Lrp1/LDL Receptor Play Critical Roles in Mannose 6-Phosphate-Independent Lysosomal Enzyme Targeting

Abstract

Most lysosomal enzymes require mannose 6-phosphate (M6P) residues for efficient receptor-mediated lysosomal targeting. Although the lack of M6P residues results in missorting and hypersecretion, selected lysosomal enzymes reach normal levels in lysosomes of various cell types, suggesting the existence of M6P-independent transport routes. Here, we quantify the lysosomal proteome in M6P-deficient mouse fibroblasts (PTki) using Stable Isotope Labeling by Amino acids in Cell culture (SILAC)-based comparative mass spectrometry, and find unchanged amounts of 20% of lysosomal enzymes, including cathepsins D and B (Ctsd and Ctsb). Examination of fibroblasts from a new mouse line lacking both M6P and sortilin, a candidate for M6P-independent transport of lysosomal enzymes, revealed that sortilin does not act as cargo receptor for Ctsb and Ctsd. Using fibroblast lines deficient for endocytic lipoprotein receptors, we could demonstrate that both LDL receptor and Lrp1 mediate the internalization of non-phosphorylated Ctsb and Ctsd. Furthermore, the presence of Lrp1 inhibitor increased the secretion of Ctsd from PTki cells. These findings establish Lrp1 and LDL receptors in M6P-independent secretion-recapture targeting mechanism for lysosomal enzymes. Stable Isotope Labeling by Amino acids in Cell culture (SILAC)-based quantitative analysis of the lysosomal proteome of mouse fibroblasts allowed the identification of lysosomal enzymes whose localization was unaffected by the lack of mannose 6-phosphate (M6P) targeting signals. Among these, the sorting of cathepsins D and B is independent of sortilin but involves secretion-recapture mechanisms mediated by Lrp1 and LDL receptors.