Mitochondrial health index correlates with plasma circulating cell-free mitochondrial DNA in bipolar disorder

Although mitochondrial dysfunction is known to play an essential role in the pathophysiology of bipolar disorder (BD), there is a glaring gap in our understanding of how mitochondrial dysfunction can modulate clinical phenotypes. An emerging paradigm suggests mitochondria play an important non-energetic role in adaptation to stress, impacting cellular resilience and acting as a source of systemic allostatic load. Known as mitochondrial allostatic load, this (phenomenon) occurs when mitochondria are unable to recalibrate and maintain cell homeostasis. This study aimed to evaluate the composite mitochondrial health index (MHI) in BD subjects and non-psychiatry controls. We will also explore whether lower MIH will be related to higher cell-free mtDNA (ccf-mtDNA) levels and poor clinical outcomes. In this study, 14 BD-I patients and 16 age- and sex-matched non-psychiatry controls were enrolled. Peripheral blood mononuclear cells (PBMCs) were used to measure the enzymatic activities of citrate synthase and complexes I, II, and IV and mtDNA copy number. Ccf-mtDNA was evaluated by qPCR in plasma. Mitochondrial quality control (MQC) proteins were evaluated by western blotting. After adjusting for confounding variables, such as age, sex, body mass index (BMI), and smoking status, patients with BD presented lower MHI compared to non-psychiatry controls, as well as higher ccf-mtDNA levels that negatively correlated with MHI. Because the MQC network is essential to maintain mitochondrial health, MHI and ccf-mtDNA were also examined in relation to several MQC-related proteins, such as Fis-1, Opa-1, and LC3. Our results showed that MHI correlated negatively with Fis-1 and positively with Opa-1 and LC3. Accordingly, ccf-mtDNA had a positive correlation with Fis-1 and a negative correlation with Opa-1 and LC3. Furthermore, we found a noteworthy inverse correlation between illness severity and MHI, with lower MHI and higher ccf-mtDNA levels in subjects with a longer illness duration, worse functional status, and higher depressive symptoms. Our findings indicate that mitochondrial allostatic load contributes to BD, suggesting mitochondria represent a potential biological intersection point that could contribute to impaired cellular resilience and increased vulnerability to stress and mood episodes. Ultimately, by linking mitochondrial dysfunction to disease progression and poor outcomes, we might be able to build a predictive marker that explains how mitochondrial function and its regulation contribute to BD development and that may eventually serve as a treatment guide for both old and new therapeutic targets.