Voltage-Gated Calcium Channels

Classification, structure, inactivation, modulation, exocytosis, and pharmacology.

T-type: Low-threshold, low-conductance.
L-type: High-threshold, large-conductance, slowly inactivating.
N-, P-, Q-, R-type: Intermediate conductances, inactivation kinetics, and activation thresholds.

α1 Subunit:
- 24 transmembrane α-helices organized into four homologous repeats (Domains I–IV).
- S4 segment: Voltage-sensing domain with positively charged amino acids.
- S5 and S6: Pore-forming domain.
Composition: Single α1 subunit (~2000 amino acids, ~170-250 kDa), 10 genes.
Tetramer-mimicking structures, like NaV channels.
Auxiliary Subunits:
- α2δ dimer: 170 kDa, 4 isoforms
- β subunit: ~55 kDa, 4 isoforms
- γ subunit: 8 isoforms, only for CaV1.1 channels
- Modulate membrane trafficking, current kinetics, and gating properties.

HVA channels: Heteromultimers of α1, β, α2δ, and γ subunits.
LVA channels: Contain only a single α1 subunit.
Functions:
- Increase membrane expression of α1 subunit.
- Facilitate channel opening.
- Affect the affinity of Ca2+ channels for channel blockers.
β subunit expression: Increases current density, modulates activation and inactivation kinetics, affects pharmacological properties and second-messenger regulation.

Bundle-crossing region at the lower third of the S6 segments: Forms the activation gate.
Closed state: Pore-lining S6 helices converge intracellularly, obstructing ion flow.
Selectivity filter: Formed by EEEE residues (Glu292, Glu614, Glu1014, Glu1323).

Senses depolarization via positively charged arginine or lysine residues on helix S4.
Segments S1–S4: Form the VSD.
Segments S5 and S6: Contribute to the Ca2+-conductive pore.
The α1 subunit: Consists of four repeated motifs (I–IV), each with six membrane-spanning segments (S1–S6).

At rest: Voltage sensors (VSs) pulled down by the electrical field, locking the channel closed.
Depolarization: Releases VSs, causing upward movement and releasing the closed channel gates.
Channel opening and inactivation: Enabled when all four S4 segments have left their resting position.
Arginine and lysine residues: Form a positively-charged band along the length of the alpha helix.

Voltage-dependent inactivation: Domain I–II linker acts as an inactivation particle, occluding the pore; influenced by the Ca2+ channel β subunit.
Calcium-dependent inactivation (CDI):
- Essential for CaV autoregulation.
- CaV selectivity filter: Forms the CDI gate.

Closed channels are activated by depolarization including VSD activation and pore opening.
$Ca^{2+}$ flow through the channel leads to $Ca^{2+}$ binding to CaM that initiates CDI.
CDI results in a selectivity filter conformational change that obstructs ion flow.

L-Type CaV Channels by PKA:
- Forskolin (cAMP activator): Enhances current flow via PKA activation.
- A-kinase anchoring proteins (AKAPs): Direct PKA to the channel.
- AKAP79/150: Binds both PKA and calcineurin (CaN).
- CaN: Opposes L channel phosphorylation by PKA.
G-proteins:
- G-protein βγ-binding pocket in CaV2 subunits.
- Carboxy-terminal region of CaV2 α subunit: Couples the channel with the GPCR.
Auxiliary Subunit Modulation:
- β subunits: Affect peak current.
- α2δ subunits: Influence inactivation.

Located pre- and postsynaptically.
N- and P/Q-types: Located presynaptically.
Presynaptic Ca2+: Interacts with the SNARE complex, facilitating neurotransmitter release.
Postsynaptic activation of AMPARs: Depolarizes cells, leading to Ca2+ entry through NMDARs and postsynaptic CaVs.

Calcium entry through presynaptic CaVs: Links membrane depolarization to exocytosis of synaptic vesicles.
Neurotransmitter release: Dependent on presynaptic $Ca^{2+}$ concentrations.

Vesicle docks along the presynaptic membrane.
Action potential depolarizes the terminal, opening voltage-gated $Ca^{2+}$ channels.
$Ca^{2+}$ floods into the terminal.
$Ca^{2+}$ interacts with SNARE proteins, causing vesicle fusion and neurotransmitter release.

Calcium binds to synaptotagmin: Stimulates v- and t-SNAREs to combine.
SNARE complex: Forces membranes together (fusion) and pulls them apart (exocytosis).

Rab3, synaptotagmin, and synaptobrevin: Anchor CaV2 channels near synaptic vesicles, forming a $Ca^{2+}$ nanodomain.

GPCRs inhibit via Gβγ subunits, requiring a RAR motif in the N-terminal sequence.
CaVβ binds to the I–II linker.
GPCRs coupled to Gq/11 inhibit by reducing PIP2 levels and activating PKC.
Phosphorylation of the α1-interaction domain (AID).
Modulation of the C-terminal domain increases channel open probability and CDI.

L-type $Ca^{2+}$ channel blockers:
- Dihydropyridines (DHP, e.g., Nifedipine): Block by binding to the pore domain.
Phenylalkylamines (Verapamil) and Benzothiazepines:
- Bind to the central cavity of the pore.
- State-dependent block of L-type channels.
ω-Conotoxins:
- N-type $Ca^{2+}$ channel blockers.
- Block CaV2.2, mediating neurotransmitter release.
- Bind to the outer vestibule, blocking ion conductance.
- Analgesics for neuropathic pain.
ω-Agatoxins:
- P/Q-type $Ca^{2+}$ channel blockers.
- Block presynaptic $Ca^{2+}$ channels, reducing neurotransmitter release.
- Bind reversibly to the outside of the pore region.
- Cause spasms leading to paralysis.