Schizophrenia-associated complement component C4 induces maturation and reactivity in human astrocytes

Beyond its canonical role in innate immunity, the complement system (CS) has been increasingly implicated in brain development and disease. Elevated expression of complement component C4, in particular, is strongly associated with an increased risk of schizophrenia (SCZ), potentially by promoting excessive synaptic pruning. Astrocytes, the primary producers of CS components in the brain and expressers of multiple CS receptors, play crucial roles in synaptic formation and refinement. Dysregulated C4 levels may therefore impair astrocyte function, disrupt synaptic development, and contribute to SCZ pathogenesis. In this study, we investigated the direct effects of exogenous C4 on astrocyte differentiation, maturation, and reactivity using human induced pluripotent stem cell (hiPSC)-derived astrocytes. Astrocytes were treated with recombinant human C4 protein either chronically during differentiation or acutely after maturation. While chronic C4 exposure did not significantly affect astrocyte differentiation, both treatment regimens induced phenotypic features of astrocyte maturation and reactivity. These included upregulation of reactive astrocyte markers (CD44 and GFAP), morphological changes consistent with mature and reactive states, and nuclear translocation of NF-κB. Mechanistically, these effects may be mediated, at least in part, by activation of the MAPK/ERK pathway and suppression of mTORC1 signaling. Together, our findings suggest that elevated C4 levels promote astrocyte maturation and reactivity, providing new insights into the mechanisms by which complement dysregulation may contribute to SCZ pathogenesis.