Role of the PLC-related, catalytically inactive protein p130 in GABA(A) receptor function

Role of PRIP-1, a novel Ins(1,4,5)P3 binding protein, in Ins(1,4,5)P3-mediated Ca2+ signaling

PRIP-1 was isolated as a novel inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] binding protein with a domain organization similar to phospholipase C-delta1 (PLC-delta1) but lacking the enzymatic activity. Further studies revealed that the pleckstrin homology (PH) domain of PRIP-1 is the region responsible for binding Ins(1,4,5)P3. In this study we aimed to clarify the role of PRIP-1 at the physiological concentration in Ins(1,4,5)P3-mediated Ca2+ signaling, as we had previously used COS-1 cells overexpressing PRIP-1 (Takeuchi et al., 2000, Biochem J 349:357-368). For this purpose we employed PRIP-1 knock out (PRIP-1-/-) mice generated previously (Kanematsu et al., 2002, EMBO J 21:1004-1011). The increase in free Ca2+ concentration in response to purinergic receptor stimulation was lower in primary cultured cortical neurons prepared from PRIP-1-/- mice than in those from wild type mice. The relative amounts of [3H]Ins(1,4,5)P3 measured in neurons labeled with [3H]inositol was also lower in cells from PRIP-1-/- mice. In contrast, PLC activities in brain cortex samples from PRIP-1-/- mice were not different from those in the wild type mice, indicating that the hydrolysis of Ins(1,4,5)P3 is enhanced in cells from PRIP-1-/- mice. In vitro analyses revealed that type1 inositol polyphosphate 5-phosphatase physically interacted with a PH domain of PRIP-1 (PRIP-1PH) and its enzyme activity was inhibited by PRIP-1PH. However, physical interaction with these two proteins did not appear to be the reason for the inhibition of enzyme activity, indicating that binding of Ins(1,4,5)P3 to the PH domain prevented its hydrolyzation. Together, these results indicate that PRIP-1 plays an important role in regulating the Ins(1,4,5)P3-mediated Ca2+ signaling by modulating type1 inositol polyphosphate 5-phosphatase activity through binding to Ins(1,4,5)P3.

Modulation of GABAA receptor activity by phosphorylation and receptor trafficking: implications for the efficacy of synaptic inhibition

Fast synaptic inhibition in the brain is largely mediated by GABA(A) receptors. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their function or cell surface stability will have major consequences for neuronal excitation. It has become clear that the stability and activity of GABA(A) receptors at synapses can be dynamically modulated by receptor trafficking and phosphorylation. Here, we discuss these regulatory mechanisms, and their consequences for the efficacy of GABA(A) receptor mediated synaptic inhibition.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

GATE-16 and GABARAP are authentic modifiers mediated by Apg7 and Apg3

GATE-16, GABARAP, and LC3 are three mammalian counterparts of yeast Apg8p/Aut7p. Here, we show that GATE-16 and GABARAP are authentic modifiers, as is the case of LC3 modification. The C-terminal Phe(117) of proGATE-16 and the C-terminal Leu(117) of proGABARAP are post-translationally cleaved to expose an essential Gly(116) within GATE-16 and GABARAP, with the products designated GATE-16-I and GABARAP-I, respectively. The Gly(116) within GATE-16 and GABARAP are essential for further formation of the intermediates between them and Apg7p(C572S) and Apg3p(C264S). When Apg7p and Apg3p are expressed, GATE-16-I and GABARAP-I are modified to a secondary ubiquitin-like modified form, GATE-16-II and GABARAP-II, respectively. GATE-16-I and GABARAP-I, but not LC3-I, localize to membrane compartments before their modification. These results indicate that GATE-16 and GABARAP are authentic modifiers, but that they have different biochemical characteristics from those of LC3.

Molecules interacting with PRIP-2, a novel Ins(1,4,5)P3 binding protein type 2: Comparison with PRIP-1

A family of phospholipase C-related, catalytically inactive proteins (designated PRIP) have been identified as a group of novel inositol 1,4,5-trisphosphate binding proteins with a domain organization similar to phospholipase C-delta but lacking the enzymatic activity. The PRIP family consists of at least two types of proteins (PRIP-1 and PRIP-2 subfamilies). In the present study, we examined the tissue distribution of PRIP-2, its expression in rat brain at the mRNA level, and the characteristics of its binding to inositol compounds, protein phosphatase 1, and gamma-amino butyric acid receptor associated protein. We also compared these characteristics with those of PRIP-1. Northern blot analysis and reverse-transcription polymerase chain reaction showed that PRIP-1 was present mainly in the brain, whereas PRIP-2 was expressed ubiquitously. In situ hybridization studies using rat brain revealed that the mRNA for both PRIP-1 and PRIP-2 was similarly expressed; it was detected in the granular cell and Purkinje cell layers in the cerebellum, and in the hippocampal pyramidal cells, dentate granule cells, and pyramidal and/or granule cells of the cerebral cortex in the cerebrum. PRIP-2 bound inositol 1,4,5-trisphosphate and its parent lipid, phosphatidylinositol 4,5-bisphosphate, with a similar affinity, while PRIP-1 preferentially bound the former ligand by about 10-fold. PRIP-1 and PRIP-2 interacted with protein phosphatase 1 and gamma-amino butyric acid receptor associated protein in a similar manner. These results indicate that, similar to PRIP-1, PRIP-2 may be involved in both inositol 1,4,5-trisphosphate-mediated and gamma-amino butyric acid-related signaling.

Mechanisms of GABAA receptor assembly and trafficking: implications for the modulation of inhibitory neurotransmission

Fast synaptic inhibition in the brain is largely mediated by ionotropic GABA receptors, which can be subdivided into GABAA and GABAC receptors based on pharmacological and molecular criteria. GABAA receptors are important therapeutic targets for a range of sedative, anxiolytic, and hypnotic agents and are implicated in several diseases including epilepsy, anxiety, depression, and substance abuse. In addition, modulating the efficacy of GABAergic neurotransmission may play a key role in neuronal plasticity. Recent studies have begun to reveal that the accumulation of ionotropic GABAA receptors at synapses is a highly regulated process that is facilitated by receptor-associated proteins and other cell-signaling molecules. This review focuses on recent experimental evidence detailing the mechanisms that control the assembly and transport of functional ionotropic GABAA receptors to cell surface sites, in addition to their stability at synaptic sites. These regulatory processes will be discussed within the context of the dynamic modulation of synaptic inhibition in the central nervous system (CNS).

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

[The analysis of protein-protein interaction with special reference to PRIP-1]

Analysis of protein-protein interaction is one of the powerful methods for elucidating the new functions of functionally unknown proteins. Using this approach, we isolated two proteins interacting with PRIP-1, which was isolated as a new Ins(1,4,5) P3 binding protein from brain. One was protein phosphatase 1 catalytic subunit (PP1c) and the other was GABARAP (GABAA-receptor-associated protein). The region of PRIP-1 responsible for their interaction was the site preceding to the pleckstrin homology domain of PRIP-1 for PP1c and the EF-hand motifs of PRIP-1 for GABARAP, which were determined by beta-galactosidase assay of yeast two-hybrid system. The association between PRIP-1 and PP1c was confirmed in vitro by a pull-down assay, a far-western assay, an immunoprecipitation analysis and a surface plasmon resonance analysis. The interaction of PRIP-1 with PP1c resulted in inhibition of the catalytic activity of PP1c in a PRIP-1 concentration-dependent manner. The association between PRIP-1 and GABARAP was also confirmed by a pull-down assay, and we found that PRIP-1 competitively inhibited the binding of the gamma 2 subunit of the GABAA receptor to GABARAP in vitro. Our electrophysiological and behavioral analysis of PRIP-1 knockout mice revealed that PRIP-1 is essential for the function of GABAA receptors, especially in response to the agents acting on the gamma 2 subunit.