Effects of methylmercury on Ephs and ephrins during central nervous system development

Neuronal differentiation and migration, critical events during central nervous system (CNS) development, rely on multiple factors that ensure properly coordinated brain structuring. Disruption of expression or function of these tightly regulated factors leads to morphological abnormalities that potentially result in intellectual and behavioral deficiencies. One family of guidance molecules involved in CNS development is the Eph tyrosine kinase receptors and their ligands, the ephrins. We investigated the effects of methylmercury, a well-known developmental neurotoxicant, on expression of Ephs and ephrins in brains of neonatal mice exposed via maternal dosing, using an RNAse protection assay and immunoblotting. Results demonstrate that McHg alters expression of Ephs and ephrins in vivo. Brains from pups exposed to McHg during gestation and lactation showed increased ephrin-A5 mRNA and protein in day 10 hippocampus and decreased EphA4 expression in day 15 cerebellum. Brains from pups exposed only prenatally to McHg showed significant increases in mRNA for EphA8, B6, and ephrin-B2 on PND 5 along with significant decreases in EphA3, A4, A5, A6, A7, and B3 in day 15 rest-of-brain samples.; Expression of Eph and ephrin mRNA and protein during neuronal differentiation was characterized using the P19 embryonal carcinoma cell culture model. Undifferentiated cells expressed negligible levels of Ephs and ephrins. Expression of the B-class Eph receptors increased by 2 days after neuronal induction with retinoic acid, while A-class receptors were not elevated until day 6, suggesting separate roles of the receptor classes during neuronal commitment and differentiation. Both A and B class ligands were increased 2 days following neuronal induction and remained expressed at days 6 and 10.; Finally, P19 cells or their neuronal derivatives were treated with various concentrations of McHg for 24 hours on different days of commitment and analyzed for Eph/ephrin mRNA and protein expression. Increases in many of the receptors and ligands were seen at several neuronal stages, with day 6 neurons showing the most robust response. We conclude that alterations in Eph and ephrin expression contribute to the path-finding errors and cognitive deficits seen in humans and animals exposed to MeHg.