The anti-leprosy drug clofazimine reduces polyQ toxicity through activation of PPARγ.

BACKGROUND: PolyQ diseases are autosomal dominant neurodegenerative disorders caused by the expansion of CAG repeats. While of slow progression, these diseases are ultimately fatal and lack effective therapies. METHODS: A high-throughput chemical screen was conducted to identify drugs that lower the toxicity of a protein containing the first exon of Huntington's disease (HD) protein huntingtin (HTT) harbouring 94 glutamines (Htt-Q94). Candidate drugs were tested in a wide range of in vitro and in vivo models of polyQ toxicity. FINDINGS: The chemical screen identified the anti-leprosy drug clofazimine as a hit, which was subsequently validated in several in vitro models. Computational analyses of transcriptional signatures revealed that the effect of clofazimine was due to the stimulation of mitochondrial biogenesis by peroxisome proliferator-activated receptor gamma (PPARγ). In agreement with this, clofazimine rescued mitochondrial dysfunction triggered by Htt-Q94 expression. Importantly, clofazimine also limited polyQ toxicity in developing zebrafish and neuron-specific worm models of polyQ disease. INTERPRETATION: Our results support the potential of repurposing the antimicrobial drug clofazimine for the treatment of polyQ diseases. FUNDING: A full list of funding sources can be found in the acknowledgments section.

The emerging role and therapeutic implications of bacterial and parasitic deubiquitinating enzymes.

Deubiquitinating enzymes (DUBs) are emerging as key factors for the infection of human cells by pathogens such as bacteria and parasites. In this review, we discuss the most recent studies on the role of deubiquitinase activity in exploiting and manipulating ubiquitin (Ub)-dependent host processes during infection. The studies discussed here highlight the importance of DUB host-pathogen research and underscore the therapeutic potential of inhibiting pathogen-specific DUB activity to prevent infectious diseases.

In planta expression of human polyQ-expanded huntingtin fragment reveals mechanisms to prevent disease-related protein aggregation.

In humans, aggregation of polyglutamine repeat (polyQ) proteins causes disorders such as Huntington's disease. Although plants express hundreds of polyQ-containing proteins, no pathologies arising from polyQ aggregation have been reported. To investigate this phenomenon, we expressed an aggregation-prone fragment of human huntingtin (HTT) with an expanded polyQ stretch (Q69) in Arabidopsis thaliana plants. In contrast to animal models, we find that Arabidopsis sp. suppresses Q69 aggregation through chloroplast proteostasis. Inhibition of chloroplast proteostasis diminishes the capacity of plants to prevent cytosolic Q69 aggregation. Moreover, endogenous polyQ-containing proteins also aggregate on chloroplast dysfunction. We find that Q69 interacts with the chloroplast stromal processing peptidase (SPP). Synthetic Arabidopsis SPP prevents polyQ-expanded HTT aggregation in human cells. Likewise, ectopic SPP expression in Caenorhabditis elegans reduces neuronal Q67 aggregation and subsequent neurotoxicity. Our findings suggest that synthetic plant proteins, such as SPP, hold therapeutic potential for polyQ disorders and other age-related diseases involving protein aggregation.

Neuroprotective effects of hepatoma-derived growth factor in models of Huntington's disease.

Huntington's disease (HD) is a movement disorder caused by a mutation in the Huntingtin gene that leads to severe neurodegeneration. Molecular mechanisms of HD are not sufficiently understood, and no cure is currently available. Here, we demonstrate neuroprotective effects of hepatoma-derived growth factor (HDGF) in cellular and mouse HD models. We show that HD-vulnerable neurons in the striatum and cortex express lower levels of HDGF than resistant ones. Moreover, lack of endogenous HDGF exacerbated motor impairments and reduced the life span of R6/2 Huntington's disease mice. AAV-mediated delivery of HDGF into the brain reduced mutant Huntingtin inclusion load, but had no significant effect on motor behavior or life span. Interestingly, both nuclear and cytoplasmic versions of HDGF were efficient in rescuing mutant Huntingtin toxicity in cellular HD models. Moreover, extracellular application of recombinant HDGF improved viability of mutant Huntingtin-expressing primary neurons and reduced mutant Huntingtin aggregation in neural progenitor cells differentiated from human patient-derived induced pluripotent stem cells. Our findings provide new insights into the pathomechanisms of HD and demonstrate neuroprotective potential of HDGF in neurodegeneration.

Arabidopsis metacaspase MC1 localizes in stress granules, clears protein aggregates, and delays senescence.

Stress granules (SGs) are highly conserved cytoplasmic condensates that assemble in response to stress and contribute to maintaining protein homeostasis. These membraneless organelles are dynamic, disassembling once the stress is no longer present. Persistence of SGs due to mutations or chronic stress has been often related to age-dependent protein-misfolding diseases in animals. Here, we find that the metacaspase MC1 is dynamically recruited into SGs upon proteotoxic stress in Arabidopsis (Arabidopsis thaliana). Two predicted disordered regions, the prodomain and the 360 loop, mediate MC1 recruitment to and release from SGs. Importantly, we show that MC1 has the capacity to clear toxic protein aggregates in vivo and in vitro, acting as a disaggregase. Finally, we demonstrate that overexpressing MC1 delays senescence and this phenotype is dependent on the presence of the 360 loop and an intact catalytic domain. Together, our data indicate that MC1 regulates senescence through its recruitment into SGs and this function could potentially be linked to its remarkable protein aggregate-clearing activity.

Kinesin-5 inhibition improves neural regeneration in experimental autoimmune neuritis.

BACKGROUND: Autoimmune neuropathies can result in long-term disability and incomplete recovery, despite adequate first-line therapy. Kinesin-5 inhibition was shown to accelerate neurite outgrowth in different preclinical studies. Here, we evaluated the potential neuro-regenerative effects of the small molecule kinesin-5 inhibitor monastrol in a rodent model of acute autoimmune neuropathies, experimental autoimmune neuritis. METHODS: Experimental autoimmune neuritis was induced in Lewis rats with the neurogenic P2-peptide. At the beginning of the recovery phase at day 18, the animals were treated with 1 mg/kg monastrol or sham and observed until day 30 post-immunisation. Electrophysiological and histological analysis for markers of inflammation and remyelination of the sciatic nerve were performed. Neuromuscular junctions of the tibialis anterior muscles were analysed for reinnervation. We further treated human induced pluripotent stem cells-derived secondary motor neurons with monastrol in different concentrations and performed a neurite outgrowth assay. RESULTS: Treatment with monastrol enhanced functional and histological recovery in experimental autoimmune neuritis. Motor nerve conduction velocity at day 30 in the treated animals was comparable to pre-neuritis values. Monastrol-treated animals showed partially reinnervated or intact neuromuscular junctions. A significant and dose-dependent accelerated neurite outgrowth was observed after kinesin-5 inhibition as a possible mode of action. CONCLUSION: Pharmacological kinesin-5 inhibition improves the functional outcome in experimental autoimmune neuritis through accelerated motor neurite outgrowth and histological recovery. This approach could be of interest to improve the outcome of autoimmune neuropathy patients.

Experiences of insecurity among non-standard workers across different welfare states: A qualitative cross-country study.

In recent decades, economic crises and political reforms focused on employment flexibilization have increased the use of non-standard employment (NSE). National political and economic contexts determine how employers interact with labour and how the state interacts with labour markets and manages social welfare policies. These factors influence the prevalence of NSE and the level of employment insecurity it creates, but the extent to which a country's policy context mitigates the health influences of NSE is unclear. This study describes how workers experience insecurities created by NSE, and how this influences their health and well-being, in countries with different welfare states: Belgium, Canada, Chile, Spain, Sweden, and the United States. Interviews with 250 workers in NSE were analysed using a multiple-case study approach. Workers in all countries experienced multiple insecurities (e.g., income and employment insecurity) and relational tension with employers/clients, with negative health and well-being influences, in ways that were shaped by social inequalities (e.g., related to family support or immigration status). Welfare state differences were reflected in the level of workers' exclusion from social protections, the time scale of their insecurity (threatening daily survival or longer-term life planning), and their ability to derive a sense of control from NSE. Workers in Belgium, Sweden, and Spain, countries with more generous welfare states, navigated these insecurities with greater success and with less influence on health and well-being. Findings contribute to our understanding of the health and well-being influences of NSE across different welfare regimes and suggest the need in all six countries for stronger state responses to NSE. Increased investment in universal and more equal rights and benefits in NSE could reduce the widening gap between standard and NSE.

Cold temperature extends longevity and prevents disease-related protein aggregation through PA28γ-induced proteasomes.

Aging is a primary risk factor for neurodegenerative disorders that involve protein aggregation. Because lowering body temperature is one of the most effective mechanisms to extend longevity in both poikilotherms and homeotherms, a better understanding of cold-induced changes can lead to converging modifiers of pathological protein aggregation. Here, we find that cold temperature (15 °C) selectively induces the trypsin-like activity of the proteasome in Caenorhabditis elegans through PSME-3, the worm orthologue of human PA28γ/PSME3. This proteasome activator is required for cold-induced longevity and ameliorates age-related deficits in protein degradation. Moreover, cold-induced PA28γ/PSME-3 diminishes protein aggregation in C. elegans models of age-related diseases such as Huntington's and amyotrophic lateral sclerosis. Notably, exposure of human cells to moderate cold temperature (36 °C) also activates trypsin-like activity through PA28γ/PSME3, reducing disease-related protein aggregation and neurodegeneration. Together, our findings reveal a beneficial role of cold temperature that crosses evolutionary boundaries with potential implications for multi-disease prevention.

A tale of two pathways: Regulation of proteostasis by UPRmt and MDPs.

Mitochondrial fitness is critical to organismal health and its impairment is associated with aging and age-related diseases. As such, numerous quality control mechanisms exist to preserve mitochondrial stability, including the unfolded protein response of the mitochondria (UPRmt). The UPRmt is a conserved mechanism that drives the transcriptional activation of mitochondrial chaperones, proteases, autophagy (mitophagy), and metabolism to promote restoration of mitochondrial function under stress conditions. UPRmt has direct ramifications in aging, and its activation is often ascribed to improve health whereas its dysfunction tends to correlate with disease. This review pairs a description of the most recent findings within the field of UPRmt with a more poorly understood field: mitochondria-derived peptides (MDPs). Similar to UPRmt, MDPs are microproteins derived from the mitochondria that can impact organismal health and longevity. We then highlight a tantalizing interconnection between UPRmt and MDPs wherein both mechanisms may be efficiently coordinated to maintain organismal health.

G3BP1-dependent mechanism suppressing protein aggregation in Huntington's models and its demise upon stress granule assembly.

Stress granules are membrane-less ribonucleoprotein organelles that assemble upon exposure to stress conditions, but rapidly disassemble upon removal of stress. However, chronic stress can lead to persistent stress granules, a feature of distinct age-related neurodegenerative disorders. Among them, Huntington's disease (HD), which is caused by mutant expansion of the polyglutamine (polyQ) repeats of huntingtin protein (HTT), leading to its aggregation. To identify modulators of mutant HTT aggregation, we define its interactome in striatal neurons differentiated from patient-derived induced pluripotent stem cells (HD-iPSCs). We find that HTT interacts with G3BP1, a characteristic component of stress granules. Knockdown of G3BP1 increases mutant HTT protein levels and abolishes the ability of iPSCs as well as their differentiated neural counterparts to suppress mutant HTT aggregation. Moreover, loss of G3BP1 hastens polyQ-expanded aggregation and toxicity in the neurons of HD C. elegans models. Likewise, the assembly of G3BP1 into stress granules upon distinct stress conditions also reduces its interaction with HTT in human cells, promoting mutant HTT aggregation. Notably, enhancing the levels of G3BP1 is sufficient to induce proteasomal degradation of mutant HTT and prevent its aggregation, whereas the formation of stress granules blocks these ameliorative effects. In contrast, a mutant G3BP1 variant that cannot accumulate into granules retains its capacity to prevent mutant HTT aggregation even when the cells assemble stress granules. Thus, our findings indicate a direct role of G3BP1 and stress granule assembly in mutant HTT aggregation that may have implications for HD.

A dimer-monomer switch controls CHIP-dependent substrate ubiquitylation and processing.

The high substrate selectivity of the ubiquitin/proteasome system is mediated by a large group of E3 ubiquitin ligases. The ubiquitin ligase CHIP regulates the degradation of chaperone-controlled and chaperone-independent proteins. To understand how CHIP mediates substrate selection and processing, we performed a structure-function analysis of CHIP and addressed its physiological role in Caenorhabditis elegans and human cells. The conserved function of CHIP in chaperone-assisted degradation requires dimer formation to mediate proteotoxic stress resistance and to prevent protein aggregation. The CHIP monomer, however, promotes the turnover of the membrane-bound insulin receptor and longevity. The dimer-monomer transition is regulated by CHIP autoubiquitylation and chaperone binding, which provides a feedback loop that controls CHIP activity in response to cellular stress. Because CHIP also binds other E3 ligases, such as Parkin, the molecular switch mechanism described here could be a general concept for the regulation of substrate selectivity and ubiquitylation by combining different E3s.

Insights Into the Links Between Proteostasis and Aging From C. elegans.

Protein homeostasis (proteostasis) is maintained by a tightly regulated and interconnected network of biological pathways, preventing the accumulation and aggregation of damaged or misfolded proteins. Thus, the proteostasis network is essential to ensure organism longevity and health, while proteostasis failure contributes to the development of aging and age-related diseases that involve protein aggregation. The model organism Caenorhabditis elegans has proved invaluable for the study of proteostasis in the context of aging, longevity and disease, with a number of pivotal discoveries attributable to the use of this organism. In this review, we discuss prominent findings from C. elegans across the many key aspects of the proteostasis network, within the context of aging and disease. These studies collectively highlight numerous promising therapeutic targets, which may 1 day facilitate the development of interventions to delay aging and prevent age-associated diseases.

Protein clearance strategies for disease intervention.

Protein homeostasis, or proteostasis, is essential for cell function and viability. Unwanted, damaged, misfolded and aggregated proteins are degraded by the ubiquitin-proteasome system (UPS) and the autophagy-lysosome pathway. Growing evidence indicates that alterations in these major proteolytic mechanisms lead to a demise in proteostasis, contributing to the onset and development of distinct diseases. Indeed, dysregulation of the UPS or autophagy is linked to several neurodegenerative, infectious and inflammatory disorders as well as cancer. Thus, modulation of protein clearance pathways is a promising approach for therapeutics. In this review, we discuss recent findings and open questions on how targeting proteolytic mechanisms could be applied for disease intervention.

Rewiring of the ubiquitinated proteome determines ageing in C. elegans.

Ageing is driven by a loss of cellular integrity1. Given the major role of ubiquitin modifications in cell function2, here we assess the link between ubiquitination and ageing by quantifying whole-proteome ubiquitin signatures in Caenorhabditis elegans. We find a remodelling of the ubiquitinated proteome during ageing, which is ameliorated by longevity paradigms such as dietary restriction and reduced insulin signalling. Notably, ageing causes a global loss of ubiquitination that is triggered by increased deubiquitinase activity. Because ubiquitination can tag proteins for recognition by the proteasome3, a fundamental question is whether deficits in targeted degradation influence longevity. By integrating data from worms with a defective proteasome, we identify proteasomal targets that accumulate with age owing to decreased ubiquitination and subsequent degradation. Lowering the levels of age-dysregulated proteasome targets prolongs longevity, whereas preventing their degradation shortens lifespan. Among the proteasomal targets, we find the IFB-2 intermediate filament4 and the EPS-8 modulator of RAC signalling5. While increased levels of IFB-2 promote the loss of intestinal integrity and bacterial colonization, upregulation of EPS-8 hyperactivates RAC in muscle and neurons, and leads to alterations in the actin cytoskeleton and protein kinase JNK. In summary, age-related changes in targeted degradation of structural and regulatory proteins across tissues determine longevity.

The intrinsic chaperone network of Arabidopsis stem cells confers protection against proteotoxic stress.

The biological purpose of plant stem cells is to maintain themselves while providing new pools of differentiated cells that form organs and rejuvenate or replace damaged tissues. Protein homeostasis or proteostasis is required for cell function and viability. However, the link between proteostasis and plant stem cell identity remains unknown. In contrast to their differentiated counterparts, we find that root stem cells can prevent the accumulation of aggregated proteins even under proteotoxic stress conditions such as heat stress or proteasome inhibition. Notably, root stem cells exhibit enhanced expression of distinct chaperones that maintain proteome integrity. Particularly, intrinsic high levels of the T-complex protein-1 ring complex/chaperonin containing TCP1 (TRiC/CCT) complex determine stem cell maintenance and their remarkable ability to suppress protein aggregation. Overexpression of CCT8, a key activator of TRiC/CCT assembly, is sufficient to ameliorate protein aggregation in differentiated cells and confer resistance to proteotoxic stress in plants. Taken together, our results indicate that enhanced proteostasis mechanisms in stem cells could be an important requirement for plants to persist under extreme environmental conditions and reach extreme long ages. Thus, proteostasis of stem cells can provide insights to design and breed plants tolerant to environmental challenges caused by the climate change.

Systemic regulation of mitochondria by germline proteostasis prevents protein aggregation in the soma of C. elegans.

Protein aggregation causes intracellular changes in neurons, which elicit signals to modulate proteostasis in the periphery. Beyond the nervous system, a fundamental question is whether other organs also communicate their proteostasis status to distal tissues. Here, we examine whether proteostasis of the germ line influences somatic tissues. To this end, we induce aggregation of germline-specific PGL-1 protein in germline stem cells of Caenorhabditis elegans Besides altering the intracellular mitochondrial network of germline cells, PGL-1 aggregation also reduces the mitochondrial content of somatic tissues through long-range Wnt signaling pathway. This process induces the unfolded protein response of the mitochondria in the soma, promoting somatic mitochondrial fragmentation and aggregation of proteins linked with neurodegenerative diseases such as Huntington's and amyotrophic lateral sclerosis. Thus, the proteostasis status of germline stem cells coordinates mitochondrial networks and protein aggregation through the organism.

Resveratrol confers neuroprotection against high-fat diet in a mouse model of Alzheimer's disease via modulation of proteolytic mechanisms.

Cumulative evidence indicates that excessive consumption of calories from saturated fat contributes to the development of Alzheimer's disease (AD). Here, we assess the triggering and progression of AD pathology induced by a high-fat diet (HFD), and the effects of resveratrol, a polyphenol found in common dietary sources with pleiotropic neuroprotective activities. Over 16 weeks, male wild type (WT) and AD transgenic 5XFAD mice were fed a control diet, HFD (60% kcal from fat), or HFD supplemented with 0.1% resveratrol. Resveratrol protected against HFD-induced memory loss in WT mice and prevented memory loss in 5XFAD mice. Resveratrol also reduced the amyloid burden aggravated by HFD in 5XFAD, and protected against HFD-induced tau pathology in both WT and 5XFAD strains. At the mechanistic level, resveratrol inhibited the HFD-increased amyloidogenic processing of the amyloid precursor protein in both strains; it also restored abnormal high levels in the proteolytic activity of the ubiquitin-proteasome system induced by HFD, suggesting the presence of a compensatory mechanism to counteract the accumulation of aberrant proteins. Thus, our data suggest that resveratrol can correct the harmful effects of HFD in the brain and may be a potential therapeutic agent against obesity-related disorders and AD pathology.

The intrinsic proteostasis network of stem cells.

The proteostasis network adjusts protein composition and maintains protein integrity, which are essential processes for cell function and viability. Current efforts, given their intrinsic characteristics, regenerative potential and fundamental biological functions, have been directed to define proteostasis of stem cells. These insights demonstrate that embryonic stem cells and induced pluripotent stem cells exhibit an endogenous proteostasis network that not only modulates their pluripotency and differentiation but also provides a striking ability to suppress aggregation of disease-related proteins. Moreover, recent findings establish a central role of enhanced proteostasis to prevent the aging of somatic stem cells in adult organisms. Notably, proteostasis is also required for the biological purpose of adult germline stem cells, that is to be passed from one generation to the next. Beyond these links between proteostasis and stem cell function, we also discuss the implications of these findings for disease, aging, and reproduction.

Author Correction: Neuronal activity regulates DROSHA via autophagy in spinal muscular atrophy.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.