Conserved biophysical features of the CaV2 presynaptic Ca2+ channel homologue from the early-diverging animal Trichoplax adhaerens

Abstract

The dominant role of Ca_V2 voltage-gated calcium channels for driving neurotransmitter release is broadly conserved. Given the overlapping functional properties of Ca_V2 and Ca_V1 channels, and less so Ca_V3 channels, it is unclear why there have not been major shifts toward dependence on other Ca_V channels for synaptic transmission. Here, we provide a structural and functional profile of the Ca_V2 channel cloned from the early-diverging animal Trichoplax adhaerens, which lacks a nervous system but possesses single gene homologues for Ca_V1-Ca_V3 channels. Remarkably, the highly divergent channel possesses similar features as human Ca_V2.1 and other Ca_V2 channels, including high voltage-activated currents that are larger in external Ba²⁺ than in Ca²⁺; voltage-dependent kinetics of activation, inactivation, and deactivation; and bimodal recovery from inactivation. Altogether, the functional profile of Trichoplax Ca_V2 suggests that the core features of presynaptic Ca_V2 channels were established early during animal evolution, after Ca_V1 and Ca_V2 channels emerged via proposed gene duplication from an ancestral Ca_V1/2 type channel. The Trichoplax channel was relatively insensitive to mammalian Ca_V2 channel blockers ω-agatoxin-IVA and ω-conotoxin-GVIA and to metal cation blockers Cd²⁺ and Ni²⁺ Also absent was the capacity for voltage-dependent G-protein inhibition by co-expressed Trichoplax Gβγ subunits, which nevertheless inhibited the human Ca_V2.1 channel, suggesting that this modulatory capacity evolved via changes in channel sequence/structure, and not G proteins. Last, the Trichoplax channel was immunolocalized in cells that express an endomorphin-like peptide implicated in cell signaling and locomotive behavior and other likely secretory cells, suggesting contributions to regulated exocytosis.

Keywords: CaV2 presynaptic Ca2+ channels; G protein; Gβγ inhibition; Trichoplax adhaerens; Voltage-gated Ca2+ channels; calcium channel; evolution; exocytosis; ion channel; patch clamp; patch clamp electrophysiology; pharmacology; synapse; synapse evolution.