Genetics and early disturbances of breathing control

Further delineation of Auriculocondylar syndrome based on 14 novel cases and reassessment of 25 published cases

Auriculocondylar syndrome (ACS) is a rare craniofacial disorder characterized by mandibular hypoplasia and an auricular defect at the junction between the lobe and helix, known as a "Question Mark Ear" (QME). Several additional features, originating from the first and second branchial arches and other tissues, have also been reported. ACS is genetically heterogeneous with autosomal dominant and recessive modes of inheritance. The mutations identified to date are presumed to dysregulate the endothelin 1 signalling pathway. Here we describe 14 novel cases and reassess 25 published cases of ACS through a questionnaire for systematic data collection. All patients harbour mutation(s) in PLCB4, GNAI3 or EDN1. This series of patients contributes to the characterization of additional features occasionally associated with ACS such as respiratory, costal, neurodevelopmental and genital anomalies, and provides management and monitoring recommendations. This article is protected by copyright. All rights reserved.

Generation and characterization of iPSC lines (BCH001) from a boy with intron 14 mutation in the ret proto-oncogene (RET) gene

Congenital central hypoventilation syndrome (CCHS) is characterized by an alteration of the ventilatory response to hypercapnia and hypoxia, and is classically presented in neonates with abnormalities of the autonomic nervous system. Here, we generated human induced pluripotent stem cell (iPSC) lines from peripheral blood mononuclear cells (PBMCs) isolated from a male patient clinically diagnosed with CCHS. These iPSC lines carry a heterozygous RET mutation (c.2608-125C > T), express pluripotency markers, have the capacity to differentiate into the normal teratoma tissue, retain the RET mutation and display the normal karyotype, which will also provide a useful resource to study the pathogenesis of CCHS.

Molecular Mechanisms of Acute Oxygen Sensing by Arterial Chemoreceptor Cells. Role of Hif2α

Carotid body glomus cells are multimodal arterial chemoreceptors able to sense and integrate changes in several physical and chemical parameters in the blood. These cells are also essential for O₂ homeostasis. Glomus cells are prototypical peripheral O₂ sensors necessary to detect hypoxemia and to elicit rapid compensatory responses (hyperventilation and sympathetic activation). The mechanisms underlying acute O₂ sensing by glomus cells have been elusive. Using a combination of mouse genetics and single-cell optical and electrophysiological techniques, it has recently been shown that activation of glomus cells by hypoxia relies on the generation of mitochondrial signals (NADH and reactive oxygen species), which modulate membrane ion channels to induce depolarization, Ca²⁺ influx, and transmitter release. The special sensitivity of glomus cell mitochondria to changes in O₂ tension is due to Hif2α-dependent expression of several atypical mitochondrial subunits, which are responsible for an accelerated oxidative metabolism and the strict dependence of mitochondrial complex IV activity on O₂ availability. A mitochondrial-to-membrane signaling model of acute O₂ sensing has been proposed, which explains existing data and provides a solid foundation for future experimental tests. This model has also unraveled new molecular targets for pharmacological modulation of carotid body activity potentially relevant in the treatment of highly prevalent medical conditions.

The human ventilatory response to stress: rate or depth?

Many stressors cause an increase in ventilation in humans. This is predominantly reported as an increase in minute ventilation (V̇E). But, the same V̇E can be achieved by a wide variety of changes in the depth (tidal volume, V_T ) and number of breaths (respiratory frequency, ƒ_R ). This review investigates the impact of stressors including: cold, heat, hypoxia, pain and panic on the contributions of ƒ_R and V_T to V̇E to see if they differ with different stressors. Where possible we also consider the potential mechanisms that underpin the responses identified, and propose mechanisms by which differences in ƒ_R and V_T are mediated. Our aim being to consider if there is an overall differential control of ƒ_R and V_T that applies in a wide range of conditions. We consider moderating factors, including exercise, sex, intensity and duration of stimuli. For the stressors reviewed, as the stress becomes extreme V̇E generally becomes increased more by ƒ_R than V_T . We also present some tentative evidence that the pattern of ƒ_R and V_T could provide some useful diagnostic information for a variety of clinical conditions. In The Physiological Society's year of 'Making Sense of Stress', this review has wide-ranging implications that are not limited to one discipline, but are integrative and relevant for physiology, psychophysiology, neuroscience and pathophysiology.

A Phox2b BAC Transgenic Rat Line Useful for Understanding Respiratory Rhythm Generator Neural Circuitry

The key role of the respiratory neural center is respiratory rhythm generation to maintain homeostasis through the control of arterial blood pCO₂/pH and pO₂ levels. The neuronal network responsible for respiratory rhythm generation in neonatal rat resides in the ventral side of the medulla and is composed of two groups; the parafacial respiratory group (pFRG) and the pre-Bötzinger complex group (preBötC). The pFRG partially overlaps in the retrotrapezoid nucleus (RTN), which was originally identified in adult cats and rats. Part of the pre-inspiratory (Pre-I) neurons in the RTN/pFRG serves as central chemoreceptor neurons and the CO₂ sensitive Pre-I neurons express homeobox gene Phox2b. Phox2b encodes a transcription factor and is essential for the development of the sensory-motor visceral circuits. Mutations in human PHOX2B cause congenital hypoventilation syndrome, which is characterized by blunted ventilatory response to hypercapnia. Here we describe the generation of a novel transgenic (Tg) rat harboring fluorescently labeled Pre-I neurons in the RTN/pFRG. In addition, the Tg rat showed fluorescent signals in autonomic enteric neurons and carotid bodies. Because the Tg rat expresses inducible Cre recombinase in PHOX2B-positive cells during development, it is a potentially powerful tool for dissecting the entire picture of the respiratory neural network during development and for identifying the CO₂/O₂ sensor molecules in the adult central and peripheral nervous systems.

Vocal development in dystonic rats

Vocal production, which requires the generation and integration of laryngeal and respiratory motor patterns, can be impaired in dystonia, a disorder believed due to dysfunction of sensorimotor pathways in the central nervous system. Herein, we analyze vocal and respiratory abnormalities in the dystonic (dt) rat, a well-characterized model of generalized dystonia. The dt rat is a recessive mutant with haploinsufficiency of Atcay which encodes the neuronally restricted protein caytaxin. Olivocerebellar functional abnormalities are central to the dt rat's truncal and appendicular dystonia and could also contribute to vocal and respiratory abnormalities in this model system. Differences in vocal repertoire composition were found between homozygote and wild-type dt rat pups developing after 3 weeks of life. Those spectro-temporal differences were not paralleled by differences in vocal activity or maximum lung pressures during quiet breathing and vocalization. However, breathing rhythm was slower in homozygote pups. This slower breathing rhythm persisted into adulthood. Given that cerebellectomy eliminates truncal and appendicular dystonia in the dt rat, we hypothesize that the altered breathing patterns stem either from a disturbance in the maturation of respiratory pattern generators or from deficient extracerebellar caytaxin expression affecting normal respiratory pattern generation. The altered breathing rhythm associated with vocal changes in the murine model resembles aspects of vocal dysfunction that are seen in humans with sporadic dystonia.

PHOX2B polyalanine repeat length is associated with sudden infant death syndrome and unclassified sudden infant death in the Dutch population

Unclassified sudden infant death (USID) is the sudden and unexpected death of an infant that remains unexplained after thorough case investigation including performance of a complete autopsy and review of the circumstances of death and the clinical history. When the infant is below 1 year of age and with onset of the fatal episode apparently occurring during sleep, this is referred to as sudden infant death syndrome (SIDS). USID and SIDS remain poorly understood despite the identification of several environmental and some genetic risk factors. In this study, we investigated genetic risk factors involved in the autonomous nervous system in 195 Dutch USID/SIDS cases and 846 Dutch, age-matched healthy controls. Twenty-five DNA variants from 11 genes previously implicated in the serotonin household or in the congenital central hypoventilation syndrome, of which some have been associated with SIDS before, were tested. Of all DNA variants considered, only the length variation of the polyalanine repeat in exon 3 of the PHOX2B gene was found to be statistically significantly associated with USID/SIDS in the Dutch population after multiple test correction. Interestingly, our data suggest that contraction of the PHOX2B exon 3 polyalanine repeat that we found in six of 160 SIDS and USID cases and in six of 814 controls serves as a probable genetic risk factor for USID/SIDS at least in the Dutch population. Future studies are needed to confirm this finding and to understand the functional effect of the polyalanine repeat length variation, in particular contraction, in exon 3 of the PHOX2B gene.

Understanding the basis of auriculocondylar syndrome: Insights from human, mouse and zebrafish genetic studies

Among human birth defect syndromes, malformations affecting the face are perhaps the most striking due to cultural and psychological expectations of facial shape. One such syndrome is auriculocondylar syndrome (ACS), in which patients present with defects in ear and mandible development. Affected structures arise from cranial neural crest cells, a population of cells in the embryo that reside in the pharyngeal arches and give rise to most of the bone, cartilage and connective tissue of the face. Recent studies have found that most cases of ACS arise from defects in signaling molecules associated with the endothelin signaling pathway. Disruption of this signaling pathway in both mouse and zebrafish results in loss of identity of neural crest cells of the mandibular portion of the first pharyngeal arch and the subsequent repatterning of these cells, leading to homeosis of lower jaw structures into more maxillary-like structures. These findings illustrate the importance of endothelin signaling in normal human craniofacial development and illustrate how clinical and basic science approaches can coalesce to improve our understanding of the genetic basis of human birth defect syndromes. Further, understanding the genetic basis for ACS that lies outside of known endothelin signaling components may help elucidate unknown aspects critical to the establishment of neural crest cell patterning during facial morphogenesis.

Congenital central hypoventilation syndrome: four families

BACKGROUND

Congenital central hypoventilation syndrome (CCHS) is a rare condition that usually presents soon after birth and is potentially life-shortening if not treated. The defining abnormality is hypoventilation during sleep which requires life-long treatment with artificial ventilation. This syndrome may also be associated with generalised dysfunction of the autonomic nervous system and a sub-group with associated Hirschsprung's disease. The genetic basis of CCHS has been identified as mutations in the PHOX2B gene.

METHODS

We present four families, three with autosomal dominant inheritance and familial clustering, and one with a de novo mutation resulting in CCHS.

CONCLUSIONS

We demonstrate that nasal mask ventilation from birth can provide adequate treatment and improved quality of life for these children. Phenotypic variability in expression of disease is seen in families with the same mutations in PHOX2B gene. The psychosocial costs of the disease and the unrecognised 'morbidity barter' that is part of current management needs to be factored into in all stages of management from childhood to adolescence to adulthood.

Hypoventilation syndromes

A wide variety of mechanisms can lead to the hypoventilation associated with various medical disorders, including derangements in central ventilatory control, mechanical impediments to breathing, and abnormalities in gas exchange leading to increased dead space ventilation. The pathogenesis of hypercapnia in obesity hypoventilation syndrome remains somewhat obscure, although in many patients comorbid obstructive sleep apnea appears to play an important role. Hypoventilation in neurologic or neuromuscular disorders is primarily explained by weakness of respiratory muscles, although some central nervous system diseases may affect control of breathing. In other chest wall disorders, obstructive airways disease, and cystic fibrosis, much of the pathogenesis is explained by mechanical impediments to breathing, but an element of increased dead space ventilation also often occurs. Central alveolar hypoventilation syndrome involves a genetically determined defect in central respiratory control. Treatment in all of these disorders involves coordinated management of the primary disorder (when possible) and, increasingly, the use of noninvasive positive pressure ventilation.

Neurological suppression of diaphragm electromyographs in hamsters infected with West Nile virus

To address the hypothesis that respiratory distress associated with West Nile virus (WNV) is neurologically caused, electromyographs (EMGs) were measured longitudinally from the diaphragms of alert hamsters infected subcutaneously (s.c.) with WNV. The EMG activity in WNV-infected hamsters was consistently and significantly (P Collapse

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MESH Headings

• Afferent Pathways/physiopathology
• Afferent Pathways/virology
• Animals
• Brain Stem/physiopathology
• Brain Stem/virology
• Cervical Vertebrae
• Cricetinae
• Diaphragm/innervation
• Diaphragm/physiopathology
• Electromyography
• Evoked Potentials, Auditory
• Female
• Immunohistochemistry
• Mesocricetus
• Microscopy, Confocal
• Neurons/virology
• Spinal Cord/virology
• West Nile Fever/physiopathology
• West Nile virus

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Grants

• P20 RR020146-08 NCRR NIH HHS
• P20 RR020146 NCRR NIH HHS
• P30 GM110702 NIGMS NIH HHS
• N01 AI050036 NIAID NIH HHS
• N01AI30063 NIAID NIH HHS
• U54 AI065357 NIAID NIH HHS
• RR020146 NCRR NIH HHS
• R01 NS020246 NINDS NIH HHS
• U54 AI065357-05 NIAID NIH HHS
• 1 U54 AI-065357-04 NIAID NIH HHS

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Affiliation(s)

John D Morrey Institute for Antiviral Research, Department of Animal, Dairy, and Veterinary Sciences, Utah State University, Logan, Utah 84341, USA.
Venkatraman Siddharthan
Hong Wang
Jeffery O Hall
Neil E Motter
Robert D Skinner
Ramona T Skirpstunas

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Cerebral MRI abnormalities in a premature infant with later confirmed congenital central hypoventilation syndrome

We present a premature infant with an inability to ventilate spontaneously during sleep periods. In addition, the patient showed general hypotonia. The child had a delayed passage of stool and increased anal muscle tone, indicating Hirschsprung's disease. The combination of these symptoms suggested congenital central hypoventilation syndrome, which was confirmed postmortem by DNA analysis showing a mutation in the PHOX2B gene. MRI of the brain showed damage to the white matter, including the internal capsula. This type of damage to the white matter has not been described before in a premature infant, who did not experience birth asphyxia.

Autosomal recessive diseases among the Athabaskans of the southwestern United States: recent advances and implications for the future

Genetic and linguistic data suggest that the Na-Dene, of which the Athabaskans are the largest group, are part of a later immigration into the Americas than the first Amerind immigration. Whether a second and third immigration can be separated seems unlikely but continued cross-Bering Strait exchanges may have masked what was a greater separation in the past. The movement of tribes into Siberia appears to have involved a genetic bottleneck leading to at least one disease allele shared by Eskimo/Aleuts and Navajos and a second possibly shared by the Navajo and a Siberian population, but not the same Siberian population that share deep linguistic affinities with the Navajo. A second bottleneck appears to have occurred with the migration of Athabaskans from Northwest North America to the Southwestern United States along the Rocky Mountains. This bottleneck is reflected in several rare recessive diseases shared by the Navajo and Apache. Finally, the Navajo were captured and imprisoned under conditions which led to severe population loss. This, and the "hiding away" of a small number of Navajos in what is now the Western portion of the reservation, led to a Navajo-specific bottleneck(s) resulting in an increased frequency of several rare recessive diseases among the Navajo. Prejudice against human genetic research is high among the Southwestern Athabaskans but attempts to bridge the gap are now occurring. The involvement of Navajo scientists in this process is especially encouraging.

Noradrenergic modulation of the respiratory neural network

Noradrenergic dysregulation has been reported in human pathologies affecting the control of breathing, such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome. Noradrenergic neurons, located predominantly in pontine nuclei, are among the earliest to arise within the hindbrain and play an essential role in the maturation of the respiratory network. Noradrenergic neurons also play a major role in the modulation of the respiratory motor pattern from birth through adulthood. The critical importance of this signaling system in respiratory control is illustrated by the severe respiratory disturbances associated with gene mutations affecting noradrenergic neurons (Phox2 and Mecp2). Here, the role of catecholaminergic pontine nuclei in the control of breathing, the cellular effects of norepinephrine on the respiratory network and the pathological consequence to breathing of abnormalities in this signaling system will be discussed.

Abnormalities of respiratory control and the respiratory motor unit

BACKGROUND

Control of ventilation depends on a brainstem neuronal network that controls activity of the motor neurons innervating the respiratory muscles. This network includes the pontine respiratory group and the dorsal and ventral respiratory groups in the medulla. Neurologic disorders affecting these areas or the respiratory motor unit may lead to abnormal breathing.

REVIEW SUMMARY

The brainstem respiratory network contains neurons critical for respiratory rhythmogenesis; this network receives inputs from peripheral and central chemoreceptors sensitive to levels of carbon dioxide (PaCO2) and oxygen (PaO2) and from forebrain structures that control respiration as part of integrated behaviors such as speech or exercise. Manifestations associated with disorders of this network include sleep apnea and dysrhythmic breathing frequently associated with disturbances of cardiovagal and sympathetic vasomotor control. Common disorders associated with impaired cardiorespiratory control include brainstem stroke or compression, syringobulbia, Chiari malformation, high cervical spinal cord injuries, and multiple system atrophy. By far, neuromuscular disorders are the more common neurologic conditions leading to respiratory failure.

CONCLUSIONS

Respiratory dysfunction constitute an early and relatively major manifestation of several neurologic disorders and may be due to an abnormal breathing pattern generation due to involvement of the cardiorespiratory network or more frequently to respiratory muscle weakness.

Neonatal apnea: what's new?

Apnea of prematurity (AOP) remains a major clinical problem in present day neonatology that warrants frequent evaluations and imposes challenges in therapeutic strategies. Although the pathogenesis of AOP is poorly understood, it is probably a manifestation of physiologic immaturity of breathing control rather than a pathologic disorder. Immature breathing responses to hypoxia, hypercapnia and exaggerated inhibitory pulmonary reflexes in preterm infants might also contribute to the occurrence or severity of AOP. Recent data suggest a role for genetic predisposition. Although typically resolve with maturation, the role of bradycardia and desaturation episodes associated with AOP in the development of sleep disorder breathing and neurodevelopmental delay needs further clarification. Pharmacological treatment with methylxanthines and CPAP remain the mainstay for treatment of AOP. However, recent studies have implicated central inhibitory neuromodulators including prostaglandins, GABA and adenosine in its pathogenesis, the fact that might provide future specific targets for treatment. This review will summarize new insights involving these issues as well as others involving the pathogenesis, treatment strategies and consequences of apnea in premature infants.

Central nervous system distribution of the transcription factor Phox2b in the adult rat

Phox2b is required for development of the peripheral autonomic nervous system and a subset of cranial nerves and lower brainstem nuclei. Phox2b mutations in man cause diffuse autonomic dysfunction and deficits in the automatic control of breathing. Here we study the distribution of Phox2b in the adult rat hindbrain to determine whether this protein is selectively expressed by neurons involved in respiratory and autonomic control. In the medulla oblongata, Phox2b-immunoreactive nuclei were present in the dorsal vagal complex, intermediate reticular nucleus, dorsomedial spinal trigeminal nucleus, nucleus ambiguus, catecholaminergic neurons, and retrotrapezoid nucleus (RTN). Phox2b was expressed by both central excitatory relays of the sympathetic baroreflex (nucleus of the solitary tract and C1 neurons) but not by the inhibitory relay of this reflex. Phox2b was absent from the ventral respiratory column (VRC) caudal to RTN and rare within the parabrachial nuclei. In the pons, Phox2b was confined to cholinergic efferent neurons (salivary, vestibulocochlear) and noncholinergic peritrigeminal neurons. Rostral to the pons, Phox2b was detected only in the oculomotor complex. In adult rats, Phox2b is neither a comprehensive nor a selective marker of hindbrain autonomic pathways. This marker identifies a subset of hindbrain neurons that control orofacial movements (dorsomedial spinal trigeminal nucleus, pontine peritrigeminal neurons), balance and auditory function (vestibulocochlear efferents), the eyes, and both divisions of the autonomic efferent system. Phox2b is virtually absent from the respiratory rhythm and pattern generator (VRC and dorsolateral pons) but is highly expressed by neurons involved in the chemical drive and reflex regulation of this oscillator.

Sleep-related hypoventilation/hypoxemic syndromes

The latest edition of The International Classification of Sleep Disorders: Diagnostic and Coding Manual subsumes a broad range of disorders under the heading "Sleep Related Hypoventilation/Hypoxemic Syndromes." Some are quite common, such as COPD with worsening gas exchange during sleep; while some are exceedingly rare, such as congenital central hypoventilation syndrome. All share the attribute of abnormal gas exchange that worsens, or may only be present, during sleep. The sleep state, the sleeping posture, and the circadian rhythm driving sleep all may affect respiration by altering control of breathing and/or pulmonary mechanics. These changes are largely inconsequential in the normal individual but interact with respiratory, neurologic, or neuromuscular disease to manifest as the sleep-related hypoventilation/hypoxemic syndromes. In addition to optimal treatment of the underlying disorder (when known and when possible), treatment usually involves nocturnal ventilatory support that is now most commonly provided by noninvasive positive pressure ventilation.

Acute role of the brain-derived neurotrophic factor (BDNF) on the respiratory neural network activity in mice in vitro

In humans, several pathologies are associated with disturbances of the respiratory control, some of them including alteration in the brain-derived neurotrophic factor (BDNF) signalling pathway. BDNF has long been known as a neurotrophic factor involved in survival, differentiation and maintenance of neuronal populations in the peripheral and central nervous system. More recently BDNF has also been discovered to be a potent neuromodulator with acute effects on neuronal excitability and synaptic plasticity. Animals deleted for the gene encoding BDNF exhibit respiratory alteration suggesting an important but yet undefined role of the neurotrophin in respiratory rhythmogenesis either by a trophic and/or an acute action. The possibility that BDNF might exert an acute regulatory role on the rhythmic activity of the respiratory generator of the pre-Bötzinger complex has been recently examined in newborn mice in vitro. Results obtained, reviewed in the present paper, will help getting insights in respiratory rhythm regulatory mechanisms that involve BDNF signalling.

Paroxysmal nonepileptic events of childhood

The paroxysmal nonepileptic events of childhood are a group of disorders, syndromes, and phenomena that mimic true epileptic seizures. Clinical experience and a clear description of the event in question will usually lead to a correct categorization. They span in age from neonate to young adult and are apt to be the most common diagnostic challenges clinicians face regularly. The key to diagnosis is a detailed history and careful observation. Despite the large number of discrete entities enumerated herein, common principles in clinical approach are successful and described. Each entity can pose a significant clinical challenge in identification, etiologic pathophysiology, genetics, and management. A simple division is offered here separating those episodes that are associated with an altered mental status or occurring during sleep and those without an altered mental status or occurring while awake.

Evolution of the GDNF family ligands and receptors

Four different ligand-receptor binding pairs of the GDNF (glial cell line-derived neurotrophic factor) family exist in mammals, and they all signal via the transmembrane RET receptor tyrosine kinase. In addition, GRAL (GDNF Receptor Alpha-Like) protein of unknown function and Gas1 (growth arrest specific 1) have GDNF family receptor (GFR)-like domains. Orthologs of the four GFRalpha receptors, GRAL and Gas1 are present in all vertebrate classes. In contrast, although bony fishes have orthologs of all four GDNF family ligands (GFLs), one of the ligands, neurturin, is absent in clawed frog and another, persephin, is absent in the chicken genome. Frog GFRalpha2 has selectively evolved possibly to accommodate GDNF as a ligand. The key role of GDNF and its receptor GFRalpha1 in enteric nervous system development is conserved from zebrafish to humans. The role of neurturin, signaling via GFRalpha2, for parasympathetic neuron development is conserved between chicken and mice. The role of artemin and persephin that signal via GFRalpha3 and GFRalpha4, respectively, is unknown in non-mammals. The presence of RET- and GFR-like genes in insects suggests that a ProtoGFR and a ProtoRET arose early in the evolution of bilaterian animals, but when the ProtoGFL diverged from existing transforming growth factor (TGFbeta)-like proteins remains unclear. The four GFLs and GFRalphas were presumably generated by genome duplications at the origin of vertebrates. Loss of neurturin in frog and persephin in chicken suggests functional redundancy in early tetrapods. Functions of non-mammalian GFLs and prechordate RET and GFR-like proteins remain to be explored.

Expression of Phox2b by brainstem neurons involved in chemosensory integration in the adult rat

Central congenital hypoventilation syndrome is caused by mutations of the gene that encodes the transcription factor Phox2b. The syndrome is characterized by a severe form of sleep apnea attributed to greatly compromised central and peripheral chemoreflexes. In this study, we analyze whether Phox2b expression in the brainstem respiratory network is preferentially associated with neurons involved in chemosensory integration in rats. At the very rostral end of the ventral respiratory column (VRC), Phox2b was present in many VGlut2 (vesicular glutamate transporter 2) mRNA-containing neurons. These neurons were functionally identified as the respiratory chemoreceptors of the retrotrapezoid nucleus (RTN). More caudally in the VRC, many fewer neurons expressed Phox2b. These cells were not part of the central respiratory pattern generator (CPG), because they were typically cholinergic visceral motor neurons or catecholaminergic neurons (presumed C1 neurons). Phox2b was not detected in serotonergic neurons, in the A5, A6, and A7 noradrenergic cell groups nor within the main cardiorespiratory centers of the dorsolateral pons. Phox2b was expressed by many solitary tract nucleus (NTS) neurons including those that relay peripheral chemoreceptor information to the RTN. These and previous observations by others suggest that Phox2b is expressed by an uninterrupted chain of neurons involved in the integration of peripheral and central chemoreception (carotid bodies, chemoreceptor afferents, chemoresponsive NTS neurons projecting to VRC, RTN chemoreceptors). The presence of Phox2b in this circuit and its apparent absence from the respiratory CPG could explain why Phox2b mutations disrupt breathing automaticity during sleep without causing major impairment of respiration during waking.

Endogenous noradrenaline affects the maturation and function of the respiratory network: Possible implication for SIDS

Breathing is a vital, rhythmic motor act that is required for blood oxygenation and oxygen delivery to the whole body. Therefore, the brainstem network responsible for the elaboration of the respiratory rhythm must function from the very first moments of extrauterine life. In this review, it is shown that the brainstem noradrenergic system plays a pivotal role in both the modulation and the maturation of the respiratory rhythm generator. Compelling evidence are reported demonstrating that genetically induced alterations of the noradrenergic system in mice affect the prenatal maturation and the perinatal function of the respiratory rhythm generator and have drastic consequences on postnatal survival. Sudden Infant Death Syndrome (SIDS), the leader cause of infant death in industrialised countries, may result from cardiorespiratory disorders during sleep. As several cases of SIDS have been observed in infants having noradrenergic deficits, a possible link between prenatal alteration of the noradrenergic system, altered maturation and function of the respiratory network and SIDS is suggested.

Lung-function tests in neonates and infants with chronic lung disease: tidal breathing and respiratory control

This paper is the fourth in a series of reviews that will summarize available data and critically discuss the potential role of lung-function testing in infants with acute neonatal respiratory disorders and chronic lung disease of infancy. The current paper addresses information derived from tidal breathing measurements within the framework outlined in the introductory paper of this series, with particular reference to how these measurements inform on control of breathing. Infants with acute and chronic respiratory illness demonstrate differences in tidal breathing and its control that are of clinical consequence and can be measured objectively. The increased incidence of significant apnea in preterm infants and infants with chronic lung disease, together with the reportedly increased risk of sudden unexplained death within the latter group, suggests that control of breathing is affected by both maturation and disease. Clinical observations are supported by formal comparison of tidal breathing parameters and control of breathing indices in the research setting.

Case report of Haddad syndrome in a newborn: congenital central hypoventilation syndrome and Hirschsprung's disease

Congenital central hypoventilation syndrome (CCHS) is a rare disorder characterized by failure of automatic control of breathing. Diagnosis is made by exclusion of other causes of hypoventilation. Genetic etiology is strongly suspected. Other autonomic nervous system dysfunctions, tumors of neural crest origin and Hirschsprung's disease are often found in affected children. Association with Hirschsprung's disease is known as Haddad syndrome. We present a newborn with respiratory distress since birth and Hirschprung's disease subsequently diagnosed with Haddad syndrome.

Sleep-related breathing disorders

Sleep-related breathing disorders are a heterogeneous group of conditions that may be associated with alterations in the structure of sleep, in sleep quality, and in gas exchange during sleep. Obstructive sleep apnea represents the most frequent cause of sleep-related breathing disorders, which encompass a diversity of conditions that either complicate coexisting disease or present as primary disorders. Many of these disorders have consequences during both sleep and wakefulness and may produce substantial burden of symptoms and disease in untreated individuals.

Ret deficiency in mice impairs the development of A5 and A6 neurons and the functional maturation of the respiratory rhythm

Although a normal respiratory rhythm is vital at birth, little is known about the genetic factors controlling the prenatal maturation of the respiratory network in mammals. In Phox2a mutant mice, which do not express A6 neurons, we previously hypothesized that the release of endogenous norepinephrine by A6 neurons is required for a normal respiratory rhythm to occur at birth. Here we investigated the role of the Ret gene, which encodes a transmembrane tyrosine kinase receptor, in the maturation of norepinephrine and respiratory systems. As Ret-null mutants (Ret-/-) did not survive after birth, our experiments were performed in wild-type (wt) and Ret-/- fetuses exteriorized from pregnant heterozygous mice at gestational day 18. First, in wt fetuses, quantitative in situ hybridization revealed high levels of Ret transcripts in the pontine A5 and A6 areas. Second, in Ret-/- fetuses, high-pressure liquid chromatography showed significantly reduced norepinephrine contents in the pons but not the medulla. Third, tyrosine hydroxylase immunocytochemistry revealed a significantly reduced number of pontine A5 and A6 neurons but not medullary norepinephrine neurons in Ret-/- fetuses. Finally, electrophysiological and pharmacological experiments performed on brainstem 'en bloc' preparations demonstrated impaired resting respiratory activity and abnormal responses to central hypoxia and norepinephrine application in Ret-/- fetuses. To conclude, our results show that Ret gene contributes to the prenatal maturation of A6 and A5 neurons and respiratory system. They support the hypothesis that the normal maturation of the respiratory network requires afferent activity corresponding to the A6 excitatory and A5 inhibitory input balance.

Ventilatory response to hyperoxia in newborn mice heterozygous for the transcription factor Phox2b

Heterozygous mutations of the transcription factor PHOX2B have been found in most patients with central congenital hypoventilation syndrome, a rare disease characterized by sleep-related hypoventilation and impaired chemosensitivity to sustained hypercapnia and sustained hypoxia. PHOX2B is a master regulator of autonomic reflex pathways, including peripheral chemosensitive pathways. In the present study, we used hyperoxic tests to assess the strength of the peripheral chemoreceptor tonic drive in Phox2b+/-newborn mice. We exposed 69 wild-type and 67 mutant mice to two hyperoxic tests (12-min air followed by 3-min 100% O2) 2 days after birth. Breathing variables were measured noninvasively using whole body flow plethysmography. The initial minute ventilation decrease was larger in mutant pups than in wild-type pups: -37% (SD 13) and -25% (SD 18), respectively, P<0.0001. Furthermore, minute ventilation remained depressed throughout O2 exposure in mutants, possibly because of their previously reported impaired CO2 chemosensitivity, whereas it returned rapidly to the normoxic level in wild-type pups. Hyperoxia considerably increased total apnea duration in mutant compared with wild-type pups (P=0.0001). A complementary experiment established that body temperature was not influenced by hyperoxia in either genotype group and, therefore, did not account for genotype-related differences in the hyperoxic ventilatory response. Thus partial loss of Phox2b function by heterozygosity did not diminish the tonic drive from peripheral chemoreceptors.

Transgenic Models to Study Disorders of Respiratory Control in Newborn Mice

Recent studies described the in vivo respiratory phenotype of mutant newborn mice with targeted deletions of genes involved in respiratory control development. Whole-body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. The main characteristics of the early postnatal development of respiratory control in mice are reviewed, including available data on breathing patterns and on hypoxic and hypercapnic ventilatory responses. Mice are very immature at birth, and their instable breathing is similar to that of preterm infants. Breathing pattern abnormalities with prolonged apneas occur in newborn mice that lack genes involved in the development of rhythmogenesis. Some mutant newborn mice have blunted hypoxic and hypercapnic ventilatory responses whereas others exhibit impairments in responses to hypoxia or hypercapnia. Furthermore, combined studies in mutant newborn mice and in humans have helped to provide pathogenic information on genetically determined developmental disorders of respiratory control in humans.

Development of respiratory control: Evolving concepts and perspectives

The mechanisms underlying respiratory system immaturity in newborns have been investigated, both in vivo and in vitro, in humans and in animals. Immaturity affects breathing rhythmicity and its modulation by suprapontine influences and by afferents from central and peripheral chemoreceptors. Recent research has moved from bedside tools to sophisticated technologies, bringing new insights into the plasticity and genetics of respiratory control development. Genetic research has benefited from investigations of newborn mice having targeted deletions of genes involved in respiratory control. Genetic variability may govern the normal programming of development and the processes underlying adaptation to homeostasis disturbances induced by prenatal and postnatal insults. Studies of plasticity have emphasized the role of neurotrophic factors. Improvements in our understanding of the mechanistic effects of these factors should lead to new neuroprotective strategies for infants at risk for early respiratory control disturbances, such as apnoeas of prematurity, sudden infant death syndrome and congenital central hypoventilation syndrome.

A whole-genome scan for 24-hour respiration rate: a major locus at 10q26 influences respiration during sleep

Identification of genes causing variation in daytime and nighttime respiration rates could advance our understanding of the basic molecular processes of human respiratory rhythmogenesis. This could also serve an important clinical purpose, because dysfunction of such processes has been identified as critically important in sleep disorders. We performed a sib-pair-based linkage analysis on ambulatory respiration rate, using the data from 270 sibling pairs who were genotyped at 374 markers on the autosomes, with an average distance of 9.65 cM. Uni- and multivariate variance-components-based multipoint linkage analyses were performed for respiration rate during three daytime periods (morning, afternoon, and evening) and during nighttime sleep. Evidence of linkage was found at chromosomal locations 3q27, 7p22, 10q26, and 22q12. The strongest evidence of linkage was found for respiration rate during sleep, with LOD scores of 2.36 at 3q27, 3.86 at 10q26, and 1.59 at 22q12. In a simultaneous analysis of these three loci, >50% of the variance in sleep respiration rate could be attributed to a quantitative-trait loci near marker D10S1248 at 10q. Genes in this area (GFRA1, ADORA2L, FGR2, EMX2, and HMX2) can be considered promising positional candidates for genetic association studies of respiratory control during sleep.

Automatic classification of activity and apneas using whole body plethysmography in newborn mice

An increasing number of studies in newborn mice are being performed to determine the mechanisms of sleep apnea, which is the hallmark of early breathing disorders. Whole body plethysmography is the method of choice, as it does not require immobilization, which affects behavioral states and breathing. However, activity inside the plethysmograph may disturb the respiratory signal. Visual classification of the respiratory signal into ventilatory activity, activity-related disturbances, or apneas is so time-consuming as to considerably hamper the phenotyping of large pup samples. We propose an automatic classification of activity based on respiratory disturbances and of apneas based on spectral analysis. This method was validated in newborn mice on the day of birth and on postnatal days 2, 5, and 10, under normoxic and hypoxic (5% O2) conditions. For both activity and apneas, visual and automatic scores showed high Pearson's correlation coefficients (0.92 and 0.98, respectively) and high intraclass correlation coefficients (0.96–0.99), supporting strong agreement between the two methods. The present results suggest that breathing disturbances may provide a valid indirect index of activity in freely moving newborn mice and that automatic apnea classification based on spectral analysis may be efficient in terms of precision and of time saved.