Transient pharmacologic lowering of Aβ production prior to deposition results in sustained reduction of amyloid plaque pathology

Abstract

Background: Alzheimer's disease (AD) is the leading cause of dementia among the elderly. Disease modifying therapies targeting Aβ that are in development have been proposed to be more effective if treatment was initiated prior to significant accumulation of Aβ in the brain, but optimal timing of treatment initiation has not been clearly established in the clinic. We compared the efficacy of transient pharmacologic reduction of brain Aβ with a γ-secretase inhibitor (GSI ) for 1-3 months (M) treatment windows in APP Tg2576 mice and subsequent aging of the mice to either 15M or 18M.

Results: These data show that reducing Aβ production in a 2-3M windows both initiated and discontinued before detectable Aβ deposition has the most significant impact on Aβ loads up to 11M after treatment discontinuation. In contrast, initiation of treatment for 3M windows from 7-10M or 12-15M shows progressively decreasing efficacy.

Conclusions: These data have major implications for clinical testing of therapeutics aimed at lowering Aβ production, indicating that; i) these therapies may have little efficacy unless tested as prophylactics or in the earliest preclinical stage of AD where there is no or minimal Aβ accumulation and ii) lowering Aβ production transiently during a critical pre-deposition window potentially provides long-lasting efficacy after discontinuation of the treatment.

Figure 1

Effects of LY window therapy on amyloid deposition. A. Time course of Aβ accumulation in Tg2576 mice. FA solubilized brain Aβ42 + Aβ40 levels measured by ELISA are shown (n = 3–5 mice/age group). B. Schematic of the various transient LY dosing strategies. Dotted lines represent the transient LY treatment time points. C. Aβ plaque burden analysis of treated cohorts compared to controls (*p < 0.05, ** ns, ANOVA). D. Biochemical analyses (FA solubilized Aβ42 + Aβ40 levels) by ELISA showing Aβ levels. (*p < 0.05, **ns, ANOVA). E. Representative brain sections stained with anti-Aβ mAb showing Aβ plaques at 15M of age in the LY treated cohorts. Scale bar = 100 μm.

Figure 2

Effects of LY window therapy on amyloid deposition. A. Quantitative burden analysis of cored amyloid deposits and CAA in the 4-7M LY treated group compared to controls. (*p < 0.05, Student’s t test). B. Representative brain sections stained with mAb 13.1.1 showing cored plaque and CAA immune-reactivity at 15M in the brains of Control and 4-7M LY treated Tg2576 mice. Arrows indicate CAA in meningeal vessels. Scale bar = 80 μm. C. Biochemical analyses of FA solubilized Aβ levels and plaque burden analysis from Tg2576 mice transiently dosed with LY (2.5mpk/day) from 4-7M, and aged to 18M. (*p < 0.05, Student’s t test). Tg2576 mice (n = 5-6/group) were used, data represented is from one independent experiment. D. Natural logarithm transformed Aβ values of untreated Tg2576 mice (at indicated ages) and Aβ levels from 4-7M LY treatment measured at 15M and 18M of age. E. Biochemical analyses of FA solubilized Aβ levels of Tg2576 mice transiently dosed with LY (2.5mpk/day) at 1, 2, and 3M intervals and then aged till 15M. (*p < 0.05, ANOVA). Tg2576 mice (n = 5-8/group) were used, data represented is from one independent experiment. F. Graph representing Aβ values of untreated Tg2576 mice (from indicated ages) and Aβ levels from 4-5M, 4-6M and 4-7M LY transient treatment measured at 15M. FA solubilized brain Aβ42 + Aβ40 levels measured by ELISA are shown.

Figure 3

Long term LY treatment does not permanently affect APP levels or processing. A. Plasma Aβ40 levels by ELISA after the end of 4–7M LY treatment, (7M), 1 week after termination of treatment, and 2 weeks after termination of treatment. B. Representative anti-APP CT20 immunoblot shows APP levels and accumulation of CTFs after 4-7M LY treatment (tested at 7M). C. Representative anti-APP CT20 immunoblot showing full length APP levels and CTFs after transient 4-7M LY treatment (tested at 15M). D. Quantitative intensity analysis of anti-CT20 immuno-reactive full length APP and APP CTF protein levels normalized to actin.

Figure 4

Effect of LY treatment on peripheral immune cells.A. Distribution of splenic B cell (IgD, CD19) and T cells (CD4 and CD8) from control and Tg2576 mice treated with LY (4-7M). Percent positive cells are shown (n = 4 mice/group). B and C. IFN production (B) and IL-4 production (C) by CD4+ T cells from LY (4-7M) treated Tg2576 mice and untreated control mice were measured by ELISA. Equal numbers of purified, splenic CD4+ T cells (pooled from n =4 mice/group) were used. ND = not determined.