Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC15
Cornichon proteins modify functional properties of recombinant and native AMPA receptors
I. Coombs1, D. Soto1, M. Renzi1, C. Shelley1, M. Zonouzi1, M. Farrant1, S. G. Cull-Candy1
1. Neuroscience, Physiology and Pharmacology, University College London, London, United Kingdom.
Fast excitatory synaptic transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs). These ligand-gated ion channels are formed from hetero- or homomeric combinations of four pore-forming subunits (GluA1-4). Functional diversity among AMPARs is determined by their subunit composition, and by associated proteins that act as trafficking molecules and auxiliary subunits. A family of transmembrane AMPAR regulatory proteins (TARPs) regulate various key AMPAR properties (Nicoll et al, 2006). These include agonist affinity, single-channel conductance, desensitization and deactivation kinetics and, for calcium-permeable (CP-) GluA2-lacking AMPARs, block by intracellular polyamines and relative calcium permeability (Soto et al, 2007). Recently, another family of transmembrane proteins capable of interacting with and modifying AMPARs was identified, when proteomic analysis revealed cornichon (CNIH) -2 and -3 as partners of GluA2 in brain (Schwenk et al, 2009). We found that co-expression in tsA-201 cells of CNIH-2 or -3 with GluA1 or GluA2 subunits affected many of the same channel properties that are modified by TARPs. Thus, desensitization (τdes) of homomeric GluA1 receptors was slowed by CNIH-2 (from 2.9 ± 0.2 to 5.9 ± 1.0 ms; n = 8 and 11). The mean channel conductance, measured using non-stationary fluctuation analysis, was also increased (from 20.0 ± 2.5 to 30.9 ± 3.6 pS; n = 6 and 7). TARPs greatly decrease the block of CP-AMPARs by intracellular polyamines, reducing their characteristic inward rectification. CNIH-2 similarly caused a rightward shift in the conductance-voltage plot for GluA1 receptors (depolarization in V1/2 of Boltzmann function from -67.3 ± 3.5 to -55.2 ± 3.0 mV; n = 7 and 6). There is no consensus as to the role of cornichons in neurons (Shi et al, 2010; Kato et al, 2010). This issue has not been addressed in non-neuronal cells. In the original study (Schwenk et al, 2009), CNIH-2/3 immunoreactivity was observed in various glial cells, but functional effects were not investigated. To address this, we examined oligodendrocyte precursor cells (OPCs) - immature glia that give rise to oligodendrocytes responsible for myelination. Non-permeabilized OPCs cultured from rat optic nerve showed intense labelling with a CNIH-2/3 antibody. Over-expression of CNIH 3 in OPCs slowed the τdes of glutamate-evoked current (from 6.1 ± 0.8 to 10.0 ± 0.8 ms; n = 10 and 12), indicating incorporation of CNIH-3 in functional AMPARs. Together, our data demonstrate that cornichons assemble with both CP- and calcium-impermeable AMPARs in expression systems, altering their properties. The presence of CNIH-2/3 at the surface of OPCs and the effects of CNIH over-expression suggest that these effects may influence AMPAR signalling in vivo.
Where applicable, experiments conform with Society ethical requirements