Increased Complement C4 in a Sparse Neuronal Subset Induces Network-Wide Transcriptomic Alterations in the Prefrontal Cortex

The complement (C) pathway, a vital part of innate immunity, defends against pathogens and supports tissue surveillance. While local activation in the periphery enhances immune protection, dysregulation can trigger a self-amplifying cascade that spreads beyond the initial site, resulting in tissue injury. In the brain, complement proteins regulate synaptic plasticity and connectivity, raising the possibility that similar mechanisms of maladaptive propagation may disrupt neural circuits under pathological conditions. Although complement dysregulation is linked to neurological and psychiatric disorders, the impact of localized upregulation in a small subset of cells on broader cortical networks remains unclear. To examine this, we overexpressed the schizophrenia (SCZ) risk gene C4 (C4-OE) in approximately 2% of prefrontal cortex neurons in mice using in utero electroporation. Bulk RNA sequencing of microdissected tissue revealed widespread transcriptional changes in a network predominantly comprising untransfected cells. C4-OE induced the upregulation of genes involved in cholesterol biosynthesis, axon guidance, synaptic plasticity, cytoprotection, and neurogenesis—suggesting a response consistent with compensatory remodeling or aberrant plasticity. Co-expression analysis identified a C4b-containing module enriched for dendritic development and cell cycle regulation, indicating alterations in circuit maturation. In contrast, immune and inflammatory genes were broadly downregulated, suggesting homeostatic suppression in response to sustained complement activity. Comparisons with human SCZ proteomic datasets revealed conserved gene signatures, highlighting the potential of our model to capture disease-relevant mechanisms. These findings demonstrate that sparse C4 overexpression can trigger widespread transcriptional changes across neural circuits, suggesting that local complement dysregulation may spread through cortical networks and drive broader functional disruption.