Cellular and molecular analyses of olfactory behavior in C. elegans

C. elegans responds to and discriminates among a large number of volatile and water-soluble chemicals using a few defined chemosensory neurons. The functions of individual sensory neurons have been defined by cell killing experiments, and genes required for responses to subsets of chemicals have been identified. C. elegans has several large families of putative chemosensory receptor genes, and one of these genes has been demonstrated to encode a receptor for a specific odorant. Current work is aimed at identifying additional components of chemosensory neuron development and function.     

Serotonin regulates C. elegans fat and feeding through independent molecular mechanisms

We investigated serotonin signaling in C. elegans as a paradigm for neural regulation of energy balance and found that serotonergic regulation of fat is molecularly distinct from feeding regulation. Serotonergic feeding regulation is mediated by receptors whose functions are not required for fat regulation. Serotonergic fat regulation is dependent on a neurally expressed channel and a G protein-coupled receptor that initiate signaling cascades that ultimately promote lipid breakdown at peripheral sites of fat storage. In turn, intermediates of lipid metabolism generated in the periphery modulate feeding behavior. These findings suggest that, as in mammals, C. elegans feeding behavior is regulated by extrinsic and intrinsic cues. Moreover, obesity and thinness are not solely determined by feeding behavior. Rather, feeding behavior and fat metabolism are coordinated but independent responses of the nervous system to the perception of nutrient availability.     

The neural circuits and synaptic mechanisms underlying motor initiation in C. elegans

C. elegans is widely used to dissect how neural circuits and genes generate behavior. During locomotion, worms initiate backward movement to change locomotion direction spontaneously or in response to sensory cues; however, the underlying neural circuits are not well defined. We applied a multidisciplinary approach to map neural circuits in freely behaving worms by integrating functional imaging, optogenetic interrogation, genetic manipulation, laser ablation, and electrophysiology. We found that a disinhibitory circuit and a stimulatory circuit together promote initiation of backward movement and that circuitry dynamics is differentially regulated by sensory cues. Both circuits require glutamatergic transmission but depend on distinct glutamate receptors. This dual mode of motor initiation control is found in mammals, suggesting that distantly related organisms with anatomically distinct nervous systems may adopt similar strategies for motor control. Additionally, our studies illustrate how a multidisciplinary approach facilitates dissection of circuit and synaptic mechanisms underlying behavior in a genetic model organism.     

Deciphering the molecular mechanisms of anti-tubulin plant derived drugs

Even though commercialized anticancer drugs are now produced by pharmaceutical companies, most of them were originally obtained from natural sources, and more particularly from plants. Indeed, many structurally diverse compounds isolated from plants or marine flora have been purified and synthesized for their anticancer bioactivity. Among these, several molecules belong to the class of anticancer drugs which target the microtubule cytoskeleton, either by stabilizing it or destabilizing it. To characterize the activity of these drugs and to understand in which physiological context they are more likely to be used as therapeutic agents, it is necessary to fully determine their interaction with tubulin. Understanding the molecular basis of their effects on microtubule cytoskeleton is an important step in designing analogs with greater pharmacological activity and with fewer side effects. In addition, knowing the molecular mechanism of action of each drug that is already used in chemotherapy protocols will also help to find strategies to circumvent resistance. By taking examples of known anti-tubulin plant derived drugs, we show how identification of microtubule targeting agents and further characterization of their activity can be achieved combining biophysical and biochemical approaches. We also illustrate how continuing in depth study of molecules with already known primary mechanisms of action can lead to the discovery of new targets or biomarkers which can open new perspectives in anticancer strategies.     

Distinct cellular and molecular mechanisms mediate initial axon development and adult-stage axon regeneration in C. elegans

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-beta 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.     

Axon regeneration mechanisms: insights from C. elegans

Understanding the mechanisms of axon regeneration is of great importance to the development of therapeutic treatments for spinal cord injury or stroke. Axon regeneration has long been studied in diverse vertebrate and invertebrate models, but until recently had not been analyzed in the genetically tractable model organism Caenorhabditis elegans. The small size, simple neuroanatomy, and transparency of C. elegans allows single fluorescently labeled axons to be severed in live animals using laser microsurgery. Many neurons in C. elegans are capable of regenerative regrowth, and can in some cases re-establish functional connections. Large-scale genetic screens have begun to elucidate the genetic basis of axon regrowth.     

Molecular mechanisms of developmental timing in C. elegans and Drosophila.

Characterization of the heterochronic genes has provided a strong foundation for understanding the molecular mechanisms of developmental timing in C. elegans. In apparent contrast, studies of developmental timing in Drosophila have demonstrated a central role for gene cascades triggered by the steroid hormone ecdysone. In this review, I survey the molecular mechanisms of developmental timing in C. elegans and Drosophila and outline how common regulatory pathways are beginning to emerge.     

C. elegans as a genetic model to identify novel cellular and molecular mechanisms underlying nervous system regeneration

Research into conditions that improve axon regeneration has the potential to open a new door for treatment of brain injury caused by stroke and neurodegenerative diseases of aging, such as Alzheimer, by harnessing intrinsic neuronal ability to reorganize itself. Elucidating the molecular mechanisms of axon regeneration should shed light on how this process becomes restricted in the postnatal stage and in the CNS and therefore could provide therapeutic targets for developing strategies to improve axon regeneration in the adult CNS. In this review, we first discuss the general view about nerve regeneration and the advantages of using C. elegans as a model system to study axon regeneration. We then compare the conserved regeneration patterns and molecular mechanisms between C. elegans and vertebrates. Lastly, we discuss the power of femtosecond laser technology and its application in axon regeneration research.Key words: axon regeneration, C. elegans, genetics, femtosecond laser, neuronal circuits     

Experience-dependent modulation of C. elegans behavior by ambient oxygen

BACKGROUND: Ambient oxygen (O2) influences the behavior of organisms from bacteria to man. In C. elegans, an atypical O2 binding soluble guanylate cyclase (sGC), GCY-35, regulates O2 responses. However, how acute and chronic changes in O2 modify behavior is poorly understood. RESULTS: Aggregating C. elegans strains can respond to a reduction in ambient O2 by a rapid, reversible, and graded inhibition of roaming behavior. This aerokinetic response is mediated by GCY-35 and GCY-36 sGCs, which appear to become activated as O2 levels drop and to depolarize the AQR, PQR, and URX neurons. Coexpression of GCY-35 and GCY-36 is sufficient to transform olfactory neurons into O2 sensors. Natural variation at the npr-1 neuropeptide receptor alters both food-sensing and O2-sensing circuits to reconfigure the salient features of the C. elegans environment. When cultivated in 1% O2 for a few hours, C. elegans reset their preferred ambient O2, seeking instead of avoiding 0%-5% O2. This plasticity involves reprogramming the AQR, PQR, and URX neurons. CONCLUSIONS: To navigate O2 gradients, C. elegans can modulate turning rates and speed of movement. Aerotaxis can be reprogrammed by experience or engineered artificially. We propose a model in which prolonged activation of the AQR, PQR, and URX neurons by low O2 switches on previously inactive O2 sensors. This enables aerotaxis to low O2 environments and may encode a memory of previous cultivation in low O2.     

A self-regulating feed-forward circuit controlling C. elegans egg-laying behavior

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.     

Cell signals allow the expression of a pre-existent neural pattern in C. elegans.

In C. elegans, a simple pattern develops within a row of epidermal precursor cells, V1-V6. One cell, V5, gives rise to a neuroblast called the postdeirid neuroblast, while the other V cells produce epidermal cells instead. Here we describe experiments suggesting that in order for V5 to produce the postdeirid it must be in close or direct contact with neighboring V cells. Signaling between V cells is required for the formation of the neuroblast; however, which of the V cells can make a postdeirid is not determined by these signals but rather by the action of the homeotic lin-22 and pal-1 genes. These genes prevent V cells in specific body regions from responding to intercellular signals and producing postdeirids. This is a clear example of cell signals playing a permissive rather than an instructive role in neuroblast induction.     

秀丽线虫衰老调控的生理与分子机制

秀丽线虫(Caenorhabditis elegans)是目前研究动物衰老调控机制重要模型之一.在线虫中,众多的信号通路经相互协同形成的复杂网络接受来自感觉神经元的各种环境信息,进而通过信号传递作用于各组织器官而影响衰老的诸多方面.线虫衰老的生理与分子调控主要涉及三个调节系统,即发挥神经内分泌功能的insulin/IGF-1体系、进食限制延缓衰老的调节系统和线粒体呼吸链/ATP合成体系.本文主要针对这三类体系的生理与分子调控机制予以论述.     

Computational inference of the molecular logic for synaptic connectivity in C. elegans

MOTIVATION: The nematode C. elegans is an ideal model organism in which to investigate the biomolecular mechanisms underlying the connectivity of neurons, because synaptic connections are described in a comprehensive wiring diagram and methods for defining gene expression profiles of individual neurons are now available. RESULTS: Here we present computational techniques linking these two types of information. A systems-based approach (EMBP: Entropy Minimization and Boolean Parsimony) identifies sets of synergistically interacting genes whose joint expression predicts neural connectivity. We introduce an information theoretic measure of the multivariate synergy, a fundamental concept in systems biology, connecting the members of these gene sets. We present and validate our preliminary results based on publicly available information, and demonstrate that their synergy is exceptionally high indicating joint involvement in pathways. Our strategy provides a robust methodology that will yield increasingly more accurate results as more neuron-specific gene expression data emerge. Ultimately, we expect our approach to provide important clues for universal mechanisms of neural interconnectivity.