Alkaloids Modulate the Functioning of Ion Channels Produced by Antimicrobial Agents via an Influence on the Lipid Host

The effects of plant flavones on the membrane boundary potential and lipid packing stress

Here we have revealed the effects of different plant flavones on the physicochemical properties of model lipid membranes. We have demonstrated that baicalein increases the boundary potential of membranes composed of phosphatidylcholine, while wogonin does not affect it. Other flavones tested reduce membrane boundary potential, with this ability increasing among scutellarein, chrysin, apigenin, morin, fisetin, and luteolin. Molecular dynamics simulations demonstrate connection of alteration in boundary potential with the preferential orientation of intrinsic flavone dipole moments in membranes. We have also shown that flavones reduce the melting point of phosphatidylcholine, and this ability increases in the series of luteolin, morin, wogonin, scutellarein, apigenin, baicalein, chrysin, and fisetin. The introduction of baicalein, chrysin and fisetin also leads to a significant decrease in the sharpness of the lipid phase transition. We have hypothesized that the localization of flavones in the glycerol backbone or in the C1-C8 methylene region of lipid hydrocarbon chains leads to an increase in the area per lipid and, as a consequence, to an expansion of the lipid melting peak. Replacement of neutral phosphatidylcholine with negatively charged phosphatidylserine affects the membrane-modifying activity of flavones which given the externalization of phosphatidylserine on the surface of cancer cells may be crucial in the flavone anticancer effects.

Binding and sequestration of poison frog alkaloids by a plasma globulin

Alkaloids are important bioactive molecules throughout the natural world, and in many animals they serve as a source of chemical defense against predation. Dendrobatid poison frogs bioaccumulate alkaloids from their diet to make themselves toxic or unpalatable to predators. Despite the proposed roles of plasma proteins as mediators of alkaloid trafficking and bioavailability, the responsible proteins have not been identified. We use chemical approaches to show that a ~50 kDa plasma protein is the principal alkaloid-binding molecule in blood of poison frogs. Proteomic and biochemical studies establish this plasma protein to be a liver-derived alkaloid-binding globulin (ABG) that is a member of the serine-protease inhibitor (serpin) family. In addition to alkaloid-binding activity, ABG sequesters and regulates the bioavailability of 'free' plasma alkaloids in vitro. Unexpectedly, ABG is not related to saxiphilin, albumin, or other known vitamin carriers, but instead exhibits sequence and structural homology to mammalian hormone carriers and amphibian biliverdin-binding proteins. ABG represents a new small molecule binding functionality in serpin proteins, a novel mechanism of plasma alkaloid transport in poison frogs, and more broadly points toward serpins acting as tunable scaffolds for small molecule binding and transport across different organisms.

Deciphering the Broad Antimicrobial Activity of Melaleuca alternifolia Tea Tree Oil by Combining Experimental and Computational Investigations

Tea Tree Oil (TTO) is an essential oil obtained from the distillation of Melaleuca alternifolia leaves and branches. Due to its beneficial properties, TTO is widely used as an active ingredient in antimicrobial preparations for topical use or in cosmetic products and contains about 100 different compounds, with terpinen-4-ol, γ-terpinene and 1,8-cineole (or eucalyptol) being the molecules most responsible for its biological activities. In this work, the antimicrobial activity of whole TTO and these three major components was evaluated in vitro against fungi, bacteria and viruses. Molecular dynamics simulations were carried out on a bacterial membrane model and a Coxsackievirus B4 viral capsid, to propose an atomistic explanation of their mechanism of action. The obtained results indicate that the strong antimicrobial activity of TTO is attributable to the induction of an altered membrane functionality, mediated by the incorporation of its components within the lipid bilayer, and to a possible ability of the compounds to bind and alter the structural properties of the viral capsid.

Triggering the Amphotericin B Pore-Forming Activity by Phytochemicals

The macrolide polyene antibiotic amphotericin B (AmB), remains a valuable drug to treat systemic mycoses due to its wide antifungal activity and low probability of developing resistance. The high toxicity of AmB, expressed in nephropathy and hemolysis, could be partially resolved by lowering therapeutic AmB concentration while maintaining efficacy. This work discusses the possibility of using plant polyphenols and alkaloids to enhance the pore-forming and consequently antifungal activity of AmB. We demonstrated that phloretin, phlorizin, naringenin, taxifolin, quercetin, biochanin A, genistein, resveratrol, and quinine led to an increase in the integral AmB-induced transmembrane current in the bilayers composed of palmitoyloleoylphosphocholine and ergosterol, while catechin, colchicine, and dihydrocapsaicin did not practically change the AmB activity. Cardamonin, 4'-hydroxychalcone, licochalcone A, butein, curcumin, and piperine inhibited AmB-induced transmembrane current. Absorbance spectroscopy revealed no changes in AmB membrane concentration with phloretin addition. A possible explanation of the potentiation is related to the phytochemical-produced changes in the elastic membrane properties and the decrease in the energy of formation of the lipid mouth of AmB pores, which is partially confirmed by differential scanning microcalorimetry. The possibility of AmB interaction with cholesterol in the mammalian cell membranes instead of ergosterol in fungal membranes, determines its high toxicity. The replacement of ergosterol with cholesterol in the membrane lipid composition led to a complete loss or a significant decrease in the potentiating effects of tested phytochemicals, indicating low potential toxicity of these compounds and high therapeutic potential of their combinations with the antibiotic. The discovered combinations of AmB with plant molecules that enhance its pore-forming ability in ergosterol-enriched membranes, seem to be promising for further drug development in terms of the toxicity decrease and efficacy improvement.

Modulation of the Dipole Potential of Model Lipid Membranes with Phytochemicals: Molecular Mechanisms, Structure-Activity Relationships, and Implications in Reconstituted Ion Channels

Phytochemicals, such as flavonoids, stilbenoids, alkaloids, terpenoids, and related compounds, have a wide range of useful pharmacological properties which cannot be ascribed to binding to a single peptide or protein target alone. Due to the relatively high lipophilicity of phytochemicals, the lipid membrane is thought to mediate their effects via changes in the properties of the lipid matrix, in particular, by modulating the transmembrane distribution of the electrical potential and, consequently, the formation and functioning of the ion channels reconstituted in the lipid bilayers. Therefore, biophysical studies on the interactions between plant metabolites and model lipid membranes are still of interest. This review represents an attempt to provide a critical analysis of a variety of studies on altering membranes and ion channels with phytochemicals via disturbing the potential drop at the membrane-aqueous solution interface. Critical structural motifs and functioning groups in the molecules of plant polyphenols (alkaloids and saponins are identified) and the possible mechanisms of dipole potential modulation with phytochemicals are discussed.

Transient receptor potential vanilloid 1 (TRPV1)-independent actions of capsaicin on cellular excitability and ion transport

Capsaicin is a naturally occurring alkaloid derived from chili pepper that is responsible for its hot pungent taste. Capsaicin is known to exert multiple pharmacological actions, including analgesia, anticancer, anti-inflammatory, antiobesity, and antioxidant effects. The transient receptor potential vanilloid subfamily member 1 (TRPV1) is the main receptor mediating the majority of the capsaicin effects. However, numerous studies suggest that the TRPV1 receptor is not the only target for capsaicin. An increasing number of studies indicates that capsaicin, at low to mid µM ranges, not only indirectly through TRPV1-mediated Ca²⁺ increases, but also directly modulates the functions of voltage-gated Na⁺ , K⁺ , and Ca²⁺ channels, as well as ligand-gated ion channels and other ion transporters and enzymes involved in cellular excitability. These TRPV1-independent effects are mediated by alterations of the biophysical properties of the lipid membrane and subsequent modulation of the functional properties of ion channels and by direct binding of capsaicin to the channels. The present study, for the first time, systematically categorizes this diverse range of non-TRPV1 targets and discusses cellular and molecular mechanisms mediating TRPV1-independent effects of capsaicin in excitable, as well as nonexcitable cells.

Discovery of the Potentiator of the Pore-Forming Ability of Lantibiotic Nisin: Perspectives for Anticancer Therapy

This study was focused on the action of lantibiotic nisin on the phospholipid membranes. Nisin did not produce ion-permeable pores in the membranes composed of DOPC or DOPE. The introduction of DOPS into bilayer lipid composition led to a decrease in the threshold detergent concentration of nisin. An addition of nisin to DOPG- and TOCL-enriched bilayers caused the formation of well-defined ion pores of various conductances. The transmembrane macroscopic current increased with the second power of the lantibiotic aqueous concentration, suggesting that the dimer of nisin was at least involved in the formation of conductive subunit. The pore-forming ability of lantibiotic decreased in the series: DOPC/TOCL ≈ DOPE/TOCL >> DOPC/DOPG ≥ DOPE/DOPG. The preferential interaction of nisin to cardiolipin-enriched bilayers might explain its antitumor activity by pore-formation in mitochondrial membranes. Small natural molecules, phloretin and capsaicin, were found to potentiate the membrane activity of nisin in the TOCL-containing membranes. The effect was referred to as changes in the membrane boundary potential at the adsorption of small molecules. We concluded that the compounds diminishing the membrane boundary potential should be considered as the potentiator of the nisin pore-forming ability that can be used to develop innovative formulations for anticancer therapy.

Chromone-Containing Allylmorpholines Influence Ion Channels in Lipid Membranes via Dipole Potential and Packing Stress

Herein, we report that chromone-containing allylmorpholines can affect ion channels formed by pore-forming antibiotics in model lipid membranes, which correlates with their ability to influence membrane boundary potential and lipid-packing stress. At 100 µg/mL, allylmorpholines 1, 6, 7, and 8 decrease the boundary potential of the bilayers composed of palmitoyloleoylphosphocholine (POPC) by about 100 mV. At the same time, the compounds do not affect the zeta-potential of POPC liposomes, but reduce the membrane dipole potential by 80-120 mV. The allylmorpholine-induced drop in the dipole potential produce 10-30% enhancement in the conductance of gramicidin A channels. Chromone-containing allylmorpholines also affect the thermotropic behavior of dipalmytoylphosphocholine (DPPC), abolishing the pretransition, lowering melting cooperativity, and turning the main phase transition peak into a multicomponent profile. Compounds 4, 6, 7, and 8 are able to decrease DPPC's melting temperature by about 0.5-1.9 °C. Moreover, derivative 7 is shown to increase the temperature of transition of palmitoyloleoylphosphoethanolamine from lamellar to inverted hexagonal phase. The effects on lipid-phase transitions are attributed to the changes in the spontaneous curvature stress. Alterations in lipid packing induced by allylmorpholines are believed to potentiate the pore-forming ability of amphotericin B and gramicidin A by several times.

Capsaicin Inhibits Multiple Voltage-Gated Ion Channels in Rabbit Ventricular Cardiomyocytes in TRPV1-Independent Manner

Capsaicin is a naturally occurring alkaloid derived from chili pepper which is responsible for its hot, pungent taste. It exerts multiple pharmacological actions, including pain-relieving, anti-cancer, anti-inflammatory, anti-obesity, and antioxidant effects. Previous studies have shown that capsaicin significantly affects the contractility and automaticity of the heart and alters cardiovascular functions. In this study, the effects of capsaicin were investigated on voltage-gated ion currents in rabbit ventricular myocytes. Capsaicin inhibited rapidly activated (I_Kr) and slowly activated (I_Ks) K⁺ currents and transient outward (I_to) K⁺ current with IC₅₀ values of 3.4 µM,14.7 µM, and 9.6 µM, respectively. In addition, capsaicin, at higher concentrations, suppressed voltage-gated Na⁺ and Ca²⁺ currents and inward rectifier I_K1 current with IC₅₀ values of 42.7 µM, 34.9 µM, and 38.8 µM, respectively. Capsaicin inhibitions of I_Na, I_L-Ca, I_Kr, I_Ks, I_to, and I_K1 were not reversed in the presence of capsazepine (3 µM), a TRPV1 antagonist. The inhibitory effects of capsaicin on these currents developed gradually, reaching steady-state levels within 3 to 6 min, and the recoveries were usually incomplete during washout. In concentration-inhibition curves, apparent Hill coefficients higher than unity suggested multiple interaction sites of capsaicin on these channels. Collectively, these findings indicate that capsaicin affects cardiac electrophysiology by acting on a diverse range of ion channels and suggest that caution should be exercised when capsaicin is administered to carriers of cardiac channelopathies or to individuals with arrhythmia-prone conditions, such as ischemic heart diseases.

Prosopis juliflora piperidine alkaloid extract levels in diet for sheep change energy and nitrogen metabolism and affect enteric methane yield

BACKGROUND

Ionophore antibiotics improve the efficiency of energy metabolism, which has driven their use as a feed additive in ruminants for decades. Currently, they have not been approved in many countries, generating a challenge for the immediate search for plant extracts with a similar mode of action on rumen metabolism. This study evaluated the effects of enriched Prosopis juliflora (mesquite) piperidine alkaloid extract (MPA) levels as an alternative phytoadditive to sodium monensin (MON) in sheep.

RESULTS

The MPA diet did not differ from MON with regard to nutrient intake. A quadratic effect (P < 0.05) was observed for organic matter and neutral detergent fibre digestibility, with respective maximum point at 25.40 and minimum point at 0.95 mg kg^-1 MPA. The MPA levels linearly decreased (P < 0.05) faecal nitrogen loss. MPA did not differ from MON with regard to nutrient digestibility, and MPA levels increased (P < 0.05) the proportion of digestible energy and metabolizability from dietary gross energy. The MPA levels linearly decreased (P < 0.05) enteric CH₄ production, the yield showing lower (P < 0.05) energy loss as CH₄ than MON.

CONCLUSION

The results show that MPA levels of 17.3 and 27.8 mg kg^-1 are enteric CH₄ inhibitors and enhance energy and protein utilization, indicating a promising alternative to MON for ruminants. © 2022 Society of Chemical Industry.

Interaction of Pelargonium sidoides Compounds with Lactoferrin and SARS-CoV-2: Insights from Molecular Simulations

(1) Background: Pelargonium sidoides extracts and lactoferrin are two important natural, anti-inflammatory, and antiviral agents, which can interfere with the early stages of SARS-CoV-2 infection. Molecular docking and molecular dynamics simulation approaches have been applied to check for the occurrence of interactions of the Pelargonium sidoides compounds with lactoferrin and with SARS-CoV-2 components. (2) Methods: Computational methods have been applied to confirm the hypothesis of a direct interaction between PEL compounds and the lactoferrin protein and between Pelargonium sidoides compounds and SARS-CoV-2 Spike, 3CLPro, RdRp proteins, and membrane. Selected high-score complexes were structurally investigated through classical molecular dynamics simulation, while the interaction energies were evaluated using the molecular mechanics energies combined with generalized Born and surface area continuum solvation method. (3) Results: Computational analyses suggested that Pelargonium sidoides extracts can interact with lactoferrin without altering its structural and dynamical properties. Furthermore, Pelargonium sidoides compounds should have the ability to interfere with the Spike glycoprotein, the 3CLPro, and the lipid membrane, probably affecting the functional properties of the proteins inserted in the double layer. (4) Conclusion: Our findings suggest that Pelargonium sidoides may interfere with the mechanism of infection of SARS-CoV-2, especially in the early stages.

Phosphodiesterase Type 5 Inhibitors Greatly Affect Physicochemical Properties of Model Lipid Membranes

Although phosphodiesterase type 5 inhibitors are widely used and well-studied drugs, the potential benefits of their application in the treatment of various diseases and new drug delivery systems, including liposome forms, are still being discussed. In this regard, the role of the lipid matrix of cell membranes in the pharmacological action of the inhibitors is of special interest. It was shown that sildenafil, vardenafil, and tadalafil caused a significant decrease in the boundary potential of model membranes composed of palmitoyloleoylphosphatidylcholine or its mixture with cholesterol, by 70–80 mV. The reduction in the membrane dipole potential induced by inhibitors led to a 20–25% increase in the conductance of cation-selective pores formed by the antimicrobial peptide gramicidin A. The addition of sildenafil or vardenafil also led to a significant decrease in the temperature of the main phase transition of dipalmytoylphosphatidylcholine, by about 1.5 °C, while tadalafil did not change the melting temperature. Sildenafil, vardenafil, and tadalafil enhanced the pore-forming activity of the antifungal polyene antibiotic nystatin by 11, 13, and 2 times, respectively. This fact might indicate the induction of membrane curvature stress by the inhibitors. The data obtained might be of special interest for the development of lipid-mediated forms of drugs.

Plant Alkaloids Inhibit Membrane Fusion Mediated by Calcium and Fragments of MERS-CoV and SARS-CoV/SARS-CoV-2 Fusion Peptides

To rationalize the antiviral actions of plant alkaloids, the ability of 20 compounds to inhibit calcium-mediated fusion of lipid vesicles composed of phosphatidylglycerol and cholesterol was investigated using the calcein release assay and dynamic light scattering. Piperine, tabersonine, hordenine, lupinine, quinine, and 3-isobutyl-1-methylxanthine demonstrated the most potent effects (inhibition index greater than 50%). The introduction of phosphatidylcholine into the phosphatidylglycerol/cholesterol mixture led to significant changes in quinine, hordenine, and 3-isobutyl-1-methylxanthine efficiency. Comparison of the fusion inhibitory ability of the tested alkaloids, and the results of the measurements of alkaloid-induced alterations in the physical properties of model membranes indicated a potent relationship between a decrease in the cooperativity of the phase transition of lipids and the ability of alkaloids to prevent calcium-mediated vesicle fusion. In order to use this knowledge to combat the novel coronavirus pandemic, the ability of the most effective compounds to suppress membrane fusion induced by fragments of MERS-CoV and SARS-CoV/SARS-CoV-2 fusion peptides was studied using the calcein release assay and confocal fluorescence microscopy. Piperine was shown to inhibit vesicle fusion mediated by both coronavirus peptides. Moreover, piperine was shown to significantly reduce the titer of SARS-CoV2 progeny in vitro in Vero cells when used in non-toxic concentrations.

Is the Membrane Lipid Matrix a Key Target for Action of Pharmacologically Active Plant Saponins?

This study was focused on the molecular mechanisms of action of saponins and related compounds (sapogenins and alkaloids) on model lipid membranes. Steroids and triterpenes were tested. A systematic analysis of the effects of these chemicals on the physicochemical properties of the lipid bilayers and on the formation and functionality of the reconstituted ion channels induced by antimicrobial agents was performed. It was found that digitonin, tribulosin, and dioscin substantially reduced the boundary potential of the phosphatidylcholine membranes. We concluded that saponins might affect the membrane boundary potential by restructuring the membrane hydration layer. Moreover, an increase in the conductance and lifetime of gramicidin A channels in the presence of tribulosin was due to an alteration in the membrane dipole potential. Differential scanning microcalorimetry data indicated the key role of the sapogenin core structure (steroid or triterpenic) in affecting lipid melting and disordering. We showed that an alteration in pore forming activity of syringomycin E by dioscin might be due to amendments in the lipid packing. We also found that the ability of saponins to disengage the fluorescent marker calcein from lipid vesicles might be also determined by their ability to induce a positive curvature stress.