These results extend previous reports demonstrating alterations in energy metabolism in subjects with BPD. Using whole genome microarrays, we identified a large number of genes that were differentially expressed in peripheral blood of subjects with BPD compared to healthy controls. Pathway analysis using GeneGo MetaCore software showed that the most significantly affected pathway was the mitochondrial ETC (p < 8.6 × 10−7
), suggesting that this pathway may play an important role in the pathophysiology of depression in patients with BPD. Moreover, this number is probably an underestimate of the number of dysregulated genes in the ETC pathway, since the arrays used did not include any of the mitochondrially encoded components of the ETC (13 genes) (59
Combining the results from the microarray and qRT-PCR studies, it appears that there is a general increase in the expression of both nuclear and mitochondrially encoded ETC genes in subjects with bipolar depression. Moreover, all of the top 10 functional pathways identified in the microarray analysis are interconnected, and relate directly or indirectly to mitochondrial functions, including energy metabolism and the regulation of apoptosis by mitochondrial proteins. In particular, production of energy via the ETC (the top-ranked pathway) is directly coupled to the regulation of apoptosis and cell survival (, pathways 6–8, 10) since mitochondrially formed oxidants activate a number of signaling pathways related to apoptosis. In particular, activation of the mitochondrial death pathways mediated by Bax and Bak is dependent upon oxidative phosphorylation (60
). There is also evidence that the activity of the mitochondrial ETC is regulated directly by the Notch signaling pathway (, pathways 2, 4) (62
) and indirectly by the pro-inflammatory/anti-apoptotic regulatory protein, nuclear factor kappa B (NF-κB) (, pathways 3–5) (64
). The macrophage migration inhibitory pathway (, pathway 9) also interacts with the NF-κB pathway and has been implicated as a regulator of apoptosis (66
). Thus, it appears likely that a dysregulation of mitochondrial function is central to the large number of gene expression changes observed in the current study.
Limitations of this study include the small sample size and possible confounding effects of differences among specific subpopulations of white blood cells between subjects with BPD and controls. Such differences could affect the relative expression of some genes, i.e., those specific to the over- or under-represented cell types. However, since ETC genes are expressed in all cell types, it is unlikely that that the observed differences in ETC gene expression are related to differences in subpopulations of white cells between the two groups.
Several previous studies have examined the expression of mitochondrial genes, most notably NDUFV2, in transformed lymphocytes from patients with BPD. Washizuka et al. examined the expression of NDUFV2 (67
) and 11 other mitochondria-related genes (68
) in lymphoblastoid cells derived from a cohort of 21 Japanese subjects with BPD and 11 controls. They found that components of complex I and complex IV were down-regulated in the subset of subjects with BPD-I (n =13), although only the change in NDUFV2 was significant after correction for multiple testing. In contrast, Xu et al. (69
) reported no difference in the level of expression of NDUFV2 in transformed lymphocytes from a group of 178 subjects with BPD of European Caucasian ancestry compared to a group of 120 controls. In a follow-up study, Washizuka et al. (70
) confirmed the decrease of NDUFV2 in subjects with BPD-I of Japanese ancestry (n =25), but found increased expression of NDUFV2 in Japanese subjects with BPD-II (n =10) and no change in BPD (either BPD-I or BPD-II) in subjects of Caucasian ancestry. However, the significance of these studies with regard to the expression of ETC genes in lymphocytes in vivo
is unclear, since transformation with Epstein-Barr virus has been shown to affect the expression of multiple genes in lymphocytes, including pathways regulating cell growth and division (71
In contrast to our current findings, a number of postmortem studies (24
) have suggested that the expression of ETC genes may be decreased in patients with BPD. There are several possible explanations for the difference between those findings and our findings in the current study. First, it is possible that there are state-related differences in the expression of ETC genes in subjects with BPD. In the current study, all BPD subjects were depressed at the time of study entry, while the postmortem studies may have included subjects who were depressed, manic, or euthymic at the time of death. Consistent with that hypothesis, Dror et al. (75
) found evidence of state-dependent changes in mitochondrial complex I activity in platelets of schizophrenic patients, with increased activity in patients with active psychotic symptoms but decreased activity in patients with residual schizophrenia. Further studies will be needed to determine if there is a similar state-dependence to the changes in ETC gene expression observed in this study. Second, the majority of patients included in the previous studies were receiving mood stabilizers and/or antipsychotic medications at the time of death. Thus, the decreased expression of ETC genes in those studies may be reflective of either acute or chronic effects of these medications on gene expression. Consistent with that hypothesis, Iwamoto et al. (25
) found that each of the medication groups examined (antidepressants, valproate, and antipsychotics) appeared to have a significant global repressive effect on the expression of mitochondria-related genes. Moreover, in the same study, the small group of unmedicated subjects with BPD (n =4) showed an up-regulation of ETC genes consistent with the findings we have reported here. Third, the previous reports are based on postmortem brain samples, while our findings are based on blood samples isolated from living subjects with BPD. Thus, there may be a tissue-specific difference with ETC genes being up-regulated in blood, but down-regulated in some or all regions of the CNS of patients with BPD. Consistent with that hypothesis, Ben-Shachar and Karry (28
) found that there was region-specific variation in the expression of mitochondrial genes in patients with BPD, with increased expression in the parieto-occipital cortex but reduced expression in the cerebellum. Finally, differences in brain pH at the time of death may explain a large part of the previously reported decrease in ETC gene expression (30
). Further studies will be needed to clarify more precisely the relationships among diagnosis, mood state, treatment status, and ETC gene expression in BPD.
Studies in cell culture and animal models have shown that chronic treatment with lithium or valproate can enhance mitochondrial function and protect against mitochondrially mediated toxicity (76
). Lithium has also been shown to increase the activity of ETC complexes I+ III and II + III in extracts from human postmortem brain tissue at therapeutically relevant concentrations (77
). Conversely, rats subjected to an experimental model of depression showed impaired mitochondrial function (78
). Thus, enhancing mitochondrial ETC function may play a role in the therapeutic effects of lithium and other mood stabilizers. In the current study, we found that depressed BPD subjects had elevated levels of mRNA for several of the genes in ETC complexes I, III, IV, and V. Increased expression of these genes suggests that there may be increased mitochondrial turnover in depressed subjects with BPD (79
). Consistent with the hypothesis that bipolar depression is characterized by impaired mitochondrial function, Andreazza et al.(80
) have recently demonstrated that the activity of mitochondrial complex I is decreased and oxidative damage is increased in the postmortem prefrontal cortex of patients with BPD.
Abnormal expression of mitochondrial genes has also been suggested to play a role in other disorders, including schizophrenia (18
) and type 2 diabetes (83
). An emerging challenge for the field will be to establish how changes in mitochondrial gene expression, and by implication changes in mitochondrial function, relate to the specific symptoms observed in each of these disorders.