Name: refined clarity-based tissue clearing for three-dimensional fibroblast organization in healthy and injured mouse hearts
Uploaded: 2021-05-16T14:00:00
Description: Cardiovascular disease is the most prevalent cause of mortality worldwide and is often marked by heightened cardiac fibrosis that can lead to increased ...

The authors would like to acknowledge the CCHMC Confocal Imaging Core for their assistance and guidance in development of this model, as well as Matt Batie from Clinical Engineering for the design of all 3D printed parts. Demetria Fischesser was supported by a training grant from the National Institutes of Health, (NHLBI, T32 HL125204) and Jeffery D. Molkentin was supported by the Howard Hughes Medical Institute.

All experiments involving mice were approved by the Institutional Animal Care and Use Committee (IACUC) at Cincinnati Children&#8217;s Hospital Medical Center. The institution is also AAALAC (American Association for Accreditation of Laboratory Animal Care) certified. Mice were euthanized via cervical dislocation, and mice undergoing survival surgical procedures were given pain relief (see below). All methods used for pain management and euthanasia are based on recommendations of the Panel on Euthanasia of the American V...

<ol>
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This article presents a refined method for tissue clearing that allows for visualization of cardiac fibroblasts in vivo, both at baseline and following injury, to characterize and better understand fibroblasts in the mouse heart. This enhanced protocol addresses limitations in existing tissue clearing protocols that have attempted to identify specific cell types in the adult or neonatal heart12,13,14,<sup class="xref"...

Although cardiomyocytes comprise the greatest volume fraction in the heart, cardiac fibroblasts are more plentiful and are critically involved in regulating the baseline structural and reparative features of this organ. Cardiac fibroblasts are highly mobile, mechanically responsive, and phenotypically ranging depending on the extent of their activation. Cardiac fibroblasts are necessary to maintain normal levels of extracellular matrix (ECM), and too little or too much ECM production by these cells can lead to disease1,2,3. Given their importance in disease, cardiac fibroblasts have become an increasingly important topic of investigation towards identifying novel treatment strategies, especially in attempting to limit excessive fibrosis4,5,6,7. Upon injury, fibroblasts activate and differentiate into a more synthetic cell type known as a myofibroblast, which can be proliferative and secrete abundant ECM, as well as have contractile activity that helps remodel the ventricles.
While cardiac fibroblasts have been extensively evaluated for their properties in 2-D cultures6,8,9,10, much less is understood of their properties and dynamics in the 3-D living heart, either at baseline or with disease stimulation. Here, a refined method has been described to tissue clear the adult mouse heart while maintaining the fluorescence of fibroblasts labeled with a Rosa26-loxP-eGFP x Tcf21-MerCreMer genetic reporter system. Within the heart, Tcf21 is a relatively specific marker of quiescent fibroblasts4. After tamoxifen is given to activate the inducible MerCreMer protein, essentially all quiescent fibroblasts will permanently express enhanced green fluorescent protein (eGFP) from the Rosa26 locus, which allows for their tracking in vivo.
Numerous well-established tissue clearing protocols exist, some of which have been applied to the heart11,12,13,14,15,16,17. However, many of the reagents used in different tissue clearing protocols have been found to quench endogenous fluorescence signals18. Additionally, the adult heart is difficult to clear due to abundant heme group-containing proteins that generate autofluorescence19. Therefore, the goal of this protocol was to preserve fibroblast marker fluorescence with the simultaneous inhibition of heme autofluorescence in the injured adult heart for optimal 3-D visualization in vivo12,13,14,16,17,20.
Previous studies attempting to examine the cardiac fibroblast in vivo employed perfused antibodies to label these cells, although such studies were limited by antibody penetration and cardiac vascular structure14,16,17,20. Although Salamon et al. have shown tissue clearing with maintenance of topical neuronal fluorescence in the neonatal heart, and Nehrhoff et al. have shown maintenance of fluorescence marking myeloid cells, maintenance of endogenous fluorescence through the entire ventricular wall has not yet been demonstrated, including the visualization of adult cardiac fibroblasts at baseline or following injury13,20. This tissue clearing protocol refines a mixture of previous protocols based on the CLARITY method (clear lipid-exchanged acrylamide-hybridized rigid Imaging/immunostaining/in situ-hybridization-compatible tissue hydrogel) and PEGASOS (polyethylene glycol (PEG)-associated solvent system). This refined protocol permitted a more robust examination of cardiac fibroblasts in the mouse heart at baseline and of how they respond to different types of injury. The protocol is straightforward and reproducible and will help characterize the behavior of cardiac fibroblasts in vivo.

<table>
	<tbody>
		<tr>
			<td>4-0 braided silk</td>
			<td>Ethicon</td>
			<td>K871H</td>
			<td></td>
		</tr>
		<tr>
			<td>8-0 prolene</td>
			<td>Ethicon</td>
			<td>8730H</td>
			<td></td>
		</tr>
		<tr>
			<td>40% Acrylamide Solution</td>
			<td>Bio-Rad</td>
			<td>1610140</td>
			<td></td>
		</tr>
		<tr>
			<td>Angiotensin II</td>
			<td>Sigma</td>
			<td>A9525-50G</td>
			<td></td>
		</tr>
		<tr>
			<td>Artificial Tear Ointment</td>
			<td>Covetrus</td>
			<td>048272</td>
			<td></td>
		</tr>
		<tr>
			<td>DABCO (1,4-diazabicyclo[2.2. 2]octane)</td>
			<td>Millipore Sigma</td>
			<td>D27802-25G</td>
			<td></td>
		</tr>
		<tr>
			<td>GLUture topical tissue adhesive</td>
			<td>World Precision Instruments</td>
			<td>503763</td>
			<td></td>
		</tr>
		<tr>
			<td>Heparin</td>
			<td>Sigma</td>
			<td>H0777</td>
			<td></td>
		</tr>
		<tr>
			<td>Imaris Start Analysis Software</td>
			<td>Oxford Instruments</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro-osmotic pumps</td>
			<td>Alzet</td>
			<td>Model 1002</td>
			<td></td>
		</tr>
		<tr>
			<td>Nikon Elements Analysis Software</td>
			<td>Nikon</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Nikon A1R HD upright microscope</td>
			<td>Nikon</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Normal autoclaved chow</td>
			<td>Labdiet</td>
			<td>5010</td>
			<td></td>
		</tr>
		<tr>
			<td>Nycodenz, 5- (N-2, 3-dihydroxypropylacetamido)-2, 4, 6-tri-iodo-N, 
			N&#39;-bis (2, 3 dihydroxypropyl) isophthalamide</td>
			<td>CosmoBio</td>
			<td>AXS-1002424</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraformaldehyde</td>
			<td>Electron Microscopy Sciences</td>
			<td>15710</td>
			<td></td>
		</tr>
		<tr>
			<td>Phenylephrine Hydrochloride</td>
			<td>Sigma</td>
			<td>P6126-10G</td>
			<td></td>
		</tr>
		<tr>
			<td>Photoinitiator</td>
			<td>Wako Chemicals</td>
			<td>VA-044</td>
			<td></td>
		</tr>
		<tr>
			<td>Rosa26-nLacZ [FVB.Cg-Gt(ROSA)26Sortm1 (CAG-lacZ,-EGFP)Glh/J]</td>
			<td>Jackson Laboratories</td>
			<td>Jax Stock No:012429</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Azide</td>
			<td>Sigma Aldrich</td>
			<td>S2002-5G</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Chloride solution</td>
			<td>Hospira, Inc.</td>
			<td>NDC 0409-4888-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Tamoxifen</td>
			<td>Sigma Aldrich</td>
			<td>T5648</td>
			<td></td>
		</tr>
		<tr>
			<td>Tamoxifen food</td>
			<td>Envigo</td>
			<td>TD.130860</td>
			<td></td>
		</tr>
		<tr>
			<td>Tween-20</td>
			<td>Thermo Fisher Scientific</td>
			<td>BP337-500</td>
			<td></td>
		</tr>
		<tr>
			<td>Quadrol, N,N,N&#8242;,N&#8242;-Tetrakis(2-Hydroxypropyl)ethylenediamine, decolorizing agent</td>
			<td>Millipore Sigma</td>
			<td>122262-1L</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Clarity electrophoretic clearing chamber</td>
			<td>Logos Biosystems</td>
			<td>C30001</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Clarity electrophoretic clearing solution</td>
			<td>Logos Biosystems</td>
			<td>C13001</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Clarity electrophoresis tissue basket</td>
			<td>Logos Biosystems</td>
			<td>C12001</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Clarity electrophoresis tissue basket holder</td>
			<td>Logos Biosystems</td>
			<td>C12002</td>
			<td></td>
		</tr>
	</tbody>
</table>

refined clarity-based tissue clearing for three-dimensional fibroblast organization in healthy and injured mouse hearts

Cardiovascular disease is the most prevalent cause of mortality worldwide and is often marked by heightened cardiac fibrosis that can lead to increased ventricular stiffness with altered cardiac function. This increase in cardiac ventricular fibrosis is due to activation of resident fibroblasts, although how these cells operate within the 3-dimensional (3-D) heart, at baseline or after activation, is not well understood. To examine how fibroblasts contribute to heart disease and their dynamics in the 3-D heart, a refined CLARITY-based tissue clearing and imaging method was developed that shows fluorescently labeled cardiac fibroblasts within the entire mouse heart. Tissue resident fibroblasts were genetically labeled using Rosa26-loxP-eGFP florescent reporter mice crossed with the cardiac fibroblast expressing Tcf21-MerCreMer knock-in line. This technique was used to observe fibroblast localization dynamics throughout the entire adult left ventricle in healthy mice and in fibrotic mouse models of heart disease. Interestingly, in one injury model, unique patterns of cardiac fibroblasts were observed in the injured mouse heart that followed bands of wrapped fibers in the contractile direction. In ischemic injury models, fibroblast death occurred, followed by repopulation from the infarct border zone. Collectively, this refined cardiac tissue clarifying technique and digitized imaging system allows for 3-D visualization of cardiac fibroblasts in the heart without the limitations of antibody penetration failure or previous issues surrounding lost fluorescence due to tissue processing.

Cardiac fibroblasts are essential for baseline function of the heart as well as for response to cardiac injury. Previous attempts to understand the arrangement and morphology of these cells have been conducted largely in 2-D settings. However, a refined cardiac tissue clearing (Figure 2) and 3-D imaging technique has been published, which allows for the advanced, more detailed visualization of cardiac fibroblasts. With this imaging technique, fibroblasts were found to be densely packed and h...

A refined method of tissue clearing was developed and applied to the adult mouse heart. This method was designed to clear dense, autofluorescent cardiac tissue, while maintaining labeled fibroblast fluorescence attributed to a genetic reporter strategy.

Refined CLARITY-Based Tissue Clearing for Three-Dimensional Fibroblast Organization in Healthy and Injured Mouse Hearts

Cardiovascular disease is the most prevalent cause of mortality worldwide and is often marked by heightened cardiac fibrosis that can lead to increased ...

This novel tissue clearing protocol improves existing technology to allow for a more accurate in vivo look at specific cell types in the heart and their behavior under injury conditions. This protocol is quick and fairly simple. It allows for the maintenance of marca fluorescence with minimal interference from background tissue auto fluorescence without requiring cell or tissue staining.Begin by using a 70%ethanol soaked gauze pad to clean the ventral surface of the experimental mouse. Use surgical scissors to make a three centimeter transverse incision, approximately three meters below the xiphoid process and deglove the mouse from the abdomen to the xiphoid process to separate the skin from the underlying abdominal wall tissue. Make a two centimeter transverse incision in the subcutaneous abdominal wall tissue, three centimeters below the xiphoid process and make a vertical cut from this incision up the midline through the rib cage.Pin the rib cage back to expose the heart. And use a 10-milliliter syringe equipped with a 27 gauge needle to inject cold PBS into the superior vena cava and aorta. When all of the blood has been flushed from the heart, deliver 10 milliliters of cold 4%PFA into the superior vena cava and aorta to begin the fixation process.When all of the fixative has been flashed, excise the heart and use a straight blade scalpel to separate the atria, right ventricle and septum and left ventricle. Place the heart into a 15-milliliter conical tube of cold 4%PFA for an overnight incubation at four degrees celsius on a nutator. The next morning, add a 0.25%photo initiator solution to a freshly prepared hydrogels solution and transfer the heart to the hydro gel.Wrap the conical tube in foil for an overnight incubation at four degrees celsius without physical disturbance. The next morning warm the tube in a 37 degrees celsius bead bath for 2.5 hours before using forceps to carefully transfer the heart into a 15-milliliter tube of PBS. Wash the heart three times in PBS for one hour per wash at 37 degrees celsius on a nutator and transfer the heart into the basket of an active electrophoresis machine.Secure the lid in place and fill the active electrophoresis chamber reservoir with electrophoresis clearing solution. Once the chamber is full, submerge the basket in the solution and tighten the cap on the electrophoresis machine into place. Then run the electrophoresis machine at 1.5 amps and 37 degrees celsius for 1.5 hours.After electrophoresis, check the heart's visually to ensure that no opaque tissue remains. Wash the cleared heart three times in a 15-milliliter two for one hour in fresh PBS at 37 degrees celsius per wash on a nutator. Before submerging the heart in decolorizing agent to reduce heme auto fluorescence.After 48 hours, wash the heart three times in PBS as demonstrated and equilibrate the heart in freshly prepared RIMS for 48 hours prior to imaging. For 3D imaging of the cleared heart, first, place a thin layer of vacuum grease onto the bottom of a 3D printed bottom reservoir, the same height as the sample being imaged. Fill the reservoir with RIMS and use a pipette tip to remove any bubbles.Carefully place the heart in the RIMS with the left ventricular wall facing up, taking care of that no bubbles are introduced into the solution. Use vacuum grease to attach a glass cover slip to the bottom surface of a 3D printed top reservoir piece and place the top reservoir coverslip down onto the bottom reservoirs, taking care to avoid bubbles. Then fill the top reservoirs with glycerol.Use a confocal microscope to image the cleared hearts. Combining refined cardiac tissue clearing with 3D imaging allows an advanced detailed visualization of cardiac fibroblasts. Using this imaging technique, fibroblasts are observed to be densely packed and to have a spindled morphology in uninjured hearts.After ischemia reperfusion injury, a loss of cardiac fibroblasts is seen in the ischemic region for the first three days after injury. By day seven and 14, fibroblasts have migrated or proliferated to repopulate the injured tissue area. By day 28, the cardiac fibroblast population surrounding the injured areas of the heart are at their greatest density.One and a half days after myocardial infarction injury, few fibroblasts remain within the injured left ventricle. By day three however, the fibroblasts expand and are present in most of the left ventricle. In contrast to ischemic surgery, infusion of angiotensin two and phenolefrin over several weeks does not result in cardiac tissue loss and wall thinning.Instead, the fibroblast align along the axis of the right handed helix contraction pattern as seen when using the refined to tissue clearing protocol. In addition, after angiotensin two and phenolefrin treatment, the fibroblasts appear small and rounded as opposed to the spindle shape observed in models of ischemia reprofusion and myocardial infarction injury. The electrophoretic clearing needs to be performed carefully especially when working with hearts that have undergone injury protocols.This protocol can be used to assess how cardiac fibroblasts react to injury in vivo. Different fluorescent markers can also be used to understand how the heart responds to injury at the cellular level.

Research

JoVE Journal

Medicine

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