Effects of heat shock, heat shock protein 40 (HDJ-2), and proteasome inhibition on protein aggregation in cellular models of Huntington's disease

Type I Hsp40s/DnaJs aggregates exhibit features reminiscent of amyloidogenic structures

A rise in temperature triggers a structural change in the human Type I 40 kDa heat shock protein (Hsp40/DnaJ), known as DNAJA1. This change leads to a less compact structure, characterized by an increased presence of solvent-exposed hydrophobic patches and β-sheet-rich regions. This transformation is validated by circular dichroism, thioflavin T binding, and Bis-ANS assays. The formation of this β-sheet-rich conformation, which is amplified in the absence of zinc, leads to protein aggregation. This aggregation is induced not only by high temperatures but also by low ionic strength and high protein concentration. The aggregated conformation exhibits characteristics of an amyloidogenic structure, including a distinctive X-ray diffraction pattern, seeding competence (which stimulates the formation of amyloid-like aggregates), cytotoxicity, resistance to SDS, and fibril formation. Interestingly, the yeast Type I Ydj1 also tends to adopt a similar β-sheet-rich structure under comparable conditions, whereas Type II Hsp40s, whether human or from yeast, do not. Moreover, Ydj1 aggregates were found to be cytotoxic. Studies using DNAJA1- and Ydj1-deleted mutants suggest that the zinc-finger region plays a crucial role in amyloid formation. Our discovery of amyloid aggregation in a C-terminal deletion mutant of DNAJA1, which resembles a spliced homolog expressed in the testis, implies that Type I Hsp40 co-chaperones may generate amyloidogenic species in vivo.

Diamond Quantum Sensing Revealing the Relation between Free Radicals and Huntington's Disease

Huntington's disease (HD) is a well-studied yet rare disease caused by a specific mutation that results in the expression of polyglutamine (PolyQ). The formation of aggregates of PolyQ leads to disease and increases the level of free radicals. However, it is unclear where free radicals are generated and how they impact cells. To address this, a new method called relaxometry was used to perform nanoscale MRI measurements with a subcellular resolution. The method uses a defect in fluorescent nanodiamond (FND) that changes its optical properties based on its magnetic surroundings, allowing for sensitive detection of free radicals. To investigate if radical generation occurs near PolyQ aggregates, stable tetracycline (tet)-inducible HDQ119-EGFP-expressing human embryonic kidney cells (HEK PQ) were used to induce the PolyQ formation and Huntington aggregation. The study found that NDs are highly colocalized with PolyQ aggregates at autolysosomes, and as the amount of PolyQ aggregation increased, so did the production of free radicals, indicating a relationship between PolyQ aggregation and autolysosome dysfunction.

Genome-wide screening in pluripotent cells identifies Mtf1 as a suppressor of mutant huntingtin toxicity

Huntington's disease (HD) is a neurodegenerative disorder caused by CAG-repeat expansions in the huntingtin (HTT) gene. The resulting mutant HTT (mHTT) protein induces toxicity and cell death via multiple mechanisms and no effective therapy is available. Here, we employ a genome-wide screening in pluripotent mouse embryonic stem cells (ESCs) to identify suppressors of mHTT toxicity. Among the identified suppressors, linked to HD-associated processes, we focus on Metal response element binding transcription factor 1 (Mtf1). Forced expression of Mtf1 counteracts cell death and oxidative stress caused by mHTT in mouse ESCs and in human neuronal precursor cells. In zebrafish, Mtf1 reduces malformations and apoptosis induced by mHTT. In R6/2 mice, Mtf1 ablates motor defects and reduces mHTT aggregates and oxidative stress. Our screening strategy enables a quick in vitro identification of promising suppressor genes and their validation in vivo, and it can be applied to other monogenic diseases.

Implications of specific lysine residues within ataxin-3 for the molecular pathogenesis of Machado-Joseph disease

Lysine residues are one of the main sites for posttranslational modifications of proteins, and lysine ubiquitination of the Machado-Joseph disease protein ataxin-3 is implicated in its cellular function and polyglutamine expansion-dependent toxicity. Despite previously undertaken efforts, the individual roles of specific lysine residues of the ataxin-3 sequence are not entirely understood and demand further analysis. By retaining single lysine residues of otherwise lysine-free wild-type and polyglutamine-expanded ataxin-3, we assessed the effects of a site-limited modifiability on ataxin-3 protein levels, aggregation propensity, localization, and stability. We confirmed earlier findings that levels of lysine-free ataxin-3 are reduced due to its decreased stability, which led to a diminished load of SDS-insoluble species of its polyglutamine-expanded form. The isolated presence of several single lysine residues within the N-terminus of polyglutamine-expanded ataxin-3 significantly restored its aggregate levels, with highest fold changes induced by the presence of lysine 8 or lysine 85, respectively. Ataxin-3 lacking all lysine residues presented a slightly increased nuclear localization, which was counteracted by the reintroduction of lysine 85, whereas presence of either lysine 8 or lysine 85 led to a significantly higher ataxin-3 stability. Moreover, lysine-free ataxin-3 showed increased toxicity and binding to K48-linked polyubiquitin chains, whereas the reintroduction of lysine 85, located between the ubiquitin-binding sites 1 and 2 of ataxin-3, normalized its binding affinity. Overall, our data highlight the relevance of lysine residues 8 and 85 of ataxin-3 and encourage further analyses, to evaluate the potential of modulating posttranslational modifications of these sites for influencing pathophysiological characteristics of the Machado-Joseph disease protein.

DNAJA1- and conformational mutant p53-dependent inhibition of cancer cell migration by a novel compound identified through a virtual screen

Cancers are frequently addicted to oncogenic missense mutant p53 (mutp53). DNAJA1, a member of heat shock protein 40 (HSP40), also known as J-domain proteins (JDPs), plays a crucial role in the stabilization and oncogenic activity of misfolded or conformational mutp53 by binding to and preventing mutp53 from proteasomal degradation. However, strategies to deplete mutp53 are not well-established, and no HSP40/JDPs inhibitors are clinically available. To identify compounds that bind to DNAJA1 and induce mutp53 degradation, we performed an in silico docking study of ~10 million of compounds from the ZINC database for the J-domain of DNAJA1. A compound 7-3 was identified, and its analogue A11 effectively reduced the levels of DNAJA1 and conformational mutp53 with minimal effects on the levels of wild-type p53 and DNA-contact mutp53. A11 suppressed migration and filopodia formation in a manner dependent on DNAJA1 and conformational mutp53. A mutant DNAJA1 with alanine mutations at predicted amino acids (tyrosine 7, lysine 44, and glutamine 47) failed to bind to A11. Cells expressing the mutant DNAJA1 became insensitive to A11-mediated depletion of DNAJA1 and mutp53 as well as A11-mediated inhibition of cell migration. Thus, A11 is the first HSP40/JDP inhibitor that has not been previously characterized for depleting DNAJA1 and subsequently conformational mutp53, leading to inhibition of cancer cell migration. A11 can be exploited for a novel treatment against cancers expressing conformational mutp53.

Hsp40 overexpression in pacemaker neurons delays circadian dysfunction in a Drosophila model of Huntington's disease

Circadian disturbances are early features of neurodegenerative diseases, including Huntington's disease (HD). Emerging evidence suggests that circadian decline feeds into neurodegenerative symptoms, exacerbating them. Therefore, we asked whether known neurotoxic modifiers can suppress circadian dysfunction. We performed a screen of neurotoxicity-modifier genes to suppress circadian behavioural arrhythmicity in a Drosophila circadian HD model. The molecular chaperones Hsp40 and HSP70 emerged as significant suppressors in the circadian context, with Hsp40 being the more potent mitigator. Upon Hsp40 overexpression in the Drosophila circadian ventrolateral neurons (LNv), the behavioural rescue was associated with neuronal rescue of loss of circadian proteins from small LNv soma. Specifically, there was a restoration of the molecular clock protein Period and its oscillations in young flies and a long-lasting rescue of the output neuropeptide Pigment dispersing factor. Significantly, there was a reduction in the expanded Huntingtin inclusion load, concomitant with the appearance of a spot-like Huntingtin form. Thus, we provide evidence implicating the neuroprotective chaperone Hsp40 in circadian rehabilitation. The involvement of molecular chaperones in circadian maintenance has broader therapeutic implications for neurodegenerative diseases.

This article has an associated First Person interview with the first author of the paper.

Summary: This study shows, for the first time, a neuroprotective role of chaperone Hsp40 in suppressing circadian dysfunction associated with Huntington's disease in a Drosophila model.

Heat shock proteins in cell signaling and cancer

Heat Shock Proteins (HSPs) and their co-chaperones have well-established roles in regulating proteostasis within the cell, the nature of which continues to emerge with further study. To date, HSPs have been shown to be integral to protein folding and re-folding, protein transport, avoidance of protein aggregation, and modulation of protein degradation. Many cell signaling events are mediated by the chemical modification of proteins post-translationally that can alter protein conformation and activity, although it is not yet known whether the changes in protein conformation induced by post-translational modifications (PTMs) are also dependent upon HSPs and their co-chaperones for subsequent protein re-folding. We discuss what is known regarding roles for HSPs and other molecular chaperones in cell signaling events with a focus on oncogenic signaling. We also propose a hypothesis by which Hsp70 and Hsp90 may co-operate to facilitate cell signaling events that may link PTMs with the cellular protein folding machinery.

Regulation of p53 and Cancer Signaling by Heat Shock Protein 40/J-Domain Protein Family Members

Heat shock proteins (HSPs) are molecular chaperones that assist diverse cellular activities including protein folding, intracellular transportation, assembly or disassembly of protein complexes, and stabilization or degradation of misfolded or aggregated proteins. HSP40, also known as J-domain proteins (JDPs), is the largest family with over fifty members and contains highly conserved J domains responsible for binding to HSP70 and stimulation of the ATPase activity as a co-chaperone. Tumor suppressor p53 (p53), the most frequently mutated gene in human cancers, is one of the proteins that functionally interact with HSP40/JDPs. The majority of p53 mutations are missense mutations, resulting in acquirement of unexpected oncogenic activities, referred to as gain of function (GOF), in addition to loss of the tumor suppressive function. Moreover, stability and levels of wild-type p53 (wtp53) and mutant p53 (mutp53) are crucial for their tumor suppressive and oncogenic activities, respectively. However, the regulatory mechanisms of wtp53 and mutp53 are not fully understood. Accumulating reports demonstrate regulation of wtp53 and mutp53 levels and/or activities by HSP40/JDPs. Here, we summarize updated knowledge related to the link of HSP40/JDPs with p53 and cancer signaling to improve our understanding of the regulation of tumor suppressive wtp53 and oncogenic mutp53 GOF activities.

Potentiation of the activity of Escherichia coli chaperone DnaJ by tailing hyper-acidic minipeptides

The chaperone network plays an essential role in cellular protein homeostasis. However, some core components often coaggregate with misfolded proteins for sequestration and dysfunction, leading to abnormal cell proteostasis, aggregation-associated disorders, and poor solubility of overexpressed recombinant proteins. Among them, DnaJ or its ortholog, an obligate co-chaperone in the tripartite DnaK-DnaJ-GrpE system, is of more implications, probably due to its intrinsic propensity for aggregation. Herein, we potentiated the activity of Escherichia coli DnaJ by using hyper-acidified protein fusion strategy. We found DnaJ did possess only a moderate solubility that could be remarkably improved by fusing hyper-acidic minipeptides. Most importantly, we revealed the hyper-acidified DnaJ with a fusion tail could outperform its native form (significantly up to 2.1-fold) to enhance the solubility of target proteins and meanwhile appropriately impart them an elevated activity. These results suggest the hyper-acidified DnaJs can chaperone target proteins with correct folding into a truly soluble and active form. Moreover, we showed these hyper-acidified DnaJ variants could surpass its prototype to confer E. coli or yeast an enhanced heat tolerance, and DnaJ itself could be solubilized by its hyper-acidified fusion cognates. Finally, we discussed the overall mechanism for DnaJ activity potentiation mediated by hyper-acidic tailing fusion.

Emerging Roles of Exosomes in Huntington's Disease

Huntington’s disease (HD) is a rare hereditary autosomal dominant neurodegenerative disorder, which is caused by expression of mutant huntingtin protein (mHTT) with an abnormal number of glutamine repeats in its N terminus, and characterized by intracellular mHTT aggregates (inclusions) in the brain. Exosomes are small extracellular vesicles that are secreted generally by all cell types and can be isolated from almost all body fluids such as blood, urine, saliva, and cerebrospinal fluid. Exosomes may participate in the spreading of toxic misfolded proteins across the central nervous system in neurodegenerative diseases. In HD, such propagation of mHTT was observed both in vitro and in vivo. On the other hand, exosomes might carry molecules with neuroprotective effects. In addition, due to their capability to cross blood-brain barrier, exosomes hold great potential as sources of biomarkers available from periphery or carriers of therapeutics into the central nervous system. In this review, we discuss the emerging roles of exosomes in HD pathogenesis, diagnosis, and therapy.

J-domain proteins interaction with neurodegenerative disease-related proteins

HSP70 chaperones, J-domain proteins (JDPs) and nucleotide exchange factors (NEF) form functional networks that have the ability to prevent and reverse the aggregation of proteins associated with neurodegenerative diseases. JDPs can interact with specific substrate proteins, hold them in a refolding-competent conformation and target them to specific HSP70 chaperones for remodeling. Thereby, JDPs select specific substrates and constitute an attractive target for pharmacological intervention of neurodegenerative diseases. This, under the condition that the exact mechanism of JDPs interaction with specific substrates is unveiled. In this review, we provide an overview of the structural and functional variety of JDPs that interact with neurodegenerative disease-associated proteins and we highlight those studies that identified specific residues, domains or regions of JDPs that are crucial for substrate binding.

Therapeutic and Mechanistic Effects of Curcumin in Huntington's Disease

Curcumin is a spice derived nutraceutical which gained tremendous attention because of its profound medicinal values. It alters a number of molecular pathways such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), signal transducer and activator of transcription 3 (STAT3), nuclear factor erythroid 2-related factor 2 (Nrf2) and cyclooxygenases-2 (COX-2), which make it potential therapeutic choice in treating multiple disorders. It also possesses the potential to prevent protein aggregation and thus protect against degeneration of neurons in neurodegenerative disorders including Huntington's disease (HD). HD is an autosomal dominant disorder linked with altered gene expression which leads to an increase in the size of cytosine, adenine and guanine (CAG) trinucleotide repeats, aids in protein aggregation throughout the brain and thus damages neurons. Upstream regulation of oxidative stress and inflammatory cascade are two important factors that drive HD progression. Available therapies just suppress the severity of symptoms with a number of side effects. Curcumin targets multiple mechanisms in treating or preventing HD including antioxidant and anti-inflammatory potential, metal ion chelation, transcriptional alterations and upregulating activity of molecular chaperons, heat shock proteins (HSPs). Having a favorable safety profile, curcumin can be an alternative therapeutic choice in treating neurodegenerative disorders like HD. This review will focus on mechanistic aspects of curcumin in treating or preventing HD and its potential to arrest disease progression and will open new dimensions for safe and effective therapeutic agents in diminishing HD.

Comparative interactomes of HSF1 in stress and disease reveal a role for CTCF in HSF1-mediated gene regulation

Heat shock transcription factor 1 (HSF1) orchestrates cellular stress protection by activating or repressing gene transcription in response to protein misfolding, oncogenic cell proliferation, and other environmental stresses. HSF1 is tightly regulated via intramolecular repressive interactions, post-translational modifications, and protein-protein interactions. How these HSF1 regulatory protein interactions are altered in response to acute and chronic stress is largely unknown. To elucidate the profile of HSF1 protein interactions under normal growth and chronic and acutely stressful conditions, quantitative proteomics studies identified interacting proteins in the response to heat shock or in the presence of a poly-glutamine aggregation protein cell-based model of Huntington's disease. These studies identified distinct protein interaction partners of HSF1 as well as changes in the magnitude of shared interactions as a function of each stressful condition. Several novel HSF1-interacting proteins were identified that encompass a wide variety of cellular functions, including roles in DNA repair, mRNA processing, and regulation of RNA polymerase II. One HSF1 partner, CTCF, interacted with HSF1 in a stress-inducible manner and functions in repression of specific HSF1 target genes. Understanding how HSF1 regulates gene repression is a crucial question, given the dysregulation of HSF1 target genes in both cancer and neurodegeneration. These studies expand our understanding of HSF1-mediated gene repression and provide key insights into HSF1 regulation via protein-protein interactions.

Axonal mRNA translation in neurological disorders

It is increasingly recognized that local protein synthesis (LPS) contributes to fundamental aspects of axon biology, in both developing and mature neurons. Mutations in RNA-binding proteins (RBPs), as central players in LPS, and other proteins affecting RNA localization and translation are associated with a range of neurological disorders, suggesting disruption of LPS may be of pathological significance. In this review, we substantiate this hypothesis by examining the link between LPS and key axonal processes, and the implicated pathophysiological consequences of dysregulated LPS. First, we describe how the length and autonomy of axons result in an exceptional reliance on LPS. We next discuss the roles of LPS in maintaining axonal structural and functional polarity and axonal trafficking. We then consider how LPS facilitates the establishment of neuronal connectivity through regulation of axonal branching and pruning, how it mediates axonal survival into adulthood and its involvement in neuronal stress responses.

Dynamic neural and glial responses of a head-specific model for traumatic brain injury in Drosophila

Traumatic brain injury (TBI) is the strongest environmental risk factor for the accelerated development of neurodegenerative diseases. There are currently no therapeutics to address this due to lack of insight into mechanisms of injury progression, which are challenging to study in mammalian models. Here, we have developed and extensively characterized a head-specific approach to TBI in Drosophila, a powerful genetic system that shares many conserved genes and pathways with humans. The Drosophila TBI (dTBI) device inflicts mild, moderate, or severe brain trauma by precise compression of the head using a piezoelectric actuator. Head-injured animals display features characteristic of mammalian TBI, including severity-dependent ataxia, life span reduction, and brain degeneration. Severe dTBI is associated with cognitive decline and transient glial dysfunction, and stimulates antioxidant, proteasome, and chaperone activity. Moreover, genetic or environmental augmentation of the stress response protects from severe dTBI-induced brain degeneration and life span deficits. Together, these findings present a tunable, head-specific approach for TBI in Drosophila that recapitulates mammalian injury phenotypes and underscores the ability of the stress response to mitigate TBI-induced brain degeneration.

Strategies to Investigate Ubiquitination in Huntington's Disease

Many neurodegenerative disorders including Huntington's Disease are hallmarked by intracellular protein aggregates that are decorated by ubiquitin and different ubiquitin ligases and deubiquitinating enzymes. The protein aggregates observed in Huntington's Disease are caused by a polyglutamine expansion in the N-terminus of the huntingtin protein (Htt). Improving the degradation of mutant Htt via the Ubiquitin Proteasome System prior to aggregation would be a therapeutic strategy to delay or prevent the onset of Huntington's Disease for which there is currently no cure. Here we examine the current approaches used to study the ubiquitination of both soluble Htt as well as insolubilized Htt present in aggregates, and we describe what is known about involved (de)ubiquitinating enzymes. Furthermore, we discuss novel methodologies to study the dynamics of Htt ubiquitination in living cells using fluorescent ubiquitin probes, to identify and quantify Htt ubiquitination by mass spectrometry-based approaches, and various approaches to identify involved ubiquitinating enzymes.

Amyloid cross-seeding raises new dimensions to understanding of amyloidogenesis mechanism

Hallmarks of most of the amyloid pathologies are surprisingly found to be heterocomponent entities such as inclusions and plaques which contain diverse essential proteins and metabolites. Experimental studies have already revealed the occurrence of coaggregation and cross-seeding during amyloid formation of several proteins and peptides, yielding multicomponent assemblies of amyloid nature. Further, research reports on the co-occurrence of more than one type of amyloid-linked pathologies in the same individual suggest the possible cross-talk among the disease related amyloidogenic protein species during their amyloid growth. In this review paper, we have tried to gain more insight into the process of coaggregation and cross-seeding during amyloid aggregation of proteins, particularly focusing on their relevance to the pathogenesis of the protein misfolding diseases. Revelation of amyloid cross-seeding and coaggregation seems to open new dimensions in our mechanistic understanding of amyloidogenesis and such knowledge may possibly inspire better designing of anti-amyloid therapeutics.

Extracellular heat shock proteins in neurodegenerative diseases: New perspectives

One pathological hallmark of neurodegenerative diseases and CNS trauma is accumulation of insoluble, hydrophobic molecules and protein aggregations found both within and outside cells. These may be the consequences of an inadequate or overburdened cellular response to stresses resulting from potentially toxic changes in extra- and intracellular environments. The upregulated expression of heat shock proteins (HSPs) is one example of a highly conserved cellular response to both internal and external stress. Intracellularly these proteins act as chaperones, playing vital roles in the folding of nascent polypeptides, the translocation of proteins between subcellular locations, and the disaggregation of misfolded or aggregated proteins in an attempt to maintain cellular proteostasis during both homeostatic and stressful conditions. While the predominant study of the HSPs has focused on their intracellular chaperone functions, it remains unclear if all neuronal populations can mount a complete stress response. Alternately, it is now well established that some members of this family of proteins can be secreted by nearby, non-neuronal cells to act in the extracellular environment. This review addresses the current literature detailing the use of exogenous and extracellular HSPs in the treatment of cellular and animal models of neurodegenerative disease. These findings offer a new measure of therapeutic potential to the HSPs, but obstacles must be overcome before they can be efficiently used in a clinical setting.

Quality Control in Huntington's Disease: a Therapeutic Target

Huntington's disease (HD) is a fatal autosomal dominantly inherited brain disease caused by excessively expanded CAG repeats in gene which encodes huntingtin protein. These abnormally encoded huntingtin proteins and their truncated fragments result in disruption of cellular quality mechanism ultimately triggering neuronal death. Despite great efforts, a potential causative agent leading to genetic mutation in HTT, manifesting the neurons more prone to oxidative stress, cellular inflammation, energy depletion and apoptotic death, has not been established yet. Current scenario concentrates on symptomatic pathologies to improvise the disease progression and to better the survival. Most of the therapeutic developments have been converged to rescue the protein homeostasis. In HD, abnormal expansion of glutamine repeats in the protein huntingtin leads to toxic aggregation of huntingtin which in turn impairs the quality control mechanism of cells through damaging the machineries involved in removal of aggregated abnormal protein. Therapeutic approaches to improve the efficiency of aggregate clearance through quality control mechanisms involve protein folding machineries such as chaperones and protein degradation machineries such as proteasome and autophagy. Also, to reduce protein aggregation by enhancing proper folding, to degrade and eliminate the aggregates are suggested to negatively regulate the HD progression associated with the disruption of protein homeostasis. This review focuses on the collection of therapeutic strategies targeting enhancement of protein quality control activity to delay the HD pathogenesis.

Cell-Based Screen Using Amyloid Mimic β23 Expression Identifies Peucedanocoumarin III as a Novel Inhibitor of α-Synuclein and Huntingtin Aggregates

Aggregates of disease-causing proteins dysregulate cellular functions, thereby causing neuronal cell loss in diverse neurodegenerative diseases. Although many in vitro or in vivo studies of protein aggregate inhibitors have been performed, a therapeutic strategy to control aggregate toxicity has not been earnestly pursued, partly due to the limitations of available aggregate models. In this study, we established a tetracycline (Tet)-inducible nuclear aggregate (β23) expression model to screen potential lead compounds inhibiting β23-induced toxicity. Highthroughput screening identified several natural compounds as nuclear β23 inhibitors, including peucedanocoumarin III (PCIII). Interestingly, PCIII accelerates disaggregation and proteasomal clearance of both nuclear and cytosolic β23 aggregates and protects SH-SY5Y cells from toxicity induced by β23 expression. Of translational relevance, PCIII disassembled fibrils and enhanced clearance of cytosolic and nuclear protein aggregates in cellular models of huntingtin and α-synuclein aggregation. Moreover, cellular toxicity was diminished with PCIII treatment for polyglutamine (PolyQ)-huntingtin expression and α-synuclein expression in conjunction with 6-hydroxydopamine (6-OHDA) treatment. Importantly, PCIII not only inhibited α-synuclein aggregation but also disaggregated preformed α-synuclein fibrils in vitro . Taken together, our results suggest that a Tet-Off β23 cell model could serve as a robust platform for screening effective lead compounds inhibiting nuclear or cytosolic protein aggregates. Brain-permeable PCIII or its derivatives could be beneficial for eliminating established protein aggregates.

Substrate selection by the proteasome through initiation regions

Proteins in the cell have to be eliminated once their function is no longer desired or they become damaged. Most regulated protein degradation is achieved by a large enzymatic complex called the proteasome. Many proteasome substrates are targeted for degradation by the covalent attachment of ubiquitin molecules. Ubiquitinated proteins can be bound by the proteasome, but for proteolysis to occur the proteasome needs to find a disordered tail somewhere in the target at which it initiates degradation. The initiation step contributes to the specificity of proteasomal degradation. Here, we review how the proteasome selects initiation sites within its substrates and discuss how the initiation step affects physiological processes.

A Sentinel in the Crosstalk Between the Nervous and Immune System: The (Immuno)-Proteasome

The wealth of recent evidence about a bi-directional communication between nerve- and immune- cells revolutionized the traditional concept about the brain as an “immune-privileged” organ while opening novel avenues in the pathophysiology of CNS disorders. In fact, altered communication between the immune and nervous system is emerging as a common hallmark in neuro-developmental, neurodegenerative, and neuro-immunological diseases. At molecular level, the ubiquitin proteasome machinery operates as a sentinel at the crossroad between the immune system and brain. In fact, the standard proteasome and its alternative/inducible counterpart, the immunoproteasome, operate dynamically and coordinately in both nerve- and immune- cells to modulate neurotransmission, oxidative/inflammatory stress response, and immunity. When dysregulations of the proteasome system occur, altered amounts of standard- vs. immune-proteasome subtypes translate into altered communication between neurons, glia, and immune cells. This contributes to neuro-inflammatory pathology in a variety of neurological disorders encompassing Parkinson's, Alzheimer's, and Huntingtin's diseases, brain trauma, epilepsy, and Multiple Sclerosis. In the present review, we analyze those proteasome-dependent molecular interactions which sustain communication between neurons, glia, and brain circulating T-lymphocytes both in baseline and pathological conditions. The evidence here discussed converges in that upregulation of immunoproteasome to the detriment of the standard proteasome, is commonly implicated in the inflammatory- and immune- biology of neurodegeneration. These concepts may foster additional studies investigating the role of immunoproteasome as a potential target in neurodegenerative and neuro-immunological disorders.

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.

Regulation of Antimicrobial Pathways by Endogenous Heat Shock Proteins in Gastrointestinal Disorders

Heat shock proteins (HSPs) are essential mediators of cellular homeostasis by maintaining protein functionality and stability, and activating appropriate immune cells. HSP activity is influenced by a variety of factors including diet, microbial stimuli, environment and host immunity. The overexpression and down-regulation of HSPs is associated with various disease phenotypes, including the inflammatory bowel diseases (IBD) such as Crohn’s disease (CD). While the precise etiology of CD remains unclear, many of the putative triggers also influence HSP activity. The development of different CD phenotypes therefore may be a result of the disease-modifying behavior of the environmentally-regulated HSPs. Understanding the role of bacterial and endogenous HSPs in host homeostasis and disease will help elucidate the complex interplay of factors. Furthermore, discerning the function of HSPs in CD may lead to therapeutic developments that better reflect and respond to the gut environment.

Proteome changes in the aging Drosophila melanogaster head

A combination of liquid chromatography, ion mobility spectrometry, mass spectrometry, and database searching techniques were used to characterize the proteomes of four biological replicates of adult Drosophila melanogaster heads at seven time points across their lifespans. Based on the detection of tryptic peptides, the identities of 1281 proteins were determined. An estimate of the abundance of each protein, based on the three most intense peptide ions, shows that the quantified species vary in concentration over a factor of ~103. Compared to initial studies in the field of Drosophila proteomics, our current results show an eight-fold higher temporal protein coverage with increased quantitative accuracy. Across the lifespan, we observe a range of trends in the abundance of different proteins, including: an increase in abundance of proteins involved in oxidative phosphorylation, and the tricarboxylic acid cycle; a decrease in proteasomal proteins, as well as ribosomal proteins; and, many types of proteins, which remain relatively unchanged. For younger flies, proteomes are relatively similar within their age group. For older flies, proteome similarity decreases within their age group. These combined results illustrate a correlation between increasing age and decreasing proteostasis.

DNAJ Proteins in neurodegeneration: essential and protective factors

Maintenance of protein homeostasis is vitally important in post-mitotic cells, particularly neurons. Neurodegenerative diseases such as polyglutamine expansion disorders-like Huntington's disease or spinocerebellar ataxia (SCA), Alzheimer's disease, fronto-temporal dementia (FTD), amyotrophic lateral sclerosis (ALS) and Parkinson's disease-are often characterized by the presence of inclusions of aggregated protein. Neurons contain complex protein networks dedicated to protein quality control and maintaining protein homeostasis, or proteostasis. Molecular chaperones are a class of proteins with prominent roles in maintaining proteostasis, which act to bind and shield hydrophobic regions of nascent or misfolded proteins while allowing correct folding, conformational changes and enabling quality control. There are many different families of molecular chaperones with multiple functions in proteostasis. The DNAJ family of molecular chaperones is the largest chaperone family and is defined by the J-domain, which regulates the function of HSP70 chaperones. DNAJ proteins can also have multiple other protein domains such as ubiquitin-interacting motifs or clathrin-binding domains leading to diverse and specific roles in the cell, including targeting client proteins for degradation via the proteasome, chaperone-mediated autophagy and uncoating clathrin-coated vesicles. DNAJ proteins can also contain ER-signal peptides or mitochondrial leader sequences, targeting them to specific organelles in the cell. In this review, we discuss the multiple roles of DNAJ proteins and in particular focus on the role of DNAJ proteins in protecting against neurodegenerative diseases caused by misfolded proteins. We also discuss the role of DNAJ proteins as direct causes of inherited neurodegeneration via mutations in DNAJ family genes.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Molecular signatures of differential responses to exercise trainings during rehabilitation

The loss and recovery of muscle mass and function following injury and during rehabilitation varies among individuals. While recent expression profiling studies have illustrated transcriptomic responses to muscle disuse and remodeling, how these changes contribute to the physiological responses are not clear. In this study, we quantified the effects of immobilization and subsequent rehabilitation training on muscle size and identified molecular pathways associated with muscle responsiveness in an orthopaedic patient cohort study. The injured leg of 16 individuals with ankle injury was immobilized for a minimum of 4 weeks, followed by a 6-week rehabilitation program. The maximal cross-sectional area (CSA) of the medial gastrocnemius muscle of the immobilized and control legs were determined by T1-weighted axial MRI images. Genome-wide mRNA profiling data were used to identify molecular signatures that distinguish the patients who responded to immobilization and rehabilitation and those who were considered minimal responders. RESULTS: Using 6% change as the threshold to define responsiveness, a greater degree of changes in muscle size was noted in high responders (−14.9 ± 3.6%) compared to low responders (0.1 ± 0.0%) during immobilization. In addition, a greater degree of changes in muscle size was observed in high responders (20.5 ± 3.2%) compared to low responders (2.5 ± 0.9%) at 6-week rehabilitation. Microarray analysis showed a higher number of genes differentially expressed in the responders compared to low responders in general; with more expression changes observed at the acute stage of rehabilitation in both groups. Pathways analysis revealed top molecular pathways differentially affected in the groups, including genes involved in mitochondrial function, protein turn over, integrin signaling and inflammation. This study confirmed the extent of muscle atrophy due to immobilization and recovery by exercise training is associated with distinct remodeling signature, which can potentially be used for evaluating and predicting clinical outcomes.

Proteostasis of Huntingtin in Health and Disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by motor dysfunction, cognitive deficits and psychosis. HD is caused by mutations in the Huntingtin (HTT) gene, resulting in the expansion of polyglutamine (polyQ) repeats in the HTT protein. Mutant HTT is prone to aggregation, and the accumulation of polyQ-expanded fibrils as well as intermediate oligomers formed during the aggregation process contribute to neurodegeneration. Distinct protein homeostasis (proteostasis) nodes such as chaperone-mediated folding and proteolytic systems regulate the aggregation and degradation of HTT. Moreover, polyQ-expanded HTT fibrils and oligomers can lead to a global collapse in neuronal proteostasis, a process that contributes to neurodegeneration. The ability to maintain proteostasis of HTT declines during the aging process. Conversely, mechanisms that preserve proteostasis delay the onset of HD. Here we will review the link between proteostasis, aging and HD-related changes.

Abnormal degradation of the neuronal stress-protective transcription factor HSF1 in Huntington's disease

Huntington's Disease (HD) is a neurodegenerative disease caused by poly-glutamine expansion in the Htt protein, resulting in Htt misfolding and cell death. Expression of the cellular protein folding and pro-survival machinery by heat shock transcription factor 1 (HSF1) ameliorates biochemical and neurobiological defects caused by protein misfolding. We report that HSF1 is degraded in cells and mice expressing mutant Htt, in medium spiny neurons derived from human HD iPSCs and in brain samples from patients with HD. Mutant Htt increases CK2α' kinase and Fbxw7 E3 ligase levels, phosphorylating HSF1 and promoting its proteasomal degradation. An HD mouse model heterozygous for CK2α' shows increased HSF1 and chaperone levels, maintenance of striatal excitatory synapses, clearance of Htt aggregates and preserves body mass compared with HD mice homozygous for CK2α'. These results reveal a pathway that could be modulated to prevent neuronal dysfunction and muscle wasting caused by protein misfolding in HD.

Establishment of a suite of assays that support the discovery of proteasome stimulators

BACKGROUND

The proteasome catalyzes the degradation of many mis-folded proteins, which are otherwise cytotoxic. There is interest in the discovery of proteasome agonists, but previous efforts to do so have been disappointing.

METHODS

The cleavage of small fluorogenic peptides is used routinely as an assay to screen for proteasome modulators. We have developed follow-on assays that employ more physiologically relevant substrates.

RESULTS

To demonstrate the efficacy of this workflow, the NIH Clinical Collection (NCC) was screened. While many compounds stimulated proteasome-mediated proteolysis of the pro-fluorogenic peptide substrates, most failed to evince activity in assays with larger peptide or protein substrates. We also show that two molecules claimed previously to be proteasome agonists, oleuropein and betulinic acid, indeed accelerate hydrolysis of the fluorogenic substrate, but have no effect on the turnover of a mis-folded protein in vitro or in cellulo. However, two small molecules from the NCC, MK-866 and AM-404, stimulate the proteasome-mediated turnover of a mis-folded protein in living cells by 3- to 4-fold.

CONCLUSION

Assays that monitor the proteasome-mediated degradation of larger peptides and proteins can distinguish bona fide agonists from compounds only able to stimulate the cleavage of short, non-physiologically relevant peptides.

GENERAL SIGNIFICANCE

A suite of assays has been established that allows the discovery of bona fide proteasome agonists. AM-404 and MK-866 can be useful tools for cell culture experiments, and can serve as scaffolds to generate more potent 20S stimulators.

BGP-15 Improves Aspects of the Dystrophic Pathology in mdx and dko Mice with Differing Efficacies in Heart and Skeletal Muscle

Duchenne muscular dystrophy is a severe and progressive striated muscle wasting disorder that leads to premature death from respiratory and/or cardiac failure. We have previously shown that treatment of young dystrophic mdx and dystrophin/utrophin null (dko) mice with BGP-15, a coinducer of heat shock protein 72, ameliorated the dystrophic pathology. We therefore tested the hypothesis that later-stage BGP-15 treatment would similarly benefit older mdx and dko mice when the dystrophic pathology was already well established. Later stage treatment of mdx or dko mice with BGP-15 did not improve maximal force of tibialis anterior (TA) muscles (in situ) or diaphragm muscle strips (in vitro). However, collagen deposition (fibrosis) was reduced in TA muscles of BGP-15-treated dko mice but unchanged in TA muscles of treated mdx mice and diaphragm of treated mdx and dko mice. We also examined whether BGP-15 treatment could ameliorate aspects of the cardiac pathology, and in young dko mice it reduced collagen deposition and improved both membrane integrity and systolic function. These results confirm BGP-15's ability to improve aspects of the dystrophic pathology but with differing efficacies in heart and skeletal muscles at different stages of the disease progression. These findings support a role for BGP-15 among a suite of pharmacological therapies for Duchenne muscular dystrophy and related disorders.

Identification of DNAJA1 as a novel interacting partner and a substrate of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed multifunctional member of the transglutaminase enzyme family. It has been implicated to have roles in many physiological and pathological processes such as differentiation, apoptosis, signal transduction, adhesion and migration, wound healing and inflammation. Previous studies revealed that TG2 has various intra- and extra-cellular interacting partners, which contribute to these processes. In the present study, we identified a molecular co-chaperone, DNAJA1, as a novel interacting partner of human TG2 using a GST pull-down assay and subsequent mass spectrometry analysis, and further confirmed this interaction via ELISA and surface plasmon resonance measurements. Interaction studies were also performed with domain variants of TG2 and results suggest that the catalytic core domain of TG2 is essential for the TG2-DNAJA1 interaction. Cross-linking activity was not essential for the interaction since DNAJA1 was also found to interact with the catalytically inactive form of TG2. Furthermore, we have showed that DNAJA1 interacts with the open form of TG2 and regulates its transamidation activity under both in vitro and in situ conditions. We also found that DNAJA1 is a glutamine donor substrate of TG2. Since DNAJA1 and TG2 are reported to regulate common pathological conditions such as neurodegenerative disorders and cancer, the findings in the present paper open up possibilities to explore molecular mechanisms behind TG2-regulated functions.

NMNAT2:HSP90 Complex Mediates Proteostasis in Proteinopathies

Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) is neuroprotective in numerous preclinical models of neurodegeneration. Here, we show that brain nmnat2 mRNA levels correlate positively with global cognitive function and negatively with AD pathology. In AD brains, NMNAT2 mRNA and protein levels are reduced. NMNAT2 shifts its solubility and colocalizes with aggregated Tau in AD brains, similar to chaperones, which aid in the clearance or refolding of misfolded proteins. Investigating the mechanism of this observation, we discover a novel chaperone function of NMNAT2, independent from its enzymatic activity. NMNAT2 complexes with heat shock protein 90 (HSP90) to refold aggregated protein substrates. NMNAT2’s refoldase activity requires a unique C-terminal ATP site, activated in the presence of HSP90. Furthermore, deleting NMNAT2 function increases the vulnerability of cortical neurons to proteotoxic stress and excitotoxicity. Interestingly, NMNAT2 acts as a chaperone to reduce proteotoxic stress, while its enzymatic activity protects neurons from excitotoxicity. Taken together, our data indicate that NMNAT2 exerts its chaperone or enzymatic function in a context-dependent manner to maintain neuronal health.

This study reveals NMNAT2 to be a dual-function neuronal maintenance factor that not only generates NAD to protect neurons from excitotoxicity but also moonlights as a chaperone to combat protein toxicity.

Pathological protein aggregates are found in many neurodegenerative diseases, and it has been hypothesized that these protein aggregates are toxic and cause neuronal death. Little is known about how neurons protect against pathological protein aggregates to maintain their health. Nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) is a newly identified neuronal maintenance factor. We found that in humans, levels of NMNAT2 transcript are positively correlated with cognitive function and are negatively correlated with pathological features of neurodegenerative disease like plaques and tangles. In this study, we demonstrate that NMNAT2 can act as a chaperone to reduce protein aggregates, and this function is independent from its known function in the enzymatic synthesis of nicotinamide adenine dinucleotide (NAD). We find that NMNAT2 interacts with heat shock protein 90 (HSP90) to refold protein aggregates, and that deleting NMNAT2 in cortical neurons renders them vulnerable to protein stress or excitotoxicity. Interestingly, the chaperone function of NMNAT2 protects neurons from protein toxicity, while its enzymatic function is required to defend against excitotoxicity. Our work here suggests that NMNAT2 uses either its chaperone or enzymatic function to combat neuronal insults in a context-dependent manner. In Alzheimer disease brains, NMNAT2 levels are less than 50% of control levels, and we propose that enhancing NMNAT2 function may provide an effective therapeutic intervention to reserve cognitive function.

Heat shock proteins in the retina: Focus on HSP70 and alpha crystallins in ganglion cell survival

Heat shock proteins (HSPs) belong to a superfamily of stress proteins that are critical constituents of a complex defense mechanism that enhances cell survival under adverse environmental conditions. Cell protective roles of HSPs are related to their chaperone functions, antiapoptotic and antinecrotic effects. HSPs' anti-apoptotic and cytoprotective characteristics, their ability to protect cells from a variety of stressful stimuli, and the possibility of their pharmacological induction in cells under pathological stress make these proteins an attractive therapeutic target for various neurodegenerative diseases; these include Alzheimer's, Parkinson's, Huntington's, prion disease, and others. This review discusses the possible roles of HSPs, particularly HSP70 and small HSPs (alpha A and alpha B crystallins) in enhancing the survival of retinal ganglion cells (RGCs) in optic neuropathies such as glaucoma, which is characterized by progressive loss of vision caused by degeneration of RGCs and their axons in the optic nerve. Studies in animal models of RGC degeneration induced by ocular hypertension, optic nerve crush and axotomy show that upregulation of HSP70 expression by hyperthermia, zinc, geranyl-geranyl acetone, 17-AAG (a HSP90 inhibitor), or through transfection of retinal cells with AAV2-HSP70 effectively supports the survival of injured RGCs. RGCs survival was also stimulated by overexpression of alpha A and alpha B crystallins. These findings provide support for translating the HSP70- and alpha crystallin-based cell survival strategy into therapy to protect and rescue injured RGCs from degeneration associated with glaucomatous and other optic neuropathies.

αB-Crystallin overexpression in astrocytes modulates the phenotype of the BACHD mouse model of Huntington's disease

Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the huntingtin (htt) protein. The polyQ expansion increases the propensity of htt to aggregate and accumulate, and manipulations that mitigate protein misfolding or facilitate the clearance of misfolded proteins are predicted to slow disease progression in HD models. αB-crystallin (αBc) or HspB5 is a well-characterized member of the small heat shock protein (sHsp) family that reduces mutant htt (mhtt) aggregation and toxicity in vitro and in Drosophila models of HD. Here, we determined if overexpressing αBc in vivo modulates aggregation and delays the onset and progression of disease in a full-length model of HD, BACHD mice. Expression of sHsps in neurodegenerative disease predominantly occurs in non-neuronal cells, and in the brain, αBc is mainly found in astrocytes and oligodendrocytes. Here, we show that directed αBc overexpression in astrocytes improves motor performance in rotarod and balance beam tests and improves cognitive function in the BACHD mice. Improvement in behavioral deficits correlated with mitigation of neuropathological features commonly observed in HD. Interestingly, astrocytic αBc overexpression was neuroprotective against neuronal cell loss in BACHD brains, suggesting αBc might be acting in a non-cell-autonomous manner. At the protein level, αBc decreased the level of soluble mhtt and decreased the size of mhtt inclusions in BACHD brain. Our results support a model in which elevating astrocytic αBc confers neuroprotection through a potential non-cell-autonomous pathway that modulates mhtt aggregation and protein levels.

Chaperone-like protein HYPK and its interacting partners augment autophagy

To decipher the function(s) of HYPK, a huntingtin (HTT)-interacting protein with chaperone-like activity, we had previously identified 36 novel interacting partners of HYPK. Another 13 proteins were known earlier to be associated with HYPK. On the basis of analysis of the interacting partners of HYPK, it has been shown that HYPK may participate in diverse cellular functions relevant to Huntington's disease. In the present study, we identified additional 5 proteins by co-immunoprecipitation and co-localization. As of now we have 54 primary interactors of HYPK. From the database we collected 1026 unique proteins (secondary interactors) interacting with these 54 primary HYPK interacting partners. We observed that 10 primary and 91 secondary interacting proteins of HYPK are associated with two types of autophagy processes. We next tested the hypothesis that the hub, HYPK, might itself be involved in autophagy. Using mouse striatal STHdh(Q7)/Hdh(Q7) cell lines, we observed that over expression of HYPK significantly increased background cellular autophagy, while knock down of endogenous HYPK decreased the autophagy level as detected by altered LC3I conversion, BECN1 expression, cleavage of GFP from LC3-GFP, ATG5-ATG12 conjugate formation and expression of transcription factors like Tfeb, Srebp2 and Zkscan3. This result shows that HYPK, possibly with its interacting partners, induces autophagy. We further observed that N-terminal mutant HTT reduced the cellular levels of LC3II and BECN1, which could be recovered significantly upon over expression of HYPK in these cells. This result further confirms that HYPK could also be involved in clearing mutant HTT aggregates by augmenting autophagy pathway.

Graded Proteasome Dysfunction in Caenorhabditis elegans Activates an Adaptive Response Involving the Conserved SKN-1 and ELT-2 Transcription Factors and the Autophagy-Lysosome Pathway

The maintenance of cellular proteins in a biologically active and structurally stable state is a vital endeavor involving multiple cellular pathways. One such pathway is the ubiquitin-proteasome system that represents a major route for protein degradation, and reductions in this pathway usually have adverse effects on the health of cells and tissues. Here, we demonstrate that loss-of-function mutants of the Caenorhabditis elegans proteasome subunit, RPN-10, exhibit moderate proteasome dysfunction and unexpectedly develop both increased longevity and enhanced resistance to multiple threats to the proteome, including heat, oxidative stress, and the presence of aggregation prone proteins. The rpn-10 mutant animals survive through the activation of compensatory mechanisms regulated by the conserved SKN-1/Nrf2 and ELT-2/GATA transcription factors that mediate the increased expression of genes encoding proteasome subunits as well as those mediating oxidative- and heat-stress responses. Additionally, we find that the rpn-10 mutant also shows enhanced activity of the autophagy-lysosome pathway as evidenced by increased expression of the multiple autophagy genes including atg-16.2, lgg-1, and bec-1, and also by an increase in GFP::LGG-1 puncta. Consistent with a critical role for this pathway, the enhanced resistance of the rpn-10 mutant to aggregation prone proteins depends on autophagy genes atg-13, atg-16.2, and prmt-1. Furthermore, the rpn-10 mutant is particularly sensitive to the inhibition of lysosome activity via either RNAi or chemical means. We also find that the rpn-10 mutant shows a reduction in the numbers of intestinal lysosomes, and that the elt-2 gene also plays a novel and vital role in controlling the production of functional lysosomes by the intestine. Overall, these experiments suggest that moderate proteasome dysfunction could be leveraged to improve protein homeostasis and organismal health and longevity, and that the rpn-10 mutant provides a unique platform to explore these possibilities.

Role of Carbonyl Modifications on Aging-Associated Protein Aggregation

Protein aggregation is a common biological phenomenon, observed in different physiological and pathological conditions. Decreased protein solubility and a tendency to aggregate is also observed during physiological aging but the causes are currently unknown. Herein we performed a biophysical separation of aging-related high molecular weight aggregates, isolated from the bone marrow and splenic cells of aging mice and followed by biochemical and mass spectrometric analysis. The analysis indicated that compared to younger mice an increase in protein post-translational carbonylation was observed. The causative role of these modifications in inducing protein misfolding and aggregation was determined by inducing carbonyl stress in young mice, which recapitulated the increased protein aggregation observed in old mice. Altogether our analysis indicates that oxidative stress-related post-translational modifications accumulate in the aging proteome and are responsible for increased protein aggregation and altered cell proteostasis.

Cancer associated missense mutations in BAP1 catalytic domain induce amyloidogenic aggregation: A new insight in enzymatic inactivation

BRCA1 associated protein 1 (BAP1) is a nuclear deubiquitinase that regulates tumor suppressor activity and widely involves many cellular processes ranging from cell cycle regulation to gluconeogenesis. Impairment of enzymatic activity and nuclear localization induce abnormal cell proliferation. It is considered to be an important driver gene, which undergoes frequent mutations in several cancers. However the role of mutation and oncogenic gain of function of BAP1 are poorly understood. Here, we investigated cellular localization, enzymatic activity and structural changes for four missense mutants of the catalytic domain of BAP1, which are prevalent in different types of cancer. These mutations triggered cytoplasmic/perinuclear accumulation in BAP1 deficient cells, which has been observed in proteins that undergo aggregation in cellular condition. Amyloidogenic activity of mutant BAP1 was revealed from its reactivity towards anti oligomeric antibody in HEK293T cells. We have also noted structural destabilization in the catalytic domain mutants, which eventually produced beta amyloid structure as indicated in atomic force microscopy study. The cancer associated mutants up-regulate heat shock response and activates transcription of genes normally co-repressed by BAP1. Overall, our results unambiguously demonstrate that structural destabilization and subsequent aggregation abrogate its cellular mechanism leading to adverse outcome.

Pathology and function of nuclear amyloid. Protein homeostasis matters

In aging societies increasing cases of neurodegenerative protein deposit diseases urge for the identification of the underlying mechanisms. Expectations are that in 2050 the percentage of population over age 60 is 42% in Japan, 34% in China, and 27% in the US. The cell nucleus is a major target of amyloid-like protein fibrillation in a variety of disorders that are characterized by widespread aggregation of proteins with instable homopolymeric amino acid repeats, ubiquitin, and other proteinaceous components. Additionally, accumulation of insoluble, SDS-resistant proteins has been identified as an intrinsic property of organismal aging. This review collects current knowledge about the composition and function of insoluble, nuclear protein inclusions from the protein homeostasis perspective. It discusses the occurrence and role of nuclear amyloid in the diseased as well as the healthy cell. Features of nuclear inclusions such as protein composition and locally active protein degradation may predict neural fitness and survival in a variety of health or disease settings.

Ubiquitin-Proteasome System in Neurodegenerative Disorders

Cellular proteostasis is a highly dynamic process and is primarily carried out by the degradation tools of ubiquitin-proteasome system (UPS). Abnormalities in UPS function result in the accumulation of damaged or misfolded proteins which can form intra- and extracellular aggregated proteinaceous deposits leading to cellular dysfunction and/or death. Deposition of abnormal protein aggregates and the cellular inability to clear them have been implicated in the pathogenesis of a number of neurodegenerative disorders such as Alzheimer's and Parkinson's. Contrary to the upregulation of proteasome function in oncogenesis and the use of proteasome inhibition as a therapeutic strategy, activation of proteasome function would serve therapeutic objectives of treatment of neurodegenerative diseases. This review describes the current understanding of the role of the proteasome in neurodegenerative disorders and potential utility of proteasomal modulation therein.

siRNA screen identifies QPCT as a druggable target for Huntington's disease

Huntington's disease (HD) is a currently incurable neurodegenerative condition caused by an abnormally expanded polyglutamine tract in huntingtin (HTT). We identified new modifiers of mutant HTT toxicity by performing a large-scale 'druggable genome' siRNA screen in human cultured cells, followed by hit validation in Drosophila. We focused on glutaminyl cyclase (QPCT), which had one of the strongest effects on mutant HTT-induced toxicity and aggregation in the cell-based siRNA screen and also rescued these phenotypes in Drosophila. We found that QPCT inhibition induced the levels of the molecular chaperone αB-crystallin and reduced the aggregation of diverse proteins. We generated new QPCT inhibitors using in silico methods followed by in vitro screening, which rescued the HD-related phenotypes in cell, Drosophila and zebrafish HD models. Our data reveal a new HD druggable target affecting mutant HTT aggregation and provide proof of principle for a discovery pipeline from druggable genome screen to drug development.

The role of HSP70 and its co-chaperones in protein misfolding, aggregation and disease

Molecular chaperones and their associated co-chaperones are essential in health and disease as they are key facilitators of protein folding, quality control and function. In particular, the HSP70 molecular chaperone networks have been associated with neurodegenerative diseases caused by aberrant protein folding. The pathogenesis of these disorders usually includes the formation of deposits of misfolded, aggregated protein. HSP70 and its co-chaperones have been recognised as potent modulators of inclusion formation and cell survival in cellular and animal models of neurodegenerative disease. In has become evident that the HSP70 chaperone machine functions not only in folding, but also in proteasome mediated degradation of neurodegenerative disease proteins. Thus, there has been a great deal of interest in the potential manipulation of molecular chaperones as a therapeutic approach for many neurodegenerations. Furthermore, mutations in several HSP70 co-chaperones and putative co-chaperones have been identified as causing inherited neurodegenerative and cardiac disorders, directly linking the HSP70 chaperone system to human disease.

Molecular chaperones and neuronal proteostasis

Protein homeostasis (proteostasis) is essential for maintaining the functionality of the proteome. The disruption of proteostasis, due to genetic mutations or an age-related decline, leads to aberrantly folded proteins that typically lose their function. The accumulation of misfolded and aggregated protein is also cytotoxic and has been implicated in the pathogenesis of neurodegenerative diseases. Neurons have developed an intrinsic protein quality control network, of which molecular chaperones are an essential component. Molecular chaperones function to promote efficient folding and target misfolded proteins for refolding or degradation. Increasing molecular chaperone expression can suppress protein aggregation and toxicity in numerous models of neurodegenerative disease; therefore, molecular chaperones are considered exciting therapeutic targets. Furthermore, mutations in several chaperones cause inherited neurodegenerative diseases. In this review, we focus on the importance of molecular chaperones in neurodegenerative diseases, and discuss the advances in understanding their protective mechanisms.

Sequence composition of disordered regions fine-tunes protein half-life

The proteasome controls the concentrations of most proteins in eukaryotic cells. It recognizes its protein substrates through ubiquitin tags and initiates degradation at disordered regions within the substrate. Here we find that the proteasome has pronounced preferences for the amino acid sequence composition of the regions at which it initiates degradation. Specifically, proteins where the initiation regions have biased amino acid compositions show longer half-lives in yeast. The relationship is also observed on a genomic scale in mouse cells. These preferences affect the degradation rates of proteins in vitro, can explain the unexpected stability of natural proteins in yeast, and may affect the accumulation of toxic proteins in disease. We propose that the proteasome’s sequence preferences provide a second component to the degradation code and may fine-tune protein half-life in cells.

Detection of ubiquitinated huntingtin species in intracellular aggregates

Protein conformation diseases, including polyglutamine (polyQ) diseases, result from the accumulation and aggregation of misfolded proteins. Huntington’s disease (HD) is one of nine diseases caused by an expanded polyQ repeat within the affected protein and is hallmarked by intracellular inclusion bodies composed of aggregated N-terminal huntingtin (Htt) fragments and other sequestered proteins. Fluorescence microscopy and filter trap assay are conventional methods to study protein aggregates, but cannot be used to analyze the presence and levels of post-translational modifications of aggregated Htt such as ubiquitination. Ubiquitination of proteins can be a signal for degradation and intracellular localization, but also affects protein activity and protein-protein interactions. The function of ubiquitination relies on its mono- and polymeric isoforms attached to protein substrates. Studying the ubiquitination pattern of aggregated Htt fragments offers an important possibility to understand Htt degradation and aggregation processes within the cell. For the identification of aggregated Htt and its ubiquitinated species, solubilization of the cellular aggregates is mandatory. Here we describe methods to identify post-translational modifications such as ubiquitination of aggregated mutant Htt. This approach is specifically described for use with mammalian cell culture and is suitable to study other disease-related proteins prone to aggregate.

Hydrodynamic size-based separation and characterization of protein aggregates from total cell lysates

Herein we describe a protocol that uses hollow-fiber flow field-flow fractionation (FFF) coupled with multiangle light scattering (MALS) for hydrodynamic size-based separation and characterization of complex protein aggregates. The fractionation method, which requires 1.5 h to run, was successfully modified from the analysis of protein aggregates, as found in simple protein mixtures, to complex aggregates, as found in total cell lysates. In contrast to other related methods (filter assay, analytical ultracentrifugation, gel electrophoresis and size-exclusion chromatography), hollow-fiber flow FFF coupled with MALS allows a flow-based fractionation of highly purified protein aggregates and simultaneous measurement of their molecular weight, r.m.s. radius and molecular conformation (e.g., round, rod-shaped, compact or relaxed). The polyethersulfone hollow fibers used, which have a 0.8-mm inner diameter, allow separation of as little as 20 μg of total cell lysates. In addition, the ability to run the samples in different denaturing and nondenaturing buffer allows defining true aggregates from artifacts, which can form during sample preparation. The protocol was set up using Paraquat-induced carbonylation, a model that induces protein aggregation in cultured cells. This technique will advance the biochemical, proteomic and biophysical characterization of molecular-weight aggregates associated with protein mutations, as found in many CNS degenerative diseases, or chronic oxidative stress, as found in aging, and chronic metabolic and inflammatory conditions.

Molecular chaperone dysfunction in neurodegenerative diseases and effects of curcumin

The intra- and extracellular accumulation of misfolded and aggregated amyloid proteins is a common feature in several neurodegenerative diseases, which is thought to play a major role in disease severity and progression. The principal machineries maintaining proteostasis are the ubiquitin proteasomal and lysosomal autophagy systems, where heat shock proteins play a crucial role. Many protein aggregates are degraded by the lysosomes, depending on aggregate size, peptide sequence, and degree of misfolding, while others are selectively tagged for removal by heat shock proteins and degraded by either the proteasome or phagosomes. These systems are compromised in different neurodegenerative diseases. Therefore, developing novel targets and classes of therapeutic drugs, which can reduce aggregates and maintain proteostasis in the brains of neurodegenerative models, is vital. Natural products that can modulate heat shock proteins/proteosomal pathway are considered promising for treating neurodegenerative diseases. Here we discuss the current knowledge on the role of HSPs in protein misfolding diseases and knowledge gained from animal models of Alzheimer's disease, tauopathies, and Huntington's diseases. Further, we discuss the emerging treatment regimens for these diseases using natural products, like curcumin, which can augment expression or function of heat shock proteins in the cell.