Ion channel function in the absence of ion-flow
Two recent papers from Dr Roberto Malinow’s lab at the University of California, San Diego show that agonist binding to the NMDA receptor (NMDAR) triggers conformational movement of the cytoplasmic domain (NMDARcd) in the absence of ion flow.
The change drives conformational signalling required for synaptic plasticity by the NMDA complex.
The NMDA receptor is a ligand-gated ion channel that performs many important roles in brain physiology and pathology. It is known to communicate information by allowing an influx of intracellular Calcium ([Ca2+]I), but other studies have suggested that activated NMDARs can trigger synaptic plasticity without any ion flow.
In the first paper, the researchers used fluorescence lifetime imaging microscopy (FLIM) to measure Förster Resonance Energy Transfer (FRET) between GluN1-GFP and GluN1-mCherry. FRET between the fluorescently tagged cytoplasmic domains of the GluN1 subunits of NMDARs is reduced rapidly and transiently with agonist binding. These results are consistent with a conformational change on the NMDARcd.
In the second paper, the researchers used the same technique to examine the downstream signalling of the conformational change required for synaptic depression. Ligand binding produces a temporary shift in the interaction between NMDARcd and protein phosphatase 1 (PP1) and calcium/calmodulin-dependent protein kinase II, as measured by a reduction in FRET, a process that requires NMDARcd movement. These proteins are known to mediate synaptic depression.
The complementary papers are available in the same issue of the Proceedings of the National Academy of Sciences.
FLIM-FRET was carried out on a custom built Scientifica Multiphoton Imaging System.
Paper references:
Dore K., Aow J., Malinow R. Agonist binding to the NMDA receptor drives movement of its cytoplasmic domain without ion flow Proceedings of the National Academy of Sciences (2015) doi: 10.1073/pnas.1520023112
Aow J., Dore K., Malinow R. Conformational signalling required for synaptic plasticity by the NMDA receptor complex Proceedings of the National Academy of Sciences (2015) doi: 10.1073/pnas.1520029112
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