Plasma BDNF and Cytokines Correlated with Protein Biomarkers for Bipolar II Disorder

We have previously identified five candidate proteins (matrix metallopeptidase 9 (MMP9), phenylalanyl-TRNA synthetase subunit beta (FARSB), peroxiredoxin 2 (PRDX2), carbonic anhydrase 1 (CA-1), and proprotein convertase subtilisin/kexin Type 9 (PCSK9)) as potential biomarkers for bipolar II disorder (BD-II). These candidate proteins have been associated with neuroprotective factors (BDNF) and inflammatory factors (cytokines, C-reactive protein (CRP), and tumor necrosis factor-α (TNF-α)). However, the correlations between these proteins with plasma BDNF and inflammatory factors remain unknown. We recruited a total of 185 patients with BD-II and 186 healthy controls. Plasma levels of candidate proteins, BDNF, cytokines (TNF-α, CRP, and interleukin-8 (IL-8)) were assessed from each participant. The correlations between levels of candidate proteins, BDNF, and cytokines were analyzed. In the BD-II group, we found that the level of FARSB was positively correlated with the BDNF level (r = 0.397, p < 0.001) and IL-8 (r = 0.320, p < 0.001). The CA-1 level positively correlated with IL-8 (r = 0.318, p < 0.001). In the control group, we found that the FARSB level positively correlated with the BDNF level (r = 0.648, p < 0.001). The CA-1 level positively correlated with TNF-α (r = 0.231, p = 0.002), while the MMP-9 level positively correlated with the CRP level (r = 0.227, p = 0.002). Our results may help in clarifying the underlying mechanism of these candidate proteins for BD-II.