Human eHAND, but not dHAND, is down-regulated in cardiomyopathies

The progression of cardiomyopathy to congestive heart failure is often associated with the expression of fetal cardiac-specific genes. In mice, the basic helix-loop-helix transcription factors, dHAND and eHAND, are expressed in a cardiac chamber-specific fashion and are essential for fetal cardiac development, but are down-regulated in the adult. Their expression in specific chambers of healthy and diseased human hearts has not been studied previously. Human dHAND and eHAND were mapped to human chromosomes 4q33 and 5q33, respectively, by fluorescent in situ hybridization. RNA from the four chambers of healthy human adult hearts, and from hearts of patients with several forms of cardiomyopathy, was obtained and assayed for dHAND and eHAND expression. Unlike in mice, dHAND expression was observed in all four chambers of the healthy human adult heart, but was diminished in the right atrium. In contrast, eHAND was expressed in the right and left ventricles, but was downregulated in both atrial chambers. We examined tissue from 15 human cardiomyopathic hearts obtained during cardiac transplantation or by endomyocardial biopsy for alterations in HAND gene expression. dHAND expression was unchanged in all forms of cardiomyopathy tested. However, cardiac expression of eHAND was severely down-regulated in six of six patients with ischemic cardiomyopathy and six of six patients with dilated cardiomyopathy. This study demonstrates that human dHAND and eHAND have unique spatial patterns of expression within human cardiac chambers. Downregulation of eHAND in ischemic and dilated cardiomyopathy suggests a correlation between eHAND dysregulation and the evolution of a subset of cardiomyopathies.

Rectification properties of carbon nanotube "Y-junctions"

Quantum conductivity of single-wall carbon nanotube Y-junctions is calculated. The current versus voltage characteristics of these junctions show asymmetry and rectification, in agreement with recent experimental results. Furthermore, rectification is found to be independent of the angle between the branches of these junctions, indicating this to be an intrinsic property of symmetric Y-junctions. The implications for the Y-junction to function as a nanoscale molecular electronic switch are investigated.

Tbx1, a DiGeorge syndrome candidate gene, is regulated by sonic hedgehog during pharyngeal arch development

Appropriate interactions between the epithelium and adjacent neural crest-derived mesenchyme are necessary for normal pharyngeal arch development. Disruption of pharyngeal arch development in humans underlies many of the craniofacial defects observed in the 22q11.2 deletion syndrome (del22q11), but the genes responsible remain unknown. Tbx1 is a T-box transcription factor that lies in the 22q11.2 locus. Tbx1 transcripts were found to be localized to the pharyngeal endoderm and the mesodermal core of the pharyngeal arches, but were not present in the neural crest-derived mesenchyme of the pharyngeal arches. Sonic hedgehog (Shh) is also expressed in the pharyngeal arches and is necessary for normal craniofacial development. We found that Tbx1 expression was dependent upon Shh signaling in mouse embryos, consistent with their overlapping expression in the pharyngeal arches. Furthermore, Shh was sufficient to induce Tbx1 expression when misexpressed in selected regions of chick embryos. These studies reveal a Shh-mediated pathway that regulates Tbx1 during pharyngeal arch development.

Genetic assembly of the heart: implications for congenital heart disease

More children die from congenital heart defects (CHD) each year than are diagnosed with childhood cancer, yet the causes remain unknown. The remarkable conservation of genetic pathways regulating cardiac development in species ranging from flies to humans provides an opportunity to experimentally dissect the role of critical cardiogenic factors. Utilization of model biological systems has resulted in a molecular framework in which to consider the etiology of CHD. As whole genome sequencing and single nucleotide polymorphism data become available, identification of genetic mutations predisposing to CHD may allow preventive measures by modulation of secondary genetic or environmental factors. In this review, genetic pathways regulating cardiogenesis revealed by cross-species studies are reviewed and correlated with human CHD.

CHAMP, a novel cardiac-specific helicase regulated by MEF2C

MEF2C is a MADS-box transcription factor required for cardiac myogenesis and morphogenesis. In MEF2C mutant mouse embryos, heart development arrests at the looping stage (embryonic day 9.0), the future right ventricular chamber fails to form, and cardiomyocyte differentiation is disrupted. To identify genes regulated by MEF2C in the developing heart, we performed differential array analysis coupled with subtractive cloning using RNA from heart tubes of wild-type and MEF2C-null embryos. Here, we describe a novel MEF2C-dependent gene that encodes a cardiac-restricted protein, called CHAMP (cardiac helicase activated by MEF2 protein), that contains seven conserved motifs characteristic of helicases involved in RNA processing, DNA replication, and transcription. During mouse embryogenesis, CHAMP expression commences in the linear heart tube at embryonic day 8.0, shortly after initiation of MEF2C expression in the cardiogenic region. Thereafter, CHAMP is expressed specifically in embryonic and postnatal cardiomyocytes. At the trabeculation stage of heart development, CHAMP expression is highest in the trabecular region in which cardiomyocytes have exited the cell cycle and is lowest in the proliferative compact zone. These findings suggest that CHAMP acts downstream of MEF2C in a cardiac-specific regulatory pathway for RNA processing and/or transcriptional control.

endo-Fullerene and doped diamond nanocrystallite-based models of qubits for solid-state quantum computers

Models of encapsulated nuclear spin 1/2 1H and 31P atoms in fullerene and diamond nanocrystallite, respectively, are proposed and examined with an ab initio local density functional method for possible applications as single quantum bits (qubits) in solid-state quantum computers. A 1H atom encapsulated in a fully deuterated fullerene, C20D20, forms the first model system and ab initio calculation shows that the 1H atom is stable in its atomic state at the center of the fullerene with a barrier of about 1 eV to escape. A 31P atom positioned at the center of a diamond nanocrystallite is the second model system, and 31P atom is found to be stable at the substitutional site relative to interstitial sites by 15 eV. Vacancy formation energy is 6 eV in diamond, so the substitutional 31P atom will be stable against diffusion during the formation mechanisms within the nanocrystallite. The coupling between the nuclear spin and the weakly bound (valance) donor electron in both systems is found to be suitable for single qubit applications, whereas the spatial distributions of (valance) donor electron wave functions are found to be preferentially spread along certain lattice directions, facilitating two or more qubit applications. The feasibility of the fabrication pathways for both model solid-state qubit systems within practical quantum computers is discussed within the context of our proposed solid-state qubits.

A GATA-dependent right ventricular enhancer controls dHAND transcription in the developing heart

Heart formation in vertebrates is believed to occur in a segmental fashion, with discreet populations of cardiac progenitors giving rise to different chambers of the heart. However, the mechanisms involved in specification of different chamber lineages are unclear. The basic helix-loop-helix transcription factor dHAND is expressed in cardiac precursors throughout the cardiac crescent and the linear heart tube, before becoming restricted to the right ventricular chamber at the onset of looping morphogenesis. dHAND is also expressed in the branchial arch neural crest, which contributes to craniofacial structures and the aortic arch arteries. Using a series of dHAND-lacZ reporter genes in transgenic mice, we show that cardiac and neural crest expression of dHAND are controlled by separate upstream enhancers and we describe a composite cardiac-specific enhancer that directs lacZ expression in a pattern that mimics that of the endogenous dHAND gene throughout heart development. Deletion analysis reduced this enhancer to a 1.5 kb region and identified subregions responsible for expression in the right ventricle and cardiac outflow tract. Comparison of mouse regulatory elements required for right ventricular expression to the human dHAND upstream sequence revealed two conserved consensus sites for binding of GATA transcription factors. Mutation of these sites abolished transgene expression in the right ventricle, identifying dHAND as a direct transcriptional target of GATA factors during right ventricle development. Since GATA factors are not chamber-restricted, these findings suggest the existence of positive and/or negative coregulators that cooperate with GATA factors to control right ventricular-specific gene expression in the developing heart.

Members of the HRT family of basic helix-loop-helix proteins act as transcriptional repressors downstream of Notch signaling

The Hairy-related transcription-factor (HRT) genes encode three related basic helix-loop-helix transcription factors that show sequence similarity to the Hairy and Enhancer of split family of transcriptional repressors. HRT proteins are expressed in specific regions of the developing heart, vasculature, pharyngeal arches and somites, and the periodicity of their expression in somitic precursors mirrors that of Notch signaling-related molecules. In the present study, we show that the intracellular domain of the Notch1 receptor (Notch1 IC), which is constitutively active, up-regulates HRT2 expression in 10T(1/2) fibroblasts. Luciferase reporter assays using the regulatory regions of the mouse HRT genes revealed that transcription of all three genes is stimulated by Notch1 IC. The promoters of the HRT genes share homology in a binding site for Suppressor of Hairless [Su(H)], a transcriptional mediator of Notch signaling. A dominant-negative Su(H) mutant abolished Notch-activated HRT2 expression, and mutation of the conserved Su(H) consensus site in the HRT2 promoter attenuated transcriptional activation by Notch. Ectopic expression of HRT proteins also blocked activation of HRT2 expression by Notch1 IC through a mechanism requiring the basic region, but not the conserved carboxyl-terminal YQPW-TEVGAF motif of HRT2. These findings identify HRT genes as downstream targets for Notch signaling and reveal a negative autoregulatory loop whereby HRT proteins repress their own expression through interference with Notch signaling.

Apoptosis during cardiovascular development

Morphogenesis and developmental remodeling of cardiovascular tissues involve coordinated regulation of cell proliferation and apoptosis. In the heart, clear evidence points toward focal apoptosis as a contributor to development of the embryonic outflow tract, cardiac valves, conducting system, and the developing coronary vasculature. Apoptosis in the heart is likely regulated by survival and death signals that are also present in many other tissues. Cell type-specific regulation may be superimposed on general cell death/survival machinery through tissue-specific transcriptional pathways. In the vasculature, apoptosis almost certainly contributes to developmental vessel regression, and it is of proven importance in remodeling of arterial structure in response to local changes in hemodynamics. Physical forces, growth factors, and extracellular matrix drive vascular cell survival pathways, and considerable evidence points to local nitric oxide production as an important but complex regulator of vascular cell death. In both the heart and vasculature, progress has been impeded by inadequate information concerning the incidence of apoptosis, its relative importance compared with the diverse cell behaviors that remodel developing tissues, and by our primitive knowledge concerning regulation of cell death in these tissues. However, tools are now available to better understand apoptosis in normal and abnormal development of cardiovascular structures, and a framework has been established that should lead to considerable progress in the coming years.

A genetic blueprint for cardiac development

Congenital heart disease is the leading non-infectious cause of death in children. It is becoming increasingly clear that many cardiac abnormalities once thought to have multifactorial aetiologies are attributable to mutations in developmental control genes. The consequences of these mutations can be manifest at birth as life-threatening cardiac malformations or later as more subtle cardiac abnormalities. Understanding the genetic underpinnings of cardiac development has important implications not only for understanding congenital heart disease, but also for the possibility of cardiac repair through genetic reprogramming of non-cardiac cells to a cardiogenic fate.

Conservation of sequence and expression of Xenopus and zebrafish dHAND during cardiac, branchial arch and lateral mesoderm development

dHAND and eHAND are related basic helix-loop-helix transcription factors that are expressed in the cardiac mesoderm and in numerous neural crest-derived cell types in chick and mouse. To better understand the evolutionary development of overlapping expression and function of the HAND genes during embryogenesis, we cloned the zebrafish and Xenopus orthologues. Comparison of dHAND sequences in zebrafish, Xenopus, chick, mouse and human demonstrated conservation throughout the protein. Expression of dHAND in zebrafish was seen in the earliest precursors of all lateral mesoderm at early gastrulation stages. At neurula and later stages, dHAND expression was observed in lateral precardiac mesoderm, branchial arch neural crest derivatives and posterior lateral mesoderm. At looping heart stages, cardiac dHAND expression remained generalized with no apparent regionalization. Interestingly, no eHAND orthologue was found in zebrafish. In Xenopus, dHAND and eHAND were co-expressed in the cardiac mesoderm without the segmental restriction seen in mice. Xenopus dHAND and eHAND were also expressed bilaterally in the lateral mesoderm without any left-right asymmetry. Within the branchial arches, XdHAND was expressed in a broader domain than XeHAND, similar to their mouse counterparts. Together, these data demonstrate conservation of HAND structure and expression across species.

Role of dHAND in the anterior-posterior polarization of the limb bud: implications for the Sonic hedgehog pathway

dHAND is a basic helix-loop-helix (bHLH) transcription factor essential for cardiovascular development. Here we analyze its pattern of expression and functional role during chick limb development. dHAND expression was observed in the lateral plate mesoderm prior to emergence of the limb buds. Coincident with limb initiation, expression of dHAND became restricted to the posterior half of the limb bud. Experimental procedures that caused mirror-image duplications of the limb resulted in mirror-image duplications of the pattern of dHAND expression along the anterior-posterior axis. Retroviral overexpression of dHAND in the limb bud produced preaxial polydactyly, corresponding to mild polarizing activity at the anterior border. At the molecular level, misexpression of dHAND caused ectopic activation of members of the Sonic hedgehog (Shh) pathway, including Gli and Patched, in the anterior limb bud. A subset of infected embryos displayed ectopic anterior activation of Shh. Other factors implicated in anterior-posterior polarization of the bud such as the most 5′ Hoxd genes and Bmp2 were also ectopically activated at the anterior border. Our results indicate a role for dHAND in the establishment of anterior-posterior polarization of the limb bud.

The basic helix-loop-helix transcription factor, dHAND, is required for vascular development

Reciprocal interactions between vascular endothelial cells and vascular mesenchymal cells are essential for angiogenesis. Here we show that the basic helix-loop-helix transcription factor, dHAND/Hand2, is expressed in the developing vascular mesenchyme and its derivative, vascular smooth muscle cells (VSMCs). Targeted deletion of the dHAND gene in mice revealed severe defects of embryonic and yolk sac vascular development by embryonic day 9.5. Vascular endothelial cells expressed most markers of differentiation. Vascular mesenchymal cells migrated appropriately but failed to make contact with vascular endothelial cells and did not differentiate into VSMCs. In a screen for genes whose expression was dependent upon dHAND (using subtractive hybridization comparing wild-type and dHAND-null hearts), the VEGF(165) receptor, neuropilin-1, was found to be downregulated in dHAND mutants. These results suggest that dHAND is required for vascular development and regulates angiogenesis, possibly through a VEGF signaling pathway.

Left-right asymmetry and cardiac looping: implications for cardiac development and congenital heart disease

Proper morphogenesis and positioning of internal organs requires delivery and interpretation of precise signals along the anterior-posterior, dorsal-ventral, and left-right axes. An elegant signaling cascade determines left- versus right-sided identity in visceral organs in a concordant fashion, resulting in a predictable left-right (LR) organ asymmetry in all vertebrates. The complex morphogenesis of the heart and its connections to the vasculature are particularly dependent upon coordinated LR signaling pathways. Disorganization of LR signals can result in myriad congenital heart defects that are a consequence of abnormal looping and remodeling of the primitive heart tube into a multi-chambered organ. A framework for understanding how LR asymmetric signals contribute to normal organogenesis has emerged and begins to explain the basis of many human diseases of LR asymmetry. Here we review the impact of LR signaling pathways on cardiac development and congenital heart disease.

HRT1, HRT2, and HRT3: a new subclass of bHLH transcription factors marking specific cardiac, somitic, and pharyngeal arch segments

Members of the Hairy/Enhancer of Split family of basic helix-loop-helix (bHLH) transcription factors are regulated by the Notch signaling pathway in vertebrate and Drosophila embryos and control cell fates and establishment of sharp boundaries of gene expression. Here, we describe a new subclass of bHLH proteins, HRT1 (Hairy-related transcription factor 1), HRT2, and HRT3, that share high homology with the Hairy family of proteins yet have characteristics that are distinct from those of Hairy and other bHLH proteins. Each HRT gene was expressed in distinct cell types within numerous organs, particularly in those patterned along the anterior-posterior axis. HRT1 and HRT2 were expressed in atrial and ventricular precursors, respectively, and were also expressed in the cardiac outflow tract and aortic arch arteries. HRT1 and HRT2 transcripts were also detected in precursors of the pharyngeal arches and subsequently in the pharyngeal clefts. Within somitic precursors, HRT1 and HRT3 exhibited dynamic expression in the presomitic mesoderm, mirroring the expression of other components of Notch-Delta signaling pathways. The HRT genes were expressed in other sites of epithelial-mesenchymal interactions, including the developing kidneys, brain, limb buds, and vasculature. The unique and complementary expression patterns of this novel subfamily of bHLH proteins suggest a previously unrecognized role for Hairy-related pathways in segmental patterning of the heart and pharyngeal arches, among other organs.

Developmental and genetic aspects of congenital heart disease

Congenital heart defects (CHDs) are the result of abnormal cardiac mesoderm or cardiac neural crest development. The molecular cause of most congenital heart disease remains unknown, although numerous cardiac regulatory factors have recently been described. dHAND and eHAND are basic helix-loop-helix transcription factors expressed differentially in the right and left ventricles, respectively, and in the cardiac neural crest. Mice lacking dHAND have a hypoplastic right ventricle and abnormal development of vessels arising from the heart and cell death of craniofacial precursors. By searching for dHAND-dependent genes, a gene likely responsible for the cardiac and craniofacial defects associated with chromosome 22q11 deletion has been identified. A systematic dissection of molecular pathways involved in cardiogenesis should allow for further identification of genes responsible for CHD.

A molecular pathway revealing a genetic basis for human cardiac and craniofacial defects

Microdeletions of chromosome 22q11 are the most common genetic defects associated with cardiac and craniofacial anomalies in humans. A screen for mouse genes dependent on dHAND, a transcription factor implicated in neural crest development, identified Ufd1, which maps to human 22q11 and encodes a protein involved in degradation of ubiquitinated proteins. Mouse Ufd1 was specifically expressed in most tissues affected in patients with 22q11 deletion syndrome. The human UFD1L gene was deleted in all 182 patients studied with 22q11 deletion, and a smaller deletion of approximately 20 kilobases that removed exons 1 to 3 of UFD1L was found in one individual with features typical of 22q11 deletion syndrome. These data suggest that UFD1L haploinsufficiency contributes to the congenital heart and craniofacial defects seen in 22q11 deletion.

HAND proteins: molecular mediators of cardiac development and congenital heart disease

Congenital heart defects are the clinical manifestation of anomalies in embryonic cardiac development. Such defects occur in distinct regions or chambers of the heart. A molecular framework in which to consider cardiac development and congenital heart disease in a segmental fashion has begun to emerge. dHAND and eHAND are two related basic helix-loop-helix transcription factors that are expressed in a complementary fashion in the developing right and left ventricles, respectively. They are also expressed in the neural crest-derived cardiac outflow tract and aortic arch arteries. Targeted mutations of dHAND and eHAND in mice have revealed novel pathways of organogenesis in mesodermal and neural crest derivatives. dHAND mutants exhibit hypoplasia of the right ventricle, branchial arches, and aortic arch arteries. The distinct nature of cardiac defects in dHAND mutants provides an entry into dissecting molecular pathways governing morphogenesis of specific components of the heart. Congenital heart disease is considered as a defect in segmental development of the heart and the role of dHAND and eHAND in regulating such developmental pathways in normal and abnormal cardiogenesis is examined.

Spatial and temporal expression of AP-1 responsive rod photoreceptor genes and bZIP transcription factors during development of the rat retina

PURPOSE

The promoter region of the rod-specific beta subunit of cGMP PDE (beta-PDE) and opsin genes contains highly conserved cis-acting elements, which include an AP-1 and/or Nrl response element (NRE: An extended AP-1 like sequence). Transactivation of AP-1 or NRE appears necessary to drive expression of these rod-specific genes during adulthood, however, their role during development is relatively unknown. Therefore, we determined the spatial and temporal relationships between rod morphological and functional development, rod-specific gene expression, and expression of the bZIP transcription factors c-fos, junD and Nrl.

METHODS

Retinas from 0-45 day old (PN0-45) dark- and light-adapted Long-Evans rats were used. Morphological development was monitored by light and electron microscopy. Whole retinal trypsin-activated cGMP-PDE activity and rhodopsin content were measured biochemically. The expression of opsin, beta-PDE, c-fos, junD and Nrl mRNAs were determined by Northern blot analysis. The cellular localization of Nrl was examined with in situ hybridization.

RESULTS

The mRNAs for opsin, beta-PDE and c-fos were observed at PN0-2, while cGMP-PDE activity and rhodopsin were detected first at PN5: coincident with rod outer segment development. The developmental pattern of cGMP-PDE activity and rhodopsin accumulation paralleled the expression of beta-PDE and opsin mRNA and all reached their maximal levels by PN45. Nrl expression, for all three transcripts found in the rat retina, was low on PN2 and reached its maximal level at PN14. The c-fos and Nrl expression preceded beta-PDE and opsin mRNA expression by 1-2 days. Nrl expression was detected first in the distal post-mitotic retina at PN5 and then in all nuclear layers during retinal development. Maximal expression shifted from the ganglion cells to the outer nuclear layer as the neural retina matured. In contrast, junD expression was highest at PN0 and declined to a stable level by PN10.

CONCLUSIONS

Colocalization of Nrl and c-Fos suggests that expression of rod-specific genes, which utilize AP-1 or NRE sites in their promoter, could be regulated through the formation of Nrl-Fos dimers. We hypothesize that Nrl and c-Fos play a fundamental role in the initiation and regulation of the rod-specific gene expression in developing and adult rod photoreceptors.