The bHLH transcription factor dHAND controls Sonic hedgehog expression and establishment of the zone of polarizing activity during limb development

The basic helix-loop-helix factor, HAND2, functions as a transcriptional activator by binding to E-boxes as a heterodimer

HAND2 (dHAND) is a basic helix-loop-helix (bHLH) transcription factor expressed in numerous tissues during development including the heart, limbs, and a subset of neural crest derivatives. Functional analysis has shown that HAND2 is involved in development of the branchial arches, heart, limb, vasculature, and nervous system. Although it is essential for development of numerous tissues, little is known about its mode of action. To this end, we have characterized HAND2 transcriptional regulatory mechanisms. Using mammalian one-hybrid analysis we show that HAND2 contains a strong transcriptional activation domain in the amino-terminal third of the protein. Like most tissue-restricted bHLH factors, HAND2 heterodimerizes with the broadly expressed bHLH factors, the E-proteins. We determined the consensus DNA binding site of HAND2 and show that HAND2 binds a subset of E-boxes as a heterodimer with E12. Yeast two-hybrid screening of a neuroblastoma cDNA library for HAND2-interacting proteins selected HAND2 and numerous additional members of the E-protein family. Although HAND2 homodimer formation was confirmed by in vitro analysis, HAND2 fails to homodimerize in a mammalian two-hybrid assay but demonstrates robust HAND2/E12 interaction. We conclude that HAND2 functions as a transcription activator by binding a subset of E-boxes as a heterodimer with E-proteins.

Mutual genetic antagonism involving GLI3 and dHAND prepatterns the vertebrate limb bud mesenchyme prior to SHH signaling

The bHLH transcription factor dHAND is required for establishment of SHH signaling by the limb bud organizer in posterior mesenchyme, a step crucial to development of vertebrate paired appendages. We show that the transcriptional repressor GLI3 restricts dHAND expression to posterior mesenchyme prior to activation of SHH signaling in mouse limb buds. dHAND, in turn, excludes anterior genes such as Gli3 and Alx4 from posterior mesenchyme. Furthermore, genetic interaction of GLI3 and dHAND directs establishment of the SHH/FGF signaling feedback loop by restricting the BMP antagonist GREMLIN posteriorly. These interactions polarize the nascent limb bud mesenchyme prior to SHH signaling.

Patterning mechanisms controlling vertebrate limb development

Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anterior-posterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-beta/BMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.

"Fingering" the vertebrate limb

A detailed and precise picture is being pieced together about how the pattern of digits develops in vertebrate limbs. What is particularly exciting is that it will soon be possible to trace the process all the way from establishment of a signalling centre in a small bud of undifferentiated cells right through to final limb anatomy. The development of the vertebrate limb is a traditional model in which to explore mechanisms involved in pattern formation, and there is accelerating knowledge about the genes involved. One reason why the limb is holding its place in the post-genomic age is that it is rich in pre-genomic embryology. Here, we will focus on recent findings about the aspect of vertebrate limb development concerned with digit pattern across the anteroposterior axis of the limb. This process is controlled by a signalling region in the early limb bud known as the polarizing region. Interactions between polarizing region cells and other cells in the limb bud ensure that a thumb develops at one edge of the hand (anterior) and a little finger at the other (posterior).

The combinatorial activities of Nkx2.5 and dHAND are essential for cardiac ventricle formation

Nkx2.5/Csx and dHAND/Hand2 are conserved transcription factors that are coexpressed in the precardiac mesoderm and early heart tube and control distinct developmental events during cardiogenesis. To understand whether Nkx2.5 and dHAND may function in overlapping genetic pathways, we generated mouse embryos lacking both Nkx2.5 and dHAND. Mice heterozygous for mutant alleles of Nkx2.5 and dHAND were viable. Although single Nkx2.5 or dHAND mutants have a morphological atrial and single ventricular chamber, Nkx2.5(-/-)dHAND(-/-) mutants had only a single cardiac chamber which was molecularly defined as the atrium. Complete ventricular dysgenesis was observed in Nkx2.5(-/-)dHAND(-/-) mutants; however, a precursor pool of ventricular cardiomyocytes was identified on the ventral surface of the heart tube. Because Nkx2.5 mutants failed to activate eHAND expression even in the early precardiac mesoderm, the Nkx2.5(-/-)dHAND(-/-) phenotype appears to reflect an effectively null state of dHAND and eHAND. Cell fate analysis in dHAND mutants suggests a role of HAND genes in survival and expansion of the ventricular segment, but not in specification of ventricular cardiomyocytes. Our molecular analyses also revealed the cooperative regulation of the homeodomain protein, Irx4, by Nkx2.5 and dHAND. These studies provide the first demonstration of gene mutations that result in ablation of the entire ventricular segment of the mammalian heart, and reveal essential transcriptional pathways for ventricular formation.

Role of Dlx6 in regulation of an endothelin-1-dependent, dHAND branchial arch enhancer

Neural crest cells play a key role in craniofacial development. The endothelin family of secreted polypeptides regulates development of several neural crest sublineages, including the branchial arch neural crest. The basic helix-loop-helix transcription factor dHAND is also required for craniofacial development, and in endothelin-1 (ET-1) mutant embryos, dHAND expression in the branchial arches is down-regulated, implicating it as a transcriptional effector of ET-1 action. To determine the mechanism that links ET-1 signaling to dHAND transcription, we analyzed the dHAND gene for cis-regulatory elements that control transcription in the branchial arches. We describe an evolutionarily conserved dHAND enhancer that requires ET-1 signaling for activity. This enhancer contains four homeodomain binding sites that are required for branchial arch expression. By comparing protein binding to these sites in branchial arch extracts from endothelin receptor A (EdnrA) mutant and wild-type mouse embryos, we identified Dlx6, a member of the Distal-less family of homeodomain proteins, as an ET-1-dependent binding factor. Consistent with this conclusion, Dlx6 was down-regulated in branchial arches from EdnrA mutant mice. These results suggest that Dlx6 acts as an intermediary between ET-1 signaling and dHAND transcription during craniofacial morphogenesis.

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.

Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function

The secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior-posterior limb polarity. In the Shh(-/-) mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. In this study, we report the analysis of Shh(-/-) mutant limbs in detail. Each mutant embryo has four limbs with recognizable humerus/femur bones that have anterior-posterior polarity. Distal to the elbow/knee joints, skeletal elements representing the zeugopod form but lack identifiable anterior-posterior polarity. Therefore, Shh specifically becomes necessary for normal limb development at or just distal to the stylopod/zeugopod junction (elbow/knee joints) during mouse limb development. The forelimb autopod is represented by a single distal cartilage element, while the hindlimb autopod is invariably composed of a single digit with well-formed interphalangeal joints and a dorsal nail bed at the terminal phalanx. Analysis of GDF5 and Hoxd11-13 expression in the hindlimb autopod suggests that the forming digit has a digit-one identity. This finding is corroborated by the formation of only two phalangeal elements which are unique to digit one on the foot. The apical ectodermal ridge (AER) is induced in the Shh(-/-) mutant buds with relatively normal morphology. We report that the architecture of the Shh(-/-) AER is gradually disrupted over developmental time in parallel with a reduction of Fgf8 expression in the ridge. Concomitantly, abnormal cell death in the Shh(-/-) limb bud occurs in the anterior mesenchyme of both fore- and hindlimb. It is notable that the AER changes and mesodermal cell death occur earlier in the Shh(-/-) forelimb than the hindlimb bud. This provides an explanation for the hindlimb-specific competence to form autopodial structures in the mutant. Finally, unlike the wild-type mouse limb bud, the Shh(-/-) mutant posterior limb bud mesoderm does not cause digit duplications when grafted to the anterior border of chick limb buds, and therefore lacks polarizing activity. We propose that a prepattern exists in the limb field for the three axes of the emerging limb bud as well as specific limb skeletal elements. According to this model, the limb bud signaling centers, including the zone of polarizing activity (ZPA) acting through Shh, are required to elaborate upon the axial information provided by the native limb field prepattern.

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.

Cholesterol modification of sonic hedgehog is required for long-range signaling activity and effective modulation of signaling by Ptc1

Sonic hedgehog (Shh) signaling from the posterior zone of polarizing activity (ZPA) is the primary determinant of anterior-posterior polarity in the vertebrate limb field. An active signal is produced by an autoprocessing reaction that covalently links cholesterol to the N-terminal signaling moiety (N-Shh(p)), tethering N-Shh(p) to the cell membrane. We have addressed the role played by this lipophilic modification in Shh-mediated patterning of mouse digits. Both the distribution and activity of N-Shh(p) indicate that N-Shh(p) acts directly over a few hundred microns. In contrast, N-Shh, a form that lacks cholesterol, retains similar biological activity to N-Shh(p), but signaling is posteriorly restricted. Thus, cholesterol modification is essential for the normal range of signaling. It also appears to be necessary for appropriate modulation of signaling by the Shh receptor, Ptc1.

Identification and characterization of Lbh, a novel conserved nuclear protein expressed during early limb and heart development

We report the cloning, protein characterization, and expression of a novel vertebrate gene, termed Lbh (Limb-bud-and-heart), with a spatiotemporal expression pattern that marks embryologically significant domains in the developing limbs and heart. Lbh encodes a highly conserved nuclear protein, which in tissue culture cells possesses a transcriptional activator function. During limb development, expression of Lbh initiates in the ectoderm of the presumptive limb territory in the lateral body wall. As the limb buds appear, Lbh expression is restricted primarily to the distal ventral limb ectoderm and the apical ectodermal ridge, and overlaps in these ectodermal compartments with En1 and Fgf8 expression. During heart formation, Lbh is expressed as early as Nkx2.5 and dHand in the bilateral heart primordia, with the highest levels in the anterior promyocardium. After heart tube fusion and looping, Lbh expression is confined to the ventricular myocardium, with the highest intensity in the right ventricle and atrioventricular canal, as well as in the sinus venosus. Based on the molecular characteristics and the domain-specific expression pattern, it is possible that Lbh functions in synergy with other genes known to be required for heart and limb development.

Some distal limb structures develop in mice lacking Sonic hedgehog signaling

Patterning of the limb is coordinated by the complex interplay of three signaling regions: the apical ectodermal ridge (AER), the zone of polarizing activity (ZPA), and the non-ridge limb ectoderm. Complex feedback loops exist between Shh in the ZPA, Bmps and their antagonists in the adjacent mesenchyme, Wnt7a in the dorsal ectoderm and Fgfs in the AER. In contrast to the previously reported complete absence of digits in Shh(-/-) mice, we show that one morphologically distinct digit, with a well-delineated nail and phalanges, forms in Shh(-/-) hindlimbs, while intermediate structures are severely truncated and fused. The presence of distal autopod elements is consistent with weak expression of Hoxd13 in Shh(-/-) hindlimbs. Shh(-/-) forelimbs in contrast have one distal cartilage element, a less-well differentiated nail and fused intermediate bones. Interestingly, Ihh is expressed at the tip of Shh mutant limbs and could account for formation of distal structures. In contrast to previous studies we also demonstrate that Shh signaling is required for maintenance of normal Fgf8 expression, since expression of Fgf8, unlike some other AER marker genes, is rapidly lost from anterior to posterior after E10.5, with only a small domain of Fgf8 expression remaining posteriorly. Furthermore, loss of expanded Fgf8 expression is paralleled by a collapse of the handplate. Our data show that development of most intermediate elements of the hindlimb skeleton are Shh-dependent, and that Shh signaling is required for anterior-posterior expansion of the AER in both limbs and for the subsequent branching of zeugopod and autopod elements. Finally, we show that Shh is also required for outgrowth of the limb ectoderm and thus for the formation of a distinct limb compartment.

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.

The basic helix-loop-helix transcription factors dHAND and eHAND exhibit dimerization characteristics that suggest complex regulation of function

dHAND and eHAND are basic helix-loop-helix (bHLH) transcription factors expressed during embryogenesis and are required for the proper development of cardiac and extraembryonic tissues. HAND genes, like the myogenic bHLH genes, are classified as class B bHLH genes, which are expressed in a tissue-restricted pattern and function by forming heterodimers with class A bHLH proteins. Myogenic bHLH genes are shown not to form homodimers efficiently, suggesting that their activity is dependent on their E-protein partners. To identify HIPs (HAND-interacting proteins) that regulate the activity of the HAND genes, we screened an 9.5-10.5-day-old mouse embryonic yeast two-hybrid library with eHAND. Several HIPs held high sequence identity to eHAND, indicating that eHAND could form and function as a homodimer. Based on the high degree of amino acid identity between eHAND and dHAND, it is possible that dHAND could also form homodimers and heterodimers with eHAND. We show using yeast and mammalian two-hybrid assays as well as biochemical pull-down assays that eHAND and dHAND are capable of forming both HAND homo- and heterodimers in vivo. To investigate whether HAND genes form heterodimers with other biologically relevant bHLH proteins, we tested and show HAND heterodimerization with the recently identified Hairy-related transcription factors, HRT1-3. This finding is exciting, because both HRT and HAND genes are coexpressed in the developing heart and limb and both have been implicated in establishing tissue boundaries and pattern formation. Moreover, competition gel shift analysis demonstrates that dHAND and eHAND can negatively regulate the DNA binding of MyoD/E12 heterodimers in a manner similar to MISTI and Id proteins, suggesting a possible transcriptional inhibitory role for HAND genes. Taken together, these results show that dHAND and eHAND can form homo- and heterodimer combinations with multiple bHLH partners and that this broad dimerization profile reflects the mechanisms by which HAND genes regulate transcription.