Stem Cell Research Review

Published: May 31, 2011  |  Source: newswire.net
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Once researchers are able refine techniques to derive pluripotent cells and utilize them to regenerate tissue, there will be vast applications in a clinical setting. Such advances could forever change the face of human health care.

Introduction/Background

Stem cells are biological cells that have two very important qualities, first, potency, or the ability to differentiate into several cell types, and second, self-renewal, the ability to replicate themselves creating more stem cells that maintain their same qualities. There are several types of stem cells and they differ from each other in their potency, or the potential kinds and number of cells into which they can differentiate. Totipotent cells are able to differentiate into full organisms, pluripotent cells are able to differentiate into all three germ layers critical to development, namely ectoderm, endoderm and mesoderm, multipotent can differentiate into only a certain group or family of cells, and unipotent cells only differentiate into one type of cell but can still self-replicate. Most of the research done concerning tissue regeneration, to this point, is done with pluripotent cells, due to the large variety of cells into which they can differentiate.

Embryonic Stem cells or ES cells, are taken from the inner cell material of blastocysts, and are pluripotent.[1] This means they have ability to differentiate into any possible human cell. The work of Li et al. has made it possible to guide the differentiation of the cells, effectively causing ES cells to differentiate into the cells of choice with up to 90% of cells expressing desired traits.[2] This ability makes their potential for use in medical therapies vast, spanning from repairing damaged heart and spinal tissue to treating sickle cell anemia and Parkinson’s diseases.

The upside and tremendous potential of pluripotent stem cells is unlimited, however serious ethical issues surround human ES (HES) stem cell research and clinical use. HES cells are taken from the inner cells of blastocysts. This means that human embryos are destroyed in order to harvest embryonic stem cells. For this reason HES stem cell research is on hold in many countries. The majority of ES cell research is presently done on mice or rats using non-human ES cells.[3] There is, however, a possible solution to the ethical dilemma surrounding HES cells. Induced Pluripotent Stem (IPS) cells are a relatively new discovery. Stem cells are taken from somatic tissue in adults and reprogrammed to induce pluripotency.[4] The fact that IPS cells can be derived from the recipients existing cells virtually eliminates the chances of the body rejecting tissue implants from cultured cells, and effectively quells any ethical conflicts surrounding origin of HES cells.

Once researchers are able refine techniques to derive pluripotent cells and utilize them to regenerate tissue, there will be vast applications in a clinical setting. Such advances could forever change the face of human health care.

Research

Among their currently understood qualities, human embryonic stem cells will readily differentiate into cardiomyocytes, the cells that make up myocardium or the muscular walls of the heart. Linda W. van Laake et al. explored the possibility of using these HES derived cardiomyocytes (HES-CM) cells as a treatment for hearts damaged by myocardial infarction. They did this by differentiating HES-CM cells until they showed beating colonies of heart tissue. The colonies, on average, contained 20-25% cardiomycytes. Van Laake et al. injected the hearts of several mice who had suffered from myocardial infarction with these cell containing 20-25% HES-CM cells. As a control they also injected the HES-CM cells into the hearts of mice that had not suffered myocardial infarction and injected non-CM cells into mice who had suffered infarction. They saw moderate success with the transplants, following the mice for 12 weeks prior to the injection with HES-CM cells, observing the health of the grafts, the presence of HES-CM cells, and the overall health of the hearts. In 2.5 days there was no noticeable improvement in the function, but HES-CM cells seemed to start fusing. In control experiments with non-CM cells the results were the same. After 4 weeks the hearts of mice injected with HES-CM cells showed noticeable improvement in function while the HES-CM cell population had increased. The non-CM cell injected mice showed no improvement in heart function. These results demonstrate that not only were the HES-CM cells helping the heart, but that they were continuing to differentiate in vivo. After 12 weeks, the results were less encouraging. The hearts function ceased to benefit from the presence of the HES-CM cells, but the population of the cells continued to grow and the grafts remained healthy.[5] The reason for the loss of efficacy of the implants is unknown but it may be due to one of two reasons: first, the small percentage of actual CM cells present in the injections (20-25%), and second the immaturity of the colonies. Further maturing these colonies and refining or purifying the colonies to higher percentage of CM cells should be explored, and the injection of HES-CM cells as a long-term treatment for damaged hearts should not be disregarded.

McDonald et al. conducted similar research, regarding implantation of embryonic stem cells into injured tissue. McDonald et al. used mice embryonic stem (MES) cells to differentiate into glial cells, fundamental components in brain and spinal cord matter. They then implanted these MES glial cells into the syrinxes of several rats 9 days after they had experienced spinal injury (spinal injury was forced by dropping weight onto the spines of rats). As a control, they also injected non-ES cells into rats in otherwise identical circumstances. They then observed the growth and differentiation of the MES cells along with the motor ability of the rats and the weight bearing ability of their hind legs. Their results were extremely successful. After 2 weeks the MES cells and the cells they generated, specifically oligodendrocytes (43%), astrocytes (19%) and neurons (8%) completely occupied the area previously occupied by the syrinx. The rats also showed an improvement in motor functions and showed some weight bearing capability in their hind legs. In contrast the control rats injected with non-ES cells showed no improvement in motor function or weight bearing in hind legs. Furthering the success of the results, no tumors were found in any part of the tested rats.[6] This is another example of stem cells potential ability to repair damaged tissue and treat injuries and disease. Their results, while promising, are not conclusive. They did not show complete recovery in hind legs and they were unable to identify exactly what caused the improvement of motor function, etc. in the subjects.

The studies and research of Takahashi et al. was centered on induced pluripotent or IPS cells. They set out to determine which factors were essential for maintaining pluripotency in these IPS cells. While starting their research they relied on several other studies for information, specifically the studies of Boyer et al. and Loh et al. whom determined that it was possible to induce cells that were pluripotent or able to differentiate into all three germ layers, and hypothesized, that Oct3/4, Sox2, and Nanog were essential for pluripotency. Takahashi et al. hypothesized that some of 24 genes were essential factors in forming IPS cells and in maintaining pluripotency, but they weren’t necessarily Oct3/4, Sox2, and Nanog. To test their hypothesis they first used all 24 genes to generate IPS cells and determined which maintained the ES properties. They then tested the reliance of the cells on each gene individually. Their main finding was that Oct3/4 and Sox2 were essential while, surprisingly, Nanog was not. In addition they found that c-Myc and Klf4 are essential for maintaining pluripotency. Using these four factors, Takahashi et al. were able to generate IPS cells from mice somatic tissue that could differentiate into each germ layer, both in vitro and in vivo.[7] Their findings gave great insight into what makes IPS cell formation work, however many questions are left unanswered. The exact origin of these IPS cells is still uncertain. This lack of knowledge makes the actual percentage of IPS cells generated very low, and their origin must be determined in order for the research to advance. In addition, there are some problems with tumor formation when using the IPS cells. This may be solved if the exact origin of the cells and the best way in which to transcribe them is determined.

Discussion

Upon reviewing the research pertaining to Embryonic Stem cells, it is apparent that this field has tremendous potential with regards to the regeneration of tissue. The work of McDonald et al. shows that there are substantial benefits that can be a direct result simple implantation of ES cells.[6] The fact that the cells were able to fill the void of the syrinx left from spinal cord injury and improve the rats motor functions leads me to believe that this can eventually be implemented in a clinical setting. There is certainly a correlation between the addition of the ES cells and the rat’s improvement in condition. The work of van Laake et al. further confirms this relationship. They saw positive results with implantation ES cells, the mice’s hearts function improved with the addition of ES cells just as the rat’s motor functions did.[5] Furthermore van Laake et al. used human ES cells in their research instead of mice or rat ES cells. The fact that their results were achieved using human and not rat or mouse cells is extremely encouraging for future human applications.

The results of these studies are certainly promising, however they are not yet conclusive. It is hard to deny that the implantation of ES cells benefitted both the rats and mice in their respective experiments, but neither study was conclusive, or developed a method that is ready to be implemented with humans. It is still necessary to further refine the ways in which cells are made to differentiate into their desired cells. This will potentially give more pure samples of the desired cells. In addition there is still necessary research involving the optimal maturity of the differentiating cells to be done. Improvement of these things may be a good start solving the unsustainability of the heart tissue transplant as seen in the work of van Laake et al. In addition the issues concerning sustainability may be fixed if the stem cells in use are consistent with species. In other words, ES cell transplants to mice hearts may be more successful using mouse ES cells rather than human, and I would like to see experiments testing this thesis.

The research on IPS cells is also encouraging. From the work of Yu et al. and Takahashi et al. it is apparent that induction of pluripotency using somatic cells is very possible. They, perhaps most importantly, showed that IPS cells can differentiate into each germ layer both in a culture and in a living organism (mouse).[7] Although these IPS cells were not derived or tested on humans, the similarities between mouse and human stem cells leads me to believe that human IPS cells are indeed possible. Finally, by identifying the 4 essential factors to be Oct3/4, Sox2, c-Myc and Klf4 [7] Takahashi et al. provided great insight into what causes and maintains the pluripotency of these cells. This is the first step to refining and perfecting procedures for inducing and using IPS cells.

There is still much work to be done with IPS cells. Although they were able to see differentiation to each germ layer with the IPS cells, they are still very imperfect. The actual percentage of IPS cells they are able to generate at this time is very small. There are also problems with tumors forming when using these cells. These problems, I believe, can be fixed with further experimentation. The first priority in IPS cell research should be to understand the exact pathways through which these cells become pluripotent. If this process is understood, it may be possible to eliminate tumor-forming anomalies, and to generate samples with higher percentages of pluripotent cells. From there issues that ES cell research encounters should become relevant, and can help reform techniques into clinic-ready procedures.

I believe that the future of stem cell research and its application is bright. ES cells have been used in almost all research to date, however I believe that future research most likely will involve induced pluripotent cells. Not only will IPS research avoid the serious ethical issues that surround ES cell research, but it is also possible for these cells to foster even more sustainable treatments than ES cells. If the stem cells are indeed pluripotent, they should be able to differentiate into the entire gambit of tissues observed in ES cells. In addition, the use of autologous parent cells will likely yield compatibility benefits once tissues are available for human implantation.

References

[1] Itskovitz-Eldor et al.(1999) Differentiation of Human Embryonic Stem Cells into Embryoid Bodies Comprising the Three Embryonic Germ Layers, Molecular Medicine, Vol. 6(2)

[2] Li et al.(1998) Generation of purified neural precursors from embryonic stem cells by lineage selection, Current Biology, Vol. 8(17)

[3] Robin Lovell-Badge(2001) The future for stem cell research, Nature Vol. 414

[4] Yu et al.(2007) Induced Pluripotent Stem Cell Lines Derived from Human Somatic Cells, Science Vol. 318

[5] L.W. van Laake et al.(2007), Human embryonic stem cell-derived cardiomyocytes survive and mature in the mouse heart and transiently improve function after myocardial infarction, Stem Cell Research, Vol. 1(19-24)

[6] McDonald et al.(1999) Transplanted embryonic stem cells survive, differentiate and promote recovery in injured rat spinal cord, Nature America, Vol. 5(12)

[7] Takahashi et al.(2006) Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors, Cell, Vol. 126

Douglas Brown