A new tool for quantifying the nanoscale dynamics of liquids at the interface with fluid biological membranes

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Biological membranes (biomembranes) are highly sophisticated structures that play a major role in the cell function. Aside from acting as a physical barrier between the inside from the outside of the cell, biomembranes control countless biological processes ranging from molecular trafficking to cell signalling, bioenergetics and biochemical function, endocytosis, surface compartmentalisation, and membrane shape regulation. Biomembranes have hence evolved into dynamical entities that can actively re-adjust their local composition and physical properties so as to best support the cell's biological function. This regulation is known to involve specialised proteins and markers, but increasing evidence also points to well controlled local variations in the membrane's mechanical properties, in particular stiffness and fluidity. The liquid (water and hydrated ions) adjacent to the membrane is also believed to play an important role in shaping the membrane, regulating biomolecules' dynamics and controlling interactions and diffusion along the membrane. This interfacial liquid can strongly interact with the biomembrane and locally alter its mechanical properties. To date, most results addressing the properties of the interfacial liquid or of the membrane fluidity at the nanoscale are either indirect or derived from theoretical consideration. This is largely due to the lack of experimental techniques able to operate locally with sufficient spatial and temporal resolution.Recent advances in the field of atomic force microscopy (AFM) have made it possible to probe the interfacial liquid, including at the surface of biomembranes. Significantly, results are acquired locally, in seconds, and can provide sub-nanometre precision maps of the equilibrium structure adopted by the liquid adjacent to the membrane. At the present time, however, it is not possible to deduce any information about the interfacial liquid dynamics on that scale. It is, for example, not possible to determine the directional flow patterns adopted by the liquid at the interface. Part of the problem comes from the impossibility to quantify the nanoscale fluidity of the biomembranes, a factor that influence the dynamics of the contacting liquid. Furthermore, the mutual influence that the interfacial liquid and the biomembrane mechanical properties have on each other has never been explored at that scaleThis proposal aims at developing a new tool, the vortex dissipation microscope (VDM) that can quantify both the equilibrium solvation structure and the lateral dynamics of water at biointerfaces, locally and with nanometre precision. The VDM builds on recent AFM developments and will be able to provide nanoscale charts of the flow patterns naturally adopted by the interfacial water near singularities such as edges of membrane domains or protein assemblies. Significantly, the technique will also be able to quantify the lateral fluidity within the membranes, over the same area and with the same resolution.The VDM will subsequently be applied to the study of bovine eye lens membranes. It will be used to quantify the flow patterns adjacent to the protein channels aquaporin 0 and connexins that are responsible for the transport of water and small molecules through lens cell membranes. Aside from providing a natural test platform for the VDM's capabilities, the results will bring new insight into the problem of lens membrane aging. Aging is known to be coupled with changes of the membrane mechanical properties, but the underlying molecular mechanisms are still poorly understood. This problem has important medical and hence societal consequences.