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Published on: December 26, 2016
Human brain sialoglycan ligand for CD33, a microglial inhibitory Siglec implicated in Alzheimer's disease
Anabel Gonzalez-Gil1, Ryan N Porell1, Steve M Fernandes1
1Department of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
Abstract:
Alzheimer's disease (AD) is characterized by accumulation of misfolded proteins. Genetic studies implicate microglia, brain-resident phagocytic immune cells, in AD pathogenesis. As positive effectors, microglia clear toxic proteins, whereas as negative effectors, they release proinflammatory mediators. An imbalance of these functions contributes to AD progression. Polymorphisms of human CD33, an inhibitory microglial receptor, are linked to AD susceptibility; higher CD33 expression correlates with increased AD risk. CD33, also called Siglec-3, is a member of the sialic acid-binding immunoglobulin-type lectin (Siglec) family of immune regulatory receptors. Siglec-mediated inhibition is initiated by binding to complementary sialoglycan ligands in the tissue environment. Here, we identify a single sialoglycoprotein in human cerebral cortex that binds CD33 as well as Siglec-8, the most abundant Siglec on human microglia. The ligand, which we term receptor protein tyrosine phosphatase zeta (RPTPζ)S3L, is composed of sialylated keratan sulfate chains carried on a minor isoform/glycoform of RPTPζ (phosphacan) and is found in the extracellular milieu of the human brain parenchyma. Brains from human AD donors had twofold higher levels of RPTPζS3L than age-matched control donors, raising the possibility that RPTPζS3L overexpression limits misfolded protein clearance contributing to AD pathology. Mice express the same structure, a sialylated keratan sulfate RPTPζ isoform, that binds mouse Siglec-F and crossreacts with human CD33 and Siglec-8. Brains from mice engineered to lack RPTPζ, the sialyltransferase St3gal4, or the keratan sulfate sulfotransferase Chst1 lacked Siglec binding, establishing the ligand structure. The unique CD33 and Siglec-8 ligand, RPTPζS3L, may contribute to AD progression.
Insights
Researchers identified a novel brain protein, RPTPζS3L, that binds to microglial receptors like CD33. Elevated levels of this protein in Alzheimer's disease (AD) brains suggest it may impair the clearance of toxic proteins, contributing to AD progression.
Area of Science:
- Neuroscience
- Immunology
- Biochemistry
Background:
- Alzheimer's disease (AD) is linked to microglial dysfunction and the accumulation of misfolded proteins.
- The inhibitory microglial receptor CD33 is associated with increased AD risk, with higher expression correlating to greater susceptibility.
- CD33, a sialic acid-binding immunoglobulin-type lectin (Siglec), regulates microglial immune responses through binding to specific sialoglycan ligands.
Purpose of the Study:
- To identify the specific sialoglycan ligand that binds to CD33 and Siglec-8, the primary Siglec on human microglia.
- To investigate the role of this identified ligand in Alzheimer's disease pathogenesis.
- To characterize the structural basis of Siglec-ligand interactions in the brain.
Main Methods:
- Mass spectrometry and biochemical assays were used to identify and characterize the sialoglycoprotein ligand.
- Analysis of human cerebral cortex tissue from AD and control donors.
- Genetic engineering of mice to lack specific genes involved in ligand synthesis (RPTPζ, St3gal4, Chst1) to assess the necessity of these components for Siglec binding.
Main Results:
- A novel sialoglycoprotein, receptor protein tyrosine phosphatase zeta (RPTPζ)S3L, was identified as a ligand for human CD33 and Siglec-8.
- RPTPζS3L is composed of sialylated keratan sulfate chains on a minor isoform of RPTPζ (phosphacan) and is present in the brain's extracellular milieu.
- Brains from human AD donors exhibited a twofold increase in RPTPζS3L levels compared to controls.
- Mouse models confirmed the structural basis of the ligand and its interaction with Siglecs, demonstrating that RPTPζ, St3gal4, and Chst1 are essential for Siglec binding.
Conclusions:
- The unique CD33 and Siglec-8 ligand, RPTPζS3L, is upregulated in Alzheimer's disease brains.
- Overexpression of RPTPζS3L may inhibit the clearance of misfolded proteins by microglia, thereby contributing to AD pathology.
- Targeting the RPTPζS3L-Siglec interaction could offer a novel therapeutic strategy for Alzheimer's disease.
