Discovery-driven Label-free Quantitative Proteomics Study to Understand Estradiol-mediated Neuronal Processes in the Hippocampus and its Implication in Alzheimer’s Disease in Ovariectomized Rats

Purpose: Alzheimer’s disease (AD) is a debilitating neurodegenerative disorder that affects millions of people globally. Studies report an increased susceptibility to the development of AD in post-menopausal women. There is renewed interest in utilizing estrogen therapies due to its neuroprotective effects on the brain; however, the mechanisms of these neuroprotective effects are poorly understood. The hippocampus is involved in memory formation and is a critical region where early damage in AD is often seen. This discovery-driven proteomics study elucidates several candidate proteins and biological pathways mediated via E2 and implicated in neurological signaling in the hippocampus. Methods: Ovariectomized female Sprague-Dawley rats were treated with daily subcutaneous injections of either vehicle or 50μg/kg E2 for five days before the rats were sacrificed with the hippocampus collected for proteomics. Protein extracts were taken from centrifugated hippocampal tissue and prepared via a series of steps including urea incubation, disulfide bond reduction, carbamidomethylation of thiol groups, and digestion via trypsin with subsequent quenching. The digested proteins were dried, reconstituted in solvent, and processed via nano-LC-MS/MS. The MS/MS spectra were searched against a Rattus norvegicus proteome database for peptide fragment and protein identification via ProteomeDiscover (Thermo Fisher Scientific) using Mascot as a search engine and validated using Scaffold (Proteome Software). Bioinformatic analysis using Ingenuity Pathway Analysis (Quiagen) allowed the construction of associative and predicted protein networks. Results: The processed MS/MS data proteins revealed several candidates for future targeted validation. Among those proteins, calcium/calmodulin-protein kinase II implicated in memory and learning processes, such as long-term signal potentiation in the hippocampus, was more abundant in the treatment group (p = 0.00052) with fold changes in the protein cluster abundances ranging from 1.3 to 1.5 versus control. Other proteins, such as microtubule-associated protein tau, implicated in AD, also had a marked fold change of –2.5 abundance in treatment versus control (p = 0.00014). There substantial overall difference in protein abundances for neurological disease pathways, including AD, identified in Ingenuity Pathway Analysis (p = 8.9e-7) as well as in pathways involving nervous system development and function (p= 4.57e-7) between E2-treated and vehicle-treated rats. Conclusion: This dataset analysis aims to evaluate the effects of E2 on the proteome of the hippocampus in ovariectomized rats. The evident increased fold changes in calcium-dependent and calcium-associated proteins in the context of neuronal processes suggest increased downstream modulation of synaptic signaling, which could be further examined by microdialysis assay of neurotransmitters. Future studies utilizing microdialysis may examine E2’s estrogen receptor-mediated effects on cholinergic neuronal signaling in the hippocampus, which could further understand AD in the context of cholinergic neuron loss.