Singh, Karandeep (2018). Particles at complex membranes: receptors, ligands, and cytoskeleton. PhD thesis, Universität zu Köln.


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Cells internalize cargo via many biological pathways. In all cases, the cargo is encapsulated within a carrier that interacts with the plasma membrane of a cell via wrapping. Unravelling the interactions of nanoparticles with living cells is therefore fundamental for nanomedicine and nanotoxicology. Red blood cells serve as model cells with well-characterized properties. While the lipids in the bilayer are a two-dimensional fluid, the spectrin filaments of the cytoskeleton form a network with fixed connections; while the fluid membrane is characterized by its bending rigidity, the strength of the cytoskeleton can be quantified by its shear modulus. The cytoskeleton in particular reinforces the mechanical strength of the membrane and may also contribute actively to the engulfment of nanoparticles. We use a continuum membrane model with membrane bending energy, membrane adhesion energy, and membrane tension to study the membrane-nano-object interactions, and calculate the membrane deformation energy costs using Helfrich Hamiltonian. The adhesion can be mediated via different mechanisms, like homogeneous van der Waals adhesion, or inhomogeneous receptor-mediated adhesion. We apply simulation techniques used for biomechanics, such as molecular dynamics and Brownian dynamics simulations. Quantitative experiments addressing the binding of carboxylated polystyrene nanoparticles to human red blood cells reveal saturated adsorption with only sparse coverage of the cell membrane by partial-wrapped nanoparticles independent of particle size. This suggests a restricted number of adhesive sites on the membrane. For adhesion mediated by receptor-ligand bonds, we find partial wrapping of nanoparticles for high bond energies and low receptor densities. We determine sets of receptor densities, receptor diffusion coefficients, minimum numbers of receptors required for multivalent binding of a nanoparticle, and maximum number of receptors that can bind to a nanoparticle based on experimental data and computer simulations. Furthermore, we provide quantitative characterisation and interpretation for both nanoparticle binding and red blood cell shape and deformability changes upon nanoparticle binding. Designing optimal implants, like brain and cochlear implants, requires high signal-to-noise ratios for electrical signals from cells. This can be achieved extracellularly via microelectrode arrays, where the nanostructure geometries influence their wrapping. We find partial-wrapped and complete-wrapped states for the nanopillar geometries used in the experiments. Using a combination of theory and experiments, we predict that nanopillars with small radii and arrays with large pitches are the most favourable geometries to get wrapped by cell membranes. Our model provides an informed estimate of the optimal nanostructure geometries that maximize the adhered area to design efficient geometries for microelectrode arrays. Both membrane and cortical cytoskeleton are key players in the passive wrapping process for cell membranes. We study the effect of a cortical spectrin cytoskeleton on nanoparticle wrapping using Brownian dynamics simulations. In particular, we investigate different particle sizes, different persistence lengths of the spectrin filaments, and the effect of the presence of ankyrin protein complexes that additionally bind the cytoskeleton to the lipid bilayer. We find five particle-wrapping regimes with metastable and stable non-wrapped, partial-wrapped and complete-wrapped states. Ankyrin complexes favour wrapping by effectively softening the spectrin network. Smaller particles travel faster through the red blood cell membrane. We also investigate the entry of malaria-sized particles into red blood cells. The particle-red blood cell interactions depend on the force due to motor vertex, particle-membrane adhesion strength, breaking length of the spectrin bonds, and radius of the particle. In particular, we study the dynamics of the cytoskeletal network allowing cortical cytoskeletal bonds to break. We find that depending on the spectrin breaking length, we observe two types of partial wrapping for the particle: partial-wrapped states with an intact spectrin network, and partial-wrapped states with a broken spectrin network. For short spectrin breaking lengths, the cytoskeletal bonds break on the fly while the particle is traversing through the spectrin network, hence a transition from a partial-wrapped state to a complete-wrapped state. For longer breaking lengths, the particle gets stuck in the network, and after breaking the first spectrin bond it induces an avalanche of breaking bonds. The particle jumps from a partial-wrapped state to a complete-wrapped state. The cytoskeletal crack patterns suggest that longer spectrin breaking lengths facilitate `healing' of the cytoskeleton. Our calculations offer insights into the interaction of cell membranes with nanostructures, including the aspects of a multi-component lipid bilayer membrane and a cortical cytoskeleton. Furthermore, nanoparticle-covered red blood cells may be exploited as drug delivery systems that circulate in blood.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
Singh, Karandeepkarandeep.singh2427@gmail.comUNSPECIFIEDUNSPECIFIED
URN: urn:nbn:de:hbz:38-82613
Date: 15 March 2018
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects: Natural sciences and mathematics
Uncontrolled Keywords:
Red blood cellsEnglish
Date of oral exam: 8 May 2018
NameAcademic Title
Gompper, GerhardProf. Dr.
Lässig, MichaelProf. Dr.
Refereed: Yes


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