Development and characterization of a novel transgenic strain to selectively label neurons that degenerate in 5xFAD mice.

This paper describes a novel double transgenic-based platform developed by crossing a murine model of Alzheimer's disease (AD), 5xFAD mice with RosatdTomato (tdT) reporter mice, to track degeneration of specific populations of neurons. 5xFAD+/-/RosatdT mice received intra-spinal cord injections of AAV-retrograde (rg)/Cre at 2-4 months of age to retrogradely transduce and induce tdT expression by corticospinal neurons (CSNs) in layer V of the sensorimotor cortex as well as neurons in the red nucleus and reticular formation that project to the spinal cord. Brains and spinal cords were collected 2-3 weeks post-injection or between 6-10 and 11-15 months of age. Immunohistochemical studies of transgene expression throughout the brain and spinal cord using an antibody selective for human APP (hAPP) revealed age-dependent accumulation of clusters of hAPP-positive granules in areas containing hAPP-labeled neuronal cell bodies. Surprisingly, there were also hAPP-positive granules in regions containing axons and synaptic terminals from hAPP expressing neurons. Moreover, tdT expressed by CSNs accumulated in the same granules as hAPP, and both tdT and hAPP were present in clusters of granules with other markers of AD pathology. Quantitative assessments confirmed age-related degeneration of layer V CSNs accompanied by progressive accumulation of clusters of tdT and hAPP-positive granules. Overall, our results indicate that accumulation of aggregated hAPP in areas containing axons and synaptic terminals from hAPP expressing neurons is a prominent feature of AD pathophysiology in 5xFAD mice and that accumulation of clusters of hAPP granules provides a secondary measure to track neurodegeneration of identified populations of genetically labeled neurons.