Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors

Heterodimers Revolutionize the Field of Metabotropic Glutamate Receptors

Identified 40 years ago, the metabotropic glutamate (mGlu) receptors play key roles in modulating many synapses in the brain, and are still considered as important drug targets to treat various brain diseases. Eight genes encoding mGlu subunits have been identified. They code for complex receptors composed of a large extracellular domain where glutamate binds, connected to a G protein activating membrane domain. They are covalently linked dimers, a quaternary structure needed for their activation by glutamate. For many years they have only been considered as homodimers, then limiting the number of mGlu receptors to 8 subtypes composed of twice the same subunit. Twelve years ago, mGlu subunits were shown to also form heterodimers with specific subunits combinations, increasing the family up to 19 different potential dimeric receptors. Since then, a number of studies brought evidence for the existence of such heterodimers in the brain, through various approaches. Structural and molecular dynamic studies helped understand their peculiar activation process. The present review summarizes the approaches used to study their activation process and their pharmacological properties and to demonstrate their existence in vivo. We will highlight how the existence of mGlu heterodimers revolutionizes the mGlu receptor field, opening new possibilities for therapeutic intervention for brain diseases. As illustrated by the number of possible mGlu heterodimers, this study will highlight the need for further research to fully understand their role in physiological and pathological conditions, and to develop more specific therapeutic tools.

Emerging and Re-emerging Warheads for Targeted Covalent Inhibitors: An Update

Covalent inhibitors and other types of covalent modalities have seen a revival in the past two decades, with a variety of new targeted covalent drugs having been approved in recent years. A key feature of such molecules is an intrinsically reactive group, typically a weak electrophile, which enables the irreversible or reversible formation of a covalent bond with a specific amino acid of the target protein. This reactive group, often called the "warhead", is a critical determinant of the ligand's activity, selectivity, and general biological properties. In 2019, we summarized emerging and re-emerging warhead chemistries to target cysteine and other amino acids (Gehringer, M.; Laufer, S. A. J. Med. Chem. 2019, 62, 5673-5724; DOI: 10.1021/acs.jmedchem.8b01153). Since then, the field has rapidly evolved. Here we discuss the progress on covalent warheads made since our last Perspective and their application in medicinal chemistry and chemical biology.

Therapeutic Potential for Metabotropic Glutamate Receptor 7 Modulators in Cognitive Disorders

Metabotropic glutamate receptor 7 (mGlu7) is the most highly conserved and abundantly expressed mGlu receptor in the human brain. The presynaptic localization of mGlu7, coupled with its low affinity for its endogenous agonist, glutamate, are features that contribute to the receptor's role in modulating neuronal excitation and inhibition patterns, including long-term potentiation, in various brain regions. These characteristics suggest that mGlu7 modulation may serve as a novel therapeutic strategy in disorders of cognitive dysfunction, including neurodevelopmental disorders that cause impairments in learning, memory, and attention. Primary mutations in the GRM7 gene have recently been identified as novel causes of neurodevelopmental disorders, and these patients exhibit profound intellectual and cognitive disability. Pharmacological tools, such as agonists, antagonists, and allosteric modulators, have been the mainstay for targeting mGlu7 in its endogenous homodimeric form to probe effects of its function and modulation in disease models. However, recent research has identified diversity in dimerization, as well as trans-synaptic interacting proteins, that also play a role in mGlu7 signaling and pharmacological properties. These novel findings represent exciting opportunities in the field of mGlu receptor drug discovery and highlight the importance of further understanding the functions of mGlu7 in complex neurologic conditions at both the molecular and physiologic levels. SIGNIFICANCE STATEMENT: Proper expression and function of mGlu7 is essential for learning, attention, and memory formation at the molecular level within neural circuits. The pharmacological targeting of mGlu7 is undergoing a paradigm shift by incorporating an understanding of receptor interaction with other cis- and trans- acting synaptic proteins, as well as various intracellular signaling pathways. Based upon these new findings, mGlu7's potential as a drug target in the treatment of cognitive disorders and learning impairments is primed for exploration.

Constitutive activation mechanism of a class C GPCR

Class C G-protein-coupled receptors (GPCRs) are activated through binding of agonists to the large extracellular domain (ECD) followed by rearrangement of the transmembrane domains (TMDs). GPR156, a class C orphan GPCR, is unique because it lacks an ECD and exhibits constitutive activity. Impaired GPR156-Gi signaling contributes to loss of hearing. Here we present the cryo-electron microscopy structures of human GPR156 in the Go-free and Go-coupled states. We found that an endogenous phospholipid molecule is located within each TMD of the GPR156 dimer. Asymmetric binding of Gα to the phospholipid-bound GPR156 dimer restructures the first and second intracellular loops and the carboxy-terminal part of the elongated transmembrane 7 (TM7) without altering dimer conformation. Our findings reveal that GPR156 is a transducer for phospholipid signaling. Constant binding of abundant phospholipid molecules and the G-protein-induced reshaping of the cytoplasmic face provide a basis for the constitutive activation of GPR156.

Small vs. Large Library Docking for Positive Allosteric Modulators of the Calcium Sensing Receptor

Drugs acting as positive allosteric modulators (PAMs) to enhance the activation of the calcium sensing receptor (CaSR) and to suppress parathyroid hormone (PTH) secretion can treat hyperparathyroidism but suffer from side effects including hypocalcemia and arrhythmias. Seeking new CaSR modulators, we docked libraries of 2.7 million and 1.2 billion molecules against transforming pockets in the active-state receptor dimer structure. Consistent with simulations suggesting that docking improves with library size, billion-molecule docking found new PAMs with a hit rate that was 2.7-fold higher than the million-molecule library and with hits up to 37-fold more potent. Structure-based optimization of ligands from both campaigns led to nanomolar leads, one of which was advanced to animal testing. This PAM displays 100-fold the potency of the standard of care, cinacalcet, in ex vivo organ assays, and reduces serum PTH levels in mice by up to 80% without the hypocalcemia typical of CaSR drugs. Cryo-EM structures with the new PAMs show that they induce residue rearrangements in the binding pockets and promote CaSR dimer conformations that are closer to the G-protein coupled state compared to established drugs. These findings highlight the promise of large library docking for therapeutic leads, especially when combined with experimental structure determination and mechanism.

Structural studies of serotonin receptor family

Serotonin receptors, also known as 5-HT receptors, belong to the G protein-coupled receptors (GPCRs) superfamily. They mediate the effects of serotonin, a neurotransmitter that plays a key role in a wide range of functions including mood regulation, cognition and appetite. The functions of serotonin are mediated by a family of 5-HT receptors including 12 GPCRs belonging to six major families: 5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6 and 5-HT7. Despite their distinct characteristics and functions, these receptors' subtypes share common structural features and signaling mechanisms. Understanding the structure, functions and pharmacology of the serotonin receptor family is essential for unraveling the complexities of serotonin signaling and developing targeted therapeutics for neuropsychiatric disorders. However, developing drugs that selectively target specific receptor subtypes is challenging due to the structural similarities in their orthosteric binding sites. This review focuses on the recent advancements in the structural studies of 5-HT receptors, highlighting the key structural features of each subtype and shedding light on their potential as targets for mental health and neurological disorders (such as depression, anxiety, schizophrenia, and migraine) drugs. [BMB Reports 2023; 56(10): 527-536].

Heterodimerization of Chemoreceptors TAS1R3 and mGlu2 in Human Blood Leukocytes

The expression of canonical chemosensory receptors of the tongue, such as the heteromeric sweet taste (TAS1R2/TAS1R3) and umami taste (TAS1R1/TAS1R3) receptors, has been demonstrated in many extra-oral cells and tissues. Gene expression studies have revealed transcripts for all TAS1 and metabotropic glutamate (mGlu) receptors in different types of immune cells, where they are involved, for example, in the chemotaxis of human neutrophils and the protection of T cells from activation-induced cell death. Like other class-C G protein-coupling receptors (GPCRs), TAS1Rs and mGlu receptors form heteromers within their families. Since mGlu receptors and TAS1R1/TAS1R3 share the same ligand, monosodium glutamate (MSG), we hypothesized their hitherto unknown heteromerization across receptor families in leukocytes. Here we show, by means of immunocytochemistry and co-IP/Western analysis, that across class-C GPCR families, mGlu2 and TAS1R3 co-localize and heterodimerize in blood leukocytes. Expressing the recombinant receptors in HEK-293 cells, we validated their heterodimerization by bioluminescence resonance energy transfer. We demonstrate MSG-induced, mGlu2/TAS1R3 heteromer-dependent gain-of-function and pertussis toxin-sensitive signaling in luminescence assays. Notably, we show that mGlu2/TAS1R3 is necessary and sufficient for MSG-induced facilitation of N-formyl-methionyl-leucyl-phenylalanine-stimulated IL-8 secretion in neutrophils, using receptor-specific antagonists. In summary, our results demonstrate mGlu2/TAS1R3 heterodimerization in leukocytes, suggesting cellular function-tailored chemoreceptor combinations to modulate cellular immune responses.

Metabotropic Glutamate Receptor Subtype 5 Positron-Emission-Tomography Radioligands as a Tool for Central Nervous System Drug Development: Between Progress and Setbacks

The metabotropic glutamate receptor subtype 5 (mGluR5) is a class C G-protein-coupled receptor (GPCR) that has been implicated in various neuronal processes and, consequently, in several neuropsychiatric or neurodevelopmental disorders. Over the past few decades, mGluR5 has become a major focus for pharmaceutical companies, as an attractive target for drug development, particularly through the therapeutic potential of its modulators. In particular, allosteric binding sites have been targeted for better specificity and efficacy. In this context, Positron Emission Tomography (PET) appears as a useful tool for making decisions along a drug candidate's development process, saving time and money. Thus, PET provides quantitative information about a potential drug candidate and its target at the molecular level. However, in this area, particular attention has to be given to the interpretation of the PET signal and its conclusions. Indeed, the complex pharmacology of both mGluR5 and radioligands, allosterism, the influence of endogenous glutamate and the choice of pharmacokinetic model are all factors that may influence the PET signal. This review focuses on mGluR5 PET radioligands used at several stages of central nervous system drug development, highlighting advances and setbacks related to the complex pharmacology of these radiotracers.

The G protein-coupled receptor GPRC5C is a saccharide sensor with a novel 'off' response

GPRC5C is an orphan G protein-coupled receptor (GPCR) that belongs to the class C GPCR family. Although GPRC5C is expressed in various organs, its function and ligand are still undetermined. We found that GPRC5C is expressed in mouse taste cells, enterocytes, and pancreatic α-cells. In functional imaging assays, HEK293 cells heterologously expressing GPRC5C and the chimeric G protein α subunit Gα16-gust44 showed robust intracellular Ca2+ increases in response to monosaccharides, disaccharides, and a sugar alcohol, but not an artificial sweetener or sweet-tasting amino acid. Notably, Ca2+ increases occurred after washout, not during stimulation. Our findings suggest that GPRC5C has receptor properties which lead to novel 'off' responses to saccharide detachment and may work as an internal or external chemosensor specifically tuned to natural sugars.

Prediction of dynamic allostery for the transmembrane domain of the sweet taste receptor subunit, TAS1R3

The sweet taste receptor plays an essential role as an energy sensor by detecting carbohydrates. However, the dynamic mechanisms of receptor activation remain unclear. Here, we describe the interactions between the transmembrane domain of the G protein-coupled sweet receptor subunit, TAS1R3, and allosteric modulators. Molecular dynamics simulations reproduced species-specific sensitivity to ligands. We found that a human-specific sweetener, cyclamate, interacted with the mouse receptor as a negative allosteric modulator. Agonist-induced allostery during receptor activation was found to destabilize the intracellular part of the receptor, which potentially interfaces with the Gα subunit, through ionic lock opening. A common human variant (R757C) of the TAS1R3 exhibited a reduced response to sweet taste, in support of our predictions. Furthermore, histidine residues in the binding site acted as pH-sensitive microswitches to modulate the sensitivity to saccharin. This study provides important insights that may facilitate the prediction of dynamic activation mechanisms for other G protein-coupled receptors.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

Coevolutionary signals in metabotropic glutamate receptors capture residue contacts and long-range functional interactions

Upon ligand binding to a G protein-coupled receptor, extracellular signals are transmitted into a cell through sets of residue interactions that translate ligand binding into structural rearrangements. These interactions needed for functions impose evolutionary constraints so that, on occasion, mutations in one position may be compensated by other mutations at functionally coupled positions. To quantify the impact of amino acid substitutions in the context of major evolutionary divergence in the G protein-coupled receptor subfamily of metabotropic glutamate receptors (mGluRs), we combined two phylogenetic-based algorithms, Evolutionary Trace and covariation Evolutionary Trace, to infer potential structure-function couplings and roles in mGluRs. We found a subset of evolutionarily important residues at known functional sites and evidence of coupling among distinct structural clusters in mGluR. In addition, experimental mutagenesis and functional assays confirmed that some highly covariant residues are coupled, revealing their synergy. Collectively, these findings inform a critical step toward understanding the molecular and structural basis of amino acid variation patterns within mGluRs and provide insight for drug development, protein engineering, and analysis of naturally occurring variants.

Systematic analysis reveals novel insight into the molecular determinants of function, diversity and evolution of sweet taste receptors T1R2/T1R3 in primates

Sweet taste is a primary sensation for the preference and adaption of primates to diet, which is crucial for their survival and fitness. It is clear now that the sweet perception is mediated by a G protein-coupled receptor (GPCR)-sweet taste receptor T1R2/T1R3, and many behavioral or physiological experiments have described the diverse sweet taste sensitivities in primates. However, the structure-function relationship of T1R2s/T1R3s in primates, especially the molecular basis for their species-dependent sweet taste, has not been well understood until now. In this study, we performed a comprehensive sequence, structural and functional analysis of sweet taste receptors in primates to elucidate the molecular determinants mediating their species-dependent sweet taste recognition. Our results reveal distinct taxonomic distribution and significant characteristics (interaction, coevolution and epistasis) of specific key function-related residues, which could partly account for the previously reported behavioral results of taste perception in primates. Moreover, the prosimians Lemuriformes species, which were reported to have no sensitivity to aspartame, could be proposed to be aspartame tasters based on the present analysis. Collectively, our study provides new insights and promotes a better understanding for the diversity, function and evolution of sweet taste receptors in primates.

Design and Synthesis of New Quinazolin-4-one Derivatives with Negative mGlu7 Receptor Modulation Activity and Antipsychotic-Like Properties

Following the glutamatergic theory of schizophrenia and based on our previous study regarding the antipsychotic-like activity of mGlu₇ NAMs, we synthesized a new compound library containing 103 members, which were examined for NAM mGlu₇ activity in the T-REx 293 cell line expressing a recombinant human mGlu₇ receptor. Out of the twenty-two scaffolds examined, active compounds were found only within the quinazolinone chemotype. 2-(2-Chlorophenyl)-6-(2,3-dimethoxyphenyl)-3-methylquinazolin-4(3H)-one (A9-7, ALX-171, mGlu₇ IC₅₀ = 6.14 µM) was selective over other group III mGlu receptors (mGlu₄ and mGlu₈), exhibited satisfactory drug-like properties in preliminary DMPK profiling, and was further tested in animal models of antipsychotic-like activity, assessing the positive, negative, and cognitive symptoms. ALX-171 reversed DOI-induced head twitches and MK-801-induced disruptions of social interactions or cognition in the novel object recognition test and spatial delayed alternation test. On the other hand, the efficacy of the compound was not observed in the MK-801-induced hyperactivity test or prepulse inhibition. In summary, the observed antipsychotic activity profile of ALX-171 justifies the further development of the group of quinazolin-4-one derivatives in the search for a new drug candidate for schizophrenia treatment.

Evaluation of [18F]FMTEB in Sprague Dawley rats as a PET tracer for metabotropic glutamate receptor 5

INTRODUCTION

[¹⁸F]FMTEB, along with other tracers, was developed as a promising PET radioligand for imaging metabotropic glutamate receptor subtype 5 (mGluR5). Despite favorable preliminary results, it has not been used further for studies of mGluR5. This paper presents an in-depth preclinical evaluation of [¹⁸F]FMTEB in healthy Sprague Dawley rats.

METHODS

[¹⁸F]FMTEB was synthesized from a boronic ester precursor using copper-mediated fluorination. In vivo PET imaging was performed on six rats, of which three were pre-treated with a high affinity mGluR5 receptor antagonist. An additional 18 rats were used for ex vivo experiments for metabolite analyses in plasma, brain and urine, and for biodistribution and ex vivo brain autoradiography at different time points.

RESULTS

[¹⁸F]FMTEB was synthesized in adequate radiochemical yield and a molar activity of 154 ± 64 GBq/μmol. Both in vivo imaging and ex vivo brain autoradiography showed high specificity for mGluR5, and the blocking experiments showed a clear decrease in radioactivity in mGluR5-rich brain areas. Metabolite analyses confirmed fast metabolism of the tracer in plasma. The percentage of parent compound in brain tissue exceeded 90 % up to 90 min after injection.

CONCLUSION

[¹⁸F]FMTEB produced via copper-mediated ¹⁸F-fluorination fulfilled the requirements for preclinical evaluation in rats. The absence of specific uptake in cerebellum and absence of defluorination of the tracer allowed cerebellum to be used as a reference tissue. Due to the fast kinetics in rats, the region-to-cerebellum ratios equilibrated within 30 min. These results prove [¹⁸F]FMTEB to be a good candidate for mapping mGluR5 in rat brain and a suitable alternative to [¹⁸F]FPEB.

Functional optimization of light-activatable Opto-GPCRs: Illuminating the importance of the proximal C-terminus in G-protein specificity

Introduction: G-protein coupled receptors (GPCRs) are the largest family of human receptors that transmit signals from natural ligands and pharmaceutical drugs into essentially every physiological process. One main characteristic of G-protein coupled receptors is their ability to specifically couple with different families of G-proteins, thereby triggering specific downstream signaling pathways. While an abundance of structural information is available on G-protein coupled receptorn interactions with G-proteins, little is known about the G-protein coupled receptor domains functionally mediating G-protein specificity, in particular the proximal C-terminus, the structure which cannot be predicted with high confidentiality due to its flexibility. Methods: In this study, we exploited OptoGPCR chimeras between lightgated G-protein coupled receptors (opsins) and ligand-gated G-protein coupled receptors to systematically investigate the involvement of the C-terminus steering G-protein specificity. We employed rhodopsin-beta2-adrenoceptor and melanopsin-mGluR6 chimeras in second messenger assays and developed structural models of the chimeras. Results: We discovered a dominant role of the proximal C-terminus, dictating G-protein selectivity in the melanopsin-mGluR6 chimera, whereas it is the intracellular loop 3, which steers G-protein tropism in the rhodopsin-beta2-adrenoceptor. From the functional results and structural predictions, melanopsin and mGluR6 use a different mechanism to bovine rhodopsin and b2AR to couple to a selective G-protein. Discussion: Collectively, this work adds knowledge to the G-protein coupled receptor domains mediating G-protein selectivity, ultimately paving the way to optogenetically elicited specific G-protein signaling on demand.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

Sex-specific expression of pheromones and other signals in gravid starfish

BACKGROUND

Many echinoderms form seasonal aggregations prior to spawning. In some fecund species, a spawning event can lead to population outbreaks with detrimental ecosystem impacts. For instance, outbreaks of crown-of-thorns starfish (COTS), a corallivore, can destroy coral reefs. Here, we examine the gene expression in gravid male and female COTS prior to spawning in the wild, to identify genome-encoded factors that may regulate aggregation and spawning. This study is informed by a previously identified exoproteome that attracts conspecifics. To capture the natural gene expression profiles, we isolated RNAs from gravid female and male COTS immediately after they were removed from the Great Barrier Reef.  RESULTS: Sexually dimorphic gene expression is present in all seven somatic tissues and organs that we surveyed and in the gonads. Approximately 40% of the exoproteome transcripts are differentially expressed between sexes. Males uniquely upregulate an additional 68 secreted factors in their testes. A suite of neuropeptides in sensory organs, coelomocytes and gonads is differentially expressed between sexes, including the relaxin-like gonad-stimulating peptide and gonadotropin-releasing hormones. Female sensory tentacles-chemosensory organs at the distal tips of the starfish arms-uniquely upregulate diverse receptors and signalling molecules, including chemosensory G-protein-coupled receptors and several neuropeptides, including kisspeptin, SALMFamide and orexin.

CONCLUSIONS

Analysis of 103 tissue/organ transcriptomes from 13 wild COTS has revealed genes that are consistently differentially expressed between gravid females and males and that all tissues surveyed are sexually dimorphic at the molecular level. This finding is consistent with female and male COTS using sex-specific pheromones to regulate reproductive aggregations and synchronised spawning events. These pheromones appear to be received primarily by the sensory tentacles, which express a range of receptors and signalling molecules in a sex-specific manner. Furthermore, coelomocytes and gonads differentially express signalling and regulatory factors that control gametogenesis and spawning in other echinoderms.

A metabotropic glutamate receptor affects the growth and development of Schistosoma japonicum

Schistosomiasis is a zoonotic parasitic disease caused by schistosome infection that severely threatens human health. Therapy relies mainly on single drug treatment with praziquantel. Therefore, there is an urgent need to develop alternative medicines. The glutamate neurotransmitter in helminths is involved in many physiological functions by interacting with various cell-surface receptors. However, the roles and detailed regulatory mechanisms of the metabotropic glutamate receptor (mGluR) in the growth and development of Schistosoma japonicum remain poorly understood. In this study, we identified two putative mGluRs in S. japonicum and named them SjGRM7 (Sjc_001309, similar to GRM7) and SjGRM (Sjc_001163, similar to mGluR). Further validation using a calcium mobilization assay showed that SjGRM7 and SjGRM are glutamate-specific. The results of in situ hybridization showed that SjGRM is mainly located in the nerves of both males and gonads of females, and SjGRM7 is principally found in the nerves and gonads of males and females. In a RNA interference experiment, the results showed that SjGRM7 knockdown by double-stranded RNA (dsRNA) in S. japonicum caused edema, chassis detachment, and separation of paired worms in vitro. Furthermore, dsRNA interference of SjGRM7 could significantly affect the development and egg production of male and female worms in vivo and alleviate the host liver granulomas and fibrosis. Finally, we examined the molecular mechanisms underlying the regulatory function of mGluR using RNA sequencing. The data suggest that SjGRM7 propagates its signals through the G protein-coupled receptor signaling pathway to promote nervous system development in S. japonicum. In conclusion, SjGRM7 is a potential target for anti-schistosomiasis. This study enables future research on the mechanisms of action of Schistosomiasis japonica drugs.

The rs35874116 single nucleotide polymorphism increases sweet intake and the risk of severe early childhood caries: a case–control study

Background

Dental caries is one of the most prevalent chronic diseases worldwide among children. The rs35874116 single nucleotide polymorphism (SNP) in the sweet receptor gene TAS1R2 has been associated with dental caries at a high risk in permanent teeth among school children and adults. To date, little is known about the association of this SNP with sweet intake and caries risk in the primary school children.

Methods

Total of 236 children were included, namely 118 subjects in the non-caries (NC) group and severe early childhood caries (S-ECC) group, respectively. Oral mucosa cells were collected from all the selected children, and the full length of exon 3 in TAS1R2 was sequenced to analyse rs35874116 polymorphism. A questionnaire was used to collect information about socio-demographic information, frequency of sweet intake and oral hygiene habits. Multiple logistic regression models were applied to assess the relationship of rs35874116 polymorphism with frequency of sweet intake and S-ECC among the five-year-old children.

Results

Children with the TT genotype of rs35874116 had a higher frequency of sweet intake than CT/CC carriers (51.3% vs. 32.7%; x2 = 5.436, p = 0.020), and S-ECC individuals were more likely to be TT genotype carriers than NC individuals (53.5% vs. 46.5%; x2 = 4.353, p = 0.037). Multiple logistic regression analyses showed that the TT genotype of rs35874116 was not only significantly related to the frequency of sweet intake (OR = 2.25, 95% CI = 1.14–4.44) but also significantly associated with S-ECC (OR = 2.11, 95% CI = 1.01 ~ 4.42).

Conclusions

The rs35874116 polymorphism might increase sweet intake and the risk of S-ECC among five-year-old children in Nanning, China.

Metabotropic glutamate receptor 3 as a potential therapeutic target for psychiatric and neurological disorders

Glutamate is a major excitatory neurotransmitter in the central nervous system (CNS) and abnormalities in the glutamatergic system underlie various CNS disorders. As metabotropic glutamate receptor 3 (mGlu3 receptor) regulates glutamatergic transmission in various brain areas, emerging literature suggests that targeting mGlu3 receptors can be a novel approach to the treatment of psychiatric and neurological disorders. For example, mGlu3 receptor negative allosteric modulators (NAMs) induce rapid antidepressant-like effects in both acute and chronic stress models. Activation of mGlu3 receptors can enhance cognition in the rodents modeling schizophrenia-like pathophysiology. The mGlu3 receptors expressed in the astrocytes induce neuroprotective effects. Although polymorphisms in GRM3 have been shown to be associated with addiction, there is not significant evidence about the efficacy of mGlu3 receptor ligands in rodent models of addiction. Collectively, drugs targeting mGlu3 receptors may provide an alternative approach to fill the unmet clinical need for safer and more efficacious therapeutics for CNS disorders.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Blocked metabotropic glutamate receptor 5 enhances chemosensitivity in hepatocellular carcinoma and attenuates chemotoxicity in the normal liver by regulating DNA damage

DNA damaging agents are used as chemotherapeutics in many cancers, including hepatocellular carcinoma (HCC). However, they are associated with problems such as low sensitivity to chemotherapy and the induction of liver injury, underscoring the need to identify new therapies. Here, we investigated the differential regulatory effect of metabotropic glutamate receptor 5 (mGlu5) on chemosensitivity in HCC and chemotoxicity to the normal liver. The expression of mGlu5 was higher in HCC than in the normal liver, and correlated with poor prognosis according to The Cancer Genome Atlas database and Integrative Molecular Database of Hepatocellular Carcinoma. Cisplatin, oxaliplatin or methyl methanesulfonate (MMS) caused cell death by decreasing mGlu5 expression in HCC cells and increased mGlu5 expression in hepatic cells. In HCC cells, inhibition of mGlu5 aggravated MMS-induced DNA damage by increasing intracellular Ca2+ overload and mitogen-activated protein kinase (MAPK) activation, thereby promoting cell death, and activation of mGlu5 rescued the effect of MMS. However, in hepatic cells, mGlu5 inhibition alleviated MMS-induced DNA damage by downregulating Ca2+-derived MAPK pathways to advance hepatic cell survival. The opposite effects of mGlu5 overexpression or knockdown on MMS-induced DNA damage supported that cell death is a result of the differential regulation of mGlu5 expression. Inhibition of mGlu5 increased chemosensitivity and decreased chemotoxicity in a rat tumor model. This study suggests that mGlu5 inhibition could act synergistically with HCC chemotherapeutics with minimal side effects, which may improve the treatment of patients with HCC in the future.

An Insight into Animal Glutamate Receptors Homolog of Arabidopsis thaliana and Their Potential Applications-A Review

Most excitatory impulses received by neurons are mediated by ionotropic glutamate receptors (iGluRs). These receptors are located at the apex and play an important role in memory, neuronal development, and synaptic plasticity. These receptors are ligand-dependent ion channels that allow a wide range of cations to pass through. Glutamate, a neurotransmitter, activates three central ionotropic receptors: N-methyl-D-aspartic acid (NMDA), -amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), and kainic acid (KA). According to the available research, excessive glutamate release causes neuronal cell death and promotes neurodegenerative disorders. Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGL-rec genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology. In spite of these, the iGluRs, Arabidopsis glutamate receptors (AtGluR), is associated with the ligand binding activity, which confirms the evolutionary relationship between animal and plant glutamate receptors. Along with the above activities, the impact of mammalian agonists and antagonists on Arabidopsis suggests a correlation between plant and animal glutamate receptors. In addition, these glutamate receptors (plant/animal) are being utilized for the early detection of neurogenerative diseases using the fluorescence resonance energy transfer (FRET) approach. However, a number of scientific laboratories and institutes are consistently working on glutamate receptors with different aspects. Currently, we are also focusing on Arabidopsis glutamate receptors. The current review is focused on updating knowledge on AtGluR genes, their evolution, functions, and expression, and as well as in comparison with iGluRs. Furthermore, a high throughput approach based on FRET nanosensors developed for understanding neurotransmitter signaling in animals and plants via glutamate receptors has been discussed. The updated information will aid in the future comprehension of the complex molecular dynamics of glutamate receptors and the exploration of new facts in plant/animal biology.

Structural basis for strychnine activation of human bitter taste receptor TAS2R46

Taste sensing is a sophisticated chemosensory process, and bitter taste perception is mediated by type 2 taste receptors (TAS2Rs), or class T G protein–coupled receptors. Understanding the detailed molecular mechanisms behind taste sensation is hindered by a lack of experimental receptor structures. Here, we report the cryo–electron microscopy structures of human TAS2R46 complexed with chimeric mini–G protein gustducin, in both strychnine-bound and apo forms. Several features of TAS2R46 are disclosed, including distinct receptor structures that compare with known GPCRs, a new “toggle switch,” activation-related motifs, and precoupling with mini–G protein gustducin. Furthermore, the dynamic extracellular and more-static intracellular parts of TAS2R46 suggest possible diverse ligand-recognition and activation processes. This study provides a basis for further exploration of other bitter taste receptors and their therapeutic applications.

The Statistical Trends of Protein Evolution: A Lesson from AlphaFold Database

The recent development of artificial intelligence provides us with new and powerful tools for studying the mysterious relationship between organism evolution and protein evolution. In this work, based on the AlphaFold Protein Structure Database (AlphaFold DB), we perform comparative analyses of the proteins of different organisms. The statistics of AlphaFold-predicted structures show that, for organisms with higher complexity, their constituent proteins will have larger radii of gyration, higher coil fractions, and slower vibrations, statistically. By conducting normal mode analysis and scaling analyses, we demonstrate that higher organismal complexity correlates with lower fractal dimensions in both the structure and dynamics of the constituent proteins, suggesting that higher functional specialization is associated with higher organismal complexity. We also uncover the topology and sequence bases of these correlations. As the organismal complexity increases, the residue contact networks of the constituent proteins will be more assortative, and these proteins will have a higher degree of hydrophilic-hydrophobic segregation in the sequences. Furthermore, by comparing the statistical structural proximity across the proteomes with the phylogenetic tree of homologous proteins, we show that, statistical structural proximity across the proteomes may indirectly reflect the phylogenetic proximity, indicating a statistical trend of protein evolution in parallel with organism evolution. This study provides new insights into how the diversity in the functionality of proteins increases and how the dimensionality of the manifold of protein dynamics reduces during evolution, contributing to the understanding of the origin and evolution of lives.

G protein coupling and activation of the metabotropic GABAB heterodimer

Metabotropic γ-aminobutyric acid receptor (GABA_BR), a class C G protein-coupled receptor (GPCR) heterodimer, plays a crucial role in the central nervous system. Cryo-electron microscopy studies revealed a drastic conformational change upon activation and a unique G protein (GP) binding mode. However, little is known about the mechanism for GP coupling and activation for class C GPCRs. Here, we use molecular metadynamics computations to predict the mechanism by which the inactive GP induces conformational changes in the GABA_BR transmembrane domain (TMD) to form an intermediate pre-activated state. We find that the inactive GP first interacts with TM3, which further leads to the TMD rearrangement and deeper insertion of the α5 helix that causes the Gα subunit to open, releasing GDP, and forming the experimentally observed activated structure. This mechanism provides fresh insights into the mechanistic details of class C GPCRs activation expected to be useful for designing selective agonists and antagonists.

Despite its crucial role in the central nervous system, little is known about the activation mechanism of GABA_B receptor. Here, the authors predict that the inactive G protein induces conformational changes of the receptor to form an intermediate state.

Umami polypeptide detection system targeting the human T1R1 receptor and its taste-presenting mechanism

Umami is one of five basic tastes, the elucidation of its mechanism by the study of the interaction between umami polypeptides and hT1R1 umami receptors is of great significance. However, research on umami peptides targeting human T1R1 receptors is lacking, and the molecular mechanism remains elusive. Here, we successfully established a system to detect umami peptides targeting human T1R1 receptors by fluorescence spectroscopy, Surface Plasmon Resonance (SPR) and computational simulation. The sensory evaluation, calculated Kd value, and experimental affinity results between the four selected umami peptides (GRVSNCAA, KGDEESLA, KGGGGP, and TGDPEK) and glutamate were tested using this system, and all matched well. The maximum Ka value of GRVSNCAA was 479.55 M-1, and the minimum affinity of TGDPEK was 2.67 M-1. Computational simulations showed that the different peptide binding sites in the hT1R1 binding pocket occupied due to conformational changes are important factors for different taste thresholds, and that peptide hydrophobicity plays an important role in regulating affinity. Thus, our study enables rapid screening of high-intensity umami peptides and the development of T1R1 receptor-based umami detection sensors.

Blocking Metabotropic Glutamate Receptor Subtype 7 via the Venus Flytrap Domain Promotes a Chronic Stress-Resilient Phenotype in Mice

Chronic psychosocial stress participates prominently in the etiology of various psychiatric conditions and comorbid somatic pathologies; however, suitable pharmacotherapy of these disorders is still of high medical need. During the last few decades, research on mGlu receptors advanced remarkably and much attention was given to the mGlu7 subtype. Here, genetic mGlu7 ablation, short-term pharmacological mGlu7 blockade, as well as siRNA-mediated knockdown of mGlu7 were shown to result in an acute anti-stress, antidepressant- and anxiolytic-like phenotype in mice. Moreover, we recently revealed a prominent stress-protective effect of genetic mGlu7 ablation also with respect to chronic psychosocial stress. In addition, we are able to demonstrate in the present study that the chronic pharmacological blockade of mGlu7 interferes with various chronic stress-induced alterations. For this, we used the chronic subordinate colony housing (CSC), a mouse model of chronic male subordination, in combination with chronic treatment with the mGlu7-selective orthosteric-like antagonist XAP044 (7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one). Interestingly, XAP044 dose-dependently ameliorates hypothalamic–pituitary–adrenal axis dysfunctions, thymus atrophy, as well as the CSC-induced increase in innate anxiety. Taken together, our findings provide further evidence for the role of mGlu7 in chronic psychosocial stress-induced alterations and suggests the pharmacological blockade of mGlu7 as a promising therapeutic approach for the treatment of chronic stress-related pathologies in men.

Molecular Characterization and Expression Analysis of Putative Class C (Glutamate Family) G Protein-Coupled Receptors in Ascidian Styela clava

In this study, we performed the genome-wide domain analysis and sequence alignment on the genome of Styela clava, and obtained a repertoire of 204 putative GPCRs, which exhibited a highly reduced gene number compared to vertebrates and cephalochordates. In this repertoire, six Class C GPCRs, including four metabotropic glutamate receptors (Sc-GRMs), one calcium-sensing receptor (Sc-CaSR), and one gamma-aminobutyric acid (GABA) type B receptor 2-like (Sc-GABABR2-like) were identified, with the absence of type 1 taste and vomeronasal receptors. All the Sc-GRMs and Sc-CaSR contained the typical "Venus flytrap" and cysteine-rich domains required for ligand binding and subsequent propagation of conformational changes. In swimming larvae, Sc-grm3 and Sc-casr were mainly expressed at the junction of the sensory vesicle and tail nerve cord while the transcripts of Sc-grm4, Sc-grm7a, and Sc-grm7b appeared at the anterior trunk, which suggested their important functions in neurotransmission. The high expression of these Class C receptors at tail-regression and metamorphic juvenile stages hinted at their potential involvement in regulating metamorphosis. In adults, the transcripts were highly expressed in several peripheral tissues, raising the possibility that S. clava Class C GPCRs might function as neurotransmission modulators peripherally after metamorphosis. Our study systematically characterized the ancestral chordate Class C GPCRs to provide insights into the origin and evolution of these receptors in chordates and their roles in regulating physiological and morphogenetic changes relevant to the development and environmental adaption.

The pocketome of G-protein-coupled receptors reveals previously untargeted allosteric sites

G-protein-coupled receptors do not only feature the orthosteric pockets, where most endogenous agonists bind, but also a multitude of other allosteric pockets that have come into the focus as potential binding sites for synthetic modulators. Here, to better characterise such pockets, we investigate 557 GPCR structures by exhaustively docking small molecular probes in silico and converting the ensemble of binding locations to pocket-defining volumes. Our analysis confirms all previously identified pockets and reveals nine previously untargeted sites. In order to test for the feasibility of functional modulation of receptors through binding of a ligand to such sites, we mutate residues in two sites, in two model receptors, the muscarinic acetylcholine receptor M₃ and β₂-adrenergic receptor. Moreover, we analyse the correlation of inter-residue contacts with the activation states of receptors and show that contact patterns closely correlating with activation indeed coincide with these sites.

Group I mGluRs in Therapy and Diagnosis of Parkinson’s Disease: Focus on mGluR5 Subtype

Metabotropic glutamate receptors (mGluRs; members of class C G-protein-coupled receptors) have been shown to modulate excitatory neurotransmission, regulate presynaptic extracellular glutamate levels, and modulate postsynaptic ion channels on dendritic spines. mGluRs were found to activate myriad signalling pathways to regulate synapse formation, long-term potentiation, autophagy, apoptosis, necroptosis, and pro-inflammatory cytokines release. A notorious expression pattern of mGluRs has been evident in several neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and schizophrenia. Among the several mGluRs, mGluR5 is one of the most investigated types of considered prospective therapeutic targets and potential diagnostic tools in neurodegenerative diseases and neuropsychiatric disorders. Recent research showed mGluR5 radioligands could be a potential tool to assess neurodegenerative disease progression and trace respective drugs’ kinetic properties. This article provides insight into the group I mGluRs, specifically mGluR5, in the progression and possible therapy for PD.

Molecular insights into human taste perception and umami tastants: A review

Understanding taste is key for optimizing the palatability of seaweeds and other non‐animal‐based foods rich in protein. The lingual papillae in the mouth hold taste buds with taste receptors for the five gustatory taste qualities. Each taste bud contains three distinct cell types, of which Type II cells carry various G protein‐coupled receptors that can detect sweet, bitter, or umami tastants, while type III cells detect sour, and likely salty stimuli. Upon ligand binding, receptor‐linked intracellular heterotrimeric G proteins initiate a cascade of downstream events which activate the afferent nerve fibers for taste perception in the brain. The taste of amino acids depends on the hydrophobicity, size, charge, isoelectric point, chirality of the alpha carbon, and the functional groups on their side chains. The principal umami ingredient monosodium l‐glutamate, broadly known as MSG, loses umami taste upon acetylation, esterification, or methylation, but is able to form flat configurations that bind well to the umami taste receptor. Ribonucleotides such as guanosine monophosphate and inosine monophosphate strongly enhance umami taste when l‐glutamate is present. Ribonucleotides bind to the outer section of the venus flytrap domain of the receptor dimer and stabilize the closed conformation. Concentrations of glutamate, aspartate, arginate, and other compounds in food products may enhance saltiness and overall flavor. Umami ingredients may help to reduce the consumption of salts and fats in the general population and increase food consumption in the elderly.

Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Strontium (Sr) is a trace element taken with nutrition and found in bone in close connection to native hydroxyapatite. Sr is involved in a dual mechanism of coupling the stimulation of bone formation with the inhibition of bone resorption, as reported in the literature. Interest in studying Sr has increased in the last decades due to the development of strontium ranelate (SrRan), an orally active agent acting as an anti-osteoporosis drug. However, the use of SrRan was subjected to some limitations starting from 2014 due to its negative side effects on the cardiac safety of patients. In this scenario, an interesting perspective for the administration of Sr is the introduction of Sr ions in biomaterials for bone tissue engineering (BTE) applications. This strategy has attracted attention thanks to its positive effects on bone formation, alongside the reduction of osteoclast activity, proven by in vitro and in vivo studies. The purpose of this review is to go through the classes of biomaterials most commonly used in BTE and functionalized with Sr, i.e., calcium phosphate ceramics, bioactive glasses, metal-based materials, and polymers. The works discussed in this review were selected as representative for each type of the above-mentioned categories, and the biological evaluation in vitro and/or in vivo was the main criterion for selection. The encouraging results collected from the in vitro and in vivo biological evaluations are outlined to highlight the potential applications of materials’ functionalization with Sr as an osteopromoting dopant in BTE.

Metabotropic glutamate receptor orthosteric ligands and their binding sites

Metabotropic glutamate receptors (mGluRs) have been discovered almost four decades ago. Since then, their pharmacology has been largely developed as well as their structural organization. Indeed mGluRs are attractive therapeutic targets for numerous psychiatric and neurological disorders because of their modulating role of synaptic transmission. The more recent drug discovery programs have mostly concentrated on allosteric modulators. However, orthosteric agonists and antagonists have remained unavoidable pharmacological tools as, although not expected, many of them can reach the brain, or can be modified to reach the brain. This review focuses on the most common orthosteric ligands as well as on the few allosteric modulators interacting with the glutamate binding domain. The 3D-structures of these ligands at their binding sites are reported. For most of them, X-Ray structures or docked homology models are available. Because of the high conservation of the binding site, subtype selective agonists were not easy to find. Yet, some were discovered when extending their chemical structures in order to reach selective sites of the receptors.

Ultrasensitive Bioelectronic Tongue Based on the Venus Flytrap Domain of a Human Sweet Taste Receptor

Sweet taste is an important factor that regulates calorie intake and contributes to food preferences in humans and animals. Therefore, the evaluation of sweet substances is essential for various fields such as healthcare, food, and pharmaceutical industries. Sweet tastants are detected by sweet taste receptors which are class C G-protein-coupled receptors. T1R2 venus flytrap (VFT) of the sweet taste receptor is known as a primary ligand-binding domain for sweet tastants. In this study, we developed an ultrasensitive artificial sweet taste bioelectronic tongue based on the T1R2 VFT of a human sweet taste receptor. Here, the T1R2 VFT of a human sweet taste receptor was successfully overexpressed in a bacterial expression system. A T1R2 VFT-immobilized carbon nanotube field-effect transistor with floating electrodes was exploited as an artificial sweet taste sensory system. Significantly, our T1R2 VFT-functionalized bioelectronic tongue could be used to detect solutions of sweet tastants down to 0.1 fM and selectively discriminate sweet substances from other taste substances. Furthermore, our device could be used to monitor the response of the T1R2 VFT domain of a sweet taste receptor to sweet substances in real food environments such as apple juice and chamomile herb tea. Moreover, our device was used to evaluate the inhibition and enhancement effects on sweet taste receptors by zinc ions and chamomile tea, respectively. In addition, our device demonstrated long-term storability and reusability. In this respect, our sweet taste bioelectronic tongue could be a promising tool for various basic research and industrial applications.

GPCR heteromers: An overview of their classification, function and physiological relevance

G protein-coupled receptors (GPCRs) are capable of interacting to form higher order structures such as homomers and heteromers. Heteromerisation in particular has implications for receptor function, with research showing receptors can attain unique expression, ligand binding, signalling and intracellular trafficking upon heteromerisation. As such, GPCR heteromers represent novel drug targets with extensive therapeutic potential. Changes to ligand affinity, efficacy and G protein coupling have all been described, with alterations to these pharmacological aspects now well accepted as common traits for heteromeric complexes. Changes in internalisation and trafficking kinetics, as well as β-arrestin interactions are also becoming more apparent, however, few studies to date have explicitly looked at the implications these factors have upon the signalling profile of a heteromer. Development of ligands to target GPCR heteromers both experimentally and therapeutically has been mostly concentrated on bivalent ligands due to difficulties in identifying and developing heteromer-specific ligands. Improving our understanding of the pharmacology and physiology of GPCR heteromers will enable further development of heteromer-specific ligands with potential to provide therapeutics with increased efficacy and decreased side effects.

Structure of the class C orphan GPCR GPR158 in complex with RGS7-Gβ5

GPR158, a class C orphan GPCR, functions in cognition, stress-induced mood control, and synaptic development. Among class C GPCRs, GPR158 is unique as it lacks a Venus flytrap-fold ligand-binding domain and terminates Gαi/o protein signaling through the RGS7-Gβ5 heterodimer. Here, we report the cryo-EM structures of GPR158 alone and in complex with one or two RGS7-Gβ5 heterodimers. GPR158 dimerizes through Per-Arnt-Sim-fold extracellular and transmembrane (TM) domains connected by an epidermal growth factor-like linker. The TM domain (TMD) reflects both inactive and active states of other class C GPCRs: a compact intracellular TMD, conformations of the two intracellular loops (ICLs) and the TMD interface formed by TM4/5. The ICL2, ICL3, TM3, and first helix of the cytoplasmic coiled-coil provide a platform for the DHEX domain of one RGS7 and the second helix recruits another RGS7. The unique features of the RGS7-binding site underlie the selectivity of GPR158 for RGS7.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

In silico mutational analyses reveal different ligand-binding abilities of double pockets of medaka fish taste receptor type 1 essential for efficient taste recognition

Taste receptors are important sensors for the detection of nutrient concentrations in animals. Tastes are recognized by interactions between chemical substances and taste receptors. Recently, the high-resolution X-ray crystal structure of the extracellular ligand-binding domains (LBDs) of medaka fish (Oryzias latipes) taste receptor type 1 (T1r) complexed with ligands (amino acids) was determined. Medaka fish T1r is a heterodimer composed of two different LBDs, T1r2aLBD and T1r3LBD. In this study, we performed all-atom molecular dynamics (MD) simulations on this heterodimer (T1r2aLBD-T1r3LBD) to address mutational effects on key residues near each ligand-binding pocket in recognizing one of the ligands (L-Gln). For T1r2aLBD, Ser165 is important in ligand recognition due to its direct hydrogen bonding with the ligand. After mutating Ser165 to Ile or Ala, the direct hydrogen bonds between the ligand and the binding pocket were weakened, which destabilized the ligand-binding form of T1r2aLBD. For T1r3LBD, Ser300 is important in ligand recognition. The water-mediated hydrogen bond with the side-chain hydroxyl group of Ser300 is a single interaction that maintains the ligand-binding form of T1r3LBD. After mutating Ser300 to Glu or Ala, both mutant systems almost maintained their ligand-binding form. As a mechanism for maintaining the binding form of T1r3LBD, alternative hydrogen bonds were formed as direct interactions instead of the indirect water-mediated interactions found in the wild-type system, which stabilized the binding form of T1r3LBD. Judging from our in silico mutational analyses, T1r2aLBD was structurally destabilized by the amino acid mutations. Therefore, it might be required that the ligand-binding pocket of T1r2aLBD is composed of a set of specific residues to maintain its ligand-binding form. On the contrary, T1r3LBD was robust enough to withstand the amino acid mutations. These different ligand-binding abilities of both LBDs provide multiple binding modes, which might be helpful for discriminating various taste substances or detecting concentrations of nutrients efficiently.

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. β-Amyloid 42 (Aβ42) oligomers interact with a mGluR5/cellular prion protein (PrP^C) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Scaffold Hopping to Imidazo[1,2-a]pyrazin-8-one Positive Allosteric Modulators of Metabotropic Glutamate 2 Receptor

Glutamate hyperfunction is implicated in multiple neurological and psychiatric diseases. Activation of the mGlu2 receptor results in reduced glutamate release and decreased excitability representing a promising novel therapeutic agent for the treatment of disorders such as epilepsy, schizophrenia, mood, anxiety, and other neuropsychiatric disorders. We have previously reported substantial efforts leading to potent and selective mGlu2 PAMs from different chemical series. Herein, the discovery and optimization of a novel series of imidazopyrazinone mGlu2 PAMs are reported. This new scaffold originated from computational searching of fragment databases and comparison with our previously explored scaffolds. Optimization guided by our robust understanding of SAR from former series led to potent, selective, and brain-penetrant compounds.

Agonists and allosteric modulators promote signaling from different metabotropic glutamate receptor 5 conformations

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors activated by the main excitatory neurotransmitter, L-glutamate. mGluR activation by agonists binding in the venus flytrap domain is regulated by positive (PAM) or negative (NAM) allosteric modulators binding to the 7-transmembrane domain (7TM). We report the cryo-electron microscopy structures of fully inactive and intermediate-active conformations of mGlu₅ receptor bound to an antagonist and a NAM or an agonist and a PAM, respectively, as well as the crystal structure of the 7TM bound to a photoswitchable NAM. The agonist induces a large movement between the subunits, bringing the 7TMs together and stabilizing a 7TM conformation structurally similar to the inactive state. Using functional approaches, we demonstrate that the PAM stabilizes a 7TM active conformation independent of the conformational changes induced by agonists, representing an alternative mode of mGlu activation. These findings provide a structural basis for different mGluR activation modes.

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Cryo-EM analysis of thermostabilized mGlu₅ receptor bound to inhibitors or activators

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X-ray structure of trans-Alloswitch-1 bound to thermostable mGlu₅ 7TMs

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Photopharmacology provides insight into allosteric regulation of mGlu₅ 7TMs

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Multiple conformations of mGlu₅ receptor activate G protein

Emerging roles of adhesion G protein-coupled receptors

Adhesion G protein-coupled receptors (aGPCRs) form a sub-group within the GPCR superfamily. Their distinctive structure contains an abnormally large N-terminal, extracellular region with a GPCR autoproteolysis-inducing (GAIN) domain. In most aGPCRs, the GAIN domain constitutively cleaves the receptor into two fragments. This process is often required for aGPCR signalling. Over the last two decades, much research has focussed on aGPCR-ligand interactions, in an attempt to deorphanize the family. Most ligands have been found to bind to regions N-terminal to the GAIN domain. These receptors may bind a variety of ligands, ranging across membrane-bound proteins and extracellular matrix components. Recent advancements have revealed a conserved method of aGPCR activation involving a tethered ligand within the GAIN domain. Evidence for this comes from increased activity in receptor mutants exposing the tethered ligand. As a result, G protein-coupling partners of aGPCRs have been more extensively characterised, making use of their tethered ligand to create constitutively active mutants. This has led to demonstrations of aGPCR function in, for example, neurodevelopment and tumour growth. However, questions remain around the ligands that may bind many aGPCRs, how this binding is translated into changes in the GAIN domain, and the exact mechanism of aGPCR activation following GAIN domain conformational changes. This review aims to examine the current knowledge around aGPCR activation, including ligand binding sites, the mechanism of GAIN domain-mediated receptor activation and how aGPCR transmembrane domains may relate to activation. Other aspects of aGPCR signalling will be touched upon, such as downstream effectors and physiological roles.

Structures of Gi-bound metabotropic glutamate receptors mGlu2 and mGlu4

The metabotropic glutamate receptors (mGlus) have key roles in modulating cell excitability and synaptic transmission in response to glutamate (the main excitatory neurotransmitter in the central nervous system)¹. It has previously been suggested that only one receptor subunit within an mGlu homodimer is responsible for coupling to G protein during receptor activation². However, the molecular mechanism that underlies the asymmetric signalling of mGlus remains unknown. Here we report two cryo-electron microscopy structures of human mGlu2 and mGlu4 bound to heterotrimeric G_i protein. The structures reveal a G-protein-binding site formed by three intracellular loops and helices III and IV that is distinct from the corresponding binding site in all of the other G-protein-coupled receptor (GPCR) structures. Furthermore, we observed an asymmetric dimer interface of the transmembrane domain of the receptor in the two mGlu-G_i structures. We confirmed that the asymmetric dimerization is crucial for receptor activation, which was supported by functional data; this dimerization may provide a molecular basis for the asymmetric signal transduction of mGlus. These findings offer insights into receptor signalling of class C GPCRs.

Therapeutic Opportunities of Targeting Allosteric Binding Sites on the Calcium-Sensing Receptor

The CaSR is a class C G protein-coupled receptor (GPCR) that acts as a multimodal chemosensor to maintain diverse homeostatic functions. The CaSR is a clinical therapeutic target in hyperparathyroidism and has emerged as a putative target in several other diseases. These include hyper- and hypocalcaemia caused either by mutations in the CASR gene or in genes that regulate CaSR signaling and expression, and more recently in asthma. The development of CaSR-targeting drugs is complicated by the fact that the CaSR possesses many different binding sites for endogenous and exogenous agonists and allosteric modulators. Binding sites for endogenous and exogenous ligands are located throughout the large CaSR protein and are interconnected in ways that we do not yet fully understand. This review summarizes our current understanding of CaSR physiology, signaling, and structure and how the many different binding sites of the CaSR may be targeted to treat disease.