A.F.-P. was supported by the National Science Foundation Graduate Research Fellowship under Grant No.2015165336. N.V.M was supported by grants from the NIH (HL094699), Burroughs Wellcome Fund (1009838), and the March of Dimes (#5-FY14-203). We acknowledge Young-Jae Nam, Christina Lubczyk, and Minoti Bhakta for their important contributions to protocol development and data analysis. We also thank John Shelton for valuable technical input and members of the Munshi lab for scientific discussion.

All experimental procedures involving animal practices were approved by the Institutional Animal Care and Use Committee at UT Southwestern Medical Center.
1. Isolation of Hcn4-GFP E12.5 Mouse Embryonic Fibroblast (MEFs)
<ol>
	<li>Set up timed matings between homozygous Hcn4-GFP males and CD-1 females.</li>
	<li>Sacrifice pregnant female at E12.5 by carbon dioxide euthanasia and subsequent cervical dislocation.
	<ol>
		<li>Remove uterine horns with dissecting forceps as described previously<s...

<ol>
	<li>Sissman, N. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Developmental+landmarks+in+cardiac+morphogenesis:+comparative+chronology.">Developmental landmarks in cardiac morphogenesis: comparative chronology.</a> Am J Cardiol. 25 (2), 141-148 (1970).</li><li>Buckingham, M., Meilhac, S., Zaffran, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Building+the+mammalian+heart+from+two+sources+of+myocardial+cells.">Building the mammalian heart from two sources of myocardial cells.</a> Nat Rev Genet. 6 (11), 826-835 (2005).</li><li>Ali, S. R., 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=Existing+cardiomyocytes+generate+cardiomyocytes+at+a+low+rate+after+birth+in+mice.">Existing cardiomyocytes generate cardiomyocytes at a low rate after birth in mice.</a> Proc Natl Acad Sci U S A. 111 (24), 8850-8855 (2014).</li><li>Bergmann, O., 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=Evidence+for+cardiomyocyte+renewal+in+humans.">Evidence for cardiomyocyte renewal in humans.</a> Science. 324 (5923), 98-102 (2009).</li><li>Senyo, S. 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=Mammalian+heart+renewal+by+pre-existing+cardiomyocytes.">Mammalian heart renewal by pre-existing cardiomyocytes.</a> Nature. 493 (7432), 433-436 (2013).</li><li>Lin, Z., Pu, W. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strategies+for+cardiac+regeneration+and+repair.">Strategies for cardiac regeneration and repair.</a> Sci Transl Med. 6 (239), 239rv231 (2014).</li><li>Writing Group, 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=Heart+Disease+and+Stroke+Statistics-2016+Update:+A+Report+From+the+American+Heart+Association.">Heart Disease and Stroke Statistics-2016 Update: A Report From the American Heart Association.</a> Circulation. 133 (4), e38-e360 (2016).</li><li>Takahashi, K., Yamanaka, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Induction+of+pluripotent+stem+cells+from+mouse+embryonic+and+adult+fibroblast+cultures+by+defined+factors.">Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors.</a> Cell. 126 (4), 663-676 (2006).</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 (7400), 599-604 (2012).</li><li>Ieda, 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=Direct+reprogramming+of+fibroblasts+into+functional+cardiomyocytes+by+defined+factors.">Direct reprogramming of fibroblasts into functional cardiomyocytes by defined factors.</a> Cell. 142 (3), 375-386 (2010).</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 (7400), 593-598 (2012).</li><li>Fu, J. D., 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=Direct+reprogramming+of+human+fibroblasts+toward+a+cardiomyocyte-like+state.">Direct reprogramming of human fibroblasts toward a cardiomyocyte-like state.</a> Stem Cell Reports. 1 (3), 235-247 (2013).</li><li>Zhou, H., Dickson, M. E., Kim, M. S., Bassel-Duby, R., Olson, E. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Akt1/protein+kinase+B+enhances+transcriptional+reprogramming+of+fibroblasts+to+functional+cardiomyocytes.">Akt1/protein kinase B enhances transcriptional reprogramming of fibroblasts to functional cardiomyocytes.</a> Proc Natl Acad Sci U S A. 112 (38), 11864-11869 (2015).</li><li>Zhou, Y., 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=Bmi1+Is+a+Key+Epigenetic+Barrier+to+Direct+Cardiac+Reprogramming.">Bmi1 Is a Key Epigenetic Barrier to Direct Cardiac Reprogramming.</a> Cell Stem Cell. 18 (3), 382-395 (2016).</li><li>Zhao, Y., 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=High-efficiency+reprogramming+of+fibroblasts+into+cardiomyocytes+requires+suppression+of+pro-fibrotic+signalling.">High-efficiency reprogramming of fibroblasts into cardiomyocytes requires suppression of pro-fibrotic signalling.</a> Nat Commun. 6, 8243 (2015).</li><li>Miki, K., Yoshida, Y., Yamanaka, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+steady+progress+on+direct+cardiac+reprogramming+toward+clinical+application.">Making steady progress on direct cardiac reprogramming toward clinical application.</a> Circ Res. 113 (1), 13-15 (2013).</li><li>Atkinson, 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=Anatomical+and+molecular+mapping+of+the+left+and+right+ventricular+His-Purkinje+conduction+networks.">Anatomical and molecular mapping of the left and right ventricular His-Purkinje conduction networks.</a> J Mol Cell Cardiol. 51 (5), 689-701 (2011).</li><li>Bootman, M. D., Smyrnias, I., Thul, R., Coombes, S., Roderick, H. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atrial+cardiomyocyte+calcium+signalling.">Atrial cardiomyocyte calcium signalling.</a> Biochim Biophys Acta. 1813 (5), 922-934 (2011).</li><li>Miquerol, L., Beyer, S., Kelly, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Establishment+of+the+mouse+ventricular+conduction+system.">Establishment of the mouse ventricular conduction system.</a> Cardiovasc Res. 91 (2), 232-242 (2011).</li><li>Sp&#228;ter, D., Hansson, E. M., Zangi, L., Chien, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+make+a+cardiomyocyte.">How to make a cardiomyocyte.</a> Development. 141 (23), 4418-4431 (2014).</li><li>Nam, Y. 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=Induction+of+diverse+cardiac+cell+types+by+reprogramming+fibroblasts+with+cardiac+transcription+factors.">Induction of diverse cardiac cell types by reprogramming fibroblasts with cardiac transcription factors.</a> Development. 141 (22), 4267-4278 (2014).</li><li>Conner, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Current Protocols in Molecular Biology. , (2001).</li><li>Jozefczuk, J., Drews, K., Adjaye, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+mouse+embryonic+fibroblast+cells+suitable+for+culturing+human+embryonic+and+induced+pluripotent+stem+cells.">Preparation of mouse embryonic fibroblast cells suitable for culturing human embryonic and induced pluripotent stem cells.</a> J Vis Exp. (64), (2012).</li><li>Burns, J. C., Friedmann, T., Driever, W., Burrascano, M., Yee, J. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vesicular+stomatitis+virus+G+glycoprotein+pseudotyped+retroviral+vectors:+concentration+to+very+high+titer+and+efficient+gene+transfer+into+mammalian+and+nonmammalian+cells.">Vesicular stomatitis virus G glycoprotein pseudotyped retroviral vectors: concentration to very high titer and efficient gene transfer into mammalian and nonmammalian cells.</a> Proc Natl Acad Sci U S A. 90 (17), 8033-8037 (1993).</li><li>Ichim, C. V., Wells, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+of+high-titer+viral+preparations+by+concentration+using+successive+rounds+of+ultracentrifugation.">Generation of high-titer viral preparations by concentration using successive rounds of ultracentrifugation.</a> J Transl Med. 9, 137 (2011).</li><li>Qian, L., Berry, E. C., Fu, J. 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 (6), 1204-1215 (2013).</li><li>Yang, R., 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=Direct+conversion+of+mouse+and+human+fibroblasts+to+functional+melanocytes+by+defined+factors.">Direct conversion of mouse and human fibroblasts to functional melanocytes by defined factors.</a> Nat Commun. 5, 5807 (2014).</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=Direct+reprogramming+of+fibroblasts+into+myocytes+to+reverse+fibrosis.">Direct reprogramming of fibroblasts into myocytes to reverse fibrosis.</a> Annu Rev Physiol. 76, 21-37 (2014).</li><li>Coffin, J. M., Hughes, S. H., Varmus, H. E., Coffin, J. M., Hughes, S. H., Varmus, H. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Retroviruses. , (1997).</li><li>Bass, G. 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=Automated+image+analysis+identifies+signaling+pathways+regulating+distinct+signatures+of+cardiac+myocyte+hypertrophy.">Automated image analysis identifies signaling pathways regulating distinct signatures of cardiac myocyte hypertrophy.</a> J Mol Cell Cardiol. 52 (5), 923-930 (2012).</li></ol>

The authors have nothing to disclose.

The present study provides a direct-reprogramming strategy for conversion of MEFs into a diverse set of cardiac subtypes via retrovirus-mediated expression of the cardiac transcription factors Gata4, Mef2c, Tbx5, and Hand2 (GHMT). Using a multiplex immunostaining approach in combination with a PM-specific reporter mouse, we are able to identify iAM, iVMs, and iPMs at single cell resolution. Such an assay allows for an experimental in vitro system capable of isolating the contributions of individual transcription...

The heart is the first functional organ to develop in the embryo1,2. In conjunction with the circulatory system, it supplies oxygen, nutrients, and a waste disposal mechanism during development. Three weeks after fertilization, the human heart beats for the first time and its proper regulation is maintained by cardiomyocytes (CMs). The irreversible loss of these specialized cells is therefore the fundamental issue underlying progressive heart failure. While some organisms such as the zebrafish and Xenopus have the potential for cardiac regeneration, the adult mammalian heart is more limited3,5,6. Thus, given the critical function of the heart, it is not astonishing that heart disease is the leading cause of death in the world, accounting for 600,000 deaths in the United States alone7. Therefore, cell-based therapies to efficiently repair or replace the injured myocardium are of great clinical interest.
The seminal study of Yamanaka and colleagues8 showed that forced expression of four transcription factors is sufficient to convert fully differentiated fibroblast cells to pluripotent stem cells. However, the tumorigenic capacity of all pluripotent stem cell strategies has been a critical concern in their use for therapeutic purposes. This motivated the scientific field to search for alternative methods to transdifferentiate cells while avoiding a pluripotent stage. Recently, several groups have shown the feasibility of this strategy by displaying direct conversion of mouse fibroblasts to induced cardiomyocyte-like cells (iCLMs) with the ectopic expression of the transcription factors Gata4, Mef2c, Tbx5, and later on, Hand2 (GMT and GHMT, respectively)9,10. Furthermore, the same strategy can be performed in vivo and in human-derived tissues9,11,12. Recent studies have highlighted additional factors or signaling pathways that can be modulated to further improve cardiac reprogramming efficiency13,14,15. Taken together, these studies demonstrate the potential of directed transdifferentiation for regenerative therapies. However, the low efficiency of CM reprogramming, the unknown molecular mechanisms, inconsistent reproducibility due to methodological differences16, and the heterogeneous nature of iCLMs remain unaddressed.
In order to directly evaluate iCLM heterogeneity, we designed a discrete and robust single-cell assay for the identification of sarcomere development and cardiac lineage specification-two necessary characteristics of functional cardiomyocytes. There are at least three major types of CM in the heart as defined by their location and unique electrical properties: atrial (AM), ventricular (VM) and pacemaker (PM)17,18,19,20. In an orchestrated combination, they allow the proper pumping of blood. During heart injury, one or all subtypes might be affected, and the type of cell therapy would need to be addressed on a case-by-case basis. Currently, most strategies focus on the overall generation of cardiomyocytes, while little work is being done to study the molecular mechanisms that regulates subtype specification.
The following study details how to properly quantify well-organized sarcomeres and identify a diverse set of cardiomyocyte subtypes. Using a pacemaker (PM)-specific reporter mouse, we are able to apply an immunocytochemical approach to distinguish induced atrial-like myocytes (iAM), induced ventricular-like myocytes (iVM), and induced PM-like myocytes (iPMs)21. Based on our observations, only cells that exhibit sarcomere organization are capable of spontaneous beating. This unique reprogramming platform allows for assessing the role of certain parameters in sarcomere organization, subtype specification, and efficiency of CM reprogramming at single-cell resolution.

<table border="0" cellpadding="0" cellspacing="0" width="873">
	<tbody>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">DMEM</td>
			<td style="width:91px;">Sigma</td>
			<td style="width:119px;">D5796</td>
			<td style="width:347px;">Component of iCLM media, Plat-E media, fibroblast, and Transfection media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Medium 199</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">11150059</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Fetal bovine serrum (FBS)</td>
			<td style="width:91px;">Sigma</td>
			<td style="width:119px;">F2442</td>
			<td style="width:347px;">Component of iCLM media, Plat-E media, fibroblast, and Transfection media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Insulin-Transferrin-Selenium G</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">41400-045</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">MEM vitamin solution</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">11120-052</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">MEM amino acids</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">1601149</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Non-Essential amino acids</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">11140-050</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Antibiotic-Antimycotics</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">15240062</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">B-27 supplement</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">17504044</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Heat-Inactivated Horse Serum</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">26050-088</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">NaPyruvate</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">11360-70</td>
			<td style="width:347px;">Component of iCLM media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;">Penicillin/Streptomycin</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">1514022</td>
			<td style="width:347px;">Component of Plat-E media and fibroblast media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;">Puromycin&#160;</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>A11139-03</td>
			<td>Component of Plat-E media</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;">Blasticidin&#160;&#160;</td>
			<td style="width:91px;">Gemini Bio-Products</td>
			<td>400-128P</td>
			<td>Component of Plat-E media</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Glutamax</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>35050-061</td>
			<td>Component of Fibroblast media</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">Confocal laser scanning LSM700</td>
			<td style="width:91px;">Zeiss</td>
			<td style="width:119px;"></td>
			<td>For confocal analysis</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">FuGENE 6 transfection Reagent</td>
			<td style="width:91px;">Promega</td>
			<td style="width:119px;">E2692</td>
			<td>Transfection reagent&#160;</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Opti-MEM Reduced Serum Medium</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">31985-070</td>
			<td>Transfection reagent&#160;</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">Polybrene</td>
			<td style="width:91px;">Millipore</td>
			<td style="width:119px;">TR-1003-G</td>
			<td>Induction reagent. Use at a final concentration of 8um/mL</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">Platinium-E (PE) Retroviral Packagin Cell Line, Ecotropic</td>
			<td style="width:91px;">CellBiolabs</td>
			<td style="width:119px;">RV-101</td>
			<td>Retroviral pacaking cell line</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Trypsin 0.25% EDTA</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;"></td>
			<td>For MEFs and Plat-E dissociation</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Mouse anti &#945;-Actinin (Clone EA-53)</td>
			<td style="width:91px;">Sigma</td>
			<td>A7811</td>
			<td>Antibody for confocal analysis. Use at 1:200</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;">Chicken anti-GFP IgY</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>A10262</td>
			<td>Antibody for confocal analysis. Use at 1:200</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Rabbit Pab anti-NPPA&#160;</td>
			<td style="width:91px;">Abgent</td>
			<td>AP8534A</td>
			<td>Antibody for confocal analysis. Use at 1:400</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Rabbit Pab anti Myl2 IgG&#160;</td>
			<td style="width:91px;">ProteinTech</td>
			<td>10906-1-AP</td>
			<td>Antibody for confocal analysis. Use at 1:200</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">Vectashield solution with DAPI (4&#39;,6-Diamidino-2-Phenylindole, Dihydrochloride)</td>
			<td style="width:91px;">Vector Labs</td>
			<td style="width:119px;">H-1500</td>
			<td>Dye for confocal analysis</td>
		</tr>
		<tr height="34">
			<td height="34" style="height:34px;width:319px;">Superfrost Plus Microscope slides</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">12-550-15</td>
			<td>25 x 75 x 1.0 mm</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">BioCoat Fibronectin 12mm coverslips</td>
			<td style="width:91px;">NeuVitro Corp</td>
			<td>GG-12-1.5</td>
			<td>Coverslips for confocal analysis</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">100um cell strainer</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>08-771-19</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">0.45um Syringes filters SFCA 25MM</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">09-740-106</td>
			<td>For virus filtration</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">6ml Syringes</td>
			<td style="width:91px;">Covidien</td>
			<td>8881516937</td>
			<td>For virus filtration</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Goat anti-Chicken IgY (H&#38;L) A488</td>
			<td style="width:91px;">Abcam</td>
			<td>AB150169</td>
			<td style="width:347px;">Secondary antibody for confocal analysis. Use at 1:400</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;">Donkey anti-rabbit A647 IgG(H+L)&#160;</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>A31573</td>
			<td style="width:347px;">Secondary antibody for confocal analysis. Use at 1:400</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;">Goat anti-mouse IgG(H+L) A555</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>A21422</td>
			<td style="width:347px;">Secondary antibody for confocal analysis. Use at 1:400</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">Triton X-100</td>
			<td style="width:91px;">Sigma</td>
			<td style="width:119px;">93443-100ml</td>
			<td>For cell permeabilization&#160;</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">Dulbecco&#39;s PBS without CaCl2 and MgCl2 (D-PBS)</td>
			<td style="width:91px;">Sigma</td>
			<td style="width:119px;">D8537</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">Power Block 10X Universal Blocking reagent</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">NC9495720</td>
			<td>Dilute to 1X in H20</td>
		</tr>
		<tr height="52">
			<td height="52" style="height:52px;width:319px;">16% Paraformaldehyde aqueous solution (PFA)</td>
			<td style="width:91px;">Electro Microscopy Sciences&#160;</td>
			<td style="width:119px;">15710</td>
			<td>Use at 4% diluted in dH20</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">6 cm&#160; plates</td>
			<td style="width:91px;">Olympus</td>
			<td style="width:119px;">25-260</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">6-well plates</td>
			<td style="width:91px;">Genesee Scientific</td>
			<td style="width:119px;">25-105</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">24-well plates</td>
			<td style="width:91px;">Genesee Scientific</td>
			<td style="width:119px;">25-107</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">10 cm Tissue culture dishes</td>
			<td style="width:91px;">Corning</td>
			<td>4239</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">15 cm&#160; Tissue culture dishes</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td>5442</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">15 ml Conical tubes</td>
			<td style="width:91px;">Corning</td>
			<td style="width:119px;">4308</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">50 ml Conical tubes</td>
			<td style="width:91px;">Corning</td>
			<td style="width:119px;">4249</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:319px;">0.4% Trypan blue solution</td>
			<td style="width:91px;">Sigma</td>
			<td style="width:119px;">T8154</td>
			<td>For viability</td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">Ethyl Alcohol 200 proof</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">7005</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:319px;">Bleach</td>
			<td style="width:91px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">6009</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

Taking advantage of the PM-specific reporter mouse, we developed a multiplex immunostaining strategy to identify diverse endogenous myocytes as depicted in Figure 1. Following the reprogramming steps shown in Figure 2, induction of subtype-specific CMs can be detected as early as day 421, albeit at a low-rate. By day 14, the experiment can be stopped and assessed for sarcomere organization (Figure 3) and subtype-sp...

Watch this Scientific Journal Video about Assessing Cardiomyocyte Subtypes Following Transcription Factor-mediated Reprogramming of Mouse Embryonic Fibroblasts at JoVE.com

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

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 ...

The overall goal of this procedure is to generate and quantify a diverse set of induced cardiomyocyte subtypes. This method can help answer key, fundamental questions regarding subtype-specific lineage conversion during cardiac reprogramming such as the necessary cues that regulate sarcomere assembly and subtype specification. The main advantage of this technique is that it provides the method for quantified effects on cardio subtype specification which is required to perform mechanisis studies.To begin, isolate mouse embryonic fiberblasts from stage E 12.5 embryos. Store them in liquid nitrogen as described in the accompanying text protocol. Next, harvest plat-E cells and plate them at one million cells per well into a six well plate so that the cells reach 70 to 80%confluency in 24 hours.Cover the cells with transfaction media and incubate them under standard culture conditions. On experiment day minus one, take a 15 milliliter conical polystyrene tube and mix 60 microliters of reduced serum media with six microliters of transfection reagent per reaction. Incubate the mixture for five minutes at room temperature.Since the transfection reagent use your add your reagent directly to the reduced serum media to avoid any decrease in transfection efficiency. Next, add a total of two micrograms of GHMT cocktail per reaction, and gently tap the tube to mix it. Incubate the reaction for 15 minutes at room temperature and then add the mixture to the plat-E cells in the drop wise manner.Record the date and time of transfection. Incubate the transfected plat-E cells overnight. One hour before plating the mouse embryonic fibroblast, prepare a 24 well plate for imunno-cytochemistry by first adding 12 millimeter fibronectin coated cover slips to each well.Next, add 300 microliters of bovine collagen solution to each well and incubate the plate for one hour. Near the end of the incubation, thaw a frozen vial of HCN4GFP mouse embryonic fibroblast. Quick thaw the cells, wash them once with pre-warmed fiberblast media and count the concentration of live cells.Then aspirate the coating solution in the 24 well plate and immediately plate 30, 000 cells per well onto the collagen and fibronectin coated cover slips. 24 hours post-transfection, filter the retro-viral medium through a 0.45 micron pore size surfectant free cellulose acetate filter, and transfer it to a 15 milliliter conical tube. Then, add polybrene to a final concentration of eight micrograms per milliliter.Carefully replenish the plat-E cells with two milliliters of fresh transfection medium. Changing the media too rapidly may cause the cells to detatch. Properly maintained plat-E cells produce an average of ten million infection units per milliliter.High GFP expression and intensity in more than 95%of the cells, typically correlates with a successful GHNT mediated generation of induced cardiomyocyte-like myocytes. Next, aspirate the medium of the cultured embryonic fibroblasts and add the freshly collected retroviral medium. Then return meth plate to the incubator and incubate overnight.The next day, again transfer the media from the plat-E cells to the embryonic fibroblasts. Discard the plat-E cells after the second virus collection. 48 hours post induction, aspirate the cell condition media and wash the cells once with PBS.Then add 500 microliters of pre-warmed induced cardio myocyte-like myocyite media per well of the 24 well plate. Replace the media every two to three days for the next 12 days. 14 days post induction, carefully aspirate the media and rinse each well with 300 microliters of ice cold PBS.Then removed the PBS and add 250 microliters of a 4%paraformaldehyde solution per well and incubate the cells for 15 minutes at room temperature. Next, permeabalize the cells by washing the wells three times for five minutes with 300 microliters of 0.1%tritonex 100 and PBS. Aspirate the excess permiableization solution after the last wash, and then add a universal blocking buffer at 300 microliters per well.Block for 10 minutes at room temperature. Stain one pair of slides overnight with mouse anti-alpha actenin, chicken anti-GFP and rabbit anti-NPPA antibodies to identify induced pacemaker-like myocytes in induced atrial-like myocytes. The following day, wash the wells and add the appropriate secondary antibody as described in the accompanying text protocol.Following additional washes, add 2.4 microliters of mounting media to a glass microscope slide. Take a cover slip from the 24 well plate and remove the excess solution. Then transfer it to the glass slide with the mounting media.For each slide, start from one edge and start scanning up and down in the red fluorescent channel to identify alpha-actinin positive sarcomere positive cells. Sarcomere striations are easiest to identify in the 555 nanometer wavelength. Once identified, switch to the green channel and keep note if the cells are positive.Then switch to the computer to asses the far red 647 nanometer wavelength to look for NPPA or MYL2 staining. Cells that are alpha-actinin positive, sarcomere positive, Hnc4-GFP negative and Nppa positive are induced atrial-like myocytes. Nppa staining will appear paranuclear and punctate.First, count the number of cells 24 hours after seeding as described in the accompanying text protocol. Next, visually inspect each cell on a cover slip for proper alpha-actinin and sacromere staining and record the number of cells positively stained for both markers. Then, divide by the actual total cells plated.An average reprogramming experiment will yield approximately one percent of cells that are both alpha-actinin and sarcomere positive. By day 14, the experiment can be stopped and assessed for sarcomere organization. This is identified by the positive alpha-actinin staining shown here in red.Only cells that exhibit sarcomere organization are capable of spontaneous beating, so this is a good first check. Shown here are induced pacemaker-like cardiomyocites from Hcn4-GFP reporter mice. Three cells from this view exhibit sarcomere organization.Three cell clusters positive for Hcn4-GFP are present in the center of the image and can be idenfied as a pacemaker cell type. Once cell at the bottom left can be observed with alpha-actinin staining but no sarcomere organization or identifiable subtype marker. In this population of induced ventricular like cardiomyocytes, the central cell exhibits sarcomere organization, this cell also stains positive for the myocin light chain marker MYL-2 which is a marker for ventricular cardio myocytes.Once mastered, the plating and the transfection of the plat-E cells can be performed in one hour. The induction of the mouse embryonic fibroblasts in one to two hours and the myto quantification of a full 24 well plate in about eight hours. While attempting this procedure, it's important to remember that healthy mouse embryonic fibroblasts and high titer viruses are key for a successful reprogramming experiment.After watching this video, you should have a good understanding of how to generate and quantify individual induced cardiomyocyte subtypes.

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JoVE Journal

Developmental Biology

7.3K Görüntüleme.  UT Southwestern Medical Center. The overall goal of this procedure is to generate and quantify a diverse set of induced cardiomyocyte subtypes. This method can help answer key, fundamental questions regarding subtype-specific lineage conversion during cardiac reprogramming such as the necessary cues that regulate sarcomere assembly and subtype specification. The main advantage of this technique is that it provides the method for quantified effects on cardio subtype specification which is required to perform mechanisis studi...