Cerebellar and cerebral abnormalities in Rett syndrome: a quantitative MR analysis

Structural and functional changes in the brains of patients with Rett syndrome: A multimodal MRI study

OBJECTIVE

To clarify the relationship between structural and functional changes in the brains of patients with Rett syndrome (RTT) using multimodal magnetic resonance imaging (MRI).

METHODS

Nine subjects with typical RTT (RTTs) and an equal number of healthy controls (HCs) underwent structural MRI, diffusion tensor imaging (DTI), and resting-state functional MRI (rs-fMRI). The measurements obtained from each modality were statistically compared between RTTs and HCs and examined for their correlation with the clinical severity of RTTs.

RESULTS

Structural MRI imaging revealed volume reductions in most cortical and subcortical regions of the brain. Remarkable volume reductions were observed in the frontal and parietal lobes, cerebellum, and subcortical regions including the putamen, hippocampus, and corpus callosum. DTI analysis revealed decreased white matter integrity in broad regions of the brain. Fractional anisotropy values were greatly decreased in the superior longitudinal fasciculus, corpus callosum, and middle cerebellar peduncle. Rs-fMRI analysis showed decreased functional connectivity in the interhemispheric dorsal attention network, and between the visual and cerebellar networks. The clinical severity of RTTs correlated with the volume reduction of the frontal lobe and cerebellum, and with changes in DTI indices in the fronto-occipital fasciculus, corpus callosum, and cerebellar peduncles.

CONCLUSION

Regional volume and white matter integrity of RTT brains were reduced in broad areas, while most functional connections remained intact. Notably, two functional connectivities, between cerebral hemispheres and between the cerebrum and cerebellum, were decreased in RTT brains, which may reflect the structural changes in these brain regions.

Comprehensive Volumetric Analysis of Mecp2-Null Mouse Model for Rett Syndrome by T2-Weighted 3D Magnetic Resonance Imaging

Rett syndrome (RTT) is a severe progressive neurodevelopmental disorder characterized by various neurological symptoms. Almost all RTT cases are caused by mutations in the X-linked methyl-CpG-binding protein 2 (MeCP2) gene, and several mouse models have been established to understand the disease. However, the neuroanatomical abnormalities in each brain region of RTT mouse models have not been fully understood. Here, we investigated the global and local neuroanatomy of the Mecp2 gene-deleted RTT model (Mecp2-KO) mouse brain using T2-weighted 3D magnetic resonance imaging with different morphometry to clarify the brain structural abnormalities that are involved in the pathophysiology of RTT. We found a significant reduction in global and almost all local volumes in the brain of Mecp2-KO mice. In addition, a detailed comparative analysis identified specific volume reductions in several brain regions in the Mecp2-deficient brain. Our analysis also revealed that the Mecp2-deficient brain shows changes in hemispheric asymmetry in several brain regions. These findings suggest that MeCP2 affects not only the whole-brain volume but also the region-specific brain structure. Our study provides a framework for neuroanatomical studies of a mouse model of RTT.

Quantitative Structural Brain Magnetic Resonance Imaging Analyses: Methodological Overview and Application to Rett Syndrome

Congenital genetic disorders often present with neurological manifestations such as neurodevelopmental disorders, motor developmental retardation, epilepsy, and involuntary movement. Through qualitative morphometric evaluation of neuroimaging studies, remarkable structural abnormalities, such as lissencephaly, polymicrogyria, white matter lesions, and cortical tubers, have been identified in these disorders, while no structural abnormalities were identified in clinical settings in a large population. Recent advances in data analysis programs have led to significant progress in the quantitative analysis of anatomical structural magnetic resonance imaging (MRI) and diffusion-weighted MRI tractography, and these approaches have been used to investigate psychological and congenital genetic disorders. Evaluation of morphometric brain characteristics may contribute to the identification of neuroimaging biomarkers for early diagnosis and response evaluation in patients with congenital genetic diseases. This mini-review focuses on the methodologies and attempts employed to study Rett syndrome using quantitative structural brain MRI analyses, including voxel- and surface-based morphometry and diffusion-weighted MRI tractography. The mini-review aims to deepen our understanding of how neuroimaging studies are used to examine congenital genetic disorders.

The Cerebellar Involvement in Autism Spectrum Disorders: From the Social Brain to Mouse Models

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders that include a variety of forms and clinical phenotypes. This heterogeneity complicates the clinical and experimental approaches to ASD etiology and pathophysiology. To date, a unifying theory of these diseases is still missing. Nevertheless, the intense work of researchers and clinicians in the last decades has identified some ASD hallmarks and the primary brain areas involved. Not surprisingly, the areas that are part of the so-called “social brain”, and those strictly connected to them, were found to be crucial, such as the prefrontal cortex, amygdala, hippocampus, limbic system, and dopaminergic pathways. With the recent acknowledgment of the cerebellar contribution to cognitive functions and the social brain, its involvement in ASD has become unmistakable, though its extent is still to be elucidated. In most cases, significant advances were made possible by recent technological developments in structural/functional assessment of the human brain and by using mouse models of ASD. Mouse models are an invaluable tool to get insights into the molecular and cellular counterparts of the disease, acting on the specific genetic background generating ASD-like phenotype. Given the multifaceted nature of ASD and related studies, it is often difficult to navigate the literature and limit the huge content to specific questions. This review fulfills the need for an organized, clear, and state-of-the-art perspective on cerebellar involvement in ASD, from its connections to the social brain areas (which are the primary sites of ASD impairments) to the use of monogenic mouse models.

The Cell Adhesion Molecule L1 Interacts with Methyl CpG Binding Protein 2 via Its Intracellular Domain

Cell adhesion molecule L1 regulates multiple cell functions, and L1 deficiency is linked to several neural diseases. Recently, we have identified methyl CpG binding protein 2 (MeCP2) as a potential binding partner of the intracellular L1 domain. By ELISA we show here that L1's intracellular domain binds directly to MeCP2 via the sequence motif KDET. Proximity ligation assay with cultured cerebellar and cortical neurons suggests a close association between L1 and MeCP2 in nuclei of neurons. Immunoprecipitation using MeCP2 antibodies and nuclear mouse brain extracts indicates that MeCP2 interacts with an L1 fragment of ~55 kDa (L1-55). Proximity ligation assay indicates that metalloproteases, β-site of amyloid precursor protein cleaving enzyme (BACE1) and ɣ-secretase, are involved in the generation of L1-55. Reduction in MeCP2 expression by siRNA decreases L1-dependent neurite outgrowth from cultured cortical neurons as well as the migration of L1-expressing HEK293 cells. Moreover, L1 siRNA, MeCP2 siRNA, or a cell-penetrating KDET-containing L1 peptide leads to reduced levels of myocyte enhancer factor 2C (Mef2c) mRNA and protein in cortical neurons, suggesting that the MeCP2/L1 interaction regulates Mef2c expression. Altogether, the present findings indicate that the interaction of the novel fragment L1-55 with MeCP2 affects L1-dependent functions, such as neurite outgrowth and neuronal migration.

Multimodal Neuroimaging in Rett Syndrome With MECP2 Mutation

Rett syndrome (RTT) is a rare neurodevelopmental disorder characterized by severe cognitive, social, and physical impairments resulting from de novo mutations in the X-chromosomal methyl-CpG binding protein gene 2 (MECP2). While there is still no cure for RTT, exploring up-to date neurofunctional diagnostic markers, discovering new potential therapeutic targets, and searching for novel drug efficacy evaluation indicators are fundamental. Multiple neuroimaging studies on brain structure and function have been carried out in RTT-linked gene mutation carriers to unravel disease-specific imaging features and explore genotype-phenotype associations. Here, we reviewed the neuroimaging literature on this disorder. MRI morphologic studies have shown global atrophy of gray matter (GM) and white matter (WM) and regional variations in brain maturation. Diffusion tensor imaging (DTI) studies have demonstrated reduced fractional anisotropy (FA) in left peripheral WM areas, left major WM tracts, and cingulum bilaterally, and WM microstructural/network topology changes have been further found to be correlated with behavioral abnormalities in RTT. Cerebral blood perfusion imaging studies using single-photon emission CT (SPECT) or PET have evidenced a decreased global cerebral blood flow (CBF), particularly in prefrontal and temporoparietal areas, while magnetic resonance spectroscopy (MRS) and PET studies have contributed to unraveling metabolic alterations in patients with RTT. The results obtained from the available reports confirm that multimodal neuroimaging can provide new insights into a complex interplay between genes, neurotransmitter pathway abnormalities, disease-related behaviors, and clinical severity. However, common limitations related to the available studies include small sample sizes and hypothesis-based and region-specific approaches. We, therefore, conclude that this field is still in its early development phase and that multimodal/multisequence studies with improved post-processing technologies as well as combined PET–MRI approaches are urgently needed to further explore RTT brain alterations.

Cerebellar Dysfunction in Autism Spectrum Disorders: Deriving Mechanistic Insights from an Internal Model Framework

Autism spectrum disorders (ASD) are highly prevalent neurodevelopmental disorders; however, the neurobiological mechanisms underlying disordered behavior in ASD remain poorly understood. Notably, individuals with ASD have demonstrated difficulties generating implicitly derived behavioral predictions and adaptations. Although many brain regions are involved in these processes, the cerebellum contributes an outsized role to these behavioral functions. Consistent with this prominent role, cerebellar dysfunction has been increasingly implicated in ASD. In this review, we will utilize the foundational, theoretical contributions of the late neuroscientist Masao Ito to establish an internal model framework for the cerebellar contribution to ASD-relevant behavioral predictions and adaptations. Additionally, we will also explore and then apply his key experimental contributions towards an improved, mechanistic understanding of the contribution of cerebellar dysfunction to ASD.

Modeling Rett Syndrome with Human Pluripotent Stem Cells: Mechanistic Outcomes and Future Clinical Perspectives

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2). Among many different roles, MeCP2 has a high phenotypic impact during the different stages of brain development. Thus, it is essential to intensively investigate the function of MeCP2, and its regulated targets, to better understand the mechanisms of the disease and inspire the development of possible therapeutic strategies. Several animal models have greatly contributed to these studies, but more recently human pluripotent stem cells (hPSCs) have been providing a promising alternative for the study of RTT. The rapid evolution in the field of hPSC culture allowed first the development of 2D-based neuronal differentiation protocols, and more recently the generation of 3D human brain organoid models, a more complex approach that better recapitulates human neurodevelopment in vitro. Modeling RTT using these culture platforms, either with patient-specific human induced pluripotent stem cells (hiPSCs) or genetically-modified hPSCs, has certainly contributed to a better understanding of the onset of RTT and the disease phenotype, ultimately allowing the development of high throughput drugs screening tests for potential clinical translation. In this review, we first provide a brief summary of the main neurological features of RTT and the impact of MeCP2 mutations in the neuropathophysiology of this disease. Then, we provide a thorough revision of the more recent advances and future prospects of RTT modeling with human neural cells derived from hPSCs, obtained using both 2D and organoids culture systems, and its contribution for the current and future clinical trials for RTT.

Quantitative proteomic analysis of Rett iPSC-derived neuronal progenitors

Background

Rett syndrome (RTT) is a progressive neurodevelopmental disease that is characterized by abnormalities in cognitive, social, and motor skills. RTT is often caused by mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2). The mechanism by which impaired MeCP2 induces the pathological abnormalities in the brain is not understood. Both patients and mouse models have shown abnormalities at molecular and cellular level before typical RTT-associated symptoms appear. This implies that underlying mechanisms are already affected during neurodevelopmental stages.

Methods

To understand the molecular mechanisms involved in disease onset, we used an RTT patient induced pluripotent stem cell (iPSC)-based model with isogenic controls and performed time-series of proteomic analysis using in-depth high-resolution quantitative mass spectrometry during early stages of neuronal development.

Results

We provide mass spectrometry-based quantitative proteomic data, depth of about 7000 proteins, at neuronal progenitor developmental stages of RTT patient cells and isogenic controls. Our data gives evidence of proteomic alteration at early neurodevelopmental stages, suggesting alterations long before the phase that symptoms of RTT syndrome become apparent. Significant changes are associated with the GO enrichment analysis in biological processes cell-cell adhesion, actin cytoskeleton organization, neuronal stem cell population maintenance, and pituitary gland development, next to protein changes previously associated with RTT, i.e., dendrite morphology and synaptic deficits. Differential expression increased from early to late neural stem cell phases, although proteins involved in immunity, metabolic processes, and calcium signaling were affected throughout all stages analyzed.

Limitations

The limitation of our study is the number of RTT patients analyzed. As the aim of our study was to investigate a large number of proteins, only one patient was considered, of which 3 different RTT iPSC clones and 3 isogenic control iPSC clones were included. Even though this approach allowed the study of mutation-induced alterations due to the usage of isogenic controls, results should be validated on different RTT patients to suggest common disease mechanisms.

Conclusions

During early neuronal differentiation, there are consistent and time-point specific proteomic alterations in RTT patient cells carrying exons 3–4 deletion in MECP2. We found changes in proteins involved in pathway associated with RTT phenotypes, including dendrite morphology and synaptogenesis. Our results provide a valuable resource of proteins and pathways for follow-up studies, investigating common mechanisms involved during early disease stages of RTT syndrome.

Defective proteasome biogenesis into skin fibroblasts isolated from Rett syndrome subjects with MeCP2 non-sense mutations

Rett Syndrome (RTT) is a rare X-linked neurodevelopmental disorder which affects about 1: 10000 live births. In >95% of subjects RTT is caused by a mutation in Methyl-CpG binding protein-2 (MECP2) gene, which encodes for a transcription regulator with pleiotropic genetic/epigenetic activities. The molecular mechanisms underscoring the phenotypic alteration of RTT are largely unknown and this has impaired the development of therapeutic approaches to alleviate signs and symptoms during disease progression. A defective proteasome biogenesis into two skin primary fibroblasts isolated from RTT subjects harbouring non-sense (early-truncating) MeCP2 mutations (i.e., R190fs and R255X) is herewith reported. Proteasome is the proteolytic machinery of Ubiquitin Proteasome System (UPS), a pathway of overwhelming relevance for post-mitotic cells metabolism. Molecular, transcription and proteomic analyses indicate that MeCP2 mutations down-regulate the expression of one proteasome subunit, α7, and of two chaperones, PAC1 and PAC2, which bind each other in the earliest step of proteasome biogenesis. Furthermore, this molecular alteration recapitulates in neuron-like SH-SY5Y cells upon silencing of MeCP2 expression, envisaging a general significance of this transcription regulator in proteasome biogenesis.

Altered trajectories of neurodevelopment and behavior in mouse models of Rett syndrome

Rett Syndrome (RTT) is a genetic disorder that is caused by mutations in the x-linked gene coding for methyl-CpG-biding-protein 2 (MECP2) and that mainly affects females. Male and female transgenic mouse models of RTT have been studied extensively, and we have learned a great deal regarding RTT neuropathology and how MeCP2 deficiency may be influencing brain function and maturation. In this manuscript we review what is known concerning structural and coinciding functional and behavioral deficits in RTT and in mouse models of MeCP2 deficiency. We also introduce our own corroborating data regarding behavioral phenotype and morphological alterations in volume of the cortex and striatum and the density of neurons, aberrations in experience-dependent plasticity within the barrel cortex and the impact of MeCP2 loss on glial structure. We conclude that regional structural changes in genetic models of RTT show great similarity to the alterations in brain structure of patients with RTT. These region-specific modifications often coincide with phenotype onset and contribute to larger issues of circuit connectivity, progression, and severity. Although the alterations seen in mouse models of RTT appear to be primarily due to cell-autonomous effects, there are also non-cell autonomous mechanisms including those caused by MeCP2-deficient glia that negatively impact healthy neuronal function. Collectively, this body of work has provided a solid foundation on which to continue to build our understanding of the role of MeCP2 on neuronal and glial structure and function, its greater impact on neural development, and potential new therapeutic avenues.

Elucidation of the phenotypic spectrum and genetic landscape in primary and secondary microcephaly

Purpose

Microcephaly is a sign of many genetic conditions but has been rarely systematically evaluated. We therefore comprehensively studied the clinical and genetic landscape of an unselected cohort of patients with microcephaly.

Methods

We performed clinical assessment, high-resolution chromosomal microarray analysis, exome sequencing, and functional studies in 62 patients (58% with primary microcephaly [PM], 27% with secondary microcephaly [SM], and 15% of unknown onset).

Results

We found severity of developmental delay/intellectual disability correlating with severity of microcephaly in PM, but not SM. We detected causative variants in 48.4% of patients and found divergent inheritance and variant pattern for PM (mainly recessive and likely gene-disrupting [LGD]) versus SM (all dominant de novo and evenly LGD or missense). While centrosome-related pathways were solely identified in PM, transcriptional regulation was the most frequently affected pathway in both SM and PM. Unexpectedly, we found causative variants in different mitochondria-related genes accounting for ~5% of patients, which emphasizes their role even in syndromic PM. Additionally, we delineated novel candidate genes involved in centrosome-related pathway (SPAG5, TEDC1), Wnt signaling (VPS26A, ZNRF3), and RNA trafficking (DDX1).

Conclusion

Our findings enable improved evaluation and genetic counseling of PM and SM patients and further elucidate microcephaly pathways.

Surface- and voxel-based brain morphologic study in Rett and Rett-like syndrome with MECP2 mutation

Rett syndrome (RTT) is a rare congenital disorder which in most cases (95%) is caused by methyl-CpG binding protein 2 (MECP2) mutations. RTT is characterized by regression in global development, epilepsy, autistic features, acquired microcephaly, habitual hand clapping, loss of purposeful hand skills, and autonomic dysfunctions. Although the literature has demonstrated decreased volumes of the cerebrum, cerebellum, and the caudate nucleus in RTT patients, surface-based brain morphology including cortical thickness and cortical gyrification analyses are lacking in RTT. We present quantitative surface- and voxel-based morphological measurements in young children with RTT and Rett-like syndrome (RTT-l) with MECP2 mutations. The 8 structural T1-weighted MR images were obtained from 7 female patients with MECP2 mutations (3 classic RTT, 2 variant RTT, and 2 RTT-l) (mean age 5.2 [standard deviation 3.3] years old). Our analyses demonstrated decreased total volumes of the cerebellum in RTT/RTT-l compared to gender- and age-matched controls (t (22)=-2.93, p = .008, Cohen's d = 1.27). In contrast, global cerebral cortical surface areas, global/regional cortical thicknesses, the degree of global gyrification, and global/regional gray and white matter volumes were not statistically significantly different between the two groups. Our findings, as well as literature findings, suggest that early brain abnormalities associated with RTT/RTT-l (with MECP2 mutations) can be detected as regionally decreased cerebellar volumes. Decreased cerebellar volume may be helpful for understanding the etiology of RTT/RTT-l.

Oculomotor Function in Individuals With Rett Syndrome

BACKGROUND

Individuals with Rett syndrome (RTT) are notoriously reliant on the use of eye gaze as a primary means of communication. Underlying an ability to communicate successfully via eye gaze is a complex matrix of requirements, with an intact oculomotor system being just one element. To date, the underlying neural and motor pathways associated with eye gaze are relatively under-researched in RTT.

PURPOSE

This study was undertaken to plug this gap in knowledge and to further the understanding of RTT in one specific area of development and function, namely oculomotor function.

MATERIAL AND METHODS

The eye movements of 18 girls and young women with RTT were assessed by electronystagmography (ENG). This tested their horizontal saccadic and smooth pursuit eye movements as well as optokinetic nystagmus and vestibulo-ocular reflex. Their results were compared with normative data collected from 16 typically developing children and teenagers.

RESULTS

Overall, the individuals with RTT demonstrated a range of eye movements on a par with their typically developing peers. However, there were a number of difficulties in executing the ENG testing with the RTT cohort which made quantitative analysis tricky, such as reduced motivation and attention to test materials and low-quality electrode signals.

CONCLUSIONS

This study suggests that individuals with RTT have an intact oculomotor system. However, modifications should be made to the ENG assessment procedure to combat problems in testing and add strength to the results. Further investigation into these testing difficulties is warranted in order to inform such modifications.

Epigenetic cerebellar diseases

Epigenetics is a growing field of knowledge that is changing our understanding of pathologic processes. For many cerebellar disorders, recent discoveries of epigenetic mechanisms help us to understand their pathophysiology. In this chapter, a short explanation of each epigenetic mechanism (including methylation, histone modification, and miRNA) is followed by references to those cerebellar disorders in which relevant epigenetic advances have been made. The importance of normal timing and distribution of methylation during neurodevelopment is explained. Abnormal methylation and altered gene expression in the developing cerebellum have been related to neurodevelopmental disorders such as autism, Rett syndrome, and fragile X syndrome. DNA packaging by histones is another important epigenetic mechanism in cerebellar functioning. Current knowledge of histone abnormalities in cerebellar diseases such as Friedreich ataxia and spinocerebellar ataxias is reviewed, including implications for new therapeutic approaches to these degenerative diseases. Finally, micro RNAs, the third mechanism to modulate DNA expression, and their role in normal cerebellar development and disease are described. Understanding how genetic and epigenetic mechanisms interact not only in normal cerebellar development but also in disease is a great challenge. However, such understanding will lead to promising new therapeutic possibilities as is already occurring in other areas of medicine.

X-linked ataxias

X-linked cerebellar ataxias (XLCA) are an expanding group of genetically heterogeneous and clinically variable conditions characterized by cerebellar dysgenesis (hypoplasia, atrophy, or dysplasia) caused by gene mutations or genomic imbalances on the X chromosome. The neurologic features of XLCA include hypotonia, developmental delay, intellectual disability, ataxia, and other cerebellar signs. Normal cognitive development has also been reported. Cerebellar defects may be isolated or associated with other brain malformations or extraneurologic involvement. More than 20 genes on the X chromosome, mainly encoding for proteins involved in brain development and synaptic function that have been constantly or occasionally associated with a pathologic cerebellar phenotype, and several families with X-linked inheritance have been reported. Given the excess of males with ataxia, this group of conditions is probably underestimated and families of patients with neuroradiologic and clinical evidence of a cerebellar disorder should be counseled for high risk of X-linked inheritance.

Neuroanatomy in mouse models of Rett syndrome is related to the severity of Mecp2 mutation and behavioral phenotypes

Background

Rett syndrome (RTT) is a neurodevelopmental disorder that predominantly affects girls. The majority of RTT cases are caused by de novo mutations in methyl-CpG-binding protein 2 (MECP2), and several mouse models have been created to further understand the disorder. In the current literature, many studies have focused their analyses on the behavioral abnormalities and cellular and molecular impairments that arise from Mecp2 mutations. However, limited efforts have been placed on understanding how Mecp2 mutations disrupt the neuroanatomy and networks of the brain.

Methods

In this study, we examined the neuroanatomy of male and female mice from the Mecp2^tm1Hzo, Mecp2^tm1.1Bird/J, and Mecp2^tm2Bird/J mouse lines using high-resolution magnetic resonance imaging (MRI) paired with deformation-based morphometry to determine the brain regions susceptible to Mecp2 disruptions.

Results

We found that many cortical and subcortical regions were reduced in volume within the brains of mutant mice regardless of mutation type, highlighting regions that are susceptible to Mecp2 disruptions. We also found that the volume within these regions correlated with behavioral metrics. Conversely, regions of the cerebellum were differentially affected by the type of mutation, showing an increase in volume in the mutant Mecp2^tm1Hzo brain relative to controls and a decrease in the Mecp2^tm1.1Bird/J and Mecp2^tm2Bird/J lines.

Conclusions

Our findings demonstrate that the direction and magnitude of the neuroanatomical differences between control and mutant mice carrying Mecp2 mutations are driven by the severity of the mutation and the stage of behavioral impairments.

Cerebellar and Striatal Pathologies in Mouse Models of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition with a strong genetic component. To date, several hundred different genetic mutations have been identified to play a role in its aetiology. The heterogeneity of genetic abnormalities combined with the different brain regions where aberrations are found makes the search for causative mechanisms a daunting task. Even within a limited number of brain regions, a myriad of different neural circuit dysfunctions may lead to ASD. Here, we review mouse models that incorporate mutations of ASD risk genes causing pathologies in the cerebellum and striatum and highlight the vulnerability of related circuit dysfunctions within these brain regions in ASD pathophysiology.

The Role of MicroRNAs in Cerebellar Development and Autism Spectrum Disorder During Embryogenesis

MicroRNAs (miRNAs) are a class of small non-coding RNA molecules with wide-ranging and subtle effects on protein production. Their activity during the development of the cerebellum provides a valuable exemplar of how non-coding molecules may assist the development and function of the central nervous system and drive neurodevelopmental disorders. Three distinct aspects of miRNA contribution to early cerebellar development will here be reviewed. Aspects are the establishment of the cerebellar anlage, the generation and maturation of at least two principal cell types of the developing cerebellar microcircuit, and the etiology and early progression of autism spectrum disorder. It will be argued here that the autism spectrum is an adept model to explore miRNA impact on the cognitive and affective processes that descend from the developing cerebellum.

mGlu5 positive allosteric modulation normalizes synaptic plasticity defects and motor phenotypes in a mouse model of Rett syndrome

Rett syndrome (RS) is a neurodevelopmental disorder that shares many symptomatic and pathological commonalities with idiopathic autism. Alterations in protein synthesis-dependent synaptic plasticity (PSDSP) are a hallmark of a number of syndromic forms of autism; in the present work, we explore the consequences of disruption and rescue of PSDSP in a mouse model of RS. We report that expression of a key regulator of synaptic protein synthesis, the metabotropic glutamate receptor 5 (mGlu5) protein, is significantly reduced in both the brains of RS model mice and in the motor cortex of human RS autopsy samples. Furthermore, we demonstrate that reduced mGlu5 expression correlates with attenuated DHPG-induced long-term depression in the hippocampus of RS model mice, and that administration of a novel mGlu5 positive allosteric modulator (PAM), termed VU0462807, can rescue synaptic plasticity defects. Additionally, treatment of Mecp2-deficient mice with VU0462807 improves motor performance (open-field behavior and gait dynamics), corrects repetitive clasping behavior, as well as normalizes cued fear-conditioning defects. Importantly, due to the rationale drug discovery approach used in its development, our novel mGlu5 PAM improves RS phenotypes and synaptic plasticity defects without evoking the overt adverse effects commonly associated with potentiation of mGlu5 signaling (i.e. seizures), or affecting cardiorespiratory defects in RS model mice. These findings provide strong support for the continued development of mGlu5 PAMs as potential therapeutic agents for use in RS, and, more broadly, for utility in idiopathic autism.

Impaired hypercarbic and hypoxic responses from developmental loss of cerebellar Purkinje neurons: implications for sudden infant death syndrome

Impaired responsivity to hypercapnia or hypoxia is commonly considered a mechanism of failure in sudden infant death syndrome (SIDS). The search for deficient brain structures mediating flawed chemosensitivity typically focuses on medullary regions; however, a network that includes Purkinje cells of the cerebellar cortex and its associated cerebellar nuclei also helps mediate responses to carbon dioxide (CO2) and oxygen (O2) challenges and assists integration of cardiovascular and respiratory interactions. Although cerebellar nuclei contributions to chemoreceptor challenges in adult models are well described, Purkinje cell roles in developing models are unclear. We used a model of developmental cerebellar Purkinje cell loss to determine if such loss influenced compensatory ventilatory responses to hypercapnic and hypoxic challenges. Twenty-four Lurcher mutant mice and wild-type controls were sequentially exposed to 2% increases in CO2 (0-8%) or 2% reductions in O2 (21-13%) over 4 min, with return to room air (21% O2/79% N2/0% CO2) between each exposure. Whole body plethysmography was used to continuously monitor tidal volume (TV) and breath frequency (f). Increased f to hypercapnia was significantly lower in mutants, slower to initiate, and markedly lower in compensatory periods, except for very high (8%) CO2 levels. The magnitude of TV changes to increasing CO2 appeared smaller in mutants but only approached significance. Smaller but significant differences emerged in response to hypoxia, with mutants showing smaller TV when initially exposed to reduced O2 and lower f following exposure to 17% O2. Since cerebellar neuropathology appears in SIDS victims, developmental cerebellar neuropathology may contribute to SIDS vulnerability.

Cellular commitment in the developing cerebellum

The mammalian cerebellum is located in the posterior cranial fossa and is critical for motor coordination and non-motor functions including cognitive and emotional processes. The anatomical structure of cerebellum is distinct with a three-layered cortex. During development, neurogenesis and fate decisions of cerebellar primordium cells are orchestrated through tightly controlled molecular events involving multiple genetic pathways. In this review, we will highlight the anatomical structure of human and mouse cerebellum, the cellular composition of developing cerebellum, and the underlying gene expression programs involved in cell fate commitments in the cerebellum. A critical evaluation of the cell death literature suggests that apoptosis occurs in ~5% of cerebellar cells, most shortly after mitosis. Apoptosis and cellular autophagy likely play significant roles in cerebellar development, we provide a comprehensive discussion of their role in cerebellar development and organization. We also address the possible function of unfolded protein response in regulation of cerebellar neurogenesis. We discuss recent advancements in understanding the epigenetic signature of cerebellar compartments and possible connections between DNA methylation, microRNAs and cerebellar neurodegeneration. Finally, we discuss genetic diseases associated with cerebellar dysfunction and their role in the aging cerebellum.

β2-Adrenergic receptor agonist ameliorates phenotypes and corrects microRNA-mediated IGF1 deficits in a mouse model of Rett syndrome

Rett syndrome is a severe childhood onset neurodevelopmental disorder caused by mutations in methyl-CpG-binding protein 2 (MECP2), with known disturbances in catecholamine synthesis. Here, we show that treatment with the β2-adrenergic receptor agonist clenbuterol increases survival, rescues abnormalities in respiratory function and social recognition, and improves motor coordination in young male Mecp2-null (Mecp2(-/y)) mice. Importantly, we demonstrate that short-term treatment with clenbuterol in older symptomatic female heterozygous (Mecp2(-/+)) mice rescues respiratory, cognitive, and motor coordination deficits, and induces an anxiolytic effect. In addition, we reveal abnormalities in a microRNA-mediated pathway, downstream of brain-derived neurotrophic factor that affects insulin-like growth factor 1 (IGF1) expression in Mecp2(-/y) mice, and show that treatment with clenbuterol restores the observed molecular alterations. Finally, cotreatment with clenbuterol and recombinant human IGF1 results in additional increases in survival in male null mice. Collectively, our data support a role for IGF1 and other growth factor deficits as an underlying mechanism of Rett syndrome and introduce β2-adrenergic receptor agonists as potential therapeutic agents for the treatment of the disorder.

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.

Reorganization of circuits underlying cerebellar modulation of prefrontal cortical dopamine in mouse models of autism spectrum disorder

Imaging, clinical, and pre-clinical studies have provided ample evidence for a cerebellar involvement in cognitive brain function including cognitive brain disorders, such as autism and schizophrenia. We previously reported that cerebellar activity modulates dopamine release in the mouse medial prefrontal cortex (mPFC) via two distinct pathways: (1) cerebellum to mPFC via dopaminergic projections from the ventral tegmental area (VTA) and (2) cerebellum to mPFC via glutamatergic projections from the mediodorsal and ventrolateral thalamus (ThN md and vl). The present study compared functional adaptations of cerebello-cortical circuitry following developmental cerebellar pathology in a mouse model of developmental loss of Purkinje cells (Lurcher) and a mouse model of fragile X syndrome (Fmr1 KO mice). Fixed potential amperometry was used to measure mPFC dopamine release in response to cerebellar electrical stimulation. Mutant mice of both strains showed an attenuation in cerebellar-evoked mPFC dopamine release compared to respective wildtype mice. This was accompanied by a functional reorganization of the VTA and thalamic pathways mediating cerebellar modulation of mPFC dopamine release. Inactivation of the VTA pathway by intra-VTA lidocaine or kynurenate infusions decreased dopamine release by 50 % in wildtype and 20-30 % in mutant mice of both strains. Intra-ThN vl infusions of either drug decreased dopamine release by 15 % in wildtype and 40 % in mutant mice of both strains, while dopamine release remained relatively unchanged following intra-ThN md drug infusions. These results indicate a shift in strength towards the thalamic vl projection, away from the VTA. Thus, cerebellar neuropathologies associated with autism spectrum disorders may cause a reduction in cerebellar modulation of mPFC dopamine release that is related to a reorganization of the mediating neuronal pathways.

Neuroimaging endophenotypes in animal models of autism spectrum disorders: lost or found in translation?

RATIONALE

Autism spectrum disorder(s) (ASDs) is a neurodevelopmental disorder characterized by stereotyped behaviours and impairments in communication and social interactions. This heterogeneity has been a major obstacle in uncovering the aetiology and biomarkers of ASDs. Rodent models with genetic modifications or environmental insults have been created to study particular endophenotypes and bridge the gap between genetics and behavioural phenotypes. Translational neuroimaging modalities with their ability to screen the brain noninvasively and yield structural, biochemical and functional information provide a unique platform for discovery and evaluation of such endophenotypes in preclinical and clinical research.

OBJECTIVES

We reviewed literature on translational neuroimaging in rodent models of ASDs. The most prominent models will be described and the respective neuroimaging endophenotypes will be discussed with reference to human data. A perspective on future directions of translational neuroimaging in animal models of ASDs will be given.

RESULTS AND CONCLUSIONS

To date, we experience a proliferation of rodent models which recapitulate specific liabilities identified in ASDs patients. Translational neuroimaging in these models is emerging but is skewed towards magnetic resonance imaging (MRI) modalities. Volumetric and structural assessments of the brain are dominating and a host of endophenotypes have been reported that allude to findings in ASDs patients but with only few to converge among the models. Caveats of current studies are the diverging biological conditions related to genetic background and age of the animals. It is anticipated that longitudinal and functional assessments will gain much importance and will help elucidating mechanistic relationship between behavioural and structural endophenotypes.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

Is autism a disease of the cerebellum? An integration of clinical and pre-clinical research

Autism spectrum disorders are a group of neurodevelopmental disorders characterized by deficits in social skills and communication, stereotyped and repetitive behavior, and a range of deficits in cognitive function. While the etiology of autism is unknown, current research indicates that abnormalities of the cerebellum, now believed to be involved in cognitive function and the prefrontal cortex (PFC), are associated with autism. The current paper proposes that impaired cerebello-cortical circuitry could, at least in part, underlie autistic symptoms. The use of animal models that allow for manipulation of genetic and environmental influences are an effective means of elucidating both distal and proximal etiological factors in autism and their potential impact on cerebello-cortical circuitry. Some existing rodent models of autism, as well as some models not previously applied to the study of the disorder, display cerebellar and behavioral abnormalities that parallel those commonly seen in autistic patients. The novel findings produced from research utilizing rodent models could provide a better understanding of the neurochemical and behavioral impact of changes in cerebello-cortical circuitry in autism.

Long-term home cage activity scans reveal lowered exploratory behaviour in symptomatic female Rett mice

Numerous experimental models have been developed to reiterate endophenotypes of Rett syndrome, a neurodevelopmental disorder with a multitude of motor, cognitive and vegetative symptoms. Here, female Mecp2(Stop) mice [1] were characterised at mild symptomatic conditions in tests for anxiety (open field, elevated plus maze) and home cage observation systems for food intake, locomotor activity and circadian rhythms. Aged 8-9 months, Mecp2(Stop) mice presented with heightened body weight, lower overall activity in the open field, but no anxiety phenotype. Although home cage activity scans conducted in two different observation systems, PhenoMaster and PhenoTyper, confirmed normal circadian activity, they revealed severely compromised habituation to a novel environment in all parameters registered including those derived from a non-linear decay model such as initial exploration maximum, decay half-life of activity and span, as well as plateau. Furthermore, overall activity was significantly reduced in nocturnal periods due to reductions in both fast ambulatory movements, but also a slow lingering. In contrast, light-period activity profiles during which the amount of sleep was highest remained normal in Mecp2(Stop) mice. These data confirm the slow and progressive development of Rett-like symptoms in female Mecp2(Stop) mice resulting in a prominent reduction of overall locomotor activity, while circadian rhythms are maintained. Alterations in the time-course of habituation may indicate deficiencies in cognitive processing.

Autism spectrum disorder and the cerebellum

The cerebellum has been long known for its importance in motor learning and coordination. Recently, anatomical, clinical, and neuroimaging studies strongly suggest that the cerebellum supports cognitive functions, including language and executive functions, as well as affective regulation. Furthermore, the cerebellum has emerged as one of the key brain regions affected in autism. Here, we discuss our current understanding of the role of the cerebellum in autism, including evidence from genetic, molecular, clinical, behavioral, and neuroimaging studies. Cerebellar findings in autism suggest developmental differences at multiple levels of neural structure and function, indicating that the cerebellum is an important player in the complex neural underpinnings of autism spectrum disorder, with behavioral implications beyond the motor domain.

Stem cells and modeling of autism spectrum disorders

Human neurons, generated from reprogrammed somatic cells isolated from live patients, bring a new perspective on the understanding of Autism Spectrum Disorders (ASD). The new technology can nicely complement other models for basic research and the development of therapeutic compounds aiming to revert or ameliorate the condition. Here, we discuss recent advances on the use of stem cells and other models to study ASDs, as well as their limitations, implications and future perspectives.

X-linked disorders with cerebellar dysgenesis

X-linked disorders with cerebellar dysgenesis (XLCD) are a genetically heterogeneous and clinically variable group of disorders in which the hallmark is a cerebellar defect (hypoplasia, atrophy or dysplasia) visible on brain imaging, caused by gene mutations or genomic imbalances on the X-chromosome. The neurological features of XLCD include hypotonia, developmental delay, intellectual disability, ataxia and/or other cerebellar signs. Normal cognitive development has also been reported. Cerebellar dysgenesis may be isolated or associated with other brain malformations or multiorgan involvement. There are at least 15 genes on the X-chromosome that have been constantly or occasionally associated with a pathological cerebellar phenotype. 8 XLCD loci have been mapped and several families with X-linked inheritance have been reported. Recently, two recurrent duplication syndromes in Xq28 have been associated with cerebellar hypoplasia. Given the report of several forms of XLCD and the excess of males with ataxia, this group of conditions is probably underestimated and families of patients with neuroradiological and clinical evidence of a cerebellar disorder should be counseled for high risk of X-linked inheritance.

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.

Connecting genes to brain in the autism spectrum disorders

The autism spectrum disorders (ASDs) are a complex group of neuropsychiatric conditions involving language, social communication, and mental flexibility. Here, we attempt to place recent genetic advances within a developmental and anatomical context. Recent progress in identifying ASD candidate genes supports involvement of multiple brain regions, including the frontal lobes, anterior temporal lobes, caudate, and cerebellum. Understanding genetic data within an anatomical context will be critical to explain how individual risk factors operate to shape phenotypic presentation in patients.

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.

Widespread changes in dendritic and axonal morphology in Mecp2-mutant mouse models of Rett syndrome: evidence for disruption of neuronal networks

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene MECP2. Girls with RTT show dramatic changes in brain function, but relatively few studies have explored the structure of neural circuits. Examining two mouse models of RTT (Mecp2B and Mecp2J), we previously documented changes in brain anatomy. Herein, we use confocal microscopy to study the effects of MeCP2 deficiency on the morphology of dendrites and axons in the fascia dentata (FD), CA1 area of hippocampus, and motor cortex following Lucifer yellow microinjection or carbocyanine dye tracing. At 3 weeks of age, most (33 of 41) morphological parameters were significantly altered in Mecp2B mice; fewer (23 of 39) were abnormal in Mecp2J mice. There were striking changes in the density and size of the dendritic spines and density and orientation of axons. In Mecp2B mice, dendritic spine density was decreased in the FD (approximately 11%), CA1 (14-22%), and motor cortex (approximately 16%). A decreased spine head size (approximately 9%) and an increased spine neck length (approximately 12%) were found in Mecp2B FD. In addition, axons in the motor cortex were disorganized. In Mecp2J mice, spine density was significantly decreased in CA1 (14-26%). In both models, dendritic swelling and elongated spine necks were seen in all areas studied. Marked variation in the type and extent of changes was noted in dendrites of adjacent neurons. Electron microscopy confirmed abnormalities in dendrites and axons and showed abnormal mitochondria. Our findings document widespread abnormalities of dendrites and axons that recapitulate those seen in RTT.

Neurochemical changes in a mouse model of Rett syndrome: changes over time and in response to perinatal choline nutritional supplementation

Rett syndrome (RTT), the second leading cause of mental retardation in girls, is caused by mutations in the X-linked gene for methyl-CpG-binding protein 2 (MeCP2), a transcriptional repressor. In addition to well-documented neuroanatomical and behavioral deficits, RTT is characterized by reduced markers of cholinergic activity and general neuronal health. Previously, we have shown that early postnatal choline (Cho) supplementation improves behavioral and neuroanatomical symptoms in a mouse model of RTT (Mecp2(1lox) mice). In this study, we use NMR spectroscopy to quantify the relative amounts of Cho, Glutamate (Glu), Glutamine (Gln), and N-acetyl aspartate (NAA) in the brains of wild type and mutant mice at 21, 35, and 42 days of age and in mice receiving postnatal Cho supplementation. We find that the mutant mice have reduced levels of Cho, Glu, and NAA, but elevated Gln levels, compared with their wild type littermates. These differences emerge at different developmental ages. Cho supplementation increases NAA levels, a marker of neuronal integrity, but has no effect on Cho, Glu, or Gln. These data suggest that postnatal nutritional supplementation may improve neuronal function and could serve as a therapeutic agent for human RTT patients.

FOXG1 is responsible for the congenital variant of Rett syndrome

Rett syndrome is a severe neurodevelopmental disease caused by mutations in the X-linked gene encoding for the methyl-CpG-binding protein MeCP2. Here, we report the identification of FOXG1-truncating mutations in two patients affected by the congenital variant of Rett syndrome. FOXG1 encodes a brain-specific transcriptional repressor that is essential for early development of the telencephalon. Molecular analysis revealed that Foxg1 might also share common molecular mechanisms with MeCP2 during neuronal development, exhibiting partially overlapping expression domain in postnatal cortex and neuronal subnuclear localization.

Comparative study of brain morphology in Mecp2 mutant mouse models of Rett syndrome

Rett syndrome (RTT) is caused by mutations in the X-linked gene MECP2. While patients with RTT show widespread changes in brain function, relatively few studies document changes in brain structure and none examine in detail whether mutations causing more severe clinical phenotypes are linked to more marked changes in brain structure. To study the influence of MeCP2-deficiency on the morphology of brain areas and axonal bundles, we carried out an extensive morphometric study of two Mecp2-mutant mouse models (Mecp2B and Mecp2J) of RTT. Compared to wildtype littermates, striking changes included reduced brain weight ( approximately 13% and approximately 9%) and the volumes of cortex ( approximately 11% and approximately 7%), hippocampus (both by approximately 8%), and cerebellum ( approximately 12% and 8%) in both mutant mice. At 3 weeks of age, most (24 of 47) morphological parameters were significantly altered in Mecp2B mice; fewer (18) were abnormal in Mecp2J mice. In Mecp2B mice, significantly lower values for cortical area were distributed along the rostrocaudal axis, and there was a reduced length of the olfactory bulb ( approximately 10%) and periaqueductal gray matter ( approximately 16%). In Mecp2J mice, while there was significant reduction in rostrocaudal length of cortex, this parameter was also abnormal in hippocampus ( approximately 10%), periaqueductal gray matter ( approximately 13%), fimbria ( approximately 18%), and anterior commissure ( approximately 10%). Our findings define patterns of Mecp2 mutation-induced changes in brain structure that are widespread and show that while some changes are present in both mutants, others are not. These observations provide the underpinning for studies to further define microarchitectural and physiological consequences of MECP2 deficiency.

Longitudinal brain MRI study in a mouse model of Rett Syndrome and the effects of choline

Rett Syndrome (RTT), the second most common cause of mental retardation in girls, is associated with mutations of an X-linked gene encoding the transcriptional repressor protein MeCP2. Mecp2(1lox) mutant mice express no functional MeCP2 protein and exhibit behavioral abnormalities similar to those seen in RTT patients. Here we monitor the development of both whole brain and regional volumes between 21 and 42 days of age in this model of RTT using MRI. We see decreases in whole brain volumes in both male and female mutant mice. Cerebellar and ventricular volumes are also decreased in RTT males. Previous work has suggested that perinatal choline supplementation alleviates some of the behavioral deficits in both male and female Mecp2(1lox) mutant mice. Here we show that perinatal choline supplementation also positively affects whole brain volume in heterozygous females, and cerebellar volume in male RTT mice.

Selective cerebral volume reduction in Rett syndrome: a multiple-approach MR imaging study

BACKGROUND AND PURPOSE

Previous studies have examined volumetric abnormalities in Rett syndrome (RTT), using MR imaging and focusing on selective changes. However, these studies preceded the identification of MECP2 as the gene mutated in most RTT cases. We studied regional brain volume changes as noted by MR imaging in girls with RTT who had mutations in the MECP2 gene and more or less severe clinical outcomes to further characterize the neuroanatomy of RTT and its correlations with clinical severity.

MATERIALS AND METHODS

Complementary semiautomated Talairach- and voxel-based approaches were used to study spoiled gradient-recalled acquisition sequence MR imaging scans from 23 girls with MECP2 mutations/RTT, including a pair of discordant monozygotic twins and 25 age-matched control girls. Both absolute and relative volumetric changes were examined to account for the well-documented global reduction in brain volume seen in RTT.

RESULTS

Absolute volumetric reductions were observed throughout the brain in RTT. Selective/relative decreases in parietal lobe gray matter, particularly in the dorsal parietal region, and mild, diffuse reductions in cortical white matter were observed in the RTT group compared with control subjects. In girls with RTT and a more severe phenotype, anterior frontal lobe volumes were relatively more reduced. Twin comparisons revealed selective preservation of the occipital cortex.

CONCLUSION

Selective reductions of dorsal parietal gray matter and preservation of the occipital cortex seem to be basic neuroanatomic features of RTT, whereas preferential reduction of the anterior frontal lobe appears to be a correlate of clinical severity in this disorder. The most affected brain regions include those that may underlie key functional deficits observed in RTT.

Enhanced cell death in MeCP2 null cerebellar granule neurons exposed to excitotoxicity and hypoxia

Rett syndrome (RTT) is associated with mutations in the transcriptional repressor gene MeCP2. Although the clinical and neuropathological signs of RTT suggest disrupted synaptic function, the specific role of methyl-CpG binding protein 2 (MeCP2) in postmitotic neurons remains relatively unknown. We examined whether MeCP2 deficiency in central neurons contributes to the neuropathogenesis in RTT. Primary cerebellar granule neuronal cultures from wild-type (WT) and MeCP2-/- mice were exposed to N-methyl-d-aspartate (NMDA) and AMPA-induced excitotoxicity and hypoxic-ischemic insult. The magnitude of cell death in MeCP2-/- cells after excitotoxicity and hypoxia was greater than in the WT littermate control cultures and occurred after shorter exposures that usually, in the WT, would not cause cell death. Pretreatment with the growth factor fibroblast growth factor 1 (FGF-1) under conditions at which WT cells showed complete neuroprotection, only partially protected MeCP2-/- cells. To elucidate specifically the effects of MeCP2 knockout (KO) on cell death, we examined two death cascade pathways. MeCP2-/- neurons exposed to 6 h of hypoxia exhibited enhanced activation of the proapoptotic caspase-3 and increased mitochondrial release of apoptosis inducing factor (AIF) compared with WT neurons, which did not show significant changes. However, pretreatment with the caspase inhibitor ZVAD-FMK had little or no effect on AIF release and its subcellular translocation to the nucleus, suggesting caspase-independent AIF release and their independent contribution to hypoxia-induced cell death. Reintroduction of intact MeCP2 gene in MeCP2-/- cells or MeCP2 gene silencing by MeCP2siRNA in WT cells further confirmed the differential sensitivity of the WT and MeCP2-/- cells and suggest a direct role of MeCP2 in cell death. These results clearly demonstrate increased cell death occurred in neurons lacking MeCP2 expression via both caspase- and AIF-dependent apoptotic mechanisms. Our findings suggest a novel, yet unknown, role for MeCP2 in central neurons in the control of neuronal response to cell death.

Behavioral and anatomical abnormalities in Mecp2 mutant mice: a model for Rett syndrome

Over 90% of Rett syndrome (RTT) cases have a mutation in the X-linked gene encoding methyl CpG binding-protein 2 (MeCP2). A mouse model that reprises clinical manifestations of the disease would be valuable for examining disease mechanisms. Here, we characterize physical and behavioral measures, as well as brain region volumes in young adult mice that have mutations in mouse methyl CpG binding-protein 2 gene (Mecp2) to serve as a baseline for other studies. Hemizygous males, which produce no functional protein, exhibit hypoactivity and abnormalities in locomotion, stereotypies, and anxiety reminiscent of the clinical condition. The mutant males also exhibit cognitive deficits in fear conditioning and object recognition relative to wildtypes. Volumetric analyses of male brains revealed a 25% reduction in whole brain volume in mutants relative to wildtypes; regional differences were also apparent. Mutants had decreased volumes in three specific brain regions: the amygdala (39%), hippocampus (21%), and striatum (29%). Heterozygous females, which produce varying amounts of functional protein, displayed a less severe behavioral phenotype. The mutant females exhibit abnormalities in locomotion, anxiety measures, and cognitive deficits in object recognition in an open field. This study provides the first evidence that the abnormal motor and cognitive behavioral phenotype in Mecp2 mice is consistent with specific volume reductions in brain regions associated with these behaviors, and shows how these data parallel the human condition. The Mecp2 mutant mice provide a very good model in which to examine molecular and behavioral mechanisms, as well as potential therapeutic interventions in RTT.

Brain magnetic resonance study of Mecp2 deletion effects on anatomy and metabolism

Rett syndrome, a neurodevelopmental X-linked disorder, represents the most important genetic cause of severe mental retardation in the female population and results from a mutation in the gene encoding methyl-CpG-binding protein 2 (MECP2). We report here the first characterization of Mecp2-null mice, by in vivo magnetic resonance imaging and spectroscopy, delineating the cerebral phenotype associated with the lack of Mecp2. We performed a morphometric study that revealed a size reduction of the whole brain and of structures involved in cognitive and motor functions (cerebellum and motor cortex). Significant metabolic anomalies, including reduced N-acetylaspartate, myo-inositol, and glutamine plus glutamate, and increased choline levels were evidenced. These findings indicate that not only neuronal but also glial metabolism is affected in Mecp2-null mice. Furthermore, we uncovered an important reduction of brain ATP level, a hitherto undetected anomaly of energy metabolism that may reflect and contribute to cerebral injury and dysfunction.

Rett syndrome and neuronal development

The clinical signs of Rett syndrome, as well as neuropathology and brain imaging, suggest that the disorder disrupts neuronal circuits. Studies using receptor autoradiography demonstrate abnormalities in the density of excitatory glutamate and inhibitory gamma-aminobutyric acid (GABA) synaptic receptors in postmortem brain from young female subjects with Rett syndrome. MeCP2, the protein that is abnormal in most female individuals with Rett syndrome, is expressed predominantly in neurons and appears during development at the time of synapse formation. Studies of nasal epithelium from patients with Rett syndrome show that the maturation of olfactory receptor neurons is impeded prior to the time of synapse formation. Recent reports indicate that MeCP2 controls the expression of brain-derived neurotrophic factor and the DNA-binding homeobox protein Dlx5. Brain-derived neurotrophic factor enhances glutamate neurotransmission at excitatory synapses, whereas Dlx5 is expressed in most GABAergic neurons and stimulates the synthesis of GABA. Taken together, this information supports the hypothesis that Rett syndrome is a genetic disorder of synapse development, especially synapses that use glutamate and GABA as neurotransmitters.

Neuropathology of Rett syndrome

Rett syndrome is a sporadic disorder (except for a few familial cases) occurring in 1 in 10,000 to 1 in 23,000 girls worldwide. It is associated with profound mental and motor handicap. About 90% of cases involve a mutation in the methyl-CpG binding protein 2 gene (MECP2). The role of this gene in the pathogenesis of this enigmatic disorder is being extensively investigated in animal models. Rett syndrome is associated with a complex phenotype that is unique in every aspect of its presentation, clinical physiology, chemistry, and pathology. Years of concentrated observations have defined the clinical presentation of classic Rett syndrome and its variants and related features (eg, neurophysiologic, radiologic, chemical, metabolic, and anatomic). This article reviews the neuropathology of Rett syndrome, which involves individual neurons, perhaps selected neurons, of decreased size, dendritic branching, and numbers of spines. This article also summarizes the studies in the human and mouse brain with Rett syndrome that are beginning to reveal the disorder's pathoetiology.

Developmental expression of methyl-CpG binding protein 2 is dynamically regulated in the rodent brain

The gene encoding methyl-CpG binding protein 2 (MeCP2) is mutated in the large majority of girls that have Rett Syndrome (RTT), an X-linked neurodevelopmental disorder. To better understand the developmental role of MeCP2, we studied the ontogeny of MeCP2 expression in rat brain using MeCP2 immunostaining and Western blots. MeCP2 positive neurons were present throughout the brain at all ages examined, although expression varied by region and age. At early postnatal ages, regions having neurons that were generated early and more mature had the strongest MeCP2 expression. Late developing structures including cortex, hippocampus and cerebellum exhibited the most significant changes in MeCP2 expression. Of these regions, the cerebellum showed the most striking cell-specific changes in MeCP2 expression. For example, the early-generated Purkinje cells became MeCP2 positive by P6, while the late-generated granule cells did not express MeCP2 until the fourth postnatal week. The timing of MeCP2 expression in the granule cell layer is coincident with the onset of granule cell synapse formation. Although more subtle, the degree of MeCP2 expression in cortex and hippocampus was most closely correlated with synaptogenesis in both regions. Our finding that MeCP2 expression is correlated with synaptogenesis is consistent with the hypothesis that Rett Syndrome is caused by defects in the formation or maintenance of synapses.