Bidirectional Behavioral Selection in Mice: A Novel Pre-clinical Approach to Examining Compulsivity

Adult Offspring of Deer Mouse Breeding Pairs Selected for Normal and Compulsive-Like Large Nesting Expression Invariably Show the Same Behavior Without Prior In-Breeding

Obsessive-compulsive disorder is a neuropsychiatric condition with notable genetic involvement. Against this background, laboratory-housed deer mice of both sexes varyingly present with excessive and persistent large nesting behavior (LNB), which has been validated for its resemblance of clinical compulsivity. Although LNB differs from normal nesting behavior (NNB) on both a biological and cognitive level, it is unknown to what extent the expression of LNB and NNB is related to familial background. Here, we randomly selected 14 NNB- and 14 LNB-expressing mice (equally distributed between sexes) to constitute 7 breeding pairs of each phenotype. Pairs were allowed to breed two successive generations of offspring, which were raised until adulthood (12 weeks) and assessed for nesting expression. Remarkably, our findings show that offspring from LNB-expressing pairs build significantly larger nests compared to offspring from NNB-expressing pairs and the nesting expression of the offspring of each breeding pair, irrespective of parental phenotype or litter, is family specific. Collectively, the results of this investigation indicate that LNB can be explored for its potential to shed light on heritable neurocognitive mechanisms that may underlie the expression of specific persistent behavioral phenotypes.

Wild-type MECP2 expression coincides with age-dependent sensory phenotypes in a female mouse model for Rett syndrome

Rett syndrome is characterized by an early period of typical development and then, regression of learned motor and speech skills in girls. Loss of MECP2 protein is thought to cause Rett syndrome phenotypes. The specific underlying mechanisms from typical developmental trajectory to regression features throughout life are unclear. Lack of established timelines to study the molecular, cellular, and behavioral features of regression in female mouse models is a major contributing factor. Due to random X-chromosome inactivation, female patients with Rett syndrome and female mouse models for Rett syndrome (Mecp2Heterozygous , Het) express a functional copy of wild-type MECP2 protein in approximately half of all cells. As MECP2 expression is regulated during early postnatal development and experience, we characterized the expression of wild-type MECP2 in the primary somatosensory cortex of female Het mice. Here, we report increased MECP2 levels in non-parvalbumin-positive neurons of 6-week-old adolescent Het relative to age-matched wild-type controls, while also displaying typical levels of perineuronal net expression in the barrel field subregion of the primary somatosensory cortex, mild tactile sensory perception deficits, and efficient pup retrieval behavior. In contrast, 12-week-old adult Het express MECP2 at levels similar to age-matched wild-type mice, show increased perineuronal net expression in the cortex, and display significant tactile sensory perception deficits. Thus, we have identified a set of behavioral metrics and the cellular substrates to study regression during a specific time in the female Het mouse model, which coincides with changes in wild-type MECP2 expression. We speculate that the precocious increase in MECP2 expression within specific cell types of adolescent Het may provide compensatory benefits at the behavioral level, while the inability to further increase MECP2 levels leads to regressive behavioral phenotypes over time.