In this project, Professor Jenny Morgan and her team at University College, London used a mouse model to investigate a biological process that leads to muscle fibre death in Duchenne muscular dystrophy.
This project was co-funded by three members of the Duchenne Forum – Muscular Dystrophy UK, Duchenne Children’s Trust and The Duchenne Research Fund. The Duchenne Forum is a group of charities working together to accelerate progress in the search for treatments and eventually cures for Duchenne muscular dystrophy.
What did the research show?
Muscle cells in people with Duchenne muscular dystrophy undergo an abnormal type of cell death, which is the basis of the muscle wasting.
There are two types of cell death mechanism: apoptosis and necrosis. An apoptotic cell digests itself and a necrotic cell explodes. In Duchenne muscular dystrophy, muscle fibres die by necrosis. Necrosis was believed to be a disorganized process, but it has been recently discovered that necrotic cells can be finely organized. This organized necrosis is called necroptosis.
Professor Morgan, Dr Bencze and colleagues have investigated the role of necroptosis in Duchenne. When one of the proteins of the necroptosis mechanism was blocked in muscle cells grown in the lab, the cells were more resistant to some toxic treatments, which normally trigger cell death.
The researchers also generated a dystrophic mouse model that is missing this protein; this protected cells from necroptosis. In these mice, muscle cell death and scarring (fibrosis) were decreased, and muscle function was improved. These results demonstrate that necroptosis is an important cell mechanism that is involved in dystrophic muscle damage. Targeting necroptotic cell death could therefore be a therapeutic strategy to counteract muscle loss in people with Duchenne.
The team presented this work at conferences including the UK Neuromuscular Translational Research Conference and the 2017 World Muscle Society meeting, where they won a prize. They also published their findings in the scientific journal, Nature Communications.
Why is this research important and what are the next steps?
This work identifies for the first time necroptosis as a cell death mechanism in muscle. Although basic research such as this is a long way from a treatment, it is important to gain a greater understanding of how muscles are damaged in Duchenne muscular dystrophy. These results show that the proteins involved in this cell death pathway do have a role in the symptoms of Duchenne, but further work is needed to understand the mechanisms behind this. If a suitable molecule was identified to block the cell death mechanism, it could help to reduce the cell death of muscle cells and slow down the development of fibrosis. Such an approach could be used in combination with other treatments aiming to restore dystrophin to the muscle.
How might this research impact on other neuromuscular conditions?
It is likely that muscle fibres die by this type of cell death mechanism in other neuromuscular conditions. This would need to be investigated in cell and animal models of these conditions.
What are the researchers aiming to do in this project?
The lack of dystrophin in the muscles of boys with Duchenne muscular dystrophy eventually leads to muscle fibres being damaged and dying. Although muscle stem cells can repair this damage and replace lost muscle fibres, the stem cells are eventually exhausted and muscle damage continues. The biological mechanisms that lead to the death of muscle fibres in Duchenne muscular dystrophy are not understood, but recent evidence has suggested that a newly discovered process of regulated cell death may play a role. Such a process that is controlled by the cells themselves could offer targets for drugs that could prevent muscle fibre death as a potential therapeutic approach for Duchenne muscular dystrophy.
In this project Professor Morgan’s team will use mdx mice – an animal model of Duchenne muscular dystrophy – and human muscle cells grown in the laboratory to confirm whether a regulated process causes muscle fibre death in Duchenne muscular dystrophy. The researchers also aim to stop key molecules in this pathway from working properly to investigate whether this could prolong the life of muscle fibres and slow the decline of muscle function in the mouse model. This could lead to the identification of potential targets for future treatments.
How will the outcomes of the research benefit patients?
This project will increase our understanding of the process that leads to muscle fibre death in Duchenne muscular dystrophy. Specifically, the results will indicate whether regulated processes plays a significant role in fibre death and whether interfering with the pathways leading the cell to die could offer a useful therapeutic approach.
If the results reveal specific new potentially therapeutic compounds, further preclinical testing in animal models will be pursued; this would be a necessary step before performing future clinical trial in humans.
- Apoptosis, or a cell suicide: the cell organises its own death by digesting itself from the inside. This is a clean way to die, where there is no cell explosion. There is a great deal of knowledge on apoptosis and there are medicines that can prevent it.
- Necrosis: a disorganised process – the cell rapidly explodes because of external causes (eg cold or heat, physical trauma). The necrotic cell disintegrates, releases its contents and generates a cascade of events that are bad for the whole body. Necrosis is not well understood and is therefore very difficult to control.
In Duchenne muscular dystrophy, muscle fibres die by necrosis, not apoptosis and that is why inhibiting the loss of muscle fibres is so challenging. Our project aims to understand how a muscle fibre dies by necrosis and how this cell death can be prevented.
Inflammation is implicated in the necrosis of dystrophic muscle fibres. We have data supporting our hypothesis that pro-inflammatory cytokines (proteins released by inflammatory cells) elicit cell death by necrosis in muscle cells in culture. We have identified a specific pathway that is implicated in this cell death mechanism. Inhibiting this pathway protects cultured muscle cells from death. We are currently generating mdx mice (a mouse model of Duchenne muscular dystrophy) that are deficient in this pathway. If, as we hypothesise, this genetically-modified mouse is at least partially protected against muscle fibres loss, the next step will be to design a pharmacological agent to inhibit this pathway in mdx mice.
Duchenne muscular dystrophy is characterised by degeneration (necrosis) of muscle fibres. The mechanisms underlying this muscle cell death are poorly understood and therefore need to be investigated in order to generate new therapeutic avenues for Duchenne muscular dystrophy. Proteins released by inflammatory cells are known to be involved in muscle cell death in the mdx mouse model of Duchenne muscular dystrophy. The aim of Professor Morgan and her team is to understand how inflammation induces necrotic death in muscle cells.
In year 2, Professor Morgan’s team have identified proteins that may be involved in the death of muscle fibres that is caused by inflammation. The researchers are investigating these proteins and how they are involved in cell death in muscle cells in the laboratory. Their findings suggest that a specific signalling pathway is responsible for at least some of the muscle necrosis that occurs in Duchenne muscular dystrophy. To confirm this, the researchers have generated a genetically modified mdx mouse model in which this pathway is blocked. The muscles of these mice are being analysed to determine if they have less necrosis than mdx mice.
Their findings will tell us if the pathway they are investigating might be a new therapeutic target for Duchenne muscular dystrophy.
Data from this project were presented in a poster at the Ninth UK Neuromuscular Research Conference (March 2016) and in a talk by Dr Maximilien Bencze at the London Myology Forum in December 2015.
Project leader: Professor Jenny Morgan
Location: University College, London
Conditions: Duchenne muscular dystrophy
Duration: three years, starting 2014
Total project cost: £154,065
Official title: Mechanisms of myonecrosis in Duchenne Muscular Dystrophy: can we control the death of muscle fibres?
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