Estrogen replacement profoundly facilitates nerve regeneration after injury in animal models. Estrogen replacement also protects against smell loss in humans and animals. Our long term goal is to elucidate the molecular mechanisms underlying estrogen's neuroprotective effects, specifically, in the olfactory system, and more generally, in the nervous system. The hypothesis is that apolipoprotein E (apoE, a lipid transporting protein), is a critical intermediary for the beneficial effects of estrogens on nerve repair. This hypothesis is based on the following observations. First, olfactory nerve regeneration was severely delayed in apoE-gene knockout (KO) mice as compared to wild-type (WT) litermates. Second, 17 2-estradiol replacement in ovariectomized mice resulted in a significant increase in levels of apoE and its receptor, LRP, in the olfactory bulb (OB). Third, estradiol treatment increased both apoE and neurite outgrowth in cortical and olfactory neuronal cultures. Fourth, estradiol treatment had no effect on neurite outgrowth in cultures derived from apoE knockout mice. The experimental focus of this proposal is to extend the results from culture studies to in vivo studies.
The specific aims are: 1. To determine the time course of estradiol's effect on apoE expression in the olfactory epithelium and bulb following olfactory nerve injury in ovariectomized wild-type mice. We will lesion the OE by nasal irrigation of Triton X-100 (TX) in ovariectomized (OVX) WT mice replaced with either an estradiol- or placebo- pellet. We will assess apoE levels in the OB and OE at varying time-points post-lesion in these two groups by standard immunoblotting and immunohistochemical staining. 2. To determine if estradiol treatment in ovariectomized wild-type mice improves the rate of olfactory nerve recovery following injury. We will test the hypothesis that estradiol replacement in OVX WT mice will facilitate olfactory nerve regeneration after injury. We will evaluate estradiol's effect on basal cell proliferation, recovery and axonal growth of the olfactory receptor neuron (ORN), and synapse formation of the newly generated ORN with bulbar neurons post injury. 3. To determine if estradiol's effect on olfactory nerve recovery following injury require the presence of apoE. We wil test the hypothesis that apoE is necessary for estradiol to have an effect on olfactory nerve regeneration post injury. We will evaluate in apoE KO mice the same parameters of regeneration identified in aim 2 for WT littermates. In that way, we will be able to determine if apoE is critical for all or only some measures of regeneration. ? ?
Recent studies have shown that the beneficial effects of estrogen/hormone treatment on chronic neurological diseases may depend on the inherited variant of apoE (a lipid transporting protein). Unfortunately, the role of apoE in estrogen's protection of the nervous system is unclear. This project will examine the interaction between estrogen and apoE in the nervous system in the presence and absence of injury. A better understanding of the apoE-estrogen interaction is vital to the therapeutic usefulness of estrogens in postmenopausal women. ? ? ?
