Characterization of the Aquaporin-9 Inhibitor RG100204 In Vitro and in db/db Mice

Inhibition of aquaporin-9 ameliorates severe acute pancreatitis and associated lung injury by NLRP3 and Nrf2/HO-1 pathways

Inflammation, apoptosis and oxidative stress play crucial roles in the deterioration of severe acute pancreatitis-associated acute respiratory distress syndrome (SAP-ARDS). Unfortunately, despite a high mortality rate of 45 %[1], there are limited treatment options available for ARDS outside of last resort options such as mechanical ventilation and extracorporeal support strategies[2]. This study investigated the potential therapeutic role and mechanisms of AQP9 inhibitor RG100204 in two animal models of severe acute pancreatitis, inducing acute respiratory distress syndrome: 1) a sodium-taurocholate induced rat model, and 2) and Cerulein and lipopolysaccharide induced mouse model. RG100204 treatment led to a profound reduction in inflammatory cytokine expression in pancreatic, and lung tissue, in both models. In addition, infiltration of CD68 + and CD11b + cells into these tissues were reduced in RG100204 treated SAP animals, and edema and SAP associated tissue damage were improved. Moreover, we demonstrate that RG100204 reduced apoptosis in the lungs of rat SAP animals, and reduces NF-κB signaling, NLRP3, expression, while profoundly increasing the Nrf2-dependent anti oxidative stress response. We conclude that AQP9 inhibition is a promising strategy for the treatment of pancreatitis and its systemic complications, such as ARDS.

Sex-specific effect of AQP9 deficiency on hepatic triglyceride metabolism in mice with diet-induced obesity

Studies in obese rats and human cell models of non-alcoholic fatty liver disease have indicated that knockdown of the hepatic glycerol channel aquaporin 9 (AQP9) leads to decreased hepatic steatosis. However, a study in leptin receptor-deficient mice did not find that knockout (KO) of AQP9 alleviated hepatic steatosis. The aim of this study was to investigate the effect of high-fat diet (HFD) on hepatic glycerol and triglyceride metabolism in male and female AQP9 KO mice. Male and female AQP9 KO mice and wild-type (WT) littermates were fed a HFD for 12 weeks. Weight, food intake and blood glucose were monitored throughout the study and tissue analysis included determination of hepatic triglyceride content and triglyceride secretion. The expression of key molecules for hepatic glycerol and triglyceride metabolism was evaluated using qPCR and western blotting. AQP9 KO and WT mice demonstrated a similar weight gain throughout the study period, and we found no evidence for AQP9 deficiency being associated with a reduced hepatic accumulation of triglyceride or a reduced blood glucose level. Instead, we show that the effect of AQP9 deficiency on hepatic lipid metabolism is sex-specific, with only male AQP9 KO mice having a reduced hepatic secretion of triglycerides and an elevated expression of peroxisome proliferator-activated receptor α. Male AQP9 KO mice had an elevated blood glucose level after 12 weeks of HFD when compared to baseline levels. Thus, we found no evidence for AQP9 inhibition being a target for alleviating the development of hepatic steatosis in mice with diet-induced obesity. KEY POINTS: This study investigates the effect of AQP9 deficiency on hepatic triglyceride metabolism in both male and female mice fed a high-fat diet (HFD) for 12 weeks. No evidence was found for AQP9 deficiency being associated with a reduced hepatic accumulation of triglyceride or a reduced blood glucose level. The effect of AQP9 deficiency on hepatic triglyceride metabolism is sex-specific. Male AQP9 KO mice had a reduced hepatic secretion of triglycerides and an elevated expression of peroxisome proliferator-activated receptor α, which likely promotes an increased hepatic fatty acid oxidation. Male AQP9 KO had an elevated blood glucose level after 12 weeks of HFD when compared to baseline levels.

Methods for studying mammalian aquaporin biology

Aquaporins (AQPs), transmembrane water-conducting channels, have earned a great deal of scrutiny for their critical physiological roles in healthy and disease cell states, especially in the biomedical field. Numerous methods have been implemented to elucidate the involvement of AQP-mediated water transport and downstream signaling activation in eliciting whole cell, tissue, and organ functional responses. To modulate these responses, other methods have been employed to investigate AQP druggability. This review discusses standard in vitro, in vivo, and in silico methods for studying AQPs, especially for biomedical and mammalian cell biology applications. We also propose some new techniques and approaches for future AQP research to address current gaps in methodology.

Cell-free production of fluorescent proteins for the discovery of novel ribosome-targeting antibiotics

Various issues including the overuse of antibiotics has led to the development of threatening multidrug-resistant bacterial strains urging development of novel anti-infectives. One quarter of current clinical phase III antibiotic drug candidates address ribosomal protein translation as a target. Here, we describe an effective cell-free in vitro screening system for inhibitors of bacterial ribosome activity with direct fluorescence read-out. Using ribosomal S30 extracts from Escherichia coli, Salmonella enterica, and Pseudomonas putida, the validity of this system is demonstrated by concentration-dependent inhibition of translation by a set of different classes of translation-targeting drugs. The single-compartment cell-free translation reaction is compatible with multi-well formats. Fluorophore formation of green fluorescent protein or monomeric NeonGreen occurs in an hour time frame without the need of adding reagents for secondary enzymatic detection saving handling time, and prohibiting false positives. As label-free readout, the dose response further allows for IC50 determination in the same setup. Together, we show that cell-free production of fluorescent proteins for the discovery of ribosome-targeting antibiotics is feasible and amenable to high-throughput applications.

Insights into the Function of Aquaporins in Gastrointestinal Fluid Absorption and Secretion in Health and Disease

Aquaporins (AQPs), transmembrane proteins permeable to water, are involved in gastrointestinal secretion. The secretory products of the glands are delivered either to some organ cavities for exocrine glands or to the bloodstream for endocrine glands. The main secretory glands being part of the gastrointestinal system are salivary glands, gastric glands, duodenal Brunner's gland, liver, bile ducts, gallbladder, intestinal goblet cells, exocrine and endocrine pancreas. Due to their expression in gastrointestinal exocrine and endocrine glands, AQPs fulfill important roles in the secretion of various fluids involved in food handling. This review summarizes the contribution of AQPs in physiological and pathophysiological stages related to gastrointestinal secretion.

Methylthiosulfonate-Based Cysteine Modifiers as Alternative Inhibitors of Mercurial-Sensitive Aquaporins

(1) Background: Several members of the ubiquitous aquaporin family, AQP, of water and neutral solute channels carry a cysteine residue in the selectivity filter region. Traditionally, toxic mercury-containing compounds are used to bind to the cysteine as covalent AQP inhibitors for physiological studies or analysis of structure-function relationships. (2) Methods: We tested thiol-reactive methylthiosulfonate reagents, MTS, as alternative Cys modifiers for AQP inhibition. Three MTS reagents transferring S-alkyl moieties of increasing size, i.e., S-methyl, S-n-propyl, and S-benzyl, were used with yeast-expressed water-selective AQP1 and the aquaglyceroporin AQP9. Respective Cys-to-Ala variants and mouse erythrocytes that naturally express AQP1 and AQP9 served as controls. (3) Results: Both wildtype AQP isoforms were inhibited by the Cys modifiers in a size-dependent manner, whereas the Cys-to-Ala-variants exhibited resistance. Sub-millimolar concentrations and incubation times in the minute range were sufficient. The modifications were reversible by treatment with the thiol reagents acetylcysteine, ACC, and dithiothreitol, DTT. (4) Conclusions: MTS reagents represent a valid alternative of low toxicity for the inhibition of mercurial-sensitive AQPs.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.