Interfacial interaction between dextran sulfate and lipid monolayers: an electrochemical study

Mechanistic Insights and Therapeutic Delivery through Micro/Nanobubble-Assisted Ultrasound

Ultrasound with low frequency (20–100 kHz) assisted drug delivery has been widely investigated as a non-invasive method to enhance the permeability and retention effect of drugs. The functional micro/nanobubble loaded with drugs could provide an unprecedented opportunity for targeted delivery. Then, ultrasound with higher intensity would locally burst bubbles and release agents, thus avoiding side effects associated with systemic administration. Furthermore, ultrasound-mediated destruction of micro/nanobubbles can effectively increase the permeability of vascular membranes and cell membranes, thereby not only increasing the distribution concentration of drugs in the interstitial space of target tissues but also promoting the penetration of drugs through cell membranes into the cytoplasm. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theragnostic tool. In this review, we first discuss the structure and generation of micro/nanobubbles. Second, ultrasound parameters and mechanisms of therapeutic delivery are discussed. Third, potential biomedical applications of micro/nanobubble-assisted ultrasound are summarized. Finally, we discuss the challenges and future directions of ultrasound combined with micro/nanobubbles.

Investigation on the Interactions between Self-Assembled β-Sheet Peptide Nanofibers and Model Cell Membranes

Self-assembled peptide nanofibers (NFs) obtained from β-sheet peptides conjugated with drugs, including antigenic peptides, have recently attracted significant attention. However, extensive studies on the interactions of β-sheet peptide NFs with model cell membranes have not been reported. In this study, we investigated the interactions between three types of NFs, composed of PEG-peptide conjugates with different ethylene glycol (EG) lengths (6-, 12- and 24-mer), and dipalmitoylphosphatidylcholine (DPPC) Langmuir membranes. When increasing the EG chain length, those interactions significantly decreased considering measurements in the presence of the NFs of: (i) changes in surface pressure of the DPPC Langmuir monolayers and (ii) surface pressure-area (π-A) compression isotherms of DPPC. Because the observed trend was similar to the EG length dependency with regard to cellular association and cytotoxicity of the NFs that was reported previously, the interaction of NFs with phospholipid membranes represented a crucial factor to determine the cellular association and toxicity of the NFs. In contrast to NFs, no changes were observed with varying EG chain length on the interaction of the building block peptide with the DPPC membrane. The results obtained herein can provide a design guideline on the formulation of β-sheet peptide NFs, which may broaden its potential.

The Effect of Dextran Sulfate-as Model Glycosaminoglycan Analogue-on Membrane Lipids: DPPC, Cholesterol, and DPPC-Cholesterol Mixture. The Monolayer Study

Glycosaminoglycans (GAGs) are essential components of the extracellular matrices (ECMs) located on the outer surface of cellular membranes. They belong to the group of polysaccharides involved in diverse biological processes acting on the surface and across natural lipid membranes. Recently, particular attention has been focused on possible role of GAGs in the amyloid deposits. The amyloid formation is related to a disorder in protein folding, causing that soluble-in normal conditions-peptides become deposited extracellularly as insoluble fibrils, impairing tissue structure and its function. One of the hypothesis holds that GAGs may inhibit amyloid formation by interacting with the lipid membrane by blocking the accumulation of protein aggregates on the membrane surface. Although the biophysical properties of GAGs are described rather well, little is known about the nature of association between these polysaccharides and components of natural cell membranes. Therefore, a study of GAGs influence on membrane lipids is of particular importance. The aim of the present work is to get insight into the effect of hydrophilic dextran sulfate (DS)-that can be considered as GAG analogue-on membrane lipids organization. This study was based on examining interactions between DS sodium salt of molecular weight equal to about 40 kDa (DS40), dissolved in water subphase, and a model membrane, mimicked as Langmuir monolayer, formed by representative natural membrane lipids: cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as well as their mixtures. Due to the fact that calcium ions in excess may accumulate in the lipid membrane, attracting high molecular weight molecules to their surface, the influence of calcium ions present in the subphase on the DS40 activity has also been examined. It has been found that negatively charged DS, forming a sublayer underneath the monolayer, barely interacts with membrane lipids; however, in the presence of calcium ions the electrostatic interactions between DS40 and lipid membrane are significantly enhanced, leading to the formation of network-like crystalline structures at the surface of model membrane, which can prevent incorporation and interaction with other extracellular molecules, e.g., proteins.

Mechanical Stability of Lipid Membranes Decorated with Dextran Sulfate

Lipid vesicles decorated with polysaccharides have been proposed as vehicles for drug delivery because the polymers confer to the vesicles an enhanced stability, increasing the probability of the drug for reaching the target cell. Here, we first test the affinity of dextran sulfate (DS) for two different vesicle composition, and afterward, we study the effect of DS on the liposome mechanical properties. We found that DS binds to both tested membrane compositions. The interaction of DS with the anionic membranes studied here is mediated by the metal ions present in the aqueous solution (Na⁺ and Ca²⁺), being higher in the presence of Ca²⁺. Binding occurs preferentially in regions of closely packed lipids. Strikingly, DS did not affect the stability against detergent and the membrane rigidity of none of the vesicles. Thus, the proposed stability increase induced by this kind of polymers in drug delivery systems is not related with a modulation of the membrane thermodynamic properties but to other biochemical factors.

From Micro- to Nano-Multifunctional Theranostic Platform: Effective Ultrasound Imaging Is Not Just a Matter of Scale

Ultrasound Contrast Agents (UCAs) consisting of gas-filled-coated Microbubbles (MBs) with diameters between 1 and 10 µm have been used for a number of decades in diagnostic imaging. In recent years, submicron contrast agents have proven to be a viable alternative to MBs for ultrasound (US)-based applications for their capability to extravasate and accumulate in the tumor tissue via the enhanced permeability and retention effect. After a short overview of the more recent approaches to ultrasound-mediated imaging and therapeutics at the nanoscale, phase-change contrast agents (PCCAs), which can be phase-transitioned into highly echogenic MBs by means of US, are here presented. The phenomenon of acoustic droplet vaporization (ADV) to produce bubbles is widely investigated for both imaging and therapeutic applications to develop promising theranostic platforms.

Electrochemical behaviour at a liquid-organogel microinterface array of fucoidan extracted from algae

The electrochemical behaviour of fucoidan, a sulfated polysaccharide, was investigated, leading to a detection strategy by adsorptive stripping voltammetry.

Chondroitin sulfate interacts mainly with headgroups in phospholipid monolayers

Sulfated glycosaminoglycans are precursors of the extracellular matrix used to treat diseases related to blood clotting and degenerative joint diseases. These medical applications have been well established, but the mode of action at the molecular level, which depends on the interaction with cell membranes, is not known in detail. In this study, we investigated the interaction between chondroitin sulfate (CS) and phospholipid monolayers that mimic cell membranes. From surface pressure isotherms and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), CS was found to interact mainly with the polar groups of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylglycerol (DPPG), with negligible penetration into the hydrophobic tails and only small changes in monolayer elasticity for the packing corresponding to a real cell membrane. The changes in surface pressure and surface potential isotherms depended on CS concentration and on the time allowed for its adsorption onto the monolayer, which points to a dynamic adsorption-desorption process. The charge of the phospholipid was also relevant, since CS induced order into DPPC monolayers while the opposite occurred for DPPG, according to the PM-IRRAS spectra. In summary, interaction with polar groups is responsible for the CS effects on model cell membranes.

Nanobubbles: a promising efficienft tool for therapeutic delivery

In recent decades ultrasound-guided delivery of drugs loaded on nanocarriers has been the focus of increasing attention to improve therapeutic treatments. Ultrasound has often been used in combination with microbubbles, micron-sized spherical gas-filled structures stabilized by a shell, to amplify the biophysical effects of the ultrasonic field. Nanometer size bubbles are defined nanobubbles. They were designed to obtain more efficient drug delivery systems. Indeed, their small sizes allow extravasation from blood vessels into surrounding tissues and ultrasound-targeted site-specific release with minimal invasiveness. Additionally, nanobubbles might be endowed with improved stability and longer residence time in systemic circulation. This review will describe the physico-chemical properties of nanobubbles, the formulation parameters and the drug loading approaches, besides potential applications as a therapeutic tool.

Probing the interaction between heparan sulfate proteoglycan with biologically relevant molecules in mimetic models for cell membranes: a Langmuir film study

Investigating the role of proteoglycans associated to cell membranes is fundamental to comprehend biochemical process that occurs at the level of membrane surfaces. In this paper, we exploit syndecan-4, a heparan sulfate proteoglycan obtained from cell cultures, in lipid Langmuir monolayers at the air-water interface. The monolayer served as a model for half a membrane, and the molecular interactions involved could be evaluated with tensiometry and vibrational spectroscopy techniques. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) employed in a constant surface pressure regime showed that the main chemical groups for syndecan-4 were present at the air-water interface. Subsequent monolayer decompression and compression showed surface pressure-area isotherms with a large expansion for the lipid monolayers interacting with the cell culture reported to over-express syndecan-4, which was also an indication that the proteoglycan was inserted in the lipid monolayer. The introduction of biological molecules with affinity for syndecam-4, such as growth factors, which present a key role in biochemical process of cell signaling, changed the surface properties of the hybrid film, leading to a model, by which the growth factor binds to the sulfate groups present in the heparan sulfate chains. The polypeptide moiety of syndecan-4 responds to this interaction changing its conformation, which leads to lipid film relaxation and further monolayer condensation.

Reduced diffusion of charge-modified, conformationally intact anionic Ficoll relative to neutral Ficoll across the rat glomerular filtration barrier in vivo

The glomerular filtration barrier (GFB) is commonly conceived as a negatively charged sieve to proteins. Recent studies, however, indicate that glomerular charge effects are small for anionic, carboxymethylated (CM) dextran vs. neutral dextran. Furthermore, two studies assessing the glomerular sieving coefficients (θ) for negative CM-Ficoll vs. native Ficoll have demonstrated an increased glomerular permeability for CM-Ficoll (Asgeirsson D, Venturoli D, Rippe B, Rippe C. Am J Physiol Renal Physiol 291: F1083-F1089, 2006; Guimarães M, Nikolovski J, Pratt L, Greive K, Comper W. Am Physiol Renal Physiol 285: F1118-F1124, 2003.). The CM-Ficoll used, however, showed a larger Stokes-Einstein radius (a(e)) than neutral Ficoll, and it was proposed that the introduction of negative charges in the Ficoll molecule had made it more flexible and permeable. Recently, a negative FITC-labeled CM-Ficoll (CMI-Ficoll) was produced with a conformation identical to that of neutral FITC-Ficoll. Using these probes, we determined their θ:s in anesthetized Wistar rats (259 ± 2.5 g). After blood access had been achieved, the left ureter was cannulated for urine sampling. Either polysaccharide was infused (iv) together with a filtration marker, and urine and plasma were collected. Assessment of θ FITC-Ficoll was achieved by high-performance size-exclusion chromatography (HPSEC). CMI-Ficoll and native Ficoll had identical elugrams on the HPSEC. Diffusion of anionic Ficoll was significantly reduced compared with that of neutral Ficoll across the GFB for molecules of a(e) ∼20-35 Å, while there were no charge effects for Ficoll of a(e) = 35-80 Å. The data are consistent with a charge effect present in "small pores," but not in "large pores," of the GFB and mimicked those obtained for anionic membranes in vitro for the same probes.

Electrochemical properties of phospholipid monolayers at liquid-liquid interfaces

Biomembrane models built at the interface between two immiscible electrolytes (ITIES) are useful systems to study phenomena of biological relevance by means of their electrochemical processes. The unique properties of ITIES allow one either to control or measure the potential difference across the biomimetic membranes. Herein we focus on phospholipid monolayers adsorbed at liquid-liquid interfaces, and besides discussing recent developments on the subject, we describe electrochemical techniques that can be used to get insight on the interfacial processes and electrostatic properties of phospholipid membranes at the ITIES. In particular, we examine the electrochemical and physicochemical properties of (modified) phospholipid monolayers and their interaction with other biologically relevant compounds. The use of liquid-liquid electrochemistry as a powerful tool to characterize drug properties is outlined. Although this review is not a survey of all the work in the field, it provides a comprehensive referencing to current research.

Effect of molecular structure of cationic surfactants on biophysical interactions of surfactant-modified nanoparticles with a model membrane and cellular uptake

The aim of this study was to test the hypothesis that the molecular structure of cationic surfactants at the nanoparticle (NP) interface influences the biophysical interactions of NPs with a model membrane and cellular uptake of NPs. Polystyrene NPs (surfactant-free, 130 nm) were modified with cationic surfactants. These surfactants were of either dichained (didodecyldimethylammonium bromide [DMAB]) or single-chained (cetyltrimethylammonium bromide [CTAB] and dodecyltrimethylammonium bromide [DTAB]) forms, with the latter two having different hydrophobic chain lengths. Biophysical interactions of these surfactant-modified NPs with an endothelial cell model membrane (EMM) were studied using a Langmuir film balance. Changes in surface pressure (SP) of EMM as a function of time following interaction with NPs and in the compression isotherm (pi-A) of the lipid mixture of EMM in the presence of NPs were analyzed. Langmuir-Schaeffer (LS) films, which are EMMs that have been transferred onto a suitable substrate, were imaged by atomic force microscopy (AFM), and the images were analyzed to determine the mechanisms of the NP-EMM interaction. DMAB-modified NPs showed a greater increase in SP and a shift toward higher mean molecular area (mmA) than CTAB- and DTAB-modified NPs, indicating stronger interactions of DMAB-modified NPs with the EMM. However, analysis of the AFM phase and height images of the LS films revealed that both DMAB- and CTAB-modified NPs interacted with the EMM but via different mechanisms: DMAB-modified NPs penetrated the EMM, thus explaining the increase in SP, whereas CTAB-modified NPs anchored onto the EMM's condensed lipid domains and hence did not cause any significant change in SP. Human umbilical vein endothelial cells showed greater uptake of DMAB- and CTAB-modified NPs than of DTAB-modified or unmodified NPs. We conclude that (i) the dichained and single-chained cationic surfactants on NPs have different mechanisms of interaction with the model membrane and that (ii) NPs that demonstrate greater biophysical interactions with the membrane also show greater cellular uptake. Biophysical interactions of NPs with a model membrane thus could be effectively used for developing nanocarriers with optimized surface properties for drug delivery and imaging applications.

Thermodynamic analysis of binding between drugs and glycosaminoglycans by isothermal titration calorimetry and fluorescence spectroscopy

The thermodynamics of the interaction of positively charged drug molecules with negatively charged glycosaminoglycans (GAGs) is investigated by isothermal titration calorimetry (ITC) and fluorescence spectroscopy. The drugs considered are propranolol hydrochloride, tacrine, and aminacrine, and the polymers used as model GAGs are dextran sulfate, chondroitin sulfate, and hyaluronic acid. The ITC results show that the interaction between drugs and GAGs is via direct binding and that GAGs bind to drugs at one set of sites. Large negative values of heat capacity change (DeltaC(p)) are observed upon binding of GAGs to drugs. Such negative DeltaC(p) is not expected for purely electrostatic interactions and suggests that hydrophobic and other interactions may be also involved in the binding process. These results are corroborated by fluorescence spectroscopy measurements, which show that specific drug/GAG complex formation is accompanied by a clear enhancement of the fluorescence intensity. The results highlight the importance of the formation of drug/GAG complexes as a primary step for the drug delivery process into cell membranes. It is concluded that the interactions are dependent on the nature of both GAG and drug and this is a fact to be taken into account when new drugs are designed.

Adsorption–Penetration Studies of Glucose Oxidase into Phospholipid Monolayers at the 1,2-Dichloroethane/Water Interface

The interaction between glucose oxidase (GOx) and phospholipid monolayers is studied at the 1,2-dichloroethane/water interface by electrochemical impedance spectroscopy. Electrochemical experiments show that the presence of GOx induces changes in the capacitance curves at both negative and positive potentials, which are successfully explained by a theoretical model based on the solution of the Poisson-Boltzmann equation. These changes are ascribed to a reduced partition coefficient of GOx and an increase of the permittivity of the lipid hydrocarbon domain. Our results show that the presence of lipid molecules enhances the adsorption of GOx molecules at the liquid/liquid interface. At low lipid concentrations, the adsorption of GOx is probably the first step preceding its penetration into the lipid monolayer. The experimental results indicate that GOx penetrates better and forms more stable monolayers for lipids with longer hydrophobic tails. At high GOx concentrations, the formation of multilayers is observed. The phenomenon described here is strongly dependent on 1) the GOx and lipid concentrations, 2) the nature of the lipid, and 3) the potential drop across the interface.

Effect of Gramicidin on Phospholipid-Modified Monolayers and on Ion Transfer at a Liquid–Liquid Interface

The interaction of hybrid lipid/gramicidin A (gA) monolayers with dextran sulfate (DS) and the effect of this interaction on ion transfer at a liquid-liquid interface is reported. The interfacial and physicochemical properties are studied with Langmuir-Blodgett (LB) and electrochemical techniques. The results obtained from compression isotherms demonstrate that the interactions between the different species in the hybrid monolayer vary according to the chemical nature of the lipid (hydrocarbon region and charge of the head group). Interfacial capacitance measured with AC voltammetry indicates that the DS chains form a rather flat and compact layer when adsorbed to either zwitterionic or negatively charged phospholipid monolayers, and that calcium, even at low concentrations, interacts with the monolayers. These results are successfully described by a model based on the solution of the Poisson-Boltzmann equation in the interfacial region. Ion transfer and interactions with the lipid/gA/DS-modified monolayers were also studied with electrochemical techniques. Admittance data show that although the studied ions are not using gA channels for the transfer through the lipid membranes, the incorporation of gA in the lipid domain and the adsorption of DS at the interface have a significant effect on ion transfer across the monolayers. This effect can be explained as a consequence of the modified surface charge and of the compactness of the lipid domain due to its interaction with gA and to calcium and DS adsorption at the interface. The ion-transfer rate, therefore, depends on the composition of the monolayer and the chemical nature of the ion.

Increased glomerular permeability to negatively charged Ficoll relative to neutral Ficoll in rats

It is established that the glomerular filter sieves macromolecules based on their size, shape, and charge. Anionic proteins are thus retarded compared with their neutral or cationic counterparts. However, recent studies have indicated that charge effects are small, or even “anomalous,” for polysaccharides. We therefore investigated the impact of charge on the glomerular permeability to polysaccharides by comparing sieving coefficients (θ; primary urine-to-plasma concentration ratio) for negatively charged, carboxymethylated (CM) FITC-Ficoll and FITC-dextran with their neutral counterparts. For these probes, θ were determined in anesthetized Wistar rats [269 ± 2.7 g (±SE; n = 36)], whose ureters were cannulated for urine sampling. The glomerular filtration rate was assessed using FITC-inulin. Polysaccharides were constantly infused, and after equilibration, urine was collected and a midpoint plasma sample was taken. Size and concentration determinations of the FITC-labeled polysaccharides were achieved by size-exclusion HPLC (HPSEC). For CM-Ficoll, θ was significantly increased (32 times at 55 Å) compared with that of uncharged Ficoll. A small increase in θ for CM-dextran compared with neutral dextran was also observed (1.8 times at 55 Å). In conclusion, negatively charged Ficoll relative to neutral Ficoll was found to be markedly hyperpermeable across the glomerular filter. Furthermore, negatively charged Ficoll was observed to be larger on HPSEC compared with its neutral counterpart of the same molecular weight. It is proposed that the introduction of negative charges in the “dendrimeric,” cross-linked Ficoll molecule may alter its configuration, so as to make it more extended, and conceivably, more flexible, thereby increasing its glomerular permeability.

Improving membrane activity of oligonucleotides by cetylpyridinium chloride: an electrochemical study

The interaction of phospholipid membranes and oligonucleotides complexed with a positively charged surfactant is reported. Phospholipid membranes were assembled at the interface between an immobilized organic phase and an aqueous phase using the Langmuir-Blodgett (L-B) technique. The interaction and adsorption of the naked oligonucleotides and oligonucleotides complexed with cetylpyridinium chloride (CP) was studied electrochemically using cyclic voltammetry (CV) and ac-voltammetry. Interfacial capacitance, obtained indirectly from ac-voltammetry as a function of interfacial potential, was fitted to the theory based on the solution of the Poisson-Boltzmann equation. It was shown that both types of naked oligonucleotides (phosphoromonothioates and phosphodiesters) adsorb on the lipid monolayers poorly. The introduction of CP to the system increases the adsorption efficiency significantly. However, one phosphoromonothioate appeared to form a compact globule with CP instead of adsorbing to the lipid membrane. These results demonstrate that electrochemical methods are a powerful tool for probing the behavior of drugs in the vicinity of model cell membranes.