The Role of Mga in the Survival of Pluripotent Cells During Peri-implantation Development

The dual specificity transcription factor Mga contains both a T-box binding domain and a basic helix-loop-helix zipper (bHLHZip) domain. Loss of Mga leads to embryonic lethality by E5.5. In vitro blastocyst culture and embryonic stem (ES) cell culture identify a lack of pluripotent inner cell mass (ICM) derived cells as the cause of embryonic lethality. Loss of Mga leads to increased apoptosis in E4.5 embryos, though there is no decrease in the amount of cell proliferation. Embryos with mutant Mga have fewer pluripotent ICM cells during delayed implantation, though the number of differentiated primitive endoderm cells remained initially stable. Despite the loss of pluripotent cells, there is no change in the pattern of expression of Nanog or Oct4, pluripotent cell markers, or Gata4, a primitive endoderm marker. Expression of Ornithine Decarboxylase (ODC), the rate-limiting enzyme in the synthesis of cellular polyamines, was identified as a possible cause of embryonic lethality based on a similar mutant phenotype as well as the presence of E-box sequences in genetic regulation loci. ODC is expressed at lower levels in the ICM of Mga mutants. Blastocyst and ES cell culture defects were rescued when cultured in the presence of exogenous putrescine, the metabolic product of ODC. These results suggest a mechanism for Mga to influence pluripotent cell survival through interactions with other bHLHZip domain proteins in the regulation of the polyamine pool in pluripotent cells of the embryo.