Expression and regulation of type I BMP receptors during early avian sympathetic ganglion development

Interplay: The Essential Role between INSM1 and N-Myc in Aggressive Neuroblastoma

Simple Summary

Neuroblastoma (NB) is a cancer that starts in certain very early forms of nerve cells of the sympathetic nervous system, most often found in an embryo or fetus. Symptoms may include bone pain, an abdominal mass, frequent urination, limping, anemia, spinal cord weakness, or bruising of the eye area. N-Myc is a key driver of high-risk NB. An elevated expression of N-Myc often predicts a poorer prognosis, in both time to tumor progression and overall survival rate. We discovered a transcription factor, insulinoma-associated-1 (INSM1), as the downstream target gene of N-Myc. INSM1 has emerged as a novel NB biomarker that plays a critical role in facilitating NB tumor cell development. Both N-Myc and INSM1 demonstrate high clinical relevance to NB. Therefore, further understanding the association of INSM1 and N-Myc functions in aggressive NB should be beneficial for future NB treatment.

Abstract

An aggressive form of neuroblastoma (NB), a malignant childhood cancer derived from granule neuron precursors and sympathoadrenal lineage, frequently comprises MYCN amplification/elevated N-Myc expression, which contributes to the development of neural crest-derived embryonal malignancy. N-Myc is an oncogenic driver in NB. Persistent N-Myc expression during the maturation of SA precursor cells can cause blockage of the apoptosis and induce abnormal proliferation, resulting in NB development. An insulinoma-associated-1 (INSM1) zinc-finger transcription factor has emerged as an NB biomarker that plays a critical role in facilitating tumor cell growth and transformation. INSM1 plays an essential role in sympathoadrenal cell differentiation. N-Myc activates endogenous INSM1 through an E2-box of the INSM1 proximal promoter, whereas INSM1 enhances N-Myc stability via RAC-α-serine/threonine protein kinase (AKT) phosphorylation in NB. The ectopic expression of INSM1 stimulates NB tumor growth in contrast to the knockdown of INSM1 that inhibits NB cell proliferation. The clinical pathological result and bioinformatics analysis show that INSM1 is a strong diagnostic and a prognostic biomarker for the evaluation of NB progression. The INSM1/N-Myc expression shows high clinical relevance in NB. Therefore, targeting the INSM1/N-Myc-associated signaling axis should be a feasible approach to identifying new drugs for the suppression of NB tumor growth.

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.

The Candidate Neuroprotective Agent Artemin Induces Autonomic Neural Dysplasia without Preventing Peripheral Nerve Dysfunction

Artemin (ART) signals through the GFR α—3/RET receptor complex to support sympathetic neuron development. Here we show that ART also influences autonomic elements in adrenal medulla and enteric and pelvic ganglia. Transgenic mice over-expressing Art throughout development exhibited systemic autonomic neural lesions including fusion of adrenal medullae with adjacent paraganglia, adrenal medullary dysplasia, and marked enlargement of sympathetic (superior cervical and sympathetic chain ganglia) and parasympathetic (enteric, pelvic) ganglia. Changes began by gestational day 12.5 and formed progressively larger masses during adulthood. Art supplementation in wild type adult mice by administering recombinant protein or an Art-bearing retroviral vector resulted in hyperplasia or neuronal metaplasia at the adrenal corticomedullary junction. Expression data revealed that Gfr α—3 is expressed during development in the adrenal medulla, sensory and autonomic ganglia and their projections, while Art is found in contiguous mesenchymal domains (especially skeleton) and in certain nerves. Intrathecal Art therapy did not reduce hypalgesia in rats following nerve ligation. These data (1) confirm that ART acts as a differentiation factor for autonomic (chiefly sympathoadrenal but also parasympathetic) neurons, (2) suggest a role for ART overexpression in the genesis of pheochromocytomas and paragangliomas, and (3) indicate that ART is not a suitable therapy for peripheral neuropathy.

Vascular Mural Cells Promote Noradrenergic Differentiation of Embryonic Sympathetic Neurons

The sympathetic nervous system controls smooth muscle tone and heart rate in the cardiovascular system. Postganglionic sympathetic neurons (SNs) develop in close proximity to the dorsal aorta (DA) and innervate visceral smooth muscle targets. Here, we use the zebrafish embryo to ask whether the DA is required for SN development. We show that noradrenergic (NA) differentiation of SN precursors temporally coincides with vascular mural cell (VMC) recruitment to the DA and vascular maturation. Blocking vascular maturation inhibits VMC recruitment and blocks NA differentiation of SN precursors. Inhibition of platelet-derived growth factor receptor (PDGFR) signaling prevents VMC differentiation and also blocks NA differentiation of SN precursors. NA differentiation is normal in cloche mutants that are devoid of endothelial cells but have VMCs. Thus, PDGFR-mediated mural cell recruitment mediates neurovascular interactions between the aorta and sympathetic precursors and promotes their noradrenergic differentiation.

Dendrite complexity of sympathetic neurons is controlled during postnatal development by BMP signaling

Dendrite development is controlled by the interplay of intrinsic and extrinsic signals affecting initiation, growth, and maintenance of complex dendrites. Bone morphogenetic proteins (BMPs) stimulate dendrite growth in cultures of sympathetic, cortical, and hippocampal neurons but it was unclear whether BMPs control dendrite morphology in vivo. Using a conditional knock-out strategy to eliminate Bmpr1a and Smad4 in immature noradrenergic sympathetic neurons we now show that dendrite length, complexity, and neuron cell body size are reduced in adult mice deficient of Bmpr1a. The combined deletion of Bmpr1a and Bmpr1b causes no further decrease in dendritic features. Sympathetic neurons devoid of Bmpr1a/1b display normal Smad1/5/8 phosphorylation, which suggests that Smad-independent signaling paths are involved in dendritic growth control downstream of BMPR1A/B. Indeed, in the Smad4 conditional knock-out dendrite and cell body size are not affected and dendrite complexity and number are increased. Together, these results demonstrate an in vivo function for BMPs in the generation of mature sympathetic neuron dendrites. BMPR1 signaling controls dendrite complexity postnatally during the major dendritic growth period of sympathetic neurons.

Genetics of human enteric neuropathies

Knowledge of molecular mechanisms that underlie development of the enteric nervous system has greatly expanded in recent decades. Enteric neuropathies related to aberrant genetic development are thus becoming increasingly recognized. There has been no recent review of these often highly morbid disorders. This review highlights advances in knowledge of the molecular pathogenesis of these disorders from a clinical perspective. It includes diseases characterized by an infantile aganglionic Hirschsprung phenotype and those in which structural abnormalities are less pronounced. The implications for diagnosis, screening and possible reparative approaches are presented.

An anteroposterior wave of vascular inhibitor downregulation signals aortae fusion along the embryonic midline axis

Paracrine signals, both positive and negative, regulate the positioning and remodeling of embryonic blood vessels. In the embryos of mammals and birds, the first major remodeling event is the fusion of bilateral dorsal aortae at the midline to form the dorsal aorta. Although the original bilaterality of the dorsal aortae occurs as the result of inhibitory factors (antagonists of BMP signaling) secreted from the midline by the notochord, it is unknown how fusion is later signaled. Here, we report that dorsal aortae fusion is tightly regulated by a change in signaling by the notochord along the anteroposterior axis. During aortae fusion, the notochord ceases to exert its negative influence on vessel formation. This is achieved by a transcriptional downregulation of negative regulators while positive regulators are maintained at pre-fusion levels. In particular, Chordin, the most abundant BMP antagonist expressed in the notochord prior to fusion, undergoes a dramatic downregulation in an anterior to posterior wave. With inhibitory signals diminished and sustained expression of the positive factors SHH and VEGF at the midline, fusion of the dorsal aortae is signaled. These results demonstrate a novel mechanism by which major modifications of the vascular pattern can occur through modulation of vascular inhibitors without changes in the levels of positive vascular regulators.

The migration of autonomic precursor cells in the embryo

The neural crest is an excellent model system to study cell fate and cell guidance signaling. Neural crest cells emerge from a common multipotent subpopulation and follow stereotypical migratory pathways to contribute to many diverse peripheral structures throughout the vertebrate embryo. The neural tube and diverse embryonic microenvironments from which the neural crest originate and migrate through are important sources of signals, yet it is still unclear how a common pool of neural crest stem and progenitor cells diversify and become distributed along specific stereotypical migratory paths. In the post-otic hindbrain and trunk, the neural crest emerge and contribute to the autonomic nervous system, and failure of proper cell navigation and differentiation often leads to congenital disorders that include dysautonomias, Hirschprung's disease, and neuroblastoma cancer. Recent exciting studies of neural crest cell behaviors have revealed the interplay of several molecular signaling pathways that guide and shape autonomic precursor cells to and into proper target structures, suggesting further work may help to better understand autonomic nervous system assembly, derived from a convergence of time-lapse imaging and molecular analyses. In this mini-review, we summarize recent fluorescent cell labeling strategies and cell behavior analyses that elucidate the role of molecular signals on the migration of autonomic precursor cells. We highlight advances in our understanding of the autonomic precursor cell behaviors and fate determination studied within the embryonic microenvironment.

Role of PACAP in the physiology and pathology of the sympathoadrenal system

Sympathetic neurons and chromaffin cells derive from common sympathoadrenal precursors which arise from the neural crest. Cells from this lineage migrate to their final destination and differentiate by acquiring a catecholaminergic phenotype in response to different environmental factors. It has been shown that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its PAC1 receptor are expressed at early stages of sympathetic development, and participate to the control of neuroblast proliferation and differentiation. PACAP also acts as a neurotransmitter to stimulate catecholamine and neuropeptide biosynthesis and release from sympathetic neurons and chromaffin cells, during development and in adulthood. In addition, PACAP and its receptors have been described in neuroblastoma and pheochromocytoma, and the neuropeptide regulates the differentiation and activity of sympathoadrenal-derived tumoral cell lines, suggestive of an important role in the pathophysiology of the sympathoadrenal lineage. Transcriptome studies uncovered genes and pathways of known and unknown roles that underlie the effects of PACAP in the sympathoadrenal system.

Neural crest-specific removal of Zfhx1b in mouse leads to a wide range of neurocristopathies reminiscent of Mowat-Wilson syndrome

Mowat-Wilson syndrome is a recently delineated autosomal dominant developmental anomaly, whereby heterozygous mutations in the ZFHX1B gene cause mental retardation, delayed motor development, epilepsy and a wide spectrum of clinically heterogeneous features, suggestive of neurocristopathies at the cephalic, cardiac and vagal levels. However, our understanding of the etiology of this condition at the cellular level remains vague. This study presents the Zfhx1b protein expression domain in mouse embryos and correlates this with a novel mouse model involving a conditional mutation in the Zfhx1b gene in neural crest precursor cells. These mutant mice display craniofacial and gastrointestinal malformations that show resemblance to those found in human patients with Mowat-Wilson syndrome. In addition to these clinically recognized alterations, we document developmental defects in the heart, melanoblasts and sympathetic and parasympathetic anlagen. The latter observations in our mouse model for Mowat-Wilson suggest a hitherto unknown role for Zfhx1b in the development of these particular neural crest derivatives, which is a set of observations that should be acknowledged in the clinical management of this genetic disorder.

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.

Reiterated Wnt and BMP signals in neural crest development

Common signaling pathways such as those for Wnts and BMPs are used many times during embryogenesis. During the development of the neural crest, Wnt and BMP signals are used repeatedly at different stages to influence initial induction, segregation from the neuroepithelium and cell fate determination. This review considers how specificity is generated within the neural crest for these reiterated signals, discussing examples of how the outcomes of signaling events are modulated by context.

BMPRIA is a promising marker for evaluating ganglion cells in the enteric nervous system--a pilot study

Congenital disorders of the enteric nervous system (ENS) comprise a large group of conditions characterized by abnormalities in the number, size, or location of enteric ganglia. Their diagnosis requires careful histological evaluation of intestinal biopsies to determine the presence and morphology of these cells. Based on the recently discovered role of bone morphogenetic proteins (BMPs) in ENS development, we examined the expression of the ligands, BMP2 and BMP4, and their receptors, BMPRIA, BMPRIB, and BMPRII, during formation of the human ENS. The spatiotemporal expression pattern of these proteins suggests a role for BMP signaling in human ENS formation. We find BMPRIA, in particular, strongly and specifically expressed in all ganglion cells of the ENS at every age examined, from fetus to adult. Moreover, BMPRIA immunohistochemistry consistently allowed the identification of ganglion cells in rectal biopsies from patients with Hirschsprung disease, intestinal neuronal dysplasia, and immature ganglion cells. We propose that BMPRIA immunohistochemistry may be a promising new tool for the identification of enteric ganglion cells in the evaluation of patients with neurointestinal disorders.

BMP signaling is necessary for neural crest cell migration and ganglion formation in the enteric nervous system

The enteric nervous system (ENS) is derived from neural crest cells that migrate along the gastrointestinal tract to form a network of neurons and glia that are essential for regulating intestinal motility. Despite the number of genes known to play essential roles in ENS development, the molecular etiology of congenital disorders affecting this process remains largely unknown. To determine the role of bone morphogenetic protein (BMP) signaling in ENS development, we first examined the expression of bmp2, bmp4, and bmprII during hindgut development and find these strongly expressed in the ENS. Moreover, functional BMP signaling, demonstrated by the expression of phosphorylated Smad1/5/8, is present in the enteric ganglia. Inhibition of BMP activity by noggin misexpression within the developing gut, both in ovo and in vitro, inhibits normal migration of enteric neural crest cells. BMP inhibition also leads to hypoganglionosis and failure of enteric ganglion formation, with crest cells unable to cluster into aggregates. Abnormalities of migration and ganglion formation are the hallmarks of two human intestinal disorders, Hirschsprung's disease and intestinal neuronal dysplasia. Our results support an essential role for BMP signaling in these aspects of ENS development and provide a basis for further investigation of these proteins in the etiology of neuro-intestinal disorders.

BMP4 and noggin control embryonic blood vessel formation by antagonistic regulation of VEGFR-2 (Quek1) expression

Regulation of VEGFR-2 (Quek1) is an important mechanism during blood vessel formation. In the paraxial mesoderm, Quek1 expression is restricted to the lateral portion of the somite and later to sclerotomal cells surrounding the neural tube. By grafting of either intermediate mesoderm or BMP4 beads into the paraxial mesoderm, we show that BMP4 is a positive regulator of VEGFR-2 (Quek1) expression in the quail embryo. Separation of somites from intermediate mesoderm leads to down-regulation of Quek1 expression. The expression of Quek1 in the medial somite half is normally repressed by the notochord and becomes up-regulated and lateromedially expanded after separation of the notochord. Our results show that up-regulation of BMP4 leads to an increase of the number of blood vessels, whereas inhibition of BMP4 by noggin results in a reduction of blood vessels.

Imaging neural crest cell dynamics during formation of dorsal root ganglia and sympathetic ganglia

The neural crest is a migratory population of cells that produces many diverse structures within the embryo. Trunk neural crest cells give rise to such structures as the dorsal root ganglia (DRG) and sympathetic ganglia (SG),which form in a metameric pattern along the anterior-posterior axis of the embryo. While static analyses have provided invaluable information concerning the development of these structures, time-lapse imaging of neural crest cells navigating through their normal environment could potentially reveal previously unidentified cellular and molecular interactions integral to DRG and SG development. In this study, we follow fluorescently labeled trunk neural crest cells using a novel sagittal explant and time-lapse confocal microscopy. We show that along their dorsoventral migratory route, trunk neural crest cells are highly motile and interact extensively with neighboring cells and the environment, with many cells migrating in chain-like formations. Surprisingly, the segregated pattern of crest cell streams through the rostral somite is not maintained once these cells arrive alongside the dorsal aorta. Instead, neural crest cells disperse along the ventral outer border of the somite, interacting extensively with each other and their environment via dynamic extension and retraction of filopodia. Discrete sympathetic ganglia arise as a consequence of intermixing and selective reorganization of neural crest cells at the target site. The diverse cell migratory behaviors and active reorganization at the target suggest that cell-cell and cell-environment interactions are coordinated with dynamic molecular processes.

Mechanisms and perspectives on differentiation of autonomic neurons

Neurons share many features in common but are distinguished by expression of phenotypic characteristics that define their specific function, location, or connectivity. One aspect of neuronal fate determination that has been extensively studied is that of neurotransmitter choice. The generation of diversity of neuronal subtypes within the developing nervous system involves integration of extrinsic and intrinsic instructive cues resulting in the expression of a core set of regulatory molecules. This review focuses on mechanisms of growth and transcription factor regulation in the generation of peripheral neural crest-derived neurons. Although the specification and differentiation of noradrenergic neurons are the focus, I have tried to integrate these into a larger picture providing a general roadmap for development of autonomic neurons. There is a core of DNA binding proteins required for the development of sympathetic, parasympathetic, and enteric neurons, including Phox2 and MASH1, whose specificity is regulated by the recruitment of additional transcriptional regulators in a subtype-specific manner. For noradrenergic neurons, the basic helix-loop-helix DNA binding protein HAND2 (dHAND) appears to serve this function. The studies reviewed here support the notion that neurotransmitter identity is closely linked to other aspects of neurogenesis and reveal a molecular mechanism to coordinate expression of pan-neuronal genes with cell type-specific genes.

The Booroola (FecB) mutation is associated with smaller adrenal glands in young adult ewes

The Booroola (FecB) phenotype is associated with a mutation in the bone morphogenetic protein (BMP) receptor 1B. The BMP action is important during development; surprisingly the only differences so far observed in adult animals are restricted to the ovaries where precocious development of the antral follicles and increased ovulation rate of mutant ewes is observed. The internal organs of 17 ewes homozygous for the mutation (BB) and 18 wild-type ewes (++) were macroscopically examined and weighed. No macroscopic differences were found, and the weight of the heart, liver, lungs, kidneys, and spleen were similar for both genotypes (P > 0.05). In contrast, the adrenals of BB ewes were lighter than those of ++ ewes (P < 0.05). The effect of the mutation on the adrenal function of cortisol secretion was measured at basal level and after an adrenocorticotrophic hormone challenge, before and after dexamethasone suppression. The Booroola mutation had no effect (P > 0.05) in any of these conditions. These findings indicate that the Booroola mutation also affects the size of the adrenal glands and suggest that the mutated gene could be important in the development of other organs in addition to the ovary. However, in the mutant ewes the function of the adrenal glands is not compromised or it is compensated.

Mechanisms of pattern formation in development and evolution

We present a classification of developmental mechanisms that have been shown experimentally to generate pattern and form in metazoan organisms. We propose that all such mechanisms can be organized into three basic categories and that two of these may act as composite mechanisms in two different ways. The simple categories are cell autonomous mechanisms in which cells enter into specific arrangements ('patterns') without interacting, inductive mechanisms in which cell communication leads to changes in pattern by reciprocal or hierarchical alteration of cell phenotypes ('states') and morphogenetic mechanisms in which pattern changes by means of cell interactions that do not change cell states. The latter two types of mechanism can be combined either morphostatically, in which case inductive mechanisms act first, followed by the morphogenetic mechanism, or morphodynamically, in which case both types of mechanisms interact continuously to modify each other's dynamics. We propose that this previously unexplored distinction in the operation of composite developmental mechanisms provides insight into the dynamics of many developmental processes. In particular, morphostatic and morphodynamic mechanisms respond to small changes in their genetic and microenvironmental components in dramatically different ways. We suggest that these differences in 'variational properties' lead to morphostatic and morphodynamic mechanisms being represented to different extents in early and late stages of development and to their contributing in distinct ways to morphological transitions in evolution.

Activin and bone morphogenetic proteins are present in perinatal sensory neuron target tissues that induce neuropeptides

Previous studies have shown that sensory target tissues induce neuropeptides in naïve sensory neurons, and that activin and bone morphogenetic proteins (BMPs) are capable of inducing neuropeptides associated with nociception in embryonic sensory neurons in vitro. The goal of the present study was to learn if these ligands were available in native sensory neuron target tissues at correct developmental periods to play this inductive role in vivo. Sensory neurons initially contact their peripheral target tissues and begin to express neuropeptides during late embryogenesis, and we demonstrate that activin and BMPs are present in the embryo and neonate to regulate sensory neuron differentiation. Native embryonic and neonatal target tissues were analyzed by immunoblot and immunohistochemical studies using ligand-specific antibodies. Although activin was easily solubilized, BMPs were detected only after high salt extraction, suggesting that BMPs were bound to extracellular moieties and were capable of acting only locally in native tissues. One inhibitor, noggin, was present in both embryonic skin and muscle. In combination, these data suggest that neuronal differentiation is unlikely to be regulated by simple expression of ligand, but that the functional availability of ligand is a critical component confering biological activity.

BMP growth factors and Phox2 transcription factors can induce synaptotagmin I and neurexin I during sympathetic neuron development

Synaptotagmin I and neurexin I mRNAs, coding for proteins involved in neurotransmitter secretion, become detectable in primary sympathetic ganglia shortly after initial induction of the noradrenergic transmitter phenotype. To test whether the induction of these more general neuronal genes is mediated by signals known to initiate noradrenergic differentiation in a neuronal subpopulation, we examined their expression in noradrenergic neurons induced by ectopic overexpression of growth and transcription factors. Overexpression of BMP4 or Phox2a in vivo results in synaptotagmin I and neurexin I expression in ectopically located noradrenergic cells. In vitro, BMP4 initiates synaptotagmin I and neurexin I expression in addition to tyrosine hydroxylase induction. Thus, the induction of synaptotagmin I and neurexin I, which are expressed in a large number of different neuron populations, can be accomplished by growth and transcription factors available only to a subset of neurons. These findings suggest that the initial expression of proteins involved in neurotransmitter secretion is regulated by different signals in different neuron populations.

Bone morphogenetic protein-5 (BMP-5) promotes dendritic growth in cultured sympathetic neurons

BACKGROUND

BMP-5 is expressed in the nervous system throughout development and into adulthood. However its effects on neural tissues are not well defined. BMP-5 is a member of the 60A subgroup of BMPs, other members of which have been shown to stimulate dendritic growth in central and peripheral neurons. We therefore examined the possibility that BMP-5 similarly enhances dendritic growth in cultured sympathetic neurons.

RESULTS

Sympathetic neurons cultured in the absence of serum or glial cells do not form dendrites; however, addition of BMP-5 causes these neurons to extend multiple dendritic processes, which is preceded by an increase in phosphorylation of the Smad-1 transcription factor. The dendrite-promoting activity of BMP-5 is significantly inhibited by the BMP antagonists noggin and follistatin and by a BMPR-IA-Fc chimeric protein. RT-PCR and immunocytochemical analyses indicate that BMP-5 mRNA and protein are expressed in the superior cervical ganglia (SCG) during times of initial growth and rapid expansion of the dendritic arbor.

CONCLUSIONS

These data suggest a role for BMP-5 in regulating dendritic growth in sympathetic neurons. The signaling pathway that mediates the dendrite-promoting activity of BMP-5 may involve binding to BMPR-IA and activation of Smad-1, and relative levels of BMP antagonists such as noggin and follistatin may modulate BMP-5 signaling. Since BMP-5 is expressed at relatively high levels not only in the developing but also the adult nervous system, these findings suggest the possibility that BMP-5 regulates dendritic morphology not only in the developing, but also the adult nervous system.

Evidence for an evolutionary conserved role of bone morphogenetic protein growth factors and phox2 transcription factors during noradrenergic differentiation of sympathetic neurons. Induction of a putative synexpression group of neurotransmitter-synthesizing enzymes

The noradrenergic transmitter phenotype in postganglionic sympathetic neurons is induced early during embryonic development in avian and mammalian primary sympathetic ganglia. The simultaneous expression of tyrosine hydroxylase and dopamine beta-hydroxylase, enzymes of the noradrenaline biosynthesis pathway, indicates that different genes contributing to the noradrenergic transmitter phenotype are regulated as a synexpression group. This conclusion is supported by the demonstration of bone morphogenetic protein (BMP) growth factors and Phox2 transcription factors being necessary for the expression of both tyrosine hydroxylase and dopamine beta-hydroxylase in differentiating sympathetic neurons. The close similarity in the expression patterns of the relevant genes as well as in the function of BMPs and Phox2s between avian and mammalian embryos strongly suggests that noradrenergic induction occurs along a conserved signalling pathway in these vertebrate classes.

Postmigratory enteric and sympathetic neural precursors share common, developmentally regulated, responses to BMP2

The development of enteric and sympathetic neurons from neural crest precursor cells is regulated by signals produced by the embryonic environments to which the cells migrate. Bone morphogenetic proteins (BMPs) are present in the developing embryo and act to induce neuronal differentiation and noradrenergic properties of neural crest cells. We have investigated the role of BMP2 in regulating the appearance of distinct populations of autonomic neurons from postmigratory, HNK-1-positive neural crest precursor cells. BMP2 promotes neuronal differentiation of sympathetic and enteric precursor cells isolated from E14.5 rat. The effects of BMP2 change over time, resulting in a decrease in neuron number that can be attributed to apoptotic cell death. BMP2-dependent neuron death is rescued by gut-derived factors that provide trophic support to maturing neurons, indicating that BMP2 regulates the acquisition of trophic dependence of developing peripheral neurons. In addition to regulating neuron number, BMP2 promotes both panneuronal maturation and the acquisition of an enteric phenotype, as measured by lineage-specific changes in the expression of tyrosine hydroxylase and MASH-1. While BMP2 is sufficient to induce neuronal differentiation and panneuronal development, these results suggest that additional factors in the environment must collaborate with BMP2 to promote the final noradrenergic phenotype of sympathetic neurons.