Report by Chris McMurran, PhD Student, Franklin Lab - 20/7/16
Myelin sheaths are an essential component of our nervous system. Whilst neurons send electrical messages to one another, allowing us to feel, think and move, the sheaths wrap around these “cables”, providing electrical insulation and aiding their function. Myelin sheaths become damaged in inflammatory diseases such as Multiple Sclerosis or in genetic diseases including the leukodystrophies. The symptoms of these can be devastating for the patient and their loved ones.
We know that in some cases myelin sheaths regenerate in a process called remyelination, and this can help to restore function. However, remyelination is not always successful - for example, the work of our lab and others has shown that the regeneration becomes less efficient in old age. The ongoing goal of our lab is to understand the biology of remyelination, and thus provide novel ideas for therapies that can be taken into the clinic.
One avenue we are currently exploring is the influence of the many microbes that live in our gut, skin and elsewhere: the microbiome. The number of bacteria that inhabit our bodies in fact outnumber the number of human cells, so these can have substantial effects on our physiology. For example, gut microbes help us to digest certain nutrients, and are important for the development of our immune system during childhood and throughout adult life. Several papers in the last year have linked gut microbes to some of the cells that are necessary for remyelination. Our project aims to discover whether the gut microbiome is a viable target for enhancing remyelination.
Antibiotics and inflammation during remyelination
Work of Ofra Zidon
Most of our studies on remyelination use mice or rats, as these mammals, like humans, rely on myelin in their nervous systems. In the initial studies into the microbiome we gave mice antibiotics to deplete their bacteria. This showed some interesting changes when the mice were remyelinating.
In particular, there were changes in inflammatory cells called microglia, which we know to be important for remyelination. Efficient remyelination requires a shift from “M1” microglia to “M2” microglia, which have different properties. Treating with antibiotics caused a reduction in M1 microglia (Fig 1), which is associated with improved remyelination.
Figure 1: Antibiotic treatment effects inflammation. Microglia are inflammatory cells that help to coordinate remyelination, and are identified by a protein Iba1. Antibiotics did not alter the number of microglia, but caused a reduction in the “M1” markers iNOS and MHCII, which are associated with poor remyelination.
Antibiotics and stem cells during remyelination
Work of Yvonne Ofra Zidon, Chris McMurran and Yvonne Dombrowski
Whilst microglia are important cells in providing the right environment for remyelination, the central process involves oligodendrocyte progenitor cells (OPCs). These are stem cells in the brain, which can differentiate to oligodendrocytes and produce new myelin, allowing recovery. This differentiation is essential for remyelination to take place, and is often the limiting step when remyelination fails.
We looked at how well OPCs differentiate after mice are treated with antibiotics to deplete the bacteria. Our results show that the antibiotic treatment impairs the ability of OPCs to differentiate, suggesting that a healthy microbiome might be essential for efficient remyelination (Fig 2).
Other possible explanations for these results are that antibiotics are having a direct effect on remyelination, rather than through changes in the microbiome. However, experiments using isolated microglia and OPCs in the laboratory have shown that these antibiotics have little effect on these cell types at the doses used. This finding gives further weight to our microbiome hypothesis.
Figure 2: Antibiotic treatment effects stem cell differentiation. All OPCs are identified by the marker Olig2, but only display the marker CC1 when they differentiate. Cells from an area of the spinal cord that was remyelinating (red) showed a reduction in differentiation in mice treated with antibiotics.
Plans and preliminary work for germ-free study
Work of Chris McMurran
The gold standard for microbiome research is to use a germ-free model. Mice live inside special sterile isolators (Fig 3) so never become colonised with microbes. Whilst costly, this gives several advantages over our antibiotics treatments. It will rule out any possibility of off-target effects from antibiotics. Additionally, any differences will be stronger, so we will be able to tell for certain whether the microbiome truly has an effect. Finally, as the animals are microbe-free from birth, we will be able to tell whether any differences we see occur during development, or if they are maintained throughout adult life.
On the basis of our results using antibiotic treatments, we have arranged a collaboration with the Institute for Food Research in Norwich, which gives us access to these isolators. The past few months have been spent running a preliminary study in preparation for this, ensuring that the model we are using will cause demyelination and remyelination for us to investigate. The start date for the full study will be in early August, and we hope to have some interesting results to discuss in a few months time.
Figure 3: A germ-free isolator, similar to what will be used for our study.
Our results so far suggest that the microbiome may be an interesting target for influencing key processes in remyelination: inflammation and stem cell differentiation. The microbiome is an attractive target for treating human clinical disease as it can be easily manipulated by antibiotics, or by introducing new bacterial strains as an isolated culture or through faecal transplant. Our germ-free study will show truly whether the microbiome is important for remyelination, and whether these effects are restricted to development or continue to be useful therapeutic targets throughout adult life.