TREM2 activation by vitamin D3 in the pathogenesis of Alzheimer's disease - Project Summary The purpose of this proposal is to add to prior in silico and in vitro results, to support our novel finding that vitamin D3 is a ligand for TREM2, by conducting experiments in vivo. Alzheimer’s disease (AD) is the most common neurodegenerative disease and affects more than 6 million Americans. A rare loss-of-function-like variant in triggering receptor expressed on myeloid cells 2 (TREM2), TREM2R47H, causes a 2–4-fold increased risk of developing AD. TREM2 is a receptor on microglia, and other myeloid cells, that binds notable molecules involved in AD pathogenesis such as apoE and Aβ. Additionally, TREM2 plays an important role in microglial response to Aβ plaques by increasing microglial plaque coverage, phagocytosis, and proliferation while also altering their transcriptional response. We identified, via a virtual screen, and supported, via biolayer interferometry, that D3 and several of its derivatives bind TREM2 and, with less affinity, its deleterious variant TREM2R47H. Next, we conducted in vitro experiments on different myeloid cells, derived from rats and mice, that supported that D3 signals through TREM2. We observed an increase in downstream effects associated with TREM2 signaling like increased phagocytosis of Aβ and increased cell survival against toxic Aβ oligomers. To support these results, we propose experiments using hAPPJ20 mice, a transgenic AD mouse model that overexpresses human amyloid precursor protein (hAPP) and has increased Aβ production. Prior research has also established a link between D3 and AD. Studies have found that D3 deficiency increases, while supplementation decreases, risk of developing AD. D3 deficiency and supplementation in Aβ mouse models of AD have similarly found that lower D3 levels can exacerbate Aβ pathology. We will use normal, deficient, and supplemented D3 diets to alter D3 levels in hAPPJ20 mice over a ~5.5-month treatment period to examine interactions between Trem2 genotype and D3 level. Our hypothesis is that D3 protects against Aβ-induced dysfunction by increasing Trem2 signaling in vivo. We have planned two aims to test our hypothesis. Aim 1 determines if D3 deficiency exacerbates Aβ-induced dysfunction by decreasing Trem2 signaling while aim 2 determines if D3 supplementation ameliorates the detrimental loss-of-function-like effects of TREM2R47H by increasing Trem2 signaling. Trem2-targeted outcome measures were selected and grouped into three categories: Trem2 signaling, Trem2-dependent microglial functions, and neuronal health. Experiments utilize methods such as Western blots, immunohistochemistry, and behavioral assays to examine Trem2 signaling outcomes. This project and training plan are sponsored by Dr. Erik Roberson and Dr. Yuhua Song, who have a history of mentoring other successful MD/PhD students. The training for this grant provides me with the skill set and experience to help me achieve my career goal of becoming a physician-scientist who identifies and validates therapeutic targets to help the patients I treat.
