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

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.

Transitional care and clinical management of adolescents, young adults, and suspected new adult patients with congenital central hypoventilation syndrome

PURPOSE

With contemporaneous advances in congenital central hypoventilation syndrome (CCHS), recognition, confirmatory diagnostics with PHOX2B genetic testing, and conservative management to reduce the risk of early morbidity and mortality, the prevalence of identified adolescents and young adults with CCHS and later-onset (LO-) CCHS has increased. Accordingly, there is heightened awareness and need for transitional care of these patients from pediatric medicine into a multidisciplinary adult medical team. Hence, this review summarizes key clinical and management considerations for patients with CCHS and LO-CCHS and emphasizes topics of particular importance for this demographic.

METHODS

We performed a systematic review of literature on diagnostics, pathophysiology, and clinical management in CCHS and LO-CCHS, and supplemented the review with anecdotal but extensive experiences from large academic pediatric centers with expertise in CCHS.

RESULTS

We summarized our findings topically for an overview of the medical care in CCHS and LO-CCHS specifically applicable to adolescents and adults. Care topics include genetic and embryologic basis of the disease, clinical presentation, management, variability in autonomic nervous system dysfunction, and clarity regarding transitional care with unique considerations such as living independently, family planning, exposure to anesthesia, and alcohol and drug use.

CONCLUSIONS

While a lack of experience and evidence exists in the care of adults with CCHS and LO-CCHS, a review of the relevant literature and expert consensus provides guidance for transitional care areas.

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.

Developmental disorders affecting the respiratory system: CCHS and ROHHAD

Rapid-onset Obesity with Hypothalamic dysfunction, Hypoventilation, and Autonomic Dysregulation (ROHHAD) and Congenital Central Hypoventilation Syndrome (CCHS) are ultra-rare distinct clinical disorders with overlapping symptoms including altered respiratory control and autonomic regulation. Although both disorders have been considered for decades to be on the same spectrum with necessity of artificial ventilation as life-support, recent acquisition of specific knowledge concerning the genetic basis of CCHS coupled with an elusive etiology for ROHHAD have definitely established that the two disorders are different. CCHS is an autosomal dominant neurocristopathy characterized by alveolar hypoventilation resulting in hypoxemia/hypercarbia and features of autonomic nervous system dysregulation (ANSD), with presentation typically in the newborn period. It is caused by paired-like homeobox 2B (PHOX2B) variants, with known genotype-phenotype correlation but pathogenic mechanism(s) are yet unknown. ROHHAD is characterized by rapid weight gain, followed by hypothalamic dysfunction, then hypoventilation followed by ANSD, in seemingly normal children ages 1.5-7 years. Postmortem neuroanatomical studies, thorough clinical characterization, pathophysiological assessment, and extensive genetic inquiry have failed to identify a cause attributable to a traditional genetic basis, somatic mosaicism, epigenetic mechanism, environmental trigger, or other. To find the key to the ROHHAD pathogenesis and to improve its clinical management, in the present chapter, we have carefully compared CCHS and ROHHAD.

Intrinsic properties and synaptic connectivity of Phox2b-expressing neurons in rat rostral parvocellular reticular formation

The paired-like homeobox 2b gene (Phox2b) is critical for the development of the autonomic nervous system. We have previously demonstrated the distinct characteristics of Phox2b-expressing (Phox2b⁺) neurons in the reticular formation dorsal to the trigeminal motor nucleus (RdV), which are likely related to jaw movement regulation. In this study, we focused on Phox2b⁺ neurons in the rostral parvocellular reticular formation (rPCRt), a critical region for controlling orofacial functions, using 2-11-day-old Phox2b-EYFP rats. Most Phox2b⁺ rPCRt neurons were glutamatergic, but not GABAergic or glycinergic. Approximately 65 % of Phox2b⁺ rPCRt neurons fired at a low frequency, and approximately 24 % of Phox2b⁺ rPCRt neurons fired spontaneously, as opposed to Phox2b⁺ RdV neurons. Stimulation of the RdV evoked inward postsynaptic currents in more than 50 % of Phox2b⁺ rPCRt neurons, while only one Phox2b⁺ rPCRt neuron responded to stimulation of the nucleus of the solitary tract. Five of the 10 Phox2b⁺ neurons sent their axons that ramified within the trigeminal motor nucleus (MoV). Of these, the axons of the two neurons terminated within both the MoV and rPCRt. Our findings suggest that Phox2b⁺ rPCRt neurons have distinct electrophysiological and synaptic properties that may be involved in the motor control of feeding behavior.

Brain-Body Control of Glucose Homeostasis-Insights From Model Organisms

Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.

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

KEY POINTS

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

ABSTRACT

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

Analysis of the neuronal network of the medullary respiratory center in transgenic rats expressing archaerhodopsin-3 in Phox2b-expressing cells

Preinspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG) comprise one of the respiratory rhythm generators in the medulla of the neonatal rat. A subgroup of pFRG/Pre-I neurons expresses the transcription factor Phox2b. To further analyze detailed neuronal mechanisms of respiratory rhythm generation in the neonatal rat, we developed a transgenic (Tg) rat line in which Phox2b-positive cells expressed archaerhodopsin-3 (Arch). Brainstem-spinal cord preparations were isolated from 0-2-day-old Tg newborn rats and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26 °C. Inspiratory fourth cervical ventral root (C4) activity was monitored, and membrane potentials of neurons in the pFRG including Pre-I and inspiratory neurons were recorded. Phox2b-positive cells in the Tg rats were essentially positive for enhanced green fluorescent protein (EGFP) signals (reporter for Arch) in the pFRG. Continuous photo-stimulation of the rostral ventral medulla for up to 90 s by covering the pFRG with green laser light (532 nm) induced a decrease of respiratory rate measured at C4 accompanied by membrane hyperpolarization of Phox2b-positive pFRG/Pre-I neurons. In contrast, Phox2b-negative inspiratory neurons were not hyperpolarized during the photo-stimulation. Our findings showed that Phox2b-expressing pFRG/Pre-I neurons are involved in the maintenance of the basic respiratory rhythm in neonatal rat.

Birthdate-dependent heterogeneity of oculomotor neurons is involved in transmedian migration in the developing mouse midbrain

During the formation of the oculomotor nucleus (nIII), a subset of cells undergoes transmedian migration, crossing the midline to join the contralateral nucleus. A recent study reported that the onset of transmedian migration of nIII neurons is regulated by Slit/Robo signaling. However, developmental programs that differentiate migratory subpopulations of the nIII remain elusive. Here, we identified cellular and molecular characteristics of nIII neurons that are correlated with their migratory behaviors. Birthdate analysis revealed that contralaterally migrating neurons in the caudal part of the nIII are generated at later stages than uncrossed neurons in the rostral part of the nIII. Furthermore, we found that Slit2 is expressed in the ventral midline of the midbrain and contralaterally migrating neurons. On the other hand, Robo2, a receptor of Sli2, is differentially expressed in subpopulations of rostral and caudal parts of the nIII: uncrossed neurons expressed Robo2 in the developing nIII. These results suggest that spatio-temporal regulation of developmental timings and the molecular signatures of oculomotor neurons are crucial for transmedian migration, which underlies appropriate positioning and stereotyped circuit formation of the nIII in the developing mouse midbrain.

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

Genetic approaches to access cell types in mammalian nervous systems

Understanding brain circuit organization and function requires systematic dissection of its cellular components. With vast cell number and diversity, mammalian nervous systems present a daunting challenge for achieving specific and comprehensive cell type access-prerequisite to circuit analysis. Genetic approaches in the mouse have relied on germline engineering to access marker-defined cell populations. Combinatorial strategies that engage marker intersection, anatomy and projection pattern (e.g. antero-grade and retro-grade viral vectors), and developmental lineage substantially increase the specificity of cell type targeting. While increasing number of mouse cell types are becoming experimentally accessible, comprehensive coverage requires larger coordinated efforts with strategic infrastructural and fiscal planning. CRISPR-based genome editing may enable cell type access in other species, but issues of time, cost and ethics remain, especially for primates. Novel approaches that bypass the germline, such as somatic cell engineering and cell surface-based gene delivery, may reduce the barrier of genetic access to mammalian cell types.

Confocal calcium imaging analysis of respiratory-related burst activity in the parafacial region

The parafacial respiratory group (pFRG) surrounding the ventrolateral part of the facial motor nucleus is one of respiratory rhythm generators that consists of pre-inspiratory (Pre-I) neurons. Previous studies showed that most of the Pre-I neurons locating in the Phox2b cluster of the rostral ventral medulla were also Phox2b positive and intrinsically CO₂ sensitive. However, it is not clear what percentage of Phox2b-expressing cells in the pFRG of the ventral medulla are Pre-I neurons. To address this issue, we analyzed the activity of Phox2b-positive cells by calcium imaging using a confocal laser microscope in transgenic rats in which Phox2b-positive cells expressed EYFP. We found that more than 60% of the EYFP/Phox2b-positive cells showed Pre-I neuron-like rhythmic burst activity in the parafacial region of newborn rat.

Genetic factors in sleep-disordered breathing

Sleep-disordered breathing (SDB) is characterized by repetitive episodes of decreased or arrested respiratory airflow during sleep. SDB is common and affects approximately 20% of the Japanese general population. Most traits of normal sleep and SDB show familial aggregation, suggesting significant effects of genetic factors. Obstructive sleep apnea (OSA) is the most common type of SDB and has a high heritability. Regardless of high heritability, no risk locus for OSA has reached a genome-wide level of significance (P < 5×10^-8) in linkage or candidate gene analysis. However, a recent genome-wide association study identified some genetic risks for OSA with P < 5×10^-8 for the first time. The identified genes are associated with inflammation, hypoxia signaling, and sleep pathways. The effects of genetic factors on the consequences of OSA has not been determined, although a correlation between OSA and cardiovascular disease may differ across races. Congenital central hypoventilation syndrome (CCHS) is a genetically inherited disorder caused by mutations in the paired-like homeobox 2B (PHOX2B) gene of polyalanine repeat mutations in the 20-alanine repeat or non-polyalanine repeat mutations. PHOX2B genotypes are also associated with clinical phenotypes of CCHS, including severity of hypoventilation. SDB, including obesity hypoventilation syndrome, is often seen in genetic obesity-associated disorders such as Prader-Willi syndrome. Although advances in genetics have resulted in identification of some genetic causes of SDB, further studies are required to elucidate the cellular and molecular mechanisms between genetic risks and clinical manifestations.

Distinctive features of Phox2b-expressing neurons in the rat reticular formation dorsal to the trigeminal motor nucleus

Phox2b encodes a paired-like homeodomain-containing transcription factor essential for development of the autonomic nervous system. Phox2b-expressing (Phox2b⁺) neurons are present in the reticular formation dorsal to the trigeminal motor nucleus (RdV) as well as the nucleus of the solitary tract and parafacial respiratory group. However, the nature of Phox2b⁺ RdV neurons is still unclear. We investigated the physiological and morphological properties of Phox2b⁺ RdV neurons using postnatal day 2-7 transgenic rats expressing yellow fluorescent protein under the control of Phox2b. Almost all of Phox2b⁺ RdV neurons were glutamatergic, whereas Phox2b-negative (Phox2b^-) RdV neurons consisted of a few glutamatergic, many GABAergic, and many glycinergic neurons. The majority (48/56) of Phox2b⁺ neurons showed low-frequency firing (LF), while most of Phox2b^- neurons (35/42) exhibited high-frequency firing (HF) in response to intracellularly injected currents. All, but one, Phox2b⁺ neurons (55/56) did not fire spontaneously, whereas three-fourths of the Phox2b^- neurons (31/42) were spontaneously active. K⁺ channel and persistent Na⁺ current blockers affected the firing of LF and HF neurons. The majority of Phox2b⁺ (35/46) and half of the Phox2b^- neurons (19/40) did not respond to stimulations of the mesencephalic trigeminal nucleus, the trigeminal tract, and the principal sensory trigeminal nucleus. Biocytin labeling revealed that about half of the Phox2b⁺ (5/12) and Phox2b^- RdV neurons (5/10) send their axons to the trigeminal motor nucleus. These results suggest that Phox2b⁺ RdV neurons have distinct neurotransmitter phenotypes and firing properties from Phox2b^- RdV neurons and might play important roles in feeding-related functions including suckling and possibly mastication.

The respiratory control mechanisms in the brainstem and spinal cord: integrative views of the neuroanatomy and neurophysiology

Respiratory activities are produced by medullary respiratory rhythm generators and are modulated from various sites in the lower brainstem, and which are then output as motor activities through premotor efferent networks in the brainstem and spinal cord. Over the past few decades, new knowledge has been accumulated on the anatomical and physiological mechanisms underlying the generation and regulation of respiratory rhythm. In this review, we focus on the recent findings and attempt to elucidate the anatomical and functional mechanisms underlying respiratory control in the lower brainstem and spinal cord.

Respiratory and autonomic dysfunction in congenital central hypoventilation syndrome

The developmental lineage of the PHOX2B-expressing neurons in the retrotrapezoid nucleus (RTN) has been extensively studied. These cells are thought to function as central respiratory chemoreceptors, i.e., the mechanism by which brain Pco2 regulates breathing. The molecular and cellular basis of central respiratory chemoreception is based on the detection of CO2 via intrinsic proton receptors (TASK-2, GPR4) as well as synaptic input from peripheral chemoreceptors and other brain regions. Murine models of congenital central hypoventilation syndrome designed with PHOX2B mutations have suggested RTN neuron agenesis. In this review, we examine, through human and experimental animal models, how a restricted number of neurons that express the transcription factor PHOX2B play a crucial role in the control of breathing and autonomic regulation.

A Novel Reporter Rat Strain That Conditionally Expresses the Bright Red Fluorescent Protein tdTomato

Despite the strength of the Cre/loxP recombination system in animal models, its application in rats trails that in mice because of the lack of relevant reporter strains. Here, we generated a floxed STOP tdTomato rat that conditionally expresses a red fluorescent protein variant (tdTomato) in the presence of exogenous Cre recombinase. The tdTomato signal vividly visualizes neurons including their projection fibers and spines without any histological enhancement. In addition, a transgenic rat line (FLAME) that ubiquitously expresses tdTomato was successfully established by injecting intracytoplasmic Cre mRNA into fertilized ova. Our rat reporter system will facilitate connectome studies as well as the visualization of the fine structures of genetically identified cells for long periods both in vivo and ex vivo. Furthermore, FLAME is an ideal model for organ transplantation research owing to improved traceability of cells/tissues.

Drug gastrointestinal absorption in rat: Strain and gender differences

Predictive animal models of intestinal drug absorption are essential tools in drug development to identify compounds with promising biopharmaceutical properties. In situ perfusion absorption studies are routinely used in the preclinical setting to screen drug candidates. The objective of this work is to explore the differences in magnitude and variability on intestinal absorption associated with rat strain and gender. Metoprolol and Verapamil absorption rate coefficients were determined using the in situ closed loop perfusion model in four strains of rats and in both genders. Strains used were Sprague-Dawley, Wistar-Han, Wistar-Unilever, Long-Evans and CD∗IGS. In the case of Metoprolol only CD∗IGS and Wistar Unilever showed differences between males and females. For Verapamil, Wistar Han and Sprague-Dawley strains do not show differences between male and female rats. That means that in these strains permeability data from male and female could be combined. In male rats, which are commonly used for permeability estimation, there were differences for Metoprolol permeability between Sprague-Dawley (with lower permeability values) and the other strains, while for Verapamil Sprague-Dawley and Wistar-Han showed the lower permeability values. In conclusion, the selection of rat's strain and gender for intestinal absorption experiments is a relevant element during study design and data from different strains may not be always comparable.