Control of noradrenergic differentiation and Phox2a expression by MASH1 in the central and peripheral nervous system

Differential regulation of distinct phenotypic features of serotonergic neurons by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), growth and differentiation factor 5 (GDF5) and glial cell line-derived neurotrophic factor (GDNF) are members of the transforming growth factor-beta superfamily that have been implicated in tissue growth and differentiation. Several BMPs are expressed in embryonic and adult brain. We show now that BMP-2, -6 and -7 and GDF5 are expressed in the embryonic rat hindbrain raphe. To start to define roles for BMPs in the regulation of serotonergic (5-HT) neuron development, we have generated serum-free cultures of 5-HT neurons isolated from the embryonic (E14) rat raphe. Addition of saturating concentrations (10 ng/mL) of BMP-6 and GDF5 augmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons and cells specifically taking up 5, 7-dihydroxytryptamine (5,7-DHT) by about two-fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of short-term treatments. Importantly, BMP-7 selectively induced 5, 7-DHT uptake without affecting TpOH immunoreactivity. BMP-6 and -7 also promoted DNA synthesis and increased numbers of cells immunoreactive for vimentin and glial fibrillary acidic protein (GFAP). Pharmacological suppression of cell proliferation or glial development abolished the induction of serotonergic markers by BMP-6 and -7, suggesting that BMPs act indirectly by stimulating synthesis or release of glial-derived serotonergic differentiation factors. Receptor bodies for the neurotrophin receptor trkB, but not trkC, abolished the BMP-mediated effects on serotonergic development, suggesting that the glia-derived factor is probably brain-derived neurotrophic factor (BDNF) or neurotrophin-4. In support of this notion, we detected increased levels of BDNF mRNA in BMP-treated cultures. Together, these data suggest both distinct and overlapping roles of several BMPs in regulating 5-HT neuron development.

Restriction of developmental potential during divergence of the enteric and sympathetic neuronal lineages

In the peripheral nervous system, enteric and sympathetic neurons develop from multipotent neural crest cells. While local environmental signals in the gut and in the region of the sympathetic ganglia play a role in the choice of cell fate, little is known about the mechanisms that underlie restriction to specific neuronal phenotypes. We investigated the divergence and restriction of the enteric and sympathetic neuronal lineages using immuno-isolated neural crest-derived cells from the gut and sympathetic ganglia. Analysis of neuronal and lineage-specific mRNAs and proteins indicated that neural crest-derived cells from the gut and sympathetic ganglia had initiated neuronal differentiation and phenotypic divergence by E14.5 in the rat. We investigated the developmental potential of these cells using expression of tyrosine hydroxylase as a marker for a sympathetic phenotype. Tyrosine hydroxylase expression was examined in neurons that developed from sympathetic and enteric neuroblasts under the following culture conditions: culture alone; coculture with gut monolayers to promote enteric differentiation; or coculture with dorsal aorta monolayers to promote noradrenergic differentiation. Both enteric and sympathetic neuroblasts displayed developmental plasticity at E14.5. Sympathetic neuroblasts downregulated tyrosine hydroxylase in response to signals from the gut environment and enteric neuroblasts increased expression of tyrosine hydroxylase when grown on dorsal aorta or in the absence of other cell types. Tracking of individual sympathetic cells displaying a neuronal morphology at the time of plating indicated that neuroblasts retained phenotypic plasticity even after initial neuronal differentiation had occurred. By E19.5 both enteric and sympathetic neuroblasts had undergone a significant loss of their developmental potential, with most neuroblasts retaining their lineage-specific phenotype in all environments tested. Together our data indicate that the developmental potential of enteric and sympathetic neuroblasts becomes restricted over time and that this restriction takes place not as a consequence of initial neuronal differentiation but during the period of neuronal maturation. Further, we have characterized a default pathway of adrenergic differentiation in the enteric nervous system and have defined a transient requirement for gut-derived factors in the maintenance of the enteric neuronal phenotype.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

The homeobox gene Phox2b is essential for the development of autonomic neural crest derivatives

The sympathetic, parasympathetic and enteric ganglia are the main components of the peripheral autonomic nervous system, and are all derived from the neural crest. The factors needed for these structures to develop include the transcription factor Mash1, the glial-derived neurotrophic factor GNDF and its receptor subunits, and the neuregulin signalling system, each of which is essential for the differentiation and survival of subsets of autonomic neurons. Here we show that all autonomic ganglia fail to form properly and degenerate in mice lacking the homeodomain transcription factor Phox2b, as do the three cranial sensory ganglia that are part of the autonomic reflex circuits. In the anlagen of the enteric nervous system and the sympathetic ganglia, Phox2b is needed for the expression of the GDNF-receptor subunit Ret and for maintaining Mash1 expression. Mutant ganglionic anlagen also fail to switch on the genes that encode two enzymes needed for the biosynthesis of the neurotransmitter noradrenaline, dopamine-beta-hydroxylase and tyrosine hydroxylase, demonstrating that Phox2b regulates the noradrenergic phenotype in vertebrates.

Innervation of the esophagus in mice that lack MASH1

The striated muscle of the esophagus differs from other striated muscle, because it develops by the transdifferentiation of smooth muscle, and the motor end plates receive a dual innervation from vagal (cholinergic) motor neurons and nitric oxide synthase (NOS)-containing enteric neurons. Mash1-/- mice have no enteric neurons in their esophagus and die within 48 hours of birth without milk in their stomachs (Guillemot et al. [1993] Cell 75:463-476). In this study, the innervation of the esophagus of newborn Mash1-/-, Mash1+/- and wild type mice was examined. There was no difference between Mash1-/-, Mash1+/-, and wild type mice in the transdifferentiation of the muscle and the development of nicotinic receptor clusters. However, there were significantly more cholinergic nerve terminals per motor end plate in Mash1-/- mice than Mash1+/- or wild type mice. Each of the Mash1-/- mice had fewer than 50 NOS neurons per esophagus, compared with approximately 3,000 in wild type mice. Newborn Mash1+/- mice also contained significantly fewer NOS neurons than wild type mice. In Mash1-/- mice, NOS nerve fibers were virtually absent from the external muscle but were present at the myenteric plexus. Unlike that of newborn wild type mice, the lower esophageal sphincter of Mash 1-/- mice lacked NOS nerve fibers; this may explain the absence of milk in the stomach. We conclude that 1) the transdifferentiation of the esophageal muscle and the development of the extrinsic innervation do not require enteric neurons or MASH1, 2) extrinsic NOS neurons only innervate the myenteric plexus.

Regulation of hippocampal neuronal differentiation by the basic helix-loop-helix transcription factors HES-1 and MASH-1

HES-1 is a vertebrate homologue of the Drosophila basic helix-loop-helix (bHLH) protein Hairy, a transcriptional repressor that negatively regulates neuronal differentiation. HES-1 expression in neuronal precursors precedes and represses the expression of the neuronal commitment gene MASH-1, a bHLH activator homologous to the proneural Achaete-Scute genes in Drosophila. Down-regulation of HES-1 expression in developing neuroblasts may be necessary for the induction of a regulatory cascade of bHLH activator proteins that controls the commitment and progression of neuronal differentiation. Here we show that the differentiation of embryonic day-17 rat hippocampal neurons in culture was coincident with a decline in HES-1 expression and DNA binding. Therefore, we examined the effect of forced expression of HES-1 and MASH-1 upon nerve growth factor (NGF) -induced differentiation in TrkA transfected hippocampal neurons. Expression of HES-1 inhibited both the intrinsic and NGF-induced neurite outgrowth, whereas MASH-1 expression increased neurite outgrowth. Strikingly, the increased hippocampal differentiation observed with MASH-1 expression is completely blocked by coexpression of HES-1. Furthermore, both wild-type HES-1 and a non-DNA binding mutant of HES-1 repressed MASH-1-dependent transcription activation. These results suggest that down-regulation of HES-1 is necessary for autonomous, growth factor-induced and MASH-1-activated hippocampal differentiation.

Defects in thalamocortical axon pathfinding correlate with altered cell domains in Mash-1-deficient mice

We have analyzed the pathfinding of thalamocortical axons (TCAs) from dorsal thalamus to neocortex in relation to specific cell domains in the forebrain of wild-type and Mash-1-deficient mice. In wild-type mice, we identified four cell domains that constitute the proximal part of the TCA pathway. These domains are distinguished by patterns of gene expression and by the presence of neurons retrogradely labeled from dorsal thalamus. Since the cells that form these domains are generated in forebrain proliferative zones that express high levels of Mash-1, we studied Mash-1 mutant mice to assess the potential roles of these domains in TCA pathfinding. In null mutants, each of the domains is altered: the two Pax-6 domains, one in ventral thalamus and one in hypothalamus, are expanded in size; a complementary RPTP(delta) domain in ventral thalamus is correspondingly reduced and the normally graded expression of RPTP(delta) in that domain is no longer apparent. In ventral telencephalon, a domain characterized in the wild type by Netrin-1 and Nkx-2.1 expression and by retrogradely labeled neurons is absent in the mutant. Defects in TCA pathfinding are localized to the borders of each of these altered domains. Many TCAs fail to enter the expanded, ventral thalamic Pax-6 domain that constitutes the most proximal part of the TCA pathway, and form a dense whorl at the border between dorsal and ventral thalamus. A proportion of TCAs do extend further distally into ventral thalamus, but many of these stall at an aberrant, abrupt border of high RPTP(delta) expression. A small proportion of TCAs extend around the RPTP(delta) domain and reach the ventral thalamic-hypothalamic border, but few of these axons turn at that border to extend into the ventral telencephalon. These findings demonstrate that Mash-1 is required for the normal development of cell domains that in turn are required for normal TCA pathfinding. In addition, these findings support the hypothesis that ventral telencephalic neurons and their axons guide TCAs through ventral thalamus and into ventral telencephalon.

Early specification of sensory neuron fate revealed by expression and function of neurogenins in the chick embryo

The generation of sensory and autonomic neurons from the neural crest requires the functions of two classes of basic helix-loop-helix (bHLH) transcription factors, the Neurogenins (NGNs) and MASH-1, respectively (Fode, C., Gradwohl, G., Morin, X., Dierich, A., LeMeur, M., Goridis, C. and Guillemot, F. (1998) Neuron 20, 483–494; Guillemot, F., Lo, L.-C., Johnson, J. E., Auerbach, A., Anderson, D. J. and Joyner, A. L. (1993) Cell 75, 463–476; Ma, Q., Chen, Z. F., Barrantes, I. B., de la Pompa, J. L. and Anderson, D. J. (1998 Neuron 20, 469–482). We have cloned two chick NGNs and found that they are expressed in a subset of neural crest cells early in their migration. Ectopic expression of the NGNs in vivo biases migrating neural crest cells to localize in the sensory ganglia, and induces the expression of sensory neuron-appropriate markers in non-sensory crest derivatives. Surprisingly, the NGNs can also induce the expression of multiple pan-neuronal and sensory-specific markers in the dermomyotome, a mesodermal derivative. Taken together, these data suggest that a subset of neural crest cells may already be specified for a sensory neuron fate early in migration, as a consequence of NGN expression.

Cellular and molecular determinants of sympathetic neuron development

The development of the sympathetic nervous system can be divided into three overlapping stages. First, the precursors of sympathetic neurons arise from undifferentiated neural crest cells that migrate ventrally, aggregate adjacent to the dorsal aorta, and ultimately differentiate into catecholaminergic neurons. Second, cell number is refined during a period of cell death when neurotrophic factors determine the number of neuronal precursors and neurons that survive. The final stage of sympathetic development is the establishment and maturation of synaptic connections, which for sympathetic neurons can include alterations in neurotransmitter phenotype. Considerable progress has been made recently in elucidating the cellular and molecular mechanisms that direct each of these developmental decisions. We review the current understanding of each of these, focusing primarily on events in the peripheral nervous system of rodents.

Deconstructing cell determination: proneural genes and neuronal identity

Vertebrates express scores of bHLH proteins during neural development. Earlier studies inspired by the established role of "proneural" genes in fly neurogenesis, as well as by the vertebrate bHLH myogenic program, focused on the reconstruction of bHLH gene cascades, which are thought to control successive steps leading to neuronal differentiation. Little attention has been paid thus far to the relationship between the diversity of neural bHLH genes and the diversity of neuronal phenotypes. This article reviews recent evidence that, akin to their fly counterparts, vertebrate neural bHLH genes probably confer not only "generic" neuronal properties, but also neuronal type-specific properties, inextricably linking neural determination and the specification of neuronal identity. We also speculate on the relations between positional information and gene activity, and on the evolutionary significance of the diversity of bHLH genes.

Hoxa2 and Hoxb2 control dorsoventral patterns of neuronal development in the rostral hindbrain

Little is known about how the generation of specific neuronal types at stereotypic positions within the hindbrain is linked to Hox gene-mediated patterning. Here, we show that during neurogenesis, Hox paralog group 2 genes control both anteroposterior (A-P) and dorsoventral (D-V) patterning. Hoxa2 and Hoxb2 differentially regulate, in a rhombomere-specific manner, the expression of several genes in broad D-V-restricted domains or narrower longitudinal columns of neuronal progenitors, immature neurons, and differentiating neuronal subtypes. Moreover, Hoxa2 and Hoxb2 can functionally synergize in controlling the development of ventral neuronal subtypes in rhombomere 3 (r3). Thus, in addition to their roles in A-P patterning, Hoxa2 and Hoxb2 have distinct and restricted functions along the D-V axis during neurogenesis, providing insights into how neuronal fates are assigned at stereotypic positions within the hindbrain.

Specification of neurotransmitter identity by Phox2 proteins in neural crest stem cells

We have investigated the specification of noradrenergic neurotransmitter identity in neural crest stem cells (NCSCs). Retroviral expression of both wild-type and dominant-negative forms of the paired homeodomain transcription factor Phox2a indicates a crucial and direct role for this protein (and/or the closely related Phox2b) in the regulation of endogenous tyrosine hydroxylase (TH) and dopamine-beta hydroxylase (DBH) gene expression in these cells. In collaboration with cAMP, Phox2a can induce expression of TH but not of DBH or of panneuronal genes. Phox2 proteins are, moreover, necessary for the induction of both TH and DBH by bone morphogenetic protein 2 (BMP2) (which induces Phox2a/b) and forskolin. They are also necessary for neuronal differentiation. These data suggest that Phox2a/b coordinates the specification of neurotransmitter identity and neuronal fate by cooperating environmental signals in sympathetic neuroblasts.

Transcription factors Mash-1 and Prox-1 delineate early steps in differentiation of neural stem cells in the developing central nervous system

Like other tissues and organs in vertebrates, multipotential stem cells serve as the origin of diverse cell types during genesis of the mammalian central nervous system (CNS). During early development, stem cells self-renew and increase their total cell numbers without overt differentiation. At later stages, the cells withdraw from this self-renewal mode, and are fated to differentiate into neurons and glia in a spatially and temporally regulated manner. However, the molecular mechanisms underlying this important step in cell differentiation remain poorly understood. In this study, we present evidence that the expression and function of the neural-specific transcription factors Mash-1 and Prox-1 are involved in this process. In vivo, Mash-1- and Prox-1-expressing cells were defined as a transient proliferating population that was molecularly distinct from self-renewing stem cells. By taking advantage of in vitro culture systems, we showed that induction of Mash-1 and Prox-1 coincided with an initial step of differentiation of stem cells. Furthermore, forced expression of Mash-1 led to the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells, and up-regulation of Prox-1, suggesting that Mash-1 positively regulates cell differentiation. In support of these observations in vitro, we found specific defects in cellular differentiation and loss of expression of Prox-1 in the developing brain of Mash-1 mutant mice in vivo. Thus, we propose that induction of Mash-1 and Prox-1 is one of the critical molecular events that control early development of the CNS.

Mash1 regulates neurogenesis in the ventral telencephalon

Previous studies have shown that mice mutant for the gene Mash1 display severe neuronal losses in the olfactory epithelium and ganglia of the autonomic nervous system, demonstrating a role for Mash1 in development of neuronal lineages in the peripheral nervous system. Here, we have begun to analyse Mash1 function in the central nervous system, focusing our studies on the ventral telencephalon where it is expressed at high levels during neurogenesis. Mash1 mutant mice present a severe loss of progenitors, particularly of neuronal precursors in the subventricular zone of the medial ganglionic eminence. Discrete neuronal populations of the basal ganglia and cerebral cortex are subsequently missing. An analysis of candidate effectors of Mash1 function revealed that the Notch ligands Dll1 and Dll3, and the target of Notch signaling Hes5, fail to be expressed in Mash1 mutant ventral telencephalon. In the lateral ganglionic eminence, loss of Notch signaling activity correlates with premature expression of a number of subventricular zone markers by ventricular zone cells. Therefore, Mash1 is an important regulator of neurogenesis in the ventral telencephalon, where it is required both to specify neuronal precursors and to control the timing of their production.

Transcriptional control of neurotransmitter phenotype

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Recent studies have begun to define essential transcriptional regulators involved in controlling the mode of neurotransmission in vertebrates and invertebrates, and to examine their regulation by cell-extrinsic factors. An emerging concept is that the control of transmitter choice is intimately linked to that of other aspects of the neuronal phenotype.

A single rostrocaudal colonization of the rodent intestine by enteric neuron precursors is revealed by the expression of Phox2b, Ret, and p75 and by explants grown under the kidney capsule or in organ culture

The colonization of the rodent gastrointestinal tract by enteric neuron precursors is controversial due to the lack of specific cellular markers at early stages. The transcription factor, Phox2b, is expressed by enteric neuron precursors (Pattyn et al. Development 124, 4065-4075, 1997). In this study, we have used an antiserum to Phox2b to characterize in detail the spatiotemporal expression of Phox2b in the gastrointestinal tract of adult mice and embryonic mice and rats. In adult mice, all enteric neurons (labeled with neuron-specific enolase antibodies), and a subpopulation of glial cells (labeled with GFAP antibodies), showed immunoreactivity to Phox2b. In embryonic mice, the appearance of Phox2b-immunoreactive cells was mapped during development of the gastrointestinal tract. At Embryonic Days 9.5-10 (E9.5-10), Phox2b-labeled cells were present only in the stomach, and during subsequent development, labeled cells appeared as a single rostrocaudal wave along the gastrointestinal tract; at E14 Phox2b-labeled cells were present along the entire length of the gastrointestinal tract. Ret and p75 have also been reported to label migratory-stage enteric neuron precursors. A unidirectional, rostral-to-caudal colonization of the gastrointestinal tract of embryonic mice by Ret- and p75-immunoreactive cells was also observed, and the locations of Ret- and p75-positive cells within the gut were very similar to that of Phox2b-positive cells. To verify the location of enteric neuron precursors within the gut, explants from spatiotemporally defined regions of embryonic intestine, 0.3-3 mm long, were grown in the kidney subcapsular space, or in catenary organ culture, and examined for the presence of neurons. The location and sequence of appearance of enteric neuron precursors deduced from the explants grown under the kidney capsule or in organ culture was very similar to that seen with the Phox2b, Ret, and p75 antisera. Previous studies have mapped the rostrocaudal colonization of the rat intestine by enteric neuron precursors using HNK-1 as a marker. In the current study, all HNK-1-labeled cells in the gastrointestinal tract of rat embryos showed immunoreactivity to Phox2b, but HNK-1 cells comprised only a small subpopulation of the Phox2b-labeled cells. In addition, in rats, Phox2b-labeled cells were present in advance of (more caudal to) the most caudal HNK-1-labeled cells by 600-700 microm in the hindgut at E15. We conclude that the neural crest cell population that arises from the vagal level of the neural axis and that populates the stomach, midgut, and hindgut expresses Phox2b, Ret, and p75. In contrast, the sacral-level neural crest cells that populate the hindgut either do not express, or show a delayed expression of, all of the known markers of vagal- and trunk-level neural crest cells.

MASH1 activates expression of the paired homeodomain transcription factor Phox2a, and couples pan-neuronal and subtype-specific components of autonomic neuronal identity

We have investigated the genetic circuitry underlying the determination of neuronal identity, using mammalian peripheral autonomic neurons as a model system. Previously, we showed that treatment of neural crest stem cells (NCSCs) with bone morphogenetic protein-2 (BMP-2) leads to an induction of MASH1 expression and consequent autonomic neuronal differentiation. We now show that BMP2 also induces expression of the paired homeodomain transcription factor Phox2a, and the GDNF/NTN signalling receptor tyrosine kinase c-RET. Constitutive expression of MASH1 in NCSCs from a retroviral vector, in the absence of exogenous BMP2, induces expression of both Phox2a and c-RET in a large fraction of infected colonies, and also promotes morphological neuronal differentiation and expression of pan-neuronal markers. In vivo, expression of Phox2a in autonomic ganglia is strongly reduced in Mash1 −/− embryos. These loss- and gain-of-function data suggest that MASH1 positively regulates expression of Phox2a, either directly or indirectly. Constitutive expression of Phox2a, by contrast to MASH1, fails to induce expression of neuronal markers or a neuronal morphology, but does induce expression of c-RET. These data suggest that MASH1 couples expression of pan-neuronal and subtype-specific components of autonomic neuronal identity, and support the general idea that identity is established by combining subprograms involving cascades of transcription factors, which specify distinct components of neuronal phenotype.