Identification of a novel cardiac-specific transcript critical for cardiac myocyte differentiation

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.

Loss of CENP-F results in distinct microtubule-related defects without chromosomal abnormalities

Microtubule (MT)-binding centromere protein F (CENP-F) was previously shown to play a role exclusively in chromosome segregation during cellular division. Many cell models of CENP-F depletion show a lag in the cell cycle and aneuploidy. Here, using our novel genetic deletion model, we show that CENP-F also regulates a broader range of cellular functions outside of cell division. We characterized CENP-F(+/+) and CENP-F(-/-) mouse embryonic fibroblasts (MEFs) and found drastic differences in multiple cellular functions during interphase, including cell migration, focal adhesion dynamics, and primary cilia formation. We discovered that CENP-F(-/-) MEFs have severely diminished MT dynamics, which underlies the phenotypes we describe. These data, combined with recent biochemical research demonstrating the strong binding of CENP-F to the MT network, support the conclusion that CENP-F is a powerful regulator of MT dynamics during interphase and affects heterogeneous cell functions.

Dual function of the UNC-45b chaperone with myosin and GATA4 in cardiac development

Cardiac development requires interplay between the regulation of gene expression and the assembly of functional sarcomeric proteins. We report that UNC-45b recessive loss-of-function mutations in C3H and C57BL/6 inbred mouse strains cause arrest of cardiac morphogenesis at the formation of right heart structures and failure of contractile function. Wild-type C3H and C57BL/6 embryos at the same stage, E9.5, form actively contracting right and left atria and ventricles. The known interactions of UNC-45b as a molecular chaperone are consistent with diminished accumulation of the sarcomeric myosins, but not their mRNAs, and the resulting decreased contraction of homozygous mutant embryonic hearts. The novel finding that GATA4 accumulation is similarly decreased at the protein but not mRNA levels is also consistent with the function of UNC-45b as a chaperone. The mRNAs of known downstream targets of GATA4 during secondary cardiac field development, the cardiogenic factors Hand1, Hand2 and Nkx-2.5, are also decreased, consistent with the reduced GATA4 protein accumulation. Direct binding studies show that the UNC-45b chaperone forms physical complexes with both the alpha and beta cardiac myosins and the cardiogenic transcription factor GATA4. Co-expression of UNC-45b with GATA4 led to enhanced transcription from GATA promoters in naïve cells. These novel results suggest that the heart-specific UNC-45b isoform functions as a molecular chaperone mediating contractile function of the sarcomere and gene expression in cardiac development.

Involvement of Cenp-F in interphase chromatin organization possibly through association with DNA-dependent protein kinase

Cenp-F (also named mitosin) is a 350-kDa human kinetochore protein important for the mitotic progression. It is also a nuclear matrix protein in interphase cells. Here, we showed that overexpression of N-terminal deletion mutants of Cenp-F containing the C-terminal 112 residues induced chromatin condensation into numerous aggregates of varying sizes in interphase nucleus, colocalizing with the exogenous proteins. In situ hybridization using whole chromosome painting probes indicated that the chromatin aggregates were not prematurely condensed individual chromosomes. Neither were they due to apoptosis. We provided evidence showing association of Cenp-F with certain regions of interphase chromatin fibers. Cenp-F associated with the DNA-dependent protein kinase (DNA-PK), a trimeric protein complex critical for genome homeostasis. Moreover, the DNA-PK association activity of Cenp-F mutants correlated with their ability to induce chromatin aggregation. These results imply a role of Cenp-F in organization of interphase chromatin through association and possibly regulation of DNA-PK.

Involvement of CENP-F in histone methylation

CENP-F (also named mitosin) is a multifunctional protein of 350 kDa. In interphase, it is a nuclear protein, whereas in M phase it localizes to the kinetochore, the major microtubule-binding structure on chromosomes essential for chromosome segregation. CENP-F is also critical for myocyte differentiation through the interaction with Rb. It binds to ATF4 and negatively regulates the transcriptional activity of ATF4. It is also important for mitotic progression. Here we show that depletion of CENP-F by RNAi markedly downregulated the methylation of histone H3 at K4 and K9. Consistently, association of HP1a with mitotic chromosomes was largely decreased. These results uncover a novel role of CENP-F in regulation of epigenetic modification on histone H3.

Murine CENP-F regulates centrosomal microtubule nucleation and interacts with Hook2 at the centrosome

The microtubule (MT) network is essential in a broad spectrum of cellular functions. Many studies have linked CENP-F to MT-based activities as disruption of this protein leads to major changes in MT structure and function. Still, the basis of CENP-F regulation of the MT network remains elusive. Here, our studies reveal a novel and critical localization and role for CENP-F at the centrosome, the major MT organizing center (MTOC) of the cell. Using a yeast two-hybrid screen, we identify Hook2, a linker protein that is essential for regulation of the MT network at the centrosome, as a binding partner of CENP-F. With recently developed immunochemical reagents, we confirm this interaction and reveal the novel localization of CENP-F at the centrosome. Importantly, in this first report of CENP-F(-/-) cells, we demonstrate that ablation of CENP-F protein function eliminates MT repolymerization after standard nocodazole treatment. This inhibition of MT regrowth is centrosome specific because MT repolymerization is readily observed from the Golgi in CENP-F(-/-) cells. The centrosome-specific function of CENP-F in the regulation of MT growth is confirmed by expression of truncated CENP-F containing only the Hook2-binding domain. Furthermore, analysis of partially reconstituted MTOC asters in cells that escape complete repolymerization block shows that disruption of CENP-F function impacts MT nucleation and anchoring rather than promoting catastrophe. Our study reveals a major new localization and function of CENP-F at the centrosome that is likely to impact a broad array of MT-based actions in the cell.

Silencing CENPF in bovine preimplantation embryo induces arrest at 8-cell stage

Identification of genes that are important for normal preimplantation development is essential for understanding the basics of early mammalian embryogenesis. In our previous study, we have shown that CENPF (mitosin) is differentially expressed during preimplantation development of bovine embryos. CENPF is a centromere-kinetochore complex protein that plays a crucial role in the cell division of somatic cells. To our best knowledge, no study has yet been done on either bovine model, or oocytes and preimplantation embryos. In this study, we focused on the fate of bovine embryos after injection of CENPF double-stranded RNA (dsRNA) into the zygotes. An average decrease of CENPF mRNA abundance by 94.9% or more and an extensive decline in immunofluorescence staining intensity was detected relative to controls. There was no disparity between individual groups in the developmental competence before the 8-cell stage. However, the developmental competence rapidly decreased then and only 28.1% of CENPF dsRNA injected 8-cell embryos were able to develop further (uninjected control: 71.8%; green fluorescent protein dsRNA injected control: 72.0%). In conclusion, these results show that depletion of CENPF mRNA in preimplantation bovine embryos leads to dramatic decrease of developmental competence after embryonic genome activation.

CMF1-Rb interaction promotes myogenesis in avian skeletal myoblasts

CMF1 protein is expressed in developing striated muscle before the expression of contractile proteins, and depletion of CMF1 in myoblasts results in inability to express muscle-specific proteins. Previous studies of CMF1 identify a functional Rb-binding domain, which is conserved in the murine and human homologues. Here, we show that CMF1 binds Rb family members, while a CMF1 protein with deletion of the Rb-binding domain (Rb-del CMF1) does not. Myogenic cell lines over-expressing Rb-del CMF1 proliferate normally, but exhibit markedly impaired differentiation, including dramatically reduced contractile proteins gene expression and failure to fuse into myotubes. Furthermore, by quantitative real-time polymerase chain reaction, MyoD and Myf5 mRNA levels are comparable to wild-type, while myogenin and contractile protein mRNA levels are significantly attenuated. These data demonstrate that CMF1 regulates myocyte differentiation by interaction with Rb family members to induce expression of myogenic regulatory factors.

Specific deletion of CMF1 nuclear localization domain causes incomplete cell cycle withdrawal and impaired differentiation in avian skeletal myoblasts

CMF1 is a protein expressed in embryonic striated muscle with onset of expression preceding that of contractile proteins. Disruption of CMF1 in myoblasts disrupts muscle-specific protein expression. Preliminary studies indicate both nuclear and cytoplasmic distribution of CMF1 protein, suggesting functional roles in both cellular compartments. Here we examine the nuclear function of CMF1, using a newly characterized antibody generated against the CMF1 nuclear localization domain and a CMF1 nuclear localization domain-deleted stable myocyte line. The antibody demonstrates nuclear distribution of the CMF1 protein both in vivo and in cell lines, with clustering of CMF1 protein around chromatin during mitosis. In more differentiated myocytes, the protein shifts to the cytoplasm. The CMF1 NLS-deleted cell lines have markedly impaired capacity to differentiate. Specifically, these cells express less contractile protein than wild-type or full-length CMF1 stably transfected cells, and do not fuse properly into multinucleate syncytia with linear nuclear alignment. In response to low serum medium, a signal to differentiate, CMF1 NLS-deleted cells enter G0, but continue to express proliferation markers and will reenter the cell cycle when stimulated by restoring growth medium. These data suggest that CMF1 is involved in regulation the transition from proliferation to differentiation in embryonic muscle.

Mitosin/CENP-F in mitosis, transcriptional control, and differentiation

Mitosin/CENP-F is a large nuclear/kinetochore protein containing multiple leucine zipper motifs potentially for protein interactions. Its expression levels and subcellular localization patterns are regulated in a cell cycle-dependent manner. Recently, accumulating lines of evidence have suggested it a multifunctional protein involved in mitotic control, microtubule dynamics, transcriptional regulation, and muscle cell differentiation. Consistently, it is shown to interact directly with a variety of proteins including CENP-E, NudE/Nudel, ATF4, and Rb. Here we review the current progress and discuss possible mechanisms through which mitosin may function.

LEK1 protein expression in normal and dysregulated cardiomyocyte mitosis

A defining characteristic of embryonic cells is their ability to divide rapidly, even in tissues such as cardiac muscle, which cannot divide once fully differentiated. This suggests that regulators of cell division differ in embryonic and differentiated cells. LEK1 is a member of an emerging family of proteins with diverse functions but shared structural domains, including numerous leucine zippers, a nuclear localization site, and a functional Rb-binding domain. LEK1 is expressed ubiquitously in the developing mouse embryo from the earliest stages of differentiation through birth. It is absent in adult tissues, even those that maintain active cell division. We hypothesize that LEK1 is a regulator of mitosis restricted to the developing embryo and early neonate. Here, using BrdU incorporation, we show that LEK1 protein downregulation in cardiac myocytes correlates directly with cessation of DNA synthesis between neonatal days 6 and 10. In contrast, in an immortalized cardiac cell line (HL1 cells), both BrdU incorporation and LEK1 protein expression persist, and actively dividing cells express LEK1. However, BrdU incorporation can be decreased in these cells by treatment with a morpholino targeting LEK1 mRNA. These data suggest a role for LEK1 in regulating the normal embryonic cardiomyocyte cell cycle and in promoting continued mitosis in transformed, abnormally dividing cardiomyocytes.

Les cellules souches embryonnaires : Du développement myocardique à la médecine régénératrice

Embryonic stem cells are capable to recapitulate the first stages of myocardial development. Using mouse embryonic stem cells, transcriptional networks specifying the cardiac fate can be delineated. Furthermore, using members of the TGFbeta superfamily to commit mouse ES cells toward a cardiac lineage, recent studies showed that ESC-derived cardiomyocytes were capable to repair post-infarcted myocardium of small and large animals. The next challenges are to validate such results using human ESCs in order to better comprehend cardiac congenital diseases and to foresee a cell therapy of heart failure. double dagger.

Interplay between the retinoblastoma protein and LEK1 specifies stem cells toward the cardiac lineage

The molecular mechanisms governing early cardiogenesis are still largely unknown. Interestingly, the retinoblastoma protein (Rb), a regulator of cell cycle, has recently emerged as a new candidate regulating cell differentiation. Rb-/- mice die at midgestation and mice lacking E2f1/E2f3, downstream components of the Rb-dependent transcriptional pathway, die of heart failure. To gain insight into the function of Rb pathway in early cardiogenesis, we used Rb-/- embryonic stem (ES) cells differentiating into cardiomyocytes. Rb-/- cells displayed a dramatic delay in expression of cardiac-specific transcription factors and in turn in the whole process of cardiac differentiation. The phenotype of Rb-/- ES cell-derived cardiomyocytes was rescued by reintroducing Rb in cardiac progenitors, by stimulating the BMP-dependent cardiogenic pathway or by overexpression of Nkx2.5. ES cells deficient in the recently identified factor LEK1, a murine homolog of the cardiomyogenic factor 1, or specific disruption of Rb-LEK1 interaction into the nucleus of differentiating ES cells recapitulated the delay in cardiac differentiation of Rb-/- ES cells. Thus, we provide evidence for a novel Rb/LEK1-dependent and BMP-independent transcriptional program, which plays a pivotal role in priming ES cells toward a cardiac fate.

Mitosin/CENP-F as a negative regulator of activating transcription factor-4

Mitosin/CENP-F is a human nuclear matrix protein with multiple leucine zipper motifs. Its accumulation in S-G2 phases and transient kinetochore localization in mitosis suggest a multifunctional protein for cell proliferation. Moreover, its murine and avian orthologs are implicated in myocyte differentiation. Here we report its interaction with activating transcription factor-4 (ATF4), a ubiquitous basic leucine zipper transcription factor important for proliferation, differentiation, and stress response. The C-terminal portion of mitosin between residues 2488 and 3113 bound to ATF4 through two distinct domains, one of which was a leucine zipper motif. Mitosin mutants containing these domains were able to either supershift or disrupt the ATF4-DNA complex. On the other hand, ATF4, but not ATF1-3 or ATF6, interacted with mitosin through a region containing the basic leucine zipper motif. Moreover, overexpression of full-length mitosin repressed the transactivation activity of ATF4 in dual luciferase-based reporter assays, while knocking down mitosin expression manifested the opposite effects. These findings suggest mitosin to be a negative regulator of ATF4 in interphase through direct interaction.

LEK1 is a potential inhibitor of pocket protein-mediated cellular processes

LEK1, a member of the LEK family of proteins, is ubiquitously expressed in developing murine tissues. Our current studies are aimed at identifying the role of LEK1 during cell growth and differentiation. Little is known about the function of LEK proteins. Recent studies in our laboratory have focused on the characterization of the LEK1 atypical Rb-binding domain that is conserved among all LEK proteins. Our findings suggest that LEK1 potentially functions as a universal regulator of pocket protein activity. Pocket proteins exhibit distinct expression patterns during development and function to regulate cell cycle, apoptosis, and tissue-specific gene expression. We show that LEK1 interacts with all three pocket proteins, p107, p130, and pRb. Additionally, this interaction occurs specifically between the LEK1 Rb-binding motif and the "pocket domain" of Rb proteins responsible for Rb association with other targets. Analyses of the effects of disruption of LEK1 protein expression by morpholino oligomers demonstrate that LEK1 depletion decreases cell proliferation, disrupts cell cycle progression, and induces apoptosis. Given its expression in developing cells, its association with pocket proteins, and its effects on proliferation, cell cycle, and viability of cells, we suggest that LEK1 functions in a similar manner to phosphorylation to disrupt association of Rb proteins with appropriate binding targets. Thus, the LEK1/Rb interaction serves to retain cells in a pre-differentiative, actively proliferative state despite the presence of Rb proteins during development. Our data suggest that LEK1 is unique among LEK family members in that it specifically functions during murine development to regulate the activity of Rb proteins during cell division and proliferation. Furthermore, we discuss the distinct possibility that a yet unidentified splice variant of the closely related human CENP-F, serves a similar function to LEK1 in humans.

Mitosin/CENP-F is a conserved kinetochore protein subjected to cytoplasmic dynein-mediated poleward transport

Mitosin/CENP-F is a human nuclear protein transiently associated with the outer kinetochore plate in M phase and is involved in M phase progression. LEK1 and CMF1, which are its murine and chicken orthologs, however, are implicated in muscle differentiation and reportedly not distributed at kinetochores. We therefore conducted several assays to clarify this issue. The typical centromere staining patterns were observed in mitotic cells from both human primary culture and murine, canine, and mink cell lines. A C-terminal portion of LEK1 also conferred centromere localization. Our analysis further suggests conserved kinetochore localization of mammalian mitosin orthologs. Moreover, mitosin was associated preferentially with kinetochores of unaligned chromosomes. It was also constantly transported from kinetochores to spindle poles by cytoplasmic dynein. These properties resemble those of other kinetochore proteins important for the spindle checkpoint, thus implying a role of mitosin in this checkpoint. Therefore, mitosin family may serve as multifunctional proteins involved in both mitosis and differentiation.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

[Regulation of myocardial gene expression during heart development]

The heart is an organ with special significance in medicine and developmental biology. The development of the heart and its vessels during embryogenesis is the result of numerous and complex processes. At present, our understanding is based on decades of meticulous anatomical studies. However, the spectacular progress of modern molecular biology and developmental biology has marked the beginning of a new era in embryology. The molecular bases for cardiogenesis are just emerging. Several families of genes with restricted expression to the heart have been identified in the last years, including genes encoding for contractile proteins, ion channels as well as transcription factors involved in tissue specific gene expression. Likewise, the analyses of regulatory elements have increased our understanding of the molecular mechanisms directing gene expression. In this review, we illustrate the different patterns of gene and transgene expression in the developing myocardium. These data demonstrate that the wide molecular heterogeneity observed in the developing myocardium is not restricted to embryogenesis but it also remains in the adulthood. Therefore, such molecular diversity should be taken into account on the design of future gene therapy approaches, having thus direct clinical implications.

Characterization of cardiac muscle factor 1 sequence motifs: retinoblastoma protein binding and nuclear localization

Cardiac muscle factor 1 (CMF1) is characterized as a protein important in cardiac and skeletal myocyte differentiation and is expressed in a developmentally regulated manner. Sequence analysis data showed that CMF1 has crucial protein-protein interaction domains, has a retinoblastoma protein-binding site which interacts with RB directly in vitro and in the embryo, has a functional nuclear localization signal and is highly homologous to other cell cycle regulatory proteins such as mitosin and centromere protein F, which suggests that CMF1 may be involved in regulating the cell cycle.

Anterior expression of the caudal homologue cCdx-B activates a posterior genetic program in avian embryos

Several families of regulatory genes have been implicated in anteroposterior patterning of gastrulation-stage vertebrate embryos. Members of the Drosophila caudal family of homeobox genes (Cdx) are among the earliest regulators of posterior cell fates. The regulatory cascade initiated by the caudal homologue, cCdx-B, was examined in avian embryos. During gastrulation, cCdx-B is expressed with other posterior patterning genes. In the posterior primitive streak, cCdx-B expression coincides with posteriorly expressed Hox cluster genes and Wnt family members such as Wnt-8c. The hierarchical relationship between these patterning genes was examined after anterior ectopic expression of cCdx-B. cCdx-B expression in anterior cardiogenic cells by means of adenoviral infection leads to the induction of Wnt-8c and the posterior Hox genes, Hoxa-7, Hoxc-6, and Hoxc-8. Cardiogenesis is not inhibited in cCdx-B expressing anterior lateral mesoderm, indicating that anterior cell fates are not respecified with the activation of posterior patterning genes after gastrulation. These results support an important role for cCdx-B in initiating a posterior program of gene expression that includes Wnt signaling molecules and the Hox cluster genes.

Control of cardiac myosin heavy chain gene expression

The alpha- and beta-myosin genes extend over 51 kb on chromosome 14 in human and 11 in mouse separated by about 4.5 kb of intergenic sequence. They are located in tandem in the order of their expression during development. Transcription of each gene is independently controlled but coordinately regulated. During each embryogenesis, the beta-MHC gene is expressed as part of the cardiac myogenic program under the control of NKX-2.5, MEF-2C, and GATA-4/5/6. After birth, thyroid hormone induces expression of alpha-MHC mRNA and inhibits expression of the beta-MHC gene. While a large number of physiological stimuli are capable of modifying this basic paradigm, thyroid hormone is required for expression of alpha-MHC in ventricular muscle. The positive TRE for T(3)-stimulation of alpha-MHC is an imperfect direct repeat located in the proximal promoter of the gene. The negative TRE for the beta-MHC gene is probably a binding half-site that is located adjacent to the TATA box. Binding of TEF-1 to a strong positive element in the proximal promoter is important in basal expression of beta-MHC gene and in the response to alpha(1)-adrenergic stimulation. The beta-MHC gene also is induced together with several other "fetal" genes during cardiac hypertrophy by a mechanism involving Ca(2+)-mediated activation of calcineurin and NF-AT3. Upon activation, NF-AT3 translocates to the nucleus and interacts with GATA-4 to stimulate beta-MHC expression. Changes in chromatin structure mediated by the association of histone acetylases and deacetylases with transcription factors are essential in regulating cell-specific expression of MHC genes.

Analysis of CMF1 reveals a bone morphogenetic protein-independent component of the cardiomyogenic pathway

Disruption of the CMF1 function in anterior mesoderm inhibits cardiac myogenesis in avian embryos. In the present study, we show that CMF1 is a member of an emerging family of proteins that includes centromeric protein-F, mitosin, and LEK1. These proteins are characterized by their large size (350 kDa), dynamic subcellular distribution, and potential functions in cell division and differentiation. The current data suggest that CMF1 is a unique member of this family by virtue of its restricted protein expression and variant subcellular distribution. Immunochemical analysis demonstrates that CMF1 protein is expressed in cardiogenic cells prior to the activation of cardiac structural gene products. In addition, we show that expression of CMF1 is not dependent on the bone morphogenetic protein (BMP) signaling pathway during development. Still, CMF1 cannot direct cardiomyogenesis in the absence of such factors as NKX-2.5. Taken with our previous data, this study suggests that CMF1 is a BMP-independent component of the cardiomyogenic pathway.

BMP2 is required for early heart development during a distinct time period

BMP2, like its Drosophila homologue dpp, is an important signaling molecule for specification of cardiogenic mesoderm in vertebrates. Here, we analyzed the time-course of BMP2-requirement for early heart formation in whole chick embryos and in explants of antero-lateral plate mesoderm. Addition of Noggin to explants isolated at stage 4 and cultured for 24 h resulted in loss of NKX2.5, GATA4, eHAND, Mef2A and vMHC expression. At stages 5-8 the individual genes showed differential sensitivity to Noggin addition. While expression of eHAND, NKX2.5 and Mef2A was clearly reduced by Noggin vMHC was only marginally affected. In contrast, GATA4 expression was enhanced after Noggin treatment. The developmental period during which cardiac mesoderm required the presence of BMP signaling in vivo was assessed by implantation of Noggin expressing cells into stage 4-8 embryos which were then cultured until stage 10-11. Complete loss of NKX2.5 and eHAND expression was observed in embryos implanted at stages 4-6, and expression was still suppressed in stages 7 and 8 implanted embryos. GATA4 expression was also blocked by Noggin at stage 4, however increased at stages 5, 6 and 7. Explants of central mesendoderm, that normally do not form heart tissue were employed to study the time-course of BMP2-induced cardiac gene expression. The induction of cardiac lineage markers in central mesendoderm of stage 5 embryos was distinct for different genes. While GATA4, -5, -6 and MEF2A were induced to maximal levels within 6 h after BMP2 addition, eHAND and dHAND required 12 h to reach maximum levels of expression. NKX2.5 was induced by 6 h and accumulated over 48 h. vMHC and titin were induced at significant levels only after 48 h of BMP2 addition. These results indicate that cardiac marker genes display distinct expression kinetics after BMP2 addition and differential response to Noggin treatment suggesting complex regulation of myocardial gene expression in the early tubular heart.

The cloning and analysis of LEK1 identifies variations in the LEK/centromere protein F/mitosin gene family

We report the cloning of a novel murine cDNA, LEK1, that is related to human CENP-F and mitosin and more distantly to chicken CMF1. The proteins from these three organisms have significant homology, yet differ in their temporal, spatial, and subcellular localizations. The human proteins bind the kinetochore in mitotic cells, whereas the chicken protein is found only in skeletal and cardiac muscle and is developmentally regulated. Mouse LEK1 is a single copy gene that codes for two developmentally regulated transcripts. The LEK1 protein is expressed early and ubiquitously in mouse development and is generally down-regulated as development proceeds in a manner that correlates to a cessation of mitosis. In adult tissues, the LEK1 protein is detected exclusively in the pronucleus of the oocyte and was not observed in other actively dividing tissues. Subcellular localization revealed that the LEK1 protein in mitotic cells does not bind the kinetochore. From these data, we hypothesize that chicken CMF1, human CENP-F, mitosin, and mouse LEK1 are members of an emerging family of genes that have important and functionally distinct roles in development and cell division.

bves: A novel gene expressed during coronary blood vessel development

We have used a subtractive method to clone novel messages enriched in the heart. Here we show that one such message, bves (blood vessel/epicardial substance) is a novel protein that is highly conserved between chicken and mouse. The bves message is detected at high levels in early chick hearts. Using anti-Bves antibodies, we show expression in cells of the proepicardial organ, migrating epicardium, epicardial-derived mesenchyme, and smooth muscle of the developing intracardiac arterial system, including the coronary arteries. Our data suggest that Bves is an early marker of developing vascular smooth muscle cells. In addition, the expression pattern of Bves protein reveals the patterning of intracardiac vascular smooth muscle and possible insights into the cellular regulation of smooth muscle differentiation during vasculogenesis.

Ventrally emigrating neural tube cells differentiate into heart muscle

A population of ventrally emigrating neural tube cells has been shown to migrate along the vagus nerve and contribute to the development of the gastrointestinal tract. Since the vagus also goes to the heart, we sought to determine if these cells migrated into the heart. Neural tube cells were tagged with replication-deficient retroviral vectors containing the LacZ gene, to permanently label their progeny. The virus was microinjected into the lumen of the caudal hindbrain of chick embryos on day 2. Embryos were later processed for the detection of LacZ positive cells. Labeled cells were initially confined to the neural tube. Later, they migrated in association with the vagus nerve into the heart, where they were located in the myocardium. Labeled cells were identified as cardiac muscle cells of non-neural crest origin, with specific markers. It is concluded that some cardiac muscle cells differentiate from the neural tube cells.

Analysis of Hox gene expression during early avian heart development

The anteroposterior (A-P) patterning of the developing heart underlies atrial and ventricular lineage specification and heart chamber morphogenesis. The posteriorization of cardiomyogenic phenotype with retinoic acid (RA) treatment of primitive streak stage chicken embryos is suggestive of a role for the clustered homeobox (Hox) genes in early heart patterning (Yutzey et al. [1994] Development 120:871-873; [1995] Dev. Biol. 170:531-541). A screen for Hox genes expressed in chick heart primordia and primitive heart led to the isolation of anterior genes of the Hox clusters expressed during cardiogenesis. Specific hoxd-3, hoxa-4, and hoxd-4 transcripts were detected at the early stages of heart formation and full-length cDNA clones were isolated. Expression of hoxd-3 was detected in the heart forming region of embryos prior to heart tube formation. Expression of hoxa-4, hoxd-3, and hoxb-5 was increased in cardiogenic tissue treated with RA in culture conditions that also produced changes in positionally restricted cardiomyogenic phenotypes. Hox genes expressed in cardiac explants exhibited distinct sensitivities to RA and ouabain treatment when compared to genes, such as nkx-2.5, that are involved in cardiac commitment and differentiation. These studies support a role for Hox genes in early heart patterning and suggest that positional information in the cardiogenic region is established by regulatory mechanisms distinct from early heart lineage specification.

Nonradioactive In Situ Hybridization: Recent Techniques and Applications

In situ hybridization (ISH) has become a standard method for the localization of nucleic acid sequences in chromosomes, single cells, and tissue sections. Nonradioactive ISH has not only eliminated the problems associated with radioactive probes but has also achieved a higher degree of resolution. Advances in probe preparation and labeling methods have facilitated the general application of ISH. In combination with immunohistochemistry, ISH can provide histological information on gene activity at the DNA, mRNA, and protein levels. Some nonradioactive ISH can simultaneously detect nucleic acid sequences in the same tissue or in a chromosome spread. Advances in ISH technology, including use of the polymerase chain reaction offer both a high sensitivity allowing detection of low levels of gene expression and the cytological localization of gene sequences.

The chicken GATA-6 locus contains multiple control regions that confer distinct patterns of heart region-specific expression in transgenic mouse embryos

The GATA-6 transcription factor is expressed in cardiogenic cells and during subsequent stages of heart development in diverse vertebrate species. To gain insights into the molecular events that govern this heart-restricted expression, we isolated the chicken GATA-6 gene and used several approaches to screen for associated control regions. Our analysis of two chicken GATA-6/lacZ constructs in transgenic mouse embryos was particularly revealing. One GATA-6/lacZ construct, which has 1.5 kilobase pairs of upstream sequences along with the promoter and first intron, was expressed exclusively in the atrioventricular canal region of the heart. This expression pattern is novel and appears to mark specialized myocardial cells that induce underlying endocardial cells to initiate valve formation. The other GATA-6/lacZ construct, which has an additional 7.7 kilobase pairs of upstream sequences, was expressed in the ventricle and outflow tract in addition to the atrioventricular canal. The failure of these GATA-6 control regions to function as enhancers in transfected cardiac myocyte cultures underscores the importance of using transgenic approaches to elucidate transcriptional controls that function in the developing heart. Although the endogenous GATA-6 gene is expressed throughout the heart, our results indicate that this is effected in a heart region-specific manner.