AIM: To summarize the evidence from randomized controlled trials (RCTs) regarding the effect of probiotics by using a meta-analytic approach.
METHODS: In July 2013, we searched PubMed, EMBASE, Ovid, the Cochrane Library, and three Chinese databases (Chinese Biomedical Literature Database, Chinese Medical Current Content, and Chinese Scientific Journals database) to identify relevant RCTs. We included RCTs investigating the effect of a combination of probiotics and standard therapy (probiotics group) with standard therapy alone (control group). Risk ratios (RRs) were used to measure the effect of probiotics plus standard therapy on Helicobacter pylori (H. pylori) eradication rates, adverse events, and patient compliance using a random-effect model.
RESULTS: We included data on 6997 participants from 45 RCTs, the overall eradication rates of the probiotic group and the control group were 82.31% and 72.08%, respectively. We noted that the use of probiotics plus standard therapy was associated with an increased eradication rate by per-protocol set analysis (RR = 1.11; 95%CI: 1.08-1.15; P < 0.001) or intention-to-treat analysis (RR = 1.13; 95%CI: 1.10-1.16; P < 0.001). Furthermore, the incidence of adverse events was 21.44% in the probiotics group and 36.27% in the control group, and it was found that the probiotics plus standard therapy significantly reduced the risk of adverse events (RR = 0.59; 95%CI: 0.48-0.71; P < 0.001), which demonstrated a favorable effect of probiotics in reducing adverse events associated with H. pylori eradication therapy. The specific reduction in adverse events ranged from 30% to 59%, and this reduction was statistically significant. Finally, probiotics plus standard therapy had little or no effect on patient compliance (RR = 0.98; 95%CI: 0.68-1.39; P = 0.889).
CONCLUSION: The use of probiotics plus standard therapy was associated with an increase in the H. pylori eradication rate, and a reduction in adverse events resulting from treatment in the general population. However, this therapy did not improve patient compliance.
Probiotics; Helicobacter pylori; Eradication; Systematic review; Meta-analysis
Inhibitors of the neuronal voltage-gated sodium channel subtype NaV1.3 are of interest as pharmacological tools for the study of neuropathic pain associated with spinal cord injury and have potential therapeutic applications. The recently described μ-conotoxin BuIIIB from Conus bullatus (μ-BuIIIB) was shown to block NaV1.3 with sub-micromolar potency (Kd = 0.2 μM), making it one of the most potent peptidic inhibitors of this subtype described to date. However, oxidative folding of μ-BuIIIB results in numerous folding isoforms, making it difficult to obtain sufficient quantities of the active form of the peptide for detailed structure-activity studies. Here we report the synthesis and characterization of μ-BuIIIB analogs incorporating a disulfide-deficient, diselenide-containing scaffold designed to simplify synthesis and facilitate structure-activity studies directed at identifying amino acid residues involved in NaV1.3 blockade. Our results indicate that, like other μ-conotoxins, the C-terminal residues (Trp16, Arg18 and His20) are most crucial for NaV1 block. At the N-terminus, replacement of Glu3 by Ala resulted in an analog with increased potency for NaV1.3 (Kd = 0.07 μM), implicating this position as a potential site for modification for increased potency and/or selectivity. Further examination of this position showed that increased negative charge, through γ-carboxyglutamate replacement, decreased potency (Kd = 0.33 μM), while replacement with positively-charged 2,4-diamonobutyric acid increased potency (Kd = 0.036 μM). These results provide a foundation for the design and synthesis of μ-BuIIIB-based analogs with increased potency against NaV1.3.
Conotoxin; disulfide; neuropathic pain; selenocysteine; voltage-gated sodium channel
The µ-conotoxin KIIIA is a three disulfide-bridged blocker of voltage-gated sodium channels (VGSCs). The Lys7 residue in KIIIA is an attractive target for manipulating the selectivity and efficacy of this peptide. Here, we report the design and chemical synthesis of µ-conopeptoid analogues (peptomers) in which we replaced Lys7 with peptoid monomers of increasing side-chain size: N-methylglycine, N-butylglycine and N-octylglycine. In the first series of analogues, the peptide core contained all three disulfide bridges; whereas in the second series, a disulfide-depleted selenoconopeptide core was used to simplify oxidative folding. The analogues were tested for functional activity in blocking the Nav1.2 subtype of mammalian VGSCs exogenously expressed in Xenopus oocytes. All six analogues were active, with the N-methylglycine analogue, [Sar7]KIIIA, the most potent in blocking the channels while favoring lower efficacy. Our findings demonstrate that the use of N-substituted Gly residues in conotoxins show promise as a tool to optimize their pharmacological properties as potential analgesic drug leads.
µ-conotoxins; KIIIA, peptomers; selenocysteine; diselenide; sodium channels; electrophysiology
Among the μ-conotoxins that block vertebrate voltage-gated sodium channels (VGSCs), some have been shown to be potent analgesics following systemic administration in mice. We have determined the solution structure of a new representative of this family, μ-BuIIIB, and established its disulfide connectivities by direct mass spectrometric collision induced dissociation fragmentation of the peptide with disulfides intact. The major oxidative folding product adopts a 1-4/2-5/3-6 pattern with the following disulfide bridges: Cys5-Cys17, Cys6-Cys23 and Cys13-Cys24. The solution structure reveals that the unique N-terminal extension in μ-BuIIIB, which is also present in μ-BuIIIA and μ-BuIIIC but absent in other μ-conotoxins, forms part of a short α-helix encompassing Glu3 to Asn8. This helix is packed against the rest of the toxin and stabilized by the Cys5-Cys17 and Cys6-Cys23 disulfide bonds. As such, the side chain of Val1 is located close to the aromatic rings of Trp16 and His20, which are located on the canonical helix that displays several residues found to be essential for VGSC blockade in related μ-conotoxins. Mutations of residues 2 and 3 in the N-terminal extension enhanced the potency of μ-BuIIIB for NaV1.3. One analog, [d-Ala2]BuIIIB, showed a 40-fold increase, making it the most potent peptide blocker of this channel characterized to date and thus a useful new tool with which to characterize this channel. Based on previous results for related μ-conotoxins, the dramatic effects of mutations at the N-terminus were unanticipated, and suggest that further gains in potency might be achieved by additional modifications of this region.
The μ-conotoxin μ-KIIIA, from Conus kinoshitai, blocks mammalian neuronal voltage-gated sodium channels (VGSCs) and is a potent analgesic following systemic administration in mice. We have determined its solution structure using NMR spectroscopy. Key residues identified previously as being important for activity against VGSCs (Lys7, Trp8, Arg10, Asp11, His12 and Arg14) all reside on an α-helix with the exception of Arg14. To further probe structure-activity relationships of this toxin against VGSC subtypes, we have characterised the analogue μ-KIIIA[C1A,C9A], in which the Cys residues involved in one of the three disulfides in μ-KIIIA were replaced with Ala. Its structure is quite similar to that of μ-KIIIA, indicating that the Cys1-Cys9 disulfide bond could be removed without any significant distortion of the α-helix bearing the key residues. Consistent with this, μ-KIIIA[C1A,C9A] retained activity against VGSCs, with its rank order of potency being essentially the same as that of μ-KIIIA, namely, NaV1.2 > NaV1.4 > NaV1.7 ≥ NaV1.1 > NaV1.3 > NaV1.5. Kinetics of block were obtained for NaV1.2, NaV1.4 and NaV1.7, and in each case both kon and koff values of μ-KIIIA[C1A,C9A] were larger than those of μ-KIIIA. Our results show that the key residues for VGSC binding lie mostly on an α-helix and that the first disulfide bond can be removed without significantly affecting the structure of this helix, although the modification accelerates the on- and off-rates of the peptide against all tested VGSC subtypes. These findings lay the groundwork for the design of minimized peptides and helical mimetics as novel analgesics.
In the preparation of synthetic conotoxins containing multiple disulfide bonds, oxidative folding can produce numerous permutations of disulfide bond connectivities. Establishing the native disulfide connectivities thus presents a significant challenge when the venom-derived peptide is not available, as is increasingly the case when conotoxins are identified from cDNA sequences. Here, we investigate the disulfide connectivity of μ-conotoxin KIIIA, which was predicted originally to have a [C1-C9,C2-C15,C4-C16] disulfide pattern based on homology with closely-related μ-conotoxins. The two major isomers of synthetic μ-KIIIA formed during oxidative folding were purified and their disulfide connectivities mapped by direct mass spectrometric CID fragmentation of the disulfide-bonded polypeptides. Our results show that the major oxidative folding product adopts a [C1-C15,C2-C9,C4-C16] disulfide connectivity, while the minor product adopts a [C1-C16,C2-C9,C4-C15] connectivity. Both of these peptides were potent blockers of NaV1.2 (Kd 5 and 230 nM, respectively). The solution structure for μ-KIIIA based on NMR data was recalculated with the [C1-C15,C2-C9,C4-C16] disulfide pattern; its structure was very similar to the μ-KIIIA structure calculated with the incorrect [C1-C9,C2-C15,C4-C16] disulfide pattern, with an α-helix spanning residues 7–12. In addition, the major folding isomers of μ-KIIIB, an N-terminally extended isoform of μ-KIIIA identified from its cDNA sequence, were isolated. These folding products had the same disulfide connectivities as for μ-KIIIA, and both blocked NaV1.2 (Kd 470 and 26 nM, respectively). Our results establish that the preferred disulfide pattern of synthetic μ-KIIIA/μ-KIIIB folded in vitro is 1-5/2-4/3-6 but that other disulfide isomers are also potent sodium channel blockers. These findings raise questions about the disulfide pattern(s) of μ-KIIIA in the venom of Conus kinoshitai; indeed, the presence of multiple disulfide isomers in the venom could provide a means to further expand the snail's repertoire of active peptides.
Disulfide bridges, which stabilize the native conformation of conotoxins impose a challenge in the synthesis of smaller analogs. In this work, we describe the synthesis of a minimized analog of the analgesic μ-conotoxin KIIIA that blocks two sodium channel subtypes, the neuronal NaV1.2 and skeletal muscle NaV1.4. Three disulfide-deficient analogs of KIIIA were initially synthesized in which the native disulfide bridge formed between either C1-C9, C2-C15 or C4-C16 was removed. Deletion of the first bridge only slightly affected the peptide’s bioactivity. To further minimize this analog, the N-terminal residue was removed and two non-essential Ser residues were replaced by a single 5-amino-3-oxapentanoic acid residue. The resulting “polytide” analog retained the ability to block sodium channels and to produce analgesia. Until now, the peptidomimetic approach applied to conotoxins has progressed only modestly at best; thus, the disulfide-deficient analogs containing backbone spacers provide an alternative advance toward the development of conopeptide-based therapeutics.
conopeptide; conotoxin; sodium channels; backbone spacers; disulfide bridges
Background and Purpose
Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive NaV1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV1-isoforms expressed in Xenopus laevis oocytes to conclude that action potentials of A- and C-fibres in rat sciatic nerve were, respectively, mediated primarily by NaV1.6 and NaV1.7.
We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive INa of voltage-clamped neurons acutely dissociated from adult rat DRG. We examined only small and large neurons whose respective INa were >50% and >80% TTX-sensitive.
In both small and large neurons, the ability of the toxins to block TTX-sensitive INa was μ-TIIIA < μ-PIIIA < μ-SmIIIA, with the latter blocking ≳90%. Comparison of the toxin-susceptibility profiles of the neuronal INa with recently acquired profiles of rat NaV1-isoforms, co-expressed with various NaVβ-subunits in X. laevis oocytes, were consistent: NaV1.1, 1.6 and 1.7 could account for all of the TTX-sensitive INa, with NaV1.1 < NaV1.6 < NaV1.7 for small neurons and NaV1.7 < NaV1.1 < NaV1.6 for large neurons.
Conclusions and Implications
Combinations of μ-conotoxins can be used to determine the probable NaV1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.
μ-conotoxin PIIIA; μ-conotoxin SmIIIA; μ-conotoxin TIIIA; dorsal root ganglion; superior cervical ganglion; tetrodotoxin; voltage-gated sodium channel; whole-cell patch clamp
Background and Purpose
Voltage-gated sodium channels (VGSCs) are assembled from two classes of subunits, a pore-bearing α-subunit (NaV1) and one or two accessory β-subunits (NaVβs). Neurons in mammals can express one or more of seven isoforms of NaV1 and one or more of four isoforms of NaVβ. The peptide μ-conotoxins, like the guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX), inhibit VGSCs by blocking the pore in NaV1. Hitherto, the effects of NaVβ-subunit co-expression on the activity of these toxins have not been comprehensively assessed.
Four μ-conotoxins (μ-TIIIA, μ-PIIIA, μ-SmIIIA and μ-KIIIA), TTX and STX were tested against NaV1.1, 1.2, 1.6 or 1.7, each co-expressed in Xenopus laevis oocytes with one of NaVβ1, β2, β3 or β4 and, for NaV1.7, binary combinations of thereof.
Co-expression of NaVβ-subunits modifies the block by μ-conotoxins: in general, NaVβ1 or β3 co-expression tended to increase kon (in the most extreme instance by ninefold), whereas NaVβ2 or β4 co-expression decreased kon (in the most extreme instance by 240-fold). In contrast, the block by TTX and STX was only minimally, if at all, affected by NaVβ-subunit co-expression. Tests of NaVβ1 : β2 chimeras co-expressed with NaV1.7 suggest that the extracellular portion of the NaVβ subunit is largely responsible for altering μ-conotoxin kinetics.
Conclusions and Implications
These results are the first indication that NaVβ subunit co-expression can markedly influence μ-conotoxin binding and, by extension, the outer vestibule of the pore of VGSCs. μ-Conotoxins could, in principle, be used to pharmacologically probe the NaVβ subunit composition of endogenously expressed VGSCs.
μ-conotoxin KIIIA; μ-conotoxin PIIIA; μ-conotoxin SmIIIA; μ-conotoxin TIIIA; NaVβ-subunit; saxitoxin; site 1; tetrodotoxin; voltage-gated sodium channel; Xenopus oocytes
Starting from the active ingredient shikimic acid (SA) of traditional Chinese medicine and NH2(CH2)nOH, (n = 2–6), we have synthesized a series of new water-soluble Pt(II) complexes PtLa–eCl2, where La–e are chelating diamine ligands with carbon chain covalently attached to SA (La–e = SA-NH(CH2)nNHCH2CH2NH2; La, n = 2; Lb, n = 3; Lc, n = 4; Ld, n = 5; Le, n = 6). The results of the elemental analysis, LC-MS, capillary electrophoresis, and 1H, 13C NMR indicated that there was only one product (isomer) formed under the present experimental conditions, in which the coordinate mode of PtLa–eCl2 was two-amine bidentate. Their in vitro cytotoxic activities were evaluated by MTT method, where these compounds only exhibited low cytotoxicity towards BEL7404, which should correlate their low lipophilicity. The interactions of the five Pt(II) complexes with DNA were investigated by agarose gel electrophoresis, which suggests that the Pt(II) complexes could induce DNA alteration. We also studied the interactions of the Pt(II) complexes with 5′-GMP with ESI-MS and 1H NMR and found that PtLbCl2, PtLcCl2, and PtLdCl2 could react with 5′-GMP to form mono-GMP and bis-GMP adducts. Furthermore, the cell-cycle analysis revealed that PtLbCl2, PtLcCl2 cause cell G2-phase arrest after incubation for 72 h. Overall, these water-soluble Pt(II) complexes interact with DNA mainly through covalent binding, which blocks the DNA synthesis and replication and thus induces cytotoxicity that weakens as the length of carbon chain increases.
μ-Conotoxin KIIIA (μ-KIIIA) blocks mammalian voltage-gated sodium channels (VGSCs) and is a potent analgesic following systemic administration in mice. Previous structure-activity studies of μ-KIIIA identified a helical pharmacophore for VGSC blockade. This suggested a route for designing truncated analogues of μ-KIIIA by incorporating the key residues into an α-helical scaffold. As (i, i+4) lactam bridges constitute a proven approach for stabilizing α-helices, we designed and synthesized six truncated analogues of μ-KIIIA containing single lactam bridges at various locations. The helicity of these lactam analogues was analysed by NMR spectroscopy, and their activities were tested against mammalian VGSC subtypes NaV1.1 through 1.7. Two of the analogues, Ac-cyclo9/13[Asp9,Lys13]KIIIA7–14 and Ac-cyclo9/13[Lys9,Asp13]KIIIA7–14, displayed µM activity against VGSC subtypes NaV1.2 and NaV1.6; importantly, the subtype selectivity profile for these peptides matched that of μ-KIIIA. Our study highlights structure-activity relationships within these helical mimetics and provides a basis for the design of additional truncated peptides as potential analgesics.
The structure, assembly, and function of the bacterial flagellum involves about 60 different proteins, many of which are selectively secreted via a specific type III secretion system (T3SS) (J. Frye et al., J. Bacteriol. 188:2233–2243, 2006). The T3SS is reported to secrete proteins at rates of up to 10,000 amino acid residues per second. In this work, we showed that the flagellar T3SS of Salmonella enterica serovar Typhimurium could be manipulated to export recombinant nonflagellar proteins through the flagellum and into the surrounding medium. We translationally fused various neuroactive peptides and proteins from snails, spiders, snakes, sea anemone, and bacteria to the flagellar secretion substrate FlgM. We found that all tested peptides of various sizes were secreted via the bacterial flagellar T3SS. We subsequently purified the recombinant μ-conotoxin SIIIA (rSIIIA) from Conus striatus by affinity chromatography and confirmed that T3SS-derived rSIIIA inhibited mammalian voltage-gated sodium channel NaV1.2 comparably to chemically synthesized SIIIA.
Manipulation of the flagellar secretion system bypasses the problems of inclusion body formation and cellular degradation that occur during conventional recombinant protein expression. This work serves as a proof of principle for the use of engineered bacterial cells for rapid purification of recombinant neuroactive peptides and proteins by exploiting secretion via the well-characterized flagellar type III secretion system.
Autoimmune pancreatitis (AIP) is a rare disorder frequently manifesting as a mass-like lesion that may lead to obstructive jaundice. We report here a case of pancreatic obstruction with painless jaundice, and elevation of CA 19-9 without elevation of serum IgG4. Contrast enhanced ultrasonography (CE US) revealed the possibility of AIP, and the final pathological findings confirmed the diagnosis.
AIP; CEUS; IgG4; diagnosis; case report
Despite the therapeutic promise of disulfide-rich, peptidic natural products, their discovery and structure/function studies have been hampered by inefficient oxidative folding methods for their synthesis. Here we report that converting the three disulfide-bridged μ-conopeptide KIIIA into a disulfide-depleted selenoconopeptide (by removal of a noncritical disulfide bridge and substitution of another disulfide bridge with a diselenide bridge) dramatically simplified its oxidative folding while preserving the peptide’s ability to block voltage-gated sodium channels. The simplicity of synthesizing disulfide-depleted selenopeptide analogues containing a single disulfide bridge allowed rapid positional scanning at Lys7 of μ-KIIIA, resulting in the identification of K7L as a mutation that improved the peptide’s selectivity in blocking a neuronal (Nav1.2) over a muscle (Nav1.4) subtype of sodium channel. The disulfide-depleted selenopeptide strategy offers regioselective folding compatible with high-throughput chemical synthesis and on-resin oxidation methods, and thus shows great promise to accelerate the use of disulfide-rich peptides as research tools and drugs.
Conotoxins; diselenide bridges; selenocysteines; oxidative folding; disulfide-rich peptides
The possibility of independently manipulating the affinity and efficacy of pore-blocking ligands of sodium channels is of interest for the development of new drugs for the treatment of pain. The analgesic µ-conotoxin KIIIA, a 16-residue peptide with three disulfide bridges, is a pore-blocker of voltage-gated sodium channels, including the neuronal subtype NaV1.2 (Kd of 5 nM). At saturating concentrations, µ-KIIIA incompletely blocks the sodium current of NaV1.2, leaving a 5% residual current (rINa). Lys7 is an important residue: the mutation K7A decreases both the efficacy (i.e., increases rINa to 23%) and the affinity of the peptide (Kd, 115 nM). In this report, various replacements of residue 7 were examined to determine whether affinity and efficacy were inexorably linked. Because of their facile chemical synthesis, KIIIA analogs were used that had as a core structure the disulfide-depleted KIIIA[C1A,C2U,C9A,C5U] (where U is selenocysteine) or ddKIIIA. The analogs ddKIIIA and ddKIIIA[K7X], where X represents one of nine different amino acids, were tested on voltage-clamped Xenopus oocytes expressing rat NaV1.2 or NaV1.4. Their affinities ranged from 0.01 to 36 µM and rINa's from 2 to 42%, and these two variables appeared uncorrelated. Instead, rINa varied inversely with side chain size, and remarkably charge and hydrophobicity appeared inconsequential. The ability to manipulate a µ-conopeptide's affinity and efficacy, as well as its capacity to interfere with subsequent tetrodotoxin-binding, greatly expands its scope as a reagent to probe sodium channel structure and function, and may also lead to the development of µ-conotoxins as safe analgesics.
Despite the therapeutic promise of disulfide-rich, peptidic natural products, their discovery and structure/function studies have been hampered by inefficient oxidative folding methods for their synthesis. Here we report that converting the three disulfide-bridged μ-conopeptide KIIIA into a disulfide-depleted selenoconopeptide (by removal of a noncritical disulfide bridge and substitution of a disulfide- with a diselenide-bridge) dramatically simplified its oxidative folding while preserving the peptide’s ability to block voltage-gated sodium channels. The simplicity of synthesizing disulfide-depleted selenopeptide analogs containing a single disulfide bridge allowed rapid positional scanning at Lys7 of μ-KIIIA, resulting in the identification of K7L as a mutation that improved the peptide’s selectivity in blocking a neuronal (Nav1.2) over a muscle (Nav1.4) subtype of sodium channel. The disulfide-depleted selenopeptide strategy offers regioselective folding compatible with high throughput chemical synthesis and on-resin oxidation methods, and thus shows great promise to accelerate the use of disulfide-rich peptides as research tools and drugs.
conotoxins; diselenide bridges; selenocysteines; oxidative folding; disulfide-rich peptides
Chemical synthesis of disulfide-rich peptides requires improvements in oxidative folding and disulfide mapping. To address these challenges, we combined the use of diselenide and selectively (15N/ 13C)-labeled disulfide bridges. Conotoxin analogs, each with a pair of selenocysteines and labeled cysteines, exhibited significantly improved folding while the labeled cysteines allowed correctly folded species to be rapidly identified by NMR.
diselenide; disulfide; oxidative folding; NMR; conotoxin
Tetrodotoxin (TTX) is the quintessential ligand of voltage-gated sodium channels (NaVs). Like TTX, μ-conotoxin peptides are pore blockers, and both toxins have helped to define the properties of neurotoxin receptor Site 1 of NaVs. Here, we report unexpected results showing that the recently discovered μ-conotoxin KIIIA and TTX can simultaneously bind to Site 1 and act in concert. Results with saturating concentrations of peptide applied to voltage-clamped Xenopus oocytes expressing brain NaV1.2, and single-channel recordings from brain channels in lipid bilayers, show that KIIIA or its analog, KIIIA[K7A], block partially, with a residual current that can be completely blocked by TTX. In addition, the kinetics of block by TTX and peptide are each affected by the prior presence of the other toxin. For example, bound peptide slows subsequent binding of TTX (an antagonistic interaction) and slows TTX dissociation when both toxins are bound (a synergistic effect on block). The overall functional consequence resulting from the combined action of the toxins depends on the quantitative balance between these opposing actions. The results lead us to postulate that in the bi-liganded NaV complex, TTX is bound between the peptide and the selectivity filter. These observations refine our view of Site 1 and open new possibilities in NaV pharmacology.
conotoxin; contratoxin; NaV1.2; oocyte; sodium channel; site 1; syntoxin; tetrodotoxin; voltage clamp
Described herein is a general approach to identify novel compounds using the biodiversity of a megadiverse group of animals; specifically, the phylogenetic lineage of the venomous gastropods that belong to the genus Conus (“cone snails”). Cone snail biodiversity was exploited to identify three new μ-conotoxins, BuIIIA, BuIIIB and BuIIIC, encoded by the fish-hunting species Conus bullatus. BuIIIA, BuIIIB and BuIIIC are strikingly divergent in their amino acid composition compared to previous μ-conotoxins known to target the voltage-gated Na channel skeletal muscle subtype Nav1.4. Our preliminary results indicate that BuIIIB and BuIIIC are potent inhibitors of Nav1.4 (average block ~96%, at a 1 μM concentration of peptide), displaying a very slow off-rate not seen in previously characterized μ-conotoxins that block Nav1.4. In addition, the three new Conus bullatus μ-conopeptides help to define a new branch of the M-superfamily of conotoxins, namely M-5. The exogene strategy used to discover these Na channel-inhibiting peptides was based on both understanding the phylogeny of Conus, as well as the molecular genetics of venom μ-conotoxin peptides previously shown to generally target voltage-gated Na channels. The discovery of BuIIIA, BuIIIB and BuIIIC Na channel blockers expands the diversity of ligands useful in determining the structure-activity relationship of voltage-gated sodium channels.
Biodiversity-derived compounds; Sodium channel ligands; exogenes
Disulfide-rich peptides represent a megadiverse group of natural products with very promising therapeutic potential. To accelerate their functional characterization, high-throughput chemical synthesis and folding methods are required, including efficient mapping of multiple disulfide bridges. Here, we describe a novel approach for such mapping and apply it to a three-disulfide bridged conotoxin, μ-SxIIIA (from the venom of Conus striolatus) whose discovery is also reported here for the first time. μ-SxIIIA was chemically synthesized with three cysteine residues labeled 100% with 15N/13C, while the remaining three cysteine residues were incorporated using a mixture of 70%:30% unlabeled:labeled Fmoc-protected residues. After oxidative folding, the major product was analyzed by NMR spectroscopy. Sequence-specific resonance assignments for the isotope-enriched Cys residues were determined with 2D versions of standard triple resonance (1H,13C,15N) NMR experiments and 2D [13C,1H] HSQC. Disulfide patterns were directly determined with cross-disulfide NOEs confirming that the oxidation product had the disulfide connectivities characteristic of μ-conotoxins. μ-SxIIIA was found to be a potent blocker of the sodium channel subtype NaV1.4 (IC50 = 7 nM). These results suggest that differential incorporation of isotope-labeled cysteine residues is an efficient strategy to map disulfides and should facilitate the discovery and structure-function studies of many bioactive peptides.
The excitotoxic conopeptide ι-RXIA induces repetitive action potentials in frog motor axons and seizures upon intracranial injection into mice. We recently discovered that ι-RXIA shifts the voltage-dependence of activation of voltage-gated sodium channel NaV1.6 to a more hyperpolarized level. Here, we performed voltage-clamp experiments to examine its activity against rodent NaV1.1 through NaV1.7 co-expressed with the β1 subunit in Xenopus oocytes and NaV1.8 in dissociated mouse DRG neurons. The order of sensitivity to ι-RXIA was NaV1.6 > 1.2 > 1.7, and the remaining subtypes were insensitive. The time course of ι-RXIA-activity on NaV1.6 during exposure to different peptide concentrations were well fit by single-exponential curves that provided kobs. The plot of kobs versus [ι-RXIA] was linear, consistent with a bimolecular reaction with a Kd of ~3 μM, close to the steady-state EC50 of ~2 μM. ι-RXIA has an unusual residue, D-Phe, and the analog with an L-Phe instead, ι-RXIA[L-Phe44], had a two-fold lower affinity and two-fold faster off-rate than ι-RXIA on NaV1.6 and furthermore was inactive on NaV1.2. ι-RXIA induced repetitive action potentials in mouse sciatic nerve with conduction velocities of both A- and C-fibers, consistent with the presence of NaV1.6 at nodes of Ranvier as well as in unmyelinated axons. Sixteen peptides homologous to ι-RXIA have been identified from a single species of Conus, so these peptides represent a rich family of novel sodium channel-targeting ligands.
channel-activation; conopeptide; excitotoxin; iota-conotoxin RXIA; neurotoxin; voltage-gated sodium channel