Utility of endoscopic biopsy samples to quantitate human duodenal ion transport

DRA involvement in linaclotide-stimulated bicarbonate secretion during loss of CFTR function

Duodenal bicarbonate secretion is critical to epithelial protection, as well as nutrient digestion and absorption, and is impaired in cystic fibrosis (CF). We examined if linaclotide, typically used to treat constipation, may also stimulate duodenal bicarbonate secretion. Bicarbonate secretion was measured in vivo and in vitro using mouse and human duodenum (biopsies and enteroids). Ion transporter localization was identified with confocal microscopy, and de novo analysis of human duodenal single-cell RNA sequencing (scRNA-Seq) data sets was performed. Linaclotide increased bicarbonate secretion in mouse and human duodenum in the absence of cystic fibrosis transmembrane conductance regulator (CFTR) expression (Cftr-knockout mice) or function (CFTRinh-172). Na+/H+ exchanger 3 inhibition contributed to a portion of this response. Linaclotide-stimulated bicarbonate secretion was eliminated by down-regulated in adenoma (DRA, SLC26A3) inhibition during loss of CFTR activity. ScRNA-Seq identified that 70% of villus cells expressed SLC26A3, but not CFTR, mRNA. Loss of CFTR activity and linaclotide increased apical brush border expression of DRA in non-CF and CF differentiated enteroids. These data provide further insights into the action of linaclotide and how DRA may compensate for loss of CFTR in regulating luminal pH. Linaclotide may be a useful therapy for CF individuals with impaired bicarbonate secretion.

Key role of down-regulated in adenoma (SLC26A3) chloride/bicarbonate exchanger in linaclotide-stimulated intestinal bicarbonate secretion upon loss of CFTR function

Duodenal bicarbonate secretion is critical to epithelial protection, nutrient digestion/absorption and is impaired in cystic fibrosis (CF). We examined if linaclotide, typically used to treat constipation, may also stimulate duodenal bicarbonate secretion. Bicarbonate secretion was measured in vivo and in vitro using mouse and human duodenum (biopsies and enteroids). Ion transporter localization was identified with confocal microscopy and de novo analysis of human duodenal single cell RNA sequencing (sc-RNAseq) datasets was performed. Linaclotide increased bicarbonate secretion in mouse and human duodenum in the absence of CFTR expression (Cftr knockout mice) or function (CFTRinh-172). NHE3 inhibition contributed to a portion of this response. Linaclotide-stimulated bicarbonate secretion was eliminated by down-regulated in adenoma (DRA, SLC26A3) inhibition during loss of CFTR activity. Sc-RNAseq identified that 70% of villus cells expressed SLC26A3, but not CFTR, mRNA. Loss of CFTR activity and linaclotide increased apical brush border expression of DRA in non-CF and CF differentiated enteroids. These data provide further insights into the action of linaclotide and how DRA may compensate for loss of CFTR in regulating luminal pH. Linaclotide may be a useful therapy for CF individuals with impaired bicarbonate secretion.

Simultaneous Prediction Method for Intestinal Absorption and Metabolism Using the Mini-Ussing Chamber System

Many evaluation tools for predicting human absorption are well-known for using cultured cell lines such as Caco-2, MDCK, and so on. Since the combinatorial chemistry and high throughput screening system, pharmacological assay, and pharmaceutical profiling assay are mainstays of drug development, PAMPA has been used to evaluate human drug absorption. In addition, cultured cell lines from iPS cells have been attracting attention because they morphologically resemble human intestinal tissues. In this review, we used human intestinal tissues to estimate human intestinal absorption and metabolism. The Ussing chamber uses human intestinal tissues to directly assay a drug candidate's permeability and determine the electrophysiological parameters such as potential differences (PD), short circuit current (Isc), and resistance (R). Thus, it is an attractive tool for elucidating human intestinal permeability and metabolism. We have presented a novel prediction method for intestinal absorption and metabolism by utilizing a mini-Ussing chamber using human intestinal tissues and animal intestinal tissues, based on the transport index (TI). The TI value was calculated by taking the change in drug concentrations on the apical side due to precipitation and the total amounts accumulated in the tissue (Tcorr) and transported to the basal side (Xcorr). The drug absorbability in rank order, as well as the fraction of dose absorbed (Fa) in humans, was predicted, and the intestinal metabolism of dogs and rats was also predicted, although it was not quantitative. However, the metabolites formation index (MFI) values, which are included in the TI values, can predict the evaluation of intestinal metabolism and absorption by using ketoconazole. Therefore, the mini-Ussing chamber, equipped with human and animal intestinal tissues, would be an ultimate method to predict intestinal absorption and metabolism simultaneously.

Impaired distal colonic pH in adults with cystic fibrosis

Previous wireless motility capsule (WMC) studies demonstrated decreased small intestinal pH in people with CF (PwCF) however the data is lacking on the colonic pH profile. We re-analyzed previously published WMC data to determine colonic pH/bicarbonate concentration and single cell RNA sequencing (sc-RNAseq) to examine the normal expression of acid-base transporters in the colon/rectum.CF patients showed significantly lower pH and bicarbonate concentration values, particularly in the distal rectosigmoid region. There was no difference in colonic motility parameters between CF and non-CF subjects. SLC26A3 is highly expressed bicarbonate transporter in the colon and rectum, more so than CFTR. While dysmotility can alter intraluminal pH, observed changes likely originate from alterations in intestinal ion transport rather than colonic dysmotility. SLC26A3 is abundantly expressed in the human colon and rectum and may be a therapeutic target for restoration of bicarbonate transport. These findings may help better understand the gastrointestinal symptoms in PwCF.

The human duodenal mucosa harbors all components for a local renin angiotensin system

The renin-angiotensin system (RAS) is present in the gastrointestinal (GI) tract but remains to be fully characterized, particularly in man. The duodenum plays a role in both the upper and lower GI regulation, as well as in distant organs. The present study investigates the presence and functional potential of RAS in the human duodenal mucosa of healthy individuals. Endoscopically acquired mucosal biopsies from healthy volunteers were examined using western blot, immunohistochemistry, and ELISA. Functionality was examined by using Ussing chambers and recording duodenal transmucosal potential difference (PD) and motility in vivo Angiotensinogen, Angiotensin II (AngII) and its receptors (AT1R, AT2R) as well as to the RAS associated enzymes renin, ACE, and neprylisin were detected in all samples of duodenal mucosa. Migrating motility complex induced elevations of transmucosal PD were significantly larger after per-oral administration of the AT1R receptor antagonist candesartan. Fasting duodenal motility per se was not influenced by candesartan. The epithelial current produced by duodenal mucosae mounted in Ussing chambers increased significantly after addition of AngII to specimens where the AT1R was blocked using losartan. The epithelial current also increased after addition of the AT2R-selective agonist C21. Immunostaining and pharmacological data demonstrate the presence of a local RAS in the human duodenal mucosa with capacity to influence epithelial ion transport by way of particulary the AT2R.

Oxygen in the regulation of intestinal epithelial transport

The transport of fluid, nutrients and electrolytes to and from the intestinal lumen is a primary function of epithelial cells. Normally, the intestine absorbs approximately 9 l of fluid and 1 kg of nutrients daily, driven by epithelial transport processes that consume large amounts of cellular energy and O2. The epithelium exists at the interface of the richly vascularised mucosa, and the anoxic luminal environment and this steep O2 gradient play a key role in determining the expression pattern of proteins involved in fluid, nutrient and electrolyte transport. However, the dynamic nature of the splanchnic circulation necessitates that the epithelium can evoke co-ordinated responses to fluctuations in O2 availability, which occur either as a part of the normal digestive process or as a consequence of several pathophysiological conditions. While it is known that hypoxia-responsive signals, such as reactive oxygen species, AMP-activated kinase, hypoxia-inducible factors, and prolyl hydroxylases are all important in regulating epithelial responses to altered O2 supply, our understanding of the molecular mechanisms involved is still limited. Here, we aim to review the current literature regarding the role that O2 plays in regulating intestinal transport processes and to highlight areas of research that still need to be addressed.

Evaluation of endoscopically obtained duodenal biopsy samples from cats and dogs in an adapter-modified Ussing chamber

This study was conducted to evaluate an adapter-modified Ussing chamber for assessment of transport physiology in endoscopically obtained duodenal biopsies from healthy cats and dogs, as well as dogs with chronic enteropathies. 17 duodenal biopsies from five cats and 51 duodenal biopsies from 13 dogs were obtained. Samples were transferred into an adapter-modified Ussing chamber and sequentially exposed to various absorbagogues and secretagogues. Overall, 78.6% of duodenal samples obtained from cats responded to at least one compound. In duodenal biopsies obtained from dogs, the rate of overall response ranged from 87.5% (healthy individuals; n = 8), to 63.6% (animals exhibiting clinical signs of gastrointestinal disease and histopathological unremarkable duodenum; n = 15), and 32.1% (animals exhibiting clinical signs of gastrointestinal diseases and moderate to severe histopathological lesions; n = 28). Detailed information regarding the magnitude and duration of the response are provided. The adapter-modified Ussing chamber enables investigation of the absorptive and secretory capacity of endoscopically obtained duodenal biopsies from cats and dogs and has the potential to become a valuable research tool. The response of samples was correlated with histopathological findings.

Colonic epithelial ion transport is not affected in patients with diverticulosis

Background

Colonic diverticular disease is a bothersome condition with an unresolved pathogenesis. It is unknown whether a neuroepithelial dysfunction is present. The aim of the study was two-fold; (1) to investigate colonic epithelial ion transport in patients with diverticulosis and (2) to adapt a miniaturized Modified Ussing Air-Suction (MUAS) chamber for colonic endoscopic biopsies.

Methods

Biopsies were obtained from the sigmoid part of the colon. 86 patients were included. All patients were referred for colonoscopy on suspicion of neoplasia and they were without pathological findings at colonoscopy (controls) except for diverticulosis in 22 (D-patients). Biopsies were mounted in MUAS chambers with an exposed area of 5 mm². Electrical responses to various stimulators and inhibitors of ion transport were investigated together with histological examination. The MUAS chamber was easy to use and reproducible data were obtained.

Results

Median basal short circuit current (SCC) was 43.8 μA·cm^-2 (0.8 – 199) for controls and 59.3 μA·cm^-2 (3.0 – 177.2) for D-patients. Slope conductance was 77.0 mS·cm^-2 (18.6 – 204.0) equal to 13 Ω·cm² for controls and 96.6 mS·cm^-2 (8.4 – 191.4) equal to 10.3 Ω·cm² for D-patients. Stimulation with serotonin, theophylline, forskolin and carbachol induced increases in SCC in a range of 4.9 – 18.6 μA·cm^-2, while inhibition with indomethacin, bumetanide, ouabain and amiloride decreased SCC in a range of 6.5 – 27.4 μA·cm^-2, and all with no significant differences between controls and D-patients. Histological examinations showed intact epithelium and lamina propria before and after mounting for both types of patients.

Conclusion

We conclude that epithelial ion transport is not significantly altered in patients with diverticulosis and that the MUAS chamber can be adapted for studies of human colonic endoscopic biopsies.

Electrical parameters and ion species for active transport in human esophageal stratified squamous epithelium and Barrett's specialized columnar epithelium

The human esophagus is lined by stratified squamous epithelium (ESSE), and in some subjects with reflux disease the distal esophagus becomes lined by Barrett's specialized columnar epithelium (BSCE). ESSE and BSCE differ both histologically and functionally, the latter evident by differences in their in vivo transmural electrical potential difference (PD), ESSE averaging -15 mV and BSCE being greater than -25 mV. In this report we examine the basis for this difference in PD. This is done by mounting endoscopic biopsies of ESSE from 25 subjects without esophageal disease and BSCE from 19 with Barrett's esophagus in mini-Ussing chambers for electrical recordings basally and after bathing solution ion replacement. The results show that the PD of human ESSE reflects a low level of active ion transport (5.1 +/- 0.8 muA/cm(2)) combined with a high level of tissue (electrical) resistance (344 +/- 34 Omega.cm(2)) and that of BSCE reflects a high level of active transport (43.6 +/- 11.6 muA/cm(2)) combined with a low level of resistance (69 +/- 8 Omega.cm(2)). Furthermore, active transport in ESSE was principally due to sodium absorption whereas in BSCE it was equally divided between sodium absorption and anion (chloride/bicarbonate) secretion, the latter through an apical membrane, 4-acetamido4'-isothiocyano-2,2'-stilbenedisulfonic acid-sensitive anion channel. As an anion-secreting tissue with bicarbonate secretory capacity more than fivefold greater than ESSE, BSCE is better suited than ESSE for defense of the esophagus against reflux disease.

Duodenal secretion in humans mediated by the EP4 receptor subtype

AIM

Assessment of functional EP receptor subtypes involved in PGE2-induced secretion in human duodenum. The spectrum of activities by PGE2 in mammals, including cytoprotective bicarbonate secretion in duodenum, is mediated through four G protein-coupled receptor subtypes (EP1-EP4).

METHODS

Biopsies from the second part of duodenum from patients undergoing endoscopy were mounted in modified Ussing chambers. Basal and stimulated short circuit current (SCC) and slope conductance (SG) were measured. Dose-response relations for PGE2 and subtype receptors EP1/EP3 (sulprostone), EP2 (butaprost), and EP4 (1-OH PGE1) were assessed by cumulated doses of single agonists.

RESULTS

PGE2 caused a dose-dependent increase in SCC, maximum 101 +/- 20 microA cm(-2) with an EC50 of 35.6 +/- 5.8 nm (n = 3). Likewise 1-OH PGE1 caused a dose-dependent SCC increase, maximum 63.3 +/- 28.6 microA cm(-2) with an EC50 of 56.7 +/- 7.2 nm (n = 3). 1-OH PGE1 at 500 nm increased SCC by 18.0 +/- 3.0 microA cm(-2) (n = 10) and SG by 2.9 +/- 0.4 mS cm(-2) (n = 6). Sulprostone (n = 6) and butaprost (n = 6) had no effects on SCC or SG. SCC was inhibited 31.4 +/- 13.2% (n = 10) by bumetanide (25 microM serosa) and 18.6 +/- 5.8% (n = 10) by acetazolamide (250 microM lumen). Diphenylamine-2-carboxylate (DPC) (250 microM mucosa) and SITS (10 microM mucosa) had almost no effect.

CONCLUSIONS

Effects of PGE2 on secretion in the second part of human duodenum is mediated through the EP4 receptor and not through EP1, EP2, or EP3.

The Dynamic Effects of Breastfeeding on Intestinal Development and Host Defense

In this review, evidence is provided to support the hypothesis that human milk provides a link between the mother and her newborn infant in the extrauterine environment in a manner similar to the placental link between mother and fetus in utero. In addition, breastfeeding helps prevent age-related diseases affecting the gastrointestinal tract during the newborn period. To provide evidence to support this hypothesis, anecdotal clinical studies are sited to suggest that human milk contains factors that may be missing in inherited diseases of inborn errors in metabolism and provide passive protective factors that lessen the expression of neonatal allergic and infectious diseases. In some instances, by providing the missing factor in an inherited disease, the newborn may be protected from serious damage to its developing brain. A second line of evidence to support this hypothesis is the observation that the composition of human milk varies with the infant's needs. To illustrate this principal, the composition of milk from mothers delivering prematurely and milk of mothers of full-term infants were compared, and the differences in trophic and protective factors in colostrum versus mature milk from mothers delivering full-term are cited. Finally, using observations from the laboratory that define the immaturities in neonatal and premature human intestinal defenses as the neonate's host defense deficiency, the specific effect that anti-inflammatory and maturational factors in human milk has on these immaturities is discussed. The active stimulus of maternal milk on the rapid development of host defenses is underscored. These cited examples of human milk effects in the newborn help support the stated hypothesis. Additional studies of human immature gut function along with translational and clinical studies are necessary to provide further objective evidence in support of breastfeeding for all neonates, particularly premature neonates.

Development of microbial-human enterocyte interaction: cholera toxin

Diarrhea in infants and children is a major health hazard worldwide. Certain toxigenic diarrheas occur more commonly and are manifested more severely during the neonatal period. We have previously studied the regulation of cholera toxin-induced secretion in animal models during development. In those studies we have shown that cholera toxin stimulates a much greater secretion by immature compared with mature small intestine, and the mechanism appears to be an up-regulation of postreceptor signal transduction molecules (adenyl cyclase and Gsalpha) leading to an elevated cAMP level. In this study, using experimental models of human intestinal development (fetal cell lines, a micro-Ussing chamber, organ cultures, and fetal intestinal xenograft transplants), we provide preliminary evidence that cholera toxin induces an enhanced secretion mediated in part by a developmental up-regulation of the cAMP response in immature versus mature human small intestine. Additional studies are needed, however, to further define whether other developmental events (e.g. receptor expression) also regulate cholera toxin-enterocyte-enhanced interaction. Nonetheless, this approach to determining the role of development in the pathophysiology of cholera in infants may help in strategies to prevent and treat this condition and other age-related intestinal infectious diseases.

Methods used to study intestinal nutrient transport: past and present

BACKGROUND

Vitally important to the future of surgical care is the study of nutrition and nutrient uptake. Advances in this field of research have become increasingly dependent upon the disciplines of immunology, histology, and molecular biology. The fusion of these sciences has deepened our insight into the relationship between molecular structure and physiologic function. The ability to apply new technologies to this endeavor will enable the surgeon-investigator to further widen our understanding of nutrient transport.

MATERIALS AND METHODS

Medline and current literature review.

RESULTS AND CONCLUSIONS

We summarize many of the methods used to measure the uptake of nutrients by the intestinal epithelium, providing a historical perspective.

Novel modified Ussing chamber for the study of absorption and secretion in human endoscopic biopsies

The aim of this study was to design and evaluate a modified Ussing chamber, that makes use of constant air suction (modified Ussing air suction chamber, MUAS) for fixation of biopsy specimens. Standard size forceps biopsies were taken from the descending part of duodenum from patients undergoing endoscopy. Short circuit current (SCC) and conductance (G) were measured during basal conditions and after addition of different sugars and secretagogues. Histologic examination was performed to determine the degree of tissue damage after study in the chamber. Basal SCC was 54.7 +/- 4.3 microA x cm(-2) and G was 58.7 +/- 4.7 mS x cm(-2) (mean +/- SEM, n=48) and steady values of these parameters were observed for at least 2 h. Reproducible and steady responses in SCC were obtained with D-glucose (SCCmax=172 +/- 22.1 microA x cm(-2); EC50=6.9 +/- 0.7 mM, n=5) and D-galactose (SCCmax=233 +/- 55.7 microA x cm(-2); EC50=9.2 +/- 0.7 mM, n=3), and secretory responses with 5-hydroxytryptamine, 100 microM (DeltaSCC= 16.1 +/- 3.8 microA x cm(-2), n=10), histamine, 100 microM (DeltaSCC=24.0 +/- 4.1 microA x cm(-2), n=10) and prostaglandin E2, 1 microM (DeltaSCC=30.3 +/- 5.4 microA x cm(-2), n=6). Experimental biopsy specimens showed intact surface epithelium by histologic examination and did not differ from controls apart from minor indications of edge damage. No difference in basal electrical parameters and D-glucose fluxes were found between Helicobacter pylori positive and negative patients. Our data suggests that the MUAS chamber represents a promising alternative approach to measure transport processes in intestinal endoscopic biopsies.

Advances in methods to evaluate gastrointestinal transport function

Malnutrition is a serious problem, and malabsorption of nutrients is believed to be partially responsible for its prevalence. A wide variety of innovative methods have been developed to study gastrointestinal transport function. Some of the first research into gastrointestinal function was conducted in the 1700's with animal and human models. Methodological advancements continue to allow scientists to innovatively assess gastrointestinal function in animal models, cellular preparations and clinical settings. For this update, the methods are divided into in vivo, ex vivo, isolated cells and membranes, and molecular biology approaches. The in vivo methods discussed include animal and human models to measure nutrient disappearance, catheterized animal models, models with isolated intestinal segments, and a new procedure for sampling luminal fluid from patients. The ex vivo approaches discussed obtain measurements with intact tissue, such as the everted sleeves method and Ussing chambers. The utility of novel cellular preparations, membrane fractionation procedures and various molecular biology techniques is included. Various aspects of these methods are evaluated to provide a detailed overview of recent methodological developments.

Identification of transport abnormalities in duodenal mucosa and duodenal enterocytes from patients with cystic fibrosis

BACKGROUND & AIMS

The duodenum is a cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelium with high bicarbonate secretory capacity. We aimed to define the role of CFTR in human duodenal epithelial bicarbonate secretion in normal (NL) subjects and patients with cystic fibrosis (CF).

METHODS

Endoscopic biopsy specimens of the duodenal bulb were obtained from 9 CF patients and 16 volunteers. Tissues were mounted in modified Ussing chambers. Bicarbonate secretion and short-circuit current (Isc) were quantitated under basal conditions and in response to dibutyryl adenosine 3',5'-cyclic monophosphate (db-cAMP), carbachol, and the heat-stable toxin of Escherichia coli (STa). Duodenocytes were also isolated and loaded with the pH-sensitive fluoroprobe BCECF/AM, and intracellular pH (pH(i)) was measured at rest and after intracellular acidification and alkalinization.

RESULTS

Basal HCO(3)(-) secretion and Isc were significantly lower in the CF vs. NL duodenal mucosa. In contrast to NL, db-cAMP failed to alter either HCO(3)(-) or Isc in CF tissues. However, in CF, carbachol resulted in an electroneutral HCO(3)(-) secretion, whereas STa induced electrogenic HCO(3)(-) secretion that was similar to NL. In CF and NL duodenocytes, basal pH(i) and recovery from an acid load were comparable, but pH(i) recovery after an alkaline load in CF duodenocytes was Cl(-) dependent, whereas in NL duodenocytes it was Cl(-) independent.

CONCLUSIONS

These findings implicate CFTR in NL duodenal alkaline transport and its absence in CF. Although duodenal bicarbonate secretion is impaired in CF tissues, alternate pathway(s) likely exist that can be activated by carbachol and STa.