Neuroprotective Effects of Selective Estrogen Receptor Modulators Against Amyloid Beta Toxicity and the Pathways That Provide Protection

Alzheimer’s disease is the sixth leading cause of death in the United States. The leading hypothesis to explain the prevalence of the disease in the brain is the aggregation of Amyloid Beta peptides in the brain, which form senile plaques and suppress neuronal function. Selective estrogen receptor modulators (SERMs) have been found to provide protective effects against the neurotoxic effects of Amyloid Beta. This experiment was conducted in two distinct phases: the experimental phase and the literature review phase. The experimental phase sought to determine if Amyloid Beta was neurotoxic to SH-SY5Y neuroblastoma cells, and if STX—an SERM—was able to rescue cell viability after exposure to Amyloid Beta toxicity. During the literature review phase, potential pathways already described in the literature were identified, which might be able to account for how STX was able to rescue cell viability after exposure to Amyloid Beta toxicity. Amyloid Beta was found to have a significant toxic effect on cell viability when compared to a control assay of cells grown in DMSO. STX, however, was not found to have a significant rescue effect against Amyloid Beta toxicity. The literature review identified several pathways that were likely candidates for neuroprotection, including MAPK, ERK1, ERK2, and PI3K. The future direction of this experiment would be to determine if STX is as effective at providing neuroprotection against Amyloid Beta as previously suggested by similar experiments. the next stage of investigation would then be to inhibit different pathways that have been identified as possible pathways for neuroprotection. INTRODUCTION Alzheimer’s disease is a progressive brain disorder that causes impairment in memory and thinking skills (1). It was first described by Dr. Alois Alzheimer in 1906, who noted memory loss, paranoia, and psychological changes in his patient. Today, Alzheimer’s disease is the sixth leading cause of death in the United States (2). The factors that lead to Alzheimer’s disease are still unknown, though there are many hypotheses which seek to explain the etiology of cognitive impairments associated with the disorder. One of the leading explanations is the Amyloid Cascade hypothesis, proposed by J.A. Hardy and G.A. Higgins in 1992. Amyloid precursor protein (APP) is expressed by neuronal cells and extraneuronal tissues. Hardy and Higgins argue that the proteolytic cleavage of the membrane glycoprotein amyloid precursor protein into neurotoxic amyloid beta-peptide (Amyloid β) fragments was the cause of the cellular death process in the central nervous system (3). In the brain, APP is expressed in three isoforms: APP695, APP751, and APP770. APP695 is the most abundant isoform in neurons and is expressed at relatively high levels (4). As mentioned previously, APP is a membrane-bound glycoprotein. It is processed by membrane bound aspartic proteinases: α and β secretases. Each of the products generated by these proteinases is processed by γ secretase and the fragments are secreted into the cytoplasm. When the Amyloid β domain is processed by α secretase, it is cleaved within the Amyloid β domain, between residues Lys16 and Leu17; this is referred to as the non-amyloidogenic pathway, as it inhibits the generation of intact Amyloid β peptides. When processed by β secretase, it is cleaved at Asp1, which is referred to as the amyloidogenic pathway, as it forms intact Amyloid β peptides (40-42 amino acids long) that are released into the cytosol. These peptides will frequently aggregate in the brain and cause the development of senile plaques (5). Research into the function of APP secretases, specifically γ secretase, has been of interest to many investigators. By understanding this mechanism, it is though that we could stop the production and release of Amyloid β peptide, which would prevent plaque formation all together. Unfortunately, γ secretase plays a critical role in the cleavage of other membrane proteins, such as the Notch1 receptor. Because of this, research into alternative methods of neuroprotection from Amyloid β peptide is necessary (6). Estrogens have been shown to have neuroprotective effects against brain ischemia, as well as reduce memory impairment after an ischemic event (7). This makes estrogen a compelling compound of study for other conditions that cause cognitive impairment (7,8). There are significant risks, however, associated with 17-β estradiol (E2) treatments; possible side effects include but are not limited to: cardiovascular disease, thrombosis, cerebral vascular accident, and hormone-sensitive cancers, among others. Selective estrogen receptor modulators (SERMs) are synthetic or natural compounds that provide the positive neuroprotective effects of estrogen without the negative side effects. STX is one example of a SERM. STX is able to bind to the G protein coupled estrogen receptor GqmER in the plasma membrane instead of the nuclear estrogen receptors. This eliminates the negative transcriptional responses generated by E2 that lead to negative side effects (9). Building on the research developed by Gray et al. (9), my research will seek to understand the pathway that produces the protective effects of STX against Amyloid β toxicity. My project will be undertaken through a mixed methodology of primary research and literature review. In the research phase of my experiment, I will establish that Amyloid βis toxic to SH-SY5Y neuroblastoma cells, which I will use as my cell culture assay of amyloid toxicity (9). My literature review will be focused on previous research conducted on the different signaling mechanisms of STX in the cell. This combined approach will address my research question: what is the pathway that provides STX its neuroprotective abilities against the harmful effects of Amyloid β?