title: 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-P-containing 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.
authors.affiliation: Department of Cell Biology; University Medical Center Utrecht ; Utrecht, The Netherlands.
authors.forename: Eline
authors.initials: E
authors.lastname: van Meel
authors.affiliation: Department of Cell Biology; University Medical Center Utrecht ; Utrecht, The Netherlands.
authors.forename: Judith
authors.initials: J
authors.lastname: Klumperman
keywords: B cells; CI-MPR, cation-independent MPR; FBS, fetal bovine serum; GA, glutaraldehyde; IGF-II, insulin-like growth factor II; MLII, Mucolipidosis type II; MPR, mannose 6-phosphate receptor; Mucolipidosis type II; PB, phosphate buffer; PFA, paraformaldehyde; TGN, trans-Golgi network; cation-independent mannose 6-phosphate receptor; electron microscopy; lysosomal acid hydrolases; mannose 6-phosphate modification; trans-Golgi network