Mitochondrial Structure And Function Is Impaired In The Beta-Amyloidosis 5xfad Mouse At An Extreme Early Age Prior To Neurocognitive Decline

Alzheimer's disease (AD) is the fifth leading cause of death in adults older than 65 years of age. It is also expensive, costing a predicted $305 billion in 2020. AD incidence is expected to double by 2050. AD is characterized by β-amyloid plaques, tau neurofibrillary tangles, loss of neurons, hippocampal and whole brain atrophy, and long-term memory loss. The 5xFAD mouse is a model of amyloidosis, but not tau pathology, associated with AD. Intra-neural amyloid plaques in these mice appear at 2 months of age near the dentate gyrus of the hippocampus with coincident neuro-cognitive defects evident by 4 months of age. The goal of the present work was to explore extreme early events associated with amyloid-related neuronal damage associated with amyloidosis by examination of hippocampal structure and function in 1-month old mice. This age is considered to be a quiescent period prior to deposition of amyloid plaques and neuronal damage. Following euthanasia by approved guidelines, hippocampi were excised from brains of 5xFAD mice or wild-type (WT) littermate mice (1-month old). Potential ultrastructural alterations were analyzed using transmission electron microscopy (Helios 650 Ultra) of fixed tissue sections. Results showed little evidence of cellular abnormalities except for severe mitochondrial enlargement and signs of decay in 5xFAD samples. Follow-up analysis (100 µm resolution and FIJI software) quantified significant mitochondrial cristae breakage and dissolution (N=5; p=0.0087) where nearly 75% of the mitochondria were either graded as severely or moderately damaged. Outer membrane breakage was also routinely observed. In contrast, little to no mitochondrial damage was observed in samples from WT mice. Subsequent measurement of mitochondrial function using high resolution respirometry (Oroboros) showed a significant (p<0.05) loss in succinate dehydrogenase activity in the 5xFAD samples, suggesting loss of electron transport function. Because of the severe mitochondrial oxidative damage evident in this supposed “quiescent” period, we also hypothesized that mitochondrial damage may be associated with abnormal neuronal development at this early age. Golgi staining and Sholl analysis revealed that arborization of hippocampal neurons were more simplified in 5xFAD vs. WT mice but with more mushroom-type spines. These results indicate potential changes in neuronal development and connectivity associated with inherent mitochondrial decay at 1-month of age. Overall, our work thus shows that significant neuronal alterations begin occurring in 5xFAD mice prior to appearance of plaque formation. This indicates that a potential treatment window has been discovered to test therapies to slow or limit damage associated with amyloidosis.