Mechanosensation in Caenorhabditis elegans

Chemosensation: Corollary discharge filters out self-generated chemical cues

Corollary discharge allows organisms to discriminate external sensory inputs from self-generated cues. However, the underlying synaptic and molecular mechanisms are not well understood. A new study has identified a tyraminergic corollary discharge signal that extrasynaptically modulates chemosensory neurons in Caenorhabditis elegans.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

An Aversive Response to Osmotic Upshift in Caenorhabditis elegans

Environmental osmolarity presents a common type of sensory stimulus to animals. While behavioral responses to osmotic changes are important for maintaining a stable intracellular osmolarity, the underlying mechanisms are not fully understood. In the natural habitat of Caenorhabditis elegans, changes in environmental osmolarity are commonplace. It is known that the nematode acutely avoids shocks of extremely high osmolarity. Here, we show that C. elegans also generates gradually increased aversion of mild upshifts in environmental osmolarity. Different from an acute avoidance of osmotic shocks that depends on the function of a transient receptor potential vanilloid channel, the slow aversion to osmotic upshifts requires the cGMP-gated sensory channel subunit TAX-2. TAX-2 acts in several sensory neurons that are exposed to body fluid to generate the aversive response through a motor network that underlies navigation. Osmotic upshifts activate the body cavity sensory neuron URX, which is known to induce aversion upon activation. Together, our results characterize the molecular and cellular mechanisms underlying a novel sensorimotor response to osmotic stimuli and reveal that C. elegans engages different behaviors and the underlying mechanisms to regulate responses to extracellular osmolarity.

Fluid dynamics alter Caenorhabditis elegans body length via TGF-β/DBL-1 neuromuscular signaling

Skeletal muscle wasting is a major obstacle for long-term space exploration. Similar to astronauts, the nematode Caenorhabditis elegans displays negative muscular and physical effects when in microgravity in space. It remains unclear what signaling molecules and behavior(s) cause these negative alterations. Here we studied key signaling molecules involved in alterations of C. elegans physique in response to fluid dynamics in ground-based experiments. Placing worms in space on a 1G accelerator increased a myosin heavy chain, myo-3, and a transforming growth factor-β (TGF-β), dbl-1, gene expression. These changes also occurred when the fluid dynamic parameters viscosity/drag resistance or depth of liquid culture were increased on the ground. In addition, body length increased in wild type and body wall cuticle collagen mutants, rol-6 and dpy-5, grown in liquid culture. In contrast, body length did not increase in TGF-β, dbl-1, or downstream signaling pathway, sma-4/Smad, mutants. Similarly, a D1-like dopamine receptor, DOP-4, and a mechanosensory channel, UNC-8, were required for increased dbl-1 expression and altered physique in liquid culture. As C. elegans contraction rates are much higher when swimming in liquid than when crawling on an agar surface, we also examined the relationship between body length enhancement and rate of contraction. Mutants with significantly reduced contraction rates were typically smaller. However, in dop-4, dbl-1, and sma-4 mutants, contraction rates still increased in liquid. These results suggest that neuromuscular signaling via TGF-β/DBL-1 acts to alter body physique in response to environmental conditions including fluid dynamics.

Developmental Function of the PHR Protein RPM-1 Is Required for Learning in Caenorhabditis elegans

The PAM/Highwire/RPM-1 (PHR) proteins are signaling hubs that function as important regulators of neural development. Loss of function in Caenorhabditis elegans rpm-1 and Drosophila Highwire results in failed axon termination, inappropriate axon targeting, and abnormal synapse formation. Despite broad expression in the nervous system and relatively dramatic defects in synapse formation and axon development, very mild abnormalities in behavior have been found in animals lacking PHR protein function. Therefore, we hypothesized that large defects in behavior might only be detected in scenarios in which evoked, prolonged circuit function is required, or in which behavioral plasticity occurs. Using quantitative approaches in C. elegans, we found that rpm-1 loss-of-function mutants have relatively mild abnormalities in exploratory locomotion, but have large defects in evoked responses to harsh touch and learning associated with tap habituation. We explored the nature of the severe habituation defects in rpm-1 mutants further. To address what part of the habituation circuit was impaired in rpm-1 mutants, we performed rescue analysis with promoters for different neurons. Our findings indicate that RPM-1 function in the mechanosensory neurons affects habituation. Transgenic expression of RPM-1 in adult animals failed to rescue habituation defects, consistent with developmental defects in rpm-1 mutants resulting in impaired habituation. Genetic analysis showed that other regulators of neuronal development that function in the rpm-1 pathway (including glo-4, fsn-1, and dlk-1) also affected habituation. Overall, our findings suggest that developmental defects in rpm-1 mutants manifest most prominently in behaviors that require protracted or plastic circuit function, such as learning.

Identification of nonviable genes affecting touch sensitivity in Caenorhabditis elegans using neuronally enhanced feeding RNA interference

Caenorhabditis elegans senses gentle touch along the body via six touch receptor neurons. Although genetic screens and microarray analyses have identified several genes needed for touch sensitivity, these methods miss pleiotropic genes that are essential for the viability, movement, or fertility of the animals. We used neuronally enhanced feeding RNA interference to screen genes that cause lethality or paralysis when mutated, and we identified 61 such genes affecting touch sensitivity, including five positive controls. We confirmed 18 genes by using available alleles, and further studied one of them, tag-170, now renamed txdc-9. txdc-9 preferentially affects anterior touch response but is needed for tubulin acetylation and microtubule formation in both the anterior and posterior touch receptor neurons. Our results indicate that neuronally enhanced feeding RNA interference screens complement traditional mutageneses by identifying additional nonviable genes needed for specific neuronal functions.

Tangling of tethered swimmers: interactions between two nematodes

The tangling of two tethered microswimming worms serving as the ends of "active strings" is investigated experimentally and modeled analytically. C. elegans nematodes of similar size are caught by their tails using micropipettes and left to swim and interact at different separations over long times. The worms are found to tangle in a reproducible and statistically predictable manner, which is modeled based on the relative motion of the worm heads. Our results provide insight into the intricate tangling interactions present in active biological systems.

Sensory mechanotransduction at membrane-matrix interfaces

Sensory cells specialized to detect extremely small mechanical changes are common to the auditory and somatosensory systems. It is widely accepted that mechanosensitive channels form the core of the mechanoelectrical transduction in hair cells as well as the somatic sensory neurons that underlie the sense of touch and mechanical pain. Here, we will review how the activation of such channels can be measured in a meaningful physiological context. In particular, we will discuss the idea that mechanosensitive channels normally occur in transmembrane complexes that are anchored to extracellular matrix components (ECM) both in vitro and in vivo. One component of such complexes in sensory neurons is the integral membrane scaffold protein STOML3 which is a robust physiological regulator of native mechanosensitive currents. In order to better characterize such channels in transmembrane complexes, we developed a new electrophysiological method that enables the quantification of mechanosensitive current amplitude and kinetics when activated by a defined matrix movement in cultured cells. The results of such studies strongly support the idea that ion channels in transmembrane complexes are highly tuned to detect movement of the cell membrane in relation to the ECM.

Coupling of proton binding in extracellular domain to channel gating in acid-sensing ion channel

Protonation of several amino acid residues in the extracellular domain (ECD) of acid-sensing ion channel (ASIC) causes conformational changes that lead to opening of the channel. It is not clear how conformational changes in ECD are coupled to channel gating. Here, we show that the loop connecting β9 and α4 at the base of the thumb region of ECD interacts with post-TM1 to stabilize the channel in the closed state. Flexibility of these two regions is important for optimum gating of the channel. In ASIC1a, when Y71 (post-TM1) and W287 (β9-α4 loop) were mutated to cysteine, they formed disulfide bond in the closed state. Breaking of the disulfide bond by reducing agent dithiothreitol (DTT) or tris(2-carboxyethyl)phosphine (TCEP) potentiated the current significantly. Engineered cysteine G288C reacted with sulfhydryl-specific methanethiosulfonate ethyltrimethylammonium (MTSET) in the open state but not in closed/steady desensitized state, suggesting gating-associated conformational movement of this loop. We also identified a salt bridge between highly conserved R64 at TM1 and D432 at TM2 that is important for optimum gating. Based on our results and other published work, we propose that proton binding in ECD is followed by the displacement of the β9-α4 loop of the thumb, leading to the rotation of TM1. Conformational movement propagates to TM2 and the channel gate opens by the concomitant movement of TM2 and breaking of the salt bridge between R64 and D432.

Locomotion control of Caenorhabditis elegans through confinement

The model organism Caenorhabditis elegans shows two distinct locomotion patterns in laboratory situations: it swims in low viscosity liquids and it crawls on the surface of an agar gel. This provides a unique opportunity to discern the respective roles of mechanosensation (perception and proprioception) and mechanics in the regulation of locomotion and in the gait selection. Using an original device, we present what to our knowledge are new experiments where the confinement of a worm between a glass plate and a soft agar gel is controlled while recording the worm's motion. We observed that the worm continuously varied its locomotion characteristics from free swimming to slow crawling with increasing confinement so that it was not possible to discriminate between two distinct intrinsic gaits. This unicity of the gait is also proved by the fact that wild-type worms immediately adapted their motion when the imposed confinement was changed with time. We then studied locomotory deficient mutants that also exhibited one single gait and showed that the light touch response was needed for the undulation propagation and that the ciliated sensory neurons participated in the joint selection of motion period and undulation-wave velocity. Our results reveal that the control of maximum curvature, at a sensory or mechanical level, is a key ingredient of the locomotion regulation.

Gait Modulation in C. elegans: An Integrated Neuromechanical Model

Equipped with its 302-cell nervous system, the nematode Caenorhabditis elegans adapts its locomotion in different environments, exhibiting so-called swimming in liquids and crawling on dense gels. Recent experiments have demonstrated that the worm displays the full range of intermediate behaviors when placed in intermediate environments. The continuous nature of this transition strongly suggests that these behaviors all stem from modulation of a single underlying mechanism. We present a model of C. elegans forward locomotion that includes a neuromuscular control system that relies on a sensory feedback mechanism to generate undulations and is integrated with a physical model of the body and environment. We find that the model reproduces the entire swim-crawl transition, as well as locomotion in complex and heterogeneous environments. This is achieved with no modulatory mechanism, except via the proprioceptive response to the physical environment. Manipulations of the model are used to dissect the proposed pattern generation mechanism and its modulation. The model suggests a possible role for GABAergic D-class neurons in forward locomotion and makes a number of experimental predictions, in particular with respect to non-linearities in the model and to symmetry breaking between the neuromuscular systems on the ventral and dorsal sides of the body.

The ankyrin repeat domain of the TRPA protein painless is important for thermal nociception but not mechanical nociception

The Drosophila TRPA channel Painless is required for the function of polymodal nociceptors which detect noxious heat and noxious mechanical stimuli. These functions of Painless are reminiscent of mammalian TRPA channels that have also been implicated in thermal and mechanical nociception. A popular hypothesis to explain the mechanosensory functions of certain TRP channels proposes that a string of ankyrin repeats at the amino termini of these channels acts as an intracellular spring that senses force. Here, we describe the identification of two previously unknown Painless protein isoforms which have fewer ankyrin repeats than the canonical Painless protein. We show that one of these Painless isoforms, that essentially lacks ankyrin repeats, is sufficient to rescue mechanical nociception phenotypes of painless mutant animals but does not rescue thermal nociception phenotypes. In contrast, canonical Painless, which contains Ankyrin repeats, is sufficient to largely rescue thermal nociception but is not capable of rescuing mechanical nociception. Thus, we propose that in the case of Painless, ankryin repeats are important for thermal nociception but not for mechanical nociception.

Effect of pulse direct current signals on electrotactic movement of nematodes Caenorhabditis elegans and Caenorhabditis briggsae

The nematodes (worms) Caenorhabditiselegans and Caenorhabditisbriggsae are well-known model organisms to study the basis of animal development and behaviour. Their sinusoidal pattern of movement is highly stereotypic and serves as a tool to monitor defects in neurons and muscles that control movement. Until recently, a simple yet robust method to initiate movement response on-demand did not exist. We have found that the electrical stimulation in a microfluidic channel, using constant DC electric field, induces movement (termed electrotaxis) that is instantaneous, precise, sensitive, and fully penetrant. We have further characterized this behaviour and, in this paper, demonstrate that electrotaxis can also be induced using a pulse DC electric signal. Worms responded to pulse DC signals with as low as 30% duty cycle by moving towards the negative electrode at the same speed as constant DC fields (average speed of C. elegans = 296 ± 43 μm/s and C. briggsae = 356 ± 20 μm/s, for both constant and pulse DC electric fields with various frequencies). C. briggsae was found to be more sensitive to electric signals compared to C. elegans. We also investigated the turning response of worms to a change in the direction of constant and pulse DC signals. The response for constant DC signal was found to be instantaneous and similar for most worms. However, in the case of pulse DC signal, alterations in duty cycle affected the turning response time as well as the number of responding worms. Our findings show that pulse DC method allows quantitative measurement of response behaviour of worms and suggest that it could be used as a tool to study the neuronal basis of such a behaviour that is not observed under constant DC conditions.

Caenorhabditis elegans behavioral genetics: where are the knobs?

Thousands of behavioral mutants of Caenorhabditis elegans have been studied. I suggest a set of criteria by which some genes important in the evolution of behavior might be recognized, and identify neuropeptide signaling pathways as candidates.

C. elegans ten-1 is synthetic lethal with mutations in cytoskeleton regulators, and enhances many axon guidance defective mutants

BACKGROUND

Teneurins are transmembrane proteins that assist morphogenetic processes in many organisms. ten-1 is the C. elegans teneurin homolog with two transcripts, ten-1a and ten-1b, that respectively encode a long (TEN-1L) and short (TEN-1S) form of the protein. We previously isolated a C. elegans mutant where one pharyngeal neuron was frequently misplaced, and now show that it corresponds to a novel allele of ten-1.

RESULTS

The novel ten-1(et5) allele is a hypomorph since its post-embryonic phenotype is weaker than the null alleles ten-1(ok641) and ten-1(tm651). ten-1 mutants have defects in all pharyngeal neurons that we examined, and in vivo reporters show that only the long form of the ten-1 gene is expressed in the pharynx, specifically in six marginal cells and the M2 neurons. Defects in the pharyngeal M2 neurons were enhanced when the ten-1(ok641) mutation was combined with mutations in the following genes: mig-14, unc-5, unc-51, unc-52 and unc-129. None of the body neurons examined show any defects in the ten-1(ok641) mutant, but genetic interaction studies reveal that ten-1(ok641) is synthetic lethal with sax-3, unc-34 and unc-73, and examination of the hypodermal cells in embryos of the ten-1(ok641) mutant point to a role of ten-1 during hypodermal cell morphogenesis.

CONCLUSIONS

Our results are consistent with ten-1 normally providing a function complementary to the cytoskeletal remodeling processes that occur in migrating cells or cells undergoing morphogenesis. It is possible that ten-1 influences the composition/distribution of extracellular matrix.

Evaluation of the role of nitric oxide in acid sensing ion channel mediated cell death

Acid sensing ion channels (ASICs) are widely expressed in central and peripheral nervous system. They are involved in a variety of physiological and pathophysiological processes: synaptic transmission, learning and memory, pain perception, ischemia, etc. During ischemia, metabolic acidosis causes the drop of extracellular pH (pHe) which in turn activates ASICs. Activation of calcium permeable ASIC1a has been implicated in neuronal death. ASICs are modulated by several redox reagents, divalent cations and nitric oxide (NO). Although NO potentiates ASIC mediated currents, the physiological significance of such modulation has not been studied in detail. We have evaluated the role of endogenous NO in cell death at different pH, mediated by the activation of ASICs. At pH 6.1, death rates of ASIC1 expressing Neuro2A (N2A) cells are significantly higher in comparison to the cells that do not express ASICs. Amiloride, a blocker of ASICs protects the cell from acid-injury. Sodium nitroprusside, a potent NO donor not only increases the ASIC mediated currents but also increases cell death at low pH. L-Arg, the precursor of NO also potentiates ASICs in a pH dependent manner. L-Arg-induced NO production and potentiation of ASICs were observed at pHs 7.4, 7.2, 7.0 and 6.8. Lowering the pH below 6.8 did not result in significant production of NO or potentiation of ASICs upon L-Arg stimulation. Our results suggest that potentiation of ASICs by NO and subsequent cell death in vivo depends on the severity of acidosis. During mild and moderate acidosis, NO promotes cell death by potentiating ASICs, whereas this potentiation subsides in severe acidosis due to inhibition of NO synthase.

Movement encoding by a stretch receptor in the soft-bodied caterpillar, Manduca sexta

In a wide variety of animals, stretch receptors provide proprioceptive feedback for motion control. However, for animals that lack a stiff skeleton, it is unclear what information is being detected and how this is incorporated into behavior. Because such animals can change their body shape from moment-to-moment, information about body configuration could be particularly important for coordination. This study uses larval stage Lepidoptera (Manduca sexta) to examine how the longitudinal stretch receptor organ (SRO) responds to behaviorally appropriate movements. We characterized the responses of the SRO to changes in strain using magnitudes and velocities matching those seen physiologically. We found that the SRO response characteristics are compatible with the regulation of stance and with the defensive response to noxious stimuli. However, we also found that movements during crawling produce SRO responses that are dominated by the interdependence of phasic, tonic and slowly adaptive components. Ablation of stretch receptors in the proleg-bearing, fourth abdominal segment did not have any observable effect on behaviors, which suggests that the SROs are not essential for coordinating overt movements. We discuss the implications of these findings in the context of specific behaviors, and explore how the SRO response might be utilized during animal behavior.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

Structure of acid-sensing ion channel 1 at 1.9 Å resolution and low pH

Acid-sensing ion channels (ASICs) are voltage-independent, proton-activated receptors that belong to the epithelial sodium channel/degenerin family of ion channels and are implicated in perception of pain, ischaemic stroke, mechanosensation, learning and memory. Here we report the low-pH crystal structure of a chicken ASIC1 deletion mutant at 1.9 A resolution. Each subunit of the chalice-shaped homotrimer is composed of short amino and carboxy termini, two transmembrane helices, a bound chloride ion and a disulphide-rich, multidomain extracellular region enriched in acidic residues and carboxyl-carboxylate pairs within 3 A, suggesting that at least one carboxyl group bears a proton. Electrophysiological studies on aspartate-to-asparagine mutants confirm that these carboxyl-carboxylate pairs participate in proton sensing. Between the acidic residues and the transmembrane pore lies a disulphide-rich 'thumb' domain poised to couple the binding of protons to the opening of the ion channel, thus demonstrating that proton activation involves long-range conformational changes.

Gravity force transduced by the MEC-4/MEC-10 DEG/ENaC channel modulates DAF-16/FoxO activity in Caenorhabditis elegans

The gravity response is an array of behavioral and physiological plasticity elicited by changes in ambient mechanical force and is an evolutionarily ancient adaptive mechanism. We show in Caenorhabditis elegans that the force of hypergravity is translated into biological signaling via a genetic pathway involving three factors: the degenerin/epithelial Na(+) channel (DEG/ENaC) class of mechanosensory channels of touch receptor neurons, the neurotransmitter serotonin, and the FoxO transcription factor DAF-16 known to regulate development, energy metabolism, stress responses, and aging. After worms were exposed to hypergravity for 3 hr, their muscular and neuronal functions were preserved, but they exhibited DAF-16::GFP nuclear accumulation in cells throughout the body and accumulated excess fat. Mutations in MEC-4/MEC-10 DEG/ENaC or its partners MEC-6, MEC-7, and MEC-9 blocked DAF-16::GFP nuclear accumulation induced by hypergravity but did not affect DAF-16 response to other stresses. We show that exogenous serotonin and the antidepressant fluoxetine can attenuate DAF-16::GFP nuclear accumulation in WT animals exposed to hypergravity. These results reveal a novel physiological role of the mechanosensory channel, showing that the perception of mechanical stress controls FoxO signaling pathways and that inactivation of DEG/ENaC may decouple mechanical loading and physiological responses.

Touch sensitivity in Caenorhabditis elegans

The nematode Caenorhabditis elegans was the first organism for which touch insensitive mutants were obtained. The study of the genes defective in these mutants has led to the identification of components of a mechanosensory complex needed for specific cells to sense gentle touch to the body. Multiple approaches using genetics, cell biology, biochemistry, and electrophysiology have characterized a channel complex, containing two DEG/ENaC pore-forming subunits and several other proteins, that transduces the touch response. Other mechanical responses, sensed by other cells using a variety of other components, are less well understood in C. elegans. Many of these other senses may use TRP channels, although DEG/ENaC channels have also been implicated.

Hair cell mechanotransduction: the dynamic interplay between structure and function

Hair cells are capable of detecting mechanical vibrations of molecular dimensions at frequencies in the 10s to 100s of kHz. This remarkable feat is accomplished by the interplay of mechanically gated ion channels located near the top of a complex and dynamic sensory hair bundle. The hair bundle is composed of a series of actin-filled stereocilia that has both active and passive mechanical components as well as a highly active turnover process, whereby the components of the hair bundle are rapidly and continually recycled. Hair bundle mechanical properties have significant impact on the gating of the mechanically activated channels, and delineating between attributes intrinsic to the ion channel and those imposed by the channel's microenvironment is often difficult. This chapter describes what is known and accepted regarding hair-cell mechanotransduction and what remains to be explored, particularly, in relation to the interplay between hair bundle properties and mechanotransducer channel response. The interplay between hair bundle dynamics and mechanotransduction are discussed.

Genetic control of sex differences in C. elegans neurobiology and behavior

As a well-characterized, genetically tractable animal, the nematode Caenorhabditis elegans is an ideal model to explore the connections between genes and the sexual regulation of the nervous system and behavior. The two sexes of C. elegans, males and hermaphrodites, have precisely defined differences in neuroanatomy: superimposed onto a "core" nervous system of exactly 294 neurons, hermaphrodites and males have 8 and 89 sex-specific neurons, respectively. These sex-specific neurons are essential for cognate sex-specific behaviors, including hermaphrodite egg-laying and male mating. In addition, regulated sex differences in the core nervous system itself may provide additional, though poorly understood, controls on behavior. These differences in the nervous system and behavior, like all known sex differences in the C. elegans soma, are controlled by the master regulator of C. elegans sex determination, tra-1. Downstream of tra-1 lie specific effectors of sex determination, including genes controlling sex-specific cell death and a family of regulators, the DM-domain genes, related to Drosophila doublesex and the vertebrate DMRT genes. There is no central (i.e., gonadal) regulator of sexual phenotype in the C. elegans nervous system; instead, tra-1 acts cell-autonomously in nearly all sexually dimorphic somatic cells. However, recent results suggest that the status of the gonad can be communicated to the nervous system to modulate sex-specific behaviors. Continuing research into the genetic control of neural sex differences in C. elegans is likely to yield insight into conserved mechanisms of cell-autonomous cross talk between cell fate patterning and sexual differentiation pathways.