Cortical Deficits of Glutamic Acid Decarboxylase 67 Expression in Schizophrenia: Clinical, Protein, and Cell Type-Specific Features

Abstract

Objective Cognitive deficits in schizophrenia are associated with altered activity of the dorsolateral prefrontal cortex, which has been attributed to lower expression of the 67 kDa isoform of glutamic acid decarboxylase (GAD67), the major γ-aminobutyric acid (GABA)-synthesizing enzyme. However, little is know n about the relationship of prefrontal GAD67 m RNA levels and illness severity, translation of the transcript into protein, and protein levels in axon terminals, the key site of GABA production and function. Method Quantitative polymerase chain reaction was used to measure GAD67 m RNA levels in postmortem specimens of dorsolateral prefrontal cortex from subjects with schizophrenia and matched comparison subjects with no know n history of psychiatric or neurological disorders (N=42 pairs). In a subset of this cohort in which potential confounds of protein measures were controlled (N=19 pairs), Western blotting was used to quantify tissue levels of GAD67 protein in tissue. In five of these pairs, multilabel confocalimm unofluorescence was used to quantify GAD67 protein levels in the axon terminals of parvalbumin-containing GABA neurons, which are know n to have low levels of GAD67 m RNA in schizophrenia. Results GAD67 m RNA levels were significantly lower in schizophrenia subjects (by 15%), but transcript levels were not associated with predictors or measures of illness severity or chronicity. In schizophrenia subjects, GAD67 protein levels were significantly lower in total gray matter (by 10%) and in parvalbumin axon terminals (by 49%). Conclusions The findings that lower GAD67 m RNA expression is com m on in schizophrenia, that it is not a consequence of having the illness, and that it leads to less translation of the protein, especially in the axon terminals of parvalbumin-containing neurons, support the hypothesis that lower GABA synthesis in parvalbumin neurons contributes to dorsolateral prefrontal cortex dysfunction and impaired cognition in schizophrenia.

Reduced Labeling of Parvalbumin Neurons and Perineuronal Nets in the Dorsolateral Prefrontal Cortex of Subjects with Schizophrenia

“…Images were subjected to deconvolution and intensity histograms for each channel were independently adjusted to identical settings across all subjects allowing PV-positive cells and PNNs to be masked under identical conditions across all subjects. These approaches have been previously used to obtain quantitative measures of immunofluorescence intensity across a wide dynamic range (Curley et al, 2011;Fish et al, 2011;Glausier et al, 2015). See Supplementary Methods for details.…”

Neurodevelopment, GABA System Dysfunction, and Schizophrenia

“…Deficiencies in GAD1 expression, the enzyme responsible for producing the majority of the GABA in the brain, are commonly found in many brain regions in post-mortem tissue from patients with schizophrenia (Akbarian and Huang, 2006;Akbarian et al, 1995;Costa et al, 2004;Curley et al, 2011;Fatemi et al, 2005;Guidotti et al, 2000a;Hashimoto et al, 2003Hashimoto et al, , 2008aHuang and Akbarian, 2007;Impagnatiello et al, 1998;Kalkman and Loetscher, 2003;Knable et al, 2002;Lewis et al, 2005;Mirnics et al, 2000;Thompson Ray et al, 2011;Volk et al, 2000;Volk and Lewis, 2002a). Interestingly, GAD1 mRNA was not detectable in approximately 30% of GABAergic interneurons in the cortex of post-mortem brains from individuals with schizophrenia (Akbarian et al, 1995;Volk et al, 2000), whereas cells with detectable GAD1 appeared to have normal levels (Volk et al, 2000), suggesting dysregulation of GABAergic gene expression is cell type-specific.…”

Will the Treatment Protocols for Schizophrenia be Changing Soon?

“…Attempts to identify differences in the dopaminergic system have failed to reveal differences (Grace, 2012) and some of the gene candidates identified as risk factors for schizophrenia are not related to dopamine function (Howes & Kapur, 2009). A variety of findings have converged on differences in the cells that Schiz 13 control the dopamine system: the parvalbumin positive (PV +), GABA producing, fast-spiking, interneurons (Curley et al, 2011).…”