Endocardial cushion and myocardial defects after cardiac myocyte-specific conditional deletion of the bone morphogenetic protein receptor ALK3

Functional implications of mitofusin 2-mediated mitochondrial-SR tethering

Cardiomyocyte mitochondria have an intimate physical and functional relationship with sarcoplasmic reticulum (SR). Under normal conditions mitochondrial ATP is essential to power SR calcium cycling that drives phasic contraction/relaxation, and changes in SR calcium release are sensed by mitochondria and used to modulate oxidative phosphorylation according to metabolic need. When perturbed, mitochondrial-SR calcium crosstalk can evoke programmed cell death. Physical proximity and functional interplay between mitochondria and SR are maintained in part through tethering of these two organelles by the membrane protein mitofusin 2 (Mfn2). Here we review and discuss findings from our two laboratories that derive from genetic manipulation of Mfn2 and closely related Mfn1 in mouse hearts and other experimental systems. By comparing the findings of our two independent research efforts we arrive at several conclusions that appear to be strongly supported, and describe a few areas of incomplete understanding that will require further study. In so doing we hope to clarify some misconceptions regarding the many varied roles of Mfn2 as both physical trans-organelle tether and mitochondrial fusion protein. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease."

Bone Morphogenetic Protein (BMP) signaling in development and human diseases

Bone Morphogenetic Proteins (BMPs) are a group of signaling molecules that belongs to the Transforming Growth Factor-β (TGF-β) superfamily of proteins. Initially discovered for their ability to induce bone formation, BMPs are now known to play crucial roles in all organ systems. BMPs are important in embryogenesis and development, and also in maintenance of adult tissue homeostasis. Mouse knockout models of various components of the BMP signaling pathway result in embryonic lethality or marked defects, highlighting the essential functions of BMPs. In this review, we first outline the basic aspects of BMP signaling and then focus on genetically manipulated mouse knockout models that have helped elucidate the role of BMPs in development. A significant portion of this review is devoted to the prominent human pathologies associated with dysregulated BMP signaling.

Cardiomyocyte-specific deficiency of ketone body metabolism promotes accelerated pathological remodeling

OBJECTIVE

Exploitation of protective metabolic pathways within injured myocardium still remains an unclarified therapeutic target in heart disease. Moreover, while the roles of altered fatty acid and glucose metabolism in the failing heart have been explored, the influence of highly dynamic and nutritionally modifiable ketone body metabolism in the regulation of myocardial substrate utilization, mitochondrial bioenergetics, reactive oxygen species (ROS) generation, and hemodynamic response to injury remains undefined.

METHODS

Here we use mice that lack the enzyme required for terminal oxidation of ketone bodies, succinyl-CoA:3-oxoacid CoA transferase (SCOT) to determine the role of ketone body oxidation in the myocardial injury response. Tracer delivery in ex vivo perfused hearts coupled to NMR spectroscopy, in vivo high-resolution echocardiographic quantification of cardiac hemodynamics in nutritionally and surgically modified mice, and cellular and molecular measurements of energetic and oxidative stress responses are performed.

RESULTS

While germline SCOT-knockout (KO) mice die in the early postnatal period, adult mice with cardiomyocyte-specific loss of SCOT (SCOT-Heart-KO) remarkably exhibit no overt metabolic abnormalities, and no differences in left ventricular mass or impairments of systolic function during periods of ketosis, including fasting and adherence to a ketogenic diet. Myocardial fatty acid oxidation is increased when ketones are delivered but cannot be oxidized. To determine the role of ketone body oxidation in the remodeling ventricle, we induced pressure overload injury by performing transverse aortic constriction (TAC) surgery in SCOT-Heart-KO and αMHC-Cre control mice. While TAC increased left ventricular mass equally in both groups, at four weeks post-TAC, myocardial ROS abundance was increased in myocardium of SCOT-Heart-KO mice, and mitochondria and myofilaments were ultrastructurally disordered. Eight weeks post-TAC, left ventricular volume was markedly increased and ejection fraction was decreased in SCOT-Heart-KO mice, while these parameters remained normal in hearts of control animals.

CONCLUSIONS

These studies demonstrate the ability of myocardial ketone metabolism to coordinate the myocardial response to pressure overload, and suggest that the oxidation of ketone bodies may be an important contributor to free radical homeostasis and hemodynamic preservation in the injured heart.

Kruppel-like factor 4 protein regulates isoproterenol-induced cardiac hypertrophy by modulating myocardin expression and activity

Kruppel-like factor 4 (KLF4) plays an important role in vascular diseases, including atherosclerosis and vascular injury. Although KLF4 is expressed in the heart in addition to vascular cells, the role of KLF4 in cardiac disease has not been fully determined. The goals of this study were to investigate the role of KLF4 in cardiac hypertrophy and to determine the underlying mechanisms. Cardiomyocyte-specific Klf4 knockout (CM Klf4 KO) mice were generated by the Cre/LoxP technique. Cardiac hypertrophy was induced by chronic infusion of the β-adrenoreceptor agonist isoproterenol (ISO). Results showed that ISO-induced cardiac hypertrophy was enhanced in CM Klf4 KO mice compared with control mice. Accelerated cardiac hypertrophy in CM Klf4 KO mice was accompanied by the augmented cellular enlargement of cardiomyocytes as well as the exaggerated expression of fetal cardiac genes, including atrial natriuretic factor (Nppa). Additionally, induction of myocardin, a transcriptional cofactor regulating fetal cardiac genes, was enhanced in CM Klf4 KO mice. Interestingly, KLF4 regulated Nppa expression by modulating the expression and activity of myocardin, providing a mechanical basis for accelerated cardiac hypertrophy in CM Klf4 KO mice. Moreover, we showed that KLF4 mediated the antihypertrophic effect of trichostatin A, a histone deacetylase inhibitor, because ISO-induced cardiac hypertrophy in CM Klf4 KO mice was attenuated by olmesartan, an angiotensin II type 1 antagonist, but not by trichostatin A. These results provide novel evidence that KLF4 is a regulator of cardiac hypertrophy by modulating the expression and the activity of myocardin.

Epicardium is required for sarcomeric maturation and cardiomyocyte growth in the ventricular compact layer mediated by transforming growth factor β and fibroblast growth factor before the onset of coronary circulation

The epicardium, which is derived from the proepicardial organ (PE) as the third epithelial layer of the developing heart, is crucial for ventricular morphogenesis. An epicardial deficiency leads to a thin compact layer for the developing ventricle; however, the mechanisms leading to the impaired development of the compact layer are not well understood. Using chick embryonic hearts, we produced epicardium-deficient hearts by surgical ablation or blockade of the migration of PE and examined the mechanisms underlying a thin compact myocardium. Sarcomeric maturation (distance between Z-lines) and cardiomyocyte growth (size) were affected in the thin compact myocardium of epicardium-deficient ventricles, in which the amounts of phospho-smad2 and phospho-ERK as well as expression of transforming growth factor (TGF)β2 and fibroblast growth factor (FGF)2 were reduced. TGFβ and FGF were required for the maturation of sarcomeres and growth of cardiomyocytes in cultured ventricles. In ovo co-transfection of dominant negative (dN)-Alk5 (dN-TGFβ receptor I) and dN-FGF receptor 1 to ventricles caused a thin compact myocardium. Our results suggest that immature sarcomeres and small cardiomyocytes are the causative architectures of an epicardium-deficient thin compact layer and also that epicardium-dependent signaling mediated by TGFβ and FGF plays a role in the development of the ventricular compact layer before the onset of coronary circulation.

Depletion of lamina-associated polypeptide 1 from cardiomyocytes causes cardiac dysfunction in mice

We previously showed that striated muscle-selective depletion of lamina-associated polypeptide 1 (LAP1), an integral inner nuclear membrane protein, leads to profound muscular dystrophy with premature death in mice. As LAP1 is also depleted in hearts of these mice, we examined their cardiac phenotype. Striated muscle-selective LAP1 knockout mice display ventricular systolic dysfunction with abnormal induction of genes encoding cardiomyopathy related proteins. To eliminate possible confounding effects due to skeletal muscle pathology, we generated a new mouse line in which LAP1 is deleted in a cardiomyocyte-selective manner. These mice had no skeletal muscle pathology and appeared overtly normal at 20 weeks of age. However, cardiac echocardiography revealed that they developed left ventricular systolic dysfunction and cardiac gene expression analysis revealed abnormal induction of cardiomyopathy-related genes. Our results demonstrate that LAP1 expression in cardiomyocytes is required for normal left ventricular function, consistent with a report of cardiomyopathy in a human subject with mutation in the gene encoding LAP1.

Repression of cyclin D1 expression is necessary for the maintenance of cell cycle exit in adult mammalian cardiomyocytes

The hearts of neonatal mice and adult zebrafish can regenerate after injury through proliferation of preexisting cardiomyocytes. However, adult mammals are not capable of cardiac regeneration because almost all cardiomyocytes exit their cell cycle. Exactly how the cell cycle exit is maintained and how many adult cardiomyocytes have the potential to reenter the cell cycle are unknown. The expression and activation levels of main cyclin-cyclin-dependent kinase (CDK) complexes are extremely low or undetectable at adult stages. The nuclear DNA content of almost all cardiomyocytes is 2C, indicating the cell cycle exit from G1-phase. Here, we induced expression of cyclin D1, which regulates the progression of G1-phase, only in differentiated cardiomyocytes of adult mice. In these cardiomyocytes, S-phase marker-positive cardiomyocytes and the expression of main cyclins and CDKs increased remarkably, although cyclin B1-CDK1 activation was inhibited in an ATM/ATR-independent manner. The phosphorylation pattern of CDK1 and expression pattern of Cdc25 subtypes suggested that a deficiency in the increase in Cdc25 (a and -b), which is required for M-phase entry, inhibited the cyclin B1-CDK1 activation. Finally, analysis of cell cycle distribution patterns showed that >40% of adult mouse cardiomyocytes reentered the cell cycle by the induction of cyclin D1. The cell cycle of these binucleated cardiomyocytes was arrested before M-phase, and many mononucleated cardiomyocytes entered endoreplication. These data indicate that silencing the cyclin D1 expression is necessary for the maintenance of the cell cycle exit and suggest a mechanism that involves inhibition of M-phase entry.

Disruption of myocardial Gata4 and Tbx5 results in defects in cardiomyocyte proliferation and atrioventricular septation

Mutations in GATA4 and TBX5 are associated with congenital heart defects in humans. Interaction between GATA4 and TBX5 is important for normal cardiac septation, but the underlying molecular mechanisms are not well understood. Here, we show that Gata4 and Tbx5 are co-expressed in the embryonic atria and ventricle, but after E15.5, ventricular expression of Tbx5 decreases. Co-localization and co-immunoprecipitation studies demonstrate an interaction of Gata4 and Tbx5 in the developing atria and ventricles, but the ventricular interaction declines after E14.5. Gata4(+/-);Tbx5(+/-) mouse embryos display decreased atrial and ventricular myocardial thickness at E11.5, prior to cardiac septation. To determine the cell lineage in which the interaction was functionally significant in vivo, mice heterozygous for Gata4 in the myocardium or endocardium and heterozygous for Tbx5 (Gata4(MyoDel/wt);Tbx5(+/-) and Gata4(EndoDel/wt);Tbx5(+/-), respectively) were generated. Gata4(MyoDel/wt);Tbx5(+/-) mice displayed embryonic lethality, thin myocardium with reduced cell proliferation, and atrioventricular septation defects similar to Gata4;Tbx5 compound heterozygotes while Gata4(EndoDel/wt);Tbx5(+/-) embryos were normal. Cdk4 and Cdk2, cyclin-dependent kinases required for myocardial development and septation were reduced in Gata4(+/-);Tbx5(+/-) hearts. Cdk4 is a known direct target of Gata4 and the regulation of Cdk2 in the developing heart has not been studied. Chromatin immunoprecipitation and transactivation studies demonstrate that Gata4 and Tbx5 directly regulate Cdk4 while only Tbx5 activates Cdk2 expression. These findings highlight the mechanisms by which disruption of the Gata4 and Tbx5 interaction in the myocardium contributes to cardiac septation defects in humans.

Abnormal muscle mechanosignaling triggers cardiomyopathy in mice with Marfan syndrome

Patients with Marfan syndrome (MFS), a multisystem disorder caused by mutations in the gene encoding the extracellular matrix (ECM) protein fibrillin 1, are unusually vulnerable to stress-induced cardiac dysfunction. The prevailing view is that MFS-associated cardiac dysfunction is the result of aortic and/or valvular disease. Here, we determined that dilated cardiomyopathy (DCM) in fibrillin 1-deficient mice is a primary manifestation resulting from ECM-induced abnormal mechanosignaling by cardiomyocytes. MFS mice displayed spontaneous emergence of an enlarged and dysfunctional heart, altered physical properties of myocardial tissue, and biochemical evidence of chronic mechanical stress, including increased angiotensin II type I receptor (AT1R) signaling and abated focal adhesion kinase (FAK) activity. Partial fibrillin 1 gene inactivation in cardiomyocytes was sufficient to precipitate DCM in otherwise phenotypically normal mice. Consistent with abnormal mechanosignaling, normal cardiac size and function were restored in MFS mice treated with an AT1R antagonist and in MFS mice lacking AT1R or β-arrestin 2, but not in MFS mice treated with an angiotensin-converting enzyme inhibitor or lacking angiotensinogen. Conversely, DCM associated with abnormal AT1R and FAK signaling was the sole abnormality in mice that were haploinsufficient for both fibrillin 1 and β1 integrin. Collectively, these findings implicate fibrillin 1 in the physiological adaptation of cardiac muscle to elevated workload.

CHD7 interacts with BMP R-SMADs to epigenetically regulate cardiogenesis in mice

Haploinsufficiency for CHD7, an ATP-dependent nucleosome remodeling factor, is the leading cause of CHARGE syndrome. While congenital heart defects (CHDs) are major clinical features of CHARGE syndrome, affecting >75% of patients, it remains unclear whether CHD7 can directly regulate cardiogenic genes in embryos. Our complementary yeast two-hybrid and biochemical assays reveal that CHD7 is a novel interaction partner of canonical BMP signaling pathway nuclear mediators, SMAD1/5/8, in the embryonic heart. Moreover, CHD7 associates in a BMP-dependent manner with the enhancers of a critical cardiac transcription factor, Nkx2.5, that contain functional SMAD1-binding elements. Both the active epigenetic signature of Nkx2.5 regulatory elements and its proper expression in cardiomyocytes require CHD7. Finally, inactivation of Chd7 in mice impairs multiple BMP signaling-regulated cardiogenic processes. Our results thus support the model that CHD7 is recruited by SMAD1/5/8 to the enhancers of BMP-targeted cardiogenic genes to epigenetically regulate their expression. Impaired BMP activities in embryonic hearts may thus have a major contribution to CHDs in CHARGE syndrome.

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

Congenital heart defects affect approximately 1-5 % of human newborns each year, and of these cardiac defects 20-30 % are due to heart valve abnormalities. Recent literature indicates that the key factors and pathways that regulate valve development are also implicated in congenital heart defects and valve disease. Currently, there are limited options for treatment of valve disease, and therefore having a better understanding of valve development can contribute critical insight into congenital valve defects and disease. There are three major signaling pathways required for early specification and initiation of endothelial-to-mesenchymal transformation (EMT) in the cardiac cushions: BMP, TGF-β, and Notch signaling. BMPs secreted from the myocardium set up the environment for the overlying endocardium to become activated; Notch signaling initiates EMT; and both BMP and TGF-β signaling synergize with Notch to promote the transition of endothelia to mesenchyme and the mesenchymal cell invasiveness. Together, these three essential signaling pathways help form the cardiac cushions and populate them with mesenchyme and, consequently, set off the cascade of events required to develop mature heart valves. Furthermore, integration and cross-talk between these pathways generate highly stratified and delicate valve leaflets and septa of the heart. Here, we discuss BMP, TGF-β, and Notch signaling pathways during mouse cardiac cushion formation and how they together produce a coordinated EMT response in the developing mouse valves.

Ion channel-kinase TRPM7 is required for maintaining cardiac automaticity

Sick sinus syndrome and atrioventricular block are common clinical problems, often necessitating permanent pacemaker placement, yet the pathophysiology of these conditions remains poorly understood. Here we show that Transient Receptor Potential Melastatin 7 (TRPM7), a divalent-permeant channel-kinase of unknown function, is highly expressed in embryonic myocardium and sinoatrial node (SAN) and is required for cardiac automaticity in these specialized tissues. TRPM7 disruption in vitro, in cultured embryonic cardiomyocytes, significantly reduces spontaneous Ca(2+) transient firing rates and is associated with robust down-regulation of Hcn4, Cav3.1, and SERCA2a mRNA. TRPM7 knockdown in zebrafish, global murine cardiac Trpm7 deletion (KO(αMHC-Cre)), and tamoxifen-inducible SAN restricted Trpm7 deletion (KO(HCN4-CreERT2)) disrupts cardiac automaticity in vivo. Telemetered and sedated KO(αMHC-Cre) and KO(HCN4-CreERT2) mice show episodes of sinus pauses and atrioventricular block. Isolated SAN from KO(αMHC-Cre) mice exhibit diminished Ca(2+) transient firing rates with a blunted diastolic increase in Ca(2+). Action potential firing rates are diminished owing to slower diastolic depolarization. Accordingly, Hcn4 mRNA and the pacemaker current, I(f), are diminished in SAN from both KO(αMHC-Cre) and KO(HCN4-CreERT2) mice. Moreover, heart rates of KO(αMHC-Cre) mice are less sensitive to the selective I(f) blocker ivabradine, and acute application of the recently identified TRPM7 blocker FTY720 has no effect on action potential firing rates of wild-type SAN cells. We conclude that TRPM7 influences diastolic membrane depolarization and automaticity in SAN indirectly via regulation of Hcn4 expression.

Timing of myocardial trpm7 deletion during cardiogenesis variably disrupts adult ventricular function, conduction, and repolarization

BACKGROUND

Transient receptor potential (TRP) channels are a superfamily of broadly expressed ion channels with diverse physiological roles. TRPC1, TRPC3, and TRPC6 are believed to contribute to cardiac hypertrophy in mouse models. Human mutations in TRPM4 have been linked to progressive familial heart block. TRPM7 is a divalent-permeant channel and kinase of unknown function, recently implicated in the pathogenesis of atrial fibrillation; however, its function in ventricular myocardium remains unexplored.

METHODS AND RESULTS

We generated multiple cardiac-targeted knockout mice to test the hypothesis that TRPM7 is required for normal ventricular function. Early cardiac Trpm7 deletion (before embryonic day 9; TnT/Isl1-Cre) results in congestive heart failure and death by embryonic day 11.5 as a result of hypoproliferation of the compact myocardium. Remarkably, Trpm7 deletion late in cardiogenesis (about embryonic day 13; αMHC-Cre) produces viable mice with normal adult ventricular size, function, and myocardial transcriptional profile. Trpm7 deletion at an intermediate time point results in 50% of mice developing cardiomyopathy associated with heart block, impaired repolarization, and ventricular arrhythmias. Microarray analysis reveals elevations in transcripts of hypertrophy/remodeling genes and reductions in genes important for suppressing hypertrophy (Hdac9) and for ventricular repolarization (Kcnd2) and conduction (Hcn4). These transcriptional changes are accompanied by action potential prolongation and reductions in transient outward current (Ito; Kcnd2). Similarly, the pacemaker current (If; Hcn4) is suppressed in atrioventricular nodal cells, accounting for the observed heart block.

CONCLUSIONS

Trpm7 is dispensable in adult ventricular myocardium under basal conditions but is critical for myocardial proliferation during early cardiogenesis. Loss of Trpm7 at an intermediate developmental time point alters the myocardial transcriptional profile in adulthood, impairing ventricular function, conduction, and repolarization.

Expression of the BMP receptor Alk3 in the second heart field is essential for development of the dorsal mesenchymal protrusion and atrioventricular septation

RATIONALE

The dorsal mesenchymal protrusion (DMP) is a prong of mesenchyme derived from the second heart field (SHF) located at the venous pole of the developing heart. Recent studies have shown that perturbation of its development is associated with the pathogenesis of atrioventricular (AV) septal defect. Although the importance of the DMP to AV septation is now established, the molecular and cellular mechanisms underlying its development are far from fully understood. Prior studies have demonstrated that bone morphogenetic protein (BMP) signaling is essential for proper formation of the AV endocardial cushions and the cardiac outflow tract. A role for BMP signaling in regulation of DMP development remained to be elucidated.

OBJECTIVE

To determine the role of BMP signaling in DMP development.

METHODS AND RESULTS

Conditional deletion of the BMP receptor Alk3 from venous pole SHF cells leads to impaired formation of the DMP and a completely penetrant phenotype of ostium primum defect, a hallmark feature of AV septal defects. Analysis of mutants revealed decreased proliferative index of SHF cells and, consequently, reduced number of SHF cells at the cardiac venous pole. In contrast, volume and expression of markers associated with proliferation and active BMP/transforming growth factor β signaling were not significantly altered in the AV cushions of SHF-Alk3 mutants.

CONCLUSIONS

BMP signaling is required for expansion of the SHF-derived DMP progenitor population at the cardiac venous pole. Perturbation of Alk3-mediated BMP signaling from the SHF results in impaired development of the DMP and ostium primum defects.

Endocardial to myocardial notch-wnt-bmp axis regulates early heart valve development

Endocardial to mesenchymal transformation (EMT) is a fundamental cellular process required for heart valve formation. Notch, Wnt and Bmp pathways are known to regulate this process. To further address how these pathways coordinate in the process, we specifically disrupted Notch1 or Jagged1 in the endocardium of mouse embryonic hearts and showed that Jagged1-Notch1 signaling in the endocardium is essential for EMT and early valvular cushion formation. qPCR and RNA in situ hybridization assays reveal that endocardial Jagged1-Notch1 signaling regulates Wnt4 expression in the atrioventricular canal (AVC) endocardium and Bmp2 in the AVC myocardium. Whole embryo cultures treated with Wnt4 or Wnt inhibitory factor 1 (Wif1) show that Bmp2 expression in the AVC myocardium is dependent on Wnt activity; Wnt4 also reinstates Bmp2 expression in the AVC myocardium of endocardial Notch1 null embryos. Furthermore, while both Wnt4 and Bmp2 rescue the defective EMT resulting from Notch inhibition, Wnt4 requires Bmp for its action. These results demonstrate that Jagged1-Notch1 signaling in endocardial cells induces the expression of Wnt4, which subsequently acts as a paracrine factor to upregulate Bmp2 expression in the adjacent AVC myocardium to signal EMT.

Successful adaptation to ketosis by mice with tissue-specific deficiency of ketone body oxidation

During states of low carbohydrate intake, mammalian ketone body metabolism transfers energy substrates originally derived from fatty acyl chains within the liver to extrahepatic organs. We previously demonstrated that the mitochondrial enzyme coenzyme A (CoA) transferase [succinyl-CoA:3-oxoacid CoA transferase (SCOT), encoded by nuclear Oxct1] is required for oxidation of ketone bodies and that germline SCOT-knockout (KO) mice die within 48 h of birth because of hyperketonemic hypoglycemia. Here, we use novel transgenic and tissue-specific SCOT-KO mice to demonstrate that ketone bodies do not serve an obligate energetic role within highly ketolytic tissues during the ketogenic neonatal period or during starvation in the adult. Although transgene-mediated restoration of myocardial CoA transferase in germline SCOT-KO mice is insufficient to prevent lethal hyperketonemic hypoglycemia in the neonatal period, mice lacking CoA transferase selectively within neurons, cardiomyocytes, or skeletal myocytes are all viable as neonates. Like germline SCOT-KO neonatal mice, neonatal mice with neuronal CoA transferase deficiency exhibit increased cerebral glycolysis and glucose oxidation, and, while these neonatal mice exhibit modest hyperketonemia, they do not develop hypoglycemia. As adults, tissue-specific SCOT-KO mice tolerate starvation, exhibiting only modestly increased hyperketonemia. Finally, metabolic analysis of adult germline Oxct1(+/-) mice demonstrates that global diminution of ketone body oxidation yields hyperketonemia, but hypoglycemia emerges only during a protracted state of low carbohydrate intake. Together, these data suggest that, at the tissue level, ketone bodies are not a required energy substrate in the newborn period or during starvation, but rather that integrated ketone body metabolism mediates adaptation to ketogenic nutrient states.

Mechanistic target of rapamycin (Mtor) is essential for murine embryonic heart development and growth

Mechanistic target of rapamycin (Mtor) is required for embryonic inner cell mass proliferation during early development. However, Mtor expression levels are very low in the mouse heart during embryogenesis. To determine if Mtor plays a role during mouse cardiac development, cardiomyocyte specific Mtor deletion was achieved using α myosin heavy chain (α-MHC) driven Cre recombinase. Initial mosaic expression of Cre between embryonic day (E) 10.5 and E11.5 eliminated a subset of cardiomyocytes with high Cre activity by apoptosis and reduced overall cardiac proliferative capacity. The remaining cardiomyocytes proliferated and expanded normally. However loss of 50% of cardiomyocytes defined a threshold that impairs the ability of the embryonic heart to sustain the embryo's circulatory requirements. As a result 92% of embryos with cardiomyocyte Mtor deficiency died by the end of gestation. Thus Mtor is required for survival and proliferation of cardiomyocytes in the developing heart.

Myocardial Mycn is essential for mouse ventricular wall morphogenesis

MYCN is a highly conserved transcription factor with multifaceted roles in development and disease. Mutations in MYCN are associated with Feingold syndrome, a developmental disorder characterized in part by congenital heart defects. Mouse models have helped elucidate MYCN functions; however its cardiac-specific roles during development remain unclear. We employed a Cre/loxp strategy to uncover the specific activities of MYCN in the developing mouse myocardium. Myocardial deletion of Mycn resulted in a thin-myocardial wall defect with dramatically reduced trabeculation. The mutant heart defects strongly resemble the phenotype caused by disruption of BMP10 and Neuregulin-1 (NRG1) signaling pathways, two central mediators of myocardial wall development. Our further examination showed that expression of MYCN is regulated by both BMP and NRG1 signaling. The thin-wall defect in mutant hearts is caused by a reduction in both cell proliferation and cell size. MYCN promotes cardiomyocyte proliferation through regulating expression of cell cycle regulators (including CCND1, CCND2, and ID2) and promotes cardiomyocyte growth through regulating expression of p70S6K. In addition, expression of multiple sarcomere proteins is altered in Mycn myocardial-inactivation embryos, indicating its essential role for proper cardiomyocyte differentiation. In summary, Mycn acts downstream of BMP and NRG1 cardiogenic signaling pathways to promote normal myocardial wall morphogenesis.

Multiple essential roles for primary cilia in heart development

Background

The primary cilium is a microtubule-based, plasma membrane-ensheathed protrusion projecting from the basal bodies of almost all cell types in the mammalian body. In the past several years a plethora of papers has indicated a crucial role for primary cilia in the development of a wide variety of organs. We have investigated heart development in cobblestone, a hypomorphic allele of the gene encoding the intraflagellar transport protein Ift88, and uncovered a number of the most common congenital heart defects seen in newborn humans.

Methods

We generated serial sections of mutant cobblestone and wild type embryos in the region encompassing the heart and the cardiac outflow tract. The sections were further processed to generate three-dimensional reconstructions of these structures, and immunofluorescence confocal microscopy, transmission electron microscopy, and in situ hybridization were used to examine signal transduction pathways in the relevant areas. Whole mount in situ hybridization was also employed for certain developmental markers.

Results

In addition to an enlarged pericardium and failure of both ventricular and atrial septum formation, the cobblestone mutants displayed manifold defects in outflow tract formation, including persistent truncus arteriosus, an overriding aorta, and abnormal transformation of the aortic arches. To discern the basis of these anomalies we examined both the maintenance of primary cilia as well as endogenous and migratory embryonic cell populations that contribute to the outflow tract and atrioventricular septa. The colonization of the embryonic heart by cardiac neural crest occurred normally in the cobblestone mutant, as did the expression of Sonic hedgehog. However, with the loss of primary cilia in the mutant hearts, there was a loss of both downstream Sonic hedgehog signaling and of Islet 1 expression in the second heart field, a derivative of the pharyngeal mesoderm. In addition, defects were recorded in development of atrial laterality and ventricular myocardiogenesis. Finally, we observed a reduction in expression of Bmp4 in the outflow tract, and complete loss of expression of both Bmp2 and Bmp4 in the atrioventricular endocardial cushions. Loss of BMP2/4 signaling may result in the observed proliferative defect in the endocardial cushions, which give rise to both the atrioventricular septa as well as to the septation of the outflow tract.

Conclusions

Taken together, our results potentially identify a novel link between Sonic hedgehog signaling at the primary cilium and BMP-dependent effects upon cardiogenesis. Our data further point to a potential linkage of atrioventricular septal defects, the most common congenital heart defects, to genes of the transport machinery or basal body of the cilia.

Mitofusins 1 and 2 are essential for postnatal metabolic remodeling in heart

RATIONALE

At birth, there is a switch from placental to pulmonary circulation and the heart commences its aerobic metabolism. In cardiac myocytes, this transition is marked by increased mitochondrial biogenesis and remodeling of the intracellular architecture. The mechanisms governing the formation of new mitochondria and their expansion within myocytes remain largely unknown. Mitofusins (Mfn-1 and Mfn-2) are known regulators of mitochondrial networks, but their role during perinatal maturation of the heart has yet to be examined.

OBJECTIVE

The objective of this study was to determine the significance of mitofusins during early postnatal cardiac development.

METHODS AND RESULTS

We genetically inactivated Mfn-1 and Mfn-2 in midgestational and postnatal cardiac myocytes using a loxP/Myh6-cre approach. At birth, cardiac morphology and function of double-knockout (DKO) mice are normal. At that time, DKO mitochondria increase in numbers, appear to be spherical and heterogeneous in size, but exhibit normal electron density. By postnatal day 7, the mitochondrial numbers in DKO myocytes remain abnormally expanded and many lose matrix components and membrane organization. At this time point, DKO mice have developed cardiomyopathy. This leads to a rapid decline in survival and all DKO mice die before 16 days of age. Gene expression analysis of DKO hearts shows that mitochondria biogenesis genes are downregulated, the mitochondrial DNA is reduced, and mitochondrially encoded transcripts and proteins are also reduced. Furthermore, mitochondrial turnover pathways are dysregulated.

CONCLUSIONS

Our findings establish that Mfn-1 and Mfn-2 are essential in mediating mitochondrial remodeling during postnatal cardiac development, a time of dramatic transitions in the bioenergetics and growth of the heart.

Mitofusin 2-containing mitochondrial-reticular microdomains direct rapid cardiomyocyte bioenergetic responses via interorganelle Ca(2+) crosstalk

RATIONALE

Mitochondrial Ca(2+) uptake is essential for the bioenergetic feedback response through stimulation of Krebs cycle dehydrogenases. Close association of mitochondria to the sarcoplasmic reticulum (SR) may explain efficient mitochondrial Ca(2+) uptake despite low Ca(2+) affinity of the mitochondrial Ca(2+) uniporter. However, the existence of such mitochondrial Ca(2+) microdomains and their functional role are presently unresolved. Mitofusin (Mfn) 1 and 2 mediate mitochondrial outer membrane fusion, whereas Mfn2 but not Mfn1 tethers endoplasmic reticulum to mitochondria in noncardiac cells.

OBJECTIVE

To elucidate roles for Mfn1 and 2 in SR-mitochondrial tethering, Ca(2+) signaling, and bioenergetic regulation in cardiac myocytes.

METHODS AND RESULTS

Fruit fly heart tubes deficient of the Drosophila Mfn ortholog MARF had increased contraction-associated and caffeine-sensitive Ca(2+) release, suggesting a role for Mfn in SR Ca(2+) handling. Whereas cardiac-specific Mfn1 ablation had no effects on murine heart function or Ca(2+) cycling, Mfn2 deficiency decreased cardiomyocyte SR-mitochondrial contact length by 30% and reduced the content of SR-associated proteins in mitochondria-associated membranes. This was associated with decreased mitochondrial Ca(2+) uptake (despite unchanged mitochondrial membrane potential) but increased steady-state and caffeine-induced SR Ca(2+) release. Accordingly, Ca(2+)-induced stimulation of Krebs cycle dehydrogenases during β-adrenergic stimulation was hampered in Mfn2-KO but not Mfn1-KO myocytes, evidenced by oxidation of the redox states of NAD(P)H/NAD(P)(+) and FADH(2)/FAD.

CONCLUSIONS

Physical tethering of SR and mitochondria via Mfn2 is essential for normal interorganelle Ca(2+) signaling in the myocardium, consistent with a requirement for SR-mitochondrial Ca(2+) signaling through microdomains in the cardiomyocyte bioenergetic feedback response to physiological stress.

TGFβ and BMP signaling in cardiac cushion formation: lessons from mice and chicken

Cardiac cushion formation is crucial for both valvular and septal development. Disruption in this process can lead to valvular and septal malformations, which constitute the largest part of congenital heart defects. One of the signaling pathways that is important for cushion formation is the TGFβ superfamily. The involvement of TGFβ and BMP signaling pathways in cardiac cushion formation has been intensively studied using chicken in vitro explant assays and in genetically modified mice. In this review, we will summarize and discuss the role of TGFβ and BMP signaling components in cardiac cushion formation.

Congenital heart disease-causing Gata4 mutation displays functional deficits in vivo

Defects of atrial and ventricular septation are the most frequent form of congenital heart disease, accounting for almost 50% of all cases. We previously reported that a heterozygous G296S missense mutation of GATA4 caused atrial and ventricular septal defects and pulmonary valve stenosis in humans. GATA4 encodes a cardiac transcription factor, and when deleted in mice it results in cardiac bifida and lethality by embryonic day (E)9.5. In vitro, the mutant GATA4 protein has a reduced DNA binding affinity and transcriptional activity and abolishes a physical interaction with TBX5, a transcription factor critical for normal heart formation. To characterize the mutation in vivo, we generated mice harboring the same mutation, Gata4 G295S. Mice homozygous for the Gata4 G295S mutant allele have normal ventral body patterning and heart looping, but have a thin ventricular myocardium, single ventricular chamber, and lethality by E11.5. While heterozygous Gata4 G295S mutant mice are viable, a subset of these mice have semilunar valve stenosis and small defects of the atrial septum. Gene expression studies of homozygous mutant mice suggest the G295S protein can sufficiently activate downstream targets of Gata4 in the endoderm but not in the developing heart. Cardiomyocyte proliferation deficits and decreased cardiac expression of CCND2, a member of the cyclin family and a direct target of Gata4, were found in embryos both homozygous and heterozygous for the Gata4 G295S allele. To further define functions of the Gata4 G295S mutation in vivo, compound mutant mice were generated in which specific cell lineages harbored both the Gata4 G295S mutant and Gata4 null alleles. Examination of these mice demonstrated that the Gata4 G295S protein has functional deficits in early myocardial development. In summary, the Gata4 G295S mutation functions as a hypomorph in vivo and leads to defects in cardiomyocyte proliferation during embryogenesis, which may contribute to the development of congenital heart defects in humans.

Cardiac malformations occur due to abnormal heart development and are the most prevalent human birth defect. Defects of atrial and ventricular septation are the most common type of congenital heart defect and are the result of incomplete closure of the atrial and ventricular septa, a process required for formation of a four-chambered heart. The molecular mechanisms that underlie atrial and ventricular septal defects are unknown. We previously published a highly penetrant autosomal dominant mutation (G296S) in GATA4, which was associated with atrial and ventricular septal defects in a large kindred. The disease-causing mutation has a spectrum of biochemical deficits affecting both DNA binding and protein–protein interactions. Here, we report the generation and phenotypic characterization of mice harboring the orthologous mutation in Gata4 (G295S). While homozygous mutant mice display embryonic lethality and cardiac defects, the phenotype is less severe than Gata4-null mice. A subset of Gata4 G295S heterozygote mice display a persistent interatrial communication (patent foramen ovale) and stenosis of the semilunar valves. Molecular characterization of the mutant mice suggests that the Gata4 G295S mutant protein results in diminished expression of Gata4 target genes in the heart and functional deficits in cardiomyocyte proliferation. Thus, cardiomyocyte proliferation defects may contribute to defects of cardiac septation found in humans with GATA4 mutations.

Coordinating tissue interactions: Notch signaling in cardiac development and disease

The Notch pathway is a crucial cell-fate regulator in the developing heart. Attention in the past centered on Notch function in cardiomyocytes. However, recent advances demonstrate that region-specific endocardial Notch activity orchestrates the patterning and morphogenesis of cardiac chambers and valves through regulatory interaction with multiple myocardial and neural crest signals. Notch also regulates cardiomyocyte proliferation and differentiation during ventricular chamber development and is required for coronary vessel specification. Here, we review these data and highlight disease connections, including evidence that Notch-Hey-Bmp2 interplay impacts adult heart valve disease and that Notch contributes to cardiac arrhythmia and pre-excitation syndromes.

Epicardially derived fibroblasts preferentially contribute to the parietal leaflets of the atrioventricular valves in the murine heart

The importance of the epicardium for myocardial and valvuloseptal development has been well established; perturbation of epicardial development results in cardiac abnormalities, including thinning of the ventricular myocardial wall and malformations of the atrioventricular valvuloseptal complex. To determine the spatiotemporal contribution of epicardially derived cells to the developing fibroblast population in the heart, we have used a mWt1/IRES/GFP-Cre mouse to trace the fate of EPDCs from embryonic day (ED)10 until birth. EPDCs begin to populate the compact ventricular myocardium around ED12. The migration of epicardially derived fibroblasts toward the interface between compact and trabecular myocardium is completed around ED14. Remarkably, epicardially derived fibroblasts do not migrate into the trabecular myocardium until after ED17. Migration of EPDCs into the atrioventricular cushion mesenchyme commences around ED12. As development progresses, the number of EPDCs increases significantly, specifically in the leaflets which derive from the lateral atrioventricular cushions. In these developing leaflets the epicardially derived fibroblasts eventually largely replace the endocardially derived cells. Importantly, the contribution of EPDCs to the leaflets derived from the major AV cushions is very limited. The differential contribution of EPDCs to the various leaflets of the atrioventricular valves provides a new paradigm in valve development and could lead to new insights into the pathogenesis of abnormalities that preferentially affect individual components of this region of the heart. The notion that there is a significant difference in the contribution of epicardially and endocardially derived cells to the individual leaflets of the atrioventricular valves has also important pragmatic consequences for the use of endocardial and epicardial cre-mouse models in studies of heart development.

Roles of TGF-β signals in endothelial-mesenchymal transition during cardiac fibrosis

Most cardiac diseases caused by inflammation are associated with fibrosis in the heart. Fibrosis is characterized by the accumulation of fibroblasts and excess deposition of extracellular matrix (ECM), which results in the distorted organ architecture and function. Recent studies revealed that cardiac fibroblasts are heterogeneous with multiple origins. Endothelial-mesenchymal transition (EndMT) plays important roles in the formation of cardiac fibroblasts during pathological settings. EndMT is regulated by signaling pathways mediated by cytokines including transforming growth factor (TGF)-β. Better understanding of the mechanisms of the formation of cardiac fibroblasts via EndMT may provide an opportunity to develop therapeutic strategies to cure heart diseases.

Nephronectin regulates atrioventricular canal differentiation via Bmp4-Has2 signaling in zebrafish

The extracellular matrix is crucial for organogenesis. It is a complex and dynamic component that regulates cell behavior by modulating the activity, bioavailability and presentation of growth factors to cell surface receptors. Here, we determined the role of the extracellular matrix protein Nephronectin (Npnt) in heart development using the zebrafish model system. The vertebrate heart is formed as a linear tube in which myocardium and endocardium are separated by a layer of extracellular matrix termed the cardiac jelly. During heart development, the cardiac jelly swells at the atrioventricular (AV) canal, which precedes valve formation. Here, we show that Npnt expression correlates with this process. Morpholino-mediated knockdown of Npnt prevents proper valve leaflet formation and trabeculation and results in greater than 85% lethality at 7 days post-fertilization. The earliest observed phenotype is an extended tube-like structure at the AV boundary. In addition, the expression of myocardial genes involved in cardiac valve formation (cspg2, fibulin 1, tbx2b, bmp4) is expanded and endocardial cells along the extended tube-like structure exhibit characteristics of AV cells (has2, notch1b and Alcam expression, cuboidal cell shape). Inhibition of has2 in npnt morphants rescues the endocardial, but not the myocardial, expansion. By contrast, reduction of BMP signaling in npnt morphants reduces the ectopic expression of myocardial and endocardial AV markers. Taken together, our results identify Npnt as a novel upstream regulator of Bmp4-Has2 signaling that plays a crucial role in AV canal differentiation.

Few Smad proteins and many Smad-interacting proteins yield multiple functions and action modes in TGFβ/BMP signaling in vivo

Signaling by the many ligands of the TGFβ family strongly converges towards only five receptor-activated, intracellular Smad proteins, which fall into two classes i.e. Smad2/3 and Smad1/5/8, respectively. These Smads bind to a surprisingly high number of Smad-interacting proteins (SIPs), many of which are transcription factors (TFs) that co-operate in Smad-controlled target gene transcription in a cell type and context specific manner. A combination of functional analyses in vivo as well as in cell cultures and biochemical studies has revealed the enormous versatility of the Smad proteins. Smads and their SIPs regulate diverse molecular and cellular processes and are also directly relevant to development and disease. In this survey, we selected appropriate examples on the BMP-Smads, with emphasis on Smad1 and Smad5, and on a number of SIPs, i.e. the CPSF subunit Smicl, Ttrap (Tdp2) and Sip1 (Zeb2, Zfhx1b) from our own research carried out in three different vertebrate models.

Cardiomyocyte deletion of mitofusin-1 leads to mitochondrial fragmentation and improves tolerance to ROS-induced mitochondrial dysfunction and cell death

Molecular studies examining the impact of mitochondrial morphology on the mammalian heart have previously focused on dynamin related protein-1 (Drp-1) and mitofusin-2 (Mfn-2), while the role of the other mitofusin isoform, Mfn-1, has remained largely unexplored. In the present study, we report the generation and initial characterization of cardiomyocyte-specific Mfn-1 knockout (Mfn-1 KO) mice. Using electron microscopic analysis, we detect a greater prevalence of small, spherical mitochondria in Mfn-1 KO hearts, indicating that the absence of Mfn-1 causes a profound shift in the mitochondrial fusion/fission balance. Nevertheless, Mfn-1 KO mice exhibit normal left-ventricular function, and isolated Mfn-1 KO heart mitochondria display a normal respiratory repertoire. Mfn-1 KO myocytes are protected from mitochondrial depolarization and exhibit improved viability when challenged with reactive oxygen species (ROS) in the form of hydrogen peroxide (H(2)O(2)). Furthermore, in vitro studies detect a blunted response of KO mitochondria to undergo peroxide-induced mitochondrial permeability transition pore opening. These data suggest that Mfn-1 deletion confers protection against ROS-induced mitochondrial dysfunction. Collectively, we suggest that mitochondrial fragmentation in myocytes is not sufficient to induce heart dysfunction or trigger cardiomyocyte death. Additionally, our data suggest that endogenous levels of Mfn-1 can attenuate myocyte viability in the face of an imminent ROS overload, an effect that could be associated with the ability of Mfn-1 to remodel the outer mitochondrial membrane.

BMP signaling in congenital heart disease: new developments and future directions

Congenital heart malformations are the most common of all congenital human birth anomalies. During the past decade, research with zebrafish, chick, and mouse models have elucidated many fundamental genetic pathways that govern early cardiac patterning and differentiation. This review highlights the roles of the bone morphogenetic protein (BMP) signaling pathway in cardiogenesis and how defective BMP signals can disrupt the intricate steps of cardiac formation and cause congenital heart defects.

Mitofusin-2 maintains mitochondrial structure and contributes to stress-induced permeability transition in cardiac myocytes

Mitofusin-2 (Mfn-2) is a dynamin-like protein that is involved in the rearrangement of the outer mitochondrial membrane. Research using various experimental systems has shown that Mfn-2 is a mediator of mitochondrial fusion, an evolutionarily conserved process responsible for the surveillance of mitochondrial homeostasis. Here, we find that cardiac myocyte mitochondria lacking Mfn-2 are pleiomorphic and have the propensity to become enlarged. Consistent with an underlying mild mitochondrial dysfunction, Mfn-2-deficient mice display modest cardiac hypertrophy accompanied by slight functional deterioration. The absence of Mfn-2 is associated with a marked delay in mitochondrial permeability transition downstream of Ca(2+) stimulation or due to local generation of reactive oxygen species (ROS). Consequently, Mfn-2-deficient adult cardiomyocytes are protected from a number of cell death-inducing stimuli and Mfn-2 knockout hearts display better recovery following reperfusion injury. We conclude that in cardiac myocytes, Mfn-2 controls mitochondrial morphogenesis and serves to predispose cells to mitochondrial permeability transition and to trigger cell death.

Dynamic analysis of BMP-responsive smad activity in live zebrafish embryos

Bone morphogenetic proteins (BMPs) are critical players in development and disease, regulating such diverse processes as dorsoventral patterning, palate formation, and ossification. These ligands are classically considered to signal via BMP receptor-specific Smad proteins 1, 5, and 8. To determine the spatiotemporal pattern of Smad1/5/8 activity and thus canonical BMP signaling in the developing zebrafish embryo, we generated a transgenic line expressing EGFP under the control of a BMP-responsive element. EGFP is expressed in many established BMP signaling domains and is responsive to alterations in BMP type I receptor activity and smad1 and smad5 expression. This transgenic Smad1/5/8 reporter line will be useful for determining ligand and receptor requirements for specific domains of BMP activity, as well as for genetic and pharmacological screens aimed at identifying enhancers or suppressors of canonical BMP signaling.

Transcription factor genes Smad4 and Gata4 cooperatively regulate cardiac valve development. [corrected]

We report that the dominant human missense mutations G303E and G296S in GATA4, a cardiac-specific transcription factor gene, cause atrioventricular septal defects and valve abnormalities by disrupting a signaling cascade involved in endocardial cushion development. These GATA4 missense mutations, but not a mutation causing secundum atrial septal defects (S52F), demonstrated impaired protein interactions with SMAD4, a transcription factor required for canonical bone morphogenetic protein/transforming growth factor-β (BMP/TGF-β) signaling. Gata4 and Smad4 genetically interact in vivo: atrioventricular septal defects result from endothelial-specific Gata4 and Smad4 compound haploinsufficiency. Endothelial-specific knockout of Smad4 caused an absence of valve-forming activity: Smad4-deficient endocardium was associated with acellular endocardial cushions, absent epithelial-to-mesenchymal transformation, reduced endocardial proliferation, and loss of Id2 expression in valve-forming regions. We show that Gata4 and Smad4 cooperatively activated the Id2 promoter, that human GATA4 mutations abrogated this activity, and that Id2 deficiency in mice could cause atrioventricular septal defects. We suggest that one determinant of the phenotypic spectrum caused by human GATA4 mutations is differential effects on GATA4/SMAD4 interactions required for endocardial cushion development.

The phenotype of recurrent 10q22q23 deletions and duplications

The genomic architecture of the 10q22q23 region is characterised by two low-copy repeats (LCRs3 and 4), and deletions in this region appear to be rare. We report the clinical and molecular characterisation of eight novel deletions and six duplications within the 10q22.3q23.3 region. Five deletions and three duplications occur between LCRs3 and 4, whereas three deletions and three duplications have unique breakpoints. Most of the individuals with the LCR3-4 deletion had developmental delay, mainly affecting speech. In addition, macrocephaly, mild facial dysmorphisms, cerebellar anomalies, cardiac defects and congenital breast aplasia were observed. For congenital breast aplasia, the NRG3 gene, known to be involved in early mammary gland development in mice, is a putative candidate gene. For cardiac defects, BMPR1A and GRID1 are putative candidate genes because of their association with cardiac structure and function. Duplications between LCRs3 and 4 are associated with variable phenotypic penetrance. Probands had speech and/or motor delays and dysmorphisms including a broad forehead, deep-set eyes, upslanting palpebral fissures, a smooth philtrum and a thin upper lip. In conclusion, duplications between LCRs3 and 4 on 10q22.3q23.2 may lead to a distinct facial appearance and delays in speech and motor development. However, the phenotypic spectrum is broad, and duplications have also been found in healthy family members of a proband. Reciprocal deletions lead to speech and language delay, mild facial dysmorphisms and, in some individuals, to cerebellar, breast developmental and cardiac defects.

Cardiopoietic factors: extracellular signals for cardiac lineage commitment

Cardiac muscle creation during embryogenesis requires extracellular instructive signals that are regulated precisely in time and space, intersecting with intracellular genetic programs that confer or fashion the ability of the cells to respond. Unmasking the essential signals for cardiac lineage decisions has paramount importance for cardiac development and regenerative medicine, including the directed differentiation of progenitor and stem cells to a cardiac muscle fate.

Cell autonomous requirement of endocardial Smad4 during atrioventricular cushion development in mouse embryos

Atrioventricular (AV) cushions are the precursors of AV septum and valves. In this study, we examined roles of Smad4 during AV cushion development using a conditional gene inactivation approach. We found that endothelial/endocardial inactivation of Smad4 led to the hypocellular AV cushion defect and that both reduced cell proliferation and increased apoptosis contributed to the defect. Expression of multiple genes critical for cushion development was down-regulated in mutant embryos. In collagen gel assays, the number of mesenchymal cells formed is significantly reduced in mutant AV explants compared to that in control explants, suggesting that the reduction of cushion mesenchyme formation in mutants is unlikely secondary to their gross vasculature abnormalities. Using a previously developed immortal endocardial cell line, we showed that Smad4 is required for BMP signaling- stimulated upregulation of Tbx20 and Gata4. Therefore, our data collectively support the cell-autonomous requirement of endocardial Smad4 in regulating AV cushion development.

Myocardial deletion of Smad4 using a novel α skeletal muscle actin Cre recombinase transgenic mouse causes misalignment of the cardiac outflow tract

SMAD4 acts as the converging point for TGFβ and BMP signaling in heart development. Here, we investigated the role of SMAD4 in heart development using a novel α skeletal muscle actin Cre recombinase (MuCre) transgenic mouse strain. Lineage tracing using MuCre/ROSA26^LacZ reporter mice indicated strong Cre-recombinase expression in developing and adult heart and skeletal muscles. In heart development, significant MuCre expression was noted at E11.5 in the atrial, ventricular, outflow tract and atrioventricular canal myocardium, but not in the endocardial cushions. MuCre-driven conditional deletion of Smad4 in mice caused double outlet right ventricle (DORV), ventricular septal defect (VSD), impaired trabeculation and thinning of ventricular myocardium, and mid-gestational embryonic lethality. In conclusion, MuCre mice effectively delete genes in both heart and skeletal muscles, thus enabling the discovery that myocardial Smad4 deletion causes misalignment of the outflow tract and DORV.

The defects in development and apoptosis of cardiomyocytes in mice lacking the transcriptional factor Pax-8

BACKGROUND

Cardiac-specific deletion of ALK3 is lethal in mid-gestation with ventricular septum malformations (VSM). This study was designed to define the Pax-8's role in heart development and cardiomyocyte apoptosis.

METHODS

Pathologic changes in the hearts of Pax-8 or ALK3 knockout and wild type control mice were determined by light and electron microscopy. Analysis of cardiomyocyte apoptosis was performed by TUNEL. The effect of Pax-8 gene deficiency on caspase-3 activity was examined after transfecting Pax-8 siRNA into cultured myoblast cell line.

RESULTS

Mice with ALK3 or Pax-8 gene knockout but not wild type control animals showed the development of VSM. Increased cardiomyocyte apoptosis was found in homozygotes. Echocardiography showed that Pax-8 homozygote mice developed malfunction of the heart. Furthermore, the caspase-3 activity was significantly higher in the cells treated with Pax-8 siRNA as compared to those treated with negative control siRNA in H9C2 (2-1) cell line.

CONCLUSIONS

The Pax-8 gene may play a crucial role in heart development and regulating cardiocyte apoptosis. Knockout of Pax-8 may exert a similar effect on myocardial morphology and apoptosis as those seen in ALK3 knockouts. Furthermore, the ventricular septum malformations could be partially attributed to accelerated cardiomyocyte apoptosis.

The TGFβ type II receptor plays a critical role in the endothelial cells during cardiac development

TGFβ signalling is required for normal cardiac development. To investigate which cell types are involved, we used mice carrying a floxed Type II TGFβ receptor (Tgfbr2fl) allele and Cre-lox genetics to deplete this receptor in different regions of the heart. The three target tissues and corresponding Cre transgenic lines were atrioventricular myocardium (using cGata6-Cre), ventricular myocardium (using Mlc2v-Cre), and vascular endothelium (using tamoxifen-activated Cdh5(PAC)-CreERT2). Spatio-temporal Cre activity in each case was tracked via lacZ activation from the Rosa26R locus. Atrioventricular-myocardial-specific Tgfbr2 knockout (KO) embryos had short septal leaflets of the tricuspid valve, whereas ventricular myocardial-specific KO embryos mainly exhibited a normal cardiac phenotype. Inactivation of Tgfbr2 in endothelial cells from E11.5 resulted in deficient ventricular septation, accompanied by haemorrhage from cerebral blood vessels. We conclude that TGFβ signalling through the Tgfbr2 receptor, in endothelial cells, plays an important role in cardiac development, and is essential for cerebral vascular integrity.

Preserved heart function and maintained response to cardiac stresses in a genetic model of cardiomyocyte-targeted deficiency of cyclooxygenase-2

Cyclooxygenase-1 and -2 are rate-limiting enzymes in the formation of a wide array of bioactive lipid mediators collectively known as prostanoids (prostaglandins, prostacyclins, and thromboxanes). Evidence from clinical trials shows that selective inhibition of the second isoenzyme (cyclooxygenase-2, or Cox-2) is associated with increased risk for serious cardiovascular events and findings from animal-based studies have suggested protective roles of Cox-2 for the heart. To further characterize the function of Cox-2 in the heart, mice with loxP sites flanking exons 4 and 5 of Cox-2 were rendered knockout specifically in cardiac myocytes (Cox-2 CKO mice) via cre-mediated recombination. Baseline cardiac performance of CKO mice remained unchanged and closely resembled that of control mice. Furthermore, myocardial infarct size induced after in vivo ischemia/reperfusion (I/R) injury was comparable between CKO and control mice. In addition, cardiac hypertrophy and function four weeks after transverse aortic constriction (TAC) was found to be similar between the two groups. Assessment of Cox-2 expression in purified adult cardiac cells isolated after I/R and TAC suggests that the dominant source of Cox-2 is found in the non-myocyte fraction. In conclusion, our animal-based analyses together with the cell-based observations portray a limited role of cardiomyocyte-produced Cox-2 at baseline and in the context of ischemic or hemodynamic challenge.

The (pro)renin receptor/ATP6AP2 is essential for vacuolar H+-ATPase assembly in murine cardiomyocytes

RATIONALE

The (pro)renin receptor [(P)RR], encoded in ATP6AP2, plays a key role in the activation of local renin-angiotensin system (RAS). A truncated form of (P)RR, termed M8.9, was also found to be associated with the vacuolar H(+)-ATPase (V-ATPase), implicating a non-RAS-related function of ATP6AP2.

OBJECTIVE

We investigated the role of (P)RR/ATP6AP2 in murine cardiomyocytes.

METHODS AND RESULTS

Cardiomyocyte-specific ablation of Atp6ap2 resulted in lethal heart failure; the cardiomyocytes contained RAB7- and lysosomal-associated membrane protein 2 (LAMP2)-positive multivesicular vacuoles, especially in the perinuclear regions. The myofibrils and mitochondria remained at the cell periphery. Cardiomyocyte death was accompanied by numerous autophagic vacuoles that contained undigested cellular constituents, as a result of impaired autophagic degradation. Notably, ablation of Atp6ap2 selectively suppressed expression of the V(O) subunits of V-ATPase, resulting in deacidification of the intracellular vesicles. Furthermore, the inhibition of intracellular acidification by treatment with bafilomycin A1 or chloroquine reproduced the phenotype observed for the (P)RR/ATP6AP2-deficient cardiomyocytes.

CONCLUSIONS

Genetic ablation of Atp6ap2 created a loss-of-function model for V-ATPase. The gene product of ATP6AP2 is considered to act as in 2 ways: (1) as (P)RR, exerting a RAS-related function; and (2) as the V-ATPase-associated protein, exerting a non-RAS-related function that is essential for cell survival.

Tissue-tissue interactions during morphogenesis of the outflow tract

The heart forms as a linear heart tube that loops and septates to produce a mature four-chambered structure. The single vessel emerging from the embryonic heart, the truncus arteriosus, divides into the aorta and the pulmonary artery as part of this septation process, and a series of additional morphogenetic events result in the proper alignment and orientation of the cardiac outflow tract. Recent evidence indicates that this process involves the complex interactions of multiple cell types including primary and secondary heart fields, neural crest, pharyngeal mesenchyme, endoderm, and endothelium. Among the many signals that mediate tissue-tissue interactions during the formation of the outflow tract, we have focused on the role of the Notch signaling pathway. Here, we focus on recent advances in our understanding of Notch-mediated regulation of cardiac development with specific attention to the formation of the cardiac outflow tract.

Notch and cardiac outflow tract development

Congenital heart disease represents the most common form of human birth defect, occurring in nearly 1 in 100 live births. An increasing number of patients with these defects are surviving infancy. Approximately one-third of congenital heart defects involve malformations of the outflow tract. Related defects are found in isolation and as part of common human syndromes. Our laboratory has investigated mechanisms of cardiac morphogenesis with particular attention to outflow tract formation. During cardiogenesis, neural crest cells interact with second heart field myocardium and endocardial cushion mesenchyme. Our recent work demonstrates that Jagged1/Notch signaling within the second heart field initiates a signaling cascade involving Fgf8, Bmp4, and downstream effectors that modulate outflow tract development and aortic arch artery patterning. Hence, complex tissue-tissue interactions and integration of multiple pathways converge to orchestrate proper patterning of the outflow region. The role of Notch signaling in adult cardiac homeostasis and disease is an area of active investigation.

Design of a miniature tissue culture system to culture mouse heart valves

Valvular heart disease is a leading cause of morbidity and mortality in adults but little is known about the underlying etiology. A better understanding of the genetic and hemodynamic mechanisms involved in growth and remodeling of heart valves during physiological and pathological conditions is needed for a better understanding of valvular heart disease. Here, we report the design of a miniature tissue culture system (MTCS) that allows the culture of mitral valves from perinatal to adult mice. The design of the MTCS is novel in that fine positioning and cannulation can be conducted with hearts of different sizes (perinatal to adult). Perfusion of the heart and hence, culture of the mitral valve in its natural position, occurs in a hydraulically sealed culture bath environment. Using the MTCS, we successfully cultured the mitral valve of adult mouse hearts for 3 days. Histological analysis indicated that the cultured valves remained viable and their extracellular matrix organization was similar to age-matched native valves. Gene expression could also be modified in cultured valves by perfusion with medium containing beta-galactosidase-expressing adenovirus. Thus, the MTCS is a new tool to study the genetic and hemodynamic mechanisms underlying the three-dimensional organization of the heart valves, which could provide insights in the pathology of valvular heart disease and be used in animal models for the development of tissue-engineered heart valves.

The type I BMP receptors, Bmpr1a and Acvr1, activate multiple signaling pathways to regulate lens formation

BMPs play multiple roles in development and BMP signaling is essential for lens formation. However, the mechanisms by which BMP receptors function in vertebrate development are incompletely understood. To determine the downstream effectors of BMP signaling and their functions in the ectoderm that will form the lens, we deleted the genes encoding the type I BMP receptors, Bmpr1a and Acvr1, and the canonical transducers of BMP signaling, Smad4, Smad1 and Smad5. Bmpr1a and Acvr1 regulated cell survival and proliferation, respectively. Absence of both receptors interfered with the expression of proteins involved in normal lens development and prevented lens formation, demonstrating that BMPs induce lens formation by acting directly on the prospective lens ectoderm. Remarkably, the canonical Smad signaling pathway was not needed for most of these processes. Lens formation, placode cell proliferation, the expression of FoxE3, a lens-specific transcription factor, and the lens protein, alphaA-crystallin were regulated by BMP receptors in a Smad-independent manner. Placode cell survival was promoted by R-Smad signaling, but in a manner that did not involve Smad4. Of the responses tested, only maintaining a high level of Sox2 protein, a transcription factor expressed early in placode formation, required the canonical Smad pathway. A key function of Smad-independent BMP receptor signaling may be reorganization of actin cytoskeleton to drive lens invagination.

T-box 2, a mediator of Bmp-Smad signaling, induced hyaluronan synthase 2 and Tgfbeta2 expression and endocardial cushion formation

During early heart development, Tbx2 gene expression is initiated in the cardiac crescent and then becomes restricted to the outflow tract and the atrioventricular region. We identified a Tbx2 regulatory region, enriched in multiple Smad sites, sufficient to reproduce Tbx2 expression patterns overlapping Bmp2 and Bmp4 gene activity in the heart. The role of Tbx2 in cardiogenesis was analyzed by using Cre-LoxP activated Tbx2 transgenic misexpression in chamber myocardium. Ventricular Tbx2 misexpression exhibited an abnormally narrow chamber lumen owing to the expansion of Hyaluronan synthase 2 expression in the ECM or cardiac jelly and the appearance of the endocardial cushions (ECs). Excessive Tbx2 also induced Tgfbeta2, which coincided with the outgrowth epithelial-mesenchymal transformed cells in ventricular and atrial tissues modifying cardiomyocyte identity from chamber type to non-chamber type. Tbx2, a central intermediary of Bmp-Smad signaling, has a central part in directing Has2 and Tgfbeta2 expression, facilitating EC formation.