Development of Chemical Probes to Investigate Adhesion GPCR Tethered Agonism

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Summary/Abstract: GPR56/ADGRG1 is an Adhesion G protein Coupled Receptor (AGPCR) that is required for brain development and function by acting as a critical regulator of the cellular pathway responsible for myelin deposition in the brain. GPR56 mutations cause a debilitating human brain malformation disease. Our recent work unveiled the general AGPCR activation mechanism. AGPCRs are two-fragment receptors that arise from a single polypeptide following a defined self-cleavage event. The physical arrangement of the two bound receptor fragments conceals a peptide sequence, that when revealed by fragment dissociation, serves as a tethered-peptide self-activator (agonist) of the GPCR-like fragment. We used information from our delineation of the AGPCR mechanism to engineer model GPR56 receptors in high and low activity states. These receptors will be used to find small molecule compounds, through screening diverse chemical libraries, that inhibit the high-activity receptors or activate the low-activity receptors. The immediate application of our work will be to develop chemical probes that bind GPR56 for use in investigative studies of physiological AGPCR activation mechanism(s). In the longer term, our chemical scaffolds may be developed as therapeutics to correct defective brain myelination in a variety of disease contexts, including myelin-deficient schizophrenia patients. GPR56 promotes nerve myelination by controlling the growth of oligodendrocyte precursor cells (OPCs). OPCs become mature oligodendrocytes, which are the cells that wrap processes around nerves in the brain to produce the protective myelin coat. Past studies in which OPCs were injected into myelin-deficient rodent brains promoted nerve re-myelination and partial restoration of brain functions. OPC injection into human brains would not be a tenable therapy at present, but use of a new GPR56 activating drug to stimulate growth of natural OPCs might be a more realistic approach. The prospect of directing re- myelination programs in damaged brains by artificially activating GPR56 is a highly appealing goal for patients afflicted with diseases including schizophrenia or multiple sclerosis. We are confident that our endeavors to develop probes for GPR56 will be successful, as our pilot screening efforts with small compound libraries (5346 total compounds from four small libraries) identified a low-affinity, first-in-class GPR56 antagonist and a partial agonist. It is the combination of our well-honed screen and counter-screen assays and our exquisitely tuned G protein reconstitution secondary assays that made our pilot work successful. We are eager to apply our established assays to the large chemical libraries and natural product extract collections at the University of Michigan Center for Chemical Genomics to fulfill the critical, unmet need for AGPCR modulatory compounds.