Developmental expression of fibroblast growth factor receptor-1 (cek-1; flg) during heart development

Optimizing Cardiomyocyte Differentiation: Comparative Analysis of Bone Marrow and Adipose-Derived Mesenchymal Stem Cells in Rats Using 5-Azacytidine and Low-Dose FGF and IGF Treatment

Mesenchymal stem cells (MSCs) exhibit multipotency, self-renewal, and immune-modulatory properties, making them promising in regenerative medicine, particularly in cardiovascular treatments. However, optimizing the MSC source and induction method of cardiac differentiation is challenging. This study compares the cardiomyogenic potential of bone marrow (BM)-MSCs and adipose-derived (AD)-MSCs using 5-Azacytidine (5-Aza) alone or combined with low doses of Fibroblast Growth Factor (FGF) and Insulin-like Growth Factor (IGF). BM-MSCs and AD-MSCs were differentiated using two protocols: 10 μmol 5-Aza alone and 10 μmol 5-Aza with 1 ng/mL FGF and 10 ng/mL IGF. Morphological, transcriptional, and translational analyses, along with cell viability assessments, were performed. Both the MSC types exhibited similar morphological changes; however, AD-MSCs achieved 70-80% confluence faster than BM-MSCs. Surface marker profiling confirmed CD29 and CD90 positivity and CD45 negativity. The differentiation protocols led to cell flattening and myotube formation, with earlier differentiation in AD-MSCs. The combined protocol reduced cell mortality in BM-MSCs and enhanced the expression of cardiac markers (MEF2c, Troponin I, GSK-3β), particularly in BM-MSCs. Immunofluorescence confirmed cardiac-specific protein expression in all the treated groups. Both MSC types exhibited the expression of cardiac-specific markers indicative of cardiomyogenic differentiation, with the combined treatment showing superior efficiency for BM-MSCs.

Genome-wide association study of copy number variations with shank traits in a F2 crossbred chicken population

Copy number variations (CNVs) are large-scale changes in the DNA sequence that can affect the genetic structure and phenotype of an organism. The purpose of this study was to investigate the existing CNVs and their associations with the shank diameter (ShD) and shank length (ShL) traits using data from an F2 crossbred chicken population. To carry out the study, 312 chickens were genotyped using the Illumina 60k SNP Beadchip. The shank traits of the birds were measured from day 1 to 12 weeks of age. penncnv and cnvruler tools were used to find copy numbers and regions with copy number changes (CNVR), respectively. The CNVRanger package was used to perform a genome-wide association study between shank traits and CNVs. Gene ontology research in CNVRs was carried out using the david database. In this investigation, 966 CNVs and 606 regions with copy number changes were discovered. The copy number states and variations were randomly distributed along the length of the autosomal chromosomes. Weeks 1-4, 9 and 12 of growth revealed a significant association of copy number variations with shank traits, false discovery rate (FDR-corrected p-value < 0.01), and the majority of CNVs that were statistically significant were found on chromosomes 1-3. These CNV segments are nearby genes such as KCNJ12, FGF6 and MYF5, which are fundamental to growth and development. In addition, gene set analyses revealed terms related to muscle physiology, regulation of cellular processes and potassium channels.

Cardiac Development and Factors Influencing the Development of Congenital Heart Defects (CHDs): Part I

The traditional description of cardiac development involves progression from a cardiac crescent to a linear heart tube, which in the phase of transformation into a mature heart forms a cardiac loop and is divided with the septa into individual cavities. Cardiac morphogenesis involves numerous types of cells originating outside the initial cardiac crescent, including neural crest cells, cells of the second heart field origin, and epicardial progenitor cells. The development of the fetal heart and circulatory system is subject to regulatation by both genetic and environmental processes. The etiology for cases with congenital heart defects (CHDs) is largely unknown, but several genetic anomalies, some maternal illnesses, and prenatal exposures to specific therapeutic and non-therapeutic drugs are generally accepted as risk factors. New techniques for studying heart development have revealed many aspects of cardiac morphogenesis that are important in the development of CHDs, in particular transposition of the great arteries.

Cardiac Differentiation of Mesenchymal Stem Cells: Impact of Biological and Chemical Inducers

Cardiovascular disorders (CVDs) are the leading cause of global death, widely occurs due to irreparable loss of the functional cardiomyocytes. Stem cell-based therapeutic approaches, particularly the use of Mesenchymal Stem Cells (MSCs) is an emerging strategy to regenerate myocardium and thereby improving the cardiac function after myocardial infarction (MI). Most of the current approaches often employ the use of various biological and chemical factors as cues to trigger and modulate the differentiation of MSCs into the cardiac lineage. However, the recent advanced methods of using specific epigenetic modifiers and exosomes to manipulate the epigenome and molecular pathways of MSCs to modify the cardiac gene expression yield better profiled cardiomyocyte like cells in vitro. Hitherto, the role of cardiac specific inducers triggering cardiac differentiation at the cellular and molecular level is not well understood. Therefore, the current review highlights the impact and recent trends in employing biological and chemical inducers on cardiac differentiation of MSCs. Thereby, deciphering the interactions between the cellular microenvironment and the cardiac inducers will help us to understand cardiomyogenesis of MSCs. Additionally, the review also provides an insight on skeptical roles of the cell free biological factors and extracellular scaffold assisted mode for manipulation of native and transplanted stem cells towards translational cardiac research.

Physiological role of FGF signaling in growth and remodeling of developing cardiovascular system

Fibroblast growth factor (FGF) signaling plays an important role during embryonic induction and patterning, as well as in modulating proliferative and hypertrophic growth in fetal and adult organs. Hemodynamically induced stretching is a powerful physiological stimulus for embryonic myocyte proliferation. The aim of this study was to assess the effect of FGF2 signaling on growth and vascularization of chick embryonic ventricular wall and its involvement in transmission of mechanical stretch-induced signaling to myocyte growth in vivo. Myocyte proliferation was significantly higher at the 48 h sampling interval in pressure-overloaded hearts. Neither Western blotting, nor immunohistochemistry performed on serial paraffin sections revealed any changes in the amount of myocardial FGF2 at that time point. ELISA showed a significant increase of FGF2 in the serum. Increased amount of FGF2 mRNA in the heart was confirmed by real time PCR. Blocking of FGF signaling by SU5402 led to decreased myocyte proliferation, hemorrhages in the areas of developing vasculature in epicardium and digit tips. FGF2 synthesis is increased in embryonic ventricular cardiomyocytes in response to increased stretch due to pressure overload. Inhibition of FGF signaling impacts also vasculogenesis, pointing to partial functional redundancy in paracrine control of cell proliferation in the developing heart.

Cardiomyogenic differentiation of human sternal bone marrow mesenchymal stem cells using a combination of basic fibroblast growth factor and hydrocortisone

The alarming rate of increase in myocardial infarction and marginal success in efforts to regenerate the damaged myocardium through conventional treatments creates an exceptional avenue for cell-based therapy. Adult bone marrow mesenchymal stem cells (MSCs) can be differentiated into cardiomyocytes, by treatment with 5-azacytidine, thus, have been anticipated as a therapeutic tool for myocardial infarction treatment. In this study, we investigated the ability of basic fibroblastic growth factor (bFGF) and hydrocortisone as a combined treatment to stimulate the differentiation of MSCs into cardiomyocytes. MSCs were isolated from sternal marrow of patients undergoing heart surgery (CABG). The isolated cells were initially monitored for the growth pattern, followed by characterization using ISCT recommendations. Cells were then differentiated using a combination of bFGF and hydrocortisone and evaluated for the expression of characteristic cardiac markers such as CTnI, CTnC, and Cnx43 at protein level using immunocytochemistry and flow cytometry, and CTnC and CTnT at mRNA level. The expression levels and pattern of the cardiac markers upon analysis with ICC and qRT-PCR were similar to that of 5-azacytidine induced cells and cultured primary human cardiomyocytes. However, flow cytometric evaluation revealed that induction with bFGF and hydrocortisone drives MSC differentiation to cardiomyocytes with a marginally higher efficiency. These results indicate that combination treatment of bFGF and hydrocortisone can be used as an alternative induction method for cardiomyogenic differentiation of MSCs for future clinical applications.

Reciprocal repression between Fgf8 and miR-133 regulates cardiac induction through Bmp2 signaling

This data article contains complementary figures and results related to the research article entitled “Negative Fgf8-Bmp2 feed-back is controlled by miR-130 during early cardiac specification” [15], which reveals what specific role miR-130 plays during the cardiac induction process. This study evidenced miR-130 a putative microRNA that targets Erk1/2 (Mapk1) 3′UTR- as a necessary linkage in the control of Fgf8 signaling, mediated by Bmp2. Thus, miR-130 regulates a negative Fgf8-Bmp2 feed-back loop responsible to achieve early cardiac specification. A significant aspect supporting our conclusions is given by the expression pattern of miR-130 during early cardiac specification, as well as by those results obtained after the designed experimental procedures. The data presented here reveal that miR-133 is also expressed within the precardiac areas during early cardiogenesis, pattern which is comparable to that of FGFR1, receptor involved in the Fgf8/ERK signaling pathway. Interestingly, our miR-133 overexpression experiments resulted in a decrease of Fgf8 expression, whereas we observed an increase of Bmp2 and subsequently of cardiac specific markers Nkx-2.5 and Gata4. Additionally, our loss-of-function experiments -through Fgf8 siRNA electroporation- showed an increase of miR-133 expression. Finally, after our Bmp2 experiments, we observed that miR-133 is upstream-regulated by Bmp2. All those results suggest that miR-133 also constitutes a crucial linkage in the crosstalk between Fgf8 and Bmp2 signaling by regulating the Fgf8/ERK pathway during cardiac induction.

Negative Fgf8-Bmp2 feed-back is regulated by miR-130 during early cardiac specification

It is known that secreted proteins from the anterior lateral endoderm, FGF8 and BMP2, are involved in mesodermal cardiac differentiation, which determines the first cardiac field, defined by the expression of the earliest specific cardiac markers Nkx-2.5 and Gata4. However, the molecular mechanisms responsible for early cardiac development still remain unclear. At present, microRNAs represent a novel layer of complexity in the regulatory networks controlling gene expression during cardiovascular development. This paper aims to study the role of miR130 during early cardiac specification. Our model is focused on developing chick at gastrula stages. In order to identify those regulatory factors which are involved in cardiac specification, we conducted gain- and loss-of-function experiments in precardiac cells by administration of Fgf8, Bmp2 and miR130, through in vitro electroporation technique and soaked beads application. Embryos were subjected to in situ hybridization, immunohistochemistry and qPCR procedures. Our results reveal that Fgf8 suppresses, while Bmp2 induces, the expression of Nkx-2.5 and Gata4. They also show that Fgf8 suppresses Bmp2, and vice versa. Additionally, we observed that Bmp2 regulates miR-130 -a putative microRNA that targets Erk1/2 (Mapk1) 3'UTR, recognizing its expression in precardiac cells which overlap with Erk1/2 pattern. Finally, we evidence that miR-130 is capable to inhibit Erk1/2 and Fgf8, resulting in an increase of Bmp2, Nkx-2.5 and Gata4. Our data present miR-130 as a necessary linkage in the control of Fgf8 signaling, mediated by Bmp2, establishing a negative feed-back loop responsible to achieve early cardiac specification.

Cellular and molecular basis of liver development

Liver is a prime organ responsible for synthesis, metabolism, and detoxification. The organ is endodermal in origin and its development is regulated by temporal, complex, and finely balanced cellular and molecular interactions that dictate its origin, growth, and maturation. We discuss the relevance of endoderm patterning, which truly is the first step toward mapping of domains that will give rise to specific organs. Once foregut patterning is completed, certain cells within the foregut endoderm gain competence in the form of expression of certain transcription factors that allow them to respond to certain inductive signals. Hepatic specification is then a result of such inductive signals, which often emanate from the surrounding mesenchyme. During hepatic specification bipotential hepatic stem cells or hepatoblasts become apparent and undergo expansion, which results in a visible liver primordium during the stage of hepatic morphogenesis. Hepatoblasts next differentiate into either hepatocytes or cholangiocytes. The expansion and differentiation is regulated by cellular and molecular interactions between hepatoblasts and mesenchymal cells including sinusoidal endothelial cells, stellate cells, and also innate hematopoietic elements. Further maturation of hepatocytes and cholangiocytes continues during late hepatic development as a function of various growth factors. At this time, liver gains architectural novelty in the form of zonality and at cellular level acquires polarity. A comprehensive elucidation of such finely tuned developmental cues have been the basis of transdifferentiation of various types of stem cells to hepatocyte-like cells for purposes of understanding health and disease and for therapeutic applications.

Myocyte proliferation in the developing heart

Regulation of organ growth is critical during embryogenesis. At the cellular level, mechanisms controlling the size of individual embryonic organs include cell proliferation, differentiation, migration, and attrition through cell death. All these mechanisms play a role in cardiac morphogenesis, but experimental studies have shown that the major determinant of cardiac size during prenatal development is myocyte proliferation. As this proliferative capacity becomes severely restricted after birth, the number of cell divisions that occur during embryogenesis limits the growth potential of the postnatal heart. We summarize here current knowledge concerning regional control of myocyte proliferation as related to cardiac morphogenesis and dysmorphogenesis. There are significant spatial and temporal differences in rates of cell division, peaking during the preseptation period and then gradually decreasing toward birth. Analysis of regional rates of proliferation helps to explain the mechanics of ventricular septation, chamber morphogenesis, and the development of the cardiac conduction system. Proliferation rates are influenced by hemodynamic loading, and transduced by autocrine and paracrine signaling by means of growth factors. Understanding the biological response of the developing heart to such factors and physical forces will further our progress in engineering artificial myocardial tissues for heart repair and designing optimal treatment strategies for congenital heart disease.

Biological functions of the low and high molecular weight protein isoforms of fibroblast growth factor-2 in cardiovascular development and disease

Fibroblast growth factor 2 (FGF2) consists of multiple protein isoforms (low molecular weight, LMW, and high molecular weight, HMW) produced by alternative translation from the Fgf2 gene. These protein isoforms are localized to different cellular compartments, indicating unique biological activity. FGF2 isoforms in the heart have distinct roles in many pathological circumstances in the heart including cardiac hypertrophy, ischemia-reperfusion injury, and atherosclerosis. These studies suggest distinct biological activities of FGF2 LMW and HMW isoforms both in vitro and in vivo. Yet, due to the limitations that only the recombinant FGF2 LMW isoform is readily available and that the FGF2 antibody is nonspecific with regards to its isoforms, much remains to be determined regarding the role(s) of the FGF2 LMW and HMW isoforms in cellular behavior and in cardiovascular development and pathophysiology. This review summarizes the activities of LMW and HMW isoforms of FGF2 in cardiovascular development and disease.

Origin and fate of cardiac mesenchyme

The development of the embryonic heart is dependent upon the generation and incorporation of different mesenchymal subpopulations that derive from intra- and extra-cardiac sources, including the endocardium, epicardium, neural crest, and second heart field. Each of these populations plays a crucial role in cardiovascular development, in particular in the formation of the valvuloseptal apparatus. Notwithstanding shared mechanisms by which these cells are generated, their fate and function differ profoundly by their originating source. While most of our early insights into the origin and fate of the cardiac mesenchyme has come from experimental studies in avian model systems, recent advances in transgenic mouse technology has enhanced our ability to study these cell populations in the mammalian heart. In this article, we will review the current understanding of the role of cardiac mesenchyme in cardiac morphogenesis and discuss several new paradigms based on recent studies in the mouse.

Fibroblast growth factor receptor 1 gene expression is required for cardiomyocyte proliferation and is repressed by Sp3

Fibroblast growth factor receptor 1 (FGFR1) is the only high-affinity FGFR in the vertebrate myocardium. FGFR1 is a tyrosine kinase receptor and has a non-redundant role in proliferation and differentiation of cardiomyocytes during embryogenesis. Results presented here demonstrate that FGFR1 gene expression declines as neonatal cardiomyocytes develop into adult cardiomyocytes. Furthermore, silencing FGFR1 gene expression reduced neonatal cardiomyocyte proliferation, indicating that FGFR1 gene expression is required for the optimal proliferative capacity of cardiomyocytes. To determine the mechanism that governs FGFR1 gene expression in cardiomyocytes, sequence analysis of the proximal mouse FGFR1 promoter identified a potential binding site for Sp transcription factors. Mutation of this site increased FGFR1 promoter activity compared to the wild-type promoter, indicating the presence of a negative transcriptional regulator of the FGFR1 promoter at this site in cardiomyocytes. Sp3 expression in neonatal cardiomyocytes and Drosophila SL2 cells reduced FGFR1 promoter activity in a dose-dependent manner. Western blots and immunocytochemistry indicated that Sp3 was present in the nuclear and cytoplasmic compartments of neonatal cardiomyocytes. Chromatin-immunoprecipitation studies verified that endogenous Sp3 in cardiomyocytes interacts with the FGFR1 promoter. Transient chromatin-immunoprecipitation studies using wild-type and mutated FGFR1 promoter constructs in SL2 cells identified the specific Sp3 binding site within the FGFR1 promoter. These studies implicate Sp3 as a negative transcriptional regulator of FGFR1 promoter activity in cardiomyocytes and as a suppressor of cardiomyocyte proliferation.

Sp1 is required for transcriptional activation of the fibroblast growth factor receptor 1 gene in neonatal cardiomyocytes

Fibroblast growth factor receptor 1 (FGFR1) is the predominant FGFR in cardiac tissue and regulates proliferation, differentiation, and maintenance of normal myocardium. During development of cardiac tissue, FGFR1 gene expression regulates cardiomyocyte proliferation. The focus of this study was to determine the molecular mechanism of transcriptional activation of the FGFR1 gene in proliferating neonatal cardiomyocytes. Analysis of DNA sequence of the FGFR1 gene identified three potential Sp factor binding sites located at 49 bp, 68 bp, and 100 bp upstream from the 3' end of the promoter segment. Mutation of each of these sites resulted in a significant decline in FGFR1 promoter activity compared to wild type promoter activity, and combinatorial mutation of all three sites completely abrogated promoter activity to background levels. In addition, overexpression of Sp1 in neonatal cardiomyocytes resulted in a dose-dependent increase in wild type FGFR1 promoter activity. However, Sp1-mediated up-regulation of promoter activity was abrogated when all three Sp interacting sites were mutated. Chromatin immunoprecipitation (ChIP) assays were used to demonstrate direct interactions of Sp1 with the proximal promoter region of the FGFR1 gene in neonatal cardiomyocytes. ChIP assays using Drosophila Schneider Line 2 (SL2) cells transiently transfected with wild type or mutant FGFR1 promoter constructs verified the direct interaction between Sp1 and the three Sp1 interacting sites of the promoter. Western blot analyses indicated that Sp1 was present in cytoplasmic and nuclear extracts of neonatal myocardium. These results indicate that Sp1 is a necessary positive regulator of FGFR1 gene transcription in neonatal cardiomyocytes.

Early temporal-specific responses and differential sensitivity to lithium and Wnt-3A exposure during heart development

Members of both Wnt and bone morphogenetic protein (BMP) families of signaling molecules are important in heart development. We previously demonstrated that beta-catenin, a key downstream intermediary of the canonical Wnt signaling pathway, delineates the dorsal boundary of the cardiac compartments in an anteroposterior progression. We hypothesized the progression involves canonical Wnt signaling and reflects development of the primary body axis of the embryo. A similar anteroposterior signaling wave leading to cardiac cell specification involves inductive signaling by BMP-2 synthesized by the underlying endoderm in anterior bilateral regions. Any molecule that disrupts the normal balance of Wnt and BMP concentrations within the heart field may be expected to affect early heart development. The canonical Wnt signaling step mimicked by lithium involves inactivation of glycogen synthase kinase-3beta (GSK-3beta; Klein and Melton [1996] Proc. Natl. Acad. Sci. U. S. A. 93:8455-8459). We show that lithium, Wnt-3A, and an inhibitor of GSK-3beta, SB415286, affect early heart development at the cardiac specification stages. We demonstrate that normal expression patterns of key signaling molecules as Notch-1 and Dkk-1 are altered in the anterior mesoderm within the heart fields by a one-time exposure to lithium, or by noggin inhibition of BMP, at Hamburger and Hamilton (HH) stage 3 during chick embryonic development. The severity of developmental defects is greatest with exposure to lithium or Wnt-3A at HH stage 3 and decreases at HH stage 4. Taken together, our results demonstrate that there are temporal-specific responses and differential sensitivities to lithium/Wnt-3A exposure during early heart development.

Different thresholds of fibroblast growth factors pattern the ventral foregut into liver and lung

Cell fate and morphogenesis within the embryo is dependent upon secreted molecules that transduce signals between neighboring tissues. Reciprocal mesenchymal-epithelial interactions have proven essential during branching morphogenesis and cell differentiation within the lung; however, the interactions that result in lung specification from the foregut endoderm, prior to lung bud formation, are poorly understood. In this study, we investigate the tissue requirements and signals necessary for specification of a pulmonary cell fate using embryo tissue explants. We show that NKX2.1, an early transcription factor crucial for lung development, is expressed in the ventral foregut endoderm shortly after albumin and Pdx1, early markers of the liver and pancreas lineages, respectively. Similar to hepatic specification, direct contact of cardiac mesoderm with ventral endoderm is required to induce in vitro expression of NKX2.1 and downstream lung target genes including surfactant protein C and Clara cell secretory protein. In the absence of cardiac mesoderm, ventral foregut endoderm explants respond to exogenous fibroblast growth factor (FGF) 1 and FGF2 in a dose-dependent manner, with lower concentrations activating liver specific genes and higher concentrations activating lung specific genes. This signaling appears to be instructive, as the prospective dorsal midgut endoderm, which predominantly gives rise to the intestinal tract, is competent to respond to FGFs by inducing NKX2.1. Furthermore, the temporal expression and selective inhibition of FGF receptors 1 and 4 present within the endoderm implies that signaling through FGFR4 is involved in specifying lung versus liver. Together, the findings suggest that a concentration threshold of FGFs emanating from the cardiac mesoderm are involved in patterning the foregut endoderm.

Avian precardiac endoderm/mesoderm induces cardiac myocyte differentiation in murine embryonic stem cells

The ability to regenerate damaged myocardium with tissue derived from embryonic stem (ES) cells is currently undergoing extensive investigation. As a prerequisite to transplantation therapy, strategies must be developed to induce ES cells to the cardiac phenotype. Toward this end, cues from mechanisms of embryonic induction have been exploited, based on previous findings that anterior lateral endoderm (precardiac endoderm) from gastrulation-stage chick embryos potently induces cardiac myocyte differentiation in both precardiac and nonprecardiac mesoderm. Hypothesizing that avian precardiac endoderm acting as feeder/inducer cells would induce high percentage conversion of murine ES (mES) cells into cardiac myocytes, it was observed that the majority (approximately 65%) of cocultured ES cell-derived embryoid bodies (EBs) were enriched in cardiac myocytes and exhibited rhythmic contractions. By contrast, mouse EBs cultured alone, or on feeder layers of mouse embryonic fibroblasts or avian nonprecardiac posterior endoderm, contained only 7% to 16% cardiac myocytes while exhibiting a relatively low incidence (<10%) of beating. When mES cells were cocultured with a bilayer of explanted precardiac endoderm/mesoderm, the incidence of rhythmically contractile EBs increased to 100%. To verify that the rhythmically contractile cells were derived from murine ES cells, cell-free medium conditioned by avian precardiac endoderm/mesoderm was used to induce myocyte differentiation in a mES cell-line containing a nuclear LacZ reporter marker gene under control of the cardiac-specific alpha-myosin heavy chain promoter, resulting in rhythmic contractility in 92% of EBs in which the majority of cells (average=86%) were identified as cardiac myocytes. The inductive efficacy of medium conditioned by avian precardiac endoderm/mesoderm may provide an opportunity to biochemically define factors that induce cardiac myocyte differentiation in ES cells. The full text of this article is available online at http://circres.ahajournals.org.

Fibroblast growth factor (FGF)-4 can induce proliferation of cardiac cushion mesenchymal cells during early valve leaflet formation

While much has been learned about how endothelial cells transform to mesenchyme during cardiac cushion formation, there remain fundamental questions about the developmental fate of cushions. In the present work, we focus on the growth and development of cushion mesenchyme. We hypothesize that proliferative expansion and distal elongation of cushion mesenchyme mediated by growth factors are the basis of early valve leaflet formation. As a first step to test this hypothesis, we have localized fibroblast growth factor (FGF)-4 protein in cushion mesenchymal cells at the onset of prevalve leaflet formation in chick embryos (Hamburger and Hamilton stage 20-25). Ligand distribution was correlated with FGF receptor (FGFR) expression. In situ hybridization data indicated that FGFR3 mRNA was confined to the endocardial rim of the atrioventricular (AV) cushion pads, whereas FGFR2 was expressed exclusively in cushion mesenchymal cells. FGFR1 expression was detected in both endocardium and cushion mesenchyme as well as in myocardium. To determine whether the FGF pathways play regulatory roles in cushion mesenchymal cell proliferation and elongation into prevalvular structure, FGF-4 protein was added to the cushion mesenchymal cells explanted from stage 24-25 chick embryos. A significant increase in proliferative ability was strongly suggested in FGF-4-treated mesenchymal cells as judged by the incorporation of 5'-bromodeoxyuridine (BrdU). To determine whether cushion cells responded similarly in vivo, a replication-defective retrovirus encoding FGF-4 with the reporter, bacterial beta-galactosidase was microinjected into stage 18 chick cardiac cushion mesenchyme along the inner curvature where AV and outflow cushions converge. As compared with vector controls, overexpression of FGF-4 clearly induced expansion of cushion mesenchyme toward the lumen. To further test the proliferative effect of FGF-4 in cardiac cushion expansion in vivo (ovo), FGF-4 protein was microinjected into stage 18 chick inner curvature. An assay for BrdU incorporation indicated a significant increase in proliferative ability in FGF-4 microinjected cardiac cushion mesenchyme as compared with BSA-microinjected controls. Together, these results suggest a role of FGF-4 for cardiac valve leaflet formation through proliferative expansion of cushion mesenchyme.

Inhibition of the cardiac alpha-actin gene in embryonic cardiac myocytes by dominant-negative serum response factor

Serum response factor (SRF), a transcription factor ubiquitously involved in the processes of cellular proliferation and differentiation, has been implicated in cardiac and skeletal muscle development because of its strong expression in embryonic muscle lineages, and its necessity for the transcription of transiently transfected muscle genes that contain SRF binding sites. This study was designed to ascertain whether SRF is required for the expression of an endogenous SRF-dependent gene during differentiation of early embryonic cardiac myocytes by introducing a dominant-negative SRF construct via retroviral delivery. Although no effect on overt cellular differentiation was detected, semi-quantitative RT-PCR revealed that expression of the SRF-dependent gene cardiac alpha-actin was inhibited, whereas expression of the non-SRF-dependent genes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cardiac troponin-C was unaffected. No effect on myocyte proliferation was detected. Curiously, immunohistochemical localization of SRF protein suggested that whereas endogenous SRF was homogeneously dispersed throughout the cytoplasm and nucleus, the dominant-negative SRF protein was concentrated in the nucleus. These results extend previous findings using transiently transfected genes to the endogenous level, indicating that SRF is required for the full expression of muscle genes that contain SRF binding sites during cardiac myocyte differentiation.

Molecular embryogenesis of the heart

Development of the heart is a complex process involving primary and secondary heart fields that are set aside to generate myocardial and endocardial cell lineages. The molecular inductions that occur in the primary heart field appear to be recapitulated in induction and myocardial differentiation of the secondary heart field, which adds the conotruncal segments to the primary heart tube. While much is now known about the initial steps and factors involved in induction of myocardial differentiation, little is known about induction of endocardial development. Many of the genes expressed by nascent myocardial cells, which then become committed to a specific heart segment, have been identified and studied. In addition to the heart fields, several other "extracardiac" cell populations contribute to the fully functional mature heart. Less is known about the genetic programs of extracardiac cells as they enter the heart and take part in cardiogenesis. The molecular/genetic basis of many congenital cardiac defects has been elucidated in recent years as a result of new insights into the molecular control of developmental events.

Regulatory phases of early liver development: paradigms of organogenesis

Genetic analysis, embryonic tissue explantation and in vivo chromatin studies have together identified the distinct regulatory steps that are necessary for the development of endoderm into a bud of liver tissue and, subsequently, into an organ. In this review, I discuss the acquisition of competence to express liver-specific genes by the endoderm, the control of early hepatic growth, the coordination of hepatic and vascular development and the cell differentiation that is necessary to generate a functioning liver. The regulatory mechanisms that underlie these phases are common to the development of many organ systems and might be recapitulated or disrupted during stem-cell differentiation and adult tissue pathogenesis.

A sensory neuron subpopulation with unique sequential survival dependence on nerve growth factor and basic fibroblast growth factor during development

We characterized a subpopulation of dorsal root ganglion (DRG) sensory neurons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially required nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neurons required NGF, but not bFGF. NGF continued to promote their survival, although less potently, up to postnatal day 2 (P2). Conversely, at P5, they needed bFGF but not NGF, with either factor having similar effects at P2. This trophic switch was unique to that DRG neuronal group. In addition, neither neurotrophin-3 (NT-3) nor brain-derived neurotrophic factor influenced their survival during embryonic and postnatal stages, respectively. The expression of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in trophic requirement. No single P-neuron appeared to coexpress both Trk-A and flg. In contrast, all of them coexpressed flg and substance P, providing a specific marker of these cells. Immunosuppression of bFGF in newborn animals greatly reduced their number, suggesting that the factor was required in vivo. bFGF was present in the DRG and spinal cord, as well as in skeletal muscle, the peripheral projection site of P-neurons, as revealed by tracer DiIC(18)3. The lack of requirement of NT-3 for survival and immunoreactivity for the neurofilament of 200 kDa distinguished them from muscle proprioceptors, suggesting that they are likely to be unmyelinated muscle fibers. Collectively, their properties indicate that P-neurons constitute a distinct subpopulation of sensory neurons for which the function may be modulated by enkephalins.

Role of heparan sulfate as a tissue-specific regulator of FGF-4 and FGF receptor recognition

FGF signaling uses receptor tyrosine kinases that form high-affinity complexes with FGFs and heparan sulfate (HS) proteoglycans at the cell surface. It is hypothesized that assembly of these complexes requires simultaneous recognition of distinct sulfation patterns within the HS chain by FGF and the FGF receptor (FR), suggesting that tissue-specific HS synthesis may regulate FGF signaling. To address this, FGF-2 and FGF-4, and extracellular domain constructs of FR1-IIIc (FR1c) and FR2-IIIc (FR2c), were used to probe for tissue-specific HS in embryonic day 18 mouse embryos. Whereas FGF-2 binds HS ubiquitously, FGF-4 exhibits a restricted pattern, failing to bind HS in the heart and blood vessels and failing to activate signaling in mouse aortic endothelial cells. This suggests that FGF-4 seeks a specific HS sulfation pattern, distinct from that of FGF-2, which is not expressed in most vascular tissues. Additionally, whereas FR2c binds all FGF-4-HS complexes, FR1c fails to bind FGF-4-HS in most tissues, as well as in Raji-S1 cells expressing syndecan-1. Proliferation assays using BaF3 cells expressing either FR1c or FR2c support these results. This suggests that FGF and FR recognition of specific HS sulfation patterns is critical for the activation of FGF signaling, and that synthesis of these patterns is regulated during embryonic development.

Trk C receptor signaling regulates cardiac myocyte proliferation during early heart development in vivo

Neurotrophin-3 (NT-3) is a member of the neurotrophin family of growth factors, best characterized by its survival- and differentiation-inducing effects on developing neurons bearing the trk C receptor tyrosine kinase. Through analysis of NT-3 and trk C gene-targeted mice we have identified NT-3 as critically regulating cardiac septation, valvulogenesis, and conotruncal formation. Although these defects could reflect cardiac neural crest dysfunction, the expression of NT-3 and trk C by cardiac myocytes prior to neural crest migration prompted analysis of cell-autonomous actions of NT-3 on cardiac myocytes. Retroviral-mediated overexpression of truncated trk C receptor lacking kinase activity was used to inhibit activation of trk C by endogenous NT-3, during early heart development in ovo. During the first week of chicken development, expression of truncated trk C reduced myocyte clone size by more than 60% of control clones. Direct mitogenic actions of NT-3 on embryonic cardiac myocytes were demonstrated by analysis of BrdU incorporation or PCNA immunoreactivity in control and truncated trk C-expressing clones. Inhibition of trk C signaling reduced cardiac myocyte proliferation during the first week of development, but had no effect at later times. These studies demonstrate that endogenous NT-3:trk C signaling regulates cardiac myocyte proliferation during cardiac looping and the establishment of ventricular trabeculation but that myocyte proliferation becomes NT-3 independent during the second week of embryogenesis.

Endoderm and heart development

Since the first half of the 20th century, experimental embryologists have noted a relationship between endoderm cells and the development of cardiac tissue from mesoderm. During the past decade, the accumulation of evidence for an obligatory interaction between endoderm and mesoderm during the specification and terminal differentiation of myocardial, and more recently endocardial, cells has markedly accelerated. Moreover, the endoderm-derived molecules that may regulate these processes are being identified. It now appears that endoderm-derived growth factors regulate the formation of both myocardial and endocardial cells during specification, terminal differentiation, and perhaps morphogenesis of cells in the developing embryonic heart.

Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate mesoderm that expresses the Nkx2.5 and GATA4 transcription factors, but the extracellular signals specifying heart precursor gene expression are not known. We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularly during initiation of cardiac gene expression. fgf8 is mutated in acerebellar (ace) mutants, and homozygous mutant embryos do not establish normal circulation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac precursors and later in the heart ventricle. Fgf8 is required for the earliest stages of nkx2.5 and gata4, but not gata6, expression in cardiac precursors. Cardiac gene expression is restored in acerebellar mutant embryos by injecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primordium. Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming that Fgf signaling is required independently of earlier functions during gastrulation. These findings show that fgf8/acerebellar is required for induction and patterning of myocardial precursors.

A novel role for cardiac neural crest in heart development

It is well known that cardiac neural crest participates in development of the cardiac outflow septation and patterning of the great arteries. Less well known is that ablation of the cardiac neural crest leads to a primary myocardial dysfunction. Recent data suggests that the myocardial dysfunction occurs because of the absence of an interaction of neural crest and pharyngeal endoderm to alter signaling from the endoderm. Continuation of an FGF-like signal from the endoderm past a precise time in development appears to be detrimental to myocardial maturation.

Initiation of mammalian liver development from endoderm by fibroblast growth factors

The signaling molecules that elicit embryonic induction of the liver from the mammalian gut endoderm or induction of other gut-derived organs are unknown. Close proximity of cardiac mesoderm, which expresses fibroblast growth factors (FGFs) 1, 2, and 8, causes the foregut endoderm to develop into the liver. Treatment of isolated foregut endoderm from mouse embryos with FGF1 or FGF2, but not FGF8, was sufficient to replace cardiac mesoderm as an inducer of the liver gene expression program, the latter being the first step of hepatogenesis. The hepatogenic response was restricted to endoderm tissue, which selectively coexpresses FGF receptors 1 and 4. Further studies with FGFs and their specific inhibitors showed that FGF8 contributes to the morphogenetic outgrowth of the hepatic endoderm. Thus, different FGF signals appear to initiate distinct phases of liver development during mammalian organogenesis.

Evidence that FGF receptor signaling is necessary for endoderm-regulated development of precardiac mesoderm

Endoderm cells in the heart forming region (HFR endoderm) of stage 6 chicken embryos are required to support the proliferation and terminal differentiation of precardiac mesoderm cells in vitro. The endoderm's effect can be substituted by growth factors, including members of the fibroblast growth factor (FGF) family. However, direct implication of FGFs in this process requires evidence that inhibition of FGF signaling interferes with proliferation and/or terminal differentiation. This report examines the consequences of treating endoderm/precardiac mesoderm co-explants with agents that inactivate FGF receptors. Using sodium chlorate, which prevents FGF ligand-receptor interaction, it was observed that the percentage of S-phase precardiac mesoderm cells was markedly reduced, suggesting that cell proliferation was inhibited. To more specifically affect FGF signaling, the explants were treated with an antibody that recognizes an extracellular domain of FGF receptor-1 (FGFR-1). This treatment similarly inhibited cell proliferation. Although both agents modestly delayed cardiac myocyte differentiation as indicated by the contractile function, expression of alpha-sarcomeric actin was not affected. These findings provide additional evidence that an intact FGF signaling pathway is required during heart development.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

VEGF and bFGF stimulate myocardial vascularization in embryonic chick

We tested the hypothesis that early vascularization of the embryonic heart is enhanced after bolus injections of vascular, endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) into the vitelline vein before the onset of myocardial vasculogenesis (3.5 days, stage 21). Electron and light microscopy were utilized to obtain morphometric data. At stages 29 and 31, myocardial vessel volume or numerical density were higher in embryos injected with 50 ng bFGF than in the saline-injected controls. A VEGF injection increased vascular volume density at stage 29 and both volume and numerical, density at stage 31, bFGF, but not VEGF, was associated with an enhancement of the sinusoidal system (spongy layer of the ventricle) at stage 29. This effect disappeared by stage 31. In conclusion, 1) enhancement of bFGF or VEGF before myocardial vascularization increases vascular growth, but the initial effect of bFGF is greater; 2) the effects of these growth factors on vascular volume and numerical density are temporally dependent; and 3) bFGF, in addition to its effects on the coronary vasculature, influences ventricular modeling by apparently acting on myocytes as well as endothelial cells.

Fibroblast growth factor receptor 1 in skeletal and heart muscle cells: expression during early avian development and regulation after notochord transplantation

Basic fibroblast growth factor (bFGF, FGF-2) mediates several biological functions during embryonic development. With regard to skeletal muscle formation, it has been suggested that FGF-2 is involved in the growth and differentiation of myogenic precursor cells. To identify the FGF-responsive cells we studied the expression of FGF receptor type I (FGFR-1) during early embryonic development of the chick. FGFR-1 immunoreactivity is present at all stages examined (embryonic day [E] 2-E5). Expression of FGFR-1 is found in the somite myotome, limb bud muscle cells, eye and tongue muscle cells, and myocardium. Transplantation of an additional notochord into the paraxial mesoderm, which prevents the formation of a myotome, reveals the absence of FGFR-1 immunoreactivity on the operated side. The distinct expression pattern of FGFR-1 in migrating and differentiating muscle cells indicates that in addition to the stimulation of proliferation of myoblasts, FGF-2 exerts other (nonmitogenic) effects on postmitotic myocytes.

Fibroblast growth factor-2 stimulates embryonic cardiac mesenchymal cell proliferation

The proliferation response of stage 36 chick atrioventricular valve mesenchymal cells to fibroblast growth factor-2 (FGF-2) was studied in the tissue-like environment of three-dimensional cell aggregates maintained in organ culture. The mitogenic effects of FGF-2 on mesenchymal tissue depended on the FGF-2-stimulated formation of a fibronectin-containing extracellular matrix. The matrix was absent in unstimulated aggregates, and co-localized with regions of actively proliferating cells in stimulated aggregates. Inhibition of fibronectin matrix formation by the inclusion of Arg-Gly-Asp-containing peptides, which compete with fibronectin for binding to the cell surface alpha 5 beta 1 integrin receptors, abolished the proliferation effects of FGF-2. Inhibition of sulfation of cell surface glycosaminoglycans by treatment with sodium chlorate significantly reduced both the formation of the fibronectin matrix and cell proliferation in response to FGF-2, suggesting an involvement of the low-affinity sulfated glycosaminoglycan FGF receptor system. Thus, the FGF-stimulated growth of embryonic atrioventricular valve mesenchyme in vitro involves the production of a fibronectin matrix. We suggest that the stimulation of the fibronectin matrix represents an essential element in growth factor signaling of mesenchymal tissue, with the matrix serving as an anchorage substratum for the proliferating cells.