Functional Proteomics Backed by Genetics and Reverse Genetic Engineering: A Novel Successful Approach to Identify Schizophrenia Subsets

Abstract Background Disruption of proteostasis is a common cellular phenotype after a gentic or exogenous lesion of postmitotic neurons. In the most extreme examples, the neurodegenerative diseases, proteostasis disturbance leads to microscopically visible protein deposits. However, it is reasonable to assume that also in other chronic brain conditions, for example mental illnesses like residual schizophrenia or chronic depression, proteostasis occurs, even though clearly not accompanied by massive neuronal loss. The hypothesis of my laboratory is therefore to investigate the dsturbance of proteostasis in the context of chronic mental illnesses like schizphrenia, exemlified by the occurrence of protein pathology, ie. proteins insoluble in ionic detergents. Methods Post mortem brains from patients with schizophrenia, bipolar disorder, depression or healthy controls were obtained from the Stanley Research Foundation (Consortium collection; n = 60), and the insoluble proteome purified by biochemical fractionation which involves several ultracentrifugation steps, the last in cold ionic detergent. The insoluble proteome of each patient was then either immunoblotted for candidate genes or pooled by diagnosis (n = 15) and injected into mice for the generation of monoclonal antibodies that would selectively recognize only schizophrenia brains but not healthy brain (epitope discovery); epitopes of such antibodies were determined on protein arrays. Finally, we also performed proteomics of the insoluble proteome by LC-MS/MS. For positive hits, genetic studies were performed to gather independent evidence. Results For the rare candidate protein Disrupted-in-Schizophrenia 1 (DISC1), we could show insolubility in a subset of patients with mental illness crossing clinical diagnoses. When we modeled in a novel transgenic rat modestly overexpressing human full length DISC1, we observed disruption of dopamine homeostasis with amphetamine hypersensitivity, aberrant dopamine reuptake (Trossbach et al., 2016, Molecular Psychiatry), and aberrant dopamine as well as interneurron neuroanatomy, validating the functional notion of DISC1 protein pathology for chronic mental illness. Using epitope discovery we identified two candidate proteins, CRMP1 and TRIOBP1 as aggregated in subsets of cases with chronic mental illness. Systematic proteomics of the insoluble proteome of schizophrenia yielded more novel candidates like NKCC1 that are currently validated. Discussion Protein pathology is a novel way of classifying chronic mental illnesses such as schizophrenia, complementing genetic analyses. Specific signatures of insoluble proteins correspond to a disease-specific failure in proteostasis. More specifically, insoluble DISC1 assemblies seem to regulate dopamine homeostasis, a brain central metabolic disturbance during psychosis