Impaired Glucose Tolerance and Reduced Plasma Insulin Precede Decreased AKT Phosphorylation and GLUT3 Translocation in the Hippocampus of Old 3xTg-AD Mice.

Several studies have demonstrated that mouse models of Alzheimer's disease (AD) can exhibit impaired peripheral glucose tolerance. Further, in the APP/PS1 mouse model, this is observed prior to the appearance of AD-related neuropathology (e.g., amyloid-ß plaques; Aß) or cognitive impairment. In the current study, we examined whether impaired glucose tolerance also preceded AD-like changes in the triple transgenic model of AD (3xTg-AD). Glucose tolerance testing (GTT), insulin ELISAs, and insulin tolerance testing (ITT) were performed at ages prior to (1-3 months and 6-8 months old) and post-pathology (16-18 months old). Additionally, we examined for altered insulin signaling in the hippocampus. Western blots were used to evaluate the two-primary insulin signaling pathways: PI3K/AKT and MAPK/ERK. Since the PI3K/AKT pathway affects several downstream targets associated with metabolism (e.g., GSK3, glucose transporters), western blots were used to examine possible alterations in the expression, translocation, or activation of these targets. We found that 3xTg-AD mice display impaired glucose tolerance as early as 1 month of age, concomitant with a decrease in plasma insulin levels well prior to the detection of plaques (∼14 months old), aggregates of hyperphosphorylated tau (∼18 months old), and cognitive decline (≥18 months old). These alterations in peripheral metabolism were seen at all time points examined. In comparison, PI3K/AKT, but not MAPK/ERK, signaling was altered in the hippocampus only in 18-20-month-old 3xTg-AD mice, a time point at which there was a reduction in GLUT3 translocation to the plasma membrane. Taken together, our results provide further evidence that disruptions in energy metabolism may represent a foundational step in the development of AD.

Differential Fasting Plasma Glucose and Ketone Body Levels in GHRKO versus 3xTg-AD Mice: A Potential Contributor to Aging-Related Cognitive Status?

Cognitive function declines with age and appears to correlate with decreased cerebral metabolic rate (CMR). Caloric restriction, an antiaging manipulation that extends life-span and can preserve cognitive function, is associated with decreased glucose uptake, decreased lactate levels, and increased ketone body (KB) levels in the brain. Since the majority of brain nutrients come from the periphery, this study examined whether the capacity to regulate peripheral glucose levels and KB production differs in animals with successful cognitive aging (growth hormone receptor knockouts, GHRKOs) versus unsuccessful cognitive aging (the 3xTg-AD mouse model of Alzheimer's disease). Animals were fasted for 5 hours with their plasma glucose and KB levels subsequently measured. Intriguingly, in GHRKO mice, compared to those in controls, fasting plasma glucose levels were significantly decreased while their KB levels were significantly increased. Conversely, 3xTg-AD mice, compared to controls, exhibited significantly elevated plasma glucose levels and significantly reduced plasma KB levels. Taken together, these results suggest that the capacity to provide the brain with KBs versus glucose throughout an animal's life could somehow help preserve cognitive function with age, potentially through minimizing overall brain exposure to reactive oxygen species and advanced glycation end products and improving mitochondrial function.

Glucose tolerance and insulin sensitivity are impaired in APP/PS1 transgenic mice prior to amyloid plaque pathogenesis and cognitive decline.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by beta-amyloid (Aß) deposition, neurofibrillary tangles and cognitive decline. Clinical data suggests that both type 1 and type 2 diabetes are risk factors for AD-related dementia and several clinical studies have demonstrated that AD patients show alterations in peripheral glucose regulation characterized by insulin resistance (hyperinsulinemia) or hypoinsulinemia. Whether animal models of AD exhibit a pre-diabetic phenotype without additional dietary or experimental manipulation is unclear however, with contradictory data available. Further, most studies have not examined the time course of potential pre-diabetic changes relative to AD pathogenesis and cognitive decline. Thus, in this study we tested the hypothesis that a pre-diabetic phenotype (peripheral metabolic dysregulation) exists in the APP/PS1 transgenic model of AD under normal conditions and precedes AD-related pathology. Specifically, we examined glucose tolerance in male APP/PS1 mice on a C57BL/6J congenic background at 2, 4-6 and 8-9months of age by assessing fasting glucose levels, glucose tolerance, plasma insulin levels and insulin sensitivity as well as the development of pathological characteristics of AD and verified that our APP/PS1 mice develop cognitive impairment. Here we show that APP/PS1 mice, compared to wild-type controls, exhibit a significant impairment in glucose tolerance during an intraperitoneal glucose tolerance test (ipGTT) and a trend for increased fasting plasma insulin concentrations as early as 2months of age, while extracellular Aß1-42 deposition occurs later and cognitive decline exists at 8-9months of age. Moreover, APP/PS1 mice did not respond as well to exogenous insulin as the wild-type controls during an intraperitoneal insulin tolerance test (ipITT). Taken together, these data reveal that male APP/PS1 mice on a C57BL/6J congenic background exhibit a pre-diabetic phenotype prior to the development of AD-like pathology and that this metabolic deficit persists when they exhibit neuropathology and cognitive decline. This raises the question of whether altered glucose regulation and insulin production/secretion could contribute to AD pathogenesis.

BACE1 elevation is involved in amyloid plaque development in the triple transgenic model of Alzheimer's disease: differential Aß antibody labeling of early-onset axon terminal pathology.

ß-amyloid precursor protein (APP) and presenilins mutations cause early-onset familial Alzheimer's disease (FAD). Some FAD-based mouse models produce amyloid plaques, others do not. ß-Amyloid (Aß) deposition can manifest as compact and diffuse plaques; it is unclear why the same Aß molecules aggregate in different patterns. Is there a basic cellular process governing Aß plaque pathogenesis? We showed in some FAD mouse models that compact plaque formation is associated with a progressive axonal pathology inherent with increased expression of ß-secretase (BACE1), the enzyme initiating the amyloidogenic processing of APP. A monoclonal Aß antibody, 3D6, visualized distinct axon terminal labeling before plaque onset. The present study was set to understand BACE1 and axonal changes relative to diffuse plaque development and to further characterize the novel axonal Aß antibody immunoreactivity (IR), using triple transgenic AD (3xTg-AD) mice as experimental model. Diffuse-like plaques existed in the forebrain in aged transgenics and were regionally associated with increased BACE1 labeled swollen/sprouting axon terminals. Increased BACE1/3D6 IR at axon terminals occurred in young animals before plaque onset. These axonal elements were also co-labeled by other antibodies targeting the N-terminal and mid-region of Aß domain and the C-terminal of APP, but not co-labeled by antibodies against the Aß C-terminal and APP N-terminal. The results suggest that amyloidogenic axonal pathology precedes diffuse plaque formation in the 3xTg-AD mice, and that the early-onset axonal Aß antibody IR in transgenic models of AD might relate to a cross-reactivity of putative APP ß-carboxyl terminal fragments.