The genetic inhibition of GPR17 affects myelination by influencing important players in oligodendrocyte maturation, including purinergic receptors

Oligodendrocyte precursor cells (OPCs) are the primary source of myelinating oligodendrocytes in the central nervous system (CNS). Both OPCs and mature oligodendrocytes express purinergic receptors, and for some of them, their expression is restricted to specific differentiation stages, suggesting key roles in OPC maturation and myelination. At early differentiation stages, OPCs also start to express GPR17, a P2Y-like G protein-coupled receptor activated by uracil-nucleotides and other non-purinergic ligands [1,2]. Although the endogenous ligands of GPR17 are still debated, in the last decade, we [3] and others [4,5] have contributed to highlight this receptor as a key actor in oligodendroglial differentiation and maturation. After reaching its maximal expression in pre-oligodendrocytes, GPR17 has to be downregulated to allow the correct expression of myelin proteins and cells' terminal maturation. Any interference in this time-regulated pattern result in myelination impairment [6]. This is also supported by the findings that in several models of disease, GPR17 is up-regulated, contributing to a blockade of OPC maturation program [3,7]. Here we aimed at identifying the genes downstream Of GPR17 to understand how the receptor influences oligodendrocyte differentiation. To this purpose, we transfected a specific GPR17 siRNA in rat OPCs during their differentiation if vitro in the absence of growth factors. In these conditions, significant reduction in the expression of the myelin oligodendrocyte glycoprotein (MOG) was found. We then analyzed the transcriptome of the cells by microarray hybridization and observed that 812 genes were significantly up- or down-regulated after GPR17 gene ablation. Results were clustered in gene ontology categories, pathways and biological processes. Among the most influenced pathways, we found the downstream signalling of the mammalian target of rapamycin (mTOR), important players in cytoskeletal rearrangements, several chemokines such as the stromal derived factor 1 (SDF1), which also act as a GPR17 ligand [8], and outward conductance potassium channels, previously described by us as effectors of GPR17 signalling [9]. Interestingly, also the expression of some purinergic receptors such as P2Y1, P2Y13, P2X2 and P2X3, was significantly altered, suggesting that different P2 receptors are recruited to complete maturation. Except for the P2Y1 receptor, which is known to contribute to ADP-mediated OPC chemotaxis in vitro [10], the functional role of the other affected purinergic receptors in oligodendrocyte physiology is currently unknown. Taken together, these data confirm that GPR17 absence profoundly affects the biological processes typical of oligodendrocyte differentiation and set the basis for a more detailed analysis of the interplay beteween distinct P2 receptors in mediating OPC maturation and myelination.