Degradation of Tau by Lysosomal Enzyme Cathepsin D: Implication for Alzheimer Neurofibrillary Degeneration

Abstract

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‐Leu428‐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 amino acids 34–161, 200–257, and 267–358. The cleavage sites at amino acids 34–161 and 267–358 were observed at pH 3.5, whereas that at amino 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.

Microinjection of Cathepsin D Induces Caspase-Dependent Apoptosis in Fibroblasts

“…26,27 In light of those findings, and because cat D released from lysosomes is exposed to a higher pH in the cytosol, we felt that it was important to determine whether cat D could induce apoptosis at pH values higher than 5.5. Our results indicate that cat D also induces apoptosis at pH 7.0, and therefore it might be able to proteolytically cleave substrates outside the lysosomal compartment.…”

Cathepsin D—Many functions of one aspartic protease

“…CD is optimally active against most substrates at pH 3 to 4. However, CD is able to cleave Bid at pH 6.2 and tau protein at pH 7 in vitro [57,69]. Bidere et al found that Bafilomycin A, an inhibitor of lysosomal acidification, did not inhibit CD mediated apoptosis of T lymphocytes induced by staurosporine.…”

“…The CD level also increases in cerebrospinal fluid [137][138][139][140][141]. CD has been implicated in the processing of amyloid precursor protein (APP) [56], apolipoprotein E (apoE) [61] and the tau protein [57], which are important factors of AD pathogenesis [136,142]. However, the study of CD-knockout mice revealed that CD is not essential for APP processing [143].…”

“…General protein degradation and turnover Many (including ↓) [36] Activation and degradation of polypeptide hormones, FGF [37] chemokines, growth factors and their receptors Plasminogen [38] Prolactin [39,40] Endostatin [41] Osteocalcin [42] Thyroglobulin [43] Insulin [44] Glucagon [45] IL-1 [46] IGFBP [47] Androgen receptor [48] Parathyroid hormone [49] MIP-1 alpha [50] MIP-1 beta [50] SLC [50] Activation of enzymatic precursors Cathepsin B [51,52] Cathepsin L [51,53] Transglutaminase 1 [54] Brain antigen processing Amyloid beta A4 protein [55,56] Tau [57] Tubulin [58] Myelin basic protein [59] Mhtt [60] Apolipoprotein E [61] Alpha-synuclein [62] Degradation of cytoskeletal proteins Neurofilaments [63] Actin [64,65] Myosin [64,66] Tropomyosin [65] Monocyte-mediated fibrinolysis Fibrinogen [67,68] Fibrin [67,68] Regulation of apoptosis Bid [69] Thiredoxin-1 [70] Processing of enzyme activators and inhibitors Prosaposin …”

Caffeine blocks cholesterol-enriched diet induced AD-like pathology in rabbits

“…In support, (1) endolysosomes are the major site, where Aβ is produced [52,53], and endolysosome dysfunction leads to brain deposition of Aβ [17]; (2) Endolysosomes are responsible for synaptic protein recycling [54][55][56], and endolysosome dysfunction leads to disruption of synaptic integrity in brain [57][58][59][60][61]; (3) Tau can be degraded by cathepsin D in autophagosomes-lysosomes [62][63][64][65] and increased levels of endolysosome cholesterol, as occurs in Niemann-Pick type C disease, leads to lysosome dysfunction and tau-pathology [66][67][68][69][70][71]. Substantial evidence from human epidemiological studies and from experimental studies conducted in animals and cultured cell models indicate that caffeine, when ingested chronically, can decrease Aβ levels, protect against the onset and severity of AD, and in some cases it can reverse behavioral and pathological features of AD [19,21,24,29].…”