The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

De Novo Variant in the KCNJ9 Gene as a Possible Cause of Neonatal Seizures

BACKGROUND

The reduction in next-generation sequencing (NGS) costs allows for using this method for newborn screening for monogenic diseases (MDs). In this report, we describe a clinical case of a newborn participating in the EXAMEN project (ClinicalTrials.gov Identifier: NCT05325749).

METHODS

The child presented with convulsive syndrome on the third day of life. Generalized convulsive seizures were accompanied by electroencephalographic patterns corresponding to epileptiform activity. Proband WES expanded to trio sequencing was performed.

RESULTS

A differential diagnosis was made between symptomatic (dysmetabolic, structural, infectious) neonatal seizures and benign neonatal seizures. There were no data in favor of the dysmetabolic, structural, or infectious nature of seizures. Molecular karyotyping and whole exome sequencing were not informative. Trio WES revealed a de novo variant in the KCNJ9 gene (1:160087612T > C, p.Phe326Ser, NM_004983), for which, according to the OMIM database, no association with the disease has been described to date. Three-dimensional modeling was used to predict the structure of the KCNJ9 protein using the known structure of its homologs. According to the predictions, Phe326Ser change possibly disrupts the hydrophobic contacts with the valine side chain. Destabilization of the neighboring structures may undermine the formation of GIRK2/GIRK3 tetramers necessary for their proper functioning.

CONCLUSIONS

We believe that the identified variant may be the cause of the disease in this patient but further studies, including the search for other patients with the KCNJ9 variants, are needed.

Trace amine-associated receptor 1 (TAAR1): Potential application in mood disorders: A systematic review

There is a need for innovation with respect to therapeutics in psychiatry. Available evidence indicates that the trace amine-associated receptor 1 (TAAR1) agonist SEP-363856 is promising, as it improves measures of cognitive and reward function in schizophrenia. Hedonic and cognitive impairments are transdiagnostic and constitute major burdens in mood disorders. Herein, we systematically review the behavioural and genetic literature documenting the role of TAAR1 in reward and cognitive function, and propose a mechanistic model of TAAR1's functions in the brain. Notably, TAAR1 activity confers antidepressant-like effects, enhances attention and response inhibition, and reduces compulsive reward seeking without impairing normal function. Further characterization of the responsible mechanisms suggests ion-homeostatic, metabolic, neurotrophic, and anti-inflammatory enhancements in the limbic system. Multiple lines of evidence establish the viability of TAAR1 as a biological target for the treatment of mood disorders. Furthermore, the evidence suggests a role for TAAR1 in reward and cognitive function, which is attributed to a cascade of events that are relevant to the cellular integrity and function of the central nervous system.

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.

Novel Roles of Non-Coding RNAs in Opioid Signaling and Cardioprotection

Cardiovascular disease (CVD) is a significant cause of morbidity and mortality across the world. A large proportion of CVD deaths are secondary to coronary artery disease (CAD) and myocardial infarction (MI). Even though prevention is the best strategy to reduce risk factors associated with MI, the use of cardioprotective interventions aimed at improving patient outcomes is of great interest. Opioid conditioning has been shown to be effective in reducing myocardial ischemia-reperfusion injury (IRI) and cardiomyocyte death. However, the molecular mechanisms behind these effects are under investigation and could provide the basis for the development of novel therapeutic approaches in the treatment of CVD. Non-coding RNAs (ncRNAs), which are functional RNA molecules that do not translate into proteins, are critical modulators of cardiac gene expression during heart development and disease. Moreover, ncRNAs such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are known to be induced by opioid receptor activation and regulate opioid signaling pathways. Recent advances in experimental and computational tools have accelerated the discovery and functional characterization of ncRNAs. In this study, we review the current understanding of the role of ncRNAs in opioid signaling and opioid-induced cardioprotection.

DNA Origami Scaffolds as Templates for Functional Tetrameric Kir3 K+ Channels

In native systems, scaffolding proteins play important roles in assembling proteins into complexes to transduce signals. This concept is yet to be applied to the assembly of functional transmembrane protein complexes in artificial systems. To address this issue, DNA origami has the potential to serve as scaffolds that arrange proteins at specific positions in complexes. Herein, we report that Kir3 K⁺ channel proteins are assembled through zinc-finger protein (ZFP)-adaptors at specific locations on DNA origami scaffolds. Specific binding of the ZFP-fused Kir3 channels and ZFP-based adaptors on DNA origami were confirmed by atomic force microscopy and gel electrophoresis. Furthermore, the DNA origami with ZFP binding sites nearly tripled the K⁺ channel current activity elicited by heterotetrameric Kir3 channels in HEK293T cells. Thus, our method provides a useful template to control the oligomerization states of membrane protein complexes in vitro and in living cells.

A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ

G protein-gated K⁺ channels (GIRK; Kir3), activated by Gβγ subunits derived from G_i/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (I_evoked) and neurotransmitter-independent basal (I_basal) GIRK activities are physiologically important, but mechanisms of I_basal and its relation to I_evoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that I_basal and I_evoked are interrelated: the extent of activation by neurotransmitter (activation index, R_a) is inversely related to I_basal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates I_basal and I_evoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse I_basal-R_a correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gα_i/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gα_i/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα.

Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K⁺ channel, GIRK (G protein-gated Inwardly Rectifying K⁺ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.

Anti-incretin, Anti-proliferative Action of Dopamine on β-Cells

Human islet β-cells exploit an autocrine dopamine (DA)-mediated inhibitory circuit to regulate insulin secretion. β-Cells also express the DA active transporter and the large neutral amino acid transporter heterodimer enabling them to import circulating DA or its biosynthetic precursor, L-3,4-dihydroxyphenylalanine (L-DOPA). The capacity to import DA or L-DOPA from the extracellular space possibly indicates that DA may be an endocrine signal as well. In humans, a mixed meal stimulus is accompanied by contemporary serum excursions of incretins, DA and L-DOPA, suggesting that DA may act as an anti-incretin as postulated by the foregut hypothesis proposed to explain the early effects of bariatric surgery on type 2 diabetes. In this report, we take a translational step backwards and characterize the kinetics of plasma DA and incretin production after a mixed meal challenge in a rat model and study the integration of incretin and DA signaling at the biochemical level in a rodent β-cell line and islets. We found that there are similar excursions of incretins and DA in rats, as those reported in humans, after a mixed meal challenge and that DA counters incretin enhanced glucose-stimulated insulin secretion and intracellular signaling at multiple points from dampening calcium fluxes to inhibiting proliferation as well as apoptosis. Our data suggest that DA is an important regulator of insulin secretion and may represent 1 axis of a gut level circuit of glucose and β-cell mass homeostasis.

Localization and Targeting of GIRK Channels in Mammalian Central Neurons

G protein-gated inwardly rectifying K(+) (GIRK/K(ir)3) channels are critical to brain function. They hyperpolarize neurons in response to activation of different G protein-coupled receptors, reducing cell excitability. Molecular cloning has revealed four distinct mammalian genes (GIRK1-4), which, with the exception of GIRK4, are broadly expressed in the central nervous system (CNS) and have been implicated in a variety of neurological disorders. Although the molecular structure and composition of GIRK channels are key determinants of their biophysical properties, their cellular and subcellular localization patterns and densities on the neuronal surface are just as important to nerve function. Current data obtained with high-resolution quantitative localization techniques reveal complex, subcellular compartment-specific distribution patterns of GIRK channel subunits. Recent efforts have focused on determining the associated proteins that form macromolecular complexes with GIRK channels. Demonstration of the precise subcellular compartmentalization of GIRK channels and their associated proteins represents a crucial step in understanding the contribution of these channels to specific aspects of neuronal function under both physiological and pathological conditions. Here, we present an overview of studies aimed at determining the cellular and subcellular localization of GIRK channel subunits in mammalian brain neurons and discuss implications for neuronal physiology.

Recruitment of Gβγ controls the basal activity of G-protein coupled inwardly rectifying potassium (GIRK) channels: crucial role of distal C terminus of GIRK1

The G-protein coupled inwardly rectifying potassium (GIRK, or Kir3) channels are important mediators of inhibitory neurotransmission via activation of G-protein coupled receptors (GPCRs). GIRK channels are tetramers comprising combinations of subunits (GIRK1-4), activated by direct binding of the Gβγ subunit of Gi/o proteins. Heterologously expressed GIRK1/2 exhibit high, Gβγ-dependent basal currents (Ibasal) and a modest activation by GPCR or coexpressed Gβγ. Inversely, the GIRK2 homotetramers exhibit low Ibasal and strong activation by Gβγ. The high Ibasal of GIRK1 seems to be associated with its unique distal C terminus (G1-dCT), which is not present in the other subunits. We investigated the role of G1-dCT using electrophysiological and fluorescence assays in Xenopus laevis oocytes and protein interaction assays. We show that expression of GIRK1/2 increases the plasma membrane level of coexpressed Gβγ (a phenomenon we term 'Gβγ recruitment') but not of coexpressed Gαi3. All GIRK1-containing channels, but not GIRK2 homomers, recruited Gβγ to the plasma membrane. In biochemical assays, truncation of G1-dCT reduces the binding between the cytosolic parts of GIRK1 and Gβγ, but not Gαi3. Nevertheless, the truncation of G1-dCT does not impair activation by Gβγ. In fluorescently labelled homotetrameric GIRK1 channels and in the heterotetrameric GIRK1/2 channel, the truncation of G1-dCT abolishes Gβγ recruitment and decreases Ibasal. Thus, we conclude that G1-dCT carries an essential role in Gβγ recruitment by GIRK1 and, consequently, in determining its high basal activity. Our results indicate that G1-dCT is a crucial part of a Gβγ anchoring site of GIRK1-containing channels, spatially and functionally distinct from the site of channel activation by Gβγ.

Kir3 channel ontogeny - the role of Gβγ subunits in channel assembly and trafficking

The role of Gβγ subunits in Kir3 channel gating is well characterized. Here, we have studied the role of Gβγ dimers during their initial contact with Kir3 channels, prior to their insertion into the plasma membrane. We show that distinct Gβγ subunits play an important role in orchestrating and fine-tuning parts of the Kir3 channel life cycle. Gβ1γ2, apart from its role in channel opening that it shares with other Gβγ subunit combinations, may play a unique role in protecting maturing channels from degradation as they transit to the cell surface. Taken together, our data suggest that Gβ1γ2 prolongs the lifetime of the Kir3.1/Kir3.2 heterotetramer, although further studies would be required to shed more light on these early Gβγ effects on Kir3 maturation and trafficking.

Differential effects of genetically-encoded Gβγ scavengers on receptor-activated and basal Kir3.1/Kir3.4 channel current in rat atrial myocytes

Opening of G-protein-activated inward-rectifying K(+) (GIRK, Kir3) channels is regulated by interaction with βγ-subunits of Pertussis-toxin-sensitive G proteins upon activation of appropriate GPCRs. In atrial and neuronal cells agonist-independent activity (I(basal)) contributes to the background K(+) conductance, important for stabilizing resting potential. Data obtained from the Kir3 signaling pathway reconstituted in Xenopus oocytes suggest that I(basal) requires free G(βγ). In cells with intrinsic expression of Kir3 channels this issue has been scarcely addressed experimentally. Two G(βγ)-binding proteins (myristoylated phosducin - mPhos - and G(αi1)) were expressed in atrial myocytes using adenoviral gene transfer, to interrupt G(βγ)-signaling. Agonist-induced and basal currents were recorded using whole cell voltage-clamp. Expression of mPhos and G(αi1) reduced activation of Kir3 current via muscarinic M(2) receptors (IK(ACh)). Inhibition of IK(ACh) by mPhos consisted of an irreversible component and an agonist-dependent reversible component. Reduction in density of IK(ACh) by overexpressed Gαi1, in contrast to mPhos, was paralleled by substantial slowing of activation, suggesting a reduction in density of functional M2 receptors, rather than G(βγ)-scavenging as underlying mechanism. In line with this notion, current density and activation kinetics were rescued by fusing the αi1-subunit to an Adenosine A(1) receptor. Neither mPhos nor G(αi1) had a significant effect on I(basal), defined by the inhibitory peptide tertiapin-Q. These data demonstrate that basal Kir3 current in a native environment is unrelated to G-protein signaling or agonist-independent free G(βγ). Moreover, our results illustrate the importance of physiological expression levels of the signaling components in shaping key parameters of the response to an agonist.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

Ionic conductances regulating the excitability of colonic smooth muscles

The tunica muscularis of the gastrointestinal (GI) tract contains two layers of smooth muscle cells (SMC) oriented perpendicular to each other. SMC express a variety of voltage-dependent and voltage-independent ionic conductance(s) that develop membrane potential and control excitability. Resting membrane potentials (RMP) vary through the GI tract but generally are within the range of -80 to -40 mV. RMP sets the 'gain' of smooth muscle and regulates openings of voltage-dependent Ca(2+) channels. A variety of K(+) channels contribute to setting RMP of SMC. In most regions, RMP is considerably less negative than the K(+) equilibrium potential, due to a finely tuned balance between background K(+) channels and non-selective cation channels (NSCC). Variations in expression patterns and openings of K(+) channels and NSCC account for differences of the RMP in different regions of the GI tract. Smooth muscle excitability is also regulated by interstitial cells (interstitial cells of Cajal (ICC) and PDGFRα(+) cells) that express additional conductances and are electrically coupled to SMC. Thus, 'myogenic' activity results from the integrated behavior of the SMC/ICC/PDGFRα(+) cell (SIP) syncytium. Inputs from excitatory and inhibitory motor neurons are required to produce the complex motor patterns of the gut. Motor neurons innervate three cell types in the SIP, and receptors, second messenger pathways, and ion channels in these cells mediate postjunctional responses. Studies of isolated SIP cells have begun to unravel the mechanisms responsible for neural responses. This review discusses ion channels that set and regulate RMP of SIP cells and how neurotransmitters regulate membrane potential.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.

Two distinct aspects of coupling between Gα(i) protein and G protein-activated K+ channel (GIRK) revealed by fluorescently labeled Gα(i3) protein subunits

G protein-activated K(+) channels (Kir3 or GIRK) are activated by direct interaction with Gβγ. Gα is essential for specific signaling and regulates basal activity of GIRK (I(basal)) and kinetics of the response elicited by activation by G protein-coupled receptors (I(evoked)). These regulations are believed to occur within a GIRK-Gα-Gβγ signaling complex. Fluorescent energy resonance transfer (FRET) studies showed strong GIRK-Gβγ interactions but yielded controversial results regarding the GIRK-Gα(i/o) interaction. We investigated the mechanisms of regulation of GIRK by Gα(i/o) using wild-type Gα(i3) (Gα(i3)WT) and Gα(i3) labeled at three different positions with fluorescent proteins, CFP or YFP (xFP). Gα(i3)xFP proteins bound the cytosolic domain of GIRK1 and interacted with Gβγ in a guanine nucleotide-dependent manner. However, only an N-terminally labeled, myristoylated Gα(i3)xFP (Gα(i3)NT) closely mimicked all aspects of Gα(i3)WT regulation except for a weaker regulation of I(basal). Gα(i3) labeled with YFP within the Gα helical domain preserved regulation of I(basal) but failed to restore fast I(evoked). Titrated expression of Gα(i3)NT and Gα(i3)WT confirmed that regulation of I(basal) and of the kinetics of I(evoked) of GIRK1/2 are independent functions of Gα(i). FRET and direct biochemical measurements indicated much stronger interaction between GIRK1 and Gβγ than between GIRK1 and Gα(i3). Thus, Gα(i/o)βγ heterotrimer may be attached to GIRK primarily via Gβγ within the signaling complex. Our findings support the notion that Gα(i/o) actively regulates GIRK. Although regulation of I(basal) is a function of Gα(i)(GDP), our new findings indicate that regulation of kinetics of I(evoked) is mediated by Gα(i)(GTP).