Posttranslational Proteolytic Cleavage of α2δ Subunits: Functional Implications for High Voltage-Gated Calcium Channels

Abstract

CaV1 and CaV2 voltage-gated calcium channels contain an α1 subunit, which forms the selective pore and determines the functional properties of the channel. The α1 subunit is associated with auxiliary β and α2δ subunits, which modulate the trafficking and functional properties of the channels.α2δ subunits are membrane-associated proteins, which are highly glycosylated and possess multiple disulfide bonds. The α2 and δ peptides are encoded by a single gene as an uninterrupted α2δ pre-protein, which is cleaved post-translationally by a yet unidentified protease. The highly glycosylated α2 polypeptide, which is entirely extracellular, remains disulfide-bonded to the smaller δ moiety, which links the protein to the plasma membrane. All four known α2δ subtypes are cleaved into α2 and δ in vivo, but the significance of the proteolytic processing with regard of the function of α2δs as voltage-gated calcium channel subunits has remained elusive.We now provide evidence that preventing the cleavage, by mutating the proteolytic site in α2δ-1 and α2δ-3 subtypes, abolishes any effect of α2δ on CaV currents in tsA −201 cells expressing CaV2.2/β. In contrast, our experiments indicate that the trafficking of Cav2.2 channels by α2δ-1 is not affected by the lack of cleavage. In neurons, we find that all α2δ-1 in DRG neuronal axons is proteolytically cleaved; however, in cell bodies not all α2δ-1 is cleaved, indicating that there is a potential modulatory role for unprocessed α2δ.We are currently examining the nature of the protease(s) involved in the processing of α2δ-1 by application of selected chemical protease inhibitors.Targeting the protease responsible for the cleavage of α2δ-1 could provide a novel avenue for therapy of neuropathic pain, which would act on the same target as gabapentinoids, but by a completely different mechanism.