Insulin/IGF-1 signaling regulates proteasome activity through the deubiquitinating enzyme UBH-4

Impairment of proteasome-associated deubiquitinating enzyme Uchl5/UBH-4 affects autophagy

The autophagy-lysosomal pathway (ALP) and the ubiquitin-proteasome system (UPS) are the two major intracellular proteolytic systems that mediate protein turnover in eukaryotes. Although a crosstalk exists between these two systems, it is still unclear how UPS and ALP interact in vivo. Here, we investigated how impaired function of the proteasome-associated deubiquitinating enzyme (DUB) Uchl5/UBH-4 affects autophagy in human cells and in a multicellular organism. We show that downregulation of Uchl5 by siRNA reduces autophagy by partially blocking the fusion of autophagosomes with the lysosomes in HeLa cells, which is similar to a previously reported role of the proteasome-associated DUB Usp14 on autophagy. However, exposure of Caenorhabditis elegans to ubh-4 or usp-14 RNAi, or to their pharmacological inhibitors, results in diverse effects on numbers of autophagosomes and autolysosomes, without blocking the lysosomal fusion, in the intestine, hypodermal seam cells and the pharynx. Our results reveal that impairment of Uchl5/UBH-4 and Usp14 affects autophagy in a tissue context manner. A deeper insight into the interplay between UPS and ALP in various tissues in vivo has the potential to promote development of therapeutic approaches for disorders associated with proteostasis dysfunction.

UCHL5 promotes hepatocellular carcinoma progression by promoting glycolysis through activating Wnt/β-catenin pathway

BACKGROUND

Hepatocellular carcinoma (HCC) is highly malignant with a dismal prognosis, although the available therapies are insufficient. No efficient ubiquitinase has been identified as a therapeutic target for HCC despite the complicating role that of proteins ubiquitination plays in the malignant development of HCC.

METHODS

The expression of ubiquitin carboxyl terminal hydrolase L5 (UCHL5) in HCC tumor tissue and adjacent normal tissue was determined using the cancer genome atlas (TCGA) database and was validated using real-time quantitative polymerase chain reaction (RT-qRCR), Western blot and immunohistochemistry (IHC), and the relation of UCHL5 with patient clinical prognosis was explored. The expression of UCHL5 was knocked down and validated, and the effect of UCHL5 on the biological course of HCC was explored using cellular assays. To clarify the molecular mechanism of action of UCHL5 affecting HCC, expression studies of Adenosine triphosphate adenosine triphosphate (ATP), extracellular acidification (ECAR), and glycolysis-related enzymes were performed. The effects of UCHL5 on β-catenin ubiquitination and Wnt signaling pathways were explored in depth and validated using cellular functionalities. Validation was also performed in vivo.

RESULTS

In the course of this investigation, we discovered that UCHL5 was strongly expressed in HCC at both cellular and tissue levels. The prognosis of patients with high UCHL5 expression is considerably worse than that of those with low UCHL5 expression. UCHL5 has been shown to increase the degree of glycolysis in HCC cells with the impact of stimulating the proliferation and metastasis of HCC cells in both in vivo and in vitro. UCHL5 downregulates its degree of ubiquitination by binding to β-catenin, which activates the Wnt/β-catenin pathway and accelerates HCC cell glycolysis. Thereby promoting the growth of the HCC.

CONCLUSIONS

In summary, we have demonstrated for the first time that UCHL5 is a target of HCC and promotes the progression of hepatocellular carcinoma by promoting glycolysis through the activation of the Wnt/β-catenin pathway. UCHL5 may thus serve as a novel prognostic marker and therapeutic target for the treatment of HCC.

AKIR-1 regulates proteasome subcellular function in Caenorhabditis elegans

Polyubiquitinated proteins are primarily degraded by the ubiquitin-proteasome system (UPS). Proteasomes are present both in the cytoplasm and nucleus. Here, we investigated mechanisms coordinating proteasome subcellular localization and activity in a multicellular organism. We identified the nuclear protein-encoding gene akir-1 as a proteasome regulator in a genome-wide Caenorhabditis elegans RNAi screen. We demonstrate that depletion of akir-1 causes nuclear accumulation of endogenous polyubiquitinated proteins in intestinal cells, concomitant with slower in vivo proteasomal degradation in this subcellular compartment. Remarkably, akir-1 is essential for nuclear localization of proteasomes both in oocytes and intestinal cells but affects differentially the subcellular distribution of polyubiquitinated proteins. We further reveal that importin ima-3 genetically interacts with akir-1 and influences nuclear localization of a polyubiquitin-binding reporter. Our study shows that the conserved AKIR-1 is an important regulator of the subcellular function of proteasomes in a multicellular organism, suggesting a role for AKIR-1 in proteostasis maintenance.

BAP1 Malignant Pleural Mesothelioma Mutations in Caenorhabditis elegans Reveal Synthetic Lethality between ubh-4/BAP1 and the Proteasome Subunit rpn-9/PSMD13

The deubiquitinase BAP1 (BRCA1-associated protein 1) is associated with BAP1 tumor predisposition syndrome (TPDS). BAP1 is a tumor suppressor gene whose alterations in cancer are commonly caused by gene mutations leading to protein loss of function. By CRISPR-Cas, we have generated mutations in ubh-4, the BAP1 ortholog in Caenorhabditis elegans, to model the functional impact of BAP1 mutations. We have found that a mimicked BAP1 cancer missense mutation (UBH-4 A87D; BAP1 A95D) resembles the phenotypes of ubh-4 deletion mutants. Despite ubh-4 being ubiquitously expressed, the gene is not essential for viability and its deletion causes only mild phenotypes without affecting 20S proteasome levels. Such viability facilitated an RNAi screen for ubh-4 genetic interactors that identified rpn-9, the ortholog of human PSMD13, a gene encoding subunit of the regulatory particle of the 26S proteasome. ubh-4[A87D], similarly to ubh-4 deletion, cause a synthetic genetic interaction with rpn-9 inactivation affecting body size, lifespan, and the development of germ cells. Finally, we show how ubh-4 inactivation sensitizes animals to the chemotherapeutic agent Bortezomib, which is a proteasome inhibitor. Thus, we have established a model to study BAP1 cancer-related mutations in C. elegans, and our data points toward vulnerabilities that should be studied to explore therapeutic opportunities within the complexity of BAP1 tumors.

Ageing at Molecular Level: Role of MicroRNAs

The progression of age triggers a vast number of diseases including cardiovascular, cancer, and neurodegenerative disorders. Regardless of our plentiful knowledge about age-related diseases, little is understood about molecular pathways that associate the ageing process with various diseases. Several cellular events like senescence, telomere dysfunction, alterations in protein processing, and regulation of gene expression are common between ageing and associated diseases. Accumulating information on the role of microRNAs (miRNAs) suggests targeting miRNAs can aid our understanding of the interplay between ageing and associated diseases. In the present chapter, we have attempted to explore the information available on the role of miRNAs in ageing of various tissues/organs and diseases and understand the molecular mechanism of ageing.

Metabolic transitions define spermatogonial stem cell maturation

STUDY QUESTION

Do spermatogonia, including spermatogonial stem cells (SSCs), undergo metabolic changes during prepubertal development?

SUMMARY ANSWER

Here, we show that the metabolic phenotype of prepubertal human spermatogonia is distinct from that of adult spermatogonia and that SSC development is characterized by distinct metabolic transitions from oxidative phosphorylation (OXPHOS) to anaerobic metabolism.

WHAT IS KNOWN ALREADY

Maintenance of both mouse and human adult SSCs relies on glycolysis, while embryonic SSC precursors, primordial germ cells (PGCs), exhibit an elevated dependence on OXPHOS. Neonatal porcine SSC precursors reportedly initiate a transition to an adult SSC metabolic phenotype at 2 months of development. However, when and if such a metabolic transition occurs in humans is ambiguous.

STUDY DESIGN, SIZE, DURATION

To address our research questions: (i) we performed a meta-analysis of publicly available and newly generated (current study) single-cell RNA sequencing (scRNA-Seq) datasets in order to establish a roadmap of SSC metabolic development from embryonic stages (embryonic week 6) to adulthood in humans (25 years of age) with a total of ten groups; (ii) in parallel, we analyzed single-cell RNA sequencing datasets of isolated pup (n = 3) and adult (n = 2) murine spermatogonia to determine whether a similar metabolic switch occurs; and (iii) we characterized the mechanisms that regulate these metabolic transitions during SSC maturation by conducting quantitative proteomic analysis using two different ages of prepubertal pig spermatogonia as a model, each with four independently collected cell populations.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Single testicular cells collected from 1-year, 2-year and 7-year-old human males and sorted spermatogonia isolated from 6- to 8-day (n = 3) and 4-month (n = 2) old mice were subjected to scRNA-Seq. The human sequences were individually processed and then merged with the publicly available datasets for a meta-analysis using Seurat V4 package. We then performed a pairwise differential gene expression analysis between groups of age, followed by pathways enrichment analysis using gene set enrichment analysis (cutoff of false discovery rate < 0.05). The sequences from mice were subjected to a similar workflow as described for humans. Early (1-week-old) and late (8-week-old) prepubertal pig spermatogonia were analyzed to reveal underlying cellular mechanisms of the metabolic shift using immunohistochemistry, western blot, qRT-PCR, quantitative proteomics, and culture experiments.

MAIN RESULTS AND THE ROLE OF CHANCE

Human PGCs and prepubertal human spermatogonia show an enrichment of OXPHOS-associated genes, which is downregulated at the onset of puberty (P < 0.0001). Furthermore, we demonstrate that similar metabolic changes between pup and adult spermatogonia are detectable in the mouse (P < 0.0001). In humans, the metabolic transition at puberty is also preceded by a drastic change in SSC shape at 11 years of age (P < 0.0001). Using a pig model, we reveal that this metabolic shift could be regulated by an insulin growth factor-1 dependent signaling pathway via mammalian target of rapamycin and proteasome inhibition.

LARGE SCALE DATA

New single-cell RNA sequencing datasets obtained from this study are freely available through NCBI GEO with accession number GSE196819.

LIMITATIONS, REASONS FOR CAUTION

Human prepubertal tissue samples are scarce, which led to the investigation of a low number of samples per age. Gene enrichment analysis gives only an indication about the functional state of the cells. Due to limited numbers of prepubertal human spermatogonia, porcine spermatogonia were used for further proteomic and in vitro analyses.

WIDER IMPLICATIONS OF THE FINDINGS

We show that prepubertal human spermatogonia exhibit high OXHPOS and switch to an adult-like metabolism only after 11 years of age. Prepubescent cancer survivors often suffer from infertility in adulthood. SSC transplantation could provide a powerful tool for the treatment of infertility; however, it requires high cell numbers. This work provides key insight into the dynamic metabolic requirements of human SSCs across development that would be critical in establishing ex vivo systems to support expansion and sustained function of SSCs toward clinical use.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by the NIH/NICHD R01 HD091068 and NIH/ORIP R01 OD016575 to I.D. K.E.O. was supported by R01 HD100197. S.K.M. was supported by T32 HD087194 and F31 HD101323. The authors declare no conflict of interest.

HSF-1: Guardian of the Proteome Through Integration of Longevity Signals to the Proteostatic Network

Discoveries made in the nematode Caenorhabditis elegans revealed that aging is under genetic control. Since these transformative initial studies, C. elegans has become a premier model system for aging research. Critically, the genes, pathways, and processes that have fundamental roles in organismal aging are deeply conserved throughout evolution. This conservation has led to a wealth of knowledge regarding both the processes that influence aging and the identification of molecular and cellular hallmarks that play a causative role in the physiological decline of organisms. One key feature of age-associated decline is the failure of mechanisms that maintain proper function of the proteome (proteostasis). Here we highlight components of the proteostatic network that act to maintain the proteome and how this network integrates into major longevity signaling pathways. We focus in depth on the heat shock transcription factor 1 (HSF1), the central regulator of gene expression for proteins that maintain the cytosolic and nuclear proteomes, and a key effector of longevity signals.

Transcriptome Analysis of the Nematodes Caenorhabditis elegans and Litoditis marina in Different Food Environments

Diets regulate animal development, reproduction, and lifespan. However, the underlying molecular mechanisms remain elusive. We previously showed that a chemically defined CeMM diet attenuates the development and promotes the longevity of C. elegans, but whether it impacts other nematodes is unknown. Here, we studied the effects of the CeMM diet on the development and longevity of the marine nematode Litoditis marina, which belongs to the same family as C. elegans. We further investigated genome-wide transcriptional responses to the CeMM and OP50 diets for both nematodes, respectively. We observed that the CeMM diet attenuated L. marina development but did not extend its lifespan. Through KEEG enrichment analysis, we found that many of the FOXO DAF-16 signaling and lysosome and xenobiotic metabolism related genes were significantly increased in C. elegans on the CeMM diet, which might contribute to the lifespan extension of C. elegans. Notably, we found that the expression of lysosome and xenobiotic metabolism pathway genes was significantly down-regulated in L. marina on CeMM, which might explain why the CeMM diet could not promote the lifespan of L. marina compared to bacterial feeding. Additionally, the down-regulation of several RNA transcription and protein generation and related processes genes in C. elegans on CeMM might not only be involved in extending longevity, but also contribute to attenuating the development of C. elegans on the CeMM diet, while the down-regulation of unsaturated fatty acids synthesis genes in L. marina might contribute to slow down its growth while on CeMM. This study provided important insights into how different diets regulate development and lifespan, and further genetic analysis of the candidate gene(s) of development and longevity will facilitate exploring the molecular mechanisms underlying how diets regulate animal physiology and health in the context of variable nutritional environments.

How to resist soil desiccation: Transcriptional changes in a Mediterranean earthworm during aestivation

Earthworms have a central role in ministering the terrestrial ecosystems and are proving to have an important role in modulating the effects climate change has on soil. Aestivation is a form of dormancy employed by the organisms living in deserts and arid environments, when confronted with prolonged periods of drought. Understanding global metabolic adjustments required for withstanding the harsh conditions of the ever more severe Iberian drought, we performed a global transcriptomic exploration of the endogeic earthworm Carpetania matritensis during aestivation. There were a total of 6352 differentially expressed transcripts in the aestivating group, with 65% being downregulated. Based on GO and KEGG enrichment analyses, downregulated genes seem to be indicative of an overall metabolic depression during aestivation. Indeed we noted a reduction of protein turnover and macromolecule metabolism coupled with suppression of genes involved in digestion. Upregulated genes, namely antioxidant genes and DNA repair genes showed clear signs of abiotic stress caused by ROS generation. Abiotic stress led to transcriptomic changes of genes involved in immune response, mostly affecting the NF-kb signaling pathway as well as changes in apoptotic genes indicating the necessity of investigating these processes in a tissue specific manner. Lastly we uncovered a possible mechanism for water retention by nitrogenous waste accumulation. This study provides the first ever transcriptomic investigation done on aestivating earthworms and as such serves as a general framework for investigation on other earthworm species and other soil invertebrates, which is becoming increasingly important with the current scenario of climate change.

Proteasome activity contributes to pro-survival response upon mild mitochondrial stress in Caenorhabditis elegans

Defects in mitochondrial function activate compensatory responses in the cell. Mitochondrial stress that is caused by unfolded proteins inside the organelle induces a transcriptional response (termed the "mitochondrial unfolded protein response" [UPRmt]) that is mediated by activating transcription factor associated with stress 1 (ATFS-1). The UPRmt increases mitochondrial protein quality control. Mitochondrial dysfunction frequently causes defects in the import of proteins, resulting in the accumulation of mitochondrial proteins outside the organelle. In yeast, cells respond to mistargeted mitochondrial proteins by increasing activity of the proteasome in the cytosol (termed the "unfolded protein response activated by mistargeting of proteins" [UPRam]). The presence and relevance of this response in higher eukaryotes is unclear. Here, we demonstrate that defects in mitochondrial protein import in Caenorhabditis elegans lead to proteasome activation and life span extension. Both proteasome activation and life span prolongation partially depend on ATFS-1, despite its lack of influence on proteasomal gene transcription. Importantly, life span prolongation depends on the fully assembled proteasome. Our data provide a link between mitochondrial dysfunction and proteasomal activity and demonstrate its direct relevance to mechanisms that promote longevity.

Modulation of the ubiquitin-proteasome system by marine natural products

The ubiquitin-proteasome system (UPS) is a key player in the maintenance of cellular protein homeostasis (proteostasis). Since proteasome function declines upon aging leading to the acceleration of its progression and the manifestation of age-related pathologies, many attempts have been performed towards proteasome activation as a strategy to promote healthspan and longevity. The marine environment hosts a plethora of organisms that produce a vast array of primary and secondary metabolites, the majority of which are unique, exhibiting a wide spectrum of biological activities. The fact that these biologically important compounds are also present in edible marine organisms has sparked the interest for elucidating their potential health-related applications. In this review, we focus on the antioxidant, anti-aging, anti-aggregation and anti-photoaging properties of various marine constituents. We further discuss representatives of marine compounds classes with regard to their potential (direct or indirect) action on UPS components that could serve as UPS modulators and exert beneficial effects on conditions such as oxidative stress, aging and age-related diseases.

Differential Thresholds of Proteasome Activation Reveal Two Separable Mechanisms of Sensory Organ Polarization in C. elegans

Cephalization is a major innovation of animal evolution and implies a synchronization of nervous system, mouth, and foregut polarization to align alimentary tract and sensomotoric system for effective foraging. However, the underlying integration of morphogenetic programs is poorly understood. Here, we show that invagination of neuroectoderm through de novo polarization and apical constriction creates the mouth opening in the Caenorhabditis elegans embryo. Simultaneously, all 18 juxta-oral sensory organ dendritic tips become symmetrically positioned around the mouth: While the two bilaterally symmetric amphid sensilla endings are towed to the mouth opening, labial and cephalic sensilla become positioned independently. Dendrite towing is enabled by the pre-polarized sensory amphid pores intercalating into the leading edge of the anteriorly migrating epidermal sheet, while apical constriction-mediated cell–cell re-arrangements mediate positioning of all other sensory organs. These two processes can be separated by gradual inactivation of the 26S proteasome activator, RPN-6.1. Moreover, RPN-6.1 also shows a dose-dependent requirement for maintenance of coordinated apical polarization of other organs with apical lumen, the pharynx, and the intestine. Thus, our data unveil integration of morphogenetic programs during the coordination of alimentary tract and sensory organ formation and suggest that this process requires tight control of ubiquitin-dependent protein degradation.

Type 2 Diabetes, Obesity, and Cancer Share Some Common and Critical Pathways

Diabetes and cancer are among the most frequent and complex diseases. Epidemiological evidence showed that the patients suffering from diabetes are significantly at higher risk for a number of cancer types. There are a number of evidence that support the hypothesis that these diseases are interlinked, and obesity may aggravate the risk(s) of type 2 diabetes and cancer. Multi-level unwanted alterations such as (epi-)genetic alterations, changes at the transcriptional level, and altered signaling pathways (receptor, cytoplasmic, and nuclear level) are the major source which promotes a number of complex diseases and such heterogeneous level of complexities are considered as the major barrier in the development of therapeutic agents. With so many known challenges, it is critical to understand the relationships and the commonly shared causes between type 2 diabetes and cancer, which is difficult to unravel and understand. Furthermore, the real complexity arises from contended corroborations that specific drug(s) (individually or in combination) during the treatment of type 2 diabetes may increase or decrease the cancer risk or affect cancer prognosis. In this review article, we have presented the recent and most updated evidence from the studies where the origin, biological background, the correlation between them have been presented or proved. Furthermore, we have summarized the methodological challenges and tasks that are frequently encountered. We have also outlined the physiological links between type 2 diabetes and cancers. Finally, we have presented and summarized the outline of the hallmarks for both these diseases, diabetes and cancer.

Build-UPS and break-downs: metabolism impacts on proteostasis and aging

Perturbation of metabolism elicits cellular stress which profoundly modulates the cellular proteome and thus protein homeostasis (proteostasis). Consequently, changes in the cellular proteome due to metabolic shift require adaptive mechanisms by molecular protein quality control. The mechanisms vitally controlling proteostasis embrace the entire life cycle of a protein involving translational control at the ribosome, chaperone-assisted native folding, and subcellular sorting as well as proteolysis by the proteasome or autophagy. While metabolic imbalance and proteostasis decline have been recognized as hallmarks of aging and age-associated diseases, both processes are largely considered independently. Here, we delineate how proteome stability is governed by insulin/IGF1 signaling (IIS), mechanistic target of Rapamycin (TOR), 5′ adenosine monophosphate-activated protein kinase (AMPK), and NAD-dependent deacetylases (Sir2-like proteins known as sirtuins). This comprehensive overview is emphasizing the regulatory interconnection between central metabolic pathways and proteostasis, indicating the relevance of shared signaling nodes as targets for future therapeutic interventions.

Tissue-Specific Impact of Autophagy Genes on the Ubiquitin-Proteasome System in C. elegans

The ubiquitin–proteasome system (UPS) and the autophagy–lysosomal pathway (ALP) are the two main eukaryotic intracellular proteolytic systems involved in maintaining proteostasis. Several studies have reported on the interplay between the UPS and ALP, however it remains largely unknown how compromised autophagy affects UPS function in vivo. Here, we have studied the crosstalk between the UPS and ALP by investigating the tissue-specific effect of autophagy genes on the UPS at an organismal level. Using transgenic Caenorhabditis elegans expressing fluorescent UPS reporters, we show that the downregulation of the autophagy genes lgg-1 and lgg-2 (ATG8/LC3/GABARAP), bec-1 (BECLIN1), atg-7 (ATG7) and epg-5 (mEPG5) by RNAi decreases proteasomal degradation, concomitant with the accumulation of polyubiquitinated proteasomal substrates in a tissue-specific manner. For some of these genes, the changes in proteasomal degradation occur without a detectable alteration in proteasome tissue expression levels. In addition, the lgg-1 RNAi-induced reduction in proteasome activity in intestinal cells is not dependent on sqst-1/p62 accumulation. Our results illustrate that compromised autophagy can affect UPS in a tissue-specific manner, and demonstrate that UPS does not function as a direct compensatory mechanism in an animal. Further, a more profound understanding of the multilayered crosstalk between UPS and ALP can facilitate the development of therapeutic options for various disorders linked to dysfunction in proteostasis.

Tissue-specific effects of temperature on proteasome function

Variation in ambient growth temperature can cause changes in normal animal physiology and cellular functions such as control of protein homeostasis. A key mechanism for maintaining proteostasis is the selective degradation of polyubiquitinated proteins, mediated by the ubiquitin-proteasome system (UPS). It is still largely unsolved how temperature changes affect the UPS at the organismal level. Caenorhabditis elegans nematodes are normally bred at 20 °C, but for some experimental conditions, 25 °C is often used. We studied the effect of 25 °C on C. elegans UPS by measuring proteasome activity and polyubiquitinated proteins both in vitro in whole animal lysates and in vivo in tissue-specific transgenic reporter strains. Our results show that an ambient temperature shift from 20 to 25 °C increases the UPS activity in the intestine, but not in the body wall muscle tissue, where a concomitant accumulation of polyubiquitinated proteins occurs. These changes in the UPS activity and levels of polyubiquitinated proteins were not detectable in whole animal lysates. The exposure of transgenic animals to 25 °C also induced ER stress reporter fluorescence, but not the fluorescence of a heat shock responsive reporter, albeit detection of a mild induction in hsp-16.2 mRNA levels. In conclusion, C. elegans exhibits tissue-specific responses of the UPS as an organismal strategy to cope with a rise in ambient temperature.

Low Concentrations of Caffeine and Its Analogs Extend the Lifespan of Caenorhabditis elegans by Modulating IGF-1-Like Pathway

Caffeine has been reported to delay aging and protect aging-associated disorders in Caenorhabditis elegans. However, the effects of low concentration of caffeine and its analogs on lifespan are currently missing. Herein, we report that at much lower concentrations (as low as 10 μg/ml), caffeine extended the lifespan of C. elegans without affecting food intake and reproduction. The effect of caffeine was dependent on IGF-1-like pathway, although the insulin receptor homolog, daf-2 allele, e1371, was dispensable. Four caffeine analogs, 1-methylxanthine, 7-methylxanthine, 1,3-dimethylxanthine, and 1,7-dimethylxanthine, also extended lifespan, whereas 3-methylxanthine and 3,7-dimethylxanthine did not exhibit lifespan-extending activity.

Ubiquitylation Pathways In Insulin Signaling and Organismal Homeostasis

The insulin/insulin-like growth factor-1 (IGF-1) signaling (IIS) pathway is a pivotal genetic program regulating cell growth, tissue development, metabolic physiology, and longevity of multicellular organisms. IIS integrates a fine-tuned cascade of signaling events induced by insulin/IGF-1, which is precisely controlled by post-translational modifications. The ubiquitin/proteasome-system (UPS) influences the functionality of IIS through inducible ubiquitylation pathways that regulate internalization of the insulin/IGF-1 receptor, the stability of downstream insulin/IGF-1 signaling targets, and activity of nuclear receptors for control of gene expression. An age-related decline in UPS activity is often associated with an impairment of IIS, contributing to pathologies such as cancer, diabetes, cardiovascular, and neurodegenerative disorders. Recent findings identified a key role of diverse ubiquitin modifications in insulin signaling decisions, which governs dynamic adaption upon environmental and physiological changes. In this review, we discuss the mutual crosstalk between ubiquitin and insulin signaling pathways in the context of cellular and organismal homeostasis.

Reduction of RPT6/S8 (a Proteasome Component) and Proteasome Activity in the Cortex is Associated with Cognitive Impairment in Lewy Body Dementia

Lewy body dementia is the second most common neurodegenerative dementia and is pathologically characterized by α-synuclein positive cytoplasmic inclusions, with varying amounts of amyloid-β (Aβ) and hyperphosphorylated tau (tau) aggregates in addition to synaptic loss. A dysfunctional ubiquitin proteasome system (UPS), the major proteolytic pathway responsible for the clearance of short lived proteins, may be a mediating factor of disease progression and of the development of α-synuclein aggregates. In the present study, protein expression of a key component of the UPS, the RPT6 subunit of the 19S regulatory complex was determined. Furthermore, the main proteolytic-like (chymotrypsin- and PGPH-) activities have also been analyzed. The middle frontal (Brodmann, BA9), inferior parietal (BA40), and anterior cingulate (BA24) gyrus' cortex were selected as regions of interest from Parkinson's disease dementia (PDD, n = 31), dementia with Lewy bodies (DLB, n = 44), Alzheimer's disease (AD, n = 16), and control (n = 24) brains. Clinical and pathological data available included the MMSE score. DLB, PDD, and AD were characterized by significant reductions of RPT6 (one-way ANOVA, p < 0.001; Bonferroni post hoc test) in prefrontal cortex and parietal cortex compared with controls. Strong associations were observed between RPT6 levels in prefrontal, parietal cortex, and anterior cingulate gyrus and cognitive impairment (p = 0.001, p = 0.001, and p = 0.008, respectively). These findings highlight the involvement of the UPS in Lewy body dementia and indicate that targeting the UPS may have the potential to slow down or reduce the progression of cognitive impairment in DLB and PDD.

Positive cytoplasmic UCHL5 tumor expression in gastric cancer is linked to improved prognosis

Gastric cancer is the second most common cause of cancer-related mortality worldwide. Accurate prediction of disease progression is difficult, and new biomarkers for clinical use are essential. Recently, we reported that the proteasome-associated deubiquitinating enzyme UCHL5/Uch37 is a new prognostic marker in both rectal cancer and pancreatic ductal adenocarcinoma. Here, we have assessed by immunohistochemistry UCHL5 tumor expression in gastric cancer. The study cohort comprised 650 patients, who underwent surgery in Helsinki University Hospital, Finland, between 1983 and 2009. We investigated the association of cytoplasmic UCHL5 tumor expression to assess clinicopathological parameters and patient survival. Positive cytoplasmic UCHL5 tumor immunoexpression is linked to increased survival of patients with small (<5 cm) tumors (p = 0.001), disease stages I-II (p = 0.025), and age 66 years or older (p = 0.037). UCHL5 is thus a potential marker in gastric cancer with new prognostic relevance.

Fluorescent Tools for In Vivo Studies on the Ubiquitin-Proteasome System

The ubiquitin-proteasome system (UPS) plays a key role in maintaining proteostasis by degrading most of the cellular proteins. Traditionally, UPS activity is studied in vitro, in yeast, or in mammalian cell cultures by using short-lived GFP-based UPS reporters. Here, we present protocols for two fluorescent tools facilitating real-time imaging of UPS activity in living animals. We have generated transgenic Caenorhabditis elegans (C. elegans) expressing a photoconvertible UbG76V-Dendra2 UPS reporter, which permits measurement of reporter degradation by the proteasome independently of reporter protein synthesis, and a fluorescent polyubiquitin-binding reporter for detection of the endogenous pool of Lys48-linked polyubiquitinated proteasomal substrates. These reporter systems facilitate cell- and tissue-specific analysis of UPS activity especially in young adult animals, but can also be used for studies during development, aging, and for example stress conditions.

Real-time detection of N-end rule-mediated ubiquitination via fluorescently labeled substrate probes

The N-end rule pathway has emerged as a major system for regulating protein functions by controlling their turnover in medical, animal and plant sciences as well as agriculture. Although novel functions and enzymes of the pathway have been discovered, the ubiquitination mechanism and substrate specificity of N-end rule pathway E3 ubiquitin ligases have remained elusive. Taking the first discovered bona fide plant N-end rule E3 ligase PROTEOLYSIS1 (PRT1) as a model, we used a novel tool to molecularly characterize polyubiquitination live, in real time. We gained mechanistic insights into PRT1 substrate preference and activation by monitoring live ubiquitination using a fluorescent chemical probe coupled to artificial substrate reporters. Ubiquitination was measured by rapid in-gel fluorescence scanning as well as in real time by fluorescence polarization. The enzymatic activity, substrate specificity, mechanisms and reaction optimization of PRT1-mediated ubiquitination were investigated ad hoc instantaneously and with significantly reduced reagent consumption. We demonstrated that PRT1 is indeed an E3 ligase, which has been hypothesized for over two decades. These results demonstrate that PRT1 has the potential to be involved in polyubiquitination of various substrates and therefore pave the way to understanding recently discovered phenotypes of prt1 mutants.

Meddling with Fate: The Proteasomal Deubiquitinating Enzymes

Three deubiquitinating enzymes-Rpn11, Usp14, and Uch37-are associated with the proteasome regulatory particle. These enzymes allow proteasomes to remove ubiquitin from substrates before they are translocated into the core particle to be degraded. Although the translocation channel is too narrow for folded proteins, the force of translocation unfolds them mechanically. As translocation proceeds, ubiquitin chains bound to substrate are drawn to the channel's entry port, where they can impede further translocation. Rpn11, situated over the port, can remove these chains without compromising degradation because substrates must be irreversibly committed to degradation before Rpn11 acts. This coupling between deubiquitination and substrate degradation is ensured by the Ins-1 loop of Rpn11, which controls ubiquitin access to its catalytic site. In contrast to Rpn11, Usp14 and Uch37 can rescue substrates from degradation by promoting substrate dissociation from the proteasome prior to the commitment step. Uch37 is unique in being a component of both the proteasome and a second multisubunit assembly, the INO80 complex. However, only recruitment into the proteasome activates Uch37. Recruitment to the proteasome likewise activates Usp14. However, the influence of Usp14 on the proteasome depends on the substrate, due to its marked preference for proteins that carry multiple ubiquitin chains. Usp14 exerts complex control over the proteasome, suppressing proteasome activity even when inactive in deubiquitination. A major challenge for the field will be to elucidate the specificities of Rpn11, Usp14, and Uch37 in greater depth, employing not only model in vitro substrates but also their endogenous targets.

Immunohistochemical analysis reveals variations in proteasome tissue expression in C. elegans

The ubiquitin-proteasome system (UPS) plays a crucial part in normal cell function by mediating intracellular protein clearance. We have previously shown that UPS-mediated protein degradation varies in a cell type-specific manner in C. elegans. Here, we use formalin-fixed, paraffin-embedded C. elegans sections to enable studies on endogenous proteasome tissue expression. We show that the proteasome immunoreactivity pattern differs between cell types and within subcellular compartments in adult wild-type (N2) C. elegans. Interestingly, widespread knockdown of proteasome subunits by RNAi results in tissue-specific changes in proteasome expression instead of a uniform response. In addition, long-lived daf-2(e1370) mutants with impaired insulin/IGF-1 signaling (IIS) display similar proteasome tissue expression as aged-matched wild-type animals. Our study emphasizes the importance of alternate approaches to the commonly used whole animal lysate-based methods to detect changes in proteasome expression occurring at the sub-cellular, cell or tissue resolution level in a multicellular organism.

Proteostasis and ageing: insights from long-lived mutant mice

The global increase in life expectancy is creating significant medical, social and economic challenges to current and future generations. Consequently, there is a need to identify the fundamental mechanisms underlying the ageing process. This knowledge should help develop realistic interventions capable of combatting age‐related disease, and thus improving late‐life health and vitality. While several mechanisms have been proposed as conserved lifespan determinants, the loss of proteostasis – where proteostasis is defined here as the maintenance of the proteome – appears highly relevant to both ageing and disease. Several studies have shown that multiple proteostatic mechanisms, including the endoplasmic reticulum (ER)‐induced unfolded protein response (UPR), the ubiquitin–proteasome system (UPS) and autophagy, appear indispensable for longevity in many long‐lived invertebrate mutants. Similarly, interspecific comparisons suggest that proteostasis may be an important lifespan determinant in vertebrates. Over the last 20 years a number of long‐lived mouse mutants have been described, many of which carry single‐gene mutations within the growth‐hormone, insulin/IGF‐1 or mTOR signalling pathways. However, we still do not know how these mutations act mechanistically to increase lifespan and healthspan, and accordingly whether mechanistic commonality occurs between different mutants. Recent evidence supports the premise that the successful maintenance of the proteome during ageing may be linked to the increased lifespan and healthspan of long‐lived mouse mutants.

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.

Nuclear ubiquitin C-terminal hydrolase L5 expression associates with increased patient survival in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma is a lethal disease with an overall 5-year survival of less than 5%. Prognosis among surgically treated patients is difficult and identification of new biomarkers is essential for accurate prediction of patient outcome. As part of one of the major cellular protein degradation systems, the proteasome plays a fundamental role in both physiological and pathophysiological conditions including cancer. The proteasome-associated deubiquitinating enzyme ubiquitin C-terminal hydrolase L5 (UCHL5)/Uch37 is a modulator of proteasome activity with cancer prognostic marker potential. Cytoplasmic and nuclear immunoexpression of UCHL5 was evaluated in 154 surgical specimens from pancreatic ductal adenocarcinoma patients treated at Helsinki University Hospital, Finland, in 2000-2011. UCHL5 expression in relation to clinicopathological parameters and the association between UCHL5 In this study, positive expression and patient survival were assessed. Positive nuclear UCHL5 expression was associated with increased patient survival ( p = 0.005). A survival benefit was also detectable in these subgroups of patients: over 65 years ( p < 0.001), at tumor stages IIB to III ( p = 0.007), or with lymph-node positivity ( p = 0.006). In stages IIB to III disease, patients with positive nuclear UCHL5 expression showed a twofold increase in 5-year cancer-specific survival compared to those with negative expression. Multivariate analysis identified positive nuclear UCHL5 expression as an independent prognostic factor ( p = 0.012). In conclusion, UCHL5 expression could function as a prognostic marker in pancreatic ductal adenocarcinoma, particularly at disease stages IIB to III. As UCHL5 is one of the few markers predicting increased survival, our results may be of clinical relevance.

UCHL5 expression associates with improved survival in lymph-node-positive rectal cancer

Colorectal cancer is among the three most common cancer types for both genders, with a rising global incidence. To date, prognostic evaluation is difficult and largely dependent on early detection and successful surgery. UCHL5/Uch37 is an integral part of the protein homeostasis network as one of the three deubiquitinating enzymes associated with the 26S proteasome. Here, we have investigated in colorectal cancer the possible association of UCHL5 tumor expression and patient survival. UCHL5 tumor expression was evaluated by immunohistochemistry in 779 surgically treated colorectal cancer patients from Helsinki University Hospital, Finland, with assessment of clinicopathological parameters and the effect of UCHL5 expression on patient survival. High and undetectable UCHL5 expression both correlated with increased overall disease-specific survival in the subgroup of patients with lymph-node-positive (Dukes C/stage III) rectal cancer. Within this subgroup of 105 stage-III rectal cancer patients, none of the 7 with high UCHL5 expression died of colorectal cancer within 10 years after surgery ( p = 0.012). A similar, though less prominent, survival trend occurred throughout the whole patient cohort. In conclusion, UCHL5 is a promising novel prognostic marker in lymph-node-positive rectal cancer. Our results also advance the currently limited knowledge of biomarkers in colorectal cancer treatment.

Proteasome expression and activity in cancer and cancer stem cells

Proteasome is a multi-protein organelle that participates in cellular proteostasis by destroying damaged or short-lived proteins in an organized manner guided by the ubiquitination signal. By being in a central place in the cellular protein complement homeostasis, proteasome is involved in virtually all cell processes including decisions on cell survival or death, cell cycle, and differentiation. These processes are important also in cancer, and thus, the proteasome is an important regulator of carcinogenesis. Cancers include a variety of cells which, according to the cancer stem cell theory, descend from a small percentage of cancer stem cells, alternatively termed tumor-initiating cells. These cells constitute the subsets that have the ability to propagate the whole variety of cancer and repopulate tumors after cytostatic therapies. Proteasome plays a role in cellular processes in cancer stem cells, but it has been found to have a decreased function in them compared to the rest of cancer cells. This article will discuss the transcriptional regulation of proteasome sub-unit proteins in cancer and in particular cancer stem cells and the relationship of the proteasome with the pluripotency that is the defining characteristic of stem cells. Therapeutic opportunities that present from the understanding of the proteasome role will also be discussed.

18α-Glycyrrhetinic Acid Proteasome Activator Decelerates Aging and Alzheimer's Disease Progression in Caenorhabditis elegans and Neuronal Cultures

AIMS

Proteasomes are constituents of the cellular proteolytic networks that maintain protein homeostasis through regulated proteolysis of normal and abnormal (in any way) proteins. Genetically mediated proteasome activation in multicellular organisms has been shown to promote longevity and to exert protein antiaggregation activity. In this study, we investigate whether compound-mediated proteasome activation is feasible in a multicellular organism and we dissect the effects of such approach in aging and Alzheimer's disease (AD) progression.

RESULTS

Feeding of wild-type Caenorhabditis elegans with 18α-glycyrrhetinic acid (18α-GA; a previously shown proteasome activator in cell culture) results in enhanced levels of proteasome activities that lead to a skinhead-1- and proteasome activation-dependent life span extension. The elevated proteasome function confers lower paralysis rates in various AD nematode models accompanied by decreased Aβ deposits, thus ultimately decelerating the progression of AD phenotype. More importantly, similar positive results are also delivered when human and murine cells of nervous origin are subjected to 18α-GA treatment.

INNOVATION

This is the first report of the use of 18α-GA, a diet-derived compound as prolongevity and antiaggregation factor in the context of a multicellular organism.

CONCLUSION

Our results suggest that proteasome activation with downstream positive outcomes on aging and AD, an aggregation-related disease, is feasible in a nongenetic manipulation manner in a multicellular organism. Moreover, they unveil the need for identification of antiaging and antiamyloidogenic compounds among the nutrients found in our normal diet. Antioxid. Redox Signal. 25, 855-869.

Mechanisms of aging-related proteinopathies in Caenorhabditis elegans

Aging is the most important risk factor for human neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Pathologically, these diseases are characterized by the deposition of specific protein aggregates in neurons and glia, representing the impairment of neuronal proteostasis. However, the mechanism by which aging affects the proteostasis system and promotes protein aggregation remains largely unknown. The short lifespan and ample genetic resources of Caenorhabditis elegans (C. elegans) have made this species a favorite model organism for aging research, and the development of proteinopathy models in this organism has helped us to understand how aging processes affect protein aggregation and neurodegeneration. Here, we review the recent literature on proteinopathies in C. elegans models and discuss the insights we have gained into the mechanisms of how aging processes are integrated into the pathogenesis of various neurodegenerative diseases.

The cell biology of aging

One of the original hypotheses of organismal longevity posits that aging is the natural result of entropy on the cells, tissues, and organs of the animal—a slow, inexorable slide into nonfunctionality caused by stochastic degradation of its parts. We now have evidence that aging is instead at least in part genetically regulated. Many mutations have been discovered to extend lifespan in organisms of all complexities, from yeast to mammals. The study of metazoan model organisms, such as Caenorhabditis elegans, has been instrumental in understanding the role of genetics in the cell biology of aging. Longevity mutants across the spectrum of model organisms demonstrate that rates of aging are regulated through genetic control of cellular processes. The regulation and subsequent breakdown of cellular processes represent a programmatic decision by the cell to either continue or abandon maintenance procedures with age. Our understanding of cell biological processes involved in regulating aging have been particularly informed by longevity mutants and treatments, such as reduced insulin/IGF-1 signaling and dietary restriction, which are critical in determining the distinction between causes of and responses to aging and have revealed a set of downstream targets that participate in a range of cell biological activities. Here we briefly review some of these important cellular processes.

Abnormal Osmotic Avoidance Behavior in C. elegans Is Associated with Increased Hypertonic Stress Resistance and Improved Proteostasis

Protein function is controlled by the cellular proteostasis network. Proteostasis is energetically costly and those costs must be balanced with the energy needs of other physiological functions. Hypertonic stress causes widespread protein damage in C. elegans. Suppression and management of protein damage is essential for optimal survival under hypertonic conditions. ASH chemosensory neurons allow C. elegans to detect and avoid strongly hypertonic environments. We demonstrate that mutations in osm-9 and osm-12 that disrupt ASH mediated hypertonic avoidance behavior or genetic ablation of ASH neurons are associated with enhanced survival during hypertonic stress. Improved survival is not due to altered systemic volume homeostasis or organic osmolyte accumulation. Instead, we find that osm-9(ok1677) mutant and osm-9(RNAi) worms exhibit reductions in hypertonicity induced protein damage in non-neuronal cells suggesting that enhanced proteostasis capacity may account for improved hypertonic stress resistance in worms with defects in osmotic avoidance behavior. RNA-seq analysis revealed that genes that play roles in managing protein damage are upregulated in osm-9(ok1677) worms. Our findings are consistent with a growing body of work demonstrating that intercellular communication between neuronal and non-neuronal cells plays a critical role in integrating cellular stress resistance with other organismal physiological demands and associated energy costs.

Increased Protein Stability and Decreased Protein Turnover in the Caenorhabditis elegans Ins/IGF-1 daf-2 Mutant

In Caenorhabditis elegans, cellular proteostasis is likely essential for longevity. Autophagy has been shown to be essential for lifespan extension of daf-2 insulin/IGF mutants. Therefore, it can be hypothesized that daf-2 mutants achieve this phenotype by increasing protein turnover. However, such a mechanism would exert a substantial energy cost. By using classical ³⁵S pulse-chase labeling, we observed that protein synthesis and degradation rates are decreased in young adults of the daf-2 insulin/IGF mutants. Although reduction of protein turnover may be energetically favorable, it may lead to accumulation and aggregation of damaged proteins. As this has been shown not to be the case in daf-2 mutants, another mechanism must exist to maintain proteostasis in this strain. We observed that proteins isolated from daf-2 mutants are more soluble in acidic conditions due to increased levels of trehalose. This suggests that trehalose may decrease the potential for protein aggregation and increases proteostasis in the daf-2 mutants. We postulate that daf-2 mutants save energy by decreasing protein turnover rates and instead stabilize their proteome by trehalose.

UPS Activation in the Battle Against Aging and Aggregation-Related Diseases: An Extended Review

Aging is a biological process accompanied by gradual increase of damage in all cellular macromolecules, i.e., nucleic acids, lipids, and proteins. When the proteostasis network (chaperones and proteolytic systems) cannot reverse the damage load due to its excess as compared to cellular repair/regeneration capacity, failure of homeostasis is established. This failure is a major hallmark of aging and/or aggregation-related diseases. Dysfunction of the major cellular proteolytic machineries, namely the proteasome and the lysosome, has been reported during the progression of aging and aggregation-prone diseases. Therefore, activation of these pathways is considered as a possible preventive or therapeutic approach against the progression of these processes. This chapter focuses on UPS activation studies in cellular and organismal models and the effects of such activation on aging, longevity and disease prevention or reversal.

The inhibition of IGF-1 signaling promotes proteostasis by enhancing protein aggregation and deposition

The discovery that the alteration of aging by reducing the activity of the insulin/IGF-1 signaling (IIS) cascade protects nematodes and mice from neurodegeneration-linked, toxic protein aggregation (proteotoxicity) raises the prospect that IIS inhibitors bear therapeutic potential to counter neurodegenerative diseases. Recently, we reported that NT219, a highly efficient IGF-1 signaling inhibitor, protects model worms from the aggregation of amyloid β peptide and polyglutamine peptides that are linked to the manifestation of Alzheimer's and Huntington's diseases, respectively. Here, we employed cultured cell systems to investigate whether NT219 promotes protein homeostasis (proteostasis) in mammalian cells and to explore its underlying mechanisms. We found that NT219 enhances the aggregation of misfolded prion protein and promotes its deposition in quality control compartments known as "aggresomes." NT219 also elevates the levels of certain molecular chaperones but, surprisingly, reduces proteasome activity and impairs autophagy. Our findings show that IGF-1 signaling inhibitors in general and NT219 in particular can promote proteostasis in mammalian cells by hyperaggregating hazardous proteins, thereby bearing the potential to postpone the onset and slow the progression of neurodegenerative illnesses in the elderly.-Moll, L., Ben-Gedalya, T., Reuveni, H., Cohen, E. The inhibition of IGF-1 signaling promotes proteostasis by enhancing protein aggregation and deposition.

IGF-1 promotes the growth and metastasis of hepatocellular carcinoma via the inhibition of proteasome-mediated cathepsin B degradation

AIM: To investigate the molecular mechanisms of the high IGF-1 level linking diabetes and cancers, which is a risk factor.

METHODS: We used cell growth, wound healing and transwell assay to evaluate the proliferation and metastasis ability of the hepatocellular carcinoma (HCC) cells. Western blot and reverse transcription polymerase chain reaction were used to assess a previously identified lysosomal protease, cathepsin B (CTSB) expression in the HCC cell lines. C57 BL/6J and KK-Ay diabetic mice are used to detect the growth and metastasis of HCC cells that were depleted with or without CTSB shRNA in vivo. Statistical significance was determined by Student’s t-test.

RESULTS: IGF-1 promoted the growth and metastasis of the HCC cell lines via its ability to enhance CTSB expression in both a time-dependent and concentration-dependent manner. HCC cells grew much faster in diabetic KK-Ay mice than in C57 BL/6J mice. Additionally, more metastatic nodules were found in the lungs of KK-Ay mice than the lungs of C57 BL/6J mice. CTSB depletion protects against the tumor-promoting actions of IGF-1 in HCC cells, as well tumor growth and metastasis both in vitro and in vivo. IGF-1 did not change the mRNA levels of CTSB but prolonged the half-life of cathepsin B in Hepa 1-6 and H22 cells. Our results showed that IGF-1 promotes the growth and metastasis of the HCC cells most likely by hindering CTSB degradation mediated by the ubiquitin-proteasome system (UPS), but not autophagy. Overexpression of proteasome activator 28, a family of activators of the 20S proteasome, could not only restore IGF-1-inhibited UPS activity but also decrease IGF-1-induced CTSB accumulation.

CONCLUSION: Our study demonstrates that IGF-1 promotes the growth and metastasis of hepatocellular carcinoma by inhibition of proteasome-mediated CTSB degradation.

Proteasome activation: An innovative promising approach for delaying aging and retarding age-related diseases

Aging is a natural process accompanied by a progressive accumulation of damage in all constituent macromolecules (nucleic acids, lipids and proteins). Accumulation of damage in proteins leads to failure of proteostasis (or vice versa) due to increased levels of unfolded, misfolded or aggregated proteins and, in turn, to aging and/or age-related diseases. The major cellular proteolytic machineries, namely the proteasome and the lysosome, have been shown to dysfunction during aging and age-related diseases. Regarding the proteasome, it is well established that it can be activated either through genetic manipulation or through treatment with natural or chemical compounds that eventually result to extension of lifespan or deceleration of the progression of age-related diseases. This review article focuses on proteasome activation studies in several species and cellular models and their effects on aging and longevity. Moreover, it summarizes findings regarding proteasome activation in the major age-related diseases as well as in progeroid syndromes.

Regulation of mechanosensation in C. elegans through ubiquitination of the MEC-4 mechanotransduction channel

In Caenorhabditis elegans, gentle touch is sensed by the anterior (ALM and AVM) and posterior (PLM) touch receptor neurons. Anterior, but not posterior, touch is affected by several stress conditions via the action of AKT kinases and the DAF-16/FOXO transcription factor. Here we show that a ubiquitination-dependent mechanism mediates such effects. AKT-1/AKT kinase and DAF-16 alter the transcription of mfb-1, which encodes an E3 ubiquitin ligase needed for the ubiquitination of the mechanosensory channel subunit MEC-4. Ubiquitination of MEC-4 reduces the amount of MEC-4 protein in the processes of ALM neurons and, consequently, the mechanoreceptor current. Even under nonstress conditions, differences in the amount of MFB-1 appear to cause the PLM neurons to be less sensitive to touch than the ALM neurons. These studies demonstrate that modulation of surface mechanoreceptors can regulate the sensitivity to mechanical signals.

The impact of host diet on Wolbachia titer in Drosophila

While a number of studies have identified host factors that influence endosymbiont titer, little is known concerning environmental influences on titer. Here we examined nutrient impact on maternally transmitted Wolbachia endosymbionts in Drosophila. We demonstrate that Drosophila reared on sucrose- and yeast-enriched diets exhibit increased and reduced Wolbachia titers in oogenesis, respectively. The yeast-induced Wolbachia depletion is mediated in large part by the somatic TOR and insulin signaling pathways. Disrupting TORC1 with the small molecule rapamycin dramatically increases oocyte Wolbachia titer, whereas hyper-activating somatic TORC1 suppresses oocyte titer. Furthermore, genetic ablation of insulin-producing cells located in the Drosophila brain abolished the yeast impact on oocyte titer. Exposure to yeast-enriched diets altered Wolbachia nucleoid morphology in oogenesis. Furthermore, dietary yeast increased somatic Wolbachia titer overall, though not in the central nervous system. These findings highlight the interactions between Wolbachia and germline cells as strongly nutrient-sensitive, and implicate conserved host signaling pathways by which nutrients influence Wolbachia titer.

Mechanism of UCH-L5 activation and inhibition by DEUBAD domains in RPN13 and INO80G

Deubiquitinating enzymes (DUBs) control vital processes in eukaryotes by hydrolyzing ubiquitin adducts. Their activities are tightly regulated, but the mechanisms remain elusive. In particular, the DUB UCH-L5 can be either activated or inhibited by conserved regulatory proteins RPN13 and INO80G, respectively. Here we show how the DEUBAD domain in RPN13 activates UCH-L5 by positioning its C-terminal ULD domain and crossover loop to promote substrate binding and catalysis. The related DEUBAD domain in INO80G inhibits UCH-L5 by exploiting similar structural elements in UCH-L5 to promote a radically different conformation, and employs molecular mimicry to block ubiquitin docking. In this process, large conformational changes create small but highly specific interfaces that mediate activity modulation of UCH-L5 by altering the affinity for substrates. Our results establish how related domains can exploit enzyme conformational plasticity to allosterically regulate DUB activity. These allosteric sites may present novel insights for pharmaceutical intervention in DUB activity.

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The RPN13 DEUBAD domain activates UCH-L5 by positioning its CL and ULD domain

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The INO80G DEUBAD domain inhibits UCH-L5 by blocking ubiquitin binding

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The FRF hairpin in the DEUBAD domain of INO80G drives UCH-L5 inhibition

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DEUBAD domains regulate UCH-L5 activity by tuning UCH-L5 substrate affinity

20S proteasome activation promotes life span extension and resistance to proteotoxicity in Caenorhabditis elegans

Protein homeostasis (proteostasis) is one of the nodal points that need to be preserved to retain physiologic cellular/organismal balance. The ubiquitin-proteasome system (UPS) is responsible for the removal of both normal and damaged proteins, with the proteasome being the downstream effector. The proteasome is the major cellular protease with progressive impairment of function during aging and senescence. Despite the documented age-retarding properties of proteasome activation in various cellular models, simultaneous enhancement of the 20S core proteasome content, assembly, and function have never been reported in any multicellular organism. Consequently, the possible effects of the core proteasome modulation on organismal life span are elusive. In this study, we have achieved activation of the 20S proteasome at organismal level. We demonstrate enhancement of proteasome levels, assembly, and activity in the nematode Caenorhabditis elegans, resulting in life span extension and increased resistance to stress. We also provide evidence that the observed life span extension is dependent on the transcriptional activity of Dauer formation abnormal/Forkhead box class O (DAF-16/FOXO), skinhead-1 (SKN-1), and heat shock factor-1 (HSF-1) factors through regulation of downstream longevity genes. We further show that the reported beneficial effects are not ubiquitous but they are dependent on the genetic context. Finally, we provide evidence that proteasome core activation might be a potential strategy to minimize protein homeostasis deficiencies underlying aggregation-related diseases, such as Alzheimer's disease (AD) or Huntington's disease (HD). In summary, this is the first report demonstrating that 20S core proteasome up-regulation in terms of both content and activity is feasible in a multicellular eukaryotic organism and that in turn this modulation promotes extension of organismal health span and life span.

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.