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

250 words Background Neuroinflammation is a key neuropathological finding in schizophrenia and bipolar disorder, as increased cytokines are found in the midbrain of these individuals. Yet, the most upregulated inflammatory cytokine and most activated downstream signalling pathway(s) are unidentified. Methods We aimed to identify the most robust transcriptional change in the schizophrenia midbrain by bulk RNA-seq, and to confirm the cellular source and magnitude of change by single-nuclei RNA-seq, RT-PCR, and immunohistochemistry in 61 healthy controls, 63 schizophrenia cases, and 33 bipolar disorder cases stratified into low and high-inflammation. Results By RNA seq, the TNF superfamily (TNFSF) pathway mRNAs were among the most changed in high-inflammation schizophrenia (all p ≤0.01), with TNFSF receptors ( TNFR1 , TNFR2 and FAS ) being most highly expressed in astrocytes and microglia. By RT-PCR, we confirmed that five TNFSF receptor mRNAs ( TNFR1/2 , DR4 , FAS and TWEAKR , all p ≤0.01) were increased in high-inflammation schizophrenia/bipolar disorder compared to low-inflammation controls. Further, mRNA encoding cell death-related proteins acting downstream of TNFRs ( P53, CASP1, 7, 8, all p ≤0.05) were increased in high-inflammation schizophrenia, as were mRNAs encoding proteins regulating cell survival ( BCL2 and MCL1, all p ≤0.01). All five TNFSF receptor mRNAs positively correlated with effector protein mRNAs (all p ≤0.05) and with the astrocyte-related marker GFAP mRNA (all p ≤0.001). Conclusions Our results suggest TNFSF transcripts represent the main activated inflammatory pathway in the midbrains of people with schizophrenia, which overlaps 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.