Nanoparticle-induced biomembrane fusion: unraveling the effect of core size on stalk formation

Design of engineered nanoparticles for biomedical applications by computational modeling

Engineered nanoparticles exhibit superior physicochemical, antibacterial, optical, and sensing properties compared to their bulk counterparts, rendering them attractive for biomedical applications. However, given that nanoparticle properties are sensitive to their nanostructural characteristics and their chemical stability is largely affected by physiological conditions, nanoparticle behavior can be unpredictable in vivo, requiring careful surface modification to ensure biocompatibility, prevent rapid aggregation, and maintain functionality under biological environments. Therefore, understanding the mechanisms of nanoparticle formation and macroscopic behavior in physiological media is essential for the development of structure-property relationships and, their rational design for biomedical applications. Computational simulations provide insight into nanoscale phenomena and nanoparticle dynamics, expediting material discovery and innovation. This review provides an overview of the process design and characterization of metallic and metal oxide nanoparticles with an emphasis on atomistic and mesoscale simulations for their application in bionanomedicine.

Physical determinants of nanoparticle-mediated lipid membrane fusion

A wide range of fundamental cellular activities rely on lipid membrane fusion. Membrane fusion processes can be mimicked by synthetic approaches to understand fusion mechanisms and develop novel drug delivery systems and therapeutic agents. Recently, membrane-embedded amphiphilic gold nanoparticles (AuNPs) have been employed as artificial fusogens to induce finely tuned membrane fusion in vitro. However, the physical determinants driving and regulating the fusion process mediated by AuNPs remain largely unexplored, thus limiting the application potential of this synthetic fusion system. Herein, we focus on unraveling the effect of the interplay between the curvature of the lipid membrane and the size of amphiphilic AuNPs during fusion events. We employed AuNPs with the same surface chemistry but different core diameters (∼2 nm and ∼4 nm) interacting with phosphatidylcholine unilamellar vesicles of different membrane curvatures containing a biologically relevant percentage of cholesterol. Based on a combination of fluorescence spectroscopy assays, dissipative quartz microbalance, and molecular dynamics simulations, our findings reveal that small AuNPs promote vesicle fusion regardless of the membrane curvature. In contrast, large AuNPs do not exhibit fusogenic properties with low curvature membranes and can induce fusion events only with significantly curved membranes. Large NPs impede the progression from the stalk state to the hemifused state via steric hindrance, an effect that is only partially compensated by the membrane curvature. These results offer novel insights into the role of AuNP core size and membrane curvature in mediating the interaction between the vesicles during fusion and highlight how understanding these physical determinants has broad implications in fully exploiting the application potential of novel synthetic fusion approaches.

Myomerger Induces Membrane Hemifusion and Regulates Fusion Pore Expansion

Membrane fusion is a crucial mechanism in a wide variety of important events in cell biology from viral infection to exocytosis. However, despite many efforts and much progress, cell-cell fusion has remained elusive to our understanding. Along the life of the fusion pore, large conformational changes take place from the initial lipid bilayer bending, passing through the hemifusion intermediates, and ending with the formation of the first nascent fusion pore. In this sense, computer simulations are an ideal technique for describing such complex lipid remodeling at the molecular level. In this work, we studied the role played by the muscle-specific membrane protein Myomerger during the formation of the fusion pore. We have conducted μs length atomistic and coarse-grained molecular dynamics, together with free-energy calculations using ad hoc collective variables. Our results show that Myomerger favors the hemifusion diaphragm-stalk transition, reduces the nucleation-expansion energy difference, and promotes the formation of nonenlarging fusion pores.

Ligand Lipophilicity Determines Molecular Mechanisms of Nanoparticle Adsorption to Lipid Bilayers

The interactions of ligand-functionalized nanoparticles with the cell membrane affect cellular uptake, cytotoxicity, and related behaviors, but relating these interactions to ligand properties remains challenging. In this work, we perform coarse-grained molecular dynamics simulations to study how the adsorption of ligand-functionalized cationic gold nanoparticles (NPs) to a single-component lipid bilayer (as a model cell membrane) is influenced by ligand end group lipophilicity. A set of 2 nm diameter NPs, each coated with a monolayer of organic ligands that differ only in their end groups, was simulated to mimic NPs recently studied experimentally. Metadynamics calculations were performed to determine key features of the free energy landscape for adsorption as a function of the distance of the NP from the bilayer and the number of NP-lipid contacts. These simulations revealed that NP adsorption is thermodynamically favorable for all NPs due to the extraction of lipids from the bilayer and into the NP monolayer. To resolve ligand-dependent differences in adsorption behavior, string method calculations were performed to compute minimum free energy pathways for adsorption. These calculations revealed a surprising nonmonotonic dependence of the free energy barrier for adsorption on ligand end group lipophilicity. Large free energy barriers are predicted for the least lipophilic end groups because favorable NP-lipid contacts are initiated only through the unfavorable protrusion of lipid tail groups out of the bilayer. The smallest free energy barriers are predicted for end groups of intermediate lipophilicity which promote NP-lipid contacts by intercalating within the bilayer. Unexpectedly, large free energy barriers are also predicted for the most lipophilic end groups which remain sequestered within the ligand monolayer rather than intercalating within the bilayer. These trends are broadly in agreement with past experimental measurements and reveal how subtle variations in ligand lipophilicity dictate adsorption mechanisms and associated kinetics by influencing the interplay of lipid-ligand interactions.