CD33 and SIGLECL1 Immunoglobulin Superfamily Involved in Dementia

Therapeutic potential to target sialylation and SIGLECs in neurodegenerative and psychiatric diseases

Sialic acids, commonly found as the terminal carbohydrate on the glycocalyx of mammalian cells, are pivotal checkpoint inhibitors of the innate immune system, particularly within the central nervous system (CNS). Sialic acid-binding immunoglobulin-like lectins (SIGLECs) expressed on microglia are key players in maintaining microglial homeostasis by recognizing intact sialylation. The finely balanced sialic acid-SIGLEC system ensures the prevention of excessive and detrimental immune responses in the CNS. However, loss of sialylation and SIGLEC receptor dysfunctions contribute to several chronic CNS diseases. Genetic variants of SIGLEC3/CD33, SIGLEC11, and SIGLEC14 have been associated with neurodegenerative diseases such as Alzheimer's disease, while sialyltransferase ST8SIA2 and SIGLEC4/MAG have been linked to psychiatric diseases such as schizophrenia, bipolar disorders, and autism spectrum disorders. Consequently, immune-modulatory functions of polysialic acids and SIGLEC binding antibodies have been exploited experimentally in animal models of Alzheimer's disease and inflammation-induced CNS tissue damage, including retinal damage. While the potential of these therapeutic approaches is evident, only a few therapies to target either sialylation or SIGLEC receptors have been tested in patient clinical trials. Here, we provide an overview of the critical role played by the sialic acid-SIGLEC axis in shaping microglial activation and function within the context of neurodegeneration and synaptopathies and discuss the current landscape of therapies that target sialylation or SIGLECs.

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Epigenome-Wide Analysis of DNA Methylation and Optimism in Women and Men

OBJECTIVE

Higher optimism is associated with reduced mortality and a lower risk of age-related chronic diseases. DNA methylation (DNAm) may provide insight into mechanisms underlying these relationships. We hypothesized that DNAm would differ among older individuals who are more versus less optimistic.

METHODS

Using cross-sectional data from two population-based cohorts of women with diverse races/ethnicities ( n = 3816) and men (only White, n = 667), we investigated the associations of optimism with epigenome-wide leukocyte DNAm. Random-effects meta-analyses were subsequently used to pool the individual results. Significantly differentially methylated cytosine-phosphate-guanines (CpGs) were identified by the "number of independent degrees of freedom" approach: effective degrees of freedom correction using the number of principal components (PCs), explaining >95% of the variation of the DNAm data (PC-correction). We performed regional analyses using comb-p and pathway analyses using the Ingenuity Pathway Analysis software.

RESULTS

We found that essentially all CpGs (total probe N = 359,862) were homogeneous across sex and race/ethnicity in the DNAm-optimism association. In the single CpG site analyses based on homogeneous CpGs, we identified 13 significantly differentially methylated probes using PC-correction. We found four significantly differentially methylated regions and two significantly differentially methylated pathways. The annotated genes from the single CpG site and regional analyses are involved in psychiatric disorders, cardiovascular disease, cognitive impairment, and cancer. Identified pathways were related to cancer, and neurodevelopmental and neurodegenerative disorders.

CONCLUSION

Our findings provide new insights into possible mechanisms underlying optimism and health.

Semantic and right temporal variant of FTD: Next generation sequencing genetic analysis on a single-center cohort

Semantic and right temporal variant of frontotemporal dementia (svFTD and rtvFTD) are rare clinical phenotypes in which, in most cases, the underlying pathology is TDP-43 proteinopathy. They are usually sporadic disorders, but recent evidences suggest a higher frequency of genetic mutations for the right temporal versus the semantic variant. However, the genetic basis of these forms is not clear. In this study we performed a genetic screening of a single-center cohort of svFTD and rtvFTD patients, aiming at identifying the associated genetic variants. A panel of 73 dementia candidate genes has been analyzed by NGS target sequencing including both causal and risk/modifier genes in 23 patients (15 svFTD and 8 rtvFTD) and 73 healthy age-matched controls. We first performed a single variant analysis considering rare variants and then a gene-based aggregation analysis to evaluate the cumulative effects of multiple rare variants in a single gene. We found 12 variants in nearly 40% of patients (9/23), described as pathogenic or classified as VUS/likely pathogenic. The overall rate was higher in svFTD than in rtvFTD. Three mutations were located in MAPT gene and single mutations in the following genes: SQSTM1, VCP, PSEN1, TBK1, OPTN, CHCHD10, PRKN, DCTN1. Our study revealed the presence of variants in genes involved in pathways relevant for the pathology, especially autophagy and inflammation. We suggest that molecular analysis should be performed in all svFTD and rtvFTD patients, to better understand the genotype-phenotype correlation and the pathogenetic mechanisms that could drive the clinical phenotypes in FTD.

CD33 rs2455069 SNP: Correlation with Alzheimer's Disease and Hypothesis of Functional Role

The CD33 gene encodes for a member of the sialic-acid-binding immunoglobulin-type lectin (Siglec) family, and is one of the top-ranked Alzheimer’s disease (AD) risk genes identified by genome-wide association studies (GWAS). Many CD33 polymorphisms are associated with an increased risk of AD, but the function and potential mechanism of many CD33 single-nucleotide polymorphisms (SNPs) in promoting AD have yet to be elucidated. We recently identified the CD33 SNP rs2455069-A>G (R69G) in a familial form of dementia. Here, we demonstrate an association between the G allele of the rs2455069 gene variant and the presence of AD in a cohort of 195 patients from southern Italy. We carried out in silico analysis of the 3D structures of CD33 carrying the identified SNP to provide insights into its functional effect. Structural models of the CD33 variant carrying the R69G amino acid change were compared to the CD33 wild type, and used for the docking analysis using sialic acid as the ligand. Our analysis demonstrated that the CD33-R69G variant may bind sialic acid at additional binding sites compared to the wild type, thus potentially increasing its affinity/specificity for this molecule. Our results led to a new hypothesis of rs2455069-A>G SNP as a risk factor for AD, suggesting that a long-term cumulative effect of the CD33-R69G variant results from the binding of sialic acid, acting as an enhancer of the CD33 inhibitory effects on amyloid plaque degradation.