EGF signalling activates the ubiquitin proteasome system to modulate C. elegans lifespan

Dopamine signaling promotes the xenobiotic stress response and protein homeostasis

Multicellular organisms encounter environmental conditions that adversely affect protein homeostasis (proteostasis), including extreme temperatures, toxins, and pathogens. It is unclear how they use sensory signaling to detect adverse conditions and then activate stress response pathways so as to offset potential damage. Here, we show that dopaminergic mechanosensory neurons in C. elegans release the neurohormone dopamine to promote proteostasis in epithelia. Signaling through the DA receptor DOP-1 activates the expression of xenobiotic stress response genes involved in pathogenic resistance and toxin removal, and these genes are required for the removal of unstable proteins in epithelia. Exposure to a bacterial pathogen (Pseudomonas aeruginosa) results in elevated removal of unstable proteins in epithelia, and this enhancement requires DA signaling. In the absence of DA signaling, nematodes show increased sensitivity to pathogenic bacteria and heat-shock stress. Our results suggest that dopaminergic sensory neurons, in addition to slowing down locomotion upon sensing a potential bacterial feeding source, also signal to frontline epithelia to activate the xenobiotic stress response so as to maintain proteostasis and prepare for possible infection.

Lifespan-regulating genes in C. elegans

The molecular mechanisms underlying the aging process have garnered much attention in recent decades because aging is the most significant risk factor for many chronic diseases such as type 2 diabetes and cancer. Until recently, the aging process was not considered to be an actively regulated process; therefore, discovering that the insulin/insulin-like growth factor-1 signaling pathway is a lifespan-regulating genetic pathway in Caenorhabditis elegans was a major breakthrough that changed our understanding of the aging process. Currently, it is thought that animal lifespans are influenced by genetic and environmental factors. The genes involved in lifespan regulation are often associated with major signaling pathways that link the rate of aging to environmental factors. Although many of the major mechanisms governing the aging process have been identified from studies in short-lived model organisms such as yeasts, worms and flies, the same mechanisms are frequently observed in mammals, indicating that the genes and signaling pathways that regulate lifespan are highly conserved among different species. This review summarizes the lifespan-regulating genes, with a specific focus on studies in C. elegans.

Regulation of 26S Proteasome Activity in Pulmonary Fibrosis

RATIONALE

The ubiquitin-proteasome system is critical for maintenance of protein homeostasis by degrading polyubiquitinated proteins in a spatially and temporally controlled manner. Cell and protein homeostasis are altered upon pathological tissue remodeling. Dysregulation of the proteasome has been reported for several chronic diseases of the heart, brain, and lung. We hypothesized that proteasome function is altered upon fibrotic lung remodeling, thereby contributing to the pathogenesis of idiopathic pulmonary fibrosis (IPF).

OBJECTIVES

To investigate proteasome function during myofibroblast differentiation.

METHODS

We treated lung fibroblasts with transforming growth factor (TGF)-β and examined proteasome composition and activity. For in vivo analysis, we used mouse models of lung fibrosis and fibrotic human lung tissue.

MEASUREMENTS AND MAIN RESULTS

We demonstrate that induction of myofibroblast differentiation by TGF-β involves activation of the 26S proteasome, which is critically dependent on the regulatory subunit Rpn6. Silencing of Rpn6 in primary human lung fibroblasts counteracted TGF-β-induced myofibroblast differentiation. Activation of the 26S proteasome and increased expression of Rpn6 were detected during bleomycin-induced lung remodeling and fibrosis. Importantly, Rpn6 is overexpressed in myofibroblasts and basal cells of the bronchiolar epithelium in lungs of patients with IPF, which is accompanied by enhanced protein polyubiquitination.

CONCLUSIONS

We identified Rpn6-dependent 26S proteasome activation as an essential feature of myofibroblast differentiation in vitro and in vivo, and our results suggest it has an important role in IPF pathogenesis.

Evolutionarily conserved autoregulation of alternative pre-mRNA splicing by ribosomal protein L10a

Alternative splicing of pre-mRNAs can regulate expression of protein-coding genes by generating unproductive mRNAs rapidly degraded by nonsense-mediated mRNA decay (NMD). Many of the genes directly regulated by alternative splicing coupled with NMD (AS-NMD) are related to RNA metabolism, but the repertoire of genes regulated by AS-NMD in vivo is to be determined. Here, we analyzed transcriptome data of wild-type and NMD-defective mutant strains of the nematode worm Caenorhabditis elegans and demonstrate that eight of the 82 cytoplasmic ribosomal protein (rp) genes generate unproductively spliced mRNAs. Knockdown of any of the eight rp genes exerted a dynamic and compensatory effect on alternative splicing of its own transcript and inverse effects on that of the other rp genes. A large subunit protein L10a, termed RPL-1 in nematodes, directly and specifically binds to an evolutionarily conserved 39-nt stretch termed L10ARE between the two alternative 5′ splice sites in its own pre-mRNA to switch the splice site choice. Furthermore, L10ARE-mediated splicing autoregulation of the L10a-coding gene is conserved in vertebrates. These results indicate that L10a is an evolutionarily conserved splicing regulator and that homeostasis of a subset of the rp genes are regulated at the level of pre-mRNA splicing in vivo.

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.

Naturally Occurring Isothiocyanates Exert Anticancer Effects by Inhibiting Deubiquitinating Enzymes

The anticancer properties of cruciferous vegetables are well known and attributed to an abundance of isothiocyanates such as benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC). While many potential targets of isothiocyanates have been proposed, a full understanding of the mechanisms underlying their anticancer activity has remained elusive. Here we report that BITC and PEITC effectively inhibit deubiquitinating enzymes (DUB), including the enzymes USP9x and UCH37, which are associated with tumorigenesis, at physiologically relevant concentrations and time scales. USP9x protects the antiapoptotic protein Mcl-1 from degradation, and cells dependent on Mcl-1 were especially sensitive to BITC and PEITC. These isothiocyanates increased Mcl-1 ubiquitination and either isothiocyanate treatment, or RNAi-mediated silencing of USP9x decreased Mcl-1 levels, consistent with the notion that USP9x is a primary target of isothiocyanate activity. These isothiocyanates also increased ubiquitination of the oncogenic fusion protein Bcr-Abl, resulting in degradation under low isothiocyanate concentrations and aggregation under high isothiocyanate concentrations. USP9x inhibition paralleled the decrease in Bcr-Abl levels induced by isothiocyanate treatment, and USP9x silencing was sufficient to decrease Bcr-Abl levels, further suggesting that Bcr-Abl is a USP9x substrate. Overall, our findings suggest that USP9x targeting is critical to the mechanism underpinning the well-established anticancer activity of isothiocyanate. We propose that the isothiocyanate-induced inhibition of DUBs may also explain how isothiocyanates affect inflammatory and DNA repair processes, thus offering a unifying theme in understanding the function and useful application of isothiocyanates to treat cancer as well as a variety of other pathologic conditions.

The Four Layers of Aging

Instead of considering aging in terms of discrete hallmarks, we suggest that it operates in four layers, each at a different biological scale. Malfunctions within each layer-and connections between them-produce the aged phenotype and its associated susceptibility to disease.

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.

Genetics and pharmacology of longevity: the road to therapeutics for healthy aging

Aging can be defined as the progressive decline in tissue and organismal function and the ability to respond to stress that occurs in association with homeostatic failure and the accumulation of molecular damage. Aging is the biggest risk factor for human disease and results in a wide range of aging pathologies. Although we do not completely understand the underlying molecular basis that drives the aging process, we have gained exceptional insights into the plasticity of life span and healthspan from the use of model organisms such as the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Single-gene mutations in key cellular pathways that regulate environmental sensing, and the response to stress, have been identified that prolong life span across evolution from yeast to mammals. These genetic manipulations also correlate with a delay in the onset of tissue and organismal dysfunction. While the molecular genetics of aging will remain a prosperous and attractive area of research in biogerontology, we are moving towards an era defined by the search for therapeutic drugs that promote healthy aging. Translational biogerontology will require incorporation of both therapeutic and pharmacological concepts. The use of model organisms will remain central to the quest for drug discovery, but as we uncover molecular processes regulated by repurposed drugs and polypharmacy, studies of pharmacodynamics and pharmacokinetics, drug-drug interactions, drug toxicity, and therapeutic index will slowly become more prevalent in aging research. As we move from genetics to pharmacology and therapeutics, studies will not only require demonstration of life span extension and an underlying molecular mechanism, but also the translational relevance for human health and disease prevention.

Current aging research in China

The mini-review stemmed from a recent meeting on national aging research strategies in China discusses the components and challenges of aging research in China. Highlighted are the major efforts of a number of research teams, funding situations and outstanding examples of recent major research achievements. Finally, authors discuss potential targets and strategies of aging research in China.

The biology of proteostasis in aging and disease

Loss of protein homeostasis (proteostasis) is a common feature of aging and disease that is characterized by the appearance of nonnative protein aggregates in various tissues. Protein aggregation is routinely suppressed by the proteostasis network (PN), a collection of macromolecular machines that operate in diverse ways to maintain proteome integrity across subcellular compartments and between tissues to ensure a healthy life span. Here, we review the composition, function, and organizational properties of the PN in the context of individual cells and entire organisms and discuss the mechanisms by which disruption of the PN, and related stress response pathways, contributes to the initiation and progression of disease. We explore emerging evidence that disease susceptibility arises from early changes in the composition and activity of the PN and propose that a more complete understanding of the temporal and spatial properties of the PN will enhance our ability to develop effective treatments for protein conformational diseases.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

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.

Royalactin extends lifespan of Caenorhabditis elegans through epidermal growth factor signaling

Royalactin is a glycoprotein essential for the development of long-lived queen honeybees. Only larvae fed with royal jelly, containing royalactin, develop into queens. Royalactin plays a central role in this process by switching on the epidermal growth factor (EGF) receptor signaling pathway which ultimately leads to epigenetic changes and a long-lived queen phenotype. Recently it was shown that royalactin by itself also extends lifespan in Drosophila melanogaster. Yet, the mechanism by which royalactin promotes longevity remains largely unknown. We set out to characterize the effects of royalactin on Caenorhabditis elegans lifespan, and clarify the possible involvement of EGF signaling in this process. We demonstrate that royalactin extends lifespan of this nematode and that both EGF (LIN-3) and its receptor (LET-23) are essential to this process. To our knowledge, this is the first report of royalactin-mediated lifespan extension in a non-insect species. Additionally, we show that royalactin enhances locomotion in adult nematodes, implying that royalactin also influences healthspan. Our results suggest that royalactin is an important lifespan-extending factor in royal jelly and acts by promoting EGF signaling in C. elegans. Further work will now be needed to clarify which (secondary) signaling pathways are activated by royalactin, and how this ultimately translates into an extended health- and lifespan.

Epidermal growth factor/epidermal growth factor receptor signaling axis is a significant regulator of the proteasome expression and activity in colon cancer cells

Colon cancer is the third most common type of cancer worldwide. Epidermal growth factor receptor (EGFR) plays a crucial role in the (patho)physiology of the disease. EGFR controls vital cellular processes, while this action is associated with poor prognosis. In addition, K-Ras mutations are associated with the promotion of the disease and the anti-EGFR resistance. The ubiquitin-proteasome system plays also a very important role in cancer, modulating cell cycle and other cellular processes such as the growth and the survival of cancer cells. Proteasome inhibition affects, in several cases, the action and the protein levels of EGFR. Nevertheless, little is known whether the reversed option is possible. In this study, we, therefore, investigated the impact of epidermal growth factor (EGF)/EGFR signaling axis on gene expression and the proteolytic activity of the proteasome subunits, as well as whether Nrf2, an activator of proteasome expression, plays a role in this process. Moreover, we evaluated whether EGF regulates the expression of its own receptor and the proliferation rate of DLD-1 (K-Ras mutated) colon cancer cells. The obtained data showed that, although EGF has no significant effect on the proliferation of DLD-1 colon cancer cells, it significantly upregulates the expression of EGFR as well as the expression and the activity of the proteasome, suggesting that the EGF-mediated proteasome activation could possibly lead to enhanced EGFR degradation leading to autoregulation of EGF–EGFR pathway. Nrf2 activation did not induce proteasome gene expression in DLD-1 colon cancer cells.

Autophagy protects C. elegans against necrosis during Pseudomonas aeruginosa infection

Autophagy, a conserved pathway that delivers intracellular materials into lysosomes for degradation, is involved in development, aging, and a variety of diseases. Accumulating evidence demonstrates that autophagy plays a protective role against infectious diseases by diminishing intracellular pathogens, including bacteria, viruses, and parasites. However, the mechanism by which autophagy regulates innate immunity remains largely unknown. Here, we show that autophagy is involved in host defense against a pathogenic bacterium Pseudomonas aeruginosa in the metazoan Caenorhabditis elegans. P. aeruginosa infection induces autophagy via a conserved extracellular signal-regulated kinase (ERK). Intriguingly, impairment of autophagy does not influence the intestinal accumulation of P. aeruginosa, but instead induces intestinal necrosis. Inhibition of necrosis results in the survival of autophagy-deficient worms after P. aeruginosa infection. These findings reveal a previously unidentified role for autophagy in protection against necrosis triggered by pathogenic bacteria in C. elegans and implicate that such a function of autophagy may be conserved through the inflammatory response in diverse organisms.

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.

EGF/EGFR signaling axis is a significant regulator of the proteasome expression and activity in colon cancer cells

Colon cancer is the third most common type of cancer worldwide. Epidermal growth factor receptor (EGFR) plays a crucial role in the (patho)physiology of the disease. EGFR controls vital cellular processes, while this action is associated with poor prognosis. In addition, K-Ras mutations are associated with the promotion of the disease and the anti-EGFR resistance. The ubiquitin-proteasome system plays also a very important role in cancer, modulating cell cycle and other cellular processes such as the growth and the survival of cancer cells. Proteasome inhibition affects, in several cases, the action and the protein levels of EGFR. Nevertheless, little is known whether the reversed option is possible. In this study, we, therefore, investigated the impact of epidermal growth factor (EGF)/EGFR signaling axis on gene expression and the proteolytic activity of the proteasome subunits, as well as whether Nrf2, an activator of proteasome expression, plays a role in this process. Moreover, we evaluated whether EGF regulates the expression of its own receptor and the proliferation rate of DLD-1 (K-Ras mutated) colon cancer cells. The obtained data showed that, although EGF has no significant effect on the proliferation of DLD-1 colon cancer cells, it significantly upregulates the expression of EGFR as well as the expression and the activity of the proteasome, suggesting that the EGF-mediated proteasome activation could possibly lead to enhanced EGFR degradation leading to autoregulation of EGF–EGFR pathway. Nrf2 activation did not induce proteasome gene expression in DLD-1 colon cancer cells.

Remodeling of Proteostasis Upon Transition to Adulthood is Linked to Reproduction Onset

Protein folding and clearance networks sense and respond to misfolded and aggregation-prone proteins by activating cytoprotective cell stress responses that safeguard the proteome against damage, maintain the health of the cell, and enhance lifespan. Surprisingly, cellular proteostasis undergoes a sudden and widespread failure early in Caenorhabditis elegans adulthood, with marked consequences on proteostasis functions later in life. These changes in the regulation of quality control systems, such as chaperones, the ubiquitin proteasome system and cellular stress responses, are controlled cell-nonautonomously by the proliferation of germline stem cells. Here, we review recent studies examining changes in proteostasis upon transition to adulthood and how proteostasis is modulated by reproduction onset, focusing on C. elegans. Based on these and our own findings, we propose that the regulation of quality control systems is actively remodeled at the point of transition between development and adulthood to influence the subsequent course of aging.

Pathogenesis of human mitochondrial diseases is modulated by reduced activity of the ubiquitin/proteasome system

Mitochondria maintain cellular homeostasis by coordinating ATP synthesis with metabolic activity, redox signaling, and apoptosis. Excessive levels of mitochondria-derived reactive oxygen species (ROS) promote mitochondrial dysfunction, triggering numerous metabolic disorders. However, the molecular basis for the harmful effects of excessive ROS formation is largely unknown. Here, we identify a link between mitochondrial stress and ubiquitin-dependent proteolysis, which supports cellular surveillance both in Caenorhabditis elegans and humans. Worms defective in respiration with elevated ROS levels are limited in turnover of a GFP-based substrate protein, demonstrating that mitochondrial stress affects the ubiquitin/proteasome system (UPS). Intriguingly, we observed similar proteolytic defects for disease-causing IVD and COX1 mutations associated with mitochondrial failure in humans. Together, these results identify a conserved link between mitochondrial metabolism and ubiquitin-dependent proteostasis. Reduced UPS activity during pathological conditions might potentiate disease progression and thus provides a valuable target for therapeutic intervention.

The Mechanistic Links Between Proteasome Activity, Aging and Age-related Diseases

Damaged and misfolded proteins accumulate during the aging process, impairing cell function and tissue homeostasis. These perturbations to protein homeostasis (proteostasis) are hallmarks of age-related neurodegenerative disorders such as Alzheimer’s, Parkinson’s or Huntington’s disease. Damaged proteins are degraded by cellular clearance mechanisms such as the proteasome, a key component of the proteostasis network. Proteasome activity declines during aging, and proteasomal dysfunction is associated with late-onset disorders. Modulation of proteasome activity extends lifespan and protects organisms from symptoms associated with proteostasis disorders. Here we review the links between proteasome activity, aging and neurodegeneration. Additionally, strategies to modulate proteasome activity and delay the onset of diseases associated to proteasomal dysfunction are discussed herein.

Regulation of PSMB5 protein and β subunits of mammalian proteasome by constitutively activated signal transducer and activator of transcription 3 (STAT3): potential role in bortezomib-mediated anticancer therapy

The ubiquitin-proteasome system facilitates the degradation of ubiquitin-tagged proteins and performs a regulatory role in cells. Elevated proteasome activity and subunit expression are found in several cancers. However, the inherent molecular mechanisms responsible for increased proteasome function in cancers remain unclear despite the well investigated and defined role of the mammalian proteasome. This study was initiated to elucidate the mechanisms involved in the regulation of β subunits of the mammalian proteasome. Suppression of STAT3 tyrosine phosphorylation coordinately decreased the mRNA and protein levels of the β subunits of the 20 S core complex in DU145 cells. Notably, PSMB5, a molecular target of bortezomib, was shown to be a target of STAT3. Knockdown of STAT3 decreased PSMB5 protein. Inhibition of phospho-STAT3 substantially reduced PSMB5 protein levels in cells expressing constitutively active-STAT3. Accumulation of activated STAT3 resulted in the induction of PSMB5 promoter and protein levels. In addition, a direct correlation was observed between the endogenous levels of PSMB5 and constitutively active STAT3. PSMB5 and STAT3 protein levels remained unaltered following the inhibition of proteasome activity. The EGF-induced concerted increase of β subunits was blocked by inhibition of the EGF receptor or STAT3 but not by the PI3K/AKT or MEK/ERK pathways. Decreased proteasome activities were due to reduced protein levels of catalytic subunits of the proteasome in STAT3-inhibited cells. Combined treatments with bortezomib and inhibitor of STAT3 abrogated proteasome activity and enhanced cellular apoptosis. Overall, we demonstrate that aberrant activation of STAT3 regulates the expression of β subunits, in particular PSMB5, and the catalytic activity of the proteasome.

Proteostasis and longevity: when does aging really begin?

Aging is a complex process regulated by multiple cellular pathways, including the proteostasis network. The proteostasis network consists of molecular chaperones, stress-response transcription factors, and protein degradation machines that sense and respond to proteotoxic stress and protein misfolding to ensure cell viability. A loss of proteostasis is associated with aging and age-related disorders in diverse model systems, moreover, genetic or pharmacological enhancement of the proteostasis network has been shown to extend lifespan and suppress age-related disease. However, our understanding of the relationship between aging, proteostasis, and the proteostasis network remains unclear. Here, we propose, from studies in Caenorhabditis elegans, that proteostasis collapse is not gradual but rather a sudden and early life event that triggers proteome mismanagement, thereby affecting a multitude of downstream processes. Furthermore, we propose that this phenomenon is not stochastic but is instead a programmed re-modeling of the proteostasis network that may be conserved in other species. As such, we postulate that changes in the proteostasis network may be one of the earliest events dictating healthy aging in metazoans.

Fluorodeoxyuridine improves Caenorhabditis elegans proteostasis independent of reproduction onset

Protein homeostasis (proteostasis) networks are dynamic throughout the lifespan of an organism. During Caenorhabditis elegans adulthood, the maintenance of metastable proteins and the activation of stress responses are inversely associated with germline stem cell proliferation. Here, we employed the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine (FUdR) to chemically inhibit reproduction, thus allowing for examination of the interplay between reproduction and somatic proteostasis. We found that treatment with FUdR modulates proteostasis decline both before and after reproduction onset, such that effective induction of the heat shock response was maintained during adulthood and that metastable temperature-sensitive mutant phenotypes were rescued under restrictive conditions. However, FUdR treatment also improved the folding capacity of germline- and gonadogenesis-defective mutants, suggesting that proteostasis modulation by FUdR is independent of germline stem cell proliferation or inhibition of reproduction. Our data, therefore, indicate that FUdR converges on alternative regulatory signals that modulate C. elegans proteostasis capacity during development and adulthood.

The ubiquitin proteasome system in Caenorhabditis elegans and its regulation

Protein degradation constitutes a major cellular function that is responsible for maintenance of the normal cellular physiology either through the degradation of normal proteins or through the elimination of damaged proteins. The Ubiquitin–Proteasome System (UPS)¹ is one of the main proteolytic systems that orchestrate protein degradation. Given that up- and down- regulation of the UPS system has been shown to occur in various normal (such as ageing) and pathological (such as neurodegenerative diseases) processes, the exogenous modulation of the UPS function and activity holds promise of (a) developing new therapeutic interventions against various diseases and (b) establishing strategies to maintain cellular homeostasis. Since the proteasome genes are evolutionarily conserved, their role can be dissected in simple model organisms, such as the nematode, Caenorhabditis elegans. In this review, we survey findings on the redox regulation of the UPS in C. elegans showing that the nematode is an instrumental tool in the identification of major players in the UPS pathway. Moreover, we specifically discuss UPS-related genes that have been modulated in the nematode and in human cells and have resulted in similar effects thus further exhibiting the value of this model in the study of the UPS.

•

UPS is one of the main proteolytic systems that orchestrate protein degradation.

•

Proteasome function can be dissected in Caenorhabditis elegans.

•

Nematodes can be used in the identification of major players in the UPS pathway.

Germline stem cell arrest inhibits the collapse of somatic proteostasis early in Caenorhabditis elegans adulthood

All cells rely on highly conserved protein folding and clearance pathways to detect and resolve protein damage and to maintain protein homeostasis (proteostasis). Because age is associated with an imbalance in proteostasis, there is a need to understand how protein folding is regulated in a multicellular organism that undergoes aging. We have observed that the ability of Caenorhabditis elegans to maintain proteostasis declines sharply following the onset of oocyte biomass production, suggesting that a restricted protein folding capacity may be linked to the onset of reproduction. To test this hypothesis, we monitored the effects of different sterile mutations on the maintenance of proteostasis in the soma of C. elegans. We found that germline stem cell (GSC) arrest rescued protein quality control, resulting in maintenance of robust proteostasis in different somatic tissues of adult animals. We further demonstrated that GSC-dependent modulation of proteostasis requires several different signaling pathways, including hsf-1 and daf-16/kri-1/tcer-1, daf-12, daf-9, daf-36, nhr-80, and pha-4 that differentially modulate somatic quality control functions, such that each signaling pathway affects different aspects of proteostasis and cannot functionally complement the other pathways. We propose that the effect of GSCs on the collapse of proteostasis at the transition to adulthood is due to a switch mechanism that links GSC status with maintenance of somatic proteostasis via regulation of the expression and function of different quality control machineries and cellular stress responses that progressively lead to a decline in the maintenance of proteostasis in adulthood, thereby linking reproduction to the maintenance of the soma.

Toward a unified model of developmental timing: A "molting" approach

Animal development requires temporal coordination between recurrent processes and sequential events, but the underlying timing mechanisms are not yet understood. The molting cycle of C. elegans provides an ideal system to study this basic problem. We recently characterized LIN-42, which is related to the circadian clock protein PERIOD, as a key component of the developmental timer underlying rhythmic molting cycles. In this context, LIN-42 coordinates epithelial stem cell dynamics with progression of the molting cycle. Repeated actions of LIN-42 may enable the reprogramming of seam cell temporal fates, while stage-specific actions of LIN-42 and other heterochronic genes select fates appropriate for upcoming, rather than passing, life stages. Here, we discuss the possible configuration of the molting timer, which may include interconnected positive and negative regulatory loops among lin-42, conserved nuclear hormone receptors such as NHR-23 and -25, and the let-7 family of microRNAs. Physiological and environmental conditions may modulate the activities of particular components of this molting timer. Finding that LIN-42 regulates both a sleep-like behavioral state and epidermal stem cell dynamics further supports the model of functional conservation between LIN-42 and mammalian PERIOD proteins. The molting timer may therefore represent a primitive form of a central biological clock and provide a general paradigm for the integration of rhythmic and developmental processes.

The genome of the hydatid tapeworm Echinococcus granulosus

Cystic echinococcosis (hydatid disease), caused by the tapeworm E. granulosus, is responsible for considerable human morbidity and mortality. This cosmopolitan disease is difficult to diagnose, treat and control. We present a draft genomic sequence for the worm comprising 151.6 Mb encoding 11,325 genes. Comparisons with the genome sequences from other taxa show that E. granulosus has acquired a spectrum of genes, including the EgAgB family, whose products are secreted by the parasite to interact and redirect host immune responses. We also find that genes in bile salt pathways may control the bidirectional development of E. granulosus, and sequence differences in the calcium channel subunit EgCavβ1 may be associated with praziquantel sensitivity. Our study offers insights into host interaction, nutrient acquisition, strobilization, reproduction, immune evasion and maturation in the parasite and provides a platform to facilitate the development of new, effective treatments and interventions for echinococcosis control.

Canonical RTK-Ras-ERK signaling and related alternative pathways

Receptor Tyrosine Kinase (RTK)-Ras-Extracellular signal-regulated kinase (ERK) signaling pathways control many aspects of C. elegans development and behavior. Studies in C. elegans helped elucidate the basic framework of the RTK-Ras-ERK pathway and continue to provide insights into its complex regulation, its biological roles, how it elicits cell-type appropriate responses, and how it interacts with other signaling pathways to do so. C. elegans studies have also revealed biological contexts in which alternative RTK- or Ras-dependent pathways are used instead of the canonical pathway.

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

The proteasome plays an important role in proteostasis by carrying out controlled protein degradation in the cell. Impairments in proteasome function are associated with severe and often age-related diseases. Here, we have characterized a molecular mechanism linking insulin/IGF-1 signaling (IIS) to proteasome activity. We show that decreased IIS, which promotes proteostasis and longevity, increases proteasome activity through the FOXO transcription factor DAF-16 in C. elegans. Furthermore, we reveal that DAF-16 represses expression of the proteasome-associated deubiquitinating enzyme ubh-4, which we suggest functions as a tissue-specific proteasome inhibitor. Finally, we demonstrate that proteasome activation through downregulation of the ubh-4 human ortholog uchl5 increases degradation of proteotoxic proteins in mammalian cells. In conclusion, we have established a mechanism by which the evolutionarily conserved IIS contributes to the regulation of proteasome activity in a multicellular organism.

The hallmarks of aging

Aging is characterized by a progressive loss of physiological integrity, leading to impaired function and increased vulnerability to death. This deterioration is the primary risk factor for major human pathologies, including cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. Aging research has experienced an unprecedented advance over recent years, particularly with the discovery that the rate of aging is controlled, at least to some extent, by genetic pathways and biochemical processes conserved in evolution. This Review enumerates nine tentative hallmarks that represent common denominators of aging in different organisms, with special emphasis on mammalian aging. These hallmarks are: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. A major challenge is to dissect the interconnectedness between the candidate hallmarks and their relative contributions to aging, with the final goal of identifying pharmaceutical targets to improve human health during aging, with minimal side effects.

Genome-scale studies of aging: challenges and opportunities

Whole-genome studies involving a phenotype of interest are increasingly prevalent, in part due to a dramatic increase in speed at which many high throughput technologies can be performed coupled to simultaneous decreases in cost. This type of genome-scale methodology has been applied to the phenotype of lifespan, as well as to whole-transcriptome changes during the aging process or in mutants affecting aging. The value of high throughput discovery-based science in this field is clearly evident, but will it yield a true systems-level understanding of the aging process? Here we review some of this work to date, focusing on recent findings and the unanswered puzzles to which they point. In this context, we also discuss recent technological advances and some of the likely future directions that they portend.

Neuronal Cbl controls biosynthesis of insulin-like peptides in Drosophila melanogaster

The Cbl family proteins function as both E3 ubiquitin ligases and adaptor proteins to regulate various cellular signaling events, including the insulin/insulin-like growth factor 1 (IGF1) and epidermal growth factor (EGF) pathways. These pathways play essential roles in growth, development, metabolism, and survival. Here we show that in Drosophila melanogaster, Drosophila Cbl (dCbl) regulates longevity and carbohydrate metabolism through downregulating the production of Drosophila insulin-like peptides (dILPs) in the brain. We found that dCbl was highly expressed in the brain and knockdown of the expression of dCbl specifically in neurons by RNA interference increased sensitivity to oxidative stress or starvation, decreased carbohydrate levels, and shortened life span. Insulin-producing neuron-specific knockdown of dCbl resulted in similar phenotypes. dCbl deficiency in either the brain or insulin-producing cells upregulated the expression of dilp genes, resulting in elevated activation of the dILP pathway, including phosphorylation of Drosophila Akt and Drosophila extracellular signal-regulated kinase (dERK). Genetic interaction analyses revealed that blocking Drosophila epidermal growth factor receptor (dEGFR)-dERK signaling in pan-neurons or insulin-producing cells by overexpressing a dominant-negative form of dEGFR abolished the effect of dCbl deficiency on the upregulation of dilp genes. Furthermore, knockdown of c-Cbl in INS-1 cells, a rat β-cell line, also increased insulin biosynthesis and glucose-stimulated secretion in an ERK-dependent manner. Collectively, these results suggest that neuronal dCbl regulates life span, stress responses, and metabolism by suppressing dILP production and the EGFR-ERK pathway mediates the dCbl action. Cbl suppression of insulin biosynthesis is evolutionarily conserved, raising the possibility that Cbl may similarly exert its physiological actions through regulating insulin production in β cells.

Integrated control of protein degradation in C. elegans muscle

Protein degradation is a fundamental cellular process, the genomic control of which is incompletely understood. The advent of transgene-coded reporter proteins has enabled the development of C. elegans into a model for studying this problem. The regulation of muscle protein degradation is surprisingly complex, integrating multiple signals from hypodermis, intestine, neurons and muscle itself. Within the muscle, degradation is executed by separately regulated autophagy-lysosomal, ubiquitin-proteasome and calpain-mediated systems. The signal-transduction mechanisms, in some instances, involve modules previously identified for their roles in developmental processes, repurposed in terminally differentiated muscle to regulate the activities of pre-formed proteins. Here we review the genes, and mechanisms, which appear to coordinately control protein degradation within C. elegans muscle. We also consider these mechanisms in the context of development, physiology, pathophysiology and disease models.

Epidermal growth factor-induced vacuolar (H+)-atpase assembly: a role in signaling via mTORC1 activation

Using proteomics and immunofluorescence, we demonstrated epidermal growth factor (EGF) induced recruitment of extrinsic V(1) subunits of the vacuolar (H(+))-ATPase (V-ATPase) to rat liver endosomes. This was accompanied by reduced vacuolar pH. Bafilomycin, an inhibitor of V-ATPase, inhibited EGF-stimulated DNA synthesis and mammalian target of rapamycin complex 1 (mTORC1) activation as indicated by a decrease in eukaryotic initiation factor 4E-binding 1 (4E-BP1) phosphorylation and p70 ribosomal S6 protein kinase (p70S6K) phosphorylation and kinase activity. There was no corresponding inhibition of EGF-induced Akt and extracellular signal-regulated kinase (Erk) activation. Chloroquine, a neutralizer of vacuolar pH, mimicked bafilomycin effects. Bafilomycin did not inhibit the association of mTORC1 with Raptor nor did it affect AMP-activated protein kinase activity. Rather, the intracellular concentrations of essential but not non-essential amino acids were decreased by bafilomycin in EGF-treated primary rat hepatocytes. Cycloheximide, a translation elongation inhibitor known to augment intracellular amino acid levels, prevented the effect of bafilomycin on amino acids levels and completely reversed its inhibition of EGF-induced mTORC1 activation. In vivo administration of EGF stimulated the recruitment of Ras homologue enriched in brain (Rheb) but not mammalian target of rapamycin (mTOR) to endosomes and lysosomes. This was inhibited by chloroquine treatment. Our results suggest a role for vacuolar acidification in EGF signaling to mTORC1.

Protein homeostasis and aging: role of ubiquitin protein ligases

Protein homeostasis is fundamental in normal cellular function and cell survival. The ubiquitin-proteasome system (UPS) plays a central role in maintaining the protein homeostasis network through selective elimination of misfolded and damaged proteins. Impaired function of UPS is implicated in normal aging process and also in several age-related neurodegenerative disorders that are characterized by increased accumulation oxidatively modified proteins and protein aggregates. Growing literature also indicate the potential role of various ubiquitin protein ligases in the regulation of aging process by enhancing the degradation of either central lifespan regulators or abnormally folded and damaged proteins. This review mainly focuses on our current understanding of the importance of UPS function in the regulation of normal aging process.

Epidermal growth factor and aging: a signaling molecule reveals a new eye opening function

Epidermal Growth Factor (EGF) is known for its role in promoting cell division and cellular differentiation in developing animals, but we know surprising little about what EGF does in vivo in mature adult animals. Here I review EGF signaling, emphasizing several recent studies that uncovered an unexpected role for EGF in promoting longevity and healthspan in mature adult C. elegans. EGF, acting through phospholipase Cγ and the IP3 receptor signaling, maintains pharyngeal and body wall muscle function in aging adults, and delays the accumulation of lipofuscin-enriched aging pigments within intestinal cells. EGF also acts through the Ras/ERK pathway to regulate protein homeostasis by promoting the expression of antioxidant genes, stimulating the activity of the Ubiquitin Proteasome System (UPS), and repressing the expression of small heat shock protein chaperones. The effects of EGF signaling on lifespan are largely independent of Insulin/IGF-like Signaling (IIS), as the effects of EGF signaling mutants on lifespan and heathspan are not affected by mutations in the DAF-2 insulin receptor or the DAF-16 FOXO transcription factor. Nevertheless, these two signal pathways have multiple points of overlap, coordination, and cross regulation. I propose that the IIS and EGF signaling pathways respond to environment and to developmental timing, respectively, so as to coordinate the appropriate physiological strategy that cells use to maintain protein homeostasis.