Down syndrome critical region-1 is a transcriptional target of nuclear factor of activated T cells-c1 within the endocardium during heart development

RCAN1 in cardiovascular diseases: molecular mechanisms and a potential therapeutic target

Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide. Considerable efforts are needed to elucidate the underlying mechanisms for the prevention and treatment of CVDs. Regulator of calcineurin 1 (RCAN1) is involved in both development/maintenance of the cardiovascular system and the pathogenesis of CVDs. RCAN1 reduction protects against atherosclerosis by reducing the uptake of oxidized low-density lipoproteins, whereas RCAN1 has a protective effect on myocardial ischemia/reperfusion injury, myocardial hypertrophy and intramural hematoma/aortic rupture mainly mediated by maintaining mitochondrial function and inhibiting calcineurin and Rho kinase activity, respectively. In this review, the regulation and the function of RCAN1 are summarized. Moreover, the dysregulation of RCAN1 in CVDs is reviewed. In addition, the beneficial role of RCAN1 reduction in atherosclerosis and the protective role of RCAN1 in myocardial ischemia/reperfusion injury, myocardial hypertrophy and intramural hematoma /aortic rupture are discussed, as well as underlying mechanisms. Furthermore, the therapeutic potential and challenges of targeting RCAN1 for CVDs treatment are also discussed.

Regulator of Calcineurin (RCAN): Beyond Down Syndrome Critical Region

The regulator of calcineurin (RCAN) was first reported as a novel gene called DSCR1, encoded in a region termed the Down syndrome critical region (DSCR) of human chromosome 21. Genome sequence comparisons across species using bioinformatics revealed three members of the RCAN gene family, RCAN1, RCAN2, and RCAN3, present in most jawed vertebrates, with one member observed in most invertebrates and fungi. RCAN is most highly expressed in brain and striated muscles, but expression has been reported in many other tissues, as well, including the heart and kidneys. Expression levels of RCAN homologs are responsive to external stressors such as reactive oxygen species, Ca2+, amyloid β, and hormonal changes and upregulated in pathological conditions, including Alzheimer's disease, cardiac hypertrophy, diabetes, and degenerative neuropathy. RCAN binding to calcineurin, a Ca2+/calmodulin-dependent phosphatase, inhibits calcineurin activity, thereby regulating different physiological events via dephosphorylation of important substrates. Novel functions of RCANs have recently emerged, indicating involvement in mitochondria homeostasis, RNA binding, circadian rhythms, obesity, and thermogenesis, some of which are calcineurin-independent. These developments suggest that besides significant contributions to DS pathologies and calcineurin regulation, RCAN is an important participant across physiological systems, suggesting it as a favorable therapeutic target.

The Population Prevalence, Associations of Congenital Heart Defect and Mortality Risk for Down's Syndrome in South Korea Based on National Health Insurance Service (NHIS) Data

Background

In the present study, we estimated the population prevalence, associations of congenital heart defect (CHD) and mortality risk for DS using data from National Health Insurance Service (NHIS) and Rare Diseases Registry (RDR).

Methods

We collected data on subjects with DS who were registered in the RDR between 2010 and 2015. To estimate associations of CHD and mortality risk of DS, the data of DS subjects were compared with 1:5 age- and sex-matched controls.

Results

In 2015, 2077 individuals with DS were identified out of the total population of 51,574,044 South Koreans and the prevalence was 4.03 per 100,000 persons. The trend of DS population prevalence across 10-year-old intervals showed a peak in the group under the age of 10 years (26.0 per 100,000 persons) and then declined sharply after the age of 20 years (0.98 per 100,000 persons at 30-39 years of age). In subjects with DS, the frequencies of atrial septal defect [odds ratios (OR) =65.9; 95% CI, 84.1-99.1], ventricular septal defect (OR = 88.1, 95% CI, 57.9-134.1), patent ductus arteriosus (OR = 56.9, 95% CI, 40.1-80.8), tetralogy of fallot (OR = 42.1, 95% CI, 19.3-92.3), or atrioventricular septal defect (OR = 510.0, 95% CI, 126.7-999.0) were higher than those of age- and sex-matched controls. The risk of death in patients with DS was significantly higher than that of age- and sex-matched controls [hazard ratio (HR) =41.7, 95% CI 20.0-87.0].

Conclusion

In South Korea, the DS population prevalence was 4.03 per 100,000 persons in 2015. The subjects with DS were more likely to accompany CHD and have higher mortality risk than healthy controls.

Mitral valve leaflet response to ischaemic mitral regurgitation: from gene expression to tissue remodelling

Ischaemic mitral regurgitation (IMR), a frequent complication following myocardial infarction (MI), leads to higher mortality and poor clinical prognosis if untreated. Accumulating evidence suggests that mitral valve (MV) leaflets actively remodel post MI, and this remodelling increases both the severity of IMR and the occurrence of MV repair failures. However, the mechanisms of extracellular matrix maintenance and modulation by MV interstitial cells (MVICs) and their impact on MV leaflet tissue integrity and repair failure remain largely unknown. Herein, we sought to elucidate the multiscale behaviour of IMR-induced MV remodelling using an established ovine model. Leaflet tissue at eight weeks post MI exhibited significant permanent plastic radial deformation, eliminating mechanical anisotropy, accompanied by altered leaflet composition. Interestingly, no changes in effective collagen fibre modulus were observed, with MVICs slightly rounder, at eight weeks post MI. RNA sequencing indicated that YAP-induced genes were elevated at four weeks post MI, indicating elevated mechanotransduction. Genes related to extracellular matrix organization were downregulated at four weeks post MI when IMR occurred. Transcriptomic changes returned to baseline by eight weeks post MI. This multiscale study suggests that IMR induces plastic deformation of the MV with no functional damage to the collagen fibres, providing crucial information for computational simulations of the MV in IMR.

Nfatc1 Promotes Interstitial Cell Formation During Cardiac Valve Development in Zebrafish

RATIONALE

The transcription factor NFATC1 (nuclear factor of activated T-cell 1) has been implicated in cardiac valve formation in humans and mice, but we know little about the underlying mechanisms. To gain mechanistic understanding of cardiac valve formation at single-cell resolution and insights into the role of NFATC1 in this process, we used the zebrafish model as it offers unique attributes for live imaging and facile genetics.

OBJECTIVE

To understand the role of Nfatc1 in cardiac valve formation.

METHODS AND RESULTS

Using the zebrafish atrioventricular valve, we focus on the valve interstitial cells (VICs), which confer biomechanical strength to the cardiac valve leaflets. We find that initially atrioventricular endocardial cells migrate collectively into the cardiac jelly to form a bilayered structure; subsequently, the cells that led this migration invade the ECM (extracellular matrix) between the 2 endocardial cell monolayers, undergo endothelial-to-mesenchymal transition as marked by loss of intercellular adhesion, and differentiate into VICs. These cells proliferate and are joined by a few neural crest-derived cells. VIC expansion and a switch from a promigratory to an elastic ECM drive valve leaflet elongation. Functional analysis of Nfatc1 reveals its requirement during VIC development. Zebrafish nfatc1 mutants form significantly fewer VICs due to reduced proliferation and impaired recruitment of endocardial and neural crest cells during the early stages of VIC development. With high-speed microscopy and echocardiography, we show that reduced VIC formation correlates with valvular dysfunction and severe retrograde blood flow that persist into adulthood. Analysis of downstream effectors reveals that Nfatc1 promotes the expression of twist1b-a well-known regulator of epithelial-to-mesenchymal transition.

CONCLUSIONS

Our study sheds light on the function of Nfatc1 in zebrafish cardiac valve development and reveals its role in VIC formation. It also further establishes the zebrafish as a powerful model to carry out longitudinal studies of valve formation and function.

Limited synergy of obesity and hypertension, prevalent risk factors in onset and progression of heart failure with preserved ejection fraction

Obesity and hypertension are prevalent comorbidities in heart failure with preserved ejection fraction. To clarify if and how interaction between these comorbidities contributes to development of diastolic dysfunction, lean and obese ZSF1 rats were treated with deoxycorticosterone acetate implants and a high‐salt diet (DS) to induce severe hypertension, or with placebo. In addition to echocardiographic, metabolic and hemodynamic analyses, immunohistochemistry and RNAseq were performed on left ventricular tissue. Obesity negatively affected cardiac output, led to an elevated E/e’ ratio and mildly reduced ejection fraction. DS‐induced hypertension did not affect cardiac output and minimally elevated E/e’ ratio. Diastolic derangements in placebo‐treated obese rats developed in absence of inflammation and fibrosis, yet in presence of oxidative stress and hypertrophic remodelling. In contrast, hypertension triggered apoptosis, inflammation and fibrosis, with limited synergy of the comorbidities observed for inflammation and fibrosis. Transcriptional data suggested that these comorbidities exerted opposite effects on mitochondrial function. In placebo‐treated obese rats, genes involved in fatty acid metabolism were up‐regulated, whereas DS‐induced a down‐regulation of genes involved in oxidative phosphorylation. Overall, limited interaction was observed between these comorbidities in development of diastolic dysfunction. Importantly, differences in obesity‐ and hypertension‐induced cardiac remodelling emphasize the necessity for comorbidity‐specific phenotypical characterization.

Association Between DSCR1 Variations and Congenital Heart Disease Susceptibility

Background

The objective of this study was aimed to detect the association of Down syndrome critical region 1 (DSCR1) gene polymorphisms (rs149048873 and rs143081213) and congenital heart disease (CHD) susceptibility.

Material/Methods

This case-control study included 102 CHD patients and 113 healthy controls. Cases and controls were matched in age and gender. Genotypes of DSCR1 gene polymorphisms were detected by TaqMan method in cases and controls. Hardy-Weinberg equilibrium (HWE) examination was performed by PLINK 1.0 software. Chi square test was utilized to assess the distribution of the genotypes and the alleles. Relative risk of CHD was presented by odds ratios (ORs) with 95% confidence intervals (CIs). All of the calculations were implemented using SPSS 18.0.

Results

Variant genotype distribution of rs149048873 and rs143081213 mutations were higher in cases than in controls, but the differences were not statistically obvious (P>0.05). Additionally, frequencies of mutant allele of the two polymorphisms were also significantly different in case and control groups (P>0.05).

Conclusions

No significant associations existed between DSCR1 gene rs149048873 and rs143081213 polymorphisms and CHD susceptibility.

Murine Creld1 controls cardiac development through activation of calcineurin/NFATc1 signaling

Calcineurin is a heteromeric Ca(2+)-dependent serine/threonine phosphatase. It dephosphorylates the transcription factor nuclear factor of activated T cells (NFAT) in the cytoplasm, which subsequently undergoes nuclear translocation. NFAT regulates numerous biological processes, including inflammatory T cell responses and cardiac development. Our study identifies the Cysteine-Rich with EGF-Like Domains 1 (Creld1) gene as a regulator of calcineurin/NFATc1 signaling. We show that Creld1 is sufficient to promote NFATc1 dephosphorylation and translocation to the nucleus. Creld1 is contained in a joint protein complex with the regulatory subunit of calcineurin, CnB, thereby controlling calcineurin function. Localization of Creld1 at the endoplasmic reticulum (ER) is important to exert its action on calcineurin. By using Creld1KO mice, we demonstrate that Creld1 is essential for heart development. Creld1 function is required for the VEGF-dependent proliferation of endocardial cells by promoting the expression of NFATc1 target-genes. Collectively, our study identifies Creld1 as an important regulator of calcineurin/NFATc1 signaling.

Aberrant expression of RCAN1 in Alzheimer's pathogenesis: a new molecular mechanism and a novel drug target

AD, a devastating neurodegenerative disorder, is the most common cause of dementia in the elderly. Patients with AD are characterized by three hallmarks of neuropathology including neuritic plaque deposition, neurofibrillary tangle formation, and neuronal loss. Growing evidences indicate that dysregulation of regulator of calcineurin 1 (RCAN1) plays an important role in the pathogenesis of AD. Aberrant RCAN1 expression facilitates neuronal apoptosis and Tau hyperphosphorylation, leading to neuronal loss and neurofibrillary tangle formation. This review aims to describe the recent advances of the regulation of RCAN1 expression and its physiological functions. Moreover, the AD risk factors-induced RCAN1 dysregulation and its role in promoting neuronal loss, synaptic impairments and neurofibrillary tangle formation are summarized. Furthermore, we provide an outlook into the effects of RCAN1 dysregulation on APP processing, Aβ generation and neuritic plaque formation, and the possible underlying mechanisms, as well as the potential of targeting RCAN1 as a new therapeutic approach.

Pbx1 activates Fgf10 in the mesenchyme of developing lungs

Insufficiency of surfactants is a core factor in respiratory distress syndrome, which causes apnea and neonatal death, particularly in preterm infants. Surfactant proteins are secreted by alveolar type II cells in the lung epithelium, the differentiation of which is regulated by Fgf10 elaborated by the adjacent mesenchyme. However, the molecular regulation of mesenchymal Fgf10 during lung development has not been fully understood. Here, we show that Pbx1, a homeodomain transcription factor, is required in the lung mesenchyme for the expression of Fgf10. Mouse embryos lacking Pbx1 in the lung mesenchyme show compact terminal saccules and perinatal lethality with failure of postnatal alveolar expansion. Mutant embryos had severely reduced expression of Fgf10 and surfactant genes (Spa, Spb, Spc, and Spd) that are essential for alveolar expansion for gas exchange at birth. Molecularly, Pbx1 directly binds to the Fgf10 promoter and cooperates with Meis and Hox proteins to transcriptionally activate Fgf10. Our results thus show how Pbx1 controls Fgf10 in the developing lung.

The vertebrate RCAN gene family: novel insights into evolution, structure and regulation

Recently there has been much interest in the Regulators of Calcineurin (RCAN) proteins which are important endogenous modulators of the calcineurin-NFATc signalling pathway. They have been shown to have a crucial role in cellular programmes such as the immune response, muscle fibre remodelling and memory, but also in pathological processes such as cardiac hypertrophy and neurodegenerative diseases. In vertebrates, the RCAN family form a functional subfamily of three members RCAN1, RCAN2 and RCAN3 whereas only one RCAN is present in the rest of Eukarya. In addition, RCAN genes have been shown to collocate with RUNX and CLIC genes in ACD clusters (ACD21, ACD6 and ACD1). How the RCAN genes and their clustering in ACDs evolved is still unknown. After analysing RCAN gene family evolution using bioinformatic tools, we propose that the three RCAN vertebrate genes within the ACD clusters, which evolved from single copy genes present in invertebrates and lower eukaryotes, are the result of two rounds of whole genome duplication, followed by a segmental duplication. This evolutionary scenario involves the loss or gain of some RCAN genes during evolution. In addition, we have analysed RCAN gene structure and identified the existence of several characteristic features that can be involved in RCAN evolution and gene expression regulation. These included: several transposable elements, CpG islands in the 5′ region of the genes, the existence of antisense transcripts (NAT) associated with the three human genes, and considerable evidence for bidirectional promoters that regulate RCAN gene expression. Furthermore, we show that the CpG island associated with the RCAN3 gene promoter is unmethylated and transcriptionally active. All these results provide timely new insights into the molecular mechanisms underlying RCAN function and a more in depth knowledge of this gene family whose members are obvious candidates for the development of future therapies.

The regulator of calcineurin (RCAN1) an important factor involved in atherosclerosis and cardiovascular diseases development

Atherosclerosis, one of the main causes of cardiovascular diseases, is a complex process that involves manifold factors. Besides the vascular lipids accumulation, inflammatory factors could be considered as a proatherogenic factor - RCAN1. RCAN1 is a regulator of calcineurin, both of them being calcium dependent proteins. Recent studies have shown that RCAN1 has an important role in heart valve development. In the same time researchers found that, the atherosclerotic plaques have an up-regulated RCAN1 gene expression. In the near future, it is desirable to elucidate the RCAN1 function and classify it as a possible biochemical marker to diagnose infancy atherosclerosis.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Endocardial and epicardial epithelial to mesenchymal transitions in heart development and disease

Epithelial to mesenchymal transition (EMT) converts epithelial cells to mobile and developmentally plastic mesenchymal cells. All cells in the heart arise from one or more EMTs. Endocardial and epicardial EMTs produce most of the noncardiomyocyte lineages of the mature heart. Endocardial EMT generates valve progenitor cells and is necessary for formation of the cardiac valves and for complete cardiac septation. Epicardial EMT is required for myocardial growth and coronary vessel formation, and it generates cardiac fibroblasts, vascular smooth muscle cells, a subset of coronary endothelial cells, and possibly a subset of cardiomyocytes. Emerging studies suggest that these developmental mechanisms are redeployed in adult heart valve disease, in cardiac fibrosis, and in myocardial responses to ischemic injury. Redirection and amplification of disease-related EMTs offer potential new therapeutic strategies and approaches for treatment of heart disease. Here, we review the role and molecular regulation of endocardial and epicardial EMT in fetal heart development, and we summarize key literature implicating reactivation of endocardial and epicardial EMT in adult heart disease.

The secondary heart field is a new site of calcineurin/Nfatc1 signaling for semilunar valve development

Semilunar valve malformations are common human congenital heart defects. Bicuspid aortic valves occur in 2-3% of the population, and pulmonic valve stenosis constitutes 10% of all congenital heart disease in adults (Brickner et al., 2000) [1]. Semilunar valve defects cause valve regurgitation, stenosis, or calcification, leading to endocarditis or congestive heart failure. These complications often require prolonged medical treatment or surgical intervention. Despite the medical importance of valve disease, the regulatory pathways governing semilunar valve development are not entirely clear. In this report we investigated the spatiotemporal role of calcineurin/Nfatc1 signaling in semilunar valve development. We generated conditional knockout mice with calcineurin gene disrupted in various tissues during semilunar valve development. Our studies showed that calcineurin/Nfatc1 pathway signals in the secondary heart field (SHF) but not in the outflow tract myocardium or neural crest cells to regulate semilunar valve morphogenesis. Without SHF calcineurin/Nfatc1 signaling, the conal endocardial cushions-the site of prospective semilunar valve formation--first develop and then regress due to apoptosis, resulting in a striking phenotype with complete absence of the aortic and pulmonic valves, severe valve regurgitation, and perinatal lethality. This role of calcineurin/Nfatc1 signaling in the SHF is different from the requirement of calcineurin/Nfatc1 in the endocardium for semilunar valve formation (Chang et al., 2004) [2], indicating that calcineurin/Nfatc1 signals in multiple tissues to organize semilunar valve development. Also, our studies suggest distinct mechanisms of calcineurin/Nfat signaling for semilunar and atrioventricular valve morphogenesis. Therefore, we demonstrate a novel developmental mechanism in which calcineurin signals through Nfatc1 in the secondary heart field to promote semilunar valve morphogenesis, revealing a new supportive role of the secondary heart field for semilunar valve formation.

VEGF signaling has distinct spatiotemporal roles during heart valve development

Heart valve malformations are one of the most common types of birth defects, illustrating the complex nature of valve development. Vascular endothelial growth factor (VEGF) signaling is one pathway implicated in valve formation, however its specific spatial and temporal roles remain poorly defined. To decipher these contributions, we use two inducible dominant negative approaches in mice to disrupt VEGF signaling at different stages of embryogenesis. At an early step in valve development, VEGF signals are required for the full transformation of endocardial cells to mesenchymal cells (EMT) at the outflow tract (OFT) but not atrioventricular canal (AVC) endocardial cushions. This role likely involves signaling mediated by VEGF receptor 1 (VEGFR1), which is highly expressed in early cushion endocardium before becoming downregulated after EMT. In contrast, VEGFR2 does not exhibit robust cushion endocardium expression until after EMT is complete. At this point, VEGF signaling acts through VEGFR2 to direct the morphogenesis of the AVC cushions into mature, elongated valve leaflets. This latter role of VEGF requires the VEGF-modulating microRNA, miR-126. Thus, VEGF roles in the developing valves are dynamic, transitioning from a differentiation role directed by VEGFR1 in the OFT to a morphogenetic role through VEGFR2 primarily in the AVC-derived valves.

The CCAAT/enhancer binding protein beta (C/EBPbeta) cooperates with NFAT to control expression of the calcineurin regulatory protein RCAN1-4

Regulator of calcineurin 1 (RCAN1) inhibits the protein phosphatase calcineurin and is required for appropriate immune responses, synaptic plasticity, vascular tone, angiogenesis, and cardiac remodeling. Expression of the RCAN1-4 isoform is under the control of the calcineurin-responsive transcription factor NFAT. Typically, NFATs act in cooperation with other transcription factors to achieve maximal activation of gene expression. In this study, we identify the CCAAT/enhancer binding protein beta (C/EBPbeta) as an NFAT binding partner that cooperates with NFAT to regulate RCAN1-4 expression. Numerous C/EBPbeta binding sites are conserved in the RCAN1-4 proximal promoter. Overexpression of C/EBPbeta increased activity of both the endogenous mouse Rcan1-4 gene and a human RCAN1-4 luciferase reporter. Binding of C/EBPbeta to multiple sites in the promoter was verified using electrophoretic mobility shift assays and chromatin immunoprecipitation. A direct interaction between C/EBPbeta and NFAT was demonstrated by co-immunoprecipitation of proteins and complex formation at NFAT-C/EBPbeta composite sites. Depletion of endogenous C/EBPbeta decreased maximal activation of RCAN1-4 expression by calcineurin, whereas inhibition of calcineurin did not alter the ability of C/EBPbeta to activate RCAN1-4 expression. Together, these findings suggest that calcineurin/NFAT activation of RCAN1-4 expression is in part dependent upon C/EBPbeta, whereas activation by C/EBPbeta is not dependent on calcineurin and may provide a calcineurin-independent pathway for regulating RCAN1-4 expression. Importantly, nuclear localization, C/EBPbeta DNA binding activity and occupancy of the Rcan1-4 promoter increased in mouse models of heart failure demonstrating in vivo activation of this pathway to regulate Rcan1-4 expression and ultimately shape the dynamics of calcineurin-dependent signaling.

Differential functions of genes regulated by VEGF-NFATc1 signaling pathway in the migration of pulmonary valve endothelial cells

We have reported that vascular endothelial growth factor (VEGF)-A induces the proliferation of human pulmonary valve endothelial cells (HPVECs) through nuclear factor in activated T cells (NFAT)c1 activation. Here we show that VEGF-A increases the migration of HPVECs through NFATc1 activation, suggesting that VEGF-A/NFATc1 regulates the migration of HPVECs. To learn how this pathway may be involved in post-natal valvular repair, HPVECs were treated with VEGF-A, with or without cyclosporine A to selectively block VEGF-NFATc1 signaling. Down Syndrome critical region 1 (DSCR1) and heparin-binding EGF-like growth factor (HB-EGF) are two genes identified by DNA microarray as being up-regulated by VEGF-A in a cyclosporine-A-sensitive manner. DSCR1 silencing increased the migration of ovine valve endothelial cells, whereas HB-EGF silencing inhibited migration. This differential effect suggests that VEGF-A/NFATc1 signaling might be a crucial coordinator of endothelial cell migration in post-natal valves.

Spatial and temporal regulation of coronary vessel formation by calcineurin-NFAT signaling

Formation of the coronary vasculature requires reciprocal signaling between endothelial, epicardially derived smooth muscle and underlying myocardial cells. Our studies show that calcineurin-NFAT signaling functions in endothelial cells within specific time windows to regulate coronary vessel development. Mouse embryos exposed to cyclosporin A (CsA), which inhibits calcineurin phosphatase activity, failed to develop normal coronary vasculature. To determine the cellular site at which calcineurin functions for coronary angiogenesis, we deleted calcineurin in endothelial, epicardial and myocardial cells. Disruption of calcineurin-NFAT signaling in endothelial cells resulted in the failure of coronary angiogenesis, recapitulating the coronary phenotype observed in CsA-treated embryos. By contrast, deletion of calcineurin in either epicardial or myocardial cells had no effect on coronary vasculature during early embryogenesis. To define the temporal requirement for NFAT signaling, we treated developing embryos with CsA at overlapping windows from E9.5 to E12.5 and examined coronary development at E12.5. These experiments demonstrated that calcineurin-NFAT signaling functions between E10.5 and E11.5 to regulate coronary angiogenesis. Consistent with these in vivo observations, endothelial cells exposed to CsA within specific time windows in tissue culture were unable to form tubular structures and their cellular responses to VEGF-A were blunted. Thus, our studies demonstrate specific temporal and spatial requirements of NFAT signaling for coronary vessel angiogenesis. These requirements are distinct from the roles of NFAT signaling in the angiogenesis of peripheral somatic vessels, providing an example of the environmental influence of different vascular beds on the in vivo endothelial responses to angiogenic stimuli.

Calcineurin/NFAT signaling is required for neuregulin-regulated Schwann cell differentiation

Schwann cells develop from multipotent neural crest cells and form myelin sheaths around axons that allow rapid transmission of action potentials. Neuregulin signaling through the ErbB receptor regulates Schwann cell development; however, the downstream pathways are not fully defined. We find that mice lacking calcineurin B1 in the neural crest have defects in Schwann cell differentiation and myelination. Neuregulin addition to Schwann cell precursors initiates an increase in cytoplasmic Ca2+, which activates calcineurin and the downstream transcription factors NFATc3 and c4. Purification of NFAT protein complexes shows that Sox10 is an NFAT nuclear partner and synergizes with NFATc4 to activate Krox20, which regulates genes necessary for myelination. Our studies demonstrate that calcineurin and NFAT are essential for neuregulin and ErbB signaling, neural crest diversification, and differentiation of Schwann cells.

Pbx1 functions in distinct regulatory networks to pattern the great arteries and cardiac outflow tract

The patterning of the cardiovascular system into systemic and pulmonic circulations is a complex morphogenetic process, the failure of which results in clinically important congenital defects. This process involves extensive vascular remodeling and coordinated division of the cardiac outflow tract (OFT). We demonstrate that the homeodomain transcription factor Pbx1 orchestrates separate transcriptional pathways to control great-artery patterning and cardiac OFT septation in mice. Pbx1-null embryos display anomalous great arteries owing to a failure to establish the initial complement of branchial arch arteries in the caudal pharyngeal region. Pbx1 deficiency also results in the failure of cardiac OFT septation. Pbx1-null embryos lose a transient burst of Pax3 expression in premigratory cardiac neural crest cells (NCCs) that ultimately specifies cardiac NCC function for OFT development, but does not regulate NCC migration to the heart. We show that Pbx1 directly activates Pax3, leading to repression of its target gene Msx2 in NCCs. Compound Msx2/Pbx1-null embryos display significant rescue of cardiac septation, demonstrating that disruption of this Pbx1-Pax3-Msx2 regulatory pathway partially underlies the OFT defects in Pbx1-null mice. Conversely, the great-artery anomalies of compound Msx2/Pbx1-null embryos remain within the same spectrum as those of Pbx1-null embryos. Thus, Pbx1 makes a crucial contribution to distinct regulatory pathways in cardiovascular development.

NFATc1 in mice represses osteoprotegerin during osteoclastogenesis and dissociates systemic osteopenia from inflammation in cherubism

Osteoporosis results from an imbalance in skeletal remodeling that favors bone resorption over bone formation. Bone matrix is degraded by osteoclasts, which differentiate from myeloid precursors in response to the cytokine RANKL. To gain insight into the transcriptional regulation of bone resorption during growth and disease, we generated a conditional knockout of the transcription factor nuclear factor of activated T cells c1 (Nfatc1). Deletion of Nfatc1 in young mice resulted in osteopetrosis and inhibition of osteoclastogenesis in vivo and in vitro. Transcriptional profiling revealed NFATc1 as a master regulator of the osteoclast transcriptome, promoting the expression of numerous genes needed for bone resorption. In addition, NFATc1 directly repressed osteoclast progenitor expression of osteoprotegerin, a decoy receptor for RANKL previously thought to be an osteoblast-derived inhibitor of bone resorption. "Cherubism mice", which carry a gain-of-function mutation in SH3-domain binding protein 2 (Sh3bp2), develop osteoporosis and widespread inflammation dependent on the proinflammatory cytokine, TNF-alpha. Interestingly, deletion of Nfatc1 protected cherubism mice from systemic bone loss but did not inhibit inflammation. Taken together, our study demonstrates that NFATc1 is required for remodeling of the growing and adult skeleton and suggests that NFATc1 may be an effective therapeutic target for osteoporosis associated with inflammatory states.

NFAT signaling in neural development and axon growth

The NFAT (nuclear factor of activated T-cells) family of transcription factors functions as integrators of multiple signaling pathways by binding to chromatin in combination with other transcription factors and coactivators to regulate genes central for many developmental systems. Recent experimental evidence has shown that the calcineurin/NFAT signaling pathway is important in axonal growth and guidance during vertebrate development. In fact, studies with triple NFATc2/c3/c4 mutant mice demonstrate that the extension and organization of sensory axon projection and commissural axon growth are both dependent upon NFAT activity. Neurotrophin and L-type calcium channel signaling modulate intracellular calcium levels to regulate the nuclear import and transcriptional activity of NFAT by activating the phosphatase calcineurin. The rephosphorylation and subsequent export of NFAT from the nucleus is mediated by several kinases, including GSK-3beta, which contribute to the fine tuning of NFAT transcriptional activity in neurons. However, currently, no direct transcriptional targets for NFAT have been identified in a chromatin environment in the nervous system. Undiscovered are also the binding partners of NFAT that might combinatorially regulate specific genes important for neuronal development. This review will discuss the current knowledge related to NFAT signaling in the nervous system development and the potential for future research directions.