Bringing optogenetics to light with Dr Barney Bryson and Professor Linda Greensmith
Motor Neurone Disease (MND) describes a group of related diseases affecting the motor nerves or neurons in the brain and spinal cord, which pass messages to the muscles telling them what to do. In the UK, a person’s lifetime risk of developing MND is up to 1 in 300 and six people are diagnosed or die daily; this is just under 2,200 per year. In fact, MND affects up to 5,000 adults in the UK at any one time. With an ongoing surge of cases predicted due to an ageing population (people are generally living for longer and MND is more common in older people) the development of new and better treatments is crucial to enhance human health.
Supported by leading charities the Motor Neurone Disease Association and Thierry Latran Foundation, Dr Barney Bryson and Professor Linda Greensmith from University College London (UCL), are currently conducting ground-breaking research into the use of optogenetics and testing this technique as a potential therapy for MND.
Research Features caught up with Dr Bryson and Prof Greensmith to discuss their latest findings in more detail, clarifying the importance of this research in the overall fight against MND. They also outline the development of their research into therapeutic applications for optogenetics for neurodegenerative conditions from the very beginning stages and their foresight for what the future may hold.
Hi Barney and Linda, can you both tell us about optogenetics? What is the background to this technique and what does it do?
The name optogenetics comes from the fact that scientists are using optics (light) and genetics (the ability to modify the DNA of a cell or organism), together. Optogenetics is a technique that has revolutionised the ability of neuroscientists to finely control the electrical signalling activity of nerve cells using precise bursts of light. Similar to the way the nerve cells at the back of the retina convert light that enters the eye into electrical signals (relayed to, and interpreted by, the brain), optogenetics enables us to artificially induce electrical signals in any type of nerve cell by genetically manipulating that cell to produce a light-sensitive protein. In our research group, we make specialised nerve cells from stem cells that produce a light-sensitive protein normally found in algae, called channelrhodopsin-2. This enables us to initiate electrical signals in these specialised nerve cells using pulses of blue light. In theory, this can be applied to any type of nerve cell. However, we have taken advantage of this technique to control the electrical signals of specialised nerve cells that normally control muscle contraction, called motor neurons. This essentially enables us to artificially control the contraction of muscles that they form connections with.
How did you initially start researching into translational optogenetics?
From early on, we had a clear future goal of employing this pioneering technique to control the activity of motor neurons as a way of restoring muscle function in patients whose muscles had become paralysed due to injury or disease. In most cases, this paralysis is permanent and can have extremely debilitating and even life-threatening consequences, particularly in degenerative conditions such as Motor Neurone Disease (MND), where muscles that control breathing eventually become paralysed.
Since stem cells represent an extremely promising therapeutic approach to provide a source of any specialised cell type in the body, including motor neurons, we began to collaborate with Dr Ivo Lieberam, a developmental neurobiologist at King’s College London. Dr Lieberam generated “optogenetically” modified mouse stem cells, from which we produced specialised motor neurons, capable of replacing those that were damaged due to injury or disease. Afterwards, we tackled the task of determining whether these tailor-made motor neurons could restore control of muscle function in mice that had undergone injury- induced muscle paralysis.
In your latest research, you have been exploring the use of optogenetics in the treatment of MND. Can you describe the research process?
Although our earlier “proof-of-principle” experiments were carried out in mice that had undergone a nerve injury, it has long been our intention and major focus to assess the ability of optogenetics as a potential therapy for MND. However, several essential steps had to be tested and established before we could directly address this goal.
Firstly, we had to develop the customised stem cells and prove that we could convert them into motor neurons whose activity could be controlled using light. Next, we had to demonstrate that it was possible to engraft and implant these customised motor neurons into peripheral nerves (which connect the central nervous system, consisting of the brain and spinal cord, to the various muscles and organs of the body) in normal mice whose own motor neurons had been damaged by injury; and to ensure that they could grow along the nerve to contract and form new connections with specific paralysed muscles.
This was by far the most challenging part of the process, but we eventually overcame this by further genetically modifying the motor neurons so that they produced a survival protein called, Glial cell-derived neurotrophic factor (Gdnf). Gdnf enhanced their ability to survive in this abnormal environment.
Having demonstrated that the transplanted motor neurons could form new connections with paralysed muscles, we also demonstrated that we could finely control the contraction of the muscle using optical stimulation, providing the essential “proof-of-concept” that we had sought. We are currently in the process of testing this approach in mice that reproduce many of the traits of MND, since it is essential to prove that the transplanted motor neurons will not be killed-off by the same processes which cause the body’s own motor neurons to die in the disease. This essential part of the project is currently being supported by leading charities, the MND Association and the Thierry Latran Foundation – and we’re very pleased to report that our recent results are very encouraging. However, a great deal remains to be done before this strategy could be successfully employed as a therapy for MND patients.
What is neural engraftment and what is its relationship to optogenetics?
In our approach, neural engraftment is simply the process of injecting replacement motor neurons into peripheral nerve branches, such as those in the arm or leg that control muscles in the hand or foot. Normally, motor neurons are located in the spinal cord but they extend a long fine process out of the spinal cord and along the peripheral nerve along which electrical signals are transmitted to the muscle. However, by engrafting the motor neurons directly into the peripheral nerve, where we have shown that they are still able to survive, this greatly reduces the time and distance that they must grow to reach the target muscle – in the longest human nerves, this process could take years!
Can you explain why flashes of light are necessary to stimulate modified neurons?
Flashes of light are necessary to stimulate the engrafted motor neurons as they are outside their normal environment in the spinal cord, meaning that they are cut off from electrical signals from the brain that normally controls their activity and, in turn, allows motor neurons to activate muscle contraction. Therefore, an artificial means of controlling their activity and the contraction of the muscle that they connect with, is required. One way to stimulate the grafted motor neurons would be to electrically stimulate them, but this approach has significant disadvantages. To overcome these disadvantages, we decided to employ optogenetic stimulation, enabling the activity of the engrafted motor neurons to be controlled using flashes of lights. This carries a significant number of advantages.
What are the advantages of using optogenetic stimulation over electrical stimulation, which has been trialled as a treatment for MND?
Optogenetic stimulation has several critical advantages over electrical stimulation. Firstly, electrical stimulation is unable to selectively activate one type of neuron within a nerve, and since all nerves contain a mix of motor and sensory fibres, non-specific stimulation of sensory fibres will cause pain. Since sensory fibres are not affected in MND, this would be a major problem. Furthermore, electrical stimulation activates motor neurons in an abnormal manner, meaning that they cause abnormal contraction of the muscle that leads to rapid muscle fatigue.
In contrast, optogenetic stimulation completely avoids accidental activation of sensory fibres and does not result in muscle fatigue. The reason why this is so important was highlighted by two recent multicentre clinical trials conducted in the UK and France, where electrical stimulation was used on MND patients to control contractions of the diaphragm muscle that is largely responsible for breathing. Sadly, this intervention accelerated the course of the disease in these patients, leading to early death.
What challenges need to be overcome before the first successful optogenetic therapy can be realised?
There are several critical challenges remaining to successfully implement this strategy as a therapy for MND patients, or people suffering from paralysis due to other conditions, such as spinal cord injury. Firstly, to engraft motor neurons into humans, we need to be able to generate human motor neurons, which will entail genetically modifying human adult stem cells in a similar manner to the mouse stem cells that we currently use. However, these will need to be extensively tested for safety purposes before patient grafts can begin. Additionally, more sophisticated light stimulators need to be developed to control the engrafted motor neurons to help produce functionally useful muscle contraction and movement.
What are the implications of the findings of your research for the treatment of other forms of nervous pathology?
We believe that this strategy can be employed to restore artificial control over any muscle, or group of muscles, that has been paralysed due to nerve damage. This not only includes traumatic damage to nerve cells, such as in spinal cord, brain and peripheral nerve injury, but also other paralysing conditions such as stroke. In theory, this approach can bypass damage to any part of the neuronal circuits that normally control muscle function, albeit in an artificial manner.
In what direction would you like to develop this research over the next decade?
The complexity of muscle function that can be controlled using this strategy is only limited by the type of light stimulator used to control the activity of engrafted motor neurons, which could be placed at multiple sites to control groups of muscles. In conjunction with developing the human motor neurons necessary for translation of this strategy in clinical trials, we are also forging collaborations with leading engineering researchers in the field of bioelectronic and optoelectronic devices, including Prof Andreas Demosthenous and Dr Dai Jiang at UCL. This will provide light stimulator devices that are suitable for human use and capable of controlling useful function movements.
Finally, we see the ultimate application of this strategy coming to fruition through the integration of this approach with Brain-Computer Interface (BCI) technology, which enables the decoding of electrical signals from the brain and can interpret intended movements. By coupling BCI technology with our strategy, it will be possible to enable paralysed patients to exert voluntary control over their own musculature again.
• For more information about optogentics research at UCL, please visit their website at iris.ucl.ac.uk/iris/
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