Rainer Kaufmann

Dual color localization microscopy of cellular nanostructures

The dual color localization microscopy (2CLM) presented here is based on the principles of spectral precision distance microscopy (SPDM) with conventional autofluorescent proteins under special physical conditions. This technique allows us to measure the spatial distribution of single fluorescently labeled molecules in entire cells with an effective optical resolution comparable to macromolecular dimensions. Here, we describe the application of the 2CLM approach to the simultaneous nanoimaging of cellular structures using two fluorochrome types distinguished by different fluorescence emission wavelengths. The capabilities of 2CLM for studying the spatial organization of the genome in the mammalian cell nucleus are demonstrated for the relative distributions of two chromosomal proteins labeled with autofluorescent GFP and mRFP1 domains. The 2CLM images revealed quantitative information on their spatial relationships down to length‐scales of 30 nm.

Using conventional fluorescent markers for far-field fluorescence localization nanoscopy allows resolution in the 10-nm range

We present a novel technique of far‐field localization nanoscopy combining spectral precision distance microscopy with widely used fluorochromes like the Green Fluorescent Protein (GFP) derivatives eGFP, EmGFP, Yellow Fluorescent Protein (YFP) and eYFP, synthetic dyes like Alexa 488 and Alexa 568, as well as fluoresceine derivates. Spectral precision distance microscopy allows the surpassing of conventional resolution limits in fluorescence far‐field microscopy by precise object localization after the optical isolation of single signals in time. Based on the principles of this technique, our novel nanoscopic method was realized for laser optical precision localization and image reconstruction with highly enhanced optical resolution in intact cells. This allows for spatial assignment of individual fluorescent molecules with nanometre precision. The technique is based on excitation intensity dependent reversible photobleaching of the molecules used combined with fast time sequential imaging under appropriate focusing conditions. A meaningful advantage of the technique is the simple applicability as a universal tool for imaging and investigations to the major part of already available preparations according to standard protocols. Using the above mentioned fluorophores, the positions of single molecules within cellular structures were determined by visible light with an estimated localization precision down to 3 nm; hence distances in the range of 10–30 nm were resolved between individual fluorescent molecules allowing to apply different quantitative structure analysis tools.

Analysis of Her2/neu membrane protein clusters in different types of breast cancer cells using localization microscopy

The Her2/neu tyrosine kinase receptor is a member of the epidermal growth factor family. It plays an important role in tumour genesis of certain types of breast cancer and its overexpression correlates with distinct diagnostic and therapeutic decisions. Nevertheless, it is still under intense investigation to improve diagnostic outcome and therapy control. In this content, we applied spectral precision distance/position determination microscopy, a technique based on the general principles of localization microscopy in order to study tumour typical conformational changes of receptor clusters on cell membranes. We examined two different mamma carcinoma cell lines as well as cells of a breast biopsy of a healthy donor. The Her2/neu receptor sites were labelled by immunofluorescence using conventional fluorescent dyes (Alexa conjugated antibodies). The characterization of the Her2/neu distribution on plasma membrane sections of 176 different cells yielded a total amount of 20 637 clusters with a mean diameter of 67 nm. Statistical analysis on the single molecule level revealed differences in clustering of Her2/neu between all three different cell lines. We also showed that using spectral precision distance/position determination microscopy, a dual colour reconstruction of the 3D spatial arrangement of Her2/neu and Her3 is possible. This indicates that spectral precision distance/position determination microscopy could be used as an enhanced tool offering additional information of Her2/neu receptor status.

Localization Microscopy Reveals Expression-Dependent Parameters of Chromatin Nanostructure

A combined approach of 2D high-resolution localization light microscopy and statistical methods is presented to infer structural features and density fluctuations at the nuclear nanoscale. Hallmarks of nuclear nanostructure are found on the scale below 100 nm for both human fibroblast and HeLa cells. Mechanical measures were extracted as a quantitative tool from the histone density fluctuations inside the cell to obtain structural fluctuations on the scale of several micrometers. Results show that different mechanisms of expression of the same nuclear protein type lead to significantly different patterns on the nanoscale and to pronounced differences in the detected compressibility of chromatin. The observed fluctuations, including the experimental evidence for dynamic looping, are consistent with a recently proposed chromatin model.

SIMcheck: a Toolbox for Successful Super-resolution Structured Illumination Microscopy.

Three-dimensional structured illumination microscopy (3D-SIM) is a versatile and accessible method for super-resolution fluorescence imaging, but generating high-quality data is challenging, particularly for non-specialist users. We present SIMcheck, a suite of ImageJ plugins enabling users to identify and avoid common problems with 3D-SIM data, and assess resolution and data quality through objective control parameters. Additionally, SIMcheck provides advanced calibration tools and utilities for common image processing tasks. This open-source software is applicable to all commercial and custom platforms, and will promote routine application of super-resolution SIM imaging in cell biology.

High-precision correlative fluorescence and electron cryo microscopy using two independent alignment markers.

Correlative light and electron microscopy (CLEM) is an emerging technique which combines functional information provided by fluorescence microscopy (FM) with the high-resolution structural information of electron microscopy (EM). So far, correlative cryo microscopy of frozen-hydrated samples has not reached better than micrometre range accuracy. Here, a method is presented that enables the correlation between fluorescently tagged proteins and electron cryo tomography (cryoET) data with nanometre range precision. Specifically, thin areas of vitrified whole cells are examined by correlative fluorescence cryo microscopy (cryoFM) and cryoET. Novel aspects of the presented cryoCLEM workflow not only include the implementation of two independent electron dense fluorescent markers to improve the precision of the alignment, but also the ability of obtaining an estimate of the correlation accuracy for each individual object of interest. The correlative workflow from plunge-freezing to cryoET is detailed step-by-step for the example of locating fluorescence-labelled adenovirus particles trafficking inside a cell.

Superresolution imaging of biological nanostructures by spectral precision distance microscopy

For the improved understanding of biological systems on the nanoscale, it is necessary to enhance the resolution of light microscopy in the visible wavelength range beyond the limits of conventional epifluorescence microscopy (optical resolution of about 200 nm laterally, 600 nm axially). Recently, various far‐field methods have been developed allowing a substantial increase of resolution (“superresolution microscopy”, or “lightoptical nanoscopy”). This opens an avenue to ‘nano‐image’ intact and even living cells, as well as other biostructures like viruses, down to the molecular detail. Thus, it is possible to combine light optical spatial nanoscale information with ultrastructure analyses and the molecular interaction information provided by molecular cell biology. In this review, we describe the principles of spectrally assigned localization microscopy (SALM) of biological nanostructures, focusing on a special SALM approach, spectral precision distance/position determination microscopy (SPDM) with physically modified fluorochromes (SPDMPhymod. Generally, this SPDM method is based on high‐precision localization of fluorescent molecules, which can be discriminated using reversibly bleached states of the fluorophores for their optical isolation. A variety of application examples is presented, ranging from superresolution microscopy of membrane and cytoplasmic protein distribution to dual‐color SPDM of nuclear proteins. At present, we can achieve an optical resolution of cellular structures down to the 20‐nm range, with best values around 5 nm (∼1/100 of the exciting wavelength).

Structurally encoded intraclass differences in EphA clusters drive distinct cell responses

Functional outcomes of ephrin binding to Eph receptors (Ephs) range from cell repulsion to adhesion. Here we used cell collapse and stripe assays, showing contrasting effects of human ephrinA5 binding to EphA2 and EphA4. Despite equivalent ligand binding affinities, EphA4 triggered greater cell collapse, whereas EphA2-expressing cells adhered better to ephrinA5-coated surfaces. Chimeric receptors showed that the ectodomain is a major determinant of cell response. We report crystal structures of EphA4 ectodomain alone and in complexes with ephrinB3 and ephrinA5. These revealed closed clusters with a dimeric or circular arrangement in the crystal lattice, contrasting with extended arrays previously observed for EphA2 ectodomain. Localization microscopy showed that ligand-stimulated EphA4 induces smaller clusters than does EphA2. Mutant Ephs link these characteristics to interactions observed in the crystal lattices, suggesting a mechanism by which distinctive ectodomain surfaces determine clustering, and thereby signaling, properties.

Super-Resolution Microscopy Using Standard Fluorescent Proteins in Intact Cells under Cryo-Conditions

We introduce a super-resolution technique for fluorescence cryo-microscopy based on photoswitching of standard genetically encoded fluorescent marker proteins in intact mammalian cells at low temperature (81 K). Given the limit imposed by the lack of cryo-immersion objectives, current applications of fluorescence cryo-microscopy to biological specimens achieve resolutions between 400–500 nm only. We demonstrate that the single molecule characteristics of reversible photobleaching of mEGFP and mVenus at liquid nitrogen temperature are suitable for the basic concept of single molecule localization microscopy. This enabled us to perform super-resolution imaging of vitrified biological samples and to visualize structures in unperturbed fast frozen cells for the first time with a structural resolution of ∼125 nm (average single molecule localization accuracy ∼40 nm), corresponding to a 3–5 fold resolution improvement.

Correlative in-resin super-resolution and electron microscopy using standard fluorescent proteins

We introduce a method for correlative in-resin super-resolution fluorescence and electron microscopy (EM) of biological structures in mammalian culture cells. Cryo-fixed resin embedded samples offer superior structural preservation, performing in-resin super-resolution, however, remains a challenge. We identified key aspects of the sample preparation procedure of high pressure freezing, freeze substitution and resin embedding that are critical for preserving fluorescence and photo-switching of standard fluorescent proteins, such as mGFP, mVenus and mRuby2. This enabled us to combine single molecule localization microscopy with transmission electron microscopy imaging of standard fluorescent proteins in cryo-fixed resin embedded cells. We achieved a structural resolution of 40–50 nm (~17 nm average single molecule localization accuracy) in the fluorescence images without the use of chemical fixation or special fluorophores. Using this approach enabled the correlation of fluorescently labeled structures to the ultrastructure in the same cell at the nanometer level and superior structural preservation.