Undine Dietrich

Investigation of Phospholipid Area Compression Induced by Calcium-Mediated Dextran Sulfate Interaction

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca(2+) bridges results in a perturbation of lipid packing at physiologically relevant Ca(2+) concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by (2)H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca(2+) results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca(2+) concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of DeltaA = 2.7 A(2) per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca(2+)/DS-complexes. Lipid area compression after addition of DS and Ca(2+) to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca(2+) bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca(2+) concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Interaction of the MARCKS peptide with PIP2 in phospholipid monolayers

In this present work we have studied the effect of MARCKS (151-175) peptide on a mixed DPPC/PIP2 monolayer. By means of film balance, fluorescence microscopy, x-ray reflection/diffraction and neutron reflection measurements we detected changes in the lateral organization of the monolayer and changes in the perpendicular orientation of the PIP2 molecules depending on the presence of MARCKS (151-175) peptide in the subphase. In the mixed monolayer, the PIP2 molecules are distributed uniformly in the disordered phase of the monolayer, whereas the PI(4,5) groups elongate up to 10 A below the phosphodiester groups. This elongation forms the precondition for the electrostatic interaction of the MARCKS peptide with the PIP2 molecules. Due to the enrichment of PIP2 in the disordered phase, the interaction with the peptide occurs primarily in this phase, causing the PI(4,5) groups to tilt toward the monolayer interface.

Cell membrane softening in human breast and cervical cancer cells

Biomechanical properties are key to many cellular functions such as cell division and cell motility and thus are crucial in the development and understanding of several diseases, for instance cancer. The mechanics of the cellular cytoskeleton have been extensively characterized in cells and artificial systems. The rigidity of the plasma membrane, with the exception of red blood cells, is unknown and membrane rigidity measurements only exist for vesicles composed of a few synthetic lipids. In this study, thermal fluctuations of giant plasma membrane vesicles (GPMVs) directly derived from the plasma membranes of primary breast and cervical cells, as well as breast cell lines, are analyzed. Cell blebs or GPMVs were studied via thermal membrane fluctuations and mass spectrometry. It will be shown that cancer cell membranes are significantly softer than their non-malignant counterparts. This can be attributed to a loss of fluid raft forming lipids in malignant cells. These results indicate that the reduction of membrane rigidity promotes aggressive blebbing motion in invasive cancer cells.

Oscillations in the Lateral Pressure of Lipid Monolayers Induced by Nonlinear Chemical Dynamics of the Second Messengers MARCKS and Protein Kinase C

The binding of the MARCKS peptide to the lipid monolayer containing PIP(2) increases the lateral pressure of the monolayer. The unbinding dynamics modulated by protein kinase C leads to oscillations in lateral pressure of lipid monolayers. These periodic dynamics can be attributed to changes in the crystalline lipid domain size. We have developed a mathematical model to explain these observations based on the changes in the physical structure of the monolayer by the translocation of MARCKS peptide. The model indicates that changes in lipid domain size drives these oscillations. The model is extended to an open system that sustains chemical oscillations.

The effect of incorporation of phosphonic acid dibutyl ester in POPC model membranes on the hydration force

Abstract The hydration and swelling properties of lipid model membranes formed by mixtures of 1-palmitoyl-2-oleoylglycerophosphatidyl-choline (POPC) and different molar ratios R a l of dodecyl phosphonic acid dibutyl ester (PAB 12 ) in water were studied at 25°C. It is shown by X-ray diffraction and 31 P NMR spectroscopy that the system exists in a fluid lamellar phase at R a l = 0.25 and 0.5 over a wide range of water concentration. Intermembrane forces were determined by the osmotic stress technique. The hydration repulsion was found to decrease exponentially with increasing intermembrane water layer thickness. The values of the pre-exponential factor of the hydration forces which slightly decrease with increasing R a l were discussed according to the theory of water polarization near structured water/lipid interfaces (Cevc).

Structural investigation on the adsorption of the MARCKS peptide on anionic lipid monolayers - effects beyond electrostatic.

The presence of charged lipids in the cell membrane constitutes the background for the interaction with numerous membrane proteins. As a result, the valence of the lipids plays an important role concerning their lateral organization in the membrane and therefore the very manner of this interaction. This present study examines this aspect, particularly regarding to the interaction of the anionic lipid DPPS with the highly basic charged effector domain of the MARCKS protein, examined in monolayer model systems. Film balance, fluorescence microscopy and X-ray reflection/diffraction measurements were used to study the behavior of DPPS in a mixture with DPPC for its dependance on the presence of MARCKS (151-175). In the mixed monolayer, both lipids are completely miscible therefore DPPS is incorporated in the ordered crystalline DPPC domains as well. The interaction of MARCKS peptide with the mixed monolayer leads to the formation of lipid/peptide clusters causing an elongation of the serine group of the DPPS up to 7Å in direction to surface normal into the subphase. The large cationic charge of the peptide pulls out the serine group of the interface which simultaneously causes an elongation of the phosphodiester group of the lipid fraction too. The obtained results were used to compare the interaction of MARCKS peptide with the polyvalent PIP(2) in mixed monolayers. On this way we surprisingly find out, that the relative small charge difference of the anionic lipids causes a significant different interaction with MARCKS (151-175). The lateral arrangement of the anionic lipids depends on their charge values and determines the diffusion of the electrostatic binding clusters within the membrane.

Dynamic membrane structure induces temporal pattern formation.

The understanding of temporal pattern formation in biological systems is essential for insights into regulatory processes of cells. Concerning this problem, the present work introduces a model to explain the attachment/detachment cycle of MARCKS and PKC at the cell membrane, which is crucial for signal transduction processes. Our model is novel with regard to its driving mechanism: Structural changes within the membrane fuel an activator-inhibitor based global density oscillation of membrane related proteins. Based on simulated results of our model, phase diagrams were generated to illustrate the interplay of MARCKS and PKC. They predict the oscillatory behavior in the form of the number of peaks, the periodic time, and the damping constant depending on the amounts of MARCKS and PKC, respectively. The investigation of the phase space also revealed an unexpected intermediate state prior to the oscillations for high amounts of MARCKS in the system. The validation of the obtained results was carried out by stability analysis, which also accounts for further enhanced understanding of the studied system. It was shown, that the occurrence of the oscillating behavior is independent of the diffusion and the consumption of the reactants. The diffusion terms in the used reaction-diffusion equations only act as modulating terms and are not required for the oscillation. The hypothesis of our work suggests a new mechanism of temporal pattern formation in biological systems. This mechanism includes a classical activator-inhibitor system, but is based on the modifications of the membrane structure, rather than a reaction-diffusion system.