Limits...
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.

Show MeSH
Coarse-grained models of the system components.DPPC molecule (top, left), cholesterol molecule (bottom, left), nanoparticle simulated in the water phase (center), and nanoparticle simulated in a membrane containing 30% cholesterol (right) (final snapshots). The molecules or CG beads are not shown to scale. Colors: Negative beads bearing -1e charge = purple; hydrophobic beads = cyan; positive beads = blue; cholesterol hydroxyl bead = gray; glycerol backbone beads = white; cholesterol sterol body beads = lime. The core of the NP is shown in gray and surface representation, whereas the hydrophobic parts of NP surface ligands are shown in a bead-spring representation. Arrows indicate the tendency of the surface charged ligands to associate with the DPPC head groups and their hydrophobic parts to interact with the hydrophobic core, inducing a snorkeling effect.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4256007&req=5

pcbi-1003917-g001: Coarse-grained models of the system components.DPPC molecule (top, left), cholesterol molecule (bottom, left), nanoparticle simulated in the water phase (center), and nanoparticle simulated in a membrane containing 30% cholesterol (right) (final snapshots). The molecules or CG beads are not shown to scale. Colors: Negative beads bearing -1e charge = purple; hydrophobic beads = cyan; positive beads = blue; cholesterol hydroxyl bead = gray; glycerol backbone beads = white; cholesterol sterol body beads = lime. The core of the NP is shown in gray and surface representation, whereas the hydrophobic parts of NP surface ligands are shown in a bead-spring representation. Arrows indicate the tendency of the surface charged ligands to associate with the DPPC head groups and their hydrophobic parts to interact with the hydrophobic core, inducing a snorkeling effect.

Mentions: We consider an anionic NP with a core diameter of 4.3 nm coated with regular, striped patterns of hydrophobic (octanethiol, OT) and hydrophilic, negatively charged (11-mercapto-1-undecanesulphonate, MUS) domains in a 2∶1 MUS∶OT ratio, inspired by recent studies [17], [24], [27]. In our model, the bilayer, water phase, and NP are modeled with a CG representation using the MARTINI force field (Figure 1) [19]. The NP is coated with surface ligands that are represented explicitly as flexible chains (Figure S1 in Text S1, Supporting Information (SI)). The modeled NP is negatively charged with a charge density of 1.19 e/nm2 and total charge of −134. Charged ligands are often used as capping agents on NPs to keep them separated via electrostatic repulsion and prevent their aggregation, which has been linked to cytotoxic effects [28].


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)

Coarse-grained models of the system components.DPPC molecule (top, left), cholesterol molecule (bottom, left), nanoparticle simulated in the water phase (center), and nanoparticle simulated in a membrane containing 30% cholesterol (right) (final snapshots). The molecules or CG beads are not shown to scale. Colors: Negative beads bearing -1e charge = purple; hydrophobic beads = cyan; positive beads = blue; cholesterol hydroxyl bead = gray; glycerol backbone beads = white; cholesterol sterol body beads = lime. The core of the NP is shown in gray and surface representation, whereas the hydrophobic parts of NP surface ligands are shown in a bead-spring representation. Arrows indicate the tendency of the surface charged ligands to associate with the DPPC head groups and their hydrophobic parts to interact with the hydrophobic core, inducing a snorkeling effect.
© Copyright Policy
Related In: Results  -  Collection

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

pcbi-1003917-g001: Coarse-grained models of the system components.DPPC molecule (top, left), cholesterol molecule (bottom, left), nanoparticle simulated in the water phase (center), and nanoparticle simulated in a membrane containing 30% cholesterol (right) (final snapshots). The molecules or CG beads are not shown to scale. Colors: Negative beads bearing -1e charge = purple; hydrophobic beads = cyan; positive beads = blue; cholesterol hydroxyl bead = gray; glycerol backbone beads = white; cholesterol sterol body beads = lime. The core of the NP is shown in gray and surface representation, whereas the hydrophobic parts of NP surface ligands are shown in a bead-spring representation. Arrows indicate the tendency of the surface charged ligands to associate with the DPPC head groups and their hydrophobic parts to interact with the hydrophobic core, inducing a snorkeling effect.
Mentions: We consider an anionic NP with a core diameter of 4.3 nm coated with regular, striped patterns of hydrophobic (octanethiol, OT) and hydrophilic, negatively charged (11-mercapto-1-undecanesulphonate, MUS) domains in a 2∶1 MUS∶OT ratio, inspired by recent studies [17], [24], [27]. In our model, the bilayer, water phase, and NP are modeled with a CG representation using the MARTINI force field (Figure 1) [19]. The NP is coated with surface ligands that are represented explicitly as flexible chains (Figure S1 in Text S1, Supporting Information (SI)). The modeled NP is negatively charged with a charge density of 1.19 e/nm2 and total charge of −134. Charged ligands are often used as capping agents on NPs to keep them separated via electrostatic repulsion and prevent their aggregation, which has been linked to cytotoxic effects [28].

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