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

最近，越来越多的证据表明神经元内淀粉样蛋白-β (Aβ)积累在阿尔茨海默病的病理级联中起着初始作用。另一方面，在阿尔茨海默病（AD）的易损神经元中已经报道了细胞周期失调，即异常细胞周期再进入（CCRE）。为了阐明阿尔茨海默病大脑中神经元内淀粉样蛋白-β (a β)积累与CCRE之间的可能联系，我们使用原位方法在阿尔茨海默病死后大脑中鉴定了神经元内a β和增殖细胞核抗原（PCNA）， CCRE的标记物。免疫细胞化学结果显示，在阿尔茨海默病海马和枕颞回连续切片的相同神经元群中（n=10）观察到神经元内Aβ和PCNA的阳性反应，而在年龄匹配的对照脑（n=5）中，这两种免疫反应都很微弱。为了进一步研究CCRE在伴有a β病理的神经变性中的作用，我们建立了双转基因（CaMKII- myc）小鼠模型，该模型通过CaMKII启动子下的四环素控制的反激活子系统，在前脑神经元中表达强大的细胞周期诱导剂人c-MYC。在MYC诱导4周或8周后，与MYC- off小鼠的基础水平相比，MYC- on小鼠的脑部分显示MYC和CCRE标记物如PCNA的强烈表达以及海马神经元中溴脱氧尿苷（BrdU）的结合。特别有趣的是，在MYC-On小鼠（而非MYC-Off小鼠）中，主要在CA1区域的海马神经元中，通过Aβ1-42的c端特异性抗体检测到神经元内Aβ免疫反应。这些结果表明，细胞周期失调可能是阿尔茨海默病神经退行性变的基本机制之一。

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.