The Zinc-sensing Receptor (GPR39) Modulates Declarative Memory and Age-related Hippocampal Gene Expression in Male Mice

The pathophysiological functions and therapeutic potential of GPR39: Focus on agonists and antagonists

G protein-coupled receptor 39 (GPR39), a member of the growth hormone-releasing peptide family, exhibits widespread expression across various tissues and demonstrates high constitutive activity, primarily activated by zinc ions. It plays critical roles in cell proliferation, differentiation, survival, apoptosis, and ion transport through the recruitment of Gq/11, Gs, G12/13, and β-arrestin proteins. GPR39 is involved in anti-inflammatory and antioxidant responses, highlighting its diverse pathophysiological functions. Recent discoveries of endogenous ligands have enhanced our understanding of GPR39's physiological roles. Aberrant expression and reactivation of GPR39 have been implicated in a range of diseases, particularly central nervous system disorders, endocrine disruptions, cardiovascular diseases, cancers, and liver conditions. These findings position GPR39 as a promising therapeutic target, with the efficacy of synthetic ligands validated in various in vivo models. Nonetheless, their clinical applicability remains uncertain, necessitating further exploration of novel agonists-especially biased agonists-and antagonists. This review examines the unique residues contributing to the high constitutive activity of GPR39, its endogenous and synthetic ligands, and its pathophysiological implications, aiming to elucidate its pharmacological potential for clinical application in disease treatment.

Loss of the zinc receptor ZnR/GPR39 in mice enhances anxiety-related behavior and motor deficits, and modulates KCC2 expression in the amygdala

BACKGROUND

Mood disorders, particularly depression and anxiety, are associated with zinc dyshomeostasis and aberrant GABAergic signaling. Activation of ZnR/GPR39 by synaptic zinc in the hippocampus triggers phosphorylation of extracellular regulated kinase (ERK1/2), which regulates the K+/Cl- cotransporter (KCC2) and thereby GABAergic inhibitory neurotransmission and seizure activity. Therefore, we studied whether impaired ZnR/GPR39 signaling is linked to anxiety-related behavior in male or female mice.

RESULTS

Using the acoustic startle response, elevated plus maze, and open field test, we found increased anxiety-related behavior in ZnR/GPR39 knockout (KO) mice. Despite a well-established sex difference, where females are typically more prone to anxiety, both male and female ZnR/GPR39 KO mice exhibited increased anxiety-related behavior compared to wildtype (WT) mice. Additionally, ZnR/GPR39 KO mice displayed impaired motor coordination in the pole and rotarod tests but did not show reduced muscle strength, as indicated by a grip test. Finally, we found intrinsic alterations in the expression level of KCC2, a major Cl- transporter regulating GABAergic signaling, in the amygdala of naïve ZnR/GPR39 KO mice compared to controls.

CONCLUSIONS

Our findings indicate that loss of ZnR/GPR39 enhances anxiety-related behavior in both male and female mice. Moreover, ZnR/GPR39 KO mice exhibit impaired motor coordination, which may be associated with increased anxiety. Finally, we demonstrate that loss of ZnR/GPR39 modulates the expression of KCC2 in the amygdala. Thus, we propose that ZnR/GPR39 can serve as a target for regulating GABAergic signaling in anxiety treatment.

GPR39: An orphan receptor begging for ligands

Progress in the understanding of the receptor GPR39 is held up by inconsistent pharmacological data. First, the endogenous ligand(s) remain(s) contentious. Data pointing to zinc ions (Zn2+) and/or eicosanoids as endogenous ligands are a matter of debate. Second, there are uncertainties in the specificity of the widely used synthetic ligand (agonist) TC-G 1008. Third, activation of GPR39 has been often proposed as a novel treatment strategy, but new data also support that inhibition might be beneficial in certain disease contexts. Constitutive activity/promiscuous signaling suggests the need for antagonists/inverse agonists in addition to (biased) agonists. Here, we scrutinize data on the signaling and functions of GPR39 and critically assess factors that might have contributed to divergent outcomes and interpretations of investigations on this important receptor.

Chronic memantine disrupts spatial memory and up-regulates Htr1a gene expression in the hippocampus of GPR39 (zinc-sensing receptor) KO male mice

GPR39 is a receptor involved in zincergic neurotransmission, and its role in regulating psychological functions is an active area of research. The purported roles of GPR39 at the cellular level include regulation of inflammatory and oxidative stress response, and modulation of GABAergic and endocannabinoid neurotransmission. GPR39 knock-out (KO) mice exhibit episodic-like and spatial memory (ELM and SM, respectively) deficits throughout their lifetime, and are similar in that respect to senescent wild-type (WT) conspecifics. Since a role for zinc has been postulated in neurodegenerative disorders, in this study we investigated the possibility of a pharmacological rescue of both types of declarative memory with memantine - a noncompetitive NMDAR antagonist used for slowing down dementia; or, a putative GPR39 agonist - TC-G 1008. First, we tested adult WT and GPR39KO male mice under acute 5 mg/kg memantine or vehicle treatment in an object recognition task designed to simultaneously probe the "what?", "where?" and "when?" components of ELM. Next, we investigated the impact of chronic memantine or TC-G 1008 on ELM and SM (Morris water maze, MWM) in both WT and GPR39KO mice. Following chronic experiments, we assessed with qRT-PCR hippocampal gene expression of targets previously associated with GPR39. We report: no effects of acute memantine on ELM; a tendency to improve the "where?" component of ELM in both WT and GPR39 KO mice following 12 days of memantine; and, a disruption of SM in GPR39KO mice after 24 days of memantine treatment. The latter result was associated with upregulation of Htr1a hippocampal expression.

Signaling pathway mechanisms of neurological diseases induced by G protein-coupled receptor 39

BACKGROUND

G protein-coupled receptor 39 (GPR39) is a transmembrane zinc receptor with two splice variants, which belongs to the G-protein-coupled receptor growth hormone-releasing peptide family. Its expression is induced by zinc, which activates GPR39, and its activation mediates cell proliferation, ion homeostasis, and anti-inflammatory, antioxidant, and other pathophysiological effects via different signaling pathways.

AIMS

The article reviews the latest literature in this field. In particular, the role of GPR39 in nervous system is discussed.

MATERIALS AND METHODS

GPR39 can be a promising target in neurological diseases for targeted therapy, which will help doctors overcome the associated problems.

DISCUSSION

GPR39 is expressed in vivo at several sites. Increasing evidence suggests that GPR39 plays an important role as a neuroprotective agent in vivo and regulates various neurological functions, including neurodegeneration, neuroelectrophysiology, and neurovascular homeostasis.

CONCLUSION

This review aims to provide an overview of the functions, signal transduction pathways, and pathophysiological role of GPR39 in neurological diseases and summarize the GPR39 agonists that have been identified in the recent years.