Association of Genes Involved in the Circadian Clock With Polysomes in the Bovine Preimplantation Embryo

The circadian clock is not functional during preimplantation development because lack of transcription prevents the feedback loop required for circadian cyclicity. Moreover, transcript abundance for the core clock genes (CLOCK, ARNTL, PER1, PER2, CRY1, and CRY2) declines in the embryo after embryonic genome activation. Nonetheless, transcripts for each of the clock genes are present in the embryo. The potential for translation of these transcripts in the bovine embryo was evaluated by assessing whether circadian clock genes are associated with polysomes and whether these transcripts are preferentially retained as development proceeds to the blastocyst stage. Transcript abundance declined at the eight-cell stage for BMAL1 and CRY1 and at the morula stage for PER1, PER2, and CRY2. Before embryonic genome activation at the eight-cell stage, a large fraction of transcripts for each of the genes was associated with polysomes. At specific later stages of development, there was less transcript associated with polysomes than with other fractions. This was true for PER1 at the morula and blastocyst stage, PER2 at the morula stage, and CRY1 at the eight-cell stage. The percent of transcripts associated with polysomes was also calculated. This value was lower after the two-cell stage for CLOCK and PER1. Based on the decrease in transcript abundance and proportional association with polysomes after the two-cell stage, it was concluded that capacity for translation of circadian clock genes declines in the preimplantation embryo as development proceeds. Thus, de novo synthesized proteins involved in the circadian clock mechanism are unlikely to play an important function in the preimplantation embryo following embryonic genome activation.