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Antibody to mannose 6-phosphate specific receptor induces receptor deficiency in human fibroblasts
Abstract:
Polyclonal antibodies to the mannose 6-phosphate specific receptor from human liver inhibited the endocytosis of lysosomal enzymes in fibroblasts by greater than 95% and enhanced 3-20-fold the secretion of precursors of lysosomal enzymes in these cells. Exposing fibroblasts for 4 h to antibody resulted in loss of greater than 90% of the membrane-bound receptors. If fibroblasts were treated with the antibody in the presence of CBZ-Phe-Ala-CHN2, an inhibitor of lysosomal cysteine proteinases, the receptor and smaller degradation products are recovered in dense lysosomes. In treated cells 18-58% of total receptor-related polypeptides were recovered in dense lysosomes. In control cells less than 4% of the receptor was found in the lysosomal fraction. We conclude from these results that normally the receptor is spared from lysosomal degradation. When tagged with antibody, however, the receptor is transported into lysosomes and degraded. The loss of intracellular receptors involved in segregation of newly synthesized lysosomal enzymes indicates an exchange between the former and the plasma membrane-bound receptors.
Insights
Antibodies targeting the mannose 6-phosphate receptor block lysosomal enzyme uptake in fibroblasts. This leads to receptor degradation, suggesting a role in receptor trafficking and turnover.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- The mannose 6-phosphate (M6P) receptor is crucial for targeting lysosomal enzymes to lysosomes.
- Understanding M6P receptor trafficking is key to lysosomal storage diseases.
Purpose of the Study:
- To investigate the role of M6P receptor antibodies in endocytosis and degradation.
- To elucidate the fate of the M6P receptor upon antibody binding.
Main Methods:
- Utilized polyclonal antibodies against the human liver M6P receptor.
- Studied endocytosis inhibition and enzyme secretion in fibroblasts.
- Analyzed receptor localization and degradation using a lysosomal cysteine proteinase inhibitor (CBZ-Phe-Ala-CHN2).
Main Results:
- Antibodies inhibited M6P receptor-mediated endocytosis of lysosomal enzymes by >95%.
- Antibody treatment enhanced secretion of lysosomal enzyme precursors 3-20 fold.
- Exposure to antibodies led to >90% loss of membrane-bound receptors.
- In the presence of an inhibitor, M6P receptor and degradation products accumulated in dense lysosomes (18-58% of polypeptides).
- Control cells showed minimal receptor presence (<4%) in lysosomes.
Conclusions:
- Normally, the M6P receptor evades lysosomal degradation.
- Antibody tagging redirects the M6P receptor to lysosomes for degradation.
- This suggests an exchange mechanism between intracellular and plasma membrane-bound M6P receptors.