The differentiation of the stem cells in to progeny is a feature that may exhibit itself either very frequently or very sparingly depending on the type of the cell stem line. This is essentially a shift in the activity position from a non committed state to the entry into the developmental pathway. The factors that help the stem cell exit its non committed state include the PIE-1 and SKN-1 proteins. Many scientists agree that this differentiation and exit from the quiescent state may actually be due to a default pathway which was maintaining its uncommitted state.
The role of environmental factors in either progressing or repressing the differentiating state become evident here and the conflicting roles that they may play become under scrutiny. (Morrison, Shah and Anderson, 1990, pp. 292) Regulation of cell cycle, apoptosis, and senescence by Bmi1. In normal stem cells, p16Ink4a and p19Arf genes are repressed in a Bmi1-dependent manner. In the absence of p16Ink4a, the cyclin D/Cdk4/6 complex can phosphorylate pRB, allowing the E2F-dependent transcription that leads to cell cycle progression and DNA synthesis.
In addition, MDM2-mediated p53 degradation causes low p53 levels in the absence of p19Arf, thus preventing cell cycle arrest and apoptosis. The absence of Bmi1 relieves the repression of the Ink4a locus, resulting in the expression of p16Ink4a and p19Arf. p16Ink4a inhibits binding of cyclin D to Cdk4/6, resulting in inhibition of the kinase activity. This leads to a hypophosphorylated pRB, which then can bind E2F and inhibit E2F-dependent transcription, resulting in cell cycle arrest and senescence.
p19Arf inhibits MDM2, which mediates ubiquitin-dependent degradation of p53, thus leading to accumulation of p53 protein in the cell. This leads to induction of various p53 target genes involved in cell cycle arrest and apoptosis. Proteins affected by high and low levels of Bmi1 are shown by black and red arrows, respectively. *Sites of frequent mutations associated with cancer. J Clin Invest. 2004 January 15; 113(2): 175–179. Bone, fat and cartilage cells for example, yield multipotent cells with varying proliferation and differentiation potentials. (Kolf, Cho and Tuan, 2007, pp.
204) A number of new techniques have started helping in understanding the differentiation of the stem cells and the factors affecting them. The introduction of the individual cell-based biochemical models is one of the current most widely used methods to understand stem cell regulation in vivo. These models are different from other methods as they are able to study all changes that take place during the whole stem cell formation and differentiation. However, scientists insist that these models are under the influence of the regulatory mechanisms. (Roeder, Galle and Loeffler, 2006, pp.
3) The phenotypic expression and the future of a particular cell is therefore, programmed into the cell’s genome and through this complex array arises the spiral model of cell and tissue and organization, showing varying degrees of cooperation and coordination between the different stages of progression and maturation of various cells. (Potten and Leoffler, 1990, pp. 1018) This phenotypic expression is dependant on the type of tissue being formed. For example, the genes that take active part in the formation of the kidney cells have been described as Xsal-3, SALL1.
These molecules are thought to cause induction as well as differentiation of the kidney cells into its types. The cells or the meristems which are the pool of provision of new cells in the infant and adult, require a fine balance and activity in order to differentiate. While stimuli given to the progenitor cells increase the cell differentiation, there is also the need for negative stimuli to control unnecessary proliferation as well. Some of these inhibitory factors include the WUSCHEL or WUS and the CLAVATA3 or CLV3. (Eckardt, 2006, pp.
275) WUS has shown intense activity on the ARR regulator genes, making it one of the key factors affecting proliferation. (Eckardt, 2006, pp. 276) The ID inhibitor family or the inhibitor of DNA binding transcription family is a very important family which is involved in the differentiation, cell cycle control and senescence of cells. (Kulterer et al, 2007, pp. 354) Of the many genes responsible for the various stages of stem cell proliferation and maturation, the ones involved in its differentiation include the Hox genes which also are important regulators in the proliferation cascade.
Its types are specific for different entities of the stem cells, and their effects are similarly different for the different type of the gene. (Ivanova et al, 2002, pp. 603) The key feature of these stem cells is the lack of differentiation markers in the initial stages, or the lack of the pluripotent transcription factor marker in the adult stem cell regulation. This finding leads to many questions about what regulates these cells in the early stages of development. Researches are underway to study the role of factors and genes such as Oct-4 in the stem cells and whether they are the markers for “stemness” in the cells.
(Roberts, 2007) Other undifferentiated markers include Oct 3/ 4, Sox 2, Rex1, UTF1, hTERT, ABCG2, CD24, Cx43 and Cx45. (Bhattacharya et al, 2004, pp. 2958) Venn diagrams showing overlap of “stemness” genes and stem cell–enriched genes among studies by Ramalho-Santos et al. (1), Ivanova et al. (2), and Fortunel et al. (this study). Ivanova et al. used three different Affymetrix chips (U74v2 A, B, and C); Fortunel et al. and Ramalho-Santos et al. used only the U74v2 A chip (Fig. 3 shows same comparison, limiting Ivanova et al.
results to the A chip). (A) “Stemness” genes found by the three groups overlap by only one gene. (B) ESC-enriched genes identified by each study overlap by 332 genes; the probability that such overlap occurs by chance is extremely low (P < 10–8). (C) NPC-enriched genes overlapping by 236 genes between the three groups (P < 10–8). (D) Overlap of “stemness” genes—two types of stem cell (ESC/NPC)-enriched genes—is limited to 10 genes. The probability of this number of genes overlapping by chance is greatly increased.
P > 10–4 is not significant because there are more than 104 genes studied (8). (Fortunel et al, 2003) While the influence of the exogenous factors was established in the early stages of research on the stem cells, the influence of the endogenous factors remains to be in the early stages of research. The intrinsic of endogenous factors defined include proteins that cause asymmetric cell divisions of the stem cell, nuclear factors that regulate the expression of the gene, chromosomal modifications and the number of divisions of the cells in the genomic makeup.
Here may be the answer to the key ingredients of the regulatory systems that define the cell’s phenotype. Cells such as the neural cells are capable of undergoing both symmetric as well as asymmetric differentiation and division, a pathway rarely used in these cells but nevertheless present. Other lineages may be able to undergo only one type of cell division, either symmetric or asymmetric. Most of these factors are thought to be dictated by the external factors which contain the TGF? and the Wnt families.
Other external factors include the cell-cell interactions which are thought to contribute to the differentiation, the integrins and the formation of extra cellular matrices, and the various homeostatic controls. All these factors are mainly responsible for the proper differentiating of the stem cells. (Watt and Hogan, 2000, pp. 1427) These findings have been supported by the fact that different stem cell populations may have different stem cell markers, which may not even affect the outcomes of the stem cell proliferation.
These cells hold the capability of reversibility and therefore pose a challenge for scientists to identify markers are essential for which kinds of differentiation, and which are essential markers while others are not. Scientists agree that globin in the adult type may revert itself back to the embryonic type, raising the question if adult cell types retain some kind of “stemness” within them, or is it other factors that are affecting the changes. Similarly, as thought of before, there are no “dormant stem cell” populations as each is in some stage of cell differentiation or activity.
Similarly, these cells can enter in to prolonged rest phases during the processes of differentiation. This flexibility within the cells leads to confusion about the actual genes and regulatory proteins involved in the various stem cell proliferation and phenotypicity. (Roedder, Galle and Loeffler, 2005, pp. 13) Experts now also agree that spatio temporal organization is another key factor of stem cell organization, which was previously not considered in the stem cell area. (Roedder, Galle and Loeffler, 2005, pp.
10) Among the more prominent players in the differentiation of the stem cells, is the TGF ? family of proteins of which BMP-2 or bone morphogenetic protein 2 is a vivid example. This protein has been found to be active in the early stages of the embryonic formation as well as in the differentiation of the extraembryonic endoderm. (Pera et al, 2003, pp. 1270) A similar protein the BMP-4 has been shown to cause differentiation of the stem cells, and in the formation of trophoblast lineage of cells.
Almost all of these proteins initiate differentiation by the by the process of embryoid body formation, which can exhibit a variety of different cell markers. This gene expression can in some instances resemble the extraembryonic endoderm. BMP-2 has been specifically seen to help in the spontaneous differentiation of the stem cells. It can also “induce its own expression in the in human EC cells. ” BMP-2 therefore is one of the major proteins that help in the differentiation of the stem cells in to different lineages. (Pera et al, 2003,1273)
In conclusion, the differentiation of the stem cells in to its various progenies is very much under the influence of the regulatory mechanisms of the cells. However, there is still lack of information about the different genetic factors that contribute in this complex pattern. While we have much information regarding the murine models and they are used consistently in order to understand the human models, there are still many differences that can only be understood when we are able to distinguish the human stem cell mechanisms exclusively.