Lysosomal Clearance Capacity Determines Therapeutic Window—Autophagy Enhancemen…

The therapeutic hypothesis centers on the kinetic constraints governing autophagy-lysosomal degradation of pathological protein aggregates, specifically targeting the mTORC1/ULK1 autophagy initiation pathway and lysosomal processing capacity. At the molecular level, this mechanism involves mTORC1-mediated phosphorylation of ULK1 at Ser757, which inhibits autophagy initiation under nutrient-rich conditions. Upon cellular stress or aggregate accumulation, mTORC1 inhibition allows ULK1 autophosphorylation at Ser317 and subsequent activation of the autophagy cascade through Beclin-1/VPS34 complex recruitment and LC3 lipidation. The kinetic model predicts that aggregate clearance follows Michaelis-Menten kinetics, where Vmax represents the maximum lysosomal degradation capacity determined by lysosome number, cathepsin activity levels (particularly cathepsin B and L), and autophagosome-lysosome fusion efficiency mediated by SNARE proteins and Rab7. The Km reflects the aggregate concentration required for half-maximal clearance, influenced by selective autophagy receptor binding (p62/SQSTM1, NBR1) to LC3-decorated autophagosomes. Critical threshold effects emerge when aggregate influx exceeds lysosomal processing capacity, leading to lysosomal dysfunction, reduced acidification, and impaired cathepsin activity. This creates a positive feedback loop where decreased clearance capacity allows further aggregate accumulation, ultimately resulting in lysosomal membrane permeabilization and cytotoxic cathepsin release. Unlike chaperone-mediated disaggregation, this pathway provides complete substrate degradation but requires functional lysosomal biogenesis through TFEB/TFE3 transcriptional programs, making early intervention essential before lysosomal capacity becomes overwhelmed.