Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulation in the brain o aggregated forms of the 40- and 42-amino acid amyloid about peptide (AB340, AB42). Epidemiological studie indicate that estrogen replacement therapy (ERT) in postmenopausal women appears to decrease the risk of AD or to delay its onset, but the mechanism by which estrogen exerts this neuroprotective role remains elusive Evidence suggests that physiological concentrations of 176-estradiol reduce the release of AB40 and AB42 peptides by primary neuronal cultures from rat, mouse or human embryonic cerebral cortex, providing a possible mechanism by which ERT may act. In order to test whether estrogen modulates the metabolism of AB peptides in vivo, we propose in Aim 1 of this application to investigate the effects of ovariectomy (ovx) and estrogen replacement on brain AB40 and At342 levels in guinea pigs. We also propose to determine whether estrogen action on brain is likely to involve conventional estrogen receptors by treating ovx animals with estrogen during the concomitant administration of the estrogen receptor antagonist tamoxifen or its active metabolite, 4 hydroxytamoxifen. We will also assess the potential """"""""agonist"""""""" or """"""""antagonist"""""""" activity of the enantiomer, 17 alpha-estradiol, which is inactive at the best studied conventional estrogen receptor, estrogen receptor alpha. Finally, since hormonal treatment of cultured cells has been demonstrated to potentiate the nonamyloidogenic processing of the the Alzbeimer amyloid precursor when cholinergic stimuli are applied, we will also test whether brain AB340 and AB42 levels are (as predicted by the cell culture model) sharply diminished when 17B-estradiol and the clinically relevant cholinesterase inhibitor donepezil are administered simultaneously. Since one molecular explanation for estrogen action on brain AB could involve the inhibition of B-secretase activity, we will measure soluble APP-D and BACE mRNA levels in the brains of control, ovx and pharmacologically treated guinea pigs.
In Aim 2, we propose to generate transgenic mice utilizing genetically engineered chimeri molecules from which AD is generated only in restricted subcellular compartments. These mice will be subjected to ovariectomy or sham surgery, and their brain AB40 and AB42 ievels will be determined. This study will implicate the subcellular compartment(s) which contain hormone-sensitive AB-generating machinery. Taken together, these data will test whether the currently available neuronal cell culture data on hormonal regulation of AB metabolism are relevant in the brain of living experimental animals. In addition, novel insights will be gained into the roles of certain subcellular compartments as sites of estrogen-sensitive AB metabolism and into the roles (if any) in estrogen-sensitive AB metabolism which are played by the conventional estrogen receptor (estrogen receptor alpha) and/or the expression or activity of B-secretase (the rate-limiting enzyme for AD generation).
