GABA _B receptor phosphorylation regulates KCTD12-induced K ⁺ current desensitization

Abstract

GABA _B receptors assemble from GABA _B1 and GABA _B2 subunits. GABA _B2 additionally associates with auxiliary KCTD subunits (named after their K ⁺ channel tetramerization-domain). GABA _B receptors couple to heterotrimeric G–proteins and activate inwardly-rectifying K ⁺ channels through the βγ subunits released from the G-protein. Receptor-activated K ⁺ currents desensitize in the sustained presence of agonist to avoid excessive effects on neuronal activity. Desensitization of K ⁺ currents integrates distinct mechanistic underpinnings. GABA _B receptor activity reduces protein kinase-A activity, which reduces phosphorylation of serine-892 in GABA _B2 and promotes receptor degradation. This form of desensitization operates on the time scale of several minutes to hours. A faster form of desensitization is induced by the auxiliary subunit KCTD12, which interferes with channel activation by binding to the G-protein βγ subunits. Here we show that the two mechanisms of desensitization influence each other. Serine-892 phosphorylation in heterologous cells rearranges KCTD12 at the receptor and slows KCTD12-induced desensitization. Likewise, protein kinase-A activation in hippocampal neurons slows fast desensitization of GABA _B receptor-activated K ⁺ currents while protein kinase-A inhibition accelerates fast desensitization. Protein kinase-A fails to regulate fast desensitization in KCTD12 knock-out mice or knock-in mice with a serine-892 to alanine mutation, thus demonstrating that serine-892 phosphorylation regulates KCTD12-induced desensitization in vivo. Fast current desensitization is accelerated in hippocampal neurons carrying the serine-892 to alanine mutation, showing that tonic serine-892 phosphorylation normally limits KCTD12-induced desensitization. Tonic serine-892 phosphorylation is in turn promoted by assembly of receptors with KCTD12. This cross-regulation of serine-892 phosphorylation and KCTD12 activity sharpens the response during repeated receptor activation.