Publications by Year: 2008

Zhang, M., et al. A Self-Regulating Feed-Forward Circuit Controlling C. elegans Egg-Laying Behavior. Current Biology 18, 19, 1445-1455 (2008). Publisher's VersionAbstract
Summary Background Egg laying in Caenorhabditis elegans has been well studied at the genetic and behavioral levels. However, the neural basis of egg-laying behavior is still not well understood; in particular, the roles of specific neurons and the functional nature of the synaptic connections in the egg-laying circuit remain uncharacterized. Results We have used in vivo neuroimaging and laser surgery to address these questions in intact, behaving animals. We have found that the HSN neurons play a central role in driving egg-laying behavior through direct excitation of the vulval muscles and VC motor neurons. The VC neurons play a dual role in the egg-laying circuit, exciting the vulval muscles while feedback-inhibiting the HSNs. Interestingly, the HSNs are active in the absence of synaptic input, suggesting that egg laying may be controlled through modulation of autonomous HSN activity. Indeed, body touch appears to inhibit egg laying, in part by interfering with HSN calcium oscillations. Conclusions The egg-laying motor circuit comprises a simple three-component system combining feed-forward excitation and feedback inhibition. This microcircuit motif is common in the C. elegans nervous system, as well as in the mammalian cortex; thus, understanding its functional properties in C. elegans may provide insight into its computational role in more complex brains.
Jung, S., Lee, S. & Samuel, A. Swimming C. elegans in a wet granular medium. Chaos: An Interdisciplinary Journal of Nonlinear Science 18, 4, 041106 (2008). Publisher's Version
Gabel, C., Antoine, F., Chuang, C., Samuel, A.D. & Chang, C. Distinct cellular and molecular mechanisms mediate initial axon development and adult-stage axon regeneration in C. elegans. Development 135, 21, 1129-1136 (2008). WebsiteAbstract
The molecular and cellular mechanisms that allow adult-stage neurons to regenerate following damage are poorly understood. Recently, axons of motoneurons and mechanosensory neurons in adult C. elegans were found to regrow after being snipped by femtosecond laser ablation. Here, we explore the molecular determinants of adult-stage axon regeneration using the AVM mechanosensory neurons. The first step in AVM axon development is a pioneer axonal projection from the cell body to the ventral nerve cord. We show that regeneration of the AVM axon to the ventral nerve cord lacks the deterministic precision of initial axon development, requiring competition and pruning of unwanted axon branches. Nevertheless, axons of injured AVM neurons regrow to the ventral nerve cord with over 60% reliability in adult animals. In addition, in contrast to initial development, axon guidance during regeneration becomes heavily dependent on cytoplasmic protein MIG-10/Lamellipodin but independent of UNC-129/TGF-β repellent and UNC-40/DCC receptor, and axon growth during regeneration becomes heavily dependent on UNC-34/Ena and CED-10/Rac actin regulators. Thus, C. elegans may be used as a genetic system to characterize novel cellular and molecular mechanisms underlying adult-stage nervous system regeneration.
Luo, L., Gabel, C., Ha, H., Zhang, Y. & Samuel, A. Olfactory behavior of swimming C. elegans analyzed by measuring motile responses to temporal variations of odorants. Journal of Neurophysiology 99, 5, 2617-2625 (2008). Publisher's VersionAbstract
Caenorhabditis elegans responds to chemical cues using a small number of chemosensory neurons that detect a large variety of molecules in its environment. During chemotaxis, C. elegans biases its migration in spatial chemical gradients by lengthening (/shortening) periods of forward movement when it happens to be moving toward (/away) from preferred locations. In classical assays of chemotactic behavior, a group of crawling worms is placed on an agar plate containing a point source of chemical, the group is allowed to navigate for a period of time, and aggregation of worms near the source is quantified. Here we show that swimming worms exhibit acute motile responses to temporal variations of odor in their surrounding environment, allowing our development of an automated assay of chemotactic behavior with single-animal resolution. By placing individual worms in small microdroplets and quantifying their movements as they respond to the addition and removal of odorized airstreams, we show that the sensorimotor phenotypes of swimming worms (wild-type behavior, the effects of certain mutations, and the effects of laser ablation of specific olfactory neurons) are consistent with aggregation phenotypes previously obtained in crawling assays. The microdroplet swimming assay has certain advantages over crawling assays, including flexibility and precision in defining the stimulus waveform and automated quantification of motor response during stimulus presentation. In this study, we use the microdroplet assay to quantify the temporal dynamics of the olfactory response, the sensitivity to odorant concentration, combinations, and gradients, and the contribution of specific olfactory neurons to overall behavior.
Hulme, S., Shevkopylas, S. & Samuel, A. Microfluidics: streamlining discovery in worm biology. Nature Methods 5, 7, 589-590 (2008). Publisher's VersionAbstract
Advances in the application of microfluidics technology to biological assays using the model organism Caenorhabditis elegans help to automate otherwise time-consuming experiments.
Biron, D., Wasserman, S., Thomas, J., Samuel, A.D. & Sengupta, P. An olfactory neuron responds stochastically to temperature and modulates Caenorhabditis elegans thermotactic behavior. Proceedings of the National Academy of Sciences USA 105, 31, 11002-11007 (2008). Publisher's VersionAbstract
Caenorhabditis elegans navigates thermal gradients by using a behavioral strategy that is regulated by a memory of its cultivation temperature (T c). At temperatures above or around the T c, animals respond to temperature changes by modulating the rate of stochastic reorientation events. The bilateral AFD neurons have been implicated as thermosensory neurons, but additional thermosensory neurons are also predicted to play a role in regulating thermotactic behaviors. Here, we show that the AWC olfactory neurons respond to temperature. Unlike AFD neurons, which respond to thermal stimuli with continuous, graded calcium signals, AWC neurons exhibit stochastic calcium events whose frequency is stimulus-correlated in a T c-dependent manner. Animals lacking the AWC neurons or with hyperactive AWC neurons exhibit defects in the regulation of reorientation rate in thermotactic behavior. Our observations suggest that the AFD and AWC neurons encode thermal stimuli via distinct strategies to regulate C. elegans thermotactic behavior.