Identification of Siglecs as molecular signatures of Disease-Associated Microglia during AD Progression

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PROJECT SUMMARY Microglia are implicated in the initiation and progression of Alzheimer's disease (AD), making their regulation a therapeutic target. As positive effectors, microglia phagocytose and clear toxic proteins; as negative effectors they release inflammatory mediators. Imbalance of microglial function is believed to contribute to AD progression. Among microglial regulatory proteins linked to AD susceptibility are immune inhibitory members of the Siglec family, sialic acid binding immunoglobulin-like lectins. Siglecs became a special focus of AD research when genome-wide association studies (GWAS) discovered that a Siglec family member, CD33 (Siglec-3), was associated with AD. Siglec-3 overexpression results in increased susceptibility to AD, and depletion decreases susceptibility, supporting the view that Siglec-mediated inhibition restricts microglial phagocytosis and exacerbates AD proteinopathy. As multiple inhibitory Siglecs are expressed on human (and mouse) microglia, understanding the regulation by Siglec pathways will give us opportunity to modulate the microglia function and prevent or slowdown the progression of AD. Transcriptome analyses has been widely used to revealed changes in gene expression of microglia in disease. However, since different subsets of microglia display a wide range of responses and functions, whole transcriptome analyses alone could not fully capture the context-dependent microglia variety during disease. Recent studies with single-cell analysis have provided a high-resolution view of the transcriptional landscape of microglia subtypes of the murine CNS during development and disease. Our preliminary scRNA-seq and immunocytochemical analysis demonstrated that Siglecs, especially Siglec-F, and SiglecG are associated with a specific subset of microglia that are activated at different stage of AD, which support our hypothesis that Siglec genes are specifically expressed in subsets of microglia at particular stages of AD. Here, we propose to use a combination of scRNA-seq and whole transcriptome sequencing analysis to understand the relationship between Siglecs signaling and the microglial phenotypes in AD models to elucidate the impact of microglia subtypes on AD progression. We have developed a mouse model (Tau4RΔK-AP mice) that could mimic different stages of AD development from pre-symptomatic early stage to advance stage of AD with widespread neuronal loss and brain atrophy, which is instrumental for the current proposal. In two aims, we will take advantage of Tau4RΔK-AP mice and mice expressing either only Aβ plaques (APP;PS1 mice) or only tau tangles (Tau4RΔK mice) to test the hypothesis that Siglecs regulate microglial function during AD progression in two Aims. Aim 1: To determine whether Siglecs are specifically expressed in the subset of microglia that are activated by AD pathologies at different stages of the disease. Aim 2: To determine the regulation of Siglecs as signature genes in DAMs. Successful outcome from proposed study will help define Siglecs and other microglia specific genes in AD progression, and reveal new strategies for development of new AD therapy.