Carbon monoxide-releasing molecules inhibit the cystic fibrosis transmembrane conductance regulator Cl- channel

Crucial role of carbon monoxide as a regulator of diarrhea caused by cholera toxin: Evidence of direct interaction with toxin

The present study evaluated the role of heme oxygenase 1 (HO-1)/carbon monoxide (CO) pathway in the cholera toxin-induced diarrhea and its possible action mechanism. The pharmacological modulation with CORM-2 (a CO donor) or Hemin (a HO-1 inducer) decreased the intestinal fluid secretion and Cl- efflux, altered by cholera toxin. In contrast, ZnPP (a HO-1 inhibitor) reversed the antisecretory effect of Hemin and potentiated cholera toxin-induced intestinal secretion. Moreover, CORM-2 also prevented the alteration of intestinal epithelial architecture and local vascular permeability promoted by cholera toxin. The intestinal absorption was not altered by any of the pharmacological modulators. Cholera toxin inoculation also increased HO-1 immunoreactivity and bilirubin levels, a possible protective physiological response. Finally, using fluorometric technique, ELISA assay and molecular docking simulations, we show evidence that CO directly interacts with cholera toxin, forming a complex that affects its binding to GM1 receptor, which help explain the antisecretory effect. Thus, CO is an essential molecule for protection against choleric diarrhea and suggests its use as a possible therapeutic tool.

Plight of CORMs: The unreliability of four commercially available CO-releasing molecules, CORM-2, CORM-3, CORM-A1, and CORM-401, in studying CO biology

Carbon monoxide (CO) is an endogenously produced gaseous signaling molecule with demonstrated pharmacological effects. In studying CO biology, three delivery forms have been used: CO gas, CO in solution, and CO donors of various types. Among the CO donors, four carbonyl complexes with either a transition metal ion or borane (BH3) (termed CO-releasing molecules or CORMs) have played the most prominent roles appearing in over 650 publications. These are CORM-2, CORM-3, CORM-A1, and CORM-401. Intriguingly, there have been unique biology findings that were only observed with these CORMs, but not CO gas; yet these properties were often attributed to CO, raising puzzling questions as to why CO source would make such a fundamental difference in terms of CO biology. Recent years have seen a large number of reports of chemical reactivity (e.g., catalase-like activity, reaction with thiol, and reduction of NAD(P)+) and demonstrated CO-independent biological activity for these four CORMs. Further, CORM-A1 releases CO in an idiosyncratic fashion; CO release from CORM-401 is strongly influenced or even dependent on reaction with an oxidant and/or a nucleophile; CORM-2 mostly releases CO2, not CO, after a water-gas shift reaction except in the presence of a strong nucleophile; and CORM-3 does not release CO except in the presence of a strong nucleophile. All these beg the question as to what constitutes an appropriate CO donor for studying CO biology. This review critically summarizes literature findings related to these aspects, with the aim of helping result interpretation when using these CORMs and development of essential criteria for an appropriate donor for studying CO biology.

A topological switch in CFTR modulates channel activity and sensitivity to unfolding

The cystic fibrosis transmembrane conductance regulator (CFTR) anion channel is essential to maintain fluid homeostasis in key organs. Functional impairment of CFTR due to mutations in the cftr gene leads to cystic fibrosis. Here, we show that the first nucleotide-binding domain (NBD1) of CFTR can spontaneously adopt an alternate conformation that departs from the canonical NBD fold previously observed. Crystallography reveals that this conformation involves a topological reorganization of NBD1. Single-molecule fluorescence resonance energy transfer microscopy shows that the equilibrium between the conformations is regulated by adenosine triphosphate binding. However, under destabilizing conditions, such as the disease-causing mutation F508del, this conformational flexibility enables unfolding of the β-subdomain. Our data indicate that, in wild-type CFTR, this conformational transition of NBD1 regulates channel function, but, in the presence of the F508del mutation, it allows domain misfolding and subsequent protein degradation. Our work provides a framework to design conformation-specific therapeutics to prevent noxious transitions.