The effects of prenatal bisphenol a exposure on the expression of MIRNA and NMDA receptor subunits in learning and memory function

Bisphenol A (BPA) is one of the most widely used chemicals in the production of plastic and epoxy resin. BPA reportedly can be leached and migrate into the foodstuff and the environment, potentially posing a health hazard to humans. Early life exposure to BPA reportedly causes cognitive impairment in young children. The effects are believed to be derived from epigenetic modification since the early neurodevelopmental stage. In the hippocampus, miR-19a and miR-539 are recognized to regulate the N-methyl-D Aspartate (NMDA) receptor subunits, GluN2A and GluN2B. However, the impact of BPA-induced miRNA alteration remains elusive. This study was done to investigate the effects of prenatal BPA exposure on the expression of microRNA (miRNA) and NMDA receptor subunits in male rat hippocampus as well as its memory retention ability. Pregnant Sprague Dawley rats were orally exposed to 5 mg/kg/day of BPA with 0.5% Tween 80 in RO water from pregnancy day 1 to day 21. Whereas, the control group were without BPA. The pregnant rats were subjected to cesarean section or spontaneous delivery. At GD21, cesarean section was performed and the placenta and fetus hippocampus were dissected. The remaining mother that undergoes spontaneous delivery, the male pups were raised until postnatal day 7, 14, 21 (PND7, 14 and 21) and adolescent day 35 (AD35) where their hippocampus was collected. The expression of miR-19a, miR-539, GRIN2A, and GRIN2B in the hippocampus was measured by qRT- PCR. The level of ERα and ERβ in placenta and synaptic GluN2A, GluN2B, PSD-95 and synapsin I in the hippocampus were measured in ELISA. The extrasynaptic GluN2A and GluN2B were next measured in the Western Blot. At AD35, neurobehavioral tests were also assessed to evaluate the anxiety-related behavior and memory retention ability of the rat. The findings showed that the levels of ERα (p<0.05) and ERβ (p<0.05) were significantly increased in the placenta. In the hippocampus, prenatal BPA exposure at 5 mg/kg/day significantly reduced the expression of miR-19a (p<0.001), miR-539 (p<0.01), GRIN2A (p<0.001) and GRIN2B (p<0.001) at all stages of age. The levels of synaptic and extrasynaptic GluN2A (p<0.01) and GluN2B (p<0.001, p<0.01), PSD-95 (p<0.05) and synapsin I (p<0.05) were also significantly reduced when reaching adolescent age. Consequently, the adolescent male rats experienced anxiety-related behavior and impairments in fear and spatial memory. These findings suggest that the increment of ERα and ERβ in the placenta of a BPA- exposed pregnany rats may increase the susceptibility of fetus to the free BPA. In the hippocampus, the changes in miR-19a and miR-539 expression were postulated to influence the expression of GRIN2A and GRIN2B along the development. Further, the decrement of GluN2A, GluN2B, PSD-95 and synapsin I suggested to cause learning and memory impairment. In conclusion, BPA-induced miRNA alteration in regulating NMDA receptor subunits suggested to increase the susceptibility of the offspring to have impairment in learning and memory function. These findings suggest that miR- 19a and miR-539 as epigenetic markers, may act as future therapeutic targets in highlighting the disruption in hippocampal function at an early stage of life.