UCLA

Preimplantation mammalian embryo is a critical stage in embryonic development, during which the totipotent zygote goes through zygotic genome activation (ZGA) at 2-cell stage, and then generates the first two cell lineages, trophectodem (TE), the inner cell mass (ICM) at the blastocyst stage. The nutritional requirements of the preimplantation embryo are minimal and are largely derived from the oviductal fluid in which it floats. An in vitro culture medium with only pyruvate, lactate, and glucose as nutrients, but lacking any amino acids, fats or proteins supports normal development through the 4.5 days of preimplantation stages.

Extensive studies in the past have shown that lack of these metabolites during culture result in specific developmental phenotypes. Zygote cultured in the medium lacking pyruvate is viable but fails to develop beyond the 2-cell stage. Lack of glucose in the culture medium blocks preimplantation development at the morula stage. The mechanism by which specific nutrients control different developmental processes is still unclear.

In this thesis, I interrogated the roles of pyruvate and glucose during mammalian embryogenesis. We found pyruvate mediated O-glycosylation and mitochondrial enzymes nuclear localization are critical steps in mammalian ZGA at 2-cell stage (Chapter 2), while glucose metabolism distinguishes TE from ICM fate at 8- to 16-cell stages (Chapter 3).

In Chapter 2, we made the novel and surprising finding that a number of enzymatically active mitochondrial enzymes associated with the TCA cycle are transiently localized to the nucleus where they directly make the metabolites that are essential for ZGA and the associated global genome organization. We also found that pyruvate, O-glycosylation, and chaperones are essential for this nuclear localization. In Chapter 3, we found that at morula stage critical pathways of glucose catabolism are the pentose pathway (PPP) and the hexosamine biosynthetic pathway (HBP) and blocking these pathways recapitulate distinct aspects of the glucose phenotype. Analysis of the roles of the PPP and the HBP further showed that these pathways have non-over lapping roles in the regulation of specific transcription factors that are essential for the establishment of the TE fate.