This work was supported by the China Scholarship Council (CSC, to R. Xia), the German Centre for Cardiovascular Research (DZHK; 81X2600255 to S. Clauss, 81Z0600206 to S. K&#228;&#228;b), the Corona Foundation (S199/10079/2019 to S. Clauss), the SFB 914 (project Z01 to H. Ishikawa-Ankerhold and S. Massberg and project A10 to C. Schulz), the ERA-NET on Cardiovascular Diseases (ERA-CVD; 01KL1910 to S. Clauss) and the Heinrich-and-Lotte-M&#252;hlfenzl Stiftung (to S. Clauss). The funders had no role in manuscript preparation.

Animal care and all experimental procedures were conducted in accordance with the guidelines of the Animal Care and Ethics committee of the University of Munich, and all the procedures undertaken on mice were approved by the Government of Bavaria, Munich, Germany (ROB-55.2-2532.Vet_02-16-106, ROB-55.2-2532.Vet_02-19-86). C57BL6/J mice were purchased from Jackson Laboratory.
NOTE: Figure 1 shows the instruments needed for the experiment. <s...

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Cardiac anatomy has traditionally been studied using thin histological sections11. However, these methods do not preserve the three-dimensional structure of the conduction system and thus, only provides 2D information. The whole-mount immunofluorescence staining protocol described here allows to overcome these limitations and can be routinely used for SAN and AVN imaging.
In comparison to standard methods such as conventional immunohistochemistry that require paraffin-e...

An erratum was issued for:&#160;<a href="https://www.jove.com/t/62058">Whole-Mount Immunofluorescence Staining, Confocal Imaging and 3D Reconstruction of the Sinoatrial and Atrioventricular Node in the Mouse</a>. The Authors section was updated from:
Ruibing Xia1,2,3 
Julia Vlcek1,2 
Julia Bauer1,2,3 
Stefan K&#228;&#228;b1,3 
Hellen Ishikawa-Ankerhold1,2 
Dominic Adam van den Heuvel1,2 
Christian Schulz1,2,3 
Steffen Massberg1,2,3 
Sebastian Clauss1,2,3 
1University Hospital Munich, Department of Medicine I, Ludwig Maximilian University Munich 
2Walter Brendel Center of Experimental Medicine, Ludwig Maximilian University Munich 
3German Center for Cardiovascular Research (DZHK), Partner Site Munich, Munich Heart Alliance
to:
Ruibing Xia1,2,3 
Julia Vlcek1,2 
Julia Bauer1,2,3 
Stefan K&#228;&#228;b1,3 
Hellen Ishikawa-Ankerhold1,2 
Dominic Adam van den Heuvel1,2 
Christian Schulz1,2,3 
Steffen Massberg1,2,3 
Sebastian Clauss1,2,3 
1University Hospital Munich, Department of Medicine I, Ludwig Maximilians University (LMU) Munich 
2Institute of Surgical Research at the Walter Brendel Center of Experimental Medicine, University Hospital Munich, Ludwig Maximilians University (LMU) Munich 
3German Center for Cardiovascular Research (DZHK), Partner Site Munich, Munich Heart Alliance

An erratum was issued for:&#160;<a href="https://www.jove.com/t/62058">Whole-Mount Immunofluorescence Staining, Confocal Imaging and 3D Reconstruction of the Sinoatrial and Atrioventricular Node in the Mouse</a>. The Authors section was updated.

Erratum: Whole-Mount Immunofluorescence Staining, Confocal Imaging and 3D Reconstruction of the Sinoatrial and Atrioventricular Node in the Mouse

Arrhythmias are common diseases affecting millions of people, and are the cause of significant morbidity and mortality worldwide. Despite enormous advances in treatment and prevention, such as the development of cardiac pacemakers, treatment of arrhythmias remains challenging, primarily due to the very limited knowledge regarding underlying disease mechanisms1,2,3. A better understanding of both the normal electrophysiology and the pathophysiology of arrhythmias may help to develop novel, innovative and causal treatment strategies in the future. Additionally, to comprehensively study arrhythmogenesis, it is important to localize and visualize the specific cardiac conduction system in animal models such as the mouse, as mice are widely used in electrophysiology research.
The major parts of the cardiac conduction system are the sinoatrial node (SAN), where the electrical impulse is generated in specialized pacemaker cells, and the atrioventricular node (AVN), which is the only electrical connection between the atria and the ventricles4. Whenever the electrophysiological properties of SAN and AVN are altered, arrhythmias such as sick sinus syndrome or atrioventricular block can occur which may lead to hemodynamic deterioration, syncope and even death, and thus underline the essential role of both SAN and AVN in electrophysiology and arrhythmogenesis5.
Comprehensive studies on SAN or AVN require a precise localization and visualization of both structures, ideally within their physiologic environment. However, due to their small size and location within the working myocardium, without establishing a clear macroscopically visible structure, studying the anatomy and electrophysiology of SAN and AVN is challenging. Anatomical landmarks can be used to roughly identify the region that contains SAN and AVN6,7,8. In brief, SAN is located in the inter-caval region of the right atrium adjacent to the muscular crista terminalis (CT), AVN is located within the triangle of Koch established by the tricuspid valve, the ostium of the coronary sinus and the tendon of Todaro. Thus far, these anatomical landmarks were mainly used to localize, remove and then study SAN and AVN as individual structures (e.g., by conventional histology). To better understand the complex electrophysiology of SAN and AVN (e.g., regulatory effects of adjacent cells of the working myocardium), however, studying the conduction systems within the physiologic 3D environment is necessary.
Whole-mount immunofluorescence staining is a method that is used to study anatomical structures in situ while preserving the integrity of the surrounding tissue9. Taking advantage of confocal microscopy and image analysis software, SAN and AVN can be visualized with fluorescently labeled antibodies targeting ion channels specifically expressed in these regions.
This following protocol explains the necessary steps to perform a well-established whole-mount staining method for SAN and AVN microscope localization and visualization. Specifically, this protocol describes how (1) to localize SAN and AVN by anatomical landmarks to prepare these samples for staining and microscopy analysis (2) to perform whole-mount immunofluorescence staining of the reference markers HCN4 and Cx43 (3) to prepare SAN and AVN samples for confocal microscopy (4) to perform confocal imaging of SAN and AVN. We also describe how this protocol can be modified to include additional staining of surrounding working myocardium or non-myocyte cells such as autonomous nerve fibers which allows a thorough investigation of the cardiac conduction system within the heart.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td colspan="4">Anesthesia</td>
		</tr>
		<tr>
			<td>Isoflurane vaporizer system&#160;</td>
			<td>Hugo Sachs Elektronik</td>
			<td>34-0458, 34-1030, 73-4911, 34-0415, 73-4910</td>
			<td>Includes an induction chamber, a gas evacuation unit and charcoal filters</td>
		</tr>
		<tr>
			<td>Modified Bain circuit</td>
			<td>Hugo Sachs Elektronik</td>
			<td>73-4860</td>
			<td>Includes an anesthesia mask for mice</td>
		</tr>
		<tr>
			<td>Surgical Platform</td>
			<td>Kent Scientific</td>
			<td>SURGI-M</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">In vivo instrumentation</td>
		</tr>
		<tr>
			<td>Fine forceps</td>
			<td>Fine Science Tools</td>
			<td>11295-51</td>
			<td></td>
		</tr>
		<tr>
			<td>Iris scissors</td>
			<td>Fine Science Tools</td>
			<td>14084-08</td>
			<td></td>
		</tr>
		<tr>
			<td>Spring scissors</td>
			<td>Fine Science Tools</td>
			<td>91500-09</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue forceps</td>
			<td>Fine Science Tools</td>
			<td>11051-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue pins</td>
			<td>Fine Science Tools</td>
			<td>26007-01</td>
			<td>Could use 27G needles as a substitute</td>
		</tr>
		<tr>
			<td colspan="4">General lab instruments</td>
		</tr>
		<tr>
			<td>Orbital shaker</td>
			<td>Sunlab</td>
			<td>D-8040</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stirrer</td>
			<td>IKA&#160;</td>
			<td>RH basic</td>
			<td></td>
		</tr>
		<tr>
			<td>Pipette,volume 10 &#181;L, 100 &#181;L, 1000 &#181;L</td>
			<td>Eppendorf</td>
			<td>Z683884-1EA</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">Microscopes</td>
		</tr>
		<tr>
			<td>Dissection stereo- zoom microscope&#160;</td>
			<td>VWR</td>
			<td>10836-004</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser Scanning Confocal microscope</td>
			<td>Zeiss</td>
			<td>LSM 800</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">Software</td>
		</tr>
		<tr>
			<td>Imaris 8.4.2</td>
			<td>Oxford instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>ZEN 2.3 SP1 black</td>
			<td>Zeiss</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">General Lab Material</td>
		</tr>
		<tr>
			<td>0.2 &#181;m syringe filter</td>
			<td>Sartorius</td>
			<td>17597</td>
			<td></td>
		</tr>
		<tr>
			<td>100 mm petri dish</td>
			<td>Falcon</td>
			<td>351029</td>
			<td></td>
		</tr>
		<tr>
			<td>27G needle</td>
			<td>BD Microlance 3</td>
			<td>300635</td>
			<td></td>
		</tr>
		<tr>
			<td>50 ml Polypropylene conical Tube</td>
			<td>Falcon</td>
			<td>352070</td>
			<td></td>
		</tr>
		<tr>
			<td>5ml Syringe</td>
			<td>Braun</td>
			<td>4606108V</td>
			<td></td>
		</tr>
		<tr>
			<td>Cover slips</td>
			<td>Thermo Scientific</td>
			<td>7632160</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Tubes</td>
			<td>Eppendorf</td>
			<td>30121872</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">Chemicals</td>
		</tr>
		<tr>
			<td>0.5 M EDTA</td>
			<td>Sigma</td>
			<td>20-158</td>
			<td>Components of TEA</td>
		</tr>
		<tr>
			<td>16% Formaldehyde Solution</td>
			<td>Thermo Scientific&#160;</td>
			<td>28908</td>
			<td>use as a 4% solution&#160;</td>
		</tr>
		<tr>
			<td>Acetic acid</td>
			<td>Merck</td>
			<td>100063</td>
			<td>Components of TEA</td>
		</tr>
		<tr>
			<td>Agarose</td>
			<td>Biozym</td>
			<td>850070</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine Serum Albumin</td>
			<td>Sigma</td>
			<td>A2153-100G</td>
			<td></td>
		</tr>
		<tr>
			<td>DPBS (1X) Dulbecco&#39;s Phosphate Buffered Saline</td>
			<td>Gibco</td>
			<td>14190-094</td>
			<td></td>
		</tr>
		<tr>
			<td>Normal goat serum</td>
			<td>Sigma</td>
			<td>NS02L</td>
			<td></td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma</td>
			<td>S1888-1kg</td>
			<td></td>
		</tr>
		<tr>
			<td>Tris-base</td>
			<td>Roche</td>
			<td>TRIS-RO</td>
			<td>Components of TEA</td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Sigma</td>
			<td>T8787-250ml</td>
			<td>Diluted to 1% in PBS</td>
		</tr>
		<tr>
			<td>Tween 20</td>
			<td>Sigma</td>
			<td>P2287-500ml</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">Drugs</td>
		</tr>
		<tr>
			<td>Fentanyl 0.5&#160;mg/10&#160;mL</td>
			<td>Braun Melsungen</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Isoflurane 1 mL/mL</td>
			<td>Cp-pharma</td>
			<td>31303</td>
			<td></td>
		</tr>
		<tr>
			<td>Oxygen 5 L</td>
			<td>Linde</td>
			<td>2020175</td>
			<td>Includes a pressure regulator</td>
		</tr>
		<tr>
			<td colspan="4">Antibodies</td>
		</tr>
		<tr>
			<td>Goat anti-Rabbit IgG Alexa Fluor&#160;488&#160;</td>
			<td>Cell Signaling Technology</td>
			<td>#4412</td>
			<td>diluted to 1:200</td>
		</tr>
		<tr>
			<td>Goat anti-Rat IgG Alexa Fluor 647</td>
			<td>Invitrogen</td>
			<td>#A-21247</td>
			<td>diluted to 1:200</td>
		</tr>
		<tr>
			<td>Hoechst 33342, Trihydrochloride, Trihydrate (DAPI)</td>
			<td>Invitrogen</td>
			<td>H3570</td>
			<td>diluted to 1:1000</td>
		</tr>
		<tr>
			<td>Rabbit Anti-Connexin-43</td>
			<td>Sigma</td>
			<td>C6219</td>
			<td>diluted to 1:200</td>
		</tr>
		<tr>
			<td>Rat anti-HCN4 (SHG 1E5)</td>
			<td>Invitrogen</td>
			<td>MA3-903</td>
			<td>diluted to 1:200</td>
		</tr>
		<tr>
			<td colspan="4">Other</td>
		</tr>
		<tr>
			<td>Plastic ring</td>
			<td></td>
			<td></td>
			<td>Self-designed and 3D printed</td>
		</tr>
		<tr>
			<td>Plasticine</td>
			<td>Cernit</td>
			<td>49655005</td>
			<td></td>
		</tr>
		<tr>
			<td>Silikonpasten, Baysilone</td>
			<td>VWR</td>
			<td>291-1220</td>
			<td></td>
		</tr>
		<tr>
			<td colspan="4">Animals</td>
		</tr>
		<tr>
			<td>Mouse, C57BL/6</td>
			<td>The Jackson Laboratory</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

whole-mount immunofluorescence staining, confocal imaging and 3d reconstruction of the sinoatrial and atrioventricular node in the mouse

The electrical signal physiologically generated by pacemaker cells in the sinoatrial node (SAN) is conducted through the conduction system, which includes the atrioventricular node (AVN), to allow excitation and contraction of the whole heart. Any dysfunction of either SAN or AVN results in arrhythmias, indicating their fundamental role in electrophysiology and arrhythmogenesis. Mouse models are widely used in arrhythmia research, but the specific investigation of SAN and AVN remains challenging.
The SAN is located at the junction of the crista terminalis with the superior vena cava and AVN is located at the apex of the triangle of Koch, formed by the orifice of the coronary sinus, the tricuspid annulus, and the tendon of Todaro. However, due to the small size, visualization by conventional histology remains challenging and it does not allow the study of SAN and AVN within their 3D environment.
Here we describe a whole-mount immunofluorescence approach that allows the local visualization of labelled mouse SAN and AVN. Whole-mount immunofluorescence staining is intended for smaller sections of tissue without the need for manual sectioning. To this purpose, the mouse heart is dissected, with unwanted tissue removed, followed by fixation, permeabilization and blocking. Cells of the conduction system within SAN and AVN are then stained with an anti-HCN4 antibody. Confocal laser scanning microscopy and image processing allow differentiation between nodal cells and working cardiomyocytes, and to clearly localize SAN and AVN. Furthermore, additional antibodies can be combined to label other cell types as well, such as nerve fibers.
Compared to conventional immunohistology, whole-mount immunofluorescence staining preserves the anatomical integrity of the cardiac conduction system, thus allowing the investigation of AVN; especially so into their anatomy and interactions with the surrounding working myocardium and non-myocyte cells.

By using the protocol outlined above, confocal microscopy imaging of both SAN and AVN can be reliably performed. Specific staining of the conduction system using fluorescent antibodies targeting HCN4 and staining of the working myocardium using fluorescent antibodies targeting Cx43 allows the clear identification of SAN (Figure 5, Video 1) and AVN (Figure 6, Video 2)&#160;within the intact anatom...

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Whole-Mount Immunofluorescence Staining, Confocal Imaging and 3D Reconstruction of the Sinoatrial and Atrioventricular Node in the Mouse

We provide a step-by-step protocol for whole-mount immunofluorescence staining of the sinoatrial node (SAN) and atrioventricular node (AVN) in murine hearts.

The electrical signal physiologically generated by pacemaker cells in the sinoatrial node (SAN) is conducted through the conduction system, which includes ...

This protocol explains the necessary steps to perform micro dissection whole-mount immunofluorescence staining and microscopy specifically for the sinus node and AV node in the mouse. The whole-mount methodology is advantageous for addressing the exact 3D localization and morphology of the sinus and AV node and to examine the relationship with the surrounding tissue. The tissue morphology is considerably preserved with only minimum tissue dehydration and physical rupture.To begin, puncture the heart with a 27 gauge needle in the area of the apex by carefully pushing the needle into the left ventricle and gently injecting five to 10 milliliters of ice cold PBS to profuse the heart. Carefully lift the heart using a tweezer and cut the large vessels to remove the heart. Put the heart into a dissection dish filled with ice cold PBS under the dissecting microscope.After determination of the orientation of the heart turn the heart around so that the front of the heart is at the bottom of the dish. Immobilize the heart by putting little pins through the apex and the atrial appendage into the agarose at the bottom of the dissection dish. Using fine tweezers and scissors carefully remove non-cardiac tissue around the superior and inferior vena cava.To expose the intercaval region, remove the majority of the ventricles by cutting parallel to the groove between the ventricles and the atria with a micro scissors. For fixation and dehydration put the sample in 4%paraformaldehyde overnight at four degrees Celsius. On the next day, transfer the heart to 15%sucrose solution and incubate it for 24 hours at four degrees Celsius.After the incubation transfer the heart to 30%sucrose solution for another 24 hours at four degrees Celsius. Wash the heart in 1%Triton X-100 diluted in PBS. Then block and permeabilize it in blocking solution overnight at four degrees Celsius.Place the heart in a 1.5 milliliter tube and incubate it with rabbit anti-mouse Connexin 43 and rat anti-mouse HTN4 antibodies diluted with blocking solution for seven days at four degrees Celsius. After seven days, remove the solution containing primary antibodies using a pipette. Wash the heart with 1%Triton X-100 solution three times for one hour per wash on the orbital shaker.Repeat the washing steps after the incubation with secondary antibodies and DAPI. After preparing plastic rings and filling them with plasticine, place the heart into the plasticine groove with the back of the heart facing up. Identify the SAN, which is located on the dorsal side of the heart with the intercaval region.Add PBS to the heart to displace all air within the cavity until the heart is fully covered with PBS. Apply silicone to the edges of the plastic ring. Gently press the surrounding part of the sample into the plasticine to fix the heart and cover the heart with a cover slip.Then gently press the backside of the plasticine to squeeze out some of the PBS and attach the heart to the cover slip taking care to avoid any air bubbles within the imaging area. Position the whole-mount staining samples upside down on the platform of the confocal microscope and proceed with imaging as described in the text manuscript. The parameters for each channel of the confocal microscope are set using the respective software.This protocol was used to perform confocal microscopy imaging of both the sinoatrial node and atrioventricular node in murine hearts. Specific staining of the conduction system and working myocardium with fluorescent antibodies, targeting HCN4 and Connexin 43 respectively makes it possible to identify the sinoatrial node and the atrioventricular node within the intact anatomy. This whole-mount staining protocol makes it possible to study the interaction of different cell types by using various antibodies.

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4.8K vistas.  Ludwig Maximilians University (LMU) Munich. This protocol explains the necessary steps to perform micro dissection whole-mount immunofluorescence staining and microscopy specifically for the sinus node and AV node in the mouse. The whole-mount methodology is advantageous for addressing the exact 3D localization and morphology of the sinus and AV node and to examine the relationship with the surrounding tissue. The tissue morphology is considerably preserved with only minimum tissue dehydration and physical rupture.To begin, punc...