title: Differential sorting of lysosomal enzymes in mannose 6-phosphate receptor-deficient fibroblasts.
abstract: In higher eukaryotes, the transport of soluble lysosomal enzymes involves the recognition of their mannose 6-phosphate signal by two receptors: the cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor (CD-MPR). It is not known why these two different proteins are present in most cell types. To investigate their relative function in lysosomal enzyme targeting, we created cell lines that lack either or both MPRs. This was accomplished by mating CD-MPR-deficient mice with Thp mice that carry a CI-MPR deleted allele. Fibroblasts prepared from embryos that lack the two receptors exhibit a massive missorting of multiple lysosomal enzymes and accumulate undigested material in their endocytic compartments. Fibroblasts that lack the CI-MPR, like those lacking the CD-MPR, exhibit a milder phenotype and are only partially impaired in sorting. This demonstrates that both receptors are required for efficient intracellular targeting of lysosomal enzymes. More importantly, comparison of the phosphorylated proteins secreted by the different cell types indicates that the two receptors may interact in vivo with different subgroups of hydrolases. This observation may provide a rational explanation for the existence of two distinct mannose 6-phosphate binding proteins in mammalian cells.
authors.affiliation: European Molecular Biology Laboratory, Heidelberg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Ludwig
authors.forename: H
authors.initials: H
authors.lastname: Munier-Lehmann
authors.forename: U
authors.initials: U
authors.lastname: Bauer
authors.forename: M
authors.initials: M
authors.lastname: Hollinshead
authors.forename: C
authors.initials: C
authors.lastname: Ovitt
authors.forename: P
authors.initials: P
authors.lastname: Lobel
authors.forename: B
authors.initials: B
authors.lastname: Hoflack
mesh_terms: D000818:Animals; D002402:Cathepsin D / Q000378:metabolism; D002478:Cells, Cultured; D004705:Endocytosis; D005260:Female; D005347:Fibroblasts / Q000378:metabolism*; D005838:Genotype; D005966:Glucuronidase / Q000378:metabolism; D037181:Lectins, C-Type; D008247:Lysosomes / Q000201:enzymology*; D008297:Male; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D008360:Mannosephosphates / Q000378:metabolism*; D051379:Mice; D010750:Phosphoproteins / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000235:genetics / Q000502:physiology*; D001616:beta-Galactosidase / Q000378:metabolism; D001619:beta-N-Acetylhexosaminidases / Q000378:metabolism
chemical_list: D037181:Lectins, C-Type; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D008360:Mannosephosphates; D010750:Phosphoproteins; D011498:Protein Precursors; D011956:Receptors, Cell Surface; C027693:mannose-6-phosphate; D001616:beta-Galactosidase; D005966:Glucuronidase; D001619:beta-N-Acetylhexosaminidases; D002402:Cathepsin D