世界生命科学前沿动态周报（四十五）

【点评】
胰腺β细胞转变为α细胞的机制说明DNA甲基化等表观调控是决定细胞生命属性的重要因素，单只DNA本身并不能决定细胞的生命属性。这也为合理调节表观调控以影响细胞生命属性提供了科学依据。

【参考论文】Developmental Cell, 2011; 20 (4): 419 DOI:10.1016/j.devcel.2011.03.012
Pancreatic β Cell Identity Is Maintained by DNA Methylation-Mediated Repression of Arx
Sangeeta Dhawan, Senta Georgia, Shuen-ing Tschen, Guoping Fan, Anil Bhushan
Adult pancreatic β cells can replicate during growth and after injury to maintain glucose homeostasis. Here, we report that β cells deficient in Dnmt1, an enzyme that propagates DNA methylation patterns during cell division, were converted to α cells. We identified the lineage determination gene aristaless-related homeobox (Arx), as methylated and repressed in β cells, and hypomethylated and expressed in α cells and Dnmt1-deficient β cells. We show that the methylated region of the Arx locus in β cells was bound by methyl-binding protein MeCP2, which recruited PRMT6, an enzyme that methylates histone H3R2 resulting in repression of Arx. This suggests that propagation of DNA methylation during cell division also ensures recruitment of enzymatic machinery capable of modifying and transmitting histone marks. Our results reveal that propagation of DNA methylation during cell division is essential for repression of α cell lineage determination genes to maintain pancreatic β cell identity.

3.通过激活成熟神经元中的BMP/Smad1信号通路再生受伤脊椎中的轴突神经
【动态】
　　轴突生长潜力在年轻神经元中最高但随年龄增长而消失，因而成为成体受伤后轴突再生的显著障碍。一项新研究表明在DRG神经元中Smad1依赖的骨成型蛋白（BMP）逐步被调节控制轴突生长。下调该途径造成年龄相关的轴突生长潜力的下降。在成熟的DRG神经元中选择性重新激活Smad1导致脊椎损伤的老鼠模型感觉神经再生。Smad1信号可以通过临床适用的最小侵入技术运送编码BMP4的AAV载体来有效的操控。重要的是，即使是脊椎损伤后实施AAV治疗，断了的轴突也能再生，因此很像相应的临床事件。

【点评】
　 这一神经再生的研究也说明DNA并非决定细胞生命活动的关键，对基因表达的调控更重要。

【参考论文】PNAS 2011, doi: 10.1073/pnas.1100426108
Regeneration of axons in injured spinal cord by activation of bone morphogenetic protein/Smad1 signaling pathway in adult neurons
Pranav Parikha, Yuhan Haoa, Mohsen Hosseinkhania.
Axon growth potential is highest in young neurons but diminishes with age, thus becoming a significant obstacle to axonal regeneration after injury in maturity. The mechanism for the decline is incompletely understood, and no effective clinical treatment is available to rekindle innate growth capability. Here, we show that Smad1-dependent bone morphogenetic protein (BMP) signaling is developmentally regulated and governs axonal growth in dorsal root ganglion (DRG) neurons. Down-regulation of the pathway contributes to the age-related decline of the axon growth potential. Reactivating Smad1 selectively in adult DRG neurons results in sensory axon regeneration in a mouse model of spinal cord injury (SCI). Smad1 signaling can be effectively manipulated by an adeno-associated virus (AAV) vector encoding BMP4 delivered by a clinically applicable and minimally invasive technique, an approach devoid of unwanted abnormalities in mechanosensation or pain perception. Importantly, transected axons are able to regenerate even when the AAV treatment is delivered after SCI, thus mimicking a clinically relevant scenario. Together, our results identify a therapeutic target to promote axonal regeneration after SCI.

4.  线粒体作治疗癌症的靶点
【动态】  
有关癌细胞线粒体的一些研究引导了发展针对线粒体蛋白和功能的无遗传毒性的抗癌疗法。目前已经明确的线粒体在细胞凋亡中的作用提供了肿瘤细胞自杀的新靶点。线粒体的作用是作为对细胞压力和损伤做出反应的中央转换器。可以针对癌细胞中导致免于凋亡的那些变化来抑制其增殖。因为癌细胞代谢的重组涉及到增加糖酵解，看起来通过针对HIF-1 或HIF-1基因编码的代谢酶来阻断InsP3R Ca2+的释放或响应缺氧的适应性途径代表了一种可行的癌症治疗方法。在体外向癌细胞添加白藜芦醇后很快发生的是细胞内数秒内即可测到的Ca2+的增多。在添加无毒的类黄酮时也观察到Ca2+的释放，看来值得找出钙活化中涉及的模型化合物白藜芦醇的作用靶点。

【点评】
针对癌细胞线粒体与正常细胞线粒体的差异进行癌症治疗的思路是对的,但是目前这种差异的细节还不是很清楚。

【参考论文】Journal of cellular physiology. 2011 DOI 10.1002/jcp.22788
Targeting mitochondria as a therapeutic target in cancer
Wenner CE
Knowledge of re-programming in cancer cells with metabolic differences from their normal counterparts has resulted in new examination of therapeutic approaches. Several studies of the role of tumor mitochondria in cancer have led to the development of non-genotoxic therapies which target mitochondrial proteins, function. The now well-established functions of mitochondria in apoptosis provide novel targets for tumor cell suicide. Mitochondria serve as a central hub for responses to cellular stress as well as injury. The alterations in cancer cells which result in protection from apoptosis can be targeted to inhibit proliferation. Because of the reprogramming of cancer cell metabolism involving increased glycolysis, it appears that blocking InsP3R Ca2+ release or adaptive pathways in response to hypoxia by targeting HIF-1 or metabolic enzymes encoded by the HIF-1 gene represents a feasible therapeutic approach to cancer. A very early in vitro event found in tumor cells following resveratrol addition is an increase in intracellular Ca2+, measurable within seconds. Ca2+ release is also observed with non-toxic flavonoids and a goal to identify the sentinel targets of resveratrol as a model compound involved in calcium activation seems worthwhile. New findings of the relationship between autophagy and apoptosis are discussed. The contribution of ROS generated by mitochondria is also considered. New data as to how cyclophilins and VDAC are involved in mitochondrial hexokinase protection of factors that induce apoptosis are reviewed. In addition, chemotherapeutic approaches based on Aktactivated mTORC1 are described, and their relationship to the role of aerobic glycolysis in this protection.

5.  小分子协同作用维持人体胚胎干细胞早期分化后的长期稳定自我更新
【动态】
    小分子抑制剂迅速诱导人体胚胎干细胞转变为原始神经祖细胞并维持长期自我更新。人体胚胎干细胞赋予了再生医学巨大希望。通常人体胚胎干细胞的应用需要其在体外分化成期望的均一细胞群。目前的人体胚胎干细胞分化模式面临的一个主要挑战是不能有效锁定并长期稳定扩增那些保有广泛的分化能力，更重要的是发育阶段特异性的分化倾向的原始的细胞系特异性干细胞/祖细胞群。一项新的研究报告了用小分子协同抑制糖原合成酶激酶3（GSK3）,转化生长因子β (TGF-β)，和Notch信号途径能够在特定化学条件下于一周内有效的将单层培养的人体胚胎干细胞转变成均一的原始神经上皮细胞。这些原始神经上皮能够在白细胞抑制因子，GSK3抑制剂（CHIR99021）和TGF-β受体抑制剂（SB431542）存在的情况下稳定的自我更新；保持高潜力的神经生长能力和对中脑后脑神经亚型引导性神经模式线索的反应性；并表现出体内的整体性。

【点评】
通过小分子抑制剂协同作用调控不同的信号途径，来达到阻止干细胞进一步分化和维持干细胞长期稳定扩增，对于干细胞体外培养还是有些价值的，只是这种人为规定的环境下培育的干细胞能否发挥体内正常生理条件下的干细胞功能还有很大疑问。

【参考论文】PNAS 2011 ; doi:10.1073/pnas.1014041108
Rapid induction and long-term self-renewal of primitive neural precursors from human embryonic stem cells by small molecule inhibitors
Wenlin Li, Woong Sun, Yu Zhang, et al.
Human embryonic stem cells (hESCs) hold enormous promise for regenerative medicine. Typically, hESC-based applications would require their in vitro differentiation into a desirable homogenous cell population. A major challenge of the current hESC differentiation paradigm is the inability to effectively capture and, in the long-term, stably expand primitive lineage-specific stem/precursor cells that retain broad differentiation potential and, more importantly, developmental stage-specific differentiation propensity. Here, we report synergistic inhibition of glycogen synthase kinase 3 (GSK3), transforming growth factor β (TGF-β), and Notch signaling pathways by small molecules can efficiently convert monolayer cultured hESCs into homogenous primitive neuroepithelium within 1 wk under chemically defined condition. These primitive neuroepithelia can stably self-renew in the presence of leukemia inhibitory factor, GSK3 inhibitor (CHIR99021), and TGF-β receptor inhibitor (SB431542); retain high neurogenic potential and responsiveness to instructive neural patterning cues toward midbrain and hindbrain neuronal subtypes; and exhibit in vivo integration. Our work uniformly captures and maintains primitive neural stem cells from hESCs.

6.  鉴别出与肌肉修复有关的关键基因
【动态】
    作为胚胎骨骼肌发育的关键，卫星细胞（SCs）直至婴儿期还在继续活跃的增加肌肉质量。之后，他们数量逐渐减少并沉寂下来，直到损伤或退化激活它们增殖。卫星细胞维持肌肉生长并使成熟的骨骼肌具有强大的再生能力。一项新的研究报告了Polycomb抑制性复合物2（PRC2）的酶亚基EZH2，在Pax7+/Myf5− 干细胞和 Pax7+/Myf5+ 肌原性前体细胞中都有表达，并且是保持卫星细胞池稳定所需要的。有条件的切除SCs的Ezh2的老鼠出生后Pax7+细胞很少且肌肉数量下降还不能适当再生。这些缺陷与受损的SC增殖及非肌细胞中表达的基因的去抑制有联系。因此，EZH2在SCs中控制自我更新和增殖以及维持合适的转录程序。

【点评】
该研究对卫星细胞的增殖规律以及肌肉损伤后修复有了更深入的了解，有助于损伤和衰老肌肉复原的研究。