Ilaria Testa

Endocytic Trafficking of Rac Is Required for the Spatial Restriction of Signaling in Cell Migration

The small GTPases, Rab5 and Rac, are essential for endocytosis and actin remodeling, respectively. Coordination of these processes is critical to achieve spatial restriction of intracellular signaling, which is essential for a variety of polarized functions. Here, we show that clathrin- and Rab5-mediated endocytosis are required for the activation of Rac induced by motogenic stimuli. Rac activation occurs on early endosomes, where the RacGEF Tiam1 is also recruited. Subsequent recycling of Rac to the plasma membrane ensures localized signaling, leading to the formation of actin-based migratory protrusions. Thus, membrane trafficking of Rac is required for the spatial resolution of Rac-dependent motogenic signals. We further demonstrate that a Rab5-to-Rac circuitry controls the morphology of motile mammalian tumor cells and primordial germinal cells during zebrafish development, suggesting that this circuitry is relevant for the regulation of migratory programs in various cells, in both in vitro settings and whole organisms.

Diffraction-unlimited all-optical imaging and writing with a photochromic GFP

Lens-based optical microscopy failed to discern fluorescent features closer than 200 nm for decades, but the recent breaking of the diffraction resolution barrier by sequentially switching the fluorescence capability of adjacent features on and off is making nanoscale imaging routine. Reported fluorescence nanoscopy variants switch these features either with intense beams at defined positions or randomly, molecule by molecule. Here we demonstrate an optical nanoscopy that records raw data images from living cells and tissues with low levels of light. This advance has been facilitated by the generation of reversibly switchable enhanced green fluorescent protein (rsEGFP), a fluorescent protein that can be reversibly photoswitched more than a thousand times. Distributions of functional rsEGFP-fusion proteins in living bacteria and mammalian cells are imaged at <40-nanometre resolution. Dendritic spines in living brain slices are super-resolved with about a million times lower light intensities than before. The reversible switching also enables all-optical writing of features with subdiffraction size and spacings, which can be used for data storage.

A reversibly photoswitchable GFP-like protein with fluorescence excitation decoupled from switching.

Photoswitchable fluorescent proteins have enabled new approaches for imaging cells, but their utility has been limited either because they cannot be switched repeatedly or because the wavelengths for switching and fluorescence imaging are strictly coupled. We report a bright, monomeric, reversibly photoswitchable variant of GFP, Dreiklang, whose fluorescence excitation spectrum is decoupled from that for optical switching. Reversible on-and-off switching in living cells is accomplished at illumination wavelengths of ∼365 nm and ∼405 nm, respectively, whereas fluorescence is elicited at ∼515 nm. Mass spectrometry and high-resolution crystallographic analysis of the same protein crystal in the photoswitched on- and off-states demonstrate that switching is based on a reversible hydration/dehydration reaction that modifies the chromophore. The switching properties of Dreiklang enable far-field fluorescence nanoscopy in living mammalian cells using both a coordinate-targeted and a stochastic single molecule switching approach.

Multicolor fluorescence nanoscopy in fixed and living cells by exciting conventional fluorophores with a single wavelength.

Current far-field fluorescence nanoscopes provide subdiffraction resolution by exploiting a mechanism of fluorescence inhibition. This mechanism is implemented such that features closer than the diffraction limit emit separately when simultaneously exposed to excitation light. A basic mechanism for such transient fluorescence inhibition is the depletion of the fluorophore ground state by transferring it (via a triplet) in a dark state, a mechanism which is workable in most standard dyes. Here we show that microscopy based on ground state depletion followed by individual molecule return (GSDIM) can effectively provide multicolor diffraction-unlimited resolution imaging of immunolabeled fixed and SNAP-tag labeled living cells. Implemented with standard labeling techniques, GSDIM is demonstrated to separate up to four different conventional fluorophores using just two detection channels and a single laser line. The method can be expanded to even more colors by choosing optimized dichroic mirrors and selecting marker molecules with negligible inhomogeneous emission broadening.

The 2015 super-resolution microscopy roadmap.

Far-field optical microscopy using focused light is an important tool in a number of scientific disciplines including chemical, (bio) physical and biomedical research, particularly with respect to ...

rsEGFP2 enables fast RESOLFT nanoscopy of living cells.

The super-resolution microscopy called RESOLFT relying on fluorophore switching between longlived states, stands out by its coordinate-targeted sequential sample interrogation using low light levels. While RESOLFT has been shown to discern nanostructures in living cells, the reversibly photoswitchable green fluorescent protein (rsEGFP) employed in these experiments was switched rather slowly and recording lasted tens of minutes. We now report on the generation of rsEGFP2 providing faster switching and the use of this protein to demonstrate 25–250 times faster recordings. DOI: http://dx.doi.org/10.7554/eLife.00248.001

Nanoscopy of living brain slices with low light levels.

Lens-based fluorescence microscopy, which has long been limited in resolution to about 200 nanometers by diffraction, is rapidly evolving into a nanoscale imaging technique. Here, we show that the superresolution fluorescence microscopy called RESOLFT enables comparatively fast and continuous imaging of sensitive, nanosized features in living brain tissue. Using low-intensity illumination to switch photochromic fluorescent proteins reversibly between a fluorescent and a nonfluorescent state, we increased the resolution more than 3-fold over that of confocal microscopy in all dimensions. Dendritic spines located 10-50 μm deep inside living organotypic hippocampal brain slices were recorded for hours without signs of degradation. Using a fast-switching protein increased the imaging speed 50-fold over reported RESOLFT schemes, which in turn enabled the recording of spontaneous and stimulated changes of dendritic actin filaments and spine morphology occurring on time scales from seconds to hours.

CRISPR/Cas9-mediated endogenous protein tagging for RESOLFT super-resolution microscopy of living human cells

Overexpression is a notorious concern in conventional and especially in super-resolution fluorescence light microscopy studies because it may cause numerous artifacts including ectopic sub-cellular localizations, erroneous formation of protein complexes, and others. Nonetheless, current live cell super-resolution microscopy studies generally rely on the overexpression of a host protein fused to a fluorescent protein. Here, we establish CRISPR/Cas9-mediated generation of heterozygous and homozygous human knockin cell lines expressing fluorescently tagged proteins from their respective native genomic loci at close to endogenous levels. We tagged three different proteins, exhibiting various localizations and expression levels, with the reversibly switchable fluorescent protein rsEGFP2. We demonstrate the benefit of endogenous expression levels compared to overexpression and show that typical overexpression-induced artefacts were avoided in genome-edited cells. Fluorescence activated cell sorting analysis revealed a narrow distribution of fusion protein expression levels in genome-edited cells, compared to a pronounced variability in transiently transfected cells. Using low light intensity RESOLFT (reversible saturable optical fluorescence transitions) nanoscopy we show sub-diffraction resolution imaging of living human knockin cells. Our strategy to generate human cell lines expressing fluorescent fusion proteins at endogenous levels for RESOLFT nanoscopy can be extended to other fluorescent tags and super-resolution approaches.

A New FRAP/FRAPa Method for Three-Dimensional Diffusion Measurements Based on Multiphoton Excitation Microscopy

We present a new convenient method for quantitative three-dimensionally resolved diffusion measurements based on the photobleaching (FRAP) or photoactivation (FRAPa) of a disk-shaped area by the scanning laser beam of a multiphoton microscope. Contrary to previously reported spot-photobleaching protocols, this method has the advantage of full scalability of the size of the photobleached area and thus the range of diffusion coefficients, which can be measured conveniently. The method is compatible with low as well as high numerical aperture objective lenses, allowing us to perform quantitative diffusion measurements in three-dimensional extended samples as well as in very small volumes, such as cell nuclei. Furthermore, by photobleaching/photoactivating a large area, diffusion along the optical axis can be measured separately, which is convenient when studying anisotropic diffusion. First, we show the rigorous mathematical derivation of the model, leading to a closed-form formula describing the fluorescence recovery/redistribution phase. Next, the ability of the multiphoton FRAP method to correctly measure absolute diffusion coefficients is tested thoroughly on many test solutions of FITC-dextrans covering a wide range of diffusion coefficients. The same is done for the FRAPa method on a series of photoactivatable green fluorescent protein solutions with different viscosities. Finally, we apply the method to photoactivatable green fluorescent protein diffusing freely in the nucleus of living NIH-3T3 mouse embryo fibroblasts.

Nanoscale separation of molecular species based on their rotational mobility

We combine far-field fluorescence nanoscopy through serialized recording of switchable emitters with polarization-sensitive fluorescence detection. In addition to imaging with nanoscale spatial resolution, this technique allows determination of the fluorescence anisotropy of each detected dipole emitter and thus an estimate of its rotational mobility. Sub-populations of fluorescent markers can thus be separated based on their interaction with the sample. We applied this new functional nanoscopy to imaging of living mammalian cells.