room of Molecular and Cellular Biology, Harvard University, united States; room of Biology, Brandeis University, joined States;
All animals detect and integrate diverse environmental signals to mediate behavior. Cnidarians, including jellyfish and also sea anemones, both detect and also capture prey making use of stinging cells called nematocytes which fire a venom-covered barb via an unknown triggering mechanism. Here, we present that nematocytes from Nematostella vectensis usage a dedicated voltage-gated calcium channel (nCaV) to differentiate salient sensory cues and control the explosive discharge response. Adaptations in nCaV confer person who is abnormal sensitive, voltage-dependent inactivation come inhibit responses come non-prey signals, such as mechanical water turbulence. Prey-derived chemosensory signals are synaptically transmitted to acutely relieve nCaV inactivation, allowing mechanosensitive-triggered predatory attack. This findings expose a molecule basis because that the cnidarian stinging solution and to mark general ethics by which solitary proteins combine diverse signals to elicit discrete animal behaviors.

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Introduction


Jellyfish, sea anemones, and also hydrozoans the the Cnidarian phylum use committed cells referred to as cnidocytes come facilitate both sensation and also secretion required for prey capture and defense (Watson and also Mire-Thibodeaux, 1994b). Two major types of cnidocytes contribute to prey capture by the tentacles of the starlet sea anemone (Nematostella vectensis, figure 1A): (1) spirocytes, anthozoan-specific cells that extrude a thread-like organelle come ensnare prey, and (2) nematocytes, pan-cnidarian cells which eject a single-use venom-covered barb to mediate stinging (Babonis and Martindale, 2017). Sensory cues from food act ~ above nematocytes to trigger the explosive discharge of a committed organelle (nematocyst) at an acceleration of as much as 5.41 × 106 g, among the more quickly of any kind of biological procedure (Holstein and Tardent, 1984; Nüchter et al., 2006; Figure 1B). The nematocyst have the right to only it is in discharged once and therefore stinging represents an energetically expensive procedure that is likely tightly regulation (Watson and also Mire-Thibodeaux, 1994b, Babonis and also Martindale, 2014). Indeed, simultaneously presented chemical and also mechanical (chemo-tactile) cues are forced to elicit nematocyte discharge (Pantin, 1942a; Watson and also Hessinger, 1989; Watson and Hessinger, 1992; Anderson and also Bouchard, 2009). Electric stimulation of nematocytes rises the probability of discharge in a calcium (Ca2+)-dependent manner (Anderson and Mckay, 1987; McKay and Anderson, 1988; Santoro and also Salleo, 1991; Gitter et al., 1994; Watson and Hessinger, 1994a; Anderson and also Bouchard, 2009), but direct recordings native nematocytes are minimal and therefore mechanisms whereby environmental signals regulate discharge are not fine studied.


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Nematocyte voltage-gated Ca2+ currents exhibit sensitive voltage-dependent inactivation.
(A) Starlet sea anemone (Nematostella vectensis). Scale bar = 3 mm. (B) undamaged (yellow) and discharged nematocyte (blue). Scale bar = 20 μm. (C) Left: Representative spot clamp experiment from a nematocyte and tentacle neuron. Scale bar = 10 μm. Right: Nematocyte or neuron voltage-gated currents elicited by a maximally activating voltage pulse complying with 1 s pre-pulses come −110 mV (max current), −90 mV (colored), or 0 mV (inactivated, no current). (D) Nematocyte voltage-gated currents inactivated at very an unfavorable voltages contrasted with neurons. Nematocyte inactivation occurred at voltages an ext negative than can be measured contrasted with a sigmoidal inactivation relationship in neurons: nematocyte estimated Vi1/2 = -100.2 ± 0.4mV, n = 13 and neuron Vi1/2 = -70.8 ± 1.0mV, n = 9. Apparent activation thresholds were similar (Figure 1—figure supplement 1A). (E) Nematocyte voltage-gated currents elicited through −40 mV and also 0 mV pulses to be abolished in absence of exterior Ca2+ and blocked through cadmium (Cd2+). Representative that n = 4 for 0 Ca2+ and three for Cd2+, p2+-sensitive. Substitution of extracellular Ca2+, yet not Na+ because that NMDG+, affected the reversal potential. N = 3–4, p2+ versus other conditions, one-way ANOVA with post-hoc Tukey test. (G) Nematocyte discharge was minimal or lacking in an answer to mechanical stimulation alone (n = 11, 3.3 mM Ca2+). In the visibility of food extract, mechanically evoked discharge was comparable in typical and higher concentration the extracellular Ca2+ (n = 8 because that 3.3 mM Ca2+, n = 5 for 10 mM) and also blocked through Cd2+ (n = 8) or the removal of extracellular Ca2+ (n = 15). Discharged nematocysts installed in presented gelatin-coated coverslips to be quantified. P2+ versus various other conditions, one-way ANOVA through post-hoc Bonferroni test. Data stood for as mean ± sem.


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Properties that nematocytes and also neurons.

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Here, we show that nematocytes from Nematostella vectensis use a committed Cav2.1 voltage-gated calcium channel orthologue (nCaV) to combine dynamic voltage signals developed by distinctive sensory stimuli. We present nematocytes are intrinsically mechanosensitive yet nCaV exhibits unique voltage-dependent inactivation the basally inhibits moving activity, thereby preventing responses to extraneous mechanically stimuli, such as background water turbulence. We further show that sensory neurons do synaptic call with nematocytes, and also the neurotransmitter acetylcholine (ACh) elicits a hyperpolarizing response that relieves nCaV inactivation to enable for subsequent cellular stimulation and chemo-tactile-elicited discharge. Thus, us propose that the devoted voltage dependence of nCaV acts as a molecular filter because that sensory discrimination.


Results


Nematocyte CaV channels


We an initial obtained whole-cell job clamp recordings indigenous acutely dissociated nematocytes to inspection nematocyte signal transduction. Utilizing intracellular cesium (Cs+) come block potassium (K+) currents revealed a voltage-gated inward current that was caused by optimistic or depolarized membrane voltages (ICaV, figure 1C). In an answer to sustained hopeful voltage, voltage-activated ion channels enter a non-conducting, inactivated state and cannot be activated until returned to a resting state by an adverse membrane voltage. This property generally serves to border responses to recurring or lengthy stimulation, comparable to receptor desensitization. Remarkably, ICaV started to inactivate in ~ voltages an ext negative 보다 we might technically measure, thus demonstrating an inexplicable voltage sensitivity the this conductance (Figure 1C,D). To identify whether these properties were details to nematocytes, we used a transgenic sea anemone with fluorescently labeled neurons to facilitate straight comparison between these excitable cell varieties (Nakanishi et al., 2012; Figure 1C). Neuronal voltage-gated currents had actually a lower threshold because that activation and also exhibited much weaker voltage-dependent inactivation (Figure 1C,D, figure 1—figure complement 1A–D), similar to currents found in neurons of other animals (Hille, 2001), indicating that nematocytes exhibit inexplicable voltage-dependent properties. Ion substitution and pore blocker experiments confirmed ICaV is a Ca2+-sensitive present (Figure 1E,F), constant with the donation of extracellular Ca2+ come chemo-tactile-induced discharge (Watson and Hessinger, 1994a; Gitter et al., 1994; Figure 1G). Boosted concentrations that extracellular Ca2+ walk not affect inactivation the ICaV (Figure 1—figure supplement 1E), suggesting the enhanced voltage-dependent inactivation is intrinsic to the channel complex. This observation is important since it argues ICaV makes nematocytes completely inactivated at common resting membrane voltages and also thus cells might not be created from resting state.

To determine the ion channel mediating ICaV, we generated a tentacle-specific transcriptome and also aligned reads native nematocyte-enriched cells (Sunagar et al., 2018). This strategy enabled us to search for differentially express transcripts that could encode CaV channel subunits (pore-forming α and also auxiliary β and also α2δ subunits). The orthologue of cacnb2, a β subunit that CaV channels, to be the highest expressed CaV transcript in nematocyte-enriched cells, v levels 14-fold higher than various other cells in the sea anemone (Figure 2A). β subunits deserve to modulate voltage-dependence and trafficking in diverse ways relying on their splice isoform, connecting subunits, and also cellular context (Buraei and Yang, 2010). Importantly, β subunits only connect with α subunits the high voltage-activated (HVA) calcium channels (Perez-Reyes, 2003). In covenant with robust β subunit expression, us found far-reaching enrichment for cacna1a, the pore-forming subunit the HVA CaV2.1, and also high expression that cacna2d1 (Figure 2—figure supplement 1A,B). These monitorings are continual with a previous report demonstrating details expression the cacna1a in nematocytes the sea anemone tentacles and expression that β subunits in nematocytes native jellyfish (Bouchard et al., 2006; Moran and Zakon, 2014; Bouchard and Anderson, 2014). Expression that cacna1h, which does not connect with assistant subunits (Buraei and Yang, 2010), was likewise observed, albeit at reduced levels and across all cells (Figure 2—figure supplement 1A). Thus, the remains feasible that voltage-gated currents in nematocytes room not brought exclusively by one CaV subtype.


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Nematostella CaV exhibits distinctive voltage-dependent nature conferred through its β subunit.
(A) CaV channel facility with α, β, and α2δ subunits. MRNA expression (transcripts every million, TPM) the voltage-gated calcium (CaV) channel β subunits in nematocyte-enriched cell (blue) and also non-enriched cell (grey). N = 6, pcacnb2.1 in nematocytes versus various other cells, two-way ANOVA v post-hoc Bonferroni test. (B) Heterologously-expressed nCav channels (Nematostella cacna1a, cacnb2, cacna2d1) inactivated in ~ very an adverse voltages (estimated Vi1/2 = -101.5 ± 1.6mV, n = 5) versus mammalian orthologues (mCaV, Vi1/2 = -20.9 ± 3.4mV, n = 10). Noticeable activation thresholds were the same: nCaV Va1/2 = -9.8 ± 0.3mV, n = 5, mCaV Va1/2 = -10.4 ± 0.5mV, n = 9. Inactivation was measured in an answer to 1 s pre-pulses native −110 mV to 10 mV v an inter-sweep holding potential of −90 mV. (C) nCaV showed slow inactivation v −70 mV holding potential (0.2 Hz stimulation, 5 s inter-pulse interval) the was ideal fit by two exponential attributes with time constants of 10.0 and 369.5 s. N = 6, multiple heat two-tailed student’s t-test with meaning of pV, n = 6 because that mCaV. (E) Voltage-gated currents taped from nCaV or mCaV complying with a −110 mV pre-pulse, −50 mV pre-pulse (colored), and 20 mV pre-pulse. CaV β subunits were substituted as suggested (mammalian β in red and Nematostella β in blue). Range bars = 100 pA, 25 ms. (F) Mammalian β move nCaV voltage-dependent inactivation to optimistic voltages. NCaV Vi1/2 = -73.2 ± 1.2mV, n = 6. NCaV + mβ=−16.9 ± 1.9 mV, n = 6. (G) fifty percent maximal inactivation voltage (Vi1/2) for CaV chimeras. PV + mβ, mCaV matches mCaV + nβ, one-way ANOVA with post-hoc Tukey test. Inactivation was measured in response to pre-pulses native −100 mV come 10 mV v an inter-sweep holding potential that −110 mV to alleviate slow inactivation. Data stood for as mean ±sem.


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Properties that nCaV.

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Heterologous expression the Nematostella CaV (nCaV: cacna1a, cacnb2, and also cacna2d1) produced voltage-gated currents with an obvious activation threshold nearly identical come the CaV complicated made from particular mammalian orthologues (mCaV, number 2B, figure 2—figure supplement 1C,D). Both channels had comparable activation kinetics, however fast inactivation was significantly pronounced in nCaV, resembling native ICaV (Figure 2E, figure 2—figure supplement 1E). Importantly, nCaV voltage-dependent inactivation to be greatly intensified compared v mCaV, nevertheless of the fee carrier (Figure 2B, number 2—figure supplement 1F,G). Similar to ICaV, nCaV showed unusually-sensitive voltage-dependence and also began come inactivate at voltages an ext negative 보다 we could measure through an approximated midpoint inactivation voltage (Vi1/2)~80 mV an ext negative than mCaV (Figure 2B). Also with a stop potential that −70 mV, nCaV showed slow inactivation leading to a correctly decrease in responses come depolarizing stimuli gradually (Figure 2C). This sluggish inactivation was mostly prevented through adjusting the hold potential come −110 mV, suggesting inactivation wake up when networks are in a closed-state at potentials close to or more negative than usual resting membrane potential (Figure 2—figure complement 1H). Importantly, nCaV promptly recovered from inactivation, demonstrating that networks could it is in reset for succeeding activation following quick exposure to an unfavorable voltage (Figure 2D). This distinctive features carefully match the distinct properties of indigenous ICaV, arguing nCaV creates the primary CaV channel in nematocytes.

To determine the molecule basis for nCaV inactivation, us analyzed chimeric CaV complexes containing particular α, β, and α2δ1 subunits native nCaV or mCaV orthologues. Utilizing a stop potential that −110 mV to compare voltage-dependent inactivation, we uncovered that only transfer that the β subunit significantly affected voltage-dependent inactivation, if α or α2δ1 subunits created minimal results on voltage-dependent activation, inactivation, or kinetics (Figure 2E, figure 2—figure supplement 1C–E). Indeed, other β subunits have the right to induce far-reaching hyperpolarized shifts in inactivation that HVA CaV networks (Yasuda et al., 2004). In this case, the mCaV β subunit considerably shifted nCaV inactivation by ~56 mV in the positive direction, prevented finish inactivation, and also produced slower quick inactivation (Figure 2E–G, number 2—figure complement 1E). Furthermore, nCaV β was adequate to confer greatly magnified voltage-dependent inactivation come mCaV (Figure 2E,G). From these results, us conclude that nCaV β, the most enriched CaV subunit in nematocytes, confers nCaV’s uniquely-sensitive voltage-dependent inactivation.


Because electrical stimulation has been implicated in nematocyte discharge and some nematocytes can produce activity potentials (Anderson and also Mckay, 1987; McKay and Anderson, 1988; Anderson and Bouchard, 2009), we provided current-clamp to record the electric responses of nematocytes come depolarizing stimuli. Under our conditions, nematocytes had a resting potential the −64.8 ± 8.9 mV and did not create a voltage spike once injected with present from remainder (Figure 3A). We further considered that the strong voltage-dependent inactivation that ICaV can prevent excitability. Continuous with this idea, once nematocytes were first hyperpolarized to −90 mV and also subsequently created by present injection, us observed a singular long voltage spike (Figure 3B,C). In contrast, tentacle neurons developed multiple small spikes when injected through equivalent current amplitudes indigenous a comparable resting voltage, consistent with various other neural systems (Figure 3A–C). Differences in spike width and frequency appear suited come mediate distinctive moving functions: dynamic details processing in neurons and also a solitary robust discharge event in nematocytes. Furthermore, these outcomes indicate strong voltage-dependent inactivation prevents nematocyte activation from rest.


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Nematocyte excitability needs hyperpolarized voltages.

Nematocyte sensory transduction



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Nematocytes are intrinsically mechanosensitive and also indirectly chemosensitive.
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Mechano- and chemosensory properties.

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Nematocyte voltage-dependence facilitates signal integration required for stinging.
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Immunohistochemistry


Adult Nv-Elav1::mOrange Nematostella were paralyzed in anesthetic solution, then put in a 4% solution of PFA overnight. Animals were cryoprotected making use of a gradient of boosting sucrose concentrations (10% come 50%) in PBS over two days. Cryostat part (20 µm thick) to be permeabilized through 0.2% Triton-X and 4% typical goat serum (NGS) at room temperature because that 1 hr, followed by incubation through DsRed Polyclonal Antibody (Takara Bio Cat# 632496, RRID:AB_10013483) overnight in PBST (0.2%) and NGS (4%) at 4°C. Organization was rinsed three times v PBST before secondary was applied (Goat anti-rabbit 647, Abcam in PBST + NGS) for 2 hr in ~ room temperature. Organization was rinsed v PBS and mounted with Vectashield comprise DAPI (Novus Biologicals).