Myocardial Progenitor Research Articles

Stem cells and cardiac arrhythmias.

Recent discoveries in the physiology and pathology of myocardial progenitor cells have allowed researchers to better understand a variety of cardiac pathologies and look at the pathophysiology of arrhythmias from a different perspective. Since the myocardium is composed of a syncytium of electrically interconnected cells, the process of incorporation of newly formed or imported cells into its structure is particularly important. Progenitor cells are stimulated by spontaneous electrical activity, a lengthy action potential, and easily induced triggering activity. All these can lead to arrhythmias development via the three classical mechanisms (reentrancy, automatism, or triggering activity). Transplanted stem cells can influence the electrophysiological properties of cardiomyocytes, thus creating a proarrhythmic substrate. The islets of unbound cells can form an anatomical block, causing unidirectional blockages and recurrent arrhythmias. Similarly, stem cells are capable of establishing heterotopic excitation foci with cardiac stimulatory activity. Finally, the paracrine factors produced by stem cells can also cause proarrhythmic effects. The review examines the factors that influence the proarrhythmic properties of administered stem cells and the mechanisms of arrhythmia development. The results indicate that further research should be carried out to establish the possible impact of stem cells on the development of arrhythmias.

Isolation and phenotyping of cardiac-derived progenitor cells from neonatal mice

Dysfunctions of resident progenitor cells play a significant role in the pathogenesis of decreased myocardial contractility in heart failure, so the most promising approaches for the treatment of heart disease are cardiac-derived stem/progenitor cells (CSCs). Materials and methods. Protocols for progenitor cell cultures from different parts of the heart of newborn FVB/N mice have been developed and their proliferative potential has been characterized. Comparative analysis of the expression of CD31, CD34, CD44, CD45, CD73, CD90, CD105, CD117, CD309 and troponin I by cells from native myocardial biopsies and in the obtained cultures was performed by flow cytometric immunophenotyping. Results. The expression of mesenchymal markers CD44 and CD90 in the absence of the hematopoietic marker CD45 was demonstrated in early passages in mouse myocardial progenitor cell cultures. Relatively high expression of CD34 and CD31 was found. The presence of a minor population of CD44+117+ cells which correspond to the phenotype of cardiac progenitor cells, was detected. Expression of troponin I as one of the key markers of cardiomyocytes as well as the vascular endothelial growth factor receptor has been confirmed in terminally differentiated cultures of cells with contractile activity. Conclusions. It was found that newborn mice in the myocardial tissue contain more cells with the expression of markers of cardiac progenitors than in adult animals. The relative content of such cells is higher in the atria than in the ventricles. Cardiac progenitor cells in neonatal mice derived from the atrial appendages have better proliferative potential than cell cultures isolated from the ventricles.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish.

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Chicken Second Branchial Arch Progenitor Cells Contribute to Heart Musculature in vitro and in vivo

In the past, the heart muscle was thought to originate from a single source of myocardial progenitor cells. More recently, however, an additional source of myocardial progenitors has been revealed to be the second heart field, and chicken embryos were important for establishing this concept. However, there have been few studies in chicken on how this field contributes to heart muscles in vitro. We have developed an ex vivo experimental system from chicken embryos between stages HH17–20 to investigate how mesodermal progenitors in the second branchial arch (BA2) differentiate into cardiac muscles. Using this method, we presented evidence that the progenitor cells within the BA2 arch differentiated into beating cardiomyocytes in vitro. The beating explant cells were positive for cardiac actin, Nkx2.5, and ventricular myosin heavy chain. In addition, we performed a time course for the expression of second heart field markers (Isl1 and Nkx2.5) in the BA2 from stage HH16 to stage HH21 using in situ hybridization. Accordingly, using EGFP-based cell labeling techniques and quail-chicken cell injection, we demonstrated that mesodermal cells from the BA2 contributed to the outflow tract and ventricular myocardium in vivo. Thus, our findings highlight the cardiogenic potential of chicken BA2 mesodermal cells in vitro and in vivo.

Perspectives of cell therapy for myocardial infarction and heart failure based on cardiosphere cells

Cardiovascular diseases are the leading cause of morbidity and mortality worldwide. In recent years, researchers are attracted to the use of cell therapy based on stem cell and progenitor cells, which has been a promising strategy for cardiac repair after injury. However, conducted research using intracoronary or intramyocardial transplantation of various types of stem/progenitor cells as a cell suspension showed modest efficiency. This is due to the low degree of integration and cell survival after transplantation. To overcome these limitations, the concept of the use of multicellular spheroids modeling the natural microenvironment of cells has been proposed, which allows maintaining their viability and therapeutic properties. It is of great interest to use so-called cardial spheroids (cardiospheres) spontaneously forming three-dimensional structures under low-adhesive conditions, consisting of a heterogeneous population of myocardial progenitor cells and extracellular matrix proteins. This review presents data on methods for creating cardiospheres, directed regulation of their properties and reparative potential, as well as the results of preclinical and clinical studies on their use for the treatment of heart diseases.

Loss of Asb2 Impairs Cardiomyocyte Differentiation and Leads to Congenital Double Outlet Right Ventricle.

SummaryDefining the pathways that control cardiac development facilitates understanding the pathogenesis of congenital heart disease. Herein, we identify enrichment of a Cullin5 Ub ligase key subunit, Asb2, in myocardial progenitors and differentiated cardiomyocytes. Using two conditional murine knockouts, Nkx+/Cre.Asb2fl/fl and AHF-Cre.Asb2fl/fl, and tissue clarifying technique, we reveal Asb2 requirement for embryonic survival and complete heart looping. Deletion of Asb2 results in upregulation of its target Filamin A (Flna), and concurrent Flna deletion partially rescues embryonic lethality. Conditional AHF-Cre.Asb2 knockouts harboring one Flna allele have double outlet right ventricle (DORV), which is rescued by biallelic Flna excision. Transcriptomic and immunofluorescence analyses identify Tgfβ/Smad as downstream targets of Asb2/Flna. Finally, using CRISPR/Cas9 genome editing, we demonstrate Asb2 requirement for human cardiomyocyte differentiation suggesting a conserved mechanism between mice and humans. Collectively, our study provides deeper mechanistic understanding of the role of the ubiquitin proteasome system in cardiac development and suggests a previously unidentified murine model for DORV.

412-P: Oleate Prevents Palmitate-Induced Apoptosis and Autophagy in Human Cardiac Progenitor Cells by Inhibiting p38 MAPK and c-Jun Phosphorylation

Elevated circulating levels of saturated fatty acids (SFA; e.g., palmitate) are associated with increased cardiovascular risk in diabetes. In contrast, monounsaturated fatty acids (e.g., oleate) prevent the toxic effects of SFA in multiple cell types. The viability of cardiac progenitor cells (CPC) is essential for myocardial tissue homeostasis. In this study, the ability of palmitate and oleate to induce apoptosis and autophagy, as well as the effects of oleate on the palmitate-induced damage, were investigated in human CPC isolated from right auricle biopsies. Palmitate, but not oleate, induced apoptosis in a dose- and time-dependent manner in human CPC, as assessed by caspase-3 cleavage and ELISA assay for cytoplasmic oligonucleosomes (p<0.05). Exposure of CPC to 0.25 mM palmitate for 16 h also resulted in increased autophagy, evidenced by autophagosome labeling with monodansyl cadaverine and light chain 3 (LC3)-II immunoblotting (p<0.05). In contrast, oleate did not induce autophagy. Palmitate, but not oleate, also induced phosphorylation of both p38 MAPK and the JNK substrate, c-Jun (p<0.05). Pretreatment with the p38 MAPK inhibitors, SB202190 and SB203580, significantly inhibited palmitate-induced apoptosis and autophagy (p<0.05), whereas the JNK inhibitor SP600125 was able to prevent palmitate-induced apoptosis, but not autophagy (p<0.05). Interestingly, palmitate-induced apoptosis and autophagy were prevented when human CPC were co-treated with 0.1 mM oleate for 16 h (p<0.05). Co-incubation with oleate also abolished the ability of palmitate to induce p38 MAPK and c-Jun phosphorylation. In conclusion, oleate prevents the palmitate-induced increase in apoptosis and autophagy of human CPC by counteracting the activation of p38 MAPK and JNK. Hence, oleate supplementation might limit the lipotoxic damage of human heart by preserving the viability of myocardial progenitors. Disclosure R. Doria: None. R. Schipani: None. C. Caccioppoli: None. M. Incalza: None. A. Leonardini: None. A. Natalicchio: Other Relationship; Self; Novo Nordisk Inc., Sanofi-Aventis. S. Perrini: None. A. Cignarelli: Consultant; Self; Eli Lilly and Company. Speaker's Bureau; Self; Merck Sharp & Dohme Corp., Novo Nordisk A/S. L. Laviola: Advisory Panel; Self; Abbott, Boehringer Ingelheim Pharmaceuticals, Inc., Lilly Diabetes, Novo Nordisk Inc. Board Member; Self; AstraZeneca, Roche Diabetes Care, Sanofi-Aventis. Speaker's Bureau; Self; Medtronic, Mundipharma, Takeda Pharmaceutical Company Limited. F. Giorgino: Advisory Panel; Self; Aegerion Pharmaceuticals, AstraZeneca, Boehringer Ingelheim International GmbH, Eli Lilly and Company, MedImmune, Merck Sharp & Dohme Corp., Novo Nordisk A/S, Roche Diabetes Care, Sanofi. Consultant; Self; Roche Diabetes Care, Sanofi. Research Support; Self; Eli Lilly and Company, LifeScan, Inc., Takeda Pharmaceutical Company Limited. Other Relationship; Self; AstraZeneca, Eli Lilly and Company, Sanofi.

Alogliptin and Pioglitazone, Alone or in Combination, Prevent Palmitate-Induced Apoptosis and Autophagy in Human Cardiac Progenitor Cells

Physiological tissue turnover in the heart requires proper activation and viability of multipotent cardiac progenitor cells (CPCs). A defective CPC compartment, in terms of CPC number and maturation capacity, contributes to diabetes- and hyperglycemia-related heart failure in humans. GLP-1-based therapies, including the DPP-4 inhibitors, improve cardiac function in vivo and ameliorate myocardial and endothelial dysfunction in vitro. Pioglitazone has also demonstrated pleiotropic anti-oxidant and anti-atherogenic effects. The aim of this study was to evaluate the effects of alogliptin and pioglitazone, alone or in combination, on the viability of human CPCs exposed to 0.25 mM palmitate for 16 h. Human CPCs were obtained from patients undergoing cardiac surgery for coronary artery bypass grafting and/or valve surgery. Exposure of human CPCs to both alogliptin (10 µM) and pioglitazone (10 µM) for 16 h resulted in Akt, but not Erk, activation. Exposure to palmitate was associated with increased human CPC apoptosis and autophagy evaluated by ELISA assay and LC3-II immunoblotting, respectively (p<0.05). Pretreatment with alogliptin, alone or in combination with pioglitazone for 1 h before exposure to palmitate, reduced palmitate-induced apoptosis and autophagy (p<0.05). In conclusion, palmitate induces apoptosis and autophagy in human CPCs, and alogliptin and pioglitazone, as well as their combination, prevent the effects of palmitate, likely through enhancement of pro-survival pathways. Hence, DPP-4 inhibitors and thiazolidinediones, alone or in combination, may limit the lipotoxic damage of the human heart by preserving the viability of myocardial progenitor cells. Disclosure M. Incalza: None. R. D'Oria: None. C. Caccioppoli: None. A. Leonardini: None. R. Schipani: None. A. Cignarelli: Advisory Panel; Self; Aegerion Pharmaceuticals. Consultant; Self; Roche Diagnostics Corporation, Eli Lilly and Company. A. Natalicchio: None. S. Perrini: None. V. Margari: None. D. Paparella: None. L. Laviola: Advisory Panel; Self; AstraZeneca, Animas Corporation. Speaker's Bureau; Self; A. Menarini Diagnostics. Advisory Panel; Self; Boehringer Ingelheim GmbH, Eli Lilly and Company. Speaker's Bureau; Self; Medtronic, Merck Sharp & Dohme Corp.. Advisory Panel; Self; Novo Nordisk Inc., Roche Diabetes Care Health and Digital Solutions, Sanofi-Aventis, Takeda Development Centre Europe Ltd. F. Giorgino: Consultant; Self; Abbott, AstraZeneca, Boehringer Ingelheim GmbH. Research Support; Self; Eli Lilly and Company, Johnson & Johnson Services, Inc.. Consultant; Self; MedImmune, Merck Sharp & Dohme Corp., Roche Diabetes Care Health and Digital Solutions, Sanofi. Research Support; Self; Takeda Development Centre Europe Ltd..

Abstract 240: Asb2-dependent Proteolysis of the Cytoskeleton Directs Cardiac Development and Disease

Congenital heart diseases (CHDs) account for 25% of birth defects and are major risk factors for adult cardiovascular problems. Partial disease penetrance is seen even in autosomal dominant disorders and genotype/phenotype correlations remain a clinical challenge; thus, the need to understand different regulators of cardiac formation. The Ubiquitin-Proteasome System (UPS) is important in controlling protein turnover during organ development but its role in the mammalian heart remains ambiguous. We have identified a specificity subunit of ubiquitin-mediated proteolysis (Asb2) as being specific for the cardiac myogenic lineage. Asb2 was previously shown to regulate hematopoietic and skeletal muscle cell differentiation through targeting filamin proteins (FlnA, B and C), actin-binding proteins important for cytoskeleton stabilization. In our present study, we show that Asb2 is markedly enriched in myocardial progenitor cells and cardiomyocytes. To investigate the role of Asb2 and UPS dependent proteolysis in heart development, we generated two cardiac-specific murine knockouts (KOs): Nkx Cre .Asb2 -/- and Mef2c Cre .Asb2 -/- (deleting Asb2 in early cardiomyocyte progenitors and anterior heart field progenitors, respectively). Both KOs are embryonic lethal with pericardial edema. We used tissue clarifying and confocal microscopy to define the morphological defects of the Asb2 null heart. Moreover, we found that FlnA is overexpressed in the hearts of these mice and its deletion therein partially rescues their lethality. In addition, using transcriptomic analysis on Asb2-null e9.5 hearts, we identified novel potential Asb2 targets in the heart. Finally, to understand the role of Asb2 in the differentiation and function of human cardiomyocytes, we used CRISPR/Cas9 genome editing technique to generate Asb2-null human induced pluripotent stem cells. Collectively, our study provides a novel mechanistic understanding of the role of the UPS proteasome in cardiac development, myocardial function, and disease pathogenesis. Given recent interests in both the UPS and the cytoskeleton as therapeutic targets, our study provides an innovative platform for the development of pharmacotherapy for cardiac disease.

Irx4 Marks a Multipotent, Ventricular-Specific Progenitor Cell.

While much progress has been made in the resolution of the cellular hierarchy underlying cardiogenesis, our understanding of chamber-specific myocardium differentiation remains incomplete. To better understand ventricular myocardium differentiation, we targeted the ventricle-specific gene, Irx4, in mouse embryonic stem cells to generate a reporter cell line. Using an antibiotic-selection approach, we purified Irx4+ cells in vitro from differentiating embryoid bodies. The isolated Irx4+ cells proved to be highly proliferative and presented Cxcr4, Pdgfr-alpha, Flk1, and Flt1 on the cell surface. Single Irx4+ ventricular progenitor cells (VPCs) exhibited cardiovascular potency, generating endothelial cells, smooth muscle cells, and ventricular myocytes in vitro. The ventricular specificity of the Irx4+ population was further demonstrated in vivo as VPCs injected into the cardiac crescent subsequently produced Mlc2v+ myocytes that exclusively contributed to the nascent ventricle at E9.5. These findings support the existence of a newly identified ventricular myocardial progenitor. This is the first report of a multipotent cardiac progenitor that contributes progeny specific to the ventricular myocardium. Stem Cells 2016;34:2875-2888.

Abstract 322: Postnatal Islet-1 Cardioblasts Are of Neural Crest and Not Second Heart-Field Lineage

Introduction: The transcription factor Islet-1 (Isl1) is expressed in cardiac mesodermal and neural crest (CNC) lineages during cardiogenesis. A pool of Isl1 + cells persist in the perinatal heart (Isl1 + CPCs), some of which are touted as residual second heart-field (SHF)- derived cardioblasts. However, direct lineage-tracing evidence, supporting a SHF over a CNC origin of Isl1 + CPCs, are lacking. Hypothesis: Isl1 + CPCs are of CNC and not SHF lineage. Methods: The Isl1-nLacZ, Wnt1-Cre;tdTomato and Wnt1::FlpE;RC::Fela mice, and iPSCs derived from Wnt1-Cre;tdTomato mice (iPSC Wnt1 ) were employed to lineage-trace Isl1 + CPCs. Results: Temporal analysis of Isl1-nLacZ embryos illustrated a transient, stage-specific reporter gene activity in mesendodermal and neuroectodermal cells. Particularly, at embryonic day (E)9.5, reporter gene activity (x-gal), was strong in the outflow tract (OFT), and exhibited a weak, spotty pattern in the heart. At E12.5, x-gal was strong in the neural tube (NT); reduced in the OFT; and undetectable in the heart. Isl1 immunohistochemistry (IHC) illustrated similar activity of x-gal with endogenous Isl1 expression. Importantly, x-gal remained permanently undetected in the heart. Isl1 IHC in E12.5 Wnt1-Cre;tdTomato and Wnt1::FlpE;RC::Fela embryos indicated that Isl1 + cells in the OFT and NT colocalized with Wnt1 reporter transgenes. At E18.5 and postnatal day 1 (PN1), Isl1 + cells were exclusively of Wnt1 lineage, indicating that Isl1 + CPCs are CNC- and not SHF-derived. Since CNCs minimally contribute cardiomyocytes in mammals, a presumptive full cardiomyogenic capacity of Isl1 + CPCs potentially contrasts with a CNC origin. To address this controversy, we differentiated iPSC Wnt1 toward the CNC lineage, using a previously established embryoid body (EB) differentiation protocol involving transient BMP antagonism. At ~EB-day 9, tdTomato + /Isl1 + CNCs emerged, which progressively differentiated into spontaneously beating, Nkx2.5 + cardiomyocytes. Conclusions: Our findings clarify that Isl1 + CPCs are of CNC and not SHF origin, and suggest a novel role of the mammalian CNC, as a contributor of long-lived myocardial progenitors that could be targeted for heart-related therapeutic purposes.

Myocardial progenitor cells as an option for cell therapy for congenital heart disease

Myocardial progenitor cells represent a perspective source for cell therapy of cardiovascular disorders and are intensively studied. However, mainly the cells from adult patients are studied whereas the cells derived from children with congenital heart diseases remained poorly investigated. The aim of the present work was to obtain progenitor cells from intraoperatively obtained cardiac tissue from infants and children undergoing repair of congenital cardiac defects. Following isolation, the cells gave rise to a clonogenic, highly proliferative spindle-shaped cell population. The cells expressed markers of cardiogenic origin and were shown to differentiate towards cardiogenic lineage. This resident myocardial progenitor cells obtained from infant myocardial tissue demonstrate similar characteristics to previously described cells derived from adult myocardial tissue. This study confirms the possibility of obtaining a pool of progenitor cells from tiny tissue fragments and opens a new perspective of using these cells in regenerative medicine and further research of congenital heart disease pathogenesis.

Abstract 394: Transient Bone Morphogenic Protein Antagonism Directs Differentiation of iPSCs into the Cardiac Neural Crest and cKit+ Myocardial Progenitor Lineages

Introduction: The capability of cKit+ cardiac progenitor cells (CPCs) to participate in cardiomyocyte regeneration remains controversial, despite basic and clinical studies supporting such a role. Hypothesis: A non-permissive cardiac milieu minimizes the generation of cardiomyocytes from CPCs. Methods: We lineage-traced CPCs using novel dual-recombinase responsive indicator mice (cKitCreERT2;Wnt1::Flpe;RC::Fela) and iPSCs derived from cKitCreERT2;IRG (iPSCKit) mice. Results: Intersectional genetic fate-mapping of cKitCreERT2;Wnt1:: Flpe;RC::Fela embryos supported that cKit marks Wnt1-expressing cardiac neural crest (CNC) progenitors, emerging at ~E9.5 and contributing a limited number of cardiomyocytes. To decipher the mechanisms underlying cardiomyocyte differentiation of CPCs, we lineage-traced CPCs during stage-specific cardiogenic differentiation of iPSCKit. Ascorbate treatment promoted differentiation of cKit+ iPSC-derived embryoid bodies (EBs) into Nkx2.5+ myocardium, 45.5%±6.7% of which co-expressed the Cre-reporter EGFP (n=154 EBs; 12 preparations), suggesting that CPCs encompass fully competent cardiomyogenic progenitors. Noggin (or Dorsomorphin), a BMP antagonist transiently expressed in the heart at E7.5-E8.5 but not during CNC invasion, directed the differentiation of iPSCkit-EBs into Mesp1+/Isl1+/Nkx2.5+ cardiac mesoderm progenitors (p≤0.0001). Remarkably, the same signaling pathway subsequently directed EBs into the cKit+/Wnt1+/Pax3+/Mitf-H+/Isl1+/Nkx2.5+ CNC lineage (p≤0.0001), while suppressing the generation of WT1+/Tbx18+ epicardium (p<0.05). Stage-specific induction of Cre-recombination delineated that iPSCkit-derived CPCs encompass Mesp1–/cKit+/Nkx2.5+ CNC progenitors which contributed EGFP+ CNC derivatives, including Nkx2-5+ cardiomyocytes, to 60.7%±7.3% of spontaneously beating EBs (n=147 EBs; 12 preparations). Conclusions: Collectively, our data show that CPCkit are fully competent CNC-derived cardiomyogenic progenitors, whose differentiation to cardiomyocytes is minimized by a latent Noggin-mediated signaling pathway. Therefore exploiting CPCkit therapeutically, provides an important strategy for maximizing myocardial regeneration.

The endoderm and myocardium join forces to drive early heart tube assembly

Formation of the muscular layer of the heart, the myocardium, involves the medial movement of bilateral progenitor fields; driven primarily by shortening of the endoderm during foregut formation. Using a combination of time-lapse imaging, microsurgical perturbations and computational modeling, we show that the speed of the medial-ward movement of the myocardial progenitors is similar, but not identical to that of the adjacent endoderm. Further, the extracellular matrix microenvironment separating the two germ layers also moves with the myocardium, indicating that collective tissue motion and not cell migration drives tubular heart assembly. Importantly, as myocardial cells approach the midline, they perform distinct anterior-directed movements relative to the endoderm. Based on the analysis of microincision experiments and computational models, we propose two characteristic, autonomous morphogenetic activities within the early myocardium: 1) an active contraction of the medial portion of the heart field and 2) curling- the tendency of the unconstrained myocardial tissue to form a spherical surface with a concave ventral side. In the intact embryo, these deformations are constrained by the endoderm and the adjacent mesoderm, nevertheless the corresponding mechanical stresses contribute to the proper positioning of myocardial primordia.

Clonal analysis reveals a common origin between nonsomite-derived neck muscles and heart myocardium.

Neck muscles constitute a transition zone between somite-derived skeletal muscles of the trunk and limbs, and muscles of the head, which derive from cranial mesoderm. The trapezius and sternocleidomastoid neck muscles are formed from progenitor cells that have expressed markers of cranial pharyngeal mesoderm, whereas other muscles in the neck arise from Pax3-expressing cells in the somites. Mef2c-AHF-Cre genetic tracing experiments and Tbx1 mutant analysis show that nonsomitic neck muscles share a gene regulatory network with cardiac progenitor cells in pharyngeal mesoderm of the second heart field (SHF) and branchial arch-derived head muscles. Retrospective clonal analysis shows that this group of neck muscles includes laryngeal muscles and a component of the splenius muscle, of mixed somitic and nonsomitic origin. We demonstrate that the trapezius muscle group is clonally related to myocardium at the venous pole of the heart, which derives from the posterior SHF. The left clonal sublineage includes myocardium of the pulmonary trunk at the arterial pole of the heart. Although muscles derived from the first and second branchial arches also share a clonal relationship with different SHF-derived parts of the heart, neck muscles are clonally distinct from these muscles and define a third clonal population of common skeletal and cardiac muscle progenitor cells within cardiopharyngeal mesoderm. By linking neck muscle and heart development, our findings highlight the importance of cardiopharyngeal mesoderm in the evolution of the vertebrate heart and neck and in the pathophysiology of human congenital disease.

Myocardium and BMP signaling are required for endocardial differentiation.

Endocardial and myocardial progenitors originate in distinct regions of the anterior lateral plate mesoderm and migrate to the midline where they coalesce to form the cardiac tube. Endocardial progenitors acquire a molecular identity distinct from other vascular endothelial cells and initiate expression of specific genes such as nfatc1. Yet the molecular pathways and tissue interactions involved in establishing endocardial identity are poorly understood. The endocardium develops in tight association with cardiomyocytes. To test for a potential role of the myocardium in endocardial morphogenesis, we used two different zebrafish models deficient in cardiomyocytes: the hand2 mutant and a myocardial-specific genetic ablation method. We show that in hand2 mutants endocardial progenitors migrate to the midline but fail to assemble into a cardiac cone and do not express markers of differentiated endocardium. Endocardial differentiation defects were rescued by myocardial but not endocardial-specific expression of hand2. In metronidazole-treated myl7:nitroreductase embryos, myocardial cells were targeted for apoptosis, which resulted in the loss of endocardial nfatc1 expression. However, endocardial cells were present and retained expression of general vascular endothelial markers. We further identified bone morphogenetic protein (BMP) as a candidate myocardium-derived signal required for endocardial differentiation. Chemical and genetic inhibition of BMP signaling at the tailbud stage resulted in severe inhibition of endocardial differentiation while there was little effect on myocardial development. Heat-shock-induced bmp2b expression rescued endocardial nfatc1 expression in hand2 mutants and in myocardium-depleted embryos. Our results indicate that the myocardium is crucial for endocardial morphogenesis and differentiation, and identify BMP as a signal involved in endocardial differentiation.

Overlapping and Opposing Functions of G Protein-coupled Receptor Kinase 2 (GRK2) and GRK5 during Heart Development

G protein-coupled receptor kinases 2 (GRK2) and 5 (GRK5) are fundamental regulators of cardiac performance in adults but are less well characterized for their function in the hearts of embryos. GRK2 and -5 belong to different subfamilies and function as competitors in the control of certain receptors and signaling pathways. In this study, we used zebrafish to investigate whether the fish homologs of GRK2 and -5, Grk2/3 and Grk5, also have unique, complementary, or competitive roles during heart development. We found that they differentially regulate the heart rate of early embryos and equally facilitate heart function in older embryos and that both are required to develop proper cardiac morphology. A loss of Grk2/3 results in dilated atria and hypoplastic ventricles, and the hearts of embryos depleted in Grk5 present with a generalized atrophy. This Grk5 morphant phenotype was associated with an overall decrease of early cardiac progenitors as well as a reduction in the area occupied by myocardial progenitor cells. In the case of Grk2/3, the progenitor decrease was confined to a subset of precursor cells with a committed ventricular fate. We attempted to rescue the GRK loss-of-function heart phenotypes by downstream activation of Hedgehog signaling. The Grk2/3 loss-of-function embryos were rescued by this approach, but Grk5 embryos failed to respond. In summary, we found that GRK2 and GRK5 control cardiac function as well as morphogenesis during development although with different morphological outcomes.

Transcriptional inhibition of etv2 expression is essential for embryonic cardiac development

E-twenty six variant 2 (Etv2) transcription factor participates in cardiac, vascular-endothelial and blood cell lineage specification decisions during embryonic development. Previous studies have identified genomic elements in the etv2 locus responsible for vascular endothelial cell specification. Using transgenic analysis in zebrafish, we report here an etv2 proximal promoter fragment that prevents transgene misexpression in myocardial progenitor cells. This inhibition of etv2 expression in the cardiac progenitor population is partly mediated by Scl and Nkx2.5, likely through direct binding to the etv2 promoter, and cis-regulatory elements located in the first and second introns. The results identify an etv2 cis-regulatory mechanism controlling cardiovascular fate choice implying that etv2 participates in a transcriptional network mediating developmental plasticity of endothelial progenitor cells during embryonic development.

Cyp26 enzymes are required to balance the cardiac and vascular lineages within the anterior lateral plate mesoderm

Normal heart development requires appropriate levels of retinoic acid (RA) signaling. RA levels in embryos are dampened by Cyp26 enzymes, which metabolize RA into easily degraded derivatives. Loss of Cyp26 function in humans is associated with numerous developmental syndromes that include cardiovascular defects. Although previous studies have shown that Cyp26-deficient vertebrate models also have cardiovascular defects, the mechanisms underlying these defects are not understood. Here, we found that in zebrafish, two Cyp26 enzymes, Cyp26a1 and Cyp26c1, are expressed in the anterior lateral plate mesoderm (ALPM) and predominantly overlap with vascular progenitors (VPs). Although singular knockdown of Cyp26a1 or Cyp26c1 does not overtly affect cardiovascular development, double Cyp26a1 and Cyp26c1 (referred to here as Cyp26)-deficient embryos have increased atrial cells and reduced cranial vasculature cells. Examining the ALPM using lineage tracing indicated that in Cyp26-deficient embryos the myocardial progenitor field contains excess atrial progenitors and is shifted anteriorly into a region that normally solely gives rise to VPs. Although Cyp26 expression partially overlaps with VPs in the ALPM, we found that Cyp26 enzymes largely act cell non-autonomously to promote appropriate cardiovascular development. Our results suggest that localized expression of Cyp26 enzymes cell non-autonomously defines the boundaries between the cardiac and VP fields within the ALPM through regulating RA levels, which ensures a proper balance of myocardial and endothelial lineages. Our study provides novel insight into the earliest consequences of Cyp26 deficiency that underlie cardiovascular malformations in vertebrate embryos.

Abstract 18448: Ckit Marks Cardiac Neural Crest Progenitors in the Developing Mouse Heart

Introduction: Cardiac stem cells expressing the receptor tyrosine kinase c-Kit are a promising cell-based therapy for heart regeneration. However, there is ongoing controversy regarding the embryogenic origins and identity of these cells. We addressed this issue by using a tamoxifen-inducible cKitCreERT2/+ allele to establish the genetic fate-map of c-Kit in the developing mouse heart. Hypothesis: c-Kit marks an embryonic myocardial progenitor cell population which contributes extensively to mammalian cardiogenesis. Methods: cKitCreERT2/+ mice were crossed with R26RLacZ or IRG Cre-reporter mice. To induce recombination in c-Kit expressing cells, females pregnant with cKitCreERT2/R26RLacZ or cKitCreERT2/IRG fetuses were administered intraperitoneal injections of tamoxifen (100μl of 20mg/ml) from 7.5-8.5 (n=10) or 9.5-12.5 (n=15) days post coitum (dpc). Embryos were analyzed at 18.5dpc. Results: Confocal analysis, as well as x-gal staining, revealed that during 7.5-8.5dpc (the period which first and second heart field myocardial progenitors contribute to cardiogenesis) few to no labeled cells were seen in the heart, indicating that c-Kit does not mark progenitors of the cardiogenic mesoderm. In contrast, there was extensive cardiomyogenic contribution from cells expressing c-Kit during 9.5-12.5dpc. A mean of 171.4±21 c-Kit derivatives/field were detected throughout the myocardium [cells/field: 15.2±5.5 in outflow tract (OFT); 5.8±1.0 in left atrium; 16.1±4.7 in right atrium; 39.4±6.3 in right ventricle; 30.1±3.8 in left ventricle; 72.6±11.0 in interventricular septum], ~30% of which were cardiomyocytes (48.1±5.8 per field). Importantly, c-Kit contributed to all expected cardiac neural crest derivatives, including OFT, cardiac and aortic valves, cardiac conduction system, epicardium, endocardium and myocardium. Consistent with a neural crest origins, contribution to endothelium and smooth muscle of the OFT was documented, although coronary vascular cell differentiation was not observed Conclusions: Collectively, our findings strongly support that adult c-Kit+ CSCs are postnatal derivatives of c-Kit+ cardioblasts of neural crest origin, an original source of cardiomyocytes during cardiogenesis.