ReviewTransgenic AD model mice, effects of potential anti-AD treatments on inflammation and pathology
Introduction
Alzheimer disease (AD), the most common cause of dementia in the elderly [8], is characterized pathologically by the presence of large numbers of neuritic plaques and neurofibrillary tangles [2], [3]. Neuritic plaques primarily consist of deposits of amyloid β (Aβ), a 39–43 amino acid peptide that is derived through the processing of amyloid precursor protein (APP), i.e., when APP is sequentially cleaved by the β- and γ-secretase (e.g., [11], [28]). Most AD cases are sporadic (e.g., [12], [24]); however, approximately 5% of AD cases are familial. Some of these familial cases are early-onset, and those are caused by mutations in the genes for APP, presenilin 1 and 2 (PS1 and PS2 [24]). These mutations have been shown to alter APP metabolism and they lead to an increase of the Aβ levels in the brain [12], [24], [27], implicating a central role for APP processing and the concomitant increase in Aβ levels in the pathogenesis of AD.
Transgenic mice expressing mutated human genes associated with familial forms of AD offer a powerful model to study the role of Aβ in the development of AD pathology (e.g., [15], [25]). The present study employs double transgenic mice expressing both human APPswe and PS1-A246E mutations (APP/PS1 mice [1]). These mice develop elevated levels of the highly fibrillogenic Aβ42 peptide, and get amyloid plaques starting around the age of 9 months [1], [20], [32]. The first amyloid plaques are present in the subiculum (and caudal cortex), and later these deposits extend to the hippocampus and all cortical areas [20], [32]. This feature is somewhat similar to the early stages of AD pathology, in which the amyloid plaques are also largely restricted to the medial temporal cortical structures [3]. We noted that in these mice, early in the development of pathology there are predominantly dense plaques, and that diffuse deposits develop later [31], [32]. However, it is still unknown how this amyloid deposition and the changes in the neuropathology are related to the change(s) in the processing of APP, and the development of AD. It has been suggested that the earliest changes in AD are present in the cholinergic basal forebrain (e.g., [4], [9]); therefore, we hypothesized that lesioning the cholinergic input to the hippocampal formation in these mice would allow us to examine the possible correlation between Aβ deposition, inflammation and the level of acetylcholine. Thus, we transected the fimbria-fornix (FFX-lesion); our results show that FFX-lesions did not change the deposition of Aβ [21], neither did it change the number of activated glial cells surrounding amyloid plaques in the brains of these animals.
In our experiments, we noted that there were significant differences in the Aβ load between males in female mice [32]; therefore, we hypothesized that manipulating the levels of estrogen in female mice would affect amyloid pathology. Thus, we ovariectomized female mice and gave them estrogen replacement therapy (i.e., zero, low, and high estrogen levels); our results indicated that this did not change amyloid load pathology [13], [16]. Our analysis of the number of activated glial cells surrounding amyloid plaques in these mice also showed no significant changes in inflammation.
We noted that in these mice, early in the development of pathology, there are predominantly dense plaques, and that diffuse deposits develop later [30], [31]. Whereas plaques are surrounded by activated astrocytes and microglial cell, the diffuse deposits do not show this activation of glial cells [30], [31]. Furthermore, even the earliest plaques are surrounded by activated astrocytes and microglia; therefore, we hypothesized that treatment of these mice with a non-steroidal anti-inflammatory drug (NSAID) from an early age would decrease amyloid pathology. For this study, we used four groups of female mice; they were treated with the NSAID from 8 months of age (i.e., before pathology is present) for 6 months, one group was on the Control diet, one group on Flurbiprofen, one group on a low HCT 1026 (a NO donating form of flurbiprofen), and one group was on a high HCT 1026 diet. Both the pathology, i.e., the Aβ load, and the amount of activated microglia were significantly reduced in the mice treated with high dose of HCT 1026 compared to the other groups.
Section snippets
Animals
Male and female APP and PS1 double transgenic mice (AP mice, n = 48) and non-transgenic littermate controls (Control mice, n = 48) were used in the present study. The mice were generated from matings between APPswe transgenic mice and HuPS1-A246E transgenic mice. These mice were originally produced at the Johns Hopkins University ([1]; Baltimore, MD, USA) and are now bred locally. Throughout the experiments, the animals were housed individually in a controlled environment (temperature 22 °C,
Experiment 1, cholinergic manipulations
Part of the data (Aβ load and AChE measurements) from the FFX lesion experiments have been published before [20], [21]; therefore, we will only shortly describe those data and focus on the results from the stainings with astrocytic and microglial markers. At 17 months of age (i.e., 11 months post FFX lesion), the lesion resulted in a nearly complete loss of the hippocampal AChE-positive fibers (Fig. 1B). The decrease in AChE staining was higher in the hippocampus than in the subiculum; further,
Discussion
We investigated the effects of different anti-AD treatments on the amount of amyloid deposition, and on the inflammation surrounding Aβ deposits in transgenic mice coexpressing mutated human APP and PS1 genes [1].
We found that a near-complete cholinergic denervation of the hippocampus [19] did not significantly affect hippocampal APP levels and Aβ deposition in the hippocampus in our APP + PS1 transgenic mice. Further, our data show that the transection of the fimbria-fornix pathway does not
Acknowledgments
We thank Dr. Egon von Schnier for his excellent comments on an earlier version of this manuscript, Pasi Miettinen for his assistance with the histology. We thank K. Beyreuther for the W0-2 antibody; flurbiprofen and 2-fluoro-a-methyl[1.1′-biphenyl]-4-acetic acid 4-(nitrooxy)butyl ester (HCT 1026) were provided by the Nicox Research Institute (Bresso, Milan, Italy). This study was supported by a grant from the Pääivikki and Sakari Sohlberg Foundation and TEKES project 40043/01.
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