Phox2b, congenital central hypoventilation syndrome and the control of respiration

Knockdown of PHOX2B in the retrotrapezoid nucleus reduces the central CO2 chemoreflex in rats

PHOX2B is a transcription factor essential for the development of different classes of neurons in the central and peripheral nervous system. Heterozygous mutations in the PHOX2B coding region are responsible for the occurrence of Congenital Central Hypoventilation Syndrome (CCHS), a rare neurological disorder characterised by inadequate chemosensitivity and life-threatening sleep-related hypoventilation. Animal studies suggest that chemoreflex defects are caused in part by the improper development or function of PHOX2B expressing neurons in the retrotrapezoid nucleus (RTN), a central hub for CO2 chemosensitivity. Although the function of PHOX2B in rodents during development is well established, its role in the adult respiratory network remains unknown. In this study, we investigated whether reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the RTN altered respiratory function. Four weeks following local RTN injection of a lentiviral vector expressing the short hairpin RNA (shRNA) targeting Phox2b mRNA, a reduction of PHOX2B expression was observed in Nmb neurons compared to both naive rats and rats injected with the non-target shRNA. PHOX2B knockdown did not affect breathing in room air or under hypoxia, but ventilation was significantly impaired during hypercapnia. PHOX2B knockdown did not alter Nmb expression but it was associated with reduced expression of both Task2 and Gpr4, two CO2/pH sensors in the RTN. We conclude that PHOX2B in the adult brain has an important role in CO2 chemoreception and reduced PHOX2B expression in CCHS beyond the developmental period may contribute to the impaired central chemoreflex function.

Organoid models of breathing disorders reveal patterning defect of hindbrain neurons caused by PHOX2B-PARMs

Retrotrapezoid nucleus (RTN) neurons in the brainstem regulate the ventilatory response to hypercarbia. It is unclear how PHOX2B-polyalanine repeat mutations (PHOX2B-PARMs) alter the function of PHOX2B and perturb the formation of RTN neurons. Here, we generated human brainstem organoids (HBSOs) with RTN-like neurons from human pluripotent stem cells. Single-cell transcriptomics revealed that expression of PHOX2B+7Ala PARM alters the differentiation trajectories of the hindbrain neurons and hampers the formation of the RTN-like neurons in HBSOs. With the unguided cerebral organoids (HCOs), PHOX2B+7Ala PARM interrupted the patterning of PHOX2B+ neurons with dysregulation of Hedgehog pathway and HOX genes. With complementary use of HBSOs and HCOs with a patient and two mutant induced pluripotent stem cell lines carrying different polyalanine repetition in PHOX2B, we further defined the association between the length of polyalanine repetition and malformation of RTN-respiratory center and demonstrated the potential toxic gain of function of PHOX2B-PARMs, highlighting the uniqueness of these organoid models for disease modeling.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

Rostral ventrolateral medulla, retropontine region and autonomic regulations

The rostral half of the ventrolateral medulla (RVLM) and adjacent ventrolateral retropontine region (henceforth RVLMRP) have been divided into various sectors by neuroscientists interested in breathing or autonomic regulations. The RVLMRP regulates respiration, glycemia, vigilance and inflammation, in addition to blood pressure. It contains interoceptors that respond to acidification, hypoxia and intracranial pressure and its rostral end contains the retrotrapezoid nucleus (RTN) which is the main central respiratory chemoreceptor. Acid detection by the RTN is an intrinsic property of the principal neurons that is enhanced by paracrine influences from surrounding astrocytes and CO₂-dependent vascular constriction. RTN mediates the hypercapnic ventilatory response via complex projections to the respiratory pattern generator (CPG). The RVLM contributes to autonomic response patterns via differential recruitment of several subtypes of adrenergic (C1) and non-adrenergic neurons that directly innervate sympathetic and parasympathetic preganglionic neurons. The RVLM also innervates many brainstem and hypothalamic nuclei that contribute, albeit less directly, to autonomic responses. All lower brainstem noradrenergic clusters including the locus coeruleus are among these targets. Sympathetic tone to the circulatory system is regulated by subsets of presympathetic RVLM neurons whose activity is continuously restrained by the baroreceptors and modulated by the respiratory CPG. The inhibitory input from baroreceptors and the excitatory input from the respiratory CPG originate from neurons located in or close to the rhythm generating region of the respiratory CPG (preBötzinger complex).

Chemoreceptor mechanisms regulating CO2 -induced arousal from sleep

Arousal from sleep in response to CO₂ is a life-preserving reflex that enhances ventilatory drive and facilitates behavioural adaptations to restore eupnoeic breathing. Recurrent activation of the CO₂ -arousal reflex is associated with sleep disruption in obstructive sleep apnoea. In this review we examine the role of chemoreceptors in the carotid bodies, the retrotrapezoid nucleus and serotonergic neurons in the dorsal raphe in the CO₂ -arousal reflex. We also provide an overview of the supra-medullary structures that mediate CO₂ -induced arousal. We propose a framework for the CO₂ -arousal reflex in which the activity of the chemoreceptors converges in the parabrachial nucleus to trigger cortical arousal.

Ablation of Zfhx4 results in early postnatal lethality by disrupting the respiratory center in mice

Breathing is an integrated motor behavior that is driven and controlled by a network of brainstem neurons. Zfhx4 is a zinc finger transcription factor and our results showed that it was specifically expressed in several regions of the mouse brainstem. Mice lacking Zfhx4 died shortly after birth from an apparent inability to initiate respiration. We also found that the electrical rhythm of brainstem‒spinal cord preparations was significantly depressed in Zfhx4-null mice compared to wild-type mice. Immunofluorescence staining revealed that Zfhx4 was coexpressed with Phox2b and Math1 in the brainstem and that Zfhx4 ablation greatly decreased the expression of these proteins, especially in the retrotrapezoid nucleus. Combined ChIP‒seq and mRNA expression microarray analysis identified Phox2b as the direct downstream target gene of Zfhx4, and this finding was validated by ChIP‒qPCR. Previous studies have reported that both Phox2b and Math1 play key roles in the development of the respiratory center, and Phox2b and Math1 knockout mice are neonatal lethal due to severe central apnea. On top of this, our study revealed that Zfhx4 is a critical regulator of Phox2b expression and essential for perinatal breathing.

Breathing regulation and blood gas homeostasis after near complete lesions of the retrotrapezoid nucleus in adult rats

KEY POINTS

The retrotrapezoid nucleus (RTN) drives breathing proportionally to brain PCO2 but its role during various states of vigilance needs clarification. Under normoxia, RTN lesions increased the arterial PCO2 set-point, lowered the PO2 set-point and reduced alveolar ventilation relative to CO₂ production. Tidal volume was reduced and breathing frequency increased to a comparable degree during wake, slow-wave sleep and REM sleep. RTN lesions did not produce apnoeas or disordered breathing during sleep. RTN lesions in rats virtually eliminated the central respiratory chemoreflex (CRC) while preserving the cardiorespiratory responses to hypoxia; the relationship between CRC and number of surviving RTN Nmb neurons was an inverse exponential. The CRC does not function without the RTN. In the quasi-complete absence of the RTN and CRC, alveolar ventilation is reduced despite an increased drive to breathe from the carotid bodies.

ABSTRACT

The retrotrapezoid nucleus (RTN) is one of several CNS nuclei that contribute, in various capacities (e.g. CO₂ detection, neuronal modulation) to the central respiratory chemoreflex (CRC). Here we test how important the RTN is to PCO₂ homeostasis and breathing during sleep or wake. RTN Nmb-positive neurons were killed with targeted microinjections of substance P-saporin conjugate in adult rats. Under normoxia, rats with large RTN lesions (92 ± 4% cell loss) had normal blood pressure and arterial pH but were hypoxic (-8 mmHg PaO₂ ) and hypercapnic (+10 mmHg ). In resting conditions, minute volume (V_E ) was normal but breathing frequency (f_R ) was elevated and tidal volume (V_T ) reduced. Resting O₂ consumption and CO₂ production were normal. The hypercapnic ventilatory reflex in 65% FiO₂ had an inverse exponential relationship with the number of surviving RTN neurons and was decreased by up to 92%. The hypoxic ventilatory reflex (HVR; FiO₂ 21-10%) persisted after RTN lesions, hypoxia-induced sighing was normal and hypoxia-induced hypotension was reduced. In rats with RTN lesions, breathing was lowest during slow-wave sleep, especially under hyperoxia, but apnoeas and sleep-disordered breathing were not observed. In conclusion, near complete RTN destruction in rats virtually eliminates the CRC but the HVR persists and sighing and the state dependence of breathing are unchanged. Under normoxia, RTN lesions cause no change in V_E but alveolar ventilation is reduced by at least 21%, probably because of increased physiological dead volume. RTN lesions do not cause sleep apnoea during slow-wave sleep, even under hyperoxia.

The Retrotrapezoid Nucleus: Central Chemoreceptor and Regulator of Breathing Automaticity

The ventral surface of the rostral medulla oblongata has been suspected since the 1960s to harbor central respiratory chemoreceptors [i.e., acid-activated neurons that regulate breathing to maintain a constant arterial PCO₂ (PaCO₂)]. The key neurons, a.k.a. the retrotrapezoid nucleus (RTN), have now been identified. In this review we describe their transcriptome, developmental lineage, and anatomical projections. We also review their contribution to CO₂ homeostasis and to the regulation of breathing automaticity during sleep and wake. Finally, we discuss several mechanisms that contribute to the activation of RTN neurons by CO₂in vivo: cell-autonomous effects of protons; paracrine effects of pH mediated by surrounding astrocytes and blood vessels; and excitatory inputs from other CO₂-responsive CNS neurons.

Chemosensitivity of Phox2b-expressing retrotrapezoid neurons is mediated in part by input from 5-HT neurons

KEY POINTS

Neurons of the retrotrapezoid nucleus (RTN) and medullary serotonin (5-HT) neurons are both candidates for central CO2 /pH chemoreceptors, but it is not known how interactions between them influence their responses to pH. We found that RTN neurons in brain slices were stimulated by exogenous 5-HT and by heteroexchange release of endogenous 5-HT, and these responses were blocked by antagonists of 5-HT7 receptors. The pH response of RTN neurons in brain slices was markedly reduced by the same antagonists of 5-HT7 receptors. Similar results were obtained in dissociated, primary cell cultures prepared from the ventral medulla, where it was also found that the pH response of RTN neurons was blocked by preventing 5-HT synthesis and enhanced by blocking 5-HT reuptake. Exogenous 5-HT did not enable latent intrinsic RTN chemosensitivity. RTN neurons may play more of a role as relays from other central and peripheral chemoreceptors than as CO2 sensors.

ABSTRACT

Phox2b-expressing neurons in the retrotrapezoid nucleus (RTN) and serotonin (5-HT) neurons in the medullary raphe have both been proposed to be central respiratory chemoreceptors. How interactions between these two sets of neurons influence their responses to acidosis is not known. Here we recorded from mouse Phox2b+ RTN neurons in brain slices, and found that their response to moderate hypercapnic acidosis (pH 7.4 to ∼7.2) was markedly reduced by antagonists of 5-HT7 receptors. RTN neurons were stimulated in response to heteroexchange release of 5-HT, indicating that RTN neurons are sensitive to endogenous 5-HT. This electrophysiological behaviour was replicated in primary, dissociated cell cultures containing 5-HT and RTN neurons grown together. In addition, pharmacological inhibition of 5-HT synthesis in culture reduced RTN neuron chemosensitivity, and blocking 5-HT reuptake enhanced chemosensitivity. The effect of 5-HT on RTN neuron chemosensitivity was not explained by a mechanism whereby activation of 5-HT7 receptors enables or potentiates intrinsic chemosensitivity of RTN neurons, as exogenous 5-HT did not enhance the pH response. The ventilatory response to inhaled CO2 of mice was markedly decreased in vivo after systemic treatment with ketanserin, an antagonist of 5-HT2 and 5-HT7 receptors. These data indicate that 5-HT and RTN neurons may interact synergistically in a way that enhances the respiratory chemoreceptor response. The primary role of RTN neurons may be as relays and amplifiers of the pH response from 5-HT neurons and other chemoreceptors rather than as pH sensors themselves.

Mutation in LBX1/Lbx1 precludes transcription factor cooperativity and causes congenital hypoventilation in humans and mice

Maintaining low CO₂ levels in our bodies is critical for life and depends on neurons that generate the respiratory rhythm and monitor tissue gas levels. Inadequate response to increasing levels of CO₂ is common in congenital hypoventilation diseases. Here, we identified a mutation in LBX1, a homeodomain transcription factor, that causes congenital hypoventilation in humans. The mutation alters the C terminus of the protein without disturbing its DNA-binding domain. Mouse models carrying an analogous mutation recapitulate the disease. The mutation spares most Lbx1 functions, but selectively affects development of a small group of neurons central in respiration. Our work reveals a very unusual pathomechanism, a mutation that hampers a small subset of functions carried out by a transcription factor.

The respiratory rhythm is generated by the preBötzinger complex in the medulla oblongata, and is modulated by neurons in the retrotrapezoid nucleus (RTN), which are essential for accelerating respiration in response to high CO₂. Here we identify a LBX1 frameshift (LBX1^FS) mutation in patients with congenital central hypoventilation. The mutation alters the C-terminal but not the DNA-binding domain of LBX1. Mice with the analogous mutation recapitulate the breathing deficits found in humans. Furthermore, the mutation only interferes with a small subset of Lbx1 functions, and in particular with development of RTN neurons that coexpress Lbx1 and Phox2b. Genome-wide analyses in a cell culture model show that Lbx1^FS and wild-type Lbx1 proteins are mostly bound to similar sites, but that Lbx1^FS is unable to cooperate with Phox2b. Thus, our analyses on Lbx1^FS (dys)function reveals an unusual pathomechanism; that is, a mutation that selectively interferes with the ability of Lbx1 to cooperate with Phox2b, and thus impairs the development of a small subpopulation of neurons essential for respiratory control.

Structural and functional differences in PHOX2B frameshift mutations underlie isolated or syndromic congenital central hypoventilation syndrome

Heterozygous mutations in the PHOX2B gene are causative of congenital central hypoventilation syndrome (CCHS), a neurocristopathy characterized by defective autonomic control of breathing due to the impaired differentiation of neural crest cells. Among PHOX2B mutations, polyalanine (polyAla) expansions are almost exclusively associated with isolated CCHS, whereas frameshift variants, although less frequent, are often more severe than polyAla expansions and identified in syndromic CCHS. This article provides a complete review of all the frameshift mutations identified in cases of isolated and syndromic CCHS reported in the literature as well as those identified by us and not yet published. These were considered in terms of both their structure, whether the underlying indels induced frameshifts of either 1 or 2 steps ("frame 2" and "frame 3" mutations respectively), and clinical associations. Furthermore, we evaluated the structural and functional effects of one "frame 3" mutation identified in a patient with isolated CCHS, and one "frame 2" mutation identified in a patient with syndromic CCHS, also affected with Hirschsprung's disease and neuroblastoma. The data thus obtained confirm that the type of translational frame affects the severity of the transcriptional dysfunction and the predisposition to isolated or syndromic CCHS.

Neuromedin B Expression Defines the Mouse Retrotrapezoid Nucleus

The retrotrapezoid nucleus (RTN) consists, by definition, of Phox2b-expressing, glutamatergic, non-catecholaminergic, noncholinergic neurons located in the parafacial region of the medulla oblongata. An unknown proportion of RTN neurons are central respiratory chemoreceptors and there is mounting evidence for biochemical diversity among these cells. Here, we used multiplexed in situ hybridization and single-cell RNA-Seq in male and female mice to provide a more comprehensive view of the phenotypic diversity of RTN neurons. We now demonstrate that the RTN of mice can be identified with a single and specific marker, Neuromedin B mRNA (Nmb). Most (∼75%) RTN neurons express low-to-moderate levels of Nmb and display chemoreceptor properties. Namely they are activated by hypercapnia, but not by hypoxia, and express proton sensors, TASK-2 and Gpr4. These Nmb-low RTN neurons also express varying levels of transcripts for Gal, Penk, and Adcyap1, and receptors for substance P, orexin, serotonin, and ATP. A subset of RTN neurons (∼20-25%), typically larger than average, express very high levels of Nmb mRNA. These Nmb-high RTN neurons do not express Fos after hypercapnia and have low-to-undetectable levels of Kcnk5 or Gpr4 transcripts; they also express Adcyap1, but are essentially devoid of Penk and Gal transcripts. In male rats, Nmb is also a marker of the RTN but, unlike in mice, this gene is expressed by other types of nearby neurons located within the ventromedial medulla. In sum, Nmb is a selective marker of the RTN in rodents; Nmb-low neurons, the vast majority, are central respiratory chemoreceptors, whereas Nmb-high neurons likely have other functions.SIGNIFICANCE STATEMENT Central respiratory chemoreceptors regulate arterial PCO₂ by adjusting lung ventilation. Such cells have recently been identified within the retrotrapezoid nucleus (RTN), a brainstem nucleus defined by genetic lineage and a cumbersome combination of markers. Using single-cell RNA-Seq and multiplexed in situ hybridization, we show here that a single marker, Neuromedin B mRNA (Nmb), identifies RTN neurons in rodents. We also suggest that >75% of these Nmb neurons are chemoreceptors because they are strongly activated by hypercapnia and express high levels of proton sensors (Kcnk5 and Gpr4). The other RTN neurons express very high levels of Nmb, but low levels of Kcnk5/Gpr4/pre-pro-galanin/pre-pro-enkephalin, and do not respond to hypercapnia. Their function is unknown.

Chemosensitive Phox2b-expressing neurons are crucial for hypercapnic ventilatory response in the nucleus tractus solitarius

KEY POINTS

Central hypercapnic hypoventilation is highly prevalent in children suffering from congenital central hypoventilation syndrome (CCHS). Mutations of the gene for paired-like homeobox 2b (Phox2b) are aetiologically associated with CCHS and Phox2b is present in central components of respiratory chemoreflex, such as the nucleus tractus solitarius (NTS). Injection of the neurotoxin substance P-saporin into NTS destroys Phox2b-expressing neurons. Impaired hypercapnic ventilatory response caused by this neurotoxin is attributable to a loss of CO₂ -sensitive Phox2b-expressing NTS neurons. A subgroup of Phox2b-expressing neurons exhibits intrinsic chemosensitivity. A background K⁺ channel-like current is partially responsible for such chemosensitivity in Phox2b-expressing neurons. The present study helps us better understand the mechanism of respiratory deficits in CCHS and potentially locates a brainstem site for development of precise clinical intervention.

ABSTRACT

The nucleus tractus solitarius (NTS) neurons have been considered to function as central respiratory chemoreceptors. However, the common molecular marker defined for these neurons remains unknown. The present study investigated whether paired-like homeobox 2b (Phox2b)-expressing NTS neurons are recruited in hypercapnic ventilatory response (HCVR) and whether these neurons exhibit intrinsic chemosensitivity. HCVR was assessed using whole body plethysmography and neuronal chemosensitivity was examined by patch clamp recordings in brainstem slices or dissociated neurons from Phox2b-EGFP transgenic mice. Injection of the neurotoxin substance P-saporin (SSP-SAP) into NTS destroyed Phox2b-expressing neurons. Minute ventilation and tidal volume were both reduced by 13% during exposure to 8% CO₂ in inspired air when ∼13% of the Phox2b-expressing neurons were eliminated. However, a loss of ∼18% of these neurons was associated with considerable decreases in minute ventilation by ≥18% and in tidal volume by≥22% when challenged by ≥4% CO₂ . In both cases, breathing frequency was unaffected. Most CO₂ -activated neurons were immunoreactive to Phox2b. In brainstem slices, ∼43% of Phox2b-expressing neurons from Phox2b-EGFP mice displayed a sustained or transient increase in firing rate during physiological acidification (pH 7.0 or 8% CO₂ ). Such a response was also present in dissociated neurons in favour of an intrinsic property. In voltage clamp recordings, a background K⁺ channel-like current was found in a subgroup of Phox2b-expressing neurons. Thus, the respiratory deficits caused by injection of SSP-SAP into the NTS are attributable to proportional lesions of CO₂ /H⁺ -sensitive Phox2b-expressing neurons.

Nonsense pathogenic variants in exon 1 of PHOX2B lead to translational reinitiation in congenital central hypoventilation syndrome

Pathogenic variants in PHOX2B lead to congenital central hypoventilation syndrome (CCHS), a rare disorder of the nervous system characterized by autonomic dysregulation and hypoventilation typically presenting in the neonatal period, although a milder late-onset (LO) presentation has been reported. More than 90% of cases are caused by polyalanine repeat mutations (PARMs) in the C-terminus of the protein; however non-polyalanine repeat mutations (NPARMs) have been reported. Most NPARMs are located in exon 3 of PHOX2B and result in a more severe clinical presentation including Hirschsprung disease (HSCR) and/or peripheral neuroblastic tumors (PNTs). A previously reported nonsense pathogenic variant in exon 1 of a patient with LO-CCHS and no HSCR or PNTs leads to translational reinitiation at a downstream AUG codon producing an N-terminally truncated protein. Here we report additional individuals with nonsense pathogenic variants in exon 1 of PHOX2B. In vitro analyses were used to determine if these and other reported nonsense variants in PHOX2B exon 1 produced N-terminally truncated proteins. We found that all tested nonsense variants in PHOX2B exon 1 produced a truncated protein of the same size. This truncated protein localized to the nucleus and transactivated a target promoter. These data suggest that nonsense pathogenic variants in the first exon of PHOX2B likely escape nonsense mediated decay (NMD) and produce N-terminally truncated proteins functionally distinct from those produced by the more common PARMs.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Neural Control of Breathing and CO2 Homeostasis

Recent advances have clarified how the brain detects CO2 to regulate breathing (central respiratory chemoreception). These mechanisms are reviewed and their significance is presented in the general context of CO2/pH homeostasis through breathing. At rest, respiratory chemoreflexes initiated at peripheral and central sites mediate rapid stabilization of arterial PCO2 and pH. Specific brainstem neurons (e.g., retrotrapezoid nucleus, RTN; serotonergic) are activated by PCO2 and stimulate breathing. RTN neurons detect CO2 via intrinsic proton receptors (TASK-2, GPR4), synaptic input from peripheral chemoreceptors and signals from astrocytes. Respiratory chemoreflexes are arousal state dependent whereas chemoreceptor stimulation produces arousal. When abnormal, these interactions lead to sleep-disordered breathing. During exercise, central command and reflexes from exercising muscles produce the breathing stimulation required to maintain arterial PCO2 and pH despite elevated metabolic activity. The neural circuits underlying central command and muscle afferent control of breathing remain elusive and represent a fertile area for future investigation.

CO₂ in the spotlight

Optogenetic techniques have revealed that retrotrapezoid neurons are essential for sensitivity to carbon dioxide.

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

Emotional disorders in adult mice heterozygous for the transcription factor Phox2b

Phox2b is an essential transcription factor for the development of the autonomic nervous system. Mice carrying one invalidated Phox2b allele (Phox2b(+/-)) show mild autonomic disorders including sleep apneas, and impairments in chemosensitivity and thermoregulation that recover within 10days of postnatal age. Because Phox2b is not expressed above the pons nor in the cerebellum, this mutation is not expected to affect brain development and cognitive functioning directly. However, the transient physiological disorders in Phox2b(+/-) mice might impair neurodevelopment. To examine this possibility, we conducted a behavioral test battery of emotional, motor, and cognitive functioning in adult Phox2b(+/-) mice and their wildtype littermates (Phox2b(+/+)). Adult Phox2b(+/-) mice showed altered exploratory behavior in the open field and in the elevated plus maze, both indicative of anxiety. Phox2b(+/-) mice did not show cognitive or motor impairments. These results suggest that also mild autonomic control deficits may disturb long-term emotional development.

Inactivity-induced respiratory plasticity: protecting the drive to breathe in disorders that reduce respiratory neural activity

Multiple forms of plasticity are activated following reduced respiratory neural activity. For example, in ventilated rats, a central neural apnea elicits a rebound increase in phrenic and hypoglossal burst amplitude upon resumption of respiratory neural activity, forms of plasticity called inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF), respectively. Here, we provide a conceptual framework for plasticity following reduced respiratory neural activity to guide future investigations. We review mechanisms giving rise to iPMF and iHMF, present new data suggesting that inactivity-induced plasticity is observed in inspiratory intercostals (iIMF) and point out gaps in our knowledge. We then survey conditions relevant to human health characterized by reduced respiratory neural activity and discuss evidence that inactivity-induced plasticity is elicited during these conditions. Understanding the physiological impact and circumstances in which inactivity-induced respiratory plasticity is elicited may yield novel insights into the treatment of disorders characterized by reductions in respiratory neural activity.

Rodent models of sleep apnea

Rodent models of sleep apnea have long been used to provide novel insight into the generation and predisposition to apneas as well as to characterize the impact of sleep apnea on cardiovascular, metabolic, and psychological health in humans. Given the significant body of work utilizing rodent models in the field of sleep apnea, the aims of this review are three-fold: first, to review the use of rodents as natural models of sleep apnea; second, to provide an overview of the experimental interventions employed in rodents to simulate sleep apnea; third, to discuss the refinement of rodent models to further our understanding of breathing abnormalities that occur during sleep. Given mounting evidence that sleep apnea impairs cognitive function, reduces quality of life, and exacerbates the course of multiple chronic diseases, rodent models will remain a high priority as a tool to interrogate both the pathophysiology and sequelae of breathing related abnormalities during sleep and to improve approaches to diagnosis and therapy.

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.

Congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) is characterized by hypoventilation during sleep and impaired ventilatory responses to hypercapnia and hypoxemia. Most cases are sporadic and caused by de novo PHOX2B gene mutations, which are usually polyalanine repeat expansions. Physiological and neuroanatomical studies of genetically engineered mice and analyses of cellular responses to mutated Phox2b have shed light on the pathophysiological mechanisms of CCHS. Findings in Phox2b(27Ala/+) knock-in mice consisted of unstable breathing with apneas, absence of the ventilatory response to hypercapnia, death within a few hours after birth, and absence of the retrotrapezoid nucleus (RTN). Conditional mouse mutants in which Phox2b(27Ala) was targeted to the RTN also lacked the ventilatory response to hypercapnia at birth but survived to adulthood and developed a partial hypercapnia response. The therapeutic effects of desogestrel are being evaluated in clinical trials, and recent analyses of cellular responses to polyAla Phox2b aggregates have suggested new pharmacological approaches designed to counteract the toxic effects of mutated Phox2b.

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.

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Transcriptional dysregulation and impairment of PHOX2B auto-regulatory mechanism induced by polyalanine expansion mutations associated with congenital central hypoventilation syndrome

The PHOX2B transcription factor plays a crucial role in autonomic nervous system development. In humans, heterozygous mutations of the PHOX2B gene lead to congenital central hypoventilation syndrome (CCHS), a rare disorder characterized by a broad variety of symptoms of autonomic nervous system dysfunction including inadequate control of breathing. The vast majority of patients with CCHS are heterozygous for a polyalanine repeat expansion mutation involving a polyalanine tract of twenty residues in the C-terminus of PHOX2B. Although several lines of evidence support a dominant-negative mechanism for PHOX2B mutations in CCHS, the molecular effects of PHOX2B mutant proteins on the transcriptional activity of the wild-type protein have not yet been elucidated. As one of the targets of PHOX2B is the PHOX2B gene itself, we tested the transcriptional activity of wild-type and mutant proteins on the PHOX2B gene promoter, and found that the transactivation ability of proteins with polyalanine expansions decreased as a function of the length of the expansion, whereas DNA binding was severely affected only in the case of the mutant with the longest polyalanine tract (+13 alanine). Co-transfection experiments using equimolar amounts of PHOX2B wild-type and mutant proteins in order to simulate a heterozygous state in vitro and four different PHOX2B target gene regulatory regions (PHOX2B, PHOX2A, DBH, TLX2) clearly showed that the polyalanine expanded proteins alter the transcriptional activity of wild-type protein in a promoter-specific manner, without any clear correlation with the length of the expansion. Moreover, although reduced transactivation may be caused by retention of the wild-type protein in the cytoplasm or in nuclear aggregates, this mechanism can only be partially responsible for the pathogenesis of CCHS because of the reduction in cytoplasmic and nuclear accumulation when the +13 alanine mutant is co-expressed with wild-type protein, and the fact that the shortest polyalanine expansions do not form visible cytoplasmic aggregates. Deletion of the C-terminal of PHOX2B leads to a protein that correctly localizes in the nucleus but impairs PHOX2B wild-type transcriptional activity, thus suggesting that protein mislocalization is not the only mechanism leading to CCHS. The results of this study provide novel in vitro experimental evidence of a transcriptional dominant-negative effect of PHOX2B polyalanine mutant proteins on wild-type protein on two different PHOX2B target genes.

Breathing without CO(2) chemosensitivity in conditional Phox2b mutants

Breathing is a spontaneous, rhythmic motor behavior critical for maintaining O(2), CO(2), and pH homeostasis. In mammals, it is generated by a neuronal network in the lower brainstem, the respiratory rhythm generator (Feldman et al., 2003). A century-old tenet in respiratory physiology posits that the respiratory chemoreflex, the stimulation of breathing by an increase in partial pressure of CO(2) in the blood, is indispensable for rhythmic breathing. Here we have revisited this postulate with the help of mouse genetics. We have engineered a conditional mouse mutant in which the toxic PHOX2B(27Ala) mutation that causes congenital central hypoventilation syndrome in man is targeted to the retrotrapezoid nucleus, a site essential for central chemosensitivity. The mutants lack a retrotrapezoid nucleus and their breathing is not stimulated by elevated CO(2) at least up to postnatal day 9 and they barely respond as juveniles, but nevertheless survive, breathe normally beyond the first days after birth, and maintain blood PCO(2) within the normal range. Input from peripheral chemoreceptors that sense PO(2) in the blood appears to compensate for the missing CO(2) response since silencing them by high O(2) abolishes rhythmic breathing. CO(2) chemosensitivity partially recovered in adulthood. Hence, during the early life of rodents, the excitatory input normally afforded by elevated CO(2) is dispensable for life-sustaining breathing and maintaining CO(2) homeostasis in the blood.

Impaired ventilatory and thermoregulatory responses to hypoxic stress in newborn phox2b heterozygous knock-out mice

The Phox2b genesis necessary for the development of the autonomic nervous system, and especially, of respiratory neuronal circuits. In the present study, we examined the role of Phox2b in ventilatory and thermoregulatory responses to hypoxic stress, which are closely related in the postnatal period. Hypoxic stress was generated by strong thermal stimulus, combined or not with reduced inspired O(2). To this end, we exposed 6-day-old Phox2b(+/-) pups and their wild-type littermates (Phox2b(+/+)) to hypoxia (10% O(2)) or hypercapnia (8% CO(2)) under thermoneutral (33°C) or cold (26°C) conditions. We found that Phox2b(+/-) pups showed less normoxic ventilation (V(E)) in the cold than Phox2b(+/+) pups. Phox2b(+/-) pups also showed lower oxygen consumption (VO(2)) in the cold, reflecting reduced thermogenesis and a lower body temperature. Furthermore, while the cold depressed ventilatory responses to hypoxia and hypercapnia in both genotype groups, this effect was less pronounced in Phox2b(+/-) pups. Finally, because serotonin (5-HT) neurons are pivotal to respiratory and thermoregulatory circuits and depend on Phox2b for their differentiation, we studied 5-HT metabolism using high pressure liquid chromatography, and found that it was altered in Phox2b(+/-) pups. We conclude that Phox2b haploinsufficiency alters the ability of newborns to cope with metabolic challenges, possibly due to 5-HT signaling impairments.

Julius H. Comroe, Jr., distinguished lecture: central chemoreception: then ... and now

The 2010 Julius H. Comroe, Jr., Lecture of the American Physiological Society focuses on evolving ideas in chemoreception for CO₂/pH in terms of what is "sensed," where it is sensed, and how the sensed information is used physiologically. Chemoreception is viewed as involving neurons (and glia) at many sites within the hindbrain, including, but not limited to, the retrotrapezoid nucleus, the medullary raphe, the locus ceruleus, the nucleus tractus solitarius, the lateral hypothalamus (orexin neurons), and the caudal ventrolateral medulla. Central chemoreception also has an important nonadditive interaction with afferent information arising at the carotid body. While ventilation has been viewed as the primary output variable, it appears that airway resistance, arousal, and blood pressure can also be significantly affected. Emphasis is placed on the importance of data derived from studies performed in the absence of anesthesia.