Biophysics of membranes

Academic lead: Dr Peter Petrov

Investigating the relationships between composition, structure, lateral order, physical properties and function of biological membranes.

We use and develop a wide range of techniques to investigate the elastic and electrical properties of plasma membrane and their modification in disease. In particular, we are interested in the effects of oxidative stress on membrane mechanics, since there is overwhelming evidence that oxidative stress contributes critically to the development and detrimental effects of many diseases, including major health threats such as diabetes mellitus, atherosclerosis, arthritis, and a number of age related conditions. This approach also allows us to assess the anti-oxidative potential of drugs.

Another area of research in our laboratory is the interaction between proteins and lipid bilayer membranes. We investigate how the membrane lateral microdomain organisation and viscoelastic and electrical properties are modified as a result of the interactions with various classes of proteins (e.g. cytosekeltal and elastic proteins as well as bacterial toxins). Along with colleagues from the Medical School, we also work on protein-membrane interactions for proteins involved in the process of apoptosis.

We were the first to establish the presence of long-range lateral order in the tear film lipid layer using Grazing Incidence X-ray Diffraction. This is a part of a wider project aimed at detailed understanding of the relations between the mesoscopic organisation and the functional performance of the pre-ocular tear film lipid layer, and how they depend on the molecular diversity found in the layer.

Much work, in collaboration with Julian Moger, is carried out on the use of nonlinear optical imaging techniques, such as Coherent Anti-Stokes Raman Scattering (CARS), Stimulated Raman Scattering (SRS) and Multiphoton Fluorescence (MF) to investigate protein-membrane interactions and uptake of fatty acids by cells.

Outside the immediate area of membrane biophysics, we work on the problem of swimming at low Reynolds numbers and the possibility to construct working prototypes of viscous swimmers.

Current projects

  • Analysis of the relationships between membrane lipid composition and rheoviscous properties.
  • Characterisation of the interactions between membrane lipids and the cytoskeletal protein, spectrin.
  • Investigation of the changes in the mechanical and rheological properties of blood cells in diabetes.
  • Investigation of the mechanics and rheological properties of the cammelid erythrocyte.
  • Characterisation of the physical properties of the endothelial cell glycocalyx.
  • The development of theoretical models of membrane mechanics.
  • The role of endothelial cells in mechanotransduction

In this research we employ a variety of approaches ranging from micropipette aspiration and fluctuation spectroscopy of cells and vesicles to the study of Langmuir monolayers using classical approaches, microscopy and glancing angle X-ray diffraction.

Langmuir monolayers

Current studies include:

  • Structure and properties of the pre-ocular tear film
  • Lipid monolayers from normal and abnormal cell membranes
  • Interactions between skeletal proteins and lipid monolayers

Research techniques:

These classical techniques give information about the overall phase behaviour and the electrostatic properties of the monomolecular layer spread on the liquid surface.

Pressure-area isotherm of a DMPC monolayer

Fluorescence microscopy can be used to visualise domain structures in lipid monomolecular layers formed on the water/air interface at different surface pressures, and to investigate the effects of various solutes such as ions and proteins in the aqueous subphase on domain structure. The domain structure of membrane lipids is believed to be important to the function. We use fluorescence microscopy to detect lipid domain formation and dynamics.

Fluorescence microscopy image from a DPPC monolayer doped with NBD-PC. The domain size is approximately 35 µm.‌

The 2D order in the monolayer can be resolved using GIXD. The methods gives valuable information about change in molecular ordering due to interactions with solutes such as skeletal proteins or changes in the surface pressure.

One of our Langmuir troughs mounted in Station 16.2 at the Daresbury Synchrotron Radiation Source.

Giant lipid vesicles and native cell membranes

Current studies include:

 

  • Elastic properties of normal and abnormal cell membranes
  • Elasticity of giant lipid vesicles as cell membrane mimics

Research techniques:

Depending on the thermodynamical state of the membrane, this method can be used to evaluate the three main elastic characteristics of the membrane (bending, linear shear and the compressibility moduli). Stability (breaking stress) and shear viscosity are also open to exploration by this technique.

We are particularly interested in the mechanical properties of Red Blood Cells. In disease, membrane elasticity could change leading to circulatory problems.

Aspiration of a vacuole in a micropipette a few micrometres in diameter.

Giant lipid vesicles (typically tens of micrometres in size) are one of the best model systems for biological membranes. In their liquid state they are extremely soft and exhibit constant thermal shape fluctuations. We use an advanced method for analysis of the shape fluctuations (flicker spectroscopy) in order to measure the bending elastic modulus of this type of membranes. In this way, giant vesicles can be used as elastic probes to evaluate the contribution of different biologically relevant additives to membrane elasticity.

Thermal shape fluctuations of a prolate vesicle.

Raman microspectroscopy can provide information about the composition and structure of cell membranes: lipid composition, protein content, molecular conformations and protein-lipid interactions.