Formation of toxic oligomers of polyQ-expanded Huntingtin by prion-mediated cross-seeding.

Manifestation of aggregate pathology in Huntington's disease is thought to be facilitated by a preferential vulnerability of affected brain cells to age-dependent proteostatic decline. To understand how specific cellular backgrounds may facilitate pathologic aggregation, we utilized the yeast model in which polyQ-expanded Huntingtin forms aggregates only when the endogenous prion-forming protein Rnq1 is in its amyloid-like prion [PIN+] conformation. We employed optogenetic clustering of polyQ protein as an orthogonal method to induce polyQ aggregation in prion-free [pin-] cells. Optogenetic aggregation circumvented the prion requirement for the formation of detergent-resistant polyQ inclusions but bypassed the formation of toxic polyQ oligomers, which accumulated specifically in [PIN+] cells. Reconstitution of aggregation in vitro suggested that these polyQ oligomers formed through direct templating on Rnq1 prions. These findings shed light on the mechanism of prion-mediated formation of oligomers, which may play a role in triggering polyQ pathology in the patient brain.

Sis1 potentiates the stress response to protein aggregation and elevated temperature.

Cells adapt to conditions that compromise protein conformational stability by activating various stress response pathways, but the mechanisms used in sensing misfolded proteins remain unclear. Moreover, aggregates of disease proteins often fail to induce a productive stress response. Here, using a yeast model of polyQ protein aggregation, we identified Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress. At elevated levels, Sis1 prevented the formation of dense polyQ inclusions and directed soluble polyQ oligomers towards the formation of permeable condensates. Hsp70 accumulated in a liquid-like state within this polyQ meshwork, resulting in a potent activation of the HSF1 dependent stress response. Sis1, and the homologous DnaJB6 in mammalian cells, also regulated the magnitude of the cellular heat stress response, suggesting a general role in sensing protein misfolding. Sis1/DnaJB6 functions as a limiting regulator to enable a dynamic stress response and avoid hypersensitivity to environmental changes.