Interactions between age, stress and insulin on cognition: implications for Alzheimer's disease

Abstract

There is much interest in understanding the mechanisms responsible for interactions among stress, aging, memory and Alzheimer's disease. Glucocorticoid secretion associated with early life stress may contribute to the variability of the aging process and to the development of neuro- and psychopathologies. Maternal separation (MS), a model of early life stress in which rats experience 3 h of daily separation from the dam during the first 3 weeks of life, was used to study the interactions between stress and aging. Young (3 months) MS rats showed an altered hypothalamic-pituitary-adrenal (HPA) axis reactivity, depressive-like behavior in the Porsolt swimming test and cognitive impairments in the Morris water maze and new object recognition test that persisted in aged (18 months) rats. Levels of insulin receptor, phosphorylated insulin receptor and markers of downstream signaling pathways (pAkt, pGSK3 beta, pTau, and pERK1 levels) were significantly decreased in aged rats. There was a significant decrease in pERK2 and in the plasticity marker ARC in MS aged rats compared with single MS or aged rats. It is interesting to note that there was a significant increase in the C99 : C83 ratio, A beta levels, and BACE1 levels the hippocampus of MS aged rats, suggesting that in aged rats subjected to early life stress, there was an increase in the amyloidogenic processing of amyloid precursor protein (APP). These results are integrated in a tentative mechanism through which aging interplay with stress to influence cognition as the basis of Alzheimer disease (AD). The present results may provide the proof-of-concept for the use of glucocorticoid-/insulin-related drugs in the treatment of AD.

Figure 1

Effects of age and maternal separation (MS) on cognition. (a) Retention phase of the Morris water maze, data are presented as distance swam in the quadrant where platform used to be located. There was a main effect of rearing (F_1,55=7.199, p<0.05) and age (F_1,55=14.220, p<0.001). (b) New object recognition test, data presented as discrimination index: time exploring the new object/total exploration time × 100. There was a main effect of rearing (F_1,58=8.964, p<0.01). 3 m: 3 months, young rats; and 18 m: 18 months, aged rats ^*p<0.001 main effect of rearing, ^†p<0.05 or better, main effect of age.

Figure 2

Age and maternal separation (MS) interact to affect insulin levels and intracellular insulin pathways. (a) Phosphorylated Akt levels (active form of the enzyme, 60 kDa) normalized to total Akt, main effect of age (F_1,41=9.606, p<0.01; n=8–12); (b) phosphorylated (inactive) GSK3β levels (46 kDa) normalized to total GSK3β, main effect of age (F_1,32=33.318, p<0.001; n=6–10); (c) Phosphorylated Tau levels (40–50 kDa) normalized to total Tau, main effect of age (F_1,35=25.622, p<0.001; n=8–10); (d) Phosphorylated (active) ERK1 (44 kDa) normalized to total ERK1, main effect of age (F_1,33=20.429, p<0.001; n=6–10) and phosphorylated (active) ERK2 levels (42 kDa) normalized to total ERK2, interaction (rearing × age) (F_1,35=6.054, p<0.05). Figure shows percentage of optical density (O.D.) values of control young rats and representative picture of the blotting. No differences were found in the non-phosphorylated (total) levels of the enzymes. 3 m: 3 months, young rats; and 18 m: 18 months, aged rats. ^*p<0.01 or better, main effect of age. ^†p<0.05 or better vs control young rats, ^#p<0.05 vs MS young rats.

Figure 3

Neonatal stress favors the amyloidogenic processing of amyloid precursor protein (APP) in aged rats. (a) APP protein (105 kDa) levels (no effects of age or rearing) and C99 : C83 (10 kDa) ratio, main effect of rearing (F_1,26=4.164, p<0.05) and age (F_1,26=5.158, p<0.05). (b) Aβ levels (pmol/mg) ( × 10⁸), main effect of age (F_1,22=11.116, p<0.01; n=6), main effect of rearing (F_1,22=5.723, p<0.05; n=6). (c) BACE1 (70 kDa) levels, main effect of rearing (F_1,22=7.390, p<0.05) and age (F_1,22=10.380, p<0.01). a and c show percentage of optical density (O.D.) values of control young rats and representative picture of the blotting. 3 m: 3 months, young rats; and 18 m: 18 months, aged rats; MS: maternal separation rats. ^*p<0.05 main effect of rearing, ^†p<0.05 or better, main effect of age.

Figure 4

Effects of age and maternal separation (MS) on synaptic plasticity. Figure shows percentage of optical density (O.D.) values of control young rats and representative pictures of autoradiograms. (a) BDNF mRNA expression. (b) ARC mRNA expression. 3 m: 3 months, young rats; and 18 m: 18 months, aged rats. ^*p<0.01 or better, main effect of rearing, two-way ANOVA (rearing × age), ^†p<0.01 or better, main effect of age, two-way ANOVA (rearing × age), ^#p<0.05 or better vs control young (3 months) rats, ^‡p<0.001 vs MS young (3 months) rats or control aged (18 months) rats.

Figure 5

Mechanism through which stress and/or glucocorticoids exerts their effects on aging/Alzheimer's disease (AD). Glucocorticoids by inhibiting the insulin pathway, would lead to a decrease in pERK2 levels, as well as affecting synaptic plasticity. Therefore neuronal ability would be compromised to survive insults, such as increases in glucocorticoid-induced C99 or Aβ production.