Abstract
The elemental anion chloride is generally considered a passive participant in neuronal excitability, and has never been shown to function as an agonist in its own right. We show that the antagonist-mediated, glutamate-independent inverse agonism of group II and III metabotropic glutamate (mGlu) receptors results from inhibition of chloride-mediated activation. In silico molecular modeling, site-directed mutagenesis, and functional assays demonstrate (1) that chloride is an agonist of mGlu3, mGlu4, mGlu6, and mGlu8 receptors with its own orthosteric site, and (2) that chloride is not an agonist of mGlu2 receptors. Molecular modeling–predicted and site-directed mutagenesis supported that this unique property of mGlu2 receptors results from a single divergent amino acid, highlighting a molecular switch for chloride insensitivity that is transduced through an arginine flip. Ultimately, these results suggest that activation of group II and III mGlu receptors is mediated not only by glutamate, but also by physiologically relevant concentrations of chloride.
Footnotes
- Received October 15, 2014.
- Accepted June 18, 2015.
Some information presented herein is in the Ph.D. thesis of J.O.D. (unpublished until July 2015): DiRaddo JO (2013) New Molecular Properties of Metabotropic Glutamate Receptors. Doctoral dissertation, Georgetown University, Washington, DC; and the Ph.D. thesis of E.J.M. (scheduled for August 2015): Miller EJ (2015) Chapter 2: Preferential Activation of Metabotropic Glutamate Receptor 3 Over Metabotropic Glutamate Receptor 2. Ph.D. thesis.
Some information was presented by J.O.D. and E.J.M. on September 30, 2014 during a poster session at the 8th International Meeting on Metabotropic Glutamate Receptors. DiRaddo JO, Miller EJ, Bowman-Dalley C, Javidnia M, Grajkowska E, Wroblewska B, Wolfe BB, Liotta DC, Wroblewski JT (2014) Chloride is an Agonist of mGlu3, mGlu4, mGlu6, and mGlu8 Receptors, in 8th International Meeting on Metabotropic Glutamate Receptors; 2014 Sept 28–Oct 3; Taormina, Italy. Miller EJ, DiRaddo JO, Grajkowska E, Wroblewska B, Wolfe BB, Snyder JP, Wroblewski JT, Liotta DC (2014) Structural Basis of Chloride Selectivity Between Group II Metabotropic Glutamate Receptors, in 8th International Meeting on Metabotropic Glutamate Receptors; 2014 Sept 28–Oct 3; Taormina, Italy.
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- Copyright © 2015 by The American Society for Pharmacology and Experimental Therapeutics
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