Alterations in β-amyloid production and deposition in brain regions of two transgenic models
Introduction
Alzheimer’s disease (AD) is defined by neuropathological hallmarks including intracellular accumulation of phosphorylated tau protein in neurofibrillary tangles and extracellular deposits of the amyloid beta (Aβ) peptide in senile plaques. AD neuropathology proceeds in a characteristic pattern with Aβ deposition first appearing in the basal neocortex, later in the hippocampus and eventually throughout the cortical layers [3], [4]. The biological pathways that regulate this regional pattern of Aβ deposition remain to be elucidated.
Aβ is derived from APP, a protein of unknown function with at least four tissue-specific protein isoforms of 695, 714, 751 and 770 amino acids [10], [18]. APP is proteolytically processed by α-secretase at Lys-16 of Aβ generating a C-terminal fragment (CTFα) which lacks the complete Aβ sequence. Alternatively, β-secretase cleavage at the N-terminus of Aβ generates a C-terminal fragment (CTFβ) that can be cleaved by γ-secretase to release full-length Aβ. APP and many of the secretases are expressed in a variety of tissues and throughout the brain, suggesting that unknown factors regulate the regional pattern of Aβ deposition in AD.
The most widely utilized transgenic mouse model of AD, Tg2576, expresses the 695 amino acid form of human APP containing the K670N/M671L mutation [23] found in a large Swedish familial AD pedigree, under the control of the hamster prion promoter. Tg2576 animals develop Aβ deposits in the brain at 9–11 months of age [13]. While Tg2576 and other cDNA-based transgenic models of AD have provided evidence that transgenic mice can develop Aβ deposits (reviewed by Hock and Lamb [12]), the different APP cDNAs in these models are not expressed in a temporal and spatial pattern that recapitulates the expression pattern of endogenous App.
In contrast to the cDNA-based models, we have focused on developing an accurate genomic-based model of AD that contains all the regulatory elements required for appropriate spatial and temporal expression of the transgene and for generation of alternatively-spliced transcripts and protein isoforms. Introduction of a genomic copy of APP with the Swedish mutation, yielded line R1.40 which develops Aβ deposits by 14 months of age in the homozygous state [19], [20], [21]. Importantly, the pattern of Aβ deposition and associated pathology in R1.40 mice closely resembles that observed in human AD.
The current manuscript characterizes tissue-specific and brain region-specific APP processing in the genomic-based R1.40 transgenic mice and compares these results to those obtained with the cDNA-based Tg2576 transgenic animals. In addition, we have examined the brains of aged R1.40 and Tg2576 animals for regional Aβ deposition. Our results demonstrate that the different APP transgenic models of AD have unique biochemical and pathological attributes that likely reflect their experimental design. These findings may have substantial implications for the testing of therapeutic approaches in transgenic models of AD.
Section snippets
Transgenic mice
R1.40 YAC transgenic mice were previously described [21]. For these studies the R1.40 line was maintained by crossing to the C57BL/6J inbred strain for two generations, and animals homozygous for the transgene were generated by intercrossing animals hemizygous for the transgene. Homozygotes were identified by Southern blotting and confirmed by progeny testing. All R1.40 animals used in these studies were homozygous for the transgene. The Tg2576 transgenics were kindly provided by Hsiao-Ashe
Tissue-specificity of APP processing in R1.40 transgenic mice
To further examine the tissue-specificity of APP processing, we analyzed the levels of APP processing products in tissues from R1.40 transgenic mice, where APP is expressed as specified by the endogenous human promoter. A number of tissues, including brain, heart, kidney, testes, lung, stomach, large intestine, small intestine, liver, uterus, ear, skeletal muscle, eye and pancreas were isolated from R1.40 homozygous transgenic mice, non-transgenic mice and App knockout mice. The App knockout
Discussion
Our results suggest that APP processing is a complex process regulated uniquely in various tissues of the body and in different regions within the brain. Furthermore, we demonstrate that the transgenic approach utilized to model AD impacts APP processing and AD-related neuropathological abnormalities including Aβ deposition.
Acknowledgements
We thank K. Hsiao-Ashe for the generous gift of Tg2576 animals and S. Gandy for the generous gift of 369 antibody. We also thank K. Herrup and P. Gambetti for their helpful discussions and critical reading of the manuscript. This work was supported by NIH Grant AG14451 and Alzheimer’s Association grant IIRG-99-1517 to B.T. Lamb as well as support from the University Alzheimer Center (AG08012) and the Ireland Cancer Center (CA43703). E.J.H. Lehman was supported in part by NIH training grant
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