Molecular control of ciliary neuron development: BMPs and downstream transcriptional control in the parasympathetic lineage

A systematic review and embryological perspective of pluripotent stem cell-derived autonomic postganglionic neuron differentiation for human disease modeling

Human autonomic neuronal cell models are emerging as tools for modeling diseases such as cardiac arrhythmias. In this systematic review, we compared 33 articles applying 14 different protocols to generate sympathetic neurons and 3 different procedures to produce parasympathetic neurons. All methods involved the differentiation of human pluripotent stem cells, and none employed permanent or reversible cell immortalization. Almost all protocols were reproduced in multiple pluripotent stem cell lines, and over half showed evidence of neural firing capacity. Common limitations in the field are a lack of three-dimensional models and models that include multiple cell types. Sympathetic neuron differentiation protocols largely mirrored embryonic development, with the notable absence of migration, axon extension, and target-specificity cues. Parasympathetic neuron differentiation protocols may be improved by including several embryonic cues promoting cell survival, cell maturation, or ion channel expression. Moreover, additional markers to define parasympathetic neurons in vitro may support the validity of these protocols. Nonetheless, four sympathetic neuron differentiation protocols and one parasympathetic neuron differentiation protocol reported more than two-thirds of cells expressing autonomic neuron markers. Altogether, these protocols promise to open new research avenues of human autonomic neuron development and disease modeling.

Self-organizing human heart assembloids with autologous and developmentally relevant cardiac neural crest-derived tissues

Neural crest cells (NCCs) are a multipotent embryonic cell population of ectodermal origin that extensively migrate during early development and contribute to the formation of multiple tissues. Cardiac NCCs play a critical role in heart development by orchestrating outflow tract septation, valve formation, aortic arch artery patterning, parasympathetic innervation, and maturation of the cardiac conduction system. Abnormal migration, proliferation, or differentiation of cardiac NCCs can lead to severe congenital cardiovascular malformations. However, the complexity and timing of early embryonic heart development pose significant challenges to studying the molecular mechanisms underlying NCC-related cardiac pathologies. Here, we present a sophisticated functional model of human heart assembloids derived from induced pluripotent stem cells, which, for the first time, recapitulates cardiac NCC integration into the human embryonic heart in vitro . NCCs successfully integrated at developmentally relevant stages into heart organoids, and followed developmental trajectories known to occur in the human heart. They demonstrated extensive migration, differentiated into cholinergic neurons capable of generating nerve impulses, and formed mature glial cells. Additionally, they contributed to the mesenchymal populations of the developing outflow tract. Through transcriptomic analysis, we revealed that NCCs acquire molecular features of their cardiac derivatives as heart assembloids develop. NCC-derived parasympathetic neurons formed functional connections with cardiomyocytes, promoting the maturation of the cardiac conduction system. Leveraging this model's cellular complexity and functional maturity, we uncovered that early exposure of NCCs to antidepressants harms the development of NCC derivatives in the context of the developing heart. The commonly prescribed antidepressant Paroxetine disrupted the expression of a critical early neuronal transcription factor, resulting in impaired parasympathetic innervation and functional deficits in cardiac tissue. This advanced heart assembloid model holds great promise for high-throughput drug screening and unraveling the molecular mechanisms underlying NCC-related cardiac formation and congenital heart defects.

IN BRIEF

Human neural crest heart assembloids resembling the major directions of neural crest differentiation in the human embryonic heart, including parasympathetic innervation and the mesenchymal component of the outflow tract, provide a human-relevant embryonic platform for studying congenital heart defects and drug safety.

The differences in the anatomy of the thoracolumbar and sacral autonomic outflow are quantitative

PURPOSE

We have re-evaluated the anatomical arguments that underlie the division of the spinal visceral outflow into sympathetic and parasympathetic divisions.

METHODOLOGY

Using a systematic literature search, we mapped the location of catecholaminergic neurons throughout the mammalian peripheral nervous system. Subsequently, a narrative method was employed to characterize segment-dependent differences in the location of preganglionic cell bodies and the composition of white and gray rami communicantes.

RESULTS AND CONCLUSION

One hundred seventy studies were included in the systematic review, providing information on 389 anatomical structures. Catecholaminergic nerve fibers are present in most spinal and all cranial nerves and ganglia, including those that are known for their parasympathetic function. Along the entire spinal autonomic outflow pathways, proximal and distal catecholaminergic cell bodies are common in the head, thoracic, and abdominal and pelvic region, which invalidates the "short-versus-long preganglionic neuron" argument. Contrary to the classically confined outflow levels T1-L2 and S2-S4, preganglionic neurons have been found in the resulting lumbar gap. Preganglionic cell bodies that are located in the intermediolateral zone of the thoracolumbar spinal cord gradually nest more ventrally within the ventral motor nuclei at the lumbar and sacral levels, and their fibers bypass the white ramus communicans and sympathetic trunk to emerge directly from the spinal roots. Bypassing the sympathetic trunk, therefore, is not exclusive for the sacral outflow. We conclude that the autonomic outflow displays a conserved architecture along the entire spinal axis, and that the perceived differences in the anatomy of the autonomic thoracolumbar and sacral outflow are quantitative.

Trunk Neural Crest Migratory Position and Asymmetric Division Predict Terminal Differentiation

The generation of complex structures during embryogenesis requires the controlled migration and differentiation of cells from distant origins. How these processes are coordinated and impact each other to form functional structures is not fully understood. Neural crest cells migrate extensively giving rise to many cell types. In the trunk, neural crest cells migrate collectively forming chains comprised of cells with distinct migratory identities: one leader cell at the front of the group directs migration, while followers track the leader forming the body of the chain. Herein we analysed the relationship between trunk neural crest migratory identity and terminal differentiation. We found that trunk neural crest migration and fate allocation is coherent. Leader cells that initiate movement give rise to the most distal derivativities. Interestingly, the asymmetric division of leaders separates migratory identity and fate. The distal daughter cell retains the leader identity and clonally forms the Sympathetic Ganglia. The proximal sibling migrates as a follower and gives rise to Schwann cells. The sympathetic neuron transcription factor phox2bb is strongly expressed by leaders from early stages of migration, suggesting that specification and migration occur concomitantly and in coordination. Followers divide symmetrically and their fate correlates with their position in the chain.

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.

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.

Selective Induction of Human Autonomic Neurons Enables Precise Control of Cardiomyocyte Beating

The autonomic nervous system (ANS) regulates tissue homeostasis and remodelling through antagonistic effects of noradrenergic sympathetic and cholinergic parasympathetic signalling. Despite numerous reports on the induction of sympathetic neurons from human pluripotent stem cells (hPSCs), no induction methods have effectively derived cholinergic parasympathetic neurons from hPSCs. Considering the antagonistic effects of noradrenergic and cholinergic inputs on target organs, both sympathetic and parasympathetic neurons are expected to be induced. This study aimed to develop a stepwise chemical induction method to induce sympathetic-like and parasympathetic-like ANS neurons. Autonomic specification was achieved through restricting signals inducing sensory or enteric neurogenesis and activating bone morphogenetic protein (BMP) signals. Global mRNA expression analyses after stepwise induction, including single-cell RNA-seq analysis of induced neurons and functional assays revealed that each induced sympathetic-like or parasympathetic-like neuron acquired pharmacological and electrophysiological functional properties with distinct marker expression. Further, we identified selective induction methods using appropriate seeding cell densities and neurotrophic factor concentrations. Neurons were individually induced, facilitating the regulation of the beating rates of hiPSC-derived cardiomyocytes in an antagonistic manner. The induction methods yield specific neuron types, and their influence on various tissues can be studied by co-cultured assays.

Dual Sympathetic Input into Developing Salivary Glands

Neuronal signaling is known to be required for salivary gland development, with parasympathetic nerves interacting with the surrounding tissues from early stages to maintain a progenitor cell population and control morphogenesis. In contrast, postganglionic sympathetic nerves arrive late in salivary gland development to perform a secretory function; however, no previous report has shown their role during development. Here, we show that a subset of neuronal cells within the parasympathetic submandibular ganglion (PSG) express the catecholaminergic marker tyrosine hydroxylase (TH) in developing murine and human submandibular glands. This sympathetic phenotype coincided with the expression of transcription factor Hand2 within the PSG from the bud stage (E12.5) of mouse embryonic salivary gland development. Hand2 was previously associated with the decision of neural crest cells to become sympathetic in other systems, suggesting a role in controlling neuronal fate in the salivary gland. The PSG therefore provides a population of TH-expressing neurons prior to the arrival of the postganglionic sympathetic axons from the superior cervical ganglion at E15.5. In culture, in the absence of nerves from the superior cervical ganglion, these PSG-derived TH neurons were clearly evident forming a network around the gland. Chemical ablation of dopamine receptors in explant culture with the neurotoxin 6-hydroxydopamine at early stages of gland development resulted in specific loss of the TH-positive neurons from the PSG, and subsequent branching was inhibited. Taken altogether, these results highlight for the first time the detailed developmental time course of TH-expressing neurons during murine salivary gland development and suggest a role for these neurons in branching morphogenesis.

Schwann cell precursors contribute to skeletal formation during embryonic development in mice and zebrafish

Multipotent Schwann cell precursors (SCPs) generate numerous cell types. Here, in both mouse and zebrafish, SCPs contributed to the generation of mesenchymal, chondroprogenitor, and osteoprogenitor cells during embryonic development. These findings reveal a source of cartilage and bone cells and previously unanticipated interactions between the nervous system and skeleton during development.

Immature multipotent embryonic peripheral glial cells, the Schwann cell precursors (SCPs), differentiate into melanocytes, parasympathetic neurons, chromaffin cells, and dental mesenchymal populations. Here, genetic lineage tracing revealed that, during murine embryonic development, some SCPs detach from nerve fibers to become mesenchymal cells, which differentiate further into chondrocytes and mature osteocytes. This occurred only during embryonic development, producing numerous craniofacial and trunk skeletal elements, without contributing to development of the appendicular skeleton. Formation of chondrocytes from SCPs also occurred in zebrafish, indicating evolutionary conservation. Our findings reveal multipotency of SCPs, providing a developmental link between the nervous system and skeleton.

Induced pluripotent stem cells for disease modeling, cell therapy and drug discovery in genetic autonomic disorders: a review

The autonomic nervous system (ANS) regulates all organs in the body independent of consciousness, and is thus essential for maintaining homeostasis of the entire organism. Diseases of the ANS can arise due to environmental insults such as injury, toxins/drugs and infections or due to genetic lesions. Human studies and animal models have been instrumental to understanding connectivity and regulation of the ANS and its disorders. However, research into cellular pathologies and molecular mechanisms of ANS disorders has been hampered by the difficulties in accessing human patient-derived ANS cells in large numbers to conduct meaningful research, mainly because patient neurons cannot be easily biopsied and primary human neuronal cultures cannot be expanded.Human-induced pluripotent stem cell (hiPSC) technology can elegantly bridge these issues, allowing unlimited access of patient-derived ANS cell types for cellular, molecular and biochemical analysis, facilitating the discovery of novel therapeutic targets, and eventually leading to drug discovery. Additionally, such cells may provide a source for cell replacement therapy to replenish lost or injured ANS tissue in patients.Here, we first review the anatomy and embryonic development of the ANS, as this knowledge is crucial for understanding disease modeling approaches. We then review the current advances in human stem cell technology for modeling diseases of the ANS, recent strides toward cell replacement therapy and drug discovery initiatives.

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.

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.

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.

The Nervous System Orchestrates and Integrates Craniofacial Development: A Review

Development of a head is a dazzlingly complex process: a number of distinct cellular sources including cranial ecto- and endoderm, mesoderm and neural crest contribute to facial and other structures. In the head, an extremely fine-tuned developmental coordination of CNS, peripheral neural components, sensory organs and a musculo-skeletal apparatus occurs, which provides protection and functional integration. The face can to a large extent be considered as an assembly of sensory systems encased and functionally fused with appendages represented by jaws. Here we review how the developing brain, neurogenic placodes and peripheral nerves influence the morphogenesis of surrounding tissues as a part of various general integrative processes in the head. The mechanisms of this impact, as we understand it now, span from the targeted release of the morphogens necessary for shaping to providing a niche for cellular sources required in later development. In this review we also discuss the most recent findings and ideas related to how peripheral nerves and nerve-associated cells contribute to craniofacial development, including teeth, during the post- neural crest period and potentially in regeneration.

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.

Prox1 identifies proliferating neuroblasts and nascent neurons during neurogenesis in sympathetic ganglia

Neurogenesis in embryonic sympathetic ganglia involves neuroblasts that resume proliferation following neuronal differentiation. As cell cycle exit is not associated with neuronal differentiation, the identity of proliferating neuroblasts is incompletely understood. Here, we use sympathetic ganglia of chick embryos to define the timing of neurogenesis and neuroblast identity focusing on the expression and function of the transcription factor Prox1. We show that a large fraction of neuroblasts has initially withdrawn from the cell cycle at embryonic day 3 (E3), which is reflected by a high proportion of p27(+)/Islet1(+) neuroblasts (63%) and low numbers of EdU(+)/Islet1(+) cells (12%). The proportion of proliferating Islet1(+) neuroblasts, identified by EdU pulse labeling and by the absence of the postmitotic marker p27 increases to reach maximal levels at E5, when virtually all neuroblasts are in the cell cycle (95%). Subsequently, the proportion of EdU-labeled and p27(-) neuroblasts is reduced to reach low levels at E11. Interestingly, the expression of the transcription factor Prox1 is restricted to the neuronal lineage, that is, Sox10(+)/Phox2b(+) neuron progenitors, proliferating p27(-)/Islet1(+) neuroblasts and nascent neurons but is rapidly lost in postmitotic neurons. In vitro and in vivo knockdown and overexpression experiments demonstrate effects of Prox1 in the support of neuroblast proliferation and survival. Taken together, these results define the neurogenesis period in the chick paravertebral sympathetic ganglia including an initial cell cycle withdrawal and identify Prox1 as a marker and regulator of proliferating sympathetic neuroblasts.

RARβ regulates neuronal cell death and differentiation in the avian ciliary ganglion

Programmed cell death during chicken ciliary ganglion (CG) development is mostly discussed as an extrinsically regulated process, guided either by the establishment of a functional balance between preganglionic and postganglionic activity or the availability of target‐derived neurotrophic factors. We found that the expression of the gene coding for the nuclear retinoic acid receptor β (RARB) is transiently upregulated prior to and during the execution phase of cell death in the CG. Using retroviral vectors, the expression of RARB was knocked down during embryonic development in ovo. The knockdown led to a significant increase in CG neuron number after the cell death phase. BrdU injections and active caspase‐3 staining revealed that this increase in neuron number was due to an inhibition of apoptosis during the normal cell death phase. Furthermore, apoptotic neuron numbers were significantly increased at a stage when cell death is normally completed. While the cholinergic phenotype of the neurons remained unchanged after RARB knockdown, the expression of the proneural gene Cash1 was increased, but somatostatin‐like immunoreactivity, a hallmark of the mature choroid neuron population, was decreased. Taken together, these results point toward a delay in neuronal differentiation as well as cell death. The availability of nuclear retinoic acid receptor β (RARβ) and RARβ‐induced transcription of genes could therefore be a new intrinsic cue for the maturation of CG neurons and their predisposition to undergo cell death. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 75: 1204–1218, 2015

Signaling via the transcriptionally regulated activin receptor 2B is a novel mediator of neuronal cell death during chicken ciliary ganglion development

The TGF-β ligand superfamily members activin A and BMP control important aspects of embryonic neuronal development and differentiation. Both are known to bind to activin receptor subtypes IIA (ActRIIA) and IIB, while in the avian ciliary ganglion (CG), so far only ActRIIA-expression has been described. We show that the expression of ACVR2B, coding for the ActRIIB, is tightly regulated during CG development and the knockdown of ACVR2B expression leads to a deregulation in the execution of neuronal apoptosis and therefore affects ontogenetic programmed cell death in vivo. While the differentiation of choroid neurons was impeded in the knockdown, pointing toward a reduction in activin A-mediated neural differentiation signaling, naturally occurring neuronal cell death in the CG was not prevented by follistatin treatment. Systemic injections of the BMP antagonist noggin, on the other hand, reduced the number of apoptotic neurons to a similar extent as ACVR2B knockdown. We therefore propose a novel pathway in the regulation of CG neuron ontogenetic programmed cell death, which could be mediated by BMP and signals via the ActRIIB.

Neurodevelopment. Parasympathetic neurons originate from nerve-associated peripheral glial progenitors

The peripheral autonomic nervous system reaches far throughout the body and includes neurons of diverse functions, such as sympathetic and parasympathetic. We show that the parasympathetic system in mice--including trunk ganglia and the cranial ciliary, pterygopalatine, lingual, submandibular, and otic ganglia--arise from glial cells in nerves, not neural crest cells. The parasympathetic fate is induced in nerve-associated Schwann cell precursors at distal peripheral sites. We used multicolor Cre-reporter lineage tracing to show that most of these neurons arise from bi-potent progenitors that generate both glia and neurons. This nerve origin places cellular elements for generating parasympathetic neurons in diverse tissues and organs, which may enable wiring of the developing parasympathetic nervous system.

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.

A genome-wide screen to identify transcription factors expressed in pelvic Ganglia of the lower urinary tract

Relative positions of neurons within mature murine pelvic ganglia based on expression of neurotransmitters have been described. However the spatial organization of developing innervation in the murine urogenital tract (UGT) and the gene networks that regulate specification and maturation of neurons within the pelvic ganglia of the lower urinary tract (LUT) are unknown. We used whole-mount immunohistochemistry and histochemical stains to localize neural elements in 15.5 days post coitus (dpc) fetal mice. To identify potential regulatory factors expressed in pelvic ganglia, we surveyed expression patterns for known or probable transcription factors (TF) annotated in the mouse genome by screening a whole-mount in situ hybridization library of fetal UGTs. Of the 155 genes detected in pelvic ganglia, 88 encode TFs based on the presence of predicted DNA-binding domains. Neural crest (NC)-derived progenitors within the LUT were labeled by Sox10, a well-known regulator of NC development. Genes identified were categorized based on patterns of restricted expression in pelvic ganglia, pelvic ganglia and urethral epithelium, or pelvic ganglia and urethral mesenchyme. Gene expression patterns and the distribution of Sox10+, Phox2b+, Hu+, and PGP9.5+ cells within developing ganglia suggest previously unrecognized regional segregation of Sox10+ progenitors and differentiating neurons in early development of pelvic ganglia. Reverse transcription-PCR of pelvic ganglia RNA from fetal and post-natal stages demonstrated that multiple TFs maintain post-natal expression, although Pax3 is extinguished before weaning. Our analysis identifies multiple potential regulatory genes including TFs that may participate in segregation of discrete lineages within pelvic ganglia. The genes identified here are attractive candidate disease genes that may now be further investigated for their roles in malformation syndromes or in LUT dysfunction.

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.

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.

Generation of the tamoxifen-inducible DBH-Cre transgenic mouse line DBH-CT

We generated transgenic mice bearing a tamoxifen-dependent Cre recombinase expressed under the control of the dopamine-β-hydroxylase promoter. By crossing to the ROSA26 reporter mice we show that tamoxifen-induced Cre recombinase in adult mice specifically activates β-galactosidase expression in differentiated noradrenergic neurons of the central and peripheral nervous system. Tamoxifen application in adult mice did not induce β-galactosidase activity in parasympathetic neurons that transiently express DBH during development. Thus, this transgenic mouse line represents a valuable tool to study gene function in mature noradrenergic neurons by conditional inactivation.

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.

Ongoing roles of Phox2 homeodomain transcription factors during neuronal differentiation

Transcriptional determinants of neuronal identity often stay expressed after their downstream genetic program is launched. Whether this maintenance of expression plays a role is for the most part unknown. Here, we address this question for the paralogous paired-like homeobox genes Phox2a and Phox2b, which specify several classes of visceral neurons at the progenitor stage in the central and peripheral nervous systems. By temporally controlled inactivation of Phox2b, we find that the gene, which is required in ventral neural progenitors of the hindbrain for the production of branchio-visceral motoneuronal precursors, is also required in these post-mitotic precursors to maintain their molecular signature - including downstream transcription factors - and allow their tangential migration and the histogenesis of the corresponding nuclei. Similarly, maintenance of noradrenergic differentiation during embryogenesis requires ongoing expression of Phox2b in sympathetic ganglia, and of Phox2a in the main noradrenergic center, the locus coeruleus. These data illustrate cases where the neuronal differentiation program does not unfold as a transcriptional `cascade' whereby downstream events are irreversibly triggered by an upstream regulator, but instead require continuous transcriptional input from it.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Neuroblastoma phox2b variants stimulate proliferation and dedifferentiation of immature sympathetic neurons

Neuroblastoma is a pediatric tumor that is thought to arise from autonomic precursors in the neural crest. Mutations in the PHOX2B gene have been observed in familial and sporadic forms of neuroblastoma and represent the first defined genetic predisposition for neuroblastoma. Here, we address the mechanisms that may underlie this predisposition, comparing the function of wild-type and mutant Phox2b proteins ectopically expressed in proliferating, embryonic sympathetic neurons. Phox2b displays a strong antiproliferative effect, which is lost in all Phox2b neuroblastoma variants analyzed. In contrast, an increase in sympathetic neuron proliferation is elicited by Phox2b variants with mutations in the homeodomain when endogenous Phox2b levels are lowered by siRNA-mediated knockdown to mimic the situation of heterozygous PHOX2B mutations in neuroblastoma. The increased proliferation is blocked by Hand2 knockdown and the antiproliferative Phox2b effects are rescued by Hand2 overexpression, implying Hand2 in Phox2b-mediated proliferation control. A Phox2b variant with a nonsense mutation in the homeodomain elicits, in addition, a decreased expression of characteristic marker genes. Together, these results suggest that PHOX2B mutations predispose to neuroblastoma by increasing proliferation and promoting dedifferentiation of cells in the sympathoadrenergic lineage.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

The bHLH transcription factor Hand2 is essential for the maintenance of noradrenergic properties in differentiated sympathetic neurons

The basic helix-loop-helix transcription factor Hand2 is essential for the proliferation and noradrenergic differentiation of sympathetic neuron precursors during development. Here we address the function of Hand2 in postmitotic, differentiated sympathetic neurons. Knockdown of endogenous Hand2 in cultured E12 chick sympathetic neurons by siRNA results in a significant (about 60%) decrease in the expression of the noradrenergic marker genes dopamine-beta-hydroxylase (DBH) and tyrosine hydroxylase (TH). In contrast, expression of the pan-neuronal genes TuJ1, HuC and SCG10 was not affected. To analyze the in vivo role of Hand2 in differentiated sympathetic neurons we used mice harboring a conditional Hand2-null allele and excised the gene by expression of Cre recombinase under control of the DBH promotor. Mouse embryos homozygous for Hand2 gene deletion showed decreased sympathetic neuron number and TH expression was strongly reduced in the residual neuron population. The in vitro Hand2 knockdown also enhances the CNTF-induced expression of the cholinergic marker genes vesicular acetylcholine transporter (VAChT) and choline acetyltransferase (ChAT). Taken together, these findings demonstrate that the Hand2 transcription factor plays a key role in maintaining noradrenergic properties in differentiated neurons.

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.

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.

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.

Adult neurogenesis requires Smad4-mediated bone morphogenic protein signaling in stem cells

In the mammalian brain, neurogenesis continues only in few regions of the forebrain. The molecular signals governing neurogenesis in these unique neurogenic niches, however, are still ill defined. Here, we show that bone morphogenic protein (BMP)-mediated signaling is active in adult neural stem cells and is crucial to initiate the neurogenic lineage in the adult mouse subependymal zone. Conditional deletion of Smad4 in adult neural stem cells severely impairs neurogenesis, and this is phenocopied by infusion of Noggin, an extracellular antagonist of BMP. Smad4 deletion in stem, but not progenitor cells, as well as Noggin infusion lead to an increased number of Olig2-expressing progeny that migrate to the corpus callosum and differentiate into oligodendrocytes. Transplantation experiments further verified the cell-autonomous nature of this phenotype. Thus, BMP-mediated signaling via Smad4 is required to initiate neurogenesis from adult neural stem cells and suppress the alternative fate of oligodendrogliogenesis.

Bone morphogenetic proteins specify the retinal pigment epithelium in the chick embryo

In vertebrates, the neuroepithelium of the optic vesicle is initially multipotential, co-expressing a number of transcription factors that are involved in retinal pigment epithelium (RPE) and neural retina (NR)development. Subsequently, extrinsic signals emanating from the surrounding tissues induce the separation of the optic vesicle into three domains: the optic stalk/nerve, the NR and the RPE. Here, we show that bone morphogenetic proteins (BMPs) are sufficient and essential for RPE development in vivo. Bmp4 and Bmp7 are expressed in the surface ectoderm overlying the optic vesicle, the surrounding mesenchyme and/or presumptive RPE during the initial stages of eye development. During the initial stages of chick eye development the microphthalmia-associated transcription factor(Mitf), important for RPE development, is expressed in the optic primordium that is covered by the BMP-expressing surface ectoderm. Following BMP application, the optic neuroepithelium, including the presumptive optic stalk/nerve and NR domain, develop into RPE as assessed by the expression of Otx2, Mitf, Wnt2b and the pigmented cell marker MMP115. By contrast, interfering with BMP signalling prevents RPE development in the outer layer of the optic cup and induces NR-specific gene expression (e.g. Chx10). Our results show that BMPs are sufficient and essential for RPE development during optic vesicle stages. We propose a model in which the BMP-expressing surface ectoderm initiates RPE specification by inducing Mitf expression in the underlying neuroepithelium of the optic vesicle.

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.

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.

Plasticity of pelvic autonomic ganglia and urogenital innervation

Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons and provide most of the motor innervation of the urogenital organs. They show a remarkable sensitivity to androgens and estrogens, which impacts on their development into sexually dimorphic structures and provide an array of mechanisms by which plasticity of these neurons can occur during puberty and adulthood. The structure of pelvic ganglia varies widely among species, ranging from rodents, which have a pair of large ganglia, to humans, in whom pelvic ganglion neurons are distributed in a large, complex plexus. This plexus is frequently injured during pelvic surgical procedures, yet strategies for its repair have yet to be developed. Advances in this area will come from a better understanding of the effects of injury on the cellular signaling process in pelvic neurons and also the role of neurotrophic factors during development, maintenance, and repair of these axons.

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.

Phenotypes of neural-crest-derived cells in vagal and sacral pathways

Enteric neurons arise from vagal and sacral level neural crest cells. To examine the phenotype of neural-crest-derived cells in vagal and sacral pathways, we used antisera to Sox10, p75, Phox2b, and Hu, and transgenic mice in which the expression of green fluorescent protein was under the control of the Ret promoter. Sox10 was expressed prior to the emigration of vagal cells, whereas p75 was expressed shortly after their emigration. Most crest-derived cells that emigrated adjacent to somites 1-4 migrated along a pathway that was later followed by the vagus nerve. A sub-population of these vagal cells coalesced to form vagal ganglia, whereas others continued their migration towards the heart and gut. Cells that coalesced into vagal ganglia showed a different phenotype from cells in the migratory streams proximal and distal to the ganglia. Only a sub-population of the vagal cells that first entered the foregut expressed Phox2b or Ret. Sacral neural crest cells gave rise to pelvic ganglia and some neurons in the hindgut. The pathways of sacral neural crest cells were examined by using DbetaH-nlacZ mice. Sacral cells appeared to enter the distal hindgut around embryonic day 14.5. Very few of the previously demonstrated, but rare, neurons that were present in the large intestine of Ret null mutants and that presumably arose from the sacral neural crest expressed nitric oxide synthase, unlike their counterparts in Ret heterozygous mice.