Accumulation of intraneuronal amyloid β and cell cycle dysregulation in the brains of Alzheimer's disease

近年来，越来越多的证据表明，神经元内β淀粉样蛋白（amyloid-β，Aβ）的积累在AD的病理级联反应中起着重要作用。另一方面，细胞周期失调，为阐明阿尔茨海默病（AD）神经元内β淀粉样蛋白（Aβ）的积聚与CCRE的可能联系，我们采用原位方法检测了AD脑组织中神经元内Aβ和CCRE标志物增殖细胞核抗原（PCNA）的表达。免疫细胞化学结果显示，在AD患者海马和枕颞回的连续切片中，神经元内Aβ和PCNA呈阳性反应（n=10），而在年龄匹配的对照组中，两者均呈微弱的免疫反应（n=5）。我们已经开发了双转基因（CaMKII-MYC）小鼠模型，其通过在CaMKII启动子下使用四环素控制的反式激活因子系统，在前脑神经元中特异性地表达强有力的细胞周期诱导剂人c-MYC。在4或8周MYC诱导后，MYC-On小鼠的脑切片与MYC-Off小鼠中的基础水平相比，在海马神经元中显示出MYC和CCLE标记物如PCNA的稳健表达和溴脱氧尿苷（BrdU）的掺入。特别值得注意的是，在MYC-On小鼠中，而在MYC-Off小鼠中，通过C-末端特异性Aβ1-42抗体检测到神经元内Aβ免疫反应，主要在CA 1区。

Recently, growing body of evidence suggests an initial role of intraneuronal amyloid-β (Aβ) accumulation in the pathological cascade of AD. On the other hand, cell cycle dysregulation, i.e., abnormal cell cycle re-entry (CCRE) has been reported in vulnerable neurons in Alzheimer disease (AD).To elucidate a possible connection between intraneuronal amyloid-β (Aβ) accumulation and CCRE in the brains of AD, we used an in situ approach to identify intraneuronal Aβ and proliferating cell nuclear antigen (PCNA), a marker of CCRE, in the postmortem brains of AD. Immunocytochemically, positive reactions with intraneuronal Aβ and PCNA were observed in the same neuronal populations in the serial sections of the hippocampus and occipitotemporal gyrus of AD (n=10), while both of the immnoreactions were faint in the age-matched control brains (n=5).To further investigate an involvement of CCRE in neurodegeneration accompanied by Aβ pathology, we have developed double transgenic (CaMKII-MYC) mouse model that expresses a powerful cell cycle inducer, human c-MYC, specifically in forebrain neurons by using the tetracycline-controlled transactivator system under the CaMKII promoter. After 4 or 8-week MYC induction, brain sections of MYC-On mice displayed robust expression of MYC and CCRE markers such as PCNA and incorporation of bromodeoxyuridine (BrdU) in the hippocampal neurons in comparison to the basal levels in MYC-Off mice. Of particular interest, in MYC-On mice, but not MYC-Off mice, intraneuronal Aβ immunoreaction was detected by C-terminal specific antibodies for Aβ1-42 in the hippocampal neurons predominantly in the CA1 region.These results suggest that cell cycle dysregulation may be one of the basic mechanisms underlying the neurodegeneration in AD.