Functional Defects in Lysosomal Enzymes in Autosomal Dominant Polycystic Kidney Disease (ADPKD): Abnormalities in Synthesis, Molecular Processing, Polarity, and Secretion

Abstract

The phenotype of autosomal dominant polycystic kidney disease (ADPKD) is characterized by basement membrane abnormalities, hyperproliferation, and alterations in epithelial cell polarity. Since proteinases have been implicated in matrix degradation and growth factor activation, lysosomal enzymes were compared in normal and ADPKD tissues and cell cultures. Acidic proteolytic activity (azocasein) was reduced in ADPKD, and specific enzymatic assays detected disease-dependent decreases in the specific activities of β-galactosidase, β-hexosaminidase, and cathepsins, B, L, and H. Cathepsin D-specific activities were unchanged. Lucifer yellow fluorescence in ADPKD cells was consistent with an alteration in heterogeneity of lysosomal enzyme content in ADPKD rather than a decrease in total lysosomal number. Western analysis, metabolic labeling, and immunoprecipitation analysis confirmed decreases in the expression and synthesis of the major normal molecular immunoreactive species of β-galactosidase and cathepsins B and H in ADPKD tissue and cells but no changes in cathepsin D. In addition, ADPKD-specific high-molecular-weight species of cathepsin H were seen and abnormal forms of cathepsin B and β-galactosidase were common in ADPKD, suggesting abnormal molecular processing and posttranslational modifications. In addition, immunolocalization studies showed abnormal apical plasma-membrane localization of cathepsins B and H in ADPKD cyst epithelial cells, consistent with a protein sorting defect in ADPKD. Increased extracellular secretion of lysosomal enzymes was also measured in ADPKD cultured cells and in filter-grown epithelia shown to be predominantly directed to the basal compartment. These results demonstrate that lysosomal enzyme alterations in ADPKD may play a role in aberrant processing of the basement membrane. Alterations in the polarized secretion of lysosomal enzymes by ADPKD epitheliain vitrowere also detected. Whereas all normal epithelia cells secreted lysosomal enzymes predominantly to the apical medium compartments, basally directed secretion was increased in all ADPKD epithelia and attained an overall reversal of polarity for cathepsins B + L. It is concluded that alterations in lysosomal enzyme function in ADPKD are the result of alterations in synthesis, molecular processing, and polarized secretion of specific enzymes and may have impact on proliferative and basement membrane abnormalities in this genetic disease. These results are consistent with a fundamental defect in protein processing, sorting, and trafficking in ADPKD.