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

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.