Alzheimer’s disease presents a complex challenge for researchers, largely due to the difficulties in directly observing its progression within the human brain. This limitation has led to the development of various animal models, which serve as controlled environments for studying disease mechanisms and evaluating potential treatments. Transgenic mouse models have become valuable tools in this research. Among these, the 5xFAD mouse model is widely recognized and frequently utilized in understanding Alzheimer’s disease.
Genetic Construction of the 5xFAD Model
The “5xFAD” designation refers to the five familial Alzheimer’s disease (FAD) mutations incorporated into this mouse model’s genetic makeup. These mice carry and co-express two human transgenes. One transgene is the human amyloid precursor protein (APP) gene, which includes three mutations: Swedish (K670N/M671L), Florida (I716V), and London (V717I).
The other co-expressed transgene is the human presenilin 1 (PSEN1) gene, which carries two additional mutations: M146L and L286V. These five mutations were chosen because they cause aggressive forms of familial Alzheimer’s disease in humans. The combined expression of these mutated genes within the mouse brain leads to an accelerated overproduction of the amyloid-beta 42 (Aβ42) peptide. This overproduction drives the formation of amyloid plaques, a hallmark pathological feature of Alzheimer’s disease.
Timeline of Pathological Development
The 5xFAD mouse model exhibits an accelerated progression of amyloid-related pathology, making it suitable for rapid experimental studies. Early signs of disease pathology become evident around two months of age, with intraneuronal Aβ42 accumulation. Soon after, amyloid plaques can be observed within various brain regions.
By four to six months of age, plaque burden increases throughout the cortex and hippocampus. This period also marks the onset of gliosis, characterized by the activation and proliferation of brain immune cells like microglia and astrocytes. As the mice age, between nine and twelve months, progressive amyloid pathology leads to widespread synaptic dysfunction. This stage is also associated with neuronal loss, particularly in regions such as the subiculum and specific cortical areas. While this model develops amyloid pathology, it does not spontaneously develop neurofibrillary tangles, which are composed of hyperphosphorylated tau protein and represent another primary pathological hallmark of human Alzheimer’s disease.
Manifestation of Cognitive Deficits
The progressive accumulation of amyloid pathology and subsequent neuronal changes in 5xFAD mice lead to impairments in cognitive functions, including memory and learning. These deficits begin to manifest around four to six months of age, aligning with the period of increasing plaque burden and gliosis. Cognitive decline worsens as the mice age, reflecting ongoing neuropathological processes.
Researchers employ various behavioral tests to assess these cognitive impairments. For instance, the Y-maze test evaluates working memory, observing the mouse’s ability to remember previously visited arms of the maze. The Morris water maze is another test, which measures spatial learning and long-term memory by assessing how well a mouse learns to locate a hidden platform. These tests provide quantifiable data on the behavioral outcomes in the 5xFAD model.
Applications in Preclinical Studies
The 5xFAD mouse model serves as a valuable tool in the preclinical phase of Alzheimer’s disease research, particularly in the development of new therapies. Its rapid formation of amyloid plaques makes it well-suited for evaluating compounds designed to target amyloid-beta. This includes testing drugs aimed at reducing Aβ production, preventing its aggregation, or enhancing its clearance from the brain.
Researchers also use this model to investigate other aspects of Alzheimer’s disease progression, such as the role of neuroinflammation. The gliosis observed in 5xFAD mice allows for studies on how activated microglia and astrocytes contribute to or modify the disease process. The model’s predictable pathology and cognitive decline provide a consistent platform for screening potential therapeutic interventions before human clinical trials.