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  • An important landmark in CF research has been the

    2019-07-08

    An important landmark in CF research has been the cloning of the CFTR gene in 1989 [16]. Soon later, several CF mutant mouse models were generated [17]. Initially researchers concentrated their efforts on the production of null mice by deleting specific portions of the murine homolog gene to CFTR (cftr). In these mice, the CFTR protein is not detectable and therefore they are comparable to a stop codon (Class I) mutation [18]. CFTR null mice were generated in different genetic backgrounds [19]. Since these models were not completely representative of the complexity of the CF human disease with the different spectrum of mutations, other models were created that mimic the class II mutations F508del and G480C and the class III mutation G551D [[20], [21], [22]]. The use of the mouse to model CF has certain advantages including the short time to generate the animals and the relatively contained cost of maintenance. However, physiological differences between the human and the mouse species and the restricted environment to which mice are exposed to has been a major challenge in the attempt to reproduce a complex multi-organ disease as CF. Indeed, CF mice present a mild (i.e. G480C, G551D) to severe (i.e. null, F508del) intestinal phenotype, similar to human patients, with intestinal obstructions at the weaning, nutrient malabsorption, mucus accumulation and small intestinal bacterial overgrowth (SIBO) [17,[20], [21], [22]]. The incidence of developing intestinal obstruction (i.e. meconium ileum, MI) at the time of weaning is quite high and to decrease the rate of mortality in these mice, their diet is supplemented with laxatives (i.e. Poly Ethylene Glycol; PEG) or substituted with a complete liquid diet (Peptamen, Nestle) [17]. To overcome this complication, Whitsett lab was able to introduce the expression of human CFTR using a fatty acid-binding protein promoter in intestinal villus Bradykinin (acetate) in null mice and generated a gut-corrected strain where the intestinal phenotype is completely recovered [23]. Although not many studies have been performed using the gut corrected strain, this is the first proof that transfer of the human CFTR gene in epithelia that lack the expression of the protein can correct the physiologic defect. Despite the intestinal disease, CF mice lack the spontaneous development of lung, pancreas and liver phenotypes unless they are aged (12–24 months) or challenged with toxic agents [24]. However, the G551D mice represent an exception; in fact, a liver phenotype is described although in a small percentage of animals [21]. The failure of these models to reproduce the human CF disease has been mainly attributed to the physiological differences in species. Specifically, mice show a higher presence of CFTR-independent Calcium activated Chloride Conductance (CaCC) that can mediate chloride efflux when CFTR is not functional [11]. More recently, a discovery of TMEM16A as the possible chloride channel that mediates CaCC [25] and its expression has also been confirmed in biliary cells [26]. Studies of the lung have shown that when CF mice are challenged with either bacterial pathogens or their components (i.e. Pseudomonas aeruginosa, LPS), they develop an excessive inflammatory response with a decreased clearance of bacteria as compared to normal mice [27]. This observation suggests that the inflammatory response to pathogens in the lung of CF mice is altered compared to normal mice, and that external challenges are essential toward the development of a phenotype. Bile efflux and bicarbonate secretion are both decreased in CFTR null and ΔF508 mice confirming the presence of a secretory defect [28,29] and providing an in vivo model to test molecules or drugs able to correct the defect. In this regard, our group has identified glibenclamide, a compound belonging to the family of sulfonylureas and known to potentiate insulin secretion in β-cells, as a choleretic agent that stimulates bile flow with a mechanism independent of CFTR function. Our findings suggest that glibenclamide acts by stimulating a Ca2+ and cAMP independent vesicular transport pathway that is preserved in the biliary epithelium of CFTR null mice [29].