Analysis of the Putative Role of CR1 in Alzheimer's Disease: Genetic Association, Expression and Function

• Published in: PloS one Vol. 11; no. 2; p. e0149792
• Main Authors: Fonseca, Maria I, Chu, Shuhui, Pierce, Aimee L, Brubaker, William D, Hauhart, Richard E, Mastroeni, Diego, Clarke, Elizabeth V, Rogers, Joseph, Atkinson, John P, Tenner, Andrea J
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 25.02.2016; Public Library of Science (PLoS)
• Subjects: Aged; Aged, 80 and over; Alzheimer Disease - blood; Alzheimer Disease - genetics; Alzheimer Disease - metabolism; Alzheimer's disease; Alzheimers disease; Analysis; Antibodies; Astrocytes; Astrocytes - metabolism; Binding; Biochemistry; Biology and Life Sciences; Blood; Blood cells; Blood vessels; Brain; Brain - pathology; Brain research; Cell receptors; Complement activation; Complement C1q - metabolism; Complement C3b - metabolism; Complement component C1q; Complement component C3b; Development and progression; Diagnosis; Enzyme-linked immunosorbent assay; Epitopes; Erythrocytes; Erythrocytes - metabolism; Female; Gene Expression Regulation; Genetic aspects; Genetic Predisposition to Disease - genetics; Genomes; Genotypes; Humans; Immune system; Male; Medicine; Medicine and Health Sciences; Molecular biology; Neurodegenerative diseases; Neurological disorders; Neurosciences; Parenchyma; Physical Sciences; Physiological aspects; Plasma levels; Polymorphism, Single Nucleotide; Protein Transport; Proteins; Receptors, Complement 3b - blood; Receptors, Complement 3b - genetics; Receptors, Complement 3b - metabolism; Red blood cells; Research and Analysis Methods; Rheumatology; Single nucleotide polymorphisms; Single-nucleotide polymorphism; Staining; Statistical analysis; Statistical methods; United States; Oregon; California; United States > US; Arizona; Missouri
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0149792

Chronic activation of the complement system and induced inflammation are associated with neuropathology in Alzheimer's disease (AD). Recent large genome wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) in the C3b/C4b receptor (CR1 or CD35) that are associated with late onset AD. Here, anti-CR1 antibodies (Abs) directed against different epitopes of the receptor, were used to localize CR1 in brain, and relative binding affinities of the CR1 ligands, C1q and C3b, were assessed by ELISA. Most Abs tested stained red blood cells in blood vessels but showed no staining in brain parenchyma. However, two monoclonal anti-CR1 Abs labeled astrocytes in all of the cases tested, and this reactivity was preabsorbed by purified recombinant human CR1. Human brain-derived astrocyte cultures were also reactive with both mAbs. The amount of astrocyte staining varied among the samples, but no consistent difference was conferred by diagnosis or the GWAS-identified SNPs rs4844609 or rs6656401. Plasma levels of soluble CR1 did not correlate with diagnosis but a slight increase was observed with rs4844609 and rs6656401 SNP. There was also a modest but statistically significant increase in relative binding activity of C1q to CR1 with the rs4844609 SNP compared to CR1 without the SNP, and of C3b to CR1 in the CR1 genotypes containing the rs6656401 SNP (also associated with the larger isoform of CR1) regardless of clinical diagnosis. These results suggest that it is unlikely that astrocyte CR1 expression levels or C1q or C3b binding activity are the cause of the GWAS identified association of CR1 variants with AD. Further careful functional studies are needed to determine if the variant-dictated number of CR1 expressed on red blood cells contributes to the role of this receptor in the progression of AD, or if another mechanism is involved.