High caloric intake improves neuronal metabolism and functional hyperemia in a rat model of early AD pathology

Abstract

Introduction: While obesity has been linked to both increased and decreased rate of cognitive decline in Alzheimer's Disease (AD) patients, there is no consensus on the interaction between obesity and AD. Methods: The TgF344-AD rat model was used to investigate the effects of high carbohydrate, high fat (HCHF) diet on brain glucose metabolism and hemodynamics in the presence or absence of AD transgenes, in presymptomatic (6-month-old) vs. symptomatic (12-month-old) stages of AD progression using non-invasive neuroimaging. Results: In presymptomatic AD, HCHF exerted detrimental effects, attenuating both hippocampal glucose uptake and resting perfusion in both non-transgenic and TgAD cohorts, when compared to CHOW-fed cohorts. In contrast, HCHF consumption was beneficial in established AD, resolving the AD-progression associated attenuation in hippocampal glucose uptake and functional hyperemia. Discussion: Whereas HCHF was harmful to the presymptomatic AD brain, it ameliorated deficits in hippocampal metabolism and neurovascular coupling in symptomatic TgAD rats.

Keywords: Alzheimer's disease; CEST; MRI; TgF344-AD; diet; functional hyperemia; glucose metabolism; obesity.

Figure 1

Study Timeline. A total of five groups were used in the study. Animals were scanned at 6 months and 12 months of age after feeding them either CHOW alone or CHOW and HCHF food items for 3 months. An additional group was scanned at 15 months of age after these diets were administered for 6 months. Blood glucose and BHB were measured before fasting, after fasting, and at the end of MRI scan. Rats were fasted during their inactive phase, to administer the 2DG uptake in the active phase, thereby enhancing the 2DG uptake and potentiating the MRI contrast (relative to those achievable in the inactive phase).

Figure 2

HCHF elicits weight gain, particularly in females. (A) Caloric intake was higher in HCHF-fed rats than in their CHOW counterparts (6 months: 206 ± 17 kcal/week, P < 0.001; 12 months: 213 ± 33 kcal/week, P < 0.001). HCHF-fed rats consumed lower CHOW food compared to CHOW-fed rats (6 months: -165 ± 32 kcal/week, P = 0.009; 12 months: -169 ± 29 kcal/week, P = 0.004). Compared to CHOW-fed rats, HCHF-fed rats consumed higher (B) fats (6 months: 127 ± 29 kcal/week, P = 0.033; 12 months: 111 ± 27 kcal/week, P = 0.043), and (C) carbohydrates (6 months: 165 ± 32 kcal/week, P = 0.009; 12 months: 169 ± 29 kcal/week, P = 0.004). However, the total carbohydrates consumed by HCHF-fed rats was comparable to CHOW-fed rats without the consumption of 12% sucrose. (D) Sodium intake of HCHF-fed male rats was higher than CHOW-fed male rats (6 months: 117 ± 19 mg/week, P = 0.027; 12 months: 155 ± 26 mg/week, P = 0.011). (E) Weight at baseline and after 3 months of either CHOW or HCHF diet. (F) HCHF-fed rats gained more weight compared to their CHOW-fed littermates (6 months: 27 ± 7% higher weight gain, P < 0.001; 12 months: 23 ± 5% higher weight gain, P < 0.001). At 12 months, TgAD rats gained more weight than nTg rats (CHOW-fed rats: 15 ± 4%, P = 0.047; HCHF-fed rats: 16 ± 4%, P = 0.025). The average weight of CHOW-fed nTg cohorts was considered as the normal weight for each age and used as the reference in computing the weight elevation for each rat. (G) The gray-shaded regions indicate the weight gain level classified as overweight [14%, 38%], obese class 1 [38%, 62%], or obese class 2 [62%, 84%], as defined from human BMI ranges . At 12 months of age, HCHF-fed female rats had higher increase in weight (by 15 ± 4% in nTg, P = 0.016; by 29 ± 4% higher weight gain in TgAD, P < 0.001) compared to HCHF-fed male rats. At 6 months of age, n=20, where n=9 [nTg(1 female, 2 male); TgAD(4 female, 2 male)] were CHOW-fed and n=11 [nTg(3 female, 2 male); TgAD(2 female, 4 male)] were HCHF-fed. At 12 months of age, n=43, where n=21 [nTg(6 female, 5 male); TgAD(5 female, 5 male)] were CHOW-fed and n=22 [nTg(5 female, 6 male); TgAD(5 female, 6 male)] were HCHF-fed. *P < 0.05, **P < 0.01.

Figure 3

HCHF attenuates hippocampal glucose uptake in presymptomatic AD, but elevates it in symptomatic AD. (A) Representative time-resolved MTR change map at baseline and subsequent changes observed after the infusion of 2DG (indicated by the gray bar) at 20-minute intervals, illustrating the dynamics of 2DG uptake. (B) Dynamic CEST ΔMTR (mean and SEM) time courses in 6-, 12-, and 15-month-old nTg and TgAD cohorts fed with CHOW or HCHF diets. The time origin is the start of the 6-min 2DG infusion, indicated by the gray-shaded rectangle. (C) Area under the curve (AUC) of all scanned rats and (D) AUC of female rats over 60 minutes after the onset of 2DG infusion for 6-, 12-, and 15-month-old cohorts. There was a significant interaction of diet and genotype in 12-month-old cohorts (P < 0.001). At 6 months of age, there was a significant diet effect (P < 0.001) where HCHF-fed nTg rats (-0.09 ± 0.09 AUC units) had lower AUC units (by -153%) compared to CHOW-fed nTg (0.2 ± 0.3 AUC units); while HCHF-fed TgAD rats (-0.2 ± 0.2 AUC units) had lower AUC units (by -169%) compared to CHOW-fed TgAD rats (0.3 ± 0.4 AUC units). However, at 12 months of age, these contrasts were reversed: HCHF-fed nTg rats (-0.002 ± 0.20 AUC units) had higher AUC compared to CHOW-fed nTg (-0.1 ± 0.3 AUC units, by 2x), while HCHF-fed TgAD rats had higher AUC (0.4 ± 0.3 AUC units, by 20x) when compared to CHOW-fed TgAD (-0.02 ± 0.40 AUC units). Considering the HCHF-fed cohorts, 12-month-old TgAD rats (0.4 ± 0.3 AUC units) showed highest levels of hippocampal glucose uptake. They exhibited higher AUC compared to 12-month-old nTg rats (by 0.4 ± 0.1 AUC units, P < 0.001), to 6-month-old TgAD rats (by 0.6 ± 0.1 AUC units, by 4x, P < 0.001), and to 15-month-old nTg rats (by 0.5 ± 0.1, by 2x, P < 0.05). (D) After stratification by sex and pairwise comparisons, the differences across the groups remain evident when female rats were considered in isolation (initial uptake phase: age-gn-sex, P < 0.05; gn-sex, P < 0.001 | peak uptake phase: age-gn-sex, P < 0.05 and gn-sex, P < 0.05). In addition, the attenuation of glucose uptake of HCHF-fed 15-month-old TgAD rats compared to 12-month-old TgAD rats was driven by females (initial uptake phase: sex, P < 0.05), but not statistically significant after pairwise comparison. At 6 months of age, n=20, where n=9 [nTg(1 female, 2 male); TgAD(4 female, 2 male)] are CHOW-fed and n=11 [nTg (3 female, 2 male); TgAD (2 female, 4 male)] are HCHF-fed. At 12 months of age, n=41, where n=21 [nTg (6 female, 5 male); TgAD (5 female, 5 male)] were CHOW-fed and n=20 [nTg (5 female, 6 male); TgAD (5 female, 4 male)] were HCHF-fed. At 15 months of age, n=18, where n=7 [nTg (2 female, 2 male); TgAD (2 female, 1 male)] were CHOW-fed and n=11 [nTg (4 female, 2 male); TgAD (3 female, 2 male)] were HCHF-fed. *P < 0.05, **P < 0.01.

Figure 4

HCHF elevates functional hyperemia in presymptomatic and symptomatic AD; and decreases resting perfusion in presymptomatic AD. (A) Representative maps of CBF changes, and average hippocampal ASL time courses normalized by the mean resting ASL in response to bilateral forepaw stimulation. (B) The resting CBF of cohorts at 6 and 12 months were indistinguishable from each other. However at 15 months, lower resting CBF in HCHF-fed rats (nTg: 136 ± 36 ml/100g/min; TgAD: 136 ± 12 ml/100g/min) by 47% in nTg and 41% in TgAD rats was observed compared to CHOW-fed rats (nTg: 258 ± 107 ml/100g/min; TgAD: 232 ± 100 ml/100g/min) mainly in females (diet, P < 0.001; diet-sex, P < 0.05). At 6 months, HCHF-fed rats had significantly lower resting CBF compared to CHOW-fed rats (diet, P < 0.001). (C) At 6 months, HCHF-fed rats (nTg: 17 ± 4%; TgAD: 22 ± 1%) showed a greater increase in CBF in response to forepaw electric stimulation by 51% in nTg and 40% in TgAD compared to CHOW-fed rats (nTg: 11 ± 2%; TgAD: 16 ± 7%) (diet, P < 0.01). However, the effect of HCHF in 12 and 15 months of age was genotype specific (12 months: diet-genotype, P < 0.001; 15 months: diet-genotype-sex, P < 0.05). The CBF response to forepaw stimulation of HCHF-fed nTg rats (12 months: 13 ± 11%; 15 months: 17 ± 9%) are lower (by 17% in 12 months and 4% in 15 months) compared to CHOW-fed nTg rats (12 months: 16 ± 6%; 15 months: 18 ± 13%) while HCHF-fed TgAD rats (12 months: 25 ± 11%; 15 months: 18 ± 5%) have higher CBF response (by 114% in 12 months and 37% in 15 months) compared to CHOW-fed TgAD rats (12 months: 12 ± 9%; 15 months: 13 ± 2%). (D) At 6 months of age, HCHF-fed rats had a larger area of activation by 154% in nTg and by 141% in TgAD rats when compared to CHOW-fed rats (diet, P < 0.001); whereas HCHF-fed TgAD rats (0.37 ± 0.10) had significantly higher area of activation compared to CHOW-fed nTg rats (0.1 ± 0.1, P < 0.001). At 12 months of age, there was a significant interaction of diet and genotype (P < 0.001): HCHF-fed nTg (0.2 ± 0.2) rats exhibited a trend toward lower area of activation by 14% when compared to CHOW-fed nTg (0.3 ± 0.2). In contrast, HCHF-fed TgAD rats (0.3 ± 0.1) showed a trend of higher area of activation by 400% compared to that of CHOW-fed TgAD rats (0.07 ± 0.07). At 15 months of age, there was no difference in the areas of activation between cohorts. (E) The observed difference in resting perfusion and (F) CBF change was driven by female rats while the (G) area of activation was more prominent in male rats. In evaluating resting perfusion at 6 months of age, n=17, where n=10 [nTg (3 female, 3 male); TgAD (2 female, 2 male)] were CHOW-fed and n=7 [nTg (2 female, 2 male); TgAD (1 female, 2 male)] were HCHF-fed. At 12 months of age, n=41, where 22 [nTg (6 female, 5 male); TgAD (6 female, 5 male)] were CHOW-fed and 19 [nTg (5 female, 5 male); TgAD (6 female, 3 male)] were HCHF-fed. At 15 months of age, n=21, where n=8 [nTg (2 female, 2 male); TgAD (3 female, 1 male)] were CHOW-fed and n=13 [nTg (4 female, 3 male); TgAD (3 female, 3 male)] were HCHF-fed. *P < 0.05, **P < 0.01.

Figure 5

Prolonged HCHF diet attenuates lectin density in TgAD rats. (A-B) Representative tomato lectin-Texas Red-stained images of hippocampal capillaries in CHOW- and HCHF-fed nTg and TgAD 12-month-old rats. (C) 12-month-old TgAD rats had higher lectin density compared to nTg rats. HCHF attenuated the lectin density in TgAD rats, with further decreases seen with prolonged HCHF diet. (D) Both male and female TgAD and nTg rats had comparable lectin density at 15 months. At 12 months, n=50, where n=26 [nTg (7 female, 5 male), TgAD (7 female, 7 male)] were fed with HCHF diet and n=24 [nTg (5 female, 4 male), TgAD (7 female, 8 male)] were fed with CHOW. At 15 months, n=37, where n=17 [nTg (4 female, 3 male), TgAD (5 female, 5 male)] were fed with HCHF and n=20 [nTg (6 female, 4 male), TgAD (5 female , 5 male)] were fed with CHOW. Scale bar: 1mm. *P < 0.05, **P < 0.01.

Figure 6

Prolonged HCHF diet rescued elevated hippocampal AQP4 density in symptomatic AD. (A-B) Representative stained images of AQP4 in both CHOW- and HCHF-fed 12- and 15-month-old (A) male and (B) female rats. (C) At 12 months of age, there was a significant interaction of genotype and diet (P < 0.001), whereby AQP4 density was elevated in TgAD rats, but rescued with HCHF diet and maintained with extended HCHF diet at 15 months. (D) The attenuation of hippocampal AQP4 density of HCHF-fed TgAD rats (28 ± 8%) compared to CHOW-fed TgAD rats (34 ± 5%, P < 0.001) at 12 months was driven by males (diet-genotype-sex, P < 0.01; genotype-sex, P < 0.001). With prolonged exposure to the HCHF diet, there was a female-driven attenuation of AQP4 density (age-genotype-sex, P < 0.001; genotype-sex, P < 0.001). At 15 months, CHOW-fed male TgAD rats maintained an elevated hippocampal AQP4 density (age-genotype-sex, P < 0.05). At 12 months, n=50, where n=26 [nTg (7 female, 5 male), TgAD (7 female, 7 male)] were fed with HCHF diet and n=24 [nTg (5 female, 4 male), TgAD (7 female, 8 male)] were fed with CHOW. At 15 months, n=37, where n=17 [nTg (4 female, 3 male), TgAD (5 female, 5 male)] were fed with HCHF and n=20 [nTg (6 female, 4 male), TgAD (5 female, 5 male)] were fed with CHOW. Scale bar: 1mm. *P < 0.05, **P < 0.01.

Figure 7

Prolonged HCHF exacerbated cerebral amyloid angiopathy in females. (A) Representative stained (tomato lectin-Texas Red) images of hippocampal CAA in TgAD rats fed with CHOW and HCHF diet for 3 months (12-month-old rats) and 6 months (15-month-old rats). (B) Quantification of hippocampal CAA density showed a significant interaction of age and sex (P < 0.01) and a significant effect of age (P < 0.001). (C) the increase in CAA coverage with age was driven by female rats (sex, P < 0.001). At 12 months, n=29, where n=14 (7 female, 7 male) was fed with HCHF diet and n=15 (7 female, 8 male) was fed with CHOW. At 15 months, n=20, where n=10 (5 female, 5 male) was fed with HCHF and n=10 (5 female, 5 male) was fed with CHOW. Scale bar: 0.07 mm. *P < 0.05, **P < 0.01.