Does genetic risk for depression influence microglial function? analyses in human induced pluripotent stem cell-derived microglia

Depression is a common psychiatric disorder, affecting around 280 million people worldwide. Both genetic and environmental factors contribute to disease liability, likely via gene-environment interactions. Clinical observations as well as genetic findings suggest that inflammatory processes are involved in the pathophysiology of depression. One potential cell type implicated are microglia - the resident immune cells of the brain. While there is substantial evidence for a role of microglia from animal models of depression (e.g. chronic stress), data on human microglia is limited. Recent advances in induced pluripotent stem cell (iPSC) research provide new possibilities to study human microglia in vitro. In this study, we generated iPSC lines of patients with major depressive disorder (MDD) and healthy controls (HC) selected from the extreme ends of the genetic risk spectrum to study a potential influence of genetic risk for depression on microglial function and their responses to relevant environmental stimuli. Peripheral blood mononuclear cells for iPSC generation were selected from unrelated individuals from the German FOR2107 cohort. Patients with recurrent MDD and HC were selected from the tail-ends of the distribution of polygenic risk scores (PRS) for depression, based on the genome-wide association study (GWAS) by Howard et al. (2019). Additionally, only donors with at least one first degree relative with MDD or no family history of psychiatric disorders were selected in the high and low PRS group, respectively. iPSC lines were generated for six male donors (n = 3 per group) at the Cell Programming Core Facility of the University of Bonn and will be differentiated into iPSC-derived microglia (iMG) in the upcoming months. In pilot experiments, sequential differentiation of iPSCs into primitive macrophage precursors and iMG was established using a control iPSC line (HPSI0514i-letw_5). Cell-type identity was monitored throughout differentiation using flow cytometry, immunocytochemistry and transcriptome analysis (3’mRNA-Seq). Furthermore, iMG were stimulated with lipopolysaccharide (LPS) or dexamethasone (Dex) to study inflammatory and glucocorticoid (“stress”) signalling. Results from pilot experiments indicate successful differentiation from iPSCs into macrophage precursors and iMG, based on the expression of macrophage/microglia-specific proteins and gene expression signatures. Transcriptome analysis further revealed that 217 out of 269 GWAS candidate genes for depression are expressed in at least one of the differentiation stages. When stimulated with LPS or Dex, iMG showed differential expression of inflammatory and glucocorticoid-response genes, respectively. For example, FKBP5, a negative regulator of glucocorticoid signalling and known candidate gene to mediate interactions between stress and depression liability, was the most strongly upregulated gene in Dex-treated iMG. Additionally, secretion of pro-inflammatory cytokines by LPS-treated iMG confirmed a functional response to inflammatory stimulation. In summary, these preliminary analyses suggest that iMG are a valid model to study inflammatory and glucocorticoid signalling. The generation of iMG from our newly established MDD and HC cell lines will provide a valuable tool to test whether genetic risk for depression affects human microglia function and enable analyses of gene-environment interactions (e.g. “stress”) in a defined cellular model.