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

Possible New Candidates Involved to Thalidomide-Related Limbs and Cardiac Defects: A Systems Biology Approach

Thalidomide is a known teratogen that causes malformations especially in heart and limbs. Its mechanism of teratogenicity is still not fully elucidated. Recently, a new target of thalidomide was described, TBX5, and was observed a new interaction between HAND2 and TBX5 that is disrupted in the presence of thalidomide. Therefore, our study aimed to raise potential candidates for thalidomide teratogenesis, through systems biology, evaluating HAND2 and TBX5 interaction and heart and limbs malformations of thalidomide. Genes and proteins related to TBX5 and HAND2 were selected through TF2DNA, REACTOME, Human Phenotype Ontology, and InterPro databases. Networks were assembled using STRING © database. Network analysis were performed in Cytoscape © and R v3.6.2. Differential gene expression (DGE) analysis was performed through gene expression omnibus. We constructed a network for HAND2 and TBX5 interaction; a network for heart and limbs malformations of TE; and the two joined networks. We observed that EP300 protein seemed to be important in all networks. We also looked for proteins containing C2H2 domain in the assembled networks. ZIC3, GLI1, GLI3, ZNF148, and PRDM16 were the ones present in both heart and limbs malformations of TE networks. Furthermore, in the DGE analysis after treatment with thalidomide, we observed that FANCB, ESCO2, and XRCC2 were downregulated and present both in heart and limbs networks. Through systems biology, we were able to point to different new proteins and genes, and selected specially EP300, which was important in all the analyzed networks, to be further evaluated in the TE teratogenicity.

Hand2 represses non-cardiac cell fates through chromatin remodeling at cis- regulatory elements

Developmental studies have revealed the importance of the transcription factor Hand2 in cardiac development. Hand2 promotes cardiac progenitor differentiation and epithelial maturation, while repressing other tissue types. The mechanisms underlying the promotion of cardiac fates are far better understood than those underlying the repression of alternative fates. Here, we assess Hand2-dependent changes in gene expression and chromatin remodeling in cardiac progenitors of zebrafish embryos. Cell-type specific transcriptome analysis shows a dual function for Hand2 in activation of cardiac differentiation genes and repression of pronephric pathways. We identify functional cis- regulatory elements whose chromatin accessibility are increased in hand2 mutant cells. These regulatory elements associate with non-cardiac gene expression, and drive reporter gene expression in tissues associated with Hand2-repressed genes. We find that functional Hand2 is sufficient to reduce non-cardiac reporter expression in cardiac lineages. Taken together, our data support a model of Hand2-dependent coordination of transcriptional programs, not only through transcriptional activation of cardiac and epithelial maturation genes, but also through repressive chromatin remodeling at the DNA regulatory elements of non-cardiac genes.

Genetic evaluation of HAND2 gene and its effects on thalidomide embryopathy

BACKGROUND

HAND2 is a transcription factor important for embryonic development, required for limbs and cardiovascular development. Thalidomide is a drug responsible to a spectrum of congenital anomalies known as Thalidomide Embryopathy (TE), which includes mainly limb and heart defects. It is known that HAND2 interaction with TBX5, an important protein for limbs and heart development, is inhibited by Thalidomide. The aim of this study was to evaluate and characterize HAND2 in the context of TE, and to evaluate its variability in TE individuals.

METHODS

DNA from 35 TE subjects was extracted from saliva samples and PCR was performed for amplification and Sanger sequencing of HAND2 coding sequence.

RESULTS

The analysis showed only one variant; a synonymous variant p.P51 (rs59621536) in exon 1 found in three individuals. Further in silico evaluation confirmed highly HAND2 conservation, being the 3'UTR the most polymorphic region of the gene. Additional computational analyses classified the variant as neutral, without alteration in splicing and miRNA sites. In silico predictions pointed to alteration of two CpG islands adjacent to the variant; however, we did not observe any alterations on the methylation pattern of HAND2 gene in our sample. Moreover, alteration of the binding site of MeCP2, a nuclear protein involved in DNA methylation, was predicted along with alteration in HAND2 mRNA structure.

CONCLUSIONS

Considering HAND2 being a well conserved gene, further studies with a larger sample should be performed to evaluate the role this gene on genetic susceptibility to TE.

Somatic mutations enriched in cis-regulatory elements affect genes involved in embryonic development and immune system response in neuroblastoma

Noncoding cis-regulatory variants have gained interest as cancer drivers, yet progress in understanding their significance is hindered by the numerous challenges and limitations of variant prioritization. To overcome these limitations, we focused on active cis-regulatory elements (aCRE) in order to design a customized panel for the deep sequencing of 56 neuroblastoma tumor and normal DNA sample pairs. In order to search for driver mutations, aCREs were defined by reanalysis of H3K27ac ChiP-seq peaks in 25 neuroblastoma cell lines. These regulatory genomic regions were tested for an excess of somatic mutations and assessed for statistical significance using a global approach that accounted for chromatin accessibility and replication timing. Additional validation was provided by whole genome sequence analysis of 151 neuroblastomas. Analysis of Hi-C data determined the presence of candidate target genes interacting with mutated regions. An excess of somatic mutations in aCREs of diverse genes were identified, including IPO7, HAND2, and ARID3A. CRISPR-Cas9 editing was utilized to assess the functional consequences of mutations in the IPO7 aCRE. Patients with noncoding mutations in aCREs showed inferior overall and event-free survival independent of age at diagnosis, stage, risk stratification, and MYCN status. Expression of aCRE-interacting genes correlated strongly with negative prognostic markers and low survival rates. Moreover, a convergence between the biological functions of aCRE target genes and transcription factors with mutated binding motifs was associated with embryonic development and immune system response. Overall, this strategy enabled the identification of somatic mutations in regulatory elements that collectively promote neuroblastoma tumorigenesis.

Homodimeric and Heterodimeric Interactions among Vertebrate Basic Helix-Loop-Helix Transcription Factors

The basic helix–loop–helix transcription factor (bHLH TF) family is involved in tissue development, cell differentiation, and disease. These factors have transcriptionally positive, negative, and inactive functions by combining dimeric interactions among family members. The best known bHLH TFs are the E-protein homodimers and heterodimers with the tissue-specific TFs or ID proteins. These cooperative and dynamic interactions result in a complex transcriptional network that helps define the cell’s fate. Here, the reported dimeric interactions of 67 vertebrate bHLH TFs with other family members are summarized in tables, including specifications of the experimental techniques that defined the dimers. The compilation of these extensive data underscores homodimers of tissue-specific bHLH TFs as a central part of the bHLH regulatory network, with relevant positive and negative transcriptional regulatory roles. Furthermore, some sequence-specific TFs can also form transcriptionally inactive heterodimers with each other. The function, classification, and developmental role for all vertebrate bHLH TFs in four major classes are detailed.

Hand2 Selectively Reorganizes Chromatin Accessibility to Induce Pacemaker-like Transcriptional Reprogramming

Gata4, Hand2, Mef2c, and Tbx5 (GHMT) can reprogram transduced fibroblasts into induced pacemaker-like myocytes (iPMs), but the underlying mechanisms remain obscure. Here, we explore the role of Hand2 in iPM formation by using a combination of transcriptome, genome, and biochemical as-says. We found many shared transcriptional signatures between iPMs and the endogenous sinoatrial node (SAN), yet key regulatory networks remain missing. We demonstrate that Hand2 augments chromatin accessibility at loci involved in sarcomere organization, electrical coupling, and membrane depolarization. Focusing on an established cardiac Hand2 cistrome, we observe selective reorganization of chromatin accessibility to promote pacemaker-specific gene expression. Moreover, we identify a Hand2 cardiac subtype diversity (CSD) domain through biochemical analysis of the N terminus. By integrating our RNA-seq and ATAC-seq datasets, we highlight desmosome organization as a hallmark feature of iPM formation. Collectively, our results illuminate Hand2-dependent mechanisms that may guide future efforts to rationally improve iPM formation.

Gata4, Hand2, Mef2c, and Tbx5 can reprogram fibroblasts into cardiomyocyte-like cells, including induced pacemakers (iPMs). Fernandez-Perez et al. show that Hand2 coordinates this process by influencing chromatin accessibility and gene expression in fibroblasts undergoing iPM lineage conversion. These insights could eventually inform the production of superior replacement cells.

Establishing cell lines for canine tonsillar and non-tonsillar oral squamous cell carcinoma and identifying characteristics associated with malignancy

Canine tonsillar squamous cell carcinoma (TSCC) shows a higher metastasis rate than non-tonsillar oral SCC (NTSCC). The mechanisms of metastasis for TSCC have been less studied, because both TSCC and NTSCC cell lines are few. In this study, 6 cloned TSCC (TSCCLN#1-#6), which were from a metastatic lymph node, and 2 cloned NTSCC (oSCC-1 and -4) cell lines, which were from the primary lesion, were established, and their characteristics were evaluated in vitro and in vivo. Results showed that increased expression level of Vimentin in TSCC cell lines and increased expression levels of mesenchymal markers including Vimentin, Snail, and Slug in NTSCC cell lines corelated with the malignant phenotypes such as the cell growth and colony formation abilities in vitro. However, expression levels of mesenchymal markers and in vitro characteristics were unrelated to tumorigenic ability in nude mice. Additionally, the expression levels of E-cadherin and Vimentin were also evaluated by immunohistochemistry using the formalin-fixed paraffin embedded canine oral SCC tissues, and the results show that the expression level of Vimentin in TSCC was higher than in NTSCC. In conclusion, the cell lines established in this study might contribute to elucidating the mechanisms involved in TSCC metastasis.

Intronic Polymorphisms in Gene of Second Heart Field as Risk Factors for Human Congenital Heart Disease in a Chinese Population

Transcriptional factors and signaling factors in the second heart field (SHF) contribute to cardiac development. However, the associations of intronic gene variants in the SHF with congenital heart disease (CHD) remain ununderstood. Ten single nucleotide polymorphisms (SNPs) from our previous sequencing data were selected and then genotyped in 383 CHD patients and 384 healthy controls in a Chinese population. Genotype analyses revealed that minor alleles in TBX1: rs12165908 C > G [odds ratio (OR) = 2.64; 95% confidence interval (CI) = 1.87-3.73, p = 3.03 × 10^-8] and GATA6: rs143085291 C > T (OR = 2.49; 95% CI = 1.18-5.29, p = 0.01) increased CHD risk significantly. Meanwhile, FGF10: rs78454549 T > C and GATA4: rs13275657 A>G polymorphisms were significantly associated with increased risk of simple CHDs. The minor allele C in GATA4: rs17153694 T > C increased the risk of tetralogy of Fallot, whereas minor alleles in TBX1: rs41298006 G>A, FGF10: rs75629618 C>T, FGF10: rs10461755 G>A, FGF10: rs75632187 A>G, and FGF10: rs12518964 G > A were associated with increased risk of single ventricle. The minor allele T in rs143085291 in GATA6 enhancer decreased the transcription level in luciferase assay. Our findings suggest that intronic SNPs in transcriptional factors and signaling factors in the SHF are significantly associated with increased risk of different CHD types.

HAND2 Target Gene Regulatory Networks Control Atrioventricular Canal and Cardiac Valve Development

The HAND2 transcriptional regulator controls cardiac development, and we uncover additional essential functions in the endothelial to mesenchymal transition (EMT) underlying cardiac cushion development in the atrioventricular canal (AVC). In Hand2-deficient mouse embryos, the EMT underlying AVC cardiac cushion formation is disrupted, and we combined ChIP-seq of embryonic hearts with transcriptome analysis of wild-type and mutants AVCs to identify the functionally relevant HAND2 target genes. The HAND2 target gene regulatory network (GRN) includes most genes with known functions in EMT processes and AVC cardiac cushion formation. One of these is Snai1, an EMT master regulator whose expression is lost from Hand2-deficient AVCs. Re-expression of Snai1 in mutant AVC explants partially restores this EMT and mesenchymal cell migration. Furthermore, the HAND2-interacting enhancers in the Snai1 genomic landscape are active in embryonic hearts and other Snai1-expressing tissues. These results show that HAND2 directly regulates the molecular cascades initiating AVC cardiac valve development.

Zone of Polarizing Activity Regulatory Sequence Mutations/Duplications with Preaxial Polydactyly and Longitudinal Preaxial Ray Deficiency in the Phenotype: A Review of Human Cases, Animal Models, and Insights Regarding the Pathogenesis

Clinicians and scientists interested in developmental biology have viewed preaxial polydactyly (PPD) and longitudinal preaxial ray deficiency (LPAD) as two different entities. Point mutations and duplications in the zone of polarizing activity regulatory sequence (ZRS) are associated with anterior ectopic expression of Sonic Hedgehog (SHH) in the limb bud and usually result in a PPD phenotype. However, some of these mutations/duplications also have LPAD in the phenotype. This unusual PPD-LPAD association in ZRS mutations/duplications has not been specifically reviewed in the literature. The author reviews this unusual entity and gives insights regarding its pathogenesis.

Specification of jaw identity by the Hand2 transcription factor

Acquisition of the lower jaw (mandible) was evolutionarily important for jawed vertebrates. In humans, syndromic craniofacial malformations often accompany jaw anomalies. The basic helix-loop-helix transcription factor Hand2, which is conserved among jawed vertebrates, is expressed in the neural crest in the mandibular process but not in the maxillary process of the first branchial arch. Here, we provide evidence that Hand2 is sufficient for upper jaw (maxilla)-to-mandible transformation by regulating the expression of homeobox transcription factors in mice. Altered Hand2 expression in the neural crest transformed the maxillae into mandibles with duplicated Meckel's cartilage, which resulted in an absence of the secondary palate. In Hand2-overexpressing mutants, non-Hox homeobox transcription factors were dysregulated. These results suggest that Hand2 regulates mandibular development through downstream genes of Hand2 and is therefore a major determinant of jaw identity. Hand2 may have influenced the evolutionary acquisition of the mandible and secondary palate.

A HAND2 Loss-of-Function Mutation Causes Familial Ventricular Septal Defect and Pulmonary Stenosis

Congenital heart disease (CHD) is the most common developmental abnormality, and is the leading noninfectious cause of mortality in neonates. Increasing evidence demonstrates that genetic defects play an important role in the pathogenesis of CHD. However, CHD exhibits substantial heterogeneity, and the genetic determinants for CHD remain unknown in the overwhelming majority of cases. In the current study, the coding exons and flanking introns of the HAND2 gene, which encodes a basic helix-loop-helix transcription factor essential for normal cardiovascular development, were sequenced in 192 unrelated patients with CHD, and a novel heterozygous mutation, p.S65I, was identified in a patient with congenital ventricular septal defect (VSD). Genetic analysis of the index patient’s pedigree revealed that the mutation was present in all seven affected family members available, but absent in the 13 unaffected family members examined. Besides, in addition to VSD, five of the proband’s close relatives also had pulmonary stenosis (PS), and the proband’s son also had double outlet right ventricle (DORV). The missense mutation, which altered an evolutionarily conserved amino acid, was absent in 300 unrelated, ethnically matched healthy individuals. Biological analyses using a dual-luciferase reporter assay system showed that the mutant HAND2 was associated with significantly diminished transcriptional activity. Furthermore, the mutation abolished the synergistic activation between HAND2 and GATA4, as well as NKX2.5—two other cardiac core transcriptional factors that have been causally linked to CHD. These findings indicate that HAND2 loss-of-function mutation contributes to human CHD, perhaps via its interaction with GATA4 and NKX2.5.

The bHLH Transcription Factor Hand Regulates the Expression of Genes Critical to Heart and Muscle Function in Drosophila melanogaster

Hand proteins belong to the highly conserved family of basic Helix-Loop-Helix transcription factors and are critical to distinct developmental processes, including cardiogenesis and neurogenesis in vertebrates. In Drosophila melanogaster a single orthologous hand gene is expressed with absence of the respective protein causing semilethality during early larval instars. Surviving adult animals suffer from shortened lifespan associated with a disorganized myofibrillar structure being apparent in the dorsal vessel, the wing hearts and in midgut tissue. Based on these data, the major biological significance of Hand seems to be related to muscle development, maintenance or function; however, up to now the physiological basis for Hand functionality remains elusive. Thus, the identification of genes whose expression is, directly or indirectly, regulated by Hand has considerable relevance with respect to understanding its biological functionality in flies and vertebrates. Beneficially, hand mutants are viable and exhibit affected tissues, which renders Drosophila an ideal model to investigate up- or downregulated target genes by a comparative microarray approach focusing on the respective tissues from mutant specimens. Our present work reveals for the first time that Drosophila Hand regulates the expression of numerous genes of diverse physiological relevancy, including distinct factors required for proper muscle development and function such as Zasp52 or Msp-300. These results relate Hand activity to muscle integrity and functionality and may thus be highly beneficial to the evaluation of corresponding hand phenotypes.

HAND2-mediated proteolysis negatively regulates the function of estrogen receptor α

A previous study demonstrated that the progesterone‑inducible HAND2 gene product is a basic helix‑loop‑helix transcription factor and prevents mitogenic effects of estrogen receptor α (ERα) by inhibiting fibroblast growth factor signalling in mouse uteri. However, whether HAND2 directly affects the transcriptional activation function of ERα remains to be elucidated. In the present study, the physical interaction between HAND2 and ERα was investigating by performing an immunoprecipitation assay and an in vitro pull‑down assay. The results demonstrated that HAND2 and ERα interacted in a ligand‑independent manner. The in vitro pull‑down assays revealed a direct interaction between HAND2 and the amino‑terminus of ERα, termed the activation function‑1 domain. To determine the physiological significance of this interaction, the role of HAND2 as a cofactor of ERα was investigated, which revealed that HAND2 inhibited the ligand‑dependent transcriptional activation function of ERα. This result was further confirmed and the mRNA expression of vascular endothelial growth factor, an ERα‑downstream factor, was decreased by the overexpression of HAND2. This inhibition of ligand‑dependent transcriptional activation function of ERα was possibly attributed to the proteasomic degradation of ERα by HAND2. These results indicate a novel anti‑tumorigenic function of HAND2 in regulating ERα‑dependent gene expression. Considering that HAND2 is commonly hypermethylated and silenced in endometrial cancer, it is hypothesized that HAND2 may serve as a possible tumor suppressor, particularly in uterine tissue.

HAND2 targets define a network of transcriptional regulators that compartmentalize the early limb bud mesenchyme

The genetic networks that govern vertebrate development are well studied, but how the interactions of trans-acting factors with cis-regulatory modules (CRMs) are integrated into spatiotemporal regulation of gene expression is not clear. The transcriptional regulator HAND2 is required during limb, heart, and branchial arch development. Here, we identify the genomic regions enriched in HAND2 chromatin complexes from mouse embryos and limb buds. Then we analyze the HAND2 target CRMs in the genomic landscapes encoding transcriptional regulators required in early limb buds. HAND2 controls the expression of genes functioning in the proximal limb bud and orchestrates the establishment of anterior and posterior polarity of the nascent limb bud mesenchyme by impacting Gli3 and Tbx3 expression. TBX3 is required downstream of HAND2 to refine the posterior Gli3 expression boundary. Our analysis uncovers the transcriptional circuits that function in establishing distinct mesenchymal compartments downstream of HAND2 and upstream of SHH signaling.

Preaxial polydactyly of the upper limb viewed as a spectrum of severity of embryonic events

Preaxial polydactyly (PPD) is a common congenital abnormality and its classification varies among geneticists and hand surgeons. For example, the triphalangeal thumb, preaxial polysyndactyly, and the mirror hand deformity are considered as forms of PPD only in the genetics literature. Preaxial polydactyly is an error in the anteroposterior axis of the development of the upper limb. In this paper, the development of this axis is detailed and all molecular events that are known to lead to PPD are reviewed. Finally, based on the review, PPD is viewed as a spectrum of severity of embryonic events.

Epithelial to mesenchymal transition in head and neck squamous cell carcinoma

Epithelial to mesenchymal transition (EMT) is a dynamic cellular process that is essential for the development of metastatic disease. During EMT, a tumor cell with epithelial characteristics transitions to a tumor cell with mesenchymal characteristics through modulation of cell polarity and adhesion. Two hallmark EMT proteins, E-Cadherin and Vimentin, are tightly controlled during EMT through multiple signal transduction pathways. Epidermal growth factor (EGF) and transforming growth factorβ (TGFβ) promote EMT by regulating a distinct set of transcription factors, including Snail and Twist. Snail, Twist, and Slug are integral to the induction of EMT through direct regulation of genes involved in cellular adhesion, migration, and invasion. This review highlights the current literature on EMT in HNSCC. Understanding the role of EMT will provide insight to the pathogenesis of disease progression and may lead to the development of novel anti-cancer therapeutics for metastatic HNSCC.

Downregulation of Dlx5 and Dlx6 expression by Hand2 is essential for initiation of tongue morphogenesis

Lower jaw development is a complex process in which multiple signaling cascades establish a proximal-distal organization. These cascades are regulated both spatially and temporally and are constantly refined through both induction of normal signals and inhibition of inappropriate signals. The connective tissue of the tongue arises from cranial neural crest cell-derived ectomesenchyme within the mandibular portion of the first pharyngeal arch and is likely to be impacted by this signaling. Although the developmental mechanisms behind later aspects of tongue development, including innervation and taste acquisition, have been elucidated, the early patterning signals driving ectomesenchyme into a tongue lineage are largely unknown. We show here that the basic helix-loop-helix transcription factor Hand2 plays key roles in establishing the proximal-distal patterning of the mouse lower jaw, in part through establishing a negative-feedback loop in which Hand2 represses Dlx5 and Dlx6 expression in the distal arch ectomesenchyme following Dlx5- and Dlx6-mediated induction of Hand2 expression in the same region. Failure to repress distal Dlx5 and Dlx6 expression results in upregulation of Runx2 expression in the mandibular arch and the subsequent formation of aberrant bone in the lower jaw along with proximal-distal duplications. In addition, there is an absence of lateral lingual swelling expansion, from which the tongue arises, resulting in aglossia. Hand2 thus appears to establish a distal mandibular arch domain that is conducive for lower jaw development, including the initiation of tongue mesenchyme morphogenesis.

Interaction of Hand2 and E2a is important for transcription of Phox2b in sympathetic nervous system neuron differentiation

Transcription factors play a crucial role in the development of various tissues. In particular, the transcription factors of the basic helix-loop-helix (bHLH) family are crucial regulators of neurodifferentiation. Previous studies suggested that the bHLH transcription factor Hand2 is essential for sympathetic nervous system neuron differentiation in vivo, but the molecular mechanisms involved have not been well elucidated. It is important for understanding their mode of action in cellular differentiation to clarify how these bHLH factors regulate distinct transcriptional targets in a temporally and spatially controlled manner. Recent reports on ES cell differentiation suggested that its molecular mechanism mimics that of in vivo neurogenesis. However, the diverse nature of ES cell populations has prevented efficient analysis. To address this issue, we previously established a cell line in P19 embryonal carcinoma (EC) cells. Efficient sympathetic nervous system (SNS) neuron differentiation is induced in the cell line. Using this cell line, we succeeded in showing that the interaction of bHLH transcription factor Hand2 with E2a is required for transcription of Phox2b, which is essential for autonomic nervous system neuron development, and this binding activates this expression in SNS differentiation. Moreover, we also demonstrated that Hes5 regulated the transcription of Phox2b as a negative regulator and it inhibited the SNS differentiation. These findings have enabled us to determine the novel regulatory mechanism of Phox2b in SNS differentiation.

Transferase activity function and system development process are critical in cattle embryo development

Microarray gene expression experiments often consider specific developmental stages, tissue sources, or reproductive technologies. This focus hinders the understanding of the cattle embryo transcriptome. To address this, four microarray experiments encompassing three developmental stages (7, 25, 280 days), two tissue sources (embryonic or extra-embryonic), and two reproductive technologies (artificial insemination or AI and somatic cell nuclear transfer or NT) were combined using two sets of meta-analyses. The first set of meta-analyses uncovered 434 genes differentially expressed between AI and NT (regardless of stage or source) that were not detected by the individual-experiment analyses. The molecular function of transferase activity was enriched among these genes that included ECE2, SLC22A1, and a gene similar to CAMK2D. Gene POLG2 was over-expressed in AI versus NT 7-day embryos and was under-expressed in AI versus NT 25-day embryos. Gene HAND2 was over-expressed in AI versus NT extra-embryonic samples at 280 days yet under-expressed in AI versus NT embryonic samples at 7 days. The second set of meta-analyses uncovered enrichment of system, organ, and anatomical structure development among the genes differentially expressed between 7- and 25-day embryos from either reproductive technology. Genes PRDX1and SLC16A1 were over-expressed in 7- versus 25-day AI embryos and under-expressed in 7- versus 25-day NT embryos. Changes in stage were associated with high number of differentially expressed genes, followed by technology and source. Genes with transferase activity may hold a clue to the differences in efficiency between reproductive technologies.

Distinct roles of Hand2 in initiating polarity and posterior Shh expression during the onset of mouse limb bud development

The polarization of nascent embryonic fields and the endowment of cells with organizer properties are key to initiation of vertebrate organogenesis. One such event is antero-posterior (AP) polarization of early limb buds and activation of morphogenetic Sonic Hedgehog (SHH) signaling in the posterior mesenchyme, which in turn promotes outgrowth and specifies the pentadactylous autopod. Inactivation of the Hand2 transcriptional regulator from the onset of mouse forelimb bud development disrupts establishment of posterior identity and Shh expression, which results in a skeletal phenotype identical to Shh deficient limb buds. In wild-type limb buds, Hand2 is part of the protein complexes containing Hoxd13, another essential regulator of Shh activation in limb buds. Chromatin immunoprecipitation shows that Hand2-containing chromatin complexes are bound to the far upstream cis-regulatory region (ZRS), which is specifically required for Shh expression in the limb bud. Cell-biochemical studies indicate that Hand2 and Hoxd13 can efficiently transactivate gene expression via the ZRS, while the Gli3 repressor isoform interferes with this positive transcriptional regulation. Indeed, analysis of mouse forelimb buds lacking both Hand2 and Gli3 reveals the complete absence of antero-posterior (AP) polarity along the entire proximo-distal axis and extreme digit polydactyly without AP identities. Our study uncovers essential components of the transcriptional machinery and key interactions that set-up limb bud asymmetry upstream of establishing the SHH signaling limb bud organizer.

During early limb bud development, posterior mesenchymal cells are selected to express Sonic Hedgehog (Shh), which controls antero-posterior (AP) limb axis formation (axis from thumb to little finger). We generated a conditional loss-of-function Hand2 allele to inactivate Hand2 specifically in mouse limb buds. This genetic analysis reveals the pivotal role of Hand2 in setting up limb bud asymmetry as initiation of posterior identity and establishment of the Shh expression domain are completely disrupted in Hand2 deficient limb buds. The resulting loss of the ulna and digits mirror the skeletal malformations observed in Shh-deficient limbs. We show that Hand2 is part of the chromatin complexes that are bound to the cis-regulatory region that controls Shh expression specifically in limb buds. In addition, we show that Hand2 is part of a protein complex containing Hoxd13, which also participates in limb bud mesenchymal activation of Shh expression. Indeed, Hand2 and Hoxd13 stimulate ZRS–mediated transactivation in cells, while the Gli3 repressor form (Gli3R) interferes with this up-regulation. Interestingly, limb buds lacking both Hand2 and Gli3 lack AP asymmetry and are severely polydactylous. Molecular analysis reveals some of the key interactions and hierarchies that govern establishment of AP limb asymmetries upstream of SHH.

Nicotiana tabacum EIL2 directly regulates expression of at least one tobacco gene induced by sulphur starvation

Sulphur deficiency severely affects plant growth and their agricultural productivity leading to diverse changes in development and metabolisms. Molecular mechanisms regulating gene expression under low sulphur conditions remain largely unknown. AtSLIM1, a member of the EIN3-like (EIL) family was reported to be a central transcriptional regulator of the plant sulphur response, however, no direct interaction of this protein with any sulphur-responsive promoters was demonstrated. The focus of this study was on the analysis of a promoter region of UP9C, a tobacco gene strongly induced by sulphur limitation. Cloning and subsequent examination of this promoter resulted in the identification of a 20-nt sequence (UPE-box), also present in the promoters of several Arabidopsis genes, including three out of four homologues of UP9C. The UPE-box, consisting of two parallel tebs sequences (TEIL binding site), proved to be necessary to bind the transcription factors belonging to the EIL family and of a 5-nt conserved sequence at the 3'-end. The yeast one-hybrid analysis resulted in the identification of one transcription factor (NtEIL2) capable of binding to the UPE-box. The interactions of NtEIL2, and its homologue from Arabidopsis, AtSLIM1, with DNA were affected by mutations within the UPE-box. Transient expression assays in Nicotiana benthamiana have further shown that both factors, NtEIL2 and AtSLIM1, activate the UP9C promoter. Interestingly, activation by NtEIL2, but not by AtSLIM1, was dependent on the sulphur-deficiency of the plants.

Analyzing mRNA expression identifies Smad3 as a microRNA-140 target regulated only at protein level

mRNA profiling is routinely used to identify microRNA targets, however, this high-throughput technology is not suitable for identifying targets regulated only at protein level. Here, we have developed and validated a novel methodology based on computational analysis of promoter sequences combined with mRNA microarray experiments to reveal transcription factors that are direct microRNA targets at the protein level. Using this approach we identified Smad3, a key transcription factor in the TGFbeta signaling pathway, as a direct miR-140 target. We showed that miR-140 suppressed the TGFbeta pathway through repression of Smad3 and that TGFbeta suppressed the accumulation of miR-140 forming a double negative feedback loop. Our findings establish a valid strategy for the discovery of microRNA targets regulated only at protein level, and we propose that additional targets could be identified by re-analysis of existing microarray datasets.

Hand2 controls osteoblast differentiation in the branchial arch by inhibiting DNA binding of Runx2

Members of the basic helix-loop-helix (bHLH) family of transcription factors regulate the specification and differentiation of numerous cell types during embryonic development. Hand1 and Hand2 are expressed by a subset of neural crest cells in the anterior branchial arches and are involved in craniofacial development. However, the precise mechanisms by which Hand proteins mediate biological actions and regulate downstream target genes in branchial arches is largely unknown. Here, we report that Hand2 negatively regulates intramembranous ossification of the mandible by directly inhibiting the transcription factor Runx2, a master regulator of osteoblast differentiation. Hand proteins physically interact with Runx2, suppressing its DNA binding and transcriptional activity. This interaction is mediated by the N-terminal domain of the Hand protein and requires neither dimerization with other bHLH proteins nor DNA binding. We observed partial colocalization of Hand2 and Runx2 in the mandibular primordium of the branchial arch, and downregulation of Hand2 precedes Runx2-driven osteoblast differentiation. Hand2 hypomorphic mutant mice display insufficient mineralization and ectopic bone formation in the mandible due to accelerated osteoblast differentiation, which is associated with the upregulation and ectopic expression of Runx2 in the mandibular arch. Here, we show that Hand2 acts as a novel inhibitor of the Runx2-DNA interaction and thereby regulates osteoblast differentiation in branchial arch development.

A twist of insight - the role of Twist-family bHLH factors in development

Members of the Twist-family of bHLH proteins play a pivotal role in a number of essential developmental programs. Twist-family bHLH proteins function by dimerizing with other bHLH members and binding to cis- regulatory elements, called E-boxes. While Twist-family members may simply exhibit a preference in terms of high-affinity binding partners, a complex, multilevel cascade of regulation creates a dynamic role for these bHLH proteins. We summarize in this review information on each Twist-family member concerning expression pattern, function, regulation, downstream targets, and interactions with other bHLH proteins. Additionally, we focus on the phospho-regulatory mechanisms that tightly control posttranslational modification of Twist-family member bHLH proteins.

Effects of methotrexate on the developments of heart and vessel in zebrafish

Methotrexate (MTX), an antagonist of folic acid, can inhibit dihydrofolate reductase (DHFR) which is of great importance in the synthesis of tetrahydrofolic acid and embryonic development. In this study, we found that after being exposed to 1.5 mM MTX at 6-10 hours post-fertilization, zebrafish embryos fail to form normal cardiovascular system. In MTX-treated embryos, the morphological development of ventricle and atrium was disrupted, the cardiac twist was abnormal, the heart rate and ventricular shortening fraction were reduced, and the vascular development was disrupted. We also found that either microinjection with dhfr-gfp mRNA or treatment with folinic acid calcium salt pentahydrate (CF) could cause improved development in the heart and vessels in MTX-treated embryos, which proved that MTX induced the malformations by inhibiting DHFR. The transcript levels of genes such as hand2, mef2a, mef2c, and flk-1 were reduced in MTXtreated embryos. Compared with the MTX-treated group, the transcript levels of hand2, mef2a, mef2c, and flk-1 were increased in the MTX 1 dhfr-gfp mRNA injected group and in the MTX 1 CF group. Our results indicated that the disrupted development of the heart and vessels in MTX-treated embryos is related to the reduced transcript levels of hand2, mef2a, mef2c, and flk-1.

Target-dependent inhibition of sympathetic neuron growth via modulation of a BMP signaling pathway

Target-derived factors modulate many aspects of peripheral neuron development including neuronal growth, survival, and maturation. Less is known about how initial target contact regulates changes in gene expression associated with these developmental processes. One early consequence of contact between growing sympathetic neurons and their cardiac myocyte targets is the inhibition of neuronal outgrowth. Analysis of neuronal gene expression following this contact revealed coordinate regulation of a bone morphogenetic protein (BMP)-dependent growth pathway in which basic helix-loop-helix transcription factors and downstream neurofilament expression contribute to the growth dynamics of developing sympathetic neurons. BMP2 had dose-dependent growth-promoting effects on sympathetic neurons cultured in the absence, but not the presence, of myocyte targets, suggesting that target contact alters neuronal responses to BMP signaling. Target contact also induced the expression of matrix Gla protein (MGP), a regulator of BMP function in the vascular system. Increased MGP expression inhibited BMP-dependent neuronal growth and MGP expression increased in sympathetic neurons during the period of target contact in vivo. These experiments establish MGP as a novel regulator of BMP function in the nervous system, and define developmental transitions in BMP responses during sympathetic development.

Mutations within helix I of Twist1 result in distinct limb defects and variation of DNA binding affinities

Twist1 is a basic helix-loop-helix (bHLH) factor that plays an important role in limb development. Haploinsufficiency of Twist1 results in polydactyly via the inability of Twist1 to antagonistically regulate the related factor Hand2. The mechanism modulating Twist1-Hand2 antagonism is via phosphoregulation of conserved threonine and serine residues in helix I of the bHLH domain. Phosphoregulation alters the dimerization affinities for both proteins. Here we show that the expression of Twist1 and Twist1 phosphoregulation mutants results in distinct limb phenotypes in mice. In addition to dimer regulation, Twist1 phosphoregulation affects the DNA binding affinities of Twist1 in a partner-dependent and cis-element-dependent manner. In order to gain a better understanding of the specific Twist1 transcriptional complexes that function during limb morphogensis, we employ a series of Twist1-tethered dimers that include the known Twist1 partners, E12 and Hand2, as well as a tethered Twist1 homodimer. We show that these dimers behave in a manner similar to monomerically expressed bHLH factors and result in distinct limb phenotypes that correlate well with those observed from the limb expression of Twist1 and Twist1 phosphoregulation mutants. Taken together, this study shows that the Twist1 dimer affinity for a given partner can modulate the DNA binding affinity and that Twist1 dimer choice determines phenotypic outcome during limb development.

Hand2 is necessary for terminal differentiation of enteric neurons from crest-derived precursors but not for their migration into the gut or for formation of glia

Hand genes encode basic helix-loop-helix transcription factors that are expressed in the developing gut, where their function is unknown. We now report that enteric Hand2 expression is limited to crest-derived cells, whereas Hand1 expression is restricted to muscle and interstitial cells of Cajal. Hand2 is developmentally regulated and is intranuclear in precursors but cytoplasmic in neurons. Neurons develop in explants from wild-type but not Hand2(-/-) bowel, although, in both, crest-derived cells are present and glia arise. Similarly, small interfering RNA (siRNA) silencing of Hand2 in enteric crest-derived cells prevents neuronal development. Terminally differentiated enteric neurons do not develop after conditional inactivation of Hand2 in migrating crest-derived cells; nevertheless, conditional Hand2 inactivation does not prevent precursors from expressing early neural markers. We suggest that enteric neuronal development occurs in stages and that Hand2 expression is required for terminal differentiation but not for precursors to enter the neuronal lineage.

Molecular basis of pregnancy-induced breast cancer protection

We have postulated that the lifetime protective effect of an early pregnancy against breast cancer is due to the complete differentiation of the mammary gland characterized by a specific genomic signature imprinted by the physiological process of pregnancy. In the present work, we show evidence that the breast tissue of postmenopausal parous women has had a shifting of stem cell 1 to stem cell 2 with a genomic signature different from similar structures derived from postmenopausal nulliparous women that have stem cell 1. Those genes that are significantly different are grouped in major categories on the basis of their putative functional significance. Among them are those gene transcripts related to immune surveillance, DNA repair, transcription, chromatin structure/activators/co-activators, growth factor and signal transduction pathway, transport and cell trafficking, cell proliferation, differentiation, cell adhesion, protein synthesis and cell metabolism. From these data, it was concluded that during pregnancy there are significant genomic changes that reflect profound alterations in the basic physiology of the mammary gland that explain the protective effect against carcinogenesis. The implication of this knowledge is that when the genomic signature of protection or refractoriness to carcinogenesis is acquired by the shifting of stem cell 1 to stem cell 2, the hormonal milieu induced by pregnancy or pregnancy-like conditions is no longer required. This is a novel concept that challenges the current knowledge that a chemopreventive agent needs to be given for a long period to suppress a metabolic pathway or abrogate the function of an organ.

The genomic signature of breast cancer prevention

Early pregnancy imprints in the breast permanent genomic changes or a signature that reduces the susceptibility of this organ to cancer. The breast attains its maximum development during pregnancy and lactation. After menopause, the breast regresses in both nulliparous and parous women containing lobular structures designated Lob.1. The Lob 1 found in the breast of nulliparous women and of parous women with breast cancer never went through the process of differentiation, retaining a high concentration of epithelial cells that are targets for carcinogens and therefore susceptible to undergoing neoplastic transformation, these cell are called Stem cells 1, whereas Lob 1 structures found in the breast of early parous postmenopausal women free of mammary pathology, on the other hand, are composed of an epithelial cell population that is refractory to transformation called Stem cells 2. The degree of differentiation acquired through early pregnancy has changed the genomic signature that differentiates the Lob 1 from the early parous women from that of the nulliparous women by shifting the Stem cell 1 to a Stem cell 2, making this the postulated mechanism of protection conferred by early full-term pregnancy. The identification of a putative breast stem cell (Stem cell 1) has reached in the last decade a significant impulse and several markers also reported for other tissues have been found in the mammary epithelial cells of both rodents and humans. The data obtained thus far is supporting the concept that the lifetime protective effect of an early pregnancy against breast cancer is due to the complete differentiation of the mammary gland, which results in the replacement of the Stem cell 1 that is a component of the nulliparous breast epithelium with a new stem cell, called Stem cell 2, which is characterized by a specific genomic signature. The pattern of gene expression of the stem cell 2 could potentially be used as useful intermediate end points for evaluating the degree of mammary gland differentiation and for evaluating preventive agents such as human chorionic gonadotropin.

Congenital heart diseases in small animals: part I. Genetic pathways and potential candidate genes

Proper cardiac morphogenesis requires a series of specific cell and tissue interactions driven by several cardiac transcription factors and downstream cardiac genes. To date, a number of genetic aetiologies responsible for human congenital heart defects (CHDs) have been identified, although none has been found for CHDs in small animals. Most gene mutations responsible for human CHDs exist in genetic pathways associated with cardiomorphogenesis. Insights into cardiomorphogenesis from human and mouse genetic studies will help us to identify potential genetic aetiologies in CHDs in small animals. Therefore, in this first part of a two-part review, the major genetic pathways for cardiomorphogenesis and important candidate genes for CHDs, based on mouse knock-out and human genetic studies are discussed.

The basic helix-loop-helix factor Hand 2 regulates autonomic nervous system development

Mammalian autonomic nervous system (ANS) development requires the combinatorial action of a number of transcription factors, which include Mash 1, Phox 2b, and GATA 3. Here we show that the bHLH transcription factor, Hand 2 (dHAND), is expressed concurrently with Mash 1 during sympathetic nervous system (SNS) development and that the expression of Hand 2 is not dependent on Mash 1. This suggests that these two bHLH factors work in parallel during SNS development. We also show that ectopic expression of Hand 2 activates the neuronal program and promotes the acquisition of a phenotype corresponding to peripheral neurons including neurons of the SNS lineage in P19 embryonic carcinoma cells. We propose that Hand 2 works in parallel with other members of the transcriptional network to regulate ANS developmental but can ectopically activate the program by a cross-regulatory mechanism that includes the activation of Mash 1. We show that this function is dependent on its interaction with the histone acetyltransferase p300/CBP, indicating that Hand 2 functions to promote ANS development as part of a larger transcriptional complex.

MEF2-dependent Recruitment of the HAND1 Transcription Factor Results in Synergistic Activation of Target Promoters

HAND proteins are tissue-restricted members of the basic helix-loop-helix transcription factor family that play critical roles in cell differentiation and organogenesis including placental, cardiovascular, and craniofacial development. Nevertheless, the molecular basis underlying the developmental action of HAND proteins remains undefined. Within the embryo, HAND1 is first detected in the developing heart where it becomes restricted to the atrial and left ventricular compartments, a pattern identical to that of the Nppa gene, which encodes atrial natriuretic factor, the major secretory product of the heart. We hereby report that the cardiac atrial natriuretic factor promoter is directly activated by HAND1, making it the first known HAND1 transcriptional target. The action of HAND1 does not require heterodimerization with class I basic helix-loop-helix factors or DNA binding through E-box elements. Instead, HAND1 is recruited to the promoter via physical interaction with MEF2 proteins. MEF2/HAND1 interaction results in synergistic activation of MEF2-dependent promoters, and MEF2 binding sites are sufficient to mediate this synergy. MEF2 binding to DNA is not enhanced in the presence of HAND1. Instead, cooperativity likely results from corecruitment of co-activators such as CREB-binding protein. The related HAND2 protein can also synergize with MEF2. Thus, HAND proteins act as cell-specific developmental co-activators of the MEF2 family of transcription factors. These findings identify a novel mechanism for HAND action in the heart and provide a general paradigm to understand the mechanism of HAND action in organogenesis.

Tbx3 can alter limb position along the rostrocaudal axis of the developing embryo

The limbs of the vertebrate embryo form at precise locations along the body and these positions are fixed across different species. The mechanisms that control this process are not understood. Ectopic expression of Tbx3, a transcriptional repressor that belongs to the Tbx2/3/4/5 subfamily of T-box transcriptional regulators, in the forelimb results in a rostral shift in the position of the limb along the main body axis. By contrast, a transcriptional activator form of Tbx3 shifts the limb to more caudal locations. We also show that dHand and Gli3, genes previously implicated in anteroposterior pre-patterning of the limb-forming region, are also involved in refining the position of the limbs. Our data suggest a new role for Tbx3 in positioning the limb along the main body axis through a genetic interplay between dHand and Gli3.

Hand2 regulates epithelial formation during myocardial diferentiation

Myocardial differentiation is initiated by the activation of terminal-differentiation gene expression within a subset of cells in the anterior lateral plate mesoderm. We have previously shown that shortly after this activation, myocardial cells undergo epithelial maturation [1], suggesting that myocardial differentiation encompasses both molecular and cellular changes. To address the question of how the molecular programs driving myocardial gene expression and the formation of the myocardial epithelium are integrated, we analyzed the role of two essential myocardial terminal-differentiation factors, Hand2 and Gata5, in myocardial epithelia formation. hand2 and gata5 mutants exhibit a much-reduced number of myocardial cells and defects in myocardial gene expression [2,3]. We find that the few myocardial precursors that are present in hand2 mutants do not polarize. In contrast, embryos with reduced Gata5 function exhibit polarized myocardial epithelia despite a similar reduction in myocardial precursor number, indicating that proper cell number is not required for epithelial formation. Taken thogether, these results indicate that Hand2 is uniquely required for myocardial polarization, a previously unappreciated role for this critical transcription factor. Furthermore, these results demonstrate that two independent processes, the polarizaton of myocardial precursors and the allocation of proper cell number, contribute to myocardial development.

Cooperative activation of atrial naturetic peptide promoter by dHAND and MEF2C

An intricate array of cell-specific multiprotein complexes participate in programs of cell-specific gene expression through combinatorial interaction with different transcription factors and cofactors. The dHAND basic helix-loop-helix (bHLH) transcription factor, which is essential for heart development and extra embryonic structures, is thought to regulate cardiomyocyte-specific gene expression through combinatorial interactions with other cardiac-restricted transcription factors such as GATA4 and NKX2.5. Here, we determine that dHAND also interacts with the myocyte enhancer binding factor-2c (MEF2C) protein, which belongs to MADS-box transcription factors and is essential for heart development. dHAND and MEF2C synergistically activated expression of the atrial naturetic peptide gene (ANP) in transfected HeLa cells. GST-pulldown and immunoprecipitation assay demonstrate that full-length MEF2C protein is able to interact with dHAND in vitro and in vivo, just like MEF2A and bHLH transcription factors MyoD in skeletal muscle cells. In addition, electrophoretic mobility shift assays (EMSAs) demonstrate that MEF2C and dHAND do not influence each other's DNA binding activity. Using chromatin immunoprecipitation (ChIP) analysis in H9c2 cells we show that dHAND interact with MEF2C to form protein complex and bind A/T sequence in promoter of ANP. Taken together with previous observations, these results suggest the existence of large multiprotein transcriptional complex with core DNA binding proteins that physically interact with other transcriptional factors to form favorable conformation to potentiate transcription.

Combinatorial transcriptional interaction within the cardiac neural crest: a pair of HANDs in heart formation

The cardiac neural crest cells migrate from the rostral dorsal neural folds and populate the branchial arches, which contribute directly to the cardiac-outflow structures. Although neural crest cell specification is associated with a number of morphogenic factors, little is understood about the mechanisms by which transcription factors actually implement the transcriptional programs that dictate cell migration and later the differentiation into the proper cell types within the great vessels and the heart. It is clear from genetic evidence that members of the paired box family and basic helix-loop-helix (bHLH) transcription factors from the twist family of proteins are expressed in and play an important function in cardiac neural crest specification and differentiation. Interestingly, both paired box and bHLH factors can function as dimers and, in the case of twist family bHLH factors, partner choice can clearly dictate a change in transcriptional program. The focus of this review is to consider what role the protein-protein interactions of these transcription factors may play in determining cardiac neural crest specification and differentiation, and how genetic alteration of transcription factor stoichiometry within the cell may reflect more than a simple null event.

A twisted hand: bHLH protein phosphorylation and dimerization regulate limb development

Saethre-Chotzen syndrome (SCS), a human autosomal dominant condition with limb defects and craniosynostosis, is caused by haploinsufficiency of TWIST1, a basic helix-loop-helix (bHLH) transcription factor. Until recently, the molecular pathogenesis of the limb defects in SCS has not been well understood. Now, Firulli et al.1 show in mouse and chick that ectopic expression of a related bHLH protein, Hand2, results in phenocopies of the limb defects caused by Twist1 loss-of-function mutations. These two proteins interact in a dosage-dependent antagonistic manner, and both can be regulated through phosphorylation at conserved helix I amino acid residues. These findings provide an important link between the misregulation of Twist1 dimerization and the limb phenotypes observed in SCS.

Differential cooperation between dHAND and three different E-proteins

dHAND is a transcription factor belonging to the class B basic helix-loop-helix protein family and is expressed during embryogenesis in the heart, branchial arches, limb buds, and neural crest derivatives. Despite much study, the molecular mechanisms involved in the regulation of dHAND activity are not well understood. We therefore carried out yeast two-hybrid screening using full-length dHAND as bait, which led to identification of several dHAND-binding proteins, including three E-proteins: E2A, ME2, and ALF1. Subsequent analysis revealed that although their heterodimerization and transcriptional activities were similar, dHAND/E-protein heterodimers bind to an E-box element with differing affinities, suggesting they have distinct DNA binding specificities. Moreover, in situ hybridization showed that E-protein genes are expressed fairly ubiquitously among embryonic tissues, including the branchial arches and limb buds. By contrast, little signal was detected in the heart, suggesting that dHAND complexes with partners other than E-proteins in cardiac tissue.

Cooperative interaction between the basic helix-loop-helix transcription factor dHAND and myocyte enhancer factor 2C regulates myocardial gene expression

Cardiac-restricted transcription factors dHAND and myocyte enhancer factor 2C are expressed in the developing heart and activate several cardiac promoters. However, their regulatory mechanisms are still to be understood. To elucidate their exact regulatory functions, we have developed an RNA interference strategy to specifically inhibit dHAND and myocyte enhancer factor 2C protein production in H9c2 cells, which are derived from rat embryonic heart. Expression of endogenous cardiac genes atrial natriuretic peptide and alpha-myosin heavy chain was down-regulated in H9c2 cells lacking both dHAND and myocyte enhancer factor 2C, indicating that these factors are required for the maintenance of the cardiac genetic program. Consistent with these, expression of atrial natriuretic peptide and alpha-myosin heavy chain was up-regulated in H9c2 cells, which overexpressed dHAND and myocyte enhancer factor 2C. In addition, dHAND and myocyte enhancer factor 2C interact to synergistically activate atrial natriuretic peptide and alpha-myosin heavy chain transcription. Furthermore, chromatin immunoprecipitation analysis in H9c2 cells treated with phenylephrine showed that dHAND and myocyte enhancer factor 2C protein complex bind to the A/T sequence on atrial natriuretic peptide promoter. Taken together, these results not only suggest that the complex cis-trans interaction of dHAND, myocyte enhancer factor 2C, and the target gene may fine-tune gene expression in cardiac myocytes but also provide a molecular paradigm to elucidate the mechanisms of action of dHAND and myocyte enhancer factor 2C in the developing heart.

Basic helix-loop-helix proteins expressed during early embryonic organogenesis

The basic helix-loop-helix proteins form a special group of transcription factors unique for the eukaryotic organisms. They are crucial for the embryonic development of many fundamental organ systems such as muscle, heart, central nervous system, hematopoiteic system, and many others. They are very flexible in terms of regulating transcription in that they can either promote or repress transcription, and do so in many different ways. Basic helix-loop-helix proteins can form homo- or heterodimers with other members of the group, and are subject to post-transcriptional modifications. In this review, an overview of basic helix-loop-helix protein classification, biochemical function, and examples of past and recent advances in our understanding of embryonic development are presented, with emphasis on the vertebrate muscle, heart, brain, and eye.