Dengue and Zika virus capsid proteins bind to membranes and self-assemble into liquid droplets with nucleic acids.

Dengue virus (DENV) and Zika virus (ZIKV) capsid proteins efficiently recruit and surround the viral RNA at the endoplasmic reticulum (ER) membrane to yield nascent viral particles. However, little is known either about the molecular mechanisms by which multiple copies of capsid proteins assemble into nucleocapsids (NCs) or how the NC is recruited and wrapped by the ER membrane during particle morphogenesis. Here, we measured relevant interactions concerning this viral process using purified DENV and ZIKV capsid proteins, membranes mimicking the ER lipid composition, and nucleic acids in in vitro conditions to understand the biophysical properties of the RNA genome encapsidation process. We found that both ZIKV and DENV capsid proteins bound to liposomes at liquid-disordered phase regions, docked exogenous membranes, and RNA molecules. Liquid-liquid phase separation is prone to occur when positively charged proteins interact with nucleic acids, which is indeed the case for the studied capsids. We characterized these liquid condensates by measuring nucleic acid partition constants and the extent of water dipolar relaxation, observing a cooperative process for the formation of the new phase that involves a distinct water organization. Our data support a new model in which capsid-RNA complexes directly bind the ER membrane, seeding the process of RNA recruitment for viral particle assembly. These results contribute to our understanding of the viral NC formation as a stable liquid-liquid phase transition, which could be relevant for dengue and Zika gemmation, opening new avenues for antiviral intervention.

Bioactivity and action mechanism of green propolis against Pythium aphanidermatum.

We have established how natural compounds from green propolis collected by the species Apis mellifera act against the growth of Pythium aphanidermatum. On the basis of mass spectrometry (Q-ToF MS), we determined that Artepillin C, the major constituent of green propolis, underlies the effect and displays activity against P. aphanidermatum at a minimal inhibitory concentration of 750 µg.mL-1. Biophysical studies based on model membranes showed that this inhibitory effect may be linked with a membrane-related phenomenon: Artepillin C increases the permeability of membranes with relatively high fluidity in their lateral structure, a feature that is in line with the lipid composition reported for the cytoplasmic membrane of P. aphanidermatum. Therefore, the present study supports the use of the effective and inexpensive green propolis to control the impact of the dangerous phytopathogen P. aphanidermatum on agriculture.

Direct visualization of the lateral structure of giant vesicles composed of pseudo-binary mixtures of sulfatide, asialo-GM1 and GM1 with POPC.

We compared the lateral structure of giant unilamellar vesicles (GUVs) composed of three pseudo binary mixtures of different glycosphingolipid (GSL), i.e. sulfatide, asialo-GM1 or GM1, with POPC. These sphingolipids possess similar hydrophobic residues but differ in the size and charge of their polar head group. Fluorescence microscopy experiments using LAURDAN and DiIC18 show coexistence of micron sized domains in a molar fraction range that depends on the nature of the GSLs. In all cases, experiments with LAURDAN show that the membrane lateral structure resembles the coexistence of solid ordered and liquid disordered phases. Notably, the overall extent of hydration measured by LAURDAN between the solid ordered and liquid disordered membrane regions show marked similarities and are independent of the size of the GSL polar head group. In addition, the maximum amount of GSL incorporated in the POPC bilayer exhibits a strong dependence on the size of the GSL polar head group following the order sulfatide>asialo-GM1>GM1. This observation is in full harmony with previous experiments and theoretical predictions for mixtures of these GSL with glycerophospholipids. Finally, compared with previous results reported in GUVs composed of mixtures of POPC with the sphingolipids cerebroside and ceramide, we observed distinctive curvature effects at particular molar fraction regimes in the different mixtures. This suggests a pronounced effect of these GSL on the spontaneous curvature of the bilayer. This observation may be relevant in a biological context, particularly in connection with the highly curved structures found in neural cells.

Is a constant low-entropy process at the root of glycolytic oscillations?

We measured temporal oscillations in thermodynamic variables such as temperature, heat flux, and cellular volume in suspensions of non-dividing yeast cells which exhibit temporal glycolytic oscillations. Oscillations in these variables have the same frequency as oscillations in the activity of intracellular metabolites, suggesting strong coupling between them. These results can be interpreted in light of a recently proposed theoretical formalism in which isentropic thermodynamic systems can display coupled oscillations in all extensive and intensive variables, reminiscent of adiabatic waves. This interpretation suggests that oscillations may be a consequence of the requirement of living cells for a constant low-entropy state while simultaneously performing biochemical transformations, i.e., remaining metabolically active. This hypothesis, which is in line with the view of the cellular interior as a highly structured and near equilibrium system where energy inputs can be low and sustain regular oscillatory regimes, calls into question the notion that metabolic processes are essentially dissipative.

Spectral phasor analysis of LAURDAN fluorescence in live A549 lung cells to study the hydration and time evolution of intracellular lamellar body-like structures.

Using LAURDAN spectral imaging and spectral phasor analysis we concurrently studied the growth and hydration state of subcellular organelles (lamellar body-like, LB-like) from live A549 lung cancer cells at different post-confluence days. Our results reveal a time dependent two-step process governing the size and hydration of these intracellular LB-like structures. Specifically, a first step (days 1 to 7) is characterized by an increase in their size, followed by a second one (days 7 to 14) where the organelles display a decrease in their global hydration properties. Interestingly, our results also show that their hydration properties significantly differ from those observed in well-characterized artificial lamellar model membranes, challenging the notion that a pure lamellar membrane organization is present in these organelles at intracellular conditions. Finally, these LB-like structures show a significant increase in their hydration state upon secretion, suggesting a relevant role of entropy during this process.

Spatial distribution and activity of Na(+)/K(+)-ATPase in lipid bilayer membranes with phase boundaries.

We have reconstituted functional Na(+)/K(+)-ATPase (NKA) into giant unilamellar vesicles (GUVs) of well-defined binary and ternary lipid composition including cholesterol. The activity of the membrane system can be turned on and off by ATP. The hydrolytic activity of NKA is found to depend on membrane phase, and the water relaxation in the membrane on the presence of NKA. By collapsing and fixating the GUVs onto a solid support and using high-resolution atomic-force microscopy (AFM) imaging we determine the protein orientation and spatial distribution at the single-molecule level and find that NKA is preferentially located at lo/ld interfaces in two-phase GUVs and homogeneously distributed in single-phase GUVs. When turned active, the membrane is found to unbind from the support suggesting that the protein function leads to softening of the membrane.

Enzymatic studies on planar supported membranes using a widefield fluorescence LAURDAN Generalized Polarization imaging approach.

We introduce a custom-built instrument designed to perform fast LAURDAN Generalized Polarization (GP) imaging on planar supported membranes. It is mounted on a widefield fluorescence microscope and allows kinetic analysis of the GP function in the millisecond time scale, largely improving the temporal resolution previously achieved using laser scanning based microscopes. A dedicated protocol to calibrate LAURDAN GP data obtained with charge-coupled device (CCD) cameras as detectors is also presented, enabling reliable assignment of GP values in the field of view. Using this methodology we studied structural and dynamical transformations induced by Sphingomyelinase D (SM-D) on planar supported membranes composed of N-lauroyl sphingomyelin (C12SM). GP data show the evolution of an initially compositionally homogeneous symmetric bilayer existing in a single liquid disordered phase, to an intermediate configuration showing coexistence of liquid disordered and solid ordered domains, which are not always in-register across the axial plane of the bilayer. This intermediate state, caused by the transformation of C12SM to C12-ceramide-1-phosphate in the distal leaflet of the bilayer, evolved to a single solid ordered phase at longer time scales. Additionally, we comparatively studied this system using the membrane fluorophore DiIC18. The advantages and limitations of both fluorescent dyes are discussed, emphasizing the adequacy of LAURDAN GP imaging to explore this type of membrane phenomena.

Topographic analysis by atomic force microscopy of proteoliposomes matrix vesicle mimetics harboring TNAP and AnxA5.

Atomic force microscopy (AFM) is one of the most commonly used scanning probe microscopy techniques for nanoscale imaging and characterization of lipid-based particles. However, obtaining images of such particles using AFM is still a challenge. The present study extends the capabilities of AFM to the characterization of proteoliposomes, a special class of liposomes composed of lipids and proteins, mimicking matrix vesicles (MVs) involved in the biomineralization process. To this end, proteoliposomes were synthesized, composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), with inserted tissue-nonspecific alkaline phosphatase (TNAP) and/or annexin V (AnxA5), both characteristic proteins of osteoblast-derived MVs. We then aimed to study how TNAP and AnxA5 insertion affects the proteoliposomes' membrane properties and, in turn, interactions with type II collagen, thus mimicking early MV activity during biomineralization. AFM images of these proteoliposomes, acquired in dynamic mode, revealed the presence of surface protrusions with distinct viscoelasticity, thus suggesting that the presence of the proteins induced local changes in membrane fluidity. Surface protrusions were measurable in TNAP-proteoliposomes but barely detectable in AnxA5-proteoliposomes. More complex surface structures were observed for proteoliposomes harboring both TNAP and AnxA5 concomitantly, resulting in a lower affinity for type II collagen fibers compared to proteoliposomes harboring AnxA5 alone. The present study achieved the topographic analysis of lipid vesicles by direct visualization of structural changes, resulting from protein incorporation, without the need for fluorescent probes.

Imaging and modeling of acute pressure-induced changes of collagen and elastin microarchitectures in pig and human resistance arteries.

The impact of disease-related changes in the extracellular matrix (ECM) on the mechanical properties of human resistance arteries largely remains to be established. Resistance arteries from both pig and human parietal pericardium (PRA) display a different ECM microarchitecture compared with frequently used rodent mesenteric arteries. We hypothesized that the biaxial mechanics of PRA mirror pressure-induced changes in the ECM microarchitecture. This was tested using isolated pig PRA as a model system, integrating vital imaging, pressure myography, and mathematical modeling. Collagenase and elastase digestions were applied to evaluate the load-bearing roles of collagen and elastin, respectively. The incremental elastic modulus linearly related to the straightness of adventitial collagen fibers circumferentially and longitudinally (both R2 ≥ 0.99), whereas there was a nonlinear relationship to the internal elastic lamina elastin fiber branching angles. Mathematical modeling suggested a collagen recruitment strain (means ± SE) of 1.1 ± 0.2 circumferentially and 0.20 ± 0.01 longitudinally, corresponding to a pressure of ~40 mmHg, a finding supported by the vital imaging. The integrated method was tested on human PRA to confirm its validity. These showed limited circumferential distensibility and elongation and a collagen recruitment strain of 0.8 ± 0.1 circumferentially and 0.06 ± 0.02 longitudinally, reached at a distending pressure below 20 mmHg. This was confirmed by vital imaging showing negligible microarchitectural changes of elastin and collagen upon pressurization. In conclusion, we show here, for the first time in resistance arteries, a quantitative relationship between pressure-induced changes in the extracellular matrix and the arterial wall mechanics. The strength of the integrated methods invites for future detailed studies of microvascular pathologies.NEW & NOTEWORTHY This is the first study to quantitatively relate pressure-induced microstructural changes in resistance arteries to the mechanics of their wall. Principal findings using a pig model system were confirmed in human arteries. The combined methods provide a strong tool for future hypothesis-driven studies of microvascular pathologies.

Preparing giant unilamellar vesicles (GUVs) of complex lipid mixtures on demand: Mixing small unilamellar vesicles of compositionally heterogeneous mixtures.

Giant unilamellar vesicles (GUVs) are simple model membrane systems of cell-size, which are instrumental to study the function of more complex biological membranes involving heterogeneities in lipid composition, shape, mechanical properties, and chemical properties. We have devised a method that makes it possible to prepare a uniform sample of ternary GUVs of a prescribed composition and heterogeneity by mixing different populations of small unilamellar vesicles (SUVs). The validity of the protocol has been demonstrated by applying it to ternary lipid mixture of DOPC, DPPC, and cholesterol by mixing small unilamellar vesicles (SUVs) of two different populations and with different lipid compositions. The compositional homogeneity among GUVs resulting from SUV mixing is quantified by measuring the area fraction of the liquid ordered-liquid disordered phases in giant vesicles and is found to be comparable to that in GUVs of the prescribed composition produced from hydration of dried lipids mixed in organic solvent. Our method opens up the possibility to quickly increase and manipulate the complexity of GUV membranes in a controlled manner at physiological buffer and temperature conditions. The new protocol will permit quantitative biophysical studies of a whole new class of well-defined model membrane systems of a complexity that resembles biological membranes with rafts.

Monitoring Membrane Hydration with 2-(Dimethylamino)-6-Acylnaphtalenes Fluorescent Probes.

A family of polarity sensitive fluorescent probes (2-(dimethylamino)-6-acylnaphtalenes, i.e. LAURDAN, PRODAN, ACDAN) was introduced by Gregorio Weber in 1979, with the aim to monitor solvent relaxation phenomena on protein matrices. In the following years, however, PRODAN and particularly LAURDAN, were used to study membrane lateral structure and associated dynamics. Once incorporated into membranes, the (nanosecond) fluorescent decay of these probes is strongly affected by changes in the local polarity and relaxation dynamics of restricted water molecules existing at the membrane/water interface. For instance, when glycerophospholipid containing membranes undertake a solid ordered (gel) to liquid disordered phase transition the fluorescence emission maximum of these probes shift ~ 50 nm with a significant change in their fluorescence lifetime. Furthermore, the fluorescence parameters of LAURDAN and PRODAN are exquisitely sensitive to cholesterol effects, allowing interpretations that correlate changes in membrane packing with membrane hydration. Different membrane model systems as well as innate biological membranes have been studied with this family of probes allowing interesting comparative studies. This chapter presents a short historical overview about these fluorescent reporters, discusses on different models proposed to explain their sensitivity to membrane hydration, and includes relevant examples from experiments performed in artificial and biological membranes.

Fluid domain patterns in free-standing membranes captured on a solid support.

We devise a methodology to fixate and image dynamic fluid domain patterns of giant unilamellar vesicles (GUVs) at sub-optical length scales. Individual GUVs are rapidly transferred to a solid support forming planar bilayer patches. These are taken to represent a fixated state of the free standing membrane, where lateral domain structures are kinetically trapped. High-resolution images of domain patterns in the liquid-ordered (lo) and liquid-disordered (ld) co-existence region in the phase-diagram of ternary lipid mixtures are revealed by atomic force microscopy (AFM) scans of the patches. Macroscopic phase separation as known from fluorescence images is found, but with superimposed fluctuations in the form of nanoscale domains of the lo and ld phases. The size of the fluctuating domains increases as the composition approaches the critical point, but with the enhanced spatial resolution, such fluctuations are detected even deep in the coexistence region. Agreement between the area-fraction of domains in GUVs and the patches respectively, supports the assumption that the thermodynamic state of the membrane remains stable. The approach is not limited to specific lipid compositions, but could potentially help uncover lateral structures in highly complex membranes.

Elastin organization in pig and cardiovascular disease patients' pericardial resistance arteries.

Peripheral vascular resistance is increased in essential hypertension. This involves structural changes of resistance arteries and stiffening of the arterial wall, including remodeling of the extracellular matrix. We hypothesized that biopsies of the human parietal pericardium, obtained during coronary artery bypass grafting or cardiac valve replacement surgeries, can serve as a source of resistance arteries for structural research in cardiovascular disease patients. We applied two-photon excitation fluorescence microscopy to study the parietal pericardium and isolated pericardial resistance arteries with a focus on the collagen and elastin components of the extracellular matrix. Initial findings in pig tissue were confirmed in patient biopsies. The microarchitecture of the internal elastic lamina in both the pig and patient pericardial resistance arteries (studied at a transmural pressure of 100 mm Hg) is fiber like, and no prominent external elastic lamina could be observed. This microarchitecture is very different from that in rat mesenteric arteries frequently used for resistance artery research. In conclusion, we add three-dimensional information on the structure of the extracellular matrix in resistance arteries from cardiovascular disease patients and propose further use of patient pericardial resistance arteries for studies of the human microvasculature.

Compositional and structural characterization of monolayers and bilayers composed of native pulmonary surfactant from wild type mice.

This work comprises a structural and dynamical study of monolayers and bilayers composed of native pulmonary surfactant from mice. Spatially resolved information was obtained using fluorescence (confocal, wide field and two photon excitation) and atomic force microscopy methods. Lipid mass spectrometry experiments were also performed in order to obtain relevant information on the lipid composition of this material. Bilayers composed of mice pulmonary surfactant showed coexistence of distinct domains at room temperature, with morphologies and lateral packing resembling the coexistence of liquid ordered (lo)/liquid disordered (ld)-like phases reported previously in porcine lung surfactant. Interestingly, the molar ratio of saturated (mostly DPPC)/non-saturated phospholipid species and cholesterol measured in the innate material corresponds with that of a DOPC/DPPC/cholesterol mixture showing lo/ld phase coexistence at a similar temperature. This suggests that at quasi-equilibrium conditions, key lipid classes in this complex biological material are still able to produce the same scaffold observed in relevant but simpler model lipid mixtures. Also, robust structural and dynamical similarities between mono- and bi-layers composed of mice pulmonary surfactant were observed when the monolayers reach a surface pressure of 30mN/m. This value is in line with theoretically predicted and recently measured surface pressures, where the monolayer-bilayer equivalence occurs in samples composed of single phospholipids. Finally, squeezed out material attached to pulmonary surfactant monolayers was observed at surface pressures near the beginning of the monolayer reversible exclusion plateau (~40mN/m). Under these conditions this material adopts elongated tubular shapes and displays ordered lateral packing as indicated by spatially resolved LAURDAN GP measurements.

Phosphatidylethanolamine binding is a conserved feature of cyclotide-membrane interactions.

Cyclotides are bioactive cyclic peptides isolated from plants that are characterized by a topologically complex structure and exceptional resistance to enzymatic or thermal degradation. With their sequence diversity, ultra-stable core structural motif, and range of bioactivities, cyclotides are regarded as a combinatorial peptide template with potential applications in drug design. The mode of action of cyclotides remains elusive, but all reported biological activities are consistent with a mechanism involving membrane interactions. In this study, a diverse set of cyclotides from the two major subfamilies, Möbius and bracelet, and an all-d mirror image form, were examined to determine their mode of action. Their lipid selectivity and membrane affinity were determined, as were their toxicities against a range of targets (red blood cells, bacteria, and HIV particles). Although they had different membrane-binding affinities, all of the tested cyclotides targeted membranes through binding to phospholipids containing phosphatidylethanolamine headgroups. Furthermore, the biological potency of the tested cyclotides broadly correlated with their ability to target and disrupt cell membranes. The finding that a broad range of cyclotides target a specific lipid suggests their categorization as a new lipid-binding protein family. Knowledge of their membrane specificity has the potential to assist in the design of novel drugs based on the cyclotide framework, perhaps allowing the targeting of peptide drugs to specific cell types.

Morphometric image analysis of giant vesicles: a new tool for quantitative thermodynamics studies of phase separation in lipid membranes.

We have developed a strategy to determine lengths and orientations of tie lines in the coexistence region of liquid-ordered and liquid-disordered phases of cholesterol containing ternary lipid mixtures. The method combines confocal-fluorescence-microscopy image stacks of giant unilamellar vesicles (GUVs), a dedicated 3D-image analysis, and a quantitative analysis based in equilibrium thermodynamic considerations. This approach was tested in GUVs composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-palmitoyl-sn-glycero-3-phosphocholine/cholesterol. In general, our results show a reasonable agreement with previously reported data obtained by other methods. For example, our computed tie lines were found to be nonhorizontal, indicating a difference in cholesterol content in the coexisting phases. This new, to our knowledge, analytical strategy offers a way to further exploit fluorescence-microscopy experiments in GUVs, particularly retrieving quantitative data for the construction of three lipid-component-phase diagrams containing cholesterol.

A method for analysis of lipid vesicle domain structure from confocal image data.

Quantitative characterization of the lateral structure of curved membranes based on fluorescence microscopy requires knowledge of the fluorophore distribution on the surface. We present an image analysis approach for extraction of the fluorophore distribution on a spherical lipid vesicle from confocal imaging stacks. The technique involves projection of volumetric image data onto a triangulated surface mesh representation of the membrane, correction of photoselection effects and global motion of the vesicle during image acquisition and segmentation of the surface into domains using histograms. The analysis allows for investigation of the morphology and size distribution of domains on the surface.

The innards of the cell: studies of water dipolar relaxation using the ACDAN fluorescent probe.

This article reviews the use of the 6-acetyl-2-(dimethylamino)naphthalene (ACDAN) fluorophore to study dipolar relaxation in cells, tissues, and biomimetic systems. As the most hydrophilic member of the 6-acyl-2-(dimethylamino)naphthalene series, ACDAN markedly partitions to aqueous environments. In contrast to 6-lauroyl-2-(dimethylamino)naphthalene (LAURDAN), the hydrophobic and best-known member of the series used to explore relaxation phenomena in biological (or biomimetic) membranes, ACDAN allows mapping of spatial and temporal water dipolar relaxation in cytosolic and intra-organelle environments of the cell. This is also true for the 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) derivative which, unlike LAURDAN, partitions to both hydrophobic and aqueous environments. We will (i) summarize the mechanism which underlies the solvatochromic properties of the DAN probes, (ii) expound on the importance of water relaxation to understand the intracellular environment, (iii) discuss technical aspects of the use of ACDAN in eukaryotic cells and some specialized structures, including liquid condensates arising from processes leading to liquid immiscibility and, (iv) present some novel studies in plant cells and tissues which demonstrate the kinds of information that can be uncovered using this approach to study dipolar relaxation in living systems.

Lipid lateral organization on giant unilamellar vesicles containing lipopolysaccharides.

We developed a new (to our knowledge) protocol to generate giant unilamellar vesicles (GUVs) composed of mixtures of single lipopolysaccharide (LPS) species and Escherichia coli polar lipid extracts. Four different LPSs that differed in the size of the polar headgroup (i.e., LPS smooth > LPS-Ra > LPS-Rc > LPS-Rd) were selected to generate GUVs composed of different LPS/E. coli polar lipid mixtures. Our procedure consists of two main steps: 1), generation and purification of oligolamellar liposomes containing LPSs; and 2), electroformation of GUVs using the LPS-containing oligolamellar vesicles at physiological salt and pH conditions. Analysis of LPS incorporation into the membrane models (both oligolamellar vesicles and GUVs) shows that the final concentration of LPS is lower than that expected from the initial E. coli lipids/LPS mixture. In particular, our protocol allows incorporation of no more than 15 mol % for LPS-smooth and LPS-Ra, and up to 25 mol % for LPS-Rc and LPS-Rd (with respect to total lipids). We used the GUVs to evaluate the impact of different LPS species on the lateral structure of the host membrane (i.e., E. coli polar lipid extract). Rhodamine-DPPE-labeled GUVs show the presence of elongated micrometer-sized lipid domains for GUVs containing either LPS-Rc or LPS-Rd above 10 mol %. Laurdan GP images confirm this finding and show that this particular lateral scenario corresponds to the coexistence of fluid disordered and gel (LPS-enriched)-like micron-sized domains, in similarity to what is observed when LPS is replaced with lipid A. For LPSs containing the more bulky polar headgroup (i.e., LPS-smooth and LPS-Ra), an absence of micrometer-sized domains is observed for all LPS concentrations explored in the GUVs (up to ∼15 mol %). However, fluorescence correlation spectroscopy (using fluorescently labeled LPS) and Laurdan GP experiments in these microscopically homogeneous membranes suggests the presence of LPS clusters with dimensions below our microscope's resolution (∼380 nm radial). Our results indicate that LPSs can cluster into gel-like domains in these bacterial model membranes, and that the size of these domains depends on the chemical structure and concentration of the LPSs.

Impact of membrane-anchored fluorescent probes on the mechanical properties of lipid bilayers.

Fluorescent probes are used in membrane biophysics studies to provide information about physical properties such as lipid packing, polarity and lipid diffusion or to visualize membrane domains. However, our understanding of the effects the dyes themselves may induce on the membrane structure and properties are sparse. As mechanical properties like bending elasticity were already shown to be highly sensitive to the addition of "impurities" into the membranes, we have investigated the impact of six different commonly used fluorescent membrane probes (LAURDAN, TR-DPPE, Rh-DPPE, DiIC18, Bodipy-PC and NBD-PC) on the bending elasticity of dye containing POPC GUVs as compared to single component POPC GUVs. Small changes in the membrane bending elasticity compared to single POPC bilayers are observed when 2 mol% of Rh-DPPE, Bodipy-PC or NBD-PC are added in POPC membranes. These binary membranes are showing non reproducible mechanical properties attributed to a photo-induced peroxidation processes that may be controlled by a reduction of the fluorescent dye concentration. For TR-DPPE, a measurable decrease of the bending elasticity is detected with reproducible bending elasticity measurements. This is a direct indication that this dye, when exposed to illumination by a microscope lamp and contrary to Rh-DPPE, does not induce chemical degradation. At last, LAURDAN and DiIC18 probes mixed with POPC do not significantly affect the bending elasticity of pure POPC bilayers, even at 2 mol%, suggesting these latter probes do not induce major perturbations on the structure of POPC bilayers.