Characterization of the proteostasis roles of glycerol accumulation, protein degradation and protein synthesis during osmotic stress in C. elegans

An enteric neuron-expressed variant ionotropic receptor detects ingested salts to regulate salt stress resistance

The detection of internal chemicals by interoceptive chemosensory pathways is critical for regulating metabolism and physiology. The molecular identities of interoceptors, and the functional consequences of chemosensation by specific interoceptive neurons remain to be fully described. The C. elegans pharyngeal neuronal network is anatomically and functionally homologous to the mammalian enteric nervous system. Here, we show that the I3 pharyngeal enteric neuron responds to cations via an I3-specific variant ionotropic receptor (IR) to regulate salt stress tolerance. The GLR-9 IR, located at the gut lumen-exposed sensory end of I3, is necessary and sufficient for salt sensation, establishing a chemosensory function for IRs beyond insects. Salt detection by I3 protects specifically against high salt stress, as glr-9 mutants show reduced tolerance of hypertonic salt but not sugar solutions, with or without prior acclimation. While cholinergic signaling from I3 promotes tolerance to acute high salt stress, peptidergic signaling from I3 during acclimation is essential for resistance to a subsequent high salt challenge. Transcriptomic analyses show that I3 regulates salt tolerance in part via regulating the expression of osmotic stress response genes in distal tissues. Our results describe the mechanisms by which chemosensation mediated by a defined enteric neuron regulates physiological homeostasis in response to a specific abiotic stress.

Anti-Aging Effects and Mechanisms of Cod Collagen Peptides (CCPs) in Caenorhabditis elegans

Given the growing interest in natural compounds for promoting healthy aging, this study aimed to investigate the potential of cod collagen peptides (CCPs), a readily available marine resource, to extend lifespan and improve health. Lifespan assays were performed on C. elegans treated with different concentrations of CCPs. Furthermore, various stress resistance assays, including those evaluating oxidative and thermal stress, were conducted. To elucidate the underlying mechanisms, gene expression analysis of key aging-related genes was performed. The results demonstrated that treatment with 25 mg/mL of CCPs extended the lifespan of C. elegans by 13.2%, increased body length and width by 14.8% and 20.6%, respectively, and enhanced head-swing and body-bending frequencies by 66.9% and 80.4%. Lipofuscin content and apoptosis were reduced by 45.9% and 34.1%, respectively. C. elegans treated with 25 mg/mL of CCPs also showed improved stress resistance, a 90.7% increase in glutathione peroxidase (GPX) activity, and a 147.4% increase in glutathione (GSH) content. Transcriptomic analysis showed that CCPs enhanced anti-aging activity by activating the MAPK pathway and inhibiting the IIS pathway, which was associated with protein aggregation. It also reduced lipid synthesis and regulated lipid metabolism through the fat-6 pathway. The results indicated that CCPs could be employed as a valuable ingredient in the food and pharmaceutical fields.

Structural and physiological functions of Caenorhabditis elegans epidermis

Research on the skin is continuously evolving, and it is imperative to select a streamlined and efficient research model. Caenorhabditis elegans is a free-leaving nematode whose epidermis serves as the primary barrier epithelium, composed of a collagen matrix. Differentiation of the epidermis begins in the middle of embryonic development, including polarization of the cytoskeleton and formation of cell junctions. Cuticle secretion is one of the main developmental and physiological features of the epidermis. Mutations in the collagen genes of individual worms lead to cuticle defects, thereby changing the shape of the animals. The complete genome sequence of C. elegans indicates that more than 170 different collagen genes may be related to this structure. Collagen is a structural protein that plays an important role in the development of extracellular matrix. Different collagen genes are expressed at different stages of matrix synthesis, which may help form specific interactions between different collagens. The differentiated epidermis also plays a key role in the transmission of hormonal signals, fat storage, and ion homeostasis and is closely related to the development and function of the nervous system. The epidermis also provides passive and active defenses against pathogens that penetrate the skin and can repair wounds. In addition, age-dependent epidermal degeneration is a prominent feature of aging and may affect aging and lifespan. This review we highlight recent findings of the structure and related physiological functions of the cuticle of C. elegans. In contrast to previous studies, we offer novel insights into the utilization of C. elegans as valuable models for skin-related investigations. It also encourages the use of C. elegans as a skin model, and its high-throughput screening properties facilitate the acceleration of fundamental research in skin-related diseases.

Regulation of the hypertonic stress response by the 3' mRNA cleavage and polyadenylation complex

Maintenance of osmotic homeostasis is one of the most aggressively defended homeostatic set points in physiology. One major mechanism of osmotic homeostasis involves the upregulation of proteins that catalyze the accumulation of solutes called organic osmolytes. To better understand how osmolyte accumulation proteins are regulated, we conducted a forward genetic screen in Caenorhabditis elegans for mutants with no induction of osmolyte biosynthesis gene expression (Nio mutants). The nio-3 mutant encoded a missense mutation in cpf-2/CstF64, while the nio-7 mutant encoded a missense mutation in symk-1/Symplekin. Both cpf-2 and symk-1 are nuclear components of the highly conserved 3' mRNA cleavage and polyadenylation complex. cpf-2 and symk-1 block the hypertonic induction of gpdh-1 and other osmotically induced mRNAs, suggesting they act at the transcriptional level. We generated a functional auxin-inducible degron (AID) allele for symk-1 and found that acute, post-developmental degradation in the intestine and hypodermis was sufficient to cause the Nio phenotype. symk-1 and cpf-2 exhibit genetic interactions that strongly suggest they function through alterations in 3' mRNA cleavage and/or alternative polyadenylation. Consistent with this hypothesis, we find that inhibition of several other components of the mRNA cleavage complex also cause a Nio phenotype. cpf-2 and symk-1 specifically affect the osmotic stress response since heat shock-induced upregulation of a hsp-16.2::GFP reporter is normal in these mutants. Our data suggest a model in which alternative polyadenylation of 1 or more mRNAs is essential to regulate the hypertonic stress response.

Increased alcohol dehydrogenase 1 activity promotes longevity

Several molecules can extend healthspan and lifespan across organisms. However, most are upstream signaling hubs or transcription factors orchestrating complex anti-aging programs. Therefore, these molecules point to but do not reveal the fundamental mechanisms driving longevity. Instead, downstream effectors that are necessary and sufficient to promote longevity across conditions or organisms may reveal the fundamental anti-aging drivers. Toward this goal, we searched for effectors acting downstream of the transcription factor EB (TFEB), known as HLH-30 in C. elegans, because TFEB/HLH-30 is necessary across anti-aging interventions and its overexpression is sufficient to extend C. elegans lifespan and reduce biomarkers of aging in mammals including humans. As a result, we present an alcohol-dehydrogenase-mediated anti-aging response (AMAR) that is essential for C. elegans longevity driven by HLH-30 overexpression, caloric restriction, mTOR inhibition, and insulin-signaling deficiency. The sole overexpression of ADH-1 is sufficient to activate AMAR, which extends healthspan and lifespan by reducing the levels of glycerol-an age-associated and aging-promoting alcohol. Adh1 overexpression is also sufficient to promote longevity in yeast, and adh-1 orthologs are induced in calorically restricted mice and humans, hinting at ADH-1 acting as an anti-aging effector across phyla.

Disexcitation in the ASH/RIM/ADL negative feedback circuit fine-tunes hyperosmotic sensation and avoidance in Caenorhabditis elegans

Sensations, especially nociception, are tightly controlled and regulated by the central and peripheral nervous systems. Osmotic sensation and related physiological and behavioral reactions are essential for animal well-being and survival. In this study, we find that interaction between secondary nociceptive ADL and primary nociceptive ASH neurons upregulates Caenorhabditis elegans avoidance of the mild and medium hyperosmolality of 0.41 and 0.88 Osm but does not affect avoidance of high osmolality of 1.37 and 2.29 Osm. The interaction between ASH and ADL is actualized through a negative feedback circuit consisting of ASH, ADL, and RIM interneurons. In this circuit, hyperosmolality-sensitive ADL augments the ASH hyperosmotic response and animal hyperosmotic avoidance; RIM inhibits ADL and is excited by ASH; thus, ASH exciting RIM reduces ADL augmenting ASH. The neuronal signal integration modality in the circuit is disexcitation. In addition, ASH promotes hyperosmotic avoidance through ASH/RIC/AIY feedforward circuit. Finally, we find that in addition to ASH and ADL, multiple sensory neurons are involved in hyperosmotic sensation and avoidance behavior.

RNAi screening for modulators of an osmo-sensitive gene response to extracellular matrix damage reveals negative feedback and interactions with translation inhibition

In epidermal tissues, extracellular matrices (ECMs) function as barriers between the organism and environment. Despite being at the interface with the environment, little is known about the role of animal barrier ECMs in sensing stress and communicating with cytoprotective gene pathways in neighboring cells. We and others have identified a putative damage sensor in the C. elegans cuticle that regulates osmotic, detoxification, and innate immune response genes. This pathway is associated with circumferential collagen bands called annular furrows; mutation or loss of furrow collagens causes constitutive activation of osmotic, detoxification, and innate immune response genes. Here, we performed a genome-wide RNAi screen for modulators of osmotic stress response gene gpdh-1 in a furrow collagen mutant strain. RNAi of six genes identified in this screen were tested under other conditions and for effects on other stress responses. The functions of these genes suggest negative feedback within osmolyte accumulation pathways and interactions with ATP homeostasis and protein synthesis. Loss of these gpdh-1 modulators had distinct effects on canonical detoxification and innate immune response genes.

Neuronal IRE-1 coordinates an organism-wide cold stress response by regulating fat metabolism

Cold affects many aspects of biology, medicine, agriculture, and industry. Here, we identify a conserved endoplasmic reticulum (ER) stress response, distinct from the canonical unfolded protein response, that maintains lipid homeostasis during extreme cold. We establish that the ER stress sensor IRE-1 is critical for resistance to extreme cold and activated by cold temperature. Specifically, neuronal IRE-1 signals through JNK-1 and neuropeptide signaling to regulate lipid composition within the animal. This cold-response pathway can be bypassed by dietary supplementation with unsaturated fatty acids. Altogether, our findings define an ER-centric conserved organism-wide cold stress response, consisting of molecular neuronal sensors, effectors, and signaling moieties, which control adaptation to cold conditions in the organism. Better understanding of the molecular basis of this stress response is crucial for the optimal use of cold conditions on live organisms and manipulation of lipid saturation homeostasis, which is perturbed in human pathologies.

Sexual dimorphism in Caenorhabditis elegans stress resistance

Physiological responses to the environment, disease, and aging vary by sex in many animals, but mechanisms of dimorphism have only recently begun to receive careful attention. The genetic model nematode Caenorhabditis elegans has well-defined mechanisms of stress response, aging, and sexual differentiation. C. elegans has males, but the vast majority of research only uses hermaphrodites. We found that males of the standard N2 laboratory strain were more resistant to hyperosmolarity, heat, and a natural pro-oxidant than hermaphrodites when in mixed-sex groups. Resistance to heat and pro-oxidant were also male-biased in three genetically and geographically diverse C. elegans strains consistent with a species-wide dimorphism that is not specific to domestication. N2 males were also more resistant to heat and pro-oxidant when keep individually indicating that differences in resistance do not require interactions between worms. We found that males induce canonical stress response genes by similar degrees and in similar tissues as hermaphrodites suggesting the importance of other mechanisms. We find that resistance to heat and pro-oxidant are influenced by the sex differentiation transcription factor TRA-1 suggesting that downstream organ differentiation pathways establish differences in stress resistance. Environmental stress influences survival in natural environments, degenerative disease, and aging. Understanding mechanisms of stress response dimorphism can therefore provide insights into sex-specific population dynamics, disease, and longevity.

An extracellular matrix damage sensor signals through membrane-associated kinase DRL-1 to mediate cytoprotective responses in Caenorhabditis elegans

We and others previously identified circumferential bands of collagen named annular furrows as key components of a damage sensor in the cuticle of Caenorhabditis elegans that regulates cytoprotective genes. Mutation or loss of noncollagen secreted proteins OSM-7, OSM-8, and OSM-11 activate the same cytoprotective responses without obvious changes to the cuticle indicating that other extracellular proteins are involved. Here, we used RNAi screening to identify protein kinase DRL-1 as a key modulator of cytoprotective gene expression and stress resistance in furrow and extracellular OSM protein mutants. DRL-1 functions downstream from furrow disruption and is expressed in cells that induce cytoprotective genes. DRL-1 is not required for the expression of cytoprotective genes under basal or oxidative stress conditions consistent with specificity to extracellular signals. DRL-1 was previously shown to regulate longevity via a "Dietary Restriction-Like" state, but it functions downstream from furrow disruption by a distinct mechanism. The kinase domain of DRL-1 is related to mammalian MEKK3, and MEKK3 is recruited to a plasma membrane osmosensor complex by a scaffold protein. In C. elegans, DRL-1 contains an atypical hydrophobic C-terminus with predicted transmembrane domains and is constitutively expressed at or near the plasma membrane where it could function to receive extracellular damage signals for cells that mount cytoprotective responses.

Phosphoglycolate phosphatase homologs act as glycerol-3-phosphate phosphatase to control stress and healthspan in C. elegans

Metabolic stress due to nutrient excess and lipid accumulation is at the root of many age-associated disorders and the identification of therapeutic targets that mimic the beneficial effects of calorie restriction has clinical importance. Here, using C. elegans as a model organism, we study the roles of a recently discovered enzyme at the heart of metabolism in mammalian cells, glycerol-3-phosphate phosphatase (G3PP) (gene name Pgp) that hydrolyzes glucose-derived glycerol-3-phosphate to glycerol. We identify three Pgp homologues in C. elegans (pgph) and demonstrate in vivo that their protein products have G3PP activity, essential for glycerol synthesis. We demonstrate that PGPH/G3PP regulates the adaptation to various stresses, in particular hyperosmolarity and glucotoxicity. Enhanced G3PP activity reduces fat accumulation, promotes healthy aging and acts as a calorie restriction mimetic at normal food intake without altering fertility. Thus, PGP/G3PP can be considered as a target for age-related metabolic disorders.

The C. elegans Hypertonic Stress Response: Big Insights from Shrinking Worms

Cell volume is one of the most aggressively defended physiological set points in biology. Changes in intracellular ion and water concentrations, which are induced by changes in metabolism or environmental exposures, disrupt protein folding, enzymatic activity, and macromolecular assemblies. To counter these challenges, cells and organisms have evolved multifaceted, evolutionarily conserved molecular mechanisms to restore cell volume and repair stress induced damage. However, many unanswered questions remain regarding the nature of cell volume 'sensing' as well as the molecular signaling pathways involved in activating physiological response mechanisms. Unbiased genetic screening in the model organism C. elegans is providing new and unexpected insights into these questions, particularly questions relating to the hypertonic stress response (HTSR) pathway. One surprising characteristic of the HTSR pathway in C. elegans is that it is under strong negative regulation by proteins involved in protein homeostasis and the extracellular matrix (ECM). The role of the ECM in particular highlights the importance of studying the HTSR in the context of a live organism where native ECM-tissue associations are preserved. A second novel and recently discovered characteristic is that the HTSR is regulated at the post-transcriptional level. The goal of this review is to describe these discoveries, to provide context for their implications, and to raise outstanding questions to guide future research.

Dehydrated Caenorhabditis elegans Stocks Are Resistant to Multiple Freeze-Thaw Cycles

Ultracold preservation is widely used for storage of genetic stocks of Caenorhabditis elegans. Current cryopreservation protocols are vulnerable to refrigeration failures, which can result in the loss of stock viability due to damage during re-freezing. Here we present a method for preserving worms in a dehydrated and frozen form that retains viability after multiple freeze-thaw cycles. After dehydration in the presence of trehalose or glycerol, C. elegans stocks can be frozen and thawed multiple times while maintaining viability. While both dauer and non-dauer larvae survive desiccation and freezing, the dauer defective mutant daf-16 does not survive desiccation. Our technique is useful for storing stocks in a manner robust to freezer failures, and potentially for shipping strains between laboratories.

Black tea increases hypertonic stress resistance in C. elegans

Here we identified that BTE (black tea extract), within the studied concentration range, is more effective than GTE (green tea extract) in protecting C. elegans against hypertonic stress, by enhancing survival after exposure to various salts, and alleviating suffered motility loss and body shrinkage. The mechanism of such protection may be due to the ability of black tea to induce the conserved WNK/GCK signaling pathway and down-regulation of the expression levels of nlp-29. Intriguingly, black tea does not relieve hypertonicity-induced protein damage. The findings implicate the potential health benefits of black tea consumed worldwide.

Regulation of Lysosomal Function by the DAF-16 Forkhead Transcription Factor Couples Reproduction to Aging in Caenorhabditis elegans

Aging in eukaryotes is accompanied by widespread deterioration of the somatic tissue. Yet, abolishing germ cells delays the age-dependent somatic decline in Caenorhabditis elegans In adult worms lacking germ cells, the activation of the DAF-9/DAF-12 steroid signaling pathway in the gonad recruits DAF-16 activity in the intestine to promote longevity-associated phenotypes. However, the impact of this pathway on the fitness of normally reproducing animals is less clear. Here, we explore the link between progeny production and somatic aging and identify the loss of lysosomal acidity-a critical regulator of the proteolytic output of these organelles-as a novel biomarker of aging in C. elegans The increase in lysosomal pH in older worms is not a passive consequence of aging, but instead is timed with the cessation of reproduction, and correlates with the reduction in proteostasis in early adult life. Our results further implicate the steroid signaling pathway and DAF-16 in dynamically regulating lysosomal pH in the intestine of wild-type worms in response to the reproductive cycle. In the intestine of reproducing worms, DAF-16 promotes acidic lysosomes by upregulating the expression of v-ATPase genes. These findings support a model in which protein clearance in the soma is linked to reproduction in the gonad via the active regulation of lysosomal acidification.

The cAMP-PKA pathway-mediated fat mobilization is required for cold tolerance in C. elegans

Low temperature has a great impact on animal life. Homoiotherms such as mammals increase their energy expenditure to produce heat by activating the cAMP-protein kinase A (PKA)-hormone-sensitive lipase (HSL) pathway under cold stress. Although poikilothermic animals do not have the ability to regulate body temperature, whether this pathway is required for cold tolerance remains unknown. We have now achieved this using the genetically tractable model animal Caenorhabditis elegans. We demonstrate that cold stress activates PKA signaling, which in turn up-regulates the expression of a hormone-sensitive lipase hosl-1. The lipase induces fat mobilization, leading to glycerol accumulation, thereby protecting worms against cold stress. Our findings provide an example of an evolutionarily conserved mechanism for cold tolerance that has persisted in both poikilothermic and homoeothermic animals.

Osmotic Stress-Induced Defective Glial Proteostasis Contributes to Brain Demyelination after Hyponatremia Treatment

Adequate protein folding is necessary for normal cell function and a tightly regulated process that requires proper intracellular ionic strength. In many cell types, imbalance between protein synthesis and degradation can induce endoplasmic reticulum (ER) stress, which if sustained, can in turn lead to cell death. In nematodes, osmotic stress induces massive protein aggregation coupled with unfolded protein response and ER stress. In clinical practice, patients sustaining rapid correction of chronic hyponatremia are at risk of osmotic demyelination syndrome. The intense osmotic stress sustained by brain cells is believed to be the major risk factor for demyelination resulting from astrocyte death, which leads to microglial activation, blood-brain barrier opening, and later, myelin damage. Here, using a rat model of osmotic demyelination, we showed that rapid correction of chronic hyponatremia induces severe alterations in proteostasis characterized by diffuse protein aggregation and ubiquitination. Abrupt correction of hyponatremia resulted in vigorous activation of both the unfolded protein response and ER stress accompanied by increased autophagic activity and apoptosis. Immunofluorescence revealed that most of these processes occurred in astrocytes within regions previously shown to be demyelinated in later stages of this syndrome. These results identify osmotic stress as a potent protein aggregation stimuli in mammalian brain and further suggest that osmotic demyelination might be a consequence of proteostasis failure on severe osmotic stress.

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.

Morphological remodeling of C. elegans neurons during aging is modified by compromised protein homeostasis

Understanding cellular outcomes, such as neuronal remodeling, that are common to both healthy and diseased aging brains is essential to the development of successful brain aging strategies. Here, we used Caenorhabdits elegans to investigate how the expression of proteotoxic triggers, such as polyglutamine (polyQ)-expanded huntingtin and silencing of proteostasis regulators, such as the ubiquitin–proteasome system (UPS) and protein clearance components, may impact the morphological remodeling of individual neurons as animals age. We examined the effects of disrupted proteostasis on the integrity of neuronal cytoarchitecture by imaging a transgenic C. elegans strain in which touch receptor neurons express the first 57 amino acids of the human huntingtin (Htt) gene with expanded polyQs (128Q) and by using neuron-targeted RNA interference in adult wild-type neurons to knockdown genes encoding proteins involved in proteostasis. We found that proteostatic challenges conferred by polyQ-expanded Htt and knockdown of specific genes involved in protein homeostasis can lead to morphological changes that are restricted to specific domains of specific neurons. The age-associated branching of PLM neurons is suppressed by N-ter polyQ-expanded Htt expression, whereas ALM neurons with polyQ-expanded Htt accumulate extended outgrowths and other soma abnormalities. Furthermore, knockdown of genes important for ubiquitin-mediated degradation, lysosomal function, and autophagy modulated these age-related morphological changes in otherwise normal neurons. Our results show that the expression of misfolded proteins in neurodegenerative disease such as Huntington’s disease modifies the morphological remodeling that is normally associated with neuronal aging. Our results also show that morphological remodeling of healthy neurons during aging can be regulated by the UPS and other proteostasis pathways. Collectively, our data highlight a model in which morphological remodeling during neuronal aging is strongly affected by disrupted proteostasis and expression of disease-associated, misfolded proteins such as human polyQ-Htt species.

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.

Dynamic Sumoylation of a Conserved Transcription Corepressor Prevents Persistent Inclusion Formation during Hyperosmotic Stress

Cells are often exposed to physical or chemical stresses that can damage the structures of essential biomolecules. Stress-induced cellular damage can become deleterious if not managed appropriately. Rapid and adaptive responses to stresses are therefore crucial for cell survival. In eukaryotic cells, different stresses trigger post-translational modification of proteins with the small ubiquitin-like modifier SUMO. However, the specific regulatory roles of sumoylation in each stress response are not well understood. Here, we examined the sumoylation events that occur in budding yeast after exposure to hyperosmotic stress. We discovered by proteomic and biochemical analyses that hyperosmotic stress incurs the rapid and transient sumoylation of Cyc8 and Tup1, which together form a conserved transcription corepressor complex that regulates hundreds of genes. Gene expression and cell biological analyses revealed that sumoylation of each protein directs distinct outcomes. In particular, we discovered that Cyc8 sumoylation prevents the persistence of hyperosmotic stress-induced Cyc8-Tup1 inclusions, which involves a glutamine-rich prion domain in Cyc8. We propose that sumoylation protects against persistent inclusion formation during hyperosmotic stress, allowing optimal transcriptional function of the Cyc8-Tup1 complex.

Cells have evolved complex stress responses to cope with environmental challenges that could otherwise inflict severe damage on the molecules essential for life. Stress responses must ameliorate the immediate damage caused by stress exposure and also adjust metabolic capacity, gene expression output, and other cellular functions to protect against further damage that could be incurred by prolonged exposure to stress. Posttranslational protein modifications are a major means by which cells respond to changing environmental conditions. These modifications can alter the function, localization, and molecular interactions of their target proteins. In addition, evidence is emerging that some posttranslational modifications may also change the physical characteristics of target proteins. In this study, we present evidence that during hyperosmotic stress, a condition known to induce protein misfolding, cells rapidly but transiently use the small ubiquitin-modifier SUMO to protect against persistent inclusion formation of a conserved transcriptional repressor complex. We propose that this rapid protective action via posttranslational modification enables optimal gene regulation during the cellular response to hyperosmotic stress.

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.

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.

Neuronal responses to stress and injury in C. elegans

The nervous system plays critical roles in the stress response. Animals can survive and function under harsh conditions, and resist and recover from injuries because neurons perceive and respond to various stressors through specific regulatory mechanisms. Caenorhabditis elegans has served as an excellent model to discover fundamental mechanisms underlying the neuronal response to stress. The basic physiological processes that C. elegans exhibits under stress conditions are similar to those observed in higher organisms. Many molecular pathways activated by environmental and cellular stresses are also conserved. In this review, we summarize major findings in examining neuronal responses to hypoxia, oxidative stress, osmotic stress, and traumatic injury. These studies from C. elegans have provided novel insights into our understanding of neuronal responses to stress at the molecular, cellular, and circuit levels.

Asymmetric damage segregation at cell division via protein aggregate fusion and attachment to organelles

The segregation of damaged components at cell division determines the survival and aging of cells. In cells that divide asymmetrically, such as Saccharomyces cerevisiae, aggregated proteins are retained by the mother cell. Yet, where and how aggregation occurs is not known. Recent work by Zhou and collaborators shows that the birth of protein aggregates, under specific stress conditions, requires active translation, and occurs mainly at the endoplasmic reticulum. Later, aggregates move to the mitochondrial surface through fis1-dependent association. During replicative aging, aggregate association with the mother-cell mitochondria contributes to the asymmetric segregation of aggregates, because mitochondria in the daughter cell do not carry aggregates. With increasing age of mother cells, aggregates lose their connection to the mitochondria, and segregation is less asymmetric. Relating these findings to other mechanisms of aggregate segregation in different organisms, we postulate that fusion between aggregates and their tethering to organelles such as the vacuole, nucleus, ER, or mitochondria are common principles that establish asymmetric segregation during stress resistance and aging.

Molecular basis of chill resistance adaptations in poikilothermic animals

Chill and freeze represent very different components of low temperature stress. Whilst the principal mechanisms of tissue damage and of acquired protection from freeze-induced effects are reasonably well established, those for chill damage and protection are not. Non-freeze cold exposure (i.e. chill) can lead to serious disruption to normal life processes, including disruption to energy metabolism, loss of membrane perm-selectivity and collapse of ion gradients, as well as loss of neuromuscular coordination. If the primary lesions are not relieved then the progressive functional debilitation can lead to death. Thus, identifying the underpinning molecular lesions can point to the means of building resistance to subsequent chill exposures. Researchers have focused on four specific lesions: (i) failure of neuromuscular coordination, (ii) perturbation of bio-membrane structure and adaptations due to altered lipid composition, (iii) protein unfolding, which might be mitigated by the induced expression of compatible osmolytes acting as 'chemical chaperones', (iv) or the induced expression of protein chaperones along with the suppression of general protein synthesis. Progress in all these potential mechanisms has been ongoing but not substantial, due in part to an over-reliance on straightforward correlative approaches. Also, few studies have intervened by adoption of single gene ablation, which provides much more direct and compelling evidence for the role of specific genes, and thus processes, in adaptive phenotypes. Another difficulty is the existence of multiple mechanisms, which often act together, thus resulting in compensatory responses to gene manipulations, which may potentially mask disruptive effects on the chill tolerance phenotype. Consequently, there is little direct evidence of the underpinning regulatory mechanisms leading to induced resistance to chill injury. Here, we review recent advances mainly in lower vertebrates and in arthropods, but increasingly in genetic model species from a broader range of taxa.

Inherent instability of the retinitis pigmentosa P23H mutant opsin

The P23H opsin mutation is the most common cause of autosomal dominant retinitis pigmentosa. Even though the pathobiology of the resulting retinal degeneration has been characterized in several animal models, its complex molecular mechanism is not well understood. Here, we expressed P23H bovine rod opsin in the nervous system of Caenorhabditis elegans. Expression was low due to enhanced protein degradation. The mutant opsin was glycosylated, but the polysaccharide size differed from that of the normal protein. Although P23H opsin aggregated in the nervous system of C. elegans, the pharmacological chaperone 9-cis-retinal stabilized it during biogenesis, producing a variant of rhodopsin called P23H isorhodopsin. In vitro, P23H isorhodopsin folded correctly, formed the appropriate disulfide bond, could be photoactivated but with reduced sensitivity, and underwent Meta II decay at a rate similar to wild type isorhodopsin. In worm neurons, P23H isorhodopsin initiated phototransduction by coupling with the endogenous Gi/o signaling cascade that induced loss of locomotion. Using pharmacological interventions affecting protein synthesis and degradation, we showed that the chromophore could be incorporated either during or after mutant protein translation. However, regeneration of P23H isorhodopsin with chromophore was significantly slower than that of wild type isorhodopsin. This effect, combined with the inherent instability of P23H rhodopsin, could lead to the structural cellular changes and photoreceptor death found in autosomal dominant retinitis pigmentosa. These results also suggest that slow regeneration of P23H rhodopsin could prevent endogenous chromophore-mediated stabilization of rhodopsin in the retina.

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.

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

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Proteasome function can be dissected in Caenorhabditis elegans.

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

Changes in translation rate modulate stress-induced damage of diverse proteins

Proteostasis is the maintenance of the proper function of cellular proteins. Hypertonic stress disrupts proteostasis and causes rapid and widespread protein aggregation and misfolding in the nematode Caenorhabditis elegans. Optimal survival in hypertonic environments requires degradation of damaged proteins. Inhibition of protein synthesis occurs in response to diverse environmental stressors and may function in part to minimize stress-induced protein damage. We recently tested this idea directly and demonstrated that translation inhibition by acute exposure to cycloheximide suppresses hypertonicity-induced aggregation of polyglutamine::YFP (Q35::YFP) in body wall muscle cells. In this article, we further characterized the relationship between protein synthesis and hypertonic stress-induced protein damage. We demonstrate that inhibition of translation reduces hypertonic stress-induced formation and growth of Q35::YFP, Q44::YFP, and α-synuclein aggregates; misfolding of paramyosin and ras GTPase; and aggregation of multiple endogenous proteins expressed in diverse cell types. Activation of general control nonderepressible-2 (GCN-2) kinase signaling during hypertonic stress inhibits protein synthesis via phosphorylation of eukaryotic initiation factor-2α (eIF-2α). Inhibition of GCN-2 activation prevents the reduction in translation rate and greatly exacerbates the formation and growth of Q35::YFP aggregates and the aggregation of endogenous proteins. The current studies together with our previous work provide the first direct demonstration that hypertonic stress-induced reduction in protein synthesis minimizes protein aggregation and misfolding. Reduction in translation rate also serves as a signal that activates osmoprotective gene expression. The cellular proteostasis network thus plays a critical role in minimizing hypertonic stress-induced protein damage, in degrading stress-damaged proteins, and in cellular osmosensing and signaling.

Salinity-induced regulation of the myo-inositol biosynthesis pathway in tilapia gill epithelium

The myo-inositol biosynthesis (MIB) pathway converts glucose-6-phosphate to the compatible osmolyte myo-inositol that protects cells from osmotic stress. Using proteomics, the enzymes that constitute the MIB pathway, myo-inositol phosphate synthase (MIPS) and inositol monophosphatase 1 (IMPA1), are identified in tilapia (Oreochromis mossambicus) gill epithelium. Targeted, quantitative, label-free proteomics reveals that they are both upregulated during salinity stress. Upregulation is stronger when fish are exposed to severe (34 ppt acute and 90 ppt gradual) relative to moderate (70 ppt gradual) salinity stress. IMPA1 always responds more strongly than MIPS, suggesting that MIPS is more stable during salinity stress. MIPS is N-terminally acetylated and the corresponding peptide increases proportionally to MIPS protein, while non-acetylated N-terminal peptide is not detectable, indicating that MIPS acetylation is constitutive and may serve to stabilize the protein. Hyperosmotic induction of MIPS and IMPA1 is confirmed using western blot and real-time qPCR and is much higher at the mRNA than at the protein level. Two distinct MIPS mRNA variants are expressed in the gill, but one is more strongly regulated by salinity than the other. A single MIPS gene is encoded in the tilapia genome whereas the zebrafish genome lacks MIPS entirely. The genome of euryhaline tilapia contains four IMPA genes, two of which are expressed, but only one is salinity regulated in gill epithelium. The genome of stenohaline zebrafish contains a single IMPA gene. We conclude that the MIB pathway represents a major salinity stress coping mechanism that is regulated at multiple levels in euryhaline fish but absent in stenohaline zebrafish.

Physiological and molecular mechanisms of salt and water homeostasis in the nematode Caenorhabditis elegans

Intracellular salt and water homeostasis is essential for all cellular life. Extracellular salt and water homeostasis is also important for multicellular organisms. Many fundamental mechanisms of compensation for osmotic perturbations are well defined and conserved. Alternatively, molecular mechanisms of detecting salt and water imbalances and regulating compensatory responses are generally poorly defined for animals. Throughout the last century, researchers studying vertebrates and vertebrate cells made critical contributions to our understanding of osmoregulation, especially mechanisms of salt and water transport and organic osmolyte accumulation. Researchers have more recently started using invertebrate model organisms with defined genomes and well-established methods of genetic manipulation to begin defining the genes and integrated regulatory networks that respond to osmotic stress. The nematode Caenorhabditis elegans is well suited to these studies. Here, I introduce osmoregulatory mechanisms in this model, discuss experimental advantages and limitations, and review important findings. Key discoveries include defining genetic mechanisms of osmolarity sensing in neurons, identifying protein damage as a sensor and principle determinant of hypertonic stress resistance, and identification of a putative sensor for hypertonic stress associated with the extracellular matrix. Many of these processes and pathways are conserved and, therefore, provide new insights into salt and water homeostasis in other animals, including mammals.

GCN-2 dependent inhibition of protein synthesis activates osmosensitive gene transcription via WNK and Ste20 kinase signaling

Increased gpdh-1 transcription is required for accumulation of the organic osmolyte glycerol and survival of Caenorhabditis elegans during hypertonic stress. Our previous work has shown that regulators of gpdh-1 (rgpd) gene knockdown constitutively activates gpdh-1 expression. Fifty-five rgpd genes play essential roles in translation suggesting that inhibition of protein synthesis is an important signal for regulating osmoprotective gene transcription. We demonstrate here that translation is reduced dramatically by hypertonic stress or knockdown of rgpd genes encoding aminoacyl-tRNA synthetases and eukaryotic translation initiation factors (eIFs). Toxin-induced inhibition of translation also activates gpdh-1 expression. Hypertonicity-induced translation inhibition is mediated by general control nonderepressible (GCN)-2 kinase signaling and eIF-2α phosphoryation. Loss of gcn-1 or gcn-2 function prevents eIF-2α phosphorylation, completely blocks reductions in translation, and inhibits gpdh-1 transcription. gpdh-1 expression is regulated by the highly conserved with-no-lysine kinase (WNK) and Ste20 kinases WNK-1 and GCK-3, which function in the GCN-2 signaling pathway downstream from eIF-2α phosphorylation. Our previous work has shown that hypertonic stress causes rapid and dramatic protein damage in C. elegans and that inhibition of translation reduces this damage. The current studies demonstrate that reduced translation also serves as an essential signal for activation of WNK-1/GCK-3 kinase signaling and subsequent transcription of gpdh-1 and possibly other osmoprotective genes.