Using nitric oxide to sustain muscle regeneration
What is the idea behind this study?
Muscular dystrophy involves muscle weakness and loss of muscle tissue, getting worse over time. Muscle fibres are repeatedly injured, and this reduces the muscle’s ability to repair and regenerate itself. Normally, the body uses a muscle stem cell called a satellite cell to repair muscle fibres. This happens during normal muscle growth or after injury. When needed, the satellite cells multiply, become active, and change into healthy replacement muscle cells. After this, some go back to an inactive form to renew themselves as a reserve supply for future regeneration. However, with repeated damage in muscular dystrophy, this supply runs out over time.
Previous research shows that several substances control whether satellite cells can renew themselves. However, this knowledge has not led to any developments in increasing satellite cell renewal to treat muscular dystrophy.
It is known that production of nitric oxide (a cell signalling molecule controlling the way muscle is arranged and how it works) is altered in muscular dystrophies. Research has shown that restoring nitric oxide signalling improves muscle function and limits damage, but it is unknown how it affects satellite cells. Therefore, the researchers behind this study investigated how nitric oxide maintains the satellite cell reserve supply, and if it could be used in muscular dystrophy therapy.
When satellite cells are inactive, they can be identified because they possess certain proteins. An important one is Pax7, which sustains the reserve supply of satellite cells and stops them changing into muscle cells. Equally, there are different proteins that identify activated satellite cells. One of these is Myf5, and it reduces Pax7 levels. Therefore, when you test the reserve supply of satellite cells (inactive cells) you find that they do have Pax7, but don’t have Myf5. This is described as Pax7+/Myf5-. To study the effect of nitric oxide on satellite cell renewal and the available reserve supply, levels of these proteins were tested.
What did this study show
Firstly, the scientists compared the effects of a chemical called SIN-1 that increased levels of nitric oxide in muscle with one called L-NAME that decreased levels. This was carried out in normal muscles extracted from mice. They found that SIN-1 increased the number of inactive Pax7+/Myf5- satellite cells (cells that do have Pax7, but don’t have Myf5). L-NAME decreased the number of inactive Pax7+/Myf5- satellite cells. Therefore, increasing levels of nitric oxide increases the number of satellite cells in the reserve supply (inactive Pax7+/Myf5- cells) for muscle regeneration.
Next, they looked at the effect of nitric oxide on muscle that had been repeatedly damaged, as occurs in muscular dystrophy. The nitric oxide drug molsidomine was compared with L-NAME, and these were given to mice before muscle damage occurred. Molsidomine increased the number of Pax7+/Myf5- satellite cells, delaying the reduction of the satellite cell supply and sustaining muscle regeneration.
The researchers also found that SIN-1 stimulates multiplication of satellite cells. Their research showed two possible pathways for this, involving different signalling molecules:
- Cyclic guanosine monophosphate pathway – nitric oxide increases the number of activated satellite cells.
- Wnt noncanonical pathway – nitric oxide increases the number of inactive (reserve supply) satellite cells, dependent on a protein called Vangl2.
Finally, when examining the effect of nitric oxide on muscular dystrophy symptoms, they found that molsidomine increased the number of Pax7+/Myf5- satellite cells in a mouse model. Also, these mice had more regenerating muscle fibres, less signs of muscle fibre cell death, and better muscle function.
What does this mean for patients?
There is not yet an effective drug treatment for muscular dystrophy, and current therapies have severe side effects. Recent developments in research with stem cells and exon skipping are promising, but expensive. Drug treatments could be cheaper and help a wide range of people. In this case, the effect of the drug would be to support and enhance the body’s own ability to heal itself.
Nitric oxide drugs such as molsidomine could become important in the treatment of muscular dystrophy, either on their own or in conjunction with other therapies. This study showed that nitric oxide controls the number of satellite cells in muscles, and whether they renew themselves. Therefore, it could prevent the reserve supply of satellite cells from running out in conditions with severe muscle damage, like muscular dystrophy.
However, while these researchers showed two potential pathways for the effect of nitric oxide on satellite cell multiplication, there may be others that have not been discovered. The precise effects of nitric oxide on satellite cells are not yet fully understood.
While these results are encouraging, this study was carried out in mice and effects in humans may be different. It will take time for any developments to come through into medical treatment.
Further information and links
The full original paper was published in the journal Stem Cells in 2012. It can be accessed at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3378700/
Muscular Dystrophy Campaign website: http://www.muscular-dystrophy.org/
This scientific work and summary were supported by funding from the European Community’s Seventh Framework Programme project OPTISTEM. You can find out more about the scientists in this programme and the work they do at www.optistem.org
This summary was written by Heather Kennedy