Mice deficient for G‐protein‐coupled receptor 75 display altered presynaptic structural protein expression and disrupted fear conditioning recall

• Published in: Journal of neurochemistry 2023-06, Vol.165 (6), p.827-841
• Main Authors: Speidell, Andrew, Walton, Sofia, Campbell, Lee A., Tomassoni‐Ardori, Francesco, Tessarollo, Lino, Corbo, Claudia, Taraballi, Francesca, Mocchetti, Italo
• Format: Article
• Language: English
• Subjects: Animals; anxiety; CCL5; Chemokines; Down-regulation; Fear; Fear conditioning; GPR75; Hippocampus; Hippocampus - metabolism; Insulin; Insulin secretion; Ligands; Mental task performance; Mice; Mice, Knockout; Orphan receptors; Proteins; Proteomics; Receptors; Receptors, G-Protein-Coupled - genetics; Receptors, G-Protein-Coupled - metabolism; Synapses; Synapsin I; synapsin I and II
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1111/jnc.15818

There are a number of G‐protein‐coupled receptors (GPCRs) that are considered “orphan receptors” because the information on their known ligands is incomplete. Yet, these receptors are important targets to characterize, as the discovery of their ligands may lead to potential new therapies. GPR75 was recently deorphanized because at least two ligands appear to bind to it, the chemokine CCL5 and the eicosanoid 20‐Hydroxyeicosatetraenoic acid. Recent reports suggest that GPR75 may play a role in regulating insulin secretion and obesity. However, little is known about the function of this receptor in the brain. To study the function of GPR75, we have generated a knockout (KO) mouse model of this receptor and we evaluated the role that this receptor plays in the adult hippocampus by an array of histological, proteomic, and behavioral endpoints. Using RNAscope® technology, we identified GPR75 puncta in several Rbfox3‐/NeuN‐positive cells in the hippocampus, suggesting that this receptor has a neuronal expression. Proteomic analysis of the hippocampus in 3‐month‐old GPR75 KO animals revealed that several markers of synapses, including synapsin I and II are downregulated compared with wild type (WT). To examine the functional consequence of this down‐regulation, WT and GPR75 KO mice were tested on a hippocampal‐dependent behavioral task. Both contextual memory and anxiety‐like behaviors were significantly altered in GPR75 KO, suggesting that GPR75 plays a role in hippocampal activity. GPR75 is a recently deorphanized G‐protein‐coupled receptor that is suspected to mediate unique properties of its cognate ligand, CCL5. In the present study, we examined the neural expression of GPR75 transcripts with a special emphasis on the hippocampus. We next employed CRISPR/Cas9 to generate a novel GPR75 knockout model and examined alterations in the hippocampal proteome in the absence of GPR75. Knockout mice were found to have down‐regulation of major presynaptic neurotransmitter docking proteins versus wild‐type mice. We also report that knockout mice exhibit an impaired hippocampal‐dependent recall of a trace interval.