KEYWORDS: Point spread functions, 3D image processing, Microscopes, Biological imaging, 3D modeling, Molecules, Biological samples, Imaging systems, Education and training
Single Molecule Localization Microscopy (SMLM) and its ability to resolve < 100 nm structures has generated an evergrowing demand in biomedical research. This technique is highly relevant when trying to gain better understanding of biological structures details or cellular machinery in infectious models. The Imaging and Modelling Unit led by C. Zimmer developed an open optical and computational method based on Zernike Optimised Localisation Approach (ZOLA) enabling 3D localization of single molecules using point spread function (PSF) engineering in the detection path. This technique offers different performances and trade-off depending on the required application. This unique flexibility is relevant when dealing with various types of samples and models as those presented to an Imaging core facility. We will present how the Unit of Technology and Services (UTechS) Photonic Bio Imaging (PBI), the imaging platform of Institut Pasteur in Paris has conducted the technological transfer of ZOLA 3D from a research laboratory to a Bio Safety Level 2 (BSL2) ISO 9001 core facility. This will make flexible 3D super-resolution imaging accessible to a wide range of biological projects, including the study of pathogens.
Micromirror arrays (MMA) are spatial light modulators (SLM) used in a wide variety of applications for structured light manipulation i.e. structured illumination microscopy.
In our setup, we use a combination of two micromirror arrays, which allow not only to spatially structure the light in the field of view, but also to control the direction and angle of the incident light. In order to achieve this, a first MMA is imaged in the focal plane and used as a black and white (or even greyscale) mask. With a fully illuminated objective, this image would normally be formed from the complete light cone. By imaging the second MMA onto the backfocal plane of the objective only a portion of the light cone is used to form the image. This enables avoiding the unwanted illumination of out of focus objects. The MMAs in our setup consist of an array of 256x256 micromirrors, that can each be individually and continuously tilted up to 450nm, allowing the creation of greyscale images in real time in the illumination pattern. The mirrors themselves can be tilted for times as short as 10μs up to several seconds. This gives unprecedented control over the illumination times and intensities in the sample. Furthermore, our enhanced coating technology yields a high reflectivity over a broad optical spectrum (240- 1000nm).
Overall, the setup allows targetted illumination of subcellular regions enabling the precise, localized activation of optogenetic probes or the activation and deactivation of signaling cascades using photo-activated ion-channels.
We present a method for Z-super-localization in fluorescence microscopy, based on conical diffraction. By using a thin biaxial crystal, the Point Spread Function (PSF) shape of an objective is made to depend strongly on the z coordinate. This z dependence is then exploited to localize fluorescent emitters axially with a great precision. We study how this method can be used for single molecule imaging with a global assessment by Fisher information analysis. Preliminary experiments demonstrate that this technique can obtain resolutions of tens of nm with the use of high NA objectives.
Diffractive micromirror arrays (MMA) are a special class of optical MEMS, serving as spatial light modulators (SLM)
that control the phase of reflected light. Since the surface profile is the determining factor for an accurate phase
modulation, high-precision topographic characterization techniques are essential to reach highest optical performance.
While optical profiling techniques such as white-light interferometry are still considered to be most suitable to this task,
the practical limits of interferometric techniques start to become apparent with the current state of optical MEMS
technology. Light scatter from structured surfaces carries information about their topography, making scatter techniques
a promising alternative. Therefore, a spatially resolved scatter measurement technique, which takes advantage of the
MMA’s diffractive principle, has been implemented experimentally. Spectral measurements show very high contrast
ratios (up to 10 000 in selected samples), which are consistent with calculations from micromirror roughness parameters
obtained by white-light interferometry, and demonstrate a high sensitivity to changes in the surface topography. The
technique thus seems promising for the fast and highly sensitive characterization of diffractive MMAs.
We present a new technology for super-resolution fluorescence imaging, based on conical diffraction. Conical
diffraction is a linear, singular phenomenon taking place when a polarized beam is diffracted through a biaxial
crystal. The illumination patterns generated by conical diffraction are more compact than the classical Gaussian
beam; we use them to generate a super-resolution imaging modality. Conical Diffraction Microscopy (CODIM)
resolution enhancement can be achieved with any type of objective on any kind of sample preparation and standard
fluorophores. Conical diffraction can be used in multiple fashion to create new and disruptive technologies
for super-resolution microscopy. This paper will focus on the first one that has been implemented and give a
glimpse at what the future of microscopy using conical diffraction could be.
Photoactivation and “optogenetics” require the precise control of the illumination path in optical microscopes. It is equally important to shape the illumination spatially as well as to have control over the intensity and the duration of the illumination. In order to achieve these goals we use programmable, diffractive Micro Mirror Arrays (MMA) as fast spatial light modulators for beam steering. By combining two MMAs with 256×256 mirrors each, our illumination setup allows for fast angular and spatial control at a wide spectral range (260-1000 nm). Illumination pulses can be as short as 50 μs, or can also extend to several seconds. The specific illumination modes of the individual areas results in a precise control over the light dose to the sample, giving significant advantage when investigating dosage dependent activation inasmuch as both the duration and the intensity can be controlled distinctly. The setup is integrated to a microscope and allows selective illumination of regions in the sample, enabling the precise, localized activation of fluorescent probes and the activation and deactivation of cellular and subcellular signaling cascades using photo activated ion channels. The high reflectivity in the UV range (up to 260nm) further allows gene silencing using UV activated constructs (e.g. caged morpholinos).
Quantification of cell proliferation and monitoring its kinetics are essential in fields of research such as developmental biology, oncology, etc. Although several proliferation assays exist, monitoring cell proliferation kinetics remains challenging. We present a novel cell proliferation assay based on real-time monitoring of cell culture inside a standard incubator using a lensfree video-microscope, combined with automated detection of single cell divisions over a population of several thousand cells. Since the method is based on direct visualization of dividing cells, it is label-free, continuous, and not sample destructive. Kinetics of cell proliferation can be monitored from a few hours to several days. We compare our method to a standard assay, the EdU proliferation assay, and as proof of principle, we demonstrate concentration-dependent and time-dependent effect of actinomycin D—a cell proliferation inhibitor.
Innovative imaging methods are continuously developed to investigate the function of biological systems at the microscopic scale. As an alternative to advanced cell microscopy techniques, we are developing lensfree video microscopy that opens new ranges of capabilities, in particular at the mesoscopic level. Lensfree video microscopy allows the observation of a cell culture in an incubator over a very large field of view (24 mm2) for extended periods of time. As a result, a large set of comprehensive data can be gathered with strong statistics, both in space and time. Video lensfree microscopy can capture images of cells cultured in various physical environments. We emphasize on two different case studies: the quantitative analysis of the spontaneous network formation of HUVEC endothelial cells, and by coupling lensfree microscopy with 3D cell culture in the study of epithelial tissue morphogenesis. In summary, we demonstrate that lensfree video microscopy is a powerful tool to conduct cell assays in 2D and 3D culture experiments. The applications are in the realms of fundamental biology, tissue regeneration, drug development and toxicology studies.
The ability to control the illumination and imaging paths of optical microscopes is an essential part of advanced
fluorescence microscopy, and a powerful tool for optogenetics. In order to maximize the visualization and the image
quality of the objects under observation we use programmable, fast Micro Mirror Arrays (MMAs) as high-resolution
Spatial Light Modulators (SLMs). Using two 256x256 MMAs in a mirror-based illumination setup allows for fast
angular-spatial control at a wide range of wavelengths (300-1000nm). Additionally, the illumination intensity can be
controlled at 10-bit resolution. The setup allows selective illumination of subcellular regions of interest enabling the
precise, localized activation of fluorescent probes and the activation and deactivation of subcellular and cellular
signaling cascades using photo-activated ion-channels. Furthermore, inasmuch as phototoxicity is dependent on the rate
of photo illumination [1] we show that our system, which provides fast, compartmentalized illumination is minimally
phototoxic.
Bioluminescence Imaging (BLI) is an increasingly useful and applicable technique that allows for the non-invasive
observation of biological events in intact living organisms, ranging from single cells to small rodents. Though the
photon production occurs within the host, significant exposure times can be necessary due to the low photon flux
compared to fluorescence imaging. The optical absorption spectrum of haemoglobin strongly overlaps most
bioluminescent emission spectra, greatly attenuating the total detectable photons in animal models. We have
developed and validated a technique that is able to red-shift the bioluminescent photons to the more desirable optical
region of > 650 nm, a region of minimal absorbance by hemoglobin. This red-shift occurs by using bioluminescence
as an internal light source capable of exciting a fluorophore, such as a fluorescent protein or a quantum dot, that
emits in the red. Interestingly, in the absence of an absorber, this excitation can occur over substantial distances
(microns to centimeters), far exceeding distances associated to, and thereby precluding, resonance energy transfer
phenomena. We show this novel technique yields a substantial increase in the number of red photons for in vitro and
ex vivo conditions, suggesting eventually utility for in vivo studies on, for example, intact living mice.
Fluorophore concentration, the surrounding microenvironment, and photobleaching greatly influence the fluorescence
intensity of a fluorophore, increasing the difficulty to directly observe micro-environmental factors such as pH and
oxygen. However, the fluorescence lifetime of a fluorophore is essentially independent of both the fluorophore
concentration and photobleaching, providing a viable alternative to intensity measurements. The development of
fluorescence lifetime imaging (FLI) allows for the direct measurement of the microenvironment surrounding a
fluorophore. Pt-porphyrin is a fluorophore whose optical properties include a very stable triplet excited state. This energy
level overlaps strongly with the ground triplet state of oxygen, making the phosphorescent lifetime directly proportional
to the surrounding oxygen concentration. Initial experiments using this fluorophore involved the use of individual microwells
coated with the porphyrin. Cells were allowed to enter the micro-wells before being sealed to create a diffusionally
isolated volume. The decrease in the extracellular oxygen concentration was observed using FLI. However, this isolation
technique provides only the consumption rate but cannot indicate the subcellular oxygen distribution. To improve upon
this, live macrophages are loaded with the porphyrin and the fluorescence lifetime determined using a Lambert
Instruments Lifa-X FLI system. Initial results indicate that an increase in subcellular oxygen is observed upon initial
exposure to invasive bacteria. A substantial decrease in oxygen is observed after about 1 hour of exposure. The cells
remain in this deoxygenated state until the bacteria are removed or cell death occurs.
KEYWORDS: Calcium, Bioluminescence, Electron multiplying charge coupled devices, Sensors, Green fluorescent protein, Microscopes, Signal detection, Luminescence, Acquisition tracking and pointing, Video
The construction and application of genetically encoded intracellular calcium concentration ([Ca2+]i) indicators has a checkered history. Excitement raised over the creation of new probes is often followed by disappointment when it is found that the initial demonstrations of [Ca2+]i sensing capability cannot be leveraged into real scientific advances. Recombinant apo-aequorin cloned from Aequorea victoria was the first Ca2+ sensitive protein genetically targeted to subcellular compartments. In the jellyfish, bioluminescence resonance energy transfer (BRET) between Ca2+ bound aequorin and green fluorescent protein (GFP) emits green light. Similarly, Ca2+ sensitive bioluminescent reporters undergoing BRET have been constructed between aequorin and GFP, and more recently with other fluorescent protein variants. These hybrid proteins display red-shifted spectrums and have higher light intensities and stability compared to aequorin alone. We report BRET measurement of single-cell [Ca2+]i based on the use of electron-multiplying charge-coupled-detector (EMCCD) imaging camera technology, mounted on either a bioluminescence or conventional microscope. Our results show for the first time how these new technologies make facile long-term monitoring of [Ca2+]i at the single-cell level, obviating the need for expensive, fragile, and sophisticated equipment based on image-photon-detectors (IPD) that were until now the only technical recourse to dynamic BRET experiments of this type.
Three dimensional imaging provides high-content information from living intact biology, and can serve as a visual
screening cue. In the case of single cell imaging the current state of the art uses so-called "axial through-stacking".
However, three-dimensional axial through-stacking requires that the object (i.e. a living cell) be adherently stabilized on
an optically transparent surface, usually glass; evidently precluding use of cells in suspension. Aiming to overcome this
limitation we present here the utility of dielectric field trapping of single cells in three-dimensional electrode cages. Our
approach allows gentle and precise spatial orientation and vectored rotation of living, non-adherent cells in fluid
suspension. Using various modes of widefield, and confocal microscope imaging we show how so-called "microrotation"
can provide a unique and powerful method for multiple point-of-view (three-dimensional) interrogation of
intact living biological micro-objects (e.g. single-cells, cell aggregates, and embryos). Further, we show how visual
screening by micro-rotation imaging can be combined with micro-fluidic sorting, allowing selection of rare phenotype
targets from small populations of cells in suspension, and subsequent one-step single cell cloning (with high-viability).
Our methodology combining high-content 3D visual screening with one-step single cell cloning, will impact diverse
paradigms, for example cytological and cytogenetic analysis on haematopoietic stem cells, blood cells including
lymphocytes, and cancer cells.
KEYWORDS: Fluorescence resonance energy transfer, Deconvolution, Microscopes, Microscopy, Signal detection, Proteins, 3D image processing, Data modeling, Luminescence, Confocal microscopy
A complete understanding of cellular behavior will require precise temporal and spatial measurement of protein-protein interactions inside living cells. FRET Stoichiometry (Hoppe, A.D. et al., 2002 Biophys. J. 83:3652) has been used to measure the timing and spatial organization of protein-protein interactions in cells expressing yellow fluorescent protein (YFP)-labeled proteins and cyan fluorescent protein (CFP)-labeled proteins. However, all FRET data
collected in a single plane of a widefield microscope is a distorted 2D representation of a 3D object. Here we show that image blurring in the widefield microscope dramatically reduces sensitivity and spatial discrimination of FRET-based measurements of protein interactions. We present an algorithm for 3D restoration and calculation of FRET data that greatly increases signal-to-noise ratio and accuracy. The approach uses maximum likelihood deconvolution to quantitatively reassign out-of-focus light in 3D-FRET data sets. FRET Stoichiometry calculations performed on test constructs of linked YFP-CFP produced images that displayed uniform apparent FRET efficiencies (both EA and ED) and molar ratio of 1. 3D images of cells expressing free YFP and free CFP indicated apparent FRET efficiencies of 0%. Furthermore, 3D-FRET Stoichiometry imaging of the interaction of activated YFP-Rac1 with CFP-PBD in living cells produced superior detail with maximal apparent FRET efficiencies that were consistent with in vitro data. Together, these data demonstrated 3D-FRET Stoichiometry could accurately measure the fractions of interacting molecules and their molar ratios with high 3D spatial resolution.
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