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Heat shock protein 90 (Hsp90) is a highly conserved molecular chaperone that regulates the maturation of various client proteins. Most therapeutic studies have focused on N-terminal Hsp90 inhibitors, but these are limited by dose-escalating toxicities that are caused by induction of the heat shock response. Leonard Neckers’discovery of novobiocin as a Hsp90 C-terminal inhibitor revealed an alternative mode to Hsp90 inhibition and established the C-terminal domain (CTD) as a therapeutic target. This review highlights recent advances in Hsp90 CTD inhibition and summarizes the evolution of novobiocin-based C-terminal inhibitors. Structure-activity relationship studies are discussed, demonstrating how medicinal chemistry optimization has produced CTD modulators with selective anti-proliferative or neuroprotective activities.

Hsp90 is a dimeric molecular chaperone essential for the maturation, activation, stabilization, and folding of numerous clients required for cellular functions. Hsp90 progresses through a dynamic ATP-driven conformational cycle that is precisely regulated by accessory proteins known as co-chaperones. Here, we show that the isolated N-domain of Aha1 (Aha1N156) binds the apo state of Hsp90 but fails to associate with the closed, nucleotide-bound state. In contrast, the full-length Aha1 binds Hsp90 in both conformational states, suggesting a key role for the Aha1 C domain in binding to the nucleotide-bound, closed state of Hsp90. Surprisingly, the Aha1 paralogue Hch1, which corresponds to the Aha1 N domain, was capable of binding to Hsp90 in both the apo and nucleotide-bound states. Interestingly, the addition of a 14 amino acid residues section of the linker to the Aha1 N domain restores closedstate binding, indicating an unexpected role for the linker in stabilizing nucleotide-dependent interactions. Analysis of yeast-human Aha1 chimeras further demonstrates that the C-terminal domain of Aha-type co-chaperones serves as an evolutionarily conserved anchoring module, enabling stable engagement of the ATP-bound state despite significant sequence divergence. This work allows us to propose a model in which the Aha1 C domain allows for the repositioning of the Aha1 N domain that occurs during the transition from the apo to the ATP-bound state of Hsp90.

LA1011 (dimethyl 4-(4-Trifluoro-methyl-phenyl)-2,6-bis (2-dimethylamino-ethyl)-1-methyl-1-4 dihydropyridine-3-5-dicarboxylate dihydrochloride) has been shown to improve the prognosis of Alzheimer’s disease (AD) in an APPxPS1 mouse model. The target for LA1011 is the C-terminal domain of Hsp90, where it was shown previously to reduce the interaction between FKBP51 and Hsp90. FKBP51 is a Hsp90 co-chaperone that promotes the trans to cis isomerization of proline at multiple tau pSer/pThr-pro sites, thus preventing their dephosphorylation. Potentially this leads to the hyperphosphorylation of tau and the formation of neurofibrillary tangles that eventually lead to the development of AD. In this study, we demonstrate that LA1011 affects the FKBP51-mediated regulation of Hsp90 but also potentially modulates the regulation Hsp90 by the co-chaperones FKBP52, CHIP, Aha1, Hch1 and PP5. We also show that the co-chaperones HOP, CDC37 and Sgt1 appear to enhance mildly the binding of LA1011. In contrast, nucleotide alone or nucleotide with Aha1 or p23, which promote the closed conformation of Hsp90, reduce the affinity for LA1011. We conclude that LA1011 can modulate the regulatory landscape of the Hsp90 co-chaperone network, which in turn appears to improve the prognosis of Alzheimer’s disease.

Maintenance of protein homeostasis, also known as proteostasis, is essential for cellular survival under both basal and stress conditions. Proteostasis relies on a coordinated action between molecular chaperones, such as heat shock proteins (HSPs), and stress-responsive transcription factors. HSP90 is an abundant and functionally central ATP-dependent chaperone that supports the stability and function of a great variety of client proteins, while specific members of the heat shock factor (HSF) family orchestrate transcriptional programs in cells exposed to proteotoxic stress. According to the established chaperone titration model, HSP90, together with other chaperones, represses the master regulator HSF1 by maintaining it in an inactive monomeric state. Emerging evidence, however, indicates that also other HSFs, especially HSF2, can form a complex with HSP90 and contribute to constitutive and stress-inducible HSP gene regulation, thereby expanding the HSF1-centric view of the chaperone titration model. This review discusses the current understanding of the HSP90-HSF interplay and highlights the recent advances in targeting HSP90 for therapeutic purposes. Together, these insights underscore the HSP90-HSF axis as a regulatory hub of proteostasis in health and disease.

Purpose: The aim of this experimental study was to investigate the pathogenesis of Terson syndrome (TS), which currently is controversial. Methods: The central retinal artery (in 39 orbits), posterior ciliary arteries (in 8 orbits), and central retinal vein (CRV in 21 orbits) were occluded in rhesus monkeys by exposing them to lateral orbitotomy. Fundus examination and fluorescein fundus angiography were performed before and immediately after cutting the vessels and serially thereafter during the follow-up period. The rationale of the experimental study design is discussed. Results: In eyes with central retinal artery occlusion, retinal hemorrhages were seen soon after the procedure in 7 eyes, and on follow-up in a total of 15 eyes. In posterior ciliary artery occlusion, retinal hemorrhages were seen soon after the procedure in one eye, and on follow-up in a total of three eyes. In eyes with CRV, all eyes had extensive scattered retinal hemorrhages. Conclusion: The findings of this experimental study, and my basic, experimental, and comprehensive clinical studies on CRVO, suggest the following concept of the pathogenesis of TS: Compression of the CRV plays a crucial role in the development of TS. The CRV is compressed, as it lies in the subarachnoid space of the optic nerve sheath, by raised cerebrospinal fluid pressure and/or accumulated blood. This results in retinal venous stasis and raised venous pressure in the retinal veins, leading to venous engorgement, rupture of the retinal capillaries and retinal hemorrhages. The clinical importance of compression of the CRV and not occlusion of CRV in TS is that optic nerve sheath decompression by opening it and releasing the blood and raised cerebrospinal fluid (CSF) pressure, would result in immediate decompressing of the CRV in the subarachnoid space and restoration of normal circulation and prevent visual loss.