Effect of chain unsaturation on bilayer response to pressure

Residual Membrane Fluidity in Mycobacterial Cell Envelope Layers under Extreme Conditions Underlines Membrane-Centric Adaptation

One of the routes for adaptation to extreme environments is via remodeling of cell membrane structure, composition, and biophysical properties rendering a functional membrane. Collective studies suggest some form of membrane feedback in mycobacterial species that harbor complex lipids within the outer and inner cell wall layers. Here, we study the homeostatic membrane landscape of mycobacteria in response to high hydrostatic pressure and temperature triggers using high pressure fluorescence, mass and infrared spectroscopies, NMR, SAXS, and molecular dynamics simulations. Our findings reveal that mycobacterial membrane possesses unique and lipid-specific pressure-induced signatures that attenuate progression to highly ordered phases. Both inner and outer membrane layers exhibit phase coexistence of nearly identical lipid phases keeping residual fluidity over a wide range of temperature and pressure, but with different sensitivities. Lipidomic analysis of bacteria grown under pressure revealed lipidome remodeling in terms of chain length, unsaturation, and specific long-chained characteristic mycobacterial lipids, rendering a fluid bacterial membrane. These findings could help understand how bacteria may adapt to a broad spectrum of harsh environments by modulating their lipidome to select lipids that enable the maintenance of a fluid functional cell envelope.

The effect of hydrostatic pressure on lipid membrane lateral structure

High pressure is both an environmental challenge to which deep sea biology has to adapt, and a highly sensitive thermodynamic tool that can be used to trigger structural changes in biological molecules and assemblies. Lipid membranes are amongst the most pressure sensitive biological assemblies and pressure can have a large influence on their structure and properties. In this chapter, we will explore the use of high pressure small angle X-ray diffraction and high pressure microscopy to measure and quantify changes in the lateral structure of lipid membranes under both equilibrium high pressure conditions and in response to pressure jumps.

Effects of lipid saturation on bicelle to vesicle transition of a binary phospholipid mixture: a molecular dynamics simulation study

Controlling the transition from lipid bicelles to vesicles is essential for producing engineered vesicles. We perform coarse-grained molecular dynamics (CGMD) simulations of unsaturated/saturated lipid mixtures to clarify the effects of lipid unsaturation on vesiculation at the molecular scale. The results demonstrate that vesiculation depends on the concentration of unsaturated lipids and the degree of unsaturation. The probability of vesiculation increases linearly with the apparent unsaturated lipid concentration at a low degree of unsaturation. Higher degrees of unsaturation lead to phase segregation within the binary bicelles, reducing the probability of vesiculation. A comparison between CGMD simulations and the conventional theory of vesiculation shows that the theoretical predictions of binary lipid systems must explicitly include phase segregation effects. Furthermore, simulations with biased lipid distributions reveal that vesiculation is facilitated by the preconcentration of unsaturated lipids in the core region of the bicelle but is then temporally limited as the unsaturated lipids move to the bicelle edges. These findings advance theoretical and experimental studies on binary lipid systems and promote the development of tailor-made vesicles.

Cations Do Not Alter the Membrane Structure of POPC—A Lipid With an Intermediate Area

Combining small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and densitometric measurements, we have studied the interactions of the divalent cations Ca²⁺ and Mg²⁺ with the lipid vesicles prepared of a mixed-chain palmitoyl-oleoyl-phosphatidylcholine (POPC) at 25°C. The structural parameters of the POPC bilayer, such as the bilayer thickness, lateral area, and volume per lipid, displayed no changes upon the ion addition at concentrations up to 30 mM and minor changes at > 30 mM Ca²⁺ and Mg²⁺, while some decrease in the vesicle radius was observed over the entire concentration range studied. This examination allows us to validate the concept of lipid–ion interactions governed by the area per lipid suggested previously and to propose the mixed mode of those interactions that emerge in the POPC vesicles. We speculate that the average area per POPC lipid that corresponds to the cutoff length of lipid–ion interactions generates an equal but opposite impact on ion bridges and separate lipid–ion pairs. As a result of the dynamic equilibrium, the overall structural properties of bilayers are not affected. As the molecular mechanism proposed is affected by the structural properties of a particular lipid, it might help us to understand the fundamentals of processes occurring in complex multicomponent membrane systems.

Biomolecules under Pressure: Phase Diagrams, Volume Changes, and High Pressure Spectroscopic Techniques

Pressure is an equally important thermodynamical parameter as temperature. However, its importance is often overlooked in the biophysical and biochemical investigations of biomolecules and biological systems. This review focuses on the application of high pressure (>100 MPa = 1 kbar) in biology. Studies of high pressure can give insight into the volumetric aspects of various biological systems; this information cannot be obtained otherwise. High-pressure treatment is a potentially useful alternative method to heat-treatment in food science. Elevated pressure (up to 120 MPa) is present in the deep sea, which is a considerable part of the biosphere. From a basic scientific point of view, the application of the gamut of modern spectroscopic techniques provides information about the conformational changes of biomolecules, fluctuations, and flexibility. This paper reviews first the thermodynamic aspects of pressure science, the important parameters affecting the volume of a molecule. The technical aspects of high pressure production are briefly mentioned, and the most common high-pressure-compatible spectroscopic techniques are also discussed. The last part of this paper deals with the main biomolecules, lipids, proteins, and nucleic acids: how they are affected by pressure and what information can be gained about them using pressure. I I also briefly mention a few supramolecular structures such as viruses and bacteria. Finally, a subjective view of the most promising directions of high pressure bioscience is outlined.

Ordering of Saturated and Unsaturated Lipid Membranes near Their Phase Transitions Induced by an Amphiphilic Cyclodextrin and Cholesterol

When inserted in membranes of dimyristoyl phosphatidylcholine (DMPC), methylated β-cyclodextrins with one (TrimβMLC) or two (TrimβDLC) lauryl acyl chains grafted onto the hydrophilic cavity exert a "cholesterol-like ordering effect", by straightening the acyl chains in the fluid phase at temperatures near the chain melting transition. This effect may be related to pretransitional events such as the "anomalous swelling" known to occur with saturated phosphatidylcholine membranes. To investigate this model, order profiles and bilayer thicknesses of DMPC and unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) membranes containing amphiphilic cyclodextrins or cholesterol were determined by deuterium NMR. The pure lipid membranes display both a qualitatively similar chain ordering upon cooling in the fluid phase, more important at the chain extremity, which gets more pronounced near their fluid-to-gel transitions. Both membranes show a bilayer thickness increase by ∼0.5 Å just above their transition, as observed previously with saturated phosphatidylcholines of various chain lengths. Membrane-insertion of 5% TrimβMLC or cholesterol induces an important ordering of the DMPC acyl chains just above the transition, which is also more pronounced at the chain extremity. There is an additional increase of the bilayer thickness, most probably due to a deep insertion of these amphiphilic molecules, facilitated by increased bilayer softness in the anomalous swelling regime. These effects are more important with TrimβMLC than with cholesterol. By contrast, no enhanced acyl chain ordering was observed when approaching the transition of TrimβMLC-containing POPC membranes, as a possible consequence of an eventual lack of anomalous swelling in unsaturated lipid membranes. Insertion of higher concentrations of TrimβMLC was found to induce a magnetic orientation of the DMPC membranes in the fluid phase with 10% of this derivative, coupled with the appearance of a broad isotropic component when the concentration is raised to 20%. No membrane orientation or isotropic component was detected with TrimβMLC-containing POPC membranes.

Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Effects of High Pressure on Phospholipid Bilayers

The response of lipid membranes to changes in external pressure is important for many biological processes, and it can also be exploited for technological applications. In this work, we employ all-atom molecular dynamics simulations to characterize the changes in the physical properties of phospholipid bilayers brought about by high pressure (1000 bar). In particular, we study how the response differs, in relation to different chain unsaturation levels, by comparing monounsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and biunsaturated dioleoyl-phosphatidylcholine (DOPC) bilayers. Various structural, mechanical, and dynamical features are found to be altered by the pressure increase in both bilayers. Notably, for most properties, including bilayer area and thickness, lipid order parameters, lateral pressure profile, and curvature frustration energy, we observe significantly more pronounced effects for monounsaturated POPC than biunsaturated DOPC. Possible biological implications of the results obtained are discussed, especially in relation to how different lipids can control the structure and function of membrane proteins.

Role of TRPC1 channels in pressure-mediated activation of murine pancreatic stellate cells

The tumor environment contributes importantly to tumor cell behavior and cancer progression. Aside from biochemical constituents, physical factors of the environment also influence the tumor. Growing evidence suggests that mechanics [e.g., tumor (stroma) elasticity, tissue pressure] are critical players of cancer progression. Underlying mechanobiological mechanisms involve among others the regulation of focal adhesion molecules, cytoskeletal modifications, and mechanosensitive (MS) ion channels of cancer- and tumor-associated cells. After reviewing the current concepts of cancer mechanobiology, we will focus on the canonical transient receptor potential 1 (TRPC1) channel and its role in mechano-signaling in tumor-associated pancreatic stellate cells (PSCs). PSCs are key players of pancreatic fibrosis, especially in cases of pancreatic ductal adenocarcinoma (PDAC). PDAC is characterized by the formation of a dense fibrotic stroma (desmoplasia), primarily formed by activated PSCs. Desmoplasia contributes to high pancreatic tissue pressure, which in turn activates PSCs, thereby perpetuating matrix deposition. Here, we investigated the role of the putatively mechanosensitive TRPC1 channels in murine PSCs exposed to elevated ambient pressure. Pressurization leads to inhibition of mRNA expression of MS ion channels. Migration of PSCs representing a readout of their activation is enhanced in pressurized PSCs. Knockout of TRPC1 leads to an attenuated phenotype. While TRPC1-mediated calcium influx is increased in wild-type PSCs after pressure incubation, loss of TRPC1 abolishes this effect. Our findings provide mechanistic insight how pressure, an important factor of the PDAC environment, contributes to PSC activation. TRPC1-mediated activation could be a potential target to disrupt the positive feedback of PSC activation and PDAC progression.

Pressure effects on lipids and bio-membrane assemblies

Membranes are amongst the most important biological structures; they maintain the fundamental integrity of cells, compartmentalize regions within them and play an active role in a wide range of cellular processes. Pressure can play a key role in probing the structure and dynamics of membrane assemblies, and is also critical to the biology and adaptation of deep-sea organisms. This article presents an overview of the effect of pressure on the mesostructure of lipid membranes, bilayer organization and lipid-protein assemblies. It also summarizes recent developments in high-pressure structural instrumentation suitable for experiments on membranes.

Elastic deformation and area per lipid of membranes: atomistic view from solid-state deuterium NMR spectroscopy

This article reviews the application of solid-state ²H nuclear magnetic resonance (NMR) spectroscopy for investigating the deformation of lipid bilayers at the atomistic level. For liquid-crystalline membranes, the average structure is manifested by the segmental order parameters (SCD) of the lipids. Solid-state ²H NMR yields observables directly related to the stress field of the lipid bilayer. The extent to which lipid bilayers are deformed by osmotic pressure is integral to how lipid-protein interactions affect membrane functions. Calculations of the average area per lipid and related structural properties are pertinent to bilayer remodeling and molecular dynamics (MD) simulations of membranes. To establish structural quantities, such as area per lipid and volumetric bilayer thickness, a mean-torque analysis of ²H NMR order parameters is applied. Osmotic stress is introduced by adding polymer solutions or by gravimetric dehydration, which are thermodynamically equivalent. Solid-state NMR studies of lipids under osmotic stress probe membrane interactions involving collective bilayer undulations, order-director fluctuations, and lipid molecular protrusions. Removal of water yields a reduction of the mean area per lipid, with a corresponding increase in volumetric bilayer thickness, by up to 20% in the liquid-crystalline state. Hydrophobic mismatch can shift protein states involving mechanosensation, transport, and molecular recognition by G-protein-coupled receptors. Measurements of the order parameters versus osmotic pressure yield the elastic area compressibility modulus and the corresponding bilayer thickness at an atomistic level. Solid-state ²H NMR thus reveals how membrane deformation can affect protein conformational changes within the stress field of the lipid bilayer.

Solid-state ²H NMR shows equivalence of dehydration and osmotic pressures in lipid membrane deformation

Lipid bilayers represent a fascinating class of biomaterials whose properties are altered by changes in pressure or temperature. Functions of cellular membranes can be affected by nonspecific lipid-protein interactions that depend on bilayer material properties. Here we address the changes in lipid bilayer structure induced by external pressure. Solid-state ²H NMR spectroscopy of phospholipid bilayers under osmotic stress allows structural fluctuations and deformation of membranes to be investigated. We highlight the results from NMR experiments utilizing pressure-based force techniques that control membrane structure and tension. Our ²H NMR results using both dehydration pressure (low water activity) and osmotic pressure (poly(ethylene glycol) as osmolyte) show that the segmental order parameters (S(CD)) of DMPC approach very large values of ≈ 0.35 in the liquid-crystalline state. The two stresses are thermodynamically equivalent, because the change in chemical potential when transferring water from the interlamellar space to the bulk water phase corresponds to the induced pressure. This theoretical equivalence is experimentally revealed by considering the solid-state ²H NMR spectrometer as a virtual osmometer. Moreover, we extend this approach to include the correspondence between osmotic pressure and hydrostatic pressure. Our results establish the magnitude of the pressures that lead to significant bilayer deformation including changes in area per lipid and volumetric bilayer thickness. We find that appreciable bilayer structural changes occur with osmotic pressures in the range of 10-100 atm or lower. This research demonstrates the applicability of solid-state ²H NMR spectroscopy together with bilayer stress techniques for investigating the mechanism of pressure sensitivity of membrane proteins.

Pressure effects on lipid membrane structure and dynamics

The effect of hydrostatic pressure on lipid structure and dynamics is highly important as a tool in biophysics and bio-technology, and in the biology of deep sea organisms. Despite its importance, high hydrostatic pressure remains significantly less utilised than other thermodynamic variables such as temperature and chemical composition. Here, we give an overview of some of the theoretical aspects which determine lipid behaviour under pressure and the techniques and technology available to study these effects. We also summarise several recent experiments which highlight the information available from these approaches.