Protective Effects of Positive Lysosomal Modulation in Alzheimer's Disease Transgenic Mouse Models

Alzheimer's disease (AD) is an age-related neurodegenerative pathology in which defects in proteolytic clearance of amyloid beta peptide (A beta) likely contribute to the progressive nature of the disorder. Lysosomal proteases of the cathepsin family exhibit up-regulation in response to accumulating proteins including A beta(1-42). Here, the lysosomal modulator Z-Phe-Ala-diazomethylketone (PADK) was used to test whether proteolytic activity can be enhanced to reduce the accumulation events in AD mouse models expressing different levels of Ab pathology. Systemic PADK injections in APP(SwInd) and APPswe/PS1 Delta E9 mice caused 3-to 8-fold increases in cathepsin B protein levels and 3- to 10-fold increases in the enzyme's activity in lysosomal fractions, while neprilysin and insulin-degrading enzyme remained unchanged. Biochemical analyses indicated the modulation predominantly targeted the active mature forms of cathepsin B and markedly changed Rab proteins but not LAMP1, suggesting the involvement of enhanced trafficking. The modulated lysosomal system led to reductions in both A beta immunostaining as well as A beta(x-42) sandwich ELISA measures in APP(SwInd) mice of 10-11 months. More extensive A beta deposition in 20-22-month APPswe/PS1 Delta E9 mice was also reduced by PADK. Selective ELISAs found that a corresponding production of the less pathogenic A beta(1-38) occurs as A beta(1-42) levels decrease in the mouse models, indicating that PADK treatment leads to A beta truncation. Associated with A beta clearance was the elimination of behavioral and synaptic protein deficits evident in the two transgenic models. These findings indicate that pharmacologically-controlled lysosomal modulation reduces A beta(1-42) accumulation, possibly through intracellular truncation that also influences extracellular deposition, and in turn offsets the defects in synaptic composition and cognitive functions. The selective modulation promotes clearance at different levels of A beta pathology and provides proof-of-principle for small molecule therapeutic development for AD and possibly other protein accumulation disorders.