Nf1 has an essential role in endothelial cells

Controlled delivery of PDGF-BB for myocardial protection using injectable self-assembling peptide nanofibers

Endothelial cells can protect cardiomyocytes from injury, but the mechanism of this protection is incompletely described. Here we demonstrate that protection of cardiomyocytes by endothelial cells occurs through PDGF-BB signaling. PDGF-BB induced cardiomyocyte Akt phosphorylation in a time- and dose-dependent manner and prevented apoptosis via PI3K/Akt signaling. Using injectable self-assembling peptide nanofibers, which bound PDGF-BB in vitro, sustained delivery of PDGF-BB to the myocardium at the injected sites for 14 days was achieved. A blinded and randomized study in 96 rats showed that injecting nanofibers with PDGF-BB, but not nanofibers or PDGF-BB alone, decreased cardiomyocyte death and preserved systolic function after myocardial infarction. A separate blinded and randomized study in 52 rats showed that PDGF-BB delivered with nanofibers decreased infarct size after ischemia/reperfusion. PDGF-BB with nanofibers induced PDGFR-beta and Akt phosphorylation in cardiomyocytes in vivo. These data demonstrate that endothelial cells protect cardiomyocytes via PDGF-BB signaling and that this in vitro finding can be translated into an effective in vivo method of protecting myocardium after infarction. Furthermore, this study shows that injectable nanofibers allow precise and sustained delivery of proteins to the myocardium with potential therapeutic benefits.

Atrioventricular cushion transformation is mediated by ALK2 in the developing mouse heart

Developmental abnormalities in endocardial cushions frequently contribute to congenital heart malformations including septal and valvular defects. While compelling evidence has been presented to demonstrate that members of the TGF-beta superfamily are capable of inducing endothelial-to-mesenchymal transdifferentiation in the atrioventricular canal, and thus play a key role in formation of endocardial cushions, the detailed signaling mechanisms of this important developmental process, especially in vivo, are still poorly known. Several type I receptors (ALKs) for members of the TGF-beta superfamily are expressed in the myocardium and endocardium of the developing heart, including the atrioventricular canal. However, analysis of their functional role during mammalian development has been significantly complicated by the fact that deletion of the type I receptors in mouse embryos often leads to early embryonal lethality. Here, we used the Cre/loxP system for endothelial-specific deletion of the type I receptor Alk2 in mouse embryos. The endothelial-specific Alk2 mutant mice display defects in atrioventricular septa and valves, which result from a failure of endocardial cells to appropriately transdifferentiate into the mesenchyme in the AV canal. Endocardial cells deficient in Alk2 demonstrate decreased expression of Msx1 and Snail, and reduced phosphorylation of BMP and TGF-beta Smads. Moreover, we show that endocardial cells lacking Alk2 fail to delaminate from AV canal explants. Collectively, these results indicate that the BMP type I receptor ALK2 in endothelial cells plays a critical non-redundant role in early phases of endocardial cushion formation during cardiac morphogenesis.

Noonan syndrome mutation Q79R in Shp2 increases proliferation of valve primordia mesenchymal cells via extracellular signal-regulated kinase 1/2 signaling

The molecular pathways regulating valve development are only partially understood. Recent studies indicate that dysregulation of mitogen-activated protein kinase (MAPK) signaling might play a major role in the pathogenesis of congenital valvular malformations, and, in this study, we explored the role of extracellular signal-regulated kinase (ERK) 1/2 activation in valve primordia expressing the Noonan syndrome mutation Q79R-Shp2. Noonan syndrome is an autosomal dominant disease characterized by dysmorphic features and cardiac abnormalities, with frequent pulmonic stenosis. The Q79R mutation of PTPN11 previously identified in Noonan syndrome families results in a gain-of-function of the encoded protein tyrosine phosphatase Shp2. We compared the effects of wild-type Shp2 and Q79R-Shp2 on endocardial cushion development. Atrioventricular and outflow tract endocardial cushions were excised from chick embryos, infected with wild-type Shp2 or Q79R-Shp2 adenovirus and embedded in a gel matrix. Q79R-Shp2, but not wild-type-Shp2, expression resulted in increased outgrowth of cells into the gel. The dependence of the Q79R-Shp2 effect on ERK1/2 and p38 MAPK signaling was then determined. The MAPK/ERK kinase (MEK)-1 inhibitor U0126, but not the p38-MAPK pathway inhibitor SB203580, abolished the effect of Q79R-Shp2 on cushion outgrowth. Coinfection with Q79R-Shp2 and dominant negative MEK-1 prevented enhanced endocardial cushion outgrowth, whereas expression of constitutively active MEK-1 mimicked the effect of Q79R-Shp2. Furthermore, dissociated cushion cells displayed increased 5-bromodeoxyuridine incorporation when infected with Q79R-Shp2 but not with wild-type Shp2. This promitotic effect was eliminated by U0126. Our results demonstrate that ERK1/2 activation is both necessary and sufficient to mediate the hyperproliferative effect of a gain-of-function mutation of Shp2 on mesenchymal cells in valve primordia.

Cardiac neural crest

Neural crest cells (NCCs) contribute to many organs and tissues during embryonic development. Amongst these, the cardiovascular system represents a fascinating example. In this review, recent advances in our understanding of the developmental biology and molecular genetics regulating cardiac NCC maturation will be summarized. While the existence of a significant neural crest (NC) contribution to the developing heart has been appreciated for more than 20 years, only in the last few years have molecular pathways regulating this process been elucidated and the significant contribution of these mechanisms to the etiology of congenital heart disease in man become apparent. Emerging data suggest that ongoing studies will reveal complex inductive interactions between cardiac NC and a series of other cell types contributing to the developing cardiovascular system.

Targeting neovascular pericytes in neurofibromatosis type 1

Apart from tumor-driven neovascularization, a less-appreciated consequence of neurofibromatosis type 1 (NF1) is the hyperproliferation of vascular mural cells (pericytes). This study aims at establishing a role for pericytes in NF1, and determining whether interference with the function of a key pericyte component (NG2 proteoglycan) inhibits NF1 tumor neovascularization. Neovascularization in NF1 was studied in Nf+/+(control), Nf1+/-, and Nf1-/-embryos at E-10, ischemia-induced retinal angiogenesis model in 24 eyes of Nf1+/-, Nf1+/+mice, and in malignant peripheral nerve sheath tumors (MPNSTs) derived from NF1 patients (ST88-14, NMS-2PC) orthotopically grown in nude mice (Crl: nu/nu). The anti-angiogenic effect of intracorneal polymer pellets containing anti-NG2 neutralizing antibody was quantified in the nude-mouse corneal angiogenesis model in which angiogenesis was induced by xenografting NMS-2PC tumor into the corneal stroma of 22 eyes. By using confocal microscopy, immunohistochemistry, and BrdU proliferation assay, the pericyte/endothelium ratios and proliferation rates were measured. Activated pericytes were present at the leading tip of the angiogenic sprouts. Pericytes showed continuous investment of endothelium in both NMS-2PC and ST88-14 MPNST tumor xenografts. Mean corneal angiogenesis induced by NMS-2PC tumor grafts in NG2-antibody treated eyes was 1.491 and 3.186 mm2 in isotype-matched non-immunoglobulin treated eyes (control) (P=0.0002). A total of 193.8 vascular nuclei (a measure of ischemia-induced retinal angiogenesis) was present in angiogenic retinal tufts in Nf1+/- mice compared to 89.23 in Nf1+/+ mice (control) (P<0.0001). Mean pericyte/endothelium investment ratios were 1.015, 1.380, and 2.084 in control, Nf1+/-, and Nf1-/-embryos, respectively. Pericytes were 23% (control), 49% (Nf1+/-), and 69% (Nf1-/-) BrdU-positive. Endothelial cells from the same embryos were 29% (control), 47% (Nf1+/-), and 62% (Nf1-/-) BrdU-positive. Angiogenesis is accelerated in NF1 due to hyperproliferation of pericytes and endothelial cells. Mitotically activated NG2-positive pericytes, and endothelial cells may serve as potential therapeutic targets in NF1.

NF1 tumor suppressor in epidermal wound healing with special focus on wound healing in patients with type 1 neurofibromatosis

Type 1 neurofibromatosis syndrome (NF1) has been linked with mutations of the NF1 gene which encodes tumor suppressor neurofibromin, a regulator of Ras-MAPK signaling. In human epidermis, keratinocytes express NF1 tumor suppressor and it may have a distinctive function in these cells during wound healing, such as regulating Ras activity. NF1 expression was first studied during the epidermal wound healing using suction blister method. NF1 gene expression increased both in hypertrophic and migrating zones of the healing epidermis, and also in dermal fibroblasts underneath the injury. This prompted us to study epidermal wound healing in NF1 patients. Wound healing efficiency was evaluated 4 days after blister induction by clinical, physiological and histological methods. Epidermal wound healing was equally effective in NF1 patients and healthy controls. In addition, dermal wound healing appears to function normally in NF1 patients based on retrospective and follow-up study of biopsy scars. Furthermore, the healing wounds were analyzed immunohistochemically for cell proliferation rate and Ras-MAPK activity. Neither epidermal keratinocytes nor dermal fibroblasts showed difference in the cell proliferation rate or Ras-MAPK activity between NF1 patients and controls. Interestingly, NF1 patients displayed increased cell proliferation rate and Ras-MAPK activity in periarteriolar tissue underneath the wound. The results of the study suggest that epidermal wound healing is not markedly altered in NF1 patients. Furthermore, NF1 protein seems not to have an important function as a Ras-MAPK regulator in epidermal keratinocytes or dermal fibroblasts but instead appears to be regulator of Ras-MAPK signaling in vascular tissues.

Heart valve development: endothelial cell signaling and differentiation

During the past decade, single gene disruption in mice and large-scale mutagenesis screens in zebrafish have elucidated many fundamental genetic pathways that govern early heart patterning and differentiation. Specifically, a number of genes have been revealed serendipitously to play important and selective roles in cardiac valve development. These initially surprising results have now converged on a finite number of signaling pathways that regulate endothelial proliferation and differentiation in developing and postnatal heart valves. This review highlights the roles of the most well-established ligands and signaling pathways, including VEGF, NFATc1, Notch, Wnt/beta-catenin, BMP/TGF-beta, ErbB, and NF1/Ras. Based on the interactions among and relative timing of these pathways, a signaling network model for heart valve development is proposed.

Severe Aortic Regurgitation in Von Recklinghausen’s Disease

A 71-year-old man with von Recklinghausen’s disease presented with NYHA class II exertional dyspnea and easy fatigability since about 20 days. Echocardiography and angiography demonstrated dilatation of aortic root, ascending aorta, and left ventricle, together with severe aortic regurgitation. Because of very few cases of cardiovascular abnormalities having been reported with von Recklinghausen’s disease, it is logical to concur that this is simply co-existence, whereas other possible mechanisms are endothelial dysfunction or sympathetic denervation.

Role for the epidermal growth factor receptor in neurofibromatosis-related peripheral nerve tumorigenesis

Benign neurofibromas and malignant peripheral nerve sheath tumors are serious complications of neurofibromatosis type 1. The epidermal growth factor receptor is not expressed by normal Schwann cells, yet is overexpressed in subpopulations of Nf1 mutant Schwann cells. We evaluated the role of EGFR in Schwann cell tumorigenesis. Expression of EGFR in transgenic mouse Schwann cells elicited features of neurofibromas: Schwann cell hyperplasia, excess collagen, mast cell accumulation, and progressive dissociation of non-myelin-forming Schwann cells from axons. Mating EGFR transgenic mice to Nf1 hemizygotes did not enhance this phenotype. Genetic reduction of EGFR in Nf1(+/-);p53(+/-) mice that develop sarcomas significantly improved survival. Thus, gain- and loss-of-function experiments support the relevance of EGFR to peripheral nerve tumor formation.

Cell Biology of Cardiac Cushion Development

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Frequency of incidental intracranial aneurysms in neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is often mentioned among the heritable connective tissue disorders associated with intracranial aneurysms, but the association has not been firmly established. We therefore reviewed a contemporary series of hospitalized patients with NF1, many of whom underwent brain magnetic resonance imaging (MRI). We identified patients with NF1 who were hospitalized at Cedars-Sinai Medical Center, Los Angeles, California, between January 1, 1997 and December 31, 2001 through the hospital's centralized medical records system using DRG codes. The mean age of the 39 patients was 30.4 years, and 22 patients had undergone MRI of the brain for the evaluation of symptoms due to the presence of central or peripheral nervous system tumors. Incidental intracranial aneurysms were detected in 2 (5%) of the 39 patients. Limiting the patient population to the 22 patients who had undergone MRI examination, the detection rate was 9%. This was significantly (P < 0.005) higher than the aneurysm detection rate in a control population (0/526 [0%]) of patients hospitalized for primary or secondary brain tumors, all of whom had undergone MRI examination. Our study suggests that patients with NF1 are at an increased risk of developing intracranial aneurysms.

A vascular gene trap screen defines RasGRP3 as an angiogenesis-regulated gene required for the endothelial response to phorbol esters

We identified Ras guanine-releasing protein 3 (RasGRP3) as a guanine exchange factor expressed in blood vessels via an embryonic stem (ES) cell-based gene trap screen to identify novel vascular genes. RasGRP3 is expressed in embryonic blood vessels, down-regulated in mature adult vessels, and reexpressed in newly formed vessels during pregnancy and tumorigenesis. This expression pattern is consistent with an angiogenic function for RasGRP3. Although a loss-of-function mutation in RasGRP3 did not affect viability, RasGRP3 was up-regulated in response to vascular endothelial growth factor (VEGF) stimulation of human umbilical vein endothelial cells, placing RasGRP3 regulation downstream of VEGF signaling. Phorbol esters mimic the second messenger diacylglycerol (DAG) in activating both protein kinase C (PKC) and non-PKC phorbol ester receptors such as RasGRP3. ES cell-derived wild-type blood vessels exposed to phorbol myristate acetate (PMA) underwent extensive aberrant morphogenesis that resulted in the formation of large endothelial sheets rather than properly branched vessels. This response to PMA was completely dependent on the presence of RasGRP3, as mutant vessels were refractory to the treatment. Taken together, these findings show that endothelial RasGRP3 is up-regulated in response to VEGF stimulation and that RasGRP3 functions as an endothelial cell phorbol ester receptor in a pathway whose stimulation perturbs normal angiogenesis. This suggests that RasGRP3 activity may exacerbate vascular complications in diseases characterized by excess DAG, such as diabetes.

A field of myocardial-endocardial NFAT signaling underlies heart valve morphogenesis

The delicate leaflets that make up vertebrate heart valves are essential for our moment-to-moment existence. Abnormalities of valve formation are the most common serious human congenital defect. Despite their importance, relatively little is known about valve development. We show that the initiation of heart valve morphogenesis in mice requires calcineurin/NFAT to repress VEGF expression in the myocardium underlying the site of prospective valve formation. This repression of VEGF at E9 is essential for endocardial cells to transform into mesenchymal cells. Later, at E11, a second wave of calcineurin/NFAT signaling is required in the endocardium, adjacent to the earlier myocardial site of NFAT action, to direct valvular elongation and refinement. Thus, NFAT signaling functions sequentially from myocardium to endocardium within a valvular morphogenetic field to initiate and perpetuate embryonic valve formation. This mechanism also operates in zebrafish, indicating a conserved role for calcineurin/NFAT signaling in vertebrate heart valve morphogenesis.

Mouse model of Noonan syndrome reveals cell type- and gene dosage-dependent effects of Ptpn11 mutation

Noonan syndrome is a common human autosomal dominant birth defect, characterized by short stature, facial abnormalities, heart defects and possibly increased risk of leukemia. Mutations of Ptpn11 (also known as Shp2), which encodes the protein-tyrosine phosphatase Shp2, occur in approximately 50% of individuals with Noonan syndrome, but their molecular, cellular and developmental effects, and the relationship between Noonan syndrome and leukemia, are unclear. We generated mice expressing the Noonan syndrome-associated mutant D61G. When homozygous, the D61G mutant is embryonic lethal, whereas heterozygotes have decreased viability. Surviving Ptpn11(D61G/+) embryos ( approximately 50%) have short stature, craniofacial abnormalities similar to those in Noonan syndrome, and myeloproliferative disease. Severely affected Ptpn11(D61G/+) embryos ( approximately 50%) have multiple cardiac defects similar to those in mice lacking the Ras-GAP protein neurofibromin. Their endocardial cushions have increased Erk activation, but Erk hyperactivation is cell and pathway specific. Our results clarify the relationship between Noonan syndrome and leukemia and show that a single Ptpn11 gain-of-function mutation evokes all major features of Noonan syndrome by acting on multiple developmental lineages in a gene dosage-dependent and pathway-selective manner.

Hereditary pancreatic endocrine tumours

The two main types of hereditary pancreatic neuroendocrine tumours are found in multiple endocrine neoplasia type 1 (MEN-1) and von Hippel-Lindau disease (VHL), but also in the rarer disorders of neurofibromatosis type 1 and tuberous sclerosis. This review considers the major advances that have been made in genetic diagnosis, tumour localization, medical and surgical treatment and palliation with systemic chemotherapy and radionuclides. With the exception of the insulinoma syndrome, all of the various hormone excess syndromes of MEN-1 can be treated medically. The role of surgery however remains controversial ranging from no intervention (except enucleation for insulinoma), intervening for tumours diagnosed only by biochemical criteria, intervening in those tumours only detected radiologically (1-2 cm in diameter) or intervening only if the tumour diameter is > 3 cm in diameter. The extent of surgery is also controversial, although radical lymphadenectomy is generally recommended. Pancreatic tumours associated with VHL are usually non-functioning and tumours of at least 2 cm in diameter should be resected. Practice guidelines recommend that screening in patients with MEN-1 should commence at the age of 5 years for insulinoma and at the age of 20 years for other pancreatic neuroendocrine tumours and variously at 10-20 years of age for pancreatic tumours in patients with VHL. The evidence is increasing that the life span of patients may be significantly improved with surgical intervention, mandating the widespread use of tumour surveillance and multidisciplinary team management.

NF1 tumor suppressor protein and mRNA in skeletal tissues of developing and adult normal mouse and NF1-deficient embryos

NF1 is a heritable disease with multiple osseous lesions. The expression of the NF1 gene was studied in embryonic and adult rodent skeleton and in NF1-deficient embryos. The NF1 gene was expressed intensely in the cartilage and the periosteum. Impaired NF1 expression may lead to inappropriate development and dynamics of bones and ultimately to the osseous manifestations of the disease.

INTRODUCTION

Neurofibromatosis type 1 is caused by mutations in the NF1 gene encoding the Ras GTPase activating protein (Ras-GAP) neurofibromin. Skeletal ailments such as short stature, kyphoscoliosis, and tibial bowing and pseudarthrosis are common osseous manifestations of NF1. These symptoms are congenital, implying a role for neurofibromin in proper bone growth. However, little is known about its expression in skeletal tissues during their development.

MATERIALS AND METHODS

The expression of the NF1 gene was studied in normal and NF1+/- mouse fetuses at embryonic days 12.5-15.5 and in skeletal tissues of adult mice and rats. In situ hybridization, immunohistochemistry, and Western blot analysis were used to identify the NF1 gene expression profile.

RESULTS

NF1 mRNA and protein were elevated in resting, maturation, and hypertrophic chondrocytes at the growth plate. Parallel studies on NF1+/- embryos showed expression patterns identical to wildtype. The periosteum, including osteoblasts and osteoclasts, and osteocytes of the cortical bone of adult mice were also intensely labeled for NF1 protein and mRNA. Western transfer analysis detected NF1 protein in the respective rat tissues. Phosphorylation of p42 and p44 MAP kinases, the downstream consequence of Ras activation, was elevated in hypertrophic chondrocytes of NF1+/- embryos.

CONCLUSIONS

The results suggest that neurofibromin may act as a Ras-GAP in skeletal cells to attenuate Ras transduced growth signals and thus play a role during ossification and dynamics of bone. Loss of NF1 function may therefore lead to dysplastic bone growth, thereby causing the debilitating osseous symptoms of NF1.

Essential role of Sox9 in the pathway that controls formation of cardiac valves and septa

Epithelial-mesenchymal transformation is a critical developmental process reiterated in multiple organs throughout embryogenesis. Formation of endocardial cushions, primordia of valves and septa, is a classic example of epithelial-mesenchymal transformation. Several gene mutations are known to affect cardiac valve formation. Sox9 is activated when endocardial endothelial cells undergo mesenchymal transformation and migrate into an extracellular matrix, called cardiac jelly, to form endocardial cushions. In Sox9-null mutants, endocardial cushions are markedly hypoplastic. In these mutants, Nfatc1 is ectopically expressed and no longer restricted to endothelial cells. Further, Sox9-deficient endocardial mesenchymal cells fail to express ErbB3, which is required for endocardial cushion cell differentiation and proliferation. Our results reveal a succession of molecular steps in the pathway of endocardial cushion development. We propose that loss of Sox9 inhibits epithelial-mesenchymal transformation after delamination and initial migration, but before definitive mesenchymal transformation.

DSCR1 gene expression is dependent on NFATc1 during cardiac valve formation and colocalizes with anomalous organ development in trisomy 16 mice

The Down syndrome critical region 1 (DSCR1) gene is present in the region of human chromosome 21 and the syntenic region of mouse chromosome 16, trisomy of which is associated with congenital heart defects observed in Down syndrome. DSCR1 encodes a regulatory protein in the calcineurin/NFAT signal transduction pathway. During valvuloseptal development in the heart, DSCR1 is expressed in the endocardium of the developing atrioventricular and semilunar valves, the muscular interventricular septum, and the ventricular myocardium. Human DSCR1 contains an NFAT-rich calcineurin-responsive element adjacent to exon 4. Transgenic mice generated with a homologous regulatory region of the mouse DSCR1 gene linked to lacZ (DSCR1(e4)/lacZ) show gene activation in the endocardium of the developing valves and aorticopulmonary septum of the heart, recapitulating a specific subdomain of endogenous DSCR1 cardiac expression. DSCR1(e4)/lacZ expression in the developing valve endocardium colocalizes with NFATc1 and, endocardial DSCR1(e4)/lacZ, is notably reduced or absent in NFATc1(-/-) embryos. Furthermore, expression of the endogenous DSCR1(e4) isoform is decreased in the outflow tract of NFATc1(-/-) hearts, and the DSCR1(e4) intragenic element is trans-activated by NFATc1 in cell culture. In trisomy 16 (Ts16) mice, expression of endogenous DSCR1 and DSCR1(e4)/lacZ colocalizes with anomalous valvuloseptal development, and transgenic Ts16 hearts have increased beta-galactosidase activity. DSCR1 and DSCR1(e4)/lacZ also are expressed in other organ systems affected by trisomy 16 in mice or trisomy 21 in humans including the brain, eye, ear, face, and limbs. Together, these results show that DSCR1(e4) expression in the developing valve endocardium is dependent on NFATc1 and support a role for DSCR1 in normal cardiac valvuloseptal formation as well as the abnormal development of several organ systems affected in individuals with Down syndrome.

Tie2-Cre-induced inactivation of a conditional mutant Nf1 allele in mouse results in a myeloproliferative disorder that models juvenile myelomonocytic leukemia

Neurofibromatosis type one (NF1) is a common genetic disorder affecting 1:4000 births and is characterized by benign and malignant tumors. Children with NF1 are predisposed to juvenile myelomonocytic leukemia. The Nf1 gene encodes neurofibromin, which can function as a Ras GTPase-activating protein. Neurofibromin deficiency in mice leads to mid-gestation lethality due to cardiovascular defects. We have previously shown that conditional inactivation of Nf1 using Tie2-Cre recapitulates the heart defects seen in Nf1(-/-) embryos. Tie2-Cre transgenic mice express Cre recombinase in all endothelial cells. Here, we show that Tie2-Cre-mediated deletion of Nf1 also leads to excision of Nf1 in the hematopoietic lineage. Surviving mice exhibit a myeloproliferative disorder similar to juvenile myelomonocytic leukemia seen in NF1 patients. These mice provide a useful model to study neurofibromin deficiency in hematopoiesis. Furthermore, defects in Tie2-Cre-expressing progenitors that result in heart and blood defects suggest that related heart and blood disorders in NF1 and other syndromes represent disorders of the hemangioblast.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.

Development Gone Awry

Significant advances in the understanding of the molecular and genetic basis of congenital heart disease have emerged from gene inactivation studies in mice and from human genetic investigations. However, the ability to utilize information gleaned from animal models to inform clinical care of patients depends on an accurate anatomic analysis and presentation in terms that are meaningful to the clinical pediatric cardiologist. Likewise, the enormous depth and breadth of accumulated clinical experience can inform the developmental biologist and can highlight the importance and interrelationships of particular phenotypes. The explosion of potentially informative genetic tools demands that basic scientists and clinicians concerned with congenital cardiac disease enhance the ongoing bidirectional dialogue. In some cases, categories of congenital disease familiar to clinicians are not recognized by developmental biologists, and mechanisms accepted by the biologist seem inconsistent with clinical experience. In this review, we summarize some of the more clinically significant forms of congenital heart disease, and we highlight relevant genetic and developmental pathways.

Activated Ras induces a proangiogenic phenotype in primary endothelial cells

Angiogenic factors alter endothelial cell phenotype to promote the formation of new blood vessels, a process critical for a number of normal and pathological conditions. Ras is required for the induction of the angiogenic phenotype in response to vascular endothelial growth factor (VEGF). However, VEGF generates many signals, several of which are not dependent upon Ras activation. Our current study investigates the sufficiency of Ras activation for driving angiogenic responses. An activated Ras(V12) mutant induces prominent membrane ruffling, branching morphogenesis on three-dimensional collagen, DNA synthesis, and cell migration in primary endothelial cells. An upregulation of PI3K/AKT, Erk, and Jnk signaling pathways accompany these phenotypic changes. The inhibition of Erk blocked cell proliferation, but only partially attenuated migration. Blocking PI3K had no effect on DNA synthesis, but caused a modest reduction in cell migration. Lastly, Jnk played a significant role in both the proliferation and migration response. These effects of Ras(V12) are not the result of increased autocrine secretion of VEGF. These data suggest that the acquisition of activating Ras mutations can lead to a proangiogenic conversion in the phenotype of primary endothelial cells. Furthermore, these data raise the possibility that chronic Ras activation in endothelial cells may be sufficient to promote angiogenesis and the development of vascular anomalies.

MAP kinase activation in avian cardiovascular development

Signaling pathways mediated by receptor tyrosine kinases (RTK) and mitogen-activated protein kinase (MAPK) activation have multiple functions in the developing cardiovascular system. The localization of diphosphorylated extracellular signal regulated kinase (dp-ERK) was monitored as an indicator of MAPK activation in the forming heart and vasculature of avian embryos. Sustained dp-ERK expression was observed in vascular endothelial cells of embryonic and extraembryonic origins. Although dp-ERK was not detected during early cardiac lineage induction, MAPK activation was observed in the epicardial, endocardial, and myocardial compartments during heart chamber formation. Endocardial expression of dp-ERK in the valve primordia and heart chambers may reflect differential cell growth associated with RTK signaling in the heart. dp-ERK localization in the epicardium, subepicardial fibroblasts, myocardial fibroblasts, and coronary vessels is consistent with MAPK activation in epicardial-derived cell lineages. The complex temporal-spatial regulation of dp-ERK in the heart supports diverse regulatory functions for RTK signaling in different cell populations, including the endocardium, myocardium, and epicardial-derived cells during cardiac organogenesis.

Molecular markers of cardiac endocardial cushion development

Endocardial cushions are precursors of mature heart valves. They form within the looped heart tube as discrete swellings and develop into thin, pliable leaflets that prevent regurgitation of blood. The embryonic origins of cardiac valves include endothelial, myocardial, and neural crest cells. Recently, an increasing number of animal models derived from mutational screens, gene inactivation, and transgenic studies have identified specific molecules required for normal development of the cardiac valves, and critical molecular pathways are beginning to emerge. To further this process, we have sought to assemble a diverse set of molecular markers encompassing all stages of cardiac valve development. Here, we provide a detailed comparative gene expression analysis of thirteen endocardial cushion markers. We identify endocardial cushion expression of the transcription factor Fog1, and we demonstrate active Wnt/beta-catenin signaling in developing endocardial cushions suggesting pathways that have not been previously appreciated to participate in cardiac valve formation.

Endothelial lineage-mediated loss of the GATA cofactor Friend of GATA 1 impairs cardiac development

GATA transcription factors, together with Friend of GATA (FOG) cofactors, are required for the differentiation of diverse cell types. Multiple aspects of hematopoiesis are controlled by the interaction of FOG-1 with the GATA-1/2/3 subfamily. Likewise, FOG-2 is coexpressed with the GATA-4/5/6 subfamily at other sites, including the heart and gonads. FOG-2 and GATA-4 are required for cardiac development. Through transgenic rescue of hematopoietic defects of FOG-1-/- embryos we define an unsuspected role for FOG-1 in heart development. In particular, rescued FOG-1-/- mice die at embryonic day (E) 14.5 with cardiac defects that include double outlet right ventricle and a common atrioventricular valve. Using conditional inactivation of Fog-1 we assign the cell of origin in which FOG-1 function is required. Neural crest cells migrate properly into FOG-1-/- hearts and mice with FOG-1 conditionally excised from neural crest derivatives fail to develop cardiac abnormalities. In contrast, conditional inactivation of FOG-1 in endothelial-derived tissues by means of Tie-2-expressed Cre recapitulates the rescue-knockout defects. These findings establish a nonredundant requirement for FOG-1 in the outlet tract and atrioventricular valves of the heart that depend on expression in endothelial-derived tissue and presumably reflect cooperation with the GATA-4/5/6 subfamily.