Abstract
Introduction: Tauopathies involve progressive accumulation of abnormal tau species that disrupt the autophagy-lysosomal pathway (ALP), critical for degrading intracellular macromolecules and aggregates, leading to toxicity and cell death. This study examines how overexpression of the N-terminally truncated Tau35 protein affects proteolytic pathways, including autophagy and endo-lysosomal processes. Methods: Using the Tau35 mouse model and SH-SY5Y human neuroblastoma cells stably expressing Tau35 or full-length tau, we assessed protein degradation and lysosomal function via Western blotting, proteomics of lysosome-enriched brain fractions, cathepsin activity assays, endocytosis/proteolysis assays, and live-cell imaging using LysoTracker. Results: We identified early endo-lysosomal alterations associated with Tau35 expression, including increased endocytosis, disrupted autophagic flux, proteolytic impairment, and lysosomal motility defects. Discussion: These findings extend previous research by elucidating Tau35-induced dysfunction in intracellular degradation systems and offer mechanistic insight into tauopathy progression. This work provides a foundation for developing targeted therapies to restore acidification, proteostasis, and lysosomal function in tauopathies. Highlights: Tau35, an N-terminally truncated tau fragment, disrupts proteolytic pathways: We show that Tau35 overexpression leads to significant alterations in autophagy and endo-lysosomal function. Endo-lysosomal dysfunction is an early pathological event: Our findings demonstrate early-stage increases in endocytosis, impaired proteolytic activity, altered autophagic flux, and disrupted lysosomal motility in Tau35-expressing models. In vivo and in vitro models confirm consistent pathogenic signatures: Parallel studies in a Tau35 mouse model and SH-SY5Y cells reveal converging cellular and molecular dysfunctions. Lysosome-enriched proteomics reveals novel pathway alterations: Proteomic profiling of lysosomal fractions identifies Tau35-specific protein dysregulation contributing to disease pathology. Mechanistic insights into tauopathy progression: These results provide a mechanistic understanding of how truncated tau species contribute to neuronal dysfunction, offering a rationale for targeting endo-lysosomal pathways in therapeutic development.