Nuclei of embryonic stem cells reprogram somatic cells.

The restricted potential of a differentiated cell can be reverted back to a pluripotent state by cell fusion; totipotency can even be regained after somatic cell nuclear transfer. To identify factors involved in resetting the genetic program of a differentiated cell, we fused embryonic stem cells (ESCs) with neurosphere cells (NSCs). The fusion activated Oct4, a gene essential for pluripotency, in NSCs. To further identify whether cytoplasmic or nuclear factors are responsible for its reactivation, we fused either karyoplasts or cytoplasts of ESCs with NSCs. Our results show that ESC karyoplasts could induce Oct4 expression in the somatic genome, but cytoplasts lacked this ability. In addition, mitomycin C-treated ESCs, although incapable of DNA replication and cell division, could reprogram 5-azacytidine-treated NSCs. We therefore conclude that the Oct4 reprogramming capacity resides in the ESC karyoplast and that gene reactivation is independent of DNA replication and cell division.