MODULATION OF CHICKEN ASIC1 BY 2-GUANIDINE-4-METHYLQUINAZOLINE (GMQ) IN THE ABSENCE AND PRESENCE OF PSALMOTOXIN-1

Ion channels are an important means of communication between the outside and inside of the cell. One family of ion channels, the acid-sensing ion channels (ASICs) are activated by a drop in pH surrounding the cell. Their activation allows the movement of ions from the outside and inside compartment of the cell, and specifically with ASICs, this exchange of ions can be involved many pathophysiological conditions. However, one shortcoming is that there remains a lack of specific agonists (channel activators) or antagonists (channel inactivators) for the different ASIC subtypes. Recent studies have indicated that there are natural venom toxins derived from organisms like sea anemones and tarantulas that can interact with ASICs. Additionally, compounds containing a guanidinium group, like the synthetic compound GMQ, can enhance channel activation in some of the ASIC subtypes. We sought to determine if the combination of these natural venom toxins and guanidine compounds can simultaneously interact with ASICs using a cellular model that contains no ASIC subtypes. We record the channel activity following the addition of the compounds both individually and simultaneously. Understanding the interaction of these compounds with the ASIC subtypes will provide the foundation for the design of novel drugs to influence the activity of these channels. Purpose (a): Acid-sensing ion channels (ASICs) are trimeric, sodium-selective channels that sense changes in extracellular acidity and are part of the epithelium sodium channel/degenerin (ENaC/DEG) family of ion channels. ASICs are sensitive to an increasing number of nonproton ligands that include natural venom peptides and guanidine compounds, such as amiloride and 2-guanidine-4-methylquinazoline (GMQ). The nonproton ligand GMQ has been shown to stimulate ASIC3 by expanding the pH range of the ASIC window current, but decreased the sensitivity of other ASIC subtypes to protons. The effect of GMQ on chicken ASIC1 (cASIC1), which has been used to elucidate the protein crystal structures, is unknown. Furthermore, cASIC1 exhibits unique channel gating properties, including the spider toxin ASIC1a Psalmotoxin-1 (PcTx1) induced activation. Methods (b): We sought to elucidate the interaction of GMQ, PcTx1, and cASIC1 using whole-cell and outside-out patch clamp electrophysiology to provide additional insight into the nonproton ligand interaction with a structurally characterized ASIC construct. Results (c): Our studies revealed GMQ increases the cASIC1 proton sensitivity, as observed by a leftward shift in the proton activation curve. When alone, the nonproton ligand failed to activate cASIC1. Additionally, we observed GMQ concentration-dependent enhancement of the cASIC1 PcTx1 persistent current. Conclusions (d): Our data suggests that GMQ may have multiple sites of action on cASIC1 and may act as a “molecular wedge” that forces the desensitized ASIC into an open state. We anticipate that the revelation of GMQ stimulation in the cASIC1 subtype will warrant further investigations into nonproton ligand sensitivity in other ASIC subtypes and provide the foundation for the design of novel ligands that exploit the nonproton ligand site to influence ASIC activity.