Elementary Mechanisms Producing Facilitation of Ca_v2.1 (P/Q-type) Channels

The regulation of Ca_V2.1 (P/Q-type) channels by calmodulin (CaM) showcases the powerful Ca²⁺ decoding capabilities of CaM in complex with the family of Ca_V1-2 Ca²⁺ channels. Throughout this family, CaM does not simply exert a binary on/off regulatory effect; rather, Ca²⁺ binding to either the C- or N-terminal lobe of CaM alone can selectively trigger a distinct form of channel modulation. Additionally, Ca²⁺ binding to the C-terminal lobe triggers regulation that appears preferentially responsive to local Ca²⁺ influx through the channel to which CaM is attached (local Ca²⁺ preference), whereas Ca²⁺ binding to the N-terminal lobe triggers modulation that favors activation via Ca²⁺ entry through channels at a distance (global Ca²⁺ preference). Ca_V2.1 channels fully exemplify these features; Ca²⁺ binding to the C-terminal lobe induces Ca²⁺-dependent facilitation of opening (CDF), whereas the N-terminal lobe yields Ca²⁺-dependent inactivation of opening (CDI). In mitigation of these interesting indications, support for this local/global Ca²⁺ selectivity has been based upon indirect inferences from macroscopic recordings of numerous channels. Nagging uncertainty has also remained as to whether CDF represents a relief of basal inhibition of channel open probability (P_o) in the presence of external Ca²⁺, or an actual enhancement of P_o over a normal baseline seen with Ba²⁺ as the charge carrier. To address these issues, we undertake the first extensive single-channel analysis of Ca_V2.1 channels with Ca²⁺ as charge carrier. A key outcome is that CDF persists at this level, while CDI is entirely lacking. This result directly upholds the local/global Ca²⁺ preference of the lobes of CaM, because only a local (but not global) Ca²⁺ signal is here present. Furthermore, direct single-channel determinations of P_o and kinetic simulations demonstrate that CDF represents a genuine enhancement of open probability, without appreciable change of activation kinetics. This enhanced-opening mechanism suggests that the CDF evoked during action-potential trains would produce not only larger, but longer-lasting Ca²⁺ responses, an outcome with potential ramifications for short-term synaptic plasticity.