Differential Activation of Cultured Neonatal Cardiomyocytes by Plasmalemmal Versus Intracellular G Protein-coupled Receptor 55

The L-α-lysophosphatidylinositol (LPI)-sensitive receptor GPR55 is coupled to Ca2+ signaling. Low levels of GPR55 expression in the heart have been reported. Similar to other G protein-coupled receptors involved in cardiac function, GPR55 may be expressed both at the sarcolemma and intracellularly. Thus, to explore the role of GPR55 in cardiomyocytes, we used calcium and voltage imaging and extracellular administration or intracellular microinjection of GPR55 ligands. We provide the first evidence that, in cultured neonatal ventricular myocytes, LPI triggers distinct signaling pathways via GPR55, depending on receptor localization. GPR55 activation at the sarcolemma elicits, on one hand, Ca2+ entry via L-type Ca2+ channels and, on the other, inositol 1,4,5-trisphosphate-dependent Ca2+ release. The latter signal is further amplified by Ca2+-induced Ca2+ release via ryanodine receptors. Conversely, activation of GPR55 at the membrane of intracellular organelles promotes Ca2+ release from acidic-like Ca2+ stores via the endolysosomal NAADP-sensitive two-pore channels. This response is similarly enhanced by Ca2+-induced Ca2+ release via ryanodine receptors. Extracellularly applied LPI produces Ca2+-independent membrane depolarization, whereas the Ca2+ signal induced by intracellular microinjection of LPI converges to hyperpolarization of the sarcolemma. Collectively, our findings point to GPR55 as a novel G protein-coupled receptor regulating cardiac function at two cellular sites. This work may serve as a platform for future studies exploring the potential of GPR55 as a therapeutic target in cardiac disorders.