ABSTRACT
Embryonic stem cells (ESCs) are metabolically distinct from their differentiated counterparts. ESC mitochondria are less complex and fewer in number than their differentiated progeny. However, few studies have examined the proteins responsible for differences in mitochondrial structure and function between ESCs and somatic cells. Therefore, in this study, we aimed to investigate the differences between mitochondrial proteins in these two cell types. We demonstrate that HSP60 is more abundant in mouse ESC mitochondria than in mouse embryonic fibroblasts. Depletion of HSP60 inhibited mouse ESC proliferation and self-renewal, characterized by decreased OCT4 expression. HSP60 depletion also enhanced apoptosis during mouse ESC differentiation into embryoid bodies. Our results suggest that HSP60 expression has an essential role in ESC self-renewal and survival of differentiated cells from ESCs.
ABSTRACT
The molecular mechanism underlying the initiation of somatic cell reprogramming into induced pluripotent stem cells (iPSCs) has not been well described. Thus, we generated single-cell-derived clones by using a combination of drug-inducible vectors encoding transcription factors (Oct4, Sox2, Klf4 and Myc) and a single-cell expansion strategy. This system achieved a high reprogramming efficiency after metabolic and epigenetic remodeling. Functional analyses of the cloned cells revealed that extracellular signal-regulated kinase (ERK) signaling was downregulated at an early stage of reprogramming and that its inhibition was a driving force for iPSC formation. Among the reprogramming factors, Myc predominantly induced ERK suppression. ERK inhibition upregulated the conversion of somatic cells into iPSCs through concomitant suppression of serum response factor (SRF). Conversely, SRF activation suppressed the reprogramming induced by ERK inhibition and negatively regulated embryonic pluripotency by inducing differentiation via upregulation of immediate early genes, such as c-Jun, c-Fos and EGR1. These data reveal that suppression of the ERK-SRF axis is an initial molecular event that facilitates iPSC formation and may be a useful surrogate marker for cellular reprogramming.