Straightforward approach to produce recombinant scorpion toxins-Pore blockers of potassium channels

Interactions of the Kv1.1 Channel with Peptide Pore Blockers: A Fluorescent Analysis on Mammalian Cells

The voltage-gated potassium channel Kv1.1, which is abundant in the CNS and peripheral nervous system, controls neuronal excitability and neuromuscular transmission and mediates a number of physiological functions in non-excitable cells. The development of some diseases is accompanied by changes in the expression level and/or activity of the channels in particular types of cells. To meet the requirements of studies related to the expression and localization of the Kv1.1 channels, we report on the subnanomolar affinity of hongotoxin 1 N-terminally labeled with Atto 488 fluorophore (A-HgTx) for the Kv1.1 channel and its applicability for fluorescent imaging of the channel in living cells. Taking into consideration the pharmacological potential of the Kv1.1 channel, a fluorescence-based analytical system was developed for the study of peptide ligands that block the ion conductivity of Kv1.1 and are potentially able to correct abnormal activity of the channel. The system is based on analysis of the competitive binding of the studied compounds and A-HgTx to the mKate2-tagged human Kv1.1 (S369T) channel, expressed in the plasma membrane of Neuro2a cells. The system was validated by measuring the affinities of the known Kv1.1-channel peptide blockers, such as agitoxin 2, kaliotoxin 1, hongotoxin 1, and margatoxin. Peptide pore blocker Ce1, from the venom of the scorpion Centruroides elegans, was shown to possess a nanomolar affinity for the Kv1.1 channel. It is reported that interactions of the Kv1.1 channel with the studied peptide blockers are not affected by the transition of the channel from the closed to open state. The conclusion is made that the structural rearrangements accompanying the channel transition into the open state do not change the conformation of the P-loop (including the selectivity filter) involved in the formation of the binding site of the peptide pore blockers.

AgTx2-GFP, Fluorescent Blocker Targeting Pharmacologically Important Kv1.x (x = 1, 3, 6) Channels

The growing interest in potassium channels as pharmacological targets has stimulated the development of their fluorescent ligands (including genetically encoded peptide toxins fused with fluorescent proteins) for analytical and imaging applications. We report on the properties of agitoxin 2 C-terminally fused with enhanced GFP (AgTx2-GFP) as one of the most active genetically encoded fluorescent ligands of potassium voltage-gated K_v1.x (x = 1, 3, 6) channels. AgTx2-GFP possesses subnanomolar affinities for hybrid KcsA-K_v1.x (x = 3, 6) channels and a low nanomolar affinity to KcsA-K_v1.1 with moderate dependence on pH in the 7.0-8.0 range. Electrophysiological studies on oocytes showed a pore-blocking activity of AgTx2-GFP at low nanomolar concentrations for K_v1.x (x = 1, 3, 6) channels and at micromolar concentrations for K_v1.2. AgTx2-GFP bound to K_v1.3 at the membranes of mammalian cells with a dissociation constant of 3.4 ± 0.8 nM, providing fluorescent imaging of the channel membranous distribution, and this binding depended weakly on the channel state (open or closed). AgTx2-GFP can be used in combination with hybrid KcsA-K_v1.x (x = 1, 3, 6) channels on the membranes of E. coli spheroplasts or with K_v1.3 channels on the membranes of mammalian cells for the search and study of nonlabeled peptide pore blockers, including measurement of their affinity.

Combining mKate2-Kv1.3 Channel and Atto488-Hongotoxin for the Studies of Peptide Pore Blockers on Living Eukaryotic Cells

The voltage-gated potassium Kv1.3 channel is an essential component of vital cellular processes which is also involved in the pathogenesis of some autoimmune, neuroinflammatory and oncological diseases. Pore blockers of the Kv1.3 channel are considered as potential drugs and are used to study Kv1 channels' structure and functions. Screening and study of the blockers require the assessment of their ability to bind the channel. Expanding the variety of methods used for this, we report on the development of the fluorescent competitive binding assay for measuring affinities of pore blockers to Kv1.3 at the membrane of mammalian cells. The assay constituents are hongotoxin 1 conjugated with Atto488, fluorescent mKate2-tagged Kv1.3 channel, which was designed to improve membrane expression of the channel in mammalian cells, confocal microscopy, and a special protocol of image processing. The assay is implemented in the "mix and measure", format and allows the screening of Kv1.3 blockers, such as peptide toxins, that bind to the extracellular vestibule of the K⁺-conducting pore, and analyzing their affinity.

Atto488-Agitoxin 2—A Fluorescent Ligand with Increased Selectivity for Kv1.3 Channel Binding Site

Fluorescently labeled peptide blockers of ion channels are useful probes in studying the localization and functioning of the channels and in the performance of a search for new channel ligands with bioengineering screening systems. Here, we report on the properties of Atto488-agitoxin 2 (A-AgTx2), a derivative of the Kv1 channel blocker agitoxin 2 (AgTx2), which was N-terminally labeled with Atto 488 fluorophore. The interactions of A-AgTx2 with the outer binding sites of the potassium voltage-gated Kv1.x (x = 1, 3, 6) channels were studied using bioengineered hybrid KcsA–Kv1.x (x = 1, 3, 6) channels. In contrast to AgTx2, A-AgTx2 was shown to lose affinity for the Kv1.1 and Kv1.6 binding sites but to preserve it for the Kv1.3 site. Thus, Atto488 introduces two new functionalities to AgTx2: fluorescence and the selective targeting of the Kv1.3 channel, which is known for its pharmacological significance. In the case of A-AgTx2, fluorescent labeling served as an alternative to site-directed mutagenesis in modulating the pharmacological profile of the channel blocker. Although the affinity of A-AgTx2 for the Kv1.3 binding site was decreased as compared to the unlabeled AgTx2, its dissociation constant value was within a low nanomolar range (4.0 nM). The properties of A-AgTx2 allow one to use it for the search and study of Kv1.3 channel blockers as well as to consider it for the imaging of the Kv1.3 channel in cells and tissues.

GFP-Margatoxin, a Genetically Encoded Fluorescent Ligand to Probe Affinity of Kv1.3 Channel Blockers

Peptide pore blockers and their fluorescent derivatives are useful molecular probes to study the structure and functions of the voltage-gated potassium Kv1.3 channel, which is considered as a pharmacological target in the treatment of autoimmune and neurological disorders. We present Kv1.3 fluorescent ligand, GFP-MgTx, constructed on the basis of green fluorescent protein (GFP) and margatoxin (MgTx), the peptide, which is widely used in physiological studies of Kv1.3. Expression of the fluorescent ligand in E. coli cells resulted in correctly folded and functionally active GFP-MgTx with a yield of 30 mg per 1 L of culture. Complex of GFP-MgTx with the Kv1.3 binding site is reported to have the dissociation constant of 11 ± 2 nM. GFP-MgTx as a component of an analytical system based on the hybrid KcsA-Kv1.3 channel is shown to be applicable to recognize Kv1.3 pore blockers of peptide origin and to evaluate their affinities to Kv1.3. GFP-MgTx can be used in screening and pre-selection of Kv1.3 channel blockers as potential drug candidates.

Bioengineered System for High Throughput Screening of Kv1 Ion Channel Blockers

Screening drug candidates for their affinity and selectivity for a certain binding site is a crucial step in developing targeted therapy. Here, we created a screening assay for receptor binding that can be easily scaled up and automated for the high throughput screening of Kv channel blockers. It is based on the expression of the KcsA-Kv1 hybrid channel tagged with a fluorescent protein in the E. coli membrane. In order to make this channel accessible for the soluble compounds, E. coli were transformed into spheroplasts by disruption of the cellular peptidoglycan envelope. The assay was evaluated using a hybrid KcsA-Kv1.3 potassium channel tagged with a red fluorescent protein (TagRFP). The binding of Kv1.3 channel blockers was measured by flow cytometry either by using their fluorescent conjugates or by determining the ability of unconjugated compounds to displace fluorescently labeled blockers with a known affinity. A fraction of the occupied receptor was calculated with a dedicated pipeline available as a Jupyter notebook. Measured binding constants for agitoxin-2, charybdotoxin and kaliotoxin were in firm agreement with the earlier published data. By using a mid-range flow cytometer with manual sample handling, we measured and analyzed up to ten titration curves (eight data points each) in one day. Finally, we considered possibilities for multiplexing, scaling and automation of the assay.

N-Terminal Tagging with GFP Enhances Selectivity of Agitoxin 2 to Kv1.3-Channel Binding Site

Recently developed fluorescent protein-scorpion toxin chimeras (FP-Tx) show blocking activities for potassium voltage-gated channels of Kv1 family and retain almost fully pharmacological profiles of the parental peptide toxins (Kuzmenkov et al., Sci Rep. 2016, 6, 33314). Here we report on N-terminally green fluorescent protein (GFP)-tagged agitoxin 2 (GFP-L2-AgTx2) with high affinity and selectivity for the binding site of Kv1.3 channel involved in the pathogenesis of various (primarily of autoimmune origin) diseases. The basis for this selectivity relates to N-terminal location of GFP, since transposition of GFP to the C-terminus of AgTx2 recovered specific interactions with the Kv1.1 and Kv1.6 binding sites. Competitive binding experiments revealed that the binding site of GFP-L2-AgTx2 overlaps that of charybdotoxin, kaliotoxin 1, and agitoxin 2, the known Kv1.3-channel pore blockers. GFP-L2-AgTx2 was demonstrated to be applicable as a fluorescent probe to search for Kv1.3 pore blockers among individual compounds and in complex mixtures, to measure blocker affinities, and to visualize Kv1.3 distribution at the plasma membrane of Kv1.3-expressing HEK293 cells. Our studies show that definite combinations of fluorescent proteins and peptide blockers can result in considerable modulation of the natural blocker-channel binding profile yielding selective fluorescent ligands of certain channels.

Scorpion toxins interact with nicotinic acetylcholine receptors

Neurotoxins are among the main components of scorpion and snake venoms. Scorpion neurotoxins affect voltage-gated ion channels, while most snake neurotoxins target ligand-gated ion channels, mainly nicotinic acetylcholine receptors (nAChRs). We report that scorpion venoms inhibit α-bungarotoxin binding to both muscle-type nAChR from Torpedo californica and neuronal human α7 nAChR. Toxins inhibiting nAChRs were identified as OSK-1 (α-KTx family) from Orthochirus scrobiculosus and HelaTx1 (κ-KTx family) from Heterometrus laoticus, both being blockers of voltage-gated potassium channels. With an IC₅₀ of 1.6 μm, OSK1 inhibits acetylcholine-induced current through mouse muscle-type nAChR heterologously expressed in Xenopus oocytes. Other well-characterized scorpion toxins from these families also bind to Torpedo nAChR with micromolar affinities. Our results indicate that scorpion neurotoxins present target promiscuity.

KV1.2 channel-specific blocker from Mesobuthus eupeus scorpion venom: Structural basis of selectivity

Scorpion venom is an unmatched source of selective high-affinity ligands of potassium channels. There is a high demand for such compounds to identify and manipulate the activity of particular channel isoforms. The objective of this study was to obtain and characterize a specific ligand of voltage-gated potassium channel K_V1.2. As a result, we report the remarkable selectivity of the peptide MeKTx11-1 (α-KTx 1.16) from Mesobuthus eupeus scorpion venom to this channel isoform. MeKTx11-1 is a high-affinity blocker of K_V1.2 (IC₅₀ ∼0.2 nM), while its activity against K_V1.1, K_V1.3, and K_V1.6 is 10 000, 330 and 45 000 fold lower, respectively, as measured using the voltage-clamp technique on mammalian channels expressed in Xenopus oocytes. Two substitutions, G9V and P37S, convert MeKTx11-1 to its natural analog MeKTx11-3 (α-KTx 1.17) having 15 times lower activity and reduced selectivity to K_V1.2. We produced MeKTx11-1 and MeKTx11-3 as well as their mutants MeKTx11-1(G9V) and MeKTx11-1(P37S) recombinantly and demonstrated that point mutations provide an intermediate effect on selectivity. Key structural elements that explain MeKTx11-1 specificity were identified by molecular modeling of the toxin-channel complexes. Confirming our molecular modeling predictions, site-directed transfer of these elements from the pore region of K_V1.2 to K_V1.3 resulted in the enhanced sensitivity of mutant K_V1.3 channels to MeKTx11-1. We conclude that MeKTx11-1 may be used as a selective tool in neurobiology.

Recombinant scorpion toxins: Focus on four-disulfide peptide blockers of Kv1-channels

We have recently developed a simple and effective bioengineering approach to large-scale production of alpha-KTx, peptide toxins from scorpion venoms, that block voltage-gated potassium channels with high affinity and specificity. This approach was successfully approved for different peptides containing three disulfide bonds. To extend this method to production of peptide toxins with four disulfide bridges, in particular, maurotoxin and hetlaxin, appropriate conditions of a cleavage reaction with tobacco etch virus (TEV) protease need to be found. For this, the interplay between efficiency of TEV hydrolysis and sensitivity of the target peptides to disulfide reducing agents was studied, and optimized protocols of TEV cleavage reaction were worked out. Maurotoxin and hetlaxin were produced in a folded form avoiding in vitro renaturation step with yields of 14 and 12 mg/liter of culture, respectively.