RGS4 regulates partial agonism of the M2 muscarinic receptor-activated K+ currents.

Partial agonists are used clinically to avoid overstimulation of receptor-mediated signalling, as they produce a submaximal response even at 100% receptor occupancy. The submaximal efficacy of partial agonists is due to conformational change of the agonist-receptor complex, which reduces effector activation. In addition to signalling activators, several regulators help control intracellular signal transductions. However, it remains unclear whether these signalling regulators contribute to partial agonism. Here we show that regulator of G-protein signalling (RGS) 4 is a determinant for partial agonism of the M2 muscarinic receptor (M2R). In rat atrial myocytes, pilocarpine evoked smaller G-protein-gated K(+) inwardly rectifying (KG) currents than those evoked by ACh. In a Xenopus oocyte expression system, pilocarpine acted as a partial agonist in the presence of RGS4 as it did in atrial myocytes, while it acted like a full agonist in the absence of RGS4. Functional couplings within the agonist-receptor complex/G-protein/RGS4 system controlled the efficacy of pilocarpine relative to ACh. The pilocarpine-M2R complex suppressed G-protein-mediated activation of KG currents via RGS4. Our results demonstrate that partial agonism of M2R is regulated by the RGS4-mediated inhibition of G-protein signalling. This finding helps us to understand the molecular components and mechanism underlying the partial agonism of M2R-mediated physiological responses

Conformational changes underlying pore dilation in the cytoplasmic domain of mammalian inward rectifier K^+ channels

Inanobe A, Nakagawa A, Kurachi Y (2013) Conformational Changes Underlying Pore Dilation in the Cytoplasmic Domain of Mammalian Inward Rectifier K^+ Channels. PLOS ONE 8(11): e79844. https://doi.org/10.1371/journal.pone.007984

Interactions of Cations with the Cytoplasmic Pores of Inward Rectifier K^+ Channels in the Closed State

This research was originally published in Journal of Biological Chemistry. Atsushi Inanobe, Atsushi Nakagawa, and Yoshihisa Kurachi. Interactions of Cations with the Cytoplasmic Pores of Inward Rectifier K^+ Channels in the Closed State. Journal of Biological Chemistry. 2011; 286, 41801-41811. © the American Society for Biochemistry and Molecular Biology

A structural determinant for the control of PIP_2 sensitivity in G protein-gated inward rectifier K^+ channels

Inward rectifier K^+ (Kir) channels are activated by phosphatidylinositol-( 4,5)-bisphosphate (PIP_2), but G protein-gated Kir (K_G) channels further require either G protein βγ subunits (Gβγ) or intracellular Na^+ for their activation. To reveal the mechanism(s) underlying this regulation, we compared the crystal structures of the cytoplasmic domain of K_G channel subunit Kir3.2 obtained in the presence and the absence of Na^+. The Na^+ -free Kir3.2, but not the Na^+ -plus Kir3.2, possessed an ionic bond connecting the N terminus and the CD loop of the C terminus. Functional analyses revealed that the ionic bond between His-69 on theNterminus and Asp-228 on the CD loop, which are known to be critically involved in Gβγ- and Na^+ -dependent activation, lowered PIP_2 sensitivity. The conservation of these residues within the K_G channel family indicates that the ionic bond is a character that maintains the channels in a closed state by controlling the PIP_2 sensitivity.This research was originally published in Journal of Biological Chemistry. Atsushi Inanobe, Atsushi Nakagawa, Takanori Matsuura and Yoshihisa Kurachi. A structural determinant for the control of PIP2 sensitivity in G protein-gated inward rectifier K^+ channels. Journal of Biological Chemistry. 2010; 285, 38517-38523. © the American Society for Biochemistry and Molecular Biology

Somatostatin induces hyperpolarization in pancreatic islet α cells by activating a G protein-gated K+ channel

AbstractSomatostatin inhibits glucagon-secretion from pancreatic α cells but its underlying mechanism is unknown. In mouse α cells, we found that somatostatin induced prominent hyperpolarization by activating a K+ channel, which was unaffected by tolbutamide but prevented by pre-treating the cells with pertussis toxin. The K+ channel was activated by intracellular GTP (with somatostatin), GTPγS or Gβγ subunits. It was thus identified as a G protein-gated K+ (KG) channel. RT-PCR and immunohistochemical analyses suggested the KG channel to be composed of Kir3.2c and Kir3.4. This study identified a novel ionic mechanism involved in somatostatin-inhibition of glucagon-secretion from pancreatic α cells

KCTD12 Auxiliary Proteins Modulate Kinetics of GABAB Receptor-Mediated Inhibition in Cholecystokinin-Containing Interneurons

Cholecystokinin-expressing interneurons (CCK-INs) mediate behavior state-dependent inhibition in cortical circuits and themselves receive strong GABAergic input. However, it remains unclear to what extent GABAB receptors (GABABRs) contribute to their inhibitory control. Using immunoelectron microscopy, we found that CCK-INs in the rat hippocampus possessed high levels of dendritic GABABRs and KCTD12 auxiliary proteins, whereas postsynaptic effector Kir3 channels were present at lower levels. Consistently, whole-cell recordings revealed slow GABABR-mediated inhibitory postsynaptic currents (IPSCs) in most CCK-INs. In spite of the higher surface density of GABABRs in CCK-INs than in CA1 principal cells, the amplitudes of IPSCs were comparable, suggesting that the expression of Kir3 channels is the limiting factor for the GABABR currents in these INs. Morphological analysis showed that CCK-INs were diverse, comprising perisomatic-targeting basket cells (BCs), as well as dendrite-targeting (DT) interneurons, including a previously undescribed DT type. GABABR-mediated IPSCs in CCK-INs were large in BCs, but small in DT subtypes. In response to prolonged activation, GABABR-mediated currents displayed strong desensitization, which was absent in KCTD12-deficient mice. This study highlights that GABABRs differentially control CCK-IN subtypes, and the kinetics and desensitization of GABABR-mediated currents are modulated by KCTD12 proteins

Control of KirBac3.1 potassium channel gating at the interface between cytoplasmic domains.

KirBac channels are prokaryotic homologs of mammalian inwardly rectifying potassium (Kir) channels, and recent structures of KirBac3.1 have provided important insights into the structural basis of gating in Kir channels. In this study, we demonstrate that KirBac3.1 channel activity is strongly pH-dependent, and we used x-ray crystallography to determine the structural changes that arise from an activatory mutation (S205L) located in the cytoplasmic domain (CTD). This mutation stabilizes a novel energetically favorable open conformation in which changes at the intersubunit interface in the CTD also alter the electrostatic potential of the inner cytoplasmic cavity. These results provide a structural explanation for the activatory effect of this mutation and provide a greater insight into the role of the CTD in Kir channel gating

Upregulation of basolateral small conductance potassium channels (KCNQ1/KCNE3) in ulcerative colitis

Background Basolateral K+ channels hyperpolarize colonocytes to ensure Na+ (and thus water) absorption. Small conductance basolateral (KCNQ1/KCNE3) K+ channels have never been evaluated in human colon. We therefore evaluated KCNQ1/KCNE3 channels in distal colonic crypts obtained from normal and active ulcerative colitis (UC) patients. Methods KCNQ1 and KCNE3 mRNA levels were determined by qPCR, and KCNQ1/KCNE3 channel activity in normal and UC crypts, and the effects of forskolin (activator of adenylate cyclase) and UC-related proinflammatory cytokines on normal crypts, studied by patch clamp recording. Results Whereas KCNQ1 and KCNE3 mRNA expression was similar in normal and UC crypts, single 6.8 pS channels were seen in 36% of basolateral patches in normal crypts, and to an even greater extent (74% of patches, P < 0.001) in UC crypts, with two or more channels per patch. Channel activity was 10-fold higher (P < 0.001) in UC crypts, with a greater contribution to basolateral conductance (5.85 ± 0.62 mS cm−2) than in controls (0.28 ± 0.04 mS cm−2, P < 0.001). In control crypts, forskolin and thromboxane A2 stimulated channel activity 30-fold and 10-fold respectively, while PGE2, IL-1β, and LTD4 had no effect. Conclusions KCNQ1/KCNE3 channels make only a small contribution to basolateral conductance in normal colonic crypts, with increased channel activity in UC appearing insufficient to prevent colonic cell depolarization in this disease. This supports the proposal that defective Na+ absorption rather than enhanced Cl− secretion, is the dominant pathophysiological mechanism of diarrhea in UC

Inwardly rectifying potassium channels (KIR) in GtoPdb v.2021.3

The 2TM domain family of K channels are also known as the inward-rectifier K channel family. This family includes the strong inward-rectifier K channels (Kir2.x) that are constitutively active, the G-protein-activated inward-rectifier K channels (Kir3.x) and the ATP-sensitive K channels (Kir6.x, which combine with sulphonylurea receptors (SUR1-3)). The pore-forming &#945; subunits form tetramers, and heteromeric channels may be formed within subfamilies (e.g. Kir3.2 with Kir3.3)

Inwardly rectifying potassium channels (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

The 2TM domain family of K channels are also known as the inward-rectifier K channel family. This family includes the strong inward-rectifier K channels (Kir2.x) that are constitutively active, the G-protein-activated inward-rectifier K channels (Kir3.x) and the ATP-sensitive K channels (Kir6.x, which combine with sulphonylurea receptors (SUR1-3)). The pore-forming &#945; subunits form tetramers, and heteromeric channels may be formed within subfamilies (e.g. Kir3.2 with Kir3.3)