Increased Human like Alzheimer features in a Novel Mouse Model with Oxidative DNA Repair Dysfunction

The decline of cognitive function is a feature of normal human aging and is exacerbated in Alzheimer’s disease (AD). DNA repair declines in brain cells during normal aging and even more so in AD. Here we show that experimental reduction in levels of the base excision repair enzyme, DNA polymerase β (Polb) renders neurons vulnerable to age-related dysfunction and degeneration in a mouse model of AD. Whereas 3xTgAD mice exhibit age-related extracellular amyloid b-peptide (Ab) accumulation and cognitive deficits, but no neuronal death, 3xTg/Polb+/- mice accumulates intracellular Ab and neurons die in the hippocampus and cerebral cortex. The DNA repair-deficient 3xTgAD mice exhibited increased DNA strand breaks and apoptotic caspase activation with loss of hippocampal volume, and impaired synaptic plasticity and memory retention. Molecular profiling revealed remarkable similarities in gene expression alterations in brain cells of AD patients and 3xTgAD/Polb+/- mice including multiple abnormalities suggestive of impaired cellular bioenergetics. Our findings demonstrate that a modest decrement in oxidative DNA damage processing is sufficient to render neurons vulnerable to AD-related pathogenic molecular and cellular alterations that result in the dysfunction and death of neurons, and associated cognitive deficits. 4 mouse strains were used in these experiments, the 3xTgAD and Pol β (+/-) mice were bred at the National Institute on Aging (Baltimore, Maryland). The original line 3xTgAD line was generated as described previously (Oddo, et. al 2003) and possess APPswe, PS1M146V, and tauP301L mutations. DNA polymerase beta heterozygous mice, Pol β (+/-), were crossed with the 3xTgAD mice to generate a 3xTgAD/Pol β (+/-) mouse. The Wt strain is C57Bl/6. At 20 months of age these mice were euthanized by cervical dislocation, the brain removed from the skull and dissected into regions of interest, the prefrontal cortex was used for the microarray studies.认知功能衰退是人类正常衰老的典型特征，而阿尔茨海默病（Alzheimer’s disease, AD）会使该进程进一步恶化。正常衰老过程中，脑细胞的DNA修复能力会出现下降，在AD患者体内这一下降幅度更为显著。 本研究证实，在阿尔茨海默病小鼠模型中，实验性降低碱基切除修复（base excision repair）酶DNA聚合酶β（DNA polymerase β, Polβ）的表达水平，会使神经元更易受到衰老相关的功能异常与变性损伤。相较于仅表现出衰老相关细胞外淀粉样β肽（amyloid β-peptide, Aβ）沉积与认知缺陷，但无神经元死亡的3xTgAD小鼠，3xTgAD/Polβ+/-小鼠则会出现细胞内Aβ沉积，并在海马体与大脑皮层中发生神经元死亡。 DNA修复缺陷型3xTgAD小鼠表现出DNA链断裂增多、凋亡半胱天冬酶（caspase）激活，同时伴随海马体积缩减，并且存在突触可塑性受损与记忆保留能力下降的问题。 分子谱分析显示，AD患者脑细胞与3xTgAD/Polβ+/-小鼠脑细胞的基因表达改变存在显著相似性，其中包含多项提示细胞生物能量代谢受损的异常特征。 本研究结果表明，氧化DNA损伤修复过程的轻度减退，足以使神经元更容易受到AD相关的致病分子与细胞改变影响，进而引发神经元功能异常、死亡，并伴随相关认知缺陷。 本实验共使用4种小鼠品系：3xTgAD小鼠与Polβ(+/-)小鼠均在美国国家衰老研究所（马里兰州巴尔的摩）繁育。原始3xTgAD品系的构建方法已在既往研究中发表（Oddo等，2003），该品系携带APPswe、PS1M146V以及tauP301L突变。DNA聚合酶β杂合子小鼠（Polβ(+/-)）与3xTgAD小鼠交配，从而获得3xTgAD/Polβ(+/-)小鼠。野生型（Wild type, Wt）品系为C57Bl/6。在小鼠饲养至20月龄时，通过颈椎脱臼法实施安乐死，将脑组织从颅骨中取出并分离至目标脑区，本研究选取前额叶皮层进行微阵列（microarray）研究。