Oral treatment with desipramine improves breathing and life span in Rett syndrome mouse model

Rett syndrome: from bed to bench

Rett syndrome (RTT), a neurodevelopmental condition characterized by delayed-onset loss of spoken language and the development of distinctive hand stereotypies, affects approximately 1 in 10,000 live female births. Clinical diagnosis has been based on symptoms such as loss of acquired purposeful hand skills, autistic behaviors, motor dysfunctions, seizure disorders, and gait abnormalities. RTT is a genetic disease and is caused almost exclusively by mutations in the X-linked gene, MECP2, to produce a phenotype that is thought to be primarily of neurological origin. Clinical reports show RTT patients to have a smaller brain volume, especially in the cerebral hemispheres, and alterations in various neurotransmitter systems, including acetylcholine, dopamine, serotonin, glutamate, substance P, and various trophic factors. Because of its monogenetic characteristic, disruption of Mecp2 is readily recapitulated in mice to produce a prominent RTT-like phenotype and provide an excellent platform for understanding the pathogenesis of RTT. As shown in human studies, Mecp2 mutants also display subtle alterations in neuronal morphology, including smaller cortical neurons with a higher-packing density and reduced dendritic complexity. Neurophysiological studies in Mecp2-mutant mice consistently report alterations in synaptic function, notably, defects in synaptic plasticity. These data suggest that RTT might be regarded as a synaptopathy (disease of the synapse) and thus potentially amenable to rational therapeutic intervention.

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.

The disruption of central CO2 chemosensitivity in a mouse model of Rett syndrome

People with Rett syndrome (RTT) have breathing instability in addition to other neuropathological manifestations. The breathing disturbances contribute to the high incidence of unexplained death and abnormal brain development. However, the cellular mechanisms underlying the breathing abnormalities remain unclear. To test the hypothesis that the central CO(2) chemoreception in these people is disrupted, we studied the CO(2) chemosensitivity in a mouse model of RTT. The Mecp2-null mice showed a selective loss of their respiratory response to 1-3% CO(2) (mild hypercapnia), whereas they displayed more regular breathing in response to 6-9% CO(2) (severe hypercapnia). The defect was alleviated with the NE uptake blocker desipramine (10 mg·kg(-1)·day(-1) ip, for 5-7 days). Consistent with the in vivo observations, in vitro studies in brain slices indicated that CO(2) chemosensitivity of locus coeruleus (LC) neurons was impaired in Mecp2-null mice. Two major neuronal pH-sensitive Kir currents that resembled homomeric Kir4.1 and heteromeric Ki4.1/Kir5.1 channels were identified in the LC neurons. The screening of Kir channels with real-time PCR indicated the overexpression of Kir4.1 in the LC region of Mecp2-null mice. In a heterologous expression system, an overexpression of Kir4.1 resulted in a reduction in the pH sensitivity of the heteromeric Kir4.1-Kir5.1 channels. Given that Kir4.1 and Kir5.1 subunits are also expressed in brain stem respiration-related areas, the Kir4.1 overexpression may not allow CO(2) to be detected until hypercapnia becomes severe, leading to periodical hyper- and hypoventilation in Mecp2-null mice and, perhaps, in people with RTT as well.

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.

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.

Emerging pharmacotherapies for neurodevelopmental disorders

A growing and interdisciplinary translational neuroscience research effort for neurodevelopmental disorders (NDDs) is investigating the mechanisms of dysfunction and testing effective treatment strategies in animal models and, when possible, in the clinic. NDDs with a genetic basis have received particular attention. Transgenic animals that mimic genetic insults responsible for disease in man have provided insight about mechanisms of dysfunction, and, surprisingly, have shown that cognitive deficits can be addressed in adult animals. This review will present recent translational research based on animal models of genetic NDDs, as well as pharmacotherapeutic strategies under development to address deficits of brain function for Down syndrome, fragile X syndrome, Rett syndrome, neurofibromatosis-1, tuberous sclerosis, and autism. Although these disorders vary in underlying causes and clinical presentation, common pathways and mechanisms for dysfunction have been observed. These include abnormal gene dosage, imbalance among neurotransmitter systems, and deficits in the development, maintenance and plasticity of neuronal circuits. NDDs affect multiple brain systems and behaviors that may be amenable to drug therapies that target distinct deficits. A primary goal of translational research is to replace symptomatic and supportive drug therapies with pharmacotherapies based on a principled understanding of the causes of dysfunction. Based on this principle, several recently developed therapeutic strategies offer clear promise for clinical development in man.

Reversibility of functional deficits in experimental models of Rett syndrome

Mutations in the X-linked MECP2 gene are the primary cause of the severe autism spectrum disorder RTT (Rett syndrome). Deletion of Mecp2 in mice recapitulates many of the overt neurological features seen in humans, and the delayed onset of symptoms is accompanied by deficits in neuronal morphology and synaptic physiology. Recent evidence suggests that reactivation of endogenous Mecp2 in young and adult mice can reverse aspects of RTT-like pathology. In the current perspective, we discuss these findings as well as other genetic, pharmacological and environmental interventions that attempt phenotypic rescue in RTT. We believe these studies provide valuable insights into the tractability of RTT and related conditions and are useful pointers for the development of future therapeutic strategies.

Pontine norepinephrine defects in Mecp2-null mice involve deficient expression of dopamine beta-hydroxylase but not a loss of catecholaminergic neurons

Rett syndrome is a neurodevelopmental disorder caused by Mecp2 gene mutations. In RTT patients and Mecp2-null (Mecp2(-/Y)) mice, norepinephrine (NE) content drops significantly, which may play a role in breathing arrhythmia, sleep disorders and sudden death. However, the underlying mechanisms for the NE defect are not fully understood. The NE defect may result from decreased NE biosynthesis, loss of catecholaminergic neurons or both. Although deficiency in tyrosine hydroxylase (TH) has been demonstrated, it is possible that dopamine beta-hydroxylase (DBH), the critical enzyme converting dopamine to NE, is also affected. To test these possibilities, we studied DBH expressions in pontine catecholaminergic neurons of Mecp2(-/Y) mice identified with breathing abnormalities. In comparison to the wild type, Mecp2(-/Y) mice at 2months of age showed approximately 50% decrease in the expressions of DBH and TH, at both protein and mRNA levels in the locus coeruleus (LC) region. Consistently, DBH and TH immunoreactivity was markedly decreased in LC neurons of Mecp2(-/Y) mice. No evidence was found for selective deficiency in TH- or DBH-containing neurons in Mecp2(-/Y) mice, as almost all TH-positive cells expressed DBH. By counting TH-immunoreactive cells in the LC, we found that the Mecp2(-/Y) mice lost only approximately 5% of the catecholaminergic neurons as compared to wild-type, although their LC volume shrank by approximately 15%. These results strongly suggest that the NE defect in Mecp2(-/Y) mice is likely to result from deficient expression of not only TH but also DBH without significant loss of catecholaminergic neurons in the LC.

Alterations of cortical and hippocampal EEG activity in MeCP2-deficient mice

Rett syndrome is a pediatric neurological condition caused by mutations of the gene encoding the transcriptional regulator MECP2. In this study, we examined cortical and hippocampal electroencephalographic (EEG) activity in male and female MeCP2-deficient mice at symptomatic stages during different behavioral states. During acute sleep, MeCP2-deficient mice displayed normal delta-like activity in cortex and sharp-wave activity in hippocampus. However, when the mice were awake but immobile, abnormal spontaneous, rhythmic EEG discharges of 6-9 Hz were readily detected in the somatosensory cortex. During exploratory activity, MeCP2-deficient mice displayed clear theta rhythm activity in hippocampus, but its peak frequency was significantly attenuated compared to wild type. Collectively, these findings indicate that a deficiency in MeCP2 function in mice leads to alterations in EEG activity with similarities to what has been observed clinically in Rett syndrome patients.

Intrinsic membrane properties of locus coeruleus neurons in Mecp2-null mice

Rett syndrome caused by mutations in methyl-CpG-binding protein 2 (Mecp2) gene shows abnormalities in autonomic functions in which brain stem norepinephrinergic systems play an important role. Here we present systematic comparisons of intrinsic membrane properties of locus coeruleus (LC) neurons between Mecp2(-/Y) and wild-type (WT) mice. Whole cell current clamp was performed in brain slices of 3- to 4-wk-old mice. Mecp2(-/Y) neurons showed stronger inward rectification and had shorter time constant than WT cells. The former was likely due to overexpression of inward rectifier K(+) (K(ir))4.1 channel, and the latter was attributable to the smaller cell surface area. The action potential duration was prolonged in Mecp2(-/Y) cells with an extended rise time. This was associated with a significant reduction in the voltage-activated Na(+) current density. After action potentials, >60% Mecp2(-/Y) neurons displayed fast and medium afterhyperpolarizations (fAHP and mAHP), while nearly 90% WT neurons showed only mAHP. The mAHP amplitude was smaller in Mecp2(-/Y) neurons. The firing frequency was higher in neurons with mAHP, and the frequency variation was greater in cells with both fAHP and mAHP in Mecp2(-/Y) mice. Small but significant differences in spike frequency adaptation and delayed excitation were found in Mecp2(-/Y) neurons. These results indicate that there are several electrophysiological abnormalities in LC neurons of Mecp2(-/Y) mice, which may contribute to the dysfunction of the norepinephrine system in Rett syndrome.

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

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

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.

Bioaminergic neuromodulation of respiratory rhythm in vitro

Bioamines, such as norepinephrine and serotonin are key neurotransmitters implicated in multiple physiological and pathological brain mechanisms. Evolutionarily, the bioaminergic neuromodulatory system is widely distributed throughout the brain and is among the earliest neurotransmitters to arise within the hindbrain. In both vertebrates and invertebrates, monoamines play a critical role in the control of respiration. In mammals, both norepinephrine and serotonin are involved in the maturation of the respiratory network, as well as in the neuromodulation of intrinsic and synaptic properties, that not only differentially alters the activity of individual respiratory neurons but also the activity of the network during normoxic and hypoxic conditions. Here, we review the basic noradrenergic and serotonergic pathways and their impact on the activity of the pre-Bötzinger Complex inspiratory neurons and network activity.

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.

Neurochemical phenotypes of cardiorespiratory neurons

Interactions between the cardiovascular and respiratory systems have been known for many years but the functional significance of the interactions is still widely debated. Here I discuss the possible role of metabotropic receptors in regulating cardiorespiratory neurons in the brainstem and spinal cord. It is clear that, although much has been discovered, cardiorespiratory regulation is certainly one area that still has a long way to go before its secrets are fully divulged and their function in controlling circulatory and respiratory function is revealed.

Mouse models of Rett syndrome: from behavioural phenotyping to preclinical evaluation of new therapeutic approaches

Rett syndrome (RTT) is a neurodevelopmental disorder, primarily affecting girls. RTT causes severe cognitive, social, motor and physiological impairments and no cure currently exists. The discovery of a monogenic origin for RTT and the subsequent generation of RTT mouse models provided a major breakthrough for RTT research. Although the characterization of these mutant mice is far from complete, they recapitulate several RTT symptoms. This review provides an overview of the behavioural domains so far investigated in these models, including the very few mouse data concerning the developmental course of RTT. Both clinical and animal studies support the presence of early defects and highlight the importance of probing the presymptomatic phase for both the precocious identification of biomarkers and the early assessment of potential therapies. Preclinical evaluations of pharmacological and nonpharmacological interventions so far carried out are also illustrated. In addition, genetic manipulations are reported that demonstrate rescue from the damage caused by the absence of the methyl-CpG-binding protein 2 (MeCP2) gene even at a mature stage. Given the rare occurrence of RTT cases, transnational collaborative networks are expected to provide a deeper understanding of aetiopathology and the development of new therapeutic approaches.

Breathing dysfunction in Rett syndrome: understanding epigenetic regulation of the respiratory network

Severely arrhythmic breathing is a hallmark of Rett syndrome (RTT) and profoundly affects quality of life for patients and their families. The last decade has seen the identification of the disease-causing gene, methyl-CpG-binding protein 2 (Mecp2) and the development of mouse models that phenocopy many aspects of the human syndrome, including breathing dysfunction. Recent studies have begun to characterize the breathing phenotype of Mecp2 mutant mice and to define underlying electrophysiological and neurochemical deficits. The picture that is emerging is one of defects in synaptic transmission throughout the brainstem respiratory network associated with abnormal expression in several neurochemical signaling systems, including brain-derived neurotrophic factor (BDNF), biogenic amines and gamma-amino-butyric acid (GABA). Based on such findings, potential therapeutic strategies aimed at improving breathing by targeting deficits in neurochemical signaling are being explored. This review details our current understanding of respiratory dysfunction and underlying mechanisms in RTT with a particular focus on insights gained from mouse models.

Neural control of breathing: insights from genetic mouse models

Recent studies described the in vivo ventilatory phenotype of mutant newborn mice with targeted deletions of genes involved in the organization and development of the respiratory-neuron network. Whole body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. Breathing-pattern abnormalities with apneas occur in mutant newborn mice that lack genes involved in the development and modulation of rhythmogenesis. Studies of deficits in ventilatory responses to hypercapnia and/or hypoxia helped to identify genes involved in chemosensitivity to oxygen and carbon dioxide. Combined studies in mutant newborn mice and in humans have shed light on the pathogenesis of genetically determined respiratory-control abnormalities such as congenital central hypoventilation syndrome, Rett syndrome, and Prader-Willi syndrome. The development of mouse models has opened up the field of research into new treatments for respiratory-control disorders in humans.