Novel Approaches to Track Neurodegeneration in Murine Models of Alzheimer's Disease Reveal Previously Unknown Aspects of Extracellular Aggregate Deposition

This paper describes a novel transgenic-based platform to track degeneration of specific populations of neurons in 5xFAD mice, a murine model of Alzheimers disease. We created a new double transgenic model by crossing 5xFAD mice with RosatdT reporter mice. 5xFAD+/-/RosatdT mice received intra-spinal cord injections of AAV-retrograde (rg)/Cre at 2-3 months of age to permanently label corticospinal neurons (CSNs). Brains and spinal cords were retrieved 2-3 weeks post-injection or at 12-14 months of age. Immunohistochemical studies of transgene expression throughout the brain and spinal cord, using an antibody selective for hAPP, revealed age-dependent accumulation of hAPP in extracellular aggregates in regions containing hAPP expressing neuronal cell bodies and in regions containing axons and synaptic terminals from hAPP expressing neurons. Permanent labeling of CSNs with tdT confirmed extensive loss of CSNs in old mice. Surprisingly, we discovered that tdT expressed by CSNs accumulated in extracellular aggregates that persisted after the neurons that expressed tdT degenerated. Extracellular aggregates of tdT also contained hAPP and co-localized with other markers of AD pathology. Overall, deposition of hAPP in extracellular aggregates in areas containing axons and synaptic terminals from hAPP expressing neurons is a prominent feature of AD pathophysiology in 5xFAD mice. In addition, accumulation of hAPP and reporter proteins in extracellular aggregates provides a secondary measure to track neurodegeneration of identified populations of neurons in these mice. Highlights- Characterization of a new double transgenic strain allowing Cre-dependent labeling of populations of neurons that degenerate in 5xFAD mice. - Selective labeling of layer V corticospinal neurons (CSNs) via retrograde transduction with AAV-rg allows quantification of previously un-recognized aspects of age-dependent CSN degeneration. - Age-dependent deposition of extracellular hAPP by axons and synaptic terminals revealed by immunohistochemistry for mutant human APP in 5xFAD mice - Petal-shaped clusters of hAPP originate mainly due to axonal degeneration and fragmentation. - Surprisingly, tdTomato expressed by neurons that degenerate, persists in extracellular deposits that co-localize with extracellular deposits of hAPP.