Neuronal activity regulates phosphorylation-dependent surface delivery of G protein-activated inwardly rectifying potassium channels

Abstract

G protein-activated inwardly rectifying K ؉ (GIRK) channels regulate neuronal excitability by mediating inhibitory effects of G protein-coupled receptors for neurotransmitters and neuromodulators. Notwithstanding many studies reporting modulation of GIRK channel function, whether neuronal activity regulates GIRK channel trafficking remains an open question. Here we report that NMDA receptor activation in cultured dissociated hippocampal neurons elevates surface expression of the GIRK channel subunits GIRK1 and GIRK2 in the soma, dendrites, and dendritic spines within 15 min. This activity-induced increase in GIRK surface expression requires protein phosphatase-1-mediated dephosphorylation of a serine residue (Ser-9) preceding the GIRK2 Val-13/ Leu-14 (VL) internalization motif, thereby promoting channel recycling. Because activation of GIRK channels hyperpolarizes neuronal membranes, the NMDA receptor-induced regulation of GIRK channel trafficking may represent a dynamic adjustment of neuronal excitability in response to inhibitory neurotransmitters and/or neuromodulators.GIRK ͉ NMDA receptor ͉ trafficking ͉ protein phosphatase-1 ͉ dendrites G protein-activated inwardly rectifying K ϩ (GIRK) channels belong to the Kir3.x subfamily of inwardly rectifying potassium channels, with each subunit containing 2 transmembrane segments and cytoplasmic N-and C-terminal domains (1). They regulate neuronal excitability in response to neurotransmitters and neuromodulators that activate G protein-coupled receptors (GPCRs) coupled to pertussis toxin-sensitive Gi/o proteins, inducing the exchange of GDP for GTP on the G␣ and dissociation of G␣-GTP from G␤␥ (1). GIRK channel activation by direct binding of G␤␥ causes hyperpolarization, thus reducing neuronal excitability (1). GIRK channels are also modulated by intracellular Na ϩ , Mg 2ϩ , phosphatidylinositol 4,5-bisphosphate, G␣i, and regulators of G protein signaling (2).Neuronal GIRK channels in the central nervous system are mostly heterotetramers of GIRK1 and GIRK2 subunits, whereas midbrain dopaminergic neurons express homomeric GIRK2 channels (1). GIRK channels reside predominantly in the soma and dendrites of pyramidal neurons (1), where their current (3, 4) dampens the effects of excitatory synaptic input (5, 6). They also mediate slow inhibitory postsynaptic current upon GABA B receptor activation and account for the hyperpolarization induced by adenosine and serotonin receptors (7). Underscoring the physiologic importance of GIRK channels, mice lacking GIRK2 display sporadic seizures, increased susceptibility to convulsant agents, and hyperactivity, as well as abnormality in cocaine self-administration, pain threshold, response to analgesics including opioids, and sensitivity to ethanol's motivational effects (8).Dynamic regulation of GIRK channel number affords a powerful way to modulate neuronal activity. Because GIRK1 (9) and GIRK2 (10) reside in the dendrites and dendritic spines that harbor the majority of the excitatory synapses, we wondered whether activation o...

NMDA receptor-dependent GABA B receptor internalization via CaMKII phosphorylation of serine 867 in GABA B1

“…This provides a means to keep individual glutamatergic synapses modifiable (38,39) and to spare them from inhibition through spillover of GABA. A previous study reported that NMDA receptor activation promotes surface expression of Kir3 channels in hippocampal neurons (40), which was paralleled with an increase in basal Kir3 currents and adenosine A 1 -mediated Kir3 currents (41). However, GABA B mediated Kir3 currents were not altered, in apparent conflict with an earlier report (42) and our own findings.…”

Synaptic NMDA receptor stimulation activates PP1 by inhibiting its phosphorylation by Cdk5

“…dephosphorylation at T320 induced by NMDA receptor signaling, while a PP1/PP2A inhibitor did. This is in agreement with two recent studies showing that (1) GIRK channels are dephosphorylated by PP1 in response to NMDA receptor signaling in a calcineurin-independent manner (Chung et al, 2009), and (2) LTD of NMDA receptor responses, as opposed to LTD of AMPA receptor responses, is dependent on PP1, but not on calcineurin . Our results thus suggest that the phosphatase-cascade model of PP1 activation, or I-1 disinhibition by calcineurin proposed in skeletal muscle cell types and in classical models of LTD in CA1 hippocampal neurons (Lisman, 1989;Mulkey et al, 1993), appear not to be the dominant mode of PP1 activation in cortical neurons.…”

“…In neurons, an attractive model for hippocampal LTD has been suggested whereby PP1 is activated via dis-inhibition of I-1 after calcineurin-mediated dephosphorylation of pT35-I-1. However, PP1 activation independent of calcineurin has been reported both in CA3-CA1 synapses and hippocampal neurons (Chung et al, 2009). Consistent with this, LTD at CA3-CA1 synapses was found to be normal in I-1 KO mice (Allen et al, 2000).…”

“…Stimulation of synaptic and extrasynaptic NMDA receptors Synaptic and extrasynaptic NMDAR stimulations were performed according to published protocols (Hardingham et al, 2001(Hardingham et al, , 2002Ivanov et al, 2006;Chung et al, 2009;Hoey et al, 2009;Xu et al, 2009). Three protocols were used for synaptic NMDAR stimulation.…”

α-Actinin2 cytoskeletal protein is required for the functional membrane localization of a Ca2+-activated K+channel (SK2 channel)

“…2C). To further test whether the regulation involves SK2 channel trafficking from recycling endosomes, we examined SK2 channel surface expression in the presence of primaquine, which acutely and selectively inhibits recycling but not endocytosis (20,21). Pretreatment with primaquine dramatically abolished surface expression of SK2 channel, even in the presence of ␣-actinin2 (Fig.…”