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Mr 46,000 mannose 6-phosphate specific receptor: its role in targeting of lysosomal enzymes
M Stein1, J E Zijderhand-Bleekemolen, H Geuze
1Physiologisch-Chemisches Institut, Universität Münster, FRG.
Abstract:
Antibodies that block the ligand binding site of the cation-dependent mannose 6-phosphate specific receptor (Mr 46,000 MPR) were used to probe the function of the receptor in transport of lysosomal enzymes. Addition of the antibodies to the medium of Morris hepatoma 7777 cells, which express only the Mr 46,000 MPR, resulted in a decreased intracellular retention and increased secretion of newly synthesized lysosomal enzymes. In fibroblasts and HepG2 cells that express the cation-independent mannose 6-phosphate specific receptor (Mr 215,000 MPR) in addition to the Mr 46,000 MPR, antibodies against the Mr 46,000 MPR inhibited the intracellular retention of newly synthesized lysosomal enzymes only when added to the medium together with antibodies against the Mr 215,000 MPR. Morris hepatoma (M.H.) 7777 did not endocytose lysosomal enzymes, while U937 monocytes, which express both types of MPR, internalized lysosomal enzymes. The uptake was inhibited by antibodies against the Mr 215,000 MPR, but not by antibodies against the Mr 46,000 MPR. These observations suggest that Mr 46,000 MPR mediates transport of endogenous but not endocytosis of exogenous lysosomal enzymes. Internalization of receptor antibodies indicated that the failure to mediate endocytosis of lysosomal enzymes is due to an inability of surface Mr 46,000 MPR to bind ligands rather than its exclusion from the plasma membrane or from internalization.
Insights
The cation-dependent mannose 6-phosphate receptor (MPR) transports internal lysosomal enzymes but does not endocytose external ones. Antibodies targeting this MPR revealed its specific role in endogenous protein transport.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Lysosomal enzymes are crucial for cellular waste degradation.
- Mannose 6-phosphate receptors (MPRs) mediate lysosomal enzyme trafficking.
- Two main MPRs exist: cation-dependent (46 kDa) and cation-independent (215 kDa).
Purpose of the Study:
- To investigate the distinct roles of the cation-dependent MPR (46 kDa) in lysosomal enzyme transport.
- To differentiate the MPRs' functions in endogenous protein retention versus exogenous enzyme endocytosis.
Main Methods:
- Utilized antibodies to block the ligand-binding site of the 46 kDa MPR.
- Studied lysosomal enzyme transport in cell lines expressing different MPR combinations (Morris hepatoma 7777, fibroblasts, HepG2, U937 monocytes).
- Assessed intracellular enzyme retention, secretion, and endocytosis rates.
Main Results:
- Blocking the 46 kDa MPR in cells expressing only it decreased enzyme retention and increased secretion.
- In cells with both MPRs, blocking the 46 kDa MPR only affected enzyme retention when the 215 kDa MPR was also blocked.
- Morris hepatoma cells lacking the 215 kDa MPR did not endocytose lysosomal enzymes.
- U937 monocytes, expressing both MPRs, internalized enzymes via the 215 kDa MPR, not the 46 kDa MPR.
- The 46 kDa MPR's inability to mediate endocytosis stems from ligand-binding impairment, not membrane exclusion.
Conclusions:
- The cation-dependent 46 kDa MPR primarily mediates the transport of endogenous lysosomal enzymes.
- The cation-independent 215 kDa MPR is responsible for the endocytosis of exogenous lysosomal enzymes.
- The 46 kDa MPR's function is specific to intracellular trafficking, distinct from exogenous uptake mechanisms.