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

Abstract

UBB{superscript 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{superscript 1} disrupts the ubiquitin-proteasome system while inducing autophagy. Notably, UBB{superscript 1} itself is secreted via autophagosome-like vesicles. Here, we demonstrate that UBB{superscript 1} can be removed from the cell by degradative and secretory autophagy. SQSTM1/p62 functions as a pivotal ubiquitin receptor for UBB{superscript 1}, recognizing its ubiquitin domain and facilitating loading into autophagosomes. Oligomerization of SQSTM1/p62 was critical to isolate UBB{superscript 1} in bodies preventing its aggregation. Intriguingly, both gain- and loss-of-function SQSTM1/p62 suppressed UBB{superscript 1} secretion, causing intracellular retention: SQSTM1/p62 knockout led to UBB{superscript 1} accumulation in insoluble aggregates, while its overexpression promoted the formation of p62-UBB{superscript 1} bodies. We further identified distinct roles for SNARE-mediated membrane fusion in secretory autophagy of UBB{superscript 1}. Specifically, the R-SNARE SEC22B and the Q-SNAREs Syntaxin-4 (STX4) and SNAP23 participated in UBB{superscript 1} exocytosis. Disruption of SEC22B impaired the fusion of UBB{superscript 1}-containing autophagosomes with the plasma membrane, reducing UBB{superscript 1} secretion without affecting its intracellular turnover. Inhibition of lysosomes partially stabilized UBB{superscript 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{superscript 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. SignificanceTo maintain health, cells must remove toxic proteins, typically by ubiquitin-dependent degradation. Neurons are particularly sensitive since they are not dividing and cannot replace damaged cells. This study suggests that autophagy, a pathway for degrading proteins, can also secrete harmful proteins, highlighting a new pathway for tackling neurodegenerative diseases. Specifically, UBB+1, a variant of the ubiquitin protein linked to Alzheimers disease, is recognized by a central autophagy component, p62, packaged into vesicles and secreted from cells. When p62 is absent, UBB+1 accumulates inside cells, which can be harmful. This work identifies UBB{superscript 1} as a novel cargo for secretory autophagy, extending the understanding of how cells may handle proteotoxic stress beyond classical degradation pathways.