Ion conduction and selectivity in acid-sensing ion channel 1

Area of Science:

• Molecular Biology
• Neuroscience
• Ion Channel Physiology

Background:

• Acid-sensing ion channels (ASICs) are crucial for neuronal function, but their ion selectivity remains incompletely understood.
• Understanding ASIC ion permeability is key to elucidating their role in physiological and pathological processes.
• Previous studies have hinted at cation selectivity but lacked detailed characterization of ASIC1a's pore properties.

Purpose of the Study:

• To comprehensively assess the cation selectivity of human ASIC1a.
• To investigate the influence of specific amino acid residues and pore vestibule charges on ion permeation.
• To determine the binding affinity and interactions of various cations within the ASIC1a pore.

Main Methods:

• Expressed human ASIC1a in Xenopus laevis oocytes.
• Measured acid-induced currents using two-electrode voltage clamp.
• Performed ion substitution experiments and analyzed conductance and reversal potentials.
• Utilized site-directed mutagenesis (D434N) and excised outside-out patch-clamp recordings.

Main Results:

• ASIC1a exhibits a selectivity sequence for monovalent cations: Li+ ∼ Na+ > K+ > Rb+ > Cs+.
• Permeability decreased more rapidly than conductance with increasing atomic size.
• ASIC1a showed finite permeability to divalent cations (Ca2+, Sr2+, Ba2+) and permeation of nitrogen-containing cations.
• The D434N mutation altered channel conductance, Na+ interaction (Km), and cation selectivity, indicating the role of negative charges in the outer vestibule.

Conclusions:

• ASIC1a selectivity is determined by interactions with binding sites within and internal to the outer pore vestibule.
• Negative charges in the outer vestibule play a significant role in modulating ASIC1a ion selectivity and Na+ affinity.
• These findings provide critical insights into the molecular mechanisms governing ion permeation through ASICs.