Rex1 (Zfp-42) is a known marker of
pluripotency, and is usually found in undifferentiated
embryonic stem cells. In addition to being a marker for
pluripotency, its regulation is also critical in maintaining a pluripotent state.[5] As the cells begin to
differentiate, Rex1 is severely and abruptly downregulated.[6]
Discovery
Rex1 was discovered by Hosler, BA et al. in 1989 when studying F9
murine teratocarcinoma
stem cells. They found that these teratocarcinoma stem cells expressed high levels of Rex1, and that they resembled pluripotent stem cells of the
inner cell mass (ICM).[7] Hosler, BA et al. found that these teratocarcinoma stem cells, when in the presence of
retinoic acid (RA), differentiated into nontumorigenic cells resembling extraembryonic
endoderm of early mouse embryos.[8] They were able to isolate the nucleotide sequence for Rex1 using
differential hybridization of an F9 cell. They named it Rex1 for reduced expression 1 because there was a steady decline of its
mRNA levels within 12 hours of the addition of RA.[8]
Structure
Rex1 is a
protein that in humans is encoded by the ZFP42gene.[9][10] The Rex1 protein is 310 amino acids long, and has four closely spaced zinc fingers at 188–212, 217–239, 245–269, and 275–299.[7]
p38 MAPK & Mesenchymal Stem Cells
Rex1 has been found to be critically important in maintaining proliferative state in
mesenchymal stem cells (MSC), while simultaneously preventing differentiation. Both
umbilical cord blood MSC and
adipose MSC express high levels of Rex1, while
bone marrow MSC expressed low levels of Rex1. Proliferation rates are highly correlated with Rex1 expression levels, meaning high Rex1 expression is correlated with high levels of proliferation. The MSCs with weak Rex1 expression, have activated
p38 MAPK and high expression levels of
MKK3. Thus, Rex1 expression is inversely correlated with
p38 MAPK activation, and positively correlated with high proliferation rates.[11] Rex1 was found to inhibit
MKK3 expression, which activates
p38 MAPK. Activated
p38 MAPK, in turn, inhibits proliferation. Rex1 was also found to inhibit
NOTCH and
STAT3, two transcription factors which lead to differentiation.[11] Therefore, Rex1 expression allows for high levels of proliferation, and prevents differentiation through a network of various transcription factors and protein kinases.
Embryo Development
Tissue Derivation
During
embryogenesis, the
inner cell mass (ICM) is separated from the
trophoblast. The stem cells derived from the ICM and trophectoderm have been found to express high levels of
Oct3/4 and Rex1.[12] As the ICM matures and begins to form the
epiblast, and primitive
ectoderm, the cells in the ICM have been found to be a heterogenous population, with varying levels of Rex1 expression. Rex1−/Oct3/4− triggers trophectoderm differentiation, while Rex1+/Oct3/4+ cells predominantly differentiate into primitive endoderm and
mesoderm.[13] Also, Rex1−/Oct3/4+ cells differentiate into cells of primitive ectoderm, the
somatic cell lineage.[14]
Gene Control
Studies have shown that
PEG3 and Nespas are downstream targets of Rex1.[15] Rex1 can control the expression of Peg3 via
epigenetic changes.
YY1 has been shown to be involved in setting up DNA
methylation on the maternal allele of
PEG3 during
oogenesis.[16] Rex1 was found to protect the paternal
allele from being methylated, and keep the
PEG3 gene unmethylated during early embryogenesis.[15] Rex1 exhibits gene control in developing embryos via its epigenetic control on genes such as
PEG3, which has been identified as playing a key role in fetal growth rates [17]
Expression in Adult Tissues
The only adult tissue Rex1 has been identified in are the
testicles. Using
in situ hybridization it was determined that the
spermatocytes in the more inner layers of the testicles are expressing Rex1.[18] Thus, the male germ cells undergoing
meiosis are the specific cells in the testicles that express Rex1. It has not been observed, however, that Rex1 is expressed in the female germ cells.
Rex1 Interactions with Other Transcription Factors
Rex1 participates in a network of transcription factors that all work to regulate each other via varying expression levels.
Nanog
The
Nanog protein has been found to be a transcriptional activator for the Rex-1 promoter, playing a key role in sustaining Rex1 expression. Knockdown of
Nanog in
embryonic stem cells results in a reduction of Rex-1 expression, while forced expression of
Nanog stimulates Rex-1 expression.[5] Nanog regulates the transcription of Rex1 through 2 strong transactivation domains on the C-terminus which are required to activate the Rex1
promoter.[5]
NOTCH
Rex1 has been found to inhibit the expression of
NOTCH, thus preventing differentiation.[11]
STAT3
Rex1 has been found to inhibit the expression of
STAT3, thus preventing differentiation.[11]
Sox2
Cooperative regulation of Rex1 is seen with
Sox2 and
Nanog.[5]
Oct3/4
Oct3/4 can both repress and activate the Rex1 promoter. In cells that already express high level of Oct3/4, exogenously transfected
Oct3/4 will lead to the repression of Rex1.[19] However, in cells that are not actively expressing
Oct3/4, an exogenous transfection of
Oct3/4 will lead to the activation of Rex1.[19] This implies a dual regulatory ability of
Oct3/4 on Rex1. At low levels of the
Oct3/4 protein, the Rex1 promoter is activated, while at high levels of the Oct3/4 protein, the Rex1 promoter is repressed.
^Niwa H, Miyazaki J, Smith AG (April 2000). "Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells". Nature Genetics. 24 (4): 372–376.
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^Toyooka Y, Shimosato D, Murakami K, Takahashi K, Niwa H (March 2008). "Identification and characterization of subpopulations in undifferentiated ES cell culture". Development. 135 (5): 909–918.
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10.1242/dev.017400.
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Mongan NP, Martin KM, Gudas LJ (December 2006). "The putative human stem cell marker, Rex-1 (Zfp42): structural classification and expression in normal human epithelial and carcinoma cell cultures". Molecular Carcinogenesis. 45 (12): 887–900.
doi:
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