ligation. Data suggest that GPR55 and GPR119 participate in the processing of neuropathic pain and could be useful targets to manage neuropathic pain disorders.

The Endocannabinoid System in Glial Cells and Their Profitable Interactions to Treat Epilepsy: Evidence from Animal Models

Epilepsy is one of the most common neurological conditions. Yearly, five million people are diagnosed with epileptic-related disorders. The neuroprotective and therapeutic effect of (endo)cannabinoid compounds has been extensively investigated in several models of epilepsy. Therefore, the study of specific cell-type-dependent mechanisms underlying cannabinoid effects is crucial to understanding epileptic disorders. It is estimated that about 100 billion neurons and a roughly equal number of glial cells co-exist in the human brain. The glial population is in charge of neuronal viability, and therefore, their participation in brain pathophysiology is crucial. Furthermore, glial malfunctioning occurs in a wide range of neurological disorders. However, little is known about the impact of the endocannabinoid system (ECS) regulation over glial cells, even less in pathological conditions such as epilepsy. In this review, we aim to compile the existing knowledge on the role of the ECS in different cell types, with a particular emphasis on glial cells and their impact on epilepsy. Thus, we propose that glial cells could be a novel target for cannabinoid agents for treating the etiology of epilepsy and managing seizure-like disorders.

Expression of cannabinoid and cannabinoid-related receptors in the oral mucosa of healthy cats and cats with chronic gingivostomatitis

OBJECTIVES

Feline chronic gingivostomatitis (FCGS) is an oral disease. Cats with FCGS experience intense oral pain. Some cats remain refractory to current therapies based on dental extraction and adjuvant medical treatment; it is therefore necessary to investigate alternative therapeutic targets involved in inflammatory mechanisms and pain, namely the endocannabinoid system (ECS). The present study investigated the expression of cannabinoid receptors type 1 (CB1R) and 2 (CB2R), and cannabinoid-related receptors G protein-coupled receptor 55 (GPR55), transient receptor potential ankyrin 1 (TRPA1) and serotonin 1a receptor (5-HT1aR), in the oral mucosa of healthy cats to determine whether there was altered expression and distribution in cats with FCGS.

METHODS

Samples of caudal oral mucosa were collected from eight control cats (CTRL cats) and from eight cats with FCGS (FCGS cats). Tissue samples were processed using an immunofluorescence assay with cat-specific antibodies, and the immunolabelling of the receptors studied was semiquantitatively evaluated.

RESULTS

The mucosal epithelium of the CTRL cats showed CB1R, TRPA1 and 5-HT1aR immunoreactivity (IR), while CB2R and GPR55 IR were generally not expressed. In the CTRL cats, the subepithelial inflammatory cells expressed CB2R, GPR55 and 5-HT1aR IR. In the FCGS cats, all the receptors studied were markedly upregulated in the epithelium and inflammatory infiltrate.

CONCLUSIONS AND RELEVANCE

Cannabinoid and cannabinoid-related receptors are widely expressed in the oral mucosa of healthy cats and are upregulated during the course of FCGS. The presence of cannabinoid and cannabinoid-related receptors in healthy tissues suggests the possible role of the ECS in the homeostasis of the feline oral mucosa, while their overexpression in the inflamed tissues of FCGS cats suggests the involvement of the ECS in the pathogenesis of this disease, with a possible role in the related inflammation and pain. Based on the present findings, ECS could be considered a potential therapeutic target for patients with FCGS.

GPCRs get fatty: the role of G protein-coupled receptor signaling in the development and progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), characterized by the abnormal deposition of lipids within the liver not due to alcohol consumption, is a growing epidemic affecting over 30% of the United States population. Both simple fatty liver and its more severe counterpart, nonalcoholic steatohepatitis, represent one of the most common forms of liver disease. Recently, several G protein-coupled receptors have emerged as targets for therapeutic intervention for these disorders. These include those with known hepatic function as well as those involved in global metabolic regulation. In this review, we highlight these emerging therapeutic targets, focusing on several common themes including their activation by microbial metabolites, stimulatory effect on insulin and incretin secretion, and contribution to glucose tolerance. The overlap in ligands, localization, and downstream effects of activation indicate the interdependent nature of these receptors and highlight the importance of this signaling family in the development and prevention of NAFLD.

Cannabinoids in the descending pain modulatory circuit: Role in inflammation

The legalization of cannabis in some states has intensified interest in the potential for cannabis and its constituents to lead to novel therapeutics for pain. Our understanding of the cellular mechanisms underlying cannabinoid actions in the brain have lagged behind opioids; however, the current opioid epidemic has also increased attention on the use of cannabinoids as alternatives to opioids for pain, especially chronic pain that requires long-term use. Endogenous cannabinoids are lipid signaling molecules that have complex roles in modulating neuronal function throughout the brain. In this review, we discuss cannabinoid functions in the descending pain modulatory pathway, a brain circuit that integrates cognitive and emotional processing of pain to modulate incoming sensory inputs. In addition, we highlight areas where further studies are necessary to understand cannabinoid regulation of descending pain modulation.

Cannabinoids, Chemical Senses, and Regulation of Feeding Behavior

The herb Cannabis sativa has been traditionally used in many cultures and all over the world for thousands of years as medicine and recreation. However, because it was brought to the Western world in the late 19th century, its use has been a source of controversy with respect to its physiological effects as well as the generation of specific behaviors. In this regard, the CB1 receptor represents the most relevant target molecule of cannabinoid components on nervous system and whole-body energy homeostasis. Thus, the promotion of CB1 signaling can increase appetite and stimulate feeding, whereas blockade of CB1 suppresses hunger and induces hypophagia. Taste and flavor are sensory experiences involving the oral perception of food-derived chemicals and drive a primal sense of acceptable or unacceptable for what is sampled. Therefore, research within the last decades focused on deciphering the effect of cannabinoids on the chemical senses involved in food perception and consequently in the pattern of feeding. In this review, we summarize the data on the effect of cannabinoids on chemical senses and their influences on food intake control and feeding behavior.

GPCRs and Insulin Receptor Signaling in Conversation: Novel Avenues for Drug Discovery

Type 2 diabetes is a major health issue worldwide with complex metabolic and endocrine abnormalities. Hyperglycemia, defects in insulin secretion and insulin resistance are classic features of type 2 diabetes. Insulin signaling regulates metabolic homeostasis by regulating glucose and lipid turnover in the liver, skeletal muscle and adipose tissue. Major treatment modalities for diabetes include the drugs from the class of sulfonyl urea, Insulin, GLP-1 agonists, SGLT2 inhibitors, DPP-IV inhibitors and Thiazolidinediones. Emerging antidiabetic therapeutics also include classes of drugs targeting GPCRs in the liver, adipose tissue and skeletal muscle. Interestingly, recent research highlights several shared intermediates between insulin and GPCR signaling cascades opening potential novel avenues for diabetic drug discovery.

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.