Abstract
Recent studies highlighted the importance of loop 2 of α1 glycine receptors (GlyRs) in the propagation of ligand-binding energy to the channel gate. Mutations that changed polarity at position 52 in the β hairpin of loop 2 significantly affected sensitivity to ethanol. The present study extends the investigation to charged residues. We found that substituting alanine with the negative glutamate at position 52 (A52E) significantly left-shifted the glycine concentration response curve and increased sensitivity to ethanol, whereas the negative aspartate substitution (A52D) significantly right-shifted the glycine EC50 but did not affect ethanol sensitivity. It is noteworthy that the uncharged glutamine at position 52 (A52Q) caused only a small right shift of the glycine EC50 while increasing ethanol sensitivity as much as A52E. In contrast, the shorter uncharged asparagine (A52N) caused the greatest right shift of glycine EC50 and reduced ethanol sensitivity to half of wild type. Collectively, these findings suggest that charge interactions determined by the specific geometry of the amino acid at position 52 (e.g., the 1-Å chain length difference between aspartate and glutamate) play differential roles in receptor sensitivity to agonist and ethanol. We interpret these results in terms of a new homology model of GlyR based on a prokaryotic ion channel and propose that these mutations form salt bridges to residues across the β hairpin (A52E-R59 and A52N-D57). We hypothesize that these electrostatic interactions distort loop 2, thereby changing agonist activation and ethanol modulation. This knowledge will help to define the key physical-chemical parameters that cause the actions of ethanol in GlyRs.
Footnotes
This work was supported in part by the National Institutes of Health National Institute on Alcohol Abuse and Alcoholism [Grants F31-AA017569 (to D.I.P.), RO-1-AA03972 (to R.L.A.), RO-1-AA013922 (to D.L.D.), RO-1-AA013378 to J.R.T.]; the Southern California Translational Research Institute [Pilot Award 1UL1RR031986-01] (to R.L.A.); and the University of Southern California School of Pharmacy.
This work was conducted as partial fulfillment of the requirements for the Ph.D degree in Molecular Pharmacology and Toxicology at the University of Southern California for D.I.P.
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS:
- GlyR
- glycine receptor
- GLIC
- Gloeobacter violaceus pentameric ligand-gated ion channel homolog
- MTS
- methanethiosulfonate
- MTSES
- MTS sodium salt
- WT
- wild type
- TM
- transmembrane
- GABAAR
- γ-aminobutyric acid type A receptor.
- Received December 9, 2011.
- Accepted February 21, 2012.
- Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics
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