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We have synthesized a series of fluorescent acylcholine derivatives carrying different linkers that vary in length and structure and connect the acylcholine unit to the environment-sensitive fluorophores 7-(diethylamino)coumarin-3-carbonyl (DEAC) or N-(7-nitrobenz-2-oxa-1,3-diazol-yl) (NBD). The pharmacological properties of the fluorescent analogues were investigated on heterologously expressed nicotinic acetylcholine receptor (nAChR) from Torpedo californica and on oocytes transplanted with nAChR-rich Torpedo marmorata membranes. Agonist action strongly depends on the length and the structure of the linker. One particular analogue, DEAC-Gly-C6-choline, showed partial agonist behavior with about half of the maximum response of acetylcholine, which is at least 20 times higher than those observed with previously described fluorescent dansyl- and NBD-acylcholine analogues. Binding of DEAC-Gly-C6-choline to Torpedo nAChR induces a strong enhancement of fluorescence intensity. Association and displacement kinetic experiments revealed dissociation constants of 0.5 nM for the alphadelta-binding site and 15.0 nM for the alphagamma-binding site. Both the pharmacological and the spectroscopic properties of this agonist show great promise for characterizing the allosteric mechanism behind the function of the Torpedo nAChR, as well as for drug-screening studies.

The structural reorganizations occurring on the nicotinic acetylcholine receptor (nAChR) during activation and subsequent desensitization have been investigated through time-resolved photoaffinity labeling using a photoactivatable nicotinic agonist. [(3)H]AC5 is a photosensitive nicotinic probe with high affinity for the desensitized state of the Torpedo marmorata receptor (K(D) = 5 nM) that displays full agonist activity on the Torpedo californica receptor expressed in oocytes (EC(50) = 1.2 microM). Photoaffinity labeling of this receptor in the desensitized state showed a predominant specific labeling of gamma and delta subunits, whereas the alpha subunit was barely labeled. Using a stopped-flow device combined with a flash photolysis quenching system, we investigated the covalent mapping of the subunits as a function of incubation time of the receptor with [(3)H]AC5 (17 ms-1.25 h). During agonist-induced desensitization, specific labeling increased substantially, with similar time constants for gamma and delta subunits (0.016 s(-1)), whereas labeling of the alpha subunit remained relatively low. Therefore, the repartition of radioactivity shifted during desensitization from a weak but predominant labeling of the alpha and gamma subunits toward a substantial labeling of gamma and delta subunits. The observed time-dependent labeling pattern together with AC5 docking into a homology model of the T. californica nAChR suggest a subunit reorganization during agonist-induced desensitization, leading to a tightly packed arrangement that corresponds to a stable high affinity state for agonists.

We developed an engineered site-directed labeling method (Foucaud et al., 2001) to investigate ligand receptor interactions on the acetylcholine (ACh)- binding site of nicotinic acetylcholine receptors (nAChRs). The method uses cysteine receptor mutants, together with cysteine-reactive ligand analogs, to generate a site-directed covalent reaction within the binding site. We selected epibatidine (EPB) as a prototypical ligand, acting at all types of nAChRs with sufficient affinity to allow this study. Accordingly, we synthesized three cysteine-reactive derivatives, all modified at the C-3 of the pyridine ring of the alkaloid with NCS; -NHCOCH2Cl, and -CH2Cl groups, respectively (Fig. 1). The binding properties have been established on rat brain, alpha7-5HT3 chimera, and Torpedo membranes, respectively, whereas the functional properties were tested on alpha4beta2 and alpha7 receptor expressed in oocytes and Cys-less muscular receptor expressed in HEK cells (Sakr et al., 2005).

The synthesis of epibatidine derivatives modified at the 2-position of the pyridine or pyrimidine rings by reactive functions are described for potential irreversible site-directed coupling reactions on cysteine mutants of neuronal nicotinic acetylcholine receptors. An improved synthesis of the 7-azabicyclo[2,2,1]hepta-2,5-diene key intermediate has been developed to allow reproducible syntheses of the epibatidine derivatives. Binding tests and electrophysiological experiments allowed to select the 2-substituted alpha-chloroacetamido 13 and the chloropyrimidine derivative 11 as potential site-directed probes for the epibatidine binding site.

Torpedo marmorata acetylcholine binding sites were photolabeled using 360 nm light, at equilibrium in the desensitized state, with the agonist [3H]DCTA utilizing the CeIV/glutathione procedure described previously (Grutter, et al. (1999) Biochemistry 38, 7476-7484). Photoincorporation of [3H]DCTA was concentration-dependent with a maximum of 7.5% specific labeling on the alpha-subunit and 1.2% on the gamma-subunit. The apparent dissociation constants for labeling of the alpha- and gamma-subunits were 2.2 +/- 1.1 and 3.6 +/- 2.8 microM, respectively. The alpha-chains isolated from receptor-rich membranes photolabeled in the absence or in the presence of carbamylcholine were cleaved with CNBr using an efficient "in gel" procedure. The resulting peptide fragments were purified by HPLC and further submitted to trypsinolysis. The digest was analyzed by HPLC leading to a single radioactive peak which, by microsequencing, revealed two sequences extending from alpha Lys-179 and from alpha His-186, respectively. Radioactive signals could be unambiguously attributed to positions corresponding to residues alpha Tyr-190, alpha Cys-192, alpha Cys-193, and alpha Tyr-198. These four identified [3H]DCTA-labeled residues, which have been also labeled with other affinity and photoaffinity probes including the agonist [3H]nicotine, belong to loop C of the ACh binding site. The chemical structure of [3H]DCTA, together with its well-defined and powerful photochemical reactivity, provides convincing evidence that loop C-labeled residues are primarily involved in the interaction with the ester moiety of acetylcholine.

The molecular structure of Torpedo marmorata acetylcholine binding sites has been investigated previously by photoaffinity labeling. However, besides the nicotine molecule [Middleton et al. (1991) Biochemistry 30, 6987-6997], all other photosensitive probes used for this purpose interacted only with closed receptor states. In the perspective of mapping the functional activated state, we synthesized and developed a new photoactivatable agonist of nAChR capable of alkylation of the acetylcholine (ACh) binding sites, as reported previously [Kotzyba-Hibert et al. (1997) Bioconjugate Chem. 8, 472-480]. Here, we describe the setup of experimental conditions that were made in order to optimize the photolabeling reaction and in particular its specificity. We found that subsequent addition of the oxidant ceric ion (CeIV) and reduced glutathione before the photolabeling step lowered considerably nonspecific labeling (over 90% protection with d-tubocurarine) without affecting the binding properties of the ACh binding sites. As a consequence, irradiation at 360 nm for 20 min in these new conditions gave satisfactory coupling yields (7.5%). A general mechanism was proposed to explain the successive reactions occurring and their drastic effect on the specificity of the labeling reaction. Last, these incubation conditions can be extended to nanosecond pulsed laser photolysis leading to the same specific photoincorporation as for usual irradiations (8.5% coupling yield of ACh binding sites, 77% protection with carbamylcholine). Laser flash photocoupling of a diazocyclohexadienoyl probe on nAChR was achieved for the first time. Taken together, these data indicate that future investigation of the molecular dynamics of allosteric transitions occurring at the activated ACh binding sites should be possible.

The nicotinic acetylcholine receptor (AChR) exhibits at least four different conformational states varying in affinity for agonists such as acetylcholine (ACh). Photoaffinity labeling has been previously used to elucidate the topography of the AChR. However, to date, the photosensitive probes used to explore the cholinergic binding site photolabeled only closed or desensitized states of the receptor. To identify the structural modifications occurring at the ACh binding site on allosteric transition associated with receptor activation, we have investigated novel photoactivatable 4-diazocyclohexa-2,5-dienone derivatives as putative cholinergic agonists. Such compounds are fairly stable in the dark and generate highly reactive carbenic species on irradiation. In binding experiments using AChRs from Torpedo marmorata, these ligands had affinities for the ACh binding site in the micromolar range and did not interact with the noncompetitive blocker site (greater than millimolar affinity). Irreversible photoinactivation of ACh binding sites was obtained with the ligand 1b (up to 42% at 500 microM) in a protectable manner. In patch-clamp studies, 1b was shown to be a functional agonist of peripheral AChR in TE 671 cells, with the interesting property of exhibiting no or very little desensitization even at high concentrations.