The mammalian target of rapamycin (mTOR) controls oligodendrocyte maturation by fine-tuning the activity of GPR17 receptor via G protein-coupled receptor kinases.

Background and Purpose - In the adult central nervous system (CNS), oligodendrocyte precursor cells (OPCs) dispersed throughout the parenchyma serve as a primary source of myelinating cells in demyelinated lesions. The Gi-protein-coupled receptor GPR17 is activated by both uracil nucleotides (e.g. UDP-glucose) and cysteinyl-leukotrienes (e.g. LTD4). We previously reported that GPR17 (i) is transiently expressed by early OPCs (ii) its activation by endogenous ligands promotes OPC differentiation1, and that, (iii) its forced over-expression at late differentiation stages inhibits cell maturation, suggesting that the receptor needs to be down-regulated to allow terminal OPC maturation. Physiologically, GPR17 down-regulation may occur through agonist-induced receptor phosphorylation via G-protein coupled receptor kinases (GRKs)2, and may, in turn, be controlled by the mTOR pathway, that indeed plays a pivotal role in oligodendrocyte maturation3. Here, we used primary OPC cultures to investigate the mechanisms underlying GPR17 down-regulation/desensitization. Their characterization is quite important, since alterations leading to prolonged GPR17 expression in OPCs may contribute to the limited remyelination observed in demyelinating diseases. Methods and Results - To assess whether GPR17 desensitization occurs through agonist-induced receptor phosphorylation via G-protein coupled receptor kinases (GRKs), OPCs were pre-treated with GPR17 ligands for different time periods and GPR17 ability to inhibit forskolin-induced cAMP formation was measured with a cAMP assay. Through this assay, we demonstrated a loss of GPR17 responsiveness after prolonged exposure to both UDP-glucose and LTD4. Moreover, we also showed that the same agonists induce the direct physical association of GPR17 with GRK2, which in turn phosphorylates the receptor, suggesting a role for GRK2 in GPR17 regulation. Interestingly, we also demonstrated that 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. Conclusions - Globally, these data suggest that dysregulation of the pathways controlling GPR17 desensitization leads to aberrant GPR17 overexpression, which, in turn, may prematurely block OPC differentiation at a pre-immature stage. New pharmacological or biotechnological strategies able to re-normalize GPR17 function in demyelinating diseases will help implementing the reparative potential of the OPCs that are still present in the adult CNS. References [1] M. Fumagalli, S. Daniele,D. Lecca, P.R. Lee, C. Parravicini, R.D. Fields, P. Rosa, F. Antonucci, C. Verderio, M.L. Trincavelli, P. Bramanti, C. Martini, M.P. Abbracchio, Phenotypic changes, signaling pathway, and functional correlates of GPR17-expressing neural precursor cells during oligodendrocyte differentiation. J Biol Chem. 286(12):10593-604, 2011. [2] S. Daniele, M.L. Trincavelli, P. Gabelloni, D. Lecca, P. Rosa, M.P. Abbracchio, C. Martini, Agonist-induced desensitization/resensitization of human G protein-coupled receptor 17: a functional cross-talk between purinergic and cysteinyl-leukotriene ligands. J Pharmacol Exp Ther. 338(2):559-67, 2011. [3] W.A. Tyler, N. Gangoli, P. Gokina, H.A. Kim, M. Covey, S.W. Levison, T.L. Wood, Activation of the mammalian target of rapamycin (mTOR) is essential for oligodendrocyte differentiation. J Neurosci. 29(19):6367-78, 2009.