Chemoreceptor-mediated polymerization and depolymerization of actin in hair bundles of sea anemones

Divergence in metabolomic profile in clownfish and damselfish skin mucus

Introduction

The clownfish - sea anemone mutualism was suggested to have triggered the adaptive radiation of clownfishes, but the origin of clownfish resistance to stinging tentacles of host anemones remains unclear. The presence of specific compounds in the mucus of clownfishes conferring them the unique ability to prevent nematocyst discharge from their hosts has been the most supported hypothesis. Yet the mystery regarding the types of compounds found in clownfish skin mucus remains unsolved.

Methods

We analyzed the chemical composition of clownfish and damselfish mucus using an untargeted metabolomics (HILIC-HRMS) and lipidomics (RPLC-HRMS) approach.

Results and Discussion

The polar and lipid metabolome signatures were highly specific and allowed to discriminate between the clownfish and damselfish clades. The most discriminative part of the signature was the sphingolipid profile, displaying a broader diversity of ceramides present in significantly higher levels in clownfish mucus. Importantly, the inter-specific variability of metabolic signature was significantly higher in clownfishes, although their diversification is evolutionarily more recent, thus implying the impact of symbiosis on metabolic variability and adaptation. Furthermore, specialists and generalists clownfish species displayed distinctive metabolite signature. Two strict clownfish specialists, which are phylogenetically distant but share the same host species, clustered together based on their molecular signature, suggesting a link with their mutualistic nature. Overall, comparative analyses of metabolic signatures highlight differences in chemical composition of clownfish mucus and provide insight into biochemical pathways potentially implicated in clownfish adaptation to inhabit sea anemones and consequently diversify.

Cnidarian hair cell development illuminates an ancient role for the class IV POU transcription factor in defining mechanoreceptor identity

Although specialized mechanosensory cells are found across animal phylogeny, early evolutionary histories of mechanoreceptor development remain enigmatic. Cnidaria (e.g. sea anemones and jellyfishes) is the sister group to well-studied Bilateria (e.g. flies and vertebrates), and has two mechanosensory cell types – a lineage-specific sensory effector known as the cnidocyte, and a classical mechanosensory neuron referred to as the hair cell. While developmental genetics of cnidocytes is increasingly understood, genes essential for cnidarian hair cell development are unknown. Here, we show that the class IV POU homeodomain transcription factor (POU-IV) – an indispensable regulator of mechanosensory cell differentiation in Bilateria and cnidocyte differentiation in Cnidaria – controls hair cell development in the sea anemone cnidarian Nematostella vectensis. N. vectensis POU-IV is postmitotically expressed in tentacular hair cells, and is necessary for development of the apical mechanosensory apparatus, but not of neurites, in hair cells. Moreover, it binds to deeply conserved DNA recognition elements, and turns on a unique set of effector genes – including the transmembrane receptor-encoding gene polycystin 1 – specifically in hair cells. Our results suggest that POU-IV directs differentiation of cnidarian hair cells and cnidocytes via distinct gene regulatory mechanisms, and support an evolutionarily ancient role for POU-IV in defining the mature state of mechanosensory neurons.

Sialic acid and biology of life: An introduction

Sialic acids are important molecule with high structural diversity. They are known to occur in higher animals such as Echinoderms, Hemichordata, Cephalochorda, and Vertebrata and also in other animals such as Platyhelminthes, Cephalopoda, and Crustaceae. Plants are known to lack sialic acid. But they are reported to occur in viruses, bacteria, protozoa, and fungi. Deaminated neuraminic acid although occurs in vertebrates and bacteria, is reported to occur in abundance in the lower vertebrates. Sialic acids are mostly located in terminal ends of glycoproteins and glycolipids, capsular and tissue polysialic acids, bacterial lipooligosaccharides/polysaccharides, and in different forms that dictate their role in biology. Sialic acid play important roles in human physiology of cell-cell interaction, communication, cell-cell signaling, carbohydrate-protein interactions, cellular aggregation, development processes, immune reactions, reproduction, and in neurobiology and human diseases in enabling the infection process by bacteria and virus, tumor growth and metastasis, microbiome biology, and pathology. It enables molecular mimicry in pathogens that allows them to escape host immune responses. Recently sialic acid has found role in therapeutics. In this chapter we have highlighted the (i) diversity of sialic acid, (ii) their occurrence in the diverse life forms, (iii) sialylation and disease, and (iv) sialic acid and therapeutics.

Rho-family G-proteins are required for the recovery of traumatized hair bundle mechanoreceptors in the sea anemone, Nematostella vectensis

Immersing anemones in calcium-free seawater disorganizes hair bundle mechanoreceptors on tentacles of sea anemones while causing a loss of vibration sensitivity. Remarkably, anemone hair bundles recover after being returned to calcium-containing seawater. Reorganization of actin in stereocilia likely follows during the recovery of normal morphology of hair bundles after such immersion. Previous studies have reported that Rho G-proteins are located in the stereocilia of hair bundles in sea anemones where they participate in polymerizing actin in stereocilia upon activation of specific chemoreceptors. We here find that immersing anemones in calcium-free seawater significantly reduces the abundance of hair bundles. A partial recovery of abundance of hair bundles occurs within 3 h post-immersion, but a full recovery of abundance does not occur even 6 h after specimens are returned to calcium-containing seawater. Anemones recovering from immersion in calcium-free seawater feature hair bundles that are significantly wider at their tips than in controls. The hair bundles subsequently narrow at their tips, becoming comparable to those of untreated controls within 6 h. Stereocilia of hair bundles are significantly longer in experimental animals than in controls at 2 h of recovery before shortening to lengths comparable to untreated controls at 6 h. In the presence of Rho inhibitors, the recovery in abundance of hair bundles through 6 h is delayed or inhibited. Likewise, in the presence of Rho inhibitors, stereocilia fail to significantly elongate within 2 h of recovery. These data suggest that Rho G-proteins participate in the normal recovery of abundance and recovery of normal morphology of experimentally damaged hair bundle mechanoreceptors.

Periodic, moderate water flow reversibly increases hair bundle density and size in Nematostella vectensis

Animals employ hair bundles on hair cells to detect flow, vibrations and gravity. Hair bundles on sea anemone tentacles detect nearby vibrations in the water column produced by prey movements and then regulate discharge of cnidae to capture prey. This study investigated: (1) the progressive effects of periodic water flow on hair bundle morphology and density of hair bundles and cnidae in sea anemones, (2) the reversibility of the flow response and (3) the ability of the response to be expedited with increased flow duration. Linear density of hair bundles along tentacles and each hair bundle's dimensions was measured in anemones exposed to flow and in the absence of flow. With increasing numbers of days of flow, hair bundles in anemones exposed to flow for 1 h every weekday for 20 days increased in density and grew longer and wider at bases and middles, whereas controls did not. Time courses fit to a linear function exhibited significantly larger positive slopes from animals exposed to flow compared with controls. Hair bundles in anemones exposed to flow for 3 h each day increased in linear density, length, base width and middle width after 10 days of flow and returned to control levels after 10 days following cessation of flow. In addition, there was a trend for an increase in density of cnidae with flow. Therefore, anemone hair bundles are dynamically and reversibly modified by periodic, moderate flow to become more abundant and robust. These findings may have relevance to hair cells in acoustico-lateralis systems of higher animals.

Cadherin-23 may be dynamic in hair bundles of the model sea anemone Nematostella vectensis

Cadherin 23 (CDH23), a component of tip links in hair cells of vertebrate animals, is essential to mechanotransduction by hair cells in the inner ear. A homolog of CDH23 occurs in hair bundles of sea anemones. Anemone hair bundles are located on the tentacles where they detect the swimming movements of nearby prey. The anemone CDH23 is predicted to be a large polypeptide featuring a short exoplasmic C-terminal domain that is unique to sea anemones. Experimentally masking this domain with antibodies or mimicking this domain with free peptide rapidly disrupts mechanotransduction and morphology of anemone hair bundles. The loss of normal morphology is accompanied, or followed by a decrease in F-actin in stereocilia of the hair bundles. These effects were observed at very low concentrations of the reagents, 0.1-10 nM, and within minutes of exposure. The results presented herein suggest that: (1) the interaction between CDH23 and molecular partners on stereocilia of hair bundles is dynamic and; (2) the interaction is crucial for normal mechanotransduction and morphology of hair bundles.

Force-dependent discharge of nematocysts in the sea anemone Haliplanella luciae (Verrill)

Sea anemones discharge cnidae ('stinging capsules' including nematocysts) to capture prey and to defend themselves. In the present study, we tested the relationship between the force of test probes striking feeding tentacles and discharge of microbasic p-mastigophore nematocysts into the test probes. In seawater alone, the response curve is bimodal with maximal discharge observed at 0.33 and 1.10 millinewtons (mN) and with minimal discharge at 1.50 mN. Upon activating chemoreceptors for N-acetylated sugars, maximal discharge is observed across a broad range of smaller forces from 0.16 to 0.9 mN before decreasing to a minimum at 1.50 mN. Likewise, in the presence of nearby vibrations at key frequencies, maximal discharge is observed over a broad range of smaller forces before decreasing to a minimum at 1.50 mN. It appears that sensory input indicating proximity of potential prey expands the range of small forces of impact that stimulate maximal discharge (i.e. to less than 1.10 mN) but not at larger forces of impact (i.e. at approximately 1.50 mN). Thus, contact by small prey would stimulate maximal discharge, and all the more so if such contact is accompanied by specific odorants or by vibrations at specific frequencies. Nevertheless, anemones would not maximally discharge nematocysts into large animals that blunder into contact with their tentacles.

Nematocytes' activation in Pelagia noctiluca (Cnidaria, Scyphozoa) oral arms

Nematocytes' discharge is triggered to perform both defense and predation strategies in cnidarians and occurs under chemico-physical stimulation. In this study, different compounds such as amino acids and proteins (mucin, albumin, poly-L: -lysine, trypsin), sugars and N-acetylate sugars (N-acetyl neuraminic acid, N-acetyl galactosamine, sucrose, glucose, agarose and trehalose), nucleotides (ATP and cAMP), were tested as chemosensitizers of nematocyte discharge in the oral arms of the scyphozoan Pelagia noctiluca, particularly abundant in the Strait of Messina (Italy). Excised oral arms were submitted to a combined chemico-physical stimulation by treatment with different compounds followed by mechanical stimulation by a non-vibrating test probe. Discharge induced by a chemico-physical stimulation was more significant than that obtained after mechanical stimulation alone. A chemosensitizing mechanism, with a dose-dependent effect, was observed after treatment with sugars, amino compounds such as glutathione, nucleotides and mucin, according to that already seen in sea anemones. Such findings suggest that, though Anthozoa and Scyphozoa exhibit different divergence times during the evolutionary process, the discharge activation exhibits common features, probably derived from their last common ancestor.

Cnidocyte discharge is regulated by light and opsin-mediated phototransduction

BACKGROUND

Cnidocytes, the eponymous cell type of the Cnidaria, facilitate both sensory and secretory functions and are among the most complex animal cell types known. In addition to their structural complexity, cnidocytes display complex sensory attributes, integrating both chemical and mechanical cues from the environment into their discharge behavior. Despite more than a century of work aimed at understanding the sensory biology of cnidocytes, the specific sensory receptor genes that regulate their function remain unknown.

RESULTS

Here we report that light also regulates cnidocyte function. We show that non-cnidocyte neurons located in battery complexes of the freshwater polyp Hydra magnipapillata specifically express opsin, cyclic nucleotide gated (CNG) ion channel and arrestin, which are all known components of bilaterian phototransduction cascades. We infer from behavioral trials that different light intensities elicit significant effects on cnidocyte discharge propensity. Harpoon-like stenotele cnidocytes show a pronounced diminution of discharge behavior under bright light conditions as compared to dim light. Further, we show that suppression of firing by bright light is ablated by cis-diltiazem, a specific inhibitor of CNG ion channels.

CONCLUSIONS

Our results implicate an ancient opsin-mediated phototransduction pathway and a previously unknown layer of sensory complexity in the control of cnidocyte discharge. These findings also suggest a molecular mechanism for the regulation of other cnidarian behaviors that involve both photosensitivity and cnidocyte function, including diurnal feeding repertoires and/or substrate-based locomotion. More broadly, our findings highlight one novel, non-visual function for opsin-mediated phototransduction in a cnidarian, the origins of which might have preceded the evolution of cnidarian eyes.

Evidence for involvement of TRPA1 in the detection of vibrations by hair bundle mechanoreceptors in sea anemones

A homolog of TRPA1 was identified in the genome of the anemone, Nematostella vectensis (nv-TRPA1a), and predicted to possess six ankyrin repeat domains at the N-terminus and an ion channel domain near the C-terminus. Transmembrane segments of the ion channel domain are well conserved among several known TRPA1 polypeptides. Inhibitors of TRPA1 including ruthenium red decrease vibration-dependent discharge of nematocysts in N. vectensis and Haliplanella luciae. Activators of TRPA1 including URB-597 and polygodial increase nematocyst discharge in the absence of vibrations. Co-immunoprecipitation yields a band on SDS-PAGE gels at the predicted mass of the nv-TRPA1a polypeptide among other bands. Co-immunoprecipitation performed in the presence of antigenic peptide decreases the yield of this and several other polypeptides. In untreated controls, anti-nv-TRPA1a primarily labels the base of the hair bundle with some labeling also distributed along the length of stereocilia. Tissue immunolabeled in the presence of the antigenic peptide exhibits reduced labeling. Activating chemoreceptors for N-acetylated sugars induce immunolabel to distribute distally in stereocilia. In anemones, activating chemoreceptors for N-acetylated sugars induce hair bundles to elongate among several other structural and functional changes. Taken together, these results are consistent with the possibility that nv-TRPA1a participates in signal transduction of anemone hair bundles.

Cloning and functional expression of voltage-gated ion channel subunits from cnidocytes of the Portuguese Man O'War Physalia physalis

Cnidocytes were dissociated from the tentacles of the Portuguese Man O'War Physalia physalis using heat treatment, and purified using density centrifugation. Visual observation confirmed that these cnidocytes contained a nucleus, a cnidocyst and an apical stereocilium, confirming that the cells were intact. A cnidocyte-specific amplified cDNA library was then prepared using RNA isolated from the cnidocytes, and screened for voltage-gated ion channel subunits using conventional molecular cloning techniques. A variety of channel proteins were identified and full-length sequence obtained for two of them, a Ca(2+) channel beta subunit (PpCa(V)beta) and a Shaker-like K(+) channel (PpK(V)1). The location of the transcripts was confirmed by RT-PCR of total RNA isolated from individually selected and rinsed cnidocytes. The functional properties of these two channel proteins were characterized electrophysiologically using heterologous expression. PpCa(V)beta modulates currents carried by both cnidarian and mammalian alpha(1) subunits although the specifics of the modulation differ. PpK(V)1 produces fast transient outward currents that have properties typical of other Shaker channels. The possible role of these channel proteins in the behavior of cnidocytes is discussed.

A randomized, controlled field trial for the prevention of jellyfish stings with a topical sting inhibitor

BACKGROUND

Jellyfish stings are a common occurrence among ocean goers worldwide with an estimated 150 million envenomations annually. Fatalities and hospitalizations occur annually, particularly in the Indo-Pacific regions. A new topical jellyfish sting inhibitor based on the mucous coating of the clown fish prevents 85% of jellyfish stings in laboratory settings. The field effectiveness is unknown. The objective is to evaluate the field efficacy of the jellyfish sting inhibitor, Safe Sea.

METHODS

A double-blind, randomized, placebo-controlled trial occurred at the Dry Tortugas National Park, FL, USA and Sapodilla Cayes, Belize. Participants were healthy volunteers planning to snorkel for 30 to 45 minutes. Ten minutes prior to swimming, each participant was directly observed applying a blinded sample of Safe Sea (Nidaria Technology Ltd, Jordan Valley, Israel) to one side of their body and a blinded sample of Coppertone (Schering-Plough, Kenilworth, NJ, USA) to the contralateral side as placebo control. Masked 26 g samples of both Safe Sea SPF15 and Coppertone SPF15 were provided in identical containers to achieve 2 mg/cm(2) coverage. Sides were randomly chosen by participants. The incidence of jellyfish stings was the main outcome measure. This was assessed by participant interview and examination as subjects exited the water.

RESULTS

A total of 82 observed water exposures occurred. Thirteen jellyfish stings occurred during the study period for a 16% incidence. Eleven jellyfish stings occurred with placebo, two with the sting inhibitor, resulting in a relative risk reduction of 82% (95% confidence interval: 21%-96%; p= 0.02). No seabather's eruption or side effects occurred.

CONCLUSIONS

Safe Sea is a topical barrier cream effective at preventing >80% jellyfish stings under real-world conditions.

Developmental and evolutionary aspects of the basic helix–loop–helix transcription factors Atonal-like 1 and Achaete-scute homolog 2 in the jellyfish

The close functional link of nerve and muscle cells in neuromuscular units has led to the hypothesis of a common evolutionary origin of both cell types. Jellyfish are well suited to evaluate this theory since they represent the most basal extant organisms featuring both striated muscle and a nervous system. Here we describe the structure and expression of two novel genes for basic helix-loop-helix (bHLH) transcription factors, the Achaete-scute B family member Ash2 and the Atonal-like gene Atl1, in the hydrozoan jellyfish Podocoryne carnea. Ash2 is expressed exclusively in larval and adult endoderm cells and may be involved in differentiation of secretory cells. Atl1 expression is more widespread and includes the developing striated muscle as well as mechanosensory and nerve cell precursors in the medusa tentacles. Moreover, Atl1 expression is upregulated in proliferating nerve cell precursors arising from adult striated muscle cells by transdifferentiation in vitro. Likewise, the neuronal marker gene NP coding for the RFamide neuropeptide is expressed not only in mature nerve cells but also transiently in the developing muscle. The molecular evidence is concurrent to the hypothesis that muscle and nerve cells are closely linked in evolution and derive from a common myoepithelial precursor.

AA. The behavioral and developmental physiology of nematocysts

Nematocysts are the nonliving secretions of specialized cells, the nematocytes, which develop from multipotent stem cells. Nematocysts are the means by which coelenterates capture prey and defend against predation. The 25 or more known types of nematocysts can be divided into to four functional categories: those that pierce, ensnare, or adhere to prey, and those that adhere to the substrate. During development a collagenous cyst, which may contain toxins, forms; a hollow thread, which becomes coiled as it invaginates, develops. Maturing nematocyte–nematocyst complexes migrate to their discharge sites and are deployed in specific patterns. The mechanisms of pattern determination are not clear. Discharge of nematocysts appears to involve increases in intracapsular osmotic pressure consequent upon release of bound calcium within the capsule; the eversion of the filament may depend upon release of structural tension consequent upon a loss of zinc from the thread. Evidence exists that discharge is initiated as a calcium-dependent exocytosis, triggered by an electrical signal resulting from the transduction of mechanical stimuli received at the nematocyte's cnidocil. Chemical signals transduced in adjacent sensory cells alter the frequency response of the nematocyte. In opposition to the nematocyte–nematocyst independent effector hypothesis, excitatory and inhibitory neuronal input appears to regulate discharge.

Hair bundle motility induced by chemoreceptors in anemones

Most hair bundles are essentially fixed with respect to frequency specificity. However, hair bundles in sea anemones are dynamically tuned by actin-dependent changes in length. Tuning to low frequencies is accomplished by activation of chemoreceptors to N-acetylated sugars resulting in hair bundle elongation. We report here that following sugar-induced tuning of hair bundles, membrane currents reverse polarity in hair cells during unidirectional mechanical stimulation. Reversal in polarity of currents with sugar stimulation is inhibited if hair bundle elongation is blocked by pretreatment with cytochalasin D. A re-examination of morphological changes to hair bundles reveals a sugar-induced reorientation of stereocilia in addition to elongation with chemosensitization. In controls, hair bundles are noticeably twisted. With sugar stimulation stereocilia become oriented more parallel to the long axis of the hair bundle. This sugar-induced change in orientation is similarly inhibited by cytochalasin D pretreatment. Based on these results, we present a model wherein anemone hair bundle twisting serves as a built-in safety mechanism to preserve linkages likely to be subjected to potentially damaging tension during tuning. The twisted hair bundles can untwist while elongating to relieve excessive tension on extracellular linkages between stereocilia critical to mechanosensitivity.

Immunological evidence that anemone repair proteins include replacement linkages

In response to damage to hair bundles caused by exposure to calcium free buffers, sea anemones secrete large protein complexes named 'repair proteins' that rapidly restore structural integrity and function to hair bundles. A specific chromatographic fraction of the repair protein mixture, named 'fraction beta', has biological activity comparable to the complete repair protein mixture (Watson et al., 1998, Hear. Res. 115, 119-128). In this study, we find that polyclonal antibodies raised against deglycosylated fraction beta specifically bind fraction beta on Western blots. Anti-fraction beta delays the normal recovery of vibration sensitivity in experimental animals (i.e., those with hair bundles damaged by calcium free buffers). Moreover, anti-fraction beta disrupts vibration sensitivity in control animals (i.e., those with healthy hair bundles). Experimentally damaged hair bundles subsequently exposed to repair protein and then processed for immunoelectron microscopy show labeled linkages interconnecting stereocilia of the hair bundle. Immunofluorescence microscopy confirms strong labeling of hair bundles treated with repair proteins and only weak labeling of tips of hair bundles from control animals. Immunofluorescence microscopy indicates stores of repair proteins in gland cells of the body column in control animals and in gland cells of the mouth in experimental animals. Repair biological activity is confirmed in column purified homogenates of these tissues. Apparently repair proteins are delivered to damaged hair bundles in mucus carried by beating cilia.

A comparison of hair bundle mechanoreceptors in sea anemones and vertebrate systems

Hair bundle mechanoreceptors of the acousticolateralis system of vertebrates are similar to hair bundles found on tentacles of sea anemones, primitive marine invertebrates. In each case, hair bundles consist of actin-based stereocilia interconnected by extracellular linkages. Recently, considerable attention has been directed to one class of linkages called "tip links." Tip links interconnect the tip of one stereocilium to the adjacent, taller stereocilium. According to the currently favored hypothesis for signal transduction, tip links may be "gating springs" that gate cation channels opened during deflection of the hair bundle. To depolarize the membrane, deflections must be directed so as to induce strain on the tip links. Deflections in the opposite direction lead to hyperpolarization of the membrane. Hair cells adapt to prolonged deflection of hair bundles. Whereas in some vertebrates, adaptation is incomplete (i.e., the current fails to return to baseline), adaptation in anemones appears to be complete. Signal transduction is reversibly blocked by agents thought to interact with the transduction channel including streptomycin. In addition, signal transduction is abolished following exposure to agents thought to attack tip links including calcium-depleted buffers or elastase. Hair cells of lower vertebrates can be replaced by division and differentiation of supporting cells. In chickens, a repair system exists wherein tip links are replaced via a mechanism that does not involve protein synthesis. The repair mechanism of anemones involves synthesis of new proteins that may constitute replacement linkages and accessory proteins that attach the linkages to appropriate integral proteins.

Repair of hair bundles in sea anemones by secreted proteins

Sea anemones are sessile invertebrates that detect movements of prey using numerous hair bundles located on tentacles surrounding their mouth. Previously we found that hair bundles of anemones are structurally and functionally similar to those of vertebrates. After 10-15 min exposure to calcium depleted buffers, hair bundles in chickens suffer moderate damage from which they recover in 12 h without requiring new protein synthesis [Zhao, Yamoah and Gillespie, Proc. Natl. Acad. Sci. USA 94 (1996) 15469-15474]. We find that after 1 h exposure to calcium free seawater, hair bundles of anemones suffer extensive damage from which they recover in 4 h, apparently because of newly synthesized, secretory proteins called 'repair proteins'. Recovery is delayed in a dose dependent fashion by cycloheximide. In the presence of exogenously added repair proteins, recovery occurs within 8 min and is cycloheximide insensitive. Recovery is ascertained by a bioassay performed on intact specimens, by electrophysiology, and by timelapse video microscopy. Fraction beta, a chromatographic fraction with bioactivity comparable to the complete mixture of repair proteins, consists of complexes having an estimated mass of 2000 kDa. Avidin based cytochemistry suggests that biotinylated fraction beta binds to damaged hair bundles. SDS-PAGE gel electrophoresis demonstrates that fraction beta contains 8-10 polypeptides of 90 kDa or smaller. At least four of these polypeptides apparently are consumed during the repair process. Negatively stained samples of fraction beta are shown by transmission electron microscopy to include filamentous structures similar in length (150 nm) and width (6 nm) to linkages between stereocilia. The filamentous structures can be associated with globular structures (20 nm in diameter). A model is presented wherein repair proteins comprise replacement linkages and enzymes that attach linkages to appropriate membrane proteins.

Mechanotransduction of hair bundles arising from multicellular complexes in anemones

Sea anemones are among the simplest animals to use hair bundles to detect vibrations. Although we previously found anemone bundles to be morphologically similar to vertebrate hair bundles, only indirect evidence implicated anemone bundles in mechanotransduction. Here, we test mechanotransduction of these bundles using loose-patch current recording from apical membranes of cells at the base of deflected bundles. Step bundle deflection results in graded membrane currents that are inward in some cells (positive) and outward in other cells (negative). Positive responses range from 5 to 30 pA, abruptly saturate with stronger stimuli, and increase in duration with prolonged deflections. Negative responses range from 10 to 150 pA, show a logarithmic relation to stimulus strength, and attenuate with prolonged deflections. Additionally, responses are reversibly inhibited by streptomycin. We present a model for anemone bundle mechanotransduction modified from the gating spring model for vertebrate mechanotransduction. Because anemone bundles comprise stereocilia arising from a multicellular complex, we propose that supporting cells on opposite sides of a bundle function as oppositely polarized hair cells. Thus, deflection induces ion channels to open in cells on one side of the complex, while allowing channels to close in cells on the opposite side of the complex.

Hair bundles of sea anemones as a model system for vertebrate hair bundles

Sea anemones are marine invertebrates that use hair bundles to detect swimming movements of prey. Prey are captured by nematocysts (stinging capsules) that discharge into the prey. To further characterize anemone hair bundles and to compare hair bundles in anemones with hair bundles in vertebrates, we investigated fine structure and cytochemistry of anemone hair bundles. In addition, using a biological assay based on counting nematocysts discharged into vibrating test probes, we examined sensitivity of vibration detection to aminoglycoside antibiotics, Ca(2+)-free seawater, and amiloride. Like vertebrate hair bundles, anemone hair bundles are composed of stereocilia, possess lateral linkages between stereocilia whose preservation for transmission electron microscopy is enhanced by ruthenium red, and possess tip links morphologically similar to vertebrate tip links. Furthermore, vibration-dependent discharge of nematocysts is reversibly inhibited by 10(-4) M streptomycin and abolished by brief exposure to Ca(2+)-free seawater. However, unlike vertebrate hair bundles, anemone hair bundles appear to be insensitive to amiloride since vibration-dependent discharge of nematocysts is unaffected by up to mM amiloride. Thus, anemone hair bundles may serve as a useful model system for vertebrate hair bundles with the interesting feature of being insensitive to amiloride.