Sustained Structural Change of GABAA Receptor-Associated Protein Underlies Long-Term Potentiation at Inhibitory Synapses on a Cerebellar Purkinje Neuron

Abstract

Fast inhibitory synaptic transmission is predominantly mediated by GABA A receptor (GABA A R) in the CNS. Although several types of neuronal activity-dependent plasticity at GABAergic synapses have been reported, the detailed mechanism is elusive. Here we show that binding of structurally altered GABA A R-associated protein (GABARAP) to GABA A R ␥2 subunit and to tubulin is critical for long-term potentiation [called rebound potentiation (RP)] at inhibitory synapses on a cerebellar Purkinje neuron (PN). Either inhibition of GABARAP association with GABA A R␥2 or deletion of tubulin binding region of GABARAP impaired RP. Inhibition of tubulin polymerization also suppressed RP. Thus, precise regulation of GABA A R␥2-GABARAP-microtubule interaction is critical for RP. Furthermore, competitive inhibition of GABARAP binding to GABA A R␥2 after the RP establishment attenuated the potentiated response, suggesting that GABARAP is critical not only for the induction but also for the maintenance of RP. Fluorescence resonance energy transfer analysis revealed that GABARAP underwent sustained structural alteration after brief depolarization of a PN depending on the activity of Ca 2ϩ / calmodulin-dependent protein kinase II (CaMKII), which is required for the RP induction. The susceptibility of GABARAP to undergo structural alteration was abolished by an amino acid replacement in GABARAP. Furthermore, RP was impaired by expression of the mutant GABARAP with the replacement. Together, we conclude that GABA A R association with structurally altered GABARAP downstream of CaMKII activation is essential for RP.

NMDA Receptor Activation Potentiates Inhibitory Transmission through GABA Receptor-Associated Protein-Dependent Exocytosis of GABAAReceptors

“…Although a scrambled control peptide had no effect on the NMDA-mediated increase in GABA A R surface expression (48.9 Ϯ 9.18% increase from control; n ϭ 5; p Ͻ 0.05), when cells were pretreated with the GABARAP inhibitory peptide for 30 min, NMDA induced a significant decrease in GABA A R surface levels (Ϫ38.17 Ϯ 5.15%; n ϭ 5; p Ͻ 0.05). As reported previously (Kawaguchi and Hirano, 2007), disruption of the GABARAP-GABA A R interaction with the GABARAP inhibitory peptide had no effect on basal surface GABA A R expression (1.42 Ϯ 13.03% change from control; n ϭ 5; p Ͼ 0.1).…”

“…Interestingly, it is not clear that expression of RP, although similar to the NMDAR-dependent potentiation described here, is caused by trafficking of GABA A Rs to synapses. In fact, Kawaguchi and Hirano (2007) reported no change in surface GABA A R expression after RP induction. It remains possible that GABARAP has multiple functions in regulating GABAergic signaling: through altered GABA A R function as seen in Purkinje neurons and through enhanced GABA A R numbers as we found in the hippocampus.…”

“…Kawaguchi and Hirano (2007) showed that the interaction of GABARAP with the ␥2 subunit of the GABA A R is necessary for rebound potentiation (RP), a form of inhibitory synaptic potentiation that occurs in cerebellar Purkinje neurons after postsynaptic depolarization (Kano et al, 1992). RP was found to require a structural change in GABARAP mediated by CaMKII activity (Kawaguchi and Hirano, 2007).…”

Regulation of GABAARs by Phosphorylation

“…This “rebound potentiation” (RP) occurs through activation of voltage-gated Ca 2+ channels (VGCCs) resulting in a transient Ca 2+ influx and is dependent upon CamKII (Kano, Kano, Fukunaga, & Konnerth, 1996) and γ2 subunit association with GABARAP (Kawaguchi & Hirano, 2007). Further, RP can be suppressed by GABA type B receptor (GABA B R) activation through decreasing levels of PKA.…”

LTD, RP, and Motor Learning

“…Molecular regulation mechanisms of RP induction has been studied extensively [13], and it was revealed that interaction of GABA A receptor binding protein (GABARAP) and GABA A receptor is necessary for RP induction [16]. This result prompted us to examine physiological roles of RP using RP-deficient mice.…”