The Phox2b transcription factor coordinately regulates neuronal cell cycle exit and identity

Making neurons, made easy: The use of Neurogenin-2 in neuronal differentiation

Directed neuronal differentiation of human pluripotent stem cells (hPSCs), neural progenitors, or fibroblasts using transcription factors has allowed for the rapid and highly reproducible differentiation of mature and functional neurons. Exogenous expression of the transcription factor Neurogenin-2 (NGN2) has been widely used to generate different populations of neurons, which have been used in neurodevelopment studies, disease modeling, drug screening, and neuronal replacement therapies. Could NGN2 be a “one-glove-fits-all” approach for neuronal differentiations? This review summarizes the cellular roles of NGN2 and describes the applications and limitations of using NGN2 for the rapid and directed differentiation of neurons.

Deep analysis of neuroblastoma core regulatory circuitries using online databases and integrated bioinformatics shows their pan-cancer roles as prognostic predictors

AIM

Neuroblastoma is a heterogeneous childhood cancer derived from the neural crest. The dual cell identities of neuroblastoma include Mesenchymal (MES) and Adrenergic (ADRN). These identities are conferred by a small set of tightly-regulated transcription factors (TFs) binding super enhancers, collectively forming core regulatory circuitries (CRCs). The purpose of this study was to gain a deep understanding of the role of MES and ADRN TFs in neuroblastoma and other cancers as potential indicators of disease prognosis, progression, and relapse.

METHODS

To that end, we first investigated the expression and mutational profile of MES and ADRN TFs in neuroblastoma. Moreover, we established their correlation with neuroblastoma risk groups and overall survival while establishing their extended networks with long non-coding RNAs (lncRNAs). Furthermore, we analysed the pan-cancer expression and mutational profile of these TFs and their correlation with patient survival and finally their network connectivity, using a panel of bioinformatic tools including GEPIA2, human pathology atlas, TIMER2, Omicsnet, and Cytoscape.

RESULTS

We show the association of multiple MES and ADRN TFs with neuroblastoma risk groups and overall survival and find significantly higher expression of various MES and ADRN TFs compared to normal tissues and their association with overall survival and disease-free survival in multiple cancers. Moreover, we report the strong correlation of the expression of these TFs with the infiltration of stromal and immune cells in the tumour microenvironment and with stemness and metastasis-related genes. Furthermore, we reveal extended pan-cancer networks comprising these TFs that influence the tumour microenvironment and metastasis and may be useful indicators of cancer prognosis and patient survival.

CONCLUSION

Our meta-analysis shows the significance of MES and ADRN TFs as indicators of patient prognosis and the putative utility of these TFs as potential novel biomarkers.

An epigenetic circuit controls neurogenic programs during neocortex development

The production and expansion of intermediate progenitors (IPs) are essential for neocortical neurogenesis during development and over evolution. Here, we have characterized an epigenetic circuit that precisely controls neurogenic programs, particularly properties of IPs, during neocortical development. The circuit comprises a long non-coding RNA (LncBAR) and the BAF (SWI/SNF) chromatin-remodeling complex, which transcriptionally maintains the expression of Zbtb20. LncBAR knockout neocortex contains more deep-layer but fewer upper-layer projection neurons. Intriguingly, loss of LncBAR promotes IP production, but paradoxically prolongs the duration of the cell cycle of IPs during mid-later neocortical neurogenesis. Moreover, in LncBAR knockout mice, depletion of the neural progenitor pool at embryonic stage results in fewer adult neural progenitor cells in the subventricular zone of lateral ventricles, leading to a failure in adult neurogenesis to replenish the olfactory bulb. LncBAR binds to BRG1, the core enzymatic component of the BAF chromatin-remodeling complex. LncBAR depletion enhances association of BRG1 with the genomic locus of, and suppresses the expression of, Zbtb20, a transcription factor gene known to regulate both embryonic and adult neurogenesis. ZBTB20 overexpression in LncBAR-knockout neural precursors reverses compromised cell cycle progressions of IPs.

Modeling Motor Neuron Resilience in ALS Using Stem Cells

Oculomotor neurons, which regulate eye movement, are resilient to degeneration in the lethal motor neuron disease amyotrophic lateral sclerosis (ALS). It would be highly advantageous if motor neuron resilience could be modeled in vitro. Toward this goal, we generated a high proportion of oculomotor neurons from mouse embryonic stem cells through temporal overexpression of PHOX2A in neuronal progenitors. We demonstrate, using electrophysiology, immunocytochemistry, and RNA sequencing, that in vitro-generated neurons are bona fide oculomotor neurons based on their cellular properties and similarity to their in vivo counterpart in rodent and man. We also show that in vitro-generated oculomotor neurons display a robust activation of survival-promoting Akt signaling and are more resilient to the ALS-like toxicity of kainic acid than spinal motor neurons. Thus, we can generate bona fide oculomotor neurons in vitro that display a resilience similar to that seen in vivo.

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Bona fide oculomotor neurons can be derived from stem cells by PHOX2A overexpression

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In vitro- and in vivo-generated oculomotor neurons are transcriptionally similar

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Stem cell-derived oculomotor neurons display a robust activation of Akt signaling

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In vitro-generated oculomotor neurons are relatively resilient to ALS-like toxicity

The role of PHOX2B-derived astrocytes in chemosensory control of breathing and sleep homeostasis

KEY POINTS

The embryonic PHOX2B-progenitor domain generates neuronal and glial cells which together are involved in chemosensory control of breathing and sleep homeostasis. Ablating PHOX2B-derived astrocytes significantly contributes to secondary hypoxic respiratory depression as well as abnormalities in sleep homeostasis. PHOX2B-derived astrocyte ablation results in axonal pathologies in the retrotrapezoid nucleus.

ABSTRACT

We identify in mice a population of ∼800 retrotrapezoid nucleus (RTN) astrocytes derived from PHOX2B-positive, OLIG3-negative progenitor cells, that interact with PHOX2B-expressing RTN chemosensory neurons. PHOX2B-derived astrocyte ablation during early life results in adult-onset O2 chemoreflex deficiency. These animals also display changes in sleep homeostasis, including fragmented sleep and disturbances in delta power after sleep deprivation, all without observable changes in anxiety or social behaviours. Ultrastructural evaluation of the RTN demonstrates that PHOX2B-derived astrocyte ablation results in features characteristic of degenerative neuro-axonal dystrophy, including abnormally dilated axon terminals and increased amounts of synapses containing autophagic vacuoles/phagosomes. We conclude that PHOX2B-derived astrocytes are necessary for maintaining a functional O2 chemosensory reflex in the adult, modulate sleep homeostasis, and are key regulators of synaptic integrity in the RTN region, which is necessary for the chemosensory control of breathing. These data also highlight how defects in embryonic development may manifest as neurodegenerative pathology in an adult.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

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.

Targeting of PHOX2B expression allows the identification of drugs effective in counteracting neuroblastoma cell growth

The pathogenic role of the PHOX2B gene in neuroblastoma is indicated by heterozygous mutations in neuroblastoma patients and by gene overexpression in both neuroblastoma cell lines and tumor samples.

PHOX2B encodes a transcription factor which is crucial for the correct development and differentiation of sympathetic neurons.

PHOX2B overexpression is considered a prognostic marker for neuroblastoma and it is also used by clinicians to monitor minimal residual disease. Furthermore, it has been observed that neuronal differentiation in neuroblastoma is dependent on down-regulation of PHOX2B expression, which confirms that PHOX2B expression may be considered a target in neuroblastoma.

Here, PHOX2B promoter or 3′ untranslated region were used as molecular targets in an in vitro high-throughput approach that led to the identification of molecules able to decrease PHOX2B expression at transcriptional and likely even at post-transcriptional levels. Further functional investigations carried out on PHOX2B mRNA levels and biological consequences, such as neuroblastoma cell apoptosis and growth, showed that chloroquine and mycophenolate mofetil are most promising agents for neuroblastoma therapy based on down-regulation of PHOX2B expression.

Finally, a strong correlation between the effect of drugs in terms of down-regulation of PHOX2B expression and of biological consequences in neuroblastoma cells confirms the role of PHOX2B as a potential molecular target in neuroblastoma.

Motor neuron vulnerability and resistance in amyotrophic lateral sclerosis

In the fatal disease-amyotrophic lateral sclerosis (ALS)-upper (corticospinal) motor neurons (MNs) and lower somatic MNs, which innervate voluntary muscles, degenerate. Importantly, certain lower MN subgroups are relatively resistant to degeneration, even though pathogenic proteins are typically ubiquitously expressed. Ocular MNs (OMNs), including the oculomotor, trochlear and abducens nuclei (CNIII, IV and VI), which regulate eye movement, persist throughout the disease. Consequently, eye-tracking devices are used to enable paralysed ALS patients (who can no longer speak) to communicate. Additionally, there is a gradient of vulnerability among spinal MNs. Those innervating fast-twitch muscle are most severely affected and degenerate first. MNs innervating slow-twitch muscle can compensate temporarily for the loss of their neighbours by re-innervating denervated muscle until later in disease these too degenerate. The resistant OMNs and the associated extraocular muscles (EOMs) are anatomically and functionally very different from other motor units. The EOMs have a unique set of myosin heavy chains, placing them outside the classical characterization spectrum of all skeletal muscle. Moreover, EOMs have multiple neuromuscular innervation sites per single myofibre. Spinal fast and slow motor units show differences in their dendritic arborisations and the number of myofibres they innervate. These motor units also differ in their functionality and excitability. Identifying the molecular basis of cell-intrinsic pathways that are differentially activated between resistant and vulnerable MNs could reveal mechanisms of selective neuronal resistance, degeneration and regeneration and lead to therapies preventing progressive MN loss in ALS. Illustrating this, overexpression of OMN-enriched genes in spinal MNs, as well as suppression of fast spinal MN-enriched genes can increase the lifespan of ALS mice. Here, we discuss the pattern of lower MN degeneration in ALS and review the current literature on OMN resistance in ALS and differential spinal MN vulnerability. We also reflect upon the non-cell autonomous components that are involved in lower MN degeneration in ALS.

Common PHOX2B poly-alanine contractions impair RET gene transcription, predisposing to Hirschsprung disease

HSCR is a congenital disorder of the enteric nervous system, characterized by the absence of neurons along a variable length of the gut resulting from loss-of-function RET mutations. Congenital Central Hypoventilation Syndrome (CCHS) is a rare neurocristopathy characterized by impaired response to hypercapnia and hypoxemia caused by heterozygous mutations of the PHOX2B gene, mostly polyalanine (polyA) expansions but also missense, nonsense, and frameshift mutations, while polyA contractions are common in the population and believed neutral. HSCR associated CCHS can present in patients carrying PHOX2B mutations. Indeed, RET expression is orchestrated by different transcriptional factors among which PHOX2B, thus suggesting its possible role in HSCR pathogenesis. Following the observation of HSCR patients carrying in frame trinucleotide deletions within the polyalanine stretch in exon 3 (polyA contractions), we have verified the hypothesis that these PHOX2B variants do reduce its transcriptional activity, likely resulting in a down-regulation of RET expression and, consequently, favouring the development of the HSCR phenotype. Using proper reporter constructs, we show here that the in vitro transactivation of the RET promoter by different HSCR-associated PHOX2B polyA variants has resulted significantly lower compared to the effect of PHOX2B wild type protein. In particular, polyA contractions do induce a reduced transactivation of the RET promoter, milder compared to the severe polyA expansions associated with CCHS+HSCR, and correlated with the length of the deleted trait, with a more pronounced effect when contractions are larger.

Transcriptomic changes throughout post-hatch development in Gallus gallus pituitary

The pituitary gland is a neuroendocrine organ that works closely with the hypothalamus to affect multiple processes within the body including the stress response, metabolism, growth and immune function. Relative tissue expression (rEx) is a transcriptome analysis method that compares the genes expressed in a particular tissue to the genes expressed in all other tissues with available data. Using rEx, the aim of this study was to identify genes that are uniquely or more abundantly expressed in the pituitary when compared to all other collected chicken tissues. We applied rEx to define genes enriched in the chicken pituitaries at days 21, 22 and 42 post-hatch. rEx analysis identified 25 genes shared between all time points, 295 genes shared between days 21 and 22 and 407 genes unique to day 42. The 25 genes shared by all time points are involved in morphogenesis and general nervous tissue development. The 295 shared genes between days 21 and 22 are involved in neurogenesis and nervous system development and differentiation. The 407 unique day 42 genes are involved in pituitary development, endocrine system development and other hormonally related gene ontology terms. Overall, rEx analysis indicates a focus on nervous system/tissue development at days 21 and 22. By day 42, in addition to nervous tissue development, there is expression of genes involved in the endocrine system, possibly for maturation and preparation for reproduction. This study defines the transcriptome of the chicken pituitary gland and aids in understanding the expressed genes critical to its function and maturation.

OCT4 Acts as an Integrator of Pluripotency and Signal-Induced Differentiation

Cell type specification relies on the capacity of undifferentiated cells to properly respond to specific differentiation-inducing signals. Using genomic approaches along with loss- and gain-of-function genetic models, we identified OCT4-dependent mechanisms that provide embryonic stem cells with the means to customize their response to external cues. OCT4 binds a large set of low-accessible genomic regions. At these sites, OCT4 is required for proper enhancer and gene activation by recruiting co-regulators and RAR:RXR or β-catenin, suggesting an unexpected collaboration between the lineage-determining transcription factor and these differentiation-initiating, signal-dependent transcription factors. As a proof of concept, we demonstrate that overexpression of OCT4 in a kidney cell line is sufficient for signal-dependent activation of otherwise unresponsive genes in these cells. Our results uncover OCT4 as an integral and necessary component of signal-regulated transcriptional processes required for tissue-specific responses.

PHOX2A and PHOX2B are differentially regulated during retinoic acid-driven differentiation of SK-N-BE(2)C neuroblastoma cell line

PHOX2B and its paralogue gene PHOX2A are two homeodomain proteins in the network regulating the development of autonomic ganglia that have been associated with the pathogenesis of neuroblastoma (NB), because of their over-expression in different NB cell lines and tumour samples. We used the SK-N-BE(2)C cell line to show that all-trans retinoic acid (ATRA), a drug that is widely used to inhibit growth and induce differentiation in NBs, regulates both PHOX2A and PHOX2B expression, albeit by means of different mechanisms: it up-regulates PHOX2A and down-regulates PHOX2B. Both mechanisms act at transcriptional level, but prolonged ATRA treatment selectively degrades the PHOX2A protein, whereas the corresponding mRNA remains up-regulated. Further, we show that PHOX2A is capable of modulating PHOX2B expression, but this mechanism is not involved in the PHOX2B down-regulation induced by retinoic acid. Our findings demonstrate that PHOX2A expression is finely controlled during retinoic acid differentiation and this, together with PHOX2B down-regulation, reinforces the idea that they may be useful biomarkers for NB staging, prognosis and treatment decision making.

MycN Is Critical for the Maintenance of Human Embryonic Stem Cell-Derived Neural Crest Stem Cells

The biologic studies of human neural crest stem cells (hNCSCs) are extremely challenging due to the limited source of hNCSCs as well as ethical and technical issues surrounding isolation of early human embryonic tissues. On the other hand, vast majority of studies on MycN have been conducted in human tumor cells, thus, the role of MycN in normal human neural crest development is completely unknown. In the present study, we determined the role of MycN in hNCSCs isolated from in vitro-differentiating human embryonic stem cells (hESCs). For the first time, we show that suppression of MycN in hNCSCs inhibits cell growth and cell cycle progression. Knockdown of MycN in hNCSCs increases the expression of Cdkn1a, Cdkn2a and Cdkn2b, which encodes the cyclin-dependent kinases p21^CIP1, p16 ^INK4a and p15^INK4b. In addition, MycN is involved in the regulation of human sympathetic neurogenesis, as knockdown of MycN enhances the expression of key transcription factors involved in sympathetic neuron differentiation, including Phox2a, Phox2b, Mash1, Hand2 and Gata3. We propose that unlimited source of hNCSCs provides an invaluable platform for the studies of human neural crest development and diseases.

Functional Diversification of Motor Neuron-specific Isl1 Enhancers during Evolution

Functional diversification of motor neurons has occurred in order to selectively control the movements of different body parts including head, trunk and limbs. Here we report that transcription of Isl1, a major gene necessary for motor neuron identity, is controlled by two enhancers, CREST1 (E1) and CREST2 (E2) that allow selective gene expression of Isl1 in motor neurons. Introduction of GFP reporters into the chick neural tube revealed that E1 is active in hindbrain motor neurons and spinal cord motor neurons, whereas E2 is active in the lateral motor column (LMC) of the spinal cord, which controls the limb muscles. Genome-wide ChIP-Seq analysis combined with reporter assays showed that Phox2 and the Isl1-Lhx3 complex bind to E1 and drive hindbrain and spinal cord-specific expression of Isl1, respectively. Interestingly, Lhx3 alone was sufficient to activate E1, and this may contribute to the initiation of Isl1 expression when progenitors have just developed into motor neurons. E2 was induced by onecut 1 (OC-1) factor that permits Isl1 expression in LMCm neurons. Interestingly, the core region of E1 has been conserved in evolution, even in the lamprey, a jawless vertebrate with primitive motor neurons. All E1 sequences from lamprey to mouse responded equally well to Phox2a and the Isl1-Lhx3 complex. Conversely, E2, the enhancer for limb-innervating motor neurons, was only found in tetrapod animals. This suggests that evolutionarily-conserved enhancers permit the diversification of motor neurons.

During evolution, motor neurons became specialized to control movements of different body parts including head, trunk and limbs. Here we report that two enhancers of Isl1, E1 and E2, are active together with transcription factors in motor neurons. Surprisingly, E1 and its response to transcription factors has been conserved in evolution from the lamprey to man, whereas E2 is only found in animals with limbs. Our study provides an evolutionary example of how functional diversification of motor neurons is achieved by a dynamic interplay between enhancers and transcription factors.

Identification of novel pathways and molecules able to down-regulate PHOX2B gene expression by in vitro drug screening approaches in neuroblastoma cells

PHOX2B is a transcription factor involved in the regulation of neurogenesis and in the correct differentiation of the autonomic nervous system. The pathogenetic role of PHOX2B in neuroblastoma (NB) is supported by mutations in familial, sporadic and syndromic cases of NB and overexpression of PHOX2B and its target ALK in tumor samples and NB cell lines. Starting from these observations, we have performed in vitro drug screening approaches targeting PHOX2B overexpression as a potential pharmacological means in NB. In particular, in order to identify molecules able to decrease PHOX2B expression, we have evaluated the effects of 70 compounds in IMR-32 cell line stably expressing the luciferase gene under the control of the PHOX2B promoter. Curcumin, SAHA and trichostatin A showed to down-regulate the PHOX2B promoter activity which resulted in a decrease of both protein and mRNA expressions. In addition, we have observed that curcumin acts by interfering with PBX-1/MEIS-1, NF-κB and AP-1 complexes, in this work demonstrated for the first time to regulate the transcription of the PHOX2B gene. Finally, combined drug treatments showed successful effects in down-regulating the expression of both PHOX2B and its target ALK genes, thus supporting the notion of the effectiveness of molecule combination in tumor therapy.

Shox2 is required for the proper development of the facial motor nucleus and the establishment of the facial nerves

Background

Axons from the visceral motor neurons (vMNs) project from nuclei in the hindbrain to innervate autonomic ganglia and branchial arch-derived muscles. Although much is known about the events that govern specification of somatic motor neurons, the genetic pathways responsible for the development of vMNs are less well characterized. We know that vMNs, like all motor neurons, depend on sonic hedgehog signaling for their generation. Similarly, the paired-like homeobox 2b (Phox2b) gene, which is expressed in both proliferating progenitors and post-mitotic motor neurons, is essential for the development of vMNs. Given that our previous study identified a novel role for the short stature homeobox 2 (Shox2) gene in the hindbrain, and since SHOX2 has been shown to regulate transcription of islet 1 (Isl1), an important regulator of vMN development, we sought to determine whether Shox2 is required for the proper development of the facial motor nucleus.

Results

Using a Nestin-Cre driver, we show that elimination of Shox2 throughout the brain results in elevated cell death in the facial motor nucleus at embryonic day 12.5 (E12.5) and E14.5, which correlates with impaired axonal projection properties of vMNs. We also observed changes in the spatial expression of the vMN cell fate factors Isl1 and Phox2b, and concomitant defects in Shh and Ptch1 expression in Shox2 mutants. Furthermore, we demonstrate that elimination of Shox2 results in the loss of dorsomedial and ventromedial subnuclei by postnatal day 0 (P0), which may explain the changes in physical activity and impaired feeding/nursing behavior in Shox2 mutants.

Conclusions

Combined, our data show that Shox2 is required for development of the facial motor nucleus and its associated facial (VII) nerves, and serves as a new molecular tool to probe the genetic programs of this understudied hindbrain region.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-015-0176-0) contains supplementary material, which is available to authorized users.

Nkx2.2 and Nkx2.9 are the key regulators to determine cell fate of branchial and visceral motor neurons in caudal hindbrain

Cranial motor nerves in vertebrates are comprised of the three principal subtypes of branchial, visceral, and somatic motor neurons, which develop in typical patterns along the anteroposterior and dorsoventral axes of hindbrain. Here we demonstrate that the formation of branchial and visceral motor neurons critically depends on the transcription factors Nkx2.2 and Nkx2.9, which together determine the cell fate of neuronal progenitor cells. Disruption of both genes in mouse embryos results in complete loss of the vagal and spinal accessory motor nerves, and partial loss of the facial and glossopharyngeal motor nerves, while the purely somatic hypoglossal and abducens motor nerves are not diminished. Cell lineage analysis in a genetically marked mouse line reveals that alterations of cranial nerves in Nkx2.2; Nkx2.9 double-deficient mouse embryos result from changes of cell fate in neuronal progenitor cells. As a consequence progenitors of branchiovisceral motor neurons in the ventral p3 domain of hindbrain are transformed to somatic motor neurons, which use ventral exit points to send axon trajectories to their targets. Cell fate transformation is limited to the caudal hindbrain, as the trigeminal nerve is not affected in double-mutant embryos suggesting that Nkx2.2 and Nkx2.9 proteins play no role in the development of branchiovisceral motor neurons in hindbrain rostral to rhombomere 4.

The retrotrapezoid nucleus neurons expressing Atoh1 and Phox2b are essential for the respiratory response to CO₂

Maintaining constant CO₂ and H⁺ concentrations in the arterial blood is critical for life. The principal mechanism through which this is achieved in mammals is the respiratory chemoreflex whose circuitry is still elusive. A candidate element of this circuitry is the retrotrapezoid nucleus (RTN), a collection of neurons at the ventral medullary surface that are activated by increased CO₂ or low pH and project to the respiratory rhythm generator. Here, we use intersectional genetic strategies to lesion the RTN neurons defined by Atoh1 and Phox2b expression and to block or activate their synaptic output. Photostimulation of these neurons entrains the respiratory rhythm. Conversely, abrogating expression of Atoh1 or Phox2b or glutamatergic transmission in these cells curtails the phrenic nerve response to low pH in embryonic preparations and abolishes the respiratory chemoreflex in behaving animals. Thus, the RTN neurons expressing Atoh1 and Phox2b are a necessary component of the chemoreflex circuitry.

DOI:http://dx.doi.org/10.7554/eLife.07051.001

An adult at rest will typically breathe in and out up to 20 times per minute, inhaling oxygen and exhaling carbon dioxide in a process that, for the most part, occurs automatically. While we can choose to override this process and exert voluntary control over our breathing, we cannot suppress it indefinitely. Attempting to do so will ultimately trigger a reflex that forces us to start breathing again.

This reflex is mostly a response to the rise of carbon dioxide (CO₂) in the blood, which lowers the pH of the blood. This rise in CO₂ is toxic and triggers an increase in breathing so that the excess CO₂ is exhaled. The majority of the sensors that detect CO₂ are in the brainstem, which is at the junction of the brain and the spinal cord. However, the precise location of these sensors is not clear. Ruffault et al. now argue that the sensors are in a region called the ‘retrotrapezoid nucleus’, and that they can be identified by the presence of two proteins, Atoh1 and Phox2b.

In the brains of foetal mice, Ruffault et al. recorded cells in the retrotrapezoid nucleus and found that they fired in a rhythmic pattern, as would be expected for cells that control breathing. Moreover, the firing rate of these cells increased when the pH was lowered.

Ruffault et al. then created genetically modified mice with mutations in genes for Atoh1 or Phox2b. The retrotrapezoid nucleus was either absent or abnormal in these mutant mice. Moreover, new-born pups with these mutations were not able to increase their breathing when the level of CO₂ in their blood rose.

These results shed light on the respiratory distress experienced by patients with a rare disorder called congenital central hypoventilation syndrome (CCHS) that is caused by mutations in Phox2b. More commonly, unstable or irregular breathing is seen in human infants that are born prematurely, and sometimes in infants born at full term. In the light of the new findings by Ruffault et al., it is possible that abnormal development or immaturity of the retrotrapezoid nucleus is the cause.

DOI:http://dx.doi.org/10.7554/eLife.07051.002

Directed midbrain and spinal cord neurogenesis from pluripotent stem cells to model development and disease in a dish

Induction of specific neuronal fates is restricted in time and space in the developing CNS through integration of extrinsic morphogen signals and intrinsic determinants. Morphogens impose regional characteristics on neural progenitors and establish distinct progenitor domains. Such domains are defined by unique expression patterns of fate determining transcription factors. These processes of neuronal fate specification can be recapitulated in vitro using pluripotent stem cells. In this review, we focus on the generation of dopamine neurons and motor neurons, which are induced at ventral positions of the neural tube through Sonic hedgehog (Shh) signaling, and defined at anteroposterior positions by fibroblast growth factor (Fgf) 8, Wnt1, and retinoic acid (RA). In vitro utilization of these morphogenic signals typically results in the generation of multiple neuronal cell types, which are defined at the intersection of these signals. If the purpose of in vitro neurogenesis is to generate one cell type only, further lineage restriction can be accomplished by forced expression of specific transcription factors in a permissive environment. Alternatively, cell-sorting strategies allow for selection of neuronal progenitors or mature neurons. However, modeling development, disease and prospective therapies in a dish could benefit from structured heterogeneity, where desired neurons are appropriately synaptically connected and thus better reflect the three-dimensional structure of that region. By modulating the extrinsic environment to direct sequential generation of neural progenitors within a domain, followed by self-organization and synaptic establishment, a reductionist model of that brain region could be created. Here we review recent advances in neuronal fate induction in vitro, with a focus on the interplay between cell intrinsic and extrinsic factors, and discuss the implications for studying development and disease in a dish.

Mutations that disrupt PHOXB interaction with the neuronal calcium sensor HPCAL1 impede cellular differentiation in neuroblastoma

Heterozygous germline mutations in PHOX2B, a transcriptional regulator of sympathetic neuronal differentiation, predispose to diseases of the sympathetic nervous system, including neuroblastoma and congenital central hypoventilation syndrome (CCHS). Although the PHOX2B variants in CCHS largely involve expansions of the second polyalanine repeat within the C-terminus of the protein, those associated with neuroblastic tumors are nearly always frameshift and truncation mutations. To test the hypothesis that the neuroblastoma-associated variants exert their effects through loss or gain of protein-protein interactions, we performed a large-scale yeast two-hybrid screen using both wild-type (WT) and six different mutant PHOX2B proteins against over 10 000 human genes. The neuronal calcium sensor protein HPCAL1 (VILIP-3) exhibited strong binding to WT PHOX2B and a CCHS-associated polyalanine expansion mutant but only weakly or not at all to neuroblastoma-associated frameshift and truncation variants. We demonstrate that both WT PHOX2B and the neuroblastoma-associated R100L missense and the CCHS-associated alanine expansion variants induce nuclear translocation of HPCAL1 in a Ca(2+)-independent manner, while the neuroblastoma-associated 676delG frameshift and K155X truncation mutants impair subcellular localization of HPCAL1, causing it to remain in the cytoplasm. HPCAL1 did not appreciably influence the ability of WT PHOX2B to transactivate the DBH promoter, nor did it alter the decreased transactivation potential of PHOX2B variants in 293T cells. Abrogation of the PHOX2B-HPCAL1 interaction by shRNA knockdown of HPCAL1 in neuroblastoma cells expressing PHOX2B led to impaired neurite outgrowth with transcriptional profiles indicative of inhibited sympathetic neuronal differentiation. Our results suggest that certain PHOX2B variants associated with neuroblastoma pathogenesis, because of their inability to bind to key interacting proteins such as HPCAL1, may predispose to this malignancy by impeding the differentiation of immature sympathetic neurons.

Distinct neuroblastoma-associated alterations of PHOX2B impair sympathetic neuronal differentiation in zebrafish models

Heterozygous germline mutations and deletions in PHOX2B, a key regulator of autonomic neuron development, predispose to neuroblastoma, a tumor of the peripheral sympathetic nervous system. To gain insight into the oncogenic mechanisms engaged by these changes, we used zebrafish models to study the functional consequences of aberrant PHOX2B expression in the cells of the developing sympathetic nervous system. Allelic deficiency, modeled by phox2b morpholino knockdown, led to a decrease in the terminal differentiation markers th and dbh in sympathetic ganglion cells. The same effect was seen on overexpression of two distinct neuroblastoma-associated frameshift mutations, 676delG and K155X - but not the R100L missense mutation - in the presence of endogenous Phox2b, pointing to their dominant-negative effects. We demonstrate that Phox2b is capable of regulating itself as well as ascl1, and that phox2b deficiency uncouples this autoregulatory mechanism, leading to inhibition of sympathetic neuron differentiation. This effect on terminal differentiation is associated with an increased number of phox2b⁺, ascl1⁺, elavl3⁻ cells that respond poorly to retinoic acid. These findings suggest that a reduced dosage of PHOX2B during development, through either a heterozygous deletion or dominant-negative mutation, imposes a block in the differentiation of sympathetic neuronal precursors, resulting in a cell population that is likely to be susceptible to secondary transforming events.

Neuroblastoma, a tumor of the peripheral sympathetic nervous system, is the most common cancer diagnosed in infancy. Although most cases arise sporadically, familial predisposition also occurs in association with mutations in a single copy of the PHOX2B gene, a “master regulator” of sympathetic neuronal development. The exact mechanisms by which these mutations increase susceptibility to neuroblastoma are unclear, primarily because of the paucity of optimal models in which to study very early development of the sympathetic nervous system. We took advantage of the ex vivo development and transparent nature of zebrafish embryos to study the roles of both normal and mutated PHOX2B in development of the sympathetic nervous system. We present data indicating that aberrant PHOX2B expression causes an arrest in the normal maturation of sympathetic neurons, leading to immature cells that are resistant to drug-induced differentiation. Indeed, we demonstrate that phox2b gene “dosage” is important for normal differentiation of sympathetic neurons in the zebrafish and suggest that the population of immature cells resulting from a decreased dosage of this pivotal factor may be susceptible to secondary mutations that could ultimately lead to neuroblastoma.

Phox2b influences the development of a caudal dopaminergic subset

The developing mesodiencephalic dopaminergic (mdDA) neuronal field can be subdivided into several molecularly distinct domains that arise due to spatiotemporally distinct origins of the neurons and distinct transcriptional pathways controlling these neuronal subsets. Two large anatomically and functionally different subdomains are formed that eventually give rise to the SNc and VTA, but more subsets exist which require detailed characterization in order to better understand the development of the functionally different mdDA subsets, and subset-specific vulnerability. In this study, we aimed to characterize the role of transcription factor Phox2b in the development of mdDA neurons. We provide evidence that Phox2b is co-expressed with TH in a dorsal-caudal subset of neurons in the mdDA neuronal field during embryonic development. Moreover, Phox2b transcripts were identified in FAC-sorted Pitx3 positive neurons. Subsequent analysis of Phox2b mutant embryos revealed that in the absence of Phox2b, a decrease of TH expression occurred specifically in the midbrain neuronal subset that normally co-expresses Phox2b with TH. Our data suggest that Phox2b is, next to the known role in the development of the oculomotor complex, involved in the development of a specific caudal mdDA neuronal subset.

Expression of delta-protocadherins in the spinal cord of the chicken embryo

Protocadherins constitute the largest subfamily of cadherin genes and are widely expressed in the nervous system. In the present study, we cloned eight members of the delta-protocadherin subfamily of cadherins (Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh17, Pcdh18, and Pcdh19) from the chicken, and investigated their expression in the developing chicken spinal cord by in situ hybridization. Our results showed that each of the investigated delta-protocadherins exhibits a spatially restricted and temporally regulated pattern of expression. Pcdh1, Pcdh8, Pcdh18, and Pcdh19 are expressed in restricted dorsoventral domains of the neuroepithelial layer at early developmental stages (E2.5–E4). In the differentiating mantle layer, specific expression profiles are observed for all eight delta-protocadherins along the dorsoventral, mediolateral, and rostrocaudal dimensions at intermediate stages of development (E6–E10). Expression profiles are especially diverse in the motor column, where different pools of motor neurons exhibit signal for subsets of delta-protocadherins. In the dorsal root ganglion, subpopulations of cells express combinations of Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, and Pcdh17. The ventral boundary cap cells are positive for Pcdh7, Pcdh9, and Pcdh10. Signals for Pcdh8, Pcdh18, and Pcdh19 are found in the meninges. Surrounding tissues, such as the notochord, dermomyotome, and sclerotome also exhibit differential expression patterns. The highly regulated spatiotemporal expression patterns of delta-protocadherins suggest that they have multiple and diverse functions during development of the spinal cord and its surrounding tissues.

Neuroblastoma progress on many fronts: the Neuroblastoma Research Symposium

Neuroblastoma (NBL) is a pediatric tumor of infancy derived from precursor cells of the sympathetic nervous system. Clinicians and researchers in developmental biology and genetics recently met to facilitate meaningful crosstalk and to discuss considerable progress made in the clinical treatment and basic biology of NBL. For instance, discoveries in familial NBL have identified genetic aberrations in Phox2b and Alk that predispose to NBL, while advances in epigenetics and MYCN regulation have also offered insight into NBL pathogenesis and future treatment. Moreover, novel therapeutic avenues are also being explored, including targeted immunotherapies, and innovative radiotherapeutic and chemotherapeutic approaches. This multi-disciplinary meeting was convened to aid the transfer of new biological findings into the clinic and to use clinical advances to inform the basic biological understanding of this devastating disease.

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.

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.

[Malformation syndromes associated with childhood cancer: an update]

Biology, genetics and environment of childhood solid tumours set them apart from adult solid tumours. The nature of the progenitor cells from which these tumours arise, and their immature tissue environment, allows childhood solid tumours to develop with fewer defects in cell regulatory processes. Constitutional molecular defects are known to play a role in childhood solid tumours, as shown by the increased incidence of embryonic cancers in children carrying malformations associated with childhood cancer. These rare diagnoses are commonly missed. In this article, we reviewed the spectrum of these tumour predisposition syndromes.

PHOX2A regulation of oculomotor complex nucleogenesis

Brain nuclei are spatially organized collections of neurons that share functional properties. Despite being central to vertebrate brain circuitry, little is known about how nuclei are generated during development. We have chosen the chick midbrain oculomotor complex (OMC) as a model with which to study the developmental mechanisms of nucleogenesis. The chick OMC comprises two distinct cell groups: a dorsal Edinger-Westphal nucleus of visceral oculomotor neurons and a ventral nucleus of somatic oculomotor neurons. Genetic studies in mice and humans have established that the homeobox transcription factor gene PHOX2A is required for midbrain motoneuron development. We probed, in forced expression experiments, the capacity of PHOX2A to generate a spatially organized midbrain OMC. We found that exogenous Phox2a delivery to embryonic chick midbrain can drive a complete OMC molecular program, including the production of visceral and somatic motoneurons. Phox2a overexpression was also able to generate ectopic motor nerves. The exit points of such auxiliary nerves were invested with ectopic boundary cap cells and, in four examples, the ectopic nerves were seen to innervate extraocular muscle directly. Finally, Phox2a delivery was able to direct ectopic visceral and somatic motoneurons to their correct native spatial positions, with visceral motoneurons settling close to the ventricular surface and somatic motoneurons migrating deeper into the midbrain. These findings establish that in midbrain, a single transcription factor can both specify motoneuron cell fates and orchestrate the construction of a spatially organized motoneuron nuclear complex.

Zinc finger protein 191 (ZNF191/Zfp191) is necessary to maintain neural cells as cycling progenitors

The identification of the factors that allow better monitoring of stem cell renewal and differentiation is of paramount importance for the implementation of new regenerative therapies, especially with regard to the nervous and hematopoietic systems. In this article, we present new information on the function of zinc finger protein 191 (ZNF/Zfp191), a factor isolated in hematopoietic cell lines, within progenitors of the central nervous system (CNS). ZNF/Zfp191 has been found to be principally expressed in progenitors of the developing CNS of humans and mice. Such an overlap of the expression patterns in addition to the high homology of the protein in mammals suggested that ZNF/Zfp191 exerts a conserved function within such progenitors. Indeed, ZNF191 knockdown in human neural progenitors inhibits proliferation and leads to the exit of the cell cycle. Conversely, ZNF191 misexpression maintains progenitors in cycle and exerts negative control on the Notch pathway, which prevents them from differentiating. The present data, together with the fact that the inactivation of Zfp191 leads to embryonic lethality, confirm ZNF191 as an essential factor acting for the promotion of the cell cycle and thus maintenance in the progenitor stage. On the bases of expression data, such a function can be extended to progenitor cells of other tissues such as the hematopoietic system, which emphasizes the important issue of further understanding the molecular events controlled by ZNF/Zfp191.

Association analysis of the PHOX2B gene with Hirschsprung disease in the Han Chinese population of Southeastern China

BACKGROUND

Hirschsprung disease (HSCR, OMIM 142623) is a complex congenital disorder characterized by intestinal obstructions caused by the absence of the intestinal ganglion cells of the nerve plexuses in variable lengths of the digestive tract. The PHOX2B gene is involved in neurogenesis and disruption of Phox2b in mice results in a HSCR-like phenotype. The first association study of the PHOX2B gene with HSCR derived from Chinese population in Hong Kong; here, we address the question of whether PHOX2B acts as a predisposing factor in HSCR pathogenesis in Chinese population in mainland.

METHODS

To investigate the contribution of PHOX2B to the HSCR phenotype, polymerase chain reaction amplification and direct sequencing were used to screen PHOX2B coding regions and intron/exon boundaries for mutations and polymorphisms in 102 patients with HSCR and 96 ethnically matched controls, in Han Chinese populations of Southeastern China.

RESULTS

In this study, we genotyped 4 single nucleotide polymorphisms (SNPs) (including 1 novel SNP) located within the PHOX2B gene. Statistically significant differences were found for c.701 A > G and IVS2 + 100 A > G, and the log-additive model was accepted as the best inheritance model (odds ratio [OR], 1.79; 95% confidence interval [CI], 1.11-2.87) for IVS2 + 100 A > G. We also showed that the haplotype-A G A N composed of 4 SNPs exhibited significant association with the disease (P = .03); this haplotype was more frequently observed in cases than in controls (OR, 2.31; 95% CI, 1.11-4.82).

CONCLUSIONS

Our study provided further evidence that the PHOX2B gene is involved in the susceptibility to HSCR in the Han Chinese population. Our findings are in accordance with the involvement of PHOX2B in the signaling pathways governing the development of enteric neurons.

Genetic basis of Hirschsprung's disease

Hirschsprung's disease (HSCR) is a developmental disorder characterized by the absence of ganglion cells in the lower digestive tract. Aganglionosis is attributed to a disorder of the enteric nervous system (ENS) whereby ganglion cells fail to innervate the lower gastrointestinal tract during embryonic development. HSCR is a complex disease that results from the interaction of several genes and manifests with low, sex-dependent penetrance and variability in the length of the aganglionic segment. The genetic complexity observed in HSCR can be conceptually understood in light of the molecular and cellular events that take place during the ENS development. DNA alterations in any of the genes involved in the ENS development may interfere with the colonization process, and represent a primary etiology for HSCR. This review will focus on the genes known to be involved in HSCR pathology, how they interact, and on how technology advances are being employed to uncover the pathological processes underlying this disease.

Cytokinesis of neuroepithelial cells can divide their basal process before anaphase

Neuroepithelial (NE) cells, the primary stem and progenitor cells of the vertebrate central nervous system, are highly polarized and elongated. They retain a basal process extending to the basal lamina, while undergoing mitosis at the apical side of the ventricular zone. By studying NE cells in the embryonic mouse, chick and zebrafish central nervous system using confocal microscopy, electron microscopy and time-lapse imaging, we show here that the basal process of these cells can split during M phase. Splitting occurred in the basal-to-apical direction and was followed by inheritance of the processes by either one or both daughter cells. A cluster of anillin, an essential component of the cytokinesis machinery, appeared at the distal end of the basal process in prophase and was found to colocalize with F-actin at bifurcation sites, in both proliferative and neurogenic NE cells. GFP-anillin in the basal process moved apically to the cell body prior to anaphase onset, followed by basal-to-apical ingression of the cleavage furrow in telophase. The splitting of the basal process of M-phase NE cells has implications for cleavage plane orientation and the relationship between mitosis and cytokinesis.

Congenital central hypoventilation syndrome with hyperinsulinism in a preterm infant

Congenital central hypoventilation syndrome (CCHS), a rare disorder typically presenting in the newborn period, results in over 90% of cases from PHOX2B polyalanine repeat mutations. It is characterized by alveolar hypoventilation, symptoms of autonomic nervous system dysregulation, and in a subset of cases Hirschsprung's disease and, later, tumors of neural crest origin. We describe a preterm infant with severe phenotype of CCHS and hyperinsulinism. A novel de novo heterozygote missence mutation (Gly68Cys) in the PHOX2B gene could be identified. Based on the observation of three patients presenting with the combination of congenital hyperinsulinism and CCHS, hyperinsulinism might represent an additional clinical feature of CCHS.

BM88/CEND1 coordinates cell cycle exit and differentiation of neuronal precursors

During development, coordinate regulation of cell cycle exit and differentiation of neuronal precursors is essential for generation of appropriate number of neurons and proper wiring of neuronal circuits. BM88 is a neuronal protein associated in vivo with terminal neuron-generating divisions, marking the exit of proliferative cells from the cell cycle. Here, we provide functional evidence that BM88 is sufficient to initiate the differentiation of spinal cord neural precursors toward acquisition of generic neuronal and subtype-specific traits. Gain-of-function approaches show that BM88 negatively regulates proliferation of neuronal precursors, driving them to prematurely exit the cell cycle, down-regulate Notch1, and commit to a neuronal differentiation pathway. The combined effect on proliferation and differentiation results in precocious induction of neurogenesis and generation of postmitotic neurons within the ventricular zone. The dual action of BM88 is not recapitulated by the cell cycle inhibitor p27Kip1, suggesting that cell cycle exit does not induce differentiation by default. Mechanistically, induction of endogenous BM88 by forced expression of the proneural gene Mash1 indicates that BM88 is part of the differentiation program activated by proneural genes. Furthermore, BM88 gene silencing conferred by small interfering RNA in spinal cord neural progenitor cells enhances cell cycle progression and impairs neuronal differentiation. Our results implicate BM88 in the synchronization of cell cycle exit and differentiation of neuronal precursors in the developing nervous system.

Methylation-associated PHOX2B gene silencing is a rare event in human neuroblastoma

Neuroblastoma (NB), an embryonic tumour originating from neural crest cells, is one of the most common solid tumours in childhood. Although NB is characterised by numerous recurrent, large-scale chromosome rearrangements, the genes targeted by these imbalances have remained elusive. We recently identified the paired-like homeobox 2B (PHOX2B, MIM 603851) gene as disease-causing in dysautonomic disorders including Congenital Central Hypoventilation Syndrome (CCHS), Hirschsprung disease (HSCR) and NB in various combinations. Most patients with NB due to a germline heterozygous PHOX2B gene mutation are familial and/or syndromic. PHOX2B, at chromosome 4p12, does not lie in a commonly rearranged locus in NB. To evaluate the role of PHOX2B in sporadic, isolated NB, we analysed 13 NB cell lines and 45 tumours for expression, mutations of coding and promoter sequences, loss of heterozygosity (LOH), or aberrant hypermethylation of PHOX2B (13 cell lines and 18 tumours). We didn't identify any mutation but LOH in about 10% of the cases and aberrant CpG dinucleotide methylation of the 500 bp PHOX2B promoter region in 4/31 tumours and cell lines (12.9%). Altogether, both germinal and somatic anomalies at the PHOX2B locus are found in NB.

Prevalence and functional consequence of PHOX2B mutations in neuroblastoma

PHOX2B is a homeodomain-containing protein that is involved in the development of the peripheral nervous system and is the major disease gene for the rare congenital breathing disorder congenital central hypoventilation syndrome (CCHS). Germline PHOX2B alterations were also recently discovered in neuroblastoma cases with CCHS and/or Hirschsprung disease, but a comprehensive survey for mutational frequency and functional consequence has not been performed. We therefore studied a large panel of hereditary neuroblastomas to understand the frequency and functional effects of PHOX2B mutations. Three of 47 individuals with presumed genetic predisposition to neuroblastoma showed a germline PHOX2B mutation (6.4%). Mutations were also discovered in 2 of 30 human neuroblastoma-derived cell lines, but none of 86 primary tumors from patients with sporadically occurring neuroblastoma. The vast majority of primary tumors showed abundant PHOX2B mRNA expression relative to the remainder of the transcriptome. Consistent with its role as an important neurodevelopmental gene, forced overexpression of wild-type PHOX2B in neuroblastoma cell lines suppressed cell proliferation and synergized with all-trans retinoic acid to promote differentiation. Patient-derived mutant PHOX2B constructs retained the ability to suppress cellular proliferation, but were not able to promote differentiation or activate expression of a known PHOX2B target gene in vitro. These findings show that PHOX2B alterations are a rare cause of hereditary neuroblastoma, but disruption of this neurodevelopmental pathway can interfere with transcription-dependent terminal differentiation. These data also suggest that the genetics of neuroblastoma initiation are complex, and highlight genes involved in normal noradrenergic development as candidate predisposition genes.

Forced expression of Phox2 homeodomain transcription factors induces a branchio-visceromotor axonal phenotype

What causes motor neurons to project into the periphery is not well understood. We here show that forced expression of the homeodomain protein Phox2b, shown previously to be necessary and sufficient for branchio-visceromotor neuron development, and of its paralogue Phox2a imposes a branchiomotor-like axonal phenotype in the spinal cord. Many Phox2-transfected neurons, whose axons would normally stay within the confines of the neural tube, now project into the periphery. Once outside the neural tube, a fraction of the ectopic axons join the spinal accessory nerve, a branchiomotor nerve which, as shown here, does not develop in the absence of Phox2b. Explant studies show that the axons of Phox2-transfected neurons need attractive cues to leave the neural tube and that their outgrowth is promoted by tissues, to which branchio-visceromotor fibers normally grow. Hence, Phox2 expression is a key step in determining the peripheral axonal phenotype and thus the decision to stay within the neural tube or to project out of it.

Homeodomain transcription factor Phox2a, via cyclic AMP-mediated activation, induces p27Kip1 transcription, coordinating neural progenitor cell cycle exit and differentiation

Mechanisms coordinating neural progenitor cell cycle exit and differentiation are incompletely understood. The cyclin-dependent kinase inhibitor p27(Kip1) is transcriptionally induced, switching specific neural progenitors from proliferation to differentiation. However, neuronal differentiation-specific transcription factors mediating p27(Kip1) transcription have not been identified. We demonstrate the homeodomain transcription factor Phox2a, required for central nervous system (CNS)- and neural crest (NC)-derived noradrenergic neuron differentiation, coordinates cell cycle exit and differentiation by inducing p27(Kip1) transcription. Phox2a transcription and activation in the CNS-derived CAD cell line and primary NC cells is mediated by combined cyclic AMP (cAMP) and bone morphogenetic protein 2 (BMP2) signaling. In the CAD cellular model, cAMP and BMP2 signaling initially induces proliferation of the undifferentiated precursors, followed by p27(Kip1) transcription, G(1) arrest, and neuronal differentiation. Small interfering RNA silencing of either Phox2a or p27(Kip1) suppresses p27(Kip1) transcription and neuronal differentiation, suggesting a causal link between p27(Kip1) expression and differentiation. Conversely, ectopic Phox2a expression via the Tet-off expression system promotes accelerated CAD cell neuronal differentiation and p27(Kip1) transcription only in the presence of cAMP signaling. Importantly, endogenous or ectopically expressed Phox2a activated by cAMP signaling binds homeodomain cis-acting elements of the p27(Kip1) promoter in vivo and mediates p27(Kip1)-luciferase expression in CAD and NC cells. We conclude that developmental cues of cAMP signaling causally link Phox2a activation with p27(Kip1) transcription, thereby coordinating neural progenitor cell cycle exit and differentiation.

Reciprocal gene replacements reveal unique functions for Phox2 genes during neural differentiation

The paralogous paired-like homeobox genes Phox2a and Phox2b are involved in the development of specific neural subtypes in the central and peripheral nervous systems. The different phenotypes of Phox2 knockout mutants, together with their asynchronous onset of expression, prompted us to generate two knock-in mutant mice, in which Phox2a is replaced by the Phox2b coding sequence, and vice versa. Our results indicate that Phox2a and Phox2b are not functionally equivalent, as only Phox2b can fulfill the role of Phox2a in the structures that depend on both genes. Furthermore, we demonstrate unique roles of Phox2 genes in the differentiation of specific motor neurons. Whereas the oculomotor and the trochlear neurons require Phox2a for their proper development, the migration of the facial branchiomotor neurons depends on Phox2b. Therefore, our analysis strongly indicates that biochemical differences between the proteins rather than temporal regulation of their expression account for the specific function of each paralogue.

Molecular consequences of PHOX2B missense, frameshift and alanine expansion mutations leading to autonomic dysfunction

Heterozygous mutations of the PHOX2B gene account for a broad variety of disorders of the autonomic nervous system, either isolated or combined, including congenital central hypoventilation syndrome (CCHS), tumours of the sympathetic nervous system and Hirschsprung disease. In CCHS, the prevalent mutation is an expansion of a 20-alanine stretch ranging from +5 to +13 alanines, whereas frameshift and missense mutations are found occasionally. To determine the molecular basis of impaired PHOX2B function, we assayed the transactivation and DNA binding properties of wild-type and mutant PHOX2B proteins. Furthermore, we investigated aggregate formation by proteins with polyalanine tract expansions ranging from +5 to +13 alanines using immunofluorescence of transfected cells and gel filtration of in vitro translated proteins. We found that transactivation of the dopamine beta-hydroxylase promoter by PHOX2B proteins with frameshift and missense mutations was abolished or severely curtailed, as was in vitro DNA binding although the proteins localized to the nucleus. The transactivation potential of proteins with polyalanine tract expansions declined with increasing length of the polyalanine stretch, and DNA binding was affected for an expansion of +9 alanines and above. Cytoplasmic aggregation in transfected cells was only observed for the longest expansions, whereas even the short expansion mutants were prone to form multimers in vitro. Such a tendency to protein misfolding could explain loss of transactivation for alanine expansion mutations. However, additional mechanisms such as toxic gain-of-function may play a role in the pathogenic process.

Characterization of Xenopus Phox2a and Phox2b defines expression domains within the embryonic nervous system and early heart field

The closely related homeodomain containing genes, Phox2a and Phox2b, are essential for neuronal specification and differentiation within discrete subsets of neurons during vertebrate embryogenesis. We have isolated Xenopus Phox2 homologs, termed Xphox2a and Xphox2b, and characterized their expression during early development. In addition, we have characterized a Phox2a splice variant, termed Xphox2a.2, which lacks homeo- and C-terminal protein coding domains. Xphox2a, Xphox2a.2 and Xphox2b transcripts are expressed in dynamic temporal and regional patterns during nervous system development. The expression of Xphox2a and Xphox2b is only partially overlapping and includes cranial motor and interneuron populations as well as peripheral sympathetic and cranial ganglion neurons, sites linked to Phox2 expression in other species. In addition, we have identified an early domain of Xphox2a and subsequent Xphox2b expression in ventral regions of the embryo, within the developing heart field. XPhox2 expression within this domain is preceded by the gastrula-stage expression of the proneural basic helix-loop-helix transcription factor, Xash1, pointing to a new region of action for this group transcription factors during vertebrate development.

BM88 is an early marker of proliferating precursor cells that will differentiate into the neuronal lineage

Progression of progenitor cells towards neuronal differentiation is tightly linked with cell cycle control and the switch from proliferative to neuron-generating divisions. We have previously shown that the neuronal protein BM88 drives neuroblastoma cells towards exit from the cell cycle and differentiation into a neuronal phenotype in vitro. Here, we explored the role of BM88 during neuronal birth, cell cycle exit and the initiation of differentiation in vivo. By double- and triple-labelling with the S-phase marker BrdU or the late G2 and M-phase marker cyclin B1, antibodies to BM88 and markers of the neuronal or glial cell lineages, we demonstrate that in the rodent forebrain, BM88 is expressed in multipotential progenitor cells before terminal mitosis and in their neuronal progeny during the neurogenic interval, as well as in the adult. Further, we defined at E16 a cohort of proliferative progenitors that exit S phase in synchrony, and by following their fate for 24 h we show that BM88 is associated with the dynamics of neuron-generating divisions. Expression of BM88 was also evident in cycling cortical radial glial cells, which constitute the main neurogenic population in the cerebral cortex. In agreement, BM88 expression was markedly reduced and restricted to a smaller percentage of cells in the cerebral cortex of the Small eye mutant mice, which lack functional Pax6 and exhibit severe neurogenesis defects. Our data show an interesting correlation between BM88 expression and the progression of progenitor cells towards neuronal differentiation during the neurogenic interval.

SPECIFICATION OF TEMPORAL IDENTITY IN THE DEVELOPING NERVOUS SYSTEM

The nervous system of higher organisms exhibits extraordinary cellular diversity owing to complex spatial and temporal patterning mechanisms. The role of spatial patterning in generating neuronal diversity is well known; here we discuss how neural progenitors change over time to contribute to cell diversity within the central nervous system (CNS). We focus on five model systems: the vertebrate retina, cortex, hindbrain, spinal cord, and Drosophila neuroblasts. For each, we address the following questions: Do multipotent progenitors generate different neuronal cell types in an invariant order? Do changes in progenitor-intrinsic factors or progenitor-extrinsic signals regulate temporal identity (i.e., the sequence of neuronal cell types produced)? What is the mechanism that regulates temporal identity transitions; i.e., what triggers the switch from one temporal identity to the next? By applying the same criteria to analyze each model system, we try to highlight common themes, point out unique attributes of each system, and identify directions for future research.

Ascl1/Mash1 is required for the development of central serotonergic neurons

The transcriptional control of the differentiation of central serotonergic (5-HT) neurons in vertebrates has recently come under scrutiny and has been shown to involve the homeobox genes Nkx2-2 and Lmx1b, the Ets-domain gene Pet1 (also known as Fev) and the zinc-finger gene Gata3. The basic helix-loop-helix (bHLH) gene Ascl1 (also known as Mash1) is coexpressed with Nkx2-2 in the neuroepithelial domain of the hindbrain, which gives rise to 5-HT neurons. Here we show in the mouse that Ascl1 is essential for the birth of 5-HT neurons, both as a proneural gene for the production of postmitotic neuronal precursors and as a determinant of the serotonergic phenotype for the parallel activation of Gata3, Lmx1b and Pet1. Thus Ascl1, which is essential for noradrenergic differentiation, is also a determinant of the serotonergic phenotype.

Germline mutations of the paired-like homeobox 2B (PHOX2B) gene in neuroblastoma

Neuroblastoma (NB) is a frequent pediatric tumor for which recurrent somatic rearrangements are known. Germline mutations of predisposing gene(s) are suspected on the basis of rare familial cases and the association of NB with other genetically determined congenital malformations of neural crest-derived cells--namely, Hirschsprung disease (HSCR) and/or congenital central hypoventilation syndrome (CCHS). We recently identified the paired-like homeobox 2B (PHOX2B) gene as the major disease-causing gene in isolated and syndromic CCHS, which prompted us to regard it as a candidate gene in NB. Here, we report on germline mutations of PHOX2B in both a familial case of NB and a patient with the HSCR-NB association. PHOX2B, therefore, stands as the first gene for which germline mutations predispose to NB.