GABAergic regulation of glutamine synthetase and its role in preventing epilepsy

描述（由申请人提供）：成人大脑中的快速神经元抑制严重依赖于神经元合成抑制性神经递质的能力？-氨基丁酸（GABA），通过嗜离子性GABAA和代谢性GABAB受体介导其作用。gaba能抑制的缺陷是癫痫和大量其他神经精神疾病的核心。神经元合成GABA的主要代谢前体是谷氨酰胺，而谷氨酰胺又由星形胶质细胞提供。星形胶质细胞输出谷氨酰胺的能力取决于星形胶质细胞特异性谷氨酰胺合成酶（GS）的活性。GS对脑功能的重要意义已通过使用特异性抑制剂和基因删除被揭示。这些操作会导致癫痫发作和死亡，这是由于突触抑制减弱造成的。与此一致的是，在癫痫患者和癫痫动物模型的大脑中发现GS表达缺陷。然而，到目前为止，还没有系统的实验来评估GS的活性如何被调节以满足神经元对谷氨酰胺的需求，以及这些过程中的缺陷是否有助于癫痫发生。这些问题将在这里讨论。初步结果表明，GS的表达受星形细胞GABABRs的强大调控。2+ GABABRs是异二聚体g蛋白偶联受体，与Gi/o偶联，调节Ca瞬态，抑制腺苷酸环化酶活性。初步研究表明，星形细胞GABABRs通过减少GS的泛素化和随后的降解来稳定GS。为了理解这一发现的意义，我们创造了一种小鼠，可以特异性地消融星形细胞GABABRs的表达。这些小鼠GS的稳态表达水平下降，自发性癫痫发作和过早死亡。基于这些观察，我们假设：星形胶质细胞GABABRs阻止了GS的泛素依赖性降解，从而确保了神经元合成GABA所需的谷氨酰胺的持续可用性。该提案将围绕以下三个目标展开：目标1。验证星形细胞GABABRs调控GS稳定性的假说。目标2。为了验证星形细胞GABABRs表达减少导致GS降解、自发性癫痫发作和死亡的假说。目标3。为了验证减少星形胶质细胞GABABR表达损害突触抑制和神经元活力的假设。这些实验将为星形细胞GABABRs在调节GS表达、快速突触抑制和癫痫发生中的作用提供独特的见解。总的来说，这些研究可能会导致新疗法的发展，以增加GS的活动，以减轻癫痫的负担

DESCRIPTION (provided by applicant): Fast neuronal inhibition in the adult brain is critically dependent on the ability of neurons to synthesize the inhibitory neurotransmitter ?-aminobutyric acid (GABA) that mediates its actions via ionotropic GABAA and metabotropic GABAB receptors. Deficits in GABAergic inhibition are central to epilepsy and a plethora of other neuropsychiatric disorders. The major metabolic precursor for GABA synthesis by neurons is glutamine, which in turn is supplied by astrocytes. The ability of astrocytes to export glutamine i dependent upon the activity of the astrocyte-specific enzyme glutamine synthetase (GS). The significance of GS for brain function has been revealed by the use of specific inhibitors and gene deletion. These manipulations lead to seizures and death that result from decreased synaptic inhibition. Consistent with this, deficits in GS expression are found in the brains of epileptics and animal models of epilepsy. To date, however, there have been no systematic experiments to evaluate how the activity of GS is regulated to meet the demands of neurons for glutamine, and if deficits in these processes contribute to epileptogenesis. These issues will be addressed here. Preliminary results suggest that GS expression is subject to powerful regulation by astrocytic GABABRs. 2+ GABABRs are heterodimeric G-protein coupled receptors which couple to Gi/o, to modulate Ca transients, and inhibit the activity of adenylate cyclase. Preliminary studies have revealed that astrocytic GABABRs act to stabilize GS by reducing its ubiquitination and subsequent degradation. To understand the significance of this finding, we have created a mouse in which the expression of astrocytic GABABRs can be specifically ablated. These mice have decreased steady state expression levels of GS, spontaneous seizures, and premature death. Based on these observations we hypothesize that: Astrocytic GABABRs prevent the ubiquitin-dependent degradation of GS and thereby ensure the continued availability of glutamine for neuronal GABA synthesis. This proposal will center on three aims that are detailed below: Aim 1. To test the hypothesis that astrocytic GABABRs regulate the stability of GS. Aim 2. To test the hypothesis that ablating the expression of astrocytic GABABRs results in GS degradation, spontaneous seizures and death. Aim 3. To test the hypothesis that reducing astrocytic GABABR expression compromises synaptic inhibition and neuronal viability. Together these experiments will provide unique insights into the role that astrocytic GABABRs play in regulating GS expression, fast synaptic inhibition and epileptogenesis. Collectively these studies may lead to the development of novel therapies to increase the activity of GS to alleviate the burdens of epilepsy PUBLIC HEALTH RELEVANCE: Our studies are intended to provide insights into the role that gamma aminobutyric acid type B receptors play in regulating a specific enzyme found only in astrocytes. These insights have the potential to lead to the development of novel therapies to modulate the activity of this enzyme and ultimately to alleviate the burdens of epilepsy.