Chaperone-dependent Neurodegeneration: A Molecular Perspective on Therapeutic Intervention

Geldanamycin, a Naturally Occurring Inhibitor of Hsp90 and a Lead Compound for Medicinal Chemistry

Geldanamycin remains a driver in the medicinal chemistry of heat shock protein 90 (Hsp90) inhibition, even half a century after its original isolation from nature. This Perspective focuses on the properties of the benzoquinone ring of the natural product that enable a range of functionalization reactions to take place. Therefore, inherent reactivity at C-17, where the methoxy group serves as a vinylogous ester, and at C-19 that demonstrates nucleophilic, enamide-type character toward electrophiles, and also as a conjugate acceptor to react with nucleophiles, has facilitated the synthesis of semisynthetic derivatives. Thus, a range of C-17-substituted amine derivatives has been investigated in oncology applications, with a number of compounds in this series reaching clinical trials. In contrast, the 19-position of geldanamycin has received less attention, although 19-substituted derivatives offer promise with markedly reduced toxicity compared to geldanamycin itself, while retaining Hsp90 inhibitory activity albeit with diminished potency in cellular studies.

Natural Products Targeting Hsp90 for a Concurrent Strategy in Glioblastoma and Neurodegeneration

Glioblastoma multiforme (GBM) is one of the most common aggressive, resistant, and invasive primary brain tumors that share neurodegenerative actions, resembling many neurodegenerative diseases. Although multiple conventional approaches, including chemoradiation, are more frequent in GBM therapy, these approaches are ineffective in extending the mean survival rate and are associated with various side effects, including neurodegeneration. This review proposes an alternative strategy for managing GBM and neurodegeneration by targeting heat shock protein 90 (Hsp90). Hsp90 is a well-known molecular chaperone that plays essential roles in maintaining and stabilizing protein folding to degradation in protein homeostasis and modulates signaling in cancer and neurodegeneration by regulating many client protein substrates. The therapeutic benefits of Hsp90 inhibition are well-known for several malignancies, and recent evidence highlights that Hsp90 inhibitors potentially inhibit the aggressiveness of GBM, increasing the sensitivity of conventional treatment and providing neuroprotection in various neurodegenerative diseases. Herein, the overview of Hsp90 modulation in GBM and neurodegeneration progress has been discussed with a summary of recent outcomes on Hsp90 inhibition in various GBM models and neurodegeneration. Particular emphasis is also given to natural Hsp90 inhibitors that have been evidenced to show dual protection in both GBM and neurodegeneration.

C9orf72-Related Neurodegenerative Diseases: From Clinical Diagnosis to Therapeutic Strategies

Hexanucleotide expansion in C9orf72 has been related to several phenotypes to date, complicating the clinical recognition of these neurodegenerative disorders. An early diagnosis can improve the management of patients, promoting early administration of therapeutic supportive strategies. Here, we report known clinical presentations of C9orf72-related neurodegenerative disorders, pointing out suggestive phenotypes that can benefit the genetic characterization of patients. Considering the high variability of C9orf72-related disorder, frequent and rare manifestations are described, with detailed clinical, instrumental evaluation, and supportive therapeutical approaches. Furthermore, to improve the understanding of molecular pathways of the disease and potential therapeutical targets, a detailed description of the cellular mechanisms related to the pathological effect of C9orf72 is reported. New promising therapeutical strategies and ongoing studies are reported highlighting their molecular role in cellular pathological pathways of C9orf72. These therapeutic approaches are particularly promising because they seem to stop the disease before neuronal damage. The knowledge of clinical and molecular features of C9orf72-related neurodegenerative disorders improves the therapeutical application of known strategies and will lay the basis for the development of new potential therapies.

Preclinical Evaluation of [11C]YC-72-AB85 for In Vivo Visualization of Heat Shock Protein 90 in Brain and Cancer with Positron Emission Tomography

Aberrant Hsp90 has been implied in cancer and neurodegenerative disorders. The development of a suitable Hsp90 Positron emission tomography (PET) probe can provide in vivo quantification of the expression levels of Hsp90 as a biomarker for diagnosis and follow-up of cancer and central nervous system (CNS) disease progression. In this respect, [¹¹C]YC-72-AB85 was evaluated as an Hsp90 PET probe in B16.F10 melanoma bearing mice and its brain uptake was determined in rats and nonhuman primate. In vitro binding of [¹¹C]YC-72-AB85 to tissue slices of mouse B16.F10 melanoma, PC3 prostate carcinoma, and rodent brain was evaluated using autoradiography. Biodistribution of [¹¹C]YC-72-AB85 was evaluated in healthy and B16.F10 melanoma mice. In vivo brain uptake was assessed by μPET studies in rats and a rhesus monkey. In vitro binding was deemed Hsp90-specific by blocking studies with heterologous Hsp90 inhibitors onalespib and SNX-0723. Saturable Hsp90 binding was observed in brain, tumor, blood, and blood-rich organs in mice. In combined pretreatment and displacement studies, reversible and Hsp90-specific binding of [¹¹C]YC-72-AB85 was observed in rat brain. Dynamic μPET brain scans in baseline and blocking conditions in a rhesus monkey indicated Hsp90-specific binding. [¹¹C]YC-72-AB85 is a promising PET tracer for in vivo visualization of Hsp90 in tumor and brain. Clear differences of Hsp90 binding to blood and blood-rich organs were observed in tumor vs control mice. Further, we clearly demonstrate, for the first time, binding to a saturable Hsp90 pool in brain of rats and a rhesus monkey.

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.

TAS-116, a Well-Tolerated Hsp90 Inhibitor, Prevents the Activation of the NLRP3 Inflammasome in Human Retinal Pigment Epithelial Cells

Chronic inflammation has been associated with several chronic diseases, such as age-related macular degeneration (AMD). The NLRP3 inflammasome is a central proinflammatory signaling complex that triggers caspase-1 activation leading to the maturation of IL-1β. We have previously shown that the inhibition of the chaperone protein, Hsp90, prevents NLRP3 activation in human retinal pigment epithelial (RPE) cells; these are cells which play a central role in the pathogenesis of AMD. In that study, we used a well-known Hsp90 inhibitor geldanamycin, but it cannot be used as a therapy due to its adverse effects, including ocular toxicity. Here, we have tested the effects of a novel Hsp90 inhibitor, TAS-116, on NLRP3 activation using geldanamycin as a reference compound. Using our existing protocol, inflammasome activation was induced in IL-1α-primed ARPE-19 cells with the proteasome and autophagy inhibitors MG-132 and bafilomycin A1, respectively. Intracellular caspase-1 activity was determined using a commercial caspase-1 activity kit and the FLICA assay. The levels of IL-1β were measured from cell culture medium samples by ELISA. Cell viability was monitored by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test and lactate dehydrogenase (LDH) measurements. Our findings show that TAS-116 could prevent the activation of caspase-1, subsequently reducing the release of mature IL-1β. TAS-116 has a better in vitro therapeutic index than geldanamycin. In summary, TAS-116 appears to be a well-tolerated Hsp90 inhibitor, with the capability to prevent the activation of the NLRP3 inflammasome in human RPE cells.

Hsp70 affects memory formation and behaviorally relevant gene expression in Drosophila melanogaster

Heat shock proteins, in particular Hsp70, play a central role in proteostasis in eukaryotic cells. Due to its chaperone properties, Hsp70 is involved in various processes after stress and under normal physiological conditions. In contrast to mammals and many Diptera species, inducible members of the Hsp70 family in Drosophila are constitutively synthesized at a low level and undergo dramatic induction after temperature elevation or other forms of stress. In the courtship suppression paradigm used in this study, Drosophila males that have been repeatedly rejected by mated females during courtship are less likely than naive males to court other females. Although numerous genes with known function were identified to play important roles in long-term memory, there is, to the best of our knowledge, no direct evidence implicating Hsp70 in this process. To elucidate a possible role of Hsp70 in memory formation, we used D. melanogaster strains containing different hsp70 copy numbers, including strains carrying a deletion of all six hsp70 genes. Our investigations exploring the memory of courtship rejection paradigm demonstrated that a low constitutive level of Hsp70 is apparently required for learning and the formation of short and long-term memories in males. The performed transcriptomic studies demonstrate that males with different hsp70 copy numbers differ significantly in the expression of a few definite groups of genes involved in mating, reproduction, and immunity in response to rejection. Specifically, our analysis reveals several major pathways that depend on the presence of hsp70 in the genome and participate in memory formation and consolidation, including the cAMP signaling cascade.

Could Heat Therapy Be an Effective Treatment for Alzheimer's and Parkinson's Diseases? A Narrative Review

Neurodegenerative diseases involve the progressive deterioration of structures within the central nervous system responsible for motor control, cognition, and autonomic function. Alzheimer's disease and Parkinson's disease are among the most common neurodegenerative disease and have an increasing prevalence over the age of 50. Central in the pathophysiology of these neurodegenerative diseases is the loss of protein homeostasis, resulting in misfolding and aggregation of damaged proteins. An element of the protein homeostasis network that prevents the dysregulation associated with neurodegeneration is the role of molecular chaperones. Heat shock proteins (HSPs) are chaperones that regulate the aggregation and disaggregation of proteins in intracellular and extracellular spaces, and evidence supports their protective effect against protein aggregation common to neurodegenerative diseases. Consequently, upregulation of HSPs, such as HSP70, may be a target for therapeutic intervention for protection against neurodegeneration. A novel therapeutic intervention to increase the expression of HSP may be found in heat therapy and/or heat acclimation. In healthy populations, these interventions have been shown to increase HSP expression. Elevated HSP may have central therapeutic effects, preventing or reducing the toxicity of protein aggregation, and/or peripherally by enhancing neuromuscular function. Broader physiological responses to heat therapy have also been identified and include improvements in muscle function, cerebral blood flow, and markers of metabolic health. These outcomes may also have a significant benefit for people with neurodegenerative disease. While there is limited research into body warming in patient populations, regular passive heating (sauna bathing) has been associated with a reduced risk of developing neurodegenerative disease. Therefore, the emerging evidence is compelling and warrants further investigation of the potential benefits of heat acclimation and passive heat therapy for sufferers of neurodegenerative diseases.

Chaperome heterogeneity and its implications for cancer study and treatment

The chaperome is the collection of proteins in the cell that carry out molecular chaperoning functions. Changes in the interaction strength between chaperome proteins lead to an assembly that is functionally and structurally distinct from each constituent member. In this review, we discuss the epichaperome, the cellular network that forms when the chaperome components of distinct chaperome machineries come together as stable, functionally integrated, multimeric complexes. In tumors, maintenance of the epichaperome network is vital for tumor survival, rendering them vulnerable to therapeutic interventions that target critical epichaperome network components. We discuss how the epichaperome empowers an approach for precision medicine cancer trials where a new target, biomarker, and relevant drug candidates can be correlated and integrated. We introduce chemical biology methods to investigate the heterogeneity of the chaperome in a given cellular context. Lastly, we discuss how ligand-protein binding kinetics are more appropriate than equilibrium binding parameters to characterize and unravel chaperome targeting in cancer and to gauge the selectivity of ligands for specific tumor-associated chaperome pools.

Heat shock promotes inclusion body formation of mutant huntingtin (mHtt) and alleviates mHtt-induced transcription factor dysfunction

PolyQ-expanded huntingtin (mHtt) variants form aggregates, termed inclusion bodies (IBs), in individuals with and models of Huntington's disease (HD). The role of IB versus diffusible mHtt in neurotoxicity remains unclear. Using a ponasterone (PA)-inducible cell model of HD, here we evaluated the effects of heat shock on the appearance and functional outcome of Htt103QExon1-EGFP expression. Quantitative image analysis indicated that 80-90% of this mHtt protein initially appears as "diffuse" signals in the cytosol, with IBs forming at high mHtt expression. A 2-h heat shock during the PA induction reduced the diffuse signal, but greatly increased mHtt IB formation in both cytosol and nucleus. Dose- and time-dependent mHtt expression suggested that nucleated polymerization drives IB formation. RNA-mediated knockdown of heat shock protein 70 (HSP70) and heat shock cognate 70 protein (HSC70) provided evidence for their involvement in promoting diffuse mHtt to form IBs. Reporter gene assays assessing the impacts of diffuse versus IB mHtt showed concordance of diffuse mHtt expression with the repression of heat shock factor 1, cAMP-responsive element-binding protein (CREB), and NF-κB activity. CREB repression was reversed by heat shock coinciding with mHtt IB formation. In an embryonic striatal neuron-derived HD model, the chemical chaperone sorbitol similarly promoted the structuring of diffuse mHtt into IBs and supported cell survival under stress. Our results provide evidence that mHtt IB formation is a chaperone-supported cellular coping mechanism that depletes diffusible mHtt conformers, alleviates transcription factor dysfunction, and promotes neuron survival.

7B2 chaperone knockout in APP model mice results in reduced plaque burden

Impairment of neuronal proteostasis is a hallmark of Alzheimer's and other neurodegenerative diseases. However, the underlying molecular mechanisms leading to pathogenic protein aggregation, and the role of secretory chaperone proteins in this process, are poorly understood. We have previously shown that the neural-and endocrine-specific secretory chaperone 7B2 potently blocks in vitro fibrillation of Aβ42. To determine whether 7B2 can function as a chaperone in vivo, we measured plaque formation and performed behavioral assays in 7B2-deficient mice in an hAPPswe/PS1dE9 Alzheimer's model mouse background. Surprisingly, immunocytochemical analysis of cortical levels of thioflavin S- and Aβ-reactive plaques showed that APP mice with a partial or complete lack of 7B2 expression exhibited a significantly lower number and burden of thioflavin S-reactive, as well as Aβ-immunoreactive, plaques. However, 7B2 knockout did not affect total brain levels of either soluble or insoluble Aβ. While hAPP model mice performed poorly in the Morris water maze, their brain 7B2 levels did not impact performance. Since 7B2 loss reduced amyloid plaque burden, we conclude that brain 7B2 can impact Aβ disposition in a manner that facilitates plaque formation. These results are reminiscent of prior findings in hAPP model mice lacking the ubiquitous secretory chaperone clusterin.

Hsp90 inhibition as a means to inhibit activation of the NLRP3 inflammasome

Once activated, the intracellular receptor NLRP3 assembles an inflammasome protein complex that facilitates the caspase-1-mediated maturation of IL-1β and IL-18. Inactive NLRP3 is guarded by a protein complex containing Hsp90. In response to stress stimuli, Hsp90 is released, and NLRP3 can be activated to promote inflammation. In this study, we blocked Hsp90 with geldanamycin and studied the fate of NLRP3 in human retinal pigment epithelial (RPE) cells. RPE cells play a central role in the development of age-related macular degeneration (AMD), a progressive eye disease causing severe vision loss in the elderly. IL-1α-primed ARPE-19 cells, human embryonal stem cell (hESC)-derived RPE cells, and primary human RPE cells were exposed to MG-132 and bafilomycin A to activate NLRP3 via the inhibition of proteasomes and autophagy, respectively. Additionally, RPE cells were treated with geldanamycin at different time points and the levels of NLRP3 and IL-1β were determined. Caspase-1 activity was measured using a commercial assay. Geldanamycin prevented the activation of the inflammasome in human RPE cells. NLRP3 released from its protective complex became degraded by autophagy or secreted from the cells. Controlled destruction of NLRP3 is a potential way to regulate the inflammation associated with chronic diseases, such as AMD.

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.

Proteomic interrogation of HSP90 and insights for medical research

INTRODUCTION

Heat shock protein 90 (HSP90) regulates protein homeostasis in eukaryotes. As a 'professional interactor', HSP90 binds to and chaperones many proteins and has both housekeeping and disease-related functions but its regulation remains in part elusive. HSP90 complexes are a target for therapy, notably against cancer, and several inhibitors are currently in clinical trials. Proteomic studies have revealed the vast interaction network of HSP90 and, in doing so, the extent of cellular processes the chaperone takes part in, especially in yeast and human cells. Furthermore, small-molecule inhibitors were used to probe the global impact of its inhibition on the proteome. Areas covered: We review here recent HSP90-related interactomics and total proteome studies and their relevance for research on cancer, neurodegenerative and pathogen diseases. Expert commentary: Proteomics experiments are our best chance to identify the context-dependent global proteome of HSP90 and thus uncover and understand its disease-specific biology. However, understanding the complexity of HSP90 will require multiple complementary, quantitative approaches and novel bioinformatics to translate interactions into ordered functional networks and pathways. Developing therapies will necessitate more knowledge on HSP90 complexes and networks with disease relevance and on total proteome changes induced by their perturbation. Most work has been done in cancer, thus a lot remains to be done in the context of other diseases.

Protein Quality Control by Molecular Chaperones in Neurodegeneration

Protein homeostasis (proteostasis) requires the timely degradation of misfolded proteins and their aggregates by protein quality control (PQC), of which molecular chaperones are an essential component. Compared with other cell types, PQC in neurons is particularly challenging because they have a unique cellular structure with long extensions. Making it worse, neurons are postmitotic, i.e., cannot dilute toxic substances by division, and, thus, are highly sensitive to misfolded proteins, especially as they age. Failure in PQC is often associated with neurodegenerative diseases, such as Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), and prion disease. In fact, many neurodegenerative diseases are considered to be protein misfolding disorders. To prevent the accumulation of disease-causing aggregates, neurons utilize a repertoire of chaperones that recognize misfolded proteins through exposed hydrophobic surfaces and assist their refolding. If such an effort fails, chaperones can facilitate the degradation of terminally misfolded proteins through either the ubiquitin (Ub)-proteasome system (UPS) or the autophagy-lysosome system (hereafter autophagy). If soluble, the substrates associated with chaperones, such as Hsp70, are ubiquitinated by Ub ligases and degraded through the proteasome complex. Some misfolded proteins carrying the KFERQ motif are recognized by the chaperone Hsc70 and delivered to the lysosomal lumen through a process called, chaperone-mediated autophagy (CMA). Aggregation-prone misfolded proteins that remain unprocessed are directed to macroautophagy in which cargoes are collected by adaptors, such as p62/SQSTM-1/Sequestosome-1, and delivered to the autophagosome for lysosomal degradation. The aggregates that have survived all these refolding/degradative processes can still be directly dissolved, i.e., disaggregated by chaperones. Studies have shown that molecular chaperones alleviate the pathogenic symptoms by neurodegeneration-causing protein aggregates. Chaperone-inducing drugs and anti-aggregation drugs are actively exploited for beneficial effects on symptoms of disease. Here, we discuss how chaperones protect misfolded proteins from aggregation and mediate the degradation of terminally misfolded proteins in collaboration with cellular degradative machinery. The topics also include therapeutic approaches to improve the expression and turnover of molecular chaperones and to develop anti-aggregation drugs.

Mechanistic basis for the recognition of a misfolded protein by the molecular chaperone Hsp90

The critical toxic species in over 40 human diseases are misfolded proteins. Their interaction with molecular chaperones such as Hsp90, which preferentially interacts with metastable proteins, is essential for the blocking of disease progression. Here we used nuclear magnetic resonance (NMR) spectroscopy to determine the three-dimensional structure of the misfolded cytotoxic monomer of the amyloidogenic human protein transthyretin, which is characterized by the release of the C-terminal β-strand and perturbations of the A-B loop. The misfolded transthyretin monomer, but not the wild-type protein, binds to human Hsp90. In the bound state, the Hsp90 dimer predominantly populates an open conformation, and transthyretin retains its globular structure. The interaction surface for the transthyretin monomer comprises the N-terminal and middle domains of Hsp90 and overlaps with that of the Alzheimer's-disease-related protein tau. Taken together, the data suggest that Hsp90 uses a mechanism for the recognition of aggregation-prone proteins that is largely distinct from those of other Hsp90 clients.

Stressing Out Hsp90 in Neurotoxic Proteinopathies

A toxic accumulation of proteins is the hallmark pathology of several neurodegenerative disorders. Protein accumulation is regularly prevented by the network of molecular chaperone proteins, including and especially Hsp90. For reasons not yet elucidated, Hsp90 and the molecular chaperones interact with, but do not degrade, these toxic proteins resulting in the pathogenic accumulation of proteins such as tau, in Alzheimer's Disease, and α-synuclein, in Parkinson's Disease. In this review, we describe the associations between Hsp90 and the pathogenic and driver proteins of several neurodegenerative disorders. We additionally describe how the inhibition of Hsp90 promotes the degradation of both mutant and pathogenic protein species in models of neurodegenerative diseases. We also examine the current state of Hsp90 inhibitors capable of crossing the blood-brain barrier; compounds which may be capable of slowing, preventing, and possible reversing neurodegenerative diseases.

Molecular Chaperones in Neurodegenerative Diseases: A Short Review

Stress and misfolded proteins result to dysfunction in the cell, often leading to neurodegenerative diseases and aging. Misfolded proteins form toxic aggregates that threaten cell's stability and normal functions. In order to restore its homeostasis, the cell activates the UPR system. Leading role in the restoration play the molecular chaperones which target the misfolded proteins with the purpose of either helping them to unfold and refold to their natural state or lead them degradation. This paper aims to present some of the most known molecular chaperones and their relation with diseases associated to protein misfolding and neurodegeneration, as well as the role of chaperones in proteostasis.

Chemical Tools to Investigate Mechanisms Associated with HSP90 and HSP70 in Disease

The chaperome is a large and diverse protein machinery composed of chaperone proteins and a variety of helpers, such as the co-chaperones, folding enzymes, and scaffolding and adapter proteins. Heat shock protein 90s and 70s (HSP90s and HSP70s), the most abundant chaperome members in human cells, are also the most complex. As we have learned to appreciate, their functions are context dependent and manifested through a variety of conformations that each recruit a subset of co-chaperone, scaffolding, and folding proteins and which are further diversified by the posttranslational modifications each carry, making their study through classic genetic and biochemical techniques quite a challenge. Chemical biology tools and techniques have been developed over the years to help decipher the complexities of the HSPs and this review provides an overview of such efforts with focus on HSP90 and HSP70.

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.

The neural chaperone proSAAS blocks α-synuclein fibrillation and neurotoxicity

Emerging evidence strongly suggests that chaperone proteins are cytoprotective in neurodegenerative proteinopathies involving protein aggregation; for example, in the accumulation of aggregated α-synuclein into the Lewy bodies present in Parkinson's disease. Of the various chaperones known to be associated with neurodegenerative disease, the small secretory chaperone known as proSAAS (named after four residues in the amino terminal region) has many attractive properties. We show here that proSAAS, widely expressed in neurons throughout the brain, is associated with aggregated synuclein deposits in the substantia nigra of patients with Parkinson's disease. Recombinant proSAAS potently inhibits the fibrillation of α-synuclein in an in vitro assay; residues 158-180, containing a largely conserved element, are critical to this bioactivity. ProSAAS also exhibits a neuroprotective function; proSAAS-encoding lentivirus blocks α-synuclein-induced cytotoxicity in primary cultures of nigral dopaminergic neurons, and recombinant proSAAS blocks α-synuclein-induced cytotoxicity in SH-SY5Y cells. Four independent proteomics studies have previously identified proSAAS as a potential cerebrospinal fluid biomarker in various neurodegenerative diseases. Coupled with prior work showing that proSAAS blocks β-amyloid aggregation into fibrils, this study supports the idea that neuronal proSAAS plays an important role in proteostatic processes. ProSAAS thus represents a possible therapeutic target in neurodegenerative disease.

The ubiquitin proteasomal system: a potential target for the management of Alzheimer's disease

The cellular quality control system degrades abnormal or misfolded proteins and consists of three different mechanisms: the ubiquitin proteasomal system (UPS), autophagy and molecular chaperones. Any disturbance in this system causes proteins to accumulate, resulting in neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease (AD), Parkinson's disease, Huntington's disease and prion or polyglutamine diseases. Alzheimer's disease is currently one of the most common age-related neurodegenerative diseases. However, its exact cause and pathogenesis are unknown. Currently approved medications for AD provide symptomatic relief; however, they fail to influence disease progression. Moreover, the components of the cellular quality control system represent an important focus for the development of targeted and potent therapies for managing AD. This review aims to evaluate whether existing evidence supports the hypothesis that UPS impairment causes the early pathogenesis of neurodegenerative disorders. The first part presents basic information about the UPS and its molecular components. The next part explains how the UPS is involved in neurodegenerative disorders. Finally, we emphasize how the UPS influences the management of AD. This review may help in the design of future UPS-related therapies for AD.

Pharmacological Chaperones: Design and Development of New Therapeutic Strategies for the Treatment of Conformational Diseases

Errors in protein folding may result in premature clearance of structurally aberrant proteins, or in the accumulation of toxic misfolded species or protein aggregates. These pathological events lead to a large range of conditions known as conformational diseases. Several research groups have presented possible therapeutic solutions for their treatment by developing novel compounds, known as pharmacological chaperones. These cell-permeable molecules selectively provide a molecular scaffold around which misfolded proteins can recover their native folding and, thus, their biological activities. Here, we review therapeutic strategies, clinical potentials, and cost-benefit impacts of several classes of pharmacological chaperones for the treatment of a series of conformational diseases.

Protein homeostasis gene dysregulation in pretangle-bearing nucleus basalis neurons during the progression of Alzheimer's disease

Conformational phosphorylation and cleavage events drive the tau protein from a soluble, monomeric state to a relatively insoluble, polymeric state that precipitates the formation of neurofibrillary tangles (NFTs) in projection neurons in Alzheimer's disease (AD), including the magnocellular perikarya located in the nucleus basalis of Meynert (NBM) complex of the basal forebrain. Whether these structural changes in the tau protein are associated with pathogenic changes at the molecular and cellular level remains undetermined during the onset of AD. Here, we examined alterations in gene expression within individual NBM neurons immunostained for pS422, an early tau phosphorylation event, or dual labeled for pS422 and TauC3, a later stage tau neoepitope, from tissue obtained postmortem from subjects who died with an antemortem clinical diagnosis of no cognitive impairment, mild cognitive impairment, or mild/moderate AD. Specifically, pS422-positive pretangles displayed an upregulation of select gene transcripts subserving protein quality control. On the other hand, late-stage TauC3-positive NFTs exhibited upregulation of messenger RNAs involved in protein degradation but also cell survival. Taken together, these results suggest that molecular pathways regulating protein homeostasis are altered during the evolution of NFT pathology in the NBM. These changes likely contribute to the disruption of protein turnover and neuronal survival of these vulnerable NBM neurons during the progression of AD.

Chaperones in Neurodegeneration

Cellular protein homeostasis (proteostasis) maintains the integrity of the proteome and includes protein synthesis, folding, oligomerization, and turnover; chaperone proteins assist with all of these processes. Neurons appear to be especially susceptible to failures in proteostasis, and this is now increasingly recognized as a major origin of neurodegenerative disease. This review, based on a mini-symposium presented at the 2015 Society for Neuroscience meeting, describes new work in the area of neuronal proteostasis, with a specific focus on the roles and therapeutic uses of protein chaperones. We first present a brief review of protein misfolding and aggregation in neurodegenerative disease. We then discuss different aspects of chaperone control of neuronal proteostasis on topics ranging from chaperone engineering, to chaperone-mediated blockade of protein oligomerization and cytotoxicity, to the potential rescue of neurodegenerative processes using modified chaperone proteins.

SIGNIFICANCE STATEMENT

Aberrant protein homeostasis within neurons results in protein misfolding and aggregation. In this review, we discuss specific roles for protein chaperones in the oligomerization, assembly, and disaggregation of proteins known to be abnormally folded in neurodegenerative disease. Collectively, our goal is to identify therapeutic mechanisms to reduce the cellular toxicity of abnormal aggregates.