No abstract

Alzheimer’s disease (AD) is the most common progressive neurodegenerative disorder causing dementia. Massive deposition of amyloid β peptide (Aβ) as senile plaques in the brain is the pathological hallmark of AD, but oligomeric, soluble forms of Aβ have been implicated as the synaptotoxic component. The apolipoprotein E epsilon 4 (apoE ε4) allele is known to be a genetic risk factor for developing AD. However it is still unknown how apoE impacts the process of Aβ oligomerization. Here, we found that the level of Aβ oligomers in APOEε4/ε4 AD patient brains is 2.7 times higher than those in APOEε3/ε3 AD patient brains, matched for total plaque burden, suggesting that apoE4 impacts the metabolism of Aβ oligomers. To test this hypothesis, we examined apoE’s effect on Aβ oligomer formation. Using both synthetic Aβ and a split-luciferase method for monitoring Aβ oligomers, we observed that apoE increased the level of Aβ oligomers in an isoform dependent manner (E2 < E3 < E4). This effect appears to be dependent on the ApoE carboxy-terminal domain. Moreover, these results were confirmed using endogenous apoE isolated from the TBS-soluble fraction of human brain, which increased the formation of Aβ oligomers. Taken together, these data show that lipidated apoE, especially apoE4, increases Aβ oligomers in the brain. Higher levels of Aβ oligomers in the brains of APOEε4/ε4 carriers compared to APOEε3/ε3 carriers may increase the loss of dendritic spines and accelerate memory impairments, leading to earlier cognitive decline in AD.

Sphingosine kinases (SphK) 1 and 2 phosphorylate sphingosine to generate sphingosine-1-phosphate (S1P), a pluripotent lipophilic mediator implicated in a variety of cellular events. Here we show that the activity of β-site APP cleaving enzyme-1 (BACE1), the rate limiting enzyme for amyloid-β peptide (Aβ) production, is modulated by S1P in neurons. Treatment by SphK inhibitor, RNAi knockdown of SphK or overexpression of S1P degrading enzymes decreased BACE1 activity to reduce Aβ production. S1P specifically bound to full-length BACE1 and increased its proteolytic activity, suggesting that the cellular S1P directly modulates BACE1 activity. Notably, the relative activity of SphK2 was upregulated in the brains of patients with Alzheimer disease. The unique modulatory effect of cellular S1P on BACE1 activity is a novel potential therapeutic target for Alzheimer disease.

A subset of Alzheimer disease cases is caused by autosomal dominant mutations in genes encoding the amyloid ␤-protein precursor or presenilins. Whereas some amyloid ␤-protein precursor mutations alter its metabolism through effects on A␤ production, the pathogenic effects of those that alter amino acid residues within the A␤ sequence are not fully understood. Here we examined the biophysical effects of two recently described intra-A␤ mutations linked to early-onset familial Alzheimer disease, the D7N Tottori-Japanese and H6R English mutations. Although these mutations do not affect A␤ production, synthetic A␤(1-42) peptides carrying D7N or H6R substitutions show enhanced fibril formation. In vitro analysis using A␤(1-40)-based mutant peptides reveal that D7N or H6R mutations do not accelerate the nucleation phase but selectively promote the elongation phase of amyloid fibril formation. Notably, the levels of protofibrils generated from D7N or H6R A␤ were markedly inhibited despite enhanced fibril formation. These N-terminal A␤ mutations may accelerate amyloid fibril formation by a unique mechanism causing structural changes of A␤ peptides, specifically promoting the elongation process of amyloid fibrils without increasing metastable intermediates. Alzheimer disease (AD)3 is characterized pathologically by deposition of the amyloid ␤-protein (A␤) in the form of amyloid plaques in the brain (1). A␤ is produced through proteolytic cleavage of the amyloid ␤-protein precursor (APP) through sequential cleavages by two proteases, ␤-and ␥-secretase (2, 3). A subset of AD cases is inherited in an autosomal-dominant manner. Three genes, APP and presenilins 1 and 2, have been linked to familial AD (FAD) (4 -6). Missense mutations in presenilin 1, presenilin 2, or APP generally increase the proportion of the 42-residue form of 〈␤, A␤(1-42), relative to the 40-residue form of the peptide, A␤(1-40). 〈␤(1-42) is the species initially deposited in the brain in AD and has been shown to aggregate more readily than A␤(1-40) (7,8). A Swedish APP double mutation increases the overall production of A␤ by enhancing ␤-secretase cleavage of the 〈␤ N terminus.In contrast, the pathological phenotypes caused by mutations that alter amino acid residues within the A␤ sequence are variable, and the underlying pathogenetic mechanisms are not fully understood. Recent data support the notion that intra-A␤ amino acid substitutions affect peptide self-association. For example, the Arctic mutation that causes an E22G substitution and early-onset FAD enhances the formation of protofibrils (9) and fibrils of A␤ in vitro (10) and the deposition of A␤ in vivo in the brains of transgenic mice (11,12). The Dutch (E22Q), Italian (E22K), or Iowa (D23N) mutations, linked to hereditary cerebral hemorrhage with amyloidosis or AD with severe amyloid angiopathy, have been shown to enhance the formation of protofibrils or fibrils from A␤ in vitro (10, 13-15) and produce parenchymal or vascular A␤ deposits in the brains of transgenic mice (16,17). Taken together,...

Background:Cromolyn sodium is an FDA-approved drug structurally similar to fisetin, an antiamyloidogenic molecule. Results: Cromolyn sodium interferes with amyloid ␤ (A␤) aggregation in vitro while rapidly decreasing the levels of soluble A␤ peptides in vivo after a week. Conclusion: Cromolyn sodium may have an impact on amyloid economy. Significance: Developing new disease-modifying therapeutics remains an urgent need in the treatment of Alzheimer disease.