Hand2 determines the noradrenergic phenotype in the mouse sympathetic nervous system

A knock-in allele of Hand2 expressing Dre recombinase

HAND2 is a basic helix-loop-helix transcription factor with diverse functions during development. To facilitate the investigation of genetic and functional diversity among Hand2-expressing cells in the mouse, we have generated Hand2Dre, a knock-in allele expressing Dre recombinase. To avoid disrupting Hand2 function, the Dre cDNA is inserted at the 3' end of the Hand2 coding sequence following a viral 2A peptide. Hand2Dre homozygotes can therefore be used in complex crosses to increase the proportion of useful genotypes among offspring. Dre expression in mid-gestation Hand2Dre embryos is indistinguishable from wild-type Hand2 expression, and HandDre efficiently recombines rox target sites in vivo. In combination with existing Cre and Flp mouse lines, Hand2Dre will therefore extend the ability to perform genetic intersectional labeling, fate mapping, and functional manipulation of subpopulations of cells characterized by developmental expression of Hand2.

Hand2 Immunohistochemistry in the Diagnosis of Paragangliomas and Other Neuroendocrine Neoplasms

Hand2 is a core transcription factor responsible for chromaffin cell differentiation. However, its potential utility in surgical pathology has not been studied. Thus, we aimed to investigate its expression in paragangliomas, other neuroendocrine neoplasms (NENs), and additional non-neuroendocrine tumors. We calibrated Hand2 immunohistochemistry on adrenal medulla cells and analyzed H-scores in 46 paragangliomas (PGs), 9 metastatic PGs, 21 cauda equina neuroendocrine tumors (CENETs), 48 neuroendocrine carcinomas (NECs), 8 olfactory neuroblastomas (ONBs), 110 well-differentiated NETs (WDNETs), 10 adrenal cortical carcinomas, 29 adrenal cortical adenomas, 8 melanomas, 41 different carcinomas, and 10 gastrointestinal stromal tumors (GISTs). Both tissue microarrays (TMAs) and whole sections (WSs) were studied. In 171 NENs, previously published data on Phox2B and GATA3 were correlated with Hand2. Hand2 was positive in 98.1% (54/55) PGs, but only rarely in WDNETs (9.6%, 10/104), CENETs (9.5%, 2/21), NECs (4.2%, 2/48), or ONBs (12.5%, 1/8). Any Hand2 positivity was 98.1% sensitive and 91.7% specific for the diagnosis of PG. The Hand2 H-score was significantly higher in primary PGs compared to Hand2-positive WDNETs (median 166.3 vs. 7.5; p < 0.0001). Metastatic PGs were positive in 88.9% (8/9). No Hand2 positivity was observed in any adrenal cortical neoplasm or other non-neuroendocrine tumors, with exception of 8/10 GISTs. Parasympathetic PGs showed a higher Hand2 H-score compared to sympathetic PGs (median H-scores 280 vs. 104, p < 0.0001). Hand2 positivity in NENs serves as a reliable marker of primary and metastatic PG, since other NENs only rarely exhibit limited Hand2 positivity.

Comparative single-cell analyses identify shared and divergent features of human and mouse kidney development

Mammalian kidneys maintain fluid homeostasis through the cellular activity of nephrons and the conjoined collecting system. Each epithelial network originates from distinct progenitor cell populations that reciprocally interact during development. To extend our understanding of human and mouse kidney development, we profiled chromatin organization (ATAC-seq) and gene expression (RNA-seq) in developing human and mouse kidneys. Data were analyzed at a species level and then integrated into a common, cross-species multimodal data set. Comparative analysis of cell types and developmental trajectories identified conserved and divergent features of chromatin organization and linked gene activity, revealing species- and cell-type specific regulatory programs. Identification of human-specific enhancer regions linked through GWAS studies to kidney disease highlights the potential of developmental modeling to provide clinical insight.

Single cell evaluation of endocardial Hand2 gene regulatory networks reveals HAND2-dependent pathways that impact cardiac morphogenesis

The transcription factor HAND2 plays essential roles during cardiogenesis. Hand2 endocardial deletion (H2CKO) results in tricuspid atresia or double inlet left ventricle with accompanying intraventricular septum defects, hypo-trabeculated ventricles and an increased density of coronary lumens. To understand the regulatory mechanisms of these phenotypes, single cell transcriptome analysis of mouse E11.5 H2CKO hearts was performed revealing a number of disrupted endocardial regulatory pathways. Using HAND2 DNA occupancy data, we identify several HAND2-dependent enhancers, including two endothelial enhancers for the shear-stress master regulator KLF2. A 1.8 kb enhancer located 50 kb upstream of the Klf2 TSS imparts specific endothelial/endocardial expression within the vasculature and endocardium. This enhancer is HAND2-dependent for ventricular endocardium expression but HAND2-independent for Klf2 vascular and valve expression. Deletion of this Klf2 enhancer results in reduced Klf2 expression within ventricular endocardium. These data reveal that HAND2 functions within endocardial gene regulatory networks including shear-stress response.

The proneural transcription factor ASCL1 regulates cell proliferation and primes for differentiation in neuroblastoma

Neuroblastoma is believed to arise from sympathetic neuroblast precursors that fail to engage the neuronal differentiation programme, but instead become locked in a pro-proliferative developmental state. Achaete-scute homolog 1 (ASCL1) is a proneural master regulator of transcription which modulates both proliferation and differentiation of sympathetic neuroblast precursor cells during development, while its expression has been implicated in the maintenance of an oncogenic programme in MYCN-amplified neuroblastoma. However, the role of ASCL1 expression in neuroblastoma is not clear, especially as its levels vary considerably in different neuroblastoma cell lines. Here, we have investigated the role of ASCL1 in maintaining proliferation and controlling differentiation in both MYCN amplified and Anaplastic Lymphoma Kinase (ALK)-driven neuroblastoma cells. Using CRISPR deletion, we generated neuroblastoma cell lines lacking ASCL1 expression, and these grew more slowly than parental cells, indicating that ASCL1 contributes to rapid proliferation of MYCN amplified and non-amplified neuroblastoma cells. Genome-wide analysis after ASCL1 deletion revealed reduced expression of genes associated with neuronal differentiation, while chromatin accessibility at regulatory regions associated with differentiation genes was also attenuated by ASCL1 knock-out. In neuroblastoma, ASCL1 has been described as part of a core regulatory circuit of developmental regulators whose high expression is maintained by mutual cross-activation of a network of super enhancers and is further augmented by the activity of MYC/MYCN. Surprisingly, ASCL1 deletion had little effect on the transcription of CRC gene transcripts in these neuroblastoma cell lines, but the ability of MYC/MYCN and CRC component proteins, PHOX2B and GATA3, to bind to chromatin was compromised. Taken together, our results demonstrate several roles for endogenous ASCL1 in neuroblastoma cells: maintaining a highly proliferative phenotype, regulating DNA binding of the core regulatory circuit genes to chromatin, while also controlling accessibility and transcription of differentiation targets. Thus, we propose a model where ASCL1, a key developmental regulator of sympathetic neurogenesis, plays a pivotal role in maintaining proliferation while simultaneously priming cells for differentiation in neuroblastoma.

Serotonin limits generation of chromaffin cells during adrenal organ development

Adrenal glands are the major organs releasing catecholamines and regulating our stress response. The mechanisms balancing generation of adrenergic chromaffin cells and protecting against neuroblastoma tumors are still enigmatic. Here we revealed that serotonin (5HT) controls the numbers of chromaffin cells by acting upon their immediate progenitor "bridge" cells via 5-hydroxytryptamine receptor 3A (HTR3A), and the aggressive HTR3Ahigh human neuroblastoma cell lines reduce proliferation in response to HTR3A-specific agonists. In embryos (in vivo), the physiological increase of 5HT caused a prolongation of the cell cycle in "bridge" progenitors leading to a smaller chromaffin population and changing the balance of hormones and behavioral patterns in adulthood. These behavioral effects and smaller adrenals were mirrored in the progeny of pregnant female mice subjected to experimental stress, suggesting a maternal-fetal link that controls developmental adaptations. Finally, these results corresponded to a size-distribution of adrenals found in wild rodents with different coping strategies.

The adult heart requires baseline expression of the transcription factor Hand2 to withstand RV pressure overload

AIMS

Research on the pathophysiology of right ventricular (RV) failure has, in spite of the associated high mortality and morbidity, lagged behind compared to the left ventricle (LV).Previous work from our lab revealed that the embryonic basic helix-loop-helix transcription factor heart and neural crest derivatives expressed-2 (Hand2) is re-expressed in the adult heart and activates a 'fetal gene program' contributing to pathological cardiac remodeling under conditions of LV pressure overload. As such, ablation of cardiac expression of Hand2 conferred protection to cardiac stress and abrogated the maladaptive effects that were observed upon increased expression levels. In this study, we aimed to understand the contribution of Hand2 to RV remodeling in response to pressure overload induced by pulmonary artery banding (PAB).

METHODS AND RESULTS

In the present study, Hand2F/F and MCM- Hand2F/F mice were treated with tamoxifen (control and knockout, respectively) and subjected to six weeks of RV pressure overload induced by PAB. Echocardiographic- and MRI-derived hemodynamic parameters as well as molecular remodeling were assessed for all experimental groups and compared to sham-operated controls. Six weeks after PAB, levels of Hand2 expression increased in the control banded animals but, as expected, remained absent in the knockout hearts. Despite the dramatic differences in Hand2 expression, pressure overload resulted in impaired cardiac function independently of the genotype. In fact, Hand2 depletion seems to sensitize the RV to pressure overload as these mice develop more hypertrophy and more severe cardiac dysfunction. Higher expression levels of HAND2 were also observed in RV samples of human hearts from patients with pulmonary hypertension. In turn, the LV of RV-pressure overloaded hearts was also dramatically affected as reflected by changes in shape, decreased LV mass and impaired cardiac function. RNA sequencing revealed a distinct set of genes that are dysregulated in the pressure-overloaded RV, compared to the previously described pressure-overloaded LV.

CONCLUSIONS

Cardiac-specific depletion of Hand2 is associated with severe cardiac dysfunction in conditions of RV pressure overload. While inhibiting Hand2 expression can prevent cardiac dysfunction in conditions of LV pressure overload, the same does not hold true for conditions of RV pressure overload. This study highlights the need to better understand the molecular mechanisms driving pathological remodeling of the RV in contrast to the LV, in order to better diagnose and treat patients with RV or LV failure.

TRANSLATIONAL PERSPECTIVE

RV failure associated with pulmonary hypertension reduces long-term survival rate to 55% within 3 years, suggesting that 3 years after diagnosis almost half of the patients will die. To revert these numbers an adequate RV-specific and, therefore, more efficient treatment is needed. Our work suggests that current therapies and potential mechanisms underlying LV failure may not be suitable for RV failure. While Hand2 deletion is favorable in LV response to stress, it is particularly detrimental in the RV under similar conditions, and thus, highlighting potential severe consequences of not differentiating therapeutic targets or treatment for RV or LV failure.

Comparative morphological and molecular studies on the oxygen-chemoreceptive cells in the carotid body and fish gills

Oxygen-chemoreceptive cells play critical roles for the respiration control. This review summarizes the chemoreceptive cells in the carotid body and fish gills from a morphological and molecular perspective. The cells synthesize and secrete biogenic amines, neuropeptides, and neuroproteins and also express many signaling molecules and transcription factors. In mammals, birds, reptiles, and amphibians, the carotid body primordium is consistently formed in the wall of the third arch artery which gives rise to the common carotid artery and the basal portion of the internal carotid artery. Consequently, the carotid body is located in the carotid bifurcation region, except birds in which the organ is situated at the lateral side of the common carotid artery. The carotid body receives branches of the cranial nerves IX and/or X dependent on the location of the organ. The glomus cell progenitors in mammals and birds are derived from the neighboring ganglion, i.e., the superior cervical sympathetic ganglion and the nodose ganglion, respectively, and immigrate into the carotid body primordium, constituting a solid cell cluster. In other animal species, the glomus cells are dispersed singly or forming small cell groups in intervascular stroma of the carotid body. In fishes, the neuroepithelial cells, corresponding to the glomus cells, are distributed in the gill filaments and lamellae. All oxygen-chemoreceptive cells sensitively respond to acute or chronic hypoxia, exhibiting degranulation, hypertrophy, hyperplasia, and upregulated expression of many genes.

HAND transcription factors cooperatively specify the aorta and pulmonary trunk

Congenital heart defects (CHDs) affecting the cardiac outflow tract (OFT) constitute a significant cause of morbidity and mortality. The OFT develops from migratory cell populations which include the cardiac neural crest cells (cNCCs) and secondary heart field (SHF) derived myocardium and endocardium. The related transcription factors HAND1 and HAND2 have been implicated in human CHDs involving the OFT. Although Hand1 is expressed within the OFT, Hand1 NCC-specific conditional knockout mice (H1CKOs) are viable. Here we show that these H1CKOs present a low penetrance of OFT phenotypes, whereas SHF-specific Hand1 ablation does not reveal any cardiac phenotypes. Further, HAND1 and HAND2 appear functionally redundant within the cNCCs, as a reduction/ablation of Hand2 on an NCC-specific H1CKO background causes pronounced OFT defects. Double conditional Hand1 and Hand2 NCC knockouts exhibit persistent truncus arteriosus (PTA) with 100% penetrance. NCC lineage-tracing and Sema3c in situ mRNA expression reveal that Sema3c-expressing cells are mis-localized, resulting in a malformed septal bridge within the OFTs of H1CKO;H2CKO embryos. Interestingly, Hand1 and Hand2 also genetically interact within the SHF, as SHF H1CKOs on a heterozygous Hand2 background exhibit Ventricular Septal Defects (VSDs) with incomplete penetrance. Previously, we identified a BMP, HAND2, and GATA-dependent Hand1 OFT enhancer sufficient to drive reporter gene expression within the nascent OFT and aorta. Using these transcription inputs as a probe, we identify a novel Hand2 OFT enhancer, suggesting that a conserved BMP-GATA dependent mechanism transcriptionally regulates both HAND factors. These findings support the hypothesis that HAND factors interpret BMP signaling within the cNCCs to cooperatively coordinate OFT morphogenesis.

The Intrinsic Cardiac Nervous System and Its Role in Cardiac Pacemaking and Conduction

The cardiac autonomic nervous system (CANS) plays a key role for the regulation of cardiac activity with its dysregulation being involved in various heart diseases, such as cardiac arrhythmias. The CANS comprises the extrinsic and intrinsic innervation of the heart. The intrinsic cardiac nervous system (ICNS) includes the network of the intracardiac ganglia and interconnecting neurons. The cardiac ganglia contribute to the tight modulation of cardiac electrophysiology, working as a local hub integrating the inputs of the extrinsic innervation and the ICNS. A better understanding of the role of the ICNS for the modulation of the cardiac conduction system will be crucial for targeted therapies of various arrhythmias. We describe the embryonic development, anatomy, and physiology of the ICNS. By correlating the topography of the intracardiac neurons with what is known regarding their biophysical and neurochemical properties, we outline their physiological role in the control of pacemaker activity of the sinoatrial and atrioventricular nodes. We conclude by highlighting cardiac disorders with a putative involvement of the ICNS and outline open questions that need to be addressed in order to better understand the physiology and pathophysiology of the ICNS.

Gli3 utilizes Hand2 to synergistically regulate tissue-specific transcriptional networks

Despite a common understanding that Gli TFs are utilized to convey a Hh morphogen gradient, genetic analyses suggest craniofacial development does not completely fit this paradigm. Using the mouse model (Mus musculus), we demonstrated that rather than being driven by a Hh threshold, robust Gli3 transcriptional activity during skeletal and glossal development required interaction with the basic helix-loop-helix TF Hand2. Not only did genetic and expression data support a co-factorial relationship, but genomic analysis revealed that Gli3 and Hand2 were enriched at regulatory elements for genes essential for mandibular patterning and development. Interestingly, motif analysis at sites co-occupied by Gli3 and Hand2 uncovered mandibular-specific, low-affinity, 'divergent' Gli-binding motifs (dGBMs). Functional validation revealed these dGBMs conveyed synergistic activation of Gli targets essential for mandibular patterning and development. In summary, this work elucidates a novel, sequence-dependent mechanism for Gli transcriptional activity within the craniofacial complex that is independent of a graded Hh signal.

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.

MicroRNA-92b-3p suppresses angiotensin II-induced cardiomyocyte hypertrophy via targeting HAND2

AIMS

The pathological cardiac hypertrophy will develop into heart failure, which has no effective treatment currently. Previous studies have proved that microRNAs (miRNAs) participate in the development of cardiac hypertrophy and regulate the pathological progress. In this study, we want to investigate the role of microRNA-92b-3p (miR-92b-3p) in cardiomyocyte hypertrophy and the mechanisms involved.

MATERIALS AND METHODS

Neonatal mouse ventricular cells (NMVCs) were isolated from the hearts of 1-3-d-old newborn C57BL6 mice. The isolated NMVCs were induced hypertrophic phenotype by Angiotensin-II (Ang-II) and the cell size was examined by FITC-phalloidin staining assay. The expression of miR-92b-3p was determined by quantitative real-time PCR (qRT-qPCR). MRNA and protein level of β-MHC, ACTA1 and HAND2 in NMVCs transfected with miR-92b-3p mimic and inhibition were assessed by RT-qPCR assay and western blot assay, respectively. Dual luciferase assay was used to verify the interaction between miR-92b-3p and the 3'-untranslated region (UTR) of HAND2 gene.

KEY FINDINGS

MiR-92b-3p and HAND2 were significantly increased in Ang-II-induced NMVCs. Overexpression of miR-92b-3p can ameliorate Ang-II-induced cardiomyocyte hypertrophy. MiR-92b-3p negatively regulated HAND2 expression at the transcriptional level. Both miR-92b-3p mimic and HAND2 siRNA could efficiently inhibit Ang-II-induced hypertrophy in mouse cardiomyocytes.

SIGNIFICANCE

MiR-92b-3p inhibits Ang-II-induced cardiomyocyte hypertrophy via targeting HAND2.

The lncRNA Hand2os1/Uph locus orchestrates heart development through regulation of precise expression of Hand2

Exploration and dissection of potential actions and effects of long noncoding RNA (lncRNA) in animals remain challenging. Here, using multiple knockout mouse models and single cell RNA sequencing, we demonstrate that the divergent lncRNA Hand2os1/Uph has a key complex modulatory effect on the expression of its neighboring gene HAND2 and subsequently on heart development and function. Short deletion of the Hand2os1 promoter in mouse diminishes Hand2os1 transcription to ∼8-32%, but fails to affect HAND2 expression and yields no discernable heart phenotypes. Interestingly, full-length deletion of Hand2os1 in mouse causes moderate yet prevalent upregulation of HAND2 in hundreds of cardiac cells, leading to profound biological consequences, including dysregulated cardiac gene programs, congenital heart defects and perinatal lethality. We propose that the Hand2os1 locus dampens HAND2 expression to restrain cardiomyocyte proliferation, thereby orchestrating a balanced development of cardiac cell lineages. This study highlights the regulatory complexity of the lncRNA Hand2os1 on HAND2 expression, emphasizing the need for complementary genetic and single cell approaches to delineate the function and primary molecular effects of an lncRNA in animals.

Wnt signaling in orofacial clefts: crosstalk, pathogenesis and models

Diverse signaling cues and attendant proteins work together during organogenesis, including craniofacial development. Lip and palate formation starts as early as the fourth week of gestation in humans or embryonic day 9.5 in mice. Disruptions in these early events may cause serious consequences, such as orofacial clefts, mainly cleft lip and/or cleft palate. Morphogenetic Wnt signaling, along with other signaling pathways and transcription regulation mechanisms, plays crucial roles during embryonic development, yet the signaling mechanisms and interactions in lip and palate formation and fusion remain poorly understood. Various Wnt signaling and related genes have been associated with orofacial clefts. This Review discusses the role of Wnt signaling and its crosstalk with cell adhesion molecules, transcription factors, epigenetic regulators and other morphogenetic signaling pathways, including the Bmp, Fgf, Tgfβ, Shh and retinoic acid pathways, in orofacial clefts in humans and animal models, which may provide a better understanding of these disorders and could be applied towards prevention and treatments.

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.

Significant transcriptional changes in 15q duplication but not Angelman syndrome deletion stem cell-derived neurons

Background

The inability to analyze gene expression in living neurons from Angelman (AS) and Duplication 15q (Dup15q) syndrome subjects has limited our understanding of these disorders at the molecular level.

Method

Here, we use dental pulp stem cells (DPSC) from AS deletion, 15q Duplication, and neurotypical control subjects for whole transcriptome analysis. We identified 20 genes unique to AS neurons, 120 genes unique to 15q duplication, and 3 shared transcripts that were differentially expressed in DPSC neurons vs controls.

Results

Copy number correlated with gene expression for most genes across the 15q11.2-q13.1 critical region. Two thirds of the genes differentially expressed in 15q duplication neurons were downregulated compared to controls including several transcription factors, while in AS differential expression was restricted primarily to the 15q region. Here, we show significant downregulation of the transcription factors FOXO1 and HAND2 in neurons from 15q duplication, but not AS deletion subjects suggesting that disruptions in transcriptional regulation may be a driving factor in the autism phenotype in Dup15q syndrome. Downstream analysis revealed downregulation of the ASD associated genes EHPB2 and RORA, both genes with FOXO1 binding sites. Genes upregulated in either Dup15q cortex or idiopathic ASD cortex both overlapped significantly with the most upregulated genes in Dup15q DPSC-derived neurons.

Conclusions

Finding a significant increase in both HERC2 and UBE3A in Dup15q neurons and significant decrease in these two genes in AS deletion neurons may explain differences between AS deletion class and UBE3A specific classes of AS mutation where HERC2 is expressed at normal levels. Also, we identified an enrichment for FOXO1-regulated transcripts in Dup15q neurons including ASD-associated genes EHPB2 and RORA indicating a possible connection between this syndromic form of ASD and idiopathic cases.

Neuroblast differentiation during development and in neuroblastoma requires KIF1Bβ-mediated transport of TRKA

Fell et al. deleted KIF1Bβ in the mouse sympathetic nervous system and observed impaired sympathetic nervous function and misexpression of genes required for sympathoadrenal lineage differentiation. They discovered that KIF1Bβ is required for NGF-dependent neuronal differentiation through anterograde transport of the NGF receptor TRKA.

We recently identified pathogenic KIF1Bβ mutations in sympathetic nervous system malignancies that are defective in developmental apoptosis. Here we deleted KIF1Bβ in the mouse sympathetic nervous system and observed impaired sympathetic nervous function and misexpression of genes required for sympathoadrenal lineage differentiation. We discovered that KIF1Bβ is required for nerve growth factor (NGF)-dependent neuronal differentiation through anterograde transport of the NGF receptor TRKA. Moreover, pathogenic KIF1Bβ mutations identified in neuroblastoma impair TRKA transport. Expression of neuronal differentiation markers is ablated in both KIF1Bβ-deficient mouse neuroblasts and human neuroblastomas that lack KIF1Bβ. Transcriptomic analyses show that unfavorable neuroblastomas resemble mouse sympathetic neuroblasts lacking KIF1Bβ independent of MYCN amplification and the loss of genes neighboring KIF1B on chromosome 1p36. Thus, defective precursor cell differentiation, a common trait of aggressive childhood malignancies, is a pathogenic effect of KIF1Bβ loss in neuroblastomas. Furthermore, neuropathy-associated KIF1Bβ mutations impede cargo transport, providing a direct link between neuroblastomas and neurodegeneration.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

Advances in understanding functional variations in the Hirschsprung disease spectrum (variant Hirschsprung disease)

Hirschsprung disease (HSCR) is a fairly well understood congenital, genetically based functional obstruction due to the congenital absence of ganglion cells in the distal bowel. However, although over 90% of Hirschsprung cases conform to the normally accepted histological diagnostic criteria, it has become increasingly clear that in addition to HSCR, there is a group of functional disturbances relating to a number of other congenital neurodysplastic conditions causing some degree of gastrointestinal tract malfunction. Although these represent a variety of possibly separate conditions of the enteric nervous system, this spectrum it would appear to be also influenced by similar developmental processes. The term "variant Hirschsprung" is commonly used to describe these conditions, but ganglion cells are mostly present if abnormal in number and distribution. These conditions are a problem group being amongst the most difficult to diagnose and treat with possible practical and legal consequences. The problem appears to be possibly one of definition which has proven difficult in the relative paucity of normal values, especially when correlated to age and gestation. It is the purpose of this paper to review the current position on these conditions and to explore possible shared common pathogenetic and genetic mechanisms. This article explores those conditions where a similar pathogenetic mechanisms to HSCR can be demonstrated (e.g. hypoganglionosis) as well as other neural features, which appear to represent separate conditions possibly linked to certain syndromes.

Part and Parcel of the Cardiac Autonomic Nerve System: Unravelling Its Cellular Building Blocks during Development

The autonomic nervous system (cANS) is essential for proper heart function, and complications such as heart failure, arrhythmias and even sudden cardiac death are associated with an altered cANS function. A changed innervation state may underlie (part of) the atrial and ventricular arrhythmias observed after myocardial infarction. In other cardiac diseases, such as congenital heart disease, autonomic dysfunction may be related to disease outcome. This is also the case after heart transplantation, when the heart is denervated. Interest in the origin of the autonomic nerve system has renewed since the role of autonomic function in disease progression was recognized, and some plasticity in autonomic regeneration is evident. As with many pathological processes, autonomic dysfunction based on pathological innervation may be a partial recapitulation of the early development of innervation. As such, insight into the development of cardiac innervation and an understanding of the cellular background contributing to cardiac innervation during different phases of development is required. This review describes the development of the cANS and focuses on the cellular contributions, either directly by delivering cells or indirectly by secretion of necessary factors or cell-derivatives.

Transcriptomic analyses of Hand2 transgenic embryos

In this article, we further provide the data generated for the previously published research article “Specification of jaw identity by the Hand2 transcription factor.” To better understand the downstream genes of the basic helix-loop-helix transcription factor Hand2, we generated double-transgenic mice (Hand2^NC) by intercrossing CAG-floxed CAT-Hand2 mice with Wnt1-Cre mice for conditional activation of Hand2 expression in the neural crest. Altered expression of Hand2 induces transformation of the upper jaw to the lower jaw in Hand2^NC mutant mice. This data article provides Tables detailing the differentially expressed genes between wild-type and Hand2^NC mutant embryos. The raw array data of our transcriptomes as generated using Affymetrix microarrays are available on the NCBI Gene Expression Omnibus (GEO) browser (Reference number GSE75805).

Mouse models of Hirschsprung disease and other developmental disorders of the enteric nervous system: Old and new players

Hirschsprung disease (HSCR, intestinal aganglionosis) is a multigenic disorder with variable penetrance and severity that has a general population incidence of 1/5000 live births. Studies using animal models have contributed to our understanding of the developmental origins of HSCR and the genetic complexity of this disease. This review summarizes recent progress in understanding control of enteric nervous system (ENS) development through analyses in mouse models. An overview of signaling pathways that have long been known to control the migration, proliferation and differentiation of enteric neural progenitors into and along the developing gut is provided as a framework for the latest information on factors that influence enteric ganglia formation and maintenance. Newly identified genes and additional factors beyond discrete genes that contribute to ENS pathology including regulatory sequences, miRNAs and environmental factors are also introduced. Finally, because HSCR has become a paradigm for complex oligogenic diseases with non-Mendelian inheritance, the importance of gene interactions, modifier genes, and initial studies on genetic background effects are outlined.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

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.

Differences in CART expression and cell cycle behavior discriminate sympathetic neuroblast from chromaffin cell lineages in mouse sympathoadrenal cells

Adrenal medullary chromaffin cells and peripheral sympathetic neurons originate from a common sympathoadrenal (SA) progenitor cell. The timing and phenotypic changes that mark this lineage diversification are not fully understood. The present study investigated the expression patterns of phenotypic markers, and cell cycle dynamics, in the adrenal medulla and the neighboring suprarenal ganglion of embryonic mice. The noradrenergic marker, tyrosine hydroxylase (TH), was detected in both presumptive adrenal medulla and sympathetic ganglion cells, but with significantly stronger immunostaining in the former. There was intense cocaine and amphetamine-regulated transcript (CART) peptide immunostaining in most neuroblasts, whereas very few adrenal chromaffin cells showed detectable CART immunostaining. This phenotypic segregation appeared as early as E12.5, before anatomical segregation of the two cell types. Cell cycle dynamics were also examined. Initially, 88% of Sox10 positive (+) neural crest progenitors were proliferating at E10.5. Many SA progenitor cells withdrew from the cell cycle at E11.5 as they started to express TH. Whereas 70% of neuroblasts (TH+/CART+ cells) were back in the cell cycle at E12.5, only around 20% of chromaffin (CART negative) cells were in the cell cycle at E12.5 and subsequent days. Thus, chromaffin cell and neuroblast lineages showed differences in proliferative behavior from their earliest appearance. We conclude that the intensity of TH immunostaining and the expression of CART permit early discrimination of chromaffin cells and sympathetic neuroblasts, and that developing chromaffin cells exhibit significantly lower proliferative activity relative to sympathetic neuroblasts.

Zebrafish as a systems toxicology model for developmental neurotoxicity testing

The developing brain is extremely sensitive to many chemicals. Exposure to neurotoxicants during development has been implicated in various neuropsychiatric and neurological disorders, including autism spectrum disorder, attention deficit hyperactive disorder, schizophrenia, Parkinson's disease, and Alzheimer's disease. Although rodents have been widely used for developmental neurotoxicity testing, experiments using large numbers of rodents are time-consuming, expensive, and raise ethical concerns. Using alternative non-mammalian animal models may relieve some of these pressures by allowing testing of large numbers of subjects while reducing expenses and minimizing the use of mammalian subjects. In this review, we discuss some of the advantages of using zebrafish in developmental neurotoxicity testing, focusing on central nervous system development, neurobehavior, toxicokinetics, and toxicodynamics in this species. We also describe some important examples of developmental neurotoxicity testing using zebrafish combined with gene expression profiling, neuroimaging, or neurobehavioral assessment. Zebrafish may be a systems toxicology model that has the potential to reveal the pathways of developmental neurotoxicity and to provide a sound basis for human risk assessments.

Segregation of neuronal and neuroendocrine differentiation in the sympathoadrenal lineage

Neuronal and neuroendocrine cells possess the capacity for Ca(2+)-regulated discharge of messenger molecules, which they release into synapses or the blood stream, respectively. The neural-crest-derived sympathoadrenal lineage gives rise to the sympathetic neurons of the autonomic nervous system and the neuroendocrine chromaffin cells of the adrenal medulla. These cells provide an excellent model system for studying common and distinct developmental mechanisms underlying the acquisition of neuroendocrine and neuronal properties. As catecholaminergic cells, they possess common markers related to noradrenaline synthesis, storage and release, but they also display diverging gene expression patterns and are morphologically and functionally different. The precise mechanisms that underlie the diversification of sympathoadrenal cells into neurons and neuroendocrine cells are not fully understood. However, in the past we could show that the establishment of a chromaffin phenotype does not depend on signals from the adrenal cortex and that chromaffin cells and sympathetic neurons apparently differ from the onset of their catecholaminergic differentiation. Nevertheless, the cues that specifically induce neuroendocrine features remain elusive. The early development of the progenitors of chromaffin cells and sympathetic neurons depends on a common set of transcription factors with overlapping but distinct influences on their development. In addition to the well-defined role of transcription factors as developmental regulators, our understanding of post-transcriptional gene regulation by microRNAs has substantially increased within the last few decades. This review highlights the major similarities and differences between chromaffin cells and sympathetic neurons, summarizes our current knowledge of the roles of selected transcription factors, microRNAs and environmental signals for the neuroendocrine differentiation of sympathoadrenal cells, and draws comparisons with the development of other endocrine and neuronal cells.

Signaling molecules and transcription factors involved in the development of the sympathetic nervous system, with special emphasis on the superior cervical ganglion

The cells that constitute the sympathetic nervous system originate from the neural crest. This review addresses the current understanding of sympathetic ganglion development viewed from molecular and morphological perspectives. Development of the sympathetic nervous system is categorized into three main steps, as follows: (1) differentiation and migration of cells in the neural crest lineage for formation of the primary sympathetic chain, (2) differentiation of sympathetic progenitors, and (3) growth and survival of sympathetic ganglia. The signaling molecules and transcription factors involved in each of these developmental stages are elaborated mostly on the basis of the results of targeted mutation of respective genes. Analyses in mutant mice revealed differences between the superior cervical ganglion (SCG) and the other posterior sympathetic ganglia. This review provides a summary of the similarities and differences in the development of the SCG and other posterior sympathetic ganglia. Relevant to the development of sympathetic ganglia is the demonstration that neuroendocrine cells, such as adrenal chromaffin cells and carotid body glomus cells, share a common origin with the sympathetic ganglia. Neural crest cells at the trunk level give rise to common sympathoadrenal progenitors of sympathetic neurons and chromaffin cells, while progenitors segregated from the SCG give rise to glomus cells. After separation from the sympathetic primordium, the progenitors of both chromaffin cells and glomus cells colonize the anlage of the adrenal gland and carotid body, respectively. This review highlights the biological properties of chromaffin cells and glomus cells, because, although both cell types are derivatives of sympathetic primordium, they are distinct in many respects.

Ascl1 as a novel player in the Ptf1a transcriptional network for GABAergic cell specification in the retina

In contrast with the wealth of data involving bHLH and homeodomain transcription factors in retinal cell type determination, the molecular bases underlying neurotransmitter subtype specification is far less understood. Using both gain and loss of function analyses in Xenopus, we investigated the putative implication of the bHLH factor Ascl1 in this process. We found that in addition to its previously characterized proneural function, Ascl1 also contributes to the specification of the GABAergic phenotype. We showed that it is necessary for retinal GABAergic cell genesis and sufficient in overexpression experiments to bias a subset of retinal precursor cells towards a GABAergic fate. We also analysed the relationships between Ascl1 and a set of other bHLH factors using an in vivo ectopic neurogenic assay. We demonstrated that Ascl1 has unique features as a GABAergic inducer and is epistatic over factors endowed with glutamatergic potentialities such as Neurog2, NeuroD1 or Atoh7. This functional specificity is conferred by the basic DNA binding domain of Ascl1 and involves a specific genetic network, distinct from that underlying its previously demonstrated effects on catecholaminergic differentiation. Our data show that GABAergic inducing activity of Ascl1 requires the direct transcriptional regulation of Ptf1a, providing therefore a new piece of the network governing neurotransmitter subtype specification during retinogenesis.

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.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

The LIM-Homeodomain transcription factor Islet-1 is required for the development of sympathetic neurons and adrenal chromaffin cells

Islet-1 is a LIM-Homeodomain transcription factor with important functions for the development of distinct neuronal and non-neuronal cell populations. We show here that Islet-1 acts genetically downstream of Phox2B in cells of the sympathoadrenal cell lineage and that the development of sympathetic neurons and chromaffin cells is impaired in mouse embryos with a conditional deletion of Islet-1 controlled by the wnt1 promotor. Islet-1 is not essential for the initial differentiation of sympathoadrenal cells, as indicated by the correct expression of pan-neuronal and catecholaminergic subtype specific genes in primary sympathetic ganglia of Islet-1 deficient mouse embryos. However, our data indicate that the subsequent survival of sympathetic neuron precursors and their differentiation towards TrkA expressing neurons depends on Islet-1 function. In contrast to spinal sensory neurons, sympathetic neurons of Islet-1 deficient mice did not display ectopic expression of genes normally present in the CNS. In Islet-1 deficient mouse embryos the numbers of chromaffin cells were only mildly reduced, in contrast to that of sympathetic neurons, but the initiation of the adrenaline synthesizing enzyme PNMT was abrogated and the expression level of chromogranin A was diminished. Microarray analysis revealed that developing chromaffin cells of Islet-1 deficient mice displayed normal expression levels of TH, DBH and the transcription factors Phox2B, Mash-1, Hand2, Gata3 and Insm1, but the expression levels of the transcription factors Gata2 and Hand1, and AP-2β were significantly reduced. Together our data indicate that Islet-1 is not essentially required for the initial differentiation of sympathoadrenal cells, but has an important function for the correct subsequent development of sympathetic neurons and chromaffin cells.

The transcription factor Hmx1 and growth factor receptor activities control sympathetic neurons diversification

The sympathetic nervous system relies on distinct populations of neurons that use noradrenaline or acetylcholine as neurotransmitter. We show that fating of the sympathetic lineage at early stages results in hybrid precursors from which, genetic cell-lineage tracing reveals, all types progressively emerge by principal mechanisms of maintenance, repression and induction of phenotypes. The homeobox transcription factor HMX1 represses Tlx3 and Ret, induces TrkA and maintains tyrosine hydroxylase (Th) expression in precursors, thus driving segregation of the noradrenergic sympathetic fate. Cholinergic sympathetic neurons develop through cross-regulatory interactions between TRKC and RET in precursors, which lead to Hmx1 repression and sustained Tlx3 expression, thereby resulting in failure of TrkA induction and loss of maintenance of Th expression. Our results provide direct evidence for a model in which diversification of noradrenergic and cholinergic sympathetic neurons is based on a principle of cross-repressive functions in which the specific cell fates are directed by an active suppression of the expression of transcription factors and receptors that direct the alternative fate.

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.

Transcriptional control of differentiation and neurogenesis in autonomic ganglia

Autonomic neuron development is controlled by a network of transcription factors, which is induced by bone morphogenetic protein signalling in neural crest progenitor cells. This network intersects with a transcriptional program in migratory neural crest cells that pre-specifies autonomic neuron precursor cells. Recent findings demonstrate that the transcription factors acting in the initial specification and differentiation of sympathetic neurons are also important for the proliferation of progenitors and immature neurons during neurogenesis. Elimination of Phox2b, Hand2 and Gata3 in differentiated neurons affects the expression of subtype-specific and/or generic neuronal properties or neuron survival. Taken together, transcription factors previously shown to act in initial neuron specification and differentiation display a much broader spectrum of functions, including control of neurogenesis and the maintenance of subtype characteristics and survival of mature neurons.

Targeted deletion of Hand2 in enteric neural precursor cells affects its functions in neurogenesis, neurotransmitter specification and gangliogenesis, causing functional aganglionosis

Targeted deletion of the bHLH DNA-binding protein Hand2 in the neural crest, impacts development of the enteric nervous system (ENS), possibly by regulating the transition from neural precursor cell to neuron. We tested this hypothesis by targeting Hand2 deletion in nestin-expressing neural precursor (NEP) cells. The mutant mice showed abnormal ENS development, resulting in lethal neurogenic pseudo-obstruction. Neurogenesis of neurons derived from NEP cells identified a second nestin non-expressing neural precursor (NNEP) cell in the ENS. There was substantial compensation for the loss of neurons derived from the NEP pool by the NNEP pool but this was insufficient to abrogate the negative impact of Hand2 deletion. Hand2-mediated regulation of proliferation affected both neural precursor and neuron numbers. Differentiation of glial cells derived from the NEP cells was significantly decreased with no compensation from the NNEP pool of cells. Our data indicate differential developmental potential of NEPs and NNEPs; NNEPs preferentially differentiate as neurons, whereas NEPs give rise to both neurons and glial cells. Deletion of Hand2 also resulted in complete loss of NOS and VIP and a significant decrease in expression of choline acetyltransferase and calretinin, demonstrating a role for Hand2 in neurotransmitter specification and/or expression. Loss of Hand2 resulted in a marked disruption of the developing neural network, exemplified by lack of a myenteric plexus and extensive overgrowth of fibers. Thus, Hand2 is essential for neurogenesis, neurotransmitter specification and neural network patterning in the developing ENS.

Expression level of Hand2 affects specification of enteric neurons and gastrointestinal function in mice

BACKGROUND & AIMS

Hand2 is a basic helix-loop-helix transcription factor required for terminal differentiation of enteric neurons. We studied Hand2 haploinsufficient mice, to determine whether reduced expression of Hand2 allows sufficient enteric neurogenesis for survival, but not for development of a normal enteric nervous system (ENS).

METHODS

Enteric transcripts that encode Hand2 and the neuron-specific embryonic lethal abnormal vision proteins HuB, HuC, and HuD were quantified. Immunocytochemistry was used to identify and quantify neurons. Apoptosis was analyzed with the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling procedure. Intracellular microelectrodes were used to record inhibitory junction potentials. Gastrointestinal transit and colonic motility were measured in vivo.

RESULTS

Levels of of enteric Hand2 transcripts were associated with genotypes of mice, in the following order: Hand2(+/+) > Hand2(LoxP/+) > Hand2(+/-) > Hand2(LoxP/-). Parallel reductions were found in expression of HuD and in regional and phenotypic manners. Numbers of neurons, numbers of neuronal nitric oxide synthase(+) and calretinin(+), but not substance P(+) or vasoactive intestinal peptide(+) neurons, decreased. No effects were observed in stomach or cecum. Apoptosis was not detected, consistent with the concept that Hand2 inhibits neuronal differentiation, rather than regulates survival. The amplitude of inhibitory junction potentials in colonic circular muscle was similar in Hand2 wild-type and haploinsufficient mice, although in haploinsufficient mice, the purinergic component was reduced and a nitrergic component appeared. The abnormal ENS of haploinsufficient mice slowed gastrointestinal motility but protected mice against colitis.

CONCLUSIONS

Reduced expression of factors required for development of the ENS can cause defects in the ENS that are subtle enough to escape detection yet cause significant abnormalities in bowel function.

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.

Hand factors as regulators of cardiac morphogenesis and implications for congenital heart defects

Almost 15 years of careful study have established the related basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 as critical for heart development across evolution. Hand factors make broad contributions, revealed through animal models, to the development of multiple cellular lineages that ultimately contribute to the heart. They perform critical roles in ventricular cardiomyocyte growth, differentiation, morphogenesis, and conduction. They are also important for the proper development of the cardiac outflow tract, epicardium, and endocardium. Molecularly, they function both through DNA binding and through protein-protein interactions, which are regulated transcriptionally, posttranscriptionally by microRNAs, and posttranslationally through phosphoregulation. Although direct Hand factor transcriptional targets are progressively being identified, confirmed direct targets of Hand factor transcriptional activity in the heart are limited. Identification of these targets will be critical to model the mechanisms by which Hand factor bHLH interactions affect developmental pathways. Improved understanding of Hand factor-mediated transcriptional cascades will be necessary to determine how Hand factor dysregulation translates to human disease phenotypes. This review summarizes the insight that animal models have provided into the regulation and function of these factors during heart development, in addition to the recent findings that suggest roles for HAND1 and HAND2 in human congenital heart disease.

A Tlx2-Cre mouse line uncovers essential roles for hand1 in extraembryonic and lateral mesoderm

Hand1 regulates development of numerous tissues within the embryo, extraembryonic mesoderm, and trophectoderm. Systemic loss of Hand1 results in early embryonic lethality but the cause has remained unknown. To determine if Hand1 expression in extraembryonic mesoderm is essential for embryonic survival, Hand1 was conditionally deleted using the HoxB6-Cre mouse line that expresses Cre in extraembryonic and lateral mesoderm. Deletion of Hand1 using HoxB6-Cre resulted in embryonic lethality identical to systemic knockout. To determine if lethality is due to Hand1 function in extraembryonic mesoderm or lateral mesoderm, we generated a Tlx2-Cre mouse line expressing Cre in lateral mesoderm but not extraembryonic tissues. Deletion of Hand1 using the Tlx2-Cre line results in embryonic survival with embryos exhibiting herniated gut and thin enteric smooth muscle. Our results show that Hand1 regulates development of lateral mesoderm derivatives and its loss in extraembryonic mesoderm is the primary cause of lethality in Hand1-null embryos.

Analysis of the Hand1 cell lineage reveals novel contributions to cardiovascular, neural crest, extra-embryonic, and lateral mesoderm derivatives

The basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 play critical roles in the development of multiple organ systems during embryogenesis. The dynamic expression patterns of these two factors within developing tissues obfuscate their respective unique and redundant organogenic functions. To define cell lineages potentially dependent upon Hand gene expression, we generated a mutant allele in which the coding region of Hand1 is replaced by Cre recombinase. Subsequent Cre-mediated activation of β-galactosidase or eYFP reporter alleles enabled lineage trace analyses that clearly define the fate of Hand1-expressing cells. Hand1-driven Cre marks specific lineages within the extra embryonic tissues, placenta, sympathetic nervous system, limbs, jaw, and several cell types within the cardiovascular system. Comparisons between Hand1 expression and Hand1-lineage greatly refine our understanding of its dynamic spatial-temporal expression domains and raise the possibility of novel Hand1 functions in structures not thought to be Hand1-dependent.

Cytokines inhibit norepinephrine transporter expression by decreasing Hand2

Functional noradrenergic transmission requires the coordinate expression of enzymes involved in norepinephrine (NE) synthesis, as well as the norepinephrine transporter (NET) which removes NE from the synapse. Inflammatory cytokines acting through gp130 can suppress the noradrenergic phenotype in sympathetic neurons. This occurs in a subset of sympathetic neurons during development and also occurs in adult neurons after injury. For example, cytokines suppress noradrenergic function in sympathetic neurons after axotomy and during heart failure. The molecular basis for suppression of noradrenergic genes is not well understood, but previous studies implicated a reduction of Phox2a in cytokine suppression of dopamine beta hydroxylase. We used sympathetic neurons and neuroblastoma cells to investigate the role of Phox2a in cytokine suppression of NET transcription. Chromatin immunoprecipitation experiments revealed that Phox2a did not bind the NET promoter, and overexpression of Phox2a did not prevent cytokine suppression of NET transcription. Hand2 and Gata3 are transcription factors that induce noradrenergic genes during development and are present in mature sympathetic neurons. Both Hand2 and Gata3 were decreased by cytokines in sympathetic neurons and neuroblastoma cells. Overexpression of either Hand2 or Gata3 was sufficient to rescue NET transcription following suppression by cytokines. We examined expression of these genes following axotomy to determine if their expression was altered following nerve injury. NET and Hand2 mRNAs decreased significantly in sympathetic neurons 48 h after axotomy, but Gata3 mRNA was unchanged. These data suggest that cytokines can inhibit NET expression through downregulation of Hand2 or Gata3 in cultured sympathetic neurons, but axotomy in adult animals selectively suppresses Hand2 expression.