Phospholipase C-related inactive protein is involved in trafficking of gamma2 subunit-containing GABA(A) receptors to the cell surface

Phospholipase C-related but catalytically inactive protein is required for insulin-induced cell surface expression of gamma-aminobutyric acid type A receptors

The gamma-aminobutyric acid type A (GABA(A)) receptors play a pivotal role in fast synaptic inhibition in the central nervous system. One of the key factors for determining synaptic strength is the number of receptors on the postsynaptic membrane, which is maintained by the balance between cell surface insertion and endocytosis of the receptors. In this study, we investigated whether phospholipase C-related but catalytically inactive protein (PRIP) is involved in insulin-induced GABA(A) receptor insertion. Insulin potentiated the GABA-induced Cl(-) current (I(GABA)) by about 30% in wild-type neurons, but not in PRIP1 and PRIP2 double-knock-out (DKO) neurons, suggesting that PRIP is involved in insulin-induced potentiation. The phosphorylation level of the GABA(A) receptor beta-subunit was increased by about 30% in the wild-type neurons but not in the mutant neurons, which were similar to the changes observed in I(GABA). We also revealed that PRIP recruited active Akt to the GABA(A) receptors by forming a ternary complex under insulin stimulation. The disruption of the binding between PRIP and the GABA(A) receptor beta-subunit by PRIP interference peptide attenuated the insulin potentiation of I(GABA). Taken together, these results suggest that PRIP is involved in insulin-induced GABA(A) receptor insertion by recruiting active Akt to the receptor complex.

Structure and potential function of gamma-aminobutyrate type A receptor-associated protein

The gamma-aminobutyrate type A receptor-associated protein (GABARAP) is a ubiquitin-like modifier, and is implicated in a variety of membrane trafficking and fusion events that are crucial to synaptic plasticity, autophagy and apoptosis. However, important aspects of GABARAP function and regulation remain poorly understood. We review the current state of knowledge about GABARAP, highlighting newly-identified GABARAP ligands, and discuss the possible physiological relevance of each ligand interaction.

Activity-dependent tuning of inhibitory neurotransmission based on GABAAR diffusion dynamics

An activity-dependent change in synaptic efficacy is a central tenet in learning, memory, and pathological states of neuronal excitability. The lateral diffusion dynamics of neurotransmitter receptors are one of the important parameters regulating synaptic efficacy. We report here that neuronal activity modifies diffusion properties of type-A GABA receptors (GABA(A)R) in cultured hippocampal neurons: enhanced excitatory synaptic activity decreases the cluster size of GABA(A)Rs and reduces GABAergic mIPSC. Single-particle tracking of the GABA(A)R gamma2 subunit labeled with quantum dots reveals that the diffusion coefficient and the synaptic confinement domain size of GABA(A)R increases in parallel with neuronal activity, depending on Ca(2+) influx and calcineurin activity. These results indicate that GABA(A)R diffusion dynamics are directly linked to rapid and plastic modifications of inhibitory synaptic transmission in response to changes in intracellular Ca(2+) concentration. This transient activity-dependent reduction of inhibition would favor the onset of LTP during conditioning.

GABA(A) receptor trafficking and its role in the dynamic modulation of neuronal inhibition

GABA (gamma-aminobutyric acid) type A receptors (GABA(A)Rs) mediate most fast synaptic inhibition in the mammalian brain, controlling activity at both the network and the cellular levels. The diverse functions of GABA in the CNS are matched not just by the heterogeneity of GABA(A)Rs, but also by the complex trafficking mechanisms and protein-protein interactions that generate and maintain an appropriate receptor cell-surface localization. In this Review, we discuss recent progress in our understanding of the dynamic regulation of GABA(A)R composition, trafficking to and from the neuronal surface, and lateral movement of receptors between synaptic and extrasynaptic locations. Finally, we highlight a number of neurological disorders, including epilepsy and schizophrenia, in which alterations in GABA(A)R trafficking occur.

The role of transcriptional and translational mechanisms in flumazenil-induced up-regulation of recombinant GABA(A) receptors

The aim of this study was to further elucidate the mechanisms involved in adaptive changes of GABA(A) receptors following prolonged exposure to flumazenil, the antagonist of benzodiazepine binding sites on GABA(A) receptors. The effects of prolonged flumazenil treatment were studied on recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors stably expressed in human embryonic kidney (HEK 293) cells. Using radioligand binding experiments we found enhancement in the maximum number of [(3)H]muscimol labeled binding sites in different preparations of HEK 293 cells. The parallel increase of [(3)H]flunitrazepam binding sites in the membranes was reduced in the presence of actinomycin D and cycloheximide, inhibitors of RNA and protein synthesis, respectively. Chronic flumazenil also raised the steady-state level of mRNA encoding alpha(1) receptor subunit. The results suggest that the up-regulation of GABA(A) receptors, observed after prolonged flumazenil treatment is at least partly due to increased de novo synthesis of receptor proteins at both transcriptional and translational level.

Identification of clathrin heavy chain as a direct interaction partner for the gamma-aminobutyric acid type A receptor associated protein

Gamma-aminobutyric acid type A receptors (GABAA receptors) are the major sites of GABA-mediated fast synaptic inhibition in the central nervous system. Variation of the cell surface receptor count is postulated to be of importance in modulating inhibitory synaptic transmission. The GABAA receptor associated protein (GABARAP) is a ubiquitin-like modifier, implicated in GABAA receptor clustering, trafficking, and turnover. GABARAP pull-down experiments with brain lysate identified clathrin heavy chain to be GABARAP-associated. Phage display screening of a randomized peptide library for GABARAP ligands yielded a sequence motif which characterizes the peptide binding specificity of GABARAP. Sequence database searches with this motif revealed clathrin heavy chain as a protein containing the identified sequence motif within its residues 510-522, supporting the result of the pull-down experiments. Calreticulin, which was identified recently as a GABARAP ligand, contains a very similar sequence motif. We demonstrate that calreticulin indeed competes with clathrin heavy chain for GABARAP binding. Finally, employing nuclear magnetic resonance spectroscopy, we mapped the GABARAP residues responsible for binding to clathrin. The hereby mapped GABARAP regions overlap very well with the homologue residues in yeast Atg8 that were recently shown to be important for autophagy. Together with the knowledge that GABARAP and clathrin are known to be involved in GABAA receptor trafficking within the cell, this strongly suggests a clear physiological relevance of the direct interaction of GABARAP with clathrin heavy chain.

Regulation of GABAA-Receptor Surface Expression With Special Reference to the Involvement of GABARAP (GABAA Receptor-Associated Protein) and PRIP (Phospholipase C-Related, but Catalytically Inactive Protein)

GABA(A) receptors are heteropentameric ligand-gated chloride channels composed of a variety of subunits, including alpha1 - 6, beta1 - 3, gamma1 - 3, delta, epsilon, theta, and pi, and play a key role in controlling inhibitory neuronal activity. Modification of the efficacy of the synaptic strength is produced by changes in both the number of neuronal surface receptors and pentameric molecular assembly, leading to differences of sensitivity to neurotransmitters and neuromimetic drugs. Therefore, it is important to understand the molecular mechanisms regulating the so-called "life cycle of GABA(A) receptors" including sequential pentameric assembly at the site synthesized, intracellular transport through the Golgi apparatus and the cytoplasm, insertion into the cell membrane, functional modulation at the cell surface, and finally internalization, followed by either recycling back to the surface membrane or lysosomal degradation. This review is focused on events related to the surface expression of the receptor containing the gamma2 subunit and clathrin/AP2 complex-mediated phospho-regulated endocytosis of the receptor, with special reference to the function of novel GABA(A) receptor modulators, GABARAP (GABA(A) receptor-associated protein) and PRIP (phospholipase C-related, but catalytically inactive protein).