RET proto-oncogene is important for the development of respiratory CO2 sensitivity

Central and peripheral chemoreceptors in sudden infant death syndrome

The pathogenesis of sudden infant death syndrome (SIDS) has been ascribed to an underlying biological vulnerability to stressors during a critical period of development. This paper reviews the main data in the literature supporting the role of central (e.g. retrotrapezoid nucleus, serotoninergic raphe nuclei, locus coeruleus, orexinergic neurons, ventral medullary surface, solitary tract nucleus) and peripheral (e.g. carotid body) chemoreceptors in the pathogenesis of SIDS. Clinical and experimental studies indicate that central and peripheral chemoreceptors undergo critical development during the initial postnatal period, consistent with the age range of SIDS (<1 year). Most of the risk factors for SIDS (gender, genetic factors, prematurity, hypoxic/hyperoxic stimuli, inflammation, perinatal exposure to cigarette smoke and/or substance abuse) may structurally and functionally affect the developmental plasticity of central and peripheral chemoreceptors, strongly suggesting the involvement of these structures in the pathogenesis of SIDS. Morphometric and neurochemical changes have been found in the carotid body and brainstem respiratory chemoreceptors of SIDS victims, together with functional signs of chemoreception impairment in some clinical studies. However, the methodological problems of SIDS research will have to be addressed in the future, requiring large and highly standardized case series. Up-to-date autopsy protocols should be produced, involving substantial, and exhaustive sampling of all potentially involved structures (including peripheral arterial chemoreceptors). Morphometric approaches should include unbiased stereological methods with three-dimensional probes. Prospective clinical studies addressing functional tests and risk factors (including genetic traits) would probably be the gold standard, allowing markers of intrinsic or acquired vulnerability to be properly identified.

Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Role of TrkB during the postnatal development of the rat carotid body

Brain-derived neurotrophic factor (BDNF) supports innervation of the carotid body by neurons projecting from the petrosal ganglion. Although carotid body glomus cells also express TrkB, BDNF's high affinity receptor, the role of BDNF in carotid body growth and O2 sensitivity has not been studied. Neonatal rats were treated with the TrkB antagonist K252a (100 μg kg(-1), i.p., b.i.d.) or vehicle on postnatal days P0-P6 and studied on P7. Carotid body volume was decreased by 35% after chronic K252a (P<0.001); a reduction in carotid body size was also elicited using the more selective TrkB antagonist ANA-12 (500 μg kg(-1), i.p., b.i.d.). In contrast, single-unit chemoafferent responses to 5% O2, measured in vitro, were unaffected by chronic K252a administration. Normoxic and hypoxic ventilation, measured by head-body plethysmography, were also normal after chronic K252a administration, but acute K252a administration produced a slower, deeper breathing pattern during the transition into hypoxia. These data suggest that BDNF regulates postnatal carotid body growth but does not influence the development of glomus cell O2 sensitivity.

Insights into obstructive sleep apnea research

Moderate to severe obstructive sleep apnea (OSA) occurs in 10-17% of middle aged men and 3-9% of middle-aged women with a higher prevalence among obese subjects. This condition is an independent risk factor for many cardiovascular diseases. Intermittent hypoxia is a major pathophysiologic character of OSA; it can lead to oxidative stress and inflammation, which in their turn cause endothelial dysfunction, a hallmark of atherosclerosis. Many animal models have been designed to mimic OSA in human patients to allow more in-depth investigation of biological and cellular mechanisms of this condition. This review discusses the cardiovascular outcomes of OSA and some of the animal models that are being used to investigate it.

Genetic diseases: congenital central hypoventilation, Rett, and Prader-Willi syndromes

The present review summarizes current knowledge on three rare genetic disorders of respiratory control, congenital central hypoventilation syndrome (CCHS), Rett syndrome (RTT), and Prader-Willi syndrome (PWS). CCHS is characterized by lack of ventilatory chemosensitivity caused by PHOX2B gene abnormalities consisting mainly of alanine expansions. RTT is associated with episodes of tachypneic and irregular breathing intermixed with breathholds and apneas and is caused by mutations in the X-linked MECP2 gene encoding methyl-CpG-binding protein. PWS manifests as sleep-disordered breathing with apneas and episodes of hypoventilation and is caused by the loss of a group of paternally inherited genes on chromosome 15. CCHS is the most specific disorder of respiratory control, whereas the breathing disorders in RTT and PWS are components of a more general developmental disorder. The main clinical features of these three disorders are reviewed with special emphasis on the associated brain abnormalities. In all three syndromes, disease-causing genetic defects have been identified, allowing the development of genetically engineered mouse models. New directions for future therapies based on these models or, in some cases, on clinical experience are delineated. Studies of CCHS, RTT, and PWS extend our knowledge of the molecular and cellular aspects of respiratory rhythm generation and suggest possible pharmacological approaches to respiratory control disorders. This knowledge is relevant for the clinical management of many respiratory disorders that are far more prevalent than the rare diseases discussed here.

Neurotrophic factors in development and regulation of respiratory control

Neurotrophic factors (NTFs) are a heterogeneous group of extracellular signaling molecules that play critical roles in the development, maintenance, modulation and plasticity of the central and peripheral nervous systems. A subset of these factors, including members of three multigene families-the neurotrophins, neuropoetic cytokines and the glial cell line-derived neurotrophic factor ligands-are particularly important for development and regulation of neurons involved in respiratory control. Here, we review the functional biology of these NTFs and their receptors, as well as their roles in regulating survival, maturation, synaptic strength and plasticity in respiratory control pathways. In addition, we highlight recent progress in identifying the role of abnormal NTF signaling in the molecular pathogenesis of respiratory dysfunction in Rett syndrome and in the development of potential new NTF-targeted therapeutic strategies.

ASCL1 and RET expression defines a clinically relevant subgroup of lung adenocarcinoma characterized by neuroendocrine differentiation

ASCL1 is an important regulatory transcription factor in pulmonary neuroendocrine (NE) cell development, but its value as a biomarker of NE differentiation in lung adenocarcinoma (AD) and as a potential prognostic biomarker remains unclear. We examined ASCL1 expression in lung cancer samples of varied histologic subtype, clinical outcome and smoking status and compared with expression of traditional NE markers. ASCL1 mRNA expression was found almost exclusively in smokers with AD, in contrast to non-smokers and other lung cancer subtypes. ASCL1 protein expression by immunohistochemical (IHC) analysis correlated best with synaptophysin compared with chromogranin and CD56/NCAM. Analysis of a compendium of 367 microarray-based gene expression profiles in stage I lung adenocarcinomas identified significantly higher expression levels of the RET oncogene in ASCL1-positive tumors (ASCL1(+)) compared with ASCL1(-) tumors (q-value <10(-9)). High levels of RET expression in ASCL1(+) but not in ASCL1(-) tumors was associated with significantly shorter overall survival (OS) in stage 1 (P=0.007) and in all AD (P=0.037). RET protein expression by IHC had an association with OS in the context of ASCL1 expression. In silico gene set analysis and in vitro experiments by ASCL1 shRNA in AD cells with high endogenous expression of ASCL1 and RET implicated ASCL1 as a potential upstream regulator of the RET oncogene. Also, silencing ASCL1 in AD cells markedly reduced cell growth and motility. These results suggest that ASCL1 and RET expression defines a clinically relevant subgroup of ∼10% of AD characterized by NE differentiation.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.

Cardiovascular consequences of sleep apnea

Sleep apnea is a common health concern that is characterized by repetitive episodes of asphyxia. This condition has been linked to serious long-term adverse effects such as hypertension, metabolic dysregulation, and cardiovascular disease. Although the mechanism for the initiation and aggravation of cardiovascular disease has not been fully elucidated, oxidative stress and subsequent endothelial dysfunction play major roles. Animal models, which have the advantage of being free of comorbidities and/or behavioral variables (that commonly occur in humans), allow invasive measurements under well-controlled experimental conditions, and as such are useful tools in the study of the pathophysiological mechanisms of sleep apnea. This review summarizes currently available information on the cardiovascular consequences of sleep apnea and briefly describes common experimental approaches useful to sleep apnea in different animal models.

Obstructive sleep apnea syndrome: from phenotype to genetic basis

Obstructive sleep apnea syndrome (OSAS) is a complex chronic clinical syndrome, characterized by snoring, periodic apnea, hypoxemia during sleep, and daytime hypersomnolence. It affects 4-5% of the general population. Racial studies and chromosomal mapping, familial studies and twin studies have provided evidence for the possible link between the OSAS and genetic factors and also most of the risk factors involved in the pathogenesis of OSAS are largely genetically determined. A percentage of 35-40% of its variance can be attributed to genetic factors. It is likely that genetic factors associated with craniofacial structure, body fat distribution and neural control of the upper airway muscles interact to produce the OSAS phenotype. Although the role of specific genes that influence the development of OSAS has not yet been identified, current researches, especially in animal model, suggest that several genetic systems may be important. In this chapter, we will first define the OSAS phenotype, the pathogenesis and the risk factors involved in the OSAS that may be inherited, then, we will review the current progress in the genetics of OSAS and suggest a few future perspectives in the development of therapeutic agents for this complex disease entity.

The intrinsic innervation of the lung is derived from neural crest cells as shown by optical projection tomography in Wnt1-Cre;YFP reporter mice

Within the embryonic lung, intrinsic nerve ganglia, which innervate airway smooth muscle, are required for normal lung development and function. We studied the development of neural crest-derived intrinsic neurons within the embryonic mouse lung by crossing Wnt1-Cre mice with R26R-EYFP reporter mice to generate double transgenic mice that express yellow fluorescent protein (YFP) in all neural crest cells (NCCs) and their derivatives. In addition to utilizing conventional immunohistochemistry on frozen lung sections, the complex organization of lung innervation was visualized in three dimensions by combining the genetic labelling of NCCs with optical projection tomography, a novel imaging technique that is particularly useful for the 3D examination of developing organs within embryos. YFP-positive NCCs migrated into the mouse lung from the oesophagus region at embryonic day 10.5. These cells subsequently accumulated around the bronchi and epithelial tubules of the lung and, as shown by 3D lung reconstructions with optical projection tomography imaging, formed an extensive, branching network in association with the developing airways. YFP-positive cells also colonized lung maintained in organotypic culture, and responded in a chemoattractive manner to the proto-oncogene, rearranged during transfection (RET) ligand, glial-cell-line-derived neurotrophic factor (GDNF), suggesting that the RET signalling pathway is involved in neuronal development within the lung. However, when the lungs of Ret(-/-) and Gfrα1(-/-) embryos, deficient in the RET receptor and GDNF family receptor α 1 (GFRα1) co-receptor respectively, were examined, no major differences in the extent of lung innervation were observed. Our findings demonstrate that intrinsic neurons of the mouse lung are derived from NCCs and that, although implicated in the development of these cells, the role of the RET signalling pathway requires further investigation.

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.

PHOX2B mutations and ventilatory control

The transcription factor PHOX2B is essential for the development of the autonomic nervous system. In humans, polyalanine expansion mutations in PHOX2B cause Congenital Central Hypoventilation Syndrome (CCHS), a rare life-threatening disorder characterized by hypoventilation during sleep and impaired chemosensitivity. CCHS is combined with comparatively less severe impairments of autonomic functions including thermoregulation, cardiac rhythm, and digestive motility. Respiratory phenotype analyses of mice carrying an invalidated Phox2b allele (Phox2b+/- mutant mice) or the Phox2b mutation (+7 alanine expansion) found in patients with CCHS (Phox2b(27Ala/+) mice) have shed light on the role for PHOX2B in breathing control and on the pathophysiological mechanisms underlying CCHS. Newborn mice that lacked one Phox2b allele (Phox2b+/-) had sleep apneas and depressed sensitivity to hypercapnia. However, these impairments resolved rapidly, whereas the CCHS phenotype is irreversible. Heterozygous Phox2b(27Ala/+) pups exhibited a lack of responsiveness to hypercapnia and unstable breathing; they died within the first few postnatal hours. The generation of mouse models of CCHS provides tools for evaluating treatments aimed at alleviating both the respiratory symptoms and all other autonomic symptoms of CCHS.

Congenital central hypoventilation syndrome (CCHS) and sudden infant death syndrome (SIDS): kindred disorders of autonomic regulation

Congenital central hypoventilation syndrome (CCHS) and sudden infant death syndrome (SIDS) were long considered rare disorders of respiratory control and more recently have been highlighted as part of a growing spectrum of disorders within the rubric of autonomic nervous system (ANS) dysregulation (ANSD). CCHS typically presents in the newborn period with a phenotype including alveolar hypoventilation, symptoms of ANSD and, in a subset of cases, Hirschsprung disease and later tumors of neural crest origin. Study of genes related to autonomic dysregulation and the embryologic origin of the neural crest led to the discovery of PHOX2B as the disease-defining gene for CCHS. Like CCHS, SIDS is thought to result from central deficits in control of breathing and ANSD, although SIDS risk is most likely defined by complex multifactorial genetic and environmental interactions. Some early genetic and neuropathological evidence is emerging to implicate serotonin systems in SIDS risk. The purpose of this article is to review the current understanding of the genetic basis for CCHS and SIDS, and discuss the impact of this information on clinical practice and future research directions.

Neural control of breathing: insights from genetic mouse models

Recent studies described the in vivo ventilatory phenotype of mutant newborn mice with targeted deletions of genes involved in the organization and development of the respiratory-neuron network. Whole body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. Breathing-pattern abnormalities with apneas occur in mutant newborn mice that lack genes involved in the development and modulation of rhythmogenesis. Studies of deficits in ventilatory responses to hypercapnia and/or hypoxia helped to identify genes involved in chemosensitivity to oxygen and carbon dioxide. Combined studies in mutant newborn mice and in humans have shed light on the pathogenesis of genetically determined respiratory-control abnormalities such as congenital central hypoventilation syndrome, Rett syndrome, and Prader-Willi syndrome. The development of mouse models has opened up the field of research into new treatments for respiratory-control disorders in humans.

Development of the neural crest-derived intrinsic innervation of the human lung

The formation of neural tissue, in association with airway smooth muscle (ASM), is a feature of normal lung development and function. Intrinsic neuronal tissue has recently been shown, in animal models, to be derived from neural crest cells (NCC). Since defects in NCC development underlie a range of disease states (neurocristopathies), it is important to determine the spatiotemporal development of NCC in the human lung, as defects in their development could have pathophysiologic implications. The aims of this study were to: (1) establish a time course for the formation of ASM and neural tissue within the embryonic and fetal human lung, (2) investigate whether intrinsic neural tissue within the lung is derived from NCC, and (3) gain insight into the possible signaling mechanisms underlying the development of the intrinsic lung innervation. Using human lung tissue from Weeks 6 to 12 of gestation, we analyzed the formation of ASM, NCC, neuronal and glial tissue, and the expression of Gfralpha1, a receptor component of the RET (rearranged during transfection) tyrosine kinase signaling pathway. Our results showed that NCC accumulated along the branching airways, in close association with the ASM, and differentiated into neurons and glia. Neural crest-derived neural tissue within the lung strongly expressed membrane-bound Gfralpha1, and soluble Gfralpha1 was expressed within the lung mesenchyme, but only at early developmental stages. Together these findings indicate that the intrinsic innervation of the human lung is derived from the neural crest.

Neuroendocrine functions of melanocytes: beyond the skin-deep melanin maker

The skin is armored with "dead cells", the stratum corneum, and is continuously exposed to external stressful environments, such as atmospheric oxygen, solar radiations, and thermal and chemical insults. Melanocytes of neural crest origin are located in the skin, eye, inner ear, and leptomeninges. Melanin pigment in the skin is produced by melanocytes under the influence of various endogenous factors, derived from neighboring keratinocytes and underlying fibroblasts. The differentiation and functions of melanocytes are regulated at multiple processes, including transcription, RNA editing, melanin synthesis, and the transport of melanosomes to keratinocytes. Impairment at each step causes the pigmentary disorders in humans, with the historical example of oculocutaneous albinism. Moreover, heterozygous mutations in the gene coding for microphthalmia-associated transcription factor, a key regulator for melanocyte development, are associated with Waardenburg syndrome type 2, an auditory-pigmentary disorder. Sun tanning, melasma, aging spots (lentigo senilis), hair graying, and melanoma are well-known melanocyte-related pathologies. Melanocytes therefore have attracted much attention of many ladies, makeup artists and molecular biologists. More recently, we have shown that lipocalin-type prostaglandin D synthase (L-PGDS) is expressed in melanocytes but not in other skin cell types. L-PGDS generates prostaglandin D2 and also functions as an inter-cellular carrier protein for lipophilic ligands, such as bilirubin and thyroid hormones. Thus, melanocytes may exert hitherto unknown functions through L-PGDS and prostaglandin D2. Here we update the neuroendocrine functions of melanocytes and discuss the possible involvement of melanocytes in the control of the central chemosensor that generates respiratory rhythm.

Sudden Infant Death Syndrome: review of implicated genetic factors

Genetic studies in Sudden Infant Death Syndrome (SIDS) have been motivated by clinical, epidemiological, and/or neuropathological observations in SIDS victims, with subsequent pursuit of candidate genes in five categories: (1) genes for ion channel proteins based on electrocardiographic evidence of prolonged QT intervals in SIDS victims, (2) gene for serotonin transporter based on decreased serotonergic receptor binding in brainstems of SIDS victims, (3) genes pertinent to the early embryology of the autonomic nervous system (ANS) (and with a link to the 5-HT system) based on reports of ANS dysregulation in SIDS victims, (4) genes for nicotine metabolizing enzymes based on evidence of cigarette smoking as a modifiable risk factor for SIDS, and (5) genes regulating inflammation, energy production, hypoglycemia, and thermal regulation based on reports of postnatal infection, low birth weight, and/or overheating in SIDS victims. Evidence for each of these classes of candidate genes is reviewed in detail. As this review indicates, a number of genetically controlled pathways appear to be involved in at least some cases of SIDS. Given the diversity of results to date, genetic studies support the clinical impression that SIDS is heterogeneous with more than one entity and with more than one possible genetic etiology. Future studies should consider expanded phenotypic features that might help clarify the heterogeneity and improve the predictive value of the identified genetic factors. Such features should be evaluated to the extent possible in both SIDS victims and their family members. With 2,162 infants dying from SIDS in 2003 in the U.S. alone, and improved but still imperfect parent and caretaker compliance with known modifiable risk factors for SIDS, it behooves clinicians, researchers, and parents to combine efforts to reach a common goal. The message of the "Back to Sleep" campaign needs to be re-introduced/re-engineered to reach families and caretakers of all ethnic groups. Clinicians and researchers need to gently inform new SIDS parents about the opportunity to contribute tissue to the NICHD-funded University of Maryland Brain and Tissue Bank. By expanding the network of clinicians, scientists, and families working together, and by combined efforts in a collaborative multi-center study of candidate genes and/or genomics, the discovery of the genetic profile of the infant at risk for SIDS can ultimately be determined.

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.

Regulation of respiratory neuron development by neurotrophic and transcriptional signaling mechanisms

Functionally diverse populations of respiratory neurons appear to be targets of common neurotrophic and transcriptional signaling pathways. For example, peripheral chemoafferent neurons and noradrenergic neurons in the pontine A5 cell group both require co-signaling by brain derived neurotrophic factor (BDNF) and glial cell line derived neurotrophic factor (GDNF) for survival, growth and/or phenotypic differentiation. Moreover, these same cell groups are dependent on the Phox2 family of transcription factors for early cell type specification. In addition, BDNF and its receptor, TrkB, are expressed in the pre-Botzinger complex (pBC), a critical site for respiratory rhythm generation, and exogenous BDNF can modulate the activity of pBC neurons. This convergence of BDNF, GDNF and Phox2 dependencies may help to explain how mutations in each of these pathways can result in human developmental disorders of breathing.

[Ondine syndrome: diagnosis and management]

Ondine's syndrome is a congenital central hypoventilation syndrome due to a disorder in the autonomic control of breathing in the absence of any primary disease that would explain it. Although the incidence of this entity is low, it may be underestimated due to the variable clinical expression of this syndrome, depending on its severity. Early diagnosis is of great importance to provide appropriate management to prevent the acute and chronic asphyxia that determines the long-term prognosis of this disease. This review aims to present practical management guidelines to provide doctors unfamiliar with this syndrome with basic knowledge of the diagnosis, treatment and follow-up of these patients.

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.

Ablation of vagal preganglionic neurons innervating the extra-thoracic trachea affects ventilatory responses to hypercapnia and hypoxia

This study tested the hypothesis that during hypercapnia or hypoxia, airway-related vagal preganglionic neurons (AVPNs) of the nucleus ambiguus (NA) release acetylcholine (ACh), which in a paracrine fashion, activates ACh receptors expressed by inspiratory rhythm generating cells. AVPNs in the NA were ablated by injecting a saporin- (SA) cholera toxin b subunit (CTb-SA) conjugate into the extra-thoracic trachea (n=6). Control animals were injected with free CTb (n=6). In CTb treated rats, baseline ventilation and ventilatory responses to hypercapnia (5 and 12% CO(2) in O(2)) or hypoxia (8% O(2) in N(2)) were similar (p>0.05) prior to and 5 days after injection. CTb-SA injected rats maintained rhythmic breathing patterns 5 days post injection, however, tachypneic responses to hypercapnia or hypoxia were significantly reduced. The number of choline acetyltransferase (ChAT) immunoreactive cells in the NA was much lower (p<0.05) in CTb-SA rats as compared to animals receiving CTb only. These results suggest that AVPNs participate in the respiratory frequency response to hypercapnia or hypoxia.

Sodium/Proton Exchanger 3 in the Medulla Oblongata and Set Point of Breathing Control

RATIONALE

In vivo inhibition of the sodium/proton exchanger 3 (NHE3) in chemosensitive neurons of the ventrolateral brainstem augments central respiratory drive in anesthetized rabbits.

OBJECTIVES

To further explore the possible role of this exchanger for the control of breathing, we examined the individual relationship between brainstem NHE3 abundance and ventilation in rabbits during wakefulness.

METHODS

In 32 adult male rabbits on standard nutritional alkali load, alveolar ventilation, metabolic CO2 production, and blood gases were determined, together with arterial and urinary acid-base status and renal base control functions. Expression of NHE3 in brainstem tissue from the obex region was determined by quantitative real-time reverse-transcription polymerase chain reaction analysis.

MEASUREMENTS AND MAIN RESULTS

Regarding the distribution above and below the median, we classified high and low brainstem NHE3 animals, expressing a mean (+/- SEM) NHE3 mRNA of 2.08 +/- 0.28 and 0.72 +/- 0.06 fg cDNA/mg RNA, respectively. Alveolar ventilation of high brainstem NHE3 animals was lower than that of low brainstem NHE3 animals (715 +/- 36 vs. 919 +/- 41 ml . minute(-1); p < 0.01), a finding also reflected by a marked difference in Pa(CO2) (5.24 +/- 0.16 vs. 4.44 +/- 0.15 kPa; p < 0.01). Among possible secondary factors, CO2 production, systemic base excess, and fractional renal base reabsorption were not found to be different.

CONCLUSIONS

We conclude that the level of brainstem NHE3 expression-most likely via intracellular pH modulation-contributes to the individual control of breathing and Pa(CO2) in conscious rabbits by adjusting the set point and the loop gain of the system.

Glial cell line-derived neurotrophic factor (GDNF) is required for differentiation of pontine noradrenergic neurons and patterning of central respiratory output

Genetic mutations affecting signaling by glial cell line-derived neurotrophic factor (GDNF) perturb development of breathing in mice and are associated with congenital central hypoventilation syndrome in humans. However, the role of GDNF in development of brainstem neurons that control breathing is largely unknown. The present study demonstrates that genetic loss of GDNF decreases the number of tyrosine hydroxylase (TH) neurons in the pontine A5 noradrenergic cell group, a major source of inhibitory input to the medullary respiratory pattern generator. This phenotype is associated with a significant increase in the frequency of central respiratory output recorded from the fetal medulla-spinal cord in vitro. In dissociate cultures of the A5 region from rat embryos, GDNF increases TH cell number and neurite growth without affecting total neuronal survival or proliferation of TH neurons. These effects of GDNF are inhibited by function blocking antibodies against endogenous brain-derived neurotrophic factor (BDNF), indicating that GDNF requires BDNF as a cofactor to stimulate differentiation of A5 neurons. Our findings demonstrate that GDNF is required for development of pontine noradrenergic neurons in vivo and indicate that defects in the A5 cell group may contribute to the effects of genetic disruption of GDNF signaling on respiratory control.

A human yeast artificial chromosome containing the multiple endocrine neoplasia type 2B Ret mutation does not induce medullary thyroid carcinoma but does support the growth of kidneys and partially rescues enteric nervous system development in Ret-deficient mice

We generated a line of transgenic mice using a yeast artificial chromosome containing the Ret mutation responsible for the multiple endocrine neoplasia type 2B syndrome (MEN 2B). The resulting animals did not develop any of the expected neoplasms associated with MEN 2B. Transgenic animals were then bred with animals lacking murine Ret (Ret(M)) to further evaluate the function of human mutated Ret (Ret(H)(2B)) in the murine context. Whereas mice lacking Ret(M) exhibit intestinal aganglionosis and the absence of kidneys with other genitourinary anomalies, expression of the Ret(H)(2B) transgene in Ret(M)-deficient mice allowed significant renal development with a partial rescue of the enteric nervous system. These Ret(H)(2B)-positive/Ret(M)-deficient mice exhibit normal Ret expression and survive longer than Ret(M)-deficient mice, but still die at 3 to 5 days of age with evidence of enterocolitis. We conclude that the normal expression of a human Ret proto-oncogene with the MEN 2B mutation does not cause any features of MEN 2B in mice. Although the gene is normally expressed in the appropriate target tissues, there is incomplete phenotypic rescue in mice lacking murine Ret. These results suggest important interspecies differences between humans and mice in the function of the Ret oncogene.

Increased ventilation and CO2 chemosensitivity in acetylcholinesterase knockout mice

To investigate the effects of a permanent excess of acetylcholine (AChE) on respiration, breathing and chemosensitivity were analyzed from birth to adulthood in mice lacking the AChE gene (AChE-/-), in heterozygotes, and in control wild-type (AChE+/+) littermates. Breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O2) and hypercapnia (3-5% CO2) were measured by whole-body plethysmography. At rest AChE-/- mice show larger tidal volumes (VT, + 96% in adults), overall ventilation (VE, + 70%), and mean inspiratory flow (+270%) than wild-type mice, with no change in breathing frequency (fR). AChE-/- mice have a slightly blunted response to hypoxia, but increased VE and fR responses to hypercapnia. Heterozygous animals present no consistent alterations of breathing at rest and chemosensitivity is normal. Adult AChE-/- mice have an increased VE/VO2 and a marginally higher normalized VO2. The results suggest that the hyperventilation and altered chemosensitivity in AChE-/- mice largely reflect alterations of central respiratory control.

Sudden infant death syndrome: case-control frequency differences at genes pertinent to early autonomic nervous system embryologic development

We have previously identified polymorphisms in the serotonin transporter gene promoter region and in intron 2 that were more common among sudden infant death syndrome (SIDS) cases compared with control subjects. To elucidate further the genetic profile that might increase an infant's vulnerability to SIDS, we focused on the recognized relationship between autonomic nervous system (ANS) dysregulation and SIDS. We therefore studied genes pertinent to early embryologic development of the ANS, including MASH1, BMP2, PHOX2a, PHOX2b, RET, ECE1, EDN1, TLX3, and EN1 in 92 probands with SIDS and 92 gender- and ethnicity-matched control subjects. Eleven protein-changing rare mutations were identified in 14 of 92 SIDS cases among the PHOX2a, RET, ECE1, TLX3, and EN1 genes. Only 1 of these mutations (TLX3) was identified in 2 of 92 control subjects. Black infants accounted for 10 of these mutations in SIDS cases and 2 control subjects. Four protein-changing common polymorphisms were identified in BMP2, RET, ECE1, and EDN1, but the allele frequency did not differ between SIDS cases and control subjects. However, among SIDS cases, the allele frequency for the BMP2 common polymorphism demonstrated ethnic differences; among control subjects, the allele frequency for the BMP2 and the ECE1 common polymorphisms also demonstrated ethnic differences. These data represent further refinement of the genetic profile that might place an infant at risk for SIDS.

Genes and genetics in respiratory control

The genetic approach to respiratory control is opening up new paths for research into developmental respiratory control disorders. Despite the identification of numerous genes involved in respiratory control, none of the genetically engineered mice developed to date fully replicate the human respiratory phenotype of human developmental respiratory disorders. However, combining studies in humans and studies in mouse models has proved useful in identifying candidate genes for human developmental respiratory control disorders and providing pathogenic information. In clinical practice, the development of databases that incorporate clinical phenotypes and genetic samples from patients would facilitate further genetic studies. International multicentre studies would advance the area of respiratory control research.

Genetics and early disturbances of breathing control

Early disturbances in breathing control, including apneas of prematurity and apparently life-threatening events, account for some cases of sudden infant death syndrome and for a rare disorder called congenital central hypoventilation syndrome (CCHS). Data suggesting a genetic basis for CCHS have been obtained. Recently, we found heterozygous de novo mutations of the PHOX2B gene in 18 of 29 individuals with CCHS. Most mutations consisted of five to nine alanine expansions within a 20-residue polyalanine tract, probably resulting from nonhomologous recombination. Other mutations, generally inherited from one of the parents, in the coding regions of genes involved in the endothelin and RET signaling pathways and in the brain-derived-neurotrophic factor (BDNF) gene have been found in a few CCHS patients. Interestingly, all these genes are involved in the development of neural crest cells. Targeted disruption of these genes in mice has provided information on the pathophysiological mechanisms underlying CCHS. Despite the identification of these genes involved in breathing control, none of the genetically engineered mice developed to date replicate the full human CCHS respiratory phenotype. Recent insights into the genetic basis for CCHS may shed light on the genetics of other early disturbances in breathing control, such as apnea of prematurity and sudden infant death syndrome.

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.

Sleep apnea in mice: a useful animal model for study of SIDS?

Although the incidence of sudden infant death syndrome (SIDS) has been decreased by education programs to avoid sleeping in prone position, the pathological mechanisms of SIDS have not fully been understood. Basic research on sleep apnea using experimental animals may help further understanding and prevention of SIDS because the syndrome is thought as inability to wake up from respiratory arrest (apnea) during sleep. Although several animal models of sleep apnea have been described previously, mice would be useful experimental animals in that these animals are frequently used in genetic engineering. Those considerations prompted us to establish a method for measuring ventilation of mice concomitantly with electroencephalography and electromyography for assessing sleep-wake states. Normal wild-type mice developed two types of central sleep apneas (CSA), that is, post-sigh and spontaneous apneas, as normal humans do. Moreover, post-sigh apneas in mice were observed exclusively during slow-wave sleep (SWS) while spontaneous apneas were seen in both SWS and rapid eye movement (REM) sleep. These characteristics are very similar to those of sleep apneas in healthy human infants and children. Therefore, mice seem to be a promising experimental animal model for studying the genetic and molecular basis of respiratory regulation and dysregulation during sleep in humans, especially infants and children. However, we should keep in mind limitations in studying mice as an animal model of SIDS, since they are nocturnal rodents and they sleep in the prone position.

Genes modulating chemical breathing control: lessons from mutant animals

Genetic factors influence breathing control. Respiratory phenotypes of mutant mice may help to better understand these factors. Congenital central hypoventilation syndrome (CCHS) is a rare disorder defined as failure of chemical control of breathing causing central alveolar hypoventilation, especially during sleep. A genetic basis for CCHS is supported by several arguments, mainly the identification, in a few CCHS patients, of heterozygous mutations of genes contributing to neural crest cell development, namely, genes involved in the endothelin and c-ret pathways. Furthermore, plethysmography studies of the respiratory phenotypes of newborn heterozygous mutant mice have shown that genes in both pathways are involved in breathing control at birth. Nevertheless, no single gene mutation in newborn mice reproduces the human CCHS phenotype. Avenues for future research into the genetics of CCHS include (i) testing of mutant newborn mice for genes in other pathways and (ii) use of microarrays to identify gene clusters that should be associated with abnormal chemical breathing control.

Genomic approaches to understanding obstructive sleep apnea

Obstructive sleep apnea (OSA) is an increasingly recognized, common chronic disease in the developed nations and is a complex disease that has high social and economic costs. OSA and its associated 'intermediate' phenotypes-craniofacial structure, body fat distribution and metabolism, and neurological control of the upper airway muscles and of sleep and circadian rhythm-are under a substantial degree of genetic control. Investigating the genetic aetiology of OSA offers a means of better understanding its pathogenesis, with the goal of improving preventive strategies, diagnostic tools and therapies. Molecular studies of OSA itself are in their infancy, but considerable effort and expense has already been expended in attempts to detect genetic loci contributing to OSA-associated intermediate phenotypes, such as obesity. However, many of the fundamental questions relating to the genetic epidemiology of OSA and associated factors remain unanswered. This chapter reviews the current state of knowledge of the genetics of OSA, with a focus on genomic approaches to understanding sleep disorders.

Do genes on rat chromosomes 9, 13, 16, 18, and 20 contribute to regulation of breathing?

As part of a large scale, high through-put physiologic genomics study, we sought to determine whether genes on rat chromosomes 9, 13, 16, 18, and 20 contribute to phenotypic differences in the control of breathing between two inbred rat strains (SS/Mcw and BN/Mcw). Through a chromosomal substitution breeding strategy, we created 5 consomic rat strains (SS.BN9, SS.BN13, SS.BN16, SS.BN18, and SS.BN20), which were BN/Mcw homozygous at only one chromosome and SS/Mcw homozygous at all other chromosomes. Standard plethsmography was used to assess eupneic breathing and ventilatory responses to CO(2) (FI(CO(2))=0.07) and hypoxia (FI(CO(2))=0.12), and Pa(CO(2)) during treadmill exercises provided the index of the exercise hyperpnea. There were no robust differences in eupneic breathing between any strains. The ventilatory response to CO(2) was 150% greater (P<0.001) in the SS/Mcw rats than in the BN/Mcw rats and all consomic strains had the SS/Mcw phenotype. Hyperventilation during hypoxia did not differ between the parental and the consomic strains, but ventilation during hypoxia was greater (P<0.001) in the SS/Mcw than in the BN/Mcw, and the SS.BN9, and SS.BN18 appeared to acquire this BN/Mcw phenotype. The hyperventilation during treadmill walking was greater (P<0.006) in the BN/Mcw and the SS.BN18 rats than in the SS/Mcw rats. Finally, the duration of the apnea following an augmented breath (post sigh apnea, PSA) was greater (P<0.001) in the BN/Mcw and the SS.BN9 rats than all other strains. We conclude that the robust difference between the parental strains in ventilatory CO(2) sensitivity is not due to genotypic differences on the 5 chromosomes studied to date, but genotypic differences on chromosomes 9 and 18 contribute to differences in ventilatory responses to hypoxia, exercise, and/or to the differences in the PSA.

Neuronal growth factors and development of respiratory control

Neurotrophic molecules, released by neurons and neural target tissues, play a pivotal role in regulating neuronal development and plasticity. This article reviews recent work demonstrating the pivotal role of two such molecules, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), in the growth and maturation of respiratory neurons and the expression of normal ventilatory behavior. Although BDNF and GDNF are structurally dissimilar and signal through wholly distinct receptors, they are both required for development of peripheral chemoafferent neurons that provide hypoxic drive to the brainstem respiratory network. Studies of genetically engineered mice carrying targeted deletions in the genes encoding BDNF and GDNF, as well as genetic linkage analysis in humans, indicate that these trophic molecules may be candidate genes for human developmental disorders of breathing.

Genetic aspects of breathing: on interactions between hypercapnia and hypoxia

Indeed, specific genes in humans and mice regulate breathing pattern at baseline and breathing control during chemical stimulation. The current review addresses the question of coupling plausible candidate genes to physiological variation in control of breathing. That is, can genes discovered in mice be candidates assigned to similar physiological mechanisms as genetic control of breathing in humans? As an illustration, this review examines the interaction of hypoxia in affecting the hypercapnic ventilatory sensitivity (HCVS) curve in humans and mice. Strain distribution patterns (SDPs) incorporating ten inbred mouse strains demonstrate that hypoxic stimulation likely influences HVCS via an additive mechanism rather than synergy between hypercapnia and hypoxia (i.e. CO(2) potentiation). As a mechanism associated with the chemical control of breathing in humans, the absence of CO(2) potentiation in mice suggests that specific genes interact to establish variation in complex breathing traits among mouse strains and between species. If future studies support the current evidence, the absence of CO(2) potentiation in mice compared with humans suggest a clearly defined species difference, which may depend on alternative hypoxic interactions such as hypometabolic and central neuronal depressive mechanisms in mice. Because the complexity of breathing mechanisms varies with modest adjustments in the environment, gene-targeting strategies that achieve 'one-gene, one-phenotype' results must be complimented with alternative strategies that consider integrating complex respiratory mechanisms with gene-to-gene interactions.

Developmental plasticity in respiratory control

Development of the mammalian respiratory control system begins early in gestation and does not achieve mature form until weeks or months after birth. A relatively long gestation and period of postnatal maturation allows for prolonged pre- and postnatal interactions with the environment, including experiences such as episodic or chronic hypoxia, hyperoxia, and drug or toxin exposures. Developmental plasticity occurs when such experiences, during critical periods of maturation, result in long-term alterations in the structure or function of the respiratory control neural network. A critical period is a time window during development devoted to structural and/or functional shaping of the neural systems subserving respiratory control. Experience during the critical period can disrupt and alter developmental trajectory, whereas the same experience before or after has little or no effect. One of the clearest examples to date is blunting of the adult ventilatory response to acute hypoxia challenge by early postnatal hyperoxia exposure in the newborn. Developmental plasticity in neural respiratory control development can occur at multiple sites during formation of brain stem neuronal networks and chemoafferent pathways, at multiple times during development, by multiple mechanisms. Past concepts of respiratory control system maturation as rigidly predetermined by a genetic blueprint have now yielded to a different view in which extremely complex interactions between genes, transcriptional factors, growth factors, and other gene products shape the respiratory control system, and experience plays a key role in guiding normal respiratory control development. Early-life experiences may also lead to maladaptive changes in respiratory control. Pathological conditions as well as normal phenotypic diversity in mature respiratory control may have their roots, at least in part, in developmental plasticity.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Ventilatory responses to hypercapnia and hypoxia in heterozygous c-ret newborn mice

The c-ret proto-oncogene encodes a tyrosine-kinase receptor involved in survival and differentiation of neural crest cell lineages. Previous studies have shown that homozygous c-ret-/- mice die soon after birth and have impaired ventilatory responses to hypercapnia. Heterozygous c-ret +/- mice develop normally, but their respiratory phenotype has not been described in detail. We used whole-body flow plethysmography to compare baseline breathing and ventilatory and arousal responses to chemical stimuli in unrestrained heterozygous c-ret +/- newborn mice and their wild-type c-ret +/+ littermates at 10-12 h of postnatal age. The hyperpnoeic and arousal responses to hypoxia and hypercapnia were not significantly different in these two groups. However, the number and total duration of apnoeas and periodic breathing episodes were significantly higher in c-ret +/- than in c-ret +/+ pups during hypoxia and post-hypoxic normoxia. These results are further evidence that respiratory control at birth is heavily dependent on genes involved in the neural determination of neural crest cells.

Abnormal esophageal motility in children with congenital central hypoventilation syndrome

BACKGROUND & AIMS

Congenital central hypoventilation syndrome, an unexplained disorder of the central control of breathing that may reflect widespread dysfunction of brainstem structures, is regarded as a form of neuro cristopathy. Because swallowing-induced peristalsis is centrally controlled and depends on neural crest-derived esophageal innervation, we looked for esophageal dysmotility in patients with congenital central hypoventilation syndrome.

METHODS

Seven patients without dysphagia or any other upper gastrointestinal tract symptoms were studied prospectively (5 girls and 2 boys; median age, 14 years; range, 11-18 years). They were compared with 7 age- and sex-matched controls. Esophageal manometry was performed using a low-compliance infusion system and the station pull-through technique. At least 10 wet swallows were analyzed in each subject.

RESULTS

Pressure wave propagation was abnormal in all 7 patients (median percentage of swallows propagated, 18%, and range, 0-66; controls, 90% and 80-100; P < 0.001). Lower esophageal sphincter relaxation was abnormal in 5 patients (patients, 73% and 53-100; controls, 95% and 90-100; P = 0.01). In 2 patients, lower esophageal sphincter pressure was above the 95th percentile of control values.

CONCLUSIONS

These abnormalities are strong evidence of lower esophageal dysfunction in congenital central hypoventilation syndrome. We speculate that the underlying mechanism may be dysfunction of the central structures that control swallowing.

Congenital central hypoventilation syndrome: a novel mutation of the RET gene in an isolated case

Recently, a few genetic abnormalities were identified in congenital central hypoventilation syndrome (CCHS or Ondine's curse). CCHS is often associated with other neurocristopathies, especially with Hirschsprung's disease (HSCR). Mutations of the genes involved in the receptor tyrosine kinase RET (REarranged during Transfection) (RET)-glial cell line-derived neurotrophic factor (GDNF) and/or endothelin 3 (EDN3)-endothelin receptor-B (EDNRB) signaling pathway have been found in some of HSCR patients. In this study, we analyzed candidates for HSCR, namely the RET, GDNF, EDN3 and EDNRB genes in three isolated CCHS patients to confirm the hypothesis that some CCHS patients have a common genetic abnormality with patients having HSCR or other neurocristopathies. We found a novel R114H mutation of the RET gene in one patient. The R114H mutation is unlikely to be a polymorphism and appears to be associated with CCHS. In addition, we also examined the HOX11L2 (RNX) gene, for which knock-out mice showed CCHS-like syndrome in these isolated CCHS patients and did not detected any mutation. Further cases should be analyzed for more candidates to clarify the pathophysiology of CCHS.

Congenital central hypoventilation syndrome associated with Hirschsprung's disease and neuroblastoma: case of multiple neurocristopathies

We report on a male infant with the rare combined occurrence of congenital central hypoventilation syndrome (CCHS or Ondine's curse), Hirschsprung's disease (HD), and neuroblastoma. Current therapeutical options leave no doubt that children with isolated forms of CCHS, HD, or neuroblastoma must be treated, but management decisions and the ethical dilemma become more difficult with the presence of multiple neurocristopathies. Our patient was dependent on mechanical ventilation and total parenteral nutrition, when a neuroblastoma was diagnosed at age 5 months. We initiated an attempt at curative chemotherapy. The tumor failed to respond to recommended chemotherapeutic regimens, and the patient died at 11 months of age. We emphasize the importance of screening CCHS patients for associated illnesses such as neuroblastoma and ganglioneuroblastoma at time of diagnosis.

MASH-1/RET pathway involvement in development of brain stem control of respiratory frequency in newborn mice

Respiratory abnormalities have been described in MASH-1 (mammalian achaete-scute homologous gene) and c-RET ("rearranged during transfection") mutant newborn mice. However, the neural mechanisms underlying these abnormalities have not been studied. We tested the hypothesis that the MASH-1 mutation may impair c-RET expression in brain stem neurons involved in the control of breathing. To do this, we analyzed brain stem c-RET expression and respiratory phenotype in MASH-1 +/+ wild-type, MASH-1 +/- heterozygous, and MASH-1 -/- knock-out newborn mice during the first 2 h of life. In MASH-1 -/- newborns, c-RET gene expression was absent in the noradrenergic nuclei (A2, A5, A6, A7) that contribute to modulate respiratory frequency and in scattered cells of the rostral ventrolateral medulla. The c-RET transcript levels measured by quantitative RT-PCR were lower in MASH-1 -/- and MASH-1 +/- than in MASH-1 +/+ brain stems (P = 0.001 and P = 0.003, respectively). Breath durations were shorter in MASH-1 -/- and MASH-1 +/- than in MASH-1 +/+ mice (P = 0.022) and were weakly correlated with c-RET transcript levels (P = 0.032). Taken together, these results provide evidence that MASH-1 is upstream of c-RET in noradrenergic brain stem neurons important for respiratory rhythm modulation.

Chemoreceptive mechanisms elucidated by studies of congenital central hypoventilation syndrome

Humans born with the condition of central hypoventilation during non-rapid eye movement sleep, termed congenital central hypoventilation syndrome (CCHS), invariably have absent or greatly diminished central hypercapnic ventilatory chemosensitivity. Genetic and pathological studies of CCHS may enable identification of the genes or areas of the central nervous system involved in the syndrome and thus implicated in central hypercapnic ventilatory chemosensitivity. Functional studies of CCHS permit a more quantitative assessment of the importance of ventilatory chemosensitivity in the regulation of breathing during wakefulness and sleep. The experimental evidence suggests that central hypercapnic ventilatory chemosensitivity is crucial in regulating alveolar ventilation during non-rapid eye movement sleep but not during rapid eye movement sleep or during many of the behaviors occurring during wakefulness. Presumably, other neural drives to breathe supervene to enable adequate ventilation. However, although physiological studies in CCHS subjects have been greatly instructive, their accurate interpretation will have to await future determination of the potential genetic and/or neuroanatomic basis of the syndrome.

Orphan nuclear receptor Nurr1 is essential for Ret expression in midbrain dopamine neurons and in the brain stem

The orphan nuclear receptor Nurr1 is essential for development of midbrain dopamine (DA) cells. In Nurr1-deficient mice, DA precursor cells fail to migrate normally, are unable to innervate target areas, and only transiently express DA cell marker genes. In the search for Nurr1-regulated genes that might explain this developmental phenotype, we found that expression of the receptor tyrosine kinase Ret is deregulated in these cells of Nurr1-deficient embryos. In addition, our analyses establish Nurr1 as an early marker for the dorsal motor nucleus (DMN) of the vagus nerve. Interestingly, Ret expression is absent also in these cells in Nurr1-targeted mice. Neuronal innervation of vagus nerve target areas appeared normal apart from a subtle disorganization of the DMN-derived nerve fibers. In conclusion, regulation of Ret by Nurr1 in midbrain DA neurons and in the DMN has implications for both embryonal development and adult physiology in which signaling by neurotrophic factors plays important roles.

Maturation of baseline breathing and of hypercapnic and hypoxic ventilatory responses in newborn mice

Breathing during the first postnatal hours has not been examined in mice, the preferred mammalian species for genetic studies. We used whole body plethysmography to measure ventilation (VE), breath duration (T(TOT)), and tidal volume (VT) in mice delivered vaginally (VD) or by cesarean section (CS). In experiment 1, 101 VD and 100 CS pups aged 1, 6, 12, 24, or 48 h were exposed to 8% CO2 or 10% O2 for 90 s. In experiment 2, 31 VD pups aged 1, 12, or 24 h were exposed to 10% O2 for 5 min. Baseline breathing maturation was delayed in CS pups, but VE responses to hypercapnia and hypoxia were not significantly different between VD and CS pups [at postnatal age of 1 h (H1): 48 +/- 44 and 18 +/- 32%, respectively, in VD and CS pups combined]. The VE increase induced by hypoxia was greater at H12 (46 +/- 27%) because of T(TOT) response maturation. At all ages, hypoxic decline was ascribable mainly to a VT decrease, and posthypoxic decline was ascribable to a T(TOT) increase with apneas, suggesting different underlying neuronal mechanisms.