A photonic atom probe coupling 3D atomic scale analysis with in situ photoluminescence spectroscopy

Laser enhanced field evaporation of surface atoms in laser-assisted Atom Probe Tomography (APT) can simultaneously excite photoluminescence in semiconductor or insulating specimens. An atom probe equipped with appropriate focalization and collection optics has been coupled with an in situ micro-photoluminescence (μPL) bench that can be operated during APT analysis. The photonic atom probe instrument we have developed operates at frequencies up to 500 kHz and is controlled by 150 fs laser pulses tunable in energy in a large spectral range (spanning from deep UV to near IR). Micro-PL spectroscopy is performed using a 320 mm focal length spectrometer equipped with a CCD camera for time-integrated and with a streak camera for time-resolved acquisitions. An example of application of this instrument on a multi-quantum well oxide heterostructure sample illustrates the potential of this new generation of tomographic atom probes.

Quantum interface of an electron and a nuclear ensemble

Coherent excitation of an ensemble of quantum objects underpins quantum many-body phenomena and offers the opportunity to realize a memory that stores quantum information. Thus far, a deterministic and coherent interface between a spin qubit and such an ensemble has remained elusive. In this study, we first used an electron to cool the mesoscopic nuclear spin ensemble of a semiconductor quantum dot to the nuclear sideband–resolved regime. We then implemented an all-optical approach to access individual quantized electronic-nuclear spin transitions. Lastly, we performed coherent optical rotations of a single collective nuclear spin excitation—a spin wave. These results constitute the building blocks of a dedicated local memory per quantum-dot spin qubit and promise a solid-state platform for quantum-state engineering of isolated many-body systems.

Assignment of 1H, 13C and 15N resonances and secondary structure of the Rgd1-RhoGAP domain

The protein Rgd1 is involved in the regulation of cytoskeleton formation and in signalling pathways that control cell polarity and growth in Saccharomyces cerevisiae. Rgd1p is composed of a F-BAR domain required for membrane binding and a RhoGAP domain responsible for activating Rho3p and Rho4p, two GTPases respectively involved in bud growth and cytokinesis. Rgd1p is recruited to the membrane through interactions with phosphoinositide lipids, which bind the two isolated domains and stimulate the RhoGAP activity on Rho4p. As previously shown by crystallography, the membrane-binding F-BAR domain contains a conserved inositol phosphate binding site, which explains the preferential binding of phosphoinositides. In contrast, RhoGAP domains are not expected to bind lipids. In order to unravel this puzzling feature, we solved the three-dimensional structure of the isolated protein and found a cryptic phosphoinositide binding site involving non conserved residues (Martinez et al. 2017). The assignment of the resonances and secondary structure of Rgd1-RhoGAP (aa 450-666) is presented here.

Improving a Solid-State Qubit through an Engineered Mesoscopic Environment

A controlled quantum system can alter its environment by feedback, leading to reduced-entropy states of the environment and to improved system coherence. Here, using a quantum-dot electron spin as a control and probe, we prepare the quantum-dot nuclei under the feedback of coherent population trapping and observe their evolution from a thermal to a reduced-entropy state, with the immediate consequence of extended qubit coherence. Via Ramsey interferometry on the electron spin, we directly access the nuclear distribution following its preparation and measure the emergence and decay of correlations within the nuclear ensemble. Under optimal feedback, the inhomogeneous dephasing time of the electron, T_{2}^{*}, is extended by an order of magnitude to 39 ns. Our results can be readily exploited in quantum information protocols utilizing spin-photon entanglement and represent a step towards creating quantum many-body states in a mesoscopic nuclear-spin ensemble.

Phase-Tuned Entangled State Generation between Distant Spin Qubits

Quantum entanglement between distant qubits is an important feature of quantum networks. Distribution of entanglement over long distances can be enabled through coherently interfacing qubit pairs via photonic channels. Here, we report the realization of optically generated quantum entanglement between electron spin qubits confined in two distant semiconductor quantum dots. The protocol relies on spin-photon entanglement in the trionic Λ system and quantum erasure of the Raman-photon path information. The measurement of a single Raman photon is used to project the spin qubits into a joint quantum state with an interferometrically stabilized and tunable relative phase. We report an average Bell-state fidelity for |ψ^{(+)}⟩ and |ψ^{(-)}⟩ states of 61.6±2.3% and a record-high entanglement generation rate of 7.3 kHz between distant qubits.

Quantum dot spin coherence governed by a strained nuclear environment

The interaction between a confined electron and the nuclei of an optically active quantum dot provides a uniquely rich manifestation of the central spin problem. Coherent qubit control combines with an ultrafast spin–photon interface to make these confined spins attractive candidates for quantum optical networks. Reaching the full potential of spin coherence has been hindered by the lack of knowledge of the key irreversible environment dynamics. Through all-optical Hahn echo decoupling we now recover the intrinsic coherence time set by the interaction with the inhomogeneously strained nuclear bath. The high-frequency nuclear dynamics are directly imprinted on the electron spin coherence, resulting in a dramatic jump of coherence times from few tens of nanoseconds to the microsecond regime between 2 and 3 T magnetic field and an exponential decay of coherence at high fields. These results reveal spin coherence can be improved by applying large magnetic fields and reducing strain inhomogeneity.

Spins confined to quantum dots are a possible qubit, but the mechanism that limits their coherence is unclear. Here, the authors use an all-optical Hahn-echo technique to determine the intrinsic coherence time of such spins set by its interaction with the inhomogeneously strained nuclear bath.

Production, in Pichia pastoris, of a recombinant monomeric mapacalcine, a protein with anti-ischemic properties

Mapacalcine is a small homodimeric protein of 19 kDa with 9 disulfide bridges extracted from the Cliona vastifica sponge (Red Sea). It selectively blocks a calcium current insensitive to most calcium blockers. Specific receptors for mapacalcine have been described in a variety of tissues such as brain, smooth muscle, liver, and kidney. Previous works achieved on hepatocytes and nervous cells demonstrated that this protein selectively blocks a calcium influx triggered by an ischemia/reperfusion (I/R) shock and efficiently protects cells from death after I/R. The aim of this work was to produce the recombinant mapacalcine in the yeast Pichia pastoris. Mass spectrometry, light scattering analysis and biological characterization demonstrated that the recombinant mapacalcine obtained was a monomeric form with 4 disulfide bridges which retains the biological activity of the natural protein.

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Mapacalcine is a homodimeric protein extracted from the Cliona vastifica sponge.

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Mapacalcine significantly increases cell survival after ischemia/reperfusion.

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We expressed a recombinant mapacalcine in the yeast Pichia pastoris.

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The expressed protein retains the biological properties of the natural mapacalcine.

Mapacalcine protects mouse neurons against hypoxia by blocking cell calcium overload

Stroke is one of a major cause of death and adult disability. Despite intense researches, treatment for stroke remains reduced to fibrinolysis, a technique useful for less than 10% of patients. Finding molecules able to treat or at least to decrease the deleterious consequences of stroke is an urgent need. Here, we showed that mapacalcine, a homodimeric peptide purified from the marine sponge Cliona vastifica, is able to protect mouse cortical neurons against hypoxia. We have also identified a subtype of L-type calcium channel as a target for mapacalcine and we showed that the channel has to be open for mapacalcine binding. The two main L-type subunits at the brain level are CaV1.3 and CaV1.2 subunits but mapacalcine was unable to block these calcium channels.Mapacalcine did not interfere with N-, P/Q- and R-type calcium channels. The protective effect was studied by measuring internal calcium level variation triggered by Oxygen Glucose Deprivation protocol, which mimics stroke, or glutamate stimulation. We showed that NMDA/AMPA receptors are not involved in the mapacalcine protection. The protective effect was confirmed by measuring the cell survival rate after Oxygen Glucose Deprivation condition. Our data indicate that mapacalcine is a promising molecule for stroke treatment.

Interfacing spins in an InGaAs quantum dot to a semiconductor waveguide circuit using emitted photons

An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.

Population inversion in a single InGaAs quantum dot using the method of adiabatic rapid passage

Preparation of a specific quantum state is a required step for a variety of proposed quantum applications. We report an experimental demonstration of optical quantum state inversion in a single semiconductor quantum dot using adiabatic rapid passage. This method is insensitive to variation in the optical coupling in contrast with earlier work based on Rabi oscillations. We show that when the pulse power exceeds a threshold for inversion, the final state is independent of power. This provides a new tool for preparing quantum states in semiconductor dots and has a wide range of potential uses.

The transport of high amounts of vascular endothelial growth factor by blood platelets underlines their potential contribution in systemic sclerosis angiogenesis

OBJECTIVES

Altered angiogenesis is a characteristic feature in SSc and remains ill-understood. VEGF is believed to play a central role. Serum VEGF is elevated in SSc patients but questions remain concerning the source of circulating VEGF. Here we investigated platelet activation and the role of platelets as a source of VEGF and other angiogenic mediators in this disease.

METHODS

A cohort of 40 patients with SSc was included. Age- and sex-matched healthy subjects and subjects presenting a primary RP were included as controls. Platelets were isolated, activated with thrombin and the secretion of VEGF, platelet derived growth factor, homodimeric form BB (PDGF-BB), TGF-beta1 and angiopoietins-1 and -2 measured. Plasma concentrations of these mediators and the functionality of platelet-derived VEGF were also studied. Platelet activation was assayed by measuring plasma beta-thromboglobulin and expression of P-selectin on platelets. The effect of iloprost on VEGF secretion by platelets was studied.

RESULTS

Platelets from SSc patients, in contrast to controls, secreted large amounts of VEGF when activated, but not PDGF-BB, TGF-beta1 or angiopoietins. Increased expression of membrane P-selectin confirmed platelet activation in the patients. Iloprost inhibited VEGF secretion by platelets both in vivo and in vitro, through inhibition of platelet activation.

CONCLUSIONS

Platelets transport high levels of VEGF in SSc. They may contribute to circulating VEGF because of ongoing activation in the course of the disease. If activated at the contact of injured endothelium, platelets may be important in the altered angiogenesis associated with the disease through the secretion of high levels of VEGF.

Overexpression and role of the ATPase and putative DNA helicase RuvB-like 2 in human hepatocellular carcinoma

Using a proteomic analysis of human hepatocellular carcinoma (HCC), we identified the overexpression in 4 tumors of RuvB-like 2 (RUVBL2), an ATPase and putative DNA helicase known to interact with beta-catenin and cellular v-myc myelocytomatosis viral oncogene homolog (c-myc). RUVBL2 expression was further analyzed in tumors with quantitative reverse-transcription polymerase chain reaction analysis and immunohistochemistry; in addition, RUVBL2 expression in a HuH7 cell line was silenced by small interfering RNA or increased with a lentiviral vector. RUVBL2 messenger RNA overexpression was confirmed in 72 of 96 HCC cases, and it was associated with poorly differentiated tumors (P = 0.02) and a poor prognosis (P = 0.02) but not with beta-catenin mutations or c-myc levels. Although RUVBL2 was strictly nuclear in normal hepatocytes, tumoral hepatocytes exhibited additional cytoplasmic staining. There was no mutation in the coding sequence of RUVBL2 in 10 sequenced cases. Silencing RUVBL2 in HuH7 HCC cells reduced cell growth (P < 0.001) and increased apoptosis, as shown by DNA fragmentation (P < 0.001) and caspase 3 activity (P < 0.005). This was associated with an increased expression of several proapoptotic genes and with an increased conformational activation of Bak-1 and Bax. On the other hand, HuH7 cells with an overexpression of RUVBL2 grew better in soft agar (P < 0.03), had increased resistance to C2 ceramide-induced apoptosis (P < 0.001), and gave rise to significantly larger tumors when injected into immunodeficient Rag2/gammac mice (P = 0.016).

CONCLUSION

RUVBL2 is overexpressed in a large majority of HCCs. RUVBL2 overexpression enhances tumorigenicity, and RUVBL2 is required for tumor cell viability. These results argue for a major role of RUVBL2 in liver carcinogenesis.

Characterization of mapacalcine-sensitive Ca(2+) channels in rat kidney

Mapacalcine receptors have been found to be associated with a Ca(2+) permeability insensitive to all known calcium blockers. Recently, high densities of mapacalcine receptors have been detected in the choroid plexus of rat brain. To determine a possible role for these channels, we have investigated their presence on other structures which, like choroid plexus, are involved in the secretion of biological fluids. Our data demonstrate that there are specific mapacalcine receptors on kidney membranes and glomeruli preparations. The mapacalcine receptors were present in all structures of the kidney. However, autoradiographic data demonstrated that superficial part of the cortex was more labeled than the other part of the kidney. These data would suggest that mapacalcine receptors could play a role in calcium homeostasis.

Mapacalcine specifically blocks hypoxia-induced calcium influx in rat hepatocytes

Post ischaemic cell calcium invasion has been described as one of the main causes of graft failure. Protective effects of calcium antagonists have been investigated but are not convincing and their mechanisms of action remain unclear. In this work we tested the protective effect of a new calcium inhibitor described to block a calcium current insensitive to all known calcium blockers. Specific mapacalcine receptors were first characterized on rat hepatocytes membranes using the 125I-labeled mapacalcine. 45Ca fluxes were then measured on cultured hepatocytes submitted (or not) to an hypoxic period. The action of mapacalcine was investigated on the ischaemia-induced calcium influx. We demonstrate here that: (a) there are specific receptors for mapacalcine in rat hepatocytes; (b) Mapacalcine is able to specifically block ischaemia-induced calcium influx with an IC50 of 0.3 micro m and does not significantly interact with the basal calcium flux. Our work demonstrates that the mapacalcine receptor is a cellular structure directly involved in the phenomenon of postischaemic cell invasion by calcium. Specific block of ischaemia-induced Ca2+ influx by mapacalcine suggests that the development of a panel of pharmacological drugs acting on this receptor could lead to the discovery of therapeutic agents able to protect cells against one of the events responsible for organ failure after transplantation or simply after an ischaemic period. Moreover, identification of the cellular protein which binds mapacalcine may become an important step in the research of mechanisms involved in postischaemic cell invasion by calcium.

Distribution of mapacalcine receptors in the central nervous system of rat using the [125I]-labeled mapacalcine derivative

Mapacalcine is a dimeric protein of Mr 19041 extracted from the marine sponge Cliona vastifica. Electrophysiological and pharmacological approaches have demonstrated that mapacalcine was blocking a calcium channel different from N-, L-, P-, T- or Q-type calcium channels on mouse intestinal smooth muscle. Recently a [125I]-labeled derivative of mapacalcine has been synthesized and characterized as a tool usable as a probe to investigate mapacalcine receptors. On rat brain membranes, it binds to its receptor with a K(d)=0.35 nM and a maximal binding capacity of 706 fmol/mg protein. We use here [125I]-mapacalcine to study the mapping of its receptors in the rat brain. Data obtained show a practically homogeneous labeling of the brain. Our experiments suggest that mapacalcine receptors are present on neuronal and glial cells. Interestingly, choroid plexus demonstrates a high density of mapacalcine receptors. These data would suggest that mapacalcine sensitive calcium channels could be involved in the control of calcium homeostasis of the cerebrospinal fluid.

Inhibition of calcium influx during hypoxia/reoxygenation in primary cultured rat hepatocytes

Calcium has been demonstrated to play an important role in hepatocyte damage during ischemia/reperfusion phases. Calcium influx was determined in primary cultured rat hepatocytes submitted to a succession of warm hypoxia and reoxygenation phases in the presence of diltiazem, gallopamil and a Na+/H+ antiport inhibitor, HOE-694. Only diltiazem significantly inhibited calcium influx with higher potency after reoxygenation than after hypoxia only, suggesting a complex mechanism of action of diltiazem which could act on different physiological functions involved in Ca2+ invasion of hepatocytes after hypoxic insult.

[The role of peptidic toxins in the pharmacological approach of the diversity of calcium channels]

Peptidic toxins extracted from spider, marine snails or snakes venoms, have considerably helped the pharmacological characterization of calcium channels. They have successfully been used for calcium channels mapping. However, the actual situation remains unclear. Genetic investigations demonstrated the existence of a great number of types or sub-types of calcium channels. In recent year a large number of toxins have been purified. Many of these toxins have specific actions on calcium channels and have been used as powerful tools in pharmacological approaches of calcium channels. However the pharmacology of the calcium channels remains very limited, many of them are waiting for the discovery of pharmacological tools allowing their molecular approach in order to determinate their biological implications. In this paper we describe the different families of calcium channels and toxins that interact with these channels. We also recapitulate the "non defined" calcium channels i.e. calcium channel which does not correspond to a N, L, P/Q, R or T type channel and for which no effector are available. We report the discovery and characterization of mapacalcine, a toxin extracted for a marine sponge, as an example of an approach of an undefined calcium channels first characterized by electrophysiological techniques and for which a specific toxin has been purified allowing its pharmacological approach. We also state the possible role of calcium channel toxins in the domain of therapeutic applications.

125I-Labelled mapacalcine: a specific tool for a pharmacological approach to a receptor associated with a new calcium channel on mouse intestinal membranes

Mapacalcine is a small protein (Mr=19041) composed of two homologous chains purified from the marine sponge Cliona vastifica. Recently, we demonstrated that it was able to specifically block a Ca2+ channel which could not be related to already described channels on mouse intestinal myocytes. This Ca2+ current was insensitive to the known peptidic and organic calcium channel blockers. Mapacalcine was ineffective on T-type and L-type Ca2+ currents present on rat portal vein myocytes [Morel, Drobecq, Sautière, Tartar, Mironneau, Qar, Lavie, and Hugues (1997) Mol. Pharmacol. 51, 1042-1052]. We report here the preparation and purification of a monoiodo-derivative of mapa-calcine which retains its biological properties. Binding parameters of mapacalcine to its receptors have been characterized on mouse intestinal membranes. It binds to its receptors with a Kd=0. 8 nM, and a maximal binding capacity of 171 fmol/mg of protein on membrane preparations. Our data show that we have prepared a tool that is usable for pharmacological studies of a receptor associated with a new type of calcium channel for which no ligand was available until now.

Purification of a new dimeric protein from Cliona vastifica sponge, which specifically blocks a non-L-type calcium channel in mouse duodenal myocytes

Marine sponges are synthesizing a wide variety of peptidic and organic molecules with biological activities. Multiple-step purification of Cliona vastifica extract led to a new dimeric peptide (mapacalcine; M(r) = 19,064) that is composed of two homologous chains, each containing nine cysteins. This protein has been found to selectively block a new calcium conductance characterized in mouse duodenal myocytes with an IC50 value of approximately 0.2 microM. The mapacalcine-sensitive current was a non-L-type calcium current activated from a holding potential of -80 mV that persisted during stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10 min. Time constants of inactivation were similar for both L-type and non-L-type calcium currents. The non-L-type calcium current of duodenal myocytes was not blocked by the pharmacological agents specific for N-, L-, P-, or Q-type calcium channels. Mapacalcine was unable to block T-type calcium current in portal vein myocytes as well as voltage-dependent potassium currents and calcium-activated chloride currents in duodenal and portal vein cells. Mapacalcine did not affect caffeine-induced calcium responses, indicating that it did not interfere with intracellular calcium stores. Competition experiments on mouse intestinal membranes showed that mapacalcine did not interact with dihydropyridines receptors. These data suggest that mapacalcine may be a specific inhibitor of a new type of calcium current, first identified in duodenal myocytes.

Solubilization and characterization of functionally coupled Escherichia coli heat-stable toxin receptors and particulate guanylate cyclase associated with the cytoskeleton compartment of intestinal membranes

1. Particulate guanylate cyclase and receptors for E. coli heat-stable enterotoxin were solubilized from the rat intestinal cytoskeletal compartment using Lubrol-PX and KCl. 2. Thirty to forty percent of the ST receptor and guanylate cyclase activities were extracted from the lipid layer with Lubrol-PX alone. 2. Seventy percent of the remaining activities were solubilized from the cytoskeleton with Lubrol-PX and KCl. 3. Guanylate cyclase solubilized from either compartment exhibited similar reaction kinetics. 4. Both high- and low-affinity classes of ST receptors were solubilized from the lipid and cytoskeleton compartments. 5. In the presence of ATP gamma S, ST selectively activated the guanylate cyclase solubilized from the cytoskeleton compared to that solubilized from the lipid bilayer. 6. Crosslinking experiments demonstrated a preferential solubilization of the 130 kDa receptor subunit from the cytoskeleton and the 56 kDa subunit from the lipid bilayer. 7. Development of a procedure to solubilize ST receptors and guanylate cyclase from the intestinal membrane cytoskeleton will permit purification and further detailed studies of the coupling of these activities.

Characterization and partial purification from pheochromocytoma cells of an endogenous equivalent of scyllatoxin, a scorpion toxin which blocks small conductance Ca(2+)-activated K+ channels

This work describes the partial purification of a heat-stable peptide which has the same properties as the scorpion toxin, scyllatoxin, a specific blocker of one class of Ca(2+)-activated K+ channels: (i) it competes with [125I]apamin for binding to the same site, (ii) like apamin and scyllatoxin, it blocks the after-potential hyperpolarization in skeletal muscle cells in culture, (iii) like apamin and scyllatoxin, it contracts guinea-pig taenia coli relaxed by epinephrine, (iv) it cross-reacts with antibodies raised against scyllatoxin but not with antibodies raised against apamin.

Identification of two affinity states of low affinity receptors for Escherichia coli heat-stable enterotoxin: correlation of occupation of lower affinity state with guanylate cyclase activation

Two distinct affinity states of low affinity Escherichia coli heat-stable enterotoxin (ST) receptors in rat intestinal membranes, with dissociation constants of 0.12 and 2.5 nM, were identified. Kinetic binding studies demonstrated biphasic association kinetics, whereas dissociation was unimodal. These studies also confirmed that ligand bound to each receptor state in an independent bimolecular reaction. In contrast, equilibrium binding studies yielded linear Scatchard plots, indicative of a single class of noninteractive binding sites, with a Kd = 2.3 nM. Close agreement of the dissociation constants determined by kinetic and equilibrium methods suggested that receptors were in the lower affinity state at equilibrium. Several models, including binding site heterogeneity, cooperativity, and ligand-induced alterations in receptor conformation were inconsistent with these observations. Indeed, these data were most consistent with a two-step binding process involving a third component. Comparison of the ligand dependence of enzyme activation (EC50 = 124 nM) and the calculated fractional receptor occupancy of the lower affinity component at 5 min (EC50 = 40 nM) demonstrated that occupation of the lower affinity state of low affinity ST receptors correlated with guanylate cyclase activation. The close correlation between receptor occupation and enzyme activation suggests that there are no spare receptors for ST in intestinal membranes. These data resolve the previously observed discrepancy between the affinity of receptors for ST and the potency of this ligand for activating guanylate cyclase. Receptor affinity state alterations may represent a common mechanism for receptor-effector coupling of particulate guanylate cyclases.

Scyllatoxin, a blocker of Ca(2+)-activated K+ channels: structure-function relationships and brain localization of the binding sites

Chemical modifications of scyllatoxin (leiurustoxin I) have shown that two arginines in the sequence, Arg6 and Arg13, are essential both for binding to the Ca(2+)-activated K+ channel protein and for the functional effect of the toxin. His31 is important both for the binding activity of the toxin and for the induction of contractions on taenia coli. However, although its iodination drastically decreases the toxin activity, it does not abolish it. Chemical modification of lysine residues or of Glu27 does not significantly alter toxin binding, but it drastically decreases potency with respect to contraction of taenia coli. The same observation has been made after chemical modification of the lysine residues. The brain distribution of scyllatoxin binding sites has been analyzed by quantitative autoradiographic analysis. It indicates that apamin (a bee venom toxin) binding sites are colocalized with scyllatoxin binding sites. The results are consonant with the presence of apamin/scyllatoxin binding sites associated with Ca(2+)-activated K+ channels. High-affinity binding sites for apamin can be associated with very-high-affinity (less than 70 pM), high-affinity (approximately 100-500 pM), or moderate-affinity (greater than 800 pM) binding sites for scyllatoxin.

Affinity purification of functional receptors for Escherichia coli heat-stable enterotoxin from rat intestine

Active receptors for Escherichia coli heat-stable enterotoxin (ST) were partially purified by ligand-affinity chromatography. The affinity column was prepared by coupling ST to biotin derivatized with an extended N-hydroxysuccinylated spacer arm prior to binding to monomeric avidin immobilized on agarose. Detergent extracts of rat intestinal mucosa membranes were quantitatively depleted of ST binding activity when chromatographed on this affinity matrix. Biotinylated ST-receptor complexes were eluted from affinity columns with 2 mM biotin and these complexes quantitatively dissociated with bile salts. Using this technique, functional ST receptors were purified maximally about 2000-fold, with about 3% of the total activity in crude extracts recovered in these purified preparations. Analysis of affinity-purified preparations by polyacrylamide gel electrophoresis and silver staining demonstrated a major protein subunit of 74 kDa. Affinity cross-linking of these preparations to 125I-ST demonstrated specific labeling predominantly of the 74-kDa subunit. In addition, lower amounts of labeled ST were incorporated into subunits of 164 and 45 kDa, confirming the heterogeneous nature of ST receptors. Purified receptors bound ST in a concentration-dependent fashion, with an IC50 of 10(-9) M. These studies demonstrate that ligand-affinity chromatography can be employed to purify ST receptors. The availability of purified receptors will facilitate further studies of mechanisms underlying ST-induced intestinal secretion.

Identification and characterization of a new family of high-affinity receptors for Escherichia coli heat-stable enterotoxin in rat intestinal membranes

Novel high-affinity, low-capacity binding sites in intestinal membranes for the heat-stable toxin produced by Escherichia coli have been defined. The appearance of these sites is observed in the presence of physiological concentrations of NaCl in binding reactions. Scatchard analyses of equilibrium binding in the absence of NaCl demonstrated a single class of binding sites with KD = 1.9 x 10(-9) M and Bmax = 0.75 pmol/mg of protein. In contrast, similar experiments in the presence of NaCl demonstrated, in addition to the previously described low-affinity site, a high-affinity site with a KD of 2.1 x 10(-11) M and a Bmax of 73 fmol/mg of protein. Confirmation of the presence of high- and low-affinity sites was obtained in studies of the kinetics of ST binding. These sites exhibited similar dissociation but markedly different association kinetics. Determination of the association and dissociation constants permitted calculation of the KD's for the high- and low-affinity sites, which were 1.15 x 10(-11) M and 1.89 x 10(-9) M, respectively. These data agree closely with those obtained in studies of equilibrium binding. Furthermore, similar values for the KD's of these sites were obtained in experiments of competitive displacement of labeled ST, confirming the presence of two receptors for this toxin. Binding of ST to high-affinity sites is completely reversible and does not appear to be coupled to activation of particulate guanylate cyclase. In contrast, binding of ST to low-affinity sites appears to be partially reversible and may be coupled to activation of guanylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Leiurotoxin I (scyllatoxin), a peptide ligand for Ca2(+)-activated K+ channels. Chemical synthesis, radiolabeling, and receptor characterization

Leiurotoxin I (scyllatoxin) is a 31-amino acid polypeptide from the venom of the scorpion Leiurus quinquestriatus hebraeus which has been previously isolated and sequenced by others. This paper reports (i) the total synthesis of this scorpion neurotoxin as well as some aspects of its structure-function relationships; (ii) the synthesis of the analog [Tyr2]leiurotoxin I (scyllatoxin) that has been monoiodinated at high specific radioactivity (2000 Ci/mmol) and has served for the characterization of the properties of 125I-[Tyr2]leiurotoxin I binding sites (Kd = 80 pM, molecular mass of 27 and 57 kDa for two polypeptides in the leiurotoxin I binding protein); (iii) the similarity of physiological actions between leiurotoxin I and apamin. Both toxins contract Taenia coli previously relaxed with epinephrine, both toxins block the after-hyperpolarization due to Ca2(+)-activated K+ channel activity in muscle cells in culture; (iv) the probable identity of binding sites for apamin and leiurotoxin I. In spite of a different chemical structure apamin competitively inhibits 125I-[Tyr2] leiurotoxin I binding and vice versa. Moreover, the peculiar effects of K+ on 125I-[Tyr2]leiurotoxin I binding are identical to those previously observed for 125I-apamin binding.

Molecular properties of potassium channels

The paper describes the molecular pharmacology and biochemistry of three types of K+ channels, the calcium-activated potassium channels, ATP-regulated potassium channels and voltage-sensitive potassium channels.

Quantitative autoradiographic mapping in rat brain of the receptor of apamin, a polypeptide toxin specific for one class of Ca2+-dependent K+ channels

The localization of the receptor for apamin, a specific toxin for one class of sensitive Ca2+-dependent K+ channel, was studied in rat brain using an in vitro autoradiographic technique. Radiolabeled monoiodoapamin binds specifically to rat brain sections with a high affinity (Kd = 25 pM) to a single class of sites. Autoradiograms demonstrated a very heterogeneous distribution of the apamin receptor throughout the brain. Very high grain densities were localized on the habenula, lateral septum, supraoptic and suprachiasmatic nuclei. Areas containing high levels of apamin binding sites included anterior olfactory nucleus, stratum oriens of hippocampus, pontine nuclei and granular layer of the cerebellar cortex and inferior olive. The thalamus, some nuclei of hypothalamus, hippocampus, tegmental area, red and oculomotor nuclei, vestibular nuclei and superior olive, among others, presented intermediate grain densities. In the other main areas, in particular basal ganglia, raphe, low to very low levels of apamin binding sites have been observed.

Properties of the apamin-sensitive Ca2+-activated K+ channel in PC12 pheochromocytoma cells which hyper-produce the apamin receptor

Undifferentiated PC12 cell produce high levels of apamin receptors (measured with 125I-apamin) after 7 days in culture. These levels are at least 50 times higher than those found in other cellular types which are also known to have apamin receptors and apamin-sensitive Ca2+-activated K+ channels in their membranes. Treatment of undifferentiated PC12 cells with nerve growth factor maintains these cells in a state having a low level (10 times less after 7 days of culture) of apamin receptors. Ca2+ injection into PC12 cells with the calcium ionophore A23187 has been used to monitor the activity of the Ca2+-activated K+ channel following 86Rb+ efflux. A large component of this Ca2+-activated 86Rb+ efflux is inhibited by apamin. Half-maximum inhibition by apamin of both 86Rb+ efflux and 125I-apamin binding was observed at 240 pM apamin. Another component of 86Rb+ efflux is due to another type of Ca2+-activated K+ channel which is resistant to apamin and sensitive to tetraethylammonium. The Ca2+ channel activator Bay K8644 also triggers an apamin-sensitive Ca2+-dependent 86Rb+ efflux. Bay K8644 has been used to analyze the internal Ca2+ concentration dependence of the apamin-sensitive channel activity. Under normal conditions, the internal Ca2+ concentration is 109 +/- 17 nM, and the apamin-sensitive channel is not activated. The channel is fully activated at an internal Ca2+ concentration of 320 +/- 20 nM.

Expression of apamin receptor in muscles of patients with myotonic muscular dystrophy

Myotonic muscular dystrophy, or Steinert disease, is a dominantly inherited disease of muscle which occurs with a frequency of between 1 in 18,000 and 1 in 7,500 people (refs 1, 2). One of the prominent clinical manifestations is muscle stiffness and difficulty in relaxation of muscles after voluntary contractions. Electrophysiological signs of myotonia include increased excitability with a tendency to fire trains of repetitive action potentials in response to direct electrical and mechanical stimulation. Most experimental and clinical data suggest that myotonic muscular dystrophy arises from genetically induced alterations of the muscle membrane. We show here for the first time that muscle membranes of patients with myotonic muscular dystrophy contain the receptor for apamin, a bee venom toxin known to be a specific and high-affinity blocker of one class of Ca2+-activated K+ channels in mammalian muscle. The apamin receptor is completely absent in normal human muscle as well as in muscles of patients with spinal anterior horn disorders.

The all-or-none role of innervation in expression of apamin receptor and of apamin-sensitive Ca2+-activated K+ channel in mammalian skeletal muscle

The long-lasting after-hyperpolarization(s) (AHP) that follows the action potential in rat myotubes differentiated in culture is due to Ca2+-activated K+ channels. These channels have the property to be specifically blocked by the bee venom toxin apamin at low concentrations. Apamin has been used in this work to analyze, by electrophysiological and biochemical techniques, the role of innervation in expression of these important channels. The main results are as follows: (i) Long-lasting AHP that follows the action potential in rat myotubes in culture disappears when myotubes are cocultured with nerve cells from the spinal cord under the conditions of in vitro innervation. (ii) Extensor digitorum longus muscles from adult rats have action potentials that are not followed by AHP but AHP are systematically recorded after muscle denervation and they are blocked by apamin. (iii) Specific 125I-labeled apamin binding is undetectable in innervated muscle fibers but it becomes detectable 2-4 days after muscle denervation to be maximal 10 days after denervation. (iv) Apamin receptors detected with 125I-labeled apamin are present at fetal stages with biochemical characteristics identical to those found in myotubes in culture. The receptor number decreases as maturation proceeds and 125I-labeled apamin receptors completely disappear after the first week of postnatal life, in parallel with the disappearance of multi-innervation. All these results taken together strongly suggest an all-or-none effect of innervation on the expression of apamin-sensitive Ca2+-activated K+ channels.

The presence in pig brain of an endogenous equivalent of apamin, the bee venom peptide that specifically blocks Ca2+-dependent K+ channels

An apamin-like factor has been isolated from pig brain after extraction of the tissue and purification on sulfopropyl-Sephadex C-25 and on reversed-phase high pressure liquid chromatography. The apamin-like factor has the following properties: (i) it prevents 125I-labeled apamin binding to its specific receptor site present on rat brain synaptosomes, (ii) it is active in the radioimmunoassay for apamin (i.e., it prevents 125I-labeled apamin precipitation by anti-apamin antibodies), (iii) it induces contraction of guinea pig intestinal smooth muscle previously relaxed with epinephrine, and (iv) it blocks Ca2+-dependent K+ channels responsible for the long-lasting afterpotential hyperpolarization following the action potential in rat skeletal muscle cells in culture. All these properties are those of apamin itself. The apamin-like factor is a peptide that, like apamin, is destroyed by trypsin and unaffected by chymotrypsin. These results suggest the presence in mammalian brain of a potent Ca2+-dependent K+-channel modulator.

Molecular properties of the apamin-binding component of the Ca2+-dependent K+ channel. Radiation-inactivation, affinity labelling and solubilization

Radiation-inactivation was used to assess the functional size of the apamin-binding component of the Ca2+-dependent K+ channel. The amount of specific binding of 125I-apamin to receptors in synaptic membranes of rat cortex decayed exponentially with increasing doses of ionizing radiation and target size analysis was consistent with a relative molecular mass of 250 000 +/- 20 000 for the 125I-apamin receptor. Analysis on sodium dodecyl sulfate gels following covalent cross-linking of 125I-apamin to its receptor in a synaptosomal membrane preparation from rat cortex revealed a single labelled polypeptide chain of Mr = 33 000 +/- 2000 in the presence of protease inhibitors. Our results suggest that the Ca2+-dependent K+ channel from rat cortex is an oligomeric structure of Mr = 250 000 +/- 20 000 containing an apamin-binding subunit of Mr = 33 000 +/- 2000. The apamin-binding component of the Ca2+-dependent K+ channel from rat synaptosomes was solubilized using detergents such as sodium cholate or 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate. Phospholipids did not increase the stability of the apamin-binding component during the solubilization. Binding of apamin to its solubilized receptor is reversible and saturable. The dissociation constant of the apamin-receptor complex is 40-150 pM, the rates constants of association and dissociation being 3.2 X 10(6) M-1s-1 and 1.4 X 10(-4)s-1 respectively. These binding characteristics are similar to those found for the membrane-bound apamin receptor.

The Ca2+-dependent slow K+ conductance in cultured rat muscle cells: characterization with apamin

The interaction of apamin, a bee venom neurotoxin, with rat skeletal muscle cell membranes has been followed using both an electrophysiological and a biochemical approach. Voltage-clamp analyses have shown that apamin, at low concentrations, specifically blocks the Ca2+-dependent slow K+ conductance in rat myotubes and myosacs . A specific binding site for apamin in rat muscle cell membranes has been characterized with the use of a highly radiolabelled apamin derivative [( 125I]apamin). The dissociation constant for the apamin-receptor complex is 36-60 pM and the maximal binding capacity is 3.5 fmol/mg of protein. [125I]Apamin binding to rat muscle membranes is displaced by quinine and quinidine with K0.5 values of 110 microM and 200 microM, respectively.

Specific binding and pharmacological interactions of apamin, the neurotoxin from bee venom, with guinea pig colon

This paper describes the interaction of apamin, the bee venom neurotoxin, with its receptor in the guinea pig colon. The pharmacological activity of the toxin was assayed by measuring its contracting effect on guinea pig colon preparations that had been previously relaxed by neurotensin. The IC50 value of apamin in this in vitro bioassay is 7 nM. These pharmacological data are compared to the binding properties of apamin to smooth muscle membranes prepared from guinea pig colon. The highly radiolabeled monoiododerivative of apamin binds to its colon receptor with a dissociation constant Kd* = 36 pM. The maximal binding capacity of colonic membranes is 30dfmol/mg of protein. The dissociation constant of the unmodified toxin is 23 pM. The difference between the toxin concentrations that produce half-maximal effects in the binding and pharmacological studies arises from the different experimental conditions used for the two assays.

Preparation of a pure monoiodo derivative of the bee venom neurotoxin apamin and its binding properties to rat brain synaptosomes

The preparation and purification of an active monoiodo derivative of apamin is described. Radiolabeled monoiodoapamin (2000 Ci/mmol) binds specifically to rat brain synaptosomes at 0 degrees C and pH 7.5 with a second order rate constant of association (ka = 2.6 x 10(7) M-1 s-1) and a first order rate constant of dissociation (kd = 3.8 x 10(-4) s-1). The maximal binding capacity is 12.5 fmol/mg of protein and the dissociation constant is 15-25 pM for the monoiodo derivative and 10 pM for the native toxin. The apamin receptor is destroyed by proteases suggesting that it is of a proteic nature. Neurotensin and its COOH-terminal partial sequences are the only molecules unrelated to apamin that are able to displace monoiodoapamin from its receptor at low concentrations. Half-displacement occurs at 170 nM neurotensin. This property is due to the presence in the COOH-terminal sequence of neurotensin of two contiguous arginine residues, a structure analogous to that of the apamin active site. The binding of monoiodoapamin to its receptor is sensitive to cations. Increasing K+ or Rb+ concentrations from 10 microM to 5 mM selectively enhances the binding by a factor of 1.8. Increasing the concentration of any cation from 1 to 100 mM completely inhibits iodoapamin binding. Both effects are due to a cation-induced modulation of the affinity of monoidoapamin for its receptor without any change of the maximal toxin binding capacity of synaptosomes. Guanidinium and molecules containing a guanidinium group are better inhibitors of iodoapamin binding than other inorganic cations or positively charged organic molecules.

Apamin as a selective blocker of the calcium-dependent potassium channel in neuroblastoma cells: voltage-clamp and biochemical characterization of the toxin receptor

This paper describes the interaction of apamin, a bee venom neurotoxin, with the mouse neuroblastoma cell membrane. Voltage-clamp analyses have shown that apamin at low concentrations specifically blocks the Ca2+-dependent K+ channel in differentiated neuroblastoma cells. Binding experiments with highly radiolabeled toxin indicate that the dissociation constant of the apamin-receptor complex in differentiated neuroblastoma cells is 15-22 pM and the maximal binding capacity is 12 fmol/mg of protein. The receptor is destroyed by proteases, suggesting that it is a protein. The binding capacity of neuroblastoma cells for radiolabeled apamin dramatically increases during the transition from the nondifferentiated to the differentiated state. The number of Ca2+-dependent K+ channels appears to be at most 1/5th the number of fast Na+ channels in differentiated neuroblastoma. The binding of radiolabeled apamin to its receptor is antagonized by monovalent and divalent cations. Na+ inhibition of the binding of 125I-labeled apamin is of the competitive type (Kd(Na+) = 44 mM). Guanidinium and guanidinated compounds such as amiloride or neurotensin prevent binding of 125I-labeled apamin, the best antagonist being neurotensin.

Purification and pharmacological properties of eight sea anemone toxins from Anemonia sulcata, Anthopleura xanthogrammica, Stoichactis giganteus, and Actinodendron plumosum

Eight different polypeptide toxins from sea anemones of four different origins (Anemonia sulcata, Anthopleura xanthogrammica, Stoichactis giganteus, and Actinodendron plumosum) have been studied. Three of these toxins are new; the purification procedure for the five other ones has been improved. Sea anemone toxins were assayed (i) for their toxicity to crabs and mice, (ii) for their affinity for the specific sea anemone toxin receptor situated on the Na+ channels of rat brain synaptosomes, and (iii) for their capacity to increase, in synergy with veratridine, the rate of 22Na+ entry into neuroblastoma cells via the Na+ channel. Some of the toxins are more active on crustaceans, whereas others are more toxic to mammals. A very good correlation exists between the toxic activity to mice, the affinity of the toxin for the Na+ channel in rat brain synaptosomes, and the stimulating effect on 22 Na+ uptake by neuroblastoma cells. The observation has also been made that the most cationic toxins are also the most active on mammals and the least active on crustaceans. Toxicities (LD50) to mice of the most active sea anemone toxins and of the most active scorpion toxins are similar, and sea anemone toxins at high enough concentrations prevent binding of scorpion toxins to their receptor. However, scorpion toxins have affinities for the Na+ channel which are approximately 60 times higher than those found for the most active sea anemone toxins. Three sea anemone toxins appear to be more interesting than toxin II from A. sulcata (the "classical" sea anemone toxin) for studies of the Na+ channel structure and mechanism when the source of the channel is of a mammalian origin. Two of these three toxins can be radiolabeled with iodine while retaining their toxic activity; they appear to be useful tools for future biochemical studies of the Na+ channel.

Structure-function relationships of sea anemone toxin II from Anemonia sulcata

Chemical modifications of sea anemone toxin II from Anemonia sulcata have been used to study the residues involved in its toxic action on crabs and mice and in its binding properties to the Na+ channel of rat brain synaptosomes. Guanidination of th epsilon-amino groups of lysines 35, 36, and 46 with O-methylisourea hydrogen sulfate did not change the net charge of the toxin molecule and had no effect upon its toxic and binding properties. Either acetylation or fluorescamine treatment of the toxin that destroyed the positive charges of the three epsilon-amino groups and of the alpha-amino function of Gly produced an almost complete loss of toxicity and a considerable decrease in the binding activity. Iodination of the toxin on His induced practically no loss of toxic or binding properties. Carbethoxylation of both histidines 32 and 37 with diethyl pyrocarbonate provoked an important decrease of both the toxicity and the binding activity. Modifications of the guanidine side chain of Arg with 1,2-cyclohexanedione fully destroyed both toxicity and binding of the toxin to the Na+ channel. Modification of the carboxylate functions of Asp, Asp, and of the COOH-terminal Gln with glycine ethyl ester in the presence of a soluble carbodiimide completely abolished the toxicity but left the affinity for the sea anemone toxin receptor unchanged. The antagonist character of this carboxylate-modified derivative was further confirmed by electrophysiological and Na+ flux experiments. The theoretical and practical significance of these results are discussed.