Molecular chaperones and regulation of tau quality control: strategies for drug discovery in tauopathies

Modulation of cholesterol metabolism with Phytoremedies in Alzheimer's disease: A comprehensive review

Alzheimer's disease (AD) is a complex neurological ailment that causes cognitive decline and memory loss. Cholesterol metabolism dysregulation has emerged as a crucial element in AD pathogenesis, contributing to the formation of amyloid-beta (Aβ) plaques and tau tangles, the disease's hallmark neuropathological characteristics. Thus, targeting cholesterol metabolism has gained attention as a potential therapeutic method for Alzheimer's disease. Phytoremedies, which are generated from plants and herbs, have shown promise as an attainable therapeutic option for Alzheimer's disease. These remedies contain bioactive compounds like phytochemicals, flavonoids, and polyphenols, which have demonstrated potential in modulating cholesterol metabolism and related pathways. This comprehensive review explores the modulation of cholesterol metabolism by phytoremedies in AD. It delves into the role of cholesterol in brain function, highlighting disruptions observed in AD. Additionally, it examines the underlying molecular mechanisms of cholesterol-related pathology in AD. The review emphasizes the significance of phytoremedies as a potential therapeutic intervention for AD. It discusses the drawbacks of current treatments and the need for alternative strategies addressing cholesterol dysregulation and its consequences. Through an in-depth analysis of specific phytoremedies, the review presents compelling evidence of their potential benefits. Molecular mechanisms underlying phytoremedy effects on cholesterol metabolism are examined, including regulation of cholesterol-related pathways, interactions with Aβ pathology, influence on tau pathology, and anti-inflammatory effects. The review also highlights challenges and future perspectives, emphasizing standardization, clinical evidence, and personalized medicine approaches to maximize therapeutic potential in AD treatment. Overall, phytoremedies offer promise as a potential avenue for AD management, but further research and collaboration are necessary to fully explore their efficacy, safety, and mechanisms of action.

EFhd2 co-aggregates with monomeric and filamentous tau in vitro

Tauopathies are characterized by the abnormal buildup of tau protein, with early oligomeric forms associated with neurodegeneration and the later neurofibrillary tangles possibly conferring neuroprotection. The molecular mechanisms governing the formation of these tau species are unclear. Lately, there has been an increased focus on examining the interactions between tau and other proteins, along with their influence on the aggregation of tau. Our previous work revealed EFhd2's association with pathological tau in animal models and tauopathy brains. Herein, we examined the impact of EFhd2 on monomeric and filamentous tau in vitro. The results demonstrated that EFhd2 incubation with monomeric full length human tau (hTau40) formed amorphous aggregates, where both EFhd2 and hTau40 colocalized. Moreover, EFhd2 is entangled with arachidonic acid (ARA)-induced filamentous hTau40. Furthermore, EFhd2-induced aggregation with monomeric and filamentous hTau40 is EFhd2 concentration dependent. Using sandwich ELISA assays, we assessed the reactivity of TOC1 and Alz50-two conformation-specific tau antibodies-to EFhd2-hTau40 aggregates (in absence and presence of ARA). No TOC1 signal was detected in EFhd2 aggregates with monomeric hTau40 whereas EFhd2 aggregates with hTau in the presence of ARA showed a higher signal compared to hTau40 filaments. In contrast, EFhd2 aggregates with both monomeric and filamentous hTau40 reduced Alz50 reactivity. Taken together, our results illustrate for the first time that EFhd2, a tau-associated protein, interacts with monomeric and filamentous hTau40 to form large aggregates that are starkly different from tau oligomers and filaments. Given these findings and previous research, we hypothesize that EFhd2 may play a role in the formation of tau aggregates. Nevertheless, further in vivo studies are imperative to test this hypothesis.

Heat shock proteins: Biological functions, pathological roles, and therapeutic opportunities

The heat shock proteins (HSPs) are ubiquitous and conserved protein families in both prokaryotic and eukaryotic organisms, and they maintain cellular proteostasis and protect cells from stresses. HSP protein families are classified based on their molecular weights, mainly including large HSPs, HSP90, HSP70, HSP60, HSP40, and small HSPs. They function as molecular chaperons in cells and work as an integrated network, participating in the folding of newly synthesized polypeptides, refolding metastable proteins, protein complex assembly, dissociating protein aggregate dissociation, and the degradation of misfolded proteins. In addition to their chaperone functions, they also play important roles in cell signaling transduction, cell cycle, and apoptosis regulation. Therefore, malfunction of HSPs is related with many diseases, including cancers, neurodegeneration, and other diseases. In this review, we describe the current understandings about the molecular mechanisms of the major HSP families including HSP90/HSP70/HSP60/HSP110 and small HSPs, how the HSPs keep the protein proteostasis and response to stresses, and we also discuss their roles in diseases and the recent exploration of HSP related therapy and diagnosis to modulate diseases. These research advances offer new prospects of HSPs as potential targets for therapeutic intervention.

Synthesis, in vitro biological evaluation and molecular modelling of new 2-chloro-3-hydrazinopyrazine derivatives as potent acetylcholinesterase inhibitors‏ on PC12 cells

Background

The loss of cholinergic neurotransmission in Alzheimer's disease (AD) patients' brain is accompanied by a reduced concentration of Acetylcholine (ACh) within synaptic clefts. Thus, the use of acetylcholinesterase inhibitors (AChEIs) to block the cholinergic degradation of ACh is a promising approach for AD treatment. In the present study, a series of 2-chloro-3-hydrazinopyrazine derivatives (CHP1-5) were designed, synthesized, and biologically evaluated as potential multifunctional anti-AD agents.

Methods

In addition, the chemical structures and purity of the synthesized compounds were elucidated through using IR, ¹H and ¹³C NMR, and elemental analyses. Further, the intended compounds were assessed in vitro for their AChE inhibitory and neuroprotective effects. Furthermore, DPPH, FRAP and ABTS assays were utilized to determine their antioxidant activity. The statistical analysis was performed using one-way ANOVA.

Results

Based on the results, CHP4 and CHP5 exhibited strong AChE inhibitory effects with the IC₅₀ values of 3.76 and 4.2 µM compared to the donepezil (0.53 µM), respectively. The study examined the effect and molecular mechanism of CHP4 on the Ab1–42-induced cytotoxicity in differentiated PC12 cells. At concentrations of 0–100 μM, CHP4 was non-toxic in PC12. Additionally, Ab1–42 significantly stimulated tau hyperphosphorylation and induced differentiated PC12 cell death. Further, CHP4 resulted in diminishing the Ab1–42-induced toxicity in PC12 cell significantly. CHP4 at 30 μM concentration significantly increased the Ab1–42-induced HSP70 expression and decreased tau hyperphosphorylation.

Conclusions

According to the results of our studies CHP4 can be considered as safe and efficient AChEI and employed as a potential multifunctional anti-AD agent.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13065-022-00799-w.

Recent advances in cellular biosensor technology to investigate tau oligomerization

Tau is a microtubule binding protein which plays an important role in physiological functions but it is also involved in the pathogenesis of Alzheimer's disease and related tauopathies. While insoluble and β-sheet containing tau neurofibrillary tangles have been the histopathological hallmark of these diseases, recent studies suggest that soluble tau oligomers, which are formed prior to fibrils, are the primary toxic species. Substantial efforts have been made to generate tau oligomers using purified recombinant protein strategies to study oligomer conformations as well as their toxicity. However, no specific toxic tau species has been identified to date, potentially due to the lack of cellular environment. Hence, there is a need for cell-based models for direct monitoring of tau oligomerization and aggregation. This review will summarize the recent advances in the cellular biosensor technology, with a focus on fluorescence resonance energy transfer, bimolecular fluorescence complementation, and split luciferase complementation approaches, to monitor formation of tau oligomers and aggregates in living cells. We will discuss the applications of the cellular biosensors in examining the heterogeneous tau conformational ensembles and factors affecting tau self-assembly, as well as detecting cell-to-cell propagation of tau pathology. We will also compare the advantages and limitations of each type of tau biosensors, and highlight their translational applications in biomarker development and therapeutic discovery.

Hsp40s play complementary roles in the prevention of tau amyloid formation

The microtubule-associated protein, tau, is the major subunit of neurofibrillary tangles associated with neurodegenerative conditions, such as Alzheimer's disease. In the cell, however, tau aggregation can be prevented by a class of proteins known as molecular chaperones. While numerous chaperones are known to interact with tau, though, little is known regarding the mechanisms by which these prevent tau aggregation. Here, we describe the effects of ATP-independent Hsp40 chaperones, DNAJA2 and DNAJB1, on tau amyloid-fiber formation and compare these to the small heat shock protein HSPB1. We find that the chaperones play complementary roles, with each preventing tau aggregation differently and interacting with distinct sets of tau species. Whereas HSPB1 only binds tau monomers, DNAJB1 and DNAJA2 recognize aggregation-prone conformers and even mature fibers. In addition, we find that both Hsp40s bind tau seeds and fibers via their C-terminal domain II (CTDII), with DNAJA2 being further capable of recognizing tau monomers by a second, distinct site in CTDI. These results lay out the mechanisms by which the diverse members of the Hsp40 family counteract the formation and propagation of toxic tau aggregates and highlight the fact that chaperones from different families/classes play distinct, yet complementary roles in preventing pathological protein aggregation.

Several neurological conditions, such as Alzheimer’s and Parkinson’s disease, are characterized by the build-up of protein clumps known as aggregates. In the case of Alzheimer’s disease, a key protein, called tau, aggregates to form fibers that are harmful to neuronal cells in the brain. One of the ways our cells can prevent this from occurring is through the action of proteins known as molecular chaperones, which can bind to tau proteins and prevent them from sticking together.

Tau can take on many forms. For example, a single tau protein on its own, known as a monomer, is unstructured. In patients with Alzheimer’s, these monomers join together into small clusters, known as seeds, that rapidly aggregate and accumulate into rigid, structured fibers. One chaperone, HSPB1, is known to bind to tau monomers and prevent them from being incorporated into fibers. Recently, another group of chaperones, called J-domain proteins, was also found to interact with tau. However, it was unclear how these chaperones prevent aggregation and whether they bind to tau in a similar manner as HSPB1.

To help answer this question, Irwin, Faust et al. studied the effect of two J-domain proteins, as well as the chaperone HSBP1, on tau aggregation. This revealed that, unlike HSBP1, the two J-domain proteins can bind to multiple forms of tau, including when it has already aggregated in to seeds and fibers. This suggests that these chaperones can stop the accumulation of fibers at several different stages of the aggregation process. Further experiments examining which sections of the J-domain proteins bind to tau, showed that both attach to fibers via the same region. However, the two J-domain proteins are not identical in their interaction with tau. While one of them uses a distinct region to bind to tau monomers, the other does not bind to single tau proteins at all.

These results demonstrate how different cellular chaperones can complement one another in order to inhibit harmful protein aggregation. Further studies will be needed to understand the full role of J-domain proteins in preventing tau from accumulating into fibers, as well as their potential as drug targets for developing new treatments.

Interplay of isoform 1N4R tau protein and amyloid-β peptide fragment 25-35 in reducing and non-reducing conditions

Amyloid-β (Aβ) peptide and tau protein are two hallmark proteins in Alzheimer's disease (AD); however, the parameters, which mediate the abnormal aggregation of Aβ and tau, have not been fully discovered. Here, we have provided an optimum method to purify tau protein isoform 1N4R by using nickel-nitrilotriacetic acid agarose chromatography under denaturing condition. The biochemical and biophysical properties of the purified protein were further characterized using in vitro tau filament assembly, tubulin polymerization assay, circular dichroism (CD) spectroscopy and atomic force microscopy. Afterwards, we investigated the effect of tau protein on aggregation of Aβ (25-35) peptide using microscopic imaging and cell viability assay. Incubation of tau at physiologic and supra-physiologic concentrations with Aβ25-35 for 40 days under reducing and non-reducing conditions revealed formation of two types of aggregates with distinct morphologies and dimensions. In non-reducing condition, the co-incubated sample showed granular aggregates, while in reducing condition, they formed annular protofibrils. Results from cell viability assay revealed the increased cell viability for the co-incubated sample. Therefore, the disassembling action shown by tau protein on Aβ25-35 suggests the possibility that tau may have a protective role in preventing Aβ peptide from acquiring the cytotoxic, aggregated form against oxidative stress damages.

Inhibitors of heat shock protein 70 (Hsp70) with enhanced metabolic stability reduce tau levels

The molecular chaperone, Heat Shock Protein 70 (Hsp70), is an emerging drug target for neurodegenerative diseases, because of its ability to promote degradation of microtubule-associated protein tau (MAPT/tau). Recently, we reported YM-08 as a brain penetrant, allosteric Hsp70 inhibitor, which reduces tau levels. However, the benzothiazole moiety of YM-08 is vulnerable to metabolism by CYP3A4, limiting its further application as a chemical probe. In this manuscript, we designed and synthesized seventeen YM-08 derivatives by systematically introducing halogen atoms to the benzothiazole ring and shifting the position of the heteroatom in a distal pyridine. In microsome assays, we found that compound JG-23 has 12-fold better metabolic stability and it retained the ability to reduce tau levels in two cell-based models. These chemical probes of Hsp70 are expected to be useful tools for studying tau homeostasis.

Hsp90 co-chaperones, FKBP52 and Aha1, promote tau pathogenesis in aged wild-type mice

The microtubule associated protein tau is an intrinsically disordered phosphoprotein that accumulates under pathological conditions leading to formation of neurofibrillary tangles, a hallmark of Alzheimer's disease (AD). The mechanisms that initiate the accumulation of phospho-tau aggregates and filamentous deposits are largely unknown. In the past, our work and others' have shown that molecular chaperones play a crucial role in maintaining protein homeostasis and that imbalance in their levels or activity can drive tau pathogenesis. We have found two co-chaperones of the 90 kDa heat shock protein (Hsp90), FK506-binding protein 52 (FKBP52) and the activator of Hsp90 ATPase homolog 1 (Aha1), promote tau aggregation in vitro and in the brains of tau transgenic mice. Based on this, we hypothesized that increased levels of these chaperones could promote tau misfolding and accumulation in the brains of aged wild-type mice. We tested this hypothesis by overexpressing Aha1, FKBP52, or mCherry (control) proteins in the hippocampus of 9-month-old wild-type mice. After 7 months of expression, mice were evaluated for cognitive and pathological changes. Our results show that FKBP52 overexpression impaired spatial reversal learning, while Aha1 overexpression impaired associative learning in aged wild-type mice. FKBP52 and Aha1 overexpression promoted phosphorylation of distinct AD-relevant tau species. Furthermore, FKBP52 activated gliosis and promoted neuronal loss leading to a reduction in hippocampal volume. Glial activation and phospho-tau accumulation were also detected in areas adjacent to the hippocampus, including the entorhinal cortex, suggesting that after initiation these pathologies can propagate through other brain regions. Overall, our findings suggest a role for chaperone imbalance in the initiation of tau accumulation in the aging brain.

Liver Growth Factor "LGF" as a Therapeutic Agent for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive degenerative disorder and the most common cause of dementia in aging populations. Although the pathological hallmarks of AD are well defined, currently no effective therapy exists. Liver growth factor (LGF) is a hepatic albumin-bilirubin complex with activity as a tissue regenerating factor in several neurodegenerative disorders such as Parkinson's disease and Friedreich's ataxia. Our aim here was to analyze the potential therapeutic effect of LGF on the APPswe mouse model of AD. Twenty-month-old mice received intraperitoneal (i.p.) injections of 1.6 µg LGF or saline, twice a week during three weeks. Mice were sacrificed one week later, and the hippocampus and dorsal cortex were prepared for immunohistochemical and biochemical studies. LGF treatment reduced amyloid-β (Aβ) content, phospho-Tau/Tau ratio and the number of Aβ plaques with diameter larger than 25 µm. LGF administration also modulated protein ubiquitination and HSP70 protein levels, reduced glial reactivity and inflammation, and the expression of the pro-apoptotic protein Bax. Because the administration of this factor also restored cognitive damage in APPswe mice, we propose LGF as a novel therapeutic tool that may be useful for the treatment of AD.

Molecular Chaperones: A Double-Edged Sword in Neurodegenerative Diseases

Aberrant accumulation of misfolded proteins into amyloid deposits is a hallmark in many age-related neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS). Pathological inclusions and the associated toxicity appear to spread through the nervous system in a characteristic pattern during the disease. This has been attributed to a prion-like behavior of amyloid-type aggregates, which involves self-replication of the pathological conformation, intercellular transfer, and the subsequent seeding of native forms of the same protein in the neighboring cell. Molecular chaperones play a major role in maintaining cellular proteostasis by assisting the (re)-folding of cellular proteins to ensure their function or by promoting the degradation of terminally misfolded proteins to prevent damage. With increasing age, however, the capacity of this proteostasis network tends to decrease, which enables the manifestation of neurodegenerative diseases. Recently, there has been a plethora of studies investigating how and when chaperones interact with disease-related proteins, which have advanced our understanding of the role of chaperones in protein misfolding diseases. This review article focuses on the steps of prion-like propagation from initial misfolding and self-templated replication to intercellular spreading and discusses the influence that chaperones have on these various steps, highlighting both the positive and adverse consequences chaperone action can have. Understanding how chaperones alleviate and aggravate disease progression is vital for the development of therapeutic strategies to combat these debilitating diseases.

Changes in proteome solubility indicate widespread proteostatic disruption in mouse models of neurodegenerative disease

The deposition of pathologic misfolded proteins in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, frontotemporal dementia and amyotrophic lateral sclerosis is hypothesized to burden protein homeostatic (proteostatic) machinery, potentially leading to insufficient capacity to maintain the proteome. This hypothesis has been supported by previous work in our laboratory, as evidenced by the perturbation of cytosolic protein solubility in response to amyloid plaques in a mouse model of Alzheimer's amyloidosis. In the current study, we demonstrate changes in proteome solubility are a common pathology to mouse models of neurodegenerative disease. Pathological accumulations of misfolded tau, α-synuclein and mutant superoxide dismutase 1 in CNS tissues of transgenic mice were associated with changes in the solubility of hundreds of CNS proteins in each model. We observed that changes in proteome solubility were progressive and, using the rTg4510 model of inducible tau pathology, demonstrated that these changes were dependent upon sustained expression of the primary pathologic protein. In all of the models examined, changes in proteome solubility were robust, easily detected, and provided a sensitive indicator of proteostatic disruption. Interestingly, a subset of the proteins that display a shift towards insolubility were common between these different models, suggesting that a specific subset of the proteome is vulnerable to proteostatic disruption. Overall, our data suggest that neurodegenerative proteinopathies modeled in mice impose a burden on the proteostatic network that diminishes the ability of neural cells to prevent aberrant conformational changes that alter the solubility of hundreds of abundant cellular proteins.

Azure C Targets and Modulates Toxic Tau Oligomers

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder affecting millions of people worldwide. Therefore, finding effective interventions and therapies is extremely important. AD is one of over 20 different disorders known as tauopathies, characterized by the pathological aggregation and accumulation of tau, a microtubule-associated protein. Tau aggregates are heterogeneous and can be divided into two major groups: large metastable fibrils, including neurofibrillary tangles, and oligomers. The smaller, soluble and dynamic tau oligomers have been shown to be more toxic with more proficient seeding properties for the propagation of tau pathology as compared to the fibrillar Paired Helical Filaments (PHFs). Therefore, developing small molecules that target and interact with toxic tau oligomers can be beneficial to modulate their aggregation pathways and toxicity, preventing progression of the pathology. In this study, we show that Azure C (AC) is capable of modulating tau oligomer aggregation pathways at micromolar concentrations and rescues tau oligomers-induced toxicity in cell culture. We used both biochemical and biophysical in vitro techniques to characterize preformed tau oligomers in the presence and absence of AC. Interestingly, AC prevents toxicity not by disassembling the oligomers but rather by converting them into clusters of aggregates with nontoxic conformation.

Hsp70 Inhibits the Nucleation and Elongation of Tau and Sequesters Tau Aggregates with High Affinity

As a key player of the protein quality control network of the cell, the molecular chaperone Hsp70 inhibits the aggregation of the amyloid protein tau. To date, the mechanism of this inhibition and the tau species targeted by Hsp70 remain unknown. This is partly due to the inherent difficulty of studying amyloid aggregates because of their heterogeneous and transient nature. Here, we used ensemble and single-molecule fluorescence measurements to dissect how Hsp70 counteracts the self-assembly process of the K18 ΔK280 tau variant. We found that Hsp70 blocks the early stages of tau aggregation by suppressing the formation of tau nuclei. Additionally, Hsp70 sequesters oligomers and mature tau fibrils with nanomolar affinity into a protective complex, efficiently neutralizing their ability to damage membranes and seed further tau aggregation. Our results provide novel insights into the molecular mechanisms by which the chaperone Hsp70 counteracts the formation, propagation, and toxicity of tau aggregates.

Effects of celastrol on Tau hyperphosphorylation and expression of HSF-1 and HSP70 in SH-SY5Y neuroblastoma cells induced by amyloid-β peptides

To observe the effects of celastrol on Tau hyperphosphorylation induced by amyloid-β peptides (Aβ) in SH-SY5Y neuroblastoma cells, the changes of Tau hyperphosphorylation and the expression of heat shock protein 90 (HSP90), HSP70, and heat shock factor 1 (HSF-1) in SH-SY5Y cells treated with Aβ_1-42 and celastrol were measured. Tau hyperphosphorylation and HSP90 expression induced by Aβ_1-42 was also measured by Western blotting after HSP70 or HSF-1 knockdown by siRNA. The interaction between HSP70 and Tau or HSP70 and carboxyl terminus of HSP70 interacting protein (CHIP) was measured by co-immunoprecipitation. Compared with the control group, the expressions of HSP70 and HSF-1 were markedly decreased after the induction of Aβ_1-42 , whereas the expressions of HSP90, Tau phospho S199/202, and Tau phospho S396 were markedly increased. Meanwhile, both celastrol treatment and knockdown of HSP70 or HSF-1 in SH-SY5Y cells significantly inhibited the Tau hyperphosphorylation and HSP90 expression induced by Aβ_1-42 . Moreover, celastrol treatment had no effects on Aβ_1-42 -induced decreased expression of HSP70 and HSF-1, Tau ubiquitination, and the interaction of HSP70/Tau and HSP70/CHIP. These results suggest that celastrol_- inhibited Tau hyperphosphorylation may not be dependent on the cause of HSF-1/HSP70/CHIP-mediated ubiquitination of Tau.

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.

Imbalances in the Hsp90 Chaperone Machinery: Implications for Tauopathies

The ATP-dependent 90 kDa heat shock protein, Hsp90, is a major regulator of protein triage, from assisting in nascent protein folding to refolding or degrading aberrant proteins. Tau, a microtubule associated protein, aberrantly accumulates in Alzheimer's disease (AD) and other neurodegenerative diseases, deemed tauopathies. Hsp90 binds to and regulates tau fate in coordination with a diverse group of co-chaperones. Imbalances in chaperone levels and activity, as found in the aging brain, can contribute to disease onset and progression. For example, the levels of the Hsp90 co-chaperone, FK506-binding protein 51 kDa (FKBP51), progressively increase with age. In vitro and in vivo tau models demonstrated that FKBP51 synergizes with Hsp90 to increase neurotoxic tau oligomer production. Inversely, protein phosphatase 5 (PP5), which dephosphorylates tau to restore microtubule-binding function, is repressed with aging and activity is further repressed in AD. Similarly, levels of cyclophilin 40 (CyP40) are reduced in the aged brain and further repressed in AD. Interestingly, CyP40 was shown to breakup tau aggregates in vitro and prevent tau-induced neurotoxicity in vivo. Moreover, the only known stimulator of Hsp90 ATPase activity, Aha1, increases tau aggregation and toxicity. While the levels of Aha1 are not significantly altered with aging, increased levels have been found in AD brains. Overall, these changes in the Hsp90 heterocomplex could drive tau deposition and neurotoxicity. While the relationship of tau and Hsp90 in coordination with these co-chaperones is still under investigation, it is clear that imbalances in these proteins with aging can contribute to disease onset and progression. This review highlights the current understanding of how the Hsp90 family of molecular chaperones regulates tau or other misfolded proteins in neurodegenerative diseases with a particular emphasis on the impact of aging.

Molecular analysis of Hsp70 mechanisms in plants and their function in response to stress

Studying the strategies of improving abiotic stress tolerance is quite imperative and research under this field will increase our understanding of response mechanisms to abiotic stress such as heat. The Hsp70 is an essential regulator of protein having the tendency to maintain internal cell stability like proper folding protein and breakdown of unfolded proteins. Hsp70 holds together protein substrates to help in movement, regulation, and prevent aggregation under physical and or chemical pressure. However, this review reports the molecular mechanism of heat shock protein 70 kDa (Hsp70) action and its structural and functional analysis, research progress on the interaction of Hsp70 with other proteins and their interaction mechanisms as well as the involvement of Hsp70 in abiotic stress responses as an adaptive defense mechanism.

Stabilizing the Hsp70-Tau Complex Promotes Turnover in Models of Tauopathy

Heat shock protein 70 (Hsp70) is a chaperone that normally scans the proteome and initiates the turnover of some proteins (termed clients) by linking them to the degradation pathways. This activity is critical to normal protein homeostasis, yet it appears to fail in diseases associated with abnormal protein accumulation. It is not clear why Hsp70 promotes client degradation under some conditions, while sparing that protein under others. Here, we used a combination of chemical biology and genetic strategies to systematically perturb the affinity of Hsp70 for the model client, tau. This approach revealed that tight complexes between Hsp70 and tau were associated with enhanced turnover while transient interactions favored tau retention. These results suggest that client affinity is one important parameter governing Hsp70-mediated quality control.

Heat shock proteins as potential targets for protective strategies in neurodegeneration

Protein aggregates are hallmarks of nearly all age-related neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and several polyglutamine diseases such as Huntington's disease and different forms of spinocerebellar ataxias (SCA; SCA1-3, SCA6, and SCA7). The collapse of cellular protein homoeostasis can be both a cause and a consequence of this protein aggregation. Boosting components of the cellular protein quality control system has been widely investigated as a strategy to counteract protein aggregates or their toxic consequences. Heat shock proteins (HSPs) play a central part in regulating protein quality control and contribute to protein aggregation and disaggregation. Therefore, HSPs are viable targets for the development of drugs aimed at reducing pathogenic protein aggregates that are thought to contribute to the development of so many neurodegenerative disorders.

Determinants of amyloid fibril degradation by the PDZ protease HTRA1

Excessive aggregation of proteins has a major impact on cell fate and is a hallmark of amyloid diseases in humans. To resolve insoluble deposits and to maintain protein homeostasis, all cells use dedicated protein disaggregation, protein folding and protein degradation factors. Despite intense recent research, the underlying mechanisms controlling this key metabolic event are not well understood. Here, we analyzed how a single factor, the highly conserved serine protease HTRA1, degrades amyloid fibrils in an ATP-independent manner. This PDZ protease solubilizes protein fibrils and disintegrates the fibrillar core structure, allowing productive interaction of aggregated polypeptides with the active site for rapid degradation. The aggregate burden in a cellular model of cytoplasmic tau aggregation is thus reduced. Mechanistic aspects of ATP-independent proteolysis and its implications in amyloid diseases are discussed.

Hsp90 oligomerization process: How can p23 drive the chaperone machineries?

The 90-kDa heat shock protein (Hsp90) is a highly flexible dimer that is able to self-associate in the presence of divalent cations or under heat shock. In a previous work, we focused on the Mg2+-induced oligomerization process of Hsp90, and characterized the oligomers. Combining analytical ultracentrifugation, size-exclusion chromatography coupled to multi-angle laser light scattering and high-mass matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we studied the interaction of p23 with both Hsp90 dimer and oligomers. Even if p23 predominantly binds the Hsp90 dimer, we demonstrated, for the first time, that p23 is also able to interact with Hsp90 oligomers, shifting the Hsp90 dimer-oligomers equilibrium toward dimer. Our results showed that the Hsp90:p23 binding stoichiometry decreases with the Hsp90 oligomerization degree. Therefore, we propose a model in which p23 would act as a "protein wedge" regarding the Hsp90 dimer closure and the Hsp90 oligomerization process.

Hsp90 Oligomers Interacting with the Aha1 Cochaperone: An Outlook for the Hsp90 Chaperone Machineries

The 90-kDa heat shock protein (Hsp90) is a highly flexible dimer able to self-associate in the presence of divalent cations or under heat shock. This study investigated the relationship between Hsp90 oligomers and the Hsp90 cochaperone Aha1 (activator of Hsp90 ATPase). The interactions of Aha1 with Hsp90 dimers and oligomers were evaluated by ultracentrifugation, size-exclusion chromatography coupled to multiangle laser light scattering and high-mass matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hsp90 dimer was able to bind up to four Aha1 molecules, and Hsp90 oligomers are also able to interact with Aha1. The binding of Aha1 did not interfere with the Hsp90 oligomerization process. Except for Hsp90 dimer, the stoichiometry of the interaction remained constant, at 2 Aha1 molecules per Hsp90 dimer, regardless of the degree of Hsp90 oligomerization. Moreover, Aha1 predominantly bound to Hsp90 oligomers. Thus, the ability of Hsp90 oligomers to bind the Aha1 ATPase activator reinforces their role within the Hsp90 chaperone machineries.

The Chemical Biology of Molecular Chaperones--Implications for Modulation of Proteostasis

Protein homeostasis (proteostasis) is inextricably tied to cellular health and organismal lifespan. Aging, exposure to physiological and environmental stress, and expression of mutant and metastable proteins can cause an imbalance in the protein-folding landscape, which results in the formation of non-native protein aggregates that challenge the capacity of the proteostasis network (PN), increasing the risk for diseases associated with misfolding, aggregation, and aberrant regulation of cell stress responses. Molecular chaperones have central roles in each of the arms of the PN (protein synthesis, folding, disaggregation, and degradation), leading to the proposal that modulation of chaperone function could have therapeutic benefits for the large and growing family of diseases of protein conformation including neurodegeneration, metabolic diseases, and cancer. In this review, we will discuss the current strategies used to tune the PN through targeting molecular chaperones and assess the potential of the chemical biology of proteostasis.

Comparative analysis of autophagy and tauopathy related markers in cerebral ischemia and Alzheimer's disease animal models

Alzheimer's disease (AD) and cerebral ischemia (CI) are neuropathologies that are characterized by aggregates of tau protein, a hallmark of cognitive disorder and dementia. Protein accumulation can be induced by autophagic failure. Autophagy is a metabolic pathway involved in the homeostatic recycling of cellular components. However, the role of autophagy in those tauopathies remains unclear. In this study, we performed a comparative analysis to identify autophagy related markers in tauopathy generated by AD and CI during short-term, intermediate, and long-term progression using the 3xTg-AD mouse model (aged 6,12, and 18 months) and the global CI 2-VO (2-Vessel Occlusion) rat model (1,15, and 30 days post-ischemia). Our findings confirmed neuronal loss and hyperphosphorylated tau aggregation in the somatosensory cortex (SS-Cx) of the 3xTg-AD mice in the late stage (aged 18 months), which was supported by a failure in autophagy. These results were in contrast to those obtained in the SS-Cx of the CI rats, in which we detected neuronal loss and tauopathy at 1 and 15 days post-ischemia, and this phenomenon was reversed at 30 days. We proposed that this phenomenon was associated with autophagy induction in the late stage, since the data showed a decrease in p-mTOR activity, an association of Beclin-1 and Vps34, a progressive reduction in PHF-1, an increase in LC3B puncta and autophago-lysosomes formation were observed. Furthermore, the survival pathways remained unaffected. Together, our comparative study suggest that autophagy could ameliorates tauopathy in CI but not in AD, suggesting a differential temporal approach to the induction of neuroprotection and the prevention of neurodegeneration.

Screening strategies to identify HSP70 modulators to treat Alzheimer's disease

Alzheimer’s disease, the most common type of dementia, is a progressive brain disease that destroys cognitive function and eventually leads to death. In patients with Alzheimer’s disease, beta amyloids and tau proteins form plaques/oligomers and oligomers/tangles that affect the ability of neurons to function properly. Heat shock protein 70 (HSP70) has the ability to prevent aggregation/oligomerization of beta amyloid/tau proteins, making it a potential drug target. To determine this potential, it is essential that we have appropriate in vitro and cell-based assays that help identify specific molecules that affect this aggregation or oligomerization through HSP70. Potential drug candidates could be identified through a series of assays, starting with ATPase assays, followed by aggregation assays with enzymes/proteins and cell-based systems. ATPase assays are effective in identification of ATPase modulators but do not determine the effect of the molecule on beta amyloid and tau proteins. Molecules identified through ATPase assays are validated by thioflavin T aggregation assays in the presence of HSP70. These assays help uncover if a molecule affects beta amyloid and tau through HSP70, but are limited by their in vitro nature. Potential drug candidates are further validated through cell-based assays using mammalian, yeast, or bacterial cultures. However, while these assays are able to determine the effect of a specific molecule on beta amyloid and tau, they fail to determine whether the action is HSP70-dependent. The creation of a novel, direct assay that can demonstrate the antiaggregation effect of a molecule as well as its action through HSP70 would reduce the number of false-positive drug candidates and be more cost-effective and time-effective.

Heat shock protein 70 in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease that caused dementia which has no effective treatment. Growing evidence has demonstrated that AD is a “protein misfolding disorder” that exhibits common features of misfolded, aggregation-prone proteins and selective cell loss in the mature nervous system. Heat shock protein 70 (HSP70) attracts extensive attention worldwide, because it plays a crucial role in preventing protein misfolding and inhibiting aggregation and represents a class of proteins potentially involved in AD pathogenesis. Numerous studies have indicated that HSP70 could suppress the progression of AD with in vitro and in vivo experiments. Thus, targeting HSP70 and the related compounds might represent a promising strategy for the treatment of AD.

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.

Tau protein modifications and interactions: their role in function and dysfunction

Tau protein is abundant in the central nervous system and involved in microtubule assembly and stabilization. It is predominantly associated with axonal microtubules and present at lower level in dendrites where it is engaged in signaling functions. Post-translational modifications of tau and its interaction with several proteins play an important regulatory role in the physiology of tau. As a consequence of abnormal modifications and expression, tau is redistributed from neuronal processes to the soma and forms toxic oligomers or aggregated deposits. The accumulation of tau protein is increasingly recognized as the neuropathological hallmark of a number of dementia disorders known as tauopathies. Dysfunction of tau protein may contribute to collapse of cytoskeleton, thereby causing improper anterograde and retrograde movement of motor proteins and their cargos on microtubules. These disturbances in intraneuronal signaling may compromise synaptic transmission as well as trophic support mechanisms in neurons.

Ethoxyquin prevents chemotherapy-induced neurotoxicity via Hsp90 modulation

OBJECTIVE

Peripheral neurotoxicity is a major dose-limiting side effect of many chemotherapeutic drugs. Currently there are no effective disease-modifying therapies for chemotherapy-induced peripheral neuropathies, but these side effects of chemotherapy are potentially ideal targets for development of neuroprotective therapies, because candidate drugs can be co- or preadministered before the injury to peripheral axons takes place.

METHODS

We used a phenotypic drug screening approach to identify ethoxyquin as a potential neuroprotective drug and carried out additional biochemical experiments to identify its mechanism of action.

RESULTS

We validated the screening results with ethoxyquin and its derivatives and showed that they prevented paclitaxel-induced peripheral neuropathy without blocking paclitaxel's ability to kill tumor cells. Furthermore, we demonstrated that ethoxyquin acts by modulating the chaperone activity of heat shock protein 90 (Hsp90) and blocking the binding of 2 of its client proteins, ataxin-2 and Sf3b2. Ethoxyquin-induced reduction in levels of both of these proteins resulted in prevention of axonal degeneration caused by paclitaxel.

INTERPRETATION

Ethoxyquin and its novel derivatives as well as other classes of small molecules that act as Hsp90 modulators may offer a new opportunity for development of drugs to prevent chemotherapy-induced axonal degeneration.

Hsp90-Tau complex reveals molecular basis for specificity in chaperone action

Protein folding in the cell relies on the orchestrated action of conserved families of molecular chaperones, the Hsp70 and Hsp90 systems. Hsp70 acts early and Hsp90 late in the folding path, yet the molecular basis of this timing is enigmatic, mainly because the substrate specificity of Hsp90 is poorly understood. Here, we obtained a structural model of Hsp90 in complex with its natural disease-associated substrate, the intrinsically disordered Tau protein. Hsp90 binds to a broad region in Tau that includes the aggregation-prone repeats. Complementarily, a 106-Å-long substrate-binding interface in Hsp90 enables many low-affinity contacts. This allows recognition of scattered hydrophobic residues in late folding intermediates that remain after early burial of the Hsp70 sites. Our model resolves the paradox of how Hsp90 specifically selects for late folding intermediates but also for some intrinsically disordered proteins-through the eyes of Hsp90 they look the same.

Potential synergy between tau aggregation inhibitors and tau chaperone modulators

Tau is a soluble, microtubule-associated protein known to aberrantly form amyloid-positive aggregates. This pathology is characteristic for more than 15 neuropathies, the most common of which is Alzheimer’s disease. Finding therapeutics to reverse or remove this non-native tau state is of great interest; however, at this time only one drug is entering phase III clinical trials for treating tauopathies. Generally, tau manipulation by therapeutics can either directly or indirectly alter tau aggregation and stability. Drugs that bind and change the conformation of tau itself are largely classified as aggregation inhibitors, while drugs that alter the activity of a tau-effector protein fall into several categories, such as kinase inhibitors, microtubule stabilizers, or chaperone modulators. Chaperone inhibitors that have proven effective in tau models include heat shock protein 90 inhibitors, heat shock protein 70 inhibitors and activators, as well as inducers of heat shock proteins. While many of these compounds can alter tau levels and/or aggregation states, it is possible that combining these approaches may produce the most optimal outcome. However, because many of these compounds have multiple off-target effects or poor blood–brain barrier permeability, the development of this synergistic therapeutic strategy presents significant challenges. This review will summarize many of the drugs that have been identified to alter tau biology, with special focus on therapeutics that prevent tau aggregation and regulate chaperone-mediated clearance of tau.

Opportunities and challenges for molecular chaperone modulation to treat protein-conformational brain diseases

A common pathological hallmark of protein-conformational brain diseases is the formation of disease-specific protein aggregates. In Alzheimer's disease, these are comprised of amyloid-β and Tau as opposed to α-synuclein in Parkinson's disease and N-terminal fragments of mutant huntingtin in Huntington's disease. Most aggregates also sequester molecular chaperones, a protein family that assists in the folding, refolding, stabilization, and processing of client proteins, including misfolded proteins in brain diseases. Molecular chaperone modulation has achieved remarkable therapeutic effects in some cellular and preclinical animal models of protein-conformational diseases. This has raised hope for chaperone-based strategies to combat these diseases. Here, we review briefly the functional diversity and medical significance of molecular chaperones, their therapeutic potential, and common and specific challenges towards clinical application.

Synthesis and initial evaluation of YM-08, a blood-brain barrier permeable derivative of the heat shock protein 70 (Hsp70) inhibitor MKT-077, which reduces tau levels

The molecular chaperone, heat shock protein 70 (Hsp70), is an emerging drug target for treating neurodegenerative tauopathies. We recently found that one promising Hsp70 inhibitor, MKT-077, reduces tau levels in cellular models. However, MKT-077 does not penetrate the blood-brain barrier (BBB), limiting its use as either a clinical candidate or probe for exploring Hsp70 as a drug target in the central nervous system (CNS). We hypothesized that replacing the cationic pyridinium moiety in MKT-077 with a neutral pyridine might improve its clogP and enhance its BBB penetrance. To test this idea, we designed and synthesized YM-08, a neutral analogue of MKT-077. Like the parent compound, YM-08 bound to Hsp70 in vitro and reduced phosphorylated tau levels in cultured brain slices. Pharmacokinetic evaluation in CD1 mice showed that YM-08 crossed the BBB and maintained a brain/plasma (B/P) value of ∼0.25 for at least 18 h. Together, these studies suggest that YM-08 is a promising scaffold for the development of Hsp70 inhibitors suitable for use in the CNS.

Cysteine reactivity distinguishes redox sensing by the heat-inducible and constitutive forms of heat shock protein 70

The heat shock protein 70 (Hsp70) family of molecular chaperones has important functions in maintaining proteostasis under stress conditions. Several Hsp70 isoforms, especially Hsp72 (HSPA1A), are dramatically upregulated in response to stress; however, it is unclear whether these family members have biochemical properties that are specifically adapted to these scenarios. The redox-active compound, methylene blue (MB), has been shown to inhibit the ATPase activity of Hsp72 in vitro, and it promotes degradation of the Hsp72 substrate, tau, in cellular and animal models. Here, we report that MB irreversibly inactivates Hsp72 but not the nearly identical, constitutively expressed isoform, heat shock cognate 70 (Hsc70; HSPA8). Mass spectrometry results show that MB oxidizes Cys306, which is not conserved in Hsc70. Molecular models suggested that oxidation of Cys306 exposes Cys267 to modification and that both events contribute to loss of ATP binding in response to MB. Consistent with this model, mutating Cys267 and Cys306 to serine made Hsp72 largely resistant to MB in vitro, and overexpression of the C306S mutant blocked MB-mediated loss of tau in a cellular model. Furthermore, mutating Cys267 and Cys306 to the pseudo-oxidation mimic, aspartic acid, mirrored MB treatment: the C267D and C306D mutants had reduced ATPase activity in vitro, and overexpression of the C267/306D double mutant significantly reduced tau levels in cells. Together, these results suggest that redox sensing by specific cysteine residues in Hsp72, but not Hsc70, may be an important component of the chaperone response to oxidative stress.

FKBP10 depletion enhances glucocerebrosidase proteostasis in Gaucher disease fibroblasts

Lysosomal storage diseases (LSDs) are often caused by mutations compromising lysosomal enzyme folding in the endoplasmic reticulum (ER), leading to degradation and loss of function. Mass spectrometry analysis of Gaucher fibroblasts treated with mechanistically distinct molecules that increase LSD enzyme folding, trafficking, and function resulted in the identification of nine commonly downregulated and two jointly upregulated proteins, which we hypothesized would be critical proteostasis network components for ameliorating loss-of-function diseases. LIMP-2 and FK506 binding protein 10 (FKBP10) were validated as such herein. Increased FKBP10 levels accelerated mutant glucocerebrosidase degradation over folding and trafficking, whereas decreased ER FKBP10 concentration led to more LSD enzyme partitioning into the calnexin profolding pathway, enhancing folding and activity to levels thought to ameliorate LSDs. Thus, targeting FKBP10 appears to be a heretofore unrecognized therapeutic strategy to ameliorate LSDs.

Hsp70 protein complexes as drug targets

Heat shock protein 70 (Hsp70) plays critical roles in proteostasis and is an emerging target for multiple diseases. However, competitive inhibition of the enzymatic activity of Hsp70 has proven challenging and, in some cases, may not be the most productive way to redirect Hsp70 function. Another approach is to inhibit Hsp70's interactions with important co-chaperones, such as J proteins, nucleotide exchange factors (NEFs) and tetratricopeptide repeat (TPR) domain-containing proteins. These co-chaperones normally bind Hsp70 and guide its many diverse cellular activities. Complexes between Hsp70 and co-chaperones have been shown to have specific functions, including roles in pro-folding, pro-degradation and pro-trafficking pathways. Thus, a promising strategy may be to block protein- protein interactions between Hsp70 and its co-chaperones or to target allosteric sites that disrupt these contacts. Such an approach might shift the balance of Hsp70 complexes and re-shape the proteome and it has the potential to restore healthy proteostasis. In this review, we discuss specific challenges and opportunities related to these goals. By pursuing Hsp70 complexes as drug targets, we might not only develop new leads for therapeutic development, but also discover new chemical probes for use in understanding Hsp70 biology.

Chaperone-dependent Neurodegeneration: A Molecular Perspective on Therapeutic Intervention

Maintenance of cellular homeostasis is regulated by the molecular chaperones. Under pathogenic conditions, aberrant proteins are triaged by the chaperone network. These aberrant proteins, known as "clients," have major roles in the pathogenesis of numerous neurological disorders, including tau in Alzheimer's disease, α-synuclein and LRRK2 in Parkinson's disease, SOD-1, TDP-43 and FUS in amyotrophic lateral sclerosis, and polyQ-expanded proteins such as huntingtin in Huntington's disease. Recent work has demonstrated that the use of chemical compounds which inhibit the activity of molecular chaperones subsequently alter the fate of aberrant clients. Inhibition of Hsp90 and Hsc70, two major molecular chaperones, has led to a greater understanding of how chaperone triage decisions are made and how perturbing the chaperone system can promote clearance of these pathogenic clients. Described here are major pathways and components of several prominent neurological disorders. Also discussed is how treatment with chaperone inhibitors, predominately Hsp90 inhibitors which are selective for a diseased state, can relieve the burden of aberrant client signaling in these neurological disorders.

Analysis of the tau-associated proteome reveals that exchange of Hsp70 for Hsp90 is involved in tau degradation

The microtubule associated protein tau (MAPT/tau) aberrantly accumulates in 15 neurodegenerative diseases, termed tauopathies. One way to treat tauopathies may be to accelerate tau clearance, but the molecular mechanisms governing tau stability are not yet clear. We recently identified chemical probes that markedly accelerate the clearance of tau in cellular and animal models. In the current study, we used one of these probes in combination with immunoprecipitation and mass spectrometry to identify 48 proteins whose association with tau changes during the first 10 min after treatment. These proteins included known modifiers of tau proteotoxicity, such as ILF-2 (NFAT), ILF-3, and ataxin-2. A striking observation from the data set was that tau binding to heat shock protein 70 (Hsp70) decreased, whereas binding to Hsp90 significantly increased. Both chaperones have been linked to tau homeostasis, but their mechanisms have not been established. Using peptide arrays and binding assays, we found that Hsp70 and Hsp90 appeared to compete for binding to shared sites on tau. Further, the Hsp90-bound complex proved to be important in initiating tau clearance in cells. These results suggest that the relative levels of Hsp70 and Hsp90 may help determine whether tau is retained or degraded. Consistent with this model, analysis of reported microarray expression data from Alzheimer's disease patients and age-matched controls showed that the levels of Hsp90 are reduced in the diseased hippocampus. These studies suggest that Hsp70 and Hsp90 work together to coordinate tau homeostasis.

Features of protein-protein interactions that translate into potent inhibitors: topology, surface area and affinity

Protein-protein interactions (PPIs) control the assembly of multi-protein complexes and, thus, these contacts have enormous potential as drug targets. However, the field has produced a mix of both exciting success stories and frustrating challenges. Here, we review known examples and explore how the physical features of a PPI, such as its affinity, hotspots, off-rates, buried surface area and topology, might influence the chances of success in finding inhibitors. This analysis suggests that concise, tight binding PPIs are most amenable to inhibition. However, it is also clear that emerging technical methods are expanding the repertoire of 'druggable' protein contacts and increasing the odds against difficult targets. In particular, natural product-like compound libraries, high throughput screens specifically designed for PPIs and approaches that favour discovery of allosteric inhibitors appear to be attractive routes. The first group of PPI inhibitors has entered clinical trials, further motivating the need to understand the challenges and opportunities in pursuing these types of targets.

A chaperone trap contributes to the onset of cystic fibrosis

Protein folding is the primary role of proteostasis network (PN) where chaperone interactions with client proteins determine the success or failure of the folding reaction in the cell. We now address how the Phe508 deletion in the NBD1 domain of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein responsible for cystic fibrosis (CF) impacts the binding of CFTR with cellular chaperones. We applied single ion reaction monitoring mass spectrometry (SRM-MS) to quantitatively characterize the stoichiometry of the heat shock proteins (Hsps) in CFTR folding intermediates in vivo and mapped the sites of interaction of the NBD1 domain of CFTR with Hsp90 in vitro. Unlike folding of WT-CFTR, we now demonstrate the presence of ΔF508-CFTR in a stalled folding intermediate in stoichiometric association with the core Hsps 40, 70 and 90, referred to as a ‘chaperone trap’. Culturing cells at 30 C resulted in correction of ΔF508-CFTR trafficking and function, restoring the sub-stoichiometric association of core Hsps observed for WT-CFTR. These results support the interpretation that ΔF508-CFTR is restricted to a chaperone-bound folding intermediate, a state that may contribute to its loss of trafficking and increased targeting for degradation. We propose that stalled folding intermediates could define a critical proteostasis pathway branch-point(s) responsible for the loss of function in misfolding diseases as observed in CF.

Hsp90: structure and function

Hsp90 is a highly abundant and ubiquitous molecular chaperone which plays an essential role in many cellular processes including cell cycle control, cell survival, hormone and other signalling pathways. It is important for the cell's response to stress and is a key player in maintaining cellular homeostasis. In the last ten years, it has become a major therapeutic target for cancer, and there has also been increasing interest in it as a therapeutic target in neurodegenerative disorders, and in the development of anti-virals and anti-protozoan infections. The focus of this review is the structural and mechanistic studies which have been performed in order to understand how this important chaperone acts on a wide variety of different proteins (its client proteins) and cellular processes. As with many of the other classes of molecular chaperone, Hsp90 has a critical ATPase activity, and ATP binding and hydrolysis known to modulate the conformational dynamics of the protein. It also uses a host of cochaperones which not only regulate the ATPase activity and conformational dynamics but which also mediate interactions with Hsp90 client proteins. The system is also regulated by post-translational modifications including phosphorylation and acetylation. This review discusses all these aspects of Hsp90 structure and function.