2025 Theses Doctoral

Targeting the genetic susceptibility of CD33 in Alzheimer's disease

Hall, Jennifer

Genetic studies have indicated a causal role for microglia, the innate immune cells of the central nervous system (CNS), in Alzheimer’s disease (AD). The AD- associated rs3865444CC risk variant reduces splicing of the sialic acid-binding domain and increases expression of the full-length (sialic acid-binding) CD33 isoform seven-fold compared to the rs3865444AA protective genotype, suggesting that CD33’s sialic acid-dependent binding may be critical to its genetic association to AD. Here, we identify CD45 as an immune cell-specific sialic acid-dependent cis CD33 binding partner, whose phosphatase activity is inhibited by CD33.

Furthermore, we establish a genetic and functional interaction between CD33 and CD45 relevant to AD susceptibility and systemic myeloid dysfunction in this disease. Using a competitive inhibitor peptide, IMAD-001, we demonstrate that this interaction can be physically and functionally inhibited. By disrupting CD33’s interaction with its binding partners, we demonstrate inhibition of CD33 activity, which enhances uptake and clearance of amyloid beta in vitro and in in vivo mouse and zebrafish models of AD. Furthermore, IMAD-001 driven increases in uptake of amyloid beta monomers were positively correlated with the cell surface expression of CD33 on monocytes, further validating its effects are driven through inhibition of the CD33 receptor.

We also observed a reduction in cell surface triggering receptor expressed on myeloid 2 (TREM2), likely due to increased cleavage of the receptor resulting in increased levels of the soluble ectodomain (sTREM2) in the supernatant. Finally using single cell RNA sequencing, we show that IMAD-001 mediated inhibition of CD33 in 5xFAD mice results in an upregulation of homeostatic marker genes and down-regulation of pro-inflammatory genes.

Together, our findings indicate that inhibition of CD33 activity by disrupting its interactions with its binding partners is a promising strategy to reprogram microglia towards a homeostatic state to enhance protective microglial function, including clearance of pathologic amyloid beta, and ultimately confer neuroprotection in Alzheimer’s disease.