Inhibition of Neuronal Nicotinic Acetylcholine Receptor Subtypes by α-Conotoxin GID and Analogues

Emma L Millard,Simon T Nevin,Marion L Loughnan,Annette Nicke,Richard J Clark,Paul F Alewood,Richard J Lewis,David J Adams,David J Craik,Norelle L Daly

doi:10.1074/jbc.m804950200

Emma L Millard, Simon T Nevin + Show 8 more

Open Access

https://doi.org/10.1074/jbc.m804950200

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Abstract

alpha-Conotoxins are small disulfide-rich peptides from the venom of the Conus species that target the nicotinic acetylcholine receptor (nAChR). They are valuable pharmacological tools and also have potential therapeutic applications particularly for the treatment of chronic pain. alpha-Conotoxin GID is isolated from the venom of Conus geographus and has an unusual N-terminal tail sequence that has been shown to be important for binding to the alpha4beta2 subtype of the nAChR. To date, only four conotoxins that inhibit the alpha4beta2 subtype have been characterized, but they are of considerable interest as it is the most abundant nAChR subtype in the mammalian brain and has been implicated in a range of diseases. In this study, analysis of alanine-scan and truncation mutants of GID reveals that a conserved proline in alpha-conotoxins is important for activity at the alpha7, alpha3beta2, and alpha4beta2 subtypes. Although the proline residue was the most critical residue for activity at the alpha3beta2 subtype, Asp(3), Arg(12), and Asn(14) are also critical at the alpha7 subtype. Interestingly, very few of the mutations tested retained activity at the alpha4beta2 subtype indicating a tightly defined binding site. This lack of tolerance to sequence variation may explain the lack of selective ligands discovered for the alpha4beta2 subtype to date. Overall, our findings contribute to the understanding of the structure-activity relationships of alpha-conotoxins and may be beneficial for the ongoing attempts to exploit modulators of the neuronal nAChRs as therapeutic agents.

Highlights

␣-Conotoxins are small, disulfide-rich peptide toxins isolated from the venom of predatory marine snails of the Conus genus and are generally 12–19 amino acids in length [1, 2]
We synthesized and analyzed a series of alanine and truncation mutants of ␣-conotoxin GID to gain an understanding of the importance of the N-terminal tail and the individual residues on both structure and activity at the ␣7, ␣3␤2, and ␣4␤2 subtypes of the neuronal nicotinic acetylcholine receptor (nAChR)
This study has provided the first significant information on ␣-conotoxin interaction with the ␣4␤2 subtype of the nAChR in addition to significant information on the factors influencing activity at the ␣7 and ␣3␤2 nAChR subtypes

Summary

Synthesis of GID Mutants

GID and analogues were manually synthesized by t-butoxycarbonyl chemistry, deprotected, and cleaved from the resin as described previously [11]. The HPLCpurified reduced peptides (100 ␮M) were oxidized in 100 mM ammonium bicarbonate at pH 7.5– 8.2 with stirring for 48 h at 4 °C. The oxidized peptides were purified by preparative RPHPLC. GID was quantified in triplicate by amino acid analysis [12] and RP-HPLC using an external reference standard for each peptide. All other mutants were quantified by absorbance using GID as a reference

Functional Characterization

NMR Solution Structures

Sequences of GID and analogues

Hill slope n

DISCUSSION