PolyQ-expanded proteins impair cellular proteostasis of ataxin-3 through sequestering the co-chaperone HSJ1 into aggregates

Abstract

Polyglutamine (polyQ) expansion of proteins can trigger protein misfolding and amyloid-like aggregation, which thus lead to severe cytotoxicities and even the respective neurodegenerative diseases. However, why polyQ aggregation is toxic to cells is not fully elucidated. Here, we took the fragments of polyQ-expanded (PQE) ataxin-7 (Atx7) and huntingtin (Htt) as models to investigate the effect of polyQ aggregates on the cellular proteostasis of endogenous ataxin-3 (Atx3), a protein that frequently appears in diverse inclusion bodies. We found that PQE Atx7 and Htt impair the cellular proteostasis of Atx3 by reducing its soluble as well as total Atx3 level but enhancing formation of the aggregates. Expression of these polyQ proteins promotes proteasomal degradation of endogenous Atx3 and accumulation of its aggregated form. Then we verified that the co-chaperone HSJ1 is an essential factor that orchestrates the balance of cellular proteostasis of Atx3; and further discovered that the polyQ proteins can sequester HSJ1 into aggregates or inclusions in a UIM domain-dependent manner. Thereby, the impairment of Atx3 proteostasis may be attributed to the sequestration and functional loss of cellular HSJ1. This study deciphers a potential mechanism underlying how PQE protein triggers proteinopathies, and also provides additional evidence in supporting the hijacking hypothesis that sequestration of cellular interacting partners by protein aggregates leads to cytotoxicity or neurodegeneration.

Targeting DNA topoisomerases or checkpoint kinases results in an overload of chaperone systems, triggering aggregation of a metastable subproteome

“…We find that all aggregating fractions, including U2OS ATM KO aggregation, are enriched for (co)chaperone interactors (Figure 5 Intriguingly, the (co)chaperones that we found to aggregate themselves are among the most frequent interactors. This suggests that they were sequestered by protein aggregates as they engaged their client proteins, in line with what has been reported for disease-associated aggregation (Hipp et al 2019;Jana 2000;Kim et al 2013;Mogk et al 2018;Yu et al 2019;Yue et al 2021). Overall, we find that the relative levels of chaperone engagement of the different aggregating fractions largely reflect their respective supersaturation and LLPSpropensities.…”

Targeting DNA topoisomerases or checkpoint kinases results in an overload of chaperone systems, triggering aggregation of a metastable subproteome

“…Intriguingly, several (co)chaperones that we found to aggregate themselves are among the most frequent interactors ( Figure 5D ). This suggests that they were sequestered by protein aggregates as they engaged their client proteins, in line with what has been reported for disease-associated aggregation ( Hipp et al, 2019 ; Jana et al, 2000 ; Kim et al, 2013 ; Mogk et al, 2018 ; Yu et al, 2019 ; Yue et al, 2021 ). Overall, we find that the relative levels of chaperone engagement of the different aggregating fractions largely reflect their respective supersaturation and LLPS propensities.…”

Aptamer Reduces Aggregation of Mutant Huntingtin and Rescues Proteostasis Network in Non-Neuronal and Neuronal Cells

“…In HD, the interaction of aggregated mutant huntingtin with cytosolic proteins leads to sequestration of chaperones and impairs the cellular proteostasis network. [13][14][15][16][17]66 Previous studies have shown reduced levels of HSP70 and HSP40 in brain tissues of HD mice 18,67 and other HD models. 19 Sequestration of HSP70 and TATA-binding protein in the intracellular aggregates of 96QN (with nuclear localization signal) in HEK293 cells and intracellular aggregates in human post-mortem tissue from HD individuals was also observed.…”

“…Mutant huntingtin aggregates cause increased oxidative stress and impair mitochondrial biogenesis and ROS metabolism. − The binding of mutant huntingtin to the promoter region of peroxisome proliferator-activated receptor-γ coactivator (PGC-1α), a transcription co-regulator of mitochondrial biogenesis, downregulates synthesis of PGC-1α. − In addition, direct interaction of mHTT with PGC-1α reduces its function and affects the synthesis of downstream effectors of the cellular antioxidant system. The growing huntingtin aggregate also sequesters several cellular proteins, including chaperones, so free chaperones are not available to take part in cellular processes. − As molecular chaperones play a vital role in a large number of protein folding processes, sequestration of chaperones in the cells induces proteostasis imbalance, which further alleviates the cytotoxicity due to the accumulation of the aggregated and misfolded protein. Progressive decline in HSP40 and HSP70 levels was observed in the brain of R6/2 HD mice .…”