Characterisation of novel beta 2 and beta 3 gamma-aminobutyric acidA receptor antibodies

Identification, molecular cloning, and characterization of a novel GABAA receptor-associated protein, GRIF-1

A novel 913-amino acid protein, gamma-aminobutyric acid type A (GABA(A)) receptor interacting factor-1 (GRIF-1), has been cloned and identified as a GABA(A) receptor-associated protein by virtue of its specific interaction with the GABA(A) receptor beta 2 subunit intracellular loop in a yeast two-hybrid assay. GRIF-1 has no homology with proteins of known function, but it is the rat orthologue of the human ALS2CR3/KIAA0549 gene. GRIF-1 is expressed as two alternative splice forms, GRIF-1a and a C-terminally truncated form, GRIF-1b. GRIF-1 mRNA has a wide distribution with a major transcript size of 6.2 kb. GRIF-1a protein is only expressed in excitable tissues, i.e. brain, heart, and skeletal muscle major immunoreactive bands of M(r) approximately 115 and 106 kDa and, in muscle and heart only, an additional 88-kDa species. When expressed in human embryonic kidney 293 cells, GRIF-1a yielded three immunoreactive bands with M(r) approximately 115, 106, and 98 kDa. Co-expression of GRIF-1a and alpha 1 beta 2 gamma 2 GABA(A) receptors in mammalian cells revealed some co-localization in the cell cytoplasm. Anti-FLAG-agarose specifically precipitated GRIF-1(FLAG) and GABA(A) receptor beta 2 subunits from human embryonic kidney 293 cells co-transfected with GRIF-1a(FLAG) and beta 2 subunit clones. Further, immobilized GRIF-1-(8-633) specifically precipitated in vitro GABA(A) receptor alpha 1 and beta 2 subunit immunoreactivities from detergent extracts of adult rat brain. The respective GABA(A) receptor beta 2 subunit/GRIF-1 binding domains were mapped using the yeast two-hybrid reporter gene assays. A possible role for GRIF-1 as a GABA(A) receptor beta 2 subunit trafficking factor is proposed.

Cyclic AMP-mediated regulation of GABA(A) receptor subunit expression in mature rat cerebellar granule cells: evidence for transcriptional and translational control

Exposure of rat cerebellar granule cells cultured to maturity in vitro to forskolin, N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP), and 3-isobutyl-1-methylxanthine (IBMX) down-regulated GABA(A) receptor alpha6 and beta3 subunits but up-regulated alpha1 and beta2 subunits with respect to vehicle-treated controls. Dideoxyforskolin had no effect on subunit expression. Protein kinase A inhibitors, H-89 and Rp-adenosine 3',5'cyclic monophosphothioate, prevented these effects on alpha1 but not alpha6 subunit expression. Flunitrazepam-sensitive [3H]Ro 15-4513 binding sites were increased by 144 +/- 20% following forskolin treatment. [3H]Ro 15-4513 photoaffinity labelling showed that the GABA(A) receptor alpha1 subunit was the principal locus of the increased flunitrazepam-sensitive [3H]Ro 15-4513 binding. Forskolin decreased flunitrazepam-insensitive [3H]Ro 15-4513 binding sites by 25 +/- 8% and resulted in a 20% decrease in the irreversible incorporation of radioactivity in the alpha6 subunit. Steady-state levels of GABA(A) receptor subunit mRNAs were determined by semiquantitative RT-PCR in forskolin-treated cultures. Forskolin, Bt2cAMP, and IBMX down-regulated GABA(A) receptor alpha6 subunit mRNA expression; alpha1 and beta3 mRNA levels were unaffected, whereas beta2 subunit mRNA was up-regulated. Dideoxyforskolin had no significant effect on alpha1, alpha6, beta2, and beta3 mRNA levels. Thus, in mature cerebellar granule cells, GABA(A) receptor expression can be regulated by intracellular cyclic AMP levels. This occurs at the level of gene transcription and/or translation by mechanisms that are only partially governed by protein kinase A.

Decreased expression of GABAA receptor alpha6 and beta3 subunits in stargazer mutant mice: a possible role for brain-derived neurotrophic factor in the regulation of cerebellar GABAA receptor expression?

The cerebellar granule cells of the spontaneous recessive mutant mouse strain, stargazer (stg/stg), fail to express brain-derived neurotrophic factor mRNA. This deficit is exclusive to these neurons and is believed to underlie the motor irregularities displayed by stg/stg, though the molecular basis for their phenotype has still to be resolved. Brain-derived neurotrophic factor has been shown to play a role in the postnatal maturation of cerebellar granule cells. Differentiation of these neurons, postnatally, is characterised by a switch in their GABAA receptor subunit expression profile. Notably, the GABAA receptor alpha6 subunit, which is specific to these neurons, becomes detectable at postnatal days 10-14 (P10-14). To determine whether cerebellar GABAA receptor expression has been compromised in stg/stg mice, the expression levels of GABAA receptor alpha1, alpha6, beta2 and beta3 subunits were compared between stg/stg mice and the appropriate wild-type background strain, C57BL/6J (+/+). By quantitative immunoblotting, it was found that the expression of the alpha6 and beta3 subunits was 23+/-8% and 38+/-12% (mean+/-S.E.M., n=6) of control (+/+) levels, respectively. In contrast, the expression of the alpha1 and beta2 subunits was not significantly different from controls, being 116+/-11% and 87+/-24% (mean+/-S.E.M., n=6) of +/+ levels, respectively. Total specific [3H]Ro15-4513 binding activity detected in cerebellar membranes prepared from stg/stg was not significantly different from +/+ mice. However, the benzodiazepine agonist-insensitive subtype of [3H]Ro15-4513 binding activity, a pharmacological motif of alpha6 subunit-containing GABAA receptors, was lower in stg/stg mice relative to the +/+ strain which correlated with the lowered level of alpha6 subunit expression. Thus, we have identified an abnormality in the GABAA receptor profile of stg/stg mutant mice that might underpin its irregular phenotype.