No abstract

Local surface charge density of lipid membranes influences membrane–protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values.

Understanding the mechanics and dynamics of active matter at high density is indispensable to a range of physical and biological processes such as swarm dynamics, tissue formation and cancer metastasis. Here, we study the dynamics and mechanics of an MCF-10A epithelial cell monolayer on the multi-cellular and single-cell scales and over a wide density range. We show that the dynamics and Young's modulus of the monolayer are spatially heterogeneous on the multi-cellular scale. With increasing cell density, the monolayer approached kinetic arrest and the Young's modulus scaled critically. On the single-cell scale, as the cell density increased, cells were intermittently trapped in cages formed by their neighbors and their motion evolved from a ballistic motion to a sub-diffusive motion. Furthermore, the relaxation time and inverse self-diffusivity increased exponentially with the cell density. These findings provide a mechanism for long-ranged mechanical stress propagation, tissue remodeling and patterning at very high cell densities.

No abstract

The local surface charge density of the cell membrane influences regulation and localization of membrane proteins. The local surface charge density could, until recently, not be measured directly under physiological conditions, and it was largely a hypothetical yet very important parameter. Here we use unsaturated lipids of a distinct charge (DOTAP, DOPC, and DOPG) and a neutral fully saturated lipid (DPPC) to create model membranes with phase separating domains of a defined charge. We then apply quantitative surface charge microscopy (QSCM) to investigate the local surface charge density; this is a technique based on a scanning ion conductance microscope (SICM) capable of measuring surface charge density with nanoscale lateral resolution. We are able to clearly distinguish lipid domains from charge and topography in all three model membranes. The measured surface charge densities furthermore reveal that disordered domains formed by charged lipids are in fact not only impure, but also incorporate uncharged saturated lipids. We estimate that at least 30% of disordered domains in DOPG : DPPC and DOTAP : DPPC will be DPPC. These ratios could present a limit for the formation of charged domains in lipid membranes.