Abstract
The glutamate receptor AMPAR (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor) is responsible for fast excitatory neurotransmission and is implicated in long term potentiation (LTP) which is now commonly accepted to be the cellular mechanism that underlies memory formation and learning. This enhanced synaptic transmission is associated with an increase in activity of receptors in the post-synaptic membrane and involves an increased receptor density in specific membrane rafts which then act as signalling platforms. While underlying biochemical processes such as phosphorylation and local dendritic synthesis of AMPARs appear to be critical in LTP, interactions of receptors with the cytoskeleton and the lipid membrane itself are believed to be important contributory factors. In order to investigate the latter behaviour we reconstituted homomeric GluR3 receptors into artificial raft-like domains of supported lipid membranes (SLM).
In our work we have used atomic force microscopy (AFM) to probe the structure and function of these neural receptors under experimental conditions that are close to physiological. AFM imaging in liquid is capable of achieving sub-nanometre spatial resolution, and dynamic processes can be followed in real time using the latest high-speed AFM. Using AFM we have managed to image:
(i) lipid bilayers (supported on atomically flat mica) using phospholipid mixtures that approximate the composition of synaptic membranes;
(ii) lipid microdomains or rafts that form spontaneously: these domains mimic the raft-like structures that form in native membranes;
(iii) purified protein reconstituted into the membranes: preferential insertion into specific microdomains was observed, and was strongly influenced by added cholesterol;
(iv) conformational changes of the receptor extracellular domains: photolysis of caged-glutamate was employed bind to the receptor under observation..
(v) the mobility of receptors within and between domains which may shed light on receptor trafficking.
The use of AFM gives a new insight into the structure, function and dynamics of these vital neural receptors. This insight could be the key to understanding the processes of memory, learning, ageing and a number of neurological ailments that have been linked to AMPARs. This novel use of the AFM could also be a powerful tool in the field of drug discovery.
Short Biography
Chandra Ramanujan received a D.Phil. degree in atomic force microscopy and nanomechanics from the University of Oxford and is currently a member of the joint bionanoscience collaboration between NTT Basic Research Laboratories and Oxford University. This unique collaboration brings together one of Japan’s foremost industrial research centres with the bionanotechnology expertise found at Oxford. Bionanotechnology is a new field that aims to understand the structures and functions of biological devices and to utilise Nature’s solutions in advancing science and engineering in areas as diverse as biosensors, the discovery of new medicines, diagnostics and drug delivery. These advances are the subject of a number of patent applications that are led by Dr. Ramanujan. Prior to pursuing a DPhil., Chandra worked as an industrial research engineer in Singapore.