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Membrane partitioning of anionic, ligand-coated nanoparticles is accompanied by ligand snorkeling, local disordering, and cholesterol depletion.

Gkeka P, Angelikopoulos P, Sarkisov L, Cournia Z - PLoS Comput. Biol. (2014)

Bottom Line: This picture is supported by the free energy analysis, revealing a considerable free energy barrier for NP translocation across the lipid bilayer. 5-µs unbiased MD simulations with the NP inserted in the bilayer core reveal that the hydrophobic and hydrophilic ligands of the NP surface rearrange to form optimal contacts with the lipid bilayer, leading to the so-called snorkeling effect.Inside cholesterol-containing bilayers, the NP induces rearrangement of the structure of the lipid bilayer in its vicinity from the liquid-ordered to the liquid phase spanning a distance almost twice its core radius (8-10 nm).Based on the physical insights obtained in this study, we propose a mechanism of cellular anionic NP partitioning, which requires structural rearrangements of both the NP and the bilayer, and conclude that the translocation of anionic NPs through cholesterol-rich membranes must be accompanied by formation of cholesterol-lean regions in the proximity of NPs.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Research Foundation, Academy of Athens, Athens, Greece.

ABSTRACT
Intracellular uptake of nanoparticles (NPs) may induce phase transitions, restructuring, stretching, or even complete disruption of the cell membrane. Therefore, NP cytotoxicity assessment requires a thorough understanding of the mechanisms by which these engineered nanostructures interact with the cell membrane. In this study, extensive Coarse-Grained Molecular Dynamics (MD) simulations are performed to investigate the partitioning of an anionic, ligand-decorated NP in model membranes containing dipalmitoylphosphatidylcholine (DPPC) phospholipids and different concentrations of cholesterol. Spontaneous fusion and translocation of the anionic NP is not observed in any of the 10-µs unbiased MD simulations, indicating that longer timescales may be required for such phenomena to occur. This picture is supported by the free energy analysis, revealing a considerable free energy barrier for NP translocation across the lipid bilayer. 5-µs unbiased MD simulations with the NP inserted in the bilayer core reveal that the hydrophobic and hydrophilic ligands of the NP surface rearrange to form optimal contacts with the lipid bilayer, leading to the so-called snorkeling effect. Inside cholesterol-containing bilayers, the NP induces rearrangement of the structure of the lipid bilayer in its vicinity from the liquid-ordered to the liquid phase spanning a distance almost twice its core radius (8-10 nm). Based on the physical insights obtained in this study, we propose a mechanism of cellular anionic NP partitioning, which requires structural rearrangements of both the NP and the bilayer, and conclude that the translocation of anionic NPs through cholesterol-rich membranes must be accompanied by formation of cholesterol-lean regions in the proximity of NPs.

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Number density maps of the negatively charged end-terminal groups of the NP ligands.The calculation was performed over the last 500 ns of the simulation for the different systems. The snapshots correspond to the final frame of the simulation and are depicted to indicate the relative position of the NP with respect to the lipid bilayer at the end of the simulation. White corresponds to zero density of the negatively-charged moieties, indicating the snorkeling effect, where charged-end terminal groups orient themselves towards the lipid head groups and outside of the bilayer core. A typical RGB color scale is used to show increasing occupancy. Only the NP core is shown for clarity. The coloring is the same as in Figure 1.
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pcbi-1003917-g002: Number density maps of the negatively charged end-terminal groups of the NP ligands.The calculation was performed over the last 500 ns of the simulation for the different systems. The snapshots correspond to the final frame of the simulation and are depicted to indicate the relative position of the NP with respect to the lipid bilayer at the end of the simulation. White corresponds to zero density of the negatively-charged moieties, indicating the snorkeling effect, where charged-end terminal groups orient themselves towards the lipid head groups and outside of the bilayer core. A typical RGB color scale is used to show increasing occupancy. Only the NP core is shown for clarity. The coloring is the same as in Figure 1.

Mentions: Therefore, to investigate the structure of the NP-bilayer system upon NP partitioning, we performed a series of unbiased 5 µs CG-MD simulations with the NP placed inside the hydrophobic core of preassembled and equilibrated bilayers. After equilibration, in all considered systems, the NP positions itself either in the core of the bilayer or close to the bilayer-water interface (Figure S5 in Text S1). We observe that the hydrophobic ligands on the NP surface ligands rearrange in order to associate with the bilayer interior, maximizing hydrophobic and minimizing polar contacts. At the same time, the negatively charged MUS termini form salt bridges with the positively charged choline group of the DPPC molecules inducing the so-called “snorkeling effect” (Figure S5 in Text S1). This effect was also observed in the study of Ref. [24]. In Figure 2, the “snorkeling” effect is presented using the number density maps of the negatively charged end-terminal groups of the NP ligands over the last 500 ns of the simulation for each system.


Membrane partitioning of anionic, ligand-coated nanoparticles is accompanied by ligand snorkeling, local disordering, and cholesterol depletion.

Gkeka P, Angelikopoulos P, Sarkisov L, Cournia Z - PLoS Comput. Biol. (2014)

Number density maps of the negatively charged end-terminal groups of the NP ligands.The calculation was performed over the last 500 ns of the simulation for the different systems. The snapshots correspond to the final frame of the simulation and are depicted to indicate the relative position of the NP with respect to the lipid bilayer at the end of the simulation. White corresponds to zero density of the negatively-charged moieties, indicating the snorkeling effect, where charged-end terminal groups orient themselves towards the lipid head groups and outside of the bilayer core. A typical RGB color scale is used to show increasing occupancy. Only the NP core is shown for clarity. The coloring is the same as in Figure 1.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4256007&req=5

pcbi-1003917-g002: Number density maps of the negatively charged end-terminal groups of the NP ligands.The calculation was performed over the last 500 ns of the simulation for the different systems. The snapshots correspond to the final frame of the simulation and are depicted to indicate the relative position of the NP with respect to the lipid bilayer at the end of the simulation. White corresponds to zero density of the negatively-charged moieties, indicating the snorkeling effect, where charged-end terminal groups orient themselves towards the lipid head groups and outside of the bilayer core. A typical RGB color scale is used to show increasing occupancy. Only the NP core is shown for clarity. The coloring is the same as in Figure 1.
Mentions: Therefore, to investigate the structure of the NP-bilayer system upon NP partitioning, we performed a series of unbiased 5 µs CG-MD simulations with the NP placed inside the hydrophobic core of preassembled and equilibrated bilayers. After equilibration, in all considered systems, the NP positions itself either in the core of the bilayer or close to the bilayer-water interface (Figure S5 in Text S1). We observe that the hydrophobic ligands on the NP surface ligands rearrange in order to associate with the bilayer interior, maximizing hydrophobic and minimizing polar contacts. At the same time, the negatively charged MUS termini form salt bridges with the positively charged choline group of the DPPC molecules inducing the so-called “snorkeling effect” (Figure S5 in Text S1). This effect was also observed in the study of Ref. [24]. In Figure 2, the “snorkeling” effect is presented using the number density maps of the negatively charged end-terminal groups of the NP ligands over the last 500 ns of the simulation for each system.

Bottom Line: This picture is supported by the free energy analysis, revealing a considerable free energy barrier for NP translocation across the lipid bilayer. 5-µs unbiased MD simulations with the NP inserted in the bilayer core reveal that the hydrophobic and hydrophilic ligands of the NP surface rearrange to form optimal contacts with the lipid bilayer, leading to the so-called snorkeling effect.Inside cholesterol-containing bilayers, the NP induces rearrangement of the structure of the lipid bilayer in its vicinity from the liquid-ordered to the liquid phase spanning a distance almost twice its core radius (8-10 nm).Based on the physical insights obtained in this study, we propose a mechanism of cellular anionic NP partitioning, which requires structural rearrangements of both the NP and the bilayer, and conclude that the translocation of anionic NPs through cholesterol-rich membranes must be accompanied by formation of cholesterol-lean regions in the proximity of NPs.

View Article: PubMed Central - PubMed

Affiliation: Biomedical Research Foundation, Academy of Athens, Athens, Greece.

ABSTRACT
Intracellular uptake of nanoparticles (NPs) may induce phase transitions, restructuring, stretching, or even complete disruption of the cell membrane. Therefore, NP cytotoxicity assessment requires a thorough understanding of the mechanisms by which these engineered nanostructures interact with the cell membrane. In this study, extensive Coarse-Grained Molecular Dynamics (MD) simulations are performed to investigate the partitioning of an anionic, ligand-decorated NP in model membranes containing dipalmitoylphosphatidylcholine (DPPC) phospholipids and different concentrations of cholesterol. Spontaneous fusion and translocation of the anionic NP is not observed in any of the 10-µs unbiased MD simulations, indicating that longer timescales may be required for such phenomena to occur. This picture is supported by the free energy analysis, revealing a considerable free energy barrier for NP translocation across the lipid bilayer. 5-µs unbiased MD simulations with the NP inserted in the bilayer core reveal that the hydrophobic and hydrophilic ligands of the NP surface rearrange to form optimal contacts with the lipid bilayer, leading to the so-called snorkeling effect. Inside cholesterol-containing bilayers, the NP induces rearrangement of the structure of the lipid bilayer in its vicinity from the liquid-ordered to the liquid phase spanning a distance almost twice its core radius (8-10 nm). Based on the physical insights obtained in this study, we propose a mechanism of cellular anionic NP partitioning, which requires structural rearrangements of both the NP and the bilayer, and conclude that the translocation of anionic NPs through cholesterol-rich membranes must be accompanied by formation of cholesterol-lean regions in the proximity of NPs.

Show MeSH