α-Elapitoxin-Aa2a, a long-chain snake α-neurotoxin with potent actions on muscle (α1)(2)βγδ nicotinic receptors, lacks the classical high affinity for neuronal α7 nicotinic receptors.
In contrast to all classical long-chain α-neurotoxins possessing the critical fifth disulfide bond, α-elapitoxin-Aa2a (α-EPTX-Aa2a), a novel long-chain α-neurotoxin from the common death adder Acanthophis antarcticus, lacks affinity for neuronal α7-type nicotinic acetylcholine receptors (nAChRs). α-EPTX-Aa2a (8850Da; 0.1-1μM) caused a concentration-dependent inhibition of indirect twitches, and blocked contractures to cholinergic agonists in the isolated chick biventer cervicis nerve-muscle preparation, consistent with a postsynaptic curaremimetic mode of action. α-EPTX-Aa2a (1-10nM) produced a potent pseudo-irreversible antagonism of chick muscle nAChRs, with an estimated pA(2) value of 8.311±0.031, which was not reversed by monovalent death adder antivenom. This is only 2.5-fold less potent than the prototypical long-chain α-neurotoxin, α-bungarotoxin. In contrast, α-EPTX-Aa2a produced complete, but weak, inhibition of (125)I-α-bungarotoxin binding to rat hippocampal α7 nAChRs (pK(I)=3.670), despite high sequence homology and similar mass to a wide range of long-chain α-neurotoxins. The mostly likely cause for the loss of α7 binding affinity is a leucine substitution, in loop II of α-EPTX-Aa2a, for the highly conserved Arg(33) in long-chain α-neurotoxins. Arg(33) has been shown to be critical for both neuronal and muscle activity. Despite this substitution, α-EPTX-Aa2a retains high affinity for muscle (α1)(2)βγδ nAChRs. This is probably as a result of an Arg(29) residue, previously shown to be critical for muscle (α1)(2)βγδ nAChR affinity, and highly conserved across all short-chain, but not long-chain, α-neurotoxins. α-EPTX-Aa2a therefore represents a novel atypical long-chain α-neurotoxin that includes a fifth disulfide but exhibits differential affinity for nAChR subtypes.
Blacklow
Benjamin
B
Neurotoxin Research Group, Department of Medical & Molecular Biosciences, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia.
Kornhauser
Rachelle
R
Hains
Peter G
PG
Loiacono
Richard
R
Escoubas
Pierre
P
Graudins
Andis
A
Nicholson
Graham M
GM
D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004546:Elapid Venoms / Q000737:chemistry* / Q000494:pharmacology; D017815:Elapidae / Q000502:physiology*; D005260:Female; D018482:Muscle, Skeletal / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009498:Neurotoxins / Q000737:chemistry* / Q000494:pharmacology*; D054467:Phospholipases A2 / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011978:Receptors, Nicotinic / Q000378:metabolism*
D004546:Elapid Venoms; D009498:Neurotoxins; D011978:Receptors, Nicotinic; C555064:alpha-elapitoxin-Aa2a, Acanthophis antarcticus; D002217:Carbachol; D054467:Phospholipases A2