Fast and slow biomembrane solubilizing detergents: Insights into their mechanism of action

In Situ Monitoring of Morphology Changes and Oxygenation State of Human Erythrocytes During Surfactant-Induced Hemolysis

Erythrocytes, the most abundant blood cells, are a prevalent cell model for the analysis of the membrane-damaging effects of different molecules, including drugs. In response to stimuli, erythrocytes can change their morphology, e.g., shape or volume, which in turns influences their main function to transport oxygen. Membrane active molecules can induce hemolysis, i.e., release of hemoglobin into the blood plasma. Free hemoglobin in the blood circulation is toxic causing serious health problems including vasoconstriction, high blood pressure and kidney damage. Therefore, early recognition of the risk of massive hemolysis is highly important. Here, we investigated surfactant induced hemolysis applying UV-vis spectrophotometry. Saponin, sodium dodecyl sulfate and Triton X-100, detergents known to provoke hemolysis at different concentrations and by different mechanisms, were applied to initiate the process. Whole absorption spectra of erythrocyte suspensions in the range 300-750 nm were recorded every 15 s for following the process in real-time. The hemolysis process, with respect to morphological changes in the erythrocytes and their influence on the oxygenation state of hemoglobin, was characterized by the absorbance at 700 nm, the height relative to the background and the wavelength of the Soret peak. The results suggest that these UV-vis spectrophotometry parameters provide reliable information in real-time; not only about the process of hemolysis itself, but also about pre-hemolytic changes in the erythrocytes, even at sub-hemolytic surfactant concentrations.

Interaction of biomimetic lipid membranes with detergents with different physicochemical characteristics

Membrane solubilization by detergents is routinely performed to separate membrane components, and to extract and purify membrane proteins. This process depends both on the characteristics of the detergent and properties of the membrane. Here we investigate the interaction of eight detergents with very distinct physicochemical features with model membranes in different biologically relevant phases. The detergents chosen were the non-ionic Triton X-100, Triton X-165, C10E5, octyl glucopyranoside (OG) and dodecyl maltoside (DDM) and the ionic sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium bromide (CTAB) and Chaps. Three lipid compositions were explored: pure palmitoyl oleoyl phosphatidylcholine (POPC), in the liquid-disordered (Ld) phase, sphingomyelin (SM)/cholesterol 7:3 (chol) in the liquid-ordered (Lo) phase and the biomimetic POPC/SM/chol 2:1:2, which might exhibit Lo/Ld phase separation. Turbidity measurements of small liposomes were performed along the titration with the detergents to obtain the overall solubilization profiles and optical microscopy of giant unilamellar vesicles (GUVs) was used to reveal the mechanism of interaction of the detergents. The presence of cholesterol renders the membranes partly/fully insoluble in all detergents, and the charged detergents are the least effective to solubilize POPC. The non-ionic detergents, with exception of DDM, with the bulkiest headgroup, caused a substantial increase in surface area of POPC, which was quantified directly on single GUVs. The other detergents induced mainly vesicle burst. Detergents that caused some increase in area induced Lo/Ld phase separation in the ternary mixture, with preferential solubilization of the latter. The insoluble area fraction left intact was quantified. Overall, the non-ionic detergents were the most effective in solubilizing lipid membranes.

A Roadmap of Responses to Asymmetry Stress in Lipid Membranes

The selective insertion of membrane-impermeant amphiphiles such as detergents, (lipo)peptides, drugs, etc. into the cis leaflet of a membrane causes an imbalance between the intrinsic areas of the cis and trans leaflet, referred to as asymmetry stress or differential stress. The literature provides individual mechanisms of how membranes respond to such stress, which are relevant to membrane remodeling processes and leakage phenomena. By studying vesicle budding, membrane leakage, and isothermal titration calorimetry of liposomes interacting with digitonin, alkyl maltosides, miltefosine, and octyl glucoside, we developed a roadmap linking the stress-response mechanisms to each other. Initially, lateral compression or stretching of the leaflets accommodates a minor asymmetry stress. Then, either molecules flip to the trans leaflet or the membrane bends to form buds. Fast flip leads to the classic three-stage model. Budding proceeds up to its limit at 20-40% of the lipid. Beyond, insertion of further detergent is opposed by the pressure in the overpopulated leaflet. This "staying out" state can persist over hours or days and up to high detergent concentrations before detergent micelles induce "micellar solubilization". Alternatively, the stress can be reduced by a transient failure of the membrane, allowing for "cracking in" of molecules, transferring them to the trans side.

Methyl-β-cyclodextrin asymmetrically extracts phospholipid from bilayers, granting tunable control over differential stress in lipid vesicles

Lipid asymmetry - that is, a nonuniform lipid distribution between the leaflets of a bilayer - is a ubiquitous feature of biomembranes and is implicated in several cellular phenomena. Differential tension - that is, unequal lateral monolayer tensions comparing the leaflets of a bilayer- is closely associated with lipid asymmetry underlying these varied roles. Because differential tension is not directly measurable in combination with the fact that common methods to adjust this quantity grant only semi-quantitative control over it, a detailed understanding of lipid asymmetry and differential tension are impeded. To overcome these challenges, we leveraged reversible complexation of phospholipid by methyl-β-cyclodextrin (mbCD) to tune the direction and magnitude of lipid asymmetry in synthetic vesicles. Lipid asymmetry generated in our study induced (i) vesicle shape changes and (ii) gel-liquid phase coexistence in 1-component vesicles. By applying mass-action considerations to interpret our findings, we discuss how this approach provides access to phospholipid thermodynamic potentials in bilayers containing lipid asymmetry (which are coupled to the differential tension of a bilayer). Because lipid asymmetry yielded by our approach is (i) tunable and (ii) maintained over minute to hour timescales, we anticipate that this approach will be a valuable addition to the experimental toolbox for systematic investigation into the biophysical role(s) of lipid asymmetry (and differential tension).

Solubilization of biomimetic lipid mixtures by some commonly used non-ionic detergents

Detergents are amphiphilic molecules often used to solubilize biological membranes and separate their components. Here we investigate the solubilization of lipid vesicles by the commonly used non-ionic detergents polyoxyethylene (20) oleyl ether (Brij 98), n-octyl-β-D-glucoside (OG), and n-dodecyl β-D maltoside (DDM) and compare the results with the standard detergent Triton X-100 (TX-100). The vesicles were composed of palmitoyl oleoyl phosphatidylcholine (POPC) or of a biomimetic ternary mixture of POPC, egg sphingomyelin (SM) and cholesterol (2:1:2 molar ratio). To follow the solubilization profile of large unilamellar vesicles (LUVs), 90° light scattering measurements were done along the titration of LUVs with the detergents. Then, giant unilamellar vesicles (GUVs) were observed with optical microscopy during exposure to the detergents, to allow direct visualization of the solubilization process. Isothermal titration calorimetry (ITC) was used to assess the binding constant of the detergents in POPC bilayers. The results show that the incorporation of TX-100, Brij 98 and, to a lesser extent, OG in the pure POPC liposomes leads to an increase in the vesicle area, which indicates their ability to redistribute between the two leaflets of the membrane in a short scale of time. On the other hand, DDM incorporates mainly in the external leaflet causing an increase in vesicle curvature/tension leading ultimately to vesicle burst. Only TX-100 and OG were able to completely solubilize the POPC vesicles, whereas the biomimetic ternary mixture was partially insoluble in all detergents tested. TX-100 and OG were able to incorporate in the bilayer of the ternary mixture and induce macroscopic phase separation of liquid-ordered (Lo) and liquid-disordered (Ld) domains, with selective solubilization of the latter. Combination of ITC data with turbidity results showed that TX-100 and OG can be incorporated up to almost 0.3 detergent/lipid, significantly more than Brij 98 and DDM. This fact seems to be directly related to their higher capacity to solubilize POPC membranes and their ability to induce macroscopic phase separation in the biomimetic lipid mixture.

Critical Role of Molecular Packing in Lo Phase Membrane Solubilization

Membrane solubilization induced by Triton X-100 (TX-100) was investigated. Different membrane compositions and phase states were studied along the detergent titration. Expected solubilization profiles were obtained but new information is provided. The fluorescence of nitrobenzoxadiazole (NBD)-labeled lipids indicates that the liquid-ordered (Lo)/liquid-disordered (Ld) phase coexistence is barely unaffected at sub-solubilizing detergent concentrations and highlights the vesicle-to-micelle transition. Moreover, the location of the NBD group in the bilayer emphasizes a detergent-membrane interaction in the case of the insoluble Lo phase membrane. It has also been shown that the molecular packing of the membrane loosens in the presence of TX-100, regardless of the solubilization profile. Motivated by studies on GPMVs, the solubilization of less ordered Lo phase membranes was considered in order to improve the effect of molecular packing on the extent of solubilization. Membranes composed of SM and Chol in an equimolar ratio doped with different amounts of PC were studied. The more ordered the Lo phase membrane is in the absence of detergent, the less likely it is to be solubilized. Furthermore, and in contrast to what is observed for membranes exhibiting an Lo/Ld phase coexistence, a very small decrease in the molecular packing of the Lo phase membrane radically modifies the extent of solubilization. These results have implications for the reliability of TX-100 insolubility as a method to detect ordered domains.

Pathways of Membrane Solubilization: A Structural Study of Model Lipid Vesicles Exposed to Classical Detergents

Understanding the pathways of solubilization of lipid membranes is of high importance for their use in biotechnology and industrial applications. Although lipid vesicle solubilization by classical detergents has been widely investigated, there are few systematic structural and kinetic studies where different detergents are compared under varying conditions. This study used small-angle X-ray scattering to determine the structures of lipid/detergent aggregates at different ratios and temperatures and studied the solubilization in time using the stopped-flow technique. Membranes composed of either of two zwitterionic lipids, DMPC or DPPC, and their interactions with three different detergents, sodium dodecyl sulfate (SDS), n-dodecyl-beta-maltoside (DDM), and Triton X-100 (TX-100), were tested. The detergent TX-100 can cause the formation of collapsed vesicles with a rippled bilayer structure that is highly resistant to TX-100 insertion at low temperatures, while at higher temperatures, it partitions and leads to the restructuring of vesicles. DDM also causes this restructuring into multilamellar structures at subsolubilizing concentrations. In contrast, partitioning of SDS does not alter the vesicle structure below the saturation limit. Solubilization is more efficient in the gel phase for TX-100 but only if the cohesive energy of the bilayer does not prevent sufficient partitioning of the detergent. DDM and SDS show less temperature dependence compared to TX-100. Kinetic measurements reveal that solubilization of DPPC largely occurs through a slow extraction of lipids, whereas DMPC solubilization is dominated by fast and burst-like solubilization of the vesicles. The final structures obtained seem to preferentially be discoidal micelles where the detergent can distribute in excess along the rim of the disc, although we do observe the formation of worm- and rodlike micelles in the case of solubilization of DDM. Our results are in line with the suggested theory that bilayer rigidity is the main factor influencing which aggregate is formed.

Beyond the standard model of solubilization: Non-ionic surfactants induce collapse of lipid vesicles into rippled bilamellar nanodiscs

HYPOTHESIS

Although solubilization of lipid membranes has been studied extensively, questions remain regarding the structural pathways and metastable structures involved. This study investigated whether the non-ionic detergent Triton X-100 follows the classical solubilization pathway or if intermediate nanostructures are formed.

EXPERIMENTS

Small angle X-ray and neutron scattering (SAXS/SANS) was used in combination with transmission electron cryo-microscopy and cryo-tomography to deduce the structure of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles and Triton X-100. Time-resolved SAXS and dynamic light scattering were used to investigate the kinetics of the process.

FINDINGS

Upon addition of moderate detergent amounts at low temperatures, the lipid vesicles implode into ordered rippled bilamellar disc structures. The bilayers arrange in a ripple phase to accommodate packing constraints caused by inserted TX-100 molecules. The collapse is suggested to occur through a combination of water structure destabilization by detergents flipping across the membrane and osmotic pressure causing interbilayer attraction internally. The subsequently induced ripples then stabilize the aggregates and prevent solubilization, supported by the observation that negatively charged vesicles undergo a different pathway upon TX-100 addition, forming large bicelles. The findings demonstrate the richness in assembly pathways of simple lipids and detergents and stimulate considerations for the use of certain detergents in membrane solubilization.

Studying the Mechanics of Membrane Permeabilization through Mechanoelectricity

In this research, real-time monitoring of lipid membrane disruption is made possible by exploiting the dynamic properties of model lipid bilayers formed at oil-water interfaces. This involves tracking an electrical signal generated through rhythmic membrane perturbation translated into the adsorption and penetration of charged species within the membrane. Importantly, this allows for the detection of membrane surface interactions that occur prior to pore formation that may be otherwise undetected. The requisite dynamic membranes for this approach are made possible through the droplet interface bilayer (DIB) technique. Membranes are formed at the interface of lipid monolayer-coated aqueous droplets submerged in oil. We present how cyclically alternating the membrane area leads to the generation of mechanoelectric current. This current is negligible without a transmembrane voltage until a composition mismatch between the membrane monolayers is produced, such as a one-sided accumulation of disruptive agents. The generated mechanoelectric current is then eliminated when an applied electric field compensates for this asymmetry, enabling measurement of the transmembrane potential offset. Tracking the compensating voltage with respect to time then reveals the gradual accumulation of disruptive agents prior to membrane permeabilization. The innovation of this work is emphasized in its ability to continuously track membrane surface activity, highlighting the initial interaction steps of membrane disruption. In this paper, we begin by validating our proposed approach against measurements taken for fixed composition membranes using standard electrophysiological techniques. Next, we investigate surfactant adsorption, including hexadecyltrimethylammonium bromide (CTAB, cationic) and sodium decyl sulfate (SDS, anionic), demonstrating the ability to track adsorption prior to disruption. Finally, we investigate the penetration of lipid membranes by melittin, confirming that the peptide insertion and disruption mechanics are, in part, modulated by membrane composition.

Effectiveness of dual-detergent strategy using Triton X-100 in membrane protein purification

Membrane solubilization by detergents is a critical step for successful membrane protein purification. Alkyl maltoside detergents such as DDM and DM are very expensive and are commonly used to produce most of the high-quality proteins in stable and functional form. Recently, dual-detergent strategy using inexpensive detergents for membrane solubilization step has been shown to be highly effective in purifying different classes of membrane proteins in a cost-effective manner. In this work, we have monitored the effectiveness of 'dual-detergent strategy' towards successful purification of the isolated voltage sensing domain (VSD) of KvAP and the inward rectifying K⁺ channel, KirBac1.1. We demonstrate that the inexpensive detergent Triton X-100 extracts the activated conformation of the KvAP-VSD well without compromising the structural integrity of the sensor, and also retains its proper structural dynamics. Importantly, the cost associated with solubilizing the KvAP sensor can be reduced by ∼2000 fold. To the best of our knowledge, our results constitute the first report characterizing the purification of KvAP voltage sensor using an inexpensive detergent. However, the dual-detergent strategy using Triton X-100 for membrane solubilization is not effective for the purification of inward rectifying K⁺ channel, KirBac1.1 even in presence of high salt concentration during solubilization. We propose that the dual-detergent strategy will be useful for extracting stable and functional proteins that are both DDM- and DM-extractable, but will be ineffective if the protein is only DM-extractable. The relevance of the effectiveness of dual-detergent strategy with respect to the hydrophobic thickness of proteins is discussed.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

Soap, water, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): an ancient handwashing strategy for preventing dissemination of a novel virus

Public Health Agencies worldwide (World Health Organization, United States Centers for Disease Prevention & Control, Chinese Center for Disease Control and Prevention, European Centre for Disease Prevention and Control, etc.) are recommending hand washing with soap and water for preventing the dissemination of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. In this review, we have discussed the mechanisms of decontamination by soap and water (involving both removal and inactivation), described the contribution of the various components of formulated soaps to performance as cleansers and to pathogen inactivation, explained why adherence to recommended contact times is critical, evaluated the possible contribution of water temperature to inactivation, discussed the advantages of antimicrobial soaps vs. basic soaps, discussed the differences between use of soap and water vs. alcohol-based hand sanitizers for hand decontamination, and evaluated the limitations and advantages of different methods of drying hands following washing. While the paper emphasizes data applicable to SARS-CoV-2, the topics discussed are germane to most emerging and re-emerging enveloped and non-enveloped viruses and many other pathogen types.

Cholesterol-phospholipid interactions resist the detergent effect of bovine bile

Sphingomyelin (SM) and cholesterol complexation gives rise to detergent-resistant liquid-ordered domains. The persistence of these domains and subsequent mixed micelle formation was examined in the presence of bile under physiological digestive in vitro conditions for vesicles comprising either SM/cholesterol, porcine brain phosphatidylcholine (BPC)/cholesterol, or soy phosphatidylcholine (SPC)/cholesterol bilayers, the latter two systems having no liquid-ordered domains. Micellization of these digested phospholipid multilamellar vesicle systems was confirmed by transmission electron microscopy. Bovine bile was found to consist of large multilamellar sheets which subsumed phospholipid vesicles to form aggregated superstructures. Budding off from these superstructures were vesicle-to-micelle transition intermediates: unilamellar vesicles and cylindrical micelles. The presence of cholesterol (60/40 phospholipid/cholesterol mol/mol) delayed the initial rapid onset of digestion, but not for BPC and SPC vesicle systems. Acyl chain order/disorder before and after vesicle-to-micelle transition of all three phospholipid/cholesterol systems was examined using Raman spectroscopy. The addition of bovine bile to both PC/cholesterol vesicle systems reduced the overall ratio of acyl chain disorder to order. In SM/cholesterol vesicles with ≤ 20% mol cholesterol, only the lateral inter-acyl chain packing was reduced, whereas for SM/cholesterol vesicles with ≥ 30% mol cholesterol, a higher proportion of gauche-to-trans isomerization was apparent, demonstrating that SM/cholesterol complexes modify the acyl chain structure of micelles.

Interactions of the Lysosomotropic Detergent O-Methyl-Serine Dodecylamide Hydrochloride (MSDH) with Lipid Bilayer Membranes-Implications for Cell Toxicity

O-methyl-serine dodecylamine hydrochloride (MSDH) is a detergent that accumulates selectively in lysosomes, a so-called lysosomotropic detergent, with unexpected chemical properties. At physiological pH, it spontaneously forms vesicles, which disassemble into small aggregates (probably micelles) below pH 6.4. In this study, we characterize the interaction between MSDH and liposomes at different pH and correlate the findings to toxicity in human fibroblasts. We find that the effect of MSDH on lipid membranes is highly pH-dependent. At neutral pH, the partitioning of MSDH into the liposome membrane is immediate and causes the leakage of small fluorophores, unless the ratio between MSDH and lipids is kept low. At pH 5, the partitioning of MSDH into the membrane is kinetically impeded since MSDH is charged and a high ratio between MSDH and the lipids is required to permeabilize the membrane. When transferred to cell culture conditions, the ratio between MSDH and plasma membrane lipids must therefore be low, at physiological pH, to maintain plasma membrane integrity. Transmission electron microscopy suggests that MSDH vesicles are taken up by endocytosis. As the pH of the endosomal compartment progressively drops, MSDH vesicles disassemble, leading to a high concentration of increasingly charged MSDH in small aggregates inside the lysosomes. At sufficiently high MSDH concentrations, the lysosome is permeabilized, the proteolytic content released to the cytosol and apoptotic cell death is induced.