Small Molecule Binding to Alzheimer Risk Factor CD33 Promotes Aβ Phagocytosis.

Luke A Miles,Tracy L Nero,Ines Royaux,Daniel Oehlrich,Andrés A Trabanco,Michael W Parker,Gabriela A.N Crespi,Stefan J Hermans,Jonathan H Gooi,Andreas Ebneth,Jasmina Markulić,Nancy C Hancock,Berthold Wroblowski,Larissa Doughty,Marie-Laure Rives

doi:10.1016/j.isci.2019.07.023

Abstract

SummaryPolymorphism in the microglial receptor CD33 gene has been linked to late-onset Alzheimer disease (AD), and reduced expression of the CD33 sialic acid-binding domain confers protection. Thus, CD33 inhibition might be an effective therapy against disease progression. Progress toward discovery of selective CD33 inhibitors has been hampered by the absence of an atomic resolution structure. We report here the crystal structures of CD33 alone and bound to a subtype-selective sialic acid mimetic called P22 and use them to identify key binding residues by site-directed mutagenesis and binding assays to reveal the molecular basis for its selectivity toward sialylated glycoproteins and glycolipids. We show that P22, when presented on microparticles, increases uptake of the toxic AD peptide, amyloid-β (Aβ), into microglial cells. Thus, the sialic acid-binding site on CD33 is a promising pharmacophore for developing therapeutics that promote clearance of the Aβ peptide that is thought to cause AD.

Highlights

Microglia, resident immune cells in the brain, can be activated in response to misfolded proteins found in neurodegenerative diseases leading to neuroinflammation and the release of neurotoxic substances
Concluding Remarks Structural insights presented in this study reveal the molecular interactions that underpin sialic acid ligand recognition by CD33
We have shown that the sialic acid-based compound P22, when presented on microparticles, can increase phagocytosis of microglial cells and increase Ab uptake into these cells

Summary

Introduction

Resident immune cells in the brain, can be activated in response to misfolded proteins found in neurodegenerative diseases leading to neuroinflammation and the release of neurotoxic substances. Tanzi and co-workers showed that knocking out the CD33 gene could mitigate amyloid-b (Ab42) pathology (Griciuc et al, 2013) They reported that numbers of CD33-immunoreactive microglia correlated positively with both insoluble Ab42 levels and amyloid plaque burden in AD brain, that CD33 inhibited clearance of Ab42 in microglial cell cultures, and that brain levels of insoluble Ab42/plaque burden were markedly reduced in APPSwe/PS1DE9/CD33À/À mice. The rs386544 allele is in perfect linkage disequilibrium with rs12459419, which is located at a splice site of the sialic acid-binding site containing exon 2 (Raj et al, 2014) Both alleles lead to lower expression levels of functional CD33 (Malik et al, 2013; Walker et al, 2015), enhanced phagocytic activity of microglial cells, and uptake of Ab42 (Griciuc et al, 2013; Bradshaw et al, 2013). Small, drug-like inhibitors of CD33 to promote amyloid clearance could represent a novel class of therapeutics for the prevention and treatment of AD

Methods

Results

Conclusion