Lrp1/LDL Receptor Play Critical Roles in Mannose 6‐Phosphate‐Independent Lysosomal Enzyme Targeting |
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| Authors: | Sandra Markmann Melanie Thelen Kerstin Cornils Michaela Schweizer Nahal Brocke‐Ahmadinejad Thomas Willnow Joerg Heeren Volkmar Gieselmann Thomas Braulke Katrin Kollmann |
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| Institution: | 1. Department for Biochemistry, Children's Hospital, University Medical Center Hamburg‐Eppendorf, Hamburg, Germany;2. Institute of Biochemistry and Molecular Biology, University of Bonn, Bonn, Germany;3. Research Department Cell and Gene Therapy, Clinic for Stem Cell Transplantation, University Medical Center Hamburg‐Eppendorf, Hamburg, Germany;4. Center for Molecular Neurobiology Hamburg, ZMNH, University Medical Center Hamburg‐Eppendorf, Hamburg, Germany;5. Max Delbrück Center for Molecular Medicine, Berlin‐Buch, Germany;6. Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg‐Eppendorf, Hamburg, Germany |
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| 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.  |
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| Keywords: | low‐density lipoprotein‐related receptor lysosomal storage disorder lysosomal targeting mannose 6‐phosphate receptor‐mediated endocytosis sortilin |
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