Sugar-studded protein linked to Alzheimer’s disease

Summary: Glycan, a special sugar protein, appears to play an important role in the development of Alzheimer’s disease.

Source: Johns Hopkins University

In a bit of ‘reverse-engineered’ research using brain tissue from five people who died of Alzheimer’s disease, Johns Hopkins Medicine researchers say they discovered that a special sugar molecule could play a key role in the development of Alzheimer’s disease.

If further research confirms the discovery, the molecule, known as a glycan, could serve as a new target for early diagnostic tests, treatments and possibly prevention of Alzheimer’s disease, the researchers say.

The study was published online April 20 in the Journal of Biological Chemistry.

Alzheimer’s disease is the most common form of dementia in the United States. Affecting about 5.8 million Americans, the progressive disorder occurs when nerve cells in the brain die due to the buildup of harmful forms of proteins called amyloid and tau.

Cleaning up pathogenic forms of amyloid and tau is the job of immune cells in the brain, called microglia. Previous studies have found that when cleansing is impaired, Alzheimer’s disease is more likely to occur. In some people, this is caused by an overabundance of a receptor on microglia cells called CD33.

“Receptors are not active by themselves. Something has to connect with them to keep microglia from cleaning up these toxic proteins in the brain, says Ronald Schnaar, Ph.D., John Jacob Abel Professor of Pharmacology at Johns Hopkins University School of Medicine and director of the lab who led the study.

Previous studies by the researchers have shown that for CD33, these “connector” molecules are particular sugars. Known to scientists as glycans, these molecules are carried around the cell by specialized proteins that help them find their appropriate receptors. The protein-glycan combination is called glycoprotein.

In an effort to find out which specific glycoprotein connects to CD33, Schnaar’s research team obtained brain tissue from five people who died of Alzheimer’s disease and five people who died of other causes from Johns Hopkins Alzheimer’s Disease Research. Center. Of the thousands of glycoproteins they collected from brain tissue, only one was connected to CD33.

To identify this mysterious glycoprotein, researchers first had to separate it from other brain glycoproteins. Since it was the only one in the brain that attached to CD33, they used this function to “catch up” with it and pull it apart.

Glycans are made up of various sugar building blocks that influence the interactions of the molecule. These sugars can be identified by their components.

The researchers used chemical tools to deconstruct the glycan step by step, establishing the identity and order of its building blocks. The researchers identified the glycan portion of the glycoprotein as sialylated keratan sulfate.

Next, the researchers determined the identity of the protein component by taking its ‘fingerprint’ using mass spectroscopy, which identifies the building blocks of proteins.

By comparing the molecular makeup of the protein with a database of known protein structures, the research team was able to conclude that the protein part of the glycoprotein was receptor tyrosine phosphatase (RPTP) zeta.

The researchers named the combined structure of the RPTP zeta glycoprotein S3L.

Cleaning up pathogenic forms of amyloid and tau is the job of immune cells in the brain, called microglia. Image is in public domain

The group had previously found the same glycan “signature” on a protein that controls allergic responses in the airways, and that disruption of the glycan dampened allergic responses in mice.

“We suspect that the glycan signature carried by RPTP zeta may have a similar role in microglia silencing via CD33,” says Anabel Gonzalez-Gil Alvarenga, Ph.D., postdoctoral fellow in the Schnaar lab and first author of the study.

Other experiments showed that the brain tissue of the five people who died of Alzheimer’s disease contained more than twice as much RPTP zeta S3L as the donors who did not have the disease.

This implies that this glycoprotein can connect to more CD33 receptors than a healthy brain, limiting the brain’s ability to clean up harmful proteins.

“The identification of this unique glycoprotein is a step towards finding new drug targets and potentially early diagnostics of Alzheimer’s disease,” says Gonzalez-Gil.

Next, the researchers plan to further study the structure of RPTP zeta S3L to determine how its attached glycans give the glycoprotein its unique ability to interact with CD33.

See also

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About this Alzheimer’s disease research news

Author: Press office
Source: Johns Hopkins University
Contact: Press Office – Johns Hopkins University
Image: Image is in public domain

Original research: Free access.
Human brain sialoglycan ligand for CD33, a Siglec microglia inhibitor implicated in Alzheimer’s disease” by Anabel Gonzalez-Gil et al. Journal of Biological Chemistry


Summary

Human brain sialoglycan ligand for CD33, a Siglec microglia inhibitor implicated in Alzheimer’s disease

Alzheimer’s disease (AD) is characterized by an accumulation of misfolded proteins. Genetic studies implicate microglia, resident phagocytic immune cells in the brain, in the pathogenesis of AD. As positive effectors, microglia remove toxic proteins, while as negative effectors, they release pro-inflammatory mediators. An imbalance of these functions contributes to the progression of AD.

Polymorphisms of human CD33, an inhibitory microglial receptor, are linked to AD susceptibility; higher expression of CD33 is correlated with an increased risk of AD. CD33, also known as Siglec-3, is a member of the sialic acid-binding immunoglobulin (Siglec)-like lectin family of immunoregulatory 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 to CD33 as well as Siglec-8, the most abundant Siglec on human microglia. The ligand, which we call receptor protein tyrosine phosphatase zeta (RPTPζ)S3Lis composed of sialylated keratan sulfate chains carried by a minor isoform/glycoform of RPTPζ (phosphacan) and is found in the extracellular medium of human brain parenchyma.

AD human donor brains had twice the levels of RPTPζS3L than age-matched control donors, raising the possibility that RPTPζS3L overexpression limits clearance of misfolded proteins contributing to AD pathology.

The mice express the same structure, an RPTPζ isoform of sialylated keratan sulfate, which binds mouse Siglec-F and cross-reacts with human CD33 and Siglec-8.

Mouse brains engineered to lack RPTPζ, the sialyltransferase St3gal4or keratan sulfate sulfotransferase Chst1 lacked a Siglec bond, establishing the structure of the ligand.

The unique ligand CD33 and Siglec-8, RPTPζS3Lmay contribute to the progression of AD.

Rachel J. Bradford