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 aren’t active on their own. Something has to connect with them to prevent microglia from cleaning up these toxic proteins in the brain,” says Ronald Schnaar, Ph.D., John Jacob Abel Professor of Pharmacology at the Johns Hopkins University School. of medicine and director of the laboratory that conducted 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 glycoprotein structure RPTP zeta S3L.
The group had previously found the same glycan “signature” on a protein that controls allergic responses in the airways, and disrupts allergic glycan responses in mice.
We suspect that the glycan signature carried on RPTP zeta may have a similar role in microglia silencing via CD33. »
Anabel Gonzalez-Gil Alvarenga, Ph.D., Schnaar Lab Postdoctoral Fellow and Study First Author
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 diagnosis 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.
Gonzalez-Gil, A., et al. (2022) Human brain sialoglycan ligand for CD33, a microglia inhibitor Siglec implicated in Alzheimer’s disease. Journal of Biological Chemistry. doi.org/10.1016/j.jbc.2022.101960.