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Mar Drugs
2018 Mar 31;164:. doi: 10.3390/md16040112.
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Cloning, Synthesis and Functional Characterization of a Novel α-Conotoxin Lt1.3.
Chen J, Liang L, Ning H, Cai F, Liu Z, Zhang L, Zhou L, Dai Q.
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α-Conotoxins (α-CTxs) are small peptides composed of 11 to 20 amino acid residues with two disulfide bridges. Most of them potently and selectively target nicotinic acetylcholine receptor (nAChR) subtypes, and a few were found to inhibit the GABAB receptor (GABABR)-coupled N-type calcium channels (Cav2.2). However, in all of α-CTxs targeting both receptors, the disulfide connectivity arrangement "C¹-C³, C²-C⁴" is present. In this work, a novel α4/7-CTx named Lt1.3 (GCCSHPACSGNNPYFC-NH₂) was cloned from the venom ducts of Conus litteratus (C. litteratus) in the South China Sea. Lt1.3 was then chemically synthesized and two isomers with disulfide bridges "C¹-C³, C²-C⁴" and "C¹-C⁴, C²-C³" were found and functionally characterized. Electrophysiological experiments showed that Lt1.3 containing the common disulfide bridges "C¹-C³, C²-C⁴" potently and selectively inhibited α3β2 nAChRs and not GABABR-coupled Cav2.2. Surprisingly, but the isomer with the disulfide bridges "C¹-C⁴, C²-C³" showed exactly the opposite inhibitory activity, inhibiting only GABABR-coupled Cav2.2 and not α3β2 nAChRs. These findings expand the knowledge of the targets and selectivity of α-CTxs and provide a new structural motif to inhibit the GABABR-coupled Cav2.2.
Figure 1. The partial cDNA sequence and predicted translation product of Lt1.3. The primer sequences are shaded. The codons of mature peptides are underlined. The nucleotide sequence data are available in the GenBank database under the accession numbers KF414121 for Lt1.3.
Figure 2. HPLC analyses of one-step folding products of linear Lt1.3. Traces from bottom to top: (a) the linear peptide; (b) one-step oxidized products; (c) the purified product of Lt1.3-I; and (d) the purified product of Lt1.3-II. Samples were applied to a Kromasil C18 column (5 μm, 4.6 mm × 250 mm) and eluted with a linear gradient of 5–10% B for 0–1 min; 10–50% B (B is acetonitrile containing 0.1% TFA) for 1–25 min. Absorbance was monitored at 214 nm. The flow rate was 1.0 mL/min.
Figure 3. HPLC analyses of the folded products of linear Lt1.3 with Acm modification. Determination of the disulfide bond connectivity of Lt1.3-I (A) and Lt1.3-II (B). Traces from bottom to top: (a) linear peptide with Acm modifications at Cys1 and Cys3 or Cys 1 and Cys 4; (b) the first oxidized product; (c) the second oxidized product; and (d) the co-elution of the two-step folding products plus the purified product Lt1.3-I or Lt1.3-II (Figure 2). *: C-terminal caboxamide. Analytical conditions were the same as those described in Figure 2.
Figure 4. CD spectra of Lt1.3 in 0.01 M phosphate buffer solution (pH = 7.2, 50% TFE (2,2,2-trifluoroethanol).
Figure 5. Effects of Lt1.3-II and variants on rat nAChRs expressed in Xenopus oocytes. (A) A bar graph of the mean ACH-evoked current amplitude of various rat nAChR subtypes in the presence of 10 μM Lt1.3-II (n = 3–4). (B) Concentration-dependent response curves of the rat α3β2 nAChRs (n = 4–6). (C) IC50 of peptides on various nAChR subtypes. The control peptide of α3β2 was RegIIA (IC50 = 34.5 (26.0–46.7) nM). IC50 of Lt1.3-II and its Ala variants for α3β2 was analyzed by GraphPad Prism and listed in Table 1. Data represent mean ± SEM.
Figure 6. α-Conotoxin Lt1.3 inhibits Cav2.2 channels by activating GABABR in HEK293T cells. (A) Representative superimposed current traces from HEK293T cells co-expressing human GABABR and rat Cav2.2 channels in the absence and presence of Lt1.3-I (100 nM) and baclofen (10 μM, β-(4-chlorophenyl)-γ-aminobutyric acid). (B) Typical peak current amplitude plotted as a function of time in the inhibition of GABABR-coupled Cav2.2 by Lt1.3-I (100 nM) and baclofen (10 μM). (C) Bar graph of inhibition of peak current amplitude by 10 μM Lt1.3-II (3.2 ± 1.8%), 100 nM Lt1.3-I (29.6 ± 4.9%), 100 nM Lt1.3-I + 10 nM Vc1.1 (28.0 ± 3.8%), 10 μM baclofen (64.9 ± 5.1%), 10 μM GABA (75.3 ± 6.7%), CPG55845 + 100 nM Lt1.3-I (2.3 ± 1.6%), 10 μM Lt1.3-I on Cav2.2 alone (4.0 ± 3.0%). Data represent the mean ± SEM. ** p < 0.01, *** p < 0.001 versus Lt1.3-II, one-way analysis of variance. (D) Concentration-response relationship for peptide inhibition of peak current in HEK293T cells co-expressing GABABR and Cav2.2 channels. Data points represent averaged peak Ica amplitudes (I/Icontrol ± SEM); Values of IC50 Vc1.1 (2.4 nM (0.8–7.0)), Lt1.3-I (33.9 nM (10.7–107.1)) (n = 4–6 cells per data point).
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