Expression of HAND gene products may be sufficient for the differentiation of avian neural crest-derived cells into catecholaminergic neurons in culture

Generation of self-organized autonomic ganglion organoids from fibroblasts

Neural organoids have been shown to serve as powerful tools for studying the mechanism of neural development and diseases as well as for screening drugs and developing cell-based therapeutics. Somatic cells have previously been reprogrammed into scattered autonomic ganglion (AG) neurons but not AG organoids. Here we have identified a combination of triple transcription factors (TFs) Ascl1, Phox2a/b, and Hand2 (APH) capable of efficiently reprogramming mouse fibroblasts into self-organized and networked induced AG (iAG) organoids, and characterized them by immunostaining, qRT-PCR, patch-clamping, and scRNA-seq approaches. The iAG neurons exhibit molecular properties, subtype diversity, and electrophysiological characteristics of autonomic neurons. Moreover, they can integrate into the superior cervical ganglia following transplantation and innervate and control the beating rate of co-cultured ventricular myocytes. Thus, iAG organoids may provide a valuable tool to study the pathogenesis of autonomic nervous system diseases and screen for drugs, as well as a source for cell-based therapies.

Neonatal Deletion of Hand1 and Hand2 within Murine Cardiac Conduction System Reveals a Novel Role for HAND2 in Rhythm Homeostasis

The cardiac conduction system, a network of specialized cells, is required for the functioning of the heart. The basic helix loop helix factors Hand1 and Hand2 are required for cardiac morphogenesis and have been implicated in cardiac conduction system development and maintenance. Here we use embryonic and post-natal specific Cre lines to interrogate the role of Hand1 and Hand2 in the function of the murine cardiac conduction system. Results demonstrate that loss of HAND1 in the post-natal conduction system does not result in any change in electrocardiogram parameters or within the ventricular conduction system as determined by optical voltage mapping. Deletion of Hand2 within the post-natal conduction system results in sex-dependent reduction in PR interval duration in these mice, suggesting a novel role for HAND2 in regulating the atrioventricular conduction. Surprisingly, results show that loss of both HAND factors within the post-natal conduction system does not cause any consistent changes in cardiac conduction system function. Deletion of Hand2 in the embryonic left ventricle results in inconsistent prolongation of PR interval and susceptibility to atrial arrhythmias. Thus, these results suggest a novel role for HAND2 in homeostasis of the murine cardiac conduction system and that HAND1 loss potentially rescues the shortened HAND2 PR phenotype.

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.

Placental mammals acquired functional sequences in NRK for regulating the CK2-PTEN-AKT pathway and placental cell proliferation

The molecular evolution processes underlying the acquisition of the placenta in eutherian ancestors are not fully understood. Mouse NCK-interacting kinase (NIK)-related kinase (NRK) is expressed highly in the placenta and plays a role in preventing placental hyperplasia. Here, we show the molecular evolution of NRK, which confers its function for inhibiting placental cell proliferation. Comparative genome analysis identified NRK orthologs across vertebrates, which share the kinase and citron homology (CNH) domains. Evolutionary analysis revealed that NRK underwent extensive amino acid substitutions in the ancestor of placental mammals and has been since conserved. Biochemical analysis of mouse NRK revealed that the CNH domain binds to phospholipids, and a region in NRK binds to and inhibits casein kinase-2 (CK2), which we named the CK2-inhibitory region (CIR). Cell culture experiments suggest the following: 1) Mouse NRK is localized at the plasma membrane via the CNH domain, where the CIR inhibits CK2. 2) This mitigates CK2-dependent phosphorylation and inhibition of PTEN and 3) leads to the inhibition of AKT signaling and cell proliferation. Nrk deficiency increased phosphorylation levels of PTEN and AKT in mouse placenta, supporting our hypothesis. Unlike mouse NRK, chicken NRK did not bind to phospholipids and CK2, decrease phosphorylation of AKT, or inhibit cell proliferation. Both the CNH domain and CIR have evolved under purifying selection in placental mammals. Taken together, our study suggests that placental mammals acquired the phospholipid-binding CNH domain and CIR in NRK for regulating the CK2–PTEN–AKT pathway and placental cell proliferation.

Deep analysis of neuroblastoma core regulatory circuitries using online databases and integrated bioinformatics shows their pan-cancer roles as prognostic predictors

AIM

Neuroblastoma is a heterogeneous childhood cancer derived from the neural crest. The dual cell identities of neuroblastoma include Mesenchymal (MES) and Adrenergic (ADRN). These identities are conferred by a small set of tightly-regulated transcription factors (TFs) binding super enhancers, collectively forming core regulatory circuitries (CRCs). The purpose of this study was to gain a deep understanding of the role of MES and ADRN TFs in neuroblastoma and other cancers as potential indicators of disease prognosis, progression, and relapse.

METHODS

To that end, we first investigated the expression and mutational profile of MES and ADRN TFs in neuroblastoma. Moreover, we established their correlation with neuroblastoma risk groups and overall survival while establishing their extended networks with long non-coding RNAs (lncRNAs). Furthermore, we analysed the pan-cancer expression and mutational profile of these TFs and their correlation with patient survival and finally their network connectivity, using a panel of bioinformatic tools including GEPIA2, human pathology atlas, TIMER2, Omicsnet, and Cytoscape.

RESULTS

We show the association of multiple MES and ADRN TFs with neuroblastoma risk groups and overall survival and find significantly higher expression of various MES and ADRN TFs compared to normal tissues and their association with overall survival and disease-free survival in multiple cancers. Moreover, we report the strong correlation of the expression of these TFs with the infiltration of stromal and immune cells in the tumour microenvironment and with stemness and metastasis-related genes. Furthermore, we reveal extended pan-cancer networks comprising these TFs that influence the tumour microenvironment and metastasis and may be useful indicators of cancer prognosis and patient survival.

CONCLUSION

Our meta-analysis shows the significance of MES and ADRN TFs as indicators of patient prognosis and the putative utility of these TFs as potential novel biomarkers.

The diversity of neuronal phenotypes in rodent and human autonomic ganglia

Selective sympathetic and parasympathetic pathways that act on target organs represent the terminal actors in the neurobiology of homeostasis and often become compromised during a range of neurodegenerative and traumatic disorders. Here, we delineate several neurotransmitter and neuromodulator phenotypes found in diverse parasympathetic and sympathetic ganglia in humans and rodent species. The comparative approach reveals evolutionarily conserved and non-conserved phenotypic marker constellations. A developmental analysis examining the acquisition of selected neurotransmitter properties has provided a detailed, but still incomplete, understanding of the origins of a set of noradrenergic and cholinergic sympathetic neuron populations, found in the cervical and trunk region. A corresponding analysis examining cholinergic and nitrergic parasympathetic neurons in the head, and a range of pelvic neuron populations, with noradrenergic, cholinergic, nitrergic, and mixed transmitter phenotypes, remains open. Of particular interest are the molecular mechanisms and nuclear processes that are responsible for the correlated expression of the various genes required to achieve the noradrenergic phenotype, the segregation of cholinergic locus gene expression, and the regulation of genes that are necessary to generate a nitrergic phenotype. Unraveling the neuron population-specific expression of adhesion molecules, which are involved in axonal outgrowth, pathway selection, and synaptic organization, will advance the study of target-selective autonomic pathway generation.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

Striking parallels between carotid body glomus cell and adrenal chromaffin cell development

Carotid body glomus cells mediate essential reflex responses to arterial blood hypoxia. They are dopaminergic and secrete growth factors that support dopaminergic neurons, making the carotid body a potential source of patient-specific cells for Parkinson's disease therapy. Like adrenal chromaffin cells, which are also hypoxia-sensitive, glomus cells are neural crest-derived and require the transcription factors Ascl1 and Phox2b; otherwise, their development is little understood at the molecular level. Here, analysis in chicken and mouse reveals further striking molecular parallels, though also some differences, between glomus and adrenal chromaffin cell development. Moreover, histology has long suggested that glomus cell precursors are ‘émigrés’ from neighbouring ganglia/nerves, while multipotent nerve-associated glial cells are now known to make a significant contribution to the adrenal chromaffin cell population in the mouse. We present conditional genetic lineage-tracing data from mice supporting the hypothesis that progenitors expressing the glial marker proteolipid protein 1, presumably located in adjacent ganglia/nerves, also contribute to glomus cells. Finally, we resolve a paradox for the ‘émigré’ hypothesis in the chicken - where the nearest ganglion to the carotid body is the nodose, in which the satellite glia are neural crest-derived, but the neurons are almost entirely placode-derived - by fate-mapping putative nodose neuronal 'émigrés' to the neural crest.

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Glomus cell precursors express the neuron-specific marker Elavl3/4 (HuC/D).

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Developing glomus cells express multiple ‘sympathoadrenal' genes.

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Glomus cell development requires Hand2 and Sox4/11, but not Ret or Tfap2b.

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Multipotent progenitors with a glial phenotype contribute to glomus cells.

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Fate-mapping resolves a paradox for the ganglionic 'émigré' hypothesis in birds.

Distinct roles of hand2 in developing and adult autonomic neurons

The bHLH transcription factor Hand2 is essential for the acquisition and maintenance of noradrenergic properties of embryonic sympathetic neurons and controls neuroblast proliferation. Hand2 is also expressed in embryonic and postnatal parasympathetic ganglia and remains expressed in sympathetic neurons up to the adult stage. Here, we address its function in developing parasympathetic and adult sympathetic neurons. We conditionally deleted Hand2 in the parasympathetic sphenopalatine ganglion by crossing a line of floxed Hand2 mice with DbhiCre transgenic mice, taking advantage of the transient Dbh expression in parasympathetic ganglia. Hand2 elimination does not affect Dbh expression and sphenopalatine ganglion size at E12.5 and E16.5, in contrast to sympathetic ganglia. These findings demonstrate different functions for Hand2 in the parasympathetic and sympathetic lineage. Our previous Hand2 knockdown in postmitotic, differentiated chick sympathetic neurons resulted in decreased expression of noradrenergic marker genes but it was unclear whether Hand2 is required for maintaining noradrenergic neuron identity in adult animals. We now show that Hand2 elimination in adult Dbh-expressing sympathetic neurons does not decrease the expression of Th and Dbh, in contrast to the situation during development. However, gene expression profiling of adult sympathetic neurons identified 75 Hand2-dependent target genes. Interestingly, a notable proportion of down-regulated genes (15%) encode for proteins with synaptic and neurotransmission functions. These results demonstrate a change in Hand2 target genes during maturation of sympathetic neurons. Whereas Hand2 controls genes regulating noradrenergic differentiation during development, Hand2 seems to be involved in the regulation of genes controlling neurotransmission in adult sympathetic neurons. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1111-1124, 2016.

Neuroblastoma and Its Zebrafish Model

Neuroblastoma, an important developmental tumor arising in the peripheral sympathetic nervous system (PSNS), accounts for approximately 10 % of all cancer-related deaths in children. Recent genomic analyses have identified a spectrum of genetic alterations in this tumor. Amplification of the MYCN oncogene is found in 20 % of cases and is often accompanied by mutational activation of the ALK (anaplastic lymphoma kinase) gene, suggesting their cooperation in tumor initiation and spread. Understanding how complex genetic changes function together in oncogenesis has been a continuing and daunting task in cancer research. This challenge was addressed in neuroblastoma by generating a transgenic zebrafish model that overexpresses human MYCN and activated ALK in the PSNS, leading to tumors that closely resemble human neuroblastoma and new opportunities to probe the mechanisms that underlie the pathogenesis of this tumor. For example, coexpression of activated ALK with MYCN in this model triples the penetrance of neuroblastoma and markedly accelerates tumor onset, demonstrating the interaction of these modified genes in tumor development. Further, MYCN overexpression induces adrenal sympathetic neuroblast hyperplasia, blocks chromaffin cell differentiation, and ultimately triggers a developmentally-timed apoptotic response in the hyperplastic sympathoadrenal cells. In the context of MYCN overexpression, activated ALK provides prosurvival signals that block this apoptotic response, allowing continued expansion and oncogenic transformation of hyperplastic neuroblasts, thus promoting progression to neuroblastoma. This application of the zebrafish model illustrates its value in rational assessment of the multigenic changes that define neuroblastoma pathogenesis and points the way to future studies to identify novel targets for therapeutic intervention.

Neural crest migration is driven by a few trailblazer cells with a unique molecular signature narrowly confined to the invasive front

Neural crest (NC) cell migration is crucial to the formation of peripheral tissues during vertebrate development. However, how NC cells respond to different microenvironments to maintain persistence of direction and cohesion in multicellular streams remains unclear. To address this, we profiled eight subregions of a typical cranial NC cell migratory stream. Hierarchical clustering showed significant differences in the expression profiles of the lead three subregions compared with newly emerged cells. Multiplexed imaging of mRNA expression using fluorescent hybridization chain reaction (HCR) quantitatively confirmed the expression profiles of lead cells. Computational modeling predicted that a small fraction of lead cells that detect directional information is optimal for successful stream migration. Single-cell profiling then revealed a unique molecular signature that is consistent and stable over time in a subset of lead cells within the most advanced portion of the migratory front, which we term trailblazers. Model simulations that forced a lead cell behavior in the trailing subpopulation predicted cell bunching near the migratory domain entrance. Misexpression of the trailblazer molecular signature by perturbation of two upstream transcription factors agreed with the in silico prediction and showed alterations to NC cell migration distance and stream shape. These data are the first to characterize the molecular diversity within an NC cell migratory stream and offer insights into how molecular patterns are transduced into cell behaviors.

MicroRNAs define distinct human neuroblastoma cell phenotypes and regulate their differentiation and tumorigenicity

Background

Neuroblastoma (NB) is the most common extracranial solid tumor in children. NB tumors and derived cell lines are phenotypically heterogeneous. Cell lines are classified by phenotype, each having distinct differentiation and tumorigenic properties. The neuroblastic phenotype is tumorigenic, has neuronal features and includes stem cells (I-cells) and neuronal cells (N-cells). The non-neuronal phenotype (S-cell) comprises cells that are non-tumorigenic with features of glial/smooth muscle precursor cells. This study identified miRNAs associated with each distinct cell phenotypes and investigated their role in regulating associated differentiation and tumorigenic properties.

Methods

A miRNA microarray was performed on the three cell phenotypes and expression verified by qRT-PCR. miRNAs specific for certain cell phenotypes were modulated using miRNA inhibitors or stable transfection. Neuronal differentiation was induced by RA; non-neuronal differentiation by BrdU. Changes in tumorigenicity were assayed by soft agar colony forming ability. N-myc binding to miR-375 promoter was assayed by chromatin-immunoprecipitation.

Results

Unsupervised hierarchical clustering of miRNA microarray data segregated neuroblastic and non-neuronal cell lines and showed that specific miRNAs define each phenotype. qRT-PCR validation confirmed that increased levels of miR-21, miR-221 and miR-335 are associated with the non-neuronal phenotype, whereas increased levels of miR-124 and miR-375 are exclusive to neuroblastic cells. Downregulation of miR-335 in non-neuronal cells modulates expression levels of HAND1 and JAG1, known modulators of neuronal differentiation. Overexpression of miR-124 in stem cells induces terminal neuronal differentiation with reduced malignancy. Expression of miR-375 is exclusive for N-myc-expressing neuroblastic cells and is regulated by N-myc. Moreover, miR-375 downregulates expression of the neuronal-specific RNA binding protein HuD.

Conclusions

Thus, miRNAs define distinct NB cell phenotypes. Increased levels of miR-21, miR-221 and miR-335 characterize the non-neuronal, non-malignant phenotype and miR-335 maintains the non-neuronal features possibly by blocking neuronal differentiation. miR-124 induces terminal neuronal differentiation with reduction in malignancy. Data suggest N-myc inhibits neuronal differentiation of neuroblastic cells possibly by upregulating miR-375 which, in turn, suppresses HuD. As tumor differentiation state is highly predictive of patient survival, the involvement of these miRNAs with NB differentiation and tumorigenic state could be exploited in the development of novel therapeutic strategies for this enigmatic childhood cancer.

Loss of Hand2 in a population of Periostin lineage cells results in pronounced bradycardia and neonatal death

The Periostin Cre (Postn-Cre) lineage includes endocardial and neural crest derived mesenchymal cells of the cardiac cushions, neural crest-derived components of the sympathetic and enteric nervous systems, and cardiac fibroblasts. In this study, we use the Postn-Cre transgenic allele to conditionally ablate Hand2 (H2CKO). We find that Postn-Cre H2CKOs die shortly after birth despite a lack of obvious cardiac structural defects. To ascertain the cause of death, we performed a detailed comparison of the Postn-Cre lineage and Hand2 expression at mid and late stages of embryonic development. Gene expression analyses demonstrate that Postn-Cre ablates Hand2 from the adrenal medulla as well as the sphenopalatine ganglia of the head. In both cases, Hand2 loss-of-function dramatically reduces expression of Dopamine Beta Hydroxylase (Dbh), a gene encoding a crucial catecholaminergic biosynthetic enzyme. Expression of the genes Tyrosine Hydroxylase (Th) and Phenylethanolamine N-methyltransferase (Pnmt), which also encode essential catecholaminergic enzymes, were severely reduced in postnatal adrenal glands. Electrocardiograms demonstrate that 3-day postnatal Postn-Cre H2CKO pups exhibit sinus bradycardia. In conjunction with the aforementioned gene expression analyses, these results strongly suggest that the observed postnatal lethality occurs due to a catecholamine deficiency and subsequent heart failure.

Distinct neuroblastoma-associated alterations of PHOX2B impair sympathetic neuronal differentiation in zebrafish models

Heterozygous germline mutations and deletions in PHOX2B, a key regulator of autonomic neuron development, predispose to neuroblastoma, a tumor of the peripheral sympathetic nervous system. To gain insight into the oncogenic mechanisms engaged by these changes, we used zebrafish models to study the functional consequences of aberrant PHOX2B expression in the cells of the developing sympathetic nervous system. Allelic deficiency, modeled by phox2b morpholino knockdown, led to a decrease in the terminal differentiation markers th and dbh in sympathetic ganglion cells. The same effect was seen on overexpression of two distinct neuroblastoma-associated frameshift mutations, 676delG and K155X - but not the R100L missense mutation - in the presence of endogenous Phox2b, pointing to their dominant-negative effects. We demonstrate that Phox2b is capable of regulating itself as well as ascl1, and that phox2b deficiency uncouples this autoregulatory mechanism, leading to inhibition of sympathetic neuron differentiation. This effect on terminal differentiation is associated with an increased number of phox2b⁺, ascl1⁺, elavl3⁻ cells that respond poorly to retinoic acid. These findings suggest that a reduced dosage of PHOX2B during development, through either a heterozygous deletion or dominant-negative mutation, imposes a block in the differentiation of sympathetic neuronal precursors, resulting in a cell population that is likely to be susceptible to secondary transforming events.

Neuroblastoma, a tumor of the peripheral sympathetic nervous system, is the most common cancer diagnosed in infancy. Although most cases arise sporadically, familial predisposition also occurs in association with mutations in a single copy of the PHOX2B gene, a “master regulator” of sympathetic neuronal development. The exact mechanisms by which these mutations increase susceptibility to neuroblastoma are unclear, primarily because of the paucity of optimal models in which to study very early development of the sympathetic nervous system. We took advantage of the ex vivo development and transparent nature of zebrafish embryos to study the roles of both normal and mutated PHOX2B in development of the sympathetic nervous system. We present data indicating that aberrant PHOX2B expression causes an arrest in the normal maturation of sympathetic neurons, leading to immature cells that are resistant to drug-induced differentiation. Indeed, we demonstrate that phox2b gene “dosage” is important for normal differentiation of sympathetic neurons in the zebrafish and suggest that the population of immature cells resulting from a decreased dosage of this pivotal factor may be susceptible to secondary mutations that could ultimately lead to neuroblastoma.

Twist1 controls a cell-specification switch governing cell fate decisions within the cardiac neural crest

Neural crest cells are multipotent progenitor cells that can generate both ectodermal cell types, such as neurons, and mesodermal cell types, such as smooth muscle. The mechanisms controlling this cell fate choice are not known. The basic Helix-loop-Helix (bHLH) transcription factor Twist1 is expressed throughout the migratory and post-migratory cardiac neural crest. Twist1 ablation or mutation of the Twist-box causes differentiation of ectopic neuronal cells, which molecularly resemble sympathetic ganglia, in the cardiac outflow tract. Twist1 interacts with the pro-neural factor Sox10 via its Twist-box domain and binds to the Phox2b promoter to repress transcriptional activity. Mesodermal cardiac neural crest trans-differentiation into ectodermal sympathetic ganglia-like neurons is dependent upon Phox2b function. Ectopic Twist1 expression in neural crest precursors disrupts sympathetic neurogenesis. These data demonstrate that Twist1 functions in post-migratory neural crest cells to repress pro-neural factors and thereby regulate cell fate determination between ectodermal and mesodermal lineages.

During vertebrate development, a unique population of cells, termed neural crest cells, migrates throughout the developing embryo, generating various cell types, for example, the smooth muscle that divides the aorta and pulmonary artery where they connect to the heart, and the autonomic neurons, which coordinate organ function. The distinctions between neural crest cells that will form smooth muscle and those that will become neurons are thought to occur prior to migration. Here, we show that, in mice with mutations of the transcription factor Twist1, a subpopulation of presumptive smooth muscle cells, following migration to the heart, instead mis-specify to resemble autonomic neurons. Twist1 represses transcription of the pro-neural factor Phox2b both through antagonism of its upstream effector, Sox10, and through direct binding to its promoter. Phox2b is absolutely required for autonomic neuron development, and indeed, the aberrant neurons in Twist1 mutants disappear when Phox2b is also mutated. Ectopic Twist1 expression within all neural crest cells disrupts the specification of normal autonomic neurons. Collectively, these data reveal that neural crest cells can alter their cell fate from mesoderm to ectoderm after they have migrated and that Twist1 functions to maintain neural crest cell potency during embryonic development.

Ontogeny of cardiac sympathetic innervation and its implications for cardiac disease

The vertebrate heart is innervated by the sympathetic and parasympathetic components of the peripheral autonomic nervous system, which regulates its contractile rate and force. Understanding the mechanisms that control sympathetic neuronal growth, differentiation, and innervation of the heart may provide insight into the etiology of cardiac arrhythmogenesis. This review provides an overview of the cell signaling pathways and transcriptional effectors that regulate both the noradrenergic gene program during sympathetic neurogenesis and regional nerve density during cardiac innervation. Recent studies exploring transcriptional regulation of the bHLH transcription factor Hand1 in developing sympathetic neurons are explored, and how the Hand1 sympathetic neuron-specific cis-regulatory element may be used further to assess the contribution of altered sympathetic innervation to human cardiac disease is discussed.

A Phox2- and Hand2-dependent Hand1 cis-regulatory element reveals a unique gene dosage requirement for Hand2 during sympathetic neurogenesis

Neural crest cell specification and differentiation to a sympathetic neuronal fate serves as an important model for neurogenesis and depends upon the function of both bHLH transcription factors, notably Hand2, and homeodomain transcription factors, including Phox2b. Here, we define a 1007 bp cis-regulatory element 5' of the Hand1 gene sufficient to drive reporter expression within the sympathetic chain of transgenic mice. Comparative genomic analyses uncovered evolutionarily conserved consensus-binding sites within this element, which chromatin immunoprecipitation and electrophoretic mobility shift assays confirm are bound by Hand2 and Phox2b. Mutational analyses revealed that the conserved Phox2 and E-box binding sites are necessary for proper cis-regulatory element activity, and expression analyses on both Hand2 conditionally null and hypomorphic backgrounds demonstrate that Hand2 is required for reporter activation in a gene dosage-dependent manner. We demonstrate that Hand2 and Hand1 differentially bind the E-boxes in this cis-regulatory element, establishing molecular differences between these two factors. Finally, we demonstrate that Hand1 is dispensable for normal tyrosine hydroxylase (TH) and dopamine β-hydroxylase (DBH) expression in sympathetic neurons, even when Hand2 gene dosage is concurrently reduced by half. Together, these data define a tissue-specific Hand1 cis-regulatory element controlled by two factors essential for the development of the sympathetic nervous system and provide in vivo regulatory evidence to support previous findings that Hand2, rather than Hand1, is predominantly responsible for regulating TH, DBH, and Hand1 expression in developing sympathetic neurons.

HoxB8 in noradrenergic specification and differentiation of the autonomic nervous system

Different prespecification of mesencephalic and trunk neural crest cells determines their response to environmental differentiation signals and contributes to the generation of different autonomic neuron subtypes, parasympathetic ciliary neurons in the head and trunk noradrenergic sympathetic neurons. The differentiation of ciliary and sympathetic neurons shares many features, including the initial BMP-induced expression of noradrenergic characteristics that is, however, subsequently lost in ciliary but maintained in sympathetic neurons. The molecular basis of specific prespecification and differentiation patterns has remained unclear. We show here that HoxB gene expression in trunk neural crest is maintained in sympathetic neurons. Ectopic expression of a single HoxB gene, HoxB8, in mesencephalic neural crest results in a strongly increased expression of sympathetic neuron characteristics like the transcription factor Hand2, tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) in ciliary neurons. Other subtype-specific properties like RGS4 and RCad are not induced. HoxB8 has only minor effects in postmitotic ciliary neurons and is unable to induce TH and DBH in the enteric nervous system. Thus, we conclude that HoxB8 acts by maintaining noradrenergic properties transiently expressed in ciliary neuron progenitors during normal development. HoxC8, HoxB9, HoxB1 and HoxD10 elicit either small and transient or no effects on noradrenergic differentiation, suggesting a selective effect of HoxB8. These results implicate that Hox genes contribute to the differential development of autonomic neuron precursors by maintaining noradrenergic properties in the trunk sympathetic neuron lineage.

Neuroblastoma aggressiveness in relation to sympathetic neuronal differentiation stage

Neuroblastoma is a childhood malignancy of the sympathetic neuronal lineage. It is a rare disease, but since it is frequently diagnosed during infancy, neuroblastoma causes life-long medical follow up of those children that survive the disease. It was early recognized that a high tumor cell differentiation stage correlates to favorable clinical stage and positive clinical outcome. Today, highly differentiated tumors are surgically removed and not further treated. Cells of many established human neuroblastoma cell lines have the capacity to differentiate when stimulated properly, and these cell lines have been used as models for studying and understanding central concepts of tumor cell differentiation. One recent aspect of this issue is the observation that tumor cells can dedifferentiate and gain a stem cell-like phenotype during hypoxic conditions, which was first shown in neuroblastoma. Aberrant or blocked differentiation is a central aspect of neuroblastoma genesis. In this review we summarize known genetic and non-genetic events in neuroblastoma that might be coupled to an aberrant sympathetic neuronal differentiation and thereby indirectly influencing tumorigenesis and/or aggressive neuroblastoma behavior.

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.

Targeted deletion of Hand2 in cardiac neural crest-derived cells influences cardiac gene expression and outflow tract development

The basic helix-loop-helix DNA binding protein Hand2 has critical functions in cardiac development both in neural crest-derived and mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest has allowed us to genetically dissect Hand2-dependent defects specifically in outflow tract and cardiac cushion independent of Hand2 functions in mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest results in misalignment of the aortic arch arteries and outflow tract, contributing to development of double outlet right ventricle (DORV) and ventricular septal defects (VSD). These neural crest-derived developmental anomalies are associated with altered expression of Hand2-target genes we have identified by gene profiling. A number of Hand2 direct target genes have been identified using ChIP and ChIP-on-chip analyses. We have identified and validated a number of genes related to cell migration, proliferation/cell cycle and intracellular signaling whose expression is affected by Hand2 deletion in the neural crest and which are associated with development of VSD and DORV. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting expression of target genes associated with a number of functional interactions in neural crest-derived cells required for proper patterning of the outflow tract, generation of the appropriate number of neural crest-derived cells for elongation of the conotruncus and cardiac cushion organization. Our genetic model has made it possible to investigate the molecular genetics of neural crest contributions to outflow tract morphogenesis and cell differentiation.

Notch mediates Wnt and BMP signals in the early separation of smooth muscle progenitors and blood/endothelial common progenitors

During embryonic development in amniotes, the extraembryonic mesoderm,where the earliest hematopoiesis and vasculogenesis take place, also generates smooth muscle cells (SMCs). It is not well understood how the differentiation of SMCs is linked to that of blood (BCs) and endothelial (ECs) cells. Here we show that, in the chick embryo, the SMC lineage is marked by the expression of a bHLH transcription factor, dHand. Notch activity in nascent ventral mesoderm cells promotes SMC progenitor formation and mediates the separation of SMC and BC/EC common progenitors marked by another bHLH factor, Scl. This is achieved by crosstalk with the BMP and Wnt pathways,which are involved in mesoderm ventralization and SMC lineage induction,respectively. Our findings reveal a novel role of the Notch pathway in early ventral mesoderm differentiation, and suggest a stepwise separation among its three main lineages, first between SMC progenitors and BC/EC common progenitors, and then between BCs and ECs.

Genetic model system studies of the development of the enteric nervous system, gut motility and Hirschsprung's disease

The enteric nervous system (ENS) is the largest and most complicated subdivision of the peripheral nervous system. Its action is necessary to regulate many of the functions of the gastrointestinal tract including its motility. Whilst the ENS has been studied extensively by developmental biologists, neuroscientists and physiologists for several decades it has only been since the early 1990s that the molecular and genetic basis of ENS development has begun to emerge. Central to this understanding has been the use of genetic model organisms. In this article, we will discuss recent advances that have been achieved using both mouse and zebrafish model genetic systems that have led to new insights into ENS development and the genetic basis of Hirschsprung's disease.

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.

Induction of mirror-image supernumerary jaws in chicken mandibular mesenchyme by Sonic Hedgehog-producing cells

Previous studies have shown that Sonic Hedgehog (Shh) signaling is crucial for the development of the first branchial arch (BA1) into a lower-jaw in avian and mammalian embryos. We have already shown that if Shh expression is precociously inhibited in pharyngeal endoderm, neural crest cells migrate to BA1 but fail to survive, and Meckel's cartilage and associated structures do not develop. This phenotype can be rescued by addition of an exogenous source of Shh. To decipher the role of Shh, we explored the consequences of providing an extra source of Shh to the presumptive BA1 territory. Grafting quail fibroblasts engineered to produce Shh (QT6-Shh), at the 5- to 8-somite stage, resulted in the induction of mirror-image extra lower jaws, caudolateral to the normal one. It turns out that the oral opening epithelium, in which Shh, Fgf8 and Bmp4 are expressed in a definite pattern, functions as an organizing center for lower-jaw development. In our experimental design, the extra source of Shh activates Fgf8, Bmp4 and Shh genes in caudal BA1 ectoderm in a spatial pattern similar to that of the oral epithelium, and regularly leads to the formation of two extra lower-jaw-organizing centers with opposite rostrocaudal polarities. These results emphasize the similarities between the developmental processes of the limb and mandibular buds, and show that in both cases Shh-producing cells create a zone of polarizing activity for the structures deriving from them.

HAND transcription factors are required for neonatal sympathetic neuron survival

Expression of the basic helix-loop-helix transcription factor HAND2 begins early in sympathetic neuron development and is essential for the differentiation of noradrenergic neurons. Here, we show that the expression of HAND2 and related HAND1 are maintained in sympathetic neurons throughout fetal and postnatal development when these neurons depend on target-derived nerve growth factor (NGF) for survival. Short interfering RNA knockdown of endogenous HAND2 and, to a lesser extent, HAND1 in neonatal sympathetic neurons cultured with NGF, reduced the expression of the NGF receptor tyrosine kinase TrkA (tropomyosin-related kinase A), as well as neuronal survival. Chromatin immunoprecipitation analysis showed that NGF promotes HAND2 binding to the TrkA minimal enhancer and that transfection of sympathetic neurons with a TrkA expression plasmid rescued the neurons from HAND knockdown. These findings show that HAND transcription factors have a crucial function in sustaining the survival of neonatal sympathetic neurons with NGF by a feed-forward loop that maintains the expression of TrkA.

Endoderm-derived Sonic hedgehog and mesoderm Hand2 expression are required for enteric nervous system development in zebrafish

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest cells (NCC). The developmental controls that govern the migration, proliferation and patterning of the ENS precursors are not well understood. We have investigated the roles of endoderm and Sonic hedgehog (SHH) in the development of the ENS. We show that endoderm is required for the migration of ENS NCC from the vagal region to the anterior end of the intestine. We show that the expression of shh and its receptor ptc-1 correlate with the development of the ENS and demonstrate that hedgehog (HH) signaling is required in two phases, a pre-enteric and an enteric phase, for normal ENS development. We show that HH signaling regulates the proliferation of vagal NCC and ENS precursors in vivo. We also show the zebrafish hand2 is required for the normal development of the intestinal smooth muscle and the ENS. Furthermore we show that endoderm and HH signaling, but not hand2, regulate gdnf expression in the intestine, highlighting a central role of endoderm and SHH in patterning the intestine and the ENS.

Conditional deletion of Hand2 reveals critical functions in neurogenesis and cell type-specific gene expression for development of neural crest-derived noradrenergic sympathetic ganglion neurons

Neural crest-derived structures that depend critically upon expression of the basic helix-loop-helix DNA binding protein Hand2 for normal development include craniofacial cartilage and bone, the outflow tract of the heart, cardiac cushion, and noradrenergic sympathetic ganglion neurons. Loss of Hand2 is embryonic lethal by E9.5, obviating a genetic analysis of its in-vivo function. We have overcome this difficulty by specific deletion of Hand2 in neural crest-derived cells by crossing our line of floxed Hand2 mice with Wnt1-Cre transgenic mice. Our analysis of Hand2 knock-out in neural crest-derived cells reveals effects on development in all neural crest-derived structures where Hand2 is expressed. In the autonomic nervous system, conditional disruption of Hand2 results in a significant and progressive loss of neurons as well as a significant loss of TH expression. Hand2 affects generation of the neural precursor pool of cells by affecting both the proliferative capacity of the progenitors as well as affecting expression of Phox2a and Gata3, DNA binding proteins important for the cell autonomous development of noradrenergic neurons. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting differentiation and cell type-specific gene expression in neural crest-derived noradrenergic sympathetic ganglion neurons. Hand2 has a pivotal function in a non-linear cross-regulatory network of DNA binding proteins that affect cell autonomous control of differentiation and cell type-specific gene expression.

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.

Insm1 (IA-1) is a crucial component of the transcriptional network that controls differentiation of the sympatho-adrenal lineage

Insm1 (IA-1) encodes a Zn-finger factor that is expressed in the developing nervous system. We demonstrate here that the development of the sympatho-adrenal lineage is severely impaired in Insm1 mutant mice. Differentiation of sympatho-adrenal precursors, as assessed by the expression of neuronal subtype-specific genes such as Th and Dbh, is delayed in a pronounced manner, which is accompanied by a reduced proliferation. Sympathetic neurons eventually overcome the differentiation blockade and mature correctly, but sympathetic ganglia remain small. By contrast, terminal differentiation of adrenal chromaffin cells does not occur. The transcription factors Mash1 (Ascl1), Phox2a, Gata3 and Hand2 (previously dHand) control the differentiation of sympatho-adrenal precursor cells, and their deregulated expression in Insm1 mutant mice demonstrates that Insm1 acts in the transcriptional network that controls differentiation of this lineage. Pronounced similarities between Mash1 and Insm1 phenotypes are apparent, which suggests that Insm1 might mediate aspects of Mash1 function in the subtype-specific differentiation of sympatho-adrenal precursors. Noradrenaline is the major catecholamine produced by developing sympatho-adrenal cells and is required for fetal survival. We demonstrate that the fetal lethality of Insm1 mutant mice is caused by catecholamine deficiency, which highlights the importance of Insm1 in the development of the sympatho-adrenal lineage.

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.

Hand2 determines the noradrenergic phenotype in the mouse sympathetic nervous system

The basic helix-loop-helix (bHLH) transcription factor Hand2 has been shown to play a role in the development of the mammalian sympathetic nervous system (SNS); however, its precise role could not be uncovered because Hand2 is required for early embryonic survival. We therefore generated a conditional Hand2 knockout mouse line by excising Hand2 in Wnt1-Cre-expressing neural crest-derived cells. These mice die at 12.5 dpc with embryos showing severe cardiovascular and facial defects. Crest-derived cells, however, populate sites of SNS development and proliferate normally. Sympathetic precursors differentiate into neurons and express the pan-neuronal markers, beta3-tubulin (Tuj1) and Hu showing that Hand2 is not essential for SNS neuronal differentiation. To determine whether Hand2 regulates noradrenergic differentiation, the levels of the norepinephrine biosynthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) was examined. Both enzymes were dramatically reduced in mutant embryos suggesting that the primary role of Hand2 in the SNS is determination of neuronal phenotype. Loss of Hand2 did not affect the expression of other members of the transcriptional circuit regulating SNS development, including Phox2a/b, Mash1 and Gata2/3; however, Hand2 was required for Hand1 expression. Our data suggest that the major role of Hand2 during SNS development is to permit sympathetic neurons to acquire a catecholaminergic phenotype.

The bHLH transcription factor hand2 is essential for noradrenergic differentiation of sympathetic neurons

The basic helix-loop-helix transcription factor Hand2, together with Ascl1, Phox2a, Phox2b and Gata2/Gata3, is induced by bone morphogenetic proteins in neural crest-derived precursor cells during sympathetic neuron generation. Hand2 overexpression experiments and the analysis of its function at the Dbh promotor implicated Hand2 in the control of noradrenergic gene expression. Using the zebrafish hand2 deletion mutant hands off, we have now investigated the physiological role of hand2 in the development of sympathetic ganglia. In hands off mutant embryos, sympathetic precursor cells aggregate to form normal sympathetic ganglion primordia characterized by the expression of phox2b, phox2a and the achaete-scute family member zash1a/ascl1. The expression of the noradrenergic marker genes th and dbh is strongly reduced, as well as the transcription factors gata2 and tfap2a (Ap-2alpha). By contrast, generic neuronal differentiation seems to be unaffected, as the expression of elavl3 (HuC) is not reduced in hands off sympathetic ganglia. These results demonstrate in vivo an essential and selective function of hand2 for the noradrenergic differentiation of sympathetic neurons, and implicates tfap2a and gata2 as downstream effectors.

The sympathoadrenal cell lineage: specification, diversification, and new perspectives

During the past years considerable progress has been made in understanding the generation of cell diversity in the neural crest (NC). Sympathoadrenal (SA) cells constitute a major lineage among NC derivatives; they give rise to sympathetic neurons, neuroendocrine chromaffin cells, and the intermediate small intensely fluorescent (SIF) cells. The classic perception of how this diversification is achieved implies that (i) there is a common progenitor cell for sympathetic neurons and chromaffin cells, (ii) NC cells are instructed to a SA cell fate by signals derived from the wall of the dorsal aorta, especially bone morphogenetic proteins (BMP), and (iii) the local environments of secondary sympathetic ganglia and adrenal gland, respectively, are crucial for inducing differentiation of SA cells into sympathetic neurons and adrenal chromaffin cells. However, recent studies have suggested that the adrenal cortex is dispensable for the acquisition of a chromaffin cell fate. This review summarizes the current understanding of the development of SA cells. It covers the specification of SA cells from multipotent NC crest cells, the role of transcription factors during their development, the classic model of their subsequent diversification as well as alternative views for explaining the generation of endocrine versus neuronal SA derivatives.

Delays in neuronal differentiation in Mash1/Ascl1 mutants

The inactivation of a developmental transcription factor may lead to the complete absence of a specific cell type. More commonly, though, it only partially impairs its generation. The modalities of this partial effect have rarely been documented in any detail. Here, we report a novel function for the bHLH transcription factor Ascl1/Mash1 in the generation of the nucleus of the solitary tract (nTS). In Mash1(-/-) late embryos, the nTS is markedly atrophic. Tracing back the origin of this atrophy, we show that nTS precursors appear in the mutants 1 day later than in the wild type and then accumulate at a slower pace. We also show that the previously reported atrophy of the sympathetic chain in Mash1 mutants is similarly preceded by a delay of 1 to 2 days in the appearance of differentiated ganglionic cells. Finally, we provide evidence that the acceleration imposed by Mash1, regardless of the production of post-mitotic cells, affects differentiation itself, both generic and type-specific.

Perspectives on integration of cell extrinsic and cell intrinsic pathways of signaling required for differentiation of noradrenergic sympathetic ganglion neurons

This review presents an analysis of current research aimed at deciphering the interplay of cell extrinsic and intrinsic signals required for specification and differentiation of noradrenergic sympathetic ganglion neurons. The development of noradrenergic sympathetic ganglion neurons depends upon expression of a core set of DNA regulatory molecules, including the Phox2 homeodomain proteins and the basic helix-loop-helix proteins, HAND2 and MASH1 whose expression is dependent upon cell extrinsic cues. Both bone morphogenetic protein(s) and cAMP have an integral role in the specification/differentiation of noradrenergic sympathetic ganglion neurons but how signaling downstream of these molecules is integrated and identification of their particular functions is just beginning to be elucidated. Data currently available suggests a model with BMP providing both instructive and permissive cues in a pathway integrated by cAMP and MAPK by activation of both canonical and non-canonical intracellular signaling cascades.

Dynamics of heart differentiation, visualized utilizing heart enhancer elements of the Drosophila melanogaster bHLH transcription factor Hand

Drosophila melanogaster has become one of the important model systems to investigate the development and differentiation of the heart. After 24h after egg deposition (h AED), a simple tube-like organ is formed, consisting of essentially only two cell types, the contractile cardioblasts and non-myogenic pericardial cells. In contrast to the detailed knowledge of heart formation during embryogenesis, only a few studies deal with later changes in heart morphology and/or function. This is mainly due to the difficulties to carry out whole mount stainings in later stages without complicated dissections or treatments of the cuticle and puparium. In this paper we describe the identification of a hand genomic region, which is fully sufficient to drive GFP expression in heart cells of embryos, larvae, and adults. This serves as an initial step to understand the position of hand in the early regulatory network in heart development. Furthermore, we demonstrate that our newly created GFP reporter line is extremely useful to study postembryonic heart differentiation. For the first time we document heart differentiation in living animals throughout all developmental stages of Drosophila melanogaster, including embryogenesis, all three larval stages, metamorphosis, and the adult life with respect to pericardial cells and cardiomyocytes.

Phox2 and dHAND transcription factors select shared and unique target genes in the noradrenergic cell type

The noradrenergic cell type is characterized by the expression of proteins involved in the biosynthesis, transport, and secretion of noradrenaline and is dependent on the sequential and combinatorial expression of numerous transcription factors, including Phox2a, Phox2b, dHAND, GATA2, GATA3, and MASH1. Phox2a and Phox2b transactivate the promoter of the gene encoding the noradrenergic biosynthetic enzyme, dopamine beta-hydroxylase (DBH), and dHAND potentiates the activity of Phox2a. In this study, we use chromatin immunoprecipitation assays to identify target genes of the Phox2 proteins and dHAND. All three proteins are bound to the DBH and PHOX2B promoter regions in SH-SY5Y neuroblastoma cells. The interaction between Phox2a and dHAND is analyzed by fluorescent anisotropy, which demonstrates that dHAND causes an eightfold increase in the affinity of Phox2a for its recognition sites on the DBH promoter region. The Phox2 proteins are not found on the genes encoding other noradrenergic enzymatic or transport proteins but are reciprocally bound to each other's promoters in SH-SY5Y cells. Together with Phox2a and Phox2b, dHAND is bound to the PHOX2B promoter and is also associated with the GATA2 and eHAND genes in the absence of the Phox2 proteins. These results demonstrate the direct interactions of the Phox2 and dHAND transcription factors within a noradrenergic cell type. The Phox2 proteins were found to share all target genes, whereas dHAND binds to genes independently of Phox2a.

lessen encodes a zebrafish trap100 required for enteric nervous system development

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest. The developmental controls that govern the specification and patterning of the ENS are not well understood. To identify genes required for the formation of the vertebrate ENS, we preformed a genetic screen in zebrafish. We isolated the lessen (lsn) mutation that has a significant reduction in the number of ENS neurons as well as defects in other cranial neural crest derived structures. We show that the lsn gene encodes a zebrafish orthologue of Trap100, one of the subunits of the TRAP/mediator transcriptional regulation complex. A point mutation in trap100 causes a premature stop codon that truncates the protein, causing a loss of function. Antisense-mediated knockdown of trap100 causes an identical phenotype to lsn. During development trap100 is expressed in a dynamic tissue-specific expression pattern consistent with its function in ENS and jaw cartilage development. Analysis of neural crest markers revealed that the initial specification and migration of the neural crest is unaffected in lsn mutants. Phosphohistone H3 immunocytochemistry revealed that there is a significant reduction in proliferation of ENS precursors in lsn mutants. Using cell transplantation studies, we demonstrate that lsn/trap100 acts cell autonomously in the pharyngeal mesendoderm and influences the development of neural crest derived cartilages secondarily. Furthermore, we show that endoderm is essential for ENS development. These studies demonstrate that lsn/trap100 is not required for initial steps of cranial neural crest development and migration, but is essential for later proliferation of ENS precursors in the intestine.

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.

Expression of Hand2 is sufficient for neurogenesis and cell type–specific gene expression in the enteric nervous system

The basic helix-loop-helix DNA binding protein Hand2 is expressed in neural crest-derived precursors of enteric neurons and has been shown to affect both neurogenesis and neurotransmitter specification of noradrenergic sympathetic ganglion neurons. In the current study, our goal was to determine whether Hand2 affects neurogenesis and/or expression of vasoactive intestinal polypeptide and choline acetyltransferase in developing enteric neurons. Gain-of-function of Hand2 in HNK-1(+) immmunoselected precursor cells resulted in increased neurogenesis. The number of neurons expressing vasoactive intestinal polypeptide increased in response to Hand2 overexpression although choline acetyltransferase was not affected. Targeted deletion of Hand2 in neural crest cells resulted in loss of all neurons expressing vasoactive intestinal polypeptide along the length of the gastrointestinal tract, patterning defects in the myenteric plexus of the stomach, and altered number and morphology of neurons expressing TH. Our data demonstrate that expression of Hand2 is sufficient and necessary for neurogenesis and expression of a subset of cell type-specific markers in the developing enteric nervous system.

BMP4 supports noradrenergic differentiation by a PKA-dependent mechanism

Differentiation of neural crest-derived noradrenergic neurons depends upon signaling mediated downstream of BMP binding to cognate receptors and involving cAMP. Compiled data from many groups suggest that neurogenesis and cell type-specific noradrenergic marker gene regulation is coordinated through the expression and function of the basic helix-loop-helix DNA binding protein HAND2 and the homeodomain DNA binding protein Phox2a. However, information detailing how BMP-mediated signaling and signaling through cAMP are coordinated has been lacking. We now provide compelling data suggesting that differentiation of noradrenergic sympathetic ganglion neurons depends upon both canonical and non-canonical pathways of BMP-mediated signaling. The non-canonical pathway involves the activation of protein kinase A (PKA) independent of cAMP. This is a novel mechanism in neural crest-derived cells and is necessary to support neurogenesis as well as aspects of DBH promoter regulation involving HAND2 phosphorylation and dimerization. The expression of transcripts encoding HAND2 and Phox2a is regulated via canonical BMP signaling and thus affects both neurogenesis and cell type-specific gene expression. Interestingly, cAMP- and MapK-mediated signaling modulate specific target sites in both the canonical and non-canonical BMP pathways. Activity of MapK is required for HAND2 transcription and thus affects neurogenesis. Signaling affected by cAMP is necessary for the transcription of Phox2a as well as regulation of DBH promoter transactivation by Phox2a and HAND2. We suggest a comprehensive model that shows how BMP- and cAMP-mediated intracellular signaling integrate neurogenesis and cell type-specific noradrenergic marker gene expression and function.

ZENON, a novel POZ Kruppel-like DNA binding protein associated with differentiation and/or survival of late postmitotic neurons

The rat tyrosine hydroxylase gene promoter contains an E-box/dyad motif and an octameric and heptameric element that may be recognized by classes of transcription factors highly expressed during nervous system development. In a one-hybrid genetic screen, we used these sites as targets to isolate cDNAs encoding new transcription factors present in the brain. We identified ZENON, a novel rat POZ protein that contains two clusters of Kruppel-like zinc fingers and that presents several features of a transcription factor. ZENON is found in nuclei following transient transfection with the cDNA. The N-terminal zinc finger cluster contains a DNA binding domain that interacts with the E box. Cotranfection experiments revealed that ZENON induces tyrosine hydroxylase promoter activity. Unlike other POZ proteins, the ZENON POZ domain is not required for either activation of transcription or self-association. In the embryonic neural tube, ZENON expression is restricted to neurons that have already achieved mitosis and are engaged in late stages of neuronal differentiation (late postmitotic neurons). ZENON neuronal expression persists in the adult brain; therefore, ZENON can be considered a marker of mature neurons. We propose that ZENON is involved in the maintenance of panneuronal features and/or in the survival of mature neurons.

Restricted response of mesencephalic neural crest to sympathetic differentiation signals in the trunk

Lineage diversification in the vertebrate neural crest may occur via instructive signals acting on pluripotent cells, and/or via early specification of subpopulations towards particular lineages. Mesencephalic neural crest cells normally form cholinergic parasympathetic neurons in the ciliary ganglion, while trunk neural crest cells normally form both catecholaminergic and cholinergic neurons in sympathetic ganglia. In contrast to trunk neural crest cells, mesencephalic neural crest cells apparently fail to express the catecholaminergic transcription factor dHAND in response to BMPs in the head environment. Here, we show that migrating quail mesencephalic neural crest cells grafted into the trunk of host chick embryos colonise the sympathetic ganglia. While many express dHAND and form tyrosine hydroxylase (TH)-positive catecholaminergic neurons, the proportion that expresses either dHAND or TH is significantly smaller than that of quail trunk neural crest cells under the same conditions. Furthermore, the proportion of quail mesencephalic neural crest cells that is TH+ in the sympathetic ganglia decreases with time, while the proportion of TH+ quail trunk neural crest-derived cells increases. Thus, a subset of mesencephalic neural crest cells fails to express dHAND or TH in the sympathetic ganglia, while a further subset initiates but fails to maintain TH expression. Taken together, our results suggest that a subpopulation of migrating mesencephalic neural crest cells is refractory to catecholaminergic differentiation signals in the trunk. We suggest that this heterogeneity, together with local signals that repress catecholaminergic differentiation, may ensure that most ciliary neurons adopt a cholinergic fate.

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.