GPR55: Current knowledge and future perspectives of a purported "Type-3" cannabinoid receptor

In the last decade, accumulated evidence highlighted that GPR55 might be activated by several classical cannabinoid ligands, making this orphan receptor the main candidate to be considered as the "third" cannabinoid receptor. The investigation of its pharmacology has often provided divergent and more intricate results that have complicated the understanding of the physiological role of GPR55. Nevertheless, the patent analysis regarding GPR55 outlines the fair interest of big pharmaceutical companies, especially in the first years of this decade. This investigation provides a brief overview of the current "state of the art" of our knowledge of GPR55, giving particular emphasis to its functional selectivity. This property could account for controversial roles of GPR55, whose pharmacology and downstream signaling is known to vary significantly both in ligand- and system-dependent manners. In addition, we gain insights into the challenging aspect of finding out novel GPR55 modulators, by analyzing conserved structural and functional motifs that, together with future studies, could help to elucidate its mechanism of action and to design more selective and potent small-molecules directed towards GPR55. Preliminary data highlight remarkable differences, but also intriguing commonalities, between GPR55 and other members of class A G protein-coupled receptors. It is anticipated that, in the next future, novel lead candidates targeting GPR55 could represent new tools to better understand GPR55-mediated human diseases and, hopefully, generate an innovative class of effective next-generation therapeutics.