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

Abstract

UBB⁺¹, a ubiquitin variant protein resulting from a frameshift in the ubiquitin-B gene, is a pathological hallmark of Alzheimer disease (AD). At the cellular level, UBB⁺¹ disrupts the ubiquitin-proteasome system while inducing autophagy. Notably, UBB⁺¹ itself is secreted via autophagosome-like vesicles. Here, we demonstrate that UBB⁺¹ can be removed from the cell by degradative and secretory autophagy. Sequestosome 1 (SQSTM1)/p62 functions as a pivotal ubiquitin receptor for UBB⁺¹, recognizing its ubiquitin domain and facilitating loading into autophagosomes. Oligomerization of SQSTM1/p62 was critical to isolate UBB⁺¹ in bodies preventing its aggregation. Intriguingly, both gain- and loss-of-function SQSTM1/p62 suppressed UBB⁺¹ secretion, causing intracellular retention: SQSTM1/p62 knockout led to UBB⁺¹ accumulation in insoluble aggregates, while its overexpression promoted the formation of p62-UBB⁺¹ bodies. We further identified distinct roles for SNARE-mediated membrane fusion in secretory autophagy of UBB⁺¹. Specifically, the R-SNARE SEC22B and the Q-SNAREs Syntaxin-4 and SNAP23 participated in UBB⁺¹ exocytosis. Disruption of SEC22B impaired the fusion of UBB⁺¹-containing autophagosomes with the plasma membrane, reducing UBB⁺¹ secretion without affecting its intracellular turnover. Inhibition of lysosomes partially stabilized UBB⁺¹ indicating that degradation and secretion are complementary processes that determine the fate of UBB⁺¹. This study elucidates the dual roles of autophagy in managing neurotoxic proteins, highlighting SQSTM1/p62 as a key mediator of UBB⁺¹ trafficking and secretion. Although ubiquitin typically acts as a degradation signal, our findings reveal a rare instance of a ubiquitin-related protein driving secretory autophagy. These findings advance our understanding of cellular mechanisms underlying the clearance of misfolded proteins in neurodegenerative diseases.