Bipolar disorder (BD) is a chronic and severe mental disorder affecting more than 1% of the global population. BD causes considerable disability, whose pathophysiology is still poorly understood. Studies suggest that BD individuals present a chronic inflammation. However, whether this chronic inflammation is associated with pathophysiological mechanisms in the central nervous system (CNS) is less clear.
This work, supported by the Swiss National Science Foundation (No. 184713), hypothesizes alterations in glial cells in the CNS of BD patients. To evaluate proteomic and transcriptomic alterations in microglia and astrocytes in BD, proteomics, single-nucleus RNA sequencing (snRNAseq), spatial transcriptomics, and bulk RNA sequencing were employed.
Proteins were extracted from the anterior cingulate cortex (aCC) of postmortem human tissue samples and quantified with nanoLC-MS/MS. The identification of co-expressed proteins revealed the presence of two significantly differentially co-expressed modules. Nuclei from BD and control subjects were isolated from the aCC of the same human tissue samples. Cell sorting was employed to enrich the samples for microglia and astrocytes, which were then processed with the 10X Genomics Chromium Controller and sequenced. Differential gene expression (DGE) was performed separately in microglia and astrocytes. Additionally, spatial transcriptomics was conducted on formalin-fixed paraffin-embedded sections from the same subjects. The sequencing procedure was performed on areas surrounding GFAP and IBA1 staining to obtain transcripts from astrocytic, microglial, and spatially adjacent cells in both grey and white matter. Finally, bulk RNA sequencing was conducted on a panel of glial genes in both grey and white matter. Some of the results were further confirmed with immunohistology (IH).
Alterations in glial cells was identified in microglia, proinflammatory and phagocytosis-related pathways were found to be downregulated, as evidenced by both snRNAseq and bulk RNAseq results. This was further corroborated by a reduced amount of IBA1 in the grey matter of BD subjects with IH, suggesting downregulated immune response in BD. It is noteworthy that almost all of the 30 genes associated with GWAS loci were expressed in astrocytes and microglia. Of particular interest are NCAN and ADCY2, which were significantly upregulated in astrocytes of BD subjects, as also confirmed by IH. Our findings indicate that GWAS genes map onto glial cells and are implicated in the impaired energetic metabolism of astrocytes. Therefore, they may play a role in the pathophysiology of BD and may serve as biomarkers, in addition to representing potential targets for inflammation-related therapeutic approaches.
