Abstract W P242: Role of Hippocampal Estrogen Receptor α and Src in Sexually Differential Phosphorylation of Tyrosine kinase-B receptors

Introduction: Male neonate brains are more susceptible to the effects of hypoxic ischemic encephalopathy (HI) as compared to females with comparable brain injury. Our recent findings reveal that HI induces an increase in phosphorylation of tyrosine kinase-B receptors (TrkB) in female hippocampi and TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), exerts profound early and late neuroprotective effects in female but not in male neonates. We hypothesized that differential TrkB phosphorylation seen in females is associated with increased hippocampal estrogen receptor α (ERα) expression and src (Src family kinase) phosphorylation resulting in decreased apoptosis post-HI. Methods: WT (ERα+/+) and KO (ERα-/-) P9 male and female mice pups were subjected to Vannucci’s neonatal HI model by left common carotid artery cauterization and exposure of the pups to 10% O2 at 37°C for 50 min. Pups were either sham or HI operated and given PBS or 7,8-DHF (5 mg/kg in PBS, ip.) starting at 10 min, then at 24 h, and 48 h post HI. Hippocampi were dissected out for either immunoblotting or qRTPCR 3d post-HI. We assessed the effects of sex (M vs F), treatment (PBS vs 7,8-DHF) and HI (sham vs HI) on immunoblotting band densities [pTrkB)/full length TrkB, ERα/actin, p-src/total src, cleaved caspase 3 (cc3)/actin ratios] and qRTPCR results using ANOVA analysis. Results: HI and 7,8-DHF differentially increase p-TrkB and p-src in WT female hippocampi 3 d post-HI (p < 0.0001). HI induces a 2.3 fold increase in ERα mRNA and protein expression in female hippocampi compared to the males 3d post-HI (p=0.002). HI and 7,8-DHF fail to increase p-TrkB and p-src in KO male and female mice suggesting that the TrkB and src phosphorylation are dependent on the ERα signaling in the neonatal hippocampi post-HI. Cc3 levels are found to be increased in KO mice while the 7,8-DHF treated WT female hippocampi had decreased cc3 expression. Conclusion: Sexually differential phosphorylation of TrkB and src are abolished in KO mice suggesting an important role of ERα in TrkB phosphorylation and src activation resulting in decreased apoptosis in female hippocampi post-HI. This mechanism might be one of the mechanisms that makes the male neonate brains more susceptible to the effects of HI.