Single-cell multiomics study of cell-type-specific neuron activation unravels context-dependent gene regulation of brain disorders

Genome-wide association studies (GWAS) have identified many risk genes for neuropsychiatric disorders (NPD) such as schizophrenia (SCZ). However, functional interpretation of these GWAS findings remains challenging; a major hurdle is that polygenic risk may manifest its effect conditionally upon neural activation, i.e., context-dependent. Human induced pluripotent stem cell (hiPSC)-derived neurons are a tractable cellular model for ascertaining context-dependent polygenic effects, e.g., neural activation that may mimic the physiological effects of environmental stimuli. Here, we modelled neural activation by depolarisation using high potassium chloride (KCl) in co-cultured excitatory/inhibitory neurons of 100 hiPSC lines (28 from SCZ donors), followed by assaying single-cell multiomes (scRNA/ATAC-seq) of over 700,000 nuclei from neurons at 0 hr (unstimulated), 1 hr (early response), and 6 hr (later response) of high KCI exposure. We linked genes with open chromatin peaks for each main neural subtype (GABAergic inhibitory, NEFM+ or NEFM-excitatory neurons) and confirmed a significant correlation between peak accessibility and target gene expression. Compared to static genes/peaks, dynamic ones, specifically those activity-upregulated, were more enriched for NPD GWAS risk, with the most robust enrichment for SCZ. Based on the dynamic gene expression pattern across the three time points, we found NPD risk genes tend to be continuously upregulated (i.e., with up-up dynamics). We further analysed context-specific SCZ-associated differentially expressed genes (DEGs) using MAST for single neurons of 28 SCZ cases and 72 controls. We found that 3-5%, 4-6%, and 15-23% of genes were SCZ-associated DEGs in NEFM+ excitatory, NEFM- excitatory, and GABAergic neurons, respectively, most of which were cell activity-specific. Notably, we found that SCZ-associated DEGs in high-activity neurons were more enriched for synapse-related GO terms and NPD risk genes. Interestingly, upstream regulatory sequences of the stimulation-specific SCZ-associated DEGs were enriched for binding sites of TCF4, which is a strong GWAS risk gene and was considered the master regulator of other SCZ-related genes. Moreover, the upregulated genes in SCZ cases in the stimulated (only at 6hr) NEFM+ excitatory neurons showed strong enrichment for GO terms related to cholesterol synthesis. Single-neuron DEG analysis for 28 SCZ cases and 28 matched controls gave similar results. Our study suggests that many NPD genes may only elicit disease-relevant effects upon neuronal activation, providing novel insights into how polygenic risk factors interact with environmental stimuli for NPD.