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

Conditional ablation of nonmuscle myosin II-B delineates heart defects in adult mice

RATIONALE

Germline ablation of the cytoskeletal protein nonmuscle myosin II (NMII)-B results in embryonic lethality, with defects in both the brain and heart. Tissue-specific ablation of NMII-B by a Cre recombinase strategy should prevent embryonic lethality and permit study of the function of NMII-B in adult hearts.

OBJECTIVE

We sought to understand the function of NMII-B in adult mouse hearts and to see whether the brain defects found in germline-ablated mice influence cardiac development.

METHODS AND RESULTS

We used a loxP/Cre recombinase strategy to specifically ablate NMII-B in the brains or hearts of mice. Mice ablated for NMII-B in neural tissues die between postnatal day 12 and 22 without showing cardiac defects. Mice deficient in NMII-B only in cardiac myocytes (B(alphaMHC)/B(alphaMHC) mice) do not show brain defects. However, B(alphaMHC)/B(alphaMHC) mice display novel cardiac defects not seen in NMII-B germline-ablated mice. Most of the B(alphaMHC)/B(alphaMHC) mice are born with enlarged cardiac myocytes, some of which are multinucleated, reflecting a defect in cytokinesis. Between 6 to 10 months, they develop a cardiomyopathy that includes interstitial fibrosis and infiltration of the myocardium and pericardium with inflammatory cells. Four of 5 B(alphaMHC)/B(alphaMHC) hearts develop marked widening of intercalated discs.

CONCLUSIONS

By avoiding the embryonic lethality found in germline-ablated mice, we were able to study the function of NMII-B in adult mice and show that absence of NMII-B in cardiac myocytes results in cardiomyopathy in the adult heart. We also define a role for NMII-B in maintaining the integrity of intercalated discs.

The transforming growth factor-beta type III receptor mediates distinct subcellular trafficking and downstream signaling of activin-like kinase (ALK)3 and ALK6 receptors

Bone morphogenetic proteins (BMPs) signal through the BMP type I and type II receptors to regulate cellular processes, including embryonic development. The type I BMP receptors activin-like kinase (ALK)3 and ALK6 share a high degree of homology, yet possess distinct signaling roles. Here, we report that although the transforming growth factor (TGF)-beta type III receptor (TbetaRIII) enhanced both ALK3 and ALK6 signaling, TbetaRIII more potently enhanced ALK6-mediated stimulation of the BMP-responsive promoters XVent2 and 3GC2, and up-regulation of the early response gene Smad6. In contrast, TbetaRIII specifically enhanced ALK3-mediated up-regulation of the early response gene ID-1. TbetaRIII associated with ALK3 primarily through their extracellular domains, whereas its interaction with ALK6 required both the extracellular and cytoplasmic domains. TbetaRIII, along with its interacting scaffolding protein beta-arrestin2, induced the internalization of ALK6. In contrast, TbetaRIII colocalized with and resulted in the cell surface retention of ALK3, independently of beta-arrestin2. Although complex formation between TbetaRIII, ALK6, and beta-arrestin2 and TbetaRIII/ALK6 internalization resulted in maximal BMP signaling, the TbetaRIII mutant unable to interact with beta-arrestin2, TbetaRIII-T841A, was unable to do so. These studies support a novel role for TbetaRIII in mediating differential ALK3 and ALK6 subcellular trafficking resulting in distinct signaling downstream of ALK3 and ALK6.

Outflow tract cushions perform a critical valve-like function in the early embryonic heart requiring BMPRIA-mediated signaling in cardiac neural crest

Neural crest-specific ablation of BMP type IA receptor (BMPRIA) causes embryonic lethality by embryonic day (E) 12.5, and this was previously postulated to arise from a myocardial defect related to signaling by a small population of cardiac neural crest cells (cNCC) in the epicardium. However, as BMP signaling via cNCC is also required for proper development of the outflow tract cushions, precursors to the semilunar valves, a plausible alternate or additional hypothesis is that heart failure may result from an outflow tract cushion defect. To investigate whether the outflow tract cushions may serve as dynamic valves in regulating hemodynamic function in the early embryo, in this study we used noninvasive ultrasound biomicroscopy-Doppler imaging to quantitatively assess hemodynamic function in mouse embryos with P0-Cre transgene mediated neural crest ablation of Bmpr1a (P0 mutants). Similar to previous studies, the neural crest-deleted Bmpr1a P0 mutants died at approximately E12.5, exhibiting persistent truncus arteriosus, thinned myocardium, and congestive heart failure. Surprisingly, our ultrasound analyses showed normal contractile indices, heart rate, and atrioventricular conduction in the P0 mutants. However, reversed diastolic arterial blood flow was detected as early as E11.5, with cardiovascular insufficiency and death rapidly ensuing by E12.5. Quantitative computed tomography showed thinning of the outflow cushions, and this was associated with a marked reduction in cell proliferation. These results suggest BMP signaling to cNCC is required for growth of the outflow tract cushions. This study provides definitive evidence that the outflow cushions perform a valve-like function critical for survival of the early mouse embryo.

Endoglin is dispensable for angiogenesis, but required for endocardial cushion formation in the midgestation mouse embryo

Vascular patterning depends on precisely coordinated timing of endothelial cell differentiation and onset of cardiac function. Endoglin is a transmembrane receptor for members of the TGF-beta superfamily that is expressed on endothelial cells from early embryonic gestation to adult life. Heterozygous loss of function mutations in human ENDOGLIN cause Hereditary Hemorrhagic Telangiectasia Type 1, a vascular disorder characterized by arteriovenous malformations that lead to hemorrhage and stroke. Endoglin null mice die in embryogenesis with numerous lesions in the cardiovascular tree including incomplete yolk sac vessel branching and remodeling, vessel dilation, hemorrhage and abnormal cardiac morphogenesis. Since defects in multiple cardiovascular tissues confound interpretations of these observations, we performed in vivo chimeric rescue analysis using Endoglin null embryonic stem cells. We demonstrate that Endoglin is required cell autonomously for endocardial to mesenchymal transition during formation of the endocardial cushions. Endoglin null cells contribute widely to endothelium in chimeric embryos rescued from cardiac development defects, indicating that Endoglin is dispensable for angiogenesis and vascular remodeling in the midgestation embryo, but is required for early patterning of the heart.

Fog2 is critical for cardiac function and maintenance of coronary vasculature in the adult mouse heart

Aberrant transcriptional regulation contributes to the pathogenesis of both congenital and adult forms of heart disease. While the transcriptional regulator friend of Gata 2 (FOG2) is known to be essential for heart morphogenesis and coronary development, its tissue-specific function has not been previously investigated. Additionally, little is known about the role of FOG2 in the adult heart. Here we used spatiotemporally regulated inactivation of Fog2 to delineate its function in both the embryonic and adult mouse heart. Early cardiomyocyte- restricted loss of Fog2 recapitulated the cardiac and coronary defects of the Fog2 germline murine knockouts. Later cardiomyocyte-restricted loss of Fog2 (Fog2MC) did not result in defects in cardiac structure or coronary vessel formation. However, Fog2MC adult mice had severely depressed ventricular function and died at 8-14 weeks. Fog2MC adult hearts displayed a paucity of coronary vessels, associated with myocardial hypoxia, increased cardiomyocyte apoptosis, and cardiac fibrosis. Induced inactivation of Fog2 in the adult mouse heart resulted in similar phenotypes, as did ablation of the FOG2 interaction with the transcription factor GATA4. Loss of the FOG2 or FOG2-GATA4 interaction altered the expression of a panel of angiogenesis-related genes. Collectively, our data indicate that FOG2 regulates adult heart function and coronary angiogenesis.

Tissue-specific deletion of the coxsackievirus and adenovirus receptor protects mice from virus-induced pancreatitis and myocarditis

In cultured cells, infection by group B coxsackievirus (CVB) is mediated by the coxsackievirus and adenovirus receptor (CAR), but the importance of this molecule in CVB-induced disease has not been determined. We generated mice with tissue-specific ablation of CAR within each of two major CVB target organs, the pancreas and heart. In the pancreas, deletion of CAR resulted in a significant reduction in both virus titers and virus-induced tissue damage. Similarly, cardiomyocyte-specific CAR deletion resulted in a marked reduction in virus titer, infection-associated cytokine production, and histopathology within the heart. Consistent with the in vivo phenotype, CAR-deficient cardiomyocytes resisted infection in vitro. These results demonstrate a critical function for CAR in the pathogenesis of CVB infection in vivo and in virus tropism for the heart and pancreas.

BMP type II receptor regulates positioning of outflow tract and remodeling of atrioventricular cushion during cardiogenesis

Signaling of bone morphogenetic protein (BMP) via type I and type II receptors is involved in multiple processes contributing to cardiogenesis. To investigate the role of the BMP type II receptor (BMPRII) in heart development, the BMPRII gene was deleted throughout the embryo during gastrulation using a Mox2-Cre transgene. BMPRII(flox/-);Mox2-Cre mice exhibited cardiac defects including double-outlet right ventricle, ventricular septal defect (VSD), atrioventricular (AV) cushion defects, and thickened valve leaflets. To characterize the tissue-specific functions of BMPRII in cardiogenesis, a series of Cre transgenes (alphaMHC-, Tie2-, Wnt1-, and SM22alpha-Cre) was employed. Interestingly, myocardial development was normal when the BMPRII gene was deleted in myocardial cells using Mox2-Cre, alphaMHC-Cre, or SM22alpha-Cre transgenes, suggesting that signaling by other BMP type II receptors may compensate for the absence of BMPRII in the myocardial cells. AV cushion defects including atrial septal defect, membranous VSD, and thickened valve leaflets were found in BMPRII(flox/-);Tie2-Cre mice. Abnormal positioning of the aorta was observed in BMPRII(flox/-);Wnt1-Cre and BMPRII(flox/-);SM22alpha-Cre mice. Taken together, these results demonstrate that endocardial BMPRII expression is required for septal formation and valvulogenesis. Moreover, mesenchymal BMPRII expression in the outflow tract cushion is required for proper positioning of the aorta.

Murine Jagged1/Notch signaling in the second heart field orchestrates Fgf8 expression and tissue-tissue interactions during outflow tract development

Notch signaling is vital for proper cardiovascular development and function in both humans and animal models. Indeed, mutations in either JAGGED or NOTCH cause congenital heart disease in humans and NOTCH mutations are associated with adult valvular disease. Notch typically functions to mediate developmental interactions between adjacent tissues. Here we show that either absence of the Notch ligand Jagged1 or inhibition of Notch signaling in second heart field tissues results in murine aortic arch artery and cardiac anomalies. In mid-gestation, these mutants displayed decreased Fgf8 and Bmp4 expression. Notch inhibition within the second heart field affected the development of neighboring tissues. For example, faulty migration of cardiac neural crest cells and defective endothelial-mesenchymal transition within the outflow tract endocardial cushions were observed. Furthermore, exogenous Fgf8 was sufficient to rescue the defect in endothelial-mesenchymal transition in explant assays of endocardial cushions following Notch inhibition within second heart field derivatives. These data support a model that relates second heart field, neural crest, and endocardial cushion development and suggests that perturbed Notch-Jagged signaling within second heart field progenitors accounts for some forms of congenital and adult cardiac disease.

Developmental signaling in myocardial progenitor cells: a comprehensive view of Bmp- and Wnt/beta-catenin signaling

The tight regulation of different signaling systems and the transcriptional and translational networks during embryonic development have been the focus of embryologists in recent decades. Defective developmental signaling due to genetic mutation or temporal and region-specific alteration of gene expression causes embryonic lethality or accounts for birth defects (e.g., congenital heart disease). The formation of the heart requires the coordinated integration of multiple cardiac progenitor cell populations derived from the first and second heart fields and from cardiac neural crest cells. This article summarizes what has been learned from conditional mutagenesis of Bmp pathway components and the Wnt effector, beta-catenin, in the developing heart of mice. Although Bmp signaling is required for cardiac progenitor cell specification, proliferation, and differentiation, recent studies have demonstrated distinct functions of Wnt/beta-catenin signaling at various stages of heart development.

Bmp2 and Bmp4 genetically interact to support multiple aspects of mouse development including functional heart development

Bone morphogenetic proteins (BMPs) have multiple roles during embryogenesis. Current data indicate that the dosage of BMPs is tightly regulated for normal development in mice. Since Bmp2 or Bmp4 homozygous mutant mice show early embryonic lethality, we generated compound heterozygous mice for Bmp2 and Bmp4 to explore the impact of lowered dosage of these BMP ligands. Genotyping pups bred between Bmp2 and Bmp4 heterozygous mice revealed that the ratio of adult compound heterozygous mice for Bmp2 and Bmp4 is much lower than expected. During embryogenesis, the compound heterozygous embryos showed several abnormalities, including defects in eye formation, body wall closure defects, and ventricular septal defects (VSD) in the heart. However, the ratio of the compound heterozygous embryos was the same as expected. Caesarean sections at E18.5 revealed that half of the compound heterozygotes died soon after birth, and the majority of the dead individuals exhibited VSD. Survivors were able to grow to adults, but their body weight was significantly lower than control littermates. They demonstrated progressive abnormalities in the heart, eventually showing a branched leaflet in atrioventricular valves. These results suggest that the dosage of both BMP2 and 4 is critical for functional heart formation during embryogenesis and after birth.

FGF-regulated BMP signaling is required for eyelid closure and to specify conjunctival epithelial cell fate

There are conflicting reports about whether BMP signaling is required for eyelid closure during fetal development. This question was addressed using mice deficient in BMP or TGFbeta signaling in prospective eyelid and conjunctival epithelial cells. Genes encoding two type I BMP receptors, the type II TGFbeta receptor, two BMP- or two TGFbeta-activated R-Smads or the co-Smad Smad4 were deleted from the ocular surface ectoderm using Cre recombinase. Only mice with deletion of components of the BMP pathway had an 'eyelid open at birth' phenotype. Mice lacking Fgf10 or Fgfr2 also have open eyelids at birth. To better understand the pathways that regulate BMP expression and function during eyelid development, we localized BMPs and BMP signaling intermediates in Fgfr2 and Smad4 conditional knockout (CKO) mice. We found that Fgfr2 was required for the expression of Bmp4, the normal distribution of Shh signaling and for preserving the differentiation of the conjunctival epithelium. FGF signaling also promoted the expression of the Wnt antagonist Sfrp1 and suppressed Wnt signaling in the prospective eyelid epithelial cells, independently of BMP function. Transcripts encoding Foxc1 and Foxc2, which were previously shown to be necessary for eyelid closure, were not detectable in Smad4(CKO) animals. c-Jun, another key regulator of eyelid closure, was present and phosphorylated in eyelid periderm cells at the time of fusion, but failed to translocate to the nucleus in the absence of BMP function. Smad4(CKO) mice also showed premature differentiation of the conjunctival epithelium, conjunctival hyperplasia and the acquisition of epidermal characteristics, including formation of an ectopic row of hair follicles in place of the Meibomian glands. A second row of eyelashes is a feature of human lymphedema-distichiasis syndrome, which is associated with mutations in FOXC2.

Genetic interaction between Bmp2 and Bmp4 reveals shared functions during multiple aspects of mouse organogenesis

Vertebrate Bmp2 and Bmp4 diverged from a common ancestral gene and encode closely related proteins. Mice homozygous for null mutations in either gene show early embryonic lethality, thereby precluding analysis of shared functions. In the current studies, we present phenotypic analysis of compound mutant mice heterozygous for a null allele of Bmp2 in combination with null or hypomorphic alleles of Bmp4. Whereas mice lacking a single copy of Bmp2 or Bmp4 are viable and have subtle developmental defects, compound mutants show embryonic and postnatal lethality due to defects in multiple organ systems including the allantois, placental vasculature, ventral body wall, skeleton, eye and heart. Within the heart, BMP2 and BMP4 function coordinately to direct normal lengthening of the outflow tract, proper positioning of the outflow vessels, and septation of the atria, ventricle and atrioventricular canal. Our results identify numerous BMP4-dependent developmental processes that are also very sensitive to BMP2 dosage, thus revealing novel functions of Bmp2.

Noonan syndrome cardiac defects are caused by PTPN11 acting in endocardium to enhance endocardial-mesenchymal transformation

Noonan syndrome (NS), the most common single-gene cause of congenital heart disease, is an autosomal dominant disorder that also features proportionate short stature, facial abnormalities, and an increased risk of myeloproliferative disease. Germline-activating mutations in PTPN11, which encodes the protein tyrosine phosphatase SHP2, cause about half of NS cases; other causative alleles include KRAS, SOS1, and RAF1 mutants. We showed previously that knock-in mice bearing the NS mutant Ptpn11(D61G) on a mixed 129S4/SvJae X C57BL6/J background exhibit all major NS features, including a variety of cardiac defects, with variable penetrance. However, the cellular and molecular mechanisms underlying NS cardiac defects and whether genetic background and/or the specific NS mutation contribute to the NS phenotype remained unclear. Here, using an inducible knock-in approach, we show that all cardiac defects in NS result from mutant Shp2 expression in the endocardium, not in the myocardium or neural crest. Furthermore, the penetrance of NS defects is affected by genetic background and the specific Ptpn11 allele. Finally, ex vivo assays and pharmacological approaches show that NS mutants cause cardiac valve defects by increasing Erk MAPK activation, probably downstream of ErbB family receptor tyrosine kinases, extending the interval during which cardiac endocardial cells undergo endocardial-mesenchymal transformation. Our data provide a mechanistic underpinning for the cardiac defects in this disorder.

Interaction of Gata4 and Gata6 with Tbx5 is critical for normal cardiac development

Congenital heart disease is the most common type of birth defect with an incidence of 1%. Previously, we described a point mutation in GATA4 that segregated with cardiac defects in a family with autosomal dominant disease. The mutation (G296S) exhibited biochemical deficits and disrupted a novel interaction between Gata4 and Tbx5. To determine if Gata4 and Tbx5 genetically interact in vivo, we generated mice heterozygous for both alleles. We found that nearly 100% of mice heterozygous for Gata4 and Tbx5 were embryonic or neonatal lethal and had complete atrioventricular (AV) septal defects with a single AV valve and myocardial thinning. Consistent with this phenotype, Gata4 and Tbx5 are co-expressed in the developing endocardial cushions and myocardium. In mutant embryos, cardiomyocyte proliferation deficits were identified compatible with the myocardial hypoplasia. Similar to Gata4, Gata6 and Tbx5 are co-expressed in the embryonic heart, and the transcription factors synergistically activate the atrial natiuretic factor promoter. We demonstrate a genetic interaction between Gata6 and Tbx5 with an incompletely penetrant phenotype of neonatal lethality and thin myocardium. Gene expression analyses were performed on both sets of compound heterozygotes and demonstrated downregulation of alpha-myosin heavy chain only in Gata4/Tbx5 heterozygotes. These findings highlight the unique genetic interactions of Gata4 and Gata6 with Tbx5 for normal cardiac morphogenesis in vivo.

Smad7 is required for the development and function of the heart

Transforming growth factor-beta (TGF-beta) family members, including TGF-betas, activins, and bone morphogenetic proteins, exert diverse biological activities in cell proliferation, differentiation, apoptosis, embryonic development, and many other processes. These effects are largely mediated by Smad proteins. Smad7 is a negative regulator for the signaling of TGF-beta family members. Dysregulation of Smad7 is associated with pathogenesis of a variety of human diseases. However, the in vivo physiological roles of Smad7 have not been elucidated due to the lack of a mouse model with significant loss of Smad7 function. Here we report generation and initial characterization of Smad7 mutant mice with targeted deletion of the indispensable MH2 domain. The majority of Smad7 mutant mice died in utero due to multiple defects in cardiovascular development, including ventricular septal defect and non-compaction, as well as outflow tract malformation. The surviving adult Smad7 mutant mice had impaired cardiac functions and severe arrhythmia. Further analyses suggest that Smad2/3 phosphorylation was elevated in atrioventricular cushion in the heart of Smad7 mutant mice, accompanied by increased apoptosis in this region. Taken together, these observations pinpoint an important role of Smad7 in the development and function of the mouse heart in vivo.

MEKK3 initiates transforming growth factor beta 2-dependent epithelial-to-mesenchymal transition during endocardial cushion morphogenesis

Congenital heart defects occur at a rate of 5% and are the most prevalent birth defects. A better understanding of the complex signaling networks regulating heart development is necessary to improve repair strategies for congenital heart defects. The mitogen-activated protein 3 kinase (MEKK3) is important to early embryogenesis, but developmental processes affected by MEKK3 during heart morphogenesis have not been fully examined. We identify MEKK3 as a critical signaling molecule during endocardial cushion development. We report the detection of MEKK3 transcripts to embryonic hearts before, during, and after cardiac cushion cells have executed epithelial-to-mesenchymal transition (EMT). MEKK3 is observed to endocardial cells of the cardiac cushions with a diminishing gradient of expression into the cushions. These observations suggest that MEKK3 may function during production of cushion mesenchyme as required for valvular development and septation of the heart. We used a kinase inactive form of MEKK3 (MEKK3(KI)) in an in vitro assay that recapitulates in vivo EMT and show that MEKK3(KI) attenuates mesenchyme formation. Conversely, constitutively active MEKK3 (ca-MEKK3) triggers mesenchyme production in ventricular endocardium, a tissue that does not normally undergo EMT. MEKK3-driven mesenchyme production is further substantiated by increased expression of EMT-relevant genes, including TGFbeta(2), Has2, and periostin. Furthermore, we show that MEKK3 stimulates EMT via a TGFbeta(2)-dependent mechanism. Thus, the activity of MEKK3 is sufficient for developmental EMT in the heart. This knowledge provides a basis to understand how MEKK3 integrates signaling cascades activating endocardial cushion EMT.

Periostin as a heterofunctional regulator of cardiac development and disease

Periostin (Postn) is a heterofunctional secreted extracellular matrix (ECM) protein comprised of four fasciclin domains that promotes cellular adhesion and movement, as well as collagen fibrillogenesis. Postn is expressed in unique growth centers during embryonic development where it facilitates epithelial-mesenchymal transition (EMT) of select cell populations undergoing reorganization. In the heart, Postn is expressed in the developing valves, cardiac fibroblasts and in regions of the outflow track. In the adult, Postn expression is specifically induced in areas of tissue injury or areas with ongoing cellular re-organization. In the adult heart Postn is induced in the ventricles following myocardial infarction, pressure overload stimulation, or generalized cardiomyopathy. Here we will review the functional consequences associated with Postn induction in both the developing and adult heart. The majority of data collected to date suggest a common function for Postn in both development and disease as a potent inducible regulator of cellular reorganization and extracellular matrix homeostasis, although some alternate and controversial functions have also been ascribed to Postn, the validity of which will be discussed here.

Signaling via the Tgf-beta type I receptor Alk5 in heart development

Trophic factors secreted both from the endocardium and epicardium regulate appropriate growth of the myocardium during cardiac development. Epicardially-derived cells play also a key role in development of the coronary vasculature. This process involves transformation of epithelial (epicardial) cells to mesenchymal cells (EMT). Similarly, a subset of endocardial cells undergoes EMT to form the mesenchyme of endocardial cushions, which function as primordia for developing valves and septa. While it has been suggested that transforming growth factor-betas (Tgf-beta) play an important role in induction of EMT in the avian epi- and endocardium, the function of Tgf-betas in corresponding mammalian tissues is still poorly understood. In this study, we have ablated the Tgf-beta type I receptor Alk5 in endo-, myo- and epicardial lineages using the Tie2-Cre, Nkx2.5-Cre, and Gata5-Cre driver lines, respectively. We show that while Alk5-mediated signaling does not play a major role in the myocardium during mouse cardiac development, it is critically important in the endocardium for induction of EMT both in vitro and in vivo. Moreover, loss of epicardial Alk5-mediated signaling leads to disruption of cell-cell interactions between the epicardium and myocardium resulting in a thinned myocardium. Furthermore, epicardial cells lacking Alk5 fail to undergo Tgf-beta-induced EMT in vitro. Late term mutant embryos lacking epicardial Alk5 display defective formation of a smooth muscle cell layer around coronary arteries, and aberrant formation of capillary vessels in the myocardium suggesting that Alk5 is controlling vascular homeostasis during cardiogenesis. To conclude, Tgf-beta signaling via Alk5 is not required in myocardial cells during mammalian cardiac development, but plays an irreplaceable cell-autonomous role regulating cellular communication, differentiation and proliferation in endocardial and epicardial cells.

Cardiomyogenic stem and progenitor cell plasticity and the dissection of cardiopoiesis

Cell-based therapies hold promise of repairing an injured heart, and the description of stem and progenitor cells with cardiomyogenic potential is critical to its realization. At the vanguard of these efforts are analyses of embryonic stem cells, which clearly have the capacity to generate large numbers of cardiomyocytes in vitro. Through the use of this model system, a number of signaling mechanisms have been worked out that describes at least partially the process of cardiopoiesis. Studies on adult stem and on progenitor cells with cardiomyogenic potential are still in their infancy, and much less is known about the molecular signals that are required to induce the differentiation to cardiomyocytes. It is also unclear whether the pathways are similar or different between embryonic and adult cell-induced cardiomyogenesis, partly because of the continued controversies that surround the stem cell theory of cardiac self-renewal. Irrespective of any perceived or actual limitations, the study of stem and progenitor cells has provided important insights into the process of cardiomyogenesis, and it is likely that future research in this area will turn the promise of repairing an injured heart into a reality.

Real time monitoring of BMP Smads transcriptional activity during mouse development

SUMMARY

Bone morphogenetic protein (BMP) signaling is a key pathway in the patterning and development of organisms as diverse as fruit fly and humans. However, the determination of net BMP signaling, paramount to understanding organogenesis, is limited to the analysis of fixed material. We generated a transgenic mouse that reports the transcriptional response of BMP Smad activation by coupling a well established BMP response element (BRE), isolated from the Id1 promoter, to green fluorescent protein (BRE:gfp). We monitored BMP Smad transcriptional activity in fresh and fixed BRE:gfp embryos. GFP expression was observed where expected on the basis of known signaling sites, but also in specific cell populations in which BMP signaling had been implicated but not directly demonstrated. Deletion of Smad5 reduced GFP in vivo as expected. The BRE:gfp transgenic mice are a novel tool, which will facilitate the identification of specific BMP Smad responsive cell types and allow BMP Smad signaling to be monitored in real time, supporting studies in development and disease.

An in vivo reporter of BMP signaling in organogenesis reveals targets in the developing kidney

Background

Bone morphogenetic proteins (BMPs) regulate essential processes during organogenesis, and a functional understanding of these secreted proteins depends on identification of their target cells. In this study, we generate a transgenic reporter for organogenesis studies that we use to define BMP pathway activation in the developing kidney.

Results

Mouse strains reporting on BMP pathway activation were generated by transgenically expressing β-galactosidase under the control of BMP responsive elements from Id1. Reporter expression corresponds well with immunoassays for pathway activation in all organs studied, validating the model. Using these reporters we have generated a detailed map of cellular targets of BMP signaling in the developing kidney. We find that SMAD dependent BMP signaling is active in collecting duct trunks, but not tips. Furthermore, glomerular endothelial cells, and proximal nephron tubules from the renal vesicle stage onward show pathway activation. Surprisingly, little activation is detected in the nephrogenic zone of the kidney, and in organ culture BMP treatment fails to activate SMAD dependent BMP signaling in nephron progenitor cells. In contrast, signaling is efficiently induced in collecting duct tips.

Conclusion

Transgenic reporters driven by control elements from BMP responsive genes such as Id1 offer significant advantages in sensitivity and consistency over immunostaining for studies of BMP pathway activation. They also provide opportunities for analysis of BMP signaling in organ and primary cell cultures subjected to experimental manipulation. Using such a reporter, we made the surprising finding that SMAD dependent BMP signaling is inactive in nephron progenitors, and that these cells are refractory to activation by applied growth factors. Furthermore, we find that the BMP pathway is not normally active in collecting duct tips, but that it can be ectopically activated by BMP treatment, offering a possible explanation for the inhibitory effects of BMP treatment on collecting duct growth and branching.

SM22alpha-targeted deletion of bone morphogenetic protein receptor 1A in mice impairs cardiac and vascular development, and influences organogenesis

Expression of bone morphogenetic protein receptor 1A (BMPR1A) is attenuated in the lung vessels of patients with pulmonary arterial hypertension, but the functional impact of this abnormality is unknown. We ablated Bmpr1a in cardiomyocytes and vascular smooth muscle cells (VSMCs) by breeding mice possessing a loxP allele of Bmpr1a (Bmpr1aflox) expressing R26R with SM22alpha-Cre mice. SM22alpha-Cre;R26R;Bmpr1aflox/flox mice died soon after embryonic day 11 (E11) with massive vascular and pericardial hemorrhage and impaired brain development. At E10.5, SM22alpha-Cre;R26R;Bmpr1aflox/flox embryos showed thinning of the myocardium associated with reduced cell proliferation. These embryos also had severe dilatation of the aorta and large vessels with impaired investment of SMCs that was also related to reduced proliferation. SM22alpha-Cre;R26R;Bmpr1aflox/flox mice showed collapsed telencephalon in association with impaired clearing of brain microvessels in areas where reduced apoptosis was observed. Transcript and protein levels of matrix metalloproteinase (MMP) 2 and 9 were reduced in E9.5 and E10.5 SM22alpha-Cre;R26R;Bmpr1aflox/flox embryos, respectively. Knock-down of BMPR1A by RNA interference in human pulmonary artery SMCs reduced MMP2 and MMP9 activity, attenuated serum-induced proliferation, and impaired PDGF-BB-directed migration. RNA interference of MMP2 or MMP9 recapitulated these abnormalities, supporting a functional interaction between BMP signaling and MMP expression. In human brain microvascular pericytes, knock-down of BMPR1A reduced MMP2 activity and knock-down of either BMPR1A or MMP2 caused resistance to apoptosis. Thus, loss of Bmpr1a, by decreasing MMP2 and/or MMP9 activity, can account for vascular dilatation and persistence of brain microvessels, leading to the impaired organogenesis documented in the brain.

ROCK1 expression is regulated by TGFbeta3 and ALK2 during valvuloseptal endocardial cushion formation

During early heart development at the looped heart stage, endothelial cells in the outflow tract and atrioventricular (AV) regions transform into mesenchyme to generate endocardial cushion tissue. This endocardial epithelial-mesenchymal transition (EMT) is regulated by several regulatory pathways, including the transforming growth factor-beta (TGFbeta), bone morphogenetic protein (BMP), and Rho-ROCK pathways. Here, we investigated the spatiotemporal expression pattern of ROCK1 mRNA during EMT in chick and examined whether TGFbeta or BMP could induce the expression of ROCK1. At the onset of EMT, ROCK1 expression was up-regulated in endothelial/mesenchymal cells. A three-dimensional collagen gel assay was used to examine the mechanisms regulating the expression of ROCK1. In AV endocardium co-cultured with associated myocardium, ROCK1 expression was inhibited by either anti-TGFbeta3 antibody, anti-ALK2 antibody or noggin, but not SB431542 (ALK5 inhibitor). In cultured preactivated AV endocardium, TGFbeta3 protein induced the expression of ROCK1, but BMP did not. AV endothelial cells that were cultured in medium supplemented with TGFbeta3 plus anti-ALK2 antibody failed to express ROCK1. These results suggest that the expression of ROCK1 is up-regulated at the onset of EMT and that signaling mediated by TGFbeta3/ALK2 together with BMP is involved in the expression of ROCK1.

Deletion of the NADPH-cytochrome P450 reductase gene in cardiomyocytes does not protect mice against doxorubicin-mediated acute cardiac toxicity

A genetic mouse model (designated cardiomyocyte-Cpr-null) with cardiomyocyte-specific deletion of the cytochrome P450 (P450) reductase (Cpr) gene was generated in this study. CPR protein levels, as well the enzyme activity of P450s, were greatly reduced in heart microsomes from the null mice compared with wild-type mice, whereas CPR expression in other organs remained unchanged. Nonetheless, homozygous null mice were normal in appearance, gross anatomy, tissue morphology, and general cardiac functional parameters, and there was no indication of embryonic lethality or premature mortality in contrast to the recognized role of CPR in embryonic development. Thus, this new mouse model should be useful for determination of the in vivo roles of cardiomyocyte CPR and CPR-dependent enzymes, including microsomal P450s, not only in the metabolism and toxicity of numerous xenobiotic compounds but also in cardiac pathophysiology. As a first application, we studied the role of cardiomyocyte CPR and CPR-dependent enzymes in doxorubicin (Dox)-mediated acute cardiotoxicity. Wild-type and null mice were treated with a single i.p. dose of Dox at 5, 10, or 20 mg/kg. The Dox treatment caused apoptosis and vacuolization in cardiomyocytes at the dose of 20 mg/kg and a significant increase in the levels of serum creatine kinase at 10 and 20 mg/kg in both wild-type and null mice. However, there was no significant difference in the extent of Dox-induced cardiac injury between the two strains; this lack of difference suggests that cardiomyocyte CPR and CPR-dependent enzymes do not play critical roles in the acute cardiotoxicity induced by Dox.

Recent advances in cardiovascular regenerative medicine: the induced pluripotent stem cell era

Induced pluripotent stem (iPS) cells have recently been established by transfecting mouse and human fibroblasts with the transcription factors Oct3/4, Sox2, Klf4 and c-Myc, known to be expressed at high levels in embryonic stem (ES) cells. These cells have great potential in regenerative medicine as they have the capacity to differentiate into all three germ layer-derived cells and are syngeneic. The differentiation of ES cells into cardiomyocytes mimics the early processes involved in heart development. Recent studies describe the contribution of various growth factors and corresponding inhibitors to heart development during embryogenesis. Bone morphogenetic proteins, Wnt protein and Notch signals play critical roles in heart development in a context- and time-dependent manner. Consistent with ES cells, the exposure of iPS cells to such growth factors is hypothesized to augment differentiation into cardiomyocytes. The combination of iPS cells and appropriate developmental signal information has the potential for providing the foundations for future regenerative medicine.

BMP signaling through BMPRIA in astrocytes is essential for proper cerebral angiogenesis and formation of the blood-brain-barrier

Bone morphogenetic protein (BMP) signaling is involved in differentiation of neural precursor cells into astrocytes, but its contribution to angiogenesis is not well characterized. This study examines the role of BMP signaling through BMP type IA receptor (BMPRIA) in early neural development using a conditional knockout mouse model, in which Bmpr1a is selectively disrupted in telencephalic neural stem cells. The conditional mutant mice show a significant increase in the number of cerebral blood vessels and the level of vascular endothelial growth factor (VEGF) is significantly upregulated in the mutant astrocytes. The mutant mice also show leakage of immunoglobulin around cerebral microvessels in neonatal mice, suggesting a defect in formation of the blood-brain-barrier. In addition, astrocytic endfeet fail to encircle cortical blood vessels in the mutant mice. These results suggest that BMPRIA signaling in astrocytes regulates the expression of VEGF for proper cerebrovascular angiogenesis and has a role on in the formation of the blood-brain-barrier.

Endothelial expression of bone morphogenetic protein receptor type 1a is required for atrioventricular valve formation

BACKGROUND

Atrioventricular canal defects account for 4% of all congenital heart anomalies. They arise from failure of endocardial cushion formation, a process dependent on transition of endothelial cells into clustered mesenchymal cells in the mid-atrioventricular septum. To date, the genetic signals necessary for atrioventricular canal defects are poorly understood. We hypothesized that bone morphogenetic protein signaling in cardiac endothelial cells may be crucial to this process.

METHODS

To study the role of bone morphogenetic protein receptors (Bmpr) in the developing heart, we created knockout mice with inactivation of Bmpr1a selectively in endocardium. Two strains of null mice were created: one with constitutive endothelial-specific knockout of Bmpr1a and one with time-inducible, endothelial-specific knockout of Bmpr1a. Embryos and animals were analyzed by microscopy, RNA in situ hybridization, and microangiography.

RESULTS

Animals with null mutation of Bmpr1a in endothelium were embryonic lethal at E11.5 to 12.0 and demonstrated absence of endocardial cushion formation. Embryos failed to form atrioventricular valves and adjacent septa. Endocardial knockout of Bmpr1a did not affect development of the outflow tract or aortic arches. Using time-inducible, cell-specific knockout mice, we show that Bmpr1a has two functions in the developing atrioventricular canal: to induce endocardial endothelial-mesenchymal transition, and to pattern the septal mesenchyme into endocardial cushions. We demonstrate that these processes are temporally linked to expression of the transcription factors Id1 and Id3.

CONCLUSIONS

Endocardial cushion formation is dependent on cell-specific expression of Bmpr1a. Our results suggest that Bmpr1a-mediated signaling is a crucial pathway involved in pathogenesis of atrioventricular septal and valve malformations, which are among the most common congenital heart defects in humans.

BMPR IA downstream genes related to VSD

Cardiac-specific deletion of the receptor IA of bone morphogenetic protein (BMP) (ALK3) by Cre recombinase driven under the [alpha]-MHC promoter is lethal in mid-gestation with defects in the interventricular septum [ventricular septum defect (VSD)]. Analysis of expression of the ALK3 downstream genes is important to identify the signaling pathway for interventricular septum development. The mRNA expression level of a control group was compared with that of a test group. ALK3 downstream genes were screened using polymerase chain reaction (PCR)-select cDNA subtraction and microarray. It was found that the mice with an ALK3 knockout gene produced a VSD. The expression of some genes such as platelet-activating factor acetylhydrolase (PAF) and Pax-8 was down-regulated in the test group. Pax-8 gene expression was down-regulated by 7.1 times in the test group and expressed specifically in the 11.5-d embryonic (E11.5) heart. Furthermore, the expression of the protein-tyrosine kinase of the focal adhesion kinase subfamily (PTK) and [beta] subtype protein 14-3-3 was up-regulated in the test group. PTK gene expression was up-regulated by 3.7 times in the test group. These data provided support that the ALK3 gene plays an important role during heart development. The PAF and Pax-8 genes could be important ALK3 downstream genes in the BMP signaling pathway during interventricular septum development. PTK and [beta] subtype protein 14-3-3 might be regulatory factors in this pathway.

Myocardial Pitx2 differentially regulates the left atrial identity and ventricular asymmetric remodeling programs

The Pitx2 gene regulates left-right (L/R) asymmetrical cardiac morphogenesis. Constitutive Pitx2 knock out (ko) mice die before birth and display, among other defects, right atrial isomerism, atrial and ventricular septal defects, and double outlet right ventricle. The myocardial role of the gene has not been dissected. In particular, how Pitx2 regulates the differential L/R cardiac identity program is not clear. Additionally, the relation between Pitx2 ko ventricular defects and the gene expression pattern is not understood. In this article we analyze Pitx2 myocardial function during mouse heart development. By in situ hybridization analysis we show that myocardial Pitx2 expression delineates the remodeling of the left atrioventricular canal, the inner curvature, the ventral part of the interventricular ring, and the ventral portion of the right and left ventricle. By genetic analysis using an allelic series of Pitx2 mutants, among which a myocardial specific ko (ko(myo)) we show it has a crucial role in this process. Pitx2 ko(myo) mutants survive to adulthood, when they present strong cardiac morphological and functional defects. Confocal analysis of embryonic Pitx2 ko(myo) hearts reveals delayed cardiomyocyte development in the ventricular but not in the atrial Pitx2 null areas. Conversely, selective left atrial BMP10 mRNA downregulation which normally occurs at fetal stages is not found in the Pitx2 ko(myo) mice. This is the first evidence for distinct Pitx2 action in mediating L/R atrial identity and asymmetrical ventricular remodeling.

Genes in congenital heart disease: atrioventricular valve formation

Through the use of animal studies, many candidate genes (mainly encoding transcriptional factors and receptors) have been implicated in the development of congenital heart disease. Thus far, only a minority of these genes have been shown to carry mutations associated with congenital disease in humans, e.g., GATA 4, TBX-5, NOTCH1 and NKX2-5. Mutations in these genes can cause a variety of cardiac defects even within the same family. Conversely, similar phenotypes are observed for different gene mutations suggesting a common pathway. Multiple genes and genetic pathways have been related to atrioventricular valve formation, although most of these genes have not yet been demonstrated as causative in human atrioventricular valve defects. Key pathways include the epidermal growth factor receptor pathway and related interacting pathways, most importantly the pathway of UDP-glucose dehydrogenase, resulting ultimately in activation of Ras. Other examples of interacting pathways include that of Nodal/Cited2/Pitx2, Wnt, Notch and ECE. Further studies are needed to investigate the pathways which are crucial for atrioventricular valve formation in humans. Understanding the underlying molecular process of abnormal atrioventricular valve formation in patients with congenital heart disease may provide important insight, in the etiology and possibly into preventive or treatment regimes.

BMP-2 induces cell migration and periostin expression during atrioventricular valvulogenesis

Atrioventricular (AV) endocardium transforms into the cushion mesenchyme, the primordia of the valves and membranous septa, through epithelial-mesenchymal transformation (EMT). While bone morphogenetic protein (BMP)-2 is known to be critical for AV EMT, the role of BMP-2 in post-EMT AV valvulogenesis remains to be elucidated. To find BMP signaling loops, we first localized Type I BMP receptors (BMPRs), BMPR-1A (ALK3), -1B (ALK6) and ALK2 in AV cushion mesenchyme in stage-24 chick embryos. Based on the BMP receptor expression pattern, we examined the functional roles of BMP-2 and BMP signaling in post-EMT valvulogenesis by using stage-24 AV cushion mesenchymal cell aggregates cultured on 3D-collagen gels. Exogenous BMP-2 or constitutively active (ca) BMPR-1B (ALK6)-virus treatments induced migration of the mesenchymal cells into the collagen gels, whereas noggin, an antagonist of BMPs, or dominant-negative (dn) BMPR-1 B (ALK6)-virus treatments reduced cell migration from the mesenchymal cell aggregates. Exogenous BMP-2 or caBMPR-1B (ALK6) treatments significantly promoted expression of an extracellular matrix (ECM) protein, periostin, a known valvulogenic matrix maturation mediator, at both mRNA and protein levels, whereas periostin expression was repressed by adding noggin or dnBMPR-1B (ALK6)-virus to the culture. Moreover, transcripts of Twist and Id1, which have been implicated in cell migration in embryogenesis and activation of the periostin promoter, were induced by BMP-2 but repressed by noggin in cushion mesenchymal cell cultures. These data provide evidence that BMP-2 and BMP signaling induce biological processes involved in early AV valvulogenesis, i.e. mesenchymal cell migration and expression of periostin, indicating critical roles for BMP signaling in post-EMT AV cushion tissue maturation and differentiation.

Smooth muscle protein 22alpha-mediated patchy deletion of Bmpr1a impairs cardiac contractility but protects against pulmonary vascular remodeling

Vascular expression of bone morphogenetic type IA receptor (Bmpr1a) is reduced in lungs of patients with pulmonary arterial hypertension, but the significance of this observation is poorly understood. To elucidate the role of Bmpr1a in the vascular pathology of pulmonary arterial hypertension and associated right ventricular (RV) dysfunction, we deleted Bmpr1a in vascular smooth muscle cells and in cardiac myocytes in mice using the SM22alpha;TRE-Cre/LoxP;R26R system. The LacZ distribution reflected patchy deletion of Bmpr1a in the lung vessels, aorta, and heart of SM22alpha;TRE-Cre;R26R;Bmpr1a(flox/+) and flox/flox mutants. This reduction in BMPR-IA expression was confirmed by Western immunoblot and immunohistochemistry in the flox/flox group. This did not affect pulmonary vasoreactivity to acute hypoxia (10% O2) or the increase in RV systolic pressure and RV hypertrophy following 3 weeks in chronic hypoxia. However, both SM22alpha;TRE-Cre;R26R;Bmpr1a(flox/+) and flox/flox mutant mice had fewer muscularized distal pulmonary arteries and attenuated loss of peripheral pulmonary arteries compared with age-matched control littermates in hypoxia. When Bmpr1a expression was reduced by short interference RNA in cultured pulmonary arterial smooth muscle cells, serum-induced proliferation was attenuated explaining decreased hypoxia-mediated muscularization of distal vessels. When Bmpr1a was reduced in cultured microvascular pericytes by short interference RNA, resistance to apoptosis was observed and this could account for protection against hypoxia-mediated vessel loss. The similar elevation in RV systolic pressure and RV hypertrophy, despite the attenuated remodeling with chronic hypoxia in the flox/flox mutants versus controls, was not a function of elevated left ventricular end diastolic pressure but was associated with increased periadventitial deposition of elastin and collagen, potentially influencing vascular stiffness.

Abnormal conduction and morphology in the atrioventricular node of mice with atrioventricular canal targeted deletion of Alk3/Bmpr1a receptor

BACKGROUND

The atrioventricular (AV) node is essential for the sequential excitation and optimized contraction of the adult multichambered heart; however, relatively little is known about its formation from the embryonic AV canal. A recent study demonstrated that signaling by Alk3, the type 1a receptor for bone morphogenetic proteins, in the myocardium of the AV canal was required for the development of both the AV valves and annulus fibrosus. To test the hypothesis that bone morphogenetic protein signaling also plays a role in AV node formation, we investigated conduction system function and AV node morphology in adult mice with conditional deletion of Alk3 in the AV canal.

METHODS AND RESULTS

High-resolution optical mapping with correlative histological analysis of 28 mutant hearts revealed 4 basic phenotypic classes based on electrical activation patterns and volume-conducted ECGs. The frequency of AV node conduction and morphological abnormalities increased from no detectable anomalies (class I) to severe defects (class IV), which included the presence of bypass tracts, abnormal ventricular activation patterns, fibrosis of the AV node, and twin AV nodes.

CONCLUSIONS

The present findings demonstrate that bone morphogenetic protein signaling is required in the myocardium of the AV canal for proper AV junction development, including the AV node.

Distinct roles of Wnt/beta-catenin and Bmp signaling during early cardiogenesis

Heart formation requires the coordinated recruitment of multiple cardiac progenitor cell populations derived from both the first and second heart fields. In this study, we have ablated the Bmp receptor 1a and the Wnt effector beta-catenin in the developing heart of mice by using MesP1-cre, which acts in early mesoderm progenitors that contribute to both first and second heart fields. Remarkably, the entire cardiac crescent and later the primitive ventricle were absent in MesP1-cre; BmpR1a(lox/lox) mutants. Although myocardial progenitor markers such as Nkx2-5 and Isl1 and the differentiation marker MLC2a were detected in the small, remaining cardiac field in these mutants, the first heart field markers, eHand and Tbx-5, were not expressed. We conclude from these results that Bmp receptor signaling is crucial for the specification of the first heart field. In MesP1-cre; beta-catenin(lox/lox) mutants, cardiac crescent formation, as well as first heart field markers, were not affected, although cardiac looping and right ventricle formation were blocked. Expression of Isl1 and Bmp4 in second heart field progenitors was strongly reduced. In contrast, in a gain-of-function mutation of beta-catenin using MesP1-cre, we revealed an expansion of Isl1 and Bmp4 expressing cells, although the heart tube was not formed. We conclude from these results that Wnt/beta-catenin signaling regulates second heart-field development, and that a precise amount and/or timing of Wnt/beta-catenin signaling is required for proper heart tube formation and cardiac looping.

Essential role of Smad4 in maintaining cardiomyocyte proliferation during murine embryonic heart development

Transforming growth factor-beta/bone morphogenetic protein (TGF-beta/BMP) signaling pathway is essential for embryonic and postnatal heart development and remodeling. The intracellular factor Smad4 plays a pivotal role in mediating TGF-beta/BMP signal transduction in the nucleus. To examine the function of Smad4 in embryonic cardiac development during mid-gestation, we specifically deleted the Smad4 gene in embryonic cardiomyocytes using the Cre-LoxP system. Deletion of Smad4 as early as E9.5, led to embryonic lethality between E12.5 and E15.5, and embryos exhibited severe morphological defects in the heart, including a thin compact layer, disorganized trabeculae, and ventricular septum defects (VSD). Smad4 deletion also led to a dramatic decrease in cardiomyocyte proliferation accompanied by downregulation of contractile protein-encoding genes such as alpha-myosin heavy chain, beta-myosin heavy chain, ventricular myosin light chain 2, and alpha-cardiac actin. In addition, deletion of Smad4 resulted in perturbation of TGF-beta/BMP ligand expression and signaling, and defects in expression of several cardiac transcription factor genes such as Nkx2.5, GATA4, and MEF2c. These results provide direct genetic evidences that Smad4 is essential for regulating cardiomyocyte proliferation and differentiation during murine cardiogenesis, and provides new insights into potential causes of congenital heart disease.

Regulation of cardiomyocyte differentiation of embryonic stem cells by extracellular signalling

Investigating the signalling pathways that regulate heart development is essential if stem cells are to become an effective source of cardiomyocytes that can be used for studying cardiac physiology and pharmacology and eventually developing cell-based therapies for heart repair. Here, we briefly describe current understanding of heart development in vertebrates and review the signalling pathways thought to be involved in cardiomyogenesis in multiple species. We discuss how this might be applied to stem cells currently thought to have cardiomyogenic potential by considering the factors relevant for each differentiation step from the undifferentiated cell to nascent mesoderm, cardiac progenitors and finally a fully determined cardiomyocyte. We focus particularly on how this is being applied to human embryonic stem cells and provide recent examples from both our own work and that of others.

Bmp2 is essential for cardiac cushion epithelial-mesenchymal transition and myocardial patterning

Cardiac cushion development provides a valuable system to investigate epithelial to mesenchymal transition (EMT), a fundamental process in development and tumor progression. In the atrioventricular (AV) canal, endocardial cells lining the heart respond to a myocardial-derived signal, undergo EMT, and contribute to cushion mesenchyme. Here, we inactivated bone morphogenetic protein 2 (Bmp2) in the AV myocardium of mice. We show that Bmp2 has three functions in the AV canal: to enhance formation of the cardiac jelly, to induce endocardial EMT and to pattern the AV myocardium. Bmp2 is required for myocardial expression of Has2, a crucial component of the cardiac jelly matrix. During EMT, Bmp2 promotes expression of the basic helix-loop-helix factor Twist1, previously implicated in EMT in cancer metastases, and the homeobox genes Msx1 and Msx2. Deletion of the Bmp type 1A receptor, Bmpr1a, in endocardium also resulted in failed cushion formation, indicating that Bmp2 signals directly to cushion-forming endocardium to induce EMT. Lastly, we show that Bmp2 mutants failed to specify the AV myocardium with loss of Tbx2 expression uncovering a myocardial, planar signaling function for Bmp2. Our data indicate that Bmp2 has a crucial role in coordinating multiple aspects of AV canal morphogenesis.

Cardiac progenitors and the embryonic cell cycle

Despite the critical importance of proper cell cycle regulation in establishing the correct morphology of organs and tissues during development, relatively little is known about how cell proliferation is regulated in a tissue-specific manner. The control of cell proliferation within the developing heart is of considerable interest, given the high prevalence of congenital cardiac abnormalities among humans, and recent interest in the isolation of cardiac progenitor populations. We therefore review studies exploring the contribution of cell proliferation to overall cardiac morphology and the molecular mechanisms regulating this process. In addition, we also review recent studies that have identified progenitor cell populations within the adult myocardium, as well as those exploring the capability of differentiated myocardial cells to proliferate post-natally. Thus, the exploration of cardiomyocyte cell cycle regulation, both during development as well as in the adult heart, promises to yield many exciting and important discoveries over the coming years.

Myocardial smad4 is essential for cardiogenesis in mouse embryos

Congenital heart diseases are the most commonly observed human birth defects and are the leading cause of infant morbidity and mortality. Accumulating evidence indicates that transforming growth factor-beta/bone morphogenetic protein signaling pathways play critical roles during cardiogenesis. Smad4 encodes the only common Smad protein in mammals, which is a critical nuclear mediator of transforming growth factor-beta/bone morphogenetic protein signaling. The aim of this work was to investigate the roles of Smad4 during heart development. To overcome the early embryonic lethality of Smad4(-/-) mice, we specifically disrupted Smad4 in the myocardium using a Cre/loxP system. We show that myocardial-specific inactivation of Smad4 caused heart failure and embryonic lethality at midgestation. Histological analysis revealed that mutant mice displayed a hypocellular myocardial wall defect, which is likely the primary cause for heart failure. Both decreased cell proliferation and increased apoptosis contributed to the myocardial wall defect in mutant mice. Data presented in this article contradict a previous report showing that Smad4 is dispensable for heart development. Our further molecular characterization showed that expression of Nmyc and its downstream targets, including cyclin D1, cyclin D2, and Id2, were downregulated in mutant embryos. Reporter analysis indicated that the transcriptional activity of the 351-bp Nmyc promoter can be positively regulated by bone morphogenetic protein stimulation and negatively regulated by transforming growth factor-beta stimulation. Chromatin immunoprecipitation analysis revealed that the Nmyc promoter can form a complex with Smad4, suggesting that Nmyc is a direct downstream target of Smad4. In conclusion, this study provides the first mouse model showing that Smad4 plays essential roles during cardiogenesis.

Conditional deletion of focal adhesion kinase leads to defects in ventricular septation and outflow tract alignment

To examine a role for focal adhesion kinase (FAK) in cardiac morphogenesis, we generated a line of mice with a conditional deletion of FAK in nkx2-5-expressing cells (herein termed FAKnk mice). FAKnk mice died shortly after birth, likely resulting from a profound subaortic ventricular septal defect and associated malalignment of the outflow tract. Additional less penetrant phenotypes included persistent truncus arteriosus and thickened valve leaflets. Thus, conditional inactivation of FAK in nkx2-5-expressing cells leads to the most common congenital heart defect that is also a subset of abnormalities associated with tetralogy of Fallot and the DiGeorge syndrome. No significant differences in proliferation or apoptosis between control and FAKnk hearts were observed. However, decreased myocardialization was observed for the conal ridges of the proximal outflow tract in FAKnk hearts. Interestingly, chemotaxis was significantly attenuated in isolated FAK-null cardiomyocytes in comparison to genetic controls, and these effects were concomitant with reduced tyrosine phosphorylation of Crk-associated substrate (CAS). Thus, it is possible that ventricular septation and appropriate outflow tract alignment is dependent, at least in part, upon FAK-dependent CAS activation and subsequent induction of polarized myocyte movement into the conal ridges. Future studies will be necessary to determine the precise contributions of the additional nkx2-5-derived lineages to the phenotypes observed.

BMP and FGF regulatory pathways in semilunar valve precursor cells

In the developing atrioventricular (AV) valve, limb bud, and somites, cartilage cell lineage differentiation is regulated by bone morphogenetic protein (BMP), while fibroblast growth factor (FGF) controls tendon cell fate. We observed aggrecan and sox9, characteristic of cartilage cell types, and scleraxis and tenascin, characteristic of tendon cell types, in developing avian semilunar valves. Addition of BMP4 to outflow tract (OFT) precursor cells of young (E4.5) but not older (E6) chick embryos activated Smad1/5/8 and induced sox9 and aggrecan expression, while FGF4 treatment increased phosphorylated MAPK (dpERK) signaling and promoted expression of scleraxis and tenascin. These results identify BMP and FGF pathways that promote expression of cartilage- or tendon-like characteristics in semilunar valve precursor cells. In contrast to AV valve precursor cells, which diversify into leaflets (cartilage-like) or chordae tendineae (tendon-like), semilunar valve cells exhibit both cartilage- and tendon-like characteristics in the developing and mature valve cusp.

Inactivation of Smad5 in endothelial cells and smooth muscle cells demonstrates that Smad5 is required for cardiac homeostasis

Smads are intracellular signaling proteins that transduce signals elicited by members of the transforming growth factor (TGF)-beta superfamily. Smad5 and Smad1 are highly homologous, and they mediate primarily bone morphogenetic protein (Bmp) signals. We used the Cre-loxP system and Sm22-Cre and Tie-1-Cre mice to study the function of Smad5 in the developing blood vessel wall. Analysis of embryos demonstrated that deletion of Smad5 in endothelial or smooth muscle cells resulted in a normal organization of embryonic and extra-embryonic vasculature. Angiogenic assays performed in adult mice revealed that mutant mice display a comparable angiogenic and vascular remodeling response to control mice. In Sm22-Cre; Smad5(fl/-) mice, Smad5 is also deleted in cardiomyocytes. Echocardiographic analysis on those 9-month-old female mice demonstrated larger left ventricle internal diameters and decreased fractional shortening compared with control littermates without signs of cardiac hypertrophy. The decreased cardiac contractility was associated with a decreased performance in a treadmill experiment. In isolated cardiomyocytes, fractional shortening was significantly reduced compared with control cells. These data demonstrate that restricted deletion of Smad5 in the blood vessel wall results in viable mice. However, loss of Smad5 in cardiomyocytes leads to a mild heart defect.

Survival pathways in hypertrophy and heart failure: the gp130-STAT3 axis

Circulating levels of interleukin (IL)-6 and related cytokines are elevated in patients with congestive heart failure and after myocardial infarction. Serum IL-6 concentrations are related to decreasing functional status of these patients and provide important prognostic information.Moreover, in the failing human heart, multiple components of the IL-6- glycoprotein (gp)130 receptor system are impaired, implicating an important role of this system in cardiac pathophysiology.Experimental studies have shown that the common receptor subunit of IL-6 cytokines is phosphorylated in response to pressure overload and myocardial infarction and that it subsequently activates at least three different downstream signaling pathways, the signal transducers and activators of transcription 1 and 3 (STAT1/3), the Src-homology tyrosine phosphatase 2 (SHP2)-Ras-ERK, and the PI3K-Akt system. Gp130 receptor mediated signaling promotes cardiomyocyte survival, induces hypertrophy, modulates cardiac extracellular matrix and cardiac function. In this regard, the gp130 receptor system and its main downstream mediator STAT3 play a key role in cardioprotection. This review summarizes the current knowledge of IL-6 cytokines, gp130 receptor and STAT3 signaling in the heart exposed to physiological (aging, pregnancy) and pathophysiological stress (ischemia, pressure overload, inflammation and cardiotoxic agents) with a special focus on the potential role of individual IL-6 cytokines.

Isolated left ventricular non-compaction: the case for abnormal myocardial development

Isolated ventricular non-compaction is an increasingly commonly diagnosed myocardial disorder characterised by excessive and prominent trabeculation of the morphologically left, and occasionally the right, ventricle. This is associated with high rates of thromboembolism, cardiac failure, and cardiac arrhythmia. Recent improvements in understanding the embryonic processes underlying ventricular formation have led to the hypothesis that ventricular non-compaction is due to a failure of normal ventriculogenesis, leading to abnormal myocardium which may present clinically many years later. Experimental work in animal models provides several candidate transcription factors and signalling molecules that could, in theory, cause ventricular non-compaction if disrupted.

Functional BMP receptor in endocardial cells is required in atrioventricular cushion mesenchymal cell formation in chick

Transformation of atrioventricular (AV) canal endocardium into invasive mesenchyme correlates spatially and temporally with the expression of bone morphogenetic protein (BMP)-2 in the AV myocardium. We revealed the presence of mRNA of Type I BMP receptors, BMPR-1A (ALK3), BMPR-1B (ALK6) and ALK2 in chick AV endocardium at stage-14(-), the onset of epithelial to mesenchymal transformation (EMT), by RT-PCR and localized BMPR-1B mRNA in the endocardium by in situ hybridization. To circumvent the functional redundancies among the Type I BMP receptors, we applied dominant-negative (dn) BMPR-1B-viruses to chick AV explants and whole-chick embryo cultures to specifically block BMP signaling in AV endocardium during EMT. dnBMPR-1B-virus infection of AV endocardial cells abolished BMP-2-supported AV endocardial EMT. Conversely, caBMPR-1B-virus infection promoted AV endocardial EMT in the absence of AV myocardium. Moreover, dnBMPR-1B-virus treatments significantly reduced myocardially supported EMT in AV endocardial-myocardial co-culture. AV cushion mesenchymal cell markers, alpha-smooth muscle actin (SMA), and TGFbeta3 in the endocardial cells were promoted by caBMPR-1B and reduced by dnBMPR-1B infection. Microinjection of the virus into the cardiac jelly in the AV canal at stage-13 in vivo (ovo) revealed that the dnBMPR-1B-virus-infected cells remained in the endocardial epithelium, whereas caBMPR-1B-infected cells invaded deep into the cushions. These results provide evidence that BMP signaling through the AV endocardium is required for the EMT and the activation of the BMP receptor in the endocardium can promote AV EMT in the chick.