Friday 3 August 2012
Protein degradation: a new drug target for Duchenne muscular dystrophy?
In muscle cells, dystrophin forms part of a group of proteins which acts as a scaffold to link the inside and outside of the cell. When dystrophin is not in the group (for instance in Duchenne muscular dystrophy) the cell degrades the rest of the proteins. Now, scientists have shown in a mouse model that blocking this degradation can increase levels of the other proteins and help to stabilise the scaffold - leading to reduced muscle damage.
The dystrophin glycoprotein complex (DGC) is a group of several proteins, including dystrophin, which works to stabilise muscle structure. The complex acts as a scaffold, linking the internal skeleton of the muscle to structures outside the cell.
In healthy muscle cells, the dystrophin protein hides a "signal" that tells the cell to degrade the DGC. The lack of dystrophin in Duchenne muscular dystrophy can uncover this "signal" and lead to the loss of the DGC resulting in a weaker structural scaffold inside the muscle cell. This means the muscle is more likely to be damaged when it contracts and eventually leads to the muscle weakness and wasting seen in Duchenne muscular dystrophy.
A team of researchers led by Prof Steve Winder at the University of Sheffield has tested whether blocking the "signal" for degradation could prevent the DGC being degraded in the absence of dystrophin. They created modified mdx mice (an animal model of Duchenne muscular dystrophy) that were missing the signal and found that this prevented the loss of the DGC. They found that preventing the loss of the DGC led to an improvement in the muscle structure, and a reduction in muscle damage.
When the researchers examined the proteins that were present in the DGC of the modified mdx mice, they found that a protein called plectin was doing the job that dystrophin would normally do. Plectin is protein that plays a similar role to dystrophin, acting as a piece of scaffold to give cells stability and structure. The researchers do not know why plectin, rather than one of the other scaffold proteins in the cell, was taking the place of dystrophin in the DGC and this will need further investigation.
This research has increased our understanding of how a lack of dystrophin, leads to muscle damage. The researchers demonstrated that when dystrophin is missing, a "signal" is uncovered which tells the cell to degrade the rest of the DGC. The lack of a DGC means that the muscle has less protection from damage caused by the muscle contracting. If the "signal" is blocked, or switched off, then the DGC is not degraded by the cell and plectin can partially compensate for the lack of dystrophin, reducing muscle damage. This "signal" represents a new target for potential drugs for Duchenne muscular dystrophy and, like potential drugs which aim to increase utrophin production, chemicals that target this pathway may have the potential to treat all people with Duchenne muscular dystrophy, regardless of their mutation.
Although this work is promising, it is still at a very early stage and has been carried out in an animal model. The techniques used to block the "signal" in mice could not be used in a clinical setting and so further research will be needed to identify candidate drugs which could specifically block the "signal" to degrade the DGC in the muscles of people with Duchenne muscular dystrophy.
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