Site-specific fluorescein-labeled cobra alpha-toxin. Biochemical and spectroscopic characterization

Snake Toxins Labeled by Green Fluorescent Protein or Its Synthetic Chromophore are New Probes for Nicotinic acetylcholine Receptors

Fluorescence can be exploited to monitor intermolecular interactions in real time and at a resolution up to a single molecule. It is a method of choice to study ligand-receptor interactions. However, at least one of the interacting molecules should possess good fluorescence characteristics, which can be achieved by the introduction of a fluorescent label. Gene constructs with green fluorescent protein (GFP) are widely used to follow the expression of the respective fusion proteins and monitor their function. Recently, a small synthetic analogue of GFP chromophore (p-HOBDI-BF₂) was successfully used for tagging DNA molecules, so we decided to test its applicability as a potential fluorescent label for proteins and peptides. This was done on α-cobratoxin (α-CbTx), a three-finger protein used as a molecular marker of muscle-type, neuronal α7 and α9/α10 nicotinic acetylcholine receptors (nAChRs), as well as on azemiopsin, a linear peptide neurotoxin selectively inhibiting muscle-type nAChRs. An activated N-hydroxysuccinimide ester of p-HOBDI-BF₂ was prepared and utilized for toxin labeling. For comparison we used a recombinant α-CbTx fused with a full-length GFP prepared by expression of a chimeric gene. The structure of modified toxins was confirmed by mass spectrometry and their activity was characterized by competition with iodinated α-bungarotoxin in radioligand assay with respective receptor preparations, as well as by thermophoresis. With the tested protein and peptide neurotoxins, introduction of the synthetic GFP chromophore induced considerably lower decrease in their affinity for the receptors as compared with full-length GFP attachment. The obtained fluorescent derivatives were used for nAChR visualization in tissue slices and cell cultures.

Binding of long-chain alpha-neurotoxin would stabilize the resting state of nAChR: a comparative study with alpha-conotoxin

Background

The details of interaction in a complex between potent antagonists such as long chain α-neurotoxins and α-conotoxins with nicotinic acetylcholine receptor (nAChR), and conformational changes induced by these antagonists, are not yet clear.

Modeling

In order to uncover some of these critical structural features, we conducted a docking simulation and a molecular dynamics simulation (MD) of a model of the ligand binding domain of nAChR in complex with a long-chain α-neurotoxin and an α-conotoxin.

Results

Our docking results confirm the claim that T.nAChR is in the basal or resting state, which favors binding to the alpha-neurotoxins. Moreover, more correct "hits" for the α/γ interface upon docking for conotoxin-nAChR confirm the preference of conotoxin GI for the α/γ interface. More importantly, upon binding of α-neurotoxin, ligand-bonded nAChR is less dynamic in certain domains than the apo form of the conotoxin-AChR complex. Some critical interactions in the binding site such as the salt bridge formed between K145/D200 in the neurotoxin-nAChR complex is further stabilized during the MD simulation, while it is obviously more labile in the apo form.

Conclusion

These observations could support the claim that alpha neurotoxins stabilize the nAChR resting state.

Impaired neutrophils in children with the typical form of hemolytic uremic syndrome

Experimental and clinical evidence suggest that activated neutrophils (PMN) could contribute to endothelial damage in Hemolytic Uremic Syndrome (D+HUS). Additionally, while PMN-activating cytokines and PMN-derived products have been found in D+HUS sera, we have demonstrated phenotypic alterations in D+HUS PMN compatible with a deactivation state. Here, we investigated whether D+HUS PMN were actually hyporesponsive, and explored some of the mechanisms probably involved in their derangement. Twenty-two D+HUS children were bled in the acute period, and blood samples from healthy, acute uremic and neutrophilic children were obtained as controls. We evaluated degranulation markers in response to cytokines, intracellular granule content, and reactive oxygen species (ROS) generation in circulating D+HUS and control PMN. The influence of D+HUS-derived plasma and the direct effects of Stx in vitro were evaluated on healthy donors' PMN. We found that D+HUS PMN presented reduced degranulatory capacity in response to cytokines and intracellular granule content, and decreased ROS generation. D+HUS plasma or Stx did not affect the phenotype and function of healthy donors' PMN. These results suggest that upon hospitalization D+HUS PMN are functionally impaired and show features of previous degranulation, indicating a preceding process of activation with release of ROS and proteases involved in endothelial damage.

A homogeneous, fluorescence polarization assay for src-family tyrosine kinases

A nonradioactive, simple, sensitive fluorescence polarization assay was developed to assay protein tyrosine kinase activity. This assay involves incubation of a fluorescenylated peptide substrate with the kinase, ATP, and anti-phosphotyrosine antibody. The phosphorylated peptide product is immunocomplexed with the anti-phosphotyrosine antibody resulting in an increase in the polarization signal as measured in a fluorescence polarization analyzer. Among several anti-phosphotyrosine antibodies examined, monoclonal antibody PY54 was found to give the best polarization signal with the test peptide. For validation of the fluorescence polarization assay, Lck activity was compared with a 32PO4 transfer assay. In both the fluorescence polarization and 32PO4 transfer assays, Lck activity showed a similar dependence on ATP, Lck enzyme, and peptide substrate concentrations. Both assays gave similar inhibition constants with a known tyrosine kinase inhibitor staurosporine and the Lck inhibitor, 4-amino-5-(methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine. These results show that the fluorescence polarization assay can detect inhibitors and is comparable to the 32PO4 transfer assay. The fluorescence polarization method is advantageous compared to the 32PO4 transfer assay or ELISA or DELFIA because it is a one-step assay that does not involve several washings, liquid transfer, and sample preparation steps. It has the added advantage of using nonisotopic substrates. The fluorescence polarization assay thus is environmentally safe and minimizes handling problems. The homogeneous nature of the assay makes it readily adaptable to high-throughput screening for small-molecule drug discovery.

Detection of nicotinic receptor ligands with a light addressable potentiometric sensor

The nicotinic acetylcholine receptor, purified from Torpedo electric organ, was coupled to a light addressable potentiometric sensor (LAPS) to form a LAPS-receptor biosensor. Receptor-ligand complexes containing biotin and urease were captured on a biotinylated nitrocellulose membrane via a streptavidin bridge and detected with a silicon-based sensor. Competition between biotinylated alpha-bungarotoxin and nonbiotinylated ligands formed the basis of this assay. This biosensor detected both agonists (acetylcholine, carbamylcholine, succinylcholine, suberyldicholine, and nicotine) and competitive antagonists (d-tubocurarine, alpha-bungarotoxin, and alpha-Naja toxin) of the receptor with affinities comparable to those obtained using radioactive ligand binding assays. Consistent with agonist-induced desensitization of the receptor, the LAPS-receptor biosensor reported a time-dependent increase in affinity for the agonist carbamylcholine as expected, but not for the antagonists.

Pharmacological specificity of a nicotinic acetylcholine receptor optical sensor

The pharmacological specificity of a nicotinic acetylcholine receptor (nAChR) optical biosensor was investigated using three fluorescein isothiocyanate (FITC)-tagged neurotoxic peptides that vary in the reversibility of their receptor inhibition: alpha-bungarotoxin (alpha-BGT), alpha-Naja toxin (alpha-NT), and alpha-conotoxin (GI) (alpha-CNTX). Kinetic analysis of the time course of binding of FITC-neurotoxins to the nAChR-coated fiber gave association rate constants (k+1) of 8.4 x 10(6) M-1 min-1 for FITC-alpha-BGT, 6.0 x 10(6) M-1 min-1 for FITC-alpha-NT and 1.4 x 10(6) M-1 min-1 for FITC-alpha-CNTX. The dissociation rate constants (k-1) for the three neurotoxins were 7.9 x 10(-3) min-1. 4.8 x 10(-2) min-1 and 8.0 x 10(-1) min-1 for FITC-alpha-BGT. FITC-alpha-NT and FITC-alpha-CNTX, respectively. The equilibrium dissociation constant (Kd) values for the three toxins. calculated from these rare constants, were similar to published values obtained from tissue responses or ligand binding assays. The optical signal generated by FITC-alpha-NT binding to the nAChR-coated fiber was effectively quenched by agonists and antagonists of the nAChR but not by most of the tested agonists and antagonists of muscarinic cholinergic, adrenergic, glutamatergic, serotonergic, dopaminergic or GABAergic receptors. Interestingly, 5-hydroxy-tryptamine, haloperidol and (+)cis-methyldioxolane gave significant inhibition of FITC-alpha-NT binding to the immobilized receptor. Equilibrium constants of inhibition (Ki) for d-tubocurarine (d-TC) and carbamylcholine (carb) were determined from competition studies using FITC-alpha-CNTX. FITC-alpha-NT or FITC-alpha-BGT as probes for receptor occupancy. When the more reversible probe FITC-alpha-CNTX was used, the Ki value for d-TC was an order of magnitude lower than those determined using the less reversible probes. Ki values for carb however, were independent of the FITC-toxin probe used.

Selective labeling of alpha-bungarotoxin with fluorescein isothiocyanate and its use for the study of toxin-acetylcholine receptor interactions

The main product of the reaction of fluorescein isothiocyanate (FITC) and bungarotoxin (Bgt) under near stoichiometric conditions is a monofluorescein derivative preferentially labeled at Lys 26, a highly conserved residue known to be involved in the binding (McDaniel, C.S., Manshouri, T., and Atassi, M.Z. (1987) J. Prot. Chem. 6, 455-461; Garcia-Borron, J.C., Bieber, A.L., and Martinez-Carrion, M. (1987) Biochemistry 26, 4295-4303) of postsynaptic neurotoxins specific for the nicotinic acetylcholine receptor (AcChR). The fluorescently labeled toxin retains a high affinity for the AcChR, and an unaltered specificity. Binding of FITC-Bgt to AcChR results in a significant decrease in the fluorescence intensity of the probe. This AcChR-mediated quenching of FITC-Bgt fluorescence allows for a continuous monitoring of the binding process. The quenching of free and bound FITC-Bgt by charged and neutral quenchers shows few fluorophore accessibility changes as induced by the toxin-bound state. The results are consistent with a model in which the positively charged concave surface of the toxin interacts with a negatively charged complementary surface in the receptor molecule.

Preparation and characterization of monoclonal antibody specific for alpha-bungarotoxin and localization of the epitope

We prepared a monoclonal antibody (mAb) specific for alpha-bungarotoxin (alpha-BuTX) which can neutralize the lethal toxicity of the toxin and inhibit the binding of [3H]-alpha-BuTX to the nicotinic acetylcholine receptor. The radiolabelled toxin has a high affinity for the mAb. alpha-BuTX was digested with acid protease A and the resulting peptide fragments were isolated by reverse-phase HPLC. The epitope recognized by the mAb has been localized on the basis of competition radioimmunoassay between [3H]-alpha-BuTX and the peptide fragments of alpha-BuTX towards the antibody. The epitope specific for the mAb may be located in the second loop of alpha-BuTX, probably involving residues 34-41.