State-dependent dynamics of extramembrane domains in the β2 -adrenergic receptor

G protein-coupled receptors (GPCRs) are membrane-bound signaling proteins that play an essential role in cellular signaling processes. Due to their intrinsic function of transmitting internal signals in response to external cues, these receptors are adapted to be highly dynamic in nature. The β2 -adrenergic receptor (β2 AR) is a representative member of the family that has been extensively analyzed in terms of its structure and activation. Although the structure of the transmembrane domain has been characterized in the different functional states of the receptor, the conformational dynamics of the extramembrane domains, especially the intrinsically disordered regions are still emerging. In this study, we analyze the state-dependent dynamics of extramembrane domains of β2 AR using atomistic molecular dynamics simulations. We introduce a parameter, the residue excess dynamics that allows us to better quantify receptor dynamics. Using this measure, we show that the dynamics of the extramembrane domains are sensitive to the receptor state. Interestingly, the ligand-bound intermediateR ' state shows the maximal dynamics compared to either the active R*G or inactive R states. Ligand binding appears to be correlated with high residue excess dynamics that are dampened upon G protein coupling. The intracellular loop-3 (ICL3) domain has a tendency to flip towards the membrane upon ligand binding, which could contribute to receptor "priming." We highlight an important ICL1-helix-8 interplay that is broken in the ligand-bound state but is retained in the active state. Overall, our study highlights the importance of characterizing the functional dynamics of the GPCR loop domains.

Neuromodulation and the toolkit for behavioural evolution: can ecdysis shed light on an old problem?

The geneticist Thomas Dobzhansky famously declared: 'Nothing in biology makes sense except in the light of evolution'. A key evolutionary adaptation of Metazoa is directed movement, which has been elaborated into a spectacularly varied number of behaviours in animal clades. The mechanisms by which animal behaviours have evolved, however, remain unresolved. This is due, in part, to the indirect control of behaviour by the genome, which provides the components for both building and operating the brain circuits that generate behaviour. These brain circuits are adapted to respond flexibly to environmental contingencies and physiological needs and can change as a function of experience. The resulting plasticity of behavioural expression makes it difficult to characterize homologous elements of behaviour and to track their evolution. Here, we evaluate progress in identifying the genetic substrates of behavioural evolution and suggest that examining adaptive changes in neuromodulatory signalling may be a particularly productive focus for future studies. We propose that the behavioural sequences used by ecdysozoans to moult are an attractive model for studying the role of neuromodulation in behavioural evolution.

Heterotrimeric G protein signaling without GPCRs: The Gα-binding-and-activating (GBA) motif

Heterotrimeric G proteins (Gαβγ) are molecular switches that relay signals from 7-transmembrane receptors located at the cell surface to the cytoplasm. The function of these receptors is so intimately linked to heterotrimeric G proteins that they are named G protein-coupled receptors (GPCRs), showcasing the interdependent nature of this archetypical receptor-transducer axis of transmembrane signaling in eukaryotes. It is generally assumed that activation of heterotrimeric G protein signaling occurs exclusively by the action of GPCRs, but this idea has been challenged by the discovery of alternative mechanisms by which G proteins can propagate signals in the cell. This review will focus on a general principle of G protein signaling that operates without the direct involvement of GPCRs. The mechanism of G protein signaling reviewed here is mediated by a class of G protein regulators defined by containing an evolutionarily conserved sequence named the Gα-binding-and-activating (GBA) motif. Using the best characterized proteins with a GBA motif as examples, Gα-interacting vesicle-associated protein (GIV)/Girdin and dishevelled-associating protein with a high frequency of leucine residues (DAPLE), this review will cover (i) the mechanisms by which extracellular cues not relayed by GPCRs promote the coupling of GBA motif-containing regulators with G proteins, (ii) the structural and molecular basis for how GBA motifs interact with Gα subunits to facilitate signaling, (iii) the relevance of this mechanism in different cellular and pathological processes, including cancer and birth defects, and (iv) strategies to manipulate GBA-G protein coupling for experimental therapeutics purposes, including the development of rationally engineered proteins and chemical probes.

Blockade of vasoactive intestinal peptide receptor 2 (VIPR2) signaling suppresses cyclin D1-dependent cell-cycle progression in MCF-7 cells

Vasoactive intestinal peptide (VIP) receptor 2 (VIPR2) is a G protein-coupled receptor that binds to Gαs, Gαi, and Gαq proteins to regulate various downstream signaling molecules, such as protein kinase A (PKA), phosphatidylinositol 3-kinase (PI3K), and phospholipase C. In this study, we examined the role of VIPR2 in cell cycle progression. KS-133, a newly developed VIPR2-selective antagonist peptide, attenuated VIP-induced cell proliferation in MCF-7 cells. The percentage of cells in the S-M phase was decreased in MCF-7 cells treated with KS-133. KS-133 in the presence of VIP decreased the phosphorylation of extracellular signal-regulated kinase (ERK), AKT, and glycogen synthase kinase-3β (GSK3β), resulting in a decrease in cyclin D1 levels. In MCF-7 cells stably-expressing VIPR2, KS-133 decreased PI3K activity and cAMP levels. Treatment with the ERK-specific kinase (MEK) inhibitor U0126 and the class I PI3K inhibitor ZSTK474 decreased the percentage of cells in the S phase. KS-133 reduced the percentage of cells in the S phase more than treatment with U0126 or ZSTK474 alone and did not affect the effect of the mixture of these inhibitors. Our findings suggest that VIPR2 signaling regulates cyclin D1 levels through the cAMP/PKA/ERK and PI3K/AKT/GSK3β pathways, and mediates the G1/S transition to control cell proliferation.

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

Evaluating conserved domains and motifs of decapod gonadotropin-releasing hormone G protein-coupled receptor superfamily

G protein-coupled receptors (GPCRs) are an ancient family of signal transducers that are both abundant and consequential in metazoan endocrinology. The evolutionary history and function of the GPCRs of the decapod superfamilies of gonadotropin-releasing hormone (GnRH) are yet to be fully elucidated. As part of which, the use of traditional phylogenetics and the recycling of a diminutive set of mis-annotated databases has proven insufficient. To address this, we have collated and revised eight existing and three novel GPCR repertoires for GnRH of decapod species. We developed a novel bioinformatic workflow that included clustering analysis to capture likely GnRH receptor-like proteins, followed by phylogenetic analysis of the seven transmembrane-spanning domains. A high degree of conservation of the sequences and topology of the domains and motifs allowed the identification of species-specific variation (up to ~70%, especially in the extracellular loops) that is thought to be influential to ligand-binding and function. Given the key functional role of the DRY motif across GPCRs, the classification of receptors based on the variation of this motif can be universally applied to resolve cryptic GPCR families, as was achieved in this work. Our results contribute to the resolution of the evolutionary history of invertebrate GnRH receptors and inform the design of bioassays in their deorphanization and functional annotation.

Fine-tuning activation specificity of G-protein-coupled receptors via automated path searching

Physics-based simulation methods can grant atomistic insights into the molecular origin of the function of biomolecules. However, the potential of such approaches has been hindered by their low efficiency, including in the design of selective agonists where simulations of myriad protein-ligand combinations are necessary. Here, we describe an automated input-free path searching protocol that offers (within 14 d using Graphics Processing Unit servers) a minimum free energy path (MFEP) defined in high-dimension configurational space for activating sphingosine-1-phosphate receptors (S1PRs) by arbitrary ligands. The free energy distributions along the MFEP for four distinct ligands and three S1PRs reached a remarkable agreement with Bioluminescence Resonance Energy Transfer (BRET) measurements of G-protein dissociation. In particular, the revealed transition state structures pointed out toward two S1PR3 residues F263/I284, that dictate the preference of existing agonists CBP307 and BAF312 on S1PR1/5. Swapping these residues between S1PR1 and S1PR3 reversed their response to the two agonists in BRET assays. These results inspired us to design improved agonists with both strong polar head and bulky hydrophobic tail for higher selectivity on S1PR1. Through merely three in silico iterations, our tool predicted a unique compound scaffold. BRET assays confirmed that both chiral forms activate S1PR1 at nanomolar concentration, 1 to 2 orders of magnitude less than those for S1PR3/5. Collectively, these results signify the promise of our approach in fine agonist design for G-protein-coupled receptors.

Optical Control of Cell-Surface and Endomembrane-Exclusive β-Adrenergic Receptor Signaling

Beta-adrenergic receptors (βARs) are G protein-coupled receptors (GPCRs) that mediate catecholamine-induced stress responses, such as heart rate increase and bronchodilation. In addition to signals from the cell surface, βARs also broadcast non-canonical signaling activities from the cell interior membranes (endomembranes). Dysregulation of these receptor pathways underlies severe pathological conditions. Excessive βAR stimulation is linked to cardiac hypertrophy, leading to heart failure, while impaired stimulation causes compromised fight or flight stress responses and homeostasis. In addition to plasma membrane βAR, emerging evidence indicates potential pathological implications of deeper endomembrane βARs, such as inducing cardiomyocyte hypertrophy and apoptosis, underlying heart failure. However, the lack of approaches to control their signaling in subcellular compartments exclusively has impeded linking endomembrane βAR signaling with pathology. Informed by the β1AR-catecholamine interactions, we engineered an efficiently photo-labile, protected hydroxy β1AR pro-ligand (OptoIso) to trigger βAR signaling at the cell surface, as well as exclusive endomembrane regions upon blue light stimulation. Not only does OptoIso undergo blue light deprotection in seconds, but it also efficiently enters cells and allows examination of G protein heterotrimer activation exclusively at endomembranes. In addition to its application in the optical interrogation of βARs in unmodified cells, given its ability to control deep organelle βAR signaling, OptoIso will be a valuable experimental tool.

Why classical receptor theory, which ignores allostery, can effectively measure the strength of an allosteric effect as agonist's efficacy

BACKGROUND AND PURPOSE

The classical theory of receptor action has been used for decades as a powerful tool to estimate molecular determinants of ligand-induced receptor activation (i.e., affinity and efficacy) from experimentally observable biological responses. However, it is also a well-recognized fact that the receptor-binding and activation mechanisms, and the parameters thereof, described in the classical theory contradict with the modern view of receptor activation based on allosteric principles.

EXPERIMENTAL APPROACH

We used mathematical analysis, along with some numerical simulations, to answer the key question as to what extent the classical theory is compatible-if at all-with the modern understanding of receptor activation.

KEY RESULTS

Here, we showed conclusively that (1) receptor activation equations based on allosteric principles contain the logic of the classical theory in disguise, and therefore, (2) estimates of "intrinsic efficacy" (ε) obtained by means of classical techniques (i.e., null methods or fitting the operational model to concentration-response data) are equivalent to the allosteric coupling factors that represent the molecular efficacy of ligands.

CONCLUSION AND IMPLICATIONS

Thus, we conclude that despite the justified criticisms it has received so far, the classical theory may continue to be useful in estimating ligand efficacy from experimental data, if used properly. Here, we also provide rigorous criteria for the proper use of the theory. These findings not only have implications for ligand classification but also resolve some long lasting discussions in the field of bias agonism in GPCR, which requires reasonable estimates of relative ligand efficacies at different signalling pathways.

Direct modulation of TRPC ion channels by Gα proteins

GPCR-Gi protein pathways are involved in the regulation of vagus muscarinic pathway under physiological conditions and are closely associated with the regulation of internal visceral organs. The muscarinic receptor-operated cationic channel is important in GPCR-Gi protein signal transduction as it decreases heart rate and increases GI rhythm frequency. In the SA node of the heart, acetylcholine binds to the M2 receptor and the released Gβγ activates GIRK (I(K,ACh)) channel, inducing a negative chronotropic action. In gastric smooth muscle, there are two muscarinic acetylcholine receptor (mAChR) subtypes, M2 and M3. M2 receptor activates the muscarinic receptor-operated nonselective cationic current (mIcat, NSCC(ACh)) and induces positive chronotropic effect. Meanwhile, M3 receptor induces hydrolysis of PIP2 and releases DAG and IP3. This IP3 increases intracellular Ca2+ and then leads to contraction of GI smooth muscles. The activation of mIcat is inhibited by anti-Gi/o protein antibodies in GI smooth muscle, indicating the involvement of Gαi/o protein in the activation of mIcat. TRPC4 channel is a molecular candidate for mIcat and can be directly activated by constitutively active Gαi QL proteins. TRPC4 and TRPC5 belong to the same subfamily and both are activated by Gi/o proteins. Initial studies suggested that the binding sites for G protein exist at the rib helix or the CIRB domain of TRPC4/5 channels. However, recent cryo-EM structure showed that IYY58-60 amino acids at ARD of TRPC5 binds with Gi3 protein. Considering the expression of TRPC4/5 in the brain, the direct G protein activation on TRPC4/5 is important in terms of neurophysiology. TRPC4/5 channels are also suggested as a coincidence detector for Gi and Gq pathway as Gq pathway increases intracellular Ca2+ and the increased Ca2+ facilitates the activation of TRPC4/5 channels. More complicated situation would occur when GIRK, KCNQ2/3 (IM) and TRPC4/5 channels are co-activated by stimulation of muscarinic receptors at the acetylcholine-releasing nerve terminals. This review highlights the effects of GPCR-Gi protein pathway, including dopamine, μ-opioid, serotonin, glutamate, GABA, on various oragns, and it emphasizes the importance of considering TRPC4/5 channels as crucial players in the field of neuroscience.

Optical Control of Adenosine A2A Receptor Using Istradefylline Photosensitivity

In recent years, there has been growing interest in the potential therapeutic use of inhibitors of adenosine A2A receptors (A2AR) for the treatment of neurodegenerative diseases and cancer. Nevertheless, the widespread expression of A2AR throughout the body emphasizes the importance of temporally and spatially selective ligands. Photopharmacology is an emerging strategy that utilizes photosensitive ligands to attain high spatiotemporal precision and regulate the function of biomolecules using light. In this study, we combined photochemistry and cellular and in vivo photopharmacology to investigate the light sensitivity of the FDA-approved antagonist istradefylline and its potential use as an A2AR photopharmacological tool. Our findings reveal that istradefylline exhibits rapid trans-to-cis isomerization under near-UV light, and prolonged exposure results in the formation of photocycloaddition products. We demonstrate that exposure to UV light triggers a time-dependent decrease in the antagonistic activity of istradefylline in A2AR-expressing cells and enables real-time optical control of A2AR signaling in living cells and zebrafish. Together, these data demonstrate that istradefylline is a photoinactivatable A2AR antagonist and that this property can be utilized to perform photopharmacological experiments in living cells and animals.

Role of lipids in the control of autophagy and primary cilium signaling in neurons

The brain is, after the adipose tissue, the organ with the greatest amount of lipids and diversity in their composition in the human body. In neurons, lipids are involved in signaling pathways controlling autophagy, a lysosome-dependent catabolic process essential for the maintenance of neuronal homeostasis and the function of the primary cilium, a cellular antenna that acts as a communication hub that transfers extracellular signals into intracellular responses required for neurogenesis and brain development. A crosstalk between primary cilia and autophagy has been established; however, its role in the control of neuronal activity and homeostasis is barely known. In this review, we briefly discuss the current knowledge regarding the role of autophagy and the primary cilium in neurons. Then we review the recent literature about specific lipid subclasses in the regulation of autophagy, in the control of primary cilium structure and its dependent cellular signaling in physiological and pathological conditions, specifically focusing on neurons, an area of research that could have major implications in neurodevelopment, energy homeostasis, and neurodegeneration.

Identification of neuropeptides and their G protein-coupled receptors in the predatory stink bug, Arma custos (Hemiptera: Pentatomidae)

The predatory stink bug Arma custos has been selected as an effective biological control agent and has been successfully massly bred and released into fields for the control of a diverse insect pests. As a zoophytophagous generalist, A. custos relies on a complex neuropeptide signaling system to prey on distinct food and adapt to different environments. However, information about neuropeptide signaling genes in A. custos has not been reported to date. In the present study, a total of 57 neuropeptide precursor transcripts and 41 potential neuropeptide G protein-coupled receptor (GPCR) transcripts were found mainly using our sequenced transcriptome data. Furthermore, a number of neuropeptides and their GPCR receptors that were enriched in guts and salivary glands of A. custos were identified, which might play critical roles in feeding and digestion. Our study provides basic information for an in-depth understanding of biological and ecological characteristics of the predatory bug and would aid in the development of better pest management strategies based on the effective utilization and protection of beneficial natural enemies.

Advancements in the use of xenopus oocytes for modelling neurological disease for novel drug discovery

INTRODUCTION

Introduced about 50 years ago, the model of Xenopus oocytes for the expression of recombinant proteins has gained a broad spectrum of applications. The authors herein review the benefits brought from using this model system, with a focus on modeling neurological disease mechanisms and application to drug discovery.

AREAS COVERED

Using multiple examples spanning from ligand gated ion channels to transporters, this review presents, in the light of the latest publications, the benefits offered from using Xenopus oocytes. Studies range from the characterization of gene mutations to the discovery of novel treatments for disorders of the central nervous system (CNS).

EXPERT OPINION

Development of new drugs targeting CNS disorders has been marked by failures in the translation from preclinical to clinical studies. As progress in genetics and molecular biology highlights large functional differences arising from a single to a few amino acid exchanges, the need for drug screening and functional testing against human proteins is increasing. The use of Xenopus oocytes to enable precise modeling and characterization of clinically relevant genetic variants constitutes a powerful model system that can be used to inform various aspects of CNS drug discovery and development.

Unraveling the G protein-coupled receptor superfamily in aphids: Contractions and duplications linked to phloem feeding

The G Protein-Coupled Receptor (GPCR) superfamily is the largest and most diverse transmembrane receptor family, playing crucial roles in regulating various physiological processes. As one of the most destructive pests, aphids have been subject to previous studies, which revealed fewer GPCR superfamily members in Acyrthosiphon pisum and Aphis gossypii and the loss of multiple neuropeptide GPCRs. To elucidate the contraction patterns and evolutionary features of the aphid GPCR superfamily, we identified 97, 105, and 95 GPCR genes in Rhopalosiphum maidis, A. pisum, and A. gossypii, respectively. Comparative analysis and phylogenetic investigations with other hemipteran insects revealed a contracted GPCR superfamily in aphids. This contraction mainly occurred in biogenic amine receptors, GABA-B-R, and fz families, and several neuropeptide receptors such as ACPR, CrzR, and PTHR were completely lost. This phenomenon may be related to the parasitic nature of aphids. Additionally, several GPCRs associated with aphid feeding and water balance underwent duplication, including Lkr, NPFR, CCHa1-R, and DH-R, Type A LGRs, but the SK/CCKLR that inhibits feeding was completely lost, indicating changes in feeding genes that underpin the aphid's prolonged phloem feeding behavior. Furthermore, we observed fine-tuning in opsins, with reduced long-wavelength opsins and additional duplications of short-wavelength opsin, likely associated with daytime activity. Lastly, we found variations in the number of mthl genes in aphids. In conclusion, our investigation sheds light on the GPCR superfamily in aphids, revealing its association with diet lifestyle and laying the foundation for understanding and developing control strategies for the aphid GPCR superfamily.

Intensive receptor blockade and plasma exchange to treat refractory scleroderma renal crisis in patients with agonistic autoantibodies targeting the angiotensin II type 1 and endothelin-1 type A receptors

Scleroderma renal crisis is a rare complication of systemic sclerosis characterized by a rapid decline in kidney function due to acute renal vascular injury. Recently, activating autoantibodies targeting the angiotensin II type 1 receptor and the endothelin-1 type A receptor have been implicated in the pathophysiology of scleroderma renal crisis by sensitizing the angiotensin II type 1 receptor and endothelin-1 type A receptor in renal resistance arteries to their natural ligands. Here, we describe a cohort of 10 patients with scleroderma renal crisis refractory to standard treatment, including blockade of the renin-angiotensin system. Multimodal therapy was initiated, targeting at the removal of anti-angiotensin II type 1 receptor and anti-endothelin-1 type A receptor autoantibodies by plasma exchange and the reduction of vasoconstrictive activity. Further treatment options included angiotensin II type 1 receptor and endothelin-1 type A receptor blockade, iloprost, intravenous immunoglobulins, and immunosuppression. Six patients were hypertensive. On kidney biopsy, concentric intimal sclerosis was present in all patients, whereas acute vascular injury was evident in eight. Levels of anti-angiotensin II type 1 receptor and anti-endothelin-1 type A receptor autoantibodies were significantly reduced by multimodal treatment. Kidney function improved in three patients with histological signs of severe acute renal vascular damage. This report demonstrates that intensive multimodal therapy taking account of potentially pathogenic anti-angiotensin II type 1 receptor and anti-endothelin-1 type A receptor autoantibodies in concert with other vasodilatory interventions provides a salvage option for patients with refractory scleroderma renal crisis.

Hepatic regulator of G protein signaling 14 ameliorates NAFLD through activating cAMP-AMPK signaling by targeting Giα1/3

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) is an emerging public health threat as the most common chronic liver disease worldwide. However, there remains no effective medication to improve NAFLD. G protein-coupled receptors (GPCRs) are the most frequently investigated drug targets family. The Regulator of G protein signaling 14 (RGS14), as an essential negative modulator of GPCR signaling, plays important regulatory roles in liver damage and inflammatory responses. However, the role of RGS14 in NAFLD remains largely unclear.

METHODS AND RESULTS

In this study, we found that RGS14 was decreased in hepatocytes in NAFLD individuals in a public database. We employed genetic engineering technique to explore the function of RGS14 in NAFLD. We demonstrated that RGS14 overexpression ameliorated lipid accumulation, inflammatory response and liver fibrosis in hepatocytes in vivo and in vitro. Whereas, hepatocyte specific Rgs14-knockout (Rgs14-HKO) exacerbated high fat high cholesterol diet (HFHC) induced NASH. Further molecular experiments demonstrated that RGS14 depended on GDI activity to attenuate HFHC-feeding NASH. More importantly, RGS14 interacted with Guanine nucleotide-binding protein (Gi) alpha 1 and 3 (Giα1/3, gene named GNAI1/3), promoting the generation of cAMP and then activating the subsequent AMPK pathways. GNAI1/3 knockdown abolished the protective role of RGS14, indicating that RGS14 binding to Giα1/3 was required for prevention against hepatic steatosis.

CONCLUSIONS

RGS14 plays a protective role in the progression of NAFLD. RGS14-Giα1/3 interaction accelerated the production of cAMP and then activated cAMP-AMPK signaling. Targeting RGS14 or modulating the RGS14-Giα1/3 interaction may be a potential strategy for the treatment of NAFLD in the future.

Whole Brain Mapping of Orexin Receptor mRNA Expression Visualized by Branched In Situ Hybridization Chain Reaction

Orexins, which are produced within neurons of the lateral hypothalamic area, play a pivotal role in the regulation of various behaviors, including sleep/wakefulness, reward behavior, and energy metabolism, via orexin receptor type 1 (OX1R) and type 2 (OX2R). Despite the advanced understanding of orexinergic regulation of behavior at the circuit level, the precise distribution of orexin receptors in the brain remains unknown. Here, we develop a new branched in situ hybridization chain reaction (bHCR) technique to visualize multiple target mRNAs in a semiquantitative manner, combined with immunohistochemistry, which provided comprehensive distribution of orexin receptor mRNA and neuron subtypes expressing orexin receptors in mouse brains. Only a limited number of cells expressing both Ox1r and Ox2r were observed in specific brain regions, such as the dorsal raphe nucleus and ventromedial hypothalamic nucleus. In many brain regions, Ox1r-expressing cells and Ox2r-expressing cells belong to different cell types, such as glutamatergic and GABAergic neurons. Moreover, our findings demonstrated considerable heterogeneity in Ox1r- or Ox2r-expressing populations of serotonergic, dopaminergic, noradrenergic, cholinergic, and histaminergic neurons. The majority of orexin neurons did not express orexin receptors. This study provides valuable insights into the mechanism underlying the physiological and behavioral regulation mediated by the orexin system, as well as the development of therapeutic agents targeting orexin receptors.

Tools for investigating O-GlcNAc in signaling and other fundamental biological pathways

Cells continuously fine-tune signaling pathway proteins to match nutrient and stress levels in their local environment by modifying intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) sugars, an essential process for cell survival and growth. The small size of these monosaccharide modifications poses a challenge for functional determination, but the chemistry and biology communities have together created a collection of precision tools to study these dynamic sugars. This review presents the major themes by which O-GlcNAc influences signaling pathway proteins, including G-protein coupled receptors, growth factor signaling, mitogen-activated protein kinase (MAPK) pathways, lipid sensing, and cytokine signaling pathways. Along the way, we describe in detail key chemical biology tools that have been developed and applied to determine specific O-GlcNAc roles in these pathways. These tools include metabolic labeling, O-GlcNAc-enhancing RNA aptamers, fluorescent biosensors, proximity labeling tools, nanobody targeting tools, O-GlcNAc cycling inhibitors, light-activated systems, chemoenzymatic labeling, and nutrient reporter assays. An emergent feature of this signaling pathway meta-analysis is the intricate interplay between O-GlcNAc modifications across different signaling systems, underscoring the importance of O-GlcNAc in regulating cellular processes. We highlight the significance of O-GlcNAc in signaling and the role of chemical and biochemical tools in unraveling distinct glycobiological regulatory mechanisms. Collectively, our field has determined effective strategies to probe O-GlcNAc roles in biology. At the same time, this survey of what we do not yet know presents a clear roadmap for the field to use these powerful chemical tools to explore cross-pathway O-GlcNAc interactions in signaling and other major biological pathways.

Decrypting orphan GPCR drug discovery via multitask learning

The drug discovery of G protein-coupled receptors (GPCRs) superfamily using computational models is often limited by the availability of protein three-dimensional (3D) structures and chemicals with experimentally measured bioactivities. Orphan GPCRs without known ligands further complicate the process. To enable drug discovery for human orphan GPCRs, multitask models were proposed for predicting half maximal effective concentrations (EC50) of the pairs of chemicals and GPCRs. Protein multiple sequence alignment features, and physicochemical properties and fingerprints of chemicals were utilized to encode the protein and chemical information, respectively. The protein features enabled the transfer of data-rich GPCRs to orphan receptors and the transferability based on the similarity of protein features. The final model was trained using both agonist and antagonist data from 200 GPCRs and showed an excellent mean squared error (MSE) of 0.24 in the validation dataset. An independent test using the orphan dataset consisting of 16 receptors associated with less than 8 bioactivities showed a reasonably good MSE of 1.51 that can be further improved to 0.53 by considering the transferability based on protein features. The informative features were identified and mapped to corresponding 3D structures to gain insights into the mechanism of GPCR-ligand interactions across the GPCR family. The proposed method provides a novel perspective on learning ligand bioactivity within the diverse human GPCR superfamily and can potentially accelerate the discovery of therapeutic agents for orphan GPCRs.

Optimizing cryo-EM structural analysis of Gi-coupling receptors via engineered Gt and Nb35 application

Cryo-EM single particle analysis has recently facilitated the high-resolution structural determination of numerous GPCR-G complexes. Diverse methodologies have been devised with this trend, and in the case of GPCR-Gi complexes, scFv16, an antibody that recognizes the intricate interface of the complex, has been mainly implemented to stabilize the complex. However, owing to their flexibility and heterogeneity, structural determinations of GPCR-Gi complexes remain both challenging and resource-intensive. By employing eGαt, which exhibits binding affinity to modified nanobody Nb35, the cryo-EM structure of Rhodopsin-eGαt complex was previously reported. Using this modified G protein, we determined the structure of the ETB-eGt complex bound to the modified Nb35. The determined structure of ETB receptor was the same as the previously reported ETB-Gi complex, and the resulting dataset demonstrated significantly improved anisotropy. This modified G protein will be utilized for the structural determination of other GPCR-Gi complexes.

Spatiotemporal Optical Control of Gαq-PLCβ Interactions

Cells experience time-varying and spatially heterogeneous chemokine signals in vivo, activating cell surface proteins including G protein-coupled receptors (GPCRs). The Gαq pathway activation by GPCRs is a major signaling axis with broad physiological and pathological significance. Compared with other Gα members, GαqGTP activates many crucial effectors, including PLCβ (Phospholipase Cβ) and Rho GEFs (Rho guanine nucleotide exchange factors). PLCβ regulates many key processes, such as hematopoiesis, synaptogenesis, and cell cycle, and is therefore implicated in terminal-debilitating diseases, including cancer, epilepsy, Huntington's Disease, and Alzheimer's Disease. However, due to a lack of genetic and pharmacological tools, examining how the dynamic regulation of PLCβ signaling controls cellular physiology has been difficult. Since activated PLCβ induces several abrupt cellular changes, including cell morphology, examining how the other pathways downstream of Gq-GPCRs contribute to the overall signaling has also been difficult. Here we show the engineering, validation, and application of a highly selective and efficient optogenetic inhibitor (Opto-dHTH) to completely disrupt GαqGTP-PLCβ interactions reversibly in user-defined cellular-subcellular regions on optical command. Using this newly gained PLCβ signaling control, our data indicate that the molecular competition between RhoGEFs and PLCβ for GαqGTP determines the potency of Gq-GPCR-governed directional cell migration.

Hepatocyte GPCR signaling regulates IRF3 to control hepatic stellate cell transdifferentiation

BACKGROUND

Interferon Regulatory Factor 3 (IRF3) is a transcription factor that plays a crucial role in the innate immune response by recognizing and responding to foreign antigens. Recently, its roles in sterile conditions are being studied, as in metabolic and fibrotic diseases. However, the search on the upstream regulator for efficient pharmacological targeting is yet to be fully explored. Here, we show that G protein-coupled receptors (GPCRs) can regulate IRF3 phosphorylation through of GPCR-Gα protein interaction.

RESULTS

IRF3 and target genes were strongly associated with fibrosis markers in liver fibrosis patients and models. Conditioned media from MIHA hepatocytes overexpressing IRF3 induced fibrogenic activation of LX-2 hepatic stellate cells (HSCs). In an overexpression library screening using active mutant Gα subunits and Phos-tag immunoblotting, Gαs was found out to strongly phosphorylate IRF3. Stimulation of Gαs by glucagon or epinephrine or by Gαs-specific designed GPCR phosphorylated IRF3. Protein kinase A (PKA) signaling was primarily responsible for IRF3 phosphorylation and Interleukin 33 (IL-33) expression downstream of Gαs. PKA phosphorylated IRF3 on a previously unrecognized residue and did not require reported upstream kinases such as TANK-binding kinase 1 (TBK1). Activation of Gαs signaling by glucagon induced IL-33 production in hepatocytes. Conditioned media from the hepatocytes activated HSCs, as indicated by α-SMA and COL1A1 expression, and this was reversed by pre-treatment of the media with IL-33 neutralizing antibody.

CONCLUSIONS

Gαs-coupled GPCR signaling increases IRF3 phosphorylation through cAMP-mediated activation of PKA. This leads to an increase of IL-33 expression, which further contributes to HSC activation. Our findings that hepatocyte GPCR signaling regulates IRF3 to control hepatic stellate cell transdifferentiation provides an insight for understanding the complex intercellular communication during liver fibrosis progression and suggests therapeutic opportunities for the disease. Video Abstract.

Structural and functional characterization of an egg-laying hormone signaling system in a lophotrochozoan - The pacific oyster (Crassostrea gigas)

The egg-laying hormones (ELHs) of gastropod mollusks were characterized more than forty years ago. Yet, they have remained little explored in other mollusks. To gain insights into the functionality of the ELH signaling system in a bivalve mollusk - the oyster Crassostrea gigas, this study investigates the processing of its ELH precursor (Cragi-ELH) by mass spectrometry. Some of the ELH mature peptides identified in this study were subsequently investigated by nuclear magnetic resonance and shown to adopt an extended alpha-helix structure in a micellar medium mimicking the plasma membrane. To further characterize the ELH signaling system in C. gigas, a G protein-coupled receptor phylogenetically related to ecdysozoan diuretic hormone DH44 and corticotropin-releasing hormone (CRH) receptors named Cragi-ELHR was also characterized functionally and shown to be specifically activated by the two predicted mature ELH peptides and their N-terminal fragments. Both Cragi-ELH and Cragi-ELHR encoding genes were mostly expressed in the visceral ganglia (VG). Cragi-ELH expression was significantly increased in the VG of both fully mature male and female oysters at the spawning stage. When the oysters were submitted to a nutritional or hyposaline stress, no change in the expression of the ligand or receptor genes was recorded, except for Cragi-ELHR only during a mild acclimation episode to brackish water. These results suggest a role of Cragi-ELH signaling in the regulation of reproduction but not in mediating the stress response in our experimental conditions.

ThermoBRET: A Ligand-Engagement Nanoscale Thermostability Assay Applied to GPCRs

Measurements of membrane protein thermostability reflect ligand binding. Current thermostability assays often require protein purification or rely on pre-existing radiolabelled or fluorescent ligands, limiting their application to established targets. Alternative methods, such as fluorescence-detection size exclusion chromatography thermal shift, detect protein aggregation but are not amenable to high-throughput screening. Here, we present a ThermoBRET method to quantify the relative thermostability of G protein coupled receptors (GPCRs), using cannabinoid receptors (CB1 and CB2 ) and the β2 -adrenoceptor (β2 AR) as model systems. ThermoBRET reports receptor unfolding, does not need labelled ligands and can be used with non-purified proteins. It uses Bioluminescence Resonance Energy Transfer (BRET) between Nanoluciferase (Nluc) and a thiol-reactive fluorescent dye that binds cysteines exposed by unfolding. We demonstrate that the melting point (Tm ) of Nluc-fused GPCRs can be determined in non-purified detergent solubilised membrane preparations or solubilised whole cells, revealing differences in thermostability for different solubilising conditions and in the presence of stabilising ligands. We extended the range of the assay by developing the thermostable tsNLuc by incorporating mutations from the fragments of split-Nluc (Tm of 87 °C versus 59 °C). ThermoBRET allows the determination of GPCR thermostability, which is useful for protein purification optimisation and drug discovery screening.

Morphine- and foot shock-responsive neuronal ensembles in the VTA possess different connectivity and biased GPCR signaling pathway

Background: Neurons in the ventral tegmental area (VTA) are sensitive to stress and their maladaptation have been implicated in the psychiatric disorders such as anxiety and addiction, etc. The cellular properties of the VTA neurons in response to different stressors related to different emotional processing remain to be investigated. Methods: By combining immediate early gene (IEG)-dependent labeling, rabies virus tracing, ensemble-specific transcriptomic analysis and fiber photometry recording in the VTA of male mice, the spatial distribution, brain-wide connectivity and cellular signaling pathways in the VTA neuronal ensembles in response to morphine (Mor-Ens) or foot shock (Shock-Ens) stimuli were investigated. Results: Optogenetic activation of the Mor-Ens drove approach behavior, whereas chemogenetic activation of the Shock-Ens increased the anxiety level in mice. Mor-Ens were clustered and enriched in the ventral VTA, contained a higher proportion of dopaminergic neurons, received more inputs from the dorsal medial striatum and the medial hypothalamic zone, and exhibited greater axonal arborization in the zona incerta and ventral pallidum. Whereas Shock-Ens were more dispersed, contained a higher proportion of GABAergic neurons, and received more inputs from the ventral pallidum and the lateral hypothalamic area. The downstream targets of the G protein and β-arrestin pathways, PLCβ3 and phosphorylated AKT1Thr308, were relatively enriched in the Mor-Ens and Shock-Ens, respectively. Cariprazine, the G-protein-biased agonist for the dopamine D2 receptor, increased the response of Mor-Ens to sucrose water and decreased the anxiety-like behavior during morphine withdrawal, whereas the β-arrestin-biased agonist UNC9994 decreased the response of Shock-Ens to tail suspension. Conclusions: Taken together, these findings reveal the heterogeneous connectivity and signaling pathways of the VTA neurons in response to morphine and foot shock, providing new insights for development of specific interventions for psychiatric disorders caused by various stressors associated with different VTA neuronal functions.

The Na+ leak channel NALCN controls spontaneous activity and mediates synaptic modulation by α2-adrenergic receptors in auditory neurons

Cartwheel interneurons of the dorsal cochlear nucleus (DCN) potently suppress multisensory signals that converge with primary auditory afferent input, and thus regulate auditory processing. Noradrenergic fibers from locus coeruleus project to the DCN, and α2-adrenergic receptors inhibit spontaneous spike activity but simultaneously enhance synaptic strength in cartwheel cells, a dual effect leading to enhanced signal-to-noise for inhibition. However, the ionic mechanism of this striking modulation is unknown. We generated a glycinergic neuron-specific knockout of the Na+ leak channel NALCN in mice and found that its presence was required for spontaneous firing in cartwheel cells. Activation of α2-adrenergic receptors inhibited both NALCN and spike generation, and this modulation was absent in the NALCN knockout. Moreover, α2-dependent enhancement of synaptic strength was also absent in the knockout. GABAB receptors mediated inhibition through NALCN as well, acting on the same population of channels as α2 receptors, suggesting close apposition of both receptor subtypes with NALCN. Thus, multiple neuromodulatory systems determine the impact of synaptic inhibition by suppressing the excitatory leak channel, NALCN.

Investigating G-protein coupled receptor signalling with light-emitting biosensors

G protein-coupled receptors (GPCRs) are the most frequent target of currently approved drugs and play a central role in both physiological and pathophysiological processes. Beyond the canonical understanding of GPCR signal transduction, the importance of receptor conformation, beta-arrestin (β-arr) biased signalling, and signalling from intracellular locations other than the plasma membrane is becoming more apparent, along with the tight spatiotemporal compartmentalisation of downstream signals. Fluorescent and bioluminescent biosensors have played a pivotal role in elucidating GPCR signalling events in live cells. To understand the mechanisms of action of the GPCR-targeted drugs currently available, and to develop new and better GPCR-targeted therapeutics, understanding these novel aspects of GPCR signalling is critical. In this review, we present some of the tools available to interrogate each of these features of GPCR signalling, we illustrate some of the key findings which have been made possible by these tools and we discuss their limitations and possible developments.

A peroxidase-derived ligand that induces Fusarium graminearum Ste2 receptor-dependent chemotropism

Introduction

The fungal G protein-coupled receptors Ste2 and Ste3 are vital in mediating directional hyphal growth of the agricultural pathogen Fusarium graminearum towards wheat plants. This chemotropism is induced by a catalytic product of peroxidases secreted by the wheat. Currently, the identity of this product, and the substrate it is generated from, are not known.

Methods and results

We provide evidence that a peroxidase substrate is derived from F. graminearum conidia and report a simple method to extract and purify the FgSte2-activating ligand for analyses by mass spectrometry. The mass spectra arising from t he ligand extract are characteristic of a 400 Da carbohydrate moiety. Consistent with this type of molecule, glycosidase treatment of F. graminearum conidia prior to peroxidase treatment significantly reduced the amount of ligand extracted. Interestingly, availability of the peroxidase substrate appears to depend on the presence of both FgSte2 and FgSte3, as knockout of one or the other reduces the chemotropism-inducing effect of the extracts.

Conclusions

While further characterization is necessary, identification of the F. graminearum-derived peroxidase substrate and the FgSte2-activating ligand will unearth deeper insights into the intricate mechanisms that underlie fungal pathogenesis in cereal crops, unveiling novel avenues for inhibitory interventions.

Dissecting the molecular basis for the modulation of neurotransmitter GPCR signaling by GINIP

It is well established that G-protein-coupled receptors (GPCRs) stimulated by neurotransmitters are critical for neuromodulation. Much less is known about how heterotrimeric G-protein (Gαβγ) regulation after receptor-mediated activation contributes to neuromodulation. Recent evidence indicates that the neuronal protein GINIP shapes GPCR inhibitory neuromodulation via a unique mechanism of G-protein regulation that controls pain and seizure susceptibility. However, the molecular basis of this mechanism remains ill-defined because the structural determinants of GINIP responsible for binding and regulating G proteins are not known. Here, we combined hydrogen-deuterium exchange mass spectrometry, computational structure predictions, biochemistry, and cell-based biophysical assays to demonstrate an effector-like binding mode of GINIP to Gαi. Specific amino acids of GINIP's PHD domain first loop are essential for G-protein binding and subsequent regulation of Gαi-GTP and Gβγ signaling upon neurotransmitter GPCR stimulation. In summary, these findings shed light onto the molecular basis for a post-receptor mechanism of G-protein regulation that fine-tunes inhibitory neuromodulation.

Functional profiling of the G protein-coupled receptor C3aR1 reveals ligand-mediated biased agonism

G protein-coupled receptors (GPCRs) are leading druggable targets for several medicines, but many GPCRs are still untapped for their therapeutic potential due to poor understanding of specific signaling properties. The complement C3a receptor 1 (C3aR1) has been extensively studied for its physiological role in C3a-mediated anaphylaxis/inflammation, and in TLQP-21-mediated lipolysis, but direct evidence for the functional relevance of the C3a and TLQP-21 ligands and signal transduction mechanisms are still limited. In addition, C3aR1 G protein coupling specificity is still unclear, and whether endogenous ligands, or drug-like compounds, show ligand-mediated biased agonism is unknown. Here, we demonstrate that C3aR1 couples preferentially to Gi/o/z proteins and can recruit β-arrestins to cause internalization. Furthermore, we showed that in comparison to C3a63-77, TLQP-21 exhibits a preference toward Gi/o-mediated signaling compared to β-arrestin recruitment and internalization. We also show that the purported antagonist SB290157 is a very potent C3aR1 agonist, where antagonism of ligand-stimulated C3aR1 calcium flux is caused by potent β-arrestin-mediated internalization. Finally, ligand-mediated signaling bias impacted cell function as demonstrated by the regulation of calcium influx, lipolysis in adipocytes, phagocytosis in microglia, and degranulation in mast cells. Overall, we characterize C3aR1 as a Gi/o/z-coupled receptor and demonstrate the functional relevance of ligand-mediated signaling bias in key cellular models. Due to C3aR1 and its endogenous ligands being implicated in inflammatory and metabolic diseases, these results are of relevance toward future C3aR1 drug discovery.

Chlorinated Enyne Fatty Acid Amides from a Marine Cyanobacterium: Discovery of Taveuniamides L-M and Pharmacological Characterization of Taveuniamide F as a GPCR Antagonist with CNR1 Selectivity

NMR and MS/MS-based metabolomics of a cyanobacterial extract from Piti Bomb Holes, Guam, indicated the presence of unique enyne-containing halogenated fatty acid amides. We isolated three new compounds of this class, taveuniamides L-N (1-3), along with the previously reported taveuniamide F (4), which was the most abundant analog. The planar structures of the new compounds were established using 1D and 2D NMR as well as mass spectrometry. We established the configuration of this chemical class to be R at C-8 via Mosher's analysis of 4 after reduction of the carboxamide group. Our biological investigations with 4 revealed that the compound binds to the cannabinoid receptor CNR1, acting as an antagonist/inverse agonist in the canonical G-protein signaling pathways. In selectivity profiling against 168 GPCR targets using the β-arrestin functional assay, we found that 4 antagonizes GPR119, NPSR1b, CCR9, CHRM4, GPR120, HTR2A, and GPR103, in addition to CNR1. Interestingly, 4 showed a 6.8-fold selectivity for CNR1 over CNR2. The binding mode of 4 to CNR1 was investigated using docking and molecular dynamics simulations with both natural and unnatural stereoisomers, revealing important CNR1 residues for the interaction and also providing a possible reasoning for the observed CNR1/CNR2 selectivity.

GPCR-G protein selectivity revealed by structural pharmacology

Receptor-G protein promiscuity is frequently observed in class A G protein-coupled receptors (GPCRs). In particular, GPCRs can couple with G proteins from different families (Gαs, Gαq/11, Gαi/o, and Gα12/13) or the same family subtypes. The molecular basis underlying the selectivity/promiscuity is not fully revealed. We recently reported the structures of kappa opioid receptor (KOR) in complex with the Gi/o family subtypes [Gαi1, GαoA, Gαz, and Gustducin (Gαg)] determined by cryo-electron microscopy (cryo-EM). The structural analysis, in combination with pharmacological studies, provides insights into Gi/o subtype selectivity. Given the conserved sequence identity and activation mechanism between different G protein families, the findings within Gi/o subtypes could be likely extended to other families. Understanding the KOR-Gi/o or GPCR-G protein selectivity will facilitate the development of more precise therapeutics targeting a specific G protein subtype.

How accurately can one predict drug binding modes using AlphaFold models?

Computational prediction of protein structure has been pursued intensely for decades, motivated largely by the goal of using structural models for drug discovery. Recently developed machine-learning methods such as AlphaFold 2 (AF2) have dramatically improved protein structure prediction, with reported accuracy approaching that of experimentally determined structures. To what extent do these advances translate to an ability to predict more accurately how drugs and drug candidates bind to their target proteins? Here, we carefully examine the utility of AF2 protein structure models for predicting binding poses of drug-like molecules at the largest class of drug targets, the G-protein-coupled receptors. We find that AF2 models capture binding pocket structures much more accurately than traditional homology models, with errors nearly as small as differences between structures of the same protein determined experimentally with different ligands bound. Strikingly, however, the accuracy of ligand-binding poses predicted by computational docking to AF2 models is not significantly higher than when docking to traditional homology models and is much lower than when docking to structures determined experimentally without these ligands bound. These results have important implications for all those who might use predicted protein structures for drug discovery.

Snapshot of the cannabinoid receptor 1-arrestin complex unravels the biased signaling mechanism

Cannabis activates the cannabinoid receptor 1 (CB1), which elicits analgesic and emotion regulation benefits, along with adverse effects, via Gi and β-arrestin signaling pathways. However, the lack of understanding of the mechanism of β-arrestin-1 (βarr1) coupling and signaling bias has hindered drug development targeting CB1. Here, we present the high-resolution cryo-electron microscopy structure of CB1-βarr1 complex bound to the synthetic cannabinoid MDMB-Fubinaca (FUB), revealing notable differences in the transducer pocket and ligand-binding site compared with the Gi protein complex. βarr1 occupies a wider transducer pocket promoting substantial outward movement of the TM6 and distinctive twin toggle switch rearrangements, whereas FUB adopts a different pose, inserting more deeply than the Gi-coupled state, suggesting the allosteric correlation between the orthosteric binding pocket and the partner protein site. Taken together, our findings unravel the molecular mechanism of signaling bias toward CB1, facilitating the development of CB1 agonists.

Structure of the dopamine D3 receptor bound to a bitopic agonist reveals a new specificity site in an expanded allosteric pocket

Although aminergic GPCRs are the target for ~25% of approved drugs, developing subtype selective drugs is a major challenge due to the high sequence conservation at their orthosteric binding site. Bitopic ligands are covalently joined orthosteric and allosteric pharmacophores with the potential to boost receptor selectivity, driven by the binding of the secondary pharmacophore to non-conserved regions of the receptor. Although bitopic ligands have great potential to improve current medications by reducing off-target side effects, the lack of structural information on their binding mode impedes rational design. Here we determine the cryo-EM structure of the hD3R coupled to a GO heterotrimer and bound to the D3R selective bitopic agonist FOB02-04A. Structural, functional and computational analyses provide new insights into its binding mode and point to a new TM2-ECL1-TM1 region, which requires the N-terminal ordering of TM1, as a major determinant of subtype selectivity in aminergic GPCRs. This region is underexploited in drug development, expands the established secondary binding pocket in aminergic GPCRs and could potentially be used to design novel and subtype selective drugs.

Reshaping the yeast galactose regulon via GPCR signaling cascade

Dynamically regulated systems are preferable to control metabolic pathways for an improved strain performance with better productivity. Here, we harnessed to the G protein-coupled receptor (GPCR) signaling pathway to reshape the yeast galactose regulon. The galactose-regulated (GAL) system was coupled with the GPCR signaling pathway for mating pheromone via a synthetic transcription factor. In this study, we refabricated the dynamic range, sensitivity, and response time of the GAL system to α factor by modulating the key components of the GPCR signaling cascade. A series of engineered yeasts with self-secretion of α factor were constructed to achieve quorum-sensing behaviors. In addition, we also repurposed the GAL system to make it responsive to heat shock. Taken together, our work showcases the great potential of synthetic biology in creating user-defined metabolic controls. We envision that the plasticity of our genetic design would be of significant interest for the future fabrication of novel gene expression systems.

GPCR Screening Reveals that the Metabolite Receptor HCAR3 Regulates Epithelial Proliferation, Migration, and Cellular Respiration

Epithelial cells in the skin and other tissues rely on signals from their environment to maintain homeostasis and respond to injury, and GPCRs play a critical role in this communication. A better understanding of the GPCRs expressed in epithelial cells will contribute to understanding the relationship between cells and their niche and could lead to developing new therapies to modulate cell fate. This study used human primary keratinocytes as a model to investigate the specific GPCRs regulating epithelial cell proliferation and differentiation. We identified 3 key receptors-HCAR3, LTB4R, and GPR137-and found that knockdown of these receptors led to changes in numerous gene networks that are important for maintaining cell identity and promoting proliferation while inhibiting differentiation. Our study also revealed that the metabolite receptor HCAR3 regulates keratinocyte migration and cellular metabolism. Knockdown of HCAR3 led to reduced keratinocyte migration and respiration, which could be attributed to altered metabolite use and aberrant mitochondrial morphology caused by the absence of the receptor. This study contributes to understanding the complex interplay between GPCR signaling and epithelial cell fate decisions.

Research progress on the role of orphan receptor GPR139 in neuropsychiatric behaviours

The study of orphan G protein-coupled receptors (GPCRs) holds much promise for increasing our understanding of neuropsychiatric diseases and for the development of new therapeutic strategies for these diseases. GPR139 is an orphan GPCR expressed in the central nervous system, especially in areas of the brain that control movement, motivation, and reward, and those that regulate neuropsychiatric behaviour. This review provides information about the discovery, tissue expression, signal transduction pathways, and physiological functions of GPR139, as well as how GPR139 interacts with other GPCRs, which form heteromeric complexes that affect their pharmacology and function. We also discuss the utility and therapeutic potential of ligands that target GPR139, including the pharmacological properties of reported agonists and antagonists. Finally, we highlight the pathologic role of GPR139 in neuropsychiatric behaviour and its potential as a therapeutic target in neuropsychiatric disorders.

Histamine Receptors: Ex Vivo Functional Studies Enabling the Discovery of Hits and Pathways

Histamine receptors (HRs) are G-protein-coupled receptors involved in diverse responses triggered by histamine release during inflammation or by encounters with venomous creatures. Four histamine receptors (H1R-H4R) have been cloned and extensively characterized. These receptors are distributed throughout the body and their activation is associated with clinical manifestations such as urticaria (H1R), gastric acid stimulation (H2R), regulation of neurotransmitters in neuronal diseases (H3R), and immune responses (H4R). Despite significant homologous overlap between H3R and H4R, much remains unknown about their precise roles. Even though some drugs have been developed for H1R, H2R, and H3R, not a single H4R antagonist has been approved for clinical use. To enhance our understanding and advance innovative therapeutic targeting of H1R, H2R, H3R, and H4R, we established a robust ex vivo functional platform. This platform features the successful heterologous expression of H1R-H4R in Xenopus laevis oocytes, utilizing an electrophysiological readout. Our findings contribute to a deeper understanding of the function and pharmacological properties of the histamine receptors. Researchers can benefit from the utility of this platform when investigating the effects of histamine receptors and exploring potential therapeutic targets. In doing so, it broadens the horizon of drug discovery, offering new perspectives for therapeutic interventions.

Neuronal survival factor TAFA2 suppresses apoptosis through binding to ADGRL1 and activating cAMP/PKA/CREB/BCL2 signaling pathway

AIMS

TAFA2, a cytokine specifically expressed in the central nervous system, plays a vital role in neuronal cell survival. TAFA2 deficiency has been correlated to various neurological disorders in mice and humans. However, the underlying mechanism remains elusive, especially its membrane-binding receptor through which TAFA2 functions. This study aimed to identify the specific binding receptor responsible for the anti-apoptotic effects of TAFA2.

MAIN METHOD

Co-immunoprecipitation (Co-IP) and quantitative mass spectrometry-based proteomic analysis were employed to identify potential TAFA2 binding proteins in V5 knockin mouse brain lysates. Subsequent validation involved in vitro and in vivo Co-IP and pull-down using specific antibodies. The functional analysis included evaluating the effects of ADGRL1 knockout, overexpression, and Lectin-like domain (Lec) deletion mutant on TAFA2's anti-apoptotic activity and analyzing the intracellular signaling pathways mediated by TAFA2 through ADGRL1.

KEY FINDINGS

Our study identified ADGRL1 as a potential receptor for TAFA2, which directly binds to TAFA2 through its lectin-like domain. Overexpression ADGRL1, but not ADGRL1ΔLec, induced apoptosis, which could be effectively suppressed by recombinant TAFA2 (rTAFA2). In ADGRL1-/- cells or re-introducing with ADGRL1ΔLec, responses to rTAFA2 in suppressing cell apoptosis were compromised. Increased cAMP, p-PKA, p-CREB, and BCL2 levels were also observed in response to rTAFA2 treatment, with these responses attenuated in ADGRL1-/- or ADGRL1ΔLec-expressing cells.

SIGNIFICANCE

Our results demonstrated that TAFA2 directly binds to the lectin-like domain of ADGRL1, activating cAMP/PKA/CREB/BCL2 signaling pathway, which is crucial in preventing cell death. These results implicate TAFA2 and its receptor ADGRL1 as potential therapeutic targets for neurological disorders.

Structural determinants of sphingosine-1-phosphate receptor selectivity

Fingolimod, the prodrug of fingolimod-1-phosphate (F1P), was the first sphingosine-1-phosphate receptor (S1PR) modulator approved for multiple sclerosis. F1P unselectively targets all five S1PR subtypes. While agonism (functional antagonism via receptor internalization) at S1PR1 leads to the desired immune modulatory effects, agonism at S1PR3 is associated with cardiac adverse effects. This motivated the development of S1PR3 -sparing compounds and led to a second generation of S1PR1,5 -selective ligands like siponimod and ozanimod. Our method combines molecular dynamics simulations and three-dimensional pharmacophores (dynophores) and enables the elucidation of S1PR subtype-specific binding site characteristics, visualizing also subtle differences in receptor-ligand interactions. F1P and the endogenous ligand sphingosine-1-phosphate bind to the orthosteric pocket of all S1PRs, but show different binding mode dynamics, uncovering potential starting points for the development of subtype-specific ligands. Our study contributes to the mechanistic understanding of the selectivity profile of approved drugs like ozanimod and siponimod and pharmaceutical tool compounds like CYM5541.

AT2 receptor agonist LP2 restores respiratory function in a rat model of bleomycin-induced lung remodelling

This study aimed to evaluate the prophylactic and therapeutic potential of angiotensin II type 2 receptor peptide agonist LP2 in bleomycin-induced airway and cardiac remodeling in rats. Male Wistar rats were intratracheally instillated with bleomycin. Animals of a prophylactic arm received LP2 from day 0 at intraperitoneal doses of 1, 3 or 10 μg/kg/d, whereas animals from a therapeutic arm received this LP2 treatment from day 7. On day 28 direct lung mechanics were determined and cardiac and lung tissues were collected and (histo)morphologically assessed. Prophylactic LP2 at 1 µg/kg/d with bleomycin, versus bleomycin alone, significantly improved the airway pressure responses at fixed inflation of 4 ml (p < 0.05) and 7 ml volume (p < 0.05), static compliance (p < 0.01), inspiratory capacity (p < 0.05), lung tolerance of increased volume (p < 0.0001), right to left ventricular hypertrophy (p < 0.05). Therapeutic regime showed a similar trend as the prophylactic arm but was less effective, mostly lacking significance. However, and importantly, therapeutic LP2 at 1 µg/kg/d significantly decreased mRNA expression of collagen 1A1 (p < 0.01), of Connective Tissue Growth Factor 1 (p < 0.05) and of Tissue MetalloPeptidase inhibitor 1 (p < 0.05). In conclusion, a very low dose of 1 µg/kg/d LP2 has capacity to counter bleomycin-induced impairment of lung functioning and consequent cardiac remodeling.

Targeting mGluR group III for the treatment of neurodegenerative diseases

Glutamate, an excitatory neurotransmitter, is essential for neuronal function, and it acts on ionotropic or metabotropic glutamate receptors (mGluRs). A disturbance in glutamatergic signaling is a hallmark of many neurodegenerative diseases. Developing disease-modifying treatments for neurodegenerative diseases targeting glutamate receptors is a promising avenue. The understudied group III mGluR 4, 6-8 are commonly found in the presynaptic membrane, and their activation inhibits glutamate release. Thus, targeted mGluRs therapies could aid in treating neurodegenerative diseases. This review describes group III mGluRs and their pharmacological ligands in the context of amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. Attempts to evaluate the efficacy of these drugs in clinical trials are also discussed. Despite a growing list of group III mGluR-specific pharmacological ligands, research on the use of these drugs in neurodegenerative diseases is limited, except for Parkinson's disease. Future efforts should focus on delineating the contribution of group III mGluR to neurodegeneration and developing novel ligands with superior efficacy and a favorable side effect profile for the treatment of neurodegenerative diseases.

Novel multi-target ligands of dopamine and serotonin receptors for the treatment of schizophrenia based on indazole and piperazine scaffolds-synthesis, biological activity, and structural evaluation

Schizophrenia is a chronic mental disorder that is not satisfactorily treated with available antipsychotics. The presented study focuses on the search for new antipsychotics by optimising the compound D2AAK3, a multi-target ligand of G-protein-coupled receptors (GPCRs), in particular D₂, 5-HT_1A, and 5-HT_2A receptors. Such receptor profile may be beneficial for the treatment of schizophrenia. Compounds 1-16 were designed, synthesised, and subjected to further evaluation. Their affinities for the above-mentioned receptors were assessed in radioligand binding assays and efficacy towards them in functional assays. Compounds 1 and 10, selected based on their receptor profile, were subjected to in vivo tests to evaluate their antipsychotic activity, and effect on memory and anxiety processes. Molecular modelling was performed to investigate the interactions of the studied compounds with D₂, 5-HT_1A, and 5-HT_2A receptors on the molecular level. Finally, X-ray study was conducted for compound 1, which revealed its stable conformation in the solid state.

Modulating signalling lifetime to optimise a prototypical animal opsin for optogenetic applications

Animal opsins are light activated G-protein-coupled receptors, capable of optogenetic control of G-protein signalling for research or therapeutic applications. Animal opsins offer excellent photosensitivity, but their temporal resolution can be limited by long photoresponse duration when expressed outside their native cellular environment. Here, we explore methods for addressing this limitation for a prototypical animal opsin (human rod opsin) in HEK293T cells. We find that the application of the canonical rhodopsin kinase (GRK1)/visual arrestin signal termination mechanism to this problem is complicated by a generalised suppressive effect of GRK1 expression. This attenuation can be overcome using phosphorylation-independent mutants of arrestin, especially when these are tethered to the opsin protein. We further show that point mutations targeting the Schiff base stability of the opsin can also reduce signalling lifetime. Finally, we apply one such mutation (E122Q) to improve the temporal fidelity of restored visual responses following ectopic opsin expression in the inner retina of a mouse model of retinal degeneration (rd1). Our results reveal that these two strategies (targeting either arrestin binding or Schiff-base hydrolysis) can produce more time-delimited opsin signalling under heterologous expression and establish the potential of this approach to improve optogenetic performance.

Cardiac optogenetics: shining light on signaling pathways

In the early 2000s, the field of neuroscience experienced a groundbreaking transformation with the advent of optogenetics. This innovative technique harnesses the properties of naturally occurring and genetically engineered rhodopsins to confer light sensitivity upon target cells. The remarkable spatiotemporal precision offered by optogenetics has provided researchers with unprecedented opportunities to dissect cellular physiology, leading to an entirely new level of investigation. Initially revolutionizing neuroscience, optogenetics quickly piqued the interest of the wider scientific community, and optogenetic applications were expanded to cardiovascular research. Over the past decade, researchers have employed various optical tools to observe, regulate, and steer the membrane potential of excitable cells in the heart. Despite these advancements, achieving control over specific signaling pathways within the heart has remained an elusive goal. Here, we review the optogenetic tools suitable to control cardiac signaling pathways with a focus on GPCR signaling, and delineate potential applications for studying these pathways, both in healthy and diseased hearts. By shedding light on these exciting developments, we hope to contribute to the ongoing progress in basic cardiac research to facilitate the discovery of novel therapeutic possibilities for treating cardiovascular pathologies.

Versatile lactate signaling via HCAR1: a multifaceted GPCR involved in many biological processes

G-coupled protein receptors (GPCRs) are the ultimate refuge of pharmacology and medicine as more than 40% of all marketed drugs are directly targeting these receptors. Through cell surface expression, they are at the forefront of cellular communication with the outside world. Metabolites among the conveyors of this communication are becoming more prominent with the recognition of them as ligands for GPCRs. HCAR1 is a GPCR conveyor of lactate. It is a class A GPCR coupled to Gαi which reduces cellular cAMP along with the downstream Gβγ signaling. It was first found to inhibit lipolysis, and lately has been implicated in diverse cellular processes, including neural activities, angiogenesis, inflammation, vision, cardiovascular function, stem cell proliferation, and involved in promoting pathogenesis for different conditions, such as cancer. Other than signaling from the plasma membrane, HCAR1 shows nuclear localization with different location-biased activities therein. Although different functions for HCAR1 are being discovered, its cell and molecular mechanisms are yet ill understood. Here, we provide a comprehensive review on HCAR1, which covers the literature on the subject, and discusses its importance and relevance in various biological phenomena.