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) ...

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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 ...

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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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 t...

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 ...

The endogenous heart regeneration is a dynamic process that involves multiple cell types working together in concert to mount a robust regenerative response. Peering into the regenerating tissue via 3D imaging of the cleared heart in response to injury can aid in reconstruction of the cellular dynamics that guide this cardiac regenerative response. Our protocol is specifically designed to track cells during development and regeneration.We foresee no limitations in broadly applying our clearing, staining and imaging techniques to other organ types. For LAD-induced coronary artery occlusion, first sterilize the surgical area of an anesthetized one-day-old neonatal pup with antiseptic solution. Make a transverse skin incision in the chest region.To separate the skin from the muscle, use a pair of dressing forceps to lift the skin carefully while gently pressing the scissors against the intercostal muscles in a closed position. Locate the fourth intercostal space and use small forceps to make a small superficial puncture taking care to avoid the internal organs. Using blunt dissection and the dressing forceps, widen the area between the intercostal muscles just until a finger can be applied to the left side of the abdomen while holding the intercostal space open with dressing forceps to gently guide the heart out of the chest cavity.Once the heart has been extracted, allow the heart to rest on the intercostal muscles and locate the LAD as the region of the heart with less pooled blood. To induce myocardial infarction, use a 6-0 suture to tie a square knot tightly around the vessel two times without severing the artery. Blanching at the apex should be observed immediately.It's important to remain consistent on the suture placement along the LAD for a similar infarct size. This is best achieved by accessing the heart quickly while the blood is still flowing through the arteries. When the suture has been placed, allow the heart to slip back into the chest cavity.Suture the ribs together with one surgeon and one 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. Apply a small volume of skin glue to the upper abdomen and use fine forceps to position the skin of the lower abdomen over the exposed chest region. Then place the pup on a heating pad and perform the ligation on the next animal.When all of the surgeries have been performed, return the warm and mobile pups to the mother's home cage. For passive clarity of the harvested heart tissue, 21 days postsurgery after confirming a lack of response to toe pinch, place the mouse onto a clean surgical area in the supine position and secure the limbs with tape. Using tissue forceps, grasp the fur just below the xiphoid process and make an incision spanning the width of the ribcage.Cut alongside the distal portions of the ribcage and use the forceps to lift the xiphoid process to expose the diaphragm. Detach the diaphragm with curved forceps and pull the tissue cranially until the beating heart is accessible. Use the curved forceps to grasp the heart at the base and use iridectomy scissors to cut the aorta and superior vena cava.Place the beating heart into a Petri dish of PBS so that the heart pumps out any retained blood. The myocardial infarction can be confirmed by a correct anatomical placement of the LAD ligation. When the heart stops beating, gently squeeze the organ with forceps to expel the residual blood and transfer the specimen into a disposable 2.5 milliliter glass shell vial containing two milliliters of PBS.Place the vial onto a shaker for at least 30 minutes at room temperature changing the PBS solution every 10 minutes until the solution remains clear of blood. When the PBS is clear, replace the PBS with two milliliters of cold 4%paraformaldehyde for a four-hour incubation at room temperature followed by three 10-minute washes on the shaker with two milliliters of fresh PBS per wash. After the last wash, fill the vial with two milliliters of 4%acrylamide supplemented with 0.5%VA-044 solution for an overnight incubation at four degrees Celsius.The next day, place the vial in a 37 degrees Celsius heat block for three hours. At the end of the polymerization, transfer the heart into a new glass shell vial and wash the heart tissue in two milliliters of fresh PBS 10 minutes at a time refreshing the PBS at the end of every incubation. When the PBS is clear, replace the wash buffer with two milliliters of clearing solution.Incubate the specimen at 37 degrees Celsius for several weeks until the heart is clear, refreshing the clearing solution every two to three days. When the organ is clear, wash the specimen with PBS on the shaker as demonstrated before refilling the vial with fresh PBS for a 24-hour incubation at 37 degrees Celsius. The next day, replace the PBS with RIMS for a 24-hour incubation at 37 degrees Celsius.At the end of the incubation, the tissue is fully cleared. The tissue should become transparent after several weeks. For imaging of the cleared postnatal mouse heart after myocardial infarction induction, fill half of a chamber of a custom-made depression slide with PBS and use curved forceps to carefully transfer the cleared heart into the chamber.Fill the remaining volume of the chamber with PBS. Then fill the chamber with PBS until the surface of the liquid forms a dome above the top of the chamber and mount the cover slide before imaging the tissue by confocal microscopy according to standard imaging protocols. 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.If the heart does not easily exit the cavity, apply pressure to the left abdomen to facilitate the process. Complications may occur from resting the heart on the intercostal muscles. Therefore, it is important to keep the blunt dissection to a minimal size in a horizontal orientation to allow for a clear visualization and accessibility of the LAD.Note that a superficial ligation will have less room for adjustment in the final suture placement and that the suture tying around the LAD should be performed with controlled steady movements. By the end of the clearing step, the entire heart should be consistently opaque with no discoloration in the center. After a few days in RIMS solution at room temperature, the heart should be completely cleared and some tissue expansion may be observed.As observed by tdTomato reporter protein expression, the cardiac nerves are mainly superficial with some populations residing in the epicardial layer. Importantly, reporter protein expression confirmation is preserved after undergoing the clarity protocol as demonstrated. Clearing and imaging the 3D heart provides perspective into the location and morphology of cardiac cell populations.This allows us to identify new aspects in cell patterning and cell-cell interactions.

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

Research

JoVE Journal

Developmental Biology

5.8K 조회수.  University of Wisconsin-Madison School of Medicine and Public Health. 내인성 심장 재생은 강력한 재생 반응을 탑재하기 위해 함께 작업하는 여러 세포 유형을 포함하는 동적 과정입니다. 부상에 대한 응답으로 클리어 된 심장의 3D 이미징을 통해 재생 조직으로 피어링은이 심장 재생 반응을 안내하는 세포 역학의 재건에 도움이 될 수 있습니다. 우리의 프로토콜은 개발 및 재생 중에 세포를 추적하도록 특별히 설계되었습니다.우리는 다른 ...