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Plugs and Patches Improve Muscle Health
Much attention has been paid to gene therapy and stem cell strategies for treating muscle diseases, but several less dramatic strategies also appear to hold potential, especially if used in conjunction with more definitive therapies to enhance their effectiveness.
One such new approach proposes stopping a leak of calcium inside muscle fibers affected by Duchenne muscular dystrophy (DMD). Two others focus on repairing or shoring up the muscle-fiber membrane, a structure that’s affected in many diseases, such as DMD, Becker muscular dystrophy (BMD), some types of limb-girdle muscular dystrophy, and a type of congenital muscular dystrophy (CMD).
Plugging an internal leak
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| Repairing or reinforcing a fragile muscle-fiber membrane could be helpful in treating several muscular dystrophies. |
Investigators conducting experiments in mice with a disease resembling DMD have discovered that calcium can leak from internal storage areas in muscle fibers, and may be contributing to muscle degeneration. “Plugging” this leak could complement more definitive strategies, such as gene transfer, they say.
Andrew Marks at Columbia University in New York coordinated a team that included researchers from Montpellier (France) University and other institutions in Montpellier. They published their findings in the March 2009 issue of Nature Medicine.
The mice in these experiments lack the muscle protein dystrophin. In the mice — and in humans with DMD — a lack of dystrophin means a cluster of proteins nestled in a membrane surrounding each muscle fiber can’t preserve the integrity of the membrane. Leaks and tears in the membrane occur and are accompanied by entry of excess calcium into muscle fibers.
Excess calcium entry into cells (which is not related to dietary intake of calcium) can cause multiple types of damage, and it’s been assumed that it’s responsible for some of the fiber degeneration seen in this form of MD.
However, until recently not much attention was paid to the release of calcium from internal storage areas in muscle fibers. A burst of calcium from inside the fiber is necessary for it to contract (see In Focus: Periodic Paralysis), but a continuous leak of calcium can be damaging.
Marks and colleagues discovered internal calcium leakage in the dystrophic muscle that they say could contribute significantly to calcium-related damage in muscles and might be relatively amenable to preventive therapies.
When the researchers treated some of their mice, either orally or under the skin, with a compound called S107, they found it plugged the calcium leak without interfering with normal calcium release.
The treated mice developed better grip strength and tolerated downhill running better than their untreated counterparts, and biochemical and microscopic signs of muscle degeneration were much less severe than in the untreated group.
Improved exercise tolerance was seen after only one week of treatment, and improvement in the appearance of the muscle tissue after four weeks.
Investigators say therapeutic strategies for DMD that inhibit internal calcium leakage into the muscle fiber with a small molecule such as S107 could provide an additional way to help protect against muscle damage and improve function in this disease.
New membrane repair protein ID’d
Scientists in the United States and Japan say they’ve identified a previously unknown but crucial step in a natural muscle-cell repair process that could have implications for the treatment of muscular dystrophies, particularly those in which membrane defects are implicated.
Jianjie Ma of the Robert Wood Johnson Medical School in Piscataway, N.J. (part of the University of Medicine and Dentistry of New Jersey), with Hiroshi Takeshima of the Kyoto (Japan) University Graduate School of Pharmaceutical Sciences, and colleagues, have found that a muscle protein called mitsugumin 53 (MG53) is an essential component of the membrane repair machinery in muscle cells.
The researchers, who published their findings in the January 2009 issue of Nature Cell Biology, say the finding is relevant to both skeletal muscle fibers and cardiac muscle cells.
Muscle-membrane repair, the researchers note, is required in response to exercise, injury, aging and a variety of muscle conditions. They describe a three-part repair process in which MG53 first senses damage to the membrane; MG53 steers vesicles (bubbles) carrying repair molecules to the damage site and holds them in place; and the vesicles fuse with the membrane, forming a repair patch.
Several years ago, MDA-supported researchers identified another protein, now called dysferlin, which participates in the membrane repair process. The researchers say future studies are needed to see whether MG53 and dysferlin are part of the same or different repair pathways.
Laminin 111 protein may shore up membrane, reactivate muscle development program
A protein called laminin 111 had a marked therapeutic effect in dystrophin-deficient mice that have a DMD-like disease, say researchers at the University of Nevada School of Medicine.
Dean Burkin and colleagues, whose results were published online in Proceedings of the National Academy of Sciences on April 28, 2009, found systemic treatment with laminin 111 restored several aspects of muscle health and prevented exercise-related damage in these mice.
In their paper, Jachinta Rooney, Praveen Burpur and Burkin say laminin 111 is a “highly potent therapeutic agent” in the mouse model of DMD and could have applications for human muscle disease.
Earlier this year, Burkin and colleagues showed laminin 111 improved the muscle health of mice with an integrin-deficient form of congenital muscular dystrophy (CMD). (See Research Updates, April-June 2009.)
When the investigators gave DMD mice a systemic injection of the laminin 111 protein and analyzed their tissues a month later, they were surprised to see laminin 111 throughout limb, diaphragm and cardiac muscles. They had suspected the large size of the laminin molecule might prevent it from migrating far from the injection site.
More importantly, the laminin-treated mice showed signs that their muscle-cell membranes were intact. An enzyme called creatine kinase (CK) was not leaking from their muscle fibers into the circulation, a positive indication of membrane integrity. (Dystrophin-deficient muscles leak CK.)
And, when the muscles of laminin-treated DMD mice were examined after the mice ran downhill on a treadmill, they showed very little damage, while their untreated counterparts showed significant injury.
The laminin 111 protein normally is present in skeletal and cardiac muscles in mice and humans only during embryonic development. As tissues mature, it disappears and is replaced by other forms of laminin.
During development, it’s located just outside the membrane that surrounds each muscle cell, in a gel-like substance called the extracellular matrix. While there, it increases production of a membrane protein called alpha 7 integrin, which is known to play a role in skeletal muscle regeneration and repair. (Burkin received MDA funding from 2000 to 2003 to study the role of integrin in alleviation of muscular dystrophy.)
Earlier research by Burkin and colleagues suggests the presence of the embryonic laminin protein may activate a muscle regeneration “program” like the one used to make muscle during early development. They believe it may have played additional roles in the DMD mice, such as directly reinforcing the muscle-fiber membrane.
Success of exon skipping in dogs bodes well for human DMD treatment
Scientists at Children’s National Medical Center in Washington, Carolinas Medical Center in Charlotte, N.C., and the National Center of Neurology and Psychiatry in Tokyo, have successfully treated dogs with a disease closely resembling Duchenne muscular dystrophy (DMD), using a molecular treatment strategy called exon skipping.
Exon skipping as a strategy for treating DMD is simultaneously under development in human subjects. Estimates are that 80 percent to 90 percent of boys with DMD could potentially benefit from it. (Note: no single exon skipping drug will treat all mutations; compounds will be exon-specific.)
The investigators, who were supported in part by MDA, showed intravenous injections of a “cocktail” of laboratory-developed compounds coaxed the muscle fibers of three DMD-affected dogs to produce functional dystrophin protein, the absence of which causes the disease.
“Many efforts have focused on treating dogs with muscular dystrophy, as it is widely expected that what works in the dogs will work in humans,” said Eric Hoffman, professor of pediatrics at Children’s National Medical Center and an MDA grantee.
Exon skipping is a strategy that hides the error-containing exons (regions of a gene) from the cell’s “view” in such a way that they’re skipped over, and the remaining, correct instructions surrounding the region are spliced together. The spliced instructions allow for production of nearly normal, functional muscle protein that’s free of genetic errors.
Two clinical trials, one in the Netherlands and the other in the United Kingdom, recently have shown that intramuscular injection of either of two exon-skipping compounds appears safe in boys with DMD and that it can lead to production of dystrophin. (See “AVI BioPharma.”) These trials, which used compounds developed with MDA support, provide “proof of principle” for the strategy, but they only targeted a single muscle and weren’t designed to show functional benefit.
In contrast, the DMD dog experiments delivered exon-skipping compounds systemically, via intravenous injections, resulting in body-wide production of significant levels of dystrophin and improvement in the dogs’ functional abilities.
In addition, the human trials targeted only one exon, while the dog experiments targeted two consecutive exons.
In the experiments, for which results were published March 16, 2009, in Annals of Neurology, three dystrophin-deficient beagles each were given intravenous injections of a cocktail of exon-skipping compounds either weekly or every other week.
All three showed new dystrophin production in all examined muscles, although the degree of production varied. The average dystrophin protein production level was greatest in the dog given seven weekly doses of 200 milligrams per kilogram of the exon-skipping cocktail, causing dystrophin levels to rise from zero to 26 percent of normal.
Functional improvement was assessed by a 15-minute timed running test and by a combined functional score. All dogs that received the exon-skipping compounds ran faster after the treatment, while their untreated littermates became slower over the same period of time.
Scientists also saw marked improvements in the microscopic appearance of the muscle tissue in the treated dogs, as well as other measures of muscle health.
Utrophin injections aid dystrophin-deficient mice
MDA grantee James Ervasti and colleagues at the University of Minnesota-Twin Cities in Minneapolis have found that the muscle protein utrophin conferred significant benefits when injected into mice lacking the dystrophin protein and showing a disease resembling Duchenne muscular dystrophy (DMD).
Dystrophin is the muscle protein missing in people with DMD and partially absent in those with Becker muscular dystrophy (BMD). The utrophin protein is very similar to dystrophin and is thought to partially compensate for dystrophin’s absence.
For people with DMD and BMD, the advantage of utrophin-based therapies over dystrophin-based therapies is that utrophin is highly unlikely to provoke an unwanted response from the immune system. Dystrophin can elicit an immune response from people whose immune systems haven’t previously been exposed to it, such as those with DMD and some people with BMD. However, people with DMD and BMD already make normal utrophin, so their immune systems are more tolerant of it.
Utrophin therapies have been explored in dystrophin-deficient mice as a strategy to treat DMD or BMD for several years, and they’ve shown promise. Until now, most of the experiments have involved either transferring extra utrophin genes (gene therapy) into the mice or boosting production of utrophin from their existing utrophin genes (gene upregulation). Both those strategies are viable and continue to be the subject of experimentation.
However, the experiments Ervasti and colleagues described online May 26, 2009, in PLoS Medicine, are the first to show benefit from the direct injection of utrophin protein (protein therapy) into DMD mice.
The investigators injected miniaturized utrophin protein molecules (micro-utrophin) into the abdomens of DMD mice twice a week for three weeks, starting at 18 days after birth. They attached a cell-penetrating molecule called TAT to each utrophin protein molecule. (The investigators also tried using full-length utrophin molecules, but the micro-utrophins penetrated cells better.)
The TAT-micro-utrophin penetrated all the tissues the researchers examined. In addition, it aligned itself with the muscle-fiber membrane as part of a cluster of proteins in the way dystrophin normally would. Loss of the integrity of this cluster, and therefore of the muscle-fiber membrane itself, is a hallmark of DMD and to a lesser extent of BMD.
When compared with untreated mice, the mice that received utrophin protein injections had lower levels of a muscle enzyme called creatine kinase (CK) in their blood, which told the researchers that the muscle fibers in the treated mice were more intact and prevented CK leakage out into the bloodstream. The treated mice also showed fewer cellular signs of muscle degeneration than did their untreated counterparts, as well as better force production by the muscles and less susceptibility to contraction-related drops in force.
Scientists prevent toxic protein clumps in flies with OPMD-like disease
Scientists in France and the Netherlands recently announced they’ve identified a promising new strategy that could potentially become a therapy for oculopharyngeal muscular dystrophy (OPMD), a form of MD that primarily weakens the eyelid and throat muscles and also can affect limb muscles.
The strategy involves using an immune-system protein (antibody) derived from llamas that sticks to abnormally formed protein molecules in muscle cells and keeps them from forming large, damaging clumps.
The experiments were conducted in fruit flies with the same genetic defect that causes human OPMD. The flies cannot hold their wings in a normal position.
Martine Simonelig at the Institut de Genetique Humaine in Paris coordinated a group that included MDA research grantee Silvere van der Maarel at Leiden (Netherlands) University Medical Center. The group announced its findings online March 3, 2009, in Human Molecular Genetics.
Llamas and related animals, such as camels and alpacas, produce single-chain antibodies, which are not found in humans, whose immune systems make antibodies that consist of two chains.
Unlike double-chain antibodies, these single-chain antibodies can easily be selected and modified to enter and function inside a cell nucleus, which is where the abnormal protein molecules are located in OPMD-affected cells. Antibodies that are expressed inside a cell are known as intrabodies.
After testing several llama intrabodies by introducing the DNA for them into the muscle cells of the fruit flies, the researchers chose one, dubbed 3F5, as the most effective in allowing the flies to assume their normal wing posture. They found the 3F5 intrabody didn’t reduce the number of protein-containing clumps in the nucleus, but it markedly reduced the size of each clump, presumably reducing its toxicity to the cell.
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LGMD gene therapy trial first to show promise beyond safety alone
Researchers supported by MDA and the National Institutes of Health say results of a gene therapy trial in three people with type 2D limb-girdle muscular dystrophy (LGMD) are the first to show promise beyond safety alone. This type of LGMD is due to a deficiency of the muscle protein alpha-sarcoglycan.
Neurologist Jerry Mendell, co-director of the MDA clinic and director of the Center for Gene Therapy at Nationwide Children’s Hospital in Columbus, Ohio, received MDA support to coordinate the study team, which announced its results online April 16, 2009, in Annals of Neurology.
Although the primary goal of the small trial was to establish the safety of intramuscular injection of the alpha-sarcoglycan gene into a foot muscle, the investigators also evaluated how long gene activity persisted in the muscle, the level of alpha-sarcoglycan protein produced from the gene, and the response of the immune system to the gene.
No adverse events, such as rejection of the therapy by the immune system, occurred during the trial, reassuring researchers of the likelihood that their approach is safe in people with this form of LGMD.
Moreover, all three trial participants produced four to five times the amount of alpha-sarcoglycan protein in the gene-injected foot muscle compared to the amount in the corresponding muscle on the other foot, which received a salt solution. This level of output from the transferred gene persisted at least until the date of the last evaluation, which was six weeks after injection in one participant, seven weeks in another, and 12 weeks in a third.
In each case, the alpha-sarcoglycan protein assumed its normal position in the membrane of the muscle fiber and restored the structure of a protein cluster that’s normally found at that location but is missing in muscles that lack alpha-sarcoglycan. The cluster is crucial to the integrity of the muscle fibers.
The response of the immune system to the transferred gene and its carrier, a viral shell, was brief and minimal in all cases and did not interfere with gene activity.
No improvement in function was expected from this direct injection into a very small area. A delivery method that reaches a large muscle area will be necessary to improve function, the researchers say.
They also note that the study has potential relevance for other muscle diseases and for diseases in which muscle tissue can be used to secrete therapeutic proteins into the bloodstream.
AVI BioPharma exon-skipping trial in DMD enters systemic-delivery phase
AVI BioPharma of Portland, Ore., has started the systemic (through the blood) delivery phase of its clinical trial of AVI4658 in Duchenne muscular dystrophy (DMD). The trial is being conducted in the United Kingdom.
In January, the company announced that this laboratory-engineered molecule was safe and well tolerated when injected directly into a foot muscle in boys with DMD. More importantly, the molecule led to production of dystrophin, the necessary muscle protein missing in DMD, in all trial participants.
AVI BioPharma said the systemic-delivery phase of the trial now under way will test the safety and efficacy of administering AVI4658 intravenously into 16 boys with DMD. Systemic delivery is expected to reach several muscles and potentially could improve strength and function.
The 12-week study is being conducted in London and Newcastle Upon Tyne, United Kingdom. Francesco Muntoni at Imperial College London, who has MDA support to conduct research in another muscle disease, is the principal investigator. Support for this trial comes from AVI BioPharma and the British Medical Research Council.
AVI4658, a so-called antisense compound, is designed to cause muscle cells to skip over an error in a region (exon) of the gene for the dystrophin protein (see “Success of exon skipping.”)
In its Feb. 19, 2009, press release, AVI said the earlier results and preclinical research suggest that “by skipping [exon 51], a truncated but functional form of the dystrophin protein is produced to ameliorate the disease process, potentially prolonging and improving the quality of life in these patients.”
Prednisone dosing schedule affects behavior in DMD
A somewhat surprising result about moderate-dose, daily prednisone versus high-dose, weekend-only prednisone in boys with Duchenne muscular dystrophy (DMD) was obtained by a team of researchers from Children’s National Medical Center in Washington (CNMC) and the University of Pittsburgh.
The corticosteroid drug prednisone often is prescribed for boys with DMD because it has been shown to slow the decline of muscle strength.
The study, which was supported by MDA and the National Institutes of Health, was reported at the 61st annual meeting of the American Academy of Neurology, held in Seattle April 25-May 2, 2009.
The investigators found behavioral side effects associated with prednisone lessened over the course of a year in the daily prednisone group but stayed the same in the weekends-only prednisone group.
Twenty-eight boys with DMD were randomly assigned to receive 0.75 milligrams per kilogram of body weight of prednisone daily, while another 28 were randomly assigned to receive 10 milligrams per kilogram of prednisone weekly over two consecutive days (the weekend).
The investigators administered the Child Behavior Checklist (CBCL) rating scale at screening and after one, three, six, nine and 12 months.
Total behavioral problems at the start of the study were similar between the two treatment groups. One month into treatment, there were no changes in behavior within the weekend prednisone group and an improvement in total problems and attention in the daily prednisone group.
After a year, there were no significant differences within the weekend group. However, in the daily prednisone group, there were significant decreases in total problems, such as attention and aggression. The daily group also showed significantly fewer behavioral problems than the weekend group at one year.
At the 2008 AAN meeting, researchers announced analyses of other aspects of this study. At that time, they said the effects on maintenance of strength were similar between the daily and weekend prednisone dosing schedules, but that the time required to rise from the floor was better in the daily group.
Growth retardation, another predniosone side effect, was less severe in the weekend prednisone group. The weight gain side effect was the same in the two treatment groups after one year.
Myasthenia Gravis Studies
A variety of studies concerning myasthenia gravis (MG), a disease in which the immune system mistakenly attacks specialized parts of the junction between nerve and muscle cells, were presented at the 61st annual meeting of the American Academy of Neurology, held in Seattle April 25-May 2, 2009.
Some studies examined alternatives to long-term, high-dose prednisone. Prednisone, a corticosteroid drug that suppresses parts of the immune system, is often prescribed for this type of MG, and is fairly effective at helping people maintain strength. However, it has many side effects, such as weight gain, high blood pressure, diabetes, bone loss and psychological problems, if taken for long periods of time at high doses.
Mycophenolate mofetil allowed decreased prednisone dose
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| The acetylcholine receptors, which receive signals from nerve cells, are the typical targets of the immune system in MG. |
Michael Hehir at the University of Virginia Health System in Charlottesville, and colleagues, found a drug called mycophenolate mofetil (CellCept), when added to prednisone or substituted for prednisone, allowed people with MG to reduce or eliminate their prednisone intake during the second and third years of their therapy. There were 103 people in this study, all of whom had the type of autoimmune MG in which the acetylcholine receptors are the target of the immune system. These receptors are on the muscle fibers and receive signals from nerve fibers.
Those treated with mycophenolate mofetil alone began to improve between six months and a year after starting the drug. In the group taking mycophenolate mofetil and prednisone, the prednisone dose decreased after a year of mycophenolate mofetil. More than two years after starting mycophenolate mofetil, 53 percent of participants were off prednisone entirely, and 74.5 percent were taking less than 7.5 milligrams per day (a low dose).
Methotrexate allowed reduction of prednisone dosage
Faisal Raja at the University of Kansas Medical Center in Kansas City, Kan., and colleagues, found that eight people with MG tolerated an immunosuppressant medication called methotrexate (Rhematrex, Trexall), and four were able to reduce their prednisone dosage after an average of 8.7 weeks after starting methotrexate.
No one experienced any methotrexate-related adverse events, but no one had an improvement in their functional scores.
In seven people, the acetylcholine receptor was the autoimmune target. In one, it was a protein called muscle-specific receptor tyrosine kinase, or MUSK, which is needed at the junction of nerve and muscle fibers.
In ocular MG, prednisone reduced symptom spread
Another study, conducted by Mark Kupersmith at Roosevelt Hospital and the New York Eye and Ear Infirmary in New York, found prednisone appeared to prevent or at least delay the onset of generalized (all over the body) MG in people who only had ocular (eye-muscle) MG. It also controlled double vision resulting from ocular MG.
Eighty-seven people with ocular MG participated in this study, of whom 55 received prednisone and 32 did not.
Participants were followed for more than four years or until generalized MG developed. Generalized MG developed in seven (13 percent) of the 55 who took prednisone and in 16 (50 percent) of the 32 who did not. It typically appeared within a year in those not taking prednisone.
Double vision associated with ocular MG was present at the end of the study in 15 participants (27 percent) of the 55 who took prednisone and in 18 (57 percent) of the 32 who did not. |
Two longtime MDA grantees receive prestigious award
On May 3, 2009, molecular biologist Louis Kunkel at Children’s Hospital in Boston and Harvard University, and biophysicist Kevin Campbell at the University of Iowa, received the prestigious March of Dimes Prize in Developmental Biology. The prize includes a $250,000 cash award.
Kunkel was on the MDA-supported research team that in 1986 identified the gene for dystrophin, the protein missing in Duchenne muscular dystrophy (DMD).
Since then, his laboratory has been studying muscle stem cells and has been using dystrophin-deficient zebrafish to screen for small molecules that potentially can be developed into therapeutic agents. Much of this work has had MDA support. Kunkel now chairs MDA’s Scientific Advisory Committee, which recommends new projects for MDA funding.
Kevin Campbell headed teams that identified several of the proteins that interact with dystrophin in the muscle-fiber membrane. This research, much of which was MDA-supported, shed light on the functions and structure of dystrophin and the membrane itself and also clarified the role that other membrane-associated proteins play in causing limb-girdle muscular dystrophies (LGMD) and congenital muscular dystrophies (CMD).
More recently, Campbell’s laboratory has focused on the dystroglycan protein and its role in CMD; the role of sarcoglycan protein deficiency in skeletal and cardiac muscle abnormalities; and development of gene transfer for LGMD. Much of this work has been MDA-supported. Campbell also serves on MDA’s Scientific Advisory Committee.
Symposium honors two MDA-supported leaders in myositis research
On April 25, 2009, W. King Engel and Valerie Askanas, both neurologists and neuropathologists at the University of Southern California in Los Angeles, were honored at the USC International Neuromuscular Symposium. Engel and Askanas, who are married, co-direct the MDA clinic at Hospital of the Good Samaritan in Los Angeles, and Engel also directs the MDA/ALS Center at that institution.
Askanas has received support from MDA for many years for studies of inclusion-body myositis (IBM). She and Engel are well known for having demonstrated that amyloid-beta and several other proteins form toxic clumps in the muscle fibers in this disease. All speakers at the symposium were former trainees of Engel and Askanas.
Scientists Continue to Explore Stem Cells
Stem cells — immature cells with the potential to develop into different tissue types — have been heralded as a major advance for developing treatments for a variety of diseases. That’s true for diseases of the nerves and muscles, where such cells could potentially be transplanted into the body and either support or replace a patient’s ailing cells. Although most experts believe it will be a few years before stem cells are used for this purpose, they say another usage of stem cells already is bearing fruit: studying how a genetic disease evolves by watching it develop as the stem cell matures.
President Barack Obama announced March 9, 2009, that he will lift Bush-era restraints on federal funding for stem cell research involving human embryos. Although federally funded researchers may now move beyond the limited number of embryo-derived cell lines authorized for research under the Bush administration, restrictions remain, the details of which are as yet unclear. (Stem cell research funded by private companies and organizations in the United States has never been subject to these restraints.) Obama asked the National Institutes of Health to develop new guidelines within four months.
Although opening up federal funding for all types of stem cell development likely will speed research in the field, recent developments have shown that stem cells also can be developed from non-embryonic sources. It remains to be seen which types of cells will be best for specific applications.
Umbilical cord blood cells
Stem cells that are isolated from the umbilical cord blood of healthy babies then mixed in a lab dish with early-stage muscle cells (myoblasts) from people with Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD), can fuse. The resulting early-stage muscle fibers can produce dystrophin, researchers in Brazil have found. (Dystrophin is the protein that’s missing or deficient in DMD and BMD.)
Mayana Zatz of the Human Genome Research Center in Sao Paulo, Brazil, and colleagues published their findings Jan. 14, 2009, in the Journal of Translational Medicine.
The researchers say these very immature (“undifferentiated”) umbilical cord stem cells, known as CD34-positive stem cells, are extremely flexible. They say the cells probably responded to chemical factors released by the dystrophic muscle cells that invited fusion and formation of muscle fibers.
Stem cells that renew repair cells
This spring, MDA grantees Bradley Olwin at the University of Colorado-Boulder and Dawn Cornelison, now at the University of Missouri-Columbia, and colleagues, isolated a new type of skeletal-muscle stem cell in mice that appears to be particularly suited to repairing damaged muscle tissue.
The investigators, who published their findings March 6, 2009, in the journal Cell Stem Cell, say they believe these cells are the precursors of special muscle repair cells called “satellite cells.” Satellite cells reside near muscle fibers and move into them to compensate for damage when necessary.
The newly isolated cells are a subset of previously identified muscle precursors known as muscle SP (“side population”) cells, the researchers note. SP cells are “stemlike” in their ability to give rise to mature muscle fibers. However, in experiments in mice, relatively few of them have engrafted into existing muscle tissue after injection.
When Olwin and colleagues injected satellite SP cells into leg muscles in mice lacking dystrophin, they saw extensive muscle regeneration and replenishment of this protein.
“These cells are presumably poised to conduct repair operations when needed and can replenish the satellite cells as well as repair muscle,” Olwin said. He added that he’s encouraged at the large effect of one injection with a small number of cells.
Both Olwin and Cornelison have MDA support to continue working in this area.
Nerve cells derived from skin cells
Recently, skin cells from a child with spinal muscular atrophy (SMA) and from an 82-year-old woman with amyotrophic lateral sclerosis (ALS) have been “reprogrammed” back to a stemlike state and then coaxed to develop into SMA-affected or ALS-affected nerve cells. (See Research Updates, April 2009.)
This type of turning back the clock so that a mature cell can return to its stem cell origins and regain its ability to take a number of developmental paths is known as creating “induced pluripotent stem cells,” or iPS.
The technique has the advantage of not having to use human embryos to create this type of cell, as well as holding out the possibility of creating therapeutic cells from the cells of a patient, thus avoiding an unwanted immune response to donated cells.
So far, no one is sure of the extent to which iPS cells can actually become functioning specialized cells, such as motor neurons, the nerve cells affected in SMA and ALS. But scientists at the University of California-Los Angeles and the University of Rochester (N.Y.) say they’ve taken a step forward.
William Lowry and colleagues at UCLA, who published their findings online Feb. 23, 2009, in Stem Cells Express, say they’ve created fully functional human motor neurons from skin cells converted back to iPS cells. The motor neurons showed the typical electricity-like signaling functions of these nerve cells. In previous experiments, they say, these functions were not assessed.
New way of deriving nerve cells from stem cells
Researchers at the Burnham Institute for Medical Research in La Jolla, Calif., and the University of California at Los Angeles, say they’ve developed immature nerve cells that are flexible enough to become multiple nervous-system cell types but committed enough not to become other types of cells or form tumors.
Alexey Terskikh at the Burnham Institute, with colleagues there and at UCLA, used two different human embryonic stem cell lines previously approved by the National Institutes of Health to produce “committed neural precursor cells” using a procedure they developed. They say the technique was equally successful in both cell lines.
Their procedure for deriving the partially specialized cells is different from that of other research groups in that they omit a “priming step” in which cells are cultured in serum or serum replacement.
With this new method, the investigators say, the embryonic stem cells rapidly developed into committed neural progenitors, generally after 10 to 14 days in culture.
Unlike neural progenitor cells cultured using certain other conditions, Terskikh’s cells appear to have limited proliferation capacity (ability to divide) and instead mature into nerve cells and related cells called glia.
The researchers consider this a good sign, because too much cell division can lead to damaging over-proliferation of cells and even result in tumor formation. Tumors have been a concern when considering transplantation of embryo-derived cells into patients.
When the researchers transplanted these neural precursor cells into the brains of mice, they found they migrated to different parts of the brain and took on the characteristics of cells in their surroundings. Importantly, the transplanted human cells didn’t over proliferate or form tumors.
The investigators say they’ve described the molecular stages and pathways that normally occur as embryonic cells develop into nerve cells and have proposed genes that may play a role in the process but have not been previously recognized.
They say their cells can be best described as committed neural progenitor cells, which are on a path to becoming nerve cells but are still capable of becoming different types of nerve cells and don’t undergo dangerous proliferation or tumor formation.
Stem cells made from skin cells without help from viruses
Two scientific teams describe virus-free methods for “reprogramming” skin cells from mice and humans so that they become stem cells, with the potential to mature into multiple cell types. Until now, most methods for doing this have required the use of viruses.
Andras Nagy at the University of Toronto and colleagues, and Keisuke Kaji at the University of Edinburgh (United Kingdom) and colleagues, each described their work online March 1, 2009, in the journal Nature.
They say the new approach to cellular reprogramming is technically simpler than earlier methods and allows a range of cell types, not just those with limited susceptibility to viral infection, to be used to create stem cells. In addition, it allows the complete and accurate removal of genes inserted to accomplish the reprogramming.
Virus-free delivery of genes that reprogram cells, and an effective way to remove these genes after reprogramming has occurred, have the potential to provide stem cells that could be used to treat disease, as well as study disease development and screen potentially therapeutic compounds, the researchers say. |
