GABAA α6-containing receptors are selectively compromised in cerebellar granule cells of the ataxic mouse, stargazer

Abstract

Stargazer mice fail to express the γ2 isoform of transmembrane α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptor regulatory proteins that has been shown to be absolutely required for the trafficking and synaptic targeting of excitatory AMPA receptors in adult murine cerebellar granule cells. Here we show that 30 ± 6% fewer inhibitory γ-aminobutyric acid, type A (GABAA), receptors were expressed in adult stargazer cerebellum compared with controls because of a specific loss of GABAA receptor expression in the cerebellar granule cell layer. Radioligand binding assays allied to in situ immunogold-EM analysis and furosemide-sensitive tonic current estimates revealed that expression of the extrasynaptic (α6βxδ) α6-containing GABAA receptor were markedly and selectively reduced in stargazer. These observations were compatible with a marked reduction in expression of GABAA receptor α6, δ (mature cerebellar granule cell-specific proteins), and β3 subunit expression in stargazer. The subunit composition of the residual α6-containing GABAA receptors was unaffected by the stargazer mutation. However, we did find evidence of an ∼4-fold up-regulation of α1βδ receptors that may compensate for the loss of α6-containing GABAA receptors. PCR analysis identified a dramatic reduction in the steady-state level of α6 mRNA, compatible with α6 being the primary target of the stargazer mutation-mediated GABA A receptor abnormalities. We propose that some aspects of assembly, trafficking, targeting, and/or expression of extrasynaptic α6-containing GABAA receptors in cerebellar granule cells are selectively regulated by AMPA receptor-mediated signaling. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.