Rex1 (Zfp-42) is a known marker of
pluripotency Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
, and is usually found in undifferentiated
embryonic stem cell
Embryonic stem cells (ESCs) are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre- implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consi ...
s. In addition to being a marker for
pluripotency Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
, its regulation is also critical in maintaining a pluripotent state.
As the cells begin to
differentiate, Rex1 is severely and abruptly downregulated.
Discovery
Rex1 was discovered by Hosler, BA et al. in 1989 when studying F9
murine
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families excep ...
teratocarcinoma
stem cells
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
. They found that these teratocarcinoma stem cells expressed high levels of Rex1, and that they resembled pluripotent stem cells of the
inner cell mass
The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of ...
(ICM).
[ ] Hosler, BA et al. found that these teratocarcinoma stem cells, when in the presence of
retinoic acid
Retinoic acid (used simplified here for all-''trans''-retinoic acid) is a metabolite of vitamin A1 (all-''trans''-retinol) that mediates the functions of vitamin A1 required for growth and development. All-''trans''-retinoic acid is required in ...
(RA), differentiated into nontumorigenic cells resembling extraembryonic
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
of early mouse embryos.
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
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
levels within 12 hours of the addition of RA.
Structure
Rex1 is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
that in humans is encoded by the ''ZFP42''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
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.
p38 MAPK & Mesenchymal Stem Cells
Rex1 has been found to be critically important in maintaining proliferative state in
mesenchymal stem cell
Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage c ...
s (MSC), while simultaneously preventing differentiation. Both
umbilical cord
In placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologic ...
blood MSC and
adipose
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular ...
MSC express high levels of Rex1, while
bone marrow
Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...
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
Dual specificity mitogen-activated protein kinase kinase 3 is an enzyme that in humans is encoded by the ''MAP2K3'' gene.
The protein encoded by this gene is a dual specificity protein kinase that belongs to the MAP kinase kinase family. This k ...
. Thus, Rex1 expression is inversely correlated with
p38 MAPK activation, and positively correlated with high proliferation rates.
Rex1 was found to inhibit
MKK3
Dual specificity mitogen-activated protein kinase kinase 3 is an enzyme that in humans is encoded by the ''MAP2K3'' gene.
The protein encoded by this gene is a dual specificity protein kinase that belongs to the MAP kinase kinase family. This k ...
expression, which activates
p38 MAPK. Activated
p38 MAPK, in turn, inhibits proliferation. Rex1 was also found to inhibit
NOTCH and
STAT3
Signal transducer and activator of transcription 3 (STAT3) is a transcription factor which in humans is encoded by the ''STAT3'' gene. It is a member of the STAT protein family.
Function
STAT3 is a member of the STAT protein family. In respons ...
, two transcription factors which lead to differentiation.
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
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
, the
inner cell mass
The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of ...
(ICM) is separated from the
trophoblast
The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the pl ...
. The stem cells derived from the ICM and trophectoderm have been found to express high levels of
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
and Rex1. As the ICM matures and begins to form the
epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is t ...
, and primitive
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
, 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
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
. Also, Rex1
−/Oct3/4
+ cells differentiate into cells of primitive ectoderm, the
somatic cell
A somatic cell (from Ancient Greek σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells compo ...
lineage.
Gene Control
Studies have shown that
PEG3
Paternally-expressed gene 3 protein is a protein that in humans is encoded by the ''PEG3'' gene. PEG3 is an imprinted gene expressed exclusively from the paternal allele and plays important roles in controlling fetal growth rates and nurturing be ...
and ''Nespas'' are downstream targets of Rex1.
Rex1 can control the expression of Peg3 via
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
changes.
YY1
YY1 (Yin Yang 1) is a transcriptional repressor protein in humans that is encoded by the YY1 gene.
Function
YY1 is a ubiquitously distributed transcription factor belonging to the GLI-Kruppel class of zinc finger proteins. The protein is invo ...
has been shown to be involved in setting up DNA
methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
on the maternal allele of
PEG3
Paternally-expressed gene 3 protein is a protein that in humans is encoded by the ''PEG3'' gene. PEG3 is an imprinted gene expressed exclusively from the paternal allele and plays important roles in controlling fetal growth rates and nurturing be ...
during
oogenesis
Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.
O ...
. Rex1 was found to protect the paternal
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
from being methylated, and keep the
PEG3
Paternally-expressed gene 3 protein is a protein that in humans is encoded by the ''PEG3'' gene. PEG3 is an imprinted gene expressed exclusively from the paternal allele and plays important roles in controlling fetal growth rates and nurturing be ...
gene unmethylated during early embryogenesis.
Rex1 exhibits gene control in developing embryos via its epigenetic control on genes such as
PEG3
Paternally-expressed gene 3 protein is a protein that in humans is encoded by the ''PEG3'' gene. PEG3 is an imprinted gene expressed exclusively from the paternal allele and plays important roles in controlling fetal growth rates and nurturing be ...
, which has been identified as playing a key role in fetal growth rates
Expression in Adult Tissues
The only adult tissue Rex1 has been identified in are the
testicles
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
. Using
in situ hybridization
''In situ'' hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA, RNA or modified nucleic acids strand (i.e., probe) to localize a specific DNA or RNA sequence in a portion or section of tissue (''in situ'') or ...
it was determined that the
spermatocytes
Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and s ...
in the more inner layers of the testicles are expressing Rex1.
Thus, the male germ cells undergoing
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
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 cell
Embryonic stem cells (ESCs) are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre- implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consi ...
s results in a reduction of ''Rex-1'' expression, while forced expression of
Nanog stimulates ''Rex-1'' expression.
Nanog regulates the transcription of Rex1 through 2 strong transactivation domains on the C-terminus which are required to activate the Rex1
promoter.
NOTCH
Rex1 has been found to inhibit the expression of
NOTCH, thus preventing differentiation.
STAT3
Rex1 has been found to inhibit the expression of
STAT3
Signal transducer and activator of transcription 3 (STAT3) is a transcription factor which in humans is encoded by the ''STAT3'' gene. It is a member of the STAT protein family.
Function
STAT3 is a member of the STAT protein family. In respons ...
, thus preventing differentiation.
Sox2
Cooperative regulation of Rex1 is seen with
Sox2 and
Nanog.
Oct3/4
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
can both repress and activate the Rex1 promoter. In cells that already express high level of Oct3/4, exogenously transfected
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
will lead to the repression of Rex1.
However, in cells that are not actively expressing
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
, an exogenous transfection of
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
will lead to the activation of Rex1.
This implies a dual regulatory ability of
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
on Rex1. At low levels of the
Oct3/4
Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...
protein, the Rex1 promoter is activated, while at high levels of the Oct3/4 protein, the Rex1 promoter is repressed.
References
Further reading
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Stem cells