Disturbances of breathing in Rett syndrome: results from patients and animal models

Inhibitory Synaptic Influences on Developmental Motor Disorders

During development, GABA and glycine play major trophic and synaptic roles in the establishment of the neuromotor system. In this review, we summarise the formation, function and maturation of GABAergic and glycinergic synapses within neuromotor circuits during development. We take special care to discuss the differences in limb and respiratory neuromotor control. We then investigate the influences that GABAergic and glycinergic neurotransmission has on two major developmental neuromotor disorders: Rett syndrome and spastic cerebral palsy. We present these two syndromes in order to contrast the approaches to disease mechanism and therapy. While both conditions have motor dysfunctions at their core, one condition Rett syndrome, despite having myriad symptoms, has scientists focused on the breathing abnormalities and their alleviation-to great clinical advances. By contrast, cerebral palsy remains a scientific quagmire or poor definitions, no widely adopted model and a lack of therapeutic focus. We conclude that the sheer abundance of diversity of inhibitory neurotransmitter targets should provide hope for intractable conditions, particularly those that exhibit broad spectra of dysfunction-such as spastic cerebral palsy and Rett syndrome.

Oral Feeding of an Antioxidant Cocktail as a Therapeutic Strategy in a Mouse Model of Rett Syndrome: Merits and Limitations of Long-Term Treatment

Rett syndrome (RTT) is a severe neurodevelopmental disorder that typically arises from spontaneous germline mutations in the X-chromosomal methyl-CpG binding protein 2 (MECP2) gene. For the first 6–18 months of life, the development of the mostly female patients appears normal. Subsequently, cognitive impairment, motor disturbances, hand stereotypies, epilepsy, and irregular breathing manifest, with previously learned skills being lost. Early mitochondrial impairment and a systemic oxidative burden are part of the complex pathogenesis, and contribute to disease progression. Accordingly, partial therapeutic merits of redox-stabilizing and antioxidant (AO) treatments were reported in RTT patients and Mecp2-mutant mice. Pursuing these findings, we conducted a full preclinical trial on male and female mice to define the therapeutic value of an orally administered AO cocktail composed of vitamin E, N-acetylcysteine, and α-lipoic acid. AO treatment ameliorated some of the microcephaly-related aspects. Moreover, the reduced growth, lowered blood glucose levels, and the hippocampal synaptic plasticity of Mecp2−/y mice improved. However, the first-time detected intensified oxidative DNA damage in Mecp2-mutant cortex persisted. The behavioral performance, breathing regularity, and life expectancy of Mecp2-mutant mice did not improve upon AO treatment. Long-term-treated Mecp2+/− mice eventually became obese. In conclusion, the AO cocktail ameliorated a subset of symptoms of the complex RTT-related phenotype, thereby further confirming the potential merits of AO-based pharmacotherapies. Yet, it also became evident that long-term AO treatment may lose efficacy and even aggravate the metabolic disturbances in RTT. This emphasizes the importance of a constantly well-balanced redox balance for systemic well-being.

Mafa-dependent GABAergic activity promotes mouse neonatal apneas

While apneas are associated with multiple pathological and fatal conditions, the underlying molecular mechanisms remain elusive. We report that a mutated form of the transcription factor Mafa (Mafa^4A) that prevents phosphorylation of the Mafa protein leads to an abnormally high incidence of breath holding apneas and death in newborn Mafa^4A/4A mutant mice. This apneic breathing is phenocopied by restricting the mutation to central GABAergic inhibitory neurons and by activation of inhibitory Mafa neurons while reversed by inhibiting GABAergic transmission centrally. We find that Mafa activates the Gad2 promoter in vitro and that this activation is enhanced by the mutation that likely results in increased inhibitory drives onto target neurons. We also find that Mafa inhibitory neurons are absent from respiratory, sensory (primary and secondary) and pontine structures but are present in the vicinity of the hypoglossal motor nucleus including premotor neurons that innervate the geniohyoid muscle, to control upper airway patency. Altogether, our data reveal a role for Mafa phosphorylation in regulation of GABAergic drives and suggest a mechanism whereby reduced premotor drives to upper airway muscles may cause apneic breathing at birth.

Apneas are associated with many pathological conditions. Here, the authors show in a mouse model that stabilization of the transcription factor Mafa in brainstem GABAergic neurons may contribute to apnea, by decreasing motor drive to muscles controlling the airways.

Breathing Abnormalities During Sleep and Wakefulness in Rett Syndrome: Clinical Relevance and Paradoxical Relationship With Circulating Pro-oxidant Markers

Background

Breathing abnormalities are common in Rett syndrome (RTT), a pervasive neurodevelopmental disorder almost exclusively affecting females. RTT is linked to mutations in the methyl-CpG-binding protein 2 (MeCP2) gene. Our aim was to assess the clinical relevance of apneas during sleep-wakefulness cycle in a population with RTT and the possible impact of apneas on circulating oxidative stress markers.

Methods

Female patients with a clinical diagnosis of typical RTT (n = 66), MECP2 gene mutation, and apneas were enrolled (mean age: 12.5 years). Baseline clinical severity, arterial blood gas analysis, and red blood cell count were assessed. Breathing was monitored during the wakefulness and sleep states (average recording time: 13 ± 0.5 h) with a portable polygraphic screening device. According to prevalence of breath holdings, the population was categorized into the wakefulness apnea (WA) and sleep apnea (SA) groups, and apnea-hypopnea index (AHI) was calculated. The impact of respiratory events on oxidative stress was assessed by plasma and intra-erythrocyte non-protein-bound iron (P-NPBI and IE-NPBI, respectively), and plasma F2-isoprostane (F2-IsoP) assays.

Results

Significant prevalence of obstructive apneas with values of AHI > 15 was present in 69.7% of the population with RTT. The group with SA showed significantly increased AHI values > 15 (p = 0.0032), total breath holding episodes (p = 0.007), and average SpO2 (p = 0.0001) as well as lower nadir SpO2 (p = 0.0004) compared with the patients with WAs. The subgroups of patients with WA and SA showed no significant differences in arterial blood gas analysis variables (p > 0.089). Decreased mean cell hemoglobin (MCH) (p = 0.038) was observed in the group with WAs. P-NPBI levels were significantly higher in the group with WA than in that with SAs (p = 0.0001). Stepwise multiple linear regression models showed WA being related to nadir SpO2, average SpO2, and P-NPBI (adjusted R2 = 0.613, multiple correlation coefficient = 0.795 p < 0.0001), and P-NPBI being related to average SpO2, blood PaCO2, red blood cell mean corpuscular volume (MCV), age, and topiramate treatment (adjusted R2 = 0.551, multiple correlation coefficient = 0.765, p < 0.0001).

Conclusion

Our findings indicate that the impact of apneas in RTT is uneven according to the sleep-wakefulness cycle, and that plasma redox active iron represents a potential novel therapeutic target.

Breathing disturbances in Rett syndrome

Rett Syndrome is an X-linked neurological disorder characterized by behavioral and neurological regression, seizures, motor deficits, and dysautonomia. A particularly prominent presentation includes breathing abnormalities characterized by breathing irregularities, hyperventilation, repetitive breathholding during wakefulness, obstructive and central apneas during sleep, and abnormal responses to hypoxia and hypercapnia. The condition and pathology of the respiratory system is further complicated by dysfunctions of breathing-motor coordination, which is reflected in dysphagia. The discovery of the X-linked mutations in the MECP2 gene has transformed our understanding of the cellular and molecular mechanisms that are at the root of various clinical phenotypes. However, the genotype-phenotype relationship is complicated by various factors which include not only X-inactivation but also consequences of the intermittent hypoxia and oxidative stress associated with the breathing abnormalities.

The Pathophysiology of Rett Syndrome With a Focus on Breathing Dysfunctions

Rett syndrome (RTT), an X-chromosome-linked neurological disorder, is characterized by serious pathophysiology, including breathing and feeding dysfunctions, and alteration of cardiorespiratory coupling, a consequence of multiple interrelated disturbances in the genetic and homeostatic regulation of central and peripheral neuronal networks, redox state, and control of inflammation. Characteristic breath-holds, obstructive sleep apnea, and aerophagia result in intermittent hypoxia, which, combined with mitochondrial dysfunction, causes oxidative stress-an important driver of the clinical presentation of RTT.

Disturbed redox homeostasis and oxidative stress: Potential players in the developmental regression in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder affecting mostly girls. A seemingly normal initial development is followed by developmental stagnation and regression, leading to severe mental impairment with autistic features, motor dysfunction, irregular breathing and epilepsy. Currently, a cure does not exist. Due to the close association of RTT with mitochondrial alterations, cellular redox-impairment and oxidative stress, compounds stabilizing mitochondrial function, cellular redox-homeostasis, and oxidant detoxification are increasingly considered as treatment concepts. Indeed, antioxidants and free-radical scavengers ameliorate certain aspects of the complex and severe clinical presentation of RTT. To further evaluate these strategies, reliable biosensors are needed to quantify redox-conditions in brain and peripheral organs of mouse models or in patient-derived cells. Genetically-encoded redox-sensors meet these requirements. Expressed in transgenic mouse-models such as our unique Rett-redox indicator mice, they will report for any cell type desired the severity of oxidant stress throughout the various disease stages of RTT. Furthermore, these sensors will be crucial to evaluate in vitro and in vivo the outcome of mitochondria- and redox-balance targeted treatments.

Persistent Expression of Serotonin Receptor 5b Alters Breathing Behavior in Male MeCP2 Knockout Mice

Mutations in the transcription factor methyl-CpG-binding protein 2 (MeCP2) cause the neurodevelopmental disorder Rett syndrome (RTT). Besides many other neurological problems, RTT patients show irregular breathing with recurrent apneas or breath-holdings. MeCP2-deficient mice, which recapitulate this breathing phenotype, show a dysregulated, persistent expression of G-protein-coupled serotonin receptor 5-ht_5b (Htr5b) in the brainstem. To investigate whether the persistence of 5-ht_5b expression is contributing to the respiratory phenotype, we crossbred MeCP2-deficient mice with 5-ht_5b-deficient mice to generate double knockout mice (Mecp2^−/y;Htr5b^−/−). To compare respiration between wild type (WT), Mecp2^−/y and Mecp2^−/y;Htr5b^−/− mice, we used unrestrained whole-body plethysmography. While the breathing of MeCP2-deficient male mice (Mecp2^−/y) at postnatal day 40 is characterized by a slow breathing rate and the occurrence of prolonged respiratory pauses, we found that in MeCP2-deficient mice, which also lacked the 5-ht_5b receptor, the breathing rate and the number of pauses were indistinguishable from WT mice. To test for a potential mechanism, we also analyzed if the known coupling of 5-ht_5b receptors to G_i proteins is altering second messenger signaling. Tissue cAMP levels in the medulla of Mecp2^−/y mice were decreased as compared to WT mice. In contrast, cAMP levels in Mecp2^−/y;Htr5b^−/− mice were indistinguishable from WT mice. Taken together, our data points towards a role of 5-ht_5b receptors within the complex breathing phenotype of MeCP2-deficient mice.

Autonomic breathing abnormalities in Rett syndrome: caregiver perspectives in an international database study

BACKGROUND

Rett syndrome is a severe neurodevelopmental disorder associated with mutations in the MECP2 gene. Irregular breathing patterns and abdominal bloating are prominent but poorly understood features. Our aims were to characterize the abnormal breathing patterns and abdominal bloating, investigate the distribution of these by age and mutation type and examine their impact and management from a caregiver perspective.

METHODS

We invited previously recruited families from the International Rett Syndrome Study to complete a web-based questionnaire concerning their family member with Rett syndrome aged between 2 and 57 years. We used logistic regression to investigate presence, frequency and impact of breath-holding, hyperventilation, or abdominal bloating by age group and mutation type. Age of onset for both breathing abnormalities was investigated using time-to-onset analysis, and the Kaplan-Meier method was used to estimate the failure function for the study sample. Descriptive statistics were used to characterize the management of irregular breathing.

RESULTS

Questionnaires were returned by 413/482 (85.7%) families. Breath-holding was reported for 68.8%, hyperventilation for 46.4% and abdominal bloating for 42.4%. Hyperventilation was more prevalent and frequent in those younger than 7 years of age and abdominal bloating in those aged over 20 years. Onset of breathing irregularities usually occurred during early childhood. Caregivers perceived that daily life was considerably impacted for almost half (44.1%) of those with abdominal bloating and in just over than a third of those with breath-holding (35.8%) or hyperventilation (35.1%). Although perceived impact was broadly comparable between age and mutation groups for breath-holding, hyperventilation and abdominal bloating, girls and women with a p.Arg294* mutation were considered to be more affected by all three conditions. Only 31 individuals had received medically prescribed treatments including 12 different medications, added oxygen, rebreathing apparatus or non-invasive ventilation.

CONCLUSIONS

Autonomic disturbances are prevalent and burdensome in Rett syndrome. This information may guide the design of inclusion criteria and outcome measures for clinical intervention trials targeting autonomic abnormalities. Further investigation of available treatments is necessary to delineate evidence-based management pathways.

Analysis of the Serotonergic System in a Mouse Model of Rett Syndrome Reveals Unusual Upregulation of Serotonin Receptor 5b

Mutations in the transcription factor methyl-CpG-binding-protein 2 (MeCP2) cause a delayed-onset neurodevelopmental disorder known as Rett syndrome (RTT). Although alteration in serotonin levels have been reported in RTT patients, the molecular mechanisms underlying these defects are not well understood. Therefore, we chose to investigate the serotonergic system in hippocampus and brainstem of male Mecp2^-/y knock-out mice in the B6.129P2(C)-Mecp2(tm1.1Bird) mouse model of RTT. The serotonergic system in mouse is comprised of 16 genes, whose mRNA expression profile was analyzed by quantitative RT-PCR. Mecp2^-/y mice are an established animal model for RTT displaying most of the cognitive and physical impairments of human patients and the selected areas receive significant modulation through serotonin. Using anatomically and functional characterized areas, we found region-specific differential expression between wild type and Mecp2^-/y mice at post-natal day 40. In brainstem, we found five genes to be dysregulated, while in hippocampus, two genes were dysregulated. The one gene dysregulated in both brain regions was dopamine decarboxylase, but of special interest is the serotonin receptor 5b (5-ht_5b), which showed 75-fold dysregulation in brainstem of Mecp2^-/y mice. This dysregulation was not due to upregulation, but due to failure of down-regulation in Mecp2^-/y mice during development. Detailed analysis of 5-ht_5b revealed a receptor that localizes to endosomes and interacts with G_αi proteins.

Systemic Radical Scavenger Treatment of a Mouse Model of Rett Syndrome: Merits and Limitations of the Vitamin E Derivative Trolox

Rett syndrome (RTT) is a severe neurodevelopmental disorder typically arising from spontaneous mutations in the X-chromosomal methyl-CpG binding protein 2 (MECP2) gene. The almost exclusively female Rett patients show an apparently normal development during their first 6-18 months of life. Subsequently, cognitive- and motor-impairment, hand stereotypies, loss of learned skills, epilepsy and irregular breathing manifest. Early mitochondrial impairment and oxidative challenge are considered to facilitate disease progression. Along this line, we recently confirmed in vitro that acute treatment with the vitamin E-derivative Trolox dampens neuronal hyperexcitability, reinstates synaptic plasticity, ameliorates cellular redox balance and improves hypoxia tolerance in male MeCP2-deficient (Mecp2^-/y ) mouse hippocampus. Pursuing these promising findings, we performed a preclinical study to define the merit of systemic Trolox administration. Blinded, placebo-controlled in vivo treatment of male mice started at postnatal day (PD) 10-11 and continued for ~40 days. Compounds (vehicle only, 10 mg/kg or 40 mg/kg Trolox) were injected intraperitoneally every 48 h. Detailed phenotyping revealed that in Mecp2^-/y mice, blood glucose levels, lipid peroxidation, synaptic short-term plasticity, hypoxia tolerance and certain forms of environmental exploration were improved by Trolox. Yet, body weight and size, motor function and the rate and regularity of breathing did not improve. In conclusion, in vivo Trolox treatment partially ameliorated a subset of symptoms of the complex Rett phenotype, thereby confirming a partial merit of the vitamin E-derivative based pharmacotherapy. Yet, it also became evident that frequent animal handling and the route of drug administration are critical issues to be optimized in future trials.

Effects of ion channel noise on neural circuits: an application to the respiratory pattern generator to investigate breathing variability

Neural activity generally displays irregular firing patterns even in circuits with apparently regular outputs, such as motor pattern generators, in which the output frequency fluctuates randomly around a mean value. This "circuit noise" is inherited from the random firing of single neurons, which emerges from stochastic ion channel gating (channel noise), spontaneous neurotransmitter release, and its diffusion and binding to synaptic receptors. Here we demonstrate how to expand conductance-based network models that are originally deterministic to include realistic, physiological noise, focusing on stochastic ion channel gating. We illustrate this procedure with a well-established conductance-based model of the respiratory pattern generator, which allows us to investigate how channel noise affects neural dynamics at the circuit level and, in particular, to understand the relationship between the respiratory pattern and its breath-to-breath variability. We show that as the channel number increases, the duration of inspiration and expiration varies, and so does the coefficient of variation of the breath-to-breath interval, which attains a minimum when the mean duration of expiration slightly exceeds that of inspiration. For small channel numbers, the variability of the expiratory phase dominates over that of the inspiratory phase, and vice versa for large channel numbers. Among the four different cell types in the respiratory pattern generator, pacemaker cells exhibit the highest sensitivity to channel noise. The model shows that suppressing input from the pons leads to longer inspiratory phases, a reduction in breathing frequency, and larger breath-to-breath variability, whereas enhanced input from the raphe nucleus increases breathing frequency without changing its pattern.

NEW & NOTEWORTHY

A major source of noise in neuronal circuits is the "flickering" of ion currents passing through the neurons' membranes (channel noise), which cannot be suppressed experimentally. Computational simulations are therefore the best way to investigate the effects of this physiological noise by manipulating its level at will. We investigate the role of noise in the respiratory pattern generator and show that endogenous, breath-to-breath variability is tightly linked to the respiratory pattern.

Characterization of Rett Syndrome-like phenotypes in Mecp2-knockout rats

Background

Rett Syndrome (RTT) is a neurodevelopmental disease caused by the disruption of the MECP2 gene. Several mouse models of RTT have been developed with Mecp2 disruptions. Although the mouse models are widely used in RTT research, results obtained need to be validated in other species. Therefore, we performed these studies to characterize phenotypes of a novel Mecp2^−/Y rat model and compared them with the Mecp2^tm1.1Bird mouse model of RTT.

Methods

RTT-like phenotypes were systematically studied and compared between Mecp2^−/Y rats and Mecp2^−/Y mice. In-cage conditions of the rats were monitored. Grip strength and spontaneous locomotion were used to evaluate the motor function. Three-chamber test was performed to show autism-type behaviors. Breathing activity was recorded with the plethysmograph. Individual neurons in the locus coeruleus (LC) were studied in the whole-cell current clamp. The lifespan of the rats was determined with their survival time.

Results

Mecp2^−/Y rats displayed growth retardation, malocclusion, and lack of movements, while hindlimb clasping was not seen. They had weaker forelimb grip strength and a lower rate of locomotion than the WT littermates. Defects in social interaction with other rats were obvious. Breathing frequency variation and apnea in the null rats were significantly higher than in the WT. LC neurons in the null rats showed excessive firing activity. A half of the null rats died in 2 months. Most of the RTT-like symptoms were comparable to those seen in Mecp2^−/Y mice, while some appeared more or less severe. The findings that most RTT-like symptoms exist in the rat model with moderate variations and differences from the mouse models support the usefulness of both Mecp2^−/Y rodent models.

Conclusions

The novel Mecp2^−/Y rat model recapitulated numerous RTT-like symptoms as Mecp2^−/Y mouse models did, which makes it a valuable alternative model in the RTT studies when the body size matters.

Exploring the possible link between MeCP2 and oxidative stress in Rett syndrome

Rett syndrome (RTT, MIM 312750) is a rare and orphan progressive neurodevelopmental disorder affecting girls almost exclusively, with a frequency of 1/15,000 live births of girls. The disease is characterized by a period of 6 to 18 months of apparently normal neurodevelopment, followed by early neurological regression, with a progressive loss of acquired cognitive, social, and motor skills. RTT is known to be caused in 95% of the cases by sporadic de novo loss-of-function mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene encoding methyl-CpG binding protein 2 (MeCP2), a nuclear protein able to regulate gene expression. Despite almost two decades of research into the functions and role of MeCP2, little is known about the mechanisms leading from MECP2 mutation to the disease. Oxidative stress (OS) is involved in the pathogenic mechanisms of several neurodevelopmental and neurodegenerative disorders, although in many cases it is not clear whether OS is a cause or a consequence of the pathology. Fairly recently, the presence of a systemic OS has been demonstrated in RTT patients with a strong correlation with the patients' clinical status. The link between MECP2 mutation and the redox imbalance found in RTT is not clear. Animal studies have suggested a possible direct correlation between Mecp2 mutation and increased OS levels. In addition, the restoration of Mecp2 function in astrocytes significantly improves the developmental outcome of Mecp2-null mice and reexpression of Mecp2 gene in the brain of null mice restored oxidative damage, suggesting that Mecp2 loss of function can be involved in oxidative brain damage. Starting from the evidence that oxidative damage in the brain of Mecp2-null mice precedes the onset of symptoms, we evaluated whether, based on the current literature, the dysfunctions described in RTT could be a consequence or, in contrast, could be caused by OS. We also analyzed whether therapies that at least partially treated some RTT symptoms can play a role in defense against OS. At this stage we can propose that OS could be one of the main causes of the dysfunctions observed in RTT. In addition, the major part of the therapies recommended to alleviate RTT symptoms have been shown to interfere with oxidative homeostasis, suggesting that MeCP2 could somehow be involved in the protection of the brain from OS.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

Inflammatory lung disease in Rett syndrome

Rett syndrome (RTT) is a pervasive neurodevelopmental disorder mainly linked to mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2). Respiratory dysfunction, historically credited to brainstem immaturity, represents a major challenge in RTT. Our aim was to characterize the relationships between pulmonary gas exchange abnormality (GEA), upper airway obstruction, and redox status in patients with typical RTT (n = 228) and to examine lung histology in a Mecp2-null mouse model of the disease. GEA was detectable in ~80% (184/228) of patients versus ~18% of healthy controls, with “high” (39.8%) and “low” (34.8%) patterns dominating over “mixed” (19.6%) and “simple mismatch” (5.9%) types. Increased plasma levels of non-protein-bound iron (NPBI), F₂-isoprostanes (F₂-IsoPs), intraerythrocyte NPBI (IE-NPBI), and reduced and oxidized glutathione (i.e., GSH and GSSG) were evidenced in RTT with consequently decreased GSH/GSSG ratios. Apnea frequency/severity was positively correlated with IE-NPBI, F₂-IsoPs, and GSSG and negatively with GSH/GSSG ratio. A diffuse inflammatory infiltrate of the terminal bronchioles and alveoli was evidenced in half of the examined Mecp2-mutant mice, well fitting with the radiological findings previously observed in RTT patients. Our findings indicate that GEA is a key feature of RTT and that terminal bronchioles are a likely major target of the disease.

Rett syndrome and MeCP2

Rett syndrome (RTT) is a severe and progressive neurological disorder, which mainly affects young females. Mutations of the methyl-CpG binding protein 2 (MECP2) gene are the most prevalent cause of classical RTT cases. MECP2 mutations or altered expression are also associated with a spectrum of neurodevelopmental disorders such as autism spectrum disorders with recent links to fetal alcohol spectrum disorders. Collectively, MeCP2 relation to these neurodevelopmental disorders highlights the importance of understanding the molecular mechanisms by which MeCP2 impacts brain development, mental conditions, and compromised brain function. Since MECP2 mutations were discovered to be the primary cause of RTT, a significant progress has been made in the MeCP2 research, with respect to the expression, function and regulation of MeCP2 in the brain and its contribution in RTT pathogenesis. To date, there have been intensive efforts in designing effective therapeutic strategies for RTT benefiting from mouse models and cells collected from RTT patients. Despite significant progress in MeCP2 research over the last few decades, there is still a knowledge gap between the in vitro and in vivo research findings and translating these findings into effective therapeutic interventions in human RTT patients. In this review, we will provide a synopsis of Rett syndrome as a severe neurological disorder and will discuss the role of MeCP2 in RTT pathophysiology.

Respiratory phenotypes are distinctly affected in mice with common Rett syndrome mutations MeCP2 T158A and R168X

Respiratory disturbances are a primary phenotype of the neurological disorder, Rett syndrome (RTT), caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Mouse models generated with null mutations in Mecp2 mimic respiratory abnormalities in RTT girls. Large deletions, however, are seen in only ∼10% of affected human individuals. Here we characterized respiration in heterozygous females from two mouse models that genetically mimic common RTT point mutations, a missense mutation T158A (Mecp2(T158A/)(+)) or a nonsense mutation R168X (Mecp2(R168X/+)). MeCP2 T158A shows decreased binding to methylated DNA, while MeCP2 R168X retains the capacity to bind methylated DNA but lacks the ability to recruit complexes required for transcriptional repression. We found that both Mecp2(T158A/+) and Mecp2(R168X/+) heterozygotes display augmented hypoxic ventilatory responses and depressed hypercapnic responses, compared to wild-type controls. Interestingly, the incidence of apnea was much greater in Mecp2(R168X/+) heterozygotes, 189 per hour, than Mecp2(T158A/+) heterozygotes, 41 per hour. These results demonstrate that different RTT mutations lead to distinct respiratory phenotypes, suggesting that characterization of the respiratory phenotype may reveal functional differences between MeCP2 mutations and provide insights into the pathophysiology of RTT.

The free radical scavenger Trolox dampens neuronal hyperexcitability, reinstates synaptic plasticity, and improves hypoxia tolerance in a mouse model of Rett syndrome

Rett syndrome (RS) causes severe cognitive impairment, loss of speech, epilepsy, and breathing disturbances with intermittent hypoxia. Also mitochondria are affected; a subunit of respiratory complex III is dysregulated, the inner mitochondrial membrane is leaking protons, and brain ATP levels seem reduced. Our recent assessment of mitochondrial function in MeCP2 (methyl-CpG-binding protein 2)-deficient mouse (Mecp2^-/y) hippocampus confirmed early metabolic alterations, an increased oxidative burden, and a more vulnerable cellular redox balance. As these changes may contribute to the manifestation of symptoms and disease progression, we now evaluated whether free radical scavengers are capable of improving neuronal and mitochondrial function in RS. Acute hippocampal slices of adult mice were incubated with the vitamin E derivative Trolox for 3–5 h. In Mecp2^-/y slices this treatment dampened neuronal hyperexcitability, improved synaptic short-term plasticity, and fully restored synaptic long-term potentiation (LTP). Furthermore, Trolox specifically attenuated the increased hypoxia susceptibility of Mecp2^-/y slices. Also, the anticonvulsive effects of Trolox were assessed, but the severity of 4-aminopyridine provoked seizure-like discharges was not significantly affected. Adverse side effects of Trolox on mitochondria can be excluded, but clear indications for an improvement of mitochondrial function were not found. Since several ion-channels and neurotransmitter receptors are redox modulated, the mitochondrial alterations and the associated oxidative burden may contribute to the neuronal dysfunction in RS. We confirmed in Mecp2^-/y hippocampus that Trolox dampens neuronal hyperexcitability, reinstates synaptic plasticity, and improves the hypoxia tolerance. Therefore, radical scavengers are promising compounds for the treatment of neuronal dysfunction in RS and deserve further detailed evaluation.

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

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

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Oxidative burden and mitochondrial dysfunction in a mouse model of Rett syndrome

Rett syndrome is an X chromosome-linked neurodevelopmental disorder associated with cognitive impairment, motor dysfunction and breathing irregularities causing intermittent hypoxia. Evidence for impaired mitochondrial function is also accumulating. A subunit of complex III is among the potentially dys-regulated genes, the inner mitochondrial membrane is leaking protons, brain ATP levels seem reduced, and Rett patient blood samples confirm increased oxidative damage. We therefore screened for mitochondrial dysfunction and impaired redox balance. In hippocampal slices of a Rett mouse model (Mecp2(-/y)) we detected an increased FAD/NADH baseline-ratio indicating intensified oxidization. Cyanide-induced anoxia caused similar decreases in FAD/NADH ratio and mitochondrial membrane potential in both genotypes, but Mecp2(-/y) mitochondria seemed less polarized. Quantifying cytosolic redox balance with the genetically-encoded optical probe roGFP1 confirmed more oxidized baseline conditions, a more vulnerable redox-balance, and more intense responses of Mecp2(-/y) hippocampus to oxidative challenge and mitochondrial impairment. Trolox treatment improved the redox baseline of Mecp2(-/y) hippocampus and dampened its exaggerated responses to oxidative challenge. Microarray analysis of the hippocampal CA1 subfield did not detect alterations of key mitochondrial enzymes or scavenging systems. Yet, quantitative PCR confirmed a moderate upregulation of superoxide dismutase 1 in Mecp2(-/y) hippocampus, which might be a compensatory response to the increased oxidative burden. Since several receptors and ion-channels are redox-modulated, the mitochondrial and redox changes which already manifest in neonates could contribute to the hyperexcitability and diminished synaptic plasticity in MeCP2 deficiency. Therefore, targeting cellular redox balance might qualify as a potential pharmacotherapeutic approach to improve neuronal network function in Rett syndrome.

Altered responses of MeCP2-deficient mouse brain stem to severe hypoxia

Rett syndrome (RTT) patients suffer from respiratory arrhythmias with frequent apneas causing intermittent hypoxia. In a RTT mouse model (methyl-CpG-binding protein 2-deficient mice; Mecp2−/ y) we recently discovered an enhanced hippocampal susceptibility to hypoxia and hypoxia-induced spreading depression (HSD). In the present study we investigated whether this also applies to infant Mecp2−/ y brain stem, which could become life-threatening due to failure of cardiorespiratory control. HSD most reliably occurred in the nucleus of the solitary tract (NTS) and the spinal trigeminal nucleus (Sp5). HSD susceptibility of the Mecp2−/ y NTS and Sp5 was increased on 8 mM K+-mediated conditioning. 5-HT1A receptor stimulation with 8-hydroxy-2-(di-propylamino)tetralin (8-OH-DPAT) postponed HSD by up to 40%, mediating genotype-independent protection. The deleterious impact of HSD on in vitro respiration became obvious in rhythmically active slices, where HSD propagation into the pre-Bötzinger complex (pre-BötC) immediately arrested the respiratory rhythm. Compared with wild-type, the Mecp2−/ y pre-BötC was invaded less frequently by HSD, but if so, HSD occurred earlier. On reoxygenation, in vitro rhythms reappeared with increased frequency, which was less pronounced in Mecp2−/ y slices. 8-OH-DPAT increased respiratory frequency but failed to postpone HSD in the pre-BötC. Repetitive hypoxia facilitated posthypoxic recovery only if HSD occurred. In 57% of Mecp2−/ y slices, however, HSD spared the pre-BötC. Although this occasionally promoted residual hypoxic respiratory activity (“gasping”), it also prolonged the posthypoxic recovery, and thus the absence of central inspiratory drive, which in vivo would lengthen respiratory arrest. In view of the breathing disorders in RTTs, the increased hypoxia susceptibility of MeCP2-deficient brain stem potentially contributes to life-threatening disturbances of cardiorespiratory control.

Habituation without NMDA Receptor-Dependent Desensitization of Hering-Breuer Apnea Reflex in a Mecp2 Mutant Mouse Model of Rett Syndrome

Non-associative learning is a basic neuroadaptive behavior exhibited in almost all animal species and sensory modalities but its functions and mechanisms in the mammalian brain are poorly understood. Previous studies have identified two distinct forms of non-associative learning in the classic Hering-Breuer inflation reflex (HBIR) induced apnea in rats: NMDA receptor (NMDAR)-independent habituation in a primary vagal pathway and NMDAR-dependent desensitization in a secondary pontine pathway. Here, we show that abnormal non-associative learning of the HBIR may underlie the endophenotypic tachypnea in an animal model of Rett syndrome (RTT), an autism-spectrum disorder caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MECP2). Mecp2(+/-) symptomatic mice on a mixed-strain background demonstrated significantly increased resting respiratory frequency with shortened expiration and normal inspiratory duration compared with asymptomatic mutants and wild-type controls, a phenotype that is characteristic of girls with RTT. Low-intensity electrical stimulation of the vagus nerve elicited fictive HBIR with time-dependent habituation in both Mecp2(+/-) and wild-type mice. However, time-dependent desensitization of the HBIR was evidenced only in wild-type controls and asymptomatic mutant mice but was absent or suppressed in Mecp2(+/-) symptomatic mice or in wild-type mice after blockade of NMDAR with dizocilpine. Remarkably, ∼50% of the Mecp2(+/-) mice developed these X-linked phenotypes despite somatic mosaicism. Such RTT-like respiratory endophenotypes in mixed-strain Mecp2(+/-) mice differed from those previously reported in Mecp2(-/y) mice on pure C57BL/6J background. These findings provide the first evidence indicating that impaired NMDAR-dependent desensitization of the HBIR may contribute to the endophenotypic tachypnea in RTT.

Autonomic dysfunction with mutations in the gene that encodes methyl-CpG-binding protein 2: insights into Rett syndrome

Rett syndrome (RTT) is an autism spectrum disorder with an incidence of ~1:10,000 females (reviewed in Bird, 2008; Chahrour et al., 2007; Francke, 2006). Affected individuals are apparently normal at birth. Between 6-18 months of age, however, RTT patients begin to exhibit deceleration of head growth, replacement of purposeful hand movements with stereotypic hand wringing, loss of speech, social withdrawal and other autistic features. RTT is caused by loss of function mutations in the gene that encodes methyl-CpG-binding protein 2 (Mecp2) (Amir et al., 1999), a transcriptional repressor that targets genes essential for neuronal survival, dendritic growth, synaptogenesis, and activity dependent plasticity. MECP2 is X-linked, and males die soon after birth. Included in the RTT phenotype are cardiorespiratory disorders involving the autonomic nervous system. The respiratory disorders, including the roles of bioaminergic and brain derived neurotrophic factor (BDNF) signaling in the respiratory pathophysiology of RTT have been recently reviewed (Bissonnette et al., 2007a; Ogier et al., 2008; Katz et al., 2009). Here we will cover the work on RTT regarding respiration that has appeared since 2009 as well as cardiovascular abnormalities.

Correction of respiratory disorders in a mouse model of Rett syndrome

Rett syndrome (RTT) is an autism spectrum disorder caused by mutations in the X-linked gene that encodes the transcription factor methyl-CpG-binding protein 2 (MeCP2). A major debilitating phenotype in affected females is frequent apneas, and heterozygous Mecp2-deficient female mice mimic the human respiratory disorder. GABA defects have been demonstrated in the brainstem of Mecp2-deficient mice. Here, using an intact respiratory network, we show that apnea in RTT mice is characterized by excessive excitatory activity in expiratory cranial and spinal nerves. Augmenting GABA markedly improves the respiratory phenotype. In addition, a serotonin 1a receptor agonist that depresses expiratory neuron activity also reduces apnea, corrects the irregular breathing pattern, and prolongs survival in MeCP2 null males. Combining a GABA reuptake blocker with a serotonin 1a agonist in heterozygous females completely corrects their respiratory defects. The results indicate that GABA and serotonin 1a receptor activity are candidates for treatment of the respiratory disorders in Rett syndrome.

Pontine respiratory activity involved in inspiratory/expiratory phase transition

Control of the timing of the inspiratory/expiratory (IE) phase transition is a hallmark of respiratory pattern formation. In principle, sensory feedback from pulmonary stretch receptors (Breuer-Hering reflex, BHR) is seen as the major controller for the IE phase transition, while pontine-based control of IE phase transition by both the pontine Kölliker-Fuse nucleus (KF) and parabrachial complex is seen as a secondary or backup mechanism. However, previous studies have shown that the BHR can habituate in vivo. Thus, habituation reduces sensory feedback, so the role of the pons, and specifically the KF, for IE phase transition may increase dramatically. Pontine-mediated control of the IE phase transition is not completely understood. In the present review, we discuss existing models for ponto-medullary interaction that may be involved in the control of inspiratory duration and IE transition. We also present intracellular recordings of pontine respiratory units derived from an in situ intra-arterially perfused brainstem preparation of rats. With the absence of lung inflation, this preparation generates a normal respiratory pattern and many of the recorded pontine units demonstrated phasic respiratory-related activity. The analysis of changes in membrane potentials of pontine respiratory neurons has allowed us to propose a number of pontine-medullary interactions not considered before. The involvement of these putative interactions in pontine-mediated control of IE phase transitions is discussed.

Breathing disorders in Rett syndrome: progressive neurochemical dysfunction in the respiratory network after birth

Disorders of respiratory control are a prominent feature of Rett syndrome (RTT), a severely debilitating condition caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2). RTT patients present with a complex respiratory phenotype that can include periods of hyperventilation, apnea, breath holds terminated by Valsalva maneuvers, forced and deep breathing and apneustic breathing, as well as abnormalities of heart rate control and cardiorespiratory integration. Recent studies of mouse models of RTT have begun to shed light on neurologic deficits that likely contribute to respiratory dysfunction including, in particular, defects in neurochemical signaling resulting from abnormal patterns of neurotransmitter and neuromodulator expression. The authors hypothesize that breathing dysregulation in RTT results from disturbances in mechanisms that modulate the respiratory rhythm, acting either alone or in combination with more subtle disturbances in rhythm and pattern generation. This article reviews the evidence underlying this hypothesis as well as recent efforts to translate our emerging understanding of neurochemical defects in mouse models of RTT into preclinical trials of potential treatments for respiratory dysfunction in this disease.

Postnatal emergence of synaptic plasticity associated with dynamic adaptation of the respiratory motor pattern

The shape of the three-phase respiratory motor pattern (inspiration, postinspiration, late expiration) is controlled by a central pattern generator (CPG) located in the ponto-medullary brainstem. Synaptic interactions between and within specific sub-compartments of the CPG are subject of intensive research. This review addresses the neural control of postinspiratory activity as the essential determinant of inspiratory/expiratory phase duration. The generation of the postinspiratory phase depends on synaptic interaction between neurones of the nucleus tractus solitarii (NTS), which relay afferent inputs from pulmonary stretch receptors, and the pontine Kölliker-Fuse nucleus (KF) as integral parts of the CPG. Both regions undergo significant changes during the first three postnatal weeks in rodents. Developmental changes in glutamatergic synaptic functions and its modulation by brain-derived neurotrophic factor may have implications in synaptic plasticity within the NTS/KF axis. We propose that dependent on these developmental changes, the CPG becomes permissive for short- and long-term plasticity associated with environmental, metabolic and behavioural adaptation of the breathing pattern.

Enhanced hypoxia susceptibility in hippocampal slices from a mouse model of rett syndrome

Rett syndrome is a neurodevelopmental disorder caused by mutations in the X-chromosomal MECP2 gene encoding for the transcriptional regulator methyl CpG binding protein 2 (MeCP2). Rett patients suffer from episodic respiratory irregularities and reduced arterial oxygen levels. To elucidate whether such intermittent hypoxic episodes induce adaptation/preconditioning of the hypoxia-vulnerable hippocampal network, we analyzed its responses to severe hypoxia in adult Rett mice. The occurrence of hypoxia-induced spreading depression (HSD)--an experimental model for ischemic stroke--was hastened in Mecp2-/y males. The extracellular K+ rise during HSD was attenuated in Mecp2-/y males and the input resistance of CA1 pyramidal neurons decreased less before HSD onset. CA1 pyramidal neurons were smaller and more densely packed, but the cell swelling during HSD was unaffected. The intrinsic optical signal and the propagation of HSD were similar among the different genotypes. Basal synaptic function was intact, but Mecp2-/y males showed reduced paired-pulse facilitation and higher field potential/fiber volley ratios, but no increased seizure susceptibility. Synaptic failure during hypoxia was complete in all genotypes and the final degree of posthypoxic synaptic recovery indistinguishable. Cellular ATP content was normal in Mecp2-/y males, but their hematocrit was increased as was HIF-1alpha expression throughout the brain. This is the first study showing that in Rett syndrome, the susceptibility of telencephalic neuronal networks to hypoxia is increased; the underlying molecular mechanisms apparently involve disturbed K+ channel function. Such an increase in hypoxia susceptibility may potentially contribute to the vulnerability of male Rett patients who are either not viable or severely disabled.