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]
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]
Rex1 is a protein that in humans is encoded by the ZFP42 gene. [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]
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.
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]
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]
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 participates in a network of transcription factors that all work to regulate each other via varying expression levels.
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]
Rex1 has been found to inhibit the expression of NOTCH, thus preventing differentiation. [11]
Rex1 has been found to inhibit the expression of STAT3, thus preventing differentiation. [11]
Cooperative regulation of Rex1 is seen with Sox2 and Nanog. [5]
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.
Cellular differentiation is the process in which a stem cell changes from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Some differentiation occurs in response to antigen exposure. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. However, metabolic composition does get altered quite dramatically where stem cells are characterized by abundant metabolites with highly unsaturated structures whose levels decrease upon differentiation. Thus, different cells can have very different physical characteristics despite having the same genome.
Oct-4, also known as POU5F1, is a protein that in humans is encoded by the POU5F1 gene. Oct-4 is a homeodomain transcription factor of the POU family. It is critically involved in the self-renewal of undifferentiated embryonic stem cells. As such, it is frequently used as a marker for undifferentiated cells. Oct-4 expression must be closely regulated; too much or too little will cause differentiation of the cells.
Homeobox protein NANOG(hNanog) is a transcriptional factor that helps embryonic stem cells (ESCs) maintain pluripotency by suppressing cell determination factors. hNanog is encoded in humans by the NANOG gene. Several types of cancer are associated with NANOG.
In biology, reprogramming refers to erasure and remodeling of epigenetic marks, such as DNA methylation, during mammalian development or in cell culture. Such control is also often associated with alternative covalent modifications of histones.
Leukemia inhibitory factor, or LIF, is an interleukin 6 class cytokine that affects cell growth by inhibiting differentiation. When LIF levels drop, the cells differentiate.
The inner cell mass (ICM) or embryoblast 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 the fetus. The inner cell mass forms in the earliest stages of embryonic development, before implantation into the endometrium of the uterus. The ICM is entirely surrounded by the single layer of trophoblast cells of the trophectoderm.
In molecular genetics, the Krüppel-like family of transcription factors (KLFs) are a set of eukaryotic C2H2 zinc finger DNA-binding proteins that regulate gene expression. This family has been expanded to also include the Sp transcription factor and related proteins, forming the Sp/KLF family.
The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
p38 mitogen-activated protein kinases are a class of mitogen-activated protein kinases (MAPKs) that are responsive to stress stimuli, such as cytokines, ultraviolet irradiation, heat shock, and osmotic shock, and are involved in cell differentiation, apoptosis and autophagy. Persistent activation of the p38 MAPK pathway in muscle satellite cells due to ageing, impairs muscle regeneration.
Induced pluripotent stem cells are a type of pluripotent stem cell that can be generated directly from a somatic cell. The iPSC technology was pioneered by Shinya Yamanaka and Kazutoshi Takahashi in Kyoto, Japan, who together showed in 2006 that the introduction of four specific genes, collectively known as Yamanaka factors, encoding transcription factors could convert somatic cells into pluripotent stem cells. Shinya Yamanaka was awarded the 2012 Nobel Prize along with Sir John Gurdon "for the discovery that mature cells can be reprogrammed to become pluripotent."
Forkhead box protein P1 is a protein that in humans is encoded by the FOXP1 gene. FOXP1 is necessary for the proper development of the brain, heart, and lung in mammals. It is a member of the large FOX family of transcription factors.
Mitogen-activated protein kinase 14, also called p38-α, is an enzyme that in humans is encoded by the MAPK14 gene.
SRY -box 2, also known as SOX2, is a transcription factor that is essential for maintaining self-renewal, or pluripotency, of undifferentiated embryonic stem cells. Sox2 has a critical role in maintenance of embryonic and neural stem cells.
Nuclear factor of activated T-cells 5, also known as NFAT5 and sometimes TonEBP, is a human gene that encodes a transcription factor that regulates the expression of genes involved in the osmotic stress.
Growth arrest and DNA-damage-inducible protein GADD45 gamma is a protein that in humans is encoded by the GADD45G gene on chromosome 9. GADD45G is also known as CR6, DDIT2, GRP17, OIG37, and GADD45gamma. GADD45G is involved in several different processes, including sexual development, human-specific brain development, tumor suppression, and the cellular stress response. GADD45G interacts with several other proteins that are involved in DNA repair, cell cycle control, apoptosis, and senescence. Low expression of GADD45G has been associated with many types of cancer.
T-box transcription factor TBX3 is a protein that in humans is encoded by the TBX3 gene.
Dual specificity protein phosphatase 10 is an enzyme that in humans is encoded by the DUSP10 gene.
Developmental pluripotency-associated protein 3 is a protein that in humans is encoded by the DPPA3 gene.
In molecular biology, mir-720 microRNA is a short RNA molecule. MicroRNAs function to regulate the expression levels of other genes by several mechanisms.
Glis1 is gene encoding a Krüppel-like protein of the same name whose locus is found on Chromosome 1p32.3. The gene is enriched in unfertilised eggs and embryos at the one cell stage and it can be used to promote direct reprogramming of somatic cells to induced pluripotent stem cells, also known as iPS cells. Glis1 is a highly promiscuous transcription factor, regulating the expression of numerous genes, either positively or negatively. In organisms, Glis1 does not appear to have any directly important functions. Mice whose Glis1 gene has been removed have no noticeable change to their phenotype.
