TSPO upregulation in bipolar disorder and concomitant downregulation of mitophagic proteins and NLRP3 inflammasome activation

Bipolar disorder (BD) is a chronic, debilitating illness with a global prevalence of up to 4.8%. The importance of understanding how dysfunctional mitochondria and mitophagy contribute to cell survival and death in BD is becoming increasingly apparent. Therefore, the purpose of this study was to evaluate the mitophagic pathway and NLRP3 inflammasome activation in peripheral blood mononuclear cells (PBMCs) of patients with BD and healthy individuals. Since 18-kDa translocator protein (TSPO) plays an important role in regulating mitochondrial function and since TSPO itself impairs cellular mitophagy, we also investigated the changes in the TSPO-related pathway. Our results showed that patients with BD had lower levels of Parkin, p62/SQSTM1 and LC3A and an upregulation of TSPO pathway proteins (TSPO and VDAC), both in terms of mRNA and protein levels. Additionally, we found a negative correlation between mitophagy-related proteins and TSPO levels, while VDAC correlated negatively with p62/SQSTM1 and LC3 protein levels. Moreover, we found that the gene expression levels of the NLRP3-related proteins NLRP3, ASC, and procasp1 were upregulated in BD patients, followed by an increase in caspase-1 activity as well as IL-1 beta and IL-18 levels. As expected, there was a strong positive correlation between NLRP3-related inflammasome activation and TSPO-related proteins. The data reported here suggest that TSPO-VDAC complex upregulation in BD patients, the simultaneous downregulation of mitophagic proteins and NLRP3 inflammasome activation could lead to an accumulation of dysfunctional mitochondria, resulting in inflammation and apoptosis. In summary, the findings of this study provide novel evidence that mitochondrial dysfunction measured in peripheral blood is associated with BD.