Increased expression of schizophrenia-associated gene C4 leads to hypoconnectivity of prefrontal cortex and reduced social interaction

Abstract

Schizophrenia is a severe mental disorder with an unclear pathophysiology. Increased expression of the immune gene C4 has been linked to a greater risk of developing schizophrenia; however, it is unknown whether C4 plays a causative role in this brain disorder. Using confocal imaging and whole-cell electrophysiology, we demonstrate that overexpression of C4 in mouse prefrontal cortex neurons leads to perturbations in dendritic spine development and hypoconnectivity, which mirror neuropathologies found in schizophrenia. We find evidence that microglia-neuron interactions and microglia-mediated synaptic engulfment are enhanced with increased expression of C4. We also show that C4-dependent circuit dysfunction in the frontal cortex leads to decreased social interactions in juvenile mice. These results demonstrate that increased expression of the schizophrenia-associated gene C4 causes aberrant circuit wiring in the developing prefrontal cortex and leads to deficits in early social behavior, suggesting that altered C4 expression contributes directly to schizophrenia pathogenesis.

Systematic comparative analysis of Mendelian randomization methods for inferring causal genes of complex phenotypes and the application to psychiatric diseases

“…Figure 7a visualizes relationship of the association signals in a scatter plot. The estimated causal effects 0.156±0.0145 (p=4.45E-27, by ML-based MR) suggested its over expression increased the risk of schizophrenia, which was consistent with a very recent finding 28 . Among the 8 non-MHC genes, the most significant one was GATAD2A, p =9.18E-24 by ML-based MR method.…”

“…Eleven genes, including 4 MHC genes (C4A, HLA-B, HLA-C, and HLA-DQA1) and 7 non-MHC genes (NT5C2, FURIN, GLT8D1, NOTCH4, GATAD2A, NEK4, and MAPK3), had over 5 hit papers. The MHC gene C4A was prioritized as the top gene by both MR methods according to the p-values, which is also a well-known causal gene of schizophrenia 28 . Figure 7a visualizes relationship of the association signals in a scatter plot.…”

Complement System in Brain Architecture and Neurodevelopmental Disorders

“…In support of the first possibility are, as abovementioned, recent findings reporting multiple emerging novel noninflammatory roles of complement in every stage of brain development (Coulthard et al, 2018b), as well as data from genetic studies showing an association between risk variants in complement genes and neurodevelopmental disorders (Warren et al, 1991;Odell et al, 2005;Sekar et al, 2016). In addition, there are studies suggesting that the neuropathological and behavioral phenotypes in genetically modified mouse models with aberrant complement expression in the brain parallel known features of these human developmental brain disorders (Chu et al, 2010;Perez-Alcazar et al, 2014;Sekar et al, 2016;Comer et al, 2019).…”

“…In addition, a study using cortical wild type neurons co-cultured with astrocytes derived from IkBα knockout mice, which overexpress the transcription factor NFkB and consequently complement C3, has shown that C3 released by astrocytes acts through neuronal C3aR reducing excitatory synaptic density and dendritic length and complexity (Lian et al, 2015). Moreover, it was shown that overexpression of C4 in mouse prefrontal cortex leads to dendritic spine dysgenesis, reduced connectivity in cortical neurons, enhanced microglia-mediated engulfment of synapses and deficits in social interactions (Comer et al, 2019; Table 1).…”

“…Whereas the abnormal complement expression in the peripheral blood of both patients with schizophrenia and their mothers suggests that complement dysfunction as part of standard immune pathways may contribute to schizophrenia in a subset of patients, findings from several of the abovementioned studies also suggest that the clinical and neuropathological phenotypes of schizophrenia may also be, at least in part, due to dysfunction of locally synthesized complement in the brain during specific periods of neural development. The strong association between increased C4A expression with schizophrenia (Sekar et al, 2016), the involvement of classical complement cascade in synapse elimination (Stevens et al, 2007;Schafer et al, 2012;Bialas and Stevens, 2013;Sekar et al, 2016;Comer et al, 2019), and the decreased brain connectivity and sociability in mice overexpressing C4 in the prefrontal cortex (Comer et al, 2019) strongly suggest that enhanced complementmediated synaptic pruning contributes directly to reduction in cortical gray matter thickness and to schizophrenia pathogenesis. On the other hand, while it is highly attractive to speculate that complement-mediated dysregulation in neuronal migration may contribute to the pathogenesis of schizophrenia, further detailed studies are still required to directly establish a causal link.…”