線蟲ced-2/crkII與ced-10/rac在凋亡細胞吞噬機制中具有先前未知的負向調節作用

Abstract

在C. elegans中，兩條平行且部分重複的訊息傳遞途徑作用於凋亡細胞的吞噬機制中，至少有八個基因（ced-1, ced-6, ced-7, dyn-1 and ced-2, ced-5, ced-10, ced-12）參與其中。這些基因一旦發生突變，就會造成吞噬功能的缺陷，並產生存留的細胞屍體。CED-2是一個帶有SH2 domain和 SH3 domain的adapter protein，它可能會與CED-5/CED-12形成的heterodimeric guanine nucleotide exchange factor結合，共同活化CED-10（small Rac GTPase），造成細胞骨架的重組，使吞噬細胞能夠吞噬凋亡細胞。我們意外的發現ced-10(n3246) mutation顯著降低ced-5 mutants中的細胞屍體數目。在使用4D顯微攝影，分析AB cell lineage和MS cell lineage中最早的14個細胞死亡過程後，結果發現ced-10(n3246)部分抑制了ced-5 mutant中的吞噬缺陷。這些結果顯示在吞噬凋亡細胞的過程中，ced-10可能具有負向調節的作用。有趣的是，ced-10(n3246)並不會減低ced-12 single mutant或ced-12; ced-5 double mutants中的細胞屍體數目，顯示ced-10的負向調節作用需要ced-12。我們也發現ced-2(n1994)可以減少ced-10(n3246)中的細胞屍體數目，而ced-2(e1752)則否；細胞屍體數目減少的原因是吞噬缺陷受到部分抑制。這些發現意味著CED-2可能透過N-terminal SH3 domain進行對吞噬機制的抑制作用。進一步的遺傳分析顯示ced-2的負向調節作用需要ced-12而不需ced-5。總括來說，透過遺傳和性狀分析，我們的研究顯示ced-2和ced-10在細胞凋亡過程中，對吞噬機制具有先前未知的負向調節作用。

In Caenorhabditis elegans, two partially redundant pathways containing at least eight genes (ced-1, ced-6, ced-7, dyn-1 and ced-2, ced-5, ced-10, ced-12) function in the engulfment of apoptotic cells. Mutations in any of these genes cause engulfment defects and hence result in the persistence of cell corpses. CED-2, a SH2 and SH3 containing adapter protein, may bind to the CED-5/CED-12 heterodimeric guanine nucleotide exchange factor complex to activate CED-10, a small Rac GTPase, leading to cytoskeletal reorganization during engulfment of apoptotic cells. Surprisingly, we found ced-10(n3246) significantly reduced cell corpse number in ced-5 mutants. Using four-dimensional microscopic recording, we analyzed the first 14 cell deaths in the AB and MS cell lineages and found that ced-10(n3246) partially suppressed the engulfment defect of ced-5 mutants. These results reveal a possible negative role of ced-10 in cell-corpse engulfment. Interestingly, ced-10(n3246) did not reduce cell corpse number of ced-12 single or ced-12; ced-5 double mutants, indicating that the negative role of ced-10 may require ced-12. We also found that ced-2(n1994), but not ced-2(e1752), mutation reduced cell corpse number in the ced-10(n3246) background; the reduction was caused by a partial suppression of the Ced-10 engulfment defect. These findings suggest that CED-2 may exert an inhibitory function through its N-terminal SH3 domain. Further genetic analyses showed that the negative role of ced-2 required ced-12 but not ced-5. In summary, our genetic and phenotypic analyses reveal novel roles of ced-2 and ced-10 in negative regulation of cell-corpse engulfment during programmed cell death.