Voltage-dependent anion channel transports calcium ions through biomimetic membranes

Electrografting a Hybrid Bilayer Membrane via Diazonium Chemistry for Electrochemical Impedance Spectroscopy of Amyloid-β Aggregation

Herein, a novel hybrid bilayer membrane is introduced as a platform to study the aggregation of amyloid-β_1–42 (Aβ_1–42) peptide on surfaces. The first layer was covalently attached to a glassy carbon electrode (GCE) via diazonium electrodeposition, which provided a highly stable template for the hybrid bilayer formation. To prepare the long-chain hybrid bilayer membrane (lcHBLM)-modified electrodes, GCE surfaces were modified with 4-dodecylbenzenediazonium (DDAN) followed by the modification with dihexadecyl phosphate (DHP) as the second layer. For the preparation of short-chain hybrid bilayer membrane (scHBLM)-modified electrodes, GCE surfaces were modified with 4-ethyldiazonium (EDAN) as the first layer and bis(2-ethylhexyl) phosphate (BEHP) was utilized as the second layer. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to characterize the bilayer formation. Both positively charged [Ru(NH₃)₆]³⁺ and negatively charged ([Fe(CN)₆]^3-/4-) redox probes were used for electrochemical characterization of the modified surfaces using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS results showed a decrease in charge transfer resistance (R_ct) upon incubation of Aβ_1–42 on the hybrid bilayer-modified surfaces. This framework provides a promising electrochemical platform for designing hybrid bilayers with various physicochemical properties to study the interaction of membrane-bound receptors and biomolecules on surfaces.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

BALM: Watching the Formation of Tethered Bilayer Lipid Membranes with Submicron Lateral Resolution

Tethered bilayer lipid membranes (tBLMs) are artificial membranes largely used for the in situ study of biological membranes and membrane-associated proteins. To date, the formation of these membranes was essentially monitored by surface averaging techniques like surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCM-D), which cannot provide both local and real-time information in a single approach. Here, we report an original application of backside absorbing layer microscopy (BALM), a novel white-light wide-field optical microscopy, to study tBLMs. Thanks to the combination of sensitivity and resolution, BALM not only allowed the real-time quantitative monitoring of tBLM formation but also enabled the high-resolution visualization of local fluxes and matter exchanges taking place at each step of the process. Quantitative BALM measurements of the final layer thickness, reproduced in parallel with SPR, were consistent with the achievement of a continuous lipid bilayer. This finding was confirmed by BALM imaging, which additionally revealed the heterogeneity of the bilayer during its formation. While established real-time techniques, like SPR or QCM-D, view the surface as homogeneous, BALM showed the presence of surface patterns appearing in the first step of the tBLM formation process and governing subsequent matter adsorption or desorption steps. Finally, matter fluxes persisting even after rinsing at the end of the tBLM formation demonstrated the lasting presence of dispersed vesicular pockets with laterally fluctuating positions over the final single and continuous lipid bilayer. These new mechanistic insights into the tBLM formation process demonstrate the great potential of BALM in the study of complex biological systems.

Why Do Tethered-Bilayer Lipid Membranes Suit for Functional Membrane Protein Reincorporation?

Membrane proteins (MPs) are essential for cellular functions. Understanding the functions of MPs is crucial as they constitute an important class of drug targets. However, MPs are a challenging class of biomolecules to analyze because they cannot be studied outside their native environment. Their structure, function and activity are highly dependent on the local lipid environment, and these properties are compromised when the protein does not reside in the cell membrane. Mammalian cell membranes are complex and composed of different lipid species. Model membranes have been developed to provide an adequate environment to envisage MP reconstitution. Among them, tethered-Bilayer Lipid Membranes (tBLMs) appear as the best model because they allow the lipid bilayer to be decoupled from the support. Thus, they provide a sufficient aqueous space to envisage the proper accommodation of large extra-membranous domains of MPs, extending outside. Additionally, as the bilayer remains attached to tethers covalently fixed to the solid support, they can be investigated by a wide variety of surface-sensitive analytical techniques. This review provides an overview of the different approaches developed over the last two decades to achieve sophisticated tBLMs, with a more and more complex lipid composition and adapted for functional MP reconstitution.

Vitamin C versus Cancer: Ascorbic Acid Radical and Impairment of Mitochondrial Respiration?

Vitamin C as a cancer therapy has a controversial history. Much of the controversy arises from the lack of predictive biomarkers for stratification of patients, as well as a clear understanding of the mechanism of action and its multiple targets underlying the anticancer effect. Our review expands the analysis of cancer vulnerabilities for high-dose vitamin C, based on several facts, illustrating the cytotoxic potential of the ascorbyl free radical (AFR) via impairment of mitochondrial respiration and the mechanisms of its elimination in mammals by the membrane-bound NADH:cytochrome b5 oxidoreductase 3 (Cyb5R3). This enzyme catalyzes rapid conversion of AFR to ascorbate, as well as reduction of other redox-active compounds, using NADH as an electron donor. We propose that vitamin C can function in “protective mode” or “destructive mode” affecting cellular homeostasis, depending on the intracellular “steady-state” concentration of AFR and differential expression/activity of Cyb5R3 in cancerous and normal cells. Thus, a specific anticancer effect can be achieved at high doses of vitamin C therapy. The review is intended for a wide audience of readers—from students to specialists in the field.

Polyethyleneimine renders mitochondrial membranes permeable by interacting with negatively charged phospholipids in them

Polyethyleneimines (PEIs) are used for transfection of cells with nucleic acids. Meanwhile, the interaction of PEI with mitochondria causes cytochrome c release prior to apoptosis; the mechanisms how PEI causes this permeabilization of mitochondrial membranes and the release of cytochrome c remain unclear. To clarify these mechanisms, we examined the effects of branched-type PEI and linear-type PEI, each of which was 25 kDa in size, on mitochondria. The permeabilization potency of mitochondrial membranes by branched PEI was stronger than that by linear PEI. The permeabilization by PEIs were insensitive to permeability-transition inhibitors, indicating that PEI-induced permeabilization was not attributed to permeability transition. Meanwhile, PEIs caused permeabilization of artificial lipid vesicles; again, the permeabilization potency of branched PEI was stronger than that of linear PEI. Such a difference in this potency was close to that in the case of isolated mitochondria, signifying that the PEI-induced permeabilization of mitochondrial membranes could be attributed to PEI's interaction with the phospholipid phase. Furthermore, this PEI-induced permeabilization of the lipid vesicles was observed only in the case of lipid vesicles including negatively charged phospholipids. These results indicate that PEIs interacted with negatively charged phospholipids in the mitochondrial membranes to directly lead to their permeabilization.

Fluorescent and Photosensitizing Conjugates of Cell-Penetrating Peptide TAT(47-57): Design, Microwave-Assisted Synthesis at 60 °C, and Properties

Conjugates based on cell-penetrating peptides (CPPs) are scientifically relevant owing to their structural complexity; their ability to enter cells and deliver drugs, labels, antioxidants, bioactive compounds, or DNA fragments; and, consequently, their potential for application in research and biomedicine. In this study, carboxyamidated fluorescently labeled conjugates FAM-GG-TAT(47-57)-NH2 and FAM-PEG6-TAT(47-57)-NH2 and photosensitizer-labeled conjugate Chk-PEG6-TAT(47-57)-NH2 [where TAT(47-57) is the CPP, 5(6)-carboxyfluorescein is the (FAM) fluorophore, chlorin k (Chk) is the photosensitizer, and the dipeptide glycyl-glycine (GG) or hexaethylene glycol (PEG6) is the spacer] were originally designed, prepared, and fully characterized. Practically, all chemical reactions of the synthetic steps (peptide synthesis, spacer incorporation, and conjugation) were microwave-assisted at 60 °C using optimized protocols to give satisfying yields and high-quality products. Detailed analyses of the conjugates using spectrofluorimetry and singlet oxygen detection showed that they display photophysical properties typical of FAM or Chk. Anticandidal activity assays showed that not only this basic property of TAT(47-57) was preserved in the conjugates but also that the minimal inhibitory concentration was slightly reduced for cells incubated with PS-bearing conjugate Chk-PEG6-TAT(47-57)-NH2. Overall, these results indicated that the synthetic approach on-resin assisted by microwaves at 60 °C is simple, straightforward, selective, metal-free, sufficiently fast, cleaner, and more cost-effective than those previously used for preparing this type of macromolecule. Furthermore, such new data show that microwaves at 60 °C and/or conjugation did not harm the integrity of the conjugates' constituents. Therefore, FAM-GG-TAT(47-57)-NH2, FAM-PEG6-TAT(47-57)-NH2, and Chk-PEG6-TAT(47-57)-NH2 have high potential for practical applications in biochemistry, biophysics, and therapeutics.

New Tethered Phospholipid Bilayers Integrating Functional G-Protein-Coupled Receptor Membrane Proteins

Membrane proteins exhibiting extra- and intracellular domains require an adequate near-native lipid platform for their functional reconstitution. With this aim, we developed a new technology enabling the formation of a peptide-tethered bilayer lipid membrane (pep-tBLM), a lipid bilayer grafted onto peptide spacers, by way of a metal-chelate interaction. To this end, we designed an original peptide spacer derived from the natural α-laminin thiopeptide (P19) possessing a cysteine residue in the N-terminal extremity for grafting onto gold and a C-terminal extremity modified by four histidine residues (P19-4H). In the presence of nickel, the use of this anchor allowed us to bind liposomes of variable compositions containing a 2% molar ratio of a chelating lipid, 1,2-dioleoyl-sn-glycero-3-[(N-(5-amino-1-carboxypentyl)iminodiacetic acid)succinyl] so-called DOGS-NTA, and to form the planar bilayer by triggering liposome fusion by an α-helical (AH) peptide derived from the N-terminus of the hepatitis C virus NS5A protein. The formation of pep-tBLMs was characterized by surface plasmon resonance imaging (SPRi), and their continuity, fluidity, and homogeneity were demonstrated by fluorescence recovery after photobleaching (FRAP), with a diffusion coefficient of 2.5 × 10^-7 cm²/s, and atomic force microscopy (AFM). By using variable lipid compositions including phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol 4,5-bisphosphate (PIP₂), sphingomyelin (SM), phosphatidic acid (PA), and cholesterol (Chol) in various ratios, we show that the membrane can be formed independently from the lipid composition. We made the most of this advantage to reincorporate a transmembrane protein in an adapted complex lipid composition to ensure its functional reinsertion. For this purpose, a cell-free expression system was used to produce proteoliposomes expressing the functional C-X-C motif chemokine receptor 4 (CXCR4), a seven-transmembrane protein belonging to the large superfamily of G-protein-coupled receptors (GPCRs). We succeeded in reinserting CXCR4 in pep-tBLMs formed on P19-4H by the fusion of tethered proteoliposomes. AFM and FRAP characterization allowed us to show that pep-tBLMs inserting CXCR4 remained fluid, homogeneous, and continuous. The value of the diffusion coefficient determined in the presence of reinserted CXCR4 was 2 × 10^-7 cm²/s. Ligand binding assays using a synthetic CXCR4 antagonist, T22 ([Tyr5,12, Lys7]-polyphemusin II), revealed that CXCR4 can be reinserted in pep-tBLMs with functional folding and orientation. This new approach represents a method of choice for investigating membrane protein reincorporation and a promising way of creating a new generation of membrane biochips adapted for screening agonists or antagonists of transmembrane proteins.

Synthesis and characterization of tethered lipid assemblies for membrane protein reconstitution (Review)

Biological membranes and their related molecular mechanisms are essential for all living organisms. Membranes host numerous proteins and are responsible for the exchange of molecules and ions, cell signaling, and cell compartmentation. Indeed, the plasma membrane delimits the intracellular compartment from the extracellular environment and intracellular membranes. Biological membranes also play a major role in metabolism regulation and cellular physiology (e.g., mitochondrial membranes). The elaboration of membrane based biomimetic systems allows us to reconstitute and investigate, in controlled conditions, biological events occurring at the membrane interface. A whole variety of model membrane systems have been developed in the last few decades. Among these models, supported membranes were developed on various hydrophilic supports. The use of solid supports enables the direct use of surface sensitive techniques (e.g., surface plasmon resonance, quartz crystal microbalance, and atomic force microscopy) to monitor and quantify events occurring at the membrane surface. Tethered bilayer membranes (tBLMs) could be considered as an achievement of the first solid supported membranes described by the McConnell group. Tethered bilayers on solid supports were designed to delimit an inside compartment from an outside one. They were used for measuring interactions with ligands or incorporating large membrane proteins or complexes without interference with the support. In this context, the authors developed an easy concept of versatile tBLMs assembled on amino coated substrates that are formed upon the vesicle fusion rupture process applicable to protein-free vesicles as well as proteoliposomes. The phospholipid bilayer (natural or synthetic lipids) incorporated 5% of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly ethylene glycol-N-hydroxy succinimide to ensure the anchorage of the bilayer to the amino coated surface. The conditions for the formation of tBLMs on amino-coated gold and glass were optimized for protein-free vesicles. This biomimetic membrane delimits an inside "trans" compartment separated from an outside reservoir "cis." Using this tBLM construction, the authors were interested in deciphering two complex molecular mechanisms involving membrane-associated proteins. The first one concerns two mitochondrial proteins, i.e., the porin voltage dependent anion channel (VDAC) embedded in the outer membrane and the nucleotide transporter (adenine nucleotide translocase) that interacts dynamically during mitochondrial pathophysiology. The purified VDAC porin was first reconstituted in proteoliposomes that were subsequently assembled on an amino coated support to form a biomimetic membrane. As a major result, VDAC was reconstituted in this tBLM and calcium channeling was demonstrated across the lipid bilayer. The same two-compartment biomimetic membrane design was further engineered to study the translocation mechanism of a bacterial toxin, the adenylate cyclase toxin, CyaA, from Bordetella pertussis. As a result, the authors developed an elegant in vitro translocation toolkit applicable to potentially a large panel of proteins transported across membranes.

The BH4 domain of anti-apoptotic Bcl-XL, but not that of the related Bcl-2, limits the voltage-dependent anion channel 1 (VDAC1)-mediated transfer of pro-apoptotic Ca2+ signals to mitochondria

Excessive Ca(2+) fluxes from the endoplasmic reticulum to the mitochondria result in apoptotic cell death. Bcl-2 and Bcl-XL proteins exert part of their anti-apoptotic function by directly targeting Ca(2+)-transport systems, like the endoplasmic reticulum-localized inositol 1,4,5-trisphosphate receptors (IP3Rs) and the voltage-dependent anion channel 1 (VDAC1) at the outer mitochondrial membranes. We previously demonstrated that the Bcl-2 homology 4 (BH4) domain of Bcl-2 protects against Ca(2+)-dependent apoptosis by binding and inhibiting IP3Rs, although the BH4 domain of Bcl-XL was protective independently of binding IP3Rs. Here, we report that in contrast to the BH4 domain of Bcl-2, the BH4 domain of Bcl-XL binds and inhibits VDAC1. In intact cells, delivery of the BH4-Bcl-XL peptide via electroporation limits agonist-induced mitochondrial Ca(2+) uptake and protects against staurosporine-induced apoptosis, in line with the results obtained with VDAC1(-/-) cells. Moreover, the delivery of the N-terminal domain of VDAC1 as a synthetic peptide (VDAC1-NP) abolishes the ability of BH4-Bcl-XL to suppress mitochondrial Ca(2+) uptake and to protect against apoptosis. Importantly, VDAC1-NP did not affect the ability of BH4-Bcl-2 to suppress agonist-induced Ca(2+) release in the cytosol or to prevent apoptosis, as done instead by an IP3R-derived peptide. In conclusion, our data indicate that the BH4 domain of Bcl-XL, but not that of Bcl-2, selectively targets VDAC1 and inhibits apoptosis by decreasing VDAC1-mediated Ca(2+) uptake into the mitochondria.

External mitochondrial NADH-dependent reductase of redox cyclers: VDAC1 or Cyb5R3?

It was reported that VDAC1 possesses an NADH oxidoreductase activity and plays an important role in the activation of xenobiotics in the outer mitochondrial membrane. In the present work, we evaluated the participation of VDAC1 and Cyb5R3 in the NADH-dependent activation of various redox cyclers in mitochondria. We show that external NADH oxidoreductase caused the redox cycling of menadione ≫ lucigenin>nitrofurantoin. Paraquat was predominantly activated by internal mitochondria oxidoreductases. An increase in the ionic strength stimulated and suppressed the redox cycling of negatively and positively charged acceptors, as was expected for the Cyb5R3-mediated reduction. Antibodies against Cyb5R3 but not VDAC substantially inhibited the NADH-related oxidoreductase activities. The specific VDAC blockers G3139 and erastin, separately or in combination, in concentrations sufficient for the inhibition of substrate transport, exhibited minimal effects on the redox cycler-dependent NADH oxidation, ROS generation, and reduction of exogenous cytochrome c. In contrast, Cyb5R3 inhibitors (6-propyl-2-thiouracil, p-chloromercuriobenzoate, quercetin, mersalyl, and ebselen) showed similar patterns of inhibition of ROS generation and cytochrome c reduction. The analysis of the spectra of the endogenous cytochromes b5 and c in the presence of nitrofurantoin and the inhibitors of VDAC and Cyb5R3 demonstrated that the redox cycler can transfer electrons from Cyb5R3 to endogenous cytochrome c. This caused the oxidation of outer membrane-bound cytochrome b5, which is in redox balance with Cyb5R3. The data obtained argue against VDAC1 and in favor of Cyb5R3 involvement in the activation of redox cyclers in the outer mitochondrial membrane.

Tethered bilayer lipid membranes (tBLMs): interest and applications for biological membrane investigations

Biological membranes play a central role in the biology of the cell. They are not only the hydrophobic barrier allowing separation between two water soluble compartments but also a supra-molecular entity that has vital structural functions. Notably, they are involved in many exchange processes between the outside and inside cellular spaces. Accounting for the complexity of cell membranes, reliable models are needed to acquire current knowledge of the molecular processes occurring in membranes. To simplify the investigation of lipid/protein interactions, the use of biomimetic membranes is an approach that allows manipulation of the lipid composition of specific domains and/or the protein composition, and the evaluation of the reciprocal effects. Since the middle of the 80's, lipid bilayer membranes have been constantly developed as models of biological membranes with the ultimate goal to reincorporate membrane proteins for their functional investigation. In this review, after a brief description of the planar lipid bilayers as biomimetic membrane models, we will focus on the construction of the tethered Bilayer Lipid Membranes, the most promising model for efficient membrane protein reconstitution and investigation of molecular processes occurring in cell membranes.

Control of the morphology of lipid layers by substrate surface chemistry

In this study, surface coatings were used to control the morphology of the deposited lipid layers during vesicle spreading, i.e., to control if liposomes self-assemble on a surface into a supported lipid bilayer or a supported vesicular layer. The influence of the properties of the surface coating on formation of the deposited lipid layer was studied with quartz crystal microbalance and two-wavelength multiparametric surface plasmon resonance techniques. Control of lipid self-assembly on the surface was achieved by two different types of soft substrate materials, i.e., dextran and thiolated polyethylene glycol, functionalized with hydrophobic linkers for capturing the lipid layer. The low-molecular-weight dextran-based surface promoted formation of supported lipid bilayers, while the thiolated polyethylene glycol-based surface promoted supported vesicular layer formation. A silicon dioxide surface was used as a reference surface in both measurement techniques. In addition to promoting supported lipid bilayer formation of known lipid mixtures, the dextran surface also promoted supported lipid bilayer formation of vesicles containing the cell membrane extract of human hepatoblastoma cells. The new dextran-based surface was also capable of protecting the supported lipid bilayer against dehydration when exposed to a constant flow of air. The well-established quartz crystal microbalance technique was effective in determining the morphology of the formed lipid layer, while the two-wavelength surface plasmon resonance analysis enabled further complementary characterization of the adsorbed supported lipid bilayers and supported vesicular layers.

Voltage-dependent structural changes of the membrane-bound anion channel hVDAC1 probed by SEIRA and electrochemical impedance spectroscopy

SEIRA spectroscopy provides insight into the potential-induced structural changes of the anion channel hVDAC1 embedded in the tethered bilayer lipid membrane.

Reconstitution of purified VDAC1 into a lipid bilayer and recording of channel conductance

The functional properties of purified voltage-dependent anion-selective channel protein 1 (VDAC1) have been examined in reconstituted systems based on artificially prepared phospholipid bilayers. The most widespread method for the characterization of the pore-forming activity of the mitochondrial VDAC1 protein requires reconstitution of the channel activity into a planar lipid bilayer (PLB) that separates two aqueous compartments. This system is able to produce a refined and large set of information on channel activity. The activity of the channel is reflected in the flow of ions (i.e., current) through a membrane that otherwise represents a barrier to ion flow. The setup thus requires the use of purified protein and a source of continuous current, as well as a sophisticated detector system able to amplify and record low, picoamper-level currents. This system is so efficient that the activity of even a single channel can be detected, allowing for study of VDAC1 at the molecular level.

Glycogen synthase kinase 3-mediated voltage-dependent anion channel phosphorylation controls outer mitochondrial membrane permeability during lipid accumulation

Nonalcoholic steatosis is a liver pathology characterized by fat accumulation and severe metabolic alterations involving early mitochondrial impairment and late hepatocyte cell death. However, mitochondrial dysfunction mechanisms remain elusive. Using four models of nonalcoholic steatosis, i.e., livers from patients with fatty liver disease, ob/ob mice, mice fed a high-fat diet, and in vitro models of lipotoxicity, we show that outer mitochondrial membrane permeability is altered and identified a posttranslational modification of voltage-dependent anion channel (VDAC), a membrane channel and NADH oxidase, as a cause of early mitochondrial dysfunction. Thus, in nonalcoholic steatosis VDAC exhibits reduced threonine phosphorylation, which increases the influx of water and calcium into mitochondria, sensitizes the organelle to matrix swelling, depolarization, and cytochrome c release without inducing cell death. This also amplifies VDAC enzymatic and channel activities regulation by calcium and modifies its interaction with proteic partners. Moreover, lipid accumulation triggers a rapid lack of VDAC phosphorylation by glycogen synthase kinase 3 (GSK3). Pharmacological and genetic manipulations proved GSK3 to be responsible for VDAC phosphorylation in normal cells. Notably, VDAC phosphorylation level correlated with steatosis severity in patients.

CONCLUSION

VDAC acts as an early sensor of lipid toxicity and its GSK3-mediated phosphorylation status controls outer mitochondrial membrane permeabilization in hepatosteatosis.

ANT-VDAC1 interaction is direct and depends on ANT isoform conformation in vitro

The voltage-dependent anion channel (VDAC) and the adenine nucleotide translocase (ANT) have central roles in mitochondrial functions such as nucleotides transport and cell death. The interaction between VDAC, an outer mitochondrial membrane protein and ANT, an inner membrane protein, was studied in isolated mitochondria and in vitro. Both proteins were isolated from various mitochondrial sources and reconstituted in vitro using a biomimetic system composed of recombinant human VDAC isoform 1 (rhVDAC1) immobilized on a surface plasmon resonance (SPR) sensor chip surface. Two enriched-preparations of (H)ANT (ANT from heart, mainly ANT1) and (L)ANT (ANT from liver, mainly ANT2) isoforms interacted differently with rhVDAC1. Moreover, the pharmacological ANT inhibitors atractyloside and bongkrekic acid modulated this interaction. Thus, ANT-VDAC interaction depends both on ANT isoform identity and on the conformation of ANT.

Inhibition of the Mitochondrial Permeability Transition for Cytoprotection: Direct versus Indirect Mechanisms

Mitochondria are fascinating organelles, which fulfill multiple cellular functions, as diverse as energy production, fatty acid β oxidation, reactive oxygen species (ROS) production and detoxification, and cell death regulation. The coordination of these functions relies on autonomous mitochondrial processes as well as on sustained cross-talk with other organelles and/or the cytosol. Therefore, this implies a tight regulation of mitochondrial functions to ensure cell homeostasis. In many diseases (e.g., cancer, cardiopathies, nonalcoholic fatty liver diseases, and neurodegenerative diseases), mitochondria can receive harmful signals, dysfunction and then, participate to pathogenesis. They can undergo either a decrease of their bioenergetic function or a process called mitochondrial permeability transition (MPT) that can coordinate cell death execution. Many studies present evidence that protection of mitochondria limits disease progression and severity. Here, we will review recent strategies to preserve mitochondrial functions via direct or indirect mechanisms of MPT inhibition. Thus, several mitochondrial proteins may be considered for cytoprotective-targeted therapies.

The role of VDAC in cell death: friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Construction and structural analysis of tethered lipid bilayer containing photosynthetic antenna proteins for functional analysis

The construction and structural analysis of a tethered planar lipid bilayer containing bacterial photosynthetic membrane proteins, light-harvesting complex 2 (LH2), and light-harvesting core complex (LH1-RC) is described and establishes this system as an experimental platform for their functional analysis. The planar lipid bilayer containing LH2 and/or LH1-RC complexes was successfully formed on an avidin-immobilized coverglass via an avidin-biotin linkage. Atomic force microscopy (AFM) showed that a smooth continuous membrane was formed there. Lateral diffusion of these membrane proteins, observed by a fluorescence recovery after photobleaching (FRAP), is discussed in terms of the membrane architecture. Energy transfer from LH2 to LH1-RC within the tethered membrane was observed by steady-state fluorescence spectroscopy, indicating that the tethered membrane can mimic the natural situation.

Field-effect detection using phospholipid membranes

The application of field-effect devices to biosensors has become an area of intense research interest. An attractive feature of field-effect sensing is that the binding or reaction of biomolecules can be directly detected from a change in electrical signals. The integration of such field-effect devices into cell membrane mimics may lead to the development of biosensors useful in clinical and biotechnological applications. This review summarizes recent studies on the fabrication and characterization of field-effect devices incorporating model membranes. The incorporation of black lipid membranes and supported lipid monolayers and bilayers into semiconductor devices is described.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

Uncovering the role of VDAC in the regulation of cell life and death

Proper cell activity requires an efficient exchange of molecules between mitochondria and cytoplasm. Lying in the outer mitochondrial membrane, VDAC assumes a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. As such, it has been recognized that VDAC plays a crucial role in regulating the metabolic and energetic functions of mitochondria. Indeed, down-regulation of VDAC1 expression by shRNA leads to a decrease in energy production and cell growth. VDAC has also been recognized as a key protein in mitochondria-mediated apoptosis through its involvement in the release of apoptotic proteins located in the inter-membranal space and as the proposed target of pro- and anti-apoptotic members of the Bcl2-family and of hexokinase. Questions, however, remain as to if and how VDAC mediates the transfer of apoptotic proteins from the inter-membranal space to the cytosol. The diameter of the VDAC pore is only about 2.5-3 nm, insufficient for the passage of a folded protein like cytochrome c. New work, however, suggests that pore formation involves the assembly of homo-oligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins, such as Bax. Thus, VDAC appears to represent a convergence point for a variety of cell survival and cell death signals. This review provides insight into the central role of VDAC in mammalian cell life and death, emphasizing VDAC function in the regulation of mitochondria-mediated apoptosis and, as such, its potential as a rational target for new therapeutics.

Mitoparan and target-selective chimeric analogues: membrane translocation and intracellular redistribution induces mitochondrial apoptosis

Mastoparan, and structurally-related amphipathic peptides, may induce cell death by augmentation of necrotic and/or apoptotic pathways. To more precisely delineate cytotoxic mechanisms, we determined that [Lys(5,8)Aib(10)]mastoparan (mitoparan) specifically induces apoptosis of U373MG and ECV304 cells, as demonstrated by endonuclease and caspase-3 activation and phosphatidylserine translocation. Live cell imaging confirmed that, following translocation of the plasma membrane, mitoparan specifically co-localizes with mitochondria. Complementary studies indicated that mitoparan induces swelling and permeabilization of isolated mitochondria, through cooperation with a protein of the permeability transition pore complex VDAC, leading to the release of the apoptogenic factor, cytochrome c. N-terminal acylation of mitoparan facilitated the synthesis of chimeric peptides that incorporated target-specific address motifs including an integrin-specific RGD sequence and a Fas ligand mimetic. Significantly, these sychnologically-organised peptides demonstrated further enhanced cytotoxic potencies. We conclude that the cell penetrant, mitochondriotoxic and apoptogenic properties of mitoparan, and its chimeric analogues, offer new insights to the study and therapeutic induction of apoptosis.

Biomimetic tethered lipid membranes designed for membrane-protein interaction studies

The complexity of the biological membranes restricts their direct investigation at the nanoscale. Lipid bilayer membranes have been developed as a model of biological membranes in order to allow the interaction and insertion of peptides and membrane proteins in a functional manner. Promising models have been developed in the past two decades and tethered bilayer design traduces constant improvement of membrane models. The formation of protein free solid tethered membranes can be achieved by direct vesicle fusion, Langmuir-Blodgett, Langmuir-Schaffer transfers, self assembly of various building blocks such as thiol on gold, silane on quartz, grafting of polymers, as well as ligand receptor recognition. In this review, the current state of different tethered bilayer membrane will be described. We will focus on critical analysis of the main advantages/drawbacks of each kind of model construction and their ability to allow protein incorporation in non-denaturing conditions. Some of the current drawbacks encountered in these biomimetic models can be overcome using an innovative tethered bilayer design based on a reliable and fast formation method. The successful protein incorporation of the Adenylate Cyclase produced by Bordetella pertussis and the voltage dependent anion channel (VDAC) was demonstrated on this model.

Surface Response Methodology for the Study of Supported Membrane Formation

We report on the investigations of the formation of the tethered lipid bilayer by vesicle deposition on amine-functionalized surfaces. The tethered bilayer was created by the deposition of egg-PC vesicles containing 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly-(ethyleneglycol)-N-hydroxysuccinimide as anchoring molecules on an amine-coated surface. This approach is an easy route for the formation of a biomimetic-supported membrane. A Doelhert experimental design was applied to determine the conditions leading to the formation of a continuous and defect-free tethered bilayer on different surfaces (gold and glass). Doehlert designs allow modeling of the experimental responses by second-order polynomial equations as a function of experimental factors. Four factors expected to influence bilayer formation were studied: the lipid concentration in the vesicle suspension, the mass percentage of anchoring molecules in the vesicles, the contact time between the vesicles and the surface, and the resting time of the membrane after buffer rinse. The optimization of the membrane preparation parameters was achieved by monitoring lipid assembly formation using surface plasmon resonance spectroscopy on gold and by fluorescence recovery after photobleaching on glass. Three characteristic responses were systematically measured: the bilayer thickness, the lipid diffusion coefficient, and the lipid mobile fraction. The simultaneous inspection of the three characteristics revealed that a restricted experimental domain leads to properties that are in accordance with a bilayer presence. The factors of this domain are a lipid concentration from 0.1 to 1 mg/mL, 4-8% of anchoring molecules in the vesicles, 1-4 h of contact time between vesicles and surface, and 21-24 h of resting time after buffer rinse. Under these conditions, a membrane having a lipid mass per surface between 545 +/- 5 and 590 +/- 10 ng/cm2, a diffusion coefficient of between 2.5 +/- 0.3 x 10(-8) and 3.60 +/- 0.5 x 10(-8) cm2/s, and a mobile fraction between 94 +/- 2 and 99 +/- 1% was formed. These findings were confirmed by atomic force microscopy observations, which showed the presence of a continuous and homogeneous bilayer in the determined experimental domain. This formation procedure presents many advantages; it provides an easily obtainable biomimetic membrane model for proteins studies and offers a versatile tethered bilayer because it can be adapted easily to various types of supports.