GPCR-dependent biasing of GIRK channel signaling dynamics by RGS6 in mouse sinoatrial nodal cells

G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels: Molecular, cellular, and subcellular diversity

G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels are mainly expressed in excitable cells such as neurons and atrial myocytes, where they can respond to a wide variety of neurotransmitters. Four GIRK subunits have been found in mammals (GIRK1-4) and act as downstream targets for various Gαi/o-linked G protein-coupled receptors (GPCRs). Activation of GIRK channels produces a postsynaptic efflux of potassium from the cell, responsible for hyperpolarization/inhibition of the neuron. A growing body of evidence suggests that dysregulation of GIRK signalling can lead to excessive or deficient neuronal excitability, which contributes to neurological diseases and disorders. Therefore, GIRK channels are proposed as new pharmacological targets. The function of GIRK channels in neurons is not only determined by their biophysical properties but also by their cellular and subcellular localization patterns and densities on the neuronal surface. GIRK channels can be located within several subcellular compartments, where they have many different functional implications. This subcellular localization changes dynamically along the neuronal surface in response to drug intake. Ongoing research is focusing on determining the proteins that form macromolecular complexes with GIRK channels and are responsible for fast and precise signalling under physiological conditions, and how their alteration is implicated in pathological conditions. In this review, the distinct regional, cellular, and subcellular distribution of GIRK channel subunits in the brain will be discussed in view of their possible functional and pathological implications.

Receptor-dependent influence of R7 RGS proteins on neuronal GIRK channel signaling dynamics

Most neurons are influenced by multiple neuromodulatory inputs that converge on common effectors. Mechanisms that route these signals are key to selective neuromodulation but are poorly understood. G protein-gated inwardly rectifying K+ (GIRK or Kir3) channels mediate postsynaptic inhibition evoked by G protein-coupled receptors (GPCRs) that signal via inhibitory G proteins. GIRK-dependent signaling is modulated by Regulator of G protein Signaling proteins RGS6 and RGS7, but their selectivity for distinct GPCR-GIRK signaling pathways in defined neurons is unclear. We compared how RGS6 and RGS7 impact GIRK channel regulation by the GABAB receptor (GABABR), 5HT1A receptor (5HT1AR), and A1 adenosine receptor (A1R) in hippocampal neurons. Our data show that RGS6 and RGS7 make non-redundant contributions to GABABR- and 5HT1AR-GIRK signaling and compartmentalization and suggest that GPCR-G protein preferences and the substrate bias of RGS proteins, as well as receptor-dependent differences in Gαo engagement and effector access, shape GPCR-GIRK signaling dynamics in hippocampal neurons.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Agonist-selective activation of individual G-proteins by muscarinic receptors

Selective activation of individual subtypes of muscarinic receptors is a promising way to safely alleviate a wide range of pathological conditions in the central nervous system and the periphery as well. The flexible G-protein interface of muscarinic receptors allows them to interact with several G-proteins with various efficacy, potency, and kinetics. Agonists biased to the particular G-protein mediated pathway may result in selectivity among muscarinic subtypes and, due to the non-uniform expression of individual G-protein alpha subunits, possibly achieve tissue specificity. Here, we demonstrate that novel tetrahydropyridine-based agonists exert specific signalling profiles in coupling with individual G-protein α subunits. These signalling profiles profoundly differ from the reference agonist carbachol. Moreover, coupling with individual Gα induced by these novel agonists varies among subtypes of muscarinic receptors which may lead to subtype selectivity. Thus, the novel tetrahydropyridine-based agonist can contribute to the elucidation of the mechanism of pathway-specific activation of muscarinic receptors and serve as a starting point for the development of desired selective muscarinic agonists.

Activation of Multiple G Protein Pathways to Characterize the Five Dopamine Receptor Subtypes Using Bioluminescence Technology

G protein-coupled receptors show preference for G protein subtypes but can recruit multiple G proteins with various downstream signaling cascades. This functional selection can guide drug design. Dopamine receptors are both stimulatory (D1-like) and inhibitory (D2-like) with diffuse expression across the central nervous system. Functional selectivity of G protein subunits may help with dopamine receptor targeting and their downstream effects. Three bioluminescence-based assays were used to characterize G protein coupling and function with the five dopamine receptors. Most proximal to ligand binding was the miniG protein assay with split luciferase technology used to measure recruitment. For endogenous and selective ligands, the G-CASE bioluminescence resonance energy transfer (BRET) assay measured G protein activation and receptor selectivity. Downstream, the BRET-based CAMYEN assay quantified cyclic adenosine monophosphate (cAMP) changes. Several dopamine receptor agonists and antagonists were characterized for their G protein recruitment and cAMP effects. G protein selectivity with dopamine revealed potential Gq coupling at all five receptors, as well as the ability to activate subtypes with the "opposite" effects to canonical signaling. D1-like receptor agonist (+)-SKF-81297 and D2-like receptor agonist pramipexole showed selectivity at all receptors toward Gs or Gi/o/z activation, respectively. The five dopamine receptors show a wide range of potentials for G protein coupling and activation, reflected in their downstream cAMP signaling. Targeting these interactions can be achieved through drug design. This opens the door to pharmacological treatment with more selectivity options for inducing the correct physiological events.

Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions, including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G protein coupled receptor (GPCR) activating numerous Gα i/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based bioluminescence resonance energy transfer assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gα i/o/z family members. Our concentration-response analysis of the activation of each Gα i/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared with the reference 5-hydroxytryptamine, serotonin. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gα i3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gα i3 versus Gα i2 by F-15599, while it was -0.29 for 8-hydroxy-2-(di-n-propylamino) tetralin. Similarly, vortioxetine showed a biased factor of +1.06 for Gα z versus Gα oA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Gα protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate the molecular mechanisms underlying therapeutic profile and side effects of each drug. SIGNIFICANCE STATEMENT: Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.

RGS6 negatively regulates inhibitory G protein signaling in dopamine neurons and positively regulates binge-like alcohol consumption in mice

BACKGROUND AND PURPOSE

Drugs of abuse, including alcohol, increase dopamine in the mesocorticolimbic system via actions on dopamine neurons in the ventral tegmental area (VTA). Increased dopamine transmission can activate inhibitory G protein signalling pathways in VTA dopamine neurons, including those controlled by GABAB and D2 receptors. Members of the R7 subfamily of regulator of G protein signalling (RGS) proteins can regulate inhibitory G protein signalling, but their influence on VTA dopamine neurons is unclear. Here, we investigated the influence of RGS6, an R7 RGS family memberthat has been implicated in the regulation of alcohol consumption in mice, on inhibitory G protein signalling in VTA dopamine neurons.

EXPERIMENTAL APPROACH

We used molecular, electrophysiological and genetic approaches to probe the impact of RGS6 on inhibitory G protein signalling in VTA dopamine neurons and on binge-like alcohol consumption in mice.

KEY RESULTS

RGS6 is expressed in adult mouse VTA dopamine neurons and it modulates inhibitory G protein signalling in a receptor-dependent manner, tempering D2 receptor-induced somatodendritic currents and accelerating deactivation of synaptically evoked GABAB receptor-dependent responses. RGS6-/- mice exhibit diminished binge-like alcohol consumption, a phenotype replicated in female (but not male) mice lacking RGS6 selectively in VTA dopamine neurons.

CONCLUSIONS AND IMPLICATIONS

RGS6 negatively regulates GABAB - and D2 receptor-dependent inhibitory G protein signalling pathways in mouse VTA dopamine neurons and exerts a sex-dependent positive influence on binge-like alcohol consumption in adult mice. As such, RGS6 may represent a new diagnostic and/or therapeutic target for alcohol use disorder.

Fine-tuning GPCR-mediated neuromodulation by biasing signaling through different G protein subunits

G-protein-coupled receptors (GPCRs) mediate neuromodulation through the activation of heterotrimeric G proteins (Gαβγ). Classical models depict that G protein activation leads to a one-to-one formation of Gα-GTP and Gβγ species. Each of these species propagates signaling by independently acting on effectors, but the mechanisms by which response fidelity is ensured by coordinating Gα and Gβγ responses remain unknown. Here, we reveal a paradigm of G protein regulation whereby the neuronal protein GINIP (Gα inhibitory interacting protein) biases inhibitory GPCR responses to favor Gβγ over Gα signaling. Tight binding of GINIP to Gαi-GTP precludes its association with effectors (adenylyl cyclase) and, simultaneously, with regulator-of-G-protein-signaling (RGS) proteins that accelerate deactivation. As a consequence, Gαi-GTP signaling is dampened, whereas Gβγ signaling is enhanced. We show that this mechanism is essential to prevent the imbalances of neurotransmission that underlie increased seizure susceptibility in mice. Our findings reveal an additional layer of regulation within a quintessential mechanism of signal transduction that sets the tone of neurotransmission.

Acetylcholine Inhibits Spontaneous Firing Activity of Terminal Nerve GnRH Neurons in Medaka

Vertebrates generally possess hypophysiotropic and non-hypophysiotropic gonadotropin releasing hormone (GnRH) neurons. The terminal nerve (TN) GnRH neurons are known to belong to the non-hypophysiotropic neurons and have been suggested to modulate sexual behaviors. These neurons show spontaneous pacemaker firing activity and release neuropeptides GnRH and neuropeptide FF. Since the spontaneous firing activities of peptidergic neurons, including GnRH neurons, are believed to play important roles in the release of neuropeptides, understanding the regulatory mechanisms of these spontaneous firing activities is important. Here, we analyzed firing activities of the TN-GnRH neurons in medaka during application of acetylcholine (ACh), which is one of the essential neuromodulators in the brain. Whole cell patch clamp recording of TN-GnRH neurons demonstrated that ACh induces hyperpolarization and inhibits their pacemaker firing. Electrophysiological analysis using an antagonist for acetylcholine receptors and in situ hybridization analysis showed that firing of TN-GnRH neurons is inhibited via M2-type muscarinic acetylcholine receptor. These findings, taken together with literature from several other fish species (including teleosts and elasmobranchs), indicate that ACh may generally play an inhibitory role in modulating spontaneous activities of TN-GnRH neurons and thereby sexual behaviors in fish.

Impact of schizophrenia GWAS loci converge onto distinct pathways in cortical interneurons vs glutamatergic neurons during development

Remarkable advances have been made in schizophrenia (SCZ) GWAS, but gleaning biological insight from these loci is challenging. Genetic influences on gene expression (e.g., eQTLs) are cell type-specific, but most studies that attempt to clarify GWAS loci's influence on gene expression have employed tissues with mixed cell compositions that can obscure cell-specific effects. Furthermore, enriched SCZ heritability in the fetal brain underscores the need to study the impact of SCZ risk loci in specific developing neurons. MGE-derived cortical interneurons (cINs) are consistently affected in SCZ brains and show enriched SCZ heritability in human fetal brains. We identified SCZ GWAS risk genes that are dysregulated in iPSC-derived homogeneous populations of developing SCZ cINs. These SCZ GWAS loci differential expression (DE) genes converge on the PKC pathway. Their disruption results in PKC hyperactivity in developing cINs, leading to arborization deficits. We show that the fine-mapped GWAS locus in the ATP2A2 gene of the PKC pathway harbors enhancer marks by ATACseq and ChIPseq, and regulates ATP2A2 expression. We also generated developing glutamatergic neurons (GNs), another population with enriched SCZ heritability, and confirmed their functionality after transplantation into the mouse brain. Then, we identified SCZ GWAS risk genes that are dysregulated in developing SCZ GNs. GN-specific SCZ GWAS loci DE genes converge on the ion transporter pathway, distinct from those for cINs. Disruption of the pathway gene CACNA1D resulted in deficits of Ca2+ currents in developing GNs, suggesting compromised neuronal function by GWAS loci pathway deficits during development. This study allows us to identify cell type-specific and developmental stage-specific mechanisms of SCZ risk gene function, and may aid in identifying mechanism-based novel therapeutic targets.

RGS6 Drives Spinal Cord Injury by Inhibiting AMPK Pathway in Mice

Objective. Oxidative stress and inflammation play critical roles in the pathogenesis of spinal cord injury (SCI). Regulator of G protein signaling 6 (RGS6) is involved in controlling ROS generation and inflammatory response under different contexts. This study is aimed at investigating its role and underlying mechanism in SCI. Methods. Contusive SCI mouse models were generated, and lentiviral vectors were injected to silence or overexpress RGS6 in the spinal cord. To inhibit AMP-activated protein kinase (AMPK) activity, SCI mice were intraperitoneally injected with compound C (20 mg/kg) every two days. Oxidative and inflammatory markers were detected. Results. Spinal RGS6 expression was elevated upon SCI stimulation. RGS6 knockdown suppressed, while RGS6 overexpression aggravated oxidative stress, inflammation, and SCI in mice. Mechanistically, RGS6 elevation during SCI deactivated AMPK pathway, thereby exacerbating oxidative stress and inflammation in SCI mice. Conclusion. RGS6 is required for the initiation and progression of SCI, and knocking down RGS6 may provide promising therapeutic strategies for SCI patients.

Common coupling map advances GPCR-G protein selectivity

Two-thirds of human hormones and one-third of clinical drugs act on membrane receptors that couple to G proteins to achieve appropriate functional responses. While G protein transducers from literature are annotated in the Guide to Pharmacology database, two recent large-scale datasets now expand the receptor-G protein ‘couplome’. However, these three datasets differ in scope and reported G protein couplings giving different coverage and conclusions on G protein-coupled receptor (GPCR)-G protein signaling. Here, we report a common coupling map uncovering novel couplings supported by both large-scale studies, the selectivity/promiscuity of GPCRs and G proteins, and how the co-coupling and co-expression of G proteins compare to the families from phylogenetic relationships. The coupling map and insights on GPCR-G protein selectivity will catalyze advances in receptor research and cellular signaling toward the exploitation of G protein signaling bias in design of safer drugs.

Ligand-directed bias of G protein signaling at the dopamine D2 receptor

G-protein-coupled receptors (GPCRs) represent the largest family of drug targets. Upon activation, GPCRs signal primarily via a diverse set of heterotrimeric G proteins. Most GPCRs can couple to several different G protein subtypes. However, how drugs act at GPCRs contributing to the selectivity of G protein recognition is poorly understood. Here, we examined the G protein selectivity profile of the dopamine D₂ receptor (D₂), a GPCR targeted by antipsychotic drugs. We show that D₂ discriminates between six individual members of the G_i/o family, and its profile of functional selectivity is remarkably different across its ligands, which all engaged D₂ with a distinct G protein coupling pattern. Using structural modeling, receptor mutagenesis, and pharmacological evaluation, we identified residues in the D₂ binding pocket that shape these ligand-directed biases. We further provide pharmacogenomic evidence that natural variants in D₂ differentially affect its G protein biases in response to different ligands.

Community Guidelines for GPCR Ligand Bias: IUPHAR Review XX

G protein-coupled receptors modulate a plethora of physiological processes and mediate the effects of one-third of FDA-approved drugs. Depending on which ligand activates a receptor, it can engage different intracellular transducers. This 'biased signaling' paradigm requires that we now characterize physiological signaling not just by receptors but by ligand-receptor pairs. Ligands eliciting biased signaling may constitute better drugs with higher efficacy and fewer adverse effects. However, ligand bias is very complex, making reproducibility and description challenging. Here, we provide guidelines and terminology for any scientists to design and report ligand bias experiments. The guidelines will aid consistency and clarity, as the basic receptor research and drug discovery communities continue to advance our understanding and exploitation of ligand bias. Scientific insight, biosensors, and analytical methods are still evolving and should benefit from and contribute to the implementation of the guidelines, together improving translation from in vitro to disease-relevant in vivo models.

Characterization of VU0468554, a New Selective Inhibitor of Cardiac G Protein-Gated Inwardly Rectifying K+ Channels

G protein-gated inwardly rectifying K+ (GIRK) channels are critical mediators of excitability in the heart and brain. Enhanced GIRK-channel activity has been implicated in the pathogenesis of supraventricular arrhythmias, including atrial fibrillation. The lack of selective pharmacological tools has impeded efforts to investigate the therapeutic potential of cardiac GIRK-channel interventions in arrhythmias. Here, we characterize a recently identified GIRK-channel inhibitor, VU0468554. Using whole-cell electrophysiological approaches and primary cultures of sinoatrial nodal cells and hippocampal neurons, we show that VU0468554 more effectively inhibits the cardiac GIRK channel than the neuronal GIRK channel. Concentration-response experiments suggest that VU0468554 inhibits Gβγ-activated GIRK channels in noncompetitive and potentially uncompetitive fashion. In contrast, VU0468554 competitively inhibits GIRK-channel activation by ML297, a GIRK-channel activator containing the same chemical scaffold as VU0468554. In the isolated heart model, VU0468554 partially reversed carbachol-induced bradycardia in hearts from wild-type mice but not Girk4-/- mice. Collectively, these data suggest that VU0468554 represents a promising new pharmacological tool for targeting cardiac GIRK channels with therapeutic implications for relevant cardiac arrhythmias. SIGNIFICANCE STATEMENT: Although cardiac GIRK-channel inhibition shows promise for the treatment of supraventricular arrhythmias, the absence of subtype-selective channel inhibitors has hindered exploration into this therapeutic strategy. This study utilizes whole-cell patch-clamp electrophysiology to characterize the new GIRK-channel inhibitor VU0468554 in human embryonic kidney 293T cells and primary cultures. We report that VU0468554 exhibits a favorable pharmacodynamic profile for cardiac over neuronal GIRK channels and partially reverses GIRK-mediated bradycardia in the isolated mouse heart model.

Diversity of the Gβγ complexes defines spatial and temporal bias of GPCR signaling

The signal transduction by G-protein-coupled receptors (GPCRs) is mediated by heterotrimeric G proteins composed from one of the 16 Gα subunits and the inseparable Gβγ complex assembled from a repertoire of 5 Gβ and 12 Gγ subunits. However, the functional role of compositional diversity in Gβγ complexes has been elusive. Using optical biosensors, we examined the function of all Gβγ combinations in living cells and uncovered two major roles of Gβγ diversity. First, we demonstrate that the identity of Gβγ subunits greatly influences the kinetics and efficacy of GPCR responses at the plasma membrane. Second, we show that different Gβγ combinations are selectively dispatched from the plasma membrane to various cellular organelles on a timescale from milliseconds to minutes. We describe the mechanisms regulating these processes and document their implications for GPCR signaling via various Gα subunits, thereby illustrating a role for the compositional diversity of G protein heterotrimers.