PROJECT SUMMARY Drug discovery pipelines for neuropsychiatric disorders are dry. One approach to rejuvenating these pipelines would be to create assays based on relevant disease phenotypes in primary neurons, something that is currently lacking. However, a scalable assay development platform that is based on bona fide neurons, remains cost effective, and that can support industrial level high-throughput screening (HTS) does not currently exist. Over the past eight years (spread across different NIH-sponsored grants), our collaborative group has created a flexible and scalable primary neuron-based assay development system that is compatible with industrial-level HTS. Our long-standing goal for this project has been to optimize these procedures and workflows to support neuron-based HTS phenotypic assays so that they can support very large screening campaigns of up to 200K compounds. We are happy to report that progress over the last budget period has pushed us closer toward this stated goal. We have invented a state-of-the-art, disease-modeling assay created in primary neurons that is designed to discover compounds that reverse the cellular consequences of genetic haploinsufficiency. Indeed, a substantial proportion of childhood brain disorders are caused by single autosomal dominant variants resulting in genetic haploinsufficiency. The rare genetic brain disorders that arise from these variants offer great potential for translation because the disease mechanism is well-understood (i.e. low protein expression). Therefore, a rationale precision therapy for treating genetic haploinsufficiency disorders would be to discover “magic bullet” compounds that raise expression of functional proteins from the remaining undamaged allele (e.g. “boosting compounds”). In this renewal project, we will employ technical innovations that have unlocked the scalability of primary neurons for phenotypic HTS. As a proof-of-principle, we will scale-up and implement an assay that reports reversal of low SynGAP expression in neurons caused by genetic haploinsufficiency of the SYNGAP1/Syngap1 gene. We will miniaturize an HTS-compatible and disease- modeling steady-state endogenous SynGAP expression assay so that it is compatible with industrial scale HTS automation. Once implemented, we will then screen up to 200,000 unique substances using a completely automated version of the neuron-based SynGAP expression assay. Finally, using a comprehensive multi-stage biological validation funnel, we will identify and prioritize the most translatable chemical probes that raise SynGAP protein expression. The overall impact of this project is that discovery of multiple, validated SynGAP boosting compounds would provide proof-of-principle that our flexible platform is an effective tool for phenotypic drug discovery for nervous system disorders.