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

Proceedings of the fourth international conference on central hypoventilation

Central hypoventilation syndromes (CHS) are rare diseases of central autonomic respiratory control associated with autonomous nervous dysfunction. Severe central hypoventilation is the hallmark and the most life-threatening feature. CHS is a group of not-fully defined disorders. Congenital CHS (CCHS) (ORPHA661) is clinically and genetically well-characterized, with the disease-causing gene identified in 2003. CCHS presents at birth in most cases, and associated with Hirschsprung's disease (ORPHA99803) and neural crest tumours in 20% and 5% of cases, respectively. The incidence of CCHS is estimated to be 1 of 200,000 live births in France, yet remains unknown for the rest of the world. In contrast, late-onset CHS includes a group of not yet fully delineated diseases. Overlap with CCHS is likely, as a subset of patients harbours PHOX2B mutations. Another subset of patients present with associated hypothalamic dysfunction. The number of these patients is unknown (less than 60 cases reported worldwide). Treatment of CHS is palliative using advanced techniques of ventilation support during lifetime. Research is ongoing to better understand physiopathological mechanisms and identify potential treatment pathways.The Fourth International Conference on Central Hypoventilation was organised in Warsaw, Poland, April 13-15, 2012, under the patronage of the European Agency for Health and Consumers and Public Health European Agency of European Community. The conference provided a state-of-the-art update of knowledge on all the genetic, molecular, cellular, and clinical aspects of these rare diseases.

Primate adult brain cell autotransplantation produces behavioral and biological recovery in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonian St. Kitts monkeys

The potential for "replacement cells" to restore function in Parkinson's disease has been widely reported over the past 3 decades, rejuvenating the central nervous system rather than just relieving symptoms. Most such experiments have used fetal or embryonic sources that may induce immunological rejection and generate ethical concerns. Autologous sources, in which the cells to be implanted are derived from recipients' own cells after reprogramming to stem cells, direct genetic modifications, or epigenetic modifications in culture, could eliminate many of these problems. In a previous study on autologous brain cell transplantation, we demonstrated that adult monkey brain cells, obtained from cortical biopsies and kept in culture for 7 weeks, exhibited potential as a method of brain repair after low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused dopaminergic cell death. The present study exposed monkeys to higher MPTP doses to produce significant parkinsonism and behavioral impairments. Cerebral cortical cells were biopsied from the animals, held in culture for 7 weeks to create an autologous neural cell "ecosystem" and reimplanted bilaterally into the striatum of the same six donor monkeys. These cells expressed neuroectodermal and progenitor markers such as nestin, doublecortin, GFAP, neurofilament, and vimentin. Five to six months after reimplantation, histological analysis with the dye PKH67 and unbiased stereology showed that reimplanted cells survived, migrated bilaterally throughout the striatum, and seemed to exert a neurorestorative effect. More tyrosine hydroxylase-immunoreactive neurons and significant behavioral improvement followed reimplantation of cultured autologous neural cells as a result of unknown trophic factors released by the grafts.

A Phox2b::FLPo transgenic mouse line suitable for intersectional genetics

Phox2b is a transcription factor expressed in the central and peripheral neurons that control cardiovascular, respiratory, and digestive functions and essential for their development. Several populations known or suspected to regulate visceral functions express Phox2b in the developing hindbrain. Extensive cell migration and lack of suitable markers have greatly hampered studying their development. Reasoning that intersectional fate mapping may help to overcome these impediments, we have generated a BAC transgenic mouse line, P2b::FLPo, which expresses codon-optimized FLP recombinase in Phox2b expressing cells. By partnering the P2b::FLPo with the FLP-responsive RC::Fela allele, we show that FLP recombination switches on lineage tracers in the cells that express or have expressed Phox2b, permanently marking them for study across development. Taking advantage of the dual-recombinase feature of RC::Fela, we further show that the P2b::FLPo transgene can be partnered with Lbx1(Cre) as Cre driver to generate triple transgenics in which neurons having a history of both Phox2b and Lbx1 expression are specifically labeled. Hence, the P2b::FLPo line when partnered with a suitable Cre driver provides a tool for tracking and accessing genetically subsets of Phox2b-expressing neuronal populations, which has not been possible by Cre-mediated recombination alone.

Ancient origin of somatic and visceral neurons

Background

A key to understanding the evolution of the nervous system on a large phylogenetic scale is the identification of homologous neuronal types. Here, we focus this search on the sensory and motor neurons of bilaterians, exploiting their well-defined molecular signatures in vertebrates. Sensorimotor circuits in vertebrates are of two types: somatic (that sense the environment and respond by shaping bodily motions) and visceral (that sense the interior milieu and respond by regulating vital functions). These circuits differ by a small set of largely dedicated transcriptional determinants: Brn3 is expressed in many somatic sensory neurons, first and second order (among which mechanoreceptors are uniquely marked by the Brn3+/Islet1+/Drgx+ signature), somatic motoneurons uniquely co-express Lhx3/4 and Mnx1, while the vast majority of neurons, sensory and motor, involved in respiration, blood circulation or digestion are molecularly defined by their expression and dependence on the pan-visceral determinant Phox2b.

Results

We explore the status of the sensorimotor transcriptional code of vertebrates in mollusks, a lophotrochozoa clade that provides a rich repertoire of physiologically identified neurons. In the gastropods Lymnaea stagnalis and Aplysia californica, we show that homologues of Brn3, Drgx, Islet1, Mnx1, Lhx3/4 and Phox2b differentially mark neurons with mechanoreceptive, locomotory and cardiorespiratory functions. Moreover, in the cephalopod Sepia officinalis, we show that Phox2 marks the stellate ganglion (in line with the respiratory — that is, visceral— ancestral role of the mantle, its target organ), while the anterior pedal ganglion, which controls the prehensile and locomotory arms, expresses Mnx.

Conclusions

Despite considerable divergence in overall neural architecture, a molecular underpinning for the functional allocation of neurons to interactions with the environment or to homeostasis was inherited from the urbilaterian ancestor by contemporary protostomes and deuterostomes.

Phox2b expression in the taste centers of fish

The homeodomain transcription factor Phox2b controls the formation of the sensory-motor reflex circuits of the viscera in vertebrates. Among Phox2b-dependent structures characterized in rodents is the nucleus of the solitary tract, the first relay for visceral sensory input, including taste. Here we show that Phox2b is expressed throughout the primary taste centers of two cyprinid fish, Danio rerio and Carassius auratus, i.e., in their vagal, glossopharyngeal, and facial lobes, providing the first molecular evidence for their homology with the nucleus of the solitary tract of mammals and suggesting that a single ancestral Phox2b-positive neuronal type evolved to give rise to both fish and mammalian structures. In zebrafish larvae, the distribution of Phox2b²⁺ neurons, combined with the expression pattern of Olig4 (a homologue of Olig3, determinant of the nucleus of the solitary tract in mice), reveals that the superficial position and sheet-like architecture of the viscerosensory column in cyprinid fish, ideally suited for the somatotopic representation of oropharyngeal and bodily surfaces, arise by radial migration from a dorsal progenitor domain, in contrast to the tangential migration observed in amniotes.

Autonomic neurocristopathy-associated mutations in PHOX2B dysregulate Sox10 expression

The most common forms of neurocristopathy in the autonomic nervous system are Hirschsprung disease (HSCR), resulting in congenital loss of enteric ganglia, and neuroblastoma (NB), childhood tumors originating from the sympathetic ganglia and adrenal medulla. The risk for these diseases dramatically increases in patients with congenital central hypoventilation syndrome (CCHS) harboring a nonpolyalanine repeat expansion mutation of the Paired-like homeobox 2b (PHOX2B) gene, but the molecular mechanism of pathogenesis remains unknown. We found that introducing nonpolyalanine repeat expansion mutation of the PHOX2B into the mouse Phox2b locus recapitulates the clinical features of the CCHS associated with HSCR and NB. In mutant embryos, enteric and sympathetic ganglion progenitors showed sustained sex-determining region Y (SRY) box10 (Sox10) expression, with impaired proliferation and biased differentiation toward the glial lineage. Nonpolyalanine repeat expansion mutation of PHOX2B reduced transactivation of wild-type PHOX2B on its known target, dopamine β-hydroxylase (DBH), in a dominant-negative fashion. Moreover, the introduced mutation converted the transcriptional effect of PHOX2B on a Sox10 enhancer from repression to transactivation. Collectively, these data reveal that nonpolyalanine repeat expansion mutation of PHOX2B is both a dominant-negative and gain-of-function mutation. Our results also demonstrate that Sox10 regulation by PHOX2B is pivotal for the development and pathogenesis of the autonomic ganglia.

Short-term effects of unilateral lesion of the primary motor cortex (M1) on ipsilesional hand dexterity in adult macaque monkeys

Although the arrangement of the corticospinal projection in primates is consistent with a more prominent role of the ipsilateral motor cortex on proximal muscles, rather than on distal muscles involved in manual dexterity, the role played by the primary motor cortex on the control of manual dexterity for the ipsilateral hand remains a matter a debate, either in the normal function or after a lesion. We, therefore, tested the impact of permanent unilateral motor cortex lesion on the manual dexterity of the ipsilateral hand in 11 macaque monkeys, within a time window of 60 days post-lesion. For comparison, unilateral reversible pharmacological inactivation of the motor cortex was produced in an additional monkey. Manual dexterity was assessed quantitatively based on three motor parameters derived from two reach and grasp manual tasks. In contrast to the expected dramatic, complete deficit of manual dexterity of the contralesional hand that persists for several weeks, the impact on the manual dexterity of the ipsilesional hand was generally moderate (but statistically significant) and, when present, lasted less than 20 days. Out of the 11 monkeys, only 3 showed a deficit of the ipsilesional hand for 2 of the 3 motor parameters, and 4 animals had a deficit for only one motor parameter. Four monkeys did not show any deficit. The reversible inactivation experiment yielded results consistent with the permanent lesion data. In conclusion, the primary motor cortex exerts a modest role on ipsilateral manual dexterity, most likely in the form of indirect hand postural control.

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.

Central chemoreception: lessons from mouse and human genetics

The response to increased P(CO(2)) in the brain is an essential drive to breathe and required for CO(2) and pH homeostasis in the blood, but where and how CO(2) is sensed are still contentious issues. Here, we review evidence from mouse and human genetics that argue for the crucial role in CO(2) chemosensitivity of a limited set of central neurons that express the Phox2b transcription factor and are disabled by Phox2b mutations. A common trait of different Phox2b mutations that impair CO(2) responsiveness in the embryo and respiration in neonates is the depletion of Phox2b-expressing neurons in the retrotrapezoid nucleus, providing genetic evidence for their importance for proper breathing and central chemosensitivity at birth.

Cell locations for AQP1, AQP4 and 9 in the non-human primate brain

The presence of three water channels (aquaporins, AQP), AQP1, AQP4 and AQP9 were observed in normal brain and several rodent models of brain pathologies. Little is known about AQP distribution in the primate brain and its knowledge will be useful for future testing of drugs aimed at preventing brain edema formation. We studied the expression and cellular distribution of AQP1, 4 and 9 in the non-human primate brain. The distribution of AQP4 in the non-human primate brain was observed in perivascular astrocytes, comparable to the observation made in the rodent brain. In contrast with rodent, primate AQP1 is expressed in the processes and perivascular endfeet of a subtype of astrocytes mainly located in the white matter and the glia limitans, possibly involved in water homeostasis. AQP1 was also observed in neurons innervating the pial blood vessels, suggesting a possible role in cerebral blood flow regulation. As described in rodent, AQP9 mRNA and protein were detected in astrocytes and in catecholaminergic neurons. However additional locations were observed for AQP9 in populations of neurons located in several cortical areas of primate brains. This report describes a detailed study of AQP1, 4 and 9 distributions in the non-human primate brain, which adds to the data already published in rodent brains. This relevant species differences have to be considered carefully to assess potential drugs acting on AQPs non-human primate models before entering human clinical trials.

Effects of Unilateral Motor Cortex Lesion on Ipsilesional Hand's Reach and Grasp Performance in Monkeys: Relationship With Recovery in the Contralesional Hand

Manual dexterity, a prerogative of primates, is under the control of the corticospinal (CS) tract. Because 90–95% of CS axons decussate, it is assumed that this control is exerted essentially on the contralateral hand. Consistently, unilateral lesion of the hand representation in the motor cortex is followed by a complete loss of dexterity of the contralesional hand. During the months following lesion, spontaneous recovery of manual dexterity takes place to a highly variable extent across subjects, although largely incomplete. In the present study, we tested the hypothesis that after a significant postlesion period, manual performance in the ipsilesional hand is correlated with the extent of functional recovery in the contralesional hand. To this aim, ten adult macaque monkeys were subjected to permanent unilateral motor cortex lesion. Monkeys' manual performance was assessed for each hand during several months postlesion, using our standard behavioral test (modified Brinkman board task) that provides a quantitative measure of reach and grasp ability. The ipsilesional hand's performance was found to be significantly enhanced over the long term (100–300 days postlesion) in six of ten monkeys, with the six exhibiting the best, though incomplete, recovery of the contralesional hand. There was a statistically significant correlation ( r = 0.932; P < 0.001) between performance in the ipsilesional hand after significant postlesion period and the extent of recovery of the contralesional hand. This observation is interpreted in terms of different possible mechanisms of recovery, dependent on the recruitment of motor areas in the lesioned and/or intact hemispheres.

Glycogen metabolism as a marker of astrocyte differentiation

Glycogen is a hallmark of mature astrocytes, but its emergence during astrocytic differentiation is unclear. Differentiation of E14 mouse neurospheres into astrocytes was induced with fetal bovine serum (FBS), Leukemia Inhibitory Factor (LIF), or Ciliary Neurotrophic Factor (CNTF). Cytochemical and enzymatic analyses showed that glycogen is present in FBS- or LIF- but not in CNTF-differentiated astrocytes. Glycogenolysis was induced in FBS- and LIF-differentiated astrocytes but glycogen resynthesis was observed only with FBS. Protein targeting to glycogen mRNA expression appeared with glial fibrillary acidic protein and S100beta in FBS and LIF conditions but not with CNTF. These results show that glycogen metabolism constitutes a useful marker of astrocyte differentiation.

[Painful knee: choosing the right imaging test]

The diagnosis of non-traumatic knee complaints relies on history taking and clinical examination, joint fluid analysis and imaging tests. This paper aims at reminding the benefits of conventional radiography and clarifying the role of ultrasound and magnetic resonance imaging based on illustrated case reports.

Breathing with phox2b

In the last few years, elucidation of the architecture of breathing control centres has reached the cellular level. This has been facilitated by increasing knowledge of the molecular signatures of various classes of hindbrain neurons. Here, we review the advances achieved by studying the homeodomain factor Phox2b, a transcriptional determinant of neuronal identity in the central and peripheral nervous systems. Evidence from human genetics, neurophysiology and mouse reverse genetics converges to implicate a small population of Phox2b-dependent neurons, located in the retrotrapezoid nucleus, in the detection of CO(2), which is a paramount source of the 'drive to breathe'. Moreover, the same and other studies suggest that an overlapping or identical neuronal population, the parafacial respiratory group, might contribute to the respiratory rhythm at least in some circumstances, such as for the initiation of breathing following birth. Together with the previously established Phox2b dependency of other respiratory neurons (which we review briefly here), our new data highlight a key role of this transcription factor in setting up the circuits for breathing automaticity.

In Vitro studies of non poly alanine PHOX2B mutations argue against a loss-of-function mechanism for congenital central hypoventilation

A wide range of autonomic dysfunctions, i.e. Central Hypoventilation Syndromes, Hirschsprung disease and Tumours of the Sympathetic Nervous System have been ascribed to heterozygous PHOX2B mutations in man. The PHOX2B mutations reported include polyalanine expansions in a 20 alanines tract, missense, frameshift mutations and nonsense mutation. Some genotype/phenotype correlations have been drawn, but the molecular mechanism(s) underlying them remain(s) unclear. So far, loss-of-function, gain-of-function and dominant negative effects have been proposed as disease-causing mechanisms for polyalanine expansions. Indeed, mutant with an expanded polyalanine tract result in decreased transactivation of known target genes and protein misfolding leading to oligomerisation in vitro for all expansions and to cytoplasmic protein aggregation for longer expansions. We extended the molecular studies to other non-polyalanine expansion mutations and show that most PHOX2B protein mutants oligomerize even in the absence of the normal 20 alanines tract. Conversely, a premature stop codon mutation in a CHS patient leads to the production of an N-terminally truncated protein by re-initiation of translation that does not form oligomers. Therefore, PHOX2B misfolding is not the only mechanism leading to dysfunction of the ventilatory autonomic system.

Primate adult brain cell autotransplantation, a pilot study in asymptomatic MPTP-treated monkeys

Autologous brain cell transplantation might be useful for repairing lesions and restoring function of the central nervous system. We have demonstrated that adult monkey brain cells, obtained from cortical biopsy and kept in culture for a few weeks, exhibit neural progenitor characteristics that make them useful for brain repair. Following MPTP treatment, primates were dopamine depleted but asymptomatic. Autologous cultured cells were reimplanted into the right caudate nucleus of the donor monkey. Four months after reimplantation, histological analysis by stereology and TH immunolabeling showed that the reimplanted cells successfully survived, bilaterally migrated in the whole striatum, and seemed to have a neuroprotection effect over time. These results may add a new strategy to the field of brain neuroprotection or regeneration and could possibly lead to future clinical applications.

Protective role of early aquaporin 4 induction against postischemic edema formation

Aquaporin 4 (AQP4) is a water channel involved in water movements across the cell membrane and is spatially organized on the cell surface in orthogonal array particles (OAPs). Its role in edema formation or resolution after stroke onset has been studied mainly at late time points. We have shown recently that its expression is rapidly induced after ischemia coinciding in time with an early swelling of the ischemic hemisphere. There are two isoforms of AQP4: AQP4-M1 and AQP4-M23. The ratio of these isoforms influences the size of the OAPs but the functional impact is not known. The role of the early induction of AQP4 is not yet known. Thrombin preconditioning in mice provides a useful model to study endogenous protective mechanisms. Using this model, we provide evidence for the first time that the early induction of AQP4 may contribute to limit the formation of edema and that the AQP4-M1 isoform is predominantly induced in the ischemic tissue at this time point. Although it prevents edema formation, the early induction of the AQP4 expression does not prevent the blood-brain barrier disruption, suggesting an effect limited to the prevention of edema formation possibly by removing of water from the tissue.

Regulator of G protein signaling-4 controls fatty acid and glucose homeostasis

Circulating free fatty acids are a reflection of the balance between lipogenesis and lipolysis that takes place mainly in adipose tissue. We found that mice deficient for regulator of G protein signaling (RGS)-4 have increased circulating catecholamines, and increased free fatty acids. Consequently, RGS4-/- mice have increased concentration of circulating free fatty acids; abnormally accumulate fatty acids in liver, resulting in liver steatosis; and show a higher degree of glucose intolerance and decreased insulin secretion in pancreas. We show in this study that RGS4 controls adipose tissue lipolysis through regulation of the secretion of catecholamines by adrenal glands. RGS4 controls the balance between adipose tissue lipolysis and lipogenesis, secondary to its role in the regulation of catecholamine secretion by adrenal glands. RGS4 therefore could be a good target for the treatment of metabolic diseases.