Stem Cells: It's Good To Have Choices

Abstract

Three types of stem cells, embryonic, adult, and induced pluripotent stem cells, are currently studied by scientists. Barack Obama's presidency has opened the door for stem cell research by revoking statements and orders made during the former President Bush's administration. This provisional period will allow the National Institute of Health to rewrite policies governing how federal funds are distributed for stem cell research. These new regulations will grant more freedom to researchers wishing to use stem cells in their research and challenge them to determine the most appropriate stem cell treatment for a given disorder.

In August of 2001 then-President George W. Bush announced that federal funds could not be granted for embryonic stem cell (ESC) research except for embryonic cell lines existing prior to his statement. Researchers interested in ESCs were challenged with obtaining funds from the private research sector or using previously established ESC lines, although more than 70% of these lines failed to expand undifferentiated in cell cultures. While this announcement put a damper on ESC research throughout the United States, it fortunately did not entirely limit the use of all stem cells for regenerative medicine. Investigators still had the option of studying stem cells derived from an assortment of adult tissues. As such, these cells were deemed appropriate for use since they were isolated from adult tissue and therefore not subjected to the argument of creating nor destroying human life for the purpose of advancing science. Despite the ethical controversies surrounding the use of embryos in research, one research group from Japan under the direction of Dr. Shinya Yamanaka astonished the scientific community by generating ESC-like cells, also known as induced pluripotent stem (iPS) cells, from adult somatic cells.^1,2 The creation of this new pluripotent cell line provided researchers with an additional option in their investigations.

The recent Executive Order 13505, titled “Removing Barriers to Responsible Scientific Research Involving Human Stem Cells,” by President Obama has opened the door for researchers by repealing former President Bush's 2001 statements and Executive Order 13435 to permit the granting of federal funds from the National Institutes of Health (NIH) to those interested in using ESCs for their studies. During this provisional period, the NIH has created guidelines extending federal funds to researchers using human ESCs derived from embryos created by in vitro fertilization for reproductive purposes and no longer needed. Moreover, all procedures regarding informed consent must be followed according to the standards set forth by the NIH. At the same time, the NIH has asked the general public for suggestions about refining the regulations of ESC research.

With the potential to cure debilitating diseases or repair devastating wounds, scientists are challenged with determining which of the 3 different stem cell types will produce the best outcome for a particular disorder. All three cell types match the classical definitions of a stem cell; that is, they (1) maintain their undifferentiated state through multiple cycles of cell division and (2) are capable of differentiating into various specialized cell types.^1,3,4 Fundamentally, then, what distinguishes each of the 3 stem cell types from one another? What separates ESCs from adult stem cells and iPS cells is their origin. ESCs are the result of cells harvested from the inner mast cells of a blastocyst, an early-stage embryo, after 5 to 7 days of cell culture.⁵ ESCs are termed pluripotent because they differentiate into all 3 germ layers: endoderm, ectoderm, and mesoderm. Under defined cell culture conditions and certain growth factors, ESCs can remain pluripotent almost indefinitely. What makes using ESCs controversial is their ability to generate human life. Of course, many individuals believe it is wrong to destroy life at any stage of development. Taking it a step further, others even fear that studies with ESCs will lead to human cloning and embryo farms.

Adult stem cells avoid this ethical dispute, and studies have demonstrated their equal effectiveness as ESCs. Adult stem cells, unlike ESCs, can be derived from various tissues, such as bone marrow, adipose tissue, and skeletal muscle.^3,6,7 These cells are considered multipotent because they are capable of giving rise to a number of specialized tissues, including osteogenic, chondrogenic, adipogenic, myogenic, and neurogenic, yet still maintain a lower differentiation capacity than pluripotent cells (ESCs).

The third and newest stem cell option is iPS cells, which are just as pluripotent as ESCs and avoid the ethical dilemma. Originally, the 2 approaches used to generate pluripotent adult cells were performed by either directly transferring the nuclear contents of an adult cell into an oocyte or by fusion with an ESC.^8,9 Then in 2006, Dr. Yamanaka's research team in Japan manufactured adult pluripotent stem cells by tranfecting both mouse and human somatic fibroblasts with 4 embryonic transcription factors: Oct3/4, Sox2, c-Myc, and Klf4.^1,2 Other researchers speculated that transcriptional factors Nanog and Lin28 may play a role in creating iPS cells.¹⁰ These iPS cells were indistinguishable from ESCs based on morphology, gene expression, and even teratoma formation.

Scientific investigators have yet to identify which of the 3 cell types best suits treatments for wound repair and genetic disorders. At first glance, ESCs appeared as the premium solution for all of humankind's disorders based on their unique ability to generate every cell type. However, ESCs proved to have negative impacts on health such that transplantation studies revealed the formation of teratomas through uncontrollable differentiation and immunorejection. The discovery of adult stem cells and iPS cells minimized the problem of immune rejection because these cells were acceptable for autologous transplantation. Some researchers even preferred adult stem cells on the basis of multiple tissue sources for harvesting, which provided several approaches to treat a disease or injury. While animal studies have indicated improvement in patients' health, none demonstrated complete recovery of tissue function. A reduced differentiation capacity is not the only limitation of adult stem cells. Some studies have demonstrated erroneous differentiation such as inadequate differentiation of mesenchymal stem cells or the unwanted differentiation of muscle-derived stem cells into fibrotic cells because of stimuli in the local environment.^11,12 These factors have contributed to some hesitation in choosing ESCs or adult stem cells as treatment in clinical trials.

In this regard, the creation of iPS cells has been acknowledged as a popular alternative. As mentioned earlier, iPS cells can be generated from a patient's own cells, thus avoiding the risk of immune rejection. Because they have a differentiation capability equal to that of ESCs through gene transfer of 4 prime genes (Oct3/4, Sox2, Klf4, & c-Myc), they have also been deemed a highly valuable biological treatment option. The major drawback of iPS cells is the genome-integrating viruses used to transfect cells, which could cause undesirable effects from improperly removing harmful viral DNA or disrupting other vital genes in the genome. Other limitations of iPS cells were low efficiency of reprogramming a small population of human primary cells and the formation for teratomas from in vivo transplantation by the activation of the c-Myc retrovirus. To resolve the apparent disadvantage of viral integration, researchers have explored several other methods of manufacturing iPS cells. One group tested nonintegrating adenoviruses transiently expressing Oct4, Sox2, Klf4, and c-Myc on mouse fibroblasts and liver cells, while Yamanaka's team tested repeated transfection of 2 expression plasmids (one with Oct3/4, Sox2, & Klf4 cDNA and the other with c-Myc cDNA) in mouse embryonic fibroblasts.^13,14 Different investigators have suggested that iPS cells could be generated with a reduced number of vectors, such as without c-Myc or even Klf4, which are known oncogenes.^15,16 Regrettably, removing factors for cell reprogramming has lowered the efficiency with which these genes combine with the host cell; however, with the help of small molecular compounds, valproic acid or BIX-01294 and BahK8644 used together, reprogramming efficiency has been improved.^16,17 Despite the advances made with iPS cells, questions still remain; for instance, Are there differences between iPS cells created from different adult tissues? Will they have the same outcome when transplanted? How similar are iPS cells to ESCs?

President Obama's Executive Order has given investigators the opportunity to further our understanding of ESCs. Although there are no limitations on the extent of using adult stem cells or iPS cells, the Dickey Amendment still prevents the creation or destruction of ESCs by somatic cell nuclear transfer, parthenogenesis, or embryos created by in vitro fertilization specifically for research purposes. Investigators will have to pay close attention to evaluate which cell selection works best for a given disease, while not violating any of the terms set forth by the NIH. In addition, once a stem cell type has been selected for a therapeutic treatment, researchers must work to overcome any relevant obstacles, whether they are tumorigenicity, immunorejection, or incomplete differentiation. During a period when innovative technologies with stem cells are being pursued to improve some of the nation's most devastating conditions (ie heart disease, cancer, and neurologic disorders), it is best to have the most choices available.