Autophagy-Lysosomal Pathway

Autophagy is the cell's recycling system — degrading damaged organelles, protein aggregates, and pathogens through lysosomal machinery. Dysfunction underlies neurodegeneration, cancer, and aging.

Overview

Macroautophagy begins with formation of the phagophore (isolation membrane), which engulfs cytoplasmic cargo and closes to form a double-membrane autophagosome. The autophagosome fuses with lysosomes, forming autolysosomes where contents are degraded by acidic hydrolases. The process is initiated by ULK1 complex activation (when mTORC1 is inhibited) and regulated by ~40 ATG (autophagy-related) genes.

Key Steps

1. mTORC1 inhibition (starvation, rapamycin) dephosphorylates ULK1, activating the initiation complex
2. ULK1 phosphorylates Beclin-1/VPS34 PI3K complex, generating PI3P on phagophore membrane
3. ATG12-ATG5-ATG16L1 and LC3-II conjugation systems expand the phagophore
4. Cargo receptors (p62/SQSTM1, NBR1) bridge ubiquitinated cargo to LC3 on the autophagosome
5. SNARE proteins mediate autophagosome-lysosome fusion
6. Lysosomal hydrolases degrade contents; amino acids are recycled via mTORC1 reactivation

Disease Relevance

• Alzheimer's: Impaired autophagy fails to clear tau tangles and Aβ aggregates. Presenilin-1 mutations (familial AD) impair lysosomal acidification. Autophagy induction is a therapeutic strategy.
• Parkinson's: Alpha-synuclein aggregates overwhelm autophagy capacity. LRRK2 mutations impair autophagosome-lysosome fusion. GBA mutations reduce lysosomal function.
• ALS: TDP-43 and SOD1 aggregates accumulate when autophagy is impaired. C9orf72 repeat expansions disrupt autophagy initiation.
• Cancer: Autophagy has dual roles: tumor suppressive (removes damaged DNA/organelles) early, but tumor promoting (provides metabolic substrates) in established cancers.

Therapeutic Targets

• Rapamycin: mTORC1 inhibition derepresses ULK1 → autophagy induction
• Spermidine: Induces autophagy via EP300 acetyltransferase inhibition
• Fasting-Mimicking Diet: Nutrient deprivation activates AMPK → autophagy induction
• Metformin: AMPK-mediated mTOR inhibition promotes autophagy