ABSTRACT
Amyloid-ß (Aß) is believed to directly affect memory and learning in Alzheimer's disease (AD). It is widely suggested that there is a relationship between Aß40 and Aß42 levels and cognitive performance. In order to explore the validity of this relationship, we performed a meta-analysis of 40 peer-reviewed, published AD transgenic mouse studies that quantitatively measured Aß levels in brain tissue after assessing cognitive performance. We examined the relationship between Aß levels (Aß40, Aß42, or the ratio of Aß42 to Aß40) and cognitive function as measured by escape latency times in the Morris water maze or exploratory preference percentage in the novel object recognition test. Our systematic review examined five mouse models (Tg2576, APP, PS1, 3xTg, APP(OSK)-Tg), gender, and age. The overall result revealed no statistically significant correlation between quantified Aß levels and experimental measures of cognitive function. However, enough of the trends were of the same sign to suggest that there probably is a very weak qualitative trend visible only across many orders of magnitude. In summary, the results of the systematic review revealed that mice bred to show elevated levels of Aß do not perform significantly worse in cognitive tests than mice that do not have elevated Aß levels. Our results suggest two lines of inquiry: 1) Aß is a biochemical "side effect" of the AD pathology; or 2) learning and memory deficits in AD are tied to the presence of qualitatively "high" levels of Aß but are not quantitatively sensitive to the levels themselves.
