Histochemical Clues to the Onset and Progression of Plaques in Alzheimer's Pathology
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Contributing OfficeNational Center for Toxicological Research
Abstract
Alzheimer’s disease (AD) is a highly debilitating neurodegenerative disorder which is prevalent in older adults, affecting over six million people in the US alone. One way that AD makes itself known is through the formation of plaques. These plaques—parenchymal and vascular—are formed through the buildup of amyloid beta (Aβ) protein in the brain. Congo Red (CR) is a fluorescent dye which can be used to label these plaques because of its binding affinity to Aβ. Using AD transgenic rats, the progression of plaque buildup was quantified in different age groups. We wanted to observe when the plaques started to form, where they congregated, how they differed among the rats in different age groups and what types of plaques appeared in specific locations of the brain (cortex and hippocampus). We used Tg344-AD rat models which express mutant human amyloid precursor protein (APP) and presenilin 1 (PS1) genes. The expression of these genes drives the production and growth of Aβ to induce early-onset AD. This supports the amyloid cascade hypothesis which states that the buildup of Aβ is essential for the progression of AD. The transgenic model reliably mirrors human AD in rats in relation to the location of plaque development and plaque age-dependency. Brain samples from fifteen female transgenic rats were taken—six 6-month, four 16-to-17-month and five 20-month rats—to look at plaque progression. Brain samples were mounted to gelled slides and incubated in Sudan Black solution, to reduce lipofuscin, and Congo Red. Using a fluorescence microscope, Nikon camera and NIS-imaging software, images of each brain section were taken. The NIS-Analysis software was used to count the number of plaques (total and vascular). In addition, one non-transgenic rat from each age group was analyzed to act as a control. We found that rats as young as six months showed plaque formation in both the cortex and hippocampus. The twenty-month rats had the highest number of plaques, and the six-month rats had the lowest. This was the same for both parenchymal and vascular plaques. In each age group, parenchymal plaques appeared at a higher rate than vascular plaques. 2.74% of total plaques in twenty-month rats were vascular plaques, 4.66% in sixteen-to-seventeen-month rats and 6.18% in six-month rats. In addition, a significantly higher number of total plaques were present in the cortex than in the hippocampus. Vascular plaques were found predominantly in the cortex regions but not in the hippocampus and appeared as early as 6 months. The non-transgenic rat models showed little to no plaque buildup. In humans, Aβ production begins in the neocortex before spreading to other brain regions. Any transgenic animal model purporting to mirror this disease pathophysiology must be assessed for its validity. We have found that the TgF344 rat model reliably mirrors the staging of Aβ progression. Furthermore, this study has informed potential age points of interest for further testing in this model. Results indicated here will contribute to subsequent studies regarding the factors that cause Aβ accumulation, specifically in its early stages in the neocortex.