Department of Biology, Middle Tennessee State University, 1301 E. Main St, Murfreesboro, TN, 37132, USA.
School of Medicine and Dentistry, University of Rochester Medical Center, Rochester, NY, 14642, USA.
Dev Biol. 2019 Sep 15;453(2):168-179. doi: 10.1016/j.ydbio.2019.05.015. Epub 2019 May 30.
Elimination of paternal mitochondria after fertilization occurs in many species using the process of selective autophagy. The mechanism for targeting paternal mitochondria, but not maternal mitochondria, for elimination in the early embryo is not well understood. The results in this paper suggest that there are at least two different mechanisms for targeting paternal mitochondria for elimination: the first involving ubiquitination and a second involving a mitochondrial associated autophagy receptor, fndc-1. Elimination of paternal mitochondria can be visualized in embryos of the nematode, C. elegans. Paternal mitochondria enter the zygote at fertilization. Initially, they are closely associated with another sperm organelle, the membraneous organelle (MO). The MOs become ubiquitinated within minutes after fertilization. Simultaneous RNAi knockdown of two ubiquitin conjugating enzymes, ubc-18 and ubc-16, reduces MO ubiquitination. Loss of function of ubc-18 alone leads to loss of K48-linked polyubiquitin chains and halts the recruitment of proteasome to MOs. Interestingly, knockdown of ubc-18 or ubc-16 or the combination does not reduce the localization of K63-linked ubiquitin chains to MOs suggesting that some ubiquitin structure other than K63 chains is responsible for recruiting the autophagy machinery to MOs. Double knockdown (ubc-18/ubc-16) inhibits the recruitment of the autophagy protein, LGG-1 (homolog of LC3/GABARAP), to paternal organelles and causes the persistence of paternal mitochondria into the two cell stage. If paternal mitochondria are not eliminated via this early process, they are eventually removed from the embryo in a process that depends on the mitophagy adaptor protein, fndc-1. Thus, there are two redundant, but temporally distinct mechanisms that target paternal mitochondria for elimination in C. elegans. In addition to the involvement of ubiquitination in the elimination of paternal mitochondria, two subunits of the proteasome, rpn-10 and rad-23, are required for elimination of paternal mitochondria. These subunits are known to function as ubiquitin receptors and knockdown of either inhibits the recruitment of proteasome to ubiquitinated MOs. Their knockdown does not affect the localization of LGG-1 to paternal structures indicating that the proteasome is not required for autophagy membrane recruitment but might be involved in autophagosome maturation or its fusion with the lysosome.
在许多物种中，通过选择性自噬的过程，在受精后消除父系线粒体。然而，对于早期胚胎中针对父系线粒体而非母系线粒体进行消除的机制，目前还不是很清楚。本文的结果表明，至少存在两种针对父系线粒体进行消除的不同机制：第一种机制涉及泛素化，第二种机制涉及线粒体相关自噬受体 Fndc-1。在线虫 C. elegans 的胚胎中可以观察到父系线粒体的消除。父系线粒体在受精时进入受精卵。最初，它们与另一个精子细胞器，膜状细胞器 (MO) 紧密相关。MO 在受精后几分钟内被泛素化。同时敲低两种泛素连接酶 ubc-18 和 ubc-16，会减少 MO 的泛素化。仅缺失 ubc-18 的功能会导致 K48 连接的多泛素链丢失，并阻止蛋白酶体募集到 MO。有趣的是，敲低 ubc-18 或 ubc-16 或两者组合并不减少 K63 连接的泛素链在 MO 上的定位，这表明招募自噬机制到 MO 的泛素结构不是 K63 链。双敲低（ubc-18/ubc-16）抑制自噬蛋白 LGG-1（LC3/GABARAP 的同源物）向父系细胞器的募集，并导致父系线粒体持续存在于两个细胞阶段。如果父系线粒体不能通过这个早期过程被消除，它们最终会在一个依赖于线粒体自噬衔接蛋白 Fndc-1 的过程中从胚胎中被清除。因此，在 C. elegans 中，有两种冗余但时间上不同的机制针对父系线粒体进行消除。除了泛素化参与父系线粒体的消除外，蛋白酶体的两个亚基 rpn-10 和 rad-23 也需要消除父系线粒体。这两个亚基已知作为泛素受体，敲低任何一个都会抑制蛋白酶体募集到泛素化的 MO。它们的敲低不影响 LGG-1 向父系结构的定位，表明蛋白酶体不需要自噬膜募集，但可能参与自噬体成熟或与溶酶体融合。
