Regulation of the cellular heat shock response in Caenorhabditis elegans by thermosensory neurons

Neuronal function of the mRNA decapping complex determines survival of Caenorhabditis elegans at high temperature through temporal regulation of heterochronic gene expression

In response to adverse environmental cues, Caenorhabditis elegans larvae can temporarily arrest development at the second moult and form dauers, a diapause stage that allows for long-term survival. This process is largely regulated by certain evolutionarily conserved signal transduction pathways, but it is also affected by miRNA-mediated post-transcriptional control of gene expression. The 5'-3' mRNA decay mechanism contributes to miRNA-mediated silencing of target mRNAs in many organisms but how it affects developmental decisions during normal or stress conditions is largely unknown. Here, we show that loss of the mRNA decapping complex activity acting in the 5'-3' mRNA decay pathway inhibits dauer formation at the stressful high temperature of 27.5°C, and instead promotes early developmental arrest. Our genetic data suggest that this arrest phenotype correlates with dysregulation of heterochronic gene expression and an aberrant stabilization of lin-14 mRNA at early larval stages. Restoration of neuronal dcap-1 activity was sufficient to rescue growth phenotypes of dcap-1 mutants at both high and normal temperatures, implying the involvement of common developmental timing mechanisms. Our work unveils the crucial role of 5'-3' mRNA degradation in proper regulation of heterochronic gene expression programmes, which proved to be essential for survival under stressful conditions.

Protein Quality Control in Health and Disease

Maintaining functional protein homeostasis (proteostasis) is a constant challenge in the face of limited protein-folding capacity, environmental threats, and aging. Cells have developed several quality-control mechanisms that assist nascent polypeptides to fold properly, clear misfolded molecules, respond to the accumulation of protein aggregates, and deposit potentially toxic conformers in designated sites. Proteostasis collapse can lead to the development of diseases known as proteinopathies. Here we delineate the current knowledge on the different layers of protein quality-control mechanisms at the organelle and cellular levels with an emphasis on the prion protein (PrP). We also describe how protein quality control is integrated at the organismal level and discuss future perspectives on utilizing proteostasis maintenance as a strategy to develop novel therapies for the treatment of proteinopathies.

Proteostasis and Diseases of the Motor Unit

The accumulation in neurons of aberrant protein species, the pathological hallmark of many neurodegenerative diseases, results from a global impairment of key cellular processes governing protein synthesis/degradation and repair mechanisms, also known as the proteostasis network (PN). The growing number of connections between dysfunction of this intricate network of pathways and diseases of the motor unit, where both motor neurons and muscle are primarily affected, has provided momentum to investigate the muscle- and motor neuron-specific response to physiological and pathological stressors and to explore the therapeutic opportunities that manipulation of this process may offer. Furthermore, these diseases offer an unparalleled opportunity to deepen our understanding of the molecular mechanisms behind the intertissue communication and transfer of signals of proteostasis. The most compelling aspect of these investigations is their immediate potential for therapeutic impact: targeting muscle to stem degeneration of the motor unit would represent a dramatic paradigm therapeutic shift for treating these devastating diseases. Here we will review the current state of the art of the research on the alterations of the PN in diseases of the motor unit and its potential to result in effective treatments for these devastating neuromuscular disorders.

The struggle by Caenorhabditis elegans to maintain proteostasis during aging and disease

The presence of only small amounts of misfolded protein is an indication of a healthy proteome. Maintaining proteome health, or more specifically, “proteostasis,” is the purview of the “proteostasis network.” This network must respond to constant fluctuations in the amount of destabilized proteins caused by errors in protein synthesis and exposure to acute proteotoxic conditions. Aging is associated with a gradual increase in damaged and misfolded protein, which places additional stress on the machinery of the proteostasis network. In fact, despite the ability of the proteostasis machinery to readjust its stoichiometry in an attempt to maintain homeostasis, the capacity of cells to buffer against misfolding is strikingly limited. Therefore, subtle changes in the folding environment that occur during aging can significantly impact the health of the proteome. This decline and eventual collapse in proteostasis is most pronounced in individuals with neurodegenerative disorders such as Alzheimer’s Disease, Parkinson’s Disease, and Huntington’s Disease that are caused by the misfolding, aggregation, and toxicity of certain proteins. This review discusses how C. elegans models of protein misfolding have contributed to our current understanding of the proteostasis network, its buffering capacity, and its regulation.

Reviewers: This article was reviewed by Luigi Bubacco, Patrick Lewis and Xavier Roucou.

Neuropeptide signals cell non-autonomous mitochondrial unfolded protein response

Neurons have a central role in the systemic coordination of mitochondrial unfolded protein response (UPR^mt) and the cell non-autonomous modulation of longevity. However, the mechanism by which the nervous system senses mitochondrial stress and communicates to the distal tissues to induce UPR^mt remains unclear. Here we employ the tissue-specific CRISPR-Cas9 approach to disrupt mitochondrial function only in the nervous system of Caenorhabditis elegans, and reveal a cell non-autonomous induction of UPR^mt in peripheral cells. We further show that a neural sub-circuit composed of three types of sensory neurons, and one interneuron is required for sensing and transducing neuronal mitochondrial stress. In addition, neuropeptide FLP-2 functions in this neural sub-circuit to signal the non-autonomous UPR^mt. Taken together, our results suggest a neuropeptide coordination of mitochondrial stress response in the nervous system.

A C. elegans Thermosensory Circuit Regulates Longevity through crh-1/CREB-Dependent flp-6 Neuropeptide Signaling

Sensory perception, including thermosensation, shapes longevity in diverse organisms, but longevity-modulating signals from the sensory neurons are largely obscure. Here we show that CRH-1/CREB activation by CMK-1/CaMKI in the AFD thermosensory neuron is a key mechanism that maintains lifespan at warm temperatures in C. elegans. In response to temperature rise and crh-1 activation, the AFD neurons produce and secrete the FMRFamide neuropeptide FLP-6. Both CRH-1 and FLP-6 are necessary and sufficient for longevity at warm temperatures. Our data suggest that FLP-6 targets the AIY interneurons and engages DAF-9 sterol hormone signaling. Moreover, we show that FLP-6 signaling downregulates ins-7/insulin-like peptide and several insulin pathway genes, whose activity compromises lifespan. Our work illustrates how temperature experience is integrated by the thermosensory circuit to generate neuropeptide signals that remodel insulin and sterol hormone signaling and reveals a neuronal-endocrine circuit driven by thermosensation to promote temperature-specific longevity.

Chemical activation of a food deprivation signal extends lifespan

Model organisms subject to dietary restriction (DR) generally live longer. Accompanying this lifespan extension are improvements in overall health, based on multiple metrics. This indicates that pharmacological treatments that mimic the effects of DR could improve health in humans. To find new chemical structures that extend lifespan, we screened 30 000 synthetic, diverse drug‐like chemicals in Caenorhabditis elegans and identified several structurally related compounds that acted through DR mechanisms. The most potent of these NP1 impinges upon a food perception pathway by promoting glutamate signaling in the pharynx. This results in the overriding of a GPCR pathway involved in the perception of food and which normally acts to decrease glutamate signals. Our results describe the activation of a dietary restriction response through the pharmacological masking of a novel sensory pathway that signals the presence of food. This suggests that primary sensory pathways may represent novel targets for human pharmacology.

The genome-wide role of HSF-1 in the regulation of gene expression in Caenorhabditis elegans

BACKGROUND

The heat shock response, induced by cytoplasmic proteotoxic stress, is one of the most highly conserved transcriptional responses. This response, driven by the heat shock transcription factor HSF1, restores proteostasis through the induction of molecular chaperones and other genes. In addition to stress-dependent functions, HSF1 has also been implicated in various stress-independent functions. In C. elegans, the HSF1 homolog HSF-1 is an essential protein that is required to mount a stress-dependent response, as well as to coordinate various stress-independent processes including development, metabolism, and the regulation of lifespan. In this work, we have performed RNA-sequencing for C. elegans cultured in the presence and absence of hsf-1 RNAi followed by treatment with or without heat shock. This experimental design thus allows for the determination of both heat shock-dependent and -independent biological targets of HSF-1 on a genome-wide level.

RESULTS

Our results confirm that C. elegans HSF-1 can regulate gene expression in both a stress-dependent and -independent fashion. Almost all genes regulated by HS require HSF-1, reinforcing the central role of this transcription factor in the response to heat stress. As expected, major categories of HSF-1-regulated genes include cytoprotection, development, metabolism, and aging. Within both the heat stress-dependent and -independent gene groups, significant numbers of genes are upregulated as well as downregulated, demonstrating that HSF-1 can both activate and repress gene expression either directly or indirectly. Surprisingly, the cellular process most highly regulated by HSF-1, both with and without heat stress, is cuticle structure. Via network analyses, we identify a nuclear hormone receptor as a common link between genes that are regulated by HSF-1 in a HS-dependent manner, and an epidermal growth factor receptor as a common link between genes that are regulated by HSF-1 in a HS-independent manner. HSF-1 therefore coordinates various physiological processes in C. elegans, and HSF-1 activity may be coordinated across tissues by nuclear hormone receptor and epidermal growth factor receptor signaling.

CONCLUSION

This work provides genome-wide HSF-1 regulatory networks in C. elegans that are both heat stress-dependent and -independent. We show that HSF-1 is responsible for regulating many genes outside of classical heat stress-responsive genes, including genes involved in development, metabolism, and aging. The findings that a nuclear hormone receptor may coordinate the HS-induced HSF-1 transcriptional response, while an epidermal growth factor receptor may coordinate the HS-independent response, indicate that these factors could promote cell non-autonomous signaling that occurs through HSF-1. Finally, this work highlights the genes involved in cuticle structure as important HSF-1 targets that may play roles in promoting both cytoprotection as well as longevity.

The stress response paradox: fighting degeneration at the cost of cancer

In the modern research era, sequencing and high-throughput analysis have linked genetic factors with a multitude of disease states. Often times, the same cellular machinery is implicated in several different diseases and has made it challenging to drug a particular disease with minimal pleotropic consequences. It is intriguing to see how different fields of disease research can present such differing views when describing the same biological process, pathway, or molecule. As observations in one field converge with research in another, we gain a more complete picture of a biological system and can accurately assess the feasibility for translational science. As an example discussed here, modulating latent stress response pathways within the cell provides exciting therapeutic potential, however, opposing views have emerged in the fields of degenerative disease and cancer. This at first glance seems logical as suppression of degenerative disease entails maintaining cell viability, while cancer aims to enhance selective senescence and cell death. As both of these disciplines seek novel therapeutic interventions, we should not overlook how scientific biases involving one biological process may impact different disease paradigms.

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.

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.

Small Heat Shock Proteins Are Novel Common Determinants of Alcohol and Nicotine Sensitivity in Caenorhabditis elegans

Addiction to drugs is strongly determined by multiple genetic factors. Alcohol and nicotine produce distinct pharmacological effects within the nervous system through discrete molecular targets; yet, data from family and twin analyses support the existence of common genetic factors for addiction in general. The mechanisms underlying addiction, however, are poorly described and common genetic factors for alcohol and nicotine remain unidentified. We investigated the role that the heat shock transcription factor, HSF-1, and its downstream effectors played as common genetic modulators of sensitivity to addictive substances. Using Caenorhabditis elegans, an exemplary model organism with substance dose-dependent responses similar to mammals, we demonstrate that HSF-1 altered sensitivity to both alcohol and nicotine. Using a combination of a targeted RNAi screen of downstream factors and transgenic approaches we identified that these effects were contingent upon the constitutive neuronal expression of HSP-16.48, a small heat shock protein (HSP) homolog of human α-crystallin. Furthermore we demonstrated that the function of HSP-16.48 in drug sensitivity surprisingly was independent of chaperone activity during the heat shock stress response. Instead we identified a distinct domain within the N-terminal region of the HSP-16.48 protein that specified its function in comparison to related small HSPs. Our findings establish and characterize a novel genetic determinant underlying sensitivity to diverse addictive substances.

TRX-1 Regulates SKN-1 Nuclear Localization Cell Non-autonomously in Caenorhabditis elegans

The Caenorhabditis elegans oxidative stress response transcription factor, SKN-1, is essential for the maintenance of redox homeostasis and is a functional ortholog of the Nrf family of transcription factors. The numerous levels of regulation that govern these transcription factors underscore their importance. Here, we add a thioredoxin, encoded by trx-1, to the expansive list of SKN-1 regulators. We report that loss of trx-1 promotes nuclear localization of intestinal SKN-1 in a redox-independent, cell non-autonomous fashion from the ASJ neurons. Furthermore, this regulation is not general to the thioredoxin family, as two other C. elegans thioredoxins, TRX-2 and TRX-3, do not play a role in this process. Moreover, TRX-1-dependent regulation requires signaling from the p38 MAPK-signaling pathway. However, while TRX-1 regulates SKN-1 nuclear localization, classical SKN-1 transcriptional activity associated with stress response remains largely unaffected. Interestingly, RNA-Seq analysis revealed that loss of trx-1 elicits a general, organism-wide down-regulation of several classes of genes; those encoding for collagens and lipid transport being most prevalent. Together, these results uncover a novel role for a thioredoxin in regulating intestinal SKN-1 nuclear localization in a cell non-autonomous manner, thereby contributing to the understanding of the processes involved in maintaining redox homeostasis throughout an organism.

Molecular mechanism of thermosensory function of human heat shock transcription factor Hsf1

The heat shock response is a universal homeostatic cell autonomous reaction of organisms to cope with adverse environmental conditions. In mammalian cells, this response is mediated by the heat shock transcription factor Hsf1, which is monomeric in unstressed cells and upon activation trimerizes, and binds to promoters of heat shock genes. To understand the basic principle of Hsf1 activation we analyzed temperature-induced alterations in the conformational dynamics of Hsf1 by hydrogen exchange mass spectrometry. We found a temperature-dependent unfolding of Hsf1 in the regulatory region happening concomitant to tighter packing in the trimerization region. The transition to the active DNA binding-competent state occurred highly cooperative and was concentration dependent. Surprisingly, Hsp90, known to inhibit Hsf1 activation, lowered the midpoint temperature of trimerization and reduced cooperativity of the process thus widening the response window. Based on our data we propose a kinetic model of Hsf1 trimerization.

Experience Modulates the Reproductive Response to Heat Stress in C. elegans via Multiple Physiological Processes

Natural environments are considerably more variable than laboratory settings and often involve transient exposure to stressful conditions. To fully understand how organisms have evolved to respond to any given stress, prior experience must therefore be considered. We investigated the effects of individual and ancestral experience on C. elegans reproduction. We documented ways in which cultivation at 15°C or 25°C affects developmental time, lifetime fecundity, and reproductive performance after severe heat stress that exceeds the fertile range of the organism but is compatible with survival and future fecundity. We found that experience modulates multiple aspects of reproductive physiology, including the male and female germ lines and the interaction between them. These responses vary in their environmental sensitivity, suggesting the existence of complex mechanisms for coping with unpredictable and stressful environments.

Cell-Nonautonomous Mechanisms Underlying Cellular and Organismal Aging

Cell-autonomous mechanisms underlying cellular and organismal aging in evolutionarily distant eukaryotes have been established; these mechanisms regulate longevity-defining processes within a single eukaryotic cell. Recent findings have provided valuable insight into cell-nonautonomous mechanisms modulating cellular and organismal aging in eukaryotes across phyla; these mechanisms involve a transmission of various longevity factors between different cells, tissues, and organisms. Herein, we review such cell-nonautonomous mechanisms of aging in eukaryotes. We discuss the following: (1) how low molecular weight transmissible longevity factors modulate aging and define longevity of cells in yeast populations cultured in liquid media or on solid surfaces, (2) how communications between proteostasis stress networks operating in neurons and nonneuronal somatic tissues define longevity of the nematode Caenorhabditis elegans by modulating the rates of aging in different tissues, and (3) how different bacterial species colonizing the gut lumen of C. elegans define nematode longevity by modulating the rate of organismal aging.

Lipid signalling couples translational surveillance to systemic detoxification in Caenorhabditis elegans

Translation in eukaryotes is followed to detect toxins and virulence factors and coupled to the induction of defence pathways. Caenorhabditis elegans germline-specific mutations in translation components are detected by this system to induce detoxification and immune responses in distinct somatic cells. An RNA interference screen revealed gene inactivations that act at multiple steps in lipid biosynthetic and kinase pathways upstream of MAP kinase to mediate the systemic communication of translation defects to induce detoxification genes. Mammalian bile acids can rescue the defect in detoxification gene induction caused by C. elegans lipid biosynthetic gene inactivations. Extracts prepared from C. elegans with translation deficits but not from the wild type can also rescue detoxification gene induction in lipid-biosynthesis-defective strains. These eukaryotic antibacterial countermeasures are not ignored by bacteria: particular bacterial species suppress normal C. elegans detoxification responses to mutations in translation factors.

Stress signaling: serotonin spreads systemic stress

Cells respond to elevated temperatures through a well-characterized heat-shock response that enables short-term survival, long-term adaptation and mitigation of macromolecular damage. New work reveals a cell non-autonomous layer of stress-response regulation between neurons and the gonad involving serotonin.

Stress signaling between organs in metazoa

Many organisms have developed a robust ability to adapt and survive in the face of environmental perturbations that threaten the integrity of their genome, proteome, or metabolome. Studies in multiple model organisms have shown that, in general, when exposed to stress, cells activate a complex prosurvival signaling network that includes immune and DNA damage response genes, chaperones, antioxidant enzymes, structural proteins, metabolic enzymes, and noncoding RNAs. The manner of activation runs the gamut from transcriptional induction of genes to increased stability of transcripts to posttranslational modification of important biosynthetic proteins within the stressed tissue. Superimposed on these largely autonomous effects are nonautonomous responses in which the stressed tissue secretes peptides and other factors that stimulate tissues in different organs to embark on processes that ultimately help the organism as a whole cope with stress. This review focuses on the mechanisms by which tissues in one organ adapt to environmental challenges by regulating stress responses in tissues of different organs.

The unpredictability of prolonged activation of stress response pathways

In response to stress, cellular compartments activate signaling pathways that mediate transcriptional programs to promote survival and reestablish homeostasis. Manipulation of the magnitude and duration of the activation of stress responses has been proposed as a strategy to prevent or repair the damage associated with aging or degenerative diseases. However, as these pathways likely evolved to respond specifically to transient perturbations, the unpredictability of prolonged activation should be considered.

Heterotypic Signals from Neural HSF-1 Separate Thermotolerance from Longevity

Integrating stress responses across tissues is essential for the survival of multicellular organisms. The metazoan nervous system can sense protein-misfolding stress arising in different subcellular compartments and initiate cytoprotective transcriptional responses in the periphery. Several subcellular compartments possess a homotypic signal whereby the respective compartment relies on a single signaling mechanism to convey information within the affected cell to the same stress-responsive pathway in peripheral tissues. In contrast, we find that the heat shock transcription factor, HSF-1, specifies its mode of transcellular protection via two distinct signaling pathways. Upon thermal stress, neural HSF-1 primes peripheral tissues through the thermosensory neural circuit to mount a heat shock response. Independent of this thermosensory circuit, neural HSF-1 activates the FOXO transcription factor, DAF-16, in the periphery and prolongs lifespan. Thus a single transcription factor can coordinate different stress response pathways to specify its mode of protection against changing environmental conditions.

Proteotoxic stress and ageing triggers the loss of redox homeostasis across cellular compartments

The cellular proteostasis network integrates the protein folding and clearance machineries in multiple sub-cellular compartments of the eukaryotic cell. The endoplasmic reticulum (ER) is the site of synthesis and folding of membrane and secretory proteins. A distinctive feature of the ER is its tightly controlled redox homeostasis necessary for the formation of inter- and intra-molecular disulphide bonds. Employing genetically encoded in vivo sensors reporting on the redox state in an organelle-specific manner, we show in the nematode Caenorhabditis elegans that the redox state of the ER is subject to profound changes during worm lifetime. In young animals, the ER is oxidizing and this shifts towards reducing conditions during ageing, whereas in the cytosol the redox state becomes more oxidizing with age. Likewise, the redox state in the cytosol and the ER change in an opposing manner in response to proteotoxic challenges in C. elegans and in HeLa cells revealing conservation of redox homeostasis. Moreover, we show that organelle redox homeostasis is regulated across tissues within C. elegans providing a new measure for organismal fitness.

Repression of the Heat Shock Response Is a Programmed Event at the Onset of Reproduction

The heat shock response (HSR) is essential for proteostasis and cellular health. In metazoans, aging is associated with a decline in quality control, thus increasing the risk for protein conformational disease. Here, we show that in C. elegans, the HSR declines precipitously over a 4 hr period in early adulthood coincident with the onset of reproductive maturity. Repression of the HSR occurs due to an increase in H3K27me3 marks at stress gene loci, the timing of which is determined by reduced expression of the H3K27 demethylase jmjd-3.1. This results in a repressed chromatin state that interferes with HSF-1 binding and suppresses transcription initiation in response to stress. The removal of germline stem cells preserves jmjd-3.1 expression, suppresses the accumulation of H3K27me3 at stress gene loci, and maintains the HSR. These findings suggest that competing requirements of the germline and soma dictate organismal stress resistance as animals begin reproduction.

Thermosensation and longevity

Temperature has profound effects on behavior and aging in both poikilotherms and homeotherms. To thrive under the ever fluctuating environmental temperatures, animals have evolved sophisticated mechanisms to sense and adapt to temperature changes. Animals sense temperature through various molecular thermosensors, such as thermosensitive transient receptor potential (TRP) channels expressed in neurons, keratinocytes, and intestine. These evolutionarily conserved thermosensitive TRP channels feature distinct activation thresholds, thereby covering a wide spectrum of ambient temperature. Temperature changes trigger complex thermosensory behaviors. Due to the simplicity of the nervous system in model organisms such as Caenorhabditis elegans and Drosophila, the mechanisms of thermosensory behaviors in these species have been extensively studied at the circuit and molecular levels. While much is known about temperature regulation of behavior, it remains largely unclear how temperature affects aging. Recent studies in C. elegans demonstrate that temperature modulation of longevity is not simply a passive thermodynamic phenomenon as suggested by the rate-of-living theory, but rather a process that is actively regulated by genes, including those encoding thermosensitive TRP channels. In this review, we discuss our current understanding of thermosensation and its role in aging.

Co-chaperone p23 regulates C. elegans Lifespan in Response to Temperature

Temperature potently modulates various physiologic processes including organismal motility, growth rate, reproduction, and ageing. In ectotherms, longevity varies inversely with temperature, with animals living shorter at higher temperatures. Thermal effects on lifespan and other processes are ascribed to passive changes in metabolic rate, but recent evidence also suggests a regulated process. Here, we demonstrate that in response to temperature, daf-41/ZC395.10, the C. elegans homolog of p23 co-chaperone/prostaglandin E synthase-3, governs entry into the long-lived dauer diapause and regulates adult lifespan. daf-41 deletion triggers constitutive entry into the dauer diapause at elevated temperature dependent on neurosensory machinery (daf-10/IFT122), insulin/IGF-1 signaling (daf-16/FOXO), and steroidal signaling (daf-12/FXR). Surprisingly, daf-41 mutation alters the longevity response to temperature, living longer than wild-type at 25°C but shorter than wild-type at 15°C. Longevity phenotypes at 25°C work through daf-16/FOXO and heat shock factor hsf-1, while short lived phenotypes converge on daf-16/FOXO and depend on the daf-12/FXR steroid receptor. Correlatively daf-41 affected expression of DAF-16 and HSF-1 target genes at high temperature, and nuclear extracts from daf-41 animals showed increased occupancy of the heat shock response element. Our studies suggest that daf-41/p23 modulates key transcriptional changes in longevity pathways in response to temperature.

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.

Endocrine aspects of organelle stress—cell non-autonomous signaling of mitochondria and the ER

Organisms have to cope with an unpredictable and dynamic environment. It is crucial for any living being to respond to these changes by buffering the effects on cellular homeostasis. Failure to appropriately respond to stress can have severe consequences for health and survival. Eukaryotic cells possess several organelle-specific stress responses to cope with this challenge. Besides their central role in stress resistance, these pathways have also been shown to be important in the regulation of proteome maintenance, development and longevity. Intriguingly, many of these effects seem to be controlled by only a subset of cells implying a systemic regulation in a cell non-autonomous manner. The understanding of the nature of this stress communication across tissues, its mechanisms and impact, will be paramount in understanding disease etiology and the development of therapeutic strategies.

Rapid RNA analysis of individual Caenorhabditis elegans

Traditional RNA extraction methods rely on the use of hazardous chemicals such as phenol, chloroform, guanidinium thiocyanate to disrupt cells and inactivate RNAse simultaneously. RNA isolation from Caenorhabditis elegans presents another challenge due to its tough cuticle, therefore several repeated freeze–thaw cycles may be needed to disrupt the cuticle before the cell contents are released. In addition, a large number of animals are required for successful RNA isolation. To overcome these issues, we have developed a simple and efficient method using proteinase K and a brief heat treatment to release RNA of quality suitable for quantitative PCR analysis.The benefits of the method are:

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Faster and safer compared to conventional RNA extraction methods

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Released RNA can be used directly for cDNA synthesis without purification

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As little as a single worm is sufficient

The reprogramming of tumor stroma by HSF1 is a potent enabler of malignancy

Stromal cells within the tumor microenvironment are essential for tumor progression and metastasis. Surprisingly little is known about the factors that drive the transcriptional reprogramming of stromal cells within tumors. We report that the transcriptional regulator heat shock factor 1 (HSF1) is frequently activated in cancer-associated fibroblasts (CAFs), where it is a potent enabler of malignancy. HSF1 drives a transcriptional program in CAFs that complements, yet is completely different from, the program it drives in adjacent cancer cells. This CAF program is uniquely structured to support malignancy in a non-cell-autonomous way. Two central stromal signaling molecules-TGF-β and SDF1-play a critical role. In early-stage breast and lung cancer, high stromal HSF1 activation is strongly associated with poor patient outcome. Thus, tumors co-opt the ancient survival functions of HSF1 to orchestrate malignancy in both cell-autonomous and non-cell-autonomous ways, with far-reaching therapeutic implications.

Neuronal serotonin release triggers the heat shock response in C. elegans in the absence of temperature increase

BACKGROUND

Cellular mechanisms aimed at repairing protein damage and maintaining homeostasis, widely understood to be triggered by the damage itself, have recently been shown to be under cell nonautonomous control in the metazoan C. elegans. The heat shock response (HSR) is one such conserved mechanism, activated by cells upon exposure to proteotoxic conditions such as heat. Previously, we had shown that this conserved cytoprotective response is regulated by the thermosensory neuronal circuitry of C. elegans. Here, we investigate the mechanisms and physiological relevance of neuronal control.

RESULTS

By combining optogenetic methods with live visualization of the dynamics of the heat shock transcription factor (HSF1), we show that excitation of the AFD thermosensory neurons is sufficient to activate HSF1 in another cell, even in the absence of temperature increase. Excitation of the AFD thermosensory neurons enhances serotonin release. Serotonin release elicited by direct optogenetic stimulation of serotonergic neurons activates HSF1 and upregulates molecular chaperones through the metabotropic serotonin receptor SER-1. Consequently, excitation of serotonergic neurons alone can suppress protein misfolding in C. elegans peripheral tissue.

CONCLUSIONS

These studies imply that thermosensory activity coupled to serotonergic signaling is sufficient to activate the protective HSR prior to frank proteotoxic damage. The ability of neurosensory release of serotonin to control cellular stress responses and activate HSF1 has powerful implications for the treatment of protein conformation diseases.

MicroRNAs meet calcium: joint venture in ER proteostasis

The endoplasmic reticulum (ER) is a cellular compartment that has a key function in protein translation and folding. Maintaining its integrity is of fundamental importance for organism's physiology and viability. The dynamic regulation of intraluminal ER Ca(2+) concentration directly influences the activity of ER-resident chaperones and stress response pathways that balance protein load and folding capacity. We review the emerging evidence that microRNAs play important roles in adjusting these processes to frequently changing intracellular and environmental conditions to modify ER Ca(2+) handling and storage and maintain ER homeostasis.

HSF-1-mediated cytoskeletal integrity determines thermotolerance and life span

The conserved heat shock transcription factor-1 (HSF-1) is essential to cellular stress resistance and life-span determination. The canonical function of HSF-1 is to regulate a network of genes encoding molecular chaperones that protect proteins from damage caused by extrinsic environmental stress or intrinsic age-related deterioration. In Caenorhabditis elegans, we engineered a modified HSF-1 strain that increased stress resistance and longevity without enhanced chaperone induction. This health assurance acted through the regulation of the calcium-binding protein PAT-10. Loss of pat-10 caused a collapse of the actin cytoskeleton, stress resistance, and life span. Furthermore, overexpression of pat-10 increased actin filament stability, thermotolerance, and longevity, indicating that in addition to chaperone regulation, HSF-1 has a prominent role in cytoskeletal integrity, ensuring cellular function during stress and aging.

Heat shock in the springtime

A collaborative workshop dedicated to the discussion of heat shock factors in stress response, development, and disease was held on April 22-24, 2014 at the Université Paris Diderot in Paris, France. Recent years have witnessed an explosion of interest in these highly conserved transcription factors, with biological roles ranging from environmental sensing to human development and cancer.

HSF1 at a glance

Heat shock factor 1 (HSF1) is an evolutionarily highly conserved transcription factor that coordinates stress-induced transcription and directs versatile physiological processes in eukaryotes. The central position of HSF1 in cellular homeostasis has been well demonstrated, mainly through its strong effect in transactivating genes that encode heat shock proteins (HSPs). However, recent genome-wide studies have revealed that HSF1 is capable of reprogramming transcription more extensively than previously assumed; it is also involved in a multitude of processes in stressed and non-stressed cells. Consequently, the importance of HSF1 in fundamental physiological events, including metabolism, gametogenesis and aging, has become apparent and its significance in pathologies, such as cancer progression, is now evident. In this Cell Science at a Glance article, we highlight recent advances in the HSF1 field, discuss the organismal control over HSF1, and present the processes that are mediated by HSF1 in the context of cell type, cell-cycle phase, physiological condition and received stimuli.

It's all in your mind: determining germ cell fate by neuronal IRE-1 in C. elegans

The C. elegans germline is pluripotent and mitotic, similar to self-renewing mammalian tissues. Apoptosis is triggered as part of the normal oogenesis program, and is increased in response to various stresses. Here, we examined the effect of endoplasmic reticulum (ER) stress on apoptosis in the C. elegans germline. We demonstrate that pharmacological or genetic induction of ER stress enhances germline apoptosis. This process is mediated by the ER stress response sensor IRE-1, but is independent of its canonical downstream target XBP-1. We further demonstrate that ire-1-dependent apoptosis in the germline requires both CEP-1/p53 and the same canonical apoptotic genes as DNA damage-induced germline apoptosis. Strikingly, we find that activation of ire-1, specifically in the ASI neurons, but not in germ cells, is sufficient to induce apoptosis in the germline. This implies that ER stress related germline apoptosis can be determined at the organism level, and is a result of active IRE-1 signaling in neurons. Altogether, our findings uncover ire-1 as a novel cell non-autonomous regulator of germ cell apoptosis, linking ER homeostasis in sensory neurons and germ cell fate.

Application of functional 'Omics' in environmental stress physiology: insights, limitations, and future challenges

Omic technologies have revolutionised how environmental physiologists investigate stress response pathways. To date, however, omic screens typically constitute simple presence/absence correlations, and fall short of explaining mechanism. Disentangling function necessitates hypothesis-driven manipulation of selected molecular signals, and a systems level view will only come from more detailed tissue-specific and time series sampling. The increasing accessibility of omic applications means that species can be selected based on Krogh principles, but focus also needs to be given to core models where multi-platform approaches can be combined to provide a deeper understanding. This review highlights recent technological and intellectual advances in the application of omics to understanding insect stress adaptation, and sets out how to address remaining knowledge gaps.

Caenorhabditis elegans as a model system for studying non-cell-autonomous mechanisms in protein-misfolding diseases

Caenorhabditis elegans has a number of distinct advantages that are useful for understanding the basis for cellular and organismal dysfunction underlying age-associated diseases of protein misfolding. Although protein aggregation, a key feature of human neurodegenerative diseases, has been typically explored in vivo at the single-cell level using cells in culture, there is now increasing evidence that proteotoxicity has a non-cell-autonomous component and is communicated between cells and tissues in a multicellular organism. These discoveries have opened up new avenues for the use of C. elegans as an ideal animal model system to study non-cell-autonomous proteotoxicity, prion-like propagation of aggregation-prone proteins, and the organismal regulation of stress responses and proteostasis. This Review focuses on recent evidence that C. elegans has mechanisms to transmit certain classes of toxic proteins between tissues and a complex stress response that integrates and coordinates signals from single cells and tissues across the organism. These findings emphasize the potential of C. elegans to provide insights into non-cell-autonomous proteotoxic mechanisms underlying age-related protein-misfolding diseases.

Organismal proteostasis: role of cell-nonautonomous regulation and transcellular chaperone signaling

Protein quality control is regulated by the proteostasis network and cell stress response pathways to promote cellular health. In this review, van Oosten-Hawle and Morimoto cover recent advances in model systems that reveal how communication between subcellular compartments and across different cells and tissues maintains a functional proteome during stress. The authors propose that transcellular stress signaling provides a critical control mechanism for the proteostasis network to maintain organismal health and life span.

Protein quality control is essential in all organisms and regulated by the proteostasis network (PN) and cell stress response pathways that maintain a functional proteome to promote cellular health. In this review, we describe how metazoans employ multiple modes of cell-nonautonomous signaling across tissues to integrate and transmit the heat-shock response (HSR) for balanced expression of molecular chaperones. The HSR and other cell stress responses such as the unfolded protein response (UPR) can function autonomously in single-cell eukaryotes and tissue culture cells; however, within the context of a multicellular animal, the PN is regulated by cell-nonautonomous signaling through specific sensory neurons and by the process of transcellular chaperone signaling. These newly identified forms of stress signaling control the PN between neurons and nonneuronal somatic tissues to achieve balanced tissue expression of chaperones in response to environmental stress and to ensure that metastable aggregation-prone proteins expressed within any single tissue do not generate local proteotoxic risk. Transcellular chaperone signaling leads to the compensatory expression of chaperones in other somatic tissues of the animal, perhaps preventing the spread of proteotoxic damage. Thus, communication between subcellular compartments and across different cells and tissues maintains proteostasis when challenged by acute stress and upon chronic expression of metastable proteins. We propose that transcellular chaperone signaling provides a critical control step for the PN to maintain cellular and organismal health span.

Transcellular chaperone signaling: an organismal strategy for integrated cell stress responses

The ability of each cell within a metazoan to adapt to and survive environmental and physiological stress requires cellular stress-response mechanisms, such as the heat shock response (HSR). Recent advances reveal that cellular proteostasis and stress responses in metazoans are regulated by multiple layers of intercellular communication. This ensures that an imbalance of proteostasis that occurs within any single tissue 'at risk' is protected by a compensatory activation of a stress response in adjacent tissues that confers a community protective response. While each cell expresses the machinery for heat shock (HS) gene expression, the HSR is regulated cell non-autonomously in multicellular organisms, by neuronal signaling to the somatic tissues, and by transcellular chaperone signaling between somatic tissues and from somatic tissues to neurons. These cell non-autonomous processes ensure that the organismal HSR is orchestrated across multiple tissues and that transmission of stress signals between tissues can also override the neuronal control to reset cell- and tissue-specific proteostasis. Here, we discuss emerging concepts and insights into the complex cell non-autonomous mechanisms that control stress responses in metazoans and highlight the importance of intercellular communication for proteostasis maintenance in multicellular organisms.

Balanced trade-offs between alternative strategies shape the response of C. elegans reproduction to chronic heat stress

To ensure long-term reproductive success organisms have to cope with harsh environmental extremes. A reproductive strategy that simply maximizes offspring production is likely to be disadvantageous because it could lead to a catastrophic loss of fecundity under unfavorable conditions. To understand how an appropriate balance is achieved, we investigated reproductive performance of C. elegans under conditions of chronic heat stress. We found that following even prolonged exposure to temperatures at which none of the offspring survive, worms could recover and resume reproduction. The likelihood of producing viable offspring falls precipitously after exposure to temperatures greater than 28°C primarily due to sperm damage. Surprisingly, we found that worms that experienced higher temperatures can recover considerably better, provided they did not initiate ovulation. Therefore mechanisms controlling this process must play a crucial role in determining the probability of recovery. We show, however, that suppressing ovulation is only beneficial under relatively long stresses, whereas it is a disadvantageous strategy under shorter stresses of the same intensity. This is because the benefit of shutting down egg laying, and thus protecting the reproductive system, is negated by the cost associated with implementing this strategy--it takes considerable time to recover and produce offspring. We interpret these balanced trade-offs as a dynamic response of the C. elegans reproductive system to stress and an adaptation to life in variable and unpredictable conditions.

Control of systemic proteostasis by the nervous system

Maintenance of organismal homeostasis depends on the integration of intracellular and external signals, involving the ability to detect molecular perturbations. An explosion of studies in model organisms indicates the occurrence of dynamic communication between alarm pathways engaged by protein-folding stress in neurons that activate adaptive programs in peripheral organs to control cellular proteostasis. Here we review emerging concepts that highlight the contribution of the proteostasis network to the regulation of several aspects of animal physiology through central integration of signals spanning multiple tissues and organs. These recent findings uncover a new layer of functional interrelation between cells that handle and orchestrate the global maintenance of the proteome at the organismal level in a cell-nonautonomous manner.

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.

Sensory systems: their impact on C. elegans survival

An animal's survival strongly depends on a nervous system that can rapidly process and integrate the changing quality of its environment and promote the most appropriate physiological responses. This is amply demonstrated in the nematode worm Caenorhabditis elegans, where its sensory system has been shown to impact multiple physiological traits that range from behavior and developmental plasticity to longevity. Because of the accessibility of its nervous system and the number of tools available to study and manipulate its neural circuitry, C. elegans has thus become an important model organism in dissecting the mechanisms through which the nervous system promotes survival. Here we review our current understanding of how the C. elegans sensory system affects diverse physiological traits, whose coordination would be essential for survival under fluctuating environments. The knowledge we derive from the C. elegans studies should provide testable hypotheses in discovering similar mechanisms in higher animals.

Oxygen sensing neurons and neuropeptides regulate survival after anoxia in developing C. elegans

Hypoxic brain injury remains a major source of neurodevelopmental impairment for both term and preterm infants. The perinatal period is a time of rapid transition in oxygen environments and developmental resetting of oxygen sensing. The relationship between neural oxygen sensing ability and hypoxic injury has not been studied. The oxygen sensing circuitry in the model organism C. elegans is well understood. We leveraged this information to investigate the effects of impairments in oxygen sensing on survival after anoxia. There was a significant survival advantage in developing worms specifically unable to sense oxygen shifts below their preferred physiologic range via genetic ablation of BAG neurons, which appear important for conferring sensitivity to anoxia. Oxygen sensing that is mediated through guanylate cyclases (gcy-31, 33, 35) is unlikely to be involved in conferring this sensitivity. Additionally, animals unable to process or elaborate neuropeptides displayed a survival advantage after anoxia. Based on these data, we hypothesized that elaboration of neuropeptides by BAG neurons sensitized animals to anoxia, but further experiments indicate that this is unlikely to be true. Instead, it seems that neuropeptides and signaling from oxygen sensing neurons operate through independent mechanisms, each conferring sensitivity to anoxia in wild type animals.

Stress, inflammation, and defense of homeostasis

Inflammation is traditionally considered a defense response induced by infection or injury. However, inflammation can also be induced by tissue stress and malfunction in the absence of infection or overt tissue damage. Here we discuss the relationship between homeostasis, stress responses, and inflammation. Stress responses have cell-autonomous and cell-extrinsic components, the latter contributing to tissue level adaptation to stress conditions. Inflammation can be thought of as the extreme end of a spectrum that ranges from homeostasis to stress response to bona fide inflammatory response. Inflammation can be triggered by two types of stimuli: extreme deviations of homeostasis or challenges that cause a disruption of homeostasis. This perspective may help to explain qualitative differences and functional outcomes of diverse inflammatory responses.

Integrin-linked kinase modulates longevity and thermotolerance in C. elegans through neuronal control of HSF-1

Integrin-signaling complexes play important roles in cytoskeletal organization and cell adhesion in many species. Components of the integrin-signaling complex have been linked to aging in both Caenorhabditis elegans and Drosophila melanogaster, but the mechanism underlying this function is unknown. Here, we investigated the role of integrin-linked kinase (ILK), a key component of the integrin-signaling complex, in lifespan determination. We report that genetic reduction of ILK in both C. elegans and Drosophila increased resistance to heat stress, and led to lifespan extension in C. elegans without majorly affecting cytoskeletal integrity. In C. elegans, longevity and thermotolerance induced by ILK depletion was mediated by heat-shock factor-1 (HSF-1), a major transcriptional regulator of the heat-shock response (HSR). Reduction in ILK levels increased hsf-1 transcription and activation, and led to enhanced expression of a subset of genes with roles in the HSR. Moreover, induction of HSR-related genes, longevity and thermotolerance caused by ILK reduction required the thermosensory neurons AFD and interneurons AIY, which are known to play a critical role in the canonical HSR. Notably, ILK was expressed in neighboring neurons, but not in AFD or AIY, implying that ILK reduction initiates cell nonautonomous signaling through thermosensory neurons to elicit a noncanonical HSR. Our results thus identify HSF-1 as a novel effector of the organismal response to reduced ILK levels and show that ILK inhibition regulates HSF-1 in a cell nonautonomous fashion to enhance stress resistance and lifespan in C. elegans.