Activated L-Type Calcium Channels Inhibit Chemosensitized Nematocyst Discharge from Sea Anemone Tentacles

N-acetylated sugars in clownfish and damselfish skin mucus as messengers involved in chemical recognition by anemone host

The clownfish - sea anemone system is a great example of symbiotic mutualism where host «toxicity» does not impact its symbiont partner, although the underlying protection mechanism remains unclear. The regulation of nematocyst discharge in cnidarians involves N-acetylated sugars like sialic acid, that bind chemoreceptors on the tentacles of sea anemones, leading to the release of stings. It has been suggested that clownfish could be deprived of sialic acid on their skin surface, sparing them from being stung and facilitating mutualism with sea anemones. In this study, we sampled the skin mucus of two anemone symbionts, the clownfish Amphiprion akindynos and the juvenile damselfish Dascyllus trimaculatus, as well as two non-symbiotic adult damselfish Pomacentrus moluccensis and P. pavo. The free and total sialic acid content, including its conjugated form, and three other intermediates of this pathway were quantified using a stable isotope dilution mass spectrometry approach. We found significantly higher levels of sialic acid and its precursor in the non-symbiotic damselfishes. Concentrations of total sialic acid in anemone symbionts ranged between 13 µM and 16 µM, whereas the non-symbiotic damselfishes ranged between 21 µM and 30 µM. The presence of this metabolite and its precursors, as triggers of nematocyst discharge, in anemone symbionts, suggests that this is not the direct mechanism of protection or that the trigger is concentration dependent. This experiment demonstrates that anemone symbionts are not spared by nematocysts because of a lack of N-acetylated sugars, as previously thought, rather the biochemical mechanisms involving N-acetylated sugars are more complex than just a presence/absence of these molecules.

BK Channels Function in Nematocyst Discharge from Vibration-Sensitive Cnidocyte Supporting Cell Complexes of the Sea Anemone Diadumene lineata

AbstractIntegrated chemo- and mechanosensory pathways, along with activated calcium influxes, regulate nematocyst discharge from sea anemone tentacles. Discharge from vibration-sensitive Type A cnidocyte supporting cell complexes use calcium-conducting transient receptor potential V4-like channels. Because calcium influxes often couple with calcium-activated, large-conductance potassium (BK) channels, we hypothesized that BK channels function in nematocyst discharge. To verify this hypothesis, we first tested five selective BK channel blockers on nematocyst-mediated prey killing in Diadumene lineata (aka Haliplanella luciae). All tested BK channel blockers inhibited prey killing at concentrations comparable to their inhibition of vertebrate BK channels. In addition, the BK channel blocker paxilline selectively inhibited prey killing mediated by vibration-sensitive Type A cnidocyte supporting cell complexes. We queried a mammalian BKα amino acid sequence to the Exaiptasia diaphena database, from which we identified a putative anemone, pore-forming BKα subunit sequence. Using the E. diaphena BKα sequence as a template, we assembled a BKα transcript from our assembled D. lineata transcriptome. In addition, the hydra homolog of D. lineata BKα localizes to nematocytes on the hydra single-cell RNA sequencing map. Our findings suggest that D. lineata expresses BK channels that play a role in vibration-sensitive nematocyst discharge from Type A cnidocyte supporting cell complexes. We believe this is the first functional demonstration of BK channels in nonbilaterians. Because stimulated chemoreceptors frequency tune Type A cnidocyte supporting cell complexes to frequencies matching swimming movements of prey via a protein kinase A signaling pathway and protein kinase A generally activates BK channels, we suggest that D. lineata BK channels may participate in protein kinase A-mediated frequency tuning.

Resting Membrane Potential Modulates Chemoreceptor Sensitivity in Nematocyst Discharge of the Sea Anemone Exaiptasia diaphena

AbstractExtracellular calcium has been known to be required for in situ nematocyst discharge for more than 60 years, yet calcium's role in nematocyst discharge is poorly understood. Currently, we know that extracellular calcium plays at least two distinct roles in in situ nematocyst discharge. First, calcium plays a role in the triggering of discharge by physical contact, most likely involving transient receptor potential channels. Second, activated L-type calcium channels desensitize nematocyst discharge predisposed to discharge by stimulated chemoreceptors for N-acetylated sugars, such as N-acetylneuraminic acid (NANA). It is not known whether the stimulated NANA signaling pathway activates L-type channels electrogenically through membrane depolarization or directly by phosphorylation of the channel. We hypothesize that the activated NANA signaling pathway initiates desensitization by depolarizing cell membrane potentials to activate voltage-gated L-type calcium channels. Consistent with our hypothesis, we show that depolarization induced by blocking voltage-gated potassium channels with 4-aminopyridine selectively activates Ca2+ influx into tentacle ectodermal cells via L-type channels and inhibits in situ nematocyst discharge from chemosensitized anemones. Furthermore, preventing membrane depolarization with valinomycin or hyperpolarizing resting membrane potentials with low-potassium seawater suppresses NANA-induced Ca2+ influx, prevents desensitization of in situ nematocyst discharge, and enhances NANA sensitivity. Thus, changing resting membrane potentials modulates NANA sensitivity, and NANA-induced depolarization drives desensitization. We suggest that desensitization of the NANA signaling pathway occurs by a feedback pathway involving calcium channels that are activated by NANA-induced depolarization. Elucidating the desensitization pathway may suggest methods to protect or prevent public health cases of nematocyst stinging.

Functional analysis in a model sea anemone reveals phylogenetic complexity and a role in cnidocyte discharge of DEG/ENaC ion channels

Ion channels of the DEG/ENaC family share a similar structure but serve strikingly diverse biological functions, such as Na+ reabsorption, mechanosensing, proton-sensing, chemosensing and cell-cell communication via neuropeptides. This functional diversity raises the question of the ancient function of DEG/ENaCs. Using an extensive phylogenetic analysis across many different animal groups, we found a surprising diversity of DEG/ENaCs already in Cnidaria (corals, sea anemones, hydroids and jellyfish). Using a combination of gene expression analysis, electrophysiological and functional studies combined with pharmacological inhibition as well as genetic knockout in the model cnidarian Nematostella vectensis, we reveal an unanticipated role for a proton-sensitive DEG/ENaC in discharge of N. vectensis cnidocytes, the stinging cells typifying all cnidarians. Our study supports the view that DEG/ENaCs are versatile channels that have been co-opted for diverse functions since their early occurrence in animals and that respond to simple and ancient stimuli, such as omnipresent protons.

Divergent Ca2+/calmodulin feedback regulation of CaV1 and CaV2 voltage-gated calcium channels evolved in the common ancestor of Placozoa and Bilateria

Ca_V1 and Ca_V2 voltage-gated calcium channels evolved from an ancestral Ca_V1/2 channel via gene duplication somewhere near the stem animal lineage. The divergence of these channel types led to distinguishing functional properties that are conserved among vertebrates and bilaterian invertebrates and contribute to their unique cellular roles. One key difference pertains to their regulation by calmodulin (CaM), wherein bilaterian Ca_V1 channels are uniquely subject to pronounced, buffer-resistant Ca²⁺/CaM-dependent inactivation, permitting negative feedback regulation of calcium influx in response to local cytoplasmic Ca²⁺ rises. Early diverging, nonbilaterian invertebrates also possess Ca_V1 and Ca_V2 channels, but it is unclear whether they share these conserved functional features. The most divergent animals to possess both Ca_V1 and Ca_V2 channels are placozoans such as Trichoplax adhaerens, which separated from other animals over 600 million years ago shortly after their emergence. Hence, placozoans can provide important insights into the early evolution of Ca_V1 and Ca_V2 channels. Here, we build upon previous characterization of Trichoplax Ca_V channels by determining the cellular expression and ion-conducting properties of the Ca_V1 channel orthologue, TCa_V1. We show that TCa_V1 is expressed in neuroendocrine-like gland cells and contractile dorsal epithelial cells. In vitro, this channel conducts dihydropyridine-insensitive, high-voltage–activated Ca²⁺ currents with kinetics resembling those of rat Ca_V1.2 but with left-shifted voltage sensitivity for activation and inactivation. Interestingly, TCa_V1, but not TCa_V2, exhibits buffer-resistant Ca²⁺/CaM-dependent inactivation, indicating that this functional divergence evolved prior to the emergence of bilaterian animals and may have contributed to their unique adaptation for cytoplasmic Ca²⁺ signaling within various cellular contexts.

Functional Characterization of TRPV-Like Ion Channels Involved in Nematocyst Discharge from the Sea Anemone Diadumene lineata

AbstractCnidarians require mechanical stimuli to trigger nematocyst discharge and initiate feeding behaviors. The interval from triggering stimulus to response is tens of microseconds, making it likely that mechanically gated ion channels trigger nematocyst discharge. Because many transient receptor potential channels are mechanically gated, we hypothesized that nematocyst discharge involves transient receptor potential channels. We therefore tested various transient receptor potential channel inhibitors to determine whether they inhibit nematocyst discharge and prey killing in the acontiate sea anemone (Actinaria) Diadumene lineata (a.k.a. Haliplanella luciae). Three types of cnidocyte supporting cell complexes regulate nematocyst discharge in anemones: Types C, B, and A. Discharge from Type Cs is directly triggered by stimulation of contact-sensitive mechanoreceptors, while Type Bs require activation of chemoreceptors from prey-derived N-acetylated sugars to sensitize contact-sensitive mechanoreceptors. In Type As, activated chemoreceptors tune vibration-sensitive mechanoreceptors that predispose contact-sensitive mechanoreceptors for triggering. The non-selective transient receptor potential channel blockers lanthanum and gadolinium dose-dependently inhibited about 80% of prey killing and all nematocyst discharge from Type Bs and Type Cs, but not Type As. The selective transient receptor potential vanilloid 4 (TRPV4) blocker GSK2193874 inhibited Type As and Type Bs. However, the selective TRPV4 blockers HC-067047 and RN-1734 inhibited only Type As. Thus, three TRPV4-selective blockers implicate TRPV-like involvement in discharge from Type As, whereas GSK2193874 also affected Type Bs. Our results suggest that a TRPV-like homolog plays an essential role in nematocyst-mediated prey killing from Type As, whereas other transient receptor potential channels are likely involved in discharge from Type B and C cnidocyte supporting cell complexes.

N-Acetyl Neuraminic Acid (NANA) Activates L-Type Calcium Channels on Isolated Tentacle Supporting Cells of the Sea Anemone (Aiptasia pallida)

AbstractSensory receptors control nematocyst discharge on sea anemone tentacles. Micromolar N-acetylated sugars (e.g., N-acetyl neuraminic acid [NANA]) bind chemoreceptors on ectodermal supporting cells and predispose adjacent nematocyst discharge in response to mechanical contact via a cyclic adenosine monophosphate (cAMP)-dependent sensitization pathway, while higher NANA levels dose-dependently desensitize. Recent evidence implicates L-type calcium channels in desensitizing the pathway in aconitate sea anemones Aiptasia pallida (also known as Exaiptasia diaphana). We, therefore, hypothesize that NANA activates calcium influx via L-type calcium channels. We demonstrate a dose-dependent, NANA-activated ⁴⁵Ca influx into dissociated ectodermal cells isolated from A. pallida tentacles, with maximal influx occurring at desensitizing concentrations of NANA. The L-type calcium channel inhibitors nifedipine, diltiazem, methoxyverapamil, and cadmium blocked NANA-stimulated ⁴⁵Ca influx. Elevated extracellular KCl levels dose-dependently increased nifedipine-sensitive ⁴⁵Ca influx to implicate voltage-gated calcium channels. Forskolin, 8-bromo-cAMP, and the protein kinase A inhibitor H-8 affect NANA-stimulated calcium influx in a manner consistent with activated cAMP-dependent pathway involvement. Because NANA chemoreceptors localize to supporting cells of cnidocyte supporting cell complexes, NANA activation of ⁴⁵Ca influx into isolated tentacle ectodermal cells suggests that L-type calcium channels and NANA chemoreceptors co-localize to supporting cells. Indeed, a fluorescent marker of L-type calcium channels localizes to the apical ectoderm adjacent to nematocysts of live tentacles. We conclude that supporting cell chemoreceptors activate co-localized L-type calcium channels via a cAMP-dependent mechanism in order to initiate desensitization. We suggest that pathway desensitization may conserve nematocysts from excessive discharge during prey capture.

Cnidocyte Supporting Cell Complexes Regulate Nematocyst-Mediated Feeding Behaviors in the Sea Anemone Diadumene lineata

AbstractCnidarians, as model animals for studying conserved feeding behavior, possess the simplest nervous and digestive systems. Feeding behavior in cnidarians begins with nematocyst-mediated prey retention, proceeds to coordinated tentacle movements and mouth opening, and then proceeds to release of retained prey for ingestion. Understanding the basis of nematocyst discharge, retention, and release is central to explaining cnidarian feeding. Based on studies using artificial targets, cnidocyte supporting cell complexes (CSCCs) regulate nematocyst discharge, retention, and release in Actinaria (sea anemones); but the relevance of CSCCs to prey retention and ingestion has not yet been established. CSCCs exist as three functional types (Types A, B, and C), with a ratio of Types A∶B∶C of 2∶2∶1 in Diadumene lineata (a.k.a. Haliplanella luciae). We tested the hypothesis that CSCCs control nematocyst-mediated prey killing and ingestion. We used a quantitative feeding assay involving Artemia nauplii (prey) and monoclonal D. lineata. The ratios of Types A∶B∶C involved in prey killing and ingestion were 1∶2.5∶5 and 1∶2∶3, respectively. These findings support the CSCC hypothesis. They also indicate that Type Cs predominate in killing small, hard-surfaced, motile, crustaceous prey. Chemoreceptor-bearing Type Bs and Type As assist in prey killing and assume somewhat greater roles in ingestion. Thus, CSCC types differ with respect to their afferent sensory roles as well as their subsequent efferent roles in killing and ingestion. We conclude that CSCC types perform overlapping and complementary roles during feeding.