Cysteine [2,4] Disulfide Bond as a New Modifiable Site of α-Conotoxin TxIB

Stapling Cysteine[2,4] Disulfide Bond of α-Conotoxin LsIA and Its Potential in Target Delivery

α-Conotoxins, as selective nAChR antagonists, can be valuable tools for targeted drug delivery and fluorescent labeling, while conotoxin-drug or conotoxin-fluorescent conjugates through the disulfide bond are rarely reported. Herein, we demonstrate the [2,4] disulfide bond of α-conotoxin as a feasible new chemical modification site. In this study, analogs of the α-conotoxin LsIA cysteine[2,4] were synthesized by stapling with five linkers, and their inhibitory activities against human α7 and rat α3β2 nAChRs were maintained. To further apply this method in targeted delivery, the alkynylbenzyl bromide linker was synthesized and conjugated with Coumarin 120 (AMC) and Camptothecin (CPT) by copper-catalyzed click chemistry, and then stapled between cysteine[2,4] of the LsIA to construct a fluorescent probe and two peptide-drug conjugates. The maximum emission wavelength of the LsIA fluorescent probe was 402.2 nm, which was essentially unchanged compared with AMC. The cytotoxic activity of the LsIA peptide-drug conjugates on human A549 was maintained in vitro. The results demonstrate that the stapling of cysteine[2,4] with alkynylbenzyl bromide is a simple and feasible strategy for the exploitation and utilization of the α-conotoxin LsIA.

Molecular Determinants of Species Specificity of α-Conotoxin TxIB towards Rat and Human α6/α3β4 Nicotinic Acetylcholine Receptors

Conotoxins are widely distributed and important for studying ligand-gated ion channels. TxIB, a conotoxin consisting of 16 amino acids derived from Conus textile, is a unique selective ligand that blocks rat α6/α3β2β3 nAChR (IC₅₀ = 28 nM) without affecting other rat subtypes. However, when the activity of TxIB against human nAChRs was examined, it was unexpectedly found that TxIB had a significant blocking effect on not only human α6/α3β2β3 nAChR but also human α6/α3β4 nAChR, with an IC₅₀ of 537 nM. To investigate the molecular mechanism of this species specificity and to establish a theoretical basis for drug development studies of TxIB and its analogs, different amino acid residues between human and rat α6/α3 and β4 nAChR subunits were identified. Each residue of the human species was then substituted with the corresponding residue of the rat species via PCR-directed mutagenesis. The potencies of TxIB towards the native α6/α3β4 nAChRs and their mutants were evaluated through electrophysiological experiments. The results showed that the IC₅₀ of TxIB against h[α6_{V32L, K61R}/α3]β4_{L107V, V115I} was 22.5 μM, a 42-fold decrease in potency compared to the native hα6/α3β4 nAChR. Val-32 and Lys-61 in the human α6/α3 subunit and Leu-107 and Val-115 in the human β4 subunit, together, were found to determine the species differences in the α6/α3β4 nAChR. These results also demonstrate that the effects of species differences between humans and rats should be fully considered when evaluating the efficacy of drug candidates targeting nAChRs in rodent models.

Marine natural products

Covering: January to December 2021This review covers the literature published in 2021 for marine natural products (MNPs), with 736 citations (724 for the period January to December 2021) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1425 in 416 papers for 2021), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of the number of authors, their affiliations, domestic and international collection locations, focus of MNP studies, citation metrics and journal choices is discussed.

α-Conotoxin TxIB Inhibits Development of Morphine-Induced Conditioned Place Preference in Mice via Blocking α6β2* Nicotinic Acetylcholine Receptors

Morphine, the main component of opium, is a commonly used analgesic in clinical practice, but its abuse potential limits its clinical application. Nicotinic acetylcholine receptors (nAChRs) in the mesolimbic circuitry play an important role in the rewarding effects of abused drugs. Previous studies have showed that α6β2* (* designated other subunits) nAChRs are mainly distributed in dopaminergic neurons in the midbrain area, which regulates the release of dopamine. So α6β2* nAChRs are regarded as a new target to treat drug abuse. α-Conotoxin TxIB was discovered in our lab, which is the most selective ligand to inhibit α6β2* nAChRs only. Antagonists of α6β2* nAChRs decreased nicotine, cocaine, and ethanol rewarding effects previously. However, their role in morphine addiction has not been reported so far. Thus, it is worth evaluating the effect of α-conotoxin TxIB on the morphine-induced conditioned place preference (CPP) and its behavioral changes in mice. Our results showed that TxIB inhibited expression and acquisition of morphine-induced CPP and did not produce a rewarding effect by itself. Moreover, repeated injections of TxIB have no effect on learning, memory, locomotor activity, and anxiety-like behavior. Therefore, blocking α6/α3β2β3 nAChRs inhibits the development of morphine-induced CPP. α-Conotoxin TxIB may be a potentially useful compound to mitigate the acquisition and/or retention of drug-context associations.