Achieving synaptic stability: An investigation of processes that maintain glutamate receptor clusters at synapses

Memories, it is said can last a lifetime and yet the very molecules that form the substrate for memory are proteins that are highly labile and have a lifespan far shorter than the memory they represent. While one might imagine that if a molecule became damaged and was degraded by the cell, the solution would simply be to replace it with another molecule just like it, this is far from straight forward as the storage of a memory is thought to require these molecules to be arranged in precise patterns, with certain numbers of molecules at each point in the pattern. Thus, it is necessary to return the degraded molecule from whence it came. It would seem that what is needed is a memory for memory! As this is rather absurd then an alternative needs be sought. Ideally, a mechanism is required that self stabilizes once the memory is established. Importantly, any mechanism must have the capacity to show some adaptability, as memories are not always robust and can be augmented but can also wain. Recently, we have been exploring some theoretical ideas about the types of mechanism that might enable neurons to generate memories that become stable. Our ideas incorporate some less commonly considered features of biological processes and accept that the biological building blocks degrade and need to be replaced on a time scale far more rapid than the loss of memory. In our current work we are looking to test our theoretical ideas with experiments designed to determine whether stability arises from the behaviour of populations of molecules not the activity of any single element.

Plasticity driven changes in synaptic strength can be achieved in a number of ways including a change in the number of neurotransmitter protein receptors, specifically alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors (AMPARs) within the postsynaptic membrane of a stimulated neuron. While increasing the number of receptors at the synapse will increase transmission efficacy, this change represents just one step in understanding how a long-term memory is actually stored. One reason for this is that individual receptors have limited lifetime that is they are recycled into and out of a synapse several times an hour. The challenge is therefore to understand how a synapse is able to maintain stable transmission efficacy against the backdrop of receptor protein turnover. Most current models fail to make a clear distinction between what is occurring across a population of elements as opposed to an individual element (an individual receptor). Thus we have developed a model that offers a mechanism by which a quasi stable population of receptors can exist while individual receptors diffuse into or out of the synapse. This process offers a mechanism that would afford long-term stability of a population of receptors and moves us towards us thinking about the finite lifetime of biological structures. The experiments listed within the proposal are designed to explore the conditions required to form and stabilize a cluster of AMPA receptors at a synapse. The experimental designs are guided by our model and as such look to test its core predictions. To this end we will ask whether AMPA receptor numbers influence synapse formation. We will look to determine the sequence of events that occur in conjunction with AMPA receptor clustering. We will look to determine which scaffold proteins stabilize AMPA receptor clusters and finally assess the role of scaffold proteins following potentiation or de-potentiation of a synapse.

The research contained within this proposal looks to examine fundamental process of normal synaptic function. As such it will add to an existing body of research directed at understanding the mechanisms that mediate and modulate synaptic transmission. This is an important research area as many neurological diseases are intimately linked to synaptic dysfunction. Whether changes at the synapse are a consequence or causal for the diseases is often not clear. What is clear is that considerable therapeutic benefit can be realized in patients that receive drugs that act at the synapse. It is quite remarkable to observe the extent to which this is true. Diseases as diverse as Parkinson's, Alzheimer's, schizophrenia and depressive illness each have different etiology and yet they are all treated using drugs whose locus of action is the synapse. The research proposed here is relevant to these areas as it trades upon a simple fact, understanding a normal process gives us the opportunity to recognize what is abnormal. While it would be unrealistic to suggest that the work will yield rapid quality of life benefits, it is nonetheless targeted to understanding features of the brain that are likely to be susceptible to the ageing process and so long-term beneficiaries might include those subject to premature memory loss.
The research tools to be used in this project will have immediate impact for the neuroscience research community. Novel compounds such as ANQXF have utility for a variety of researchers, thus we are currently in discussion with Accent Scientific to make this compound available for commercial distribution. Equally, a fluorescently tagged glutamate receptor marker may have utility in therapeutic drug assays screens as it offers a method with which to measure receptor turnover.
In previous work we have developed analysis software for SPT studies (Ref). No similar resource is available publicly. The software is in Java format and is free on request. The current work will and continue to use and refine this important resource.
The PDRA employed on this grant will develop skills across a range of disciplines including imaging and molecular biology as well as generic skills such as data analysis and file management. Previous members of the group have deployed their skills in a variety of ways that include academic faculty positions as well as posts within the finance (Goldman Sachs, Berenburg) and business sector (Google).