A chimeric scorpion a-toxin displays de novo electrophysiological properties similar to those of a-like toxins Balkiss Bouhaouala-Zahar1, Rym Benkhalifa1, Najet Srairi1, Ilhem Zenouaki1,
Trang 1A chimeric scorpion a-toxin displays de novo electrophysiological properties similar to those of a-like toxins
Balkiss Bouhaouala-Zahar1, Rym Benkhalifa1, Najet Srairi1, Ilhem Zenouaki1, Caroline Ligny-Lemaire2, Pascal Drevet2, Franc¸ois Sampieri3, Marcel Pelhate4, Mohamed El Ayeb1, Andre´ Me´nez2, Habib Karoui1 and Fre´de´ric Ducancel2
1
Laboratoire des Venins et Toxines, Institut Pasteur de Tunis, Tunisia;2De´partement d’Inge´nierie et d’E´tude des Prote´ines, CEA, Saclay, France;3UMR 6560, Universite´ de la Me´diterrane´e, CNRS, Inge´nierie des Prote´ines, Laboratoire de Biochimie, IFR Jean-Roche, Faculte´ de Me´decine Nord, Marseille, France;4Laboratoire de Neurophysiologie UPRES EA 2647,
UFR Sciences, Angers, France
BotXIV and LqhaIT are two structurally related long chain
scorpion a-toxins that inhibit sodiumcurrent inactivation in
excitable cells However, while LqhaIT from Leiurus
quinquestriatus hebraeus is classified as a true and strong
insect a-toxin, BotXIV from Buthus occitanus tunetanus is
characterized by moderate biological activities To assess the
possibility that structural differences between these two
molecules could reflect the localization of particular
func-tional topographies, we compared their sequences Three
structurally deviating segments located in three distinct and
exposed loops were identified They correspond to residues
8–10, 19–22, and 38–43 To evaluate their functional role,
three BotXIV/LqhaIT chimeras were designed by
transfer-ring the corresponding LqhaIT sequences into BotXIV
Structural and antigenic characterizations of the resulting recombinant chimera show that BotXIV can accommodate the imposed modifications, confirming the structural flexi-bility of that particular a/b fold Interestingly, substitution of residues 8–10 yields to a new electrophysiological profile of the corresponding variant, partially comparable to that one
of a-like scorpion toxins Taken together, these results suggest that even limited structural deviations can reflect functional diversity, and also that the structure–function relationships between insect a-toxins and a-like scorpion toxins are probably more complex than expected
Keywords: chimeric scorpion toxin; insect sodium channel; sodium current kinetics; molecular modelling
Long-chain scorpion toxins isolated from Androctonus
australis hector and Buthus occitanus tunetanus scorpions
[1,2] are responsible for human envenomation, a public
heath problemin Tunisia [3] These small and basic
polypeptides, composed of a globular core compacted by
four disulfide bridges, bind and modulate sodium channels
in excitable cells [4,5] They have been divided into a- [6,7]
and b-toxins [8,9] according to their mode of action and
binding properties [10,11]
Whereas b-toxins interfere with the current activation
stage, a-toxins inhibit sodiumcurrent inactivation in
excitable cells (reviewed in [10,11]) a-Toxins have been
instrumental in functional mapping of voltage gated sodium
channels [12,13] and display a wide array of preferences on
interaction with sodiumchannels of different animal phyla
[10]
Scorpion a-toxins are classically divided into the
follow-ing three groups: (a) mammal a-toxins that are highly active
on mammals, and display very low toxicity to insects, e.g AahII toxin fromthe venomof the scorpion Androct-onus australis hector; (b) insect a-toxins that are highly toxic
to insect and shows weak activity in mammalian central nervous system, e.g LqhaIT from Leiurus quinquestriatus hebraeusscorpion; (c) a-like toxins, that display similar high toxicity to both mammals and insects, e.g BomIII and BomIV [11,14] from Buthus occitanus mardochei and LqhIII from Leiurus quinquestriatus hebraeus scorpions [15] Classically, binding of scorpion a-toxins to the receptor site 3 on the extracellular surface of sodiumchannels induces prolongation of action potentials due to selective inhibition or slowing of the fast inactivation process of the sodiumcurrent in vertebrate and insect electrophysiological preparations [12,16,17] Interestingly, and despite some differences in their primary structure, all scorpion a-toxins that compete for binding to receptor site 3 on sodium channel reveal similar effects of inhibition [11,12,14,18] However, some comparative studies make uncertain the strict assignment of many toxins to a particular pharmaco-logical group [11,17] Thus, scorpion a and a-like toxins share similar and competitive binding activities towards insect sodiumchannels, when this is not the case in rat brain synaptosomes [17] These observations support the existence
of two distinct receptor sites for a and a-like toxins on sodiumchannels, the latter being more or less related in mammals or insects On the other hand, LqhaIT one of the most studied insect a-toxins, seems to share some pharma-cological properties with a-like toxins [17], suggesting that the receptor sites recognized by both families of toxins either
Correspondence to B Bouhaouala-Zahar, Laboratoire des Venins
et Toxines, Institut Pasteur de Tunis, 13 Place Pasteur, Belve´de`re,
Tunis, 1002 Tunisia.
Fax: + 216 71 791 833, Tel.: + 216 71 1843 755,
E-mail: balkiss.bouhaoulala@pasteur.rns.tn
Abbreviations: BotXIV, a-toxin fromthe venomof Buthus occitanus
tunetanus; LqhaIT, a-toxin fromthe venomof Leiurus quinquestriatus
hebraeus; TSB, tryptic soy broth; AP, action potential.
(Received 26 November 2001, revised 13 March 2002,
accepted 5 April 2002)
Trang 2partially overlap, or are closely localized on insect sodium
channels Furthermore, recent data mentioned the possibility
that LqhaIT could have a weak effect on sodiumchannels
activation, an activity classically attributed to a-like toxins
even if such activity is not very significant [17] Clearly,
additional data are necessary to tentatively elucidate the
structure–function relationship of a-toxins in general, and
between insect a-toxins and a-like toxins in particular
Recently, we cloned and characterized a new insect
a-toxin fromthe venomof the scorpion Buthus occitanus
tunetanus called BotXIV [19] We showed that BotXIV
shows 49.25 and 52.23% identities with LqhaIT and
LqqIII, respectively, and is not toxic on mice even at high
concentration [up to 2.5 lg per 20 g of body weight at
intracerebroventricularly (i.c.v.) route] However, unlike
other insect a-toxins, BotXIV displays a weak anti-insect
activity, a moderate toxicity to cockroach and slows only
the inactivation process of insect sodiumchannels Also,
comparison of BotXIV with BomIV, a classical member of
a-like toxins [11] revealed 57% of amino-acid identity, as
compared to the 73% existing identity between LqhaIT and
BomIV These data suggest that a-like toxins forma large
family of structurally related compounds, displaying similar
basic biological properties, but susceptible also of expressing
particular activities
The aimof this paper was to tentatively explore the
possibility that subtle structural deviations between BotXIV
and LqhaIT, could reflect the localization of a particular
functional topography Such a relation was recently
evidenced in the case of three fingered toxins fromsnakes
[20,21] To investigate this hypothesis, we compared in
detailed the amino-acid sequences of BotXIV and LqhaIT,
and searched for significant differences Thus, we identified
three different stretches of amino acid residues located in
three distinct exposed areas on the surface of the toxins: the
first five-residue turn, the N-terminal part of a helix and the
b turn between the two last b strands Using site-directed
mutagenesis, three BotXIV variants were constructed by
replacement of residues 8–10, 19–22, or 38–43 with those
found in LqhaIT These three BotXIV/LqhaIT chimeras
together with the native BotXIV recombinant toxin were
expressed in Escherichia coli Their overall structural and
detailed electrophysiological properties were studied and
compared Interestingly, in this paper we show that
substitution of residues 8–10 is associated with de novo
electrophysiological properties partially comparable with
those of BomIV, an a-like scorpion toxin Implications of
that particular functional anatomy elucidation regarding
the classification of a-toxins of scorpions will be discussed
M A T E R I A L S A N D M E T H O D S
Materials
Enzymes were purchased from Boehringer–Mannheim and
Biolabs Oligonucleotides were synthesized by Genset
HPLC separation procedure was performed using a Merck
system(L-4250 UV/Vis detector and L-6200 intelligent
pump) N-Terminal sequencing was carried out using an
Applied Biosystemsequencer (477A protein sequencer) on
line with a phenylthiohydantoin analyser (120A analyser)
Dichroic spectra were recorded at 20C on a Jobin–Yvon
CD6 using toxin solutions which concentrations were
determined by spectrometry SDS/PAGE was performed using the Phast SystemfromPharmacia
Bacterial strains and plasmids The E coli strain MV1190 was used as the host strain for transformations by M13-derived vectors The strain CJ236 [dut–, ung–, thi1, relA1/pCJ105 (Cmr)] was used to prepare single-stranded template DNA for mutagenesis, as des-cribed by Kunkel [22] The bacterial host used for expres-sion was E coli HB101 [F–D(mcrC-mrr) leu supE44 ara14 galK2 lacY1 proA2 rpsL20 (Strr) xyl-5 mtl-1 recA13 23] Expression vector pEZZ18 was obtained fromPharmacia Molecular biology
Manipulations of DNA were performed according to published procedures [24] Single- and double-stranded DNA sequencing procedures were performed by the dideoxynucleotide method [25] using the T7 sequencing kit fromPharmacia and [35S]dATP (Amersham) Site-directed mutagenesis assays were performed according to Kunkel
et al [22] using a Bio-Rad kit The cDNA encoding the precursor of BotXIV [19] was modified as follows: a KpnI/ BamHI fragment carrying the sequence encoding BotXIV was inserted into the corresponding restriction sites of M13mp19 to produce M13mp19-BotXIV template for mutagenesis Phages from an individual lysis plaque were used to re-infect fresh host cells to produce high-titer phage stock This stock was then passed through two rounds of infection of E coli CJ236 dut–ung– host Single-stranded phage DNA was then isolated froma large volume of phage-containing supernatant Substitutions of BotXIV amino-acid stretches: Q8-P-H10, I19-S-S-G22or G38 -H-K-S-G-H43 by the corresponding sequences in LqhaIT were performed using the following oligonucleotides: 5¢-GGTTA TATTGCCAAGAACTATAACTGTGCATAC-3¢, 5¢-C ATTGTTTAAAAATCTCCTCAGGCTGCGACACTTT A-3¢, and 5¢-ACGAGTGGCCACTGCGGACATAAATC TGGACACGGAAGTGCCTGCTGG-3¢, respectively Production of recombinant chimera toxins inE coli Bacteria E coli HB101 transformed by the expression vectors pEZZ-M8-10, pEZZ-M19-22 or pEZZ-M38-43 were grown in a 5-L fermentor (LSL Biolafitte, Saint Germain en Lay, France) with an initial culture volume of 4-L of tryptic soy broth (TSB) medium (Difco) supple-mented with 5 gÆL)1of glucose and 200 lgÆmL)1ampicillin Conditions of production were performed as previously described [19] Hybrid recombinant proteins contained in extracted periplasmic fractions and in the culture medium were purified by affinity chromatography on an IgG-Sepharose column according to Ducancel et al [26], then, lyophilized The procedure followed to cleave the fusion proteins by CNBr treatment was previously described by Boyot et al [27] Purification of cleaved recombinant chimeras was performed as previously described [19] Electrophysiological techniques
Adult male cockroaches (Periplaneta americana) were used
A segment (1.5–2.5 mm) of one giant axon was isolated
Trang 3froma connective linking the fourth and fifth abdominal
ganglia The preparation was immersed in paraffin oil and
an artificial node of Ranvier was created [28] Active
membrane area of 0.01–0.02 mm2 (node) was superfused
with saline or test solutions Membrane potentials and
transmembrane currents of this small surface of axonal
membrane were recorded in current-clamp or voltage-clamp
using the double oil-gap single fiber technique as described
in detail earlier [29,30] Normal physiological saline had the
following composition (in mM): NaCl, 200; KCl, 3.1; CaCl2,
5.4; MgCl2, 5.0; Hepes buffer, 1.0; pH 7.2 Lyophilized
M8-10 and BotXIV were dissolved in the saline solution to final
concentrations of 0.5 or 2.0· 10)6M, in the presence of
bovine serumalbumin (0.25 mgÆmL)1) before tests
Potas-siumcurrents were blocked by 0.5· 10)3M
3,4-diamino-pyridine (Sigma Chemical, France), and when needed
sodiumcurrents were blocked by 5· 10)7Mtetrodotoxin
(Sigma Chemical, France)
Sodiumconductance (gNa) can be calculated as a function
of the membrane potential according to the equation:
gNa¼ INa=ðEm ENaÞ where Em and ENa are the membrane potential, and the
reversal potential for Na+ current, respectively Smooth
curves correspond to the best fit through the mean data
points according to the Boltzmann distribution:
gNa=gNamax¼ 1=f1 þ exp½ðE0:5þ EmÞ=kg
where E0.5is the potential at which 50% of the maximal
sodiumconductance are reached, k is the slope factor
Voltage-dependence of steady-state inactivation of Na+
channels was determined using a conventional two-pulses
protocol: the test pulse to )10 mV is preceded by long
(40 ms) prepulses from)80 to +30 mV, and the relative
amplitude of the peak Na+current during the test pulse is
plotted according to the prepulse value Smooth curves
correspond to the best fit through the mean data points
according to the Boltzmann distribution:
INa=INamax¼ 1=f1 þ exp½ðEmþ E0:5Þ=kg
where E0.5 is the potential at which 50% of the sodium
channels are inactivated, k is the slope factor
Enzyme-linked-immuno-sorbent-assays
ELISAs were used to assess cross antigenicity of each
purified recombinant BotXIV mutant towards different
polyclonal antibodies Some were raised against toxic
fractions BotG-50 and AahG-50 from Buthus occitanus
tunetanus and Androctonus australis hectorvenoms,
respect-ively; or against BotI and AahII purified toxins For this
purpose, optimization of the previously described
proce-dures [19,31] was carried out
In vivo insect and mammal toxicities (biological assays)
For LD50determination, groups of four female C57/B16
mice (22 ± 0.2 g) were individually i.c.v injected under
light diethyl ether anesthesia with 20 ng to 2.5 lg of
recombinant proteins Toxicity of purified BotXIV mutants
were assessed on four Blatella germanica males per dose
(50 mg body weight) A volume of 0.5–2 lL was injected in
the abdominal segments, and the lethality was monitored after 1 h For all injections, the solvent used was 0.15M NaCl containing 1 mg BSA per mL The LD50values were calculated according to Reed & Muench method [32] Molecular modelling of BotXIV and m8–10 mutant Molecular modelling of both BotXIV and its 8–10 mutant were based on the experimentally determined three-dimen-sional structures of two templates: toxin II of Androctonus australis hector (AahII) solved at 1.3 A˚ (PDB entry 1ptx [33]), and toxin V (CsV; PDB entry 1nra [34]), of Centruro-ides sculpturatus, by using the program MODELLER3 [35], running on a silicon Graphics Indigo R3000 workstation The first set of models was obtained from manual sequence alignment To limit the problems with backbone dihedrals
of nonGly residues, we avoided to aligning the Gly residues with nonGly residues The 40 first models (20 in each protein) were screened with the programs PROCHECK[36], PROSA I [37] and INSIGHT II (Molecular Simulation Inc.), fromwhich only four models for each protein were selected and then used as templates in the final subsequent series Electrostatic properties
The electrostatic potential and outside isopotential gradients
on the molecule surfaces were computed with the program GRASP[38] The ionic strength was 0.145Mand the probe radius was 1.4 A˚ The dielectric constant was 2 inside and 80 outside the solute molecules Except the His residues, all acidic and basic residues were set in their ionized form
R E S U L T S
Identification of divergent regions
To tentatively clarify the structure–function relationships existing between a-like and a-insect scorpion toxins, we compared in details the primary structures of BotXIV and representative a-toxins Thus, LqqIII, BotIT1, BomIV and BomIII display 49, 53, 54, 57 and 73% identities with LqhaIT, respectively These data suggest that BotXIV occupies an intermediate position between strictly insectici-dal a-toxins (LqhaIT, LqqIII, and BotIT1) and typical a-like toxins (BomIV and BomIII) This confirms our previous experimental results, which established that BotXIV was inactive towards mammals and weakly toxic for Blatella cockroaches [19] Thus, a precise comparison of BotXIV and LqhaIT primary sequences essentially revealed three divergent regions containing most of the amino acid variations noticed between these two functionally unrelated molecules It is noteworthy, that these divergent regions mostly correspond to three exposed to solvent loops connecting conserved a helix and b sheet elements (Fig 1) Thus, segment 8–10 is part of the b turn (8–12) following the first b strand (residues 1–5) Interestingly, the sequence found in BotXIV (Q8-P-H10) is similar to the corresponding ones in BomIII and BomIV toxins (Q8-P-E10) two typical a-like toxins, when totally different fromthose ones of true insecticidal toxins (LqhaIT, LqqIII, and BotIT1), K/Q8
-N-Y10 The second main divergent region is constituted of the loop preceding the unique a helix Classically, it displays variable length and amino acid content from one toxin to
Trang 4another Thus, the sequence I-S-S-G(19–22) found in
BotXIV is replaced by D-A-Y in LqhaIT Finally, the third
variable segment corresponds to residues 38–43, and
includes three amino acids of the second LqhaIT b strand
(34–39) and the following b turn Here also, BotXIV and
a-insect toxins display totally different sequences:
GHKSGH(38–43) vs QWAGKY in LqhaIT for instance
Based on these observations, we built three BotXIV/
LqhaIT chimeric molecules corresponding to the individual
substitution of deviating and exposed to the solvent regions
8–10, 19–22 or 38–43 in BotXIV by the equivalent sequences
found in LqhaIT (Fig 1) The latter should be noted:
BotXIVM8-10, M19-22, and M38-43
Production, purification and characterization
of recombinant chimera
Mutated DNA fragments surrounded by KpnI (5¢) and
BamHI (3¢) restriction site sequences were excised from
M13mp19-BotXIV vectors and inserted into the
corres-ponding sites in the pEZZ18 expression vector [39] The
three BotXIV variants were produced as recombinant ZZ
fusion proteins as previously described [19] Briefly, fusion
proteins were mainly found in the culture medium of
bacterial suspensions, as expected frompEZZ-18 expression
vector [39] Affinity chromatography performed on IgG–
Sepharose allowed as expected, recovery of recombinant
hybrid proteins having an apparent 22-kDa molecular mass
(not shown) We noticed also the presence of few low
molecular weight fragments resulting probably from
pro-teolytic degradation events of the toxin moiety, as
previ-ously and classically observed for such compounds
[19,26,40,41] The three IgG-purified fractions revealed
similar proteic profiles and overall yields were estimated
between 16 and 18 mg of fusion proteins per litre of culture Recombinant variant fused molecules were treated by cyanogen bromide as previously described [27] The average efficiency of the CNBr cleavage was estimated to 35% (Fig 2, lanes 3 and 4), and the products were purified by cation HPLC (data not shown) Recombinant chimera displayed an apparent 7 kDa molecular mass as showed in the case of BotXIVM8-10 variant (Fig 2, lane 2) The three recombinant chimera comigrated with recombinant BotXIV on a 20% SDS/polyacrylamide gel, and shown the expected amino acid composition and N-terminal amino-acid sequences (data not shown) The circular dichroic spectra of the recombinant and chimeric BotXIV proteins revealed similar overall profiles (Fig 3), associated however,
Fig 2 SDS/PAGE (20% Phast-gel) of a cleaved M8-10 variant Lane
2 represents the cleaved and HPLC-purified recombinant M8-10 variant Chimeric protein BotXIVM8-10 appears as a proteic band of HPLC-purified hybrid fractions Lanes 1 and 5, correspond to medium and low molecular mass markers, respectively.
Fig 1 Alignment of principal scorpion a-neurotoxins amino acid sequences The amino-acid sequences are aligned according to their cysteine residues (orange) and their three-dimensional structures (in italic for known three-dimensional structures) Disulfide bridges are indicated in dashed lines Positively charged residues (K/R) are indicated in blue, negatively charged residues (D/E) in red, and aromatic residues in green Consensus numbering is displayed under the sequences The secondary structures are indicated in the top line Deletions are indicated by (–) The three main divergent regions involved in the building of BotXIV/LqhaIT chimeric molecules are in boxes LqhaIT, Leiurus quinquestriatus hebraeus a-insect toxin; LqqIII, Leiurus quinquestriatus quinquestriatus toxin III; BotI, IT1, and XIV, Buthus occitanus tunetanus toxin I, insect toxin I, and XIV, respectively; BomIII and BomIV, Buthus occitanus mardochei toxins III and IV.
Trang 5to a weak increase in the positive 190-nmband coupled to a
more significant one in the negative 205-nm band of the CD
spectra Finally, about 1 mg of each recombinant BotXIV
variants was obtained from20 mg of initial fusion protein
Cross antigenicity and biological toxicity of purified
recombinant BotXIV mutants
To establish the antigenic profiles of the three
HPLC-purified BotXIV mutants, we performed ELISA using
various scorpion antitoxins sera [42] Thus, BotXIVM8-10,
BotXIVM19-22 and BotXIVM38-43 variants, were
simi-larly recognized by the polyclonal antibodies raised against
BotG-50 (a partially purified mixture of Buthus occitanus
tunetanusvenom) or BotI toxin On the contrary, very low
cross antigenicity was observed with anti-AahII toxin or
AahG-50 toxic fraction (data not shown) A similar result
was initially observed for recombinant BotXIV [19]
Together, these data indicate that the three substitutions
introduced within BotXIV did not modify significantly its
overall antigenic profile [19] Furthermore, they indicate
that these three BotXIV mutants as BotXIV all belong to
the same antigenic group related to Buthus occitanus
tunetanus(Bot) scorpion a-toxins, which is different from
those of Androctonus australis hector [42]
I.c.v injections in C57/Black 6 mice of purified
Bot-XIVM8-10, M19-22, or M38-43 variant ranging from2 ng
to 2.5 lg did not cause any toxic effect These results clearly
established that the three BotXIV/LqhaIT chimeric
mole-cules are devoided of any toxicity towards mammal, because
a weakly toxic LD50value in mammals corresponds on an
average to 100 ng This result was not surprising, because
the starting toxin BotXIV was already inactive towards
mammals [19], and the three chimeras were obtained by
limited substitution of equivalent regions between BotXIV
(as starting molecule) and LqhaIT (as donor molecule),
which is classically reported as a potent anti-insect toxin
[14,15,43] Unexpectedly however, injection to Blatella of
1 lg of each of the three recombinant chimera did not induce any additional insect lethality, when in the particular case of BotXIVM8-10 variant, a contractive effect of injected Blatella was noticed Thus, when substitution of 19–22 and 38–43 regions completely affected the initial insect toxicity of BotXIV, BotXIVM8-10 variant is charac-terized by an intermediate biological mode of action towards insects
Electrophysiological results
To compare the effects of native and mutated BotXIV molecules on the insect channels, we carried out standard current- and voltage-clamp experiments on cockroach axonal preparations, as described in Materials and methods
As previously reported, BotXIV has typical a-toxins effects
as it was shown to slow down the sodiumcurrent inactivation [19] First of all, both BotXIVM19-22 and M38-43 did not show any effect neither on action potential
or sodiumcurrent decay as compared to BotXIV (data not shown) These data confirmthe absence of insect toxicity noticed previously On the other hand, in the case of BotXIVM8-10 variant, modifications of the initial electro-physiological properties of BotXIV were observed in current- and voltage-clamp conditions
Current-clamp conditions When BotXIV (1.4· 10)6M) was devoid of any effect on the action potential (AP) amplitude, or on the membrane resting potential value, BotXIVM8-10 was applied at the same concentration; BotXIVM8-10 caused an unusual and slight membrane depolarization of about 5–8 mV (Fig 4A,B) Furthermore, BotXIV and BotXIVM8-10 both increase the action potential duration and lead to a progressive prolongation
of the evoked AP with a more drastic effect in the case of the native toxin In fact, for the same toxin application time (14 min) BotXIV induced a plateau potential lasting for
110 ms followed by a long lasting and a very slowly decayed depolarization In the case of the BotXIVM8-10, the evoked
plateau is only 3–6 ms duration with a normal repolariza-tion phase (Fig 4B) In addirepolariza-tion, an artificial re-polarizarepolariza-tion by-passing a constant hyperpolarizing current, did restore neither the PA amplitude nor its initial duration
Voltage-clamp conditions Membrane sodium currents were measured under voltage-clamp conditions in response
to a step depolarization froma holding potential (Eh) ¼ )60 mV to a membrane potential (Em) ¼ )10 mV, after suppression of the potassium current with 3.4-DAP 0.5· 10)3M In normal conditions, and as shown
in Fig 4C,D less than 3 ms were necessary to obtain a complete INa inactivation Application of BotXIVM8-10 induced a progressive and dose-dependent development of a maintained sodium current which was about 6% of the peak at 0.7· 10)6M (Fig 4D), and reached 12% at 1.4· 10)6M Besides, this mutant slightly decreased the initial peak amplitude of the inward sodium current by about 20–24% At the end of the voltage pulse, the maintained current returned to 0 with a slightly slowed deactivation Finally, a 10-lMtetrodotoxin post-application generated a constant but slight inward current of about)20
to )26 nA at the holding potential (Fig 4D) In another series of experiments, and after selective suppression of I
Fig 3 UV circular dichroism spectra of BotXIV toxin and its three
chimeric variants The cell path length is 0.05 cmand the recording
temperature is 20 C The different CD spectra are shown as following:
native BotXIV toxin (––), M8-10 (ÆÆÆÆ), M19-22 (- - -), and M38-43
(- Æ - Æ -) BotXIV/LqhaIT chimera.
Trang 6with 10)6M of tetrodotoxin, it was demonstrated that
BotXIVM8-10 (1.4· 10)6M) did not modify potassium
conductance (data not shown)
Effects on sodium activation and inactivation voltage
dependence Sodium peak current measurements during
voltage pulses from)80 to 10 mV allowed us to calculate
sodiumconductance (gNa) as a function of the membrane
potential as indicated in Materials and methods Figure 5B
shows that, BotXIVM8-10 shifted the relative sodium
conductance by about 5 to 10 mV towards negative
potentials, whereas BotXIV had no effect on activation
(Fig 5A) Figure 5 illustrates also the voltage dependence
of steady-state inactivation of Na+channels Under normal
conditions, the sodiumcurrent inactivation was complete at
0 to)10 mV, whereas with BotXIVM8-10 the inactivation
was almost complete but the curve is shifted by about 10–
20 mV towards negative potentials (Fig 5B) On the other
hand, under BotXIV application the sodiumcurrent
inactivation remained partial and the curve was shifted by
about 10 mV towards more positive potentials (Fig 5A)
It is important to notice that when using BotXIVM8-10,
the voltage dependence was decreased both during the
activation and inactivation mechanisms, in presence of
BotXIV the sodiumcurrent voltage dependence was
decreased only in the case of the inactivation process These
data imply that the mutated toxin opens Na+channels at very negative potential values, suggesting that sodium current is activated and inactivated earlier than in normal conditions To summarize, BotXIVM8-10 variant affects in the same time the sodium current activation and inactiva-tion mechanisms, when BotXIV only affects the inactivainactiva-tion process Together, our results clearly show that the substi-tution of segment 8–10 in BotXIV by its equivalent in LqhaIT, yields to the acquisition by BotXIVM8-10 variant
of original electrophysiological properties Molecular mode-ling and electrostatic potential studies were then carried out
to tentatively explain the biological properties displayed by BotXIVM8-10 variant
Molecular modeling of native BotXIV and BotXIVM8-10 variant
Figure 6 illustrates the best a-carbon BotXIV and M8-10 models we obtained They are similarly oriented (face A view) as the published experimental structures of CsEV3 [34] and AahII [44] Clearly, the selected models predict three-dimensional structures similar to the related a-toxins The main differences affect the C-terminal region especially
Fig 5 Effects on sodium activation and inactivation voltage dependence Sodium peak current measurements during voltage pulses from )80 to
10 mV allowed us to calculate sodium conductance (g Na ) as a function
of the membrane potential (see Method paragraph) Control meas-urements are represented by open symbols (A) 10 min after BotXIV application, the sodiumcurrent steady state inactivation curve (d) is slowed and is shifted to more positive potentials while the sodium current activation curve (m) remains unchanged (B) Ten minutes after M8-10 application, both sodiuminactivation and activation curves (d,m) are shifted to more negative potentials.
Fig 4 Effects of native and M8-10 chimeric BotXIV proteins on the
action potential and the sodium current of isolated cockroach axons.
(A,B) Current-clamp experiments: superimposed records of action
potentials evoked by a short (0.5 ms) depolarizing current pulse of 18
to 20 nA, at initial time: control (C), after BotXIV (A) and M8-10 (B)
application Note the progressive evolution of the action potential
durations under BotXIV (A) and the axonal membrane depolarization
under M8-10 associated to a slight prolongation of AP duration and a
AP amplitude decrease An artificial repolarization (AR) does not
restitute the initial AP (B) (C,D) Voltage-clamp experiments:
cock-roach axonal sodiumcurrent recordings are evoked by voltage pulses
of )10 mV from a holding potential E h ¼ )60 mV In normal
con-ditions, I Na completely inactivates in less than 3 ms (C) BotXIV
application induces a maintained inward sodium current without
affecting the inward peak sodiumcurrent (C) M8-10 application
induces a decrease of the peak sodiumcurrent amplitude associated
with a maintained inward sodium current (D) Tetrodotoxin
applica-tion reveals a slight holding inward sodiumcurrent (D).
Trang 7for BotXIV, the latter being predicted larger in the case of
BotXIVM8-10 variant It is noteworthy, that in the 40
BotXIV and M8-10 variant models, the C-terminus
extremity displayed the homogenous organization shown
on Fig 6
Electrostatic properties of the models
The existence onto the surface of scorpion a-neurotoxins of
an electrostatic potential is suspected to contribute to the
recognition of the receptor site [45] By resolving the
Boltzman–Poisson equation, the programGRASPis capable
of calculating the electrostatic potential at any point of a
grid in the space surrounding a protein molecule [38]
Applied to BotXIV and BotXIVM8-10 mutant, the
programallowed the visualization of several charged amino
acid residues susceptible to forman electrostatic potential
gradient on the surface of the molecule in the solvent space
(Fig 7) The positively and negatively charged residues are
in blue and red, respectively, whereas the neutral side-chains
are indicated in white Figure 7A shows the face A
(hydrophobic face) of BotXIV and BotXIVM8-10,
com-paratively to the published RMN structure of LqhaIT [46]
On the contrary of the clear dipolar charge repartition of
LqhaIT (with positive charges localized in C-terminal
region: R3, K42, K62 R65 and negative charges: D4,
D20, D54), BotXIV and BotXIVM8-10 display a different
and less homogenous repartition of charges However, the
global charge (+1) of the face A of these three molecules
remains unchanged BotXIVM8-10 and BotXIV display a
conserved charge distribution except for the C-terminal
region, the orientation of which was modified upon
substi-tution of segment 8–10, as predicted from modeling Finally,
analysis of the B faces (Fig 7B) revealed that the main
differences between LqhaIT, BotXIV and BotXIVM8-10
molecules, rely upon the substitutions K28/E29 in BotXIV
and BotXIVM8-10, together with K9/Q8 in the particular case of BotXIVM8-10 (Fig 7B)
D I S C U S S I O N
The aimof this paper was to explore the possibility that subtle structural deviations between BotXIV and LqhaIT, could reflect the localization of a particular functional topography Based on sequence analysis and the identifica-tion of three divergent regions between these two a-toxins,
we explored the biological implications of these segments by building corresponding chimeric molecules From a general point of view and as compared to rBotXIV, the substitu-tions performed did not modify the yield of production of the three recombinant chimera This suggests that the variants display a structural stability and a proteolytic sensitiveness similar to those of the starting recombinant molecule This is partially corroborated by the CD spectra
of the three purified recombinant variants that reveal similar, but not identical, profiles as compared to rBotXIV This observation suggests that the three recombinant BotXIV variants probably adopt a similar overall spatial arrangement as the unmodified compound, characterized
by a similar overall secondary structure content; however, the local structural features may be different Furthermore, each chimeric molecule shares an identical antigenic profile and cross-reactivity pattern with BotXIV Together, these results strongly suggest that the transfer of residues 8–10, 19–22 or 38–43 has a limited effect on the overall three-dimensional structure adopted by the three recombinant molecules generated and tested in the present study This further illustrates the stability and the structural flexibility of the a/b scorpion motif [47]
When the three segments were substituted to form the BotXIV variants, M8-10, M19-22 and M38-43 forma homogenous area largely overlapping the putative toxic
Fig 6 Homology molecular modeling of
BotXIV and M8-10 mutant Ribbon
representations of experimentally determined
structures of AahII (cyan), CsV (red), and
best models of BotXIV (orange) and its
M8-10 variant (pink).
Trang 8surface of scorpion a-neurotoxins affecting sodiumchannel
gating [45] Unexpectedly, however, when the three
substi-tutions were performed independently in BotXIV, the weak
lethality initially observed after abdominal injection to
Blatellacockroaches was totally abolished More
surpris-ingly, when BotXIVM19-22 and M38-43 mutants are
characterized by an absolute lack of electrophysiological
effects on cockroach giant axons, M8-10 variant shows
controversies effects Assuming the absence of major
structural change as discussed above, the loss of toxicity
suggests that subtle structural deviations might have
affected the structural integrity of the minimal toxic surface
characterizing BotXIV Numerous studies are consistent with a multipoint receptor recognition site onto the surface
of scorpion a-neurotoxins including residues at positions: 8,
10, 17, 18, 58, 59, 62, and 64 in interaction with the receptor [45,48–50] In addition, the spatial arrangement of the toxin polypeptide chain together with the formation of an electrostatic potential are also predicted to participate to the capacity of these compounds to interact specifically and with high affinity with voltage-sensitive sodiumchannels In this respect, our results are not surprising, because the substitutions we performed are only partial, and thus too limited to yield to the design of a LqhaIT-like toxic site
Fig 7 Electrostatic gradient potential obtained with GRASP program The gradient potential surfaces were computed from the modeled structures of BotXIV (top left), M8-10 mutant (top right), comparatively to that experimentally determined of recombinant LqhaIT (bottom) Faces A and B of the toxins are shown in (A) and (B), respectively.
Trang 9respecting the structural integrity of the transferred region.
Recently, we have shown the importance of such a
structural respect in the case of the successful design of a
recombinant fasciculin-like molecule obtained by
transfer-ring the structural-deviating segments that exist between
fasciculins and short-chain neurotoxins fromsnakes [20,21]
Clearly, substitution of residues QPH(8–10) in BotXIV
by the segment KNY (LqhaIT), which was recently
reported as playing a major role in the biological activity
of LqhaIT [45], results in de novo electrophysiological
properties of BotXIVM8-10 variant as compared to
BotXIV Indeed, and as classically observed with scorpion
a-toxins, BotXIVM8-10 variant induces a prolongation of
the action potential duration on cockroach giant axons
Furthermore under voltage-clamp conditions, the voltage
dependence is decreased both during the activation and
inactivation mechanisms when in the same conditions
BotXIV toxin only slows the sodiumcurrent inactivation
process Thus, the sodiumcurrent activation and the
inactivation mechanisms, are both and uniquely modified
in presence of BotXIVM8-10 Such results were sometimes
observed after LqhaIT application, but in a much less
significant manner [51] Sodium current activation is also
affected in the case of a-like toxins Thus, it was recently
shown that BomIV tested on the same insect preparations
inhibits the sodiumcurrent inactivation process with
additional effects on the sodiumcurrent activation [17] In
the presence of BotXIVM8-10 mutant, as with BomIV, we
observed a resting depolarization due to the induced
constant holding current, which is also responsible of the
shift of the sodiumcurrent voltage dependence to negative
potentials Furthermore, this shift induces an early
inacti-vating sodiumcurrent, which has not been seen until now
with a and a-like neurotoxins Thus, a part of the affected
sodiumchannels might be unable to participate in the fast
transient current This hypothesis could explain, at least
partially, the decrease of the peak sodiumcurrent
For the first time, a shift of inactivation curve towards
negative potentials can be unambiguously attributed to the
substitution of residues 8–10 within the first b turn of a
scorpion a-neurotoxin That result provides a second
example of how a structural deviation can be associated
to a particular functional topography It also emphasizes the
unique analytical power of a positive functional mapping
strategy based on the identification of a particular biological
property absent in the host scaffold, i.e BotXIV However,
the reasons for the dual effect observed upon transfer of the
segment 8–10 from LqhaIT to BotXIV (i.e., the loss of
insect toxicity and the de novo acquisition of particular
electrophysiological properties) remains unclear
Interest-ingly, it was recently proposed that the pharmacological
versatility displayed by scorpions a-neurotoxins might be
related to the structural configuration of the C-terminal tail
[50] Indeed, based on structure comparisons and bioactive
surface identifications, it was hypothesized that the highly
variable and dynamic C-terminal tail together with the
spatially vicinal residues, formthe interacting areas onto the
surface of scorpion a-neurotoxins Thus, the C-termini of
scorpion a-neurotoxins in general, and of LqhaIT in
particular, is positioned between the five-residue turn
(8–12) and the b turn formed by residues 40–43 If the first
one is identical between BotXIVM8-10 and LqhaIT, the
second turn displays several differences; AGK(40–43) vs
KSG(40–43) in LqhaIT and BotXIVM8-10, respectively Furthermore, Lys58, which is conserved within scorpion a-neurotoxins and predicted to forma network stabilizing the C-terminus, is replaced by an isoleucine in
BotXIVM8-10 Finally, the length and amino-acid composition of the C-termini of LqhaIT and BotXIVM8-10 are signifi-cantly different; RVPGKCR(58–66) for LqhaIT vs IVHGEKCHR(59–67) in BotXIVM8-10 Thus, the com-position and length differences between native BotXIV and M8-10 variant vs LqhaIT C-terminal tails, together with its peculiar environment in BotXIVM8-10 vs BotXIV, could
be related to the biological properties expressed by these different molecules
Finally, the fact that a limited modification can directly or indirectly be responsible for a new biological property shared between a-insect and a-like neurotoxins, suggests that BotXIV probably occupies an intermediate position within the evolutionary scheme of these molecules It also supports the hypothesis that the acquisition of such electrophysiological properties might constitute an early biological event on the way of the molecular design of potent sodiumchannel gated ligands
A C K N O W L E D G E M E N T S
We wish to thank S Pinkasfield and Dr F Bouet for technical assistance and N-terminal sequencing, Drs D Gordon, S Zinn-Justin for fruitful discussions, and Dr P Mansuelle for his help in preparation
of Figs 6 and 7 This work was supported in part by an IFS grant (International Foundation for Science).
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