Regulation of interneuron function in the C. elegans thermoregulatory pathway by the ttx-3 LIM homeobox gene

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.

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.

Specific configurations of electrical synapses filter sensory information to drive choices in behavior

Synaptic configurations in precisely wired circuits underpin how sensory information is processed by the nervous system, and the emerging animal behavior. This is best understood for chemical synapses, but far less is known about how electrical synaptic configurations modulate, in vivo and in specific neurons, sensory information processing and context-specific behaviors. We discovered that INX-1, a gap junction protein that forms electrical synapses, is required to deploy context-specific behavioral strategies during C. elegans thermotaxis behavior. INX-1 couples two bilaterally symmetric interneurons, and this configuration is required for the integration of sensory information during migration of animals across temperature gradients. In inx-1 mutants, uncoupled interneurons display increased excitability and responses to subthreshold temperature stimuli, resulting in abnormally longer run durations and context-irrelevant tracking of isotherms. Our study uncovers a conserved configuration of electrical synapses that, by increasing neuronal capacitance, enables differential processing of sensory information and the deployment of context-specific behavioral strategies.

Disexcitation in the ASH/RIM/ADL negative feedback circuit fine-tunes hyperosmotic sensation and avoidance in Caenorhabditis elegans

Sensations, especially nociception, are tightly controlled and regulated by the central and peripheral nervous systems. Osmotic sensation and related physiological and behavioral reactions are essential for animal well-being and survival. In this study, we find that interaction between secondary nociceptive ADL and primary nociceptive ASH neurons upregulates Caenorhabditis elegans avoidance of the mild and medium hyperosmolality of 0.41 and 0.88 Osm but does not affect avoidance of high osmolality of 1.37 and 2.29 Osm. The interaction between ASH and ADL is actualized through a negative feedback circuit consisting of ASH, ADL, and RIM interneurons. In this circuit, hyperosmolality-sensitive ADL augments the ASH hyperosmotic response and animal hyperosmotic avoidance; RIM inhibits ADL and is excited by ASH; thus, ASH exciting RIM reduces ADL augmenting ASH. The neuronal signal integration modality in the circuit is disexcitation. In addition, ASH promotes hyperosmotic avoidance through ASH/RIC/AIY feedforward circuit. Finally, we find that in addition to ASH and ADL, multiple sensory neurons are involved in hyperosmotic sensation and avoidance behavior.

Avoidance behavior and experience-dependent tolerance in response to bitter compounds in Caenorhabditis elegans

This study investigates the mechanisms governing experience-dependent tolerance of bitter compounds in Caenorhabditis elegans. The nematodes showed an aversion to nicotinamide, MgCl2, isoleucine, and arginine in a Gα-dependent manner. Worms furthermore displayed diminished avoidance of MgCl2 upon MgCl2-preconditioning at the larval stages. AIY interneurons have been suggested to be involved in experience-dependent behavioral plasticity.

An olfactory-interneuron circuit that drives stress-induced avoidance behavior in C. elegans

Physiological stress represents a drastic change of internal state and can drive avoidance behavior, but the neural circuits are incompletely defined. Here, we characterize a sensory-interneuron circuit for mitochondrial stress-induced avoidance behavior in C. elegans. The olfactory sensory neurons and the AIY interneuron are essential, with the olfactory neurons acting upstream of AIY. Unlike pathogen avoidance, stress-induced avoidance does not require AIB, AIZ or RIA interneurons. Ablation or inhibition of the head motor neurons SMDD/V alters the worm's locomotion and reduces avoidance. These findings substantiate our understanding of the circuit mechanisms that underlie learned avoidance behavior triggered by mitochondrial stress.

Cell Non-autonomous Proteostasis Regulation in Aging and Disease

Aging is a risk factor for a number of diseases, being the more notorious ones perhaps neurodegenerative diseases such as Alzheimer's and Parkinson's. These and other age-related pathologies are often associated with accumulation of proteotoxic material inside cells, as well as with the accumulation of protein deposits extracellularly. It is widely accepted that this accumulation of toxic proteins trails a progressive decline in the mechanisms that regulate protein homeostasis, or proteostasis, during aging. However, despite significant efforts, the progress in terms of novel or improved therapies targeting accumulation of proteotoxic material has been rather limited. For example, clinical trials for new drugs aimed at treating Alzheimer's disease, by preventing accumulation of toxic proteins, have notoriously failed. On the other hand, it is becoming increasingly apparent that regulation of proteostasis is not a cell autonomous process. In fact, cells rely on complex transcellular networks to maintain tissue and organ homeostasis involving endocrine and paracrine signaling pathways. In this review we will discuss the impact of cell non-autonomous proteostasis mechanisms and their impact in aging and disease. We will focus on how transcellular proteostasis networks can shed new light into stablished paradigms about the aging of organisms.

OLA-1, an Obg-like ATPase, integrates hunger with temperature information in sensory neurons in C. elegans

Animals detect changes in both their environment and their internal state and modify their behavior accordingly. Yet, it remains largely to be clarified how information of environment and internal state is integrated and how such integrated information modifies behavior. Well-fed C. elegans migrates to past cultivation temperature on a thermal gradient, which is disrupted when animals are starved. We recently reported that the neuronal activities synchronize between a thermosensory neuron AFD and an interneuron AIY, which is directly downstream of AFD, in well-fed animals, while this synchrony is disrupted in starved animals. However, it remained to be determined whether the disruption of the synchrony is derived from modulation of the transmitter release from AFD or from the modification of reception or signal transduction in AIY. By performing forward genetics on a transition of thermotaxis behavior along starvation, we revealed that OLA-1, an Obg-like ATPase, functions in AFD to promote disruption of AFD-AIY synchrony and behavioral transition. Our results suggest that the information of hunger is delivered to the AFD thermosensory neuron and gates transmitter release from AFD to disrupt thermotaxis, thereby shedding light onto a mechanism for the integration of environmental and internal state to modulate behavior.

Diet-responsive transcriptional regulation of insulin in a single neuron controls systemic metabolism

Metabolic homeostasis is coordinated through a robust network of signaling pathways acting across all tissues. A key part of this network is insulin-like signaling, which is fundamental for surviving glucose stress. Here, we show that Caenorhabditis elegans fed excess dietary glucose reduce insulin-1 (INS-1) expression specifically in the BAG glutamatergic sensory neurons. We demonstrate that INS-1 expression in the BAG neurons is directly controlled by the transcription factor ETS-5, which is also down-regulated by glucose. We further find that INS-1 acts exclusively from the BAG neurons, and not other INS-1-expressing neurons, to systemically inhibit fat storage via the insulin-like receptor DAF-2. Together, these findings reveal an intertissue regulatory pathway where regulation of insulin expression in a specific neuron controls systemic metabolism in response to excess dietary glucose.

Metabolic homeostasis is coordinated through a robust network of signaling pathways acting across all tissues. This study shows that Caenorhabditis elegans nematodes fed excess dietary glucose reduce the expression of insulin-1 specifically in the BAG glutamatergic sensory neurons, and that insulin-1 produced by these neurons systemically inhibits fat storage via the insulin-like receptor DAF-2.

Presynaptic autophagy is coupled to the synaptic vesicle cycle via ATG-9

Autophagy is a cellular degradation pathway essential for neuronal health and function. Autophagosome biogenesis occurs at synapses, is locally regulated, and increases in response to neuronal activity. The mechanisms that couple autophagosome biogenesis to synaptic activity remain unknown. In this study, we determine that trafficking of ATG-9, the only transmembrane protein in the core autophagy pathway, links the synaptic vesicle cycle with autophagy. ATG-9-positive vesicles in C. elegans are generated from the trans-Golgi network via AP-3-dependent budding and delivered to presynaptic sites. At presynaptic sites, ATG-9 undergoes exo-endocytosis in an activity-dependent manner. Mutations that disrupt endocytosis, including a lesion in synaptojanin 1 associated with Parkinson's disease, result in abnormal ATG-9 accumulation at clathrin-rich synaptic foci and defects in activity-induced presynaptic autophagy. Our findings uncover regulated key steps of ATG-9 trafficking at presynaptic sites and provide evidence that ATG-9 exo-endocytosis couples autophagosome biogenesis at presynaptic sites with the activity-dependent synaptic vesicle cycle.

Neuronal HSF-1 coordinates the propagation of fat desaturation across tissues to enable adaptation to high temperatures in C. elegans

To survive elevated temperatures, ectotherms adjust the fluidity of membranes by fine-tuning lipid desaturation levels in a process previously described to be cell autonomous. We have discovered that, in Caenorhabditis elegans, neuronal heat shock factor 1 (HSF-1), the conserved master regulator of the heat shock response (HSR), causes extensive fat remodeling in peripheral tissues. These changes include a decrease in fat desaturase and acid lipase expression in the intestine and a global shift in the saturation levels of plasma membrane's phospholipids. The observed remodeling of plasma membrane is in line with ectothermic adaptive responses and gives worms a cumulative advantage to warm temperatures. We have determined that at least 6 TAX-2/TAX-4 cyclic guanosine monophosphate (cGMP) gated channel expressing sensory neurons, and transforming growth factor ß (TGF-β)/bone morphogenetic protein (BMP) are required for signaling across tissues to modulate fat desaturation. We also find neuronal hsf-1 is not only sufficient but also partially necessary to control the fat remodeling response and for survival at warm temperatures. This is the first study to show that a thermostat-based mechanism can cell nonautonomously coordinate membrane saturation and composition across tissues in a multicellular animal.

On the role of dauer in the adaptation of nematodes to a parasitic lifestyle

Nematodes are presumably the most abundant Metazoa on Earth, and can even be found in some of the most hostile environments of our planet. Various types of hypobiosis evolved to adapt their life cycles to such harsh environmental conditions. The five most distal major clades of the phylum Nematoda (Clades 8-12), formerly referred to as the Secernentea, contain many economically relevant parasitic nematodes. In this group, a special type of hypobiosis, dauer, has evolved. The dauer signalling pathway, which culminates in the biosynthesis of dafachronic acid (DA), is intensively studied in the free-living nematode Caenorhabditis elegans, and it has been hypothesized that the dauer stage may have been a prerequisite for the evolution of a wide range of parasitic lifestyles among other nematode species. Biosynthesis of DA is not specific for hypobiosis, but if it results in exit of the hypobiotic state, it is one of the main criteria to define certain behaviour as dauer. Within Clades 9 and 10, the involvement of DA has been validated experimentally, and dauer is therefore generally accepted to occur in those clades. However, for other clades, such as Clade 12, this has hardly been explored. In this review, we provide clarity on the nomenclature associated with hypobiosis and dauer across different nematological subfields. We discuss evidence for dauer-like stages in Clades 8 to 12 and support this with a meta-analysis of available genomic data. Furthermore, we discuss indications for a simplified dauer signalling pathway in parasitic nematodes. Finally, we zoom in on the host cues that induce exit from the hypobiotic stage and introduce two hypotheses on how these signals might feed into the dauer signalling pathway for plant-parasitic nematodes. With this work, we contribute to the deeper understanding of the molecular mechanisms underlying hypobiosis in parasitic nematodes. Based on this, novel strategies for the control of parasitic nematodes can be developed.

Transcription Factors That Control Behavior-Lessons From C. elegans

Behavior encompasses the physical and chemical response to external and internal stimuli. Neurons, each with their own specific molecular identities, act in concert to perceive and relay these stimuli to drive behavior. Generating behavioral responses requires neurons that have the correct morphological, synaptic, and molecular identities. Transcription factors drive the specific gene expression patterns that define these identities, controlling almost every phenomenon in a cell from development to homeostasis. Therefore, transcription factors play an important role in generating and regulating behavior. Here, we describe the transcription factors, the pathways they regulate, and the neurons that drive chemosensation, mechanosensation, thermosensation, osmolarity sensing, complex, and sex-specific behaviors in the animal model Caenorhabditis elegans. We also discuss the current limitations in our knowledge, particularly our minimal understanding of how transcription factors contribute to the adaptive behavioral responses that are necessary for organismal survival.

Homeobox genes and the specification of neuronal identity

The enormous diversity of cell types that characterizes any animal nervous system is defined by neuron-type-specific gene batteries that endow cells with distinct anatomical and functional properties. To understand how such cellular diversity is genetically specified, one needs to understand the gene regulatory programmes that control the expression of cell-type-specific gene batteries. The small nervous system of the nematode Caenorhabditis elegans has been comprehensively mapped at the cellular and molecular levels, which has enabled extensive, nervous system-wide explorations into whether there are common underlying mechanisms that specify neuronal cell-type diversity. One principle that emerged from these studies is that transcription factors termed 'terminal selectors' coordinate the expression of individual members of neuron-type-specific gene batteries, thereby assigning unique identities to individual neuron types. Systematic mutant analyses and recent nervous system-wide expression analyses have revealed that one transcription factor family, the homeobox gene family, is broadly used throughout the entire C. elegans nervous system to specify neuronal identity as terminal selectors. I propose that the preponderance of homeobox genes in neuronal identity control is a reflection of an evolutionary trajectory in which an ancestral neuron type was specified by one or more ancestral homeobox genes, and that this functional linkage then duplicated and diversified to generate distinct cell types in an evolving nervous system.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

EGL-4/PKG regulates the role of an interneuron in a chemotaxis circuit of C. elegans through mediating integration of sensory signals

Interneurons, innervated by multiple sensory neurons, need to integrate information from these sensory neurons and respond to sensory stimuli adequately. Mechanisms how sensory information is integrated to form responses of interneurons are not fully understood. In Caenorhabditis elegans, loss-of-function mutations of egl-4, which encodes a cGMP-dependent protein kinase (PKG), cause a defect in chemotaxis to odorants. Our genetic and imaging analyses revealed that the response property of AIY interneuron to an odorant is reversed in the egl-4 mutant, while the responses of two upstream olfactory neurons, AWA and AWC, are largely unchanged. Cell- ablation experiments show that AIY in the egl-4 mutant functions to suppress chemotaxis. Furthermore, the reversal of AIY response occurs only in the presence of sensory signals from both AWA and AWC. These results suggest that sensory signals are inadequately integrated in the egl-4 mutant. We also show that egl-4 expression in AWA and another sensory neuron prevents the reversed AIY response and restores chemotaxis in the egl-4 mutants. We propose that EGL-4/PKG, by suppressing aberrant integration of signals from olfactory neurons, converts the response property of an interneuron to olfactory stimuli and maintains the role of the interneuron in the circuit to execute chemotactic behavior.

Golgi localization of the LIN-2/7/10 complex points to a role in basolateral secretion of LET-23 EGFR in the Caenorhabditiselegans vulval precursor cells

The evolutionarily conserved LIN-2 (CASK)/LIN-7 (Lin7A-C)/LIN-10 (APBA1) complex plays an important role in regulating spatial organization of membrane proteins and signaling components. In Caenorhabditiselegans, the complex is essential for the development of the vulva by promoting the localization of the sole Epidermal growth factor receptor (EGFR) ortholog LET-23 to the basolateral membrane of the vulva precursor cells where it can specify the vulval cell fate. To understand how the LIN-2/7/10 complex regulates receptor localization, we determined its expression and localization during vulva development. We found that LIN-7 colocalizes with LET-23 EGFR at the basolateral membrane, whereas the LIN-2/7/10 complex colocalizes with LET-23 EGFR at cytoplasmic punctae that mostly overlap with the Golgi. Furthermore, LIN-10 recruits LIN-2, which in turn recruits LIN-7. We demonstrate that the complex forms in vivo with a particularly strong interaction and colocalization between LIN-2 and LIN-7, consistent with them forming a subcomplex. Thus, the LIN-2/7/10 complex forms on the Golgi on which it likely targets LET-23 EGFR trafficking to the basolateral membrane rather than functioning as a tether.

Neuronal subclass-selective proteomic analysis in Caenorhabditis elegans

Neurons are categorised into many subclasses, and each subclass displays different morphology, expression patterns, connectivity and function. Changes in protein synthesis are critical for neuronal function. Therefore, analysing protein expression patterns in individual neuronal subclass will elucidate molecular mechanisms for memory and other functions. In this study, we used neuronal subclass-selective proteomic analysis with cell-selective bio-orthogonal non-canonical amino acid tagging. We selected Caenorhabditis elegans as a model organism because it shows diverse neuronal functions and simple neural circuitry. We performed proteomic analysis of all neurons or AFD subclass neurons that regulate thermotaxis in C. elegans. Mutant phenylalanyl tRNA synthetase (MuPheRS) was selectively expressed in all neurons or AFD subclass neurons, and azido-phenylalanine was incorporated into proteins in cells of interest. Azide-labelled proteins were enriched and proteomic analysis was performed. We identified 4,412 and 1,834 proteins from strains producing MuPheRS in all neurons and AFD subclass neurons, respectively. F23B2.10 (RING-type domain-containing protein) was identified only in neuronal cell-enriched proteomic analysis. We expressed GFP under the control of the 5' regulatory region of F23B2.10 and found GFP expression in neurons. We expect that more single-neuron specific proteomic data will clarify how protein composition and abundance affect characteristics of neuronal subclasses.

ngn-1/neurogenin Activates Transcription of Multiple Terminal Selector Transcription Factors in the Caenorhabditis elegans Nervous System

Proper nervous system development is required for an organism's survival and function. Defects in neurogenesis have been linked to neurodevelopmental disorders such as schizophrenia and autism. Understanding the gene regulatory networks that orchestrate neural development, specifically cascades of proneural transcription factors, can better elucidate which genes are most important during early neurogenesis. Neurogenins are a family of deeply conserved factors shown to be both necessary and sufficient for the development of neural subtypes. However, the immediate downstream targets of neurogenin are not well characterized. The objective of this study was to further elucidate the role of ngn-1/neurogenin in nervous system development and to identify its downstream transcriptional targets, using the nematode Caenorhabditis elegans as a model for this work. We found that ngn-1 is required for axon outgrowth, nerve ring architecture, and neuronal cell fate specification. We also showed that ngn-1 may have roles in neuroblast migration and epithelial integrity during embryonic development. Using RNA sequencing and comparative transcriptome analysis, we identified eight transcription factors (hlh-34/NPAS1, unc-42/PROP1, ceh-17/PHOX2A, lim-4/LHX6, fax-1/NR2E3, lin-11/LHX1, tlp-1/ZNF503, and nhr-23/RORB) whose transcription is activated, either directly or indirectly, by ngn-1 Our results show that ngn-1 has a role in transcribing known terminal regulators that establish and maintain cell fate of differentiated neural subtypes and confirms that ngn-1 functions as a proneural transcription factor in C. elegans neurogenesis.

Cellular damage, including wounding, drives C. elegans stress-induced sleep

Across animal phyla, sleep is associated with increased cellular repair, suggesting that cellular damage may be a core component of sleep pressure. In support of this notion, sleep in the nematode Caenorhabditis elegans can be triggered by damaging conditions, including noxious heat, high salt, and ultraviolet light exposure. It is not clear, however, whether this stress-induced sleep (SIS) is a direct consequence of cellular damage, or of a resulting energy deficit, or whether it is triggered simply by the sensation of noxious conditions. Here, we show that thermosensation is dispensable for heat-induced sleep, that osmosensation is dispensable for salt-induced sleep, and that wounding is also a sleep trigger, together indicating that SIS is not triggered by sensation of noxious environments. We present evidence that genetic variation in cellular repair pathways impacts sleep amount, and that SIS involves systemic monitoring of cellular damage. We show that the low-energy sensor AMP-activated protein kinase (AMPK) is not required for SIS, suggesting that energy deficit is not the primary sleep trigger. Instead, AMPK-deficient animals display enhanced SIS responses, and pharmacological activation of AMPK reduces SIS, suggesting that ATP-dependent repair of cellular damage mitigates sleep pressure.

Brn3/POU-IV-type POU homeobox genes-Paradigmatic regulators of neuronal identity across phylogeny

One approach to understand the construction of complex systems is to investigate whether there are simple design principles that are commonly used in building such a system. In the context of nervous system development, one may ask whether the generation of its highly diverse sets of constituents, that is, distinct neuronal cell types, relies on genetic mechanisms that share specific common features. Specifically, are there common patterns in the function of regulatory genes across different neuron types and are those regulatory mechanisms not only used in different parts of one nervous system, but are they conserved across animal phylogeny? We address these questions here by focusing on one specific, highly conserved and well-studied regulatory factor, the POU homeodomain transcription factor UNC-86. Work over the last 30 years has revealed a common and paradigmatic theme of unc-86 function throughout most of the neuron types in which Caenorhabditis elegans unc-86 is expressed. Apart from its role in preventing lineage reiterations during development, UNC-86 operates in combination with distinct partner proteins to initiate and maintain terminal differentiation programs, by coregulating a vast array of functionally distinct identity determinants of specific neuron types. Mouse orthologs of unc-86, the Brn3 genes, have been shown to fulfill a similar function in initiating and maintaining neuronal identity in specific parts of the mouse brain and similar functions appear to be carried out by the sole Drosophila ortholog, Acj6. The terminal selector function of UNC-86 in many different neuron types provides a paradigm for neuronal identity regulation across phylogeny. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Invertebrate Organogenesis > Worms Nervous System Development > Vertebrates: Regional Development.

Expanding the Organismal Proteostasis Network: Linking Systemic Stress Signaling with the Innate Immune Response

Stress response pathways regulate proteostasis and mitigate macromolecular damage to promote long-term cellular health. Intercellular signaling is an essential layer of systemic proteostasis in an organism and is facilitated via transcellular signaling molecules that orchestrate the activation of stress responses across tissues and organs. Accumulating evidence indicates that components of the immune response act as signaling factors that regulate the cell-non-autonomous proteostasis network. Here, we review emergent advances in our understanding of cell-non-autonomous regulators of proteostasis networks in multicellular settings, from the model organism, Caenorhabditis elegans, to humans. We further discuss how innate immune responses can be players of the organismal proteostasis network and discuss how both are linked in cancer.

Transcription factor autoregulation is required for acquisition and maintenance of neuronal identity

The expression of transcription factors that initiate the specification of a unique cellular identity in multicellular organisms is often maintained throughout the life of the respective cell type via an autoregulatory mechanism. It is generally assumed that such autoregulation serves to maintain the differentiated state of a cell. To experimentally test this assumption, we used CRISPR/Cas9-mediated genome engineering to delete a transcriptional autoregulatory, cis-acting motif in the che-1 zinc-finger transcription factor locus, a terminal selector required to specify the identity of the ASE neuron pair during embryonic development of the nematode Caenorhabditis elegans. We show that che-1 autoregulation is indeed required to maintain the differentiated state of the ASE neurons but that it is also required to amplify che-1 expression during embryonic development to reach an apparent minimal threshold to initiate the ASE differentiation program. We conclude that transcriptional autoregulation fulfills two intrinsically linked purposes: one in proper initiation, the other in proper maintenance of terminal differentiation programs.This article has an associated 'The people behind the papers' interview.

Molecular and cellular modulators for multisensory integration in C. elegans

In the natural environment, animals often encounter multiple sensory cues that are simultaneously present. The nervous system integrates the relevant sensory information to generate behavioral responses that have adaptive values. However, the neuronal basis and the modulators that regulate integrated behavioral response to multiple sensory cues are not well defined. Here, we address this question using a behavioral decision in C. elegans when the animal is presented with an attractive food source together with a repulsive odorant. We identify specific sensory neurons, interneurons and neuromodulators that orchestrate the decision-making process, suggesting that various states and contexts may modulate the multisensory integration. Among these modulators, we characterize a new function of a conserved TGF-β pathway that regulates the integrated decision by inhibiting the signaling from a set of central neurons. Interestingly, we find that a common set of modulators, including the TGF-β pathway, regulate the integrated response to the pairing of different foods and repellents. Together, our results provide mechanistic insights into the modulatory signals regulating multisensory integration.

The present study characterizes the modulation of a behavioral decision in C. elegans when the worm is presented with a food lawn that is paired with a repulsive smell. We show that multiple specific sensory neurons and interneurons play roles in making the decision. We also identify several modulatory molecules that are essential for the integrated decision when the animal faces a choice between the cues of opposing valence. We further show that many of these factors, which often represent different states and contexts, are common for behavioral decisions that integrate sensory information from different types of foods and repellents. Overall, our results reveal the molecular and cellular basis for integration of simultaneously present attractive and repulsive cues to fine-tune decision-making.

Olfaction regulates organismal proteostasis and longevity via microRNA-dependent signaling

The maintenance of proteostasis is crucial for any organism to survive and reproduce in an ever-changing environment, but its efficiency declines with age¹. Posttranscriptional regulators such as microRNAs control protein translation of target mRNAs with major consequences for development, physiology, and longevity^2,3. Here we show that food odor stimulates organismal proteostasis and promotes longevity in Caenorhabditis elegans through mir-71-mediated inhibition of tir-1 mRNA stability in olfactory AWC neurons. Screening a collection of microRNAs that control aging³ we find that miRNA mir-71 regulates lifespan and promotes ubiquitin-dependent protein turnover, particularly in the intestine. We show that mir-71 directly inhibits the toll receptor domain protein TIR-1 in AWC olfactory neurons and that disruption of mir-71/tir-1 or loss of AWC olfactory neurons eliminates the influence of food source on proteostasis. mir-71-mediated regulation of TIR-1 controls chemotactic behavior and is regulated by odor. Thus, odor perception influences cell-type specific miRNA-target interaction to regulate organismal proteostasis and longevity. We anticipate that the proposed mechanism of food perception will stimulate further research on neuroendocrine brain-to-gut communication and may open the possibility for therapeutic interventions to improve proteostasis and organismal health via the sense of smell, with potential implication for obesity, diabetes and aging.

daf-16/FOXO isoform b in AIY neurons is involved in low preference for Bifidobacterium infantis in Caenorhabditis elegans

The neural and molecular mechanisms underlying food preference have been poorly understood. We previously showed that Bifidobacterium infantis (B. infantis), a well-known probiotic bacterium, extends the lifespan of Caenorhabditis elegans (C. elegans) compared with a standard food, Escherichia coli (E. coli) OP50. In this study, we characterized C. elegans behavior against B. infantis and examined the neural and molecular mechanisms governing that behavior. The majority of the wild-type animals were outside of the B. infantis lawn 10 min after transfer. Although worms did not prefer B. infantis compared to E. coli OP50, they preferred the B. infantis lawn over a lawn containing M9 buffer alone, in which there was no food. Mutant analyses suggested that leaving the B. infantis lawn required daf-16/FOXO. Isoform-specific mutant phenotypes suggested that daf-16 isoform b seemed to be associated with leaving. Genetic rescue experiments demonstrated that the function of daf-16b in AIY interneurons was involved in leaving the B. infantis lawn. The daf-18/PTEN mutants were also defective in leaving. In conclusion, C. elegans showed a low preference for B. infantis, and daf-16b in AIY interneurons and daf-18 had roles in leaving B. infantis.

Phosphatidylserine exposure mediated by ABC transporter activates the integrin signaling pathway promoting axon regeneration

Following axon injury, a cascade of signaling events is triggered to initiate axon regeneration. However, the mechanisms regulating axon regeneration are not well understood at present. In Caenorhabditis elegans, axon regeneration utilizes many of the components involved in phagocytosis, including integrin and Rac GTPase. Here, we identify the transthyretin (TTR)-like protein TTR-11 as a component functioning in axon regeneration upstream of integrin. We show that TTR-11 binds to both the extracellular domain of integrin-α and phosphatidylserine (PS). Axon injury induces the accumulation of PS around the injured axons in a manner dependent on TTR-11, the ABC transporter CED-7, and the caspase CED-3. Furthermore, we demonstrate that CED-3 activates CED-7 during axon regeneration. Thus, TTR-11 functions to link the PS injury signal to activation of the integrin pathway, which then initiates axon regeneration.

Unconventional function of an Achaete-Scute homolog as a terminal selector of nociceptive neuron identity

Proneural genes are among the most early-acting genes in nervous system development, instructing blast cells to commit to a neuronal fate. Drosophila Atonal and Achaete-Scute complex (AS-C) genes, as well as their vertebrate orthologs, are basic helix-loop-helix (bHLH) transcription factors with such proneural activity. We show here that a C. elegans AS-C homolog, hlh-4, functions in a fundamentally different manner. In the embryonic, larval, and adult nervous systems, hlh-4 is expressed exclusively in a single nociceptive neuron class, ADL, and its expression in ADL is maintained via transcriptional autoregulation throughout the life of the animal. However, in hlh-4 null mutants, the ADL neuron is generated and still appears neuronal in overall morphology and expression of panneuronal and pansensory features. Rather than acting as a proneural gene, we find that hlh-4 is required for the ADL neuron to function properly, to adopt its correct morphology, to express its unusually large repertoire of olfactory receptor-encoding genes, and to express other known features of terminal ADL identity, including neurotransmitter phenotype, neuropeptides, ion channels, and electrical synapse proteins. hlh-4 is sufficient to induce ADL identity features upon ectopic expression in other neuron types. The expression of ADL terminal identity features is directly controlled by HLH-4 via a phylogenetically conserved E-box motif, which, through bioinformatic analysis, we find to constitute a predictive feature of ADL-expressed terminal identity markers. The lineage that produces the ADL neuron was previously shown to require the conventional, transient proneural activity of another AS-C homolog, hlh-14, demonstrating sequential activities of distinct AS-C-type bHLH genes in neuronal specification. Taken together, we have defined here an unconventional function of an AS-C-type bHLH gene as a terminal selector of neuronal identity and we speculate that such function could be reflective of an ancestral function of an "ur-" bHLH gene.

Zic Genes in Nematodes: A Role in Nervous System Development and Wnt Signaling

Transcription factors of the Zic family play important roles during animal development, and their misregulation has been implicated in several human diseases. Zic proteins are present in nematodes, and their function has been mostly studied in the model organism C. elegans. C. elegans possesses only one Zic family member, REF-2. Functional studies have shown that this factor plays a key role during the development of the nervous system, epidermis, and excretory system. In addition, they have revealed that the C. elegans Zic protein acts as an atypical mediator of the Wnt/β-catenin pathway. In other animals including vertebrates, Zic factors are also regulators of nervous system development and modulators of Wnt signaling, suggesting that these are evolutionary ancient functions of Zic proteins.

Integration of Plasticity Mechanisms within a Single Sensory Neuron of C. elegans Actuates a Memory

Neural plasticity, the ability of neurons to change their properties in response to experiences, underpins the nervous system's capacity to form memories and actuate behaviors. How different plasticity mechanisms act together in vivo and at a cellular level to transform sensory information into behavior is not well understood. We show that in Caenorhabditis elegans two plasticity mechanisms-sensory adaptation and presynaptic plasticity-act within a single cell to encode thermosensory information and actuate a temperature preference memory. Sensory adaptation adjusts the temperature range of the sensory neuron (called AFD) to optimize detection of temperature fluctuations associated with migration. Presynaptic plasticity in AFD is regulated by the conserved kinase nPKCε and transforms thermosensory information into a behavioral preference. Bypassing AFD presynaptic plasticity predictably changes learned behavioral preferences without affecting sensory responses. Our findings indicate that two distinct neuroplasticity mechanisms function together through a single-cell logic system to enact thermotactic behavior. VIDEO ABSTRACT.

Cholinergic Sensorimotor Integration Regulates Olfactory Steering

Sensorimotor integration regulates goal-directed movements. We study the signaling mechanisms underlying sensorimotor integration in C. elegans during olfactory steering, when the sinusoidal movements of the worm generate an in-phase oscillation in the concentration of the sampled odorant. We show that cholinergic neurotransmission encodes the oscillatory sensory response and the motor state of head undulations by acting through an acetylcholine-gated channel and a muscarinic acetylcholine receptor, respectively. These signals converge on two axonal domains of an interneuron RIA, where the sensory-evoked signal suppresses the motor-encoding signal to transform the spatial information of the odorant into the asymmetry between the axonal activities. The asymmetric synaptic outputs of the RIA axonal domains generate a directional bias in the locomotory trajectory. Experience alters the sensorimotor integration to generate specific behavioral changes. Our study reveals how cholinergic neurotransmission, which can represent sensory and motor information in the mammalian brain, regulates sensorimotor integration during goal-directed locomotions.

Interactions between LHX3- and ISL1-family LIM-homeodomain transcription factors are conserved in Caenorhabditis elegans

LIM-Homeodomain (LIM-HD) transcription factors are highly conserved in animals where they are thought to act in a transcriptional ‘LIM code’ that specifies cell types, particularly in the central nervous system. In chick and mammals the interaction between two LIM-HD proteins, LHX3 and Islet1 (ISL1), is essential for the development of motor neurons. Using yeast two-hybrid analysis we showed that the Caenorhabditis elegans orthologs of LHX3 and ISL1, CEH-14 and LIM-7 can physically interact. Structural characterisation of a complex comprising the LIM domains from CEH-14 and a LIM-interaction domain from LIM-7 showed that these nematode proteins assemble to form a structure that closely resembles that of their vertebrate counterparts. However, mutagenic analysis across the interface indicates some differences in the mechanisms of binding. We also demonstrate, using fluorescent reporter constructs, that the two C. elegans proteins are co-expressed in a small subset of neurons. These data show that the propensity for LHX3 and Islet proteins to interact is conserved from C. elegans to mammals, raising the possibility that orthologous cell specific LIM-HD-containing transcription factor complexes play similar roles in the development of neuronal cells across diverse species.

skn-1 is required for interneuron sensory integration and foraging behavior in Caenorhabditis elegans

Nrf2/skn-1, a transcription factor known to mediate adaptive responses of cells to stress, also regulates energy metabolism in response to changes in nutrient availability. The ability to locate food sources depends upon chemosensation. Here we show that Nrf2/skn-1 is expressed in olfactory interneurons, and is required for proper integration of multiple food-related sensory cues in Caenorhabditis elegans. Compared to wild type worms, skn-1 mutants fail to perceive that food density is limiting, and display altered chemo- and thermotactic responses. These behavioral deficits are associated with aberrant AIY interneuron morphology and migration in skn-1 mutants. Both skn-1-dependent AIY autonomous and non-autonomous mechanisms regulate the neural circuitry underlying multisensory integration of environmental cues related to energy acquisition.

The VPS34 PI3K negatively regulates RAB-5 during endosome maturation

The GTPase Rab5 and phosphatidylinositol-3 phosphate [PI(3)P] coordinately regulate endosome trafficking. Rab5 recruits Vps34, the class III phosphoinositide 3-kinase (PI3K), to generate PI(3)P and recruit PI(3)P-binding proteins. Loss of Rab5 and loss of Vps34 have opposite effects on endosome size, suggesting that our understanding of how Rab5 and PI(3)P cooperate is incomplete. Here, we report a novel regulatory loop whereby Caenorhabditis elegans VPS-34 inactivates RAB-5 via recruitment of the TBC-2 Rab GTPase-activating protein. We found that loss of VPS-34 caused a phenotype with large late endosomes, as with loss of TBC-2, and that Rab5 activity (mice have two Rab5 isoforms, Rab5a and Rab5b) is increased in Vps34-knockout mouse embryonic fibroblasts (Vps34 is also known as PIK3C3 in mammals). We found that VPS-34 is required for TBC-2 endosome localization and that the pleckstrin homology (PH) domain of TBC-2 bound PI(3)P. Deletion of the PH domain enhanced TBC-2 localization to endosomes in a VPS-34-dependent manner. Thus, PI(3)P binding of the PH domain might be permissive for another PI(3)P-regulated interaction that recruits TBC-2 to endosomes. Therefore, VPS-34 recruits TBC-2 to endosomes to inactivate RAB-5 to ensure the directionality of endosome maturation.

Protein Quality Control in Health and Disease

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

The RFamide receptor DMSR-1 regulates stress-induced sleep in C. elegans

In response to environments that cause cellular stress, animals engage in sleep behavior that facilitates recovery from the stress. In Caenorhabditis elegans, stress-induced sleep(SIS) is regulated by cytokine activation of the ALA neuron, which releases FLP-13 neuropeptides characterized by an amidated arginine-phenylalanine (RFamide) C-terminus motif. By performing an unbiased genetic screen for mutants that impair the somnogenic effects of FLP-13 neuropeptides, we identified the gene dmsr-1, which encodes a G-protein coupled receptor similar to an insect RFamide receptor. DMSR-1 is activated by FLP-13 peptides in cell culture, is required for SIS in vivo, is expressed non-synaptically in several wake-promoting neurons, and likely couples to a Gi/o heterotrimeric G-protein. Our data expand our understanding of how a single neuroendocrine cell coordinates an organism-wide behavioral response, and suggest that similar signaling principles may function in other organisms to regulate sleep during sickness.

DOI:http://dx.doi.org/10.7554/eLife.19837.001

People often feel fatigued and sleepy when they are sick. Other animals also show signs of sleepiness when ill – they stop eating, move less, and are less responsive to changes in their environment. Sickness-induced sleep helps both people and other animals to recover, and many scientists believe that this type of sleep is different than nightly sleep.

Studies of sickness-induced sleep have made use of a simple worm with a simple nervous system. In this worm, a single nerve cell releases chemicals that cause the worm to fall asleep in response to illness. Animals exposed to one of these chemicals, called FLP-13, fall asleep even when they are not sick. As such, scientists would like to know which cells in the nervous system FLP-13 interacts with, what receptor the cells use to recognize this chemical, and whether it turns on cells that induce sleep or turns off the cells that cause wakefulness.

Now, Iannacone et al. show that FLP-13 likely causes sleep by turning down activity in the cells in the nervous system that promote wakefulness. The experiments sifted through genetic mutations to determine which ones cause the worms not to fall asleep when FLP-13 is released. This revealed that worms with a mutation that causes them to lack a receptor protein called DMSR-1 do not become sleepy in response to FLP-13. This suggests that DMSR-1 must be essential for FLP-13 to trigger sleep. About 10% of cells in the worm’s nervous system have the DMSR-1 receptor. Some of these neurons tell the worm to move forward or to forage around for food. The experiments also showed that FLP-13 is probably not the only chemical that interacts with the DMSR-1 receptor, but the identities of these other chemicals remain unknown.

Additional experiments are now needed to determine if sickness-induced sleepiness in humans and other mammals is triggered by a similar mechanism. If it is, then drugs might be developed to treat people experiencing fatigue associated with sickness as well as other unexplained cases of fatigue.

DOI:http://dx.doi.org/10.7554/eLife.19837.002

Recent Duplication and Functional Divergence in Parasitic Nematode Levamisole-Sensitive Acetylcholine Receptors

Helminth parasites rely on fast-synaptic transmission in their neuromusculature to experience the outside world and respond to it. Acetylcholine plays a pivotal role in this and its receptors are targeted by a wide variety of both natural and synthetic compounds used in human health and for the control of parasitic disease. The model, Caenorhabditis elegans is characterized by a large number of acetylcholine receptor subunit genes, a feature shared across the nematodes. This dynamic family is characterized by both gene duplication and loss between species. The pentameric levamisole-sensitive acetylcholine receptor has been characterized from C. elegans, comprised of five different subunits. More recently, cognate receptors have been reconstituted from multiple parasitic nematodes that are found to vary in subunit composition. In order to understand the implications of receptor composition change and the origins of potentially novel drug targets, we investigated a specific example of subunit duplication based on analysis of genome data for 25 species from the 50 helminth genome initiative. We found multiple independent duplications of the unc-29, acetylcholine receptor subunit, where codon substitution rate analysis identified positive, directional selection acting on amino acid positions associated with subunit assembly. Characterization of four gene copies from a model parasitic nematode, Haemonchus contortus, demonstrated that each copy has acquired unique functional characteristics based on phenotype rescue of transgenic C. elegans and electrophysiology of receptors reconstituted in Xenopus oocytes. We found evidence that a specific incompatibility has evolved for two subunits co-expressed in muscle. We demonstrated that functional divergence of acetylcholine receptors, driven by directional selection, can occur more rapidly than previously thought and may be mediated by alteration of receptor assembly. This phenomenon is common among the clade V parasitic nematodes and this work provides a foundation for understanding the broader context of changing anthelmintic drug targets across the parasitic nematodes.

A map of terminal regulators of neuronal identity in Caenorhabditis elegans

Our present day understanding of nervous system development is an amalgam of insights gained from studying different aspects and stages of nervous system development in a variety of invertebrate and vertebrate model systems, with each model system making its own distinctive set of contributions. One aspect of nervous system development that has been among the most extensively studied in the nematode Caenorhabditis elegans is the nature of the gene regulatory programs that specify hardwired, terminal cellular identities. I first summarize a number of maps (anatomical, functional, and molecular) that describe the terminal identity of individual neurons in the C. elegans nervous system. I then provide a comprehensive summary of regulatory factors that specify terminal identities in the nervous system, synthesizing these past studies into a regulatory map of cellular identities in the C. elegans nervous system. This map shows that for three quarters of all neurons in the C. elegans nervous system, regulatory factors that control terminal identity features are known. In-depth studies of specific neuron types have revealed that regulatory factors rarely act alone, but rather act cooperatively in neuron-type specific combinations. In most cases examined so far, distinct, biochemically unlinked terminal identity features are coregulated via cooperatively acting transcription factors, termed terminal selectors, but there are also cases in which distinct identity features are controlled in a piecemeal fashion by independent regulatory inputs. The regulatory map also illustrates that identity-defining transcription factors are reemployed in distinct combinations in different neuron types. However, the same transcription factor can drive terminal differentiation in neurons that are unrelated by lineage, unrelated by function, connectivity and neurotransmitter deployment. Lastly, the regulatory map illustrates the preponderance of homeodomain transcription factors in the control of terminal identities, suggesting that these factors have ancient, phylogenetically conserved roles in controlling terminal neuronal differentiation in the nervous system. WIREs Dev Biol 2016, 5:474-498. doi: 10.1002/wdev.233 For further resources related to this article, please visit the WIREs website.

Single-Cell Memory Regulates a Neural Circuit for Sensory Behavior

Unveiling the molecular and cellular mechanisms underlying memory has been a challenge for the past few decades. Although synaptic plasticity is proven to be essential for memory formation, the significance of "single-cell memory" still remains elusive. Here, we exploited a primary culture system for the analysis of C. elegans neurons and show that a single thermosensory neuron has an ability to form, retain, and reset a temperature memory. Genetic and proteomic analyses found that the expression of the single-cell memory exhibits inter-individual variability, which is controlled by the evolutionarily conserved CaMKI/IV and Raf pathway. The variable responses of a sensory neuron influenced the neural activity of downstream interneurons, suggesting that modulation of the sensory neurons ultimately determines the behavioral output in C. elegans. Our results provide proof of single-cell memory and suggest that the individual differences in neural responses at the single-cell level can confer individuality.

The Caenorhabditis elegans Ephrin EFN-4 Functions Non-cell Autonomously with Heparan Sulfate Proteoglycans to Promote Axon Outgrowth and Branching

The Eph receptors and their cognate ephrin ligands play key roles in many aspects of nervous system development. These interactions typically occur within an individual tissue type, serving either to guide axons to their terminal targets or to define boundaries between the rhombomeres of the hindbrain. We have identified a novel role for the Caenorhabditis elegans ephrin EFN-4 in promoting primary neurite outgrowth in AIY interneurons and D-class motor neurons. Rescue experiments reveal that EFN-4 functions non-cell autonomously in the epidermis to promote primary neurite outgrowth. We also find that EFN-4 plays a role in promoting ectopic axon branching in a C. elegans model of X-linked Kallmann syndrome. In this context, EFN-4 functions non-cell autonomously in the body-wall muscle and in parallel with HS modification genes and HSPG core proteins. This is the first report of an epidermal ephrin providing a developmental cue to the nervous system.

Sonogenetics is a non-invasive approach to activating neurons in Caenorhabditis elegans

A major challenge in neuroscience is to reliably activate individual neurons, particularly those in deeper brain regions. Current optogenetic approaches require invasive surgical procedures to deliver light of specific wavelengths to target cells to activate or silence them. Here, we demonstrate the use of low-pressure ultrasound as a non-invasive trigger to activate specific ultrasonically sensitized neurons in the nematode, Caenorhabditis elegans. We first show that wild-type animals are insensitive to low-pressure ultrasound and require gas-filled microbubbles to transduce the ultrasound wave. We find that neuron-specific misexpression of TRP-4, the pore-forming subunit of a mechanotransduction channel, sensitizes neurons to ultrasound stimulus, resulting in behavioural outputs. Furthermore, we use this approach to manipulate the function of sensory neurons and interneurons and identify a role for PVD sensory neurons in modifying locomotory behaviours. We suggest that this method can be broadly applied to manipulate cellular functions in vivo.

Common optogenetic approaches require surgical procedures to deliver light of specific wavelengths to the target cells. Here the authors demonstrate the use of low-pressure ultrasound as a non-invasive trigger to activate specific neurons in Caenorhabditis elegans and find that the mechanotransduction channel TRP-4 sensitizes cells to the ultrasound stimulus.

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

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

Molecular biology of thermosensory transduction in C. elegans

As the environmental temperature prominently influences diverse biological aspects of the animals, thermosensation and the subsequent information processing in the nervous system has attracted much attention in biology. Thermotaxis in the nematode Caenorhabditis elegans is an ideal behavioral paradigm by which to address the molecular mechanism underlying thermosensory transduction. Molecular genetic analysis in combination with other physiological and behavioral studies revealed that sensation of ambient temperature is mediated mainly by cyclic guanosine monophosphate (cGMP) signaling in thermosensory neurons. The information of the previously perceived temperature is also stored within the thermosensory neurons, and the consequence of the comparison between the past and the present temperature is conveyed to the downstream interneurons to further regulate the motor-circuits that encode the locomotion.

FBN-1, a fibrillin-related protein, is required for resistance of the epidermis to mechanical deformation during C. elegans embryogenesis

During development, biomechanical forces contour the body and provide shape to internal organs. Using genetic and molecular approaches in combination with a FRET-based tension sensor, we characterized a pulling force exerted by the elongating pharynx (foregut) on the anterior epidermis during C. elegans embryogenesis. Resistance of the epidermis to this force and to actomyosin-based circumferential constricting forces is mediated by FBN-1, a ZP domain protein related to vertebrate fibrillins. fbn-1 was required specifically within the epidermis and FBN-1 was expressed in epidermal cells and secreted to the apical surface as a putative component of the embryonic sheath. Tiling array studies indicated that fbn-1 mRNA processing requires the conserved alternative splicing factor MEC-8/RBPMS. The conserved SYM-3/FAM102A and SYM-4/WDR44 proteins, which are linked to protein trafficking, function as additional components of this network. Our studies demonstrate the importance of the apical extracellular matrix in preventing mechanical deformation of the epidermis during development.

DOI:http://dx.doi.org/10.7554/eLife.06565.001

For an animal embryo to develop, its cells must organize themselves into tissues and organs. For example, skin and the lining of internal organs—such as the lungs and gut—are made from cells called epithelial cells, which are tightly linked to form flat sheets.

In a microscopic worm called Caenorhabditis elegans, the outermost layer of epithelial cells (called the epidermis) forms over the surface of the embryo early on in embryonic development. Shortly afterwards, the embryonic epidermis experiences powerful contractions along the surface of the embryo. The force generated by these contractions converts the embryo from an oval shape to a roughly cylindrical form. These contractions also squeeze the internal tissues and organs, which correspondingly elongate along with the epidermis.

It has been known for decades that such ‘mechanical’ forces are important for the normal development of embryos. However, it remains poorly understood how these forces generate tissues and organs of the proper shape—partly because it is difficult to measure forces in living embryos. It is also not clear how the mechanical properties of specific tissues are controlled.

Now, Kelley, Yochem, Krieg et al. have analyzed the development of C. elegans' embryos and discovered a novel mechanical interplay between the feeding organ (called the pharynx) and the worm's epidermis. The experiments involved studying several mutant worms that perturb epidermal contractions and disrupt the attachment of the pharynx to the epidermis. These studies suggested that the pharynx exerts a strong inward pulling force on the epidermis during development. Using recently developed methods, Kelley, Yochem, Krieg et al. then measured mechanical forces within intact worm embryos and demonstrated that greater forces were experienced in cells that were being pulled by the pharynx.

Kelley, Yochem, Krieg et al. further analyzed how the epidermis normally resists this pulling force from the pharynx and implicated a protein called FBN-1. This worm protein is structurally related to a human protein that is affected in people with a disorder called Marfan Syndrome. Worm embryos without the FBN-1 protein become severely deformed because they are unable to withstand mechanical forces at the epidermis. FBN-1 is normally synthesized and then transported to the outside of the worm embryo by epidermal cells, where it is thought to assemble into a meshwork of long fibers. This provides a strong scaffold that attaches to the epidermis to prevent the epidermis from undergoing excessive deformation while it experiences mechanical forces.

The work of Kelley, Yochem, Krieg et al. provides an opportunity to understand how FBN-1 and other fiber-forming proteins are produced and transported to the cell surface. Moreover, these findings may have implications for human diseases and birth defects that result from an inability of tissues to respond appropriately to mechanical forces.

DOI:http://dx.doi.org/10.7554/eLife.06565.002

Decoding a neural circuit controlling global animal state in C. elegans

Brains organize behavior and physiology to optimize the response to threats or opportunities. We dissect how 21% O₂, an indicator of surface exposure, reprograms C. elegans' global state, inducing sustained locomotory arousal and altering expression of neuropeptides, metabolic enzymes, and other non-neural genes. The URX O₂-sensing neurons drive arousal at 21% O₂ by tonically activating the RMG interneurons. Stimulating RMG is sufficient to switch behavioral state. Ablating the ASH, ADL, or ASK sensory neurons connected to RMG by gap junctions does not disrupt arousal. However, disrupting cation currents in these neurons curtails RMG neurosecretion and arousal. RMG signals high O₂ by peptidergic secretion. Neuropeptide reporters reveal neural circuit state, as neurosecretion stimulates neuropeptide expression. Neural imaging in unrestrained animals shows that URX and RMG encode O₂ concentration rather than behavior, while the activity of downstream interneurons such as AVB and AIY reflect both O₂ levels and the behavior being executed.

DOI:http://dx.doi.org/10.7554/eLife.04241.001

From humans to worms, animals must respond appropriately to environmental challenges to survive. Starving animals must conserve energy while they seek food; animals that encounter a predator must fight or flee. These responses involve the animals re-programming their bodies and behavior, and, in humans, are thought to coincide with feelings or emotions such as ‘hunger’ and ‘fear’. Understanding these states in humans is difficult, but studies of simpler animals may provide some insights.

The microscopic worm Caenorhabditis elegans offers a unique advantage to these studies because it has the most precisely described nervous system of any animal. The worm lives in rotting fruit, but it avoids the fruit's surface, perhaps because there is an increased risk of it drying out or being eaten by predators. Microbes that grow within the rotting fruit reduce the oxygen level below the 21% oxygen found in the surrounding air, and so one strategy that C. elegans uses to avoid surface exposure is to continuously monitor the oxygen concentration. If the worm senses that the oxygen level is approaching 21%, which suggests it is nearing the surface, it reverses and turns around. If it cannot find a lower-oxygen environment, the worm switches to continuous rapid movement until it locates such an environment, and adapts its body for surface exposure.

Laurent, Soltesz et al. sought to understand the circuit of neurons that controls this switch. Monitoring gene expression in the worms revealed that specific oxygen-sensing neurons help generate the widespread changes that occur in the worm's body. These neurons also control the switch in the worm's behavior. Sensory neurons relay signals to downstream neurons that act on muscles to alter behavior. Neurons typically communicate with other neurons via specific connections; but neurons can also release signaling molecules, which act like ‘wireless’ signals and can affect many other cells. Laurent, Soltesz et al. showed that both kinds of signaling are needed to change the worm's behavior, and suggest that the release of signaling molecules may explain the widespread effects of 21% oxygen on the worm.

Laurent, Soltesz et al. then monitored the activity of neurons in freely moving worms, and found that some neurons appear to encode and relay specific sensory information. Other neurons encode the behavior the animal is performing, and yet others can encode both kinds of information. To confirm which neurons control particular behavioral responses, Laurent, Soltesz et al. measured changes in the worm’s behavior after destroying or altering specific cells, or while they used light-based techniques to artificially excite or inhibit specific neurons.

At a simple level the worm's response to 21% oxygen resembles the response of a mammal to a dangerous environment: both become more aroused, change how they respond to other sensory cues, and adapt both their bodies and behavior. As such, C. elegans provides a great model to explore at a small and accessible scale how changes in animals' states are generated.

DOI:http://dx.doi.org/10.7554/eLife.04241.002

Vitamin E ameliorates neurodegeneration related phenotypes caused by neurotoxicity of Al2O3-nanoparticles in C. elegans

Vitamin E has the potential to ameliorate the neurotoxicity of Al₂O₃-nanoparticles that induce neurodegeneration related phenotypes inC. elegans.

Evaluation of the long-term memory for thermosensation regulated by neuronal calcium sensor-1 in Caenorhabditis elegans

OBJECTIVE

To evaluate whether the thermotaxis tracking model is suitable for assessing long-term memory (LTM) in the nematode Caenorhabditis elegans.

METHODS

Animals were trained at 20 degrees C overnight in presence of food. The percentage of animals performing isothermal tracking (IT) behavior was measured at different time intervals after the training.

RESULTS

The percentage of animals performing IT behavior, the numbers of body bends inside and outside the training temperature, and the expression patterns of AFD and AIY neurons were similar to those in control animals at 36 and 48 h after training; whereas when extending to 60, 72, and 84 h, locomotory behavior defects were observed in the assayed animals, suggesting that this thermal tracking model is feasible for analyzing LTM at 36 and 48 h after training. Moreover, the percentage of animals performing IT behavior was reduced at 18, 36, and 48 h after training in neuronal calcium sensor-1 gene (nsc-1) mutant animals compared with that in wild-type N2 animals. In addition, exposure to plumbum (Pb) significantly repressed the LTM at 18, 36, and 48 h after training in both wild-type N2 and ncs-1 mutant animals.

CONCLUSION

The thermotaxis tracking model is suitable for evaluating the LTM regulated by NCS-1, and can be employed for elucidating regulatory functions of specific genes or effects of stimuli on memory in C. elegans.