Name: Author Spotlight: Advancements in Correlative Light and Electron Microscopy with Fluorescent Protein Preservation
Uploaded: 2024-01-12T14:10:00
Description: Watch this Scientific Journal Video about Epon Post Embedding Correlative Light and Electron Microscopy at JoVE.com

This project was supported by the National Natural Science Foundation of China (32201235 to Zhifei Fu), the Natural Science Foundation of Fujian Province, China (2022J01287 to Zhifei Fu), the Research Foundation for Advanced Talents at Fujian Medical University, China (XRCZX2021013 to Zhifei Fu), the Finance Special Science Foundation of Fujian Province, China (22SCZZX002 to Zhifei Fu), Foundation of NHC Key Laboratory of Technical Evaluation of Fertility Regulation for Non-human Primate, and Fujian Maternity and Child Health Hospital (2022-NHP-04 to Zhifei Fu). We thank Linying Zhou, Minxia Wu, Xi Lin, and Yan Hu at the Public Technology Service Center, Fujian Medical University for support with EM sample preparation and EM imaging.

Animal husbandry and experiments were approved by the Institutional Animal Care and Use Committee of Fujian Medical University Medical Center. The step-by-step workflow of the current protocol is shown in Figure 1.
1. Sample preparation
<ol>
	<li>Mouse brain
	<ol>
		<li>Purchase transgenic mice (see Table of Materials) and oligonucleotide primers (see Table of Materials) to genotype these mice.</li>
		<li>Perfus...

Novel Protocol for Simultaneous Fluorescence and Ultrastructure Preservation

<ol>
	<li>de Boer, P., Hoogenboom, J. P., Giepmans, B. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Correlated+light+and+electron+microscopy:+ultrastructure+lights+up.">Correlated light and electron microscopy: ultrastructure lights up.</a> Nat Methods. 12 (6), 503-513 (2015).</li><li>Kopek, B. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diverse+protocols+for+correlative+super-resolution+fluorescence+imaging+and+electron+microscopy+of+chemically+fixed+samples.">Diverse protocols for correlative super-resolution fluorescence imaging and electron microscopy of chemically fixed samples.</a> Nat Protoc. 12 (5), 916-946 (2017).</li><li>Watanabe, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+localization+in+electron+micrographs+using+fluorescence+nanoscopy.">Protein localization in electron micrographs using fluorescence nanoscopy.</a> Nat Methods. 8 (1), 80-84 (2011).</li><li>Johnson, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Correlative+in-resin+super-resolution+and+electron+microscopy+using+standard+fluorescent+proteins.">Correlative in-resin super-resolution and electron microscopy using standard fluorescent proteins.</a> Sci Rep. 5 (1), 9583 (2015).</li><li>Paez-Segala, M. 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A., Uchiyama, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Visualization+of+cytoplasmic+organelles+via+in-resin+CLEM+using+an+osmium-resistant+far-red+protein.">Visualization of cytoplasmic organelles via in-resin CLEM using an osmium-resistant far-red protein.</a> Sci Rep. 10 (1), 11314 (2020).</li><li>MacManus, D. 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E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comprehensive+dual-+and+triple-feature+intersectional+single-vector+delivery+of+diverse+functional+payloads+to+cells+of+behaving+mammals.">Comprehensive dual- and triple-feature intersectional single-vector delivery of diverse functional payloads to cells of behaving mammals.</a> Neuron. 107 (5), 836-853 (2020).</li><li>Shcherbo, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Far-red+fluorescent+tags+for+protein+imaging+in+living+tissues.">Far-red fluorescent tags for protein imaging in living tissues.</a> Biochem J. 418 (3), 567-574 (2009).</li><li>Shen, Y., Chen, Y., Wu, J., Shaner, N. C., Campbell, R. 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S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=mScarlet:+a+bright+monomeric+red+fluorescent+protein+for+cellular+imaging.">mScarlet: a bright monomeric red fluorescent protein for cellular imaging.</a> Nat Methods. 14 (1), 53-56 (2017).</li><li>Xu, P., Xu, T., Zhang, M., Fu, Z., He, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+protocol+for+Epon-embedding-based+correlative+super-resolution+light+and+electron+microscopy.">A protocol for Epon-embedding-based correlative super-resolution light and electron microscopy.</a> Research Square. , (2019).</li><li>Johnson, E., Kaufmann, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preserving+the+photoswitching+ability+of+standard+fluorescent+proteins+for+correlative+in-resin+super-resolution+and+electron+microscopy.">Preserving the photoswitching ability of standard fluorescent proteins for correlative in-resin super-resolution and electron microscopy.</a> Methods Cell Biol. 140, 49-67 (2017).</li><li>Zhou, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Liquid-liquid+phase+separation+mediates+the+formation+of+herpesvirus+assembly+compartments.">Liquid-liquid phase separation mediates the formation of herpesvirus assembly compartments.</a> J Cell Biol. 222 (1), 17 (2023).</li><li>Tao, C. 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The protocol presented here is a versatile imaging method, which can combine the localization information of the target protein from light microscopy (LM) and the context surrounding the target protein from electron microscopy (EM)6. With the limitations of current fluorescent proteins, the widely used method is pre embedding correlative light and electron microscopy (CLEM), which means the LM imaging is done before the EM sample preparation. Almost all existing fluorescent proteins can be examine...

Fluorescence microscopy (FM) can be used to obtain the localization and distribution of the target protein. However, the context that surrounds the target protein is lost, which is crucial for investigating the target protein thoroughly. Electron microscopy (EM) has the highest imaging resolution, providing several subcellular details. Nevertheless, EM lacks target labeling. By accurately merging the fluorescence image taken by FM with the gray image acquired by EM, correlative light and electron microscopy (CLEM) can combine the information obtained by these two imaging modes1,2,3,4.
CLEM is a trade-off between fluorescence and the ultrastructure1. Because of the limitations of current fluorescent proteins and the traditional EM sample preparation procedures, especially the use of osmic acid (OsO4) and hydrophobic resins such as Epon, the ultrastructure always compromises fluorescence5. OsO4 is an indispensable reagent in EM sample preparation, which is used to improve the contrast of EM images. Compared with other embedding resins, Epon is superior in ultrastructure preservation and sectioning properties5. However, no fluorescent proteins can retain the fluorescence signal after the treatment of OsO4 and Epon embedding6. To overcome the limitations of fluorescent proteins, pre embedding CLEM was developed, in which FM imaging is done before EM sample preparation6. However, the drawback of pre embedding CLEM is the imprecise registration between the FM and the EM images5.
On the contrary, post embedding CLEM method performs the FM imaging after the EM sample preparation, the registration accuracy of which can reach 6-7 nm5,6. To retain the fluorescence of fluorescent proteins, very low concentrations of OsO4 (0.001%)3 or the high-pressure frozen (HPF) and freeze substitution (FS) EM preparation methods4,7 have been used at the expense of compromised ultrastructure or the contrast of the EM image. The development of mEos4b greatly promotes the progress of post embedding CLEM, although glycidyl methacrylate is used as the embedding resin5. With the development of mEosEM, which can survive the OsO4 staining and Epon embedding, Epon post embedding super-resolution CLEM was achieved for the first time, maintaining the fluorescence and ultrastructure simultaneously6. After mEosEM, several fluorescent proteins that can survive the OsO4 staining and the Epon embedding were developed8,9,10,11. This greatly promotes the development of CLEM.
There are three key aspects to Epon post-embedding CLEM. The first is the fluorescent protein, which should maintain the fluorescent signal after EM sample preparation. According to our experience, mScarlet is superior to other reported fluorescent proteins. The second is how to find the same cell imaged by FM imaging in EM imaging. To solve this problem, we improve the procedure for this step so that one can readily find the targeted cell. The last is the method to align the FM image with the EM image. Here, we detail the registration between the FM and the EM images. In this protocol, we express mScarlet in VGLUT2 neurons and demonstrate that mScarlet can target secondary lysosomes using Epon post-embedding CLEM. We provide step-by-step details for Epon post-embedding CLEM, without compromising the fluorescence and the ultrastructure.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>0.2 M Phosphate Buffer (PB)</td>
			<td></td>
			<td></td>
			<td>NaH2PO4 &#183; 2H2O+Na2HPO4 &#183; 12H2O</td>
		</tr>
		<tr>
			<td>0.2 M Tris-Cl (pH 8.5)</td>
			<td>Shanghai yuanye Bio-Technology</td>
			<td>R26284</td>
			<td></td>
		</tr>
		<tr>
			<td>25% Glutaraldehyde (GA)</td>
			<td>Alfa Aesar</td>
			<td>A17876</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Abbelight 3D</td>
			<td>Nanolnsights</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>SCR</td>
			<td>10000418</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium hydroxide</td>
			<td>J&#38;K Scientific</td>
			<td>335213</td>
			<td></td>
		</tr>
		<tr>
			<td>BioPhotometer D30</td>
			<td>eppendorf</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cleaning buffer of cover glasses</td>
			<td></td>
			<td></td>
			<td>50 mL Ammonium hydroxide, 50 mL Hydrogen peroxide, 250 mL H2O</td>
		</tr>
		<tr>
			<td>Coverglass</td>
			<td>Warner</td>
			<td>64-0715</td>
			<td></td>
		</tr>
		<tr>
			<td>DABCO&#160;</td>
			<td>Sigma</td>
			<td>290734</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>DDSA</td>
			<td>SPI company</td>
			<td>GS02827</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Desktop centrifuge</td>
			<td>WIGGENS</td>
			<td>MINICEN 10E</td>
			<td></td>
		</tr>
		<tr>
			<td>Diamond knife</td>
			<td>DiATOME</td>
			<td>MX6353</td>
			<td></td>
		</tr>
		<tr>
			<td>DMP-30</td>
			<td>SPI company</td>
			<td>GS02823</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>DNA transfection reagent</td>
			<td>Thermo Fisher&#160;</td>
			<td>2696953</td>
			<td>Lipofectamine 3000 Transfection Kit</td>
		</tr>
		<tr>
			<td>Epon 812&#160;</td>
			<td>SPI company</td>
			<td>GS02659</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>SCR</td>
			<td>10009218</td>
			<td></td>
		</tr>
		<tr>
			<td>Fiji image J</td>
			<td>National Institutes of Health</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fixative solution&#160;</td>
			<td></td>
			<td></td>
			<td>4% PFA+0.25% GA+0.02 M PB</td>
		</tr>
		<tr>
			<td>Formvar</td>
			<td>Sigma</td>
			<td>9823</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>SCR</td>
			<td>10010618</td>
			<td></td>
		</tr>
		<tr>
			<td>Gold nanoparticles</td>
			<td>Corpuscular</td>
			<td>790120-010</td>
			<td></td>
		</tr>
		<tr>
			<td>Gradient resin</td>
			<td></td>
			<td></td>
			<td>Acetone to resin 3:1, 1:1, 1:3</td>
		</tr>
		<tr>
			<td>Hydrofluoric acid</td>
			<td>SCR</td>
			<td>10011118</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide</td>
			<td>SCR</td>
			<td>10011218</td>
			<td></td>
		</tr>
		<tr>
			<td>ICY (https://icy.bioimageanalysis.org/about/)</td>
			<td></td>
			<td></td>
			<td>Easy CLEMv0 Plugin</td>
		</tr>
		<tr>
			<td>Imaging chamber</td>
			<td>Thermo Fisher&#160;</td>
			<td>A7816</td>
			<td></td>
		</tr>
		<tr>
			<td>Large gelatin capsules</td>
			<td>Electron Microscopy Sciences</td>
			<td>70117</td>
			<td></td>
		</tr>
		<tr>
			<td>Mounting buffer</td>
			<td></td>
			<td></td>
			<td>Mowiol 4-88, Glycerol, 0.2 M Tris-Cl (pH 8.5), DABCO</td>
		</tr>
		<tr>
			<td>Mowiol 4-88</td>
			<td>Sigma</td>
			<td>9002-89-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Na2HPO4&#160;&#382;12H2O</td>
			<td>SCR</td>
			<td>10020318</td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4&#160;&#382;2H2O</td>
			<td>SCR</td>
			<td>20040718</td>
			<td></td>
		</tr>
		<tr>
			<td>NMA</td>
			<td>SPI company</td>
			<td>GS02828</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Oligonucleotide primers</td>
			<td>Takara Biomedical Technology (Beijing)</td>
			<td></td>
			<td>Three oligonucleotides primers were used to detect Vglut2-ires-Cre and wild-type simultaneously. The primers 5,-ATCGACCGGTAATGCAGGCAA-3, and 5,-CGGTACCACCAAATCTTACGG-3, aimed to detect Vglut2-ires-Cre. The primers&#160; 5,-CGGTACCACCAAATCTTACGG-3, and 5,-CATGGTCTGTTTTGAATTCAG-3, aimed to detect wild-type.</td>
		</tr>
		<tr>
			<td>Oscillating microtome</td>
			<td>Leica</td>
			<td>VT1000S</td>
			<td></td>
		</tr>
		<tr>
			<td>Osmium tetroxide</td>
			<td>SCR</td>
			<td>L01210302</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>OsO4 solution</td>
			<td></td>
			<td></td>
			<td>1% Osmium tetroxide+1.5% K4Fe (CN)6&#183;3H2O</td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>Amcor</td>
			<td>PM-996</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraformaldehyde (PFA)</td>
			<td>SCR</td>
			<td>80096618</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Perfusion buffer</td>
			<td></td>
			<td></td>
			<td>4% PFA+0.1 M PB</td>
		</tr>
		<tr>
			<td>Pioloform</td>
			<td>Sigma</td>
			<td>63148-65-2</td>
			<td>Hazardous chemical</td>
		</tr>
		<tr>
			<td>Poly-L-lysine&#160;</td>
			<td>Sigma</td>
			<td>25986-63-0</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium ferrocyanide (K4Fe (CN)6&#183;3H2O)&#160;</td>
			<td>SCR</td>
			<td>10016818</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel blades</td>
			<td>Merck</td>
			<td>S2771</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel handles</td>
			<td>Merck</td>
			<td>S2896-1EA</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>OLYMPUS</td>
			<td>MVX10</td>
			<td></td>
		</tr>
		<tr>
			<td>Transgenic mice</td>
			<td>The Jackson Laboratory</td>
			<td></td>
			<td>Vglut2-ires-Cre mice (strain: 129S6/SvEvTac) were housed in standard conditions (25 &#176;C, a 12 h light/dark cycle, with water and food given ad libitum. Male and Female mice were used at 2&#8211;3 months old, weight range 20-30 g.&#160;&#160;</td>
		</tr>
		<tr>
			<td>Transmission electron microscope (TEM)</td>
			<td>FEI</td>
			<td></td>
			<td>TECNAL G2</td>
		</tr>
		<tr>
			<td>UA solution (2% UA)</td>
			<td></td>
			<td></td>
			<td>Aqueous solution</td>
		</tr>
		<tr>
			<td>Ultramicrotome</td>
			<td>Leica</td>
			<td>LEICA EM UC6</td>
			<td></td>
		</tr>
		<tr>
			<td>Uranyl acetate (UA)</td>
			<td>TED PELLA</td>
			<td>19481</td>
			<td>Hazardous chemical</td>
		</tr>
	</tbody>
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epon post embedding correlative light and electron microscopy

Correlative light and electron microscopy (CLEM) is a comprehensive microscopy that combines the localization information provided by fluorescence microscopy (FM) and the context of cellular ultrastructure acquired by electron microscopy (EM). CLEM is a trade-off between fluorescence and ultrastructure, and usually, ultrastructure compromises fluorescence. Compared with other hydrophilic embedding resins, such as glycidyl methacrylate, HM20, or K4M, Epon is superior in ultrastructure preservation and sectioning properties. Previously, we had demonstrated that mEosEM can survive osmium tetroxide fixation and Epon embedding. Using mEosEM, we achieved, for the first time, Epon post embedding CLEM, which maintains the fluorescence and the ultrastructure simultaneously. Here, we provide step-by-step details about the EM sample preparation, the FM imaging, the EM imaging, and the image alignment. We also improve the procedures for identifying the same cell imaged by FM imaging during the EM imaging and detail the registration between the FM and EM images. We believe one can easily achieve Epon post embedding correlative light and electron microscopy following this new protocol in traditional EM facilities.

Author Spotlight: Advancements in Correlative Light and Electron Microscopy with Fluorescent Protein Preservation

Epon Post Embedding Correlative Light and Electron Microscopy

Correlative light and electron microscopy is a completely stable microscopy technique, which can combine the localization information provided by fluorescent microscopy and is the ultra structure by electron microscopy. However, the mentality of this technique is to preserve the fluorescence of after you have a simple preparation. Using this fluorescent protein, we can achieve correlative light and the electron microscope which can maintain fluorescence and ultra structure simultaneously.

Previous reports demonstrated that mScarlet can target the lysosome15. In this protocol, AAV expressing mScarlet (rAAV-hSyn-DIO-mScarlet-WPRE-pA) was injected into the M1 (ML: &#177;1.2 AP: +1.3 DV: -1.5) of Vglut2-ires-cre mouse brain using stereotaxic instruments. Following the protocol described above, the final correlated image is shown in Figure 4A. The FM image can be accurately aligned with the EM image using gold nanoparticles (the green dots)...

Watch this Scientific Journal Video about Epon Post Embedding Correlative Light and Electron Microscopy at JoVE.com

We present a detailed protocol for Epon post-embedding correlative light and electron microscopy using a fluorescent protein called mScarlet. This method can maintain the fluorescence and the ultrastructure simultaneously. This technique is amenable to a wide variety of biological applications.