TIME-DEPENDENT AND CRITICAL ROLE OF THE GPR17 RECEPTOR DURING OLIGODENDROCYTE DIFFERENTIATION: IMPLICATIONS FOR THE DEVELOPMENT OF NEW REMYELINATION STRATEGIES

In the adult central nervous system (CNS), neural progenitor cells expressing the proteoglycan NG2 (NG2 cells, also known as Oligodendrocyte Precursor Cells, OPCs) dispersed throughout the parenchyma serve as a primary source of myelinating cells in demyelinated lesions, such as multiple sclerosis (MS). The Gi-protein-coupled receptor GPR17, activated by both uracil nucleotides (e.g. UDP-glucose) and cysteinyl-leukotrienes (e.g. LTD4) (Ciana et al., 2006), has recently emerged as an important player in oligodendrogliogenesis (Lecca et al., 2008; Chen et al., 2009). In both brain and spinal cord, GPR17 was found on parenchymal NG2+ cells in transition from precursors to premyelinating phenotypes, whereas it is not present on mature myelinating oligodendrocytes. Previous data in cultured OPCs showed that, at early differentiation stages, GPR17 activation by endogenous ligands promotes (while inhibition by antagonists or silencing RNAs impairs) OPC differentiation (Fumagalli et al., 2011). Altogether, these data point at GPR17 as a key functional modulator of oligodendrocyte maturation. Using primary OPC cultures and OPC/DRG co-cultures, we demonstrated that: (i) GPR17 activation promotes OPC differentiation (UDP, LTD4 and LTE4) and myelination (UDP-glucose and UDP); (ii) GPR17 forced over-expression at late differentiation stages, obtained by transfecting cultured OPCs with a GFP-GPR17 fusion vector, impairs cell terminal maturation, suggesting that the receptor has a stage specific function in controlling OPC differentiation and that it needs to be down-regulated/desensitized in late immature oligodendrocytes to allow their terminal maturation; (iii) GPR17 down-regulation occurs through agonist-induced receptor phosphorylation via GRK2. Interestingly, we also demonstrated that the inhibition of the mTOR pathway by rapamycin determines a significant reduction of GRK2 levels, with parallel increases in GPR17 expression and strong impairment of OPC maturation. Globally, these data suggest that dysregulation of these pathways leading to aberrant GPR17 over-expression may prematurely block OPC maturation at a pre-immature stage. Altogether these experiments shed light on the mechanisms underlying GPR17 physiological regulation that can be exploited for the development of new pharmacological and biotechnological strategies to promote/implement the reparative potential of the OPC precursors, that are still present in the adult brain.