Factors that Contribute to Huntington’s Disease Pathogenesis – Copy

Alzheimer’s Disease Awareness Month

Alzheimer’s Disease (AD) is the most common cause of dementia, affecting an estimated 5,800,000 Americans (1,2). Many drug candidates to treat AD have emerged, but effective therapies are currently unavailable (3). Scientists continue to search for pathophysiological changes that contribute to AD and could become the target of new therapies.

Microglia are innate immune cells responsible for surveilling the brain and clearing debris and toxic proteins, including amyloid b and phosphorylated tau, which play a crucial role in AD onset and progression (5–7). Modulation of microglia function can potentially become a therapeutic target in AD (4–6). Data from genome-wide association studies indicate that changes in microglial gene expression increase susceptibility to AD (5–6,8). In particular, increased expression of the immunoregulatory receptor CD33, also known as sialic acid-binding immunoglobulin-type lectin 3 (Siglec-3), correlates with an increased risk of AD (9–12). Activation of CD33 suppresses microglial phagocytosis, migration, and proliferation, and its overexpression inhibits amyloid β clearance in vitro (12–14). As the name implies, Siglecs bind to endogenous glycoproteins or glycolipids carrying a sialic acid molecule (15). Thus, understanding and characterizing CD33 ligands present in the brain milieu can help pave the way to new therapeutics for AD.

Gonzalez-Gil et al. performed a series of biochemical experiments to isolate and characterize the structure of CD33 ligands in brain samples from patients with AD (16). Results revealed that AD brains express high levels of single sialoglycoprotein that binds to CD33 and Siglec-8, the most abundant Siglec on human microglia (16). 

Keep reading to learn the details of the Gonzalez-Gil et al. study, and check out other publication highlights on the blog. 

CD33 Binds to a Single Brain Sialoglycoprotein

In the first experiment, Gonzalez-Gil et al. sought to isolate and characterize CD33 ligands present in human brain samples. They collected post-mortem brain samples from 5 patients who died from AD and 5 age-matched controls. After homogenizing the samples, Gonzalez-Gil et al. performed western blots using CD33-Fc and Siglec-8-Fc chimeras. Results showed that both probes recognized a single glycoprotein with approximately 1 MDa.

Previous data from Gonzalez-Gil et al. have revealed that Siglec-8 selectively binds to sialic acid linked to a galactose carrying a sulfate ester (15). And CD33 displays a high affinity for this same type of structure (15). Researchers then investigated whether the glycoprotein recognized by CD33-Fc and Siglec-8-Fc carries the required sialic acid structure. Pre-treatment of brain homogenates with keratanase I (cleaves monosulfated disaccharides in keratan sulfate) or sialidase (hydrolyses sialic acid) eliminated the binding of CD33-Fc and Siglec-8-Fc to the glycoprotein of interest. However, pre-treatment with PNGase F (cleaves N-linked but not O-linked glycans) shifted the glycoprotein migration but spared its binding to CD33-Fc and Siglec-8-Fc. These results confirmed that endogenous CD33 and Siglec-8 ligands are sialoglycoproteins.

CD33 Ligand Comprises a Receptor Protein Tyrosine Phosphatase Zeta

Next, Gonzalez-Gil et al. performed mass spectroscopy, and the results revealed that a receptor protein tyrosine phosphatase zeta (RPTPζ) composes the protein portion of the newly isolated sialoglycoprotein. RPTPζ is a proteoglycan found both as a transmembrane protein tyrosine phosphatase and as a released extracellular form known as phosphacan.

Electrophoretic resolution revealed 3 large and 2 small isoforms of RPTPζ, but only the largest one co-migrated with CD33-Fc and Siglec-8-Fc. The largest isoform also co-eluted with CD33-Fc and Siglec-8-Fc in size-exclusion chromatography. Additional experiments confirmed that the same isoform of RPTPζ carries CD33 and Siglec-8 ligands. Gonzalez-Gil et al. performed western blots using anti-RPTPζ, CD33-Fc, Siglec-8-Fc probes, and fluorescent secondary antibodies. After overlaying the images, they observed that the band intensity increased, suggesting that all the different probes bound to the same molecule. After confirming the identity of the CD33 ligand, the authors named it RPTPζ^S3L.

To test the hypothesis that activation of CD33 by endogenous ligands might play a role in the pathogenesis of AD, Gonzalez-Gil et al. quantified the expression of RPTPζ^S3L in brain samples from patients with AD and age-matched controls. Results revealed that the concentration of RPTPζ^S3L was 2X higher in AD than in control brains.

Siglecs on Murine Microglia Bind to the Same Sialoglycan

Researchers also screened mouse brain samples for Siglecs that bind to human RPTPζ^S3L. Results revealed that Siglec-F, the mouse correspondent of human Siglec-8, co-migrated with RPTPζ^S3L (17). However, in Ptprz1-null mice—they don’t express RPTPζ—Siglec-F-Fc and anti-RPTPζ staining and co-migration were absent. Gonzalez-Gil et al. also developed transgene mice lacking enzymes essential to synthesizing sialoglycoproteins. Western blot results revealed that brain extracts from those knockout mice don’t bind to Siglec-F-Fc and Siglec-8-Fc. Together, these data suggest that Siglecs on human and mouse microglia bind to a similar sialoglycoprotein.

CD33 Ligand is Present in the Brain Milieu

In the last set of experiments, Gonzalez-Gil et al. performed immunohistochemistry to investigate the histological distribution of RPTPζ^S3L in non-AD human brain samples. After quenching endogenous HRP and AP activity with BLOXALL® Endogenous Blocking Solution, Peroxidase and Alkaline Phosphatase (SP-6000-100), researchers incubated brain samples with anti-human RPTPζ, CD33-Fc, or Siglec-8-Fc. After incubation with secondary antibodies, they quenched endogenous fluorescence in the tissue using Vector® TrueVIEW® Autofluorescence Quenching Kit (SP-8400-15). Results revealed that RPTPζ^S3L is present in the extracellular environment of human brain parenchyma. 

Future Perspectives

As future research confirms the role of CD33 and RPTPζ^S3L in AD pathophysiology, a new target for therapeutic intervention might emerge. Blocking the interaction of CD33 and RPTPζ^S3L could potentially prevent microglia inhibition and optimize amyloid b and phosphorylated tau clearance. The fact that Gonzalez-Gil et al. demonstrated that human and mouse Siglecs bind to the same sialoglycoproteins increases the translational impact of the acquired data. Drug development benefits from molecular structures that are well-conserved across different species.

Check out other resources in the blog, and stay tuned for more insights and research. 

References

1. Breijyeh Z, et al. 2020. Comprehensive Review on Alzheimer’s Disease: Causes and Treatment. Molecules.
2. Matthews KA, et al. 2018. Racial and Ethnic Estimates of Alzheimer’s Disease and Related Dementias in the United States (2015-2060) in Adults Aged ≥ 65 Years. Alzheimer’s & Dementia.
3. Cummings J, et al. 2022. Alzheimer’s Disease Drug Development Pipeline: 2022. Translational Research & Clinical Interventions.
4. Eskandari-Sedighi G, et al. 2023. CD33 Isoforms in Microglia and Alzheimer’s Disease: Friend and Foe. Molecular Aspects of Medicine.
5. Lewcock JW, et al. 2020. Emerging Microglia Biology Defines Novel Therapeutic Approaches for Alzheimer’s Disease. Neuron.
6. Salter MW, et al. 2017. Microglia Emerge as Central Players in Brain Disease. Nature Medicine.
7. Scheltens P, et al. 2021. Alzheimer’s Disease. The Lancet.
8. Schwabe T, et al. 2020. Shifting Paradigms: The Central Role of Microglia in Alzheimer’s Disease. Neurobiology of Disease.
9. Bertram L, et al. 2008. Genome-Wide Association Analysis Reveals Putative Alzheimer’s Disease Susceptibility Loci in Addition to APOE. American Journal of Human Genetics.
10. Hollingworth P, et al. 2011. Common Variants at ABCA7, MS4A6A/MS4A4E, EPHA1, CD33 and CD2AP are Associated with Alzheimer’s Disease. Nature Genetics.
11. Naj AC, et al. 2011. Common Variants at MS4A4/MS4A6E, CD2AP, CD33 and EPHA1 are Associated With Late-Onset Alzheimer’s Disease. Nature Genetics.
12. Griciuc A, et al. 2021. The Role of Innate Immune Genes in Alzheimer’s Disease. Current Opinion in Neurology.
13. Butler CA, et al. 2021. CD33M Inhibits Microglial Phagocytosis, Migration and Proliferation, but the Alzheimer’s Disease-Protective Variant CD33m Stimulates Phagocytosis and Proliferation, and Inhibits Adhesion. Journal of Neurochemistry.
14. Griciuc A, et al. 2013. Alzheimer’s Disease Risk Gene CD33 Inhibits Microglial Uptake of Amyloid Beta. Neuron.
15. Gonzalez-Gil A, et al. 2021. Siglec Ligands. Cells.
16. Gonzalez-Gil A, et al. 2022. Human Brain Sialoglycan Ligand for CD33, a Microglial Inhibitory Siglec Implicated in Alzheimer’s Disease. Journal of Biological Chemistry.
17. Morshed N, et al. 2020. Phosphoproteomics Identifies Microglial Siglec-F Inflammatory Response During Neurodegeneration. Molecular Systems Biology.