Name: myocardial infarction in neonatal mice, a model of cardiac regeneration
Uploaded: 2016-05-24T15:00:00
Description: Watch this Scientific Journal Video about Myocardial Infarction in Neonatal Mice, A Model of Cardiac Regeneration at JoVE.com

This study was supported by an operating grant from the Canadian Institutes of Health Research (CIHR) to Q.F. (grant #MOP-119600).

Breeding pairs of C57BL/6 and CD-1 IG-S mice were purchased from Charles River. Animals used in this study were handled in accordance with the guidelines of the Canadian Council on Animal Care, and study protocols were approved by the Animal Use Subcommittee at Western University, London, Canada.
1. Animal Care
<ol>
	<li>After birthing is complete and pups have been initially breast-fed by their mother for a few hr, place them in a different cage with a CD-1 foster mother. CD-1 mothers displ...

<ol>
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The survival of patients with heart failure with preserved or reduced left ventricular ejection fraction: an individual patient data meta-analysis.</a> Eur Heart J. 33 (14), 1750-1757 (2012).</li><li>Soonpaa, M. H., Field, L. 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The surgical LAD ligation demonstrated herein is a reliable method to produce MI in neonatal mice. This model provides researchers with a reproducible model with which to study mammalian heart regeneration. Visualization of the coronary vasculature is a key component of this method, ensuring correct suture placement and thus guaranteeing reproducibility. While adult mice do not possess poikilothermic capabilities, the body temperature and metabolic rate of neonatal mice is closely associated with ambient temperature. Fur...

Myocardial infarction (MI) is a leading cause of death worldwide, and remains responsible for about one third of heart failure cases1. While the advent of percutaneous intervention and continuous optimization of the use of thrombolytics has increased reperfusion following MI, cardiomyocyte death and loss of contractile myocardium nevertheless occurs. There also remain large numbers of &#34;no-option&#34; patients who are not candidates for or do not see benefit from these interventions. These patients continue to experience disabling ischemia leading to scar formation and deleterious ventricular remodeling as a mechanism of infarct healing. This process ultimately results in heart failure, for which prognosis remains poor despite optimal pharmacologic management with angiotensin-converting enzyme (ACE) inhibitors and beta blockers. Unfortunately, the one-year mortality rate for patients with severely impaired left ventricular function still remains as high as 26%2. Heart transplant is the final treatment option for patients with heart failure. However, the limited donor pool for heart transplantation does not make this a viable option for most patients. Thus, the discovery of novel therapeutic agents to restore the damaged myocardium remains paramount to solving the cardiac disease problem. Reliable animal models of cardiac injury are therefore required as a critical component of this process.
Traditional dogma has dictated that adult cardiomyocytes are post-mitotic, terminally differentiated cells, incapable of dividing or de-differentiating to replace the damaged myocardium3. As such, an adult mammalian heart could never completely recover from injury, and lost cardiomyocytes would be replaced with fibrous tissue. Thus, research has focused primarily on therapeutic agents to minimize infarct expansion and reduce scar formation. More recently however, a paradigm shift has occurred in the thinking surrounding cardiac healing and many research efforts have been redirected to focus on the potential for cardiac regeneration4.
Until recently, in vivo study of cardiac regeneration was restricted to non-vertebrate models, such as those in urodele amphibians and teleost fish5-7. However, the discovery of the capacity for cardiac regeneration in the neonatal mouse has led to the development of two surgical models of mammalian cardiac regeneration: resection of the cardiac apex and coronary artery occlusion to induce myocardial infarction8,9. In 2011, a mouse apex resection model was used to demonstrate that complete cardiac regeneration is possible at postnatal day 1 (P1). However, this capacity declines rapidly after the initial neonatal period. The mammalian heart loses its regenerative potential shortly after birth at P7 as progenitor cell numbers decline, and cardiomyocytes become binucleated, lose their proliferative competency, and permanently exit the cell cycle10,11. Understanding the fundamental differences between the neonatal and adult mammalian heart may lead to novel insights into cardiac regeneration.
While apex resection indeed offers insight into re-growth of contractile tissue, the model does not simulate typical human cardiac injury, and thus does not lend itself as well to the development of therapeutics. The coronary artery occlusion model, however, more directly simulates the pathophysiologic aspects of MI pathology, and thus may provide more useful insights into mechanisms that may be applicable to therapeutic advancement for human use.
Surgical coronary ligation has been used as a useful experimental technique in many animal models12-14. In the adult coronary artery ligation model, animals are anesthetized and intubated to allow opening of the chest cavity while maintaining respiration. The heart continues to beat regularly, permitting visualization of the coronary vasculature and allowing for accurate suture placement. Furthermore, the heart remains pink as perfusion continues, and after ligation the ischemic myocardium appears pale, indicating successful coronary artery ligation. The protocol described for neonatal mice, however, is less reliable as the coronary artery is not visualized and the surgeon must estimate where to place the suture15. Although the general anatomy of the coronary vasculature is the same, individual animal variability in the direction and branching of the LAD exists16. Thus, when &#34;going in blind,&#34; the artery could be easily missed. Other techniques such as echocardiography are then required to confirm successful induction of MI, and to ensure all surgeries result in a similar infarct size. Described here is an improvement on a recently published method15, where the position of the LAD can be established and thus LAD may be ligated to reproducibly induce MI.
This technique does not require endotracheal intubation or mechanical ventilation, as thoracotomy in a hypothermic state in the neonatal mouse does not result in lung collapse. However, in the previously described method, severe hypothermia must be induced to the point of both complete apnea and cessation of the cardiac rhythm15. The major limitation of this approach is that the coronary artery is no longer perfused and the heart appears pale even before LAD ligation. In the approach described herein, coronary artery visualization is possible at a point of torpor before deep hypothermia and cardiac rhythm cessation, with full recovery of the neonatal mouse after the surgery. This method offers a major advantage of 100% reproducibility.

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			<td height="21" style="height:21px;width:261px;">8-0 Nylon Suture</td>
			<td style="width:244px;">Microsurgery Instruments</td>
			<td style="width:119px;">8-0 Nylon</td>
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			<td style="width:244px;">Shanghai Pudong Medical Products Co Ltd</td>
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			<td height="21" style="height:21px;width:261px;">Fine Scissors</td>
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			<td height="21" style="height:21px;width:261px;">Small forceps</td>
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			<td height="21" style="height:21px;width:261px;">Micro Needle Holder</td>
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			<td height="21" style="height:21px;width:261px;">Zeiss Opmi 6s/S3 Microscope</td>
			<td style="width:244px;">Zeiss</td>
			<td style="width:119px;">300002</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:261px;">Isoflurane</td>
			<td style="width:244px;">Baxter</td>
			<td style="width:119px;">CA2L9100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:261px;">Isoflurane Chamber</td>
			<td style="width:244px;">Made in Feng laboratory</td>
			<td style="width:119px;"></td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:261px;">Bead Sterilizer</td>
			<td style="width:244px;">Fine Science Tools</td>
			<td style="width:119px;">18000-45</td>
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			<td height="42" style="height:42px;width:261px;">2,3,5-Triphenyltetraolium chloride (TTC)</td>
			<td style="width:244px;">Sigma</td>
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			<td height="22" style="height:23px;width:261px;">Stereomicroscope SteREO Discovery. V8</td>
			<td style="width:244px;">Zeiss</td>
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			<td height="21" style="height:21px;width:261px;">AxioVision 8.0</td>
			<td style="width:244px;">Zeiss</td>
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			<td height="21" style="height:21px;width:261px;">Axiocam Icc5</td>
			<td style="width:244px;">Zeiss</td>
			<td style="width:119px;">426554</td>
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			<td height="21" style="height:21px;width:261px;">Heat pad</td>
			<td style="width:244px;">Sunbeam&#160;</td>
			<td style="width:119px;">731A0-CN</td>
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			<td height="21" style="height:21px;width:261px;">Sterile Gloves</td>
			<td style="width:244px;">VWR</td>
			<td style="width:119px;">414004-430</td>
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			<td height="21" style="height:21px;width:261px;">Gauze Sponges</td>
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			<td height="21" style="height:21px;width:261px;">Ice</td>
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myocardial infarction in neonatal mice, a model of cardiac regeneration

Myocardial infarction induced by coronary artery ligation has been used in many animal models as a tool to study the mechanisms of cardiac repair and regeneration, and to define new targets for therapeutics. For decades, models of complete heart regeneration existed in amphibians and fish, but a mammalian counterpart was not available. The recent discovery of a postnatal window during which mice possess regenerative capabilities has led to the establishment of a mammalian model of cardiac regeneration. A surgical model of mammalian cardiac regeneration in the neonatal mouse is presented herein. Briefly, postnatal day 1 (P1) mice are anesthetized by isoflurane and placed on an ice pad to induce hypothermia. After the chest is opened, and the left anterior descending coronary artery (LAD) is visualized, a suture is placed around the LAD to inflict myocardial ischemia in the left ventricle. The surgical procedure takes 10-15 min. Visualizing the coronary artery is crucial for accurate suture placement and reproducibility. Myocardial infarction and cardiac dysfunction are confirmed by triphenyl-tetrazolium chloride (TTC) staining and echocardiography, respectively. Complete regeneration 21 days post myocardial infarction is verified by histology. This protocol can be used to as a tool to elucidate mechanisms of mammalian cardiac regeneration after myocardial infarction.

The myocardial infarction procedure at P1 can be completed in 10 - 15 min and has a mortality rate of 7.8% (5 out of 64 pups). After surgery, mice recover from hypothermic anesthesia within the next 5 - 20 min (time of recovery depends on body temperature reached during anesthesia and speed of surgeon). When using P7 pups (for comparison with a non-regenerative myocardium), a longer period of cooling is required to reach torpor. P7 pups are much larger and have more difficulty recovering ...

Watch this Scientific Journal Video about Myocardial Infarction in Neonatal Mice, A Model of Cardiac Regeneration at JoVE.com

Myocardial Infarction in Neonatal Mice, A Model of Cardiac Regeneration

This protocol describes a highly reproducible model of cardiac regeneration by surgical induction of myocardial infarction in the left ventricle of postnatal day 1 mice. The method involves induction of hypothermic anesthesia and ligation of the left anterior descending coronary artery.

Myocardial infarction induced by coronary artery ligation has been used in many animal models as a tool to study the mechanisms of cardiac repair and ...

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