Role of the novel cannabinoid receptor GPR55 in islets of Langerhans

Previous studies in our lab have reported that CB1 and CB2 cannabinoid receptors are expressed by islet -cells where they regulate insulin secretion. A novel cannabinoid receptor, GPR55, is also activated by cannabinoids, but its physiological role in islets of Langerhans has not been established: this was investigated in this thesis, which described the expression and function of this novel receptor in islets. Initial studies of this PhD project demonstrated that GPR55 is expressed at both mRNA and protein levels in isolated mouse and human islets. Pharmacological activation of GPR55 using either a pharmacological agonist (O-1602) or the putative endogenous agonist lysophosphatidylinositol (LPI) elevated intracellular Ca2+ levels ([Ca2+]i) in Fura-2-loaded mouse and human islet cells. In addition, both agonists also stimulated insulin secretion from perifused mouse and human islets at a sub-stimulatory glucose concentration (2mM) and potentiated glucose-stimulated insulin secretion. The stimulatory effects of O-1602 were abolished in islets isolated from GPR55 knockout mice, while the LPI-induced effects on [Ca2+]i and insulin secretion were maintained following GPR55 deletion. The second phase of this PhD project demonstrated that cannabidiol (CBD), which competes with agonists at GPR55 receptors and is commonly used as a GPR55 antagonist, unexpectedly increased [Ca2+]i in mouse islet cells. Stimulatory effects in response to CBD were also observed in perifusion experiments using isolated mouse and human islets, with observations of both initiation of insulin secretion at 2mM glucose and potentiation of glucose-stimulated insulin secretion. The effects of CBD on insulin secretion were also observed in experiments using islets isolated from GPR55 knockout mice. The effects of pharmacological regulation of GPR55 on islet apoptosis were also investigated. Exposure of mouse islets to O-1602 and LPI reduced basal apoptosis and these agents also promoted islet survival in the presence of cytokines, and the GPR55 ‘antagonist’ CBD also reduced apoptosis in the absence and presence of cytokines. In addition, islets from GPR55 knockout mice showed elevations in basal and cytokine-induced apoptosis compared to wildtype mouse islets. In conclusion, the studies carried out during this PhD project indicate that GPR55 is expressed by islets and its activation stimulates increases in intracellular Ca2+ and insulin secretion, and promotes islet survival. Studies using islets isolated from GPR55 knockout mice have shown that the effect of O-1602 is dependent on the presence of GPR55, but LPI and CBD exert GPR55-independent effects in islets. These results suggest that GPR55 plays an important role in the regulation of islet physiology, and that it may contribute to the peripheral regulation of energy balance and glucose homeostasis by cannabinoids.