The 5xFAD Mouse Model of Alzheimer's Disease Displays Early Stunting of Neuronal Development in the Dentate Gyrus: a Potential Novel Treatment Window for Dementias Associated with Amyloidosis?

Abstract

Approximately 5.8 million people over the age of 65 are living with Alzheimer's Disease (AD), and the number of individuals afflicted with AD is projected to double by 2050. Hallmarks of AD are intraneural plaques of amyloid-beta and neurofibrillary tangles with concomitant and progressive decline in neuro-cognitive function. Currently, there is no cure for AD, which stems partly from the sporadic nature of the disease and “after-the-fact” treatments which have failed to remediate disease progression. The goal of our lab is to elucidate early markers of AD onset prior to overt disease occurrence. To this end, we are using the 5XFAD transgenic mouse model which displays an aggressive form of amyloidosis and associated dementias, which develop because of insertion of the five most common genetic mutations associated with aberrant amyloid processing found in AD. Typically, intraneural amyloid plaques develop at 2-4 months of age in 5xFAD mice and cognitive deficits are observed at 4 to 6 months of age. The objective of our current study was to determine characteristics of hippocampal neuronal arborization in 5xFAD mice and non-transgenic (WT) littermates at at 15 days, 1-month, and 3-months of age, which is thought to be a pre-symptomatic period. Mice were euthanized in strict accordance with AALAC guidelines and approved protocols. Isolated brains from male and female 5xFAD and WT mice were fixed and Golgi stained (Rapid Stain Golgi kit, FDNeurotechnologies). Fixed tissue was sectioned by a cryostat (100 um), and slides were imaged using a Leica DM6000 at 40X. Images were then reassembled in Z-stacks and analyzed with FIJI. A Sholl analysis was performed on the hippocampal subregions of the dentate gyrus (DG), cortical area 1 (CA1), and cortical area 3 (CA3) where dendritic length was the primary endpoint. Results showed that there was no shortening at 15 days and 3-months in 5XFAD versus WT, but there was a shortening of neurites in the 5XFAD versus WT at 1-month of age, mainly within the DG (p = 0.04969; N = 6). At 1-month, the dentate gyrus neurons were 108.2μm (p = 0.04969; N = 6), or 5% shorter, with observed simplification of arborization. The other hippocampal subregions did not display significant alterations in neuronal development. These results show that at an extreme early age, 5xFAD mice already show a lack of neuronal development that occurs prior to intraneuronal amyloid plaque appearance, especially in a hippocampal subregion known to be vulnerable to AD-related neuronal loss. This subregion is especially vulnerable due to the fact that amyloid beta accumulates in this region initially, which could lead to the noted shortening in neuronal length. The DG receives input from the entorhinal cortex and is relayed to CA3 via mossy fibers, which is imperative for long-term potentiation and spatial memory. This suggests that the so-called pre-symptomatic period will have to be re-evaluated. Furthermore, our data suggest that a potential window for treatments have been found which may aid in developing novel therapeutic regimens to slow or prevent progression of AD symptomology associated with amyloidosis.