Abstract
Lysosomes are involved in degrading and recycling cellular ingredients, and their disruption with age may contribute to amyloidogenesis, paired helical filaments (PHFs), and α-synuclein and mutant huntingtin aggregation. Lysosomal cathepsins are upregulated by accumulating proteins and more so by the modulator Z-Phe-Ala-diazomethylketone (PADK). Such positive modulators of the lysosomal system have been studied in the well-characterized hippocampal slice model of protein accumulation that exhibits the pathogenic cascade of tau aggregation, tubulin breakdown, microtubule destabilization, transport failure, and synaptic decline. Active cathepsins were upregulated by PADK; Rab proteins were modified as well, indicating enhanced trafficking, whereas lysosome-associated membrane protein and proteasome markers were unchanged. Lysosomal modulation reduced the pre-existing PHF deposits, restored tubulin structure and transport, and recovered synaptic components. Further proof-of-principle studies used Alzheimer disease mouse models. It was recently reported that systemic PADK administration caused dramatic increases in cathepsin B protein and activity levels, whereas neprilysin, insulin-degrading enzyme, α-secretase, and β-secretase were unaffected by PADK. In the transgenic models, PADK treatment resulted in clearance of intracellular amyloid beta (Aβ) peptide and concomitant reduction of extracellular deposits. Production of the less pathogenic Aβ(1-38) peptide corresponded with decreased levels of Aβ(1-42), supporting the lysosome's antiamyloidogenic role through intracellular truncation. Amelioration of synaptic and behavioral deficits also indicates a neuroprotective function of the lysosomal system, identifying lysosomal modulation as an avenue for disease-modifying therapies. From the in vitro and in vivo findings, unique lysosomal modulators represent a minimally invasive, pharmacologically controlled strategy against protein accumulation disorders to enhance protein clearance, promote synaptic integrity, and slow the progression of dementia.