As mentioned earlier, the kinase GSK3 induces bCat degradation. Two new mechanisms of action for nuclear bCat independent of TCF transcription have been proposed. The first is based on the finding that in bCat-negative ESCs, there is enhanced expression and over-activation of the TCF3 transcription factor. As TCF3 is a strong repressor of pluripotency genes, this alters the co-regulating network of stemness and induces commitment and differentiation.
As intracellular bCat sequesters TCF3 from nuclear binding sites, it prevents the repressive actions of TCF3 on the core group of pluripotency transcription factors. Such cells are unable to differentiate and have high intracellular bCat levels as well as enhanced levels of pluripotency factors.
It is therefore proposed that bCat binds OCT4, potentiating its transcriptional activity and enhancing pluripotency Kelly et al. It is not yet known whether these two new roles of nuclear bCat independent of TCF transcriptional binding sites might also exist in ASCs. ASCs resident in the niche, however, show a strong sub-membrane bCat expression. This can be seen in the pituitary niche Fig. If ASCs are recruited, bCat expression would transiently become nuclear. This has been partially demonstrated in the pituitary where activating bCat mutations driven under the promoter of the pituitary progenitor transcription factor Hesx1 give rise to craniopharyngiomas Gaston-Massuet et al.
The adult pituitary SC niche. ASCs in the pituitary are aligned in a compact manner at the border between the anterior and intermediate lobes. Cells are strongly attached by cytokeratins and bCat C. Taken as a whole, this work suggests that bCAT can have different functions in SCs than in differentiated cells. The next challenge will be to determine whether bCat has a common role in all types of SC or specific roles in ESCs, ASCs, committed progenitors, differentiated somatic cells, and mutated cancer cells.
Of key importance to understanding bCat's mechanisms of actions will be the elucidation of when and where its cell adhesion function and association with cadherins takes precedence over its nuclear transcription function and with which transcription factor TFC , OCT4 , or tissue specific , repressor TCF3 , or coactivator CBP or p it interacts in any given situation Fig. The proposed roles of bCat. This leads to bCat degradation by proteosomes.
Some extracellular factors e. Wnt prevent regulatory bCat phosphorylation, thus allowing its migration to the nucleus. Nuclear bCat regulates gene expression as part of a complex with other transcription factors. TCF2 activates cell division. A different mechanism is proposed for maintenance of pluripotency. During embryonic development, bCat plays a major role at the plasma membrane through adherens junctions and in combination with cadherins.
Wnt factors can also regulate sub-membrane bCat through alternative noncanonical pathways. Postnatally, resident ASCs within the niches show a high expression of sub-membrane bCat. Nuclear bCat induces somatic cell commitment and differentiation by binding to specific differentiation factors such as Sox2, Sox17, SF-1, Pitx2, or Prop1.
This last section will focus on a few aspects related to all types of SCs that are anticipated to be important in the near future. Many triggers for cellular aging have been proposed, among them, the intracellular accumulation of old or altered biomolecules such as lipids and proteins make it impossible for the cell to maintain its normal rhythm of biosynthesis.
The most widely held view is that every time a cell divides, it is more likely to pass on accumulated DNA mutations and adverse epigenetic events to its descendants, which in turn have the potential for increasingly compromised function. In an attractive hypothesis, this phenomenon would increase the ability in women to repair tissues and organs and thus contribute to their greater longevity.
Some very recent experiments not only show that the loss of proliferation associated with ageing of neural SCs is reversible but also suggest that it could be mediated by the endocrine system. Rando and colleagues have studied neural SCs NSCs in the subgranular niche of the dentate gyrus of the hippocampus, using parabiotic mice of two different ages and paired for 1 month young—old, young—young, and old—old Rando In heterochronic pairs, several interesting data were obtained Villeda et al.
First, an increase in proliferating NSCs was found in the old mouse, whereas the young mouse showed a decrease compared with control levels; isochronic pairs showed no such changes, and glia were unchanged in all cases.
Intriguingly, the NSC changes were associated with functional alterations in memory, with faster long-term potentiation and higher memory test scores in the older paired mice with increased NSC proliferation but the opposite for the younger mice.
As blood was the only shared tissue, the experiments were repeated using injections of plasma instead of parabiosis; similar results were obtained. Finally, a plasma-borne cytokine CCL11 was found to be increased in older mice and injection of this cytokine into young mice partially reproducing the NSC arrest and memory alterations. If such findings can be reproduced in other niches, it will be feasible to hypothesize that the endocrine system plays an important role in aging.
SC therapy requires a source of hundreds of millions of SCs per patient. ESCs, however, have two serious problems — biologically, they have tumorigenic potential, being highly proliferative and adaptable to different environments, while ethically they are derived from an embryo albeit at the very early blastocyst stage. While the latter is not always viewed as problematic by the patient, the emergence of iPS cells has brought substantial new hope of making SC therapy safer.
The discovery of genetic, epigenetic, and genomic alterations in reprogrammed iPS cells as described in the previous sections has led to a fundamental rethinking of in vitro strategies to maintain iPS cells in a proliferating but stable state Blasco et al. One intriguing recent discovery in the mouse has been the importance of the stoichiometry of reprogramming factors for the stability of iPS cells.
Using mRNA profiling, karyotyping, or whole-genome sequencing, good-quality iPS cells are readily distinguished. A new and very active current focus is the development of live-cell staining to detect unaltered but fully reprogrammed iPS cells, worthy of amplification and study, from the plethora of colonies.
The technique must not perturb cell viability as do labeling with anti-Tra antibodies and in vivo alkaline phosphatase staining; Baker New strategies to reprogram somatic fibroblasts directly into the differentiated cell type required by an individual patient are also being tested, in order to obviate the need for iPS cells and subsequent redifferentiation. In a recent report, a combination of six reprogramming factors was able to directly convert mouse fibroblasts into dopaminergic neurons.
In vivo , these artificial neurons were integrated in the brain and had functionality Kim et al. There is also excitement surrounding the potential application of zinc-finger-tailored ZNF and TALE nucleases, which can replace altered genes within the genome of a patient's iPS cells Hockemeyer et al.
In , the encouraging discovery that ASCs appeared to be immunotolerant was made. Mouse cardiac progenitor cells expressing GFP were injected into rats and directly observed in the weeks following their incorporation into the myocardium Saito et al. Mouse iPS cells and ESCs, by contrast, have been found to be immunogenic, generating T-cell-mediated immunity and tissue damage after injection into an allogenic mouse strain.
Worse still, iPS cells reprogrammed with viral or episome vectors have also elicited an immune response when introduced into their own syngenic strains, due to the residual foreign proteins they express following the reprogramming process.
ESCs, at least, have been found not to be immunogenic when introduced into a syngenic strain Zhao et al. MSCs are well known for their acute anti-inflammatory properties in vivo , both in syngenic animal models and after autotransplantation in humans. They are recruited to the inflammatory area and seem to exert their beneficial effect by reducing proliferation and activation of cytotoxic T, B, and natural killer cells as well as antigen-presenting cells such as dendritic cells and macrophages.
MSCs, moreover, induce regulatory T-cells that modulate the immune response Corcione et al. Some MSC anti-inflammatory properties are known to be mediated by cell—cell contact-induced pathways Duffy et al.
Owing to their immunomodulatory properties, MSCs have been proposed as a therapeutic agent in autoimmune and host-vs-graft disease. However, few clinical trials, addressing either autologous or allogeneic MSCs, or with sufficient numbers of patients, have been conducted Zapata Recently, however, the concept of MSC immunomodulation has been challenged. If validated in other systems, these data could leave ASCs as the only SC type that induces allogenic immunotolerance.
Taken together, therefore, the data seem to indicate that when using MSC or ESC derivatives, only autotranplants will be safe. In the meantime, other benefits that should be considered include recruitment of the patient's own MSCs to inflammatory sites as a potential therapy in cancer see below. Many of the early studies using autologous SC injections in patients did not have adequate controls or were not double-blinded.
Of these, nearly have now finished, although only some have presented their results. Many of these clinical studies are phase I trials relating to safety issues and many use autologous bone marrow-purified populations. A few hundred are using MSCs, although none has yet formally presented their results; some trials are using skin derivatives grown in vitro.
Success of cell therapy in patients need not necessarily be directly due to cell regeneration from the injected SCs but instead to reduction of cell damage or improvement of tissue repair by indirect mechanisms. These could include control of inflammation, immunosuppression, secretion of growth factors, and increased cell survival.
Recently, in rats and other animal models, partial locomotor recovery after spinal damage has been obtained through multiple mechanisms after SC injections Sakai et al. In models of stroke, similar functional recovery has been achieved after injection of MSCs into the carotid artery or intravenously, although substantially fewer cells incorporated into damaged brain issue via the i. Here, we will list some of the ongoing trials, starting with those that use ASC populations and finishing with those using ESCs.
While there seem to be some improvements in cardiac function, the number of patients is low and it is too soon to predict ultimate usefulness for this particular cell therapy Bolli et al. A different approach is used by NCT, where a proprietary Myc-ER viral immortalized neural SC line is being used to improve sequelae in stroke patients.
However, the abrupt closure of the Geron biotechnology company — said to be due to economical rather than scientific reasons — has suddenly attenuated the high expectations that GRNOPC ESC derivatives had raised.
The trial will remain active with respect to following up patients already injected. Concerns about safety in all these trials have increased since early reports of donor-derived leukemia after bone marrow transplants and one case of multiple neural tumors in the spinal cauda equina and brainstem meninges arising 5 years after repeated transplantation of fetal neural precursors Greaves , Amariglio et al.
It has therefore been proposed that cells used in therapy should have an inherent trigger for destruction if necessary, and a modified inactive caspase 9 transgene has recently been introduced into human T-cells. This protein is fused to an FK protein domain and needs access to a small-molecule drug to dimerize and become active, inducing apoptosis. The modified T-cells have been injected into five leukemia patients who previously received a matched isotypic bone marrow transplant and have been detected and found to be functionally active in peripheral blood.
Indeed, when four of these patients developed graft-vs-host disease, as expected in many transplant patients, a single injection of the dimerizing drug was enough to control the disease and prevent its recurrence Di Stasi et al. It will be challenging to integrate cell therapy into routine hospital procedures.
An example of this has been seen recently in a simple clinical trial using MSCs to treat perianal fistulas that are resistant to other forms of treatment. The initial study with a small group of patients in a single hospital was very promising Garcia-Olmo et al. However, in the phase II study NCT , significant variability in outcome has been seen; this has been attributed to the enrollment of surgeons from several hospitals who each injected the cells prepared in a single lead hospital in a slightly different manner.
It is important to remember that biological therapy is dynamic and multi-faceted, and not following the exact procedure for any one aspect, such as handling cell vials, can lead to significant variation in results Garcia-Olmo, personal communication. Finally, cell therapies are not applicable exclusively to regeneration but can also be useful in cancer. A recent study used an oncolytic virus, an adenovirus bearing mutated oncogenes that specifically arrests and kills cancer cells, to transduce autologous MSCs, which were then injected into four children with grade IV neuroblastoma resistant to therapy.
MSCs were recruited and migrated into the tumor. In one of the young patients, the virus destroyed enough tumor cells to generate a CD8 cytotoxic response, the tumor disappeared and the patient was free of the disease Garcia-Castro et al.
Improving oncolytic viruses and adjusting the doses of injected MSCs could therefore have potential as be an alternative therapy in some advanced cancers in the future. Both reasons have been discussed at length with a mixture of religious, political, and scientific arguments.
It is therefore understandable that moral and ethical principles have guided the design of specific legislation in this topic. Under the general directives of the European Community EC , each country has its own laws, leading to a degree of uncertainty within the international scientific community as to the detailed procedures that may or may not be carried out in any particular collaborative project.
Legislation affects not only clinical and basic research but also specifically SC-related biotechnology. This decision has led to a heated debate between scientists, industries, patients, and lawyers, each of whom are economically affected by the decision.
It therefore remains to be seen whether the decision will slow investigation or simply change the way in which the results of SC research are legally registered.
However, while basic and clinical research has been regulated and is strictly monitored, the use of autologous cell injections of more or less purified populations of HSCs or MSCs in patients in private clinics is not controlled. The current explosion in private SC therapy is a source of worry to scientists owing to the ineffectiveness and potential danger of treatment with untested therapies and, furthermore, the negative effect on public opinion of SC research in general that an adverse outcome would inevitably cause Cyranoski To conclude, we are in a new era of SC therapy in which its true efficacy will be tested.
It will still be many years before we can accurately evaluate the results of SC therapies already carried out and assess which therapy is most appropriate or how it should be further adapted for any particular disease.
But it can clearly be seen that considerable progress is being made toward understanding the potential of several types of SC, both biologically and therapeutically. The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the review reported. M G-L is a recipient of an Isabel Barreto technological grant.
PLoS Medicine 6 e doi Nature Medicine 17 — Baker M Reprogramming: faithful reporters. Nature Methods 9 — Nature — PNAS — Cell Stem Cell 2 — Stem Cells 30 33 — PNAS 99 — PLoS Genetics 7 e doi Cell — EMBO Journal 30 — Lancet — Cancer Research 71 — Nature Biotechnology 29 — Stem Cell Reviews 8 — Cell Stem Cell 9 — Endocrine Reviews 32 — Development — Stem Cells 27 — PNAS 96 — Blood — When they were no longer needed for that purpose, they were donated for research with the informed consent of the donor.
In , researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be "reprogrammed" genetically to assume a stem cell-like state. This new type of stem cell is now known as induced pluripotent stem cells iPSCs. ASCs are undifferentiated cells found living within specific differentiated tissues in our bodies that can renew themselves or generate new cells that can replenish dead or damaged tissue. ASCs are typically scarce in native tissues which have rendered them difficult to study and extract for research purposes.
Resident in most tissues of the human body, discrete populations of ASCs generate cells to replace those that are lost through normal repair, disease, or injury. ASCs are found throughout ones lifetime in tissues such as the umbilical cord, placenta, bone marrow, muscle, brain, fat tissue, skin, gut, etc.
The first ASCs were extracted and used for blood production in This procedure was expanded in when the first adult bone marrow cells were used in clinical therapies for blood disease.
Studies proving the specificity of developing ASCs are controversial; some showing that ASCs can only generate the cell types of their resident tissue whereas others have shown that ASCs may be able to generate other tissue types than those they reside in. More studies are necessary to confirm the dispute. During days following fertilization and prior to implantation, the embryo at this stage, called a blastocyst , contains an inner cell mass that is capable of generating all the specialized tissues that make up the human body.
ESCs are derived from the inner cell mass of an embryo that has been fertilized in vitro and donated for research purposes following informed consent. These pluripotent stem cells have the potential to become almost any cell type and are only found during the first stages of development.
Scientists hope to understand how these cells differentiate during development. Skip to main content. Search all BMC articles Search. Back to article page. Contact us Submission enquiries: Access here and click Contact Us General enquiries: info biomedcentral. Abstract Background The therapeutic use of multipotent stem cells depends on their differentiation potential, which has been shown to be variable for different populations.
Introduction The progressive restriction of the differentiation potential from pluripotent embryonic stem cells ESC to different populations of multipotent adult stem cells ASC depends on the orchestrated action of key transcription factors and changes in the profile of epigenetic modifications that ultimately lead to expression of different sets of genes.
Results Isolation and Characterization of Stem Cell Populations We first compared the differentiation potential of three different stem cell populations: mesenchymal stromal cells MSC isolated from human bone marrow, adipose tissue-derived stem cells ADSC and human multipotent adult progenitor cells MAPC.
Download: PPT. Figure 1. Hierarchical clustering and supervised analysis of consensus ESC genes and differentiation genes. Figure 2. Stem Cell Regulation by Polycomb Proteins and Histone Modifications Recent studies have demonstrated that Polycomb group PcG proteins participate in the repression of developmental genes that are activated during ESC differentiation [10] , [39] , [40].
Figure 3. Consensus ESC and differentiation genes with predicted Polycomb group marks, and regulation of differentiation genes by PcG proteins in stem cells. Figure 4. Figure 5. Figure 6. Discussion Stem cells are characterized by their capacity to self-renew and differentiate into committed cells.
Figure 7. Model of epigenetic regulation of ESC and differentiation genes in different populations of human stem cells. Materials and Methods Cell Populations Human mesenchymal stromal cells MSC were established from bone marrow from patients of 20—60 years of age. Chromatin Immunoprecipitation Assay A sample of each population studied in Affymetrix arrays was subjected to chromatin immunoprecipitation.
Supporting Information. Text S1. Figure S1. Figure S2. Figure S3. Figure S4. Figure S5. Table S1. Primers and probes used for PCR. Table S2. Table S3. Table S4. Table S5. Table S6. Table S7. Table S8. Table S9.
Table S References 1. Genes Dev — View Article Google Scholar 2. Nat Biotechnol — View Article Google Scholar 3. Cell Stem Cell 2: — View Article Google Scholar 4. Cell — View Article Google Scholar 5. Cell Stem Cell 1: — View Article Google Scholar 6. View Article Google Scholar 7. Nat Cell Biol 8: — View Article Google Scholar 8.
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