Date of Award
Doctor of Philosophy (PhD)
Alzheimer's disease (AD) and other neurodegenerative diseases present a large and growing challenge to global health. The immune system, particularly the innate immune system, is increasingly recognized as having a major role in these pathologies. The innate immune system is responsible to contain disease and promote healing. However, immune misregulation exacerbates disease. The innate immunomodulatory receptor Triggering receptor expressed on myeloid cells-2 (TREM2) is expressed on myeloid cells such as dendritic cells, macrophages, and in the brain, on microglia. TREM2 is a single-pass transmembrane receptor with an extracellular Ig domain that mediates ligand binding. This protein regulates inflammation in vitro and is required in vivo to sustain the microglia response during neurodegenerative diseases. Additionally, genetic studies have identified rare coding variants that increase risk for AD and separate variants that cause the severe, early fatal dementia known as Nasu-Hakola Disease (NHD). Combined, these animal and genetic studies have described a crucial role for TREM2 and identified variants that contribute disease risk. However, a full understanding of TREM2 function has been lacking due to a dearth of information regarding its structure and ligands. The goal of this study was to determine the TREM2 structure and understand how the disease variants alter structure and function. The TREM2 Ig domain was expressed, purified, and crystallized using a novel mammalian expression system. The TREM2 crystal structure and subsequent biophysical and functional assays revealed that NHD variants reduce protein stability and cause protein misfolding while the AD variants have minimal structural changes and instead impact ligand binding. TREM2 bound cell-surface sulfated proteoglycans on mammalian cells. AD-risk variants decreased binding while another variant, which is possibly protective, increased binding. These variants mapped a functional ligand-binding surface on the TREM2 protein. Functionally, chemical inhibition of nascent proteoglycan sulfation impaired TREM2 signaling, suggesting a ligand that acts in cis to position TREM2 for signaling. Additionally, TREM2 interacts with the soluble lipoprotein apolipoprotein E (ApoE) and with amyloid beta (Aβ) peptides. Intriguingly, AD-risk variants impair both of these interactions, suggesting a physiological relevance during AD. These experiments offer the first structural and mechanistic studies of TREM2 function and will engender targeted molecular therapies to restore or enhance TREM2 function during neurodegenerative disease.
Chair and Committee
Tom J. Brett
Jeffrey P. Henderson, Daved H. Fremont, Robyn S. Klein, Thaddeus S. Stappenbeck,
Kober, Daniel L., "Structural, Biophysical, and Functional Studies of TREM2 In Neurodegenerative Disease" (2017). Arts & Sciences Electronic Theses and Dissertations. 1120.