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[
J Neurosci,
1999]
To investigate the behavioral mechanism of chemotaxis in Caenorhabditis elegans, we recorded the instantaneous position, speed, and turning rate of single worms as a function of time during chemotaxis in gradients of the attractants ammonium chloride or biotin. Analysis of turning rate showed that each worm track could be divided into periods of smooth swimming (runs) and periods of frequent turning (pirouettes). The initiation of pirouettes was correlated with the rate of change of concentration (dC/dt) but not with absolute concentration. Pirouettes were most likely to occur when a worm was heading down the gradient (dC/dt < 0) and least likely to occur when a worm was heading up the gradient (dC/dt > 0). Further analysis revealed that the average direction of movement after a pirouette was up the gradient. These observations suggest that chemotaxis is produced by a series of pirouettes that reorient the animal to the gradient. We tested this idea by imposing the correlation between pirouettes and dC/dt on a stochastic point model of worm motion. The model exhibited chemotaxis behavior in a radial gradient and also in a novel planar gradient. Thus, the pirouette model of C. elegans chemotaxis is sufficient and general.
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[
J Exp Biol,
2005]
C. elegans advances up a chemical gradient by modulating the probability of occasional large, course-correcting turns called pirouettes. However, it remains uncertain whether C. elegans also uses other behavioral strategies for chemotaxis. Previous observations of the unusual, spiral-shaped chemotaxis tracks made by the bent-head mutant
unc-23 point to a different strategy in which the animal continuously makes more subtle course corrections. In the present study we have combined automated tracking of individual animals with computer modeling to test the hypothesis that the pirouette strategy is sufficient on its own to account for the spiral tracks. Tracking experiments showed that the bent-head phenotype causes a strong turning bias and disrupts pirouette execution but does not disrupt pirouette initiation. A computer simulation of disrupted pirouette behavior and turning bias reproduced the spiral tracks of
unc-23 chemotaxis behavior, showing that the pirouette strategy is sufficient to account for the mutant phenotype. In addition, the simulation reproduced higher order features of the behavior such as the relationship between the handedness of the spiral and the side to which the head was bent. Our results suggest that the pirouette mechanism is sufficient to account for a diverse range of chemotaxis trajectories.
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[
Neuronal Development, Synaptic Function and Behavior, Madison, WI,
2010]
A wide variety of animals must quickly adjust their pattern of locomotion to successfully navigate through different environmental niches. Selection and execution of the appropriate locomotory pattern is therefore paramount to survival. Although C. elegans is capable of performing many adaptive behaviors, it has been controversial whether forward crawling and swimming represent distinct gait-like forms of locomotion or the modulation of a single form of locomotion [1-3]. Biogenic amines have been shown to mediate the transition between gait-like forms of locomotion across taxa as diverse as sea slugs, leeches, lampreys and humans. We previously reported that C. elegans crawls and swims with distinct kinematics and different patterns of muscle activity [2]. We now combine quantitative behavioral analysis, optogenetic tools and neuronal ablation to show that C. elegans uses biogenic amines to switch between crawling and swimming in a gait-like manner. As in other invertebrates, we find that serotonin mediates the smooth transition from crawling to swimming in C. elegans. Serotonin is further required to inhibit motor behaviors (e.g. foraging and pharyngeal pumping) during swimming that normally only accompany crawling. Mirroring the role of dopamine in other invertebrates, C. elegans uses dopamine to successfully initiate and maintain crawling when emerging from liquid. Over 600 million years of separate evolution notwithstanding, the highly conserved role played by biogenic amines such as dopamine and serotonin across taxa attests to how vital their function is to adaptive strategies for locomotion. Korta J, Clark DA, Gabel CV, Mahadevan L, Samuel AD. J. Exp. Bio. 2007 210:2383-9.Pierce-Shimomura JT, Chen BL, Mun JJ, Ho R, Sarkis R, McIntire SL. PNAS. 2008 105:20982-7.Berri S, Boyle JH, Tassieri M, Hope IA, Cohen N. HSFP J. 2009 3:186-93.
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[
Neuronal Development, Synaptic Function and Behavior, Madison, WI,
2010]
Alzheimer's disease (AD) is the most common cause of dementia. It is characterized by selective degeneration of cholinergic neurons involved in memory. Dying neurons are surrounded by dense plaques primarily composed of beta-amyloid peptide (Aβ). Aβ is just one cleavage product from the protein APP. Mutations in APP that affect the processing of Aβ result in early-onset AD; however, a single additional wild-type copy of APP can also lead to AD, as seen in all individuals with Down syndrome. Aβ plaques were originally thought to be the primary cause of neurodegeneration, but new research suggests the role of the APP in AD is more complex than originally appreciated. The labs of Dr. Chris Li and Dr. Chris Link have pioneered the use of C. elegans to study APP function and dysfunction. For instance, overexpression of human Aβ in C. elegans muscle leads to the formation of Aβ aggregates(1). Overexpression of multiple copies of the APP-related gene,
apl-1, in C. elegans leads to gross phenotypes, including partial lethality, arrested development, and vacuolization(2). We are testing whether expression of only one additional wild-type copy of
apl-1 results in AD-related phenotypes. DIC and fluorescence microscopy were used to evaluate worms for signs of neurodegeneration. We have discovered that overexpression of
apl-1 in a single copy results in the age-related degeneration of a specific subset of cholinergic neurons in C. elegans. To quantify the impact of degeneration, we track the deterioration of two natural behaviors (swimming and egg-laying) that rely on these neurons. These worms displayed no obvious defects in early adulthood but began to display defects in egg-laying and swimming by the third day of adulthood ('middle age'). In addition, ablation of these specific neurons in wild-type individuals recapitulates the behavioral defects observed in egg-laying and swimming. These results suggest that the behavioral defects observed in the overexpression strain are primarily due to the selective degeneration of these neurons. The quantifiable link between these behaviors and neurodegeneration allows us to test strategies to recover the function mediated by the degenerated cholinergic neurons. We are now studying the mechanisms by which
apl-1 overexpression causes death of cholinergic neurons in the worm and in the process, generate novel hypotheses about the mechanism of Alzheimer's disease. 1. Link CD. (1995) PNAS 92, 9368-9372. 2. Hornsten A. et al. (2007) PNAS 104, 1971-1976.
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[
Neuronal Development, Synaptic Function and Behavior, Madison, WI,
2010]
Moisture is essential for life -- critically influencing physiology, behavior and evolution. As such, many animals have adapted different behavioral mechanisms to migrate toward their preferred level of moisture. This ability to detect moisture is called hygrosensation and the ability to migrate to a favored moisture level is called hygrotaxis. These behaviors are often critical to keep an animal within its niche and regulate essential processes like growth and reproduction. It is therefore surprising that although the basic neuromolecular mechanisms for the detection of light, sound, tastants, odorants, and even temperature have been identified, the molecular basis for how a nervous system senses moisture remains mostly unknown. In order to uncover the molecular basis for hygrosensation we have developed a novel behavioral hygrotaxis assay for C. elegans. With this assay we have shown that C. elegans has the ability to move directly to a preferred level in a moisture gradient, whereas it moves randomly when there is no moisture gradient. By assaying extant mutants, we have found that molecular pathways that are essential for taste, olfaction, thermosensation and hyperosmotic repulsion are not required for hygrotaxis. We are currently characterizing the neural network for hygrosensation and hygrotaxis by combining behavioral assays of additional mutant and transgenic worms, a forward genetic screen for hygrotaxis (htx) mutants, and functional imaging of identified neurons. Study of the molecular basis for hygrosensation in C. elegans may provide insight into this sensory modality in other organisms.
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[
Chembiochem,
2013]
An in vivo system for monitoring small-molecule-mediated neuronal branching has been developed by using C. elegans. Growth-promoting compounds can be detected by visual inspection of GFPlabeled cholinergic neurons, as axonal branching occurs following treatment with neurotrophic agents. Investigation of the structure-activity relationship of the neurotrophic natural product clovanemagnolol (1) led us to a comparable chemically edited derivative.
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[
Neuronal Development, Synaptic Function and Behavior, Madison, WI,
2010]
Down syndrome (DS) is the most common genetic cause of mental retardation, occurring at a rate of 1 in 733 live births in the United States. Individuals with DS often suffer from additional neurological and neuromuscular symptoms including congenital heart defects, hypotonia (poor muscle tone), defects in learning and memory, as well as early-onset Alzheimer's disease. While DS is known to result from an extra copy of chromosome 21, it is unknown which specific genes cause the associated phenotypes. The human 21st chromosome encodes 226 known protein-coding genes. Only a small portion of these genes have been studied for potential roles in DS. We have found that, excluding keratin genes, over 85% of the 21st chromosome genes have equivalents in the C. elegans genome suggesting that the worm can be an extremely useful model for DS. We now aim to identify the subset of these genes which may cause neural and/or muscle dysfunction in DS. To this end, we are first focused on identifying the in vivo role of these genes using RNAi. In our initial screen, we found that approximately half of the RNAi treatments caused worms to exhibit abnormal neuromuscular phenotypes. By examining the locomotion, pharyngeal pumping, and neural anatomy of RNAi-treated worms we can further describe how these genes elicit phenotypes related to DS. Furthermore, by generating strains of worms which overexpress genes of interest, we will be able to observe the effects of DS-equivalent doses of the genes and test putative treatments for DS-related phenotypes. Knowledge of the genes which contribute to the phenotypes of DS will help us understand the causes of this condition and aid in the development of novel treatments to improve the quality of life of people with DS.
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[
J Neurophysiol,
2015]
Although the ability to detect humidity (i.e., hygrosensation) represents an important sensory attribute in many animal species (including humans), the neurophysiological and molecular bases of such sensory ability remain largely unknown in many animals. Recently, Russell and colleagues (Russell J, Vidal-Gadea AG, Makay A, Lanam C, Pierce-Shimomura JT. Proc Natl Acad Sci USA 111: 8269-8274, 2014) provided for the first time neuromolecular evidence for the sensory integration of thermal and mechanical sensory cues which underpin the hygrosensation strategy of an animal (i.e., the free-living roundworm Caenorhabditis elegans) that lacks specific sensory organs for humidity detection (i.e., hygroreceptors). Due to the remarkable similarities in the hygrosensation transduction mechanisms used by hygroreceptor-provided (e.g., insects) and hygroreceptor-lacking species (e.g., roundworms and humans), the findings of Russell et al. highlight potentially universal mechanisms for humidity detection that could be shared across a wide range of species, including humans.
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[
International Worm Meeting,
2007]
C. elegans increase its frequency of reversals and turns (jointly termed pirouettes, Pierce-Shimomura et al 1999) after removal of a food stimulus. The AWC and ASK sensory neurons and the AIB interneurons stimulate pirouettes immediately after removal from food, while the AIY and AIA interneurons inhibit pirouettes (Wakabayashi et al 2004, Gray et al 2005). We have found that the sensory neuron AWC releases two neurotransmitters (glutamate and a neuropeptide, NLP-1) when the worm is removed from food. The released glutamate acts to activate AIB and inhibit AIY, promoting reversals. Strains with different reversal frequencies can be generated by manipulating the level of glutamate receptors on interneurons AIB and AIY. Decreasing receptor expression leads to fewer reversals, and increasing receptor expression results in more reversals than in wild-type. The AWC released neuropeptide NLP-1 serves to reduce reversals, suggesting that reversal frequencies are regulated by at least two opposing signaling systems. Consistent with behavioral responses, AWC and AIB respond (by increasing calcium concentration) to removal of stimulus. We plan to extend the imaging studies to other neurons in the circuit. These results provide a plausible molecular explanation that links neurotransmitters, their receptors, and neuronal circuitry to generate behavior. References: Gray, J.M., Hill, J.J., and Bargmann, C.I. (2005). A circuit for navigation in Caenorhabditis elegans. Proc. Natl. Acad. Sci. 102, 3184-3191. Pierce-Shimomura, J.T., Morse, T.M., and Lockery, S.R. (1999). The fundamental role of pirouettes in Caenorhabditis elegans chemotaxis. J. Neurosci 19, 9557-9569. Wakabayashi, T., Kitagawa, I., and Shingai, R. (2004). Neurons regulating the duration of forward locomotion in Caenorhabditis elegans. Neurosci. Res. 50, 103-111.
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[
Neuronal Development, Synaptic Function and Behavior, Madison, WI,
2010]
Humans have the ability to switch smoothly between a wide variety of movement patterns including distinct locomotory gaits. This ability is severely disrupted in certain neurological diseases such as Parkinson's disease which is caused by degeneration of dopamine neurons. We have found that C. elegans displays alternate forms of locomotion; crawling when on firm substrates and swimming when in liquid [1]. Moreover, we examined worm locomotion in different viscosities and found that C. elegans swims at low viscosities, crawls at high viscosities, and continually switches between bouts of crawl- and swim-like motions at intermediate viscosities. This suggests that the worm switches between two distinct forms of locomotion. To investigate a potential role for dopamine in this switching, we examined how activation of dopamine neurons with channelrhodopsin2 (ChR2) affected locomotion.ChR2 is a light-activated cation channel which depolarizes neurons after stimulation with blue light. Activation of dopamine neurons was sufficient to induce a switch from swimming to crawl-like behavior. Application of exogenous dopamine to swimming worms also induced bouts of crawl-like behavior suggesting that dopamine alone acts as a chemical switch sufficient to initiate crawling. Exogenous application of serotonin, which is known to encourage swimming behavior in other species [2], significantly reduced the probability of switching to crawl-like behavior during dopaminergic activation. Lastly, we also have found that ablation of dopamine neurons specifically perturbs the worm's ability to switch from swimming to crawling. Dopamine has been previously implicated in reducing the rate of crawling in the basal-slowing response [3]. Consistent with this, we found that activation of dopamine neurons in animals crawling on unseeded plates induced a transition to a slower form of crawling. This suggests that dopamine can induce context-dependent changes in locomotory patterns. The fact that dopamine is both necessary and sufficient for the swim to crawl transition suggests that C. elegans can be used to model the abnormal motor switching in human Parkinson's disease. Further investigations should shed light on the fundamental neural mechanisms that underlie the switching between distinct patterns of neural activity.1. Pierce-Shimomura JT, Chen BL, Mun JJ, Ho R, Sarkis R, McIntire SL. PNAS. 20082. Friesen WO, Kristan WB.Current Opinion in Neurobiology. 20073. Sawin ER, Ranganathan R, Horvitz HR. Neuron. 2000