Skeletal muscle differentiation induces wide-ranging nucleosome repositioning in muscle gene promoters.

In a previous report, we demonstrated that Cbx1, PurB and Sp3 inhibited cardiac muscle differentiation by increasing nucleosome density around cardiac muscle gene promoters. Since cardiac and skeletal muscle express many of the same proteins, we asked if Cbx1, PurB and Sp3 similarly regulated skeletal muscle differentiation. In a C2C12 model of skeletal muscle differentiation, Cbx1 and PurB knockdown increased myotube formation. In contrast, Sp3 knockdown inhibited myotube formation, suggesting that Sp3 played opposing roles in cardiac muscle and skeletal muscle differentiation. Consistent with this finding, Sp3 knockdown also inhibited various muscle-specific genes. The Cbx1, PurB and Sp3 proteins are believed to influence gene-expression in part by altering nucleosome position. Importantly, we developed a statistical approach to determine if changes in nucleosome positioning were significant and applied it to understanding the architecture of muscle-specific genes. Through this novel statistical approach, we found that during myogenic differentiation, skeletal muscle-specific genes undergo a set of unique nucleosome changes which differ significantly from those shown in commonly expressed muscle genes. While Sp3 binding was associated with nucleosome loss, there appeared no correlation with the aforementioned nucleosome changes. In summary, we have identified a novel role for Sp3 in skeletal muscle differentiation and through the application of quantifiable MNase-seq have discovered unique fingerprints of nucleosome changes for various classes of muscle genes during myogenic differentiation.

C166 EVs potentiate miR cardiac reprogramming via miR-148a-3p.

We have demonstrated that directly reprogramming cardiac fibroblasts into new cardiomyocytes via miR combo improves cardiac function in the infarcted heart. However, major challenges exist with delivery and efficacy. During a screening based approach to improve delivery, we discovered that C166-derived EVs were effective delivery agents for miR combo both in vitro and in vivo. In the latter, EV mediated delivery of miR combo induced significant conversion of cardiac fibroblasts into cardiomyocytes (∼20%), reduced fibrosis and improved cardiac function in a myocardial infarction injury model. When compared to lipid-based transfection, C166 EV mediated delivery of miR combo enhanced reprogramming efficacy. Improved reprogramming efficacy was found to result from a miRNA within the exosome: miR-148a-3p. The target of miR-148a-3p was identified as Mdfic. Over-expression and targeted knockdown studies demonstrated that Mdfic was a repressor of cardiomyocyte specific gene expression. In conclusion, we have demonstrated that C166-derived EVs are an effective method for delivering reprogramming factors to cardiac fibroblasts and we have identified a novel miRNA contained within C166-derived EVs which enhances reprogramming efficacy.

Modifying miRs for effective reprogramming of fibroblasts to cardiomyocytes.

Reprogramming scar fibroblasts into cardiomyocytes has been proposed to reverse the damage associated with myocardial infarction. However, the limited improvement in cardiac function calls for enhanced strategies. We reported enhanced efficacy of our miR reprogramming cocktail miR combo (miR-1, miR-133a, miR-208a, and miR-499) via RNA-sensing receptor stimulation. We hypothesized that we could combine RNA-sensing receptor activation with fibroblast reprogramming by chemically modifying miR combo. To test the hypothesis, miR combo was modified to enhance interaction with the RNA-sensing receptor Rig1 via the addition of a 5'-triphosphate (5'ppp) group. Importantly, when compared with unmodified miR combo, 5'ppp-modified miR combo markedly improved reprogramming efficacy in vitro. Enhanced reprogramming efficacy correlated with a type-I interferon immune response with strong and selective secretion of interferon ß (IFNß). Antibody blocking studies and media replacement experiments indicated that 5'ppp-miR combo utilized IFNß to enhance fibroblast reprogramming efficacy. In conclusion, miRs can acquire powerful additional roles through chemical modification that potentially increases their clinical applications.

Neonatal and adult cardiac fibroblasts exhibit inherent differences in cardiac regenerative capacity.

Directly reprogramming fibroblasts into cardiomyocytes improves cardiac function in the infarcted heart. However, the low efficacy of this approach hinders clinical applications. Unlike the adult mammalian heart, the neonatal heart has an intrinsic regenerative capacity. Consequently, we hypothesized that birth imposes fundamental changes in cardiac fibroblasts which limit their regenerative capabilities. In support, we found that reprogramming efficacy in vitro was markedly lower with fibroblasts derived from adult mice versus those derived from neonatal mice. Notably, fibroblasts derived from adult mice expressed significantly higher levels of pro-angiogenic genes. Moreover, under conditions that promote angiogenesis, only fibroblasts derived from adult mice differentiated into tube-like structures. Targeted knockdown screening studies suggested a possible role for the transcription factor Epas1. Epas1 expression was higher in fibroblasts derived from adult mice, and Epas1 knockdown improved reprogramming efficacy in cultured adult cardiac fibroblasts. Promoter activity assays indicated that Epas1 functions as both a transcription repressor and an activator, inhibiting cardiomyocyte genes while activating angiogenic genes. Finally, the addition of an Epas1 targeting siRNA to the reprogramming cocktail markedly improved reprogramming efficacy in vivo with both the number of reprogramming events and cardiac function being markedly improved. Collectively, our results highlight differences between neonatal and adult cardiac fibroblasts and the dual transcriptional activities of Epas1 related to reprogramming efficacy.

Novel method of differentiating human induced pluripotent stem cells to mature cardiomyocytes via Sfrp2.

Current methods to generate cardiomyocytes from induced pluripotent stem cells (iPSc) utilize broad-spectrum pharmacological inhibitors. These methods give rise to cardiomyocytes which are typically immature. Since we have recently demonstrated that cardiomyogenesis in vitro and in vivo requires Sfrp2, we asked if Sfrp2 would drive differentiation of human iPSc into cardiomyocytes. Indeed, we found that Sfrp2 induced robust cardiac differentiation. Importantly, replacement of broad spectrum pharmacological inhibitors with Sfrp2 gave rise to mature cardiomyocytes as evidenced by their sarcomere structure, electrophysiological profiles, and ability to form gap junctions.

Rig1 receptor plays a critical role in cardiac reprogramming via YY1 signaling.

We discovered that innate immunity plays an important role in the reprogramming of fibroblasts into cardiomyocytes. In this report, we define the role of a novel retinoic acid-inducible gene 1 Yin Yang 1 (Rig1:YY1) pathway. We found that fibroblast to cardiomyocyte reprogramming efficacy was enhanced by specific Rig1 activators. To understand the mechanism of action, we performed various transcriptomic, nucleosome occupancy, and epigenomic approaches. Analysis of the datasets indicated that Rig1 agonists had no effect on reprogramming-induced changes in nucleosome occupancy or loss of inhibitory epigenetic motifs. Instead, Rig1 agonists were found to modulate cardiac reprogramming by promoting the binding of YY1 specifically to cardiac genes. To conclude, these results show that the Rig1:YY1 pathway plays a critical role in fibroblast to cardiomyocyte reprogramming.

A novel Cbx1, PurB, and Sp3 complex mediates long-term silencing of tissue- and lineage-specific genes.

miRNA-based cellular fate reprogramming offers an opportunity to investigate the mechanisms of long-term gene silencing. To further understand how genes are silenced in a tissue-specific manner, we leveraged our miRNA-based method of reprogramming fibroblasts into cardiomyocytes. Through screening approaches, we identified three proteins that were downregulated during reprogramming of fibroblasts into cardiomyocytes: heterochromatin protein Cbx1, transcriptional activator protein PurB, and transcription factor Sp3. We show that knockdown of Cbx1, PurB, and Sp3 was sufficient to induce cardiomyocyte gene expression in fibroblasts. Similarly, gene editing to ablate Cbx1, PurB, and Sp3 expression induced fibroblasts to convert into cardiomyocytes in vivo. Furthermore, high-throughput DNA sequencing and coimmunoprecipitation experiments indicated that Cbx1, PurB, and Sp3 also bound together as a complex and were necessary to localize nucleosomes to cardiomyocyte genes on the chromosome. Finally, we found that the expression of these genes led to nucleosome modification via H3K27me3 (trimethylated histone-H3 lysine-27) deposition through an interaction with the polycomb repressive PRC2 complex. In summary, we conclude that Cbx1, PurB, and Sp3 control cell fate by actively repressing lineage-specific genes.

A role for Sfrp2 in cardiomyogenesis in vivo.

Cardiomyogenesis, the process by which the body generates cardiomyocytes, is poorly understood. We have recently shown that Sfrp2 promotes cardiomyogenesis in vitro. The objective of this study was to determine if Sfrp2 would similarly promote cardiomyogenesis in vivo. To test this hypothesis, we tracked multipotent cKit(+) cells in response to Sfrp2 treatment. In control adult mice, multipotent cKit(+) cells typically differentiated into endothelial cells but not cardiomyocytes. In contrast, Sfrp2 switched the fate of these cells. Following Sfrp2 injection, multipotent cKit(+) cells differentiated solely into cardiomyocytes. Sfrp2-derived cardiomyocytes integrated into the myocardium and exhibited identical physiological properties to preexisting native cardiomyocytes. The ability of Sfrp2 to promote cardiomyogenesis was further supported by tracking EdU-labeled cells. In addition, Sfrp2 did not promote the formation of new cardiomyocytes when the cKit(+) cell population was selectively ablated in vivo using a diphtheria toxin receptor-diphtheria toxin model. Notably, Sfrp2-induced cardiomyogenesis was associated with significant functional improvements in a cardiac injury model. In summary, our study further demonstrates the importance of Sfrp2 in cardiomyogenesis.

CRISPR/Cas9 Mediated Deletion of the Angiotensinogen Gene Reduces Hypertension: A Potential for Cure?

[Figure: see text].

Enhancing cardiac reprogramming via synthetic RNA oligonucleotides.

Reprogramming scar fibroblasts into new heart muscle cells has the potential to restore function to the injured heart. However, the effectiveness of reprogramming is notably low. We have recently demonstrated that the effectiveness of reprogramming fibroblasts into heart muscle cells (cardiomyocytes) is increased by the addition of RNA-sensing receptor ligands. Clinical use of these ligands is problematic due to their ability to induce adverse inflammatory events. To overcome this issue, we sought to determine whether synthetic analogs of natural RNA-sensing receptor ligands, which avoid generating inflammatory insults and are nuclease resistant, would similarly enhance fibroblast reprogramming into cardiomyocytes. Indeed, one such stabilized RNA, ICR2, increased the expression of cardiomyocyte-specific mRNAs in reprogrammed fibroblasts. Moreover, ICR2 enhanced the ability of reprogramming factors to produce cardiomyocytes with mature sarcomeres. Knockdown assays indicated that the effects of ICR2 were mediated by the RNA-sensing receptors Rig-I and TLR3. In addition, ICR2 reduced the effective dose and number of reprogramming factors needed for efficient reprogramming. In summary, the synthetic RNA oligonucleotide ICR2 is a potential therapeutic agent to enhance cardiac reprogramming efficiency.

Sequential paracrine mechanisms are necessary for the therapeutic benefits of stem cell therapy.

Stem cell injections are an attractive therapeutic tool. It has been demonstrated that injected stem cells promote tissue repair and regeneration via paracrine mechanisms. However, the effects of injected stem cells continue for far longer than they are present. We hypothesized that the effects of injected stem cells are prolonged because of a sequential paracrine relay mechanism. Conditioned media was collected from mesenchymal stem cells (MSCs) after 24 h. This media was then added to RAW264.7. Media was collected from the macrophages after 24 h and was then added to endothelial cells (ECs). This conditioned macrophage media, but not control media, promoted wound healing and induced EC differentiation. Similar results were observed with primary macrophages. To identify the active paracrine factors released by macrophages in response to stimulation by MSC conditioned media we used an antibody array, identifying increased expression of the angiogenesis-related proteins stromal cell-derived factor 1 (SDF1) and plasminogen activator inhibitor-1 (PAI-1). Knockdown of either protein inhibited the ability of conditioned media derived from MSC paracrine factor-stimulated macrophages to induce EC differentiation both in vitro and in vivo. Conditioned media derived from postnatal day 7 (P7) mouse macrophages induced EC differentiation. Moreover, SDF1 and PAI-1 levels were >120 higher in P7 macrophages compared with adult macrophages, suggesting that MSC paracrine factors promote adult macrophages to adopt a juvenile phenotype. These results indicate that MSC paracrine factors induce macrophages to secrete SDF1 and PAI-1, in-turn inducing endothelial cells to differentiate. Identification of a sequential paracrine mechanism opens new therapeutic avenues for stem cell therapy.

Optimizing delivery for efficient cardiac reprogramming.

Following heart injury, cardiomyocytes, are lost and are not regenerated. In their place, fibroblasts invade the dead tissue where they generate a scar, which reduces cardiac function. We and others have demonstrated that combinations of specific miRNAs (miR combo) or transcription factors (GMT), delivered by individual lenti-/retro-viruses in vivo, can convert fibroblasts into cardiomyocytes and improve cardiac function. However, the effects are relatively modest due to the low efficiency of delivery of miR combo or GMT. We hypothesized that efficiency would be improved by optimizing delivery. In the first instance, we developed a multicistronic system to express all four miRNAs of miR combo from a single construct. The order of each miRNA in the multicistronic construct gave rise to different levels of miRNA expression. A combination that resulted in equivalent expression levels of each of the four miRNAs of miR combo showed the highest reprogramming efficiency. Further efficiency can be achieved by directly targeting fibroblasts. Screening of several AAV serotypes indicated that AAV1 displayed tropism towards cardiac fibroblasts. Combining multicistronic expression with AAV1 delivery robustly reprogrammed cardiac fibroblasts into cardiomyocytes in vivo.

Insights from molecular signature of in vivo cardiac c-Kit(+) cells following cardiac injury and ß-catenin inhibition.

There is much interest over resident c-Kit(+) cells in tissue regeneration. Their role in cardiac regeneration has been controversial. In this study we aim to understand the in vivo behavior of cardiac c-Kit(+) cells at baseline and after myocardial infarction and in response to Sfrp2. This approach can accurately study the in vivo transcript expressions of these cells in temporal response to injury and overcomes the limitations of the in vitro approach. RNA-seq was performed with c-Kit(+) cells and cardiomyocytes from healthy non-injured mice as well as c-Kit(+) cells from 1 day post-MI and 12 days post-MI mice. When compared to in vivo c-Kit(+) cells isolated from a healthy non-injured mouse heart, cardiomyocytes were enriched in transcripts that express anion channels, cation channels, developmental/differentiation pathway components, as well as proteins that inhibit canonical Wnt/ß-catenin signaling. Myocardial infarction (MI) induced in vivo c-Kit(+) cells to transiently adopt the cardiomyocyte-specific signature: expression of a number of cardiomyocyte-specific transcripts was maximal 1 day post-MI and declined by 12 days post-MI. We next studied the effect of ß-catenin inhibition on in vivo c-Kit(+) cells by administering the Wnt inhibitor Sfrp2 into the infarct border zone. Sfrp2 both enhanced and sustained cardiomyocyte-specific gene expression in the in vivo c-Kit(+) cells: expression of cardiomyocyte-specific transcripts was higher and there was no decline in expression by 12 days post-MI. Further analysis of the biology of c-Kit(+) cells identified that culture induced a significant and irreversible change in their molecular signature raising questions about reliability of in vitro studies. Our findings provide evidence that MI induces in vivo c-Kit(+) cells to adopt transiently a cardiomyocyte-specific pattern of gene expression, and Sfrp2 further enhances and induces sustained gene expression. Our approach is important for understanding c-Kit(+) cells in cardiac regeneration and also has broad implications in the investigation of in vivo resident stem cells in other areas of tissue regeneration.

Cardiomyocyte Maturation Requires TLR3 Activated Nuclear Factor Kappa B.

The process by which committed precursors mature into cardiomyocytes is poorly understood. We found that TLR3 inhibition blocked cardiomyocyte maturation; precursor cells committed to the cardiomyocyte lineage failed to express maturation genes and sarcomeres did not develop. Using various approaches, we found that the effects of TLR3 upon cardiomyocyte maturation were dependent upon the RelA subunit of nuclear factor kappa B (NFκB). Importantly, under conditions that promote the development of mature cardiomyocytes NFκB became significantly enriched at the promoters of cardiomyocyte maturation genes. Furthermore, activation of the TLR3-NFκB pathway enhanced cardiomyocyte maturation. This study, therefore, demonstrates that the TLR3-NFκB pathway is necessary for the maturation of committed precursors into mature cardiomyocytes. Stem Cells 2018;36:1198-1209.

HASF (C3orf58) is a novel ligand of the insulin-like growth factor 1 receptor.

We have recently shown that hypoxia and Akt-induced stem cell factor (HASF) protects the heart from ischemia-induced damage and promotes cardiomyocyte proliferation. While we have identified certain signaling pathways responsible for these protective effects, the receptor mediating these effects was unknown. Here, we undertook studies to identify the HASF receptor. A yeast two-hybrid screen identified a partial fragment of insulin-like growth factor 1 receptor (IGF1R) as a binding partner of HASF. Subsequent co-immunoprecipitation experiments showed that HASF bound to full-length IGF1R. Binding assays revealed a high affinity of HASF for IGF1R. The treatment of neonatal ventricular cardiomyocytes with HASF resulted in the phosphorylation of IGF1R and other proteins known to be involved in IGF1R-mediated signaling pathways. HASF-mediated ERK activation was abrogated by IGF1R pharmacological inhibitors and siRNAs that targeted IGF1R. However, siRNA-mediated knockdown of either IGF2R or the insulin receptor had no effect on HASF-induced cell signaling. Additionally, pharmacologic inhibition of IGF1R impeded HASF's ability to induce cardiomyocyte proliferation. Finally, we documented that in vivo deletion of the IGF1R completely abolished the ability of HASF to promote cardiomyocyte proliferation in an overexpression mouse model providing further evidence in vivo that the IGF1R is the functional receptor for HASF.

Deletion of angiotensin II type 2 receptor accelerates adipogenesis in murine mesenchymal stem cells via Wnt10b/beta-catenin signaling.

Recent evidence suggests that the renin-angiotensin system (RAS) has a vital role in adipocyte biology and the pathophysiology of metabolic syndrome. Obesity is the main culprit of metabolic syndrome; and mesenchymal stem cells (MSCs) have been forwarded as a major source of adipocyte generation. Previously, we reported that MSCs have a local RAS and that pharmacological blockade of angiotensin II type 2 receptor (AT2R) promotes adipogenesis in human MSCs. However, the definitive roles of AT2R and how AT2R functions in adipogenesis remains unknown. To this end, we employed AT2R-null murine MSCs to characterize how AT2R affects the differentiation of MSCs into adipocytes. Murine MSCs were isolated from AT2R-null mice and wild-type littermates, grown to confluency, and then differentiated into adipocytes. Adipogenesis was quantitated by assessing the lipid droplet accumulation. Using the lipophilic fluorescent dye, the AT2R-null cells showed significantly increased total fluorescence (261.6±49.6% vs littermate) on day 7. Oil red O staining followed by extraction of the absorbed dye and measurement of the absorbance on day 14 also exhibited significantly increased lipid droplet accumulation in the AT2R-null cells (202.7±14.1% vs littermate). We also examined the expression of adipogenic marker genes by quantitative RT-PCR. The AT2R-null group exhibited significantly increased expression of PPAR-gamma, fatty acid synthase, and adiponectin (vs littermate). We further examined the role of Wnt10b/beta-catenin signaling, which reportedly has an important inhibitory role in adipogenesis. The AT2R-null group exhibited significantly decreased Wnt10b expression accompanied by decreased beta-catenin (vs littermate). Our results thus revealed that the AT2R inhibits adipogenic differentiation in murine MSCs. Moreover, this inhibitory effect is associated with Wnt10b/beta-catenin signaling. These results provide important insights into the pathophysiology of obesity and obesity-related consequences such as metabolic syndrome, hinting at possible future therapies.

Selenium Augments microRNA Directed Reprogramming of Fibroblasts to Cardiomyocytes via Nanog.

We have recently shown that a combination of microRNAs, miR combo, can directly reprogram cardiac fibroblasts into functional cardiomyocytes in vitro and in vivo. However, direct reprogramming strategies are inefficient and slow. Moving towards the eventual goal of clinical application it is necessary to develop new methodologies to overcome these limitations. Here, we report the identification of a specific media composition, reprogramming media (RM), which augmented the effect of miR combo by 5-15-fold depending upon the cardiac marker tested. RM alone was sufficient to strongly induce cardiac gene and protein expression in neonatal tail-tip as well as cardiac fibroblasts. Expression of pluripotency markers Nanog, Oct4, Sox2, and Klf4 was significantly enhanced by RM, with miR combo augmenting the effect further. Knockdown of Nanog by siRNA inhibited the effect of RM on cardiac gene expression. Removal of insulin-transferrin-selenium completely inhibited the effect of reprogramming media upon cardiac gene expression and the addition of selenium to standard culture media recapitulated the effects of RM. Moreover, selenium enhanced the reprogramming efficiency of miR combo.

Nuclear hormone receptor LXRα inhibits adipocyte differentiation of mesenchymal stem cells with Wnt/beta-catenin signaling.

Nuclear hormone receptor liver X receptor-alpha (LXRα) has a vital role in cholesterol homeostasis and is reported to have a role in adipose function and obesity although this is controversial. Conversely, mesenchymal stem cells (MSCs) are suggested to be a major source of adipocyte generation. Accordingly, we examined the role of LXRα in adipogenesis of MSCs. Adult murine MSCs (mMSCs) were isolated from wild-type (WT) and LXR-null mice. Using WT mMSCs, we further generated cell lines stably overexpressing GFP-LXRα (mMSC/LXRα/GFP) or GFP alone (mMSC/GFP) by retroviral infection. Confluent mMSCs were differentiated into adipocytes by the established protocol. Compared with MSCs isolated from WT mice, MSCs from LXR-null mice showed significantly increased adipogenesis, as determined by lipid droplet accumulation and adipogenesis-related gene expression. Moreover, mMSCs stably overexpressing GFP-LXRα (mMSC/LXRα/GFP) exhibited significantly decreased adipogenesis compared with mMSCs overexpressing GFP alone (mMSC/GFP). Since Wnt/beta-catenin signaling is reported to inhibit adipogenesis, we further examined it. The LXR-null group showed significantly decreased Wnt expression accompanied by a decrease of cellular beta-catenin (vs WT). The mMSC/LXRα/GFP group exhibited significantly increased Wnt expression accompanied by an increase of cellular beta-catenin (vs mMSC/GFP). These data demonstrate that LXRα has an inhibitory effect on adipogenic differentiation in mMSCs with Wnt/beta-catenin signaling. These results provide important insights into the pathophysiology of obesity and obesity-related consequences such as metabolic syndrome and may identify potential therapeutic targets.

Blockade of angiotensin II type 2 receptor by PD123319 inhibits osteogenic differentiation of human mesenchymal stem cells via inhibition of extracellular signal-regulated kinase signaling.

Recent evidence indicates that the vasculature contains mesenchymal stem cells (MSCs). We hypothesized that angiotensin II (Ang II) type 2 receptors (AT2Rs) play a role in the osteogenesis of MSCs and may have a role in vascular calcification. Human MSCs were differentiated into osteoblasts. Expression of AT2R was significantly increased during osteogenesis, whereas the expression of Ang II type 1 receptors was not significantly changed. Incubation with the AT2R blocker PD123319 with or without Ang II significantly inhibited calcium deposition, whereas type 1 receptor blocker valsartan had no significant effect. PD123319 inhibited extracellular signal-regulated kinase (ERK) phosphorylation in the osteogenic process, whereas valsartan had no effect. Furthermore, PD123319 combined with Ang II also inhibited acute ERK phosphorylation in MSCs induced by insulin. In conclusion, AT2R is upregulated during osteogenesis. Blockade of AT2R inhibits osteogenesis and ERK phosphorylation of human MSCs. These results provide a novel insight into the pathophysiology of calcific vascular disease.

Inhibition of Wnt6 by Sfrp2 regulates adult cardiac progenitor cell differentiation by differential modulation of Wnt pathways.

Wnt signaling has recently emerged as an important regulator of cardiac progenitor cell proliferation and differentiation, but the exact mechanisms by which Wnt signaling modulates these effects are not known. Understanding these mechanisms is essential for advancing our knowledge of cardiac progenitor cell biology and applying this knowledge to enhance cardiac therapy. Here, we explored the effects of Sfrp2, a canonical Wnt inhibitor, in adult cardiac progenitor cell (CPC) differentiation and investigated the molecular mechanisms involved. Our data show that Sfrp2 treatment can promote differentiation of CPCs after ischemia-reperfusion injury. Treatment of CPCs with Sfrp2 inhibited CPC proliferation and primed them for cardiac differentiation. Sfrp2 binding to Wnt6 and inhibition of Wnt6 canonical pathway was essential for the inhibition of CPC proliferation. This inhibition of Wnt6 canonical signaling by Sfrp2 was important for activation of the non-canonical Wnt/Planar Cell Polarity (PCP) pathway through JNK, which in turn induced expression of cardiac transcription factors and CPC differentiation. Taken together, these results demonstrate a novel role of Sfrp2 and Wnt6 in regulating the dynamic process of CPC proliferation and differentiation, as well as providing new insights into the mechanisms of Wnt signaling in cardiac differentiation.