Mechanisms that regulate the trafficking and clustering of postsynaptic GABA-A receptors (supported by NIMH, R01 MH62391)

The long-term goals of the proposed research are to unravel mechanisms that regulate trafficking and clustering of gamma-aminobutyric acid type A (GABA-A) receptors at synapses. These receptors are hetero-pentameric chloride channels and they mediate most inhibitory neurotransmission in the brain. GABA-A receptor subtypes distinguished by their subunit composition are differentially expressed during development and in different regions of the mature brain. Most GABA-A receptor subtypes are clustered at postsynaptic sites and changes in their coconcentration contribute to physiological and patholigical changes in synaptic activity and brain function. The pool of GABA-A receptors at the cell surface that is amenable to neurotransmitter (GABA) is regulated by dynamic changes in synthesis, exocytosis, and endoctytosis of GABA-A receptors, as well as interactions of GABA-A receptors with the postsynaptic cytoskeleton (gephyrin). Studies in our lab employing knock-out/gene targeting of GABA-A receptor subunit genes have identified the gamma2 subunit as an important determinant of postsynaptic localization of GABA-A receptors (Essrich et al. 1998, Schweizer et al 2003). More recently we have identified GODZ as a gamma2 subunit interacting palmitoyltransferase that palmitoylates the intracellular domain of the gamma2 subunit and thereby facilitates the accumulation of GABA-A receptors at synapses (Keller et al 2004, Fang et al 2006). Indirectly, GODZ-mediated palmitoylation also contributes to GABAergic synapse formation. Palmitoylation is a prominent posttranslational modification that similar to phosphorylation can be used to reversibly and dynamically change the function of diverse proteins. Ongoing studies explore the mechanism of GODZ-mediated palmitoylation, with special reference to dynamic changes in the function of GABAergic synapses in cultured neurons and mice.

Stressful experiences in early life are known risk factors for anxiety and depressive illnesses and inhibit hippocampal neurogenesis and the expression of GABA-A receptors in adulthood. Conversely, deficits in GABAergic neurotransmission and reduced neurogenesis are implicated in the etiology of pathological anxiety and diverse mood disorders.

Mice that are heterozygous for the gamma2 subunit of GABA-A receptors exhibit a modest functional deficit in mainly postsynaptic GABA-A receptors that is associated with a behavioral, cognitive and pharmacological phenotype indicative of heightened trait anxiety (Crestani et al. 1999). Based on these findings we have used cell type-specific and developmentally controlled inactivation of the gamma2 subunit gene to further analyze the mechanism and brain substrate underlying this phenotype. We found that gamma2 subunit deficits induced selectively in immature glutamatergic neurons of the embryonic and adult forebrain resulted in reduced adult hippocampal neurogenesis associated with behavioral indicative of anxiety-driven depression. Reduced neurogenesis was associated with normal cell proliferation, indicating a selective vulnerability of postmitotic immature neurons to modest functional deficits in gamma2 subunit-containing GABA-A receptors. By contrast, a comparable forebrain-specific GABA A receptor deficit induced selectively in mature neurons during adolescence lacked neurogenic and behavioral consequences. These results suggest that modestly reduced GABA-A receptor function in immature neurons of the developing and adult brain can serve as a common molecular substrate for deficits in adult neurogenesis and behavior indicative of anxious and depressive-like mood states (Earnheart et al 2007). Ongoing experiements are testing the relationship between chenges in neurogenesis and anxiety and mood disorders.