1
|
Abstract
The ability of cholesterol to uncoil (i.e., condense) the acyl chains of phospholipids has been known for a century. Despite extensive studies of the interactions between cholesterol and phospholipids, a molecular-level understanding of this uncoiling phenomenon has remained elusive. Equally unclear has been whether cholesterol's two different faces (i.e., its relatively smooth α face and its relatively rough β face) contribute to its condensing power. Because cholesterol's condensing effect is believed to play a major role in controlling the fluidity, structure, and functioning of all animal cell membranes, its biological importance cannot be overstated. This Perspective focuses on experimental evidence that addresses (i) the credibility of a popular "umbrella" mechanism that has been used to account for cholesterol's condensing effect, (ii) the credibility of an alternate "template" mechanism, (iii) the importance of cholesterol two-faced character with respect to its condensing power, and (iv) the viability of a surface occupancy model.
Collapse
|
2
|
Almeida PF. How to Determine Lipid Interactions in Membranes from Experiment Through the Ising Model. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:21-40. [PMID: 30589556 DOI: 10.1021/acs.langmuir.8b03054] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The determination and the meaning of interactions in lipid bilayers are discussed and interpreted through the Ising model. Originally developed to understand phase transitions in ferromagnetic systems, the Ising model applies equally well to lipid bilayers. In the case of a membrane, the essence of the Ising model is that each lipid is represented by a site on a lattice and that the interaction of each site with its nearest neighbors is represented by an energy parameter ω. To calculate the thermodynamic properties of the system, such as the enthalpy, the Gibbs energy, and the heat capacity, the partition function is derived. The calculation of the configurational entropy factor in the partition function, however, requires approximations or the use of Monte Carlo (MC) simulations. Those approximations are described. Ultimately, MC simulations are used in combination with experiment to determine the interaction parameters ω in lipid bilayers. Several experimental approaches are described, which can be used to obtain interaction parameters. They include nearest-neighbor recognition, differential scanning calorimetry, and Förster resonance energy transfer. Those approaches are most powerful when used in combination of MC simulations of Ising models. Lipid membranes of different compositions are discussed, which have been studied with these approaches. They include mixtures of cholesterol, saturated (ordered) phospholipids, and unsaturated (disordered) phospholipids. The interactions between those lipid species are examined as a function of molecular properties such as the degree of unsaturation and the acyl chain length. The general rule that emerges is that interactions between different lipids are usually unfavorable. The exception is that cholesterol interacts favorably with saturated (ordered) phospholipids. However, the interaction of cholesterol with unsaturated phospholipids becomes extremely unfavorable as the degree of unsaturation increases.
Collapse
Affiliation(s)
- Paulo F Almeida
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| |
Collapse
|
3
|
Almeida PF, Carter FE, Kilgour KM, Raymonda MH, Tejada E. Heat Capacity of DPPC/Cholesterol Mixtures: Comparison of Single Bilayers with Multibilayers and Simulations. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:9798-9809. [PMID: 30088940 DOI: 10.1021/acs.langmuir.8b01774] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The excess heat capacity (Δ C p) of mixtures of dipalmitoylphosphatidylcholine (DPPC) and cholesterol (Chol) is examined in detail in large unilamellar vesicles (LUVs), both experimentally, using differential scanning calorimetry (DSC), and theoretically, using a three-state Ising model. The model postulates that DPPC can access three conformational states: gel, liquid-disordered (Ld), and liquid-ordered (Lo). The Lo state, however, is only available if coupled with interaction with an adjacent Chol. Δ C p was calculated using Monte Carlo simulations on a lattice and compared to experiment. The DSC results in LUVs are compared with literature data on multilamellar vesicles (MLVs). The enthalpy change of the complete phase transition from gel to Ld is identical in LUVs and MLVs, and the melting temperatures ( Tm) are similar. However, the DSC curves in LUVs are significantly broader, and the maxima of Δ C p are accordingly smaller. The parameters in the Ising model were chosen to match the DSC curves in LUVs and the nearest-neighbor recognition (NNR) data. The model reproduces the NNR data very well. It also reproduces the phase transition in DPPC, the freezing point depression induced by Chol, and the broad component of Δ C p in DPPC/Chol LUVs. However, there is a sharp component, between 5 and 15 mol % Chol, that the model does not reproduce. The broad component of Δ C p becomes dominant as Chol concentration increases, indicating that it involves melting of the Lo phase. Because the simulations reproduce this component, the conclusions regarding the nature of the phase transition at high Chol concentrations and the structure of the Lo phase are important: there is no true phase separation in DPPC/Chol LUVs. There are large domains of gel and Lo phase coexisting below Tm of DPPC, but above Tm the three states of DPPC are mixed with Chol, although clusters persist.
Collapse
Affiliation(s)
- Paulo F Almeida
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| | - Faith E Carter
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| | - Katie M Kilgour
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| | - Matthew H Raymonda
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| | - Emmanuel Tejada
- Department of Chemistry and Biochemistry , University of North Carolina Wilmington , Wilmington , North Carolina 28403 , United States
| |
Collapse
|
4
|
Javanainen M, Martinez-Seara H, Vattulainen I. Nanoscale Membrane Domain Formation Driven by Cholesterol. Sci Rep 2017; 7:1143. [PMID: 28442766 PMCID: PMC5430823 DOI: 10.1038/s41598-017-01247-9] [Citation(s) in RCA: 57] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2017] [Accepted: 03/27/2017] [Indexed: 11/09/2022] Open
Abstract
Biological membranes generate specific functions through compartmentalized regions such as cholesterol-enriched membrane nanodomains that host selected proteins. Despite the biological significance of nanodomains, details on their structure remain elusive. They cannot be observed via microscopic experimental techniques due to their small size, yet there is also a lack of atomistic simulation models able to describe spontaneous nanodomain formation in sufficiently simple but biologically relevant complex membranes. Here we use atomistic simulations to consider a binary mixture of saturated dipalmitoylphosphatidylcholine and cholesterol - the "minimal standard" for nanodomain formation. The simulations reveal how cholesterol drives the formation of fluid cholesterol-rich nanodomains hosting hexagonally packed cholesterol-poor lipid nanoclusters, both of which show registration between the membrane leaflets. The complex nanodomain substructure forms when cholesterol positions itself in the domain boundary region. Here cholesterol can also readily flip-flop across the membrane. Most importantly, replacing cholesterol with a sterol characterized by a less asymmetric ring region impairs the emergence of nanodomains. The model considered explains a plethora of controversial experimental results and provides an excellent basis for further computational studies on nanodomains. Furthermore, the results highlight the role of cholesterol as a key player in the modulation of nanodomains for membrane protein function.
Collapse
Affiliation(s)
- Matti Javanainen
- Laboratory of Physics, Tampere University of Technology, Tampere, Finland.,Department of Physics, University of Helsinki, Helsinki, Finland
| | - Hector Martinez-Seara
- Laboratory of Physics, Tampere University of Technology, Tampere, Finland. .,Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague, Czech Republic.
| | - Ilpo Vattulainen
- Laboratory of Physics, Tampere University of Technology, Tampere, Finland. .,Department of Physics, University of Helsinki, Helsinki, Finland. .,MEMPHYS - Centre for Biomembrane Physics, University of Southern Denmark, Odense, Denmark.
| |
Collapse
|
5
|
Kulig W, Cwiklik L, Jurkiewicz P, Rog T, Vattulainen I. Cholesterol oxidation products and their biological importance. Chem Phys Lipids 2016; 199:144-160. [DOI: 10.1016/j.chemphyslip.2016.03.001] [Citation(s) in RCA: 98] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2016] [Revised: 03/02/2016] [Accepted: 03/03/2016] [Indexed: 12/14/2022]
|
6
|
Wang Y, Gkeka P, Fuchs JE, Liedl KR, Cournia Z. DPPC-cholesterol phase diagram using coarse-grained Molecular Dynamics simulations. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:2846-2857. [PMID: 27526680 DOI: 10.1016/j.bbamem.2016.08.005] [Citation(s) in RCA: 77] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2015] [Revised: 08/06/2016] [Accepted: 08/11/2016] [Indexed: 12/30/2022]
Abstract
Cholesterol-phospholipid bilayers continue to be the current state of the art in membrane models and serve as representative systems for studying the effect of cholesterol on the cell membrane. As the mixing of different lipid species requires long spatio-temporal scales, coarse-grained models have gained increasing popularity in modeling such membrane systems. In this paper, a systematic study of the MARTINI coarse-grained model for the DPPC-cholesterol binary system has been performed. We construct the phase diagram of DPPC lipid bilayers in the presence of different cholesterol concentrations and at different temperatures using coarse-grained Molecular Dynamics (MD) simulations with the MARTINI force field. The phase diagram based on the condensation effect is directly comparable to available experimental data and demonstrates qualitative agreement over all cholesterol concentrations. Self-assembled bilayers quantitatively reproduce experimental observables, such as lateral diffusion of lipids, electron density, area per lipid and lipid order parameters. The phase diagram of the DPPC-cholesterol binary system also reveals the profound effect of cholesterol on the physical properties of phospholipid bilayers such lipid order, diffusion, and fluidity. Cholesterol induces the liquid-ordered phase, which increases the fluidity of the phospholipid hydrocarbon chains above the gel to liquid-crystalline phase transition temperature and decreases it below the phase transition. The present study suggests that the MARTINI force field can be successfully used to obtain molecular level insights into cholesterol-DPPC model membranes.
Collapse
Affiliation(s)
- Yin Wang
- Department of Theoretical Chemistry and Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria
| | - Paraskevi Gkeka
- Biomedical Research Foundation, Academy of Athens, 4 Soranou Ephessiou, 11527, Athens, Greece
| | - Julian E Fuchs
- Department of Theoretical Chemistry and Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria
| | - Klaus R Liedl
- Department of Theoretical Chemistry and Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria.
| | - Zoe Cournia
- Biomedical Research Foundation, Academy of Athens, 4 Soranou Ephessiou, 11527, Athens, Greece.
| |
Collapse
|
7
|
Kepczynski M, Róg T. Functionalized lipids and surfactants for specific applications. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:2362-2379. [PMID: 26946243 DOI: 10.1016/j.bbamem.2016.02.038] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/15/2016] [Revised: 02/23/2016] [Accepted: 02/25/2016] [Indexed: 12/17/2022]
Abstract
Synthetic lipids and surfactants that do not exist in biological systems have been used for the last few decades in both basic and applied science. The most notable applications for synthetic lipids and surfactants are drug delivery, gene transfection, as reporting molecules, and as support for structural lipid biology. In this review, we describe the potential of the synergistic combination of computational and experimental methodologies to study the behavior of synthetic lipids and surfactants embedded in lipid membranes and liposomes. We focused on select cases in which molecular dynamics simulations were used to complement experimental studies aiming to understand the structure and properties of new compounds at the atomistic level. We also describe cases in which molecular dynamics simulations were used to design new synthetic lipids and surfactants, as well as emerging fields for the application of these compounds. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.
Collapse
Affiliation(s)
- Mariusz Kepczynski
- Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
| | - Tomasz Róg
- Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101, Tampere, Finland; Department of Physics, Helsinki University, P.O. Box 64, FI 00014 Helsinki, Finland.
| |
Collapse
|
8
|
Róg T, Pöyry S, Vattulainen I. Building Synthetic Sterols Computationally - Unlocking the Secrets of Evolution? Front Bioeng Biotechnol 2015; 3:121. [PMID: 26347865 PMCID: PMC4543873 DOI: 10.3389/fbioe.2015.00121] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2015] [Accepted: 08/07/2015] [Indexed: 01/28/2023] Open
Abstract
Cholesterol is vital in regulating the physical properties of animal cell membranes. While it remains unclear what renders cholesterol so unique, it is known that other sterols are less capable in modulating membrane properties, and there are membrane proteins whose function is dependent on cholesterol. Practical applications of cholesterol include its use in liposomes in drug delivery and cosmetics, cholesterol-based detergents in membrane protein crystallography, its fluorescent analogs in studies of cholesterol transport in cells and tissues, etc. Clearly, in spite of their difficult synthesis, producing the synthetic analogs of cholesterol is of great commercial and scientific interest. In this article, we discuss how synthetic sterols non-existent in nature can be used to elucidate the roles of cholesterol’s structural elements. To this end, we discuss recent atomistic molecular dynamics simulation studies that have predicted new synthetic sterols with properties comparable to those of cholesterol. We also discuss more recent experimental studies that have vindicated these predictions. The paper highlights the strength of computational simulations in making predictions for synthetic biology, thereby guiding experiments.
Collapse
Affiliation(s)
- Tomasz Róg
- Department of Physics, Tampere University of Technology , Tampere , Finland
| | - Sanja Pöyry
- Department of Physics, Tampere University of Technology , Tampere , Finland
| | - Ilpo Vattulainen
- Department of Physics, Tampere University of Technology , Tampere , Finland ; MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark , Odense , Denmark
| |
Collapse
|
9
|
Cis and trans unsaturated phosphatidylcholine bilayers: A molecular dynamics simulation study. Chem Phys Lipids 2015; 195:12-20. [PMID: 26187855 DOI: 10.1016/j.chemphyslip.2015.07.002] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2015] [Revised: 07/06/2015] [Accepted: 07/07/2015] [Indexed: 12/14/2022]
Abstract
Trans unsaturated lipids are uncommon in nature. In the human diet, they occur as natural products of ruminal bacteria or from industrial food processing like hydrogenation of vegetable oils. Consumption of trans unsaturated lipids has been shown to have a negative influence on human health; in particular, the risk of cardiovascular disease is higher when the amount of trans unsaturated lipids in the diet is elevated. In this study, we first performed quantum mechanical calculations to specifically and accurately parameterize cis and trans mono-unsaturated lipids and subsequently validated the newly derived parameter set. Then, we carried out molecular dynamics (MD) simulations of lipid bilayers composed of cis or trans unsaturated lipids with and without cholesterol. Our results show that trans mono-unsaturated chains are more flexible than cis mono-unsaturated chains due to lower barriers for rotation around the single bonds next to the trans double bond than those next to the cis double bond. In effect, interactions between cholesterol and trans unsaturated chains are stronger than cis unsaturated chains, which results in a higher ordering effect of cholesterol in trans unsaturated bilayers.
Collapse
|
10
|
Róg T, Vattulainen I. Cholesterol, sphingolipids, and glycolipids: what do we know about their role in raft-like membranes? Chem Phys Lipids 2014; 184:82-104. [PMID: 25444976 DOI: 10.1016/j.chemphyslip.2014.10.004] [Citation(s) in RCA: 140] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2014] [Revised: 10/24/2014] [Accepted: 10/25/2014] [Indexed: 12/14/2022]
Abstract
Lipids rafts are considered to be functional nanoscale membrane domains enriched in cholesterol and sphingolipids, characteristic in particular of the external leaflet of cell membranes. Lipids, together with membrane-associated proteins, are therefore considered to form nanoscale units with potential specific functions. Although the understanding of the structure of rafts in living cells is quite limited, the possible functions of rafts are widely discussed in the literature, highlighting their importance in cellular functions. In this review, we discuss the understanding of rafts that has emerged based on recent atomistic and coarse-grained molecular dynamics simulation studies on the key lipid raft components, which include cholesterol, sphingolipids, glycolipids, and the proteins interacting with these classes of lipids. The simulation results are compared to experiments when possible.
Collapse
Affiliation(s)
- Tomasz Róg
- Department of Physics, Tampere University of Technology, Tampere, Finland
| | - Ilpo Vattulainen
- Department of Physics, Tampere University of Technology, Tampere, Finland; MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark, Odense, Denmark.
| |
Collapse
|