Structural features of absorptive cell and microvillus membrane preparations from rat small intestine

Molecular transport through primary human small intestinal monolayers by culture on a collagen scaffold with a gradient of chemical cross-linking

BACKGROUND

The luminal surface of the small intestine is composed of a monolayer of cells overlying a lamina propria comprised of extracellular matrix (ECM) proteins. The ECM provides a porous substrate critical for nutrient exchange and cellular adhesion. The enterocytes within the epithelial monolayer possess proteins such as transporters, carriers, pumps and channels that participate in the movement of drugs, metabolites, ions and amino acids and whose function can be regulated or altered by the properties of the ECM. Here, we characterized expression and function of proteins involved in transport across the human small intestinal epithelium grown on two different culture platforms. One strategy employs a conventional scaffolding method comprised of a thin ECM film overlaying a porous membrane while the other utilizes a thick ECM hydrogel placed on a porous membrane. The thick hydrogel possesses a gradient of chemical cross-linking along its length to provide a softer substrate than that of the ECM film-coated membrane while maintaining mechanical stability.

RESULTS

The monolayers on both platforms possessed goblet cells and abundant enterocytes and were impermeable to Lucifer yellow and fluorescein-dextran (70 kD) indicating high barrier integrity. Multiple transporter proteins were present in both primary-cell culture formats at levels similar to those present in freshly isolated crypts/villi; however, expression of breast cancer resistance protein (BCRP) and multidrug resistance protein 2 (MRP2) in the monolayers on the conventional scaffold was substantially less than that on the gradient cross-linked scaffold and freshly isolated crypts/villi. Monolayers on the conventional scaffold failed to transport the BCRP substrate prazosin while cells on the gradient cross-linked scaffold successfully transported this drug to better mimic the properties of in vivo small intestine.

CONCLUSIONS

The results of this comparison highlight the need to create in vitro intestinal transport platforms whose characteristics mimic the in vivo lamina propria in order to accurately recapitulate epithelial function.

GRAPHICAL ABSTRACT

Intracellular Antioxidant Activity of Grape Skin Polyphenolic Extracts in Rat Superficial Colonocytes: In situ Detection by Confocal Fluorescence Microscopy

Colon is exposed to a number of prooxidant conditions and several colon diseases are associated with increased levels of reactive species. Polyphenols are the most abundant antioxidants in the diet, but to date no information is available about their absorption and potential intracellular antioxidant activity on colon epithelial cells. The work was addressed to study the intracellular antioxidant activity of red grape polyphenolic extracts on rat colon epithelium experimentally exposed to prooxidant conditions. The experimental model chosen was represented by freshly isolated colon explants, which closely resemble the functional, and morphological characteristics of the epithelium in vivo. The study was carried out by in situ confocal microscopy observation on CM-H2DCFDA charged explants exposed to H₂O₂ (5, 10, and 15 min). The qualitative and quantitative polyphenolic composition of the extracts as well as their in vitro oxygen radical absorbing capacity (ORAC) was determined. The incubation of the explants with the polyphenolic extracts for 1 h produced a significant decrease of the H₂O₂ induced fluorescence. This effect was more pronounced following 15 min H₂O₂ exposure with respect to 5 min and it was also more evident for extracts obtained from mature grapes, which showed an increased ORAC value and qualitative peculiarities in the polyphenolic composition. The results demonstrated the ability of red grape polyphenols to cross the plasma membrane and exert a direct intracellular antioxidant activity in surface colonocytes, inducing a protection against pro-oxidant conditions. The changes in the polyphenol composition due to ripening process was reflected in a more effective antioxidant protection.

The Na+-K+-2Cl- cotransporter and the osmotic stress response in a model salt transport epithelium

Epithelia are physiologically exposed to osmotic stress resulting in alteration of cell volume in several aspects of their functioning; therefore, the activation of 'emergency' systems of rapid cell volume regulation is fundamental in their physiology. In this review, the physiological response to osmotic stress, particularly hypertonic stress, was described in a salt-transporting epithelium, the intestine of the euryhaline teleost European eel. This epithelium is physiologically exposed to changes in extracellular osmolarity and represents a good physiological model for functional studies on cellular volume regulation, permitting the study of volume regulated ion transport mechanisms in a native tissue. An absorptive form of the cotransporter, homologue of the renal NKCC2, localized on the apical membrane, was found in the intestine of the euryhaline teleost European eel. This cotransporter accounts for the luminal uptake of Cl-; it operates in series with a basolateral Cl- conductance and presumably a basolateral electroneutral KCl cotransport and in parallel with a luminal K+ conductance. The ion transport model described for eel intestine, based on the operation of an absorptive luminal Na+-K+-2Cl-, is basically the same as the model that has been proposed for the thick ascending limb (cTAL) of the mammalian renal cortex. This paper focuses on the role of Na+-K+-2Cl- cotransport in the responses to hypertonic stress in the eel intestine and the role of cytoskeleton (either actin-based or tubulin based) is discussed.

Intestinal epithelial stem cells and progenitors

The adult intestinal epithelium contains a relatively simple, highly organized, and readily accessible stem cell system. Excellent methods exist for the isolation of intestinal epithelium from adults, and as a result collecting large quantities of intestinal stem and progenitor cells for study or culture and subsequent clinical applications should be routine. It is not, however, for two reasons: (1) adult intestinal epithelial cells rapidly initiate apoptosis on detachment from the basement membrane, and (2) in vitro conditions necessary for survival, proliferation, and differentiation are poorly understood. Thus to date the study of intestinal stem and progenitor cells has been largely dependent on in vivo approaches. We discuss existing in vivo assays for stem and progenitor cell behavior as well as current methods for isolating and culturing the intestinal epithelium.

A simple method of rat renal brush border membrane preparation using polyethylene glycol precipitation

A simple method for preparation of brush border membranes (BBM) from rat kidney using polyethylene glycol (PEG) precipitation has been described. This method avoids the use of cations for the preparation, which might alter membrane lipid composition. These preparations were assessed for enrichment of marker enzymes, contamination by subcellular structures, lipid composition and transport function. An enrichment of 11.8910-fold of alkaline phosphatase, 13.9500-fold of amino peptidase and 13.6500-fold of gamma-glutamyl transpeptidase and an approximate yield of 60% were seen in the final membrane preparation as compared to the homogenate. There was very little contamination of basolateral membranes, peroxisomes, microsomes or lysosomes in the final membrane preparation. Analysis of sugars indicated high content of fucose and sailic acid as compared to hexoses. Isolated membranes appeared as vesicles as seen by electron microscopy. Lipid analysis indicated the presence of various neutral and phospholipids with a high content of sphingomyelin along with a cholesterol/phospholipid ratio of 0.4850. The isolated membrane vesicles were able to transport glucose. This study has shown a simple method of renal brush border membrane preparation, which is comparatively pure and functionally active.

Simple method of preparation of rat colonocyte apical membranes using polyethylene glycol precipitation

BACKGROUND

Although a number of methods are available for the preparation of brush border membranes (BBM) from the small intestine, very few studies have been performed on the isolation of colonic apical membranes (CAM). Cation precipitation is one of the methods used for the isolation of these membranes, which can result in altered lipid composition as a result of activation of phospholipase. We have earlier established a method for BBM isolation using polyethylene glycol (PEG) precipitation. In the present study, CAM from isolated colonocytes were prepared using PEG and the isolated membranes were characterized.

METHODS

The CAM were prepared from isolated rat colonocytes using PEG precipitation and differential centrifugation. Purity was assessed by enrichment of the marker enzymes and contamination by other subcellular particles. Lipid composition and sugar components were analyzed in the isolated membranes.

RESULTS

The CAM showed 11-12-fold enrichment in alkaline phosphatase and 9-10-fold enrichment in gamma-glutamyl transpeptidase as compared with the cell homogenate. The final membrane preparation also showed good vesicle formation as seen by electron microscopy. There was very little contamination of basolateral membranes, microsomes or lysosomes in the final membrane preparation. Analysis of sugars indicated high content of fucose and sialic acid as compared with hexoses. Lipid analysis indicated the presence of various neutral and phospholipids and a cholesterol/phospholipid ratio of 0.65 was seen.

CONCLUSION

This study has shown a simple method of CAM preparation using PEG precipitation, which is less time-consuming than other methods and comparatively pure.

Methods for the study of ionic currents and Ca2+-signals in isolated colonic crypts

Isolated epithelial cells from intestinal mucosae are a suitable object for the study of the regulation of ion transport in the gut. This regulation possesses a great importance for human and veterinary medicine, as diarrheal diseases, which often are caused by an inadequate activation of intestinal anion secretion, are one of the major lethal diseases of children or young animals. The aim of this paper is to describe a method for the isolation of intact colonic crypts, e.g. for the subsequent investigation of the regulation of anion secretion by the intracellular second messenger, Ca(2+) using electrophysiological and imaging techniques.

A novel method of preparation of small intestinal brush border membrane vesicles by polyethylene glycol precipitation

A novel method of brush border membrane vesicle (BBMV) preparation from the small intestinal mucosa using polyethylene glycol (PEG) precipitation has been presented. This preparation is compared with calcium-precipitated BBMVs in marker enzyme enrichment, contamination by other subcellular membranes, transport of glucose, and lipid composition. PEG-precipitated BBMVs are comparable with calcium-precipitated membranes in all parameters except lipid composition and thiol content. PEG-precipitated membranes have more phosphatidylcholine and phosphatidylethanolamine and less lysophosphatidylcholine and lysophosphatidylethanolamine as compared to calcium-precipitated membranes. Diacylglycerol and triacylglycerol content are also high in PEG-precipitated membranes. Alteration in lipid composition indicate the possible activation of lipase and phospholipase by calcium during BBMV preparation, which is not seen in PEG precipitation. Thiol content is almost double in PEG-precipitated membranes as compared to calcium-precipitated membranes. These results indicate that PEG can be used for the preparation of BBMVs in native form from the intestine without any alteration in their structural components, and these membranes show comparable transport activity.

Comparative study on brush border membranes prepared from rat and monkey small intestine by Ca2+ and Mg2+ precipitation

Brush border membranes, prepared by Ca2+ or Mg2+ precipitation, from monkey or rat small intestine were compared for marker enzyme enrichment and lipid composition. Membranes prepared from rat intestine by Mg2+ precipitation were less pure in terms of their marker enzyme enrichment than Ca2+ precipitated membranes. Moreover, Mg2+ precipitated membranes were contaminated by basolateral membranes as evidenced by the enrichment of Na(+)-K+ ATPase. The content and composition of phospholipids were different in Mg2+ and Ca2+ precipitated membranes. The membranes prepared by the two methods from monkeys were identical in their marker enzyme enrichment and lipid composition and unlike those obtained from the rat, there was no basolateral membrane contamination.

Effects of acute and chronic ethanol exposure on intestinal microvillus membrane lipid composition and fluidity

Chronic ethanol consumption produces nutrient malabsorption. The mechanisms by which this occurs are poorly understood. One potential mechanism is an alteration in microvillus membrane (MVM) composition and fluidity. The effects of in vivo ethanol exposure on MVM lipid composition and fluidity were determined in rats fed either a standard diet or 15% ethanol in water for 2 months. Acute jejunal exposure to 4% ethanol was also performed in vivo in each feeding group. Acute exposure to ethanol produced an increase in static and dynamic membrane fluidity associated with a decrease in MVM cholesterol regardless of prior ethanol exposure. Chronic ethanol feeding alone did not alter membrane fluidity. Changes in membrane fatty acid composition were minor and variable after both acute and chronic ethanol exposure. Prior chronic ethanol feeding did not prevent the acute effects of ethanol on MVM composition or fluidity. These data support the theory that ethanol acutely disrupts nutrient transport by changing MVM lipid fluidity. The absence of adaptive changes in membrane composition and fluidity may also explain the persistent absorptive defects seen with chronic alcoholism.

Electron microscopic analysis of the normal and the activated pleural macrophage

Despite an apparent role in pleural pathophysiology, little information is known about pleural macrophage morphology. Intrapleural tetracycline (TCN) results in pleural macrophage influx and pleural fibrosis; intrapleural carrageenan (CAR) induces macrophage influx without ensuing fibrosis. Pleural macrophages collected from normal (NL) and TCN- or CAR-exposed rabbit pleural spaces were examined with electron microscopy. Cellular size; number of microvilli; pseudopods; coated pits (CP) and coated vesicles (CV); and prevalence of golgi, rough endoplasmic reticulum (RER), and intermediate filaments (IF) were determined. The means of each variable in each group were assessed by one-way analysis of variance, with post hoc testing performed by Scheffe F test; p < or = .05 was considered significant. TCN-stimulated pleural macrophages were characterized by their small perimeters. CAR-induced pleural macrophages were marked by their large size and abundant intracellular amorphous material. They had larger perimeters, areas, and diameters than the TCN-induced or normal macrophages and thus smaller numbers of CV + CP per area. The normal pleural macrophages were characterized by more IF, microvilli, and microvilli per perimeter than either the CAR- or TCN-induced pleural macrophages. No differences between groups were found in nuclear cytoplasmic ratios, number of pseudopods, and content of golgi or of RER. The results suggest that normal pleural macrophages and TCN- and CAR-induced pleural macrophages differ morphologically and that these morphologic differences reflect functional differences.

Enzyme changes in remodelling epithelial cells: a histochemical study of the rat jejunum in vivo during and following exposure to deoxycholic acid

Loops of rat jejunum were exposed in vivo to different concentrations of deoxycholic acid (DOC; 0, 2.5, 5, 10 and 20 mM). Following a 30 min exposure period, DOC was washed out of the loops and the intestines were allowed to recover for 15 or 150 min. Frozen tissue for enzyme histochemistry was collected during exposure and following the recovery periods. As shown previously, exposure to DOC caused a dose-dependent loss of epithelial cells at the villous tips and denudation of the lamina propria. Flattened epithelial cells bordering the denuded areas were, however, responsible for a rapid restoration of epithelial continuity, which was completed within 15 min. In the present study, these flattened cells showed normal reactivity for non-specific esterase and succinate dehydrogenase. In contrast, following a prolonged recovery period (150 min), a subpopulation of enterocytes at the villous tips that otherwise appeared normal showed decreased reactivity for brush border enzymes and non-specific esterase, and a positive reaction for mucin. A shutdown in the synthesis of cytoplasmic enzymes and redistribution of cell surface enzymes could be responsible for these late occurring enzyme changes, that were consistently observed after 150 min of recovery from DOC at 20 mM. Alternatively, retention of goblet cells and/or a modification in enzyme synthesis may explain the presence of mucin that was demonstrated in the epithelial cells which had low enzyme reactivity.

Differences in composition and fluidity of intestinal microvillus membrane vesicles prepared by different methods

Multiple methods have been developed to isolate the intestinal microvillus membrane and facilitate the study of its composition and function. Variations in membrane composition and fluidity may result from different preparative techniques. This study shows that the use of MgCl2 and/or KSCN in vesicle preparation alters phospholipid and protein composition of the membrane compared to CaCl2 precipitation. The use of MgCl2 in membrane preparation increased phosphatidylethanolamine and decreased phosphatidylinositol content. The use of KSCN in membrane preparation decreased the protein content. The structural changes seen with the use MgCl2 alone are accompanied by an increase in both static and dynamic membrane fluidity. These results suggest that different methods of membrane vesicle preparation affect membrane phospholipid and protein content as well as membrane fluidity.

Maxi K+ channels and their relationship to the apical membrane conductance in Necturus gallbladder epithelium

Using the patch-clamp technique, we have identified large-conductance (maxi) K+ channels in the apical membrane of Necturus gallbladder epithelium, and in dissociated gallbladder epithelial cells. These channels are more than tenfold selective for K+ over Na+, and exhibit unitary conductance of approximately 200 pS in symmetric 100 mM KCl. They are activated by elevation of internal Ca2+ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca2+ sensitivities of the macroscopic apical membrane conductance (Ga). Ga was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180 microS/cm2 at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 microS/cm2 at -25 mV. We have related maxi K+ channel properties and Ga quantitatively, relying on the premise that at any apical membrane voltage Ga comprises a leakage conductance and a conductance due to maxi K+ channels. Comparison between Ga and maxi K+ channels reveals that the latter are present at a surface density of 0.09/microns 2, are open approximately 15% of the time under control conditions, and account for 17% of control Ga. Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca2+ and voltage dependencies of single maxi K+ channels, results from intracellular microelectrode experiments indicate that maxi K+ channel activity in situ is higher than predicted from the measured apical membrane voltage and estimated bulk cytosolic Ca2+ activity. Mechanisms that could account for this finding are proposed.

Immunohistochemical localization of low density lipoprotein receptors in adrenal gland, liver, and intestine

The localization of LDL receptors in adrenal gland, liver, and intestine was studied using immunohistochemistry. The anti-LDL receptor antibody used was shown to be monospecific and did not react with striated muscle, a tissue which has a very low level of LDL receptors. Similarly, cerebral cortex showed only faint reactivity and that was to an area previously demonstrated to have LDL receptors. Adrenal gland was intensely reactive with the zona fasciculata, having a greater density of receptors than the zona reticularis. In normal liver, LDL receptors were present on the sinusoidal membranes and were sparse in the areas of hepatocyte-to-hepatocyte contact without an obvious portal to central gradient. LDL receptors were present throughout the intestine. In jejunum, staining was most intense at the base of the villus and extended up toward the villus tip. At the base of the villus, the receptor was primarily at the basal lateral membrane, but toward the villus tip, there was appreciable intracellular staining. Staining in crypts was more faint; in duodenum, staining in crypts equaled that in the villus region in intensity. In colon, there was intense staining throughout the epithelial cells. These results provide new information about the cellular and subcellular localization of LDL receptors and raise the interesting possibility that there is a role for LDL-derived cholesterol in new lipoprotein formation.

Single chloride channels in colon mucosa and isolated colonic enterocytes of the rat

Chloride channels from rat colonic enterocytes were studied using the patch-clamp technique. After removal of mucus, inside-out patches were excised from the apical membrane of intact epithelium located at the luminal surface. They contained spontaneously switching Cl- channels with a conductance of 35-40 pS. The channels were blocked reversibly by anthracene-9-carboxylic acid (1mM). In excised patches from single enterocytes, isolated by calcium removal, the Cl- channels were studied in more detail. The I-V relation was linear between +/- 80 mV. The selectivity was I- greater than Br- greater than Cl- = NO3- greater than F- = HCO3-. Thirty pS Cl- channels were also found on the basolateral membrane of crypts isolated by brief calcium removal. The I-V curve of these Cl- channels was also linear. The results provide direct evidence for the existence of Cl- channels in the apical membrane of surface cells in colonic mucosa. The properties of these channels are similar to those previously observed when incorporating membrane vesicles into planar lipid bilayers. Both results support the validity of the theoretical models describing intestinal secretion.

Amiloride-blockable sodium currents in isolated taste receptor cells

Isolated taste receptor cells from the frog tongue were investigated under whole-cell patch-clamp conditions. With the cytosolic potential held at -80 mV, more than 50% of the cells had a stationary inward Na current of 10 to 700 pA in Ringer's solution. This current was in some cells partially, in others completely, blockable by low concentrations of amiloride. With 110 mM Na in the external and 10 mM Na in the internal solution, the inhibition constant of amiloride was (at -80 mV) near 0.3 microM. In some cells the amiloride-sensitive conductance was Na specific; in others it passed both Na and K. The Na/K selectivity (estimated from reversal potentials) varied between 1 and 100. The blockability by small concentrations of amiloride resembled that of channels found in some Na-absorbing epithelia, but the channels of taste cells showed a surprisingly large range of ionic specificities. Receptor cells, which in situ express these channels in their apical membrane, may be competent to detect the taste quality "salty." The same cells also express TTX-blockable voltage-gated Na channels.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.