GPR37 is exceptional among GPCRs having a high propensity for intracellular receptor accumulation and aggregation leading to neurotoxicity (see Figure). However, unexpectedly, our results suggest that GPR37 is neuroprotective in dopaminergic when located at the plasma membrane. Prosaposin has been claimed to be a agonist at GPR37 and GPR37L1. The programme therefore aimed at generating mechanistic insight into the role of prosaposin, GPR37 and GPR37L1 as novel diagnostics and targets for the development of neuroprotective pharmacological therapies against synucleopathies, particularly PD. During this first period of the programme, I spent time summarizing the literature and posing key research questions related to the topic. This review has been published in Trends of Pharmacological Sciences. The research has been conducted around four specific objectives related to prosaposin, GPR37 and GPR37L1. The first objective studies the structural basis for the high propensity of GPR37 to misfold and aggregate. In collaboration, in silico models of GPR37 (see figure) and GPR37L1 have been made. These models have contributed to identify residues relevant for function and ligand binding. In particular, we found that GPR37 contains an amino acid in a TM region which is uncommon among class A GPCR’s. We have mutated this amino acid residue and expressed mutated constructs in cell lines and found that it governs GPR37 trafficking to the cell surface. Using these homology models of GPR37 and GPR37L1, we have also perform a molecular docking experiments with a large chemical library and identified compounds which we are now evaluating in cellular assays. In the second objective we investigated GPR37 and GPR37L1 dimerization and multimerization in live cells using three quantitative imaging methods: Fluorescence Cross-Correlation Spectroscopy (FCCS), Förster Resonance Energy Transfer, and Fluorescence Lifetime Imaging Microscopy. Our data show that GPR37 and GPR37L1 form homo- and heterodimers in live cells. Importantly, aggregation of GPR37, but not GPR37L1, was identified in the cytoplasm, which could be counteracted by Parkin overexpression. These data provide further evidence that GPR37 participate in cytosolic aggregation processes implicated in PD pathology. By using FCCS, we have found that GPR37 and GPR37L1 differentially interacts with the two splice forms of dopamine D2 receptors. The third objective concerns animal models with altered levels of prosaposin, GPR37 and GPR37L1. GPR37 KO mice have a dopamine neuron deficit, enhanced striatal GABA levels and deficient corticostriatal LTP. They also respond stronger to 6-OHDA-induced neurotoxicity. The data indicates that properly functional GPR37 may counteract aging processes and parkinsonism. We have generated conditional prosaposin, GPR37 KO and GPR37L1 KO mice and crossed them with DAT-CreER mice. Deficits in locomotion and emotionality are seen in prosaposin KO mice. These deficits are reversed by viral constructs overexpressing prosaposin. In fact, such viral constructs also counteract toxicity by AAV-alpha-synuclein overexpression. In a fourth aim, we proposed to use prosaposin/GPR37 as targets for biomarker development. We have reported that N-terminal fragments of GPR37 and prosaposin can aid as diagnostic cerebrospinal fluid biomarkers in PD. Several scientific publications have been reported and there will be additional publications. The data has been presented at several international conferences.
