Furutani S et al. (2018), The fungal alkaloid Okaramine-B activates an L-...

XB-ART-55071

Int J Parasitol Drugs Drug Resist 2018 Aug 01;82:350-360. doi: 10.1016/j.ijpddr.2018.06.001.

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The fungal alkaloid Okaramine-B activates an L-glutamate-gated chloride channel from Ixodes scapularis, a tick vector of Lyme disease.

Furutani S , Ihara M , Lees K , Buckingham SD , Partridge FA , David JA , Patel R , Warchal S , Mellor IR , Matsuda K , Sattelle DB .

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A novel L-glutamate-gated anion channel (IscaGluCl1) has been cloned from the black-legged tick, Ixodes scapularis, which transmits multiple pathogens including the agents of Lyme disease and human granulocytic anaplasmosis. When mRNA encoding IscaGluCl1 was expressed in Xenopus laevis oocytes, we detected robust 50-400 nA currents in response to 100 μM L-glutamate. Responses to L-glutamate were concentration-dependent (pEC50 3.64 ± 0.11). Ibotenate was a partial agonist on IscaGluCl1. We detected no response to 100 μM aspartate, quisqualate, kainate, AMPA or NMDA. Ivermectin at 1 μM activated IscaGluCl1, whereas picrotoxinin (pIC50 6.20 ± 0.04) and the phenylpyrazole fipronil (pIC50 6.90 ± 0.04) showed concentration-dependent block of the L-glutamate response. The indole alkaloid okaramine B, isolated from fermentation products of Penicillium simplicissimum (strain AK40) grown on okara pulp, activated IscaGluCl1 in a concentration-dependent manner (pEC50 5.43 ± 0.43) and may serve as a candidate lead compound for the development of new acaricides.

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Genes referenced: tpm3

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	Fig. 1. Multiple sequence alignment of the Ixodes scapularis GluCl (IscaGluCl1) with GluCls from other arthropods and C. elegans. This multiple sequence alignment indicates that IscaGluCl1 shares characteristic sequence features with known GluCls. Multiple sequence alignments were performed with the MAFFT (Yamada et al., 2016) algorithm using Geneious software versiton 9 (Kearse et al., 2012), and the details were adjusted manually - gaps originally produced by MAFFT at N-/C-terminal were removed, and alignment between TM3 and TM4, a poorly conserved region, were corrected in order to reduce gaps. Accession numbers of the GluCls depicted from C. elegans, D. melanogaster, I. scapularis and T. castaneum are AAA50785, AAG40735, ALF36853 and NP_001107775, respectively.
	Fig. 2. Phylogenetic analysis of known arthropod and helminth GluCls. This analysis indicates that IscaGluCl clusters closely with tick GluCl1 homologues, hence the receptor has been labelled IscaGluCl1. The phylogenetic tree was constructed using Geneious software version 9 (Kearse et al., 2012) with genetic distance model of Jukes-Cantor, and with Neighbour-Joining methods. Scale bar indicates substitutions per site.
	Fig. 3. Concentration-dependent responses to L-glutamate and ibotenate of recombinant homomeric IscaGluCl1 expressed in Xenopus laevis oocytes and the current-voltage relationship for the response to L-glutamate. A, rapidly activated and desensitized response to L-glutamate of IscaGluCl1 at three concentrations. B, peak current amplitude of the response to 1 mM L-glutamate at two external Cl− concentrations. C, concentration-response relationship for L-glutamate and ibotenate. Ibotenate is a partial agonist but shows higher affinity for IscaGluCl1 than L-glutamate. Data were normalised to 1 mM L-glutamate. pEC50s are given in Table 1. In B and C, each data plot represents mean ± standard error of the mean (n = 4). D, percent maximal L-glutamate responses to neurotransmitters applied at 1 mM of IscaGluCl1. Each bar graph represents mean ± standard error of the mean (n = 3–6).
	Fig. 4. Inhibition by picrotoxinin and fipronil of recombinant homomeric IscaGluCl1 expressed in Xenopus laevis oocytes. A. Block by picrotoxinin and fipronil (100 nM) of L-glutamate responses (100 μM) recorded from IscaGluCl1 heterologously expressed in Xenopus laevis oocytes. B. Concentration-inhibition relationships for the actions of picrotoxinin and fipronil on responses to L-glutamate of IscaGluCl1. Data were normalised to the response to 100 μM L-glutamate. Each data point is represented by the mean ± standard error of mean (n = 4). Curves are fits to Eq. (1) and IC50s are given in Table 2.
	Fig. 5. Concentration-dependent activation of IscaGluCl1 by ivermectin (A) and okaramine B (B). Each data point represents the mean ± standard error of the mean (n = 4, from 2 experiments). Data were normalised to the response to 1 mM L-glutamate. Curves are fits to Eq. (1). Above the concentration-response curve, individual responses of IscaGluCl1 (recorded with oocyte voltage-clamped at Eh = −100 mV) to 1 μM, 3 μM and 10 μM ivermectin (A) or okaramine B (B) are shown.
	Fig. 6. Positive allosteric modulation by Okaramine B of responses to L-glutamate mediated by IscaGluCl1. A) Responses IscGluCl1 to 100 μM L-glutamate are enhanced by preincubation in 1 μM okaramine-B. B) The enhancement by okaramine-B of responses to 100 μM L-glutamate is concentration-dependent. C) Concentration-response curves for L-glutamate in the presence and absence of 1 μM okaramine-B.
	Fig. 7. Effects of okaramine B on the concentration-response to ivermectin. Each data point represents mean ± standard error of the mean (n = 4).

References [+] :

Buckingham, Oocytes as an expression system for studying receptor/channel targets of drugs and pesticides. 2006, Pubmed, Xenbase