GABA(A) receptor alpha4 subunit in DBA/2J and C57BL/6J mice

Gene expression profile for different susceptibilities to sound stimulation: a comparative study on brainstems between two inbred laboratory mouse strains

BACKGROUND

DBA/1 mice have a higher susceptibility to generalized audiogenic seizures (AGSz) and seizure-induced respiratory arrest (S-IRA) than C57/BL6 mice. The gene expression profile might be potentially related to this difference. This study aimed to investigate the susceptibility difference in AGSz and S-IRA between DBA/1 and C57BL/6 mice by profiling long noncoding RNAs (lncRNAs) and mRNA expression.

METHODS

We compared lncRNAs and mRNAs from the brainstem of the two strains with Arraystar Mouse lncRNA Microarray V3.0 (Arraystar, Rockville, MD). Gene Ontology (GO) and pathway analyses were performed to determine the potentially related biological functions and pathways based on differentially expressed mRNAs. qRT-PCR was carried out to validate the results.

RESULTS

A total of 897 lncRNAs and 438 mRNAs were differentially expressed (fold change ≥2, P < 0.05), of which 192 lncRNAs were upregulated and 705 lncRNAs were downregulated. A total of 138 mRNAs were upregulated, and 300 mRNAs were downregulated. In terms of specific mRNAs, Htr5b, Gabra2, Hspa1b and Gfra1 may be related to AGSz or S-IRA. Additionally, lncRNA Neat1 may participate in the difference in susceptibility. GO and pathway analyses suggested that TGF-β signaling, metabolic process and MHC protein complex could be involved in these differences. Coexpression analysis identified 9 differentially expressed antisense lncRNAs and 115 long intergenic noncoding RNAs (lincRNAs), and 2010012P19Rik and its adjacent RNA Tnfsf12-Tnfsf13 may have participated in S-IRA by regulating sympathetic neuron function. The results of the qRT-PCR of five selected lncRNAs (AK038711, Gm11762, 1500004A13Rik, AA388235 and Neat1) and four selected mRNAs (Hspa1b, Htr5b, Gabra2 and Gfra1) were consistent with those obtained by microarray.

CONCLUSION

We concluded that TGF-β signaling and metabolic process may contribute to the differential sensitivity to AGSz and S-IRA. Among mRNAs, Htr5b, Gabra2, Hspa1b and Gfra1 could potentially influence the susceptibility. LncRNA Neat1 and 2010012P19Rik may also contribute to the different response to sound stimulation. Further studies should be carried out to explore the underlying functions and mechanisms of differentially expressed RNAs.

Cloning and characterization of GABAA α subunits and GABAB subunits in Xenopus laevis during development

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult nervous system, acts via two classes of receptors, the ionotropic GABA(A) and metabotropic GABA(B) receptors. During the development of the nervous system, GABA acts in a depolarizing, excitatory manner and plays an important role in various neural developmental processes including cell proliferation, migration, synapse formation, and activity-dependent differentiation. Here we describe the spatial and temporal expression patterns of the GABA(A) and GABA(B) receptors during early development of Xenopus laevis. Using in situ hybridization and qRT-PCR, GABA(A) α2 was detected as a maternal mRNA. All other α-subunits were first detected by tailbud through hatching stages. Expression of the various subunits was seen in the brain, spinal cord, cranial ganglia, olfactory epithelium, pineal, and pituitary gland. Each receptor subunit showed a distinctive, unique expression pattern, suggesting these receptors have specific functions and are regulated in a precise spatial and temporal manner.

Trace and contextual fear conditioning is enhanced in mice lacking the alpha4 subunit of the GABA(A) receptor

The GABA(A)R alpha4 subunit is highly expressed in the dentate gyrus region of the hippocampus at predominantly extra synaptic locations where, along with the GABA(A)R delta subunit, it forms GABA(A) receptors that mediate a tonic inhibitory current. The present study was designed to test hippocampus-dependent and hippocampus-independent learning and memory in GABA(A)R alpha4 subunit-deficient mice using trace and delay fear conditioning, respectively. Mice were of a mixed C57Bl/6J X 129S1/X1 genetic background from alpha4 heterozygous breeding pairs. The alpha4-knockout mice showed enhanced trace and contextual fear conditioning consistent with an enhancement of hippocampus-dependent learning and memory. These enhancements were sex-dependent, similar to previous studies in GABA(A)R delta knockout mice, but differences were present in both males and females. The convergent findings between alpha4 and delta knockout mice suggests that tonic inhibition mediated by alpha4betadelta GABA(A) receptors negatively modulates learning and memory processes and provides further evidence that tonic inhibition makes important functional contributions to learning and behavior.

Quantitative trait locus for seizure susceptibility on mouse chromosome 5 confirmed with reciprocal congenic strains

Multiple quantitative trait locus (QTL) mapping studies designed to localize seizure susceptibility genes in C57BL/6 (B6, seizure resistant) and DBA/2 (D2, seizure susceptible) mice have detected a significant effect originating from midchromosome 5. To confirm the presence and refine the position of the chromosome 5 QTL for maximal electroshock seizure threshold (MEST), reciprocal congenic strains between B6 and D2 mice were created by a DNA marker-assisted backcross breeding strategy and studied with respect to changes in MEST. A genomic interval delimited by marker D5Mit75 (proximal to the acromere) and D5Mit403 (distal to the acromere) was introgressed for 10 generations. A set of chromosome 5 congenic strains produced by an independent laboratory was also studied. Comparison of MEST between congenic and control (parental genetic background) mice indicates that genes influencing this trait were captured in all strains. Thus, mice from strains having D2 alleles from chromosome 5 on a B6 genetic background exhibit significantly lower MEST compared with control littermates, whereas congenic mice harboring B6 chromosome 5 alleles on a D2 genetic background exhibit significantly higher MEST compared with control littermates. Combining data from all congenic strains, we conclude that the gene(s) underlying the chromosome 5 QTL for MEST resides in the interval between D5Mit108 (26 cM) and D5Mit278 (61 cM). Generation of interval-specific congenic strains from the primary congenic strains described here may be used to achieve high-resolution mapping of the chromosome 5 gene(s) that contributes to the large difference in seizure susceptibility between B6 and D2 mice.

Changes in Expression of GABAA α4 Subunit mRNA in the Brain under Anesthesia Induced by Volatile and Intravenous Anesthetics

We investigated changes in levels of GABAA receptor alpha4 subunit mRNA in the mouse brain after administration of volatile or i.v. anesthetic, by performing quantitative RT-PCR. We also performed immunohistochemical assays for c-fos-like protein. During deep anesthesia (which was estimated by loss of righting reflex) after administration of propofol, levels of GABAA receptor alpha4 subunit mRNA in the hippocampus, striatum and diencephalons were significantly greater than those observed after administration of pentobarbital, midazolam or GOI (5.0% isoflurane and 70% nitrous oxide in oxygen). Under incomplete anesthesia, levels of GABAA receptor alpha4 subunit mRNA were significantly increased by midazolam in all brain regions, and were significantly increased by pentobarbital in the posterior cortex and striatum. Expression of GABAA receptor alpha4 subunit mRNA closely correlated with expression of c-fos-like protein. These results indicate that the GABAA receptor alpha4 subunit plays an important role in regulating the anesthetic stage of i.v. anesthetics.

Genetic manipulations of GABAA receptor in mice make inhibition exciting

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in brain development and behavior. GABA(A) receptor subunits knock-out and knock-in mice have proven that GABA(A) receptors are involved in control of motor coordination, learning, and memory and play a role in anxiety, panic, and epileptogenesis. In addition, these receptors are involved in the molecular mechanisms of action of many drugs and participate actively in cortical plasticity. The use of genetically engineered mice has perhaps never been as successful as in understanding the importance of the heterogeneity of GABA(A) receptors. We review these findings and speculate on the new directions that the use of mice with altered expression of GABA(A) receptor subunits may provide.

Alternative splicing of the GABAA receptor α4 subunit creates a severely truncated mRNA

GABA(A) receptors, important sites of drug action, are chloride channels composed of 5 subunits chosen from among 19 or more. Alternative splicing for alpha 5, alpha 6, and rho 1 subunits results in truncated proteins which appear to lack function. We report a similar, relatively common (about 20%) form of alternative splicing of the alpha 4 subunit mRNA in mice and humans which, remarkably, creates a severely truncated message containing only the first two and last coding exons, with a frameshift in between. The only apparent translation product includes a short piece (39 amino acids) of the N-terminus right after the signal peptide. The splicing was developmentally and regionally regulated; the highest proportions of truncated alpha 4 mRNA, about 40%, were observed in embryonic day 18 whole brain and adult cerebellum. The truncated mRNA, when coexpresssed in human embryonic kidney (HEK) 293 cells with the complete alpha 4 subunit and beta1 and gamma 2 S subunits, reduced observed GABA currents without kinetic alterations. No such effect of truncated alpha 4 was observed with alpha1 subunit-containing receptors. Thus, the truncated alpha 4 N-terminus may play a post-translational regulatory role in intracellular folding/glycosylation/assembly of the alpha 4 subunit.