Altered dendritic spines morphology in the hippocampus of MeCP2-deficent mice

Rett syndrome (RTT) is a progressive neurodevelopmental disorder that affects 1: 10000-15000 girls worldwide. Mutations of the methyl-CpG binding protein 2 (MeCP2), a gene encoding a protein involved in transcription repression, cause the majority of RTT cases. Two targets of MeCP2 have been individuated, Dlx5 a protein that induces glutamic acid decarboxylase expression and promotes the differentiation of GABAergic neurons and BDNF (brain−derived neurotrophic factor), a major neurotrophin involved in brain development. MeCP2 loss of function mice has been generated that show manifestations resembling RTT-like symptoms. However, very little is known on the cellular and molecular consequences that are associated with MeCP2 disruption in the brain of these mutants. Here, we used electron microscopy to investigate whether normal MeCP2 function is important in determining structural and molecular architecture of excitatory and inhibitory synaptic connections. Ultrastructural analysis of neuronal circuits was performed in symptomatic MeCP2-hemizygous (MeCP2-/y) mice and wild-type male littermate. Light-microscopic examination of plastic-embedded semithin sections did not reveal major neurocytological defects in the hippocampus as well as in the somatosensory cortex of these mutants. A quantitative analysis using the dissector method revealed that MeCP2 deletion induced no significant differences in asymmetrical/excitatory synapse density in CA1 stratum radiatum (2.04 ± 0.05 vs. 2,18 ± 0,03 per μm3). In contrast, the morphometric analysis of dendritic spines area and PSDs length showed a specific effect of MeCP2 mutation. The mean cross-section area of dendritic spines was decreased significantly by about 13% in MeCP2-/y compared to wild type controls (0.0823 ± 0.003 vs. 0.0948 ± 0.004 μm2, respectively; P < 0.05; n = 156 and 119). In addition, we observed a similar significant decrease of the length of the PSDs present at axo-spinous contacts of MeCP2-/y mice (181.04 ± 1.07 vs. 215.61 ± 1.52 μm, P < 0.05; n = 100 and 100). Immunohistochemical studies are underway to study the molecular organization of both excitatory and inhibitory synapses in different brain areas. Our results suggest that loss of MeCP2 expression is associated with alterations of the structure of excitatory axo-spinous synapses and support the hypothesis that RTT may be a disorder of synaptic modulation or maintenance.