Discovery of retinoic acid receptor agonists as proliferators of cardiac progenitor cells through a phenotypic screening approach

Transient stabilization of human cardiovascular progenitor cells from hPSCs in vitro reflects stage-specific heart development in vivo

AIM

Understanding the molecular identity of human pluripotent stem cell (hPSC)-derived cardiac progenitors and mechanisms controlling their proliferation and differentiation, is valuable for developmental biology and regenerative medicine.

METHODS AND RESULTS

Here we show that chemical modulation of Histone Acetyl Transferases (HATs; by IQ-1) and WNT (by CHIR99021), synergistically enable the transient and reversible block of directed cardiac differentiation progression on hPSCs. The resulting stabilized cardiovascular progenitors (SCPs) are characterized by ISL1pos/KI-67pos/NKX2-5neg expression. In the presence of the chemical inhibitors, SCPs maintain a proliferation quiescent state. Upon small molecules removal SCPs resume proliferation and concomitant NKX2-5 upregulation triggers cell-autonomous differentiation into cardiomyocytes. Directed differentiation of SCPs into the endothelial and smooth muscle lineages confirms their full developmental potential typical of bona fide cardiovascular progenitors. Single-cell RNAseq-based transcriptional profiling of our in vitro generated human SCPs notably reflects the dynamic cellular composition of E8.25-E9.25 posterior second heart field (pSHF) of mouse hearts, hallmarked by NR2F2 expression. Investigating molecular mechanisms of SCP stabilization, we found that the cell-autonomously regulated Retinoic Acid (RA) and BMP signaling is governing SCPs transition from quiescence towards proliferation and cell-autonomous differentiation, reminiscent of a niche-like behavior.

CONCLUSION

The chemically defined and reversible nature our stabilization approach provides an unprecedented opportunity to dissect mechanisms of cardiovascular progenitors' specification and reveal their cellular and molecular properties.

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.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Retinoic acid released from self-assembling peptide activates cardiomyocyte proliferation and enhances repair of infarcted myocardium

The limited cardiomyocyte proliferation is insufficient for repair of the myocardium. Therefore, activating cardiomyocyte proliferation might be a reasonable option for myocardial regeneration. Here, we investigated effect of retinoic acid (RA) on inducing adult cardiomyocyte proliferation and assessed efficacy of self-assembling peptide (SAP)-released RA in activating regeneration of the infarcted myocardium. Effect of RA on inducing cardiomyocyte proliferation was examined with the isolated cardiomyocytes. Expression of the cell cycle-associated genes and paracrine factors in the infarcted myocardium was examined at one week after treatment with SAP-carried RA. Cardiomyocyte proliferation, myocardial regeneration and improvement of cardiac function were assessed at four weeks after treatment. In the adult rat myocardium, expression of RA synthetase gene Raldh2 and RA concentration were decreased significantly. After treatment with RA, the proliferated cardiomyocytes were increased. The formulated SAP could sustainedly release RA. After treatment with SAP-carried RA, expression of the pro-proliferative genes in cell cycle and paracrine factors in the infarcted myocardium were up-regulated. Myocardial regeneration was enhanced, and cardiac function was improved significantly. These results demonstrate that RA can induce adult cardiomyocytes to proliferate effectively. The sustained release of RA with SAP is a promise strategy to enhance repair of the infarcted myocardium.

All-trans retinoic acid (ATRA) regulates key genes in the RARG-TOP2B pathway and reduces anthracycline-induced cardiotoxicity

The effectiveness of anthracycline chemotherapeutics (e.g., doxorubicin) is limited by anthracycline-induced cardiotoxicity (ACT). A nonsynonymous variant (S427L) in the retinoic acid receptor-γ (RARG) gene has been associated with ACT. This variant causes reduced RARG activity, which is hypothesized to lead to increased susceptibility to ACT through reduced activation of the retinoic acid pathway. This study explored the effects of activating the retinoic acid pathway using a RAR-agonist, all-trans retinoic acid (ATRA), in human cardiomyocytes and mice treated with doxorubicin. In human cardiomyocytes, ATRA induced the gene expression of RARs (RARG, RARB) and repressed the expression of topoisomerase II enzyme genes (TOP2A, TOP2B), which encode for the molecular targets of anthracyclines and repressed downstream ACT response genes. Importantly, ATRA enhanced cell survival of human cardiomyocytes exposed to doxorubicin. The protective effect of ATRA was also observed in a mouse model (B6C3F1/J) of ACT, in which ATRA treatment improved heart function compared to doxorubicin-only treated mice. Histological analyses of the heart also indicated that ATRA treatment reduced the pathology associated with ACT. These findings provide additional evidence for the retinoic acid pathway’s role in ACT and suggest that the RAR activator ATRA can modulate this pathway to reduce ACT.

Human-induced pluripotent stem cell-atrial-specific cardiomyocytes and atrial fibrillation

Patient-specific human-induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-aCMs) may be produced, genome-edited, and differentiated into multiple cell types for regenerative medicine, disease modeling, drug testing, toxicity screening, and three-dimensional tissue fabrication. There is presently no complete model of atrial fibrillation (AF) available for studying human pharmacological responses and evaluating the toxicity of potential medication candidates. It has been demonstrated that hiPSC-aCMs can replicate the electrophysiological disease phenotype and genotype of AF. The hiPSC-aCMs, however, are immature and do not reflect the maturity of aCMs in the native myocardium. Numerous laboratories utilize a variety of methodologies and procedures to improve and promote aCM maturation, including electrical stimulation, culture duration, biophysical signals, and changes in metabolic variables. This review covers the current methods being explored for use in the maturation of patient-specific hiPSC-aCMs and their application towards a personalized approach to the pharmacologic therapy of AF.

Phenotypic screen identifies FOXO inhibitor to counteract maturation and promote expansion of human iPS cell-derived cardiomyocytes

Achieving pharmacological control over cardiomyocyte proliferation represents a prime goal in therapeutic cardiovascular research. Here, we identify a novel chemical tool compound for the expansion of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes. The forkhead box O (FOXO) inhibitor AS1842856 was identified as a significant hit from an unbiased proliferation screen in early, immature hiPSC- cardiomyocytes (eCMs). The mitogenic effects of AS1842856 turned out to be robust, dose-dependent, sustained, and reversible. eCM numbers increased >30-fold as induced by AS1842856 over three passages. Phenotypically as well as by marker gene expression, the compound interestingly appeared to counteract cellular maturation both in immature hiPSC-CMs as well as in more advanced ones. Thus, FOXO inhibitor AS1842856 presents a novel proliferation inducer for the chemically defined, xeno-free expansion of hiPSC-derived CMs, while its de-differentiation effect might as well bear potential in regenerative medicine.

Physiologic Targets and Modes of Action for CBL0137, a Lead for Human African Trypanosomiasis Drug Development

CBL0137 is a lead drug for human African trypanosomiasis, caused by Trypanosoma brucei Herein, we use a four-step strategy to 1) identify physiologic targets and 2) determine modes of molecular action of CBL0137 in the trypanosome. First, we identified fourteen CBL0137-binding proteins using affinity chromatography. Second, we developed hypotheses of molecular modes of action, using predicted functions of CBL0137-binding proteins as guides. Third, we documented effects of CBL0137 on molecular pathways in the trypanosome. Fourth, we identified physiologic targets of the drug by knocking down genes encoding CBL0137-binding proteins and comparing their molecular effects to those obtained when trypanosomes were treated with CBL0137. CBL0137-binding proteins included glycolysis enzymes (aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphofructokinase, phosphoglycerate kinase) and DNA-binding proteins [universal minicircle sequence binding protein 2, replication protein A1 (RPA1), replication protein A2 (RPA2)]. In chemical biology studies, CBL0137 did not reduce ATP level in the trypanosome, ruling out glycolysis enzymes as crucial targets for the drug. Thus, many CBL0137-binding proteins are not physiologic targets of the drug. CBL0137 inhibited 1) nucleus mitosis, 2) nuclear DNA replication, and 3) polypeptide synthesis as the first carbazole inhibitor of eukaryote translation. RNA interference (RNAi) against RPA1 inhibited both DNA synthesis and mitosis, whereas RPA2 knockdown inhibited mitosis, consistent with both proteins being physiologic targets of CBL0137. Principles used here to distinguish drug-binding proteins from physiologic targets of CBL0137 can be deployed with different drugs in other biologic systems. SIGNIFICANCE STATEMENT: To distinguish drug-binding proteins from physiologic targets in the African trypanosome, we devised and executed a multidisciplinary approach involving biochemical, genetic, cell, and chemical biology experiments. The strategy we employed can be used for drugs in other biological systems.

High-Throughput Screening Platform in Postnatal Heart Cells and Chemical Probe Toolbox to Assess Cardiomyocyte Proliferation

Restoring lost heart muscle is an attractive goal for cardiovascular regenerative medicine. One appealing strategy is the therapeutic stimulation of cardiomyocyte proliferation, which inter alia remains challenging due to available assay technologies capturing the complex biology. Here, a high-throughput-formatted phenotypic assay platform was established using rodent whole heart-derived cells to preserve the cellular environment of cardiomyocytes. Several readouts allowed the quantification of cycling cardiomyocytes, including a transgenic H2B-mCherry system for unequivocal, automated detection of cardiomyocyte nuclei. A chemical genetics approach revealed pronounced species differences and furnished pan-kinase inhibitors 5 and 36 as potent and robust inducers of endoreplication and acytokinetic mitosis. Combined profiling of the commonly used p38 MAPK inhibitors SB203580 (1), SB239063 (2) and a novel set of skepinone-L (6) derivatives pointed to off-target effects beyond p38 that might be critical for effective cardiomyocyte cytokinesis. Kinome-focused screening eventually furnished TG003 (38) as a novel candidate for stimulating cardiomyocyte proliferation.

Cardiac stem cells: Current knowledge and future prospects

Regenerative medicine is the field concerned with the repair and restoration of the integrity of damaged human tissues as well as whole organs. Since the inception of the field several decades ago, regenerative medicine therapies, namely stem cells, have received significant attention in preclinical studies and clinical trials. Apart from their known potential for differentiation into the various body cells, stem cells enhance the organ's intrinsic regenerative capacity by altering its environment, whether by exogenous injection or introducing their products that modulate endogenous stem cell function and fate for the sake of regeneration. Recently, research in cardiology has highlighted the evidence for the existence of cardiac stem and progenitor cells (CSCs/CPCs). The global burden of cardiovascular diseases’ morbidity and mortality has demanded an in-depth understanding of the biology of CSCs/CPCs aiming at improving the outcome for an innovative therapeutic strategy. This review will discuss the nature of each of the CSCs/CPCs, their environment, their interplay with other cells, and their metabolism. In addition, important issues are tackled concerning the potency of CSCs/CPCs in relation to their secretome for mediating the ability to influence other cells. Moreover, the review will throw the light on the clinical trials and the preclinical studies using CSCs/CPCs and combined therapy for cardiac regeneration. Finally, the novel role of nanotechnology in cardiac regeneration will be explored.

Cardiac stem cells: Current knowledge and future prospects

Regenerative medicine is the field concerned with the repair and restoration of the integrity of damaged human tissues as well as whole organs. Since the inception of the field several decades ago, regenerative medicine therapies, namely stem cells, have received significant attention in preclinical studies and clinical trials. Apart from their known potential for differentiation into the various body cells, stem cells enhance the organ's intrinsic regenerative capacity by altering its environment, whether by exogenous injection or introducing their products that modulate endogenous stem cell function and fate for the sake of regeneration. Recently, research in cardiology has highlighted the evidence for the existence of cardiac stem and progenitor cells (CSCs/CPCs). The global burden of cardiovascular diseases' morbidity and mortality has demanded an in-depth understanding of the biology of CSCs/CPCs aiming at improving the outcome for an innovative therapeutic strategy. This review will discuss the nature of each of the CSCs/CPCs, their environment, their interplay with other cells, and their metabolism. In addition, important issues are tackled concerning the potency of CSCs/CPCs in relation to their secretome for mediating the ability to influence other cells. Moreover, the review will throw the light on the clinical trials and the preclinical studies using CSCs/CPCs and combined therapy for cardiac regeneration. Finally, the novel role of nanotechnology in cardiac regeneration will be explored.

Untangling the Cooperative Role of Nuclear Receptors in Cardiovascular Physiology and Disease

The heart is the first organ to acquire its physiological function during development, enabling it to supply the organism with oxygen and nutrients. Given this early commitment, cardiomyocytes were traditionally considered transcriptionally stable cells fully committed to contractile function. However, growing evidence suggests that the maintenance of cardiac function in health and disease depends on transcriptional and epigenetic regulation. Several studies have revealed that the complex transcriptional alterations underlying cardiovascular disease (CVD) manifestations such as myocardial infarction and hypertrophy is mediated by cardiac retinoid X receptors (RXR) and their partners. RXRs are members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors and drive essential biological processes such as ion handling, mitochondrial biogenesis, and glucose and lipid metabolism. RXRs are thus attractive molecular targets for the development of effective pharmacological strategies for CVD treatment and prevention. In this review, we summarize current knowledge of RXR partnership biology in cardiac homeostasis and disease, providing an up-to-date view of the molecular mechanisms and cellular pathways that sustain cardiomyocyte physiology.

Effects of vitamin A and retinoic acid on mouse embryonic stem cells and their differentiating progeny

Embryonic development is controlled by retinoids, and one approach that has been used to investigate the mechanisms for retinoid actions in developmental processes has been to study the effects of adding retinoids to cultures of pluripotent embryonic stem cells (ESC). To date, most in vitro retinoid research has been directed at deciphering the actions of all-trans retinoic acid (atRA). atRA is a derivative of all-trans retinol (a.k.a. vitamin A, VA), which mammals can generate via an enzyme-catalyzed pathway. atRA's effects on development result from its (1) activation of receptor complexes (RARs and RXRs) in the nucleus which then bind to and activate RA response elements (RAREs) in genes and (2) its interactions with processes that are initiated in the cytoplasm. While much work has focused on the impact of atRA on cell differentiation, VA, itself, has been shown to exert effects on the maintenance of ESC identity that are not dependent upon classic RA-signaling pathways. In this chapter, we present results from our laboratory and others using well-documented approaches for investigating the effects of retinoids on the differentiation of ESC in vitro and introduce a novel method that uses chemically-defined growth conditions. The merits of each approach are discussed.

Engineering spatial-organized cardiac organoids for developmental toxicity testing

Emerging technologies in stem cell engineering have produced sophisticated organoid platforms by controlling stem cell fate via biomaterial instructive cues. By micropatterning and differentiating human induced pluripotent stem cells (hiPSCs), we have engineered spatially organized cardiac organoids with contracting cardiomyocytes in the center surrounded by stromal cells distributed along the pattern perimeter. We investigated how geometric confinement directed the structural morphology and contractile functions of the cardiac organoids and tailored the pattern geometry to optimize organoid production. Using modern data-mining techniques, we found that pattern sizes significantly affected contraction functions, particularly in the parameters related to contraction duration and diastolic functions. We applied cardiac organoids generated from 600 μm diameter circles as a developmental toxicity screening assay and quantified the embryotoxic potential of nine pharmaceutical compounds. These cardiac organoids have potential use as an in vitro platform for studying organoid structure-function relationships, developmental processes, and drug-induced cardiac developmental toxicity.

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Micropattern-based geometric confinement directs cardiac organoid development

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Cardiac organoid structure-function relationships are guided by organoid size

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Cardiac organoids can be used as an in vitro embryotoxicity assessment tool

Vitamin A as a Transcriptional Regulator of Cardiovascular Disease

Vitamin A is a micronutrient and signaling molecule that regulates transcription, cellular differentiation, and organ homeostasis. Additionally, metabolites of Vitamin A are utilized as differentiation agents in the treatment of hematological cancers and skin disorders, necessitating further study into the effects of both nutrient deficiency and the exogenous delivery of Vitamin A and its metabolites on cardiovascular phenotypes. Though vitamin A/retinoids are well-known regulators of cardiac formation, recent evidence has emerged that supports their role as regulators of cardiac regeneration, postnatal cardiac function, and cardiovascular disease progression. We here review findings from genetic and pharmacological studies describing the regulation of both myocyte- and vascular-driven cardiac phenotypes by vitamin A signaling. We identify the relationship between retinoids and maladaptive processes during the pathological hypertrophy of the heart, with a focus on the activation of neurohormonal signaling and fetal transcription factors (Gata4, Tbx5). Finally, we assess how this information might be leveraged to develop novel therapeutic avenues.

Chemical Screening of Nuclear Receptor Modulators

Nuclear receptors are ligand-inducible transcriptional factors that control multiple biological phenomena, including proliferation, differentiation, reproduction, metabolism, and the maintenance of homeostasis. Members of the nuclear receptor superfamily have marked structural and functional similarities, and their domain functionalities and regulatory mechanisms have been well studied. Various modulators of nuclear receptors, including agonists and antagonists, have been developed as tools for elucidating nuclear receptor functions and also as drug candidates or lead compounds. Many assay systems are currently available to evaluate the modulation of nuclear receptor functions, and are useful as screening tools in the discovery and development of new modulators. In this review, we cover the chemical screening methods for nuclear receptor modulators, focusing on assay methods and chemical libraries for screening. We include some recent examples of the discovery of nuclear receptor modulators.

Bioactive Lipid Signaling in Cardiovascular Disease, Development, and Regeneration

Cardiovascular disease (CVD) remains a leading cause of death globally. Understanding and characterizing the biochemical context of the cardiovascular system in health and disease is a necessary preliminary step for developing novel therapeutic strategies aimed at restoring cardiovascular function. Bioactive lipids are a class of dietary-dependent, chemically heterogeneous lipids with potent biological signaling functions. They have been intensively studied for their roles in immunity, inflammation, and reproduction, among others. Recent advances in liquid chromatography-mass spectrometry techniques have revealed a staggering number of novel bioactive lipids, most of them unknown or very poorly characterized in a biological context. Some of these new bioactive lipids play important roles in cardiovascular biology, including development, inflammation, regeneration, stem cell differentiation, and regulation of cell proliferation. Identifying the lipid signaling pathways underlying these effects and uncovering their novel biological functions could pave the way for new therapeutic strategies aimed at CVD and cardiovascular regeneration.

Engineered M13 Peptide Carrier Promotes Angiogenic Potential of Patient-Derived Human Cardiac Progenitor Cells and In Vivo Engraftment

BACKGROUND

Despite promising advances in stem cell-based therapy, the treatment of ischemic cardiovascular diseases remains a big challenge due to both the insufficient in vivo viability of transplanted cells and poor angiogenic potential of stem cells. The goal of this study was to develop therapeutic human cardiac progenitor cells (hCPCs) for ischemic cardiovascular diseases with a novel M13 peptide carrier.

METHOD

In this study, an engineered M13 peptide carrier was successfully generated using a QuikChange Kit. The cellular function of M13 peptide carrier-treated hCPCs was assessed using a tube formation assay and scratch wound healing assay. The in vivo engraftment and cell survival bioactivities of transplanted cells were demonstrated by immunohistochemistry after hCPC transplantation into a myocardial infarction animal model.

RESULTS

The engineered M13^RGD+SDKP peptide carrier, which expressed RGD peptide on PIII site and SDKP peptide on PVIII site, did not affect morphologic change and proliferation ability in hCPCs. In contrast, hCPCs treated with M13^RGD+SDKP showed enhanced angiogenic capacity, including tube formation and migration capacity. Moreover, transplanted hCPCs with M13^RGD+SDKP were engrafted into the ischemic region and promoted in vivo cell survival.

CONCLUSION

Our present data provides a promising protocol for CPC-based cell therapy via short-term cell priming of hCPCs with engineered M13^RGD+SDKP before cell transplantation for treatment of cardiovascular disease.

Phenotypic Screen with the Human Secretome Identifies FGF16 as Inducing Proliferation of iPSC-Derived Cardiac Progenitor Cells

Paracrine factors can induce cardiac regeneration and repair post myocardial infarction by stimulating proliferation of cardiac cells and inducing the anti-fibrotic, antiapoptotic, and immunomodulatory effects of angiogenesis. Here, we screened a human secretome library, consisting of 923 growth factors, cytokines, and proteins with unknown function, in a phenotypic screen with human cardiac progenitor cells. The primary readout in the screen was proliferation measured by nuclear count. From this screen, we identified FGF1, FGF4, FGF9, FGF16, FGF18, and seven additional proteins that induce proliferation of cardiac progenitor cells. FGF9 and FGF16 belong to the same FGF subfamily, share high sequence identity, and are described to have similar receptor preferences. Interestingly, FGF16 was shown to be specific for proliferation of cardiac progenitor cells, whereas FGF9 also proliferated human cardiac fibroblasts. Biosensor analysis of receptor preferences and quantification of receptor abundances suggested that FGF16 and FGF9 bind to different FGF receptors on the cardiac progenitor cells and cardiac fibroblasts. FGF16 also proliferated naïve cardiac progenitor cells isolated from mouse heart and human cardiomyocytes derived from induced pluripotent cells. Taken together, the data suggest that FGF16 could be a suitable paracrine factor to induce cardiac regeneration and repair.

Discovery of retinoic acid receptor agonists as proliferators of cardiac progenitor cells through a phenotypic screening approach

Identification of small molecules with the potential to selectively proliferate cardiac progenitor cells (CPCs) will aid our understanding of the signaling pathways and mechanisms involved and could ultimately provide tools for regenerative therapies for the treatment of post‐MI cardiac dysfunction. We have used an in vitro human induced pluripotent stem cell‐derived CPC model to screen a 10,000‐compound library containing molecules representing different target classes and compounds reported to modulate the phenotype of stem or primary cells. The primary readout of this phenotypic screen was proliferation as measured by nuclear count. We identified retinoic acid receptor (RAR) agonists as potent proliferators of CPCs. The CPCs retained their progenitor phenotype following proliferation and the identified RAR agonists did not proliferate human cardiac fibroblasts, the major cell type in the heart. In addition, the RAR agonists were able to proliferate an independent source of CPCs, HuES6. The RAR agonists had a time‐of‐differentiation‐dependent effect on the HuES6‐derived CPCs. At 4 days of differentiation, treatment with retinoic acid induced differentiation of the CPCs to atrial cells. However, after 5 days of differentiation treatment with RAR agonists led to an inhibition of terminal differentiation to cardiomyocytes and enhanced the proliferation of the cells. RAR agonists, at least transiently, enhance the proliferation of human CPCs, at the expense of terminal cardiac differentiation. How this mechanism translates in vivo to activate endogenous CPCs and whether enhancing proliferation of these rare progenitor cells is sufficient to enhance cardiac repair remains to be investigated.