Identification of late larval stage developmental checkpoints in Caenorhabditis elegans regulated by insulin/IGF and steroid hormone signaling pathways

Life history in Caenorhabditis elegans: from molecular genetics to evolutionary ecology

Life history is defined by traits that reflect key components of fitness, especially those relating to reproduction and survival. Research in life history seeks to unravel the relationships among these traits and understand how life history strategies evolve to maximize fitness. As such, life history research integrates the study of the genetic and developmental mechanisms underlying trait determination with the evolutionary and ecological context of Darwinian fitness. As a leading model organism for molecular and developmental genetics, Caenorhabditis elegans is unmatched in the characterization of life history-related processes, including developmental timing and plasticity, reproductive behaviors, sex determination, stress tolerance, and aging. Building on recent studies of natural populations and ecology, the combination of C. elegans' historical research strengths with new insights into trait variation now positions it as a uniquely valuable model for life history research. In this review, we summarize the contributions of C. elegans and related species to life history and its evolution. We begin by reviewing the key characteristics of C. elegans life history, with an emphasis on its distinctive reproductive strategies and notable life cycle plasticity. Next, we explore intraspecific variation in life history traits and its underlying genetic architecture. Finally, we provide an overview of how C. elegans has guided research on major life history transitions both within the genus Caenorhabditis and across the broader phylum Nematoda. While C. elegans is relatively new to life history research, significant progress has been made by leveraging its distinctive biological traits, establishing it as a highly cross-disciplinary system for life history studies.

De novo lipid synthesis and polarized prenylation drive cell invasion through basement membrane

To breach the basement membrane, cells in development and cancer use large, transient, specialized lipid-rich membrane protrusions. Using live imaging, endogenous protein tagging, and cell-specific RNAi during Caenorhabditis elegans anchor cell (AC) invasion, we demonstrate that the lipogenic SREBP transcription factor SBP-1 drives the expression of the fatty acid synthesis enzymes POD-2 and FASN-1 prior to invasion. We show that phospholipid-producing LPIN-1 and sphingomyelin synthase SMS-1, which use fatty acids as substrates, produce lysosome stores that build the AC's invasive protrusion, and that SMS-1 also promotes protrusion localization of the lipid raft partitioning ZMP-1 matrix metalloproteinase. Finally, we discover that HMG-CoA reductase HMGR-1, which generates isoprenoids for prenylation, localizes to the ER and enriches in peroxisomes at the AC invasive front, and that the final transmembrane prenylation enzyme, ICMT-1, localizes to endoplasmic reticulum exit sites that dynamically polarize to deliver prenylated GTPases for protrusion formation. Together, these results reveal a collaboration between lipogenesis and a polarized lipid prenylation system that drives invasive protrusion formation.

Free long-chain fatty acids trigger early postembryonic development in starved Caenorhabditis elegans by suppressing mTORC1

Postembryonic development of animals has long been considered an internally predetermined program, while macronutrients were believed to be essential solely for providing biomatters and energy to support this process. However, in this study, by using a nematode Caenorhabditis elegans (abbreviated as C. elegans hereafter) model, we surprisingly discovered that dietary supplementation of palmitic acid alone, rather than other abundant essential nutrients such as glucose or amino acid mixture, was sufficient to initiate early postembryonic development even under complete macronutrient deprivation. Such a development was evidenced by changes in morphology, cellular markers in multiple tissues, behaviors, and the global transcription pattern and it occurred earlier than the well-known early L1 nutrient checkpoint. Mechanistically, palmitic acid did not function as a biomatter/energy provider, but rather as a ligand to activate the nuclear hormone receptor NHR-49/80, leading to the production of an unknown peroxisome-derived secretive hormone in the intestine. This hormonal signal was received by chemosensory neurons in the head, regulating the insulin-like neuropeptide secretion and its downstream nuclear receptor to orchestrate global development. Additionally, the nutrient-sensing hub mTORC1 played a negative role in this process. In conclusion, our data indicate that free fatty acids act as a primary nutrient signal to launch the early development in C. elegans, which suggests that specific nutrients, rather than the internal genetic program, serve as the first impetus for postembryonic development.

Essential function of transmembrane transcription factor MYRF in promoting transcription of miRNA lin-4 during C. elegans development

Precise developmental timing control is essential for organism formation and function, but its mechanisms are unclear. In C. elegans, the microRNA lin-4 critically regulates developmental timing by post-transcriptionally downregulating the larval-stage-fate controller LIN-14. However, the mechanisms triggering the activation of lin-4 expression toward the end of the first larval stage remain unknown. We demonstrate that the transmembrane transcription factor MYRF-1 is necessary for lin-4 activation. MYRF-1 is initially localized on the cell membrane, and its increased cleavage and nuclear accumulation coincide with lin-4 expression timing. MYRF-1 regulates lin-4 expression cell-autonomously and hyperactive MYRF-1 can prematurely drive lin-4 expression in embryos and young first-stage larvae. The tandem lin-4 promoter DNA recruits MYRF-1GFP to form visible loci in the nucleus, suggesting that MYRF-1 directly binds to the lin-4 promoter. Our findings identify a crucial link in understanding developmental timing regulation and establish MYRF-1 as a key regulator of lin-4 expression.

Accelerated hermaphrodite maturation on male pheromones suggests a general principle of coordination between larval behavior and development

Environment in general and social signals in particular could alter development. In Caenorhabditis elegans, male pheromones hasten development of hermaphrodite larvae. We show that this involves acceleration of growth and both somatic and germline development during the last larval stage (L4). Larvae exposed to male pheromones spend more time in L3 and less in the quiescent period between L3 and L4. This behavioral alteration improves provision in early L4, likely allowing for faster development. Larvae must be exposed to male pheromones in late L3 for behavioral and developmental effects to occur. Latter portions of other larval stages also contain periods of heightened sensitivity to environmental signals. Behavior during the early part of the larval stages is biased toward exploration, whereas later the emphasis shifts to food consumption. We argue that this organization allows assessment of the environment to identify the most suitable patch of resources, followed by acquisition of sufficient nutrition and salient information for the developmental events in the next larval stage. Evidence from other species indicates that such coordination of behavior and development may be a general feature of larval development.

Exploiting the Unique Biology of Caenorhabditis elegans to Launch Neurodegeneration Studies in Space

The 21st century is likely to be the first century in which large-scale short- and long-term space missions become common. Accordingly, an ever-increasing body of research is focusing on understanding the effects of current and future space expeditions on human physiology in health and disease. Yet the complex experimental environment, the small number of participants, and the high cost of space missions are among the primary factors that hinder a better understanding of the impact of space missions on human physiology. The goal of our research was to develop a cost-effective, compact, and easy-to-manipulate system to address questions related to human health and disease in space. This initiative was part of the Ramon SpaceLab program, an annual research-based learning program designed to cultivate high school students' involvement in space exploration by facilitating experiments aboard the International Space Station (ISS). In the present study, we used the nematode Caenorhabditis elegans (C. elegans), a well-suited model organism, to investigate the effect of space missions on neurodegeneration-related processes. Our study specifically focused on the level of aggregation of Huntington's disease-causing polyglutamine stretch-containing (PolyQ) proteins in C. elegans muscles, the canonical system for studying neurodegeneration in this organism. We compared animals expressing PolyQ proteins grown onboard the ISS with their genetically identical siblings grown on Earth and observed a significant difference in the number of aggregates between the two populations. Currently, it is challenging to determine whether this effect stems from developmental or morphological differences between the cultures or is a result of life in space. Nevertheless, our results serve as a proof of concept and open a new avenue for utilizing C. elegans to address various open questions in space studies, including the effects of space conditions on the onset and development of neurodegenerative diseases.

Timers, variability, and body-wide coordination: C. elegans as a model system for whole-animal developmental timing

Successful development requires both precise timing of cellular processes, such as division and differentiation, and tight coordination of timing between tissues and organs. Yet, how time information is encoded with high precision and synchronized between tissues, despite inherent molecular noise, is unsolved. Here, we propose the nematode C. elegans as a unique model system for studying body-wide control of developmental timing. Recent studies combining genetics, quantitative analysis, and simulations have 1) mapped core timers controlling larval development, indicating temporal gradients as an underlying mechanism, and 2) elucidated general principles that make timing insensitive to inherent fluctuations and variation in environmental conditions. As the molecular regulators of C. elegans developmental timing are broadly conserved, these mechanisms likely apply also to higher organisms.

Decoding lifespan secrets: the role of the gonad in Caenorhabditis elegans aging

The gonad has become a central organ for understanding aging in C. elegans, as removing the proliferating stem cells in the germline results in significant lifespan extension. Similarly, when starvation in late larval stages leads to the quiescence of germline stem cells the adult nematode enters reproductive diapause, associated with an extended lifespan. This review summarizes recent advancements in identifying the mechanisms behind gonad-mediated lifespan extension, including comparisons with other nematodes and the role of lipid signaling and transcriptional changes. Given that the gonad also mediates lifespan regulation in other invertebrates and vertebrates, elucidating the underlying mechanisms may help to gain new insights into the mechanisms and evolution of aging.

Developmental plasticity: a worm's eye view

Numerous examples of different phenotypic outcomes in response to varying environmental conditions have been described across phyla, from plants to mammals. Here, we examine the impact of the environment on different developmental traits, focusing in particular on one key environmental variable, nutrient availability. We present advances in our understanding of developmental plasticity in response to food variation using the nematode Caenorhabditis elegans, which provides a near-isogenic context while permitting lab-controlled environments and analysis of wild isolates. We discuss how this model has allowed investigators not only to describe developmental plasticity events at the organismal level but also to zoom in on the tissues involved in translating changes in the environment into a plastic response, as well as the underlying molecular pathways, and sometimes associated changes in behaviour. Lastly, we also discuss how early life starvation experiences can be logged to later impact adult physiological traits, and how such memory could be wired.

A circadian-like gene network programs the timing and dosage of heterochronic miRNA transcription during C. elegans development

Development relies on the exquisite control of both the timing and the levels of gene expression to achieve robust developmental transitions. How cis- and trans-acting factors control both aspects simultaneously is unclear. We show that transcriptional pulses of the temporal patterning microRNA (miRNA) lin-4 are generated by two nuclear hormone receptors (NHRs) in C. elegans, NHR-85 and NHR-23, whose mammalian orthologs, Rev-Erb and ROR, function in the circadian clock. Although Rev-Erb and ROR antagonize each other to control once-daily transcription in mammals, NHR-85/NHR-23 heterodimers bind cooperatively to lin-4 regulatory elements to induce a single pulse of expression during each larval stage. Each pulse's timing, amplitude, and duration are dictated by the phased expression of these NHRs and the C. elegans Period ortholog, LIN-42, that binds to and represses NHR-85. Therefore, during nematode temporal patterning, an evolutionary rewiring of circadian clock components couples the timing of gene expression to the control of transcriptional dosage.

Cardiac microRNAs: diagnostic and therapeutic potential

MicroRNAs are small non-coding post-translational biomolecules which, when expressed, modify their target genes. It is estimated that microRNAs regulate production of approximately 60% of all human proteins and enzymes that are responsible for major physiological processes. In cardiovascular disease pathophysiology, there are several cells that produce microRNAs, including endothelial cells, vascular smooth muscle cells, macrophages, platelets, and cardiomyocytes. There is a constant crosstalk between microRNAs derived from various cell sources. Atherosclerosis initiation and progression are driven by many pro-inflammatory and pro-thrombotic microRNAs. Atherosclerotic plaque rupture is the leading cause of cardiovascular death resulting from acute coronary syndrome (ACS) and leads to cardiac remodeling and fibrosis following ACS. MicroRNAs are powerful modulators of plaque progression and transformation into a vulnerable state, which can eventually lead to plaque rupture. There is a growing body of evidence which demonstrates that following ACS, microRNAs might inhibit fibroblast proliferation and scarring, as well as harmful apoptosis of cardiomyocytes, and stimulate fibroblast reprogramming into induced cardiac progenitor cells. In this review, we focus on the role of cardiomyocyte-derived and cardiac fibroblast-derived microRNAs that are involved in the regulation of genes associated with cardiomyocyte and fibroblast function and in atherosclerosis-related cardiac ischemia. Understanding their mechanisms may lead to the development of microRNA cocktails that can potentially be used in regenerative cardiology.

Periods of environmental sensitivity couple larval behavior and development

The typical life cycle in most animal phyla includes a larval period that bridges embryogenesis and adulthood1. Despite the great diversity of larval forms, all larvae grow, acquire adult morphology and function, while navigating their habitats to obtain resources necessary for development. How larval development is coordinated with behavior remains substantially unclear. Here, we describe features of the iterative organization of larval stages that serve to assess the environment and procure resources prior to costly developmental commitments. We found that male-excreted pheromones accelerate2-4 the onset of adulthood in C. elegans hermaphrodites by coordinately advancing multiple developmental events and growth during the last larval stage. The larvae are sensitive to the accelerating male pheromones only at the end of the penultimate larval stage, just before the acceleration begins. Other larval stages also contain windows of sensitivity to environmental inputs. Importantly, behaviors associated with search and consumption of food are distinct between early and late portions of larval stages. We infer that each larval stage in C. elegans is subdivided into two epochs: A) global assessment of the environment to identify the most suitable patch and B) consumption of sufficient food and acquisition of salient information for developmental events in the next stage. We predict that in larvae of other species behavior is also divided into distinct epochs optimized either for assessing the habitat or obtaining the resources. Thus, a major role of larval behavior is to coordinate the orderly progression of development in variable environments.

Inhibition of ribosome biogenesis in the epidermis is sufficient to trigger organism-wide growth quiescence independently of nutritional status in C. elegans

Interorgan communication is crucial for multicellular organismal growth, development, and homeostasis. Cell nonautonomous inhibitory cues, which limit tissue-specific growth alterations, are not well characterized due to cell ablation approach limitations. In this study, we employed the auxin-inducible degradation system in C. elegans to temporally and spatially modulate ribosome biogenesis, through depletion of essential factors (RPOA-2, GRWD-1, or TSR-2). Our findings reveal that embryo-wide inhibition of ribosome biogenesis induces a reversible early larval growth quiescence, distinguished by a unique gene expression signature that is different from starvation or dauer stages. When ribosome biogenesis is inhibited in volumetrically similar tissues, including body wall muscle, epidermis, pharynx, intestine, or germ line, it results in proportionally stunted growth across the organism to different degrees. We show that specifically inhibiting ribosome biogenesis in the epidermis is sufficient to trigger an organism-wide growth quiescence. Epidermis-specific ribosome depletion leads to larval growth quiescence at the L3 stage, reduces organism-wide protein synthesis, and induced cell nonautonomous gene expression alterations. Further molecular analysis reveals overexpression of secreted proteins, suggesting an organism-wide regulatory mechanism. We find that UNC-31, a dense-core vesicle (DCV) pathway component, plays a significant role in epidermal ribosome biogenesis-mediated growth quiescence. Our tissue-specific knockdown experiments reveal that the organism-wide growth quiescence induced by epidermal-specific ribosome biogenesis inhibition is suppressed by reducing unc-31 expression in the epidermis, but not in neurons or body wall muscles. Similarly, IDA-1, a membrane-associated protein of the DCV, is overexpressed, and its knockdown in epidermis suppresses the organism-wide growth quiescence in response to epidermal ribosome biogenesis inhibition. Finally, we observe an overall increase in DCV puncta labeled by IDA-1 when epidermal ribosome biogenesis is inhibited, and these puncta are present in or near epidermal cells. In conclusion, these findings suggest a novel mechanism of nutrition-independent multicellular growth coordination initiated from the epidermis tissue upon ribosome biogenesis inhibition.

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

Visualizing cytoplasmic ATP in C. elegans larvae using PercevalHR

Measuring ATP levels within the cytosol of living cells in animals is important to understand how cellular activities are energetically supported, but is challenging because of tissue complexity and ATP sensor limitations. In this protocol, we describe how to quantify ATP levels using PercevalHR in C. elegans larvae during anchor cell invasion. PercevalHR is a fluorescent biosensor that reports the cytoplasmic ATP:ADP ratio. The protocol can be adapted to analyze the ATP:ADP ratios within other cell types in C. elegans. For complete details on the use and execution of this protocol, please refer to Garde et al. (2022).

Deviations from temporal scaling support a stage-specific regulation for C. elegans postembryonic development

BACKGROUND

After embryonic development, Caenorhabditis elegans progress through for larval stages, each of them finishing with molting. The repetitive nature of C. elegans postembryonic development is considered an oscillatory process, a concept that has gained traction from regulation by a circadian clock gene homologue. Nevertheless, each larval stage has a defined duration and entails specific events. Since the overall duration of development is controlled by numerous factors, we have asked whether different rate-limiting interventions impact all stages equally.

RESULTS

We have measured the duration of each stage of development for over 2500 larvae, under varied environmental conditions known to alter overall developmental rate. We applied changes in temperature and in the quantity and quality of nutrition and analysed the effect of genetically reduced insulin signalling. Our results show that the distinct developmental stages respond differently to these perturbations. The changes in the duration of specific larval stages seem to depend on stage-specific events. Furthermore, our high-resolution measurement of the effect of temperature on the stage-specific duration of development has unveiled novel features of temperature dependence in C. elegans postembryonic development.

CONCLUSIONS

Altogether, our results show that multiple factors fine tune developmental timing, impacting larval stages independently. Further understanding of the regulation of this process will allow modelling the mechanisms that control developmental timing.

The CHARGE syndrome ortholog CHD-7 regulates TGF-β pathways in Caenorhabditis elegans

CHARGE syndrome is a complex developmental disorder caused by mutations in the chromodomain helicase DNA-binding protein-7 (CHD7) and characterized by retarded growth and malformations in the heart and nervous system. Despite the public health relevance of this disorder, relevant cellular pathways and targets of CHD7 that relate to disease pathology are still poorly understood. Here we report that chd-7, the nematode ortholog of Chd7, is required for dauer morphogenesis, lifespan determination, stress response, and body size determination. Consistent with our discoveries, we found chd-7 to be allelic to scd-3, a previously identified dauer suppressor from the DAF-7/ tumor growth factor-β (TGF-β) pathway. Epistatic analysis places CHD-7 at the level of the DAF-3/DAF-5 complex, but we found that CHD-7 also directly impacts the expression of multiple components of this pathway. Transcriptomic analysis revealed that chd-7 mutants fail to repress daf-9 for execution of the dauer program. In addition, CHD-7 regulates the DBL-1/BMP pathway components and shares roles in male tail development and cuticle synthesis. To explore a potential conserved function for chd-7 in vertebrates, we used Xenopus laevis embryos, an established model to study craniofacial development. Morpholino-mediated knockdown of Chd7 led to a reduction in col2a1 messenger RNA (mRNA) levels, a collagen whose expression depends on TGF-β signaling. Both embryonic lethality and craniofacial defects in Chd7-depleted tadpoles were partially rescued by overexpression of col2a1 mRNA. We suggest that Chd7 has conserved roles in regulation of the TGF-β signaling pathway and pathogenic Chd7 could lead to a defective extracellular matrix deposition.

Temporal scaling in C. elegans larval development

SignificanceAn enduring mystery of development is how its timing is controlled, particularly for development after birth, where timing is highly flexible and depends on environmental conditions, such as food availability and diet. We followed timing of cell- and organism-level events in individual Caenorhabditis elegans larvae developing from hatching to adulthood, uncovering widespread variations in event timing, both between isogenic individuals in the same environment and when changing conditions and genotypes. However, in almost all cases, we found that events occurred at the same time, when time was rescaled by the duration of development measured in each individual. This observation of "temporal scaling" poses strong constraints on models to explain timing of larval development.

Insulin signaling and osmotic stress response regulate arousal and developmental progression of C. elegans at hatching

The progression of animal development from embryonic to juvenile life depends on the coordination of organism-wide responses with environmental conditions. We found that two transcription factors that function in interneuron differentiation in Caenorhabditis elegans, fax-1, and unc-42, are required for arousal and progression from embryogenesis to larval life by potentiating insulin signaling. The combination of mutations in either transcription factor and a mutation in daf-2 insulin receptor results in a novel perihatching arrest phenotype; embryos are fully developed but inactive, often remaining trapped within the eggshell, and fail to initiate pharyngeal pumping. This pathway is opposed by an osmotic sensory response pathway that promotes developmental arrest and a sleep state at the end of embryogenesis in response to elevated salt concentration. The quiescent state induced by loss of insulin signaling or by osmotic stress can be reversed by mutations in genes that are required for sleep. Therefore, countervailing signals regulate late embryonic arousal and developmental progression to larval life, mechanistically linking the two responses. Our findings demonstrate a role for insulin signaling in an arousal circuit, consistent with evidence that insulin-related regulation may function in control of sleep states in many animals. The opposing quiescent arrest state may serve as an adaptive response to the osmotic threat from high salinity environments.

Monomethyl branched-chain fatty acid mediates amino acid sensing upstream of mTORC1

Animals have developed various nutrient-sensing mechanisms for survival under fluctuating environmental conditions. Although extensive cell-culture-based analyses have identified diverse mediators of amino acid sensing upstream of mTOR, studies using animal models to examine intestine-initiated amino acid sensing mechanisms under specific physiological conditions are lacking. Here, we developed a Caenorhabditis elegans model to examine the impact of amino acid deficiency on development. We discovered a leucine-derived monomethyl branched-chain fatty acid and its downstream metabolite, glycosphingolipid, which critically mediates the overall amino acid sensing by intestinal and neuronal mTORC1, which in turn regulates postembryonic development at least partly by controlling protein translation and ribosomal biogenesis. Additional data suggest that a similar mechanism may operate in mammals. This study uncovers an amino-acid-sensing mechanism mediated by a lipid biosynthesis pathway.

Human amyloid beta and α-synuclein co-expression in neurons impair behavior and recapitulate features for Lewy body dementia in Caenorhabditis elegans

Amyloid β (Aβ), a product of APP, and SNCA (α-synuclein (α-syn)) are two of the key proteins found in lesions associated with the age-related neurodegenerative disorders Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. Previous clinical studies uncovered Aβ and α-syn co-expression in the brains of patients, which lead to Lewy body dementia (LBD), a disease encompassing Dementia with Lewy bodies (DLB) and Parkinson's disease dementia (PDD). To explore the pathogenesis and define the relationship between Aβ and α-syn for LBD, we established a C. elegans model which co-expresses human Aβ and α-syn with alanine 53 to threonine mutant (α-syn(A53T)) in pan-neurons. Compared to α-syn(A53T) single transgenic animals, pan-neuronal Aβ and α-syn(A53T) co-expression further enhanced the thrashing, egg laying, serotonin and cholinergic signaling deficits, and dopaminergic neuron damage in C. elegans. In addition, Aβ increased α-syn expression in transgenic animals. Transcriptome analysis of both Aβ;α-syn(A53T) strains and DLB patients showed common downregulation in lipid metabolism and lysosome function genes, suggesting that a decrease of lysosome function may reduce the clearance ability in DLB, and this may lead to the further pathogenic protein accumulation. These findings suggest that our model can recapitulate some features in LBD and provides a mechanism by which Aβ may exacerbate α-syn pathogenesis.

Development: Keeping Time with Transcription

A new study shows that the pioneer transcription factor BLMP-1 governs the cyclic transcriptional output of hundreds of genes during Caenorhabditis elegans larval development. BLMP-1 is also critical for restarting the proper developmental dynamics of gene expression after nutritionally induced developmental arrest.

Developmental plasticity and the response to nutrient stress in Caenorhabditis elegans

Developmental plasticity refers the ability of an organism to adapt to various environmental stressors, one of which is nutritional stress. Caenorhabditis elegans require various nutrients to successfully progress through all the larval stages to become a reproductive adult. If nutritional criteria are not satisfied, development can slow or completely arrest. In poor growth conditions, the animal can enter various diapause stages, depending on its developmental progress. In C. elegans, there are three well-characterized diapauses: the L1 arrest, the dauer diapause, and adult reproductive diapause, each associated with drastic changes in metabolism and germline development. At the centre of these changes is AMP-activated protein kinase (AMPK). AMPK is a metabolic regulator that maintains energy homeostasis, particularly during times of nutrient stress. Without AMPK, metabolism is disrupted during dauer, leading to the rapid consumption of lipid stores as well as misregulation of metabolic enzymes, leading to reduced survival. During the L1 arrest and dauer diapause, AMPK is responsible for ensuring germline quiescence by modifying the germline chromatin landscape to maintain germ cell integrity until conditions improve. Similar to classic hormonal signalling, small RNAs also play a critical role in regulating development and behaviour in a cell non-autonomous fashion. Thus, during the challenges associated with developmental plasticity, AMPK summons an army of signalling pathways to work collectively to preserve reproductive fitness during these periods of unprecedented uncertainty.

An Epigenetic Priming Mechanism Mediated by Nutrient Sensing Regulates Transcriptional Output during C. elegans Development

Although precise tuning of gene expression levels is critical for most developmental pathways, the mechanisms by which the transcriptional output of dosage-sensitive molecules is established or modulated by the environment remain poorly understood. Here, we provide a mechanistic framework for how the conserved transcription factor BLMP-1/Blimp1 operates as a pioneer factor to decompact chromatin near its target loci during embryogenesis (hours prior to major transcriptional activation) and, by doing so, regulates both the duration and amplitude of subsequent target gene transcription during post-embryonic development. This priming mechanism is genetically separable from the mechanisms that establish the timing of transcriptional induction and functions to canalize aspects of cell-fate specification, animal size regulation, and molting. A key feature of the BLMP-1-dependent transcriptional priming mechanism is that chromatin decompaction is initially established during embryogenesis and maintained throughout larval development by nutrient sensing. This anticipatory mechanism integrates transcriptional output with environmental conditions and is essential for resuming normal temporal patterning after animals exit nutrient-mediated developmental arrests.

Nutritional control of postembryonic development progression and arrest in Caenorhabditis elegans

Developmental programs are under strict genetic control that favors robustness of the process. In order to guarantee the same outcome in different environmental situations, development is modulated by input pathways, which inform about external conditions. In the nematode Caenorhabditis elegans, the process of postembryonic development involves a series of stereotypic cell divisions, the progression of which is controlled by the nutritional status of the animal. C. elegans can arrest development at different larval stages, leading to cell arrest of the relevant divisions of the stage. This means that studying the nutritional control of development in C. elegans we can learn about the mechanisms controlling cell division in an in vivo model. In this work, we reviewed the current knowledge about the nutrient sensing pathways that control the progression or arrest of development in response to nutrient availability, with a special focus on the arrest at the L1 stage.

Gene expression oscillations in C. elegans underlie a new developmental clock

During C. elegans larval development, thousands of genes, accounting for >20% of the transcriptome, exhibit oscillatory expression with large amplitudes. The time of peaking varies for different genes, but expression generally peaks once per larval stage, with both the oscillation period and larval stage duration varying in concert with temperature. This and other evidence support the existence of a gene expression oscillator that functions as a developmental clock. In this article, we review what is known about the biology, architecture and possible mechanisms of this clock. We compare it to other oscillators, and highlight tools and approaches suited to its study. Finally, we point out implications of these wide-spread and dynamic changes of gene expression on any type of gene expression profiling experiment in C. elegans larvae and how such experiments need to be controlled.

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.

Cephalic Neuronal Vesicle Formation is Developmentally Dependent and Modified by Methylmercury and sti-1 in Caenorhabditis elegans

Methylmercury (MeHg) is a potent neurotoxicant. The mechanisms underlying MeHg-induced neurotoxicity are not fully understood. Several studies have shown that protein chaperones are involved in MeHg toxicity. The protein co-chaperone, stress inducible protein 1 (STI-1), has important functions in protein quality control of the chaperone pathway. In the current study, dopaminergic (DAergic) cephalic (CEP) neuronal morphology was evaluated in the Caenorhabditis elegans (C. elegans) sti-1 knockout strain. In the control OH7193 strain (dat-1::mCherry + ttx-3::mCherry), we characterized the morphology of CEP neurons by checking the presence of attached vesicles and unattached vesicles to the CEP dendrites. We showed that the attached vesicles were only present in adult stage worms; whereas they were absent in the younger L3 stage worms. In the sti-1 knockout strain, MeHg treatment significantly altered the structures of CEP dendrites with discontinuation of mCherry fluorescence and shrinkage of CEP soma, as compared to the control. 12 h post treatment on MeHg-free OP50-seeded plates, the discontinuation of mCherry fluorescence of CEP dendrites in worms treated with 0.05 or 0.5 µM MeHg returned to levels statistically indistinguishable from control, while in worms treated with 5 µM MeHg a higher percentage of discontinuation of mCherry fluorescence persisted. Despite this strong effect by 5 µM MeHg, CEP attached vesicles were increased upon 0.05 or 0.5 µM MeHg treatment, yet unaffected by 5 µM MeHg. The CEP attached vesicles of sti-1 knockout strain were significantly increased shortly after MeHg treatment, but were unaffected 48 h post treatment. In addition, there was a significant interactive effect of MeHg and sti-1 on the number of attached vesicles. Knock down sti-1 via RNAi did not alter the number of CEP attached vesicles. Taking together, our data suggests that the increased occurrence of attached vesicles in adult stage worms could initiate a substantial loss of membrane components of CEP dendrites following release of vesicles, leading to the discontinuation of mCherry fluorescence, and the formation of CEP attached vesicles could be regulated by sti-1 to remove cellular debris for detoxification.

N,N' bis-(2-mercaptoethyl) isophthalamide induces developmental delay in Caenorhabditis elegans by promoting DAF-16 nuclear localization

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NBMI induces developmental delays in C. elegans.

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The nuclear translocation of DAF-16 is involved in the developmental effects of NBMI.

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NBMI re­presses the ex­pres­sion of detox­i­fy­ing genes (skn-1, gst-4 and gcs-1).

N,N’ bis-(2-mercaptoethyl) isophthalamide (NBMI) is a lipophilic thiol-containing agent that has high affinity for toxic metal ions, such as Hg²⁺, Pb²⁺, and Cd²⁺. Studies have shown that NBMI is a potent chelator of heavy metals, yet its potential toxicity in animals has yet to be determined. Using the model organism Caenorhabditis elegans (C. elegans), we show no significant change in worms’ death rate or lifespan following NBMI treatment (10−1000 μM). However, NBMI treatment was associated with a significant developmental delay. To determine if the daf-2/age-1/daf-16 pathway is involved in NBMI toxicity, mRNA levels of these genes were assessed in worms treated with NBMI. Here, we found that while NBMI failed to significantly alter the expression of daf-16 or daf-2; age-1 was significantly downregulated by NBMI. Furthermore, NBMI significantly increased DAF-16 nuclear localization. Consistent with a role for this pathway in NBMI toxicity, the developmental arrest by NBMI was more prominent in the DAF-16 transgenic strain than in the wild type N2 strain. Moreover, in the mutant strains harboring null alleles of daf-16, NBMI had no effect on development. In addition, NBMI repressed the expression of detoxifying genes (skn-1, gst-4 and gcs-1). In summary, NBMI has a low developmental toxicity in the C. elegans model, and the nuclear translocation of DAF-16 is involved in the developmental effect of NBMI.

Developmental function and state transitions of a gene expression oscillator in Caenorhabditis elegans

Gene expression oscillators can structure biological events temporally and spatially. Different biological functions benefit from distinct oscillator properties. Thus, finite developmental processes rely on oscillators that start and stop at specific times, a poorly understood behavior. Here, we have characterized a massive gene expression oscillator comprising > 3,700 genes in Caenorhabditis elegans larvae. We report that oscillations initiate in embryos, arrest transiently after hatching and in response to perturbation, and cease in adults. Experimental observation of the transitions between oscillatory and non-oscillatory states at high temporal resolution reveals an oscillator operating near a Saddle Node on Invariant Cycle (SNIC) bifurcation. These findings constrain the architecture and mathematical models that can represent this oscillator. They also reveal that oscillator arrests occur reproducibly in a specific phase. Since we find oscillations to be coupled to developmental processes, including molting, this characteristic of SNIC bifurcations endows the oscillator with the potential to halt larval development at defined intervals, and thereby execute a developmental checkpoint function.

Cholesterol Regulates Innate Immunity via Nuclear Hormone Receptor NHR-8

Cholesterol is an essential nutrient for the function of diverse biological processes and for steroid biosynthesis across metazoans. However, the role of cholesterol in immune function remains understudied. Using the nematode Caenorhabditis elegans, which depends on the external environment for cholesterol, we studied the relationship between cholesterol and innate immunity. We found that the transporter CHUP-1 is required for the effect of cholesterol in the development of innate immunity and that the cholesterol-mediated immune response requires the nuclear hormone receptor NHR-8. Cholesterol acts through NHR-8 to transcriptionally regulate immune genes that are controlled by conserved immune pathways, including a p38/PMK-1 MAPK pathway, a DAF-2/DAF-16 insulin pathway, and an Nrf/SKN-1 pathway. Our results indicate that cholesterol plays a key role in the activation of conserved microbicidal pathways that are essential for survival against bacterial infections.

HLH-30/TFEB Is a Master Regulator of Reproductive Quiescence

All animals have evolved the ability to survive nutrient deprivation, and nutrient signaling pathways are conserved modulators of health and disease. In C. elegans, late-larval starvation provokes the adult reproductive diapause (ARD), a long-lived quiescent state that enables survival for months without food, yet underlying molecular mechanisms remain unknown. Here, we show that ARD is distinct from other forms of diapause, showing little requirement for canonical longevity pathways, autophagy, and fat metabolism. Instead it requires the HLH-30/TFEB transcription factor to promote the morphological and physiological remodeling involved in ARD entry, survival, and recovery, suggesting that HLH-30 is a master regulator of reproductive quiescence. HLH-30 transcriptome and genetic analyses reveal that Max-like HLH factors, AMP-kinase, mTOR, protein synthesis, and mitochondrial fusion are target processes that promote ARD longevity. ARD thus rewires metabolism to ensure long-term survival and may illuminate similar mechanisms acting in stem cell quiescence and long-term fasting.

A metabolic switch regulates the transition between growth and diapause in C. elegans

BACKGROUND

Metabolic activity alternates between high and low states during different stages of an organism's life cycle. During the transition from growth to quiescence, a major metabolic shift often occurs from oxidative phosphorylation to glycolysis and gluconeogenesis. We use the entry of Caenorhabditis elegans into the dauer larval stage, a developmentally arrested stage formed in response to harsh environmental conditions, as a model to study the global metabolic changes and underlying molecular mechanisms associated with growth to quiescence transition.

RESULTS

Here, we show that the metabolic switch involves the concerted activity of several regulatory pathways. Whereas the steroid hormone receptor DAF-12 controls dauer morphogenesis, the insulin pathway maintains low energy expenditure through DAF-16/FoxO, which also requires AAK-2/AMPKα. DAF-12 and AAK-2 separately promote a shift in the molar ratios between competing enzymes at two key branch points within the central carbon metabolic pathway diverting carbon atoms from the TCA cycle and directing them to gluconeogenesis. When both AAK-2 and DAF-12 are suppressed, the TCA cycle is active and the developmental arrest is bypassed.

CONCLUSIONS

The metabolic status of each developmental stage is defined by stoichiometric ratios within the constellation of metabolic enzymes driving metabolic flux and controls the transition between growth and quiescence.

The Rheb-TORC1 signaling axis functions as a developmental checkpoint

In many eukaryotes, the small GTPase Rheb functions as a switch to toggle activity of TOR complex 1 (TORC1) between anabolism and catabolism, thus controlling lifespan, development and autophagy. Our CRISPR-generated, fluorescently tagged endogenous Caenorhabditis elegans RHEB-1 and DAF-15/Raptor are expressed ubiquitously and localize to lysosomes. LET-363/TOR and DAF-15/Raptor are required for development beyond the third larval stage (L3). We observed that deletion of RHEB-1 similarly conferred L3 arrest. Unexpectedly, robust RNAi-mediated depletion of TORC1 components caused arrest at stages prior to L3. Accordingly, conditional depletion of endogenous DAF-15/Raptor in the soma revealed that TORC1 is required at each stage of the life cycle to progress to the next stage. Reversal of DAF-15 depletion permits arrested animals to recover to continue development. Our results are consistent with TORC1 functioning as a developmental checkpoint that governs the decision of the animal to progress through development.

Insights Into the Hypometabolic Stage Caused by Prolonged Starvation in L4-Adult Caenorhabditis elegans Hermaphrodites

Animals alter their reproductive cycles in response to changing nutritional conditions, to ensure that offspring production only occurs under favorable circumstances. These adaptive strategies include reversible hypometabolic states of dormancy such as “arrest” and “diapause.” The free-living nematode Caenorhabditis elegans can arrest its life cycle during some larval stages without modifying its anatomy and physiology until conditions improve but it can also modify its morphological and physiological features to cope with harsh conditions and transition into diapause. The well-defined “dauer” diapause was described more than 40 years ago and has been the subject of comprehensive investigations. The existence of another hypometabolic state, termed adult reproductive diapause (ARD), has been debated after it was first described 10 years ago. Here, we review the current knowledge regarding the effect of food deprivation during the pre-reproductive larval and adult stages on overall organismal homeostasis, highlighting the implications on germ cell maintenance and fertility preservation.

Nutrient Sensing and Response Drive Developmental Progression in Caenorhabditis elegans

In response to nutrient limitation, many animals, including Caenorhabditis elegans, slow or arrest their development. This process requires mechanisms that sense essential nutrients and induce appropriate responses. When faced with nutrient limitation, C. elegans can induce both short and long-term survival strategies, including larval arrest, decreased developmental rate, and dauer formation. To select the most advantageous strategy, information from many different sensors must be integrated into signaling pathways, including target of rapamycin (TOR) and insulin, that regulate developmental progression. Here, how nutrient information is sensed and integrated into developmental decisions that determine developmental rate and progression in C. elegans is reviewed.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Population Selection and Sequencing of Caenorhabditis elegans Wild Isolates Identifies a Region on Chromosome III Affecting Starvation Resistance

To understand the genetic basis of complex traits, it is important to be able to efficiently phenotype many genetically distinct individuals. In the nematode Caenorhabditis elegans, individuals have been isolated from diverse populations around the globe and whole-genome sequenced. As a result, hundreds of wild strains with known genome sequences can be used for genome-wide association studies (GWAS). However, phenotypic analysis of these strains can be laborious, particularly for quantitative traits requiring multiple measurements per strain. Starvation resistance is likely a fitness-proximal trait for nematodes, and it is related to metabolic disease risk in humans. However, natural variation in C. elegans starvation resistance has not been systematically characterized, and precise measurement of the trait is time-intensive. Here, we developed a population-selection-and-sequencing-based approach to phenotype starvation resistance in a pool of 96 wild strains. We used restriction site-associated DNA sequencing (RAD-seq) to infer the frequency of each strain among survivors in a mixed culture over time during starvation. We used manual starvation survival assays to validate the trait data, confirming that strains that increased in frequency over time are starvation-resistant relative to strains that decreased in frequency. Further, we found that variation in starvation resistance is significantly associated with variation at a region on chromosome III. Using a near-isogenic line (NIL), we showed the importance of this genomic interval for starvation resistance. This study demonstrates the feasibility of using population selection and sequencing in an animal model for phenotypic analysis of quantitative traits, documents natural variation of starvation resistance in C. elegans, and identifies a genomic region that contributes to such variation.

Insulin/IGF Signaling and Vitellogenin Provisioning Mediate Intergenerational Adaptation to Nutrient Stress

The roundworm C. elegans reversibly arrests larval development during starvation [1], but extended early-life starvation reduces reproductive success [2, 3]. Maternal dietary restriction (DR) buffers progeny from starvation as young larvae, preserving reproductive success [4]. However, the developmental basis of reduced fertility following early-life starvation is unknown, and it is unclear how maternal diet modifies developmental physiology in progeny. We show here that extended starvation in first-stage (L1) larvae followed by unrestricted feeding results in a variety of developmental abnormalities in the reproductive system, including proliferative germ-cell tumors and uterine masses that express neuronal and epidermal cell fate markers. We found that maternal DR and reduced maternal insulin/insulin-like growth factor (IGF) signaling (IIS) increase oocyte provisioning of vitellogenin lipoprotein, reducing penetrance of starvation-induced abnormalities in progeny, including tumors. Furthermore, we show that maternal DR and reduced maternal IIS reduce IIS in progeny. daf-16/FoxO and skn-1/Nrf, transcriptional effectors of IIS, are required in progeny for maternal DR and increased vitellogenin provisioning to suppress starvation-induced abnormalities. daf-16/FoxO activity in somatic tissues is sufficient to suppress starvation-induced abnormalities, suggesting cell-nonautonomous regulation of reproductive system development. This work reveals that early-life starvation compromises reproductive development and that vitellogenin-mediated intergenerational insulin/IGF-to-insulin/IGF signaling mediates adaptation to nutrient availability.

Apoptosis contributes to protect germ cells from the oogenic germline starvation response but is not essential for the gonad shrinking or recovery observed during adult reproductive diapause in C. elegans

When C. elegans hermaphrodites are deprived of food during the mid-L4 larval stage and throughout adulthood, they enter an alternative stage termed "adult reproductive diapause (ARD)" in which they halt reproduction and extend their lifespan. During ARD, germ cell proliferation stops; oogenesis is slowed; and the gonad shrinks progressively, which has been described as the "oogenic germline starvation response". Upon refeeding, the shrunken gonad is regenerated, and animals recover fertility and live out their remaining lifespan. Little is known about the effects of ARD on oocyte quality after ARD. Thus, the aim of this study was to determine how oocyte quality is affected after ARD by measuring brood size and embryonic lethality as a reflection of defective oocyte production. We found that ARD affects reproductive capacity. The oogenic germline starvation response protects oogenic germ cells by slowing oogenesis to prevent prolonged arrest in diakinesis. In contrast to a previous report, we found that germ cell apoptosis is not the cause of gonad shrinkage; instead, we propose that ovulation contributes to gonad shrinkage during the oogenic germline starvation response. We show that germ cell apoptosis increases and continues during ARD via lin-35/Rb and an unknown mechanism. Although apoptosis contributes to maintain germ cell quality during ARD, we demonstrated that apoptosis is not essential to preserve animal fertility. Finally, we show that IIS signaling inactivation partially participates in the oogenic germline starvation response.

Pheromones and Nutritional Signals Regulate the Developmental Reliance on let-7 Family MicroRNAs in C. elegans

Adverse environmental conditions can affect rates of animal developmental progression and lead to temporary developmental quiescence (diapause), exemplified by the dauer larva stage of the nematode Caenorhabditis elegans (C. elegans). Remarkably, patterns of cell division and temporal cell-fate progression in C. elegans larvae are not affected by changes in developmental trajectory. However, the underlying physiological and gene regulatory mechanisms that ensure robust developmental patterning despite substantial plasticity in developmental progression are largely unknown. Here, we report that diapause-inducing pheromones correct heterochronic developmental cell lineage defects caused by insufficient expression of let-7 family microRNAs in C. elegans. Moreover, two conserved endocrine signaling pathways, DAF-7/TGF-β and DAF-2/Insulin, that confer on the larva diapause and non-diapause alternative developmental trajectories interact with the nuclear hormone receptor, DAF-12, to initiate and regulate a rewiring of the genetic circuitry controlling temporal cell fates. This rewiring includes engagement of certain heterochronic genes, lin-46, lin-4, and nhl-2, that are previously associated with an altered genetic program in post-diapause animals, in combination with a novel ligand-independent DAF-12 activity, to downregulate the critical let-7 family target Hunchback-like-1 (HBL-1). Our results show how pheromone or endocrine signaling pathways can coordinately regulate both developmental progression and cell-fate transitions in C. elegans larvae under stress so that the developmental schedule of cell fates remains unaffected by changes in developmental trajectory.

Cell Non-autonomous Function of daf-18/PTEN in the Somatic Gonad Coordinates Somatic Gonad and Germline Development in C. elegans Dauer Larvae

C. elegans larvae integrate environmental information and developmental decisions [1-3]. In favorable conditions, worms develop rapidly and continuously through four larval stages into reproductive adulthood. However, if conditions are unfavorable through the second larval stage, worms enter dauer diapause, a state of global and reversible developmental arrest in which precursor cells remain quiescent and preserve developmental potential, anticipating developmental progression if conditions improve. Signaling from neurons, hypodermis, and intestine regulate the appearance and behavior of dauer larvae and many aspects of developmental arrest of the non-gonadal soma [1, 4, 5]. Here, we show that the decision of somatic gonad blast cells (SGBs) and germline stem cells (GSCs) to be quiescent or progress developmentally is regulated differently from the non-gonadal soma: daf-18/PTEN acts non-autonomously within the somatic gonad to maintain developmental quiescence of both SGBs and GSCs. Our analysis suggests that daf-18 acts in somatic gonad cells to produce a "pro-quiescence" signal (or signals) that acts inter se and between the somatic gonad and the germline. The inferred signal does not require DAF-2/insulin receptor or maintain quiescence of the nearby sex myoblasts, and developmental progression in daf-18(0) does not require dafachronic acids. Abrogating quiescence in dauer results in post-dauer sterility. Our results implicate the somatic gonad as an endocrine organ to synchronize somatic gonad and germline development during dauer diapause and recovery, and our finding that PTEN acts non-autonomously to control blast cell quiescence may be relevant to its function as a tumor suppressor in mammals and to combating parasitic nematodes.

Molecular evolution and functional characterisation of insulin related peptides in molluscs: Contributions of Crassostrea gigas genomic and transcriptomic-wide screening

Insulin Related Peptides (IRPs) belong to the insulin superfamily and possess a typical structure with two chains, B and A, linked by disulphide bonds. As the sequence conservation is usually low between members, IRPs are classified according to the number and position of their disulphide bonds. In molluscan species, the first IRPs identified, named Molluscan Insulin-related Peptides (MIPs), exhibit four disulphide bonds. The genomic and transcriptomic data screening in the Pacific oyster Crassostrea gigas (Mollusc, Bivalvia) allowed us to identify six IRP sequences belonging to three structural groups. Cg-MIP1 to 4 have the typical structure of MIPs with four disulphide bonds. Cg-ILP has three disulphide bonds like vertebrate Insulin-Like Peptides (ILPs). The last one, Cg-MILP7 has a significant homology with Drosophila ILP7 (DILP7) associated with two additional cysteines allowing the formation of a fourth disulphide bond. The phylogenetic analysis points out that ILPs may be the most ancestral form. Moreover, it appears that ILP7 orthologs are probably anterior to lophotrochozoa and ecdysozoa segregation. In order to investigate the diversity of physiological functions of the oyster IRPs, we combine in silico expression data, qPCR measurements and in situ hybridization. The Cg-ilp transcript, mainly detected in the digestive gland and in the gonadal area, is potentially involved in the control of digestion and gametogenesis. The expression of Cg-mip4 is mainly associated with the larval development. The Cg-mip transcript shared by the Cg-MIP1, 2 and 3, is mainly expressed in visceral ganglia but its expression was also observed in the gonads of mature males. This pattern suggested the key roles of IRPs in the control of sexual reproduction in molluscan species.

Nonselective autophagy reduces mitochondrial content during starvation in Caenorhabditis elegans

Starvation significantly alters cellular physiology, and signs of aging have been reported to occur during starvation. Mitochondria are essential to the regulation of cellular energetics and aging. We sought to determine whether mitochondria exhibit signs of aging during starvation and whether quality control mechanisms regulate mitochondrial physiology during starvation. We describe effects of starvation on mitochondria in the first and third larval stages of the nematode Caenorhabditis elegans. When starved, C. elegans larvae enter developmental arrest. We observed fragmentation of the mitochondrial network, a reduction in mitochondrial DNA (mtDNA) copy number, and accumulation of DNA damage during starvation-induced developmental arrest. Mitochondrial function was also compromised by starvation. Starved worms had lower basal, maximal, and ATP-linked respiration. These observations are consistent with reduced mitochondrial quality, similar to mitochondrial phenotypes during aging. Using pharmacological and genetic approaches, we found that worms deficient for autophagy were short-lived during starvation and recovered poorly from extended starvation, indicating sensitivity to nutrient stress. Autophagy mutants unc-51/Atg1 and atg-18/Atg18 maintained greater mtDNA content than wild-type worms during starvation, suggesting that autophagy promotes mitochondrial degradation during starvation. unc-51 mutants also had a proportionally smaller reduction in oxygen consumption rate during starvation, suggesting that autophagy also contributes to reduced mitochondrial function. Surprisingly, mutations in genes involved in mitochondrial fission and fusion as well as selective mitophagy of damaged mitochondria did not affect mitochondrial content during starvation. Our results demonstrate the profound influence of starvation on mitochondrial physiology with organismal consequences, and they show that these physiological effects are influenced by autophagy.

Transgenerational Effects of Extended Dauer Diapause on Starvation Survival and Gene Expression Plasticity in Caenorhabditis elegans

Phenotypic plasticity is facilitated by epigenetic regulation, and remnants of such regulation may persist after plasticity-inducing cues are gone. However, the relationship between plasticity and transgenerational epigenetic memory is not understood. Dauer diapause in Caenorhabditis elegans provides an opportunity to determine how a plastic response to the early-life environment affects traits later in life and in subsequent generations. We report that, after extended diapause, postdauer worms initially exhibit reduced reproductive success and greater interindividual variation. In contrast, F3 progeny of postdauers display increased starvation resistance and lifespan, revealing potentially adaptive transgenerational effects. Transgenerational effects are dependent on the duration of diapause, indicating an effect of extended starvation. In agreement, RNA-seq demonstrates a transgenerational effect on nutrient-responsive genes. Further, postdauer F3 progeny exhibit reduced gene expression plasticity, suggesting a trade-off between plasticity and epigenetic memory. This work reveals complex effects of nutrient stress over different time scales in an animal that evolved to thrive in feast and famine.

Reactivation of RNA metabolism underlies somatic restoration after adult reproductive diapause in C. elegans

The mechanisms underlying biological aging are becoming recognized as therapeutic targets to delay the onset of multiple age-related morbidities. Even greater health benefits can potentially be achieved by halting or reversing age-associated changes. C. elegans restore their tissues and normal longevity upon exit from prolonged adult reproductive diapause, but the mechanisms underlying this phenomenon remain unknown. Here, we focused on the mechanisms controlling recovery from adult diapause. Here, we show that functional improvement of post-mitotic somatic tissues does not require germline signaling, germline stem cells, or replication of nuclear or mitochondrial DNA. Instead a large expansion of the somatic RNA pool is necessary for restoration of youthful function and longevity. Treating animals with the drug 5-fluoro-2'-deoxyuridine prevents this restoration by blocking reactivation of RNA metabolism. These observations define a critical early step during exit from adult reproductive diapause that is required for somatic rejuvenation of an adult metazoan animal.

Hypodermal responses to protein synthesis inhibition induce systemic developmental arrest and AMPK-dependent survival in Caenorhabditis elegans

Across organisms, manipulation of biosynthetic capacity arrests development early in life, but can increase health- and lifespan post-developmentally. Here we demonstrate that this developmental arrest is not sickness but rather a regulated survival program responding to reduced cellular performance. We inhibited protein synthesis by reducing ribosome biogenesis (rps-11/RPS11 RNAi), translation initiation (ifg-1/EIF3G mutation and egl-45/EIF3A RNAi), or ribosome progression (cycloheximide treatment), all of which result in a specific arrest at larval stage 2 of C. elegans development. This quiescent state can last for weeks—beyond the normal C. elegans adult lifespan—and is reversible, as animals can resume reproduction and live a normal lifespan once released from the source of protein synthesis inhibition. The arrest state affords resistance to thermal, oxidative, and heavy metal stress exposure. In addition to cell-autonomous responses, reducing biosynthetic capacity only in the hypodermis was sufficient to drive organism-level developmental arrest and stress resistance phenotypes. Among the cell non-autonomous responses to protein synthesis inhibition is reduced pharyngeal pumping that is dependent upon AMPK-mediated signaling. The reduced pharyngeal pumping in response to protein synthesis inhibition is recapitulated by exposure to microbes that generate protein synthesis-inhibiting xenobiotics, which may mechanistically reduce ingestion of pathogen and toxin. These data define the existence of a transient arrest-survival state in response to protein synthesis inhibition and provide an evolutionary foundation for the conserved enhancement of healthy aging observed in post-developmental animals with reduced biosynthetic capacity.

Protein synthesis is an essential cellular process, but post-developmental reduction of protein synthesis across multiple species leads to improved health- and lifespan. To better understand the physiological responses to impaired protein synthesis, we characterize a novel developmental arrest state that occurs when reducing protein synthesis during C. elegans development. Arrested animals have multiple survival-promoting phenotypes that are all dependent on the cellular energy sensor, AMP kinase. This survival response acts through the hypodermis and causes a reduction in pharyngeal pumping, indicating that the animal is responding to a perceived external threat, even in adults. Furthermore, exposing animals to pathogens, or xenobiotics they produce, can recapitulate these phenotypes, providing a potential evolutionary explanation for how a beneficial response in adults could evolve through the inhibition of an essential biological process such as protein synthesis.

Effects of Larval Density on Gene Regulation in Caenorhabditis elegans During Routine L1 Synchronization

Bleaching gravid C. elegans followed by a short period of starvation of the L1 larvae is a routine method performed by worm researchers for generating synchronous populations for experiments. During the process of investigating dietary effects on gene regulation in L1 stage worms by single-worm RNA-Seq, we found that the density of resuspended L1 larvae affects expression of many mRNAs. Specifically, a number of genes related to metabolism and signaling are highly expressed in worms arrested at low density, but are repressed at higher arrest densities. We generated a GFP reporter strain based on one of the most density-dependent genes in our dataset – lips-15 – and confirmed that this reporter was expressed specifically in worms arrested at relatively low density. Finally, we show that conditioned media from high density L1 cultures was able to downregulate lips-15 even in L1 animals arrested at low density, and experiments using daf-22 mutant animals demonstrated that this effect is not mediated by the ascaroside family of signaling pheromones. Together, our data implicate a soluble signaling molecule in density sensing by L1 stage C. elegans, and provide guidance for design of experiments focused on early developmental gene regulation.

daf-16/FoxO promotes gluconeogenesis and trehalose synthesis during starvation to support survival

daf-16/FoxO is required to survive starvation in Caenorhabditis elegans, but how daf-16IFoxO promotes starvation resistance is unclear. We show that daf-16/FoxO restructures carbohydrate metabolism by driving carbon flux through the glyoxylate shunt and gluconeogenesis and into synthesis of trehalose, a disaccharide of glucose. Trehalose is a well-known stress protectant, capable of preserving membrane organization and protein structure during abiotic stress. Metabolomic, genetic, and pharmacological analyses confirm increased trehalose synthesis and further show that trehalose not only supports survival as a stress protectant but also serves as a glycolytic input. Furthermore, we provide evidence that metabolic cycling between trehalose and glucose is necessary for this dual function of trehalose. This work demonstrates that daf-16/FoxO promotes starvation resistance by shifting carbon metabolism to drive trehalose synthesis, which in turn supports survival by providing an energy source and acting as a stress protectant.

Most animals rarely have access to a constant supply of food, and so have evolved ways to cope with times of plenty and times of shortage. Insulin is a hormone that travels throughout the body to signal when an animal is well fed. Insulin signaling inhibits the activity of a protein called FoxO, which otherwise switches on and off hundreds of genes to control the starvation response.

The roundworm, Caenorhabditis elegans, has been well studied in the laboratory, and often has to cope with starvation in the wild. These worms can pause their development if no food is available, or divert to a different developmental path if they anticipate that food will be short in future. As with more complex animals, the worm responds to starvation by reducing insulin-like signaling, which in turn activates a FoxO protein called daf-16. When the worms stop feeding, daf-16 is switched on, which is crucial for survival.

It was known how daf-16 stops the roundworm’s development, but it was not known how it helps the worms to survive starvation. Now, Hibshman et al. have compared normal roundworm larvae to larvae that are missing the gene for daf-16 to determine how this protein influences the roundworm’s ability to survive starvation.

The worms were examined with and without food, to look for which genes were switched on and off by daf-16 during starvation. This revealed that daf-16 controls metabolism, activating a metabolic shortcut that makes the worms produce glucose and begin turning it into another type of sugar, called trehalose. This sugar usually promotes survival in conditions where water is limiting, like dehydration and high salt, but it can also be broken down to release energy. The levels of trehalose in the worms rose within hours of the onset of starvation.

To confirm the importance of trehalose in surviving starvation, roundworms with mutations in genes involved in glucose or trehalose production were examined, as was the effect of giving starving worms glucose or trehalose. Disrupting the production of sugars caused the worms to die sooner of starvation, while supplementing with sugar had the opposite effect meaning the worms survived for longer.

Taken together, these findings reveal that daf-16 protects against starvation by shifting metabolism towards the production of trehalose. This helps worms to survive by both protecting them from stress and providing them with a source of energy. These findings not only extend the current understanding of how animals respond to starvation, but could also lead to improved understanding of diseases where this response goes wrong, including diabetes and obesity.