Evaluation of different approaches used to study membrane permeabilization by actinoporins on model lipid vesicles

Effects of Conjugation of Ferrocene and Gallic Acid On desCys11/Lys12/Lys13-(p-BthTX-I)2K Peptide: Structure, Permeabilization and Antibacterial Activity

BACKGROUND

Antimicrobial resistance is an emerging global health challenge that has led researchers to study alternatives to conventional antibiotics. A promising alternative is antimicrobial peptides (AMPs), produced as the first line of defense by almost all living organisms. To improve its biological activity, the conjugation of AMPs is a promising approach.

OBJECTIVE

In this study, we evaluated the N-terminal conjugation of p-Bt (a peptide derived from Bothrops Jararacuçu`s venom) with ferrocene (Fc) and gallic acid (GA). Acetylated and linear versions of p-Bt were also synthesized to evaluate the importance of N-terminal charge and dimeric structure.

METHODS

The compounds were obtained using solid-phase peptide synthesis. Circular dichroism, vesicle permeabilization, antimicrobial activity, and cytotoxicity studies were conducted.

RESULTS

No increase in antibacterial activity against Escherichia coli was observed by adding either Fc or GA to p-Bt. However, Fc-p-Bt and GA-p-Bt exhibited improved activity against Staphylococcus aureus. No cytotoxicity upon fibroblast was observed for GA-p-Bt. On the other hand, conjugation with Fc increased cytotoxicity. This toxicity may be related to the membrane permeabilization capacity of this bioconjugate, which showed the highest carboxyfluorescein leakage in vesicle permeabilization experiments.

CONCLUSION

Considering these observations, our findings highlight the importance of adding bioactive organic compounds in the N-terminal position as a tool to modulate the activity of AMPs.

The Sea Anemone Neurotoxins Modulating Sodium Channels: An Insight at Structure and Functional Activity after Four Decades of Investigation

Many human cardiovascular and neurological disorders (such as ischemia, epileptic seizures, traumatic brain injury, neuropathic pain, etc.) are associated with the abnormal functional activity of voltage-gated sodium channels (VGSCs/Na_Vs). Many natural toxins, including the sea anemone toxins (called neurotoxins), are an indispensable and promising tool in pharmacological researches. They have widely been carried out over the past three decades, in particular, in establishing different Na_V subtypes functional properties and a specific role in various pathologies. Therefore, a large number of publications are currently dedicated to the search and study of the structure-functional relationships of new sea anemone natural neurotoxins-potential pharmacologically active compounds that specifically interact with various subtypes of voltage gated sodium channels as drug discovery targets. This review presents and summarizes some updated data on the structure-functional relationships of known sea anemone neurotoxins belonging to four structural types. The review also emphasizes the study of type 2 neurotoxins, produced by the tropical sea anemone Heteractis crispa, five structurally homologous and one unique double-stranded peptide that, due to the absence of a functionally significant Arg14 residue, loses toxicity but retains the ability to modulate several VGSCs subtypes.

Sea Anemones, Actinoporins, and Cholesterol

Spanish or Spanish-speaking scientists represent a remarkably populated group within the scientific community studying pore-forming proteins. Some of these scientists, ourselves included, focus on the study of actinoporins, a fascinating group of metamorphic pore-forming proteins produced within the venom of several sea anemones. These toxic proteins can spontaneously transit from a water-soluble fold to an integral membrane ensemble because they specifically recognize sphingomyelin in the membrane. Once they bind to the bilayer, they subsequently oligomerize into a pore that triggers cell-death by osmotic shock. In addition to sphingomyelin, some actinoporins are especially sensible to some other membrane components such as cholesterol. Our group from Universidad Complutense of Madrid has focused greatly on the role played by sterols in this water–membrane transition, a question which still remains only partially solved and constitutes the main core of the article below.

Structural foundations of sticholysin functionality

Actinoporins constitute a family of α pore-forming toxins produced by sea anemones. The soluble fold of these proteins consists of a β-sandwich flanked by two α-helices. Actinoporins exert their activity by specifically recognizing sphingomyelin at their target membranes. Once there, they penetrate the membrane with their N-terminal α-helices, a process that leads to the formation of cation-selective pores. These pores kill the target cells by provoking an osmotic shock on them. In this review, we examine the role and relevance of the structural features of actinoporins, down to the residue level. We look at the specific amino acids that play significant roles in the function of actinoporins and their fold. Particular emphasis is given to those residues that display a high degree of conservation across the actinoporin sequences known to date. In light of the latest findings in the field, the membrane requirements for pore formation, the effect of lipid composition, and the process of pore formation are also discussed.

Biophysical approaches to study actinoporin-lipid interactions

Protein-lipid interactions are crucial events from a biochemical point of view, like the interaction of proteins with the cell plasma membrane, and their study is of great importance. Actinoporins are a very powerful tool to study this kind of interactions, since they are soluble proteins in an aqueous environment, capable of inserting into membranes when they have the adequate composition. In fact, actinoporins have been used to study protein-lipid interactions for many years now. Sometimes it is not possible to use real biological membranes in the experiments, so model membranes need to be used. This article aims to give a thorough description of many of the techniques used to study actinoporin-lipid interactions, using both biological and model membranes: Hemolysis, release of vesicles content, surface plasmon resonance, isothermal titration calorimetry, fluorescence-based measurements, etc. Some of these techniques measure the actinoporins activity and some measure their binding properties. The combination of all the techniques described can offer valuable information about the thermodynamics and the kinetics of the actinoporin-lipid interaction.

Functional and Structural Variation among Sticholysins, Pore-Forming Proteins from the Sea Anemone Stichodactyla helianthus

Venoms constitute complex mixtures of many different molecules arising from evolution in processes driven by continuous prey-predator interactions. One of the most common compounds in these venomous cocktails are pore-forming proteins, a family of toxins whose activity relies on the disruption of the plasmatic membranes by forming pores. The venom of sea anemones, belonging to the oldest lineage of venomous animals, contains a large amount of a characteristic group of pore-forming proteins known as actinoporins. They bind specifically to sphingomyelin-containing membranes and suffer a conformational metamorphosis that drives them to make pores. This event usually leads cells to death by osmotic shock. Sticholysins are the actinoporins produced by Stichodactyla helianthus. Three different isotoxins are known: Sticholysins I, II, and III. They share very similar amino acid sequence and three-dimensional structure but display different behavior in terms of lytic activity and ability to interact with cholesterol, an important lipid component of vertebrate membranes. In addition, sticholysins can act in synergy when exerting their toxin action. The subtle, but important, molecular nuances that explain their different behavior are described and discussed throughout the text. Improving our knowledge about sticholysins behavior is important for eventually developing them into biotechnological tools.