Identifying the role of NMDA receptors in STP through investigation of synaptic plasticity and discovery of novel subtype-specific antagonists

The purpose of the proposed research is to make new chemicals that can be used as tools to study the mechanisms by which the central nervous system (CNS) exercises its multiple functions. Communication between nerve cells occurs at specialised zones called synapses. 'Messages' are sent across the synapse by the release of a chemical called a neurotransmitter. Nerve impulses in the pre-synaptic cell are conveyed 'through' the synapse by the release of neurotransmitter which binds to specialized proteins (called receptors) situated on the surface of the recipient (or postsynaptic cell) which generate small electrical impulses when activated by neurotransmitter. One such neurotransmitter is glutamate, which is present at synapses throughout the CNS. Our previous research helped establish that glutamate can interact with a family of structurally related glutamate receptor subtypes each performing different functions in the CNS. The present work aims to design and make novel chemicals (pharmacological tools) that modulate the action of glutamate at particular glutamate receptor subtypes. One particular subtype of glutamate receptor, the N-methyl-D-aspartate receptor (or NMDAR for short), named after the chemical that selectively activates it, is a tetramer made up of two types of protein subunit known as GluN1 and GluN2. In most areas of the CNS NMDARs are comprised of two GluN1 and two GluN2 subunits. Four GluN2 subunits are known, GluN2A-D, and they can be combined with GluN1 to form tetramers of different subunit composition, which are differentially expressed in the brain. It is now well established that the strength of synaptic connections between nerve cells can be regulated (a process known as synaptic plasticity). Synaptic plasticity has been demonstrated experimentally in many areas of the brain including the hippocampus, a part of the brain known to be important in learning and memory. Two major forms of plasticity are long-term potentiation (LTP), which facilitates connections, thereby enhancing communication between neurons and long-term depression (LTD), which decreases neuronal connections and communication between neurons. NMDARs are known to be involved in the mechanisms of LTP and LTD in the hippocampus. We have recently identified that different NMDAR subtypes are differentially involved in various forms of synaptic plasticity that may underline specific forms of learning and memory. What is less clear is the precise role of individual NMDAR subtypes, due in part to the lack of specific pharmacological tools that can discriminate between NMDARs with different GluN2 subunit composition. We have developed a series of novel inhibitors that can modulate NMDAR function either by binding to the same site as glutamate (competitive inhibitors) or binding to a site(s) on the receptor away from the glutamate binding site (allosteric inhibitors). The inhibitors that we have developed have various patterns of GluN2 subunit selectivity and are leads for the development of newer compounds with improved GluN2 subunit selectivity. We plan to develop and use NMDAR inhibitors with the desired GluN2 subunit selectivity to investigate the specific roles of particular NMDAR subtypes in LTP and LTD in the hippocampus. The major focus of this work will be to investigate an early form of LTP known as short-term potentiation (STP) which we hypothesise is important for short-term memory. We will test this hypothesis with behavioural experiments in collaboration with a laboratory in Korea.

NMDA receptors (NMDARs) are ligand-gated ion channels that belong to the glutamate receptor (GluR) family and are expressed throughout the central nervous system (CNS). In most areas of the CNS NMDARs are comprised of two GluN1 and two GluN2 subunits. The GluN2A-D subunits can combine with GluN1 to form functional tetramers of different subunit composition (or NMDAR subtypes), which are differentially expressed in the brain. NMDARs have been identified as key players in synaptic plasticity and many other CNS functions. We have recently discovered that different NMDAR subtypes are involved in different temporal components of long-term potentiation (LTP). Surprisingly we found that an early component of synaptic potentiation, termed short-term potentiation (STP), comprises two distinct components (which can be separated pharmacologically). This discovery is likely to impact enormously on our understanding of the roles of NMDARs in learning and memory. However, to explore this further we need more selective NMDAR antagonists. In this regard, we have recently developed a number of competitive antagonists and negative allosteric modulators of NMDARs with novel patterns of GluN2 subunit selectivity. Inhibitors with better GluN2 subunit selectivity will be developed using structure-activity relationship data combined with receptor structure based and ligand based drug design. These optimised compounds will be used for the identification of NMDAR subtypes in STP, and other forms of synaptic plasticity, and to identify the role of STP in learning and memory.

Understanding how the brain works by regulating neuronal activity is a topic of great public and academic interest. The extensively studied NMDA receptors (NMDARs) play a central role in synaptic plasticity and in several neurological disorders. Better understanding of the structure of NMDARs and their properties created exciting new opportunities for rational drug design and the development of new, more potent and selective pharmacological reagents for the analysis of endogenous NMDARs and their complex interactions in their native environment in neurones. This project is thus timely and should have major impact at both academic and social levels. To deliver that impact, it is important that the research is disseminated as effectively as possible. The target audiences are academics, pharmaceutical industry, health professionals, schools and the wider public.
ACADEMICS: The various ways chemists, structural biologists, neuroscientists, neuropharmacologists, postdoctoral scientists employed on the project and the very wide range of undergraduate and postgraduate students benefit from this project are described under 'Academic beneficiaries'.
PHARMACEUTICAL INDUSTRY: Mutually beneficial collaborations with industry are an integral part of the way that we achieve our aims. DJ has worked as a consultant for Eli Lilly's project to develop kainate receptor antagonists for the treatment of a range of neurological disorders. DJ, GC and DL have been members of Lilly's Centre for Cognitive Neuroscience (CCN) since its inception in 2008. The CCN is a major collaborative venture between UK based academics and Lilly, the one remaining Pharma with a major UK presence in the area of neuroscience. DJ and GC have close links with biotechnology companies (Abcam and Tocris Biosciences). These established collaborations will be used to commercialise and distribute new research tools developed during this project (novel subunit selective competitive antagonists and negative allosteric modulators for NMDARs) to the wider research community. Furthermore, there is significant interest in NMDARs by industry as potential drug targets for epilepsy, schizophrenia, neurodegenerative disorders, migraine and chronic pain therefore our results are likely to have an impact in this area.
HEALTH PROFESSIONALS: Because NMDARs have been linked to a range of neurological disorders where effective treatment is currently unavailable, they offer inviting possibilities for therapeutic exploitation. Two channel blockers are used clinically, memantine (moderate-severe Alzheimer's) and ketamine (dissociative anaesthetic) and some inhibitors are used as a drug of abuse (ketamine and PCP). A much better understanding of NMDAR pharmacology and the role of NMDARs in physiological processes will be essential for the development of new therapies and understanding the underlying mechanisms behind the use of inhibitors as drugs of abuse. Therefore, the medical community will also benefit from any new developments in the field.
SCHOOLS: The future of science depends on enthusiastic young scientists. All applicants are involved in University open days, making explanation of science accessible to prospective students and parents. For instance, DJ has developed an interactive tutorial "drug molecules in 3D" through which school children can learn about structures of drug molecules and how they interact with their protein targets.
THE WIDER PUBLIC: All applicants are committed to engage public interest and to shape public perception about the benefits of scientific discovery. We will take every opportunity to directly engage the public and schools through initiatives coordinated by the Bristol University Centre for Public Engagement, Bristol Neuroscience and by local attractions (e.g. the science activity centre '@Bristol'). Our published findings will be promoted to the public through Bristol Neuroscience, University web sites and the University Press Office.