Cerebral vascular amyloid seeds drive amyloid ß-protein fibril assembly with a distinct anti-parallel structure.

Cerebrovascular accumulation of amyloid ß-protein (Aß), a condition known as cerebral amyloid angiopathy (CAA), is a common pathological feature of patients with Alzheimer's disease. Familial Aß mutations, such as Dutch-E22Q and Iowa-D23N, can cause severe cerebrovascular accumulation of amyloid that serves as a potent driver of vascular cognitive impairment and dementia. The distinctive features of vascular amyloid that underlie its unique pathological properties remain unknown. Here, we use transgenic mouse models producing CAA mutants (Tg-SwDI) or overproducing human wild-type Aß (Tg2576) to demonstrate that CAA-mutant vascular amyloid influences wild-type Aß deposition in brain. We also show isolated microvascular amyloid seeds from Tg-SwDI mice drive assembly of human wild-type Aß into distinct anti-parallel ß-sheet fibrils. These findings indicate that cerebrovascular amyloid can serve as an effective scaffold to promote rapid assembly and strong deposition of Aß into a unique structure that likely contributes to its distinctive pathology.

Early-onset formation of parenchymal plaque amyloid abrogates cerebral microvascular amyloid accumulation in transgenic mice.

The fibrillar assembly and deposition of amyloid ß (Aß) protein, a key pathology of Alzheimer disease, can occur in the form of parenchymal amyloid plaques and cerebral amyloid angiopathy (CAA). Familial forms of CAA exist in the absence of appreciable parenchymal amyloid pathology. The molecular interplay between parenchymal amyloid plaques and CAA is unclear. Here we investigated how early-onset parenchymal amyloid plaques impact the development of microvascular amyloid in transgenic mice. Tg-5xFAD mice, which produce non-mutated human Aß and develop early-onset parenchymal amyloid plaques, were bred to Tg-SwDI mice, which produce familial CAA mutant human Aß and develop cerebral microvascular amyloid. The bigenic mice presented with an elevated accumulation of Aß and fibrillar amyloid in the brain compared with either single transgenic line. Tg-SwDI/Tg-5xFAD mice were devoid of microvascular amyloid, the prominent pathology of Tg-SwDI mice, but exhibited larger parenchymal amyloid plaques compared with Tg-5xFAD mice. The larger parenchymal amyloid deposits were associated with a higher loss of cortical neurons and elevated activated microglia in the bigenic Tg-SwDI/Tg-5xFAD mice. The periphery of parenchymal amyloid plaques was largely composed of CAA mutant Aß. Non-mutated Aß fibril seeds promoted CAA mutant Aß fibril formation in vitro. Further, intrahippocampal administration of biotin-labeled CAA mutant Aß peptide accumulated on and adjacent to pre-existing parenchymal amyloid plaques in Tg-5xFAD mice. These findings indicate that early-onset parenchymal amyloid plaques can serve as a scaffold to capture CAA mutant Aß peptides and prevent their accumulation in cerebral microvessels.

Cerebral microvascular rather than parenchymal amyloid-ß protein pathology promotes early cognitive impairment in transgenic mice.

Alzheimer's disease (AD) is an age-dependent neurodegenerative condition that causes a progressive decline in cognitive function. Accumulation of amyloid ß-protein (Aß) in the brain is a prominent feature of AD and related disorders. However, the levels of Aß accumulation alone are not a reliable predictor of cognitive deficits. Aß accumulates in AD brain in the form of parenchymal amyloid plaques and cerebral vascular deposits. Although both types of lesions can contribute to cognitive decline, their temporal impact remains unclear. Moreover, cerebral microvascular pathology is identified as an early driver of cognitive impairment. Here for the first time, we compared two transgenic mouse strains, Tg-5xFAD and Tg-SwDI, which exhibit similar onset and anatomical accumulation of Aß, but with distinct parenchymal and microvascular compartmental deposition, respectively, to assess their impact on cognitive impairment. Cohorts of each line were tested at 3 and 6 months of age to assess the relationship between spatial working memory performance and quantitative pathology. At 3 months of age, Tg-SwDI mice with onset of cerebral microvascular amyloid were behaviorally impaired, while the Tg-5xFAD, which had disproportionately higher levels of total Aß, soluble oligomeric Aß, and parenchymal amyloid were not. However, at 6 months of age, behavioral deficits for both groups of transgenic mice were evident, as the levels of Aß pathologies in the Tg-5xFAD accumulated to extremely high amounts. The present findings suggest early-onset cerebral microvascular amyloid deposition, that precedes high parenchymal levels of Aß, may be an important early factor in the development of cognitive deficits.

Fine mapping of the amyloid ß-protein binding site on myelin basic protein.

The assembly and deposition of amyloid ß-protein (Aß) in brain is a key pathological feature of Alzheimer's disease and related disorders. Factors have been identified that can either promote or inhibit Aß assembly in brain. We previously reported that myelin basic protein (MBP) is a potent inhibitor of Aß fibrillar assembly [Hoos, M. D., et al. (2007) J. Biol. Chem. 282, 9952-9961; Hoos, M. D., et al. (2009) Biochemistry 48, 4720-4727]. Moreover, the region on MBP responsible for this activity was localized to the 64 N-terminal amino acids (MBP1-64) [Liao, M. C., et al. (2010) J. Biol. Chem. 285, 35590-35598]. In the study presented here, we sought to further define the site on MBP1-64 involved in this activity. Deletion mapping studies showed that the C-terminal region (residues 54-64) is required for the ability of MBP1-64 to bind Aß and inhibit fibril assembly. Alanine scanning mutagenesis revealed that amino acids K54, R55, G56, and K59 within MBP1-64 are important for both Aß binding and inhibition of fibril assembly as assessed by solid phase binding, thioflavin T binding and fluorescence, and transmission electron microscopy studies. Strong spectral shifts are observed by solution nuclear magnetic resonance spectroscopy of specific N-terminal residues (E3, R5, D7, E11, and Q15) of Aß42 upon the interaction with MBP1-64. Although the C-terminal region of MBP1-64 is required for interactions with Aß, a synthetic MBP50-64 peptide was itself devoid of activity. These studies identify key residues in MBP and Aß involved in their interactions and provide structural insight into how MBP regulates Aß fibrillar assembly.