TrkB-mediated neuroprotection in female hippocampal neurons is autonomous, estrogen receptor alpha-dependent, and eliminated by testosterone: a proposed model for sex differences in neonatal hippocampal neuronal injury

Abstract

Following in vitro ischemia, the nerve growth factor receptor TrkB is activated in the presence of the TrkB agonist 7,8-DHF only in female and not in male cultured hippocampal neurons, leading to increased neuronal survival. Expression of ERα is increased following in vitro ischemia in female but not male hippocampal neurons. The female hippocampal neuronal specific responses to in vitro ischemia are blocked by pre-treatment with testosterone. The data support a model for a female-specific a neuroprotective pathway in hippocampal neurons. The pathway is activated by a TrkB agonist, dependent on ERα and blocked by testosterone. Neonatal hypoxia ischemia (HI) related brain injury is one of the major causes of learning disabilities and memory deficits in children. In both human and animal studies, female neonate brains are less susceptible to HI than male brains. Phosphorylation of the nerve growth factor receptor TrkB has been shown to provide sex-specific neuroprotection following in vivo HI in female mice in an estrogen receptor alpha (ERα)-dependent manner. However, the molecular and cellular mechanisms conferring sex-specific neonatal neuroprotection remain incompletely understood. Here, we test whether female neonatal hippocampal neurons express autonomous neuroprotective properties and assess the ability of testosterone (T) to alter this phenotype. We cultured sexed hippocampal neurons from ERα+/+ and ERα−/− mice and subjected them to 4 h oxygen glucose deprivation and 24 h reoxygenation (4-OGD/24-REOX). Sexed hippocampal neurons were treated either with vehicle control (VC) or the TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) following in vitro ischemia. End points at 24 h REOX were TrkB phosphorylation (p-TrkB) and neuronal survival assessed by immunohistochemistry. In addition, in vitro ischemia-mediated ERα gene expression in hippocampal neurons were investigated following testosterone (T) pre-treatment and TrkB antagonist therapy via q-RTPCR. Multifactorial analysis of variance was conducted to test for significant differences between experimental conditions. Under normoxic conditions, administration of 3 µM 7,8-DHF resulted an ERα-dependent increase in p-TrkB immunoexpression that was higher in female, as compared to male neurons. Following 4-OGD/24-REOX, p-TrkB expression increased 20% in both male and female ERα+/+ neurons. However, with 3 µM 7,8-DHF treatment p-TrkB expression increased further in female neurons by 2.81 ± 0.79-fold and was ERα dependent. 4-OGD/24-REOX resulted in a 56% increase in cell death, but only female cells were rescued with 3 µM 7,8-DHF, again in an ERα dependent manner. Following 4-OGD/3-REOX, ERα mRNA increased ~ 3 fold in female neurons. This increase was blocked with either the TrkB antagonist ANA-12 or pre-treatment with T. Pre-treatment with T also blocked the 7,8-DHF- dependent sex-specific neuronal survival in female neurons following 4-OGD/24-REOX. OGD/REOX results in sex-dependent TrkB phosphorylation in female neurons that increases further with 7,8-DHF treatment. TrkB phosphorylation by 7,8-DHF increased ERα mRNA expression and promoted cell survival preferentially in female hippocampal neurons. The sex-dependent neuroprotective actions of 7,8-DHF were blocked by either ANA-12 or by T pre-treatment. These results are consistent with a model for a female-specific neuroprotective pathway in hippocampal neurons in response to hypoxia. The pathway is activated by 7,8-DHF, mediated by TrkB phosphorylation, dependent on ERα and blocked by pre-exposure to T. In the clinical setting, it is well known that in neonates who suffer brain injury due to lack of blood flow and oxygen to the brain, the resulting damage is less severe in females compared to males. Furthermore, males sustaining brain injuries are more likely to exhibit learning and memory deficits as they mature. However, the underlying cellular mechanisms that lead to these sex differences in brain injury outcomes are poorly understood. In this report, we cultured hippocampal neurons from neonatal female and male mice and subjected them to reduced oxygen and glucose to mimic neonatal hypoxia ischemia. We found that the nerve growth factor receptor TrkB in females, as compared to males, is activated following the insult and more responsive to an exogenous nerve growth factor which leads to cell survival. In addition, expression of the estrogen receptor alpha is increased following the insult in females but not in males. Interestingly, the nerve growth factor receptor response in females is dependent on the presence of the estrogen receptor alpha. Both the nerve growth factor response and the increase in estrogen receptor alpha are abolished in females when treated with the male androgen, testosterone. Thus, our results support a model for a female-specific neuroprotective pathway in hippocampal neurons. The pathway is activated by nerve growth factors, dependent on estrogen receptor alpha, and is likely rendered inoperative in males by exposure to neonatal testosterone.