Apoptosis cascade progresses during turnover of human trophoblast: Analysis of villous cytotrophoblast and syncytial fragments in vitro

Growth and survival of the trophoblast layer of the human placenta depends on continuous incorporation of villous trophoblast stem cells (cytotrophobl ast), by syncytial fusion, into the syncytiotrophoblast. Descriptive studie s suggest that this process may be intimately related to apoptosis. We have analyzed the expression and activation of initiator and execution caspases , critical effecters of apoptosis, in relation to trophoblast turnover (dif ferentiation) in human placental villi. We used immunohistochemistry, caspa se enzyme histochemistry, caspase activity assays, Western blots, and autor adiography techniques on placental tissue sections, trophoblast lysates, vi llous explants, and isolated trophoblast fragments (villous cytotrophoblast and mononuclear syncytial elements) in vitro. Our data demonstrate express ion of initiator caspases 8 and 10 and activity of caspase 8 in villous cyt otrophoblast. Proforms of the execution caspases 3, 6, and 7 were also expr essed in villous cytotrophoblast, but activation of execution caspases 3 an d 6 could only be demonstrated in the syncytiotrophoblast after syncytial f usion. Down-regulation of the general transcription level (reduced incorpor ation of [H-3]uridine) as well as cleavage of the execution caspase substra tes poly-(ADP-ribose)polymerase and lamin B was confined to syncytiotrophob last and preceded the final events of apoptotic death (terminal deoxynucleo tidyl transferase-mediated deoxyuridine triphosphate nick-end labeling reac tivity and nuclear collapse). Our data confirm that the apoptosis cascade i n villous trophoblast is regulated in parallel with trophoblast differentia tion, syncytial fusion, and trophoblast turnover.