TGF344‐AD rats exhibit cognitive reserve despite continual Aβ and tau accumulation

Abstract

AbstractBackgroundPatient to patient variability in Alzheimer’s disease progression complicates treatment efficacy. Furthermore, the presence of Aβ pathology does not always indicate cognitive impairments. Epidemiological evidence of cognitive reserve suggests some individuals exhibit enhanced neuronal and behavioural resilience, protecting against neurodegeneration and cognitive decline. However, Alzheimer’s disease models typically exhibit a correlation between accumulation of pathology and behavioural impairments, making cognitive reserve difficult to investigate preclinically.MethodTgF344‐AD rats recapitulate the amyloid cascade with a long‐term accumulation of Aβ and tau pathology leading to significant neurodegeneration and behavioural impairments. TgF344‐AD rats were assessed for cognitive and neuropathological alterations compared to age‐matched non‐transgenic littermates, across disease progression at 4, 6, 9, 12 and 15 months of age. Latency to escape and complexity of search strategies utilized in the Barnes maze spatial and reversal phases were assessed to determine learning, long‐term memory, and executive function. Neuropathological assessment included GABAergic and total neuronal counts, and quantification of Aβ and tau pathology.ResultTgF344‐AD rats demonstrate deficits in spatial memory and executive function beginning at 6 months which are not exacerbated at 9 or 12 months, but are at 15 months of age. GABAergic and total neuronal counts in the hippocampus and entorhinal cortex of TgF344‐AD rats exhibit a similar trend, with deficits not progressing between 9 and 12 months of age. Conversely, Aβ and tau pathology accumulate continually across disease progression. Non‐transgenic rats exhibit slight, yet consistent age‐related decline in cognitive and neuronal measures.ConclusionDisease‐bearing TgF344‐AD rats exhibit cognitive reserve before robust cognitive decline at 15 months, despite consistent increases in Aβ and tau deposition. Maintenance, and in certain hippocampal subregions a recovery, of neuronal loss at 12 months support this observation, and suggest the involvement of compensatory mechanisms such as hippocampal neurogenesis. Understanding the mechanisms underlying cognitive reserve in Alzheimer’s disease will help explain patient variability in disease progression and elucidate novel regenerative therapeutic targets.