Degradation of tau by lysosomal enzyme cathepsin D: Implication for Alzheimer neurofibrillary degeneration

The degradation of different isoforms of human recombinant tau (R-tau; T39, T40, and T44) and fetal tau (F-tau) by cathepsin D (CD) was investigated. Gel electrophoresis and Coomassie Blue staining of different R- tau species digested at pH 3.5 showed very little differences in CD susceptibility. Immunoblotting analyses revealed that amino and carboxy termini of tau were cleaved before other regions. F-tau was most vulnerable to proteolysis at both termini. Digestion of R-tau with 0.01 unit of CD/ml at pH 3.5 resulted in cleavage between Phe⁸-Glu⁹, Met⁴¹⁹-Val⁴²⁰, Thr⁴²⁷-Leu⁴²⁶-Ala⁴²⁹, and Leu⁴³⁶-Ala⁴³⁷ as determined by amino acid sequencing and mass spectroscopy (numbering of amino acids was based on T40). With higher concentrations of CD (1 unit/ml), additional sites of digestion were detected between amine acids 34-161, 200-257, and 267-358. The cleavage sites at amine acids 34-161 and 267-358 were observed at pH 3.5, whereas that at amine acids 200-257 was detected at pH 7.0. Our results suggest that CD cleavage of tau could generate tau fragments with intact microtubule binding domains, which could have a role in the pathogenesis of paired helical filaments (PHFs) in Alzheimer's disease. Such proteolysis might also contribute to the changes of PHF phenotype observed in intracellular and extracellular tangles.