Normal Adult Heart Research Articles

GATA4 expression is primarily regulated via a miR-26b-dependent post-transcriptional mechanism during cardiac hypertrophy

GATA4 is a transcription factor that is up-regulated during cardiac hypertrophy and plays a fundamental role in myocyte growth and survival. In this study, we investigate the transcriptional vs. post-transcriptional mechanisms that are involved in regulating GATA4 in the heart during neonatal and pressure overload-induced hypertrophic growth. GATA4 protein is significantly higher during pressure overload-induced (2.9 ± 0.4-fold) and neonatal (6.8 ± 1-fold) hypertrophic growth vs. the normal adult mouse heart. Using RNA polymerase II immunoprecipitation combined with deep sequencing, we confirmed that active transcription of the Gata4 gene remained unchanged during hypertrophy, whereas it was two-fold higher in the neonatal vs. adult heart, commensurate with the mRNA levels. These results suggested a post-transcriptional mode of regulation of its expression, which prompted the identification of a conserved sequence in its 3'-untranslated region that was responsible for reduced translation via miR-26b. Overexpression of miR-26b reduced GATA4-dependent transcription, endothelin-induced hypertrophy, and sensitized the cells to apoptotic insults. Additionally, miR-26b targeted phospholipase C-β1, which, in turn, inhibited miR-26b expression, creating a double-negative feedback loop. Accordingly, overexpression of miR-26b in the heart inhibited up-regulation of its targets and the development of hypertrophy. However, knockdown of miR-26b is not sufficient for inducing hypertrophy. Down-regulation of miR-26b in the heart is required for the up-regulation of GATA4 and the induction of pressure-induced cardiac hypertrophy. The results also underscore the functional relevance of miRNAs in regulating gene expression during cardiac hypertrophy.

Autophagy Is Constitutively Active in Normal Mouse Sino-Atrial Nodal Cells

This study was designed to examine the autophagy in sino-atrial (SA) nodal cells from the normal adult mouse heart. Autophagy is the cellular process responsible for the degradation and recycling of long-lived and/or damaged cytoplasmic components by lysosomal digestion. In the heart, autophagy is known to occur at a low level under physiological conditions, but to become upregulated when cells are exposed to certain stresses, such as ischemia. We examined whether the basal level of autophagy in SA nodal cells was different from that in ventricular or atrial myocytes. An ultrastructural analysis revealed that the SA nodal cells contained a number of autophagic vacuoles (autophagosomes) with various stages of degradation by lysosomal digestion, whereas the number of those in ventricular or atrial myocytes was either negligible or very small. The immunostaining of autophagosome marker microtubule-associated protein 1 light chain 3 (LC3) and lysosome marker lysosome-associated membrane protein 1 (LAMP1) indicated that the content of both autophagosomes and lysosomes were much greater in SA nodal cells than in ordinary cardiomyocytes. Our results provide evidence that the autophagy is active in normal SA nodal cells, which is not a stress-activated process but a constitutive event in the mouse heart.

Influence of different doses of retinoic acid on cardiac remodeling

Objective The role of retinoic acid in promoting postnatal heart alterations is still unclear. The aim of this study was to evaluate whether the cardiac alterations caused by all-trans- retinoic acid (ATRA) in normal adult rat hearts are physiologic or pathologic and if these alterations are dose-dependent. Methods Rats were allocated into a control group that received a diet without ATRA ( n = 16), a group that received 0.3 mg of ATRA/kg of diet ( n = 17), a group that received a diet containing 10 mg of ATRA/kg ( n = 18), or a group that received 50 mg of ATRA/kg in the diet ( n = 18). After 4 wk, the animals were evaluated echocardiographically, morphologically, and biochemically. Results The 50-mg ATRA group presented cardiac hypertrophy with maintenance of cardiac geometry and increased systolic function, whereas diastolic function was similar to that of the control group. In addition, progressive increases in the ATRA dose resulted in gradual augmentations of left atrial diameter, left ventricular diastolic and systolic diameters, left ventricular mass index, cardiac output, cardiac index, and aortic velocity. The ATRA did not produce alterations in interferon-γ and tumor necrosis factor-α cardiac levels, interstitial collagen volume fraction, or the intensity and localization of connexin-43. In addition, no alteration was observed in β-hydroxyacyl coenzyme A dehydrogenase, lactate dehydrogenase, or citrate synthase, suggesting that cardiac energetic metabolism was preserved with ATRA. Conclusion These results suggest that ATRA produced dose-dependent effects and cardiac remodeling that is more compatible with a physiologic response.

E0139 The cardiomyogenic potential of cardiac stem cells in an in vitro coculture system

Features so far documented in reliably isolated and manipulated cardiac stem cell (CSCs) resident within the cardiac tissue suggest that innovative treatments to repair damaged myocardium could have promising clinical...

Epithelial–mesenchymal transition of epicardial mesothelium is a source of cardiac CD117-positive stem cells in adult human heart

Epithelial–mesenchymal transition is implicated in the remodelling of tissues during development and in the adult life. In the heart, it gives origin to progenitors of fibroblasts, coronary endothelium, smooth muscle cells, and cardiomyocytes. Moreover, epicardially-derived cells determine myocardial wall thickness and Purkinje fibre network. Recently, the presence of numerous cardiac stem cells in the subepicardium of the adult human heart has been described and the hypothesis that epicardially-derived cells can contribute to the population of cardiac stem cells in the adult heart has been advanced. In an effort to test this hypothesis and establish a possible link between epicardium, epicardially-derived cells and cardiac stem cells in the adult human heart we have examined epicardial mesothelial cells in the normal and pathological adult human heart with ischemic cardiomyopathy in vivo and we have induced and documented their epithelial–mesenchymal transition in vitro. Noticeably, epicardial cells were missing from the surface of pathological hearts and the cells with the expression of epithelial and mesenchymal markers populated thick subepicardial space. When the fragments of epicardium from the normal hearts were cultured on the specific substrate formed by extracellular matrix derived from cardiac fibroblasts, we obtained the outgrowth of the epithelial sheet with the mRNA and protein expression characteristic of epicardium. TGFβ induced cellular and molecular changes typical of epithelial–mesenchymal transition. Moreover, the epicardially-derived cells expressed CD117 antigen. Thus, this study provides evidence that cardiac stem cells can originate from epithelial–mesenchymal transition of the epicardial cells in the adult human heart.

Cardiomyogenic Potential of C-Kit + –Expressing Cells Derived From Neonatal and Adult Mouse Hearts

C-kit is a receptor tyrosine kinase family member expressed in hematopoietic stem cells. C-kit is also transiently expressed in cardiomyocyte precursors during development and in a rare cell population in the normal adult heart. In the present study, the cardiomyogenic potential of c-kit(+) cells isolated from normal neonatal, normal adult, and infarcted adult mouse hearts was evaluated. Magnetic activated cell sorting was used to prepare c-kit(+) cells from the hearts of ACT-EGFP/MHC-nLAC double transgenic mice. These animals exhibit widespread enhanced green fluorescent protein (EGFP) expression and cardiomyocyte-restricted nuclear beta-galactosidase activity, thus permitting simultaneous tracking of cell survival and differentiation. A subset of the c-kit(+) cells from double transgenic neonatal hearts acquired a cardiomyogenic phenotype when cocultured with fetal cardiomyocytes (2.4% of all EGFP(+) cells screened) but rarely when cultured alone or when cocultured with mouse fibroblasts (0.03% and 0.05% of the EGFP(+) cells screened, respectively). In contrast, c-kit(+) cells from normal adult double transgenic hearts failed to undergo cardiomyogenic differentiation when cocultured with nontransgenic fetal cardiomyocytes (>18 000 EGFP(+) cells screened) or when transplanted into normal or infarcted adult mouse hearts (14 EGFP(+) grafts examined). A single c-kit(+) cell from an infarcted double transgenic adult heart was observed to acquire a cardiomyogenic phenotype in coculture (>37 000 EGFP(+) cells screened). These data suggest that the ability of cardiac-resident c-kit(+) cells to acquire a cardiomyogenic phenotype is subject to temporal limitations or, alternatively, that the cardiomyogenic population is lost. Elucidation of the underlying molecular basis may permit robust cardiomyogenic induction in adult-derived cardiac c-kit(+) cells.

Tumor Necrosis Factor‐alpha Mediated Changes in Cardiomyocyte Adhesion

We have established that myocardial levels of tumor necrosis factor‐a (TNF) are acutely elevated in the aortocaval fistula model of heart failure. Furthermore, the elevations in TNF coincide with the initial development of ventricular dilatation induced by the sustained volume overload. It was our hypothesis that TNF induced alterations in integrin mediated cardiomyocyte adhesion contribute to this adverse myocardial remodeling. To test this supposition, left ventricular cardiomyocytes were isolated from normal adult hearts (n = 6) by perfusion with collagenase. Aliquots of 5×104 calcium tolerant isolated cardiomyocytes were incubated for 0.5 hr in DMEN media containing TNF (50 ng/ml), TNF + PP2 (Src kinase inhibitor), TNF + SB203580 (p38 MAP kinase inhibitor), or media alone. The cardiomyocytes were then plated on laminin coated wells for 2 hr, after which the wells were washed and the percentage of adherent cells determined. Incubation with TNF produced a significant 45% reduction in cardiomyocyte adhesion to laminin. This decrease was markedly attenuated by the Src kinase inhibitor PP2, whereas the p38 MAP kinase inhibitor did not prevent the TNF mediated decrease in adhesion. This study implicates reductions in integrin mediated cardiomyocyte adhesion induced by TNF signaling via Src kinase dependent pathways in the adverse myocardial contributing to alterations in ventricular structure and function.

Tenascin-C may aggravate left ventricular remodeling and function after myocardial infarction in mice

Tenascin-C (TN-C) is an extracellular matrix glycoprotein with high bioactivity. It is expressed at low levels in normal adult heart, but upregulated under pathological conditions, such as myocardial infarction (MI). Recently, we (Ref. 34) reported that MI patients with high serum levels of TN-C have a greater incidence of maladaptive cardiac remodeling and a worse prognosis. We hypothesized that TN-C may aggravate left ventricular remodeling. To examine the effects of TN-C, MI was induced by ligating coronary arteries of TN-C knockout (KO) mice under anesthesia and comparing them with sibling wild-type (WT) mice. In WT+MI mice, TN-C expression was upregulated at day 1, peaked at day 5, downregulated and disappeared by day 28, and the molecule was localized in the border zone between intact myocardium and infarct lesions. The morphometrically determined infarct size and survival rate on day 28 were comparable between the WT+MI and KO+MI groups. Echocardiography and hemodynamic analyses demonstrated left ventricular end-diastolic diameter, myocardial stiffness, and left ventricular end-diastolic pressure to be significantly increased in both WT+MI and KO+MI mice compared with sham-operated mice. However, end-diastolic pressure and dimension and myocardial stiffness of KO+MI were lower than those of the WT+MI mice. Histological examination revealed normal tissue healing, but interstitial fibrosis in the residual myocardium in peri-infarcted areas was significantly less pronounced in KO+MI mice than in WT+MI mice. TN-C may thus accelerate adverse ventricular remodeling, cardiac failure, and fibrosis in the residual myocardium after MI.

Anatomic variability of the coronary arterial orifices

Anatomical differences in coronary orifices (CO) are important as they are associated with myocardial ischemia and sudden death. The location of coronary orifices to the aortic valve has been studied since it is a determining point in surgical and radiological attempts. The number, position, and shape of the CO, and their relation to the sinotubular junction (SJ), were studied in 100 normal adult hearts. Student-t test, one- and two-way ANOVA with posthoc Tukey's HSD tests were applied for statistical analysis. The mean height of the right, non- and left coronary sinuses were 18.75+/- 1.71, 17.86+/- 1.55 and 16.41+/- 1.21 mm, respectively. The mean height of the right, non- and left coronary cusps were measured as 16.2+/- 1.2, 15.9+/- 1.1 and 12.3+/- 2.1 mm, respectively. The left coronary artery (LCA) was observed to arise from the lower part of the SJ in 58% of the patients, while in 29% of them it originated from the SJ, and in 13% of the cases it arose from the upper part of the SJ. The right coronary artery (RCA) arising from the lower part of the SJ was seen in 78% of the patients, while it originated from the SJ in 13% of the patients, and it was observed to arise from the upper part of the SJ in 9 specimens. The diameters of the LCA and RCA were measured as 4.22+/- 0.72 and 3.32+/- 0.82 mm, respectively. An accessory orifice was found on the left in 47 specimens, while it was seen on the right in 54 of them. The location of the CO should be identified according to the vertical and horizontal surfaces of the sinus. In this study, the frequency of the coronary artery orifices with different locations was provided. Such data will increase the success of coronary interventions and decrease complication rate.

Localization and origin of cardiac CD117-positive cells: identification of a population of epicardially-derived cells in adult human heart.

During heart morphogenesis, epicardial cells undergo epithelial-mesenchymal transition giving origin to a population of epicardially derived cells that play a crucial role in the development of most cardiac cell lineages. Considering the hypothesis that epithelial-mesenchymal transition of epicardial mesothelium can generate cardiac primitive cells in the adult heart, we have examined in vivo and in vitro the epicardium and subepicardium of normal human adult hearts and of pathological hearts from patients with chronic ischemic heart failure for the presence of CD117-positive cells with epithelial and mesenchymal markers expression. The number of CD117-positive cells increased significantly in the subepicardium of pathological hearts and sloped down towards myocardium, remaining still elevated with respect to normal hearts. While cells with typical epithelial proteins expression formed an intact layer on the surface of the normal hearts, CD117-positive cells were localized mainly in the subepicardium and expressed mesenchymal markers in the pathological hearts. Epithelial-mesenchymal transition, induced in vitro by several growth factors known to accumulate in the ischemic myocardium, gave origin to epicardially-derived cells with CD117 expression. These data support the hypothesis of epicardial origin of cardiac primitive cells in the adult human heart.

DBTSS provides a tissue specific dynamic view of Transcription Start Sites

DataBase of Transcription Start Sites (DBTSS) is a database which contains precise positional information for transcription start sites (TSSs) of eukaryotic mRNAs. In this update, we included 330 million new tags generated by massively sequencing the 5′-end of oligo-cap selected cDNAs in humans and mice. The tags were collected from normal fetal or adult human tissues, including brain, thymus, liver, kidney and heart, from 6 human cell lines in 21 diverse growth conditions as well as from mouse NIH3T3 cell line: altogether 31 different cell types or culture conditions are represented. This unprecedented increase in depth of data now allows DBTSS to faithfully represent the dynamically changing landscape of TSSs in different cell types and conditions, during development and in the course of evolution. Differential usage of alternative 5′-ends across cell types and conditions can be viewed in a series of new interfaces. Promoter sequence information is now displayed in a comparative genomics viewer where evolutionary turnover of the TSSs can be evaluated. DBTSS can be accessed at http://dbtss.hgc.jp/.

Pulmonary vein and atrial wall pathology in human total anomalous pulmonary venous connection

Background Normally, the inside of the left atrial (LA) body and pulmonary veins (PVs) is lined by vessel wall tissue covered by myocardium. In total anomalous pulmonary venous connection (TAPVC), no connection of the PVs with the LA body exists. These veins have an increased incidence of PV stenosis. We describe the consequences of the absent connection for the histopathology of the wall of the LA body and the PVs, and hypothesize on a mechanism predisposing to PV stenosis. Methods and results In 10 human neonates with TAPVC, the wall of the LA body and PVs were studied using histological and immunohistochemical techniques. As controls, 2 normal neonatal and adult hearts and 5 neonatal hearts with partial anomalous venous connection (PAPVC) or situs inversus were studied. In hearts with TAPVC no vessel wall tissue was found in the LA body and its myocardial layer was hypoplastic. No myocardial sleeve was found around the abnormally draining PVs. In hearts with PAPVC, only the non-LA draining PV lacked myocardial covering, whereas in situs inversus PVs connecting to the right-sided LA, were normally myocardialized. Conclusion An open connection of the PVs with the morphological LA is necessary for the presence of vessel wall tissue in the LA and myocardialization of the PVs. Absence of myocardium covering the PVs is hypothesized to enhance susceptibility to PV stenosis and prevent onset of PV originating arrhythmias. The embryonic posterior heart field may be responsible for the abnormal myocardialization and smooth muscle cell formation in TAPVC.

Myosin-activating protein kinases in human myocardium: Localization and content

Using the immunofluorescence approach, we have determined that the recently detected protein kinases, among which are RhoA-activated kinase, integrin-linked kinase, zipper interacting protein kinase, and death-associated protein kinase, which are capable of phosphorylating myosin, are localized in the Z-lines sarcomeres of human myocardium. Additionally, we studied the content of integrin-linked and zipper interacting protein kinases in human embryonic myocardium, as well as in normal and hypertrophic adult human heart. The content of these protein kinases in adult normal myocardium increases in comparison with the embryonic heart. The content of integrin-linked and zipper interacting protein kinases in hypertrophic myocardium is higher compared with the normal adult heart. The data obtained suggest the involvement of these protein kinases in the development and hypertrophy of human heart.

Anti-La (SS-B) but not anti-Ro52 (SS-A) antibodies cross-react with laminin--a role in the pathogenesis of congenital heart block?

Cross-reactions between maternally derived autoantibodies and fetal cardiac antigens have been postulated to play a role in the pathogenesis of congenital heart block (CHB). We have explored the cross-reactivity of autoantibodies to the small ribonuclear autoantigens, La/SS-B and Ro/SS-A, with laminin, the major component of cardiac sarcolemmal membrane using affinity-purified antibodies from patients with Sjögren's syndrome (SS). Anti-La antibodies purified from eight of 10 patients cross-reacted significantly with mouse laminin by ELISA. In contrast, purified antibodies to Ro52 from the same 10 patients showed little or no binding to laminin. Laminin inhibited up to 70% binding of anti-La antibodies to La antigen, and La inhibited up to 65% binding of anti-La antibodies to laminin. The cross-reaction was further examined on cryosections of 10 human fetal hearts aged from 8.7 to 14.9 weeks of gestation, two normal adult hearts, and one pathological adult heart with a diagnosis of dilated cardiomyopathy. Anti-Ro52 antibodies did not bind to the surface of cardiac cells. However, anti-La antibodies from seven of 10 patients tested bound to the surface of fetal myocytes from hearts aged 9.4 to 14.9 weeks of gestation, and also to the myocytes from the pathological adult heart but not to normal adult hearts. Preincubation with La antigen abolished the binding of anti-La antibodies to the surface of adult heart myocytes with dilated cardiomyopathy, and pre-incubation with mouse laminin could partially block this binding. These results suggest that molecular mimicry between laminin and La, but not Ro52, may act as a target for specific maternal autoantibodies, and contribute to the pathogenesis of CHB at a critical stage during fetal cardiac development.

The Pro-angiogenic Cytokine Pleiotrophin Potentiates Cardiomyocyte Apoptosis through Inhibition of Endogenous AKT/PKB Activity

Pleiotrophin is a development-regulated cytokine and growth factor that can promote angiogenesis, cell proliferation, or differentiation, and it has been reported to have neovasculogenic effects in damaged heart. Developmentally, it is prominently expressed in fetal and neonatal hearts, but it is minimally expressed in normal adult heart. Conversely, we show in a rat model of myocardial infarction and in human dilated cardiomyopathy that pleiotrophin is markedly up-regulated. To elucidate the effects of pleiotrophin on cardiac contractile cells, we employed primary cultures of rat neonatal and adult cardiomyocytes. We show that pleiotrophin is released from cardiomyocytes in vitro in response to hypoxia and that the addition of recombinant pleiotrophin promotes caspase-mediated genomic DNA fragmentation in a dose- and time-dependent manner. Functionally, it potentiates the apoptotic response of neonatal cardiomyocytes to hypoxic stress and to ultraviolet irradiation and of adult cardiomyocytes to hypoxia-reoxygenation. Moreover, UV-induced apoptosis in neonatal cardiomyocytes can be partially inhibited by small interfering RNA-mediated knockdown of endogenous pleiotrophin. Mechanistically, pleiotrophin antagonizes IGF-1 associated Ser-473 phosphorylation of AKT/PKB, and it concomitantly decreases both BAD and GSK3beta phosphorylation. Adenoviral expression of constitutively active AKT and lithium chloride-mediated inhibition of GSK3beta reduce the potentiated programmed cell death elicited by pleiotrophin. These latter data indicate that pleiotrophin potentiates cardiomyocyte cell death, at least partially, through inhibition of AKT signaling. In conclusion, we have uncovered a novel function for pleiotrophin on heart cells following injury. It fosters cardiomyocyte programmed cell death in response to pro-apoptotic stress, which may be critical to myocardial injury repair.

Myocardial hypertrophy in the absence of external stimuli is induced by angiogenesis in mice

Although studies have suggested a role for angiogenesis in determining heart size during conditions demanding enhanced cardiac performance, the role of EC mass in determining the normal organ size is poorly understood. To explore the relationship between cardiac vasculature and normal heart size, we generated a transgenic mouse with a regulatable expression of the secreted angiogenic growth factor PR39 in cardiomyocytes. A significant change in adult mouse EC mass was apparent by 3 weeks following PR39 induction. Heart weight; cardiomyocyte size; vascular density normalization; upregulation of hypertrophy markers including atrial natriuretic factor, beta-MHC, and GATA4; and activation of the Akt and MAP kinase pathways were observed at 6 weeks post-induction. Treatment of PR39-induced mice with the eNOS inhibitor L-NAME in the last 3 weeks of a 6-week stimulation period resulted in a significant suppression of heart growth and a reduction in hypertrophic marker expression. Injection of PR39 or another angiogenic growth factor, VEGF-B, into murine hearts during myocardial infarction led to induction of myocardial hypertrophy and restoration of myocardial function. Thus stimulation of vascular growth in normal adult mouse hearts leads to an increase in cardiac mass.

Big Cells, Little Cells, Stem Cells

See related article, pages 536–544 As of August 2003, the prevailing dogma posited that the heart is a terminally-differentiated organ, that the myocytes we have now are those we were born with (only bigger), and that the best hope for treating cardiac injury is to limit the insult (eg, by prompt reperfusion) or to block secondary maladaptive pathways. Because of pioneering work from various laboratories,1–3 the concept of cardiac plasticity has come to replace the old static-organ dogma. We now believe that the normal adult heart is the net result of a slow rate of cell loss balanced by ongoing cell replacement. The predominant mode of cell loss is via apoptosis, reflecting the normal processes of wear and tear. Cell replacement, on the other hand, taps into a reservoir of resident cardiac stem cells (CSCs). Such CSCs lurk in niches distributed inhomogeneously throughout the heart,4 and, when activated, enter the mitotic cycle to eventually become cardiomyocytes and vascular cells. Back-of-the-envelope calculations, based on the observed frequencies of apoptotic events and mitotic figure …

Fibrosis, not cell size, delineates beta-myosin heavy chain reexpression during cardiac hypertrophy and normal aging in vivo.

Reexpression of the fetally expressed beta-myosin heavy chain (beta-MHC) gene is a well documented marker of pathological cardiac hypertrophy and normal aging in many experimental models. To gain insights into factors affecting this reexpression of beta-MHC within the complex anatomical structure of the heart, we investigated the spatial pattern of its expression at the level of single cells during aging and hypertrophy. We generated mice that express yellow fluorescent protein fused to the N terminus of the beta-MHC and examined its expression pattern during normal aging and in mice with hypertrophy induced by constitutive expression of a renin transgene. The localization of fibrosis within the hearts also was determined by using a fluorescent lectin. The results show that reexpression of beta-MHC occurs in discrete subsets of myocytes within the subendocardium rather than uniformly throughout the heart, that beta-MHC induction is not an obligatory consequence of cellular hypertrophy, and that beta-MHC-expressing cells in the normal aging heart and the hypertrophic heart are distributed predominantly in clusters within and surrounding foci of fibrosis. We conclude that beta-MHC gene expression in the normal aging adult and hypertrophic mouse heart is a marker of fibrosis rather than of cellular hypertrophy.

GDF15/MIC-1 Functions As a Protective and Antihypertrophic Factor Released From the Myocardium in Association With SMAD Protein Activation

Here we identified growth-differentiation factor 15 (GDF15) (also known as MIC-1), a secreted member of the transforming growth factor (TGF)-beta superfamily, as a novel antihypertrophic regulatory factor in the heart. GDF15 is not expressed in the normal adult heart but is induced in response to conditions that promote hypertrophy and dilated cardiomyopathy. To elucidate the function of GDF15 in the heart, we generated transgenic mice with cardiac-specific overexpression. GDF15 transgenic mice were normal but were partially resistant to pressure overload-induced hypertrophy. Expression of GDF15 in neonatal cardiomyocyte cultures by adenoviral-mediated gene transfer antagonized agonist-induced hypertrophy in vitro. Transient expression of GDF15 outside the heart by intravenous adenoviral delivery, or by direct injection of recombinant GDF15 protein, attenuated ventricular dilation and heart failure in muscle lim protein gene-targeted mice through an endocrine effect. Conversely, examination of Gdf15 gene-targeted mice showed enhanced cardiac hypertrophic growth following pressure overload stimulation. Gdf15 gene-targeted mice also demonstrated a pronounced loss in ventricular performance following only 2 weeks of pressure overload stimulation, whereas wild-type controls maintained function. Mechanistically, GDF15 stimulation promoted activation of SMAD2/3 in cultured neonatal cardiomyocytes. Overexpression of SMAD2 attenuated cardiomyocyte hypertrophy similar to GDF15 treatment, whereas overexpression of the inhibitory SMAD proteins, SMAD6/7, reversed the antihypertrophic effects of GDF15. These results identify GDF15 as a novel autocrine/endocrine factor that antagonizes the hypertrophic response and loss of ventricular performance, possibly through a mechanism involving SMAD proteins.

CD117-positive cells and mast cells in adult human cardiac valves—observations and implications for the creation of bioengineered grafts

Background There is no report to date of stem cells in human cardiac valves. We examined their possible presence, number, and distribution in valves removed at cardiac surgery from patients with a variety of underlying valve pathologies. Methods Grossly normal aortic and mitral valves were obtained from live heart transplant patients. Surgically excised valves with rheumatic mitral stenosis, aortic valve age-related degeneration, aortic valve changes of aortoannular ectasia, and mitral valves with myxomatous degeneration were studied. Immunohistochemical and histochemical studies were performed on sequential valve sections, including hematoxylin and eosin, hematoxylin phloxine saffron, Movat pentachrome, toluidine blue, CD31, CD34, and CD117. Results There were small clusters of CD117-positive cells in the fibrosa and spongiosa of mitral and aortic valves from all groups of valves. Sequential sectioning and staining showed that almost all of these cells were mast cells. However, in the mitral myxomatous valves and the mitral rheumatic valves, there were rare CD117-positive cells that did not have corresponding toluidine blue staining and thus could be valve mesenchymal stem cells. Conclusions Most of the CD117-positive cells in normal and diseased adult heart valves are mast cells. These valve cells could play a role in valve pathology and injury. A very small number of possible valve stem cells were also identified. It is unlikely that these valve stem cells are sufficient in number to allow isolation and expansion for tissue engineering purposes.