Dante, S. | bioRxiv Science

bioRxiv2022

Cholesterol-containing liposomes decorated with Au nanoparticles as minimal tunable fusion machinery

Membrane fusion is essential for the basal functionality of eukaryotic cells. In physiological conditions, fusion events are regulated by a wide range of specialized proteins, as well as by a finely tuned local lipid composition and ionic environment. SNARE proteins, for example, provide the mechanical energy necessary to achieve vesicle fusion in neuromediator release, and their action is assisted by other soluble proteins, membrane cholesterol, and calcium ions. Similar cooperative effects must be explored when considering synthetic approaches to achieve controlled and selective membrane fusion. Here we show that liposomes decorated with amphiphilic Au nanoparticles (AuLips) can act as minimal tunable fusion machinery. AuLips fusion is triggered by divalent ions, while the number of fusion events dramatically depends on, and can be finely tuned by, the liposome cholesterol content. Our results, obtained via a combination of experimental (Quartz-Crystal-Microbalance with Dissipation monitoring, Fluorescence assays, Small-Angle X-ray Scattering) and computational techniques (Molecular Dynamics with coarse-grained resolution), reveal new mechanistic details on the fusogenic activity of amphiphilic Au nanoparticles in synergy with membrane cholesterol, and demonstrate the ability of these synthetic nanomaterials to induce fusion regardless of the divalent ion used (Ca2+ or Mg2+). This evidence provides a novel contribution to the development of new artificial fusogenic agents for next-generation biomedical applications that require tight control of the rate of fusion events (e.g., targeted drug delivery).

biophysics↗

bioRxiv2019

Electrostatics and Solvation: Essential Determinants of Chromatin Compaction

Chromatin compaction is a process of fundamental importance in Biology, as it greatly influences cellular function and gene expression. The dynamics of compaction is determined by the interactions between DNA and histones, which are mainly mechanical and electrostatic. The high charge of DNA makes electrostatics extremely important for chromatin topology and dynamics. Besides their mechanical and steric role in the chromatin fibre, linker DNA length and linker histone presence and binding position also bear great electrostatic consequences. Electrostatics in chromatin is also indirectly linked to the DNA sequence: the presence of high-curvature AT-rich segments in DNA can cause conformational variations with electrostatic repercussions, attesting to the fact that the role of DNA is both structural and electrostatic. Electrostatics in this system has been analysed by extensively examining at the computational level the repercussions of varying ionic concentration, using all-atom, coarse-grained, and continuum models. There have been some tentative attempts to describe the force fields governing chromatin conformational changes and the energy landscapes of these transitions, but the intricacy of the system has hampered reaching a consensus. Chromatin compaction is a very complex issue, depending on many factors and spanning orders of magnitude in space and time in its dynamics. Therefore, comparison and complementation of theoretical models with experimental results is fundamental. Here, we present existing approaches to analyse electrostatics in chromatin and the different points of view from which this issue is treated. We pay particular attention to solvation, often overlooked in chromatin studies. We also present some numerical results on the solvation of nucleosome core particles. We discuss experimental techniques that have been combined with computational approaches and present some related experimental data such as the Z-potential of nucleosomes at varying ionic concentrations. Finally, we discuss how these observations support the importance of electrostatics and solvation in chromatin models.\n\nSIGNIFICANCEThis work explores the determinants of chromatin compaction, focusing on the importance of electrostatic interactions and solvation. Chromatin compaction is an intrinsically multiscale issue, since processes concerning chromatin occur on a wide range of spatial and temporal scales. Since DNA is a highly charged macromolecule, electrostatic interactions are extremely significant for chromatin compaction, an effect examined in this work from many angles, such as the importance of ionic concentration and different ionic types, DNA-protein interactions, and solvation. Solvation is often overlooked in chromatin studies, especially in coarse-grained models, where the nucleosome core, the building block of the chromatin fibre, is represented as a rigid body, even though it has been observed that solvation influences chromatin even at the base-pair level.

biophysics↗