Characterization and partial purification from pheochromocytoma cells of an endogenous equivalent of scyllatoxin, a scorpion toxin which blocks small conductance Ca(2+)-activated K+ channels

Inhibition of CFTR channels by a peptide toxin of scorpion venom

Peptide toxins have been valuable probes in efforts to identify amino acid residues that line the permeation pathway of cation-selective channels. However, no peptide toxins have been identified that interact with known anion-selective channels such as the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR channels are expressed in epithelial cells and are associated with several genetic disorders, including cystic fibrosis and polycystic kidney disease. Several organic inhibitors have been used to investigate the structure of the Cl−permeation pathway in CFTR. However, investigations of the wider cytoplasmic vestibule have been hindered by the lack of a high-affinity blocker that interacts with residues in this area. In this study we show that venom of the scorpion Leiurus quinquestriatus hebraeus reversibly inhibits CFTR, in a voltage-independent manner, by decreasing single-channel mean burst duration and open probability only when applied to the cytoplasmic surface of phosphorylated channels. Venom was able to decrease burst duration and open probability even when CFTR channels were locked open by treatment with either vanadate or adenosine 5′-(β,γ-imido)triphosphate, and block was strengthened on reduction of extracellular Cl−concentration, suggesting inhibition by a pore-block mechanism. Venom had no effect on ATP-dependent macroscopic opening rate in channels studied by inside-out macropatches. Interestingly, the inhibitory activity was abolished by proteinase treatment. We conclude that a peptide toxin contained in the scorpion venom inhibits CFTR channels by a pore-block mechanism; these experiments provide the first step toward isolation of the active component, which would be highly valuable as a probe for CFTR structure and function.

Characterization of the outer pore region of the apamin-sensitive Ca2+-activated K+ channel rSK2

We have studied the interaction between the SK2 channel and different scorpion toxins in order to find similarity and differences to other K+ channels. Beside apamin, ScTX is a high affinity blocker of the SK2 channel, whereas CTX is unable to block current through SK2. In order to prove that the ScTX affinity can be explained by the character of the different residues in the outer pore of the SK channels we introduced point mutations that render SK2 K+ channel SK1 and SK3 like. Directed by the results of the toxin receptor on the ShakerK+ channel, we changed single amino acids of the SK2 K+ channel that should render it sensitive to other peptide toxins like CTX a blocker of the IK channel, or KTX a blocker of the voltage-dependent channel Kv1.1 and Kv1.3. Amino acids V342G, S344E, and G384D of SK2 were changed to amino acids known from ShakerK+ channel to improve Shaker K+ channel CTX sensitivity. Interestingly SK2 V342G became CTX sensitive with a Kd of 19 nM and was also KTX sensitive Kd=97 nM. SK2 S344E (KdCTX = 105 nM,KdKTX = 144 nM) and G348D (KdCTX = 31 nM,Kd KTX = 89 nM) became also CTX and KTX sensitive with a lower affinity. The mutant channels SK V342G, SK2 S344E and SK2 G348D showed reduced ScTX sensitivity (Kd = 6 nM,Kd = 48 nM, and Kd = 12 nM). Because the exchange of a single residue could create a new high affinity binding site for CTX and KTX we concluded that the outer vestibule around position V342, S344, and G348 of the SK2 K+ channel pore is very similar to those of voltage-gated K+ channels such as the Shaker K+ channel, Kv1.1 and Kv1.3 channels and also to the prokaryotic KcsA channel. From mutant cycle analysis of KTX position H34 and SK2 position V342G, S344E, and G348D we could deduce that KTX binds in a similar way to SK2 channel mutant pore than to the Kv1.1 pore. We have studied the interaction between the SK2 channel and different scorpion toxins in order to find similarity and differences to other K+ channels. Beside apamin, ScTX is a high affinity blocker of the SK2 channel, whereas CTX is unable to block current through SK2. In order to prove that the ScTX affinity can be explained by the character of the different residues in the outer pore of the SK channels we introduced point mutations that render SK2 K+ channel SK1 and SK3 like. Directed by the results of the toxin receptor on the ShakerK+ channel, we changed single amino acids of the SK2 K+ channel that should render it sensitive to other peptide toxins like CTX a blocker of the IK channel, or KTX a blocker of the voltage-dependent channel Kv1.1 and Kv1.3. Amino acids V342G, S344E, and G384D of SK2 were changed to amino acids known from ShakerK+ channel to improve Shaker K+ channel CTX sensitivity. Interestingly SK2 V342G became CTX sensitive with a Kd of 19 nM and was also KTX sensitive Kd = 97 nM. SK2 S344E (KdCTX = 105 nM,KdKTX = 144 nM) and G348D (KdCTX = 31 nM,Kd KTX = 89 nM) became also CTX and KTX sensitive with a lower affinity. The mutant channels SK V342G, SK2 S344E and SK2 G348D showed reduced ScTX sensitivity (Kd = 6 nM,Kd = 48 nM, and Kd = 12 nM). Because the exchange of a single residue could create a new high affinity binding site for CTX and KTX we concluded that the outer vestibule around position V342, S344, and G348 of the SK2 K+ channel pore is very similar to those of voltage-gated K+ channels such as the Shaker K+ channel, Kv1.1 and Kv1.3 channels and also to the prokaryotic KcsA channel. From mutant cycle analysis of KTX position H34 and SK2 position V342G, S344E, and G348D we could deduce that KTX binds in a similar way to SK2 channel mutant pore than to the Kv1.1 pore.

Synthesis, molecular modeling, and pharmacological testing of bis-quinolinium cyclophanes: potent, non-peptidic blockers of the apamin-sensitive Ca(2+)-activated K(+) channel

The synthesis and pharmacological testing of two series of novel bis-quinolinium cyclophanes as blockers of the apamin-sensitive Ca(2+)-activated K(+) (SK(Ca)) channel are presented. In these cyclophanes the two 4-aminoquinolinium groups are joined at the ring N atoms (linker L) and at the exocyclic N atoms (linker A). In those cases where A and L contain two or more aromatic rings each, the activity of the compound is not critically dependent upon the nature of the linkers. When A and L each have only one benzene ring, the blocking potency changes dramatically with simple structural variations in the linkers. One of these smaller cyclophanes having A = benzene-1,4-diylbis(methylene) and L = benzene-1, 3-diylbis(methylene) (3j, 6,10-diaza-1,5(1,4)-diquinolina-3(1,3),8(1, 4)-dibenzenacyclodecaphanedium tritrifluoroacetate, UCL 1684) has an IC(50) of 3 nM and is the most potent non-peptidic SK(Ca) channel blocker described to date. Conformational analysis on the smaller cyclophanes using molecular modeling techniques suggests that the differences in the blocking potencies of the compounds may be attributable to their different conformational preferences.

Activation of small conductance Ca(2+)-dependent K+ channels by purinergic agonists in smooth muscle cells of the mouse ileum

1. Whole-cell and single-channel K+ currents were recorded at room temperature (22-24 degrees C), from smooth muscle cells enzymatically dispersed from the mouse ileum, using variations of the patch-clamp technique. 2. Net outward K+ currents recorded through amphotericin-B-perforated patches in response to step depolarizations positive to -50 mV from a holding potential of -80 mV were decreased by up to 70% by external apamin (0.5 microM). Apamin-sensitive whole-cell currents were also recorded from cells perfused internally with 150 nM Ca2+ but not from cells perfused internally with 85 nM Ca2+. 3. Three types of non-inactivating Ca(2+)-sensitive K+ channels were identified in cell-attached and excised patches under an asymmetrical K+ gradient: (i) large conductance (BKCa; approximately 200 pS) channels blocked by 2 mM external TEA; (ii) intermediate conductance (IKCa; approximately 39 pS) channels blocked by 2 mM external TEA and inhibited by external apamin (0.5 microM); and (iii) small conductance (SKCa; approximately 10 pS) channels that were not blocked by 5 mM external TEA but were sensitive to extracellular apamin (0.5 microM). 4. The TEA-resistant SKCa channels were activated by an increase in [Ca2+]i with an EC50 of 1.5 microM and a Hill coefficient of 1.3. 5. P2 purinoceptor agonists 2-methylthioATP (2-MeSATP), 2-chloroATP and ATP (10-50 microM) increased an apamin-sensitive whole-cell outward K+ current. Extrapatch application of 2-MeSATP (20-100 microM) stimulated the apamin-sensitive IKCa and SKCa channels and activated an apamin-sensitive steady outward current at 0 mV. 6. Smooth muscle cells from the mouse ileum possess two apamin-sensitive K+ channels (IKCa and SKCa); of these, the IKCa channels are TEA sensitive while the SKCa channels are TEA resistant. These channels, along with an apamin-sensitive but TEA-resistant steady outward current, may mediate membrane hyperpolarization elicited by purinergic agonists.

Synthesis, molecular modeling, and K+ channel-blocking activity of dequalinium analogues having semirigid linkers

Dequalinium [1,1'-(decane-1, 10-diyl)bis(2-methyl-4-aminoquinolinium)] is an effective blocker of the small conductance Ca2(+)-activated K+ channel. It has been shown that the number of methylene groups in the alkyl chain linking the two quinolinium rings of this type of molecule is not critical for activity. To further investigate the role of the linker, analogues of dequalinium have been synthesized, in which the alkyl chain has been replaced by CH2XCH2 where X is a rigid or semirigid group containing aromatic rings. The compounds have been tested for blockade of the slow after-hyperpolarization on rat sympathetic neurons. The most potent compounds have X = phenanthryl, fluorenyl, cis-stilbene, and C6H4(CH2)nC6H4, where n = 0-4. The conformational preferences of the compounds were investigated using the XED/COSMIC molecular modeling system. Although there is some dependence of the potency of the analogue on the conformational properties of the linker (X), overall, X groups having substantial structural differences are tolerated. It seems that X provides a support for the two quinolinium groups and does not interact with the channel directly. The intramolecular separation between the quinolinium rings, which is provided by rigid groups X, is not critical for activity; this may be attributed to the residual conformational mobility of the heterocycles and to the extensive delocalization of the positive charge. These two factors may permit favorable contacts between the quinolinium groups and the channel over a range of intramolecular separations.