Downregulation in components of the mitochondrial electron transport chain in the postmortem frontal cortex of subjects with bipolar disorder

Abstract

Objective: Many studies indicate a genetic predisposition to bipolar disorder (BD) and suggest that a number of abnormal genes are involved in its development. In this study, we used DNA microarray technology to analyze gene-expression profiles in the postmortem frontal cortex of subjects with BD.

Methods: Microarray hybridization was performed using human 19K microarray with universal human reference RNA in each hybridization. The reference cDNA was labelled with Cy3 and experimental cDNA, with Cy5. Glass array slides were cohybridized with equal amounts of mixed reference and experimental cDNA. Selected gene targets were further verified using real-time polymerase chain reaction (PCR).

Results: We found that 831 genes were differentially expressed in subjects with BD, including a number of genes in the mitochondrial electron transport chain (ETC), phosphatidylinositol-signalling system and glycolysis/ gluconeogenesis. Eight genes coding for the components of the mitochondrial ETC were identified along with 15 others related to mitochondrial function. Downregulation of NADH-ubiquinone oxidoreductase 20-kd subunit (ETC complex I), cytochrome c oxidase polypeptide Vic (ETC complex IV) and ATP synthase lipid-binding protein (ETC complex V) were further verified by real-time PCR. We also found that the expression of the NADH-ubiquinone oxidoreductase 20-kd subunit was increased in subjects with BD who were receiving mood-stabilizing treatment with lithium at the time of death, when compared with subjects with BD who were not being treated with lithium.

Conclusions: Because the mitochondrial ETC is a major source for the generation of reactive oxygen species, these findings suggest that oxidative damage may play an important role in the pathophysiology of BD and that neuroprotection against this damage may be involved in the effect of lithium treatment.

Fig. 1: Summary of altered expression of genes involved in the pathways of the electron transport chain, the phosphatidylinositol-signalling system, and glycolysis and gluconeogenesis. Pathways of the electron transport chain: ATP5 = adenosine triphosphate (ATP) synthase; COX = cytochrome c oxidase polypeptide; NDUF = NADH-ubiquinone oxidoreductase; UQCRC = ubiquinol-cytochrome c reductase complex core protein. Pathways of the phosphatidylinositol-signalling system: CALM = calmodulin; CaMK = calmodulin-dependent kinase; DAG = diacylglycerol; Ins(1,4,5)P3 = inositol 1,4,5-trisphosphate; Ins(1,3,4,5)P4 = inositol 1,3,4,5-tetrakisphosphate; IP3K = inositol-1,4,5-trisphosphate 3-kinase; Pl(4,5)P2 = phosphatidylinositol 4,5-bisphosphate; PLC = phospholipase C; PRKC = protein kinase C. The pathways of glycolysis and gluconeogenesis: ENO2 = gamma enolase; FBPase = fructose-1,6-bisphophosphatase; GAPD = glyceraldehyde-3-phosphate dehydrogenase; GPI = glucose-6-phosphate isomerase; PFK = 6-phosphofructokinase. ↑ = increase; ↓ = decrease.

Fig. 2: The mRNA level of NADH-ubiquinone oxidoreductase 20-kd subunit (NDUFS7), ubiquinol-cytochrome c reductase complex core protein 2 (UQCRC2), cytochrome c oxidase polypeptide Vic (COX6C) and ATP synthase lipid-binding protein (ATP5G3) in control (CTL) subjects and subjects with bipolar disorder (BD). The mRNA level was measured by real-time quantitative polymerase chain reaction (PCR). rRNA was used as an endogenous control for normalization. The results presented indicate the mean mRNA level and standard error of the mean for each gene studied (n = 33). A schema illustrating the specific mitochondrial electron transport chain complexes in which gene regulation was altered in subjects with BD appears below the results of PCR analysis. Points at which electrons are transferred to molecular oxygen by complexes I, II and III are denoted by curved arrows. *indicates p < 0.05, **indicates p < 0.01 and + indicates 0.05 < p < 0.10 when compared with control subjects using the Mann–Whitney U test.

Fig. 3: Effect of lithium (Li⁺), valproate (VPA) and atypical antipsychotics (AAP) on mRNA levels of NADH-ubiquinone oxidoreductase 20-kd subunit (NDUFS7), cytochrome c oxidase polypeptide Vic (COX6C) and ATP synthase lipid-binding protein (ATP5G3) in the postmortem frontal cortex of subjects with bipolar disorder. The mRNA level was measured by real-time quantitative polymerase chain reaction. rRNA was used as an endogenous control for normalization. The results are the mean and standard error of the mean (n = 9–24). *indicates p < 0.05, **indicates p < 0.01 and + indicates 0.05 < p < 0.10 using the Mann–Whitney U test.