Calcium-permeable AMPA receptors and their auxiliary subunits: pharmacological and molecular intervention in health and disease

All of our sensations, thoughts, movements, emotions, and memories are produced by the electrical activity of neurons in our nervous system. The transfer of information between these cells relies on the release of a chemical signal at specialised points of contact - synapses. In the mammalian brain glutamate is the main excitatory transmitter, released from one neuron to activate glutamate receptors embedded in the postsynaptic membrane of neighbouring cells. By this means, electrical nerve impulses are transmitted between neurons.

While several different types of glutamate receptor exist within the brain, signalling by one family - the AMPA receptors (AMPARs) - is central to virtually all brain functions, being responsible for most fast signalling, and longer-term processes intimately associated with changes in the function of synapses and circuits. One of the defining features of the healthy brain is its ability to retain information arising from experience. Changes in AMPAR properties are important in the laying down of such memories. This process involves alterations in the number or efficiency of AMPARs, resulting in long-lasting changes in synaptic strength.

One important sub-family of AMPARs - the calcium-permeable AMPARs (CP-AMPARs) - plays a crucial role in many forms of synaptic plasticity. Their ability to allow calcium entry into cells is essential in triggering rapid chemical processes that cause long-term changes in AMPAR number, subtype and properties. In the healthy brain, activation and regulation of CP- AMPARs is a key part of normal transmission. However, forms of synaptic dysfunction that involve inappropriate calcium influx can be toxic. Indeed, failure to correctly regulate these CP-AMPARs underlies a number of neurodegenerative conditions, including neuron death following stroke, glial cell damage in infants starved of oxygen, and neuron loss in motor neuron disease. Increased expression of CP-AMPARs also underlies certain chronic pain syndromes and psychiatric disorders, including addiction. There is therefore an urgent need to understand the cellular and molecular mechanisms governing CP-AMPAR regulation - to reveal potential strategies that could be used in treatments.

AMPAR properties are dictated by their core building blocks, and also by a large number of associated proteins, many of which have only recently been discovered. Our work, and that of close colleagues, has identified some of the protein partners governing the regulation of CP-AMPARs involved in normal synaptic plasticity, and in deleterious changes. One important recent discovery, highly relevant to our work, has been the identification of novel and exciting AMPAR subtype-selective drugs that act by targeting auxiliary proteins. These suppress transmitter activation of AMPARs only when the receptor is associated with a specific type of auxiliary protein. These drugs therefore offer a powerful means of identifying those types of auxiliary subunits involved in regulating detrimental forms of CP-AMPAR plasticity, and a means of selectively suppressing dysfunctional CP-AMPAR activity. At the same time, recent work on the structure of AMPARs physically associated with their auxiliary subunits offers the possibility of real insight into understanding the structure of the binding site occupied by these highly selective drugs.

We plan to capitalise on these important new developments to elucidate the roles played by auxiliary subunits in CP-AMPAR plasticity in healthy brain, and in neurological disorders where regulation of CP-AMPARs appears to be a prominent feature. This will provide insight into ways in which harmful and damaging effects of CP-AMPAR plasticity may be suppressed or blocked.

AMPARs mediate fast excitatory transmission throughout the brain, and their dynamic regulation is essential in the expression of synaptic plasticity. The calcium permeable AMPAR subtype plays an important role in the expression of normal synaptic plasticity. In addition, its increased expression or activation is implicated in several serious neurological conditions and deleterious forms of plasticity. Our programme aims to understand the molecular mechanisms, particularly the role of auxiliary subunits, in normal regulation of CP-AMPARs, and to examine their potential for manipulation in damaging forms of synaptic plasticity.

We will use molecular biology, patch-clamp recording, structural biology and in vitro and in vivo pharmacology to address the following. First, we will use a novel intracellular NASPM-based technique to accurately monitor CP-AMPAR prevalence in living neurons. Parallel experiments will use a powerful fluorescent labelling approach to track the precise location of known TARP subtypes in the membrane of living cells. Second, we aim to elucidate how the newly discovered TARP-selective drugs alter AMPAR channel function and examine their utility as tools to dissect AMPAR-TARP interactions. Third, we will use newly developed structural and functional methods to identify auxiliary subunit C-tail residues critical in regulating CP-AMPAR plasticity. Fourth, we will examine auxiliary subunits mechanisms that drive CP-AMPAR plasticity under normal and deleterious conditions, and use novel TARP-selective drugs to intervene in these processes. Specifically, we will examine the identity of the auxiliary subunits underlying CP-AMPARs plasticity in ischemic damage, and the auxiliary subunits that drive CP-AMPAR plasticity in hyperalgesia, and the influence of TARP-selective drugs. We expect to obtain insight into the regulatory roles of auxiliary proteins, and reveal targets with translational value in enabling selective manipulation of CP-AMPARs.

AMPA-type glutamate receptors (AMPARs) are responsible for mediating most of the excitatory signalling between nerve cells in our brain. Consequently these receptor proteins, which are essential for normal cognition, shape many of our brain processes. Any failure or defect in their function gives rise to serious neurological, psychiatric and cognitive disorders. One specific type of AMPAR is of particular importance in this respect. Although normally benign (and indeed required for laying down memory and other related forms of plasticity) under certain conditions calcium-permeable types of AMPARs (CP-AMPARs) permit excessive deleterious calcium entry into nerve cells. This defective form of plasticity is thought to underlie a number of neurodegenerative conditions.

Many different types of AMPAR-associated molecules are present in the nervous system, offering attractive targets for selective drug treatments. We anticipate that identifying potential methods by which deleterious CP AMPAR plasticity may be suppressed will ultimately benefit our understanding of neurodegenerative and other conditions, including nerve cell (brain) damage following stroke, and chronic pain that accompanies inflammation.

Approximately 100,000 patients suffer strokes in the UK each year, with a cost to the NHS and to social care services of roughly £1.7 billion (per annum) as a result of subsequent brain damage. Hyperalgesia - an enhanced pain response - is one of several forms of chronic pain that represent a major and debilitating clinical problem. The social and personal cost of these conditions is, of course, immeasurable. We anticipate our Programme of work will reveal fundamental molecular mechanisms relevant to these conditions, and will help identify potential drug targets. By contributing to enhanced health and well-being, this will provide valuable societal and economic impacts in the longer term. Furthermore, we anticipate our studies will inform drug development and facilitate rational approaches to therapy.

If data from our work suggest the basis of novel therapies, we are well placed at UCL (through UCL Enterprise and UCL Business PLC) to participate in commercialisation and development of potential future treatments. We are in close contact with UCL's Contracts Department who act as liaison between UCL academics and Industry. In the event that our experiments generate information of potential commercial significance we will immediately inform colleagues in these appropriate UCL departments. An appropriate intellectual property strategy is in place in the Host Institution (UCL) to optimize the chances of downstream funding, partnering, and ultimate exploitation and utilisation.