Functions of the tubulin code in the C. elegans nervous system.

Due to their elongated and polarized morphology, neurons rely on the microtubule (MT) cytoskeleton for their shape, as well as for efficient intracellular transport that maintains neuronal function, survival, and connectivity. Although all MTs are constructed from α- and ß-tubulins that are highly conserved throughout eukaryotes, different MT networks within neurons exhibit different dynamics and functions. For example, molecular motors must be able to differentially recognize the axonal and dendritic MTs to deliver appropriate cargos to sensory endings and synaptic regions. The Tubulin Code hypothesis proposes that MTs can be specialized in form and function by chemical differences in their composition by inclusion of different α- and ß-tubulins into the MT lattice, as well as differences in post-translational enzymatic modifications. The chemical differences encode information that allow MTs to regulate interactions with various microtubule-based molecular motors such as kinesins and dyneins as well as with structural microtubule-associated proteins (MAPs), which can, in turn, modify the function or stability of MTs. Here, we review studies involving C. elegans, a model organism with a relatively simple nervous system that is amenable to genetic analysis, that have contributed to our understanding of how the Tubulin Code can specialize neuronal MT networks to establish differences in neuronal morphology and function. Such studies have revealed molecules and mechanisms that are conserved in vertebrates and have the potential to inform our understanding of neurological diseases involving defects in the cytoskeleton and intracellular transport.

Mutation of NEKL-4/NEK10 and TTLL genes suppress neuronal ciliary degeneration caused by loss of CCPP-1 deglutamylase function.

Ciliary microtubules are subject to post-translational modifications that act as a "Tubulin Code" to regulate motor traffic, binding proteins and stability. In humans, loss of CCP1, a cytosolic carboxypeptidase and tubulin deglutamylating enzyme, causes infantile-onset neurodegeneration. In C. elegans, mutations in ccpp-1, the homolog of CCP1, result in progressive degeneration of neuronal cilia and loss of neuronal function. To identify genes that regulate microtubule glutamylation and ciliary integrity, we performed a forward genetic screen for suppressors of ciliary degeneration in ccpp-1 mutants. We isolated the ttll-5(my38) suppressor, a mutation in a tubulin tyrosine ligase-like glutamylase gene. We show that mutation in the ttll-4, ttll-5, or ttll-11 gene suppressed the hyperglutamylation-induced loss of ciliary dye filling and kinesin-2 mislocalization in ccpp-1 cilia. We also identified the nekl-4(my31) suppressor, an allele affecting the NIMA (Never in Mitosis A)-related kinase NEKL-4/NEK10. In humans, NEK10 mutation causes bronchiectasis, an airway and mucociliary transport disorder caused by defective motile cilia. C. elegans NEKL-4 localizes to the ciliary base but does not localize to cilia, suggesting an indirect role in ciliary processes. This work defines a pathway in which glutamylation, a component of the Tubulin Code, is written by TTLL-4, TTLL-5, and TTLL-11; is erased by CCPP-1; is read by ciliary kinesins; and its downstream effects are modulated by NEKL-4 activity. Identification of regulators of microtubule glutamylation in diverse cellular contexts is important to the development of effective therapies for disorders characterized by changes in microtubule glutamylation. By identifying C. elegans genes important for neuronal and ciliary stability, our work may inform research into the roles of the tubulin code in human ciliopathies and neurodegenerative diseases.

The Use of Ultrasonic Communication to Maintain Social Cohesion in the Javan Slow Loris (Nycticebus javanicus).

Only a handful of primate taxa use ultrasonic vocalisations (those ≥20 kHz) to communicate. The extent and uses of ultrasonic communication remain poorly understood, potentially ranging from echolocation, advertisement of reproductive status and resource availability, social cohesion, to predator avoidance. Here, using active acoustics whereby the study subjects were observed throughout their activity period, we describe the first purely ultrasonic call from a strepsirrhine primate (family Lorisidae), recorded in a completely wild setting, and hypothesise about its function. We identified one type of ultrasonic call, the doublet click, from 14 Javan slow lorises (Nycticebus javanicus) produced by males and females of juvenile, subadult and adult ages within their social groups (n = 791, mean = 46.0 kHz). We ran quadratic discriminant function analysis, finding dominant frequency and doublet click duration as the key parameters for identifying individuals' sex and age. Significantly more vocalisations were produced during affiliative social behaviour, suggesting that the call serves a social cohesion function. Considering the range of other cryptic behaviours, including slow and silent locomotion, and the high degree of territoriality associated with venomous attacks on conspecifics, the call may also serve as a safety strategy, allowing family members to regulate distance from other slow lorises and to communicate cryptically whilst avoiding predators.

Mating behavior, male sensory cilia, and polycystins in Caenorhabditis elegans.

The investigation of Caenorhabditis elegans males and the male-specific sensory neurons required for mating behaviors has provided insight into the molecular function of polycystins and mechanisms that are needed for polycystin ciliary localization. In humans, polycystin 1 and polycystin 2 are needed for kidney function; loss of polycystin function leads to autosomal dominant polycystic kidney disease (ADPKD). Polycystins localize to cilia in C. elegans and mammals, a finding that has guided research into ADPKD. The discovery that the polycystins form ciliary receptors in male-specific neurons needed for mating behaviors has also helped to unlock insights into two additional exciting new areas: the secretion of extracellular vesicles; and mechanisms of ciliary specialization. First, we will summarize the studies done in C. elegans regarding the expression, localization, and function of the polycystin 1 and 2 homologs, LOV-1 and PKD-2, and discuss insights gained from this basic research. Molecules that are co-expressed with the polycystins in the male-specific neurons may identify evolutionarily conserved molecular mechanisms for polycystin function and localization. We will discuss the finding that polycystins are secreted in extracellular vesicles that evoke behavioral change in males, suggesting that such vesicles provide a novel form of communication to conspecifics in the environment. In humans, polycystin-containing extracellular vesicles are secreted in urine and can be taken up by cilia, and quickly internalized. Therefore, communication by polycystin-containing extracellular vesicles may also use mechanisms that are evolutionarily conserved from nematode to human. Lastly, different cilia display structural and functional differences that specialize them for particular tasks, despite the fact that virtually all cilia are built by a conserved intraflagellar transport (IFT) mechanism and share some basic structural features. Comparative analysis of the male-specific cilia with the well-studied cilia of the amphid and phasmid neurons has allowed identification of molecules that specialize the male cilia. We will discuss the molecules that shape the male-specific cilia. The cell biology of cilia in male-specific neurons demonstrates that C. elegans can provide an excellent model of ciliary specialization.

The DEG/ENaC protein MEC-10 regulates the transduction channel complex in Caenorhabditis elegans touch receptor neurons.

Gentle touch sensation in Caenorhabditis elegans is mediated by the MEC-4/MEC-10 channel complex, which is expressed exclusively in six touch receptor neurons (TRNs). The complex contains two pore-forming subunits, MEC-4 and MEC-10, as well as the accessory subunits MEC-2, MEC-6, and UNC-24. MEC-4 is essential for channel function, but beyond its role as a pore-forming subunit, the functional contribution of MEC-10 to the channel complex and to touch sensation is unclear. We addressed this question using behavioral assays, in vivo electrophysiological recordings from TRNs, and heterologous expression of mutant MEC-10 isoforms. Animals with a deletion in mec-10 showed only a partial loss of touch sensitivity and a modest decrease in the size of the mechanoreceptor current (MRC). In contrast, five previously identified mec-10 alleles acted as recessive gain-of-function alleles that resulted in complete touch insensitivity. Each of these alleles produced a substantial decrease in MRC size and a shift in the reversal potential in vivo. The latter finding indicates that these mec-10 mutations alter the ionic selectivity of the transduction channel in vivo. All mec-10 mutant animals had properly localized channel complexes, indicating that the loss of MRCs was not attributable to a dramatic mislocalization of transduction channels. Finally, electrophysiological examination of heterologously expressed complexes suggests that mutant MEC-10 proteins may affect channel current via MEC-2.

Fixation and Immunostaining of Endogenous Proteins or Post-translational Modificationsin Caenorhabditis elegans.

Although the advent of genetically-encoded fluorescent markers, such as the green fluorescent protein (GFP; Chalfie et al., 1994 ), has enabled convenient visualization of gene expression in vivo, this method is generally not effective for detecting post-translational modifications because they are not translated from DNA sequences. Genetically-encoded, fluorescently-tagged transgene products can also be misleading for observing expression patterns because transgenes may lack endogenous regulatory DNA elements needed for precise regulation of expression that could result in over or under expression. Fluorescently-tagged proteins created by CRISPR genome editing are less prone to defective expression patterns because the loci retain endogenous DNA elements that regulate their transcription (Nance and Frøkjær-Jensen, 2019). However, even CRISPR alleles encoding heritable fluorescently-tagged protein markers can result in defects in function or localization of the gene product if the fluorescent tag obstructs or otherwise interferes with important protein interaction domains or affects the protein structure. Indirect immunofluorescence is a method for detecting endogenous gene expression or post-translational modifications without the need for transgenesis or genome editing. Here, we present a reliable protocol in which C. elegans nematodes are fixed, preserved, and permeabilized for staining with a primary antibody to bind proteins or post-translational modifications, which are then labeled with a secondary antibody conjugated to a fluorescent dye. Use of this method may be limited by the availability of (or ability to generate) a primary antibody that binds the epitope of interest in fixed animals. Thousands of animals are simultaneously subjected to a series of chemical treatments and washes in a single centrifuge tube, allowing large numbers of identically-treated stained animals to be examined. We have successfully used this protocol (O' Hagan et al., 2011 and 2017; Power et al., 2020 ) to preserve and detect post-translational modifications of tubulin in C. elegans ciliated sensory neurons and to detect non-modified endogenous protein (Topalidou and Chalfie, 2011).

CCP1, a Tubulin Deglutamylase, Increases Survival of Rodent Spinal Cord Neurons following Glutamate-Induced Excitotoxicity.

Microtubules (MTs) are cytoskeletal elements that provide structural support and act as roadways for intracellular transport in cells. MTs are also needed for neurons to extend and maintain long axons and dendrites that establish connectivity to transmit information through the nervous system. Therefore, in neurons, the ability to independently regulate cytoskeletal stability and MT-based transport in different cellular compartments is essential. Posttranslational modification of MTs is one mechanism by which neurons regulate the cytoskeleton. The carboxypeptidase CCP1 negatively regulates posttranslational polyglutamylation of MTs. In mammals, loss of CCP1, and the resulting hyperglutamylation of MTs, causes neurodegeneration. It has also long been known that CCP1 expression is activated by neuronal injury; however, whether CCP1 plays a neuroprotective role after injury is unknown. Using shRNA-mediated knock-down of CCP1 in embryonic rat spinal cord cultures, we demonstrate that CCP1 protects spinal cord neurons from excitotoxic death. Unexpectedly, excitotoxic injury reduced CCP1 expression in our system. We previously demonstrated that the CCP1 homolog in Caenorhabditis elegans is important for maintenance of neuronal cilia. Although cilia enhance neuronal survival in some contexts, it is not yet clear whether CCP1 maintains cilia in mammalian spinal cord neurons. We found that knock-down of CCP1 did not result in loss or shortening of cilia in cultured spinal cord neurons, suggesting that its effect on survival of excitotoxicity is independent of cilia. Our results support the idea that enzyme regulators of MT polyglutamylation might be therapeutically targeted to prevent excitotoxic death after spinal cord injuries.

Phylogenetic conservation of the cell-type-specific Lan3-2 glycoepitope in Caenorhabditis elegans.

The biological function of a cell-type-specific glycosylation of an adhesion molecule belonging to the L1CAM immunoglobulin superfamily was previously determined in the nervous system of the embryonic leech, Hirudo medicinalis. The Lan3-2 glycoepitope is a surface marker of sensory afferent neurons and is required for their appropriate developmental collateral branching and synaptogenesis in the CNS. The chemical structure of the Lan3-2 glycoepitope consists of beta-(1,4)-linked mannopyranose. Here, we show the conservation of the cell-type-specific expression of this mannose polymer in Caenorhabditis elegans. The Lan3-2 glycoepitope is expressed on the cell surface of a subset of dissociated embryonic neurons and, in the adult worm, by the pharyngeal motor neuron, M5, and the chemosensory afferents, the amphids. Additionally, the vulval epithelium expresses the Lan3-2 glycoepitope in late L4 larvae and in adult hermaphrodites. To investigate proteins carrying this restrictively expressed glycoepitope, worm extract was immunoaffinity purified with Lan3-2 monoclonal antibody and Western blotted. A polyclonal antibody reactive with the cytoplasmic tail of LAD-1/SAX-7, a C. elegans member of the L1CAM family, recognizes a 270 kDa protein band while Lan3-2 antibody also recognizes a 190 kDa glycoform, its putative Lan3-2 ectodomain. Thus, in C. elegans, as in leech, the Lan3-2 epitope is located on a L1CAM homologue. The cell-type-specific expression of the Lan3-2 glycoepitope shared by leech and C. elegans will be useful for understanding how cell-type-specific glycoepitopes mediate cell-cell interactions during development.

The MEC-4 DEG/ENaC channel of Caenorhabditis elegans touch receptor neurons transduces mechanical signals.

Transformation of mechanical energy into ionic currents is essential for touch, hearing and nociception. Although DEG/ENaC proteins are believed to form sensory mechanotransduction channels, the evidence for this role remains indirect. By recording from C. elegans touch receptor neurons in vivo, we found that external force evokes rapidly activating mechanoreceptor currents (MRCs) carried mostly by Na(+) and blocked by amiloride-characteristics consistent with direct mechanical gating of a DEG/ENaC channel. Like mammalian Pacinian corpuscles, these neurons depolarized with both positive and negative changes in external force but not with sustained force. Null mutations in the DEG/ENaC gene mec-4 and in the accessory ion channel subunit genes mec-2 and mec-6 eliminated MRCs. In contrast, the genetic elimination of touch neuron-specific microtubules reduced, but did not abolish, MRCs. Our findings link the application of external force to the activation of a molecularly defined metazoan sensory transduction channel.

Glutamylation Regulates Transport, Specializes Function, and Sculpts the Structure of Cilia.

Ciliary microtubules (MTs) are extensively decorated with post-translational modifications (PTMs), such as glutamylation of tubulin tails. PTMs and tubulin isotype diversity act as a "tubulin code" that regulates cytoskeletal stability and the activity of MT-associated proteins such as kinesins. We previously showed that, in C. elegans cilia, the deglutamylase CCPP-1 affects ciliary ultrastructure, localization of the TRP channel PKD-2 and the kinesin-3 KLP-6, and velocity of the kinesin-2 OSM-3/KIF17, whereas a cell-specific α-tubulin isotype regulates ciliary ultrastructure, intraflagellar transport, and ciliary functions of extracellular vesicle (EV)-releasing neurons. Here we examine the role of PTMs and the tubulin code in the ciliary specialization of EV-releasing neurons using genetics, fluorescence microscopy, kymography, electron microscopy, and sensory behavioral assays. Although the C. elegans genome encodes five tubulin tyrosine ligase-like (TTLL) glutamylases, only ttll-11 specifically regulates PKD-2 localization in EV-releasing neurons. In EV-releasing cephalic male (CEM) cilia, TTLL-11 and the deglutamylase CCPP-1 regulate remodeling of 9+0 MT doublets into 18 singlet MTs. Balanced TTLL-11 and CCPP-1 activity fine-tunes glutamylation to control the velocity of the kinesin-2 OSM-3/KIF17 and kinesin-3 KLP-6 without affecting the intraflagellar transport (IFT) kinesin-II. TTLL-11 is transported by ciliary motors. TTLL-11 and CCPP-1 are also required for the ciliary function of releasing bioactive EVs, and TTLL-11 is itself a novel EV cargo. Therefore, MT glutamylation, as part of the tubulin code, controls ciliary specialization, ciliary motor-based transport, and ciliary EV release in a living animal. We suggest that cell-specific control of MT glutamylation may be a conserved mechanism to specialize the form and function of cilia.

Kymographic Analysis of Transport in an Individual Neuronal Sensory Cilium in Caenorhabditis elegans.

Intraflagellar Transport (IFT) is driven by molecular motors that travel upon microtubule-based ciliary axonemes. In the single-celled alga Chlamydomonas reinhardtii, movement of a single anterograde IFT motor, heterotrimeric kinesin-II, is required to generate two identical motile flagella. The function of this canonical anterograde IFT motor is conserved among all eukaryotes, yet multicellular organisms can generate cilia of diverse structures and functions, ranging from simple threadlike non-motile primary cilia to the elaborate cilia that make up rod and cone photoreceptors in the retina. An emerging theme is that additional molecular motors modulate the canonical IFT machinery to give rise to differing ciliary morphologies. Therefore, a complete understanding of the trafficking of ciliary receptors, as well as the biogenesis, maintenance, specialization, and function of cilia, requires the characterization of motor molecules.Here, we describe in detail our method for measuring the motility of proteins in cilia or dendrites of C. elegans male-specific CEM ciliated sensory neurons using time-lapse microscopy and kymography of green fluorescent protein (GFP)-tagged motors, receptors, and cargos. We describe, as a specific example, OSM-3::GFP puncta moving in cilia, but also include (Fig. 1) with settings that have worked well for us measuring movement of heterotrimeric kinesin-II, IFT particles, and the polycystin TRP channel PKD-2.

MEC-10 and MEC-19 Reduce the Neurotoxicity of the MEC-4(d) DEG/ENaC Channel in Caenorhabditis elegans.

The Caenorhabditis elegans DEG/ENaC proteins MEC-4 and MEC-10 transduce gentle touch in the six touch receptor neurons . Gain-of-function mutations of mec-4 and mec-4(d) result in a hyperactive channel and neurodegeneration in vivo Loss of MEC-6, a putative DEG/ENaC-specific chaperone, and of the similar protein POML-1 suppresses the neurodegeneration caused by a mec-4(d) mutation. We find that mutation of two genes, mec-10 and a new gene mec-19 (previously named C49G9.1), prevents this action of POML-1, allowing the touch receptor neurons to die in poml-1 mec-4(d) animals. The proteins encoded by these genes normally inhibit mec-4(d) neurotoxicity through different mechanisms. MEC-10, a subunit of the mechanosensory transduction channel with MEC-4, inhibits MEC-4(d) activity without affecting MEC-4 expression. In contrast, MEC-19, a membrane protein specific to nematodes, inhibits MEC-4(d) activity and reduces MEC-4 surface expression.

Caenorhabditis elegans paraoxonase-like proteins control the functional expression of DEG/ENaC mechanosensory proteins.

Caenorhabditis eleganssenses gentle touch via a mechanotransduction channel formed from the DEG/ENaC proteins MEC-4 and MEC-10. An additional protein, the paraoxonase-like protein MEC-6, is essential for transduction, and previous work suggested that MEC-6 was part of the transduction complex. We found that MEC-6 and a similar protein, POML-1, reside primarily in the endoplasmic reticulum and do not colocalize with MEC-4 on the plasma membrane in vivo. As with MEC-6, POML-1 is needed for touch sensitivity, the neurodegeneration caused by themec-4(d)mutation, and the expression and distribution of MEC-4 in vivo. Both proteins are likely needed for the proper folding or assembly of MEC-4 channels in vivo as measured by FRET. MEC-6 detectably increases the rate of MEC-4 accumulation on theXenopusoocyte plasma membrane. These results suggest that MEC-6 and POML-1 interact with MEC-4 to facilitate expression and localization of MEC-4 on the cell surface. Thus MEC-6 and POML-1 act more like chaperones for MEC-4 than channel components.

The tubulin repertoire of C. elegans sensory neurons and its context-dependent role in process outgrowth.

Microtubules contribute to many cellular processes, including transport, signaling, and chromosome separation during cell division (Kapitein and Hoogenraad, 2015). They are comprised of αß-tubulin heterodimers arranged into linear protofilaments and assembled into tubes. Eukaryotes express multiple tubulin isoforms (Gogonea et al., 1999), and there has been a longstanding debate as to whether the isoforms are redundant or perform specialized roles as part of a tubulin code (Fulton and Simpson, 1976). Here, we use the well-characterized touch receptor neurons (TRNs) of Caenorhabditis elegans to investigate this question, through genetic dissection of process outgrowth both in vivo and in vitro With single-cell RNA-seq, we compare transcription profiles for TRNs with those of two other sensory neurons, and present evidence that each sensory neuron expresses a distinct palette of tubulin genes. In the TRNs, we analyze process outgrowth and show that four tubulins (tba-1, tba-2, tbb-1, and tbb-2) function partially or fully redundantly, while two others (mec-7 and mec-12) perform specialized, context-dependent roles. Our findings support a model in which sensory neurons express overlapping subsets of tubulin genes whose functional redundancy varies between cell types and in vivo and in vitro contexts.

A motor relay on ciliary tracks.

A powerful combination of two-colour imaging in vivo, Fourier-filtered kymography and simulations provides a high-resolution view of kinesin-2 transport dynamics in cilia. This study reveals heterotrimeric kinesin-II as an 'obstacle-course runner' and homodimeric OSM-3 (KIF17) as a 'long-distance runner', and elucidates the 'baton handoff' between these two kinesin-2 motors on the microtubule track.

Regulation of tubulin glutamylation plays cell-specific roles in the function and stability of sensory cilia.

Microtubules (MTs) are post-translationally modified, but the functions of post-translational modifications (PTMs) have in many cases remained unknown. Most PTMs, such as polyglutamylation, occur on the protruding C-terminal tail (CTT) of tubulins, are reversible, and have been proposed to play a role in regulation of MT-associated proteins (MAPs), molecular motors, and MT-severing proteins. Several PTM enzymes have been identified, including a carboxypeptidase in mice known as CCP1, which reduces polyglutamylation on the CTT of MTs, and causes cell-specific neurodegeneration when mutated.

The tubulin deglutamylase CCPP-1 regulates the function and stability of sensory cilia in C. elegans.

BACKGROUND: Posttranslational modifications (PTMs) such as acetylation, detyrosination, and polyglutamylation have long been considered markers of stable microtubules and have recently been proposed to guide molecular motors to specific subcellular destinations. Microtubules can be deglutamylated by the cytosolic carboxypeptidase CCP1. Loss of CCP1 in mice causes cerebellar Purkinje cell degeneration. Cilia, which are conserved organelles that play important diverse roles in animal development and sensation, contain axonemes comprising microtubules that are especially prone to PTMs. RESULTS: Here, we report that a CCP1 homolog, CCPP-1, regulates the ciliary localization of the kinesin-3 KLP-6 and the polycystin PKD-2 in male-specific sensory neurons in C. elegans. In male-specific CEM (cephalic sensilla, male) cilia, ccpp-1 also controls the velocity of the kinesin-2 OSM-3/KIF17 without affecting the transport of kinesin-II cargo. In the core ciliated nervous system of both males and hermaphrodites, loss of ccpp-1 causes progressive defects in amphid and phasmid sensory cilia, suggesting that CCPP-1 activity is required for ciliary maintenance but not ciliogenesis. Affected cilia exhibit defective B-tubules. Loss of TTLL-4, a polyglutamylating enzyme of the tubulin tyrosine ligase-like family, suppresses progressive ciliary defects in ccpp-1 mutants. CONCLUSIONS: Our studies suggest that CCPP-1 acts as a tubulin deglutamylase that regulates the localization and velocity of kinesin motors and the structural integrity of microtubules in sensory cilia of a multicellular, living animal. We propose that the neuronal degeneration caused by loss of CCP1 in mammals may represent a novel ciliopathy in which cilia are formed but not maintained, depriving the cell of cilia-based signal transduction.

The multipurpose 15-protofilament microtubules in C. elegans have specific roles in mechanosensation.

Because microtubules perform many essential functions in neurons, delineating unique roles attributable to these organelles presents a formidable challenge. Microtubules endow neurons with shape and structure and are required for developmental processes including neurite outgrowth, intracellular transport, and synapse formation and plasticity; microtubules in sensory neurons may be required for the above processes in addition to a specific sensory function. In Caenorhabditis elegans, six touch receptor neurons (TRNs) sense gentle touch and uniquely contain 15-protofilament microtubules. Disruption of these microtubules by loss of either the MEC-7 beta-tubulin or MEC-12 alpha-tubulin or by growth in 1 mM colchicine causes touch insensitivity, altered distribution of the touch transduction channel, and a general reduction in protein levels. We show that the effect on touch sensitivity can be separated from the others; microtubule depolymerization in mature TRNs causes touch insensitivity but does not result in protein distribution and production defects. In addition, the mec-12(e1605) mutation selectively causes touch insensitivity without affecting microtubule formation and other cellular processes. Touching e1605 animals produces a reduced mechanoreceptor current that inactivates more rapidly than in wild-type, suggesting a specific role of the microtubules in mechanotransduction.

Vocal communication between male Xenopus laevis.

This study focuses on the role of male-male vocal communication in the reproductive repertoire of the South African clawed frog, Xenopus laevis. Six male and two female call types were recorded from native ponds in the environs of Cape Town, South Africa. These include all call types previously recorded in the laboratory as well as one previously unidentified male call: chirping. The amount of calling and the number of call types increased as the breeding season progressed. Laboratory recordings indicated that all six male call types were directed to males; three of these were directed to both sexes and three were directed exclusively to males. Both female call types were directed exclusively to males. The predominant call type, in both field and laboratory recordings, was the male advertisement call. Sexual state affected male vocal behaviour. Male pairs in which at least one male was sexually active (gonadotropin injected) produced all call types, whereas pairs of uninjected males rarely called. Some call types were strongly associated with a specific behaviour and others were not. Clasped males always growled and clasping males typically produced amplectant calls or chirps; males not engaged in clasping most frequently advertised. The amount of advertising produced by one male was profoundly affected by the presence of another male. Pairing two sexually active males resulted in suppression of advertisement calling in one; suppression was released when males were isolated after pairing. Vocal dominance was achieved even in the absence of physical contact (clasping). We suggest that X. laevis males gain a reproductive advantage by competing for advertisement privileges and by vocally suppressing neighbouring males.

MEC-2 regulates C. elegans DEG/ENaC channels needed for mechanosensation.

Touch sensitivity in animals relies on nerve endings in the skin that convert mechanical force into electrical signals. In the nematode Caenorhabditis elegans, gentle touch to the body wall is sensed by six mechanosensory neurons that express two amiloride-sensitive Na+ channel proteins (DEG/ENaC). These proteins, MEC-4 and MEC-10, are required for touch sensation and can mutate to cause neuronal degeneration. Here we show that these mutant or 'd' forms of MEC-4 and MEC-10 produce a constitutively active, amiloride-sensitive ionic current when co-expressed in Xenopus oocytes, but not on their own. MEC-2, a stomatin-related protein needed for touch sensitivity, increased the activity of mutant channels about 40-fold and allowed currents to be detected with wild-type MEC-4 and MEC-10. Whereas neither the central, stomatin-like domain of MEC-2 nor human stomatin retained the activity of full-length MEC-2, both produced amiloride-sensitive currents with MEC-4d. Our findings indicate that MEC-2 regulates MEC-4/MEC-10 ion channels and raise the possibility that similar ion channels may be formed by stomatin-like proteins and DEG/ENaC proteins that are co-expressed in both vertebrates and invertebrates. Some of these channels may mediate mechanosensory responses.