Mitochondrial creatine kinase adsorption to biomimetic membranes: a Langmuir monolayer study

PFAS fluidize synthetic and bacterial lipid monolayers based on hydrophobicity and lipid charge

Poly- and Perfluoroalkyl substances (PFASs) are pollutants of emerging concern that persist in nature and pose environmental health and safety risks. PFAS disrupt biological membranes resulting in cellular inhibition, but the mechanism of disruption and the role of lipid composition remain unclear. We examine the role of phospholipid saturation and headgroup charge on the interactions between PFASs and phospholipid monolayers comprised of synthetic phosphocholine (PC) and phosphoglycerol (PG) lipids and prepared from bacteria membrane extracts rich in PG lipids from an environmentally relevant marine bacterium Alcanivorax borkumensis. When deposited on a buffered subphase containing PFAS, PFAS mixed within and fluidized zwitterionic and net-anionic monolayers leading to increases in monolayer compressibility that were driven by a combination of PFAS hydrophobicity and monolayer charge density. Differences in the monolayer response using saturated or unsaturated lipids are attributed to the ability of the unsaturated lipids to accommodate PFAS within 'void space' arising from the bent lipid tails. Similar fluidization and compressibility behavior were also observed in A. borkumensis lipid monolayers. This work provides new insight into PFAS partitioning into bacterial membranes and the effect PFAS have on the physicomechanical properties of zwitterionic and charged lipid monolayers.

Behaviour of protein (BSA)-lipid (DMPA) mixed monolayer on the spreading order of the individual component

Surface pressure (π) - mean molecular area (A) isotherms of protein (BSA) - lipid (DMPA) mixed films are examined by varying their ratio and altering the spreading order of BSA and DMPA on the water surface to study the protein-lipid interactions and the corresponding structures and patterns at different interfacial conditions. π-A isotherms and compression-decompression isotherm cycles of protein-lipid mixed monolayers below and above of the isoelectric point of BSA (pI ≈ 4.8) are also examined. Below the isoelectric point of BSA (pH ≈ 4.0), i.e., when BSA is weakly hydrophobic and has net positive charge shows low hysteresis irrespective of the spreading order of the molecules. However, at pH ≈ 7.0, i.e., when the overall charge of BSA is negative and is strongly hydrophobic the protein-lipid mixed films display higher hysteresis value. Besides the properties of the isotherms, the surface morphology and secondary conformations of protein inside the mixed films are obtained from X-ray reflectivity, atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy respectively after depositing the mixed films on solid substrates. Nearly similar information is obtained after altering the spreading order of BSA and DMPA, which indicates that the spreading of molecules on the water surface is one of the better ways of forming the lipid-protein mixed film at the air-water interface.

Evidence of proteolipid domain formation in an inner mitochondrial membrane mimicking model

BACKGROUND

Mitochondrial creatine kinase (mtCK) is highly abundant in mitochondria; its quantity is equimolecular to the Adenylic Nucleotide Translocator and represents 1% of the mitochondrial proteins. It is a multitask protein localized in the mitochondria intermembrane space where it binds to the specific cardiolipin (CL) phospholipid. If mtCK was initially thought to be exclusively implicated in energy transfer between mitochondria and cytosol through a mechanism referred to as the phosphocreatine shuttle, several recent studies suggested an additional role in maintaining mitochondria membrane structure.

METHODS

To further characterized mtCK binding process we used multiphoton excitation fluorescence microscopy coupled with Giant Unilamellar Vesicles (GUV) and laurdan as fluorescence probe.

RESULTS

We gathered structural and dynamical information on the molecular events occurring during the binding of mtCK to the mitochondria inner membrane. We present the first visualization of mtCK-induced CL segregation on a bilayer model forming micrometer-size proteolipid domains at the surface of the GUV. Those microdomains, which only occurred when CL is included in the lipid mixture, were accompanied by the formation of protein multimolecular assembly, vesicle clamping, and changes in both vesicle curvature and membrane fluidity CONCLUSION: Those results highlighted the importance of the highly abundant mtCK in the lateral organization of the mitochondrial inner membrane.

GENERAL SIGNIFICANCE

Microdomains were induced in mitochondria-mimicking membranes composed of natural phospholipids without cholesterol and/or sphingolipids differing from the proposed cytoplasmic membrane rafts. Those findings as well as membrane curvature modification were discussed in relation with protein-membrane interaction and protein cluster involvement in membrane morphology.

Two-Step Membrane Binding of NDPK-B Induces Membrane Fluidity Decrease and Changes in Lipid Lateral Organization and Protein Cluster Formation

Nucleoside diphosphate kinases (NDPKs) are crucial elements in a wide array of cellular physiological or pathophysiological processes such as apoptosis, proliferation, or metastasis formation. Among the NDPK isoenzymes, NDPK-B, a cytoplasmic protein, was reported to be associated with several biological membranes such as plasma or endoplasmic reticulum membranes. Using several membrane models (liposomes, lipid monolayers, and supported lipid bilayers) associated with biophysical approaches, we show that lipid membrane binding occurs in a two-step process: first, initiation by a strong electrostatic adsorption process and followed by shallow penetration of the protein within the membrane. The NDPK-B binding leads to a decrease in membrane fluidity and formation of protein patches. The ability of NDPK-B to form microdomains at the membrane level may be related to protein-protein interactions triggered by its association with anionic phospholipids. Such accumulation of NDPK-B would amplify its effects in functional platform formation and protein recruitment at the membrane.

Protein "amyloid-like" networks at the phospholipid membrane formed by 4-hydroxy-2-nonenal-modified mitochondrial creatine kinase

4-Hydroxy-2-nonenal (4-HNE) is a reactive aldehyde and a lipid peroxidation product formed in biological tissues under physiological and pathological conditions. Its concentration increases with oxidative stress and induces deleterious modifications of proteins and membranes. Mitochondrial and cytosolic isoforms of creatine kinase were previously shown to be affected by 4-HNE. In the present study, we analyzed the effect of 4-HNE on mitochondrial creatine kinase, an abundant protein from the mitochondrial intermembrane space with a key role in mitochondrial physiology. We show that this effect is double: 4-HNE induces a step-wise loss of creatine kinase activity together with a fast protein aggregation. Protein-membrane interaction is affected and amyloid-like networks formed on the biomimetic membrane. These fibrils may disturb mitochondrial organisation both at the membrane and in the inter membrane space.

Serum albumin in 2D: a Langmuir monolayer approach

Understanding of protein interaction at the molecular level raises certain difficulties which is the reason a model membrane system such as the Langmuir monolayer technique was developed. Ubiquitous proteins such as serum albumin comprise 50% of human blood plasma protein content and are involved in many biological functions. The important nature of this class of protein demands that it be studied in detail while modifying the experimental conditions in two dimensions to observe it in all types of environments. While different from bulk colloidal solution work, the two dimensional approach allows for the observation of the interaction between molecules and subphase at the air-water interface. Compiled in this review are studies which highlight the characterization of this protein using various surroundings and also observing the types of interactions it would have when at the biomembrane interface. Free-energy changes between molecules, packing status of the bulk analyte at the interface as well as phase transitions as the monolayer forms a more organized or aggregated state are just some of the characteristics which are observed through the Langmuir technique. This unique methodology demonstrates the chemical behavior and physical behavior of this protein at the phase boundary throughout the compression of the monolayer.

Binding of a ruthenium complex to a thioether ligand embedded in a negatively charged lipid bilayer: a two-step mechanism

The interaction between the ruthenium polypyridyl complex [Ru(terpy)(dcbpy)(H2O)](2+) (terpy = 2,2';6',2"-terpyridine, dcbpy = 6,6'-dichloro-2,2'-bipyridine) and phospholipid membranes containing either thioether ligands or cholesterol were investigated using UV-visible spectroscopy, Langmuir-Blodgett monolayer surface pressure measurements, and isothermal titration calorimety (ITC). When embedded in a membrane, the thioether ligand coordinated to the dicationic metal complex only when the phospholipids of the membrane were negatively charged, that is, in the presence of attractive electrostatic interaction. In such a case coordination is much faster than in homogeneous conditions. A two-step model for the coordination of the metal complex to the membrane-embedded sulfur ligand is proposed, in which adsorption of the complex to the negative surface of the monolayers or bilayers occurs within minutes, whereas formation of the coordination bond between the surface-bound metal complex and ligand takes hours. Finally, adsorption of the aqua complex to the membrane is driven by entropy. It does not involve insertion of the metal complex into the hydrophobic lipid layer, but rather simple electrostatic adsorption at the water-bilayer interface.

Monolayer behavior of binary systems of betulinic acid and cardiolipin: thermodynamic analyses of Langmuir monolayers and AFM study of Langmuir-Blodgett monolayers

Betulinic acid (BA, a natural pentacyclic triterpene) can induce mitochondrial membrane damage and trigger the mitochondrial pathway of apoptosis in tumor cells. The monolayer behavior of binary systems of BA and cardiolipin (CL, a unique phospholipid found only in mitochondria membrane in animals) was studied by surface pressure-area (π-A) measurements and analyses and Atomic force microscopy (AFM) observation. The miscibility analysis presents that in mixed monolayers BA takes both tilted and nearly perpendicular orientations at surface pressure below 30 mN/m but only nearly perpendicular orientation at 30 mN/m. The thermodynamic stability analysis indicates that phase separation and repulsion occur in mixed BA/CL monolayers. The compressibility analysis shows that at 30 mN/m, 20% addition of BA does markedly translate the liquid-condensed CL monolayer to mixed BA/CL monolayer with the coexistence of liquid-condensed and liquid-expanded phases. The AFM images of supported monolayers give direct evidence of the conclusions obtained from the analyses of π-A isotherms. These results confirm that at high surface pressure near to real biologic situations, BA orients nearly perpendicularly with hydroxyl group toward water, causes phase separation and changes the permeability of CL film, which correlates with the mitochondrial membrane damage induced by BA.

Mitochondrial creatine kinase interaction with cardiolipin-containing biomimetic membranes is a two-step process involving adsorption and insertion

Mitochondrial creatine kinase (mtCK) binding to the mitochondrial inner membrane largely determines its biological functions in cellular energy homeostasis, mitochondrial physiology, and dynamics. The membrane binding mechanism is, however, not completely understood. Recent data suggest that a hydrophobic component is involved in mtCK binding to cardiolipin at the outer face of the inner mitochondrial membrane, in addition to the well known electrostatically driven process. In this manuscript, using an electrochemical method derived from alternating current polarography for differential capacity measurements, we distinctly reveal that protein-cardiolipin interaction has a two-step mechanism. For short incubation time, protein adsorption to the phospholipid charged headgroup was the only process detected, whereas on a longer time scale evidence of protein insertion was observed.

Active fragments from pro- and antiapoptotic BCL-2 proteins have distinct membrane behavior reflecting their functional divergence

BACKGROUND

The BCL-2 family of proteins includes pro- and antiapoptotic members acting by controlling the permeabilization of mitochondria. Although the association of these proteins with the outer mitochondrial membrane is crucial for their function, little is known about the characteristics of this interaction.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we followed a reductionist approach to clarify to what extent membrane-active regions of homologous BCL-2 family proteins contribute to their functional divergence. Using isolated mitochondria as well as model lipid Langmuir monolayers coupled with Brewster Angle Microscopy, we explored systematically and comparatively the membrane activity and membrane-peptide interactions of fragments derived from the central helical hairpin of BAX, BCL-xL and BID. The results show a connection between the differing abilities of the assayed peptide fragments to contact, insert, destabilize and porate membranes and the activity of their cognate proteins in programmed cell death.

CONCLUSION/SIGNIFICANCE

BCL-2 family-derived pore-forming helices thus represent structurally analogous, but functionally dissimilar membrane domains.

Same system-different results: the importance of protein-introduction protocols in Langmuir-monolayer studies of lipid-protein interactions

For studies of protein-lipid interactions, thin films at the air-water surface are often employed as model systems for cell membranes. A convenient manner in which to study these interactions is the Langmuir technique, which allows for formation of monolayer phospholipid films together with a choice of where and how to introduce proteins, according to the desired response variable. Here, a distinction has been made between different interaction protocols and it is also commented upon to what extent introduction of protein to a solution prior to spreading of a lipid film affects the results. This paper describes commonly used methods when working with Langmuir monolayers as membrane mimics and compares the results of four different experimental protocols: formation of a lipid film on top of a protein-containing subphase, injection of protein under an existing, semicompressed phospholipid film (surface pressure 5 mN/m), and deposition of a protein solution on top of a lipid film contained at either surface pressure 0 mN/m or at surface pressure 5 mN/m. Results obtained from Langmuir isotherms and Brewster angle microscope clearly differentiate between these methods and give insight into under which conditions and at which interfaces the protein interactions are predominant (protein-air or protein-lipid).

Morphology modifications in negatively charged lipid monolayers upon mitochondrial creatine kinase binding

Mitochondrial creatine kinase (mtCK) may participate to membrane organization at the mitochondrial level by modulating lipid state and fluidity. The effect of the protein on lipid phase behaviour of different acyl chain length phosphatidylglycerol monolayers was analyzed from pressure-area isotherms and from the compressional modulus variation with respect to the surface pressure. Monolayer morphology was visualized by Brewster angle microscopy. No condensation effect was visible on dimyristoylphosphatidylglycerol (DMPG). For the other PG monolayers tested, dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylglycerol (DSPG), mtCK facilitated the formation of a liquid condensed phase. The effect depended on the surface pressure at which transition phase occurred. The effect of mtCK was more pronounced for tetramyristoylcardiolipin (TMCL) monolayers, as liquid condensed regions appeared 10 mN/m below the transition phase of the pure TMCL monolayer. The observed domains were circular and rather uniform, indicating a stabilization of the condensed phase. The same effect, namely an overall condensation of the monolayer with formation of circular domains, was observed upon protein injection beneath TMCL monolayers in different condensation states at constant area. MtCK ability to induce and stabilize a LC phase on monolayers could have important consequences in membrane organization and emphasize its structural role at mitochondrial level.

Dibucaine effects on structural and elastic properties of lipid bilayers

In this work we report the interaction effects of the local anesthetic dibucaine (DBC) with lipid patches in model membranes by Atomic Force Microscopy (AFM). Supported lipid bilayers (egg phosphatidylcholine, EPC and dimyristoylphosphatidylcholine, DMPC) were prepared by fusion of unilamellar vesicles on mica and imaged in aqueous media. The AFM images show irregularly distributed and sized EPC patches on mica. On the other hand DMPC formation presents extensive bilayer regions on top of which multibilayer patches are formed. In the presence of DBC we observed a progressive disruption of these patches, but for DMPC bilayers this process occurred more slowly than for EPC. In both cases, phase images show the formation of small structures on the bilayer surface suggesting an effect on the elastic properties of the bilayers when DBC is present. Dynamic surface tension and dilatational surface elasticity measurements of EPC and DMPC monolayers in the presence of DBC by the pendant drop technique were also performed, in order to elucidate these results. The curve of lipid monolayer elasticity versus DBC concentration, for both EPC and DMPC cases, shows a maximum for the surface elasticity modulus at the same concentration where we observed the disruption of the bilayer by AFM. Our results suggest that changes in the local curvature of the bilayer induced by DBC could explain the anesthetic action in membranes.