The Role of Accessory 2ad Subunits in the Trafficking and Expression of Voltage Gated Calcium Channels
Voltage dependent calcium (CaV) channels act as key mediators of several fundamentally important processes in both excitable and non-excitable cells by initiating calcium entry leading to muscle contraction, changes in gene transcription, and vesicular secretion of neurotransmitters or hormones. Expression of the α1, pore forming subunit of CaV channels (as well as some of their biophysical properties: voltage dependence, kinetics, etc.) is highly dependent on the accessory proteins β and α2δ. In short, the β subunits are important in aiding the trafficking of CaV channels through the ER and to the plasma membrane, and α2δs in stabilizing them once there, probably by slowing internalization and aiding in recycling of internalized channel complexes, but specific roles of individual α2δ subunits remain unknown. A better understanding of how CaV channels are influenced by α2δ subunits has great biological significance if we are to understand the expression, localization, and function of these channels. Further, since therapeutically important compounds such as pregabalin and gabapentin are α2δ-1 and -2 ligands that work through α2δ by reducing CaV channel density, a better understanding of the role of α2δ subunits in biology may help improve the clinical utility of these drugs while reducing off target or unwanted effects.
Recent work from my lab (Scott and Kammermeier 2017) has focused on the regulation of CaV2 channel expression in adult rat sympathetic neurons. In these neurons that natively express only CaV2.2 (about 75%) and CaV2.3 (about 25%), we have found that heterologous expression of either CaV2.1 or CaV2.3 results in increased calcium current density, while heterologous expression of CaV2.2 does not. Interestingly, recombinant CaV2.1 but not CaV2.3 displaces some native CaV2.2 channels (reduces current density sensitive to the CaV2.2 selective blocker ω-conotoxin mVIIA). A closer examination of the mechanism of CaV2.2 channel displacement has revealed an interesting and selective role for α2δ subunits. Expression of individual α2δ subunits did not enhance current density or alter the ratio of native CaV2.2: CaV2.3 channels expressed. However, α2δ-1 and -2 expression promoted the displacement of native CaV2.2 by both CaV2.1 and CaV2.3. By contrast, α2δ-3 expression protected the native CaV2.2 channels from displacement by recombinant channel expression. These data suggest that individual α2δ subunits play a novel role in protecting specific CaV channels from degradation. Because selectivity of the physical interactions between α2δ subunits and CaV channels has not been demonstrated, and because, paradoxically, our preliminary data strongly suggest that interactions between distinct α2δ subunits and specific CaV isoforms must be selective, we hypothesized that their interaction is highly selective only in the endocytic compartment, where α2δ is important in promoting recycling of channel complexes. Since the endocytic compartment is acidic compared to the cytosolic or extracellular spaces, it’s plausible that the α2δ-CaV channel interaction has an altered affinity there that may have eluded detection by standard biochemical methods.
Important questions:
- Is the affinity of α2δ subunits for CaV channels more selective in acidic environments?
- Can neutralization of the endocytic pH prevent channel prioritization by α2δ subunits?
- Are similar mechanisms leading to prioritization of channel expression utilized both cell bodies and in presynaptic terminals?