Does 'sugar' permeability reflect macromolecular absorption? A comparison of the gastro-intestinal uptake of lactulose and beta-lactoglobulin in the neonatal guinea pig

The complex task of measuring intestinal permeability in basic and clinical science

Intestinal permeability is a key feature of intestinal barrier function. Altered intestinal permeability is described in many chronic diseases and may be a risk factor for disease development and a target for emerging therapeutics. Thus, reliable and sensitive methods to measure intestinal permeability in both the clinical and preclinical setting are needed. There is currently a large array of tests to choose from, each with advantages and disadvantages. When possible, a combination of methods should be used. The choice of tests should be based on a deep understanding of intestinal barrier physiology and the recognition of their limitations. This mini-review will highlight the advantages and limitations associated with intestinal permeability tests and will identify current problems in the field and how they can be addressed in the future.

A novel method for imaging sites of paracellular passage of macromolecules in epithelial sheets

Understanding the dynamics of intestinal barrier function is key to elucidating oral delivery routes of therapeutics as well as to understanding various diseases that involve the mucosal immune system. Passage of macromolecules across barrier-forming epithelia is classically analyzed by means of various tracer flux measurements. This approach averages over contributions from many cells and lacks labeling of passage-sites. Thus, abundance and nature of involved cells have remained unidentified. We present a novel method that allowed for optical analysis of passage of various macromolecules on large-scale and single-cell level. To achieve tracking of passage loci in epithelia at submicrometer resolution we used biotinylated and fluorescent macromolecules that bind to basolateral membranes pre-labeled with cell-adherent avidin. We applied this method to epithelial cell lines and isolated mucosae in order to 3-dimensionally determine barrier leak properties over time. Tracer passage was found in all epithelia examined. However, it was infrequent, strikingly inhomogeneous, depended on culture duration and tightness of the monolayer. Stimulating passage with barrier-perturbing agents increased the number of leaks exposition time-dependently in cell lines and explanted mucosae. After stepwise opening of the paracellular passage pathway, integrated tracer-signal measured by our assay strictly correlated to simultaneously performed standard fluxes. Thus, our assay allows for the study of transepithelial macromolecule passage in various physiological and pathological conditions.

Multiple facets of intestinal permeability and epithelial handling of dietary antigens

The intestinal epithelium, the largest interface between the host and environment, regulates fluxes of ions and nutrients and limits host contact with the massive load of luminal antigens. Local protective and tolerogenic immune responses toward luminal content depend on antigen sampling by the gut epithelial layer. Whether, and how exaggerated, the entrance of antigenic macromolecules across the gut epithelium might initiate and/or perpetuate chronic inflammation as well as the respective contribution of paracellular and transcellular permeability remains a matter of debate. To this extent, experimental studies involving the in vivo assessment of intestinal permeability using small inert molecules do not necessarily correlate with the uptake of larger dietary antigens. This review analyzes both the structural and functional aspects of intestinal permeability with special emphasis on antigen handling in healthy and diseased states and consequences on local immune responses to food antigens.

Gut barrier dysfunction in food allergy

Intestinal permeability tests used in the diagnosis of allergic diseases in response to oral food challenge have led to the conclusion that constitutive defects of the intestinal barrier are not the primary cause of allergic diseases. However, perturbation of environmental factors (infection, stress), by increasing intestinal permeability and enhancing danger signals, may favour food allergy in susceptible individuals. The mechanisms of enhanced permeability to specific and bystander antigens have been delineated as well as the molecular events involved in the sequential phases of allergic reactions. Intestinal absorption of food antigens and immune responses are mutually dependent, and luminal (environmental) and serosal (intrinsic) factors synergize to maintain a self-perpetuating cycle in which antigens penetrate the mucosa and induce allergic inflammation.

Epithelial permeability to proteins in the noninflamed ileum of Crohn's disease?

BACKGROUND & AIMS

Crohn's disease (CD) is associated with a disturbed intestinal barrier. Permeability studies have focused on inert molecules, but little is known about transepithelial transport of macromolecules with antigenic potential in humans. The aim of this study was to quantify permeation and to characterize passage routes for macromolecules in ileal mucosa in CD.

METHODS

Noninflamed and inflamed ileal mucosa specimens from patients with CD (n = 12) and ileal specimens from patients with colon cancer (n = 7) were studied regarding transmucosal permeation of ovalbumin, dextran (mol wt, 40,000), and 51Cr-EDTA for 90 minutes in vitro in Ussing chambers. Transepithelial passage routes for fluorescent ovalbumin and dextran 40,000 were investigated by confocal microscopy.

RESULTS

Noninflamed ileum from CD patients showed increased permeation of ovalbumin compared with ileum from colon cancer patients (P < 0.05). Dextran permeation was equal in the three groups, whereas 51Cr-EDTA permeability was increased in inflamed ileum. Ovalbumin passed both transcellularly and paracellularly, but dextran followed a strictly paracellular route. Both markers were subsequently endocytosed by cells of the lamina propria.

CONCLUSIONS

Noninflamed ileal mucosa from patients with CD shows increased epithelial permeability to ovalbumin, probably by augmented transcytosis. This increase in antigen load to the lamina propria could be an initiating pathogenic event in CD.

Gut permeability to human alpha-lactalbumin, beta-lactoglobulin, mannitol, and lactulose in celiac disease

Our objective was to examine the permeability of the gut to protein macromolecules and sugar probes and their possible association in celiac disease patients. We studied the permeability to human alpha-lactalbumin, beta-lactoglobulin, mannitol, and lactulose on 46 occasions in 33 celiac disease patients in various phases of the disease; in addition, mannitol and lactulose permeability was studied in 18 healthy controls. Lactalbumin absorption was detected in 19 of 42 patients tested, more often in celiac disease patients with villous atrophy than in those with normal jejunal biopsy (p = 0.01). Higher absorption of lactalbumin was found in patients with subtotal villous atrophy than in those with normal biopsy (p = 0.02). beta-lactoglobulin was found in four of 42 patients tested. Less mannitol was absorbed by patients with either subtotal or partial villous atrophy than by those with normal histology (p = 0.001 and 0.006, respectively). Lactulose recovery was higher in newly diagnosed patients and patients with subtotal villous atrophy than in controls (p = 0.007 and 0.03, respectively). The lactulose/mannitol ratio was higher in newly diagnosed patients and patients with villous atrophy than in controls (p = 0.002 and 0.002, respectively). The correlation between permeability to lactalbumin and mannitol and lactulose was poor. We conclude that permeability to proteins and sugar molecules is abnormal in celiac disease patients with mucosal damage and that they probably reflect different mechanisms of penetration.

Bidirectional small-intestinal permeability in the rat to some common marker molecules in vitro

BACKGROUND

The barrier properties of the small intestine were investigated by studying the bidirectional permeability to five commonly used marker molecules.

METHODS

Proximal and distal small-intestinal segments from rats were mounted in diffusion chambers, and the permeation of the markers 3H-mannitol (Mw 182), 51Cr-ethylenediamineteraacetic acid (Mw 341), [mercaptopropionic acid], D-arginine8]-vasopressin (Mw 1069), fluorescein isothiocyanate (FITC)-dextran (mean Mw 3000), and inulin (Mw 5200) was measured across the mucosa in both directions.

RESULTS

A generally increased inward (mucosa to serosa) and a decreased outward (serosa to mucosa) permeation of the markers was found in the proximal to distal direction. The inward permeability showed increasing regional differences with decreasing size of the markers. In the absence of the villous epithelium, removed by scraping the intestinal wall, 86% to 62% of the proximal and distal barrier was lost in the inward direction but only 14% to 26% in the outward direction.

CONCLUSIONS

The intestinal epithelial barrier is more permeable in the outward than in the inward direction, and regional permeability differences exist in a size-dependent fashion. The results suggest two passage routes, one for the smallest molecule, mannitol, and a second for the larger markers in the present size range, both apparently different from the route for macromolecules such as intact proteins.

Regional small-intestinal permeability in vitro to different-sized dextrans and proteins in the rat

Molecular weight-dependent passage over different regions of the rat small intestine, using different-sized proteins/peptides and fluorescein isothiocyanate-dextrans in the 1- to 70-kDa range, was studied in vitro in modified Ussing chambers. The mucosal to serosal passage was inversely related to the molecular weight. After 120 min the passage in the proximal region usually dominated, but the nonapeptide (mercaptopropionic acid1, D-arginine8)-vasopressin differed by showing a consistently higher passage in the distal region. The similar apparent permeation coefficients obtained for the two macromolecular categories of corresponding molecular weight implied that the non-degradable dextrans could be used as permeability markers reflecting the passage per se of intact proteins. Furthermore, the results indicated two different transmucosal pathways, one of low permeability for molecules > 30 kDa, in which the molecular weight was of minor importance for the passage, and another more permeable one in the 1- to 30-kDa range, in which the passage was highly influenced by the molecular weight.

Elimination diet and intestinal permeability in atopic eczema: a preliminary study

We measured small intestinal permeability to lactulose and rhamnose in 18 healthy children and in 15 children with atopic eczema, before and after a 14-day elimination diet. The children with atopic eczema had higher initial urinary lactulose/rhamnose ratios than the controls. After dietary restriction, there was no overall change in permeability. Dietary therapy did however result in a large reduction in permeability in three of the nine children whose skin disease was improved, but a statistically significant decrease in permeability was not observed in the diet-responsive group as a whole. Larger studies would be required to confirm that dietary restrictions can reduce intestinal permeability in some children with atopic eczema.

Increased protein absorption after acute gastroenteritis in children

We investigated the intestinal absorption of the macromolecule human alpha-lactalbumin during and after an episode of acute gastroenteritis in children. Twenty children were studied in the acute phase and 17 excreted rotavirus. Eleven children were studied again 5-8 weeks later (convalescent phase). Human alpha-lactalbumin serum concentrations in the acute phase were similar but in the convalescent phase they were significantly (p less than 0.001) higher than those in the reference children. The serum concentrations were also higher in the convalescent than in the acute phase (p = 0.021). This study suggests that there is an increased absorption of proteins from the gut into the circulation 5-8 weeks after rotavirus gastroenteritis.

Antigen absorption in rabbit bacterial diarrhea (RDEC-1). In vitro modifications in ileum and Peyer's patches

Intestinal absorption of antigenic (intact) and degraded beta-lactoglobulin and horseradish peroxidase was studied in rabbits experimentally infected at weaning with the rabbit-specific Escherichia coli strain RDEC-1 (015:NM). Transepithelial fluxes of both proteins were measured at four stages after infection: early, peak, late, and recovery, on segments of ileum and Peyer's patches mounted in Ussing chambers. During this period of observation, absorption of antigenic beta-lactoglobulin and intact horseradish peroxidase decreased significantly in the ileum and Peyer's patches of age-matched controls. No significant age-related decrease was observed in the controls for the transport of these proteins in degraded form. RDEC-1 infection caused, first, a rise in degraded protein fluxes across Peyer's patches only, during the early phase of the disease. Second, during the peak phase of infection, absorption of antigenic beta-lactoglobulin by the ileum rose to 74.0 ng/hr/cm2 (90% confidence interval: 63.1, 86.8) vs 15.8 (5.8, 42.8) in the controls (P = 0.07), and absorption of intact horseradish peroxidase rose to 21.8 (16.5, 26.7) ng/hr/cm2 vs 4.4 (0.7, 26.6) in control ileum, (P = 0.09). These rises, which were similar in Peyer's patches, delayed the physiologic decrease in protein absorption that occurs with age. These results indicate that increased antigen absorption is observed during bacterial diarrhea and that it might interfere with the immune responses of the host.

Early increase in intestinal permeability in patients with severe acute pancreatitis: correlation with endotoxemia, organ failure, and mortality

Sepsis accounts for 80% of deaths from acute pancreatitis. This study aimed to investigate early changes in intestinal permeability in patients with acute pancreatitis, and to correlate these changes with subsequent disease severity and endotoxemia. The renal excretion of enterally administered polyethylene glycol (PEG) 3350 and PEG 400 was measured within 72 hours of onset of acute pancreatitis to determine intestinal permeability. Severity was assessed on the basis of APACHE II scores and C-reactive protein measurements. Serum endotoxin and antiendotoxin antibodies were measured on admission. Eight-five patients with acute pancreatitis (mild in 56, severe in 29) and 25 healthy control subjects were studied. Urinary excretion of PEG 3350 (median) was significantly greater in patients who had severe attacks (0.61%) compared to those with mild disease (0.09%) and health control subjects (0.12%) (P <0. 0001), as was the permeability index (PEG 3350/400 excretion) (P <0. 00001). The permeability index was significantly greater in patients who subsequently developed multiple organ system failure and/or died compared with other severe cases (0.16 vs. 0.04) (P = 0.0005). The excretion of PEG 3350 correlated strongly with endotoxemia (r = 0.8; P = 0.002). Early increased intestinal permeability may play an important role in the pathophysiology of severe acute pancreatitis. Therapies that aim to restore intestinal barrier function may improve outcome.