[show abstract][hide abstract] ABSTRACT: The wiring of the nervous system arises from extensive directional migration of neuronal cell bodies and growth of processes that, somehow, end up forming functional circuits. Thus far, this feat of biological engineering appears to rely on sequences of pathfinding decisions upon local cues, each with little relationship to the anatomical and physiological outcome. Here, we uncover a straightforward cellular mechanism for circuit building whereby a neuronal type directs the development of its future partners. We show that visceral afferents of the head (that innervate taste buds) provide a scaffold for the establishment of visceral efferents (that innervate salivatory glands and blood vessels). In embryological terms, sensory neurons derived from an epibranchial placode-that we show to develop largely independently from the neural crest-guide the directional outgrowth of hindbrain visceral motoneurons and control the formation of neural crest-derived parasympathetic ganglia.
Proceedings of the National Academy of Sciences 02/2010; 107(5):2066-71. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: In Hirschsprung's disease (HSCR), a hypomorphic allele of a major gene, RET, accounts for most isolated (non-syndromic) cases, along with other autosomal susceptibility loci under a multiplicative model. However, some syndromic forms of HSCR are monogenic entities, for which the disease causing gene is known.
To determine whether RET could be considered a modifier gene for the enteric phenotype on the background of a monogenic trait.
The syndromic HSCR entities studied were congenital central hypoventilation (CCHS) and Mowat-Wilson syndrome (MWS), caused by PHOX2B and ZFHX1B gene mutations, respectively. The RET locus was genotyped in 143 CCHS patients, among whom 44 had HSCR, and in 30 MWS patients, among whom 20 had HSCR. The distribution of alleles, genotypes, and haplotypes was compared within the different groups. To test the interaction in vivo, heterozygous mice were bred for a null allele of Phox2b and Ret genes.
RET was shown to act as a modifier gene for the HSCR phenotype in patients with CCHS but not with MWS. The intestine of double heterozygote mice was indistinguishable from their littermates. A loss of over 50% of each gene function seemed necessary in the mouse model for an enteric phenotype to occur.
In CCHS patients, the weak predisposing haplotype of the RET gene can be regarded as a quantitative trait, being a risk factor for the HSCR phenotype, while in MWS, for which the HSCR penetrance is high, the role of the RET predisposing haplotype is not significant. It seems likely that there are both RET dependent and RET independent HSCR cases.
Journal of Medical Genetics 06/2006; 43(5):419-23. · 5.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: The inner ear (vestibular and cochlear) efferent neurons are a group of atypical motor-like hindbrain neurons which innervate inner ear hair cells and their sensory afferents. They are born in the fourth rhombomere, in close association with facial branchial motor neurons, from which they subsequently part through a specific migration route. Here, we demonstrate that the inner ear efferents depend on Phox2b for their differentiation, behaving in that respect like hindbrain visceral and branchial motor neurons. We also show that the vestibular efferent nucleus is no longer present at its usual site in mice inactivated for the bHLH transcription factor Mash 1. The concomitant appearance of an ectopic branchial-like nucleus at the location where both inner ear efferents and facial branchial motor neurons are born suggests that Mash1 is required for the migration of a subpopulation of rhombomere 4-derived efferents.
[show abstract][hide abstract] ABSTRACT: In the vertebrate neural tube, cell cycle exit of neuronal progenitors is accompanied by the expression of transcription factors that define their generic and sub-type specific properties, but how the regulation of cell cycle withdrawal intersects with that of cell fate determination is poorly understood. Here we show by both loss- and gain-of-function experiments that the neuronal-subtype-specific homeodomain transcription factor Phox2b drives progenitor cells to become post-mitotic. In the absence of Phox2b, post-mitotic neuronal precursors are not generated in proper numbers. Conversely, forced expression of Phox2b in the embryonic chick spinal cord drives ventricular zone progenitors to become post-mitotic neurons and to relocate to the mantle layer. In the neurons thus generated, ectopic expression of Phox2b is sufficient to initiate a programme of motor neuronal differentiation characterised by expression of Islet1 and of the cholinergic transmitter phenotype, in line with our previous results showing that Phox2b is an essential determinant of cranial motor neurons. These results suggest that Phox2b coordinates quantitative and qualitative aspects of neurogenesis, thus ensuring that neurons of the correct phenotype are generated in proper numbers at the appropriate times and locations.
Development 01/2001; 127(23):5191-201. · 6.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: An essential aspect of a neuron's identity is the pattern of its axonal projections. In C. elegans, axons extend either longitudinally or circumferentially in response to distinct molecular cues, some of which have been identified. It is currently unclear, however, how the differential capacity to respond to these cues is transcriptionally implemented in distinct neuronal subtypes. Here, we characterise a C. elegans paired-like homeobox gene, CePhox2/ceh-17, expressed in five head neurons, ALA and the 4 SIAs, all of which project axons towards the tail along the lateral and sublateral cords. Abrogation of ceh-17 function, while leaving intact many phenotypic traits of these neurons, disrupts their antero-posterior axonal elongation beyond the mid-body region. Conversely, ectopic expression of ceh-17 in the mechanoreceptors, several of which are known to pioneer their tract, leads to exaggerated longitudinal axonal outgrowth. Thus, ceh-17 is a novel gene involved in fasciculation-independent longitudinal axonal navigation.
Development 09/2000; 127(15):3361-71. · 6.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Axon navigation depends, in part, on guidance cues emanating from the target. We have investigated the possible role of the target in the pathfinding of visceral motor axons to cranial parasympathetic ganglia. Mice homozygous for a tau-LacZ transgene targeted in the Phox2a locus lack the sphenopalatine ganglion, which is the normal target of visceral motor axons of the facial nerve. We found that in these mutants, facial visceral motor axon pathfinding was disrupted, and some axons were misrouted to an alternative parasympathetic ganglion. Moreover, the absence of correct facial visceral motor pathways was concomitant with defects in the pathfinding of rostrally-projecting sympathetic axons.
Molecular and Cellular Neuroscience 08/2000; 16(1):14-26. · 3.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: Motor neurons are a widely studied model of vertebrate neurogenesis. They can be subdivided in somatic, branchial and visceral motor neurons. Recent studies on the dorsoventral patterning of the rhombencephalon have implicated the homeobox genes Pax6 and Nkx2.2 in the early divergence of the transcriptional programme of hindbrain somatic and visceral motor neuronal differentiation. We provide genetic evidence that the paired-like homeodomain protein Phox2b is required for the formation of all branchial and visceral, but not somatic, motor neurons in the hindbrain. In mice lacking Phox2b, both the generic and subtype-specific programs of motoneuronal differentiation are disrupted at an early stage. Most motor neuron precursors die inside the neuroepithelium while those that emigrate to the mantle layer fail to switch on early postmitotic markers and to downregulate neuroepithelial markers. Thus, the loss of function of Phox2b in hindbrain motor neurons exemplifies a novel control point in the generation of CNS neurons.
Development 05/2000; 127(7):1349-58. · 6.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The closely related homeobox genes Phox2a and Phox2b are expressed in all central and peripheral noradrenergic neurons. Our previous results have shown that Phox2a controls the differentiation of the main noradrenergic center of the brain, the locus coeruleus, but leaves unaffected the other noradrenergic centers. Here, we report that Phox2b has a wider and overlapping role, in that it is required for the differentiation of all noradrenergic centers in the brain, including the locus coeruleus. Together with the previously reported lack of dopamine-b-hydroxylase and tyrosine hydroxylase expression in the peripheral nervous system of Phox2b knock-out embryos, our present findings make Phox2b a master regulator of all central and peripheral noradrenergic differentiation. We discuss the nonredundancy of Phox2 genes and their complex partnership with the bHLH transcription factor Mash1, which is also required for the differentiation of most noradrenergic cell types.
Molecular and Cellular Neuroscience 04/2000; 15(3):235-43. · 3.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
Current Opinion in Neurobiology 03/1999; 9(1):47-53. · 7.34 Impact Factor
[show abstract][hide abstract] ABSTRACT: Mash1, a mammalian homologue of the Drosophila proneural genes of the achaete-scute complex, is transiently expressed throughout the developing peripheral autonomic nervous system and in subsets of cells in the neural tube. In the mouse, targeted mutation of Mash1 has revealed a role in the development of parts of the autonomic nervous system and of olfactory neurons, but no discernible phenotype in the brain has been reported. Here, we show that the adrenergic and noradrenergic centres of the brain are missing in Mash1 mutant embryos, whereas most other brainstem nuclei are preserved. Indeed, the present data together with the previous results show that, except in cranial sensory ganglia, Mash1 function is essential for the development of all central and peripheral neurons that express noradrenergic traits transiently or permanently. In particular, we show that, in the absence of MASH1, these neurons fail to initiate expression of the noradrenaline biosynthetic enzyme dopamine beta-hydroxylase. We had previously shown that all these neurons normally express the homeodomain transcription factor Phox2a, a positive regulator of the dopamine beta-hydroxylase gene and that a subset of them depend on it for their survival. We now report that expression of Phox2a is abolished or massively altered in the Mash1-/- mutants, both in the noradrenergic centres of the brain and in peripheral autonomic ganglia. These results suggest that MASH1 controls noradrenergic differentiation at least in part by controlling expression of Phox2a and point to fundamental homologies in the genetic circuits that determine the noradrenergic phenotype in the central and peripheral nervous system.
Development 03/1998; 125(4):599-608. · 6.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recent evidence suggests that specific families of homeodomain transcription factors control the generation and survival of distinct neuronal types. We had previously characterized the homeobox gene Phox2a, which is expressed in differentiating neurons of the central and peripheral autonomic nervous system as well as in motor nuclei of the hindbrain. Targeted deletion of the Phox2a gene affects part of the structures in which it is expressed: the locus coeruleus, visceral sensory and parasympathetic ganglia and, as we show here, the nuclei of the IIIrd and IVth cranial nerves. We now report on the characterization of Phox2b, a close relative of Phox2a, with an identical homeodomain. Phox2a and Phox2b are co-expressed at most sites, therefore suggesting a broader role for Phox2 genes in the specification of the autonomic nervous system and cranial motor nuclei than revealed by the Phox2a knock-out mice. A detailed analysis of the relative timing of Phox2a and Phox2b expression at various sites suggests positive cross-regulations, which are substantiated by the loss of Phox2b expression in cranial ganglia of Phox2a-deficient mice. In the major part of the rhombencephalon, Phox2b expression precedes that of Phox2a and starts in the proliferative neuroepithelium, in a pattern strikingly restricted on the dorsoventral axis and at rhombomeric borders. This suggests that Phox2b links early patterning events to the differentiation of defined neuronal populations in the hindbrain.
Development 11/1997; 124(20):4065-75. · 6.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The mouse Otlx2 gene is a new member of the paired-like family of homeobox genes whose human homologue, RIEG, is involved in Rieger syndrome, an autosomal-dominant disorder. One of the cardinal features of Rieger syndrome is dental hypoplasia, indicating that Otlx2/RIEG activity is essential for normal tooth development. Here, we analyzed the expression of Otlx2 during mouse tooth development and studied its regulation in dental explants. Otlx2 expression distinguishes stomatodeal from other ectoderm as early as Embryonic Day 8.5, well before tooth initiation. Thereafter, its craniofacial expression becomes restricted to the tooth-forming areas and to the epithelial components of molar and incisor primordia. Although Otlx2 induction precedes the specification of odontogenic mesenchyme, tissue recombination experiments show that the maintenance of its expression requires signals from the mesenchyme and that dental mesenchyme has the capacity to induce ectopic expression of Otlx2 in nondental epithelium. Finally, we compare Otlx2 expression with that of the recently identified homeodomain transcription factor Barx1 expressed in molar mesenchyme. Their strictly complementary expression patterns in the epithelial and mesenchymal components suggest that both genes participate in the reciprocal tissue interactions which are a hallmark of odontogenesis.
[show abstract][hide abstract] ABSTRACT: Phox2a is a vertebrate homeodomain protein expressed in subsets of differentiating neurons. Here, we show that it is essential for proper development of the locus coeruleus, a subset of sympathetic and parasympathetic ganglia and the VIIth, IXth, and Xth cranial sensory ganglia. In the sensory ganglia, we have identified two differentiation blocks in Phox2a-/- mice. First, the transient expression of dopamine-beta-hydroxylase in neuroblasts is abolished, providing evidence that Phox2a controls noradrenergic traits in vivo. Second, the expression of the GDNF receptor subunit Ret is dramatically reduced, and there is a massive increase in apoptosis of ganglion cells, which are known to depend on GDNF in vivo. Therefore, Phox2a appears to regulate conventional differentiation traits and the ability of neurons to respond to essential survival factors.
[show abstract][hide abstract] ABSTRACT: Many transcription factors, and most prominently among them, homeodomain proteins, are expressed in specific groups of cells in the developing nervous system in patterns that suggest their involvement in neural fate determination. How various aspects of neural identity are controlled by such transcription factors, or sets of them, is still mostly unknown. It has been shown previously that Phox2 is such a homeodomain protein, expressed exclusively in differentiated groups of neurons or their precursors, and that its expression correlated with that of the noradrenaline synthesis enzyme dopamine-beta-hydroxylase. Here we confirm this striking correlation at the single-cell level with the use of an anti-Phox2 antibody. Moreover, we uncover a second, nonmutually exclusive correlative clue to the Phox2 expression pattern: a high proportion of Phox2-expressing cells are involved in, or located in areas involved in, synaptic circuits, i.e., that of the medullary control reflexes of autonomic functions. This suggests that Phox2 could be involved in the establishment of these circuits.
Journal of Neuroscience 01/1997; 16(23):7649-60. · 6.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: The vertebrate nervous system forms by the specification of, successively, neuroepithelial regions and cell groups. One of the proposed major histogenic steps is the subdivision of the neural tube in compartments along its caudorostral and dorsoventral axis. This event is reflected, and may be directed, by the restricted expression of many transcription factors. Here, we report on the isolation of a new homeobox gene of the paired superclass, Otlx2, whose early expression pattern in the mesencephalon and prosencephalon is congruent with proposed neuromeric models of brain morphogenesis. In addition, its late embryonic and postnatal expression, in clear continuity with the earlier pattern, suggests a role in the neuronal differentiation and the histogenesis of several prosencephalic and mesencephalic areas. Finally, Otlx2 is expressed from the earliest morphogenetic events in the Rathke's pouch, the anlage of the adenohypophysis, an expression site shared by a very close homologue, Otlx1/Ptx1/P-Otx.
Molecular and Cellular Neuroscience 02/1996; 8(4):258-71. · 3.84 Impact Factor