Associations among Exposure to Methylmercury, Reduced Reelin Expression, and gender in the Cerebellum of Developing Mice

Genetic risk factors acting during pregnancy or early after birth have been proposed to account for the exponential increase of autism diagnoses in the past 20 years. In particular, a potential link with exposure to environmental mercury has been suggested. Male sex constitutes a second risk factor for autism. A third potential genetic risk factor is decreased Reelin expression. Male heterozygous reeler (rl(+/-)) mice show an autism-like phenotype, including Purkinje cells (PCs) loss and behavioural rigidity. We evaluated the complex interactions between 3 risk factors, i.e. genetic status, sex, and exposure to methylmercury (MeHg), in rl(+/-) mice. Mice were exposed to MeHg during the prenatal and early postnatal period, either at a subtoxic dose (2ppm in Dams' drinking water), or at a toxic dose (6ppm Dams' drinking water), based on observations in other rodent species and mice strains. We show that: a) 2ppm MeHg does not cause PCs loss in the different animal groups, and does not enhance PCs loss in rl(+/-) males; consistent with a lack of overt neurotoxicity, 2ppm MeHg per se does not cause behavioural alterations (separation-induced ultrasonic calls in newborns, or sociability and social preference in adults); b) in stark contrast, 6ppm MeHg causes a dramatic reduction of PCs number in all groups, irrespective of genotype and sex. Cytochrome C release from mitochondria of PCs is enhanced in 6ppm MeHg-exposed groups, with a concomitant increase of μ-calpain active subunit. At the behavioural level, 6ppm MeHg exposure strongly increases ultrasonic vocalizations in all animal groups. Notably, 6ppm MeHg significantly decreases sociability in rl(+/-) male mice, while the 2ppm group does not show such as decrease. At a subtoxic dose, MeHg does not enhance the autism-like phenotype of male rl(+/-) mice. At the higher MeHg dose, the scenario is more complex, with some "autism-like" features (loss of sociability, preference for sameness) being evidently affected only in rl(+/-) males, while other neuropathological and behavioural parameters being altered in all groups, independently from genotype and sex. Mitochondrial abnormalities appear to play a crucial role in the observed effects.