Increased Tumor Necrosis Factor Superfamily Members in Neuroinflammatory Schizophrenia and Bipolar Disorder Midbrains

Neuroinflammation is a key neuropathological finding in schizophrenia and bipolar disorder, as increased cytokines are found in the midbrain of these individuals. However, the most upregulated inflammatory cytokines and most activated downstream signaling pathway(s) are unidentified.

We aimed to identify the most robust transcriptional change in the schizophrenia midbrain by bulk RNA sequencing (RNA-seq) and to confirm the cellular source and magnitude of change by single-nucleus RNA-seq, reverse transcriptase-polymerase chain reaction (RT-PCR), and immunohistochemistry in 61 healthy controls, 63 schizophrenia cases, and 33 bipolar disorder cases stratified into low- and high-inflammation groups.

By RNA-seq, the TNF superfamily (TNFSF) pathway messenger RNAs (mRNAs) were among the most changed in high-inflammation schizophrenia (all ps ≤ .01), with TNFSF receptors (TNFR1, TNFR2, and FAS) being most highly expressed in astrocytes and microglia. Using RT-PCR, we confirmed that 5 TNFSF receptor mRNAs (TNFR 1, TNFR 2, DR4, FAS, and TWEAKR, all ps ≤ .01) were increased in high-inflammation schizophrenia/bipolar disorder cases compared with low-inflammation controls. Furthermore, the means for mRNA encoding cell death-related proteins acting downstream of TNF receptors (P53, CASP1, CASP7, CASP8; all ps ≤ .05) were increased in high-inflammation schizophrenia, as were mRNAs encoding proteins regulating cell survival (BCL2 and MCL1, all ps ≤ .01). All 5 TNFSF receptor mRNAs positively correlated with effector protein mRNAs (all ps ≤ .05) and with the astrocyte-related marker GFAP mRNA (all ps ≤ .001).

Our results suggest that TNFSF transcripts represent the main activated inflammatory pathway in the midbrains of people with schizophrenia, which overlaps somewhat with bipolar disorder. These findings highlight the need for anti-inflammatory interventions targeting TNF/TNFSF receptors to test for therapeutic benefits in psychiatric patients displaying elevated inflammation.