Abstract
Background
Inflammation is implicated in the pathogenesis of schizophrenia and bipolar disorder. Inflammation regulates neurogenesis, and markers for stem cells and neuronal progenitors are reduced in schizophrenia and bipolar disorder in the subependymal zone (SEZ) – the brain’s largest region of neurogenesis. This research aimed to discover core differences in gene expression and cellular composition in the SEZ in psychiatric disorders that may contribute to dysregulated neurogenesis.
Methods
We performed total RNA sequencing in the SEZ of 20 post-mortem schizophrenia and 21 control brains. Quantitative PCR (qPCR) and immunohistochemistry were performed in 32 schizophrenia and 32 control overlapping cases and 29 bipolar disorder cases. Immunohistochemistry was used for quantification and localisation of CD163+ macrophages. Cluster-analysis of IL6, IL6R, IL1R1 and SERPINA3 expression defined low and high inflammation subgroups, which were used to compare neurogenesis marker expression.
Results
Out of >60,000 genes, the most significantly differentially expressed gene in schizophrenia was CD163, a macrophage marker, which was increased 3.3 times compared to controls and confirmed by qPCR. Abundant CD163+ macrophages were located surrounding blood vessels, in the parenchyma and seem to infiltrate throughout the SEZ where neural stem and progenitor cells typically reside. Macrophage cell density was increased in schizophrenia compared to controls and bipolar disorder (by 29% and 61%; p = 0.017 and p = 0.002 respectively). CD163 expression positively correlated with the quiescent neural stem cell marker GFAPδ (r = 0.56, p = 0.001), and negatively correlated with neuronal progenitor marker ASCL1 (r = - 0.40, p = 0.032) in schizophrenia but not bipolar disorder. Cluster analysis of inflammatory gene expression revealed 40% of schizophrenia but only 10% of control cases were highly inflamed. The high inflammation schizophrenia subgroup had increased CD163 and GFAPδ expression but decreased ASCL1 expression (all p < 0.026).
Discussion
Increased macrophages in the SEZ is a key difference in schizophrenia pathology and potentially drives heightened inflammation in a subgroup. Inflammation has varied effects on different stages of neurogenesis in schizophrenia but not bipolar disorder, implicating divergent mechanisms leading to reduced neurogenesis in each psychiatric condition. In schizophrenia, macrophages and high inflammation seem to reduce neuronal differentiation and sustain neural stem cell quiescence, likely blunting stem cell proliferation. Therefore, reduced SEZ neurogenesis across the lifespan in schizophrenia may contribute to the widely reported inhibitory interneuron deficits.
Unbiased Stereological Method to Assess Proliferation Throughout the Subependymal Zone