The role of GPR55 in neuronal and immune cell signalling and function

Abstract

The orphan G-protein coupled receptor, GPR55, is widely expressed throughout the body and is responsive to cannabinoids. However, its endogenous ligand is believed to be L-a-lysophosphatidylinositol (LPI). GPR55 is expressed in the neurons and glia of the brain and therefore has been suggested to have a regulatory role in central immune processes. This makes research into GPR55 as a potential therapeutic target attractive, particularly in relation to neuropathological conditions where inflammation is a characteristic feature. Alzheimer's disease (AD) for instance is a neurodegenerative disease associated with neuroinflammation, neuronal loss and cognitive decline. The present study aimed to examine the role of GPR55 in central signalling and immune processes using a rat cortical neuron-enriched culture model, the BV2 microglial cell line and human peripheral immune cells. Cells obtained from neonatal cortical neuron-enriched cultures were immunolabelled for GPR55. They were treated with the suggested endogenous agonist for GPR55, LPI (10 uM), or the novel and selective GPR55 agonist 17g (1 uM). The cells were pre-treated with the selective antagonist, CID16020046 (20 uM), to determine selectivity of the agonists for GPR55. LPI- and 17g-induced signalling effects were assessed by examining intracellular calcium ([Ca2+]i) responses and phospho-cAMP element binding protein (pCREB) activation. Neuro-immune processes were assessed by examining BV2 microglial migration, TNF-a cytokine production, phagocytosis and the stimulation of active caspase-3 in response to A-beta. Peripheral immune cells were examined for changes in GPR55 expression levels over the course of monocyte differentiation into monocyte-derived dendritic cells. It was found that GPR55 was expressed in microglia and neurons obtained from cortical neuron-enriched cultures. Both LPI and 17g modulated [Ca2+]i activity in neurons and glia. In certain neuronal populations, 17g increased spontaneous Ca2+ activity in a GPR55-dependent manner. LPI (10 uM) diminished the frequency of spontaneous [Ca2+]i events in some neuronal populations and potentiated events in other populations independently of GPR55. Both LPI and 17g significantly increased the stimulation of CREB phosphorylation. Only 17g-induced CREB phosphorylation was inhibited by CID16020046. LPI and CID16020046 attenuated BV2 microglial migration in response to A-beta-primed neuronal medium. LPI and 17g protected against A-beta-evoked active caspase-3 induction. GPR55 expression became upregulated over the course of monocyte differentiation into monocyte-derived dendritic cells. This study suggests that GPR55 plays a possible role in the regulation of [Ca2+]i activity, gene expression, immune cell migration and neuronal apoptosis. This implicates GPR55 in having a regulatory role in processes relevant to pathological conditions such as AD, making it an attractive therapeutic target for intervention in future.