FAM134B-mediated ER-phagy degrades APP and suppresses Alzheimer's disease pathology.

Endoplasmic reticulum autophagy (ER-phagy) is a selective autophagy pathway in which receptor proteins target ER membranes and proteins for degradation, yet its role in Alzheimer's disease (AD) remains unclear. Here, we identify FAM134B/RETREG1 as a specific ER-phagy receptor mediating amyloid precursor protein (APP) degradation. FAM134B directly interacts with ER-localized wild-type and familial mutant APP via their C-terminal domains and recruits LC3 through its LC3-interacting region (LIR) to promote APP delivery to phagophores for lysosomal degradation. In AD, epigenetic silencing at the FAM134B promoter suppresses its transcription by limiting TFEB/TFE3 binding despite their nuclear enrichment. This transcriptional suppression impairs ER-phagy, leading to APP accumulation and exacerbated AD pathology. AAV-mediated hippocampal expression of wild-type, but not LIR-mutant, FAM134B in 5XFAD mice restores ER-phagy, enhances APP clearance, reduces Aβ deposition, preserves synaptic and myelin integrity, and improves cognitive performance. These findings establish FAM134B downregulation as an upstream pathogenic event in AD, suggesting ER-phagy enhancement as a promising strategy to suppress Aβ generation at its source.