APP transgenic modeling of Alzheimer's disease: mechanisms of neurodegeneration and aberrant neurogenesis

Abstract

Neurodegenerative disorders of the aging population affect over 5 million people in the US and Europe alone. The common feature is the progressive accumulation of misfolded proteins with the formation of toxic oligomers. Alzheimer's disease (AD) is characterized by cognitive impairment, progressive degeneration of neuronal populations in the neocortex and limbic system, and formation of amyloid plaques and neurofibrillary tangles. Amyloid-beta (Abeta) is the product of proteolysis of amyloid precursor protein (APP) by beta and gamma-secretase enzymes. The neurodegenerative process in AD initiates with axonal and synaptic damage and is associated with progressive accumulation of toxic Abeta oligomers in the intracellular and extracellular space. In addition, neurodegeneration in AD is associated with alterations in neurogenesis. Abeta accumulation is the consequence of an altered balance between protein synthesis, aggregation rate, and clearance. Identification of genetic mutations in APP associated with familial forms of AD and gene polymorphisms associated with the more common sporadic variants of AD has led to the development of transgenic (tg) and knock out rodents as well as viral vector driven models of AD. While APP tg murine models with mutations in the N- and C-terminal flanking regions of Abeta are characterized by increased Abeta production with plaque formation, mutations in the mid-segment of Abeta result in increased formation of oligomers, and mutations toward the C-terminus (E22Q) segment results in amyloid angiopathy. Similar to AD, in APP tg models bearing familial mutations, formation of Abeta oligomers results in defective plasticity in the perforant pathway, selective neuronal degeneration, and alterations in neurogenesis. Promising results have been obtained utilizing APP tg models of AD to develop therapies including the use of beta- and gamma-secretase inhibitors, immunization, and stimulating neurogenesis.

Fig. 1

Schematic diagram of APP processing and accumulation of toxic Aβ species. β- and γ-secretase cleavage of APP results in the production of Aβ_1–40 and Aβ_1–42, which accumulates into neurotoxic oligomers

Fig. 2

Schematic diagram of factors contributing to Aβ oligomerization. Defective cellular processes can lead to the accumulation of Aβ dimers, trimers, and oligomers, which in turn contribute to neurogenesis defects and synaptic damage

Fig. 3

Diagram showing common mutations in the APP gene that are utilized in the generation of animal models of AD. Mutations in the N- and C-terminal domains of APP result in the accumulation of intracellular and/or extracellular Aβ species, while mutations in the Aβ region lead to the development of amyloid angiopathy. Swe Swedish mutation, Lon London mutation, Ind Indiana mutation, Arc Arctic mutation, TM transmembrane domain

Fig. 4

Characterization of cognitive and neuropathological alterations in the brains of mThy1-hAPP tg mice. a Structure of mutant hAPP transgene under the control of the mThy-1 promoter. b Memory portion of the water maze behavioral test where the platform was removed (Probe test) to evaluate the number of entrances into the target quadrant where the platform was previously located (# Entrances), the number of times the animal passed over the location, where the platform was (# Passes), and time spent (Time) swimming in the target quadrant where the platform was previously located. APP tg mice exhibited reduced performance compared to non-tg controls in all three measures of memory retention in this behavioral test. c Aβ-immunoreactive deposits in the cortex of an APP tg mouse. Scale bar 50 μm. d Reduced synaptophysin immunoreactivity in the brain of an APP tg mouse. Scale bar 0.2 mm. e Degeneration of the MAP2-immunoreactive dendritic arbor in the cortex of an APP tg mouse. Scale bar 10 μm. f hAPP immunoreactivity in the dentate gyrus (DG) of an APP tg mouse. Scale bar 1 mm (left panel), 20 μm (right panel). *p < 0.05 compared to non-tg controls by Student’s t-test (n = 4 mice per group)

Fig. 5

Increased intracellular pyroglutamate-Aβ immunoreactivity and synaptic deterioration in mThy1-hAPP tg mice. a–c Sections from the brains of non-tg and APP tg mice were immunolabeled with an antibody against pyroglutamate Aβ_3–42 and developed with DAB. Increased intraneuronal immunoreactivity was detected in the cortex of APP tg mice compared to non-tg controls. Scale bar 20 μm. d–f Reduced synaptophysin (SY38) immunoreactivity in the neuropil of APP tg mice compared to non-tg controls. Scale bar 20 μm. *p < 0.05 compared to non-tg controls by Student’s t-test (n = 4 mice per group)

Fig. 6

Reduced markers of neurogenesis and increased apoptosis in the hippocampus of APP tg mice. a–c Reduced BrdU immunoreactivity in the hippocampal dentate gyrus of APP tg mice treated with BrdU compared to non-tg controls treated with BrdU. d–e Reduced doublecortin (DCX) immunoreactivity in the hippocampal dentate gyrus of APP tg mice compared to non-tg controls. g–i Reduced proliferating cell nuclear antigen (PCNA) immunoreactivity in the hippocampal dentate gyrus of APP tg mice compared to non-tg controls. j–l Increased TUNEL-positive cells in the hippocampal dentate gyrus of APP tg mice compared to non-tg controls. Scale bar 50 μm for all panels. *p < 0.05 compared to non-tg controls by Student’s t-test (n = 4 mice per group)

Fig. 7

Schematic diagram showing several factors involved in the regulation of Aβ accumulation into oligomers, including production, aggregation, and clearance