A method for measuring fatty acid oxidation in C. elegans

HLH-30-dependent rewiring of metabolism during starvation in C. elegans

One of the most fundamental challenges for all living organisms is to sense and respond to alternating nutritional conditions in order to adapt their metabolism and physiology to promote survival and achieve balanced growth. Here, we applied metabolomics and lipidomics to examine temporal regulation of metabolism during starvation in wild‐type Caenorhabditis elegans and in animals lacking the transcription factor HLH‐30. Our findings show for the first time that starvation alters the abundance of hundreds of metabolites and lipid species in a temporal‐ and HLH‐30‐dependent manner. We demonstrate that premature death of hlh‐30 animals under starvation can be prevented by supplementation of exogenous fatty acids, and that HLH‐30 is required for complete oxidation of long‐chain fatty acids. We further show that RNAi‐mediated knockdown of the gene encoding carnitine palmitoyl transferase I (cpt‐1) only impairs survival of wild‐type animals and not of hlh‐30 animals. Strikingly, we also find that compromised generation of peroxisomes by prx‐5 knockdown renders hlh‐30 animals hypersensitive to starvation, which cannot be rescued by supplementation of exogenous fatty acids. Collectively, our observations show that mitochondrial functions are compromised in hlh‐30 animals and that hlh‐30 animals rewire their metabolism to largely depend on functional peroxisomes during starvation, underlining the importance of metabolic plasticity to maintain survival.

Starvation Responses Throughout the Caenorhabditiselegans Life Cycle

Caenorhabditis elegans survives on ephemeral food sources in the wild, and the species has a variety of adaptive responses to starvation. These features of its life history make the worm a powerful model for studying developmental, behavioral, and metabolic starvation responses. Starvation resistance is fundamental to life in the wild, and it is relevant to aging and common diseases such as cancer and diabetes. Worms respond to acute starvation at different times in the life cycle by arresting development and altering gene expression and metabolism. They also anticipate starvation during early larval development, engaging an alternative developmental program resulting in dauer diapause. By arresting development, these responses postpone growth and reproduction until feeding resumes. A common set of signaling pathways mediates systemic regulation of development in each context but with important distinctions. Several aspects of behavior, including feeding, foraging, taxis, egg laying, sleep, and associative learning, are also affected by starvation. A variety of conserved signaling, gene regulatory, and metabolic mechanisms support adaptation to starvation. Early life starvation can have persistent effects on adults and their descendants. With its short generation time, C. elegans is an ideal model for studying maternal provisioning, transgenerational epigenetic inheritance, and developmental origins of adult health and disease in humans. This review provides a comprehensive overview of starvation responses throughout the C. elegans life cycle.

Lifespan-extending interventions enhance lipid-supported mitochondrial respiration in Caenorhabditis elegans

Dietary restriction and reduced reproduction have been linked to long lifespans in the vast majority of species tested. Although decreased mitochondrial mass and/or function are hallmarks of aging, little is known about the mechanisms by which these organelles contribute to physiological aging or to the effects of lifespan-extending interventions, particularly with respect to oxidative phosphorylation and energy production. Here, we employed the nematode Caenorhabditis elegans to examine the effects of inhibition of germline proliferation and dietary restriction, both of which extend the lifespan of C. elegans, on mitochondrial respiratory activity in whole animals and isolated organelles. We found that oxygen consumption rates and mitochondrial mass were reduced in wild-type (WT) C. elegans subjected to bacterial deprivation (BD) compared with animals fed ad libitum (AL). In contrast, BD decreased the rate of oxygen uptake but not mitochondrial mass in germline-less glp-1(e2144ts) mutants. Interestingly, mitochondria isolated from animals subjected to BD and/or inhibition of germline proliferation showed no differences in complex I-mediated respiratory activity compared to control mitochondria, whereas both interventions enhanced the efficiency with which mitochondria utilized lipids as respiratory substrates. Notably, the combination of BD and inhibition of germline proliferation further increased mitochondrial lipid oxidation compared to either intervention alone. We also detected a striking correlation between lifespan extension in response to BD and/or inhibition of germline proliferation and the capacity of C. elegans to generate ATP from lipids. Our results thus suggest that the ability to oxidize lipids may be determinant in enhanced longevity.

Identification of key pathways and metabolic fingerprints of longevity in C. elegans

Impaired insulin/IGF-1 signaling (IIS) and caloric restriction (CR) prolong lifespan in the nematode C. elegans. However, a cross comparison of these longevity pathways using a multi-omics integration approach is lacking. In this study, we aimed to identify key pathways and metabolite fingerprints of longevity that are shared between IIS and CR worm models using multi-omics integration. We generated transcriptomics and metabolomics data from long-lived worm strains, i.e. daf-2 (impaired IIS) and eat-2 (CR model) and compared them with the wild-type strain N2. Transcriptional profiling identified shared longevity signatures, such as an upregulation of lipid storage and defense responses, and downregulation of macromolecule synthesis and developmental processes. Metabolomics profiling identified an increase in the levels of glycerol‑3P, adenine, xanthine, and AMP, and a decrease in the levels of the amino acid pool, as well as the C18:0, C17:1, C19:1, C20:0 and C22:0 fatty acids. After we integrated transcriptomics and metabolomics data based on the annotations in KEGG, our results highlighted increased amino acid metabolism and an upregulation of purine metabolism as a commonality between the two long-lived mutants. Overall, our findings point towards the existence of shared metabolic pathways that are likely important for lifespan extension and provide novel insights into potential regulators and metabolic fingerprints for longevity.

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Multi-omics integration identified common longevity signatures.

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Amino acid metabolism was increased in both daf-2 and eat-2 mutants.

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Purine biosynthesis pathway was enhanced in the long-lived mutants.

Multilayered Reprogramming in Response to Persistent DNA Damage in C. elegans

DNA damage causally contributes to aging and age-related diseases. Mutations in nucleotide excision repair (NER) genes cause highly complex congenital syndromes characterized by growth retardation, cancer susceptibility, and accelerated aging in humans. Orthologous mutations in Caenorhabditis elegans lead to growth delay, genome instability, and accelerated functional decline, thus allowing investigation of the consequences of persistent DNA damage during development and aging in a simple metazoan model. Here, we conducted proteome, lipidome, and phosphoproteome analysis of NER-deficient animals in response to UV treatment to gain comprehensive insights into the full range of physiological adaptations to unrepaired DNA damage. We derive metabolic changes indicative of a tissue maintenance program and implicate an autophagy-mediated proteostatic response. We assign central roles for the insulin-, EGF-, and AMPK-like signaling pathways in orchestrating the adaptive response to DNA damage. Our results provide insights into the DNA damage responses in the organismal context.

Lipid and Carbohydrate Metabolism in Caenorhabditis elegans

Lipid and carbohydrate metabolism are highly conserved processes that affect nearly all aspects of organismal biology. Caenorhabditis elegans eat bacteria, which consist of lipids, carbohydrates, and proteins that are broken down during digestion into fatty acids, simple sugars, and amino acid precursors. With these nutrients, C. elegans synthesizes a wide range of metabolites that are required for development and behavior. In this review, we outline lipid and carbohydrate structures as well as biosynthesis and breakdown pathways that have been characterized in C. elegans We bring attention to functional studies using mutant strains that reveal physiological roles for specific lipids and carbohydrates during development, aging, and adaptation to changing environmental conditions.

Multi-omics Analyses of Starvation Responses Reveal a Central Role for Lipoprotein Metabolism in Acute Starvation Survival in C. elegans

Starvation causes comprehensive metabolic changes, which are still not fully understood. Here, we used quantitative proteomics and RNA sequencing to examine the temporal starvation responses in wild-type Caenorhabditis elegans and animals lacking the transcription factor HLH-30. Our findings show that starvation alters the abundance of hundreds of proteins and mRNAs in a temporal manner, many of which are involved in central metabolic pathways, including lipoprotein metabolism. We demonstrate that premature death of hlh-30 animals under starvation can be prevented by knockdown of either vit-1 or vit-5, encoding two different lipoproteins. We further show that the size and number of intestinal lipid droplets under starvation are altered in hlh-30 animals, which can be rescued by knockdown of vit-1. Taken together, this indicates that survival of hlh-30 animals under starvation is closely linked to regulation of intestinal lipid stores. We provide the most detailed poly-omic analysis of starvation responses to date, which serves as a resource for further mechanistic studies of starvation.

Investigating Connections between Metabolism, Longevity, and Behavior in Caenorhabditis elegans

An overview of Caenorhabditis elegans as an experimental organism for studying energy balance is presented. Some of the unresolved questions that complicate the interpretation of lipid measurements from C. elegans are highlighted. We review studies that show that both lipid synthesis and lipid breakdown pathways are activated and needed for the longevity of hermaphrodites that lack their germlines. These findings illustrate the heterogeneity of triglyceride-rich lipid particles in C. elegans and reveal specific lipid signals that promote longevity. Finally, we provide a brief overview of feeding behavioral responses of C. elegans to varying nutritional conditions and highlight an unanticipated metabolic pathway that allows the incorporation of experience in feeding behavior.

The nuclear receptor DAF-12 regulates nutrient metabolism and reproductive growth in nematodes

Appropriate nutrient response is essential for growth and reproduction. Under favorable nutrient conditions, the C. elegans nuclear receptor DAF-12 is activated by dafachronic acids, hormones that commit larvae to reproductive growth. Here, we report that in addition to its well-studied role in controlling developmental gene expression, the DAF-12 endocrine system governs expression of a gene network that stimulates the aerobic catabolism of fatty acids. Thus, activation of the DAF-12 transcriptome coordinately mobilizes energy stores to permit reproductive growth. DAF-12 regulation of this metabolic gene network is conserved in the human parasite, Strongyloides stercoralis, and inhibition of specific steps in this network blocks reproductive growth in both of the nematodes. Our study provides a molecular understanding for metabolic adaptation of nematodes to their environment, and suggests a new therapeutic strategy for treating parasitic diseases.

Insights and challenges in using C. elegans for investigation of fat metabolism

C. elegans provides a genetically tractable system for deciphering the homeostatic mechanisms that underlie fat regulation in intact organisms. Here, we provide an overview of the recent advances in the C. elegans fat field with particular attention to studies of C. elegans lipid droplets, the complex links between lipases, autophagy, and lifespan, and analyses of key transcriptional regulatory mechanisms that coordinate lipid homeostasis. These studies demonstrate the ancient origins of mammalian and C. elegans fat regulatory pathways and highlight how C. elegans is being used to identify and analyze novel lipid pathways that are then shown to function similarly in mammals. Despite its many advantages, study of fat regulation in C. elegans is currently faced with a number of conceptual and methodological challenges. We critically evaluate some of the assumptions in the field and highlight issues that we believe should be taken into consideration when interpreting lipid content data in C. elegans.

Visualization of lipid droplets in C. elegans by light and electron microscopy

The powerful forward and reverse genetic tools, and emerging sets of biochemical assays for fat metabolites, make Caenorhabditis elegans an attractive model organism for elucidating conserved mechanisms in fat storage. The ability to observe lipid droplets in live animals at single cell resolution offers a unique advantage for studying cellular fat storage in vivo. In this chapter, we describe transgenic technologies for expressing fluorescent lipid droplet marker proteins at near-physiological levels. Methods to visualize these markers using sensitive confocal microscopy systems are detailed. Additional methods for visualizing lipid droplets by transmission electron microscopy and detection of lipid droplet associated proteins by immunoelectron microscopy are described.

Starvation induced cholesterogenesis in hepatic and extra hepatic tissues of climbing Perch, Anabas testudineus (Bloch)

Cholesterol is a structural lipid, which may be differentially utilized or synthesized in response to stress or during insulin deficient states such as starvation. In the present investigation we estimated the levels of cholesterol in Anabas testudineus, which was subjected to brief (15 days) and prolonged fasting (60 days). Tissues such as liver, kidney, brain, accessory respiratory organ, pectoral and lateral line muscle were selected for the study. Cholesterol content was estimated by the Crawford method (1958). Both the starvation regimes showed a significant increase in cholesterol levels in almost all the tissues, but for liver, which strangely showed an insignificant decline during the short-term starvation. This overall upsurge in cholesterol levels observed in all extra hepatic tissues may be attributed to the synthesis of stress hormones such as glucocorticoids, which may promote gluconeogenesis and adrenocorticoids, which may help the animal to combat the stressful condition of starvation. Anabas adapted well to starvation stress and survived all throughout the experimental period.