Professor D Middleton
Compatibility rules for glycosaminoglycan-amyloid interactions
Several human diseases are associated with plaques which accumulate in tissue and are thought to contribute to organ damage. The most famous example of these so-called amyloidoses is Alzheimer’s disease (AD), in which the plaques identified in post-mortem brains are localised at areas of neuronal degeneration. What is less widely known is that similar plaque deposits are found in many other diseased and healthy organs including, for example, the pancreases of individuals with type II (“late-onset”) diabetes.
Amyloid was discovered over 150 years ago and the word – literally “starch-like” – was defined in recognition of the early belief that the plaques were made up of starchy carbohydrate sugars. It was later discovered that the amyloid plaques are actually deposits of fibrous proteins, but more recent evidence has vindicated the early medical pioneers and shown that amyloid is, in fact a complex mixture containing both proteins and carbohydrates called glycosaminoglycans – or GAGs. It is now known that GAGs can influence the rate at which amyloid proteins can accumulate and, importantly, can have a profound effect on amyloid toxicity.
In seeking to unravel the causes of amyloid disease, we are striving to understand how normally benign proteins of different chemical composition mis-assemble to form plaques. Thanks to sophisticated analytical techniques we now have detailed all-atom models of the multitude of interactions within amyloid fibrils/plaques. However, how GAGs alter or facilitate amyloid assembly and interact with amyloid fibrils is known, questions that are crucial to amyloid disease formation and propagation. Recent work by the applicants has produced the first experimental evidence unveiling the intimate interactions between a protein and a GAG in a fibrous amyloid deposit. This was made possible using NMR (nuclear magnetic resonance) spectroscopy, to provide atomic-level information of this crucial binding interaction.
Despite this importantbreakthrough, we are still desperately short of information on how different proteins and GAGs interact with each other in disease-related amyloid. One example is not enough. GAGs, like their amyloid fibril protein partners, are chemically variable and we do not know whether proteins and GAGs in one type of amyloid, such as in AD, interact in the same way as in another type of amyloid, such as in type 2 diabetes. In this application we propose to use NMR and other techniques to elucidate at the molecular level how a series of chemically different naturally occurring GAGs, and model GAGs of defined composition, interact with the two main proteins found in AD plaques and the main protein component of the pancreatic plaques in diabetes. We will ascertain whether each protein favours a particular type of GAG to form plaques, and whether there are general rules dictating the way in which the fibrils interact with GAGs. Furthermore, we will determine whether and how GAGs orchestrate the process of protein assembly, or whether they are just molecular “passengers”.
Endeavours to investigate the molecular structures of proteins and other biomolecules have provided information that has been invaluable in the discovery and design of pharmaceuticals and healthcare products. In amyloid research there is an intensive international effort to develop drugs which prevent or alter the way in which proteins assemble into organ-damaging plaques. By revealing how GAGs interact with proteins as they assemble into plaques, and how they bind and stabilise the final fibrillar assembly, the results generated in this proposal will help us to design molecules that mimic these interactions for use as drugs or as agents to help diagnose disease. As many amyloid diseases affect the elderly, our results could have far-reaching consequences for the quality of life of millions of people and the burden on healthcare resources in an ageing population.