Mitochondrial activity and oxidative stress markers in peripheral blood mononuclear cells of patients with bipolar disorder, schizophrenia, and healthy subjects

https://doi.org/10.1016/j.jpsychires.2013.06.018Get rights and content

Abstract

Evidence suggests that mitochondrial dysfunction is involved in the pathophysiology of psychiatric disorders such as schizophrenia (SZ) and bipolar disorder (BD). However, the exact mechanisms underlying this dysfunction are not well understood. Impaired activity of electron transport chain (ETC) complexes has been described in these disorders and may reflect changes in mitochondrial metabolism and oxidative stress markers. The objective of this study was to compare ETC complex activity and protein and lipid oxidation markers in 12 euthymic patients with BD type I, in 18 patients with stable chronic SZ, and in 30 matched healthy volunteers. Activity of complexes I, II, and III was determined by enzyme kinetics of mitochondria isolated from peripheral blood mononuclear cells (PBMCs). Protein oxidation was evaluated using the protein carbonyl content (PCC) method, and lipid peroxidation, the thiobarbituric acid reactive substances (TBARS) assay kit. A significant decrease in complex I activity was observed (p = 0.02), as well as an increase in plasma levels of TBARS (p = 0.00617) in patients with SZ when compared to matched controls. Conversely, no significant differences were found in complex I activity (p = 0.17) or in plasma TBARS levels (p = 0.26) in patients with BD vs. matched controls. Our results suggest that mitochondrial complex I dysfunction and oxidative stress play important roles in the pathophysiology of SZ and may be used in potential novel adjunctive therapy for SZ, focusing primarily on cognitive impairment and disorder progression.

Introduction

Mitochondria are responsible for many essential processes, e.g., energy production, apoptosis, intracellular calcium buffering, and reactive oxygen species (ROS) production, all potentially leading to either synaptic plasticity or cell death (Ben-Shachar, 2009). Cell energy is mainly obtained through oxidative phosphorylation. Mitochondrial adenosine-triphosphate (ATP) is produced as electrons flow through electron transport chain (ETC) complexes (I to IV), which are located in the inner mitochondrial membrane. The resulting energy, in the form of protons, reenters the mitochondrial matrix through ATP synthase (complex V), producing ATP. Mitochondria and cytosolic ROS are formed as a consequence of ETC activity, possibly affecting redox status and inducing lipid peroxidation (Blanchet et al., 2011).

Noteworthy, mitochondrial functions become less effective, including impairment of complex I and, to a lesser extent, complex III activity. The reduced activity of ETC complexes in turn leads to enhanced ROS production, reduced Ca2+ buffering capacity, and mitochondrial DNA mutations (Mattson, 2006). During the transfer along ETC complexes, single electrons may escape, encounter molecular oxygen, and eventually reduce it to form a superoxide anion (O2), especially in complex I (Green and Kroemer, 2004). Also, complex I deficiency has been associated with increased ROS levels and aberrant Ca2+/ATP homeostasis (Blanchet et al., 2011).

Oxidative stress plays an important role in the pathogenesis of several diseases, especially neurological and psychiatric disorders such as bipolar disorder (BD) and schizophrenia (SZ). Studies have described dysfunction in ETC complex activity in patients with BD and SZ, but most of the findings available derive from postmortem tissue analysis or animal models, with controversial results (Andreazza et al., 2010, Karry et al., 2004, Maurer and Möller, 1997, Valvassori et al., 2010).

As previously suggested, changes associated with psychiatric disorders are not exclusively observed in the brain, but also in peripheral markers (Kunz et al., 2011). Lymphocytes, for example, have been shown to be a suitable cell model for studies of mitochondrial dysfunction (Leuner et al., 2012). Notwithstanding, to the authors' knowledge, no previous study has examined both mitochondrial activity and oxidative stress markers in peripheral blood mononuclear cells (PBMCs) obtained from patients with SZ and BD and from healthy controls. It is our hypothesis that substantial mitochondrial dysfunction and increased ROS in peripheral tissues could be strong indicators of an important role played by these markers in the pathophysiology of the disorders assessed.

Therefore, the objective of the present study was to assess plasma oxidative stress markers, based on thiobarbituric acid reactive substances (TBARS) and protein carbonyl contents (PCC), as well as ETC complex activity (complexes I, II, and III), in PBMCs obtained from patients with chronic SZ and BD type I and from healthy controls.

Section snippets

Subjects

This study was approved by the Research Ethics Committee of Hospital de Clínicas de Porto Alegre (HCPA), southern Brazil. All procedures were performed in accordance with the ethical standards set forth in the Declaration of Helsinki. Moreover, all subjects provided written informed consent before their inclusion in the study.

We recruited 18 stabilized outpatients with SZ, all chronically medicated, among subjects attending the HCPA Schizophrenia Program, plus 12 euthymic outpatients with BD

Statistical analysis

Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS) version 18.0 for Windows. All biological variables showed a non-parametric distribution; log-transformed PCC values were back-transformed using the function Y = −1*Log(Y), to avoid negative values. After transformation, independent-samples t-test was used to evaluate differences between patients and controls. Data were presented as means ± standard deviations. Significance was set at p < 0.05.

Results

The 18 patients with SZ and 12 patients with BD included in the study were matched with 18 and 12 healthy controls, respectively. Demographic and clinical characteristics are summarized in Table 1.

All patients were in use of psychiatric medications at the time of the study. The 18 patients with SZ were receiving various medications at conventional doses, including antipsychotics such as haloperidol (n = 3), chlorpromazine (n = 2), clozapine (n = 12), risperidone (n = 2), and sulpiride (n = 2).

Discussion

In the present study, a significant decrease was observed in mitochondrial complex I activity in PBMCs obtained from patients with chronic and stabilized SZ when compared with healthy controls. An increase in plasma levels of TBARS in the same group of patients was also found, possibly indicating lipid peroxidation. Altered ETC activity may result in a reduced capacity to oxidize NADH and to transfer electrons to ubiquinone, which could expose electrons to molecular oxygen, produce ROS, and

Role of funding sources

This study was supported by grants from CNPq (Universal 470326/2011-5 and PQ 302195/2011-4), INCT Translational Medicine/CNPq (573671/2008-7), FAPERGS (PqG 1009340-06/2010) and FAPERGS/CNPq (PRONEM 11/2057-2), Brazil. These agencies had no role in the study design, in the acquisition or interpretation of the data, or in writing the report.

Contributors

CG and LS designed the study. Protocols were optimized by GTR, CG and ELS. RM and MK screened the study participants. CG, BP and BSP performed the experimental analyses. LS, CSG and MK undertook the statistical analysis. CG, LS, FK and MK interpreted the results. CG, LS and MK supervised the data analysis and writing of the paper, which was revised and approved by all authors.

Conflict of interest

CG, LS, BP, BSP, GTR, RM, ELS and MK declare no possible conflict of interest, financial or otherwise, as well as grants or other forms of financial support. CSG has been a paid speaker for Lundbeck and a consultant/speaker for Actelion Pharmaceuticals Ltd. She has received travel award from Lilly. FK has received grant/research support from Astra-Zeneca, Eli Lilly, Janssen-Cilag, Servier, CNPq, CAPES, NARSAD and Stanley Medical Research Institute; has been a member of the speakers' boards for

Acknowledgments

CG and BSP are recipients of scholarships from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). LS and BP are recipients of scholarships from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). We thank all authors of the included studies.

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