Expression of the methyl-CpG-binding protein MeCP2 in rat brain. An ontogenetic study
Introduction
The classical form of Rett syndrome (RS), found almost exclusively in females, is a disorder of arrested neuronal development Armstrong, 2001, Kerr, 2002, Percy, 2002. Affected girls are born healthy, appear to develop normally until age 6–18 months, achieving expected motor and language skills (Hagberg et al., 1983). Their neurological development is then arrested and begins regressing in a predictable pattern (Hagberg and Witt-Engerstrom, 1987). The disorder is characterized essentially by severe cognitive impairment, autistic behavior, stereotypic movements and seizures (Naidu, 1997). Functional deficits are associated with abnormal levels of neurotransmitters and peptides (Wenk, 1997). In most cases, the syndrome was found associated with mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) Amir et al., 1999, Shahbazian and Zoghbi, 2002. Once bound to methylated DNA, MeCP2 is thought to silence transcription of downstream genes by virtue of its interaction with a histone deacetylase/Sin3 complex Jones et al., 1998, Nan et al., 1997. To understand the molecular basis of RS, it is essential to identify the target genes of MeCP2 repression. Unfortunately, a recent report shows that brains of MeCP2 mutant mice have few, if any, genes that are significantly changed in their level of expression (Tudor et al., 2002).
Three MeCP2 transcripts are found at varying levels in most tissues examined, with no obvious preference for nervous tissue, except that the brain displays preferentially a 10-kb transcript compared with other transcripts (Reichwald et al., 2000). Few data are available on the expression of MeCP2 protein in the central nervous system. Unlike the mRNA, the protein is expressed at higher levels in the brain than in many other tissues LaSalle et al., 2001, Shahbazian et al., 2002b. Within the brain, MeCP2 is found in neurons but is not expressed to detectable levels in differentiated astrocytes Akbarian et al., 2001, Jung et al., 2003. Here, we studied the expression of MeCP2 protein in various brain structures from normal rat. Several cortical areas with major functional implication were selected, in addition to the septal nuclei and the hippocampus that may be involved in the cognitive deficits observed in RS, and the striatum that was also suggested to be implicated in the disease Armstrong, 2001, Kerr, 2002. Since RS is characterized as a disease of arrested development after birth, we followed the protein expression throughout rat life, from birth to old age corresponding to 112 weeks.
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Animals and tissue preparation
Female Long–Evans rats (Janvier, France) were housed with a fixed 12-h light–dark cycle and free access to food and water. All procedures involving animal care were conducted in conformity with institutional guidelines, in compliance with current laws and policies (council directive 87848, Service Vétérinaire de la Santé et de la Protection animales). Animals were sacrificed by an overdose of pentobarbital (100 mg/kg, ip) and perfused transcardially with saline followed by 4% formaldehyde in
Results
We studied MeCP2 protein expression in several brain structures of normal Long–Evans rat from birth to 2 years of age. Fig. 2 illustrates MeCP2 immunohistochemistry of cells from selected areas: the olfactory nuclei, frontal cortex, dorsal caudate and CA2/CA3 region of dorsal hippocampus of newborn rats (taken during the 5-h postnatal period) and rats sacrificed at various ages. A very large majority of cells displayed MeCP2 immunoreactivity exclusively in the nucleus, in agreement with the
Discussion
The present ontogenetic study on MeCP2 distribution demonstrates a very heterogeneous distribution of MeCP2 protein in normal rat brain, as assessed by measuring the amount of protein as well as percentages of immunoreactive cells. It has to be kept in mind, however, when interpreting the latter data, that MeCP2 is found primarily in neurons and not in glia (Akbarian et al., 2001). In effect, it was not detected in rat astrocytes (Jung et al., 2003), whereas another study reported some low
Acknowledgements
We thank Dr. P. Jouvert and A. Lazaris for help with immunohistological techniques, and Dr. K. Langley for critical reading of the manuscript.
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2010, Neurobiology of DiseaseDendritic spine pathologies in hippocampal pyramidal neurons from Rett syndrome brain and after expression of Rett-associated MECP2 mutations
2009, Neurobiology of DiseaseCitation Excerpt :The reduction in dendritic area together with the marked decrease in dendritic spine density strongly suggests that impaired synaptic transmission is a likely pathogenic consequence of MECP2 mutations causing RTT. Indeed, the increase in neuronal expression of MECP2/Mecp2 during early brain development suggests the importance of this protein in synapse formation and maintenance (Akbarian et al., 2001; Cassel et al., 2004; Jung et al., 2003; Kaufmann et al., 2005; Mullaney et al., 2004; Shahbazian et al., 2002b). While hippocampal and cortical synaptic dysfunction in Mecp2-based mouse models of RTT has been extensively studied, observations regarding neuronal, dendritic and synaptic pathologies have produced varied results (Asaka et al., 2006; Chao et al., 2007; Dani et al., 2005; Fukuda et al., 2005; Gemelli et al., 2006; Jugloff et al., 2005; Kishi and Macklis, 2004; Moretti et al., 2006; Nelson et al., 2006; Smrt et al., 2007; Zhou et al., 2006).
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2009, Neurobiology of DiseaseCitation Excerpt :Furthermore, MeCP2 expression in cortical neurons increases during brain development (Akbarian et al., 2001; Cohen et al., 2003, Jung et al., 2003; LaSalle et al., 2001; Shahbazian et al., 2002; Zoghbi, 2003). The developmental increase in its expression strongly suggests that MeCP2 plays a critical role in neuronal terminal differentiation, i.e. the development of axons, dendrites and dendritic spines leading to proper synapse formation and maturation (Cassel et al., 2004; Kaufmann et al., 2005; Kishi and Macklis, 2004; Matarazzo et al., 2004; Matarazzo and Ronnett, 2004; Mullaney et al., 2004). Missense mutations in MECP2 identified in RTT patients cluster in the methyl-binding domain (MBD) and transcriptional repressor domain (TRD) of the protein, suggesting that they represent loss-of-function mutations.
Expression of methyl CpG binding protein 2 (Mecp2) during the postnatal development of the mouse brainstem
2008, Brain ResearchCitation Excerpt :The brainstem represents the key part of the brain involved in the autonomic regulation and an immaturity of the brainstem has been proposed to explain autonomic dysfunctions observed in RS (Julu et al., 1997; Julu et al., 2005, Paterson et al., 2005). To better understand the role played by MeCP2 in the development and maturation of the brain, several studies have examined the spatiotemporal expression of Mecp2 mRNA and MeCP2 protein in different areas of the brain (Shahbazian et al., 2002; Jung et al., 2003; Cassel et al., 2004; Mullaney et al., 2004; Kishi and Macklis, 2004; Pelka et al., 2005). However, despite the growing importance of the brainstem in neurological disorders caused by MECP2 mutations, none of them was dedicated to study this area.