Tyrosine hydroxylase deficit in the chemoafferent and the sympathoadrenergic pathways of the Mecp2 deficient mouse

Loss of ganglioglomerular nerve input to the carotid body impacts the hypoxic ventilatory response in freely-moving rats

The carotid bodies are the primary sensors of blood pH, pO2 and pCO2. The ganglioglomerular nerve (GGN) provides post-ganglionic sympathetic nerve input to the carotid bodies, however the physiological relevance of this innervation is still unclear. The main objective of this study was to determine how the absence of the GGN influences the hypoxic ventilatory response in juvenile rats. As such, we determined the ventilatory responses that occur during and following five successive episodes of hypoxic gas challenge (HXC, 10% O2, 90% N2), each separated by 15 min of room-air, in juvenile (P25) sham-operated (SHAM) male Sprague Dawley rats and in those with bilateral transection of the ganglioglomerular nerves (GGNX). The key findings were that 1) resting ventilatory parameters were similar in SHAM and GGNX rats, 2) the initial changes in frequency of breathing, tidal volume, minute ventilation, inspiratory time, peak inspiratory and expiratory flows, and inspiratory and expiratory drives were markedly different in GGNX rats, 3) the initial changes in expiratory time, relaxation time, end inspiratory or expiratory pauses, apneic pause and non-eupneic breathing index (NEBI) were similar in SHAM and GGNX rats, 4) the plateau phases obtained during each HXC were similar in SHAM and GGNX rats, and 5) the ventilatory responses that occurred upon return to room-air were similar in SHAM and GGNX rats. Overall, these changes in ventilation during and following HXC in GGNX rats raises the possibility the loss of GGN input to the carotid bodies effects how primary glomus cells respond to hypoxia and the return to room-air.

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.

Rett syndrome: think outside the (skull) box

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder characterized by neurodevelopmental regression between 6 and 18 months of life and associated with multi-system comorbidities. Caused mainly by pathogenic variants in the MECP2 (methyl CpG binding protein 2) gene, it is the second leading genetic cause of intellectual disability in girls after Down syndrome. RTT affects not only neurological function but also a wide array of non-neurological organs. RTT-related disorders involve abnormalities of the respiratory, cardiovascular, digestive, metabolic, skeletal, endocrine, muscular, and urinary systems and immune response. Here, we review the different aspects of RTT affecting the main peripheral groups of organs and sometimes occurring independently of nervous system defects.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Restoration of motor learning in a mouse model of Rett syndrome following long-term treatment with a novel small-molecule activator of TrkB

Reduced expression of brain-derived neurotrophic factor (BDNF) and impaired activation of the BDNF receptor, tropomyosin receptor kinase B (TrkB; also known as Ntrk2), are thought to contribute significantly to the pathophysiology of Rett syndrome (RTT), a severe neurodevelopmental disorder caused by loss-of-function mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Previous studies from this and other laboratories have shown that enhancing BDNF expression and/or TrkB activation in Mecp2-deficient mouse models of RTT can ameliorate or reverse abnormal neurological phenotypes that mimic human RTT symptoms. The present study reports on the preclinical efficacy of a novel, small-molecule, non-peptide TrkB partial agonist, PTX-BD4-3, in heterozygous female Mecp2 mutant mice, a well-established RTT model that recapitulates the genetic mosaicism of the human disease. PTX-BD4-3 exhibited specificity for TrkB in cell-based assays of neurotrophin receptor activation and neuronal cell survival and in in vitro receptor binding assays. PTX-BD4-3 also activated TrkB following systemic administration to wild-type and Mecp2 mutant mice and was rapidly cleared from the brain and plasma with a half-life of ∼2 h. Chronic intermittent treatment of Mecp2 mutants with a low dose of PTX-BD4-3 (5 mg/kg, intraperitoneally, once every 3 days for 8 weeks) reversed deficits in two core RTT symptom domains – respiration and motor control – and symptom rescue was maintained for at least 24 h after the last dose. Together, these data indicate that significant clinically relevant benefit can be achieved in a mouse model of RTT with a chronic intermittent, low-dose treatment paradigm targeting the neurotrophin receptor TrkB.

Editor's choice: Long-term intermittent treatment with a newly developed partial agonist of the TrkB neurotrophin receptor reverses deficits in motor learning and respiration in a mouse model of Rett syndrome.

Loss of MeCP2 Function Across Several Neuronal Populations Impairs Breathing Response to Acute Hypoxia

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by loss of function of the transcriptional regulator Methyl-CpG-Binding Protein 2 (MeCP2). In addition to the characteristic loss of hand function and spoken language after the first year of life, people with RTT also have a variety of physiological and autonomic abnormalities including disrupted breathing rhythms characterized by bouts of hyperventilation and an increased frequency of apnea. These breathing abnormalities, that likely involve alterations in both the circuitry underlying respiratory pace making and those underlying breathing response to environmental stimuli, may underlie the sudden unexpected death seen in a significant fraction of people with RTT. In fact, mice lacking MeCP2 function exhibit abnormal breathing rate response to acute hypoxia and maintain a persistently elevated breathing rate rather than showing typical hypoxic ventilatory decline that can be observed among their wild-type littermates. Using genetic and pharmacological tools to better understand the course of this abnormal hypoxic breathing rate response and the neurons driving it, we learned that the abnormal hypoxic breathing response is acquired as the animals mature, and that MeCP2 function is required within excitatory, inhibitory, and modulatory populations for a normal hypoxic breathing rate response. Furthermore, mice lacking MeCP2 exhibit decreased hypoxia-induced neuronal activity within the nucleus tractus solitarius of the dorsal medulla. Overall, these data provide insight into the neurons driving the circuit dysfunction that leads to breathing abnormalities upon loss of MeCP2. The discovery that combined dysfunction across multiple neuronal populations contributes to breathing dysfunction may provide insight into sudden unexpected death in RTT.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

High-fat diet accelerates extreme obesity with hyperphagia in female heterozygous Mecp2-null mice

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused by mutation of the methyl-CpG-binding protein 2 (MECP2) gene. Although RTT has been associated with obesity, the underlying mechanism has not yet been elucidated. In this study, female heterozygous Mecp2-null mice (Mecp2^+/- mice), a model of RTT, were fed a normal chow diet or high-fat diet (HFD), and the changes in molecular signaling pathways were investigated. Specifically, we examined the expression of genes related to the hypothalamus and dopamine reward circuitry, which represent a central network of feeding behavior control. In particular, dopamine reward circuitry has been shown to regulate hedonic feeding behavior, and its disruption is associated with HFD-related changes in palatability. The Mecp2^+/- mice that were fed the normal chow showed normal body weight and food consumption, whereas those fed the HFD showed extreme obesity with hyperphagia, an increase of body fat mass, glucose intolerance, and insulin resistance compared with wild-type mice fed the HFD (WT-HFD mice). The main cause of obesity in Mecp2^+/--HFD mice was a remarkable increase in calorie intake, with no difference in oxygen consumption or locomotor activity. Agouti-related peptide mRNA and protein levels were increased, whereas proopiomelanocortin mRNA and protein levels were reduced in Mecp2^+/--HFD mice with hyperleptinemia, which play an essential role in appetite and satiety in the hypothalamus. The conditioned place preference test revealed that Mecp2^+/- mice preferred the HFD. Tyrosine hydroxylase and dopamine transporter mRNA levels in the ventral tegmental area, and dopamine receptor and dopamine- and cAMP-regulated phosphoprotein mRNA levels in the nucleus accumbens were significantly lower in Mecp2^+/--HFD mice than those of WT-HFD mice. Thus, HFD feeding induced dysregulation of food intake in the hypothalamus and dopamine reward circuitry, and accelerated the development of extreme obesity associated with addiction-like eating behavior in Mecp2^+/- mice.

D-cycloserine improves synaptic transmission in an animal model of Rett syndrome

Rett syndrome (RTT), a leading cause of intellectual disability in girls, is predominantly caused by mutations in the X-linked gene MECP2. Disruption of Mecp2 in mice recapitulates major features of RTT, including neurobehavioral abnormalities, which can be reversed by re-expression of normal Mecp2. Thus, there is reason to believe that RTT could be amenable to therapeutic intervention throughout the lifespan of patients after the onset of symptoms. A common feature underlying neuropsychiatric disorders, including RTT, is altered synaptic function in the brain. Here, we show that Mecp2^tm1.1Jae/y mice display lower presynaptic function as assessed by paired pulse ratio, as well as decreased long term potentiation (LTP) at hippocampal Schaffer–collateral-CA1 synapses. Treatment of Mecp2^tm1.1Jae/y mice with D-cycloserine (DCS), an FDA-approved analog of the amino acid D-alanine with antibiotic and glycinergic activity, corrected the presynaptic but not LTP deficit without affecting deficient hippocampal BDNF levels. DCS treatment did, however, partially restore lower BDNF levels in the brain stem and striatum. Thus, treatment with DCS may mitigate the severity of some of the neurobehavioral symptoms experienced by patients with Rett syndrome.

A codon-optimized Mecp2 transgene corrects breathing deficits and improves survival in a mouse model of Rett syndrome

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder that is primarily caused by mutations in the methyl CpG binding protein 2 gene (MECP2). RTT is the second most prevalent cause of intellectual disability in girls and there is currently no cure for the disease. The finding that the deficits caused by the loss of Mecp2 are reversible in the mouse has bolstered interest in gene therapy as a cure for RTT. In order to assess the feasibility of gene therapy in a RTT mouse model, and in keeping with translational goals, we investigated the efficacy of a self-complementary AAV9 vector expressing a codon-optimized version of Mecp2 (AAV9-MCO) delivered via a systemic approach in early symptomatic Mecp2-deficient (KO) mice. Our results show that AAV9-MCO administered at a dose of 2×10¹¹ viral genome (vg)/mouse was able to significantly increase survival and weight gain, and delay the occurrence of behavioral deficits. Apneas, which are one of the core RTT breathing deficits, were significantly decreased to WT levels in Mecp2 KO mice after AAV9-MCO administration. Semi-quantitative analysis showed that AAV9-MCO administration in Mecp2 KO mice resulted in 10 to 20% Mecp2 immunopositive cells compared to WT animals, with the highest Mecp2 expression found in midbrain regions known to regulate cardio-respiratory functions. In addition, we also found a cell autonomous increase in tyrosine hydroxylase levels in the A1C1 and A2C2 catecholaminergic Mecp2+ neurons in treated Mecp2 KO mice, which may partly explain the beneficial effect of AAV9-MCO administration on apneas occurrence.

An optogenetic mouse model of rett syndrome targeting on catecholaminergic neurons

Rett syndrome (RTT) is a neurodevelopmental disorder affecting multiple functions, including the norepinephrine (NE) system. In the CNS, NE is produced mostly by neurons in the locus coeruleus (LC), where defects in intrinsic neuronal properties, NE biosynthetic enzymes, neuronal CO2 sensitivity, and synaptic currents have been reported in mouse models of RTT. LC neurons in methyl-CpG-binding protein 2 gene (Mecp2) null mice show a high rate of spontaneous firing, although whether such hyperexcitability might increase or decrease the NE release from synapses is unknown. To activate the NEergic axonal terminals selectively, we generated an optogenetic mouse model of RTT in which NEergic neuronal excitability can be manipulated with light. Using commercially available mouse breeders, we produced a new strain of double-transgenic mice with Mecp2 knockout and channelrhodopsin (ChR) knockin in catecholaminergic neurons. Several RTT-like phenotypes were found in the tyrosine hydroxylase (TH)-ChR-Mecp2(-/Y) mice, including hypoactivity, low body weight, hindlimb clasping, and breathing disorders. In brain slices, optostimulation produced depolarization and an increase in the firing rate of LC neurons from TH-ChR control mice. In TH-ChR control mice, optostimulation of presynaptic NEergic neurons augmented the firing rate of hypoglossal neurons (HNs), which was blocked by the α-adrenoceptor antagonist phentolamine. Such optostimulation of NEergic terminals had almost no effect on HNs from two or three TH-ChR-Mecp2(-/Y) mice, indicating that excessive excitation of presynaptic neurons does not benefit NEergic modulation in mice with Mecp2 disruption. These results also demonstrate the feasibility of generating double-transgenic mice for studies of RTT with commercially available mice, which are inexpensive, labor/time efficient, and promising for cell-specific stimulation. © 2016 Wiley Periodicals, Inc.

Rett Syndrome: Reaching for Clinical Trials

Rett syndrome (RTT) is a syndromic autism spectrum disorder caused by loss-of-function mutations in MECP2. The methyl CpG binding protein 2 binds methylcytosine and 5-hydroxymethycytosine at CpG sites in promoter regions of target genes, controlling their transcription by recruiting co-repressors and co-activators. Several preclinical studies in mouse models have identified rational molecular targets for drug therapies aimed at correcting the underlying neural dysfunction. These targeted therapies are increasingly translating into human clinical trials. In this review, we present an overview of RTT and describe the current state of preclinical studies in methyl CpG binding protein 2-based mouse models, as well as current clinical trials in individuals with RTT.

A BDNF loop-domain mimetic acutely reverses spontaneous apneas and respiratory abnormalities during behavioral arousal in a mouse model of Rett syndrome

Reduced levels of brain-derived neurotrophic factor (BDNF) are thought to contribute to the pathophysiology of Rett syndrome (RTT), a severe neurodevelopmental disorder caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). In Mecp2 mutant mice, BDNF deficits have been associated with breathing abnormalities, a core feature of RTT, as well as with synaptic hyperexcitability within the brainstem respiratory network. Application of BDNF can reverse hyperexcitability in acute brainstem slices from Mecp2-null mice, suggesting that therapies targeting BDNF or its receptor, TrkB, could be effective at acute reversal of respiratory abnormalities in RTT. Therefore, we examined the ability of LM22A-4, a small-molecule BDNF loop-domain mimetic and TrkB partial agonist, to modulate synaptic excitability within respiratory cell groups in the brainstem nucleus tractus solitarius (nTS) and to acutely reverse abnormalities in breathing at rest and during behavioral arousal in Mecp2 mutants. Patch-clamp recordings in Mecp2-null brainstem slices demonstrated that LM22A-4 decreases excitability at primary afferent synapses in the nTS by reducing the amplitude of evoked excitatory postsynaptic currents and the frequency of spontaneous and miniature excitatory postsynaptic currents. In vivo, acute treatment of Mecp2-null and -heterozygous mutants with LM22A-4 completely eliminated spontaneous apneas in resting animals, without sedation. Moreover, we demonstrate that respiratory dysregulation during behavioral arousal, a feature of human RTT, is also reversed in Mecp2 mutants by acute treatment with LM22A-4. Together, these data support the hypothesis that reduced BDNF signaling and respiratory dysfunction in RTT are linked, and establish the proof-of-concept that treatment with a small-molecule structural mimetic of a BDNF loop domain and a TrkB partial agonist can acutely reverse abnormal breathing at rest and in response to behavioral arousal in symptomatic RTT mice.

Targeted pharmacological treatment of autism spectrum disorders: fragile X and Rett syndromes

Autism spectrum disorders (ASDs) are genetically and clinically heterogeneous and lack effective medications to treat their core symptoms. Studies of syndromic ASDs caused by single gene mutations have provided insights into the pathophysiology of autism. Fragile X and Rett syndromes belong to the syndromic ASDs in which preclinical studies have identified rational targets for drug therapies focused on correcting underlying neural dysfunction. These preclinical discoveries are increasingly translating into exciting human clinical trials. Since there are significant molecular and neurobiological overlaps among ASDs, targeted treatments developed for fragile X and Rett syndromes may be helpful for autism of different etiologies. Here, we review the targeted pharmacological treatment of fragile X and Rett syndromes and discuss related issues in both preclinical studies and clinical trials of potential therapies for the diseases.

Is X-linked methyl-CpG binding protein 2 a new target for the treatment of Parkinson's disease

X-linked methyl-CpG binding protein 2 mutations can induce symptoms similar to those of Parkinson's disease and dopamine metabolism disorders, but the specific role of X-linked methyl-CpG binding protein 2 in the pathogenesis of Parkinson's disease remains unknown. In the present study, we used 6-hydroxydopamine-induced human neuroblastoma cell (SH-SY5Y cells) injury as a cell model of Parkinson's disease. The 6-hydroxydopamine (50 μmol/L) treatment decreased protein levels for both X-linked methyl-CpG binding protein 2 and tyrosine hydroxylase in these cells, and led to cell death. However, overexpression of X-linked methyl-CpG binding protein 2 was able to ameliorate the effects of 6-hydroxydopamine, it reduced 6-hydroxydopamine-induced apoptosis, and increased the levels of tyrosine hydroxylase in SH-SY5Y cells. These findings suggesting that X-linked methyl-CpG binding protein 2 may be a potential therapeutic target for the treatment of Parkinson's disease.

Effect of Sarizotan, a 5-HT1a and D2-like receptor agonist, on respiration in three mouse models of Rett syndrome

Disturbances in respiration are common and debilitating features of Rett syndrome (RTT). A previous study showed that the 5-HT1a receptor agonist (R)-(+)-8-hydroxy-dipropyl-2-aminotetralin hydrobromide (8-OH-DPAT) significantly reduced the incidence of apnea and the irregular breathing pattern in a mouse model of the disorder. 8-OH-DPAT, however, is not available for clinical practice. Sarizotan, a full 5-HT1a agonist and a dopamine D2-like agonist/partial agonist, has been used in clinical trials for the treatment of l-dopa-induced dyskinesia. The purpose of this study was to evaluate the effects of sarizotan on respiration and locomotion in mouse models of RTT. Studies were performed in Bird and Jaenisch strains of methyl-CpG-binding protein 2--deficient heterozygous female and Jaenisch strain Mecp2 null male mice and in knock-in heterozygous female mice of a common nonsense mutation (R168X). Respiratory pattern was determined with body plethysmography, and locomotion was determined with open-field recording. Sarizotan or vehicle was administered 20 minutes before a 30-minute recording of respiratory pattern or motor behavior. In separate studies, a crossover design was used to administer the drug for 7 and for 14 days. Sarizotan reduced the incidence of apnea in all three RTT mouse models to approximately 15% of their pretreatment levels. The irregular breathing pattern was corrected to that of wild-type littermates. When administered for 7 or 14 days, apnea decreased to 25 to 33% of the incidence seen with vehicle. This study indicates that the clinically approved drug sarizotan is an effective treatment for respiratory disorders in mouse models of RTT.

GABA and glutamate pathways are spatially and developmentally affected in the brain of Mecp2-deficient mice

Proper brain functioning requires a fine-tuning between excitatory and inhibitory neurotransmission, a balance maintained through the regulation and release of glutamate and GABA. Rett syndrome (RTT) is a rare genetic disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene affecting the postnatal brain development. Dysfunctions in the GABAergic and glutamatergic systems have been implicated in the neuropathology of RTT and a disruption of the balance between excitation and inhibition, together with a perturbation of the electrophysiological properties of GABA and glutamate neurons, were reported in the brain of the Mecp2-deficient mouse. However, to date, the extent and the nature of the GABA/glutamate deficit affecting the Mecp2-deficient mouse brain are unclear. In order to better characterize these deficits, we simultaneously analyzed the GABA and glutamate levels in Mecp2-deficient mice at 2 different ages (P35 and P55) and in several brain areas. We used a multilevel approach including the quantification of GABA and glutamate levels, as well as the quantification of the mRNA and protein expression levels of key genes involved in the GABAergic and glutamatergic pathways. Our results show that Mecp2-deficient mice displayed regional- and age-dependent variations in the GABA pathway and, to a lesser extent, in the glutamate pathway. The implication of the GABA pathway in the RTT neuropathology was further confirmed using an in vivo treatment with a GABA reuptake inhibitor that significantly improved the lifespan of Mecp2-deficient mice. Our results confirm that RTT mouse present a deficit in the GABAergic pathway and suggest that GABAergic modulators could be interesting therapeutic agents for this severe neurological disorder.

Brain-derived neurotrophic factor and Rett syndrome

Rett syndrome (RTT) is a devastating neurodevelopmental disorder with autistic features caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2), a transcriptional regulatory protein. RTT has attracted widespread attention not only because of the urgent need for treatments, but also because it has become a window into basic mechanisms underlying epigenetic regulation of neuronal genes, including BDNF. In addition, work in mouse models of the disease has demonstrated the possibility of symptom reversal upon restoration of normal gene function. This latter finding has resulted in a paradigm shift in RTT research and, indeed, in the field of neurodevelopmental disorders as a whole, and spurred the search for potential therapies for RTT and related syndromes. In this context, the discovery that expression of BDNF is dysregulated in RTT and mouse models of the disease has taken on particular importance. This chapter reviews the still evolving story of how MeCP2 might regulate expression of BDNF, the functional consequences of BDNF deficits in Mecp2 mutant mice, and progress in developing BDNF-targeted therapies for the treatment of RTT.

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.

Neurotrophic factors in development and regulation of respiratory control

Neurotrophic factors (NTFs) are a heterogeneous group of extracellular signaling molecules that play critical roles in the development, maintenance, modulation and plasticity of the central and peripheral nervous systems. A subset of these factors, including members of three multigene families-the neurotrophins, neuropoetic cytokines and the glial cell line-derived neurotrophic factor ligands-are particularly important for development and regulation of neurons involved in respiratory control. Here, we review the functional biology of these NTFs and their receptors, as well as their roles in regulating survival, maturation, synaptic strength and plasticity in respiratory control pathways. In addition, we highlight recent progress in identifying the role of abnormal NTF signaling in the molecular pathogenesis of respiratory dysfunction in Rett syndrome and in the development of potential new NTF-targeted therapeutic strategies.

Isoform-specific anti-MeCP2 antibodies confirm that expression of the e1 isoform strongly predominates in the brain

Rett syndrome is a neurological disorder caused by mutations in the MECP2 gene.  MeCP2 transcripts are alternatively spliced to generate two protein isoforms (MeCP2_e1 and MeCP2_e2) that differ at their N-termini. Whilst mRNAs for both forms are expressed ubiquitously, the one for MeCP2_e1 is more abundant than for MeCP2_e2 in the central nervous system. In transfected cells, both protein isoforms are nuclear and colocalize with densely methylated heterochromatic foci. With a view to understanding the physiological contribution of each isoform, and their respective roles in the pathogenesis of Rett syndrome, we set out to generate isoform-specific anti-MeCP2 antibodies. To this end, we immunized rabbits against the peptides corresponding to the short amino-terminal portions that are different between the two isoforms. The polyclonal antibodies thus obtained specifically detected their respective isoforms of MeCP2 in Neuro2a (N2A) cells transfected to express either form. Both antisera showed comparable sensitivities when used for Western blot or immunofluorescence, and were highly specific for their respective isoform. When those antibodies were used on mouse tissues, specific signals were easily detected for Mecp2_e1, whilst Mecp2_e2 was very difficult to detect by Western blot, and even more so by immunofluorescence. Our results thus suggest that brain cells express low amounts of the Mecp2-e2 isoform. Our findings are compatible with recent reports showing that MeCP2_e2 is dispensable for healthy brain function, and that it may be involved in the regulation of neuronal apoptosis and embryonic development.

Preclinical research in Rett syndrome: setting the foundation for translational success

In September of 2011, the National Institute of Neurological Disorders and Stroke (NINDS), the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), the International Rett Syndrome Foundation (IRSF) and the Rett Syndrome Research Trust (RSRT) convened a workshop involving a broad cross-section of basic scientists, clinicians and representatives from the National Institutes of Health (NIH), the US Food and Drug Administration (FDA), the pharmaceutical industry and private foundations to assess the state of the art in animal studies of Rett syndrome (RTT). The aim of the workshop was to identify crucial knowledge gaps and to suggest scientific priorities and best practices for the use of animal models in preclinical evaluation of potential new RTT therapeutics. This review summarizes outcomes from the workshop and extensive follow-up discussions among participants, and includes: (1) a comprehensive summary of the physiological and behavioral phenotypes of RTT mouse models to date, and areas in which further phenotypic analyses are required to enhance the utility of these models for translational studies; (2) discussion of the impact of genetic differences among mouse models, and methodological differences among laboratories, on the expression and analysis, respectively, of phenotypic traits; and (3) definitions of the standards that the community of RTT researchers can implement for rigorous preclinical study design and transparent reporting to ensure that decisions to initiate costly clinical trials are grounded in reliable preclinical data.

Brain activity mapping in Mecp2 mutant mice reveals functional deficits in forebrain circuits, including key nodes in the default mode network, that are reversed with ketamine treatment

Excitatory-inhibitory imbalance has been identified within specific brain microcircuits in models of Rett syndrome (RTT) and other autism spectrum disorders (ASDs). However, macrocircuit dysfunction across the RTT brain as a whole has not been defined. To approach this issue, we mapped expression of the activity-dependent, immediate-early gene product Fos in the brains of wild-type (Wt) and methyl-CpG-binding protein 2 (Mecp2)-null (Null) mice, a model of RTT, before and after the appearance of overt symptoms (3 and 6 weeks of age, respectively). At 6 weeks, Null mice exhibit significantly less Fos labeling than Wt in limbic cortices and subcortical structures, including key nodes in the default mode network. In contrast, Null mice exhibit significantly more Fos labeling than Wt in the hindbrain, most notably in cardiorespiratory regions of the nucleus tractus solitarius (nTS). Using nTS as a model, whole-cell recordings demonstrated that increased Fos expression in Nulls at 6 weeks of age is associated with synaptic hyperexcitability, including increased frequency of spontaneous and miniature EPSCs and increased amplitude of evoked EPSCs in Nulls. No such effect of genotype on Fos or synaptic function was seen at 3 weeks. In the mutant forebrain, reduced Fos expression, as well as abnormal sensorimotor function, were reversed by the NMDA receptor antagonist ketamine. In light of recent findings that the default mode network is hypoactive in autism, our data raise the possibility that hypofunction within this meta-circuit is a shared feature of RTT and other ASDs and is reversible.

Synapse dysfunction in autism: a molecular medicine approach to drug discovery in neurodevelopmental disorders

Autism and autism spectrum disorders (ASDs) affect millions of individuals worldwide. Despite increased autism diagnoses over the past 30 years, therapeutic intervention is often 'trial and error'. This approach has identified some beneficial agents, but complex heterogeneous disorders require a more personalized treatment regimen. Many ASD risk factors are genetic, implicating impaired synaptic development and function. Monogenetic disorders (e.g., fragile X syndrome, Rett syndrome, and neurofibromatosis) that have phenotypic overlap with autism provide insights into ASD pathology through the identification novel drug targets (e.g., glutamatergic receptors). Encouragingly, some of these novel drug targets provide symptomatic improvement, even in patients who have lived with ASDs for protracted periods of time. Consequently, a targeted drug discovery approach is expected to deliver improved agents for the treatment and management of ASDs. Here, we review the opportunities and challenges in drug development for autism and provide insight into the neurobiology of ASDs.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.

Synaptic microcircuit dysfunction in genetic models of neurodevelopmental disorders: focus on Mecp2 and Met

Recent findings in the genetics of neurodevelopmental syndromes have ushered in an exciting era of discovery in which substrates of neurologic dysfunction are being identified at the synaptic and microcircuit levels in mouse models of these disorders. We review recent progress in this area, focusing on two examples of mouse models of autism spectrum disorders (ASDs): Mecp2 models of Rett syndrome, and a Met-knockout model of non-syndromic forms of autism. In both cases, a dominant theme is changes in synaptic strength, associated with hyper-connectivity or hypo-connectivity in specific microcircuits. Alterations in intrinsic neuronal excitability are also found, but do not appear to be as common. The microcircuit-specific nature of synaptic changes observed in these ASD models indicates that it will be necessary to define mechanisms of circuit dysfunction on a case-by-case basis, not only in neocortex but also in brainstem and other sub-cortical areas. Thus, functional microcircuit analysis is emerging as an important line of investigation, highly complementary to neurogenetic and molecular strategies, and holds promise for generating models of the underlying pathophysiology and for guiding development of novel therapeutic strategies.

Generation and characterization of rat and mouse monoclonal antibodies specific for MeCP2 and their use in X-inactivation studies

Methyl CpG binding protein 2 (MeCP2) binds DNA, and has a preference for methylated CpGs and, hence, in cells, it accumulates in heterochromatin. Even though it is expressed ubiquitously MeCP2 is particularly important during neuronal maturation. This is underscored by the fact that in Rett syndrome, a neurological disease, 80% of patients carry a mutation in the MECP2 gene. Since the MECP2 gene lies on the X chromosome and is subjected to X chromosome inactivation, affected patients are usually chimeric for wild type and mutant MeCP2. Here, we present the generation and characterization of the first rat monoclonal MeCP2 specific antibodies as well as mouse monoclonal antibodies and a rabbit polyclonal antibody. We demonstrate that our antibodies are suitable for immunoblotting, (chromatin) immunoprecipitation and immunofluorescence of endogenous and ectopically expressed MeCP2. Epitope mapping revealed that most of the MeCP2 monoclonal antibodies recognize the C-terminal domain and one the N-terminal domain of MeCP2. Using slot blot analysis, we determined a high sensitivity of all antibodies, detecting amounts as low as 1 ng of MeCP2 protein. Moreover, the antibodies recognize MeCP2 from different species, including human, mouse, rat and pig. Lastly, we have validated their use by analyzing and quantifying X chromosome inactivation skewing using brain tissue of MeCP2 heterozygous null female mice. The new MeCP2 specific monoclonal antibodies described here perform well in a large variety of immunological applications making them a very valuable set of tools for studies of MeCP2 pathophysiology in situ and in vitro.

MeCP2 and Rett syndrome: reversibility and potential avenues for therapy

Mutations in the X-linked gene MECP2 (methyl CpG-binding protein 2) are the primary cause of the neurodevelopmental disorder RTT (Rett syndrome), and are also implicated in other neurological conditions. The expression product of this gene, MeCP2, is a widely expressed nuclear protein, especially abundant in mature neurons of the CNS (central nervous system). The major recognized consequences of MECP2 mutation occur in the CNS, but there is growing awareness of peripheral effects contributing to the full RTT phenotype. MeCP2 is classically considered to act as a DNA methylation-dependent transcriptional repressor, but may have additional roles in regulating gene expression and chromatin structure. Knocking out Mecp2 function in mice recapitulates many of the overt neurological features seen in RTT patients, and the characteristic postnatally delayed onset of symptoms is accompanied by aberrant neuronal morphology and deficits in synaptic physiology. Evidence that reactivation of endogenous Mecp2 in mutant mice, even at adult stages, can reverse aspects of RTT-like pathology and result in apparently functionally mature neurons has provided renewed hope for patients, but has also provoked discussion about traditional boundaries between neurodevelopmental disorders and those involving dysfunction at later stages. In the present paper we review the neurobiology of MeCP2 and consider the various genetic (including gene therapy), pharmacological and environmental interventions that have been, and could be, developed to attempt phenotypic rescue in RTT. Such approaches are already providing valuable insights into the potential tractability of RTT and related conditions, and are useful pointers for the development of future therapeutic strategies.

MeCP2 is critical within HoxB1-derived tissues of mice for normal lifespan

Rett syndrome is a neurodevelopmental disorder caused by mutations in methyl-CpG-binding protein 2 (MECP2), a transcriptional regulator. In addition to cognitive, communication, and motor problems, affected individuals have abnormalities in autonomic function and respiratory control that may contribute to premature lethality. Mice lacking Mecp2 die early and recapitulate the autonomic and respiratory phenotypes seen in humans. The association of autonomic and respiratory deficits with premature death suggests that Mecp2 is critical within autonomic and respiratory control centers for survival. To test this, we compared the autonomic and respiratory phenotypes of mice with a null allele of Mecp2 to mice with Mecp2 removed from their brainstem and spinal cord. We found that MeCP2 is necessary within the brainstem and spinal cord for normal lifespan, normal control of heart rate, and respiratory response to hypoxia. Restoration of MeCP2 in a subset of the cells in this same region is sufficient to rescue abnormal heart rate and abnormal respiratory response to hypoxia. Furthermore, restoring MeCP2 function in neural centers critical for autonomic and respiratory function alleviates the lethality associated with loss of MeCP2 function, supporting the notion of targeted therapy toward treating 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.

Chapter 3--networks within networks: the neuronal control of breathing

Breathing emerges through complex network interactions involving neurons distributed throughout the nervous system. The respiratory rhythm generating network is composed of micro networks functioning within larger networks to generate distinct rhythms and patterns that characterize breathing. The pre-Bötzinger complex, a rhythm generating network located within the ventrolateral medulla assumes a core function without which respiratory rhythm generation and breathing cease altogether. It contains subnetworks with distinct synaptic and intrinsic membrane properties that give rise to different types of respiratory rhythmic activities including eupneic, sigh, and gasping activities. While critical aspects of these rhythmic activities are preserved when isolated in in vitro preparations, the pre-Bötzinger complex functions in the behaving animal as part of a larger network that receives important inputs from areas such as the pons and parafacial nucleus. The respiratory network is also an integrator of modulatory and sensory inputs that imbue the network with the important ability to adapt to changes in the behavioral, metabolic, and developmental conditions of the organism. This review summarizes our current understanding of these interactions and relates the emerging concepts to insights gained in other rhythm generating networks.

Monoamine deficits in the brain of methyl-CpG binding protein 2 null mice suggest the involvement of the cerebral cortex in early stages of Rett syndrome

Rett syndrome is a neurodevelopmental disorder caused by mutations in the methyl-CpG binding protein 2 gene (MECP2). Several neural systems are affected in Rett, resulting in an autonomic dysfunction, a movement disorder with characteristic loss of locomotor abilities and profound cognitive impairments. A deregulation of monoamines has been detected in the brain and cerebrospinal fluid of both Rett patients and a Rett syndrome murine model, the Mecp2 knock-out mouse. Our goal was to characterize the onset and progression of motor dysfunction in Mecp2(tm1.1Bird) knock-out mice and the possible neurochemical alterations in different brain regions potentially playing a role in Rett-like pathophysiology, at two different time-points, at weaning (3 weeks old) and in young adults when overt symptoms are observed (8 weeks old). Our results revealed significant age- and region-dependent impairments in these modulatory neurotransmitter systems that correspond well with the motor phenotype observed in these mice. At 3 weeks of age, male Mecp2 knock-out mice exhibited ataxia and delayed motor initiation. At this stage, noradrenergic and serotonergic transmission was mainly altered in the prefrontal and motor cortices, whereas during disease progression the neurochemical changes were also observed in hippocampus and cerebellum. Our data suggest that the deregulation of norepinephrine and serotonin systems in brain regions that participate in motor control are involved in the pathophysiology of Rett syndrome motor phenotypes. Moreover, we highlight the contribution of cortical regions along with the brainstem to be in the origin of the pathology and the role of hippocampus and cerebellum in the progression of the disease rather than in its establishment.

Progressive noradrenergic deficits in the locus coeruleus of Mecp2 deficient mice

Methyl-CpG binding protein 2 (MeCP2) is a transcriptional regulator. Mutations in this gene cause a wide range of neurological disorders. Mecp2 deficiency has been previously associated to catecholaminergic dysfunctions leading to autonomic defects in the brainstem and the sympathoadrenergic system of the mouse. The present study was undertaken to determine if the locus coeruleus (LC), the main noradrenergic cell group of the brain, is affected. Using real type PCR, we found a reduction of the tyrosine hydroxylase (Th) mRNA level, the rate-limiting enzyme in catecholamine synthesis, in the whole pons of P15 (-36%), P30 (-47%) and P50 (-42%) Mecp2 null male as well as in adult heterozygous female (-44%) mice. Using immunoquantification we did not observe any difference of the Th staining level in P30 null male mice. However at P50, we demonstrated a significant decrease in both the Th staining level (-24%), and the number of Th-positive neurons (-23%). We subsequently characterized a reduction (-28%) of the dendritic density of the Th-positive fibers surrounding the LC in P50 null male mice. In heterozygous female mice immunoquantification did not revealed significant modifications, but only a tendency towards reduction. Finally, we did not found any apoptotic neurons in the pons indicating that LC neurons are not dying but are more likely loosing their catecholaminergic phenotype. In conclusion, our results showing a progressive catecholaminergic deficit in the LC of Mecp2 deficient null male mice could open new perspectives to better understand the autonomic and cognitive deficits due to the lack of Mecp2.

Exogenous brain-derived neurotrophic factor rescues synaptic dysfunction in Mecp2-null mice

Postnatal deficits in brain-derived neurotrophic factor (BDNF) are thought to contribute to pathogenesis of Rett syndrome (RTT), a progressive neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). In Mecp2-null mice, a model of RTT, BDNF deficits are most pronounced in structures important for autonomic and respiratory control, functions that are severely affected in RTT patients. However, relatively little is known about how these deficits affect neuronal function or how they may be linked to specific RTT endophenotypes. To approach these issues, we analyzed synaptic function in the brainstem nucleus tractus solitarius (nTS), the principal site for integration of primary visceral afferent inputs to central autonomic pathways and a region in which we found markedly reduced levels of BDNF in Mecp2 mutants. Our results demonstrate that the amplitude of spontaneous miniature and evoked EPSCs in nTS neurons is significantly increased in Mecp2-null mice and, accordingly, that mutant cells are more likely than wild- type cells to fire action potentials in response to primary afferent stimulation. These changes occur without any increase in intrinsic neuronal excitability and are unaffected by blockade of inhibitory GABA currents. However, this synaptopathy is associated with decreased BDNF availability in the primary afferent pathway and can be rescued by application of exogenous BDNF. On the basis of these findings, we hypothesize that altered sensory gating in nTS contributes to cardiorespiratory instability in RTT and that nTS is a site at which restoration of normal BDNF signaling could help reestablish normal homeostatic controls.

Biogenic amines in Rett syndrome: the usual suspects

Rett syndrome (RTT) is a severe postnatal neurological disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. In affected children, most biological parameters, including brain structure, are normal (although acquired microcephaly is usually present). However, in recent years, a deficit in bioaminergic metabolism has been identified at the cellular and molecular levels, in more than 200 patients. Recently available transgenic mouse strains with a defective Mecp2 gene also show abnormalities, strongly suggesting that there is a direct link between the function of the MECP2 protein and the metabolism of biogenic amines. Biogenic amines appear to have an important role in the pathophysiology of Rett syndrome, for several reasons. Firstly, biogenic amines modulate a large number of autonomic and cognitive functions. Secondly, many of these functions are affected in RTT patients. Thirdly, biogenic amines are the only neurotransmitters that have repeatedly been found to be altered in RTT patients. Importantly, pharmacological interventions can be envisaged to try to counteract the deficits observed. Here, we review the available human and mouse data and present how they have been and could be used in the development of pharmacological treatments for children affected by the syndrome. Given our current knowledge and the tools available, modulating biogenic amine metabolism may prove to be the most promising strategy for improving the life quality of Rett syndrome patients in the short term.

Enhanced dense core granule function and adrenal hypersecretion in a mouse model of Rett syndrome

Rett syndrome (RTT) is a progressive developmental disorder resulting from loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcription regulatory protein. The RTT phenotype is complex and includes severe cardiorespiratory abnormalities, dysautonomia and behavioral symptoms of elevated stress. These findings have been attributed to an apparent hyperactivity of the sympathetic nervous system due to defects in brainstem development; however, the possibility that the peripheral sympathoadrenal axis itself is abnormal has not been explored. The present study demonstrates that the adrenal medulla and sympathetic ganglia of Mecp2 null mice exhibit markedly reduced catecholamine content compared with wild-type controls. Despite this, null animals exhibit significantly higher plasma epinephrine levels, suggesting enhanced secretory granule function in adrenal chromaffin cells. Indeed, we find that Mecp2 null chromaffin cells exhibit a cell autonomous hypersecretory phenotype characterized by significant increases in the speed and size of individual secretory granule fusion events in response to electrical stimulation. These findings appear to indicate accelerated formation and enhanced dilation of the secretory granule fusion pore, resulting in elevated catecholamine release. Our data therefore highlight abnormal catecholamine function in the sympathoadrenal axis as a potential source of autonomic dysfunction in RTT. These findings may help to explain the apparent 'overactivity' of the sympathetic nervous system reported in patients with RTT.