A non-toxic Hsp90 inhibitor protects neurons from Abeta-induced toxicity

Inflammatory signaling pathways in Alzheimer's disease: Mechanistic insights and possible therapeutic interventions

The complex pathophysiology of Alzheimer's disease (AD) poses challenges for the development of therapies. Recently, neuroinflammation has been identified as a key pathogenic mechanism underlying AD, while inflammation has emerged as a possible target for the management and prevention of AD. Several prior studies have demonstrated that medications modulating neuroinflammation might lessen AD symptoms, mostly by controlling neuroinflammatory signaling pathways such as the NF-κB, MAPK, NLRP3, etc, and their respective signaling cascade. Moreover, targeting these inflammatory modalities with inhibitors, natural products, and metabolites has been the subject of intensive research because of their anti-inflammatory characteristics, with many studies demonstrating noteworthy pharmacological capabilities and potential clinical applications. Therefore, targeting inflammation is considered a promising strategy for treating AD. This review comprehensively elucidates the neuroinflammatory mechanisms underlying AD progression and the beneficial effects of inhibitors, natural products, and metabolites in AD treatment.

Protection against Aβ-induced neuronal damage by KU-32: PDHK1 inhibition as important target

A feature of most neurodegenerative diseases is the presence of "mis-folded proteins" that form aggregates, suggesting suboptimal activity of neuronal molecular chaperones. Heat shock protein 90 (Hsp90) is the master regulator of cell responses to "proteotoxic" stresses. Some Hsp90 modulators activate cascades leading to upregulation of additional chaperones. Novobiocin is a modulator at the C-terminal ATP-binding site of Hsp90. Of several novobiocin analogs synthesized and tested for protection against amyloid beta (Aβ)-induced neuronal death, "KU-32" was the most potent in protecting primary neurons, but did not increase expression of other chaperones believed to help clear misfolded proteins. However, KU-32 reversed Aβ-induced superoxide formation, activated Complex I of the electron transfer chain in mitochondria, and blocked the Aβ-induced inhibition of Complex I in neuroblastoma cells. A mechanism for these effects of KU-32 on mitochondrial metabolism appeared to be the inhibition of pyruvate dehydrogenase kinase (PDHK), both in isolated brain mitochondria and in SH-SY5Y cells. PDHK inhibition by the classic enzyme inhibitor, dichloroacetate, led to neuroprotection from Aβ25-35-induced cell injury similarly to KU-32. Inhibition of PDHK in neurons would lead to activation of the PDH complex, increased acetyl-CoA generation, stimulation of the tricarboxylic acid cycle and Complex I in the electron transfer chain, and enhanced oxidative phosphorylation. A focus of future studies may be on the potential value of PDHK as a target in AD therapy.

Network pharmacology reveals that Berberine may function against Alzheimer's disease via the AKT signaling pathway

Objective

To investigate the mechanism underlying the effects of berberine (BBR) in the treatment of Alzheimer's disease (AD).

Methods

3 × Tg AD mice were treated with BBR for 3 months, then the open field test (OFT), the novel object recognition test (NOR) and the Morris water maze (MWM) test were performed to assess behavioral performance. Hematoxylin-eosin (HE) staining, Nissl staining were used to examine histopathological changes. The pharmacological and molecular properties of BBR were obtained from the TCMSP database. BBR-associated AD targets were identified using the PharmMapper (PM), the comparative toxicogenomics database (CTD), DisGeNet and the human gene database (GeneCards). Core networks and BBR targets for the treatment of AD were identified using PPI network and functional enrichment analyses. AutoDock software was used to model the interaction between BBR and potential targets. Finally, RT-qPCR, western blotting were used to validate the expression of core targets.

Results

Behavioral experiments, HE staining and Nissl staining have shown that BBR can improve memory task performance and neuronal damage in the hippocampus of AD mice. 117 BBR-associated targets for the treatment of AD were identified, and 43 genes were used for downstream functional enrichment analysis in combination with the results of protein-protein interaction (PPI) network analysis. 2,230 biological processes (BP) terms, 67 cell components (CC) terms, 243 molecular function (MF) terms and 118 KEGG terms were identified. ALB, EGFR, CASP3 and five targets in the PI3K-AKT signaling pathway including AKT1, HSP90AA1, SRC, HRAS, IGF1 were selected by PPI network analysis, validated by molecular docking analysis and RT-q PCR as core targets for further analysis. Akt1 mRNA expression levels were significantly decreased in AD mice and significantly increased after BBR treatment (p < 0.05). Besides, AKT and ERK phosphorylation decreased in the model group, and BBR significantly increased their phosphorylation levels.

Conclusion

AKT1, HSP90AA1, SRC, HRAS, IGF1 and ALB, EGFR, CASP3 were core targets of BBR in the treatment of AD. BBR may exert a neuroprotective effect by modulating the ERK and AKT signaling pathways.

Heat shock factor HSF1 regulates BDNF gene promoters upon acute stress in the hippocampus, together with pCREB

Heat shock factor 1 (HSF1) is a master stress-responsive transcriptional factor, protecting cells from death. However, its gene regulation in vivo in the brain in response to neuronal stimuli remains elusive. Here, we investigated its direct regulation of the brain-derived neurotrophic factor (BDNF) gene (Bdnf) in response to acute neuronal stress stimuli in the brain. The results of immunohistochemistry and chromatin immunoprecipitation quantitative PCR (ChIP-qPCR) showed that administration of kainic acid (a glutamate receptor agonist inducing excitotoxity) to young adult mice induced HSF1 nuclear translocation and its binding to multiple Bdnf promoters in the hippocampus. Footshock, a physical stressor used for learning, also induced HSF1 binding to selected Bdnf promoters I and IV. This is, to our knowledge, the first demonstration of HSF1 gene regulation in response to neuronal stimuli in the hippocampus in vivo. HSF1 binding sites (HSEs) in Bdnf promoters I and IV were also detected when immunoprecipitated by an antibody of phosphorylated (p)CREB (cAMP-responsive element-binding protein), suggesting their possible interplay in acute stress-induced Bdnf transcription. Interestingly, their promoter binding patterns differed by KA and footshock, suggesting that HSF1 and pCREB orchestrate to render fine-tuned promoter control depending on the types of stress. Further, HSF1 overexpression increased Bdnf promoter activity in a luciferase assay, while virus infection of constitutively active-form HSF1 increased levels of BDNF mRNA and protein in vitro in primary cultured neurons. These results indicated that HSF1 activation of Bdnf promoter was sufficient to induce BDNF expression. Taken together, these results suggest that HSF1 promoter-specific control of Bdnf gene regulation plays an important role in neuronal protection and plasticity in the hippocampus in response to acute stress, possibly interplaying with pCREB.

Dose-dependent manner of luteolin in the modulation of spatial memory with respect to the hippocampal level of HSP70 and HSP90 in sleep-deprived rats

Sleep deprivation (SD) induces a variety of deleterious effects on different cognitive functions such as memory. Elevated neuroinflammation, oxidative stress, and apoptosis, and decreased synaptic plasticity and antioxidant capacity are involved in the deleterious effects of SD on memory. On the other hand, luteolin (a flavonoid compound) has antioxidant, neuroprotective, and anti-inflammatory properties. Also, Heat shock protein 70 (HSP70) and Heat shock protein 90 (HSP90) can be involved in modulating memory. In this study, we aimed to assess the effects of SD and luteolin on spatial learning and memory using Morris Water Maze apparatus in rats, with respect to the level of HSP70 and HSP90 in the hippocampus. Luteolin was injected intracerebroventricular (i.c.v.) at the doses of 0.5, 1, and 2 µg/rat. The results showed that SD impaired spatial memory, while luteolin dose-dependently restored SD-induced spatial memory impairment. SD increased the expression level of HSP90 in the hippocampus, whereas luteolin dose-dependently reversed the effect of SD. Furthermore, SD decreased the expression level of HSP70 protein in the hippocampus, while luteolin dose-dependently reversed the effect of SD. In conclusion, HSP70 and HSP90 may be involved in the deleterious effect of SD on memory, and in the improvement effect of luteolin on memory. This is a novel study reporting novel data and we suggest further detailed studies to better understand the interactions between SD, luteolin, and Heat shock proteins.

Protein misfolding and related human diseases: A comprehensive review of toxicity, proteins involved, and current therapeutic strategies

Most of the cell's chemical reactions and structural components are facilitated by proteins. But proteins are highly dynamic molecules, where numerous modifications or changes in the cellular environment can affect their native conformational fold leading to protein aggregation. Various stress conditions, such as oxidative stress, mutations and metal toxicity may cause protein misfolding and aggregation by shifting the conformational equilibrium towards more aggregation-prone states. Most of the protein misfolding diseases (PMDs) involve aggregation of protein. We have discussed such proteins like Aβ peptide, α-synuclein, amylin and lysozyme involved in Alzheimer's, Parkinson's, type II diabetes and non-neuropathic systemic amyloidosis respectively. Till date, all advances in PMDs therapeutics help symptomatically but do not prevent the root cause of the disease, i.e., the aggregation of protein involved in the diseases. Current efforts focused on developing therapies for PMDs have employed diverse strategies; repositioning pre-existing drugs as it saves time and money; natural compounds that are touted as potential drug candidates have an advantage of being taken in diet normally and will induce lesser side effects. This review also covers recently developed therapeutic strategies like antisense drugs and disaggregases which has yielded therapeutic agents that have transitioned from preclinical studies into human clinical trials.

Hsp90 Inhibition: A Promising Therapeutic Approach for ARSACS

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a neurodegenerative disease caused by mutations in the SACS gene, encoding the 520 kDa modular protein sacsin, which comprises multiple functional sequence domains that suggest a role either as a scaffold in protein folding or in proteostasis. Cells from patients with ARSACS display a distinct phenotype including altered organisation of the intermediate filament cytoskeleton and a hyperfused mitochondrial network where mitochondrial respiration is compromised. Here, we used vimentin bundling as a biomarker of sacsin function to test the therapeutic potential of Hsp90 inhibition with the C-terminal-domain-targeted compound KU-32, which has demonstrated mitochondrial activity. This study shows that ARSACS patient cells have significantly increased vimentin bundling compared to control, and this was also present in ARSACS carriers despite them being asymptomatic. We found that KU-32 treatment significantly reduced vimentin bundling in carrier and patient cells. We also found that cells from patients with ARSACS were unable to maintain mitochondrial membrane potential upon challenge with mitotoxins, and that the electron transport chain function was restored upon KU-32 treatment. Our preliminary findings presented here suggest that targeting the heat-shock response by Hsp90 inhibition alleviates vimentin bundling and may represent a promising area for the development of therapeutics for ARSACS.

Neurobiological Opportunities in Diabetic Polyneuropathy

This review highlights a selection of potential translational directions for the treatment of diabetic polyneuropathy (DPN) currently irreversible and without approved interventions beyond pain management. The list does not include all diabetic targets that have been generated over several decades of research but focuses on newer work. The emphasis is firstly on approaches that support the viability and growth of peripheral neurons and their ability to withstand a barrage of diabetic alterations. We include a section describing Schwann cell targets and finally how mitochondrial damage has been a common element in discussing neuropathic damage. Most of the molecules and pathways described here have not yet reached clinical trials, but many trials have been negative to date. Nonetheless, these failures clear the pathway for new thoughts over reversing DPN.

Protein misfolding, ER Stress and Chaperones: An approach to develop chaperone-based therapeutics for Alzheimer's Disease

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder with complex etiology that eventually leads to dementia. The main culprit of AD is the extracellular deposition of β-amyloid (Aβ) and intracellular neurofibrillary tangles. The protein conformational change and protein misfolding are the key events of AD pathophysiology, therefore endoplasmic reticulum (ER) stress is an apparent consequence. ER, stress-induced unfolded protein response (UPR) mediators (viz. PERK, IRE1, and ATF6) have been reported widely in the AD brain. Considering these factors, preventing proteins misfolding or aggregation of tau or amyloidogenic proteins appears to be the best approach to halt its pathogenesis. Therefore, therapies through chemical and pharmacological chaperones came to light as an alternative for the treatment of AD. Diverse studies have demonstrated 4-phenylbutyric acid (4-PBA) as a potential therapeutic agent in AD. The current review outlined the mechanism of protein misfolding, different etiological features behind the progression of AD, the significance of ER stress in AD, and the potential therapeutic role of different chaperones to counter AD. The study also highlights the gaps in current knowledge of the chaperones-based therapeutic approach and the possibility of developing chaperones as a potential therapeutic agent for AD treatment.

Role of Withaferin A and Its Derivatives in the Management of Alzheimer's Disease: Recent Trends and Future Perspectives

Globally, Alzheimer's disease (AD) is one of the most prevalent age-related neurodegenerative disorders associated with cognitive decline and memory deficits due to beta-amyloid deposition (Aβ) and tau protein hyperphosphorylation. To date, approximately 47 million people worldwide have AD. This figure will rise to an estimated 75.6 million by 2030 and 135.5 million by 2050. According to the literature, the efficacy of conventional medications for AD is statistically substantial, but clinical relevance is restricted to disease slowing rather than reversal. Withaferin A (WA) is a steroidal lactone glycowithanolides, a secondary metabolite with comprehensive biological effects. Biosynthetically, it is derived from Withania somnifera (Ashwagandha) and Acnistus breviflorus (Gallinero) through the mevalonate and non-mevalonate pathways. Mounting evidence shows that WA possesses inhibitory activities against developing a pathological marker of Alzheimer's diseases. Several cellular and animal models' particulates to AD have been conducted to assess the underlying protective effect of WA. In AD, the neuroprotective potential of WA is mediated by reduction of beta-amyloid plaque aggregation, tau protein accumulation, regulation of heat shock proteins, and inhibition of oxidative and inflammatory constituents. Despite the various preclinical studies on WA's therapeutic potentiality, less is known regarding its definite efficacy in humans for AD. Accordingly, the present study focuses on the biosynthesis of WA, the epidemiology and pathophysiology of AD, and finally the therapeutic potential of WA for the treatment and prevention of AD, highlighting the research and augmentation of new therapeutic approaches. Further clinical trials are necessary for evaluating the safety profile and confirming WA's neuroprotective potency against AD.

The role and therapeutic potential of Hsp90, Hsp70, and smaller heat shock proteins in peripheral and central neuropathies

Heat shock proteins (Hsps) are molecular chaperones that also play important roles in the activation of the heat shock response (HSR). The HSR is an evolutionary conserved and protective mechanism that is used to counter abnormal physiological conditions, stressors, and disease states, such as those exemplified in cancer and/or neurodegeneration. In normal cells, heat shock factor-1 (HSF-1), the transcription factor that regulates the HSR, remains in a dormant multiprotein complex that is formed upon association with chaperones (Hsp90, Hsp70, etc.), co-chaperones, and client proteins. However, under cellular stress, HSF-1 dissociates from Hsp90 and induces the transcriptional upregulation of Hsp70 to afford protection against the encountered cellular stress. As a consequence of both peripheral and central neuropathies, cellular stress occurs and results in the accumulation of unfolded and/or misfolded proteins, which can be counterbalanced by activation of the HSR. Since Hsp90 is the primary regulator of the HSR, modulation of Hsp90 by small molecules represents an attractive therapeutic approach against both peripheral and central neuropathies.

ATP Impedes the Inhibitory Effect of Hsp90 on Aβ40 Fibrillation

Heat shock protein 90 (Hsp90) is a molecular chaperone that assists protein folding in an Adenosine triphosphate (ATP)-dependent way. Hsp90 has been reported to interact with Alzheimeŕs disease associated amyloid-β (Aβ) peptides and to suppress toxic oligomer- and fibril formation. However, the mechanism remains largely unclear. Here we use a combination of atomic force microscopy (AFM) imaging, circular dichroism (CD) spectroscopy and biochemical analysis to quantify this interaction and put forward a microscopic picture including rate constants for the different transitions towards fibrillation. We show that Hsp90 binds to Aβ₄₀ monomers weakly but inhibits Aβ₄₀ from growing into fibrils at substoichiometric concentrations. ATP impedes this interaction, presumably by modulating Hsp90's conformational dynamics and reducing its hydrophobic surface. Altogether, these results might indicate alternative ways to prevent Aβ₄₀ fibrillation by manipulating chaperones that are already abundant in the brain.

Increased levels of Stress-inducible phosphoprotein-1 accelerates amyloid-β deposition in a mouse model of Alzheimer's disease

Molecular chaperones and co-chaperones, which are part of the protein quality control machinery, have been shown to regulate distinct aspects of Alzheimer's Disease (AD) pathology in multiple ways. Notably, the co-chaperone STI1, which presents increased levels in AD, can protect mammalian neurons from amyloid-β toxicity in vitro and reduced STI1 levels worsen Aβ toxicity in C. elegans. However, whether increased STI1 levels can protect neurons in vivo remains unknown. We determined that overexpression of STI1 and/or Hsp90 protected C. elegans expressing Aβ(3-42) against Aβ-mediated paralysis. Mammalian neurons were also protected by elevated levels of endogenous STI1 in vitro, and this effect was mainly due to extracellular STI1. Surprisingly, in the 5xFAD mouse model of AD, by overexpressing STI1, we find increased amyloid burden, which amplifies neurotoxicity and worsens spatial memory deficits in these mutants. Increased levels of STI1 disturbed the expression of Aβ-regulating enzymes (BACE1 and MMP-2), suggesting potential mechanisms by which amyloid burden is increased in mice. Notably, we observed that STI1 accumulates in dense-core AD plaques in both 5xFAD mice and human brain tissue. Our findings suggest that elevated levels of STI1 contribute to Aβ accumulation, and that STI1 is deposited in AD plaques in mice and humans. We conclude that despite the protective effects of STI1 in C. elegans and in mammalian cultured neurons, in vivo, the predominant effect of elevated STI1 is deleterious in AD.

Heat shock protein 90 inhibitors block the antinociceptive effects of opioids in mouse chemotherapy-induced neuropathy and cancer bone pain models

Heat shock protein 90 (Hsp90) is a ubiquitous signal transduction regulator, and Hsp90 inhibitors are in clinical development as cancer therapeutics. However, there have been very few studies on the impact of Hsp90 inhibitors on pain or analgesia, a serious concern for cancer patients. We previously found that Hsp90 inhibitors injected into the brain block opioid-induced antinociception in tail flick, paw incision, and HIV neuropathy pain. This study extended from that initial work to test the cancer-related clinical impact of Hsp90 inhibitors on opioid antinociception in cancer-induced bone pain in female BALB/c mice and chemotherapy-induced peripheral neuropathy in male and female CD-1 mice. Mice were treated with Hsp90 inhibitors (17-AAG, KU-32) by the intracerebroventricular, intrathecal, or intraperitoneal routes, and after 24 hours, pain behaviors were evaluated after analgesic drug treatment. Heat shock protein 90 inhibition in the brain or systemically completely blocked morphine and oxymorphone antinociception in chemotherapy-induced peripheral neuropathy; this effect was partly mediated by decreased ERK and JNK MAPK activation and by increased protein translation, was not altered by chronic treatment, and Hsp90 inhibition had no effect on gabapentin antinociception. We also found that the Hsp90 isoform Hsp90α and the cochaperone Cdc37 were responsible for the observed changes in opioid antinociception. By contrast, Hsp90 inhibition in the spinal cord or systemically partially reduced opioid antinociception in cancer-induced bone pain. These results demonstrate that Hsp90 inhibitors block opioid antinociception in cancer-related pain, suggesting that Hsp90 inhibitors for cancer therapy could decrease opioid treatment efficacy.

Inhibition of Hsp90 in the spinal cord enhances the antinociceptive effects of morphine by activating an ERK-RSK pathway

Morphine and other opioids are commonly used to treat pain despite their numerous adverse side effects. Modulating μ-opioid receptor (MOR) signaling is one way to potentially improve opioid therapy. In mice, the chaperone protein Hsp90 mediates MOR signaling within the brain. Here, we found that inhibiting Hsp90 specifically in the spinal cord enhanced the antinociceptive effects of morphine in mice. Intrathecal, but not systemic, administration of the Hsp90 inhibitors 17-AAG or KU-32 amplified the effects of morphine in suppressing sensitivity to both thermal and mechanical stimuli in mice. Hsp90 inhibition enabled opioid-induced phosphorylation of the kinase ERK and increased abundance of the kinase RSK in the dorsal horns of the spinal cord, which are heavily populated with primary afferent sensory neurons. The additive effects of Hsp90 inhibition were abolished upon intrathecal inhibition of ERK, RSK, or protein synthesis. This mechanism downstream of MOR, localized to the spinal cord and repressed by Hsp90, may potentially be used to enhance the efficacy and presumably decrease the side effects of opioid therapy.

The Alpha Isoform of Heat Shock Protein 90 and the Co-chaperones p23 and Cdc37 Promote Opioid Anti-nociception in the Brain

Opioid activation of the mu opioid receptor (MOR) promotes signaling cascades that evoke both analgesic responses to pain and side effects like addiction and dependence. Manipulation of these cascades, such as by biased agonism, has great promise to improve opioid therapy. However, the signaling cascades of the MOR are in general poorly understood, providing few targets for drug development. In our earlier work, we identified Heat shock protein 90 (Hsp90) as a novel and crucial regulator of opioid anti-nociception in the brain by promoting ERK MAPK activation. In this study, we sought to identify the molecular isoforms and co-chaperones by which Hsp90 carried out this role, which could provide specific targets for future clinical intervention. We used novel selective small molecule inhibitors as well as CRISPR/Cas9 gene editing constructs delivered by the intracerebroventricular (icv) route to the brains of adult CD-1 mice to target Hsp90 isoforms (Hsp90α/β, Grp94) and co-chaperones (p23, Cdc37, Aha1). We found that inhibition of the isoform Hsp90α fully blocked morphine anti-nociception in a model of post-surgical paw incision pain, while blocking ERK and JNK MAPK activation, suggesting Hsp90α as the main regulator of opioid response in the brain. We further found that inhibition of the co-chaperones p23 and Cdc37 blocked morphine anti-nociception, suggesting that these co-chaperones assist Hsp90α in promoting opioid anti-nociception. Lastly, we used cycloheximide treatment in the brain to demonstrate that rapid protein translation within 30 min of opioid treatment is required for Hsp90 regulation of opioid response. Together these studies provide insight into the molecular mechanisms by which Hsp90 promotes opioid anti-nociception. These findings thus both improve our basic science knowledge of MOR signal transduction and could provide future targets for clinical intervention to improve opioid therapy.

The role of heat shock protein 90 in regulating pain, opioid signaling, and opioid antinociception

Heat shock protein 90 (Hsp90) is one of the central signal transduction regulators of the cell. Via client interactions with hundreds of proteins, including receptors, receptor regulatory kinases, and downstream signaling regulators, Hsp90 has a crucial and wide-ranging impact on signaling in response to numerous drugs with impacts on resultant physiology and behavior. Despite this importance, however, Hsp90 has barely been studied in the context of pain and the opioid receptor system, leaving open the possibility that Hsp90 could be manipulated to improve pain therapeutic outcomes, a current area of massive medical need. In this review, we will highlight the known roles of Hsp90 in directly regulating the initiation and maintenance of the pain state. We will also explore how Hsp90 regulates signaling and antinociceptive responses to opioid analgesic drugs, with a special emphasis on ERK MAPK signaling. Understanding this new and growing area will improve our understanding of how Hsp90 regulates signaling and physiology, and also may provide new ways to treat pain, and perhaps reduce the severe impact of the ongoing opioid addiction and overdose crisis.

Acupuncture promotes expression of Hsp84/86 and delays brain ageing in SAMP8 mice

Objective:

To study the effects of acupuncture on expression of heat shock protein (Hsp) 84 and 86, and brain ageing, in the senescence accelerated mouse prone 8 (SAMP8) model of Alzheimer’s disease.

Methods:

7-month-old male senescence resistant mouse strain 1 (SAMR1) and SAMP8 mice were assigned to the following groups, with 15 animals in each group: SAMR1 control (Rc), SAMP8 control (Pc), SAMP8 acupuncture (Pa), SAMP8 sham-acupuncture (Psa). The Pa group was given acupuncture treatment once daily for 15 days. Neuromuscular coordination and cognitive function of the mice were evaluated by the tightrope test and Morris water maze test, respectively. The number of neurons in the CA1, CA3 and dentate gyrus (DG) regions of the hippocampus were measured. The levels of oxidative stress and protein carbonyl, mRNA and protein expression levels of Hsp84 and Hsp86 in the hippocampus were detected.

Results:

Compared with the Rc group, in the Pc mice there was a lower success rate for the tightrope test, impaired cognitive abilities, a decline in neuron numbers, reduced levels of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), increased levels of superoxide anion and protein carbonyl, and decreased mRNA and protein levels of Hsp84 and Hsp86 (all P<0.05). After acupuncture treatment, the success rate for the tightrope test was elevated, cognitive function was improved, neuron numbers were enhanced, levels of SOD and GSH-Px were increased, levels of superoxide anion and protein carbonyl were decreased, and Hsp84 and Hsp86 mRNA and protein expression were increased in the Pa mice when compared with the Pc and Psa groups (all P<0.05).

Conclusion:

Acupuncture may delay brain ageing in SAMP8 mice by reducing oxidative protein damage and promoting Hsp84 and Hsp86 expression.

Prions, prionoids and protein misfolding disorders

Prion diseases are progressive, incurable and fatal neurodegenerative conditions. The term 'prion' was first nominated to express the revolutionary concept that a protein could be infectious. We now know that prions consist of PrP^Sc, the pathological aggregated form of the cellular prion protein PrP^C. Over the years, the term has been semantically broadened to describe aggregates irrespective of their infectivity, and the prion concept is now being applied, perhaps overenthusiastically, to all neurodegenerative diseases that involve protein aggregation. Indeed, recent studies suggest that prion diseases (PrDs) and protein misfolding disorders (PMDs) share some common disease mechanisms, which could have implications for potential treatments. Nevertheless, the transmissibility of bona fide prions is unique, and PrDs should be considered as distinct from other PMDs.

Stimulation of heat shock protein 90 chaperone function through binding of a novobiocin analog KU-32

Heat shock protein 90 (Hsp90) is a eukaryotic chaperone responsible for the folding and functional activation of numerous client proteins, many of which are oncoproteins. Thus, Hsp90 inhibition has been intensely pursued, resulting in the development of many potential Hsp90 inhibitors, not all of which are well-characterized. Hsp90 inhibitors not only abrogate its chaperone functions, but also could help us gain insight into the structure-function relationship of this chaperone. Here, using biochemical and cell-based assays along with isothermal titration calorimetry, we investigate KU-32, a derivative of the Hsp90 inhibitor novobiocin (NB), for its ability to modulate Hsp90 chaperone function. Although NB and KU-32 differ only slightly in structure, we found that upon binding, they induce completely opposite conformational changes in Hsp90. We observed that NB and KU-32 both bind to the C-terminal domain of Hsp90, but surprisingly, KU-32 stimulated the chaperone functions of Hsp90 via allosteric modulation of its N-terminal domain, responsible for the chaperone's ATPase activity. In vitro and in silico studies indicated that upon KU-32 binding, Hsp90 undergoes global structural changes leading to the formation of a "partially closed" intermediate that selectively binds ATP and increases ATPase activity. We also report that KU-32 promotes HeLa cell survival and enhances the refolding of an Hsp90 substrate inside the cell. This discovery explains the effectiveness of KU-32 analogs in the management of neuropathies and may facilitate the design of molecules that promote cell survival by enhancing Hsp90 chaperone function and reducing the load of misfolded proteins in cells.

The Role of Heat Shock Proteins in Regulating Receptor Signal Transduction

Heat shock proteins (Hsp) are a class of stress-inducible proteins that mainly act as molecular protein chaperones. This chaperone activity is diverse, including assisting in nascent protein folding and regulating client protein location and translocation within the cell. The main proteins within the Hsp family, particularly Hsp70 and Hsp90, also have a highly diverse and numerous set of protein clients, which when combined with the high expression levels of Hsp proteins (2%-6% of total protein content) establishes these molecules as "central regulators" of cell protein physiology. Among the client proteins, Hsps regulate numerous signal-transduction and receptor-regulatory kinases, and indeed directly regulate some receptors themselves. This also makes the Hsps, particularly Hsp90, central regulators of signal-transduction machinery, with important impacts on endogenous and drug ligand responses. Among these roles, Hsp90 in particular acts to maintain mature signaling kinases in a metastable conformation permissive for signaling activation. In this review, we will focus on the roles of the Hsps, with a special focus on Hsp90, in regulating receptor signaling and subsequent physiologic responses. We will also explore potential means to manipulate Hsp function to improve receptor-targeted therapies. Overall, Hsps are important regulators of receptor signaling that are receiving increasing interest and exploration, particularly as Hsp90 inhibitors progress toward clinical approval for the treatment of cancer. Understanding the complex interplay of Hsp regulation of receptor signaling may provide important avenues to improve patient treatment.

The emerging roles of protein homeostasis-governing pathways in Alzheimer's disease

Pathways governing protein homeostasis are involved in maintaining the structural, quantitative, and functional stability of intracellular proteins and involve the ubiquitin-proteasome system, autophagy, endoplasmic reticulum, and mTOR pathway. Due to the broad physiological implications of protein homeostasis pathways, dysregulation of proteostasis is often involved in the development of multiple pathological conditions, including Alzheimer's disease (AD). Similar to other neurodegenerative diseases that feature pathogenic accumulation of misfolded proteins, Alzheimer's disease is characterized by two pathological hallmarks, amyloid-β (Aβ) plaques and tau aggregates. Knockout or transgenic overexpression of various proteostatic components in mice results in AD-like phenotypes. While both Aβ plaques and tau aggregates could in turn enhance the dysfunction of these proteostatic pathways, eventually leading to apoptotic or necrotic neuronal death and pathogenesis of Alzheimer's disease. Therefore, targeting the components of proteostasis pathways may be a promising therapeutic strategy against Alzheimer's disease.

Synthesis and evaluation of a ring-constrained Hsp90 C-terminal inhibitor that exhibits neuroprotective activity

KU-596 is a second-generation C-terminal heat shock protein 90 KDa (Hsp90) modulator based on the natural product, novobiocin. KU-596 has been shown to induce Hsp70 levels and manifest neuroprotective activity through induction of the heat shock response. A ring-constrained analog of KU-596 was designed and synthesized to probe its binding orientation and ability to induce Hsp70 levels. Compound 2 was found to exhibit comparable or increased activity compared to KU-596, which is under clinical investigation for the treatment of neuropathy.

Heat Shock Proteins in Alzheimer's Disease: Role and Targeting

Among diseases whose cure is still far from being discovered, Alzheimer’s disease (AD) has been recognized as a crucial medical and social problem. A major issue in AD research is represented by the complexity of involved biochemical pathways, including the nature of protein misfolding, which results in the production of toxic species. Considering the involvement of (mis)folding processes in AD aetiology, targeting molecular chaperones represents a promising therapeutic perspective. This review analyses the connection between AD and molecular chaperones, with particular attention toward the most important heat shock proteins (HSPs) as representative components of the human chaperome: Hsp60, Hsp70 and Hsp90. The role of these proteins in AD is highlighted from a biological point of view. Pharmacological targeting of such HSPs with inhibitors or regulators is also discussed.

Therapeutic Strategies for Restoring Tau Homeostasis

Normal tau homeostasis is achieved when the synthesis, processing, and degradation of the protein is balanced. Together, the pathways that regulate tau homeostasis ensure that the protein is at the proper levels and that its posttranslational modifications and subcellular localization are appropriately controlled. These pathways include the enzymes responsible for posttranslational modifications, those systems that regulate mRNA splicing, and the molecular chaperones that control tau turnover and its binding to microtubules. In tauopathies, this delicate balance is disturbed. Tau becomes abnormally modified by posttranslational modification, it loses affinity for microtubules, and it accumulates in proteotoxic aggregates. How and why does this imbalance occur? In this review, we discuss how molecular chaperones and other components of the protein homeostasis (e.g., proteostasis) network normally govern tau quality control. We also discuss how aging might reduce the capacity of these systems and how tau mutations might further affect this balance. Finally, we discuss how small-molecule inhibitors are being used to probe and perturb the tau quality-control systems, playing a particularly prominent role in revealing the logic of tau homeostasis. As such, there is now interest in developing these chemical probes into therapeutics, with the goal of restoring normal tau homeostasis to treat disease.

Development of noviomimetics that modulate molecular chaperones and manifest neuroprotective effects

Heat shock protein 90 (Hsp90) is a chaperone under investigation for the treatment of cancer and neurodegenerative diseases. Neuroprotective Hsp90 C-terminal inhibitors derived from novobiocin (novologues) include KU-32 and KU-596. These novologues modulate molecular chaperones and result in an induction of Heat Shock Protein 70 (Hsp70). "Noviomimetics" replace the synthetically complex noviose sugar with a simple cyclohexyl moiety to maintain biological efficacy as compared to novologues KU-596 and KU-32. In this study, we further explore the development of noviomimetics and evaluate their efficacy using a luciferase refolding assay, immunoblot analysis, a c-jun assay, and an assay measuring mitochondrial bioenergetics. These new noviomimetics were designed and synthesized and found to induce Hsp70 and improve biological activity. Noviomimetics 39e and 40a were found to induce Hsp70 and exhibit promising effects in cellular assays.

Modified biphenyl Hsp90 C-terminal inhibitors for the treatment of cancer

Heat Shock Protein 90 (Hsp90) is a molecular chaperone under clinical investigation for the treatment of neurodegenerative diseases and cancer. Neuroprotective Hsp90 C-terminal inhibitors (novologues) contain a biaryl ring system, and include KU-596, which was modified and investigated for potential anti-cancer activity. Incorporation of a benzamide group onto the biaryl novologues in lieu of the acetamide yielded compounds that manifest anti-cancer activity. Further exploration of the central phenyl ring led to compounds with enhanced anti-proliferative activity. The design, synthesis, and evaluation of these new analogs against breast and prostate cancer cell lines is reported herein, where it was found that 8b and 10 manifest potent anti-proliferative activity and a robust degradation of Hsp90 client-dependent proteins.

A CNS-permeable Hsp90 inhibitor rescues synaptic dysfunction and memory loss in APP-overexpressing Alzheimer's mouse model via an HSF1-mediated mechanism

Induction of neuroprotective heat-shock proteins via pharmacological Hsp90 inhibitors is currently being investigated as a potential treatment for neurodegenerative diseases. Two major hurdles for therapeutic use of Hsp90 inhibitors are systemic toxicity and limited central nervous system permeability. We demonstrate here that chronic treatment with a proprietary Hsp90 inhibitor compound (OS47720) not only elicits a heat-shock-like response but also offers synaptic protection in symptomatic Tg2576 mice, a model of Alzheimer's disease, without noticeable systemic toxicity. Despite a short half-life of OS47720 in mouse brain, a single intraperitoneal injection induces rapid and long-lasting (>3 days) nuclear activation of the heat-shock factor, HSF1. Mechanistic study indicates that the remedial effects of OS47720 depend upon HSF1 activation and the subsequent HSF1-mediated transcriptional events on synaptic genes. Taken together, this work reveals a novel role of HSF1 in synaptic function and memory, which likely occurs through modulation of the synaptic transcriptome.

Targeting chaperones, heat shock factor-1, and unfolded protein response: Promising therapeutic approaches for neurodegenerative disorders

Protein misfolding, which is known to cause several serious diseases, is an emerging field that addresses multiple therapeutic areas. Misfolding of a disease-specific protein in the central nervous system ultimately results in the formation of toxic aggregates that may accumulate in the brain, leading to neuronal cell death and dysfunction, and associated clinical manifestations. A large number of neurodegenerative diseases in humans, including Alzheimer's, Parkinson's, Huntington's, and prion diseases, are primarily caused by protein misfolding and aggregation. Notably, the cellular system is equipped with a protein quality control system encompassing chaperones, ubiquitin proteasome system, and autophagy, as a defense mechanism that monitors protein folding and eliminates inappropriately folded proteins. As the intrinsic molecular mechanisms of protein misfolding become more clearly understood, the novel therapeutic approaches in this arena are gaining considerable interest. The present review will describe the chaperones network and different approaches as the therapeutic targets for neurodegenerative diseases. Current and emerging therapeutic approaches to combat neurodegenerative diseases, addressing the roles of molecular, chemical, and pharmacological chaperones, as well as heat shock factor-1 and the unfolded protein response, are also discussed in detail.

The Mechanism of Hsp90 ATPase Stimulation by Aha1

Hsp90 is a dimeric molecular chaperone responsible for the folding, maturation, and activation of hundreds of substrate proteins called ‘clients’. Numerous co-chaperone proteins regulate progression through the ATP-dependent client activation cycle. The most potent stimulator of the Hsp90 ATPase activity is the co-chaperone Aha1p. Only one molecule of Aha1p is required to fully stimulate the Hsp90 dimer despite the existence of two, presumably identical, binding sites for this regulator. Using ATPase assays with Hsp90 heterodimers, we find that Aha1p stimulates ATPase activity by a three-step mechanism via the catalytic loop in the middle domain of Hsp90. Binding of the Aha1p N domain to the Hsp90 middle domain exerts a small stimulatory effect but also drives a separate conformational rearrangement in the Hsp90 N domains. This second event drives a rearrangement in the N domain of the opposite subunit and is required for the stimulatory action of the Aha1p C domain. Furthermore, the second event can be blocked by a mutation in one subunit of the Hsp90 dimer but not the other. This work provides a foundation for understanding how post-translational modifications regulate co-chaperone engagement with the Hsp90 dimer.

Diverging Novobiocin Anti-Cancer Activity from Neuroprotective Activity through Modification of the Amide Tail

Novobiocin is a natural product that binds the Hsp90 C-terminus and manifests Hsp90 inhibitory activity. Structural investigations on novobiocin led to the development of both anti-cancer and neuroprotective agents. The varied pharmacological activity manifested by these novobiocin analogs prompted the investigation of structure-function studies to identify these contradictory effects, which revealed that modifications to the amide side chain produce either anti-cancer or neuroprotective activity. Compounds that exhibit neuroprotective activity contain a short alkyl or cycloalkyl amide side chain. In contrast, anti-cancer agents contain five or more carbons, disrupt interactions between Hsp90α and Aha1, and induce the degradation of Hsp90-dependent client proteins.

Targeting the Diabetic Chaperome to Improve Peripheral Neuropathy

The chaperome constitutes a broad family of molecular chaperones and co-chaperones that facilitate the folding, refolding, and degradation of the proteome. Heat shock protein 90 (Hsp90) promotes the folding of numerous oncoproteins to aid survival of malignant phenotypes, and small molecule inhibitors of the Hsp90 chaperone complex offer a viable approach to treat certain cancers. One therapeutic attribute of this approach is the selectivity of these molecules to target high affinity oncogenic Hsp90 complexes present in tumor cells, which are absent in nontransformed cells. This selectivity has given rise to the idea that disease may contribute to forming a stress chaperome that is functionally distinct in its ability to interact with small molecule Hsp90 modulators. Consistent with this premise, modulating Hsp90 improves clinically relevant endpoints of diabetic peripheral neuropathy but has little impact in nondiabetic nerve. The concept of targeting the "diabetic chaperome" to treat diabetes and its complications is discussed.

Domains of STIP1 responsible for regulating PrPC-dependent amyloid-β oligomer toxicity

Soluble oligomers of amyloid-beta peptide (AβO) transmit neurotoxic signals through the cellular prion protein (PrP(C)) in Alzheimer's disease (AD). Secreted stress-inducible phosphoprotein 1 (STIP1), an Hsp70 and Hsp90 cochaperone, inhibits AβO binding to PrP(C) and protects neurons from AβO-induced cell death. Here, we investigated the molecular interactions between AβO and STIP1 binding to PrP(C) and their effect on neuronal cell death. We showed that residues located in a short region of PrP (90-110) mediate AβO binding and we narrowed the major interaction in this site to amino acids 91-100. In contrast, multiple binding sites on STIP1 (DP1, TPR1 and TPR2A) contribute to PrP binding. DP1 bound the N-terminal of PrP (residues 23-95), whereas TPR1 and TPR2A showed binding to the C-terminal of PrP (residues 90-231). Importantly, only TPR1 and TPR2A directly inhibit both AβO binding to PrP and cell death. Furthermore, our structural studies reveal that TPR1 and TPR2A bind to PrP through distinct regions. The TPR2A interface was shown to be much more extensive and to partially overlap with the Hsp90 binding site. Our data show the possibility of a PrP, STIP1 and Hsp90 ternary complex, which may influence AβO-mediated cell death.

The active Hsc70/tau complex can be exploited to enhance tau turnover without damaging microtubule dynamics

The pathological accumulation of abnormally hyperphosphorylated and aggregated tau, a neuronal microtubule (MT)-associated protein that functions to maintain MT stability, is implicated in a number of hereditary and sporadic neurodegenerative diseases including frontotemporal dementia and Alzheimer's disease. Targeting tau for the treatment of these diseases is an area of intense interest and toward that end, modulation of cellular molecular chaperones is a potential therapeutic target. In particular, the constitutive Hsp70 isoform, Hsc70, seems highly interconnected with tau, preserving tau protein levels and synergizing with it to assemble MTs. But the relationship between tau and Hsc70, as well as the impact of this interaction in neurons and its therapeutic implications remain unknown. Using a human dominant negative Hsc70 that resembles isoform selective inhibition of this important chaperone, we found for the first time that Hsc70 activity is required to stimulate MT assembly in cells and brain. However, surprisingly, active Hsc70 also requires active tau to regulate MT assembly in vivo, suggesting that tau acts in some ways as a co-chaperone for Hsc70 to coordinate MT assembly. This was despite tau binding to Hsc70 as substrate, as determined biochemically. Moreover, we show that while chronic Hsc70 inhibition damaged MT dynamics, intermittent treatment with a small molecule Hsp70 inhibitor lowered tau in brain tissue without disrupting MT integrity. Thus, in tauopathies, where MT injury would be detrimental to neurons, the unique relationship of tau with the Hsc70 machinery can be exploited to deplete tau levels without damaging MT networks.

Silencing of Hsp70 intensifies 6-OHDA-induced apoptosis and Hsp90 upregulation in PC12 cells

By the current study, we tried to find out the interactive mechanisms enrolled by Hsp70 and Hsp90 following the 6-hydroxydopamine (6-OHDA)-induced oxidative stress. Of heat shock protein (Hsp) family, we have previously evaluated the effects of Hsp90 gene silencing on in vitro model of Parkinson's disease and its influence on controlling the mechanisms of cell survival. Here, we extended our study to Hsp70 silencing short interfering RNA (siRNA) oligonucleotides, transfected into Pheochromocytoma (PC12) cells with/without exposure to 6-OHDA stress. In order to determine the probable effects of Hsp70 silencing on apoptotic factors, we assessed Bcl2/Bax ratio, nuclear level of PARP, and cleavage of caspase-3 under 6-OHDA stress condition. The results showed deteriorated effect of Hsp70 siRNA on apoptosis in cells exposed to only 6-OHDA. This is, at least in part, in consequence of upregulation of Hsp90, both at messenger RNA (mRNA) and protein levels. These data highlight the critical role of Hsp70 for cell survival under 6-OHDA stress condition. It could be a suggestive issue for supervision of caspase cascades by survival roles of Hsps as Hsp70 silencing resulted in apoptosis phenomenon. Convergence of Hsp70 anti-apoptotic and 6-OHDA pro-apoptotic pathways may explain intensified apoptosis following Hsp70 silencing. In addition, nuclear factor erythroid-2-related factor 2 (Nrf2), a transcription factor, has been previously studied in detoxification of oxidative stress. For this issue, we tried to elucidate Hsp70 silencing impact on Nrf2, which has been shown to regulate the transcription of Hsp70, unspecifically. Besides, our investigations revealed that Hsp70 siRNA did not affect the level of Nrf2 during 6-OHDA exposure. But, it is still a dealing question and other investigations are needed to have a comprehensive perception of Hsp family signaling functions.

Regulation of protein turnover by heat shock proteins

Protein turnover reflects the balance between synthesis and degradation of proteins, and it is a crucial process for the maintenance of the cellular protein pool. The folding of proteins, refolding of misfolded proteins, and also degradation of misfolded and damaged proteins are involved in the protein quality control (PQC) system. Correct protein folding and degradation are controlled by many different factors, one of the most important of which is the heat shock protein family. Heat shock proteins (HSPs) are in the class of molecular chaperones, which may prevent the inappropriate interaction of proteins and induce correct folding. On the other hand, these proteins play significant roles in the degradation pathways, including endoplasmic reticulum-associated degradation (ERAD), the ubiquitin-proteasome system, and autophagy. This review focuses on the emerging role of HSPs in the regulation of protein turnover; the effects of HSPs on the degradation machineries ERAD, autophagy, and proteasome; as well as the role of posttranslational modifications in the PQC system.

Heat shock protein 90 in Alzheimer's disease

Alzheimer's disease (AD) is the first most common neurodegenerative disease. Despite a large amount of research, the pathogenetic mechanism of AD has not yet been clarified. The two hallmarks of the pathology of AD are the extracellular senile plaques (SPs) of aggregated amyloid-beta (Aβ) peptide and the accumulation of the intracellular microtubule-associated protein tau into fibrillar aggregates. Heat shock proteins (HSPs) play a key role in preventing protein misfolding and aggregation, and Hsp90 can be viewed as a ubiquitous molecular chaperone potentially involved in AD pathogenesis. A role of Hsp90 regulates the activity of the transcription factor heat shock factor-1 (HSF-1), the master regulator of the heat shock response. In AD, Hsp90 inhibitors may redirect neuronal aggregate formation, and protect against protein toxicity by activation of HSF-1 and the subsequent induction of heat shock proteins, such as Hsp70. Therefore, we review here to further discuss the recent advances and challenges in targeting Hsp90 for AD therapy.

Low dose Hsp90 inhibitor 17AAG protects neural progenitor cells from ischemia induced death

Stress adaptation effect provides cell protection against ischemia induced apoptosis. Whether this mechanism prevents other types of cell death in stroke is not well studied. This is an important question for regenerative medicine to treat stroke since other types of cell death such as necrosis are also prominent in the stroke brain apart from apoptosis. We report here that treatment with 17-N-Allylamino-17-demethoxygeldanamycin (17AAG), an Hsp90 inhibitor, protected neural progenitor cells (NPCs) against oxygen glucose deprivation (OGD) induced cell death in a dose dependent fashion. Cell death assays indicated that 17AAG not only ameliorated apoptosis, but also necrosis mediated by OGD. This NPC protection was confirmed by exposing cells to oxidative stress, a major stress signal prevalent in the stroke brain. Mechanistic studies demonstrated that 17AAG activated PI3K/Akt and MAPK cell protective pathways. More interestingly, these two pathways were activated in vivo by 17AAG and 17AAG treatment reduced infarct volume in a middle cerebral artery occlusion (MCAO) stroke model. These data suggest that 17AAG protects cells against major cell death pathways and thus might be used as a pharmacological conditioning agent for regenerative medicine for stroke.

Advances in therapeutics for neurodegenerative tauopathies: moving toward the specific targeting of the most toxic tau species

Neurodegenerative disease is one of the greatest health concerns today and with no effective treatment in sight, it is crucial that researchers find a safe and successful therapeutic. While neurofibrillary tangles are considered the primary tauopathy hallmark, more evidence continues to come to light to suggest that soluble, intermediate tau aggregates--tau oligomers--are the most toxic species in disease. These intermediate tau species may also be responsible for the spread of pathology, suggesting that oligomeric tau may be the best therapeutic target. Here, we summarize results for the modulation of tau by molecular chaperones, small molecules and aggregation inhibitors, post-translational modifications, immunotherapy, other techniques, and future directions.

Novologues containing a benzamide side chain manifest anti-proliferative activity against two breast cancer cell lines

Hsp90 represents a promising target for the development of both anti-cancer and neuroprotective agents. Structure-activity relationship studies on novobiocin and novobiocin analogues, led to the development of KU-32 and recently, KU-596, as lead compounds for the potential treatment of neurodegenerative diseases. Similar to KU-32, we have demonstrated that upon replacement of the acetamide side chain present in KU-32 with a benzamide, this neuroprotective agent was transformed into a scaffold that manifests anti-proliferative activity. To assess structure-activity relationships for this new scaffold, a library of benzamide-containing novologues was prepared and evaluated against two breast cancer cell lines. Compound 14a manifested the most potent anti-proliferative activity from these studies and induced Hsp90-dependent client protein degradation in a concentration-dependent manner.

Targeting Hsp90 and its co-chaperones to treat Alzheimer's disease

INTRODUCTION

Alzheimer's disease, characterized by the accumulation of hyperphosphorylated tau and β amyloid (Aβ), currently lacks effective treatment. Chaperone proteins, such as the heat shock protein (Hsp) 90, form macromolecular complexes with co-chaperones, which can regulate tau metabolism and Aβ processing. Although small molecule inhibitors of Hsp90 have been successful at ameliorating tau and Aβ burden, their development into drugs to treat disease has been slow due to the off- and on-target effects of this approach as well as challenges with the pharmacology of current scaffolds. Thus, other approaches are being developed to improve these compounds and to target co-chaperones of Hsp90 in an effort to limit these liabilities.

AREAS COVERED

This article discusses the most current developments in Hsp90 inhibitors including advances in blood-brain barrier permeability, decreased toxicity and homolog-specific small-molecule inhibitors. In addition, we discuss current strategies targeting Hsp90 co-chaperones rather than Hsp90 itself to reduce off-target effects.

EXPERT OPINION

Although Hsp90 inhibitors have proven their efficacy at reducing tau pathology, they have yet to meet with success in the clinic. The development of Hsp90/tau complex-specific inhibitors and further development of Hsp90 co-chaperone-specific drugs should yield more potent, less toxic therapeutics.

Identifying a C-terminal ATP binding sites-based novel Hsp90-Inhibitor in silico: a plausible therapeutic approach in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a progressive brain disorder, which gradually and irreversibly destroys the intellectual and cognitive abilities of the brain. Heat shock protein 90 (Hsp90α) is a molecular chaperone which was found to regulate the function of number of client proteins including tau that is involved in the cause of the AD. Inhibition of Hsp90α by C-Terminal domain (CTD) ATP binding-site blockage might be used as an effective treatment strategy against the disease via degradation of tau proteins that are involved in the progression of the disease. Till date, a variety of drugs have been identified as Hsp90α inhibitors, which include Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone. However, which drug among the four binds to the CTD ATP binding site strongly and what are the specific residue responsible for such binding, have not been reported so far.

HYPOTHESIS

We hypothesize that binding site for ATP of Hsp90α CTD contains multiple ATP binding sites. We also hypothesize that a drug which can bind to the ATP binding site of CTD strongly can inhibit Hsp90α function which is in turn redirects towards the proteasomal degradation of diseased client protein like tau in AD. Such inhibition will find a novel therapeutic approach in the treatment of AD.

EXPERIMENTAL DESIGN

The identification of ATP binding site of Hsp90α CTD was done using various software tools like Hex 6.3, CastP, protein Hydrophobicity plots, ATPint and LigPlot+ v.1.4.5. Docking experiments were conducted between Hsp90αCTD and its inhibitors at these ATP binding site using the Autodock 4.0. The docking energies were further compared to obtain the most effective Hsp90α inhibitor of CTD.

RESULTS

From our experiments, Leucine (Leu) 665, Leu 666 and Leu 694 were predicted to be located in CTD ATP binding site. Furthermore, docking studies were performed of various Hsp90α inhibitors like Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone with the previously recognized ATP binding residues of CTD i.e. Leu 665, Leu 666 and Leu 694. The docking results of Derrubone showed the highest binding energy at all the three sites of ATP interaction. Additionally, Derrubone showed the best binding energy at Leu 666 (-7.53kcal/mol) compared to Leu 665 (-7.20kcal/mol) and Leu 694 (-6.67kcal/mol).

CONCLUSION

Based on our findings, we propose that the recognized sites i.e. Leu665, Leu 666 and Leu694 could possibly be the binding sites of Hsp90α CTD for ATP and the Hsp90 inhibitors. It was predicted that Derrubone could bind with CTD of Hsp90α strongly and resulted tau protein degradation which might be considered to be a therapeutic approach in AD.

A mutation in the catalytic loop of Hsp90 specifically impairs ATPase stimulation by Aha1p, but not Hch1p

Heat shock protein 90 (Hsp90) is a molecular chaperone that plays a central role in maintaining cellular homeostasis by facilitating activation of a large number of client proteins. ATP-dependent client activation by Hsp90 is tightly regulated by a host of co-chaperone proteins that control progression through the activation cycle. ATPase stimulation of Hsp90 by Aha1p requires a conserved RKxK motif that interacts with the catalytic loop of Hsp90. In this study, we explore the role of this RKxK motif in the biological and biochemical properties of Hch1p. We found that this motif is required for Hch1p-mediated ATPase stimulation in vitro, but mutations that block stimulation do not impair the action of Hch1p in vivo. This suggests that the biological function of Hch1p is not directly linked to ATPase stimulation. Moreover, a mutation in the catalytic loop of Hsp90 specifically impairs ATPase stimulation by Aha1p but not by Hch1p. Our work here suggests that both Hch1p and Aha1p regulate Hsp90 function through interaction with the catalytic loop but do so in different ways.

Barcoding heat shock proteins to human diseases: looking beyond the heat shock response

There are numerous human diseases that are associated with protein misfolding and the formation of toxic protein aggregates. Activating the heat shock response (HSR)--and thus generally restoring the disturbed protein homeostasis associated with such diseases--has often been suggested as a therapeutic strategy. However, most data on activating the HSR or its downstream targets in mouse models of diseases associated with aggregate formation have been rather disappointing. The human chaperonome consists of many more heat shock proteins (HSPs) that are not regulated by the HSR, however, and researchers are now focusing on these as potential therapeutic targets. In this Review, we summarize the existing literature on a set of aggregation diseases and propose that each of them can be characterized or 'barcoded' by a different set of HSPs that can rescue specific types of aggregation. Some of these 'non-canonical' HSPs have demonstrated effectiveness in vivo, in mouse models of protein-aggregation disease. Interestingly, several of these HSPs also cause diseases when mutated--so-called chaperonopathies--which are also discussed in this Review.

Bioactive limonoids from the leaves of Azaridachta indica (Neem)

Eight new limonoids (1-8) and one new phenol glycoside (9), along with six known compounds, were isolated from the leaves of Azaridachta indica. The structures of 1-9 were elucidated on the basis of spectroscopic data analysis. Compounds isolated were assayed for their cytotoxicity against different cancer cell lines. Moreover, their ability to interact with the molecular chaperone Hsp90, affecting its biological activity, was tested.