We are grateful for expert technical assistance from the UNC Flow Cytometry Core and UNC Microscopy Core. We thank members of the Qian lab and the Liu lab for helpful discussions and critical reviews of the manuscript. This study was supported by NIH/NHLBI R00 HL109079 grant to Dr. Liu and American Heart Association (AHA) Scientist Development Grant 13SDG17060010 and the Ellison Medical Foundation (EMF) New Scholar Grant AG-NS-1064-13 to Dr. Qian.

The protocol outlined here uses neonatal mice. Animal care and experiments are performed in accordance with the guidelines established by The Division of Laboratory Animal Medicine (DLAM) at University of North Carolina, Chapel Hill.
1. Preparation of Buffers and Media
<ol>
	<li>Prepare fibroblast (FB) medium: Supplement 500 ml IMDM media with 100 ml Fetal Bovine Serum (FBS) and 6 ml penicillin/streptomycin.</li>
	<li>Prepare iCM medium: Supplement 400 ml DMEM media with 100 ml M199 media and 50 ml FBS.</li>
	<li>Prepare B27 medium: Supplement 490 ml RPMI1640 media with 10&#160;ml B27 media.</li>
	<li>Prepare Plat-E culture medium: Supplement 500 ml DMEM media with 50 ml FBS, 5 ml penicillin/streptomycin and 5 ml Non-essential amino acids.</li>
	<li>Prepare Plat-E transfection medium: Supplement 500 ml DMEM media with 50 ml FBS and 5 ml Non-essential amino acids.</li>
	<li>Prepare Gelatin coated 24 well plate: Add 0.5 ml 0.1% gelatin solution to a well of 24 well plate, incubate at 37 &#176;C for at least 10 min and aspirate right before use.</li>
	<li>Prepare FACS labeling buffer: Supplement 500 ml DPBS with 10 ml FBS and 2 ml EDTA (0.5 M) to reach a final concentration of 2 mM. Pass through a 0.22&#160;&#181;m filter and store at 4 &#176;C.</li>
	<li>Prepare MACS sorting buffer: Supplement 500 ml DPBS with 2.5 g BSA and 2 ml EDTA (0.5 M) to reach a final concentration of 2 mM. Pass through a 0.22 &#181;m filter and store at 4 &#176;C.</li>
	<li>Prepare 2x HBS buffer for transfection: supplement 500 ml HEPES buffer (50 mM) with 280 mM NaCl (8.1816 g), 10 mM KCl (0.37275 g), 1.5 mM Na2HPO4 (0.10647 g), 12 mM Glucose (1.08096 g). Add around 650 &#181;l 10 N NaOH to adjust pH to 7.05-7.12. Filter through a 0.22 &#181;m pore filter and store at 4 &#176;C.</li>
	<li>Prepare 2.5 M CaCl2 solution for transfection: Add 18.376 g CaCl2 to 50 ml ddH2O. Filter through a 0.22 &#181;m pore filter and store at 4 &#176;C.</li>
	<li>Prepare ICC (immunocytochemistry) fixation buffer: Add 5 ml paraformaldehyde (PFA, 32%) to 35 ml DPBS to reach a final concentration of 4%.</li>
	<li>Prepare ICC permeabilization buffer: Add 0.1 ml Triton to 100 ml DPBS to reach a final concentration of 0.1%.</li>
	<li>Prepare ICC blocking buffer: Add 5 g Bovine serum albumin (BSA) to 100 ml DPBS to reach a final concentration of 5%.</li>
	<li>Prepare ICC staining buffer: Add 1 g BSA to 100 ml DPBS to reach a final concentration of 1%.</li>
</ol>
2. Generation of Neonatal Mouse Cardiac Fibroblasts
<ol style="margin-left: 40px;">
	<li>Generate cardiac fibroblasts from explant culture method
	<ol>
		<li>Clean neonatal &#945;MHC-GFP transgenic mouse (P1 to P2) with 75% ethanol. Cut the head off with sterile scissors. Then make a horizontal incision from under one armpit to the other. Use a sterile blunt and bent forceps to dissect out the heart and place it in one well of the 24 well plate containing ice-cold PBS buffer.
		<ol>
			<li>Alternatively, squeeze out the heart after the horizontal incision.</li>
		</ol>
		</li>
		<li>Check GFP expression in heart by fluorescent microscope. Since the GFP expression is driven by &#945;MHC promoter that is a cardiac specific promoter, the heart from transgenic mouse should be in green15, while the negative heart is dim in any fluorescence.</li>
		<li>Take 3-4 GFP positive hearts into a 60 mm center well culture dish and mince them into small pieces less than 1 mm3 in size by sterile scissors and forceps.</li>
		<li>Place 3-4 minced hearts into one 10 cm dish with 2 ml FB media. Let the tissues settle down for 3 hr.</li>
		<li>Slowly add 8 ml pre-warmed FB media to the dishes containing heart tissues. Do not disturb the tissues for three days.</li>
		<li>Replace the media every three days.</li>
		<li>On day 7, aspirate the culture medium and wash cells with DPBS. Add 3 ml 0.05% trypsin to each plate and digest at 37 &#176;C for 5 min.</li>
		<li>Add 5 ml FB media to quench trypsin. Gently detach the cells with a cell scraper. Pipette the media up and down to further mechanically dissociate the tissue.</li>
		<li>Collect cells and filter through 40 &#181;m cell strainers to avoid contamination of heart tissue fragments, and then pellet cells by spinning at 200 x g for 5 min.</li>
		<li>Wash cells once with MACS buffer and cells are ready for sorting.</li>
	</ol>
	</li>
	<li>Generate cardiac fibroblasts from enzyme digestion method
	<ol>
		<li>Harvest GFP positive hearts as 2.1.1. Transfer all the hearts into a 10 cm dish with 10 ml ice-cold DPBS.</li>
		<li>Squeeze ventricles with sterile forceps to remove blood and rinse once with ice-cold DPBS.</li>
		<li>Trim the hearts to be free of other tissues and fat.</li>
		<li>Cut each heart into roughly 4 pieces that are still loosely connected.</li>
		<li>If less than 20 hearts, transfer the hearts into a 15 ml conical tube with 8 ml warm 0.05% Trypsin-EDTA, and incubate at 37 &#176;C for 15 min.
		<ol>
			<li>If there are 20-30 hearts, transfer the hearts into a 50 ml conical tube with 10 ml warm 0.05% Trypsin-EDTA, and incubate at 37 &#176;C for 15 min.</li>
		</ol>
		</li>
		<li>Discard the trypsin and add 5 ml (for less than 20 hearts) or 10 ml (for 20-30 hearts) warm type II collagenase (0.5 mg/ml) in HBSS.</li>
		<li>Vortex the tube on vortexer for 1 min at a speed level around 4 to 6 (if the liquid does not get up to the lid, then the speed is fine).</li>
		<li>Incubate the tube in 37 &#176;C water bath for 3-5 min.</li>
		<li>Vortex the tube for 1 min.</li>
		<li>Sediment for 1 min by gravity until the tissue pieces settle down, collect the liquid into a new tube containing 5 ml cold FB medium.</li>
		<li>Repeat steps 2.2.6-2.2.10 3-5 times.</li>
		<li>Combine all the collections by filtering through 40 &#181;m cell strainer to make single cell suspension.</li>
		<li>Centrifuge at 200 x g for 5 min at 4 &#176;C.</li>
		<li>Resuspend cells in 10 ml MACS buffer, and centrifuge at 200 x g for 5 min at 4 &#176;C.</li>
		<li>Optional: If there are a lot of blood cells, resuspend cells with 1 ml RBC lysis buffer (150 mM NH4Cl, 10 mM KHCO3, and 0.1 mM EDTA), keep on ice for 1 min, then dilute with 10 ml MACS buffer and centrifuge at 200 x g for 5 min. Wash one more time with MACS buffer.</li>
	</ol>
	</li>
	<li>Isolation of Thy1.2+ fibroblasts by MACS (magnetic-activated cell sorting)
	<ol>
		<li>Determine viable cell number through trypan blue staining.
		<ol>
			<li>Take 10 &#181;l cells out from 10 ml&#160;cell suspension in step 2.2.14. Mix it with 10 &#181;l 0.4% trypan blue solution.</li>
			<li>Add the mixture to a hemocytometer. Allow it to stand for 3-5 min and then examine immediately under a microscope. The dead cells are stained in blue and viable cells are unstained.</li>
			<li>Count all the viable cells in four 1 mm corner squares of the hemocytometer. Determine the viable cell number: the average count per square x 2 (dilution factor) x 104 x 10 (total volume of cell suspension).</li>
		</ol>
		</li>
		<li>For less than 1 x 107 cells, resuspend cells with 10 &#181;l Thy1.2 microbeads in 90 &#181;l chilled MACS buffer. Add more beads proportionally if there are more than 1 x 107 cells. Mix well and incubate in a refrigerator (2-8 &#176;C) for 30 min.</li>
		<li>Add 10 ml&#160;MACS buffer and spin samples at 200 x g for 5 min; wash once with 10 ml MACS buffer and centrifuge again.</li>
		<li>Bring volume to 2 ml in MACS buffer.</li>
		<li>Set up a MACS Separator in hood. Insert an LS column to the separator. Apply 3 ml MACS buffer to the column to equilibrate it.</li>
		<li>Pass the cell suspension through the equilibrated LS column.</li>
		<li>Wash 3 times with 2 ml MACS buffer each time.</li>
		<li>Take the LS column off the Separator and elute 3 times with 2 ml MACS buffer into a new 50 ml tube.</li>
		<li>Spin at 200 x g for 5 min.</li>
		<li>Resuspend cells in 10 ml FB media, count the cells and seed into plates at proper density. For fibroblasts generated from explant culture method, the seeding cells density could be around 2-3 x 104 cells/well of 24 well plate. For fibroblasts generated from enzyme digestion method, the cells density could be around 4-5 x 104 cells/well of 24 well plate.</li>
	</ol>
	</li>
</ol>
3. Generation of Retrovirus for iCM Reprogramming
Note: Perform the following steps in a BSL2 Biological Safety Cabinet under sterile conditions. The proper disposal of transfected cells, pipette tips and tubes is recommended to avoid risk of environmental and health hazards.
<ol style="margin-left: 40px;">
	<li>Maintain Plat-E cells in Plat-E media supplemented with 1 &#181;g/ml puromycin and 10 &#181;g/ml blasticidin at 37 &#176;C with 5% CO2.</li>
	<li>On day 2, replace the medium by Plat-E culture medium lacking puromycin and blasticidin.</li>
	<li>On day 0, split Plat-E cells into 10 cm dishes the day before transfection at approximately 4-5 x 106 cells per plate. Cells should be ~80-90% confluent on the day of transfection.</li>
	<li>On day 1, change culture media to transfection media on cells within 1 hr prior to transfection. Transfection procedure is as follows:
	<ol>
		<li>Transfection using commercial kits
		<ol>
			<li>Mix 20 &#181;l transfection reagent (e.g. Lipofectamine) and 500 &#181;l&#160;reduced serum media (e.g., Opti-MEM). Add 10 &#181;g of the retroviral plasmid to another 500 &#181;l reduced serum media. Incubate each mixture at room temperature (RT) for 5 min.</li>
			<li>Slowly add the plasmid mixture to transfection mixture. This step may take 15-30 sec. Incubate the mixture for 20 min at RT (solution may appear cloudy).</li>
			<li>Add mixture drop wise to the cells.</li>
			<li>Incubate overnight in a 37 &#176;C incubator.</li>
		</ol>
		</li>
		<li>Transfection using Calcium Phosphate.
		<ol>
			<li>Mix 40 &#181;l 2.5 M CaCl2 and 440 &#181;L ddH2O, then add 20 &#181;g of the retroviral plasmid (adjust the amount of ddH2O so that total volume is 500 &#181;l).</li>
			<li>Add 500 &#181;l 2x HBS to a 15 ml conical tube.</li>
			<li>Vortex the HBS and meanwhile add the Calcium-DNA mixture to the HBS drop wise. This step takes around 30 sec for each sample. So maximally do 3-4 samples simultaneously.</li>
			<li>Incubate at RT for 3 min Incubation for longer time leads to larger precipitate and less cells will take up the precipitate.</li>
			<li>Add the mixture drop wise to the cells and observe under 20X microscope. Fine precipitates (a lot of small ones are good but a few large ones are not good) should be seen.</li>
			<li>Incubate overnight at 37 &#176;C incubator.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>On day 2, change media on cells with 8 ml prewarmed culture media. Incubate for 24 hr.</li>
	<li>On day 3 and day 4, collect culture supernatant from the dishes by using a 10 ml sterile disposable syringe, filtering it through a 0.45 &#181;m pore size cellulose acetate filter, and transferring into a 50 ml tube. Add 2 ml of retrovirus precipitation solution to every 8 ml virus-containing supernatant. Gently mix it and incubate overnight at 4 &#176;C.</li>
	<li>On day 5, spin the viral mixture at 1,500 &#215; g at 4 &#176;C for 30 min. The viral particles should appear in white pellet at the bottom of the tube.</li>
	<li>Discard the supernatant. Spin down residual solution by centrifugation at 1,500 &#215; g for 5 min. Remove all traces of fluid by aspiration, taking great care not to disturb the precipitated retroviral particles in pellet.</li>
	<li>Resuspend retroviral pellets with 100 &#181;l DPBS buffer for every 8 ml viral supernatant. Aliquot the virus on ice. Use immediately or store at -80 &#176;C .</li>
</ol>
4. Reprogramming of Cardiac Fibroblasts

<ol>
	<li>Add 4 &#181;l of 10 mg/ml polybrene solution into the 10 ml iCM medium, and mix gently by pipetting up and down. The final concentration of polybrene is 4 &#181;g/ml.</li>
	<li>Add 500 &#181;l iCM media containing polybrene to each well of 24 well plate seeded with fibroblasts.</li>
	<li>Add 10 &#181;l retrovirus to each well containing either 2-3 x 104 cells from explant culture or 4-5 x 104 cells from enzyme digestion method. The amount of virus should be proportionally increased with increased cell number in different culture plates or dishes. Put the plate in a 37 &#176;C incubator for 24-48 hr to allow the viral infection. The time line for cardiac reprogramming is shown in Figure 1.</li>
	<li>After viral transduction, replace virus containing media with 500 &#181;l regular iCM media.</li>
	<li>Change media every 2-3 days. For positive selection of viral transduced cells, add iCM media supplemented with puromycin at 2 &#181;g/ml to target cells. Keep it for three days. After that, maintain puromycin in the iCM medium at the concentration of 1 &#181;g/ml.</li>
	<li>Three days after viral infection, take the plate out from the incubator and place it under an inverted fluorescent microscope (20X) to observe GFP expression. 
	Note: Ten days after viral infection, cells could be harvested for FACS analysis and ICC analysis to determine the reprogramming efficiency (in step 5 and 6).</li>
	<li>Fourteen days after viral infection, replace iCM media with B27 media. Change media every three days. Spontaneous beating cell loci may appear from three (cells from enzyme digestion method) or four (cells from explant culture method) weeks after viral transduction.</li>
</ol>
5. Immunocytochemical Analysis of Reprogramming Efficiency
<ol>
	<li>Rinse cells with ice cold PBS three times; remove excess solution.</li>
	<li>Add 0.5 ml ICC fixation buffer to each well of 24 well plates. Fix the cells at RT for 15-20 min.</li>
	<li>Rinse cells with PBS three times.</li>
	<li>Add 0.5 ml ICC permeabilization buffer to each well of the 24 well plates. Permeabilize the cells at RT for 15 min.</li>
	<li>Rinse cells with PBS three times.</li>
	<li>Add 0.5 ml ICC blocking buffer to each well of 24 well plate, incubate at RT for 1 hr.</li>
	<li>Diluted primary antibodies with ICC staining buffer. Add 200 &#181;l antibody solutions to each well and incubate overnight at 4 &#176;C. The antibody dilution factors are listed in Materials.</li>
	<li>On the next day, wash cells with PBS three times.</li>
	<li>Add 200 &#181;l secondary antibody solutions to the cells and incubate for 1 hr at RT in dark.</li>
	<li>Wash cells with PBS for three times.</li>
	<li>Add 150 &#181;l mounting solution containing DAPI to each well. Allow to stain nuclei for 1 min. Then cover each well with round glass cover slip and gently press the slip against the bottom surface with a forceps to remove air bubbles.</li>
	<li>Aspirate the excess solution and the samples are ready for imaging. The representative images showing the reprogrammed cells that express cardiac marker cTnT and &#945;Actinin at d14 are shown in Figure 2B.</li>
</ol>
6. FACS Analysis of Reprogramming Efficiency
<ol>
	<li>Thoroughly wash cells once with DPBS. Add 0.3 ml trypsin (0.05%) to each well and incubate at 37 &#176;C for 5 min.</li>
	<li>Gently tap the plate to facilitate lifting the cells. Check the cell detachment under microscope. Add 1 ml FACS buffer to each well when most cells are dissociated. Pipette up and down to further mechanically dissociate the cells.</li>
	<li>Transfer cells in the 24 well plate to a 96 well deep well plate well to well. Spin at 200 x g for 5 min at 4 &#176;C to collect the cells.</li>
	<li>Wash cells once with FACS buffer.</li>
	<li>Add 100 &#181;l fixation/permeabilization solutions to resuspend cells for 20 min at 4 &#176;C.</li>
	<li>Wash cells twice with 1x wash solution, pellet, and remove supernatant.</li>
	<li>Prepare primary antibody solution against GFP and cTnT in 1x wash buffer. Thoroughly resuspend each sample with 50 &#181;l antibody solution and incubate at 4 &#176;C for 30 min.</li>
	<li>Wash cells once in 1x wash solution, pellet, and remove supernatant.</li>
	<li>Add 50 &#181;l secondary antibody solution containing Alexa Fluor 488 conjugated donkey anti-rabbit IgG and Alexa Fluor 647 conjugated donkey anti-mouse IgG to each sample. Incubate at 4 &#176;C for 30 min in dark.</li>
	<li>Wash cells once in 1x wash solution, pellet, and remove supernatant.</li>
	<li>Resuspend cells in 400 &#181;l fixation buffer (DPBS with 1% PFA). Transfer cell into FACS tube with cell strainer cap. Samples are ready for FACS detection. The representative FACS analysis of reprogramming efficiency using pMxs-puromycin-MGT construct with or without puromycin selection is shown in Figure 2A.</li>
</ol>

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D., Ieda, M., Srivastava, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reprogramming+of+mouse+fibroblasts+into+cardiomyocyte-like+cells+in+vitro.">Reprogramming of mouse fibroblasts into cardiomyocyte-like cells in vitro.</a> Nat Protoc. 8, 1204-1215 (2013).</li><li>Wang, H., 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=Small+Molecules+Enable+Cardiac+Reprogramming+of+Mouse+Fibroblasts+with+a+Single+Factor,+Oct4.">Small Molecules Enable Cardiac Reprogramming of Mouse Fibroblasts with a Single Factor, Oct4.</a> Cell Rep. , (2014).</li><li>Wang, L., 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=Stoichiometry+of+Gata4,+Mef2c,+and+Tbx5+Influences+the+Efficiency+and+Quality+of+Induced+Cardiac+Myocyte+Reprogramming.">Stoichiometry of Gata4, Mef2c, and Tbx5 Influences the Efficiency and Quality of Induced Cardiac Myocyte Reprogramming.</a> Circ Res. , (2014).</li><li>Song, K., 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=Heart+repair+by+reprogramming+non-myocytes+with+cardiac+transcription+factors.">Heart repair by reprogramming non-myocytes with cardiac transcription factors.</a> Nature. 485, 599-604 (2012).</li><li>Qian, L., 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=In+vivo+reprogramming+of+murine+cardiac+fibroblasts+into+induced+cardiomyocytes.">In vivo reprogramming of murine cardiac fibroblasts into induced cardiomyocytes.</a> Nature. 485, 593-598 (2012).</li><li>Mathison, 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=In+vivo+cardiac+cellular+reprogramming+efficacy+is+enhanced+by+angiogenic+preconditioning+of+the+infarcted+myocardium+with+vascular+endothelial+growth+factor.">In vivo cardiac cellular reprogramming efficacy is enhanced by angiogenic preconditioning of the infarcted myocardium with vascular endothelial growth factor.</a> J Am Heart Assoc. 1, e005652 (2012).</li><li>Srivastava, D., Ieda, M., Fu, J., Qian, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cardiac+repair+with+thymosin+beta4+and+cardiac+reprogramming+factors.">Cardiac repair with thymosin beta4 and cardiac reprogramming factors.</a> Ann NY Acad Sci. 1270, 66-72 (2012).</li><li>Muraoka, N., Ieda, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stoichiometry+of+transcription+factors+is+critical+for+cardiac+reprogramming.">Stoichiometry of transcription factors is critical for cardiac reprogramming.</a> Circ Res. , 116-216 (2015).</li><li>Srivastava, D., Ieda, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+factors+for+cardiac+reprogramming.">Critical factors for cardiac reprogramming.</a> Circ Res. 111, 5-8 (2012).</li></ol>

The authors have nothing to disclose.

For successful iCM generation when using this protocol, there are several important factors that have an impact on the overall efficiency. Particularly the conditions of starting fibroblasts and the quality of retrovirus encoding MGT can greatly affect the reprogramming efficiency.
It is important to generate fibroblasts as fresh and healthy as possible. For explant culture method, fibroblasts can be used before seven days after the explants were plated on dishes. For enzyme digestion method, ...

Cardiovascular disease remains the leading cause of death worldwide, accounting for 17.3 million deaths per year1. Loss of cardiomyocytes resulting from myocardial infarction (MI) or progressive heart failure is a major cause of morbidity and mortality2. Due to limited regenerative capacity, adult mammalian hearts usually suffer from impaired pump function and heart failure following injury3-6. As such, efficient (re)generation of cardiomyocytes in vivo and in vitro for treatment of heart disease and for disease modeling is a critical issue needing to be addressed.
Recent development of direct reprogramming, which directly reprograms cells from one differentiated phenotype to another without transitioning through the pluripotent state, offers a promising alternative approach for regenerative medicine. The mammalian heart contains abundant cardiac fibroblasts (CFs), which account for approximately half of the cells in heart and massively proliferate upon injury7-9. Thus, the vast pool of CFs could serve as an endogenous source of new CMs for regenerative therapy if they could be directly reprogrammed into functional CMs. It has been shown that a combination of transcription factors, such as Gata4 (G), Mef2c (M) and Tbx5 (T), with or without microRNAs or small molecules can reprogram fibroblasts into iCMs10-26. Importantly, this conversion can also be induced in vivo, and results in an improvement in cardiac function and a reduction in scar size in an infarcted heart16,27-29. These studies indicate that direct cardiac reprogramming may be a potential avenue to heal an injured heart. However, the low efficiency of iCM reprogramming has become a major hurdle for further mechanistic studies. In addition, the reproducibility of cardiac reprogramming is another controversial issue of this technology11,30,31. 
Very recently, we generated a complete set of polycistronic constructs encoding G,M,T in all possible splicing orders with identical 2A sequences in a single mRNA. These polycistronic constructs yielded varied G, M and T protein expression levels, which led to significantly different reprogramming efficiency25. The most efficient construct, named MGT, which showed a relatively high Mef2c and low Gata4 and Tbx5 expression, significantly improved reprogramming efficiency and produced large amounts of iCMs with CM markers expression, robust calcium oscillation and spontaneous beating25. Moreover, by using MGT polycistronic construct, our study avoided the use of multiple vectors and generated cells with homogenous expression ratio of G,M,T, thus providing an improved platform for cardiac reprogramming research. To increase experimental reproducibility, here we describe in detail how to isolate fibroblasts, produce retrovirus carrying MGT cassette, generate iCMs and evaluate the reprogramming efficiency.

<table><tbody><tr><td>anti-cardiac troponin T</td><td>Thermo Scientific</td><td>MS-295-PO</td><td>1:200 for FACS and 1:400 for ICC</td></tr><tr><td>anti-GFP</td><td>Life Technologies</td><td>A11122</td><td>1:500 for both FACS and ICC</td></tr><tr><td>anti- aActinin</td><td>Sigma-Aldrich</td><td>A7811</td><td>1:500 for both FACS and ICC</td></tr><tr><td>anti-Connexin43</td><td>Sigma-Aldrich</td><td>C6219</td><td>1:500 for&amp;nbsp; ICC</td></tr><tr><td>anit-Mef2c</td><td>Abcam</td><td>ab64644</td><td>1:1,000 for ICC&amp;nbsp;</td></tr><tr><td>anti-Gata4</td><td>Santa Cruz Biotechnology</td><td>sc-1237</td><td>1:200 for ICC</td></tr><tr><td>anti-Tbx5</td><td>Santa Cruz Biotechnology</td><td>sc-17866</td><td>1:200 for ICC</td></tr><tr><td>Alexa Fluor 488&amp;ndash;conjugated donkey anti-rabbit IgG</td><td>Jackson ImmunoResearch Inc</td><td>711-545-152</td><td>1:500 for both FACS and ICC</td></tr><tr><td>Alexa Fluor 647&amp;ndash;conjugated donkey anti-mouse IgG</td><td>Jackson ImmunoResearch Inc</td><td>715-605-150</td><td>1:500 for both FACS and ICC</td></tr><tr><td>Cytofix/Cytoperm kit for intracellular staining</td><td>BD Biosciences</td><td>554722</td><td></td></tr><tr><td>Rhod-3 Calcium Imaging Kit</td><td>Life Technologies</td><td>R10145</td><td></td></tr><tr><td>Thy1.2 microbeads</td><td>Miltenyi Biotec</td><td>130-049-101</td><td></td></tr><tr><td>Vectashield solution with DAPI</td><td>Vector labs</td><td>H-1500</td><td></td></tr><tr><td>FBS</td><td>Sigma-Aldrich</td><td>F-2442</td><td></td></tr><tr><td>Trypsin-EDTA (0.05%)</td><td>Corning</td><td>25-052</td><td></td></tr><tr><td>PRMI1640 medium</td><td>Life Technologies</td><td>11875-093</td><td></td></tr><tr><td>B27 supplement</td><td>Life Technologies</td><td>17504-044</td><td></td></tr><tr><td>IMDM</td><td>Life Technologies</td><td>12440-053</td><td></td></tr><tr><td>Opti-MEM Reduced Serum Medium</td><td>Life Technologies</td><td>31985-070</td><td></td></tr><tr><td>M199 medium</td><td>Life Technologies</td><td>10-060</td><td></td></tr><tr><td>DMEM, high glucose</td><td>Life Technologies</td><td>10-013</td><td></td></tr><tr><td>Penicillin-streptomycin</td><td>Corning</td><td>30-002</td><td></td></tr><tr><td>Non-essential amino acids</td><td>Life Technologies</td><td>11130-050</td><td></td></tr><tr><td>Lipofectamine 2000</td><td>Life Technologies</td><td>11668500</td><td></td></tr><tr><td>blasticidin</td><td>Life Technologies</td><td>A11139-03</td><td></td></tr><tr><td>puromycin</td><td>Life Technologies</td><td>A11138-03</td><td></td></tr><tr><td>Collagenase II</td><td>Worthington</td><td>LS004176</td><td></td></tr><tr><td>polybrene</td><td>Millipore</td><td>TR-1003-G</td><td></td></tr><tr><td>Triton X-100</td><td>Fisher</td><td>BP151-100</td><td></td></tr><tr><td>CaCl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>C7902</td><td></td></tr><tr><td>HEPES</td><td>Sigma-Aldrich</td><td>H4034</td><td></td></tr><tr><td>NaCl</td><td>Sigma-Aldrich</td><td>BP358-212</td><td></td></tr><tr><td>KCl</td><td>Sigma-Aldrich</td><td>PX1405</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;HPO&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>S7907</td><td></td></tr><tr><td>Glucose</td><td>Sigma-Aldrich</td><td>G6152</td><td></td></tr><tr><td>Bovine serum albumin</td><td>Fisher</td><td>9048-46-8</td><td></td></tr><tr><td>paraformaldehyde</td><td>EMS</td><td>15714</td><td></td></tr><tr><td>Retrovirus Precipitation Solution</td><td>ALSTEM</td><td>VC-200</td><td></td></tr><tr><td>0.4% Trypan blue solution</td><td>Sigma-Aldrich</td><td>T8154</td><td></td></tr><tr><td>gelatin</td><td>Sigma-Aldrich</td><td>G1393</td><td></td></tr><tr><td>Dulbecco&#039;s PBS without CaCl&lt;sub&gt;2&lt;/sub&gt; and MgCl&lt;sub&gt;2&lt;/sub&gt; (D-PBS, 1x)</td><td>Sigma-Aldrich</td><td>D8537</td><td></td></tr><tr><td>HBSS (Hanks Balanced Salt Solution)</td><td>Corning</td><td>21022</td><td></td></tr><tr><td>LS column</td><td>Miltenyi Biotec</td><td>130-042-401</td><td></td></tr><tr><td>0.45 &amp;mu;m cellulose acetate filter&amp;nbsp;</td><td>Thermo Scientific</td><td>190-2545</td><td></td></tr><tr><td>24-well plates</td><td>Corning</td><td>3524</td><td></td></tr><tr><td>10 cm Tissue culture dishes&amp;nbsp;</td><td>Thermo Scientific</td><td>172958</td><td></td></tr><tr><td>60 mm center well culture dish&amp;nbsp;&amp;nbsp;</td><td>Corning</td><td>3260</td><td></td></tr><tr><td>96 Well Clear V-Bottom 2 ml Polypropylene Deep Well Plate</td><td>Denville Scientific</td><td>P9639</td><td></td></tr><tr><td>Polystyrene round-bottom tubes with cell-strainer cap&amp;nbsp;</td><td>BD Biosciences</td><td>352235</td><td></td></tr><tr><td>Centrifuge</td><td>Eppendorf</td><td>5810R</td><td></td></tr><tr><td>Vortexer MINI</td><td>VWR</td><td>58816-121</td><td></td></tr><tr><td>EVOS FL Auto Cell Imaging System</td><td>Life Technologies</td><td>AMAFD1000</td><td></td></tr><tr><td>MACS MultiStand</td><td>Miltenyi Biotec</td><td>130-042-303</td><td></td></tr><tr><td>MidiMACS Separator</td><td>Miltenyi Biotec</td><td>130-042-302</td><td></td></tr><tr><td>Round glass cover slip</td><td>Electron Microscopy Sciences</td><td>72195-15</td><td></td></tr></tbody></table>

improved generation of induced cardiomyocytes using a polycistronic construct expressing optimal ratio of gata4, mef2c and tbx5

Direct conversion of cardiac fibroblasts (CFs) into induced cardiomyocytes (iCMs) holds great potential for regenerative medicine by offering alternative strategies for treatment of heart disease. This conversion has been achieved by forced expression of defined factors such as Gata4 (G), Mef2c (M) and Tbx5 (T). Traditionally, iCMs are generated by a cocktail of viruses expressing these individual factors. However, reprogramming efficiency is relatively low and most of the in vitro G,M,T-transduced fibroblasts do not become fully reprogrammed, making it difficult to study the reprogramming mechanisms. We recently have shown that the stoichiometry of G,M,T is crucial for efficient iCM reprogramming. An optimal stoichiometry of G,M,T with relative high level of M and low levels of G and T achieved by using our polycistronic MGT vector (hereafter referred to as MGT) significantly increased reprogramming efficiency and improved iCM quality in vitro. Here we provide a detailed description of the methodology used to generate iCMs with MGT construct from cardiac fibroblasts. Isolation of cardiac fibroblasts, generation of virus for reprogramming and evaluation of the reprogramming process are also included to provide a platform for efficient and reproducible generation of iCMs.

The reprogramming steps are summarized by schematic in Figure 1. After MGT transduction, GFP expression in reprogramming cells could be detected as early as day 3. Puromycin selection of transduced cells starts from day 3 and is maintained during the first two weeks if pMx-puro-MGT construct is used. By day 10 to day 14, expression of cardiac markers like cTnT and &#945;Actinin could be detected by both ICC (Figure 2B, step 5) and FACS (Figure 2A, step 6), indicating tha...

Watch this Scientific Journal Video about Improved Generation of Induced Cardiomyocytes Using a Polycistronic Construct Expressing Optimal Ratio of Gata4, Mef2c and Tbx5 at JoVE.com

Improved Generation of Induced Cardiomyocytes Using a Polycistronic Construct Expressing Optimal Ratio of Gata4, Mef2c and Tbx5

We describe here a protocol for the generation of iCMs using retrovirus-mediated delivery of Gata4, Tbx5 and Mef2c in a polycistronic construct. This protocol yields a relatively homogeneous population of reprogrammed cells with improved efficiency and quality and is valuable for future studies of iCM reprogramming.

Direct conversion of cardiac fibroblasts (CFs) into induced cardiomyocytes (iCMs) holds great potential for regenerative medicine by offering alternative ...

improved-generation-induced-cardiomyocytes-using-polycistronic

Universidad de Cartagena UDEC

Research

JoVE Journal

Developmental Biology

8.6K Views.  University of North Carolina, Chapel Hill. We describe here a protocol for the generation of iCMs using retrovirus-mediated delivery of Gata4, Tbx5 and Mef2c in a polycistronic construct. This protocol yields a relatively homogeneous population of reprogrammed cells with improved efficiency and quality and is valuable for future studies of iCM reprogramming. 

Video: Improved Generation of Induced Cardiomyocytes Using a Polycistronic Construct Expressing Optimal Ratio of Gata4, Mef2c and Tbx5

Generation of Murine Cardiac Pacemaker Cell Aggregates Based on ES-Cell-Programming in Combination with Myh6-Promoter-Selection

Treatment of the &#8220;sick sinus syndrome&#8221; is based on artificial pacemakers. These bear hazards such as battery failure and infections. Moreover, they lack hormone responsiveness and the overall procedure is cost-intensive. &#8220;Biological pacemakers&#8221; generated from PSCs may become an alternative, yet the typical content of pacemaker cells in Embryoid Bodies (EBs) is extremely low. The described protocol combines &#8220;forward programming&#8221; of murine PSCs via the sinus node inducer TBX3 with Myh6-promoter based antibiotic selection. This yields cardiomyocyte aggregates consistent of &#62;80% physiologically functional pacemaker cells. These &#8220;induced-sinoatrial-bodies&#8221; (&#8220;iSABs&#8221;) are spontaneously contracting at yet unreached frequencies (400-500 bpm) corresponding to nodal cells isolated from mouse hearts and are able to pace murine myocardium ex vivo. Using the described protocol highly pure sinus nodal single cells can be generated which e.g. can be used for in vitro drug testing. Furthermore, the iSABs generated according to this protocol may become a crucial step towards heart tissue engineering.

This protocol describes how to produce functional sinus nodal tissue from murine pluripotent stem cells (PSC). T-Box3 (TBX3) overexpression plus cardiac Myosin-heavy-chain (Myh6) promoter antibiotic selection leads to highly pure pacemaker cell aggregates. These &#8220;Induced-sinoatrial-bodies&#8221; (&#8220;iSABs&#8221;) contain over 80% pacemaker cells, show highly increased beating rates and are able to pace myocardium ex vivo.

Treatment of the &#8220;sick sinus syndrome&#8221; is based on artificial pacemakers. These bear hazards such as battery failure and infections. Moreover, ...

Generation of Aligned Functional Myocardial Tissue Through Microcontact Printing

Advanced heart failure represents a major unmet clinical challenge, arising from the loss of viable and/or fully functional cardiac muscle cells. Despite optimum drug therapy, heart failure represents a leading cause of mortality and morbidity in the developed world. A major challenge in drug development is the identification of cellular assays that accurately recapitulate normal and diseased human myocardial physiology in vitro. Likewise, the major challenges in regenerative cardiac biology revolve around the identification and isolation of patient-specific cardiac progenitors in clinically relevant quantities. These cells have to then be assembled into functional tissue that resembles the native heart tissue architecture. Microcontact printing allows for the creation of precise micropatterned protein shapes that resemble structural organization of the heart, thus providing geometric cues to control cell adhesion spatially. Herein we describe our approach for the isolation of highly purified myocardial cells from pluripotent stem cells differentiating in vitro, the generation of cell growth surfaces micropatterned with extracellular matrix proteins, and the assembly of the stem cell-derived cardiac muscle cells into anisotropic myocardial tissue.

The generation of aligned myocardial tissue is a key requirement for adapting the recent advances in stem cell biology to clinically useful purposes. Herein we describe a microcontact printing approach for the precise control of cell shape and function. Using highly purified populations of embryonic stem cell derived cardiac progenitors, we then generate anisotropic functional myocardial tissue.

Advanced heart failure represents a major unmet clinical challenge,  arising from the loss of viable and/or fully functional cardiac muscle  cells. ...

Bioengineering

Assessing Cardiomyocyte Subtypes Following Transcription Factor-mediated Reprogramming of Mouse Embryonic Fibroblasts

Direct reprogramming of one cell type into another has recently emerged as a powerful paradigm for regenerative medicine, disease modeling, and lineage specification. In particular, the conversion of fibroblasts into induced cardiomyocyte-like myocytes (iCLMs) by Gata4, Hand2, Mef2c, and Tbx5 (GHMT) represents an important avenue for generating de novo cardiac myocytes in vitro and in vivo. Recent evidence suggests that GHMT generates a greater diversity of cardiac subtypes than previously appreciated, thus underscoring the need for a systematic approach to conducting additional studies. Before direct reprogramming can be used as a therapeutic strategy, however, the mechanistic underpinnings of lineage conversion must be understood in detail to generate specific cardiac subtypes. Here we present a detailed protocol for generating iCLMs by GHMT-mediated reprogramming of mouse embryonic fibroblasts (MEFs). 
We outline methods for MEF isolation, retroviral production, and MEF infection to accomplish efficient reprogramming. To determine the subtype identity of reprogrammed cells, we detail a step-by-step approach for performing immunocytochemistry on iCLMs using a defined set of compatible antibodies. Methods for confocal microscopy, identification, and quantification of iCLMs and individual atrial (iAM), ventricular (iVM), and pacemaker (iPM) subtypes are also presented. Finally, we discuss representative results of prototypical direct reprogramming experiments and highlight important technical aspects of our protocol to ensure efficient lineage conversion. Taken together, our optimized protocol should provide a stepwise approach for investigators to conduct meaningful cardiac reprogramming experiments that require identification of individual CM subtypes.

This manuscript describes a step-by-step protocol for the generation and quantification of diverse reprogrammed cardiac subtypes using a retrovirus-mediated delivery of Gata4, Hand2, Mef2c, and Tbx5.

Direct reprogramming of one cell type into another has recently emerged as a powerful paradigm for regenerative medicine, disease modeling, and lineage ...

Suppression of Pro-fibrotic Signaling Potentiates Factor-mediated Reprogramming of Mouse Embryonic Fibroblasts into Induced Cardiomyocytes

Trans-differentiation of one somatic cell type into another has enormous potential to model and treat human diseases. Previous studies have shown that mouse embryonic, dermal, and cardiac fibroblasts can be reprogrammed into functional induced-cardiomyocyte-like cells (iCMs) through overexpression of cardiogenic transcription factors including GATA4, Hand2, Mef2c, and Tbx5 both in vitro and in vivo. However, these previous studies have shown relatively low efficiency. In order to restore heart function following injury, mechanisms governing cardiac reprogramming must be elucidated to increase efficiency and maturation of iCMs.
We previously demonstrated that inhibition of pro-fibrotic signaling dramatically increases reprogramming efficiency. Here, we detail methods to achieve a reprogramming efficiency of up to 60%. Furthermore, we describe several methods including flow cytometry, immunofluorescent imaging, and calcium imaging to quantify reprogramming efficiency and maturation of reprogrammed fibroblasts. Using the protocol detailed here, mechanistic studies can be undertaken to determine positive and negative regulators of cardiac reprogramming. These studies may identify signaling pathways that can be targeted to promote reprogramming efficiency and maturation, which could lead to novel cell therapies to treat human heart disease.

Here we present a robust method to reprogram primary embryonic fibroblasts into functional cardiomyocytes through overexpression of GATA4, Hand2, Mef2c, Tbx5, miR-1, and miR-133 (GHMT2m) alongside inhibition of TGF-&#946; signaling. Our protocol generates beating cardiomyocytes as early as 7 days post-transduction with up to 60% efficiency.

Trans-differentiation of one somatic cell type into another has enormous potential to model and treat human diseases. Previous studies have shown that ...

Generation of First Heart Field-like Cardiac Progenitors and Ventricular-like Cardiomyocytes from Human Pluripotent Stem Cells

The generation of large amounts of functional human pluripotent stem cells-derived cardiac progenitors and cardiomyocytes of defined heart field origin is a pre-requisite for cell-based cardiac therapies and disease modeling. We have recently shown that Id genes are both necessary and sufficient to specify first heart field progenitors during vertebrate development. This differentiation protocol leverages these findings and uses Id1 overexpression in combination with Activin A as potent specifying cues to produce first heart field-like (FHF-L) progenitors. Importantly, resulting progenitors efficiently differentiate (~70&#8211;90%) into ventricular-like cardiomyocytes. Here we describe a detailed method to 1) generate Id1-overexpressing hPSCs and 2) differentiate scalable quantities of cryopreservable FHF-L progenitors and ventricular-like cardiomyocytes.

Here we describe a scalable method, using a simple combination of Activin A and lentivirus-mediated Id1-overexpression, to generate first heart field-like cardiac progenitors and ventricular-like cardiomyocytes from human pluripotent stem cells.

The generation of large amounts of functional human pluripotent stem cells-derived cardiac progenitors and cardiomyocytes of defined heart field origin is ...

In Vitro Generation of Heart Field-specific Cardiac Progenitor Cells

Pluripotent stem cells offer great potential for understanding heart development and disease and for regenerative medicine. While recent advances in developmental cardiology have led to generating cardiac cells from pluripotent stem cells, it is unclear if the two cardiac fields - the first and second heart fields (FHF and SHF) &#8212; are induced in pluripotent stem cells systems. To address this, we generated a protocol for in vitro specification and isolation of heart field-specific cardiac progenitor cells. We used embryonic stem cells lines carrying Hcn4-GFP and Tbx1-Cre; Rosa-RFP reporters of the FHF and the SHF, respectively, and live cell immunostaining of the cell membrane protein Cxcr4, a SHF marker. With this approach, we generated progenitor cells which recapitulate the functional properties and transcriptome of their in vivo counterparts. Our protocol can be utilized to study early specification and segregation of the two heart fields and to generate chamber-specific cardiac cells for heart disease modelling. Since this is an in vitro organoid system, it may not provide precise anatomical information. However, this system overcomes the poor accessibility of gastrulation-stage embryos and can be upscaled for high-throughput screens.

The purpose of this method is to generate heart field-specific cardiac progenitor cells in vitro in order to study the progenitor cell specification and functional properties, and to generate chamber specific cardiac cells for heart disease modelling.

Pluripotent stem cells offer great potential for understanding heart development and disease and for regenerative medicine. While recent advances in ...

Generation and Expansion of Human Cardiomyocytes from Patient Peripheral Blood Mononuclear Cells

Generating patient-specific cardiomyocytes from a single blood draw has attracted tremendous interest in precision medicine on cardiovascular disease. Cardiac differentiation from human induced pluripotent stem cells (iPSCs) is modulated by defined signaling pathways that are essential for embryonic heart development. Numerous cardiac differentiation methods on 2-D and 3-D platforms have been developed with various efficiencies and cardiomyocyte yield. This has puzzled investigators outside the field as the variety of these methods can be difficult to follow. Here we present a comprehensive protocol that elaborates robust generation and expansion of patient-specific cardiomyocytes from peripheral blood mononuclear cells (PBMCs). We first describe a high-efficiency iPSC reprogramming protocol from a patient&#39;s blood sample using non-integration Sendai virus vectors. We then detail a small molecule-mediated monolayer differentiation method that can robustly produce beating cardiomyocytes from most human iPSC lines. In addition, a scalable cardiomyocyte expansion protocol is introduced using a small molecule (CHIR99021) that could rapidly expand patient-derived cardiomyocytes for industrial- and clinical-grade applications. At the end, detailed protocols for molecular identification and electrophysiological characterization of these iPSC-CMs are depicted. We expect this protocol to be pragmatic for beginners with limited knowledge on cardiovascular development and stem cell biology.

Here, we present a protocol to robustly generate and expand human cardiomyocytes from patient peripheral blood mononuclear cells.

Generating patient-specific cardiomyocytes from a single blood draw has attracted tremendous interest in precision medicine on cardiovascular disease. ...

Electroporation-Based Transgenesis of Cardiac Cells in Mouse Hearts

An Efficient Transgenesis Approach for Gene Delivery in the Mouse Embryonic Heart

The mammalian heart is a complex organ formed during development via highly diverse populations of progenitor cells. The origin, timing of recruitment, and fate of these progenitors are vital for the proper development of this organ. The molecular mechanisms that govern the morphogenesis of the heart are essential for understanding the pathogenesis of congenital heart diseases and embryonic cardiac regeneration. Classical approaches to investigate these mechanisms employed the generation of transgenic mice to assess the function of specific genes during cardiac development. However, mouse transgenesis is a complex, time-consuming process that often cannot be performed to assess the role of specific genes during heart development. To address this, we have developed a protocol for efficient electroporation and culture of mouse embryonic hearts, enabling transient transgenesis to rapidly assess the effect of gain- or loss-of-function of genes involved in cardiac development. Using this methodology, we successfully overexpressed Meis1 in the embryonic heart, with a preference for epicardial cell transfection, demonstrating the capabilities of the technique.

Author Spotlight: Cardiac Cell Transgenesis for Rapid Gene Screening

This protocol presents a detailed methodological framework for electroporation-based transgenesis of cardiac cells in developing mouse hearts. The video assets provided here will facilitate learning of this versatile technique.

The mammalian heart is a complex organ formed during development via highly diverse populations of progenitor cells. The origin, timing of recruitment, ...

Generation of Human Cardiomyocytes: A Differentiation Protocol from Feeder-free Human Induced Pluripotent Stem Cells

In order to investigate the events driving heart development and to determine the molecular mechanisms leading to myocardial diseases in humans, it is essential first to generate functional human cardiomyocytes (CMs). The use of these cells in drug discovery and toxicology studies would also be highly beneficial, allowing new pharmacological molecules for the treatment of cardiac disorders to be validated pre-clinically on cells of human origin. Of the possible sources of CMs, induced pluripotent stem (iPS) cells are among the most promising, as they can be derived directly from readily accessible patient tissue and possess an intrinsic capacity to give rise to all cell types of the body 1. Several methods have been proposed for differentiating iPS cells into CMs, ranging from the classical embryoid bodies (EBs) aggregation approach to chemically defined protocols 2,3. In this article we propose an EBs-based protocol and show how this method can be employed to efficiently generate functional CM-like cells from feeder-free iPS cells.

Pluripotent stem cells, either embryonic or induced pluripotent stem (iPS) cells, constitute a valuable source of human differentiated cells, including cardiomyocytes. Here, we will focus on cardiac induction of iPS cells, showing how to use them to obtain functional human cardiomyocytes through an embryoid bodies-based protocol.

In order to investigate the events driving heart development and to determine the molecular mechanisms leading to myocardial diseases in humans, it is ...

Biology

In vitro Assessment of Cardiac Reprogramming by Measuring Cardiac Specific Calcium Flux with a GCaMP3 Reporter

Cardiac reprogramming has become a potentially promising therapy to repair a damaged heart. By introducing multiple transcription factors, including Mef2c, Gata4, Tbx5 (MGT), fibroblasts can be reprogrammed into induced cardiomyocytes (iCMs). These iCMs, when generated in situ in an infarcted heart, integrate electrically and mechanically with the surrounding myocardium, leading to a reduction in scar size and an improvement in heart function. Because of the relatively low reprogramming efficiency, purity, and quality of the iCMs, characterization of iCMs remains a challenge. The currently used methods in this field, including flow cytometry, immunocytochemistry, and qPCR, mainly focus on cardiac-specific gene and protein expression but not on the functional maturation of iCMs. Triggered by action potentials, the opening of voltage-gated calcium channels in cardiomyocytes leads to a rapid influx of calcium into the cell. Therefore, quantifying the rate of calcium influx is a promising method to evaluate cardiomyocyte function. Here, the protocol introduces a method to evaluate iCM function by calcium (Ca2+) flux. An &#945;MHC-Cre/Rosa26A-Flox-Stop-Flox-GCaMP3 mouse strain was established by crossing Tg(Myh6-cre)1Jmk/J (referred to as Myh6-Cre below) with Gt(ROSA)26Sortm38(CAG-GCaMP3)Hze/J (referred to as Rosa26A-Flox-Stop-Flox-GCaMP3 below) mice. Neonatal cardiac fibroblasts (NCFs) from P0-P2 neonatal mice were isolated and cultured in vitro, and a polycistronic construction of MGT was introduced to NCFs, which led to their reprogramming to iCMs. Because only successfully reprogrammed iCMs will express GCaMP3 reporter, the functional maturation of iCMs can be visually assessed by Ca2+ flux with fluorescence microscopy. Compared with un-reprogrammed NCFs, NCF-iCMs showed significant calcium transient flux and spontaneous contraction, similar to CMs. This protocol describes in detail the mouse strain establishment, isolation and selection of neonatal mice hearts, NCF isolation, production of retrovirus for cardiac reprogramming, iCM induction, the evaluation of iCM Ca2+ flux using our reporter line, and related statistical analysis and data presentation. It is expected that the methods described here will provide a valuable platform to assess the functional maturation of iCMs for cardiac reprogramming studies.

We describe here, the establishment and application of an Tg(Myh6-cre)1Jmk/J /Gt(ROSA)26Sortm38(CAG-GCaMP3)Hze/J (referred to as &#945;MHC-Cre/Rosa26A-Flox-Stop-Flox-GCaMP3 below) mouse reporter line for cardiac reprogramming assessment. Neonatal cardiac fibroblasts (NCFs) isolated from the mouse strain are converted into induced cardiomyocytes (iCMs), allowing for convenient and efficient evaluation of reprogramming efficiency and functional maturation of iCMs via calcium (Ca2+) flux.

Cardiac reprogramming has become a potentially promising therapy to repair a damaged heart. By introducing multiple transcription factors, including ...