Identification of a Novel O-Conotoxin Reveals an Unusual and Potent Inhibitor of the Human α9α10 Nicotinic Acetylcholine Receptor.

Conotoxins are a pool of disulfide-rich peptide neurotoxins produced by cone snails for predation and defense. They are a rich reservoir of novel ligands for ion channels, neurotransmitter receptors and transporters in the nervous system. In this study, we identified a novel conotoxin component, O-conotoxin GeXXVIIA, from the venom of Conus generalis. The native form of this component is a disulfide-linked homodimer of a 5-Cys-containing peptide. Surprisingly, our electrophysiological studies showed that, in comparison to the folded monomers, the linear peptide of this toxin had the highest inhibitory activity at the human α9α10 nicotinic acetylcholine receptor (nAChR), with an IC50 of 16.2 ± 1.4 nM. The activities of the N-terminal and C-terminal halves of the linear toxin are markedly reduced compared with the full-length toxin, suggesting that the intact sequence is required to potently inhibit the hα9α10 nAChR. α9α10 nAChRs are expressed not only in the nervous system, but also in a variety of non-neuronal cells, such as cochlear hair cells, keratinocytes, epithelial and immune cells. A potent inhibitor of human α9α10 nAChRs, such as GeXXVIIA, would facilitate unraveling the functions of this nAChR subtype. Furthermore, this unusual nAChR inhibitor may lead to the development of novel α9α10 nAChR-targeting drugs.

A Novel Lid-Covering Peptide Inhibitor of Nicotinic Acetylcholine Receptors Derived from αD-Conotoxin GeXXA.

Nicotinic acetylcholine receptors (nAChRs) play a fundamental role in nervous signal transmission, therefore various antagonists and agonists are highly desired to explore the structure and function of nAChRs. Recently, a novel dimeric αD-conotoxin GeXXA was identified to inhibit nAChRs by binding at the top surface of the receptors, and the monomeric C-terminal domain (CTD) of αD-GeXXA retains some inhibitory activity. In this study, the internal dimeric N-terminal domain (NTD) of this conopeptide was further investigated. We first developed a regio-selective protection strategy to chemically prepare the anti-parallel dimeric NTD, and found that the isolated NTD part of GeXXA possesses the nAChR-inhibitory activity, the subtype-dependence of which implies a preferred binding of NTD to the ß subunits of nAChR. Deletion of the NTD N-terminal residues did not affect the activity of NTD, indicating that the N-terminus is not involved in the interaction with nAChRs. By optimizing the sequence of NTD, we obtained a fully active single-chain cyclic NTD, based on which 4 Arg residues were found to interact with nAChRs. These results demonstrate that the NTD part of αD-GeXXA is a "lid-covering" nAChR inhibitor, displaying a novel inhibitory mechanism distinct from other allosteric ligands of nAChRs.

Various conotoxin diversifications revealed by a venomic study of Conus flavidus.

Conotoxins are peptide neurotoxins produced by predatory cone snails. They are mostly cysteine-rich short peptides with remarkable structural diversity. The conserved signal peptide sequences of their mRNA-encoded precursors have enabled the grouping of known conotoxins into a limited number of superfamilies. However, the conotoxins within each superfamily often present variable sequences, cysteine frameworks, and post-translational modifications. To understand better how conotoxins are diversified, we performed a venomic study with C. flavidus, an uninvestigated vermivorous Conus species, by combining transcriptomic and proteomic analyses. In order to obtain the full-length conotoxin sequences, protease digestion was not performed with the venom extraction prior to spectra acquisition via tandem mass spectrometry (MS/MS). Because conotoxins are produced from mRNA-encoded precursors by means of proteolytic cleavage, nonspecific digestion of precursors was applied during the database search. Special attention was also paid in interpreting the MS/MS spectra. All together, these analyses identified 69 nonredundant cDNA sequences and 31 conotoxin components with confident MS/MS spectra. A new Q-superfamily was also identified. More importantly, this study revealed that conotoxin-encoding transcripts are diversified by hypermutation, fragment insertion/deletion, and mutation-induced premature termination, and that a single mRNA species can produce multiple toxin products through alternative post-translational modifications and alternative cleavages of the translated precursor. These multiple diversification strategies at different levels may explain, at least in part, the diversity of conotoxins, and provide the basis for further investigation.

Mid-infrared emission and Raman spectra analysis of Er(3+)-doped oxyfluorotellurite glasses.

This paper reports on the spectroscopic and structural properties in Er(3+)-doped oxyfluorotellurite glasses. The compositional variation accounts for the evolutions of Raman spectra, Judd-Ofelt parameters, radiative properties, and fluorescent emission. It is found that, when maximum phonon energy changes slightly, phonon density plays a crucial role in quenching the 2.7 µm emission generated by the Er(3+):(4)I11/2→(4)I13/2 transition. The comparative low phonon density contributes strong 2.7 µm emission intensity. The high branching ratio (18.63%) and large emission cross section (0.95×10(-20) cm(2)) demonstrate that oxyfluorotellurite glass contained with 50 mol.% TeO2 has potential application in the mid-infrared region laser.

A helical conotoxin from Conus imperialis has a novel cysteine framework and defines a new superfamily.

Cone snail venoms are a rich source of peptides, many of which are potent and selective modulators of ion channels and receptors. Here we report the isolation and characterization of two novel conotoxins from the venom of Conus imperialis. These two toxins contain a novel cysteine framework, C-C-C-CC-C, which has not been found in other conotoxins described to date. We name it framework XXIII and designate the two toxins im23a and im23b; cDNAs of these toxins exhibit a novel signal peptide sequence, which defines a new K-superfamily. The disulfide connectivity of im23a has been mapped by chemical mapping of partially reduced intermediates and by NMR structure calculations, both of which establish a I-II, III-IV, V-VI pattern of disulfide bridges. This pattern was also confirmed by synthesis of im23a with orthogonal protection of individual cysteine residues. The solution structure of im23a reveals that im23a adopts a novel helical hairpin fold. A cluster of acidic residues on the surface of the molecule is able to bind calcium. The biological activity of the native and recombinant peptides was tested by injection into mice intracranially and intravenously to assess the effects on the central and peripheral nervous systems, respectively. Intracranial injection of im23a or im23b into mice induced excitatory symptoms; however, the biological target of these new toxins has yet to be identified.

Conotoxin αD-GeXXA utilizes a novel strategy to antagonize nicotinic acetylcholine receptors.

Nicotinic acetylcholine receptors (nAChRs) play essential roles in transmitting acetylcholine-mediated neural signals across synapses and neuromuscular junctions, and are also closely linked to various diseases and clinical conditions. Therefore, novel nAChR-specific compounds have great potential for both neuroscience research and clinical applications. Conotoxins, the peptide neurotoxins produced by cone snails, are a rich reservoir of novel ligands that target receptors, ion channels and transporters in the nervous system. From the venom of Conus generalis, we identified a novel dimeric nAChR-inhibiting αD-conotoxin GeXXA. By solving the crystal structure and performing structure-guided dissection of this toxin, we demonstrated that the monomeric C-terminal domain of αD-GeXXA, GeXXA-CTD, retains inhibitory activity against the α9α10 nAChR subtype. Furthermore, we identified that His7 of the rat α10 nAChR subunit determines the species preference of αD-GeXXA, and is probably part of the binding site of this toxin. These results together suggest that αD-GeXXA cooperatively binds to two inter-subunit interfaces on the top surface of nAChR, thus allosterically disturbing the opening of the receptor. The novel antagonistic mechanism of αD-GeXXA via a new binding site on nAChRs provides a valuable basis for the rational design of new nAChR-targeting compounds.

Critical effect of peptide cyclization on the potency of peptide inhibitors against Dengue virus NS2B-NS3 protease.

Dengue virus (DENV) infection is a serious public health threat worldwide that demands effective treatment. In the search for potent virus protease inhibitors, several cone snail venoms were screened against serotype 2 DENV NS2B-NS3 protease, and one conotoxin, MrIA, was identified to have inhibitory activity. The inhibitory activity was attributed to a disulfide bond-mediated loop, from which rational optimization was made to improve the potency and stability. An eight-residue cyclic peptide inhibitor was finally obtained with high potency (inhibitory constant 2.2 µM), stability, and cell permeability. This inhibitor can thus serve as a good lead for DENV drug development. In addition, this work highlights the critical effect of peptide cyclization on the potency of oligopeptide inhibitors against DENV protease, which may advance the design of peptide inhibitors for homologous virus proteases.

Small peptides derived from the Lys active fragment of the mung bean trypsin inhibitor are fully active against trypsin.

The Bowman-Birk protease inhibitors have recently attracted attention for their potential as cancer preventive and suppressing agents. They contain two canonical binding loops, both consisting of nine highly conserved residues capable of inhibiting corresponding serine proteases. In this study, we cloned the cDNA of the mung bean trypsin inhibitor, one of the most studied Bowman-Birk protease inhibitors. A modified peptide, Lys33GP, with 33 residues derived from the long chain of the Lys active fragment of mung bean trypsin inhibitor, was successfully expressed in Escherichia coli as a glutathione-S-transferase fusion protein. The recombinant product was obtained with a high yield, and exhibited potent inhibitory activity. Meanwhile, a shorter peptide composed of only 16 residues (the Lys16 peptide), corresponding to the active core of the fragment, was synthesized. Both the recombinant and the synthesized peptides had the same inhibitory activity toward trypsin at a molar ratio of 1 : 1, implying that the Lys16 peptide with two disulfide bonds is possibly the essential structural unit for inhibitory activity. Using site-directed mutagenesis, the P(1) position Lys was replaced by Phe, and the resulting mutant, Lys33K/F, was determined to have potent chymotrypsin inhibitory activity. Both Lys33GP and the Lys33K/F mutant may be potential pharmaceutical agents for the prevention of oncogenesis.