Colonic-crypt-derived epithelia express induced ion transport differentiation in monolayer cultures on permeable matrix substrata

Paracellular permeability is increased by basal lipopolysaccharide in a primary culture of colonic epithelial cells; an effect prevented by an activator of Toll-like receptor-2

Lipopolysaccharide (LPS), which generally activates Toll-like receptor 4 (TLR4), is expressed on commensal colonic bacteria. In a number of tissues, LPS can act directly on epithelial cells to increase paracellular permeability. Such an effect in the colon would have an important impact on the understanding of normal homeostasis and of pathology. Our aim was to use a novel primary culture of colonic epithelial cells grown on Transwells to investigate whether LPS, or Pam(3)CSK( 4), an activator of TLR2, affected paracellular permeability. Consequently, [(14)C]-mannitol transfer and transepithelial electrical resistance (TEER) were measured. The preparation consisted primarily of cytokeratin-18 positive epithelial cells that produced superoxide, stained for mucus with periodic acid-Schiff reagent, exhibited alkaline phosphatase activity and expressed TLR2 and TLR4. Tight junctions and desmosomes were visible by transmission electron microscopy. Basally, but not apically, applied LPS from Escherichia coli increased the permeability to mannitol and to a 10-kDa dextran, and reduced TEER. The LPS from Helicobacter pylori increased paracellular permeability of gastric cells when applied either apically or basally, in contrast to colon cells, where this LPS was active only from the basal aspect. A pan-caspase inhibitor prevented the increase in caspase activity caused by basal E. coli LPS, and reduced the effects of LPS on paracellular permeability. Synthetic Pam(3)CSK(4) in the basal compartment prevented all effects of basal E. coli LPS. In conclusion, LPS applied to the base of the colonic epithelial cells increased paracellular permeability by a mechanism involving caspase activation, suggesting a process by which perturbation of the gut barrier could be exacerbated. Moreover, activation of TLR2 ameliorated such effects.

Aldosterone reduces crypt colon permeability during low-sodium adaptation

Fluid and electrolyte absorption by colonic crypts depends on the transport properties of crypt cellular and paracellular routes and of the pericryptal sheath. As a low-Na(+) diet increases aldosterone and angiotensin II secretion, either hormone could affect absorption. Control and adrenalectomized (ADX) Sprague-Dawley rats were kept at a high-NaCl (HS) diet and then switched to low-NaCl (LS) diet for 3 days. Aldosterone or angiotensin II plasma concentrations were maintained using implanted osmotic mini-pumps. The extracellular Na(+) concentration in isolated rat distal colonic mucosa was determined by confocal microscopy using a low-affinity Na(+) -sensitive fluorescent dye (Sodium red, and Na(+) -insensitive BODIPY) bound to polystyrene beads. Crypt permeability to FITC-labelled dextran (10 kDa) was monitored by its rate of escape from the crypt lumen into the pericryptal space. Mucosal ion permeability was estimated by transepithelial electrical resistance (TER) and amiloride-sensitive short-circuit current (SCC). The epithelial Na(+) channel, ENaC, was determined by immunolocalization. LS diet decreased crypt wall permeability to dextran by 10-fold and doubled TER. Following ADX, aldosterone decreased crypt wall dextran permeability, increased TER, increased Na(+) accumulation in the pericryptal sheath and ENaC expression even in HS. Infusion of angiotensin II to ADX rats did not reverse the effects of aldosterone deprivation. These findings indicate that aldosterone alone is responsible for both the increase in Na(+) absorption and the decreased paracellular and pericryptal sheath permeability.

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.

Growth and characterisation of primary bovine colon epithelial cells in vitro

Epithelial crypts from the bovine colon were obtained by using a combined mechanical and enzymatic isolation method, followed by differential D-sorbitol gradient centrifugation. By using this isolation technique, a pure fraction of epithelial crypts with minimal mesenchymal contamination was obtained. The crypts were seeded in collagen-coated plastic flasks. The attached epithelial cells proliferated and formed a confluent monolayer after 6 days in culture. Under low-serum culture conditions (1% fetal calf serum), the cells had a population doubling time of 21-22 hours. During the culture period, the colonocytes were characterised morphologically and enzymatically. The morphology of the cultured cells was confirmed by scanning electron microscopy and transmission electron microscopy. The presence of microvilli, tight junctions and desmosomes demonstrated the ability of the cultured cells to restore an epithelial-like cell monolayer. The epithelial origin of the cells was demonstrated by labelling the cells with antibodies against epithelial-specific cytokeratins 7 and 13. The functional integrity of the cells was evaluated by measuring various marker enzymes (gamma-glutamyltranspeptidase, acid phosphatase, alkaline phosphatase, NADH-dehydrogenase) and membrane-associated Na+-K+-ATPase activity. Membrane integrity was determined by measuring the leakage of lactate dehydrogenase into the culture medium. This new culture system for bovine colon epithelial cells could be used as an in vitro model of the colon epithelium in physiological and toxicological studies.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Regional crypt function in rat large intestine in relation to fluid absorption and growth of the pericryptal sheath

1. Confocal microscopic studies of rat colonic mucosa showed that the pericryptal sheath surrounding distal colonic crypts is an effective barrier both to dextran and NaCl movement, whereas no such structure surrounds the caecal crypts. 2. The distal colonic pericryptal barrier was functionally demonstrated by accumulation of Sodium Green within the pericryptal space. After exposure to benzamil, Sodium Green accumulation was decreased. Fluorescein isocyanate-labelled dextran (FITC dextran; molecular mass 10000 Da) was accumulated in the crypt lumens and pericryptal spaces. Both dextran and Sodium Green accumulation were absent from the pericryptal zone surrounding caecal crypts. 3. Low dietary Na+ intake raised rat plasma aldosterone and stimulated distal pericryptal sheath growth and adhesiveness as shown by increased amounts of F-actin, smooth muscle actin, beta-catenin and E-cadherins in the pericryptal zone. It also raised the capacity of the distal colon to dehydrate against a high luminal hydraulic resistance. This linkage indicates that trophic effects on the colon resulting from a low Na+ diet are not confined solely to effects on transepithelial Na+ transport, but are observed in the pericryptal sheath. 4. A computer model of crypt function confirms that a pericryptal sheath with low permeability to NaCl is an essential component of the crypt dehydrating mechanism.

Activation of DeltaF508 CFTR in an epithelial monolayer

The DeltaF508 mutation leads to retention of cystic fibrosis transmembrane conductance regulator (CFTR) in the endoplasmic reticulum and rapid degradation by the proteasome and other proteolytic systems. In stably transfected LLC-PK1 (porcine kidney) epithelial cells, DeltaF508 CFTR conforms to this paradigm and is not present at the plasma membrane. When LLC-PK1 cells or human nasal polyp cells derived from a DeltaF508 homozygous patient are grown on plastic dishes and treated with an epithelial differentiating agent (DMSO, 2% for 4 days) or when LLC-PK1 cells are grown as polarized monolayers on permeable supports, plasma membrane DeltaF508 CFTR is significantly increased. Moreover, when confluent LLC-PK1 cells expressing DeltaF508 CFTR were treated with DMSO and mounted in an Ussing chamber, a further increase in cAMP-activated short-circuit current (i.e., approximately 7 microA/cm2; P < 0.00025 compared with untreated controls) was observed. No plasma membrane CFTR was detected after DMSO treatment in nonepithelial cells (mouse L cells) expressing DeltaF508 CFTR. The experiments describe a way to augment DeltaF508 CFTR maturation in epithelial cells that appears to act through a novel mechanism and allows insertion of functional DeltaF508 CFTR in the plasma membranes of transporting cell monolayers. The results raise the possibility that increased epithelial differentiation might increase the delivery of DeltaF508 CFTR from the endoplasmic reticulum to the Golgi, where the DeltaF508 protein is shielded from degradative pathways such as the proteasome and allowed to mature.