br Concluding Remarks Given the increasing global

Concluding Remarks Given the increasing global incidence of metabolic disorders, new drugs that lower body weight and improve glucose tolerance are desperately needed. While we still have a long way to go, GPR55 might be an interesting target to explore, given its expression in numerous metabolically important tissues in humans (Figure 1). However, prudence is required in extrapolating findings from current rodent models, given the disparity in results between human and rodent weight-gain following GPR55 perturbation. A more thorough understanding of the commonalities and differences of the GPR55–LPI signalling axis between human and rodents will be vital to allow the transition of these compounds into clinical development.
Lysophosphatidylinositols as Bioactive Lipids Lysophospholipids have long been considered as simple membrane components whose only role was to alter the mechanical properties of said membranes. Indeed, lysophospholipids alter the spontaneous curvature of the membrane, hence modulating the function of membrane proteins (Box 1) [1]. However, over the years lysophospholipids have come to be regarded as ‘bioactive lipids’. Among these lysophospholipids, some, such as lysophosphatidic acid, have been extensively studied while others, such as lysophosphatidylinositols (LPIs), are much less explored [2]. LPIs consist of a glycerol backbone, an acyl chain on the sn-1 or sn-2 position and a myo-inositol head group (Box 1 and Figure 1). Interestingly, some acyl chains are more frequent at one position compared with the other. As an example, arachidonoyl LPI and stearoyl LPI are more often detected as 2-acyl and 1-acyl LPI, respectively. LPIs are found in relatively large amounts in the murine SR 59230A hydrochloride (20–40 nmol/g of tissue), with stearoyl LPI being the most abundant, followed by arachidonoyl LPI 3, 4. Nevertheless LPIs are found ubiquitously and at similar levels in the body [4].
Metabolism and Efflux of LPIs
The GPR55 Receptor Although LPIs were identified for the first time in the 1960s, and the first evidence for receptor-mediated effects of these lipids was reported in 1995 [18], it was their identification as the endogenous ligands of the G protein-coupled receptor GPR55 in 2007 that sparked a renewed interest in the study of these lipids [19]. However, this receptor was initially reported as a putative receptor for endocannabinoids, another family of bioactive lipids [20]. Indeed, GPR55 was first shown to respond to some cannabinoid ligands, some more consistently than others. Therefore many of the reported ligands are cannabinoid-related, which raises issues in terms of selectivity when interpreting their effects as GPR55-mediated (see Table 1 for a summary of the studied ligands and Table S1 in the online supplementary material for a more comprehensive list of tested compounds). However, LPIs, and especially arachidonoyl LPI, are clearly agonists of GPR55 3, 21, 22. Indeed, several LPI species were found to induce dose-dependent ERK phosphorylation in GPR55-expressing HEK293 cells [19]. LPIs also induce rapid transient increase of intracellular Ca2+ in the same cells and stimulate [35S]GTPɣS binding. Depending on the fatty acid, the potencies of LPIs for ERK phosphorylation were different, with 2-arachidonoyl LPI showing the highest level of activity and a reported EC50 of 30nM, and 1-palmitoyl LPI being a weak partial agonist [3]. Accordingly, the International Union of Basic and Clinical Pharmacology lists LPIs as the endogenous agonists of GPR55 but this receptor still retains its orphan status due to the complex reported pharmacology and insufficient in vivo data. To date, there is no specific radioligand for GPR55 and no routine binding assay described. Therefore, most of the available data was obtained through functional assays in transfected cells monitoring intracellular calcium fluxes, ERK phosphorylation, β-arrestin recruitment, or receptor internalization in transfected cells (see Box 2 for an overview of the signaling pathways). Moreover, while some studies compared the effects of LPIs with the different cannabinoid ligands in the same assay and validated their results by using untransfected cells, this is not the case for all the studies describing agonist or antagonist ligands of GPR55.