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

Neurogenesis in Caenorhabditis elegans

Animals rely on their nervous systems to process sensory inputs, integrate these with internal signals, and produce behavioral outputs. This is enabled by the highly specialized morphologies and functions of neurons. Neuronal cells share multiple structural and physiological features, but they also come in a large diversity of types or classes that give the nervous system its broad range of functions and plasticity. This diversity, first recognized over a century ago, spurred classification efforts based on morphology, function, and molecular criteria. Caenorhabditis elegans, with its precisely mapped nervous system at the anatomical level, an extensive molecular description of most of its neurons, and its genetic amenability, has been a prime model for understanding how neurons develop and diversify at a mechanistic level. Here, we review the gene regulatory mechanisms driving neurogenesis and the diversification of neuron classes and subclasses in C. elegans. We discuss our current understanding of the specification of neuronal progenitors and their differentiation in terms of the transcription factors involved and ensuing changes in gene expression and chromatin landscape. The central theme that has emerged is that the identity of a neuron is defined by modules of gene batteries that are under control of parallel yet interconnected regulatory mechanisms. We focus on how, to achieve these terminal identities, cells integrate information along their developmental lineages. Moreover, we discuss how neurons are diversified postembryonically in a time-, genetic sex-, and activity-dependent manner. Finally, we discuss how the understanding of neuronal development can provide insights into the evolution of neuronal diversity.

Functional analysis of conserved C. elegans bHLH family members uncovers lifespan control by a peptidergic hub neuron

Throughout the animal kingdom, several members of the basic helix-loop-helix (bHLH) family act as proneural genes during early steps of nervous system development. Roles of bHLH genes in specifying terminal differentiation of postmitotic neurons have been less extensively studied. We analyze here the function of five C. elegans bHLH genes, falling into three phylogenetically conserved subfamilies, which are continuously expressed in a very small number of postmitotic neurons in the central nervous system. We show (a) that two orthologs of the vertebrate bHLHb4/b5 genes, called hlh-17 and hlh-32, function redundantly to specify the identity of a single head interneuron (AUA), as well as an individual motor neuron (VB2), (b) that the PTF1a ortholog hlh-13 acts as a terminal selector to control terminal differentiation and function of the sole octopaminergic neuron class in C. elegans , RIC, and (c) that the NHLH1/2 ortholog hlh-15 controls terminal differentiation and function of the peptidergic AVK head interneuron class, a known neuropeptidergic signaling hub in the animal. Strikingly, through null mutant analysis and cell-specific rescue experiments, we find that loss of hlh-15/NHLH in the peptidergic AVK neurons and the resulting abrogation of neuropeptide secretion causes a substantially expanded lifespan of the animal, revealing an unanticipated impact of a central, peptidergic hub neuron in regulating lifespan, which we propose to be akin to hypothalamic control of lifespan in vertebrates. Taken together, our functional analysis reveals themes of bHLH gene function during terminal differentiation that are complementary to the earlier lineage specification roles of other bHLH family members. However, such late functions are much more sparsely employed by members of the bHLH transcription factor family, compared to the function of the much more broadly employed homeodomain transcription factor family.

Aging as a Consequence of the Adaptation-Maladaptation Dilemma

In aging, the organism is unable to counteract certain harmful influences over its lifetime which leads to progressive dysfunction and eventually death, thus delineating aging as one failed process of adaptation to a set of aging stimuli. A central problem in understanding aging is hence to explain why the organism cannot adapt to these aging stimuli. The adaptation-maladaptation theory of aging proposes that in aging adaptation processes such as adaptive transcription, epigenetic remodeling, and metabolic plasticity drive dysfunction themselves over time (maladaptation) and thereby cause aging-related disorders such as cancer and metabolic dysregulation. The central dilemma of aging is thus that the set of adaptation mechanisms that the body uses to deal with internal and external stressors acts as a stressor itself and cannot be effectively counteracted. The only available option for the organism to decrease maladaptation may be a program to progressively reduce the output of adaptive cascades (e.g., via genomic methylation) which then leads to reduced physiological adaptation capacity and syndromes like frailty, immunosenescence, and cognitive decline. The adaptation-maladaptation dilemma of aging entails that certain biological mechanisms can simultaneously protect against aging as well as drive aging. The key to longevity may lie in uncoupling adaptation from maladaptation.

Biophysical modeling of the whole-cell dynamics of C. elegans motor and interneurons families

The nematode Caenorhabditis elegans is a widely used model organism for neuroscience. Although its nervous system has been fully reconstructed, the physiological bases of single-neuron functioning are still poorly explored. Recently, many efforts have been dedicated to measuring signals from C. elegans neurons, revealing a rich repertoire of dynamics, including bistable responses, graded responses, and action potentials. Still, biophysical models able to reproduce such a broad range of electrical responses lack. Realistic electrophysiological descriptions started to be developed only recently, merging gene expression data with electrophysiological recordings, but with a large variety of cells yet to be modeled. In this work, we contribute to filling this gap by providing biophysically accurate models of six classes of C. elegans neurons, the AIY, RIM, and AVA interneurons, and the VA, VB, and VD motor neurons. We test our models by comparing computational and experimental time series and simulate knockout neurons, to identify the biophysical mechanisms at the basis of inter and motor neuron functioning. Our models represent a step forward toward the modeling of C. elegans neuronal networks and virtual experiments on the nematode nervous system.

Mitochondrial stress in GABAergic neurons non-cell autonomously regulates organismal health and aging

Mitochondrial stress within the nervous system can trigger non-cell autonomous responses in peripheral tissues. However, the specific neurons involved and their impact on organismal aging and health have remained incompletely understood. Here, we demonstrate that mitochondrial stress in γ-aminobutyric acid-producing (GABAergic) neurons in Caenorhabditis elegans ( C. elegans ) is sufficient to significantly alter organismal lifespan, stress tolerance, and reproductive capabilities. This mitochondrial stress also leads to significant changes in mitochondrial mass, energy production, and levels of reactive oxygen species (ROS). DAF-16/FoxO activity is enhanced by GABAergic neuronal mitochondrial stress and mediates the induction of these non-cell-autonomous effects. Moreover, our findings indicate that GABA signaling operates within the same pathway as mitochondrial stress in GABAergic neurons, resulting in non-cell-autonomous alterations in organismal stress tolerance and longevity. In summary, these data suggest the crucial role of GABAergic neurons in detecting mitochondrial stress and orchestrating non-cell-autonomous changes throughout the organism.

Analyses of Genetic Regulation of the Nervous System in the Nematode Caenorhabditis elegans

This chapter aims to provide a comprehensive overview of the methodologies available to dissect genetic regulation of the nervous systems in the nematode Caenorhabditis elegans. These techniques encompass genetic screens and genetic tools to unravel the spatial-temporal contribution of genes on neural structure and function. Unbiased genetic screens on random mutations induced by ethyl methanesulfonate (EMS) or target gene silencing by genome-wide RNA interference (RNAi) help progress our understanding of the genetic control of neural development and functions. Complement to unbiased genetic approaches, gene- and protein-targeted manipulation by Cre/LoxP recombination system and auxin-inducible degron (AID) protein degradation system, respectively, helps identify tissues/cells and the time window critical for gene and protein function during the proper execution of a particular behavior. Considering the remarkable conservation of genetic pathways between C. elegans and mammalian systems, elucidating the genetic underpinnings of neural functions and learning behaviors in C. elegans may furnish invaluable insights into analogous processes in more complex organisms. As shown in the following chapter, leveraging these diverse methodologies enable researchers to elucidate the intricate network governing neural function and structure, laying the foundation for innovating strategies to ameliorate cognitive alterations.

GABAergic signaling between enteric neurons and intestinal smooth muscle promotes innate immunity and gut defense in Caenorhabditis elegans

The nervous system is critical for intestinal homeostasis and function, but questions remain regarding its impact on gut immune defense. By screening the major neurotransmitters of C. elegans, we found that γ-aminobutyric acid (GABA) deficiency enhanced susceptibility to pathogenic Pseudomonas aeruginosa PA14 infection. GABAergic signaling between enteric neurons and intestinal smooth muscle promoted gut defense in a PMK-1/p38-dependent, but IIS/DAF-16- and DBL-1/TGF-β-independent, pathway. Transcriptomic profiling revealed that the neuropeptide, FLP-6, acted downstream of enteric GABAergic signaling. Further data determined that FLP-6 was expressed and secreted by intestinal smooth muscle cells and functioned as a paracrine molecule on the intestinal epithelium. FLP-6 suppressed the transcription factors ZIP-10 and KLF-1 that worked in parallel and converged to the PMK-1/p38 pathway in the intestinal epithelia for innate immunity and gut defense. Collectively, these findings uncover an enteric neuron-muscle-epithelium axis that may be evolutionarily conserved in higher organisms.

Early pheromone perception remodels neurodevelopment and accelerates neurodegeneration in adult C. elegans

Age-associated neurodegenerative disorders such as Parkinson's and Alzheimer's diseases are mainly caused by protein aggregation. The etiologies of these neurodegenerative diseases share a chemical environment. However, how chemical cues modulate neurodegeneration remains unclear. Here, we found that in Caenorhabditis elegans, exposure to pheromones in the L1 stage accelerates neurodegeneration in adults. Perception of pheromones ascr#3 and ascr#10 is mediated by chemosensory neurons ASK and ASI. ascr#3 perceived by G protein-coupled receptor (GPCR) DAF-38 in ASK activates glutamatergic transmission into AIA interneurons. ascr#10 perceived by GPCR STR-2 in ASI activates the secretion of neuropeptide NLP-1, which binds to the NPR-11 receptor in AIA. Activation of both ASI and ASK is required and sufficient to remodel neurodevelopment via AIA, which triggers insulin-like signaling and inhibits autophagy in adult neurons non-cell-autonomously. Our work reveals how pheromone perception at the early developmental stage modulates neurodegeneration in adults and provides insights into how the external environment impacts neurodegenerative diseases.

frpr-18, a neuropeptide receptor, regulates organismal lifespan and stress tolerance in C. elegans

The mechanisms underlying neuropeptide signaling regulation of lifespan in Caenorhabditis elegans ( C. elegans ) remain unclear. FRPR-18 is a mammalian orexin/hypocretin-like receptor and modulates C. elegans arousal behavior by acting as a receptor for FLP-2 neuropeptide signaling, which is also associated with the systemic activation of the mitochondrial unfolded protein response (mitoUPR). Here we report our preliminary findings on the role of the frpr-18 gene in regulating lifespan and healthspan parameters, including stress resistance. Our results showed that frpr-18(ok2698) null mutants had a shorter lifespan and reduced survivability against thermal stress and paraquat treatment. On the other hand, loss of flp-2 function did not affect lifespan or paraquat tolerance but was necessary for normal thermal stress tolerance. These findings suggest that frpr-18 could play a role in regulating lifespan and stress resistance, possibly through flp-2 independent or parallel neuropeptide signaling pathways.

Multisite regulation integrates multimodal context in sensory circuits to control persistent behavioral states in C. elegans

Maintaining or shifting between behavioral states according to context is essential for animals to implement fitness-promoting strategies. How the integration of internal state, past experience and sensory inputs orchestrates persistent multidimensional behavioral changes remains poorly understood. Here, we show that C. elegans integrates environmental temperature and food availability over different timescales to engage in persistent dwelling, scanning, global or glocal search strategies matching thermoregulatory and feeding needs. Transition between states, in each case, involves regulating multiple processes including AFD or FLP tonic sensory neurons activity, neuropeptide expression and downstream circuit responsiveness. State-specific FLP-6 or FLP-5 neuropeptide signaling acts on a distributed set of inhibitory GPCR(s) to promote scanning or glocal search, respectively, bypassing dopamine and glutamate-dependent behavioral state control. Integration of multimodal context via multisite regulation in sensory circuits might represent a conserved regulatory logic for a flexible prioritization on the valence of multiple inputs when operating persistent behavioral state transitions.

The longevity response to warm temperature is neurally controlled via the regulation of collagen genes

Studies in diverse species have associated higher temperatures with shorter lifespan and lower temperatures with longer lifespan. These inverse effects of temperature on longevity are traditionally explained using the rate of living theory, which posits that higher temperatures increase chemical reaction rates, thus speeding up the aging process. Recent studies have identified specific molecules and cells that affect the longevity response to temperature, indicating that this response is regulated, not simply thermodynamic. Here, we demonstrate that in Caenorhabditis elegans, functional loss of NPR-8, a G protein-coupled receptor related to mammalian neuropeptide Y receptors, increases worm lifespan at 25°C but not at 20°C or 15°C, and that the lifespan extension at 25°C is regulated by the NPR-8-expressing AWB and AWC chemosensory neurons as well as AFD thermosensory neurons. Integrative transcriptomic analyses revealed that both warm temperature and old age profoundly alter gene expression and that genes involved in the metabolic and biosynthetic processes increase expression at 25°C relative to 20°C, indicating elevated metabolism at warm temperature. These data demonstrate that the temperature-induced longevity response is neurally regulated and also provide a partial molecular basis for the rate of living theory, suggesting that these two views are not mutually exclusive. Genetic manipulation and functional assays further uncovered that the NPR-8-dependent longevity response to warm temperature is achieved by regulating the expression of a subset of collagen genes. As increased collagen expression is a common feature of many lifespan-extending interventions and enhanced stress resistance, collagen expression could be critical for healthy aging.

Molecular encoding of stimulus features in a single sensory neuron type enables neuronal and behavioral plasticity

Neurons modify their transcriptomes in response to an animal's experience. How specific experiences are transduced to modulate gene expression and precisely tune neuronal functions are not fully defined. Here, we describe the molecular profile of a thermosensory neuron pair in C. elegans experiencing different temperature stimuli. We find that distinct salient features of the temperature stimulus, including its duration, magnitude of change, and absolute value, are encoded in the gene expression program in this single neuron type, and we identify a novel transmembrane protein and a transcription factor whose specific transcriptional dynamics are essential to drive neuronal, behavioral, and developmental plasticity. Expression changes are driven by broadly expressed activity-dependent transcription factors and corresponding cis-regulatory elements that nevertheless direct neuron- and stimulus-specific gene expression programs. Our results indicate that coupling of defined stimulus characteristics to the gene regulatory logic in individual specialized neuron types can customize neuronal properties to drive precise behavioral adaptation.

Molecular encoding of stimulus features in a single sensory neuron type enables neuronal and behavioral plasticity

Neurons modify their transcriptomes in response to an animal’s experience. How specific experiences are transduced to modulate gene expression and precisely tune neuronal functions are not fully defined. Here, we describe the molecular profile of a thermosensory neuron pair in C. elegans experiencing different temperature stimuli. We find that distinct salient features of the temperature stimulus including its duration, magnitude of change, and absolute value are encoded in the gene expression program in this single neuron, and identify a novel transmembrane protein and a transcription factor whose specific transcriptional dynamics are essential to drive neuronal, behavioral, and developmental plasticity. Expression changes are driven by broadly expressed activity-dependent transcription factors and corresponding cis -regulatory elements that nevertheless direct neuron- and stimulus-specific gene expression programs. Our results indicate that coupling of defined stimulus characteristics to the gene regulatory logic in individual specialized neuron types can customize neuronal properties to drive precise behavioral adaptation.

One-day thermal regime extends the lifespan in Caenorhabditis elegans

Environmental factors, including temperature, can modulate an animal's lifespan. However, their underlying mechanisms remain largely undefined. We observed a profound effect of temperature on the aging of Caenorhabditis elegans (C. elegans) by performing proteomic analysis at different time points (young adult, middle age, and old age) and temperature conditions (20 °C and 25 °C). Importantly, although at the higher temperature, animals had short life spans, the shift from 20 °C to 25 °C for one day during early adulthood was beneficial for protein homeostasis since; it decreased protein synthesis and increased degradation. Consistent with our findings, animals who lived longer in the 25 °C shift were also more resistant to high temperatures along with oxidative and UV stresses. Furthermore, the lifespan extension by the 25 °C shift was mediated by three important transcription factors, namely FOXO/DAF-16, HSF-1, and HIF-1. We revealed an unexpected and complicated mechanism underlying the effects of temperature on aging, which could potentially aid in developing strategies to treat age-related diseases. Our data are available in ProteomeXchange with the identifier PXD024916.

Glia of C. elegans coordinate a protective organismal heat shock response independent of the neuronal thermosensory circuit

Aging organisms lose the ability to induce stress responses, becoming vulnerable to protein toxicity and tissue damage. Neurons can signal to peripheral tissues to induce protective organelle-specific stress responses. Recent work shows that glia can independently induce such responses. Here, we show that overexpression of heat shock factor 1 (hsf-1) in the four astrocyte-like cephalic sheath cells of Caenorhabditis elegans induces a non-cell-autonomous cytosolic unfolded protein response, also known as the heat shock response (HSR). These animals have increased lifespan and heat stress resistance and decreased protein aggregation. Glial HSR regulation is independent of canonical thermosensory circuitry and known neurotransmitters but requires the small clear vesicle release protein UNC-13. HSF-1 and the FOXO transcription factor DAF-16 are partially required in peripheral tissues for non-cell-autonomous HSR, longevity, and thermotolerance. Cephalic sheath glial hsf-1 overexpression also leads to pathogen resistance, suggesting a role for this signaling pathway in immune function.

The Thermal Stress Coping Network of the Nematode Caenorhabditis elegans

Response to hyperthermia, highly conserved from bacteria to humans, involves transcriptional upregulation of genes involved in battling the cytotoxicity caused by misfolded and denatured proteins, with the aim of proteostasis restoration. C. elegans senses and responds to changes in growth temperature or noxious thermal stress by well-defined signaling pathways. Under adverse conditions, regulation of the heat shock response (HSR) in C. elegans is controlled by a single transcription factor, heat-shock factor 1 (HSF-1). HSR and HSF-1 in particular are proven to be central to survival under proteotoxic stress, with additional roles in normal physiological processes. For years, it was a common belief that upregulation of heat shock proteins (HSPs) by HSF-1 was the main and most important step toward thermotolerance. However, an ever-growing number of studies have shown that targets of HSF-1 involved in cytoskeletal and exoskeletal integrity preservation as well as other HSF-1 dependent and independent pathways are equally important. In this review, we follow the thermal stimulus from reception by the nematode nerve endings till the activation of cellular response programs. We analyze the different HSF-1 functions in HSR as well as all the recently discovered mechanisms that add to the knowledge of the heat stress coping network of C. elegans.

Neuronal temperature perception induces specific defenses that enable C. elegans to cope with the enhanced reactivity of hydrogen peroxide at high temperature

Hydrogen peroxide is the most common reactive chemical that organisms face on the microbial battlefield. The rate with which hydrogen peroxide damages biomolecules required for life increases with temperature, yet little is known about how organisms cope with this temperature-dependent threat. Here, we show that Caenorhabditis elegans nematodes use temperature information perceived by sensory neurons to cope with the temperature-dependent threat of hydrogen peroxide produced by the pathogenic bacterium Enterococcus faecium. These nematodes preemptively induce the expression of specific hydrogen peroxide defenses in response to perception of high temperature by a pair of sensory neurons. These neurons communicate temperature information to target tissues expressing those defenses via an insulin/IGF1 hormone. This is the first example of a multicellular organism inducing their defenses to a chemical when they sense an inherent enhancer of the reactivity of that chemical.

Thermosensation in Caenorhabditis elegans is linked to ubiquitin-dependent protein turnover via insulin and calcineurin signalling

Organismal physiology and survival are influenced by environmental conditions and linked to protein quality control. Proteome integrity is achieved by maintaining an intricate balance between protein folding and degradation. In Caenorhabditis elegans, acute heat stress determines cell non-autonomous regulation of chaperone levels. However, how the perception of environmental changes, including physiological temperature, affects protein degradation remains largely unexplored. Here, we show that loss-of-function of dyf-1 in Caenorhabditis elegans associated with dysfunctional sensory neurons leads to defects in both temperature perception and thermal adaptation of the ubiquitin/proteasome system centered on thermosensory AFD neurons. Impaired perception of moderate temperature changes worsens ubiquitin-dependent proteolysis in intestinal cells. Brain-gut communication regulating protein turnover is mediated by upregulation of the insulin-like peptide INS-5 and inhibition of the calcineurin-regulated forkhead-box transcription factor DAF-16/FOXO. Our data indicate that perception of ambient temperature and its neuronal integration is important for the control of proteome integrity in complex organisms.

The hypothalamus for whole-body physiology: from metabolism to aging

Obesity and aging are two important epidemic factors for metabolic syndrome and many other health issues, which contribute to devastating diseases such as cardiovascular diseases, stroke and cancers. The brain plays a central role in controlling metabolic physiology in that it integrates information from other metabolic organs, sends regulatory projections and orchestrates the whole-body function. Emerging studies suggest that brain dysfunction in sensing various internal cues or processing external cues may have profound effects on metabolic and other physiological functions. This review highlights brain dysfunction linked to genetic mutations, sex, brain inflammation, microbiota, stress as causes for whole-body pathophysiology, arguing brain dysfunction as a root cause for the epidemic of aging and obesity-related disorders. We also speculate key issues that need to be addressed on how to reveal relevant brain dysfunction that underlines the development of these disorders and diseases in order to develop new treatment strategies against these health problems.

Identification of Modulators of the C. elegans Aryl Hydrocarbon Receptor and Characterization of Transcriptomic and Metabolic AhR-1 Profiles

The Aryl hydrocarbon Receptor (AhR) is a xenobiotic sensor in vertebrates, regulating the metabolism of its own ligands. However, no ligand has been identified to date for any AhR in invertebrates. In C. elegans, the AhR ortholog, AHR-1, displays physiological functions. Therefore, we compared the transcriptomic and metabolic profiles of worms expressing AHR-1 or not and investigated the putative panel of chemical AHR-1 modulators. The metabolomic profiling indicated a role for AHR-1 in amino acids, carbohydrates, and fatty acids metabolism. The transcriptional profiling in neurons expressing AHR-1, identified 95 down-regulated genes and 76 up-regulated genes associated with neuronal and metabolic functions in the nervous system. A gene reporter system allowed us to identify several AHR-1 modulators including bacterial, dietary, or environmental compounds. These results shed new light on the biological functions of AHR-1 in C. elegans and perspectives on the evolution of the AhR functions across species.

Regulation of Aging and Longevity by Ion Channels and Transporters

Despite significant advances in our understanding of the mechanisms that underlie age-related physiological decline, our ability to translate these insights into actionable strategies to extend human healthspan has been limited. One of the major reasons for the existence of this barrier is that with a few important exceptions, many of the proteins that mediate aging have proven to be undruggable. The argument put forth here is that the amenability of ion channels and transporters to pharmacological manipulation could be leveraged to develop novel therapeutic strategies to combat aging. This review delves into the established roles for ion channels and transporters in the regulation of aging and longevity via their influence on membrane excitability, Ca2+ homeostasis, mitochondrial and endolysosomal function, and the transduction of sensory stimuli. The goal is to provide the reader with an understanding of emergent themes, and prompt further investigation into how the activities of ion channels and transporters sculpt the trajectories of cellular and organismal aging.

Temperature-Dependent Regulation of Proteostasis and Longevity

Temperature is an important environmental condition that determines the physiology and behavior of all organisms. Animals use different response strategies to adapt and survive fluctuations in ambient temperature. The hermaphrodite Caenorhabditis elegans has a well-studied neuronal network consisting of 302 neurons. The bilateral AFD neurons are the primary thermosensory neurons in the nematode. In addition to regulating thermosensitivity, AFD neurons also coordinate cellular stress responses through systemic mechanisms involving neuroendocrine signaling. Recent studies have examined the effects of temperature on altering various signaling pathways through specific gene expression programs that promote stress resistance and longevity. These studies challenge the proposed theories of temperature-dependent regulation of aging as a passive thermodynamic process. Instead, they provide evidence that aging is a well-defined genetic program. Loss of protein homeostasis (proteostasis) is one of the key hallmarks of aging. Indeed, proteostasis pathways, such as the heat shock response and aggregation of metastable proteins, are also controlled by thermosensory neurons in C. elegans. Prolonged heat stress is thought to play a critical role in the development of neurodegenerative protein misfolding diseases in humans. This review presents the latest evidence on how temperature coordinates proteostasis and aging. It also discusses how studies of poikilothermic organisms can be applied to vertebrates and provides new therapeutic strategies for human disease.

Loss of circRNAs from the crh-1 gene extends the mean lifespan in Caenorhabditis elegans

Accumulation of circular RNAs (circRNAs) during aging occurs on a genome‐wide level for multiple organisms, but its significance is unknown. Generating circRNA loss‐of‐function mutants is difficult because the vast majority of these RNAs are comprised of exons shared with protein‐coding mRNAs. In Caenorhabditis elegans, most circRNAs were previously found to accumulate during aging. Two of the most abundant, age‐accumulating circRNAs are generated from exon 4 of the crh‐1 gene (circ‐crh‐1). Here, we found that the biogenesis of circ‐crh‐1 was regulated by the double‐stranded RNA‐binding protein ADR‐1. We identified Reverse Complementary Match (RCM) sequences in introns flanking circ‐crh‐1. Using CRISPR‐Cas9, we deleted the downstream RCM and found that this completely eliminated expression of the circRNA without affecting linear mRNA expression from the crh‐1 gene. Remarkably, worms lacking circ‐crh‐1 exhibited a significantly longer mean lifespan. Lifespan was partially restored to wild type by expression of circ‐crh‐1 in neural tissues. Widespread transcriptome alterations in circ‐crh‐1 mutants were identified using RNA‐Seq. Moving forward, intronic RCM deletion using CRISPR should be a widely applicable method to identify lifespan‐regulating circRNAs in C. elegans.

Genes and Longevity of Lifespan

Aging is a complex process indicated by low energy levels, declined physiological activity, stress induced loss of homeostasis leading to the risk of diseases and mortality. Recent developments in medical sciences and an increased availability of nutritional requirements has significantly increased the average human lifespan worldwide. Several environmental and physiological factors contribute to the aging process. However, about 40% human life expectancy is inherited among generations, many lifespan associated genes, genetic mechanisms and pathways have been demonstrated during last decades. In the present review, we have evaluated many human genes and their non-human orthologs established for their role in the regulation of lifespan. The study has included more than fifty genes reported in the literature for their contributions to the longevity of life. Intact genomic DNA is essential for the life activities at the level of cell, tissue, and organ. Nucleic acids are vulnerable to oxidative stress, chemotherapies, and exposure to radiations. Efficient DNA repair mechanisms are essential for the maintenance of genomic integrity, damaged DNA is not replicated and transferred to next generations rather the presence of deleterious DNA initiates signaling cascades leading to the cell cycle arrest or apoptosis. DNA modifications, DNA methylation, histone methylation, histone acetylation and DNA damage can eventually lead towards apoptosis. The importance of calorie restriction therapy in the extension of lifespan has also been discussed. The role of pathways involved in the regulation of lifespan such as DAF-16/FOXO (forkhead box protein O1), TOR and JNK pathways has also been particularized. The study provides an updated account of genetic factors associated with the extended lifespan and their interactive contributory role with cellular pathways.

Enhancing mask activity in dopaminergic neurons extends lifespan in flies

Dopaminergic neurons (DANs) are essential modulators for brain functions involving memory formation, reward processing, and decision‐making. Here I demonstrate a novel and important function of the DANs in regulating aging and longevity. Overexpressing the putative scaffolding protein Mask in two small groups of DANs in flies can significantly extend the lifespan in flies and sustain adult locomotor and fecundity at old ages. This Mask‐induced beneficial effect requires dopaminergic transmission but cannot be recapitulated by elevating dopamine production alone in the DANs. Independent activation of Gα_s in the same two groups of DANs via the drug‐inducible DREADD system also extends fly lifespan, further indicating the connection of specific DANs to aging control. The Mask‐induced lifespan extension appears to depend on the function of Mask to regulate microtubule (MT) stability. A structure–function analysis demonstrated that the ankyrin repeats domain in the Mask protein is both necessary for regulating MT stability (when expressed in muscles and motor neurons) and sufficient to prolong longevity (when expressed in the two groups of DANs). Furthermore, DAN‐specific overexpression of Unc‐104 or knockdown of p150^Glued, two independent interventions previously shown to impact MT dynamics, also extends lifespan in flies. Together, these data demonstrated a novel DANs‐dependent mechanism that, upon the tuning of their MT dynamics, modulates systemic aging and longevity in flies.

Neuronal regulated ire-1-dependent mRNA decay controls germline differentiation in Caenorhabditis elegans

Understanding the molecular events that regulate cell pluripotency versus acquisition of differentiated somatic cell fate is fundamentally important. Studies in Caenorhabditis elegans demonstrate that knockout of the germline-specific translation repressor gld-1 causes germ cells within tumorous gonads to form germline-derived teratoma. Previously we demonstrated that endoplasmic reticulum (ER) stress enhances this phenotype to suppress germline tumor progression(Levi-Ferber et al., 2015). Here, we identify a neuronal circuit that non-autonomously suppresses germline differentiation and show that it communicates with the gonad via the neurotransmitter serotonin to limit somatic differentiation of the tumorous germline. ER stress controls this circuit through regulated inositol requiring enzyme-1 (IRE-1)-dependent mRNA decay of transcripts encoding the neuropeptide FLP-6. Depletion of FLP-6 disrupts the circuit’s integrity and hence its ability to prevent somatic-fate acquisition by germline tumor cells. Our findings reveal mechanistically how ER stress enhances ectopic germline differentiation and demonstrate that regulated Ire1-dependent decay can affect animal physiology by controlling a specific neuronal circuit.

The FMRFamide Neuropeptide FLP-20 Acts as a Systemic Signal for Starvation Responses in Caenorhabditis elegans

Most animals face frequent periods of starvation throughout their entire life and thus need to appropriately adjust their behavior and metabolism during starvation for their survival. Such adaptive responses are regulated by a complex set of systemic signals, including hormones and neuropeptides. While much progress has been made in identifying pathways that regulate nutrient-excessive states, it is still incompletely understood how animals systemically signal their nutrient-deficient states. Here, we showed that the FMRFamide neuropeptide FLP-20 modulates a systemic starvation response in Caenorhabditis elegans. We found that mutation of flp-20 rescued the starvation hypersensitivity of the G protein β-subunit gpb-2 mutants by suppressing excessive autophagy. FLP-20 acted in AIB neurons, where the metabotropic glutamate receptor MGL-2 also functions to modulate a systemic starvation response. Furthermore, FLP-20 modulated starvation-induced fat degradation in a manner dependent on the receptor-type guanylate cyclase GCY-28. Collectively, our results reveal a circuit that senses and signals nutrient-deficient states to modulate a systemic starvation response in multicellular organisms.

Insulin signaling regulates longevity through protein phosphorylation in Caenorhabditis elegans

Insulin/IGF-1 Signaling (IIS) is known to constrain longevity by inhibiting the transcription factor FOXO. How phosphorylation mediated by IIS kinases regulates lifespan beyond FOXO remains unclear. Here, we profile IIS-dependent phosphorylation changes in a large-scale quantitative phosphoproteomic analysis of wild-type and three IIS mutant Caenorhabditis elegans strains. We quantify more than 15,000 phosphosites and find that 476 of these are differentially phosphorylated in the long-lived daf-2/insulin receptor mutant. We develop a machine learning-based method to prioritize 25 potential lifespan-related phosphosites. We perform validations to show that AKT-1 pT492 inhibits DAF-16/FOXO and compensates the loss of daf-2 function, that EIF-2α pS49 potently inhibits protein synthesis and daf-2 longevity, and that reduced phosphorylation of multiple germline proteins apparently transmits reduced DAF-2 signaling to the soma. In addition, an analysis of kinases with enriched substrates detects that casein kinase 2 (CK2) subunits negatively regulate lifespan. Our study reveals detailed functional insights into longevity.

CREB mediates the C. elegans dauer polyphenism through direct and cell-autonomous regulation of TGF-β expression

Animals can adapt to dynamic environmental conditions by modulating their developmental programs. Understanding the genetic architecture and molecular mechanisms underlying developmental plasticity in response to changing environments is an important and emerging area of research. Here, we show a novel role of cAMP response element binding protein (CREB)-encoding crh-1 gene in developmental polyphenism of C. elegans. Under conditions that promote normal development in wild-type animals, crh-1 mutants inappropriately form transient pre-dauer (L2d) larvae and express the L2d marker gene. L2d formation in crh-1 mutants is specifically induced by the ascaroside pheromone ascr#5 (asc-ωC3; C3), and crh-1 functions autonomously in the ascr#5-sensing ASI neurons to inhibit L2d formation. Moreover, we find that CRH-1 directly binds upstream of the daf-7 TGF-β locus and promotes its expression in the ASI neurons. Taken together, these results provide new insight into how animals alter their developmental programs in response to environmental changes.

Sensory perception of environmental cues as a modulator of aging and neurodegeneration: Insights from Caenorhabditis elegans

Environmental stimuli such as temperature, food, and smell significantly influence the physiology and behavior of animals. Animals are differentially adapted to maintain their internal body functions in response to varied environmental conditions. These external cues are sensed by specialized neurons which are a part of the chemosensory and thermosensory systems. The inability to respond correctly to varied environmental conditions may result in compromised bodily functions and reduced longevity. For example, the ability to sense food is derived from the integrated action of olfactory and gustatory systems. The damage to the olfactory system will affect our decision of palatable food items which in turn can affect the response of the gustatory system, ultimately causing abnormal feeding habits. Recent studies have provided evidence that aging is regulated by sensory perception of environment. Aging is one of the most common causes of various neurodegenerative diseases and the perception of environmental cues is also found to regulate the development of neurodegenerative phenotype in several animal models. However, specific molecular signaling pathways involved in the process are not completely understood. The research conducted on one of the best-studied animal models of aging, Caenorhabditis elegans, has demonstrated multiple examples of gene-environment interaction at the neuronal level which affects life span. The findings may be useful to identify the key neuronal regulators of aging and age-related diseases in humans owing to conserved core metabolic and aging pathways from worms to humans.

Neuronal mitochondrial dynamics coordinate systemic mitochondrial morphology and stress response to confer pathogen resistance in C. elegans

Mitochondrial functions across different tissues are regulated in a coordinated fashion to optimize the fitness of an organism. Mitochondrial unfolded protein response (UPR^mt) can be nonautonomously elicited by mitochondrial perturbation in neurons, but neuronal signals that propagate such response and its physiological significance remain incompletely understood. Here, we show that in C. elegans, loss of neuronal fzo-1/mitofusin induces nonautonomous UPR^mt through multiple neurotransmitters and neurohormones, including acetylcholine, serotonin, glutamate, tyramine, and insulin-like peptides. Neuronal fzo-1 depletion also triggers nonautonomous mitochondrial fragmentation, which requires autophagy and mitophagy genes. Systemic activation of UPR^mt and mitochondrial fragmentation in C. elegans via perturbing neuronal mitochondrial dynamics improves resistance to pathogenic Pseudomonas infection, which is supported by transcriptomic signatures of immunity and stress-response genes. We propose that C. elegans surveils neuronal mitochondrial dynamics to coordinate systemic UPR^mt and mitochondrial connectivity for pathogen defense and optimized survival under bacterial infection.

Behavioral and molecular response of the insect parasitic nematode Steinernema carpocapsae to cues emitted by a host, the red palm weevil, Rhynchophorus ferrugineus

As the larvae of the date palm pest, the red palm weevil, Rhynchophorus ferrugineus, feeds on the host tissue, they emit a distinctive sound which can be recorded outside of the infected tree. We evaluated the response of infective juveniles (IJs) of the entomopathogenic nematodes Steinernema carpocapsae to the R. ferrugineus larvae and it's sound source, separately. In the presence of the insect larvae, 50.2 % of total IJs moved toward those larvae. Recorded insect larvae sound emitted by the speaker resulted in 7% of total IJs near the sound source. RNA-Seq data indicated that more genes were downregulated in S. carpocapsae IJs exposed to insect and speaker compared to non-stimulated IJs. IJs exposed to insect exhibited more up-regulated genes than IJs exposed to speaker. Enriched pathways and biological processes in IJs were similar for both stimuli. The inhibition of locomotion, regulation of neurotransmitter secretion, response to biotic stimulus, and cellular response to chemical stimuli were enriched with unique GO terms for speaker treatment. The regulation of localization, sodium ion transmembrane transport, regulation of response to stress and response to organic substances were the GO categories enriched unique to insect. The host-parasitic interaction was regulated by the differential expression of Ras/MAP kinase, TGF-beta signaling, insulin signaling, AMPK signaling, PPAR signaling pathways and many developmental pathways. More prominent R. ferrugineus host localization by S. carpocapsae was primarily due to the differential transcriptional regulation of olfactory signal transduction, FOXO-family proteins, calcium signaling, WNT and mTOR signaling pathway. The neural basis for the nematode attraction to insect host is based on the chemosensation and the mechanosensation. Many neuropeptides and neuromodulators are involved in regulating the foraging behavior of S. carpocapsae. The results of this study provide new insights into the molecular mechanisms that allow these nematodes to seek insect hosts. Our finding, especially the molecular ones suggest that chemical cues emitted by the active insect host are stimulants of nematodes attraction. Whereas the sound emitted by the insect has minor effects on the nematode behavior.

Toxin-Antidote Elements Across the Tree of Life

In life's constant battle for survival, it takes one to kill but two to conquer. Toxin-antitoxin or toxin-antidote (TA) elements are genetic dyads that cheat the laws of inheritance to guarantee their transmission to the next generation. This seemingly simple genetic arrangement—a toxin linked to its antidote—is capable of quickly spreading and persisting in natural populations. TA elements were first discovered in bacterial plasmids in the 1980s and have recently been characterized in fungi, plants, and animals, where they underlie genetic incompatibilities and sterility in crosses between wild isolates. In this review, we provide a unified view of TA elements in both prokaryotic and eukaryotic organisms and highlight their similarities and differences at the evolutionary, genetic, and molecular levels. Finally, we propose several scenarios that could explain the paradox of the evolutionary origin of TA elements and argue that these elements may be key evolutionary players and that the full scope of their roles is only beginning to be uncovered.

Short-Term Mild Temperature-Stress-Induced Alterations in the C. elegans Phosphoproteome

Exposure to mild early-life stresses can slow down aging, and protein phosphorylation might be an essential regulator in this process. However, the mechanisms of phosphorylation-based signaling networks during mild early-life stress remain elusive. Herein, we systematically analyzed the phosphoproteomes of Caenorhabditis elegans, which were treated with three mild temperatures (15 °C, 20 °C, and 25 °C) in two different short-term groups (10 min and 60 min). By utilizing an iTRAQ-based quantitative phosphoproteomic approach, 18,187 phosphosites from 3330 phosphoproteins were detected in this study. Volcano plots illustrated that the phosphorylation abundance of 374 proteins and 347 proteins, were significantly changed at 15 °C and 25 °C, respectively. Gene ontology, KEGG pathway and protein-protein interaction network analyses revealed that these phosphoproteins were primarily associated with metabolism, translation, development, and lifespan determination. A motif analysis of kinase substrates suggested that MAPK, CK, and CAMK were most likely involved in the adaption processes. Moreover, 16 and 14 aging-regulated proteins were found to undergo phosphorylation modifications under the mild stresses of 15 °C and 25 °C, respectively, indicating that these proteins might be important for maintaining long-term health. Further lifespan experiments confirmed that the candidate phosphoproteins, e.g., EGL-27 and XNP-1 modulated longevity at 15 °C, 20 °C, and 25 °C, and they showed increased tolerance to thermal and oxidative stresses. In conclusion, our findings offered data that supports understanding of the phosphorylation mechanisms involved in mild early-life stresses in C. elegans. Data are available via ProteomeXchange with identifier PXD021081.

Neuromodulators: an essential part of survival

The coordination between the animal's external environment and internal state requires constant modulation by chemicals known as neuromodulators. Neuromodulators, such as biogenic amines, neuropeptides and cytokines, promote organismal homeostasis. Over the past several decades, Caenorhabditiselegans has grown into a powerful model organism that allows the elucidation of the mechanisms of action of neuromodulators that are conserved across species. In this perspective, we highlight a collection of articles in this issue that describe how neuromodulators optimize C. elegans survival.

Transcriptome Profile Changes Associated With Heat Shock Reaction in the Entomopathogenic Nematode, Steinernema carpocapsae

The entomopathogenic nematode Steinernema carpocapsae has been used for control of soil insects. However, S. carpocapse is sensitive to environmental factors, particularly temperature. We studied an S. carpocapse group that was shocked with high temperature. We also studied the transcriptome-level responses associated with temperature stress using a BGIseq sequencing platform. We de novo assembled the reads from the treatment and control groups into one transcriptome consisting of 43.9 and 42.9 million clean reads, respectively. Based on the genome database, we aligned the clean reads to the Nr, Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases and analyzed the differentially expressed genes (DEGs). Compared with the control, the heat-shocked group had significant differential expression of the heat shock protein (HSP) family, antioxidase [glutathione S-transferases (GSTs) and superoxide dismutase (SOD)], monooxygenase (P450), and transcription factor genes (DAF-16 and DAF-2). These DEGs were demonstrated to be part of the Longevity pathway and insulin/insulin-like signaling pathway. The results revealed the potential mechanisms, at the transcriptional level, of S. carpocapsae under thermal stress.

Regulatory systems that mediate the effects of temperature on the lifespan of Caenorhabditis elegans

Temperature affects animal physiology, including aging and lifespan. How temperature and biological systems interact to influence aging and lifespan has been investigated using model organisms, including the nematode Caenorhabditis elegans. In this review, we discuss mechanisms by which diverse cellular factors modulate the effects of ambient temperatures on aging and lifespan in C. elegans. C. elegans thermosensory neurons alleviate lifespan-shortening effects of high temperatures via sterol endocrine signaling and probably through systemic regulation of cytosolic proteostasis. At low temperatures, C. elegans displays a long lifespan by upregulating the cold-sensing TRPA channel, lipid homeostasis, germline-mediated prostaglandin signaling, and autophagy. In addition, co-chaperone p23 amplifies lifespan changes affected by high and low temperatures. Our review summarizes how external temperatures modulate C. elegans lifespan and provides information regarding responses of biological processes to temperature changes, which may affect health and aging at an organism level.

CREB Non-autonomously Controls Reproductive Aging through Hedgehog/Patched Signaling

Evolutionarily conserved signaling pathways are crucial for adjusting growth, reproduction, and cell maintenance in response to altered environmental conditions or energy balance. However, we have an incomplete understanding of the signaling networks and mechanistic changes that coordinate physiological changes across tissues. We found that loss of the cAMP response element-binding protein (CREB) transcription factor significantly slows Caenorhabditis elegans' reproductive decline, an early hallmark of aging in many animals. Our results indicate that CREB acts downstream of the transforming growth factor β (TGF-β) Sma/Mab pathway in the hypodermis to control reproductive aging, and that it does so by regulating a Hedgehog-related signaling factor, WRT-10. Overexpression of hypodermal wrt-10 is sufficient to delay reproductive decline and oocyte quality deterioration, potentially acting via Patched-related receptors in the germline. This TGF-β-CREB-Hedgehog signaling axis allows a key metabolic tissue to communicate with the reproductive system to regulate oocyte quality and the rate of reproductive decline.

Age-dependent changes in response property and morphology of a thermosensory neuron and thermotaxis behavior in Caenorhabditis elegans

Age‐dependent cognitive and behavioral deterioration may arise from defects in different components of the nervous system, including those of neurons, synapses, glial cells, or a combination of them. We find that AFD, the primary thermosensory neuron of Caenorhabditis elegans, in aged animals is characterized by loss of sensory ending integrity, including reduced actin‐based microvilli abundance and aggregation of thermosensory guanylyl cyclases. At the functional level, AFD neurons in aged animals are hypersensitive to high temperatures and show sustained sensory‐evoked calcium dynamics, resulting in a prolonged operating range. At the behavioral level, senescent animals display cryophilic behaviors that remain plastic to acute temperature changes. Excessive cyclase activity of the AFD‐specific guanylyl cyclase, GCY‐8, is associated with developmental defects in AFD sensory ending and cryophilic behavior. Surprisingly, loss of the GCY‐8 cyclase domain reduces these age‐dependent morphological and behavioral changes, while a prolonged AFD operating range still exists in gcy‐8 animals. The lack of apparent correlation between age‐dependent changes in the morphology or stimuli‐evoked response properties of primary sensory neurons and those in related behaviors highlights the importance of quantitative analyses of aging features when interpreting age‐related changes at structural and functional levels. Our work identifies aging hallmarks in AFD receptive ending, temperature‐evoked AFD responses, and experience‐based thermotaxis behavior, which serve as a foundation to further elucidate the neural basis of cognitive aging.

How Caenorhabditis elegans Senses Mechanical Stress, Temperature, and Other Physical Stimuli

Caenorhabditis elegans lives in a complex habitat in which they routinely experience large fluctuations in temperature, and encounter physical obstacles that vary in size and composition. Their habitat is shared by other nematodes, by beneficial and harmful bacteria, and nematode-trapping fungi. Not surprisingly, these nematodes can detect and discriminate among diverse environmental cues, and exhibit sensory-evoked behaviors that are readily quantifiable in the laboratory at high resolution. Their ability to perform these behaviors depends on <100 sensory neurons, and this compact sensory nervous system together with powerful molecular genetic tools has allowed individual neuron types to be linked to specific sensory responses. Here, we describe the sensory neurons and molecules that enable C. elegans to sense and respond to physical stimuli. We focus primarily on the pathways that allow sensation of mechanical and thermal stimuli, and briefly consider this animal’s ability to sense magnetic and electrical fields, light, and relative humidity. As the study of sensory transduction is critically dependent upon the techniques for stimulus delivery, we also include a section on appropriate laboratory methods for such studies. This chapter summarizes current knowledge about the sensitivity and response dynamics of individual classes of C. elegans mechano- and thermosensory neurons from in vivo calcium imaging and whole-cell patch-clamp electrophysiology studies. We also describe the roles of conserved molecules and signaling pathways in mediating the remarkably sensitive responses of these nematodes to mechanical and thermal cues. These studies have shown that the protein partners that form mechanotransduction channels are drawn from multiple superfamilies of ion channel proteins, and that signal transduction pathways responsible for temperature sensing in C. elegans share many features with those responsible for phototransduction in vertebrates.

Differential Regulation of Innate and Learned Behavior by Creb1/Crh-1 in Caenorhabditis elegans

Memory formation is crucial for the survival of animals. Here, we study the effect of different crh-1 [Caenorhabditis elegans homolog of mammalian cAMP response element binding protein 1 (CREB1)] isoforms on the ability of C. elegans to form long-term memory (LTM). Null mutants in creb1/crh-1 are defective in LTM formation across phyla. We show that a specific isoform of CREB1/CRH-1, CRH-1e, is primarily responsible for memory related functions of the transcription factor in C. elegans. Silencing of CRH-1e-expressing neurons during training for LTM formation abolishes the LTM of the animal. Further, CRH-1e expression in RIM neurons is sufficient to rescue LTM defects of creb1/crh-1-null mutants. We go on to show that apart from being LTM defective, creb1/crh-1-null animals show defects in innate chemotaxis behavior. We further characterize the amino acids K247 and K266 as responsible for the LTM related functions of CREB1/CRH-1 while being dispensable for its innate chemotaxis behavior. These findings provide insight into the spatial and temporal workings of a crucial transcription factor that can be further exploited to find CREB1 targets involved in the process of memory formation.

SIGNIFICANCE STATEMENT This study elucidates the role of a specific isoform of CREB1/CRH-1, CRH-1e, in Caenorhabditis elegans memory formation and chemosensation. Removal of this single isoform of creb1/crh-1 shows defects in long-term memory formation in the animal and expression of CREB1/CRH-1e in a single pair of neurons is sufficient to rescue the memory defects seen in the mutant animals. We further show that two specific amino acids of CRH-1 are required for the process of memory formation in the animal.

MDT-15/MED15 permits longevity at low temperature via enhancing lipidostasis and proteostasis

Low temperatures delay aging and promote longevity in many organisms. However, the metabolic and homeostatic aspects of low-temperature–induced longevity remain poorly understood. Here, we show that lipid homeostasis regulated by Caenorhabditis elegans Mediator 15 (MDT-15 or MED15), a transcriptional coregulator, is essential for low-temperature–induced longevity and proteostasis. We find that inhibition of mdt-15 prevents animals from living long at low temperatures. We show that MDT-15 up-regulates fat-7, a fatty acid desaturase that converts saturated fatty acids (SFAs) to unsaturated fatty acids (UFAs), at low temperatures. We then demonstrate that maintaining a high UFA/SFA ratio is essential for proteostasis at low temperatures. We show that dietary supplementation with a monounsaturated fatty acid, oleic acid (OA), substantially mitigates the short life span and proteotoxicity in mdt-15(-) animals at low temperatures. Thus, lipidostasis regulated by MDT-15 appears to be a limiting factor for proteostasis and longevity at low temperatures. Our findings highlight the crucial roles of lipid regulation in maintaining normal organismal physiology under different environmental conditions.

Low temperatures delay aging and promote longevity in many organisms. This study shows that at low ambient temperatures, Mediator 15, a transcriptional coregulator, allows the nematode Caenorhabditis elegans to live longer by increasing the levels of unsaturated lipids, helping to maintain protein homeostasis.

Tks5 and Dynamin-2 enhance actin bundle rigidity in invadosomes to promote myoblast fusion

Skeletal muscle development requires the cell-cell fusion of differentiated myoblasts to form muscle fibers. The actin cytoskeleton is known to be the main driving force for myoblast fusion; however, how actin is organized to direct intercellular fusion remains unclear. Here we show that an actin- and dynamin-2-enriched protrusive structure, the invadosome, is required for the fusion process of myogenesis. Upon differentiation, myoblasts acquire the ability to form invadosomes through isoform switching of a critical invadosome scaffold protein, Tks5. Tks5 directly interacts with and recruits dynamin-2 to the invadosome and regulates its assembly around actin filaments to strengthen the stiffness of dynamin-actin bundles and invadosomes. These findings provide a mechanistic framework for the acquisition of myogenic fusion machinery during myogenesis and reveal a novel structural function for Tks5 and dynamin-2 in organizing actin filaments in the invadosome to drive membrane fusion.

Neuropeptide signaling regulates the susceptibility of developing C. elegans to anoxia

Inadequate delivery of oxygen to organisms during development can lead to cell dysfunction/death and life-long disabilities. Although the susceptibility of developing cells to low oxygen conditions changes with maturation, the cellular and molecular pathways that govern responses to low oxygen are incompletely understood. Here we show that developing Caenorhabditis elegans are substantially more sensitive to anoxia than adult animals and that this sensitivity is controlled by nervous system generated hormones (e.g., neuropeptides). A screen of neuropeptide genes identified and validated nlp-40 and its receptor aex-2 as a key regulator of anoxic survival in developing worms. The survival-promoting action of impaired neuropeptide signaling does not rely on five known stress resistance pathways and is specific to anoxic insult. Together, these data highlight a novel cell non-autonomous pathway that regulates the susceptibility of developing organisms to anoxia.

Non-canonical activation of CREB mediates neuroprotection in a Caenorhabditis elegans model of excitotoxic necrosis

Excitotoxicity, caused by exaggerated neuronal stimulation by Glutamate (Glu), is a major cause of neurodegeneration in brain ischemia. While we know that neurodegeneration is triggered by overstimulation of Glu-receptors (GluRs), the subsequent mechanisms that lead to cellular demise remain controversial. Surprisingly, signaling downstream of GluRs can also activate neuroprotective pathways. The strongest evidence involves activation of the transcription factor cAMP response element-binding protein (CREB), widely recognized for its importance in synaptic plasticity. Canonical views describe CREB as a phosphorylation-triggered transcription factor, where transcriptional activation involves CREB phosphorylation and association with CREB-binding protein. However, given CREB's ubiquitous cross-tissue expression, the multitude of cascades leading to CREB phosphorylation, and its ability to regulate thousands of genes, it remains unclear how CREB exerts closely tailored, differential neuroprotective responses in excitotoxicity. A non-canonical, alternative cascade for activation of CREB-mediated transcription involves the CREB co-factor cAMP-regulated transcriptional co-activator (CRTC), and may be independent of CREB phosphorylation. To identify cascades that activate CREB in excitotoxicity we used a Caenorhabditis elegans model of neurodegeneration by excitotoxic necrosis. We demonstrated that CREB's neuroprotective effect was conserved, and seemed most effective in neurons with moderate Glu exposure. We found that factors mediating canonical CREB activation were not involved. Instead, phosphorylation-independent CREB activation in nematode excitotoxic necrosis hinged on CRTC. CREB-mediated transcription that depends on CRTC, but not on CREB phosphorylation, might lead to expression of a specific subset of neuroprotective genes. Elucidating conserved mechanisms of excitotoxicity-specific CREB activation can help us focus on core neuroprotective programs in excitotoxicity. Cover Image for this issue: doi: 10.1111/jnc.14494.

Brain-gut communications via distinct neuroendocrine signals bidirectionally regulate longevity in C. elegans

Zhang et al. identified two distinct neuroendocrine signaling circuits by which the worm nervous system senses cool and warm environmental temperatures through cool- and warm-sensitive neurons and then signals the gut to extend and shorten life span, respectively.

Tissue–tissue communications are integral to organismal aging, orchestrating a body-wide aging process. The brain plays a key role in this process by detecting and processing signals from the environment and then communicating them to distal tissues such as the gut to regulate longevity. How this is achieved, however, is poorly understood. Here, using Caenorhabditis elegans as a model, we identified two distinct neuroendocrine signaling circuits by which the worm nervous system senses cool and warm environmental temperatures through cool- and warm-sensitive neurons and then signals the gut to extend and shorten life span, respectively. The prolongevity “cool” circuit uses the small neurotransmitters glutamate and serotonin, whereas the anti-longevity “warm” circuit is mediated by insulin-like neuropeptides. Both types of neuroendocrine signals converge on the gut through their cognate receptors to differentially regulate the transcription factor DAF-16/FOXO, leading to opposing outcomes in longevity. Our study illustrates how the brain detects and processes environmental signals to bidirectionally regulate longevity by signaling the gut.

FLP-18 Functions through the G-Protein-Coupled Receptors NPR-1 and NPR-4 to Modulate Reversal Length in Caenorhabditis elegans

Animal behavior is critically dependent on the activity of neuropeptides. Reversals, one of the most conspicuous behaviors in Caenorhabditis elegans, plays an important role in determining the navigation strategy of the animal. Our experiments on hermaphrodite C. elegans show the involvement of a neuropeptide FLP-18 in modulating reversal length in these hermaphrodites. We show that FLP-18 controls the reversal length by regulating the activity of AVA interneurons through the G-protein-coupled neuropeptide receptors, NPR-4 and NPR-1. We go on to show that the site of action of these receptors is the AVA interneuron for NPR-4 and the ASE sensory neurons for NPR-1. We further show that mutants in the neuropeptide, flp-18, and its receptors show increased reversal lengths. Consistent with the behavioral data, calcium levels in the AVA neuron of freely reversing C. elegans were significantly higher and persisted for longer durations in flp-18, npr-1, npr-4, and npr-1 npr-4 genetic backgrounds compared with wild-type control animals. Finally, we show that increasing FLP-18 levels through genetic and physiological manipulations causes shorter reversal lengths. Together, our analysis suggests that the FLP-18/NPR-1/NPR-4 signaling is a pivotal point in the regulation of reversal length under varied genetic and environmental conditions.

SIGNIFICANCE STATEMENT In this study, we elucidate the circuit and molecular machinery required for normal reversal behavior in hermaphrodite Caenorhabditis elegans. We delineate the circuit and the neuropeptide receptors required for maintaining reversal length in C. elegans. Our work sheds light on the importance of a single neuropeptide, FLP-18, and how change in levels in this one peptide could allow the animal to change the length of its reversal, thereby modulating how the C. elegans explores its environment. We also go on to show that FLP-18 functions to maintain reversal length through the neuropeptide receptors NPR-4 and NPR-1. Our study will allow for a better understanding of the complete repertoire of behaviors shown by freely moving animals as they explore their environment.

Early experiences mediate distinct adult gene expression and reproductive programs in Caenorhabditis elegans

Environmental stress during early development in animals can have profound effects on adult phenotypes via programmed changes in gene expression. Using the nematode C. elegans, we demonstrated previously that adults retain a cellular memory of their developmental experience that is manifested by differences in gene expression and life history traits; however, the sophistication of this system in response to different environmental stresses, and how it dictates phenotypic plasticity in adults that contribute to increased fitness in response to distinct environmental challenges, was unknown. Using transcriptional profiling, we show here that C. elegans adults indeed retain distinct cellular memories of different environmental conditions. We identified approximately 500 genes in adults that entered dauer due to starvation that exhibit significant opposite (“seesaw”) transcriptional phenotypes compared to adults that entered dauer due to crowding, and are distinct from animals that bypassed dauer. Moreover, we show that two-thirds of the genes in the genome experience a 2-fold or greater seesaw trend in gene expression, and based upon the direction of change, are enriched in large, tightly linked regions on different chromosomes. Importantly, these transcriptional programs correspond to significant changes in brood size depending on the experienced stress. In addition, we demonstrate that while the observed seesaw gene expression changes occur in both somatic and germline tissue, only starvation-induced changes require a functional GLP-4 protein necessary for germline development, and both programs require the Argonaute CSR-1. Thus, our results suggest that signaling between the soma and the germ line can generate phenotypic plasticity as a result of early environmental experience, and likely contribute to increased fitness in adverse conditions and the evolution of the C. elegans genome.

Environmental stress during early development in animals can have profound effects on adult behavior and physiology due to programmed changes in gene expression. However, whether different stresses result in distinct changes in traits that allow stressed animals to better survive and reproduce in future adverse conditions is largely unknown. Using the animal model system, C. elegans, we show that adults that experienced starvation exhibit opposite (“seesaw”) genome-wide gene expression changes compared to adults that experienced crowding, and are distinct from animals that experienced favorable conditions. Genes that are similarly up- or downregulated due to either starvation or crowding are located in clusters on the same chromosomes. Importantly, these gene expression changes of differently-stressed animals result in corresponding changes in progeny number, a life history trait of evolutionary significance. These distinct gene expression programs require different signaling pathways that communicate across somatic and germline tissue types. Thus, different environmental stresses experienced early in development induce distinct signaling mechanisms to result in changes in gene expression and reproduction in adults, and likely contribute to increased survival in future adverse conditions.

C. elegans Locomotion: Finding Balance in Imbalance

The excitation-inhibition (E-I) imbalance in neural circuits represents a hallmark of several neuropsychiatric disorders. The tiny nematode Caenorhabditis elegans has emerged as an excellent system to study the molecular mechanisms underlying this imbalance in neuronal circuits. The C. elegans body wall muscles receive inputs from both excitatory cholinergic and inhibitory GABAergic motor neurons at neuromuscular junctions (NMJ), making it an excellent model for studying the genetic and molecular mechanisms required for maintaining E-I balance at the NMJ. The cholinergic neurons form dyadic synapses wherein they synapse onto ipsilateral body wall muscles allowing for muscle contraction as well as onto GABAergic motor neurons that in turn synapse on the contralateral body wall muscles causing muscle relaxation. An alternating wave of contraction and relaxation mediated by excitatory and inhibitory signals maintains locomotion in C. elegans. This locomotory behavior requires an intricate balance between the excitatory cholinergic signaling and the inhibitory GABAergic signaling mechanisms.Studies on the C. elegans NMJ have provided insights into several molecular mechanisms that could regulate this balance in neural circuits. This review provides a discussion on multiple genetic factors including neuropeptides and their receptors, cell adhesion molecules, and other molecular pathways that have been associated with maintaining E-I balance in C. elegans motor circuits. Further, it also discusses the implications of these studies that could help us in understanding the role of E-I balance in mammalian neural circuits and how changes in this balance could give rise to brain disorders.

The extraordinary AFD thermosensor of C. elegans

The nematode C. elegans exhibits complex thermal experience-dependent navigation behaviors in response to environmental temperature changes of as little as 0.01°C over a > 10°C temperature range. The remarkable thermosensory abilities of this animal are mediated primarily via the single pair of AFD sensory neurons in its head. In this review, we describe the contributions of AFD to thermosensory behaviors and temperature-dependent regulation of organismal physiology. We also discuss the mechanisms that enable this neuron type to adapt to recent temperature experience and to exhibit extraordinary thermosensitivity over a wide dynamic range.