Physico-chemical characterization and synthesis of neuronally active alpha-conotoxins

Loop2 Size Modification Reveals Significant Impacts on the Potency of α-Conotoxin TxID

α4/6-conotoxin TxID, which was identified from Conus textile, simultaneously blocks rat (r) α3β4 and rα6/α3β4 nicotinic acetylcholine receptors (nAChRs) with IC₅₀ values of 3.6 nM and 33.9 nM, respectively. In order to identify the effects of loop2 size on the potency of TxID, alanine (Ala) insertion and truncation mutants were designed and synthesized in this study. An electrophysiological assay was used to evaluate the activity of TxID and its loop2-modified mutants. The results showed that the inhibition of 4/7-subfamily mutants [+9A]TxID, [+10A]TxID, [+14A]TxID, and all the 4/5-subfamily mutants against rα3β4 and rα6/α3β4 nAChRs decreased. Overall, ala-insertion or truncation of the 9th, 10th, and 11th amino acid results in a loss of inhibition and the truncation of loop2 has more obvious impacts on its functions. Our findings have strengthened the understanding of α-conotoxin, provided guidance for further modifications, and offered a perspective for future studies on the molecular mechanism of the interaction between α-conotoxins and nAChRs.

Concise synthetic routes to human insulin

We report a set of concise and efficient routes for the chemical synthesis of human insulin using a two- or three-step combination procedure that employs Trt, Acm, and t-Bu cysteine protection schemes. Starting with resin-bound assembled A and B chains, human insulin can be obtained within the span of a single work day in 5.4% overall yield based on the crude A or B chain.

Diselenium, instead of disulfide, bonded analogs of conotoxins: novel synthesis and pharmacotherapeutic potential

The venoms of the cone snail (Conus) contain toxic peptides (conotoxins) that have remarkable selectivity for subtypes of a variety of mammalian voltage- and ligand-gated ion channels, G protein-coupled receptors, and neurotransmitter transporters. They thus have tremendous potential as pharmacologic tools. Less toxic analogs or mimetics could be highly-selective pharmacotherapeutic agents at their target sites. For this reason, conopeptides have been extensively studied and have progressed to clinical trials and even regulatory approval. However, the synthesis of the peptides remains difficult and stability and toxicity remain problems. A novel synthesis and testing of analogs incorporating diselenium bonds between selenocysteine residues in place of disulfide bonds between cysteine residues has recently been reported. The technique results in analogs that retain the folding of the native peptides, are more potent, and have the same or greater biological activity.

Synthetic α-conotoxin mutants as probes for studying nicotinic acetylcholine receptors and in the development of novel drug leads

α-Conotoxins are peptide neurotoxins isolated from venomous marine cone snails that are potent and selective antagonists for different subtypes of nicotinic acetylcholine receptors (nAChRs). As such, they are valuable probes for dissecting the role that nAChRs play in nervous system function. In recent years, extensive insight into the binding mechanisms of α-conotoxins with nAChRs at the molecular level has aided in the design of synthetic analogs with improved pharmacological properties. This review examines the structure-activity relationship studies involving α-conotoxins as research tools for studying nAChRs in the central and peripheral nervous systems and their use towards the development of novel therapeutics.

Characterization of a novel alpha4/4-conotoxin, Qc1.2, from vermivorous Conus quercinus

As part of continuing studies of the identification of gene organization and cloning of novel alpha-conotoxins, the first alpha4/4-conotoxin identified in a vermivorous Conus species, designated Qc1.2, was originally obtained by cDNA and genomic DNA cloning from Conus quercinus collected in the South China Sea. The predicted mature toxin of Qc1.2 contains 14 amino acid residues with two disulfide bonds (I-III, II-IV connectivity) in a native globular configuration. The mature peptide of Qc1.2 is supposed to contain an N-terminal post-translationally processed pyroglutamate residue and a free carboxyl C-terminus. This peptide was chemically synthesized and refolded for further characterization of its functional properties. The synthetic Qc1.2 has two interconvertible conformations in aqueous solution, which may be due to the cis-trans isomerization of the two successive Pro residues in its first Cys loop. Using the Xenopus oocyte heterologous expression system, Qc1.2 was shown to selectively inhibit both rat neuronal alpha3beta2 and alpha3beta4 subtypes of nicotinic acetylcholine receptors with low potency. A block of about 63% and 37% of the ACh-evoked currents was observed, respectively, and the toxin dissociated rapidly from the receptors. Compared with other characterized alpha-conotoxin members, the unusual structural features in Qc1.2 that confer to its receptor recognition profile are addressed.

Rapid sensitive analysis of cysteine rich peptide venom components

Disulfide-rich peptide venoms from animals such as snakes, spiders, scorpions, and certain marine snails represent one of nature's great diversity libraries of bioactive molecules. The various species of marine cone shells have alone been estimated to produce >50,000 distinct peptide venoms. These peptides have stimulated considerable interest because of their ability to potently alter the function of specific ion channels. To date, only a small fraction of this immense resource has been characterized because of the difficulty in elucidating their primary structures, which range in size between 10 and 80 aa, include up to 5 disulfide bonds, and can contain extensive posttranslational modifications. The extraordinary complexity of crude venoms and the lack of DNA databases for many of the organisms of interest present major analytical challenges. Here, we describe a strategy that uses mass spectrometry for the elucidation of the mature peptide toxin components of crude venom samples. Key to this strategy is our use of electron transfer dissociation (ETD), a mass spectrometric fragmentation technique that can produce sequence information across the entire peptide backbone. However, because ETD only yields comprehensive sequence coverage when the charge state of the precursor peptide ion is sufficiently high and the m/z ratio is low, we combined ETD with a targeted chemical derivatization strategy to increase the charge state of cysteine-containing peptide toxins. Using this strategy, we obtained full sequences for 31 peptide toxins, using just 7% of the crude venom from the venom gland of a single cone snail (Conus textile).

alpha4/7-conotoxin Lp1.1 is a novel antagonist of neuronal nicotinic acetylcholine receptors

Cone snails comprise approximately 700 species of venomous molluscs which have evolved the ability to generate multiple toxins with varied and exquisite selectivity. alpha-Conotoxin is a powerful tool for defining the composition and function of nicotinic acetylcholine receptors which play a crucial role in excitatory neurotransmission and are important targets for drugs and insecticides. An alpha4/7 conotoxin, Lp1.1, originally identified by cDNA and genomic DNA cloning from Conus leopardus, was found devoid of the highly conserved Pro residue in the first intercysteine loop. To further study this toxin, alpha-Lp1.1 was chemically synthesized and refolded into its globular disulfide isomer. The synthetic Lp1.1 induced seizure and paralysis on freshwater goldfish and selectively reversibly inhibited ACh-evoked currents in Xenopus oocytes expressing rat alpha3beta2 and alpha6alpha3beta2 nAChRs. Comparing the distinct primary structure with other functionally related alpha-conotoxins could indicate structural features in Lp1.1 that may be associated with its unique receptor recognition profile.

AChBP-targeted alpha-conotoxin correlates distinct binding orientations with nAChR subtype selectivity

Neuronal nAChRs are a diverse family of pentameric ion channels with wide distribution throughout cells of the nervous and immune systems. However, the role of specific subtypes in normal and pathological states remains poorly understood due to the lack of selective probes. Here, we used a binding assay based on acetylcholine-binding protein (AChBP), a homolog of the nicotinic acetylcholine ligand-binding domain, to discover a novel alpha-conotoxin (alpha-TxIA) in the venom of Conus textile. Alpha-TxIA bound with high affinity to AChBPs from different species and selectively targeted the alpha(3)beta(2) nAChR subtype. A co-crystal structure of Ac-AChBP with the enhanced potency analog TxIA(A10L), revealed a 20 degrees backbone tilt compared to other AChBP-conotoxin complexes. This reorientation was coordinated by a key salt bridge formed between Arg5 (TxIA) and Asp195 (Ac-AChBP). Mutagenesis studies, biochemical assays and electrophysiological recordings directly correlated the interactions observed in the co-crystal structure to binding affinity at AChBP and different nAChR subtypes. Together, these results establish a new pharmacophore for the design of novel subtype-selective ligands with therapeutic potential in nAChR-related diseases.

?-Conotoxin analogs with additional positive charge show increased selectivity towards Torpedo�californica and some neuronal subtypes of nicotinic acetylcholine receptors

Alpha-conotoxins from Conus snails are indispensable tools for distinguishing various subtypes of nicotinic acetylcholine receptors (nAChRs), and synthesis of alpha-conotoxin analogs may yield novel antagonists of higher potency and selectivity. We incorporated additional positive charges into alpha-conotoxins and analyzed their binding to nAChRs. Introduction of Arg or Lys residues instead of Ser12 in alpha-conotoxins GI and SI, or D12K substitution in alpha-conotoxin SIA increased the affinity for both the high- and low-affinity sites in membrane-bound Torpedo californica nAChR. The effect was most pronounced for [D12K]SIA with 30- and 200-fold enhancement for the respective sites, resulting in the most potent alpha-conotoxin blocker of the Torpedo nAChR among those tested. Similarly, D14K substitution in alpha-conotoxin [A10L]PnIA, a blocker of neuronal alpha7 nAChR, was previously shown to increase the affinity for this receptor and endowed [A10L,D14K]PnIA with the capacity to distinguish between acetylcholine-binding proteins from the mollusks Lymnaea stagnalis and Aplysia californica. We found that [A10L,D14K]PnIA also distinguishes two alpha7-like anion-selective nAChR subtypes present on identified neurons of L. stagnalis: [D14K] mutation affected only slightly the potency of [A10L]PnIA to block nAChRs on neurons with low sensitivity to alpha-conotoxin ImI, but gave a 50-fold enhancement of blocking activity in cells with high sensitivity to ImI. Therefore, the introduction of an additional positive charge in the C-terminus of alpha-conotoxins targeting some muscle or neuronal nAChRs made them more discriminative towards the respective nAChR subtypes. In the case of muscle-type alpha-conotoxin [D12K]SIA, the contribution of the Lys12 positive charge to enhanced affinity towards Torpedo nAChR was rationalized with the aid of computer modeling.

Discovery of a novel class of conotoxin from Conus litteratus, lt14a, with a unique cysteine pattern

Conus litteratus is a worm-hunting cone snail with a highly sophisticated neuropharmacological defense strategy using small peptides in its venom. By analyzing different clones in the cDNA library of venom ducts from C. litteratus, we identified the peptide lt14a which displays a characteristic signal peptide sequence in its precursor and a unique arrangement of Cys residues (-C-C-C-C-) in its mature peptide region. RT-PCR analysis suggested that lt14a is abundantly expressed throughout the whole venom duct. An intensive analysis in sequence suggested that lt14a is similar to alpha-conotoxin qc1.1 cloned from Conus quercinus. We conducted the chemical synthesis of lt14a. The synthetic lt14a has a remarkable biological activity to suppress pain and inhibits the neuronal-type nicotinic acetylcholine receptors.

Identification of a Novel Class of Nicotinic Receptor Antagonists

The venoms of predatory marine snails (Conus spp.) contain diverse mixtures of peptide toxins with high potency and selectivity for a variety of voltage-gated and ligand-gated ion channels. Here we describe the chemical and functional characterization of three novel conotoxins, alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC, purified from the venom of Conus vexillum. Each toxin was observed as an approximately 11-kDa protein by LC/MS, size exclusion chromatography, and SDS-PAGE. After reduction, the peptide sequences were determined by Edman degradation chemistry and tandem MS. Combining the sequence data together with LC/MS and NMR data revealed that in solution these toxins are pseudo-homodimers of paired 47-50-residue peptides. The toxin subunits exhibited a novel arrangement of 10 conserved cystine residues, and additional post-translational modifications contributed heterogeneity to the proteins. Binding assays and two-electrode voltage clamp analyses showed that alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC are potent inhibitors of nicotinic acetylcholine receptors (nAChRs) with selectivity for alpha7 and beta2 containing neuronal nAChR subtypes. These dimeric conotoxins represent a fifth and highly divergent structural class of conotoxins targeting nAChRs.

Therapeutic applications of conotoxins that target the neuronal nicotinic acetylcholine receptor

Pain therapeutics discovered by molecular mining of the expressed genome of Australian predatory cone snails are providing lead compounds for the treatment of neurological diseases such as multiple sclerosis, shingles, diabetic neuropathy and other painful neurological conditions. The high specificity exhibited by these novel compounds for neuronal receptors and ion channels in the brain and nervous system indicates the high degree of selectivity that this class of neuropeptides can be expected to show when used therapeutically in humans. A lead compound, ACV1 (conotoxin Vc1.1 from Conus victoriae), has entered Phase II clinical trials and is being developed for the treatment for neuropathic pain. ACV1 will be targeted initially for the treatment of sciatica, shingles and diabetic neuropathy. The compound is a 16 amino acid peptide [Sandall et al., 2003. A novel alpha-conotoxin identified by gene sequencing is active in suppressing the vascular response to selective stimulation of sensory nerves in vivo. Biochemistry 42, 6904-6911], an antagonist of neuronal nicotinic acetylcholine receptors. It has potent analgesic activity following subcutaneous or intramuscular administration in several preclinical animal models of human neuropathic pain [Satkunanathan et al., 2005. Alpha conotoxin Vc1.1 alleviates neuropathic pain and accelerates functional recovery of injured neurons. Brain. Res. 1059, 149-158]. ACV1 may act as an analgesic by decreasing ectopic excitation in sensory nerves. In addition ACV1 appears to accelerate the recovery of injured nerves and tissues.

NMR of conotoxins: structural features and an analysis of chemical shifts of post-translationally modified amino acids

Conotoxins are small conformationally constrained peptides found in the venom of marine snails of the genus Conus. They are usually cysteine rich and frequently contain a high degree of post-translational modifications such as C-terminal amidation, hydroxylation, carboxylation, bromination, epimerisation and glycosylation. Here we review the role of NMR in determining the three-dimensional structures of conotoxins and also provide a compilation and analysis of 1H and 13C chemical shifts of post-translationally modified amino acids and compare them with data from common amino acids. This analysis provides a reference source for chemical shifts of post-translationally modified amino acids.

Sequencing and mass profiling highly modified conotoxins using global reduction/alkylation followed by mass spectrometry

A novel high-throughput method for characterizing heavily modified peptides from cone snail venom is described. Unpurified cone snail duct venom, consisting primarily of multiply disulfide-bonded peptides, is reduced and alkylated using a global procedure in order to simultaneously reduce and derivatize dozens of disulfide-bonded peptides. Samples of Conus victoriae venom are analyzed by online liquid chromatography-electrospray ionization-ion trap-mass spectrometry (LC-ESI-MS) with collisionally induced dissociation (CID). Comparison of the mass profiles of peptides and CID spectra before and after the global reduction and alkylation enables cysteine-containing conopeptides to be ascertained. In this case, over 40 conotoxins are characterized based on only two LC-ESI-MS experiments in terms of mass, number of disulfide-linked cysteine residues (and hence, potential toxin superfamilies), relative hydrophobicity, and other posttranslational modifications. Using this technique, over half of the amino acids (by mass) of several peptides are defined prior to any detailed sequencing studies. Further comparison of the mass data with previously published genetic information allows sequence verification of three novel peptides, termed vc5b, vc6b and vc6c, based on both LC-ESI-MS CID and nanoelectrospray ionization-ion trap-mass spectrometry (nanoESI-MS) experiments. This global method is ideally suited to the use of larger genetic databases in order to efficiently sequence peptides in Conus venoms and is also applicable to analysis of other disulfide-rich classes of peptides such as defensins, chemokines, and snake, spider, or other venoms.

Isolation and characterisation of conomap-Vt, ad-amino acid containing excitatory peptide from the venom of a vermivorous cone snail

Cone snail venom is a rich source of bioactives, in particular small disulfide rich peptides that disrupt synaptic transmission. Here, we report the discovery of conomap-Vt (Conp-Vt), an unusual linear tetradecapeptide isolated from Conus vitulinus venom. The sequence displays no homology to known conopeptides, but displays significant homology to peptides of the MATP (myoactive tetradecapeptide) family, which are important endogenous neuromodulators in molluscs, annelids and insects. Conp-Vt showed potent excitatory activity in several snail isolated tissue preparations. Similar to ACh, repeated doses of Conp-Vt were tachyphylactic. Since nicotinic and muscarinic antagonists failed to block its effect and Conp-Vt desensitised tissue remained responsive to ACh, it appears that Conp-Vt contractions were non-cholinergic in origin. Finally, biochemical studies revealed that Conp-Vt is the first member of the MATP family with a d-amino acid. Interestingly, the isomerization of L-Phe to D-Phe enhanced biological activity, suggesting that this post-translational modified conopeptide may have evolved for prey capture.

Isosteric replacement of sulfur with other chalcogens in peptides and proteins

The review addresses the functional and structural properties of the two series of chalcogen analogues of amino acids in peptides and proteins, the methionine and the serine/cysteine series, and discusses the synthesis of the related selenium/tellurium analogues as well as their use in peptide synthesis and protein expression. Advances in synthetic methodologies and recombinant technologies and their combined applications in native and expressed protein ligation allows the isomorphous character of selenium- and tellurium-containing amino acids to be exploited for production of heavy metal mutants of proteins and thus to facilitate the phasing problem in x-ray crystallography. In addition, selenocysteine has been recognized as an ideal tool for the production of selenoenzymes with new catalytic activities. Moreover, the fully isomorphous character of disulfide replacement with diselenide is well suited to increase the robustness of cystine frameworks in cystine-rich peptides and proteins and for the de novo design of even non-native cystine frameworks by exploiting the highly negative redox potential of selenols.

Synthesis of single- and multiple-stranded cystine-rich peptides

The large abundance of bioactive single- and multiple-stranded cystine-rich peptides in nature has fostered the development of orthogonal thiol-protection schemes and of efficient chemistries for regioselective disulfide formation in synthetic replica for decades. In parallel to these entirely synthetic strategies, an increased knowledge of oxidative refolding mechanisms of proteins has been accumulated, and the collective experience with air oxidation of cysteine-rich peptides into their native disulfide frameworks have largely confirmed Anfinsen's principle of the self-assembly of polypeptide chains. In fact, a continuously growing number of cysteine-rich bioactive peptides from the most diverse sources and with differing cysteine patterns were found to retain the critical sequence-encoded structural information for correct oxidative folding into the native structures as dominant isomers, although in the biosynthetic pathways the mature peptide forms are mostly generated by posttranslational processing of folded precursors. Such self-assembly processes can be optimized by opportune manipulation of the experimental conditions or by induction of productive intermediates. But there are also numerous cases where folding and disulfide formation are thermodynamically not coupled and where the application of a defined succession of regioselective cysteine pairings still represents the method of choice to install the desired native or non-native cystine frameworks. Among our contributions to the state of the art in the synthesis of cystine-rich peptides, we have mainly addressed the induction of correct oxidative refolding of single-stranded cysteine-rich peptides into their native structures by the use of selenocysteine and suitable strategies for disulfide-mediated assembly of monomers into defined oligomers as mimics of homo- and heterotrimeric collagens as a synthetic approach for the development of new biomaterials.

Structure-activity relationships of alpha-conotoxins targeting neuronal nicotinic acetylcholine receptors

alpha-Conotoxins that target the neuronal nicotinic acetylcholine receptor have a range of potential therapeutic applications and are valuable probes for examining receptor subtype selectivity. The three-dimensional structures of about half of the known neuronal specific alpha-conotoxins have now been determined and have a consensus fold containing a helical region braced by two conserved disulfide bonds. These disulfide bonds define the two-loop framework characteristic for alpha-conotoxins, CCX(m)CX(n)C, where loop 1 comprises four residues (m = 4) and loop 2 between three and seven residues (n = 3, 6 or 7). Structural studies, particularly using NMR spectroscopy have provided an insight into the role and spatial location of residues implicated in receptor binding and biological activity.

Computational approaches to understand alpha-conotoxin interactions at neuronal nicotinic receptors

Recent and increasing use of computational tools in the field of nicotinic receptors has led to the publication of several models of ligand-receptor interactions. These models are all based on the crystal structure at 2.7 A resolution of a protein related to the extracellular N-terminus of nicotinic acetylcholine receptors (nAChRs), the acetylcholine binding protein. In the absence of any X-ray or NMR information on nAChRs, this new structure has provided a reliable alternative to study the nAChR structure. We are now able to build homology models of the binding domain of any nAChR subtype and fit in different ligands using docking programs. This strategy has already been performed successfully for the docking of several nAChR agonists and antagonists. This minireview focuses on the interaction of alpha-conotoxins with neuronal nicotinic receptors in light of our new understanding of the receptor structure. Computational tools are expected to reveal the molecular recognition mechanisms that govern the interaction between alpha-conotoxins and neuronal nAChRs at the molecular level. An accurate determination of their binding modes on the neuronal nAChR may allow the rational design of alpha-conotoxin-based ligands with novel nAChR selectivity.

Alpha-conotoxins as tools for the elucidation of structure and function of neuronal nicotinic acetylcholine receptor subtypes

Cone snails comprise approximately 500 species of venomous molluscs, which have evolved the ability to generate multiple toxins with varied and often exquisite selectivity. One class, the alpha-conotoxins, is proving to be a powerful tool for the differentiation of nicotinic acetylcholine receptors (nAChRs). These comprise a large family of complex subtypes, whose significance in physiological functions and pathological conditions is increasingly becoming apparent. After a short introduction into the structure and diversity of nAChRs, this overview summarizes the identification and characterization of alpha-conotoxins with selectivity for neuronal nAChR subtypes and provides examples of their use in defining the compositions and function of neuronal nAChR subtypes in native vertebrate tissues.