Elsevier

Neurobiology of Aging

Volume 28, Issue 8, August 2007, Pages 1221-1230
Neurobiology of Aging

Occurrence and co-localization of amyloid β-protein and apolipoprotein E in perivascular drainage channels of wild-type and APP-transgenic mice

https://doi.org/10.1016/j.neurobiolaging.2006.05.029Get rights and content

Abstract

The deposition of the amyloid β-protein (Aβ) is a hallmark of Alzheimer's disease (AD). One reason for Aβ-accumulation and deposition in the brain may be an altered drainage along perivascular channels. Extracellular fluid is drained from the brain towards the cervical lymph nodes via perivascular channels. The perivascular space around cerebral arteries is the morphological correlative of these drainage channels. Here, we show that Aβ is immunohistochemically detectable within the perivascular space of 25 months old wild-type and amyloid precursor protein (APP)-transgenic mice harboring the Swedish double mutation driven by a neuron specific promoter. Only small amounts of Aβ can be detected immunohistochemically in the perivascular space of wild-type mice. Cerebrovascular and parenchymal Aβ-deposits were absent. In APP-transgenic mice, large amounts of Aβ were found in the perivascular drainage channels accompanied with cerebrovascular and parenchymal Aβ-deposition. The apolipoprotein E (apoE) immunostaining within the perivascular channels did not vary between wild-type and APP-transgenic mice. Almost 100% of the area that represents the perivascular space was stained with an antibody directed against apoE. Here, Aβ co-localized with apoE indicating an involvement of apoE in the perivascular clearance of Aβ. Fibrillar congophilic amyloid was not seen in wild-type mice. In APP-transgenic animals, congophilic fibrillar amyloid material was seen in the wall of cerebral blood vessels but not in the perivascular space. In conclusion, our results suggest that non-fibrillar forms of Aβ are drained along perivascular channels and that apoE is presumably involved in this clearance mechanism. Overloading such a clearance mechanism in APP-transgenic mice appears to result in insufficient Aβ-clearance, increased Aβ-levels in the brain and the perivascular drainage channels, and finally in Aβ-deposition. In so doing, our results strengthen the hypothesis that an alteration of perivascular drainage supports Aβ-deposition and the development of AD.

Introduction

Alzheimer's disease (AD) is histopathologically characterized by the deposition of amyloid β-protein (Aβ) [4], [16]. Increased levels of Aβ are considered to be responsible for neurodegeneration in AD [9]. An increase of Aβ in the brain can either result from increased production or from decreased clearance of Aβ [15]. Transgenic mice overexpressing mutant amyloid precursor protein (APP) produce increased levels of Aβ and develop Aβ-plaques and cerebral amyloid angiopathy (CAA) [7], [10], [29]. APP23 mice overexpress human APP harboring the Swedish double mutation (670/671 KM ->NL) driven by the neuron specific Thy-1 promoter [29]. APP-overexpression is not seen in other tissues except the central nervous tissue of these mice [29]. Therefore, the APP23 mouse is an ideal model for studying the mechanisms of Aβ-deposition and Aβ-clearance. One clearance mechanism for Aβ from brain is binding to apolipoprotein E (apoE) [28] and the subsequent uptake by astrocytes [13]. However, apoE is also found in senile plaques in humans [20] and mice [22] and appears to be involved in the formation of newly formed plaques [31]. In addition to the cellular clearance of Aβ by astrocytes and microglial cells [11], [13] and the enzymatic degradation by neprilysin and/or insulin degrading enzyme [11], [19], drainage of extracellular Aβ along perivascular spaces has been discussed to play a significant role in Aβ-clearance [36]. Perivascular channels represent drainage channels for extracellular fluid from the brain towards the cervical lymph nodes [35], [40]. The perivascular space around cerebral arteries is the morphological correlative of these drainage channels [35], [40] (Fig. 1A). Although the development of CAA in transgenic mice overexpressing APP through a neuron-specific promoter strongly suggests clearance of Aβ along the perivascular channels [3], Aβ has not been shown to occur physiologically in these channels and it is not clear which forms of Aβ are drained.

Therefore, the aim of this study is to address the question whether non-fibrillar Aβ is present within the perivascular space of cerebral vessels and to examine the role of apoE in the perivascular drainage of Aβ.

Section snippets

Material and methods

To demonstrate the presence of Aβ within the perivascular space and to test whether apoE is involved in this drainage process, we studied brains from 25 months old, female wild-type (n = 20) and APP23 mice (n = 16) for the presence of Aβ and apoE within the perivascular channels of cerebral vessels. Animals were treated in agreement with the German law on the use of laboratory animals. Perfusion fixation of the animals was performed transcardially with Tris buffered saline (TBS) with heparin (pH

Results

Microscopic analysis of cerebral vessels revealed visible perivascular channels in all animals (Fig. 1B and C). The perivascular space was best seen at the level of the hippocampal formation around the posterior cerebral artery and its ramifications. Near and within perivascular channels no cellular reaction was detected (Fig. 1B and C). Only amorphous proteinaceous material was visible in the Hematoxylin and Eosin and Elastica van Gieson stained sections (Fig. 1C). Nearby blood vessels did not

Discussion

The presence of Aβ within the perivascular space of wild-type and APP23 mice strongly supports the hypothesis of Weller et al. [36] that drainage along these channels contributes to the clearance of Aβ from brain. Physiologically, low amounts of Aβ were detected immunohistochemically within these channels in wild-type mice. The absence of Congo red stained material indicates the non-fibrillar nature of the Aβ-positive material in these mice. As soon as the amount of neuronal derived Aβ is

Acknowledgements

The authors gratefully acknowledge the technical assistance of N. Kolosnjaji and H. U. Klatt.

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