Molecular Characterization of an SV Capture Site in the Mid-Region of the Presynaptic CaV2.1 Calcium Channel C-Terminal

The binding of Mint/X11 PDZ domains to Ca V 2 calcium channels predates bilaterian animals

PDZ domain mediated interactions with voltage-gated calcium (Ca V ) channel C-termini play important roles in localizing and compartmentalizing membrane Ca 2+ signaling. The first such interaction discovered was between the neuronal multi-domain protein Mint-1, and the presynaptc calcium channel Ca V 2.2 in mammals. Although the physiological significance of this interaction is unclear, its occurrence in vertebrates and bilaterian invertebrates suggests important and conserved functions. In this study, we explore the evolutionary origins of Mint and its interaction with Ca V 2 channels. Phylogenetic and structural in silico analyses revealed that Mint is an animal-specific gene, like Ca V 2 channels, which bears a highly divergent N-terminus but strongly conserved C-terminus comprised of a phosphotyrosine binding domain, two tandem PDZ domains (PDZ-1 and PDZ-2), and a C-terminal auto-inhibitory element that binds and inhibits PDZ-1. Also deeply conserved are other Mint interacting proteins, namely amyloid precursor and related proteins, presenilins, neurexin, as well as CASK and Veli which form a tripartite complex with Mint in bilaterians. Through yeast 2-hybrid and bacterial 2-hybrid experiments, we show that Mint and Ca V 2 channels from cnidarians and placozoans interact in vitro , and in situ hybridization revealed co-expression of corresponding transcripts in dissociated neurons from the cnidarian Nematostella vectensis . Unexpectedly, the Mint orthologue from the ctenophore Hormiphora californiensis was able to strongly bind the divergent C-terminal ligands of cnidarian and placozoan Ca V 2 channels, despite neither the ctenophore Mint, nor the placozoan and cnidarian orthologues, binding the ctenophore Ca V 2 channel C-terminus. Altogether, our analyses provide a model for the emergence of this interaction in early animals first via adoption of a PDZ ligand by Ca V 2 channels, followed by sequence changes in the ligand that caused a modality switch for binding to Mint.

The role of specific isoforms of CaV2 and the common C-terminal of CaV2 in calcium channel function in sensory neurons of Aplysia

The presynaptic release apparatus can be specialized to enable specific synaptic functions. Habituation is the diminishing of a physiological response to a frequently repeated stimulus and in Aplysia, habituation to touch is mediated by a decrease in transmitter release from the sensory neurons that respond to touch even after modest rates of action potential firing. This synaptic depression is not common among Aplysia synaptic connections suggesting the presence of a release apparatus specialized for this depression. We found that specific splice forms of ApCaV2, the calcium channel required for transmitter release, are preferentially used in sensory neurons, consistent with a specialized release apparatus. However, we were not able to find a specific ApCaV2 splice uniquely required for synaptic depression. The C-terminus of ApCaV2 alpha1 subunit retains conserved binding to Aplysia rab-3 interacting molecule (ApRIM) and ApRIM-binding protein (ApRBP) and the C-terminus is required for full synaptic expression of ApCaV2. We also identified a splice form of ApRIM that did not interact with the ApCav2 alpha 1 subunit, but it was not preferentially used in sensory neurons.

Early Metazoan Origin and Multiple Losses of a Novel Clade of RIM Presynaptic Calcium Channel Scaffolding Protein Homologs

The precise localization of Ca_V2 voltage-gated calcium channels at the synapse active zone requires various interacting proteins, of which, Rab3-interacting molecule or RIM is considered particularly important. In vertebrates, RIM interacts with Ca_V2 channels in vitro via a PDZ domain that binds to the extreme C-termini of the channels at acidic ligand motifs of D/E-D/E/H-WC-_COOH, and knockout of RIM in vertebrates and invertebrates disrupts Ca_V2 channel synaptic localization and synapse function. Here, we describe a previously uncharacterized clade of RIM proteins bearing domain architectures homologous to those of known RIM homologs, but with some notable differences including key amino acids associated with PDZ domain ligand specificity. This novel RIM emerged near the stem lineage of metazoans and underwent extensive losses, but is retained in select animals including the early-diverging placozoan Trichoplax adhaerens, and molluscs. RNA expression and localization studies in Trichoplax and the mollusc snail Lymnaea stagnalis indicate differential regional/tissue type expression, but overlapping expression in single isolated neurons from Lymnaea. Ctenophores, the most early-diverging animals with synapses, are unique among animals with nervous systems in that they lack the canonical RIM, bearing only the newly identified homolog. Through phylogenetic analysis, we find that Ca_V2 channel D/E-D/E/H-WC-_COOH like PDZ ligand motifs were present in the common ancestor of cnidarians and bilaterians, and delineate some deeply conserved C-terminal structures that distinguish Ca_V1 from Ca_V2 channels, and Ca_V1/Ca_V2 from Ca_V3 channels.

Neurotransmitter Release Site Replenishment and Presynaptic Plasticity

An action potential (AP) triggers neurotransmitter release from synaptic vesicles (SVs) docking to a specialized release site of presynaptic plasma membrane, the active zone (AZ). The AP simultaneously controls the release site replenishment with SV for sustainable synaptic transmission in response to incoming neuronal signals. Although many studies have suggested that the replenishment time is relatively slow, recent studies exploring high speed resolution have revealed SV dynamics with milliseconds timescale after an AP. Accurate regulation is conferred by proteins sensing Ca²⁺ entering through voltage-gated Ca²⁺ channels opened by an AP. This review summarizes how millisecond Ca²⁺ dynamics activate multiple protein cascades for control of the release site replenishment with release-ready SVs that underlie presynaptic short-term plasticity.

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

The dominant role of CaV2 voltage-gated calcium channels for driving neurotransmitter release is broadly conserved. Given the overlapping functional properties of CaV2 and CaV1 channels, and less so CaV3 channels, it is unclear why there have not been major shifts toward dependence on other CaV channels for synaptic transmission. Here, we provide a structural and functional profile of the CaV2 channel cloned from the early-diverging animal Trichoplax adhaerens, which lacks a nervous system but possesses single gene homologues for CaV1-CaV3 channels. Remarkably, the highly divergent channel possesses similar features as human CaV2.1 and other CaV2 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 CaV2 suggests that the core features of presynaptic CaV2 channels were established early during animal evolution, after CaV1 and CaV2 channels emerged via proposed gene duplication from an ancestral CaV1/2 type channel. The Trichoplax channel was relatively insensitive to mammalian CaV2 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 CaV2.1 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.

Synapse and Active Zone Assembly in the Absence of Presynaptic Ca2+ Channels and Ca2+ Entry

Presynaptic Ca_V2 channels are essential for Ca²⁺-triggered exocytosis. In addition, there are two competing models for their roles in synapse structure. First, Ca²⁺ channels or Ca²⁺ entry may control synapse assembly. Second, active zone proteins may scaffold Ca_V2s to presynaptic release sites, and synapse structure is Ca_V2 independent. Here, we ablated all three Ca_V2s using conditional knockout in cultured hippocampal neurons or at the calyx of Held, which abolished evoked exocytosis. Compellingly, synapse and active zone structure, vesicle docking, and transsynaptic nano-organization were unimpaired. Similarly, long-term blockade of action potentials and Ca²⁺ entry did not disrupt active zone assembly. Although Ca_V2 knockout impaired the localization of β subunits, α2δ-1 localized normally. Rescue with Ca_V2 restored exocytosis, and Ca_V2 active zone targeting depended on the intracellular C-terminus. We conclude that synapse assembly is independent of Ca_V2s or Ca²⁺ entry through them. Instead, active zone proteins recruit and anchor Ca_V2s via Ca_V2 C-termini.