Redox Bridling of GIRK Channel Activity

G protein-gated inwardly rectifying potassium (GIRK, Kir3.x) channels belong to the large family of inwardly rectifying potassium (Kir) channels expressed throughout the body. Activation and consequent opening of GIRK channels allow inward flow of potassium (K+) ions into the cell resulting in membrane potential hyperpolarization and decreased excitability. Thus, GIRK channels play a key role in regulating the activity of neurons and controlling important physiological processes including neuronal excitability, heart rate, and pain perception.1 GIRK channels are integral membrane proteins, existing as homo- or heterotetramers. Each monomer features two membrane-spanning helices (M1 and M2), a re-entrant P-loop for controlling ion permeation and selectivity, and extensive intracellular amino- and carboxy-termini crucial for channel gating. Permeation is regulated by an inner helix gate formed by the M2 segments and a cytoplasmic G-loop gate.1 Activation of GIRK channels is mediated by the direct interaction of G?? subunits, released from various G protein-coupled receptors (GPCRs) upon the activation of inhibitory neurotransmitter receptors. However, the activity of GIRK channels depends on the presence of the membrane anionic phospholipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2 or PIP2) while it is also modulated by ubiquitously present sodium (Na+) ions. Furthermore, GIRK channels are too regulated by cholesterol, phosphorylation, ethanol, etcetera.1 The crystal structures of recombinant GIRK channels have offered valuable insights into how they are functionally regulated by various ligands. Thus, channel opening is facilitated by PIP2 at the plasma membrane, whereas G?? and Na+ modulate the channel's interaction with PIP2 through conformational changes that govern the gating process.2 The intracellular milieu is a reducing environment characterized by a balanced redox state. This state is crucial to support cellular processes while serving as a protective shield against damaging reactive oxygen species (ROS) thereby facilitating enzymatic reactions and energy production.3 GIRK channels exhibit low basal activity in reducing intracellular environments. Nevertheless, the channel's behavior in native tissues is influenced by a variety of cellular factors, which complicate the interpretation of their specific contributions to channel gating. In a recent Function publication, Lee et al. effectively bypassed the complexity of cellular systems to study the regulation of GIRK channels.4 The team employed an acellular in vitro flux assay to manipulate the redox state of the functional channel protein within liposomes made of determined phospholipid mixtures. Subsequently, they assessed GIRK2-mediated fluxes upon systematic replacement of key modulatory co-factors. This method successfully reproduced the channel's typical behavior (ie, low basal activity in reducing conditions) while also uncovering an oxidation-dependent anomalous enhancement of GIRK2 activation. It was observed that in the absence of stringent redox control, as is commonly provided by healthy cells, the GIRK2 protein undergoes oxidation at two crucial cysteine residues during purification. These cysteines have different susceptibility to environmental oxidation, and their oxidized state appears to play a pivotal role in GIRK2 gating. Two distinct effects were observed upon oxidation: (i) the loss of PIP2 and Na+-dependent activity, linked to cysteine 65 and (ii) a slower rise in basal activity associated with cysteine 190. The N-terminal cysteine at position 65 of GIRK2 is highly conserved in the Kir channel family and it is strategically located adjacent to lysine 64, which contributes to the PIP2 binding site. Moreover, C65 is positioned toward a neighb