Theory of tunable pH-sensitive vesicles of anionic and cationic lipids or anionic and neutral lipids

Lipid domain formation and non-lamellar structures associated with varied lysylphosphatidylglycerol analogue content in a model Staphylococcal plasma membrane

Dipalmitoyl-3-aza-dehydroxy-lysylphosphatidylglycerol (DP3adLPG), is a chemically stable synthetic analogue of the bacterial lipid lysylphosphatidylglycerol (LPG), designed as a substitute for the notoriously labile native lipid in biophysical investigations. In Staphylococcus aureus, LPG is known to play a role in resistance to antibiotics by altering membrane charge properties in response to environmental stress, but little is known about how LPG influences other bilayer physicochemical properties or lateral organisation, through the formation of complexes with lipids such as phosphatidylglycerol (PG). In this study we have investigated the different phases formed by biomimetic mixtures of 3adLPG and PG in different thermotropic states, using neutron diffraction and electron microscopy. In a DPPG/DP3adLPG 70:30 mol% mixture, two distinct lamellar phases were observed below the lipid melting transition: L_β' 1 and L_β' 2 with respective periodicities of 82 and 62 Å. Increasing the proportion of DP3adLPG to mimic the effects of environmental stress led to the disappearance of the L_β' 1 phase and the formation of an inverse hexagonal phase. The compositions of these different phases were identified by investigating the thermotropic properties of the two mixtures, and probing their interaction with the antimicrobial peptide magainin 2 F5W. We propose that the observed polymorphism results from the preferential formation of either triplet PG-3adLPG-PG, or paired PG-3adLPG complexes, dependent upon the mixing proportions of the two lipids. The relevance of these findings to the role native LPG in S. aureus, are discussed with respect to their influence on antibiotic resistance and lateral membrane organisation.

Pathological transitions in myelin membranes driven by environmental and multiple sclerosis conditions

In demyelination diseases, such as multiple sclerosis, the structure of the axons’ protective sheaths is disrupted. Due to the proximity of cytoplasmic myelin membrane to structural phase transition, minor alterations in the local environmental conditions can have devastating results. Using small-angle X-ray scattering and cryogenic transmission electron microscopy, we show that drastic structural reorganization and instabilities of myelin membrane are linked to specific environmental conditions and molecular composition in healthy and diseased states. These instabilities involve phase transition from the healthy lamellar membranes to pathological inverted hexagonal phase. These results highlight that local environmental conditions are critical for myelin function and should be considered as alternative routes for early pathology and as a means to avoid the initiation of demyelination.

Multiple sclerosis (MS) is an autoimmune disease, leading to the destruction of the myelin sheaths, the protective layers surrounding the axons. The etiology of the disease is unknown, although there are several postulated environmental factors that may contribute to it. Recently, myelin damage was correlated to structural phase transition from a healthy stack of lamellas to a diseased inverted hexagonal phase as a result of the altered lipid stoichiometry and low myelin basic protein (MBP) content. In this work, we show that environmental conditions, such as buffer salinity and temperature, induce the same pathological phase transition as in the case of the lipid composition in the absence of MBP. These phase transitions have different transition points, which depend on the lipid’s compositions, and are ion specific. In extreme environmental conditions, we find an additional dense lamellar phase and that the native lipid composition results in similar pathology as the diseased composition. These findings demonstrate that several local environmental changes can trigger pathological structural changes. We postulate that these structural modifications result in myelin membrane vulnerability to the immune system attacks and thus can help explain MS etiology.

Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers

A set of four structurally related glycolipids are described: two of them have one glucose unit connected to either stearic or oleic acid, and two other ones have a diglucose headgroup (sophorose) similarly connected to either stearic or oleic acid. The self-assembly properties of these compounds, poorly known, are important to know due to their use in various fields of application from cleaning to cosmetics to medical. At basic pH, they all form mainly small micellar aggregates. At acidic pH, the oleic and stearic derivatives of the monoglucose form, respectively, vesicles and bilayer, while the same derivatives of the sophorose headgroup form micelles and twisted ribbons. We use pH-resolved in situ small angle X-ray scattering (SAXS) under synchrotron radiation to characterize the pH-dependent mechanism of evolution from micelles to the more complex aggregates at acidic pH. By pointing out the importance of the COO^-/COOH ratio, the melting temperature, T_m, of the lipid moieties, hydration of the glycosidic headgroup, the packing parameter, membrane rigidity, and edge stabilization, we are now able to draw a precise picture of the full self-assembly mechanism. This work is a didactical illustration of the complexity of the self-assembly process of a stimuli-responsive amphiphile during which many concomitant parameters play a key role at different stages of the process.

Functional liposomes in the cancer-targeted drug delivery

Cancer is considered as one of the most severe health problems and is currently the third most common cause of death in the world after heart and infectious diseases. Novel therapies are constantly being discovered, developed and trialed. Many of the current anticancer agents exhibit non-ideal pharmaceutical and pharmacological properties and are distributed non-specifically throughout the body. This results in death of the both normal healthy and malignant cells and substantially leads to accruing a variety of serious toxic side effects. Therefore, the efficient systemic therapy of cancer is almost impossible due to harmful side effects of anticancer agents to the healthy organs and tissues. Furthermore, several problems such as low bioavailability of the drugs, low drug concentrations at the site of action, lack of drug specificity and drug-resistance also cause many restrictions on clinical applications of these drugs in the tumor therapy. Different types of the liposomal formulations have been used in medicine due to their distinctive advantages associated with their structural flexibility in the encapsulation of various agents with different physicochemical properties. They can also mediate delivery of the cargo to the appropriate cell type and subcellular compartment, reducing the effective dosage and possible side effects which are related to high systemic concentrations. Therefore, these novel systems were found very promising and encouraging dosage forms for the treatment of different types of cancer by increasing efficiency and reducing the systemic toxicity due to the specific drug delivery and targeting.

Efficient intracellular delivery of siRNA with a safe multitargeted lipid-based nanoplatform

AIM

The design of novel F3-targeted liposomes with adequate features for systemic administration, to enable efficient intracellular delivery of siRNA toward both cancer and endothelial cells from angiogenic blood vessels.

MATERIALS & METHODS

Cellular association studies were performed by flow cytometry. Gene silencing was evaluated with eGFP-overexpressing cells, by flow cytometry and real-time reverse-transcription PCR. Safety and immunogenicity was assessed in CD1 mice.

RESULTS

A strong improvement on siRNA internalization by the target cells was achieved, which was correlated with effective downregulation of eGFP. In addition, the F3-targeted liposomes were nonimmunogenic, even in a multiadministration schedule.

CONCLUSION

Overall, the developed F3-targeted nanocarrier constitutes a valuable tool for the specific and safe systemic delivery of siRNA to solid tumors.

Optimization of pH sensing using silicon nanowire field effect transistors with HfO2 as the sensing surface

Silicon nanowire field effect transistor sensors with SiO(2)/HfO(2) as the gate dielectric sensing surface are fabricated using a top down approach. These sensors are optimized for pH sensing with two key characteristics. First, the pH sensitivity is shown to be independent of buffer concentration. Second, the observed pH sensitivity is enhanced and is equal to the Nernst maximum sensitivity limit of 59 mV/pH with a corresponding subthreshold drain current change of ∼ 650%/pH. These two enhanced pH sensing characteristics are attributed to the use of HfO(2) as the sensing surface and an optimized fabrication process compatible with silicon processing technology.

An ion switch regulates fusion of charged membranes

Here we identify the recruitment of solvent ions to lipid membranes as the dominant regulator of lipid phase behavior. Our data demonstrate that binding of counterions to charged lipids promotes the formation of lamellar membranes, whereas their absence can induce fusion. The mechanism applies to anionic and cationic liposomes, as well as the recently introduced amphoteric liposomes. In the latter, an additional pH-dependent lipid salt formation between anionic and cationic lipids must occur, as indicated by the depletion of membrane-bound ions in a zone around pH 5. Amphoteric liposomes fuse under these conditions but form lamellar structures at both lower and higher pH values. The integration of these observations into the classic lipid shape theory yielded a quantitative link between lipid and solvent composition and the physical state of the lipid assembly. The key parameter of the new model, κ(pH), describes the membrane phase behavior of charged membranes in response to their ion loading in a quantitative way.

Spontaneous formation of bilayers and vesicles in mixtures of single-chain alkyl carboxylates: effect of pH and aging and cytotoxicity studies

We report the observation of bilayer fragments, some of which close to form vesicles, over a large range of pH at room temperature from mixtures of single-chain biocompatible commercially available nontoxic alkyl carboxylic surfactants after neutralization with HCl. The pH at which the morphological transitions occur is varied only by changing the ratio between two surfactants: the alkyloligoethyleneoxide carboxylate and sodium laurate. The effect of aging of the mixed surfactant systems in the pH region desired for dermatologic application (4.5 < pH < 7) is also studied. Finally, we show results of cytotoxicity studies on the surfactant mixtures.

C-terminal charged cluster of MscL, RKKEE, functions as a pH sensor

The RKKEE cluster of charged residues located within the cytoplasmic helix of the bacterial mechanosensitive channel, MscL, is essential for the channel function. The structure of MscL determined by x-ray crystallography and electron paramagnetic resonance spectroscopy has revealed discrepancies toward the C-terminus suggesting that the structure of the C-terminal helical bundle differs depending on the pH of the cytoplasm. In this study we examined the effect of pH as well as charge reversal and residue substitution within the RKKEE cluster on the mechanosensitivity of Escherichia coli MscL reconstituted into liposomes using the patch-clamp technique. Protonation of either positively or negatively charged residues within the cluster, achieved by changing the experimental pH or residue substitution within the RKKEE cluster, significantly increased the free energy of activation for the MscL channel due to an increase in activation pressure. Our data suggest that the orientation of the C-terminal helices relative to the aqueous medium is pH dependent, indicating that the RKKEE cluster functions as a proton sensor by adjusting the channel sensitivity to membrane tension in a pH-dependent fashion. A possible implication of our results for the physiology of bacterial cells is briefly discussed.

Understanding foods as soft materials

Foods make up some of the most complex examples of soft condensed matter (SCM) with which we interact daily. Their complexity arises from several factors: the intricacy of components, the different aggregation states in which foods are encountered, and the multitude of relevant characteristic time and length scales. Because foodstuffs are governed by the rules of SCM physics but with all the complications related to real systems, the experimental and theoretical approaches of SCM physics have deepened our comprehension of their nature and behaviour, but many questions remain. In this review we discuss the current understanding of food science, by considering established SCM methods as well as emerging techniques and theoretical approaches. With their complexity, heterogeneity and multitude of states, foods provide SCM physics with a challenge of remarkable importance.

Influence of pH on the Micelle-to-Vesicle Transition in Aqueous Mixtures of Sodium Dodecyl Benzenesulfonate with Histidine

Small unilamellar vesicles (approximately 100 nm in diameter) form spontaneously in aqueous mixtures of histidine and sodium dodecyl benzenesulfonate. By manipulating pH, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than 6.0, the pKa of the imidazole moiety on the histidine molecule. This phenomenon is likely the result of attractive interactions between the negatively charged benzenesulfonate headgroups and the positively charged imidazole group in the amino acid. Similar results are obtained when imidazole salt itself is used.

Nanopore cheminformatics

A cheminformatics method is described for classification, and biophysical examination, of individual molecules. A novel molecular detector is used--one based on current blockade measurements through a nanometer-scale ion channel (alpha-hemolysin). Classification results are described for blockades caused by DNA molecules in the alpha-hemolysin nanopore detector, with signal analysis and pattern recognition performed using a combination of methods from bioinformatics and machine learning. Due to the size of the alpha-hemolysin protein channel, the blockade events report on one DNA molecule at a time, which enables a variety of reproducible, single-molecule biophysical experiments. To capture the full sensitivity of the nanopore detector's blockade signal, Hidden Markov Models (HMMs) were used with Expectation/Maximization for denoising and for associating a feature vector with the ionic current blockade of each captured DNA molecule. Support Vector Machines (SVMs) that employ novel kernel designs were then used as discriminators. With SVM training performed off-line, and economical HMM processing on-line, blockade classification was possible during capture. HMMs were also used in conjunction with a time-domain finite state automaton (off-line) for feature discovery and kinetics analysis. Analysis of the DNA data indicates a variety of binding (DNA-protein), fraying, and conformational shifts that are consistent with data obtained from thermodynamic analyses (melting curves), X-ray crystallography, and NMR studies. The software tools are designed for analysis of generic blockades in ionic channels, including those in other biological pore-forming toxins, other biological channels in general, and semiconductor-based channels.

Cationic triple-chain amphiphiles facilitate vesicle fusion compared to double-chain or single-chain analogues

Cationic, triple-chain amphiphiles promote vesicle fusion more than structurally related double-chain or single-chain analogues. Two types of vesicle fusion experiments were conducted, mixing of oppositely charged vesicles and acid-triggered self-fusion of vesicles composed of cationic amphiphile and anionic cholesteryl hemisuccinate (CHEMS). Vesicle fusion was monitored by standard fluorescence assays for intermembrane lipid mixing, aqueous contents mixing and leakage. Differential scanning calorimetry was used to show that triple-chain amphiphiles lower the lamellar-inverse hexagonal (L(alpha)-H(II)) phase transition temperature for dipalmitoleoylphosphatidylethanolamine. The triple-chain amphiphiles may enhance vesicle fusion because they can stabilize the inversely curved membrane surfaces of the fusion intermediates, however, other factors such as extended conformation, packing defects, chain motion, or surface dehydration may also contribute. From the perspective of drug delivery, the results suggest that vesicles containing cationic, triple-chain amphiphiles (and cationic, cone-shaped amphiphiles in general) may be effective as fusogenic delivery capsules.