Lysosomal targeting via the sortilin sorting receptor

Delivery of soluble lysosomal proteins from the Golgi apparatus to the lysosomes is dependent primarily on the mannose 6-phosphate receptor (M6PR). The M6PR has been demonstrated to achieve and be recycled from the endosomal compartment through interactions with GGA, AP-1, and retromer adaptor proteins. However, in I-cell disease (ICD), in which the M6PR pathway is non-functional, many soluble lysosomal proteins continue to traffic to the lysosomes. The novel sortilin sorting receptor has been implicated in the M6PR-independent targeting of sphingolipid activator proteins. We therefore tested the hypothesis that soluble proteases that exhibit M6PR-independent trafficking are targeted to the lysosomes by sortilin in a process dependent upon AP-1 and retromer. Using a dominant-negative sortilin construct and an RNA interference (RNAi), we demonstrated that while cathepsin D transport is partially dependent upon sortilin, cathepsin H requires sortilin, whereas cathepsins K and L do not. Similarly, using RNAi, we observed that the anterograde trafficking of sortilin and its cargo required AP-1, while the retrograde recycling pathway necessitated retromer through an interaction with a YXXphi consensus site in the cytosolic tail of sortilin. Furthermore, analysis of detergent-resistant membrane (DRM) microdomains demonstrated the presence of sortilin and its ligands, but the absence of the M6PR and its ligands. Our results suggest that sortilin functions as an alternative sorting receptor to the M6PR and that uniquely sortilin and its ligands interact on sphingomyelin rich platforms in the Golgi apparatus