Identification of Various Allosteric Interaction Sites on M1Muscarinic Receptor Using125I-Met35-Oxidized Muscarinic Toxin 7

Insertion of Nanoluc into the Extracellular Loops as a Complementary Method To Establish BRET-Based Binding Assays for GPCRs

Luminescence-based techniques play an increasingly important role in all areas of biochemical research, including investigations on G protein-coupled receptors (GPCRs). One quite recent and popular addition has been made by introducing bioluminescence resonance energy transfer (BRET)-based binding assays for GPCRs, which are based on the fusion of nanoluciferase (Nluc) to the N-terminus of the receptor and the occurring energy transfer via BRET to a bound fluorescent ligand. However, being based on BRET, the technique is strongly dependent on the distance/orientation between the luciferase and the fluorescent ligand. Here we describe an alternative strategy to establish BRET-based binding assays for GPCRs, where the N-terminal fusion of Nluc did not result in functioning test systems with our fluorescent ligands (e.g., for the neuropeptide Y Y1 receptor (Y1R) and the neurotensin receptor type 1 (NTS1R)). Instead, we introduced Nluc into their second extracellular loop and we obtained binding data for the fluorescent ligands and reported standard ligands (in saturation and competition binding experiments, respectively) comparable to data from the literature. The strategy was transferred to the angiotensin II receptor type 1 (AT1R) and the M1 muscarinic acetylcholine receptor (M1R), which led to affinity estimates comparable to data from radioligand binding experiments. Additionally, an analysis of the binding kinetics of all fluorescent ligands at their respective target was performed using the newly described receptor/Nluc-constructs.

Multitargeting nature of muscarinic orthosteric agonists and antagonists

Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M1 to M5. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson's disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers' claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype's function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply "specific" agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

The Repellent DEET Potentiates Carbamate Effects via Insect Muscarinic Receptor Interactions: An Alternative Strategy to Control Insect Vector-Borne Diseases

Insect vector-borne diseases remain one of the principal causes of human mortality. In addition to conventional measures of insect control, repellents continue to be the mainstay for personal protection. Because of the increasing pyrethroid-resistant mosquito populations, alternative strategies to reconstitute pyrethroid repellency and knock-down effects have been proposed by mixing the repellent DEET (N,N-Diethyl-3-methylbenzamide) with non-pyrethroid insecticide to better control resistant insect vector-borne diseases. By using electrophysiological, biochemichal, in vivo toxicological techniques together with calcium imaging, binding studies and in silico docking, we have shown that DEET, at low concentrations, interacts with high affinity with insect M1/M3 mAChR allosteric site potentiating agonist effects on mAChRs coupled to phospholipase C second messenger pathway. This increases the anticholinesterase activity of the carbamate propoxur through calcium-dependent regulation of acetylcholinesterase. At high concentrations, DEET interacts with low affinity on distinct M1/M3 mAChR site, counteracting the potentiation. Similar dose-dependent dual effects of DEET have also been observed at synaptic mAChR level. Additionally, binding and in silico docking studies performed on human M1 and M3 mAChR subtypes indicate that DEET only displays a low affinity antagonist profile on these M1/M3 mAChRs. These results reveal a selective high affinity positive allosteric site for DEET in insect mAChRs. Finally, bioassays conducted on Aedes aegypti confirm the synergistic interaction between DEET and propoxur observed in vitro, resulting in a higher mortality of mosquitoes. Our findings reveal an unusual allosterically potentiating action of the repellent DEET, which involves a selective site in insect. These results open exciting research areas in public health particularly in the control of the pyrethroid-resistant insect-vector borne diseases. Mixing low doses of DEET and a non-pyrethroid insecticide will lead to improvement in the efficiency treatments thus reducing both the concentration of active ingredients and side effects for non-target organisms. The discovery of this insect specific site may pave the way for the development of new strategies essential in the management of chemical use against resistant mosquitoes.

Orthosteric binding of ρ-Da1a, a natural peptide of snake venom interacting selectively with the α1A-adrenoceptor

ρ-Da1a is a three-finger fold toxin from green mamba venom that is highly selective for the α_1A-adrenoceptor. This toxin has atypical pharmacological properties, including incomplete inhibition of ³H-prazosin or ¹²⁵I-HEAT binding and insurmountable antagonist action. We aimed to clarify its mode of action at the α_1A-adrenoceptor. The affinity (pKi 9.26) and selectivity of ρ-Da1a for the α_1A-adrenoceptor were confirmed by comparing binding to human adrenoceptors expressed in eukaryotic cells. Equilibrium and kinetic binding experiments were used to demonstrate that ρ-Da1a, prazosin and HEAT compete at the α_1A-adrenoceptor. ρ-Da1a did not affect the dissociation kinetics of ³H-prazosin or ¹²⁵I-HEAT, and the IC₅₀ of ρ-Da1a, determined by competition experiments, increased linearly with the concentration of radioligands used, while the residual binding by ρ-Da1a remained stable. The effect of ρ-Da1a on agonist-stimulated Ca²⁺ release was insurmountable in the presence of phenethylamine- or imidazoline-type agonists. Ten mutations in the orthosteric binding pocket of the α_1A-adrenoceptor were evaluated for alterations in ρ-Da1a affinity. The D106^3.32A and the S188^5.42A/S192^5.46A receptor mutations reduced toxin affinity moderately (6 and 7.6 times, respectively), while the F86^2.64A, F288^6.51A and F312^7.39A mutations diminished it dramatically by 18- to 93-fold. In addition, residue F86^2.64 was identified as a key interaction point for ¹²⁵I-HEAT, as the variant F86^2.64A induced a 23-fold reduction in HEAT affinity. Unlike the M1 muscarinic acetylcholine receptor toxin MT7, ρ-Da1a interacts with the human α_1A-adrenoceptor orthosteric pocket and shares receptor interaction points with antagonist (F86^2.64, F288^6.51 and F312^7.39) and agonist (F288^6.51 and F312^7.39) ligands. Its selectivity for the α_1A-adrenoceptor may result, at least partly, from its interaction with the residue F86^2.64, which appears to be important also for HEAT binding.

New α-adrenergic property for synthetic MTβ and CM-3 three-finger fold toxins from black mamba

Despite their isolation more than fifteen years ago from the venom of the African mamba Dendroaspis polylepis, very few data are known on the functional activity of MTβ and CM-3 toxins. MTβ was initially classified as a muscarinic toxin interacting non-selectively and with low affinity with the five muscarinic receptor subtypes while no biological function was determined for CM-3. Recent results highlight the multifunctional activity of three-finger fold toxins for muscarinic and adrenergic receptors and reveal some discrepancies in the pharmacological profiles of their venom-purified and synthetic forms. Here, we report the pharmacological characterization of chemically-synthesized MTβ and CM-3 toxins on nine subtypes of muscarinic and adrenergic receptors and demonstrate their high potency for α-adrenoceptors and in particular a sub-nanomolar affinity for the α1A-subtype. Strikingly, no or very weak affinity were found for muscarinic receptors, highlighting that pharmacological characterizations of venom-purified peptides may be risky due to possible contaminations. The biological profile of these two homologous toxins looks like that one previously reported for the Dendroaspis angusticeps ρ-Da1a toxin. Nevertheless, MTβ and CM-3 interact more potently than ρ-Da1a with α1B- and α1D-AR subtypes. A computational analysis of the stability of the MTβ structure suggests that mutation S38I, could be involved in this gain in function.

High throughput screening identifies disulfide isomerase DsbC as a very efficient partner for recombinant expression of small disulfide-rich proteins in E. coli

Background

Disulfide-rich proteins or DRPs are versatile bioactive compounds that encompass a wide variety of pharmacological, therapeutic, and/or biotechnological applications. Still, the production of DRPs in sufficient quantities is a major bottleneck for their complete structural or functional characterization. Recombinant expression of such small proteins containing multiple disulfide bonds in the bacteria E. coli is considered difficult and general methods and protocols, particularly on a high throughput scale, are limited.

Results

Here we report a high throughput screening approach that allowed the systematic investigation of the solubilizing and folding influence of twelve cytoplasmic partners on 28 DRPs in the strains BL21 (DE3) pLysS, Origami B (DE3) pLysS and SHuffle® T7 Express lysY (1008 conditions). The screening identified the conditions leading to the successful soluble expression of the 28 DRPs selected for the study. Amongst 336 conditions tested per bacterial strain, soluble expression was detected in 196 conditions using the strain BL21 (DE3) pLysS, whereas only 44 and 50 conditions for soluble expression were identified for the strains Origami B (DE3) pLysS and SHuffle® T7 Express lysY respectively. To assess the redox states of the DRPs, the solubility screen was coupled with mass spectrometry (MS) to determine the exact masses of the produced DRPs or fusion proteins. To validate the results obtained at analytical scale, several examples of proteins expressed and purified to a larger scale are presented along with their MS and functional characterization.

Conclusions

Our results show that the production of soluble and functional DRPs with cytoplasmic partners is possible in E. coli. In spite of its reducing cytoplasm, BL21 (DE3) pLysS is more efficient than the Origami B (DE3) pLysS and SHuffle® T7 Express lysY trxB^-/gor^- strains for the production of DRPs in fusion with solubilizing partners. However, our data suggest that oxidation of the proteins occurs ex vivo. Our protocols allow the production of a large diversity of DRPs using DsbC as a fusion partner, leading to pure active DRPs at milligram scale in many cases. These results open up new possibilities for the study and development of DRPs with therapeutic or biotechnological interest whose production was previously a limitation.

Exploration of the orthosteric/allosteric interface in human M1 muscarinic receptors by bitopic fluorescent ligands

Bitopic binding properties apply to a variety of muscarinic compounds that span and simultaneously bind to both the orthosteric and allosteric receptor sites. We provide evidence that fluorescent pirenzepine derivatives, with the M1 antagonist fused to the boron-dipyrromethene [Bodipy (558/568)] fluorophore via spacers of varying lengths, exhibit orthosteric/allosteric binding properties at muscarinic M1 receptors. This behavior was inferred from a combination of functional, radioligand, and fluorescence resonance energy transfer binding experiments performed under equilibrium and kinetic conditions on enhanced green fluorescent protein-fused M1 receptors. Although displaying a common orthosteric component, the fluorescent compounds inherit bitopic properties from a linker-guided positioning of their Bodipy moiety within the M1 allosteric vestibule. Depending on linker length, the fluorophore is allowed to reach neighboring allosteric domains, overlapping or not with the classic gallamine site, but distinct from the allosteric indolocarbazole "WIN" site. Site-directed mutagenesis, as well as molecular modeling and ligand docking studies based on recently solved muscarinic receptor structures, further support the definition of two groups of Bodipy-pirenzepine derivatives exhibiting distinct allosteric binding poses. Thus, the linker may dictate pharmacological outcomes for bitopic molecules that are hardly predictable from the properties of individual orthosteric and allosteric building blocks. Our findings also demonstrate that the fusion of a fluorophore to an orthosteric ligand is not neutral, as it may confer, unless carefully controlled, unexpected properties to the resultant fluorescent tracer. Altogether, this study illustrates the importance of a "multifacet" experimental approach to unravel and validate bitopic ligand binding mechanisms.

Engineering of three-finger fold toxins creates ligands with original pharmacological profiles for muscarinic and adrenergic receptors

Protein engineering approaches are often a combination of rational design and directed evolution using display technologies. Here, we test “loop grafting,” a rational design method, on three-finger fold proteins. These small reticulated proteins have exceptional affinity and specificity for their diverse molecular targets, display protease-resistance, and are highly stable and poorly immunogenic. The wealth of structural knowledge makes them good candidates for protein engineering of new functionality. Our goal is to enhance the efficacy of these mini-proteins by modifying their pharmacological properties in order to extend their use in imaging, diagnostics and therapeutic applications. Using the interaction of three-finger fold toxins with muscarinic and adrenergic receptors as a model, chimeric toxins have been engineered by substituting loops on toxin MT7 by those from toxin MT1. The pharmacological impact of these grafts was examined using binding experiments on muscarinic receptors M1 and M4 and on the α_1A-adrenoceptor. Some of the designed chimeric proteins have impressive gain of function on certain receptor subtypes achieving an original selectivity profile with high affinity for muscarinic receptor M1 and α_1A-adrenoceptor. Structure-function analysis supported by crystallographic data for MT1 and two chimeras permits a molecular based interpretation of these gains and details the merits of this protein engineering technique. The results obtained shed light on how loop permutation can be used to design new three-finger proteins with original pharmacological profiles.

Fluorescent derivatives of AC-42 to probe bitopic orthosteric/allosteric binding mechanisms on muscarinic M1 receptors

Two fluorescent derivatives of the M1 muscarinic selective agonist AC-42 were synthesized by coupling the lissamine rhodamine B fluorophore (in ortho and para positions) to AC42-NH(2). This precursor, prepared according to an original seven-step procedure, was included in the study together with the LRB fluorophore (alone or linked to an alkyl chain). All these compounds are antagonists, but examination of their ability to inhibit or modulate orthosteric [(3)H]NMS binding revealed that para-LRB-AC42 shared several properties with AC-42. Carefully designed experiments allowed para-LRB-AC42 to be used as a FRET tracer on EGFP-fused M1 receptors. Under equilibrium binding conditions, orthosteric ligands, AC-42, and the allosteric modulator gallamine behaved as competitors of para-LRB-AC42 binding whereas other allosteric compounds such as WIN 51,708 and N-desmethylclozapine were noncompetitive inhibitors. Finally, molecular modeling studies focused on putative orthosteric/allosteric bitopic poses for AC-42 and para-LRB-AC42 in a 3D model of the human M1 receptor.

Selective targeting of G-protein-coupled receptor subtypes with venom peptides

The G-protein-coupled receptor (GPCR) family is one of the largest gene superfamilies with approx. 370 members responding to endogenous ligands in humans and a roughly equal amount of receptors sensitive to external stimuli from the surrounding. A number of receptors from this superfamily are well recognized targets for medical treatment of various disease conditions, whereas for many others the potential medical benefit of interference is still obscure. A general problem associated with GPCR research and therapeutics is the insufficient specificity of available ligands to differentiate between closely homologous receptor subtypes. In this context, venom peptides could make a significant contribution to the development of more specific drugs. Venoms from certain animals specialized in biochemical hunting contain a mixture of molecules that are directed towards a variety of membrane proteins. Peptide toxins isolated from these mixtures usually exhibit high specificity for their targets. Muscarinic toxins found from mamba snakes attracted much attention during the 1990s. These are 65-66 amino acid long peptides with a structural three-finger folding similar to the α-neurotoxins and they target the muscarinic acetylcholine receptors in a subtype-selective manner. Recently, several members of the three-finger toxins from mamba snakes as well as conotoxins from marine cone snails have been shown to selectively interact with subtypes of adrenergic receptors. In this review, we will discuss the GPCR-directed peptide toxins found from different venoms and how some of these can be useful in exploring specific roles of receptor subtypes.

Molecular conversion of muscarinic acetylcholine receptor M(5) to muscarinic toxin 7 (MT7)-binding protein

Muscarinic toxin 7 (MT7) is a mamba venom peptide that binds selectively to the M(1) muscarinic acetylcholine receptor. We have previously shown that the second (ECL2) and third (ECL3) extracellular loops of the M(1) receptor are critically involved in binding the peptide. In this study we used a mutagenesis approach on the M(5) subtype of the receptor family to find out if this possesses a similar structural architecture in terms of toxin binding as the M(1) receptor. An M(5) receptor construct (M(5)-E(175)Y(184)E(474)), mutated at the formerly deciphered critical residues on ECL2 and 3, gained the ability to bind MT7, but with rather low affinity as determined in a functional assay (apparent K(i) = 24 nM; apparent K(i) for M(1) = 0.5 nM). After screening for different domains and residues, we found a specific residue (P(179) to L in M(5)) in the middle portion of ECL2 that was necessary for high affinity binding of MT7 (M(5)-EL(179)YE, apparent K(i) = 0.5 nM). Mutation of P(179) to A confirmed a role for the leucine side chain in the binding of MT7. Together the results reveal new binding interactions between receptors and the MT7 peptide and strengthen the hypothesis that ECL2 sequence is of utmost importance for MT binding to muscarinic receptors.

Identification of a novel snake peptide toxin displaying high affinity and antagonist behaviour for the α2-adrenoceptors

BACKGROUND AND PURPOSE Muscarinic and adrenergic G protein-coupled receptors (GPCRs) are the targets of rare peptide toxins isolated from snake or cone snail venoms. We used a screen to identify novel toxins from Dendroaspis angusticeps targeting aminergic GPCRs. These toxins may offer new candidates for the development of new tools and drugs. EXPERIMENTAL APPROACH In binding experiments with (3) H-rauwolscine, we studied the interactions of green mamba venom fractions with α(2) -adrenoceptors from rat brain synaptosomes. We isolated, sequenced and chemically synthesized a novel peptide, ρ-Da1b. This peptide was pharmacologically characterized using binding experiments and functional tests on human α(2)-adrenoceptors expressed in mammalian cells. KEY RESULTS ρ-Da1b, a 66-amino acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. Its synthetic homologue inhibited 80% of (3) H-rauwolscine binding to the three α(2)-adrenoceptor subtypes, with an affinity between 14 and 73 nM and Hill slopes close to unity. Functional experiments on α(2A) -adrenoceptor demonstrated that ρ-Da1b is an antagonist, shifting adrenaline activation curves to the right. Schild regression revealed slopes of 0.97 and 0.67 and pA(2) values of 5.93 and 5.32 for yohimbine and ρ-Da1b, respectively. CONCLUSIONS AND IMPLICATIONS ρ-Da1b is the first toxin identified to specifically interact with α(2)-adrenoceptors, extending the list of class A GPCRs sensitive to toxins. Additionally, its affinity and atypical mode of interaction open up the possibility of its use as a new pharmacological tool, in the study of the physiological roles of α(2)-adrenoceptor subtypes.

Muscarinic toxins

Muscarinic toxins isolated from the venom of Dendroaspis snakes may interact with a high affinity, large selectivity and various functional properties with muscarinic receptors. Therefore, these toxins are invaluable tools for studying the physiological role, molecular functioning and structural organization of the five subtypes of these G-Protein Coupled Receptors. We review the data on the most relevant results dealing with the isolation/identification, mode of action, structure/function and exploitation of these toxins and finally highlight the unresolved issues related to their pharmacological studies.

Structural model of ligand-G protein-coupled receptor (GPCR) complex based on experimental double mutant cycle data: MT7 snake toxin bound to dimeric hM1 muscarinic receptor

The snake toxin MT7 is a potent and specific allosteric modulator of the human M1 muscarinic receptor (hM1). We previously characterized by mutagenesis experiments the functional determinants of the MT7-hM1 receptor interaction (Fruchart-Gaillard, C., Mourier, G., Marquer, C., Stura, E., Birdsall, N. J., and Servent, D. (2008) Mol. Pharmacol. 74, 1554-1563) and more recently collected evidence indicating that MT7 may bind to a dimeric form of hM1 (Marquer, C., Fruchart-Gaillard, C., Mourier, G., Grandjean, O., Girard, E., le Maire, M., Brown, S., and Servent, D. (2010) Biol. Cell 102, 409-420). To structurally characterize the MT7-hM1 complex, we adopted a strategy combining double mutant cycle experiments and molecular modeling calculations. First, thirty-three ligand-receptor proximities were identified from the analysis of sixty-one double mutant binding affinities. Several toxin residues that are more than 25 Å apart still contact the same residues on the receptor. As a consequence, attempts to satisfy all the restraints by docking the toxin onto a single receptor failed. The toxin was then positioned onto two receptors during five independent flexible docking simulations. The different possible ligand and receptor extracellular loop conformations were described by performing simulations in explicit solvent. All the docking calculations converged to the same conformation of the MT7-hM1 dimer complex, satisfying the experimental restraints and in which (i) the toxin interacts with the extracellular side of the receptor, (ii) the tips of MT7 loops II and III contact one hM1 protomer, whereas the tip of loop I binds to the other protomer, and (iii) the hM1 dimeric interface involves the transmembrane helices TM6 and TM7. These results structurally support the high affinity and selectivity of the MT7-hM1 interaction and highlight the atypical mode of interaction of this allosteric ligand on its G protein-coupled receptor target.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

The three-finger toxin MTalpha is a selective alpha(2B)-adrenoceptor antagonist

Muscarinic toxins (MTs) are three-finger folded peptides isolated from mamba snake venoms. In this report we describe a selective antagonistic interaction of MTalpha with the human alpha(2B)-adrenoceptor. In a functional assay, measuring the alpha(2B)-adrenoceptor-induced increase in intracellular [Ca(2+)], we found that both venomous MTalpha and synthetic MTalpha inhibited the response in a concentration-dependent way. MTalpha did not affect the responses of alpha(2A)-, alpha(2C)-, alpha(1A)- or alpha(1B)-adrenoceptors. To further explore the binding of MTalpha to the alpha(2B)-adrenoceptor, we performed ligand binding experiments on Sf9 cell homogenates with [(3)H]RX821002 as reporter ligand. MTalpha bound to the receptor rather slowly requiring about 60 min to reach equilibrium. In equilibrium binding experiments, MTalpha displaced the radioligand with an IC(50) of 3.2 nM, but was not able to displace all bound radioligand. Using a saturation binding protocol, we found that MTalpha suppressed the maximum binding without any greater impact on the affinity of the radioligand, indicating a non-competitive mode of inhibition. The toxin bound reversibly to alpha(2B)-adrenoceptor, but extensive washing was needed for full recovery of binding sites at high toxin concentrations. Surprisingly, MTalpha did not affect [(3)H]-N-methylscopolamine binding to the muscarinic receptor subtypes at concentrations found to fully block alpha(2B)-adrenoceptors, showing that the toxin is a more potent antagonist for the alpha(2B)-adrenoceptor than for muscarinic receptors. These findings should open up new views in terms of selective adrenoceptor drug design as well as in elucidation of alpha(2)-adrenoceptor physiology.

Isolation and pharmacological characterization of AdTx1, a natural peptide displaying specific insurmountable antagonism of the alpha1A-adrenoceptor

BACKGROUND AND PURPOSE

Venoms are a rich source of ligands for ion channels, but very little is known about their capacity to modulate G-protein coupled receptor (GPCR) activity. We developed a strategy to identify novel toxins targeting GPCRs.

EXPERIMENTAL APPROACH

We studied the interactions of mamba venom fractions with alpha(1)-adrenoceptors in binding experiments with (3)H-prazosin. The active peptide (AdTx1) was sequenced by Edman degradation and mass spectrometry fragmentation. Its synthetic homologue was pharmacologically characterized by binding experiments using cloned receptors and by functional experiments on rabbit isolated prostatic smooth muscle.

KEY RESULTS

AdTx1, a 65 amino-acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. It has subnanomolar affinity (K(i)= 0.35 nM) and high specificity for the human alpha(1A)-adrenoceptor subtype. We showed high selectivity and affinity (K(d)= 0.6 nM) of radio-labelled AdTx1 in direct binding experiments and revealed a slow association constant (k(on)= 6 x 10(6).M(-1).min(-1)) with an unusually stable alpha(1A)-adrenoceptor/AdTx1 complex (t(1/2diss)= 3.6 h). AdTx1 displayed potent insurmountable antagonism of phenylephrine's actions in vitro (rabbit isolated prostatic muscle) at concentrations of 10 to 100 nM.

CONCLUSIONS AND IMPLICATIONS

AdTx1 is the most specific and selective peptide inhibitor for the alpha(1A)-adrenoceptor identified to date. It displays insurmountable antagonism, acting as a potent relaxant of smooth muscle. Its peptidic nature can be exploited to develop new tools, as a radio-labelled-AdTx1 or a fluoro-labelled-AdTx1. Identification of AdTx1 thus offers new perspectives for developing new drugs for treating benign prostatic hyperplasia.

Muscarinic toxins: tools for the study of the pharmacological and functional properties of muscarinic receptors

Muscarinic receptors mediate metabotropic actions of acetylcholine in the CNS and PNS and autocrine functions of acetylcholine in non-neuronal systems. Because of the lack of highly selective muscarinic ligands, the precise location, functional role, and roles in various diseases of the five muscarinic receptor subtypes remain unclear. Muscarinic toxins isolated from the venom of Dendroaspis snakes have a natural high affinity and selectivity, associated with roles as competitive antagonists, allosteric modulators, and potential agonists. These toxins may therefore be invaluable tools for studying muscarinic receptors. We review data on the structural and pharmacological characterization of the muscarinic toxins, focusing on recent structure-function studies on toxin-receptor interactions. We discuss the potential benefits of using these toxins for investigating muscarinic function in vivo.