Molecular mechanisms underlying p62-dependent secretion of the Alzheimer-associated ubiquitin variant, UBB ⁺¹

UBB⁺¹, a ubiquitin variant protein arising from a frameshift in the ubiquitin-B gene, is a pathological marker in the brains of Alzheimer disease (AD) patients. At the cellular level, UBB⁺¹ disrupts the ubiquitin-proteasome system while simultaneously activating autophagy. UBB⁺¹ is itself secreted in autophagosome-like vesicles. Secretory autophagy mediates the release of cytosolic proteins via the fusion of autophagosomes with the plasma membrane, bypassing lysosomal degradation. We demonstrate that SQSTM1/p62 plays a pivotal role in UBB⁺¹ secretion by acting as a ubiquitin receptor for its ubiquitin domain thereby facilitating UBB⁺¹ loading into autophagosome-like vesicles. Gain- and loss-of-function studies reveal that SQSTM1/p62 is critical for regulating both intracellular and extracellular UBB⁺¹ levels. Knockout of SQSTM1/p62 resulted in the accumulation of UBB⁺¹ in insoluble aggregates, while its overexpression promoted the formation of p62-UBB⁺¹ bodies. Notably, under both conditions, UBB⁺¹ secretion was suppressed, leading to its intracellular retention. This study reveals distinct roles for SNARE-mediated membrane fusion in degradative and secretory autophagy pathways. Specifically, SEC22B (an R-SNARE) and Syntaxin-4 (STX4) (a Q-SNARE) are identified as key mediators of UBB⁺¹ exocytosis. Disruption of these SNAREs impaired the fusion of UBB⁺¹-containing autophagosomes with the plasma membrane, significantly reducing UBB⁺¹ secretion while leaving intracellular turnover unaffected. This study highlights a dual role for autophagy in managing neurotoxic proteins, with SQSTM1/p62 serving as a critical ubiquitin receptor for UBB⁺¹ trafficking and secretion. These findings offer new insights into the cellular mechanisms underlying the clearance of misfolded proteins in neurodegenerative diseases.