Small bites for big problems: stepwise aggregate degradation by autophagy

Protein aggregates are a pathological hallmark of diverse disorders, including many neurodegenerative diseases, but also cardiometabolic disease and cancer. While the ubiquitin-proteasome system efficiently removes many soluble misfolded proteins, large or persistent assemblies often require the autophagy-lysosome pathway for their degradation. In the present mini-review, we summarize our knowledge of aggrephagy, the selective clearance of protein aggregates by autophagy, and discuss two recent manuscripts that argue that some aggregates must be primed for autophagosomal degradation, through chaperone-mediated remodeling. Aggrephagy substrates are defined by aggregate architecture, biophysical state, surface accessibility, and the physical constraints of membrane capture. These features help to explain why recruitment of selective autophagy receptors is necessary yet insufficient for clearance. Receptor clustering is required to concentrate early autophagy factors to establish initiation hubs, but successful degradation often requires upstream generation of smaller 'aggrephagy-competent' cargo units, which contain autophagy receptor clusters that successfully initiate autophagosome formation. Recent work supports a model in which larger aggregates are cleared through stepwise degradation enabled by prior remodeling steps that involve p97/VCP-driven disintegration or a chaperone module (DNAJB6-HSP70-HSP110) cooperating with the proteasomal 19S regulatory particle.