Intestinal surfactant-like material. A novel secretory product of the rat enterocyte

Modified-Release Phosphatidylcholine (LT-02) for Ulcerative Colitis: Two Double-Blind, Randomized, Placebo-Controlled Trials

BACKGROUND & AIMS

The aim of this study was to evaluate the efficacy of LT-02, a novel modified-release phosphatidylcholine (PC) formulation, for induction and maintenance of remission in patients with mild to moderate ulcerative colitis (UC) and inadequate response to mesalamine.

METHODS

LT-02 was evaluated in a multicenter double-blind, randomized, placebo-controlled study comprising a 12-week induction trial (PCG-2), followed by a 48-week maintenance trial (PCG-4). In PCG-2, patients were randomized 1:1:1 to treatment with 0.8 g LT-02 4 times daily (QID), 1.6 g LT-02 twice daily (BID), or placebo, respectively. All patients continued to take a standard dose of oral mesalamine (≥2.4 g/day). The primary end point in PCG-2 was deep remission. Patients achieving remission at week 12 were randomly assigned 2:1:1 to 1.6 g LT-02 BID, placebo, or 500 mg mesalamine (3 times daily), respectively, in PCG-4; the primary end point was remission at 48 weeks.

RESULTS

PCG-2 was terminated early for futility after a prespecified interim analysis; 466 patients (of 762 planned) were randomized. There was no statistically significant difference in deep remission at week 12 (placebo, 13.5%; LT-02 BID, 14.2%; LT-02 QID, 9.7%). In PCG-4, 150 patients (of approximately 400 planned) were randomized. There was no statistically significant difference in remission rates at week 48 (LT-02 BID, 49.3%; mesalamine, 50.0%; placebo, 43.2%). LT-02 was safe.

CONCLUSIONS

Despite prior evidence of beneficial effects of PC in phase 2 trials, our induction study with LT-02 in patients with mild to moderate UC was terminated prematurely for futility. Signals of efficacy in maintenance therapy require confirmation in an adequately powered maintenance trial. LT-02 was safe and well-tolerated.

CLINICALTRIALS

gov: NCT02280629, NCT02142725.

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

Biological Role of the Intercellular Transfer of Glycosylphosphatidylinositol-Anchored Proteins: Stimulation of Lipid and Glycogen Synthesis

Glycosylphosphatidylinositol-anchored proteins (GPI-APs), which are anchored at the outer leaflet of plasma membranes (PM) only by a carboxy-terminal GPI glycolipid, are known to fulfill multiple enzymic and receptor functions at the cell surface. Previous studies revealed that full-length GPI-APs with the complete GPI anchor attached can be released from and inserted into PMs in vitro. Moreover, full-length GPI-APs were recovered from serum, dependent on the age and metabolic state of rats and humans. Here, the possibility of intercellular control of metabolism by the intercellular transfer of GPI-APs was studied. Mutant K562 erythroleukemia (EL) cells, mannosamine-treated human adipocytes and methyl-ß-cyclodextrin-treated rat adipocytes as acceptor cells for GPI-APs, based on their impaired PM expression of GPI-APs, were incubated with full-length GPI-APs, prepared from rat adipocytes and embedded in micelle-like complexes, or with EL cells and human adipocytes with normal expression of GPI-APs as donor cells in transwell co-cultures. Increases in the amounts of full-length GPI-APs at the PM of acceptor cells as a measure of their transfer was assayed by chip-based sensing. Both experimental setups supported both the transfer and upregulation of glycogen (EL cells) and lipid (adipocytes) synthesis. These were all diminished by serum, serum GPI-specific phospholipase D, albumin, active bacterial PI-specific phospholipase C or depletion of total GPI-APs from the culture medium. Serum inhibition of both transfer and glycogen/lipid synthesis was counteracted by synthetic phosphoinositolglycans (PIGs), which closely resemble the structure of the GPI glycan core and caused dissociation of GPI-APs from serum proteins. Finally, large, heavily lipid-loaded donor and small, slightly lipid-loaded acceptor adipocytes were most effective in stimulating transfer and lipid synthesis. In conclusion, full-length GPI-APs can be transferred between adipocytes or between blood cells as well as between these cell types. Transfer and the resulting stimulation of lipid and glycogen synthesis, respectively, are downregulated by serum proteins and upregulated by PIGs. These findings argue for the (patho)physiological relevance of the intercellular transfer of GPI-APs in general and its role in the paracrine vs. endocrine (dys)regulation of metabolism, in particular. Moreover, they raise the possibility of the use of full-length GPI-APs as therapeutics for metabolic diseases.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

Serositis: Comparative Analysis of Histological Findings and Pathogenetic Mechanisms in Nonbacterial Serosal Inflammation

Peritonitis is the established term for infective inflammation of the peritoneum, while serositis generally refers to nonorganismal inflammation in any serous cavity, including the peritoneum. In continuous ambulatory peritoneal dialysis (CAPD) literature, however, culture-negative peritoneal inflammation is referred to as “sterile” or “chemical” peritonitis. These terms not only imply unwarranted etiologic assumptions, but may also deflect attention from the existence of medical conditions to which the peritoneum is subject. This is evident in CAPD literature where there is little recognition that the peritoneum, as a member of the serosa and a secretor of lamellar bodies, is prey to a wide range of disorders. Thus before, during, and after CAPD, the membrane is liable to fall victim to disease states unconnected with the process of dialysis. Significant peritoneal pathology occurs as part of a pan-serositis, which may be metabolic (uremia, cholesterolosis), autoimmune (systemic lupus erythematosus, rheumatoid disease, acute rheumatism, endocrinopathies), genetic (recurrent hereditary polyserositis), allergic (eosinophilic serositis), and granulomatous in nature. This paper presents a comparative analysis of histopathological presentation and pathogenetic mechanisms involved in all forms of peritoneal serositis. It incorporates recent advances in molecular biology of the membrane into a holistic reappraisal of peritoneal pathology, revealing hitherto unrecognized homologies in peritoneal reaction to diverse disorders.

Genetic Mouse Models with Intestinal-Specific Tight Junction Deletion Resemble an Ulcerative Colitis Phenotype

BACKGROUND AND AIMS

A key pathogenetic feature of ulcerative colitis [UC] is an intrinsic low mucus phosphatidylcholine[PC] content. Recently, a paracellular transport for PC across tight junctions[TJs] was described, suggesting TJ disturbance as a cause of diminished luminal PC transport. Therefore, we aimed to generate mutant mice with TJ deletion to evaluate whether a UC phenotype developed.

METHODS

CL57BL/6 control wild-type mice were compared to mutant mice with tamoxifen-induced villin-Cre-dependent intestinal deletion of kindlin 1 and 2.

RESULTS

Electron microscopy of mucosal biopsies obtained from both mutants before overt inflammation following only 2 days of tamoxifen exposure revealed a defective TJ morphology with extended paracellular space and, by light microscopy, expanded mucosal crypt lumina. PC secretion into mucus was reduced by >65% and the mucus PC content dropped by >50%, causing a >50 % decrease of mucus hydrophobicity in both mutants. Consequently, the microbiota was able to penetrate the submucosa. After 3 days of tamoxifen exposure, intestinal inflammation was present in both mutants, with loose bloody stools as well as macroscopic and histological features of colitis. Oral PC supplementation was able to suppress inflammation. By analogy, colonic biopsies obtained from patients with UC in remission also showed a defective epithelium with widened intercellular clefts, and enlarged crypt luminal diameters with functionally impaired luminal PC secretion.

CONCLUSIONS

Genetic mouse models with intestinal deletion of kindlin 1 and 2 resulted in TJ deletion and revealed pathophysiological features of impaired PC secretion to the mucus leading to mucosal inflammation compatible with human UC.

Phosphatidylcholine passes through lateral tight junctions for paracellular transport to the apical side of the polarized intestinal tumor cell-line CaCo2

Phosphatidylcholine (PC) is the most abundant phospholipid in intestinal mucus, indicative of a specific transport system across the mucosal epithelium to the intestinal lumen. To elucidate this transport mechanism, we employed a transwell tissue culture system with polarized CaCo2 cells. It was shown that PC could not substantially be internalized by the cells. However, after basal application of increasing PC concentrations, an apical transport of 47.1±6.3nmolh(-1)mMPC(-1) was observed. Equilibrium distribution studies with PC applied in equal concentrations to the basal and apical compartments showed a 1.5-fold accumulation on the expense of basal PC. Disruption of tight junctions (TJ) by acetaldehyde or PPARγ inhibitors or by treatment with siRNA to TJ proteins suppressed paracellular transport by at least 50%. Transport was specific for the choline containing the phospholipids PC, lysoPC and sphingomyelin. We showed that translocation is driven by an electrochemical gradient generated by apical accumulation of Cl(-) and HCO3(-) through CFTR. Pretreatment with siRNA to mucin 3 which anchors in the apical plasma membrane of mucosal cells inhibited the final step of luminal PC secretion. PC accumulates in intestinal mucus using a paracellular, apically directed transport route across TJs.

Regulation of the divalent metal ion transporter via membrane budding

The release of extracellular vesicles (EVs) is important for both normal physiology and disease. However, a basic understanding of the targeting of EV cargoes, composition and mechanism of release is lacking. Here we present evidence that the divalent metal ion transporter (DMT1) is unexpectedly regulated through release in EVs. This process involves the Nedd4-2 ubiquitin ligase, and the adaptor proteins Arrdc1 and Arrdc4 via different budding mechanisms. We show that mouse gut explants release endogenous DMT1 in EVs. Although we observed no change in the relative amount of DMT1 released in EVs from gut explants in Arrdc1 or Arrdc4 deficient mice, the extent of EVs released was significantly reduced indicating an adaptor role in biogenesis. Furthermore, using Arrdc1 or Arrdc4 knockout mouse embryonic fibroblasts, we show that both Arrdc1 and Arrdc4 are non-redundant positive regulators of EV release. Our results suggest that DMT1 release from the plasma membrane into EVs may represent a novel mechanism for the maintenance of iron homeostasis, which may also be important for the regulation of other membrane proteins.

First multicenter study of modified release phosphatidylcholine "LT-02" in ulcerative colitis: a randomized, placebo-controlled trial in mesalazine-refractory courses

OBJECTIVES

Phosphatidylcholine is a key component of the mucosal barrier. Treatment with modified release phosphatidylcholine aims to improve the impaired barrier function. The primary objective is to evaluate the efficacy of LT-02, a newly designed modified release phosphatidylcholine formula, in a multicenter setting.

METHODS

This is a double-blinded, randomized, placebo-controlled, superiority study conducted in 24 ambulatory referral centers in Germany, Lithuania, and Romania. A total of 156 patients with an inadequate response to mesalazine, a disease activity score (Simple Clinical Colitis Activity Index (SCCAI)) of ≥ 5, and bloody diarrhea underwent treatment with 0, 0.8, 1.6, or 3.2 g LT-02. The primary end point was defined a priori as changes in SCCAI from baseline to the end of treatment. The primary statistical model was a general linear least-squares model. The study was funded by the sponsor Lipid Therapeutics, Heidelberg, Germany, and registered at http://clinicaltrials.gov/show/NCT01011322.

RESULTS

Baseline characteristics and dropouts were well balanced between all groups. The primary analyses revealed an SCCAI drop of 33.3% in the placebo group (from 9.0 to 6.0 points) compared with 44.3% in the 0.8 g LT-02 (from 8.8 to 4.9, P>0.05) and 40.7% in the 1.6 g groups (from 8.6 to 5.1, P>0.05). The 3.2 g group improved 51.7% from 8.5 to 4.1 (P=0.030 in comparison with placebo). The remission rate was 15% (6/40) in the placebo group compared with 31.4% (11/35) in the highest LT-02 dose group (P=0.089). Mucosal healing was achieved in 32.5% of placebo patients compared with 47.4% of LT-02 patients (P=0.098); the rates for histologic remission were 20% compared with 40.5%, respectively (P=0.016). There were 17 (48.6%) treatment-emergent adverse events in the highest dose group (and 0 serious adverse events (SAEs)) compared with 22 (55%) in the placebo group (4 SAEs).

CONCLUSIONS

The primary end point analysis showed a statistically significant improvement in disease activity during LT-02 treatment in comparison with placebo. The drug was found to be very safe.

Lecithin as a therapeutic agent in ulcerative colitis

Lecithin [phosphatidylcholine (PC)] was shown to account for more than 70% of total phospholipids within the intestinal mucus layer. It is arranged in lamellar membranes (surfactant-like particles) and establishes a hydrophobic barrier preventing invasion of the colonic commensal microbiota. In ulcerative colitis (UC), the mucus PC content was demonstrated to be reduced by about 70%, irrespective of the presence of inflammation. This may be of primary pathogenetic significance allowing bacteria to enter the mucus and induce mucosal inflammation. Therefore, a new therapeutic strategy is being developed to substitute the missing mucus PC content in UC. Indeed, a delayed-release PC formulation was able to compensate the lack of PC and improve the inflammatory activity. In randomized controlled studies, delayed-release PC was proven to be clinically and endoscopically effective, which now awaits a phase III authority approval trial.

Phosphatidylcholine as a constituent in the colonic mucosal barrier--physiological and clinical relevance

Phosphatidylcholine (PC) is an important constituent of the gastrointestinal tract. PC molecules are not only important in intestinal cell membranes but also receiving increasing attention as protective agents in the gastrointestinal barrier. They are largely responsible for establishing the hydrophobic surface of the colon. Decreased phospholipids in colonic mucus could be linked to the pathogenesis of ulcerative colitis, a chronic inflammatory bowel disease. Clinical studies revealed that therapeutic addition of PC to the colonic mucus of these patients alleviated the inflammatory activity. This positive role is still elusive, however, we hypothesized that luminal PC has two possible functions: first, it is essential for surface hydrophobicity, and second, it is integrated into the plasma membrane of enterocytes and it modulates the signaling state of the mucosa. The membrane structure and lipid composition of cells is a regulatory component of the inflammatory signaling pathways. In this perspective, we will shortly summarize what is known about the localization and protective properties of PC in the colonic mucosa before turning to its evident medical importance. We will discuss how PC contributes to our understanding of the pathogenesis of ulcerative colitis and how reinforcing the luminal phospholipid monolayer can be used as a therapeutic concept in humans.

Alterations of phospholipid concentration and species composition of the intestinal mucus barrier in ulcerative colitis: a clue to pathogenesis

BACKGROUND

Phospholipids are essential for the normal function of the intestinal mucus barrier. The objective of this study was to systematically investigate phospholipids in the intestinal mucus of humans suffering from inflammatory bowel diseases, where a barrier defect is strongly supposed to be pathogenetic.

METHODS

Optimal mucus recovery was first validated in healthy mice and the method was then transferred to the endoscopic acquisition of ileal and colonic mucus from 21 patients with ulcerative colitis (UC), 10 patients with Crohn's disease (CD), and 29 healthy controls. Nano-electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to determine phosphatidylcholine (PC), lysophosphatidylcholine (LPC), and sphingomyelin (SM) in lipid extracts of mucus specimens.

RESULTS

Human and rodent mucus contained very similar phospholipid species. In the ileal and colonic mucus from patients suffering from UC, the concentration of PC was highly significantly lower (607 +/- 147 pmol/100 microg protein and 745 +/- 148 pmol/100 microg protein) compared to that of patients with CD (3223 +/- 1519 pmol/100 microg protein and 2450 +/- 431 pmol/100 microg protein) and to controls (3870 +/- 760 pmol/100 microg protein and 2790 +/- 354 pmol/100 microg protein); overall, P = 0.0002 for ileal specimens and P < 0.0001 for colonic specimens. Independent of disease activity, patients suffering from UC showed an increased saturation grade of PC fatty acid residues and a higher LPC-to-PC ratio.

CONCLUSIONS

The intestinal mucus barrier of patients with UC is significantly altered concerning its phospholipid concentration and species composition. These alterations may be very important for the pathogenesis of this disease and underline new therapeutic strategies.

The enterocyte microvillus is a vesicle-generating organelle

For decades, enterocyte brush border microvilli have been viewed as passive cytoskeletal scaffolds that serve to increase apical membrane surface area. However, recent studies revealed that in the in vitro context of isolated brush borders, myosin-1a (myo1a) powers the sliding of microvillar membrane along core actin bundles. This activity also leads to the shedding of small vesicles from microvillar tips, suggesting that microvilli may function as vesicle-generating organelles in vivo. In this study, we present data in support of this hypothesis, showing that enterocyte microvilli release unilamellar vesicles into the intestinal lumen; these vesicles retain the right side out orientation of microvillar membrane, contain catalytically active brush border enzymes, and are specifically enriched in intestinal alkaline phosphatase. Moreover, myo1a knockout mice demonstrate striking perturbations in vesicle production, clearly implicating this motor in the in vivo regulation of this novel activity. In combination, these data show that microvilli function as vesicle-generating organelles, which enable enterocytes to deploy catalytic activities into the intestinal lumen.

Dietary Fat Feeding Alters Lipid Peroxidation in Surfactant-like Particles Secreted by Rat Small Intestine

Background

Long-term feeding of fish oil (n-3) and corn oil (n-6) markedly enhances levels of lipid peroxidation within isolated rat enterocytes. The effect is 10-fold greater at the villus tip than in the crypt region, correlating with the distribution of deleterious oxidative systems (glutathione reductase) in the tip and beneficial systems (superoxide dismutase) at the base of the villus. Because of this vertical gradient of peroxidation, the process was thought to play a role in apoptosis of enterocytes at the villus tip. Surfactant-like particles (SLPs) are membranes secreted by the enterocyte and a component of these membranes is directed to the intestinal surface overlying villus tips. One suggested role for SLPs has been to protect the mucosal surface from the harsh luminal conditions that might enhance apoptotic loss of enterocytes. The hypothesis to be tested was whether SLP lipids, like those in enterocytes, were also peroxidized, although they were external to the cellular processes that seem to oxidize enterocyte lipids, or whether SLP were immune to these biological processes. Feeding with groundnut oil (n-9) was compared with fish oil (n-3) and corn oil (predominantly n-6) to determine whether oils with various lipid composition would affect peroxidation in both SLP and enterocytes.

Methods

After an overnight fast, Wistar rats were fed 2 mL of dietary oil by gavage. Five hours later SLPs and underlying microvillus membranes (MVM) were isolated and analyzed for generation of thiobarbituric acid reactive substances (TBARS) and for hydrolase activities, at baseline and after addition of an Fe⁺²/ascorbate system to induce peroxidation.

Results

In vitro lipid peroxidation using the Fe²⁺/ascorbate system produced greater peroxidation than in MVM. Intestinal alkaline phosphatase (IAP), sucrase and lactase activities were decreased in SLPs, but were unaltered in MVM except for IAP. The activities of maltase, trehalase, Leucine aminopeptidase and γ–glutamyltranspeptidase, were unaffected both in SLPs and MVM under these conditions.

Conclusions

SLPs are more susceptible to oxidative damage than are the underlying MVMs. This may reflect results of a hostile luminal environment. It is not clear whether SLPs are acting as a lipid ‘sink’ to protect the MVM from greater oxidation, or are providing an initial stimulus for apoptosis of villus tip enterocytes, or both.

Intestinal alkaline phosphatase: selective endocytosis from the enterocyte brush border during fat absorption

Absorption of dietary fat in the small intestine is accompanied by a rise of intestinal alkaline phosphatase (IAP) in the serum and of secretion of IAP-containing surfactant-like particles from the enterocytes. In the present work, fat absorption was studied in organ cultured mouse intestinal explants. By immunofluorescence microscopy, fat absorption caused a translocation of IAP from the enterocyte brush border to the interior of the cell, whereas other brush-border enzymes were unaffected. By electron microscopy, the translocation occurred by a rapid (5 min) induction of endocytosis via clathrin-coated pits. By 60 min, IAP was seen in subapical endosomes and along membranes surrounding fat droplets. IAP is a well-known lipid raft-associated protein, and fat absorption was accompanied by a marked change in the density and morphology of the detergent-resistant membranes harboring IAP. A lipid analysis revealed that fat absorption caused a marked increase in the microvillar membrane contents of free fatty acids. In conclusion, fat absorption rapidly induces a transient clathrin-dependent endocytosis via coated pits from the enterocyte brush border. The process selectively internalizes IAP and may contribute to the appearance of the enzyme in serum and surfactant-like particles.

A novel phosphatase upregulated in Akp3 knockout mice

Reexamination of the Akp3(-/-) mouse intestine showed that, despite the lack of intestinal alkaline phosphatase (IAP), the Akp3(-/-) gut still had considerable alkaline phosphatase (AP) activity in the duodenum and ileum. This activity is due to the expression of a novel murine Akp6 gene that encodes an IAP isozyme expressed in the gut in a global manner (gIAP) as opposed to duodenum-specific IAP (dIAP) isozyme encoded by the Akp3 gene. Phylogenetically, gIAP is similar to the rat IAP I isozyme. Kinetically, gIAP displays a 5.7-fold reduction in catalytic rate constant (k(cat)) and a 30% drop in K(m), leading to a 4-fold reduction k(cat)/K(m) compared with dIAP, and these changes in enzymatic properties can all be attributed to a crucial R317Q substitution. Western and Northern blot analyses document the expression of Akp6 in the gut, from the duodenum to the ileum, and it is upregulated in the jejunum and ileum of Akp3(-/-) mice. Developmentally, Akp3 expression is turned on during postnatal days 13-15 and exclusively in the duodenum, whereas Akp6 and Akp5 are expressed from birth throughout the gut with enhanced expression at weaning. Posttranslational modifications of gIAP have a pronounced effect on its catalytic properties. Given the low catalytic efficiency of gIAP, its upregulation during fat feeding, its sequence similarity with rat IAP I, and the fact that rat IAP I has been implicated in the upregulation of surfactant-like particles during fat intake, it appears likely that gIAP may have a role in mediating the accelerated fatty acid intake observed in Akp3(-/-) mice fed a high-fat diet.

Direct intestinal cholesterol secretion contributes significantly to total fecal neutral sterol excretion in mice

BACKGROUND & AIMS

Hepatobiliary secretion is generally believed to be an integral step in the pathway of cholesterol excretion from the body. Here we have investigated the validity of this paradigm in mice.

METHODS

Cholesterol balance was assessed by measuring intake, excretion, and biliary output in different mouse models. Direct secretion of cholesterol from the luminal side of enterocytes was studied by perfusion of isolated segments of the small intestine in mice.

RESULTS

Cholesterol input and output measurements in different mouse models revealed that fecal neutral sterol excretion was higher than the sum of dietary cholesterol intake and biliary cholesterol secretion indicating the existence of an alternative pathway. Here we show that substantial amounts of cholesterol can be secreted directly by enterocytes. Transintestinal cholesterol secretion is a specific process observed throughout the small intestine (proximal > medial > distal). Secretion depended on the presence of a cholesterol acceptor and was strongly stimulated by bile salts and phospholipids. The capacity of the pathway was sufficient to account for the missing cholesterol in the balance studies. The contribution of this pathway to cholesterol excretion in mice is approximately twice that of the biliary pathway.

CONCLUSIONS

In mice, the intestine plays a significant role in removal of cholesterol from the body.

Anti-inflammatory effects of phosphatidylcholine

We recently showed that mucus from patients with ulcerative colitis, a chronic inflammatory disorder of the colon, is characterized by a low level of phosphatidylcholine (PC) while clinical studies reveal that therapeutic addition of PC using slow release preparations is beneficial. The positive role of PC in this disease is still elusive. Here we tested the hypothesis that exogenous application of PC has anti-inflammatory properties using three model systems. First, human Caco-2 cells were treated with tumor necrosis factor-alpha (TNF-alpha) to induce a pro-inflammatory response via activation of NF-kappaB. Second, latex bead phagosomes were analyzed for their ability to assemble actin in vitro, a process linked to pro-inflammatory signaling and correlating with the growth versus killing of mycobacteria in macrophages. The third system used was the rapid assembly of plasma membrane actin in macrophages in response to sphingosine 1-phosphate. TNF-alpha induced a pro-inflammatory response in Caco-2 cells, including 1) assembly of plasma membrane actin; 2) activation of both MAPKs ERK and p38; 3) transport of NF-kappaB subunits to the nucleus; and 4) subsequent up-regulation of the synthesis of pro-inflammatory gene products. Exogenous addition of most PCs tested significantly inhibited these processes. Other phospholipids like sphingomyelin or phosphatidylethanolamine showed no effects in these assays. PC also inhibited latex bead phagosome actin assembly, the killing of Mycobacterium tuberculosis in macrophages, and the sphingosine 1-phosphate-induced actin assembly in macrophages. TNF-alpha induces the activation of signaling molecules and the reorganization of the actin cytoskeleton in human intestinal cells. Exogenous application of PC blocks pro-inflammatory signaling in Caco-2 cells, in phagosomes in vitro and facilitates intracellular survival of mycobacteria. We provide further evidence that actin assembly by membranes is part of the pro-inflammatory response. Collectively, these results provide a molecular foundation for the clinical studies showing a beneficial effect of PC therapy in ulcerative colitis.

Myosin-1a powers the sliding of apical membrane along microvillar actin bundles

Microvilli are actin-rich membrane protrusions common to a variety of epithelial cell types. Within microvilli of the enterocyte brush border (BB), myosin-1a (Myo1a) forms an ordered ensemble of bridges that link the plasma membrane to the underlying polarized actin bundle. Despite decades of investigation, the function of this unique actomyosin array has remained unclear. Here, we show that addition of ATP to isolated BBs induces a plus end–directed translation of apical membrane along microvillar actin bundles. Upon reaching microvillar tips, membrane is “shed” into solution in the form of small vesicles. Because this movement demonstrates the polarity, velocity, and nucleotide dependence expected for a Myo1a-driven process, and BBs lacking Myo1a fail to undergo membrane translation, we conclude that Myo1a powers this novel form of motility. Thus, in addition to providing a means for amplifying apical surface area, we propose that microvilli function as actomyosin contractile arrays that power the release of BB membrane vesicles into the intestinal lumen.

Alterations in the intestinal glycocalyx and bacterial flora in response to oral indomethacin

Nonsteroidal anti-inflammatory drugs (NSAIDs), used extensively in clinical medicine, tend to cause adverse effects in the gastrointestinal tract. Earlier work has shown that oral administration of indomethacin produced oxidative damage in the small intestine and attenuation of the glycocalyx layer of the mucosa. The present study assessed, in greater detail, the alterations produced in the glycocalyx of rat small intestinal mucosa in response to indomethacin, with specific reference to surfactant-like particles (SLP) and brush border membranes (BBM). Changes in gut flora in response to the drug were also studied, as it has been shown that luminal bacteria play a role in the pathogenesis of NSAID-induced intestinal damage. The levels of sugars such as sialic acid, fucose, hexose and hexosamine were increased in SLP and decreased in the BBM following indomethacin treatment, with the effects being maximal 24h after the administration of the drug. The composition of lipids in the SLP was also found to be altered. There was a significant increase in the number of bacteria in the luminal contents of the small intestine and caecum in these animals, as compared with controls. The number of bacteria adherent to the intestinal mucosa was also significantly higher in the drug-treated group. In vitro studies revealed that there was an increased tendency for bacteria to adhere to SLP isolated from indomethacin-treated rats. These results suggest that alterations in glycosylation of SLP and BBM in response to indomethacin, along with qualitative and quantitative changes in the luminal bacterial flora, may facilitate translocation of bacteria into the mucosa. These changes may contribute to the enteropathy observed as a result of NSAID treatment.

Retarded release phosphatidylcholine benefits patients with chronic active ulcerative colitis

BACKGROUND AND AIMS

We examined the hypothesis of an anti-inflammatory effect of phosphatidylcholine in ulcerative colitis.

METHODS

A phase IIA, double blind, randomised, placebo controlled study was performed in 60 patients with chronic active, non steroid dependent, ulcerative colitis, with a clinical activity index (CAI) of > or = 4. Retarded release phosphatidylcholine rich phospholipids and placebo were administered at a dose of 6 g daily over three months. The primary end point was a change in CAI towards clinical remission (CAI < or = 3) or CAI improvement by > or = 50%. Secondary end points included > or = 50% changes in endoscopic activity index (EAI), histology, and quality of life scores.

RESULTS

Induction of clinical remission (CAI < or = 3) as the primary outcome variable was attained by 16 (53%) patients in the phosphatidylcholine treated group compared with three (10%) in the placebo group (p<0.00001). The rate of clinical remission and CAI improvement was 90% in the phosphatidylcholine group and only 10% in the placebo group. A median drop of seven points in the CAI score (70% improvement) was recorded in the phosphatidylcholine group compared with no change in the placebo group. Secondary end point analysis revealed concomitant drops in EAI and histology scores (p = 0.00016 and p = 0.0067 compared with placebo, respectively). Improvement in quality of life was reported by 16 of 29 evaluated patients in the phosphatidylcholine group compared with two of 30 in the placebo group (p = 0.00005).

CONCLUSION

Retarded release oral phosphatidylcholine is effective in alleviating inflammatory activity caused by ulcerative colitis.

Evidence of luminal phosphatidylcholine secretion in rat ileum

BACKGROUND

Intestinal mucus not only facilitates substrate absorption, but also forms a hydrophobic, phosphatidylcholine (PC) enriched, barrier against luminal gut contents.

METHODS

For evaluation of the origin of PC in intestinal mucus, we first analyzed the mucus PC in mice with absent biliary phospholipid secretion (mdr2 (-/-) mice) using electrospray ionization (ESI) tandem mass spectroscopy (MS/MS). Second, in situ perfused rat jejunum, ileum and colon were analyzed after i.v. bolus injections of 155 pmol [(3)H]-PC. Additional in vitro experiments were performed with isolated mucosal cells after incubation with the PC precursor [(3)H]-choline.

RESULTS

In mdr2 (-/-) mice and control animals no significant quantitative difference in mucus PC was found, indicating that mucus PC is of intestinal and not biliary origin. In situ perfusion studies detected intestinal secretion of [(3)H]-PC, which was stimulated in presence of 2 mM taurocholate (TC). Secretion rates of [(3)H]-PC were highest in ileum (9.0+/-0.8 fmol h(-1)xcm(-1)), lower in jejunum (4.3+/-0.5) and minimal in colon (0.8+/-0.2). It compares to an intestinal secretion of native PC originating to 64% from bile, 9% from jejunum, 28% from ileum, and 1% from colon. Complementary in vitro studies showed 30-min secretion rates for [(3)H]-PC to be highest in enterocytes from ileum (26.5+/-5.3% of intracellular [(3)H]-PC) and jejunum (19.8+/-2.9%), and significantly lower in colonocytes (8.4+/-1.3%).

CONCLUSION

PC in the intestinal mucus originates from secretion by ileal and jejunal enterocytes.

Altered glycosylation of surfactant and brush border membrane of the small intestine in response to surgical manipulation

BACKGROUND

Surgical stress can lead to bacterial translocation from the intestine into systemic circulation. Adherence of bacteria onto the glycoconjugates of the brush border membrane (BBM) and surfactant coat (SLP) of the mucosal cells is the first step in the translocation of luminal bacteria. Our earlier study showed that surgical manipulation of the intestine results in oxidative stress leading to structural and functional alterations in the mucosa. This study looks at the effect of surgical manipulation on the glycoconjugate alterations of SLP and BBM.

MATERIALS AND METHODS

SLP and BBM were isolated from control and after surgical manipulation and the sugar composition was analyzed. Bacterial adherence using E. coli isolated from cecum was compared after coating microtiter plates with SLP or BBM isolated from control and after surgical manipulation.

RESULTS

An increase in various sugars was seen after surgical manipulation both in SLP and BBM and this increase was maximum at 12 h after surgery. These alterations increased bacterial adherence onto SLP and BBM. Inhibiting superoxide generation by allopurinol treatment prior to surgical manipulation prevented glycosylation alteration and bacterial adherence.

CONCLUSION

Surgical manipulation results in altered glycoconjugates of SLP and BBM which leads to increased bacterial adherence. These alterations are probably brought about by oxygen-free radicals. This is clinically significant because postsurgical complications such as sepsis may be brought about by altered glycosylation.

Role of intestine in postsurgical complications: involvement of free radicals

Surgery at any location in the body leads to surgical stress response and alterations in normal body homeostasis. The intestine is extremely sensitive to surgical stress even at remote locations and the gastrointestinal tract plays an important role in the development of postsurgical complications such as sepsis, the systemic immune response syndrome (SIRS), and multiple organ failure syndrome (MOFS). The generation of free radicals and subsequent biochemical alterations at the cellular and subcellular level in the intestine has been suggested to play an important role in this process. These oxidative stress-induced events in the mucosa might act as an initiator of distant organ damage and also facilitate bacterial adherence onto the epithelium and translocation into the systemic circulation. This review attempts to highlight the important role of intestine and oxygen free radicals in initiating post-surgical complications.

Rat enterocytes secrete SLPs containing alkaline phosphatase and cubilin in response to corn oil feeding

Surfactant-like particles (SLP) are unilamellar secreted membranes associated with the process of lipid absorption and isolated previously only from the apical surface of enterocytes. In this paper, the intracellular membrane has been isolated from corn oil-fed animals, identified by its content of the marker protein intestinal alkaline phosphatase (IAP). Another brush-border protein, cubilin, and its anchoring protein megalin have been identified as components of extracellular SLP, but only cubilin is present to any extent in intracellular SLP. During fat absorption, IAP is modestly enriched in intracellular SLP, but full-length cubilin (migrating at 210 kDa in fat-fed mucosal fractions) falls by one-half, although fragments of cubilin are abundant in the intracellular SLP. Both IAP and cubilin colocalize to the same cells during corn oil absorption and colocalize around lipid droplets. This localization is more intense during feeding of corn oil with Pluronic L-81, a detergent that allows uptake of fatty acids and monoglycerides from the lumen, but blocks chylomicron secretion. Confocal microscopy confirms the colocalization of IAP and the ligand for cubilin, intrinsic factor. Possible roles for cubilin in intracellular SLP include facilitating movement of the lipid droplet through the cell and binding to the basolateral membrane before reverse endocytosis.

Secreted intestinal surfactant-like particles interact with cell membranes and extracellular matrix proteins in rats

Surfactant-like particles (SLP) are secreted from enterocytes basolaterally into the lamina propria, and reach the apical surface through the intercellular tight junctions. Interactions of SLP with apical and basolateral membranes and with extracellular matrix proteins were measured using a solid-phase binding assay and gel overlays. Small-intestinal SLP bound to basolateral membranes much more than to apical membranes, and more tightly to fibronectin than to laminin (affinity constant K(a) = 1.23 x 10(-2) microg vs. 0.67 x 10(-2) microg; maximal number of binding sites 4.1 microg x ml(-1) vs. 0.32 microg x ml(-1)), but did not bind to collagen types I or IV. Small-intestinal SLP bound fibronectin more than colonic or gastric SLP. Binding to fibronectin was inhibited only partially by RGD peptide and gelatin, but not by heparin. An antibody against alpha(v) integrin also identified the fibronectin-binding component in SLP at approximately 220 kDa, which is the expected size for integrin heterodimers. SLP binding to apical microvillous membranes was weaker and was inhibited by heparin. SLP bound more strongly to heparin itself, and this binding was inhibited by glucuronic acid and chondroitin sulfate. These data are consistent with the hypothesis that the time spent by secreted SLP in the lamina propria is prolonged by strong interactions with proteins in the basolateral membranes, and in the intestinal lumen by weaker interactions with apical membrane components, including heparin. These interactions may allow SLP the time to exert their functions in each tissue compartment.

Surfactant-like particles mediate tissue-specific functions in epithelial cells

Lipid-rich, unilamellar membranes appear to be relatively common structures lining the apical or 'exposed' surface of epithelial cells. They have now been described in the intestinal tract from the esophagus to the rectum and have been isolated from tissues, such as the stomach, the small bowel, the colon, and the bladder. The presence of a lining layer in the lungs has been known for some time, and its functions, structure, and metabolism have been extensively studied, as can be gleaned from the multitude of reports presented at this symposium. The 'other' surfactants, however, have attracted far less attention and have been investigated in detail in only a few reports. This paucity of information, when compared to the pulmonary system, is most likely due to the fact that a generalized function (sufficiency state) or disease (deficiency state) has not yet been recognized for either the intestinal or urinary forms of surfactant. It seems reasonable to assume that the role of the SLP will vary, at least in part, with the organ or tissue with which it is associated, although the widespread nature of the membrane assumes that some functions (e.g. protective) will be shared. Thus, pulmonary surfactant's primary function in the lung may be to reduce surface tension and prevent lung collapse; but it also plays a significant part in the lung's defenses against bacterial and/or chemical invasion. It is hoped that future studies will shed some light on the function of the various SLPs and lead to a better appreciation for their role in both maintaining a healthy environment and contributing to the proper functioning of their host tissues.

Surfactant proteins in the digestive tract, mesentery, and other organs: evolutionary significance

For years, the so-called surfactant proteins (SPs) that were discovered in the phospholipid-rich material designated pulmonary surfactant, were considered to be lung-specific. The fact that surfactant-like materials composed of phospholipids are secreted by a number of other organs recently prompted several groups to search for SP expression in these organs also. The hydrophilic proteins SP-A and SP-D and their transcripts have been found in a number of tissues, including gastric and intestinal mucosae, mesothelial tissues (mesentery, peritoneum, and pleura), synovial cells, Eustachian tube and sinus, and possibly in salivary glands, pancreas, and urinary tract. By contrast, the hydrophobic proteins SP-B and SP-C actually appear to be expressed in lung epithelium only. SP-A and SP-D belong to the innate defence system against pathogens and play a role as opsonins for facilitating phagocytosis. Their expression appears as a general feature of organs exposed to pathogens because they present an interface with the external milieu. Although this function has thus far been investigated in the lung only through the gene-targeting approach, increased expression of SP-A in the infected middle ear and of SP-D in the Helicobacter-infected antrum argues for such a function also in other organs. In organs that are not exposed to external pathogens, their role is likely to exert anti-inflammatory and immunomodulatory functions, as suggested by increased SP-A immunoreactivity in rheumatoid disease. SP-A and SP-B have been found in association with phospholipids in the lung of all air-breathing vertebrates, including the most primitive forms represented by lungfish, which implies that the surfactant system had a single evolutionary origin. Immunochemical proximity of the proteins among vertebrates indicates considerable conservation during evolution. Moreover, the finding of an SP-A-like protein in intestine and swim bladder of actinopterygian fish implies that the ancestral form of the protein was already present before the emergence of lung structures.

Regulation of surfactant-like particle secretion by Caco-2 cells

Surfactant-like particle (SLP) is a phosphatidylcholine (PC)-rich membrane produced in the small intestine, and its secretion is increased by fat feeding. In Caco-2 cells known to produce SLP, preincubation with [(3)H]palmitate labelled the SLP and was used as a marker for newly secreted membrane. SLP-associated PC and protein (d=1.07-1.08 g/ml in a linear non-equilibrium NaBr gradient) were secreted in parallel with triacylglycerols (TG) and at a rate about twice the control rate in response to feeding cells with an oleate/egg PC mixture. Cholesterol and apolipoprotein A-I identified only a small peak corresponding to high-density lipoprotein (HDL), but the largest peak corresponded with SLP (d=1.07-1.08). Palmitate incorporation into PC showed a similar small peak migrating at the density of HDL, but most labelled PC secreted from the cells was due to SLP. PC secretion, alkaline phosphatase activity, and newly synthesized immunoprecipitated SLP proteins from conditioned serum-free media migrated together at a density of >/=1.21 g/ml in a lipoprotein NaBr step gradient, and represented SLP. Glycerol incorporated into TG migrated at a peak density of 1.12 g/ml, consistent with HDL secretion from cells incubated in serum-free media. These data confirm that the secreted PC in SLP is distinct from lipoprotein particles. Incorporation of [(3)H]palmitate into the PC fraction of either whole cell homogenate or isolated brush border membranes was not affected by oleate/egg PC feeding. Both Pluronic L-81, an inhibitor of chylomicron secretion, and BMS-197636-02, a microsomal triglyceride transfer protein inhibitor, blocked the secretion of both TG and PC. Elevation of intracellular cAMP levels that stimulate surfactant secretion from type II pneumocytes caused a 50% reduction in SLP and TG secretion from Caco-2 cells. These results confirm the SLP response to fat feeding found in vivo, further supporting a role for SLP in TG secretion from the enterocyte, and show that the regulation of SLP secretion differs from that of pulmonary surfactant.

Role of intestinal surfactant-like particles as a potential reservoir of uropathogenic Escherichia coli

The binding of uropathogenic Escherichia coli is mediated at the tips of pili by the PapG adhesin, which recognizes the Galalpha(1-4)Gal disaccharide on the uroepithelial surface. These receptors have been identified unequivocally in the human and murine urinary tracts but not in intestinal epithelium, yet uropathogenic E. coli strains are commonly found in normal colonic microflora. The gastrointestinal tract from duodenum to rectum elaborates a phospholipid-rich membrane particle with surfactant-like properties. In these studies, we report that purified murine particles contain a receptor recognized by the class I PapG adhesin because: (1) PapD-PapG complexes and class I pili bound to surfactant-like particles in a solid-phase assay, whereas binding was not detected in microvillous membranes derived from the same tissues, (2) purified PapD-PapG complex bound to a glycolipid receptor detectable in lipid extracts from the particles, and (3) soluble Galalpha(1-4)Gal inhibited the adhesin by 72% from binding to surfactant-like particles. The Galalpha(1-4)Gal receptor present in the intestinal surfactant-like particle which overlies the intestinal mucosa could provide one means to establish an intestinal habitat for uropathogenic E. coli.

The physical properties of biogels and their permeability for macromolecular drugs and colloidal drug carriers

Macromolecular drugs, either free or complexed with colloidal drug carriers, have created a great deal of interest during the last decade. If one wants to administer these new therapeutics via the oral, nasal, and cervical routes or through the conductive airways, one of the first barriers to overcome is the mucus layer that adheres to the related epithelia. In this review, the physicochemical properties of biogels, macromolecular drugs, and colloidal drug carriers that play a major role in transport through biogels are reviewed. Also, methods of studying the mobility of macromolecular drugs and colloidal drug carriers in and through biogels are addressed.

Rat intestinal alpha1-antitrypsin secretion is regulated by triacylglycerol feeding

alpha1-Antitrypsin (AAT) is secreted by the enterocyte, but its regulation of expression, intramucosal distribution, and functional status are unclear. After corn oil gavage (plus Pluronic L-81 to block chylomicron release), rat intestine was examined for mRNA encoding AAT, immunoreactivity by light and electron microscopy, and protein content by Western blot. Species-specific antisera used were raised against both AAT and surfactant-like particle (SLP), a membrane secreted by the enterocyte in response to fat feeding. Purified luminal SLP was fractionated by Bio-Gel P-200 chromatography to assess its interaction with AAT. Triacylglycerol feeding maximally increased mucosal mRNA-encoding AAT and AAT intracellular protein content by 3 and 5 h, respectively. Immunocytochemistry revealed predominance of AAT in basolateral spaces around enterocytes and Pluronic-blocked extracellular accumulation of AAT, patterns nearly identical to those of secreted SLP. About 10% of AAT was reversibly associated with SLP. Luminal AAT was smaller (51 kDa) than mature AAT (55 kDa) and did not form a complex with pancreatic elastase. When the common bile duct was tied, excluding pancreatic proteases from the lumen, mature AAT that was cleaved by pancreatic elastase was secreted. The luminal secretion of AAT and its reversible association with SLP suggest an intracellular association and a possible role for AAT during lipid digestion and absorption.

Sulphydryl blocker-induced rat colitis is ameliorated by intravenous injection of antibody to colonic surfactant-like particle

A rat model of experimental colitis and jejunitis induced by iodoacetamide (IA), a sulphydryl blocker is accompanied by increased leukotriene, prostaglandin E2 (PGE2) generation, and nitric oxide synthase (NOS) activity. Rat small intestinal and colonic surfactant-like particles (SLP) that accumulate on the apical surface of mucosal cells have been identified and specific antibodies to them have been produced. The aim of this study was to evaluate a possible role of SLP in IA-induced colitis and jejunitis. Inflammation was induced in Sprague-Dawley rats either by intracolonic administration of 3% IA (0.1 ml) or by intrajejunal administration of 2% IA (0.1 ml). Antisera raised against either colonic SLP, pulmonary SP-A (a major protein associated with colonic SLP), or small intestinal SLP were injected into the tail vein of rats 48 h before, simultaneous with, or 24 h after IA administration. Rats were killed 2 or 10 days after IA was given, their colon or small intestine was isolated and rinsed, and a segment of colon (10 cm) or small bowel (30 cm) was weighed and processed for microscopy, NOS and myeloperoxidase (MPO) activities, and PGE2 generation. Intracolonic or jejunal IA resulted after 48 h in extensive macroscopic and microscopic damage, accompanied by increased segmental weight, MPO and NOS activity, and PGE2 generation. Colonic SLP antibody administration, either 48 h before or at the time of damage induction, significantly decreased macroscopic as well as microscopic damage, segmental weight, MPO activity, and PGE2 generation, but had no effect on NOS activity. Neither control sera nor antisera against SP-A had any protective effect, nor did injection of anti-colonic SLP antisera given 24 h after IA. Small bowel SLP antibody offered no protection against IA jejunitis. IA-induced colitis but not jejunitis is ameliorated by intravenous injection of SLP antibody by a mechanism yet to be determined. These data provide further evidence of a physiologic role for gastrointestinal SLP.

Changes, with age, in the phospholipid content of the intestinal mucus layer of the newborn rabbit

BACKGROUND/PURPOSE

The high incidence of bacterial translocation in newborns is thought to be caused, in part, by the immaturity of the intestinal mucosal barrier. Recently, intestinal mucus phospholipids (PL) have been reported to be important factors in the function of this mucosal barrier. The aim of this study was to quantify changes, with age, in the intestinal mucus PL of the newborn rabbit.

METHODS

Mucus was gently scraped from the small intestinal mucosal surface of rabbits of different ages (0, 7, 14, and 28 days old and adult; n = 6 for all groups). PL was extracted from the mucus and was separated by two-dimensional thin-layer chromatography. The isolated phospholipid spots were quantified for their phosphorus content.

RESULTS

Total PL content of the mucus decreased significantly with age (day 0, 21+/-2; day 7, 16+/-4; day 14, 9+/-3; day 28, 2+/-1; adult, 1+/-1 micromol/g wet mucus; P = .0001). Phosphatidylcholine and phosphatidylethanolamine levels in the adult rabbits were significantly lower in comparison with the 0-, 7-, and 14-day-old pups (P < .05). In contrast, lysophosphatidylcholine and lysophosphatidylethanolamine were significantly higher in the 28-day-old and adult rabbits in comparison with the 0-, 7-, and 14-day-old pups (P < .05). Phosphatidylinositol + phosphatidylserine levels in 7-day-old rabbits was significantly higher compared with adult rabbits. There was no significant difference in the composition of sphingomyeline between groups.

CONCLUSION

Significant changes in the content and composition of the intestinal mucus phospholipids were observed during the first month of life in rabbits.

Expression of hydrophilic surfactant proteins by mesentery cells in rat and man

Human peritoneal dialysis effluent (PDE) contains a phosphatidylcholine-rich compound similar to the surfactant that lines lung alveoli. This material is secreted by mesothelial cells. Lung surfactant is also characterized by four proteins essential to its function. After having long been considered as lung-specific, some of them have been found in gastric and intestinal epithelial cells. To explore further the similarity between lung and peritoneal surfactants, we investigated whether mesothelial cells also produce surfactant proteins. We used rat transparent mesentery, human visceral peritoneum biopsies and PDE. Surfactant proteins were searched for after one- and two-dimensional SDS/PAGE and Western blotting. On a one-dimensional Western blot, bands at 38 and 66 kDa in rat mesentery, and at 38 and 66 kDa in human peritoneal mesothelial cells (in vivo and in vitro) and PDE, corresponded to monomeric and dimeric forms of lung surfactant protein A (SP-A). On two-dimensional Western blots, the 32 and 38 kDa spots in mesentery and PDE localized at the acidic pH appropriate to the SP-A monomer's isoelectric point. SP-D was also identified at the same 43 kDa molecular mass as in lung. SP-B was not detected in mesenteric samples. Expression of SP mRNA species was also assessed by reverse transcriptase-PCR, which was performed with specific primers of surfactant protein cDNA sequences. With primers of SP-A and SP-D, DNA fragments of the same size were amplified in lung and mesentery, indicating the presence of SP-A and SP-D mRNA species. These fragments were labelled by appropriate probes in a Southern blot. No amplification was obtained for SP-B. These results show that mesentery cells produce SP-A and SP-D, although they are of embryonic origin (mesodermal) and are different from those of the lung and digestive tract (endodermal) that secrete these surfactants.

Recognition of normal, neoplastic, and fetal airway epithelial cell membranes by two monoclonal antibodies

The reactivity of two rat monoclonal antibodies was studied. These antibodies, A2R and A2C, bind a 32 kDa alveolar type II cell membrane receptor for surfactant protein A. A2R and A2C also bind apical cell membranes of ciliated and nonciliated cells of the conducting airways. Because this reactivity suggested possible utility in targeting those cells for therapeutic gene transfer, the binding activity of these two antibodies was examined in human tissues. In conducting airways, A2R and A2C bound apical epithelial cell membranes throughout the embryologic period studied: from 15 weeks of gestation, through maturity. Reactivity was more restricted to ciliated cells of the airways as maturation progressed. In the peripheral lung, A2C and A2R only bound most cells in the early developing lung, but mainly type II cells in mature lungs. Other normal tissues recognized by these antibodies included crypt lining cells of the adult and fetal stomach, large bile duct epithelium, and pancreatic acinar cells. All of these cells derive from embryonic foregut endoderm. Other normal tissues, both of endodermal and nonendodermal origin, were negative. Pulmonary carcinomas were studied. A2C and A2R recognized all non-small cell carcinomas of the lung tested. In contrast, none of the small cell carcinomas or carcinoid tumors of the lung were recognized by these antibodies. The function of p32 in these diverse cell types is not clear, but whatever its role in these tissues, antibodies versus p32 may potentially be used to target gene or drug therapy to the normal or malignant cells they recognize.

Abundance of surfactant-like particles reflects mucosal integrity in patients with peptic ulcer disease

BACKGROUND

Surfactant-like particles, normal products of the human enterocyte, are released into the lumen and secreted into blood.

AIMS

To assess their role as markers for mucosal functional integrity, this study examined their content in biopsy specimens and serum of patients with duodenal ulcer disease, compared with non-diseased control subjects.

PATIENTS

Endoscopic biopsy specimens were taken 1-2 cm from areas of active inflammation or ulcer (peptic ulcer patients) or just beyond the duodenal bulb (normals) in 35 consecutive subjects.

METHODS

After staining for phospholipid, extracellular and intracellular particles were counted on transmission electron micrographs of coded specimens. Serum was obtained from 24 patients, and densitometry of the 59 kDa band detected on western blot by antiserum against human jejunal particle was measured.

RESULTS

Normal duodenum (n = 15) contained more particles (44 (4.7)) particles/block, mean (SD) than active duodenal ulcer (n = 13, 17 (3.9)) or gastritis/duodenitis patients (n = 4, 9 (2.7)). Three patients examined after healing of duodenal ulcers showed abundant particles (n = 3, 67 (2.2)). Similarly, the 59 kDa band was decreased in serum of patients with active peptic ulcer disease (n = 11, 0.25 (0.04) absorbance units) compared with normal patients (n = 10, 0.40 (0.03)) or healed ulcers (n = 3, 0.62 (0.04)). There was good correlation between morphological mucosal particle abundance and particle protein content of serum assayed from the same patients (r = 0.831). These changes were independent of Helicobacter pylori status.

CONCLUSION

The mucosal and serum content of surfactant-like particles may reflect general mucosal integrity of the enterocytes from which they are derived.

Microvesiculation of the microvilli provides a membranous medium within the lumen during digestion

A hypothesis is presented which states that there is a membranous medium within the intestinal lumen which occurs during digestion. The medium is thought to be generated by the microvesiculation of the microvilli.

The frog taste disc: a prototype of the vertebrate gustatory organ

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Appearance of surfactant-like particles in apical medium of Caco-2 cells may occur via tight junctions

Intestinal alkaline phosphatase (AP) is secreted by Caco-2 cells bound to surfactant-like particles (SLP), which can be localized by electron microscopy to the basolateral space and the intestinal lumen, especially over tight junctions. To investigate the hypothesis that SLP are secreted basolaterally and enter the lumen through the tight junction, Caco-2 cells were incubated with agents known to increase permeability at tight junctions. Cytochalasin D and phorbol 12-myristate 13-acetate increased Caco-2 cell monolayer permeability and the appearance of particles in apical medium two- to threefold, as monitored by mannitol movement and AP activity, respectively. Blocking the apical secretory pathway by nocodazole or colchicine had no effect on either parameter. Estimation of SLP content demonstrated an increase in apical media particles similar to that determined by AP activity. Quantitative image analysis established that apical SLP content increased 4-10 times, whereas total cell particle content remained unchanged. These data indicate that SLP may be secreted initially into the basolateral space and then transported to the intestinal lumen through the tight junctions.

Synthesis and parallel secretion of rat intestinal alkaline phosphatase and a surfactant-like particle protein

Rat intestinal microvillous alkaline phosphatases are secreted bidirectionally from the enterocyte attached to a phospholipid-rich membrane (surfactant-like particle). To determine the intracellular pathways for newly synthesized alkaline phosphatases and for the extracellular enzyme-particle complex in the intestinal mucosa, pulse-chase experiments were performed. Synthesis of both isoforms of alkaline phosphatase in fasted rats peaked in the Golgi at 15-30 min and in the microvillous membrane at 60 min, without intermediate localization in the basolateral membranes. A second peak of incorporation was found at 15-30 min in scrapings obtained from the apical surface of the enterocytes. These results demonstrate a dominant direct Golgi-to-microvillous membrane transport for newly synthesized alkaline phosphatase. An additional precursor pool(s) appears responsible for the early appearance of enzyme in the lumen. Newly synthesized alkaline phosphatase isoforms and the 97-kDa protein of surfactant-like particles showed parallel patterns of appearance in enterocytes, luminal washings, and lamina propria after triacylglycerol feeding and were preferentially secreted into the lumen and lamina propria at times (5-7 h) when enterocyte content of these newly synthesized proteins had declined toward basal rates. Enhanced secretion of newly synthesized proteins for hours after fat feeding could explain the prolonged rise in serum and luminal washings of both the enzyme and the particle.