The orphan G protein-coupled receptor GPR17: its pharmacology and function in recombinant and primary cell expression systems

Abstract

The reconstitution of myelin sheaths, so called remyelination, represents an innovative therapeutic goal in multiple sclerosis (MS), the most common inflammatory-demyelinating disease of the central nervous system (CNS). Recently, the orphan G protein-coupled receptor (GPCR) GPR17, which is predominantly expressed in oligodendrocytes, has been identified as inhibitor of oligodendroglial differentiation, arresting oligodendrocytes in an immature, non-myelinating stage. Moreover, GPR17 expression is upregulated in human MS tissues, suggesting a key role of this receptor during the remyelination impairment that occurs in MS. However, the downstream signaling pathway connecting GPR17 to oligodendroglial maturation arrest is still poorly understood. <br /> The present work confirms that GPR17 activation by the small molecule agonist MDL29,951 results in a reduction of mature oligodendrocytes in vitro, evident by their decreased intracellular myelin basic protein (MBP) levels. The GPR17-mediated maturation block is crucially triggered by the Gαi/o pathway, which leads to an inhibition of two signaling cascades: (i) the adenylyl cyclase (AC)-cyclic adenosine monophosphate (cAMP)–protein kinase A (PKA)–cAMP response element-binding protein (CREB), and (ii) the AC-cAMP-exchange protein directly activated by cAMP (EPAC). Remarkably, these findings demonstrate for the first time an involvement of EPAC on oligodendrocyte myelination. In conclusion, GPR17 but also its downstream signaling effectors as elucidated in this study provides several drug targets to influence oligodendrocyte differentiation.<br /> Furthermore, knowledge about GPR17 pharmacology would benefit greatly from the identification of both its endogenous ligand(s) and/or selective antagonists. The activation of GPR17 by cysteinyl-leukotriene and purinergic ligands (UDP, UDP-gal, UDP-glc, LTC4, and LTD4) is ten years after its initial publication still controversially discussed. In line with other laboratories that did not validate GPR17 as dualistic receptor for both ligand families, the data of the present work obtained in (i) G protein rearrangement, (ii) β-arrestin recruitment, and (iii) ERK1/2 phosphorylation assays confirm that GPR17 must still be regarded as an orphan receptor. Additionally, the present work invalidates the P2Y12 antagonists ticagrelor and cangrelor as GPR17 inhibitors. However, investigations on two structurally related CysLT2 receptor antagonists, HAMI3379 and Cay10633, strongly suggest that their basic chemical scaffolds will serve as valuable starting point for the generation of more potent and selective GPR17 antagonists to inhibit signaling of this orphan receptor in MS.