Cell adhesion and spreading at a charged interface: Insight into the mechanism using surface techniques and mathematical modelling

From algal cells to autofluorescent ghost plasma membrane vesicles

Plasma membrane vesicles can be effective, non-toxic carriers for microscale material transport, provide a convenient model for probing membrane-related processes, since intracellular biochemical processes are eliminated. We describe here a fine-tuned protocol for isolating ghost plasma membrane vesicles from the unicellular alga Dunaliella tertiolecta, and preliminary characterization of their structural features and permeability properties, with comparisons to giant unilamellar phospholipid vesicles. The complexity of the algal ghost membrane vesicles reconstructed from the native membrane material released after hypoosmotic stress lies between that of phospholipid vesicles and cells. AFM structural characterization of reconstructed vesicles shows a thick envelope and a nearly empty vesicle interior. The surface of the envelope contains a heterogeneous distribution of densely packed, nanometer-scale globules and pore-like structures which may be derived from surface coat proteins. Confocal fluorescence imaging reveals the highly pigmented photosynthetic apparatus located within the thylakoid membrane and retained in the vesicle membrane. Transport of the fluorescent dye calcein into ghost and giant unilamellar vesicles reveals significant differences in permeability. Expanded knowledge of this unique membrane system will contribute to the design of marine bio-inspired carriers for advanced biotechnological applications.

Fluorescence responsiveness of unicellular marine algae Dunaliella to stressors under laboratory conditions

We examined the responsiveness of unicellular green alga Dunalliela tertiolecta to selected stressors employing confocal- and time-resolved imaging of endogenous fluorescence. Our aim was to monitor cell endogenous fluorescence changes under exposure to heavy metal Cd, acidification, as well as light by laser-induced photobleaching. The accumulation of Cd in algae cells was confirmed by the secondary ion mass spectroscopy technique. For the first time, custom-made computational techniques were employed to evaluate separately the fluorescence in the flagella vs. the body region. In the presence of Cd, we recorded increase in the green fluorescence in the flagella region in the form of opacities, without change in the fluorescence lifetimes, suggesting higher availability of the fluorescent molecules. Under acidification, we noted significant rise in the green fluorescence in the flagella region, but associated with longer fluorescence lifetimes, pointing to changes in the algae environment. Photobleaching experiments corroborated gathered observations. Obtained data support a differential responsiveness of the flagella vs. the body region to stressors and enable us to better understand the pathophysiological changes of algal cells in culture under stress conditions.

Changes in nanomechanical properties and adhesion dynamics of algal cells during their growth

Nanomechanical and structural characterisations of algal cells are of key importance for understanding their adhesion behaviour at interfaces in the aquatic environment. We examine here the nanomechanical properties and adhesion dynamics of the algal cells during two phases of their growth using complementary surface methods and the mathematical modelling. Mechanical properties of motile cells are hard to assess while keeping cells viable, and studies to date have been limited. Immobilisation of negatively charged cells to a positively charged substrate enables high-resolution AFM imaging and nanomechanical measurements. Cells were stiffer and more hydrophobic in the exponential than in the stationary phase, suggesting molecular modification of the cell envelope during aging. The corresponding properties of algal cells were in agreement with the increase of critical interfacial tensions of adhesion, determined amperometrically. Cells in exponential phase possessed a larger cell volume, in agreement with the large amount of amperometrically measured displaced charge at the interface. Differences in the kinetics of adhesion and spreading of cells at the interface were attributed to their various volumes and nanomechanical properties that varied during cell aging. Our findings contribute to the present body of knowledge on the biophysics of algal cells on a fundamental level.

Algal cell response to laboratory-induced cadmium stress: a multimethod approach

We examined the response of algal cells to laboratory-induced cadmium stress in terms of physiological activity, autonomous features (motility and fluorescence), adhesion dynamics, nanomechanical properties, and protein expression by employing a multimethod approach. We develop a methodology based on the generalized mathematical model to predict free cadmium concentrations in culture. We used algal cells of Dunaliella tertiolecta, which are widespread in marine and freshwater systems, as a model organism. Cell adaptation to cadmium stress is manifested through cell shape deterioration, slower motility, and an increase of physiological activity. No significant change in growth dynamics showed how cells adapt to stress by increasing active surface area against toxic cadmium in the culture. It was accompanied by an increase in green fluorescence (most likely associated with cadmium vesicular transport and/or beta-carotene production), while no change was observed in the red endogenous fluorescence (associated with chlorophyll). To maintain the same rate of chlorophyll emission, the cell adaptation response was manifested through increased expression of the identified chlorophyll-binding protein(s) that are important for photosynthesis. Since production of these proteins represents cell defence mechanisms, they may also signal the presence of toxic metal in seawater. Protein expression affects the cell surface properties and, therefore, the dynamics of the adhesion process. Cells behave stiffer under stress with cadmium, and thus, the initial attachment and deformation are slower. Physicochemical and structural characterizations of algal cell surfaces are of key importance to interpret, rationalize, and predict the behaviour and fate of the cell under stress in vivo.

Phospholipid and Hydrocarbon Interactions with a Charged Electrode Interface

Using a combination of molecular dynamics simulations and experiments we examined the interactions of alkanes and phospholipids at charged interfaces in order to understand how interfacial charge densities affect the association of these two representative molecules with electrodes. Consistent with theory and experiment, these model systems reveal interfacial associations mediated through a combination of Coulombic and van der Waals forces. van der Waals forces, in particular, mediate rapid binding of decane to neutral electrodes. No decane binding was observed at high surface charge densities because of interfacial water polarization, which screens hydrophobic attractions. The positively charged choline moiety of the phospholipid palmitoyloleoylphosphatidylcholine (POPC) is primarily responsible for POPC attraction by a moderately negatively charged electrode. The hydrocarbon tails of POPC interact with the hydrophobic electrode interface similarly to decane. Previously reported electrochemical results confirm these findings by demonstrating bipolar displacement currents from PC vesicles adhering to moderately negatively charged interfaces, originating from the choline interactions observed in simulations. At more negatively charged interfaces, choline-to-surface binding was stronger. In both simulations and experiments the maximal interaction of anionic PS occurs with a positively charged interface, provided that the electrostatic forces outweigh local Lennard-Jones interactions. Direct comparisons between the binding affinities measured in experiments and those obtained in simulations reveal previously unobserved atomic interactions that facilitate lipid vesicle adhesion to charged interfaces. Moreover, the implementation of a charged interface in molecular dynamics simulations provides an alternative method for the generation of large electric fields across phospholipid bilayers, especially for systems with periodic boundary conditions, and may be useful for simulations of membrane electropermeabilization.