APP與Flotillin-1之間的交互作用對APP processing的影響

Abstract

阿茲海默症（Alzheimer’s disease）是世界上最常見的失智症。它是一漸進式的神經退化性疾病。其具有兩個特徵：一是神經纖維糾結（neurofibrillary tangles），另一則是老年斑（senile plaques）。老年斑是由不正常堆積的類澱粉β蛋白質（Aβ）構成。Aβ的產生是類澱粉前驅蛋白（amyloid precursor protein，APP）經由β-與γ-secretases依序切割而形成，此過程也會產生APP的C端片段（APP intracellular domain，AICD）。先前的研究指出脂筏（lipid rafts）可能參與Aβ的生成調控，而APP於T668位置的磷酸化可能也會增加Aβ的生成。由實驗室之前的研究發現一脂筏相關蛋白（lipid raft-associated protein）flotillin-1與AICD間有交互作用，且APP於T668位置的磷酸化也會影響AICD與flotillin-1的交互作用(未發表的數據)，因此想觀察AICD與flotillin-1的交互作用是否參與調控APP之代謝。我們藉由消耗神經母細胞瘤細胞株（SH-SY5Y）中膽固醇（cholesterol）含量的研究間接證明了lipid rafts在APP代謝中的重要性，而在利用siRNA降解(knockdown) flotillin-1的實驗中也發現C99/C83數值的下降。在以RFP-AICD融合蛋白（fusion protein）或合成的flotillin-1胜肽（peptide）進行的競爭實驗的結果也指出，當有APP-flotillin-1交互作用的競爭者存在時，會降低C99/C83值。另外，為了瞭解APP T668磷酸化在APP與flotillin-1交互作用中扮演的角色，我們自可過度表現（overexpressing）模擬不同T668磷酸化程度的APP的SH-SY5Y細胞中抽取蛋白萃取物，並藉由免疫共沉澱法與蔗糖梯度離心的方法進行分析。結果顯示模擬APP6668磷酸化的SH-SY5Y APP695T668E與flotillin-1有較好的交互作用，且在蔗糖梯度離心方法中的共分層(co-fractionation)情況也較好。我們初步的結果暗示flotillin-1可能會藉由與磷酸化的AICD的交互作用而促進APP代謝走向amyloidgenic途徑。

Alzheimer’s disease is the most common cause of dementia in the world. It is a progressive neurodegenerative disorder characterized by two hallmarks: neurofibrillary tangles (NFTs) and senile plaques (SPs), the abnormal accumulation of the β-amyloid peptide (Aβ). Aβ is generated by sequential proteolytic cleavages of amyloid precursor protein (APP) by β- and γ-secretases. This process also generates the APP intracellular domain (AICD). Previous studies have suggested that lipid rafts are probably involved in regulating Aβ generation, and the phosphorylation of APP at threonine residue 668 (Thr668) might increase Aβ level. Since our earlier observations showed that AICD is able to interact with a lipid raft-associated protein, flotillin-1, and phosphorylation of APP at Thr668 probably affect the interaction between AICD and flotillin-1 (unpublished data). It is interesting to see whether the AICD-flotillin-1 interaction plays a role in the regulation of APP processing. The importance of lipid rafts on APP processing was indirectly suggested by cholesterol depletion study in a human neuroblastoma cell line SH-SY5Y, and the ratio of C99/C83 was decreased in flotillin-1 siRNA knockdown experiment, also. Results from our RFP-AICD fusion protein and synthetic flotillin-1 peptide competition studies indicated that the presence of those binding-partner competitors would lead to a reduction in C99/C83 ratio. Additionally, to assess the effect of APPT668 phosphorylation state on the interaction between APP and flotillin-1, lysates from SH-SY5Y cells overexpressing wild-type APP and various APP mutants mimicking different T668 phosphorylation states were analysed by co-immunoprecipitation and sucrose gradient fractionation studies. The results showed that the interaction between SH-SY5Y APP695T668E, which mimics APPT668 phosphorylation and flotillin-1 is higher than others, and the co-fractionation in sucrose gradient between SH-SY5Y APP695T668E and flotillin-1 is higher, too. Our preliminary results suggest that flotillin-1 may promote amyloidogenic processing of APP through interacting with T668 phosphorylated AICD.