SNX27 is required for G protein-coupled receptor 17 recycling and oligodendrocyte differentiation: implications for Down syndrome

Background GPR17 acts as an intrinsic timer of oligodendrocyte (OL) differentiation, maintaining pre-myelinating OL into an immature state until its down-regulation allows OL maturation. The mechanisms controlling GPR17 expression/stability in developing OL are only partially known. Our previous work demonstrated that GPR17, after endocytosis, is sorted into lysosomes or recycled to the cell surface. Here we analyze the mechanisms underlying GPR17 endocytic trafficking and stability in pre-myelinating OL. Observations At first we focused our attention on GPR17 C-terminal class I PDZ-binding motif (PDZbm) and demonstrated that GPR17 mutants, obtained by site-directed mutagenesis of the PDZbm, fail to recycle to the cell surface. Thus, we analyzed the role of SNX27 (member of the so called “SNX27-retromer complex”) in GPR17 trafficking since contains a PDZ domain able to interact with class I PDZbm and has been shown to regulate receptor recycling to the plasma membrane. By means of pull-down experiments we demonstrated that SNX27 directly binds to GPR17-PDZbm. Moreover both proteins colocalize in endosomes and SNX27-knockdown affects GPR17 recycling and stability increasing receptor sorting into lysosomes. As result of this accelerated GPR17 degradation, we observed a precocious expression of myelin proteins in pre-myelinating cells and acceleration in the kinetics of OL differentiation. We next analyzed the expression of GPR17 in the Ts65Dn mouse model of Down syndrome (DS), which is known to express low levels of SNX27. Notably, we found a significant reduction in the number of GPR17+ cells paralleled by an increase in the number of more mature OL (CC1+cells) and alteration of myelin formation. Conclusions SNX27 directly interacts with GPR17 and is required for receptor recycling and stability. SNX27 down-regulation impairs cell surface expression of GPR17 and enhances the rate of OL differentiation. These results may have implications in the pathogenesis of myelination defects described in DS.