Applying the ethoexperimental approach to neurodevelopmental syndrome research reveals exaggerated defensive behavior in Mecp2 mutant mice

Abstract

Rett syndrome is a Pervasive Developmental Disorder (PDD) associated with de novo mutations of the methyl CpG-binding protein 2 (MECP2) gene. Mecp2 functions as a transcription factor that regulates the expression of hundreds of genes. Identification of the role of Mecp2 in specific neurodevelopmental symptoms remains an important research aim. We previously demonstrated that male mice possessing a truncation mutation in Mecp2 are hyper-social. We predicted that reduced fear or anxiety might underlie this enhanced affiliation. In order to probe risk assessment and anxiety-like behavior, we compared Mecp2 truncation mutants to their wild-type littermates in the elevated plus maze and elevated zero maze. Additionally, subjects were administered the mouse defense test battery to evaluate unconditioned fear- and panic-like behavior to a graded set of threat scenarios and a predator stimulus. Mutant mice showed no significant changes in anxiety-like behavior. Yet, they displayed hyper-reactive escape and defensive behaviors to an animate predatory threat stimulus. Notably, mutant mice engaged in exaggerated active defense responding to threat stimuli at nearly all phases of the fear battery. These results reveal abnormalities in emotion regulation in Mecp2 mutants particularly in response to ecologically relevant threats. This hyper-responsivity suggests that transcriptional targets of Mecp2 are critical to emotion regulation. Moreover, we suggest that detailed analysis of defensive behavior and aggression with ethologically relevant tasks provides an avenue to interrogate gene-behavior mechanisms of neurodevelopmental and other psychiatric conditions.

Figure 1. Anxiety-like behavior

Mutant wild-type (Y/+) and MeCP2^308/y (Y/−) mice exhibit similar amounts of time spent in all areas of the elevated plus-maze (A). No significant differences in mean frequencies of entries into the three zones (B), or in the frequencies of stretch attend or head dip behaviors (C) were found for the two genotypes. In the elevated zero-maze, no significant differences were found in the mean duration of time spent in the open or closed compartments (D) and although mutant mice appeared to show lower total entries into the compartments (E), this effect was not statistically significant. Mutant mice displayed comparable frequencies of stretch attend risk assessment behaviors while performing more head dips in the elevated zero-maze (F). *p<0.001. n=8-9/genotype.

Figure 2. Mouse defense test battery

In various phases of the escalating predatory threat scenario, mutant mice (Y/−) displayed increased defensive behavior relative to wild-type siblings (Y/+). Mutant mice avoided the predator from a greater distance (B). When chased by the predator, they made fewer stops and never reversed their course, and they showed higher escape velocity (C). When escape was blocked and the predator was located at the opposing end of the arena, the mutants approached less and made fewer contacts with the rat and instead engaged in more freezing (D). Upon forced contact by the rat, mutants engaged in more jump escapes (E). After the stimulus was removed, the mutant mice showed more rearing behaviors (F). *p<0.05, **p<0.01, ***p<0.001. n=9/genotype.