Mutations in the methyl-CpG binding protein 2 (MECP2) gene cause Rett syndrome (RTT), a neurodevelopmental disorder characterized by the loss of language and motor skills during early childhood. We generated mice with a truncating mutation similar to those found in RTT patients. These mice appeared normal and exhibited normal motor function for about 6 weeks, but then developed a progressive neurological disease that includes many features of RTT: tremors, motor impairments, hypoactivity, increased anxiety-related behavior, seizures, kyphosis, and stereotypic forelimb motions. Additionally, we show that although the truncated MeCP2 protein in these mice localizes normally to heterochromatic domains in vivo, histone H3 is hyperacetylated, providing evidence that the chromatin architecture is abnormal and that gene expression may be misregulated in this model of Rett syndrome.
Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by expansion of a polyglutamine tract in ataxin-1. In affected neurons of SCA1 patients and transgenic mice, mutant ataxin-1 accumulates in a single, ubiquitin-positive nuclear inclusion. In this study, we show that these inclusions stain positively for the 20S proteasome and the molecular chaperone HDJ-2/HSDJ. Similarly, HeLa cells transfected with mutant ataxin-1 develop nuclear aggregates which colocalize with the 20S proteasome and endogenous HDJ-2/HSDJ. Overexpression of wild-type HDJ-2/HSDJ in HeLa cells decreases the frequency of ataxin-1 aggregation. These data suggest that protein misfolding is responsible for the nuclear aggregates seen in SCA1, and that overexpression of a DnaJ chaperone promotes the recognition of a misfolded polyglutamine repeat protein, allowing its refolding and/or ubiquitin-dependent degradation.
Loss-of-function mutations or abnormal expression of the X-linked gene encoding methyl CpG binding protein 2 (MeCP2) cause a spectrum of postnatal neurodevelopmental disorders including Rett syndrome (RTT), nonsyndromic mental retardation, learning disability, and autism. Mice expressing a truncated allele of Mecp2 (Mecp2 308 ) reproduce the motor and social behavior abnormalities of RTT; however, it is not known whether learning deficits are present in these animals. We investigated learning and memory, neuronal morphology, and synaptic function in Mecp2 308 mice. Hippocampus-dependent spatial memory, contextual fear memory, and social memory were significantly impaired in Mecp2 308 mutant males (Mecp2 308/Y ). The morphology of dendritic arborizations, the biochemical composition of synaptosomes and postsynaptic densities, and brain-derived neurotrophic factor expression were not altered in these mice. However, reduced postsynaptic density cross-sectional length was identified in asymmetric synapses of area CA1 of the hippocampus. In the hippocampus of symptomatic Mecp2 308/Y mice, Schaffer-collateral synapses exhibited enhanced basal synaptic transmission and decreased paired-pulse facilitation, suggesting that neurotransmitter release was enhanced. Schaffer-collateral long-term potentiation (LTP) was impaired. LTP was also reduced in the motor and sensory regions of the neocortex. Finally, very early symptomatic Mecp2 308/Y mice had increased basal synaptic transmission and deficits in the induction of long-term depression. These data demonstrate a requirement for MeCP2 in learning and memory and suggest that functional and ultrastructural synaptic dysfunction is an early event in the pathogenesis of RTT.
To faithfully recreate the features of the human neurodegenerative disease spinocerebellar ataxia type 1 (SCA1) in the mouse, we targeted 154 CAG repeats into the endogenous mouse locus. Sca1(154Q/2Q) mice developed a progressive neurological disorder that resembles human SCA1, featuring motor incoordination, cognitive deficits, wasting, and premature death, accompanied by Purkinje cell loss and age-related hippocampal synaptic dysfunction. Mutant ataxin-1 solubility varied with brain region, being most soluble in the neurons most vulnerable to degeneration. Solubility decreased overall as the mice aged; Purkinje cells, the most affected in SCA1, did not form aggregates of mutant protein until an advanced stage of disease. It appears that those neurons that cannot sequester the mutant protein efficiently and thereby curb its toxicity suffer the worst damage from polyglutamine-induced toxicity.
Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Previous data have shown that MECP2 RNA is present in all mouse and human tissues tested, but the timing of expression and regional distribution have not been explored. We investigated the spatial and temporal distribution of the MeCP2 protein during mouse and human development. We found that in the adult mouse, MeCP2 is high in the brain, lung and spleen, lower in heart and kidney, and barely detectable in liver, stomach and small intestine. There was no obvious correlation between protein levels and RNA levels, suggesting that translation may be post-transcriptionally regulated by tissue-specific factors. The timing of MeCP2 expression in mouse and human correlated with the maturation of the central nervous system, with the ontogenetically older structures such as the spinal cord and brainstem becoming positive before newer structures such as the hippocampus and cerebral cortex. In the cortex, MeCP2 first appeared in the Cajal-Retzius cells, then in the neurons of the deeper, more mature cortical layers, and finally in the neurons of the more superficial layers. The MeCP2 protein was eventually present in a majority of neurons but was absent from glial cells. Our data suggest that MeCP2 may become abundant only once a neuron has reached a certain degree of maturity, and that this may explain some aspects of the RTT phenotype.
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