SignificanceLipid bilayer membranes are responsible for compartmentalization, signaling, transport, and flow of charge in living cells. Membranes self-assemble in aqueous solutions. Without a hydrating environment, membranes cannot exist. It is therefore surprising to note that the hydrating water is neglected in most membrane-related studies. We imaged membrane-bound oriented water by means of label-free second harmonic microscopy. We tracked, on millisecond time scales, membrane domain diffusion of condensed charged lipid domains, domain structure, and the spatial distribution of charge. Real-time electrostatic membrane potential maps were constructed using nonlinear optical theory. The spatiotemporal fluctuation in the membrane potential is surprisingly large and reveals the importance of charge fluctuations on membranes.

Second harmonic generation (SHG) is inherently sensitive to the absence of spatial centrosymmetry, which can render it intrinsically sensitive to interfacial processes, chemical changes and electrochemical responses. Here, we seek to improve the imaging throughput of SHG microscopy by using a wide-field imaging scheme in combination with a medium-range repetition rate amplified near infrared femtosecond laser source and gated detection. The imaging throughput of this configuration is tested by measuring the optical image contrast for different image acquisition times of BaTiO 3 nanoparticles in two different wide-field setups and one commercial point-scanning configuration. We find that the second harmonic imaging throughput is improved by 2-3 orders of magnitude compared to point-scan imaging. Capitalizing on this result, we perform low fluence imaging of (parts of) living mammalian neurons in culture. ©2014 Optical Society of America

Neurons communicate through electrochemical signaling within a complex network. These signals are composed of changes in membrane potentials and are traditionally measured with the aid of (toxic) fluorescent labels or invasive electrical probes. Here, we demonstrate an improvement in label-free second harmonic neuroimaging sensitivity by ~3 orders of magnitude using a wide-field medium repetition rate illumination. We perform a side-by-side patch-clamp and second harmonic imaging comparison to demonstrate the theoretically predicted linear correlation between whole neuron membrane potential changes and the square root of the second harmonic intensity. We assign the ion induced changes to the second harmonic intensity to changes in the orientation of membrane interfacial water, which is used to image spatiotemporal changes in the membrane potential and K+ ion flux. We observe a non-uniform spatial distribution and temporal activity of ion channels in mouse brain neurons.

Lipid membranes provide diverse and essential functions in our cells relating to transport, energy harvesting and signaling. This variety of functions is controlled by the molecular architecture, such as the presence of hydrating water, specific chemical compounds and microscopic structures, such as the local membrane curvature, as well as macroscopic properties, such as the fluidity of the membrane. To understand the chemistry of membranes, ideally one needs access to multiple length scales simultaneously, using probes that are noninvasive, label-free and membrane-interface specific. This dream is generally pursued by following either a top-down approach, introducing labels to real cell membranes or by following a bottom-up approach with well-controlled but simplified membrane monolayer or supported membrane models. This Perspective offers an alternative path that ultimately envisions bringing together both approaches. By using intermediate nano-, micro-and macroscale free-floating membrane systems in combination with novel nonlinear optical methods, one can advance the understanding of realistic membranes on a more fundamental level. Here, we describe recent advances in understanding membrane molecular structure, hydration, electrostatics and the effect of variable length scale, curvature and confinement for 3D nano-and microscale membrane systems such as lipid droplets and liposomes. We also describe an approach to image membrane hydration and membrane potentials in real time and space together with an application to neuroscience. In doing so, we consider the emerging role of interfacial transient structural heterogeneities that are apparent in both model membranes as well as in whole cells.

Ion channels are responsible for numerous physiological functions ranging from transport to chemical and electrical signaling. Although static ion channel structure has been studied following a structural biology approach, spatiotemporal investigation of the dynamic molecular mechanisms of operational ion channels has not been achieved experimentally. In particular, the role of water remains elusive. Here, we perform label-free spatiotemporal second harmonic (SH) imaging and capacitance measurements of operational voltage-gated alamethicin ion channels in freestanding lipid membranes surrounded by aqueous solution on either side. We observe changes in SH intensity upon channel activation that are traced back to changes in the orientational distribution of water molecules that reorient along the field lines of transported ions. Of the transported ions, a fraction of 10 −4 arrives at the hydrated membrane interface, leading to interfacial electrostatic changes on the time scale of a second. The time scale of these interfacial changes is influenced by the density of ion channels and is subject to a crowding mechanism. Ion transport along cell membranes is often associated with the propagation of electrical signals in neurons. As our study shows that this process is taking place over seconds, a more complex mechanism is likely responsible for the propagation of neuronal electrical signals than just the millisecond movement of ions.