Lysosomal enzymes and mannose 6-phosphate receptors in the lacrimal drainage system: Evidence and its potential implications

Abstract

Purpose: To investigate the presence and patterns of lysosomal enzymes and mannose 6-phosophate receptor (MPRs) in human lacrimal drainage system. Methods: The study was performed on healthy lacrimal sacs and nasolacrimal ducts obtained from exenteration samples immediately after surgery and frozen at -80°C for subsequent analysis. Soluble proteins' extract was used for enzyme assays, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE), native PAGE, activity staining, and western blot analysis. Membrane proteins were separately assessed for detection of mannose 6-phosphate receptors, MPR 46. Sepharose gels, 4-methylumbelliferyl substrates, and antibodies against common lysosomal enzymes and MPRs were used. Enzyme assays were carried out in triplicate to ascertain the results. Results: Differential lysosomal enzyme activities were documented, and among them acid phosphatase and β-hexosaminidase were found to be high. Western blot analysis using enzyme antibodies and subsequent activity staining confirmed strong signals for moderately expressed enzymes such as fucosidase, glucuronidase, and mannosidase. Membrane extracts demonstrated the presence of MPR 46, which indicates the possible roles of cation-dependent MPRs in lysosomal targeting in human lacrimal drainage system. Conclusion: This study provides a proof of principle for the presence of differential lysosomal activity and mannose 6-phosphate ligand transport receptors in human lacrimal drainage system and hypothesizes the potential implications of their dysfunctions.