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Robert G Kelly

Publications and source records attributed to Robert G Kelly.

15 recordsLinked to original sources
PubMed2026

Spatially resolved single-cell atlas reveals the macroevolutionary trajectory of animal hearts.

Animal hearts display diverse anatomical structures during adaptive evolution. Here, we present a multiomics atlas of adult hearts from 27 species across chordates, arthropods, and mollusks. Joint analysis indicates that Bilateria hearts share a core gene repertoire, taking a stepwise "add-on" approach as a universal evolutionary strategy. The "proto-heart" is populated by key cell types, including cardiomyocytes, fibroblasts, endothelial cells, and neural cells, which maintained core signatures while evolving with shifts in living environments and corresponding adaptations in the cardiovascular system. Additionally, we reveal an evolutionarily conserved cardiomyocyte state dynamic potentially linked to cardiac development and stress responses. Finally, we identify a common molecular program underpinning chamber evolution from a ventricular foundation. This work establishes a resource for understanding the intrinsic mechanisms of heart evolution.

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PubMed2006

Left and right ventricular contributions to the formation of the interventricular septum in the mouse heart.

Mammalian heart development involves complex morphogenetic events which lead to the formation of fully separated left and right atrial and ventricular chambers from a tubular heart. Separation of left and right ventricular chambers is dependent on a single structure, the interventricular septum (IVS), which has both muscular and mesenchymal components. Little is known about the morphogenetic events that lead to the formation of the muscular component of the IVS. We have analyzed two transgenic mouse lines that display complementary nlacZ reporter gene expression patterns in the embryonic ventricles: the Mlc1v-nlacZ-24 transgene is expressed in right ventricular myocardium and the Mlc3f-nlacZ-2 transgene in left ventricular myocardium. Detailed analysis of these transgene expression patterns during IVS formation reveals a symmetric left and right myocardial identity within the developing IVS between embryonic days 9.5 and 11.5. From embryonic day 12.5 onwards, myocytes with a left ventricular identity dominate the IVS, particularly in its dorsal aspect. The T-box transcription factor encoding gene, Tbx18, is expressed in the left ventricle and left side of the developing IVS, providing additional support for the presence of left and right ventricular identities within the IVS. Analysis of clonally related cardiomyocyte clusters confirms that both left and right ventricular myocardial cell populations contribute to the forming IVS, in similar domains to those defined by the Mlc-nlacZ transgenes. Examination of the orientation as well as the distribution of labeled cells in clusters provides new insights into the morphogenesis of the septum.

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PubMed2006

Congenital heart defects in Fgfr2-IIIb and Fgf10 mutant mice.

OBJECTIVE: Myocardial progenitor cells expressing Fgf10 give rise to the outflow tract and right ventricle of the mammalian heart. In order to define the role of fibroblast growth factor (FGF) signaling in this process we investigated whether Fgf10 or the major Fgf10 receptor Fgfr2-IIIb are required for normal heart development. METHODS: The cardiac phenotype of Fgf10 and Fgfr2-IIIb mutant mice was analysed by histology, scanning electron microscopy and gene and transgene expression studies. RESULTS: Outflow tract formation from Fgf10 expressing progenitor cells occurs normally in Fgf10 mutant embryos and in the majority of Fgfr2-IIIb mutant embryos; a proportion of Fgfr2-IIIb mutant embryos, however, display outflow tract and right ventricular hypoplasia. The predominant cardiac defects in Fgfr2-IIIb mutant embryos are ventricular septal defects associated with overriding aorta or double outlet right ventricle. In addition, loss of Fgfr2-IIIb is associated with ventricular anomalies including a thin myocardial wall, abnormal trabeculation and muscular ventricular septal defects. In contrast, Fgf10 is required to correctly position the heart in the thoracic cavity but not for outflow tract septation. Both Fgf10 and Fgfr2-IIIb mutant embryos lack pulmonary arteries and veins. CONCLUSIONS: Fgfr2-IIIb and Fgf10 mutant mice have distinct roles during cardiac morphogenesis, although neither gene is essential for outflow tract elongation from Fgf10 expressing progenitor cells. Fgfr2-IIIb and Fgf10 mutant mice provide new models for common components of congenital heart disease.

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PubMed2006

Rotation of the myocardial wall of the outflow tract is implicated in the normal positioning of the great arteries.

Congenital heart defects frequently involve a failure of outflow tract (OFT) formation during development. We analyzed the remodeling of the OFT, using the y96-Myf5-nlacZ-16 transgene, which marks a subpopulation of myocardial cells of the pulmonary trunk. Expression analyses of reporter transcript and protein suggest that the myocardial wall of the OFT rotates before and during the formation of the great arteries. Rotational movement was confirmed by Di-I injection experiments with cultured embryos. We subsequently examined the expression of the transgene in mouse models for OFT defects. In hearts with persistent truncus arteriosus (PTA), double outlet right ventricle (DORV), or transposition of the great arteries, rotation of the myocardial wall of the OFT is arrested or fails to initiate. This is observed in Splotch (Pax3) mutants with PTA or DORV and may be a result of defects in neural crest migration, known to affect OFT septation. However, in Pitx2deltac mutant embryos, where cardiac neural crest cells are present in the heart, PTA and DORV are again associated with a rotation defect. This is also seen in Pitx2deltac mutants, which have transposition of the great arteries. Because Pitx2c is involved in left-right signaling, these results suggest that embryonic laterality affects rotation of the myocardial wall during OFT maturation. We propose that failure of normal rotation of OFT myocardium may underlie major forms of congenital heart disease.

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PubMed2005

Molecular inroads into the anterior heart field.

In 2001, three research groups described a previously unrecognized population of progenitor cells in pharyngeal mesoderm that gives rise to myocardium at the arterial pole of the heart. In the last 4 years, the major importance of the cellular contribution of pharyngeal mesoderm to normal and pathologic heart development has become apparent. Lineage-tracing experiments have defined the extent to which pharyngeal progenitor cells colonize the heart, revealing a contribution to venous, as well as arterial, pole myocardium; in addition, major molecular inroads have been made into understanding gene regulation in pharyngeal myocardial progenitor cells, implicating forkhead, Gata, LIM homeodomain, MEF2, SMAD, and T-box transcription factors. The key role of the anterior heart field during normal heart development is underscored by the demonstration that both direct and indirect perturbation of myocardial progenitor cells in pharyngeal mesoderm result in congenital heart disease.

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PubMed2005

T-box genes in vertebrate development.

The myriad developmental roles served by the T-box family of transcription factor genes defy easy categorization. Present in all metazoans, the T-box genes are involved in early embryonic cell fate decisions, regulation of the development of extraembryonic structures, embryonic patterning, and many aspects of organogenesis. They are unusual in displaying dosage sensitivity in most instances. In humans, mutations in T-box genes are responsible for developmental dysmorphic syndromes, and several T-box genes have been implicated in neoplastic processes. T-box transcription factors function in many different signaling pathways, notably bone morphogenetic protein and fibroblast growth factor pathways. The few downstream target genes that have been identified indicate a wide range of downstream effectors.

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PubMed2004

The del22q11.2 candidate gene Tbx1 regulates branchiomeric myogenesis.

Formation and remodeling of the pharyngeal arches play central roles in craniofacial development. TBX1, encoding a T-box-containing transcription factor, is the major candidate gene for del22q11.2 (DiGeorge or velo-cardio-facial) syndrome, characterized by craniofacial defects, thymic hypoplasia, cardiovascular anomalies, velopharyngeal insufficiency and skeletal muscle hypotonia. Tbx1 is expressed in pharyngeal mesoderm, which gives rise to branchiomeric skeletal muscles of the head and neck. Although the genetic control of craniofacial muscle development is known to involve pathways distinct from those operational in the trunk, the regulation of branchiomeric myogenesis has remained enigmatic. Here we show that branchiomeric muscle development is severely perturbed in Tbx1 mutant mice. In the absence of Tbx1, the myogenic determination genes Myf5 and MyoD fail to be normally activated in pharyngeal mesoderm. Unspecified precursor cells expressing genes encoding the transcriptional repressors Capsulin and MyoR are present in the mandibular arch of Tbx1 mutant embryos. Sporadic activation of Myf5 and MyoD in these precursor cells results in the random presence or absence of hypoplastic mandibular arch-derived muscles at later developmental stages. Tbx1 is also required for normal expression of Tlx1 and Fgf10 in pharyngeal mesoderm, in addition to correct neural crest cell patterning in the mandibular arch. Tbx1 therefore regulates the onset of branchiomeric myogenesis and controls normal mandibular arch development, including robust transcriptional activation of myogenic determination genes. While no abnormalities in branchiomeric myogenesis were detected in Tbx1(+/-) mice, reduced TBX1 levels may contribute to pharyngeal hypotonia in del22q11.2 patients.

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PubMed2004

Right ventricular myocardium derives from the anterior heart field.

The mammalian heart develops from a primary heart tube, which is formed by fusion of bilateral cardiac territories in which myocardial and endothelial cells have already begun to differentiate from splanchnic mesoderm. A population of myocardial precursors has been identified in pharyngeal mesoderm, anterior to the early heart tube. Cell labeling studies have indicated that this novel territory, called the anterior heart field (AHF), gives rise to the myocardial wall of the outflow tract. We now report that not only the myocardium of the outflow tract but also myocardial cells of the embryonic right ventricle are derived from this source. Explants of pharyngeal mesoderm or of the early heart tube were cultured from transgenic mice in which transgene expression marks different regions of the heart. Pharyngeal mesoderm from 5 to 7 somite embryos gives rise to cardiomyocytes with right ventricular and outflow tract identities, whereas the heart tube as this stage has an essentially left ventricular identity. DiI labeling confirms that the early heart tube is destined to contribute to the embryonic left ventricle and indicates that right ventricular myocardium is added from extracardiac mesoderm. Retrospective clonal analysis of the heart at embryonic day (E) 10.5 reveals the existence of a clonal boundary in the interventricular region, which appears during ventricular septation, underlining different origins of the two ventricular compartments. This study demonstrates the differences in the embryological origin of right and left ventricular myocardium, which has important implications for congenital heart disease.

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PubMed2004

Analysis of Mlc-lacZ Met mutants highlights the essential function of Met for migratory precursors of hypaxial muscles and reveals a role for Met in the development of hyoid arch-derived facial muscles.

The Pax3 and c-met genes are necessary for the development of tongue, diaphragm, and limb muscles. These hypaxial muscles derive from precursors that migrate out of the ventrolateral lip of the somites at occipital, cervical, and limb levels. In this work, we re-examined primary myogenesis in c-met signaling mutants using a skeletal muscle-specific lacZ transgene (Mlc3f-nlacZ-2E). This strategy allowed us to identify precisely the shoulder, limb, tongue, and dermal muscles that need Met for development and to confirm that the morphological structure of epaxial and body wall muscles was normal, even in the most severe c-met mutant. Surprisingly, however, X-gal staining showed that, in this mutant, hyoid arch-derived facial muscles were either reduced or absent, thus revealing that Met also contributes to the development of muscles in the head.

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PubMed2004

The clonal origin of myocardial cells in different regions of the embryonic mouse heart.

When and how cells form and pattern the myocardium is a central issue for heart morphogenesis. Many genes are differentially expressed and function in subsets of myocardial cells. However, the lineage relationships between these cells remain poorly understood. To examine this, we have adopted a retrospective approach in the mouse embryo, based on the use of the laacZ reporter gene, targeted to the alpha-cardiac actin locus. This clonal analysis demonstrates the existence of two lineages that segregate early from a common precursor. The primitive left ventricle and the presumptive outflow tract are derived exclusively from a single lineage. Unexpectedly, all other regions of the heart, including the primitive atria, are colonized by both lineages. These results are not consistent with the prespecification of the cardiac tube as a segmented structure. They are discussed in the context of different heart fields and of the evolution of the heart.

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PubMed2004

Tbx2 is essential for patterning the atrioventricular canal and for morphogenesis of the outflow tract during heart development.

Tbx2 is a member of the T-box transcription factor gene family, and is expressed in a variety of tissues and organs during embryogenesis. In the developing heart, Tbx2 is expressed in the outflow tract, inner curvature, atrioventricular canal and inflow tract, corresponding to a myocardial zone that is excluded from chamber differentiation at 9.5 days post coitus (dpc). We have used targeted mutagenesis in mice to investigate Tbx2 function. Mice heterozygous for a Tbx2 null mutation appear normal but homozygous embryos reveal a crucial role for Tbx2 during cardiac development. Morphological defects are observed in development of the atrioventricular canal and septation of the outflow tract. Molecular analysis reveals that Tbx2 is required to repress chamber differentiation in the atrioventricular canal at 9.5 dpc. Analysis of homozygous mutants also highlights a role for Tbx2 during hindlimb digit development. Despite evidence that TBX2 negatively regulates the cell cycle control genes Cdkn2a, Cdkn2b and Cdkn1a in cultured cells, there is no evidence that loss of Tbx2 function during mouse development results in increased levels of p19(ARF), p16(INK4a), p15(INK4b) or p21 expression in vivo, nor is there evidence for a genetic interaction between Tbx2 and p53.

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PubMed2003

Cell history determines the maintenance of transcriptional differences between left and right ventricular cardiomyocytes in the developing mouse heart.

The molecular mechanisms that establish and maintain transcriptional differences between cardiomyocytes in the left and right ventricular chambers are unkown. We have previously analysed a myosin light chain 3f transgene containing an nlacZ reporter gene, which is transcribed in left but not right ventricular cardiomyocytes. In this report we examine the mechanisms involved in maintaining regionalised transgene expression. Primary cardiomyocytes prepared from left and right ventricular walls of transgenic mice were found to maintain transgene expression status in culture. However, similar cultures prepared from nontransgenic mice or rats show uniform expression after transient transfection of Mlc3f constructs, suggesting that the mechanism responsible for differential expression of the transgene between left and right ventricular cells does not operate on transiently introduced molecules. These data suggest that developmental cell history determines transgene expression status. Maintenance of transgene expression status is regulated by a cell-autonomous mechanism that is independent of DNA methylation, trichostatin A-sensitive histone deacetylation and miss-expression of transcription factors that are expressed in the left or right ventricles of the embryonic heart. Parallels between Mlc3f transgene repression in right ventricular cardiomyocytes and polycomb-mediated silencing in Drosophila suggest that Mlc3f regulatory sequences included on the transgene may contain a cellular memory module that is switched into an on or off state during early cardiogenesis. Epigenetic mechanisms may therefore be involved in maintaining patterning of the mammalian myocardium.

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PubMed2003

A retrospective clonal analysis of the myocardium reveals two phases of clonal growth in the developing mouse heart.

Key molecules which regulate the formation of the heart have been identified; however, the mechanism of cardiac morphogenesis remains poorly understood at the cellular level. We have adopted a genetic approach, which permits retrospective clonal analysis of myocardial cells in the mouse embryo, based on the targeting of an nlaacZ reporter to the alpha-cardiac actin gene. A rare intragenic recombination event leads to a clone of beta-galactosidase-positive myocardial cells. Analysis of clones at different developmental stages demonstrates that myocardial cells and their precursors follow a proliferative mode of growth, rather than a stem cell mode, with an initial dispersive phase, followed by coherent cell growth. Clusters of cells are dispersed along the venous-arterial axis of the heart tube. Coherent growth is oriented locally, with a main axis, which corresponds to the elongation of the cluster, and rows of cells, which form secondary axes. The angle between the primary and secondary axes varies, indicating independent events of growth orientation. At later stages, as the ventricular wall thickens, wedge shaped clusters traverse the wall and contain rows of cells at a progressive angle to each other. The cellular organisation of the myocardium appears to prefigure myofibre architecture. We discuss how the characteristics of myocardial cell growth, which we describe, underlie the formation of the heart tube and its subsequent regionalised expansion.

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PubMed2002

The anterior heart-forming field: voyage to the arterial pole of the heart.

Studies of vertebrate heart development have identified key genes and signalling molecules involved in the formation of a myocardial tube from paired heart-forming fields in splanchnic mesoderm. The posterior region of the paired heart-forming fields subsequently contributes myocardial precursor cells to the inflow region or venous pole of the heart. Recently, a population of myocardial precursor cells in chick and mouse embryos has been identified in pharyngeal mesoderm anterior to the early heart tube. This anterior heart-forming field gives rise to myocardium of the outflow region or arterial pole of the heart. The amniote heart is therefore derived from two myocardial precursor cell populations, which appear to be regulated by distinct genetic programmes. Discovery of the anterior heart-forming field has important implications for the interpretation of cardiac defects in mouse mutants and for the study of human congenital heart disease.

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