The life cycle of voltage-gated Ca2+ channels in neurons: an update on the trafficking of neuronal calcium channels

Abstract

Neuronal voltage-gated Ca²⁺ (Ca_V) channels play a critical role in cellular excitability, synaptic transmission, excitation-transcription coupling and activation of intracellular signaling pathways. Ca_V channels are multiprotein complexes and their functional expression in the plasma membrane involves finely tuned mechanisms, including forward trafficking from the endoplasmic reticulum (ER) to the plasma membrane, endocytosis and recycling. Whether genetic or acquired, alterations and defects in the trafficking of neuronal Ca_V channels can have severe physiological consequences. In this review, we address the current evidence concerning the regulatory mechanisms which underlie precise control of neuronal Ca_V channel trafficking and we discuss their potential as therapeutic targets.

Keywords: calcium channel; internalization; trafficking.

Figure 1. Schematic representation of the structure of Ca_V channels

The Ca_Vα₁ subunit is formed by four repeat domains (I–IV) each containing six transmembrane segments: S1–S4 constitute the voltage sensor domain (S4 segments contain positively charged residues) and S5–S6 constitute the pore domain (the P loops contain acidic residues that contribute to the selectivity filter of the channel). Ca_Vα₁ subunits can be associated with auxiliary subunits: an extracellular α₂δ subunit attached to the plasma membrane by a glycosyl phosphatidylinositol (GPI) anchor and an intracellular β subunit which contain a src homology 3 (SH3) domain and a GK domain.

Figure 2. Diagram of forward trafficking mechanisms of Ca_V channels from the ER to the plasma membrane

Newly synthesized peptides are translocated to the rough ER (RER) where they associate with auxiliary subunits and are subjected to post-translational modifications including glycosylation. Ca_V channels are then trafficked to the plasma membrane via the Golgi apparatus and trafficking endosomes. Along the way, misfolded proteins are identified by quality-control mechanisms and targeted for degradation. The association of Ca_Vα₁ with β subunits prevents the ubiquitination of the Ca_Vα₁ subunit which protect channels from degradation by the proteasome. The adaptor protein AP1 interacts with Ca_Vα₁ and contribute to the incorporation of channels to the plasma membrane via clathrin-coated vesicles.

Figure 3. Schematic depiction of the internalization of Ca_V channels

The stability of Ca_V channels at the plasma membrane is determined by the activity of the channel and by the interaction with regulatory proteins. G protein-coupled receptors (GPCRs), like the D2R dopamine receptor, have been shown to directly interact with Ca_V2.2 channels and to induce the internalization of the complex when the receptor is activated by its agonist. The adaptor protein 2 (AP2) has been implicated in this internalization process. For Ca_V3.2, the balance between ubiquitination/de-ubiquitination is key to the stability of the channels in the plasma membrane. USP5, a de-ubiquitinase, removes ubiquitin from Ca_V3.2 increasing the lifetime of the channels at the plasma membrane whereas WWP1, a ubiquitin ligase, transfers ubiquitin to Ca_V3.2 and promotes the endocytosis of the ubiquitinated channels. Endocytosed Ca_V channels are then either recycled or degraded.

Figure 4. Schematic of the recycling and degradation of Ca_V channels

Endocytosed Ca_V channels are either recycled or degraded. Rab11, a small GTPase that controls key events of vesicular transport, is suspected to be a major player in the recycling of Ca_V to the plasma membrane by interacting either with the Ca_Vα₁ subunit or with the α₂δ auxiliary subunit. Following their endocytosis Ca_V channels have been shown to be co-localized with Rab7, a marker for late endosomes and lyzosomes.