SNX27 REGULATES GPR17 RECYCLING AND THE KINETICS OF OLIGODENDROCYTE DIFFERENTIATION: IMPLICATION IN DOWN SYNDROME

Oligodendrocytes (OLs) are specialized glial cells of the central nervous system (CNS) responsible of myelin formation and trophic support of neurons. The understanding of mechanisms regulating OL differentiation may have fundamental implications in the development of therapeutic strategies for demyelinating and neurological diseases. In this respect, the G protein-coupled receptor 17 (GPR17) is considered a promising pharmacological target, in the light of its well-established role in the regulation of OL differentiation by acting as an intrinsic timer of the process. GPR17 is specifically and transiently expressed by a subset of NG2-positive OL precursors (OPCs) that exit mitosis and undergo cell differentiation. However, GPR17 expression has to be down-regulated before cells enter the terminal myelinating phases. Despite the many efforts that were made, the mechanisms controlling GPR17 expression are still unclear. To shed light on these mechanisms we have focused on the control of GPR17 intracellular trafficking. A process that regulates GPR17 levels at the cell surface and also its global expression. Our previous characterization of GPR17 endocytic trafficking had shown that upon endocytosis GPR17 is subjected to endosomal sorting between lysosomal degradation or recycling via the short loop pathway. In the light of this observation we decided to understand the molecular machineries that regulate GPR17 endosomal sorting. We here have demonstrated that the PDZ-binding motif expressed at GPR17 C-terminal is fundamental for receptor recycling, by mediating an interaction of GPR17 with SNX27. SNX27 is an accessory protein of the retromer complex that is emerging as a master regulator of receptor short loop recycling. We observed that loss of SNX27 unbalances GPR17 endosomal sorting and determines an increased receptor degradation and down-regulation. Interestingly, GPR17 precocious down-regulation was accompanied by an increased expression of myelin proteins, indicating an accelerated kinetics of OL differentiation in SNX27-silenced samples. Notably, we also found decreased GPR17 expression levels in Ts65Dn mouse brains, which are characterized by decreased levels of SNX27. Ts65Dn mice are the most used murine model of Down syndrome (DS). In accordance with what observed for DS patients, we found also a reduced amount of myelin fibers in Ts65Dn brains. The mechanism underlying myelination defects in DS brain is unknown. On the basis of the observation that SNX27 down-regulation was determined by reduced levels of C/EBPβ transcription factor, which are due to overexpression of trisomic micro-RNA 155 (mir-155), we decided to better characterize the role of mir-155 in OLs. Our preliminary results indicated that mir-155 is expressed at low levels in the oligodendroglial lineage and that its overexpression (mimicking pathological condition) determines a sustained inhibition of OL differentiation. This effect, however, is not dependent on SNX27 down-regulation, suggesting that other mechanisms may be involved.