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

Shai Berlin; Etay Artzy; Reem Handklo-Jamal; Uri Kahanovitch; Hanna Parnas; Nathan Dascal; Daniel Yakubovich

doi:10.3389/fphar.2020.01216

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

Front Pharmacol. 2020 Aug 12:11:1216. doi: 10.3389/fphar.2020.01216. eCollection 2020.

Authors

Shai Berlin¹, Etay Artzy², Reem Handklo-Jamal², Uri Kahanovitch², Hanna Parnas³, Nathan Dascal², Daniel Yakubovich^{2

4}

Affiliations

¹ Department of Neuroscience, Rappaport Faculty of Medicine, Technion, Haifa, Israel.
² Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel.
³ Silberman Institute of Life Sciences, Hebrew University, Jerusalem, Israel.
⁴ Department of Neonatology, Schneider Children's Hospital, Petah Tikva, Israel.

Abstract

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.

Keywords: G-Protein Coupled Receptor; G-protein cycle; GIRK channel; collision-coupling; kinetic model.