Auxiliary GABAB Receptor Subunits Uncouple G Protein βγ Subunits from Effector Channels to Induce Desensitization

Abstract

Activation of K(+) channels by the G protein βγ subunits is an important signaling mechanism of G-protein-coupled receptors. Typically, receptor-activated K(+) currents desensitize in the sustained presence of agonists to avoid excessive effects on cellular activity. The auxiliary GABAB receptor subunit KCTD12 induces fast and pronounced desensitization of the K(+) current response. Using proteomic and electrophysiological approaches, we now show that KCTD12-induced desensitization results from a dual interaction with the G protein: constitutive binding stabilizes the heterotrimeric G protein at the receptor, whereas dynamic binding to the receptor-activated Gβγ subunits induces desensitization by uncoupling Gβγ from the effector K(+) channel. While receptor-free KCTD12 desensitizes K(+) currents activated by other GPCRs in vitro, native KCTD12 is exclusively associated with GABAB receptors. Accordingly, genetic ablation of KCTD12 specifically alters GABAB responses in the brain. Our results show that GABAB receptors are endowed with fast and reversible desensitization by harnessing KCTD12 that intercepts Gβγ signaling.

GABAB receptor subtypes differentially regulate thalamic spindle oscillations

“…Conversely, activation of GABA B(1a,2) heteroreceptors was shown to limit the duration of the up-states, in agreement with their role in inhibiting glutamate release. We further addressed whether the kinetic effects of auxiliary KCTD subunits on the GABA B receptor response (Turecek et al, 2014;Fritzius et al, 2017) (Schwenk et al, 2016) and/or to a reduced agonist affinity at the receptor (Rajalu et al, 2015). Inactivation of N-type Ca 2þ channels, presumably at presynaptic sites, was shown to reduce excitatory synaptic transmission between thalamic neurons (Pfrieger et al, 1992) and a reduced affinity for GABA may lower activation of GABA B receptors.…”

“…GABA B(1a,2) and GABA B(1b,2) receptors were later shown to associate with the auxiliary subunits KCTD8, -12, -12b and-16 (Schwenk et al, 2010, 2016; reviewed by Pin and Bettler, 2016). The KCTD subunits influence activation/deactivation kinetics and desensitization of the receptor response (Fritzius et al, 2017;Turecek et al, 2014). Moreover, KCTD16 was shown to scaffold effector Ca 2þ and hyperpolarization-activated cyclic nucleotide gated (HCN) channels at the receptor (Schwenk et al, 2016).…”

Mechanisms of Fast Desensitization of GABAB Receptor-Gated Currents

“…Binding of Gβγ by GRK2 desensitizes K + current responses by competitively titrating Gβγ away from effector Kir3 channels (Raveh et al, 2010;Shui, Khan, Tsuga, Haga, & Boyett, 1998). This mechanism was shown to desensitize GPCR-activated K + currents in heterologous cells and cultured hippocampal neurons within seconds of agonist exposure (Raveh et al, 2010;Turecek et al, 2014). Desensitization is also induced by a kinase-deficient GRK2 mutant, clearly showing that receptor phosphorylation is not required for desensitization (Raveh et al, 2010).…”

Modulation of AMPA receptor function by auxiliary subunits

“…However, it remains to be shown whether Neto1 is a bona fide auxiliary subunit for NMDA receptors in neurons. Potassium channel tetramerization domain-containing (KCTD) proteins 8, 12, 12b, and 16 are a group of auxiliary subunits that interact with GABA(B) receptors and modulate their function [32,33]. Moreover, auxiliary subunits of ligand-gated channels have also been identified in invertebrates such as Drosophila melanogaster and Caenorhabditis elegans.…”