The Foundational Concept: Autophagy requires precise targeting autophagosomes must distinguish what to eat. Mutant huntingtin blinds this recognition system, allowing damaged mitochondria to accumulate and poison the cell.

The Mechanism:

Normal Mitophagy: Damaged mitochondria are tagged for destruction via PINK1 accumulation on outer membrane, recruiting Parkin. Ubiquitin chains signal autophagic receptors (p62, OPTN, NDP52) to link cargo to LC3 on autophagosomes.

Mutant Huntingtin Interference: Polyglutamine-expanded huntingtin binds directly to p62 and LC3, altering their conformation. This disrupts the critical protein-protein interactions required for cargo recognition.

Recognition Failure: Autophagosomes form normally but cannot identify which mitochondria are damaged. They engulf random cytoplasm while defective mitochondria persist.

Damaged Accumulation: Dysfunctional mitochondria with low membrane potential, oxidized mtDNA, and impaired ATP production accumulate in cytoplasm. They continue generating ROS.

Energy Crisis: Without clearing damaged mitochondria, cells cannot maintain ATP levels. Striatal neurons���high metabolic demand���are particularly vulnerable.

Apoptotic Trigger: Persistent damaged mitochondria release cytochrome c and activate caspase cascades. Cells die with autophagosomes full of harmless cytoplasm while true threats remain.

The Selectivity Problem:

p62 normally binds ubiquitin chains and LC3 simultaneously

Mutant huntingtin disrupts this bridging function

Cargo (damaged mitochondria) present but unrecognized

The Huntington's Specificity:

Striatal neurons show mitochondrial abnormalities early

Autophagosomes accumulate but contain inappropriate cargo

Parkin recruitment occurs but p62 fails to link to LC3

Therapeutic Implications:

p62-structure correctors restoring ubiquitin-LC3 bridging

Alternative receptor upregulation (OPTN, NDP52) bypassing p62 failure

Mitochondrial-targeted antioxidants reducing damage while recognition impaired

Autophagy induction overwhelming defective recognition with capacity

This reframes Huntington's as cargo blindness autophagosomes roam full but useless while threats multiply.