Light on the interactions between nanoparticles and lipid membranes by interface-sensitive vibrational spectroscopy

Nanoparticles are produced in natural phenomena or synthesized artificially for technological applications. Their frequent contact with humans has been judged potentially harmful for health, and numerous studies are ongoing to understand the mechanisms of the toxicity of nanoparticles. At the macroscopic level, the toxicity can be established in vitro or in vivo by measuring the survival of cells. At the sub-microscopic level, scientists want to unveil the molecular mechanisms of the first interactions of nanoparticles with cells via the cell membrane, before the toxicity cascades within the whole cell. Unveiling a molecular understanding of the nanoparticle-membrane interface is a tricky challenge, because of the chemical complexity of this system and its nanosized dimensions buried within bulk macroscopic environments. In this review, we highlight how, in the last 10 years, second-order nonlinear optical (NLO) spectroscopy, and specifically vibrational sum frequency generation (SFG), has provided a new understanding of the structural, physicochemical, and dynamic properties of these biological interfaces, with molecular sensitivity. We will show how the intrinsic interfacial sensitivity of second-order NLO and the chemical information of vibrational SFG spectroscopy have revealed new knowledge of the molecular mechanisms that drive nanoparticles to interact with cell membranes, from both sides, the nanoparticles and the membrane properties.

Viscosity-Sensitive Membrane Dyes as Tools To Estimate the Crystalline Structure of Lipid Bilayers

Lipid membranes are crucial for cellular integrity and regulation, and tight control of their structural and mechanical properties is vital to ensure that they function properly. Fluorescent probes sensitive to the membrane's microenvironment are useful for investigating lipid membrane properties; however, there is currently a lack of quantitative correlation between the exact parameters of lipid organization and a readout from these dyes. Here, we investigate this relationship for "molecular rotors", or microviscosity sensors, by simultaneously measuring their fluorescence lifetime to determine the membrane viscosity, while using X-ray diffraction to determine the membrane's structural properties. Our results reveal a phase-dependent correlation between the membrane's structural parameters and mechanical properties measured by a BODIPY-based molecular rotor, giving excellent predictive power for the structural descriptors of the lipid bilayer. We also demonstrate that differences in membrane thickness between different lipid phases are not a prerequisite for the formation of lipid microdomains and that this requirement can be disrupted by the presence of line-active molecules. Our results underpin the use of membrane-sensitive dyes as reporters of the structure of lipid membranes.

Inducible nitric oxide synthase (iNOS) is necessary for GBP-mediated T. gondii restriction in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular, protozoan pathogen of rodents and humans. T. gondii's ability to grow within cells and evade cell-autonomous immunity depends on the integrity of the parasitophorous vacuole (PV). Interferon-inducible guanylate binding proteins (GBPs) are central mediators of T. gondii clearance, however, the precise mechanism linking GBP recruitment to the PV and T. gondii restriction is not clear. This knowledge gap is linked to heterogenous GBP-targeting across a population of vacuoles and the lack of tools to selectively purify the intact PV. To identify mediators of parasite clearance associated with GBP2-positive vacuoles, we employed a novel protein discovery tool automated spatially targeted optical micro proteomics (autoSTOMP). This approach identified inducible nitric oxide synthetase (iNOS) enriched at levels similar to the GBPs in infected bone marrow-derived myeloid cells. iNOS expression on myeloid cells was necessary for mice to control T. gondii growth in vivo and survive acute infection. T. gondii infection of IFNγ-primed macrophage was sufficient to robustly induce iNOS expression. iNOS restricted T. gondii infection through nitric oxide synthesis rather than arginine depletion, leading to robust and selective nitration of the PV. Optimal parasite restriction by iNOS and vacuole nitration depended on the chromosome 3 GBPs. Notably, GBP2 recruitment and ruffling of the PV membrane occurred in iNOS knockouts, however, these vacuoles contained dividing parasites. iNOS activity was necessary for the collapse of the intravacuolar network of nanotubular membranes which connects parasites to each other and the host cytosol. Based on these data we conclude reactive nitrogen species generated by iNOS cooperate with the chromosome 3 GBPs to target distinct biology of the PV that are necessary for optimal parasite clearance in murine myeloid cells.

The peripherally membrane-attached protein MbFACL6 of Mycobacterium tuberculosis activates a broad spectrum of substrates

The infectious disease tuberculosis is one of the fifteen most common causes of death worldwide (according to the WHO). About every fourth person is infected with the main causative agent Mycobacterium tuberculosis (Mb). A characteristic of the pathogen is its entrance into a dormant state in which a phenotypic antibiotic resistance is achieved. To target resistant strains, novel dormancy-specific targets are very promising. Such a possible target is the Mb "fatty acid-CoA ligase 6" (MbFACL6), which activates fatty acids and thereby modulates the accumulation of triacylglycerol-containing lipid droplets that are used by Mb as an energy source during dormancy. We investigated the membrane association of MbFACL6 in E. coli and its specific activity towards different substrates after establishing a novel MbFACL6 activity assay. Despite a high homology to the mammalian family of fatty acid transport proteins, which are typically transmembrane proteins, our results indicate that MbFACL6 is a peripheral membrane-attached protein. Furthermore, MbFACL6 tolerates a broad spectrum of substrates including saturated and unsaturated fatty acids (C12-C20), some cholic acid derivatives, and even synthetic fatty acids, such as 9(E)-nitrooleicacid. Therefore, the substrate selectivity of MbFACL6 appears to be much broader than previously assumed.

Nitrated Fatty-Acids Distribution in Storage Biomolecules during Arabidopsis thaliana Development

The non-enzymatic interaction of polyunsaturated fatty acids with nitric oxide (NO) and derived species results in the formation of nitrated fatty acids (NO₂-FAs). These signaling molecules can release NO, reversibly esterify with complex lipids, and modulate protein function through the post-translational modification called nitroalkylation. To date, NO₂-FAs act as signaling molecules during plant development in plant systems and are involved in defense responses against abiotic stress conditions. In this work, the previously unknown storage biomolecules of NO₂-FAs in Arabidopsis thaliana were identified. In addition, the distribution of NO₂-FAs in storage biomolecules during plant development was determined, with phytosterol esters (SE) and TAGs being reservoir biomolecules in seeds, which were replaced by phospholipids and proteins in the vegetative, generative, and senescence stages. The detected esterified NO₂-FAs were nitro-linolenic acid (NO₂-Ln), nitro-oleic acid (NO₂-OA), and nitro-linoleic acid (NO₂-LA). The last two were detected for the first time in Arabidopsis. The levels of the three NO₂-FAs that were esterified in both lipid and protein storage biomolecules showed a decreasing pattern throughout Arabidopsis development. Esterification of NO₂-FAs in phospholipids and proteins highlights their involvement in both biomembrane dynamics and signaling processes, respectively, during Arabidopsis plant development.

Nitro-Oleic Acid-Mediated Nitroalkylation Modulates the Antioxidant Function of Cytosolic Peroxiredoxin Tsa1 during Heat Stress in Saccharomyces cerevisiae

Heat stress is one of the abiotic stresses that leads to oxidative stress. To protect themselves, yeast cells activate the antioxidant response, in which cytosolic peroxiredoxin Tsa1 plays an important role in hydrogen peroxide removal. Concomitantly, the activation of the heat shock response (HSR) is also triggered. Nitro-fatty acids are signaling molecules generated by the interaction of reactive nitrogen species with unsaturated fatty acids. These molecules have been detected in animals and plants. They exert their signaling function mainly through a post-translational modification called nitroalkylation. In addition, these molecules are closely related to the induction of the HSR. In this work, the endogenous presence of nitro-oleic acid (NO₂-OA) in Saccharomyces cerevisiae is identified for the first time by LC-MS/MS. Both hydrogen peroxide levels and Tsa1 activity increased after heat stress with no change in protein content. The nitroalkylation of recombinant Tsa1 with NO₂-OA was also observed. It is important to point out that cysteine 47 (peroxidatic) and cysteine 171 (resolving) are the main residues responsible for protein activity. Moreover, the in vivo nitroalkylation of Tsa1 peroxidatic cysteine disappeared during heat stress as the hydrogen peroxide generated in this situation caused the rupture of the NO₂-OA binding to the protein and, thus, restored Tsa1 activity. Finally, the amino acid targets susceptible to nitroalkylation and the modulatory effect of this PTM on the enzymatic activity of Tsa1 are also shown in vitro and in vivo. This mechanism of response was faster than that involving the induction of genes and the synthesis of new proteins and could be considered as a key element in the fine-tuning regulation of defence mechanisms against oxidative stress in yeast.

Cubosomal lipid formulation of nitroalkene fatty acids: Preparation, stability and biological effects

Lipid nitroalkenes – nitro-fatty acids (NO₂–FAs) are formed in vivo via the interaction of reactive nitrogen species with unsaturated fatty acids. The resulting electrophilic NO₂–FAs play an important role in redox homeostasis and cellular stress response. This study investigated the physicochemical properties and reactivity of two NO₂–FAs: 9/10-nitrooleic acid (1) and its newly prepared 1-monoacyl ester, (E)-2,3-hydroxypropyl 9/10-nitrooctadec-9-enoate (2), both synthesized by a direct radical nitration approach. Compounds 1 and 2 were investigated in an aqueous medium and after incorporation into lipid nanoparticles prepared from 1-monoolein, cubosomes 1@CUB and 2@CUB. Using an electrochemical analysis and LC-MS, free 1 and 2 were found to be unstable under acidic conditions, and their degradation occurred in an aqueous environment within a few minutes or hours. This degradation was associated with the production of the NO radical, as confirmed by fluorescence assay. In contrast, preparations 1@CUB and 2@CUB exhibited a significant increase in the stability of the loaded 1 and 2 up to several days to weeks. In addition to experimental data, density functional theory-based calculation results on the electronic structure and structural variability (open and closed configuration) of 1 and 2 were obtained. Finally, experiments with a human HaCaT keratinocyte cell line demonstrated the ability of 1@CUB and 2@CUB to penetrate through the cytoplasmic membrane and modulate cellular pathways, which was exemplified by the Keap1 protein level monitoring. Free 1 and 2 and the cubosomes prepared from them showed cytotoxic effect on HaCaT cells with IC₅₀ values ranging from 1 to 8 μM after 24 h. The further development of cubosomal preparations with embedded electrophilic NO₂–FAs may not only contribute to the field of fundamental research, but also to their application using an optimized lipid delivery vehicle.

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Nitro-fatty acids (NO₂–FAs) are bioactive electrophiles and new drug candidates.

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The study focused on endogenous NO₂-oleic acid and its glycerol ester.

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Cubosomes are lipid nanoparticles stabilizing the incorporated NO₂–FAs.

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Applicability of NO₂-FA-loaded cubosomes was tested on human HaCaT keratinocytes.

Synthetic Artificial Apoptosis-Inducing Receptor for On-Demand Deactivation of Engineered Cells

The design of a fully synthetic, chemical "apoptosis-inducing receptor" (AIR) molecule is reported that is anchored into the lipid bilayer of cells, is activated by the incoming biological input, and responds with the release of a secondary messenger-a highly potent toxin for cell killing. The AIR molecule has four elements, namely, an exofacial trigger group, a bilayer anchor, a toxin as a secondary messenger, and a self-immolative scaffold as a mechanism for signal transduction. Receptor installation into cells is established via a robust protocol with minimal cell handling. The synthetic receptor remains dormant in the engineered cells, but is effectively triggered externally by the addition of an activating biomolecule (enzyme) or in a mixed cell population through interaction with the surrounding cells. In 3D cell culture (spheroids), receptor activation is accessible for at least 5 days, which compares favorably with other state of the art receptor designs.

Direct Evidence That Mutations within Dysferlin's C2A Domain Inhibit Lipid Clustering

Mechanical stress on sarcolemma can create small tears in the muscle cell membrane. Within the sarcolemma resides the multidomain dysferlin protein. Mutations in this protein render it unable to repair the sarcolemma and have been linked to muscular dystrophy. A key step in dysferlin-regulated repair is the binding of the C2A domain to the lipid membrane upon increased intracellular calcium. Mutations mapped to this domain cause loss of binding ability of the C2A domain. There is a crucial need to understand the geometry of dysferlin C2A at a membrane interface as well as cell membrane lipid reorientation when compared to that of a mutant. Here, we describe a comparison between the wild-type dysferlin C2A and a mutation to the conserved aspartic acids in the domain binding loops. To identify both the geometry and the cell membrane lipid reorientation, we applied sum frequency generation (SFG) vibrational spectroscopy and coupled it with simulated SFG spectra to observe and quantify the interaction with a model cell membrane composed of phosphotidylserine and phosphotidylcholine. Observed changes in surface pressure demonstrate that calcium-bridged electrostatic interactions govern the initial interaction of the C2A domains docking with a lipid membrane. SFG spectra taken from the amide-I region for the wild type and variant contain features near 1642, 1663, and 1675 cm^-1 related to the C2A domain β-sandwich secondary structure, indicating that the domain binds in a specific orientation. Mapping simulated SFG spectra to the experimentally collected spectra indicated that both wild-type and variant domains have nearly the same orientation to the membrane surface. However, examining the ordering of the lipids that make up a model membrane using SFG, we find that the wild type clusters the lipids as seen by the increase in the ratio of the CD₃ and CD₂ symmetric intensities by 170% for the wild type and by 120% for the variant. This study highlights the capabilities of SFG to probe with great detail biological mutations in proteins at cell membrane interfaces.

Electrophilic characteristics and aqueous behavior of fatty acid nitroalkenes

Fatty acid nitroalkenes (NO2-FA) are endogenously-generated products of the reaction of metabolic and inflammatory-derived nitrogen dioxide (.NO2) with unsaturated fatty acids. These species mediate signaling actions and induce adaptive responses in preclinical models of inflammatory and metabolic diseases. The nitroalkene substituent possesses an electrophilic nature, resulting in rapid and reversible reactions with biological nucleophiles such as cysteine, thus supporting post-translational modifications (PTM) of proteins having susceptible nucleophilic centers. These reactions contribute to enzyme regulation, modulation of inflammation and cell proliferation and the regulation of gene expression responses. Herein, focus is placed on the reduction-oxidation (redox) characteristics and stability of specific NO2-FA regioisomers having biological and clinical relevance; nitro-oleic acid (NO2-OA), bis-allylic nitro-linoleic acid (NO2-LA) and the conjugated diene-containing nitro-conjugated linoleic acid (NO2-cLA). Cyclic and alternating-current voltammetry and chronopotentiometry were used to the study of reduction potentials of these NO2-FA. R-NO2 reduction was observed around -0.8 V (vs. Ag/AgCl/3 M KCl) and is related to relative NO2-FA electrophilicity. This reduction process could be utilized for the evaluation of NO2-FA stability in aqueous milieu, shown herein to be pH dependent. In addition, electron paramagnetic resonance (EPR) spectroscopy was used to define the stability of the nitroalkene moiety under aqueous conditions, specifically under conditions where nitric oxide (.NO) release could be detected. The experimental data were supported by density functional theory calculations using 6-311++G (d,p) basis set and B3LYP functional. Based on experimental and computational approaches, the relative electrophilicities of these NO2-FA are NO2-cLA >> NO2-LA > NO2-OA. Micellarization and vesiculation largely define these biophysical characteristics in aqueous, nucleophile-free conditions. At concentrations below the critical micellar concentration (CMC), monomeric NO2-FA predominate, while at greater concentrations a micellar phase consisting of self-assembled lipid structures predominates. The CMC, determined by dynamic light scattering in 0.1 M phosphate buffer (pH 7.4) at 25 °C, was 6.9 (NO2-LA) 10.6 (NO2-OA) and 42.3 μM (NO2-cLA), respectively. In aggregate, this study provides new insight into the biophysical properties of NO2-FA that are important for better understanding the cell signaling and pharmacological potential of this class of mediators.

Lipid Bilayer Interactions of Peptidic Supramolecular Polymers and Their Impact on Membrane Permeability and Stability

The synthesis and physicochemical characterization of supramolecular polymers with tunable assembly profiles offer exciting opportunities, involving the development of new biomedical carriers. Because synthetic nanocarriers aim to transport substances across or toward cellular membranes, we evaluated the interactions of amphiphilic peptide-based supramolecular polymers with lipid bilayers. Here, we focused on nanorod-like supramolecular polymers, obtained from two C₃-symmetric dendritic peptide amphiphiles with alternating Phe/His sequences, equipped with a peripheral tetraethylene glycol dendron (C3-PH) or charged ethylenediamine end groups (C3-PH+). Triggered by pH changes, these amphiphiles assemble reversibly. Our results show that the supramolecular polymers have an impact on the lipid order in model membranes. Changes in the lipid order were observed depending on the charge state of the amphiphilic building blocks, as well as the chemical composition and physical properties of the bilayer. Furthermore, we further performed cell viability assays with the C3-PH+ and C3-PH supramolecular polymers. For C3-PH, the cell viability and extent of proliferation were decreased and the membrane permeability was enhanced, indicating a strong interaction of the polymer with cellular membranes. The results have implications for the design of novel pH-switchable supramolecular drug carriers and delivery vehicles that can respond to an altered microenvironment of tumorous or inflamed tissue.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Otoferlin C2F Domain-Induced Changes in Membrane Structure Observed by Sum Frequency Generation

Proteins that contain C2 domains are involved in a variety of biological processes, including encoding of sound, cell signaling, and cell membrane repair. Of particular importance is the interface activity of the C-terminal C2F domain of otoferlin due to the pathological mutations known to significantly disrupt the protein's lipid membrane interface binding activity, resulting in hearing loss. Therefore, there is a critical need to define the geometry and positions of functionally important sites and structures at the otoferlin-lipid membrane interface. Here, we describe the first in situ probe of the protein orientation of otoferlin's C2F domain interacting with a cell membrane surface. To identify this protein's orientation at the lipid interface, we applied sum frequency generation (SFG) vibrational spectroscopy and coupled it with simulated SFG spectra to observe and quantify the otoferlin C2F domain interacting with model lipid membranes. A model cell membrane was built with equal amounts of phosphatidylserine and phosphatidylcholine. SFG measurements of the lipids that make up the model membrane indicate a 62% increase in amplitude from the SFG signal near 2075 cm^-1 upon protein interaction, suggesting domain-induced changes in the orientation of the lipids and possible membrane curvature. This increase is related to lipid ordering caused by the docking interaction of the otoferlin C2F domain. SFG spectra taken from the amide-I region contain features near 1630 and 1670 cm^-1 related to the C2F domains beta-sandwich secondary structure, thus indicating that the domain binds in a specific orientation. By mapping the simulated SFG spectra to the experimentally collected SFG spectra, we found the C2F domain of otoferlin orients 22° normal to the lipid surface. This information allows us to map what portion of the domain directly interacts with the lipid membrane.

Profile of Phosphatidylserine Modifications under Nitroxidative Stress Conditions Using a Liquid Chromatography-Mass Spectrometry Based Approach

Nitrated lipids have been detected in vitro and in vivo, usually associated with a protective effect. While nitrated fatty acids have been widely studied, few studies reported the nitration and nitroxidation of the phospholipid classes phosphatidylcholine, and phosphatidylethanolamine. However, no information regarding nitrated and nitroxidized phosphatidylserine can be found in the literature. This work aims to identify and characterize the nitrated and nitroxidized derivatives of 1-palmitoyl-2-oleoyl-sn-3-glycero-phosphoserine (POPS), obtained after incubation with nitronium tetrafluoroborate, by liquid chromatography (LC) coupled to mass spectrometry (MS) and tandem MS (MS/MS). Several nitrated and nitroxidized products were identified, namely, nitro, nitroso, nitronitroso, and dinitro derivatives, as well as some nitroxidized species such as nitrosohydroxy, nitrohydroxy, and nitrohydroperoxy. The fragmentation pathways identified were structure-dependent and included the loss of HNO and HNO₂ for nitroso and nitro derivatives, respectively. Combined losses of PS polar head group plus HNO or HNO₂ and carboxylate anions of modified fatty acyl chain were also observed. The nitrated POPS also showed antiradical potential, demonstrated by the ability to scavenge the ABTS^●+ and DPPH^● radicals. Overall, this in vitro model of nitration based on LC-MS/MS provided additional insights into the nitrated and nitroxidized derivatives of PS and their fragmentation fingerprinting. This information is a valuable tool for targeted analysis of these modified PS in complex biological samples, to further explore the new clues on the antioxidant potential of nitrated POPS.

Nitrated fatty acids in cardiovascular diseases

Cardiovascular disease (CVD) is the leading cause of death and accounts for one third of disease-related mortality worldwide. Dysregulated redox mechanisms, in particular the formation of reactive oxygen species (ROS) play a pivotal pathogenetic role in CVD. Nitro-fatty acids (NO2-FAs) are electrophilic molecules which have a NO2-group bound to one of their olefinic carbons. They are endogenously formed by the reaction of reactive nitrogen species with unsaturated fatty acids. Basal levels of NO2-FAs are in the low nanomolar range and higher concentrations can be encountered under acidic (stomach) and inflammatory (e.g. ischemia/reperfusion) conditions. Dietary intake of polyunsaturated fatty acids in combination with nitrites raises circulating NO2-FAs to a clinically relevant level in mice. NO2-FAs undergo reversible covalent binding to cysteine residues and by virtue of these posttranslational protein modifications act as potent anti-inflammatory signaling mediators via modulation of various critical pathways like nuclear factor E2-related factor 2 (Nrf2)- and peroxisome proliferator-activated receptor γ (PPARγ) activation, nuclear factor-kappa B (NF-κB) inhibition and hem oxygenase-1 (HO-1)- and heat shock protein (HSP) induction. In this review article, we summarize recent findings about the effects and underlying molecular mechanisms of NO2-FAs from a variety of pre-clinical cardiovascular disease models. The described findings suggest the potential of NO2-FAs to emerge as therapeutic agents with a broad range of potential clinical applications for CVD.