The immature, versatile precursors to human tissue known as stem cells continue to show promise in understanding and treating disease—particularly neurodegenerative diseases, in which crucial populations of brain cells begin to die. In 2007, researchers reported new ways of obtaining stem cells in quantity, without engendering ethical concerns, for use throughout the body, including the brain. Additionally, studies have revealed how stem cells can help to unravel processes of neural degeneration and be used effectively to deliver therapies to dying brain cells.
Stem Cells from Skin Tissue
In 2007, stem cell research took a giant step closer to a long-desired goal: coaxing cells from adult human tissue to behave like embryonic stem cells, thus sidestepping the ethical hurdles posed by the use of embryos. In the November 20 Cell, Shinya Yamanaka and colleagues at Kyoto University, Japan, inserted four genes that are active during embryonic development into a modified virus. The virus was then inserted into fibroblasts, which are skin cells taken from adults. These genes then “reprogrammed” the skin cells to produce a line of stem cells that could self-renew and produce as many new cells as embryonic stem cells ordinarily produce.1 Another team, led by James Thompson of the University of Wisconsin, Madison, used a slightly different combination of genes to similarly reprogram skin cells taken from newborns. Their results appeared online November 19 and in print December 21 in Science.2
Stem cells produced through this method have the same “pluripotency” of embryonic stem cells, meaning they can develop into any desired type of tissue. Two studies in the July 19 Nature, one by Yamanaka and one by Rudolph Jaenisch of the Whitehead Institute, Boston, and colleagues, demonstrated this pluripotency in cell lines produced from mouse skin cells using the same basic technique.3, 4
The most immediate use of this technique will be to produce cell lines that contain genes known to produce specific diseases, such as the inherited forms of Alzheimer’s or Parkinson’s disease. These cell lines can be used to investigate how the gene products produce neurodegeneration and to screen potential therapies. Ultimately this new stem cell technique is expected to usher in a new age of medicine in which many brain diseases can be treated by replacing damaged nerve cells with a new population of brain cells derived from the patient’s own skin cells. But many hurdles remain. For example, use of modified viruses to deliver genes into skin cells may lead to development of tumors. Additionally, the stem cells derived from skin cells are not identical to those produced by embryos, and the differences may prove significant. While these potential problems will need to be successfully addressed, the ability to produce stem cells in quantity without involving fertilized human embryos is a major step forward.
Stem Cells from Non-Viable Embryos
The successful cloning of Dolly the sheep in 1997, by a process known as somatic cell nuclear transfer, raised hopes that the same approach could produce an endless supply of stem cells—either healthy cells from the patient or, for research purposes, cells with a particular genetic disorder. The process, however, involves inserting the desired genetic material into an oocyte, or egg cell. Obtaining egg cells from humans in sufficient numbers poses technical and ethical hurdles.
A study in the June 7 Nature shows a way around many of these hurdles. Working with mice, Dieter Egli and colleagues at Harvard University showed that it is possible to introduce stem-cell material into fertilized embryos, or zygotes—something that previous research had failed to accomplish.
In one phase of the experiment the researchers took zygotes with extra chromosomes—which are non-viable and thus cannot develop into living offspring—removed the abnormal chromosomes, and inserted the DNA of the stem cells they wanted to propagate. An estimated 3 to 5 percent of the human zygotes in in vitro fertilization clinics carry such abnormalities and are usually discarded, according to a 2000 report of the American Society for Reproductive Medicine/Society for Assisted Reproductive Technology Registry.5 The study shows for the first time how these unusable zygotes—numbering in the tens of thousands—could generate a vast supply of stem cells.
This approach would not destroy a potential life, since the embryos’ chromosomal abnormalities are incompatible with life. In addition, the genetic material in the resulting stem cells would not be that of the original donors. The technique could provide an ethically acceptable way of generating stem cells in quantity for use in researching many human genetic disorders.6
Not All Neural Stem Cells Are Alike
In seeking to harness the therapeutic power of neural stem cells, researchers need a thorough understanding of the factors that control their development. A common assumption is that neural stem cells begin life in a uniform state of potential and can theoretically be nudged onto almost any developmental path.
However, this assumption is based on studies of cultured cells; less is known about how stem cells behave in the brain. A study in the July 20 Science shows that a stem cell’s fate is restricted depending on its location.7
Working with newborn and adult mice, Arturo Alvarez-Buylla of the University of California at San Francisco and colleagues tracked the progeny of small groups of stem cells. Stem cells were selectively, and permanently, labeled with green fluorescent protein. The team followed the fate of stem cells from 15 different locations of a large “germinal” brain region in the adult, where neurons and other brain cells continue to be born after birth.
Mature, green-labeled nerve cells were formed from all sites, but the types of neurons produced differed depending on the site of origin. In addition, the stem cells proved remarkably resistant to a change in environment. Even when removed from the brain and grown in culture, exposed to a variety of growth factors—or when grafted into different sites in the germinal regions of other animals—the stem cells gave rise to neurons and other brain cells, but the neurons produced were once again specific to their original location. The finding suggests that although stem cells are indeed versatile, the types of neurons an individual stem cell can generate may be specified for one part of the brain and not readily able to assume a new identity if transplanted to a different location. This region specificity might restrict the therapeutic usefulness of a given population of stem cells.
Stem Cells Protect Neurons in ALS
Stem cells are usually hailed for their potential to produce future generations of healthy replacements for cells that die in degenerative disease. But they can also be used to deliver therapeutic substances to ailing neurons.
Working with a line of embryonic stem cells, Clive Svendsen of the University of Wisconsin, Madison, and colleagues engineered stem cells to secrete a compound called glial-derived neurotrophic factor (GDNF), which nourishes and protects neurons. Reporting in the July 31 edition of PLoS One, the online journal of the Public Library of Science, the investigators implanted GDNF-secreting stem cells into the spinal cords of rats with amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease), which attacks motor neurons.8
The transplants took hold and, in rats with early-stage disease, protected virtually all of the injured neurons. The engineered cells also showed a high affinity for damaged neurons, moving directly to the injured areas and pumping out GDNF.
The procedure did not restore communication between motor neurons and muscles or improve the rats’ ability to use their limbs; as a treatment for ALS its role would be limited to keeping the neurons alive. However, the approach demonstrates a lesser-known use for stem cells that could be useful in treating a variety of disorders. This approach of using stem cells to travel to sites of damage in the brain is also being investigated for delivering targeted treatment to brain tumors.
Powerful New Tools to Study Disease
Two teams of researchers studying amyotrophic lateral sclerosis have used stem cells to provide a vital clue to this mysterious disease. More than 90 percent of cases are sporadic, meaning that the patient has no family history of the disease. However, a mutated gene that encodes an enzyme called superoxide dismutase-1 (SOD1) has been identified as a cause of the disease in a few people.
How the mutated gene damages motor neurons is not understood. In particular, it is not known if the damaged gene directly affects motor neuron function or if other cells are involved. Recent studies have found that even healthy motor neurons begin to show characteristics of ALS when cultured with non-neuronal cells carrying the mutation.
The new studies, both published in the May Nature Neuroscience, suggest that the culprit is the star-shaped cells called astrocytes, which play many supportive roles in the brain. Working with motor neurons taken directly from mouse embryos, as well as neurons derived from mouse embryonic stem cells, researchers led by Serge Przedborski at Columbia University found in the first study that motor neurons carrying the human SOD mutation showed some abnormalities, but not neurodegeneration.9
However, astrocytes with the mutation triggered motor neuron death, following the same degenerative pathway as occurs in ALS. In addition, the team found that the astrocytes cause damage by releasing a substance that is selectively toxic to motor neurons, in contrast to non-harmful substances released by other types of support cells, such as glia.
In the second study, Kevin Eggan and colleagues at Harvard University and Perugia University used embryonic stem cells from mice to create a model to study the same question.10 The researchers took stem cells of mice bred to have either the normal human SOD gene or the mutated version, then allowed them to differentiate into motor neurons in large numbers. Cells with the mutation went through the characteristic steps of the disease, leading to the death of motor neurons, which suggests that the stem-cell approach is an effective, long-term research model of ALS. In addition, both the normal and the mutated motor neurons showed signs of neurodegeneration when cultured with SOD-mutant support cells.
Both findings open up new routes to treatment by showing that ALS may result from factors, such as astrocytes, that are not intrinsic to the motor neuron but that affect it. They also show how stem cells can provide a powerful new tool for studying the process by which a disease unfolds—in the case of the latter study, the work even provides a cell-based method for screening potential new drugs.
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