分子遺傳分析線蟲cnx-1及crt-1在細胞屍體吞噬過程中的角色

Abstract

Calnexin and calreticulin are generally known as chaperons located in the ER. However, they are also involved in multiple functions other than chaperons. Overexpression of the cleavage product of calnexin in dying cells has been reported to inhibit apoptosis in mouse cells. On the other hand, calreticulin has been found on the cell surface in various mammalian cell cultures and acts as “eat-me signals” when cells undergo apoptosis. We provided the first in vivo evidence that the calnexin and calreticulin function in the engulfment of apoptotic cells in C. elegans. C. elegans calnexin and calreticulin are encoded by cnx-1 and crt-1, respectively. To examine their function in progressional cell death, we analyzed the cell-death phenotype in cnx-1 and crt-1 mutants. A significant increase of embryonic cell corpses were detected in the cnx-1; crt-1 double mutant strains while a milder increase of embryonic cell corpses were detected in their single mutant strains. This suggests that they may act on partially redundant pathways in the process. When cells undergo programmed cell death, cell corpses adopt a refractile and disc like structure. Using this morphological change as a cell death marker, we analyzed the death of the first 13 cells in the AB cell lineage of wild type and cnx-1; crt-1 mutant embryos. The 4D microscopic analysis revealed that cnx-1 and crt-1 mutations prolong cell-corpse duration time but do not affect the time when cells exhibit the death phenotype. This suggests that cnx-1 and crt-1 act in engulfment rather than in execution of apoptosis. Furthermore, from the result that cnx-1 and crt-1 can further increase the L1 persisting head cell corpse in both ced-1 and ced-5 mutants, we deduce that cnx-1 and crt-1 define a novel pathway parallel to the two classical ced-1, 6, 7 and ced-2, 5, 10 pathways. Finally, because cnx-1 can further increase the germ cell death in ced-1 mutant, and the phenotype can be rescued by introducing cnx-1::gfp into the cnx-1; ced-1 double mutants, we suggest that cnx-1 acts in the engulfing cells other than the dying cells during the engulfment process of germline apoptosis.