Funding for this project was provided by the UW School of Medicine and Public Health from the Wisconsin Partnership Program (A.I.M.), and an American Heart Association Career Development Award 19CDA34660169 (A.I.M.).

All experiments were conducted in accordance with the Guide for the Use and Care of Laboratory Animals and in compliance with the Institutional Animal Care and Use Committee in the School of Medicine and Public Health at the University of Wisconsin&#8211;Madison. All methods were performed on wild type C57BL/6J (B6) and transgenic mouse lines obtained from Jackson Laboratories.
1. Coronary Artery Occlusion (Myocardial Infarction) Induced via Ligation of the Left Anterior Descending Artery (LAD) in 1-Day-Old Neonatal Mice6
<ol>
	<li>Separate the 1-day-old neonatal pups from the mother by placing them into a clean cage along with some of the original nesting material.</li>
	<li>Place half of the cage onto a heating pad set to medium heat. The pups should remain on the unheated side of the cage, only being placed onto the heated side after surgery.</li>
	<li>Create a sterile surgical area under an operating microscope. Gather sterilized surgical equipment (Table 1).</li>
	<li>Anesthetize the pup via hypothermia: wrap the pup in gauze to avoid direct skin contact with ice and bury it in an ice bed for approximately 3&#8211;4 min. Check hypothermia of the mice periodically by performing a toe pinch. Neonates tolerate hypothermia well, however, prolonged exposure to hypothermia may result in frostbite and subsequent mortality.</li>
	<li>Once anesthetized, place the mouse onto the surgical area in the supine position, securing the arms and legs with tape. Sterilize the surgical area of the mouse with an antiseptic solution.</li>
	<li>Locate the lower chest region and make a transverse incision in the skin with small dissecting scissors. To widen the surgical view of the ribs, separate the skin from the muscle by lifting the skin gently with a pair of dressing forceps and gently press against the intercostal muscles with the small scissors in the closed position.</li>
	<li>Locate the fourth intercostal space (Figure 1A) and make a small, superficial puncture using sharp forceps, being careful not to puncture any internal organs. Perform a blunt dissection by widening the area in between the intercostal muscles with dressing forceps. Proper anatomical positioning of the incision is essential for appropriate access to the heart.</li>
	<li>Gently guide the heart out of the chest cavity by placing a finger and applying increasing pressure on the left side of the abdomen while holding the intercostal space open with dressing forceps (Figure 1B). Once the heart is outside of the chest, remove the dressing forceps, relieve pressure, and allow the heart to rest on the intercostal muscles.</li>
	<li>Locate the LAD as the area of the heart that has less pooled blood and is in the correct anatomical position (Figure 1C). The LAD can be only seen under the microscope if the heart is accessed within a few minutes of beginning surgery.</li>
	<li>Perform LAD ligation by suturing the LAD with a 6-0 suture (Figure 1D). Tie a square knot twice to induce myocardial infarction (Figure 1E). The depth of the suture into the myocardium may vary, however, proper anatomical positioning of the LAD ligation is crucial for reproducibility. When ligating the LAD, the suture should be pulled tightly but carefully so as not to sever the LAD. Blanching at the apex of the heart will be seen immediately (Figure 1E)</li>
	<li>Allow the heart to slip back into the chest cavity; this process can be gently facilitated with dressing forceps. Suture the ribs together with a surgeon&#39;s knot followed by a square knot, using blunt forceps to lift the upper set of ribs while passing a 6-0 suture through the upper and lower set of ribs.</li>
	<li>Remove the tape that was used to secure the hind legs of the pup.</li>
	<li>Adhere the skin together by placing a small amount of skin glue on the upper abdomen. Then, grab the skin of the lower abdomen with fine forceps and cover the exposed chest region. Limit the amount of excess glue that remains on the pups, as this can increase the likelihood of rejection and cannibalism by the mother.</li>
	<li>Immediately facilitate the recovery from anesthesia by placing the pup onto a heating pad set to medium heat. Periodically switch the placement of the neonates to evenly warm all parts of the body.</li>
	<li>Allow the neonate to remain directly on the heating pad for 10&#8211;15 min. Typically, movement is regained within 5 min of being placed onto heat.</li>
	<li>Clean the residual blood and glue with an alcohol wipe.</li>
	<li>Cover foreign scents on neonates by rubbing the entire body with bedding from the original cage. Place the pup into the cage on the heated side while other surgeries are being performed.</li>
	<li>Once all surgeries are completed and pups are warm and mobile, transfer the litter along with the original nesting material to the mother&#39;s cage.</li>
	<li>Monitor the mice for 30&#8211;60 min after surgery and watch for the mother&#39;s acceptance of the pups by nesting and/or grooming.</li>
	<li>Check on the mice the morning following surgery. If mother is distressed and has not nested the pups, consider a foster mother for the pups.</li>
</ol>
2. Clearing the Mouse Heart with Passive CLARITY21,22,23
<ol>
	<li>Anesthetize the mouse with isoflurane. Perform a toe pinch to ensure the mouse is fully sedated.</li>
	<li>Place the mouse onto a clean, surgical area in the supine position, securing the arms and legs with tape.</li>
	<li>Maintain isoflurane sedation on the mouse using a nose cone until the heart is extracted.</li>
	<li>Open the lower chest by holding the fur just below the xiphoid process with tissue forceps and make an incision spanning the width of the ribcage using the large dissecting scissors.</li>
	<li>Cut alongside of the distal portions of the rib cage with surgical scissors.</li>
	<li>Expose the diaphragm muscle by grasping the xiphoid process with tissue forceps. Detach the diaphragm using curved forceps.</li>
	<li>While maintaining a grasp of the xiphoid process, pull the tissue cranially until the beating heart is accessible.</li>
	<li>Grasp the heart at the base with curved forceps and dissect the heart from the chest cavity by cutting the aorta and superior vena cava with iridectomy scissors.</li>
	<li>While the heart is still beating, place the heart into a Petri dish filled with PBS so that the heart pumps out the blood inside as it keeps beating. Myocardial infarction can be confirmed by checking that the placement of the suture is in the proper anatomical position for LAD ligation.</li>
	<li>Gently squeeze the heart with forceps to allow the heart to expel the residual blood.</li>
	<li>Transfer the mouse heart into a disposable 2.5 mL glass shell vial with 2 mL of PBS. Wash away residual blood by incubating the heart on a shaker for 10 min at room temperature (RT) several times. Change the PBS solution each time until PBS remains clear.</li>
	<li>Discard the PBS and fill the vial with 2 mL of cold 4% paraformaldehyde (PFA). Incubate for 4 hours at RT (Figure 2A).</li>
	<li>After incubation, discard the PFA and the vial with 2 mL of PBS. Wash the heart on a shaker for 10 min at RT. Repeat the washing step twice, draining and filling the vial with new PBS each time to fully wash away excess PFA.</li>
	<li>Discard PBS and fill the vial with 2 mL of 4% acrylamide and 0.5% VA-044 solution. Incubate overnight at 4 &#176;C.</li>
	<li>The next day, perform polymerization by transferring the vial to a heat block set at 37 &#176;C for 3 hours.</li>
	<li>Transfer the heart into a new glass shell vial and repeat step 2.12 (PBS wash cycle).</li>
	<li>Discard PBS and fill the vial with 2 mL of Clearing Solution (Table 2). Incubate at 37 &#176;C until the heart is cleared. Change out the solution and refill with fresh Clearing Solution every 2&#8211;3 days. The clearing process could take up to several weeks (Figure 2B-C). 
	NOTE: P1 hearts typically take around 3&#8211;5 days, whereas P21 hearts can take nearly a month before Clearing Solution incubation is complete.</li>
	<li>Discard PBS and fill the vial with 2 mL of PBS and repeat step 2.12 (PBS wash cycle). Refill the vial with fresh PBS and incubate overnight at 37 &#176;C.</li>
	<li>If immunostaining will be performed, skip steps 2.21&#8211;2.22 and proceed to Section 3 for immunostaining. If relying solely on endogenous fluorescence, proceed with steps 2.21&#8211;2.22 and skip Section 3.</li>
	<li>Discard PBS and change the solution to Refractive Index Matching Solution (RIMS) (Table 3). Incubate overnight at 37 &#176;C.</li>
	<li>After incubation, the cleared tissue can be stored in RIMS solution at RT. Tissue may appear to be white and opaque in the center when first transferred into RIMS; tissue should become transparent after being incubated in RIMS at room temperature for several weeks (Figure 2D).</li>
</ol>
3. Optional: Immunohistochemistry Staining of the Whole-Mount Mouse Heart
<ol>
	<li>Remove the cleared heart from the RIMS solution and place into a clean 2.5 mL glass vial with 2 mL of PBST (PBS with 0.1% Triton-X 100)</li>
	<li>Wash the heart in PBST 3 times on an orbital rotator with 30 min incubations at RT.</li>
	<li>Block non-specific antibody binding by immersing the heart in 2 mL of 20% blocking buffer (diluted in PBST) and incubate with rotation for 3 hours at RT. Transfer to 4 &#176;C to stain with rotation overnight. 
	NOTE: Blocking buffer is made from normal serum matching the species in which the secondary antibody was raised.</li>
	<li>Wash the heart in PBST 3 times with rotation for 5 min incubations at RT.</li>
	<li>Immerse the heart in primary antibody (diluted in 2% blocking buffer with PBST) and prevent light exposure by wrapping the glass vial in aluminum foil. Incubate with rotation overnight at RT. 
	NOTE: From this point forward, aluminum foil should be continuously used to protect the secondary from ambient light exposure.</li>
	<li>Incubate for an additional 24 hours with rotation at 4 &#176;C.</li>
	<li>Following primary staining, repeat step 3.2 (long PBST wash cycle) to remove the unbound primary antibody from the tissue. Extend the wash with an overnight incubation with rotation at RT.</li>
	<li>Working in an area with limited lighting to prevent secondary antibody light exposure, immerse the heart in secondary antibody (diluted in 2% blocking buffer with PBST) and incubate with rotation for 3 hours at RT. Transfer to 4 &#176;C to stain with rotation overnight.</li>
	<li>On the next day, repeat step 3.2 (long PBST wash cycle) to remove unbound secondary antibody.</li>
	<li>Replace the solution with 2% blocking buffer (diluted in PBST). Remove residual unbound antibody by washing overnight with rotation at RT.</li>
	<li>The next day, check that excess secondary antibody has been removed using confocal microscopy. Extend the wash as needed, replacing the 2% blocking buffer solution daily. Proceed once little to no non-specific secondary is present.</li>
	<li>Store the immunostained heart in PBS at 4 &#176;C.</li>
	<li>Directly before whole-mount microscopy, incubate the heart in RIMS solution overnight at 37 &#176;C. Extend the incubation an additional 24 hours if the tissue is still not fully cleared.</li>
	<li>Store the fully cleared and immunostained heart in RIMS at RT.</li>
</ol>
4. Visualizing Non-myocyte Populations in 3D with Single-Photon Confocal Microscopy Imaging of the Cleared Mouse Heart
NOTE: If mouse hearts are harvested embryonically, continue with section 4.1. For mouse hearts harvested postnatally, continue with section 4.2.
<ol>
	<li>Imaging the Cleared Embryonic Mouse Heart
	<ol>
		<li>Fill the microscope depression slide with the PBS solution.</li>
		<li>Carefully pick up the cleared heart with curved forceps and place the tissue onto the slide.</li>
		<li>Mount the slide with a glass coverslip. The tissue can now be imaged under a confocal microscope.</li>
	</ol>
	</li>
	<li>Imaging the Cleared Postnatal Mouse Heart
	<ol>
		<li>Fill half of the chamber of the depression slide with PBS solution. In order to create a chamber large enough to fit an adult mouse heart, a 3D-printed polypropylene depression slide was custom-made (Figure 4).</li>
		<li>Carefully pick up the cleared heart with curved forceps and place the tissue into the chamber. Fill the remaining volume of the chamber with PBS.</li>
		<li>Fill the chamber with PBS so the surface of the liquid forms a dome above the top of the chamber. Mount the cover slide. The tissue can now be imaged under an upright confocal microscope.</li>
	</ol>
	</li>
</ol>

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The authors declare that they have no competing financial interests.

Cell-cell interactions between cardiomyocytes and non-myocyte populations are a determining factor of whether the heart will undergo fibrosis or repair following injury. Discoveries have been made demonstrating that a variety of cell types, including nerves14, epicardial cells24, peritoneal macrophages25, arterioles12,13, and lymphatic endothelial cells26, all play an essential role in mediating cardiac repair. These cell lineages and others of interest can be genetically traced during development, disease, and regeneration by applying Cre-lox and CRISPR-Cas9 technologies in mice. When coupled with organ clearing and advanced microscopy methods, the contributions of non-myocyte populations can be accurately assessed, opening the door to elucidate cellular and molecular targets of myocardial regeneration following injury.
The efficiency of the protocol is dependent on consistent and reproducible ligation of the LAD during coronary artery occlusion surgery. Neonatal mice are sensitive to extended exposure to hypothermia; thus, the surgery must not only be performed with accuracy but also within minutes. From start to finish, the myocardial infarction surgery should take less than 8 minutes. We recommend first practicing on several litters of the same background as the experimental mice until proficiency is achieved.
Progression of cardiac repair can be assessed by using echocardiography to measure cardiac function (i.e. fractional shortening, ejection fraction, systolic and diastolic volume) once within 3 to 7 days after surgery and again shortly before harvesting the heart, suggested between 21- and 28-days post injury. Hearts can be collected for clearing at multiple timepoints following myocardial infarction surgery.
The clearing step (step 2.17) is subject to variation in duration depending on age and strain of the mouse from which the heart was harvested, which can result in differences in heart size. For a B6 background mouse, clearing duration based on age is estimated as follows: P1 (7-10 days), P7 (14-17 days), P14 (21-24 days), P21 (28-31 days), P28 (35-38 days). Although the primary focus of our laboratory is heart clearing, our passive CLARITY method has been successful for clearing mice lungs (unpublished results) and we foresee no limit on broadly applying this to other organs. Overall, our expedited clearing process is highly valued for the ability to clear tissues rapidly and effectively.
It should be noted that complications can arise when applying tissue clearing techniques in organs with endogenous reporters in rare subpopulations. Reporter signal in dense cell populations (such as myocytes23) seem to be more resistant to the clearing process and thus the signal is typically retained; however, other more sensitive cell populations (such as cardiac nerves) are prone to having endogenous fluorescent protein signal quenched after prolonged fixation or clearing. This became apparent in the Actl6bCre;Rosa26tdT reporter mouse model, where P7 hearts fixed briefly for 15 minutes showed strong tdT signal (Figure 3A), however this fluorescent signal was quenched after fixation for 1 hour or overnight incubation in the Clearing Solution (data not shown). In the scenario of lost reporter signal, antibody staining targeting the reporter protein is compatible with tissue clearing to amplify the signal (Figure 3). The addition of an immunolabeling step can be advantageous for tissues undergoing extensive imaging, as conjugated antibodies are bleach-resistant and produce stable, long-term expression. With the ability to track proteins endogenously and through immunostaining, our protocol uniquely allows for precise localization of rare cardiac cell populations that reside deep within the heart.
Cleared hearts can undergo lineage tracing or fluorescent protein expression analysis using whole-mount 3D confocal microscopy. The confocal microscope is equipped with laser lines optimized to excite florescent reporters (or conjugated secondary antibodies), for example: BFP (DAPI, Alexa Fluor 405), EGFP (FITC, Alexa Fluor 488), DsRed (TRITC, Alexa Fluor 546/555), and APC (Cy5, Alexa Fluor 647) at 405 nm, 488 nm, 561 nm, and 683 nm, respectively. For heart samples unable to fit in a depression slide &#8211; common if the heart harvested postnatally &#8211; a custom depression slide can be made by 3D printing on polypropylene. Custom slides followed dimensions of a microscope depression slide (25 mm x 75 mm x 1 mm), varying the well depth to either 6.5 mm or 17 mm (Figure 4).
In order to visualize the reporter cell lines in 3D, the confocal microscope is set to acquire z-stacked images throughout the heart. When imaging larger hearts, the entire heart may not be able to be captured in a single z-stacked image even with a low magnification objective. In this scenario, a series of multiple z-stacked images taken at different heart regions can be stitched together using a large image function. This is accomplished by calibrating the microscope to the appropriate objective lens and specifying the field for the large image scan area. Then, z-stacked images undergo 3D reconstruction using a volume rendering program and maximum intensity projection (Figure 3). The high-resolution images acquired can be analyzed to determine precise 3D spatial cell patterning of targeted cell populations.
Collectively, this protocol provides a powerful molecular tool to understand the dynamic cellular changes that occur during cardiac repair and regeneration. This method describes the steps to induce a myocardial infarction, perform whole organ clearing, trace cell specific populations, and analyze 3D cell patterning. Together, these techniques provide access to trace cell populations that were previously inaccessible due to their sparse presence or location within the tissue. This will enable further investigation of questions paramount to advance therapeutic approaches in regenerative medicine, particularly those aimed at stimulating endogenous heart regeneration.

The heart has long been considered to be a post-mitotic organ, yet recent evidence demonstrates that cardiomyocyte renewal occurs in the adult human heart at about 1% per year1. However, these low rates of cardiomyocyte turnover are insufficient to replenish the massive loss of tissue that occurs following injury. A heart that has suffered a myocardial infarction will lose around one billion cardiomyocytes, often serving as a prelude to heart failure and sudden cardiac death2,3. With over 26 million people affected by heart failure worldwide, there is an unmet need for therapeutics that can reverse the damages inflicted by heart disease4.
In order to bridge this gap in therapeutics, scientists have begun investigating evolutionarily conserved mechanisms that underlie endogenous regeneration following injury. One model for studying mammalian cardiac regeneration is the neonatal mouse. Within the week following birth, neonatal mice have a robust regenerative response following cardiac damage5. We have previously demonstrated that neonatal mice can regenerate their heart via cardiomyocyte proliferation following an apical resection5. Although this technique can evoke cardiac regeneration in the neonates, the surgery lacks clinical relevance to human heart injuries. In order to mimic a human injury in the neonatal mouse model, we have developed a technique to induce a myocardial infarction through a coronary artery occlusion6. This technique requires surgical ligation of the left anterior descending artery (LAD), which is responsible for delivering 40%&#8211;50% of the blood to the left ventricular myocardium6,7. Thus, the surgery results in an infarct that impacts a significant portion of the left ventricular wall. This damage to the myocardium will stimulate cardiomyocyte proliferation and heart regeneration in neonates5.
The coronary artery occlusion surgery provides a highly reproducible and directly translational method to uncover the inner workings of cardiac regeneration. The neonatal surgery parallels coronary artery atherosclerosis in the human heart, where accumulation of plaque within the inner walls of the arteries can cause an occlusion and subsequent myocardial infarction8. Due to a void in therapeutic treatments for heart failure patients, an occlusion in the LAD is associated with mortality rates reaching up to 26% within a year following injury9, and consequently has been termed the &#34;widow maker.&#34; Advancements in therapeutics require a model that accurately reflects the complex physiological and pathological effects of cardiac injury. Our surgical protocol for neonatal mouse cardiac injury provides a platform that allows researchers to investigate the molecular and cellular cues that signal mammalian heart regeneration after injury.
Recent research highlights the dynamic relationship between the extracellular environment and proliferating cardiomyocytes. For example, the postnatal regenerative window can be extended by decreasing the stiffness of the extracellular matrix surrounding the heart10. Biomaterials from the neonatal extracellular matrix can also promote heart regeneration in adult mammalian hearts following cardiac injury11. Also accompanying cardiomyocyte proliferation is an angiogenic response12,13; collateral artery formation unique to the regenerating heart of the neonatal mouse was shown to be essential for stimulating cardiac regeneration12. Moreover, our lab has demonstrated that nerve signaling regulates cardiomyocyte proliferation and heart regeneration via modulation of growth factor levels, as well as the inflammatory response following injury14. These findings emphasize the need to trace non-myocyte cell populations in response to cardiac injury. In order to accomplish this goal, we have taken advantage of the Cre-lox recombination system in transgenic mice lines to incorporate constitutive or conditional expression of fluorescent reporter proteins for lineage tracing. Furthermore, we can use advanced methods to determine clonal expansion patterning with the Rainbow mouse line, which relies on stochastic expression of the Cre-dependent, multi-color fluorescent reporters to determine the clonal expansion of targeted cell populations15. Employing lineage tracing with the neonatal coronary artery occlusion surgery is a powerful tool for dissecting the intricate cellular mechanisms of cardiac regeneration.
Tracking the lineage of fluorescently labeled cells with three-dimensional (3D) whole organ imaging is difficult to achieve using traditional sectioning and reconstruction technique &#8211; especially when cell populations are fragile, such as nerve fibers or blood vessels. While direct whole-mount imaging of the organ by optical sectioning can capture superficial cell populations, structures that reside deep within the tissue remain inaccessible. To circumvent these barriers, tissue clearing techniques have been developed to reduce the opacity of whole organ tissues. Recently, significant advances have been made to Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging compatible Tissue hYdrogel (CLARITY)&#8211;based methods, which clear fixed tissue via lipid extraction16. Steps are also taken to homogenize the refractive index and subsequently reduce light scattering while imaging17. One such method is active CLARITY, which expedites lipid decomposition by using electrophoresis to penetrate the detergent throughout the tissue18. Although effective, this tissue clearing method requires expensive equipment and can cause tissue damage, making the approach incompatible with fragile cell populations such as the cardiac nerves19. Thus, we employ the passive CLARITY approach, which relies on heat to gently facilitate detergent penetration, therefore aiding in the retention of intricate cell structures20,21.
Passive CLARITY is typically thought to be less efficient than active CLARITY18, as the technique is often accompanied by two major obstacles: the inability to clear the entire organ depth and the extensive amount of time required to clear adult tissues. Our passive CLARITY approach overcomes both of these barriers with an expeditated clearing process that is capable of fully clearing neonatal and adult heart tissue. Our passive CLARITY tissue clearing technique has reached an efficiency that permits the visualization of a variety of cardiac cell populations, including rare populations distributed throughout the adult heart. When the cleared heart is imaged with confocal microscopy, the architecture of cell-specific patterning during development, disease, and regeneration can be illuminated.

<table><tbody><tr><td>1-thioglycerol</td><td></td><td></td><td></td></tr><tr><td>6-0 Prolene Sutures</td><td>Ethicon</td><td>8889H</td><td>Polypropylene Sutures</td></tr><tr><td>Acrylamide</td><td></td><td></td><td></td></tr><tr><td>Boric acid</td><td></td><td></td><td></td></tr><tr><td>Curved Forceps</td><td>Excelta</td><td>16-050-146</td><td>Half Curved, Serrated, 4 in</td></tr><tr><td>Dressing Forceps</td><td>Fisherbrand</td><td>13-812-39</td><td>Dissecting, 4.5 in</td></tr><tr><td>Glass Vial</td><td>Fisherbrand</td><td>03-339-26A</td><td>12 x 35 mm Vial with Cap</td></tr><tr><td>Histodenz</td><td>Sigma-Aldrich</td><td></td><td>Density gradient medium</td></tr><tr><td>Iridectomy Scissors</td><td>Fine Science Tools</td><td>15000-03</td><td>2 mm Cutting Edge</td></tr><tr><td>Large Dissecting Scissors</td><td>Fisherbrand</td><td>08-951-20</td><td>Straight, 6 in</td></tr><tr><td>Needle Holder</td><td>Fisherbrand</td><td>08-966</td><td>Mayo-Hegar, 6 in</td></tr><tr><td>Paraformaldehyde</td><td></td><td></td><td></td></tr><tr><td>Phosphate Buffer</td><td></td><td></td><td></td></tr><tr><td>Sharp Forceps</td><td>Sigma-Adrich</td><td>Z168777</td><td>Fine Tip, Straight, 4.25 in</td></tr><tr><td>Small Dissecting Scissor</td><td>Walter Stern Inc</td><td>25870-002</td><td>30 mm Cutting Edge</td></tr><tr><td>Sodium Azide</td><td></td><td></td><td></td></tr><tr><td>Sodium Dodecyl Sulfate (SDS)</td><td></td><td></td><td></td></tr><tr><td>Tissue Forceps</td><td>Excelta</td><td>16050133</td><td>Medium Tissue, 1X2 Teeth</td></tr><tr><td>VA-044</td><td>Wako Chemicals</td><td></td><td>Water-soluble azo initiator</td></tr></tbody></table>

capturing the cardiac injury response of targeted cell populations via cleared heart three-dimensional imaging

Cardiovascular disease outranks all other causes of death and is responsible for a staggering 31% of mortalities worldwide. This disease manifests in cardiac injury, primarily in the form of an acute myocardial infarction. With little resilience following injury, the once healthy cardiac tissue will be replaced by fibrous, non-contractile scar tissue and often be a prelude to heart failure. To identify novel treatment options in regenerative medicine, research has focused on vertebrates with innate regenerative capabilities. One such model organism is the neonatal mouse, which responds to cardiac injury with robust myocardial regeneration. In order to induce an injury in the neonatal mouse that is clinically relevant, we have developed a surgery to occlude the left anterior descending artery (LAD), mirroring a myocardial infarction triggered by atherosclerosis in the human heart. When matched with the technology to track changes both within cardiomyocytes and non-myocyte populations, this model provides us with a platform to identify the mechanisms that guide heart regeneration. Gaining insight into changes in cardiac cell populations following injury once relied heavily on methods such as tissue sectioning and histological examination, which are limited to two-dimensional analysis and often damage the tissue in the process. Moreover, these methods lack the ability to trace changes in cell lineages, instead providing merely a snapshot of the injury response. Here, we describe how technologically advanced methods in lineage tracing models, whole organ clearing, and three-dimensional (3D) whole-mount microscopy can be used to elucidate mechanisms of cardiac repair. With our protocol for neonatal mouse myocardial infarction surgery, tissue clearing, and 3D whole organ imaging, the complex pathways that induce cardiomyocyte proliferation can be unraveled, revealing novel therapeutic targets for cardiac regeneration.

Often the two most challenging steps are guiding the heart out of the chest cavity and ligating the LAD. To troubleshoot these steps, adjustments may be made in the placement of the initial puncture between the fourth intercostal muscles; if the puncture and blunt dissection are too close in proximity to the sternum, the heart may not be able to exit the chest cavity (Figure 1A).
Additionally, increased pressure on the left abdomen may be needed to facilitate this process (Figure 1B). Complications may also occur when resting the heart on the intercostal muscles. We found the heart will remain outside of the cavity without withdrawing when the blunt dissection is kept to a minimal size and performed mainly in the horizontal direction. This orientation also allows for clear visualization and accessibility to the LAD (Figure 1C).
When placing the suture needle behind the LAD, it is suggested to penetrate deep into the anterior wall of the left ventricle, as a superficial ligation has less room for adjustment in the final suture placement (Figure 1D). Tying the suture around the LAD should be performed with controlled, steady movements, as the LAD is fragile and severing this coronary artery and the anterior wall will cause mortality (Figure 1E). Within 5&#8211;10 minutes after surgery is complete, the neonate should be lively and breathing normally.
When proceeding to the passive CLARITY protocol, hearts harvested from a mouse line that incorporates an endogenous fluorescent reporter for cell lineage tracing should be protected from light (Figure 2), which can be accomplished by wrapping the glass vial in aluminum foil. During the clearing step, the time incubating in Clearing Solution is variable depending on the age of the mouse when the heart was harvested. This step is complete when the entire tissue is consistently opaque, with no discoloration in the center (Figure 2B-C). The hearts will become increasingly transparent after storage in RIMS solution at room temperature for a couple of days (Figure 2D). It should be noted that tissue expansion may occur during the clearing process.
Our passive CLARITY protocol permits effective antibody penetration and thus is compatible with immunohistochemistry methods to label protein(s) of interest. This was validated in Actl6bCre;Rosa26tdT transgenic mice, which label mature neurons with the tdTomato (tdT) reporter protein. The cardiac nerves are mainly superficial, with some populations residing in the epicardial layer, therefore we used this cell population as a proof-of-principal model for the reproducibility of endogenous reporter signal (Figure 3A), as well as preservation of reporter protein conformation before and after CLARITY (Figure 3B-C). Our representative results from Actl6bCre;Rosa26tdT mice show that the endogenous tdT protein signal seen in nerves of an uncleared P7 heart was faithfully recapitulated in nerves of both uncleared and cleared tdT immunolabeled P7 hearts (Figure 3). To immunolabel tdT-positive nerves, a Red Fluorescent Protein (RFP) primary antibody (rabbit polyclonal; 1:200 dilution; Rockland #600-401-379) was used along with an Alexa Fluor 488 secondary antibody (goat polyclonal; 1:1000 dilution; Invitrogen #A-11008). To visualize the cleared heart by eye during immunostaining and imaging steps, an ultraviolet flashlight may be briefly used to illuminate the heart.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60482/60482fig1b.jpg" /> 
Figure 1: Induction of Myocardial Infarction in the Neonatal Mouse through Coronary Artery Occlusion of the Left Anterior Descending Artery (LAD). (A) The fourth intercostal space, indicated by a single asterisk (*), is located and a blunt dissection is performed. (B) Pressure is applied on the left abdomen to guide the heart out of the chest cavity. (C) The LAD, marked by a double asterisk (**), is identified as being the predominant artery and by anatomical position. (D, E) A suture is then passed behind the LAD and a square knot is tied around the LAD to induce a coronary artery occlusion and subsequent infarct. (E) Once complete, blanching can be seen below the suture, at the apex of the heart. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60482/60482fig1blarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60482/60482fig2a.jpg" /> 
Figure 2: Progression of the Passive CLARITY Method on a P14 Mouse Heart. Hearts were harvested from P14 mice, (A) fixed, and (B&#8211;D) underwent the passive CLARITY protocol. For hearts taken at a P14 timepoint, Clearing Solution incubation step is (B) incomplete when discoloration at the center of the heart is apparent, seen after 6 days, and (C) complete when heart appears evenly opaque, seen after 10 days. (D) After the heart is stored in RIMS, the tissue is completely cleared. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60482/60482fig2alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60482/60482fig03.jpg" /> 
Figure 3: Whole-Mount 3D Imaging of P7 Mouse Hearts with Confocal Microscopy. Representative whole-mount 3D images from P7 Actl6bCre;Rosa26tdT transgenic mice hearts are shown as maximum intensity projections of z-stacked images. Hearts were imaged to show (A) endogenous tdTomato (tdT) fluorescence directly after harvesting (red) (B) immunostaining of tdT-positive nerves in an uncleared heart (Alexa Fluor 488; green) and (C) reproducible immunolabeling of tdT-positive nerves in a cleared heart (Alexa Fluor 488; green). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60482/60482fig3alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60482/60482fig4a.jpg" /> 
Figure 4: 3D Printed Polypropylene Depression Slides. To image the postnatal heart samples, custom depression slides were 3D printed on polypropylene. The slide dimensions are 25 mm x 75 mm x 1 mm, with a slide depression well 13 mm in diameter and either (A) 6.5 mm or (B) 17 mm in depth. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60482/60482fig4alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Equipment Type</td>
			<td>Description</td>
			<td>Company</td>
			<td>Catalog Number</td>
		</tr>
		<tr>
			<td>Glass Vial</td>
			<td>12 x 35mm Vial with Cap</td>
			<td>Fisherbrand</td>
			<td>03-339-26A</td>
		</tr>
		<tr>
			<td>6-0 Prolene Sutures</td>
			<td>Polypropylene Sutures</td>
			<td>Ethicon</td>
			<td>8889H</td>
		</tr>
		<tr>
			<td>Sharp Forceps</td>
			<td>Fine Tip, Straight, 4.25 in</td>
			<td>Sigma-Adrich</td>
			<td>Z168777</td>
		</tr>
		<tr>
			<td>Dressing Forceps</td>
			<td>Dissecting, 4.5 in</td>
			<td>Fisherbrand</td>
			<td>13-812-39</td>
		</tr>
		<tr>
			<td>Needle Holder</td>
			<td>Mayo-Hegar, 6 in</td>
			<td>Fisherbrand</td>
			<td>08-966</td>
		</tr>
		<tr>
			<td>Small Dissecting Scissor</td>
			<td>30 mm Cutting Edge</td>
			<td>Walter Stern Inc</td>
			<td>25870-002</td>
		</tr>
		<tr>
			<td>Tissue Forceps</td>
			<td>Medium Tissue, 1X2 Teeth</td>
			<td>Excelta</td>
			<td>16050133</td>
		</tr>
		<tr>
			<td>Large Dissecting Scissors</td>
			<td>Straight, 6 in</td>
			<td>Fisherbrand</td>
			<td>08-951-20</td>
		</tr>
		<tr>
			<td>Iridectomy Scissors</td>
			<td>2 mm Cutting Edge</td>
			<td>Fine Science Tools</td>
			<td>15000-03</td>
		</tr>
		<tr>
			<td>Curved Forceps</td>
			<td>Half Curved, Serrated, 4 in</td>
			<td>Excelta</td>
			<td>16-050-146</td>
		</tr>
	</tbody>
</table>
Table 1: Surgical Equipment.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td colspan="4">Clearing Solution</td>
		</tr>
		<tr>
			<td>Reagent</td>
			<td>Final</td>
			<td>Amount</td>
			<td>Notes</td>
		</tr>
		<tr>
			<td>Sodium Dodecyl Sulfate (SDS)</td>
			<td>8.0% w/v</td>
			<td>40 g</td>
			<td></td>
		</tr>
		<tr>
			<td>Boric acid</td>
			<td>1.25% w/v</td>
			<td>6.25 g</td>
			<td></td>
		</tr>
		<tr>
			<td>1-thioglycerol</td>
			<td>0.5% w/v</td>
			<td>5 &#956;L/mL</td>
			<td>Added as needed to solution</td>
		</tr>
		<tr>
			<td colspan="4">Add 400 ml of ultrapure H20 to 1 L beaker. Mix in SDS and Boric Acid with magnetic stirring.</td>
		</tr>
		<tr>
			<td colspan="4">Adjust pH to 8.5 using 6M NaOH. Bring volume to 500 ml and Autoclave.</td>
		</tr>
		<tr>
			<td colspan="4">Store solution at room temperature.</td>
		</tr>
		<tr>
			<td colspan="4">Other CLARITY Reagents</td>
		</tr>
		<tr>
			<td>Reagent</td>
			<td>Final</td>
			<td>Stock</td>
			<td>Notes</td>
		</tr>
		<tr>
			<td>PFA</td>
			<td>4.0%</td>
			<td>16%</td>
			<td>Diluted in PBS</td>
		</tr>
		<tr>
			<td>Acrylamide</td>
			<td>4.0%</td>
			<td>40%</td>
			<td>Diluted in PBS</td>
		</tr>
		<tr>
			<td>VA-044</td>
			<td>0.5%</td>
			<td>10%</td>
			<td>Added as needed to solution</td>
		</tr>
	</tbody>
</table>
Table 2: Clearing Solution and Other CLARITY Reagents.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Reagent</td>
			<td>Final</td>
			<td>Amount</td>
		</tr>
		<tr>
			<td>Phosphate Buffer</td>
			<td>0.02 M</td>
			<td>90 mL</td>
		</tr>
		<tr>
			<td>Histodenz</td>
			<td>133.3% w/v</td>
			<td>120 g</td>
		</tr>
		<tr>
			<td>Sodium Azide</td>
			<td>0.05% w/v</td>
			<td>45 mg</td>
		</tr>
		<tr>
			<td colspan="3">Add 90 mL of 0.02 M Phosphate Buffer to 250 mL glass media bottle wrapped in aluminum foil.</td>
		</tr>
		<tr>
			<td colspan="3">Mix in reagents listed with magnetic stirring. Allow components to dissolve overnight.</td>
		</tr>
		<tr>
			<td colspan="3">Vortex solution and filter purify with filtration system into a sterile 250 mL glass media bottle.</td>
		</tr>
		<tr>
			<td colspan="3">Wrap bottle in aluminum foil and store solution at room temperature.</td>
		</tr>
	</tbody>
</table>
Table 3: Refractive Index Matching Solution (RIMS).

Watch this Scientific Journal Video about Capturing the Cardiac Injury Response of Targeted Cell Populations via Cleared Heart Three-Dimensional Imaging at JoVE.com

Capturing the Cardiac Injury Response of Targeted Cell Populations via Cleared Heart Three-Dimensional Imaging

Cardiomyocyte proliferation following injury is a dynamic process that requires a symphony of extracellular cues from non-myocyte cell populations. Utilizing lineage tracing, passive CLARITY, and three-dimensional whole-mount confocal microscopy techniques, we can analyze the influence of a variety of cell types on cardiac repair and regeneration.

Cardiovascular disease outranks all other causes of death and is responsible for a staggering 31% of mortalities worldwide. This disease manifests in ...

capturing-cardiac-injury-response-targeted-cell-populations-via

Nanyang Technological University

Research

JoVE Journal

Developmental Biology

5.9K Views.  University of Wisconsin-Madison School of Medicine and Public Health. De endogene hartregeneratie is een dynamisch proces waarbij meerdere celtypen in overleg samenwerken om een robuuste regeneratieve respons te monteren. Turen in het regeneratieve weefsel via 3D-beeldvorming van het gewiste hart in reactie op letsel kan helpen bij de reconstructie van de cellulaire dynamiek die deze hartregeneratieve reactie begeleiden. Ons protocol is speciaal ontworpen om cellen te volgen tijdens de ontwikkeling en regeneratie.We voorzien geen beperkingen in het breed...

Video: Capturing the Cardiac Injury Response of Targeted Cell Populations via Cleared Heart Three-Dimensional Imaging

Imaging Cleared Embryonic and Postnatal Hearts at Single-cell Resolution

Single clonal tracing and analysis at the whole-heart level can determine cardiac progenitor cell behavior and differentiation during cardiac development, and allow for the study of the cellular and molecular basis of normal and abnormal cardiac morphogenesis. Recent emerging technologies of retrospective single clonal analyses make the study of cardiac morphogenesis at single cell resolution feasible. However, tissue opacity and light scattering of the heart as imaging depth is increased hinder whole-heart imaging at single cell resolution. To overcome these obstacles, a whole-embryo clearing system that can render the heart highly transparent for both illumination and detection must be developed. Fortunately, in the last several years, many methodologies for whole-organism clearing systems such as CLARITY, Scale, SeeDB, ClearT, 3DISCO, CUBIC, DBE, BABB and PACT have been reported. This lab is interested in the cellular and molecular mechanisms of cardiac morphogenesis. Recently, we established single cell lineage tracing via the ROSA26-CreERT2; ROSA26-Confetti system to sparsely label cells during cardiac development. We adapted several whole embryo-clearing methodologies including Scale and CUBIC (clear, unobstructed brain imaging cocktails and computational analysis) to clear the embryo in combination with whole mount staining to image single clones inside the heart. The heart was successfully imaged at single cell resolution. We found that Scale can clear the embryonic heart, but cannot effectively clear the postnatal heart, while CUBIC can clear the postnatal heart, but damages the embryonic heart by dissolving the tissue. The methods described here will permit the study of gene function at a single clone resolution during cardiac morphogenesis, which, in turn, can reveal the cellular and molecular basis of congenital heart defects.

We describe a protocol to volumetrically image fluorescent protein labeled cells deep inside intact embryonic and postnatal hearts. Utilizing tissue-clearing methods in combination with whole mount staining, single fluorescent protein-labeled cells inside an embryonic or postnatal heart can be imaged clearly and accurately.

Single clonal tracing and analysis at the whole-heart level can determine cardiac progenitor cell behavior and differentiation during cardiac development, ...

Pluripotent Stem Cell Derived Cardiac Cells for Myocardial Repair

Human induced pluripotent stem cells (hiPSCs) must be fully differentiated into specific cell types before administration, but conventional protocols for differentiating hiPSCs into cardiomyocytes (hiPSC-CMs), endothelial cells (hiPSC-ECs), and smooth muscle cells (SMCs) are often limited by low yield, purity, and/or poor phenotypic stability. Here, we present novel protocols for generating hiPSC-CMs, -ECs, and -SMCs that are substantially more efficient than conventional methods, as well as a method for combining cell injection with a cytokine-containing patch created over the site of administration. The patch improves both the retention of the injected cells, by sealing the needle track to prevent the cells from being squeezed out of the myocardium, and cell survival, by releasing insulin-like growth factor (IGF) over an extended period. In a swine model of myocardial ischemia-reperfusion injury, the rate of engraftment was more than two-fold greater when the cells were administered with the cytokine-containing patch comparing to the cells without patch, and treatment with both the cells and the patch, but not with the cells alone, was associated with significant improvements in cardiac function and infarct size.

We present three novel and more efficient protocols for differentiating human induced pluripotent stem cells into cardiomyocytes, endothelial cells, and smooth muscle cells and a delivery method that improves the engraftment of transplanted cells by combining cell injection with patch-mediated cytokine delivery.

Human induced pluripotent stem cells (hiPSCs) must be fully differentiated into specific cell types before administration, but conventional protocols for ...

Intramyocardial Cell Delivery: Observations in Murine Hearts

Previous studies showed that cell delivery promotes cardiac function amelioration by release of cytokines and factors that increase cardiac tissue revascularization and cell survival. In addition, further observations revealed that specific stem cells, such as cardiac stem cells, mesenchymal stem cells and cardiospheres have the ability to integrate within the surrounding myocardium by differentiating into cardiomyocytes, smooth muscle cells and endothelial cells.
Here, we present the materials and methods to reliably deliver noncontractile cells into the left ventricular wall of immunodepleted mice. The salient steps of this microsurgical procedure involve anesthesia and analgesia injection, intratracheal intubation, incision to open the chest and expose the heart and delivery of cells by a sterile 30-gauge needle and a precision microliter syringe.
Tissue processing consisting of heart harvesting, embedding, sectioning and histological staining showed that intramyocardial cell injection produced a small damage in the epicardial area, as well as in the ventricular wall. Noncontractile cells were retained into the myocardial wall of immunocompromised mice and were surrounded by a layer of fibrotic tissue, likely to protect from cardiac pressure and mechanical load.

Intramyocardial cell delivery in murine models of cardiovascular diseases, such as hypertension or myocardial infarction, is widely used to test the therapeutic potential of different cell types in regenerative studies. Therefore, a detailed description and a clear visualization of this surgical procedure will help to define the limits and advantages of cardiovascular cell therapeutic analyses in small rodents.

Previous studies showed that cell delivery promotes cardiac function amelioration by release of cytokines and factors that increase cardiac tissue ...

Medicine

Construction of Defined Human Engineered Cardiac Tissues to Study Mechanisms of Cardiac Cell Therapy

Human cardiac tissue engineering can fundamentally impact therapeutic discovery through the development of new species-specific screening systems that replicate the biofidelity of three-dimensional native human myocardium, while also enabling a controlled level of biological complexity, and allowing non-destructive longitudinal monitoring of tissue contractile function. Initially, human engineered cardiac tissues (hECT) were created using the entire cell population obtained from directed differentiation of human pluripotent stem cells, which typically yielded less than 50% cardiomyocytes. However, to create reliable predictive models of human myocardium, and to elucidate mechanisms of heterocellular interaction, it is essential to accurately control the biological composition in engineered tissues. 
To address this limitation, we utilize live cell sorting for the cardiac surface marker SIRP&#945; and the fibroblast marker CD90 to create tissues containing a 3:1 ratio of these cell types, respectively, that are then mixed together and added to a collagen-based matrix solution. Resulting hECTs are, thus, completely defined in both their cellular and extracellular matrix composition.
Here we describe the construction of defined hECTs as a model system to understand mechanisms of cell-cell interactions in cell therapies, using an example of human bone marrow-derived mesenchymal stem cells (hMSC) that are currently being used in human clinical trials. The defined tissue composition is imperative to understand how the hMSCs may be interacting with the endogenous cardiac cell types to enhance tissue function. A bioreactor system is also described that simultaneously cultures six hECTs in parallel, permitting more efficient use of the cells after sorting.

This manuscript describes the creation of defined engineered cardiac tissues using surface marker expression and cell sorting. The defined tissues can then be used in a multi-tissue bioreactor to investigate mechanisms of cardiac cell therapy in order to provide a functional, yet controlled, model system of the human heart.

Human cardiac tissue engineering can fundamentally impact therapeutic discovery through the development of new species-specific screening systems that ...

Bioengineering

3D Whole-heart Myocardial Tissue Analysis

Cardiac regenerative therapies aim to protect and repair the injured heart in patients with ischemic heart disease. By injecting stem cells or other biologicals that enhance angio- or vasculogenesis into the infarct border zone (IBZ), tissue perfusion is improved, and the myocardium can be protected from further damage. For maximum therapeutic effect, it is hypothesized that the regenerative substance is best delivered to the IBZ. This requires accurate injections and has led to the development of new injection techniques. To validate these new techniques, we have designed a validation protocol based on myocardial tissue analysis. This protocol includes whole-heart myocardial tissue processing that enables detailed two-dimensional (2D) and three-dimensional (3D) analysis of the cardiac anatomy and intramyocardial injections. In a pig, myocardial infarction was created by a 90-min occlusion of the left anterior descending coronary artery. Four weeks later, a mixture of a hydrogel with superparamagnetic iron oxide particles (SPIOs) and fluorescent beads was injected in the IBZ using a minimally-invasive endocardial approach. 1 h after the injection procedure, the pig was euthanized, and the heart was excised and embedded in agarose (agar). After the solidification of the agar, magnetic resonance imaging (MRI), slicing of the heart, and fluorescence imaging were performed. After image post-processing, 3D analysis was performed to assess the IBZ targeting accuracy. This protocol provides a structured and reproducible method for the assessment of the targeting accuracy of intramyocardial injections into the IBZ. The protocol can be easily used when the processing of scar tissue and/or validation of the injection accuracy of the whole heart is desired.

This protocol describes a novel method for the 3D comparison of whole-heart myocardial tissue with MRI. This is designed for the accurate assessment of intramyocardial injections in the infarct border zone of a chronic porcine model of myocardial infarction.

Cardiac regenerative therapies aim to protect and repair the injured heart in patients with ischemic heart disease. By injecting stem cells or other ...

Fabrication of 3D Cardiac Microtissue Arrays using Human iPSC-Derived Cardiomyocytes, Cardiac Fibroblasts, and Endothelial Cells

Generation of human cardiomyocytes (CMs), cardiac fibroblasts (CFs), and endothelial cells (ECs) from induced pluripotent stem cells (iPSCs) has provided a unique opportunity to study the complex interplay among different cardiovascular cell types that drives tissue development and disease. In the area of cardiac tissue models, several sophisticated three-dimensional (3D) approaches use induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) to mimic physiological relevance and native tissue environment with a combination of extracellular matrices and crosslinkers. However, these systems are complex to fabricate without microfabrication expertise and require several weeks to self-assemble. Most importantly, many of these systems lack vascular cells and cardiac fibroblasts that make up over 60% of the nonmyocytes in the human heart. Here we describe the derivation of all three cardiac cell types from iPSCs to fabricate cardiac microtissues. This facile replica molding technique allows cardiac microtissue culture in standard multi-well cell culture plates for several weeks. The platform allows user-defined control over microtissue sizes based on initial seeding density and requires less than 3 days for self-assembly to achieve observable cardiac microtissue contractions. Furthermore, the cardiac microtissues can be easily digested while maintaining high cell viability for single-cell interrogation with the use of flow cytometry and single-cell RNA sequencing (scRNA-seq). We envision that this in vitro model of cardiac microtissues will help accelerate validation studies in drug discovery and disease modeling.

Here, we describe an easy-to-use methodology to generate 3D self-assembled cardiac microtissue arrays composed of pre-differentiated human-induced pluripotent stem cell-derived cardiomyocytes, cardiac fibroblasts, and endothelial cells. This user-friendly and low cell requiring technique to generate cardiac microtissues can be implemented for disease modeling and early stages of drug development.

Generation of human cardiomyocytes (CMs), cardiac fibroblasts (CFs), and endothelial cells (ECs) from induced pluripotent stem cells (iPSCs) has provided ...

Light-Sheet Microscopy and Tissue Clearing for Comprehensive Cardiac Imaging

Light-Sheet Imaging to Reveal Cardiac Structure in Rodent Hearts

Light-sheet microscopy (LSM) plays a pivotal role in comprehending the intricate three-dimensional (3D) structure of the heart, providing crucial insights into fundamental cardiac physiology and pathologic responses. We hereby delve into the development and implementation of the LSM technique to elucidate the micro-architecture of the heart in mouse models. The methodology integrates a customized LSM system with tissue clearing techniques, mitigating light scattering within cardiac tissues for volumetric imaging. The combination of conventional LSM with image stitching and multiview deconvolution approaches allows for the capture of the entire heart. To address the inherent trade-off between axial resolution and field of view (FOV), we further introduce an axially swept light-sheet microscopy (ASLM) method to minimize out-of-focus light and uniformly illuminate the heart across the propagation direction. In the meanwhile, tissue clearing methods such as iDISCO enhance light penetration, facilitating the visualization of deep structures and ensuring a comprehensive examination of the myocardium throughout the entire heart. The combination of the proposed LSM and tissue clearing methods presents a promising platform for researchers in resolving cardiac structures in rodent hearts, holding great potential for the understanding of cardiac morphogenesis and remodeling.

Author Spotlight: Customized Light-Sheet Imaging for Investigating Myocardial Structures in Rodent Hearts

The protocol utilizes advanced light-sheet microscopy along with adapted tissue clearing methods to investigate intricate cardiac structures in rodent hearts, holding great potential for the understanding of cardiac morphogenesis and remodeling.

Light-sheet microscopy (LSM) plays a pivotal role in comprehending the intricate three-dimensional (3D) structure of the heart, providing crucial insights ...

Establishing a Swine Model of Post-myocardial Infarction Heart Failure for Stem Cell Treatment

Although advances have been achieved in the treatment of heart failure (HF) following myocardial infarction (MI), HF following MI remains one of the major causes of mortality and morbidity around the world. Cell-based therapies for cardiac repair and improvement of left ventricular function after MI have attracted considerable attention. Accordingly, the safety and efficacy of these cell transplantations should be tested in a preclinical large animal model of HF prior to clinical use. Pigs are widely used for cardiovascular disease research due to their similarity to humans in terms of heart size and coronary anatomy. Therefore, we sought to present an effective protocol for the establishment of a porcine chronic HF model using closed-chest coronary balloon occlusion of the left circumflex artery (LCX), followed by rapid ventricular pacing induced with pacemaker implantation. Eight weeks later, the stem cells were administered by intramyocardial injection in the peri-infarct area. Then the infarct size, cell survival, and left ventricular function (including echocardiography, hemodynamic parameters, and electrophysiology) were evaluated. This study helps establish a stable preclinical large animal HF model for stem cell treatment.

We sought to establish a swine model of heart failure induced by left circumflex artery blockage and rapid pacing to test the effect and safety of intramyocardial administration of stem cells for cell-based therapies.

Although advances have been achieved in the treatment of heart failure (HF) following myocardial infarction (MI), HF following MI remains one of the major ...

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.

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.

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

Mesoscopic Optical Imaging of Whole Mouse Heart

Both genetic and non-genetic cardiac diseases can cause severe remodeling processes in the heart. Structural remodeling, such as collagen deposition (fibrosis) and cellular misalignment, can affect electrical conduction, introduce electromechanical dysfunctions and, eventually, lead to arrhythmia. Current predictive models of these functional alterations are based on non-integrated and low-resolution structural information. Placing this framework on a different order of magnitude is challenging due to the inefficacy of standard imaging methods in performing high-resolution imaging in massive tissue. In this work, we describe a methodological framework that allows imaging of whole mouse hearts with micrometric resolution. The achievement of this goal has required a technological effort where advances in tissue transformation and imaging methods have been combined. First, we describe an optimized CLARITY protocol capable of transforming an intact heart into a nanoporous, hydrogel-hybridized, lipid-free form that allows high transparency and deep staining. Then, a fluorescence light-sheet microscope able to rapidly acquire images of a mesoscopic field of view (mm-scale) with the micron-scale resolution is described. Following the mesoSPIM project, the conceived microscope allows the reconstruction of the whole mouse heart with micrometric resolution in a single tomographic scan. We believe that this methodological framework will allow clarifying the involvement of the cytoarchitecture disarray in the electrical dysfunctions and pave the way for a comprehensive model that considers both the functional and structural data, thus enabling a unified investigation of the structural causes that lead to electrical and mechanical alterations after the tissue remodeling.

We report a method for mesoscopic reconstruction of the whole mouse heart by combining new advancements in tissue transformation and staining with the development of an axially scanned light-sheet microscope.

Both genetic and non-genetic cardiac diseases can cause severe remodeling processes in the heart. Structural remodeling, such as collagen deposition ...

Capturing the Cardiac Injury Response of Targeted Cell Populations via Cleared Heart Three-Dimensional Imaging

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