Purification of cardiomyocytes from differentiating pluripotent stem cells using molecular beacons that target cardiomyocyte-specific mRNA

Enriching Cardiomyocytes Derived from hiPSCs by Magnetic-Activated Cell Sorting (MACS)

Cardiomyocytes (CMs) derived from human-induced pluripotent stem cells (hiPSCs) are considered a promising platform for multiple applications, including disease modeling, regenerative medicine, screening of drug toxicity and investigation of cardiomyogenesis. Despite remarkable improvement in methodology enabling differentiation of hiPSCs into CMs, applied protocols generate heterogeneous cell populations composed of CMs along with differentiated non-cardiac cell-types and undifferentiated hiPSCs. Here we describea procedure of automated Magnetic-Activated Cell Sorting (autoMACS) for the purification of hiPSCs-derived CMs under sterile culture conditions. We illustrate that this approach led to a robust depletion of non-cardiac cells and enrichment of CMs, a result particularly crucial for hiPSC lines with poor cardiac differentiation efficiencies.

Cardiomyocyte precursors generated by direct reprogramming and molecular beacon selection attenuate ventricular remodeling after experimental myocardial infarction

BACKGROUND

Direct cardiac reprogramming is currently being investigated for the generation of cells with a true cardiomyocyte (CM) phenotype. Based on the original approach of cardiac transcription factor-induced reprogramming of fibroblasts into CM-like cells, various modifications of that strategy have been developed. However, they uniformly suffer from poor reprogramming efficacy and a lack of translational tools for target cell expansion and purification. Therefore, our group has developed a unique approach to generate proliferative cells with a pre-CM phenotype that can be expanded in vitro to yield substantial cell doses.

METHODS

Cardiac fibroblasts were reprogrammed toward CM fate using lentiviral transduction of cardiac transcriptions factors (GATA4, MEF2C, TBX5, and MYOCD). The resulting cellular phenotype was analyzed by RNA sequencing and immunocytology. Live target cells were purified based on intracellular CM marker expression using molecular beacon technology and fluorescence-activated cell sorting. CM commitment was assessed using 5-azacytidine-based differentiation assays and the therapeutic effect was evaluated in a mouse model of acute myocardial infarction using echocardiography and histology. The cellular secretome was analyzed using mass spectrometry.

RESULTS

We found that proliferative CM precursor-like cells were part of the phenotype spectrum arising during direct reprogramming of fibroblasts toward CMs. These induced CM precursors (iCMPs) expressed CPC- and CM-specific proteins and were selectable via hairpin-shaped oligonucleotide hybridization probes targeting Myh6/7-mRNA-expressing cells. After purification, iCMPs were capable of extensive expansion, with preserved phenotype when under ascorbic acid supplementation, and gave rise to CM-like cells with organized sarcomeres in differentiation assays. When transplanted into infarcted mouse hearts, iCMPs prevented CM loss, attenuated fibrotic scarring, and preserved ventricular function, which can in part be attributed to their substantial secretion of factors with documented beneficial effect on cardiac repair.

CONCLUSIONS

Fibroblast reprogramming combined with molecular beacon-based cell selection yields an iCMP-like cell population with cardioprotective potential. Further studies are needed to elucidate mechanism-of-action and translational potential.

Downstream bioprocessing of human pluripotent stem cell-derived therapeutics

With the advancement in lineage-specific differentiation from human pluripotent stem cells (hPSCs), downstream cell separation has now become a critical step to produce hPSC-derived products. Since differentiation procedures usually result in a heterogeneous cell population, cell separation needs to be performed either to enrich the desired cell population or remove the undesired cell population. This article summarizes recent advances in separation processes for hPSC-derived cells, including the standard separation technologies, such as magnetic-activated cell sorting, as well as the novel separation strategies, such as those based on adhesion strength and metabolic flux. Specifically, the downstream bioprocessing flow and the identification of surface markers for various cell lineages are discussed. While challenges remain for large-scale downstream bioprocessing of hPSC-derived cells, the rational quality-by-design approach should be implemented to enhance the understanding of the relationship between process and the product and to ensure the safety of the produced cells.

Enhancement strategy for effective vascular regeneration following myocardial infarction through a dual stem cell approach

Since an impaired coronary blood supply following myocardial infarction (MI) negatively affects heart function, therapeutic neovascularization is considered one of the major therapeutic strategies for cell-based cardiac repair. Here, to more effectively achieve therapeutic neovascularization in ischemic hearts, we developed a dual stem cell approach for effective vascular regeneration by utilizing two distinct types of stem cells, CD31⁺-endothelial cells derived from human induced pluripotent stem cells (hiPSC-ECs) and engineered human mesenchymal stem cells that continuously secrete stromal derived factor-1α (SDF-eMSCs), to simultaneously promote natal vasculogenesis and angiogenesis, two core mechanisms of neovascularization. To induce more comprehensive vascular regeneration, we intramyocardially injected hiPSC-ECs to produce de novo vessels, possibly via vasculogenesis, and a 3D cardiac patch encapsulating SDF-eMSCs (SDF-eMSC-PA) to enhance angiogenesis through prolonged secretion of paracrine factors, including SDF-1α, was implanted into the epicardium of ischemic hearts. We verified that hiPSC-ECs directly contribute to de novo vessel formation in ischemic hearts, resulting in enhanced cardiac function. In addition, the concomitant implantation of SDF1α-eMSC-PAs substantially improved the survival, retention, and vasculogenic potential of hiPSC-ECs, ultimately achieving more comprehensive neovascularization in the MI hearts. Of note, the newly formed vessels through the dual stem cell approach were significantly larger and more functional than those formed by hiPSC-ECs alone. In conclusion, these results provide compelling evidence that our strategy for effective vascular regeneration can be an effective means to treat ischemic heart disease.

A treatment involving two different types of stem cells leads to repairing failed hearts by making new functional blood vessels. Researchers at the City University of Hong Kong and the Catholic University of Korea induced heart attacks in rats before injecting the hearts with endothelial cells derived from human induced pluripotent stem cells, specialized to form blood vessels. These cells successfully induced the formation of new blood vessels in the damaged hearts. The researchers combined this treatment with a cardiac patch containing engineered human adult stem cells, which improved the survival and performance of the endothelial cells. And this dual stem cell treatment resulted in enhanced cardiac function and a higher number of larger and stronger new blood vessels than those produced by the single-cell treatment suggesting an effective way to repair failed hearts.

Brugada Syndrome: Different Experimental Models and the Role of Human Cardiomyocytes From Induced Pluripotent Stem Cells

Brugada syndrome (BrS) is an inherited and rare cardiac arrhythmogenic disease associated with an increased risk of ventricular fibrillation and sudden cardiac death. Different genes have been linked to BrS. The majority of mutations are located in the SCN5A gene, and the typical abnormal ECG is an elevation of the ST segment in the right precordial leads V1 to V3. The pathophysiological mechanisms of BrS were studied in different models, including animal models, heterologous expression systems, and human-induced pluripotent stem cell-derived cardiomyocyte models. Currently, only a few BrS studies have used human-induced pluripotent stem cell-derived cardiomyocytes, most of which have focused on genotype-phenotype correlations and drug screening. The combination of new technologies, such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 (CRISPR associated protein 9)-mediated genome editing and 3-dimensional engineered heart tissues, has provided novel insights into the pathophysiological mechanisms of the disease and could offer opportunities to improve the diagnosis and treatment of patients with BrS. This review aimed to compare different models of BrS for a better understanding of the roles of human-induced pluripotent stem cell-derived cardiomyocytes in current BrS research and personalized medicine at a later stage.

Regeneration of infarcted mouse hearts by cardiovascular tissue formed via the direct reprogramming of mouse fibroblasts

Fibroblasts can be directly reprogrammed into cardiomyocytes, endothelial cells or smooth muscle cells. Here we report the reprogramming of mouse tail-tip fibroblasts simultaneously into cells resembling these three cell types using the microRNA mimic miR-208b-3p, ascorbic acid and bone morphogenetic protein 4, as well as the formation of tissue-like structures formed by the directly reprogrammed cells. Implantation of the formed cardiovascular tissue into the infarcted hearts of mice led to the migration of reprogrammed cells to the injured tissue, reducing regional cardiac strain and improving cardiac function. The migrated endothelial cells and smooth muscle cells contributed to vessel formation, and the migrated cardiomyocytes, which initially displayed immature characteristics, became mature over time and formed gap junctions with host cardiomyocytes. Direct reprogramming of somatic cells to make cardiac tissue may aid the development of applications in cell therapy, disease modelling and drug discovery for cardiovascular diseases.

Atrial-specific hiPSC-derived cardiomyocytes in drug discovery and disease modeling

The discovery and application of human-induced pluripotent stem cells (hiPSCs) have been instrumental in the investigation of the pathophysiology of cardiovascular diseases. Patient-specific hiPSCs can now be generated, genome-edited, and subsequently differentiated into various cell types and used for regenerative medicine, disease modeling, drug testing, toxicity screening, and 3D tissue generation. Modulation of the retinoic acid signaling pathway has been shown to direct cardiomyocyte differentiation towards an atrial lineage. A variety of studies have successfully differentiated patient-specific atrial cardiac myocytes (hiPSC-aCM) and atrial engineered heart tissue (aEHT) that express atrial specific genes (e.g., sarcolipin and ANP) and exhibit atrial electrophysiological and contractility profiles. Identification of protocols to differentiate atrial cells from patients with atrial fibrillation and other inherited diseases or creating disease models using genetic mutation studies has shed light on the mechanisms of atrial-specific diseases and identified the efficacy of atrial-selective pharmacological compounds. hiPSC-aCMs and aEHTs can be used in drug discovery and drug screening studies to investigate the efficacy of atrial selective drugs on atrial fibrillation models. Furthermore, hiPSC-aCMs can be effective tools in studying the mechanism, pathophysiology and treatment options of atrial fibrillation and its genetic underpinnings. The main limitation of using hiPSC-CMs is their immature phenotype compared to adult CMs. A wide range of approaches and protocols are used by various laboratories to optimize and enhance CM maturation, including electrical stimulation, culture time, biophysical cues and changes in metabolic factors.

Differentiation and Application of Human Pluripotent Stem Cells Derived Cardiovascular Cells for Treatment of Heart Diseases: Promises and Challenges

Human pluripotent stem cells (hPSCs) are derived from human embryos (human embryonic stem cells) or reprogrammed from human somatic cells (human induced pluripotent stem cells). They can differentiate into cardiovascular cells, which have great potential as exogenous cell resources for restoring cardiac structure and function in patients with heart disease or heart failure. A variety of protocols have been developed to generate and expand cardiovascular cells derived from hPSCs in vitro. Precisely and spatiotemporally activating or inhibiting various pathways in hPSCs is required to obtain cardiovascular lineages with high differentiation efficiency. In this concise review, we summarize the protocols of differentiating hPSCs into cardiovascular cells, highlight their therapeutic application for treatment of cardiac diseases in large animal models, and discuss the challenges and limitations in the use of cardiac cells generated from hPSCs for a better clinical application of hPSC-based cardiac cell therapy.

Cell surface markers for immunophenotyping human pluripotent stem cell-derived cardiomyocytes

Human pluripotent stem cells (hPSC) self-renew and represent a potentially unlimited source for the production of cardiomyocytes (CMs) suitable for studies of human cardiac development, drug discovery, cardiotoxicity testing, and disease modelling and for cell-based therapies. However, most cardiac differentiation protocols yield mixed cultures of atrial-, ventricular-, and pacemaker-like cells at various stages of development, as well as non-CMs. The proportions and maturation states of these cell types result from disparities among differentiation protocols and time of cultivation, as well as hPSC reprogramming inconsistencies and genetic background variations. The reproducible use of hPSC-CMs for research and therapy is therefore limited by issues of cell population heterogeneity and functional states of maturation. A validated method that overcomes issues of cell heterogeneity is immunophenotyping coupled with live cell sorting, an approach that relies on accessible surface markers restricted to the desired cell type(s). Here we review current progress in unravelling heterogeneity in hPSC-cardiac cultures and in the identification of surface markers suitable for defining cardiac identity, subtype specificity, and maturation states.

Label-Free Isolation and Single Cell Biophysical Phenotyping Analysis of Primary Cardiomyocytes Using Inertial Microfluidics

To advance the understanding of cardiomyocyte (CM) identity and function, appropriate tools to isolate pure primary CMs are needed. A label-free method to purify viable CMs from mouse neonatal hearts is developed using a simple particle size-based inertial microfluidics biochip achieving purities of over 90%. Purified CMs are viable and retained their identity and function as depicted by the expression of cardiac-specific markers and contractility. The physico-mechanical properties of sorted cells are evaluated using downstream real-time deformability cytometry. CMs exhibited different physico-mechanical properties when compared with non-CMs. Taken together, this CM isolation and phenotyping method could serve as a valuable tool to progress the understanding of CM identity and function, and ultimately benefit cell therapy and diagnostic applications.

Cardiomyocyte Induction and Regeneration for Myocardial Infarction Treatment: Cell Sources and Administration Strategies

Occlusion of coronary artery and subsequent damage or death of myocardium can lead to myocardial infarction (MI) and even heart failure-one of the leading causes of deaths world wide. Notably, myocardium has extremely limited regeneration potential due to the loss or death of cardiomyocytes (i.e., the cells of which the myocardium is comprised) upon MI. A variety of stem cells and stem cell-derived cardiovascular cells, in situ cardiac fibroblasts and endogenous proliferative epicardium, have been exploited to provide renewable cellular sources to treat injured myocardium. Also, different strategies, including direct injection of cell suspensions, bioactive molecules, or cell-incorporated biomaterials, and implantation of artificial cardiac scaffolds (e.g., cell sheets and cardiac patches), have been developed to deliver renewable cells and/or bioactive molecules to the MI site for the myocardium regeneration. This article briefly surveys cell sources and delivery strategies, along with biomaterials and their processing techniques, developed for MI treatment. Key issues and challenges, as well as recommendations for future research, are also discussed.

Heart regeneration with human pluripotent stem cells: Prospects and challenges

Cardiovascular disease, ranging from congenital heart disease to adult myocardial infarction, is the leading cause of death worldwide. In pursuit of reliable cardiovascular regenerative medicine, human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), offer plenty of potential cell-based applications. HPSCs are capable of proliferating indefinitely in an undifferentiated state, and are also pluripotent, being able to differentiate into virtually any somatic cell types given specific stepwise cues, thus representing an unlimited source to generate functional cardiovascular cells for heart regeneration. Here we recapitulated current advances in developing efficient protocols to generate hPSC-derived cardiovascular cell lineages, including cardiomyocytes, endothelial cells, and epicardial cells. We also discussed applications of hPSC-derived cells in combination with compatible bioactive materials, promising trials of cell transplantation in animal models of myocardial infarction, and potential hurdles to bring us closer to the ultimate goal of cell-based heart repair.

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HPSCs hold tremendous therapeutic potential for treating CVDs.

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HPSCs could differentiate into multiple cardiovascular cell lineages.

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Transplantation of hPSC-derived cardiovascular cells and biomaterials shows promising results, but challenges still remain.

Isolation of Cardiomyocytes Undergoing Mitosis With Complete Cytokinesis

RATIONALE

Adult human cardiomyocytes do not complete cytokinesis despite passing through the S-phase of the cell cycle. As a result, polyploidization and multinucleation occur. To get a deeper understanding of the mechanisms surrounding division of cardiomyocytes, there is a crucial need for a technique to isolate cardiomyocytes that complete cell division/cytokinesis.

OBJECTIVE

Markers of cell cycle progression based on DNA content cannot distinguish between mitotic cardiomyocytes that fail to complete cytokinesis from those cells that undergo true cell division. With the use of molecular beacons (MBs) targeting specific mRNAs, we aimed to identify truly proliferative cardiomyocytes derived from human induced pluripotent stem cells.

METHODS AND RESULTS

Fluorescence-activated cell sorting combined with MBs was performed to sort cardiomyocyte populations enriched for mitotic cells. Expressions of cell cycle specific genes were confirmed by means of reverse transcription-quantitative polymerase chain reaction and single-cell RNA sequencing (scRNA-seq) combined with gene signatures of cell cycle progression. We characterized the sorted groups by proliferation assays and time-lapse microscopy which confirmed the proliferative advantage of MB-positive cell populations relative to MB-negative and G2/M populations. Gene expression analysis revealed that the MB-positive cardiomyocyte subpopulation exhibited patterns consistent with the processes of nuclear division, chromosome segregation, and transition from M to G1 phase. The use of dual-MBs targeting CDC20 and SPG20 mRNAs enabled the enrichment of cytokinetic events (CDC20^highSPG20^high). Interestingly, cells that did not complete cytokinesis and remained binucleated were found to be CDC20^lowSPG20^high while polyploid cardiomyocytes that replicated DNA but failed to complete karyokinesis were found to be CDC20^lowSPG20^low.

CONCLUSIONS

This study demonstrates a novel alternative to existing DNA content-based approaches for sorting cardiomyocytes with true mitotic potential that can be used to study the unique dynamics of cardiomyocyte nuclei during mitosis. Our technique for sorting live cardiomyocytes undergoing cytokinesis would provide a basis for future studies to uncover mechanisms underlying the development and regeneration of heart tissue.

Multiplex live single-cell transcriptional analysis demarcates cellular functional heterogeneity

A fundamental goal in the biological sciences is to determine how individual cells with varied gene expression profiles and diverse functional characteristics contribute to development, physiology, and disease. Here, we report a novel strategy to assess gene expression and cell physiology in single living cells. Our approach utilizes fluorescently labeled mRNA-specific anti-sense RNA probes and dsRNA-binding protein to identify the expression of specific genes in real-time at single-cell resolution via FRET. We use this technology to identify distinct myocardial subpopulations expressing the structural proteins myosin heavy chain α and myosin light chain 2a in real-time during early differentiation of human pluripotent stem cells. We combine this live-cell gene expression analysis with detailed physiologic phenotyping to capture the functional evolution of these early myocardial subpopulations during lineage specification and diversification. This live-cell mRNA imaging approach will have wide ranging application wherever heterogeneity plays an important biological role.

Quantitative Detection and Real-Time Monitoring of Endogenous mRNA at the Single Live Cell Level Using a Ratiometric Molecular Beacon

Messenger ribonucleic acid (mRNA) plays an important role in various cellular processes. however, traditional techniques cannot realize mRNA detections in live cells as they rely on mRNA purification or cell fixation. To achieve real-time and quantitative mRNA detections at a single live cell level, a single-strand stem-loop-structured ratiometric molecular beacon (RMB) composed of the phosphorothioate-modified loop domain on the 2'-O-methyl RNA backbone with a reporter dye, quencher, and reference dye is proposed to detect the Hsp27 mRNA as a modeled endogenous mRNA. When the RMB hybridizes with the target, the stem-loop structure opens, causing separation of the reporter dye and the quencher and restores the reporter fluorescent signals; therefore, the Hsp27 mRNA can be quantitatively detected according to the ratio of the reporter fluorescent signal to the reference fluorescent signal. Both the phosphorothioate and 2'-O-methyl RNA modifications obviously reduce the nonspecific opening, and the additional reference dye ensures the detection precision using co-localization analysis. Not only does this remove the false-positive signal caused by the nuclease degradation-generated RMB fragment, but it also corrects variations caused by direct measurement of reporter fluorescence intensities at a single cell level owing to inhomogeneity in probe delivery. The designed RMB could detect the Hsp27 mRNA with high signal-to-noise ratio and sensitivity as well as excellent specificity and antidegradation capability proved in vitro and in live cells. Furthermore, it was successfully adopted in subcellular localization, quantitative copy number measurements, and even real-time monitoring of Hsp27 mRNA in live cells, demonstrating that the proposed RMB can be a potential quantitative endogenous mRNA detection tool, especially at a single live cell level.

Dual stem cell therapy synergistically improves cardiac function and vascular regeneration following myocardial infarction

Since both myocardium and vasculature in the heart are excessively damaged following myocardial infarction (MI), therapeutic strategies for treating MI hearts should concurrently target both so as to achieve true cardiac repair. Here we demonstrate a concomitant method that exploits the advantages of cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs) and human mesenchymal stem cell-loaded patch (hMSC-PA) to amplify cardiac repair in a rat MI model. Epicardially implanted hMSC-PA provide a complimentary microenvironment which enhances vascular regeneration through prolonged secretion of paracrine factors, but more importantly it significantly improves the retention and engraftment of intramyocardially injected hiPSC-CMs which ultimately restore the cardiac function. Notably, the majority of injected hiPSC-CMs display adult CMs like morphology suggesting that the secretomic milieu of hMSC-PA constitutes pleiotropic effects in vivo. We provide compelling evidence that this dual approach can be a promising means to enhance cardiac repair on MI hearts.

Myocardial infarction causes damage to the myocardium and vasculature. Here the authors show in a rat model of myocardial infarction that cardiomyocytes derived from human induced pluripotent stem cells combined with a human mesenchymal stem cell-loaded patch lead to improved cardiac function and promote vessel formation.

In vivo transduction of ETV2 improves cardiac function and induces vascular regeneration following myocardial infarction

Vascular regeneration in ischemic hearts has been considered a target for new therapeutic strategies. It has been reported that ETV2 is essential for vascular development, injury-induced neovascularization and direct cell reprogramming of non-endothelial cells into endothelial cells. Thus, the objective of this study was to explore the therapeutic potential of ETV2 in murine models of myocardial infarction in vivo. Direct myocardial delivery of lentiviral ETV2 into rodents undergoing myocardial infarction dramatically upregulated the expression of markers for angiogenesis as well as anti-fibrosis and anti-inflammatory factors in vivo. Consistent with these findings, echocardiography showed significantly improved cardiac function in hearts with induced myocardial infarction upon ETV2 injection compared to that in the control virus-injected group as determined by enhanced ejection fraction and fractional shortening. In addition, ETV2-injected hearts were protected against massive fibrosis with a remarkable increase in capillary density. Interestingly, major fractions of capillaries were stained positive for ETV2. In addition, ECs infected with ETV2 showed enhanced proliferation, suggesting a direct role of ETV2 in vascular regeneration in diseased hearts. Furthermore, culture media from ETV2-overexpressing cardiac fibroblasts promoted endothelial cell migration based on scratch assay. Importantly, intramyocardial injection of the adeno-associated virus form of ETV2 into rat hearts with induced myocardial infarction designed for clinical applicability consistently resulted in significant augmentation of cardiac function. We provide compelling evidence that ETV2 has a robust effect on vascular regeneration and enhanced cardiac repair after myocardial infarction, highlighting a potential therapeutic function of ETV2 as an efficient means to treat failing hearts.

A gene therapy strategy that stimulates cardiovascular repair could improve recovery for heart attack patients. Heart attacks inflict severe damage on the heart and blood vessels, tissues with limited capacity for self-repair. Researchers led by Kiwon Ban of the City University of Hong Kong and Hun-Jun Park of the Catholic University of Korea, Seoul, have now demonstrated that a gene responsible for cardiovascular development can also efficiently stimulate heart repair. They used viruses to deliver the gene into a mouse model of heart attack, and showed that treated heart tissues exhibited strong recovery relative to untreated controls. The treatment reduced scar tissue formation and promoted proliferation of the cells lining blood vessels and blood vessel formation, measurably improving heart function. This approach could lay the groundwork for treating a common potentially fatal event.

Software to improve transfer and reproducibility of cell culture methods

Cell culture is a vital component of laboratories throughout the scientific community, yet the absence of standardized protocols and documentation practice challenges laboratory efficiency and scientific reproducibility. We examined the effectiveness of a cloud-based software application, CultureTrax^® as a tool for standardizing and transferring a complex cell culture protocol. The software workflow and template were used to electronically format a cardiomyocyte differentiation protocol and share a digitally executable copy with a different lab user. While the protocol was unfamiliar to the recipient, they executed the experiment by solely using CultureTrax and successfully derived cardiomyocytes from human induced pluripotent stem cells. This software tool significantly reduced the time and resources required to effectively transfer and implement a novel protocol.

Cardiac Regeneration with Human Pluripotent Stem Cell-Derived Cardiomyocytes

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), which are collectively called pluripotent stem cells (PSCs), have emerged as a promising source for regenerative medicine. Particularly, human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have shown robust potential for regenerating injured heart. Over the past two decades, protocols to differentiate hPSCs into CMs at high efficiency have been developed, opening the door for clinical application. Studies further demonstrated therapeutic effects of hPSC-CMs in small and large animal models and the underlying mechanisms of cardiac repair. However, gaps remain in explanations of the therapeutic effects of engrafted hPSC-CMs. In addition, bioengineering technologies improved survival and therapeutic effects of hPSC-CMs in vivo. While most of the original concerns associated with the use of hPSCs have been addressed, several issues remain to be resolved such as immaturity of transplanted cells, lack of electrical integration leading to arrhythmogenic risk, and tumorigenicity. Cell therapy with hPSC-CMs has shown great potential for biological therapy of injured heart; however, more studies are needed to ensure the therapeutic effects, underlying mechanisms, and safety, before this technology can be applied clinically.

(Re-)programming of subtype specific cardiomyocytes

Adult cardiomyocytes (CMs) possess a highly restricted intrinsic regenerative potential - a major barrier to the effective treatment of a range of chronic degenerative cardiac disorders characterized by cellular loss and/or irreversible dysfunction and which underlies the majority of deaths in developed countries. Both stem cell programming and direct cell reprogramming hold promise as novel, potentially curative approaches to address this therapeutic challenge. The advent of induced pluripotent stem cells (iPSCs) has introduced a second pluripotent stem cell source besides embryonic stem cells (ESCs), enabling even autologous cardiomyocyte production. In addition, the recent achievement of directly reprogramming somatic cells into cardiomyocytes is likely to become of great importance. In either case, different clinical scenarios will require the generation of highly pure, specific cardiac cellular-subtypes. In this review, we discuss these themes as related to the cardiovascular stem cell and programming field, including a focus on the emergent topic of pacemaker cell generation for the development of biological pacemakers and in vitro drug testing.

Enhanced Therapeutic and Long-Term Dynamic Vascularization Effects of Human Pluripotent Stem Cell-Derived Endothelial Cells Encapsulated in a Nanomatrix Gel

BACKGROUND

Human pluripotent stem cell (hPSC)-derived endothelial cells (ECs) have limited clinical utility because of undefined components in the differentiation system and poor cell survival in vivo. Here, we aimed to develop a fully defined and clinically compatible system to differentiate hPSCs into ECs. Furthermore, we aimed to enhance cell survival, vessel formation, and therapeutic potential by encapsulating hPSC-ECs with a peptide amphiphile (PA) nanomatrix gel.

METHODS

We induced differentiation of hPSCs into the mesodermal lineage by culturing on collagen-coated plates with a glycogen synthase kinase 3β inhibitor. Next, vascular endothelial growth factor, endothelial growth factor, and basic fibroblast growth factor were added for endothelial lineage differentiation, followed by sorting for CDH5 (VE-cadherin). We constructed an extracellular matrix-mimicking PA nanomatrix gel (PA-RGDS) by incorporating the cell adhesive ligand Arg-Gly-Asp-Ser (RGDS) and a matrix metalloproteinase-2-degradable sequence. We then evaluated whether the encapsulation of hPSC-CDH5⁺ cells in PA-RGDS could enhance long-term cell survival and vascular regenerative effects in a hind-limb ischemia model with laser Doppler perfusion imaging, bioluminescence imaging, real-time reverse transcription-polymerase chain reaction, and histological analysis.

RESULTS

The resultant hPSC-derived CDH5⁺ cells (hPSC-ECs) showed highly enriched and genuine EC characteristics and proangiogenic activities. When injected into ischemic hind limbs, hPSC-ECs showed better perfusion recovery and higher vessel-forming capacity compared with media-, PA-RGDS-, or human umbilical vein EC-injected groups. However, the group receiving the PA-RGDS-encapsulated hPSC-ECs showed better perfusion recovery, more robust and longer cell survival (> 10 months), and higher and prolonged angiogenic and vascular incorporation capabilities than the bare hPSC-EC-injected group. Surprisingly, the engrafted hPSC-ECs demonstrated previously unknown sustained and dynamic vessel-forming behavior: initial perivascular concentration, a guiding role for new vessel formation, and progressive incorporation into the vessels over 10 months.

CONCLUSIONS

We generated highly enriched hPSC-ECs via a clinically compatible system. Furthermore, this study demonstrated that a biocompatible PA-RGDS nanomatrix gel substantially improved long-term survival of hPSC-ECs in an ischemic environment and improved neovascularization effects of hPSC-ECs via prolonged and unique angiogenic and vessel-forming properties. This PA-RGDS-mediated transplantation of hPSC-ECs can serve as a novel platform for cell-based therapy and investigation of long-term behavior of hPSC-ECs.

Biomarkers of Human Pluripotent Stem Cell-Derived Cardiac Lineages

Human pluripotent stem cells (hPSCs) offer a practical source for the de novo generation of cardiac tissues and a unique opportunity to investigate cardiovascular lineage commitment. Numerous strategies have focused on the in vitro production of cardiomyocytes, smooth muscle, and endothelium from hPSCs. However, these differentiation protocols often yield undesired cell types. Thus, establishing a set of stage-specific markers for pure cardiac subpopulations will assist in defining the hierarchy of cardiac differentiation, aid in the development of cellular therapy, and facilitate drug screening and disease modeling. The recent characterization of many such markers is enabling the isolation of major cardiac lineages and subpopulations from differentiating hPSCs. We provide here a comprehensive review detailing the suite of biomarkers used to differentiate cardiac lineages from mixed hPSC-derived populations.

Current Strategies and Challenges for Purification of Cardiomyocytes Derived from Human Pluripotent Stem Cells

Cardiomyocytes (CMs) derived from human pluripotent stem cells (hPSCs) are considered a most promising option for cell-based cardiac repair. Hence, various protocols have been developed for differentiating hPSCs into CMs. Despite remarkable improvement in the generation of hPSC-CMs, without purification, these protocols can only generate mixed cell populations including undifferentiated hPSCs or non-CMs, which may elicit adverse outcomes. Therefore, one of the major challenges for clinical use of hPSC-CMs is the development of efficient isolation techniques that allow enrichment of hPSC-CMs. In this review, we will discuss diverse strategies that have been developed to enrich hPSC-CMs. We will describe major characteristics of individual hPSC-CM purification methods including their scientific principles, advantages, limitations, and needed improvements. Development of a comprehensive system which can enrich hPSC-CMs will be ultimately useful for cell therapy for diseased hearts, human cardiac disease modeling, cardiac toxicity screening, and cardiac tissue engineering.

CD13 and ROR2 Permit Isolation of Highly Enriched Cardiac Mesoderm from Differentiating Human Embryonic Stem Cells

The generation of tissue-specific cell types from human embryonic stem cells (hESCs) is critical for the development of future stem cell-based regenerative therapies. Here, we identify CD13 and ROR2 as cell-surface markers capable of selecting early cardiac mesoderm emerging during hESC differentiation. We demonstrate that the CD13+/ROR2+ population encompasses pre-cardiac mesoderm, which efficiently differentiates to all major cardiovascular lineages. We determined the engraftment potential of CD13+/ROR2+ in small (murine) and large (porcine) animal models, and demonstrated that CD13+/ROR2+ progenitors have the capacity to differentiate toward cardiomyocytes, fibroblasts, smooth muscle, and endothelial cells in vivo. Collectively, our data show that CD13 and ROR2 identify a cardiac lineage precursor pool that is capable of successful engraftment into the porcine heart. These markers represent valuable tools for further dissection of early human cardiac differentiation, and will enable a detailed assessment of human pluripotent stem cell-derived cardiac lineage cells for potential clinical applications.

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CD13 and ROR2 separate hESC-derived MIXL1+ mesoderm from MIXL1+ endoderm

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CD13 and ROR2 select for a population of highly enriched pre-cardiac mesoderm

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CD13+/ROR2+ cells derived from hESCs engraft into porcine, but not murine hearts

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CD13+/ROR2+ cells differentiate to all major cardiac lineages in the pig heart

Non-genetic Purification of Ventricular Cardiomyocytes from Differentiating Embryonic Stem Cells through Molecular Beacons Targeting IRX-4

Isolation of ventricular cardiomyocytes (vCMs) has been challenging due to the lack of specific surface markers. Here we show that vCMs can be purified from differentiating mouse embryonic stem cells (mESCs) using molecular beacons (MBs) targeting specific intracellular mRNAs. We designed MBs (IRX4 MBs) to target mRNA encoding Iroquois homeobox protein 4 (Irx4), a transcription factor specific for vCMs. To purify mESC vCMs, IRX4 MBs were delivered into cardiomyogenically differentiating mESCs, and IRX4 MBs-positive cells were FACS-sorted. We found that, of the cells isolated, ∼98% displayed vCM-like action potentials by electrophysiological analyses. These MB-purified vCMs continuously maintained their CM characteristics as verified by spontaneous beating, Ca²⁺ transient, and expression of vCM-specific proteins. Our study shows the feasibility of isolating pure vCMs via cell sorting without modifying host genes. The homogeneous and functional ventricular CMs generated via the MB-based method can be useful for disease investigation, drug discovery, and cell-based therapies.

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Molecular beacon (MB)-based method was developed to purify ventricular CMs from ESCs

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Ventricular CM-specific MBs targeting Irx4 mRNA were successfully generated

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About 98% of the CMs sorted via Irx4-MB displayed ventricular CM-like phenotypes

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Irx4-MB-based purified CMs continuously maintained ventricular CM characteristics

Three-dimensional cardiac tissue fabrication based on cell sheet technology

Cardiac tissue engineering is a promising therapeutic strategy for severe heart failure. However, conventional tissue engineering methods by seeding cells into biodegradable scaffolds have intrinsic limitations such as inflammatory responses and fibrosis arising from the degradation of scaffolds. On the other hand, we have developed cell sheet engineering as a scaffold-free approach for cardiac tissue engineering. Confluent cultured cells are harvested as an intact cell sheet using a temperature-responsive culture surface. By layering cardiac cell sheets, it is possible to form electrically communicative three-dimensional cardiac constructs. Cell sheet transplantation onto damaged hearts in several animal models has revealed improvements in heart functions. Because of the lack of vasculature, the thickness of viable cardiac cell sheet-layered tissues is limited to three layers. Pre-vascularized structure formation within cardiac tissue and multi-step transplantation methods has enabled the formation of thick vascularized tissues in vivo. Furthermore, development of original bioreactor systems with vascular beds has allowed reconstruction of three-dimensional cardiac tissues with a functional vascular structure in vitro. Large-scale culture systems to generate pluripotent stem cell-derived cardiac cells can create large numbers of cardiac cell sheets. Three-dimensional cardiac tissues fabricated by cell sheet engineering may be applied to treat heart disease and tissue model construction.

Large-scale production of human pluripotent stem cell derived cardiomyocytes

Regenerative medicine, including preclinical studies in large animal models and tissue engineering approaches as well as innovative assays for drug discovery, will require the constant supply of hPSC-derived cardiomyocytes and other functional progenies. Respective cell production processes must be robust, economically viable and ultimately GMP-compliant. Recent research has enabled transition of lab scale protocols for hPSC expansion and cardiomyogenic differentiation towards more controlled processing in industry-compatible culture platforms. Here, advanced strategies for the cultivation and differentiation of hPSCs will be reviewed by focusing on stirred bioreactor-based techniques for process upscaling. We will discuss how cardiomyocyte mass production might benefit from recent findings such as cell expansion at the cardiovascular progenitor state. Finally, remaining challenges will be highlighted, specifically regarding three dimensional (3D) hPSC suspension culture and critical safety issues ahead of clinical translation.

Differential Expression Levels of Integrin α6 Enable the Selective Identification and Isolation of Atrial and Ventricular Cardiomyocytes

Rationale

Central questions such as cardiomyocyte subtype emergence during cardiogenesis or the availability of cardiomyocyte subtypes for cell replacement therapy require selective identification and purification of atrial and ventricular cardiomyocytes. However, current methodologies do not allow for a transgene-free selective isolation of atrial or ventricular cardiomyocytes due to the lack of subtype specific cell surface markers.

Methods and Results

In order to develop cell surface marker-based isolation procedures for cardiomyocyte subtypes, we performed an antibody-based screening on embryonic mouse hearts. Our data indicate that atrial and ventricular cardiomyocytes are characterized by differential expression of integrin α6 (ITGA6) throughout development and in the adult heart. We discovered that the expression level of this surface marker correlates with the intracellular subtype-specific expression of MLC-2a and MLC-2v on the single cell level and thereby enables the discrimination of cardiomyocyte subtypes by flow cytometry. Based on the differential expression of ITGA6 in atria and ventricles during cardiogenesis, we developed purification protocols for atrial and ventricular cardiomyocytes from mouse hearts. Atrial and ventricular identities of sorted cells were confirmed by expression profiling and patch clamp analysis.

Conclusion

Here, we introduce a non-genetic, antibody-based approach to specifically isolate highly pure and viable atrial and ventricular cardiomyocytes from mouse hearts of various developmental stages. This will facilitate in-depth characterization of the individual cellular subsets and support translational research applications.

Detection of Intracellular Gene Expression in Live Cells of Murine, Human and Porcine Origin Using Fluorescence-labeled Nanoparticles

The reprogramming of somatic cells to induced pluripotent stem cells (iPS) has successfully been performed in different mammalian species including mouse, rat, human, pig and others. The verification of iPS clones mainly relies on the detection of the endogenous expression of different pluripotency genes. These genes mostly represent transcription factors which are located in the cell nucleus. Traditionally, the proof of their endogenous expression is supplied by immunohistochemical staining after fixation of the cells. This approach requires replicate cultures of each clone at this early stage to preserve validated clones for further experiments. The present protocol describes an approach with gene-specific nanoparticles which allows the evaluation of intracellular gene expression directly in live cells by fluorescence. The nanoparticles consist of a central gold particle coupled to a capture strand carrying a sequence complementary to the target mRNA as well as a quenched reporter strand. These nanoparticles are actively endocytosed and the target mRNA displaces the reporter strand which then start to fluoresce. Therefore, specific target gene expression can be detected directly under the microscope. In addition, the emitted fluorescence allows the identification, isolation and enrichment of cells expressing a specific gene by flow cytometry. This method can be applied directly to live cells in culture without any manipulation of the target cells.

Live fluorescent RNA-based detection of pluripotency gene expression in embryonic and induced pluripotent stem cells of different species

The generation of induced pluripotent stem (iPS) cells has successfully been achieved in many species. However, the identification of truly reprogrammed iPS cells still remains laborious and the detection of pluripotency markers requires fixation of cells in most cases. Here, we report an approach with nanoparticles carrying Cy3-labeled sense oligonucleotide reporter strands coupled to gold-particles. These molecules are directly added to cultured cells without any manipulation and gene expression is evaluated microscopically after overnight incubation. To simultaneously detect gene expression in different species, probe sequences were chosen according to interspecies homology. With a common target-specific probe we could successfully demonstrate expression of the GAPDH house-keeping gene in somatic cells and expression of the pluripotency markers NANOG and GDF3 in embryonic stem cells and iPS cells of murine, human, and porcine origin. The population of target gene positive cells could be purified by fluorescence-activated cell sorting. After lentiviral transduction of murine tail-tip fibroblasts Nanog-specific probes identified truly reprogrammed murine iPS cells in situ during development based on their Cy3-fluorescence. The intensity of Nanog-specific fluorescence correlated positively with an increased capacity of individual clones to differentiate into cells of all three germ layers. Our approach offers a universal tool to detect intracellular gene expression directly in live cells of any desired origin without the need for manipulation, thus allowing conservation of the genetic background of the target cell. Furthermore, it represents an easy, scalable method for efficient screening of pluripotency which is highly desirable during high-throughput cell reprogramming and after genomic editing of pluripotent stem cells.

Human induced pluripotent stem cell-derived cardiomyocytes: insights into molecular, cellular, and functional phenotypes

Disease models are essential for understanding cardiovascular disease pathogenesis and developing new therapeutics. The human induced pluripotent stem cell (iPSC) technology has generated significant enthusiasm for its potential application in basic and translational cardiac research. Patient-specific iPSC-derived cardiomyocytes offer an attractive experimental platform to model cardiovascular diseases, study the earliest stages of human development, accelerate predictive drug toxicology tests, and advance potential regenerative therapies. Harnessing the power of iPSC-derived cardiomyocytes could eliminate confounding species-specific and interpersonal variations and ultimately pave the way for the development of personalized medicine for cardiovascular diseases. However, the predictive power of iPSC-derived cardiomyocytes as a valuable model is contingent on comprehensive and rigorous molecular and functional characterization.

Efficient Detection and Purification of Cell Populations Using Synthetic MicroRNA Switches

Isolation of specific cell types, including pluripotent stem cell (PSC)-derived populations, is frequently accomplished using cell surface antigens expressed by the cells of interest. However, specific antigens for many cell types have not been identified, making their isolation difficult. Here, we describe an efficient method for purifying cells based on endogenous miRNA activity. We designed synthetic mRNAs encoding a fluorescent protein tagged with sequences targeted by miRNAs expressed by the cells of interest. These miRNA switches control their translation levels by sensing miRNA activities. Several miRNA switches (miR-1-, miR-208a-, and miR-499a-5p-switches) efficiently purified cardiomyocytes differentiated from human PSCs, and switches encoding the apoptosis inducer Bim enriched for cardiomyocytes without cell sorting. This approach is generally applicable, as miR-126-, miR-122-5p-, and miR-375-switches purified endothelial cells, hepatocytes, and insulin-producing cells differentiated from hPSCs, respectively. Thus, miRNA switches can purify cell populations for which other isolation strategies are unavailable.

Development of a universal RNA beacon for exogenous gene detection

Stem cell therapy requires a nontoxic and high-throughput method to achieve a pure cell population to prevent teratomas that can occur if even one cell in the implant has not been transformed. A promising method to detect and separate cells expressing a particular gene is RNA beacon technology. However, developing a successful, specific beacon to a particular transfected gene can take months to develop and in some cases is impossible. Here, we report on an off-the-shelf universal beacon that decreases the time and cost of applying beacon technology to select any living cell population transfected with an exogenous gene.

Molecular beacon-based detection and isolation of working-type cardiomyocytes derived from human pluripotent stem cells

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) provide a potential source of cells to repair injured ventricular myocardium. CM differentiation cultures contain non-cardiac cells and CMs of both nodal and working subtypes. Direct application of such cultures in clinical studies could induce arrhythmias; thus, further purification of working-type CMs from heterogeneous cultures is desirable. Here, we designed 10 molecular beacons (MBs) targeting NPPA mRNA, a marker associated with working-type CMs and highly up-regulated during differentiation. We examined these MBs by solution assays and established their specificity using NPPA-overexpressing CHO cells as well as hPSC-CMs. We selected one MB for subsequent CM subtype isolation using fluorescence-activated cell sorting because the signal-to-background ratio was the highest for this MB in solution assays and a linear correlation was observed between MB signals and the CM purity in differentiation cultures. Compared with cells with low MB signals, cells positively selected based on MB signal had higher expression levels of genes associated with working-type CMs and lower expression levels of genes associated with nodal-type CMs. Therefore, the MB-based method is capable of separating working-type CMs from nodal-type CMs with high specificity and throughput, potentially providing working-type CMs for biomedical applications.

Cell therapy with embryonic stem cell-derived cardiomyocytes encapsulated in injectable nanomatrix gel enhances cell engraftment and promotes cardiac repair

A significant barrier to the therapeutic use of stem cells is poor cell retention in vivo. Here, we evaluate the therapeutic potential and long-term engraftment of cardiomyocytes (CMs) derived from mouse embryonic stem cells (mESCs) encapsulated in an injectable nanomatrix gel consisting of peptide amphiphiles incorporating cell adhesive ligand Arg-Gly-Asp-Ser (PA-RGDS) in experimental myocardial infarction (MI). We cultured rat neonatal CMs in PA-RGDS for 7 days and found that more than 90% of the CMs survived. Next, we intramyocardially injected mouse CM cell line HL-1 CMs with or without PA-RGDS into uninjured hearts. Histologic examination and flow cytometry analysis of digested heart tissues showed approximately 3-fold higher engraftment in the mice that received CMs with PA-RGDS compared to those without PA-RGDS. We further investigated the therapeutic effects and long-term engraftment of mESC-CMs with PA-RGDS on MI in comparison with PBS control, CM-only, and PA-RGDS only. Echocardiography demonstrated that the CM-only and CM+PA-RGDS groups showed higher cardiac function at week 2 compared to other groups. However, from 3 weeks, higher cardiac function was maintained only in the CM+PA-RGDS group; this was sustained for 12 weeks. Confocal microscopic examination of the cardiac tissues harvested at 14 weeks demonstrated sustained engraftment and integration of mESC-CMs into host myocardium in the CM+PA-RGDS group only. This study for the first time demonstrated that PA-RGDS encapsulation can enhance survival of mESC-derived CMs and improve cardiac function post-MI. This nanomatrix gel-mediated stem cell therapy can be a promising option for treating MI.

Gene expression-based enrichment of live cells from adipose tissue produces subpopulations with improved osteogenic potential

Introduction

Mesenchymal stem cells have been increasingly used for cell-based therapies. Adipose-derived stem/stromal cells (ASCs) from the stromal vascular fraction (SVF) of fat tissue are a particularly attractive option for cell based therapy given their accessibility and relative abundance. However, their application in both clinical and basic science investigations is complicated by the isolation of differentiable cells within the SVF. Current enrichment strategies, such as monolayer passaging and surface marker-based sorting, can be time-consuming or overly stringent. Ideally, a population of cells with great regenerative capacity could be isolated with high yields so that extensive in vitro manipulation is not necessary. The objective of this study was to determine whether SVF cells sorted based on expression of alkaline phosphatase liver/bone/kidney (ALPL) resulted in populations with increased osteogenic differentiation potential.

Methods

SVF samples were obtained from four, human donors and processed to isolate initial, heterogeneous cell populations. These SVF cells underwent a four day osteogenic priming period, after which they were treated with a fluorescent, oligodeoxynucleotide molecular beacon probe specific for ALPL mRNA. Cells were separated into positive and negative groups using fluorescence-activated cell sorting (FACS) then differentiated down the osteogenic lineage. Differentiation was assessed by measuring calcified matrix production in each sample.

Results

Cells positive for ALPL expression (ALPL+) represented approximately 34% of the gated population, while cells negative for ALPL expression (ALPL-) represented approximately 18%. ALPL+ cells produced 3.7-fold and 2.1-fold more calcified matrix than ALPL- and unsorted SVF cells, respectively, indicating a significant improvement in osteogenic differentiation. Further, ALPL+ cells showed increases in metabolite production for both adipogenesis and chondrogenesis, suggesting that the enrichment process yields an enhanced multipotent phenotype. Osteogenic differentiation response and cell yields for ALPL+ cells were markedly improved over surface marker-sorted samples.

Conclusion

This study demonstrates a novel method to enrich heterogeneous SVF cells for increased osteogenic potential. The procedure requires less time and results in higher yields of therapeutically useful cells than other existing approaches. Gene expression-based sorting of MSCs is a potentially paradigm-shifting approach that could benefit applications spanning from basic science to clinical therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/scrt502) contains supplementary material, which is available to authorized users.

Molecular beacon-enabled purification of living cells by targeting cell type-specific mRNAs

Molecular beacons (MBs) are dual-labeled oligonucleotides that fluoresce only in the presence of complementary mRNA. The use of MBs to target specific mRNAs allows sorting of specific cells from a mixed cell population. In contrast to existing approaches that are limited by available surface markers or selectable metabolic characteristics, the MB-based method enables the isolation of a wide variety of cells. For example, the ability to purify specific cell types derived from pluripotent stem cells (PSCs) is important for basic research and therapeutics. In addition to providing a general protocol for MB design, validation and nucleofection into cells, we describe how to isolate a specific cell population from differentiating PSCs. By using this protocol, we have successfully isolated cardiomyocytes differentiated from mouse or human PSCs (hPSCs) with ∼ 97% purity, as confirmed by electrophysiology and immunocytochemistry. After designing MBs, their ordering and validation requires 2 weeks, and the isolation process requires 3 h.

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 "in a dish" 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.

Transcriptome analysis in cardiomyocyte-specific differentiation of murine embryonic stem cells reveals transcriptional regulation network

The differentiation to cardiomyocytes is a prerequisite and an important part of heart development. A good understanding of the complicated cardiomyocyte differentiation process benefits cardiogenesis study. Embryonic stem cells (ESCs), cell lines with infinite ability to proliferate and to be differentiated into all cell types of the adult body, are important research tools for investigation of differentiation and meanwhile good models for developmental research. In the current study, genome-wide gene expression of ESCs is profiled through high throughput platform during cardiomyocyte-specific differentiation and maturation. Gene expression patterns of undifferentiated ESCs and ESC-derived cardiomyocytes provide a global overview of genes involved in cardiomyocyte-specific differentiation, whereas marker gene expression profiles of both ESC-related genes and cardiac-specific genes presented the expression pattern shift during differentiation in a pure ESC-derived cardiomyocyte cell culture system. The differentiation and maturation process was completed at day 19 after initiation of differentiation, according to our gene expression profile results. Functional analysis of regulated genes reveals over-represented biological processes, molecular functions and pathways during the differentiation and maturation process. Finally, transcription factor regulation networks were engineered based on gene expression data. Within these networks, the number of identified important regulators (Trim28, E2f4, Foxm1, Myc, Hdac1, Rara, Mef2c, Nkx2-5, Gata4) and possible key co-regulation modules (Nkx2-5 - Gata4 - Tbx5, Myc - E2F4) could be expanded. We demonstrate that a more comprehensive picture of cardiomyocyte differentiation and its regulation can be achieved solely by studying gene expression patterns. The results from our study contribute to a better and more accurate understanding of the regulation mechanisms during cardiomyocyte differentiation.

Critical roles of miRNA-mediated regulation of TGFβ signalling during mouse cardiogenesis

AIMS

MicroRNAs (miRNAs) play critical roles during the development of the cardiovascular system. Blocking miRNA biosynthesis in embryonic hearts through a conditional gene inactivation approach led to differential cardiac defects depending on the Cre drivers used in different studies. The goal of this study is to reveal the cardiogenic pathway that is regulated by the miRNA mechanism at midgestation, a stage that has not been evaluated in previous publications.

METHODS AND RESULTS

We specifically inactivated Dicer1, which is essential for generation of functional mature miRNAs, in the myocardium by crossing cTnt-Cre mice with Dicer1(loxP) mice. cTnt-Cre efficiently inactivates target genes in cardiomyocytes at midgestation. All mutants died between E14.5 and E16.5 with severe myocardial wall defects, including reduced cell proliferation, increased cell death, and spongy myocardial wall. Expression of TGFβ type I receptor (Tgfbr1), which encodes the Type I receptor of TGFβ ligands, was up-regulated in mutant hearts. As expected, TGFβ activity was increased in Dicer1-inactivated hearts. Our further molecular analysis suggested that Tgfbr1 is a direct target of three miRNAs. Reducing TGFβ activities using a pharmacological inhibitor on in vitro cultured hearts, or through an in vivo genetic approach, partially rescued the cardiac defects caused by Dicer1 inactivation.

CONCLUSIONS

We show for the first time that TGFβ signalling is directly regulated by the miRNA mechanism during myocardial wall morphogenesis. Increased TGFβ activity plays a major role in the cardiac defects caused by myocardial deletion of Dicer1. Thus, miRNA-mediated regulation of TGFβ signalling is indispensable for normal cardiogenesis.

The current status of iPS cells in cardiac research and their potential for tissue engineering and regenerative medicine

The recent availability of human cardiomyocytes derived from induced pluripotent stem (iPS) cells opens new opportunities to build in vitro models of cardiac disease, screening for new drugs, and patient-specific cardiac therapy. Notably, the use of iPS cells enables studies in the wide pool of genotypes and phenotypes. We describe progress in reprogramming of induced pluripotent stem (iPS) cells towards the cardiac lineage/differentiation. The focus is on challenges of cardiac disease modeling using iPS cells and their potential to produce safe, effective and affordable therapies/applications with the emphasis of cardiac tissue engineering. We also discuss implications of human iPS cells to biological research and some of the future needs.

Dual modulation of the mitochondrial permeability transition pore and redox signaling synergistically promotes cardiomyocyte differentiation from pluripotent stem cells

Background

Cardiomyocytes that differentiate from pluripotent stem cells (PSCs) provide a crucial cellular resource for cardiac regeneration. The mechanisms of mitochondrial metabolic and redox regulation for efficient cardiomyocyte differentiation are, however, still poorly understood. Here, we show that inhibition of the mitochondrial permeability transition pore (mPTP) by Cyclosporin A (CsA) promotes cardiomyocyte differentiation from PSCs.

Methods and Results

We induced cardiomyocyte differentiation from mouse and human PSCs and examined the effect of CsA on the differentiation process. The cardiomyogenic effect of CsA mainly resulted from mPTP inhibition rather than from calcineurin inhibition. The mPTP inhibitor NIM811, which does not have an inhibitory effect on calcineurin, promoted cardiomyocyte differentiation as much as CsA did, but calcineurin inhibitor FK506 only slightly increased cardiomyocyte differentiation. CsA‐treated cells showed an increase in mitochondrial calcium, mitochondrial membrane potential, oxygen consumption rate, ATP level, and expression of genes related to mitochondrial function. Furthermore, inhibition of mitochondrial oxidative metabolism reduced the cardiomyogenic effect of CsA while antioxidant treatment augmented the cardiomyogenic effect of CsA.

Conclusions

Our data show that mPTP inhibition by CsA alters mitochondrial oxidative metabolism and redox signaling, which leads to differentiation of functional cardiomyocytes from PSCs.