Unconventional transcriptional response to environmental enrichment in a mouse model of Rett syndrome

Abstract

Background: Rett syndrome (RTT) is an X-linked postnatal neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG binding protein 2 (MeCP2) and one of the leading causes of mental retardation in females. RTT is characterized by psychomotor retardation, purposeless hand movements, autistic-like behavior and abnormal gait. We studied the effects of environmental enrichment (EE) on the phenotypic manifestations of a RTT mouse model that lacks MeCP2 (Mecp2(-/y)).

Principal findings: We found that EE delayed and attenuated some neurological alterations presented by Mecp2(-/y) mice and prevented the development of motor discoordination and anxiety-related abnormalities. To define the molecular correlate of this beneficial effect of EE, we analyzed the expression of several synaptic marker genes whose expression is increased by EE in several mouse models.

Conclusions/significance: We found that EE induced downregulation of several synaptic markers, suggesting that the partial prevention of RTT-associated phenotypes is achieved through a non-conventional transcriptional program.

Figure 1. Environmental Enrichment does not modify life span or body weight of Mecp2^−/y mice.

A, Kaplan-Meier plots showing the survival of Mecp2^−/y-SC (open circles) and Mecp2^−/y-EE (closed circles) since the initiation of the enrichment period. B, Growth curves of mice showed in A. The body weights of Mecp2^−/y mice, irrespective of housing conditions, dropped abruptly after 14 weeks and were therefore not included in the graph. Data represent mean ± SEM. The total number of animals in each group is shown (n). No significant differences were found between Mecp2^−/y-SC and Mecp2^−/y-EE according to ANOVA tukey test.

Figure 2. Environmental Enrichment improves neuromotor dysfunction in Mecp2^−/y mice.

A, Footprint analysis showed impaired walking patterns in Mecp2^−/y-SC mice compared with Mecp2^−/y-EE and Mecp2^+/y−SC control animals. B, Measures taken for footprint analysis .C, Speed of movement (cm/s) was faster for Mecp2^−/y-EE than Mecp2^−/y-SC mice. D, Mecp2^−/y-EE mice displayed improved motor coordination compared with Mecp2^−/y-SC evidenced by a shorter time to reach the platform for the first time and E, higher number of arrivals to the platform in the elevated beam test. The total number of animals in each group is shown (n). Mean ± SEM values are presented. A two-way repeated measures ANOVA (genotype×condition) demonstrated differences in the mean values among the different levels of condition (F = 8.06; p = 0.007). Student's t test indicated that the Mecp2^−/y-SC group was significantly different from the other two groups (*: p<0.05).

Figure 3. Environmental Enrichment restores a normal behavior in the plus maze test.

A, In the elevated plus maze, significant increases in the percentage of time spent in open arms were seen in Mecp2^−/y-SC (129 genetic background) compared with Mecp2^+/y-SC mice. This phenotype is reversed in the Mecp2^−/y-EE mice. B, In a B6129 F1 genetic background, Mecp2^−/y-EE mice showed a behavior indistinguishable from the Mecp2^+/y-SC mice. Mean ± SEM are presented. Two way ANOVA (genotype×condition) demonstrated differences in the mean values among the different levels of Genotype (F = 10.705; p = 0.003) and condition (F = 7.516; p = 0.011). Post hoc Student's t test detected statistical differences between groups, *: p<0.05.

Figure 4. Environmental Enrichment on Mecp2^−/y-EE mice induces a decrease in the expression of a set of synaptic plasticity-related genes.

A, A significant difference in the steady-state level of mRNA for Syp, PSD95 and SGK-1 was observed in hypothalamus of Mecp2^−/y-EE in comparison with in Mecp2^−/y-SC mice assessed by qRT-PCR. Syp WT: The expected EE-induced increase in Syp in Mecp2^+/y-EE (normalized to Mecp2^+/y-SC) is shown as a darker bar. B, In samples from cerebral cortex, differences were detected in Syt9, Stx1a and SGK-1. The results are expressed as percentage of the Mecp2^−/y-SC control group. * p<0.05 by Student's test (n = 5 for each group).