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ScienceWeek
DEVELOPMENT: EMBRYONIC STEM CELLS AND HEART REGENERATION
The following points are made by K.R. Chien et al (Science 2004 306:239):
1) Unlocking the therapeutic potential of embryonic stem (ES) cells has remained a tantalizing but elusive goal. In this new era of "regenerative medicine", the central experimental game plan has been predicated on driving the differentiation of ES cells along specific cell lineages (for example, neural, cardiac, endocrine), expansion and purification of the cell type of interest, and in vivo repopulation of damaged or degenerating organs by ES cell-derived differentiated cells. However, there are numerous hurdles to using ES cells as therapeutic tools. These include the need for reliable ES cell differentiation protocols for different cell lineages, purification techniques for the differentiated progeny, as well as ways to circumvent the immunological rejection of transplanted cells.
2) Given the complexity of these multiple steps, it is not surprising that there are few clear examples of in vivo ES cell therapy for treating disease-related phenotypes. New work (1,2) expands the potential therapeutic repertoire of ES cells. These investigators provide direct evidence that ES cells can rescue otherwise lethal cardiac defects in mouse embryos. The rescue effect is not subject to the differentiation of ES cells into the cardiac cell lineages that are normally associated with heart regeneration. Rather, the therapeutic effect of the transplanted ES cells depends on their secretion of defined factors that act either locally within the embryonic heart, or at a distance via the maternal circulation, to trigger fetal myocyte proliferation in utero.
3) In the new study, Fraidenraich et al (1) report a prominent cardiac phenotype in mouse embryos that harbor a double or triple deletion (knockout) of the Id1, Id2, and Id3 genes. The proteins encoded by these genes are transcriptional regulators that affect the differentiation of multiple cell types. The mutant Id embryos die at mid-gestation due to a marked thinning of the myocardial wall. This cardiac defect has been found in a number of mutant mouse embryos, including those lacking RXR-alpha (3-5), gp 130, or other signaling proteins. In all of these cases, the signals that link these proteins to thinning of the myocardial wall appear to arise from noncardiac muscle cells, and many of these proteins are not expressed in myocardial cells. Previously, approaches such as chimera rescue and cardiac lineage-restricted knockout of target genes indicated that a non-cell autonomous pathway causes the onset of "thin myocardial wall" syndrome (that is, the defect does not involve myocardial cells). Indeed, several of these studies implicate another section of heart tissue called the epicardium in myocardial wall thinning.
4) Given the potential of ES cells to induce the formation of teratomas (defective embryonic tissue), these findings do not necessarily suggest that administering ES cells to pregnant mothers will become a new therapeutic approach for treating congenital heart disease. However, given that a subset of maternal factors can cross the placenta, there remains a possibility that a subset of embryonic cardiac defects could be partially corrected by the careful delivery of the necessary proteins in the maternal circulation. Increasingly, congenital heart defects can be diagnosed accurately in utero with noninvasive imaging technology. In addition, ES cell-based assay systems may ultimately allow for the identification of likely candidate maternal factors that could correct a subset of severe human congenital heart defects.
References (abridged):
1. D. Fraidenraich et al., Science 306, 247 (2004)
2. W. M. Rideout III et al., Cell 109, 17 (2002)
3. H. M. Sucov et al., Genes Dev. 8, 1007 (1994)
4. P. Kastner et al., Cell 83, 859 (1995)
5. E. Dyson et al., Proc. Natl. Acad. Sci. U.S.A. 92, 7386 (1995)
Science http://www.sciencemag.org
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Related Material:
ON THE HUMAN EMBRYONIC STEM CELL DEBATE
Notes by ScienceWeek:
Human embryonic stem cells come from preimplantation embryos, most of which are generated at in vitro fertilization clinics. Within days after fertilization, the embryo consists of a hollow sphere, the blastocyst, which contains a cluster of a few hundred identical cells called the "inner cell mass" that can eventually develop into a fetus. When removed from the blastocyst, these cells can be propagated indefinitely in specialized media. When the media are changed to allow differentiation, cells continue to divide and aggregate, forming "embroid" bodies. Although these cell clusters lack the organization of an embryo, they contain all tissue types, including skin, muscle, bone, and neurons.
Because embryonic stem cells can become any cell in the body, there is hope that they will lead to treatment for diseases like diabetes, Parkinson's disease, Alzheimer's disease, and heart failure. The problem is controlling cell growth and differentiation. If large numbers of embryonic stem cells are transplanted into an organ like the brain, they grow into every cell type and form tumor-like masses called teratomas. eventually killing their host. The problem is thus to restrict embryonic stem cells to produce useful cells without overgrowing. (Curt R. Freed: Proc. Nat. Acad. Sci. 2002 99:1755,2344)
The following points are made by Irving L. Weissman (New Engl. J. Med. 2002 346:1576):
1) What if nuclear transplantation for the production of stem cells is banned in the United States but allowed in other countries (for example, China, Sweden, and the United Kingdom)? Biomedical researchers in the United States will have to learn of new advances by reading about them, rather than participating in them, or they will have to leave the United States in order to participate in research. New biomedical companies that translate these discoveries into therapies will be created in other countries, not here. And what if these companies succeed? Their products could not be imported to treat our patients (according to provisions of the Weldon bill [H.R.2505] and the Brownback bill [S.790, the "Human Cloning Prohibition Act of 2001"]), and only the wealthy would gain access to such treatments abroad. Even if these therapies could be imported, it is possible that physicians might withhold them from their patients for religious reasons.
2) Unfortunately, there are few in Congress or the President's council who can evaluate the scientific and medical issues in order to make an appropriately informed decision. Too often in recent Senate hearings, the views expressed by senators have been based on articles in newspapers and popular magazines rather than reports of the National Academies or articles in peer-reviewed journals. Some journalists are failing the public trust by publicizing findings that have not been published in the scientific literature or independently replicated.
3) In summary: Experiments in animals have shown that nuclear transplantation for the production of embryonic stem-cell lines can be accomplished with mature cell nuclei, including nuclei containing medically important genetic defects and mutations. There is already evidence that these embryonic stem-cell lines can help unlock secrets of developmental and pathogenic events that might not be revealed otherwise. The technology is ready for the production of human embryonic stem-cell lines from diverse members of our society, from somatic cells of patients with heritable diseases, and from diseased cells (for example, all cancers) whose nuclei are a repository of the history of inherited and somatic mutations that caused these diseases. The method has the potential for producing cells for the treatment of a variety of diseases. The author suggests that Congress, the President, and the medical community now face a difficult decision: to prevent the production of blastocysts by nuclear transplantation, or to pursue paths of medical research and therapies that will affect hundreds of thousands of lives.(1-5)
References (abridged):
1. Becker AJ, McCulloch EA, Till JE. Cytological demonstration of the clonal nature of spleen colonies derived from transplanted mouse marrow cells. Nature 1963;197:452-454.
2. Siminovitch L, McCulloch EA, Till JE. The distribution of colony-forming cells among spleen colonies. J Cell Comp Physiol 1963;62:327-36.
3. Weissman IL. Translating stem and progenitor cell biology to the clinic: barriers and opportunities. Science 2000;287:1442-1446.
4. Evans MJ, Kaufman MH. Establishment in culture of pluripotent cells from mouse embryos. Nature 1981;292:154-156.
5. Martin GR. Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells. Proc Natl Acad Sci U S A 1981;78:7634-7638.
New Engl. J. Med. http://www.nejm.org
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Related Material:
ON HUMAN EMBRYONIC STEM CELL RESEARCH
Notes by ScienceWeek:
In a multicellular living organism such as a human or a mouse, what differentiates one cell type from another is apparently not the genome, since the genome is the same in every cell, but which parts of the genome are operational. In other words, each cell type, skin cell, muscle cell, etc., has a particular gene profile characteristic of that cell type. Cells of a particular cell type are said to be "differentiated". Stem cells, present in all early embryos and in some tissues, are undifferentiated cells that in response to appropriate signals differentiate and give rise to a variety of cell types.
Embryonic stem cells are "totipotent", i.e., they have the potential to differentiate into any type of tissue cell. These cells form at a very early stage in human development and remain in an undifferentiated state for only a short period of time. They are first clearly recognizable approximately 5 to 7 days after fertilization, when a human embryo forms a structure called a "blastocyst", a hollow fluid-filled sphere consisting of only 140 cells.
There are two types of cells in the blastocyst at this stage: a) "trophoblast cells", which form the wall of the sphere, and which will become supporting tissues of the fetus (e.g., the placenta); b) "inner-cell-mass cells", a clump of cells located at one end within the blastocyst interior, and which are the undifferentiated cells (stem cells) that will divide and develop into the individual. The expected future medical applications of stem cells, particularly embryonic stem cells, are extremely promising, but because of the involvement of embryos and certain other considerations, basic stem cell research has provoked intense controversy.
The following points are made by Shirley J. Wright (Amer. Scientist 1999 87:352):
1) Human blastocysts -- each capable of developing into a complete human being -- are a potential source of embryonic stem cells, undifferentiated cells with the potential to develop into any cell type in the body. These cells have enormous therapeutic potential for the replacement of damaged or diseased tissues, but current legal and ethical concerns limit the nature of the research that can be performed with these cells because of their source.
2) At the 5 day stage, the human blastocyst is approximately 200 microns in diameter. Cells of the inner cell mass can give rise to all 3 germ layers -- the ectoderm, mesoderm, and endoderm --which in turn give rise to all the tissues in the body. The ectoderm cells develop into skin, nerves and eyes; the mesoderm cells develop into bone, blood, and muscles; the endoderm cells develop into the lungs, liver, and the lining of the intestines. At the 5 to 7 day stage, the inner cell mass can be removed from the blastocyst and cultured in a dish as embryonic stem cells.
3) Early human embryos can also provide undifferentiated pluripotent cells (i.e., cells capable of differentiating into certain cell types but not all cell types) in the form of primordial germ cells, the precursors of eggs and sperm cells. The primordial germ cells do not differentiate early, remaining in the yolk sac until approximately the 6th to 8th week of development, when they migrate to the developing gonads in the embryo. These primordial germ cells may be extracted as pluripotent embryonic germ cells beginning approximately 24 days after fertilization.
4) Embryonic stem cells obtained from the inner cell mass of a blastocyst can be grown in a culture dish on a layer of "feeder" cells derived from irradiated mouse *fibroblasts. The layer of feeder cells arrests the differentiation of the stem cells by releasing various inhibitory factors. Cell lines derived in this manner are immortal -- they can divide indefinitely to form more undifferentiated cells, thus providing a ready source for future research.
5) Fusing a human somatic cell (i.e., any human non-germ cell) with an enucleated egg cell allows the creation of person-specific embryonic stem cells, thus avoiding the complications of tissue incompatibility. In this technique, a patient's somatic cell is placed next to an enucleated egg cell, and the two cells are fused by application of an electric current, the somatic cell nucleus entering the egg cytoplasm. The egg is then activated and develops into a blastocyst embryo, and the blastocyst can now provide embryonic stem cells compatible with the patient. This is the technique that was used Ian Wilmut and his group to produce the cloned sheep Dolly.
6) Transfer of a human somatic-cell nucleus (such as a cheek *epithelial-cell nucleus) to an enucleated bovine egg cell produces a "*chimera" that could be the source of embryonic stem cells. Such an experiment was successfully performed by Robl and Cibelli in 1996. The embryo developed to the 32-cell stage, but was not allowed to develop further.
7) Production of human replacement tissue (e.g., neural cells, pancreatic cells, or heart-muscle cells) in a culture dish is one of the important potential clinical applications of embryonic stem-cell technology. Once cultured, the differentiated cells would be injected into the damaged organ, where they would replace the damaged tissue. But this has not yet been achieved, and the clinical technology will require years of development.
8) The author concludes: "As a society we must identify the ethical, social, legal, medical, theological, and moral issues that surround this research. People from all walks of life --scientists, lawyers, ethicists, clergy, and the general public --should be involved in making the decision. We are also at the crossroads where further scientific evidence is needed to explore the full potential of these cells, and yet many of the necessary experiments raise further ethical issues. The question of how we should use these powerful cells remains a challenging problem for the next century."
American Scientist http://www.americanscientist.org
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Notes by ScienceWeek:
fibroblasts: A type of connective tissue cell, secreting structural proteins (e.g., collagen) that form certain tissue components, including the extracellular matrix.
epithelial-cell: In animals, epithelial cells (epithelium) compose the cell layers that form the interface between a tissue and the external environment, for example, the cells of the skin, the lining of the intestinal tract, and the lung airway passages.
chimera: In general, a "chimera" is any cell or organism with genetic material from two or more genotypes (e.g., two or more species).
ScienceWeek http://scienceweek.com
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