Alcohol modulation of BK channel gating depends on β subunit composition

Area of Science:

• Molecular and Cellular Neuroscience
• Ion Channel Physiology
• Pharmacology

Background:

• Large conductance calcium- and voltage-gated potassium (BK) channels are crucial for cellular excitability in mammalian tissues.
• BK channels are composed of pore-forming slo1 subunits and auxiliary beta subunits (β1-β4), which modulate channel function.
• Ethanol's effects on BK channels are complex and depend on various factors, including Ca²⁺_i concentration and subunit composition, making specific contributions difficult to ascertain.

Purpose of the Study:

• To elucidate the specific role of different BK channel β subunits in mediating ethanol's action on channel activity.
• To investigate how ethanol influences the gating mechanisms of BK channels in the presence and absence of various β subunits.
• To understand the molecular basis for differential ethanol sensitivity of BK channels modulated by distinct β subunits.

Main Methods:

• Utilized identical experimental conditions to record single-channel and macroscopic currents of the rat cerebral artery myocyte slo1 channel (cbv1) with and without 50 mM ethanol.
• Assessed the functional impact of five different β subunits (β1, β2, β2-IR, β3-variant d, β4) on ethanol's effects across a range of Ca²⁺_i concentrations (3-20 µM).
• Applied Horrigan-Aldrich gating analysis to determine ethanol's influence on intrinsic gating, voltage dependence, Ca²⁺ binding, and allosteric coupling.

Main Results:

• Two distinct phenotypes of ethanol action were identified based on β subunit composition: potentiation at low Ca²⁺_i and inhibition at high Ca²⁺_i, with a crossover point at 20 µM Ca²⁺_i for cbv1, cbv1+β3, and cbv1+β4.
• For cbv1+β1, cbv1+wt β2, and cbv1+β2-IR, the potentiation-inhibition crossover shifted to approximately 3 µM Ca²⁺_i, indicating altered ethanol sensitivity.
• Ethanol did not alter intrinsic gating or voltage-dependent parameters but significantly increased apparent Ca²⁺ affinity (decreased K_d) and mildly decreased allosteric coupling (C) for cbv1. For cbv1+β1, ethanol decreased K_d less drastically, reduced C, and also decreased coupling between Ca²⁺ binding and voltage sensing (E).

Conclusions:

• BK channel β subunits critically determine the differential effects of ethanol on channel activity, shifting the Ca²⁺ sensitivity and modifying gating parameters.
• Ethanol's action is primarily mediated by altering Ca²⁺ binding affinity and allosteric coupling, rather than intrinsic gating or voltage dependence.
• Understanding these β subunit-dependent mechanisms provides insight into the neurobiological effects of alcohol and potential therapeutic targets.