New research by Prof David Klenerman, UK DRI Group Leader at Cambridge, and involving two other UK DRI centres, has provided better understanding of toxic protein aggregates and the disease mechanisms by which they cause damage to the brain. This new knowledge may help with the development of effective treatments for those living with dementia.
Diseases associated with the development of dementia often share a similar characteristic in the build-up of misfolded protein aggregates. In Alzheimer’s disease, the most common form of dementia, aggregates of amyloid beta (Aβ) can be seen surrounding the neurons in the brain of those living with the condition. In Dementia with Lewy bodies (closely related to Parkinson’s disease), the aggregates consist of the protein alpha-synuclein (αSyn) and form inside neurons.
Although decades of research heavily suggest these proteins play a role in disease, the exact mechanism by which this occurs, and at which stages, is still unclear. Greater understanding may open up new avenues for therapeutics.
The first of the collaborative research studies, published in Communications Biology, aimed to bring greater understanding of the molecular mechanisms behind the death of neurons and subsequent memory loss in Alzheimer’s disease. This was done in collaboration with Prof Kei Cho (UK DRI at King’s) who has extensive expertise in neuron-neuron communication.
Using cells grown in dish and slices of rat brain tissue, the team found that:
- Aβ aggregates cause an inflammatory response in two types of immune cell found in the brain – microglia and astrocytes.
- This is mediated by a receptor called Toll-like receptor 4 (TLR4) found on the immune cells.
- Aβ aggregates cause problems in the electrical signalling of neurons to one another, which can be reduced by blocking TLR4.
- Aβ aggregates cause extensive neuronal cell death, when in the presence of astrocytes, which can be reduced by blocking TLR4.
The results indicate that Aβ aggregates similar to those found in the brains of people with Alzheimer’s disease, cause an inflammatory response in immune cells through the receptor TLR4. This response appears to subsequently cause problems with neuronal signalling and eventual cell death. If this pathway of events is found to be a prominent mechanism in the Alzheimer’s brain, treatments could be developed to target it.
A Q&A with Prof David Klenerman exploring the science in more depth can be found further down the article.
UK DRI centres involved in the two studies
The second study, published in Brain Communications, aimed to identify the exact species or ‘form’ of αSyn that may be toxic to neurons in diseases such as Dementia with Lewy bodies. Using postmortem brain tissue from people who had the disease and healthy individuals for comparison, Dr Tim Bartels (UK DRI at UCL) and his team extracted different forms of the αSyn protein, for example soluble forms found inside the cell and insoluble aggregated forms. This study was done in collaboration with Prof David Klenerman.
The team found that:
- Important differences exist between the form and location of αSyn depending on the severity stage of disease, as measured by pathology.
- Using stem cells, extracted soluble αSyn causes increased permeability of cell membranes which could contribute to eventual neuronal death.
- Using stem cells, extracted insoluble aggregated αSyn causes neurons to change shape and structure, which again could contribute to eventual neuronal death.
The results show that substantial changes in the form of αSyn occur over the course of disease, and that these different forms may have toxic effects on neurons. This knowledge may be used to identify new therapeutics that target toxic αSyn or rescue dying neurons in diseases such as Dementia with Lewy bodies.
A Q&A with Dr Tim Bartels exploring the science in more depth can be found further down the article.
To find out more about how these partnerships came about and advice on how to find and maintain good collaborations, please see our associated Q&A with the three Group Leaders – "Collaborations are like experiments; it takes a few false starts before you set up a good one."
Q&A with Prof David Klenerman on
‘Beta amyloid aggregates induce sensitised TLR4 signalling causing long-term potentiation deficit and rat neuronal cell death’
Q: What do you see as the best target along this pathway?
A: The inflammatory aggregates are hard to target selectively due to the fact they are made from monomer and look like fibrils. In contrast, TLR4 is a single receptor at the start of a signalling cascade so preventing this cascade rather than the multiple downstream pathways seems a good strategy.
Q: In contrast to the previous literature, you work with very low aggregate concentration in this study. How important is the use of unphysiological conditions in research?
A: In my view this is very important. The events that occur at low physiological concentrations are likely to be the early disease. Ideally one works at these concentrations, or if not, at as low a concentration as possible, and then extrapolates to physiological concentrations. The concentration differences are not small - they are factors of 1000-1 million - so of course it is easier to see large effects quickly at high concentrations. Events may happen very differently and rapidly at such acute doses and be irrelevant to a disease that is characterised by small changes over long times.
Q: What are your future plans for this project?
A: On the basic research side, we want to study how the response of TLR4 to more than 100nm long protofibrils differs to the conventional paradigm of the molecule LPS which is from Gram negative bacteria. We think it will involve several TLR4 receptors and the formation of multiple Myddosome signalling complexes. To do this we will follow the formation of the Myddosome signalling complex in live cells in response to Ab aggregates, both synthetic and brain-derived. We also want to understand how the sensitised response that we observe occurs. There is literature to suggest that it is epigenetic - this may help identify other places for interventions. On the therapeutic side, we have a project to develop antagonists to TLR4 funded by Apollo Therapeutics and hope to have lead compounds with suitable affinity, selectivity and toxicology this year. We would be keen to see if these are effective at blocking LTP with the help of Kei and start testing in suitable in vivo models.
Q&A with Dr Tim Bartels on
‘Analysis of α-synuclein species enriched from cerebral cortex of humans with sporadic dementia with Lewy bodies’
Q: How important is it to validate findings in human tissue?
A: From my perspective, model aggregates obtained from recombinant sources are useful to set up systems and mechanistic models. However, with proteins like αSyn that form a variety of different aggregates dependent on the local environment, human samples are still the gold standard and the only way to ensure that results are actually relevant to the patient.
Q: Do you believe microheterogeneity to be a significant issue when looking at brain tissue?
A: The challenge of working with human tissue is not only the strong case-to-case variability, but also the variability of different tissue pieces from the same brain and region. Tissue samples taken 1mm apart from each other can differ in the amount of aggregate present by orders of magnitude. That usually means that high case numbers and multiple orthogonal assays are needed so we have to work closely with the respective pathologists to interpret the results accurately. In this study we worked with a several people including Dennis Dickson at Mayo Clinic and Zane Jaunmukatane and Tammaryn Lashley at Queen Square Brain Bank.
Q: Is it surprising that, in relation to soluble aggregates, Dementia with Lewy bodies appears to share similar disease characteristics to Alzheimer’s disease. What could this mean for therapies going forward?
A: It means that we should work closer with scientists from the Alzheimer’s disease field since Dementia with Lewy bodies is indeed somewhat a mix of Parkinson’s and Alzheimer’s disease. I am currently part of a larger collaborative project, National Institute of Health’s Dementia with Lewy bodies Center Without Walls, trying to figure out the synergistic actions of Aβ and αSyn. We are at the early stages of trying to address this problem.
Q. What are your future plans for this project?
A: In my mind, the most important questions going forward are addressing the brain region specificity of the aggregates and the differences in aggregate population in the synucleinopathies Parkinson's disease, Dementia with Lewy bodies and Multiple System Atrophy. Taken together, these will allow us to understand why these diseases have different clinical phenotypes and show degeneration in different brain areas. We are currently working on these topics in my lab together with Queen Square Brain Bank and the DZNE in Bonn, Germany. Also, improving aggregate species detection more specific to the disease would allow for the development of differential biomarkers for synucleinopathies - something that is sorely needed.
Banner image of pathology associated with Dementia with Lewy bodies courtesy of Dr Tim Bartels
Article published: 02 June 2020