The structural loop II of cobrotoxin is the main binding region for nAChR and epitope in the region is conformation-dependent

COVID-19 and pulmonary fibrosis: therapeutics in clinical trials, repurposing, and potential development

In 2019, an unprecedented disease named coronavirus disease 2019 (COVID-19) emerged and spread across the globe. Although the rapid transmission of COVID-19 has resulted in thousands of deaths and severe lung damage, conclusive treatment is not available. However, three COVID-19 vaccines have been authorized, and two more will be approved soon, according to a World Health Organization report on December 12, 2020. Many COVID-19 patients show symptoms of acute lung injury that eventually leads to pulmonary fibrosis. Our aim in this article is to present the relationship between pulmonary fibrosis and COVID-19, with a focus on angiotensin converting enzyme-2. We also evaluate the radiological imaging methods computed tomography (CT) and chest X-ray (CXR) for visualization of patient lung condition. Moreover, we review possible therapeutics for COVID-19 using four categories: treatments related and unrelated to lung disease and treatments that have and have not entered clinical trials. Although many treatments have started clinical trials, they have some drawbacks, such as short-term and small-group testing, that need to be addressed as soon as possible.

Loop 2 of Ophiophagus hannah toxin b binds with neuronal nicotinic acetylcholine receptors and enhances intracranial drug delivery

Three-finger snake neurotoxins have been widely investigated for their high binding affinities with nicotinic acetylcholine receptors (nAChRs), which are widely expressed in the central nervous system including the blood-brain barrier and thus mediate intracranial drug delivery. The loop 2 segments of three-finger snake neurotoxins are considered as the binding domain with nAChRs, and thus, they may have the potential to enhance drug or drug delivery system intracranial transport. In the present work, binding of the synthetic peptides to the neuronal nAChRs was assessed by measuring their ability to inhibit the binding of (125)I-α-bungarotoxin to the receptor. The loop 2 segment of Ophiophagus hannah toxin b (KC2S) showed high binding affinity, and the competitive binding IC(50) value was 32.51 nM. Furthermore, the brain targeting efficiency of KC2S had been investigated in vitro and in vivo. The specific uptake by brain capillary endothelial cells (BCECs) demonstrated that KC2S could be endocytosized after binding with nAChRs. In vivo, the qualitative and quantitative biodistribution results of fluorescent dyes (DiR or coumarin-6) indicated that KC2S modified poly(ethylene glycol)-poly(lactic acid) micelles (KC2S-PEG-PLA micelles) could enhance intracranial drug delivery. Furthermore, intravenous treatment with paclitaxel-encapsulated KC2S-PEG-PLA micelles (KC2S-PEG-PLA-PTX micelles) afforded robust inhibition of intracranial glioblastoma. The median survival time of KC2S-PEG-PLA-PTX-micelle-treated mice (47.5 days) was significantly longer than that of mice treated by mPEG-PLA-PTX micelles (41.5 days), Taxol (38.5 days), or saline (34 days). Compared with the short peptide derived from rabies virus glycoprotein (RVG29) that has been previously reported as an excellent brain targeting ligand, KC2S has a similar binding affinity with neuronal nAChRs but fewer amino acid residues. Thus, we concluded that the loop 2 segment of Ophiophagus hannah toxin b could bind with neuronal nAChRs and thus enhance intracranial drug delivery for the treatment of central nervous system diseases.

Probing the structural diversities of long alpha-neurotoxins by fluorescence quenching studies

Trp fluorescence of Ophiophagus hannah neurotoxins (Oh-4, Oh-6A, Oh-7, and Oh-8) and Bungarus multicinctus (alpha-bungarotoxin was quenched by acrylamide and iodide. Acrylamide quenching studies indicated that the degree of exposure of Trp residues in the neurotoxins followed the order Oh-8 > Oh-7 > Oh-6A > Oh-4 > alpha-bungarotoxin, as did the accessibility for iodide. These results reveal that the exposed degree of Trp residues and the microenvironment surrounding Trp residues in the neurotoxins differ, even though their Trp residues and positively charged residues are located at the same or homologous positions. In contrast to unfolded Oh-4, Oh-6A, Oh-7, and alpha-bungarotoxin, unfolding of Oh-8 by reduction and S-carboxymethylation caused a notable decrease in the susceptibility of their Trp residues for iodide. These observations support the view that the side chains of Trp residues and positively charged residues in their native structure do not point toward the same spatial positions. Computer models of the neurotoxins are in good agreement with this proposition. These results elucidate why the conserved Trp residues and cationic groups do not always play the same roles in the biological activities of the neurotoxins.

Differential contribution of the conserved tyrosine residues to activity and structural stability of Ophiophagus hannah alpha-neurotoxins

Two alpha-neurotoxins, Oh-4 and Oh-7, from king cobra (Ophiophagus hannah) venom were subjected to Tyr modification with tetranitromethane. Selective nitration of Tyr4 in Oh-4 caused a slight decrease in lethal toxicity of 11% and a decrease in nicotinic acetylcholine receptor (nAchR)-binding activity of 28%, whereas nitration of Tyr4 in Oh-7 resulted in an approximately equal 60% decrease in lethality and nAchR-binding activity. When the Tyr23 in Oh-4 or Tyr22 in Oh-7 appears to be 'buried' in the toxin following further modification, the toxins lost their biological activity and conformational change concurrently. Nevertheless, the dinitrated Oh-4 retained a beta-sheet structure as revealed by CD spectra and exhibited a precipitin reaction with anti-Oh-4 sera. These results indicate that both Tyr4 and Tyr22 play a crucial role in the neurotoxicity of Oh-7, whereas intact Tyr23 is involved in the manifestation of the toxicity of Oh-4 to a greater extent. In contrast to Oh-4, the conformational stability of Oh-7 depends heavily upon the integrity of Tyr22.

Four new postsynaptic neurotoxins from Naja naja sputatrix venom: cDNA cloning, protein expression, and phylogenetic analysis

cDNAs encoding 4 short chain alpha-neurotoxins from Malayan spitting cobra (Naja naja sputatrix) venom were cloned and expressed in Escherichia coli. The recombinant toxins possessed identical amino acid sequences to alpha-neurotoxins. This is the first report on cloning and expression of isoforms of neurotoxins from a species of spitting cobra. Two of these isoforms were also identified in the crude venom by reverse phase-high performance liquid chromatography (RP-HPLC), capillary electrophoresis followed by mass spectrometry and characterized by protein sequencing. Based on the variable amino acid residues, the neurotoxins in N. n. sputatrix could be assigned to 2 major groups, 10E11T and 10Q11A, which could be further subdivided into 10E11T28S: 10E11T28G and 10Q11A28S; 10Q11A28G respectively. These substitutions were also found to be unique to N. n. sputatrix neurotoxins. Phylogenetic analysis based on molecular properties of the toxins provided further support for the classification of N. n. sputatrix neurotoxin into 2 fundamental groups.

Role of amino and carboxyl groups of cobrotoxin in the conformational stability and the interaction with acetylcholine receptor

To study the functional involvements of the common interaction of the Leu-1 alpha-amino group and Asp-58 in cobrotoxin, the lysine epsilon-amino groups of cobrotoxin were initially guanidinated with o-methylisourea. The alpha-amino group of Leu-1 was them modified with TNBS after the guanidination of cobrotoxin. Both modified derivatives displayed no significant changes in the secondary structure and antigenicity of cobrotoxin, whereas the binding affinity for nicotinic acetylcholine receptor (nAChR) was pronouncedly decreased when Leu-1 was modified. Six out of seven free carboxyl groups and the remaining buried Glu-21 carboxyl group of cobrotoxin were modified with glycine methyl ester in the absence and presence of guanidine HCl, respectively. Alternation in the beta-sheet secondary structure of cobrotoxin was observed with the carboxyl-group modified derivatives, which caused a decrease in the binding activity of the toxin molecule to the antibody and nAChR. Moreover, modification of the Glu-21 carboxyl group of cobrotoxin further reduced the nAChR binding activity, while the antigenicity remained unchange. Thus, our results conclude that the Glu-21 residue and the common interaction of the terminal Leu-1 alpha-amino group and the Asp-58 carboxyl group are related to the nAChR-binding activity of cobrotoxin, and the free carboxyl groups in cobrotoxin are conformation-essential.