The generation of amyloid peptides (Abeta) from the amyloid precursor protein (APP) is initiated by beta-secretase (BACE), whereas subsequent gamma-secretase cleavage mediated by presenilin-1, produces Abeta peptides mainly of 40 or 42 amino acids long. In addition, alternative beta'-cleavage of APP at position 11 of the amyloid sequence results in N-truncated Abeta(11-40/42) peptides, but the functional significance or pathological impact is unknown. Here we demonstrate that in the brain of BACE x APP[V717I] double-transgenic mice, amyloidogenic processing at both Asp1 and Glu11 is increased resulting in more and different Abeta species and APP C-terminal fragments. Pathologically, BACE significantly increased the number of diffuse and senile amyloid plaques in old double-transgenic mice. Unexpectedly, vascular amyloid deposition was dramatically lower in the same BACE x APP[V717I] double-transgenic mice, relative to sex- and age-matched APP[V717I] single-transgenic mice in the same genetic background. The tight inverse relation of vascular amyloid to the levels of the less soluble N-terminally truncated Abeta peptides is consistent with the hypothesis that vascular amyloid deposition depends on drainage of excess tissue Abeta. This provides biochemical evidence in vivo for the preferential contribution of N-truncated Abeta to parenchymal amyloid deposition in contrast to vascular amyloid pathology.

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The deposition of amyloid-β protein (Aβ) in the brain is a hallmark of Alzheimer's disease (AD). Apolipoprotein E (apoE) is involved in the clearance of Aβ from brain and the APOE ε4 allele is a major risk factor for sporadic AD. We have recently shown that apoE is drained into the perivascular space (PVS), where it co-localizes with Aβ. To further clarify the role of apoE in perivascular clearance of Aβ, we studied apoE-transgenic mice over-expressing human apoE4 either in astrocytes (GE4) or in neurons (TE4). These animals were crossbred with amyloid precursor protein (APP)-transgenic mice and with APP-presenilin-1 (APP-PS1) double transgenic mice. Using an antibody that specifically detects human apoE (h-apoE), we observed that astroglial expression of h-apoE in GE4 mice leads to its perivascular drainage, whereas neuronal expression in TE4 mice does not, indicating that neuron-derived apoE is usually not the subject of perivascular drainage. However, h-apoE was observed not only in the PVS of APP-GE4 and APP-PS1-GE4 mice, but also in that of APP-TE4 and APP-PS1-TE4 mice. In all these mouse lines, we found co-localization of neuron-derived h-apoE and Aβ in the PVS. Aβ and h-apoE were also found in the cytoplasm of perivascular astrocytes indicating that astrocytes take up the neuron-derived apoE bound to Aβ, presumably prior to its clearance into the PVS. The uptake of apoE-Aβ complexes into glial cells was further investigated in glioblastoma cells. It was mediated by α(2)macroglobulin receptor/low density lipoprotein receptor-related protein (LRP-1) and inhibited by adding receptor-associated protein (RAP). It results in endosomal Aβ accumulation within these cells. These results suggest that neuronal apoE-Aβ complexes, but not neuronal apoE alone, are substrates for LRP-1-mediated astroglial uptake, transcytosis, and subsequent perivascular drainage. Thus, the production of Aβ and its interaction with apoE lead to the pathological perivascular drainage of neuronal apoE and provide insight into the pathological interactions of Aβ with neuronal apoE metabolism.

Apolipoprotein E4 (ApoE4) is associated with Alzheimer's disease by unknown mechanisms. We generated six transgenic mice strains expressing human ApoE4 in combination with mutant amyloid precursor protein (APP) and mutant presenilin-1 (PS1) in single-, double-, or triple-transgenic combinations. Diffuse, but not dense, amyloid plaque-load in subiculum and cortex was increased by neuronal but not glial ApoE4 in old (15 months) double-transgenic mice, whereas both diffuse and dense plaques formed in thalamus in both genotypes. Neuronal and glial ApoE4 promoted cerebral amyloid angiopathy as extensively as mutant PS1 but with pronounced regional differences: cortical angiopathy was induced by neuronal ApoE4 while thalamic angiopathy was again independent of ApoE4 source. Angiopathy correlated more strongly with soluble A␤40 and A␤42 levels in cortex than in thalamus throughout the six genotypes. Neither neuronal nor glial ApoE4 affected APP proteolytic processing, as opposed to mutant PS1. Neuronal ApoE4 increased soluble amyloid levels more than glial ApoE4, but the A␤42/40 ratios were similar, although significantly higher than in single APP transgenic mice. We conclude that although the cellular origin of ApoE4 differentially affects regional amyloid pathology, ApoE4 acts on the disposition of amyloid peptides downstream from their excision from APP but without induction of tauopathy. Alzheimer's disease (AD) is characterized by pathological accumulations in the brain of extracellular amyloid plaques and intraneuronal accumulations of protein tau known as neurofibrillary tangles. The amyloid peptides A␤40 and A␤42 are the major components of the amyloid deposits in parenchyma and vasculature.1 The amyloid peptides are excised from the integral membrane protein amyloid precursor protein (APP) by sequential endoproteolytic cleavages by ␤-and ␥-secretases.2 The exact causes and consequences, in terms of normal processes and mechanisms that are disturbed by the amyloid peptides and causing neurodegeneration, remain primarily unclear. 3Besides APP and presenilins (PS1, PS2), the apolipoprotein E (ApoE) is genetically linked to AD. 4 The ApoE4 lipoprotein or its encoding ⑀4 allele, are epidemiologically associated with AD and confer an increased risk and earlier age of onset relative to the more common ApoE3 protein or ⑀3 allele. [5][6][7] ApoE is a 34-kd protein abundantly expressed in liver and brain. In the circulation, ApoE-lipoproteins mediate transport of lipids and cholesterol from and to liver and extrahepatic tissues. In contrast to the peripheral functions of ApoE that are well understood, details of its actions in the central nervous system remain primarily unknown. Actually, the epidemiological association of the ⑀4-allele to AD provided a strong impetus to research, by pointing out our lack of understanding the physiology of lipid and cholesterol homeostasis and of their transport in brain, in particular the contribution of ApoE and various ApoE receptors. 8,9

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