Super-sectioning with multi-sheet reversible saturable optical fluorescence transitions (RESOLFT) microscopy.

Light-sheet fluorescence microscopy is an invaluable tool for four-dimensional biological imaging of multicellular systems due to the rapid volumetric imaging and minimal illumination dosage. However, it is challenging to retrieve fine subcellular information, especially in living cells, due to the width of the sheet of light (>1 µm). Here, using reversibly switchable fluorescent proteins (RSFPs) and a periodic light pattern for photoswitching, we demonstrate a super-resolution imaging method for rapid volumetric imaging of subcellular structures called multi-sheet RESOLFT. Multiple emission-sheets with a width that is far below the diffraction limit are created in parallel increasing recording speed (1-2 Hz) to provide super-sectioning ability (<100 nm). Our technology is compatible with various RSFPs due to its minimal requirement in the number of switching cycles and can be used to study a plethora of cellular structures. We track cellular processes such as cell division, actin motion and the dynamics of virus-like particles in three dimensions.

STED and parallelized RESOLFT optical nanoscopy of the tubular endoplasmic reticulum and its mitochondrial contacts in neuronal cells.

The classic view of organelle cell biology is undergoing a constant revision fueled by the new insights unraveled by fluorescence nanoscopy, which enable sensitive, faster and gentler observation of specific proteins in situ. The endoplasmic reticulum (ER) is one of the most challenging structure to capture due the rapid and constant restructuring of fine sheets and tubules across the full 3D cell volume. Here we apply STED and parallelized 2D and 3D RESOLFT live imaging to uncover the tubular ER organization in the fine processes of neuronal cells with focus on mitochondria-ER contacts, which recently gained medical attention due to their role in neurodegeneration. Multi-color STED nanoscopy enables the simultaneous visualization of small transversal ER tubules crossing and constricting mitochondria all along axons and dendrites. Parallelized RESOLFT allows for dynamic studies of multiple contact sites within seconds and minutes with prolonged time-lapse imaging at ~50 nm spatial resolution. When operated in 3D super resolution mode it enables a new isotropic visualization of such contacts extending our understanding of the three-dimensional architecture of these packed structures in axons and dendrites.

Fast reversibly photoswitching red fluorescent proteins for live-cell RESOLFT nanoscopy.

Reversibly photoswitchable fluorescent proteins (rsFPs) are gaining popularity as tags for optical nanoscopy because they make it possible to image with lower light doses. However, green rsFPs need violet-blue light for photoswitching, which is potentially phototoxic and highly scattering. We developed new rsFPs based on FusionRed that are reversibly photoswitchable with green-orange light. The rsFusionReds are bright and exhibit rapid photoswitching, thereby enabling nanoscale imaging of living cells.

Volumetric live cell imaging with three-dimensional parallelized RESOLFT microscopy.

Elucidating the volumetric architecture of organelles and molecules inside cells requires microscopy methods with a sufficiently high spatial resolution in all three dimensions. Current methods are limited by insufficient resolving power along the optical axis, long recording times and photobleaching when applied to live cell imaging. Here, we present a 3D, parallelized, reversible, saturable/switchable optical fluorescence transition (3D pRESOLFT) microscope capable of delivering sub-80-nm 3D resolution in whole living cells. We achieved rapid (1-2 Hz) acquisition of large fields of view (~40 × 40 µm2) by highly parallelized image acquisition with an interference pattern that creates an array of 3D-confined and equally spaced intensity minima. This allowed us to reversibly turn switchable fluorescent proteins to dark states, leading to a targeted 3D confinement of fluorescence. We visualized the 3D organization and dynamics of organelles in living cells and volumetric structural alterations of synapses during plasticity in cultured hippocampal neurons.

Predicting resolution and image quality in RESOLFT and other point scanning microscopes [Invited].

The performance of fluorescence microscopy and nanoscopy is often discussed by the effective point spread function and the optical transfer function. However, due to the complexity of the fluorophore properties such as photobleaching or other forms of photoswitching, which introduce a variance in photon emission, it is not trivial to choose optimal imaging parameters and to predict the spatial resolution. In this paper, we analytically derive a theoretical framework for estimating the achievable resolution of a microscope depending on parameters such as photoswitching, labeling densities, exposure time and sampling. We developed a numerical simulation software to analyze the impact of reversibly switchable probes in RESOLFT imaging.

Enhanced photon collection enables four dimensional fluorescence nanoscopy of living systems.

The theoretically unlimited spatial resolution of fluorescence nanoscopy often comes at the expense of time, contrast and increased dose of energy for recording. Here, we developed MoNaLISA, for Molecular Nanoscale Live Imaging with Sectioning Ability, a nanoscope capable of imaging structures at a scale of 45-65 nm within the entire cell volume at low light intensities (W-kW cm-2). Our approach, based on reversibly switchable fluorescent proteins, features three distinctly modulated illumination patterns crafted and combined to gain fluorescence ON-OFF switching cycles and image contrast. By maximizing the detected photon flux, MoNaLISA enables prolonged (40-50 frames) and large (50 × 50 µm2) recordings at 0.3-1.3 Hz with enhanced optical sectioning ability. We demonstrate the general use of our approach by 4D imaging of organelles and fine structures in epithelial human cells, colonies of mouse embryonic stem cells, brain cells, and organotypic tissues.