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A number of recent advances in stem cell biology are poised to transform therapeutic approaches to a variety of cardiovascular diseases. In the July issue of the journal Cell Stem Cell, researchers report one such advance, demonstrating that they can direct mouse embryonic stem cells to develop into an embryonic cell layer called the mesoderm, which can differentiate (meaning become different in the process of development) into the heart, blood and other tissues [1].

Animal embryos differentiate into three cellular layers called germ layers: the ectoderm, mesoderm and endoderm. Tissues and organs of the body develop from these germ layers through further differentiation. The mesoderm germ layer differentiates into circulatory and urogenital systems, connective tissue, muscle and bone.

Embryonic stem cells have the potential to develop into almost any type of cell in the body. To be used therapeutically, however, scientists have to understand how to direct stem cells to become specialized cell types, such as skin or heart cells. Why use stem cells to repair the heart? Heart muscle cells from a patient generally won’t divide in a sufficient number to replace damaged areas. Taking a part of the heart muscle from another area only creates more damage. Embryonic stem cells thus provide an external and abundant source of cells for heart muscle repair.

Scientists at Washington University School of Medicine in St. Louis found that expression of the gene Mesp1 induced expression of mesodermal markers and genetic changes associated with the transition from embryonic to epithelial tissue (termed epithelial-mesenchymal transition).

A typical epithelium is a sheet of cells, often one cell thick, held tightly together by cell-cell junctions in a uniform array. Adhesions between neighboring epithelial cells inhibit movement of individual cells. In contrast, mesenchymal cells have neither organized structure nor tight intracellular adhesion, allowing for increased migratory capacity. Cells undergoing the epithelial-mesenchymal transition (EMT) experience transient structural changes that result in a loss of contact with neighboring cells and a gain in motility. This process is vital to movements that reorganize the embryonic germ layers and to the development of other migratory cell types [2]. Many of the changes associated with cells undergoing developmental EMTs are also observed in wound healing, fibrosis and cancer.

Using mouse embryonic stem cells, researchers showed that Mesp1 expression restricted the potential fates, generating progenitors or precursor cells with the potential to differentiate into cardiovascular cells but, importantly, not into hematopoietic cells (meaning blood-forming cells). They further demonstrated that Mesp1 induces expression of genes specific to cardiovascular development. The authors suggest that Mesp1 may selectively program the development of endothelial, cardiac and smooth muscle cells.

According to senior author Kenneth Murphy, M.D., Ph.D., senior author and Professor of Pathology and Immunology at Washington University School of Medicine [3]:

That’s the challenge to realizing the potential of stem cells. We know some things about how the early embryo develops, but we need to learn a great deal more about how factors like Mesp1 control the roles that stem cells assume.

This work has the potential to one day treat cardiovascular diseases using human stem cells. Scientists next plan to identify gene programs and map out the pathways that specify development of the three cardiac cell types: endothelial, cardiac and smooth muscle cells.

For more information on stem cells and the repair of a damaged heart, see Stem Cell Information at the NIH.

References

1. Lindsley et al. Mesp1 coordinately regulates cardiovascular fate restriction and epithelial-mesenchymal transition in differentiating ES cells. Cell Stem Cell, July 3, 2008 DOI: 10.1016/j.stem.2008.04.004
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2. Shook and Keller. Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development. Mech Dev. 2003 Nov;120(11):1351-83.
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3. Gene directs stem cells to build the heart. Washington University in St. Louis Medical News Release. 2008 Jul 2.

The ethical and moral debate over the use of stem cells has taken center stage over the past decade. Stem cells are of great medical interest, since they have the potential to develop into almost any type of cell in the body. Regenerative medicine focuses on the potential uses of stem cells in medicine and how they can provide effective treatment for a range of diseases.

Stem cells have the capacity to divide indefinitely to replenish other cells in the body. When a stem cell divides, each daughter cell can remain a stem cell or become a more specialized cell, such as a red blood cell, a muscle cell or a nerve cell. An increasing body of evidence also suggests that molecular pathways and properties associated with normal stem cells is relevant to cancer development [1].

There are two types of mammalian stem cells: embryonic stem cells, which are found in blastocysts (the mammalian embryo at the stage at which it is implanted into the wall of the uterus), and adult stem cells, which are found in adult tissues.

Stem cells are undifferentiated, meaning that they aren’t committed to becoming a specific kind of cell in the body. They are characterized by the ability to self-renew (remaining undifferentiated) and the capacity to differentiate into specialized cell types (termed potency). Potency specifies a stem cell’s differentiation potential.

There are four classes of stem cell potency:

• Totipotent: A totipotent stem cell is produced from the fusion of an egg and sperm cell and has the ability to become any kind of cell in the body. For a brief period, each cell division creates identical totipotent cells — any one of these cells could develop into a fetus if placed in a woman’s uterus. By the fourth day, the totipotent cells begin to specialize, forming a blastocyte, the type of cell that forms the outer layer of a blastocyst. These cells will go on to form the placenta and other supporting tissues of the uterus necessary for fetus development.

• Pluripotent: The inner cluster of cells in a blastocyst, called inner mass cells, are pluripotent. Pluripotent stem cells have the ability of to become any kind of cell in the body other than cells of the placenta or other supporting tissues of the uterus. Because of this, pluripotent cells cannot form a fetus if placed in a woman’s uterus. Embryonic stem cells are generally considered pluripotent.

• Multipotent: As pluripotent cells continue to specialize, they become cells that only lead to the development of specific tissues. Multipotent stem cells have the ability to form several cell types of a closely related family of cells and are generally referred to by their tissue origin, e.g. cardiac stem cells, neural stem cells, bone marrow stem cells. Multipotent cells function as a repair system for damaged tissue. Adult stem cells are multipotent stem cells.

• Unipotent: Unipotent stem cells have the ability to produce only one specialized cell type. Unipotent cells maintain the property of self-renewal, thus distinguishing them from non-stem cells.

Pluripotent stem cells can be derived from a number of sources, all of which are associated with moral issues:

1. Surplus embryos that are the by-product of in vitro fertilization
2. Embryos created in the lab from donated sperm and eggs
3. Embryos created through a cloning technique called somatic cell nuclear transfer
4. Embryos from aborted fetuses

Pluripotent stem cells can also be obtained from umbilical cord blood, placenta and amniotic fluid.

Although pluripotent stem cells have greater potential, multipotent adult stem cells are currently being used therapeutically. Adult stem cells can be found in many organs and tissues, including bone marrow, peripheral blood, brain, muscle, liver, skin and heart. Nevertheless, a number of limitations exist with adult stem cells [2]. Adult stem cells are found in small quantities in adult tissues and umbilical cord blood, raising doubt that they could be grown in clinically significant quantities. Additionally, adult stem cells have not been found for all tissues of the body, necessitating the use of pluripotent stem cells for the generation of certain tissue types. Since adult stem cells are multipotent, they cannot be induced to develop into any cell type, i.e. manipulated to differentiate into a specialized cell type different than their final tissue type.

For more information on stem cell research and current federal policy on embryonic stem cell research, see Research!America.

References

1. Lobo et al. The biology of cancer stem cells. Annu Rev Cell Dev Biol. 2007;23:675-99.
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2. Vats et al. Stem cells. Lancet. 2005 Aug 13-19;366(9485):592-602.
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In 2004, Korean investigators lead by Woo Suk Hwang at Seoul National University announced the creation of the world’s first human embryonic stem cell line generated by somatic cell nuclear transfer (SCNT), which involves the transfer of DNA, usually from a skin cell, into an egg cell that has had its DNA-containing nucleus removed. The work, published in the prominent journal Science, was retracted in 2006 amidst evidence that the researchers had falsified their data.

However, a study published online August 2nd, 2007, by the journal Cell Stem Cell reports that the Koreans unintentionally created the world’s first human embryonic stem cell derived by parthenogenesis, or virgin birth. Development is triggered spontaneously from the egg alone without the need for sperm fertilization.

Parthenogenesis is a natural phenomena that occurs in some insects, including aphids, honeybees and formicine ants, as well as some reptiles, including geckos, rock lizards and Komodo dragons. The process also occurs, though more rarely, in plants. There are no known cases of naturally occurring parthenogenesis in mammals because of imprinted genes. Imprinted genes are genes whose expression is determined by the parent that contributed them instead of following the usual rule of inheritance in which both copies of an inherited gene (one from the mother and one from the father) are equally expressed. In 1899, Jacques Loeb documented the first case of artificial parthenogenesis, treating sea urchin eggs with inorganic salt solutions to initiate embryonic development instead of the sperm of the male urchin [1].

In January 2007, researchers from the Harvard Stem Cell Institute analyzed patterns of genetic recombination in parthenogenetically derived mouse embryonic stem (ES) cell lines [2]. What they found was surprising: in contrast to ES cells produced by nuclear transfer, which are homozygous at most loci (meaning they contain two copies of the same form of a given gene at a specific location on a chromosome, referred to as an allele), parthenogenetically derived ES cells show predominant heterozygosity (meaning they have different alleles at a number of chromosomal locations) as a result of meiotic recombination. Meiosis is the process of cell division in sexually reproducing organisms that results in a reduction in the number of chromosomes in reproductive cells and leads to the production of gametes in animals (male gametes are sperm and female gametes are eggs) and spores in plants. Meiotic recombination, also known as crossing over, is a process of physical rearrangement occurring between two strands of DNA. A common event during meiosis, recombination leads to offspring that has different combinations of genes from their parents.

Researchers examined Hwang’s cell line using genome-wide single nucleotide polymorphism (SNP) analysis and found recombination patterns that are consistent with its derivation from a parthenogenetically derived embryo. SNP (pronounced “snip”) analysis identifies DNA sequence variations that occur when a single nucleotide - A, T, C, or G - in the genome is changed, producing different alleles. Common SNPs only have two alleles. For example, ATCGATCG and ATCAATCG represent two alleles: G and A. SNPs make up about 90% of all human genetic variation and occur every 100 to 300 bases along the 3-billion-base human genome [3]. These small variations in DNA sequence can have a significant impact on how individuals respond to disease, drugs and other therapies.

Parthenogenesis may provide a method for the generation of stem cells that are therapeutically valuable for women. Stem cells created by parthenogenesis don’t require cloning and won’t be rejected by the host immune system. However, concerns regarding safety and differentaion efficiency exist, as mouse parthenogenetic embryos are unable to complete full development due to the absense of paternally expressed imprinted genes, and tissues derived from parthenogenetic embryonic stem cells appear to have growth defects [4]. Further study is required to characterize the stem cells generated in the study. Nevertheless, parthenogenetically derived stem cells are another step closer to patient-specific, personalized medicine.

References

1. Jacques Loeb. On the nature of the process of fertilization and the artificial production of normal larvae (plutei) from the unfertilized eggs of the sea urchin. Am J Physiol 1899 3:135-138. Reprinted in Studies in General Physiology. Chicago: The University of Chicago Press, 1905. pp. 539-543.
2. Kim et al. Histocompatible embryonic stem cells by parthenogenesis. Science 2007 315, 482–486.
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3. SNP Fact Sheet. Human Genome Project Information.
4. Hernandez et al. Paternal and maternal genomes confer opposite effects on proliferation, cell-cycle length, senescence, and tumor formation. Proc. Natl. Acad. Sci. U. S. A. 2003 100, 13344–13349.
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