VU0810464, a non-urea G protein-gated inwardly rectifying K+ (Kir 3/GIRK) channel activator, exhibits enhanced selectivity for neuronal Kir 3 channels and reduces stress-induced hyperthermia in mice

Prenatal cannabinoid exposure and the developing brain: evidence of lasting consequences in preclinical rodent models

Cannabis use by people who are pregnant is increasing. Understanding how prenatal cannabinoid exposure (PCE) affects infants and children is of high public health significance. Epidemiological studies have associated PCE with cognitive symptoms including impaired learning, memory, attention, and executive control, and affective symptoms including anxiety, emotional dysregulation, and social impairments, in children, adolescents, and young adults. PCE is also associated with neurobiological changes including decreased corticolimbic white matter and functional connectivity; however, the underlying mechanisms for these persisting effects remain unknown. Rodent models are essential for uncovering the effects of PCE on the developing brain. This review summarizes rodent studies focused on the cognitive and affective behavioral and neurobiological outcomes of PCE. Rodent studies have reported cognitive deficits including impaired learning, memory, attention, and executive control, and affect-related impairments including anxiety-like behavior, altered stress coping, social impairments, and anhedonia-like behavior, in adolescent and adult offspring. Studies have also demonstrated that PCE affects several underlying neurotransmitter systems, producing dopamine hyperactivity, glutamate and serotonin hypoactivity, and dysregulating GABA and opioid signaling. Evidence further suggests a marked difference in outcomes between males and females, with males being more susceptible to the enduring effects of PCE. However, studies that investigate female-specific outcomes or sex as a biological variable are scarce. Altogether, rodent studies provide corroborating evidence that PCE produces lasting cognitive and affective impairments underpinned by altered neurobiological mechanisms. Research is critically needed to improve our understanding of the risks associated with cannabis use during pregnancy and effects across the lifespan.

Amyloid-β oligomers trigger sex-dependent inhibition of GIRK channel activity in hippocampal neurons in mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by amyloid plaques and cognitive decline, the latter of which is thought to be driven by soluble oligomeric amyloid-β (oAβ). The dysregulation of G protein-gated inwardly rectifying K+ (GIRK; also known as Kir3) channels has been implicated in rodent models of AD. Here, seeking mechanistic insights, we uncovered a sex-dependent facet of GIRK-dependent signaling in AD-related amyloid pathophysiology. Synthetic oAβ1-42 suppressed GIRK-dependent signaling in hippocampal neurons from male mice, but not from female mice. This effect required cellular prion protein, the receptor mGluR5, and production of arachidonic acid by the phospholipase PLA2. Although oAβ suppressed GIRK channel activity only in male hippocampal neurons, intrahippocampal infusion of oAβ or genetic suppression of GIRK channel activity in hippocampal pyramidal neurons impaired performance on a memory test in both male and female mice. Moreover, genetic enhancement of GIRK channel activity in hippocampal pyramidal neurons blocked oAβ-induced cognitive impairment in both male and female mice. In APP/PS1 AD model mice, GIRK-dependent signaling was diminished in hippocampal CA1 pyramidal neurons from only male mice before cognitive deficit was detected. However, enhancing GIRK channel activity rescued cognitive deficits in older APP/PS1 mice of both sexes. Thus, whereas diminished GIRK channel activity contributes to cognitive deficits in male mice with increased oAβ burden, enhancing its activity may have therapeutic potential for both sexes.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

AsKC11, a Kunitz Peptide from Anemonia sulcata, Is a Novel Activator of G Protein-Coupled Inward-Rectifier Potassium Channels

(1) Background: G protein-coupled inward-rectifier potassium (GIRK) channels, especially neuronal GIRK1/2 channels, have been the focus of intense research interest for developing drugs against brain diseases. In this context, venom peptides that selectively activate GIRK channels can be seen as a new source for drug development. Here, we report on the identification and electrophysiological characterization of a novel activator of GIRK1/2 channels, AsKC11, found in the venom of the sea anemone Anemonia sulcata. (2) Methods: AsKC11 was purified from the sea anemone venom by reverse-phase chromatography and the sequence was identified by mass spectrometry. Using the two-electrode voltage-clamp technique, the activity of AsKC11 on GIRK1/2 channels was studied and its selectivity for other potassium channels was investigated. (3) Results: AsKC11, a Kunitz peptide found in the venom of A. sulcata, is the first peptide shown to directly activate neuronal GIRK1/2 channels independent from Gi/o protein activity, without affecting the inward-rectifier potassium channel (IRK1) and with only a minor effect on K_V1.6 channels. Thus, AsKC11 is a novel activator of GIRK channels resulting in larger K⁺ currents because of an increased chord conductance. (4) Conclusions: These discoveries provide new insights into a novel class of GIRK activators.

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.

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.

Bidirectional Influence of Limbic GIRK Channel Activation on Innate Avoidance Behavior

Systemic administration of ML297, a selective activator of G-protein-gated inwardly rectifying K+ (GIRK) channels, decreases innate avoidance behavior in male C57BL/6J mice. The cellular mechanisms mediating the ML297-induced suppression of avoidance behavior are unknown. Here, we show that systemic ML297 administration suppresses elevated plus maze (EPM)-induced neuronal activation in the ventral hippocampus (vHPC) and basolateral amygdala (BLA) and that ML297 activates GIRK1-containing GIRK channels in these limbic structures. While intracranial infusion of ML297 into the vHPC suppressed avoidance behavior in the EPM test, mirroring the effect of systemic ML297, intra-BLA administration of ML297 provoked the opposite effect. Using neuron-specific viral genetic and chemogenetic approaches, we found that the combined inhibition of excitatory neurons in CA3 and dentate gyrus (DG) subregions of the vHPC was sufficient to decrease innate avoidance behavior in the EPM, open-field, and light-dark tests in male C57BL/6J mice, while performance in the marble-burying test was not impacted. Furthermore, genetic ablation of GIRK channels in CA3/DG excitatory neurons precluded the suppression of avoidance behavior evoked by systemic ML297 in the EPM test. Thus, acute inhibition of excitatory neurons in the ventral CA3 and DG subregions of the vHPC is necessary for the apparent anxiolytic efficacy of systemic ML297 and is sufficient to decrease innate avoidance behavior in male C57BL/6J mice.SIGNIFICANCE STATEMENT We interrogated the cellular mechanisms underlying the apparent anxiolytic efficacy of ML297, a selective activator of G-protein-gated inwardly rectifying K+ (GIRK) channels and promising lead compound. Intracranial infusion of ML297 into the ventral hippocampus (vHPC) and basolateral amygdala (BLA) complex exerted opposing influence on innate avoidance behavior in male C57BL/6J mice, the former recapitulating the suppression of avoidance behavior evoked by systemic ML297. Using viral genetic and chemogenetic approaches, we showed that combined inhibition of excitatory neurons in CA3 and dentate gyrus (DG) subregions of the vHPC is sufficient to decrease innate avoidance behavior in male mice and mediates the decrease in avoidance behavior evoked by systemic ML297. These findings establish a foundation for future investigations into the therapeutic potential of GIRK channel modulation in anxiety disorders.

Next-generation inward rectifier potassium channel modulators: discovery and molecular pharmacology

Inward rectifying potassium (Kir) channels play important roles in both excitable and nonexcitable cells of various organ systems and could represent valuable new drug targets for cardiovascular, metabolic, immune, and neurological diseases. In nonexcitable epithelial cells of the kidney tubule, for example, Kir1.1 (KCNJ1) and Kir4.1 (KCNJ10) are linked to sodium reabsorption in the thick ascending limb of Henle's loop and distal convoluted tubule, respectively, and have been explored as novel-mechanism diuretic targets for managing hypertension and edema. G protein-coupled Kir channels (Kir3) channels expressed in the central nervous system are critical effectors of numerous signal transduction pathways underlying analgesia, addiction, and respiratory-depressive effects of opioids. The historical dearth of pharmacological tool compounds for exploring the therapeutic potential of Kir channels has led to a molecular target-based approach using high-throughput screen (HTS) of small-molecule libraries and medicinal chemistry to develop "next-generation" Kir channel modulators that are both potent and specific for their targets. In this article, we review recent efforts focused specifically on discovery and improvement of target-selective molecular probes. The reader is introduced to fluorescence-based thallium flux assays that have enabled much of this work and then provided with an overview of progress made toward developing modulators of Kir1.1 (VU590, VU591), Kir2.x (ML133), Kir3.X (ML297, GAT1508, GiGA1, VU059331), Kir4.1 (VU0134992), and Kir7.1 (ML418). We discuss what is known about the small molecules' molecular mechanisms of action, in vitro and in vivo pharmacology, and then close with our view of what critical work remains to be done.

Advances in Targeting GIRK Channels in Disease

G protein-gated inwardly rectifying potassium (GIRK) channels are essential regulators of cell excitability in the brain. While they are implicated in a variety of neurological diseases in both human and animal model studies, their therapeutic potential has been largely untapped. Here, we review recent advances in the development of small molecule compounds that specifically modulate GIRK channels and compare them with first-generation compounds that exhibit off-target activity. We describe the method of discovery of these small molecule modulators, their chemical features, and their effects in vivo. These studies provide a promising outlook on the future development of subunit-specific GIRK modulators to regulate neuronal excitability in a brain region-specific manner.

VU0810464, a non-urea G protein-gated inwardly rectifying K+ (Kir 3/GIRK) channel activator, exhibits enhanced selectivity for neuronal Kir 3 channels and reduces stress-induced hyperthermia in mice

BACKGROUND AND PURPOSE

G protein-gated inwardly rectifying K⁺ (K_ir 3) channels moderate the activity of excitable cells and have been implicated in neurological disorders and cardiac arrhythmias. Most neuronal K_ir 3 channels consist of K_ir 3.1 and K_ir 3.2 subtypes, while cardiac K_ir 3 channels consist of K_ir 3.1 and K_ir 3.4 subtypes. Previously, we identified a family of urea-containing K_ir 3 channel activators, but these molecules exhibit suboptimal pharmacokinetic properties and modest selectivity for K_ir 3.1/3.2 relative to K_ir 3.1/3.4 channels. Here, we characterize a non-urea activator, VU0810464, which displays nanomolar potency as a K_ir 3.1/3.2 activator, improved selectivity for neuronal K_ir 3 channels, and improved brain penetration.

EXPERIMENTAL APPROACH

We used whole-cell electrophysiology to measure the efficacy and potency of VU0810464 in neurons and the selectivity of VU0810464 for neuronal and cardiac K_ir 3 channel subtypes. We tested VU0810464 in vivo in stress-induced hyperthermia and elevated plus maze paradigms. Parallel studies with ML297, the prototypical activator of K_ir 3.1-containing K_ir 3 channels, were performed to permit direct comparisons.

KEY RESULTS

VU0810464 and ML297 exhibited comparable efficacy and potency as neuronal K_ir 3 channel activators, but VU0810464 was more selective for neuronal K_ir 3 channels. VU0810464, like ML297, reduced stress-induced hyperthermia in a K_ir 3-dependent manner in mice. ML297, but not VU0810464, decreased anxiety-related behaviour as assessed with the elevated plus maze test.

CONCLUSION AND IMPLICATIONS

VU0810464 represents a new class of K_ir 3 channel activator with enhanced selectivity for K_ir 3.1/3.2 channels. VU0810464 may be useful for examining K_ir 3.1/3.2 channel contributions to complex behaviours and for probing the potential of K_ir 3 channel-dependent manipulations to treat neurological disorders.