Wnt Signaling Inhibition Prevents Postnatal Inflammation and Disease Progression in Mouse Congenital Myxomatous Valve Disease

BACKGROUND

Myxomatous valve disease (MVD) is the most common cause of mitral regurgitation, leading to impaired cardiac function and heart failure. MVD in a mouse model of Marfan syndrome includes valve leaflet thickening and progressive valve degeneration. However, the underlying mechanisms by which the disease progresses remain undefined.

METHODS

Mice with Fibrillin 1 gene variant Fbn1C1039G/+ recapitulate histopathologic features of Marfan syndrome, and Wnt signaling activity was detected in TCF/Lef-lacZ reporter mice. Single-cell RNA sequencing was performed from mitral valves of wild-type and Fbn1C1039G/+ mice at 1 month of age. Inhibition of Wnt signaling was achieved by conditional induction of the secreted Wnt inhibitor Dkk1 expression in periostin-expressing valve interstitial cells of Periostin-Cre; tetO-Dkk1; R26rtTA; TCF/Lef-lacZ; Fbn1C1039G/+ mice. Dietary doxycycline was administered for 1 month beginning with MVD initiation (1-month-old) or MVD progression (2-month-old). Histological evaluation and immunofluorescence for ECM (extracellular matrix) and immune cells were performed.

RESULTS

Wnt signaling is activated early in mitral valve disease progression, before immune cell infiltration in Fbn1C1039G/+ mice. Single-cell transcriptomics revealed similar mitral valve cell heterogeneity between wild-type and Fbn1C1039G/+ mice at 1 month of age. Wnt pathway genes were predominantly expressed in valve interstitial cells and valve endothelial cells of Fbn1C1039G/+ mice. Inhibition of Wnt signaling in Fbn1C1039G/+ mice at 1 month of age prevented the initiation of MVD as indicated by improved ECM remodeling and reduced valve leaflet thickness with decreased infiltrating macrophages. However, later, Wnt inhibition starting at 2 months did not prevent the progression of MVD.

CONCLUSIONS

Wnt signaling is involved in the initiation of mitral valve abnormalities and inflammation but is not responsible for later-stage valve disease progression once it has been initiated. Thus, Wnt signaling contributes to MVD progression in a time-dependent manner and provides a promising therapeutic target for the early treatment of congenital MVD in Marfan syndrome.

Localized Prox1 Regulates Aortic Valve Endothelial Cell Diversity and Extracellular Matrix Stratification in Mice

BACKGROUND

Specialized valve endothelial cell (VEC) populations are localized oriented to blood flow in developing aortic and mitral valves, but their roles in valve development and disease are unknown. In the aortic valve (AoV), a population of VECs on the fibrosa side expresses the transcription factor Prox1 together with genes found in lymphatic ECs. In this study, we examine Prox1's role in regulating a lymphatic-like gene network and promoting VEC diversity required for the development of the stratified trilaminar extracellular matrix (ECM) of murine AoV leaflets.

METHODS

To determine whether disruption of Prox1 localization affects heart valve development, we generated mice (NFATc1enCre Prox1 gain-of-function) in which Prox1 is overexpressed on the ventricularis side of the AoV beginning in embryonic development. To identify potential targets of Prox1, we performed cleavage under targets and release using nuclease on wild-type and NFATc1enCre Prox1 gain-of-function AoVs with validation by colocalization in vivo using RNA in situ hybridization in NFATc1enCre Prox1 gain-of-function AoVs. Natural induction of Prox1 and target gene expression was evaluated in myxomatous AoVs in a mouse model of Marfan syndrome (Fbn1C1039G/+).

RESULTS

The overexpression of Prox1 is sufficient to cause enlargement of AoVs by postnatal day (P)0, as well as a decrease in ventricularis-specific gene expression and disorganized interstitial ECM layers at P7. We identified potential targets of Prox1 known to play roles in lymphatic ECs including Flt1, Efnb2, Egfl7, and Cx37. Ectopic Prox1 colocalized with induced Flt1, Efnb2, and Cx37 expression in NFATc1enCre Prox1 gain-of-function AoVs. Moreover, in Marfan syndrome myxomatous AoVs, endogenous Prox1, and its identified targets, were ectopically induced in ventricularis side VECs.

CONCLUSIONS

Our results support a role for Prox1 in localized lymphatic-like gene expression on the fibrosa side of the AoV. Furthermore, localized VEC specialization is required for development of the stratified trilaminar ECM critical for AoV function and is dysregulated in congenitally malformed valves.

Btg1 and Btg2 regulate neonatal cardiomyocyte cell cycle arrest

Rodent cardiomyocytes undergo mitotic arrest in the first postnatal week. Here, we investigate the role of transcriptional co-regulator Btg2 (B-cell translocation gene 2) and functionally-similar homolog Btg1 in postnatal cardiomyocyte cell cycling and maturation. Btg1 and Btg2 (Btg1/2) are expressed in neonatal C57BL/6 mouse left ventricles coincident with cardiomyocyte cell cycle arrest. Btg1/2 constitutive double knockout (DKO) mouse hearts exhibit increased pHH3+ mitotic cardiomyocytes compared to Wildtype at postnatal day (P)7, but not at P30. Similarly, neonatal AAV9-mediated Btg1/2 double knockdown (DKD) mouse hearts exhibit increased EdU+ mitotic cardiomyocytes compared to Scramble AAV9-shRNA controls at P7, but not at P14. In neonatal rat ventricular myocyte (NRVM) cultures, siRNA-mediated Btg1/2 single and double knockdown cohorts showed increased EdU+ cardiomyocytes compared to Scramble siRNA controls, without increase in binucleation or nuclear DNA content. RNAseq analyses of Btg1/2-depleted NRVMs support a role for Btg1/2 in inhibiting cell proliferation, and in modulating reactive oxygen species response pathways, implicated in neonatal cardiomyocyte cell cycle arrest. Together, these data identify Btg1 and Btg2 as novel contributing factors in mammalian cardiomyocyte cell cycle arrest after birth.

Cardiomyocyte cell cycling, maturation, and growth by multinucleation in postnatal swine

BACKGROUND

Rodent cardiomyocytes (CM) undergo mitotic arrest and decline of mononucleated-diploid population post-birth, which are implicated in neonatal loss of heart regenerative potential. However, the dynamics of postnatal CM maturation are largely unknown in swine, despite a similar neonatal cardiac regenerative capacity as rodents. Here, we provide a comprehensive analysis of postnatal cardiac maturation in swine, including CM cell cycling, multinucleation and hypertrophic growth, as well as non-CM cardiac factors such as extracellular matrix (ECM), immune cells, capillaries, and neurons. Our study reveals discordance in postnatal pig heart maturational events compared to rodents.

METHODS AND RESULTS

Left-ventricular myocardium from White Yorkshire-Landrace pigs at postnatal day (P)0 to 6 months (6mo) was analyzed. Mature cardiac sarcomeric characteristics, such as fetal TNNI1 repression and Cx43 co-localization to cell junctions, were not evident until P30 in pigs. In CMs, appreciable binucleation is observed by P7, with extensive multinucleation (4-16 nuclei per CM) beyond P15. Individual CM nuclei remain predominantly diploid at all ages. CM mononucleation at ~50% incidence is observed at P7-P15, and CM mitotic activity is measurable up to 2mo. CM cross-sectional area does not increase until 2mo-6mo in pigs, though longitudinal CM growth proportional to multinucleation occurs after P15. RNAseq analysis of neonatal pig left ventricles showed increased expression of ECM maturation, immune signaling, neuronal remodeling, and reactive oxygen species response genes, highlighting significance of the non-CM milieu in postnatal mammalian heart maturation.

CONCLUSIONS

CM maturational events such as decline of mononucleation and cell cycle arrest occur over a 2-month postnatal period in pigs, despite reported loss of heart regenerative potential by P3. Moreover, CMs grow primarily by multinucleation and longitudinal hypertrophy in older pig CMs, distinct from mice and humans. These differences are important to consider for preclinical testing of cardiovascular therapies using swine, and may offer opportunities to study aspects of heart regeneration unavailable in other models.

Scar Formation with Decreased Cardiac Function Following Ischemia/Reperfusion Injury in 1 Month Old Swine

Studies in mice show a brief neonatal period of cardiac regeneration with minimal scar formation, but less is known about reparative mechanisms in large mammals. A transient cardiac injury approach (ischemia/reperfusion, IR) was used in weaned postnatal day (P)30 pigs to assess regenerative repair in young large mammals at a stage when cardiomyocyte (CM) mitotic activity is still detected. Female and male P30 pigs were subjected to cardiac ischemia (1 h) by occlusion of the left anterior descending artery followed by reperfusion, or to a sham operation. Following IR, myocardial damage occurred, with cardiac ejection fraction significantly decreased 2 h post-ischemia. No improvement or worsening of cardiac function to the 4 week study end-point was observed. Histology demonstrated CM cell cycling, detectable by phospho-histone H3 staining, at 2 months of age in multinucleated CMs in both sham-operated and IR pigs. Inflammation and regional scar formation in the epicardial region proximal to injury were observed 4 weeks post-IR. Thus, pigs subjected to cardiac IR at P30 show myocardial damage with a prolonged decrease in cardiac function, formation of a regional scar, and increased inflammation, but do not regenerate myocardium even in the presence of CM mitotic activity.

Abstract 423: Cardiomyocyte Maturation and Multinucleation in Postnatal Swine

Objectives: Cardiomyocyte (CM) cell cycle arrest and decline of mononucleated diploid CMs have been implicated in loss of regenerative potential in postnatal mouse hearts. Sarcomeric and extracellular matrix (ECM) maturation also occur concurrently in mice, influencing CM proliferative arrest. Recent studies show a 3-day neonatal period of cardiac regeneration in pigs similar to mice, but the dynamics of postnatal pig CM growth are unknown. Our objective is to explore cardiac cell cycling, growth and maturation in postnatal pigs to understand the events guiding loss of cardiac regenerative capacity in large mammals.

Methods & Results: Left-ventricular tissue from farm pigs (White Yorkshire-Landrace) at Postnatal day (P)0, P7, P15, P30, 2 months (2mo) and 6mo were utilized. CM dissociations revealed predominant CM mononucleation at birth in swine, with persistence of ~50% (1186/2537 cells, n=5) mononucleated CMs at P15, and ~10% (227/1785 cells, n=4) at 2mo. By 6mo, pig CMs are entirely multinucleated, exhibiting 4-16 nuclei per cell. Assessing hypertrophic growth in dissociated pig CMs revealed longitudinal CM growth relative to increased nucleation at all ages. However, onset of diametric hypertrophy only occurs beyond 2mo. When nuclear pHH3 and mRNA expression of cell cycle genes was assessed, pig hearts show robust cell-cycling up to 2mo. Also, fetal TNNI1 and MYH6 are active up to 2mo in pig hearts. Ongoing studies on collagen remodeling indicate ECM remodeling in swine occurs beyond P7. Future studies are designed to identify nuclear ploidy in postnatal pig CMs and measure cytokinesis.

Conclusions: Cardiac maturational events are staggered over a 2-6 month postnatal window in pigs, with older pig hearts exhibiting extensive CM multinucleation and differences in longitudinal versus diametric CM growth. These fundamental variations in CM growth characteristics are important to consider when designing preclinical trials for cardiac regenerative strategies in pigs. Also, despite a similar period of regenerative capacity as mice, pig hearts do not undergo loss of CM cell cycling and mononucleation until 2mo after birth. Utilizing pigs may thus offer unique opportunities to study aspects of heart regeneration unavailable in other animal models.

Endothelial Cell Lineage Analysis Does Not Provide Evidence for EMT in Adult Valve Homeostasis and Disease

Epithelial-to-mesenchymal transition (EMT) enables stationary epithelial cells to exhibit migratory behavior and is the key step that initiates heart valve development. Recent studies suggest that EMT is reactivated in the pathogenesis of myxomatous valve disease (MVD), a condition that involves the progressive degeneration and thickening of valve leaflets. These studies have been limited to in vitro experimentation and reliance on histologic costaining of epithelial and mesenchymal markers as evidence of EMT in mouse and sheep models of valve disease. However, longitudinal analysis of cell lineage origins and potential pathogenic or reparative contributions of newly generated mesenchymal cells have not been reported previously. In this study, a genetic lineage tracing strategy was pursued by irreversibly labeling valve endothelial cells in the Osteogenesis imperfecta and Marfan syndrome mouse models to determine whether they undergo EMT during valve disease. Tie2-CreER ^T2 and Cdh5(PAC)-CreER ^T2 mouse lines were used in combination with colorimetric and fluorescent reporters for longitudinal assessment of endothelial cells. These lineage tracing experiments showed no evidence of EMT during adult valve homeostasis or valve pathogenesis. Additionally, CD31 and smooth muscle α-actin (αSMA) double-positive cells, used as an indicator of EMT, were not detected, and levels of EMT transcription factors were not altered. Interestingly, contrary to the endothelial cell-specific Cdh5(PAC)-CreER ^T2 driver line, Tie2-CreER ^T2 lineage-derived cells in diseased heart valves also included CD45+ leukocytes. Altogether, our data indicate that EMT is not a feature of valve homeostasis and disease but that increased immune cells may contribute to MVD. Anat Rec, 302:125-135, 2019. © 2018 Wiley Periodicals, Inc.

Notch-Tnf signalling is required for development and homeostasis of arterial valves

Aims

Congenital anomalies of arterial valves are common birth defects, leading to valvar stenosis. With no pharmaceutical treatment that can prevent the disease progression, prosthetic replacement is the only choice of treatment, incurring considerable morbidity and mortality. Animal models presenting localized anomalies and stenosis of congenital arterial valves similar to that of humans are critically needed research tools to uncover developmental molecular mechanisms underlying this devastating human condition.

Methods and results

We generated and characterized mouse models with conditionally altered Notch signalling in endothelial or interstitial cells of developing valves. Mice with inactivation of Notch1 signalling in valvar endothelial cells (VEC) developed congenital anomalies of arterial valves including bicuspid aortic valves and valvar stenosis. Notch1 signalling in VEC was required for repressing proliferation and activating apoptosis of valvar interstitial cells (VIC) after endocardial-to-mesenchymal transformation (EMT). We showed that Notch signalling regulated Tnfα expression in vivo, and Tnf signalling was necessary for apoptosis of VIC and post-EMT development of arterial valves. Furthermore, activation or inhibition of Notch signalling in cultured pig aortic VEC-promoted or suppressed apoptosis of VIC, respectively.

Conclusion

We have now met the need of critical animal models and shown that Notch-Tnf signalling balances proliferation and apoptosis for post-EMT development of arterial valves. Our results suggest that mutations in its components may lead to congenital anomaly of aortic valves and valvar stenosis in humans.

FGF23-regulated production of Fetuin-A (AHSG) in osteocytes

INTRODUCTION

AHSG, a serum glycoprotein with recognized anti-calcification activity, has also been suggested to modulate both bone formation and resorption. Though the bulk of AHSG is mostly synthesized in the liver, it has been claimed that also bone cells might produce it. However, the extent of the bone AHSG production and the potential controlling factors remain to be definitively proven. A relevant number of studies support the notion that FGF23, a bone-derived hormone, not only regulates the most important mineral metabolism (MM) related factors (phosphate, parathyroid hormone, vitamin D, etc.), but might be also involved in cardiovascular (CV) outcome, both in chronic kidney disease (CKD) patients and in the general population. Furthermore, in addition to some direct autocrine and paracrine effects in bone, FGF23 has been suggested to interact with AHSG. In this study we investigated if AHSG is really produced by bone cells, and if its bone production is related and/or controlled by FGF23, using cultured bone cells, according to a new method recently published by our group.

RESULTS

Our data show that AHSG is consistently produced in osteocytes and to a far lesser extent in osteoblasts. Both FGF23 addition to the culture medium and its over-expression in osteocytes were associated with a consistent increase of both AHSG mRNA and protein, while FGF23 silencing was followed by opposite effects. Though most of these results were largely affected by the blockage of FGF23 receptors, the role of these receptors in the different experimental sets is still not completely clarified. In addition, we found that FGF23 and AHSG proteins co-localized both in cytoplasm and nucleus, which suggests a possible reciprocal interactivity.

CONCLUSIONS

Our data not only confirm that AHSG is produced in bone, mainly in osteocytes, but show for the first time that its production is modulated by FGF23. Since both proteins play important roles in the bone and cardiovascular pathology, these results add new pieces to the puzzling relationship between bone and vascular pathology, in particular in CKD patients, prompting future investigations in this field.

Loss of β-catenin promotes chondrogenic differentiation of aortic valve interstitial cells

OBJECTIVE

The Wnt/β-catenin signaling pathway has been implicated in human heart valve disease and is required for early heart valve formation in mouse and zebrafish. However, the specific functions of Wnt/β-catenin signaling activity in heart valve maturation and maintenance in adults have not been determined previously.

APPROACH AND RESULTS

Here, we show that Wnt/β-catenin signaling inhibits Sox9 nuclear localization and proteoglycan expression in cultured chicken embryo aortic valves. Loss of β-catenin in vivo in mice, using Periostin(Postn)Cre-mediated tissue-restricted loss of β-catenin (Ctnnb1) in valvular interstitial cells, leads to the formation of aberrant chondrogenic nodules and induction of chondrogenic gene expression in adult aortic valves. These nodular cells strongly express nuclear Sox9 and Sox9 downstream chondrogenic extracellular matrix genes, including Aggrecan, Col2a1, and Col10a1. Excessive chondrogenic proteoglycan accumulation and disruption of stratified extracellular matrix maintenance in the aortic valve leaflets are characteristics of myxomatous valve disease. Both in vitro and in vivo data demonstrate that the loss of Wnt/β-catenin signaling leads to increased nuclear expression of Sox9 concomitant with induced expression of chondrogenic extracellular matrix proteins.

CONCLUSIONS

β-Catenin limits Sox9 nuclear localization and inhibits chondrogenic differentiation during valve development and in adult aortic valve homeostasis.

Placement of an elastic biodegradable cardiac patch on a subacute infarcted heart leads to cellularization with early developmental cardiomyocyte characteristics

BACKGROUND

Placement of an elastic biodegradable patch onto a subacute myocardial infarct (MI) provides temporary elastic support that may act to effectively alter adverse left ventricular (LV) remodeling processes.

METHODS

Two weeks after permanent left coronary ligation in Lewis rats, the infarcted anterior wall was covered with polyester urethane urea (MI + PEUU; n = 15) or expanded polytetrafluoroethylene (MI + ePTFE; n = 15) patches, or had no implantation (MI + sham; n = 12). Eight weeks after surgery, cardiac function and histology were assessed.

RESULTS

The ventricular wall in the MI + ePTFE and MI + sham groups was composed of fibrous tissue, whereas PEUU implantation induced α-smooth muscle actin-positive muscle bundles coexpressing sarcomeric α-actinin and cardiac-specific troponin-T. This pattern of colocalization was also found in developing embryonic myocardium. Cardiac transcription factors Nkx-2.5 and GATA-4 were strongly expressed in the muscle bundles. In the MI + sham group, end-diastolic LV cavity area (EDA) increased and the percentage of fractional area change (%FAC) decreased. For ePTFE patched animals, both EDA and %FAC decreased. In contrast, with MI + PEUU patching, %FAC increased and EDA was maintained. With dobutamine-stress echocardiography, MI + PEUU patched LVs possessed contractile reserve significantly larger than the MI + sham group.

CONCLUSIONS

MI + PEUU patch implantation onto subacute infarcted myocardium induced muscle cellularization with characteristics of early developmental cardiomyocytes as well as providing a functional reserve.

GENE EXPRESSION AND COLLAGEN FIBER MICROMECHANICAL INTERACTIONS OF THE SEMILUNAR HEART VALVE INTERSTITIAL CELL

The semilunar (aortic and pulmonary) heart valves function under dramatically different hemodynamic environments, and have been shown to exhibit differences in mechanical properties, extracellular matrix (ECM) structure, and valve interstitial cell (VIC) biosynthetic activity. However, the relationship between VIC function and the unique micromechanical environment in each semilunar heart valve remains unclear. In the present study, we quantitatively compared porcine semilunar mRNA expression of primary ECM constituents, and layer- and valve-specific VIC-collagen mechanical interactions under increasing transvalvular pressure (TVP). Results indicated that the aortic valve (AV) had a higher fibrillar collagen mRNA expression level compared to the pulmonary valve (PV). We further noted that VICs exhibited larger deformations with increasing TVP in the collagen rich fibrosa layer, with substantially smaller changes in the spongiosa and ventricularis layers. While the VIC-collagen micro-mechanical coupling varied considerably between the semilunar valves, we observed that the VIC deformations in the fibrosa layer were similar at each valve's respective peak TVP. This result suggests that each semilunar heart valve's collagen fiber microstructure is organized to induce a consistent VIC deformation under its respective diastolic TVP. Collectively, our results are consistent with higher collagen biosynthetic demands for the AV compared to the PV, and that the valvular collagen microenvironment may play a significant role in regulating VIC function.

Differential activation of valvulogenic, chondrogenic, and osteogenic pathways in mouse models of myxomatous and calcific aortic valve disease

Studies of human diseased aortic valves have demonstrated increased expression of genetic markers of valve progenitors and osteogenic differentiation associated with pathogenesis. Three potential mouse models of valve disease were examined for cellular pathology, morphology, and induction of valvulogenic, chondrogenic, and osteogenic markers. Osteogenesis imperfecta murine (Oim) mice, with a mutation in Col1a2, have distal leaflet thickening and increased proteoglycan composition characteristic of myxomatous valve disease. Periostin null mice also exhibit dysregulation of the ECM with thickening in the aortic midvalve region, but do not have an overall increase in valve leaflet surface area. Klotho null mice are a model for premature aging and exhibit calcific nodules in the aortic valve hinge-region, but do not exhibit leaflet thickening, ECM disorganization, or inflammation. Oim/oim mice have increased expression of valve progenitor markers Twist1, Col2a1, Mmp13, Sox9 and Hapln1, in addition to increased Col10a1 and Asporin expression, consistent with increased proteoglycan composition. Periostin null aortic valves exhibit relatively normal gene expression with slightly increased expression of Mmp13 and Hapln1. In contrast, Klotho null aortic valves have increased expression of Runx2, consistent with the calcified phenotype, in addition to increased expression of Sox9, Col10a1, and osteopontin. Together these studies demonstrate that oim/oim mice exhibit histological and molecular characteristics of myxomatous valve disease and Klotho null mice are a new model for calcific aortic valve disease.

Wnt signaling in heart valve development and osteogenic gene induction

Wnt signaling mediated by beta-catenin has been implicated in early endocardial cushion development, but its roles in later stages of heart valve maturation and homeostasis have not been identified. Multiple Wnt ligands and pathway genes are differentially expressed during heart valve development. At E12.5, Wnt2 is expressed in cushion mesenchyme, whereas Wnt4 and Wnt9b are predominant in overlying endothelial cells. At E17.5, both Wnt3a and Wnt7b are expressed in the remodeling atrioventricular (AV) and semilunar valves. In addition, the TOPGAL Wnt reporter transgene is active throughout the developing AV and semilunar valves at E16.5, with more localized expression in the stratified valve leaflets after birth. In chicken embryo aortic valves, genes characteristic of osteogenic cell lineages including periostin, osteonectin, and Id2 are expressed specifically in the collagen-rich fibrosa layer at E14. Treatment of E14 aortic valve interstitial cells (VICs) in culture with osteogenic media results in increased expression of multiple genes associated with bone formation. Treatment of VIC with Wnt3a leads to nuclear localization of beta-catenin and induction of periostin and matrix gla protein but does not induce genes associated with later stages of osteogenesis. Together, these studies provide evidence for Wnt signaling as a regulator of endocardial cushion maturation as well as valve leaflet stratification, homeostasis, and pathogenesis.

Mouse heart valve structure and function: echocardiographic and morphometric analyses from the fetus through the aged adult

The purpose of this study is to provide standard echocardiographic and morphometric data for normal mouse valve structure and function from late fetal to aged adult stages. Cross-sectional, two-dimensional and Doppler transthoracic echocardiography was performed in C57BL6 mice anesthetized with 1% to 2% isoflurane at embryonic day 18.5 (late fetal), 10 days (neonate), 1 mo (juvenile), 2 mo (young adult), 9 mo (old adult), and 16 mo (aged adult). Normal annulus dimensions indexed to age or weight, and selected flow velocities, were established by echocardiography. After echocardiographic imaging, hearts were harvested and histological and morphometric analyses were performed. Morphometric analysis demonstrated a progressive valve thinning and elongation during the fetal and juvenile stages that plateaued during adult stages (ANOVA, P < 0.01); however, there was increased thickening of the hinge of the aortic valve with advanced age, reminiscent of human aortic valve sclerosis. There was no age-related calcification. The results of this study provide comprehensive echocardiographic and morphometric data for normal mouse valve structure and function from late fetal to aged adult stages and should prove useful as a reference standard for future studies using mouse models of progressive valve disease.

Regulation of cardiomyocyte proliferation and myocardial growth during development by FOXO transcription factors

Cardiomyocytes actively proliferate during embryogenesis and withdraw from the cell cycle during neonatal stages. FOXO (Forkhead O) transcription factors are a direct target of phosphatidylinositol-3 kinase/AKT signaling in skeletal and smooth muscle and regulate expression of the Cip/Kip family of cyclin kinase inhibitors in other cell types; however, the interaction of phosphatidylinositol-3 kinase/AKT signaling, FOXO transcription factors, and cyclin kinase inhibitor expression has not been reported for the developing heart. Here, we show that FOXO1 and FOXO3 are expressed in the developing myocardium concomitant with increased cyclin kinase inhibitor expression from embryonic to neonatal stages. Cell culture studies show that embryonic cardiomyocytes are responsive to insulin-like growth factor 1 stimulation, which results in the induction of the phosphatidylinositol-3 kinase/AKT pathway, cytoplasmic localization of FOXO proteins, and increased myocyte proliferation. Likewise, adenoviral-mediated expression of AKT promotes cardiomyocyte proliferation and cytoplasmic localization of FOXO. In contrast, increased expression of FOXO1 negatively affects myocyte proliferation. In vivo myocyte-specific transgenic expression of FOXO1 during heart development causes embryonic lethality at embryonic day 10.5 because of severe myocardial defects that coincide with premature activation of p21(cip1), p27(kip1), and p57(kip2) and decreased myocyte proliferation. Transgenic expression of dominant negative FOXO1 in cardiomyocytes does not obviously affect heart development at embryonic day 10.5, but results in abnormal morphology of the myocardium by embryonic day 18.5 along with decreased cyclin kinase inhibitor expression and increased myocyte proliferation. These data support FOXO transcription factors as negative regulators of cardiomyocyte proliferation and promoters of neonatal cell cycle withdrawal during heart development.

Developmental regulation of the mouse IGF-I exon 1 promoter region by calcineurin activation of NFAT in skeletal muscle

Skeletal muscle development and growth are regulated through multiple signaling pathways that include insulin-like growth factor I (IGF-I) and calcineurin activation of nuclear factor of activated T cell (NFAT) transcription factors. The developmental regulation and molecular mechanisms that control IGF-I gene expression in murine embryos and in differentiating C2C12 skeletal myocytes were examined. IGF-I is expressed in developing skeletal muscle, and its embryonic expression is significantly reduced in embryos lacking both NFATc3 and NFATc4. During development, the IGF-I exon 1 promoter is active in multiple organ systems, including skeletal muscle, whereas the alternative exon 2 promoter is expressed predominantly in the liver. The IGF-I exon 1 promoter flanking sequence includes two highly conserved regions that contain NFAT consensus binding sequences. One of these conserved regions contains a calcineurin/NFAT-responsive regulatory region that is preferentially activated by NFATc3 in C2C12 skeletal muscle cells and NIH3T3 fibroblasts. This NFAT-responsive region contains three clustered NFAT consensus binding sequences, and mutagenesis experiments demonstrated the requirement for two of these in calcineurin or NFATc3 responsiveness. Chromatin immunoprecipitation analyses demonstrated that endogenous IGF-I genomic sequences containing these conserved NFAT binding sequences interact preferentially with NFATc3 in C2C12 cells. Together, these experiments demonstrated that a NFAT-rich regulatory element in the IGF-I exon 1 promoter flanking region is responsive to calcineurin signaling and NFAT activation in skeletal muscle cells. The identification of a calcineurin/NFAT-responsive element in the IGF-I gene represents a potential mechanism of intersection of these signaling pathways in the control of muscle development and homeostasis.

BMP and FGF regulatory pathways control cell lineage diversification of heart valve precursor cells

The atrioventricular heart valve leaflets and chordae tendineae are composed of diverse cell lineages and highly organized extracellular matrices that share characteristics with cartilage and tendon cell types in the limb buds and somites. During embryonic chicken valvulogenesis, aggrecan and sox9, characteristic of cartilage cells, are observed in the AV valve leaflets, in contrast to tendon-associated genes scleraxis and tenascin, present in the chordae tendineae. In the limb buds and somites, cartilage cell lineage differentiation is regulated by BMP2, while FGF4 controls tendon cell fate. The ability of BMP2 and FGF4 to induce similar patterns of gene expression in heart valve precursor cells was examined. In multiple assays of cells from prefused endocardial cushions, BMP2 is sufficient to activate Smad1/5/8 phosphorylation and induce sox9 and aggrecan expression, while FGF4 treatment increases phosphorylated MAPK (dpERK) signaling and promotes expression of scleraxis and tenascin. However, these treatments do not alter differentiated lineage gene expression in valve progenitors from fused cushions of older embryos. Together, these studies define regulatory pathways of AV valve progenitor cell diversification into leaflets and chordae tendineae that share inductive interactions and differentiation phenotypes with cartilage and tendon cell lineages.

Development of heart valve leaflets and supporting apparatus in chicken and mouse embryos

Abnormalities in valvuloseptal development significantly contribute to congenital heart defects, yet the underlying causes are complex and poorly understood. Early cardiac regulatory genes are differentially expressed during valvuloseptal development, consistent with novel functions during heart chamber formation in chicken and mouse embryos. Distinct valve cell lineages were identified in the leaflets, chordae tendineae, and myotendinous junctions with the papillary muscles based on restricted expression of extracellular matrix molecules. Specific cell types within these structures demonstrate characteristics of chondrogenesis and tendon development, identified by scleraxis, type II collagen, and tenascin expression. In chicken embryos, valve remodeling and maturation accompanies a decrease in mitotic index indicated by reduced bromodeoxyuridine incorporation. Analysis of Tie2-cre x ROSA26R mice demonstrates that mature valve structures, including the atrioventricular and outflow tract semilunar valve leaflets, chordae tendineae, and the fibrous continuity that connects the septal leaflets of mitral and tricuspid valves, arise from endothelial cells of the endocardial cushions. Together, these studies provide novel insights into the origins and cell lineage diversity of mature valve structures in the developing vertebrate heart.