This research was funded by Additional Ventures - Single Ventricle Research Fund (SVRF) and Single Ventricle Expansion Fund (to I.F.) and a Marietta Blau scholarship of the OeAD-GmbH from funds provided by the Austrian Federal Ministry of Education, Science and Research BMBWFC (to G.G.).

All the animal procedures were conducted in accordance with the National Research Council. 2011. Guide for the Care and Use of Laboratory Animals: Eighth Edition. The animal protocols were reviewed and approved by the Institutional Animal Care and Use Committee at Boston Children&#39;s Hospital.
Prior to surgery, all the surgical instruments are steam-autoclaved, and modified Krebs-Henseleit buffer, with a final concentration of 22 mmol/L KCl, is prepared as a cardioplegic solution (Table 1). The solution is filter-sterilized and stored at 4 &#176;C overnight. A surgical microscope (12.5x) is required for the heterotopic neonatal rat heart transplantation procedure.
1. Preparation and anesthesia
<ol>
	<li>Use male/female Lewis rats with a weight of around 150 g (5-6 weeks of age) as recipients.</li>
	<li>To start, generously shave the rat&#39;s abdomen with a razor.</li>
	<li>Place the rat into an isoflurane chamber, and turn on the oxygen flow at 2 L/min with 2% isoflurane until the animal is properly sedated but still spontaneously breathing. Inject 45 mg/kg ketamine and 5 mg/kg xylazine intraperitoneally (IP), as well as 300 U/kg heparin. Confirm proper anesthetization with a toe pinch test. 
	NOTE: Carefully monitor the spontaneous breathing and heart rate through palpation of the chest to assure a stable hemodynamic status throughout the entire process.</li>
	<li>For intubation, place the rat on an oblique shelf (Figure 1), secure the front teeth with a string, and place the head facing toward the surgeon.</li>
	<li>Place the light on the outside of the neck onto the area of the vocal cords, grab the tongue with two fingers, and slightly push it upward and to the left to provide optimal vision for intubation. Use an 18 G, 2 in cannula for a 100-150 g rat. Secure the intratracheal tube with tape. 
	NOTE: Surgical loups with 3.5x magnification are recommended for intubation.</li>
	<li>Connect the intubation cannula to the small animal ventilator, and adjust the settings according to the manufacturer&#39;s instructions based on the animal size. 
	NOTE: Use the following settings for a 150 g rat: volume mode; respiratory rate, 55/min; tidal volume, 1.3 mL 50 % I/E ratio, but this can be adjusted appropriately as needed. Assure proper bilateral and equal chest movement, and administer isoflurane continuously at 0.5%&#8211;2% through the ventilator.</li>
	<li>Place the rat on a heating pad (to maintain normal body temperature) in a supine position with the tail facing toward the surgeon. Sterilize the abdomen three times with betadine solution and 70% of ethanol alternatingly. Administer eye lube, and cover the rat with a sterile surgical drape, leaving the abdomen uncovered.</li>
</ol>
2. Surgical preparation and heterotopic transplantation of the neonatal donor heart in the recipient rat
<ol>
	<li>Perform a midline laparotomy using a 15 blade scalpel for the skin incision, and use scissors to open the anterior abdominal wall, followed by blunt exposure of the retroperitoneal abdominal aorta and inferior vena cava (IVC) with cotton tip applicators.</li>
	<li>Mobilize the intestines (including the descending colon), and place them toward the right upper quadrant. Cover the intestines with warm saline-soaked gauze. Use retractors to ensure optimal exposure of the IVC and abdominal aorta.</li>
	<li>Perform blunt dissection of the infrarenal IVC and abdominal aorta up toward the bifurcation. Ligate all the infrarenal branching arteries and veins (e.g., inferior mesenteric artery and lymph node arteries) with a 10-0 nylon suture. 
	NOTE: There is great variability in the anatomy of these side branches. Monitor the aorta&#39;s pulse and heart rate visually when no other hemodynamic monitoring is available. Assess the proper depth of anesthesia every 15 min through a toe pinch test. Adjust the isoflurane concentration accordingly.</li>
	<li>After the donor heart is harvested from a neonatal rat, deliver the excised heart in sterile conditions in a surgical basin containing Krebs-Henseleit buffer to the surgical field. Irrigate the donor heart intermittently with ice-cold cardioplegic solution. 
	NOTE: When a second surgeon is available, the heart should be prepared at the same time, as a second surgeon reduces the total anesthesia time of the recipient animal and the ischemia time of the donor heart. When a second surgeon is not available, cover the recipient&#39;s abdomen with warm saline, and monitor the animal during the harvesting procedure.</li>
	<li>Apply four small atraumatic vascular clamps to the distal and proximal segments of the infrarenal aorta and IVC. If needed, temporarily occlude an unfavorable renal vessel with a 7-0 silk suture, and release the suture after the procedure. Place a 10-0 nylon suture vertically onto the anterior wall of the aorta to facilitate the aortotomy. Perform an aortotomy with two small horizontal cuts (wedge-shaped) with microscissors by slightly pulling up the suture. 
	NOTE: To remove any blood clots, flushing of the aortic lumen with heparinized saline is recommended.</li>
	<li>Place the donor heart on the left side (from the animal&#8217;s perspective) of the aorta and secure the recipient&#8217;s infrarenal aorta and the donor&#8217;s ascending aorta end-to-side at the 12 o&#8217;clock and 6 o&#8217;clock positions of the aortotomy with sutures. Continue with the third and fourth sutures at the 3 o&#8217;clock and 9 o&#8217;clock positions, gently flipping the heart over to the right side of the aorta after the third suture. Complete the arterial anastomosis by adding one to two sutures to every interspace. 
	NOTE: Care should be taken to avoid touching either the donor&#39;s ascending aorta or the recipient&#39;s abdominal aorta with forceps when creating the anastomosis to avoid tissue damage.</li>
	<li>Rotate the rat counterclockwise, with the head facing toward the surgeon&#39;s left hand. Move the donor&#39;s aorta to the left side of the abdominal aorta to allow optimal sight onto the IVC.</li>
	<li>Perform a venotomy on the IVC, slightly proximal to the aortic anastomosis, using an 11 blade for puncture and microscissors for adequate size adjustment according to the diameter of the donor&#39;s pulmonary trunk. Again, flush the intracaval lumen with heparinized saline.</li>
	<li>Start with the venous anastomosis between the recipient&#39;s IVC and donor&#39;s pulmonary trunk, which is best achieved by placing interrupted 11-0 nylon sutures on the back wall of the vessel, starting at the 12 o&#39;clock and 6 o&#39;clock positions (related to the IVC), and then place a continuous 11-0 nylon suture on the front wall (from the 6 o&#39;clock toward the 12 o&#39;clock position).</li>
	<li>Cover the anastomoses with small strips of an absorbable gelatin sponge, and remove the microvascular clamps starting distally. Use a cotton tip applicator to lightly compress the sponges to obtain optimal hemostasis.</li>
	<li>Observe the graft&#39;s coronary vessels filling at the time of the release of the distal microvascular clamps, and make sure that the donor heart starts beating immediately when the proximal clamp is released. 
	NOTE: The graft&#39;s viability can be scored from 0 to 4 intraoperatively according to a modified Stanford score19 to confirm adequate graft function.</li>
	<li>Place the intestines back into the abdomen by ensuring not to distort the arterial and venous anastomosis.</li>
	<li>Administer meloxicam (1mg/kg) and ethiqa XR (0.65 mg/kg) subcutaneously while the animal is fully anesthezised to ascertain postoperative analgesia. Then, close the abdominal wall with a continuous 5-0 absorbable vicryl suture before closing the skin with a 6-0 absorbable vicryl suture intracutaneously. 
	&#8203;NOTE: Guidance regarding common failures and troubleshooting is presented in Table 2.</li>
</ol>
3. Harvesting of the neonatal donor heart
<ol>
	<li>Place the neonatal donor rat in a chamber insufflated with isoflurane (2%) for sedation. Administer ketamine (75 mg/kg) and xylazine (5 mg/kg), as well as heparin (300 U/kg) intraperitoneally.</li>
	<li>Confirm the depth of anesthesia by toe pinch, and place the rat in a supine position with the tail facing toward you. Sterilize the entire thorax and abdominal wall with betadine and 70% ethanol three times alternatively. Cover the rat with a sterile surgical drape.</li>
	<li>Using a 12.5x surgical microscope, remove the entire anterior thoracic wall by starting with a horizontal incision using a 15 blade scalpel at the xyphoid followed by vertical incisions laterally up to the axillae on both sides with scissors. The anterior thoracic wall can then be removed by continuing with another horizontal incision right beneath the neck.</li>
	<li>Dissect the IVC, right and left superior vena cavae, and pulmonary vessels with scissors, and then encircle and ligate all the vessels with a 7-0 silk suture. Administer 3 mL of ice-cold, high-potassium modified Krebs-Henseleit solution to the right atrium by puncturing the IVC with a 30 G needle and slightly pushing the diaphragm down with forceps.</li>
	<li>Cut the IVC, SVCs, pulmonary vessels, and aorta with scissors. Transect the pulmonary arteries as far as possible and the aorta distal to the brachiocephalic trunk to ensure proper length using a 11 blade scalpel.</li>
	<li>Separate the pulmonary trunk and ascending aorta with microscissors, and flush the heart with ice cold cardioplegic solution using a 3 mL syringe.</li>
</ol>
4. Recovery of the recipient and graft monitoring
<ol>
	<li>After surgery, give the rat ample time to wake up, which usually occurs in a 15 min time window, and let it recover on a heating pad. 
	NOTE: No antibiotics are necessary due to the very low risk of infection and in order to not compromise the experimental model, and no restriction to food or water is applied.</li>
	<li>Following transplantation, monitor the graft function by palpation of the transplanted heart daily, but consider that this can sometimes be difficult to assess due to intestine overlay. 
	NOTE: Abdominal echocardiography can more accurately measure the graft viability. For echocardiography, sedate the rat slightly with isoflurane (1-2%) inhaled through a nose cone, and position it on a heating pad. Echocardiography is usually performed on postoperative day (POD) 1, POD 7, and POD 14. To allow for the assessment of the heart rate and contractility, one can easily obtain long-axis and short-axis views (Figure 2A, B). To evaluate the anastomoses, use Doppler echocardiography (Figure 3A), and confirm the formation of EFE tissue as seen as an echo-bright endocardial layer within the left ventricular cavity (Figure 3B, C).</li>
</ol>

In Vivo Heart Transplantation to Study Endothelial-to-Mesenchymal Transition

<ol>
	<li>Lurie, P. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Changing+concepts+of+endocardial+fibroelastosis.">Changing concepts of endocardial fibroelastosis.</a> Cardiology in the Young. 20 (2), 115-123 (2010).</li><li>Crucean, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Re-evaluation+of+hypoplastic+left+heart+syndrome+from+a+developmental+and+morphological+perspective.">Re-evaluation of hypoplastic left heart syndrome from a developmental and morphological perspective.</a> Orphanet Journal of Rare Diseases. 12 (1), 138 (2017).</li><li>Xu, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endocardial+fibroelastosis+is+caused+by+aberrant+endothelial+to+mesenchymal+transition.">Endocardial fibroelastosis is caused by aberrant endothelial to mesenchymal transition.</a> Circulation Research. 116 (5), 857-866 (2015).</li><li>Eisenberg, L. M., Markwald, R. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+regulation+of+atrioventricular+valvuloseptal+morphogenesis.">Molecular regulation of atrioventricular valvuloseptal morphogenesis.</a> Circulation Research. 77 (1), 1-6 (1995).</li><li>Illigens, B. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vascular+endothelial+growth+factor+prevents+endothelial-to-mesenchymal+transition+in+hypertrophy.">Vascular endothelial growth factor prevents endothelial-to-mesenchymal transition in hypertrophy.</a> Annals of Thoracic Surgery. 104 (3), 932-939 (2017).</li><li>Zeisberg, E. M., Potenta, S. E., Sugimoto, H., Zeisberg, M., Kalluri, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fibroblasts+in+kidney+fibrosis+emerge+via+endothelial-to-mesenchymal+transition.">Fibroblasts in kidney fibrosis emerge via endothelial-to-mesenchymal transition.</a> Journal of the American Society of Nephrology. 19 (12), 2282-2287 (2008).</li><li>Zeisberg, E. M., Potenta, S., Xie, L., Zeisberg, M., Kalluri, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Discovery+of+endothelial+to+mesenchymal+transition+as+a+source+for+carcinoma-associated+fibroblasts.">Discovery of endothelial to mesenchymal transition as a source for carcinoma-associated fibroblasts.</a> Cancer Research. 67 (21), 10123-10128 (2007).</li><li>Souilhol, C., Harmsen, M. C., Evans, P. C., Krenning, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial-mesenchymal+transition+in+atherosclerosis.">Endothelial-mesenchymal transition in atherosclerosis.</a> Cardiovascular Research. 114 (4), 565-577 (2018).</li><li>Zeisberg, E. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial-to-mesenchymal+transition+contributes+to+cardiac+fibrosis.">Endothelial-to-mesenchymal transition contributes to cardiac fibrosis.</a> Nature Medicine. 13 (8), 952-961 (2007).</li><li>Rieder, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inflammation-induced+endothelial-to-mesenchymal+transition:+A+novel+mechanism+of+intestinal+fibrosis.">Inflammation-induced endothelial-to-mesenchymal transition: A novel mechanism of intestinal fibrosis.</a> American Journal of Pathology. 179 (5), 2660-2673 (2011).</li><li>Johnson, F. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anoxia+as+a+cause+of+endocardial+fibroelastosis+in+infancy.">Anoxia as a cause of endocardial fibroelastosis in infancy.</a> AMA Archives of Pathology. 54 (3), 237-247 (1952).</li><li>Shimada, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distention+of+the+immature+left+ventricle+triggers+development+of+endocardial+fibroelastosis:+An+animal+model+of+endocardial+fibroelastosis+introducing+morphopathological+features+of+evolving+fetal+hypoplastic+left+heart+syndrome.">Distention of the immature left ventricle triggers development of endocardial fibroelastosis: An animal model of endocardial fibroelastosis introducing morphopathological features of evolving fetal hypoplastic left heart syndrome.</a> Biomedical Research. 2015, 462-469 (2015).</li><li>Weixler, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+disturbances+and+the+development+of+endocardial+fibroelastosis.">Flow disturbances and the development of endocardial fibroelastosis.</a> Journal of Thoracic and Cardiovascular Surgery. 159 (2), 637-646 (2020).</li><li>Purevjav, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nebulette+mutations+are+associated+with+dilated+cardiomyopathy+and+endocardial+fibroelastosis.">Nebulette mutations are associated with dilated cardiomyopathy and endocardial fibroelastosis.</a> Journal of the American College of Cardiology. 56 (18), 1493-1502 (2010).</li><li>Friehs, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+animal+model+of+endocardial+fibroelastosis.">An animal model of endocardial fibroelastosis.</a> Journal of Surgical Research. 182 (1), 94-100 (2013).</li><li>Emani, S. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Staged+left+ventricular+recruitment+after+single-ventricle+palliation+in+patients+with+borderline+left+heart+hypoplasia.">Staged left ventricular recruitment after single-ventricle palliation in patients with borderline left heart hypoplasia.</a> Journal of the American College of Cardiology. 60 (19), 1966-1974 (2012).</li><li>Hickey, E. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+left+ventricular+outflow+tract+obstruction:+The+disproportionate+impact+of+biventricular+repair+in+borderline+cases.">Critical left ventricular outflow tract obstruction: The disproportionate impact of biventricular repair in borderline cases.</a> Journal of Thoracic and Cardiovascular Surgery. 134 (6), 1429-1436 (2007).</li><li>Oh, N. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Abnormal+flow+conditions+promote+endocardial+fibroelastosis+via+endothelial-to-mesenchymal+transition,+which+is+responsive+to+losartan+treatment.">Abnormal flow conditions promote endocardial fibroelastosis via endothelial-to-mesenchymal transition, which is responsive to losartan treatment.</a> JACC: Basic to Translational Science. 6 (12), 984-999 (2021).</li><li>Blanchard, J. M., Pollak, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Techniques+for+perfusion+and+storage+of+heterotopic+heart+transplants+in+mice.">Techniques for perfusion and storage of heterotopic heart transplants in mice.</a> Microsurgery. 6 (3), 169-174 (1985).</li><li>Kokudo, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Snail+is+required+for+TGFbeta-induced+endothelial-mesenchymal+transition+of+embryonic+stem+cell-derived+endothelial+cells.">Snail is required for TGFbeta-induced endothelial-mesenchymal transition of embryonic stem cell-derived endothelial cells.</a> Journal of Cell Science. 121 (20), 3317-3324 (2008).</li><li>Wu, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endocardial+cells+form+the+coronary+arteries+by+angiogenesis+through+myocardial-endocardial+VEGF+signaling.">Endocardial cells form the coronary arteries by angiogenesis through myocardial-endocardial VEGF signaling.</a> Cell. 151 (5), 1083-1096 (2012).</li><li>Clark, E. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+mouse+model+of+endocardial+fibroelastosis.">A mouse model of endocardial fibroelastosis.</a> Cardiovascular Pathology. 24 (6), 388-394 (2015).</li><li>Kovacic, J. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial+to+mesenchymal+transition+in+cardiovascular+disease:+JACC+state-of-the-art+review.">Endothelial to mesenchymal transition in cardiovascular disease: JACC state-of-the-art review.</a> Journal of the American College of Cardiology. 73 (2), 190-209 (2019).</li><li>Derynck, R., Zhang, Y. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Smad-dependent+and+Smad-independent+pathways+in+TGF-beta+family+signalling.">Smad-dependent and Smad-independent pathways in TGF-beta family signalling.</a> Nature. 425 (6958), 577-584 (2003).</li><li>Daugherty, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recommendation+on+design,+execution,+and+reporting+of+animal+atherosclerosis+studies:+A+scientific+statement+from+the+American+Heart+Association.">Recommendation on design, execution, and reporting of animal atherosclerosis studies: A scientific statement from the American Heart Association.</a> Arteriosclerosis, Thrombosis, and Vascular Biology. 37 (9), e131-e157 (2017).</li></ol>

This animal model of heterotopic transplantation of a neonatal donor rat heart into the recipient&#39;s abdomen creates the possibility to study EndMT-derived fibrosis through detailed histological tissue evaluation, identify regulatory signaling pathways, and test treatment options. Since EndMT is the underlying mechanism for fibrotic diseases of the heart, this model has great value in the field of pediatric cardiac surgery and beyond. In this model, many factors can negatively influence the outcome of the procedure. T...

Endocardial fibroelastosis (EFE), defined by the accumulation of collagen and elastic fibers in the subendocardial tissue, presents as a pearly or opaque thickened endocardium; EFE undergoes most active growth during the fetal period and early infancy1. In an autopsy study, 70% of cases with hypoplastic left heart syndrome (HLHS) were associated with the presence of EFE2.
Cells expressing markers for fibroblasts are the main cell population in EFE, but these cells also concomitantly express endocardial endothelial markers, which is an indication of the origin of these EFE cells. Our group previously established that the underlying mechanism of EFE formation involves a phenotypical change of endocardial endothelial cells to fibroblasts through endothelial-to-mesenchymal transition (EndMT)3. EndMT can be detected using immunohistochemical double-staining for endothelial markers such as cluster of differentiation (CD) 31 or vascular endothelial (VE)-cadherin (CD144) and fibroblast markers (e.g., alpha-smooth muscle actin, &#945;-SMA). Furthermore, we also previously established the regulatory role of the TGF-&#223; pathway in this process with activation of the transcription factors SLUG, SNAIL, and TWIST3.
EndMT is a physiological process that occurs during embryonic cardiac development and leads to the formation of the septa and valves from endocardial cushions4, but it also causes organ fibrosis in heart failure, kidney fibrosis, or cancer and plays a key role in vascular atherosclerosis5,6,7,8. EndMT in cardiac fibrosis is mainly regulated through the TGF-&#946; pathway, as we and others have reported3,9. Various stimuli have been described to induce EndMT: inflammation10, hypoxia11, mechanical alterations12, and flow disturbances, including alterations of the intracavitary blood flow13, and EndMT may also be a consequence of a genetic disease14.
This animal model was developed using the key components of cardiac EFE development, which are immaturity and alterations of the intracavitary blood flow, specifically flow stagnation. Immaturity was fulfilled by using neonatal rat hearts as donors, since neonatal rats are known to be developmentally immature immediately after birth. Heterotopic heart transplantation offered the provision of intracavitary flow restriction15.
From a clinical point of view, this animal model allows for better investigating the impact of EndMT on the growing left ventricle (LV). The growth restriction imposed on the fetal and neonatal heart through EndMT-induced EFE formation16 precludes patients with left ventricular outflow tract obstructions (LVOTO) such as congenital critical aortic stenosis and hypoplastic left heart syndrome (HLHS) from curative anatomical biventricular surgical repair17. This animal model facilitates the study of the cellular mechanisms and regulation of tissue formation through EndMT and allows for the testing of pharmacological treatment options3,18.
Transabdominal echocardiography is used to monitor the graft viability, contractility, and the patency of the anastomoses. Following euthanasia, EFE formation can be macroscopically observed within 3 days after transplantation. EFE tissue shows the same histopathological characteristics as human EFE tissue from patients with LVOTO.
Hence, this animal model, though developed for pediatric use in the spectrum of HLHS, can be applied when studying various diseases based on the molecular mechanism of EndMT.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Advanced Ventilator System For Rodents, SAR-1000</td>
			<td>CWE, Inc.</td>
			<td>12-03100</td>
			<td>small animal ventilator</td>
		</tr>
		<tr>
			<td>aSMA</td>
			<td>Sigma</td>
			<td>A2547</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>Axio observer Z1&#160;</td>
			<td>Carl Zeiss</td>
			<td></td>
			<td>inverted microscope</td>
		</tr>
		<tr>
			<td>Betadine&#160;Solution</td>
			<td>Avrio Health L.P.</td>
			<td>367618150092</td>
			<td></td>
		</tr>
		<tr>
			<td>CD31</td>
			<td>Invitrogen</td>
			<td>MA1-80069</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>DAPI</td>
			<td>Invitrogen</td>
			<td>D1306</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>DemeLON Nylon black 10-0</td>
			<td>DemeTECH</td>
			<td>NL76100065F0P</td>
			<td>10-0 Nylon suture</td>
		</tr>
		<tr>
			<td>ETFE IV Catheter, 18G x 2</td>
			<td>TERUMO SURFLO</td>
			<td>SR-OX1851CA</td>
			<td>intubation cannula</td>
		</tr>
		<tr>
			<td>Micro Clip 8mm</td>
			<td>Roboz Surgical Instrument Co.</td>
			<td>RS-6471</td>
			<td>microvascular clamps</td>
		</tr>
		<tr>
			<td>Nylon black monofilament 11-0</td>
			<td>SURGICAL SPECIALTIES CORP</td>
			<td>AA0130</td>
			<td>11-0 Nylon</td>
		</tr>
		<tr>
			<td>O.C.T. Compound</td>
			<td>Tissue-Tek</td>
			<td>4583</td>
			<td>Embedding medium for frozen tissue specimen</td>
		</tr>
		<tr>
			<td>p-SMAD2/3</td>
			<td>Invitrogen</td>
			<td>PA5-110155</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>Rodent, Tilting WorkStand</td>
			<td>Hallowell EMC.</td>
			<td>000A3467</td>
			<td>oblique shelf for intubation</td>
		</tr>
		<tr>
			<td>Silk Sutures, Non-absorbable, 7-0</td>
			<td>Braintree Scientific</td>
			<td>NC9201231</td>
			<td>Silk suture</td>
		</tr>
		<tr>
			<td>Slug/Snail</td>
			<td>Abcam</td>
			<td>ab180714</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>Undyed Coated Vicryl 5-0 P-3 18&#34;</td>
			<td>Ethicon</td>
			<td>J493G</td>
			<td>5-0 Vicryl</td>
		</tr>
		<tr>
			<td>Undyed Coated Vicryl 6-0 P-3 18&#34;</td>
			<td>Ethicon</td>
			<td>J492G</td>
			<td>6-0 Vicryl</td>
		</tr>
		<tr>
			<td>VE-Cadherin</td>
			<td>Abcam</td>
			<td>ab231227</td>
			<td>Antibody for Immunohistochemistry</td>
		</tr>
		<tr>
			<td>Zeiss OPMI 6-SFR</td>
			<td>Zeiss</td>
			<td></td>
			<td>Surgical microscope</td>
		</tr>
		<tr>
			<td>Zen, Blue Edition, 3.6</td>
			<td>Zen&#160;</td>
			<td></td>
			<td>inverted microscope software</td>
		</tr>
	</tbody>
</table>

a neonatal heterotopic rat heart transplantation model for the study of endothelial-to-mesenchymal transition

Endocardial fibroelastosis (EFE), defined by subendocardial tissue accumulation, has major impacts on the development of the left ventricle (LV) and precludes patients with congenital critical aortic stenosis and hypoplastic left heart syndrome (HLHS) from curative anatomical biventricular surgical repair. Surgical resection is currently the only available therapeutic option, but EFE often recurs, sometimes with an even more infiltrative growth pattern into the adjacent myocardium.
To better understand the underlying mechanisms of EFE and to explore therapeutic strategies, an animal model suitable for preclinical testing was developed. The animal model takes into consideration that EFE is a disease of the immature heart and is associated with flow disturbances, as supported by clinical observations. Thus, the heterotopic heart transplantation of neonatal rat donor hearts is the basis for this model.
A neonatal rat heart is transplanted into an adolescent rat&#39;s abdomen and connected to the recipient&#39;s infrarenal aorta and inferior vena cava. While perfusion of the coronary arteries preserves the viability of the donor heart, flow stagnation within the LV induces EFE growth in the very immature heart. The underlying mechanism of EFE formation is the transition of endocardial endothelial cells to mesenchymal cells (EndMT), which is a well-described mechanism of early embryonic development of the valves and septa but also the leading cause of fibrosis in heart failure. EFE formation can be macroscopically observed within days after transplantation. Transabdominal echocardiography is used to monitor the graft viability, contractility, and the patency of the anastomoses. Following euthanasia, the EFE tissue is harvested, and it shows the same histopathological characteristics as human EFE tissue from HLHS patients.
This in vivo model allows for studying the mechanisms of EFE development in the heart and testing treatment options to prevent this pathological tissue formation and provides the opportunity for a more generalized examination of EndMT-induced fibrosis.

Graft viability and beating 
In this work, the graft viability was visually assessed after all the clamps had been removed, and an approximate reperfusion time of 10-15 min was allowed with an open abdomen for observation of the graft. The same scoring system to objectively verify graft viability was used for visual assessment at the end of surgery and for the echocardiography on POD 1, POD 7, and POD 14.
0 = no organ function; 1 = (rest) organ function, only minimal cont...

Watch this Scientific Journal Video about A Neonatal Heterotopic Rat Heart Transplantation Model for the Study of Endothelial-to-Mesenchymal Transition at JoVE.com

Author Spotlight: A Neonatal Heterotopic Rat Heart Transplantation Model for the Study of Endothelial-to-Mesenchymal Transition  

This work presents an animal model of endothelial-to-mesenchymal transition-induced fibrosis, as seen in congenital cardiac defects such as critical aortic stenosis or hypoplastic left heart syndrome, which allows for detailed histological tissue evaluation, the identification of regulatory signaling pathways, and the testing of treatment options.

A Neonatal Heterotopic Rat Heart Transplantation Model for the Study of Endothelial-to-Mesenchymal Transition

Endocardial fibroelastosis (EFE), defined by subendocardial tissue accumulation, has major impacts on the development of the left ventricle (LV) and ...

a-neonatal-heterotopic-rat-heart-transplantation-model-for-study

Research

JoVE Journal

Medicine

1.1K Views.  Harvard Medical School. This work presents an animal model of endothelial-to-mesenchymal transition-induced fibrosis, as seen in congenital cardiac defects such as critical aortic stenosis or hypoplastic left heart syndrome, which allows for detailed histological tissue evaluation, the identification of regulatory signaling pathways, and the testing of treatment options. 

Video: Author Spotlight: A Neonatal Heterotopic Rat Heart Transplantation Model for the Study of Endothelial-to-Mesenchymal Transition  

Orthotopic Aortic Transplantation: A Rat Model to Study the Development of Chronic Vasculopathy

Research models of chronic rejection are essential to investigate pathobiological and pathophysiological processes during the development of transplant vasculopathy (TVP).
The commonly used animal model for cardiovascular chronic rejection studies is the heterotopic heart transplant model performed in laboratory rodents. This model is used widely in experiments since Ono and Lindsey (3) published their technique. To analyze the findings in the blood vessels, the heart has to be sectioned and all vessels have to be measured.
Another method to investigate chronic rejection in cardiovascular questionings is the aortic transplant model (1, 2). In the orthotopic aortic transplant model, the aorta can easily be histologically evaluated (2). The PVG-to-ACI model is especially useful for CAV studies, since acute vascular rejection is not a major confounding factor and Cyclosporin A (CsA) treatment does not prevent the development of CAV, similar to what we find in the clinical setting (4). A7-day period of CsA is required in this model to prevent acute rejection and to achieve long-term survival with the development of TVP.
This model can also be used to investigate acute cellular rejection and media necrosis in xenogeneic models (5).

This video demonstrates the orthotopic aortic transplant model as a simple model to study the development of transplant vasculopathy (TVP) in rats.

Research models of chronic rejection are essential to investigate pathobiological and pathophysiological processes during the development of transplant ...

Murine Heterotopic Heart Transplant Technique

It is now over forty years since this technique was first reported by Corry, Wynn and Russell. Although it took some years for other labs to become proficient in and utilize this technique, it is now widely used by many laboratories around the world. A significant refinement to the original technique was developed and reported in 2001 by Niimi. Described here are the techniques that have evolved over more than a decade in the hands of three surgeons (Plenter, Grazia, Pietra) in our center. These techniques are now being passed on to a younger generation of surgeons and researchers.
Based largely on the Niimi experience, the procedures used have evolved in the fine details - details which we will endeavor to relate here in such a way that others may be able to use this very useful model. Like Niimi, we have found that a video aid to learning is a priceless resource for the beginner.

The manuscript describes the steps required to perform the heterotopic heart transplant in the mouse.

It is now over forty years since this technique was first reported by Corry, Wynn and Russell. Although it took some years for other labs to become ...

Transplantation of Pulmonary Valve Using a Mouse Model of Heterotopic Heart Transplantation

Tissue engineered heart valves, especially decellularized valves, are starting to gain momentum in clinical use of reconstructive surgery with mixed results. However, the cellular and molecular mechanisms of the neotissue development, valve thickening, and stenosis development are not researched extensively. To answer the above questions, we developed a murine heterotopic heart valve transplantation model. A heart valve was harvested from a valve donor mouse and transplanted to a heart donor mouse. The heart with a new valve was transplanted heterotopically to a recipient mouse. The transplanted heart showed its own heartbeat, independent of the recipient&#8217;s heartbeat. The blood flow was quantified using a high frequency ultrasound system with a pulsed wave Doppler. The flow through the implanted pulmonary valve showed forward flow with minimal regurgitation and the peak flow was close to 100 mm/sec. This murine model of heart valve transplantation is highly versatile, so it can be modified and adapted to provide different hemodynamic environments and/or can be used with various transgenic mice to study neotissue development in a tissue engineered heart valve.

In order to understand the cellular and molecular mechanisms underlying neotissue formation and stenosis development in tissue engineered heart valves, a murine model of heterotopic heart valve transplantation was developed. A pulmonary heart valve was transplanted to recipient using the heterotopic heart transplantation technique.

Tissue engineered heart valves, especially decellularized valves, are starting to gain momentum in clinical use of reconstructive surgery with mixed ...

Method of Isolated Ex Vivo Lung Perfusion in a Rat Model: Lessons Learned from Developing a Rat EVLP Program

The number of acceptable donor lungs available for lung transplantation is severely limited due to poor quality. Ex-Vivo Lung Perfusion (EVLP) has allowed lung transplantation in humans to become more readily available by enabling the ability to assess organs and expand the donor pool. As this technology expands and improves, the ability to potentially evaluate and improve the quality of substandard lungs prior to transplant is a critical need. In order to more rigorously evaluate these approaches, a reproducible animal model needs to be established that would allow for testing of improved techniques and management of the donated lungs as well as to the lung-transplant recipient. In addition, an EVLP animal model of associated pathologies, e.g., ventilation induced lung injury (VILI), would provide a novel method to evaluate treatments for these pathologies. Here, we describe the development of a rat EVLP lung program and refinements to this method that allow for a reproducible model for future expansion. We also describe the application of this EVLP system to model VILI in rat lungs. The goal is to provide the research community with key information and &#8220;pearls of wisdom&#8221;/techniques that arose from trial and error and are critical to establishing an EVLP system that is robust and reproducible.

Method of Isolated Ex Vivo Lung Perfusion in a Rat Model: Lessons Learned from Developing a Rat EVLP Program

Ex-Vivo Lung Perfusion (EVLP) has allowed lung transplantation in humans to become more readily available by enabling the ability to assess organs and expand the donor pool. Here, we describe the development of a rat EVLP program and refinements that allow for a reproducible model for future expansion.

The number of acceptable donor lungs available for lung transplantation is severely limited due to poor quality. Ex-Vivo Lung Perfusion (EVLP) has allowed ...

Rat Heterotopic Abdominal Heart/Single-lung Transplantation in a Volume-loaded Configuration

Herein, we describe a novel technique for heterotopic abdominal heart-lung transplantation (HAHLT) in rats. The configuration of the transplant graft involves anastomosis of donor inferior vena cava (IVC) to recipient IVC, and donor ascending aorta (Ao) to recipient abdominal Ao. The right upper and middle lung lobes are preserved and function as conduits for blood flow from right heart to left heart.
There are several advantages to using this technique, and it lends itself to a broad range of applications. Because the graft is transplanted in a configuration that allows for dyamic volume-loading, cardiac function may be directly assessed in vivo. The use of pressure-volume conductance catheters permits characterization of load-dependent and load-independent hemodynamic parameters. The graft may be converted to a loaded configuration by applying a clamp to the recipient&#8217;s infra-hepatic IVC. We describe modified surgical techniques for both donor and recipient operations, and an ideal myocardial protection strategy. Depending on the experimental aim, this model may be adapted for use in both acute and chronic studies of graft function, immunologic status, and variable ventricular loading conditions. The conducting airways to the transplanted lung are preserved, and allow for acute lung re-ventilation. This facilitates analysis of the effects of the mixed venous and arterial blood providing coronary perfusion to the graft.
A limitation of this model is its technical complexity. There is a significant learning curve for new operators, who should ideally be mentored in the technique. A surgical training background is advantageous for those wishing to apply this model. Despite its complexity, we aim to present the model in a clear and easily applicable format. Because of the physiologic similarity of this model to orthotopic transplantation, and its broad range of study applications, the effort invested in learning the technique is likely to be worthwhile.

We describe a novel technique for heterotopic abdominal heart-lung transplantation (HAHLT) in rats. The transplant configuration results in a partially loaded graft circulation, allowing direct functional assessment. This model may be employed for acute or chronic studies of function and immunologic status of the transplanted graft.

Herein, we describe a novel technique for heterotopic abdominal heart-lung transplantation (HAHLT) in rats. The configuration of the transplant graft ...

Heterotopic Cervical Heart Transplantation in Mice

The heterotopic cervical heart transplantation in mice is a valuable tool in transplant and cardiovascular research. The cuff technique greatly simplifies this model by avoiding challenging suture anastomoses of small vessels thereby reducing warm ischemia time. In comparison to abdominal graft implantation the cervical model is less invasive and the implanted graft is easily accessible for further follow-up examinations. Anastomoses are performed by pulling the ascending aorta of the graft over the cuff with the recipient&#8217;s common carotid artery and by pulling the main pulmonary artery over the cuff with the external jugular vein. Selection of appropriate cuff size and complete mobilization of the vessels are important for successful revascularization. Ischemia-reperfusion (I/R) injury can be minimized by perfusing the graft with a cardioplegic solution and by hypothermia. In this article, we provide technical details for a simplified and improved cuff technique, which should allow surgeons with basic microsurgical skills to perform the procedure with a high success rate.

The cuff technique shortens and facilitates the model of heterotopic cervical heart transplantation in mice by avoiding technically challenging suture anastomoses of small vessels. The technical details shown in this video paper should allow researches to establish this model in their laboratories.

The heterotopic cervical heart transplantation in mice is a valuable tool in transplant and cardiovascular research. The cuff technique greatly simplifies ...

A Novel Microsurgical Model for Heterotopic, En Bloc Chest Wall, Thymus, and Heart Transplantation in Mice

Exploration of novel strategies in organ transplantation to prolong allograft survival and minimizing the need for long-term maintenance immunosuppression must be pursued. Employing vascularized bone marrow transplantation and co-transplantation of the thymus have shown promise in this regard in various animal models.1-11 Vascularized bone marrow transplantation allows for the uninterrupted transfer of donor bone marrow cells within the preserved donor microenvironment, and the incorporation of thymus tissue with vascularized bone marrow transplantation has shown to increase T-cell chimerism ultimately playing a supportive role in the induction of immune regulation. The combination of solid organ and vascularized composite allotransplantation can uniquely combine these strategies in the form of a novel transplant model. Murine models serve as an excellent paradigm to explore the mechanisms of acute and chronic rejection, chimerism, and tolerance induction, thus providing the foundation to propagate superior allograft survival strategies for larger animal models and future clinical application. Herein, we developed a novel heterotopic en bloc chest wall, thymus, and heart transplant model in mice using a cervical non-suture cuff technique. The experience in syngeneic and allogeneic transplant settings is described for future broader immunological investigations via an instructional manuscript and video supplement.

To study combined solid organ and vascularized composite allotransplantation, we describe a novel heterotopic en bloc chest wall, thymus, and heart transplant model in mice using a cervical non-suture cuff technique.

Exploration of novel strategies in organ transplantation to prolong allograft survival and minimizing the need for long-term maintenance immunosuppression ...

Isometric Contractility Measurement of the Mouse Mesenteric Artery Using Wire Myography

The wire myograph technique is used to assess the contractility of vascular smooth muscles in response to depolarization, GPCR agonists/inhibitors and drugs. It is widely used in many studies on the physiological functions of vascular smooth muscle, the pathogenesis of vascular diseases such as hypertension, and the development of smooth muscle relaxant drugs. The mouse is a widely used model animal with a large pool of disease models and genetically modified strains. We introduced this method to measure the isometric contraction of mouse mesenteric artery in detail. A 1.4-mm segment of mouse mesenteric resistance artery was isolated and mounted on a myograph chamber by passing two steel wires through its lumen. After equilibration and normalization steps, the vessel segment was potentiated by a high-K+ solution twice prior to the contraction assay. As an example of the application of this method in drug development, we measured the relaxant effect of a novel natural substance, neoliensinine, isolated from a Chinese herb, embryos of the lotus seed (Nelumbo nucifera Gaertn.) on mouse mesenteric arteries. The vessel segments mounted on the myograph chamber were stimulated with a high-K+ solution. When the force tension reached a stable sustained phase, cumulative doses of neoliensinine were added to the chamber. We found that neoliensinine had a dose-dependent relaxant effect on smooth muscle contraction, thus suggesting that it bears potential activity against hypertension. In addition, as the vessel segment can survive at least 4 hours after mounting and maintain contractility induced by the high-K+ solution for many times, we suggest that the wire myograph system may be used for the time-consuming process of drug screening.

The wire myograph technique is used to investigate vascular smooth muscle functions and screen new drugs. We report a detailed protocol for measuring the isometric contractility of the mouse mesenteric artery and for screening new relaxants of vascular smooth muscle.

The wire myograph technique is used to assess the contractility of vascular smooth muscles in response to depolarization, GPCR agonists/inhibitors and ...

A Simplified Model for Heterotopic Heart Valve Transplantation in Rodents

There is an urgent clinical need for heart valve replacements that can grow in children. Heart valve transplantation is proposed as a new type of transplant with the potential to deliver durable heart valves capable of somatic growth with no requirement for anticoagulation. However, the immunobiology of heart valve transplants remains unexplored, highlighting the need for animal models to study this new type of transplant. Previous rat models for heterotopic aortic valve transplantation into the abdominal aorta have been described, though they are technically challenging and costly. For addressing this challenge, a renal subcapsular transplant model was developed in rodents as a practical and more straightforward method for studying heart valve transplant immunobiology. In this model, a single aortic valve leaflet is harvested and inserted into the renal subcapsular space. The kidney is easily accessible, and the transplanted tissue is securely contained in a subcapsular space that is well vascularized and can accommodate a variety of tissue sizes. Furthermore, because a single rat can provide three donor aortic leaflets and a single kidney can provide multiple sites for transplanted tissue, fewer rats are required for a given study. Here, the transplantation technique is described, providing a significant step forward in studying the transplant immunology of heart valve transplantation.

This protocol describes a simple and efficient method for the transplantation of aortic valve leaflets under the renal capsule to allow for the study of alloreactivity of heart valves.

There is an urgent clinical need for heart valve replacements that can grow in children. Heart valve transplantation is proposed as a new type of ...

A Porcine Heterotopic Heart Transplantation Protocol for Delivery of Therapeutics to a Cardiac Allograft

Cardiac transplantation is the gold standard treatment for end-stage heart failure. However, it remains limited by the number of available donor hearts and complications such as primary graft dysfunction and graft rejection. The recent clinical use of an ex vivo perfusion device in cardiac transplantation introduces a unique opportunity for treating cardiac allografts with therapeutic interventions to improve function and avoid deleterious recipient responses. Establishing a translational, large-animal model for therapeutic delivery to the entire allograft is essential for testing novel therapeutic approaches in cardiac transplantation. The porcine, heterotopic heart transplantation model in the intraabdominal position serves as an excellent model for assessing the effects of novel interventions and the immunopathology of graft rejection. This model additionally offers long-term survival for the pig, given that the graft is not required to maintain the recipient's circulation. The aim of this protocol is to provide a reproducible and robust approach for achieving ex vivo delivery of a therapeutic to the entire cardiac allograft prior to transplantation and provide technical details to perform a survival heterotopic transplant of the ex vivo perfused heart.

We present a protocol for utilizing a normothermic ex vivo sanguinous perfusion system for the delivery of therapeutics to an entire cardiac allograft in a porcine heterotopic heart transplant model.

Cardiac transplantation is the gold standard treatment for end-stage heart failure. However, it remains limited by the number of available donor hearts ...