Clément Menuet

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

Early breathing defects after moderate hypoxia or hypercapnia in a mouse model of Rett syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disease caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2) and accompanied by complex symptoms, including erratic breathing and life-threatening apnoeas. In Mecp2-deficient male mice (Mecp2(-/y)), breathing is normal at birth but becomes altered after postnatal day 30 (P30), with erratic rhythm and apnoeas aggravating until death at around P60. Using plethysmography, we analyzed breathing of unrestrained wild type mice and Mecp2(-/y) at P15, P25 and P30 under air and under short-lasting exposure to moderate hypoxia or hypercapnia. In Mecp2(-/y) with normal resting ventilation, we report exacerbated respiratory responses to hypoxia at P30 and transient apnoeas with erratic rhythm after hypoxia and hypercapnia at P30, P25 and occasionally P15. Then environmental factors may induce breathing defects well before than expected in Mecp2(-/y) and possibly in RTT patients. We therefore suggest avoiding exposure of young RTT patients to environmental situations where they may encounter moderate hypoxia or hypercapnia.

The H3K27 Demethylase JMJD3 Is Required for Maintenance of the Embryonic Respiratory Neuronal Network, Neonatal Breathing, and Survival

JMJD3 (KDM6B) antagonizes Polycomb silencing by demethylating lysine 27 on histone H3. The interplay of methyltransferases and demethylases at this residue is thought to underlie critical cell fate transitions, and the dynamics of H3K27me3 during neurogenesis posited for JMJD3 a critical role in the acquisition of neural fate. Despite evidence of its involvement in early neural commitment, however, its role in the emergence and maturation of the mammalian CNS remains unknown. Here, we inactivated Jmjd3 in the mouse and found that its loss causes perinatal lethality with the complete and selective disruption of the pre-Bötzinger complex (PBC), the pacemaker of the respiratory rhythm generator. Through genetic and electrophysiological approaches, we show that the enzymatic activity of JMJD3 is selectively required for the maintenance of the PBC and controls critical regulators of PBC activity, uncovering an unanticipated role of this enzyme in the late structuring and function of neuronal networks.

Upper Airway Dysfunction of Tau-P301L Mice Correlates with Tauopathy in Midbrain and Ponto-Medullary Brainstem Nuclei

Tauopathy comprises hyperphosphorylation of the microtubule-associated protein tau, causing intracellular aggregation and accumulation as neurofibrillary tangles and neuropil treads. Some primary tauopathies are linked to mutations in the MAPT gene coding for protein tau, but most are sporadic with unknown causes. Also, in Alzheimers disease, the most frequent secondary tauopathy, neither the cause nor the pathological mechanisms and repercussions are understood. Transgenic mice expressing mutant Tau-P301L suffer cognitive and motor defects and die prematurely from unknown causes. Here, in situ electrophysiology in symptomatic Tau-P301L mice (7–8 months of age) revealed reduced postinspiratory discharges of laryngeal motor outputs that control laryngeal constrictor muscles. Under high chemical drive (hypercapnia), postinspiratory discharge was nearly abolished, whereas laryngeal inspiratory discharge was increased disproportionally. The latter may suggest a shift of postinspiratory laryngeal constrictor activity into inspiration. In vivo double-chamber plethysmography of Tau-P301L mice showed significantly reduced respiratory airflow but significantly increased chest movements during baseline breathing, but particularly in hypercapnia, confirming a significant increase in inspiratory resistive load. Histological analysis demonstrated hyperphosphorylated tau in brainstem nuclei, directly or indirectly involved in upper airway motor control (i.e., the Kölliker–Fuse, periaqueductal gray, and intermediate reticular nuclei). In contrast, young Tau-P301L mice did not show breathing disorders or brainstem tauopathy. Consequently, in aging Tau-P301L mice, progressive upper airway dysfunction is linked to progressive tauopathy in identified neural circuits. Because patients with tauopathy suffer from upper airway dysfunction, the Tau-P301L mice can serve as an experimental model to study disease-specific synaptic dysfunction in well defined functional neural circuits.

Increasing Brain Protein O-GlcNAc-ylation Mitigates Breathing Defects and Mortality of Tau.P301L Mice

The microtubule associated protein tau causes primary and secondary tauopathies by unknown molecular mechanisms. Post-translational O-GlcNAc-ylation of brain proteins was demonstrated here to be beneficial for Tau.P301L mice by pharmacological inhibition of O-GlcNAc-ase. Chronic treatment of ageing Tau.P301L mice mitigated their loss in body-weight and improved their motor deficits, while the survival was 3-fold higher at the pre-fixed study endpoint at age 9.5 months. Moreover, O-GlcNAc-ase inhibition significantly improved the breathing parameters of Tau.P301L mice, which underpinned pharmacologically the close correlation of mortality and upper-airway defects. O-GlcNAc-ylation of brain proteins increased rapidly and stably by systemic inhibition of O-GlcNAc-ase. Conversely, biochemical evidence for protein Tau.P301L to become O-GlcNAc-ylated was not obtained, nor was its phosphorylation consistently or markedly affected. We conclude that increasing O-GlcNAc-ylation of brain proteins improved the clinical condition and prolonged the survival of ageing Tau.P301L mice, but not by direct biochemical action on protein tau. The pharmacological effect is proposed to be located downstream in the pathological cascade initiated by protein Tau.P301L, opening novel venues for our understanding, and eventually treating the neurodegeneration mediated by protein tau.

Raphé tauopathy alters serotonin metabolism and breathing activity in terminal Tau.P301L mice: Possible implications for tauopathies and Alzheimer's disease

Tauopathies, including Alzheimers disease are the most frequent neurodegenerative disorders in elderly people. Patients develop cognitive and behaviour defects induced by the tauopathy in the forebrain, but most also display early brainstem tauopathy, with oro-pharyngeal and serotoninergic (5-HT) defects. We studied these aspects in Tau.P301L mice, that express human mutant tau protein and develop tauopathy first in hindbrain, with cognitive, motor and upper airway defects from 7 to 8 months onwards, until premature death before age 12 months. Using plethysmography, immunohistochemistry and biochemistry, we examined the respiratory and 5-HT systems of aging Tau.P301L and control mice. At 8 months, Tau.P301L mice developed upper airway dysfunction but retained normal respiratory rhythm and normal respiratory regulations. In the following weeks, Tau.P301L mice entered terminal stages with reduced body weight, progressive limb clasping and lethargy. Compared to age 8 months, terminal Tau.P301L mice showed aggravated upper airway dysfunction, abnormal respiratory rhythm and abnormal respiratory regulations. In addition, they showed severe tauopathy in Kolliker-Fuse, raphé obscurus and raphé magnus nuclei but not in medullary respiratory-related areas. Although the raphé tauopathy concerned mainly non-5-HT neurons, the 5-HT metabolism of terminal Tau.P301L mice was altered. We propose that the progressive raphé tauopathy affects the 5-HT metabolism, which affects the 5-HT modulation of the respiratory network and therefore the breathing pattern. Then, 5-HT deficits contribute to the moribund phenotype of Tau.P301L mice, and possibly in patients suffering from tauopathies, including Alzheimers disease.

Early abnormalities of post-sigh breathing in a mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disease accompanied by complex, disabling symptoms, including breathing symptoms. Because Rett syndrome is caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2), Mecp2-deficient mice have been generated as experimental model. Males of Mecp2-deficient mice (Mecp2(-/y)) breathe normally at birth but show abnormal respiratory responses to hypoxia and hypercapnia from postnatal day 25 (P25). After P30, Mecp2(-/y) mice develop breathing symptoms reminiscent of Rett syndrome, aggravating until premature death at around P60. Using plethysmography, we analyzed the sighs and the post-sigh breathing pattern of unrestrained wild type male mice (WT) and Mecp2(-/y) mice from P15 to P60. Sighs are spontaneous large inspirations known to prevent lung atelectasis and to improve alveolar oxygenation. However, Mecp2(-/y) mice show early abnormalities of post-sigh breathing, with long-lasting post-sigh apnoeas, reduced tidal volume when eupnoea resumes and lack of post-sigh bradypnoea which develop from P15, aggravate with age and possibly contribute to breathing symptoms to come.

Physiological definition of upper airway obstructions in mouse model for Rett syndrome

Rett syndrome is a neuro-developmental disease accompanied by breathing symptoms including breath-hold events, and is caused by mutation of the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2). Males of Mecp2-deficient mice (Mecp2(-/y)) also develop breathing symptoms, with erratic rhythm and life-threatening apnoeas from postnatal day 30 (P30), leading to respiratory distress and premature death at around P60. We investigated the respiratory function of conscious Mecp2(-/y) mice at P40-P60 using conventional whole-body plethysmography, double-chamber plethysmography and chest EMG recordings. Double-chamber plethysmography revealed a persistent increase in respiratory work-load with enlarged chest movements, but no subsequent increase of tidal volume thus revealing a mismatch between airflow and muscle work-load. Apnoeas occurred with cessation of both chest movements and ventilation, but some (40%) developed with persisting rhythmic chest EMG discharges or chest movements without respiratory airflow, suggesting respiratory efforts against obstructed airways. Airway obstruction was maintained even when the respiratory drive increased significantly, triggering large chest EMG discharges and movements. Whole-body plethysmography of Mecp2(-/y) mice revealed significant increases of spirograms, reflecting forced chest movements against partially obstructed airways. The persisting chest EMG discharges and rhythmic chest movements without respiratory airflow suggest that Mecp2 inactivation alters neural circuits controlling the upper airway dilator muscles. The observed breath-hold events in Mecp2(-/y) mice might imply disturbance of neural circuits attached to voluntary control of breathing.

Age-related impairment of ultrasonic vocalization in Tau.P301L mice: possible implication for progressive language disorders.

Background Tauopathies, including Alzheimers Disease, are the most frequent neurodegenerative diseases in elderly people and cause various cognitive, behavioural and motor defects, but also progressive language disorders. For communication and social interactions, mice produce ultrasonic vocalization (USV) via expiratory airflow through the larynx. We examined USV of Tau.P301L mice, a mouse model for tauopathy expressing human mutant tau protein and developing cognitive, motor and upper airway defects. Methodology/Principal Findings At age 4–5 months, Tau.P301L mice had normal USV, normal expiratory airflow and no brainstem tauopathy. At age 8–10 months, Tau.P301L mice presented impaired USV, reduced expiratory airflow and severe tauopathy in the periaqueductal gray, Kolliker-Fuse and retroambiguus nuclei. Tauopathy in these nuclei that control upper airway function and vocalization correlates well with the USV impairment of old Tau.P301L mice. Conclusions In a mouse model for tauopathy, we report for the first time an age-related impairment of USV that correlates with tauopathy in midbrain and brainstem areas controlling vocalization. The vocalization disorder of old Tau.P301L mice could be, at least in part, reminiscent of language disorders of elderly suffering tauopathy.

Isoflurane anesthesia precipitates tauopathy and upper airways dysfunction in pre-symptomatic Tau.P301L mice: Possible implication for neurodegenerative diseases

The postoperative cognitive decline resulting from volatile anesthesia is gaining acceptance as a major health problem. The common anesthetic isoflurane is suspected to precipitate neurodegeneration in Alzheimers disease by unknown mechanisms. We previously validated that 8month old Tau.P301L mice suffer upper airways defects related to tauopathy within the Kolliker-Fuse nucleus that controls upper airways function. We now report that isoflurane anesthesia in young, pre-symptomatic Tau.P301L mice triggered precocious upper airways defects and tauopathy in several brainstem nuclei, including the nucleus ambiguus that contains upper airways motor neurons and the Kolliker-Fuse. The prescription drug memantine, identified as an NMDA receptor antagonist, prevented the post-anesthesia upper airways dysfunction and alleviated tauopathy in the nucleus ambiguus and Kolliker-Fuse. We further identified protocols of anesthesia in young Tau.P301L mice that mitigated adverse effects of isoflurane anesthesia. Thus, our experimental findings in a validated mouse model for tauopathy demonstrate the link between isoflurane anesthesia, earlier onset of tauopathy and earlier onset of functional deficits, highlight the implication of NMDA-receptors in the mechanisms mediating the adverse effects of isoflurane, and potentially identify safer protocols for anesthesia in patients with tauopathy.