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

Coordinated regulation of foraging and metabolism in C. elegans by RFamide neuropeptide signaling

Animals modify food-seeking behavior and metabolism according to perceived food availability. Here we show that, in the roundworm C. elegans, release of neuropeptides from interneurons that are directly postsynaptic to olfactory, gustatory, and thermosensory neurons coordinately regulates behavior and metabolism. Animals lacking these neuropeptides, encoded by the flp-18 gene, are defective in chemosensation and foraging, accumulate excess fat, and exhibit reduced oxygen consumption. Two G protein-coupled receptors of the NPY/RFamide family, NPR-4 and NPR-5, are activated by FLP-18 peptides in vitro and exhibit mutant phenotypes that recapitulate those of flp-18 mutants. Our data suggest that sensory input can coordinately regulate behavior and metabolism via NPY/RFamide-like receptors. They suggest that peptidergic feedback from interneurons regulates sensory neuron activity, and that at least some of this communication occurs extrasynaptically. Extrasynaptic neuropeptide signaling may greatly increase the computational capacity of neural circuits.

Three distinct amine receptors operating at different levels within the locomotory circuit are each essential for the serotonergic modulation of chemosensation in Caenorhabditis elegans

Serotonin modulates behavioral plasticity in both vertebrates and invertebrates and in Caenorhabditis elegans regulates key behaviors, including locomotion, aversive learning and olfaction through at least four different 5-HT receptors. In the present study, we examined the serotonergic stimulation of aversive responses to dilute octanol in animals containing null alleles of these 5-HT receptors. Both ser-1 and mod-1 null animals failed to increase sensitivity to dilute octanol on food/5-HT, in contrast to wild-type, ser-4 or ser-7 null animals. 5-HT sensitivity was restored by the expression of MOD-1 and SER-1 in the AIB or potentially the AIY, and RIA interneurons of mod-1 and ser-1 null animals, respectively. Because none of these 5-HT receptors appear to be expressed in the ASH sensory neurons mediating octanol sensitivity, we identified a 5-HT(6)-like receptor, F16D3.7(SER-5), that was required for food/5-HT-dependent increases in octanol sensitivity. ser-5 null animals failed to increase octanol sensitivity in the presence of food/5-HT and sensitivity could be restored by expression of SER-5 in the ASHs. Similarly, the RNAi knockdown of ser-5 expression in the ASHs of wild-type animals also abolished 5-HT-dependent increases in octanol sensitivity, suggesting that SER-5 modulates the octanol responsiveness of the ASHs directly. Together, these results suggest that multiple amine receptors, functioning at different levels within the locomotory circuit, are each essential for the serotonergic modulation of ASH-mediated aversive responses.

Induction of chemotaxis to sodium chloride and diacetyl and thermotaxis defects by microcystin-LR exposure in nematode Caenorhabditis elegans

Apart from the liver disruption, embryotoxicity and genotoxicity, microcystin (MC)-LR also could cause neurotoxicity. Nematode Caenorhabditis elegans was explored as a model to study the neurotoxicity. In the present study, we provided evidence to indicate the neurotoxicity on chemotaxis to NaCl and diacetyl, and thermotaxis from MC-LR exposure to C. elegans. As a result, higher concentrations of MC-LR caused significantly severe defects of chemotaxis to NaCl and diacetyl, and thermotaxis. The neurotoxicity on chemotaxis to NaCl and diacetyl, and thermotaxis from MC-LR exposure might be largely mediated by the damage on the corresponding sensory neurons (ASE, AWA, and AFD) and interneuron AIY The expression levels of che-1 and odr-7 were significantly decreased (P < 0.01) in animals exposed to MC-LR at concentrations lower than 10 microg/L, whereas the expression levels of ttx-1 and ttx-3 could be significantly (P < 0.01) lowered in animals even exposed to 1 microg/L of MC-LR. Moreover, both the chemotaxis to NaCl and diacetyl and the thermotaxis were more significantly reduced in MC-LR exposed mutants of che-1(p674), odr-7(ky4), ttx-1(p767), and ttx-3(ks5) than those in exposed wild-type N2 animals at the same concentrations.

Regulatory logic of neuronal diversity: terminal selector genes and selector motifs

Individual neuronal cell types are defined by the expression of unique batteries of terminal differentiation genes. The elucidation of the cis-regulatory architecture of several distinct, single neuron type-specific gene batteries in Caenorhabditis elegans has revealed a strikingly simple cis-regulatory logic, in which small cis-regulatory motifs are activated in postmitotic neurons by autoregulating transcription factors (TFs). Loss of the TFs results in the loss of the identity of the individual neuron type. I propose to term these TFs "terminal selector genes" and their cognate cis-regulatory target sites "terminal selector motifs." Terminal selector genes assign individual neuronal identities by directly controlling the expression of downstream, terminal differentiation genes and act in specific regulatory network configurations. The simplicity of the cis-regulatory logic on which the terminal selector gene concept is based may contribute to the evolvability of neuronal diversity.

A potent dauer pheromone component in Caenorhabditis elegans that acts synergistically with other components

In the model organism Caenorhabditis elegans, the dauer pheromone is the primary cue for entry into the developmentally arrested, dauer larval stage. The dauer is specialized for survival under harsh environmental conditions and is considered "nonaging" because larvae that exit dauer have a normal life span. C. elegans constitutively secretes the dauer pheromone into its environment, enabling it to sense its population density. Several components of the dauer pheromone have been identified as derivatives of the dideoxy sugar ascarylose, but additional unidentified components of the dauer pheromone contribute to its activity. Here, we show that an ascaroside with a 3-hydroxypropionate side chain is a highly potent component of the dauer pheromone that acts synergistically with previously identified components. Furthermore, we show that the active dauer pheromone components that are produced by C. elegans vary depending on cultivation conditions. Identifying the active components of the dauer pheromone, the conditions under which they are produced, and their mechanisms of action will greatly extend our understanding of how chemosensory cues from the environment can influence such fundamental processes as development, metabolism, and aging in nematodes and in higher organisms.

C. elegans anaplastic lymphoma kinase ortholog SCD-2 controls dauer formation by modulating TGF-beta signaling

BACKGROUND

Different environmental stimuli, including exposure to dauer pheromone, food deprivation, and high temperature, can induce C. elegans larvae to enter the dauer stage, a developmentally arrested diapause state. Although molecular and cellular pathways responsible for detecting dauer pheromone and temperature have been defined in part, other sensory inputs are poorly understood, as are the mechanisms by which these diverse sensory inputs are integrated to achieve a consistent developmental outcome.

RESULTS

In this paper, we analyze a wild C. elegans strain isolated from a desert oasis. Unlike wild-type laboratory strains, the desert strain fails to respond to dauer pheromone at 25 degrees C, but it does respond at higher temperatures, suggesting a unique adaptation to the hot desert environment. We map this defect in dauer response to a mutation in the scd-2 gene, which, we show, encodes the nematode anaplastic lymphoma kinase (ALK) homolog, a proto-oncogene receptor tyrosine kinase. scd-2 acts in a genetic pathway shown here to include the HEN-1 ligand, the RTK adaptor SOC-1, and the MAP kinase SMA-5. The SCD-2 pathway modulates TGF-beta signaling, which mediates the response to dauer pheromone, but SCD-2 might mediate a nonpheromone sensory input, such as food.

CONCLUSIONS

Our studies identify a new sensory pathway controlling dauer formation and shed light on ALK signaling, integration of signaling pathways, and adaptation to extreme environmental conditions.

Temperature sensing by an olfactory neuron in a circuit controlling behavior of C. elegans

Temperature is an unavoidable environmental cue that affects the metabolism and behavior of any creature on Earth, yet how animals perceive temperature is poorly understood. The nematode Caenorhabditis elegans "memorizes" temperatures, and this stored information modifies its subsequent migration along a temperature gradient. We show that the olfactory neuron designated AWC senses temperature. Calcium imaging revealed that AWC responds to temperature changes and that response thresholds differ depending on the temperature to which the animal was previously exposed. In the mutant with impaired heterotrimeric guanine nucleotide-binding protein (G protein)-mediated signaling, AWC was hyperresponsive to temperature, whereas the AIY interneuron (which is postsynaptic to AWC) was hyporesponsive to temperature. Thus, temperature sensation exhibits a robust influence on a neural circuit controlling a memory-regulated behavior.

A Caenorhabditis elegans allatostatin/galanin-like receptor NPR-9 inhibits local search behavior in response to feeding cues

Movement in Caenorhabditis elegans is the result of sensory cues creating stimulatory and inhibitory output from sensory neurons. Four interneurons (AIA, AIB, AIY, and AIZ) are the primary recipients of this information that is further processed en route to motor neurons and muscle contraction. C. elegans has >1,000 G protein-coupled receptors (GPCRs), and their contribution to sensory-based movement is largely undefined. We show that an allatostatin/galanin-like GPCR (NPR-9) is found exclusively in the paired AIB interneuron. AIB interneurons are associated with local search/pivoting behavior. npr-9 mutants display an increased local search/pivoting that impairs their ability to roam and travel long distances on food. With impaired roaming behavior on food npr-9 mutants accumulate more intestinal fat as compared with wild type. Overexpression of NPR-9 resulted in a gain-of-function phenotype that exhibits enhanced forward movement with lost pivoting behavior off food. As such the animal travels a great distance off food, creating arcs to return to food. These findings indicate that NPR-9 has inhibitory effects on the AIB interneuron to regulate foraging behavior, which, in turn, may affect metabolic rate and lipid storage.

Temperature sensing across species

The ability to detect changes in temperature is a fundamental sensory mechanism for every species and provides organisms with a detailed view of the environment. This review focuses on what is known of the neuronal and molecular substrates for thermosensation across species, focusing on the three robust model systems extensively used to study sensory signaling, the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the laboratory mouse. Nematodes migrate to thermal climes that are amenable to their survival, a behavior that is regulated primarily through a single sensory neuron. Additionally, nematodes "learn" to seek out this temperate zone based upon their prior experience, a robust model of learning and memory. Drosophila larvae also prefer select thermal zones that are optimal for growth and have also developed vigorous mechanisms to avoid unfavorable conditions. In mammals, the transduction mechanisms for thermosensation have been identified primarily due to the fact that naturally occurring plant products evoke distinct psychophysical sensation of temperature change. More remarkably, the elucidation of the molecular sensors in mammals, along with those in Drosophila, has demonstrated conservation in the molecular mediators of temperature sensation across diverse species.

Neural circuits mediate electrosensory behavior in Caenorhabditis elegans

The nematode Caenorhabditis elegans deliberately crawls toward the negative pole in an electric field. By quantifying the movements of individual worms navigating electric fields, we show that C. elegans prefers to crawl at specific angles to the direction of the electric field in persistent periods of forward movement and that the preferred angle is proportional to field strength. C. elegans reorients itself in response to time-varying electric fields by using sudden turns and reversals, standard reorientation maneuvers that C. elegans uses during other modes of motile behavior. Mutation or laser ablation that disrupts the structure and function of amphid sensory neurons also disrupts electrosensory behavior. By imaging intracellular calcium dynamics among the amphid sensory neurons of immobilized worms, we show that specific amphid sensory neurons are sensitive to the direction and strength of electric fields. We extend our analysis to the motor level by showing that specific interneurons affect the utilization of sudden turns and reversals during electrosensory steering. Thus, electrosensory behavior may be used as a model system for understanding how sensory inputs are transformed into motor outputs by the C. elegans nervous system.

The molecular signature and cis-regulatory architecture of a C. elegans gustatory neuron

Taste receptor cells constitute a highly specialized cell type that perceives and conveys specific sensory information to the brain. The detailed molecular composition of these cells and the mechanisms that program their fate are, in general, poorly understood. We have generated serial analysis of gene expression (SAGE) libraries from two distinct populations of single, isolated sensory neuron classes, the gustatory neuron class ASE and the thermosensory neuron class AFD, from the nematode Caenorhabditis elegans. By comparing these two libraries, we have identified >1000 genes that define the ASE gustatory neuron class on a molecular level. This set of genes contains determinants of the differentiated state of the ASE neuron, such as a surprisingly complex repertoire of transcription factors (TFs), ion channels, neurotransmitters, and receptors, as well as seven-transmembrane receptor (7TMR)-type putative gustatory receptor genes. Through the in vivo dissection of the cis-regulatory regions of several ASE-expressed genes, we identified a small cis-regulatory motif, the "ASE motif," that is required for the expression of many ASE-expressed genes. We demonstrate that the ASE motif is a binding site for the C2H2 zinc finger TF CHE-1, which is essential for the correct differentiation of the ASE gustatory neuron. Taken together, our results provide a unique view of the molecular landscape of a single neuron type and reveal an important aspect of the regulatory logic for gustatory neuron specification in C. elegans.

Short-Term Adaptation and Temporal Processing in the Cryophilic Response ofCaenorhabditis elegans

When navigating spatial thermal gradients, the nematode C. elegans migrates toward colder temperatures until it reaches its previous cultivation temperature, exhibiting cryophilic movement. The strategy for effecting cryophilic movement is the biased random walk: C. elegans extends (shortens) periods of forward movement that are directed down (up) spatial thermal gradients by modulating the probability of reorientation. Here, we analyze the temporal sensory processor that enables cryophilic movement by quantifying the movements of individual worms subjected to defined temperature waveforms. We show that step increases in temperature as small as 0.05°C lead to transient increases in the probability of reorientation followed by gradual adaptation to the baseline level; temperature downsteps leads to similar but inverted responses. Short-term adaptation is a general property of sensory systems, allowing organisms to maintain sensitivity to sensory variations over broad operating ranges. During cryophilic movement C. elegans also uses the temporal dynamics of its adaptive response to compute the time derivative of gradual temperature variations with exquisite sensitivity. On the basis of the time derivative, the worm determines how it is oriented in spatial thermal gradients during each period of forward movement. We show that the operating range of the cryophilic response extends to lower temperatures in ttx-3 mutants, which affects the development of the AIY interneurons. We show that the temporal sensory processor for the cryophilic response is affected by mutation in the EAT-4 glutamate vesicular transporter. Regulating the operating range of the cryophilic response and executing the cryophilic response may have separate neural mechanisms.

A novel non-coding DNA family in Caenorhabditis elegans

Many repetitive elements, for example, SINEs, LINEs, LTR-retrotransposons and other SSRs are dispersed throughout eukaryotic genomes. To understand the biological function of these repetitive elements is of great current research interest. In this study, we report on the identification of a novel non-coding DNA family, designated CE1 family, in the nematode C. elegans genome. Some CE1 elements constituted a large palindrome sequence. The CE1 elements were interspersed at 95 sites in the C. elegans genome. Most of the CE1 elements were associated with, or were within, protein-coding genes. The sequence of the CE1 elements indicated that some could form a hairpin structure. One of the CE1 family, CE1(bs258), is located in the first intron of a novel gene, C46H11.6 which encodes a PDZ/DHR/GLGF domain protein. In gfp and lacZ reporter gene assays the CE1(bs258) element appeared to behave as an enhancer element for the expression of C46H11.6 but no effect on the expression of the opposite direction gene, pat-10 which encodes the body-wall muscle troponin C. The CE1(bs258) RNA transcript was detected by RT-PCR even when CE1(bs258) was located in an intron. We conclude that CE1 elements are involved in the expression of adjacent genes and are therefore selectively retained in the C. elegans genome. We discussed a biological function of the CE1(bs258) having many transcription factor-binding sites.

The serotonin receptor SER-1 (5HT2ce) contributes to the regulation of locomotion inCaenorhabditis elegans

Serotonin (5-hydroxytryptamine: 5HT) is an important neuroactive substance in the model roundworm, Caenorhabditis elegans. Aside from having effects in feeding and egg-laying, 5HT inhibits motility and also modulates several locomotory behaviors, notably food-induced slowing and foraging. Recent evidence showed that a serotonergic 5HT2-like receptor named SER-1 (also known as 5HT2ce) was responsible for the effect of 5HT on egg-laying. Here we confirm this observation and show that SER-1 also plays an important role in locomotion. A mutant lacking SER-1 was found to be highly resistant to exogenous 5HT in the absence of food and this resistant phenotype was rescued by reintroducing the SER-1 gene in a mutant background. Pharmacological studies showed that the same antagonists that blocked the activity of recombinant SER-1 in vitro also inhibited the effect of 5HT on motility, suggesting the same receptor was responsible for both effects. When tested for locomotory behaviors, the SER-1 mutant was found to be moderately defective in food-induced slowing. In addition, the mutant changed direction more frequently than the wildtype when searching for food, suggesting that SER-1 may play a role in navigational control during foraging. Both these effects required the presence of MOD-1, a 5HT gated chloride channel, and the results indicate that SER-1 and MOD-1 modulate these behaviors through a common pathway. On the basis of expression analysis of a ser-1::GFP translational fusion, SER-1 is prominently located in central, integrating neurons of the head ganglia (RIA and RIC) but not the body wall musculature. The evidence suggests that SER-1 controls locomotion through indirect modulation of neuromuscular circuits and has effects both on speed and direction of movement.

Molecular physiology of the neural circuit for calcineurin-dependent associative learning in Caenorhabditis elegans

How learning and memory is controlled at the neural circuit level is a fundamental question in neuroscience. However, molecular and cellular dissection of the neural circuits underlying learning and memory is extremely complicated in higher animals. Here, we report a simple neural circuit for learning behavior in Caenorhabditis elegans, where the calcium-activated phosphatase, calcineurin, acts as an essential modulator. The calcineurin mutant tax-6 showed defective feeding state-dependent learning behavior for temperature and salt. Surprisingly, defective associative learning between temperature and feeding state was caused by malfunctions of two pairs of directly connected interneurons, AIZ and RIA, in the mature nervous system. Monitoring temperature-evoked Ca2+ concentration changes in the AIZ-RIA neural pathway revealed that starvation, a conditioning factor, downregulated AIZ activity through calcineurin during associative learning between temperature and feeding state. Our results demonstrate the molecular and physiological mechanisms of a simple neural circuit for calcineurin-mediated associative learning behavior.

Caenorhabditis elegans MPP+ model of Parkinson's disease for high-throughput drug screenings

The neurotoxin MPTP and its active metabolite MPP+ cause Parkinson's disease (PD)-like symptoms in vertebrates by selectively destroying dopaminergic neurons in the substantia nigra. MPTP/MPP+ models have been established in rodents to screen for pharmacologically active compounds. In addition to being costly and time consuming, these animal models are not suitable for large scale testings using compound libraries. We present a novel MPP+-based model for high-throughput screenings using the nematode Caenorhabditis elegans. Incubation of C. elegans with MPTP or its active metabolite MPP+ resulted in strong symptomatic defects including reduced mobility and increased lethality, and is correlated with a specific degeneration of the dopaminergic neurons. The phenotypic consequences of MPTP/MPP+ treatments were recorded using automated hardware and software for quantification. Incubation of C. elegans with a variety of pharmacologically active components used in PD treatment reduced the MPP+-induced defects. Our data suggest that the C. elegans MPTP/MPP+ model can be used for the quantitative evaluation of anti-PD drugs.

The claudin superfamily protein nsy-4 biases lateral signaling to generate left-right asymmetry in C. elegans olfactory neurons

Early in C. elegans development, signaling between bilaterally symmetric AWC olfactory neurons causes them to express different odorant receptor genes. AWC left-right asymmetry is stochastic: in each animal, either the left or the right neuron randomly becomes AWC(ON), and the other neuron becomes AWC(OFF). Here we show that the nsy-4 gene coordinates the lateral signaling that diversifies AWC(ON) and AWC(OFF) neurons. nsy-4 mutants generate 2 AWC(OFF) neurons, as expected if communication between the AWC neurons is lost, whereas overexpression of nsy-4 results in 2 AWC(ON) neurons. nsy-4 encodes a transmembrane protein related to the gamma subunits of voltage-activated calcium channels and the claudin superfamily; it interacts genetically with calcium channels and antagonizes a calcium-to-MAP kinase cascade in the neuron that becomes AWC(ON). Genetic mosaic analysis indicates that nsy-4 functions both cell-autonomously and nonautonomously in signaling between AWC neurons, providing evidence for lateral signaling and feedback that coordinate asymmetric receptor choice.

Dietary choice behavior in Caenorhabditis elegans

Animals have evolved diverse behaviors that serve the purpose of finding food in the environment. We investigated the food seeking strategy of the soil bacteria-eating nematode Caenorhabditis elegans. C. elegans bacterial food varies in quality: some species are easy to eat and support worm growth well, while others do not. We show that worms exhibit dietary choice: they hunt for high quality food and leave hard-to-eat bacteria. This food seeking behavior is enhanced in animals that have already experienced good food. When hunting for good food, worms alternate between two modes of locomotion, known as dwelling: movement with frequent stops and reversals; and roaming: straight rapid movement. On good food, roaming is very rare, while on bad food it is common. Using laser ablations and mutant analysis, we show that the AIY neurons serve to extend roaming periods, and are essential for efficient food seeking.

The AFD sensory neurons encode multiple functions underlying thermotactic behavior in Caenorhabditis elegans

The thermotactic behaviors of Caenorhabditis elegans indicate that its thermosensory system exhibits exquisite temperature sensitivity, long-term plasticity, and the ability to transform thermosensory input into different patterns of motor output. Here, we study the physiological role of the AFD thermosensory neurons by quantifying intracellular calcium dynamics in response to defined temperature stimuli. We demonstrate that short-term adaptation allows AFD to sense temperature changes as small as 0.05 degrees C over temperature ranges as wide as 10 degrees C. We show that a bidirectional thermosensory response (increasing temperature raises and decreasing temperature lowers the level of intracellular calcium in AFD) allows the AFD neurons to phase-lock their calcium dynamics to oscillatory thermosensory inputs. By analyzing the thermosensory response of AFD dendrites severed from their cell bodies by femtosecond laser ablation, we show that long-term plasticity is encoded as shifts in the operating range of a putative thermoreceptor(s) in the AFD sensory endings. Finally, we demonstrate that AFD activity is directly coupled to stimulation of its postsynaptic partner AIY. These observations indicate that many functions underlying thermotactic behavior are properties of one sensory neuronal type. Encoding multiple functions in individual sensory neurons may enable C. elegans to perform complex behaviors with simple neuronal circuits.

Quantitative analysis of thermotaxis in the nematode Caenorhabditis elegans

Thermotaxis (TTX) is one of the sophisticated behaviors in the nematode Caenorhabditis elegans. Although the mechanisms of thermotaxis have been deduced from different studies, they are controversial. Previous studies proposed a behavioral model where thermotaxis is regulated by the counterbalance between two opposite driving forces, while recent studies proposed stochastic models. In this study, we analyzed thermotaxis by a novel quantitative population TTX assay using a gentle linear thermal gradient. Analysis of thermotaxis in wild type animals revealed a clear thermal preference to a cultivation temperature with regard to the distribution of animals and the TTX mean expressing temperature preference. A time course assay revealed that the behavioral response to a preferred temperature was initially suppressed for at least 15 min in the animals cultivated at 23 degrees C, but not in those cultivated at 17 degrees C. Our result provides a possible explanation for the inconsistency between the various studies on thermotaxis and is consistent with the early behavioral model, where thermotaxis is regulated by the counterbalance between two driving forces.

The role of the AFD neuron in C. elegans thermotaxis analyzed using femtosecond laser ablation

Background

Caenorhabditis elegans actively crawls down thermal gradients until it reaches the temperature of its prior cultivation, exhibiting what is called cryophilic movement. Implicit in the worm's performance of cryophilic movement is the ability to detect thermal gradients, and implicit in regulating the performance of cryophilic movement is the ability to compare the current temperature of its surroundings with a stored memory of its cultivation temperature. Several lines of evidence link the AFD sensory neuron to thermotactic behavior, but its precise role is unclear. A current model contends that AFD is part of a thermophilic mechanism for biasing the worm's movement up gradients that counterbalances the cryophilic mechanism for biasing its movement down gradients.

Results

We used tightly-focused femtosecond laser pulses to dissect the AFD neuronal cell bodies and the AFD sensory dendrites in C. elegans to investigate their contribution to cryophilic movement. We establish that femtosecond laser ablation can exhibit submicrometer precision, severing individual sensory dendrites without causing collateral damage. We show that severing the dendrites of sensory neurons in young adult worms permanently abolishes their sensory contribution without functional regeneration. We show that the AFD neuron regulates a mechanism for generating cryophilic bias, but we find no evidence that AFD laser surgery reduces a putative ability to generate thermophilic bias. In addition, although disruption of the AIY interneuron causes worms to exhibit cryophilic bias at all temperatures, we find no evidence that laser killing the AIZ interneuron causes thermophilic bias at any temperature.

Conclusion

We conclude that laser surgical analysis of the neural circuit for thermotaxis does not support a model in which AFD opposes cryophilic bias by generating thermophilic bias. Our data supports a model in which the AFD neuron gates a mechanism for generating cryophilic bias.

Aging-dependent and -independent modulation of associative learning behavior by insulin/insulin-like growth factor-1 signal in Caenorhabditis elegans

Mutations in the insulin/IGF-1 neuroendocrine pathway extend lifespan and affect development, metabolism, and other biological processes in Caenorhabditis elegans and in other species. In addition, they may play a role in learning and memory. Investigation of the insulin/IGF-1 pathway may provide clues for the prevention of age-related declines in cognitive functions. Here, we examined the effects of the life-extending (Age) mutations, such as the age-1 (phosphatidylinositol 3-OH kinase) and daf-2 (insulin/IGF-1 receptor) mutations, on associative learning behavior called isothermal tracking. This thermotaxis learning behavior associates paired stimuli, temperature, and food. The age-1 mutation delayed the age-related decline of isothermal tracking, resulting in a 210% extension of the period that ensures it. The effect is dramatic compared with the extension of other physiological health spans. In addition, young adults of various Age mutants (age-1, daf-2, clk-1, and eat-2) showed increased consistency of temperature-food association, which may be caused by a common feature of the mutants, such as the secondary effects of life extension (i.e., enhanced maintenance of neural mechanisms). The age-1 and daf-2 mutants but not the other Age mutants showed an increase in temperature-starvation association through a different mechanism. Increased temperature-food association of the daf-2 mutant was dependent on neuronal Ca2+-sensor ncs-1, which modulates isothermal tracking in the AIY interneuron. Interestingly, mutations in the daf-7 TGFbeta gene, which functions in parallel to the insulin/IGF-1 pathway, caused deficits in acquisition of temperature-food and temperature-starvation association. This study highlights roles of the Age mutations in modulation of certain behavioral plasticity.

Organisation of the lamprey (Lampetra fluviatilis) embryonic brain: insights from LIM-homeodomain, Pax and hedgehog genes

To investigate the embryonic development of the central nervous system of the lamprey Lampetra fluviatilis, we have isolated and analysed the expression patterns of members of the LIM-homeodomain, Pax, Hedgehog and Nkx2.1 families. Using degenerate RT-PCR, single representatives of Lhx1/Lhx5, Lhx2/Lhx9, Pax3/Pax7 and Hedgehog families could be isolated in L. fluviatilis. Expression analysis revealed that the lamprey forebrain presents a clear neuromeric pattern. We describe the existence of 4 embryonic diencephalic prosomeres whose boundaries can be identified by the combined and relative expressions of LfPax37, LfLhx15 and LfLhx29. This suggests that the embryonic lamprey and gnathostome forebrain are patterned in a highly similar manner. Moreover, analysis of the LfHh gene, which is expressed in the hypothalamus, zona limitans intrathalamica and floor plate, reveals the possible molecular origin of this neuromeric brain pattern. By contrast, LfHh and LfNkx2.1 expressions suggest major differences in patterning mechanisms of the ventral telencephalon when compared to gnathostomes. In summary, our findings highlight a neuromeric organisation of the embryonic agnathan forebrain and point to the possible origin of this organisation, which is thus a truly vertebrate character. They also suggest that Hh/Shh midline signalling might act as a driving force for forebrain evolution.

Neuronal substrates of complex behaviors in C. elegans

A current challenge in neuroscience is to bridge the gaps between genes, proteins, neurons, neural circuits, and behavior in a single animal model. The nematode Caenorhabditis elegans has unique features that facilitate this synthesis. Its nervous system includes exactly 302 neurons, and their pattern of synaptic connectivity is known. With only five olfactory neurons, C. elegans can dynamically respond to dozens of attractive and repellent odors. Thermosensory neurons enable the nematode to remember its cultivation temperature and to track narrow isotherms. Polymodal sensory neurons detect a wide range of nociceptive cues and signal robust escape responses. Pairing of sensory stimuli leads to long-lived changes in behavior consistent with associative learning. Worms exhibit social behaviors and complex ultradian rhythms driven by Ca(2+) oscillators with clock-like properties. Genetic analysis has identified gene products required for nervous system function and elucidated the molecular and neural bases of behaviors.

Neural circuit-dependent odor adaptation in C. elegans is regulated by the Ras-MAPK pathway

The molecular machinery that mediates odor adaptation in the olfactory neurons is well documented in various animal species. However, types of adaptation that depend on neural circuits are mostly unexplored. We report here that the Ras-MAPK pathway is essential for such a type of odor adaptation, called early adaptation, in C. elegans. Early adaptation requires a pair of AIY interneurons, which receive synaptic inputs from olfactory neurons. Mutants of the Ras-MAPK pathway show defects in early adaptation. Continued exposure to an odorant causes activation of MAP kinase not only in the olfactory neurons, but also in the AIY interneurons. While activity of the Ras-MAPK pathway in the olfactory neurons is important for odor perception, its activity in the AIY interneurons is important for odor adaptation. Our results thus reveal a dual role of the Ras-MAPK pathway in sensory processing in the nervous system of C. elegans.

Diverse regulation of sensory signaling by C. elegans nPKC-epsilon/eta TTX-4

Molecular and pharmacological studies in vitro suggest that protein kinase C (PKC) family members play important roles in intracellular signal transduction. Nevertheless, the in vivo roles of PKC are poorly understood. We show here that nPKC-epsilon/eta TTX-4 in the nematode Caenorhabditis elegans is required for the regulation of signal transduction in various sensory neurons for temperature, odor, taste, and high osmolality. Interestingly, the requirement for TTX-4 differs in different sensory neurons. In AFD thermosensory neurons, gain or loss of TTX-4 function inactivates or hyperactivates the neural activity, respectively, suggesting negative regulation of temperature sensation by TTX-4. In contrast, TTX-4 positively regulates the signal sensation of ASH nociceptive neurons. Moreover, in AWA and AWC olfactory neurons, TTX-4 plays a partially redundant role with another nPKC, TPA-1, to regulate olfactory signaling. These results suggest that C. elegans nPKCs regulate different sensory signaling in various sensory neurons. Thus, C. elegans provides an ideal model to reveal genetically novel components of nPKC-mediated molecular pathways in sensory signaling.

The GAR-3 muscarinic receptor cooperates with calcium signals to regulate muscle contraction in the Caenorhabditis elegans pharynx

Muscarinic acetylcholine receptors regulate the activity of neurons and muscle cells through G-protein-coupled cascades. Here, we identify a pathway through which the GAR-3 muscarinic receptor regulates both membrane potential and excitation-contraction coupling in the Caenorhabditis elegans pharyngeal muscle. GAR-3 signaling is enhanced in worms overexpressing gar-3 or lacking GPB-2, a G-protein beta-subunit involved in RGS-mediated inhibition of G(o)alpha- and G(q)alpha-linked pathways. High levels of signaling through GAR-3 inhibit pharyngeal muscle relaxation and impair feeding--but do not block muscle repolarization--when worms are exposed to arecoline, a muscarinic agonist. Loss of gar-3 function results in shortened action potentials and brief muscle contractions in the pharyngeal terminal bulb. High levels of calcium entry through voltage-gated channels also impair terminal bulb relaxation and sensitize worms to the toxic effects of arecoline. Mutation of gar-3 reverses this sensitivity, suggesting that GAR-3 regulates calcium influx or calcium-dependent processes. Because the effects of GAR-3 signaling on membrane depolarization and muscle contraction can be separated, we conclude that GAR-3 regulates multiple calcium-dependent processes in the C. elegans pharyngeal muscle.

C. elegans serine-threonine kinase KIN-29 modulates TGFbeta signaling and regulates body size formation

Background

In C. elegans there are two well-defined TGFβ-like signaling pathways. The Sma/Mab pathway affects body size morphogenesis, male tail development and spicule formation while the Daf pathway regulates entry into and exit out of the dauer state. To identify additional factors that modulate TGFβ signaling in the Sma/Mab pathway, we have undertaken a genetic screen for small animals and have identified kin-29.

Results

kin-29 encodes a protein with a cytoplasmic serine-threonine kinase and a novel C-terminal domain. The kinase domain is a distantly related member of the EMK (ELKL motif kinase) family, which interacts with microtubules. We show that the serine-threonine kinase domain has in vitro activity. kin-29 mutations result in small animals, but do not affect male tail morphology as do several of the Sma/Mab signal transducers. Adult worms are smaller than the wild-type, but also develop more slowly. Rescue by kin-29 is achieved by expression in neurons or in the hypodermis. Interaction with the dauer pathway is observed in double mutant combinations, which have been seen with Sma/Mab pathway mutants. We show that kin-29 is epistatic to the ligand dbl-1, and lies upstream of the Sma/Mab pathway target gene, lon-1.

Conclusion

kin-29 is a new modulator of the Sma/Mab pathway. It functions in neurons and in the hypodermis to regulate body size, but does not affect all TGFβ outputs, such as tail morphogenesis.

Maintenance of neuronal positions in organized ganglia by SAX-7, a Caenorhabditis elegans homologue of L1

The L1 family of cell adhesion molecules is predominantly expressed in the nervous system. Mutations in human L1 cause neuronal diseases such as HSAS, MASA, and SPG1. Here we show that sax-7 gene encodes an L1 homologue in Caenorhabditis elegans. In sax-7 mutants, the organization of ganglia and positioning of neurons are abnormal in the adult stage, but these abnormalities are not observed in early larval stage. Misplacement of neurons in sax-7 mutants is triggered by mechanical force linked to body movement. Short and long forms of SAX-7 exhibited strong and weak homophilic adhesion activities in in vitro aggregation assay, respectively, which correlated with their different activities in vivo. SAX-7 was localized on plasma membranes of neurons in vivo. Expression of SAX-7 only in a single neuron in sax-7 mutants cell-autonomously restored its normal neuronal position. Expression of SAX-7 in two different head neurons in sax-7 mutants led to the forced attachment of these neurons. We propose that both homophilic and heterophilic interactions of SAX-7 are essential for maintenance of neuronal positions in organized ganglia.

Genetic control of temperature preference in the nematode Caenorhabditis elegans

Animals modify behavioral outputs in response to environmental changes. C. elegans exhibits thermotaxis, where well-fed animals show attraction to their cultivation temperature on a thermal gradient without food. We show here that feeding-state-dependent modulation of thermotaxis is a powerful behavioral paradigm for elucidating the mechanism underlying neural plasticity, learning, and memory in higher animals. Starved experience alone could induce aversive response to cultivation temperature. Changing both cultivation temperature and feeding state simultaneously evoked transient attraction to or aversion to the previous cultivation temperature: recultivation of starved animals with food immediately induced attraction to the temperature associated with starvation, although the animals eventually exhibited thermotaxis to the new temperature associated with food. These results suggest that the change in feeding state quickly stimulates the switch between attraction and aversion for the temperature in memory and that the acquisition of new temperature memory establishes more slowly. We isolated aho (abnormal hunger orientation) mutants that are defective in starvation-induced cultivation-temperature avoidance. Some aho mutants responded normally to changes in feeding state with respect to locomotory activity, implying that the primary thermosensation followed by temperature memory formation remains normal and the modulatory aspect of thermotaxis is specifically impaired in these mutants.

Expression and regulation of an FMRFamide-related neuropeptide gene family in Caenorhabditis elegans

FMRFamide (Phe-Met-Arg-Phe-NH2) and related peptides (FaRPs) have been found throughout the animal kingdom, where they are involved in many behaviors. We previously identified 22 genes comprising the flp gene family that encodes FaRPs in Caenorhabditis elegans; in this paper we report the identification of another flp gene, flp-23. As a first step toward determining their functional roles in C. elegans, we examined the cell-specific expression pattern of the flp gene family. Of the 19 flp genes examined, each gene is expressed in a distinct set of cells; these cells include interneurons, motor neurons, and sensory neurons that are involved in multiple behaviors, as well as supporting cells, muscle cells, and epidermal cells. Several flp genes show sex-specific expression patterns. Furthermore, we find that expression of two flp genes changes in response to the developmental state of the animal. Many neurons express multiple flp genes. To investigate how flp genes are regulated in different neuronal subtypes, we examined flp expression in a small, well-defined subset of neurons, the mechanosensory neurons. Mutations in the unc-86 and mec-3 genes, which are necessary for the production and differentiation of the mechanosensory neurons, result in the complete loss of flp-4, flp-8, and flp-20 expression in mechanosensory neurons. Collectively, these data indicate that members of the flp gene family are likely to influence multiple behaviors and that their regulation can be dependent on the developmental state of the organism.

Functional domains and temperature-sensitive mutations in SPE-9, an EGF repeat-containing protein required for fertility in Caenorhabditis elegans

The spe-9 gene is required for fertility in Caenorhabditis elegans and encodes a sperm transmembrane protein with an extracellular domain (ECD) that contains 10 epidermal growth factor (EGF) repeats. Deletion analysis reveals that the EGF repeats and the transmembrane domain are required for fertilization. In contrast, the cytoplasmic region of SPE-9 is not essential for fertilization. Individual point mutations in all 10 EGF motifs uncover a differential sensitivity of these sequences to alteration. Some EGF repeats cannot tolerate mutation leading to a complete lack of fertility. Other EGF repeats can be mutated to create animals with temperature-sensitive (ts) fertility phenotypes. All ts mutations were generated by changing either conserved cysteine or glycine residues in the EGF motifs. For two endogenous ts alleles of spe-9, loss of function at nonpermissive temperatures is not due to protein mislocalization or degradation. Additionally, the proper localization of SPE-9 in sperm is not altered in a genetically interacting fertility mutant (spe-13) or a mutant that affects sperm vesicle-plasma membrane fusion (fer-1). Like the EGF repeats in the Notch/LIN-12/GLP-1 receptors and their ligands, the EGF repeats in SPE-9 may carry out different functions. Because EGF motifs are found in many proteins in different species, similar experimental strategies could be used to generate useful temperature-sensitive mutations in other EGF motif-containing molecules.

Multiple roles for the E/Daughterless ortholog HLH-2 during C. elegans gonadogenesis

HLH-2 is the Caenorhabditis elegans ortholog of the Drosophila Daughterless and mammalian E basic helix-loop-helix (bHLH) transcriptional activators that function during diverse events during animal development. HLH-2 has been implicated in cell fate specification in different neural lineages and in the LIN-12/Notch-mediated anchor cell (AC)/ventral uterine precursor cell (VU) decision in the somatic gonad. Here, we show that hlh-2 plays several distinct roles during somatic gonadogenesis. Our analysis suggests that hlh-2 is required to endow specific somatic gonadal cells with the competence to undergo the AC/VU decision, as well as functioning in the AC/VU decision per se; this novel "proAC" role appears to be analogous to the proneural role of Drosophila Daughterless. In addition to its two distinct roles in the specification of the AC, hlh-2 is also required for correct differentiation and function of the AC. hlh-2 also acts at an independent point in the gonadal lineage both to specify distal tip cells (DTCs) and in DTC differentiation and function.

The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channel regulate thermosensory neuron gene expression and function in C. elegans

The cultivation temperature (T(c)) modulates the thermosensory responses exhibited by C. elegans on thermal gradients. The AFD sensory neurons are essential for thermosensory behaviors, but the molecular mechanisms by which temperature is sensed and the memory of the T(c) is encoded are unknown. Here, we show that the CMK-1 Ca2+/calmodulin-dependent protein kinase I (CaMKI) and the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons. Mutations in cmk-1 and tax-4 result in temperature-dependent defects in AFD-specific gene expression, and TAX-4 functions are required during larval stages to maintain gene expression in the adult. CMK-1 and TAX-4 act cell autonomously to regulate AFD-mediated thermosensory behaviors. The molecular requirements for CMK-1 activity in the AFD neurons appear to be distinct from those previously described. We propose that the activation of distinct programs of AFD-specific gene expression at different temperatures by CMK-1 and TAX-4 enables C. elegans to sense and/or encode a memory for the T(c).

LIM homeobox gene-dependent expression of biogenic amine receptors in restricted regions of the C. elegans nervous system

Biogenic amines regulate a variety of behaviors. Their functions are predominantly mediated through G-protein-coupled 7-transmembrane domain receptors (GPCR), 16 of which are predicted to exist in the genome sequence of the nematode Caenorhabditis elegans. We describe here the expression pattern of several of these aminergic receptors, including two serotonin receptors (ser-1 and ser-4), one tyramine receptor (ser-2), and two dopamine receptors (dop-1 and dop-2). Moreover, we describe distinct but partially overlapping expression patterns of different splice forms of the ser-2 tyramine receptor locus. We find that each of the aminergic receptor genes is expressed in restricted regions of the nervous system and that many of them reveal significant overlap with the expression of regulatory factors of the LIM homeobox (Lhx) gene family. We demonstrate that the expression of several of the biogenic amine receptors is abrogated in specific cell types in Lhx gene mutants, thus establishing a role for these Lhx genes in regulating aspects of neurotransmission. We extend these findings with other cell fate markers and show that the lim-4 Lhx gene is required for several but not all aspects of RID motor neuron differentiation and that the lim-6 Lhx gene is required for specific aspects of RIS interneuron differentiation. We also use aminergic receptor gfp reporter fusions as tools to visualize the anatomy of specific neurons in Lhx mutant backgrounds and find that the development of the elaborate dendritic branching pattern of the PVD harsh touch sensory neuron requires the mec-3 Lhx gene. Lastly, we analyze a mutant allele of the ser-2 tyramine receptor, a target of the ttx-3 Lhx gene in the AIY interneuron class. ser-2 mutants display none of the defects previously shown to be associated with loss of AIY function.

Heat shock factor functions at the convergence of the stress response and developmental pathways in Caenorhabditis elegans

Heat shock factor (HSF) is best characterized as the transcriptional regulator of heat shock protein genes, required by all cells to survive periods of stress. Recent evidence suggests that HSF also functions to regulate the expression of genes involved in growth and development under normal physiological conditions. In this study, we used RNA interference (RNAi) assays to investigate the role of HSF in Caenorhabditis elegans. Exposure of wild-type worms to hsf dsRNAi constructs caused a temperature-sensitive developmental arrest at the L2/L3 stage. At normal growth temperatures, hsf(RNAi) worms that developed to adults were small and scrawny, largely infertile, and showed a significant reduction in life span. These results demonstrate that HSF is required for normal postembryonic development under physiological conditions. Following heat shock, hsf(RNAi) worms were thermosensitive and displayed a significant reduction of hsp16 expression. When hsf(RNAi) was carried out in various dauer-constitutive mutant backgrounds, a dramatic reversal of dauer formation was observed, indicating that HSF is also required in the dauer pathway. In its natural habitat of the soil, where C. elegans is exposed to a constantly fluctuating environment; the ability to integrate the stress response with development may be an essential element of its ecology.

Functional mapping of neurons that control locomotory behavior in Caenorhabditis elegans

One approach to understanding behavior is to define the cellular components of neuronal circuits that control behavior. In the nematode Caenorhabditis elegans, neuronal circuits have been delineated based on patterns of synaptic connectivity derived from ultrastructural analysis. Individual cellular components of these anatomically defined circuits have previously been characterized on the sensory and motor neuron levels. In contrast, interneuron function has only been addressed to a limited extent. We describe here several classes of interneurons (AIY, AIZ, and RIB) that modulate locomotory behavior in C. elegans. Using mutant analysis as well as microsurgical mapping techniques, we found that the AIY neuron class serves to tonically modulate reversal frequency of animals in various sensory environments via the repression of the activity of a bistable switch composed of defined command interneurons. Furthermore, we show that the presentation of defined sensory modalities induces specific alterations in reversal behavior and that the AIY interneuron class mediates this alteration in locomotory behavior. We also found that the AIZ and RIB interneuron classes process odorsensory information in parallel to the AIY interneuron class. AIY, AIZ, and RIB are the first interneurons directly implicated in chemosensory signaling. Our neuronal mapping studies provide the framework for further genetic and functional dissections of neuronal circuits in C. elegans.

Step response analysis of thermotaxis in Caenorhabditis elegans

The nematode Caenorhabditis elegans migrates toward a preferred temperature on a thermal gradient. A candidate neural network for thermotaxis in C. elegans has been identified, but the behavioral strategy implemented by this network is poorly understood. In this study, we tested whether thermal migration is achieved by modulating the probability of turning behavior, as in C. elegans chemotaxis. This was done by subjecting unrestrained wild-type, cryophilic, or thermophilic worms to rapid spatially uniform temperature steps (3 degrees C), up or down from the cultivation temperature. Each of the three types of worms we analyzed showed a different pair of responses to the two types of steps. Comparison of wild-type and mutant response patterns suggested a model in which thermal migration involves a unique response to the gradient depending on the orientation of the worm relative to its preferred temperature. Overall, however, turning probability was modulated in a manner consistent with a role for turning behavior in thermal migration. Our results suggest that sensory systems for thermotaxis and chemotaxis may converge on a common behavioral mechanism.

daf-28 encodes a C. elegans insulin superfamily member that is regulated by environmental cues and acts in the DAF-2 signaling pathway

In Caenorhabditis elegans, the decision to enter a developmentally arrested dauer larval stage is triggered by a combination of signals from sensory neurons in response to environmental cues, which include a dauer pheromone. These sensory inputs are coupled to the parallel DAF-2/insulin receptor-like and DAF-7/TGFbeta-like signaling pathways. Although sensory inputs have been shown to physiologically regulate DAF-7/TGFbeta expression, no such regulation of insulin-like ligands in the DAF-2 pathway has been reported. We show here that daf-28 encodes an insulin-like protein, which when mutated causes dauer arrest and down-regulation of DAF-2/IR signaling. A daf-28GFP fusion gene is expressed in ASI and ASJ, two sensory neurons that regulate dauer arrest. daf-28GFP expression in ASI and ASJ is down-regulated under dauer-inducing conditions and in mutants of DAF-11/guanylyl cyclase, a predicted component of the dauer-pheromone-sensing pathway. Thus, daf-28 expression in sensory neurons is regulated by the environmental cues that normally trigger dauer arrest. Among the 38 C. elegans insulin genes, daf-28 is so far the only insulin mutant to affect dauer arrest. daf-28 was revealed from this functional redundancy by a dominant-negative allele that disrupts a probable proteolytic processing site required for insulin maturation. This DAF-28 mutant is likely to be poisonous to wild-type DAF-28 and other insulins.

Behavioral plasticity in C. elegans: paradigms, circuits, genes

Life in the soil is an intellectual and practical challenge that the nematode Caenorhabditis elegans masters by utilizing 302 neurons. The nervous system assembled by these 302 neurons is capable of executing a variety of behaviors, some of respectable complexity. The simplicity of the nervous system, its thoroughly characterized structure, several sets of well-defined behaviors, and its genetic amenability combined with its isogenic background make C. elegans an attractive model organism to study the genetics of behavior. This review describes several behavioral plasticity paradigms in C. elegans and their underlying neuronal circuits and then goes on to review the forward genetic analysis that has been undertaken to identify genes involved in the execution of these behaviors. Lastly, the review outlines how reverse genetics and genomic approaches can guide the analysis of the role of genes in behavior and why and how they will complement the forward genetic analysis of behavior.

The Caenorhabditis elegans presenilin sel-12 is required for mesodermal patterning and muscle function

Mutations in presenilin genes impair Notch signalling and, in humans, have been implicated in the development of familial Alzheimer's disease. We show here that a reduction of the activity of the Caenorhabditis elegans presenilin sel-12 results in a late defect during sex muscle development. The morphological abnormalities and functional deficits in the sex muscles contribute to the egg-laying defects seen in sel-12 hermaphrodites and to the severely reduced mating efficiency of sel-12 males. Both defects can be rescued by expressing sel-12 from the hlh-8 promoter that is active during the development of the sex muscle-specific M lineage, but not by expressing sel-12 from late muscle-specific promoters. Both weak and strong sel-12 mutations cause defects in the sex muscles that resemble the defects we found in lin-12 hypomorphic alleles, suggesting a previously uncharacterised LIN-12 signalling event late in postembryonic mesoderm development. Together with a previous study indicating a role of lin-12 and sel-12 during the specification of the pi cell lineage required for proper vulva-uterine connection, our data suggest that the failure of sel-12 animals to lay eggs properly is caused by defects in at least two independent signalling events in different tissues during development.

Thermotaxis in Caenorhabditis elegans analyzed by measuring responses to defined Thermal stimuli

In a spatial thermal gradient, Caenorhabditis elegans migrates toward and then isothermally tracks near its cultivation temperature. A current model for thermotactic behavior involves a thermophilic drive (involving the neurons AFD and AIY) and cryophilic drive (involving the neuron AIZ) that balance at the cultivation temperature. Here, we analyze the movements of individual worms responding to defined thermal gradients. We found evidence for a mechanism for migration down thermal gradients that is active at temperatures above the cultivation temperature, and a mechanism for isothermal tracking that is active near the cultivation temperature. However, we found no evidence for a mechanism for migration up thermal gradients at temperatures below the cultivation temperature that might have supported the model of opposing drives. The mechanisms for migration down gradients and isothermal tracking control the worm's movements in different manners. Migration down gradients works by shortening (lengthening) the duration of forward movement in response to positive (negative) temperature changes. Isothermal tracking works by orienting persistent forward movement to offset temperature changes. We believe preference for the cultivation temperature is not at the balance between two drives. Instead, the worm activates the mechanism for isothermal tracking near the cultivation temperature and inactivates the mechanism for migration down gradients near or below the cultivation temperature. Inactivation of the mechanism for migration down gradients near or below the cultivation temperature requires the neurons AFD and AIY.

HEN-1, a secretory protein with an LDL receptor motif, regulates sensory integration and learning in Caenorhabditis elegans

Animals sense many environmental stimuli simultaneously and integrate various sensory signals within the nervous system both to generate proper behavioral responses and also to form relevant memories. HEN-1, a secretory protein with an LDL receptor motif, regulates such processes in Caenorhabditis elegans. The hen-1 mutants show defects in the integration of two sensory signals and in behavioral plasticity by paired stimuli, although their sensation capability seems to be identical to that of the wild-type. The HEN-1 protein is expressed in two pairs of neurons, but expression in other neurons is sufficient for wild-type behavior. In addition, expression of HEN-1 at the adult stage is sufficient. Thus, HEN-1 regulates sensory processing non-cell-autonomously in the mature neuronal circuit.

Heparan sulfate proteoglycan-dependent induction of axon branching and axon misrouting by the Kallmann syndrome gene kal-1

Kallmann syndrome is a neurological disorder characterized by various behavioral and neuroanatomical defects. The X-linked form of this disease is caused by mutations in the KAL-1 gene, which codes for a secreted molecule that is expressed in restricted regions of the brain. Its molecular mechanism of action has thus far remained largely elusive. We show here that expression of the Caenorhabditis elegans homolog of KAL-1 in selected sensory and interneuron classes causes a highly penetrant, dosage-dependent, and cell autonomous axon-branching phenotype. In a different cellular context, heterologous C. elegans kal-1 expression causes a highly penetrant axon-misrouting phenotype. The axon-branching and -misrouting activities require different domains of the KAL-1 protein. In a genetic modifier screen we isolated several loci that either suppress or enhance the kal-1-induced axonal defects, one of which codes for an enzyme that modifies specific residues in heparan sulfate proteoglycans, namely heparan-6O-sulfotransferase. We hypothesize that KAL-1 binds by means of a heparan sulfate proteoglycan to its cognate receptor or other extracellular cues to induce axonal branching and axon misrouting.

Control of neuronal subtype identity by the C. elegans ARID protein CFI-1

The Caenorhabditis elegans hermaphrodite nervous system is composed of 302 neurons that fall into at least 118 diverse classes. Here we describe cfi-1, a gene that contributes to the development of neuronal diversity. cfi-1 promotes appropriate differentiation of the URA sensory neurons and inhibits URA from expressing the male-specific CEM neuronal fate. The UNC-86 POU homeodomain protein is present in CEM and URA neurons, and can promote expression of CEM-specific genes in both CEM and URA, but CFI-1 inhibits expression of these genes in the URA cells. cfi-1 also promotes appropriate differentiation and glutamate receptor expression in the AVD and PVC interneurons. cfi-1 encodes a conserved neuron- and muscle-restricted DNA-binding protein containing an A/T rich interaction domain (ARID). ARID proteins regulate early patterning and muscle fate in Drosophila, but they have not previously been implicated in the control of neuronal subtype identity.

Negative regulation and gain control of sensory neurons by the C. elegans calcineurin TAX-6

Animals sense and adapt to variable environments by regulating appropriate sensory signal transduction pathways. Here, we show that calcineurin plays a key role in regulating the gain of sensory neuron responsiveness across multiple modalities. C. elegans animals bearing a loss-of-function mutation in TAX-6, a calcineurin A subunit, exhibit pleiotropic abnormalities, including many aberrant sensory behaviors. The tax-6 mutant defect in thermosensation is consistent with hyperactivation of the AFD thermosensory neurons. Conversely, constitutive activation of TAX-6 causes a behavioral phenotype consistent with inactivation of AFD neurons. In olfactory neurons, the impaired olfactory response of tax-6 mutants to an AWC-sensed odorant is caused by hyperadaptation, which is suppressible by a mutation causing defective olfactory adaptation. Taken together, our results suggest that stimulus-evoked calcium entry activates calcineurin, which in turn negatively regulates multiple aspects of sensory signaling.

ced-10 Rac and mig-2 function redundantly and act with unc-73 trio to control the orientation of vulval cell divisions and migrations in Caenorhabditis elegans

Vulval development in the nematode Caenorhabditis elegans can be divided into a fate specification phase controlled in part by let-60 Ras, and a fate execution phase involving stereotypical patterns of cell division and migration controlled in part by lin-17 Frizzled. Since the small GTPase Rac has been implicated as a downstream target of both Ras and Frizzled and influences cytoskeletal dynamics, we investigated the role of Rac signaling during each phase of vulval development. We show that the Rac gene ced-10 and the Rac-related gene mig-2 are redundantly required for the proper orientation of certain vulval cell divisions, suggesting a role in spindle positioning. ced-10 Rac and mig-2 are also redundantly required for vulval cell migrations and play a minor role in vulval fate specification. Constitutively active and dominant-negative mutant forms of mig-2 cause vulval defects that are very similar to those seen in ced-10;mig-2 double loss-of-function mutants, indicating that they interfere with the functions of both ced-10 Rac and mig-2. Mutations in unc-73 (a Trio-like guanine nucleotide exchange factor) cause similar vulval defects, suggesting that UNC-73 is an exchange factor for both CED-10 and MIG-2. We discuss the similarities and differences between the cellular defects seen in Rac mutants and let-60 Ras or lin-17 Frizzled mutants.

The genetics of aging

The genetic analysis of life span has only begun in mammals, invertebrates, such as Caenorhabditis elegans and Drosophila, and yeast. Even at this primitive stage of the genetic analysis of aging, the physiological observations that rate of metabolism is intimately tied to life span is supported. In many examples from mice to worms to flies to yeast, genetic variants that affect life span also modify metabolism. Insulin signaling regulates life span coordinately with reproduction, metabolism, and free radical protective gene regulation in C. elegans. This may be related to the findings that caloric restriction also regulates mammalian aging, perhaps via the modulation of insulin-like signaling pathways. The nervous system has been implicated as a key tissue where insulin-like signaling and free radical protective pathways regulate life span in C. elegans and Drosophila. Genes that determine the life span could act in neuroendocrine cells in diverse animals. The involvement of insulin-like hormones suggests that the plasticity in life spans evident in animal phylogeny may be due to variation in the timing of release of hormones that control vitality and mortality as well as variation in the response to those hormones. Pedigree analysis of human aging may reveal variations in the orthologs of the insulin pathway genes and coupled pathways that regulate invertebrate aging. Thus, genetic approaches may identify a set of circuits that was established in ancestral metazoans to regulate their longevity.

Specification of thermosensory neuron fate in C. elegans requires ttx-1, a homolog of otd/Otx

Temperature is a critical modulator of animal metabolism and behavior, yet the mechanisms underlying the development and function of thermosensory neurons are poorly understood. C. elegans senses temperature using the AFD thermosensory neurons. Mutations in the gene ttx-1 affect AFD neuron function. Here, we show that ttx-1 regulates all differentiated characteristics of the AFD neurons. ttx-1 mutants are defective in a thermotactic behavior and exhibit deregulated thermosensory inputs into a neuroendocrine signaling pathway. ttx-1 encodes a member of the conserved OTD/OTX homeodomain protein family and is expressed in the AFD neurons. Misexpression of ttx-1 converts other sensory neurons to an AFD-like fate. Our results extend a previously noted conservation of developmental mechanisms between the thermosensory circuit in C. elegans and the vertebrate photosensory circuit, suggesting an evolutionary link between thermosensation and phototransduction.