TGN exit of the cation-independent mannose 6-phosphate receptor does not require acid hydrolase binding

Abstract

The cation-independent mannose 6-phosphate (Man-6-P) receptor (CI-MPR) binds newly synthesized, Man-6-Pcontaining lysosomal acid hydrolases in the trans-Golgi network (TGN) for clathrin-mediated transport to endosomes. It has remained unresolved, however, whether acid hydrolase binding is required for exit of the CI-MPR from the TGN. To address this question we used a B cell line derived from a Mucolipidosis type II (MLII)/I-cell disease patient. In MLII patients, acid hydrolases do not acquire the Man-6-P recognition marker and as a consequence do not bind to the CI-MPR. This causes secretion of the majority of the acid hydrolases and a decreased lysosomal activity resulting in typical inclusion bodies. In agreement herewith, ultrastructural analysis of the MLII patient derived B cells showed numerous inclusion bodies with undigested material, which we defined as autolysosomes. By quantitative immuno-electron microscopy we then studied the distribution of the CI-MPR in these cells. We found that the level of co-localization of TGN-localized CI-MPR and clathrin was similar in MLII and control B cells. Moreover, the CI-MPR was readily found in endosomes of MLII cells and the TGN-to-early endosome ratio of CI-MPR labeling was unaltered. These data show that there is no block in TGN exit of the CI-MPR in the absence of Man-6-P-modified acid hydrolases. Notably, late endosomes and inclusion bodies in MLII B cells contained increased levels of the CI-MPR, which likely reflects the reduced degradative capacity of these compartments.

The mannose 6-phosphate/insulin-like growth factor 2 receptor mediates plasminogen-induced efferocytosis

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Nanoparticle entry into cells; the cell biology weak link

“…LE are enriched in the cation-independent mannose 6 phosphate receptor (ciMPR) in many, but not all cell types [397]. In some cells, the ciMPR can be more prominently enriched in the TGN [398]. LE are also generally positive for CD63, LBPA and Rab 7 whereas lysosomes are negative for these markers.…”

Loss of the HOPS complex disrupts early-to-late endosome transition, impairs endosomal recycling and induces accumulation of amphisomes

“…Another unexpected finding was that depletion of the HOPS complex caused a redistribution of CI-MPR from the TGN to HOPS bodies ( Figure 4 ). Depletion of CI-MPR from the TGN is known to increase secretion of lysosomal enzyme precursors ( Meel and Klumperman, 2014 ), which was indeed the case for HOPS KO cells (Supplementary Figure S5C). Likewise, Anderson et al.…”