Neuronal activity regulates the regional vulnerability to amyloid-β deposition

Abstract

Amyloid-β (Aβ) plaque deposition in specific brain regions is a pathological hallmark of Alzheimer's disease. However, the mechanism underlying the regional vulnerability to Aβ deposition in Alzheimer's disease is unknown. Herein, we provide evidence that endogenous neuronal activity regulates the regional concentration of interstitial fluid (ISF) Aβ, which drives local Aβ aggregation. Using in vivo microdialysis, we show that ISF Aβ concentrations in several brain regions of APP transgenic mice before plaque deposition were commensurate with the degree of subsequent plaque deposition and with the concentration of lactate, a marker of neuronal activity. Furthermore, unilateral vibrissal stimulation increased ISF Aβ, and unilateral vibrissal deprivation decreased ISF Aβ and lactate, in contralateral barrel cortex. Long-term unilateral vibrissal deprivation decreased amyloid plaque formation and growth. Our results suggest a mechanism to account for the vulnerability of specific brain regions to Aβ deposition in Alzheimer's disease.

Figure 1

Distribution of Aβ and amyloid plaque deposition in Tg2576 mouse brain. (a,b) Representative brain sections from aged (17.5 ± 0.5 months old) Tg2576 mice stained with biotinylated-3D6 antibody (anti Aβ_1–5) to visualize Aβ immunopositive plaques (n = 7 per group). Scale bar, 750 μM. (c) A stepwise increase in percent brain area occupied by Aβ deposition was present across barrel (BC), cingulate (CC) and piriform (PC) cortices. Aβ plaque burden in hippocampus (HC) was similar to barrel cortex while striatum (ST) exhibited the lowest level of plaque deposition of all regions examined (one-way ANOVA, Tukey’s post hoc test for multiple comparisons). (d–h) Representative images of barrel cortex (d), cingulate cortex (e), piriform cortex (f), hippocampus (g) and striatum (h) of aged Tg2576 mice stained with the amyloid binding dye, X-34 (n = 6 per group). Scale bar, 50 μM. (i) Percent area occupied by X-34 positive amyloid deposition was greater in piriform cortex compared to all other brain regions examined. A stepwise increase in amyloid deposition was present across barrel, cingulate and piriform cortices. Amyloid plaque burden in hippocampus was similar to barrel cortex while X-34 positive staining was not detected in striatum (one-way ANOVA, Tukey’s post hoc test for multiple comparisons). **, P < 0.01; ***, P < 0.001. Values represent mean ± SEM.

Figure 2

Steady-state ISF Aβ levels in young Tg2576 mice prior to plaque deposition are associated with the level of region-specific plaque deposition in aged Tg2576 mice. (a) In vivo microdialysis was performed to measure the steady-state concentration of ISF Aβ_x-40 in barrel cortex (BC), piriform cortex (PC), hippocampus (HC) and striatum (ST) of 3.5 ± 0.5 month-old Tg2576 mice prior to the onset of plaque deposition (n = 5–6 per group; one-way ANOVA, Tukey’s post hoc test for multiple comparisons). Steady-state levels of ISF Aβ_x-40 measured in each brain region of young Tg2576 mice were closely associated with the level of subsequent Aβ (b) and amyloid (c) plaque deposition in each brain region of aged Tg2576 mice. (d) In vivo microdialysis was performed to measure the steady-state concentration of ISF Aβ_x-42 in barrel cortex, piriform cortex, hippocampus and striatum of 3.5 ± 0.5 month-old Tg2576 mice prior to the onset of plaque deposition (n = 5–8 per group; one-way ANOVA, Tukey’s post hoc test for multiple comparisons). Steady-state levels of ISF Aβ_x-42 measured in each brain region of young Tg2576 mice were closely associated with the level of subsequent Aβ (e) and amyloid (f) plaque deposition in each brain region of aged Tg2576 mice. ◇, striatum; ■, barrel cortex, ○, hippocampus; ●, piriform cortex. *, P < 0.05; **, P < 0.01; ***, P < 0.001. Values represent mean ± SEM.

Figure 3

Neuronal activity regulates ISF lactate concentration in vivo. (a) Representative intraparenchymal EEG recordings from hippocampus of young Tg2576 mice during basal conditions, picrotoxin (PTX; 12.5 μM) treatment or tetrodotoxin (TTX; 5 μM) treatment via in vivo microdialysis (n = 4 per group). (b) Local PTX infusion increased ISF lactate levels and local TTX infusion decreased ISF lactate levels in hippocampus of young Tg2576 mice (n = 4 per group; two-tailed t-test). (c) Local PTX infusion increased ISF Aβ_x-40 levels and local TTX infusion decreased ISF Aβ_x-40 levels in hippocampus of young Tg2576 mice (n = 4 per group; two-tailed t-test). Values in b,c represent analyte levels during the final 6 hours of drug treatment. (d) ISF lactate levels were correlated with ISF Aβ_x-40 levels across treatment periods (Pearson r = 0.7645; P < 0.0001). *, P < 0.05; **, P < 0.01. Values represent mean ± SEM.

Figure 4

Steady-state ISF lactate levels in young Tg2576 mice are closely associated with regional ISF Aβ levels in young Tg2576 mice and plaque deposition in aged Tg2576 mice. (a–d) In vivo microdialysis was performed to measure steady-state ISF lactate levels in barrel cortex, piriform cortex, hippocampus and striatum of young Tg2576 mice (n = 6–7 per group). ISF lactate level in each brain region was closely associated with the concentration of ISF Aβ_x-40 (a) and ISF Aβ_x-42 (b) in each brain region of young Tg2576 mice. ISF lactate level in each brain region of young Tg2576 mice was also closely associated with the level of Aβ (c) and amyloid (d) plaque deposition in each brain region of aged Tg2576 mice. ◇, striatum; ■, barrel cortex, ○, hippocampus; ●, piriform cortex. Values represent mean ± SEM.

Figure 5

Vibrissal activity regulates ISF Aβ levels in vivo. (a–c) In vivo microdialysis was performed in posterior barrel cortex of young Tg2576 mice. After establishing stable basal ISF Aβ_x-40 and lactate levels, all vibrissae contralateral to the microdialysis probe were trimmed to within 1 mm of the facial pad and microdialysis sample collection continued for 16 hr to determine the effect of vibrissae deprivation on ISF Aβ_x-40 and lactate levels in barrel cortex. Vibrissae deprivation decreased ISF Aβ_x-40 (a, ■ in c) and lactate (b, □ in c) levels relative to baseline (n = 6–7; two-tailed t-test). (d) In vivo microdialysis was performed in barrel cortex in a separate cohort of young Tg2576 mice that underwent subsequent unilateral vibrissae stimulation. Baseline Aβ_x-40 values were obtained for a 30 minute period. Unilateral mechanical stimulation of vibrissae contralateral to the microdialysis probe (5–7 Hz, 4 s burst, 4 s inter-burst interval) was performed for 30 minutes. Vibrissae stimulation increased ISF Aβ_x-40 levels compared to control conditions (n = 4; two-tailed t-test). *, P < 0.05; **, P < 0.01. Values represent mean ± SEM.

Figure 6

Diurnal fluctuation of ISF Aβ is closely associated with ISF lactate levels. (a–c) In vivo microdialysis was performed in hippocampus of young Tg2576 mice housed in 12 hour light/12 hour dark conditions. ISF Aβ_1-x levels exhibited diurnal fluctuation (a) and were significantly greater in the dark period compared to the light period (■ in c; n = 6; two-tailed t-test). ISF lactate levels also exhibited diurnal fluctuation (b) and were significantly greater in the dark period compared to the light period (□ in c; n = 6; two-tailed t-test). (d) ISF Aβ_1-x levels were correlated with ISF lactate levels throughout the sleep/wake cycle (Pearson r = 0.6351; P < 0.0001). *, P < 0.05; **, P < 0.01. Values represent mean ± SEM.

Figure 7

Vibrissae deprivation reduces amyloid plaque growth and formation in vivo. (a,b) Representative multiphoton micrographs of individual amyloid plaques in barrel cortex in control hemisphere of APP/PS1 mice before (a) and after (b) 28 days of unilateral vibrissae deprivation. (c,d) Representative multiphoton micrographs of amyloid plaques in barrel cortex in vibrissae-deprived hemisphere of APP/PS1 mice before (c) and after (d) 28 days of vibrissae deprivation. (e) Long-term vibrissae deprivation decreased existing amyloid plaque growth compared to the control hemisphere (n = 6 mice; two-tailed t-test). (f) Long-term vibrissae deprivation also reduced new plaque formation compared to the control hemisphere (n = 6 mice; two-tailed t-test). ↑, existing amyloid plaque; △, newly formed amyloid plaque. Scale bar, 50 μM; *, P < 0.05; **, P < 0.01. Values represent mean ± SEM.