Abstract
Maternal inflammation during pregnancy affects placental function and is associated with increased risk of neurodevelopmental disorders in the offspring. The molecular mechanisms linking placental dysfunction to abnormal fetal neurodevelopment remain unclear. During typical development, serotonin (5-HT) synthesized in the placenta from maternal l-tryptophan (TRP) reaches the fetal brain. There, 5-HT modulates critical neurodevelopmental processes. We investigated the effects of maternal inflammation triggered in midpregnancy in mice by the immunostimulant polyriboinosinic-polyribocytidylic acid [poly(I:C)] on TRP metabolism in the placenta and its impact on fetal neurodevelopment. We show that a moderate maternal immune challenge upregulates placental TRP conversion rapidly to 5-HT through successively transient increases in substrate availability and TRP hydroxylase (TPH) enzymatic activity, leading to accumulation of exogenous 5-HT and blunting of endogenous 5-HT axonal outgrowth specifically within the fetal forebrain. The pharmacological inhibition of TPH activity blocked these effects. These results establish altered placental TRP conversion to 5-HT as a new mechanism by which maternal inflammation disrupts 5-HT-dependent neurogenic processes during fetal neurodevelopment. Significance statement: The mechanisms linking maternal inflammation during pregnancy with increased risk of neurodevelopmental disorders in the offspring are poorly understood. In this study, we show that maternal inflammation in midpregnancy results in an upregulation of tryptophan conversion to serotonin (5-HT) within the placenta. Remarkably, this leads to exposure of the fetal forebrain to increased concentrations of this biogenic amine and to specific alterations of crucially important 5-HT-dependent neurogenic processes. More specifically, we found altered serotonergic axon growth resulting from increased 5-HT in the fetal forebrain. The data provide a new understanding of placental function playing a key role in fetal brain development and how this process is altered by adverse prenatal events such as maternal inflammation. The results uncover important future directions for understanding the early developmental origins of mental disorders.