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.     

Isoform-specific toxicity of Mecp2 in postmitotic neurons: suppression of neurotoxicity by FoxG1

The methyl-CpG binding protein 2 (MeCP2) is a widely expressed protein, the mutations of which cause Rett syndrome. The level of MeCP2 is highest in the brain where it is expressed selectively in mature neurons. Its functions in postmitotic neurons are not known. The MeCP2 gene is alternatively spliced to generate two proteins with different N termini, designated as MeCP2-e1 and MeCP2-e2. The physiological significance of these two isoforms has not been elucidated, and it is generally assumed they are functionally equivalent. We report that in cultured cerebellar granule neurons induced to die by low potassium treatment and in Aβ-treated cortical neurons, Mecp2-e2 expression is upregulated whereas expression of the Mecp2-e1 isoform is downregulated. Knockdown of Mecp2-e2 protects neurons from death, whereas knockdown of the e1 isoform has no effect. Forced expression of MeCP2-e2, but not MeCP2-e1, promotes apoptosis in otherwise healthy neurons. We find that MeCP2-e2 interacts with the forkhead protein FoxG1, mutations of which also cause Rett syndrome. FoxG1 has been shown to promote neuronal survival and its downregulation leads to neuronal death. We find that elevated FoxG1 expression inhibits MeCP2-e2 neurotoxicity. MeCP2-e2 neurotoxicity is also inhibited by IGF-1, which prevents the neuronal death-associated downregulation of FoxG1 expression, and by Akt, the activation of which is necessary for FoxG1-mediated neuroprotection. Finally, MeCP2-e2 neurotoxicity is enhanced if FoxG1 expression is suppressed or in neurons cultured from FoxG1-haplodeficient mice. Our results indicate that Mecp2-e2 promotes neuronal death and that this activity is normally inhibited by FoxG1. Reduced FoxG1 expression frees MecP2-e2 to promote neuronal death.     

Rett syndrome

PURPOSE OF REVIEW: Nearly 70 reports on Rett syndrome were published in 2004. We have selected 51 articles, including clinical reports, on pathophysiology, genotype-phenotype correlation, and clinical and basic molecular biology studies. These articles explain how mutation of the gene (MECP2) for methyl-CpG-binding protein 2 causes the particular disorders of Rett syndrome, and also induces other neurodevelopmental disorders, clarifying the situation for future studies. RECENT FINDINGS: The role of X-chromosome inactivation has been clarified in animal experiments. New isoforms of MeCP2 have been discovered and its functional characteristics are under research. Understanding of the influence of the MECP2 mutation on other neurodevelopmental disorders has increased. However, there is no apparent progress in neurophysiological studies. SUMMARY: Clinical studies included the pathophysiology of stereotyped movement, and cardiac and respiratory disturbances, and there were four therapeutic trials including one for epilepsy. For genotype-phenotype correlation the role of X-chromosome inactivation was looked at and its basic mechanisms were studied extensively in animals. Characteristics of mutations in the C-terminus and the biological function of the new isoform, exon 1, were introduced. In studies on related neurodevelopmental disorders, a relationship is suggested between the MECP2 gene and autism-related gene, with overlapping pathways, but this is not common to other neurodevelopmental disorders. Developmental studies suggest an important role for MeCP2 in the formation and/or maintenance of synapses, and clarify the molecular biological aspects of Rett syndrome. However, early involvement of the aminergic neurons, suggested as the basic, pathognomonic lesion of Rett syndrome, has unfortunately not been investigated with the MECP2 mutation.     

The Role of MeCP2 in Brain Development and Neurodevelopmental Disorders

Methyl CpG binding protein-2 (MeCP2) is an essential epigenetic regulator in human brain development. Rett syndrome, the primary disorder caused by mutations in the X-linked MECP2 gene, is characterized by a period of cognitive decline and development of hand stereotypies and seizures following an apparently normal early infancy. In addition, MECP2 mutations and duplications are observed in a spectrum of neurodevelopmental disorders, including severe neonatal encephalopathy, X-linked mental retardation, and autism, implicating MeCP2 as an essential regulator of postnatal brain development. In this review, we compare the mutation types and inheritance patterns of the human disorders associated with MECP2. In addition, we summarize the current understanding of MeCP2 as a central epigenetic regulator of activity-dependent synaptic maturation. As MeCP2 occupies a central role in the pathogenesis of multiple neurodevelopmental disorders, continued investigation into MeCP2 function and regulatory pathways may show promise for developing broad-spectrum therapies.     

Can we relate MeCP2 deficiency to the structural and chemical abnormalities in the Rett brain?

The mutated gene for Rett syndrome, MECP2, has now been identified in ninety percent of cases. Molecular biologists are immersed in the study of this gene's biology determining how its mutation could be responsible for such an enigmatic phenotype. In this paper the same question is considered, re-examining the structural phenotype of the Rett brain and asking; is MeCP2 present at the appropriate time and place in brain development to influence the structural and chemical abnormalities which characterize the Rett brain? Data from the literature and previous research suggest that MeCP2 is expressed during critical periods of brain development at several sites and in different neurons. It supports the idea that inadequate functioning of MeCP2 alters trophic factors and raises the possibility that replacement of these factors might improve brain function. The availability of mouse models now makes it possible to test such ideas.     

Neuroanatomical distribution and functions of brain-derived neurotrophic factor in zebrafish (Danio rerio) brain

Brain-derived neurotrophic factor (BDNF) is an extensively studied protein that is evolutionarily conserved and widely distributed in the brain of vertebrates. It acts via its cognate receptors TrkB and p75^NTR and plays a central role in the developmental neurogenesis, neuronal survival, proliferation, differentiation, synaptic plasticity, learning and memory, adult hippocampal neurogenesis, and brain regeneration. BDNF has also been implicated in a plethora of neurological disorders. Hence, understanding the processes that are controlled by BDNF and their regulating mechanisms is important. Although, BDNF has been thoroughly studied in the mammalian models, contradictory effects of its functions have been reported on several occasions. These contradictory effects may be attributed to the sheer complexity of the mammalian brain. The study of BDNF and its associated functions in a simpler vertebrate model may provide some clarity about the effects of BDNF on the neurophysiology of the brain. Keeping that in mind, this review aims at summarizing the current knowledge about the distribution of BDNF and its associated functions in the zebrafish brain. The main focus of the review is to give a comparative overview of BDNF distribution and function in zebrafish and mammals with respect to distinct life stages. We have also reviewed the regulation of bdnf gene in zebrafish and discussed its role in developmental and adult neurogenesis.     

Survey of MeCP2 in the Rett syndrome and the non-Rett syndrome brain

The clinical and neuropathologic aspects of Rett syndrome suggest that an arrest of brain development produces the phenotype, but it is not understood how the gene implicated in Rett syndrome, methyl-CpG protein 2 (MeCP2), is regulated during brain development. In this study, the ontogeny of MeCP2 is examined in the developing human brain and in the female Rett syndrome brain to evaluate the relationship between MeCP2 expression and brain development in health and disease, respectively. Immunocytochemistry using an antibody to the C-terminal region of the protein was performed in paraffin sections of the developing brain to define the age and the sites of MeCP2 protein expression. In development, there is no MeCP2 expression in the germinal matrix or in the progenitor cells. At 10 to 14 weeks' gestation, the neurons of the brain stem and the Cajal-Retzius and subplate neurons of the cortex express MeCP2. By midgestation, some neurons of the basal ganglia express MeCP2, and at late gestation, the most mature cortical neurons in the lower cortical layers are positive. The postnatal cortex continues to increase its expression of neuronal MeCP2. In the Rett syndrome brain, fewer neurons express MeCP2 than in the normal brain. This reduction is most apparent in the brain stem and thalamus. The neurons of the cerebral cortex show the least reduction. We conclude that the regulation of MeCP2 abundance is related to human brain development, being expressed in neurons when they appear mature. In Rett syndrome, however, the expression pattern of MeCP2 does not completely resemble that of the normal immature brain, suggesting that the maintenance of MeCP2 might be determined in specific neurons by factors other than those controlling maturation. In the developing brain, synaptic activity and plasticity could be necessary to maintain MeCP2 in selected neuronal populations.     

Changes in Ca(2+)-binding proteins in human neurodegenerative disorders.

The cellular distribution of Ca(2+)-binding proteins has been extensively studied during the past decade. These proteins have proved to be useful neuronal markers for a variety of functional brain systems and their circuitries. Their major roles are assumed to be Ca2+ buffering and transport, and regulation of various enzyme systems. Since cellular degeneration is accompanied by impaired Ca2+ homeostasis, a protective role for Ca(2+)-binding proteins in certain neuron populations has been postulated. As massive neuronal degeneration takes place in several brain diseases of humans, such as Alzheimer's disease, Parkinson's disease and epilepsy, changes in the expression of Ca(2+)-binding proteins have therefore been studied during the course of these diseases. Although the data from these studies are inconsistent, the detection and quantification of Ca(2+)-binding proteins and the neuron populations in which they occur may nevertheless be useful to estimate, for example, the location and extent of brain damage in the various neurological disorders. If future studies advance our knowledge about the physiological functions of these proteins, the neuronal systems in which they are expressed may become important therapeutical targets for preventing neuronal death in an array of neurodegenerative diseases.     

Expression of Phospho-MeCP2s in the Developing Rat Brain and Function of Postnatal MeCP2 in Cerebellar Neural Cell Development

Abnormal expression and dysfunction of methyl-CpG binding protein 2(MeCP2) cause Rett syndrome(RTT). The diverse phosphorylation modifications modulate MeCP2 function in neural cells. Using western blot and immunohistochemistry,we examined the expression patterns of MeCP2 and three phospho-MeCP2s(pMeCP2s) in the developing rat brain. The expression of MeCP2 and phospho-S80(pS80) MeCP2 increased while pS421 MeCP2 and pS292 MeCP2 decreased with brain maturation. In contrast to the nuclear localization of MeCP2 and pS80 MeCP2,pS421 MeCP2 and pS292 MeCP2 were mainly expressed in the cytoplasmic compartment. Apart from their distribution in neurons,they were also detected at a low level in astrocytes. Postnatallyinitiated MeCP2 deficiency affected cerebellar neural cell development,as determined by the abnormal expression of GFAP,DCX,Tuj1,MAP-2,and calbindin-D28k. Together,these results demonstrate that MeCP2 and diverse pMeCP2s have distinct features of spatio-temporal expression in the rat brain,and that the precise levels of MeCP2 in the postnatal period are vital to cerebellar neural cell development.     

MeCP2 is required for activity-dependent refinement of olfactory circuits

Methyl CpG binding protein 2 (MeCP2) is a structural chromosomal protein involved in the regulation of gene expression. Alterations in the levels of MeCP2 have been related to neurodevelopmental disorders. Studies in mouse models of MeCP2 deficiency have demonstrated that this protein is important for neuronal maturation, neurite complexity, synaptogenesis, and synaptic plasticity. However, the mechanisms by which MeCP2 dysfunction leads to neurodevelopmental defects, and the role of activity, remain unclear, as most studies examine the adult nervous system, which may obfuscate the primary consequences of MeCP2 mutation. We hypothesize that MeCP2 plays a role during the formation and activity-driven maturation of neural circuits at early postnatal stages. To test this hypothesis, we use the olfactory system as a neurodevelopmental model. This system undergoes postnatal neurogenesis; axons from olfactory neurons form highly stereotyped projections to higher-order neurons, facilitating the detection of possible defects in the establishment of connectivity. In vivo olfactory stimulation paradigms were used to produce physiological synaptic activity in gene-targeted mice in which specific olfactory circuits are visualized. Our results reveal defective postnatal refinement of olfactory circuits in Mecp2 knock out (KO) mice after sensory (odorant) stimulation. This failure in refinement was associated with deficits in the normal responses to odorants, including brain-derived neurotrophic factor (BDNF) production, as well as changes in adhesion molecules known to regulate axonal convergence. The defective refinement observed in Mecp2 KO mice was prevented by daily treatment with ampakine beginning after the first postnatal week. These observations indicate that increasing synaptic activity at early postnatal stage might circumvent the detrimental effect of MeCP2 deficiency on circuitry maturation. The present results provide in vivo evidence in real time for the role of MeCP2 in activity-dependent maturation of olfactory circuitry, with implications for understanding the mechanism of MeCP2 mutations in the development of neural connectivity.