SPARSE COMPLEMENT C4 OVEREXPRESSION DRIVES NETWORK-WIDE TRANSCRIPTIONAL REWIRING IN PREFRONTAL CORTEX

The complement (C) pathway is a key component of innate immunity and plays important roles in synaptic plasticity and circuit refinement in the brain. Although complement dysregulation has been implicated in multiple neurological and psychiatric disorders, it remains unclear how localized increases in complement activity within a small subset of cells influence gene regulatory programs across broader cortical populations. To address this question, we overexpressed the schizophrenia (SCZ) risk gene C4 in approximately 2% of prefrontal cortex neurons in mice using in utero electroporation. Bulk RNA sequencing of microdissected tissue revealed widespread transcriptional changes across a network composed predominantly of untransfected cells, indicating non–cell-autonomous effects of C4 overexpression (C4-OE). C4-OE induced upregulation of genes involved in cholesterol biosynthesis, axon guidance, synaptic plasticity, cytoprotection, and neurogenesis, consistent with compensatory remodeling or aberrant plasticity. Co-expression analysis identified a C4b-containing gene module enriched for dendritic development and cell cycle regulation, suggesting altered trajectories of circuit maturation, while immune and inflammatory gene programs were broadly downregulated, consistent with homeostatic suppression in response to sustained complement activity. Comparisons with human SCZ proteomic datasets revealed conserved gene signatures, supporting the disease relevance of this model. Finally, reference-based projection of DEG signatures onto single-cell transcriptomic atlases revealed that sparse C4-OE preserves major cortical cell-type identities while fragmenting neuronal transcriptional programs and amplifying non-neuronal gene modules. Together, these findings demonstrate that localized C4 upregulation drives a systems-level reorganization of transcriptional coupling across cortical networks, extending beyond the initially affected neurons.