We have previously demonstrated that initiation of intracellular calcium ([Ca2+]i) oscillations in mouse eggs signals activation or apoptotic death depending on the age of the eggs in which the oscillations are induced. To extend these studies, mouse eggs were aged in vitro to 24, 32, and 40 h post-hCG and injected with sperm cytosolic factor (SF), adenophostin A, or sperm (intracytoplasmic sperm injection), and the times at which signs of apoptosis first appeared were examined. These treatments, which induced [Ca2+]i oscillations, caused fragmentation and other signs of programmed cell death in eggs as early as 32 h post-hCG. The susceptibility of aged eggs to apoptosis appeared to be due to cytoplasmic deficiencies, because fusion of recently ovulated eggs with aged, SF-injected eggs prevented fragmentation. Evaluation of mRNA and protein levels of the apoptotic regulatory proteins Bcl-2 and Bax showed a prominent decrease in the amounts of Bcl-2 mRNA and protein in aged eggs, whereas Bax mRNA levels did not appear to be changed. Lastly, the Ca2+ responses induced by the aforementioned Ca2+ agonists ceased in advance in aged eggs. Together, these results suggest that one or several critical cytosolic molecules involved in the regulation of Ca2+ homeostasis, and in maintaining the equilibrium between anti- and proapoptotic proteins, is either lost or inactivated during postovulatory egg aging, rendering the fertilizing Ca2+ signal into an apoptosis-inducing signal.

The preimplantation embryo development (Ped) gene that encodes the class Ib major histocompatibility complex protein Qa-2 influences the rate of embryonic cleavage during the preimplantation stages of development. Embryos from strains of mice that lack the Ped gene cleave slowly, while embryos that have a functional Ped gene cleave more rapidly. This effect is observed both in vivo and in vitro with the Ped fast haplotype showing dominance over the Ped slow haplotype. The Ped gene is associated with pleiotropic effects on reproduction. Certain strains of mice lacking the Ped gene (Ped slow) have smaller litters and the pups weigh less at birth and at weaning. Previously our laboratory reported that in litters derived from Ped fast/slow F1 mice backcrossed to the slow/slow parent, there were significantly more Ped fast pups than the 50% expected, at two months of age. This implies that there is selection in favor of the Ped fast haplotype at some point during development. The present study was designed to determine at what point during development selection occurs. Using a polymerase chain reaction assay, we determined that selection does not occur by days post coitus 14.5. However, our results show that there are significantly more Ped fast pups than Ped slow pups remaining in backcross litters just after birth, indicating that selection in favor of the Ped fast haplotype occurs between day 14.5 and birth.

Purpose:The properties of the mouse Ped gene and the genes that mediate apoptosis in mediating preimplantation embryonic survival were reviewed.

The protein product of the mouse preimplantation embryo development (Ped) gene, which controls the rate of preimplantation embryonic cleavage division and subsequent embryo survival, is the Qa-2 antigen. This major histocompatibility complex (MHC) class I b protein is encoded by four genes, Q6, Q7, Q8, and Q9. The present study was undertaken to begin to elucidate which of the four Qa-2-encoding genes are responsible for the Ped gene phenotype in the C57BL/6 mouse (H2b). First, restriction maps of the four genes, using 25 restriction enzymes, were created. The RE maps confirmed that Q6 is similar to Q8 and Q7 is similar to Q9, but that the Q6/Q8 gene pair differs from the Q7/Q9 gene pair. The genomic DNA sequences of Q6 and Q8 were determined, as well as the DNA sequences of exons 4 – 8 of Q9, and the 5′ regulatory regions of Q6, Q8, and Q9. This DNA sequence information, combined with the published DNA sequence information for the entire Q7 gene and exons 1 – 3 of Q9, allowed us to design primers for reverse transcription-polymerase chain reaction that could distinguish which of the four genes were transcribed in mouse lymphocytes and embryos. It was found that all four genes are transcribed in lymphocytes, but only Q7 and Q9 are transcribed in mouse embryos. Thus, both Q7 and Q9 are candidates for the genes responsible for the Ped gene phenotype.