Problems with the amyloid hypothesis notwithstanding, knowing where amyloid-β (Aβ) plaques form might be key to developing strategies for clearing them from the brains of those with Alzheimer disease (AD). In this month’s American Journal of Pathology, two papers report that in three different mouse models of Alzheimer disease, plaques with dense cores, which have been associated with neuritic pathology and neuronal loss, are predominantly associated with blood vessel walls, and that this association can lead to vascular damage. The findings raise questions about just where plaque formation might occur most frequently during AD.
Christine van Broeckhoven and colleagues at the Flanders Interuniversity Institute for Biotechnology and the University of Antwerp in Belgium, together with collaborators in the US, carried out a detailed and systematic study of dense core plaques in the brains of both Tg2576 (see Hsiao et al., 1996) and PSAPP (see Duff et al., 1996 /pap/annotation.asp?powID=12907) mice. Both the former, expressing human Aβ precursor protein (AβPP) with the Swedish mutations (K670N and M671L), and the latter, a double transgenic expressing mutant human presenilin 1 (M146L) and AβPP with the Swedish mutations, generate generous quantities of plaques. Both sets of transgenic animals have been used as investigational models for AD.
First author Samir Kumar-Singh and colleagues found that the vast majority of dense core plaques are found adjacent to blood vessels. Using stereological microscopy of 4 μM thin slices, the authors found that of over 2,000 plaques sampled from the neocortices and hippocampuses, 94 and 85 percent of them were associated with blood vessels in Tg2576 (n = 9) and PSAPP (n = 5) mice, respectively.
To ensure that these numbers were not arrived at by chance, Kumar-Singh studied the entire brain of a 2-year-old Tg2576 and a 5-month-old PSAPP animal—at these ages the plaque burden in the different transgenics is about the same. The findings confirmed the data from the plaque sampling experiment. Ninety-five percent of 210 plaques from the Tg2576 animal and 85 percent of 258 plaques from the PSAPP mouse were positively associated with the microvasculature. Statistical analysis based on the size and area of the blood vessels and plaques also revealed that these associations could not be incidental. In fact, even in thinner (1 μM) sections from a PSAPP animal, the authors failed to identify even one plaque that did not associate with a vessel at some level. Moreover, the plaque/vessel relationship was further strengthened by the finding that plaques that had more than one dense core were more likely to be associated with more than one blood vessel. In fact, the total number of vessels associated with a plaque turned out to be the best predictor of how many cores might be present within it.
Tackling this question from a slightly different perspective, William van Nostrand and colleagues at Stony Brook University, New York, and the University of Nijmegen, Holland, report similar findings using the Tg-SwDI transgenic mice. These animals produce human AβPP with the same Swedish mutants as the Tg2576 mice, but mutations normally found in certain Dutch and Iowan populations (E693Q and D694N) are also included in the transgene for good measure. Previously, it was shown that the amyloid generated in these animals is predominantly vascular (see Davis et al., 2004). Now, first author Jianting Miao and colleagues show that the number of blood vessels that stained positive for Aβ increases dramatically as these animals age. In the thalamus and subiculum, areas where fibrillar Aβ accumulates—but not the frontal cortex where the amyloid is mostly diffuse—the percentage of vessels staining for Aβ reached 50 by 12 months and over 80 by 2 years of age.
When Miao and colleagues examined the brain ultrastructure, they found that the accumulation of microvascular amyloid was accompanied by anatomical changes. For example, the number of microvessels fell by 16 and 27 percent in the hippocampus of 1- and 2-year-old animals, respectively. Aβ in the meningeal vessels was also associated with apoptotic cells and loss of smooth muscle cells. Miao also found a dramatic age-related increase in the numbers of reactive astrocytes and microglia in the thalamus and subiculum. Both cell types were about five times more abundant in 2-year-old animals as compared to young, 6-month-old mice.
In fact, Kumar-Singh and colleagues also found obvious morphological changes in the Tg2576 and PSAPP mice. These included loss or thinning of the endothelium, basement membrane thickening or splitting to accommodate Aβ, and degeneration of smooth muscles. They also found that the transgenic animals had more microhemorrhages than did control mice.
How these findings relate to AD is unclear. The Dutch and Iowa mutations are known to cause primarily cerebral amyloid angiopathy, or CAA (see ARF related news story), and as Kumar-Singh and colleagues point out, their mouse data is more reminiscent of Flemish familial AD, characterized by many large, dense-core plaques and CAA, than it is of sporadic AD or Down syndrome, in which plaques are more diffuse and CAA minimal.
Nonetheless, these results would seem to cement the relationship between vessel walls and Aβ deposits. And as Kumar-Singh and colleagues write, “the present study, for the first time, demonstrates that dense amyloid plaques in Tg2576 and PSAPP mice are centered on vessel walls. If a similar mechanism is also operative in AD, therapeutics targeting Aβ clearance from the vascular compartment may be most beneficial.”—Tom Fagan
- Hsiao K, Chapman P, Nilsen S, Eckman C, Harigaya Y, Younkin S, Yang F, Cole G. Correlative memory deficits, Abeta elevation, and amyloid plaques in transgenic mice. Science. 1996 Oct 4;274(5284):99-102. PubMed.
- Duff K, Eckman C, Zehr C, Yu X, Prada CM, Perez-Tur J, Hutton M, Buee L, Harigaya Y, Yager D, Morgan D, Gordon MN, Holcomb L, Refolo L, Zenk B, Hardy J, Younkin S. Increased amyloid-beta42(43) in brains of mice expressing mutant presenilin 1. Nature. 1996 Oct 24;383(6602):710-3. PubMed.
- Davis J, Xu F, Deane R, Romanov G, Previti ML, Zeigler K, Zlokovic BV, Van Nostrand WE. Early-onset and robust cerebral microvascular accumulation of amyloid beta-protein in transgenic mice expressing low levels of a vasculotropic Dutch/Iowa mutant form of amyloid beta-protein precursor. J Biol Chem. 2004 May 7;279(19):20296-306. Epub 2004 Feb 25 PubMed.
- Kumar-Singh S, Pirici D, McGowan E, Serneels S, Ceuterick C, Hardy J, Duff K, Dickson D, Van Broeckhoven C. Dense-core plaques in Tg2576 and PSAPP mouse models of Alzheimer's disease are centered on vessel walls. Am J Pathol. 2005 Aug;167(2):527-43. PubMed.
- Miao J, Xu F, Davis J, Otte-Höller I, Verbeek MM, Van Nostrand WE. Cerebral microvascular amyloid beta protein deposition induces vascular degeneration and neuroinflammation in transgenic mice expressing human vasculotropic mutant amyloid beta precursor protein. Am J Pathol. 2005 Aug;167(2):505-15. PubMed.