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
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.
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