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A Molecular Filter for the Cnidarian Stinging Response

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A Molecular Filter for the Cnidarian Stinging Response bioRxiv preprint doi: https://doi.org/10.1101/2020.04.04.025338; this version posted April 5, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. A molecular filter for the cnidarian stinging response Keiko Weir1, Christophe Dupre1, Lena van Giesen1, Amy S.Y. Lee2, Nicholas W. Bellono1* 1Department of Molecular and Cellular Biology, Harvard University, Cambridge MA 02138 USA 2Department of Biology, Brandeis University, Waltham MA 02453 USA *correspondence: [email protected] Abstract All animals detect and integrate diverse environmental signals to mediate behavior. Cnidarians, including jellyfish and sea anemones, both detect and capture prey using stinging cells called nematocytes which fire a venom-covered barb via an unknown triggering mechanism. Here, we show that nematocytes from Nematostella vectensis use a specialized voltage-gated calcium channel (nCaV) to distinguish salient sensory cues and control the eXplosive discharge response. Adaptations in nCaV confer unusually- sensitive, voltage-dependent inactivation to inhibit responses to non-prey signals, such as mechanical water turbulence. Prey-derived chemosensory signals are synaptically transmitted to acutely relieve nCaV inactivation, enabling mechanosensitive-triggered predatory attack. These findings reveal a molecular basis for the cnidarian stinging response and highlight general principles by which single proteins integrate diverse signals to elicit discrete animal behaviors. Introduction discharge (Pantin, 1942b, Watson and Hessinger, Jellyfish, sea anemones, and hydrozoans of the 1989, Watson and Hessinger, 1992, Anderson and Cnidarian phylum use specialized cells called Bouchard, 2009). Electrical stimulation of cnidocytes to facilitate both sensation and nematocytes increases the probability of discharge secretion required for prey capture and defense in a calcium (Ca2+)-dependent manner (Anderson (Watson and Mire-Thibodeaux, 1994). Two major and Mckay, 1987, Mckay and Anderson, 1988, types of cnidocytes contribute to prey capture by Santoro and Salleo, 1991, Gitter et al., 1994, the tentacles of the starlet sea anemone Watson and Hessinger, 1994, Anderson and (Nematostella vectensis, Fig. 1A): (1) spirocytes, Bouchard, 2009), but direct recordings from anthozoan-specific cells that extrude a thread-like nematocytes are limited and thus mechanisms by organelle to ensnare prey, and (2) nematocytes, which environmental signals control discharge are pan-cnidarian cells which eject a single-use not well studied. venom-covered barb to mediate stinging (Babonis and Martindale, 2017). Sensory cues from prey act Here, we demonstrate that nematocytes from on nematocytes to trigger the eXplosive discharge Nematostella vectensis use a specialized Cav2.1 of a specialized organelle (nematocyst) at an voltage-gated calcium channel orthologue (nCaV) acceleration of up to 5.41x106 g, among the fastest to integrate dynamic voltage signals produced by of any biological process (Holstein and Tardent, distinct sensory stimuli. We show nematocytes are 1984, Nuchter et al., 2006) (Fig. 1B). The intrinsically mechanosensitive but nCaV exhibits nematocyst can only be discharged once and unique voltage-dependent inactivation that basally therefore stinging represents an energetically inhibits cellular activity, thereby preventing expensive process that is likely tightly regulated responses to extraneous mechanical stimuli, such (Watson and Mire-ThibodeauX, 1994, Babonis and as background water turbulence. We further show Martindale, 2014). Indeed, simultaneously that sensory neurons make synaptic contact with presented chemical and mechanical (chemo- nematocytes, and the neurotransmitter tactile) cues are required to elicit nematocyte acetylcholine (ACh) elicits a hyperpolarizing 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.04.025338; this version posted April 5, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. response that relieves nCaV inactivation to allow for aligned reads from nematocyte-enriched cells subsequent cellular stimulation and chemo-tactile- (Sunagar et al., 2018). This strategy allowed us to elicited discharge. Thus, we propose that the search for differentially eXpressed transcripts that specialized voltage dependence of nCaV acts as a might encode CaV channel subunits (pore-forming molecular filter for sensory discrimination. a and auxiliary b and a2d subunits). The orthologue of cacnb2, a b subunit of CaV channels, Results was the highest eXpressed CaV transcript in Nematocyte CaV channels nematocyte-enriched cells, with levels 14-fold We first obtained whole-cell patch clamp higher than other cells in the sea anemone (Fig. recordings from acutely dissociated nematocytes 2A). b subunits can modulate voltage-dependence using intracellular cesium (Cs+) to block potassium and trafficking in diverse ways depending on their (K+) currents, thereby revealing a voltage-activated splice isoform, interacting subunits, and cellular inward current (ICaV, Fig. 1C). In response to context (Buraei and Yang, 2010). Importantly, b sustained positive voltage, voltage-gated ion subunits only interact with α subunits of high channels typically enter a non-conducting, voltage-activated (HVA) calcium channels (Perez- inactivated state and cannot be activated until Reyes, 2003). In agreement with robust b subunit returned to a resting state by negative membrane expression, we found significant enrichment for voltage. Remarkably, ICaV began to inactivate at cacna1a, the pore-forming subunit of HVA CaV2.1, voltages more negative than we could technically and high eXpression of cacna2d1 (Fig. 2 - measure, thus demonstrating an unusual voltage supplement 1A, B). These observations are sensitivity of this conductance (Fig. 1C, D). To consistent with a previous report demonstrating determine whether these properties were specific specific eXpression of cacna1a in nematocytes of to nematocytes, we used a transgenic sea sea anemone tentacles and expression of b anemone with fluorescently-labeled neurons to subunits in nematocytes from jellyfish (Bouchard et facilitate direct comparison between these al., 2006, Moran and Zakon, 2014, Bouchard and excitable cell types (Nakanishi et al., 2012) (Fig. Anderson, 2014). Expression of cacna1h, which 1C). Neuronal voltage-gated currents had a lower does not interact with auxiliary subunits (Buraei threshold for activation and exhibited much weaker and Yang, 2010), was also observed, albeit at voltage-dependent inactivation (Fig. 1C, D, Fig. 1 lower levels and across all cells (Fig. 2 - - supplement 1A-D), similar to currents found in supplement 1A). Thus, it remains possible that neurons of other animals (Hille, 2001), indicating voltage-gated currents in nematocytes are not that nematocytes exhibit unusual voltage- carried exclusively by one CaV subtype. dependent properties. Ion substitution and pore 2+ blocker experiments confirmed ICaV is a Ca - Heterologous expression of Nematostella CaV sensitive current (Fig. 1E, F), consistent with the (nCaV: cacna1a, cacnb2, and cacna2d1) produced contribution of extracellular Ca2+ to chemo-tactile- voltage-gated currents with an activation threshold induced discharge (Watson and Hessinger, 1994, nearly identical to the CaV compleX made from Gitter et al., 1994) (Fig. 1G). Increased respective mammalian orthologues (mCaV, Fig. concentrations of extracellular Ca2+ did not affect 2B, Fig. 2 - supplement 1C, D). Both channels inactivation (Fig. 1 - supplement 1E), suggesting had similar activation kinetics, but fast inactivation the enhanced voltage-dependent inactivation is was significantly pronounced in nCaV, resembling intrinsic to the channel complex. This observation native ICaV (Fig. 2E, Fig. 2 - supplement 1E). is important because it suggests ICaV renders Importantly, nCaV voltage-dependent inactivation nematocytes completely inactivated at typical was greatly enhanced compared with mCaV, resting membrane voltages and thus cells could regardless of the charge carrier (Fig. 2B, Fig. 2 - not be stimulated from resting state. supplement 1F, G). Similar to ICaV, nCaV exhibited unusually-sensitive voltage-dependence and To identify the ion channel mediating ICaV, we began to inactivate at voltages more negative than generated a tentacle-specific transcriptome and we could measure with an estimated midpoint 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.04.025338; this version posted April 5, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. inactivation voltage (Vi1/2) ~80mV more negative 1H). Importantly, nCaV rapidly recovered from than mCaV (Fig. 2B). Even with a 5-second inter- inactivation, demonstrating that channels could be sweep holding potential of -70mV, nCaV exhibited reset for subsequent activation following brief slow inactivation resulting in a drastic decrease in exposure to negative voltage (Fig. 2D). These channel availability over time (Fig. 2C). This slow distinctive features closely match the unique inactivation was largely prevented by adjusting the properties of native ICaV, suggesting nCaV forms the holding potential to -110mV, suggesting predominant CaV channel in nematocytes. inactivation occurs from the closed-state
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