| Summary: | 博士 === 國立成功大學 === 基礎醫學研究所 === 102 === Alzheimer’s disease (AD) is an age-related neurodegenerative disease. AD occurs gradually and results in memory loss, behavior and personality changes, and a decline in thinking abilities. Pathologically, AD is characterized by intracellular aggregation of neurofibrillary tangles and extracellular deposition of amyloid plaque. The amyloid plaques primarily consist of β-Amyloid (Aβ) peptides and the neurofibrillary tangles comprise of hyperphosphorylated tau proteins. Results from epidemiological and pathological research showed a strong link between cardiovascular diseases and AD. It has been suggested that chronic brain hypoperfusion is the common denominator among cardiovascular diseases. One major consequence of hypoperfusion is insufficient supply of glucose to brain, hence results in cerebral hypometabolism. However, whether energy deficiency contributes to the development of AD remains unclear. To investigate the causal relationship, we performed the following three experiments. 1) We cultured the differentiated N2a neuroblastoma cells in media containing no glucose or pyruvate (NGM). Shortly after the N2a cells cultured in the NGM, the mitochondria membrane potential was reduced and the AMP-activated-protein-kinase (AMPK), an energy sensor, was activated. Treatment of NGM not only increased the levels of tau phosphorylation at Ser262 and Ser396, but also increased the levels of active forms of GSK3α and GSK3β, two major tau kinases. 2) The effect of energy deficiency was further examined in vivo by intracerebroventricular (icv) injection of streptozotocin (STZ) to the Wistar rats. STZ selectively injuries glucose transporter type 2-bearing cells which are primarily astrocytes in the rat brain, hence, interrupts glucose transportation from blood vessel to neuron. STZ-icv injection induced energy crisis in the brain regions surrounding the ventricles, as indicated by elevated pAMPK levels in the hippocampus. STZ-icv treatment increased the levels of phosphorylated tau and activated GSK3β in the hippocampus. The hippocampus-dependent spatial learning and memory was impaired by the STZ-icv treatment. 3) Because the levels of Aβ in the N2a cells and Wistar rats were too low to be accurately quantified, we therefore used human amyloid precursor protein overexpressed N2a cells (APP cells) to investigate the effect of energy deficiency on Aβ production. The results showed that concentrations of glucose in culture media negatively associated with the levels of pAMPK in the APP cells, but positively correlated with the levels of Aβ in the condition media. When APP cells were cultured in glucose-containing media, drug-induced activation of AMPK decreased the levels of Aβ in the condition media. However, if APP cells were incubated in media containing no glucose, inhibition of AMPK activity increased the levels of Aβ, while the levels of full-length APP, APPα, APPβ, APP C-terminal fragment α and C-terminal fragment β were unchanged. Taken together, these studies suggest that energy deficiency increases the levels of tau phosphorylation. Furthermore, when energy is deficient and AMPK activation is inhibited, the levels of Aβ are increased, probably due to reduced clearance of Aβ. Thus, our studies support the premise that metabolic disorders contribute to AD pathogenesis.
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