Complement receptor C3ar1 deficiency does not alter brain structure or functional connectivity across early life...

Genetic deletion of the complement C3a anaphylatoxin chemotactic receptor (C3ar1), a key component of the innate immune response, is reported to induce behavioural phenotypes resembling anxiety and hyperactivity in mice, suggesting a neurodevelopmental role for this gene in health. However, it is not currently clear when and where C3ar1 is needed in the brain, which is further complicated by the fact that C3ar1 is expressed predominantly by microglia and therefore does not localize to specific brain regions, warranting exploratory and brain-wide assessment through neuroimaging. Resolving when and where C3ar1 is needed are questions of significant translational importance because, as a G-protein-coupled receptor, human C3AR1 serves as a potential therapeutic target for disorders associated with complement upregulation, such as schizophrenia. To provide a brain-wide assessment of developmental C3ar1 activity, we used longitudinal MRI in male and female adolescent and adult mice (N = 34 C3ar1^{tm1Cge/tm1Cge} and N = 35 C3ar1^+/+ ) to estimate regional brain volume using tensor based morphometry, white matter microstructure using fractional anisotropy from diffusion-weighted MRI, and functional connectivity from blood oxygen-level dependent MRI, with behavioural assessment in adulthood. We repeated structural MRI measures in this cohort ex vivo to achieve higher resolution. We further repeated in vivo structural assessment preceded by behavioural testing in adulthood in a second cohort of mice (N  = 20 C3ar1^{tm1Cge/tm1Cge} and N  = 19 C3ar1^+/+ ) to improve confidence in our findings. We achieved low regional brain volume variability, allowing us to resolve previously reported sexually dimorphic effects. We were further able to confirm a well-known developmental increase in fractional anisotropy. Despite being able to detect these established effects, we did not find a robust C3ar1-dependent phenotype in any of the measures we tested, including behaviour, which may be attributed to our study being the first behavioural study in C3ar1-deficient mice to include littermate controls. Therefore, our data do not support neurodevelopmental hypotheses for C3ar1, which is encouraging for therapeutic strategies targeting this receptor since interventions are unlikely to disrupt brain development.