Molecular basis of embryonic stem cell self-renewal: from signaling pathways to pluripotency network

Abstract

Embryonic stem cells (ESCs) can be maintained in culture indefinitely while retaining the capacity to generate any type of cell in the body, and therefore not only hold great promise for tissue repair and regeneration, but also provide a powerful tool for modeling human disease and understanding biological development. In order to fulfill the full potential of ESCs, it is critical to understand how ESC fate, whether to self-renew or to differentiate into specialized cells, is regulated. On the molecular level, ESC fate is controlled by the intracellular transcriptional regulatory networks that respond to various extrinsic signaling stimuli. In this review, we discuss and compare important signaling pathways in the self-renewal and differentiation of mouse, rat, and human ESCs with an emphasis on how these pathways integrate into ESC-specific transcription circuitries. This will be beneficial for understanding the common and conserved mechanisms that govern self-renewal, and for developing novel culture conditions that support ESC derivation and maintenance.

Fig. 1

Origin of ESCs in the mouse. After fertilization, the totipotent zygote develops into the blastocyst stage embryo (E3.5) through a series of cleavage division, compaction and finally cavitation of morula. The blastocyst further develops into egg cylinder to prepare for germ layer specification. ESCs are derived from ICM cells in the blastocyst at E3.5, while epiblast-derived stem cells (EpiSCs) are derived at E5.5–6.5

Fig. 2

LIF/JAK/STAT3 signaling pathway in mouse ESC self-renewal. a Binding of LIF to its membrane receptor results in recruitment of JAKs and phosphorylation of STAT3 at Tyrosine 705. Activated STAT3 dimerizes and translocates into nucleus to activate transcription. LIF also activates PI3K/AKT and SHP2/MAPK pathways that are not essential for mouse ESC self-renewal. b STAT3 activation level is critical for maintaining mouse ESC self-renewal. Multiple downstream target genes have been identified to connect STAT3 signaling to core pluripotency network

Fig. 3

Canonical Wnt/β-catenin signaling pathway in mouse, rat and human ESC self-renewal. a The presence of Wnt ligand prevents the formation of intracellular destruction complex (GSK3, CK1, APC and Axin) that phosphorylates β-catenin for its subsequent degradation. Thus, Wnt signaling stabilizes β-catenin and activates β-catenin-mediated transcriptional activation. b The complex role of Wnt/β-catenin signaling in self-renewal: in mouse/rat ESCs, β-catenin primarily supports ground-state pluripotency by removing TCF3-mediated transcription repression; yet in mouse EpiSCs or human ESCs, self-renewal is achieved when β-catenin transcriptional activity is blocked through its sequestration in the cytoplasm

Fig. 4

FGF signaling pathway in mouse, rat and human ESC self-renewal. Autocrine FGF4 in ESCs primarily activate RAS–MEK–ERK signaling cascade that promotes differentiation of mouse/rat ESCs into endoderm lineages. Therefore, small molecule inhibitors that block FGF/ERK signaling promote pluripotency. In contrast, mouse EpiSCs and human ESCs require FGF signaling for self-renewal

Fig. 5

BMP/SMAD and LIF/STAT3 signaling in mouse ESC self-renewal. BMP ligand-binding leads to homo-trimerization of SMAD1/5/8 and subsequent association with co-regulatory component SMAD4 to activate Id1/2/3 and DUSP9 expression, which inhibits neuroectoderm differentiation. LIF/STAT3, on the other hand, inhibits mesoderm and endoderm differentiation of mouse ESCs

Fig. 6

Activin/Nodal signaling pathway in human ESC self-renewal. Activin/Nodal treatment results in SMAD2/3-mediated transcriptional activation of Nanog, which promotes pluripotency and, at the same time, antagonizes potential differentiation effects from other unknown downstream targets