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Modulation of Voltage-Gated Na and K Channels by Pumiliotoxin 251D: A

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Modulation of Voltage-Gated Na and K Channels by Pumiliotoxin 251D: A ARTICLE IN PRESS Toxicon 51 (2008) 334–344 www.elsevier.com/locate/toxicon Modulation of voltage-gated Na+ and K+ channels by pumiliotoxin 251D: A ‘‘joint venture’’ alkaloid from arthropods and amphibians$ Thomas Vandendriesschea, Yousra Abdel-Mottaleba, Chantal Maertensa, Eva Cuypersa, Alexander Sudaub,1, Udo Nubbemeyerb, Dietrich Mebsc, Jan Tytgata,Ã aLaboratory of Toxicology, University of Leuven, Leuven, Belgium bInstitute of Organic Chemistry, University of Mainz, Mainz, Germany cZentrum der Rechtsmedizin, University of Frankfurt, Frankfurt, Germany Received 6 August 2007; received in revised form 11 October 2007; accepted 12 October 2007 Available online 24 October 2007 Abstract Certain amphibians provide themselves with a chemical defense by accumulating lipophilic alkaloids into skin glands from dietary arthropods. Examples of such alkaloids are pumiliotoxins (PTXs). In general, PTXs are known as positive modulators of voltage-gated sodium channels (VGSCs). Unlike other PTXs, PTX 251D does not share this characteristic. However, mice and insect studies showed that PTX 251D is highly toxic and to date the basis of its toxicity remains unknown. In this work, we searched for the possible target of PTX 251D. The toxin was therefore made synthetically and tested on four VGSCs (mammalian rNav1.2/b1, rNav1.4/b1, hNav1.5/b1 and insect Para/tipE) and five voltage-gated potassium channels (VGPCs) (mammalian rKv1.1-1.2, hKv1.3, hKv11.1 (hERG) and insect Shaker IR) expressed heterologously in Xenopus laevis oocytes, using the two-electrode voltage clamp technique. PTX 251D not only inhibited the Na+ influx through the mammalian VGSCs but also affected the steady-state activation and inactivation. Interestingly, in the insect ortholog, the inactivation process was dramatically affected. Additionally, PTX 251D inhibited the K+ efflux through all five tested VGPCs and slowed down the deactivation kinetics of the mammalian VGPCs. hKv1.3 was the most sensitive channel, with an IC50 value 10.870.5 mM. To the best of our knowledge this is the first report of a PTX affecting VGPCs. r 2007 Elsevier Ltd. All rights reserved. Keywords: PTX 251D; PTXs; VGSC; VGPC; Two-electrode voltage clamp technique 1. Introduction $Ethical statement: The authors declare that the article submitted for publication has not been published elsewhere and A remarkable diversity of biologically active that the guidelines for animal welfare have been followed. alkaloids has been discovered in amphibian skin ÃCorresponding author. E-mail address: [email protected] (J. Tytgat). (Daly et al., 2005). During evolution, certain 1Present address: ALTANA Pharma AG, Byk-Gulden-Street amphibians have developed an efficient system to 2, Konstanz, Germany. accumulate some of these toxic compounds from 0041-0101/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2007.10.011 ARTICLE IN PRESS T. Vandendriessche et al. / Toxicon 51 (2008) 334–344 335 dietary alkaloid-containing arthropods into their source of PTXs and other frog skin alkaloids with skin. Together with the bright warning skin colora- branches in their carbon skeletons. tion, the accumulation of toxins offers an extreme PTXs are known to be toxic. They are described evolutionary advantage against predators. Exam- as potent cardiotonic agents with positive modula- ples of such toxins are the lipophilic alkaloids tory effects on voltage-gated sodium channels pumiliotoxins (PTXs) that are very widely distrib- (VGSCs) (Daly et al., 1985). Like other PTXs, uted in alkaloid-containing anurans from the PTX 251D is highly toxic, inducing convulsions and neotropics (Dendrobates, Epipedobates, Minyobates, death to mice and insects (LD50 being, respectively, Phylobates, Oophaga, Ameerga and Ranitomeya), 10 mg/kg and 150 ng/larvae) (Bargar et al., 1995; semi-temperate South America (Melanophryniscus), Daly et al., 2003). In addition, this toxin seems to Madagascar (Mantella) and Australia (Pseudo- repel predatory and ectoparasitic arthropods, and phryne). The presence of PTX 307A and 323A hence, its function in anuran chemical defense is (Fig. 1) has also been reported in formicine ants of proved (Weldon et al., 2006). However, earlier the genera Brachymyrmex and Paratrechina (Sapor- studies in brain synaptoneurosomes showed that ito et al., 2004). Recently, in the extracts of PTX 251D only weakly stimulates the sodium flux scheloribatid mites PTX 251D (Fig. 1) was found at low concentrations (10 mM) and has inhibitory together with PTX 237A and 8-deoxyPTX 193H effects on VGSCs at higher concentrations (100 mM) (Takada et al., 2005), while PTXs 251D, 307F and (Daly et al., 1985, 1990). Therefore, the observed 307A and a homoPTX 251R were found in oribatid effects in mice and insects cannot be explained by a mites from Costa Rica and Panama (Saporito et al., modulation of VGSCs alone, making the possible 2007). It has been proposed that mites are the major physiological target(s) for this toxin unknown. Fig. 1. Structure of PTX 251D, a lipophilic alkaloid isolated from the skin of some anurans (left Epipedobates tricolor) and from Scheloribatid mites (right). Below, structures of related pumiliotoxines PTX A (307A) and B (323A). ARTICLE IN PRESS 336 T. Vandendriessche et al. / Toxicon 51 (2008) 334–344 Complex multicellular organisms require rapid VGSCs and VGPCs expressed heterologously in the and accurate transmission of information among Xenopus laevis oocyte expression system. cells and tissues, and tight coordination of distant functions. In both vertebrates and invertebrates, 2. Materials and methods electrical signals control a wide range of physiolo- gical processes. All of these processes are mediated 2.1. Toxin in part by members of the voltage-gated ion channel protein superfamily (Yu et al., 2005). VGSCs are Synthetic (+)-PTX 251D was kindly provided by transmembrane proteins responsible for action Prof. U. Nubbemeyer. The synthetic toxin was potential initiation and propagation in excitable found to be identical to the natural occurring (+)- cells, including nerve, muscle and neuroendocrine PTX 251D, based on X-ray analysis (Sudau et al., cell types. The VGSC consists of a pore-forming 2002a, b). a-subunit associated with auxiliary b-subunits. The central, pore-forming a-subunit consists of four 2.2. Heterologous expression of voltage-gated ion homologous domains each of which contains six channels in Xenopus laevis oocytes transmembrane a-helices. Although sodium chan- nels have broadly similar functional characteristics, For in vitro transcription of the tested VGSCs, the small differences in properties do distinguish differ- following a-subunits and b1-subunits were, respec- ent isoforms and contribute to their specialized tively, linearized: hNav1.5/pSP64T with XbaI and functional roles in mammalian and insect physiol- rb1/pSP64T with EcoRI. Capped cRNAs were ogy and pharmacology. Until now nine mammalian synthesized from the linearized plasmid using the (Nav1.1–Nav1.9) and three insect VGSCs have been SP6 mMESSAGE mMACHINE transcription kit cloned (Yu and Catterall, 2003; Yu et al., 2005). In (Ambion, USA). The rNav1.2/pLCT1, hb1/pGEM- excitable cells, voltage-gated potassium channels HE vector, rNav1.4/pUI-2 vector, Para/pGH19-13- (VGPCs) oppose the action of VGSCs and mediate 5 vector and tipE/pGH19 vector were linearized in this way the repolarization phase and firing with, respectively, NotI and NheI(hb1) and tran- frequency of the action potential. VGPCs are also scribed with the T7 mMESSAGE mMACHINE kit found in non-excitable cells like T-lymphocytes (Ambion, USA). where they participate in the immune response For in vitro transcription of the tested VGPCs, (Kv1.3) (Chandy et al., 2004). In contrast to VGSC, the following a-subunits were linearized, respec- a typical VGPC consists of four separate a-subunits tively, with the corresponding restriction endonu- each containing six transmembrane a-helices. The cleases: the rKv1.1/pGEM-HE, rKv1.2/pGEM-HE, first cloned VGPC was the Drosophila voltage-gated hKv1.3/pGEM-HE, hERG/pGEM-HE and the Shaker channel, and this was rapidly followed by Shaker IR (inactivation removed)/pGEM-HE vec- the identification of other VGPC in both insects and tor with PstI, SphI, NotI, EcoRI and NheI. The mammals. The VGPC superfamily can be divided cRNAs were synthesized from the linearized plas- into up to 12 families with different subunits mids using the SP6 mMESSAGE mMACHINE distinguished again by their contribution in specia- transcription kit for hERG and the T7 mMES- lized functional roles in physiology and pharmacol- SAGE mMACHINE transcription kit for rKv1.1, ogy (Gutman et al., 2005; Yu et al., 2005). rKv1.2, hKv1.3 and Shaker IR. Since PTX 251D does not share the known The harvesting of oocytes from anesthetized characteristic of PTXs to positively modulate female X. laevis frogs was as previously des- VGSCs, but is able to provoke convulsions and cribed (Abdel-Mottaleb et al. 2006). Oocytes were death in both mammalians and insects, we searched injected with 50 nl of cRNA at a concentration of in this study for a possible physiological target of 1ngnlÀ1 using a Drummond micro-injector (USA). PTX 251D. Given the crucial role that VGSCs and The solution used for incubating the oocytes VGPCs play in the central and peripheral nervous contained (in mM): NaCl 96, KCl 2, CaCl2 1.8, system, it is not surprising that a number of venoms MgCl2 2 and HEPES 5 (pH 7.4), called ND96 from poison dart frogs, scorpions, snakes, etc. solution, supplemented with 50 mg lÀ1 gentamycin target these channels. Therefore, for the very first sulfate. For expressing of the VGSCs 180 mg lÀ1 time, an electrophysiological approach was used to theophyllin was added in addition to the incubation test the synthetic PTX 251D on an extensive set of solution. ARTICLE IN PRESS T. Vandendriessche et al. / Toxicon 51 (2008) 334–344 337 2.3. Electrophysiological studies on cloned voltage- different, an unpaired t-test (GraphPad Software, gated ion channels Inc.) was used over a 95% confidence interval.
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