GPR55-dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans

AimsThe novel cannabinoid receptor GPR55 is expressed by rodent islets and it has been implicated in -cell function in response to a range of ligands. This study evaluated the effects of GPR55 ligands on intracellular calcium ([Ca2+](i)) levels and insulin secretion from islets isolated from GPR55 knockout (GPR55 (-/-)) mice, age-matched wildtype (WT) mice and human pancreas. Materials and methods GPR55 expression was determined by Western blotting and fluorescent immunohistochemistry. Changes in [Ca2+](i) were measured by Fura-2 microfluorimetry. Dynamic insulin secretion was quantified by radioimmunoassay following perifusion of isolated islets. RhoA activity was monitored using a Rho binding domain pull down assay. ResultsWestern blotting indicated that MIN6 -cells, mouse and human islets express GPR55 and its localization on human -cells was demonstrated by fluorescent immunohistochemistry. The pharmacological GPR55 agonist O-1602 (10M) significantly stimulated [Ca2+](i) and insulin secretion from WT mouse islets and these stimulatory effects were abolished in islets isolated from GPR55 (-/-) mice. In contrast, while the putative endogenous GPR55 agonist lysophosphatidylinositol (LPI, 5 mu M) and the GPR55 antagonist cannabidiol (CBD, 1 mu M) also elevated [Ca2+](i) and insulin secretion, these effects were sustained in islets from GPR55 (-/-) mice. Stimulatory effects of O-1602 on [Ca2+](i) and insulin secretion were also observed in experiments using human islets, but O-1602 did not activate RhoA in MIN6 -cells. ConclusionsOur results therefore suggest that GPR55 plays an important role in the regulation of mouse and human islet physiology, but LPI and CBD exert stimulatory effects on islet function by a GPR55-independent pathway(s).