Treatment with bexarotene, a compound that increases apolipoprotein-E, provides no cognitive benefit in mutant APP/PS1 mice

Abstract

Background: Though the precise cause(s) of Alzheimer's disease (AD) remain unknown, there is strong evidence that decreased clearance of β-amyloid (Aβ) from the brain can contribute to the disease. Therapeutic strategies to promote natural Aβ clearance mechanisms, such as the protein apolipoprotein-E (APOE), hold promise for the treatment of AD. The amount of APOE in the brain is regulated by nuclear receptors including retinoid X receptors (RXRs). Drugs that activate RXRs, including bexarotene, can increase APOE and ABCA1 production, and have been shown to decrease the Aβ burden and improve cognition in mouse models of Aβ amyloidosis. Although recent bexarotene studies failed to replicate the rapid clearance of Aβ from brains, behavioral and cognitive effects of this compound remain controversial.

Findings: In efforts to clarify these behavioral findings, mutant APP/PS1 mice were acutely dosed with bexarotene. While ABCA1 was upregulated in mutant APP/PS1 mice treated with bexarotene, this drug failed to attenuate Aβ plaques or cognitive deficits in these mice.

Conclusions: We recommend rigorous preclinical study to evaluate the mechanism and utility of such a compound for AD therapy.

Figure 1

Bexarotene significantly increases ABCA1 in the brains of APP_swe/PS1_ΔE9 mice, but does not alter Aβ plaques. (A) Representative Western blot (Vehicle (−), Bexarotene (+)) of ABCA1 in cortex of 8.5 mo. female mice, and graph of quantification normalized to β-tubulin-III. Bexarotene significantly increases ABCA1 expression by 50% after three days of gavage treatment [* F(1,13)=5.261, p=0.027]. (B) Percent brain area covered by plaques in 8 month (ubiquitin) and 11 month (7B6) female mice, calculated by stereological estimation on sections stained with the antibodies indicated. No significant differences in plaque burden were detected between treatment groups. N-values: Ubiquitin – veh=4, bex=6; 7B6 – veh=3, bex=5. Error bars represent standard deviation. (C) Gender comparison between % plaque area in 11 mo. mice, calculated by stereological estimation on sections stained with 7B6 antibody. Males had significantly lower plaque load compared to females in both treatment groups [* cortex, F(1,19)=20.177, p<0.0005 and ** hippocampus F(1,19)=14.045, p=0.001]. Error bars represent standard deviation. N-values: females- veh=3, bex5; males- veh=7, bex=8. Sample size estimates show this experiment has power of 88%. Representative images of plaques in APP_swe/PS1_ΔE9 mice, stained with 7B6 - (D) M Veh (E) M Bex (F) F Veh (G) F Bex - and ubiquitin - (H) F Veh (I) F Bex.

Figure 2

Immunoreactivity of glia is unchanged in APP_swe/PS1_ΔE9 mice following administration of bexarotene. (A) IBA1 levels in 11 month old male and female APP_swe/PS1_ΔE9 mice are unaffected by bexarotene treatment in both cortex [F(1,27)=2.231, p=0.147] and hippocampus [F(1,27)=2.127, p=0.156], and no significant effect of gender was detected [F(1,27)=0.665, p=0.422]. No significant interactions were detected. Error bars represent standard deviation. N-values: non-transgenics (veh), males=5, females=5; APP_swe/PS1_ΔE9, males: Bex =8, Veh=7, females: Bex=5, Veh=3. Representative images of IBA1 staining and microglial morphology in (B) F tg Veh, (C) F tg Bex, (D) M Tg Veh and (E) M Tg Bex. Scale bars = 500 um large images and 50 um on inserts. Morphology shows an amoeboid (active) state with clusters of activated microglia in APP_swe/PS1_ΔE9 animals (B,D). This morphology and clustering appears unchanged by treatment with bexarotene (C,E).

Figure 3

Bexarotene fails to produce an effect on context-dependent or CS-dependent freezing behavior in APP_swe/PS1_ΔE9 mice. Percent of time spent freezing was measured to quantify long term fear memory. (A) In the training session, APP_swe/PS1_ΔE9 mice receiving bexarotene treatment demonstrated similar levels of context-dependent freezing compared to their vehicle-treated counterparts [F(7,44)=3.905, p=0.002; Tukey: male nontg p=1.000, male tg p=0.182, female nontg p=0.999, female tg p=1.000]. In the context-dependent testing session, no significant differences were detected among any of the groups [F(7,44)=1.794, p=0.113]. (B) Analysis of CS-dependent fear responses show no significant effect of bexarotene on freezing behavior in APP_swe/PS1_ΔE9 male or female mice. A significant gender difference was detected between males and females in both non-transgenic and APP_swe/PS1_ΔE9 animals treated with vehicle [main effect of gender F(1,15)=23.506, p<0.0005; main effect of genotype F(1,15)=1.898, p=0.189, no significant interaction; females [M=42.006,SD=17.65] show lower levels of freezing than males [M=76.260,SD=10.50]. These gender-related differences were not observed in mice treated with bexarotene [males M=48.79, SD=20.71; females M=54.55, SD=21.48; main effect of sex F(1,27)=0.630, p=0.434; main effect of genotype F(1,27)=1.388,p=0.249; no significant interaction]. The condition of equality of variances was not met for this test. For all parts, error bars represent standard deviation. N-values females: nontg-veh=4, AP-veh=7, nontg-bex=8, AP-bex=13. N-values males: nontg-veh=4, AP-veh=4, nontg-bex=6, AP-bex=5.

Figure 4

Context-dependent freezing in APP_swe/PS1_ΔE9 female mice and non-transgenic littermates following a high-sensitivity conditioning protocol. A deficit is not yet present in APP_swe/PS1_ΔE9 females at 5 months of age [F(1,17)=0.684, p=0.420]. Context-dependent freezing deficits are detectable in APP_swe/PS1_ΔE9 females at 8 months of age [*F(1,17)=5.943, p=0.026], but do not persist to 10 months of age [F(1,18)=2.824, p=0.110]. N-values: 5 month- nontg=9, APP/PS1=10, 8 month- nontg=8, APP/PS1=11, 10 month- nontg=12, APP/PS1=8. Error bars represent standard deviation.