Serum starvation-induced cell cycle synchronization stimulated mouse rDNA transcription reactivation during somatic cell reprogramming into iPSCs.

Background: rDNA, the genes encoding ribosomal RNA (rRNA), is highly demanded for ribosome production and protein synthesis in growing cells such as pluripotent stem cells. rDNA transcription activity varies between cell types, metabolism conditions, and specific environmental challenges. Embryonic stem cells (ESCs), partially reprogrammed cells, and somatic cells reveal different epigenetic signatures, including rDNA epigenetic marks. rDNA epigenetic characteristic resetting is not quite clear during induced pluripotent stem cell (iPSC) generation. Little is known that whether the different rDNA epigenetic status in donor cells will result in different rDNA transcription activities, and furthermore affect reprogramming efficiency.

Methods: We utilized serum starvation-synchronized mouse embryonic fibroblasts (MEFs) to generate S-iPSCs. Both MEFs and serum-refeeding MEFs (S-MEFs) were reprogrammed to a pluripotent state. rDNA-related genes, UBF proteins, and rDNA methylation levels were detected during the MEF and S-MEF cell reprogramming process.

Results: We demonstrated that, after transient inhibition, retroviral induced rRNA transcriptional activity was reprogrammed towards a pluripotent state. Serum starvation would stimulate rDNA transcription reactivation during somatic cell reprogramming. Serum starvation improved the methylation status of donor cells at rRNA gene promoter regions.

Conclusions: Our results provide insight into regulation of rDNA transcriptional activity during somatic cell reprogramming and allow for comparison of rDNA regulation patterns between iPSCs and S-iPSCs. Eventually, regulation of rDNA transcriptional activity will benefit partially reprogrammed cells to overcome the epigenetic barrier to pluripotency.