Alzheimer disease (AD) is the most disabling chronic neurodegenerative disease of people over age 65. It afflicts more than 4 million Americans. Most experts agree that accumulation of oligomeric and insoluble fibrillar forms of amyloid-beta peptide (Aß) is the proximate cause of synaptic dysfunction and neuronal degeneration in AD. The findings that passive immunization of mice genetically engineered to over-express amyloid precursor protein (APP) with IgG vs. Aß, and that treatment of APP over-expressing mice with enoxaparin, a low MW heparin, leads to clearance of Aß from the brains of these mice and prevents and/or reverses their cognitive deficits, suggested a way to prevent and/or treat AD. However, the mechanism(s) by which anti-Aß IgGs and enoxaparin block Aß deposition and promote its removal were unknown. I reasoned that these mechanisms might be central to identifying methods for preventing and treating AD.
Five findings suggested to me that anti-Aß IgG and enoxaparin might block Aß binding to heparan sufate proteo glycans (HSPGs), and/or promote its dissociation from HSPGs, thereby making Aß available for clearance from the CNS by microglia and astrocytes. 1. Aß is associated with HSPGs in the CNS. 2. Proteoglycans (PGs) block microglial degradation of Aß. 3. Anti-Aß IgG and F(ab)2 fragments of these IgGs promote clearance of Aß from the brains of normal mice, and of Fc-receptor knockout mice. 4. Administration of low-molecular weight heparin promotes clearance of AA amyloid protein from spleen and kidney. 5. Mouse AA amyloid protein does not deposit in kidney or spleen of mice over-expressing heparanase.
These findings led me to explore the hypothesis that HSPGs promote oligomerization and deposition of Aß peptides and block Aß uptake and digestion by microglia. My findings suggest that substances that block interactions of Aß with HSPGs, such as anti-Aß IgGs vs. the N-terminal residues of Aß, apolipoprotein E2 (apoE2), low-molecular weight anionic sulfonates or sulfates, and specific peptides protect against AD by promoting Aß degradation. Consistent with this hypothesis, I have found that IgGs vs. N-terminal residues of Aß1-42, but not IgGs vs. its C-terminal residues, as well as various sulfated compounds, block binding of Aß to HSPG, displace Aß from Aß-HSPG complexes, and promote uptake and digestion of the displaced Aß by microglia from Fc-receptor knockout mice.
The hypothesis that Aß-HSPG interactions play a key role in Aß accumulation and clearance was reinforced by five additional observations. 1. Apolipoprotein E binds to residues in the same region of Aß as HSPG. 2. Aß forms complexes with both apoE and HSPGs. 3. The E2 isoform of apoE protects against AD, while the E4 isoform is a risk factor for AD. 4. Astrocytes are the primary source of apoE in the brain. 5. Adult astrocytes from wild type mice, but not adult astrocytes from apo-E knockout mice, remove 40% of Aß from brain slices from APP-over-expressing mice.
In exploring role(s) for apoE in regulating interactions between Aß and HSPGs, I have found that the E2, E3 and E4 isoforms of human apoE inhibit binding of Aß to HSPG in a dose-dependent manner. However, E4 is significantly less effective than E2 and E3. Moreover, apoE2 can displace Aß from Aß-HSPG complexes. These findings suggest that apoE2 protects against AD in two ways. First, it blocks Aß binding to HSPG, thereby preventing formation of Aß-HSPG complexes. Second, it promotes release of Aß from HSPG, thereby facilitating its degradation. These findings also suggest that apoE4 predisposes to AD because of its relative inefficiency in blocking Aß’s interaction with HSPG. Accordingly, in individuals expressing the E4 isoform Aß is more likely to associate with HSPG and to form Aß-HSPG complexes.
These findings provide a unifying conceptual framework for understanding the pathogenesis of Alzheimer’s disease. Taken together, they suggest mechanisms whereby the various isoforms of apoE protect against or predispose to AD. They show that HSPGs block uptake and digestion of Aß by brain microglia, and provide strong evidence that IgGs vs. the N-terminal residues of Aß promote Aß clearance from the brain by promoting its release from HSPGs. They indicate that Aß-HSPG complexes are targets for anti-Aß IgGs, sulfated compounds, peptides, and peptidomimetics that may prove useful in preventing and/or treating AD.