Expression of a familial Alzheimer's disease-linked presenilin-1 variant enhances perforant pathway lesion-induced neuronal loss in the entorhinal cortex

Abstract

Alzheimer's disease (AD) is characterized by neuronal loss in the hippocampus and entorhinal cortex that is manifested by progressive memory impairment and cognitive decline. Autosomal-dominant, familial forms of AD (FAD) are caused by mutations in genes encoding amyloid precursor protein, presenilin-1 (PS1), and presenilin 2. Although it is established that expression of mutant PS1 variants leads to increased production of highly fibrillogenic amyloidbeta42 (Abeta42) peptides that deposit in the brains of patients with AD, the mechanism(s) by which Abeta deposition and expression of mutant genes induce lamina- and region-specific vulnerability of neuronal populations is not known. We have examined the hypothesis that expression of transgene-encoded FAD-linked mutant PS1 variants in entorhinal cortex neurons exacerbates the vulnerability of these cells to lesion-induced neuronal loss. To test this notion, we transected the perforant pathway (PP) of transgenic mice harboring either wild-type human PS1 (PS1HWT) or the FAD-linked mutant PS1DeltaE9 variant and examined neuronal survival in layer II of the entorhinal cortex (ECL2). Remarkably, PP transections lead to marked reductions in the numbers of ECL2 neurons in the ECL2 of mice expressing mutant PS1, compared with ECL2 neurons in PP-lesioned PS1HWT mice. Finally, and in contrast to studies in nontransgenic mice and in mice expressing PS1HWT, ECL2 neurons that express mutant PS1 and the calcium binding protein calbindin-D28k in ECL2 are also susceptible to lesion-induced neuronal loss. We conclude that expression of FAD-linked mutant PS1 variants enhances the vulnerability of neurons in the entorhinal cortex to PP lesion-induced cytotoxicity.

Figure 1.

Design-based stereological analysis of neuronal survival in the entorhinal cortex of adult transgenic mice. Representative low-power confocal images of horizontal brain sections of sham and perforant-pathway-lesioned PS1HWT and PS1ΔE9 mice used for stereological analysis of the number of neurons in ECL2. The figure is composed of four panels: top left, brain section of a sham mouse harboring FAD-linked PS1HWT; top right, brain section of a mouse harboring FAD-linked PS1HWT 2 weeks after PP lesion; bottom left, brain section of a sham mouse harboring FAD-linked PS1ΔE9; bottom right, brain section of a mouse harboring FAD-linked PS1ΔE9 2 weeks after PP lesion. Each brain section is immunostained with antibodies raised against NeuN (left top image in each panel) and calbindin-D_28k (right top image in each panel) using multiple fluorophore-conjugated secondary antibodies. Fluoro-Ruby [cyanine 3 (Cy3) fluorophore; left bottom image in each panel] can be detected in the lesioned mice (top right, PS1HWT; bottom right, PS1ΔE9). The number of calbindin-expressing neurons was quantified using combined image stacks of NeuN and calbindin (bottom right image in each panel).

Figure 2.

Neuronal survival in ECL2 of sham and lesioned transgenic mice harboring FAD-linked PS1HWT or PS1ΔE9 after transection of the perforant pathway. High-power confocal images of ECL2 neurons as detected in brain sections of sham and lesioned PS1HWT and PS1ΔE9 mice. Top to bottom: sham PS1HWT, lesioned PS1HWT, sham PS1ΔE9, lesioned PS1ΔE9. For each animal, images show NeuN+ (left image column), calbindin-D_28k+ (middle image column), and NeuN+ calbindin-D_28k+ neurons (right image column) of ECL2. Note the reduced number of neurons in ECL2 of PS1ΔE9 transgenic mice 2 weeks after perforant pathway lesion. Scale bar, 60 μm.

Figure 3.

Reduced survival rate of neurons in ECL2 of PS1ΔE9 transgenic mice 2 weeks after perforant pathway lesion. The number of NeuN+ (A), calbindin-D_28k+ (B), and NeuN+ calbindin-D_28k+ (C) cells in ECL2 of sham and lesioned PS1HWT and PS1ΔE9 transgenic mice was quantified by unbiased stereology. The number of NeuN+ cells as well as calbindin-D_28k + cells was dramatically reduced in ECL2 of PS1ΔE9 compared with ECL2 of PS1HWT 2 weeks after lesion.