ApoE-directed therapeutics rapidly clear β-amyloid and reverse deficits in AD mouse models

Abstract

Alzheimer's disease (AD) is associated with impaired clearance of β-amyloid (Aβ) from the brain, a process normally facilitated by apolipoprotein E (apoE). ApoE expression is transcriptionally induced through the action of the nuclear receptors peroxisome proliferator-activated receptor gamma and liver X receptors in coordination with retinoid X receptors (RXRs). Oral administration of the RXR agonist bexarotene to a mouse model of AD resulted in enhanced clearance of soluble Aβ within hours in an apoE-dependent manner. Aβ plaque area was reduced more than 50% within just 72 hours. Furthermore, bexarotene stimulated the rapid reversal of cognitive, social, and olfactory deficits and improved neural circuit function. Thus, RXR activation stimulates physiological Aβ clearance mechanisms, resulting in the rapid reversal of a broad range of Aβ-induced deficits.

Fig. 1

ISF levels of Aβ decrease after bexarotene treatment. (A and B) ISF Aβ_x-40 and Aβ_x-42 levels were monitored by in vivo hippocampal microdialysis of 2-month-old APP/PS1 mice. Baseline Aβ levels were monitored for 6 hours, followed by daily orally administered bexarotene (100 mg kg⁻¹ day⁻¹) (Bex) or vehicle (Veh; water) for 3 days. Mice were coadministered compound E [20 mg kg⁻¹ intraperitoneally (i.p.)] on day 3. (C) The elimination half-life of ISF Aβ_x-40 was measured. In 2-month-old APP/PS1 mice, baseline ISF Aβ levels were sampled after administration of a single oral dose of bexarotene (100 mg kg⁻¹). (D) ISF Aβ_x-40 and Aβ_x-42 were sampled every 2 to 6 hours for 4 days after treatment. (E) Baseline ISF Aβ levels of nontransgenic (C57Bl/6) and apoE knockout (KO) mice (2 months) with and without bexarotene treatment. ISF Aβ_x-40 levels were measured between hours 7 and 12 after treatment; n = 5 mice per group (Student’s t test; mean ± SEM, *P < 0.05, **P < 0.01, ***P < 0.001).

Fig. 2

Aβ levels and plaque burden are reduced by bexarotene treatment. APP/PS1 or nontransgenic (NonTg) mice (6 months) orally gavaged for 3, 7, and 14 days with bexarotene (100 mg kg⁻¹ day⁻¹) or vehicle (water). Soluble and insoluble Aβ₄₀ and Aβ₄₂ levels were measured by enzyme-linked immunosorbent assay. (A) Fold changes based on vehicle: 7.0445 ng/mg protein and 14.529 ng/mg protein of soluble Aβ₄₀ and Aβ₄₂, respectively, and 30.349 ng/mg protein and 36.8 ng/mg protein of insoluble Aβ₄₀ and Aβ₄₂, respectively. (B and E) Representative cortex and hippocampus sections of vehicle and 14-day bexarotene-treated mice stained with antibody against Aβ (6E10) (B) or thioflavin S (E) are shown and (C and F) plaque levels quantified; n ≥ 5 animals per group (Student’s t test; mean ± SEM, *P < 0.05, **P < 0.01, ***P < 0.001). Scale bars (B and E): cortex, 100 μm; hippocampus, 200 μm. (D) Representative image of microglia in the cortex of a 6-month APP/PS1 mouse treated for 3 days with bexarotene (red: 6E10; green: Iba1; blue: DAPI). Scale bar: 10 μm.

Fig. 3

Restoration of memory and cognition with bexarotene treatment. Contextual fear-learning assayed in (A) 6-month-old and (B) 11-month-old APP/PS1 mice treated for 7 days, or (C) in 9-month-old APP/PS1 mice treated for 90 days with vehicle or bexarotene. (E) APPPS1-21 mice 7 to 8 months of age were treated for 20 days and evaluated for performance. Percent time frozen was recorded in the 5-min test trial. (D and F) Spatial memory was assessed with the Morris water maze. Time spent in the northwest quadrant in the retention probe of (D) 9-month-old, 90-day-treated APP/PS1 mice and (F) 7- to 8-month-old, 20-day-treated APPPS1-21 mice with vehicle or bexarotene (Bex) (100 mg kg⁻¹ day⁻¹). [Nontransgenic littermates were controls (NonTg), n = 7 to 14 mice per group (Student’s t test; mean ± SEM, *P < 0.05, **P < 0.01)]. (G) Nest construction was quantified in 12- to 14-month NonTg and Tg2576 mice. Baseline data were obtained on day 0, after daily drug treatment and addition of paper towels in clean cages (two-tailed t test; *P < 0.05, **P < 0.01). (H) Odor habituation behavior in 12- to 14-month Tg2576 mice tested before (baseline) and after 9 days of bexarotene treatment; n = 5 mice per group (two-tailed t test; mean ± SEM, **P < 0.01, ***P < 0.001 for Tg2576 baseline versus Tg2576 Bex).

Fig. 4

Rescue of cortical network activity with bexarotene. LFP recordings of Tg2576 or nontransgenic (NonTg) mice (12 to 14 months) gavaged with bexarotene (Bex) (100 mg kg⁻¹ day⁻¹) or vehicle (H₂O) for 3 days after implantation of electrodes into PCX. PCX LFPs in response to the odor ethyl valerate in an awake nontransgenic, bexarotene-treated mouse. (A) Fifteen-to 35-Hz beta- and 35- to 75-Hz gamma-band power traces (second-order band pass). (B) PCX odor-evoked response magnitudes (2 s odor/2 s pre-odor) (n = 5 mice per group, four odor presentations per mouse; *P < 0.05, **P < 0.01, ***P < 0.001, mean ± SEM, two-tailed t tests of mean odor-evoked magnitudes within LFP bins). ns, not significant.