“9.61x6.69” b2815 Tissue Engineering and Nanotheranostics
Directed Differentiation of Human Pluripotent Stem Cells 77
1.1. Pluripotency Markers
In order to understand the state of pluripotency, researchers studied
the genes that are upregulated in the undifferentiated ESCs. Oct4,
also called Oct3, is a Pit-Oct-Unc (POU) transcription factor shown
to be exclusively expressed in blastomeres, pluripotent stem cells, and
the germ cell lineage. Embryos deprived of Oct3/4, lost the pluripo-
tent quality of their inner-cell mass.^1 Further testing showed that
repression of Oct3/4 in ESCs induced a loss of pluripotency and
caused differentiation to trophectoderm, the tissue from the outer cell
layer of the blastocyst.^2
As with Oct3/4, Sox2 was also identified as a pluripotency marker
gene. Within the early embryos, Oct3/4 expression has been shown to
overlap with Sox2 in the pluripotent cells of the inner cell mass.^3 Many
of the shared targets of Sox2 and Oct3/4 have therefore been estab-
lished as markers for pluripotency, including Nanog and Zfp206.4,5
1.1.1. Tumorigenic pluripotency markers
Some genetic markers of stemness responsible for long-term pluripo-
tency maintenance and rapid proliferation of ESCs have been identi-
fied as tumorigenic. Several of these genes have been identified and
tested for their roles in ESC maintenance including STAT3, c-Myc,
and KLF family.
The LIF/STAT3 pathway is necessary and sufficient in self-
renewal of ESCs in mice, but not in humans.6,7 Western blot analysis
showed LIF activation of STAT3 is not as strong as in human ESCs
as in murine ECSs.^8 In human ESCs, a constitutively active STAT3,
STAT3C, was not capable of maintaining an undifferentiated state.^8
Transcription factor c-Myc has been shown to be activated by
STAT3 signaling in mice.^9 Its expression can independently adjudicate
self-renewal and maintenance of pluripotency.^9 More recently, c-Myc is
shown to play a role in generating induced pluripotent stem cells (iPSCs)
from both mouse and human fibroblasts, which will be discussed later.
The KLF family has also been used in the process of creating
iPSCs. KLF-4 overexpression was reported to reduce differentiation in
