The electrical interplay between proteins and lipids in membranes

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

Quantitative characterization of membrane-protein reversible association using FCS

Functionally meaningful reversible protein-membrane interactions mediate many biological events. Fluorescence correlation spectroscopy (FCS) is increasingly used to quantitatively study the non-reversible binding of proteins to membranes using lipid vesicles in solution. However, the lack of a complete description of the phase and statistical equilibria in the case of reversible protein-membrane partitioning has hampered the application of FCS to quantify the partition coefficient (K_x). In this work, we further extend the theory that describes membrane-protein partitioning to account for spontaneous protein-membrane dissociation and reassociation to the same or a different lipid vesicle. We derive the probability distribution of proteins on lipid vesicles for reversible binding and demonstrate that FCS is a suitable technique for accurate K_x quantification of membrane-protein reversible association. We also establish the limits to K_x determination by FCS studying the Cramer-Rao bound on the variance of the retrieved parameters. We validate the mathematical formulation against reaction-diffusion simulations to study phase and statistical equilibria and compare the K_x obtained from a computational FCS titration experiment with the experimental ground truth. Finally, we demonstrate the application of our methodology studying the association of anti-HIV broadly neutralizing antibody (10E8-3R) to the membrane.

Cyclodextrins: Only Pharmaceutical Excipients or Full-Fledged Drug Candidates?

Cyclodextrins, representing a versatile family of cyclic oligosaccharides, have extensive pharmaceutical applications due to their unique truncated cone-shaped structure with a hydrophilic outer surface and a hydrophobic cavity, which enables them to form non-covalent host-guest inclusion complexes in pharmaceutical formulations to enhance the solubility, stability and bioavailability of numerous drug molecules. As a result, cyclodextrins are mostly considered as inert carriers during their medical application, while their ability to interact not only with small molecules but also with lipids and proteins is largely neglected. By forming inclusion complexes with cholesterol, cyclodextrins deplete cholesterol from cellular membranes and thereby influence protein function indirectly through alterations in biophysical properties and lateral heterogeneity of bilayers. In this review, we summarize the general chemical principles of direct cyclodextrin-protein interactions and highlight, through relevant examples, how these interactions can modify protein functions in vivo, which, despite their huge potential, have been completely unexploited in therapy so far. Finally, we give a brief overview of disorders such as Niemann-Pick type C disease, atherosclerosis, Alzheimer's and Parkinson's disease, in which cyclodextrins already have or could have the potential to be active therapeutic agents due to their cholesterol-complexing or direct protein-targeting properties.

Membrane Dipole Potential: An Emerging Approach to Explore Membrane Organization and Function

Biological membranes are complex organized molecular assemblies of lipids and proteins that provide cells and membrane-bound intracellular organelles their individual identities by morphological compartmentalization. Membrane dipole potential originates from the electrostatic potential difference within the membrane due to the nonrandom arrangement (orientation) of amphiphile and solvent (water) dipoles at the membrane interface. In this Feature Article, we will focus on the measurement of dipole potential using electrochromic fluorescent probes and highlight interesting applications. In addition, we will focus on ratiometric fluorescence microscopic imaging technique to measure dipole potential in cellular membranes, a technique that can be used to address novel problems in cell biology which are otherwise difficult to address using available approaches. We envision that membrane dipole potential could turn out to be a convenient tool in exploring the complex interplay between membrane lipids and proteins and could provide novel insights in membrane organization and function.

Effect of Local Anesthetics on Dipole Potential of Different Phase Membranes: A Fluorescence Study

The molecular mechanism behind the action of local anesthetics is not well understood. Phenylethanol (PEtOH) is an ingredient of essential oils with a rose-like odor, and it has previously been used as a local anesthetic. In this work, we explored the effect of PEtOH on dipole potential in membranes representing biologically relevant phases, employing the dual-wavelength ratiometric method utilizing the potential-sensitive probe di-8-ANEPPS. Our results show that PEtOH reduces membrane dipole potential in membranes of all biologically relevant phases (gel, liquid-ordered, and fluid) in a concentration-dependent manner. To the best of our knowledge, these results constitute one of the early reports describing reduction of membrane dipole potential induced by local anesthetics, irrespective of membrane phase.

Differential Membrane Lipid Profiles and Vibrational Spectra of Three Edaphic Algae and One Cyanobacterium

The membrane glycerolipids of four phototrophs that were isolated from an edaphic assemblage were determined by UPLC-MS after cultivation in a laboratory growth chamber. Identification was carried out by 18S and 16S rDNA sequencing. The algal species were Klebsormidium flaccidum (Charophyta), Oocystis sp. (Chlorophyta), and Haslea spicula (Bacillariophyta), and the cyanobacterium was Microcoleus vaginatus (Cyanobacteria). The glycerolipid profile of Oocystis sp. was dominated by monogalactosyldiacylglycerol (MGDG) species, with MGDG(18:3/16:4) accounting for 68.6%, whereas MGDG(18:3/16:3) was the most abundant glycerolipid in K. flaccidum (50.1%). A ratio of digalactosyldiacylglycerol (DGDG) species to MGDG species (DGDG/MGDG) was shown to be higher in K. flaccidum (0.26) than in Oocystis sp. (0.14). This ratio increased under high light (HL) as compared to low light (LL) in all the organisms, with its highest value being shown in cyanobacterium (0.38-0.58, LL-HL). High contents of eicosapentaenoic acid (EPA, C20:5) and hexadecenoic acid were observed in the glycerolipids of H. spicula. Similar Fourier transform infrared (FTIR) and Raman spectra were found for K. flaccidum and Oocystis sp. Specific bands at 1629.06 and 1582.78 cm-1 were shown by M. vaginatus in the Raman spectra. Conversely, specific bands in the FTIR spectrum were observed for H. spicula at 1143 and 1744 cm-1. The results of this study point out differences in the membrane lipid composition between species, which likely reflects their different morphology and evolutionary patterns.

Dermal white adipose tissue: Much more than a metabolic, lipid-storage organ?

The role of dermal white adipose tissue (dWAT) has emerged in the biomedical science as an ancillary fat district in the derma without a defined and distinct function respect to the subcutaneous adipose tissue (sWAT). Despite some evidence describing dWAT as an immune-competent compartment, particularly engaged in wound repair, very few reports dealing with dWAT has elucidated its major modulatory role within the skin biology. Whereas an increasing bulk of evidence allows researcher to describe the main activity of sWAT, in humans dWAT is not properly a separated fat compartment and therefore scarcely considered in the scientific debate. Due to its strategic position between epidermis and sWAT, dermal fat might play a much more intriguing role than expected. This review tries to shed light on this issue, by expanding the debate about a possible role of dWAT in skin physiology.

An ω-3, but Not an ω-6 Polyunsaturated Fatty Acid Decreases Membrane Dipole Potential and Stimulates Endo-Lysosomal Escape of Penetratin

Although the largely positive intramembrane dipole potential (DP) may substantially influence the function of transmembrane proteins, its investigation is deeply hampered by the lack of measurement techniques suitable for high-throughput examination of living cells. Here, we describe a novel emission ratiometric flow cytometry method based on F66, a 3-hydroxiflavon derivative, and demonstrate that 6-ketocholestanol, cholesterol and 7-dehydrocholesterol, saturated stearic acid (SA) and ω-6 γ-linolenic acid (GLA) increase, while ω-3 α-linolenic acid (ALA) decreases the DP. These changes do not correlate with alterations in cell viability or membrane fluidity. Pretreatment with ALA counteracts, while SA or GLA enhances cholesterol-induced DP elevations. Furthermore, ALA (but not SA or GLA) increases endo-lysosomal escape of penetratin, a cell-penetrating peptide. In summary, we have developed a novel method to measure DP in large quantities of individual living cells and propose ALA as a physiological DP lowering agent facilitating cytoplasmic entry of penetratin.

Effects of valinomycin doping on the electrical and structural properties of planar lipid bilayers supported on polyelectrolyte multilayers

Supported Lipid Bilayers (SLBs) on Polyelectrolyte Multilayers (PEMs) have large potential as models for developing sensor devices. SLBs can be designed with receptors and channels, which benefit from the biological environment of the lipid layers, to create a sensing interface for ions and biomarkers. PEMs assembled by the Layer-by-Layer (LBL) technique and used as supports for a lipid bilayer enable an easy integration of the bilayer on almost any surface and device. For electrochemical sensors, LBL assembly enables nanoscale tunable separation of the lipid bilayer from the electrode surface, avoiding undesired effects of the electrode surface on the lipid bilayers. We study the fabrication of valinomycin-doped SLBs on PEMs as a model system for biophysical studies and for selective ion sensing. SLBs are fabricated from dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylserine (DOPS) 50:50 vesicles doped with valinomycin, as a K⁺-selective carrier. SLBs were deposited on electrodes coated with poly(allyl amine hydrochloride) (PAH) and poly(styrene sodium sulfonate) (PSS) multilayers. Lipid bilayer formation was monitored by using Quartz Crystal Microbalance with Dissipation (QCMD) technique and Atomic Force Microscopy (AFM). Electrochemical impedance spectroscopy (EIS) and potentiometric measurements were performed to assess K⁺ selectivity over other ions and the potential of valinomycin-doped SLBs for K⁺-sensing.

Structural basis of phosphatidylcholine recognition by the C2-domain of cytosolic phospholipase A2α

Ca2+-stimulated translocation of cytosolic phospholipase A2α (cPLA2α) to the Golgi induces arachidonic acid production, the rate-limiting step in pro-inflammatory eicosanoid synthesis. Structural insights into the cPLA2α preference for phosphatidylcholine (PC)-enriched membranes have remained elusive. Here, we report the structure of the cPLA2α C2-domain (at 2.2 Å resolution), which contains bound 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and Ca2+ ions. Two Ca2+ are complexed at previously reported locations in the lipid-free C2-domain. One of these Ca2+ions, along with a third Ca2+, bridges the C2-domain to the DHPC phosphate group, which also interacts with Asn65. Tyr96 plays a key role in lipid headgroup recognition via cation-π interaction with the PC trimethylammonium group. Mutagenesis analyses confirm that Tyr96 and Asn65 function in PC binding selectivity by the C2-domain and in the regulation of cPLA2α activity. The DHPC-binding mode of the cPLA2α C2-domain, which differs from phosphatidylserine or phosphatidylinositol 4,5-bisphosphate binding by other C2-domains, expands and deepens knowledge of the lipid-binding mechanisms mediated by C2-domains.

Investigation of the adsorption behavior of BSA with tethered lipid layer-modified solid-state nanopores in a wide pH range

Nanopore technology was introduced for the study of the dynamic interactions between bovine serum albumin (BSA) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) phospholipids based on a modified nanopore. The results reveal that the interaction mechanism between DOPE and BSA is affected by the pH of the subphase. Far above the BSA isoelectric point (pH > 7), a weaker hydrophobic interaction and stronger electrostatic repulsion exist between the DOPE and BSA molecules. At pH = 7, the BSA structure nearly does not change, and the interaction is weak. At pH 5 and pH 6, BSA is marginally affected by the adsorption interaction, and below pH 5, the DOPE film becomes disordered, so there is a strong repulsive force interaction between the BSA and DOPE.

Nanopore technology was introduced for the study of the interaction between bovine serum albumin (BSA) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) phospholipids.

Assessing anesthetic activity through modulation of the membrane dipole potential

There is great individual variation in response to general anesthetics (GAs) leading to difficulties in optimal dosing and sometimes even accidental awareness during general anesthesia (AAGA). AAGA is a rare, but potentially devastating, complication affecting between 0.1% and 2% of patients undergoing surgery. The development of novel personalized screening techniques to accurately predict a patient’s response to GAs and the risk of AAGA remains an unmet clinical need. In the present study, we demonstrate the principle of using a fluorescent reporter of the membrane dipole potential, di-8-ANEPPs, as a novel method to monitor anesthetic activity using a well-described inducer/noninducer pair. The membrane dipole potential has previously been suggested to contribute a novel mechanism of anesthetic action. We show that the fluorescence ratio of di-8-ANEPPs changed in response to physiological concentrations of the anesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F₃), but not the structurally similar noninducer, 1,2-dichlorohexafluorocyclobutane (F₆), to artificial membranes and in vitro retinal cell systems. Modulation of the membrane dipole provides an explanation to overcome the limitations associated with the alternative membrane-mediated mechanisms of GA action. Furthermore, by combining this technique with noninvasive retinal imaging technologies, we propose that this technique could provide a novel and noninvasive technique to monitor GA susceptibility and identify patients at risk of AAGA.

Differential Membrane Dipolar Orientation Induced by Acute and Chronic Cholesterol Depletion

Cholesterol plays a crucial role in cell membrane organization, dynamics and function. Depletion of cholesterol represents a popular approach to explore cholesterol-sensitivity of membrane proteins. An emerging body of literature shows that the consequence of membrane cholesterol depletion often depends on the actual process (acute or chronic), although the molecular mechanism underlying the difference is not clear. Acute depletion, using cyclodextrin-type carriers, is faster relative to chronic depletion, in which inhibitors of cholesterol biosynthesis are used. With the overall goal of addressing molecular differences underlying these processes, we monitored membrane dipole potential under conditions of acute and chronic cholesterol depletion in CHO-K1 cells, using a voltage-sensitive fluorescent dye in dual wavelength ratiometric mode. Our results show that the observed membrane dipole potential exhibits difference under acute and chronic cholesterol depletion conditions, even when cholesterol content was identical. To the best of our knowledge, these results provide, for the first time, molecular insight highlighting differences in dipolar reorganization in these processes. A comprehensive understanding of processes in which membrane cholesterol gets modulated would provide novel insight in its interaction with membrane proteins and receptors, thereby allowing us to understand the role of cholesterol in cellular physiology associated with health and disease.

The dipole potential correlates with lipid raft markers in the plasma membrane of living cells

The dipole potential generating an electric field much stronger than any other type of membrane potential influences a wide array of phenomena, ranging from passive permeation to voltage-dependent conformational changes of membrane proteins. It is generated by the ordered orientation of lipid carbonyl and membrane-attached water dipole moments. Theoretical considerations and indirect experimental evidence obtained in model membranes suggest that the dipole potential is larger in liquid-ordered domains believed to correspond to lipid rafts in cell membranes. Using three different dipole potential-sensitive fluorophores and four different labeling approaches of raft and nonraft domains, we showed that the dipole potential is indeed stronger in lipid rafts than in the rest of the membrane. The magnitude of this difference is similar to that observed between the dipole potential in control and sphingolipid-enriched cells characteristic of Gaucher's disease. The results established that the heterogeneity of the dipole potential in living cell membranes is correlated with lipid rafts and imply that alterations in the lipid composition of the cell membrane in human diseases can lead to substantial changes in the dipole potential.

Topical Coenzyme Q10 demonstrates mitochondrial-mediated neuroprotection in a rodent model of ocular hypertension

Coenzyme Q10 (CoQ10) is a mitochondrial-targeted antioxidant with known neuroprotective activity. Its ocular effects when co-solubilised with α-tocopherol polyethylene glycol succinate (TPGS) were evaluated. In vitro studies confirmed that CoQ10 was significantly protective in different retinal ganglion cell (RGC) models. In vivo studies in Adult Dark Agouti (DA) rats with unilateral surgically-induced ocular hypertension (OHT) treated with either CoQ10/TPGS micelles or TPGS vehicle twice daily for three weeks were performed, following which retinal cell health was assessed in vivo using DARC (Detection of Apoptotic Retinal Cells) and post-mortem with Brn3a histological assessment on whole retinal mounts. CoQ10/TPGS showed a significant neuroprotective effect compared to control with DARC (p<0.05) and Brn3 (p<0.01). Topical CoQ10 appears an effective therapy preventing RGC apoptosis and loss in glaucoma-related models.

The Dipole Potential Modifies the Clustering and Ligand Binding Affinity of ErbB Proteins and Their Signaling Efficiency

Although activation of the ErbB family of receptor tyrosine kinases (ErbB1-4) is driven by oligomerization mediated by intermolecular interactions between the extracellular, the kinase and the transmembrane domains, the transmembrane domain has been largely neglected in this regard. The largest contributor to the intramembrane electric field, the dipole potential, alters the conformation of transmembrane peptides, but its effect on ErbB proteins is unknown. Here, we show by Förster resonance energy transfer (FRET) and number and brightness (N&B) experiments that the epidermal growth factor (EGF)-induced increase in the homoassociation of ErbB1 and ErbB2 and their heteroassociation are augmented by increasing the dipole potential. These effects were even more pronounced for ErbB2 harboring an activating Val → Glu mutation in the transmembrane domain (NeuT). The signaling capacity of ErbB1 and ErbB2 was also correlated with the dipole potential. Since the dipole potential decreased the affinity of EGF to ErbB1, the augmented growth factor-induced effects at an elevated dipole potential were actually induced at lower receptor occupancy. We conclude that the dipole potential plays a permissive role in the clustering of ErbB receptors and that the effects of lipid rafts on ligand binding and receptor signaling can be partially attributed to the dipole potential.

Micellar dipole potential is sensitive to sphere-to-rod transition

Structural transitions involving shape changes play an important role in cellular physiology. Charged micelles offer a convenient model system in which structural transitions can be suitably induced by increasing the ionic strength of the medium. In this paper, we have explored sphere-to-rod transition in charged micelles of SDS and CTAB by monitoring micellar dipole potential using the dual wavelength ratiometric approach utilizing the potential-sensitive membrane probe di-8-ANEPPS. Our results show that micellar dipole potential is sensitive to sphere-to-rod transition in charged micelles. Micellar dipole potential exhibited increase with increasing ionic strength (salt), irrespective of the nature of micellar charge, implying considerable dipolar reorganization underlying structural transitions. We interpret the increase in dipole potential due to sphere-to-rod transition because of an increase in the population of confined (nonrandom) dipoles induced by micellar organizational change. This is due to the fact that dipole potential arises due to the nonrandom arrangement of micellar dipoles and water molecules at the micelle interface. Our results constitute one of the first reports describing drastic dipolar reorganization due to micellar shape (and size) change. We envision that dipole potential measurements could provide novel insights into micellar processes that are associated with dipolar reorganization.

Dipolar rearrangement during micellization explored using a potential-sensitive fluorescent probe

Dipole potential is the potential difference within the membrane bilayer, which originates due to the nonrandom arrangement of lipid dipoles and water molecules at the membrane interface. Although dipole potential is generally used in the context of bilayer membranes, the nonrandom arrangement of amphiphiles and water dipoles would also contribute to dipole potential in organized molecular assemblies such as micelles. In this work, we show that the process of micelle formation from monomers for a representative variety of detergents is associated with dipolar rearrangement. We monitor the dipolar reorganization upon micellization as a change in dipole potential, measured by the dual wavelength ratiometric approach utilizing the potential-sensitive membrane probe di-8-ANEPPS. We further utilized this phenomenon to estimate the critical micelle concentration (CMC) of a variety of detergents. CMC determined by this method are in overall agreement with the literature values of CMC for these detergents. To the best of our knowledge, these results constitute the first report showing dipolar reorientation during micellization. We conclude that dipole potential measurements could provide a novel approach to explore micellar organization.