Investigating Ionotropic Glutamate Receptor Interfaces as Novel Drug Targets.

A key goal of modern medicine is to develop better therapeutic approaches for the treatment of diseases and conditions associated with the brain. This is one of the most challenging goals of medicinal science, hampered in part by the complexity of the brain itself. Another major stumbling block is the problem of developing new drugs that do not have severe side-effects, an issue often cited as one of the reasons why patients stop taking their prescriptions.

The transmission of nerve signals in the body and brain is dependent on proteins called receptors. Most of this neurotransmission that governs memory and learning is controlled by ionotropic glutamate receptors, so-called, because upon binding of glutamate (the neurotransmitter) they open a pore that passes through the membrane of the neuron and allows positively charged ions (sodium and potassium) to pass through it. This is the basis of nerve signals in the brain. It is therefore perhaps unsurprising that glutamate receptors have been implicated in many neurological conditions of the central nervous system (CNS) ranging from epilepsy to Alzheimer's disease. There are many different subtypes of glutamate receptors and it appears that certain subtypes of these receptors are implicated to different extents in different neurological conditions. Therefore, to avoid unwanted side-effects, drugs that act at these receptors should be as specific as possible. The problem is that they all bind the same neurotransmitter, glutamate, and therefore this is actually quite difficult.

The differences between the subtypes manifest themselves away from the glutamate-binding site, and is apparent in terms of some of the properties the receptors exhibit. For example, how quickly they open or how long it takes for them to close, can be controlled by regions of the receptor away from the binding site. These regions are sometime referred to as allosteric sites and because they have not been subjected to the same evolutionary pressure as the main (orthosteric) binding site offer a potential route to greater specificity. In other words, our chances of obtaining greater specificity with compounds at these sites should be much greater than at the glutamate-binding site.

In our previous work, we were able to show how, for one particular sub-family of receptors called kainate receptors, a region away from the glutamate-binding site could control the dynamic properties of the receptor. In particular, this region contains separate binding sites for sodium and chloride ions. In this proposal we'd like to not only explore this aspect further, but also to investigate the possibility that new drugs with fewer side-effects can be targeted to this region. By targeting in this way, we are hopeful that we can develop new improved therapies for the treatment of epilepsy and neuropathic pain.

Our proposal utilizes the power of molecular simulations to provide atomic-level detail of what controls the way this binding site behaves. A full understanding of this is necessary if we are to have any chance of developing compounds that act in a predictable way. The results we will generate will be verified and tested by our collaborators in McGill University in Canada.

Ionotropic glutamate receptors (iGluRs) mediate nearly all of the fast neurotransmission in the brain and CNS and consequently are intimately linked with the processes of memory and learning. Given this, it is perhaps unsurprising that they have been implicated in a wide-range of neurological conditions ranging from stroke through to Alzheimer's. Researchers have long since recognized that these receptors could represent routes to potential therapies. However, the field has been hampered considerably by the problem of unwanted side-effects. Therefore there is clear clinical need for better, "cleaner", more-specific compounds that target the correct sub-population of receptors.

Previous research has shown that in one subtype of iGluRs, the kainate receptors (KARs), has an allosteric site that binds cations and anions and is directly associated with controlling the receptor moving from the open state into the desensitized state. Recently we also showed how the integrity of this site is controlled at the atomic level using molecular dynamics simulations in conjunction with single-channel experiments and site-directed mutagenesis. Targetting this allosteric region (ie away from the glutamate binding site) opens up new possibilities for the design of new compounds that can take advantage of differences in this region between subtypes and thus will have greater specificity.

Here we are proposing to extend our preliminary work on the factors that govern the behavior of this allosteric region, and to explore its potential as a drug-binding site via the use of molecular simulation approaches in conjunction with single-channel experiments and site-directed mutagenesis performed by our collaborators. This approach already has demonstrated proven success and we are hopeful that ultimately, new therapies can arise from this work.

The work proposed here, which is aimed at understanding how ionotropic glutamate receptors work and whether drug compounds can target them in a new way, is very much at the level of basic research. Consequently, the impact when it arises, will take time to come to fruition. There will ultimately be many different beneficiaries of the research outside of the academic circle. The most obvious group will be the general public and in particular patient groups who are suffering from neurological conditions. Tthis group would specifically include sufferers of epilepsy and chronic pain, but by extension to other subtypes, would include other neurological conditions. Thus there is a direct impact on the nation's health. Our prior work has already produced important information about the dynamics of these receptors and that understanding is critical to us designing better and improved drugs in the future.

Aside from this group, the main beneficiary will be the pharmaceutical industry, which although currently going though a period of upheaval and rationalization, particularly for neuroscience programs, will still benefit from early basic-level research that puts any drug discovery process on a firmer footing. The drug-design process is still extremely difficult and time-consuming. Drugs are developed at vast expense, typically by screening large numbers of compounds and using large numbers of experimental animals. Only a handful of drugs have been discovered by designing them to fit a particular protein target. For channel proteins like ionotropic glutamate receptors, there are two key stumbling blocks: The issue of side-effects - ie making the compounds selective enough that we don't "hit" receptors that are functioning normally and secondly that actually we don't understand channel function well enough, especially with regards to how they change in shape when they are functioning in the body, regardless of disease state. It will take a long time to get to design drugs mostly in silico, but our approach outlined here have the demonstrated power to take this forward in the right direction. The UK has a big tradition and massive knowledge in drug discovery, partly because much of the basic science discoveries occurred here.

Thus, the basic research we are proposing here will, in the long term, benefit our society through its impact both on human health and well-being (drug discovery, better understanding of physiological and pathological processes) and on economic productivity (development of novel drugs). These processes will take a long time - the drug discovery process is usually between 10 and 20 years, but the impact can be long lasting and life-changing