Alzheimer’s disease (AD) is an age-dependent neurodegenerative condition that causes a progressive decline in cognitive function. Accumulation of amyloid β-protein (Aβ) in the brain is a prominent feature of AD and related disorders. The levels of Aβ accumulation alone are not a reliable predictor of cognitive deficits, which suggests other factors such as location of deposition may be more important. Aβ 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 Aβ, but with distinct parenchymal and microvascular compartmental amyloid deposition, to assess the impact of their respective pathologies on cognitive impairment. Cohorts of each line were tested at 3 and 6 months of age to assess the relationship between spatial working memory and quantitative Aβ 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 Aβ, soluble oligomeric Aβ, 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 Aβ pathologies in the Tg-5xFAD accumulated to extremely high amounts. The present findings suggest that early-onset cerebral microvascular amyloid deposition, that precedes high parenchymal levels of Aβ, may be an important early factor in the development of cognitive deficits. Key words: amyloid β protein; cerebral microvascular, parenchymal, pathology, cognitive impairment, transgenic mice

Cerebrovascular accumulation of amyloid β-protein (Aβ), a condition known as cerebral amyloid angiopathy (CAA), is a common pathological feature of patients with Alzheimer's disease. Familial Aβ 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 Aβ (Tg2576) to demonstrate that CAA-mutant vascular amyloid influences wild-type Aβ deposition in brain. We also show isolated microvascular amyloid seeds from Tg-SwDI mice drive assembly of human wild-type Aβ into distinct anti-parallel β-sheet fibrils. These findings indicate that cerebrovascular amyloid can serve as an effective scaffold to promote rapid assembly and strong deposition of Aβ into a unique structure that likely contributes to its distinctive pathology.

The assembly and deposition of amyloid β-protein (Aβ) in brain is a key pathological feature of Alzheimer’s disease and related disorders. Factors have been identified that can either promote or inhibit Aβ assembly in brain. We previously reported that myelin basic protein (MBP) is a potent inhibitor of Aβ fibrillar assembly [Hoos et al. 2007 J. Biol. Chem. 282:9952–9961; Hoos et al. 2009 Biochemistry 48:4720–4727]. Moreover, the region on MBP responsible for this activity was localized to the N-terminal 64 amino acids (MBP1-64) [Liao et al. 2010 J. Biol. Chem. 285:35590–35598]. In the present study 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 Aβ and inhibit fibril assembly. Alanine scanning mutagenesis revealed that amino acids K54, R55, G56 and K59 within MBP1-64 are important for both Aβ 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 NMR spectroscopy of specific N-terminal residues (E3, R5, D7, E11 and Q15) of Aβ42 upon the interaction with MBP1-64. Although the C-terminal region of MBP1-64 is required for interactions with Aβ, a synthetic MBP50-64 peptide was itself devoid of activity. These studies identify key residues in MBP and Aβ involved in their interactions and provide structural insight into how MBP regulates Aβ fibrillar assembly.

Background: Fibrillar amyloid proteins deposit in plaques and blood vessels in the brain in Alzheimer disease and related disorders. Results: Early formation of amyloid plaques impedes subsequent amyloid accumulation in blood vessels. Conclusion: Amyloid deposition in one compartment can impact amyloid accumulation in another compartment in the brain. Significance: Learning how amyloid proteins influence further formation is important for understanding the progression of pathology in neurodegenerative diseases.

Background: Cerebrovascular accumulation of the amyloid betaprotein (Abeta), a condition known as cerebral amyloid angiopathy (CAA), is a common pathological feature of patients with Alzheimer's disease. Additionally, familial forms of CAA, with specific Abeta mutations such as Dutch E22Q and Iowa D23N, cause severe cerebral vascular accumulation of amyloid that serves as a potent and early driver of vascular cognitive impairment and dementia (VCID). The distinctive features of vascular amyloid that underlie its unique pathological properties remain unknown. Here we investigated how cerebral vascular fibrillar amyloid seeds influence the assembly, accumulation and structure of Abeta. Methods: A combination of biochemical and biophysical approaches were used to study amyloid fibril formation in vitro. Transgenic mice were then used in conjunction with quantitative pathological, biochemical and structural analyses to study how CAA mutant and wild-type Abeta interact in brain to drive vascular amyloid formation. Results:In the in vitro that CAA mutant amyloid fibril seeds can adopt a parallel or anti-parallel configuration and that both can promote rapid fibril assembly of wild-type Abeta peptides that adopt corresponding fibrillar signatures. In the in vivo studies we first show that intrahippocampal administration of biotin-labeled wild-type Abeta peptides strongly accumulate on pre-existing cerebral microvascular amyloid deposits in Tg-SwDI mice, a model that preferentially develops early-onset CAA mutant microvascular amyloid. Subsequently, we crossed Tg-SwDI mice with Tg2576 mice, a model that produces high amounts of human wild-type Abeta in brain. The bigenic mice exhibited markedly elevated accumulation of microvascular fibrillar amyloid in brain compared to either single transgenic line that was largely composed of human wild-type Abeta. Further, isolated microvascular amyloid seeds from Tg-SwDI mice drive assembly of human wild-type Abeta into distinct anti-parallel amyloid fibrils. Conclusions: These findings indicate that cerebral vascular amyloid can serve as an effective scaffold to promote rapid assembly and strong deposition of Abeta into a unique structure that likely contributes to its distinctive pathology.