Cholesterol induced asymmetry in DOPC bilayers probed by AFM force spectroscopy

In Situ Heterodyne-Detected Second-Harmonic Generation Study of the Influence of Cholesterol on Dye Molecule Adsorption on Lipid Membrane

Cholesterol plays an essential role in regulating the functionality of biomembranes. This study employed in situ second-harmonic generation (SHG) to investigate the adsorption behavior of the dye molecule 4-(4-(diethylamino)styryl)-N-methyl-pyridinium iodide (D289) on a biomimic membrane composed of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (DPPG) and cholesterol. The time-dependent polarization SHG intensity exhibited an initial rapid increase, followed by a subsequent decline. The initial increased SHG intensity is responsible for the electrostatic interaction-driven adsorption of D289 onto the membrane, while the decrease in the SHG signal results from the broadening of the orientation distribution within the membrane. Heterodyne-detected SHG (HD-SHG) measurements demonstrated that the adsorption of dye molecules influenced the phase of the induced electric field. The interfacial potential Φ(0) as a function of time was measured, and we found that even after reaching a stable Stern layer state, the diffusion layer continued to exhibit a dynamic change. This study offers a comprehensive understanding of the influence of cholesterol on adsorption, reorientation dynamics, and dynamic changes in the reorientation of water in the diffusion layer.

Cholesterol-Induced Nanoscale Variations in the Thickness of Phospholipid Membranes

Graphene-induced energy transfer (GIET) is a recently developed fluorescence-spectroscopic technique that achieves subnanometric optical localization of fluorophores along the optical axis of a microscope. GIET is based on the near-field energy transfer from an optically excited fluorescent molecule to a single sheet of graphene. It has been successfully used for estimating interleaflet distances of single lipid bilayers and for investigating the membrane organization of living mitochondria. In this study, we use GIET to measure the cholesterol-induced subtle changes of membrane thickness at the nanoscale. We quantify membrane thickness variations in supported lipid bilayers (SLBs) as a function of lipid composition and increasing cholesterol content. Our findings demonstrate that GIET is an extremely sensitive tool for investigating nanometric structural changes in biomembranes.

Membrane elasticity modulated by cholesterol in model of porcine eye lens-lipid membrane

Experimental evidence shows that the eye lens loses its elasticity dramatically with age. It has also been reported that the cholesterol (Chol) content in the eye lens fiber cell plasma membrane increases significantly with age. High Chol content leads to the formation of cholesterol bilayer domains (CBDs) in the lens membrane. The role of high Chol associated with lens elasticity is unclear. The purpose of this research is to investigate the membrane elasticity of the model of porcine lens-lipid (MPLL) membrane with increasing Chol content to elucidate the role of high Chol in lens membrane elasticity. In this study, we used atomic force microscopy (AFM) to study the mechanical properties (breakthrough force and area compressibility modulus (K_A)) of the MPLL membrane with increasing Chol content where K_A is the measure of membrane elasticity. We varied Chol concentration in Chol/MPLL membrane from 0 to ∼71 mol%. Supported Chol/MPLL membranes were prepared by fusion of small unilamellar vesicles (SUVs) on top of a flat mica surface. SUVs of the Chol/MPLL lipid mixture were prepared with the rapid solvent exchange method followed by probe-tip sonication. For the Chol/MPLL mixing ratio of 0, AFM image showed the formation of two distinct phases of the membrane, i.e., liquid-disordered phase (l_d) and solid-ordered phase (s_o) membrane. However, with Chol/MPLL mixing ratio of 0.5 and above, only liquid-ordered phase (l_o) membrane was formed. Also, two distinct breakthrough forces corresponding to l_d and s_o were observed for Chol/MPLL mixing ratio of 0, whereas only one breakthrough force was observed for membranes with Chol/MPLL mixing ratio of 0.5 and above. No significant difference in the membrane surface roughness was measured with increasing Chol content for these membranes; however, breakthrough force and K_A for l_o membrane increased when Chol/MPLL mixing ratio was increased from 0.5 to 1. Interestingly above the Chol/MPLL mixing ratio of 1, both breakthrough force and K_A decreased, indicating the formation of CBDs. Furthermore, these results showed that membrane elasticity increases at high Chol content, suggesting that high Chol content in lens membrane might be responsible for maintaining lens membrane elasticity.

Mechanical properties of the high cholesterol-containing membrane: An AFM study

Cholesterol (Chol) content in most cellular membranes does not exceed 50 mol%, only in the eye lens's fiber cell plasma membrane, its content surpasses 50 mol%. At this high concentration, Chol induces the formation of pure cholesterol bilayer domains (CBDs), which coexist with the surrounding phospholipid-cholesterol domain (PCD). Here, we applied atomic force microscopy to study the mechanical properties of Chol/phosphatidylcholine membranes where the Chol content was increased from 0 to 75 mol%, relevant to eye lens membranes. The surface roughness of the membrane decreases with an increase of Chol content until it reaches 60 mol%, and roughness increases with a further increment in Chol content. We propose that the increased roughness at higher Chol content results from the formation of CBDs. Force spectroscopy on the membrane with Chol content of 50 mol% or lesser exhibited single breakthrough events, whereas two distinct puncture events were observed for membranes with the Chol content greater than 50 mol%. We propose that the first puncture force corresponds to the membranes containing coexisting PCD and CBDs. In contrast, the second puncture force corresponds to the "CBD water pocket" formed due to coexisting CBDs and PCD. Membrane area compressibility modulus (K_A) increases with an increase in Chol content until it reaches 60 mol%, and with further increment in Chol content, CBDs are formed, and K_A starts to decrease. Our results report the increase in membrane roughness and decrease K_A at very high Chol content (>60 mol%) relevant to the eye lens membrane.

25-Hydroxycholesterol Effect on Membrane Structure and Mechanical Properties

Cholesterol is responsible for the plasticity of plasma membranes and is involved in physiological and pathophysiological responses. Cholesterol homeostasis is regulated by oxysterols, such as 25-hydroxycholesterol. The presence of 25-hydroxycholesterol at the membrane level has been shown to interfere with several viruses’ entry into their target cells. We used atomic force microscopy to assess the effect of 25-hydroxycholesterol on different properties of supported lipid bilayers with controlled lipid compositions. In particular, we showed that 25-hydroxycholesterol inhibits the lipid-condensing effects of cholesterol, rendering the bilayers less rigid. This study indicates that the inclusion of 25-hydroxycholesterol in plasma membranes or the conversion of part of their cholesterol content into 25-hydroxycholesterol leads to morphological alterations of the sphingomyelin (SM)-enriched domains and promotes lipid packing inhomogeneities. These changes culminate in membrane stiffness variations.

Interleaflet Decoupling in a Lipid Bilayer at Excess Cholesterol Probed by Spectroscopic Ellipsometry and Simulations

Artificial lipid membranes are often investigated as a replica of the cell membrane in the form of supported lipid bilayers (SLBs). In SLBs, the phase state of a lipid bilayer strongly depends on the presence of molecules such as cholesterol, ceramide, and physical parameters such as temperature. Cholesterol is a key molecule of biological membranes and it exerts condensing effect on lipid bilayers. In this paper, we demonstrate the influence of excess cholesterol content on a supported lipid bilayer of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (fluid-phase) using spectroscopic ellipsometry (SE) and coarse-grained (CG) molecular dynamics (MD) simulations. The results show the condensation effect due to cholesterol addition up to 30% and interleaflet decoupling at excess cholesterol beyond 30%. SE results show the separation of individual leaflets of the bilayer and influence of cholesterol on the biophysical properties such as thickness and optical index. CG simulations were performed at different ratios of DOPC:cholesterol mixtures to explore cholesterol-driven bilayer properties and stability. The simulations displayed the accumulation of cholesterol molecules at the interface of the lower and upper leaflets of the bilayer, thus leading to undulations in the bilayer. This work reports the successful application of SE technique to study lipid-cholesterol interactions for the first time.

Nano-mechanical characterization of asymmetric DLPC/DSPC supported lipid bilayers

Asymmetric distribution of lipid molecules in the inner and outer leaflets of the plasma membrane is a common occurrence in the membrane formation. Such asymmetric arrangement is a crucial parameter to manipulate the properties of the cell membrane. It controls signal transduction, endocytosis, exocytosis in the cells. The artificial membrane is often used to study the lateral and transverse arrangement of the lipid molecules in place of the cell membrane. Nano-mechanical characterization of the model membrane helps to understand the mechanical stability of the lipid bilayer. The stability is sensitive to the variations in the lipid composition and their local organization. In this article, we present both topographical and nano-mechanical properties of lipid bilayer characterized by atomic force microscopy (AFM). The results show that the asymmetric lipid bilayer formation is an intrinsic character. We have selected a bi-component fluid-gel phase 1,2-dilauroyl-sn-glycero-3-phosphocholine:1,2-disteroyl-sn-glycero-3-phosphocholine (DLPC: DSPC) system for our studies. We have observed domain formation and phase separation in the bilayer by increasing the composition of the gel phase DSPC. In force spectroscopy studies, we determine the mechanical strength of the bilayer for unique mixtures of DLPC: DSPC by measuring the breakthrough force. These results also show the effect of asymmetry in the lipid bilayer. Besides AFM studies, we have implemented a coarse-grained (CG) molecular dynamics (MD) simulation using the gromacs package at room temperature and 1 bar pressure. The results from the simulation study have been compared with AFM study. It was found that the simulation studies corroborated the findings from AFM such as an increase in the bilayer thickness, change in the phase state, asymmetric and symmetric domain formation in the lipid bilayer.

Lipid bilayers: Phase behavior and nanomechanics

Lipid membranes are involved in many physiological processes like recognition, signaling, fusion or remodeling of the cell membrane or some of its internal compartments. Within the cell, they are the ultimate barrier, while maintaining the fluidity or flexibility required for a myriad of processes, including membrane protein assembly. The physical properties of in vitro model membranes as model cell membranes have been extensively studied with a variety of techniques, from classical thermodynamics to advanced modern microscopies. Here we review the nanomechanics of solid-supported lipid membranes with a focus in their phase behavior. Relevant information obtained by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) as complementary techniques in the nano/mesoscale interface is presented. Membrane morphological and mechanical characterization will be discussed in the framework of its phase behavior, phase transitions and coexistence, in simple and complex models, and upon the presence of cholesterol.

Asymmetric bilayers mimicking membrane rafts prepared by lipid exchange: Nanoscale characterization using AFM-Force spectroscopy

Sphingolipids-enriched rafts domains are proposed to occur in plasma membranes and to mediate important cellular functions. Notwithstanding, the asymmetric transbilayer distribution of phospholipids that exists in the membrane confers the two leaflets different potentials to form lateral domains as next to no sphingolipids are present in the inner leaflet. How the physical properties of one leaflet can influence the properties of the other and its importance on signal transduction across the membrane are questions still unresolved. In this work, we combined AFM imaging and Force spectroscopy measurements to assess domain formation and to study the nanomechanical properties of asymmetric supported lipid bilayers (SLBs) mimicking membrane rafts. Asymmetric SLBs were formed by incorporating N-palmitoyl-sphingomyelin (16:0SM) into the outer leaflet of preformed 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Cholesterol SLBs through methyl-β-cyclodextrin-mediated lipid exchange. Lipid domains were detected after incorporation of 16:0SM though their phase state varied from gel to liquid ordered (Lo) phase if the procedure was performed at 24 or 37 °C, respectively. When comparing symmetric and asymmetric Lo domains, differences in size and morphology were observed, with asymmetric domains being smaller and more interconnected. Both types of Lo domains showed similar mechanical stability in terms of rupture forces and Young's moduli. Notably, force curves in asymmetric domains presented two rupture events that could be attributed to the sequential rupture of a liquid disordered (Ld) and a Lo phase. Interleaflet coupling in asymmetric Lo domains could also be inferred from those measurements. The experimental approach outlined here would significantly enhance the applicability of membrane models.

Surface effects on the degree of twist in amyloid fibril structures

Amyloid fibrils, implicated in health and diseases, commonly exhibit a periodic twist trait relevant to the structures and dynamics of the fibrils. However, the origins and modulations of fibril twist in complex in vivo environments are not yet fully understood. Here we highlight an important factor that causes twist variations in amyloid fibril structures-the presence of surrounding surfaces. Using cholesterol-containing lipid bilayers with varying cholesterol contents, we have demonstrated via atomic force microscopy that amyloid-β peptide fibrils initiated on membranes increase their average pitch size of twisting periodicity as the cholesterol content increases. These surface-induced twist variations arise from the enhanced hydrophobic interactions between the fibril and the surface distorting the torsional elastic energy of the fibril twisting as supported by a theory of an elastic model. These findings not only provide an important insight into fibril polymorphism phenomena resulting from the surface effects but also suggest a novel solution to modulate filament twisting on the nanoscale for biomaterials applications involving nanoscale features.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

Condensation of Model Lipid Films by Cholesterol: Specific Ion Effects

The condensing effect and the ability of cholesterol (CHOL) to induce ordering in lipid films is a question of relevance in biological membranes such as the milk fat globule membrane (MFGM) in which the amount of CHOL influences the phase separation and mechanical resistance to rupture of coexisting phases relevant to emulsified food systems. Here, we study the effect of different salts (NaCl, CaCl2, MgCl2, LaCl3) on monolayers made of a model mixture of lipids (DPPC:DPPS 4:1) and CHOL. To this end, we apply Langmuir Film Balance to report a combined analysis of surface pressure-area (π-A) and surface potential-area (ΔV–A) isotherms along with Micro-Brewster Angle Microscopy (Micro-BAM) images of the monolayers in the presence of the different electrolytes. We show that the condensation of lipid by CHOL depends strongly on the nature of the ions by altering the shape and features of the π-A isotherms. ΔV–A isotherms provide further detail on the ion specific interactions with CHOL. Our results show that the condensation of lipids in the presence of CHOL depends on the combined action of ions and CHOL, which can alter the physical state of the monolayer.