There are more Hsp90 chaperone mechanisms in heaven and earth, dear reader, than are dreamt of in your philosophy

Hsp90 and cochaperones have two genetically distinct roles in regulating eEF2 function

Protein homeostasis relies on the accurate translation and folding of newly synthesized proteins. Eukaryotic elongation factor 2 (eEF2) promotes GTP-dependent translocation of the ribosome during translation. eEF2 folding was recently shown to be dependent on Hsp90 as well as the cochaperones Hgh1, Cns1, and Cpr7. We examined the requirement for Hsp90 and cochaperones more closely and found that Hsp90 and cochaperones have two distinct roles in regulating eEF2 function. Yeast expressing one group of Hsp90 mutations or one group of cochaperone mutations had reduced steady-state levels of eEF2. The growth of Hsp90 mutants that affected eEF2 accumulation was also negatively affected by deletion of the gene encoding Hgh1. Further, mutations in yeast eEF2 that mimic disease-associated mutations in human eEF2 were negatively impacted by loss of Hgh1 and growth of one mutant was partially rescued by overexpression of Hgh1. In contrast, yeast expressing different groups of Hsp90 mutations or a different cochaperone mutation had altered sensitivity to diphtheria toxin, which is dictated by a unique posttranslational modification on eEF2. Our results provide further evidence that Hsp90 contributes to proteostasis not just by assisting protein folding, but also by enabling accurate translation of newly synthesized proteins. In addition, these results provide further evidence that yeast Hsp90 mutants have distinct in vivo effects that correlate with defects in subsets of cochaperones.

Structural transitions modulate the chaperone activities of Grp94

Hsp90s are ATP-dependent chaperones that collaborate with co-chaperones and Hsp70s to remodel client proteins. Grp94 is the ER Hsp90 homolog essential for folding multiple secretory and membrane proteins. Grp94 interacts with the ER Hsp70, BiP, although the collaboration of the ER chaperones in protein remodeling is not well understood. Grp94 undergoes large-scale conformational changes that are coupled to chaperone activity. Within Grp94, a region called the pre-N domain suppresses ATP hydrolysis and conformational transitions to the active chaperone conformation. In this work, we combined in vivo and in vitro functional assays and structural studies to characterize the chaperone mechanism of Grp94. We show that Grp94 directly collaborates with the BiP chaperone system to fold clients. Grp94's pre-N domain is not necessary for Grp94-client interactions. The folding of some Grp94 clients does not require direct interactions between Grp94 and BiP in vivo, suggesting that the canonical collaboration may not be a general chaperone mechanism for Grp94. The BiP co-chaperone DnaJB11 promotes the interaction between Grp94 and BiP, relieving the pre-N domain suppression of Grp94's ATP hydrolysis activity. In structural studies, we find that ATP binding by Grp94 alters the ATP lid conformation, while BiP binding stabilizes a partially closed Grp94 intermediate. Together, BiP and ATP push Grp94 into the active closed conformation for client folding. We also find that nucleotide binding reduces Grp94's affinity for clients, which is important for productive client folding. Alteration of client affinity by nucleotide binding may be a conserved chaperone mechanism for a subset of ER chaperones.

Insights into Hsp90 mechanism and in vivo functions learned from studies in the yeast, Saccharomyces cerevisiae

The molecular chaperone Hsp90 (Heat shock protein, 90 kDa) is an abundant and essential cytosolic protein required for the stability and/or folding of hundreds of client proteins. Hsp90, along with helper cochaperone proteins, assists client protein folding in an ATP-dependent pathway. The laboratory of Susan Lindquist, in collaboration with other researchers, was the first to establish the yeast Saccharomyces cerevisiae as a model organism to study the functional interaction between Hsp90 and clients. Important insights from studies in her lab were that Hsp90 is essential, and that Hsp90 functions and cochaperone interactions are highly conserved between yeast and mammalian cells. Here, we describe key mechanistic insights into the Hsp90 folding cycle that were obtained using the yeast system. We highlight the early contributions of the laboratory of Susan Lindquist and extend our analysis into the broader use of the yeast system to analyze the understanding of the conformational cycle of Hsp90 and the impact of altered Hsp90 function on the proteome.

Magnetic Nanodroplets for Enhanced Deep Penetration of Solid Tumors and Simultaneous Magnetothermal-Sensitized Immunotherapy against Tumor Proliferation and Metastasis

The central cells of solid tumors are more proliferative and metastatic than the marginal cells. Therefore, more intelligent strategies for targeting cells with deep spatial distributions in solid tumors remain to be explored. In this work, a biocompatible nanotheranostic agent with a lipid membrane-coated, Fe₃ O₄ and perfluoropentane (PFP)-loaded, cRGD peptide (specifically targeting the integrin αvβ3 receptor)-grafted, magnetic nanodroplets (MNDs) is developed. The MNDs exhibit excellent magnetothermal conversion and controllable magnetic hyperthermia (MHT) through alternating magnetic field regulation. Furthermore, MHT-mediated magnetic droplet vaporization (MDV) induces the expansion of the MNDs to transform them into ultrasonic microbubbles, increasing the permeability of tissue and the cell membrane via the ultrasound-targeted microbubble destruction (UTMD) technique and thereby promoting the deep penetration of MNDs in solid tumors. More importantly, MHT not only causes apoptotic damage by downregulating the expression of the HSP70, cyclin D1, and Bcl-2 proteins in tumor cells but also improves the response rate to T-cell-related immunotherapy by upregulating PD-L1 expression in tumor cells, thus inhibiting the growth of both primary and metastatic tumors. Overall, this work introduces a distinct application of nanoultrasonic biomedicine in cancer therapy and provides an attractive immunotherapy strategy for preventing the proliferation and metastasis of deeply distributed cells in solid tumors.

Unfolded protein response in endothelial injury

Endothelial barrier dysfunction is associated with sepsis and lung injury, both direct and indirect. We discuss the involvement of unfolded protein response in the protective effects of heat shock protein 90 inhibitors and growth hormone releasing hormone antagonists in the vascular barrier, to reveal new possibilities in acute respiratory distress syndrome treatment.