2009年10月9日，北京生命科学研究所王晓晨实验室在PLoS Genetics杂志在线发表题为“Caenorhabditis elegans Myotubularin MTM-1 Negatively Regulates the Engulfment of Apoptotic Cells”的文章。该文章报道了秀丽线虫肌管素磷酸酶MTM-1负调控凋亡细胞吞噬过程。
细胞凋亡是动物发育中的重要环节之一。其中凋亡细胞的清除能防止凋亡细胞中的有害物质外泄而伤害正常细胞，凋亡细胞清除缺陷会导致炎症和免疫紊乱。在秀丽线虫中，凋亡细胞会迅速被相邻的吞噬细胞识别、吞噬并降解。已知两条部分冗余的信号通路ced-1/6/7/；psr-1/ced-2/5/12/10正调控该过程，但其如何被负调控尚不清楚。
该文章以秀丽线虫为模式生物，通过RNA干扰筛选获得候选基因mtm-1.该基因编码一个具有双特异性的磷酸酶，可特异地水解PI3P和PI(3,5)P2生成PI和PI5P。该基因在进化上较为保守，人类mtm1基因与线虫有很高的同源性。人mtm1基因突变会导致X染色体连锁的肌管性疾病，但其致病机制尚不清楚。RNA干扰mtm-1可部分减轻ced-1/6/7/2突变体的凋亡细胞吞噬缺陷，但ced-5/12/10突变体无显著改变。表明遗传学上mtm-1作用于psr-1/ced-2/5/12/10信号通路并通过CED-5/12/10复合体起作用。
该文章利用显微成像技术，跟踪细胞凋亡的发生以及凋亡细胞的持续时间，证明mtm-1调控凋亡细胞吞噬而不影响细胞凋亡的总数。进一步，利用CED-1:GFP、ACT-5:GFP、LMP-1:GFP和GFP:RAB-7等荧光蛋白标记，证明mtm-1 RNA干扰可加快ced-1/6/7/2突变体凋亡细胞吞噬过程。
为了研究mtm-1是否通过调控PI3P而调控凋亡细胞吞噬，该文章检测了负责生成PI3P的激酶piki-1与vps-34的突变体及双突变体，发现单个突变体表现出较弱的凋亡细胞吞噬缺陷表型，而在双突变体中表型明显增强。在两个激酶都缺失的遗传背景下，mtm-1 RNA干扰不能减轻ced-1/6/7突变体的凋亡细胞吞噬缺陷表型，说明mtm-1调控该过程是依赖于其水解PI3P的磷酸酶功能。
MTM-1在数种已知的吞噬细胞中表达，并特异的定位在细胞膜上。跟踪荧光蛋白发现GFP:MTM-1可结合到凋亡细胞的吞噬体膜上并在数十分钟后与膜分离。该文章利用特异识别PI3P的荧光蛋白标记YFP:2xFYVE, 发现在mtm-1突变体的表皮细胞内的膜泡结构上PI3P水平显著增加。以上结果表明mtm-1很有可能是通过调控细胞膜上的PI3P水平而影响Rac GTPase CED-10的活性从而实现对凋亡细胞吞噬的调控。（生物谷Bioon.com）
相关链接：NIBS王晓晨实验室招聘博士后
生物谷推荐原始出处：
PLoS Genet 5(10): e1000679. doi:10.1371/journal.pgen.1000679
Caenorhabditis elegans Myotubularin MTM-1 Negatively Regulates the Engulfment of Apoptotic Cells
Wei Zou1,2#, Qun Lu1,2#, Dongfeng Zhao2, Weida Li2, James Mapes3, Yuting Xie2, Xiaochen Wang2*
1 College of Biological Sciences, China Agricultural University, Beijing, China, 2 National Institute of Biological Sciences, Zhongguancun Life Sciences Park, Beijing, China, 3 Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado, United States of America
During programmed cell death, apoptotic cells are recognized and rapidly engulfed by phagocytes. Although a number of genes have been identified that promote cell corpse engulfment, it is not well understood how phagocytosis of apoptotic cells is negatively regulated. Here we have identified Caenorhabditis elegans myotubularin MTM-1 as a negative regulator of cell corpse engulfment. Myotubularins (MTMs) constitute a large, highly conserved family of lipid phosphatases. MTM gene mutations are associated with various human diseases, but the cellular functions of MTM proteins are not clearly defined. We found that inactivation of MTM-1 caused significant reduction in cell corpses in strong loss-of-function mutants of ced-1, ced-6, ced-7, and ced-2, but not in animals deficient in the ced-5, ced-12, or ced-10 genes. In contrast, overexpression of MTM-1 resulted in accumulation of cell corpses. This effect is dependent on the lipid phosphatase activity of MTM-1. We show that loss of mtm-1 function accelerates the clearance of cell corpses by promoting their internalization. Importantly, the reduction of cell corpses caused by mtm-1 RNAi not only requires the activities of CED-5, CED-12, and CED-10, but also needs the functions of the phosphatidylinositol 3-kinases (PI3Ks) VPS-34 and PIKI-1. We found that MTM-1 localizes to the plasma membrane in several known engulfing cell types and may modulate the level of phosphatidylinositol 3-phosphate (PtdIns(3)P) in vivo. We propose that MTM-1 negatively regulates cell corpse engulfment through the CED-5/CED-12/CED-10 module by dephosphorylating PtdIns(3)P on the plasma membrane.
