Nature:重磅!清除衰老的神经胶质细胞,可以阻止tau蛋白沉积和认知能力下降(长文解析)
细胞衰老密切影响着神经神经退化(neurodegeneration),而对这一机制的研究也是长久以来的热点。细胞衰老往往是个不可逆转的过程,可以被多种胞内和胞外的因子所诱发。在细胞衰老过程中,细胞分化停止、胞内细胞死亡途径被抑制、同时有促炎性分子的释放,而这些对临近的健康细胞是有极大伤害的。然而,在对诸多神经退行性疾病的病因学研究中,细胞衰老具有何种作用仍是不清楚的【1-2】。
2018年9月20日,来自(美国)梅奥诊所生物化学和分子生物学系,与梅奥诊所儿科和青少年医学部的联合研究团队,将他们的 一项突破性研究成果 以Clearance of senescent glial cells prevents tau-dependent pathology and cognitive decline题在线发表在Nature上。
研究表明:tau突变蛋白在神经细胞中的表达会引发神经胶质细胞的衰老,阻止神经细胞的衰老,可以预防MAPT P301SPS19小鼠的认知能力下降和神经退化【3】。
衰老细胞的特征是在分子水平和基因表达水平上具有多种改变,包括细胞循环抑制蛋白p16 INK4A表达量的增加;同时,衰老的细胞可以释放炎症信号分子、生长因子以及蛋白激酶,而这些生物分子对细胞的功能和存活是有损坏作用的【4-5】。
在衰老细胞中,SA-β-Gal(senescence-associated β-galactosidase,衰老相关β-半乳糖苷酶)在pH为6.0时具有高酶活性,而正常细胞中的β-Gal在pH=6.0时是无活性的。β-Gal可以在此条件下催化底物生产蓝色的产物,因此此方法可以来鉴定细胞是否出现衰老【4】。
虽然,在脑损伤和神经退行性疾病,如阿尔兹海默症和帕金森病的患者已观察到了衰老的神经元和神经胶质细胞的存在,并且,定位和消除衰老细胞的诸多策略也被开发【6-7】。然而,在不同背景条件下,衰老细胞的精确作用是未知的。
在此研究中,研究者们将阐明细胞衰老在神经退化过程中的机制,确切的说是揭示细胞衰老与tau蛋白积累之间的关系。应用MAPT P301SPS19小鼠模型,此模型神经元中高表达人tau突变蛋白;神经胶质增生、神经纤维缠结(NFT)、神经退化、及认知功能缺失的特征。
首先,研究者们对野生型和PS19小鼠的海马和皮层内的p16 Ink4a基因进行RT-qPCR分析,发现在6月大PS19小鼠的海马内和4月大PS19小鼠的皮层内,p16 Ink4a表达显著增高,且衰老细胞会在PS19模型的病理部位聚集(Fig. 1a)。为了揭示细胞衰老在疾病发展的作用,由Fig.1b所示对小鼠杂交并喂养AP20187(AP; a ligand that can induce homodimerization of fusion proteins containing the F36V mutant of an FKBP domain (FKBPF36V), available commercially as the DmrB domain),发现AP可以有效地、特异地清理PS19;ATTAC小鼠海马和皮层内的衰老细胞(Fig. 1c) 。同时,通过对海马和皮层内染色,确定了细胞衰老机制和tau蛋白病理之间的关系,且主要是在星形胶质细胞和小神经胶质细胞中出现了衰老(Fig. 1d-e) 。
Fig. 1 Senescent astrocytes and microglia that accumulate in the brains of MAPT P301S PS19 mice can be removed using the INK-ATTAC transgene.
PS19小鼠呈现出随着疾病的进展,神经胶质增生也会加重【4】。为了确定AP是否会影响这一过程,通过对星形胶质细胞和小胶质细胞进行RT-qPCR分析,结果发现在PS19;ATTAC小鼠体内,AP可以清除衰老的神经胶质细胞和神经胶质增生( Fig. 2a-d)。
Fig.2 Senescent cells promote gliosis.
其次,6月大PS19小鼠的一个显著的特征是过度磷酸化tau蛋白的沉积【4】。研究者们分别分析了可溶性和非可溶性的tau蛋白的总量,进而确定清除衰老细胞对tau蛋白沉积的影响。数据表明衰老细胞的积累会促进过度磷酸化tau蛋白的积累(Fig. 2a-c)。另外,8月大PS19小鼠表现出神经退化【4】。对 PS19;ATTAC小鼠进行AP治疗时,发现海马和皮层内的NFT积累可以被消除,进一步佐证衰老细胞的积累会加强异常tau蛋白的聚集(Fig. 3a-c)。并且,喂养AP是否可以提升小鼠的认知功能?于是,研究者们评估了小鼠的短期记忆,实验发现衰老细胞会诱发PS19小鼠的神经退化和认知能力丧失(Fig. 4)。
Fig.3 Senescent cells drive neurodegenerative disease.
Fig.4 Vision-based novel-object discrimination remains intact in AP-treated PS19;ATTAC mice.
研究最后,衰老的神经胶质细胞和神经元之间存有极其复杂的通讯关系,基因干预操作可以清除衰老的星形胶质细胞和小神经胶质细胞,进而可以有效地降低tau蛋白聚集和NFT沉积【8】。研究者们试验了喂养“senolytic ABT263”(A senolytic is among the class of small molecules under basic research to determine if they can selectively induce death of senescent cells)是否和基因干预治疗具有类似效果,即两者都是否可以清除PS19小鼠内的衰老细胞。可喜的是,通过给PS19小鼠喂养该药剂,不但可以抑制衰老关联基因的上调( Fig. 5a),也能消除过度磷酸化的tau蛋白的沉积( Fig. 5b)。也就是说喂养“senolytic”和基因干预具有相似的治疗效果。
Fig.5 ABT263 can modulate senescent cells and attenuate tau phosphorylation.
总的来说,衰老细胞在tau-介导性疾病的起始和发展过程中起着重要作用,靶向衰老细胞可能为治疗这些病理提供治疗途径。
但同时,在接下来的研究中,仍需处理如下问题【3.8.】:
(1). 确定来自tau P301S表达的神经元在神经胶质细胞中引起衰老的信号;
(2). 衰老的星形胶质细胞和小神经胶质细胞向神经元发出信号的机制仍有待确定;
(3). 源于神经胶质细胞的相同信号是否会影响tau蛋白的病理和神经元的存活,以及星形胶质细胞和小神经胶质细胞是否会发出相同或不同的信号;
阐明这些问题对更深刻地理解神经退行性疾病具有更广泛的意义。
另外,(4). 目前,诸多研究表明“senolytic”治疗可以受益于很多患者,那么,在其他神经退化的动物模型中,移除衰老细胞是否也能缓解疾病还需进一步确定;(5). 在本文中,研究者们全面移除了小鼠衰老细胞,但“senolytic”治疗是立即生效?还是在疾病发展到有症状的阶段时才会产生有益的效果?(6). 最后,确定本文中发现的这一过程是否在人类进化中具有保守性也是至关重要的,如果是的话,也许“senolytic”治疗可以使更多的人受益。
补充阅读
【1】Neuron:前沿!Tau蛋白会破坏阿尔茨海默氏病(AD)的核质转运(新机制)
【2】Science:前沿!成人神经细胞再生与BDNF的共操作可作为阿尔兹海默症(AD)的一种强有力治疗途径
【3】JEM:前沿!TLR5诱骗受体可作为治愈阿尔兹海默症(AD)的一种新颖且安全的免疫调节剂
【4】Brain: HSPA2被确定为晚发型阿尔兹海默症的一个关键驱动因子
【5】Nature Communications:LRRK2激酶调控α-核突触蛋白的积累,可作为治疗帕金森的药物靶标
通讯作者Dr. Darren J. Baker 简介
Darren J. Baker M.S., PH.D.
Email: Baker.Darren@mayo.edu
The research interests of Darren J. Baker, M.S., Ph.D., are focused on the involvement of senescent cells in the processes of aging and cancer.
Cellular senescence, a state of irreversible cell cycle arrest that limits the ability of cells to divide, is a potent anti-tumor mechanism that is also a common feature of aged tissue. Instead of simply being resident cells within a tissue, senescent cells promote various age-related phenotypes due to components that they secrete.
Studying the role of cellular senescence in vivo, however, has been difficult due to the lack of reliable markers and the inability to selectively manipulate these cells in animals.
Dr. Baker's research team has developed a transgenic mouse model, termed INK-ATTAC, in which senescent cells can be selectively eliminated in an inducible fashion. By using the INK-ATTAC model in combination with an accelerated aging model, Dr. Baker has demonstrated the ability to significantly delay age-related pathologies and phenotypes, including cataract formation, by removing senescent cells.
Focus areas
Senescent cells in natural aging.
Dr. Baker's team has demonstrated that removal of senescent cells from a prematurely aged mouse model is effective at delaying a variety of phenotypes that are dependent on the acquisition of senescent cells. The team is now interested in extending these studies into the normal aging process that occurs in mice. The expression of p16 increases in many tissues with age, but whether this causes age-related pathologies is unclear.
Using INK-ATTAC, Dr. Baker's team is able to test, for the first time, whether a wide variety of aging-related conditions can be prevented or attenuated by clearing senescent cells from chronologically aged wild-type mice, leading to improved health span and extended life span. Since the team's approach is to interfere with senescent cell viability rather than with senescent cell formation, team members expect to fully preserve the tumor suppressive functions of the senescence program.
The role of senescent cells in Alzheimer's disease and dementia.
Alzheimer's disease, which is characterized by the presence of neurofibrillary tangles and amyloid plaques, is the leading cause of dementia in older adults. However, it is unknown whether and how these deposits contribute to Alzheimer's.
Dr. Baker is working to exploit the observation that both neurofibrillary tangles and neuritic components of plaques of patients with Alzheimer's disease show strong immunoreactivity to the cyclin-dependent kinase inhibitor p16, but not to other members of p16's cell cycle regulatory family of proteins. The p16 biomarker also exhibits increased expression in a variety of tissues with age, but not in terminally differentiated neurons.
Using mouse models of Alzheimer's, Dr. Baker uses the INK-ATTAC transgenic mice to remove p16-expressing cells. His goal is to improve understanding of how p16-expressing cells contribute to AD.
文献参考
【1】Childs, B. G. et al. Nature Rev. Drug Discov. 16,718–735 (2017).
【2】Baker, D. J. & Petersen, R. C. J. Clin. Invest. 128, 1208–1216 (2018).
【3】Bussian, T. J. et al. Nature. https://doi.org/10.1038/s41586-018-0543-y (2018).
【4】Yoshiyama, Y. et al.. Neuron 53, 337–351 (2007).
【5】Sharpless, N. E. & Sherr, C. J. Nature Rev. Cancer 15, 397–408 (2015).
【6】Baker, D. J. et al. Nature 479, 232–236 (2011).
【7】Baker, D. J. et al. Nature 530,184–189 (2016).
【8】Jay Penney, Li-Huei Tsai. Nature.https://doi.org/10.1038/d41586-018-06677-7(2018).
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