Importance of voltage-dependent inactivation in N-type calcium channel regulation by G-proteins

Abstract

Direct regulation of N-type calcium channels by G-proteins is essential to control neuronal excitability and neurotransmitter release. Binding of the G bg dimer directly onto the channel is characterized by a marked current inhibition ("ON" effect), whereas the pore opening-and time-dependent dissociation of this complex from the channel produce a characteristic set of biophysical modifications ("OFF" effects). Although G-protein dissociation is linked to channel opening, the contribution of channel inactivation to G-protein regulation has been poorly studied. Here, the role of channel inactivation was assessed by examining time-dependent G-protein de-inhibition of Ca v 2.2 channels in the presence of various inactivationaltering β subunit constructs. G-protein activation was produced via μ-opioid receptor activation using the DAMGO agonist. Whereas the "ON" effect of G-protein regulation is independent of the type of β subunit, the "OFF" effects were critically affected by channel inactivation. Channel inactivation acts as a synergistic factor to channel activation for the speed of G-protein dissociation. However, fast inactivating channels also reduce the temporal window of opportunity for G-protein dissociation, resulting in a reduced extent of current recovery, whereas slow inactivating channels undergo a far more complete recovery from inhibition. Taken together, these results provide novel insights on the role of channel inactivation in N-type channel regulation by G-proteins and contribute to the understanding of the physiological consequence of channel inactivation in the modulation of synaptic activity by G-protein coupled receptors.

Novel Mechanism of Voltage-Gated N-type (Cav2.2) Calcium Channel Inhibition Revealed through α-Conotoxin Vc1.1 Activation of the GABAB Receptor

“…This form of inhibition typically involves Gbg subunits binding to Ca v channels, shifting the voltage dependence of activation and slowing the activation kinetics. In addition, prepulse potentiation can antagonize Gbg-dependent inhibition of Ca v channels (Ikeda, 1996;Weiss et al, 2007). We next examined if these features were also observed with Vc1.1-dependent inhibition.…”

G protein inhibition of CaV2 calcium channels

“…More recently, gating current modulation by G proteins has also been reported in rat sympathetic neurons. 74,75 Gβγ can also reduce inactivation of Ca V 2.2 channels, 76,77 although this is somewhat more subtle and can be masked by concomitant voltage-dependent reversal of Gβγ-mediated inhibition (Gβγ unbinding). Inactivation of Ca 2+ channels is complex and mediated by several voltage-dependent and Ca 2+ -dependent mechanisms.…”

G Protein Regulation of Neuronal Calcium Channels: Back to the Future

“…Considering that channel activity is an important factor of the OFF G protein landmark, these results suggest that the Ca v b subunit could modulate G protein inhibition indirectly by biophysical changes induced on the channel. Consistent with this idea, the respective kinetics of channel inactivation induced by various Ca v b subunits nicely correlates with the kinetic of recovery from G protein inhibition (Weiss et al, 2007a). It was thus proposed that the Ca v b subunit, by controlling channel inactivation, indirectly influences the speed of G protein dissociation from the channel.…”

“…Hence, fastinactivating Ca v b subunits, while speeding up the rate of G protein dissociation, also reduce the maximal extent of current recovery from inhibition that leads to an apparent decrease of the prepulse facilitation. In contrast, a slow-inactivating Ca v b subunit (for instance, Ca v b 2a ) that slows down channel inactivation has minor effect on the kinetic of G protein dissociation, but prolongs the temporal window of opportunity for G protein dissociation and leads to a far more complete current recovery from inhibition, evidenced by an apparent increased prepulse facilitation (Weiss et al, 2007a). It is worth noting that the amino-terminal domain of the Ca v 2 channels that has been implicated in the direct G protein inhibition of VGCCs mediates Ca v b-dependent fast inactivation of Ca v 2.2 channel .…”

“…For example, coexpression of Rim1 significantly decreases direct [D-Ala2, N-MePhe4, Gly-ol]-enkephalin-induced G protein inhibition of Ca v 2.2 channels expressed in HEK293 cells (Weiss et al, 2011). Interestingly, careful kinetic analyses revealed that Rim1 does not prevent inhibition of the channel (ON landmark, i.e., binding of Gbg dimer to the channel), but rather favors channel recovery from inhibition (OFF landmarks, i.e., unbinding of Gbg dimer from the channel), most likely by slowing down channel inactivation (Kiyonaka et al, 2007), increasing the time window for functional recovery from G protein inhibition similarly to Ca v b subunits (Weiss et al, 2007a(Weiss et al, , 2011. Modulation of G protein inhibition of VGCCs has also been documented with cysteine string protein (CSP).…”