異時調控基因blmp-1和dre-1參與線蟲尾突細胞凋亡的調控

Abstract

計劃性細胞凋亡在生物發育及維持生理恆定上是不可或缺的機制。在線蟲中，egl-1/BH3-only gene的轉錄作用對於開啟演化中高度保留的細胞凋亡訊息傳遞鏈是相當重要的。儘管如此，特定的細胞仍會透過不同機制來活化細胞凋亡的核心路徑。在線蟲當中，尾突細胞對於它的尾部發育來說是重要的。一對尾突細胞在胚胎發育早期出生，接著經歷融合、分化出以利尾部延伸的支架結構，並且在胚胎晚期尾巴發育完成後進行細胞凋亡。然而，尾突細胞凋亡究竟是如何被調控到現在了解的仍然不多。有趣的是，有報導指出egl-1對於尾突細胞的死亡只有些許而非最主要的影響。在此我們發現，blmp-1以及dre-1這兩個對於生殖腺發育時間調控相當重要的基因也會參與尾突細胞凋亡的時間調控。blmp-1是一個zinc finger transcription factor；dre-1則是SCF E3 ubiquitin ligase複合體中的F-box protein，它的功能主要是協助辨識受質以利蛋白質降解的進行。在blmp-1以及dre-1突變株中，我們發現尾突細胞會不正常存活，這意謂著blmp-1和dre-1的功能在於促進尾突細胞凋亡，而過量表現blmp-1也會造成不正常的尾突細胞存活。從這些結果我們知道，在尾突細胞凋亡的過程中，BLMP-1蛋白的表現需要被嚴密地調控。除此之外，從免疫螢光染色的分析中，我們觀察到BLMP-1表現在胚胎晚期的尾突細胞內，但其表現僅在一小段時間區間裡，這暗示BLMP-1對於尾突細胞凋亡的時間調控相當重要。此外，透過基因轉殖的實驗方法，我們觀察到dre-1會表現在尾突細胞，但不會表現在鄰近細胞裡。這樣的結果告訴我們，dre-1在尾突細胞凋亡上的作用可能是具有細胞自主性的。而在blmp-1和dre-1突變株中，我們觀察到顯著下降的細胞屍體數目，這暗指blmp-1和dre-1對於細胞凋亡的影響可能不只侷限在尾突細胞，或許還有少數特定細胞的凋亡受到它們的調控。綜合以上所述，blmp-1和dre-1除了參與生殖腺發育的調控之外，還具備調控計劃性細胞凋亡的新功能。

Programmed cell death (PCD) is an essential process in animal development. In C. elegans, transcriptional up-regulation of egl-1/BH3-only gene is critical for activating an evolutionarily conserved signaling pathway that leads to cell death. Nonetheless, specific types of cells utilize different mechanisms to activate this core PCD pathway. In C. elegans, a pair of tail-spike cells is essential for tail development. Tail-spike cells are born during early embryogenesis, afterwards, they fuse and differentiate a long microtubule-based spike to serve as a scaffolding for tail elongation. In late embryogenesis, tail-spike cells undergo PCD after tail morphogenesis complete. The mechanism by which tail-spike cell death is regulated remains poorly understood. Interestingly, it has been reported that egl-1 plays a minor role in the death of tail-spike cell. Here, we found that blmp-1 and dre-1, which are important for temporal control of gonad development, regulate this cell death. blmp-1 encodes a zinc finger transcription factor. dre-1 encodes an F-box protein, a subunit of SCF (Skp1-Cullin-F-box protein) E3 ubiquitin ligase mediating protein degradation. We observed inappropriate tail-spike survival in blmp-1 and dre-1 mutants, indicating that blmp-1 and dre-1 function in promoting tail-spike cell death. Surprisingly, over-expression of BLMP-1 also results in abnormal tail-spike survival. These results suggest that the BLMP-1 level needs to be tightly controlled for tail-spike cell death. Additionally, BLMP-1 was detected in tail-spike cell in a narrow time window during late embryogenesis by immunofluorescence analyses, suggesting that BLMP-1 is critical for timely tail-spike cell death. Also, using transgenic approaches, we observed that dre-1 was expressed in tail-spike cell but not its neighboring cells, suggesting that dre-1 acts cell-autonomously in tail-spike cell death. Furthermore, reduced cell corpses were observed in strong loss-of-function blmp-1 and dre-1 mutants, indicating that these two genes also function in promoting cell deaths other than tail-spike cell. In summary, blmp-1 and dre-1 are the novel regulators in programmed cell death.