¹Kyushu University, Department of Neuropsychiatry, Graduate School of Medical Sciences, Fukuoka, Japan, ²Asahikawa Medical University, Department of Functional Anatomy and Neuroscience, Asahikawa, Japan, ³Hoshi University School of Pharmacy and Pharmaceutical Sciences, Department of Pharmacology, Tokyo, Japan, ⁴Hokkaido University, Department of Psychiatry, Sapporo, Japan

Background: Despite extensive research into pharmacological, genetic, and neuroimaging aspects, the pathophysiology of bipolar disorder (BD) remains not fully understood. Given its nature as a complex, multifactorial disease, integrating multidimensional data is essential. Current research supports the inflammation hypothesis, evidenced by elevated cytokine levels and microglial activation. However, the technical and ethical difficulties of analyzing live patient microglia have hindered progress. In 2014, we established human induced microglia-like (iMG) cells to overcome these limitations, and their utility has been widely reported. Using this model, Ohgidani et al. reported that CD206 expression in iMG cells could be a potential state marker in a pilot study of three patients with bipolar disorder.

Aims & Objectives: In order to validate the findings of the previous study and identify additional novel biomarkers, we expanded the sample size and performed comprehensive RNA sequencing of iMGs derived from these individuals, enabling an unbiased assessment of iMG gene expression. The objectives of this study were threefold: firstly, to identify gene expression profiles in iMGs specific to BD by comparing patients with healthy controls (HCs); secondly, to investigate state-dependent transcriptional patterns and potential state markers that fluctuate with the clinical course; and finally, to evaluate the effects of patient-derived iMGs on neurons.

Method: This study was approved by the Ethics Committee of Kyushu University. Sixteen healthy subjects and fifteen patients with BD were recruited with written informed consent. Gene expression in iMG cells from both groups was analyzed by RNA sequencing to identify differentially expressed genes (DEGs). Spearman's rank correlation analysis between changes in mood symptom scores and gene expression in iMG cells was performed on a total of thirteen patients from whom blood samples were obtained at two time points. Supernatants from iMG cells were applied to dopaminergic neurons derived from healthy human induced pluripotent stem cells (iPSCs). NicheNet analysis was performed to investigate cell-cell communication.

Results: The study cohort comprised fifteen patients and sixteen healthy controls. The mean ages of the patient and control groups were 44.7 ± 10.1 years and 43.4 ± 11.7 years, respectively. Twenty-eight DEGs were identified, some of which were microglia-related genes. Pathway analysis revealed an enrichment of inflammation-related pathways. Several novel genes, including GP***, were found to correlate with clinical symptoms. CD206 expression tended to correlate with depressive symptoms, although this did not reach statistical significance (p= 0.054). NicheNet analysis identified TG***, CX***, and IN*** as pivotal ligands upregulated in BD iMG cells.

Discussion & Conclusions: iMG cells from patients with BD showed an enrichment of inflammation- and microglia-related genes, indicating the involvement of microglia-mediated brain inflammation in the pathological background of the disorder. Several genes correlating with clinical symptoms may serve as potential state biomarkers. Humoral factors from iMG cells were found to induce gene expression changes in neurons via specific pathways, suggesting a potential link to the pathophysiological basis of bipolar disorder. However, further studies are essential to confirm these findings.
*Novel gene names are masked for intellectual property protection