MND stem cell study identifies TDP-43 astrocytes as not toxic to motor neurones

Funded by the MND Association, international researchers have used stem cell technology to learn more about the relationship between motor neurones and their support cells.

These findings highlight the potential of stem cell technology as a tool to create new human ‘in a dish’ cellular models of disease to learn more about the causes of MND.

Prof Siddharthan Chandran and Sir Prof Ian Wilmut at University of Edinburgh looking at a stem cell image

The research group included MND Association funded researchers Prof Siddharthan Chandran and Sir Prof Ian Wilmut from University of Edinburgh, Prof Chris Shaw from King’s College London and Prof Tom Maniatis from Columbia University in America.

This important finding was published in the scientific journal PNAS on 11 February 2013. This new finding follows on from previous work published by this research group in 2012 where they demonstrated the proof of principle of creating human motor neurones with MND in a dish.

Why we need an astrocyte model of MND

Astrocytes, so called because of their star-like appearance, normally act as neurone support cells to nourish and protect motor neurones. They act with motor neurones to ensure that they can continue to function.

From previous studies, we know that when these cells begin to dysfunction, they can become toxic to motor neurones to contribute to MND. Finding out why astrocytes can cause motor neurones to degenerate is an issue of ongoing debate – we recently gave an update on this from the International Symposium.

Being able to grow human astrocytes in a laboratory dish is of importance to be able to learn more about the relationship between astrocytes and motor neurones in MND.

Creating human astrocytes in a dish

Using cutting-edge stem cell technology, the research group reprogrammed skin cells into astrocytes in a laboratory dish. The skin cells were donated by people with MND who have a family history of the disease caused by known mistakes in a gene called TDP-43.

Led by Prof Chandran and colleagues, the research group aimed to identify whether these cells would develop the ‘hallmarks’ of MND in a laboratory dish.

By studying the characteristics of these human astrocytes with faults in the TDP-43 gene, the research group identified that they shared the same qualities as cells affected by MND. The astrocytes had increased levels of TDP-43 found in areas where it isn’t usually found – outside of the control centre of the cell. They also found that the astrocytes didn’t survive as long as astrocytes created from skin cells of people that didn’t have MND.

This means that the human astrocytes created by Prof Chandran and colleagues using stem cell technology develop MND-like characteristics. This new model can be used to study how motor neurones develop the disease in a system that is directly relevant to people living with MND.

Answering whether faulty astrocytes affect healthy motor neurones

The next question that this research group wanted to answer was whether these faulty astrocytes had an effect on healthy motor neurones.

By growing faulty TDP-43 astrocytes with healthy motor neurones, the research group identified that the survival of motor neurones was not adversely affected.

This was surprising as other research groups have shown that when astrocytes have faults in the SOD1 gene (which cause one in five cases of MND with a family history) that motor neurones are compromised, even if the motor neurones were originally healthy.

TDP-43 is found within tangled lumps in over 90% of cases of MND (irrespective of whether it was caused by an inherited genetic mistake). However, when MND is caused by SOD1, TDP-43 is not found in these tangled lumps. This important difference could be leading to the key difference in whether astrocytes become toxic to contribute to causing MND.

These findings will of course need to be verified by an independent research group to determine that they are valid, but the results suggests that SOD1 and TDP-43 could be causing havoc in motor neurones in slightly different ways, both avenues leading to MND.

Our Director of Research Development, Dr Brian Dickie comments: “From a therapeutic perspective this is important because it means that specific treatments targeted at astrocytes may only be relevant and effective, in specific subsets of patients who will have to be carefully selected for drug trials.”

References:

Our news release on this finding.

March 2012 finding: Association-funded stem cell study achieves milestone

Serio A et al. Astrocyte pathology and the absence of non-cell autonomy in an induced pluripotent stem cell model of TDP-43 proteinopathy. PNAS 2013

Zebrafish show that ‘connector neurons’ are the key in early stages of MND

A recent study by Motor Neurone Disease Association-funded researcher Dr Tennore Ramesh from the Sheffield Institute for Translational Neuroscience (SITraN) has shown that even before the symptoms of MND occur, at the earliest stages of the disease, ‘connector neurones’ known as interneurons are already becoming damaged in the zebrafish.

Dr Tennore Ramesh
Dr Tennore Ramesh

Zebrafish are ideal models for helping scientists understand what happens in MND. Unlike mice and fly models, zebrafish have transparent embryos which enable scientists to get a unique view of the developing neurones under a microscope! Scientists can also look at disease progression in adult zebrafish by looking at muscle strength and measuring their progress swimming against a current.

Not only are zebrafish useful for helping scientists understand what happens in MND, they are also an ideal drug screening model. Zebrafish and humans are more similar than you may think (see Kelly’s post) and potential new MND drugs can be screened quickly. Looking at how MND progresses in the zebrafish, before symptoms appear, can help us gain a better understanding of what causes the disease.

Motorways, dual carriage ways and slip roads

No, I’m not writing about travel alerts or the latest road disruptions due to flooding or snow. In fact, these road systems happen to be a perfect example of what interneurons are, how they relate to motor neurones and what goes wrong in MND.

Our body consists of two types of motor neurones, which are known as upper and lower motor neurones. The upper motor neurones are found in the motor region of our brain and connect to the spinal cord. The lower motor neurones are found between the upper motor neurones in the spinal cord and connect to the muscles (e.g. in the arms and legs). Interneurons are the vital connections between the upper and lower motor neurones.

Interneurons are the 'slip roads' between upper and lower motor neurons
Interneurons are the ‘slip roads’ between upper and lower motor neurons

When a signal is sent from our brain to bend an arm it starts by travelling down an upper motor neurone. The signal then travels to a lower motor neurone via an interneuron. When the signal from the lower motor neurone reaches the muscle in our arm it causes the muscle to contract and bend.

In MND these upper and lower motor neurones become damaged and they are unable to transport the nerve signal from the brain to the muscle in our arm. This means we are unable to contract and bend, even though the brain is telling it to.

­­­In our road system scenario the upper motor neurones are the motorways (e.g. the M1), and the lower motor neurones are the dual carriageways that link the motorways to nearby towns (e.g. the A38). In order for an upper motor neurone to send a signal (e.g. a car) to a lower motor neurone it needs to go via an interneuron, which in our road system scenario is a ‘slip road’ – making these interneurons vital connections between motor neurones.

This study has given us a better understanding of what happens in MND at the early stages of the disease (before symptoms occur). The researchers found that interneurons became damaged before the motor neurones themselves. Therefore this shows that interneurons are important in the early stages of the disease and scientists can begin to look at ways of preventing interneuron damage to see whether this has an effect on MND.

Adding more evidence to the puzzle

This study showed that, in zebrafish, interneurons are involved in the early stages of MND, which adds further evidence to previous work by another MND Association-funded researcher. Dr. Martin Turner (Oxford) also found damaged interneurons at the early stages of the disease before symptoms of MND occur in humans, with other studies showing interneuron damage in SOD1 mice models.

The next step would be to look at ways of preventing these interneurons from becoming damaged, to see whether this has any effect on the progression of MND.

This research is the first article we have paid to be made available Open Access, so that it is freely accessible to all. The article was published online in the prestigious journal ANNALS of Neurology on the 31 December 2012.

Paper reference:

McGown, A. et al. Early Interneuron Dysfunction in ALS: Insights from a mutant sod1 Zebrafish Model. ANNALS of Neurology 2012 DOI: 10.1002/ana.23780 http://onlinelibrary.wiley.com/doi/10.1002/ana.23780/abstract

Vive la difference!

It could be easy to assume that one motor neurone is pretty much like another, but a series of presentations on Thursday clearly showed that we need to be a little more sophisticated in our thinking.

Dr Hynek Wichterle (Columbia University) discussed how in the developing embryo, there are distinct regional subtypes of motor neurones in the spinal cord. For example, the motor neurones at the bottom of the spine can be easily distinguished from those at the top of the spine by the pattern of genes that are switched on and off – in a nutshell, different motor neurones have their own regional ‘postcode’.

Dr Wichterle went on to show that you can generate motor neurones with specific postcodes  in the lab, from embryonic stem cells . Having motor neurones  that  hopefully reflect different aspects of MND will allow researchers to better understand the subtleties of motor neurone function and develop more relevant treatment approaches.

It’s well known that some motor neurones are particularly resistant to the disease, such as the so called ‘oculomotor neurones’ that control eye movements. By looking at  the pattern of genes  being switched on and off in early mouse embryonic development, Dr Wichterle was able to induce the same pattern in mouse stem cells to create what look very much like oculomotor neurones in the dish. There is some further work to be done to check that they are indeed what he thinks they are, but they could be a very important tool in helping to understand why some motor neurones are tougher than others.

Continuing the theme, Dr Georg Haase  (Marseille) introduced an elegant way of separating different populations of motor neurones from mouse spinal cord. He focused on two subtypes of motor neurone – those that connect to the limb muscles  and those that connect to the main trunk of the body. After they are separated out, they can be grown in culture (in a dish in the lab) . He then showed that these two different subtypes of motor neurone acted very differently in their response to chemicals known as neurotrophic factors . The word neurotrophic can be literally translated as ‘nerve nourishing’ and these chemical compounds have attracted a lot of attention in the past as possible therapies for MND, with a couple being tested in large clinical trials . Unfortunately these trials have not shown any effect, but this could be because  – as Dr Haase showed in his lab studies – each neurotrophic factor only acts on a proportion of motor neurones. He suggested that the different ‘survival profiles’ he sees in his cellular studies  provide a rationale for selecting combinations of different neurotrophic factors for further testing in mice – and hopefully in man.

Prof Pam Shaw (University of Sheffield) also asked the question of what makes  oculomotor neurones  different, but her presentation took us from mouse to man. Using a technique called laser capture microdissection, which allows individual motor neurones to be removed from post mortem spinal cord tissue form MND patients, she and her colleagues are able to examine which genes  were switched on and off in the cells. Research like this requires very high quality tissue, removed shortly after death, but Sheffield happens to host one of the best MND tissue banks in the country.

She compared these ‘gene expression patterns’ in the disease-resistant oculomotor neurones with the more vulnerable motor neurones  from the lower spinal cord. There were  considerable differences in the patterns, with a much larger number of genes being switched on in the oculomotor neruones. Many of these genes could be dividied into functional families, which act on a similar biological pathway. Key protective processes activated included neurotransmission, mitochondrial function and proteasomal function – all of which are strongly implicated in MND. If human oculomotor neurones  survive by gearing up these  protective pathways, it provides possibilities that drugs can be found which act in the same way.

Our International Symposium website news stories:

International Symposium closes in Chicago

International Symposium focuses on clinical trials

International Symposium focuses on carer and family support

International Symposium begins in Chicago

Researchers unite at our International Symposium on MND

After you’ve finished reading the symposium articles that interest you, we’d be grateful if you could spare a few minutes to fill in our short online survey on our symposium reporting. Your comments really are useful and allow us to continually improve our symposium reporting. surveymonkey.com/s/alssymp