The existence of venomous animals represents a unique starting point for bio-discovery and drug design. Over millions of years, nature has optimized the constituents of venoms (i.e., peptide toxins) as the most selective and potent tools on Earth [1
]. Therefore, such toxins can be used as lead compounds for a novel generation of drugs. The venom peptides from cone snails (genus Conus
) are generally small cysteine-rich peptides with the unique feature of being highly selective and potent ligands for a wide range of ion channels and receptors [3
]. Consequently, they are recognized as lead compounds in the quest for novel therapeutics in diseases, such as multiple sclerosis, epilepsy, long QT syndrome and many other neurological disorders [4
Conopeptides are classified in two main groups based on the presence and number of cysteine bonds, namely disulfide-rich and disulfide-poor conopeptides. The peptides from the first category are called conotoxins and have multiple disulfide bonds. Peptides from the second category contain none or only one disulfide bond. Disulfide-poor conopeptides are subdivided into contulakines (interacting with neurotensin receptors), conantokines (interacting with N
-aspartate receptor), conorfamids (interacting with RFamide receptor), conolysines (interacting with cellular membranes), conopressins (interacting with vasopressin receptors), contryphans (interacting with CaV
channels), conophans (target unknown), conomarphines (target unknown) and conomaps (target unknown) [6
]. The disulfide-rich conotoxins are classified into superfamilies based on a conserved signal sequence and their characteristic cysteine network. Conotoxins with a similar cysteine network carry a similar signal sequence. Further subdivision in families is made based on the receptor with which they interact [6
]. These receptors are mainly voltage- and ligand-gated ion channels. Some components act on G-protein coupled receptors and neurotransmitter transporters, whereas others show enzymatic activity [2
]. To date, at least 16 superfamilies have been discovered [6
Voltage-gated sodium channels (VGSCs or NaV
s) are transmembrane proteins that are activated by depolarization of the cell membrane. In excitable cells, they play a central role in the generation and propagation of action potentials, in close collaboration with other channels like voltage-gated potassium channels [8
]. Nine mammalian channel isoforms of the NaV
1.X subfamily have been functionally characterized, and several insect channels have been successfully cloned and expressed [9
]. The mammalian NaV
isoforms have similar structural and functional properties, but are present in different cell types (neurons, neuro-endocrine cells, skeletal muscle cells, heart cells); and they possess diverse functional properties in the corresponding tissues [10
]. Furthermore, defective NaV
s cause several diseases or channelopathies, such as epileptic disorders, neuromuscular diseases, cancer and cardiomyopathies. Blocking the aberrant Na+
current can be an effective strategy in treating these disorders [11
The molecular diversity of K+
channels is larger than any other group of ion channels, with more than 80 different genes and many splice variants [12
]. Voltage-gated potassium channels (VGPC or KV
s) are responsible for the repolarization of membranes following a neuron-initiated action potential. KV
channels are specifically and widely distributed and are located in the brain, nervous system, heart, skeletal muscle, hematopoietic cells, lymphocytes and osteoclasts [13
]. They are involved in many physiological processes, such as regulation of heart rate, neuronal excitability, muscle contraction, neurotransmitter release, insulin secretion, Ca2+
signaling, cellular proliferation and migration and cell volume regulation [14
Nicotinic acetylcholine receptors (nAChRs) are a member of the ligand-gated cationic channel family and mediate fast synaptic transmission. They are broadly distributed throughout the peripheral and central nervous systems of both simple and evolutionarily complex organisms [15
]. In mammals, there are 16 different nAChR subunits: nine different α-subunits (α1–7
), four β-subunits (β1–4
), as well as γ, δ and ε subunits. Five of these subunits combine to form muscle nAChR subtypes (α1
γδ and α1
δε), which are found at neuromuscular junctions, whereas the rest (α2
) assembles in numerous homomeric (α-subunits exclusively) or heteromeric (α- and β-subunits) neuronal nAChR subtypes [16
]. The assembly of different pentamers forms a complex variety of nAChR subtypes with different pharmacological and biophysical properties [17
This study describes the isolation and purification of novel conopeptides from Conus longurionis
, Conus asiaticus
and Conus australis
that were collected from the Tamil Nadu coast, in the Indian Ocean. To the best of our knowledge, this is the first report of a conopeptide from the venom of C. asiaticus
, while other conotoxins from C. longurionis
and C. australis
have been described earlier [18
]. Toxins were electrophysiologically screened against a panel of NaV
s, as well as nAChRs. Moreover, in the quest of identifying novel pharmacological targets of conopeptides, we tested these peptides for potential antimicrobial activity.
In this report, we describe five novel conopeptides, discovered in the venom of C. longurionis, C. asiaticus and C. australis. These novel sequences have different cysteine frameworks and some of them likely represent new subgroups, based on sequence comparison with known conotoxins.
3.1. Conopeptide Alignment and Classification
Peptide Lo6/7a is a 24-residue conotoxin isolated from the venom of C. longurionis
. Depending on the target, peptides from the O-superfamily are subdivided into different families: ω-conotoxins act on CaV
channels; κ-conotoxins target KV
channels; and μO- or δ-conotoxins influence NaV
channels. By performing a Conoserver alignment search on Lo6/7a, the highest percentage of similarity (92%) was obtained for Pr6c from Conus parius
] (Figure 8
). Up to now, the target from Pr6c also remains to be discovered, but the authors suggested the peptide to be either an ω- or κ-conotoxin. Despite the high sequence identity of Lo6/7a with Pr6c, we could not demonstrate such activity on CaV
channels unequivocally. Another peptide from Conus textile
(a peptide causing convulsions in mice) has a similarity percentage of 59% [20
]. This peptide induces symptoms characterized by ‘‘sudden jumping activity followed by convulsions, stretching of limbs and jerking behavior’’. The authors predicted that this peptide belongs to a new undefined class of conotoxins. Two other peptides, Vc7.4 and Vc7.3, from Conus victoriae
were described by Robinson et al. (2014) [21
]. In this study, the precursor sequences of Vc7.4 and Vc7.3 were identified, and it was shown that these peptides, as well as the textile convulsant peptide (C. textile
), are members of a previously-undefined conotoxin superfamily, which was designated U-superfamily. This peptide superfamily shares the cysteine framework (VI/VII) of most members of the O1-, O2- and O3-superfamilies. However, the pre- and pro-peptide sequences substantially differ from other known conotoxin superfamilies. Moreover, when the O-superfamily is compared with the U-superfamily, there is little similarity in the intercysteine loop composition or length (i.e., the U-superfamily has only two residues, while the O-superfamily conotoxins have six) [21
]. The specific physiological target of these peptides has not yet been derived. However, given the similarity in the mature peptide sequence of these conotoxins with Lo6/7a, it is likely that they belong to the same superfamily and share a similar target.
Peptide Lo6/7b aligns with members of the O-superfamily, although with low percentages of similarity (Figure 9
). LtVIC is the only peptide from which a physiological target has been identified up to now. This conotoxin inhibits sodium currents in adult rat dorsal root ganglion neurons [26
]. Therefore, LtVIC is considered a μ(O
)-conotoxin. In our electrophysiological set-up, we could not identify Lo6/7b as a μ(O
)-conotoxin. Nevertheless, the similarity of Lo6/7a with LtVIC (46%) is rather low.
Asi3a is classified in the M-superfamily, acting generally on NaV
s (μ-conotoxins), KV
s (κM-conotoxins) and nAChRs (ψ-conotoxins). Asi3a shows most identity with conotoxin Pr3a from Conus parius
(87%) (Figure 10
). Jimenez et al. [22
] classified this peptide as an M-superfamily conotoxin and performed a bioassay that was carried out by intraperitoneal injection of fish. The purified peptide Pr3a (1 nmol) resulted in paralysis of the fish after ~5 min. A functional characterization of peptides similar to Asi3a has not yet been performed.
Asi14a belongs to the A-, L- or J- superfamilies acting typically on nAChRs (L- and A-superfamily). The J-superfamily characteristically targets KV channels. No meaningful alignment with other A-, L- or J-superfamily peptides could be performed. Therefore, we conclude that Asi14a probably belongs to a new subclass of framework XIV proteins. A BLAST homology search with Asi14a did not reveal similarity to any known peptide or protein.
AusB is an unusual peptide found in the venom of C. australis. Containing 18 amino acids, AusB has only one cysteine bond classifying it among the disulfide poor conopeptides. A Conoserver search resulted in a poor quality alignment, and a BLAST did not align the peptide with other relevant peptides either. Since AusB could not be matched with any disulfide-poor conotoxin, this peptide represents a new family, which we will label ConoGAY peptides, named after the first three N-terminal amino acids of this peptide.
3.2. Antagonistic Assays in the Quest of Identifying Novel Pharmacological Targets
Literature indications for conotoxins as potential antimicrobial compounds are given by Biggs et al. [34
], Jiang et al. [35
] and Takada et al. [36
]. Biggs et al. (2007) discovered conolysin-Mt, a disulfide-poor conopeptide that was initially tested on oocytes where it causes membrane potential collapse within seconds. The peptide was also evaluated for antagonism against three bacterial strains: E. coli
D21, E. coli
ATCC 25922 and S. aureus
ATCC 6538. The authors noticed low antibacterial activity against the two E. coli
strains tested, with a minimal inhibitory concentration (MIC) > 50 μM. The MIC against the Gram-positive S. aureus
was 25–50 μM [34
]. Jiang et al. (2011) tested a cysteine-rich peptide library mimicking μ-conotoxins from Conus geographus
on antiviral activity against influenza virus [35
]. Finally, Takada et al. (2006) showed that asteropine A, a sialidase-inhibiting conotoxin-like peptide from the marine sponge Asteropus simplex
, might be an important lead compound for antibacterial and antiviral drug development [36
]. This is interesting since multidrug resistant bacterial infections are a growing global health problem. Antimicrobial peptides from poisonous animals are described for a number of scorpion peptides, as well as peptides from snakes, frogs, bees (Apis
sp.), etc., as part of their host defense system [37
]. For scorpions in particular, it has been proposed that the presence of antibacterial peptides protects the venom gland from pathogenic infections or potentiates toxin action [47
]. Scorpion antimicrobial peptides (AMP) are positively-charged amphipathic peptides divided into three structural categories: (1) cysteine containing peptides with mainly three or four disulfide bridges; (2) peptides with an amphipathic α-helix, but lacking cysteine residues; and (3) peptides rich in Pro and Gly amino acids. One example of a cysteine containing scorpion AMP is scorpine, which showed activity against both Gram-positive (B. subtilis
) and Gram-negative (K. pneumonia
) bacteria (MIC 1–10 μM) [47
]. When it comes to conotoxins, this path of investigation remains underexplored.
In this work, all peptides were electrophysiologically tested on relevant ion channels predicted by their cysteine arrangement. These results indicate novel functionalities other than expected based on their cysteine framework, since no activity could be identified on all of the targets studied. In order to further determine the mode of action and the potential molecular targets of these conotoxins, in vivo or ex vivo assays should be performed. As such, symptoms observed after intracranial injection of toxins in mice may provide indications on the type of receptor or channel targeted. Furthermore experiments on neuromuscular preparations may possibly identify a pre- or post-synaptic effect or even sodium/potassium or nicotinic antagonism. In addition, a broad screening was performed against a collection of micro-organisms. Low and very specific activity was observed for Lo6/7a against Bacillus megaterium
ATCC13632. Since 1 mM is a very high test concentration and the halo was small, the inhibitory effect of Lo6/7a cannot be attributed as the main action of this peptide. Examples of scorpion antimicrobial peptides that potently target B. megaterium
are meucin-13 (MIC 0.25 μM), meucin-18 (MIC 0.25 μM) and pantinin-3 (MIC 6 μM) [47
4. Materials and Methods
4.1. Cone Snail Specimens and Venom Extraction
Specimens of C. longurionis
, C. asiaticus
and C. australis
(identified by Kiener (1845), da Motta (1985) and Holten (1802), respectively, and classified by Tucker and Tenorio [48
]) were collected from the Indian Ocean near Tamil Nadu, India. The venomous apparatuses (venom bulbs and venom ducts) were extracted from the specimens as described previously [49
]. The collected tissues were preserved in RNAlater solution (Ambion, Austin, TX, USA) and stored at −20 °C. The venomous apparatuses were used for peptide/protein extraction.
4.2. Peptide Fractionation and Purification
Two steps were followed for the separation of the venom compounds of C. longurionis. In the first step, the lyophilized crude venom powder was solubilized into 50% acetonitrile (ACN)/water and aliquots were loaded on a Gel filtration Superdex™ Peptide 10/300 GL column with 50% ACN/water as the mobile phase (flow rate 0.5 mL/min) to separate the peptides and proteins based on their size. Two sample collections obtained were stored overnight at −80 °C, freeze-dried and finally solubilized in 5% ACN/water. For the second step, an analytical Vydac C18 column (218MS54, 4.6 mm × 250 mm, 5-μm particle size; Grace, Deerfield, IL, USA) with a two solvent system was used: (A) 0.1% trifluoroacetic acid (TFA)/H2O and (B) 0.085% TFA/ACN. The sample was eluted at a constant flow rate of 1 mL/min with a 0%–80% gradient of Solvent B over 90 min (1% can per minute after 10 min of Solvent A). The HPLC column elutes were monitored by a UV/VIS-155 detector (214 nm and 280 nm; Gilson, Middleton, WI, USA).
Three steps were followed for the separation of the venom compounds of C. asiaticus. In the first step, the lyophilized crude venom powder was solubilized using 5% acetonitrile (ACN)/water and aliquots were loaded on an analytical Vydac C18 column (218MS54, 4.6 mm × 250 mm, 5-μm particle size; Grace, Deerfield, IL, USA) with a two-solvent system: (A) 0.1% trifluoroacetic acid (TFA)/H2O and (B) 0.085% TFA/ACN. The sample was eluted at a constant flow rate of 1 mL/min with a 0%–80% gradient of Solvent B over 90 min (1% ACN per minute after 10 min of Solvent A). The HPLC column elutes were monitored by a UV/VIS-155 detector (214 nm and 280 nm; Gilson, Middleton, WI, USA). The largest peak was collected and freeze-dried for further purification. This fraction collection was subjected to a second purification step, namely ion exchange chromatography, using a Luna SCX column (Phenomenex, Torrance, CA, USA, 4.6 mm × 250 mm, 5-μm particle size) at room temperature. The solutions used for ion exchange chromatography were: (A) 20 mM KH2PO4, pH 2.5: ACN (75:25) and (B) 20 mM KH2PO4/0.5 M KCl:ACN (75:25). The sample was eluted using a three-step protocol: 0% Solution B for 15 min, 0%–100% Solution B for 30 min and 100% B for 15 min at a flow rate of 1 mL/min. The collected fractions were stored overnight at −80 °C and freeze-dried. A third purification step was performed by RP-HPLC, using the same conditions as described for the first purification step.
Two steps were followed for the separation of the venom compounds of C. australis. In the first step, the lyophilized crude venom powder was solubilized into 50% acetonitrile (ACN)/water, and aliquots were loaded on a Gel filtration Superdex™ Peptide 10/300 GL column with 50% ACN/water as the mobile phase (flow rate 0.5 mL/min) to separate the peptides and proteins based on their size. Three sample collections were made that were stored overnight at −80 °C, freeze-dried and finally solubilized in 5% ACN/water. For the second step, an analytical Vydac C18 column (218MS54, 4.6 mm × 250 mm, 5-μm particle size; Grace, Deerfield, IL, USA) with a two-solvent system was used: (A) 0.1% trifluoroacetic acid (TFA)/H2O and (B) 0.085% TFA/ACN. The sample was eluted at a constant flow rate of 1 mL/min with a 0%–80% gradient of Solvent B over 90 min (1% ACN per minute after 10 min of Solvent A). The HPLC column elutes were monitored by a UV/VIS-155 detector (Gilson, Middleton, WI, USA) scanning both 214 nm and 280 nm.
4.3. Peptide Sequencing
Isolated Lo6/7a, Lo6/7b, Asi3a, Asi14a and AusB were collected and freeze-dried for direct peptide sequencing and molecular mass analysis (MALDI-TOF; 4800 Analyzer, Applied Biosystems, Foster City, CA, USA). A Protein Sequencer PPSQ-31A/33A (Shimadzu, Kyoto, Japan) was used to determine the amino acid sequence of the separated compounds. Samples were loaded onto a polybrene-pretreated, precycled glass fiber disk and Edman sequenced for 30 residue cycles.
Theoretically-calculated masses of the peptides were done with an online Peptide Mass Calculator (Peptide Protein Research Ltd., Hampshire, UK). Peptide homology search was generated online at Conoserver.org [50
] and NCBI (Rockville Pike, Bethesda MD, USA) [52
]. The CLC Main Workbench 7 software was used to align the peptide sequences (CLC bio, QIAGEN, Hilden, Germany).
4.4. Peptide Synthesis and Folding
Lo6/7a and Lo6/7b were synthesized by GeneCust (Elange, Luxemburg). Asi3a, Asi14a and AusB were synthesized by GenicBio Limited (Shanghai, China). All peptides, except AusB, were C
-terminally amidated, purified by HPLC and analyzed with LC-MS, then freeze-dried and stored at −20 °C until use. The peptides were folded using an oxidative folding solution (1 mM reduced glutathione (Sigma, Munich, Germany), 1 mM oxidized glutathione (Roche, Mannheim, Germany), 1 mM ethylenediaminetetraacetic acid (EDTA; Sigma, Munich, Germany) and 100 mM Tris/HCl (Merck, Darmstadt, Germany) [53
]). The solution was adjusted to pH 7.63 with 10 M NaOH (Merck, Darmstadt, Germany). Prior to functional characterization, the purity and folding of the synthetic peptides was validated (MALDI-TOF MS), and a chromatographic characterization was undertaken by RP-HPLC. On the basis thereof, a careful comparison of the retention time with the one of the native material was done.
4.5. Preparation of RNA for Functional Testing in Xenopus Oocytes
The RNA preparation of the different NaV, KV channels and nAChRs was done as follows:
Complementary DNA (cDNA) encoding the NaV channels was subcloned into the corresponding vector: the α-subunits rNaV1.1/pLCT1 (NotI), rNaV1.2/pLCT1 (NotI), rNaV1.3/pNa3T (NotI), rNaV1.4/pUI-2 (NotI), hNaV1.5/pcDNA3.1 (XbaI), mNaV1.6/pLCT (NotI), rNaV1.7/pBSTA.rPN1 (SacII), hNaV1.8/hPN3-pBSTAcIIR (NotI) and the corresponding β-subunits rβ1/pSP64T (EcoRI) and hβ1/pGEM-HE (NheI).
For KV channels, channel-encoding cDNA was subcloned into the corresponding vector: rKV1.1/pGEM-HE (EcoRI), rKV1.2/pGEM-HE (SphI), hKV1.3/pGEM-HE (NotI), rKV1.4/pGEM-HE (NotI), rKV1.5/pGEM-HE (SalI), rKV1.6/pGEM-HE (NdeI), hKV3.1/pGEM-HE (XbaI), hKV10.1/pSGem (SfiI), hERG/pSp64 (EcoRI).
Complementary DNA encoding the nAChR-channels was subcloned into the corresponding vector: hα3/pcDNA3 (XbaI), hα4/pGEM-HE (NheI), cα7/pBlueScript (NotI), hβ2/pSP64 (PvuII), hβ4/pcDNA3 (XbaI), rα1/pSP0oD (SalI), rβ1/pSP0oD (SalI), rδ/pSP0oD (SalI), rε/pSP0oD (SalI).
4.6. Electrophysiological Recordings
The harvesting of Stages V–VI oocytes from anaesthetized female Xenopus laevis
frogs was described previously [54
]. Using a nanoinjector (Drummond, Broomall, PA, USA), the selected oocytes (Stages V–VI) were injected with 40–70 nL mRNA (100–2000 ng/L) and 4–20 nL mRNA for KV
channels. Then, the oocytes were stored at 16 °C in a geomycin (1.25 mL/L; Rotexmedica, Trittau, Germany) and theophylline (80 mg/L; ABC chemicals, Wauthier Braine, Belgium) supplemented ND96 solution, except for KV
channels, where theophylline was not added.
Whole-cell currents from oocytes were recorded at room temperature (18–22 °C) by the two-electrode voltage clamp technique using a GeneClamp 500 amplifier (Axon Instruments, Foster City, CA, USA) controlled by a pClamp data acquisition system (Molecular Devices, Sunnyvale, CA, USA). Oocytes were placed in a bath containing ND96 solution. Voltage and current electrodes were filled with 3 M KCl, and the resistances of both electrodes were maintained as low as possible (0.5 to 1.5 MΩ). To eliminate the effect of the voltage drop across the bath grounding electrode, the bath potential was actively controlled by a two-electrode bath clamp. Leak subtraction was performed using a −P/4 protocol.
For NaV channels, whole-cell current traces were evoked every 5 s by a 100-ms depolarization to the voltage corresponding to the maximal activation of the NaV-subtype in control conditions (0 mV), starting from a holding potential of −90 mV. The elicited currents were sampled at 20 kHz and filtered at 2 kHz using a four-pole, low-pass Bessel filter. Concentration-response curves were constructed by adding different toxin concentrations directly to the bath solution.
KV1.1–KV1.6 and KV3.1 currents were evoked by 500-ms depolarizations to 0 mV followed by a 500-ms pulse to −50 mV, from a holding potential of −90 mV. The elicited currents were sampled at 2 kHz and filtered at 500 Hz using a four-pole low-pass Bessel filter. KV10.1 currents were evoked by 2-s depolarizing pulses to 0 mV from a holding potential of −90 mV. hERG or KV11.1 peak and tail currents were generated by a 2.5-s prepulse from −90 mV–40 mV followed by a 2.5-s pulse to −120 mV. KV10.1 currents were sampled at 2 kHz and filtered at 1 kHz; hERG currents were sampled at 10 kHz and filtered at 1 kHz using a four-pole low-pass Bessel filter.
For measuring nAChR currents, the following conditions were applied: during recordings, oocytes were continuously perfused with ND96 at a rate of 2 mL/min, with the conopeptides applied during 30 s before ACh was added. ACh (200 μM) was applied for 2 s at 2 mL/min, with 30-s washout periods between different ACh applications and 200 s after toxin application. The percentage response or percentage inhibition was obtained by averaging the peak amplitude of at least three control responses (two directly before exposure to the peptide and one after 200 s washout). Whole-cell current traces were evoked from a holding potential of −90 mV.
Data were analyzed using pClamp Clampfit 10.0 (Molecular Devices, Sunnyvale, CA, USA) and Origin 7.5 software (Originlab, Northampton, MA, USA) and presented as the result of at least 3 independent experiments (n ≥ 3).
4.7. Growth Media and Micro-Organisms
Growth media were prepared as follows (1 L deionized water):
LB (lysogeny broth): 0.5% yeast extract (BD Biosciences, San Jose, CA, USA); 1% bacterial peptone (International Medical); 1% NaCl (Fisher Scientific, Aalst, Belgium); MRS (Man Rogosa and Sharpe): 55 g Difco™ lactobacilli MRS broth (BD Biosciences, San Jose, CA, USA); NB (nutrient broth): 8 g nutrient broth (Oxoid); TSB (trypticase soy broth): 3%, BD Biosciences; TY (tryptone yeast extract): 3 g yeast extract, 5 g tryptone (International Medical); 7 mM CaCl2
(VWR International, Leuven, Belgium); YEP (yeast extract peptone): 10 g yeast extract, 10 g bacterial Bacto peptone; YPD (yeast extract peptone dextrose): 10 g yeast extract, 20 g bacterial peptone, 20 g dextrose. Media were solidified with agar (1.5%) For this study, 29 Gram-negative and 10 Gram-positive strains were used, together with 2 yeast strains. Strain names, growth conditions and sources are summarized in the Supplementary Table S1
4.8. Antibacterial Assays
The different bacterial and yeast strains were inoculated in 5 mL of the appropriate medium and incubated overnight at the appropriate temperatures and shaking at 200 rpm. Next, agar plates were overlaid with 5 mL soft agar (0.5%), seeded with 50 μL of the overnight cultures (~109 CFU/mL). Cell lawns were supplemented with 5-μL spots of the different conotoxins and derivatives (concentration ~1 mM) and air-dried. Plates were incubated overnight and evaluated for the presence of zones of growth inhibition or halos. ND96 buffer was used as the negative control.