Abstract

Low-voltage activated T-type calcium channels, including CaV3.1, CaV3.2 and CaV3.3 channels, are best recognized for their negative voltage of activation and inactivation thresholds that allow them to operate near the resting membrane potential of neurons. They are typically recruited by either subthreshold membrane depolarizations, or by hyperpolarizations that remove inactivation, and are therefore perfectly suited to shape action potential threshold, and to generate a low-threshold burst discharge that occurs during physiological and pathological neuronal rhythmogenesis. In addition, they support a “window current” allowing Ca2C entry at rest (for review see ). In contrast to the closely related high-voltage activated (HVA) channels whose gating is largely influenced by the coupling with auxiliary subunits, functional expression of T-type channels does not require the presence of any accessory subunits. Indeed, heterologous expression of the CaV3-subunit alone is sufficient to generate currents that mimics the currents observed in native tissue, suggesting that gating and kinetics of T-type channels are regulated by distinct mechanisms from HVA channels (for review see ). Structural and functional analyses have revealed the presence of a helix-loop-helix structure, so-called gating brake, located within the proximal 62 amino acids region of the intracellular I-II linker of the CaV3 subunit, and highly conserved among virtually all T-type channels including mammalians and their invertebrate orthologs. Deletion of this molecular determinant gives rise to channels that activate at even more hyperpolarized potentials and present faster activation and inactivation kinetics. In our recent study, we have extended the functional characterization of the gating brake by contrasting its importance within the CaV3 family members. Activation of voltage-gated calcium channels (VGCC) proceeds in 2 steps (Fig. 1). First, it requires the initial mobilization of the channel voltage-sensor (presumably formed by the S1-S4 segments), followed by the opening of the ionic pore. Activation of the voltagesensor upon depolarization of the plasma membrane triggers the outward movement of S4 segments that produces the charge movement measurable as gating currents. Consequent opening of the pore generates the ionic current. Currently, much more information is available on the regulation of the ionic current than on the charge movement. The voltage-dependence of the pore opening (G-V) that is proportional to the number of channels opened at a given voltage is virtually identical among all 3 CaV3 channel isoforms. In contrast, the time constant of current activation, which reflects the transition kinetic between closed and open states of the channel, is similar between CaV3.1 and CaV3.2 channels, but of one order magnitude slower for CaV3.3 channel. Removal of the gating brake facilitates channel activation, manifested by the opening of the pore at lower membrane potentials, and accelerated activation kinetics. Interestingly, upon deletion of the gating brake, intrinsic differences between CaV3.3 and CaV3.1 channels were abolished and the CaV3.3 deletion mutant presented a similar