Rescue of Neuroanatomical Impairments Following Mecp2 Reactivation in Adult Mice

Background: Rett syndrome is a neurodevelopment disorder caused by sporadic mutations in the X-linked gene, Mecp2¹. Interestingly, reactivation of Mecp2 leads to a rescue of the neuronal and behaviour impairments in adult mice^2,3. Although these findings are exciting, these studies quantified rescue in a limited number of brain regions and restricted their analyses to a few time points. However, to fully understand the dynamics, a non-invasive, high-throughput method is required that can quantify where the remodelling of the cellular structure occurs across the entire brain and the time course of these changes. Magnetic Resonance Imaging (MRI) is an imaging modality that has the ability to longitudinally acquire high-resolution neuroanatomical information from the mouse brain. 

Objectives: We scanned mice longitudinally with MRI to determine: a) the brain regions affected by Mecp2 silencing and b) the spatial and temporal changes that occur in the brain following Mecp2 reactivation. 

Methods: At P50, male Mecp2-STOP/Cre and Cre littermates were scanned in vivo with a Manganese-enhanced MRI protocol (MEMRI), followed by 4 weeks of treatment with oil or tamoxifen to reactivate Mecp2. At P80, a follow up MEMRI scan was conducted. The acquired images were aligned using a series of iterative linear and nonlinear registrations steps⁴. This process generates a consensus average representing each individual brain and the deformations of each image from this average. The Jacobian-determinants, which represents local volume changes, were then extracted from the deformations fields and used as the dependent variable in the statistical analyses to assess volumetric changes.

Results: At P50, Mecp2/Cre mice began to show reduced mobility and breathing impairments. At this age, total brain volume of Mecp2/Cre (n=9) mice was 14% smaller than Cre (n=5) controls (t-statistic=4.07, p < 0.001). Following normalization for brain volume, volumetric decreases were found across the cortex, striatum, hippocampus, while volumetric increases were found in the cerebellum and medulla. Following the baseline scan, Mecp2/Cre mice treated with oil showed a steady increase in body weight (beta=0.33, p < 0.05) and phenotypes reaching a score of  5 around 80 days of age. Conversely, tamoxifen treatment reduced the increase in body weight of Mecp2/Cre mice (beta=0.07, p=0.42) and reversed the increase in phenotype score from 2 at 60 days, to 0 by 80 days. At P80, the brain of Mecp2/Cre mice treated with tamoxifen (n=4) was 30% larger than Mecp2/Cre treated with oil (n=2) (t-statistic=8.71, p < 0.001). Interestingly, the tamoxifen treated Mecp2/Cre brain grew at a rate of 1.73mm³ per day (beta=1.73, p < 0.05), eventually reaching Cre control volumes at 80 days (t-statistic=0.32,  p=0.76). A regional-based analysis demonstrated a lack of regional specificity, with all brain regions increasing in volume following Mecp2 reactivation.

Conclusions: Our results demonstrate that Mecp2 mice have volumetric differences across many brains networks early in adult life. However, following Mecp2 reactivation, a substantial neuroanatomical rescue occurs across the brain eventually normalizing with controls.