Embryonic stem cells (ESCs) are pluripotent cells capable of differentiating into any cell type of the body. Human ESCs are derived from blastocysts; multicellular structures originating from four cleavages of fertilized oocytes. They are able to spontaneously give rise to different progenies of the three embryonic layers. Human ESC pluripotency is regulated by a combination of extrinsic and intrinsic factors. FGF signaling and a balance between TGF-β/Activin and BMP signaling are central to the self-renewal of human ESCs. Intrinsic factors include a group of transcription factors including Oct4, SOX2 and Nanog.TGF-β superfamily of ligands plays a major role in maintaining the Self-renewing ESCs. They signal through two main branches: the SMAD1/5 branch which transduces on behalf of BMP and GDF ligands via the type I receptors ALK1, ALK2, ALK3 and ALK6 and the TGF-β/Activin/Nodal branch, which involves the activation of SMAD2/3 via ALK4, ALK5 and ALK7. There are also two inhibitory SMADs - SMAD6, which selectively inhibits SMAD1/5; and SMAD7, which inhibits TGF-β signaling. Upon activation by phosphorylation and association with a common SMAD4, receptor activated SMADs translocate to the nucleus and in concert with other transcription factors, regulate gene expression. SMAD2/3 pathway is also required for positive regulation of several factors of TGF-β signaling. These factors include Nodal, Cripto, LEFTY1 and LEFTY2.
In contrast to TGF-β/Activin/Nodal signaling which promotes the maintenance of pluripotent human ESCs, BMP signaling is unable to support self-renewal and is associated with differentiation to trophoblast or extraembryonic endoderm cells. In human ESCs, BMP4 induces differentiation into mesoderm and ectoderm, whereas BMP2 promotes extraembryonic endoderm differentiation. Repression of BMP signaling in human ESCs by Noggin and FGF supports long term Self-renewal. Binding of FGF to its receptor and heparin leads to receptor autophosphorylation and activation of intracellular signaling cascades, including the Ras/ERK pathway, the PLC-γ/Ca2+ pathway, and the PI3K pathway. FGF2 promotes self-renewal of human ESCs by activating the PI3K pathway.
WNTs play an important role in controlling ESC maintenance. Canonical WNT signaling involves the binding of WNT to the frizzled receptors. This, in turn, activates Dsh, which displaces GSK3β from the APC/AXIN complex, preventing ubiquitin mediated degradation of Ctnn-β. Subsequently, Ctnn-β accumulates and translocates into the nucleus where it associates with TCF/LEF to activate transcription of WNT target genes. WNT signaling is known to be involved in regulating the proliferation of stem cells, including intestinal and hematopoietic cells, and the self-renewal of hematopoietic stem cells.
Sphingosine-1-Phosphate (S1P), a bioactive lysophospholipid, also supports human ESC self-renewal. S1P signals both extracellularly through EDG (Endothelial Differentiation Gene) receptors coupled to G-Proteins, and intracellularly by unidentified mechanisms. Because the prevention of apoptosis is a common self-renewal mechanism, S1P potentially aids the self-renewal process in human ESCs cells. PDGF is implicated in the prevention of apoptosis. PDGF promotes intracellular S1P signaling by activating SphK, which in turn converts sphingosine to S1P. The combination of extracellular PDGF and S1P supports human ESC self-renewal.
Major transcription factors regulating pluripotency include Oct4, SOX2 and Nanog. Oct3/4 SOX2 partnership is indispensable in the maintenance of pluripotency. Nanog is another member of the group of transcription factors whose functions are deemed essential for the process of self-renewal in Human ESCs. Thus human ESCs exhibit a number of signaling pathways involved in self-renewal and pluripotency.