Decrease in creatine kinase messenger RNA expression in the hippocampus and dorsolateral prefrontal cortex in bipolar disorder

Abstract

Objectives: Bipolar disorder (BPD) affects more than 2 million adults in the USA and ranks among the top 10 causes of worldwide disabilities. Despite its prevalence, very little is known about the etiology of BPD. Recent evidence suggests that cellular energy metabolism is disturbed in BPD. Mitochondrial function is altered, and levels of high-energy phosphates, such as phosphocreatine (PCr), are reduced in the brain. This evidence has led to the hypothesis that deficiencies in energy metabolism could account for some of the pathophysiology observed in BPD. To further explore this hypothesis, we examined levels of creatine kinase (CK) mRNA, the enzyme involved in synthesis and metabolism of PCr, in the hippocampus (HIP) and dorsolateral prefrontal cortex (DLPFC) of control, BPD and schizophrenia subjects.

Methods: Tissue was obtained from the Harvard Brain Tissue Resource Center. Real-time quantitative polymerase chain reaction (HIP, DLPFC) and gene expression microarrays (HIP) were employed to compare the brain and mitochondrial 1 isoforms of CK.

Results: Both CK isoforms were downregulated in BPD. Furthermore, mRNA transcripts for oligodendrocyte-specific proteins were downregulated in the DLPFC, whereas the mRNA for the neuron-specific protein microtubule-associated protein 2 was downregulated in the HIP.

Conclusion: Although some of the downregulation of CK might be explained by cell loss, a more general mechanism seems to be responsible. The downregulation of CK transcripts, if translated into protein levels, could explain the reduction of high-energy phosphates previously observed in BPD.

Fig. 1

Gene expression microarray data from the human hippocampus of (A) bipolar disorder (BPD) and (B) schizophrenia (SZ) subjects. CKB is represented twice on the HG-U95Av2 array and identified by the respective probe set number. Relative mRNA levels in percentage of control are shown as mean ± SEM. *p < 0.05.

Fig. 2

Real-time quantitative polymerase chain reaction data verified the gene expression microarray findings in the hippocampus in (A) bipolar disorder (BPD) and (B) schizophrenia (SZ) subjects. The downregulation of CK isoforms was furthermore extended to the dorsolateral prefrontal cortex (DLPFC) in BPD subjects (C). No significant downregulation was observed in the DLPFC in SZ subjects (D) with the sample size available. Relative mRNA levels of experiment (BPD or SZ corrected for internal standard, filamin A) over control (corrected for internal standard) are shown as mean ± SEM. *p < 0.05; **p ≤ 0.01.

Fig. 3

Real-time quantitative polymerase chain reaction experiments show that (A) the level of the MAP2 transcript, a gene specifically expressed in neurons, is downregulated in the hippocampus in bipolar disorder (BPD) subjects. Doublecortin, another neuron-specific transcript, is unchanged. (B) Neuronal markers are expressed at control level in the dorsolateral prefrontal cortex (DLPFC). The shaded table in (A) shows the gene expression data from the microarray study in the hippocampus. MAP2 was represented twice on the array, and one probe set was significantly downregulated. Relative mRNA levels of BPD (corrected for the internal standard, filamin A) over control (corrected for internal standard) are shown as mean ± SEM. *p < 0.05.

Fig. 4

Real-time quantitative polymerase chain reaction experiments were carried out for glial transcripts. (A) A downregulation of gelsolin was observed in the hippocampus, but other markers were unchanged. (B) Three of four markers were significantly downregulated in the dorsolateral prefrontal cortex (DLPFC) in bipolar disorder (BPD) subjects. The shaded table in (A) shows the gene expression data from the microarray study in the hippocampus. Relative mRNA levels of BPD (corrected for the internal standard, filamin A) over control (corrected for internal standard) are shown as mean ± SEM. *p < 0.05; **p ≤ 0.01.