Complement System Dynamics in Schizophrenia Pathogenesis

The complement cascade, a cornerstone of innate immunity, has been increasingly recognised for its non-canonical role in sculpting neural circuits through synaptic pruning. Central to this process is complement component 4A (C4A), whose upregulated expression enhances microglial removal of synapses in cortical regions critical for cognition and social function. Genetic variants that elevate C4A levels tip the balance towards excessive synaptic elimination, disrupting network connectivity and predisposing individuals to psychosis. Peripheral complement proteins mirror central immune activity and offer promise as early biomarkers. Converging evidence from animal models, neuroimaging and longitudinal cohort studies positions dysregulated complement dynamics as a mechanistic bridge between genetic risk and the emergence of schizophrenia, thereby opening avenues for targeted therapeutic intervention and risk stratification.

Research from Nature Portfolio

In a humanised mouse model, selective overexpression of C4A—but not its paralogue C4B—was shown to precipitate dramatic synaptic loss in the prefrontal cortex. Heightened C4A activity drove microglial engulfment of synapses, leading to reduced cortical synapse density and behavioural alterations paralleling schizophrenia endophenotypes. These findings establish that C4A-mediated overpruning is sufficient to induce circuit dysfunction and behavioural deficits, underscoring its causal role in disease-relevant neurodevelopmental processes.

Research from all publishers

A multi-ancestry phenome-wide association study in pre-adolescent children demonstrated that genetically regulated expression of C4A correlates with reduced surface area of the entorhinal cortex. Importantly, lower entorhinal cortex metrics at age 9–10 predicted increased number and severity of psychosis-like experiences over two years, highlighting a potential neurodevelopmental biomarker of schizophrenia risk.

In individuals at clinical high risk for psychosis, longitudinal assessments revealed that baseline serum concentrations of complement proteins (notably C3, C5 and their activation fragments) were more predictive of conversion to psychosis than traditional inflammatory cytokines. Converters exhibited significantly lower complement levels at enrolment, implicating systemic complement dysregulation in prodromal stages and suggesting a window for early intervention.

Complement System Dynamics in Schizophrenia Pathogenesis publication trend

The graph below shows the total number of articles in complement system dynamics in schizophrenia pathogenesis across all publications each year (not limited to Nature Index journals).

Technical terms

Complement system: A sequence of plasma proteins that mark pathogens or synapses for removal by immune cells.

Complement component 4A (C4A): A classical pathway protein involved in tagging synapses for microglial clearance, linked to schizophrenia risk.

Synaptic pruning: The elimination of surplus synaptic connections during development to refine neural networks.

Microglial engulfment: The process by which brain-resident immune cells ingest and remove tagged synapses.

Entorhinal cortex: A medial temporal lobe structure essential for memory and spatial navigation, vulnerable in early psychosis.

Clinical high risk (CHR): A prodromal state characterised by attenuated psychotic symptoms and elevated likelihood of developing full-blown psychosis.

Genetically regulated expression (GREx): An inferred measure of gene expression derived from individual genetic variation profiles.

References

1. Overexpression of schizophrenia susceptibility factor human complement C4A promotes excessive synaptic loss and behavioral changes in mice. Nature Neuroscience (2020).
2. Multi-ancestry phenome-wide association of complement component 4 variation with psychiatric and brain phenotypes in youth. Genome Biology (2023).
3. Serum complement proteins rather than inflammatory factors is effective in predicting psychosis in individuals at clinical high risk. Translational Psychiatry (2023).