Lysosomal Modulatory Drugs for a Broad Strategy Against Protein Accumulation Disorders

Protein accumulation leads to CNS effects in Alzheimer's disease, frontotemporal dementia, and other age-related disorders. Common mechanisms may contribute to the progressive pathology in the different protein accumulation disorders, and synergistic toxicity between dissimilar protein structures may also be involved. Among several avenues being pursued to reduce proteins prone to oligomerization and/or aggregation, a lysosomal avenue has been described that regulates the lysosomal system's broad clearance capability. Lysosomes are the primary site for protein clearance, to remove old and misfolded proteins and maintain cellular homeostasis. Small-molecule lysosomal modulators trigger a feedback response in vitro and in vivo, resulting in marked up-regulation of cathepsins and other lysosomal enzymes without any indications of synaptic pathology, behavioral abnormalities, or major organ malfunctions. For the characterization and screening of lysosomal modulatory drugs, the hippocampal slice model of protein accumulation has proved very useful. The model exhibits experimentally-induced phosphorylated tau species, paired helical filament deposits, ubiquitinated inclusions, and protein oligomers, thus providing a valuable tool to study the associated sequelae underlying progressive cellular and synaptic compromise. In the absence of modulatory drugs, the protein accumulation events lead to microtubule destabilization, transport failure, and synaptic decline. When lysosomal modulators are administered to slices with pre-existing deposits, protein accumulations are reduced causing normalization of tau chemistry, restoration of tubulin structures and tubulin-binding proteins, and recovery of synaptic composition. Thus, positive modulators of the lysosomal system represent first-in-class drugs, providing a suitable strategy to enhance protein clearance, promote synaptic health, and slow the progression of proteinopathies.