Progressive changes in synapses and glial cells in App^NL-G-F mice, a model of Alzheimer's disease

It is well documented that synapse loss correlates with cognitive decline in Alzheimer's disease. However, the mechanisms that contribute to synapse loss remain poorly understood. Studies have shown that amyloid-β directly signals to neurons to trigger changes in synaptic function leading to the subsequent loss of synapses. Other studies have demonstrated that glial cells directly target synapses in Alzheimer's disease. In this study, we determine the temporal relationship between changes in synapses and glial cells (microglia and astrocytes) in the NL-G-F knock-in mouse model of Alzheimer's disease. We evaluated synapse number and histological changes in glial cells in the hippocampus of NL-G-F mice using confocal microscopy across three timepoints, 2, 5, and 9 months, compared to their wild-type littermates. Using real-time quantitative PCR, we also evaluated molecular changes in glial cells. At 2 months of age, when very few amyloid-β plaques are present, inhibitory synapse number was transiently increased by more than 50% in NL-G-F mice, accompanied by a small increase in the microglial marker, Cx3cr1, and considerable changes in astrocyte markers, including a decreased level of Thbs1/2. At 5 months, when amyloid-β plaque load is notable, excitatory synapse number was decreased immediately proximal to plaques, whereas inhibitory synapse number was no different between NL-G-F and wild-type mice. At the cellular level, changes in microglia and astrocytes were also observed in NL-G-F mice in regions closely surrounding plaques. From 5 months, PCR analyses indicated marked and progressive changes in microglia and astrocyte markers, including increased Trem2 and Gfap expression. By 9 months, changes in excitatory synapse number and microglia at the cellular level were exacerbated, with evident synapse loss extending up to 30 µm away from plaques. Together, our data show that inhibitory synapses are the earliest change in NL-G-F mice occurring concomitantly with molecular changes in glial cells and preceding substantial plaque deposition, excitatory synapse loss, and glial cellular alterations.