Age-dependent expression of MeCP2 in a heterozygous mosaic mouse model

Interpreting the rich dialogue between astrocytes and neurons: An overview in Rett syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder primarily affecting females, with an incidence of 1 in 10,000 live births. It is caused mainly by de novo mutations in the X-linked MECP2 gene, which encodes methyl-CpG binding protein 2 (Mecp2), a key epigenetic regulator. MECP2 mutations have profound impacts on neurons, which exhibit morphological, synaptic and functional impairments. However, more recent evidence highlights a crucial role of astrocytes in RTT pathogenesis. Indeed, RTT astrocytes exhibit structural and functional impairments, failing to support neuronal growth and function through non-cell autonomous mechanisms. Studies reveal that MECP2 deficient astrocytes secrete abnormal factors that impair neuronal growth and synaptic function. Furthermore, they show dysregulated calcium signalling, disrupted glutamate and potassium homeostasis, and increased inflammatory responses, all of which contribute to neuronal dysfunction. Understanding these neuron-astrocyte interactions may offer novel therapeutic targets for RTT. In the review we aim at presenting the current knowledge of astrocyte-neuron crosstalk in RTT, describing the different mechanisms highlighted so far through which MECP2 mutant astrocytes impair neurons. Finally, we discuss existing and prospective methodological approaches for investigating cell-to-cell communication in RTT.

MeCP2 gene therapy ameliorates disease phenotype in mouse model for Pitt Hopkins syndrome

The neurodevelopmental disorder Pitt Hopkins syndrome (PTHS) causes clinical symptoms similar to Rett syndrome (RTT) patients. However, RTT is caused by MECP2 mutations whereas mutations in the TCF4 gene lead to PTHS. The mechanistic commonalities underling these two disorders are unknown, but their shared symptomology suggest that convergent pathway-level disruption likely exists. We reprogrammed patient skin derived fibroblasts into induced neuronal progenitor cells. Interestingly, we discovered that MeCP2 levels were decreased in PTHS patient iNPCs relative to healthy controls and that both iNPCs and iAstrocytes displayed defects in function and differentiation in a mutation-specific manner. When Tcf4+/- mice were genetically crossed with mice overexpressing MeCP2, molecular and phenotypic defects were significantly ameliorated, underlining and important role of MeCP2 in PTHS pathology. Importantly, post-natal intracerebroventricular gene replacement therapy with adeno-associated viral vector serotype 9 (AAV9)-expressing MeCP2 (AAV9.P546.MeCP2) significantly improved iNPC and iAstrocyte function and effectively ameliorated histological and behavioral defects in Tcf4+/- mice. Combined, our data suggest a previously unknown role of MeCP2 in PTHS pathology and common pathways that might be affected in multiple neurodevelopmental disorders. Our work highlights potential novel therapeutic targets for PTHS, including upregulation of MeCP2 expression or its downstream targets or, potentially, MeCP2-based gene therapy.

Heterozygous Nexmif female mice demonstrate mosaic NEXMIF expression, autism-like behaviors, and abnormalities in dendritic arborization and synaptogenesis

Autism spectrum disorder (ASD) is a neurodevelopmental disorder with a strong genetic basis. ASDs are commonly characterized by impairments in language, restrictive and repetitive behaviors, and deficits in social interactions. Although ASD is a highly heterogeneous disease with many different genes implicated in its etiology, many ASD-associated genes converge on common cellular defects, such as aberrant neuronal morphology and synapse dysregulation. Our previous work revealed that, in mice, complete loss of the ASD-associated X-linked gene NEXMIF results in a reduction in dendritic complexity, a decrease in spine and synapse density, altered synaptic transmission, and ASD-like behaviors. Interestingly, human females of NEXMIF haploinsufficiency have recently been reported to demonstrate autistic features; however, the cellular and molecular basis for this haploinsufficiency-caused ASD remains unclear. Here we report that in the brains of Nexmif± female mice, NEXMIF shows a mosaic pattern in its expression in neurons. Heterozygous female mice demonstrate behavioral impairments similar to those of knockout male mice. In the mosaic mixture of neurons from Nexmif± mice, cells that lack NEXMIF have impairments in dendritic arborization and spine development. Remarkably, the NEXMIF-expressing neurons from Nexmif± mice also demonstrate similar defects in dendritic growth and spine formation. These findings establish a novel mouse model of NEXMIF haploinsufficiency and provide new insights into the pathogenesis of NEXMIF-dependent ASD.

MeCP2 Deficiency Leads to Loss of Glial Kir4.1

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder usually caused by mutations in methyl-CpG-binding protein 2 (MeCP2). RTT is typified by apparently normal development until 6-18 mo of age, when motor and communicative skills regress and hand stereotypies, autonomic symptoms, and seizures present. Restoration of MeCP2 function selectively to astrocytes reversed several deficits in a murine model of RTT, but the mechanism of this rescue is unknown. Astrocytes carry out many essential functions required for normal brain functioning, including extracellular K⁺ buffering. Kir4.1, an inwardly rectifying K⁺ channel, is largely responsible for the channel-mediated K⁺ regulation by astrocytes. Loss-of-function mutations in Kir4.1 in human patients result in a severe neurodevelopmental disorder termed EAST or SESAME syndrome. Here, we evaluated astrocytic Kir4.1 expression in a murine model of Rett syndrome. We demonstrate by chromatin immunoprecipitation analysis that Kir4.1 is a direct molecular target of MeCP2. Astrocytes from Mecp2-deficient mice express significantly less Kir4.1 mRNA and protein, which translates into a >50% deficiency in Ba²⁺-sensitive Kir4.1-mediated currents, and impaired extracellular potassium dynamics. By examining astrocytes in isolation, we demonstrate that loss of Kir4.1 is cell autonomous. Assessment through postnatal development revealed that Kir4.1 expression in Mecp2-deficient animals never reaches adult, wild-type levels, consistent with a neurodevelopmental disorder. These are the first data implicating a direct MeCP2 molecular target in astrocytes and provide novel mechanistic insight explaining a potential mechanism by which astrocytic dysfunction may contribute to RTT.

Fragile X related protein 1 (FXR1P) regulates proliferation of adult neural stem cells

Fragile X related protein 1 (FXR1P) is a member of the fragile X family of RNA-binding proteins, which includes FMRP and FXR2P. Both FMRP and FXR2P regulate neurogenesis, a process affected in a number of neurological and neuropsychiatric disorders, including fragile X syndrome. Although FXR1P has been implicated in various developmental processes and neuropsychiatric diseases, its role in neurodevelopment is not well understood. The goal of the present study was to elucidate the function of FXR1P in adult neurogenesis. We used an inducible mouse model that allows us to investigate how FXR1P deficiency in adult neural stem cells (aNSCs) affects proliferation and neuronal differentiation. Deletion of FXR1 in aNSCs resulted in fewer adult-born cells in the dentate gyrus (DG) overall, reducing populations across different stages of neurogenesis, including radial glia-like cells, intermediate progenitors, neuroblasts, immature neurons and neurons. We hypothesized that this reduction in new cell numbers resulted from impaired proliferation, which we confirmed both in vivo and in vitro. We discovered that FXR1P-deficient aNSCs have altered expression of a select number of cell-cycle genes, and we identified the mRNA of cyclin-dependent kinase inhibitor 1A (Cdkn1a, p21) as a direct target of FXR1P. Restoration of p21 mRNA to wild-type levels rescued the proliferation deficit in cells lacking FXR1P, demonstrating that p21 is a mediator of FXR1P in aNSCs. These results indicate that FXR1P plays an important role in regulating aNSC self-renewal and maintenance in the adult brain, which may have implications for a number of neurodevelopmental and psychiatric disorders.

Effects of early-life exposure to THIP on phenotype development in a mouse model of Rett syndrome

Background

Rett syndrome (RTT) is a neurodevelopmental disorder caused mostly by disruptions in the MECP2 gene. MECP2-null mice show imbalances in neuronal excitability and synaptic communications. Several previous studies indicate that augmenting synaptic GABA receptors (GABA_ARs) can alleviate RTT-like symptoms in mice. In addition to the synaptic GABA_ARs, there is a group of GABA_ARs found outside the synaptic cleft with the capability to produce sustained inhibition, which may be potential therapeutic targets for the control of neuronal excitability in RTT.

Methods

Wild-type and MECP2-null mice were randomly divided into four groups, receiving the extrasynaptic GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP) and vehicle control, respectively. Low-dose THIP was administered to neonatal mice through lactation. RTT-like symptoms including lifespan, breathing, motor function, and social behaviors were studied when mice became mature. Changes in neuronal excitability and norepinephrine biosynthesis enzyme expression were studied in electrophysiology and molecular biology.

Results

With no evident sedation and other adverse side effects, early-life exposure to THIP extended the lifespan, alleviated breathing abnormalities, enhanced motor function, and improved social behaviors of MECP2-null mice. Such beneficial effects were associated with stabilization of locus coeruleus neuronal excitability and improvement of norepinephrine biosynthesis enzyme expression.

Conclusions

THIP treatment in early lives might be a therapeutic approach to RTT-like symptoms in MECP2-null mice and perhaps in people with RTT as well.

Genetics and Epigenetics in Adult Neurogenesis

The cellular basis of adult neurogenesis is neural stem cells residing in restricted areas of the adult brain. These cells self-renew and are multipotent. The maintenance of "stemness" and commitment to differentiation are tightly controlled by intricate molecular networks. Epigenetic mechanisms, including chromatin remodeling, DNA methylation, and noncoding RNAs (ncRNAs), have profound regulatory roles in mammalian gene expression. Significant advances have been made regarding the dynamic roles of epigenetic modulation and function. It has become evident that epigenetic regulators are key players in neural-stem-cell self-renewal, fate specification, and final maturation of new neurons, therefore, adult neurogenesis. Altered epigenetic regulation can result in a number of neurological and neurodevelopmental disorders. Here, we review recent discoveries that advance our knowledge in epigenetic regulation of mammalian neural stem cells and neurogenesis. Insights from studies of epigenetic gene regulation in neurogenesis may lead to new therapies for the treatment of neurodevelopmental disorders.

Inhibition of miR-15a Promotes BDNF Expression and Rescues Dendritic Maturation Deficits in MeCP2-Deficient Neurons

In both the embryonic and adult brain, a critical step in neurogenesis is neuronal maturation. Deficiency of MeCP2 leads to Rett syndrome, a severe neurodevelopmental disorder. We have previously shown that MeCP2 plays critical roles in the maturation step of new neurons during neurogenesis. MeCP2 is known to regulate the expression of brain-derived neurotrophic factor (BDNF), a potent neurotrophic factor for neuronal maturation. Nevertheless, how MeCP2 regulates BDNF expression and how MeCP2 deficiency leads to reduced BDNF expression remain unclear. Here, we show that MeCP2 regulates the expression of a microRNA, miR-15a. We find that miR-15a plays a significant role in the regulation of neuronal maturation. Overexpression of miR-15a inhibits dendritic morphogenesis in immature neurons. Conversely, a reduction in miR-15a has the opposite effect. We further show that miR-15a regulates expression levels of BDNF, and exogenous BDNF could partially rescue the neuronal maturation deficits resulting from miR-15a overexpression. Finally, inhibition of miR-15a could rescue neuronal maturation deficits in MeCP2-deficient adult-born new neurons. These results demonstrate a novel role for miR-15a in neuronal development and provide a missing link in the regulation of BDNF by MeCP2.

VPA alleviates neurological deficits and restores gene expression in a mouse model of Rett syndrome

Rett syndrome (RTT) is a devastating neurodevelopmental disorder that occurs once in every 10,000–15,000 live female births. Despite intensive research, no effective cure is yet available. Valproic acid (VPA) has been used widely to treat mood disorder, epilepsy, and a growing number of other disorders. In limited clinical studies, VPA has also been used to control seizure in RTT patients with promising albeit somewhat unclear efficacy. In this study we tested the effect of VPA on the neurological symptoms of RTT and discovered that short-term VPA treatment during the symptomatic period could reduce neurological symptoms in RTT mice. We found that VPA restores the expression of a subset of genes in RTT mouse brains, and these genes clustered in neurological disease and developmental disorder networks. Our data suggest that VPA could be used as a drug to alleviate RTT symptoms.

Sex, stroke, and inflammation: the potential for estrogen-mediated immunoprotection in stroke

Stroke is the third leading cause of death and the primary cause of disability in the developed world. Experimental and clinical data indicate that stroke is a sexually dimorphic disease, with males demonstrating an enhanced intrinsic sensitivity to ischemic damage throughout most of their lifespan. The neuroprotective role of estrogen in the female brain is well established, however, estrogen exposure can also be deleterious, especially in older women. The mechanisms for this remain unclear. Our current understanding is based on studies examining estrogen as it relates to neuronal injury, yet cerebral ischemia also induces a robust sterile inflammatory response involving local and systemic immune cells. Despite the potent anti-inflammatory effects of estrogen, few studies have investigated the contribution of estrogen to sex differences in the inflammatory response to stroke. This review examines the potential role for estrogen-mediated immunoprotection in ischemic injury.

Interactions between age, sex, and hormones in experimental ischemic stroke

Age, sex, and gonadal hormones have profound effects on ischemic stroke outcomes, although how these factors impact basic stroke pathophysiology remains unclear. There is a plethora of inconsistent data reported throughout the literature, primarily due to differences in the species examined, the timing and methods used to evaluate injury, the models used, and confusion regarding differences in stroke incidence as seen in clinical populations vs. effects on acute neuroprotection or neurorepair in experimental stroke models. Sex and gonadal hormone exposure have considerable independent impact on stroke outcome, but these factors also interact with each other, and the contribution of each differs throughout the lifespan. The contribution of sex and hormones to experimental stroke will be the focus of this review. Recent advances and our current understanding of age, sex, and hormone interactions in ischemic stroke with a focus on inflammation will be discussed.

Crosstalk among Epigenetic Pathways Regulates Neurogenesis

The process of neurogenesis includes neural stem cell proliferation, fate specification, young neuron migration, neuronal maturation, and functional integration into existing circuits. Although neurogenesis occurs largely during embryonic development, low levels but functionally important neurogenesis persists in restricted regions of the postnatal brain, including the subgranular zone of the dentate gyrus in the hippocampus and the subventricular zone of the lateral ventricles. This review will cover both embryonic and adult neurogenesis with an emphasis on the latter. Of the many endogenous mediators of postnatal neurogenesis, epigenetic pathways, such as mediators of DNA methylation, chromatin remodeling systems, and non-coding RNA modulators, appear to play an integral role. Mounting evidence shows that such epigenetic factors form regulatory networks, which govern each step of postnatal neurogenesis. In this review, we explore the emerging roles of epigenetic mechanisms particularly microRNAs, element-1 silencing transcription factor/neuron-restrictive silencing factor (REST/NRSF), polycomb proteins, and methyl-CpG bindings proteins, in regulating the entire process of postnatal and adult neurogenesis. We further summarize recent data regarding how the crosstalk among these different epigenetic proteins forms the critical regulatory network that regulates neuronal development. We finally discuss how crosstalk between these pathways may serve to translate environmental cues into control of the neurogenic process.

X chromosome dosage and the response to cerebral ischemia

Gonadal hormones contribute to ischemic neuroprotection, but cannot fully explain the observed sexual dimorphism in stroke outcomes seen during life stages with low sex steroid hormones. Sex chromosomal complement (XX in females; XY in males) may also contribute to ischemic sexual dimorphism. A transient middle cerebral artery occlusion model was used to investigate the role of X chromosome dosage in female XX and XO littermates of two mouse strains (Paf and Eda(Ta)). Cohorts of XX and XO gonadally intact, ovariectomized, and ovariectomized females supplemented with estrogen were examined. Infarct sizes were equivalent between ovariectomized XX and XO mice, between intact XX and XO mice, and between estrogen-supplemented ovariectomized XX and XO mice. This is the first study to investigate the role of sex chromosome dosage in the response to cerebral ischemia. Neither the number of X chromosomes nor the parent of origin of the remaining X chromosome had a significant effect on the degree of cerebral infarction after experimental stroke in adult female mice. Estrogen was protective against cerebral ischemia in both XX and XO mice.

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.