Central respiratory rhythmogenesis is abnormal in lbx1- deficient mice

Genetic identification of medullary neurons underlying congenital hypoventilation

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO2. The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation.

Transgenic rodents as dynamic models for the study of respiratory rhythm generation and modulation: a scoping review and a bibliometric analysis

The pre-Bötzinger complex, situated in the ventrolateral medulla, serves as the central generator for the inspiratory phase of the respiratory rhythm. Evidence strongly supports its pivotal role in generating, and, in conjunction with the post-inspiratory complex and the lateral parafacial nucleus, in shaping the respiratory rhythm. While there remains an ongoing debate concerning the mechanisms underlying these nuclei's ability to generate and modulate breathing, transgenic rodent models have significantly contributed to our understanding of these processes. However, there is a significant knowledge gap regarding the spectrum of transgenic rodent lines developed for studying respiratory rhythm, and the methodologies employed in these models. In this study, we conducted a scoping review to identify commonly used transgenic rodent lines and techniques for studying respiratory rhythm generation and modulation. Following PRISMA guidelines, we identified relevant papers in PubMed and EBSCO on 29 March 2023, and transgenic lines in Mouse Genome Informatics and the International Mouse Phenotyping Consortium. With strict inclusion and exclusion criteria, we identified 80 publications spanning 1997-2022 using 107 rodent lines. Our findings revealed 30 lines focusing on rhythm generation, 61 on modulation, and 16 on both. The primary in vivo method was whole-body plethysmography. The main in vitro method was hypoglossal/phrenic nerve recordings using the en bloc preparation. Additionally, we identified 119 transgenic lines with the potential for investigating the intricate mechanisms underlying respiratory rhythm. Through this review, we provide insights needed to design more effective experiments with transgenic animals to unravel the mechanisms governing respiratory rhythm. The identified transgenic rodent lines and methodological approaches compile current knowledge and guide future research towards filling knowledge gaps in respiratory rhythm generation and modulation.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Criteria for central respiratory chemoreceptors: experimental evidence supporting current candidate cell groups

An interoceptive homeostatic system monitors levels of CO2/H+ and provides a proportionate drive to respiratory control networks that adjust lung ventilation to maintain physiologically appropriate levels of CO2 and rapidly regulate tissue acid-base balance. It has long been suspected that the sensory cells responsible for the major CNS contribution to this so-called respiratory CO2/H+ chemoreception are located in the brainstem-but there is still substantial debate in the field as to which specific cells subserve the sensory function. Indeed, at the present time, several cell types have been championed as potential respiratory chemoreceptors, including neurons and astrocytes. In this review, we advance a set of criteria that are necessary and sufficient for definitive acceptance of any cell type as a respiratory chemoreceptor. We examine the extant evidence supporting consideration of the different putative chemoreceptor candidate cell types in the context of these criteria and also note for each where the criteria have not yet been fulfilled. By enumerating these specific criteria we hope to provide a useful heuristic that can be employed both to evaluate the various existing respiratory chemoreceptor candidates, and also to focus effort on specific experimental tests that can satisfy the remaining requirements for definitive acceptance.

Transformation of Our Understanding of Breathing Control by Molecular Tools

The rhythmicity of breath is vital for normal physiology. Even so, breathing is enriched with multifunctionality. External signals constantly change breathing, stopping it when under water or deepening it during exertion. Internal cues utilize breath to express emotions such as sighs of frustration and yawns of boredom. Breathing harmonizes with other actions that use our mouth and throat, including speech, chewing, and swallowing. In addition, our perception of breathing intensity can dictate how we feel, such as during the slow breathing of calming meditation and anxiety-inducing hyperventilation. Heartbeat originates from a peripheral pacemaker in the heart, but the automation of breathing arises from neural clusters within the brainstem, enabling interaction with other brain areas and thus multifunctionality. Here, we document how the recent transformation of cellular and molecular tools has contributed to our appreciation of the diversity of neuronal types in the breathing control circuit and how they confer the multifunctionality of breathing.

Transcription factors regulating the specification of brainstem respiratory neurons

Breathing (or respiration) is an unconscious and complex motor behavior which neuronal drive emerges from the brainstem. In simplistic terms, respiratory motor activity comprises two phases, inspiration (uptake of oxygen, O2) and expiration (release of carbon dioxide, CO2). Breathing is not rigid, but instead highly adaptable to external and internal physiological demands of the organism. The neurons that generate, monitor, and adjust breathing patterns locate to two major brainstem structures, the pons and medulla oblongata. Extensive research over the last three decades has begun to identify the developmental origins of most brainstem neurons that control different aspects of breathing. This research has also elucidated the transcriptional control that secures the specification of brainstem respiratory neurons. In this review, we aim to summarize our current knowledge on the transcriptional regulation that operates during the specification of respiratory neurons, and we will highlight the cell lineages that contribute to the central respiratory circuit. Lastly, we will discuss on genetic disturbances altering transcription factor regulation and their impact in hypoventilation disorders in humans.

Microglia shape the embryonic development of mammalian respiratory networks

Microglia, brain-resident macrophages, play key roles during prenatal development in defining neural circuitry function, including ensuring proper synaptic wiring and maintaining homeostasis. Mammalian breathing rhythmogenesis arises from interacting brainstem neural networks that are assembled during embryonic development, but the specific role of microglia in this process remains unknown. Here, we investigated the anatomical and functional consequences of respiratory circuit formation in the absence of microglia. We first established the normal distribution of microglia within the wild-type (WT, Spi1^+/+ (Pu.1 WT)) mouse (Mus musculus) brainstem at embryonic ages when the respiratory networks are known to emerge (embryonic day (E) 14.5 for the parafacial respiratory group (epF) and E16.5 for the preBötzinger complex (preBötC)). In transgenic mice depleted of microglia (Spi1^−/− (Pu.1 KO) mutant), we performed anatomical staining, calcium imaging, and electrophysiological recordings of neuronal activities in vitro to assess the status of these circuits at their respective times of functional emergence. Spontaneous respiratory-related activity recorded from reduced in vitro preparations showed an abnormally slow rhythm frequency expressed by the epF at E14.5, the preBötC at E16.5, and in the phrenic motor nerves from E16.5 onwards. These deficits were associated with a reduced number of active epF neurons, defects in commissural projections that couple the bilateral preBötC half-centers, and an accompanying decrease in their functional coordination. These abnormalities probably contribute to eventual neonatal death, since plethysmography revealed that E18.5 Spi1^−/− embryos are unable to sustain breathing activity ex utero. Our results thus point to a crucial contribution of microglia in the proper establishment of the central respiratory command during embryonic development.

Molecular Organization and Patterning of the Medulla Oblongata in Health and Disease

The medulla oblongata, located in the hindbrain between the pons and the spinal cord, is an important relay center for critical sensory, proprioceptive, and motoric information. It is an evolutionarily highly conserved brain region, both structural and functional, and consists of a multitude of nuclei all involved in different aspects of basic but vital functions. Understanding the functional anatomy and developmental program of this structure can help elucidate potential role(s) of the medulla in neurological disorders. Here, we have described the early molecular patterning of the medulla during murine development, from the fundamental units that structure the very early medullary region into 5 rhombomeres (r7–r11) and 13 different longitudinal progenitor domains, to the neuronal clusters derived from these progenitors that ultimately make-up the different medullary nuclei. By doing so, we developed a schematic overview that can be used to predict the cell-fate of a progenitor group, or pinpoint the progenitor domain of origin of medullary nuclei. This schematic overview can further be used to help in the explanation of medulla-related symptoms of neurodevelopmental disorders, e.g., congenital central hypoventilation syndrome, Wold–Hirschhorn syndrome, Rett syndrome, and Pitt–Hopkins syndrome. Based on the genetic defects seen in these syndromes, we can use our model to predict which medullary nuclei might be affected, which can be used to quickly direct the research into these diseases to the likely affected nuclei.

Are thyrotropin-releasing hormone (TRH) and analog taltirelin viable reversal agents of opioid-induced respiratory depression?

Opioid-induced respiratory depression (OIRD) is a potentially life-threatening complication of opioid consumption. Apart from naloxone, an opioid antagonist that has various disadvantages, a possible reversal strategy is treatment of OIRD with the hypothalamic hormone and neuromodulator thyrotropin-releasing hormone (TRH). In this review, we performed a search in electronic databases and retrieved 52 papers on the effect of TRH and TRH-analogs on respiration and their efficacy in the reversal of OIRD in awake and anesthetized mammals, including humans. Animal studies show that TRH and its analog taltirelin stimulate breathing via an effect at the preBötzinger complex, an important respiratory rhythm generator within the brainstem respiratory network. An additional respiratory excitatory effect may be related to TRH's analeptic effect. In awake and anesthetized rodents, TRH and taltirelin improved morphine- and sufentanil-induced respiratory depression, by causing rapid shallow breathing. This pattern of breathing increases the work of breathing, dead space ventilation, atelectasis, and hypoxia. In awake and anesthetized humans, a continuous infusion of intravenous TRH with doses up to 8 mg, did not reverse sufentanil- or remifentanil-induced respiratory depression. This is related to poor penetration of TRH into the brain compartment but also other causes are discussed. No human data on taltirelin are available. In conclusion, data from animals and human indicate that TRH is not a viable reversal agent of OIRD in awake or anesthetized humans. Further human studies on the efficacy and safety of TRH's more potent and longer lasting analog taltirelin are needed as this agent seems to be a more promising reversal drug.

Axonal Projection Patterns of the Dorsal Interneuron Populations in the Embryonic Hindbrain

Unraveling the inner workings of neural circuits entails understanding the cellular origin and axonal pathfinding of various neuronal groups during development. In the embryonic hindbrain, different subtypes of dorsal interneurons (dINs) evolve along the dorsal-ventral (DV) axis of rhombomeres and are imperative for the assembly of central brainstem circuits. dINs are divided into two classes, class A and class B, each containing four neuronal subgroups (dA1-4 and dB1-4) that are born in well-defined DV positions. While all interneurons belonging to class A express the transcription factor Olig3 and become excitatory, all class B interneurons express the transcription factor Lbx1 but are diverse in their excitatory or inhibitory fate. Moreover, within every class, each interneuron subtype displays its own specification genes and axonal projection patterns which are required to govern the stage-by-stage assembly of their connectivity toward their target sites. Remarkably, despite the similar genetic landmark of each dINs subgroup along the anterior-posterior (AP) axis of the hindbrain, genetic fate maps of some dA/dB neuronal subtypes uncovered their contribution to different nuclei centers in relation to their rhombomeric origin. Thus, DV and AP positional information has to be orchestrated in each dA/dB subpopulation to form distinct neuronal circuits in the hindbrain. Over the span of several decades, different axonal routes have been well-documented to dynamically emerge and grow throughout the hindbrain DV and AP positions. Yet, the genetic link between these distinct axonal bundles and their neuronal origin is not fully clear. In this study, we reviewed the available data regarding the association between the specification of early-born dorsal interneuron subpopulations in the hindbrain and their axonal circuitry development and fate, as well as the present existing knowledge on molecular effectors underlying the process of axonal growth.

Early development of the breathing network

Breathing (or respiration) is a complex motor behavior that originates in the brainstem. In minimalistic terms, breathing can be divided into two phases: inspiration (uptake of oxygen, O₂) and expiration (release of carbon dioxide, CO₂). The neurons that discharge in synchrony with these phases are arranged in three major groups along the brainstem: (i) pontine, (ii) dorsal medullary, and (iii) ventral medullary. These groups are formed by diverse neuron types that coalesce into heterogeneous nuclei or complexes, among which the preBötzinger complex in the ventral medullary group contains cells that generate the respiratory rhythm (Chapter 1). The respiratory rhythm is not rigid, but instead highly adaptable to the physic demands of the organism. In order to generate the appropriate respiratory rhythm, the preBötzinger complex receives direct and indirect chemosensory information from other brainstem respiratory nuclei (Chapter 2) and peripheral organs (Chapter 3). Even though breathing is a hard-wired unconscious behavior, it can be temporarily altered at will by other higher-order brain structures (Chapter 6), and by emotional states (Chapter 7). In this chapter, we focus on the development of brainstem respiratory groups and highlight the cell lineages that contribute to central and peripheral chemoreflexes.

Characterization of a novel Lbx1 mouse loss of function strain

Adolescent Idiopathic Scoliosis (AIS) is the most common type of spine deformity affecting 2-3% of the population worldwide. The etiology of this disease is still poorly understood. Several GWAS studies have identified single nucleotide polymorphisms (SNPs) located near the gene LBX1 that is significantly correlated with AIS risk. LBX1 is a transcription factor with roles in myocyte precursor migration, cardiac neural crest specification, and neuronal fate determination in the neural tube. Here, we further investigated the role of LBX1 in the developing spinal cord of mouse embryos using a CRISPR-generated mouse model expressing a truncated version of LBX1 (Lbx1^Δ). Homozygous mice died at birth, likely due to cardiac abnormalities. To further study the neural tube phenotype, we used RNA-sequencing to identify 410 genes differentially expressed between the neural tubes of E12.5 wildtype and Lbx1^Δ/Δ embryos. Genes with increased expression in the deletion line were involved in neurogenesis and those with broad roles in embryonic development. Many of these genes have also been associated with scoliotic phenotypes. In comparison, genes with decreased expression were primarily involved in skeletal development. Subsequent skeletal and immunohistochemistry analysis further confirmed these results. This study aids in understanding the significance of links between LBX1 function and AIS susceptibility.

The Lbx1 lineage differentially contributes to inhibitory cell types of the dorsal cochlear nucleus, a cerebellum-like structure, and the cerebellum

The dorsal cochlear nucleus (DCN) is a mammalian-specific nucleus of the auditory system. Anatomically, it is classified as a cerebellum-like structure. These structures are proposed to share genetic programs with the cerebellum. Previous analyses demonstrated that inhibitory serial sister cell types (SCTs) of the DCN and cerebellum are derived from the pancreatic transcription factor 1a (Ptf1a) lineage. Postmitotic neurons of the Ptf1a lineage often express the transcription factor Ladybird homeobox protein homolog 1 (Lbx1) which is involved in neuronal cell fate determination. Lbx1 is therefore an attractive candidate for a further component of the genetic program shared between the DCN and cerebellum. Here, we used cell-type specific marker analysis in combination with an Lbx1 reporter mouse line to analyze in both tissues which cell types of the Ptf1a lineage express Lbx1. In the DCN, stellate cells and Purkinje-like cartwheel cells were part of the Lbx1 lineage and Golgi cells were not, as determined by cell counts. In contrast, in the cerebellum, stellate cells and Golgi cells were part of the Lbx1 lineage and Purkinje cells were not. Hence, two out of three phenotypically similar cell types differed with respect to their Lbx1 expression. Our study demonstrates that Lbx1 is differentially recruited to the developmental genetic program of inhibitory neurons both within a given tissue and between the DCN and cerebellum. The differential expression of Lbx1 within the DCN and the cerebellum might contribute to the genetic individuation of the inhibitory SCTs to adapt to circuit specific tasks.

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

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

The Susceptibility and Potential Functions of the LBX1 Gene in Adolescent Idiopathic Scoliosis

Genome-wide association studies have identified many susceptibility genes for adolescent idiopathic scoliosis (AIS). However, most of the results are hard to be replicated in multi-ethnic populations. LBX1 is the most promising candidate gene in the etiology of AIS. We aimed to appraise the literature for the association of LBX1 gene polymorphisms with susceptibility and curve progression in AIS. We also reviewed the function of the LBX1 gene in muscle progenitor cell migration and neuronal determination processes. Three susceptibility loci (rs11190870, rs625039, and rs11598564) near the LBX1 gene, as well as another susceptibility locus (rs678741), related to LBX1 regulation, have been successfully verified to have robust associations with AIS in multi-ethnic populations. The LBX1 gene plays an essential role in regulating the migration and proliferation of muscle precursor cells, and it is known to play a role in neuronal determination processes, especially for the fate of somatosensory relay neurons. The LBX1 gene is the most promising candidate gene in AIS susceptibility due to its position and possible functions in muscle progenitor cell migration and neuronal determination processes. The causality between susceptibility loci related to the LBX1 gene and the pathogenesis of AIS deserves to be explored with further integrated genome-wide and epigenome-wide association studies.

Wt1 Positive dB4 Neurons in the Hindbrain Are Crucial for Respiration

Central pattern generator (CPG) networks coordinate the generation of rhythmic activity such as locomotion and respiration. Their development is driven by various transcription factors, one of which is the Wilms tumor protein (Wt1). It is present in dI6 neurons of the mouse spinal cord, and involved in the coordination of locomotion. Here we report about the presence of Wt1 in neurons of the caudoventral medulla oblongata and their impact on respiration. By employing immunohistofluorescence staining, we were able to characterize these Wt1 positive (+) cells as dB4 neurons. The temporal occurrence of Wt1 suggests a role for this transcription factor in the differentiation of dB4 neurons during embryonic and postnatal development. Conditional knockout of Wt1 in these cells caused an altered population size of V0 neurons already in the developing hindbrain, leading to a decline in the respiration rate in the adults. Thereby, we confirmed and extended the previously proposed similarity between dB4 neurons in the hindbrain and dI6 neurons of the spinal cord, in terms of development and function. Ablation of Wt1+ dB4 neurons resulted in the death of neonates due to the inability to initiate respiration, suggesting a vital role for Wt1+ dB4 neurons in breathing. These results expand the role of Wt1 in the CNS and show that, in addition to its function in differentiation of dI6 neurons, it also contributes to the development of dB4 neurons in the hindbrain that are critically involved in the regulation of respiration.

Development of the brainstem respiratory circuit

The respiratory circuit is comprised of over a dozen functionally and anatomically segregated brainstem nuclei that work together to control respiratory rhythms. These respiratory rhythms emerge prenatally but only acquire vital importance at birth, which is the first time the respiratory circuit faces the sole responsibility for O₂ /CO₂ homeostasis. Hence, the respiratory circuit has little room for trial-and-error-dependent fine tuning and relies on a detailed genetic blueprint for development. This blueprint is provided by transcription factors that have specific spatiotemporal expression patterns along the rostral-caudal or dorsal-ventral axis of the developing brainstem, in proliferating precursor cells and postmitotic neurons. Studying these transcription factors in mice has provided key insights into the functional segregation of respiratory control and the vital importance of specific respiratory nuclei. Many studies converge on just two respiratory nuclei that each have rhythmogenic properties during the prenatal period: the preBötzinger complex (preBötC) and retrotrapezoid nucleus/parafacial nucleus (RTN/pF). Here, we discuss the transcriptional regulation that guides the development of these nuclei. We also summarize evidence showing that normal preBötC development is necessary for neonatal survival, and that neither the preBötC nor the RTN/pF alone is sufficient to sustain normal postnatal respiratory rhythms. Last, we highlight several studies that use intersectional genetics to assess the necessity of transcription factors only in subregions of their expression domain. These studies independently demonstrate that lack of RTN/pF neurons weakens the respiratory circuit, yet these neurons are not necessary for neonatal survival because developmentally related populations can compensate for abnormal RTN/pF function at birth. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Peptidomics analysis of umbilical cord blood reveals potential preclinical biomarkers for neonatal respiratory distress syndrome

AIMS

The purpose of this study was to investigate the pathophysiology and discover novel predictors of neonatal respiratory distress syndrome (NRDS) from a peptidomics perspective.

MAIN METHODS

Comparative profiling of umbilical cord blood from NRDS and control patients was performed by liquid chromatography tandem mass spectrometry technology. The underlying biological functions of the differentially expressed peptides (DEPs) were predicted by Gene Ontology (GO) and KEGG pathway analyses. The interactions of DEPs and their precursor proteins were explored by ingenuity pathway analysis (IPA). The sources and stability of DEPs were determined by online databases, including UniProt, SMART and ProtParam tool.

KEY FINDINGS

A total of 251 DEPs were identified, of which 139 peptides were upregulated, and 112 peptides were downregulated (fold change ≥2.0, P < 0.05). These DEPs were predicted to be associated with respiratory failure, atelectasis, and morphogenesis of endothelial cells. These processes indicated that DEPs may play a role in NRDS. Among them, eleven stable DEPs might be used as preclinical biomarkers.

SIGNIFICANCE

Our findings improve our understanding of NRDS and facilitate the discovery of candidate diagnostic biomarkers for NRDS from the perspective of peptidomics.

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

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

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

Role of the K+-Cl- Cotransporter KCC2a Isoform in Mammalian Respiration at Birth

In central respiratory circuitry, synaptic excitation is responsible for synchronizing neuronal activity in the different respiratory rhythm phases, whereas chloride-mediated inhibition is important for shaping the respiratory pattern itself. The potassium chloride cotransporter KCC2, which serves to maintain low intraneuronal Cl- concentration and thus render chloride-mediated synaptic signaling inhibitory, exists in two isoforms, KCC2a and KCC2b. KCC2 is essential for functional breathing motor control at birth, but the specific contribution of the KCC2a isoform remains unknown. Here, to address this issue, we investigated the respiratory phenotype of mice deficient for KCC2a. In vivo plethysmographic recordings revealed that KCC2a-deficient pups at P0 transiently express an abnormally low breathing rate and a high occurrence of apneas. Immunostainings confirmed that KCC2a is normally expressed in the brainstem neuronal groups involved in breathing (pre-Bötzinger complex, parafacial respiratory group, hypoglossus nucleus) and is absent in these regions in the KCC2a-/- mutant. However, in variously reduced in vitro medullary preparations, spontaneous rhythmic respiratory activity is similar to that expressed in wild-type preparations, as is hypoglossal motor output, and no respiratory pauses are detected, suggesting that the rhythm-generating networks are not intrinsically affected in mutants at P0. In contrast, inhibitory neuromodulatory influences exerted by the pons on respiratory rhythmogenesis are stronger in the mutant, thereby explaining the breathing anomalies observed in vivo. Thus, our results indicate that the KCC2a isoform is important for establishing proper breathing behavior at the time of birth, but by acting at sites that are extrinsic to the central respiratory networks themselves.

Loss of Atoh1 from neurons regulating hypoxic and hypercapnic chemoresponses causes neonatal respiratory failure in mice

Atoh1-null mice die at birth from respiratory failure, but the precise cause has remained elusive. Loss of Atoh1 from various components of the respiratory circuitry (e.g. the retrotrapezoid nucleus (RTN)) has so far produced at most 50% neonatal lethality. To identify other Atoh1-lineage neurons that contribute to postnatal survival, we examined parabrachial complex neurons derived from the rostral rhombic lip (rRL) and found that they are activated during respiratory chemochallenges. Atoh1-deletion from the rRL does not affect survival, but causes apneas and respiratory depression during hypoxia, likely due to loss of projections to the preBötzinger Complex and RTN. Atoh1 thus promotes the development of the neural circuits governing hypoxic (rRL) and hypercapnic (RTN) chemoresponses, and combined loss of Atoh1 from these regions causes fully penetrant neonatal lethality. This work underscores the importance of modulating respiratory rhythms in response to chemosensory information during early postnatal life.

Respiratory consequences of targeted losses of Hoxa5 gene function in mice

Fetal development of the respiratory tract and diaphragm requires strict coordination between genetically controlled signals and mechanical forces produced by the neural network that generates breathing. HOXA5, which is expressed in the mesenchyme of the trachea, lung and diaphragm, and in phrenic motor neurons, is a key transcription factor regulating lung development and function. Consequently, most Hoxa5^-/- mutants die at birth from respiratory failure. However, the extensive effect of the null mutation makes it difficult to identify the origins of respiratory dysfunction in newborns. To address the physiological impact of Hoxa5 tissue-specific roles, we used conditional gene targeting with the Dermo1^Cre and Olig2^Cre mouse lines to produce specific Hoxa5 deletions in the mesenchyme and motor neurons, respectively. Hoxa5 expression in the mesenchyme is critical for trachea development, whereas its expression in phrenic motor neurons is essential for diaphragm formation. Breathing measurements in adult mice with whole-body plethysmography demonstrated that, at rest, only the motor neuron deletion affects respiration, resulting in higher breathing frequency and decreased tidal volume. But subsequent exposure to a moderate hypoxic challenge (Fi_O2 =0.12; 10 min) revealed that both mutant mice hyperventilate more than controls. Hoxa5^flox/flox;Dermo1^+/Cre mice showed augmented tidal volume while Hoxa5^flox/flox;Olig2^+/Cre mice had the largest increase in breathing frequency. No significant differences were observed between medulla-spinal cord preparations from E18.5 control and Hoxa5^flox/flox;Olig2^+/Cre mouse embryos that could support a role for Hoxa5 in fetal inspiratory motor command. According to our data, Hoxa5 expression in the mesenchyme and phrenic motor neurons controls distinct aspects of respiratory development.

A V0 core neuronal circuit for inspiration

Breathing in mammals relies on permanent rhythmic and bilaterally synchronized contractions of inspiratory pump muscles. These motor drives emerge from interactions between critical sets of brainstem neurons whose origins and synaptic ordered organization remain obscure. Here, we show, using a virus-based transsynaptic tracing strategy from the diaphragm muscle in the mouse, that the principal inspiratory premotor neurons share V0 identity with, and are connected by, neurons of the preBötzinger complex that paces inspiration. Deleting the commissural projections of V0s results in left-right desynchronized inspiratory motor commands in reduced brain preparations and breathing at birth. This work reveals the existence of a core inspiratory circuit in which V0 to V0 synapses enabling function of the rhythm generator also direct its output to secure bilaterally coordinated contractions of inspiratory effector muscles required for efficient breathing.

The developmental origin and functional organization of the brainstem breathing circuits are poorly understood. Here using virus-based circuit-mapping approaches in mice, the authors reveal the lineage, neurotransmitter phenotype, and connectivity patterns of phrenic premotor neurons, which are a crucial component of the inspiratory circuit.

The embryonic development of hindbrain respiratory networks is unaffected by mutation of the planar polarity protein Scribble

The central command for breathing arises mainly from two interconnected rhythmogenic hindbrain networks, the parafacial respiratory group (pFRG or epF at embryonic stages) and the preBötzinger complex (preBötC), which are comprised of a limited number of neurons located in confined regions of the ventral medulla. In rodents, both networks become active toward the end of gestation but little is known about the signaling pathways involved in their anatomical and functional establishment during embryogenesis. During embryonic development, epF and preBötC neurons migrate from their territories of origin to their final positions in ventral brainstem areas. Planar Cell Polarity (PCP) signaling, including the molecule Scrib, is known to control the developmental migration of several hindbrain neuronal groups. Accordingly, a homozygous mutation of Scrib leads to severe disruption of hindbrain anatomy and function. Here, we aimed to determine whether Scrib is also involved in the prenatal development of the hindbrain nuclei controlling breathing. We combined immunostaining, calcium imaging and electrophysiological recordings of neuronal activity in isolated in vitro preparations. In the Scrib mutant, despite severe neural tube defects, epF and preBötC neurons settled at their expected hindbrain positions. Furthermore, both networks remained capable of generating rhythmically organized, respiratory-related activities and exhibited normal sensitivity to pharmacological agents known to modify respiratory circuit function. Thus Scrib is not required for the proper migration of epF and preBötC neurons during mouse embryogenesis. Our findings thus further illustrate the robustness and specificity of the developmental processes involved in the establishment of hindbrain respiratory circuits.

Clustered Protocadherins Are Required for Building Functional Neural Circuits

Neuronal identity is generated by the cell-surface expression of clustered protocadherin (Pcdh) isoforms. In mice, 58 isoforms from three gene clusters, Pcdhα, Pcdhβ, and Pcdhγ, are differentially expressed in neurons. Since cis-heteromeric Pcdh oligomers on the cell surface interact homophilically with that in other neurons in trans, it has been thought that the Pcdh isoform repertoire determines the binding specificity of synapses. We previously described the cooperative functions of isoforms from all three Pcdh gene clusters in neuronal survival and synapse formation in the spinal cord. However, the neuronal loss and the following neonatal lethality prevented an analysis of the postnatal development and characteristics of the clustered-Pcdh-null (Δαβγ) neural circuits. Here, we used two methods, one to generate the chimeric mice that have transplanted Δαβγ neurons into mouse embryos, and the other to generate double mutant mice harboring null alleles of both the Pcdh gene and the proapoptotic gene Bax to prevent neuronal loss. First, our results showed that the surviving chimeric mice that had a high contribution of Δαβγ cells exhibited paralysis and died in the postnatal period. An analysis of neuronal survival in postnatally developing brain regions of chimeric mice clarified that many Δαβγ neurons in the forebrain were spared from apoptosis, unlike those in the reticular formation of the brainstem. Second, in Δαβγ/Bax null double mutants, the central pattern generator (CPG) for locomotion failed to create a left-right alternating pattern even in the absence of neurodegeneraton. Third, calcium imaging of cultured hippocampal neurons showed that the network activity of Δαβγ neurons tended to be more synchronized and lost the variability in the number of simultaneously active neurons observed in the control network. Lastly, a comparative analysis for trans-homophilic interactions of the exogenously introduced single Pcdh-γA3 isoforms between the control and the Δαβγ neurons suggested that the isoform-specific trans-homophilic interactions require a complete match of the expressed isoform repertoire at the contacting sites between interactive neurons. These results suggested that combinations of clustered Pcdh isoforms are required for building appropriate neural circuits.

The Onset of the Fetal Respiratory Rhythm: An Emergent Property Triggered by Chemosensory Drive?

The mechanisms responsible for the onset of respiratory activity during fetal life are unknown. The onset of respiratory rhythm may be a consequence of the genetic program of each of the constituents of the respiratory network, so they start to interact and generate respiratory cycles when reaching a certain degree of maturation. Alternatively, generation of cycles might require the contribution of recently formed sensory inputs that will trigger oscillatory activity in the nascent respiratory neural network. If this hypothesis is true, then sensory input to the respiratory generator must be already formed and become functional before the onset of fetal respiration. In this review, we evaluate the timing of the onset of the respiratory rhythm in comparison to the appearance of receptors, neurotransmitter machinery, and afferent projections provided by two central chemoreceptive nuclei, the raphe and locus coeruleus nuclei.

The dorsal spinal cord and hindbrain: From developmental mechanisms to functional circuits

Neurons of the dorsal hindbrain and spinal cord are central in receiving, processing and relaying sensory perception and participate in the coordination of sensory-motor output. Numerous cellular and molecular mechanisms that underlie neuronal development in both regions of the nervous system are shared. We discuss here the mechanisms that generate neuronal diversity in the dorsal spinal cord and hindbrain, and emphasize similarities in patterning and neuronal specification. Insight into the developmental mechanisms has provided tools that can help to assign functions to small subpopulations of neurons. Hence, novel information on how mechanosensory or pain sensation is encoded under normal and neuropathic conditions has already emerged. Such studies show that the complex neuronal circuits that control perception of somatosensory and viscerosensory stimuli are becoming amenable to investigations.

Stimulation of Respiratory Motor Output and Ventilation in a Murine Model of Pompe Disease by Ampakines

Pompe disease results from a mutation in the acid α-glucosidase gene leading to lysosomal glycogen accumulation. Respiratory insufficiency is common, and the current U.S. Food and Drug Administration-approved treatment, enzyme replacement, has limited effectiveness. Ampakines are drugs that enhance α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses and can increase respiratory motor drive. Recent work indicates that respiratory motor drive can be blunted in Pompe disease, and thus pharmacologic stimulation of breathing may be beneficial. Using a murine Pompe model with the most severe clinical genotype (the Gaa(-/-) mouse), our primary objective was to test the hypothesis that ampakines can stimulate respiratory motor output and increase ventilation. Our second objective was to confirm that neuropathology was present in Pompe mouse medullary respiratory control neurons. The impact of ampakine CX717 on breathing was determined via phrenic and hypoglossal nerve recordings in anesthetized mice and whole-body plethysmography in unanesthetized mice. The medulla was examined using standard histological methods coupled with immunochemical markers of respiratory control neurons. Ampakine CX717 robustly increased phrenic and hypoglossal inspiratory bursting and reduced respiratory cycle variability in anesthetized Pompe mice, and it increased inspiratory tidal volume in unanesthetized Pompe mice. CX717 did not significantly alter these variables in wild-type mice. Medullary respiratory neurons showed extensive histopathology in Pompe mice. Ampakines stimulate respiratory neuromotor output and ventilation in Pompe mice, and therefore they have potential as an adjunctive therapy in Pompe disease.

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

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

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

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

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

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

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

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

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

Concordance analysis for QTL detection in dairy cattle: a case study of leg morphology

BACKGROUND

The present availability of sequence data gives new opportunities to narrow down from QTL (quantitative trait locus) regions to causative mutations. Our objective was to decrease the number of candidate causative mutations in a QTL region. For this, a concordance analysis was applied for a leg conformation trait in dairy cattle. Several QTL were detected for which the QTL status (homozygous or heterozygous for the QTL) was inferred for each individual. Subsequently, the inferred QTL status was used in a concordance analysis to reduce the number of candidate mutations.

METHODS

Twenty QTL for rear leg set side view were mapped using Bayes C. Marker effects estimated during QTL mapping were used to infer the QTL status for each individual. Subsequently, polymorphisms present in the QTL regions were extracted from the whole-genome sequences of 71 Holstein bulls. Only polymorphisms for which the status was concordant with the QTL status were kept as candidate causative mutations.

RESULTS

QTL status could be inferred for 15 of the 20 QTL. The number of concordant polymorphisms differed between QTL and depended on the number of QTL statuses that could be inferred and the linkage disequilibrium in the QTL region. For some QTL, the concordance analysis was efficient and narrowed down to a limited number of candidate mutations located in one or two genes, while for other QTL a large number of genes contained concordant polymorphisms.

CONCLUSIONS

For regions for which the concordance analysis could be performed, we were able to reduce the number of candidate mutations. For part of the QTL, the concordant analyses narrowed QTL regions down to a limited number of genes, of which some are known for their role in limb or skeletal development in humans and mice. Mutations in these genes are good candidates for QTN (quantitative trait nucleotides) influencing rear leg set side view.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

Physiological and morphological properties of Dbx1-derived respiratory neurons in the pre-Botzinger complex of neonatal mice

Breathing in mammals depends on an inspiratory-related rhythm that is generated by glutamatergic neurons in the pre-Bötzinger complex (preBötC) of the lower brainstem. A substantial subset of putative rhythm-generating preBötC neurons derive from a single genetic line that expresses the transcription factor Dbx1, but the cellular mechanisms of rhythmogenesis remain incompletely understood. To elucidate these mechanisms, we carried out a comparative analysis of Dbx1-expressing neurons (Dbx1(+)) and non-Dbx1-derived (Dbx1(-)) neurons in the preBötC. Whole-cell recordings in rhythmically active newborn mouse slice preparations showed that Dbx1(+) neurons activate earlier in the respiratory cycle and discharge greater magnitude inspiratory bursts compared with Dbx1(-) neurons. Furthermore, Dbx1(+) neurons required less input current to discharge spikes (rheobase) in the context of network activity. The expression of intrinsic membrane properties indicative of A-current (IA) and hyperpolarization-activated current (Ih) tended to be mutually exclusive in Dbx1(+) neurons. In contrast, there was no such relationship in the expression of currents IA and Ih in Dbx1(-) neurons. Confocal imaging and digital morphological reconstruction of recorded neurons revealed dendritic spines on Dbx1(-) neurons, but Dbx1(+) neurons were spineless. The morphology of Dbx1(+) neurons was largely confined to the transverse plane, whereas Dbx1(-) neurons projected dendrites to a greater extent in the parasagittal plane. The putative rhythmogenic nature of Dbx1(+) neurons may be attributable, in part, to a higher level of intrinsic excitability in the context of network synaptic activity. Furthermore, Dbx1(+) neuronal morphology may facilitate temporal summation and integration of local synaptic inputs from other Dbx1(+) neurons, taking place largely in the dendrites, which could be important for initiating and maintaining bursts and synchronizing activity during the inspiratory phase.

Transcription factors define the neuroanatomical organization of the medullary reticular formation

The medullary reticular formation contains large populations of inadequately described, excitatory interneurons that have been implicated in multiple homeostatic behaviors including breathing, viserosensory processing, vascular tone, and pain. Many hindbrain nuclei show a highly stereotyped pattern of localization across vertebrates suggesting a strong underlying genetic organization. Whether this is true for neurons within the reticular regions of hindbrain is unknown. Hindbrain neurons are derived from distinct developmental progenitor domains each of which expresses distinct patterns of transcription factors (TFs). These neuronal populations have distinct characteristics such as transmitter identity, migration, and connectivity suggesting developmentally expressed TFs might identify unique subpopulations of neurons within the reticular formation. A fate-mapping strategy using perinatal expression of reporter genes within Atoh1, Dbx1, Lmx1b, and Ptf1a transgenic mice coupled with immunohistochemistry (IHC) and in situ hybridization (ISH) were used to address the developmental organization of a large subset of reticular formation glutamatergic neurons. All hindbrain lineages have relatively large populations that extend the entire length of the hindbrain. Importantly, the location of neurons within each lineage was highly constrained. Lmx1b- and Dbx1- derived populations were both present in partially overlapping stripes within the reticular formation extending from dorsal to ventral brain. Within each lineage, distinct patterns of gene expression and organization were localized to specific hindbrain regions. Rostro-caudally sub-populations differ sequentially corresponding to proposed pseudo-rhombomereic boundaries. Dorsal-ventrally, sub-populations correspond to specific migratory positions. Together these data suggests the reticular formation is organized by a highly stereotyped developmental logic.

The rhythmic, transverse medullary slice preparation in respiratory neurobiology: contributions and caveats

Our understanding of the sites and mechanisms underlying rhythmic breathing as well as the neuromodulatory control of respiratory rhythm, pattern, and respiratory motoneuron excitability during perinatal development has advanced significantly over the last 20 years. A major catalyst was the development in 1991 of the rhythmically-active medullary slice preparation, which provided precise mechanical and chemical control over the network as well as enhanced physical and optical access to key brainstem regions. Insights obtained in vitro have informed multiple mechanistic hypotheses. In vivo tests of these hypotheses, performed under conditions of reduced control and precision but more obvious physiological relevance, have clearly established the significance for respiratory neurobiology of the rhythmic slice preparation. We review the contributions of this preparation to current understanding/concepts in respiratory control, and outline the limitations of this approach in the context of studying rhythm and pattern generation, homeostatic control mechanisms and murine models of human genetic disorders that feature prominent breathing disturbances.

Brainstem respiratory networks: building blocks and microcircuits

Breathing movements in mammals are driven by rhythmic neural activity generated within spatially and functionally organized brainstem neural circuits comprising the respiratory central pattern generator (CPG). This rhythmic activity provides homeostatic regulation of gases in blood and tissues and integrates breathing with other motor acts. We review new insights into the spatial-functional organization of key neural microcircuits of this CPG from recent multidisciplinary experimental and computational studies. The emerging view is that the microcircuit organization within the CPG allows the generation of multiple rhythmic breathing patterns and adaptive switching between them, depending on physiological or pathophysiological conditions. These insights open the possibility for site- and mechanism-specific interventions to treat various disorders of the neural control of breathing.

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

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

Computational models and emergent properties of respiratory neural networks

Computational models of the neural control system for breathing in mammals provide a theoretical and computational framework bringing together experimental data obtained from different animal preparations under various experimental conditions. Many of these models were developed in parallel and iteratively with experimental studies and provided predictions guiding new experiments. This data-driven modeling approach has advanced our understanding of respiratory network architecture and neural mechanisms underlying generation of the respiratory rhythm and pattern, including their functional reorganization under different physiological conditions. Models reviewed here vary in neurobiological details and computational complexity and span multiple spatiotemporal scales of respiratory control mechanisms. Recent models describe interacting populations of respiratory neurons spatially distributed within the Bötzinger and pre-Bötzinger complexes and rostral ventrolateral medulla that contain core circuits of the respiratory central pattern generator (CPG). Network interactions within these circuits along with intrinsic rhythmogenic properties of neurons form a hierarchy of multiple rhythm generation mechanisms. The functional expression of these mechanisms is controlled by input drives from other brainstem components,including the retrotrapezoid nucleus and pons, which regulate the dynamic behavior of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple levels of circuit organization. This allows flexible, state-dependent expression of different neural pattern-generation mechanisms under various physiological conditions,enabling a wide repertoire of respiratory behaviors. Some models consider control of the respiratory CPG by pulmonary feedback and network reconfiguration during defensive behaviors such as cough. Future directions in modeling of the respiratory CPG are considered.

Irregular Breathing in Mice following Genetic Ablation of V2a Neurons

Neural networks called central pattern generators (CPGs) generate repetitive motor behaviors such as locomotion and breathing. Glutamatergic neurons are required for the generation and inhibitory neurons for the patterning of the motor activity associated with repetitive motor behaviors. In the mouse, glutamatergic V2a neurons coordinate the activity of left and right leg CPGs in the spinal cord enabling mice to generate an alternating gait. Here, we investigate the role of V2a neurons in the neural control of breathing, an essential repetitive motor behavior. We find that, following the ablation of V2a neurons, newborn mice breathe at a lower frequency. Recordings of respiratory activity in brainstem-spinal cord and respiratory slice preparations demonstrate that mice lacking V2a neurons are deficient in central respiratory rhythm generation. The absence of V2a neurons in the respiratory slice preparation can be compensated for by bath application of neurochemicals known to accelerate the breathing rhythm. In this slice preparation, V2a neurons exhibit a tonic firing pattern. The existence of direct connections between V2a neurons in the medial reticular formation and neurons of the pre-Bötzinger complex indicates that V2a neurons play a direct role in the function of the respiratory CPG in newborn mice. Thus, neurons of the embryonic V2a lineage appear to have been recruited to neural networks that control breathing and locomotion, two prominent CPG-driven, repetitive motor behaviors.

Respiratory circuits: development, function and models

Breathing is a rhythmic motor behavior generated and controlled by hindbrain neuronal networks. Respiratory motor output arises from two distinct, but functionally interacting, rhythmogenic networks: the pre-Bötzinger complex (preBötC) and the retrotrapezoïd nucleus/parafacial respiratory group (RTN/pFRG). This review outlines recent advances in delineating the genetic specification of the neuronal constituents of these two rhythmogenic networks, their respective roles in respiratory function and how they interact to constitute a functional respiratory circuit ensemble. The often lethal consequences of disruption to these networks found in naturally occurring developmental disorders, transgenic animals, and highly specific lesion studies are described. In addition, we discuss how recent computational models enhance our understanding of how respiratory networks generate and regulate respiratory behavior.

Pbx-dependent regulation of lbx gene expression in developing zebrafish embryos

Ladybird (Lbx) homeodomain transcription factors function in neural and muscle development--roles conserved from Drosophila to vertebrates. Lbx expression in mice specifies neural cell types, including dorsally located interneurons and association neurons, within the neural tube. Little, however, is known about the regulation of vertebrate lbx family genes. Here we describe the expression pattern of three zebrafish ladybird genes via mRNA in situ hybridization. Zebrafish lbx genes are expressed in distinct but overlapping regions within the developing neural tube, with strong expression within the hindbrain and spinal cord. The Hox family of transcription factors, in cooperation with cofactors such as Pbx and Meis, regulate hindbrain segmentation during embryogenesis. We have identified a novel regulatory interaction in which lbx1 genes are strongly downregulated in Pbx-depleted embryos. Further, we have produced a transgenic zebrafish line expressing dTomato and EGFP under the control of an lbx1b enhancer--a useful tool to acertain neuron location, migration, and morphology. Using this transgenic strain, we have identified a minimal neural lbx1b enhancer that contains key regulatory elements for expression of this transcription factor.

Breathing without CO(2) chemosensitivity in conditional Phox2b mutants

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

Isolated in vitro brainstem-spinal cord preparations remain important tools in respiratory neurobiology

Isolated in vitro brainstem-spinal cord preparations are used extensively in respiratory neurobiology because the respiratory network in the pons and medulla is intact, monosynaptic descending inputs to spinal motoneurons can be activated, brainstem and spinal cord tissue can be bathed with different solutions, and the responses of cervical, thoracic, and lumbar spinal motoneurons to experimental perturbations can be compared. The caveats and limitations of in vitro brainstem-spinal cord preparations are well-documented. However, isolated brainstem-spinal cords are still valuable experimental preparations that can be used to study neuronal connectivity within the brainstem, development of motor networks with lethal genetic mutations, deleterious effects of pathological drugs and conditions, respiratory spinal motor plasticity, and interactions with other motor behaviors. Our goal is to show how isolated brainstem-spinal cord preparations still have a lot to offer scientifically and experimentally to address questions within and outside the field of respiratory neurobiology.

Generation of a conditional null allele of Lbx1

The homeobox gene Lbx1 not only plays critical roles in myogenesis and neurogenesis during embryonic development but is also expressed in activated satellite cells of adult mice. To address the potential postnatal functions of Lbx1, we generated conditional Lbx1-null mice using the Cre-loxP system. We generated a mouse in which Exon 2 of Lbx1 was floxed (Lbx1flox/flox), followed by cross-breeding between the Lbx1flox/flox mouse and either a transgenic mouse where a tamoxifen-inducible Cre-recombinase (Cre) was ubiquitously expressed, or a Myf5Cre mouse where Cre was inserted into the Myf5 locus. In both Lbx1-null mouse lines generated, Pax3-expressing limb muscle precursor cells were seriously reduced during embryonic development and eventually the limb extensor muscles were lost after birth. Since the conditional Lbx1-null mice generated were viable for a prolonged time, they will be useful in the investigation of Lbx1 function throughout the lifespan of the mouse.

Prenatal development of central rhythm generation

Foetal breathing in mice results from prenatal activity of the two coupled hindbrain oscillators considered to be responsible for respiratory rhythm generation after birth: the pre-Bötzinger complex (preBötC) is active shortly before the onset of foetal breathing; the parafacial respiratory group (e-pF in embryo) starts activity one day earlier. Transcription factors have been identified that are essential to specify neural progenitors and lineages forming each of these oscillators during early development of the neural tube: Hoxa1, Egr2 (Krox20), Phox2b, Lbx1 and Atoh1 for the e-pF; Dbx1 and Evx1 for the preBötC which eventually grow contralateral axons requiring expression of Robo3. Inactivation of the genes encoding these factors leads to mis-specification of these neurons and distinct breathing abnormalities: apneic patterns and loss of central chemosensitivity for the e-pF (central congenital hypoventilation syndrome, CCHS, in humans), complete loss of breathing for the preBötC, right-left desynchronized breathing in Robo3 mutants. Mutations affecting development in more rostral (pontine) respiratory territories change the shape of the inspiratory drive without affecting the rhythm. Other (primordial) embryonic oscillators start in the mouse three days before the e-pF, to generate low frequency (LF) rhythms that are probably required for activity-dependent development of neurones at embryonic stages; in the foetus, however, they are actively silenced to avoid detrimental interaction with the on-going respiratory rhythm. Altogether, these observations provide a strong support to the previously proposed hypothesis that the functional organization of the respiratory generator is specified at early stages of development and is dual in nature, comprising two serially non-homologous oscillators.

Genetic factors determining the functional organization of neural circuits controlling rhythmic movements the murine embryonic parafacial rhythm generator

In mammals, fetal movements governed by central pattern generators are essential for the development of adaptive behaviors such as breathing, walking, and chewing, which are vital after birth. Combining targeted mutations and genetic fate mapping can help to define the molecular determinants that control the development of these central pattern generators. In this chapter, recent results are presented on the embryonic parafacial (e-pF) rhythm generator, one of the two oscillators involved in controlling the breathing behavior and chemosensitive responsiveness.

Developmental origin of preBötzinger complex respiratory neurons

A subset of preBötzinger Complex (preBötC) neurokinin 1 receptor (NK1R) and somatostatin peptide (SST)-expressing neurons are necessary for breathing in adult rats, in vivo. Their developmental origins and relationship to other preBötC glutamatergic neurons are unknown. Here we show, in mice, that the "core" of preBötC SST(+)/NK1R(+)/SST 2a receptor(+) (SST2aR) neurons, are derived from Dbx1-expressing progenitors. We also show that Dbx1-derived neurons heterogeneously coexpress NK1R and SST2aR within and beyond the borders of preBötC. More striking, we find that nearly all non-catecholaminergic glutamatergic neurons of the ventrolateral medulla (VLM) are also Dbx1 derived. PreBötC SST(+) neurons are born between E9.5 and E11.5 in the same proportion as non-SST-expressing neurons. Additionally, preBötC Dbx1 neurons are respiratory modulated and show an early inspiratory phase of firing in rhythmically active slice preparations. Loss of Dbx1 eliminates all glutamatergic neurons from the respiratory VLM including preBötC NK1R(+)/SST(+) neurons. Dbx1 mutant mice do not express any spontaneous respiratory behaviors in vivo. Moreover, they do not generate rhythmic inspiratory activity in isolated en bloc preparations even after acidic or serotonergic stimulation. These data indicate that preBötC core neurons represent a subset of a larger, more heterogeneous population of VLM Dbx1-derived neurons. These data indicate that Dbx1-derived neurons are essential for the expression and, we hypothesize, are responsible for the generation of respiratory behavior both in vitro and in vivo.

Teashirt 3 regulates development of neurons involved in both respiratory rhythm and airflow control

Neonatal breathing in mammals involves multiple neuronal circuits, but its genetic basis remains unclear. Mice deficient for the zinc finger protein Teashirt 3 (TSHZ3) fail to breathe and die at birth. Tshz3 is expressed in multiple areas of the brainstem involved in respiration, including the pre-Bötzinger complex (preBötC), the embryonic parafacial respiratory group (e-pF), and cranial motoneurons that control the upper airways. Tshz3 inactivation led to pronounced cell death of motoneurons in the nucleus ambiguus and induced strong alterations of rhythmogenesis in the e-pF oscillator. In contrast, the preBötC oscillator appeared to be unaffected. These deficits result in impaired upper airway function, abnormal central respiratory rhythm generation, and altered responses to pH changes. Thus, a single gene, Tshz3, controls the development of diverse components of the circuitry required for breathing.

The role of CO(2) and central chemoreception in the control of breathing in the fetus and the neonate

Central chemoreception is active early in development and likely drives fetal breathing movements, which are influenced by a combination of behavioral state and powerful inhibition. In the premature human infant and newborn rat ventilation increases in response to CO(2); in the rat the sensitivity of the response increases steadily after ∼P12. The premature human infant is more vulnerable to instability than the newborn rat and exhibits periodic breathing that is augmented by hypoxia and eliminated by breathing oxygen or CO(2) or the administration of respiratory stimulants. The sites of central chemoreception active in the fetus are not known, but may involve the parafacial respiratory group which may be a precursor to the adult RTN. The fetal and neonatal rat brainstem-spinal-cord preparations promise to provide important information about central chemoreception in the developing rodent and will increase our understanding of important clinical problems, including The Sudden Infant Death Syndrome, Congenital Central Hypoventilation Syndrome, and periodic breathing and apnea of prematurity.

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.