Insights into cargo sorting by SNX32 in neuronal and non-neuronal cells: physiological implications in neurite outgrowth

Abstract

Sorting nexins (SNX) are a family of proteins containing the Phox homology domain, which shows a preferential endo-membrane association and regulates cargo sorting processes. Even with the vast amount of information unveiled systematically, the underlying mechanism of sorting remains elusive. Here, we established that SNX32, a SNX-BAR (Bin/Amphiphysin/Rvs) sub-family member, is associated with SNX4 via its BAR domain. We identified A226, Q259, E256, R366 of SNX32, and Y258, S448 of SNX4 at the interface of these two SNX proteins that are important for maintaining the association. Via its PX domain, SNX32 interacts with the Transferrin receptor (TfR) and Cation Independent Mannose-6-Phosphate Receptor (CIMPR). We showed that the conserved F131 in its PX domain is important in stabilising the above interactions. Silencing of SNX32 led to a defect in intracellular trafficking of TfR and CIMPR, which could be rescued by overexpressing shRNA-resistant snx32. We also showed that both individual domains play an essential role in trafficking. Our results indicate that SNX4, SNX32 and Rab11 may participate in a common pathway regulating transferrin trafficking; however, the existence of an independent pathway for Rab11 and SNX32 could not be completely ruled out. Further, we established that the PX domain of SNX32 could bind to PI(3)P and PI(4)P, suggesting a possible explanation for its sub-cellular localization. Taken together, our study showed that SNX32 mediate the trafficking of specific cargo molecules along distinct pathway via its PX domain-directed binding to phosphoinositides and its BAR domain-mediated association with other SNX family members. Further, using SILAC-based differential proteomics of the wild type and the mutant SNX32, impaired in cargo binding, we identified Basigin (BSG), an immunoglobulin super family member, as a potential interactor of SNX32 in SHSY5Y cells. We then demonstrated that SNX32 binds to BSG through its PX domain and facilitates its trafficking to the cell surface. In Neuro-Glial cell lines, the silencing of SNX32 led to defects in neuronal differentiation. Moreover, abrogation in lactate transport in the SNX32 depleted cells led us to propose that the SNX may contribute to maintaining the neuro-glial coordination via its role in BSG trafficking and the associated Monocarboxylate transporter activity.