Partner-facilitating transmembrane penetration of nanoparticles: a biological test in silico

Selective membrane wrapping on differently sized nanoparticles regulated by clathrin assembly: A computational model

Nanoparticles (NPs) enter cells via multiple pathways, all of which are NP size dependent. Previous studies indicated that the clathrin-mediated endocytosis has different selectivity for the NP size, but the regulatory mechanism remains unclear and difficult to study at the molecular scale in vivo. By means of computer simulation, here we design membrane wrapping on differently sized NPs with mimic clathrin assembly at the opposite membrane side. With relatively large NPs readily wrapped by a pure membrane as manifested, clathrin modulates the process and tunes the size selectivity. Although finite curvature can be generated by cage-like clathrin assembly to facilitate membrane wrapping on relatively small NPs, the clathrin assemblage has a certain range of size, which is mismatched with larger NPs. Besides, the local membrane patch is rigidified by clathrin to increase the barrier of membrane wrapping on larger NPs. Competition of these items determines whether membrane wrapping on NPs is promoted or suppressed, and can be tuned by the NP-membrane adhesion strength, clathrin concentration, and inter-NP distance. Our results highlight the significance of complex environment in altering the nature of NP interaction with cell membranes, and are expected to help design NPs for biomedical applications requiring precise control of NP uptake or cell membrane attachment.

Studies on Aggregated Nanoparticles Steering during Deep Brain Membrane Crossing

Many central nervous system (CNS) diseases, such as Alzheimer's disease (AD), affect the deep brain region, which hinders their effective treatment. The hippocampus, a deep brain area critical for learning and memory, is especially vulnerable to damage during early stages of AD. Magnetic drug targeting has shown high potential in delivering drugs to a targeted disease site effectively by applying a strong electromagnetic force. This study illustrates a nanotechnology-based scheme for delivering magnetic nanoparticles (MNP) to the deep brain region. First, we developed a mathematical model and a molecular dynamic simulation to analyze membrane crossing, and to study the effects of particle size, aggregation, and crossing velocities. Then, using in vitro experiments, we studied effective parameters in aggregation. We have also studied the process and environmental parameters. We have demonstrated that aggregation size can be controlled when particles are subjected to external electromagnetic fields. Our simulations and experimental studies can be used for capturing MNPs in brain, the transport of particles across the intact BBB and deep region targeting. These results are in line with previous in vivo studies and establish an effective strategy for deep brain region targeting with drug loaded MNPs through the application of an external electromagnetic field.

Ligand-decoration determines the translational and rotational dynamics of nanoparticles on a lipid bilayer membrane

Nanoparticles (NPs) promise a huge potential for clinical diagnostic and therapeutic applications. However, nano-bio (e.g., the NP-cell membrane) interactions and underlying mechanisms are still largely elusive. In this study, two types of congeneric peptides, namely PGLa and magainin 2 (MAG2), with similar membrane activities were employed as model ligands for NP decoration, and the diffusion behaviours (including both translation and rotation) of the ligand-decorated NPs on a lipid bilayer membrane were studied via molecular dynamics simulations. It was found that, although both PGLa- and MAG2-coated NPs showed alternatively "hopping" and "jiggling" diffusions, the PGLa-coated ones had an enhanced circling at the hopping stage, while a much confined circling at the jiggling stage. In contrast, the MAG2-coated NPs demonstrated constant circling tendencies throughout the diffusion process. Such differences in the coupling between translational and rotational dynamics of these two types of NPs are ascribed to the different ligand-lipid interactions of PGLa and MAG2, in which the PGLa ligands prefer to vertically insert into the membrane, while MAG2 tends to lie flat on the membrane surface. Our results are helpful for the understanding the underlying associations between the NP motions and their interfacial membrane interactions, and shed light on the possibility of regulating NP behaviours on a cellular surface for better biomedical uses.

Membrane-curvature-mediated co-endocytosis of bystander and functional nanoparticles

Efficient cellular uptake of nanoparticles (NPs) is necessary for the development of nanomedicine in biomedical applications. Recently, the coadministration of functionalized NPs (FNPs) was shown to stimulate the cellular uptake of nonfunctionalized NPs (termed bystander NPs, BNPs), which presents a new strategy to achieve synergistic delivery. However, a mechanistic understanding of the underlying mechanism is still lacking. In this work, the bystander uptake effect was investigated at the cell membrane level by combining the coarse-grained molecular dynamics, potential of mean force calculation and theoretical energy analysis methods. The membrane internalization efficiency of BNPs was enhanced by co-administered FNPs, and such activity depends on the affinity of both NPs to the membrane and the resultant membrane deformation. The membrane-curvature-mediated attraction and aggregation of NPs facilitated the membrane uptake of BNPs. Furthermore, quantitative suggestions were given to modulate the BNP internalization through controlling the FNP properties such as size, concentration and surface-ligand density. Our results provide insight into the molecular mechanism of the bystander uptake effect, and offer a practical guide to regulate the cellular internalization of NPs for targeted and efficient delivery to cells.

Individual Roles of Peptides PGLa and Magainin 2 in Synergistic Membrane Poration

Synergy between antimicrobial peptides PGLa and Magainin 2 (MAG2) provides an efficient way to enhance their antimicrobial ability. However, the underlying molecular mechanism of such synergy, especially the individual roles of each peptide, remains poorly understood. We combined a giant unilamellar vesicle leakage assay, in situ interfacial photovoltage testing, and molecular dynamics to investigate membrane poration under the action of PGLa, MAG2, or a PGLa/MAG2 mixture. Our results clearly show the different membrane action modes of the three systems and demonstrate the importance of forming PGLa-MAG2 heterodimers in the membrane poration process. PGLa inserted into and extracted from a membrane rapidly and continually with minimal aggregation and produced only transient, small pores. In contrast, MAG2 peptides tended to aggregate together on the membrane surface or only shallowly embed in the membrane. Additionally, the PGLa and MAG2 residues were well integrated into the membrane via the formation of PGLa-MAG2 heterodimers. The membrane defect produced by the rapid insertion of PGLa was stabilized by MAG2, which further recruited other peptides for the formation of PGLa-MAG2 heterodimers and even heterodimer clusters. Growth in pore size then occurred in a step-by-step process involving the formation and assembly of heterodimer clusters within the membrane. Our results provide insight into the complicated synergy that occurs between PGLa and MAG2 during membrane poration and will assist in the design of new antimicrobial peptides.

Permeation of nanoparticles across the intestinal lipid membrane: dependence on shape and surface chemistry studied through molecular simulations

Nanoparticles are being explored for topical and oral drug delivery applications as they can cross various biological barriers, for example, the intestinal epithelium. The ability of nanoparticles to cross barriers depends on their morphological and surface properties such as size, surface chemistry and shape, among others. The effect of nanoparticle size on their membrane permeability has been well studied both experimentally and theoretically. However, less attention has been given to understand the role of nanoparticle shape in their translocation across biological barrier membranes. Here, we report on the influence of the nanoparticle's shape, surface chemistry and concentration on their permeation across a human intestinal apical cell membrane model. A representative multicomponent lipid bilayer model of the human intestinal apical membrane was built. The free energy of permeation of nanoparticles across the model lipid bilayer was calculated using multiple umbrella sampling simulations. The interaction of these nanoparticles with the model lipid bilayer was captured using extensive microsecond unrestrained molecular dynamics simulations. We observed that: (a) irrespective of the surface chemistry, the efficacy of nanoparticle penetration across the lipid layer was in the order of rod > disc > sphere; (b) irrespective of the shape, apolar and nonpolar nanoparticles were found to locate in the interior of the lipid bilayer, whereas charged and polar nanoparticles were either adsorbed on the lipid headgroups or remained in the water layer; (c) apolar and nonpolar disc shaped nanoparticles had higher efficacy in permeation across the lipid bilayer as compared to disc and sphere shaped nanoparticles; and (d) at a higher concentration of nanoparticles, sphere and disc shaped nanoparticles exhibited more agglomeration as compared to rod shaped nanoparticles. Based on these outcomes, a few nanoparticles were designed which penetrated readily into the lipid layer and these nanoparticles were also able to co-deliver a therapeutic protein inside the lipid layer. The apical model lipid membrane and protocols used in this study can thus be utilized for the in silico design of nanoparticles for the oral delivery of therapeutics.

Mechanistic modeling of spontaneous penetration of carbon nanocones into membrane vesicles

Carbon nanocones (CNCs) are promising drug delivery systems that can be functionalized with a variety of biomolecules (such as proteins, peptides, or antibodies), which allow for site-specific, targeted payload delivery to particular cells and organs. However, considerable uncertainty exists with respect to the toxicity of CNCs on their conical shape, and the underlying mechanism that leads to the penetration of CNCs (especially the truncated ones) in and through the cell membrane is not yet well understood. Using a coarse-grained dissipative particle dynamics method, we systematically investigate the spontaneous penetration of untruncated and truncated CNCs into membrane vesicles. For untruncated CNCs, the simulation results show that both pristine and oxidized ones can spontaneously penetrate across or be attached to the vesicle surface without membrane rupture, indicating low or insignificant toxicity. However, for truncated CNCs, we find that both the apex angle and aspect ratio can influence the CNC-membrane interactions and CNC-induced toxicity: a higher apex angle (and/or a lower aspect ratio) yields a higher toxicity of truncated CNCs. Further free energy analysis reveals that the lowest free energy path during the penetration is associated with CNC's orientation and rotation. For a truncated CNC with a low aspect ratio and high apex angle, it tends to rotate itself to a preferred standing-up fashion inside the vesicle membrane, posing an enhanced toxicity of CNCs. These findings may provide useful guidelines for designing effective CNC vehicles for drug delivery.

Penetration of nanoparticles across a lipid bilayer: effects of particle stiffness and surface hydrophobicity

The cellular uptake of nanoparticles (NPs) has drawn significant attention due to their great importance and potential in drug delivery, bioimaging, and specific targeting. Here, we conduct a computational study on the translocation process of soft nanoparticles with different elasticities and surface hydrophobicities through a lipid bilayer membrane. It is shown that the translocation abilities of hydrophilic NPs can be enhanced by increasing their stiffness, while the penetrability of hydrophobic NPs is weakened by increasing the particle stiffness. The free energy analysis indicates that rigid hydrophilic NPs and soft hydrophobic NPs encounter lower energy barriers during penetration. In direct translocation, different deformation modes are observed for NPs with different surface hydrophobicities during cellular internalization. Further, deformation analysis demonstrates that hydrophilic NPs are flattened in the membrane plane, while hydrophobic NPs are elongated along the membrane norm during penetration. We conclude that the elasticity of NPs has an obvious impact on their ability to penetrate across the lipid bilayer membrane through different morphological responses of hydrophilic and hydrophobic NPs. These results shed light on the coupled effects of particle elasticity and surface hydrophobicity on the cellular uptake of elastic NPs, which may provide useful guidelines for designing effective nanocarrier systems for drug delivery.