Caffeine suppresses amyloid-beta levels in plasma and brain of Alzheimer's disease transgenic mice

Abstract

Recent epidemiologic studies suggest that caffeine may be protective against Alzheimer's disease (AD). Supportive of this premise, our previous studies have shown that moderate caffeine administration protects/restores cognitive function and suppresses brain amyloid-beta (Abeta) production in AD transgenic mice. In the present study, we report that acute caffeine administration to both young adult and aged AD transgenic mice rapidly reduces Abeta levels in both brain interstitial fluid and plasma without affecting Abeta elimination. Long-term oral caffeine treatment to aged AD mice provided not only sustained reductions in plasma Abeta, but also decreases in both soluble and deposited Abeta in hippocampus and cortex. Irrespective of caffeine treatment, plasma Abeta levels did not correlate with brain Abeta levels or with cognitive performance in individual aged AD mice. Although higher plasma caffeine levels were strongly associated with lower plasma Abeta1-40 levels in aged AD mice, plasma caffeine levels were also not linked to cognitive performance. Plasma caffeine and theophylline levels were tightly correlated, both being associated with reduced inflammatory cytokine levels in hippocampus. Our conclusion is two-fold: first, that both plasma and brain Abeta levels are reduced by acute or chronic caffeine administration in several AD transgenic lines and ages, indicating a therapeutic value of caffeine against AD; and second, that plasma Abeta levels are not an accurate index of brain Abeta levels/deposition or cognitive performance in aged AD mice.

Figure 1. Diagrams depicting brain Aβ production/clearance, the suppressive actions of caffeine on Aβ production, and resultant effects on brain and plasma Aβ levels

(A) Unmodulated: Aβ is primarily produced in neurons, secreted into the brain extracellular space in soluble form, then enters a dynamic equilibrium between soluble and deposited (insoluble) Aβ. Continual transport of soluble Aβ occurs into plasma. (B) Caffeine: Short-Term: Caffeine suppression of both β- and γ-secretase activities reduces Aβ production, resulting in lower soluble Aβ in brain and plasma. The equilibrium between soluble and deposited Aβ is not impacted by this short-term reduction in brain soluble Aβ levels. (C)Caffeine: Long-Term: Continued caffeine suppression of Aβ production and resultant lower levels of brain soluble Aβ induce a flux of deposited (insoluble) Aβ to the soluble form, which is cleared from brain into plasma via soluble Aβ transport. Plasma Aβ levels may be reduced or not changed, depending on degree of caffeine-induced suppression of Aβ production. In aged APPsw mice given chronic caffeine treatment, their lower brain Aβ levels/deposition results in reversal of cognitive dysfunction.

Figure 2. Caffeine treatment induces a rapid and sustained decrease in plasma Aβ levels

(A,B,D) A single i.p. or gavage treatment with caffeine significantly reduces plasma Aβ levels in 3–4 month old (A) and 14 month old (B) APPsw mice; as well as in 19 month old APP+PS1 mice (D). In all three studies, saline vehicle treatment had no effect. (C) Plasma caffeine levels in 14M old APPsw mice are inversely correlated with plasma Aβ1-40 levels. (E) Oral caffeine treatment for one week to 15–20 month old APPsw mice results in decreased plasma levels of both Aβ1-40 and Aβ1-42 immediately following treatment, with a rebound in plasma Aβ levels occurring by 9 days after cessation of caffeine treatment. Each group in A, B, D, and E consisted of 4 – 7 mice. (F) As exemplified by two aged 20 month old APP+PS1 mice, oral caffeine administration every 4th day over a 2-month period induces a sustained and continual decrease in plasma levels of both Aβ1-40 and Aβ1-42 over the treatment period. Post-treatment versus pre- or delayed post-treatment Aβ levels were evaluated by paired t-tests. *p<0.05–0.01 versus pre-treatment levels; **p<0.05 versus both pre-treatment and 9-day post-treatment levels.

Figure 3. Caffeine lowers ISF Aβ levels

(A) Brain ISF Aβ_x-40 levels were measured by in vivo microdialysis. Prior to treatment with caffeine, ISF Aβ levels fluctuated very little. A low dose (5 mg/kg) and a higher dose (30 mg/kg) of caffeine caused ISF Aβ levels to decrease significantly compared to basal levels. (B) Following a 5 mg/kg dose of caffeine i.p., ISF Aβ_x-40 levels decreased by 19% compared to basal levels (*p<0.05), while a 30 mg/kg administration decreased Aβ levels by 33% (**p<0.01, n=6 per group). The mean concentration of ISF Aβ represents an average of hours 2–3 after each dose of caffeine when Aβ levels had stabilized. (C) Tg2576 were pre-treated with vehicle or 30 mg/kg caffeine for 3 hours (n=6 per group), followed by administration of 100 nM Compound E, a γ-secretase inhibitor, via reverse microdialysis. In both groups, ISF Aβ levels decreased rapidly when APP cleavage was blocked. (D) The elimination half-life of ISF Aβ_x-40 was comparable in vehicle-treated and caffeine-treated mice, suggesting that caffeine does not affect ISF Aβ elimination, but likely impacts Aβ production instead.

Figure 4. Long-term caffeine administration to aged, cognitively-impaired APP mice reduces soluble Aβ levels in both cortex and hippocampus, while also decreasing deposited (insoluble) Aβ in hippocampus

Caffeine was administered to 18–19 month old APPsw mice for two months in their drinking water. Both brain Aβ1-40 and Aβ1-42 were decreased in caffeine-treated Tg mice. Although plasma Aβ levels were unaffected for all Tg/Caff mice inclusively, see Figure 6B. Immunohistochemical Aβ deposition in the hippocampus was reduced by 40% in caffeine treatment mice. *p<0.05; **p<0.001. Each group consisted of 5–8 mice.

Figure 5. Plasma Aβ levels do not correlate with brain Aβ levels in aged APPsw mice

(A,B) Strong correlations are present between levels of Aβ1-40 and Aβ1-42 for both soluble and insoluble Aβ in hippocampus, irrespective of caffeine treatment. By contrast, no correlations existed between hippocampus (or cortex) and plasma for soluble Aβ levels (C) or insoluble Aβ levels (D) irrespective of caffeine treatment.

Figure 6. Plasma Aβ levels do not correlate with cognitive performance in aged APPsw mice

(A,B) No correlations were present between plasma Aβ levels and RAWM working memory. (C,D). By contrast, strong correlations were evident between brain Aβ levels/deposition and cognitive impairment, as exemplified by correlations involving soluble Aβ levels in posterior cortex (C) and Aβ deposition in entorhinal cortex (D). Each symbol represents an individual Tg mouse. All correlations involve all Tg mice collectively (Tg controls and Tg/Caff), although identical results were observed when only Tg controls were included in the analysis.

Figure 7. Effects of caffeine and theophylline on plasma, brain, and cognitive measures

In aged 20–21 month old APPsw mice that had been treated for 2 months with caffeine and cognitively evaluated (n=6), blood and brain tissues taken at euthanasia were analyzed. (A) High plasma caffeine levels correlated with lower plasma Aβ levels. (B) Mice with higher plasma caffeine levels had lower plasma Aβ1-40 levels and lower hippocampal cytokine levels than those mice with lower caffeine levels. *p<0.05; **p<0.01. (C,D) As exemplified by these 2 correlation graphs, plasma caffeine levels in individual mice were not correlated with their radial arm water maze (RAWM) working memory performance. (E) Plasma caffeine and plasma theophylline levels were strongly correlated. (F) High plasma theophylline levels correlated with lower plasma IFN-γ levels.