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.

BackgroundRecent developments in droplet-based microfluidics allow the transcriptional profiling of thousands of individual cells in a quantitative, highly parallel and cost-effective way. A critical, often limiting step is the preparation of cells in an unperturbed state, not altered by stress or ageing. Other challenges are rare cells that need to be collected over several days or samples prepared at different times or locations.MethodsHere, we used chemical fixation to address these problems. Methanol fixation allowed us to stabilise and preserve dissociated cells for weeks without compromising single-cell RNA sequencing data.ResultsBy using mixtures of fixed, cultured human and mouse cells, we first showed that individual transcriptomes could be confidently assigned to one of the two species. Single-cell gene expression from live and fixed samples correlated well with bulk mRNA-seq data. We then applied methanol fixation to transcriptionally profile primary cells from dissociated, complex tissues. Low RNA content cells from Drosophila embryos, as well as mouse hindbrain and cerebellum cells prepared by fluorescence-activated cell sorting, were successfully analysed after fixation, storage and single-cell droplet RNA-seq. We were able to identify diverse cell populations, including neuronal subtypes. As an additional resource, we provide 'dropbead', an R package for exploratory data analysis, visualization and filtering of Drop-seq data.ConclusionsWe expect that the availability of a simple cell fixation method will open up many new opportunities in diverse biological contexts to analyse transcriptional dynamics at single-cell resolution.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-017-0383-5) contains supplementary material, which is available to authorized users.

Highlights d hPSC-derived neuromesodermal progenitors generate functional NMOs in 3D d Functional NMJs are generated in NMOs supported by terminal Schwann cells d NMOs contract and develop central pattern generator-like circuits d NMOs can be used to model key aspects of myasthenia gravis

Vocalization in young mice is an innate response to isolation or mechanical stimulation. Neuronal circuits that control vocalization and breathing overlap and rely on motor neurons that innervate laryngeal and expiratory muscles, but the brain center that coordinates these motor neurons has not been identified. Here, we show that the hindbrain nucleus tractus solitarius (NTS) is essential for vocalization in mice. By generating genetically modified newborn mice that specifically lack excitatory NTS neurons, we show that they are both mute and unable to produce the expiratory drive required for vocalization. Furthermore, the muteness of these newborns results in maternal neglect. We also show that neurons of the NTS directly connect to and entrain the activity of spinal (L1) and nucleus ambiguus motor pools located at positions where expiratory and laryngeal motor neurons reside. These motor neurons control expiratory pressure and laryngeal tension, respectively, thereby establishing the essential biomechanical parameters used for vocalization. In summary, our work demonstrates that the NTS is an obligatory component of the neuronal circuitry that transforms breaths into calls.V ocalization is the primary mechanism used by many vertebrate species for communication (1). Whereas adult mice call during courtship, mating, and territorial disputes, newborn mice use vocalization to communicate with their mothers (2, 3). Newborn mice, when isolated, produce ultrasonic calls (USCs) that elicit search and retrieval behavior by their mothers. Thus, vocalizations of newborn mice represent an innate behavior that is thought to rely on a genetically determined circuit. Such innate vocalizations are reminiscent of nonverbal utterances of humans like laughing, crying, sighing, and moaning.The central circuits that control vocalization have been widely studied in adult vertebrates, where they overlap in their executive components with respiratory circuits (4). Forebrain pathways that control the frequency and sequence of ultrasounds in mice are not essential for innate vocalization (5, 6); rather, it is the periaqueductal gray in the midbrain that modulates the activity of motor neurons in the hindbrain and spinal cord to implement calls and modulate breathing (7,8). Calls are shaped through a biomechanical process that involves variations in subglottal air pressure and laryngeal muscle tension (9, 10). Expiration is an important determinant of subglottal air pressure (11), suggesting that expiratory muscle activity and laryngeal tension are highly coordinated during vocalization. However, because expiratory and laryngeal motor neurons are located at markedly different axial levels of the nervous system, in the spinal cord (T11-L1 levels, expiratory) and hindbrain (nucleus ambiguus, laryngeal), how the activities of these motor pools are coordinated is unclear (12, 13). More importantly, the identity and location of functionally important premotor neurons for vocalization are little known.Using mouse genetics to investigate the ...

SignificanceMaintaining 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.