Tatyana Volkova

Cloning and structure of delta-latroinsectotoxin, a novel insect-specific member of the latrotoxin family. Functional expression requires C-terminal truncation

The venom of the black widow spider (BWSV) (Latrodectus mactans tredecimguttatus) contains several potent, high molecular mass (>110 kDa) neurotoxins that cause neurotransmitter release in a phylum-specific manner. The molecular mechanism of action of these proteins is poorly understood because their structures are largely unknown, and they have not been functionally expressed. This study reports on the primary structure of -latroinsectotoxin (-LIT), a novel insect-specific toxin from BWSV, that contains 1214 amino acids. -LIT comprises four structural domains: a signal peptide followed by an N-terminal domain that exhibits the highest degree of identity with other latrotoxins, a central region composed of 15 ankyrin-like repeats, and a C-terminal domain. The domain organization of -LIT is similar to that of other latrotoxins, suggesting that these toxins are a family of related proteins. The predicted molecular mass and apparent mobility of the protein (130 kDa) encoded in the -LIT gene differs from that of native -LIT purified from BWSV (110 kDa), suggesting that the toxin is produced by proteolytic processing of a precursor. MALDI-MS of purified native -LIT revealed a molecular ion with m/z+ of 110916 ± 100, indicating that the native -LIT is 991 amino acids in length. When the full-length -LIT cDNA was expressed in bacteria the protein product was inactive, but expression of a C-terminally truncated protein containing 991 residues produced a protein that caused massive neurotransmitter release at the locust neuromuscular junction at nanomolar concentrations. Channels formed in locust muscle membrane and artificial lipid bilayers by the native -LIT have a high Ca permeability, whereas those formed by truncated, recombinant protein do not.

Latrophilin, Neurexin, and Their Signaling-deficient Mutants Facilitate α-Latrotoxin Insertion into Membranes but Are Not Involved in Pore Formation

Pure α-latrotoxin is very inefficient at forming channels/pores in artificial lipid bilayers or in the plasma membrane of non-secretory cells. However, the toxin induces pores efficiently in COS-7 cells transfected with the heptahelical receptor latrophilin or the monotopic receptor neurexin. Signaling-deficient (truncated) mutants of latrophilin and latrophilin-neurexin hybrids also facilitate pore induction, which correlates with toxin binding irrespective of receptor structure. This rules out the involvement of signaling in pore formation. With any receptor, the α-latrotoxin pores are permeable to Ca2+ and small molecules including fluorescein isothiocyanate and norepinephrine. Bound α-latrotoxin remains on the cell surface without penetrating completely into the cytosol. Higher temperatures facilitate insertion of the toxin into the plasma membrane, where it co-localizes with latrophilin (under all conditions) and with neurexin (in the presence of Ca2+). Interestingly, on subsequent removal of Ca2+, α-latrotoxin dissociates from neurexin but remains in the membrane and continues to form pores. These receptor-independent pores are inhibited by anti-α-latrotoxin antibodies. Our results indicate that (i) α-latrotoxin is a pore-forming toxin, (ii) receptors that bind α-latrotoxin facilitate its insertion into the membrane, (iii) the receptors are not physically involved in the pore structure, (iv) α-latrotoxin pores may be independent of the receptors, and (v) pore formation does not require α-latrotoxin interaction with other neuronal proteins.

Functional Expression of a Recombinant Unitary Glutamate Receptor from Xenopus, Which Contains N-Methyl-D-aspartate (NMDA) and Non-NMDA Receptor Subunits

A cDNA encoding a 100-kDa subunit (XenNR1) of the N-methyl-D-aspartate (NMDA) glutamate receptor type has been cloned from Xenopus central nervous system. When XenNR1 is coexpressed in a mammalian cell line with a recently cloned 51-kDa non-NMDA receptor subunit (XenU1), also from Xenopus, it forms a functional unitary receptor exhibiting the pharmacological properties characteristic of both NMDA and non-NMDA receptors. Firstly, XenU1 can replace NR2 subunits, in complementing XenNR1 to introduce the ligand binding properties of a complete NMDA receptor. Second, responses to both NMDA and non-NMDA receptor agonists and antagonists were obtained in patch-clamp recordings from the cotransfected cells, but no significant responses were recorded when the cells were singly transfected. Third, from solubilized cell membranes from the cotransfected cells, an antibody to the NR1 subunit coprecipitated the binding sites of the non-NMDA receptor subunit. The unitary glutamate receptor has a unique set of properties that denote intersubunit interaction, including a glycine requirement for the responses to non-NMDA as well as to NMDA receptor agonists and voltage-dependent block by Mg2+ of the non-NMDA agonist responses.

Low molecular weight components from black widow spider venom

Highly purified alpha-latrotoxin from the black widow spider venom (alpha-LTX) consists of two polypeptides with mol. wts of 130,000 and 8000 (LMWP). We have isolated two low mol. wt proteins LMWP and LMWP2 from the low mol. wt fraction of this venom. The chemical properties of these proteins and partial amino acid sequence of novel protein LMWP2 were studied. By means of i.v. or intracerebroventricular injections into mice it was shown that low mol. wt components of the venom at concentrations of 2.3 mg/kg and 0.8 mg/kg, respectively, did not possess any direct toxic effect on vertebrates. Injections of each protein into the third thoracic segment of cockroaches Periplaneta americana (doses up to 80 micrograms/g) did not cause lethality or paralysis of insects.

Solution spatial structure of ‘long’ neurotoxin M9 from the scorpion Buthus eupeus by 1H-NMR spectroscopy

1H-NMR spectra of Buthus eupeus neurotoxin M9 (66 amino acid residues, four disulfide bonds) reveal two slowly exchangeable conformations at acidic pH. The spatial structure of the conformer prevailing under physiologically relevant conditions has been determined from two-dimensional 1H-NMR data treated by means of a distance geometry algorithm and refined by molecular modelling. Interrelation between the structure and function of mammalian neurotoxin M9 is discussed by comparing its conformation with those of the scorpion insectotoxins which exhibit different biological specificity (insectotoxins v-2, v-3 and I5A).

Modulation of functional activities of the neurotoxin from black widow spider venom.

We have studied the action of an α‐latrotoxin (α‐LTX) complex of two polypeptides (LTX 130 kDa and low molecular weight protein (LMWP) 8 kDa) and the action of a venom fraction containing LTX with excess LMWP on calcium influx into synaptosomes and PC 12 cells as well as on [14C[GABA release from synaptosomes. Both preparations considerably activate calcium influx and stimulate [14C]GABA release from synaptosomes. Preincubation of both preparations with antibodies against a 14 amino acid residue C‐terminal peptide of LMWP differentially modulates these effects. Antibodies inhibit induced calcium influx and enhance induced GABA release.

New water-soluble dosage forms of 1,2,4-thiadiazole derivative on the basis of inclusion complexes with cyclodextrins

Effect of cyclodextrins on aqueous solubility of 1-[5-(3-chloro-phenylamino)-1,2,4-thiadiazol-3-yl]-propan-2-ol (I), which was synthesized and proposed for the treatment of Alzheimer’s disease, was studied. First of all, inclusion complex formation of I with different cyclodextrins was studied in aqueous solutions. Formation of more stable complexes with β- and hydroxypropyl-β-cyclodextrins was revealed. Next, solid inclusion complexes of I with β- and hydroxypropyl-β-cyclodextrins were prepared by the mechanical grinding, and their existence in the solid state was confirmed by powder X-ray diffraction, FTIR spectroscopy, microscopy, and thermochemical methods. Dissolution testing of the tablets of pure I and its complexes with β- and hydroxypropyl-β-cyclodextrins in the aqueous buffered solutions simulated the biological environment was carried out. The observed drastic enhancement of dissolution extent and dissolution rate of the complexes was attributed to the inclusion complex formation.

Selective binding of methotrexate to monomeric, dimeric and polymeric cyclodextrins

The difference in the complexation affinity of methotrexate (MTX) to the naturally occurring α-, β- and γ-cyclodextrins (CDs) as well as to the novel synthetic dimeric and polymeric β-cyclodextrins was revealed. The effect of CD structure complicity on the complexation with MTX was discussed. It was found that β-CD and dimeric β-CD display a higher binding ability to MTX. Dimeric β-CD acts as a ditopic host forming rather stable 1 : 2 inclusion complexes with MTX. In contrast, the binding of MTX with polymeric β-CD is not sufficient due to the possible steric hindrance. The thermodynamics and binding modes of CDs with MTX were obtained and analyzed. The influence of buffer pH on the complexation process was considered. The binding affinity of β-CD to different ionized forms of MTX was examined using the capillary electrophoresis technique.

Effects of Biorelevant Media Components on Dissolution Behaviour of 1,2,4-Thiadiazole Derivative Designed for Alzheimerʼs Disease Prevention

In this study, dissolution behaviour of 1,2,4‐thiadiazole derivative (1‐[5‐(3‐chloro‐phenylamino)‐1,2,4‐thiadiazol‐3‐yl]‐propan‐2‐ol) displaying an anti‐Alzheimer activity was examined in biorelevant media such as Simulated Gastric Fluid (SGF, pH 1.2), Fasted State Simulated Gastric Fluid (FaSSGF, pH 1.6) and Fasted State Simulated Intestinal Fluid (FaSSIF, pH 6.5). It was found that solubility and dissolution rate of 1,2,4‐thiadiazole derivative under consideration are not strongly dependent on pH, whereas these parameters are significantly affected by the buffer composition. Dissolution was found to be more effective in buffers composed of the surfactant micelles. It was demonstrated that considerable increase in solubility and dissolution rate in SGF is achieved through the interaction of 1,2,4‐thiadiazole derivative with the micelles of sodium dodecyl sulfate. On the contrary, CMC of sodium taurochalate was shifted in the presence of 1,2,4‐thiadiazole derivative, therefore, dissolution process is not so efficient in FaSSIF. Interactions occurring between 1,2,4‐thiadiazole derivative and the components of biorelevant media were investigated in detail by means of UV/VIS spectroscopy, 1H‐NMR and phase solubility methods.

Selective Interactions of Cyclodextrins with Isomeric 1,2,4-Thiadiazole Derivatives Displaying Pharmacological Activity: Spectroscopy Study

Complex formation of α-, β- and γ-cyclodextrins with 1,2,4-thiadiazole derivatives, which are structural isomers displaying an activity in the treatment of Alzheimer’s disease, was examined by the use of 1H NMR and UV-spectroscopy. Stability constants of the complexes formed between cyclodextrins and thiadiazoles in two different buffers (pH 1.2 and 7.4) were calculated and analyzed. The temperature dependences of the stability constants were used to evaluate the enthalpy and entropy changes of complex formation. It was demonstrated that selectivity of the interactions between cyclodextrins and isometric 1,2,4-thiadiazole derivatives is determined by the size of macrocyclic cavity and relative position of the side groups in the benzene ring of the guest molecules. Higher stability of the complexes in the acidic medium (pH 1.2) in comparison with the slightly alkaline solution (pH 7.4) is caused by the important role of surface interactions.