We thank S. Gr&#252;nberg (Bonn, Germany) and P. Freitag (Bonn, Germany) for their technical assistance.

<ol><li>Bergmann, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Evidence+for+cardiomyocyte+renewal+in+humans%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Evidence for cardiomyocyte renewal in humans.</a> Science. 324 (5923), 98-102 (2009).</li>
<li>Raulf, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Transgenic+systems+for+unequivocal+identification+of+cardiac+myocyte+nuclei+and+analysis+of+cardiomyocyte+cell+cycle+status%5BTitle%5D)+AND+%22Basic+Res.Cardiol%22%5BJournal%5D)">Transgenic systems for unequivocal identification of cardiac myocyte nuclei and analysis of cardiomyocyte cell cycle status.</a> Basic Res.Cardiol. 110 (3), 33(2015).</li>
<li>Field, C. M., Alberts, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Anillin%2C+a+contractile+ring+protein+that+cycles+from+the+nucleus+to+the+cell+cortex%5BTitle%5D)+AND+%22J.Cell+Biol%22%5BJournal%5D)">Anillin, a contractile ring protein that cycles from the nucleus to the cell cortex.</a> J.Cell Biol. 131 (1), 165-178 (1995).</li>
<li>Hesse, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+visualization+of+cell+division+using+high-resolution+imaging+of+M-phase+of+the+cell+cycle%5BTitle%5D)+AND+%22Nat.Commun%22%5BJournal%5D)">Direct visualization of cell division using high-resolution imaging of M-phase of the cell cycle.</a> Nat.Commun. 3, 1076(2012).</li>
<li>Eulalio, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Functional+screening+identifies+miRNAs+inducing+cardiac+regeneration%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Functional screening identifies miRNAs inducing cardiac regeneration.</a> Nature. 492 (7429), 376-381 (2012).</li>
<li>Di, S. V., Giacca, M., Capogrossi, M. C., Crescenzi, M., Martelli, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Knockdown+of+cyclin-dependent+kinase+inhibitors+induces+cardiomyocyte+re-entry+in+the+cell+cycle%5BTitle%5D)+AND+%22J.Biol.Chem%22%5BJournal%5D)">Knockdown of cyclin-dependent kinase inhibitors induces cardiomyocyte re-entry in the cell cycle.</a> J.Biol.Chem. 286 (10), 8644-8654 (2011).</li>
<li>Hosoda, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Clonality+of+mouse+and+human+cardiomyogenesis+in+vivo%5BTitle%5D)+AND+%22Proc.Natl.Acad.Sci.U.S.A%22%5BJournal%5D)">Clonality of mouse and human cardiomyogenesis in vivo.</a> Proc.Natl.Acad.Sci.U.S.A. 106 (40), 17169-17174 (2009).</li>
<li>Soonpaa, M. H., Field, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Assessment+of+cardiomyocyte+DNA+synthesis+in+normal+and+injured+adult+mouse+hearts%5BTitle%5D)+AND+%22Am.J.Physiol%22%5BJournal%5D)">Assessment of cardiomyocyte DNA synthesis in normal and injured adult mouse hearts.</a> Am.J.Physiol. 272 (1 Pt 2), H220-H226 (1997).</li>
<li>Ang, K. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Limitations+of+conventional+approaches+to+identify+myocyte+nuclei+in+histologic+sections+of+the+heart%5BTitle%5D)+AND+%22Am.J.Physiol+Cell+Physiol%22%5BJournal%5D)">Limitations of conventional approaches to identify myocyte nuclei in histologic sections of the heart.</a> Am.J.Physiol Cell Physiol. 298 (6), C1603-C1609 (2010).</li>
<li>Engel, F. B., Schebesta, M., Keating, M. T. <a target="_blank" href="https://www.ncbi.nlm.nih.gov/pubmed/16889791">Anillin localization defect in cardiomyocyte binucleation.</a> J Mol Cell Cardiol. 41 (4), 601-612 (2006).</li>
<li>Bergmann, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dynamics+of+Cell+Generation+and+Turnover+in+the+Human+Heart%5BTitle%5D)+AND+%22Cell%22%5BJournal%5D)">Dynamics of Cell Generation and Turnover in the Human Heart.</a> Cell. 161 (7), 1566-1575 (2015).</li>
</ol>

The authors declare that they have no competing financial interests.

There is a controversy over whether cardiomyocytes are able to reenter the cell cycle and divide after injury and during tissue homeostasis. Values for the basic turnover of cardiomyocytes have been given in the range between 1%1 and 80%7. Also after a cardiac lesion, the induction of cell cycle activity and the generation of new cardiomyocytes has been reported in the border zone, with values between 0.0083%8 and 25 - 40%7. T...

The correct identification of cardiomyocyte nuclei and the cell cycle status is of crucial importance for the determination of cardiac muscle turnover and regeneration. This is especially true for the use of nuclear markers, such as pHH3, Ki-67, or Thymidine analogs, for identifying cell cycle activity. As the proliferative capacity of adult mammalian cardiomyocytes is very small1, a false identification of a nucleus positive for a proliferation marker of a cardiomyocyte nucleus could make a crucial difference in the outcome of a proliferation assay. Moreover, cardiomyocytes are prone to variations in the cell cycle, such as endoreduplication and acytokinetic mitosis, which result in polyploid and multinucleated cells rather than in cell division. To this end, the interpretation of antibody staining against common cell cycle markers is not conclusive in all cases.
Here, we present methods for the straight-forward recognition of mouse cardiomyocytes and their nuclearity in native isolated cells and thick tissue sections at postnatal and adult stages by the unequivocal identification of their nuclei. For that purpose, a transgenic mouse line with cardiomyocyte-specific expression of a fusion protein consisting of human histone H2B and mCherry under the control of the Myh6 promoter (Myh6-H2B-mCh) was used2. Cross-breeding this mouse line with a transgenic proliferation indicator mouse line, in which the expression of an eGFP-anillin fusion protein is under the control of the ubiquitous chicken actin promoter with a CMV enhancer (CAG-eGFP-anillin), allows for the determination of the cell cycle status. The scaffolding protein anillin is specifically expressed in cell-cycle active cells3, and its differential subcellular localization during the cell cycle allows for live-tracking cell-cycle progression with a high resolution of M-Phase4. Therefore, the double transgenic mice can be used to discriminate between proliferating cardiomyocytes and those that undergo cell-cycle variations. This proves especially useful in screening for proliferation-inducing substances in vitro.

<table><tbody><tr><td>10 cm Petri dish</td><td>Sarstedt</td><td>821472</td><td></td></tr><tr><td>100 &amp;micro;m cell strainer</td><td>Becton Dickinson GmbH/Falcon</td><td>352360</td><td></td></tr><tr><td>2,3-Butanedione monoxime (BDM)</td><td>Sigma-Aldrich</td><td>B0753</td><td></td></tr><tr><td>G20x1 &amp;frac12; injection cannula, Sterican</td><td>Braun, Melsungen</td><td>4657519</td><td></td></tr><tr><td>20 gauge needle</td><td>Becton Dickinson GmbH</td><td>301300</td><td></td></tr><tr><td>24-well plates</td><td>Becton Dickinson GmbH/Falcon</td><td>353047</td><td></td></tr><tr><td>2-Methyl-butane</td><td>Carl Roth GmbH + Co. KG</td><td>3927.1</td><td></td></tr><tr><td>37% formaldehyde solution</td><td>AppliChem GmbH&amp;nbsp;</td><td>A0936,1000</td><td></td></tr><tr><td>3-way stopcock</td><td>B. Braun Medical Inc.</td><td>16494C</td><td></td></tr><tr><td>50 mL syringe</td><td>B. Braun Medical Inc.</td><td>8728810F</td><td></td></tr><tr><td>70% ethanol</td><td>Otto Fischar GmbH</td><td>27669</td><td></td></tr><tr><td>Alexa-Fluor-conjugated secondary antibody</td><td>Jackson ImmunoResearch</td><td>115-605-205</td><td></td></tr><tr><td>Alpha-Aktinin EA-53, Mouse IgG</td><td>Sigma-Aldrich, Steinheim</td><td>A7811</td><td></td></tr><tr><td>CaCl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>C4901</td><td></td></tr><tr><td>Cell Culture Microplate, 96 Well, Half Area</td><td>Greiner bio-one</td><td>675986</td><td></td></tr><tr><td>Collagenase B</td><td>Roche</td><td>11088815001</td><td></td></tr><tr><td>confocal microscope Eclipse Ti-E</td><td>Nikon</td><td></td><td></td></tr><tr><td>cryostat CM 3050S</td><td>Leica</td><td></td><td></td></tr><tr><td>donkey serum</td><td>Jackson Immuno Research, Suffolk, GB</td><td>017-000-121</td><td></td></tr><tr><td>Dulbecco&#039;s Phosphate Buffered Saline</td><td>Sigma-Aldrich</td><td>D8537</td><td></td></tr><tr><td>EDTA</td><td>Sigma-Aldrich</td><td>E4884</td><td></td></tr><tr><td>fetal calf serum</td><td>PromoCell, Heidelberg</td><td></td><td></td></tr><tr><td>Formaldehyde solution (4%)</td><td>PanReac AppliChem</td><td>A3697</td><td></td></tr><tr><td>Gelatine from porcine skin, Type A</td><td>Sigma-Aldrich, Steinheim</td><td>G2500</td><td></td></tr><tr><td>glass coverslips</td><td>VWR</td><td>631-0146</td><td></td></tr><tr><td>Glucose</td><td>Sigma-Aldrich</td><td>G7021</td><td></td></tr><tr><td>Heidelberger extension tube</td><td>IMPROMEDIFORM GmbH</td><td>MF 1833</td><td></td></tr><tr><td>Heparin-Natrium</td><td>Ratiopharm</td><td>5394.02.00</td><td></td></tr><tr><td>HEPES</td><td>Sigma-Aldrich</td><td>H3375</td><td></td></tr><tr><td>HistoBond microscope slides</td><td>Marienfeld</td><td>0810000</td><td></td></tr><tr><td>Hoechst 33342 (1&amp;nbsp;mg/mL)</td><td>Sigma Aldrich, Taufkirchen</td><td>B2261</td><td></td></tr><tr><td>Insulin syringe</td><td>Becton Dickinson GmbH</td><td>300334</td><td></td></tr><tr><td>Iscove&amp;rsquo;s ModifiedDulbecco&amp;rsquo;s Medium (IMDM)</td><td>Gibco/Life Technologies, Darmstadt</td><td>21980-032</td><td></td></tr><tr><td>KCl</td><td>Sigma-Aldrich</td><td>P9333</td><td></td></tr><tr><td>Laminin</td><td>Corning</td><td>354221</td><td></td></tr><tr><td>Laser Scanning Mikroskop Eclipse Ti</td><td>Nikoninstruments, D&amp;uuml;sseldorf</td><td></td><td></td></tr><tr><td>Lipofectamine RNAiMAX</td><td>Invitrogen/Life Technologies, Darmstadt</td><td>13778075</td><td></td></tr><tr><td>Mouse IgG Cy5 (donkey)</td><td>Jackson ImmunoResearch</td><td>715-175-151</td><td></td></tr><tr><td>MgCl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>M8266</td><td></td></tr><tr><td>microcentrifuge tube</td><td>Sarstedt</td><td>72690</td><td></td></tr><tr><td>Mini shaker</td><td>VWR</td><td>12620-940</td><td></td></tr><tr><td>mirVana miRNA mimic, hsa-miR199a-3p</td><td>Ambion/Thermo Fischer Scientific</td><td>4464066</td><td></td></tr><tr><td>Biopsy Mold</td><td>Sakura Finetek/ VWR</td><td>4565</td><td></td></tr><tr><td>M-slide 8-well ibiTreat</td><td>ibidi</td><td>80826</td><td></td></tr><tr><td>NaCl</td><td>Sigma-Aldrich</td><td>S9888</td><td></td></tr><tr><td>NaOH</td><td>Merck Millipore</td><td>567530</td><td></td></tr><tr><td>negative control(scrambled RNA)</td><td>Ambion/Thermo Fischer Scientific</td><td>AM4611</td><td></td></tr><tr><td>Neonatal Heart Dissociation Kit</td><td>Miltenyi Biotech, Bergisch Gladbach</td><td>130-098-373</td><td></td></tr><tr><td>NIS Elements AR 4.12.01-4.30.02-64bit</td><td>Nikoninstruments, D&amp;uuml;sseldorf</td><td></td><td></td></tr><tr><td>Non essential amino acids, NEAA</td><td>Gibco/Life Technologies, Darmstad</td><td>11140-035</td><td></td></tr><tr><td>Opti-MEM, Reduced Serum Medium</td><td>Gibco</td><td>51985-026</td><td></td></tr><tr><td>P21-siRNA</td><td>Ambion/Thermo Fischer Scientific</td><td>4390771</td><td></td></tr><tr><td>P27-siRNA</td><td>Ambion/Thermo Fischer Scientific</td><td>4390771</td><td></td></tr><tr><td>Penicillin/Streptomycin</td><td>Gibco/Life Technologies, Darmstadt</td><td>15140-122</td><td></td></tr><tr><td>Phosphate buffered saline (PBS)</td><td>Sigma-Aldrich, Steinheim</td><td>14190-094</td><td></td></tr><tr><td>Polyvinyl alcohol mounting medium with DABCO&amp;reg;, antifading</td><td>Sigma-Aldrich</td><td>10981</td><td></td></tr><tr><td>RNase A</td><td>Qiagen</td><td>1007885</td><td></td></tr><tr><td>RNaseZap</td><td>Invitrogen/Life Technologies, Darmstadt</td><td>AM9780</td><td></td></tr><tr><td>sample containers</td><td>Vitlab</td><td>80731</td><td></td></tr><tr><td>Serological pipette</td><td>Greiner</td><td>607180</td><td></td></tr><tr><td>software NIS Elements</td><td>Nikon</td><td></td><td></td></tr><tr><td>Sucrose</td><td>Sigma-Aldrich</td><td>S0389</td><td></td></tr><tr><td>Tissue-Tek O.C.T. Compound</td><td>Sakura Finetek/ VWR</td><td>25608-930</td><td></td></tr><tr><td>ToPro3 iodide (642/661)</td><td>Molecular probes/ThermoFisher Scientific</td><td>T3605</td><td></td></tr><tr><td>Tris</td><td>Sigma-Aldrich</td><td>T1503</td><td></td></tr><tr><td>Triton X</td><td>Fluka</td><td>93418</td><td></td></tr><tr><td>Triton X-100</td><td>Fluka</td><td>93418</td><td></td></tr><tr><td>Trypsin</td><td>Sigma-Aldrich</td><td>T1426</td><td></td></tr><tr><td>Wheat germ agglutinin (WGA) Fluorescein labeled</td><td>Vector Laboratories</td><td>VEC-FL-1021-5</td><td></td></tr><tr><td>&amp;alpha;-actinin antibody</td><td>Sigma-Aldrich</td><td>A7811</td><td></td></tr><tr><td>&amp;beta;-Mercaptoethanol</td><td>Sigma-Aldrich, Steinheim</td><td>M3148</td><td></td></tr></tbody></table>

visualization of cell cycle variations and determination of nucleation in postnatal cardiomyocytes

Cardiomyocytes are prone to variations of the cell cycle, such as endoreduplication (continuing rounds of DNA synthesis without karyokinesis and cytokinesis) and acytokinetic mitosis (karyokinesis but no cytokinesis). Such atypical cell cycle variations result in polyploid and multinucleated cells rather than in cell division. Therefore, to determine cardiac turnover and regeneration, it is of crucial importance to correctly identify cardiomyocyte nuclei, the number of nuclei per cell, and their cell cycle status. This is especially true for the use of nuclear markers for identifying cell cycle activity, such as thymidine analogues Ki-67, PCNA, or pHH3. Here, we present methods for recognizing cardiomyocytes and their nuclearity and for determining their cell cycle activity. We use two published transgenic systems: the Myh6-H2B-mCh transgenic mouse line, for the unequivocal identification of cardiomyocyte nuclei, and the CAG-eGFP-anillin mouse line, for distinguishing cell division from cell cycle variations. Combined together, these two systems ease the study of cardiac regeneration and plasticity.

In order to analyze the effects of siRNAs/miRNAs on the cell cycle activity of postnatal cardiomyocytes in vitro, cardiomyocytes of double-transgenic Myh6-H2B-mCh/CAG-eGFP-anillin mice were isolated on postnatal day 3 (P3) and transfected with cell cycle activity-inducing miR1995, siRNA p27, and siRNA Fzr1. Compared to the negative control (Figure 1A), the pictures of miR199- (Figure 1B) and siRNA p27- (Figure 1C</...

Watch this Scientific Journal Video about Visualization of Cell Cycle Variations and Determination of Nucleation in Postnatal Cardiomyocytes at JoVE.com

Visualization of Cell Cycle Variations and Determination of Nucleation in Postnatal Cardiomyocytes

To distinguish cell division from cell cycle variations in cardiomyocytes, we present protocols using two transgenic mouse lines: Myh6-H2B-mCh transgenic mice, for the unequivocal identification of cardiomyocyte nuclei, and CAG-eGFP-anillin mice, for distinguishing cell division from cell cycle variations.

Cardiomyocytes are prone to variations of the cell cycle, such as endoreduplication (continuing rounds of DNA synthesis without karyokinesis and ...

visualization-cell-cycle-variations-determination-nucleation

Research

JoVE Journal

Developmental Biology

8.7K Views.  University of Bonn. To distinguish cell division from cell cycle variations in cardiomyocytes, we present protocols using two transgenic mouse lines: Myh6-H2B-mCh transgenic mice, for the unequivocal identification of cardiomyocyte nuclei, and CAG-eGFP-anillin mice, for distinguishing cell division from cell cycle variations.

Video: Visualization of Cell Cycle Variations and Determination of Nucleation in Postnatal Cardiomyocytes

Analysis of Cardiomyocyte Development using Immunofluorescence in Embryonic Mouse Heart

During heart development, the generation of myocardial-specific structural and functional units including sarcomeres, contractile myofibrils, intercalated discs, and costameres requires the coordinated assembly of multiple components in time and space. Disruption in assembly of these components leads to developmental heart defects. Immunofluorescent staining techniques are used commonly in cultured cardiomyocytes to probe myofibril maturation, but this ex vivo approach is limited by the extent to which myocytes will fully differentiate in culture, lack of normal in vivo mechanical inputs, and absence of endocardial cues. Application of immunofluorescence techniques to the study of developing mouse heart is desirable but more technically challenging, and methods often lack sufficient sensitivity and resolution to visualize sarcomeres in the early stages of heart development. Here, we describe a robust and reproducible method to co-immunostain multiple proteins or to co-visualize a fluorescent protein with immunofluorescent staining in the embryonic mouse heart and use this method to analyze developing myofibrils, intercalated discs, and costameres. This method can be further applied to assess cardiomyocyte structural changes caused by mutations that lead to developmental heart defects.

Mutations that lead to congenital heart defects benefit from in vivo investigation of cardiac structure during development, but high-resolution structural studies in the mouse embryonic heart are technically challenging. Here we present a robust immunofluorescence and image analysis method to assess cardiomyocyte-specific structures in the developing mouse heart.

During heart development, the generation of myocardial-specific structural and functional units including sarcomeres, contractile myofibrils, intercalated ...

Simultaneous Assessment of Cardiomyocyte DNA Synthesis and Ploidy: A Method to Assist Quantification of Cardiomyocyte Regeneration and Turnover

Although it is accepted that the heart has a limited potential to regenerate cardiomyocytes following injury and that low levels of cardiomyocyte turnover occur during normal ageing, quantification of these events remains challenging. This is in part due to the rarity of the process and the fact that multiple cellular sources contribute to myocardial maintenance. Furthermore, DNA duplication within cardiomyocytes often leads to a polyploid cardiomyocyte and only rarely leads to new cardiomyocytes by cellular division. In order to accurately quantify cardiomyocyte turnover discrimination between these processes is essential. The protocol described here employs long term nucleoside labeling in order to label all nuclei which have arisen as a result of DNA replication and cardiomyocyte nuclei identified by utilizing nuclei isolation and subsequent PCM1 immunolabeling. Together this allows the accurate and sensitive identification of the nucleoside labeling of the cardiomyocyte nuclei population. Furthermore, 4&#8242;,6-diamidino-2-phenylindole labeling and analysis of nuclei ploidy, enables the discrimination of neo-cardiomyocyte nuclei from nuclei which have incorporated nucleoside during polyploidization. Although this method cannot control for cardiomyocyte binucleation, it allows a rapid and robust quantification of neo-cardiomyocyte nuclei while accounting for polyploidization. This method has a number of downstream applications including assessing the potential therapeutics to enhance cardiomyocyte regeneration or investigating the effects of cardiac disease on cardiomyocyte turnover and ploidy. This technique is also compatible with additional downstream immunohistological techniques, allowing quantification of nucleoside incorporation in all cardiac cell types.

Quantification of cardiomyocyte turnover is challenging. The protocol described here makes an important contribution to this challenge by enabling accurate and sensitive quantification of neo-cardiomyocyte nuclei generation and nuclei ploidy.

Although it is accepted that the heart has a limited potential to regenerate cardiomyocytes following injury and that low levels of cardiomyocyte turnover ...

Imaging Cleared Embryonic and Postnatal Hearts at Single-cell Resolution

Single clonal tracing and analysis at the whole-heart level can determine cardiac progenitor cell behavior and differentiation during cardiac development, and allow for the study of the cellular and molecular basis of normal and abnormal cardiac morphogenesis. Recent emerging technologies of retrospective single clonal analyses make the study of cardiac morphogenesis at single cell resolution feasible. However, tissue opacity and light scattering of the heart as imaging depth is increased hinder whole-heart imaging at single cell resolution. To overcome these obstacles, a whole-embryo clearing system that can render the heart highly transparent for both illumination and detection must be developed. Fortunately, in the last several years, many methodologies for whole-organism clearing systems such as CLARITY, Scale, SeeDB, ClearT, 3DISCO, CUBIC, DBE, BABB and PACT have been reported. This lab is interested in the cellular and molecular mechanisms of cardiac morphogenesis. Recently, we established single cell lineage tracing via the ROSA26-CreERT2; ROSA26-Confetti system to sparsely label cells during cardiac development. We adapted several whole embryo-clearing methodologies including Scale and CUBIC (clear, unobstructed brain imaging cocktails and computational analysis) to clear the embryo in combination with whole mount staining to image single clones inside the heart. The heart was successfully imaged at single cell resolution. We found that Scale can clear the embryonic heart, but cannot effectively clear the postnatal heart, while CUBIC can clear the postnatal heart, but damages the embryonic heart by dissolving the tissue. The methods described here will permit the study of gene function at a single clone resolution during cardiac morphogenesis, which, in turn, can reveal the cellular and molecular basis of congenital heart defects.

We describe a protocol to volumetrically image fluorescent protein labeled cells deep inside intact embryonic and postnatal hearts. Utilizing tissue-clearing methods in combination with whole mount staining, single fluorescent protein-labeled cells inside an embryonic or postnatal heart can be imaged clearly and accurately.

Single clonal tracing and analysis at the whole-heart level can determine cardiac progenitor cell behavior and differentiation during cardiac development, ...

Aggregate Size Optimization in Microwells for Suspension-based Cardiac Differentiation of Human Pluripotent Stem Cells

Cardiac differentiation of human pluripotent stems cells (hPSCs) is typically carried out in suspension cell aggregates. Conventional aggregate formation of hPSCs involves dissociating cell colonies into smaller clumps, with size control of the clumps crudely controlled by pipetting the cell suspension until the desired clump size is achieved. One of the main challenges of conventional aggregate-based cardiac differentiation of hPSCs is that culture heterogeneity and spatial disorganization lead to variable and inefficient cardiomyocyte yield. We and others have previously reported that human embryonic stem cell (hESC) aggregate size can be modulated to optimize cardiac induction efficiency. We have addressed this challenge by employing a scalable, microwell-based approach to control physical parameters of aggregate formation, specifically aggregate size and shape. The method we describe here consists of forced aggregation of defined hPSC numbers in microwells, and the subsequent culture of these aggregates in conditions that direct cardiac induction. This protocol can be readily scaled depending on the size and number of wells used. Using this method, we can consistently achieve culture outputs with cardiomyocyte frequencies greater than 70%.

Conventional methods to initiate suspension aggregate based cardiac differentiation of human pluripotent stems cells (hPSCs) are plagued with culture heterogeneity with respect to aggregate size and shape. Here, we describe a robust method for cardiac differentiation employing microwells to generate size-controlled hPSC aggregates cultured under cardiac-promoting conditions.

Cardiac differentiation of human pluripotent stems cells (hPSCs) is typically carried out in suspension cell aggregates.  Conventional aggregate formation ...

Isolation of Cardiomyocyte Nuclei from Post-mortem Tissue

Identification of cardiomyocyte nuclei has been challenging in tissue sections as most strategies rely only on cytoplasmic marker proteins1. Rare events in cardiac myocytes such as proliferation and apoptosis require an accurate identification of cardiac myocyte nuclei to analyze cellular renewal in homeostasis and in pathological conditions2. Here, we provide a method to isolate cardiomyocyte nuclei from post mortem tissue by density sedimentation and immunolabeling with antibodies against pericentriolar material 1 (PCM-1) and subsequent flow cytometry sorting. This strategy allows a high throughput analysis and isolation with the advantage of working equally well on fresh tissue and frozen archival material. This makes it possible to study material already collected in biobanks. This technique is applicable and tested in a wide range of species and suitable for multiple downstream applications such as carbon-14 dating3, cell-cycle analysis4, visualization of thymidine analogues (e.g. BrdU and IdU)4, transcriptome and epigenetic analysis.

Cardiac nuclei are isolated via density sedimentation and immunolabeled with antibodies against pericentriolar material 1 (PCM-1) to identify and sort cardiomyocyte nuclei by flow cytometry.

Identification of cardiomyocyte nuclei has been challenging in tissue sections as most strategies rely only on cytoplasmic marker proteins1. Rare events ...

Medicine

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 ...

Efficient Derivation of Human Cardiac Precursors and Cardiomyocytes from Pluripotent Human Embryonic Stem Cells with Small Molecule Induction

To date, the lack of a suitable human cardiac cell source has been the major setback in regenerating the human myocardium, either by cell-based transplantation or by cardiac tissue engineering1-3. Cardiomyocytes become terminally-differentiated soon after birth and lose their ability to proliferate. There is no evidence that stem/progenitor cells derived from other sources, such as the bone marrow or the cord blood, are able to give rise to the contractile heart muscle cells following transplantation into the heart1-3. The need to regenerate or repair the damaged heart muscle has not been met by adult stem cell therapy, either endogenous or via cell delivery1-3. The genetically stable human embryonic stem cells (hESCs) have unlimited expansion ability and unrestricted plasticity, proffering a pluripotent reservoir for in vitro derivation of large supplies of human somatic cells that are restricted to the lineage in need of repair and regeneration4,5. Due to the prevalence of cardiovascular disease worldwide and acute shortage of donor organs, there is intense interest in developing hESC-based therapies as an alternative approach. However, how to channel the wide differentiation potential of pluripotent hESCs efficiently and predictably to a desired phenotype has been a major challenge for both developmental study and clinical translation. Conventional approaches rely on multi-lineage inclination of pluripotent cells through spontaneous germ layer differentiation, resulting in inefficient and uncontrollable lineage-commitment that is often followed by phenotypic heterogeneity and instability, hence, a high risk of tumorigenicity6-8 (see a schematic in Fig. 1A). In addition, undefined foreign/animal biological supplements and/or feeders that have typically been used for the isolation, expansion, and differentiation of hESCs may make direct use of such cell-specialized grafts in patients problematic9-11. To overcome these obstacles, we have resolved the elements of a defined culture system necessary and sufficient for sustaining the epiblast pluripotence of hESCs, serving as a platform for de novo derivation of clinically-suitable hESCs and effectively directing such hESCs uniformly towards clinically-relevant lineages by small molecules12 (see a schematic in Fig. 1B). After screening a variety of small molecules and growth factors, we found that such defined conditions rendered nicotinamide (NAM) sufficient to induce the specification of cardiomesoderm direct from pluripotent hESCs that further progressed to cardioblasts that generated human beating cardiomyocytes with high efficiency (Fig. 2). We defined conditions for induction of cardioblasts direct from pluripotent hESCs without an intervening multi-lineage embryoid body stage, enabling well-controlled efficient derivation of a large supply of human cardiac cells across the spectrum of developmental stages for cell-based therapeutics.

We have established a protocol for induction of cardioblasts direct from pluripotent human embryonic stem cells maintained under defined conditions with small molecules, which enables derivation of a large supply of human cardiac progenitors and functional cardiomyocytes for cardiovascular repair.

To date, the lack of a suitable human cardiac cell source has been the major setback in regenerating the human myocardium, either by cell-based ...

Biology

High-Throughput Cardiotoxicity Screening Using Mature Human Induced Pluripotent Stem Cell-Derived Cardiomyocyte Monolayers

Human induced stem cell-derived cardiomyocytes (hiPSC-CMs) are used to replace and reduce the dependence on animals and animal cells for preclinical cardiotoxicity testing. In two-dimensional monolayer formats, hiPSC-CMs recapitulate the structure and function of the adult human heart muscle cells when cultured on an optimal extracellular matrix (ECM). A human perinatal stem cell-derived ECM (maturation-inducing extracellular matrix-MECM) matures the hiPSC-CM structure, function, and metabolic state in 7 days after plating. 
Mature hiPSC-CM monolayers also respond as expected to clinically relevant medications, with a known risk of causing arrhythmias and cardiotoxicity. The maturation of hiPSC-CM monolayers was an obstacle to the widespread adoption of these valuable cells for regulatory science and safety screening, until now. This article presents validated methods for the plating, maturation, and high-throughput, functional phenotyping of hiPSC-CM electrophysiological and contractile function. These methods apply to commercially available purified cardiomyocytes, as well as stem cell-derived cardiomyocytes generated in-house using highly efficient, chamber-specific differentiation protocols. 
High-throughput electrophysiological function is measured using either voltage-sensitive dyes (VSDs; emission: 488 nm), calcium-sensitive fluorophores (CSFs), or genetically encoded calcium sensors (GCaMP6). A high-throughput optical mapping device is used for optical recordings of each functional parameter, and custom dedicated software is used for electrophysiological data analysis. MECM protocols are applied for medication screening using a positive inotrope (isoprenaline) and human Ether-a-go-go-related gene (hERG) channel-specific blockers. These resources will enable other investigators to successfully utilize mature hiPSC-CMs for high-throughput, preclinical cardiotoxicity screening, cardiac medication efficacy testing, and cardiovascular research.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer an alternative to using animals for preclinical cardiotoxicity screening. A limitation to the widespread adoption of hiPSC-CMs in preclinical toxicity screening is the immature, fetal-like phenotype of the cells. Presented here are protocols for robust and rapid maturation of hiPSC-CMs.

Human induced stem cell-derived cardiomyocytes (hiPSC-CMs) are used to replace and reduce the dependence on animals and animal cells for preclinical ...

Bioengineering

Model of Ischemic Heart Disease and Video-Based Comparison of Cardiomyocyte Contraction Using hiPSC-Derived Cardiomyocytes

Ischemic heart disease is a significant cause of death worldwide. It has therefore been the subject of a tremendous amount of research, often with small-animal models such as rodents. However, the physiology of the human heart differs significantly from that of the rodent heart, underscoring the need for clinically relevant models to study heart disease. Here, we present a protocol to model ischemic heart disease using cardiomyocytes differentiated from human induced pluripotent stem cells (hiPS-CMs) and to quantify the damage and functional impairment of the ischemic cardiomyocytes. Exposure to 2% oxygen without glucose and serum increases the percentage of injured cells, which is indicated by staining of the nucleus with propidium iodide, and decreases cellular viability. These conditions also decrease the contractility of hiPS-CMs as confirmed by displacement vector field analysis of microscopic video images. This protocol may furthermore provide a convenient method for personalized drug screening by facilitating the use of hiPS cells from individual patients. Therefore, this model of ischemic heart disease, based on iPS-CMs of human origin, can provide a useful platform for drug screening and further research on ischemic heart disease.

We present a model of ischemic heart disease using cardiomyocytes derived from human induced pluripotent stem cells, together with a method for quantitative evaluation of tissue damage caused by ischemia. This model can provide a useful platform for drug screening and further research on ischemic heart disease.

Ischemic heart disease is a significant cause of death worldwide. It has therefore been the subject of a tremendous amount of research, often with ...

High Efficiency Differentiation of Human Pluripotent Stem Cells to Cardiomyocytes and Characterization by Flow Cytometry

There is an urgent need to develop approaches for repairing the damaged heart, discovering new therapeutic drugs that do not have toxic effects on the heart, and improving strategies to accurately model heart disease. The potential of exploiting human induced pluripotent stem cell (hiPSC) technology to generate cardiac muscle &#8220;in a dish&#8221; for these applications continues to generate high enthusiasm. In recent years, the ability to efficiently generate cardiomyogenic cells from human pluripotent stem cells (hPSCs) has greatly improved, offering us new opportunities to model very early stages of human cardiac development not otherwise accessible. In contrast to many previous methods, the cardiomyocyte differentiation protocol described here does not require cell aggregation or the addition of Activin A or BMP4 and robustly generates cultures of cells that are highly positive for cardiac troponin I and T (TNNI3, TNNT2), iroquois-class homeodomain protein IRX-4 (IRX4), myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC2v) and myosin regulatory light chain 2, atrial isoform (MLC2a) by day 10 across all human embryonic stem cell (hESC) and hiPSC lines tested to date. Cells can be passaged and maintained for more than 90 days in culture. The strategy is technically simple to implement and cost-effective. Characterization of cardiomyocytes derived from pluripotent cells often includes the analysis of reference markers, both at the mRNA and protein level. For protein analysis, flow cytometry is a powerful analytical tool for assessing quality of cells in culture and determining subpopulation homogeneity. However, technical variation in sample preparation can significantly affect quality of flow cytometry data. Thus, standardization of staining protocols should facilitate comparisons among various differentiation strategies. Accordingly, optimized staining protocols for the analysis of IRX4, MLC2v, MLC2a, TNNI3, and TNNT2 by flow cytometry are described.

The article describes the detailed methodology to efficiently differentiate human pluripotent stem cells into cardiomyocytes by selectively modulating the Wnt pathway, followed by flow cytometry analysis of reference markers to assess homogeneity and identity of the population.

There is an urgent need to develop approaches for repairing the damaged heart, discovering new therapeutic drugs that do not have toxic effects on the ...

Visualization of Cell Cycle Variations and Determination of Nucleation in Postnatal Cardiomyocytes

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