Research Updates
STEM CELLS
'Switch' May Be Key to Stem Cell Therapy in MD
The recent discovery of molecular switches that help determine whether stem cells will become muscle cells or other cell types might pave the way for effective treatments of muscular dystrophy (MD).
Stem cells - primitive, self-replacing cells that generate bodily tissues - represent one of the most promising tools for replacing muscle fibers (muscle cells) lost or damaged in MD. Although perhaps best known as the driving force behind embryonic development, stem cells are also found in adult tissues, where they can promote regeneration.
In the embryo and - to the recent surprise of scientists - in a variety of adult tissues, many stem cells are pluripotent, meaning they are able to give rise to several different cell types, including those that make up bone, blood, fat, nerve and muscle. If scientists can figure out how stem cells "choose" among these "career" options, they might be able to direct them to become specific cell types and repair specific tissues.
A team led by MDA-funded scientist Michael Rudnicki at Canada's Ottawa Hospital Research Institute has identified a chemical switch that influences the choice made by stem cells found in skeletal muscle. That switch appears required for the cells to become satellite cells, which, in adult skeletal muscle, increase muscle mass during normal growth, after exercise and in response to muscle trauma (injury).
The satellite cells are set aside during embryonic development, when muscle fibers form from the fusion of individual cells called myogenic precursor cells. During adult life, the satellite cells respond to the need for increased muscle mass by dividing. Some of the new cells become new myogenic precursor cells, and other satellite cells arise to replace them. Because satellite cells have this capacity for self-replacement, they're like stem cells with restricted potential, explained Rudnicki.
In fact, work by Rudnicki and his team, published in September in the journal Cell, suggests that muscle stem cells can become satellite cells if they activate a switch called Pax7.
Pax7 is a transcription factor (a protein that can turn genes on or off) long known to play a role in embryonic development. Patrick Seale, in Rudnicki's laboratory, discovered that Pax7 remains active in adult muscle tissue. Rudnicki's team found that, in adult mice with the Pax7 gene deleted, the muscles had no satellite cells and that stem cells in these muscles made mostly blood cells instead of dividing to produce muscle cells.
Rudnicki believes Pax7 acts as a switch that tells muscle stem cells to become satellite cells instead of blood cells. If turning on Pax7 can force the cells to make this choice, it could be used to devise treatments for MD that effectively combine gene therapy with stem cell transplantation.
If satellite cells can be directed to become muscle cells, he said, "We could then have these cells, which are committed to become muscle, repopulate the damaged muscles throughout the body in a systemic fashion." Turning on Pax7 and supplying a corrective gene (like dystrophin) in stem cells might lead to effective repair of damaged muscles in people with MD.
Other signals that influence stem cell development are cropping up. In an August issue of the journal Science, a group of researchers at the University of Michigan show that Wnts - a family of secreted proteins - are essential for preventing stem cells from becoming fat cells. If Wnt signaling is blocked, even presumptive muscle cells decide they'd rather be fat cells.
Given that muscles damaged by MD usually become surrounded by fat, this finding also could lead to improvements in stem cell transplantation. If the formation of fat cells could be blocked using Wnts, transplanted stem cells might have an easier time accessing and repairing damaged muscle fibers.
Boning Up on Stem Cells
in ALS Treatment
Two groundbreaking studies published in August suggest bone marrow might be a source for replacing nerve cells destroyed in neurodegenerative diseases like amyotrophic lateral sclerosis (ALS).
Until a few years ago, most scientists believed lost nerve cells couldn't be regenerated. But the recent discovery that the adult brain contains stem cells has overturned that idea and provided hope for repairing damaged brain tissue and nerve cells (neurons).
The recent studies show that adult bone marrow could serve as an accessible reservoir for new nerve cells. In both studies, researchers harvested stem cells from adult human bone marrow and stimulated them to produce cells that look like nerve cells.
Jeffrey Rothstein, co-director of the MDA/ALS Center at Johns Hopkins University in Baltimore and an MDA-funded ALS researcher, called the studies "an intriguing first step" to possible treatments for neurodegenerative diseases.
One study, headed by Juan Sanchez-Ramos at the University of South Florida in Tampa, isolated stem cells from adult human bone marrow and incubated them with brain-derived neurotrophic factor, a chemical known to promote nerve cell growth. This treatment caused some marrow-derived cells to make proteins found almost exclusively in nerve cells.
The other study, headed by Ira Black at the University of Medicine and Dentistry of New Jersey in Newark and MCP/Hahnemann University in Philadelphia, showed that treating the cells with an antioxidant compound had similar effects. This treatment also caused some cells to take on the shapes of nerve cells, which typically form branches that wire up with other nerve cells.
"What needs to be done next is to show that these cells really act like neurons," Sanchez-Ramos said. "There's a long way to go before humans can expect to receive any benefits from these kinds of research results."
Boys With DMD Show
Specific Memory Deficits
A study of 80 boys with Duchenne muscular dystrophy (DMD) shows that DMD often involves deficits in working verbal memory, the kind of memory needed to hold spoken verbal information in the mind and manipulate it. Such deficits come to light when subjects are asked to remember, manipulate and repeat a string of numbers; respond to a complex question; or listen to and reconstruct a story.
Other areas of cognitive functioning, including long-term memory, rote memory and visual-spatial skills, are intact in DMD, the researchers found.
MDA grantee Veronica Hinton, a neuropsychologist at the Gertrude Sergievsky Center of Columbia University in New York, led the study team, which published its results in the June 13 issue of Neurology. MDA clinic co-director Darryl DeVivo at Columbia-Presbyterian Medical Center in New York and Edward Goldstein, who co-directs the MDA clinic at Scottish Rite Children's Medical Center in Atlanta, were part of the team.
The boys studied were 6 to 16 years old. About half had siblings without DMD and, in comparisons, the siblings didn't show the same cognitive deficits as did the children with DMD.
The authors say the deficits found in the mental functioning of children with DMD put them at risk for learning disabilities, though they don't mean a child with DMD will necessarily develop learning disabilities. Children can often develop strategies to compensate for limitations imposed by a poor working verbal memory, they say.
The study confirms recent findings that the deficits in intellectual functioning are limited to a specific type. It overturns earlier speculations that DMD might cause overall mental dullness or that social or psychological factors were to blame for observed difficulties in school performance and reading.
The researchers say the reason for the deficits may be a lack of dystrophin in the brain. Brain dystrophin differs slightly from muscle dystrophin in structure and in function.
Hinton and colleagues are recruiting families with DMD, especially those who have at least one child with DMD and one or more children without it, for further study. For more information, including a newsletter, or for possible participation in the ongoing study, contact Hinton at (212) 305-2512 or hintonv@sergievsky.cpmc.columbia.edu; or Robert Fee, study coordinator, at (212) 305-2394 or feerobe@sergievsky. cpmc.columbia.edu. Because the study's travel budget is limited, subjects should be from the New York area.
Two Drugs Found to Help
in MG, One Hurts
Recent reports on the effects of three drugs used in the autoimmune disease myasthenia gravis (MG) reveal that two of them - cyclosporine (Sandimmune, Neoral) and mycophenolate mofetil (CellCept) - are helpful in this disorder, while gabapentin (Neurontin) can worsen MG symptoms.
In the Aug. 8 issue of Neurology, researchers at Duke University Medical Center in Durham, N.C., review the case records of 57 people with MG who took the immunosuppressant cyclo-sporine between 1987 and 1999 for an average of three and a half years each. Fifty-five people (96 percent) showed improvement. Of 38 people who were taking the immunosuppressant prednisone (Deltasone, Orasone), which can have serious side effects, 36 (95 percent) were able to reduce their prednisone dosage or stop the drug entirely. The study team included Donald Sanders, who directs the MDA clinic at Duke.
In the August issue of Muscle & Nerve, researchers at Rush-Presbyterian-St. Luke's Medical Center in Chicago report on a 26-year-old woman who benefited from adding mycophenolate mofetil to her drug regimen. The study's authors, who include Julie Rowin, an MDA clinic co-director at the hospital, say the patient was able to reduce dosages of all other medications to tolerable levels and return to work.
Daniel Drachman, director of the MDA clinic at Johns Hopkins University Hospital in Baltimore, has used this drug in more than 30 patients with MG and other autoimmune neuromuscular disorders and has found it safe and effective. "As a rule, it takes relatively long to work," he says, "but it's worth it most of the time, because it has relatively few side effects. It's very safe."
In the same issue of Muscle & Nerve, researchers at Hadassah Hospital in Jerusalem report on a 67-year-old woman who was given gabapentin for muscle cramps; the dosage resulted in worsening of borderline myasthenia. The researchers suggest that gabapentin can interfere with nerve-to-muscle transmission, the problem underlying the weakness in myasthenia, and say that "caution should be exercised when considering the use of gabapentin in MG patients."
Three More Genes
Linked to CMT
With an eye toward treatment, researchers have sorted out three more genetic defects that cause different types of Charcot-Marie-Tooth disease (CMT).
Schwann cells wrap around axons to form myelin. Some genetic defects interrupt this process, while others affect the axons themselves. Both defects can cause CMT. |
CMT involves damage to the peripheral nerves - bundles of nerve cell tendrils (axons) which send electrical signals to muscles and receive signals from sensory organs. For this reason, CMTs are sometimes called hereditary motor and sensory neuropathies (HMSNs).
CMTs can be caused by internal defects within the axons or by defects in Schwann cells. These supportive cells normally wrap themselves around axons within peripheral nerves to create a coating called myelin, which insulates the signals transmitted by the axons.
When the axons themselves fail to work properly (a condition called axonopathy) or when the Schwann cells fail to build myelin (demyelinating neuropathy), the axons gradually lose their ability to transmit signals and eventually shrivel up - leading to CMT.
The clinical diversity in CMT - large variability in the severity, age of onset and mode of inheritance - is thought to occur because a number of genetic defects can lead to the same result - loss of axons. Knowing the full range of these genetic flaws might be the key to fully understanding how axons are lost in CMT and how to save them.
Three studies published this summer have moved closer to that goal by identifying the defects associated with CMT type 2E, CMT type 4B and HMSN-Lom.
In one study, Russian researchers found that CMT2E - a relatively mild, adult-onset CMT so far found only in a large Russian family - appears to be caused by a mutation in the gene for neurofilament light protein (NF-L). This long, coiled protein is thought to serve dual roles as a backbone and conveyor belt within axons, so changing its function could directly interfere with axon function.
In another study, an international team of researchers discovered that CMT4B - a severe, demyelinating, infantile-onset CMT - can be caused by one of several mutations in the gene for myotubularin-related protein-2 (MTRM2). The researchers speculate that defects in MTRM2 interfere with Schwann cell development, similar to the way defects in myotubularin interrupt muscle development in the infantile-onset muscle disease, X-linked myotubular myopathy.
Finally, MDA-funded researcher Luba Kalaydjieva and her colleagues at the Western Australian Institute for Medical Research in Perth, Australia, showed that disruption of the N-myc downstream-regulated gene 1 (NDRG1) appears responsible for HMSN-Lom, a demyelinating CMT of intermediate onset and severity that occurs among descendants of Eastern European Gypsies. Based on her finding and previous studies of NDRG1, Kalaydjieva believes that - like MTRM2 - NDRG1 might be necessary for proper Schwann cell development.
Studies like these "give us a much better understanding of normal biology of the nerves as well as how [CMTs] are actually caused," said former MDA grantee Jeffrey Vance, who studies the genetics of CMT at Duke University. "Down the road, there may be treatments that may work on one type of mutation or one form of the disease and not others," added Vance.
Study Supports Exercises to
Improve Breathing in DMD and SMA
An Austrian study of 13 people with Duchenne muscular dystrophy and three with spinal muscular atrophy, ranging in age from 8 to 29, found that twice-daily performance of respiratory exercises with a specially made apparatus significantly improved respiratory strength in all patients. The study is in the August issue of Muscle & Nerve.
The exercises were performed twice a day at home and were designed to improve the strength of inspiratory muscles, those involved in breathing in air. The benefit of such exercises in the face of a progressive neuromuscular disease has been controversial, although a few small studies have previously suggested they can sometimes help.
The study divided the subjects into two groups, those whose respiratory muscle function had declined by less than 10 percent during the preceding year and those whose function had declined by 10 percent or more.
In the first group, the benefits of the exercises depended on how many exercises were performed over time (the more, the better). In the second group, there were also benefits, but they didn't correlate with the number of exercises performed. No adverse effects were seen in any participant.
Exercises to improve respiratory muscle function are generally done with an apparatus that regulates the amount of resistance against which one has to breathe to cause a measurable change. Physical and respiratory therapists, as well as pulmonologists, can advise patients about respiratory exercises that may be of benefit in their particular disorder.  |
