Ionotropic glutamate receptors (iGluRs) are transmembrane proteins ubiquitous in the mammalian brain. They are subdivided into four families: NMDA, AMPA, kainate and delta receptors. Together, these neuroreceptors mediate communication between neurons at excitatory synapses, with each of them performing a specialized role. AMPA receptors, the focus of this project, ensure the speed of synaptic transmission (1): they are the first ones to bind neurotransmitter glutamate (Glu) released from the presynaptic neuron, and the speed at which they bind Glu, activate and then deactivate dictates how fast the information is transmitted across the synapse. In the auditory neurons, for example, this transfer can reach the kilohertz range. Whenever AMPA receptors bind Glu, they open their integral ion channels allowing influx of Na+ ions and depolarizing the synaptic membrane. If the stimulus is strong enough, the membrane will depolarize sufficiently to trigger an action potential, allowing the propagation of the excitatory signal through the neuronal network.
Unsurprisingly, dysfunction of AMPA receptors has been implicated in various neurological diseases. Recently, a point mutation called Lurcher has been described in AMPA receptors of patients diagnosed with autism, schizophrenia, epilepsy and neurodevelopmental disorders (2). The mutation itself has been discovered decades ago in mice, in related delta receptors, resulting in ataxia with a characteristic “Lurcher” phenotype.
Lurcher mutation has profound effects on the activity of AMPA receptors and the main aim of the project is to characterize the activity of Lurcher receptors at the single-molecule level. The motivation to study the mutant comes from both, fundamental and applied science.
Preliminary data shows that the mutant reveals activation of individual subunits within the tetrameric AMPA receptor, giving us the unique opportunity to investigate mechanism of AMPA receptor activation, as its subunits activate one by one. This part of the project addresses one of the main open questions in the field of iGluRs: do all four subunits contribute equally to the activation of an AMPA receptor? The structural models of AMPA receptors indicate two subunits dominate the activation, but the functional data to support this is lacking, mainly because, until now, there was no way to follow activation of individual subunits. Lurcher mutant overcomes this limitation.
The applied aspect of the project relates to the fact that AMPA receptors are already a target of a commercially available drug, perampanel, used in treatments against epilepsy. The second phase of the project would test the effects of the drug and its derivatives on the Lurcher mutant. Any effects of the drugs on the mutant carry implications for the treatment of the patients carrying the mutation.
The main experimental method to be used throughout the project is patch-clamp electrophysiology, which enables us to record the activity of AMPA receptors at the macroscopic and single-molecule level (3). It is a widely used biophysical method for the study of ion channels, which are an important pharmacological target. Single-molecule recordings are accompanied by extensive data analysis, so the candidate would develop both, strong experimental and data analysis skills. https://www.ed.ac.uk/profile/jelena-baranovic
(1) Baranovic, J., Plested, A.: “How to build the fastest receptor on Earth”, Biological Chemistry, 2016, 397, 195-205
(2) Geisheker, M. R. et al.: “Hotspots of missense mutation identify neurodevelopmental disorder genesand functional domains”, Nature Neuroscience, 2017, 20, 1043–1051
(3) Baranovic, J., Plested, A.: “Auxiliary subunits keep AMPA receptors compact during activation and desensitization”, eLife, 2018