Impaired Expression of GABA Signaling Components in the Alzheimer's Disease Middle Temporal Gyrus

Abstract

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter, playing a central role in the regulation of cortical excitability and the maintenance of the excitatory/inhibitory (E/I) balance. Several lines of evidence point to a remodeling of the cerebral GABAergic system in Alzheimer's disease (AD), with past studies demonstrating alterations in GABA receptor and transporter expression, GABA synthesizing enzyme activity and focal GABA concentrations in post-mortem tissue. AD is a chronic neurodegenerative disorder with a poorly understood etiology and the temporal cortex is one of the earliest regions in the brain to be affected by AD neurodegeneration. Utilizing NanoString nCounter analysis, we demonstrate here the transcriptional downregulation of several GABA signaling components in the post-mortem human middle temporal gyrus (MTG) in AD, including the GABA_A receptor α₁, α₂, α₃, α₅, β₁, β₂, β₃, δ, γ₂, γ₃, and θ subunits and the GABA_B receptor 2 (GABA_BR2) subunit. In addition to this, we note the transcriptional upregulation of the betaine-GABA transporter (BGT1) and GABA transporter 2 (GAT2), and the downregulation of the 67 kDa isoform of glutamate decarboxylase (GAD₆₇), the primary GABA synthesizing enzyme. The functional consequences of these changes require further investigation, but such alterations may underlie disruptions to the E/I balance that are believed to contribute to cognitive decline in AD.

Keywords: Alzheimer’s disease; GABA; GABA receptors; GABA transporters; GABAergic system; middle temporal gyrus.

Figure 1

Normalized NanoString nCounter mRNA expression counts for γ-aminobutyric acid (GABA)ergic signaling components in Alzheimer’s disease (AD) and control human middle temporal gyrus (MTG) grey matter samples. Several GABAergic signaling components, including GABA_AR subunits, the GABA_BR2 subunit, GABA transporters, and the GABA synthesizing enzyme GAD₆₇, are altered in expression in the AD MTG. Raw mRNA counts were normalized to positive control and housekeeping gene counts and background-corrected using negative control counts. For each subunit, normalized mRNA counts were compared between AD and control groups using the Mann–Whitney unpaired test, * p ≤ 0.05 and ** p ≤ 0.01. Data presented as mean ± SEM, n = six AD cases and six control cases. Gene names on x-axis; GABBR1_1 and GABBR1_2 represent two distinct transcript variants encoded by the GABBR1 gene.

Figure 2

Representative fluorescent immunohistochemistry images from the human Alzheimer’s disease and control middle temporal gyrus (MTG). Post-mortem human MTG sections were labelled with antibodies against GABA_AR subunits and imaged with uniform settings for each subunit on a confocal microscope. GABA_AR subunits are presented in red and Hoechst staining is presented in blue. A reduction in fluorescent intensity across cortical layers was apparent for the α₁ subunit (cases AZ90 and H122 presented) (a–d), β₂ subunit (cases AZ90 and H122 presented) (e–h) and β₃ subunit (cases AZ90 and H181 presented) (i–l). Scale bar, 200 µm.