Mechanosensory molecules and circuits in C. elegans

Anoctamin-1 is a core component of a mechanosensory anion channel complex in C. elegans

Mechanotransduction channels are widely expressed in both vertebrates and invertebrates, mediating various physiological processes such as touch, hearing and blood-pressure sensing. While previously known mechanotransduction channels in metazoans are primarily cation-selective, we identified Anoctamin-1 (ANOH-1), the C. elegans homolog of mammalian calcium-activated chloride channel ANO1/TMEM16A, as an essential component of a mechanosensory channel complex that contributes to the nose touch mechanosensation in C. elegans. Ectopic expression of either C. elegans or human Anoctamin-1 confers mechanosensitivity to touch-insensitive neurons, suggesting a cell-autonomous role of ANOH-1/ANO1 in mechanotransduction. Additionally, we demonstrated that the mechanosensory function of ANOH-1/ANO1 relies on CIB (calcium- and integrin- binding) proteins. Thus, our results reveal an evolutionarily conserved chloride channel involved in mechanosensory transduction in metazoans, highlighting the importance of anion channels in mechanosensory processes.

Leachates from plastics and bioplastics reduce lifespan, decrease locomotion, and induce neurotoxicity in Caenorhabditis elegans

Plastic pollution continuously accumulates in the environment and poses a global threat as it fragments into microplastics and nanoplastics that can harm ecosystems. To reduce the accumulation of microplastic and nanoplastic pollution, bioplastics made from biodegradable materials are promoted as a more sustainable alternative because it can degrade faster than plastics. However, plastics also leach out chemicals as they degrade and disintegrate, but the potential toxicity of these chemicals leaching out from plastics and especially bioplastics is poorly explored. Here, we determined the composition of leachates from plastics and bioplastics and tested their toxicity in Caenorhabditis elegans. LC-MS analysis of the leachates revealed that bioplastics leached a wider array of chemicals than their counterpart plastics. Toxicity testing in our study showed that the leachates from plastics and bioplastics reduced lifespan, decreased locomotion, and induced neurotoxicity in C. elegans. Leachates from bioplastics reduced C. elegans lifespan more compared to leachates from plastics: by 7%-31% for bioplastics and by 6%-15% for plastics. Leachates from plastics decreased locomotion in C. elegans more compared to leachates from bioplastics: by 8%-34% for plastics and by 11%-24% for bioplastics. No changes were observed in the ability of the C. elegans to respond to mechanical stimuli. The leachates induced neurotoxicity in the following neurons at varying trends: cholinergic neurons by 0%-53% for plastics and by 30%-42% for bioplastics, GABAergic neurons by 3%-29% for plastics and by 10%-23% for bioplastics, and glutamatergic neurons by 3%-11% for plastics and by 15%-29% for bioplastics. Overall, our study demonstrated that chemicals leaching out from plastics and bioplastics can be toxic, suggesting that both plastics and bioplastics pose ecotoxicological and human health risks.

Behavioral plasticity

Behavioral plasticity allows animals to modulate their behavior based on experience and environmental conditions. Caenorhabditis elegans exhibits experience-dependent changes in its behavioral responses to various modalities of sensory cues, including odorants, salts, temperature, and mechanical stimulations. Most of these forms of behavioral plasticity, such as adaptation, habituation, associative learning, and imprinting, are shared with other animals. The C. elegans nervous system is considerably tractable for experimental studies-its function can be characterized and manipulated with molecular genetic methods, its activity can be visualized and analyzed with imaging approaches, and the connectivity of its relatively small number of neurons are well described. Therefore, C. elegans provides an opportunity to study molecular, neuronal, and circuit mechanisms underlying behavioral plasticity that are either conserved in other animals or unique to this species. These findings reveal insights into how the nervous system interacts with the environmental cues to generate behavioral changes with adaptive values.

Behavioral adjustment of C. elegans to mechanosensory loss requires intact mechanosensory neurons

Sensory neurons specialize in detecting and signaling the presence of diverse environmental stimuli. Neuronal injury or disease may undermine such signaling, diminishing the availability of crucial information. Can animals distinguish between a stimulus not being present and the inability to sense that stimulus in the first place? To address this question, we studied Caenorhabditis elegans nematode worms that lack gentle body touch sensation due to genetic mechanoreceptor dysfunction. We previously showed that worms can compensate for the loss of touch by enhancing their sense of smell, via an FLP-20 neuropeptide pathway. Here, we find that touch-deficient worms exhibit, in addition to sensory compensation, also cautious-like behavior, as if preemptively avoiding potential undetectable hazards. Intriguingly, these behavioral adjustments are abolished when the touch neurons are removed, suggesting that touch neurons are required for signaling the unavailability of touch information, in addition to their conventional role of signaling touch stimulation. Furthermore, we found that the ASE taste neurons, which similarly to the touch neurons, express the FLP-20 neuropeptide, exhibit altered FLP-20 expression levels in a touch-dependent manner, thus cooperating with the touch circuit. These results imply a novel form of neuronal signaling that enables C. elegans to distinguish between lack of touch stimulation and loss of touch sensation, producing adaptive behavioral adjustments that could overcome the inability to detect potential threats.

Can repetitive mechanical motion cause structural damage to axons?

Biological structures have evolved to very efficiently generate, transmit, and withstand mechanical forces. These biological examples have inspired mechanical engineers for centuries and led to the development of critical insights and concepts. However, progress in mechanical engineering also raises new questions about biological structures. The past decades have seen the increasing study of failure of engineered structures due to repetitive loading, and its origin in processes such as materials fatigue. Repetitive loading is also experienced by some neurons, for example in the peripheral nervous system. This perspective, after briefly introducing the engineering concept of mechanical fatigue, aims to discuss the potential effects based on our knowledge of cellular responses to mechanical stresses. A particular focus of our discussion are the effects of mechanical stress on axons and their cytoskeletal structures. Furthermore, we highlight the difficulty of imaging these structures and the promise of new microscopy techniques. The identification of repair mechanisms and paradigms underlying long-term stability is an exciting and emerging topic in biology as well as a potential source of inspiration for engineers.

Innate Orientating Behavior of a Multi-Legged Robot Driven by the Neural Circuits of C. elegans

The objective of this research is to achieve biologically autonomous control by utilizing a whole-brain network model, drawing inspiration from biological neural networks to enhance the development of bionic intelligence. Here, we constructed a whole-brain neural network model of Caenorhabditis elegans (C. elegans), which characterizes the electrochemical processes at the level of the cellular synapses. The neural network simulation integrates computational programming and the visualization of the neurons and synapse connections of C. elegans, containing the specific controllable circuits and their dynamic characteristics. To illustrate the biological neural network (BNN)'s particular intelligent control capability, we introduced an innovative methodology for applying the BNN model to a 12-legged robot's movement control. Two methods were designed, one involving orientation control and the other involving locomotion generation, to demonstrate the intelligent control performance of the BNN. Both the simulation and experimental results indicate that the robot exhibits more autonomy and a more intelligent movement performance under BNN control. The systematic approach of employing the whole-brain BNN for robot control provides biomimetic research with a framework that has been substantiated by innovative methodologies and validated through the observed positive outcomes. This method is established as follows: (1) two integrated dynamic models of the C. elegans' whole-brain network and the robot moving dynamics are built, and all of the controllable circuits are discovered and verified; (2) real-time communication is achieved between the BNN model and the robot's dynamical model, both in the simulation and the experiments, including applicable encoding and decoding algorithms, facilitating their collaborative operation; (3) the designed mechanisms using the BNN model to control the robot are shown to be effective through numerical and experimental tests, focusing on 'foraging' behavior control and locomotion control.

A conserved requirement for RME-8/DNAJC13 in neuronal autophagic lysosome reformation

Autophagosomes fuse with lysosomes, forming autolysosomes that degrade engulfed cargo. To maintain lysosomal capacity, autophagic lysosome reformation (ALR) must regenerate lysosomes from autolysosomes using a membrane tubule-based process. Maintaining lysosomal capacity is required to maintain cellular health, especially in neurons where lysosomal dysfunction has been repeatedly implicated in neurodegenerative disease. The DNA-J domain HSC70 co-chaperone RME-8/DNAJC13 has been linked to endosomal coat protein regulation and to neurological disease. We report new analysis of the requirements for the RME-8/DNAJC13 protein in neurons, focusing on intact C. elegans mechanosensory neurons, and primary mouse cortical neurons in culture. Loss of RME-8/DNAJC13 in both systems results in accumulation of grossly elongated autolysosomal tubules. Further C. elegans analysis revealed a similar autolysosome tubule accumulation defect in mutants known to be required for ALR in mammals, including mutants lacking bec-1/BECN1/Beclin1 and vps-15/PIK3R4/p150 that regulate the class III phosphatidylinositol 3-kinase (PtdIns3K) VPS-34, and dyn-1/dynamin that severs ALR tubules. Clathrin is also an important ALR regulator implicated in autolysosome tubule formation and release. In C. elegans we found that loss of RME-8 causes severe depletion of clathrin from neuronal autolysosomes, a phenotype shared with bec-1 and vps-15 mutants. We conclude that RME-8/DNAJC13 plays a previously unrecognized role in ALR, likely affecting autolysosome tubule severing. Additionally, in both systems, loss of RME-8/DNAJC13 reduced macroautophagic/autophagic flux, suggesting feedback regulation from ALR to autophagy. Our results connecting RME-8/DNAJC13 to ALR and autophagy provide a potential mechanism by which RME-8/DNAJC13 could influence neuronal health and the progression of neurodegenerative disease.Abbreviation: ALR, autophagic lysosome reformation; ATG-13/EPG-1, AuTophaGy (yeast Atg homolog)-13; ATG-18, AuTophaGy (yeast Atg homolog)-18; AV, autophagic vacuole; CLIC-1, Clathrin Light Chain-1; EPG-3, Ectopic P Granules-3; EPG-6, Ectopic P Granules-6; LGG-1, LC3, GABARAP and GATE-16 family-1; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; PD, Parkinson disease; PtdIns3P, phosphatidylinositol-3-phosphate; PtdIns(4,5)P2, phosphatidylinositol-4,5-bisphosphate; RME-8, Receptor Mediated Endocytosis-8; SNX-1, Sorting NeXin-1; VPS-34, related to yeast Vacuolar Protein Sorting factor-34.

Biophysical modeling of the whole-cell dynamics of C. elegans motor and interneurons families

The nematode Caenorhabditis elegans is a widely used model organism for neuroscience. Although its nervous system has been fully reconstructed, the physiological bases of single-neuron functioning are still poorly explored. Recently, many efforts have been dedicated to measuring signals from C. elegans neurons, revealing a rich repertoire of dynamics, including bistable responses, graded responses, and action potentials. Still, biophysical models able to reproduce such a broad range of electrical responses lack. Realistic electrophysiological descriptions started to be developed only recently, merging gene expression data with electrophysiological recordings, but with a large variety of cells yet to be modeled. In this work, we contribute to filling this gap by providing biophysically accurate models of six classes of C. elegans neurons, the AIY, RIM, and AVA interneurons, and the VA, VB, and VD motor neurons. We test our models by comparing computational and experimental time series and simulate knockout neurons, to identify the biophysical mechanisms at the basis of inter and motor neuron functioning. Our models represent a step forward toward the modeling of C. elegans neuronal networks and virtual experiments on the nematode nervous system.

Activity-dependent mitochondrial ROS signaling regulates recruitment of glutamate receptors to synapses

Our understanding of mitochondrial signaling in the nervous system has been limited by the technical challenge of analyzing mitochondrial function in vivo. In the transparent genetic model Caenorhabditis elegans, we were able to manipulate and measure mitochondrial reactive oxygen species (mitoROS) signaling of individual mitochondria as well as neuronal activity of single neurons in vivo. Using this approach, we provide evidence supporting a novel role for mitoROS signaling in dendrites of excitatory glutamatergic C. elegans interneurons. Specifically, we show that following neuronal activity, dendritic mitochondria take up calcium (Ca2+) via the mitochondrial Ca2+ uniporter (MCU-1) that results in an upregulation of mitoROS production. We also observed that mitochondria are positioned in close proximity to synaptic clusters of GLR-1, the C. elegans ortholog of the AMPA subtype of glutamate receptors that mediate neuronal excitation. We show that synaptic recruitment of GLR-1 is upregulated when MCU-1 function is pharmacologically or genetically impaired but is downregulated by mitoROS signaling. Thus, signaling from postsynaptic mitochondria may regulate excitatory synapse function to maintain neuronal homeostasis by preventing excitotoxicity and energy depletion.

Glia in Invertebrate Models: Insights from Caenorhabditis elegans

Glial cells modulate brain development, function, and health across all bilaterian animals, and studies in the past two decades have made rapid strides to uncover the underlying molecular mechanisms of glial functions. The nervous system of the invertebrate genetic model Caenorhabditis elegans (C. elegans) has small cell numbers with invariant lineages, mapped connectome, easy genetic manipulation, and a short lifespan, and the animal is also optically transparent. These characteristics are revealing C. elegans to be a powerful experimental platform for studying glial biology. This chapter discusses studies in C. elegans that add to our understanding of how glia modulate adult neural functions, and thereby animal behaviors, as well as emerging evidence of their roles as autonomous sensory cells. The rapid molecular and cellular advancements in understanding C. elegans glia in recent years underscore the utility of this model in studies of glial biology. We conclude with a perspective on future research avenues for C. elegans glia that may readily contribute molecular mechanistic insights into glial functions in the nervous system.

The neuropeptidergic connectome of C. elegans

Efforts are ongoing to map synaptic wiring diagrams, or connectomes, to understand the neural basis of brain function. However, chemical synapses represent only one type of functionally important neuronal connection; in particular, extrasynaptic, "wireless" signaling by neuropeptides is widespread and plays essential roles in all nervous systems. By integrating single-cell anatomical and gene-expression datasets with biochemical analysis of receptor-ligand interactions, we have generated a draft connectome of neuropeptide signaling in the C. elegans nervous system. This network is characterized by high connection density, extended signaling cascades, autocrine foci, and a decentralized topology, with a large, highly interconnected core containing three constituent communities sharing similar patterns of input connectivity. Intriguingly, several key network hubs are little-studied neurons that appear specialized for peptidergic neuromodulation. We anticipate that the C. elegans neuropeptidergic connectome will serve as a prototype to understand how networks of neuromodulatory signaling are organized.

A PDMS-Agar Hybrid Microfluidic Device for the Investigation of Chemical-Mechanical Associative Learning Behavior of C. elegans

Associative learning is a critical survival trait that promotes behavioral plasticity in response to changing environments. Chemosensation and mechanosensation are important sensory modalities that enable animals to gather information about their internal state and external environment. However, there is a limited amount of research on these two modalities. In this paper, a novel PDMS-agar hybrid microfluidic device is proposed for training and analyzing chemical-mechanical associative learning behavior in the nematode Caenorhabditis elegans. The microfluidic device consisted of a bottom agar gel layer and an upper PDMS layer. A chemical concentration gradient was generated on the agar gel layer, and the PDMS layer served to mimic mechanical stimuli. Based on this platform, C. elegans can perform chemical-mechanical associative learning behavior after training. Our findings indicated that the aversive component of training is the primary driver of the observed associative learning behavior. In addition, the results indicated that the neurotransmitter octopamine is involved in regulating this associative learning behavior via the SER-6 receptor. Thus, the microfluidic device provides a highly efficient platform for studying the associative learning behavior of C. elegans, and it may be applied in mutant screening and drug testing.

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

A Conserved Requirement for RME-8/DNAJC13 in Neuronal Autolysosome Reformation

Autophagosomes fuse with lysosomes, forming autolysosomes that degrade engulfed cargo. To maintain lysosomal capacity, autolysosome reformation (ALR) must regenerate lysosomes from autolysosomes using a membrane tubule-based process. Maintaining lysosomal capacity is required to maintain proteostasis and cellular health, especially in neurons where lysosomal dysfunction has been repeatedly implicated in neurodegenerative disease. Cell biological studies have linked the DNA-J domain Hsc70 co-chaperone RME-8/DNAJC13 to endosomal coat protein regulation, while human genetics studies have linked RME-8/DNAJC13 to neurological disease, including Parkinsonism and Essential Tremor. We report new analysis of the requirements for the RME-8/DNAJC13 protein in neurons, focusing on C. elegans mechanosensory neurons in the intact animal, and in primary mouse cortical neurons in culture. We find that loss of RME-8/DNAJC13 in both systems results in accumulation of grossly elongated autolysosomal tubules. Further C. elegans analysis revealed a similar autolysosome tubule accumulation defect in mutants known to be required for ALR in mammals, including bec-1/beclin and vps-15/PIK3R4/p150 that regulate type-III PI3-kinase VPS-34, and dyn-1/dynamin that severs ALR tubules. Clathrin is also an important ALR regulator implicated in autolysosome tubule formation and release. In C. elegans we found that loss of RME-8 causes severe depletion of clathrin from neuronal autolysosomes, a phenotype shared with bec-1 and vps-15 mutants. We conclude that RME-8/DNAJC13 plays a conserved but previously unrecognized role in autolysosome reformation, likely affecting ALR tubule initiation and/or severing. Additionally, in both systems, we found that loss of RME-8/DNAJC13 appeared to reduce autophagic flux, suggesting feedback regulation from ALR to autophagy. Our results connecting RME-8/DNAJC13 to ALR and autophagy provide a potential mechanism by which RME-8/DNAJC13 could influence neuronal health and the progression of neurodegenerative disease.

Ultrasound-driven exercise training ameliorates degeneration of ultrasonic responses in Caenorhabditis elegans

The inevitability of age-related degeneration makes research on degradation mitigation attractive to humans, while exercise is considered an effective means due to its powerful impact on life and health. Caenorhabditis elegans is a model animal with a short life cycle and is widely used in health and aging studies. In this work, ultrasonic stimuli in the form of surface acoustic waves (SAWs) were used to induce behavioral activities in worms. As the worms grew, ultrasound-elicited behavioral responses started to decrease in the early adulthood stage. However, this situation was significantly ameliorated when ultrasonic training sessions at an effective acoustic pressure of 1.1 MPa were performed four times per day for 5 or 7 days, while ultrasonic responses in trained nematodes were stronger than those in untrained ones. These results suggest that long-term ultrasonic training might positively intervene in aging-related degeneration. Besides, it was found that exercise driven by long-term ultrasonic training had insignificant effects on the lifespan of worms. A preliminary exploration of the neural mechanisms underlying the sensation of SAWs was also conducted. The results show that, apart from touch receptor neurons (TRNs), polymodal nociceptors FLP and PVD neurons may also be involved in the perception of ultrasound in C. elegans. The results of this study may inspire related studies on other animals or humans.

PEZO-1 and TRP-4 mechanosensors are involved in mating behavior in Caenorhabditis elegans

Male mating in Caenorhabditis elegans is a complex behavior with a strong mechanosensory component. C. elegans has several characterized mechanotransducer proteins, but few have been shown to contribute to mating. Here, we investigated the roles of PEZO-1, a piezo channel, and TRP-4, a mechanotransducing TRPN channel, in male mating behavior. We show that pezo-1 is expressed in several male-specific neurons with known roles in mating. We show that, among other neurons, trp-4 is expressed in the Post-Cloacal sensilla neuron type A (PCA) sensory neuron, which monitors relative sliding between the male and the hermaphrodite and inhibits neurons involved in vulva detection. Mutations in both genes compromise many steps of mating, including initial response to the hermaphrodite, scanning, turning, and vulva detection. We performed pan-neuronal imaging during mating between freely moving mutant males and hermaphrodites. Both pezo-1 and trp-4 mutants showed spurious activation of the sensory neurons involved in vulva detection. In trp-4 mutants, this spurious activation might be caused by PCA failure to inhibit vulva-detecting neurons during scanning. Indeed, we show that without functional TRP-4, PCA fails to detect the relative sliding between the male and hermaphrodite. Cell-specific TRP-4 expression restores PCA's mechanosensory function. Our results demonstrate new roles for both PEZO-1 and TRP-4 mechanotransducers in C. elegans mating behavior.

The diverse functions of the DEG/ENaC family: linking genetic and physiological insights

The DEG/ENaC family of ion channels was defined based on the sequence similarity between degenerins (DEG) from the nematode Caenorhabditis elegans and subunits of the mammalian epithelial sodium channel (ENaC), and also includes a diverse array of non-voltage-gated cation channels from across animal phyla, including the mammalian acid-sensing ion channels (ASICs) and Drosophila pickpockets. ENaCs and ASICs have wide ranging medical importance; for example, ENaCs play an important role in respiratory and renal function, and ASICs in ischaemia and inflammatory pain, as well as being implicated in memory and learning. Electrophysiological approaches, both in vitro and in vivo, have played an essential role in establishing the physiological properties of this diverse family, identifying an array of modulators and implicating them in an extensive range of cellular functions, including mechanosensation, acid sensation and synaptic modulation. Likewise, genetic studies in both invertebrates and vertebrates have played an important role in linking our understanding of channel properties to function at the cellular and whole animal/behavioural level. Drawing together genetic and physiological evidence is essential to furthering our understanding of the precise cellular roles of DEG/ENaC channels, with the diversity among family members allowing comparative physiological studies to dissect the molecular basis of these diverse functions.

Mn(II) Quinoline Complex (4QMn) Restores Proteostasis and Reduces Toxicity in Experimental Models of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, of the so-called minority diseases, due to its low prevalence. It is caused by an abnormally long track of glutamines (polyQs) in mutant huntingtin (mHtt), which makes the protein toxic and prone to aggregation. Many pathways of clearance of badly-folded proteins are disrupted in neurons of patients with HD. In this work, we show that one Mn(II) quinone complex (4QMn), designed to work as an artificial superoxide dismutase, is able to activate both the ubiquitin-proteasome system and the autophagy pathway in vitro and in vivo models of HD. Activation of these pathways degrades mHtt and other protein-containing polyQs, which restores proteostasis in these models. Hence, we propose 4QMn as a potential drug to develop a therapy to treat HD.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

A glial ClC Cl- channel mediates nose touch responses in C. elegans

In touch receptors, glia and accessory cells play a key role in mechanosensation. However, the mechanisms underlying such regulation are poorly understood. We show, for the first time, that the chloride channel CLH-1 is needed in glia of C. elegans nose touch receptors for touch responses and for regulation of excitability. Using in vivo Ca²⁺ and Cl^- imaging, behavioral assays, and combined genetic and pharmacological manipulations, we show that CLH-1 mediates Cl^- flux needed for glial GABA inhibition of ASH sensory neuron function and for regulation of cyclic AMP levels in ASH neurons. Finally, we show that the rat ClC-2 channel rescues the clh-1 nose-touch-insensitive phenotype, underscoring conservation of function across species. Our work identifies a glial Cl^- channel as a novel regulator of touch sensitivity. We propose that glial CLH-1 regulates the interplay between Ca²⁺ and cAMP signaling in ASH neurons to control the sensitivity of the worm's nose touch receptors.

Methods to Quantify and Relate Axonal Transport Defects to Changes in C. elegans Behavior

Neuronal growth, differentiation, homeostasis, viability, and injury response heavily rely on functional axonal transport (AT). Erroneous and disturbed AT may lead to accumulation of "disease proteins" such as tau, α-synuclein, or amyloid precursor protein causing various neurological disorders. Changes in AT often lead to observable behavioral consequences in C. elegans such as impeded movements, defects in touch response, chemosensation, and even egg laying. Long C. elegans neurons with clear distinguishable axons and dendrites provide an excellent platform to analyze AT. The possibility to relate changes in AT to neuronal defects that in turn lead to quantifiable changes in worm behavior allows for the advancement of neuropathological disease models. Even more, subsequent suppressor screens may aid in identifying genes responsible for observed behavioral changes providing a target for drug development to eventually delay or cure neurological diseases. Thus, in this chapter, we summarize critical methods to identify and quantify defects in axonal transport as well as exemplified behavioral assays that may relate to these defects.

Signaling via the FLP-14/FRPR-19 neuropeptide pathway sustains nociceptive response to repeated noxious stimuli in C. elegans

In order to thrive in constantly changing environments, animals must adaptively respond to threatening events. Noxious stimuli are not only processed according to their absolute intensity, but also to their context. Adaptation processes can cause animals to habituate at different rates and degrees in response to permanent or repeated stimuli. Here, we used a forward genetic approach in Caenorhabditis elegans to identify a neuropeptidergic pathway, essential to prevent fast habituation and maintain robust withdrawal responses to repeated noxious stimuli. This pathway involves the FRPR-19A and FRPR-19B G-protein coupled receptor isoforms produced from the frpr-19 gene by alternative splicing. Loss or overexpression of each or both isoforms can impair withdrawal responses caused by the optogenetic activation of the polymodal FLP nociceptor neuron. Furthermore, we identified FLP-8 and FLP-14 as FRPR-19 ligands in vitro. flp-14, but not flp-8, was essential to promote withdrawal response and is part of the same genetic pathway as frpr-19 in vivo. Expression and cell-specific rescue analyses suggest that FRPR-19 acts both in the FLP nociceptive neurons and downstream interneurons, whereas FLP-14 acts from interneurons. Importantly, genetic impairment of the FLP-14/FRPR-19 pathway accelerated the habituation to repeated FLP-specific optogenetic activation, as well as to repeated noxious heat and harsh touch stimuli. Collectively, our data suggest that well-adjusted neuromodulation via the FLP-14/FRPR-19 pathway contributes to promote nociceptive signals in C. elegans and counteracts habituation processes that otherwise tend to rapidly reduce aversive responses to repeated noxious stimuli.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

Neurobiology: From genome and connectome to understanding behavior

Many forms of synaptic plasticity are mediated by changes in the abundance, density, and expression levels of postsynaptic ionotropic receptors. A new study identifies the endogenous ligands of five 'orphan' aminergic ligand-gated ion channels in Caenorhabditis elegans, functionally characterizes these channels, and explores the role of one of them in a simple form of learning.

Transcription Factors That Control Behavior-Lessons From C. elegans

Behavior encompasses the physical and chemical response to external and internal stimuli. Neurons, each with their own specific molecular identities, act in concert to perceive and relay these stimuli to drive behavior. Generating behavioral responses requires neurons that have the correct morphological, synaptic, and molecular identities. Transcription factors drive the specific gene expression patterns that define these identities, controlling almost every phenomenon in a cell from development to homeostasis. Therefore, transcription factors play an important role in generating and regulating behavior. Here, we describe the transcription factors, the pathways they regulate, and the neurons that drive chemosensation, mechanosensation, thermosensation, osmolarity sensing, complex, and sex-specific behaviors in the animal model Caenorhabditis elegans. We also discuss the current limitations in our knowledge, particularly our minimal understanding of how transcription factors contribute to the adaptive behavioral responses that are necessary for organismal survival.

An asymmetric mechanical code ciphers curvature-dependent proprioceptor activity

A repetitive gait cycle is an archetypical component within the behavioral repertoire of many animals including humans. It originates from mechanical feedback within proprioceptors to adjust the motor program during locomotion and thus leads to a periodic orbit in a low-dimensional space. Here, we investigate the mechanics, molecules, and neurons responsible for proprioception in Caenorhabditis elegans to gain insight into how mechanosensation shapes the orbital trajectory to a well-defined limit cycle. We used genome editing, force spectroscopy, and multiscale modeling and found that alternating tension and compression with the spectrin network of a single proprioceptor encodes body posture and informs TRP-4/NOMPC and TWK-16/TREK2 homologs of mechanosensitive ion channels during locomotion. In contrast to a widely accepted model of proprioceptive “stretch” reception, we found that proprioceptors activated locally under compressive stresses in-vivo and in-vitro and propose that this property leads to compartmentalized activity within long axons delimited by curvature-dependent mechanical stresses.

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.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

Behavioral and molecular response of the insect parasitic nematode Steinernema carpocapsae to cues emitted by a host, the red palm weevil, Rhynchophorus ferrugineus

As the larvae of the date palm pest, the red palm weevil, Rhynchophorus ferrugineus, feeds on the host tissue, they emit a distinctive sound which can be recorded outside of the infected tree. We evaluated the response of infective juveniles (IJs) of the entomopathogenic nematodes Steinernema carpocapsae to the R. ferrugineus larvae and it's sound source, separately. In the presence of the insect larvae, 50.2 % of total IJs moved toward those larvae. Recorded insect larvae sound emitted by the speaker resulted in 7% of total IJs near the sound source. RNA-Seq data indicated that more genes were downregulated in S. carpocapsae IJs exposed to insect and speaker compared to non-stimulated IJs. IJs exposed to insect exhibited more up-regulated genes than IJs exposed to speaker. Enriched pathways and biological processes in IJs were similar for both stimuli. The inhibition of locomotion, regulation of neurotransmitter secretion, response to biotic stimulus, and cellular response to chemical stimuli were enriched with unique GO terms for speaker treatment. The regulation of localization, sodium ion transmembrane transport, regulation of response to stress and response to organic substances were the GO categories enriched unique to insect. The host-parasitic interaction was regulated by the differential expression of Ras/MAP kinase, TGF-beta signaling, insulin signaling, AMPK signaling, PPAR signaling pathways and many developmental pathways. More prominent R. ferrugineus host localization by S. carpocapsae was primarily due to the differential transcriptional regulation of olfactory signal transduction, FOXO-family proteins, calcium signaling, WNT and mTOR signaling pathway. The neural basis for the nematode attraction to insect host is based on the chemosensation and the mechanosensation. Many neuropeptides and neuromodulators are involved in regulating the foraging behavior of S. carpocapsae. The results of this study provide new insights into the molecular mechanisms that allow these nematodes to seek insect hosts. Our finding, especially the molecular ones suggest that chemical cues emitted by the active insect host are stimulants of nematodes attraction. Whereas the sound emitted by the insect has minor effects on the nematode behavior.

A C. elegans model for neurodegeneration in Cockayne syndrome

Cockayne syndrome (CS) is a congenital syndrome characterized by growth and mental retardation, and premature ageing. The complexity of CS and mammalian models warrants simpler metazoan models that display CS-like phenotypes that could be studied in the context of a live organism. Here, we provide a characterization of neuronal and mitochondrial aberrations caused by a mutation in the csb-1 gene in Caenorhabditis elegans. We report a progressive neurodegeneration in adult animals that is enhanced upon UV-induced DNA damage. The csb-1 mutants show dysfunctional hyperfused mitochondria that degrade upon DNA damage, resulting in diminished respiratory activity. Our data support the role of endogenous DNA damage as a driving factor of CS-related neuropathology and underline the role of mitochondrial dysfunction in the disease.

Direct glia-to-neuron transdifferentiation gives rise to a pair of male-specific neurons that ensure nimble male mating

Sexually dimorphic behaviours require underlying differences in the nervous system between males and females. The extent to which nervous systems are sexually dimorphic and the cellular and molecular mechanisms that regulate these differences are only beginning to be understood. We reveal here a novel mechanism by which male-specific neurons are generated in Caenorhabditis elegans through the direct transdifferentiation of sex-shared glial cells. This glia-to-neuron cell fate switch occurs during male sexual maturation under the cell-autonomous control of the sex-determination pathway. We show that the neurons generated are cholinergic, peptidergic, and ciliated putative proprioceptors which integrate into male-specific circuits for copulation. These neurons ensure coordinated backward movement along the mate's body during mating. One step of the mating sequence regulated by these neurons is an alternative readjustment movement performed when intromission becomes difficult to achieve. Our findings reveal programmed transdifferentiation as a developmental mechanism underlying flexibility in innate behaviour.

What can a worm learn in a bacteria-rich habitat?

With a nervous system that has only a few hundred neurons, Caenorhabditis elegans was initially not regarded as a model for studies on learning. However, the collective effort of the C. elegans field in the past several decades has shown that the worm displays plasticity in its behavioral response to a wide range of sensory cues in the environment. As a bacteria-feeding worm, C. elegans is highly adaptive to the bacteria enriched in its habitat, especially those that are pathogenic and pose a threat to survival. It uses several common forms of behavioral plasticity that last for different amounts of time, including imprinting and adult-stage associative learning, to modulate its interactions with pathogenic bacteria. Probing the molecular, cellular and circuit mechanisms underlying these forms of experience-dependent plasticity has identified signaling pathways and regulatory insights that are conserved in more complex animals.

Distinct roles for innexin gap junctions and hemichannels in mechanosensation

Mechanosensation is central to a wide range of functions, including tactile and pain perception, hearing, proprioception, and control of blood pressure, but identifying the molecules underlying mechanotransduction has proved challenging. In Caenorhabditis elegans, the avoidance response to gentle body touch is mediated by six touch receptor neurons (TRNs), and is dependent on MEC-4, a DEG/ENaC channel. We show that hemichannels containing the innexin protein UNC-7 are also essential for gentle touch in the TRNs, as well as harsh touch in both the TRNs and the PVD nociceptors. UNC-7 and MEC-4 do not colocalize, suggesting that their roles in mechanosensory transduction are independent. Heterologous expression of unc-7 in touch-insensitive chemosensory neurons confers ectopic touch sensitivity, indicating a specific role for UNC-7 hemichannels in mechanosensation. The unc-7 touch defect can be rescued by the homologous mouse gene Panx1 gene, thus, innexin/pannexin proteins may play broadly conserved roles in neuronal mechanotransduction.

How Caenorhabditis elegans Senses Mechanical Stress, Temperature, and Other Physical Stimuli

Caenorhabditis elegans lives in a complex habitat in which they routinely experience large fluctuations in temperature, and encounter physical obstacles that vary in size and composition. Their habitat is shared by other nematodes, by beneficial and harmful bacteria, and nematode-trapping fungi. Not surprisingly, these nematodes can detect and discriminate among diverse environmental cues, and exhibit sensory-evoked behaviors that are readily quantifiable in the laboratory at high resolution. Their ability to perform these behaviors depends on <100 sensory neurons, and this compact sensory nervous system together with powerful molecular genetic tools has allowed individual neuron types to be linked to specific sensory responses. Here, we describe the sensory neurons and molecules that enable C. elegans to sense and respond to physical stimuli. We focus primarily on the pathways that allow sensation of mechanical and thermal stimuli, and briefly consider this animal’s ability to sense magnetic and electrical fields, light, and relative humidity. As the study of sensory transduction is critically dependent upon the techniques for stimulus delivery, we also include a section on appropriate laboratory methods for such studies. This chapter summarizes current knowledge about the sensitivity and response dynamics of individual classes of C. elegans mechano- and thermosensory neurons from in vivo calcium imaging and whole-cell patch-clamp electrophysiology studies. We also describe the roles of conserved molecules and signaling pathways in mediating the remarkably sensitive responses of these nematodes to mechanical and thermal cues. These studies have shown that the protein partners that form mechanotransduction channels are drawn from multiple superfamilies of ion channel proteins, and that signal transduction pathways responsible for temperature sensing in C. elegans share many features with those responsible for phototransduction in vertebrates.

Brv1 Is Required for Drosophila Larvae to Sense Gentle Touch

How we sense touch is fundamental for many physiological processes. However, the underlying mechanism and molecular identity for touch sensation are largely unknown. Here, we report on defective gentle-touch behavioral responses in brv1 loss-of-function Drosophila larvae. RNAi and Ca²⁺ imaging confirmed the involvement of Brv1 in sensing touch and demonstrated that Brv1 mediates the mechanotransduction of class III dendritic arborization neurons. Electrophysiological recordings further revealed that the expression of Brv1 protein in HEK293T cells gives rise to stretch-activated cation channels. Purified Brv1 protein reconstituted into liposomes were found to sense stretch stimuli. In addition, co-expression studies suggested that Brv1 amplifies the response of mechanosensitive ion channel NOMPC (no mechanoreceptor potential C) to touch stimuli. Altogether, these findings demonstrate a molecular entity that mediates the gentle-touch response in Drosophila larvae, providing insights into the molecular mechanisms of touch sensation.

Neural circuitry of a polycystin-mediated hydrodynamic startle response for predator avoidance

Startle responses triggered by aversive stimuli including predators are widespread across animals. These coordinated whole-body actions require the rapid and simultaneous activation of a large number of muscles. Here we study a startle response in a planktonic larva to understand the whole-body circuit implementation of the behaviour. Upon encountering water vibrations, larvae of the annelid Platynereis close their locomotor cilia and simultaneously raise the parapodia. The response is mediated by collar receptor neurons expressing the polycystins PKD1-1 and PKD2-1. CRISPR-generated PKD1-1 and PKD2-1 mutant larvae do not startle and fall prey to a copepod predator at a higher rate. Reconstruction of the whole-body connectome of the collar-receptor-cell circuitry revealed converging feedforward circuits to the ciliary bands and muscles. The wiring diagram suggests circuit mechanisms for the intersegmental and left-right coordination of the response. Our results reveal how polycystin-mediated mechanosensation can trigger a coordinated whole-body effector response involved in predator avoidance.

Behavioral Mechanisms That Depend on Dopamine and Serotonin in Caenorhabditis elegans Interact With the Antipsychotics Risperidone and Aripiprazole

The neurotransmitters dopamine and serotonin participate in specific behavioral neuromuscular mechanisms in the nematode Caenorhabditis elegans. Dopamine is involved in the gentle touch response and serotonin in the pharyngeal pumping rate. In its genome, the worm presents genes encoding dopamine and serotonin receptors orthologous to those of human genes. Risperidone and aripiprazole are a class of drugs known as atypical antipsychotics commonly used to treat schizophrenia, bipolar disorder, and irritability associated with autism. Risperidone is an antagonist of the dopamine D2 and serotonin 5-HT2A receptors. Aripiprazole functions as a partial agonist of the dopamine D2 receptor and as a partial agonist and antagonist of 5-HT1A and 5-HT2A serotonin receptors, respectively. Our results show that risperidone and aripiprazole alter the touch response and pharyngeal pumping in wild-type worm animals. Furthermore, in the presence of the drugs, both behaviors change to varying degrees in dopamine (dop-1, dop-2, and dop-3), serotonin (ser-1), and tyramine (ser-2) receptor-deficient mutants. This variation in response reveals specific targets for these antipsychotics in the nematode. Interestingly, their effect on behavior persisted to some extent in successive generations, indicating that they might induce epigenetic changes throughout development. Sodium butyrate, a histone deacetylase inhibitor, eliminated the consecutive generation effect of both drugs. In addition, these transgenerational effects were also abolished after the dauer stage. These observations suggest that risperidone and aripiprazole, in addition to interacting with specific receptors impairing the function of the nervous system of the nematode, may lead to the deposition of long-lasting epigenetic marks.

The tactile receptive fields of freely moving Caenorhabditis elegans nematodes

Sensory neurons embedded in skin are responsible for the sense of touch. In humans and other mammals, touch sensation depends on thousands of diverse somatosensory neurons. By contrast, Caenorhabditis elegans nematodes have six gentle touch receptor neurons linked to simple behaviors. The classical touch assay uses an eyebrow hair to stimulate freely moving C. elegans, evoking evasive behavioral responses. This assay has led to the discovery of genes required for touch sensation, but does not provide control over stimulus strength or position. Here, we present an integrated system for performing automated, quantitative touch assays that circumvents these limitations and incorporates automated measurements of behavioral responses. The Highly Automated Worm Kicker (HAWK) unites a microfabricated silicon force sensor holding a glass bead forming the contact surface and video analysis with real-time force and position control. Using this system, we stimulated animals along the anterior-posterior axis and compared responses in wild-type and spc-1(dn) transgenic animals, which have a touch defect due to expression of a dominant-negative α-spectrin protein fragment. As expected from prior studies, delivering large stimuli anterior and posterior to the mid-point of the body evoked a reversal and a speed-up, respectively. The probability of evoking a response of either kind depended on stimulus strength and location; once initiated, the magnitude and quality of both reversal and speed-up behavioral responses were uncorrelated with stimulus location, strength, or the absence or presence of the spc-1(dn) transgene. Wild-type animals failed to respond when the stimulus was applied near the mid-point. These results show that stimulus strength and location govern the activation of a characteristic motor program and that the C. elegans body surface consists of two receptive fields separated by a gap.

Ultrasound Elicits Behavioral Responses through Mechanical Effects on Neurons and Ion Channels in a Simple Nervous System

Focused ultrasound has been shown to stimulate excitable cells, but the biophysical mechanisms behind this phenomenon remain poorly understood. To provide additional insight, we devised a behavioral-genetic assay applied to the well-characterized nervous system of Caenorhabditis elegans nematodes. We found that pulsed ultrasound elicits robust reversal behavior in wild-type animals in a pressure-, duration-, and pulse protocol-dependent manner. Responses were preserved in mutants unable to sense thermal fluctuations and absent in mutants lacking neurons required for mechanosensation. Additionally, we found that the worm's response to ultrasound pulses rests on the expression of MEC-4, a DEG/ENaC/ASIC ion channel required for touch sensation. Consistent with prior studies of MEC-4-dependent currents in vivo, the worm's response was optimal for pulses repeated 300-1000 times per second. Based on these findings, we conclude that mechanical, rather than thermal, stimulation accounts for behavioral responses. Further, we propose that acoustic radiation force governs the response to ultrasound in a manner that depends on the touch receptor neurons and MEC-4-dependent ion channels. Our findings illuminate a complete pathway of ultrasound action, from the forces generated by propagating ultrasound to an activation of a specific ion channel. The findings further highlight the importance of optimizing ultrasound pulsing protocols when stimulating neurons via ion channels with mechanosensitive properties.SIGNIFICANCE STATEMENT How ultrasound influences neurons and other excitable cells has remained a mystery for decades. Although it is widely understood that ultrasound can heat tissues and induce mechanical strain, whether or not neuronal activation depends on heat, mechanical force, or both physical factors is not known. We harnessed Caenorhabditis elegans nematodes and their extraordinary sensitivity to thermal and mechanical stimuli to address this question. Whereas thermosensory mutants respond to ultrasound similar to wild-type animals, mechanosensory mutants were insensitive to ultrasound stimulation. Additionally, stimulus parameters that accentuate mechanical effects were more effective than those producing more heat. These findings highlight a mechanical nature of the effect of ultrasound on neurons and suggest specific ways to optimize stimulation protocols in specific tissues.

Key developments that impacted the field of mechanobiology and mechanotransduction

Advances in mechanobiology have evolved through insights from multiple disciplines including structural engineering, biomechanics, vascular biology, and orthopaedics. In this paper, we reviewed the impact of key reports related to the study of applied loads on tissues and cells and the resulting signal transduction pathways. We addressed how technology has helped advance the burgeoning field of mechanobiology (over 33,600 publications from 1970 to 2016). We analyzed the impact of critical ideas and then determined how these concepts influenced the mechanobiology field by looking at the citation frequency of these reports as well as tracking how the overall number of citations within the field changed over time. These data allowed us to understand how a key publication, idea, or technology guided or enabled the field. Initial observations of how forces acted on bone and soft tissues stimulated the development of computational solutions defining how forces affect tissue modeling and remodeling. Enabling technologies, such as cell and tissue stretching, compression, and shear stress devices, allowed more researchers to explore how deformation and fluid flow affect cells. Observation of the cell as a tensegrity structure and advanced methods to study genetic regulation in cells further advanced knowledge of specific mechanisms of mechanotransduction. The future of the field will involve developing gene and drug therapies to simulate or augment beneficial load regimens in patients and in mechanically conditioning organs for implantation. Here, we addressed a history of the field, but we limited our discussions to advances in musculoskeletal mechanobiology, primarily in bone, tendon, and ligament tissues. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:605-619, 2018.

Is TMC1 the Hair Cell Mechanotransducer Channel?

Transmembrane channel-like protein isoform-1 (TMC1) has emerged over the past five years as a prime contender for the mechano-electrical transducer (MET) channel in hair cells of the inner ear. TMC1 is thought to have a six-transmembrane domain structure reminiscent of some other ion-channel subunits, and is targeted to the tips of the stereocilia in the sensory hair bundle, where the MET channel is located. Moreover, there are TMC1 mutations linked to human deafness causing loss of conventional MET currents, hair cell degeneration, and deafness in mice. Finally, mutations of Tmc1 can alter the conductance and Ca(2+) selectivity of the MET channels. For several reasons though, it is unclear that TMC1 is indeed the MET channel pore: 1) in other animals or tissues, mutations of TMC family members do not directly affect cellular mechanosensitivity; 2) there are residual manifestations of mechanosensitivity in hair cells of mouse Tmc1:Tmc2 double knockouts; 3) there is so far no evidence that expression of mammalian Tmc1 generates a mechanically sensitive ion channel in the plasma membrane when expressed in heterologous cells; and 4) there are other proteins, such as TMIE and LHFPL5, which behave similarly to TMC1, their mutation also leading to loss of MET current and deafness. This review will present these disparate lines of evidence and describes recent work that addresses the role of TMC1.

The Si elegans project at the interface of experimental and computational Caenorhabditis elegans neurobiology and behavior

OBJECTIVE

In light of recent progress in mapping neural function to behavior, we briefly and selectively review past and present endeavors to reveal and reconstruct nervous system function in Caenorhabditis elegans through simulation.

APPROACH

Rather than presenting an all-encompassing review on the mathematical modeling of C. elegans, this contribution collects snapshots of pathfinding key works and emerging technologies that recent single- and multi-center simulation initiatives are building on. We thereby point out a few general limitations and problems that these undertakings are faced with and discuss how these may be addressed and overcome.

MAIN RESULTS

Lessons learned from past and current computational approaches to deciphering and reconstructing information flow in the C. elegans nervous system corroborate the need of refining neural response models and linking them to intra- and extra-environmental interactions to better reflect and understand the actual biological, biochemical and biophysical events that lead to behavior. Together with single-center research efforts, the Si elegans and OpenWorm projects aim at providing the required, in some cases complementary tools for different hardware architectures to support advancement into this direction.

SIGNIFICANCE

Despite its seeming simplicity, the nervous system of the hermaphroditic nematode C. elegans with just 302 neurons gives rise to a rich behavioral repertoire. Besides controlling vital functions (feeding, defecation, reproduction), it encodes different stimuli-induced as well as autonomous locomotion modalities (crawling, swimming and jumping). For this dichotomy between system simplicity and behavioral complexity, C. elegans has challenged neurobiologists and computational scientists alike. Understanding the underlying mechanisms that lead to a context-modulated functionality of individual neurons would not only advance our knowledge on nervous system function and its failure in pathological states, but have directly exploitable benefits for robotics and the engineering of brain-mimetic computational architectures that are orthogonal to current von-Neumann-type machines.

Feeling force: physical and physiological principles enabling sensory mechanotransduction

Organisms as diverse as microbes, roundworms, insects, and mammals detect and respond to applied force. In animals, this ability depends on ionotropic force receptors, known as mechanoelectrical transduction (MeT) channels, that are expressed by specialized mechanoreceptor cells embedded in diverse tissues and distributed throughout the body. These cells mediate hearing, touch, and proprioception and play a crucial role in regulating organ function. Here, we attempt to integrate knowledge about the architecture of mechanoreceptor cells and their sensory organs with principles of cell mechanics, and we consider how engulfing tissues contribute to mechanical filtering. We address progress in the quest to identify the proteins that form MeT channels and to understand how these channels are gated. For clarity and convenience, we focus on sensory mechanobiology in nematodes, fruit flies, and mice. These themes are emphasized: asymmetric responses to applied forces, which may reflect anisotropy of the structure and mechanics of sensory mechanoreceptor cells, and proteins that function as MeT channels, which appear to have emerged many times through evolution.

The Power of Natural Variation for Model Organism Biology

Genetic background effects have long been recognized and, in some cases studied, but they are often viewed as a nuisance by molecular biologists. We suggest that genetic variation currently represents a critical frontier for molecular studies. Human genetics has seen a surge of interest in genetic variation and its contributions to disease, but insights into disease mechanisms are difficult since information about gene function is lacking. By contrast, model organism genetics has excelled at revealing molecular mechanisms of cellular processes, but often de-emphasizes genetic variation and its functional consequences. We argue that model organism biology would benefit from incorporating natural variation, both to capture how well laboratory lines exemplify the species they represent and to inform on molecular processes and their variability. Such a synthesis would also greatly expand the relevance of model systems for studies of complex trait variation, including disease.

Sensory-Motor Circuits: Hox Genes Get in Touch

Sensory-motor reflex circuits are the basic units from which animal nervous systems are constructed, yet little is known regarding how connections within these simple networks are established. In papers in Cell Reports and in this issue of Neuron, Zheng et al. (2015a, 2015b) demonstrate that coordinate activities of Hox genes in sensory neurons and interneurons govern connectivity within touch-reflex circuits in C. elegans.

Mechanically Activated Ion Channels

Mechanotransduction, the conversion of physical forces into biochemical signals, is essential for various physiological processes such as the conscious sensations of touch and hearing, and the unconscious sensation of blood flow. Mechanically activated (MA) ion channels have been proposed as sensors of physical force, but the identity of these channels and an understanding of how mechanical force is transduced has remained elusive. A number of recent studies on previously known ion channels along with the identification of novel MA ion channels have greatly transformed our understanding of touch and hearing in both vertebrates and invertebrates. Here, we present an updated review of eukaryotic ion channel families that have been implicated in mechanotransduction processes and evaluate the qualifications of the candidate genes according to specified criteria. We then discuss the proposed gating models for MA ion channels and highlight recent structural studies of mechanosensitive potassium channels.

Coordination and fine motor control depend on Drosophila TRPγ

Motor coordination is broadly divided into gross and fine motor control, both of which depend on proprioceptive organs. However, the channels that function specifically in fine motor control are unknown. Here we show that mutations in trpγ disrupt fine motor control while leaving gross motor proficiency intact. The mutants are unable to coordinate precise leg movements during walking, and are ineffective in traversing large gaps due to an inability in making subtle postural adaptations that are requisite for this task. TRPγ is expressed in proprioceptive organs, and is required in both neurons and glia for gap crossing. We expressed TRPγ in vitro, and found that its activity is promoted by membrane stretch. A mutation eliminating the Na(+)/Ca(2+) exchanger suppresses the gap-crossing phenotype of trpγ flies. Our findings indicate that TRPγ contributes to fine motor control through mechanical activation in proprioceptive organs, thereby promoting Ca(2+) influx, which is required for function.

Transduction and encoding sensory information by skin mechanoreceptors

Physical contact with the external world occurs through specialized neural structures called mechanoreceptors. Cutaneous mechanoreceptors provide information to the central nervous system (CNS) about touch, pressure, vibration, and skin stretch. The physiological function of these mechanoreceptors is to convert physical forces into neuronal signals. Key questions concern the molecular identity of the mechanoelectric transducer channels and the mechanisms by which the physical parameters of the mechanical stimulus are encoded into patterns of action potentials (APs). Compelling data indicate that the biophysical traits of mechanosensitive channels combined with the collection of voltage-gated channels are essential to describe the nature of the stimulus. Recent research also points to a critical role of the auxiliary cell-nerve ending communication in encoding stimulus properties. This review describes the characteristics of ion channels responsible for translating mechanical stimuli into the neural codes that underlie touch perception and pain.