Voltage-Gated K+ Channel Modulation by Marine Toxins: Pharmacological Innovations and Therapeutic Opportunities

Abstract

Bioactive compounds are abundant in animals originating from marine ecosystems. Ion channels, which include sodium, potassium, calcium, and chloride, together with their numerous variants and subtypes, are the primary molecular targets of the latter. Based on their cellular targets, these venom compounds show a range of potencies and selectivity and may have some therapeutic properties. Due to their potential as medications to treat a range of (human) diseases, including pain, autoimmune disorders, and neurological diseases, marine molecules have been the focus of several studies over the last ten years. The aim of this review is on the various facets of marine (or marine-derived) molecules, ranging from structural characterization and discovery to pharmacology, culminating in the development of some "novel" candidate chemotherapeutic drugs that target potassium channels.

Keywords: disease treating; marine toxins; peptides; voltage-gated potassium channels.

Figure 1

Schematic representation of the proposed transmembrane topology of voltage-gated ion channels. (A) The α-subunit of K⁺ channels showing the transmembrane segments (1–6) spanning the cell membrane. Segment 4 (red color) represents the voltage sensor. In the extracellular region, the S5-S6 P-loop is directly involved in the ion conduction pathway. In the intracellular region, the N and C termini ends of the polypeptide appear. The right panel shows the top and side views of homo- or heterotetramers of encircled four-fold α-subunits forming the central ion conduction pathway of a K⁺ channel. (B) A single subunit polypeptide of four homologous domains (I–IV) forming a functional pore of Ca²⁺ and Na⁺ channels. (C) K_v1.2 channel structural models in activated and resting states X-ray crystallography were used to identify the structure of the K_v1.2 channel in an activated state, and the ROSETTA membrane technique was used to predict the channel’s structure in a closed state. The voltage-sensing and pore-forming modules consist of two subunits. Take note of the labels that show how the voltage-sensing module of subunit 4 connects with the subunit 1 pore-forming module (left) and how the subunit 2 voltage-sensing module interacts with the subunit 3 pore-forming module (right). S1, dark blue; S2, light blue-green; S3, light green; S4, dark green; S5, yellow-green; and S6, orange, are the colored transmembrane segments. The S4–S5 linkers covalently connecting pore-forming and voltage-sensing modules are highlighted in magenta [7].

Figure 2

According to the “ball and chain” model of inactivation, the ion channels contain a domain (“ball”) tethered to the cytoplasmatic side of the protein. Following a conformational change, the inactivation ball is free to bind to its receptor, occluding the ion-conducting pore. This fast inactivation process (N-type inactivation) can be distinguished for K⁺ channels from a second inactivation process (C-type inactivation), which involves a conformational change in the extracellular part of the protein. The K⁺ is depicted as a green circle [12].

Figure 3

Twenty of the most abundant Conus species in the South China Sea [34].

Figure 4

A structural schematic illustration to show the classification of conotoxins into superfamilies and their ion channel targets.

Figure 5

Primary structure of conotoxins binding to voltage-gated ion channels. (A) µ- and δ-conotoxins (2EFZ, 1FUE) interact with Na⁺ channels, ω- with Ca²⁺ channels, and κ- with K⁺ channels (1TTL, 1KCP). K⁺ channel peptide blockers seem to possess a common functionally important dyad consisting of a hydrophobic residue and key lysine protruding from a relatively flat surface [45]. (B) These residues are highlighted in bold in the κ-PVIIA primary sequence [41,46].

Figure 6

A comparison of O-superfamily precursor sequences. The conserved cysteine pattern is illustrated in bold. The inferred sequence of the μO-conotoxin MrVIB precursor sequence is aligned with the prepropeptide sequences of Δ-conotoxin TxVIA (formerly called the King-Kong peptide), ω-conotoxin GVIA, and к-PVIIA. The conserved amino acids are illustrated with a grey background.

Figure 7

Sequence alignment of κM-conotoxins RIIIK and RIIIJ with other M-superfamily peptides and κ-conotoxin PVIIA. O represents 4-hydroxyproline and * an amidated C-terminal amino acid.

Figure 8

(A) Sequence alignment of chosen Kunitz-fold peptides (Conk-S1 and Conk-S2). Grey shading indicates conserved cysteine residues. Two preserved disulfide bridges are shown in solid lines. A dotted line indicates a third, non-conserved bridge. The secondary structure elements are illustrated at the bottom. A non-conserved arginine identified as a critical residue for channel block is denoted by an asterisk. The bioactive residues of both Conk-S1 and Conk-S2 from Conus Striatus, are highlighted in red. (B) Ribbon representations of Conk-S1 (PDB: 2CA7) and Conk-S2 (PDB: 2J6D). The yellow line represents disulfide bridges. The bioactive residues of Conk-S1 are highlighted in (A).

Figure 9

Molecular structures of CPY conopeptides. Tyrosine residues are evidenced in bold.

Figure 10

(A) Peptide sequence of pl14a showing the disulfide connectivity. (B) Disulfide motifs found in conotoxin superfamilies with four cysteines.

Figure 11

Comparison of peptide sr11a with other I-conotoxins from vermivorous species. γ, gamma-carboxy-glutamate; O, hydroxyPro; S, glycosylated Ser; *, amidated C-terminus [75].

Figure 12

The sea anemone mature peptide sequences resemble β-defensin. The conserved cysteine residues are marked with red writing on a grey background. Tentacles, column, mesenterial filaments, and combinations are represented by letters T, C, F, and M, which are emphasized in blue, orange, green, and yellow, respectively. Homology modeling predicts various mature peptides from sea anemones using CgNa (PDB 2H9X), BDS-I (PDB 1BDS), and APETx2 (PDB 2MUB).

Figure 13

Amino acid sequences of a new family of sea anemone peptide toxins. Charged residues are bolded in red. The sequence identities are highlighted in yellow.

Figure 14

(A) Alignment of sea anemone toxins, which most likely have an ICK fold. Predicted disulfide connectivities are illustrated with dotted lines. Amino acid identities are highlighted in yellow and similarities in bold. The toxin sequences given are BcsTx3 (κ-actitoxin-Bcs4a; UniProt C0HJC4), NvePTx1 (U-EWTX-NvePTx1; UniProt A7RMN1), MsePTx1 (U-metritoxin-Msn2a; UniProt P0DMD7), and PhcrTx1 (π-phymatoxin-Pcf1a; UniProt C0HJB1) [78]. (B) Structures of BscTx1 and BscTx2 type-1 toxins. Disulfide bridges are illustrated by dotted lines.

Figure 15

Alignment of the amino acid sequence of AETXk with those of the known type 1 potassium channel toxins from sea anemones: HmK, ShK, AeK, AsKS, and BgK. The residues identical with AETXk are highlighted. Disulfide bridges are depicted by dotted lines. Asterisks represent the amino acid dyad that is crucial for the binding to potassium channels.

Figure 16

Amino acid sequences of Kuntitz-type peptides APEKTx1, SHTX-III, and AsKC1–3. Shaded areas indicate spots that are highly conserved. α-dendrotoxin (a K⁺ channel-blocking toxin from the green mamba Dendroaspis angusticeps) and BPTI (bovine pancreatic trypsin inhibitor, the first-described Kunitz protein) are depicted as reference compounds. The three disulfide bridges are represented by a dotted line, and the cysteine residues involved are highlighted in red [119].

Figure 17

Disulfide bonds in Ate1a (PDB 6AZA) are depicted as orange tubes, with N- and C-termini labeled [55].

Figure 18

Domain structure of Ate1a and Ate1a-like contigs. Prepropeptides consist of a signal peptide (SignalP), one or two cysteine-containing propeptide domains (CysProP), and three cysteine-free propeptide domains (LinearProP), each preceding an Ate1a-like PHAB domain [95].

Figure 19

Primary sequence of the kP-crassipeptides. Cysteine residues are highlighted and aligned.

Figure 20

Structure of 4,5-dibromopyrrole-2-carboxylic acid.

Figure 21

The chemical structures of pyrrole alkaloids 1–11 from Agelas sponges.

Figure 22

Structure of Acredinones A and B.

Figure 23

Structure of gambierol toxin, showing the eight polyether rings.

Figure 24

Aplysiatoxins’ molecular structures.