Potassium channels in C. elegans

Constitutive sodium permeability in a C. elegans two-pore domain potassium channel

Two-pore domain potassium (K2P) channels play a central role in modulating cellular excitability and neuronal function. The unique structure of the selectivity filter in K2P and other potassium channels determines their ability to allow the selective passage of potassium ions across cell membranes. The nematode C. elegans has one of the largest K2P families, with 47 subunit-coding genes. This remarkable expansion has been accompanied by the evolution of atypical selectivity filter sequences that diverge from the canonical TxGYG motif. Whether and how this sequence variation may impact the function of K2P channels has not been investigated so far. Here, we show that the UNC-58 K2P channel is constitutively permeable to sodium ions and that a cysteine residue in its selectivity filter is responsible for this atypical behavior. Indeed, by performing in vivo electrophysiological recordings and Ca2+ imaging experiments, we demonstrate that UNC-58 has a depolarizing effect in muscles and sensory neurons. Consistently, unc-58 gain-of-function mutants are hypercontracted, unlike the relaxed phenotype observed in hyperactive mutants of many neuromuscular K2P channels. Finally, by combining molecular dynamics simulations with functional studies in Xenopus laevis oocytes, we show that the atypical cysteine residue plays a key role in the unconventional sodium permeability of UNC-58. As predicting the consequences of selectivity filter sequence variations in silico remains a major challenge, our study illustrates how functional experiments are essential to determine the contribution of such unusual potassium channels to the electrical profile of excitable cells.

A tonically active master neuron modulates mutually exclusive motor states at two timescales

Continuity of behaviors requires animals to make smooth transitions between mutually exclusive behavioral states. Neural principles that govern these transitions are not well understood. Caenorhabditis elegans spontaneously switch between two opposite motor states, forward and backward movement, a phenomenon thought to reflect the reciprocal inhibition between interneurons AVB and AVA. Here, we report that spontaneous locomotion and their corresponding motor circuits are not separately controlled. AVA and AVB are neither functionally equivalent nor strictly reciprocally inhibitory. AVA, but not AVB, maintains a depolarized membrane potential. While AVA phasically inhibits the forward promoting interneuron AVB at a fast timescale, it maintains a tonic, extrasynaptic excitation on AVB over the longer timescale. We propose that AVA, with tonic and phasic activity of opposite polarities on different timescales, acts as a master neuron to break the symmetry between the underlying forward and backward motor circuits. This master neuron model offers a parsimonious solution for sustained locomotion consisted of mutually exclusive motor states.

Egg-laying and locomotory screens with C. elegans yield a nematode-selective small molecule stimulator of neurotransmitter release

Nematode parasites of humans, livestock and crops dramatically impact human health and welfare. Alarmingly, parasitic nematodes of animals have rapidly evolved resistance to anthelmintic drugs, and traditional nematicides that protect crops are facing increasing restrictions because of poor phylogenetic selectivity. Here, we exploit multiple motor outputs of the model nematode C. elegans towards nematicide discovery. This work yielded multiple compounds that selectively kill and/or immobilize diverse nematode parasites. We focus on one compound that induces violent convulsions and paralysis that we call nementin. We find that nementin stimulates neuronal dense core vesicle release, which in turn enhances cholinergic signaling. Consequently, nementin synergistically enhances the potency of widely-used non-selective acetylcholinesterase (AChE) inhibitors, but in a nematode-selective manner. Nementin therefore has the potential to reduce the environmental impact of toxic AChE inhibitors that are used to control nematode infections and infestations.

A C. elegans-based screening approach identifies nementin as a nematode-selective nematicide that can be used synergistically with acetylcholinesterase inhibitors

A single-nucleotide change underlies the genetic assimilation of a plastic trait

Genetic assimilation-the evolutionary process by which an environmentally induced phenotype is made constitutive-represents a fundamental concept in evolutionary biology. Thought to reflect adaptive phenotypic plasticity, matricidal hatching in nematodes is triggered by maternal nutrient deprivation to allow for protection or resource provisioning of offspring. Here, we report natural Caenorhabditis elegans populations harboring genetic variants expressing a derived state of near-constitutive matricidal hatching. These variants exhibit a single amino acid change (V530L) in KCNL-1, a small-conductance calcium-activated potassium channel subunit. This gain-of-function mutation causes matricidal hatching by strongly reducing the sensitivity to environmental stimuli triggering egg-laying. We show that reestablishing the canonical KCNL-1 protein in matricidal isolates is sufficient to restore canonical egg-laying. While highly deleterious in constant food environments, KCNL-1 V530L is maintained under fluctuating resource availability. A single point mutation can therefore underlie the genetic assimilation-by either genetic drift or selection-of an ancestrally plastic trait.

On the Modeling of the Three Types of Non-spiking Neurons of the Caenorhabditis elegans

The nematode Caenorhabditis elegans (C. elegans) is a well-known model organism in neuroscience. The relative simplicity of its nervous system, made up of few hundred neurons, shares some essential features with more sophisticated nervous systems, including the human one. If we are able to fully characterize the nervous system of this organism, we will be one step closer to understanding the mechanisms underlying the behavior of living things. Following a recently conducted electrophysiological survey on different C. elegans neurons, this paper aims at modeling the three non-spiking RIM, AIY and AFD neurons (arbitrarily named with three upper case letters by convention). To date, they represent the three possible forms of non-spiking neuronal responses of the C. elegans. To achieve this objective, we propose a conductance-based neuron model adapted to the electrophysiological features of each neuron. These features are based on current biological research and a series of in-silico experiments which use differential evolution to fit the model to experimental data. From the obtained results, we formulate a series of biological hypotheses regarding currents involved in the neuron dynamics. These models reproduce experimental data with a high degree of accuracy while being biologically consistent with state-of-the-art research.

Identification of potassium and calcium channel inhibitors as modulators of polyomavirus endosomal trafficking

During virus entry, members of the Polyomaviridae transit the endolysosomal network en route to the endoplasmic reticulum (ER), from which degraded capsids escape into the cytoplasm and enter the nucleus. Emerging evidence suggests that viruses require both endosomal acidification and the correct ionic balance of K⁺ and Ca²⁺ ions in endosomes for correct virus trafficking and genome release. Here, using two polyomaviruses with different capsid architectures, namely Simian virus 40 (SV40) and Merkel cell polyomavirus (MCPyV), we describe methods to rapidly quantify virus infection using IncuCyte ZOOM imaging analysis, and use this system to investigate the role of both K⁺ and Ca²⁺ channels during the early stages of virus entry. Using broad spectrum blockers of both K⁺ and Ca²⁺ channels to specifically target host cell ion channel functionality, we show that MCPyV, but not SV40 can be inhibited by K⁺ channel modulators, whilst both viruses are restricted by the broad spectrum Ca²⁺ channel inhibitor verapamil. Using a panel of more specific Ca²⁺ blockers, we show that both MCPyV and SV40 are dependent on the activity of two-pore Ca²⁺ channels (TPCs), as the TPC-specific blocker tetrandrine prevented capsid disassembly and nuclear transport required for virus entry. We therefore reveal a novel target to restrict the entry of polyomaviruses, which given the known role of TPCs during endolysosomal-ER fusion, is likely to be applicable to other viruses that transit this pathway.

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We describe novel high-throughput assays to study SV40 and MCPyV infection.

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MCPyV, but not SV40, is sensitive to K⁺ channel inhibition.

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Verapamil inhibits MCPyV and SV40 infection.

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Tetrandrine is a potent inhibitor of MCPyV and SV40 infection.

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Two-pore channel 1/2 activity is essential for polyomavirus entry.

Biophysical modeling of C. elegans neurons: Single ion currents and whole-cell dynamics of AWCon and RMD

C. elegans neuronal system constitutes the ideal framework for studying simple, yet realistic, neuronal activity, since the whole nervous system is fully characterized with respect to the exact number of neurons and the neuronal connections. Most recent efforts are devoted to investigate and clarify the signal processing and functional connectivity, which are at the basis of sensing mechanisms, signal transmission, and motor control. In this framework, a refined modelof whole neuron dynamics constitutes a key ingredient to describe the electrophysiological processes, both at thecellular and at the network scale. In this work, we present Hodgkin-Huxley-based models of ion channels dynamics black, built on data available both from C. elegans and from other organisms, expressing homologous channels. We combine these channel models to simulate the electrical activity oftwo among the most studied neurons in C. elegans, which display prototypical dynamics of neuronal activation, the chemosensory AWCON and the motor neuron RMD. Our model properly describes the regenerative responses of the two cells. We analyze in detail the role of ion currents, both in wild type and in in silico knockout neurons. Moreover, we specifically investigate the behavior of RMD, identifying a heterogeneous dynamical response which includes bistable regimes and sustained oscillations. We are able to assess the critical role of T-type calcium currents, carried by CCA-1 channels, and leakage currents in the regulation of RMD response. Overall, our results provide new insights in the activity of key C. elegans neurons. The developed mathematical framework constitute a basis for single-cell and neuronal networks analyses, opening new scenarios in the in silico modeling of C. elegans neuronal system.

OpenWorm: overview and recent advances in integrative biological simulation of Caenorhabditis elegans

The adoption of powerful software tools and computational methods from the software industry by the scientific research community has resulted in a renewed interest in integrative, large-scale biological simulations. These typically involve the development of computational platforms to combine diverse, process-specific models into a coherent whole. The OpenWorm Foundation is an independent research organization working towards an integrative simulation of the nematode Caenorhabditis elegans, with the aim of providing a powerful new tool to understand how the organism's behaviour arises from its fundamental biology. In this perspective, we give an overview of the history and philosophy of OpenWorm, descriptions of the constituent sub-projects and corresponding open-science management practices, and discuss current achievements of the project and future directions.This article is part of a discussion meeting issue 'Connectome to behaviour: modelling C. elegans at cellular resolution'.

Electrophysiology of the rhythmic defecation program in nematode Heterorhabditis megidis

The nervous system controls most rhythmic behaviors, with a remarkable exception. In Caenorhabditis elegans periodic defecation rhythm does not appear to involve the nervous system. Such oscillations are studied in detail with genetic and molecular biology tools. The small size of C. elegans cells impairs the use of standard electrophysiological methods. We studied a similar rhythmic pacemaker in the noticeably larger gut cells of Heterorhabditis megidis nematode. H. megidis defecation cycle is driven by a central pattern generator (CPG) associated with unusual all-or-none hyper-polarization “action potential”. The CPG cycle period depends on the membrane potential and CPG cycling also persisted in experiments where the membrane potential of gut cells was continuously clamped at steady voltage levels. The usual excitable tissue description does not include the endoderm or imply the generation of hyper-polarization spikes. The nematode gut cells activity calls for a reevaluation of the excitable cells definition.

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.

A novel BK channel-targeted peptide suppresses sound evoked activity in the mouse inferior colliculus

Large conductance calcium-activated (BK) channels are broadly expressed in neurons and muscle where they modulate cellular activity. Decades of research support an interest in pharmaceutical applications for modulating BK channel function. Here we report a novel BK channel-targeted peptide with functional activity in vitro and in vivo. This 9-amino acid peptide, LS3, has a unique action, suppressing channel gating rather than blocking the pore of heterologously expressed human BK channels. With an IC₅₀ in the high picomolar range, the apparent affinity is higher than known high affinity BK channel toxins. LS3 suppresses locomotor activity via a BK channel-specific mechanism in wild-type or BK channel-humanized Caenorhabditis elegans. Topical application on the dural surface of the auditory midbrain in mouse suppresses sound evoked neural activity, similar to a well-characterized pore blocker of the BK channel. Moreover, this novel ion channel-targeted peptide rapidly crosses the BBB after systemic delivery to modulate auditory processing. Thus, a potent BK channel peptide modulator is open to neurological applications, such as preventing audiogenic seizures that originate in the auditory midbrain.

Gene expression changes in diapause or quiescent potato cyst nematode, Globodera pallida, eggs after hydration or exposure to tomato root diffusate

Plant-parasitic nematodes (PPN) need to be adapted to survive in the absence of a suitable host or in hostile environmental conditions. Various forms of developmental arrest including hatching inhibition and dauer stages are used by PPN in order to survive these conditions and spread to other areas. Potato cyst nematodes (PCN) (Globodera pallida and G. rostochiensis) are frequently in an anhydrobiotic state, with unhatched nematode persisting for extended periods of time inside the cyst in the absence of the host. This paper shows fundamental changes in the response of quiescent and diapaused eggs of G. pallida to hydration and following exposure to tomato root diffusate (RD) using microarray gene expression analysis encompassing a broad set of genes. For the quiescent eggs, 547 genes showed differential expression following hydration vs. hydratation and RD (H-RD) treatment whereas 708 genes showed differential regulation for the diapaused eggs following these treatments. The comparison between hydrated quiescent and diapaused eggs showed marked differences, with 2,380 genes that were differentially regulated compared with 987 genes following H-RD. Hydrated quiescent and diapaused eggs were markedly different indicating differences in adaptation for long-term survival. Transport activity is highly up-regulated following H-RD and few genes were coincident between both kinds of eggs. With the quiescent eggs, the majority of genes were related to ion transport (mainly sodium), while the diapaused eggs showed a major diversity of transporters (amino acid transport, ion transport, acetylcholine or other molecules).

SLO BK Potassium Channels Couple Gap Junctions to Inhibition of Calcium Signaling in Olfactory Neuron Diversification

The C. elegans AWC olfactory neuron pair communicates to specify asymmetric subtypes AWCOFF and AWCON in a stochastic manner. Intercellular communication between AWC and other neurons in a transient NSY-5 gap junction network antagonizes voltage-activated calcium channels, UNC-2 (CaV2) and EGL-19 (CaV1), in the AWCON cell, but how calcium signaling is downregulated by NSY-5 is only partly understood. Here, we show that voltage- and calcium-activated SLO BK potassium channels mediate gap junction signaling to inhibit calcium pathways for asymmetric AWC differentiation. Activation of vertebrate SLO-1 channels causes transient membrane hyperpolarization, which makes it an important negative feedback system for calcium entry through voltage-activated calcium channels. Consistent with the physiological roles of SLO-1, our genetic results suggest that slo-1 BK channels act downstream of NSY-5 gap junctions to inhibit calcium channel-mediated signaling in the specification of AWCON. We also show for the first time that slo-2 BK channels are important for AWC asymmetry and act redundantly with slo-1 to inhibit calcium signaling. In addition, nsy-5-dependent asymmetric expression of slo-1 and slo-2 in the AWCON neuron is necessary and sufficient for AWC asymmetry. SLO-1 and SLO-2 localize close to UNC-2 and EGL-19 in AWC, suggesting a role of possible functional coupling between SLO BK channels and voltage-activated calcium channels in AWC asymmetry. Furthermore, slo-1 and slo-2 regulate the localization of synaptic markers, UNC-2 and RAB-3, in AWC neurons to control AWC asymmetry. We also identify the requirement of bkip-1, which encodes a previously identified auxiliary subunit of SLO-1, for slo-1 and slo-2 function in AWC asymmetry. Together, these results provide an unprecedented molecular link between gap junctions and calcium pathways for terminal differentiation of olfactory neurons.

A Transparent Window into Biology: A Primer on Caenorhabditis elegans

A little over 50 years ago, Sydney Brenner had the foresight to develop the nematode (round worm) Caenorhabditis elegans as a genetic model for understanding questions of developmental biology and neurobiology. Over time, research on C. elegans has expanded to explore a wealth of diverse areas in modern biology including studies of the basic functions and interactions of eukaryotic cells, host-parasite interactions, and evolution. C. elegans has also become an important organism in which to study processes that go awry in human diseases. This primer introduces the organism and the many features that make it an outstanding experimental system, including its small size, rapid life cycle, transparency, and well-annotated genome. We survey the basic anatomical features, common technical approaches, and important discoveries in C. elegans research. Key to studying C. elegans has been the ability to address biological problems genetically, using both forward and reverse genetics, both at the level of the entire organism and at the level of the single, identified cell. These possibilities make C. elegans useful not only in research laboratories, but also in the classroom where it can be used to excite students who actually can see what is happening inside live cells and tissues.

Much more than a leak: structure and function of K₂p-channels

Over the last decade, we have seen an enormous increase in the number of experimental studies on two-pore-domain potassium channels (K2P-channels). The collection of reviews and original articles compiled for this special issue of Pflügers Archiv aims to give an up-to-date summary of what is known about the physiology and pathophysiology of K2P-channels. This introductory overview briefly describes the structure of K2P-channels and their function in different organs. Its main aim is to provide some background information for the 19 reviews and original articles of this special issue of Pflügers Archiv. It is not intended to be a comprehensive review; instead, this introductory overview focuses on some unresolved questions and controversial issues, such as: Do K2P-channels display voltage-dependent gating? Do K2P-channels contribute to the generation of action potentials? What is the functional role of alternative translation initiation? Do K2P-channels have one or two or more gates? We come to the conclusion that we are just beginning to understand the extremely complex regulation of these fascinating channels, which are often inadequately described as 'leak channels'.

SLO-2 potassium channel is an important regulator of neurotransmitter release in Caenorhabditis elegans

Slo2 channels are prominent K(+) channels in mammalian neurons but their physiological functions are not well understood. Here we investigate physiological functions and regulation of the Caenorhabditis elegans homologue SLO-2 in motor neurons through electrophysiological analyses of wild-type and mutant worms. We find that SLO-2 is the primary K(+) channel conducting delayed outward current in cholinergic motor neurons, and one of two K(+) channels with this function in GABAergic motor neurons. Loss-of-function mutation of slo-2 increases the duration and charge transfer rate of spontaneous postsynaptic current bursts at the neuromuscular junction, which are physiological signals used by motor neurons to control muscle cells, without altering postsynaptic receptor sensitivity. SLO-2 activity in motor neurons depends on Ca(2+) entry through EGL-19, an L-type voltage-gated Ca(2+) channel (CaV1), but not on other proteins implicated in either Ca(2+) entry or intracellular Ca(2+) release. Thus, SLO-2 is functionally coupled with CaV1 and regulates neurotransmitter release.

Altered and dynamic ion selectivity of K+ channels in cell development and excitability

K(+) channels play a key role in regulating cellular excitability. It was thought that the strong K(+) selectivity of these channels was static, only altered by mutations in their selectivity filter, which can cause severe genetic disorders. Recent studies demonstrate that selectivity of K(+) channels can also exhibit dynamic changes. Under acidic conditions or in low extracellular K(+) concentrations, the two-pore domain K(+) channel (K2P) TWIK1 becomes permeable to Na(+), shifting from an inhibitory role to an excitatory role. This phenomenon is responsible for the paradoxical depolarization of human cardiomyocytes in pathological hypokalemia, and therefore may contribute to cardiac arrhythmias. In other cell types, TWIK1 produces depolarizing leak currents under physiological conditions. Dynamic ion selectivity also occurs in other K2P channels. Here we review evidence that dynamic selectivity of K2P channels constitutes a new regulatory mechanism of cellular excitability, whose significance is only now becoming appreciated.

The genome and life-stage specific transcriptomes of Globodera pallida elucidate key aspects of plant parasitism by a cyst nematode

BACKGROUND

Globodera pallida is a devastating pathogen of potato crops, making it one of the most economically important plant parasitic nematodes. It is also an important model for the biology of cyst nematodes. Cyst nematodes and root-knot nematodes are the two most important plant parasitic nematode groups and together represent a global threat to food security.

RESULTS

We present the complete genome sequence of G. pallida, together with transcriptomic data from most of the nematode life cycle, particularly focusing on the life cycle stages involved in root invasion and establishment of the biotrophic feeding site. Despite the relatively close phylogenetic relationship with root-knot nematodes, we describe a very different gene family content between the two groups and in particular extensive differences in the repertoire of effectors, including an enormous expansion of the SPRY domain protein family in G. pallida, which includes the SPRYSEC family of effectors. This highlights the distinct biology of cyst nematodes compared to the root-knot nematodes that were, until now, the only sedentary plant parasitic nematodes for which genome information was available. We also present in-depth descriptions of the repertoires of other genes likely to be important in understanding the unique biology of cyst nematodes and of potential drug targets and other targets for their control.

CONCLUSIONS

The data and analyses we present will be central in exploiting post-genomic approaches in the development of much-needed novel strategies for the control of G. pallida and related pathogens.

The million mutation project: a new approach to genetics in Caenorhabditis elegans

We have created a library of 2007 mutagenized Caenorhabditis elegans strains, each sequenced to a target depth of 15-fold coverage, to provide the research community with mutant alleles for each of the worm's more than 20,000 genes. The library contains over 800,000 unique single nucleotide variants (SNVs) with an average of eight nonsynonymous changes per gene and more than 16,000 insertion/deletion (indel) and copy number changes, providing an unprecedented genetic resource for this multicellular organism. To supplement this collection, we also sequenced 40 wild isolates, identifying more than 630,000 unique SNVs and 220,000 indels. Comparison of the two sets demonstrates that the mutant collection has a much richer array of both nonsense and missense mutations than the wild isolate set. We also find a wide range of rDNA and telomere repeat copy number in both sets. Scanning the mutant collection for molecular phenotypes reveals a nonsense suppressor as well as strains with higher levels of indels that harbor mutations in DNA repair genes and strains with abundant males associated with him mutations. All the strains are available through the Caenorhabditis Genetics Center and all the sequence changes have been deposited in WormBase and are available through an interactive website.

K⁺ conduction and Mg²⁺ blockade in a shaker Kv-channel single point mutant with an unusually high conductance

Potassium channels exhibit a large diversity of single-channel conductances. Shaker is a low-conductance K-channel in which Pro475→Asp, a single-point mutation near the internal pore entrance, promotes 6- to 8-fold higher unitary current. To assess the mechanism for this higher conductance, we measured Shaker-P475D single-channel current in a wide range of symmetrical K(+) concentrations and voltages. Below 300 mM K(+), the current-to-voltage relations (i-V) showed inward rectification that disappeared at 1000 mM K(+). Single-channel conductance reached a maximum of ∼190 pS at saturating [K(+)], a value 4- to 5-fold larger than that estimated for the native channel. Intracellular Mg(2+) blocked this variant with ∼100-fold higher affinity. Near zero voltage, blockade was competitively antagonized by K(+); however, at voltages >100 mV, it was enhanced by K(+). This result is consistent with a lock-in effect in a single-file diffusion regime of Mg(2+) and K(+) along the pore. Molecular-dynamics simulations revealed higher K(+) density in the pore, especially near the Asp-475 side chains, as in the high-conductance MthK bacterial channel. The molecular dynamics also showed that K(+) ions bound distally can coexist with other K(+) or Mg(2+) in the cavity, supporting a lock-in mechanism. The maximal K(+) transport rate and higher occupancy could be due to a decrease in the electrostatic energy profile for K(+) throughout the pore, reducing the energy wells and barriers differentially by ∼0.7 and ∼2 kT, respectively.

Targeted molecular dynamics (TMD) of the full-length KcsA potassium channel: on the role of the cytoplasmic domain in the opening process

Some recent papers clearly indicate that the cytoplasmic domain of KcsA plays a role in pH sensing. We have performed, for the first time, a targeted molecular dynamics (TMD) simulation of the opening of full-length KcsA at pH 4 and pH 7, with a special interest for the cytoplasmic domain. Association energy calculations show a stabilization at pH 7 confirming that the protonation of some amino-acids at pH 4 in this domain plays a role in the opening process. A careful analysis of the pH dependent charges borne by residues in the cytoplasmic domain and their interactions confirms some literature experimental data and permits to give further insight into the role played by some of them in the opening process.

Functional transcriptomics of a migrating cell in Caenorhabditis elegans

In both metazoan development and metastatic cancer, migrating cells must carry out a detailed, complex program of sensing cues, binding substrates, and moving their cytoskeletons. The linker cell in Caenorhabditis elegans males undergoes a stereotyped migration that guides gonad organogenesis, occurs with precise timing, and requires the nuclear hormone receptor NHR-67. To better understand how this occurs, we performed RNA-seq of individually staged and dissected linker cells, comparing transcriptomes from linker cells of third-stage (L3) larvae, fourth-stage (L4) larvae, and nhr-67-RNAi-treated L4 larvae. We observed expression of 8,000-10,000 genes in the linker cell, 22-25% of which were up- or down-regulated 20-fold during development by NHR-67. Of genes that we tested by RNAi, 22% (45 of 204) were required for normal shape and migration, suggesting that many NHR-67-dependent, linker cell-enriched genes play roles in this migration. One unexpected class of genes up-regulated by NHR-67 was tandem pore potassium channels, which are required for normal linker-cell migration. We also found phenotypes for genes with human orthologs but no previously described migratory function. Our results provide an extensive catalog of genes that act in a migrating cell, identify unique molecular functions involved in nematode cell migration, and suggest similar functions in humans.

A survey of putative secreted and transmembrane proteins encoded in the C. elegans genome

BACKGROUND

Almost half of the Caenorhabditis elegans genome encodes proteins with either a signal peptide or a transmembrane domain. Therefore a substantial fraction of the proteins are localized to membranes, reside in the secretory pathway or are secreted. While these proteins are of interest to a variety of different researchers ranging from developmental biologists to immunologists, most of secreted proteins have not been functionally characterized so far.

RESULTS

We grouped proteins containing a signal peptide or a transmembrane domain using various criteria including evolutionary origin, common domain organization and functional categories. We found that putative secreted proteins are enriched for small proteins and nematode-specific proteins. Many secreted proteins are predominantly expressed in specific life stages or in one of the two sexes suggesting stage- or sex-specific functions. More than a third of the putative secreted proteins are upregulated upon exposure to pathogens, indicating that a substantial fraction may have a role in immune response. Slightly more than half of the transmembrane proteins can be grouped into broad functional categories based on sequence similarity to proteins with known function. By far the largest groups are channels and transporters, various classes of enzymes and putative receptors with signaling function.

CONCLUSION

Our analysis provides an overview of all putative secreted and transmembrane proteins in C. elegans. This can serve as a basis for selecting groups of proteins for large-scale functional analysis using reverse genetic approaches.

The Drosophila SK channel (dSK) contributes to photoreceptor performance by mediating sensitivity control at the first visual network

The contribution of the SK (small-conductance calcium-activated potassium) channel to neuronal functions in complex circuits underlying sensory processing and behavior is largely unknown in the absence of suitable animal models. Here, we generated a Drosophila line that lacks the single highly conserved SK gene in its genome (dSK). In R1-R6 photoreceptors, dSK encodes a slow Ca²⁺-activated K(+) current similar to its mammalian counterparts. Compared with wild-type, dSK(-) photoreceptors and interneurons showed accelerated oscillatory responses and adaptation. These enhanced kinetics were accompanied with more depolarized dSK(-) photoreceptors axons, assigning a role for dSK in network gain control during light-to-dark transitions. However, compensatory network adaptation, through increasing activity between synaptic neighbors, overcame many detriments of missing dSK current enabling dSK(-) photoreceptors to maintain normal information transfer rates to naturalistic stimuli. While demonstrating important functional roles for dSK channel in the visual circuitry, these results also clarify how homeostatically balanced network functions can compensate missing or faulty ion channels.

Genetic dissection of ion currents underlying all-or-none action potentials in C. elegans body-wall muscle cells

Although the neuromuscular system of C. elegans has been studied intensively, little is known about the properties of muscle action potentials (APs). By combining mutant analyses with in vivo electrophysiological recording techniques and Ca2+ imaging, we have established the fundamental properties and molecular determinants of body-wall muscle APs. We show that, unlike mammalian skeletal muscle APs, C. elegans muscle APs occur in spontaneous trains, do not require the function of postsynaptic receptors, and are all-or-none overshooting events, rather than graded potentials as has been previously reported. Furthermore, we show that muscle APs depend on Ca2+ entry through the L-type Ca2+ channel EGL-19 with a contribution from the T-type Ca2+ channel CCA-1. Both the Shaker K+ channel SHK-1 and the Ca2+/Cl−-gated K+ channel SLO-2 play important roles in controlling the speed of membrane repolarization, the amplitude of afterhyperpolarization (AHP) and the pattern of AP firing; SLO-2 is also important in setting the resting membrane potential. Finally, AP-elicited elevations of [Ca2+]i require both EGL-19 and the ryanodine receptor UNC-68. Thus, like mammalian skeletal muscle, C. elegans body-wall myocytes generate all-or-none APs, which evoke Ca2+ release from the sarcoplasmic reticulum (SR), although the specific ion channels used for AP upstroke and repolarization differ.

The intracellular localization and function of the ATP-sensitive K+ channel subunit Kir6.1

Our aim was to determine the subcellular localization and functional roles of the K(ATP) channel subunit Kir6.1 in intracellular membranes. Specifically, we focused on the potential role of Kir6.1 as a subunit of the mitochondrial ATP-sensitive K+ channel. Cell imaging showed that a major proportion of heterologously expressed Kir6.1-GFP and endogenously expressed Kir6.1 was distributed in the endoplasmic reticulum with little in the mitochondria or plasma membrane. We used pharmacological and molecular tools to investigate the functional significance of this distribution. The K(ATP) channel opener diazoxide increased reactive oxygen species production, and glibenclamide abolished this effect. However, in cells lacking Kir6.1 or expressing siRNA or dominant negative constructs of Kir6.1, the same effect was seen. Ca2+ handling was examined in the muscle cell line C2C12. Transfection of the dominant negative constructs of Kir6.1 significantly reduced the amplitude and rate of rise of [Ca2+]( c ) transients elicited by ATP. This study suggests that Kir6.1 is located in the endoplasmic reticulum and plays a role in modifying Ca2+ release from intracellular stores.

Levamisole and ryanodine receptors. II: An electrophysiological study in Ascaris suum

Resistance to antinematodal drugs like levamisole has increased and there is a need to understand what factors affect the responses to these anthelmintics. In our previous study, we examined the role of ryanodine receptors in muscle contraction pathways. Here we have examined interactions of levamisole receptors, ryanodine receptors (RyRs), the excitatory neuropeptide AF2, and coupling to electrophysiological responses. We examined the effects of a brief application of levamisole on Ascaris suum body muscle under current-clamp. The levamisole responses were characterized as an initial primary depolarization, followed by a slow secondary depolarizing response. We examined the effects of AF2 (KHEYLRFamide), 1 microM applied for 2 min. We found that AF2 potentiated the secondary response to levamisole and had no significant effect on the primary depolarization. Further, the reversal potentials observed during the secondary response suggested that more than one ion was involved in producing this potential. AF2 potentiated the secondary response in the presence of 30 microM mecamylamine suggesting the effect was independent of levamisole sensitive acetylcholine receptors. The secondary response, potentiated by AF2, appeared to be dependent on cytoplasmic events triggered by the primary depolarization. Ion-substitution experiments showed that the AF2 potentiated secondary response was dependent on extracellular calcium and chloride suggesting a role for the calcium-activated anion channel. Caffeine mimicked the AF2 potentiated secondary response and 0.1 microM ryanodine inhibited it. 1.0 microM ryanodine increased spiking showing that it affected membrane excitability. A model is proposed showing ryanodine receptors mediating effects of AF2 on levamisole responses.

The genome of the blood fluke Schistosoma mansoni

Schistosoma mansoni is responsible for the neglected tropical disease schistosomiasis that affects 210 million people in 76 countries. We report here analysis of the 363 megabase nuclear genome of the blood fluke. It encodes at least 11,809 genes, with an unusual intron size distribution, and novel families of micro-exon genes that undergo frequent alternate splicing. As the first sequenced flatworm, and a representative of the lophotrochozoa, it offers insights into early events in the evolution of the animals, including the development of a body pattern with bilateral symmetry, and the development of tissues into organs. Our analysis has been informed by the need to find new drug targets. The deficits in lipid metabolism that make schistosomes dependent on the host are revealed, while the identification of membrane receptors, ion channels and more than 300 proteases, provide new insights into the biology of the life cycle and novel targets. Bioinformatics approaches have identified metabolic chokepoints while a chemogenomic screen has pinpointed schistosome proteins for which existing drugs may be active. The information generated provides an invaluable resource for the research community to develop much needed new control tools for the treatment and eradication of this important and neglected disease.

Regulation of synaptic transmission by presynaptic CaMKII and BK channels

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the BK channel are enriched at the presynaptic nerve terminal, where CaMKII associates with synaptic vesicles whereas the BK channel colocalizes with voltage-sensitive Ca(2+) channels in the plasma membrane. Mounting evidence suggests that these two proteins play important roles in controlling neurotransmitter release. Presynaptic BK channels primarily serve as a negative regulator of neurotransmitter release. In contrast, presynaptic CaMKII either enhances or inhibits neurotransmitter release and synaptic plasticity depending on experimental or physiological conditions and properties of specific synapses. The different functions of presynaptic CaMKII appear to be mediated by distinct downstream proteins, including the BK channel.

Genetics of Egg-Laying in Worms

Genetic studies of behavior in the nematode Caenorhabditis elegans have provided an effective approach to investigate the molecular and cellular basis of nervous system function and development. Among the best studied behaviors is egg-laying, the process by which hermaphrodites deposit developing embryos into the environment. Egg-laying involves a simple motor program involving a small network of motorneurons and specialized smooth muscle cells, which is regulated by a variety of sensory stimuli. Analysis of egg-laying-defective mutants has provided insight into a number of conserved processes in nervous system development, including neurogenesis, cell migration, and synaptic patterning, as well as aspects of excitable cell signal transduction and neuromodulation.