Synaptic role of the AMPA receptor N-terminal domain

The brain is made up of nerve cells or neurons (~90 billions in the human brain, ~ 75 million in the mouse), which communicate via specific cell junctions termed synapses. Synapses are the sites where information between neurons is transmitted and stored (i.e. laid down as memories); information storage at synapses is believed to require plastic changes in synapse function and efficacy. Signaling across synapses is initiated by the release of neurotransmitter chemicals from an 'upstream' neuron, forming the pre-synaptic site, onto the post-synapse of a downstream recipient cell. A primary neurotransmitter in the mammalian brain is the amino acid glutamate, which, when released by the pre-synapse, activates recipient protein molecules on the post-synapse. These neurotransmitter recipients are termed receptors and lie at the heart of this proposal. The glutamate receptor central to the application is the AMPA-receptor (AMPA-R; based on its selective pharmacological properties). AMPA-Rs initiate excitatory neurotransmission, i.e. they depolarize the post-synaptic membrane in response to binding glutamate. Depolarization results from a glutamate-triggered flux of positively charged particles (cations), through a narrow opening in the AMPA-R channel, into the post-synaptic neuron.
Synaptic potentiation, the substrate for information storage, results from the recruitment of additional AMPA-Rs to the post-synapse - this recruitment gives rise to increased net cation-influx and augments the level of depolarization. Potentiated transmission also requires the clustering of AMPA-Rs at the synapse and their retention opposite glutamate-release sites. In this proposal we will investigate mechanisms underlying AMPA-R clustering and retention at synapses in response to potentiating stimuli. We have preliminary evidence suggesting that the distal portion of the AMPA-R, the N-terminal domain (or NTD) regulates receptor diffusion/retention and clustering at post-synaptic sites. Hence, the NTD appears to play a currently elusive role in AMPA-R-mediated synapse potentiation and therefore in synaptic learning.
AMPA-R retention also stabilizes post-synaptic structures, and our data point to a role for the NTD in this process. Since synapse elimination is a hallmark of neurodegneration, NTD-mediated receptor retention would have the potential to oppose neurological disorders such as Alzheimer's disease and related dementias. These observations, together with our accumulating evidence for the NTD as novel drug target, highlight a key role for the AMPA-R NTD in synapse function. In summary, our proposed work will provide mechanistic information into the role of AMPARs in synaptic learning and synapse stabilization.

AMPA-type glutamate receptors mediate fast excitatory neurotransmission and are instrumental for synaptic plasticity, which underlies learning and memory. AMPARs are also involved in synapse formation and stabilization, their removal can lead to synapse retraction, which is associated with dementia including Alzheimer's disease.

Neuronal signal transmission requires precise clustering of AMPARs opposite presynaptic transmitter release sites, whereas synaptic potentiation is associated with AMPAR enrichment and retention. The cytoplasmic CTD, which interacts with a variety of scaffolding proteins, has been a prime candidate for orchestrating synaptic AMPAR clustering and has been studied extensively.
By contrast, the role of the NTD is currently poorly understood. This domain encompasses ~50 percent of an AMPAR subunit and resides in close proximity to the presynapse. Our recent results imply a role for the NTD in synaptic signalling: Structural data suggest that NTDs can cluster AMPARs into a compact array. We hypothesize that this arrangement facilitates receptor enrichment that is characteristic of potentiated synapses. We also found that the NTD can substantially rearrange to interact with AMPAR auxiliary proteins (the TARPs), which is expected to stabilize AMPARs at synapses and provides a novel target for the development of specific AMPAR therapeutics.

Trans-synaptic interactions have increasingly been recognized as key elements in AMPAR-mediated synaptic plasticity but their precise role is currently unclear. In this proposal we will elucidate the synaptic function of the AMPAR NTD: its role in synapse stabilization, receptor clustering/retention and in synaptic plasticity. These questions lie at the heart of synaptic learning; they will be addressed with a combination of super-resolution microscopy and electrophysiology coupled with bio-orthogonal chemistry, permitting precise control over specific stages of AMPAR trafficking and signalling.

Who will benefit from the research?
In addition to the academic beneficiaries (listed in the 'Academic beneficiaries' section), the pharmaceutical industry and, ultimately, the public and specifically patient groups suffering from neurological disorders will benefit from this work. Hence, the UK economy is expected to benefit in the longer term.

How will they benefit?
In addition to mediating learning glutamate receptors are a central drug-target (D. Bowie CNS Neurol Disord Drug Targets 7, 2008). Positive modulators have therapeutic potential in a number of neurodegenerative diseases and psychiatric disorders. For example, AMPAR positive modulators alleviate cognitive decline in the elderly and in patients suffering from dementia, which affects close to a million people in the UK with a cost of ~ £ 26 billion (http://www.alzheimers.org.uk/dementiauk). A number of compounds have reached the clinical trial stage (see B. Pirotte et al. Expert Opin. Ther. Patents 23, 2013 and K. Partin Curr Opin Pharmacol 20, 2015). However, currently available drugs target the highly sequence-conserved ligand-binding domain or pore region of the receptor. Their action will therefore be broad-range. Similarly, non-competitive negative modulators, such as perampanel, work non-selectively (i.e. target all four AMPAR subunits with similar efficacy).
The sequence-diverse NTD, a key drug target in the related NMDAR (e.g. L. Mony et al. Br. J. Pharmacol. 157, 2009), has not been extensively studied in AMPARs. Our previous work has revealed close similarities between the AMPAR and NMDAR NTDs (ref 5 in the attached document) and revealed allosteric activity for the AMPAR NTD (references 6, 12 and J. Jensen et al. J. Mol Biol. 414, 2011). Current efforts in IGs lab are directed towards NTD drug design (in collaboration with MRC-Technology and Eli Lilly). In addition to the AMPAR core, auxiliary subunits such as the TARPs are being pursued increasingly as drug target (Gill and Bredt Neuropsychopharmacol. 36, 2011). Our recent data, showing allosteric activity of the NTD via the associated TARP (ref 12), opened avenues for the development of highly selective drugs (working at the NTD-TARP interface). This together with our observations that the NTD impacts dynamics and organization of synaptic AMPARs, which lie at the core of this proposal, highlights the need for this research. A better understanding of the inner workings of the NTD, both at the levels of synapses and as allosteric module/drug target, should permit development of therapies impacting both the health and wealth of the UK.

In addition to patients, the general public is expected to benefit. Understanding how the brain operates is a central scientific quest. Hence providing new insights into basic synaptic communication, which forms the fundament of brain function, will increase knowledge. Science activities that involve the public include the annual 'Cambridge Science Festival', which is organized together with the 'Neuroscience Seminar', organized by 'Cambridge Neuroscience' (http://www.neuroscience.cam.ac.uk/events). This year's meeting 'The Making and Breaking of the Mind' will mark the 2014 Nobel Prize in Physiology and Medicine "for the discovery of neurons that constitute a positioning system in the brain". It will be held in our institute and will permit dissemination of results (in poster format) and active engagement of the public.