Role of orphan G Protein-Coupled Receptor GPRC5B in retinoic acid induced depressive-like behaviors

Abstract

<b>Abstract ID 15567</b> <b>Poster Board 533</b> Orphan G Protein Coupled Receptors (GPCRs) are GPCRs whose endogenous ligands are unknown or still debated. Due to the lack of pharmacological modulators, the physiological function of orphan GPCRs is understudied. However, relevant physiological roles associated with orphan GPCRs have been revealed by analysis of animal models and genome-wide association studies illuminating an untapped potential for drug discovery. G Protein-coupled Receptor class C group 5 member B (GPRC5B) is an orphan GPCR originally described as a retinoic acid-inducible gene. GPRC5B is ubiquitously expressed with an enrichment in the brain where it is one of the highest expressed GPCRs. We recently generated an exhaustive profile of <i>Gprc5b</i> mRNA transcripts in mouse brains, with a detailed dissection of excitatory versus inhibitory neurons in several brain regions. However, less is known about its physiological function. Independent transcriptomic studies have shown that GPRC5B is significantly altered in the post-mortem brain of patients diagnosed with depressive disorder. Moreover, GPRC5B protein levels have been found to be significantly increased in the hippocampus and hypothalamus in a depressive mouse model. Retinoic Acid (RA) is a fat-soluble derivative of vitamin A involved in vertebrate development, vision, and mood. Besides being a well-established regulator of embryonic development, RA is also involved in adult neurogenesis, a process tightly associated with the onset of depressive symptoms. Intriguingly, both preclinical studies and clinical reports indicate that chronic treatments with RA induce depression in humans and depressive-like behaviors in rodents. Based on this, we hypothesize that GPRC5B plays a role in RA-induced depressive-like phenotypes. Interestingly, our detailed <i>in situ</i> hybridization study revealed enrichment of <i>Gprc5b</i> in the dentate gyrus and other brain regions involved in adult neurogenesis. Notably, our frequency distribution analysis showed a significantly higher expression of <i>Gprc5b</i> in dentate gyrus glutamatergic granule neurons than in GABAergic interneurons. To confirm <i>Gprc5b</i> as a RA inducible gene, we used quantitative RT-PCR and found an upregulation of <i>Gprc5b</i> after RA treatments in SH-SY5Y neuroblastoma cells. Then, to investigate how RA affects GPRC5B levels in the brain, we performed chronic intraperitoneal injections of RA on adult mice. We tracked the behavioral effects of RA with a sucrose preference test and confirmed RA-induced anhedonia. Additionally, using a validated custom polyclonal antibody, we analyzed how GPRC5B protein levels are modulated by RA in several mouse brain regions. Finally, to gain further insight into physiological functions of GPRC5B, we generated <i>GPRC5B</i>^flx/flx::<i>Nestin-Cre</i> (neuronal and glial specific <i>Gprc5b</i> KO) mice. These conditional GPRC5B KO (cKO) mice will be subjected to a battery of behavioral tests to illuminate the role of GPRC5B in affective disorders including depression. Later, the same cKO mice will be tested in our model of RA-induced depression to obtain definitive evidence supporting a role for GPRC5B as a mediator of the behavioral effects of RA as suggested by our preliminary data. In the long term, we plan to expand our behavioral characterization of the different roles played by GPRC5B in the central nervous system taking advantage of the tools we developed.