Inhibition of CFTR-mediated intestinal chloride secretion as potential therapy for bile acid diarrhea

A Novel Peptide Prevents Enterotoxin- and Inflammation-Induced Intestinal Fluid Secretion by Stimulating Sodium-Hydrogen Exchanger 3 Activity

BACKGROUND & AIMS

Acute diarrheal diseases are the second most common cause of infant mortality in developing countries. This is contributed to by lack of effective drug therapy that shortens the duration or lessens the volume of diarrhea. The epithelial brush border sodium (Na+)/hydrogen (H+) exchanger 3 (NHE3) accounts for a major component of intestinal Na+ absorption and is inhibited in most diarrheas. Because increased intestinal Na+ absorption can rehydrate patients with diarrhea, NHE3 has been suggested as a potential druggable target for drug therapy for diarrhea.

METHODS

A peptide (sodium-hydrogen exchanger 3 stimulatory peptide [N3SP]) was synthesized to mimic the part of the NHE3 C-terminus that forms a multiprotein complex that inhibits NHE3 activity. The effect of N3SP on NHE3 activity was evaluated in NHE3-transfected fibroblasts null for other plasma membrane NHEs, a human colon cancer cell line that models intestinal absorptive enterocytes (Caco-2/BBe), human enteroids, and mouse intestine in vitro and in vivo. N3SP was delivered into cells via a hydrophobic fluorescent maleimide or nanoparticles.

RESULTS

N3SP uptake stimulated NHE3 activity at nmol/L concentrations under basal conditions and partially reversed the reduced NHE3 activity caused by elevated adenosine 3',5'-cyclic monophosphate, guanosine 3',5'-cyclic monophosphate, and Ca2+ in cell lines and in in vitro mouse intestine. N3SP also stimulated intestinal fluid absorption in the mouse small intestine in vivo and prevented cholera toxin-, Escherichia coli heat-stable enterotoxin-, and cluster of differentiation 3 inflammation-induced fluid secretion in a live mouse intestinal loop model.

CONCLUSIONS

These findings suggest pharmacologic stimulation of NHE3 activity as an efficacious approach for the treatment of moderate/severe diarrheal diseases.

Intestinal secretory mechanisms and diarrhea

One of the primary functions of the intestinal epithelium is to transport fluid and electrolytes to and from the luminal contents. Under normal circumstances, absorptive and secretory processes are tightly regulated such that absorption predominates, thereby enabling conservation of the large volumes of water that pass through the intestine each day. However, in conditions of secretory diarrhea, this balance becomes dysregulated, so that fluid secretion, driven primarily by Cl- secretion, overwhelms absorptive capacity, leading to increased loss of water in the stool. Secretory diarrheas are common and include those induced by pathogenic bacteria and viruses, allergens, and disruptions to bile acid homeostasis, or as a side effect of many drugs. Here, we review the cellular and molecular mechanisms by which Cl- and fluid secretion in the intestine are regulated, how these mechanisms become dysregulated in conditions of secretory diarrhea, currently available and emerging therapeutic approaches, and how new strategies to exploit intestinal secretory mechanisms are successfully being used in the treatment of constipation.

A systems-level analysis of bile acids effects on rat colon epithelial cells

Bile acid diarrhea is a chronic condition caused by increased delivery of bile acids to the colon. The underlying mechanisms remain to be elucidated. To investigate genes involved in bile acid diarrhea, systems-level analyses were used on a rat bile acid diarrhea model. Twelve male Wistar Munich rats, housed in metabolic cages, were fed either control or bile acid-mixed (1% wt/wt) diets for 10 days. Food intake, water intake, urine volume, body weight, and fecal output were monitored daily. After euthanasia, colonic epithelial cells were isolated using calcium chelation and processed for systems-level analyses, that is, RNA-sequencing transcriptomics and mass spectrometry proteomics. Bile acid-fed rats suffered diarrhea, indicated by increased drinking, feces weight, and fecal water content compared with control rats. Urine output was unchanged. With bile acid feeding, RNA-sequencing revealed 204 increased and 401 decreased mRNAs; mass spectrometry revealed 183 increased and 111 decreased proteins. Among the altered genes were genes associated with electrolyte and water transport (including Slc12a7, Clca4, and Aqp3) and genes associated with bile acid transport (Slc2b1, Abcg2, Slc51a, Slc51b, and Fabps). Correlation analysis showed a significant positive correlation (Pearson's r = 0.28) between changes in mRNA expression and changes in protein expression. However, caution must be exercised in making a direct correlation between experimentally determined transcriptomes and proteomes. Genes associated with bile acid transport responded to bile acid feeding, suggesting that colonic bile acid transport also occur by regulated protein facilitated mechanisms in addition to passive diffusion. In summary, the study provides annotated rat colonic epithelial cell transcriptome and proteome with response to bile acid feeding.NEW & NOTEWORTHY Feeding rats with a bile acid caused changes in fecal output, underlining this bile acid diarrhea model's usefulness. Colonic epithelial expression of genes associated with facilitated transport of bile acids was altered during bile acid feeding. The study raises the possibility of regulated colonic transepithelial transport of bile acids in response to luminal bile acids. In addition, this study provides annotated rat colonic epithelial cell transcriptome and proteome with response to bile acid feeding.

Chloride transport modulators as drug candidates

Chloride transport across cell membranes is broadly involved in epithelial fluid transport, cell volume and pH regulation, muscle contraction, membrane excitability, and organellar acidification. The human genome encodes at least 53 chloride-transporting proteins with expression in cell plasma or intracellular membranes, which include chloride channels, exchangers, and cotransporters, some having broad anion specificity. Loss-of-function mutations in chloride transporters cause a wide variety of human diseases, including cystic fibrosis, secretory diarrhea, kidney stones, salt-wasting nephropathy, myotonia, osteopetrosis, hearing loss, and goiter. Although impactful advances have been made in the past decade in drug treatment of cystic fibrosis using small molecule modulators of the defective cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, other chloride channels and solute carrier proteins (SLCs) represent relatively underexplored target classes for drug discovery. New opportunities have emerged for the development of chloride transport modulators as potential therapeutics for secretory diarrheas, constipation, dry eye disorders, kidney stones, polycystic kidney disease, hypertension, and osteoporosis. Approaches to chloride transport-targeted drug discovery are reviewed herein, with focus on chloride channel and exchanger classes in which recent preclinical advances have been made in the identification of small molecule modulators and in proof of concept testing in experimental animal models.

Tandem mass tag-based quantitative proteomic analysis of the liver reveals potential protein targets of Xiaochaihutang in CUMS model of depression

Depression is a global mental disorder disease and greatly threatened human health. Xiaochaihutang (XCHT) has been used successfully in treatment of depression for many years in China, but the mechanism is unclear. Using the chronic unpredictable mild stress (CUMS) mice model of depression, the present study aimed to reveal possible antidepressant mechanisms of XCHT from the perspective of liver by analyzing hepatic proteomics in mice. Bioinformatics analysis identified 31 differentially expressed proteins (DEPs), including 5 upregulated and 26 downregulated proteins, between the CUMS model and XCHT groups. The bile secretion pathway was found by KEGG pathway analysis of these DEPs. Four of the 31 differentially expressed proteins, including 2 active proteins involved in bile secretion, carbonic anhydrase 2 (CA2) and cystic fibrosis transmembrane conductance regulator (CFTR), were selected to verify their genes. Four genes (Cyp7a1, Fxr, Shp and Ntcp) related to bile acid synthesis and transport were further investigated by quantitative real-time polymerase chain reaction (qRT-PCR). Both biochemical tests and gene studies demonstrated that CUMS affected bile acid synthesis and transport, while XCHT regulated this pathway. The results indicated that there may be a potential relationship between CUMS induced depression and hepatic injury caused by increased bile acid, and also provide a novel insight to understand the underlying anti-depression mechanisms of XCHT.

Co-Culture System of Human Enteroids/Colonoids with Innate Immune Cells

Human intestinal enteroids derived from adult stem cells offer a relevant ex vivo system to study biological processes of the human gut. They recreate cellular and functional features of the intestinal epithelium of the small intestine (enteroids) or colon (colonoids) albeit limited by the lack of associated cell types that help maintain tissue homeostasis and respond to external challenges. In the gut, innate immune cells interact with the epithelium, support barrier function, and deploy effector functions. We have established a co-culture system of enteroid/colonoid monolayers and underlying macrophages and polymorphonuclear neutrophils to recapitulate the cellular framework of the human intestinal epithelial niche. Enteroids are generated from biopsies or resected tissue from any segment of the human gut and maintained in long-term cultures as three-dimensional structures through supplementation of stem cell growth factors. Immune cells are isolated from fresh human whole blood or frozen peripheral blood mononuclear cells (PBMC). Monocytes from PBMC are differentiated into macrophages by cytokine stimulation prior to co-culture. The methods are divided into the two main components of the model: (1) generating enteroid/colonoid monolayers and isolating immune cells and (2) assembly of enteroid/colonoid-immune cell co-cultures with separate apical and basolateral compartments. Co-cultures containing macrophages can be maintained for 48 hr while those involving neutrophils, due to their shorter life span, remain viable for 4 hr. Enteroid-immune co-cultures enable multiple outcome measures, including transepithelial resistance, production of cytokines/chemokines, phenotypic analysis of immune cells, tissue immunofluorescence imaging, protein or mRNA expression, antigen or microbe uptake, and other cellular functions. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Seeding enteroid fragments onto Transwells for monolayer formation Alternate Protocol: Seeding enteroid fragments for monolayer formation using trituration Basic Protocol 2: Isolation of monocytes and derivation of immune cells from human peripheral blood Basic Protocol 3: Isolation of neutrophils from human peripheral blood Basic Protocol 4: Assembly of enteroid/macrophage or enteroid/neutrophil co-culture.

Fluid shear stress enhances differentiation of jejunal human enteroids in Intestine-Chip

There is increasing evidence that the study of normal human enteroids duplicates many known aspects of human intestinal physiology. However, this epithelial cell-only model lacks the many nonepithelial intestinal cells present in the gastrointestinal tract and exposure to the mechanical forces to which the intestine is exposed. We tested the hypothesis that physical shear forces produced by luminal and blood flow would provide an intestinal model more closely resembling normal human jejunum. Jejunal enteroid monolayers were studied in the Emulate, Inc. Intestine-Chip under conditions of constant luminal and basolateral flow that was designed to mimic normal intestinal fluid flow, with human umbilical vein endothelial cells (HUVECs) on the basolateral surface and with Wnt3A, R-spondin, and Noggin only on the luminal surface. The jejunal enteroids formed monolayers that remained confluent for 6-8 days, began differentiating at least as early as day 2 post plating, and demonstrated continuing differentiation over the entire time of the study, as shown by quantitative real-time polymerase chain reaction and Western blot analysis. Differentiation impacted villus genes and proteins differently with early expression of regenerating family member 1α (REG1A), early reduction to a low but constant level of expression of Na⁺-K⁺-2Cl^- cotransporter 1 (NKCC1), and increasing expression of sucrase-isomaltase (SI) and downregulated in adenoma (DRA). These results were consistent with continual differentiation, as was shown to occur in mouse villus enterocytes. Compared with differentiated enteroid monolayers grown on Transwell inserts, enteroids exposed to flow were more differentiated but exhibited increased apoptosis and reduced carbohydrate metabolism, as shown by proteomic analysis. This study of human jejunal enteroids-on-chip suggests that luminal and basolateral flow produce a model of continual differentiation over time and NaCl absorption that mimics normal intestine and should provide new insights in intestinal physiology.NEW & NOTEWORTHY This study showed that polarized enteroid models in which there is no basolateral Wnt3a, are differentiated, regardless of the Wnt3a status of the apical media. The study supports the concept that in the human intestine villus differentiation is not an all or none phenomenon, demonstrating that at different days after lack of basolateral Wnt exposure, clusters of genes and proteins exist geographically along the villus with different domains having different functions.

Repurposing calcium-sensing receptor agonist cinacalcet for treatment of CFTR-mediated secretory diarrheas

Diarrhea is a major cause of global mortality, and outbreaks of secretory diarrhea such as cholera remain an important problem in the developing world. Current treatment of secretory diarrhea primarily involves supportive measures, such as fluid replacement. The calcium-sensing receptor (CaSR) regulates multiple biological activities in response to changes in extracellular Ca²⁺. The FDA-approved drug cinacalcet is an allosteric activator of CaSR used for treatment of hyperparathyroidism. Here, we found by short-circuit current measurements in human colonic T84 cells that CaSR activation by cinacalcet reduced forskolin-induced Cl^– secretion by greater than 80%. Cinacalcet also reduced Cl^– secretion induced by cholera toxin, heat-stable E. coli enterotoxin, and vasoactive intestinal peptide (VIP). The cinacalcet effect primarily involved indirect inhibition of cystic fibrosis transmembrane conductance regulator–mediated (CFTR-mediated) Cl^– secretion following activation of CaSR and downstream phospholipase C and phosphodiesterases. In mice, cinacalcet reduced fluid accumulation by more than 60% in intestinal closed loop models of cholera and traveler’s diarrhea. The cinacalcet effect involved both inhibition of CFTR-mediated secretion and stimulation of sodium-hydrogen exchanger 3–mediated absorption. These findings support the therapeutic utility of the safe and commonly used drug cinacalcet in CFTR-dependent secretory diarrheas, including cholera, traveler’s diarrhea, and VIPoma.