Overview of Stem Cell Transcriptional Factors and Regulations
Development in multicellular organisms means the intricate and complex process of cell differentiation in which cells lose their developmental plasticity gradually and then take on specialized functions.
Stem cells have two remarkable features: self-renewal and multipotency. In mammals, stem cells can be divided into two broad types: embryonic stem cells which are found in the inner cell mass of blastocysts, and adult stem cells which are isolated from various tissues. Because stem cells can be found in tissues derived from all germ layers, either quiescent or cycling, and related to varied niches, different stem cells pools are regulated by different molecular mechanisms. Few generalities can be drawn regarding this regulation. What can be said in a general way is that stem cells must maintain undifferentiated if the pool is to be maintained and the molecular mechanisms which control the maintenance of stem cell state must repress differentiation, too. Following is the different stem cell population and the according molecular genetic mechanisms which have been associated with their regulation.
Transcriptional Factors and Regulations
Embryonic stem cells have the potential to differentiate into any cell types if given the right signals. A couple of signaling and intrinsic pathways involving transcriptional factors have been shown to have crucial roles in maintaining the self-renewal and pluripotent state of ES cells. It is known that Leukemia inhibitory factor (LIF) is essential for the derivation and maintenance of ES cells in the presence of serum, whose binding to the LIF receptor (LIFR)-gp130 heterodimer receptor on the cell membrane activates STAT3 (signal transducer and activator of transcription3) by phosphorylation, leading to its subsequent dimerization, nuclear translocation, and target gene activation. However, in the absence of serum, LIF is not sufficient. Bone morphogenetic proteins (BMPs) also have to work in conjunction with LIF to promote self-renewal of ES cells. With the addition of BMPs, Smad1 will be phosphorylated and the members of Id (inhibitor of differentiation) gene family, which are effectors of the BMP pathway, will be activated. Furthermore, Oct4, Sox2, and Nanog are key components of the core regulatory network which governs ES cell pluripotency.
As neural development progresses, the symmetric division of radial glial cells decreases to be replaced by asymmetric divisions and production of intermediate progenitor cells (IPCs). The switch from radial glial cells to IPCs involves the downregulation of factors important for self-renewal, such as CBF1, Emx2, Pax6 and Sox2, and the upregulation of transcriptional factors such as Tbr2, Svet1, Lmo4 and CuxI-2. While the differentiation progresses, some transcription factors, such as Pax6, which plays a part in regulation of neural stem/progenitor proliferation begin to regulate neuronal differentiation. Towards the end of the neurogenic period, a gliogenic switch occurs, leading to the production of oligodendrocytes and astrocytes. During the neurogenic phase, gliogenesis is inhibited, which is partly achieved by the high expression levels of bHLH transcription factors such as Ngns, which suppress gliogenesis by sequestering the gliogenic CBP/p300/Smad transcriptional complex and repressing the JAK/STAT pathway. In addition, the oligodendrocyte lineage is striking in its expression of a well-defined set of transcription factors including Olig1, Olig2, Sox10, Nkx2.2, Mash1/Ascl1 and upon terminal differentiation, MyRF and Nkx6.2.
As for the adult neural stem cells, there are two main regions where adult NSCs exists: the subventricular zone (SVZ) lining the lateral walls of the lateral ventricles and the subgranular zone (SGV) of the hippocampal dentate gyrus. Multiple transcription factors are involved in proliferation and maintenance of the precursor pool within the SGV. For example, Pax6 and the CCAAT/enhancer binding protein β (C/EBPβ) are involved in the proliferation of type-1 NPCS along with Sox2. Sox2 is a mediator of Notch signaling which is also involved in maintaining the precursor pool via Shh in adult SGZ. Neuronal fate specification occurs through the expression of NeuroD1, Sox3, Sox4, Sox11 and Prox1. In the SVZ, there is also many transcription factors involved, such as ARS2 (arsenite-resistant protein 2) controlling the multipotent progenitor state of NSCs through the activation of SOX2. And c-Myb is required for maintenance of the neural stem cell niche, promoting expression of Sox2 and Pax6 and subsequent proliferation.
HSCs are able to proliferate and self-renew to maintain their population for the lifetime of the organism, and keep it in balance by differentiating into the committed haematopoietic cell types to replenish physiological turnover or injury. Several transcription factors are included into this process. Scl is highly expressed in HSCs and regulates quiescence by inhibiting the G0 to G1 transition. Hox genes encode homeodomain transcription factors and are vital for developmental patterning. Several Hox genes have been implicated in HSC homeostasis. Ets transcription factors are also known to regulate HSC homeostasis and differentiation including Erg, Fli-1, Tel/Etv6, GABPα, PU.1/Spi-1 and Elf4. In addition, zinc finger protein, such as Gata2, Gata3, Gfi1, Klf4,Ikaros, Evi-1, Sall4, Zfx and Prdm16. C-Myb also plays an important role in HSC self-renewal and adult haematopoiesis. Its conditional deletion causes a defect in HSC proliferation, increases differentiation and loses the reconstitution ability. Unsurprisingly, cell cycle regulators have been identified as regulating HSC homeostasis, two of which involved in transcriptional regulation are retinoblastoma (pRB) and p53 families. Two immediate early response transcription factors, JunB and Egr1, also have been shown functions in regulating HSCs.
Mammalian skin epidermis is an excellent model system to study transcriptional regulatory mechanisms because of its easy accessibility, stereotypic spatial arrangement, and availability of well-established cell type/lineage differentiation markers. There are also a couple of transcriptional factors involved in it. p63, a transcription factor homologous to the p53 tumor suppressor, encodes two classes of protein isoforms, TAp63 and ΔNp63, with the latter being the predominant isoform expressed in epidermis. Wnt/β-catenin signaling plays an important role in the hair follicle lineage by promoting placode formation during embryogenesis, maintaining adult follicle bulge stem cell identity, activating quiescent stem cells during transition of postnatal follicle from a resting to a growing phase, and promoting terminal differentiation within the follicle. Members of the LEF/TCF family of transcription factors, which are downstream effectors of Wnt/β-catenin pathway, form bipartite transcriptional complexes withβ-catenin to regulate gene expression. Notch signaling plays complex, context-dependent roles in skin epithelial differentiation and has been implicated as an effector linking asymmetric division to differentiation of embryonic epidermal stem cells. Hes1 transcriptional repressor is a downstream target of Notch signaling. Loss of Hes1 will cause premature differentiation of suprabasal keratinocytes and is important for maintaining proliferation in both basal and spinous compartments. In addition, loss of AP-2α in the epidermis results in persistent EGFR activity in differentiating cells and localized epidermal hyperproliferation. The Ovo gene family encodes evolutionarily conserved zinc-finger transcription factors with its prototype in Drosophila being critical for epidermal denticle formation.
Figure 1. Critical morphological/molecular events and transcriptional/chromatin regulators of epidermal development
Mesenchymal stem cells (MSCs) are multipotent stromal cells existing within bone marrow and other adult tissues, which are able to differentiate into different skeletal tissue such as bone, cartilage and fat. A range of transcription factors are known to be involved in the regulation of osteogenesis, with two of the more widely studied being Runx2 (Cbfa1) and Osterix. Runx2 is regarded the major transcription factor controlling osteoblast commitment and differentiation. Peroxisome proliferator activated receptor-γ (PPARγ) is a nuclear hormone receptor, and is thought to be the master regulator of adipogenesis. And as with adipogenesis and osteogenesis, there is also an apparent master regulator of chondrogenesis: Sox9.
Hematopoietic and endothelial cells are all originated from a common progenitor cell which is hemangioblast. During the early phases of mesoderm development, hemangioblast specification occurs and is promoted by some transcription factors expression. It has been reported that Lim-only protein (LMO2) can enhance the proliferation and differentiation of hemangioblasts. Embryonic stem cells will reduce the production of hemangioblasts without RUNX1/CBFA2, which means that RUNX1/CBFA2 has important effects during hemangioblast development. In addition, stem cell leukemia (SCL), a basic helix-loop-helix (bHLH) transcription factor, is essential for the specification and function of the hemangioblast. SCL may be a direct target of hedgehog signaling during hemangioblast specification.
The hepatic and pancreatic progenitor cells that give rise to liver and pancreas are specified and regulated by some transcription factors. For example, members of the GATA and HNF-3/FoxA families have function in the early phases of both hepatic and pancreatic progenitor cell specification. In addition, PDX-1/IPF1 enhances the differentiation of pancreatic progenitor cells into all cell types of the mature pancreas, exocrine, endocrine, and ductal cells included. As for hepatic progenitor cells, they will differentiate into hepatocytes and cholangiocytes with the regulation of TBX3.
Pluripotent stem cells (PSCs), which include embryonic stem cells (ESCs), and induced PSCs (iPSCs), are able to self-renewal and differentiate into a plenty of specialized cellular lineages. Their pluripotential character is maintained by a group of transcription factors. Among these factors, SOX plays a crucial role not only in regulating pluripotency, but also in mediating self-renewal and differentiation. iPSCs are generated by a subset of transcription factors including Oct-3/4, KLF4, SOX2, and c-Myc, which are sufficient to reprogram a somatic cell back into a pluripotent state.
Cancer stem cells (CSCs) are thought to drive uncontrolled tumor growth. OCT4, NANOG, and SOX2 are three basic transcription factors that are expressed in both CSCs and ESCs. A lot of evidences indicate that the overexpression of these three genes occurs in human malignancies and are relevant to humor transformation, tumorigenicity, tumor metastasis. OCT4, NANOG, and SOX2 have been all detected as co-up-regulated in many human cancers, including oral squamous cell carcinoma, prostate cancer, and breast cancer. And the expression levels of OCT4, NANOG, and SOX2 mRNA transcripts, which are detected in tumor cells and CSC niches, are usually higher than those of nontumor tissue or stem cell markers. However, the mechanistic functions of OCT4, NANOG, and SOX2 in CSCs are a little different from their functions in ESCs. Although they both share the property of self-renewal, ESCs emphasize differentiation, whereas CSCs emphasize proliferation. OCT4, NANOG, and SOX2 together maintain the repression of lineage-specific differentiation in human ESCs. However, in CSCs, the overexpression of OCT4, NANOG, and SOX2 modulates signaling pathways to inhibit apoptosis.
Relations with Diseases
Several transcription factors are related to some human diseases. Oct4 can both activate and repress transcriptional targets in human ESCs. Loss of function pf most Oct4-associated genes studied to date result in embryonic or perinatal lethality, showing that many serve crucial functions in development. Interestingly, most Oct4-binding proteins linked to a human hereditary disorder, mostly developmental or cancer predisposition, give rise to a related phenotype when absent in the mouse. And in some neurodegenerative disorders, such as early in the Alzheimer’s disease, oligomeric amyloid-β may transiently promote the generation of immature neurons from the neural progenitor cells (NPCs). However, reduced concentrations of multiple neurotrophic factors and higher levels of FGF2 is likely to induce a developmental arrest of newly generated neurons. Further, there is a down-regulation of Olig2 and over-expression of Ascl1/Mash1 caused by amyloid-β which switches the cell fate to death. When it comes to Parkinson’s disease, it is the outcome of the loss of dopaminergic neurons in the substantia nigra of the midbrain. Alterations in neurogenesis have been linked to a decrease in Notch1 and Hes5 expression. It is certain that there will be more and more potential to cure some diseases by regulating the stem cell transcription regulatory networks with the research deepening.