Characterizing the Molecular Switch from Proteasomes to Autophagy in Aggresome Processing

Abstract

<p>Cells thrive on sustaining order and balance to maintain proper homeostatic functions. However, the primary machinery involved in protein quality control including chaperones, ubiquitin proteasome system, and autophagy all decline in function and expression with age. Failures in protein quality control lead to enhanced protein misfolding and aggregation. Efficient elimination of misfolded proteins by the proteasome system is critical for cellular proteostasis. However, inadequate proteasome capacity can lead to aberrant aggregation of misfolded proteins and inclusion body formation, which is a hallmark of numerous neurodegenerative diseases. Due to the post-mitotic nature of neurons, they are more susceptible to the collapse in proteostasis correlated with age. </p><p> </p><p>Here, we propose a cell based model of aggresome clearance using a reversible proteasome inhibitor, MG132, to identify the precise molecular machinery involved in proper processing of inclusions. It is known that once misfolded proteins are aggregated, the proteasome system can no longer degrade them. Furthermore, the continuous accumulation of aggregates often leads to aggresome formation, which results in amalgamated inclusion bodies that are simply too large for autophagosomes to engulf and degrade. Although, studies have shown that aggresomes can eventually be cleared by autophagy, the molecular mechanisms underlying this process remain unclear. </p><p>Our research reveals that regardless of impaired proteolysis, proteasomes can still stimulate autophagy-dependent aggresome clearance by producing unanchored lysine (K)63-linked ubiquitin chains via the deubiquitinating enzyme Poh1. Unanchored ubiquitin chains activate ubiquitin-binding histone deacetylase 6, which mediates actin-dependent disassembly of aggresomes. This crucial de-aggregation of aggresomes allows autophagosomes to efficiently engulf and eliminate the protein aggregates. Interestingly, the canonical function of Poh1 involves the cleavage of ubiquitin chains en bloc from proteasomal substrates prior to their degradation by the 20S core, which requires intact 26S proteasomes. In contrast, here we present evidence that during aggresome clearance, 20S proteasomes dissociate from protein aggregates, while Poh1 and selective subunits of 19S proteasomes are retained as an efficient K63 deubiquitinating enzyme complex. The dissociation of 20S proteasome components requires the molecular chaperone Hsp90. Hsp90 inhibition suppresses 26S proteasome remodeling, unanchored ubiquitin chain production, and aggresome clearance. Ultimately, we hope to apply these molecular markers of inclusion body processing to identify the underlying lesion in aggregate prone neurodegenerative disease.</p>