ARTICLE The Tarantula Toxin Psalmotoxin 1 Inhibits Acid-sensing Ion Channel (ASIC) 1a by Increasing Its Apparent Hϩ Affinity Xuanmao Chen,1,3 Hubert Kalbacher,2 and Stefan Gründer1,3 1Department of Physiology II, University of Tübingen, 72076 Tübingen, Germany 2Interfakultäres Institut für Biochemie, University of Tübingen, 72074 Tübingen, Germany 3Department of Physiology II, University of Würzburg, 97070 Würzburg, Germany Acid-sensing ion channels (ASICs) are ion channels activated by extracellular protons. They are involved in higher brain functions and perception of pain, taste, and mechanical stimuli. Homomeric ASIC1a is potently inhibited by the tarantula toxin psalmotoxin 1. The mechanism of this inhibition is unknown. Here we show that psalmotoxin 1 inhibits ASIC1a by a unique mechanism: the toxin increases the apparent affinity for Hϩ of ASIC1a. Since ASIC1a is activated by Hϩ concentrations that are only slightly larger than the resting Hϩ concentration, this Downloaded from http://rupress.org/jgp/article-pdf/126/1/71/1221003/jgp126171.pdf by guest on 29 September 2021 increase in Hϩ affinity is sufficient to shift ASIC1a channels into the desensitized state. As activation of ASIC1a has recently been linked to neurodegeneration associated with stroke, our results suggest chronic desensitization of ASIC1a by a slight increase of its Hϩ affinity as a possible way of therapeutic intervention in stroke. INTRODUCTION Hϩ is chemically the simplest transmitter. Hϩ receptor (England et al., 1998), and by glutamate (Hammerland channels have recently been characterized that open et al., 1992), either by competitive or noncompetitive upon an increase in the extracellular concentration of antagonism with the ligand. Toxins that interact with Hϩ and desensitize in the continuous presence of Hϩ voltage-gated ion channels can be divided in two groups (Waldmann et al., 1997; Waldmann and Lazdunski, based on the mechanism of interaction. One group pro- 1998). The apparent affinity of these ion channels for duces inhibition by physically occluding the ion pore ␮M, and their activation and desensitization (Catterall, 1980; Hille, 2001). The other group comprises 0.3ف ,Hϩ kinetics is comparable with other ligand-gated channels gating modifiers that interact with the voltage sensor, (Waldmann and Lazdunski, 1998; Bässler et al., 2001), shifting the voltage dependence of the channels. Some establishing Hϩ as the genuine ligand for these channels, of them (mainly spider toxins) shift the voltage de- the acid-sensing ion channels (ASICs). However, in pendence to more positive potentials, inhibiting the contrast to other ligands, resting concentrations of Hϩ channels (McDonough et al., 1997a,b; Swartz and are close to the threshold concentration needed to MacKinnon, 1997a; Chuang et al., 1998), whereas others activate ASICs. Consequently, slightly increased Hϩ (␤-toxins of scorpions) shift the voltage dependence to concentrations, for example during metabolic acidosis, more negative potentials, leading to increased channel The Journal of General Physiology chronically desensitize ASICs (Benson et al., 1999; opening at resting membrane potentials (Cestele et Alvarez De La Rosa et al., 2002; Babini et al., 2002). al., 1998; Hille, 2001). Still other toxins (␣-toxins of This steady-state desensitization of ASICs, induced by scorpions) cause a slowing of inactivation of Naϩ ligand concentrations above threshold, is analogous to channels (Catterall, 1979; Wang and Strichartz, 1985). the steady-state inactivation of voltage-gated channels Recently, a novel protein toxin from a tarantula, by continuous depolarization above the threshold psalmotoxin 1 (PcTx1), has been isolated that inhibits potential. Hϩ-gated ASIC1a (Escoubas et al., 2000) by an unknown Venomous animals, such as spiders, scorpions, and mechanism. Interestingly, PcTx1 is structurally unrelated sea anemones, contain a rich diversity of protein toxins to conotoxins but related to gating modifier toxins of that interact with different classes of ion channels. The voltage-gated ion channels (Escoubas et al., 2003). main group of toxins that interact with ligand-gated ion Here we show that PcTx1 has a unique mechanism of channels are the conotoxins produced by cone snails inhibition: it increases the apparent affinity for Hϩ of (Terlau and Olivera, 2004). They inhibit channels gated ASIC1a, leading to chronic desensitization at the resting by acetylcholine (McIntosh et al., 1999), by serotonin pH of 7.4. Correspondence to Stefan Gründer: Abbreviations used in this paper: ASIC, acid-sensing ion channel; [email protected] PcTx1, psalmotoxin 1. J. Gen. Physiol. © The Rockefeller University Press $8.00 71 Volume 126 Number 1 July 2005 71–79 http://www.jgp.org/cgi/doi/10.1085/jgp.200509303 MATERIALS AND METHODS Electrophysiology The inhibition of ASIC1a by PcTx1 was investigated by express- ing homomeric ASIC1a in Xenopus laevis oocytes. Capped ASIC1a cRNA was synthesized by SP6 RNA polymerase from linearized cDNA, using the mMessage mMachine kit (Ambion). Stage V–VI Xenopus oocytes were injected with 0.01 ng cRNA and kept in OR-2 medium (concentrations in mM: 82.5 NaCl, 2.5 KCl, 1.0 Na2HPO4, 5.0 HEPES, 1.0 MgCl2, 1.0 CaCl2, and 0.5 g/l PVP; pH 7.3) for 2–4 d. Whole cell currents were recorded at 0.1 or 1 kHz and filtered at 20 Hz with a TurboTec 03X amplifier (npi elec- tronic) using an automated, pump-driven solution exchange sys- tem together with the oocyte testing carousel controlled by the interface OTC-20 (npi electronic). Data acquisition and solution exchange were managed using the software CellWorks 5.1.1 (npi electronic). Bath solution contained (in mM) 140 NaCl, 1.8 CaCl2, 1.0 MgCl2, 10 HEPES. For the acidic test solutions, HEPES Downloaded from http://rupress.org/jgp/article-pdf/126/1/71/1221003/jgp126171.pdf by guest on 29 September 2021 was replaced by MES buffer. Conditioning solution with pH be- tween 6.45 and 7.0 was buffered with 5 mM HEPES/5 mM MES. Solutions containing 0.1 mM Ca2ϩ were supplemented with 0.1 mM flufenamic acid to block the large conductance induced in Xenopus oocytes by divalent-free extracellular solutions. Holding potential was Ϫ70 mV if not otherwise indicated. Dose–response curves were fit to the Hill equation I ϭ a ϩ (I Ϫ a)/(1 ϩ (EC /[L])n), where I is the maximal current, max 50 max Figure 1. The inhibition by PcTx1 is pH dependent. ASIC1a a is the residual current, [L] is the concentration of the ligand currents were repeatedly activated by pH 5.0. They were com- (PcTx1 or Hϩ), EC is the concentration at which half-maximal 50 pletely inhibited by application of the P. cambridgei venom (1:20,000 response occurs, and n is the Hill coefficient. Before fitting, cur- dilution) for 60 s at pH 7.4. In contrast, the venom did not inhibit rents from each measurement were normalized to the maximal the current, when it was applied at pH 7.9. Only a linear current value measured. Results are reported either, in the text, as mean Ϯ rundown can be observed that is usual for ASIC1a and does not SD or, for the figures and amplitude histograms, as mean Ϯ SEM. depend on the presence of the venom. They represent the mean of n individual measurements on differ- ent oocytes. Statistical analysis was done with the unpaired t-test. Psalmopoeus cambridgei venom was from SpiderPharm. firmed by analytical HPLC, matrix-assisted laser desorption/ion- ization time of flight mass spectrometry (MALDI-MS), and elec- Toxin Synthesis and Refolding trospray ionization mass spectrometry (ESI-MS), giving the The synthesis of native psalmotoxin 1 was performed using the correct mass of oxidized product. Fmoc/But and maximal temporary protection strategy on a Syro- The affinity of synthetic PcTx1 to ASIC1a was determined as MultiSyntec peptide synthesizer. The chemical procedure used inhibition of ASIC1a current by application of PcTx1 for 125 s in 0.05 mmol of Fmoc-Thr(OtBu)-2-chlorotrityl resin (0.50 mmol/g), the conditioning period at pH 7.4. Under these conditions, Kd an eightfold excess of each amino acid and 2-(1H-benzotriazole- was 3.7 nM and the Hill coefficient of 1.65, which is in good 1-yl)1,1,3,3-tetramethyluronium tetrafluoroborat/1-hydroxybenzo- agreement with previous published data (Escoubas et al., 2000), triazole (TBTU/HOBt) activation. Deprotection (2 h) and cleavage demonstrating the purity of our preparation. Complete block (100 mg peptide ϩ resin) were achieved using 5 ml of a mixture was achieved with a concentration of 30 nM PcTx1; this concen- of trifluoroacetic acid/thioanisole/ethandithiole (90/8/2, vol/ tration was, therefore, used for most of the experiments de- vol/vol). The acidic mixture was then precipitated three times scribed in this study. It should be emphasized, however, that the with diethylether, dissolved in 10% aqueous acetic acid and apparent affinity of ASIC1a to PxTc1, as determined above, is pH freeze dried. The crude toxin was purified by RP-HPLC on a C18 dependent. Solutions containing PcTx1 were supplemented with semi-preparative column (10 ϫ 150 mm; Nucleosil) using a 40- 0.05% BSA to avoid absorption by the tubing. min gradient of acetonitrile in 0.055% trifluoroacetic acid (10– 80% B in 40 min, where B is 80% acetonitrile/H2O/0.05% triflu- oroacetic acid). RESULTS Oxidation of the reduced toxin was achieved by dissolving the purified peptide with 2 M acetic acid, and diluted to a peptide Inhibition By PcTx1 Is Sensitive to the Conditioning pH concentration of 0.015 mM in the presence of reduced/oxidized Fig. 1 shows that the inhibition of ASIC1a by the PcTx1- glutathione (molar ratio of peptide/GSH/GSSG was 1:100:10) and 2 M guanidine hydrochloride. The solution was adjusted to containing venom of the tarantula P. cambridgei was sen- Њ pH 8.0 with aqueous NH4OH and stirred slowly at 4 C for 3 d.