Reduced neuronal size and mTOR pathway activity in the Mecp2 A140V Rett syndrome mouse model

Sampathkumar Rangasamy,Shannon Olfers,Brittany Gerald,Alex Hilbert,Sean Svejda,Vinodh Narayanan

doi:10.12688/f1000research.8156.1

Abstract

Rett syndrome(RTT) is a neurodevelopmental disorder caused by mutation in the X-linked MECP2 gene, encoding methyl-CpG-binding protein 2. We have created a mouse model ( Mecp2 A140V "knock-in" mutant) expressing the recurrent human MECP2 A140V mutation linked to an X-linked mental retardation/Rett syndrome phenotype. Morphological analyses focused on quantifying soma and nucleus size were performed on primary hippocampus and cerebellum granule neuron (CGN) cultures from mutant ( Mecp2A140V/y) and wild type ( Mecp2+/y) male mice. Cultured hippocampus and cerebellar granule neurons from mutant animals were significantly smaller than neurons from wild type animals. We also examined soma size in hippocampus neurons from individual female transgenic mice that express both a mutant (maternal allele) and a wild type Mecp2 gene linked to an eGFP transgene (paternal allele). In cultures from such doubly heterozygous female mice, the size of neurons expressing the mutant (A140V) allele also showed a significant reduction compared to neurons expressing wild type MeCP2, supporting a cell-autonomous role for MeCP2 in neuronal development. IGF-1 (insulin growth factor-1) treatment of neuronal cells from Mecp2 mutant mice rescued the soma size phenotype. We also found that Mecp2 mutation leads to down-regulation of the mTOR signaling pathway, known to be involved in neuronal size regulation. Our results suggest that i) reduced neuronal size is an important in vitro cellular phenotype of Mecp2 mutation in mice, and ii) MeCP2 might play a critical role in the maintenance of neuronal structure by modulation of the mTOR pathway. The definition of a quantifiable cellular phenotype supports using neuronal size as a biomarker in the development of a high-throughput, in vitro assay to screen for compounds that rescue small neuronal phenotype ("phenotypic assay").

Highlights

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked MECP2 gene encoding methyl-CpG binding protein 21
The Mecp[2] A140V is one such mouse model developed in our laboratory and has been previously described to have cellular abnormalities similar to those seen in RTT11
We have demonstrated a reduction in soma and nucleus size in cultured hippocampal neurons from hemizygous male Mecp[2] A140V mutant mice

Summary

Introduction

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked MECP2 gene encoding methyl-CpG binding protein 21. MECP2 gene mutations have been linked to a broad range of other clinical and neurological phenotypes One such mutation is MECP2 A140V, seen in both male and female subjects with non-classic Rett phenotypes such as intellectual disability, parkinsonism, and neuropsychiatric symptoms[6,7,8,9]. Neuropathological studies in human RTT cases have shown a reduction in brain size, increased cell packing density, and smaller neuronal size (soma)[12,13,14,15,16,17]. The most common structural abnormalities reported in Mecp[2] mutant mouse models (including the Mecp2-null mice) are thinning of the cortical layers, reduction in neuronal soma size, and decreased dendritic complexity[11,19,20,21,22]. Neuronal soma size is considered a robust and reliable marker for MeCP2 function[23]

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