Sorting nexin 17 (SNX17) is an adaptor protein present in EEA1-positive sorting endosomes that promotes the efficient recycling of low-density lipoprotein receptor-related protein 1 (LRP1) to the plasma membrane through recognition of the first NPxY motif in the cytoplasmic tail of this receptor. The interaction of LRP1 with SNX17 also regulates the basolateral recycling of the receptor from the basolateral sorting endosome (BSE). In contrast, megalin, which is apically distributed in polarized epithelial cells and localizes poorly to EEA1-positive sorting endosomes, does not interact with SNX17, despite containing three NPxY motifs, indicating that this motif is not sufficient for receptor recognition by SNX17. Here, we identified a cluster of 32 amino acids within the cytoplasmic domain of LRP1 that is both necessary and sufficient for SNX17 binding. To delineate the function of this SNX17-binding domain, we generated chimeric proteins in which the SNX17-binding domain was inserted into the cytoplasmic tail of megalin. This insertion mediated the binding of megalin to SNX17 and modified the cell surface expression and recycling of megalin in non-polarized cells. However, the polarized localization of chimeric megalin was not modified in polarized MDCK cells. These results provide evidence regarding the molecular and cellular mechanisms underlying the specificity of SNX17-binding receptors and the restricted function of SNX17 in the BSE.

ApoER2 is a member of the low density-lipoprotein receptor (LDL-R) family. As a receptor for reelin, ApoER2 participates in neuronal migration during development as well as synaptic plasticity and survival in the adult brain. A previous yeast two-hybrid screen showed that ApoER2 is a binding partner of sorting nexin 17 (SNX17) - a cytosolic adaptor protein that regulates the trafficking of several membrane proteins in the endosomal pathway, including LRP1, P-selectin and integrins. However, no further studies have been performed to investigate the role of SNX17 in ApoER2 trafficking and function. In this study, we present evidence based on GST pull-down and inmunoprecipitation assays that the cytoplasmic NPxY endocytosis motif of ApoER2 interacts with the FERM domain of SNX17. SNX17 stimulates ApoER2 recycling in different cell lines including neurons without affecting its endocytic rate and also facilitates the transport of ApoER2 from the early endosomes to the recycling endosomes. The reduction of SNX17 was associated with accumulation of an ApoER2 carboxy-terminal fragment (CTF). In addition, in SNX17 knockdown cells, constitutive ApoER2 degradation was not modified, whereas reelin-induced ApoER2 degradation was increased, implying that SNX17 is a regulator of the receptor's half-life. Finally, in SNX17 silenced hippocampal and cortical neurons, we underscored a positive role of this endosomal protein in the development of the dendritic tree and reelin signaling. Overall, these results establish the role of SNX17 in ApoER2 trafficking and function and aid in identifying new links between endocytic trafficking and receptor signaling.