measurements-of-physiological-stress-responses-in-c-elegans

Measurements of Physiological Stress Responses in <em>C. Elegans</em>

Organisms are often exposed to fluctuating environments and changes in intracellular homeostasis, which can have detrimental effects on their proteome and ...

University of California, Berkeley

FOXO family transcription factors are downstream effectors of Insulin/IGF-1 signaling (IIS) and major determinants of aging in organisms ranging from worms to man. The molecular mechanisms that actively promote DAF16/FOXO stability and function are unknown. Here we identify the deubiquitylating enzyme MATH-33 as an essential DAF-16 regulator in IIS, which stabilizes active DAF-16 protein levels and, as a consequence, influences DAF-16 functions, such as metabolism, stress response, and longevity in C. elegans. MATH-33 associates with DAF-16 in cellulo and in vitro. MATH-33 functions as a deubiquitylase by actively removing ubiquitin moieties from DAF-16, thus counteracting the action of the RLE-1 E3-ubiquitin ligase. Our findings support a model in which MATH-33 promotes DAF-16 stability in response to decreased IIS by directly modulating its ubiquitylation state, suggesting that regulated oscillations in the stability of DAF-16 protein play an integral role in controlling processes such as metabolism and longevity. 

The Deubiquitylase MATH-33 Controls DAF-16 Stability and Function in Metabolism and Longevity.

Sensory perception comprises gustatory (taste) and olfactory (smell) modalities as well as somatosensory (pain, heat, and tactile mechanosensory) inputs, which are detected by a multitude of sensory receptors. These sensory receptors are contained in specialized ciliated neurons where they detect changes in environmental conditions and participate in behavioral decisions ranging from food choice to avoiding harmful conditions, thus insuring basic survival in metazoans. Recent genetic studies, however, indicate that sensory perception plays additional physiological functions, notably influencing energy homeostatic processes and longevity through neuronal circuits originating from sensory tissues. Here we review how these findings are redefining metabolic signaling and establish a prominent role of sensory neuroendocrine processes in controlling health span and lifespan, with a goal of translating this knowledge towards managing age-associated diseases.

Emerging Role of Sensory Perception in Aging and Metabolism.

Organisms respond to mitochondrial stress through the upregulation of an array of protective genes, often perpetuating an early response to metabolic dysfunction across a lifetime. We find that mitochondrial stress causes widespread changes in chromatin structure through histone H3K9 di-methylation marks traditionally associated with gene silencing. Mitochondrial stress response activation requires the di-methylation of histone H3K9 through the activity of the histone methyltransferase met-2 and the nuclear co-factor lin-65. While globally the chromatin becomes silenced by these marks, remaining portions of the chromatin open up, at which point the binding of canonical stress responsive factors such as DVE-1 occurs. Thus, a metabolic stress response is established and propagated into adulthood of animals through specific epigenetic modifications that allow for selective gene expression and lifespan extension. 

Mitochondrial Stress Induces Chromatin Reorganization to Promote Longevity and UPR(mt).

Across eukaryotic species, mild mitochondrial stress can have beneficial effects on the lifespan of organisms. Mitochondrial dysfunction activates an unfolded protein response (UPR(mt)), a stress signaling mechanism designed to ensure mitochondrial homeostasis. Perturbation of mitochondria during larval development in C. elegans not only delays aging but also maintains UPR(mt) signaling, suggesting an epigenetic mechanism that modulates both longevity and mitochondrial proteostasis throughout life. We identify the conserved histone lysine demethylases jmjd-1.2/PHF8 and jmjd-3.1/JMJD3 as positive regulators of lifespan in response to mitochondrial dysfunction across species. Reduction of function of the demethylases potently suppresses longevity and UPR(mt) induction, while gain of function is sufficient to extend lifespan in a UPR(mt)-dependent manner. A systems genetics approach in the BXD mouse reference population further indicates conserved roles of the mammalian orthologs in longevity and UPR(mt) signaling. These findings illustrate an evolutionary conserved epigenetic mechanism that determines the rate of aging downstream of mitochondrial perturbations. 

Two Conserved Histone Demethylases Regulate Mitochondrial Stress-Induced Longevity.

The health of an organism is orchestrated by a multitude of molecular and biochemical networks responsible for ensuring homeostasis within cells and tissues. However, upon aging, a progressive failure in the maintenance of this homeostatic balance occurs in response to a variety of endogenous and environmental stresses, allowing the accumulation of damage, the physiological decline of individual tissues, and susceptibility to diseases. What are the molecular and cellular signaling events that control the aging process and how can this knowledge help design therapeutic strategies to combat age-associated diseases? Here we provide a comprehensive overview of the evolutionarily conserved biological processes that alter the rate of aging and discuss their link to disease prevention and the extension of healthy life span. 

Signaling Networks Determining Life Span.

As the first and most direct process influencing the proteostasis capacity of a cell, regulation of translation influences lifespan across taxa. Here we highlight some of the newly discovered means by which translational regulation affects cellular proteostasis, with a focus on mechanisms that may ultimately impinge upon the aging process. 

A Ribosomal Perspective on Proteostasis and Aging.

The mitochondrial unfolded protein response (UPR(mt)) promotes the recovery of dysfunctional mitochondria. Surprisingly, UPR(mt) activation inadvertently maintains and propagates the deleterious mtDNA in a heteroplasmic Caenorhabditis elegans strain, with detrimental consequences. This study extends our understanding of the UPR(mt) and provides a possible therapeutic target for diseases associated with mtDNA mutations.

Mitochondrial UPR: A Double-Edged Sword.

Defects in mitochondrial metabolism have been increasingly linked with age-onset protein-misfolding diseases such as Alzheimer's, Parkinson's, and Huntington's. In response to protein-folding stress, compartment-specific unfolded protein responses (UPRs) within the ER, mitochondria, and cytosol work in parallel to ensure cellular protein homeostasis. While perturbation of individual compartments can make other compartments more susceptible to protein stress, the cellular conditions that trigger cross-communication between the individual UPRs remain poorly understood. We have uncovered a conserved, robust mechanism linking mitochondrial protein homeostasis and the cytosolic folding environment through changes in lipid homeostasis. Metabolic restructuring caused by mitochondrial stress or small-molecule activators trigger changes in gene expression coordinated uniquely by both the mitochondrial and cytosolic UPRs, protecting the cell from disease-associated proteins. Our data suggest an intricate and unique system of communication between UPRs in response to metabolic changes that could unveil new targets for diseases of protein misfolding.

Lipid Biosynthesis Coordinates a Mitochondrial-to-Cytosolic Stress Response.

During neurodegenerative disease, the toxic accumulation of aggregates and misfolded proteins is often accompanied with widespread changes in peripheral metabolism, even in cells in which the aggregating protein is not present. The mechanism by which the central nervous system elicits a distal reaction to proteotoxic stress remains unknown. We hypothesized that the endocrine communication of neuronal stress plays a causative role in the changes in mitochondrial homeostasis associated with proteotoxic disease states. We find that an aggregation-prone protein expressed in the neurons of C. elegans binds to mitochondria, eliciting a global induction of a mitochondrial-specific unfolded protein response (UPR(mt)), affecting whole-animal physiology. Importantly, dense core vesicle release and secretion of the neurotransmitter serotonin is required for the signal's propagation. Collectively, these data suggest the commandeering of a nutrient sensing network to allow for cell-to-cell communication between mitochondria in response to protein folding stress in the nervous system.

Neuroendocrine Coordination of Mitochondrial Stress Signaling and Proteostasis.

Life is stressful. Organisms are repeatedly exposed to stressors that disrupt protein homeostasis (proteostasis), resulting in protein misfolding and aggregation. To sense and respond to proteotoxic perturbations, cells have evolved compartment-specific stress responses, such as the unfolded protein response of the endoplasmic reticulum (UPR). However, UPR function is impaired with age, which, we propose, creates a permissive environment for protein aggregation, unresolved ER stress, and chronic inflammation. Understanding age-related changes to the UPR will provide new avenues for therapeutic intervention in metabolic disease, neurodegeneration, and aging.

The UPR: Sensor and Coordinator of Organismal Homeostasis.

Olfactory inputs help coordinate food appreciation and selection, but their role in systemic physiology and energy balance is poorly understood. Here we demonstrate that mice upon conditional ablation of mature olfactory sensory neurons (OSNs) are resistant to diet-induced obesity accompanied by increased thermogenesis in brown and inguinal fat depots. Acute loss of smell perception after obesity onset not only abrogated further weight gain but also improved fat mass and insulin resistance. Reduced olfactory input stimulates sympathetic nerve activity, resulting in activation of β-adrenergic receptors on white and brown adipocytes to promote lipolysis. Conversely, conditional ablation of the IGF1 receptor in OSNs enhances olfactory performance in mice and leads to increased adiposity and insulin resistance. These findings unravel a new bidirectional function for the olfactory system in controlling energy homeostasis in response to sensory and hormonal signals.

The Sense of Smell Impacts Metabolic Health and Obesity.

There exists a phenomenon in aging research whereby early life stress can have positive impacts on longevity. The mechanisms underlying these observations suggest a robust, long-lasting induction of cellular defense mechanisms. These include the various unfolded protein responses of the endoplasmic reticulum (ER), cytosol, and mitochondria. Indeed, ectopic induction of these pathways, in the absence of stress, is sufficient to increase lifespan in organisms as diverse as yeast, worms, and flies. Here, we provide an overview of the protein quality control mechanisms that operate in the cytosol, mitochondria, and ER and discuss how they affect cellular health and viability during stress and aging.

A Futile Battle? Protein Quality Control and the Stress of Aging.

The transparent nematode Caenorhabditis elegans can sense UV and blue-violet light to alter behavior. Because high-dose UV and blue-violet light are not a common feature outside of the laboratory setting, we asked what role, if any, could low-intensity visible light play in C. elegans physiology and longevity. Here, we show that C. elegans lifespan is inversely correlated to the time worms were exposed to visible light. While circadian control, lite-1 and tax-2 do not contribute to the lifespan reduction, we demonstrate that visible light creates photooxidative stress along with a general unfolded-protein response that decreases the lifespan. Finally, we find that long-lived mutants are more resistant to light stress, as well as wild-type worms supplemented pharmacologically with antioxidants. This study reveals that transparent nematodes are sensitive to visible light radiation and highlights the need to standardize methods for controlling the unrecognized biased effect of light during lifespan studies in laboratory conditions.

Visible light reduces C. elegans longevity.

As a central hub of cellular metabolism and signaling, the mitochondrion is a crucial organelle whose dysfunction can cause disease and whose activity is intimately connected to aging. We review how the mitochondrial network maintains proteomic integrity, how mitochondrial proteotoxic stress is communicated and resolved in the context of the entire cell, and how mitochondrial systems function in the context of organismal health and aging. A deeper understanding of how mitochondrial protein quality control mechanisms are coordinated across these distinct biological levels should help explain why these mechanisms fail with age and, ultimately, how routes to intervention might be attained.

Mitochondrial proteostasis in the context of cellular and organismal health and aging.

Recently in Science, Leeman et al. find that perturbing lysosomal activity of quiescent NSCs directly impedes their ability to become activated, similar to what happens during aging. Excitingly, they could rejuvenate old quiescent NSCs by enhancing the lysosome pathway, ameliorating their ability to clear protein aggregates and become activated.

The Lysosome, Elixir of Neural Stem Cell Youth.

The mitochondrial unfolded protein response (UPR) can be triggered in a cell-non-autonomous fashion across multiple tissues in response to mitochondrial dysfunction. The ability to communicate information about the presence of mitochondrial stress enables a global response that can ultimately better protect an organism from local mitochondrial challenges. We find that animals use retromer-dependent Wnt signaling to propagate mitochondrial stress signals from the nervous system to peripheral tissues. Specifically, the polyQ40-triggered activation of mitochondrial stress or reduction of cco-1 (complex IV subunit) in neurons of C. elegans results in the Wnt-dependent induction of cell-non-autonomous UPR in peripheral cells. Loss-of-function mutations of retromer complex components that are responsible for recycling the Wnt secretion-factor/MIG-14 prevent Wnt secretion and thereby suppress cell-non-autonomous UPR. Neuronal expression of the Wnt ligand/EGL-20 is sufficient to induce cell-non-autonomous UPR in a retromer complex-, Wnt signaling-, and serotonin-dependent manner, clearly implicating Wnt signaling as a strong candidate for the "mitokine" signal.

The Mitochondrial Unfolded Protein Response Is Mediated Cell-Non-autonomously by Retromer-Dependent Wnt Signaling.

Molecular mechanisms regulating aging at the post-transcriptional level are not clear. In this issue of Molecular Cell,D'Amico et al. (2019) demonstrate that the translational inhibition of mitochondrial fission factor (MFF) regulates cellular homeostasis and aging.

Vive ut Numquam Moriturus: Tweaking Translational Control to Regulate Longevity.

Mitochondria are organelles descended from an endosymbiosed bacterium, and many bacterial toxins impair mitochondria, likely as an echo of ancient bacterial warfare. However, the signal transduction pathways that translate mitochondrial dysfunction into a transcriptional program for detoxification have not been well understood. In this issue of Cell Metabolism, Mao et al. (2019) provide insight into how mitochondrial perturbations can activate both the mitochondrial unfolded protein response (UPR) and detoxification response and, importantly, how these responses differentially affect organismal physiology under normal conditions or with pathogenic challenges.

Evolutionary Comeuppance: Mitochondrial Stress Awakens the Remnants of Ancient Bacterial Warfare.

Cells have evolved complex mechanisms to maintain protein homeostasis, such as the UPR, which are strongly associated with several diseases and the aging process. We performed a whole-genome CRISPR-based knockout (KO) screen to identify genes important for cells to survive ER-based protein misfolding stress. We identified the cell-surface hyaluronidase (HAase), Transmembrane Protein 2 (TMEM2), as a potent modulator of ER stress resistance. The breakdown of the glycosaminoglycan, hyaluronan (HA), by TMEM2 within the extracellular matrix (ECM) altered ER stress resistance independent of canonical UPR pathways but dependent upon the cell-surface receptor, CD44, a putative HA receptor, and the MAPK cell-signaling components, ERK and p38. Last, and most surprisingly, ectopic expression of human TMEM2 in C. elegans protected animals from ER stress and increased both longevity and pathogen resistance independent of canonical UPR activation but dependent on the ERK ortholog mpk-1 and the p38 ortholog pmk-1.

The Hyaluronidase, TMEM2, Promotes ER Homeostasis and Longevity Independent of the UPR.

The ability of the nervous system to sense cellular stress and coordinate protein homeostasis is essential for organismal health. Unfortunately, stress responses that mitigate disturbances in proteostasis, such as the unfolded protein response of the endoplasmic reticulum (UPR), become defunct with age. In this work, we expressed the constitutively active UPR transcription factor, XBP-1s, in a subset of astrocyte-like glia, which extended the life span in  Glial XBP-1s initiated a robust cell nonautonomous activation of the UPR in distal cells and rendered animals more resistant to protein aggregation and chronic ER stress. Mutants deficient in neuropeptide processing and secretion suppressed glial cell nonautonomous induction of the UPR and life-span extension. Thus, astrocyte-like glial cells play a role in regulating organismal ER stress resistance and longevity.

Andrew Dillin

Department of Molecular and Cell Biology

University of California, Berkeley

Measurements of Physiological Stress Responses in C. Elegans

Andrew Dillin

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Molecular and Cell Biology

University of California, Berkeley

Measurements of Physiological Stress Responses in C. Elegans

Department of Molecular and Cell Biology