Compensatory Regulation of Ca_v2.1 Ca²⁺Channels in Cerebellar Purkinje Neurons Lacking Parvalbumin and Calbindin D-28k

Abstract

Cav2.1 channels regulate Ca2+ signaling and excitability of cerebellar Purkinje neurons. These channels undergo a dual feedback regulation by incoming Ca2+ ions, Ca2+-dependent facilitation and inactivation. Endogenous Ca2+-buffering proteins, such as parvalbumin (PV) and calbindin D-28k (CB), are highly expressed in Purkinje neurons and therefore may influence Cav2.1 regulation by Ca2+. To test this, we compared Cav2.1 properties in dissociated Purkinje neurons from wild-type (WT) mice and those lacking both PV and CB (PV/CB−/−). Unexpectedly, P-type currents in WT and PV/CB−/− neurons differed in a way that was inconsistent with a role of PV and CB in acute modulation of Ca2+ feedback to Cav2.1. Cav2.1 currents in PV/CB−/− neurons exhibited increased voltage-dependent inactivation, which could be traced to decreased expression of the auxiliary Cavβ2a subunit compared with WT neurons. Although Cav2.1 channels are required for normal pacemaking of Purkinje neurons, spontaneous action potentials were not different in WT and PV/CB−/− neurons. Increased inactivation due to molecular switching of Cav2.1 β-subunits may preserve normal activity-dependent Ca2+ signals in the absence of Ca2+-buffering proteins in PV/CB−/− Purkinje neurons.

Contribution of calcium-dependent facilitation to synaptic plasticity revealed by migraine mutations in the P/Q-type calcium channel

“…P/Q-type currents in acutely dissociated cerebellar PCs (approximately 90-95% of whole cell Ca 2+ current; see refs 30, 31 and Fig. 2D) recapitulate the key features of CaMmediated CDF observed in human recombinant Ca v 2.1 channels (17,32,33). In WT mice, P/Q-type currents in PCs showed similar CDF to that for WT human recombinant Ca v 2.1 channels (Fig.…”

“…2C shows that we did not detect significant CDI of endogenous P/Q-type currents in PCs from WT or R192Q and S218L mice. Of note, CDI of P/Q-type currents in dissociated PCs has been found to be variable under different recording conditions (17,32,33).…”

“…In contrast, Ca v 2.1 channels in the facilitated state have an enhanced Ca 2+ influx during APs with durations of 1 ms or less (Figs. 1 and 2) (9,10,17,42), and as such, channels rendered toward a facilitated state by the S218L mutation would be expected to show enhanced Ca 2+ influx at PF terminals during evoked short APs. In further support, the amplitude of Ca 2+ transients in the presynaptic boutons in S218L mice were enhanced by approximately 15% (Fig.…”

Cytosolic Ca2+ Buffers

“…Contrary to the expectation based on in vitro experiments, CDI in Purkinje cells of double knockout (CB-D28k 2/2 PV 2/2 ) mice is not increased. However, P-type currents recorded in these cells exhibit increased voltage-dependent inactivation as the result of a decreased expression of the auxiliary Ca v b2a subunit compared to WT neurons (Kreiner et al 2010) (Fig. 2).…”

“…This reduction accounts for the heightened excitotoxic injury provoked by a local injection of ibotenic acid. A last example of reciprocality: elimination of PV and CB-D28k from Purkinje cells alters Ca v 2.1 channel function (Kreiner et al 2010), while a reduced Ca 2þ influx due to a mutation in the Ca v 2.1 channel down-regulates PV and CB-D28k (Dove et al 2000). The elucidation of the pathways and molecular mechanisms responsible for the regulation of the Ca 2þ homeostasome remains an exciting topic for future research.…”