RGS Redundancy and Implications in GPCR-GIRK Signaling

Elucidating the role of Rgs2 expression in the PVN for metabolic homeostasis in mice

OBJECTIVE

RGS2 is a GTPase activating protein that modulates GPCR-Gα signaling and mice lacking RGS2 globally exhibit metabolic alterations. While RGS2 is known to be broadly expressed throughout the body including the brain, the relative contribution of brain RGS2 to metabolic homeostasis remains unknown. The purpose of this study was to characterize RGS2 expression in the paraventricular nucleus of hypothalamus (PVN) and test its role in metabolic homeostasis.

METHODS

We used a combination of RNAscope in situ hybridization (ISH), immunohistochemistry, and bioinformatic analyses to characterize the pattern of Rgs2 expression in the PVN. We then created mice lacking Rgs2 either prenatally or postnatally in the PVN and evaluated their metabolic consequences.

RESULTS

RNAscope ISH analysis revealed a broad but regionally enriched Rgs2 mRNA expression throughout the mouse brain, with the highest expression being observed in the PVN along with several other brain regions, such as the arcuate nucleus of hypothalamus and the dorsal raphe nucleus. Within the PVN, we found that Rgs2 is specifically enriched in CRH+ endocrine neurons and is further increased by calorie restriction. Functionally, although Sim1-Cre-mediated prenatal deletion of Rgs2 in PVN neurons had no major effects on metabolic homeostasis, AAV-mediated adult deletion of Rgs2 in the PVN led to significantly increased food intake, body weight (both fat and fat-free masses), body length, and blood glucose levels in both male and female mice. Strikingly, we found that prolonged postnatal loss of Rgs2 leads to neuronal cell death in the PVN, while rapid body weight gain in the early phase of viral-mediated PVN Rgs2 deletion is independent of PVN neuronal loss.

CONCLUSIONS

Our results provide the first evidence to show that PVN Rgs2 expression is not only sensitive to metabolic challenge but also critically required for PVN endocrine neurons to function and maintain metabolic homeostasis.

The Relevance of GIRK Channels in Heart Function

Among the large number of potassium-channel families implicated in the control of neuronal excitability, G-protein-gated inwardly rectifying potassium channels (GIRK/Kir3) have been found to be a main factor in heart control. These channels are activated following the modulation of G-protein-coupled receptors and, although they have been implicated in different neurological diseases in both human and animal studies of the central nervous system, the therapeutic potential of different subtypes of these channel families in cardiac conditions has remained untapped. As they have emerged as a promising potential tool to treat a variety of conditions that disrupt neuronal homeostasis, many studies have started to focus on these channels as mediators of cardiac dynamics, thus leading to research into their implication in cardiovascular conditions. Our aim is to review the latest advances in GIRK modulation in the heart and their role in the cardiovascular system.

Drug-resistant epilepsy: Drug target hypothesis and beyond the receptors

Epilepsy is a chronic neurological disorder that affects more than 50 million people worldwide. Despite a recent introduction of antiseizure drugs for the treatment of epileptic seizures, one-third of these patients suffer from drug-resistant epilepsy (DRE). The therapeutic target hypothesis is a cited theory to explain DRE. According to the target hypothesis, the failure to achieve seizure freedom leads to alteration of the structure and/or function of the antiseizure medication (ASM) target. However, this hypothesis fails to explain why patients with DRE do not respond to antiseizure medications of different targets. This review presents different conditions, such as epigenetic mechanisms and protein-protein interactions that may result in alterations of diverse drug targets using different mechanisms. These novel conditions represent new targets to control DRE.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

WIN55,212-2, a Dual Modulator of Cannabinoid Receptors and G Protein-Coupled Inward Rectifier Potassium Channels

The coupling of cannabinoid receptors, CB1 and CB2, to G protein-coupled inward rectifier potassium channels, GIRK1 and GIRK2, modulates neuronal excitability in the human brain. The present study established and validated the functional expression in a Xenopus laevis oocyte expression system of CB1 and CB2 receptors, interacting with heteromeric GIRK1/2 channels and a regulator of G protein signaling, RGS4. This ex vivo system enables the discovery of a wide range of ligands interacting orthosterically or allosterically with CB1 and/or CB2 receptors. WIN55,212-2, a non-selective agonist of CB1 and CB2, was used to explore the CB1- or CB2-GIRK1/2-RGS4 signaling cascade. We show that WIN55,212-2 activates CB1 and CB2 at low concentrations whereas at higher concentrations it exerts a direct block of GIRK1/2. This illustrates a dual modulatory function, a feature not described before, which helps to explain the adverse effects induced by WIN55,212-2 in vivo. When comparing the effects with other typical cannabinoids such as Δ9-THC, CBD, CP55,940, and rimonabant, only WIN55,212-2 can significantly block GIRK1/2. Interestingly, the inward rectifier potassium channel, IRK1, a non-G protein-coupled potassium channel important for setting the resting membrane voltage and highly similar to GIRK1 and GIRK2, is not sensitive to WIN55,212-2, Δ9-THC, CBD, CP55,940, or rimonabant. From this, it is concluded that WIN55,212-2 selectively blocks GIRK1/2.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Kir3 channel blockade in the cerebellar cortex suppresses performance of classically conditioned Purkinje cell responses

In the eyeblink conditioning paradigm, cerebellar Purkinje cells learn to respond to the conditional stimulus with an adaptively timed pause in its spontaneous firing. Evidence suggests that the pause is elicited by glutamate released from parallel fibers and acting on metabotropic receptors (mGluR7) which initiates a delayed-onset suppression of firing. We suggested that G protein activation of hyperpolarizing K_ir3 channels (or ‘GIRK’, G protein-coupled inwardly-rectifying K⁺ channels) could be part of such a mechanism. Application of the K_ir3 antagonist Tertiapin-LQ locally in the superficial layers of the cerebellar cortex in decerebrate ferrets suppressed normal performance of Purkinje cell pause responses to the conditional stimulus. Importantly, there was no detectable effect on spontaneous firing. These findings suggest that intact functioning of K_ir3 channels in the cerebellar cortex is required for normal conditioned Purkinje cell responses.

A Collision Coupling Model Governs the Activation of Neuronal GIRK1/2 Channels by Muscarinic-2 Receptors

The G protein-activated Inwardly Rectifying K⁺-channel (GIRK) modulates heart rate and neuronal excitability. Following G-Protein Coupled Receptor (GPCR)-mediated activation of heterotrimeric G proteins (Gαβγ), opening of the channel is obtained by direct binding of Gβγ subunits. Interestingly, GIRKs are solely activated by Gβγ subunits released from Gα_i/o-coupled GPCRs, despite the fact that all receptor types, for instance Gα_q-coupled, are also able to provide Gβγ subunits. It is proposed that this specificity and fast kinetics of activation stem from pre-coupling (or pre-assembly) of proteins within this signaling cascade. However, many studies, including our own, point towards a diffusion-limited mechanism, namely collision coupling. Here, we set out to address this long-standing question by combining electrophysiology, imaging, and mathematical modeling. Muscarinic-2 receptors (M2R) and neuronal GIRK1/2 channels were coexpressed in Xenopus laevis oocytes, where we monitored protein surface expression, current amplitude, and activation kinetics. Densities of expressed M2R were assessed using a fluorescently labeled GIRK channel as a molecular ruler. We then incorporated our results, along with available kinetic data reported for the G-protein cycle and for GIRK1/2 activation, to generate a comprehensive mathematical model for the M2R-G-protein-GIRK1/2 signaling cascade. We find that, without assuming any irreversible interactions, our collision coupling kinetic model faithfully reproduces the rate of channel activation, the changes in agonist-evoked currents and the acceleration of channel activation by increased receptor densities.

Regulator of G-Protein Signaling (RGS) Protein Modulation of Opioid Receptor Signaling as a Potential Target for Pain Management

Opioid drugs are the gold standard for the management of pain, but their use is severely limited by dangerous and unpleasant side effects. All clinically available opioid analgesics bind to and activate the mu-opioid receptor (MOR), a heterotrimeric G-protein-coupled receptor, to produce analgesia. The activity of these receptors is modulated by a family of intracellular RGS proteins or regulators of G-protein signaling proteins, characterized by the presence of a conserved RGS Homology (RH) domain. These proteins act as negative regulators of G-protein signaling by serving as GTPase accelerating proteins or GAPS to switch off signaling by both the Gα and βγ subunits of heterotrimeric G-proteins. Consequently, knockdown or knockout of RGS protein activity enhances signaling downstream of MOR. In this review we discuss current knowledge of how this activity, across the different families of RGS proteins, modulates MOR activity, as well as activity of other members of the opioid receptor family, and so pain and analgesia in animal models, with particular emphasis on RGS4 and RGS9 families. We discuss inhibition of RGS proteins with small molecule inhibitors that bind to sensitive cysteine moieties in the RH domain and the potential for targeting this family of intracellular proteins as adjuncts to provide an opioid sparing effect or as standalone analgesics by promoting the activity of endogenous opioid peptides. Overall, we conclude that RGS proteins may be a novel drug target to provide analgesia with reduced opioid-like side effects, but that much basic work is needed to define the roles for specific RGS proteins, particularly in chronic pain, as well as a need to develop newer inhibitors.

The small molecule GAT1508 activates brain-specific GIRK1/2 channel heteromers and facilitates conditioned fear extinction in rodents

G-protein-gated inwardly-rectifying K⁺ (GIRK) channels are targets of G_i/o-protein-signaling systems that inhibit cell excitability. GIRK channels exist as homotetramers (GIRK2 and GIRK4) or heterotetramers with nonfunctional homomeric subunits (GIRK1 and GIRK3). Although they have been implicated in multiple conditions, the lack of selective GIRK drugs that discriminate among the different GIRK channel subtypes has hampered investigations into their precise physiological relevance and therapeutic potential. Here, we report on a highly-specific, potent, and efficacious activator of brain GIRK1/2 channels. Using a chemical screen and electrophysiological assays, we found that this activator, the bromothiophene-substituted small molecule GAT1508, is specific for brain-expressed GIRK1/2 channels rather than for cardiac GIRK1/4 channels. Computational models predicted a GAT1508-binding site validated by experimental mutagenesis experiments, providing insights into how urea-based compounds engage distant GIRK1 residues required for channel activation. Furthermore, we provide computational and experimental evidence that GAT1508 is an allosteric modulator of channel-phosphatidylinositol 4,5-bisphosphate interactions. Through brain-slice electrophysiology, we show that subthreshold GAT1508 concentrations directly stimulate GIRK currents in the basolateral amygdala (BLA) and potentiate baclofen-induced currents. Of note, GAT1508 effectively extinguished conditioned fear in rodents and lacked cardiac and behavioral side effects, suggesting its potential for use in pharmacotherapy for post-traumatic stress disorder. In summary, our findings indicate that the small molecule GAT1508 has high specificity for brain GIRK1/2 channel subunits, directly or allosterically activates GIRK1/2 channels in the BLA, and facilitates fear extinction in a rodent model.

Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells

Sparse neural activity in the dentate gyrus is enforced by powerful networks of inhibitory GABAergic interneurons in combination with low intrinsic excitability of the principal neurons, the dentate granule cells (GCs). Although the cellular and circuit properties that dictate synaptic inhibition are well studied, less is known about mechanisms that confer low GC intrinsic excitability. Here we demonstrate that intact G protein-mediated signaling contributes to the characteristic low resting membrane potential that differentiates mature dentate GCs from CA1 pyramidal cells and developing adult-born GCs. In mature GCs from male and female mice, intact G protein signaling robustly reduces intrinsic excitability, whereas deletion of G protein-activated inwardly rectifying potassium channel 2 (GIRK2) increases excitability and blocks the effects of G protein signaling on intrinsic properties. Similarly, pharmacological manipulation of GABA_B receptors (GABA_BRs) or GIRK channels alters intrinsic excitability and GC spiking behavior. However, adult-born new GCs lack functional GIRK activity, with phasic and constitutive GABA_BR-mediated GIRK signaling appearing after several weeks of maturation. Phasic activation is interneuron specific, arising primarily from nNOS-expressing interneurons rather than parvalbumin- or somatostatin-expressing interneurons. Together, these results demonstrate that G protein signaling contributes to the intrinsic excitability that differentiates mature and developing dentate GCs and further suggest that late maturation of GIRK channel activity is poised to convert early developmental functions of GABA_B receptor signaling into GABA_BR-mediated inhibition.SIGNIFICANCE STATEMENT The dentate gyrus exhibits sparse neural activity that is essential for the computational function of pattern separation. Sparse activity is ascribed to strong local inhibitory circuits in combination with low intrinsic excitability of the principal neurons, the granule cells. Here we show that constitutive activity of G protein-coupled inwardly rectifying potassium channels (GIRKs) underlies to the hallmark low resting membrane potential and input resistance of mature dentate neurons. Adult-born neurons initially lack functional GIRK channels, with constitutive and phasic GABA_B receptor-mediated GIRK inhibition developing in tandem after several weeks of maturation. Our results reveal that GABA_B/GIRK activity is an important determinant of low excitability of mature dentate granule cells that may contribute to sparse DG activity in vivo.

Regional Heterogeneity of D2-Receptor Signaling in the Dorsal Striatum and Nucleus Accumbens

Dopamine input to the dorsal and ventral striatum originates from separate populations of midbrain neurons. Despite differences in afferent inputs and behavioral output, little is known about how dopamine release is encoded by dopamine receptors on medium spiny neurons (MSNs) across striatal subregions. Here we examined the activation of D2 receptors following the synaptic release of dopamine in the dorsal striatum (DStr) and nucleus accumbens (NAc) shell. We found that D2 receptor-mediated synaptic currents were slower in the NAc and this difference occurred at the level of D2-receptor signaling. As a result of preferential coupling to Gαo, we also found that D2 receptors in MSNs demonstrated higher sensitivity for dopamine in the NAc. The higher sensitivity in the NAc was eliminated following cocaine exposure. These results identify differences in the sensitivity and timing of D2-receptor signaling across the striatum that influence how nigrostriatal and mesolimbic signals are encoded across these circuits.

The regulator of G-protein signaling RGS16 promotes insulin secretion and β-cell proliferation in rodent and human islets

Objective

G protein-coupled receptor (GPCR) signaling regulates insulin secretion and pancreatic β cell-proliferation. While much knowledge has been gained regarding how GPCRs are activated in β cells, less is known about the mechanisms controlling their deactivation. In many cell types, termination of GPCR signaling is controlled by the family of Regulators of G-protein Signaling (RGS). RGS proteins are expressed in most eukaryotic cells and ensure a timely return to the GPCR inactive state upon removal of the stimulus. The aims of this study were i) to determine if RGS16, the most highly enriched RGS protein in β cells, regulates insulin secretion and β-cell proliferation and, if so, ii) to elucidate the mechanisms underlying such effects.

Methods

Mouse and human islets were infected with recombinant adenoviruses expressing shRNA or cDNA sequences to knock-down or overexpress RGS16, respectively. 60 h post-infection, insulin secretion and cAMP levels were measured in static incubations in the presence of glucose and various secretagogues. β-cell proliferation was measured in infected islets after 72 h in the presence of 16.7 mM glucose ± somatostatin and various inhibitors.

Results

RGS16 mRNA levels are strongly up-regulated in islets of Langerhans under hyperglycemic conditions in vivo and ex vivo. RGS16 overexpression stimulated glucose-induced insulin secretion in isolated mouse and human islets while, conversely, insulin secretion was impaired following RGS16 knock-down. Insulin secretion was no longer affected by RGS16 knock-down when islets were pre-treated with pertussis toxin to inactivate Gαi/o proteins, or in the presence of a somatostatin receptor antagonist. RGS16 overexpression increased intracellular cAMP levels, and its effects were blocked by an adenylyl cyclase inhibitor. Finally, RGS16 overexpression prevented the inhibitory effect of somatostatin on insulin secretion and β-cell proliferation.

Conclusions

Our results identify RGS16 as a novel regulator of β-cell function that coordinately controls insulin secretion and proliferation by limiting the tonic inhibitory signal exerted by δ-cell-derived somatostatin in islets.

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RGS16 is up-regulated under hyperglycemic conditions in islets.

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RGS16 is a key regulator of insulin secretion and β-cell proliferation.

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RGS16 attenuates Gαi/o protein activity downstream of δ-cell derived SST.

Roles for Regulator of G Protein Signaling Proteins in Synaptic Signaling and Plasticity

The regulator of G protein signaling (RGS) family of proteins serves critical roles in G protein-coupled receptor (GPCR) and heterotrimeric G protein signal transduction. RGS proteins are best understood as negative regulators of GPCR/G protein signaling. They achieve this by acting as GTPase activating proteins (GAPs) for Gα subunits and accelerating the turnoff of G protein signaling. Many RGS proteins also bind additional signaling partners that either regulate their functions or enable them to regulate other important signaling events. At neuronal synapses, GPCRs, G proteins, and RGS proteins work in coordination to regulate key aspects of neurotransmitter release, synaptic transmission, and synaptic plasticity, which are necessary for central nervous system physiology and behavior. Accumulating evidence has revealed key roles for specific RGS proteins in multiple signaling pathways at neuronal synapses, regulating both pre- and postsynaptic signaling events and synaptic plasticity. Here, we review and highlight the current knowledge of specific RGS proteins (RGS2, RGS4, RGS7, RGS9-2, and RGS14) that have been clearly demonstrated to serve critical roles in modulating synaptic signaling and plasticity throughout the brain, and we consider their potential as future therapeutic targets.