Membrane Viscosity Research Articles

Shedding Light on Thermally Induced Optocapacitance at the Organic Biointerface.

Photothermal perturbation of the cell membrane is typically achieved using transducers that convert light into thermal energy, eventually heating the cell membrane. In turn, this leads to the modulation of the membrane electrical capacitance that is assigned to a geometrical modification of the membrane structure. However, the nature of such a change is not understood. In this work, we employ an all-optical spectroscopic approach, based on the use of fluorescent probes, to monitor the membrane polarity, viscosity, and order directly in living cells under thermal excitation transduced by a photoexcited polymer film. We report two major results. First, we show that rising temperature does not just change the geometry of the membrane but indeed it affects the membrane dielectric characteristics by water penetration. Second, we find an additional effect, which is peculiar for the photoexcited semiconducting polymer film, that contributes to the system perturbation and that we tentatively assigned to the photoinduced polarization of the polymer interface.

Reprogrammed lipid metabolism protects inner nuclear membrane against unsaturated fat.

SummaryThe cell nucleus is surrounded by a double membrane. The lipid packing and viscosity of membranes is critical for their function and is tightly controlled by lipid saturation. Circuits regulating the lipid saturation of the outer nuclear membrane (ONM) and contiguous endoplasmic reticulum (ER) are known. However, how lipid saturation is controlled in the inner nuclear membrane (INM) has remained enigmatic. Using INM biosensors and targeted genetic manipulations, we show that increased lipid unsaturation causes a reprogramming of lipid storage metabolism across the nuclear envelope (NE). Cells induce lipid droplet (LD) formation specifically from the distant ONM/ER, whereas LD formation at the INM is suppressed. In doing so, unsaturated fatty acids are shifted away from the INM. We identify the transcription circuits that topologically reprogram LD synthesis and identify seipin and phosphatidic acid as critical effectors. Our study suggests a detoxification mechanism protecting the INM from excess lipid unsaturation.

Lattice Boltzmann simulations on the tumbling to tank-treading transition: effects of membrane viscosity

The tumbling to tank-treading (TB-TT) transition for red blood cells (RBCs) has been widely investigated, with a main focus on the effects of the viscosity ratio [Formula: see text] (i.e., the ratio between the viscosities of the fluids inside and outside the membrane) and the shear rate [Formula: see text] applied to the RBC. However, the membrane viscosity [Formula: see text] plays a major role in a realistic description of RBC dynamics, and only a few works have systematically focused on its effects on the TB-TT transition. In this work, we provide a parametric investigation on the effect of membrane viscosity [Formula: see text] on the TB-TT transition for a single RBC. It is found that, at fixed viscosity ratios [Formula: see text], larger values of [Formula: see text] lead to an increased range of values of capillary number at which the TB-TT transition occurs; moreover, we found that increasing [Formula: see text] or increasing [Formula: see text] results in a qualitatively but not quantitatively similar behaviour. All results are obtained by means of mesoscale numerical simulations based on the lattice Boltzmann models. This article is part of the theme issue 'Progress in mesoscale methods for fluid dynamics simulation'.

(INVITED) Fluorescent probes for optical investigation of the plasma membrane

The biological functions of the cell membrane are regulated by its physicochemical properties, such as the transmembrane potential, phase state and hydration. Therefore, the in situ investigation of these properties is attracting a rising interest in the field of membrane biophysics. In these regards, a wide variety of fluorescent membrane probes has been developed, providing powerful tools for fluorescence microscopy and/or fluorescence spectroscopy to study the microscopic analogs of viscosity, polarity, and hydration, as well as the molecular order and electrostatic potential at the sites of their location. Despite the success of this approach, further developments rely on the design of fluorescence probes with optimized photophysical properties. This review discusses the most successful fluorescent molecular probes and their application to the study of the plasma membrane viscosity, lipid order and potential.

Relationship between Viscosity and Acyl Tail Dynamics in Lipid Bilayers.

Membrane viscosity is a fundamental property that controls molecular transport and structural rearrangements in lipid membranes. Given its importance in many cell processes, various experimental and computational methods have been developed to measure the membrane viscosity, yet the estimated values depend highly on the method and vary by orders of magnitude. Here we investigate the molecular origins of membrane viscosity by measuring the nanoscale dynamics of the lipid acyl tails using x-ray and neutron spectroscopy techniques. The results show that the membrane viscosity can be estimated from the structural relaxation times of the lipid tails.

Lipid Hydroperoxide Compromises the Membrane Structure Organization and Softens Bending Rigidity.

Lipid hydroperoxides are key mediators of diseases and cell death. In this work, the structural and dynamic perturbations induced by the hydroperoxidized POPC lipid (POPC-OOH) in fluid POPC membranes, at both 23 and 37 °C, were addressed using advanced small-angle X-ray scattering (SAXS) and fluorescence methodologies. Notably, SAXS reveals that the hydroperoxide group decreases the lipid bilayer bending rigidity. This alteration disfavors the bilayer stacking and increases the swelling in-between stacked bilayers. We further investigated the changes in the apolar/polar interface of hydroperoxide-containing membranes through time-resolved fluorescence/anisotropy experiments of the probe TMA-DPH and time-dependent fluorescence shifts of Laurdan. A shorter mean fluorescence lifetime for TMA-DPH was obtained in enriched POPC-OOH membranes, revealing a higher degree of hydration near the membrane interface. Moreover, a higher microviscosity near TMA-DPH and lower order are predicted for these oxidized membranes, at variance with the usual trend of variation of these two parameters. Finally, the complex relaxation process of Laurdan in pure POPC-OOH membranes also indicates a higher membrane hydration and viscosity in the close vicinity of the -OOH moiety. Altogether, our combined approach reveals that the hydroperoxide group promotes alterations in the membrane structure organization, namely, at the level of membrane order, viscosity, and bending rigidity.

Probing Metabolism and Viscosity of Cancer Cells using Fluorescence Lifetime Imaging Microscopy

Viscosity is an important physical property of a biological membrane, as it is one of the key parameters for the regulation of morphological and physiological state of living cells. Plasma membranes of tumor cells are known to have significant alterations in their composition, structure, and functional characteristics. Along with dysregulated metabolism of glucose and lipids, these specific membrane properties help tumor cells to adapt to the hostile microenvironment and develop resistance to drug therapies. Here, we demonstrate the use of fluorescence lifetime imaging microscopy (FLIM) to sequentially image cellular metabolism and plasma membrane viscosity in live cancer cell culture. Metabolic assessments are performed by detecting fluorescence of endogenous metabolic cofactors, such as reduced nicotinamide adenine dinucleotide NAD(P)H and oxidized flavins. Viscosity is measured using a fluorescent molecular rotor, a synthetic viscosity-sensitive dye, with a strong fluorescence lifetime dependence on the viscosity of the immediate environment. In combination, these techniques enable us to better understand the links between membrane state and metabolic profile of cancer cells and to visualize the changes induced by chemotherapy.

Perspective: What Should I Eat to Maintain Membrane Lipid Rafts and Avoid Metabolic Disease?

Discerning meaningful patterns in lipidomic studies of metabolic diseases is daunting. Consideration of changes in dietary lipids over the last 150 years and the effects of this variance on membrane raft formation provides a tool to better understand lipidomic data. In vitro, mixtures of low and high melting point lipids and cholesterol spontaneously form bilayers with heterologous areas of differing order, composition, and physical properties. Through dozens of studies, the general behavior of different lipid mixtures in phase separation has been mapped and displayed as ternary lipid mixture phase diagrams; areas that show both liquid order (Lo) and liquid disorder (Ld) allow raft formation (Figure 1). In vivo, ordered membrane areas composed of sterols, sphingolipids and proteins provide platforms necessary for a complex network of cellular signaling. Although there is controversy regarding the primacy of lipids versus proteins in organizing these signaling rafts, this perspective focuses on the role that variations in dietary lipids might play in raft formation. Comparison of dietary lipids in humans living before the agricultural revolution to those living in 21st century developed areas reveals major differences. Modern diets have: increased polyunsaturated fatty acids (PUFA) from vegetable oils, an increased ratio of omega 6:3 PUFA, increased trans fatty acids (TFA), and decreased cholesterol. Plotting these variations in dietary lipids on a ternary graph provides a framework to understand the theoretical effect of dietary lipid variance on phase separation and membrane behavior. For example, how might increased dietary TFA, a widely recognized risk factor in cardiovascular disease, disrupt raft formation? Substitution of TFA for PUFA at sn-2 in low melting point phospholipids would raise the melting point; substitution at sn-1 on sphingolipids would lower the melting point. Because disparity in the order of membrane domains is critical in the maintenance of phase separation, convergence of melting points of these compounds would thus impede raft formation (Figure 2). To compensate, incorporation of highly unsaturated PUFA to lower the melting point of non raft phospholipids, or incorporation of longer chain saturated fatty acids to raise the melting point of raft associated lipids would restore order differential and allow raft formation, but would require other changes to maintain optimal membrane viscosity. Elevated membrane PUFA increases oxidative damage and an elevated omega 6:3 ratio increases thrombosis and inflammation. Compared to cis forms, TFA phospholipids require eight times more cholesterol to be incorporated into a membrane; with reduced cholesterol diets, accommodation of TFAs into membranes would require stimulation of endogenous cholesterol synthesis. Increased membrane cholesterol also decreases permeability to oxygen. Because most fatty acids are incorporated into cell membranes unmodified from the form in which they were absorbed from the GI tract, changes in dietary lipids critically impact cell membrane composition. This example illustrates the theoretical impact of dietary lipids on raft formation and behavior as a means to understand lipidomic data and the pathophysiologic elements of dyslipidemia, oxidative stress, inflammation and cellular hypoxia.

Impact of the membrane viscosity on the tank-treading behavior of red blood cells

Numerical simulations are used to compare the impact of the internal fluid viscosity and the membrane viscosity on an isolated tank-treading red blood cell. Both decrease the tank-treading frequency, with moderate changes in the deformation and inclination of the red blood cell. However, it is shown that tank-treading frequencies from existing experiments are only retrieved if membrane viscosity is accounted for, and with an apparent shear-thinning of the membrane.

Optical tweezers study of membrane fluidity in small cell lung cancer cells.

The fluidity of the cell membrane is closely related to cancer metastasis/invasion. To test the relationship of membrane fluidity and invasiveness, we first demonstrated that transfection of small RNA miR-92b-3p can significantly increase invasiveness of the small cell lung cancer cell line SHP77. Then optical tweezers were used to measure membrane fluidity. This study employed continuous and step-like stretching methods to examine fluidity changes in SHP77 cell membranes before and after miR-92b-3p transfection. A newly developed physical model was used to derive the effective viscosity and static tension of the cell membrane from relaxation curves obtained via step-like stretching. Experiments showed that invasiveness and fluidity increased significantly after miR-92b-3p transfection. This study paved the way toward a better understanding of cancer cell invasion and membrane mechanical characteristics.

Melatonin and biological membrane bilayers: a never ending amity

Biological membrane, the most fluidic structure of a cell or an organelle, refrains the cells to progress toward apoptosis by sustaining their optimum environment. This bilayer-membrane equips all machineries required for cellular communication, limits the entry of foreign bodies, selectively transports molecules or ions depending on the need of the system but, it also acts as a first line defense against environmental insults. Due to the presence of a vast number of poly unsaturated fatty acids (PUFA), the biological membrane is highly prone to oxidative stress and as a consequence, acceleration in lipid peroxidation by free radicals, becomes a threat to cellular viability. Alterations in the biophysical state of bilayer caused by oxidative stress frequently occur in the in vivo as well as in vitro conditions. It has been well documented that the molecule, melatonin, exhibits profound coherence in neutralizing oxidative stress and thus, to normalize fluidity status of biological membranes. Aging associated decline in melatonin level with subsequent ascended lipid peroxidation and membrane viscosity found in almost all organisms further suggest the importance of melatonin in this context. Since disruption of membrane structure or even some modifications will cause a spectrum of diseases, keeping membrane intactness would be an adequate strategy to prevent these diseases. Considering the high permeability, safe and potent antioxidant capacity of melatonin, this molecule can be a superlative choice to alleviate membrane bilayer rigidity and its related ailments.

Lipid Bilayer Passive Area Regulation in Response to Fluid Flows

Mechanotransduction is the cellular process in which mechanical stimuli are converted into biochemical signals to induce specific cellular responses, enabling the cell to sense and respond to mechanical forces. Although many cells exhibit this behavior, vascular endothelial cells (ECs) are of considerable interest as they effectively respond to fluid shear stress, adapting their morphology, functions and gene expression to maintain the homeostasis of the vascular system. Fundamentally, these processes rely on the transduction of intracellular signals through the lipid membrane, and several studies have already demonstrated that shear stress induces significant changes in lipid order and viscosity of ECs plasma membrane. Despite such extensive study, the underlying molecular mechanisms that control this change to membrane lipid order and viscosity remain unclear. Supported lipid bilayers provide an ideal platform for the investigation of the molecular mechanisms behind shear induced biophysical changes in membrane structure. As a minimal model system, they are suited for isolating the physical role of lipid bilayer in membrane shear response, in the absence of active biological processes. Here, combining microfluidics and fluorescence microscopy, we examine how the application of controlled fluid flows on supported lipid bilayer patches alters their physical properties. We observe a reversible and significant ∼10% increase in SLB area (when physiological shear stresses of 1-2 Pa are applied). Such area fluctuations have been a previously suggested response to fluid shear stress but have not directly observed. Furthermore, we determine how these membrane area fluctuations in response to fluid flows depend on lipid composition and surface roughness. Identifying the origin of such flow-induced changes in membrane structure could reveal an underlying mechanism utilized by biology to sense and direct changes in membrane order and viscosity in response to shear flows.

Shape Fluctuations of Giant Unilamellar Vesicles and its Applications to Study Electrified Membranes and Membrane Viscosity

Bending rigidity of cellular membranes plays a key role in governing several cell remodeling processes such as endocytosis, exocytosis, cell division and viral uptake. Due to ease of implementation, fluctuation spectroscopy is a widely used tool to measure the shape fluctuations of giant unilamellar vesicles (GUVs) to determine membrane bending rigidity and compositional effects (see e.g. Faizi et al. Soft Matter 15:6006, 2019). in this approach, a time series of vesicle contour is recorded in the equatorial plane.

Systematic Measurements of Interleaflet Friction in Supported Lipid Bilayers

Along with membrane viscosity, interleaflet friction determines the dynamics of membrane deformation. The two leaflets that form a lipid membrane must slide past each other when the membrane bends, as occurs when vesicles are formed, or membrane tethers are pulled. Existing measurements of interleaflet friction are sparse, examining a single type of lipid, dye, and substrate, which does not give any conclusive answers to how changing these factors will affect the measured interleaflet friction. A recently developed method applies high shear stresses to supported lipid bilayers formed in microfluidic channels, resulting in a tank treading motion where the upper leaflet of the bilayer slides overtop of the bottom leaflet.

Membrane Fluidity: Both Fundamental and Functional

It has been known for decades that cells actively sense and regulate the fluidity of their membranes. Sinensky coined the phrase “homeoviscous adaptation” to describe the process, after observing that bacteria grown at different temperatures maintain constant membrane fluidity via changes in lipid composition. Homeostatic membrane changes have subsequently been demonstrated across the kingdoms of life and in response to other environmental changes, such as hydrostatic pressure or salinity. However, the viscosity of a lipid membrane is both phenomenologically richer and more challenging to measure than that of a fluid in three dimensions.

A Needless but Interesting Controversy with Pedagogical Overtones

A decade ago four independent groups, using four independent methods, reported the unanticipated and still surprising result that the bending modulus KC of bilayers of DOPC, unlike other standard lipids, does not increase with addition of cholesterol. A recent paper, using neutron spin echo (NSE) and nuclear magnetic resonance relaxation times (NMRτ), reports that the bending modulus of DOPC increases three fold with increasing cholesterol concentration up to 50%. It would appear that this has become a controversial topic. This controversy is needless because NSE and NMRτ do not measure the bending modulus as it has been defined for decades. Each measures important, complementary, properties that could enrich our understanding of bilayer properties, notably membrane viscosity and location of the neutral plane employed in membrane mechanics. The resolution of this controversy rests on elucidating the difference between measurements on a system in equilibrium that lead directly to equilibrium properties like the bending modulus, and measurements on the same equilibrium system that lead to more complex results involving both static moduli and transport properties. While the difference between static and non-equilibrium quantities is well understood in the physical sciences and statistical thermodynamics, this emerging controversy seems to warrant a pedagogical reminder.

Swimming of microorganisms in quasi-two-dimensional membranes

Abstract

Similar but Distinct Roles of Membrane and Interior Fluid Viscosities in Capsule Dynamics in Shear Flows.

The dynamics of biological capsules and red blood cells in shear flows has been studied extensively with experimental, analytical, and numerical methods. In particular, the effects of various parameters, including the shear rate or shear stress, membrane elasticity, capsule shape, and interior fluid viscosity, have been investigated carefully. The role of the membrane viscosity for capsule deformation dynamics has not been examined adequately. In previous studies, the so-called energy dissipation ratio has been used to account for the membrane viscosity effect by increasing the interior viscosity; however, the applicability and accuracy of this treatment have not been evaluated carefully. In this study, using the recently developed finite-difference scheme for immersed boundary simulations of viscoelastic membranes, we conduct comprehensive numerical simulations of the deformation processes of an originally spherical capsule in shear flows with various combinations of membrane and interior fluid viscosities. Our results show that the membrane and interior fluid viscosity have similar however different effects on the capsule deformation dynamics. While the capsule deformation decreases with both membrane and interior fluid viscosities, a typical decrease-then-increase variation is observed for the inclination angle as the membrane viscosity increases, instead of the monotonic decrease in the inclination angle with the interior fluid viscosity increase. Also, although both large membrane and interior fluid viscosity values can introduce oscillations in the capsule deformation and inclination, larger aptitudes and slow decay processes are noticed at larger membrane viscosities. The variations of other dynamic parameters of the capsule, including the circumference, average membrane velocity, and rotation frequency, are also analyzed, and an intuitive mechanism is proposed to relate the membrane velocity and rotation frequency to the capsule deformation and inclination angle. The simple mechanism is then applied to explain the spoon-like variation patterns for membrane velocity and rotation frequency observed in our results. Furthermore, we examine the validity of the energy dissipation ratio approach based on the mathematical functional dependence. Our results and analysis show that the dissipation ratio is a system and process dependent variable and it cannot be treated as a constant even for the same capsule. This research is valuable for a better understanding of the complex capsule dynamics in flows and also suggests that the membrane viscosity needs to be considered explicitly for accurate and reliable results in future studies.

A high-throughput method to characterize membrane viscosity of flowing microcapsules

Microcapsules have many industrial applications and also serve as a widely used mechanical model of living biological cells. Characterizing the viscosity and elasticity of capsules at a high-throughput rate has been a classical challenge, since this is a time-consuming process in which one needs to fit the time-dependent capsule deformation to theoretical predictions. In the present study, we develop a novel efficient method, by integrating a deep convolutional neural network with a high-fidelity mechanistic capsule model, to predict the membrane viscosity and elasticity of a microcapsule from its dynamic deformation when flowing in a branched microchannel. Compared with a conventional inverse method, the present approach can increase the prediction throughput rate by five orders of magnitude while maintaining the same level of prediction accuracy. We also demonstrate that the present approach can deal with capsules with large deformation in inertial flows.

A neural network-based algorithm for high-throughput characterisation of viscoelastic properties of flowing microcapsules.

Microcapsules, consisting of a liquid droplet enclosed by a viscoelastic membrane, have a wide range of biomedical and pharmaceutical applications and also serve as a popular mechanical model for biological cells. In this study, we develop a novel high throughput approach, by combining a machine learning method with a high-fidelity mechanistic capsule model, to accurately predict the membrane elasticity and viscosity of microcapsules from their dynamic deformation when flowing in a branched microchannel. The machine learning method consists of a deep convolutional neural network (DCNN) connected by a long short-term memory (LSTM) network. We demonstrate that with a superior prediction accuracy the present hybrid DCNN-LSTM network can still be faster than a conventional inverse method by five orders of magnitude, and can process thousands of capsules per second. We also show that the hybrid network has fewer restrictions compared with a simple DCNN.