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Recent evidence shows that activities of multiple pathways are altered in the etiology of schizophrenia, caused by multiple gene defects that most likely occur during neurodevelopment. Therefore, schizophrenia may also be termed a ‘signalling disease’. Thus, in addition to signals that control activities of pharmacologically relevant receptors (e.g., the dopamine D2 receptor, crucial for treating positive symptoms), intracellular signaling cascades play an important role by coordinating the maturation of neurons and concomitant synaptic plasticity. For example, signaling pathways that send signals from the synapse to the nucleus (e.g. mediated via calcium or MAP kinase signaling) were identified, causing changes of transcriptional control mechanisms. Therefore, a better understanding of how individual risk genes and signalling activities contribute to the etiology of the disease is thought to accelerate the development of drugs.


Genetically encoded sensors to monitor regulated and druggable protein-protein interactions

In previous studies, we established genetically encoded sensors to monitor dynamic protein-protein interaction events and to screen for pharmacologically active substances or genetic modulators of signalling pathways (Wehr et al., 2006 & 2016). These assays utilize the complementation-based protein-protein interaction technique split TEV, which has also been used to monitor activities of receptor tyrosine kinases, such as the ERBB family, as well as G protein coupled receptors (Wehr et al., 2008 & 2015). In addition, the split TEV technique was applied to identify modulators of Hippo signalling, a ubiquitously expressed pathway governing cell fate and cell polarity, using a genome-wide RNAi screening approach in Drosophila (Wehr et al., 2013).

Abb.  1. Applications of the split TEV system in drug development. The split TEV system has been used for a variety of applications, including the monitoring of various protein-protein interactions (PPI), high-throughput (HTP) screening, dose-response analyses, and target selectivity studies. The lower inset illustrates the use of the split TEV system in multiparametric assays that use RNA barcode reporters and Next-Generation Sequencing (NGS) as final readout. R, receptor; bc, molecular barcode. (From Wehr and Rossner, 2016)

Drug Repurposing screening in the context of psychiatric disorders

Increased levels of Neuregulin1 (NRG1) ‐ERBB4 signalling are associated with schizophrenia and corresponding mouse models display endophenotypes of the disease. Recently, we performed a drug repositioning screen using a cell-based split TEV assay to screen for modulators of the Nrg1-ERBB4 signalling pathway (Wehr et al., 2017). Spironolactone was recovered from the drug repositioning screen to inhibit NRG1‐ERBB4 signalling and improved schizophrenia‐relevant phenotypes in NRG1‐transgenic mice. Based on these pre-clinical results, a clinical trial for spironolactone was initiated by colleagues in-house using an add-on medication paradigm (pers. communication P. Falkai, A. Hasan; Together, these results indicate that cell-based assays and repositioning screening approaches provide a short-cut of standard drug discovery, providing promising therapeutic options for psychiatric diseases.

Abb.  2. Spironolactone antagonises NRG1-ERBB4 signalling and ameliorates schizophrenia-relevant endophenotypes in a NRG1 mause model. Using a split TEV-based co-culture assay that monitors ERBB4 receptor activity (measured by the NRG1 ligand-dependent association between activated ERBB4 and its adapter PIK3R1, inset top left), the compound library of the NIH Clinical Collection containing 729 approved drugs was screened for modulators that decrease ERBB4 activities. Spironolacton was recoverd as top candidate and validated using orthogonal assays (e.g. biochemical and electrophysiological validation, behavioural profiling, Inset top right). Mechanistically, it is believed that NRG1 mice treated with spironolactone show an improvement in the excitation/inhibition balance.