NF-κB pathway gene expression is elevated in the midbrain of people with high-inflammation schizophrenia

Elevated brain pro-inflammatory cytokines are found in the midbrain of about 50% of the people with schizophrenia, and increased transcription factor nuclear factor-kappa B (NF-κB) could be responsible. Here, we tested if NF-κB mRNA is increased in high-inflammation schizophrenia midbrain. We hypothesised that increased mRNA expression for distinct NF-κB pathway members may be found, which could help identify specific triggers of midbrain inflammation. Midbrain tissue RNA extracted from age-matched schizophrenia patients (n = 62) and non-psychiatric controls (n = 62) was analysed using high-throughput qPCR for 15 major NF-κB related transcripts: six activating receptors (IL-1 receptor 1 [IL1R1], toll-like receptor 4 [TLR4], TNF receptor 1 [TNFR1], cluster of differentiation 40 [CD40], lymphotoxin β receptor [LTβR], TNFR2), three inducing kinases (inhibitor of nuclear factor κ-B kinase subunit β [IKKβ], IKKα, NF-κB-inducing kinase [NIK]), three NF-κB subunits (1 [NF-κB1], 2 [NF-κB2], v-rel avian reticuloendotheliosis viral oncogene homolog A [RelA]), and three inhibitors (human immunodeficiency virus type I enhancer binding protein 2 [HIVEP2], NF-κB inhibitor α [IκBα], IκBβ). We found significantly increased mRNA levels for four activating receptors (CD40, IL1R1, TNFR1, TNFR2), one inducing kinase (NIK), all three NF-κB subunits (NF-κB1, NF-κB2, RelA), and one inhibitor (IκBα) in schizophrenia. When stratified by inflammation status based on cytokine transcript levels, all of these nine transcripts were significantly elevated in the high-inflammation schizophrenia group, as was LTβR. Only IL1R1 mRNA was elevated in the low-inflammation schizophrenia group compared to low-inflammation controls. To explore which cells may be responsible for increased NF-κB pathway activation in schizophrenia, we conducted snRNA-seq on midbrain tissue from a subgroup of the same cohort (14 controls, 20 schizophrenia). NF-κB-related mRNA levels from both the canonical and the non-canonical pathway were highest in seven cell clusters (astrocytes, microglia, macrophages, endothelial cells, oligodendrocytes, T cells, and neurons). Endothelial cells were the only cells with substantial IL1R1, NF-κB2, and IκBα expression, whereas TNFR1 was mostly found in astrocytes, microglia, macrophages, and T cells. Together, these findings support that increased NF-κB induces cytokine mRNA expression in schizophrenia midbrain via multiple upstream activators and cell types. The widespread increase in multiple NF-κB transcripts highlights both redundant activation mechanisms and a putative attempt to control NF-κB activation.