Kirill E. Volynski

Structure of α-latrotoxin oligomers reveals that divalent cation-dependent tetramers form membrane pores

We report here the first three-dimensional structure of α-latrotoxin, a black widow spider neurotoxin, which forms membrane pores and stimulates secretion in the presence of divalent cations. We discovered that α-latrotoxin exists in two oligomeric forms: it is dimeric in EDTA but forms tetramers in the presence of Ca2+ or Mg2+. The dimer and tetramer structures were determined independently at 18 Å and 14 Å resolution, respectively, using cryo-electron microscopy and angular reconstitution. The α-latrotoxin monomer consists of three domains. The N- and C-terminal domains have been identified using antibodies and atomic fitting. The C4-symmetric tetramers represent the active form of α-latrotoxin; they have an axial channel and can insert into lipid bilayers with their hydrophobic base, providing the first model of α-latrotoxin pore formation.

Latrophilin 1 and its endogenous ligand Lasso/teneurin-2 form a high-affinity transsynaptic receptor pair with signaling capabilities.

Latrophilin 1 (LPH1), a neuronal receptor of α-latrotoxin, is implicated in neurotransmitter release and control of presynaptic Ca2+. As an “adhesion G-protein-coupled receptor,” LPH1 can convert cell surface interactions into intracellular signaling. To examine the physiological functions of LPH1, we used LPH1’s extracellular domain to purify its endogenous ligand. A single protein of ∼275 kDa was isolated from rat brain and termed Lasso. Peptide sequencing and molecular cloning have shown that Lasso is a splice variant of teneurin-2, a brain-specific orphan cell surface receptor with a function in neuronal pathfinding and synaptogenesis. We show that LPH1 and Lasso interact strongly and specifically. They are always copurified from rat brain extracts. Coculturing cells expressing LPH1 with cells expressing Lasso leads to their mutual attraction and formation of multiple junctions to which both proteins are recruited. Cells expressing LPH1 form chimerical synapses with hippocampal neurons in cocultures; LPH1 and postsynaptic neuronal protein PSD-95 accumulate on opposite sides of these structures. Immunoblotting and immunoelectron microscopy of purified synapses and immunostaining of cultured hippocampal neurons show that LPH1 and Lasso are enriched in synapses; in both systems, LPH1 is presynaptic, whereas Lasso is postsynaptic. A C-terminal fragment of Lasso interacts with LPH1 and induces Ca2+ signals in presynaptic boutons of hippocampal neurons and in neuroblastoma cells expressing LPH1. Thus, LPH1 and Lasso can form transsynaptic complexes capable of inducing presynaptic Ca2+ signals, which might affect synaptic functions.

Latrophilin fragments behave as independent proteins that associate and signal on binding of LTXN4C

Heptahelical, or G‐protein‐coupled, receptors control many cellular functions and normally consist of one polypeptide chain. In contrast, heptahelical receptors that belong to the long N‐terminus, group B (LNB) family are cleaved constitutively into two fragments. The N‐terminal fragments (NTFs) resemble cell‐adhesion proteins and the C‐terminal fragments (CTFs) are typical G‐protein‐coupled receptors (GPCRs) with seven transmembrane regions. However, the functional roles of this cleavage and of any subsequent NTF–CTF interactions remain to be identified. Using latrophilin, a well‐studied member of the LNB family, we now demonstrate that cleavage is critical for delivery of this receptor to the cell surface. On the plasma membrane, NTF and CTF behave as separate membrane proteins involved, respectively, in cell‐surface reception and signalling. The two fragments can also internalise independently. However, separated NTF and CTF can re‐associate on solubilisation. Agonist binding to NTF on the cell surface also induces re‐association of fragments and provokes signal transduction via CTF. These findings define a novel principle of structural and functional organisation of the cleaved, two‐subunit GPCRs.

Differential triggering of spontaneous glutamate release by P/Q-, N- and R-type Ca2+ channels

The role of voltage-gated Ca2+ channels (VGCCs) in spontaneous miniature neurotransmitter release is incompletely understood. We found that stochastic opening of P/Q-, N- and R-type VGCCs accounts for ∼50% of all spontaneous glutamate release at rat cultured hippocampal synapses, and that R-type channels have a far greater role in spontaneous than in action potential–evoked exocytosis. VGCC-dependent miniature neurotransmitter release (minis) showed similar sensitivity to presynaptic Ca2+ chelation as evoked release, arguing for direct triggering of spontaneous release by transient spatially localized Ca2+ domains. Experimentally constrained three-dimensional diffusion modeling of Ca2+ influx–exocytosis coupling was consistent with clustered distribution of VGCCs in the active zone of small hippocampal synapses and revealed that spontaneous VGCCs openings can account for the experimentally observed VGCC-dependent minis, although single channel openings triggered release with low probability. Uncorrelated stochastic VGCC opening is therefore a major trigger for spontaneous glutamate release, with differential roles for distinct channel subtypes.

Nanoscale-Targeted Patch-Clamp Recordings of Functional Presynaptic Ion Channels

Summary Direct electrical access to presynaptic ion channels has hitherto been limited to large specialized terminals such as the calyx of Held or hippocampal mossy fiber bouton. The electrophysiology and ion-channel complement of far more abundant small synaptic terminals (≤1 μm) remain poorly understood. Here we report a method based on superresolution scanning ion conductance imaging of small synapses in culture at approximately 100–150 nm 3D resolution, which allows presynaptic patch-clamp recordings in all four configurations (cell-attached, inside-out, outside-out, and whole-cell). Using this technique, we report presynaptic recordings of K+, Na+, Cl−, and Ca2+ channels. This semiautomated approach allows direct investigation of the distribution and properties of presynaptic ion channels at small central synapses. Video Abstract

Independent Regulation of Basal Neurotransmitter Release Efficacy by Variable Ca2+ Influx and Bouton Size at Small Central Synapses

Concurrent imaging of vesicular release and calcium dynamics in small presynaptic boutons shows that the fusion probability of readily releasable vesicles is a major determinant of the overall variability in release probability.

Functional Cross-interaction of the Fragments Produced by the Cleavage of Distinct Adhesion G-protein-coupled Receptors

The unusual adhesion G-protein-coupled receptors (aGPCRs) contain large extracellular N-terminal domains, which resemble cell-adhesion receptors, and C-terminal heptahelical domains, which may couple to G-proteins. These receptors are cleaved post-translationally between these domains into two fragments (NTF and CTF). Using the aGPCR latrophilin 1, we previously demonstrated that the fragments behave as independent cell-surface proteins. Upon binding the agonist, α-latrotoxin (LTX), latrophilin fragments reassemble and induce intracellular signaling. Our observations raised important questions: is the aGPCR signaling mediated by reassembled fragments or by any non-cleaved receptors? Also, can the fragments originating from distinct aGPCRs form hybrid complexes? To answer these questions, we created two types of chimerical constructs. One contained the CTF of latrophilin joined to the NTF of another aGPCR, EMR2; the resulting protein did not bind LTX but, similar to latrophilin, could couple to G-proteins. In another construct, the NTF of latrophilin was fused with the C terminus of neurexin; this chimera bound LTX but could not signal via G-proteins. Both constructs were efficiently cleaved in cells. When the two constructs were co-expressed, their fragments could cross-interact, as shown by immunoprecipitation. Furthermore, LTXN4C induced intracellular Ca2+ signaling only in cells expressing both constructs but not each individual construct. Finally, we demonstrated that fragments of unrelated aGPCRs can be cross-immunoprecipitated from live tissues. Thus, (i) aGPCR fragments behave as independent proteins, (ii) the complementary fragments from distinct aGPCRs can cross-interact, and (iii) these cross-complexes are functionally active. This unusual cross-assembly of aGPCR fragments could couple cell-surface interactions to multiple signaling pathways.

Tetramerisation of α-latrotoxin by divalent cations is responsible for toxin-induced non-vesicular release and contributes to the Ca2+-dependent vesicular exocytosis from synaptosomes.

A novel procedure of alpha-latrotoxin (alpha LTX) purification has been developed. Pure alpha LTX has been demonstrated to exist as a very stable homodimer. Such dimers further assemble into tetramers, and Ca(2+), Mg(2+) or higher toxin concentrations facilitate this process. However, when the venom is treated with EDTA, purified alpha LTX loses the ability to tetramerise spontaneously; the addition of Mg(2+) or Ca(2+) restores this ability. This suggests that alphaLTX has some intrinsically bound divalent cation(s) that normally support its tetramerisation. Single-particle cryoelectron microscopy and statistical image analysis have shown that: 1) the toxin has a non-compact, branching structure; 2) the alpha LTX dimers are asymmetric; and 3) the tetramers are symmetric and have a 25 A-diameter channel in the centre. Both alpha LTX oligomers bind to the same receptors in synaptosomes and rat brain sections. To study the effects of the dimers and tetramers on norepinephrine release from rat cerebrocortical synaptosomes, we used the EDTA-treated and untreated toxin preparations. The number of tetramers present in a preparation correlates with alpha LTX pore formation, suggesting that the tetramers are the pore-forming species of alpha LTX. The toxin actions mediated by the pore include: 1) Ca(2+) entry from the extracellular milieu; and 2) passive efflux of neurotransmitters via the pore that occurs independently of Ca(2+). The Ca(2+)-dependent alpha LTX-stimulated secretion conforms to all criteria of vesicular exocytosis but also depends upon intact intracellular Ca(2+) stores and functional phospholipase C (PLC). The Ca(2+)-dependent effect of the toxin is stronger when dimeric alpha LTX is used, indicating that higher receptor occupancy leads to its stronger activation, which contributes to stimulation of neuroexocytosis. In contrast, the Ca(2+)-independent release measured biochemically represents leakage of neurotransmitters through the toxin pore. These results are discussed in relation to the previously published observations.

CaMKII translocation requires local NMDA receptor-mediated Ca2+ signaling

Excitatory synaptic transmission and plasticity are critically modulated by N‐methyl‐D‐aspartate receptors (NMDARs). Activation of NMDARs elevates intracellular Ca2+ affecting several downstream signaling pathways that involve Ca2+/calmodulin‐dependent protein kinase II (CaMKII). Importantly, NMDAR activation triggers CaMKII translocation to synaptic sites. NMDAR activation failed to induce Ca2+ responses in hippocampal neurons lacking the mandatory NMDAR subunit NR1, and no EGFP‐CaMKIIα translocation was observed. In cells solely expressing Ca2+‐impermeable NMDARs containing NR1N598R‐mutant subunits, prolonged NMDA application elevated internal Ca2+ to the same degree as in wild‐type controls, yet failed to translocate CaMKIIα. Brief local NMDA application evoked smaller Ca2+ transients in dendritic spines of mutant compared to wild‐type cells. CaMKIIα mutants that increase binding to synaptic sites, namely CaMKII‐T286D and CaMKII‐TT305/306VA, rescued the translocation in NR1N598R cells in a glutamate receptor‐subtype‐specific manner. We conclude that CaMKII translocation requires Ca2+ entry directly through NMDARs, rather than other Ca2+ sources activated by NMDARs. Together with the requirement for activated, possibly ligand‐bound, NMDARs as CaMKII binding partners, this suggests that synaptic CaMKII accumulation is an input‐specific signaling event.

Functional expression of α‐latrotoxin in baculovirus system

To facilitate the study of the mechanism of α‐latrotoxin action, it is necessary to create a biologically active recombinant toxin. Mature α‐latrotoxin is naturally produced by post‐translational cleavage, probably at two furin sites located at the N‐ and C‐termini of the precursor. A recombinant baculovirus has now been constructed, which encodes the melittin signal peptide fused to the 130‐kDa mature toxin between the furin sites. Insect cells, infected with this baculovirus, secreted recombinant α‐latrotoxin. This was partially purified and proved indistinguishable from the natural toxin with respect to its molecular mass, immunostaining, toxicity to mice, binding to α‐latrotoxin receptors (latrophilin or neurexin Iα) and electrophysiological recording in the mouse diaphragm. The successful expression of recombinant α‐latrotoxin permits mutational analysis of the toxin.