Name: nano-fem: protein localization using photo-activated localization microscopy and electron microscopy
Uploaded: 2012-12-03T12:00:00
Duration: 13 min 13 s
Description: Watch this Scientific Journal Video about Nano-fEM: Protein Localization Using Photo-activated Localization Microscopy and Electron Microscopy at JoVE.com

We thank Harald Hess and Eric Betzig for access to the PALM microscope for proof-of-principle experiments, Richard Fetter for sharing fixation protocols, reagents and encouragement. We thank Michael Davidson, Geraldine Seydoux, Stefan Eimer, Rudolf Leube, Keith Nehrke, Christian Fr&oslash;kjr-Jensen, Aude Ada-Nguema and Marc Hammarlund for DNA constructs. We also thank Carl Zeiss Inc. for providing access to the Zeiss PAL-M, a beta version of the Zeiss Elyra P.1 PALM microscope.

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Production and Free Access to this article is sponsored by Carl Zeiss, Inc.

Here we describe how to preserve fluorescent proteins in plastic, localize the fluorescent proteins in sections, and image the ultrastructure using electron microscopy. Proteins were localized below the diffraction limit using PALM microscopy to nanometer resolution. To adapt this protocol to particular specimens, the following parameters should be considered: fluorophore, quantification, and alignment. 
The choice of fluorescent protein or organic fluorophore depends on the application and the model system. We have tested a variety of fluorescent proteins, including EGFP, YFP, Citrine, mEosFP, mEos2, tdEos, mOrange, PA-mCherry, and Dendra12. The preservation of fluorescence from each fluorophore was similar, suggesting that all fluorescent proteins can be preserved using the described method. We chose tdEos because it expressed well in C. elegans, proteins remained functional when fused to tdEos, and because its photo-activation characteristics were optimal for PALM microscopy. However, aggregation or failed expression of tdEos has been occasionally observed12. 
Depending on the application, a different fluorophore may be better suited. In many cases, it is not necessary to use a photo-activated fluorescent protein. Simple correlative fluorescence electron microscopy does not require photo-activated fluorescent protein. GFP or organic dyes can be used to image fluorescence from tagged proteins in sections above the diffraction limit. For example, one can image an axon in a neuropil using fluorescence microscopy and correlate the fluorescence signal with a particular axon in an electron micrograph by imaging the fluorescence on a fluorescence microscope. Other super-resolution techniques, such as stimulated emission depletion microscopy (STED)12, ground state depletion microscopy followed by individual molecule return (GSDIM)13, and structured illumination microscopy (SIM)14, do not require photo-activated fluorescent proteins. Moreover, super-resolution imaging techniques that use organic dyes9,15,16 or the intrinsic property of fluorescent probes17 are readily applicable. 
In PALM, the number of molecules can be quantified because fluorescence of each molecule is separated spatially and temporally. However, quantification may be misleading for four reasons: oxidation, undercounting, overcounting, and overexpression. First, a fraction of the fluorescent proteins can be denatured or oxidized during sample processing5,12. Although ~90% of the fluorescence signal was preserved through fixation and embedding in our protocol, oxidation of the fluorescent protein may occur after the specimen has been sectioned and the surface exposed to oxygen. Second, the activation of photo-activatable proteins is stochastic, and thus multiple molecules can be activated in a given diffraction limited spot8. Fluorescence from the multiple molecules will appear as one spot, and thus the total number of proteins will be undercounted. Third, a similar problem can lead to overcounting. In PALM, each fluorescent protein is localized and then "erased" by bleaching. However, fluorescent proteins can return from the dark state without being permanently bleached18. Such molecules will then be counted multiple times. Fourth, tagged proteins are expressed as transgenes and are often present in multiple copies, which can lead to overexpression. Therefore, quantification from PALM can be used to estimate but not precisely determine the number of molecules in a given location. 
The alignment of a PALM image with an electron micrograph can also be challenging because of the resolution difference in light and electron microscopy and distortion caused by the electron beam. Gold particles serve as tightly localized fiducial markers in electron micrographs. However, fluoresecence from gold particles is not photo-activated, and appears as a large diffraction-limited spot. Thus, the placement of a fluorescence image over an electron micrograph is an estimate. Distortions can also arise from interactions of electrons with the plastic section. Acrylic resins such as GMA are less stable under the electron beam, and the dimensions of the plastic can be altered. Under these circumstances, aligning the fluorescence with ultrastructure may require non-linear transformation of the fiducial markers.

<table border="1">
<tbody>
 <tr>
 <td>Name of the reagent or equipment</td>
 <td>Company</td>
 <td>Catalog number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>High-pressure freezer</td>
 <td>ABRA</td>
 <td>HPM 010</td>
 <td>EMPact and HPM 100 from Leica or hpf-01 from Wohlwend can also 			be used.</td>
 </tr>
 <tr>
 <td>Automated freeze substitution unit</td>
 <td>Leica</td>
 <td>AFS 2</td>
 <td>AFS 1 can also be used.</td>
 </tr>
 <tr>
 <td>Zeiss PALM</td>
 <td>Zeiss</td>
 <td>ELYRA P.1</td>
 <td>Nikon and Vutara also sell commercial PALM microscopes. </td>
 </tr>
 <tr>
 <td>Scanning electron microscope</td>
 <td>FEI</td>
 <td>Nova nano</td>
 <td>Other high-resolution SEM microscopes can be used.</td>
 </tr>
 <tr>
 <td>Acetone</td>
 <td>EMS</td>
 <td>RT10016</td>
 <td></td>
 </tr>
 <tr>
 <td>Ethanol </td>
 <td>Sigma-aldrich</td>
 <td>459844-1L</td>
 <td></td>
 </tr>
 <tr>
 <td>Osmium tetroxide</td>
 <td>EMS</td>
 <td>RT19134</td>
 <td></td>
 </tr>
 <tr>
 <td>Potassium permanganate</td>
 <td>EMS</td>
 <td>RT20200</td>
 <td></td>
 </tr>
 <tr>
 <td>Albumin from bovine serum</td>
 <td>Sigma-aldrich</td>
 <td>A3059-50G</td>
 <td></td>
 </tr>
 <tr>
 <td>Uranyl acetate</td>
 <td>Polysciences</td>
 <td>21447-25</td>
 <td>pH of uranyl acetate from this company is slightly higher.</td>
 </tr>
 <tr>
 <td>Glycol methacrylate (GMA)</td>
 <td>SPI</td>
 <td>02630-AA</td>
 <td>Low acid, TEM grade.</td>
 </tr>
 <tr>
 <td>N,N-Dimethyl-p-toluidine</td>
 <td>Sigma-aldrich</td>
 <td>D9912</td>
 <td></td>
 </tr>
 <tr>
 <td>Cryo vials</td>
 <td>Nalgene</td>
 <td>5000-0020</td>
 <td></td>
 </tr>
 <tr>
 <td>Glass vials</td>
 <td>EMS</td>
 <td>72632</td>
 <td></td>
 </tr>
 <tr>
 <td>Aclar film</td>
 <td>EMS</td>
 <td>50425-10</td>
 <td></td>
 </tr>
 <tr>
 <td>BEEM capsule</td>
 <td>EBSciences</td>
 <td>TC</td>
 <td>Polypropylene</td>
 </tr>
 <tr>
 <td>3/8" DISC punches</td>
 <td>Ted Pella</td>
 <td>54741</td>
 <td></td>
 </tr>
 <tr>
 <td>Gold nanoparticles</td>
 <td>microspheres-nanospheres.com</td>
 <td>790122-010 </td>
 <td>Request 2x concentrated solution</td>
 </tr>
 </tbody>
</table>

nano-fem: protein localization using photo-activated localization microscopy and electron microscopy

Mapping the distribution of proteins is essential for understanding the function of proteins in a cell. Fluorescence microscopy is extensively used for protein localization, but subcellular context is often absent in fluorescence images. Immuno-electron microscopy, on the other hand, can localize proteins, but the technique is limited by a lack of compatible antibodies, poor preservation of morphology and because most antigens are not exposed to the specimen surface. Correlative approaches can acquire the fluorescence image from a whole cell first, either from immuno-fluorescence or genetically tagged proteins. The sample is then fixed and embedded for electron microscopy, and the images are correlated 1-3. However, the low-resolution fluorescence image and the lack of fiducial markers preclude the precise localization of proteins. 
Alternatively, fluorescence imaging can be done after preserving the specimen in plastic. In this approach, the block is sectioned, and fluorescence images and electron micrographs of the same section are correlated 4-7. However, the diffraction limit of light in the correlated image obscures the locations of individual molecules, and the fluorescence often extends beyond the boundary of the cell. 
Nano-resolution fluorescence electron microscopy (nano-fEM) is designed to localize proteins at nano-scale by imaging the same sections using photo-activated localization microscopy (PALM) and electron microscopy. PALM overcomes the diffraction limit by imaging individual fluorescent proteins and subsequently mapping the centroid of each fluorescent spot 8-10. 
We outline the nano-fEM technique in five steps. First, the sample is fixed and embedded using conditions that preserve the fluorescence of tagged proteins. Second, the resin blocks are sectioned into ultrathin segments (70-80 nm) that are mounted on a cover glass. Third, fluorescence is imaged in these sections using the Zeiss PALM microscope. Fourth, electron dense structures are imaged in these same sections using a scanning electron microscope. Fifth, the fluorescence and electron micrographs are aligned using gold particles as fiducial markers. In summary, the subcellular localization of fluorescently tagged proteins can be determined at nanometer resolution in approximately one week.

Watch this Scientific Journal Video about Nano-fEM: Protein Localization Using Photo-activated Localization Microscopy and Electron Microscopy at JoVE.com

Nano-fEM: Protein Localization Using Photo-activated Localization Microscopy and Electron Microscopy

We describe a method to localize fluorescently tagged proteins in electron micrographs. Fluorescence is first localized using photo-activated localization microscopy on ultrathin sections. These images are then aligned to electron micrographs of the same section.

Mapping the distribution of proteins is essential for understanding the function of proteins in a cell. Fluorescence microscopy is extensively used for ...

nano-fem-protein-localization-using-photo-activated-localization

Research

JoVE Journal

Biology

Simultaneous Multicolor Imaging of Biological Structures with Fluorescence Photoactivation Localization Microscopy

Localization-based super resolution microscopy can be applied to obtain a spatial map (image) of the distribution of individual fluorescently labeled single molecules within a sample with a spatial resolution of tens of nanometers. Using either photoactivatable (PAFP) or&#160;photoswitchable (PSFP) fluorescent proteins fused to proteins of interest, or organic dyes conjugated to antibodies or other molecules of interest, fluorescence photoactivation localization microscopy (FPALM) can simultaneously image multiple species of molecules within single cells. By using the following approach, populations of large numbers (thousands to hundreds of thousands) of individual molecules are imaged in single cells and localized with a precision of ~10-30 nm. Data obtained can be applied to understanding the nanoscale spatial distributions of multiple protein types within a cell. One primary advantage of this technique is the dramatic increase in spatial resolution: while diffraction limits resolution to ~200-250 nm in conventional light microscopy, FPALM can image length scales more than an order of magnitude smaller. As many biological hypotheses concern the spatial relationships among different biomolecules, the improved resolution of FPALM can provide insight into questions of cellular organization which have previously been inaccessible to conventional fluorescence microscopy. In addition to detailing the methods for sample preparation and data acquisition, we here describe the optical setup for FPALM. One additional consideration for researchers wishing to do super-resolution microscopy is cost: in-house setups are significantly cheaper than most commercially available imaging machines. Limitations of this technique include the need for optimizing the labeling of molecules of interest within cell samples, and the need for post-processing software to visualize results. We here describe the use of PAFP and PSFP expression to image two protein species in fixed cells. Extension of the technique to living cells is also described.

We demonstrate the use of fluorescence photo activation localization microscopy (FPALM) to simultaneously image multiple types of fluorescently labeled molecules&#160;within cells. The techniques described yield the localization of thousands to hundreds of thousands of individual fluorescent labeled proteins, with a precision of tens of nanometers&#160;within single cells.

Localization-based super resolution microscopy can be applied to obtain a spatial map (image) of the distribution of individual fluorescently labeled ...

Three-dimensional Super Resolution Microscopy of F-actin Filaments by Interferometric PhotoActivated Localization Microscopy (iPALM)

Fluorescence microscopy enables direct visualization of specific biomolecules within cells. However, for conventional fluorescence microscopy, the spatial resolution is restricted by diffraction to ~ 200 nm within the image plane and &#62; 500 nm along the optical axis. As a result, fluorescence microscopy has long been severely limited in the observation of ultrastructural features within cells. The recent development of super resolution microscopy methods has overcome this limitation. In particular, the advent of photoswitchable fluorophores enables localization-based super resolution microscopy, which provides resolving power approaching the molecular-length scale. Here, we describe the application of a three-dimensional super resolution microscopy method based on single-molecule localization microscopy and multiphase interferometry, called interferometric PhotoActivated Localization Microscopy (iPALM). This method provides nearly isotropic resolution on the order of 20 nm in all three dimensions. Protocols for visualizing the filamentous actin cytoskeleton, including specimen preparation and operation of the iPALM instrument, are described here. These protocols are also readily adaptable and instructive for the study of other ultrastructural features in cells.

Three-dimensional Super Resolution Microscopy of F-actin Filaments by Interferometric PhotoActivated Localization Microscopy iPALM

We present a protocol for the application of interferometric PhotoActivated Localization Microscopy (iPALM), a 3-dimensional single-molecule localization super resolution microscopy method, to the imaging of the actin cytoskeleton in adherent mammalian cells. This approach allows light-based visualization of nanoscale structural features that would otherwise remain unresolved by conventional diffraction-limited optical microscopy.

Fluorescence microscopy enables direct visualization of specific biomolecules within cells. However, for conventional fluorescence microscopy, the spatial ...

Multi-color Localization Microscopy of Single Membrane Proteins in Organelles of Live Mammalian Cells

Knowledge about the localization of proteins in cellular subcompartments is crucial to understand their specific function. Here, we present a super-resolution technique that allows for the determination of the microcompartments that are accessible for proteins by generating localization and tracking maps of these proteins. Moreover, by multi-color localization microscopy, the localization and tracking profiles of proteins in different subcompartments are obtained simultaneously. The technique is specific for live cells and is based on the repetitive imaging of single mobile membrane proteins. Proteins of interest are genetically fused with specific, so-called self-labeling tags. These tags are enzymes that react with a substrate in a covalent manner. Conjugated to these substrates are fluorescent dyes. Reaction of the enzyme-tagged proteins with the fluorescence labeled substrates results in labeled proteins. Here, Tetramethylrhodamine (TMR) and Silicon Rhodamine (SiR) are used as fluorescent dyes attached to the substrates of the enzymes. By using substrate concentrations in the pM to nM range, sub-stoichiometric labeling is achieved that results in distinct signals. These signals are localized with ~15&#8211;27 nm precision. The technique allows for multi-color imaging of single molecules, whereby the number of colors is limited by the available membrane-permeable dyes and the repertoire of self-labeling enzymes. We show the feasibility of the technique by determining the localization of the quality control enzyme (Pten)-induced kinase 1 (PINK1) in different mitochondrial compartments during its processing in relation to other membrane proteins. The test for true physical interactions between differently labeled single proteins by single molecule FRET or co-tracking is restricted, though, because the low labeling degrees decrease the probability for having two adjacent proteins labeled at the same time. While the technique is strong for imaging proteins in membrane compartments, in most cases it is not appropriate to determine the localization of highly mobile soluble proteins.

Here, we present a protocol for multi-color localization of single membrane proteins in organelles of live cells. To attach fluorophores, self-labeling proteins are used. Proteins, located in different membranes compartments of the same organelle, can be localized with a precision of ~18 nm.

Knowledge about the localization of proteins in cellular subcompartments is crucial to understand their specific function. Here, we present a ...

Biological Sample Preparation by High-pressure Freezing, Microwave-assisted Contrast Enhancement, and Minimal Resin Embedding for Volume Imaging

The described sample preparation technique is designed to combine the best quality of ultrastructural preservation with the most suitable contrast for the imaging modality in a focused ion beam scanning electron microscope (FIB-SEM), which is used to obtain stacks of sequential images for 3D reconstruction and modelling. High-pressure freezing (HPF) allows close to native structural preservation, but the subsequent freeze substitution often does not provide sufficient contrast, especially for a bigger specimen, which is needed for high-quality imaging in the SEM required for 3D reconstruction. Therefore, in this protocol, after the freeze substitution, additional contrasting steps are carried out at room temperature. Although these steps are performed in a microwave, it is also possible to follow traditional bench processing, which requires longer incubation times.The subsequent embedding in minimal amounts of resin allows for faster and more precise targeting and preparation inside the FIB-SEM. This protocol is especially useful for samples that require preparation by high-pressure freezing for a reliable ultrastructural preservation but do not gain enough contrast during the freeze substitution for volume imaging using FIB-SEM. In combination with the minimal resin embedding, this protocol provides an efficient workflow for the acquisition of high-quality volume data.

Here we present a protocol for combining two sample processing techniques, high-pressure freezing and microwave-assisted sample processing, followed by minimal resin embedding for acquiring data with a focused ion beam scanning electron microscope (FIB-SEM). This is demonstrated using a mouse tibial nerve sample and Caenorhabditis elegans.

The described sample preparation technique is designed to combine the best quality of ultrastructural preservation with the most suitable contrast for the ...

Visualizing Lymph Node Structure and Cellular Localization using Ex-Vivo Confocal Microscopy

Lymph nodes (LNs) are organs spread within the body, where the innate immune responses can connect with the adaptive immunity. In fact, LNs are strategically interposed in the path of the lymphatic vessels, allowing intimate contact of tissue antigens with all resident immune cells in the LN. Thus, understanding the cellular composition, distribution, location and interaction using ex vivo whole LN imaging will add to the knowledge on how the body coordinates local and systemic immune responses. This protocol shows an ex vivo imaging strategy following an in vivo administration of fluorescent-labeled antibodies that allows a very reproducible and easy-to-perform methodology by using conventional confocal microscopes and stock reagents. Through subcutaneous injection of antibodies, it is possible to label different cell populations in draining LNs without affecting tissue structures that can be potentially damaged by a conventional immunofluorescence microscopy technique.

This protocol describes a technique to image different cell populations in draining lymph nodes without alterations in the organ structure.

Lymph nodes (LNs) are organs spread within the body, where the innate immune responses can connect with the adaptive immunity. In fact, LNs are ...

The CryoAPEX Method for Electron Microscopy Analysis of Membrane Protein Localization Within Ultrastructurally-Preserved Cells

Key cellular events like signal transduction and membrane trafficking rely on proper protein location within cellular compartments. Understanding precise subcellular localization of proteins is thus important for answering many biological questions. The quest for a robust label to identify protein localization combined with adequate cellular preservation and staining has been historically challenging. Recent advances in electron microscopy (EM) imaging have led to the development of many methods and strategies to increase cellular preservation and label target proteins. A relatively new peroxidase-based genetic tag, APEX2, is a promising leader in cloneable EM-active tags. Sample preparation for transmission electron microscopy (TEM) has also advanced in recent years with the advent of cryofixation by high pressure freezing (HPF) and low-temperature dehydration and staining via freeze substitution (FS). HPF and FS provide excellent preservation of cellular ultrastructure for TEM imaging, second only to direct cryo-imaging of vitreous samples. Here we present a protocol for the cryoAPEX method, which combines the use of the APEX2 tag with HPF and FS. In this protocol, a protein of interest is tagged with APEX2, followed by chemical fixation and the peroxidase reaction. In place of traditional staining and alcohol dehydration at room temperature, the sample is cryofixed and undergoes dehydration and staining at low temperature via FS. Using cryoAPEX, not only can a protein of interest be identified within subcellular compartments, but also additional information can be resolved with respect to its topology within a structurally preserved membrane. We show that this method can provide high enough resolution to decipher protein distribution patterns within an organelle lumen, and to distinguish the compartmentalization of a protein within one organelle in close proximity to other unlabeled organelles. Further, cryoAPEX is procedurally straightforward and amenable to cells grown in tissue culture. It is no more technically challenging than typical cryofixation and freeze substitution methods. CryoAPEX is widely applicable for TEM analysis of any membrane protein that can be genetically tagged.

This protocol describes the cryoAPEX method, in which an APEX2-tagged membrane protein can be localized by transmission electron microscopy within optimally-preserved cell ultrastructure.

Key cellular events like signal transduction and membrane trafficking rely on proper protein location within cellular compartments. Understanding precise ...

Analyzing the &alpha;-Actinin Network in Human iPSC-Derived Cardiomyocytes Using Single Molecule Localization Microscopy

The maturation of iPSC-derived cardiomyocytes is a critical issue for their application in regenerative therapy, drug testing and disease modeling. Despite the development of multiple differentiation protocols, the generation of iPSC cardiomyocytes resembling an adult-like phenotype remains challenging. One major aspect of cardiomyocytes maturation involves the formation of a well-organized sarcomere network to ensure high contraction capacity. Here, we present a super resolution-based approach for semi-quantitative analysis of the &#945;-actinin network in cardiomyocytes. Using photoactivated localization microscopy a comparison of sarcomere length and z-disc thickness of iPSC-derived cardiomyocytes and cardiac cells isolated from neonatal tissue was performed. At the same time, we demonstrate the importance of proper imaging conditions to obtain reliable data. Our results show that this method is suitable to quantitatively monitor the structural maturity of cardiac cells with high spatial resolution, enabling the detection of even subtle changes of sarcomere organization.

Analyzing the &amp;#945;-Actinin Network in Human iPSC-Derived Cardiomyocytes Using Single Molecule Localization Microscopy

The formation of a proper sarcomere network is important for the maturation of iPSC-derived cardiomyocytes. We present a super resolution-based approach that allows for the quantitative evaluation of the structural maturation of stem cell derived cardiomyocytes, to improve culture conditions promoting cardiac development.

The maturation of iPSC-derived cardiomyocytes is a critical issue for their application in regenerative therapy, drug testing and disease modeling. ...

Cryo-Electron Microscopic Grid Preparation for Time-Resolved Studies using a Novel Robotic System, Spotiton

The capture of short-lived molecular states triggered by the early encounter of two or more interacting particles continues to be an experimental challenge of great interest to the field of cryo-electron microscopy (cryo-EM). A few methodological strategies have been developed that support these &#34;time-resolved&#34; studies, one of which, Spotiton-a novel robotic system-combines the dispensing of picoliter-sized sample droplets with precise temporal and spatial control. The time-resolved Spotiton workflow offers a uniquely efficient approach to interrogate early structural rearrangements from minimal sample volume. Fired from independently controlled piezoelectric dispensers, two samples land and rapidly mix on a nanowire EM grid as it plunges toward the cryogen. Potentially hundreds of grids can be prepared in rapid succession from only a few microliters of a sample. Here, a detailed step-by-step protocol of the operation of the Spotiton system is presented with a focus on troubleshooting specific problems that arise during grid preparation.

The protocol presented here describes the use of Spotiton, a novel robotic system, to deliver two samples of interest onto a self-wicking, nanowire grid that mix for a minimum of 90 ms prior to vitrification in liquid cryogen.

The capture of short-lived molecular states triggered by the early encounter of two or more interacting particles continues to be an experimental ...

Strategies for Optimization of Cryogenic Electron Tomography Data Acquisition

Cryogenic electron tomography (cryoET) is a powerful method to study the 3D structure of biological samples in a close-to-native state. Current state-of-the-art cryoET combined with subtomogram averaging analysis enables the high-resolution structural determination of macromolecular complexes that are present in multiple copies within tomographic reconstructions. Tomographic experiments usually require a vast amount of tilt series to be acquired by means of high-end transmission electron microscopes with important operational running-costs. Although the throughput and reliability of automated data acquisition routines have constantly improved over the recent years, the process of selecting regions of interest at which a tilt series will be acquired cannot be easily automated and it still relies on the user's manual input. Therefore, the set-up of a large-scale data collection session is a time-consuming procedure that can considerably reduce the remaining microscope time available for tilt series acquisition. Here, the protocol describes the recently developed implementations based on the SerialEM package and the PyEM software that significantly improve the time-efficiency of grid screening and large-scale tilt series data collection. The presented protocol illustrates how to use SerialEM scripting functionalities to fully automate grid mapping, grid square mapping, and tilt series acquisition. Furthermore, the protocol describes how to use PyEM to select additional acquisition targets in off-line mode after automated data collection is initiated. To illustrate this protocol, its application in the context of high-end data collection of Sars-Cov-2 tilt series is described. The presented pipeline is particularly suited to maximizing the time-efficiency of tomography experiments that require a careful selection of acquisition targets and at the same time a large amount of tilt series to be collected.

The increasing demand for large-scale data collection in cryogenic electron tomography requires high-throughput image acquisition routines. Described here is a protocol that implements the recent developments of advanced acquisition strategies aimed at maximizing the time-efficiency and throughput of tomographic data collection.

Cryogenic electron tomography (cryoET) is a powerful method to study the 3D structure of biological samples in a close-to-native state. Current ...

Visualizing Subcellular Localization of a Protein in the Heart Using Quantum Dots-Mediated Immuno-Labeling Followed by Transmission Electron Microscopy

The subcellular localization is critical to delineating proper function and determining the molecular mechanisms of a particular protein. Several qualitative and quantitative techniques are used to determine the subcellular localization of proteins. One of the emerging techniques in determining the subcellular localization of a protein is quantum dots (QD)-mediated immunolabeling of a protein followed by imaging them with transmission electron microscopy (TEM). QD is a semiconductor nanocrystal with a dual property of crystalline structure and high electron density, which makes them applicable to electron microscopy. This current method visualized the subcellular localization of Sigma 1 receptor (Sigmar1) protein using QD-TEM in the heart tissue at ultrastructural level. Small cubes of the heart tissue sections from a wild-type mouse were fixed in 3% glutaraldehyde, subsequently osmicated, stained with uranyl acetate, followed by sequential dehydration with ethanol and acetone. These dehydrated heart tissue sections were embedded in low-viscosity epoxy resins, cut into thin sections of 500 nm thickness, put on the grid, and subsequently subjected to antigen unmasking with 5% sodium metaperiodate, followed by quenching of the residual aldehydes with glycine. The tissues were blocked, followed by sequential incubation in primary antibody, biotinylated secondary antibody, and streptavidin-conjugated QD. These stained sections were blot dried and imaged at high magnification using TEM. The QD-TEM technique allowed the visualization of Sigmar1 protein&#39;s&#160;subcellular localization at the ultrastructural level in the heart. These techniques can be used to visualize the presence of any protein and subcellular localization in any organ system.

The present protocol describes a method of immuno-labeling a protein in the heart tissue sections using quantum dots. This technique provides a useful tool to visualize any protein&#39;s subcellular localization and expression at the ultrastructural level.