Expression of caspase and BCL-2 apoptotic family members in mouse preimplantation embryos

Apoptosis, as determined by blastomere and DNA fragmentation, occurs in many preimplantation mouse embryos. To investigate which genes contribute to apoptosis in preimplantation embryos, we used the reverse transcription-polymerase chain reaction to assess mRNA levels for seven genes in the caspase family and seven genes in the BCL-2 family. All caspase mRNAs were detectable in oocytes, while expression in preimplantation embryos varied in a stage-specific manner. An assay for group II caspase enzymatic activity showed that although transcripts for these caspases could not be detected in zygotes, proteolytic activity could be detected in polar bodies, fragmented zygotes, and zygotes treated with staurosporine. This suggests that maternal caspases are inherited during oogenesis. Transcripts for some members of the BCL-2 family could be detected at every stage of preimplantation development. Transcripts for other members were rarely detected. When BCL-2 and BAX protein levels were assessed using immunofluorescence, both proteins were detected in zygotes and in blastocysts. When fragmented blastocysts were compared to normal blastocysts, levels of BCL-2 immunofluorescence tended to be lower in fragmented blastocysts. This result supports a model in which the ratio of BCL-2 to BAX is altered in apoptotic embryos.