Elementary mechanisms producing facilitation of Cav2.1 (P/Q-type) channels

The regulation of Ca(V)2.1 ( P/Q-type) channels by calmodulin ( CaM) showcases the powerful Ca2+ decoding capabilities of CaM in complex with the family of Ca-V(1-2) 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). CaV2.1 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 ( Po) 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 fi rst extensive single-channel analysis of CaV2.1 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 Po 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.