Intracellular aggregation of the transthyretin protein is limited by Hsp70 chaperones

Extracellular amyloid deposits are a hallmark feature of systemic protein aggregation diseases such as transthyretin amyloidosis (ATTR). However, emerging evidence suggests that extracellular transthyretin (TTR) aggregates are internalized and result in an intracellular stress response, including elevated Hsp70 levels. While ATTR research has predominantly focused on extracellular TTR amyloid, our understanding of TTR aggregation inside the cell is poorly explored. To better understand how intracellular chaperones impact intracellular TTR, we used a yeast model that expresses TTR fused eGFP (TTR-eGFP) within the cytoplasm. Since the Hsp70 chaperone family, and co-chaperones the J-domain proteins (JDPs) and Hsp110, act as a disaggregase in vitro, we asked how these molecular chaperones impact TTR aggregation intracellularly in vivo. TTR-eGFP forms detergent-soluble high molecular weight (HMW) aggregates in yeast that have biochemistry profiles similar to human patient TTR. While knockdown of the JDP, Sis1, and deletion of the Hsp110, Sse1, appear to slightly increase TTR-eGFP aggregation, the loss of two major yeast Hsp70s, Ssa1 and Ssa2, lead to a significant increase in the size of HMW species. Taken together, our data suggest that Hsp70s limit the formation of HMW TTR aggregates in the intracellular environment. Based on our results, it is possible that the age-related decline of protein homeostasis, including Hsp70s, may promote the intracellular aggregation of TTR.