In this study we focused on analyzing the role of DmChk2 and Dmp53, apoptosis-inducing factors during oogenesis in Drosophila
. We demonstrated, for the first time in Drosophila
, that expression of DmChk2 alone induced a tissue specific massive cell death; expression of DmChk2 in the germline had a specific cell death effect on mid-oogenesis egg chambers but not on ovarian stem cells. In support of these results, it was shown that the level of DmChk2 transcripts increase in response to nutrient deprivation that leads to mid-oogenesis cell death (45
), suggesting an important role of DmChk2 in the induction of cell death in response to starvation. In contrast to DmChk2, expression of Dmp53, a Chk2-dependent effector protein, in the germline, affected stem cell survival but not mid-oogenesis egg chambers. Thus, to that end it seems that the only tissue that is refractory to DmChk2 but not to Dmp53 levels are mid-oogenesis egg chambers. Moreover, DmChk2 mid-oogenesis induced cell death is not dependent on Dmp53
. Thus, our study reveals in Drosophila
a novel role of DmChk2 in inducing cell death in a Dmp53
Surprisingly, egg chamber cell death induced by expressing DmChk2 was not prevented by co-expression of caspase inhibitors. Two other major forms of cell death are autophagic cell death and necrosis (46
). We found that DmChk2-induced cell death was associated with a delay in autophagy induction and occurred normally in an Atg7
mutant background, indicating that Chk2-induced cell death is not occurring by autophagic cell death. Further, the cell death observed was not morphologically necrotic, and looked indistinguishable from apoptosis. Taken together, Chk2-induced cell death appears to occur by a caspase-independent apoptosis-like death.
We found that over-expression of Dmp53 led to loss of ovarian stem cells. It was previously shown that during gonad formation, Dmp53 is required for programmed cell death of PGC's (14
). In Dmp53
mutant, an excess of PGC's were found ectopic of the gonads. The role of Dmp53
in this process was found to be due to germ cell death but not of PGC's migration. Interestingly enough, in contrast to our results, it was reported that p53
expression in wild-type embryos using the same Gal4 line used by us (nanos-Gal4VP16
) did not affect survival of PGCs in the gonads (14
). The difference between our results and the previous findings is likely due to the use of different pUAS vectors. In this study, we express Dmp53 using the pUASp vector, which allow expression in both somatic and germline tissues (36
), whereas Yamada et al. expressed Dmp53 in the pUAST vector which drives expression only in somatic cells.
We demonstrated that loss of ovarian stem cell by Dmp53 was not suppressed by expression of caspase inhibitors. Moreover, we were unable to detect DNA fragmentation by TUNEL in PGC's from larval ovaries expressing Dmp53. A similar type of cell death was seen with overexpression of lipid phosphate phosphatases Wun or Wun2 in somatic tissues (47
). Wun/Wun2- induced germ cell death is not affected by the expression of the inhibitor-of-apoptosis proteins. Moreover, cells dying in response to Wun/Wun2-mediated signals were negative for TUNEL staining and did not label for another marker of apoptosis, cleaved caspase 3. Thus, Dmp53 andWun/Wun2 induces cell death of the PGC's in a caspase independent manner. To check what other cell death mechanism led to the loss of PGC's by Dmp53, we stained larval ovaries with the lysosomal marker, LT and autophagy marker, GFP-LC3. Using LT and GFP-LC3 staining we detect that activation of a lysosomic process is accompanying the loss of PGC's by Dmp53. Thus, our results revealed that lysosomal cell death may contribute to loss of PGC's by Dmp53.