Extracellular purine and pyrimidine nucleotides as local extrinsic regulators of adult neural progenitor cells in the diseased CNS

Purine and pyrimidine nucleotides are signaling molecules acting via ionotropic P2X and G-protein-coupled P2Y receptors (Abbracchio et al., Trends Neurosci 32:19, 2009). Upon release by damaged cells, they accumulate at injury sites and participate to lesion repair. They induce activation, proliferation, migration and differentiation of adult neural progenitor cells (aNPCs) in both neurogenic areas (eg, lateral walls of lateral ventricles) and CNS parenchyma. Parenchymal aNPCs expressing proteoglycan NG2 (NG2 cells, also known as Oligodendrocyte Precursor Cells, OPCs) have recently attracted interest for developing new purinergic-based reparative strategies, since they (i) differentiate to mature oligodendrocytes repairing demyelinating lesions; (ii) under certain conditions, also generate neurons, and (iii) express various P2 receptors, including the new P2Y-like GPR17 receptor. We have shown that GPR17 transiently labels NG2 cells, whose activation by GPR17 ligands promotes (while inhibition by antagonists or silencing RNAs impairs) differentiation to mature oligodendrocytes (Lecca et al., PLoS One 3:e3579, 2008; Fumagalli et al., J Biol Chem 286:10593, 2011). GPR17 is maximally expressed by post-mitotic OPCs that are losing NG2 and express the pre-oligodendrocyte marker O4 (Boda et al., Glia 59:1958, 2011). GPR17 is then downregulated in mature cells. Important, abnormal GPR17 upregulation is associated to defective myelination (Chen et al., Nat. Neurosci. 12:1398, 2009). Accordingly, we recently showed that GPR17 forced over-expression at late OPC differentiation stages, obtained by transfecting OPCs with a GFP-GPR17 fusion vector, impairs terminal cell maturation. Physiological GPR17 downregulation during OPC differentiation may occur through agonist-induced receptor phosphorylation via G-protein coupled receptor kinases (GRKs). Since GRKs are altered in multiple sclerosis (Vron et al., J. Immunol. 174:4400, 2005), we are studying their role in GPR17 function during OPC maturation. Initial data show that rapamycin, an inhibitor of mTOR kinase, reduces GRK2 levels, with parallel increases in GPR17 expression and strong impairment of OPC maturation. Characterization of GPR17 regulatory mechanisms is thus pivotal to find ways to foster OPC differentiation and myelin repair in demyelinating diseases.