Molecular Dynamics Simulation Reveals Specific Interaction Sites between Scorpion Toxins and Kv1.2 Channel: Implications for Design of Highly Selective Drugs

Innovative treatments for epilepsy: Venom peptides, cannabinoids, and neurostimulation

Antiepileptic drugs have been successfully treating epilepsy and providing individuals sustained seizure freedom. However, about 30% of the patients with epilepsy present drug resistance, which means they are not responsive to the pharmacological treatment. Considering this, it becomes extremely relevant to pursue alternative therapeutic approaches, in order to provide appropriate treatment for those patients and also improve their quality of life. In the light of that, this review aims to discuss some innovative options for the treatment of epilepsy, which are currently under investigation, addressing strategies that go from therapeutic compounds to clinical procedures. For instance, peptides derived from animal venoms, such as wasps, spiders, and scorpions, demonstrate to be promising antiepileptic molecules, acting on a variety of targets. Other options are cannabinoids and compounds that modulate the endocannabinoid system, since it is now known that this network is involved in the pathophysiology of epilepsy. Furthermore, neurostimulation is another strategy, being an alternative clinical procedure for drug-resistant patients who are not eligible for palliative surgeries.

Dual-Site Binding of Quaternary Ammonium Ions as Internal K+-Ion Channel Blockers: Nonclassical (C-H···O) H Bonding vs Dispersive (C-H···H-C) Interaction

A molecular-level study of the influence of the alkyl chain length of quaternary ammonium ions (QAs) on the blocking action and the mode of binding with the bacterial KcsA K⁺-ion channel is carried out by molecular dynamics (MD) simulations as well as quantum mechanics/molecular mechanics (QM/MM) methods. The present work unveils distinct modes of binding for different QAs, due to differences in size and hydrophobicity. The QAs bind near the channel gate as well as at the central cavity, leading to a possible dual-site blocking action. Small-sized tetraethylammonium (TEA) and tetrabutylammonium (TBA) ions enter inside the channel cavity in the open state of KcsA but bind strongly in the closed state. TEA binds to the polar hydroxyl group of threonine residues situated at the channel gate via nonclassical H-bonding interaction (C-H···O), while TBA binds to a second binding site, the central cavity, with hydrophobic benzyl and sec-butyl side chains of phenylalanine and isoleucine residues via alkyl-π and hydrophobic interactions (C-H···H-C). On the contrary, large tetrahexylammonium (THA) and tetraoctylammonium (TOA) ions bind the hydrophobic side-chain methyl and isopropyl of alanine and valine at the channel gate both in the open and closed states, thereby restricting the free movement of large QAs toward the center of the cavity. However, the binding to the hydrophobic benzyl and sec-butyl side chains of phenylalanine and isoleucine residues in the closed state is thermodynamically preferable. Also, the binding energy is found to increase with an increase in the alkyl chain length from ethyl (-16.4 kcal/mol) to octyl (-65.5 kcal/mol), due to an almost linear increase in dispersive interaction.

Scorpion Toxins: Positive Selection at a Distal Site Modulates Functional Evolution at a Bioactive Site

The bioactive sites of proteins are those that directly interact with their targets. In many immunity- and predation-related proteins, they frequently experience positive selection for dealing with the changes of their targets from competitors. However, some sites that are far away from the interface between proteins and their targets are also identified to evolve under positive selection. Here, we explore the evolutionary implication of such a site in scorpion α-type toxins affecting sodium (Na⁺) channels (abbreviated as α-ScNaTxs) using a combination of experimental and computational approaches. We found that despite no direct involvement in interaction with Na⁺ channels, mutations at this site by different types of amino acids led to toxicity change on both rats and insects in three α-ScNaTxs, accompanying differential effects on their structures. Molecular dynamics simulations indicated that the mutations changed the conformational dynamics of the positively selected bioactive site-containing functional regions by allosteric communication, suggesting a potential evolutionary correlation between these bioactive sites and the distant nonbioactive site. Our results reveal for the first time the cause of fast evolution at nonbioactive sites of scorpion neurotoxins, which is presumably to adapt to the change of their bioactive sites through coevolution to maintain an active conformation for channel binding. This might aid rational design of scorpion Na⁺ channel toxins with improved phyletic selectivity via modification of a distant nonbioactive site.

Toxins in Drug Discovery and Pharmacology

Venoms from marine and terrestrial animals (cone snails, scorpions, spiders, snakes, centipedes, cnidarian, etc.) can be seen as an untapped cocktail of biologically active compounds, being increasingly recognized as a new emerging source of peptide-based therapeutics.