Upregulations of Gata4 and oxytocin receptor are important in cardiomyocyte differentiation processes of P19CL6 cells

Global profiling of Rbm24 bound RNAs uncovers a multi-tasking RNA binding protein

RNA binding proteins serve as critical molecular switches in a multitude of post-transcriptional regulatory processes. In the heart and muscles, the tissue specific RNA binding protein, Rbm24, is known to play important developmental roles via driving different post-transcriptional processes. Nonetheless, the currently identified molecular targets and regulatory pathways seem inadequate to completely explain the observed developmental effects upon Rbm24 knockdown/knockout. Here, by performing RNA Immunoprecipitation and coupling it to microarrays (RIP-Chip), we have generated an atlas of the mRNA binding repertoire of Rbm24. Further functional evaluation of its targets led to the elucidation of novel roles for Rbm24 in post-transcriptional processing, besides its already known roles in regulation of mRNA stability and alternative splicing. Interestingly, Rbm24 is found to cause the destabilization of Chrm2 via binding to an element in the coding region. In addition, Rbm24 is also found to have an uncharacterized role in driving the generation of isoforms with alternative transcriptional start sites. We have, for the first time, demonstrated that Rbm24 is a multi-tasking RNA binding protein capable of regulating its bound targets via a range of mechanisms.

All-trans retinoic acid stimulates gene expression of the cardioprotective natriuretic peptide system and prevents fibrosis and apoptosis in cardiomyocytes of obese ob/ob mice

In hypertensive rodents, retinoic acid (RA) prevents adverse cardiac remodelling and improves myocardial infarction outcome, but its role in obesity-related changes of cardiac tissue are unclear. We hypothesized that all-trans RA (ATRA) treatment will improve the cardioprotective oxytocin-natriuretic peptides (OT-NP) system, preventing apoptosis and collagen accumulation in hearts of ob/ob mice, a mouse model of obesity and insulin resistance. Female 9-week-old B6.V-Lep/J ob/ob mice (n = 16) were divided into 2 groups: 1 group (n = 8) treated with 100 μg of ATRA dissolved in 100 μL of corn oil (vehicle) delivered daily (∼2 μg·g body weight(-1)·day(-1)) by stomach intubation for 16 days, and 1 group (n = 8) that received the vehicle alone. A group of nonobese littermate mice (n = 9) served as controls. Ob/ob mice exhibited obesity, hyperglycaemia, and downregulation of the cardiac OT-NP system, including the mRNA for the transcription factor GATA4, OT receptor and brain NP, and the protein expression for endothelial nitric oxide synthase. Hearts from ob/ob mice also demonstrated increased apoptosis and collagen accumulation. ATRA treatment induced weight loss and decreased adipocytes diameter in the visceral fat, thus reducing visceral obesity, which is associated with a high risk for cardiovascular disease. RA treatment was associated with a reduction in hyperglycemia and a normalization of the OT-NP system's expression in the hearts of ob/ob mice. Furthermore, ATRA treatment prevented apoptosis and collagen accumulation in hearts of ob/ob mice. The present study indicates that ATRA treatment was effective in restoring the cardioprotective OT-NP system and in preventing abnormal cardiac remodelling in the ob/ob mice.

Oxytocin-Gly-Lys-Arg stimulates cardiomyogenesis by targeting cardiac side population cells

The functional oxytocin (OT) system is expressed in the human and rodent hearts. OT stimulates differentiation of cardiac stem cells into contracting cardiomyocytes (CM). In this study, we investigated OT receptors (OTR) expressed in the cells of cardiac side population (SP) and the abilities of these cells to differentiate into CM in response to the treatment with OT-Gly-Lys-Arg (OT-GKR), a dominant and biologically active form of OT, in the fetal rodent heart. Immunocytochemistry of whole rat embryo at mid gestation (E11) revealed parallel staining in the heart of OTR and the ATP-binding cassette sub-family G member 2 (brcp1) antigen the marker of the SP phenotype. Using flow cytometry, the SP cells were selected from the newborn CM stained with Höechst 33342: 5.32%±0.06% of SP and 15.2%±1.10 of main population expressed OTR on the cell surface. The OTR was detected in CD29 (6.6%) and then in CD31 (4.7%) but less frequently in CD45 (0.7%) positive SP cell subpopulations. Specifically, the phenotype of SP CD31- cell, but not SP CD31+ cells, proliferates in the presence of OT-GKR and develops large cell aggregates. Then, OT-GKR treatment induced the apparition of beating cell colonies after 11 days (10±2.78%), which increased until day 16 (52±1.21%). The cells in contractile colonies expressed the markers of a CM phenotype, such as troponin, cardiac myosin light chain-2, and actinin. Finally, SP cells stimulated by OT-GKR induced endothelial phenotype. These results suggest that the C-terminally extended OT molecule stimulates cardiac differentiation of SP CD31- cells and is involved in heart growth.

Expression of cardiac GATA4 and downstream genes after exercise training in the db/db mouse

GATA4 is a transcriptional factor expressed in heart that regulates the synthesis of structural and cardioprotective genes. We have demonstrated that low GATA4 expression in the db/db mouse heart is associated with reduced expression of key downstream genes, including oxytocin (OT) natriuretic peptide (A-, B-type), nitric oxide synthase (eNOS), and myosin heavy chain (α-MHC). In this study, the effect of exercise on GATA4 expression and related genes was determined in the db/db mouse, a model that represents human type 2 diabetes. Vascular endothelial growth factor (VEGF) and hypoxia-induced factor-α expression were also measured after 8 weeks of treadmill running. Compared with control littermates, db/db mice exhibited hyperglycemia and obesity, and exercise failed to improve these parameters. GATA4 expression was reduced in db/db hearts and this was associated with reduced expression of OT, OTR, ANP, BNP, eNOS, α-MHC, and ratio of α- to β-MHC, whereas mRNA expression of β-MHC and VEGF remained unchanged compared with control hearts. Exercise training increased GATA4 expression (mRNA and protein) but most genes regulated by GATA4 were not observed to increase accordingly. However, protein expression of eNOS, mRNA expression of α-MHC, ratio of α- to β-MHC, and protein expression of VEGF were increased in db/db hearts after exercise. In conclusion, while GATA4 expression is increased following exercise, not all structural and cardioprotective genes are expressed, suggesting other transcription factors may be involved in this regulation. Regardless of this effect, the positive effect of exercise training on key protective genes is evident in the db/db mouse heart.

Oxytocin revisited: its role in cardiovascular regulation

Traditionally associated with female reproduction, oxytocin (OT) was revisited recently and was revealed to have several new roles in the cardiovascular system. Functional OT receptors have been discovered in the rat and human heart, as well as in vascular beds. The cardiovascular activities of OT include: (i) lowering blood pressure; (ii) negative cardiac inotropy and chronotropy; (iii) parasympathetic neuromodulation; (iv) vasodilatation; (v) anti-inflammatory; (vi) antioxidative; and (vii) metabolic effects. These outcomes are mediated, at least in part, by stimulating cardioprotective mediators, such as nitric oxide and atrial natriuretic peptide. OT and its extended form OT-Gly-Lys-Arg have been shown to be abundant in the foetal mouse heart. OT has the capacity to generate cardiomyocytes from various types of stem cells, including the cardiac side population. Mesenchymal cells transfected with OT-Gly-Lys-Arg, or preconditioned with OT, are resistant to apoptosis and express endothelial cell markers. OT increases glucose uptake in cultured cardiomyocytes from newborn and adult rats, in normal, hypoxic and even insulin resistance conditions. In rats with experimentally-induced myocardial infarction, continuous in vivo OT delivery improves the cardiac healing process, as well as cardiac work, reduces inflammation and stimulates angiogenesis. Therefore, in pathological conditions, OT exerts anti-inflammatory and cardioprotective properties, and improves vascular and metabolic functions. Thus, OT has potential for therapeutic use.

Downregulation in GATA4 and Downstream Structural and Contractile Genes in the db/db Mouse Heart

Reduced expression of GATA4, a transcriptional factor for structural and cardioprotective genes, has been proposed as a factor contributing to the development of cardiomyopathy. We investigated whether the reduction of cardiac GATA4 expression reported in diabetes alters the expression of downstream genes, namely, atrial natriuretic peptide (ANP), B-type natriuretic, peptide (BNP), and α- and β-myosin heavy chain (MHC). db/db mice, a model of type 2 diabetes, with lean littermates serving as controls, were studied. db/db mice exhibited obesity, hyperglycemia, and reduced protein expression of cardiac GLUT4 and IRAP (insulin-regulated aminopeptidase), the structural protein cosecreted with GLUT4. Hearts from db/db mice had reduced protein expression of GATA4 (~35%) with accompanying reductions in mRNA expression of ANP (~40%), BNP (~85%), and α-MHC mRNA (~50%) whereas expression of β-MHC mRNA was increased by ~60%. Low GATA4 was not explained by an increased ligase or atrogin1 expression. CHIP protein content was modestly downregulated (27%) in db/db mice whereas mRNA and protein expression of the CHIP cochaperone HSP70 was significantly decreased in db/db hearts. Our results indicate that low GATA4 in db/db mouse heart is accompanied by reduced expression of GATA4-regulated cardioprotective and structural genes, which may explain the development of cardiomyopathy in diabetes.

Effective and steady differentiation of a clonal derivative of P19CL6 embryonal carcinoma cell line into beating cardiomyocytes

The P19CL6 cell line is a useful model to study cardiac differentiation in vitro. However, large variations were noticed in the differentiation rates among previous reports as well as our individual experiments. To overcome the unstable differentiation, we established P19CL6-A1, a new clonal derivative of P19CL6 that could differentiate into cardiomyocytes more efficiently and stably than the parent using the double stimulation with 5-Aza and DMSO based on the previous report. We also introduced a new software, Visorhythm, that can analyze the temporal variations in the beating rhythms and can chart correlograms displaying the oscillated rhythms. Using P19CL6-A1-derived cardiomyocytes and the software, we demonstrated that the correlograms could clearly display the enhancement of beating rates by cardiotonic reagents. These indicate that a combination of P19CL6-A1 and Visorhythm is a useful tool that can provide invaluable assistance in inotropic drug discovery, drug screening, and toxicity testing.

Using nucleofection of siRNA constructs for knockdown of primary cilia in P19.CL6 cancer stem cell differentiation into cardiomyocytes

Primary cilia assemble as solitary organelles in most mammalian cells during growth arrest and are thought to coordinate a series of signal transduction pathways required for cell cycle control, cell migration, and cell differentiation during development and in tissue homeostasis. Recently, primary cilia were suggested to control pluripotency, proliferation, and/or differentiation of stem cells, which may comprise an important source in regenerative biology. We here provide a method using a P19.CL6 embryonic carcinoma (EC) stem cell line to study the function of the primary cilium in early cardiogenesis. By knocking down the formation of the primary cilium by nucleofection of plasmid DNA with siRNA sequences against genes essential in ciliogenesis (IFT88 and IFT20) we block hedgehog (Hh) signaling in P19.CL6 cells as well as the differentiation of the cells into beating cardiomyocytes (Clement et al., 2009). Immunofluorescence microscopy, western blotting, and quantitative PCR analysis were employed to delineate the molecular and cellular events in cilia-dependent cardiogenesis. We optimized the nucleofection procedure to generate strong reduction in the frequency of ciliated cells in the P19.CL6 culture.

The primary cilium coordinates early cardiogenesis and hedgehog signaling in cardiomyocyte differentiation

Defects in the assembly or function of primary cilia, which are sensory organelles, are tightly coupled to developmental defects and diseases in mammals. Here, we investigated the function of the primary cilium in regulating hedgehog signaling and early cardiogenesis. We report that the pluripotent P19.CL6 mouse stem cell line, which can differentiate into beating cardiomyocytes, forms primary cilia that contain essential components of the hedgehog pathway, including Smoothened, Patched-1 and Gli2. Knockdown of the primary cilium by Ift88 and Ift20 siRNA or treatment with cyclopamine, an inhibitor of Smoothened, blocks hedgehog signaling in P19.CL6 cells, as well as differentiation of the cells into beating cardiomyocytes. E11.5 embryos of the Ift88(tm1Rpw) (Ift88-null) mice, which form no cilia, have ventricular dilation, decreased myocardial trabeculation and abnormal outflow tract development. These data support the conclusion that cardiac primary cilia are crucial in early heart development, where they partly coordinate hedgehog signaling.

GATA4 is a direct transcriptional activator of cyclin D2 and Cdk4 and is required for cardiomyocyte proliferation in anterior heart field-derived myocardium

The anterior heart field (AHF) comprises a population of mesodermal progenitor cells that are added to the nascent linear heart to give rise to the majority of the right ventricle, interventricular septum, and outflow tract in mammals and birds. The zinc finger transcription factor GATA4 functions as an integral member of the cardiac transcription factor network in the derivatives of the AHF. In addition to its role in cardiac differentiation, GATA4 is also required for cardiomyocyte replication, although the transcriptional targets of GATA4 required for proliferation have not been previously identified. In the present study, we disrupted Gata4 function exclusively in the AHF and its derivatives. Gata4 AHF knockout mice die by embryonic day 13.5 and exhibit hypoplasia of the right ventricular myocardium and interventricular septum and display profound ventricular septal defects. Loss of Gata4 function in the AHF results in decreased myocyte proliferation in the right ventricle, and we identified numerous cell cycle genes that are dependent on Gata4 by microarray analysis. We show that GATA4 is required for cyclin D2, cyclin A2, and Cdk4 expression in the right ventricle and that the Cyclin D2 and Cdk4 promoters are bound and activated by GATA4 via multiple consensus GATA binding sites in each gene's proximal promoter. These findings establish Cyclin D2 and Cdk4 as direct transcriptional targets of GATA4 and support a model in which GATA4 controls cardiomyocyte proliferation by coordinately regulating numerous cell cycle genes.

Functional arginine vasopressin system in early heart maturation

Since the neurohypophyseal hormone 8-arginine vasopressin (AVP) is involved in cardiovascular tissue hypertrophy and myocyte differentiation, it is possible that local AVP plays a role in heart maturation. AVP-specific RIA, RT-PCR, and immunoblot measurement of AVP receptors (VR) were used to investigate heart tissues from newborn and adult rats. To test AVP's role in differentiation and specialization into ventricle-like cardiomyocytes, we studied GFP-P19Cl6 stem cells, which express green fluorescence protein (GFP) reporter under transcriptional control of the myosin light chain-2v promoter. VR(1) transcripts and proteins were higher in adult than in newborn rat hearts. In contrast, VR(2) increased from postnatal day 1 to 5 and was barely detected in the adult rat heart. In cardiomyocytes expressing troponin C, immunofluorescence revealed VR(2) and VR(1). Intracellular cAMP increased 6.5- and 8.9-fold in response to the selective VR(2) agonist 1-desamino-8-D-AVP (DDAVP) after 1 and 24 h, respectively. Cardiac AVP was high in 1- and 5-day-old (330 +/- 26 and 276 +/- 53 pg/mg protein, respectively) but low in 66-day-old (98 +/- 15 pg/mg protein) rats. AVP immunostaining was detected in the tunica adventitia and endothelium of the coronary vessels. The possible role of AVP in cardiomyogenesis was indicated by DDAVP-AVP-dependent differentiation of GFP-P19Cl6 stem cells into contracting cells displaying GATA-4, a cardiac-specific marker, and ventricle-specific myosin light chain. Together, it is suggested that the AVP system is implicated in postnatal cardiac maturation.