Biomedical Engineering Reference
In-Depth Information
mESCs from the ICM, a phenotype that is rescued by an inhibitor of MAPK
(Beppu et al., 2000; Mishina et al., 1995; Qi et al., 2004). Interestingly, in the
absence of LIF, BMP4 counteracts the LIF pathway, interacting with different
R-Smad transcription factors (e.g., SMAD1, 5, and 8) that exert an inhibitory
effect on Id genes and lead ES cells into non-neural fates, such as mesoderm and
hematopoietic cells. The LIF and BMP pathways play bidirectional roles in
maintaining the ESC state. Acting through the SMAD pathway, BMP inhibits
neuroectoderm differentiation of ES cells, whereas LIF activation of the
STAT3 pathway blocks BMP-induced endoderm and mesoderm. LIF activates
pro-differentiation pathways (e.g., MAPK/ERK gene activity), while BMP
inhibits this pro-differentiation pathway to maintain self-renewal. Therefore,
a balance may exist between the effectors, STAT3, Smad, and ERK activity on
stimulation of the respective receptors.
The behavior of mouse and human ESCs differs, particularly in their require-
ments for growth factors. As given above, LIF is insufficient to maintain hESCs
(Daheron et al., 2004; Sato et al., 2004). The delicate balance and cooperation
between LIF and BMP pathway is not evident in hESCs. Unlike mESCs, BMPs
cause rapid differentiation of hESCs (Xu et al., 2002). High concentration of
bFGF (basic fibroblast growth factor) (100 ng/mL) is routinely used to main-
tain hESC in culture. However, the mechanism of bFGF function in hESCs has
not yet been elucidated. One of the effects may be related to the BMP signaling
pathway (Xu et al., 2005). Under moderate concentrations of bFGF, Noggin,
an inhibitor of BMP signaling activity, has a synergistic effect to maintain
hESCs better than bFGF alone. It was shown in another cell culture system
that bFGF interrupted BMP signaling by preventing the nuclear translocation
of phosphorylated Smad1 (Pera et al., 2003) or by repressing Smad1 activity in
the nucleus (Nakayama et al., 2003). Although bFGF does not inhibit Smad1
phosphorylation, bFGF may inhibit BMP/Smad pathway in hESCs. On the
other hand, under high concentrations of bFGF, Noggin's effect is no longer
apparent. Moreover, suppression of BMP activity by inhibitors is insufficient to
maintain hESCs. This observation suggests that bFGF not only affects BMP
pathway but also is involved in other signaling pathways.
Recent microarray gene expression profile data demonstrate the upregula-
tion of TGFb1/activin and gremlin 1 (a BMP antagonist), as well as the down-
regulation of Bmp4 by bFGF (Greber et al., 2007a). The roles of TGFb/activin/
nodal and FGF signaling pathways have been further characterized in hESCs
(James et al., 2005; Vallier et al., 2005). Both activin and TGFb have strong
positive effects in hESCs in the presence of modest concentration of bFGF.
TGFb/activin/nodal are known to activate Smad2/3 (Valdimarsdottir and
Mummery, 2005). Activation of Smad2/3 signaling is required for the main-
tenance of the undifferentiated state of ESCs (James et al., 2005). Recent
reports suggest that supplementation of culture medium with Activin A is
sufficient to promote self-renewal of hESCs (Beattie et al., 2005; Xiao et al.,
2006). Nodal have been shown to inhibit neural differentiation in hESCs (Smith
et al., 2008). TGFb1 has been suggested to prevent hESC differentiation along
