Asymmetric signal transduction in polarized ileal Na(+)-absorbing cells: carbachol activates brush-border but not basolateral-membrane PIP2-PLC and translocates PLC-gamma 1 only to the brush border

A Review on Experimental Models to Test Medicinal Plants on Postprandial Blood Glucose in Diabetes

Due to the gravity of postprandial hyperglycemia in the development of microvascular and macrovascular diseases in diabetics, many medicinal plants are tested to determine their effectiveness in glycemic control and the mechanisms of action of the products. Consequently, various diabetic models have been developed and enhanced over the years. The objective of this review is to describe some of the experimental models to study the effect of medicinal plants used to control postprandial hyperglycemia. Data was collected from PubMed, ScienceDirect, Scopus, and Google scholar (1953-2021). Fiftyseven (57) studies were included in this review article. Ten models were identified and described. For each model, we described the targets involved and their roles in postprandial blood glucose control. The experimental design and procedures described the targets such as an α-glucosidase enzyme, SGLT1, GLUT2, DPP-IV, Na+/K+ ATPase pump, or intestinal motility in the models, experiment design and procedures were described. This review will facilitate the selection of the most appropriate model for studying agents used to investigate postprandial blood glucose.

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

Effect of acute acid-base disturbances on the phosphorylation of phospholipase C-γ1 and Erk1/2 in the renal proximal tubule

The renal proximal tubule (PT) plays a major role in whole-body pH homeostasis by secreting H(+) into the tubule lumen. Previous work demonstrated that PTs respond to basolateral changes in [CO2] and [HCO3-] by appropriately altering H(+) secretion-responses blocked by the ErbB inhibitor PD168393, or by eliminating signaling through AT1 angiotensin receptors. In the present study, we analyze phosphorylation of three downstream targets of both ErbBs and AT1: phospholipase C-γ1 (PLC-γ1), extracellular-regulated kinase 1 (Erk1), and Erk2. We expose rabbit PT suspensions for 5 and 20 min to our control (Ctrl) condition (5% CO2, 22 mmol/L HCO3-, pH 7.40) or one of several conditions that mimic acid-base disturbances. We found that each disturbance produces characteristic phosphorylation patterns in the three enzymes. For example, respiratory acidosis (elevated [CO2], normal [HCO3-]) at 20 min decreases PLC-γ1 phosphorylation at tyrosine-783 (relative to Ctrl). Metabolic acidosis (normal [CO2], decreased [HCO3-]) for 5 min increases Erk1 phosphorylation (p-Erk1) but not p-Erk2, whereas metabolic alkalosis (normal [CO2], elevated [HCO3-]) for 5 min decreases p-Erk1 and p-Erk2. In the presence of CO2/HCO3-, PD168393 blocks only two of eight induced decreases in phosphorylation. In two cases in which disturbances have no remarkable effects on phosphorylation, PD168393 unmasks decreases and in two others, increases. These drug effects provide insight into the roles of PD168393-sensitive kinases. Our results indicate that PLC-γ1.pY783, p-Erk1, and p-Erk2 in the PT change in characteristic ways in response to acute acid-base disturbances, and thus presumably contribute to the transduction of acid-base signals.

Carbachol-mediated endocytosis of NHE3 involves a clathrin-independent mechanism requiring lipid rafts and Cdc42

BACKGROUND

In intestinal epithelial cells, acute regulation of the brush border Na(+)/H(+) exchanger, NHE3, usually occurs by changes in endocytosis and/or exocytosis. Constitutive NHE3 endocytosis involves clathrin. Carbachol (CCH), which elevates intracellular Ca(2+) ([Ca(2+)]i), decreases NHE3 activity and stimulates endocytosis; however, the mechanism involved in calcium-mediated endocytosis of NHE3 is unclear. A pool of NHE3 resides in lipid rafts, which contributes to basal, but not cAMP-mediated, NHE3 trafficking, suggesting that an alternative mechanism exists for NHE3 endocytosis. Cdc42 was demonstrated to play an integral role in some cases of cholesterol-sensitive, clathrin-independent endocytosis. Therefore, the current study was designed to test the hypotheses that (1) clathrin-mediated endocytosis (CME) is involved in constitutive, but not CCH-mediated, endocytosis of NHE3, and (2) CCH-mediated endocytosis of NHE3 occurs through a lipid raft, activated Cdc42-dependent pathway that does not involve clathrin.

METHODS

The role of Cdc42 and lipid rafts on NHE3 activity and endocytosis were investigated in polarized Caco-2/BBe cells using pharmacological and shRNA knockdown approaches.

RESULTS

Basal NHE3 activity was increased in the presence of CME blockers (chlorpromazine; K(+) depletion) supporting previous reports that constitutive NHE3 endocytosis is clathrin dependent. In contrast, CCH-inhibition of NHE3 activity was abolished in Caco-2/BBe cells treated with MβCD (to disrupt lipid rafts) as well as in Cdc42 knockdown cells but was unaffected by CME blockers.

CONCLUSION

CCH-mediated inhibition of NHE3 activity is not dependent on clathrin and involves lipid rafts and requires Cdc42.

PLC-γ directly binds activated c-Src, which is necessary for carbachol-mediated inhibition of NHE3 activity in Caco-2/BBe cells

Elevated levels of intracellular Ca(2+) ([Ca(2+)]i) inhibit Na(+)/H(+) exchanger 3 (NHE3) activity in the intact intestine. We previously demonstrated that PLC-γ directly binds NHE3, an interaction that is necessary for [Ca(2+)]i inhibition of NHE3 activity, and that PLC-γ Src homology 2 (SH2) domains may scaffold Ca(2+) signaling proteins necessary for regulation of NHE3 activity. [Ca(2+)]i regulation of NHE3 activity is also c-Src dependent; however, the mechanism by which c-Src is involved is undetermined. We hypothesized that the SH2 domains of PLC-γ might link c-Src to NHE3-containing complexes to mediate [Ca(2+)]i inhibition of NHE3 activity. In Caco-2/BBe cells, carbachol (CCh) decreased NHE3 activity by ∼40%, an effect abolished with the c-Src inhibitor PP2. CCh treatment increased the amount of active c-Src as early as 1 min through increased Y(416) phosphorylation. Coimmunoprecipitation demonstrated that c-Src associated with PLC-γ, but not NHE3, under basal conditions, an interaction that increased rapidly after CCh treatment and occurred before the dissociation of PLC-γ and NHE3 that occurred 10 min after CCh treatment. Finally, direct binding to c-Src only occurred through the PLC-γ SH2 domains, an interaction that was prevented by blocking the PLC-γ SH2 domain. This study demonstrated that c-Src 1) activity is necessary for [Ca(2+)]i inhibition of NHE3 activity, 2) activation occurs rapidly (∼1 min) after CCh treatment, 3) directly binds PLC-γ SH2 domains and associates dynamically with PLC-γ under elevated [Ca(2+)]i conditions, and 4) does not directly bind NHE3. Under elevated [Ca(2+)]i conditions, PLC-γ scaffolds c-Src into NHE3-containing multiprotein complexes before dissociation of PLC-γ from NHE3 and subsequent endocytosis of NHE3.

NHE3 regulatory complexes

The epithelial brush border Na/H exchanger NHE3 is active under basal conditions and functions as part of neutral NaCl absorption in the intestine and renal proximal tubule, where it accounts for the majority of total Na absorbed. NHE3 is highly regulated. Both stimulation and inhibition occur post-prandially. This digestion related regulation of NHE3 is mimicked by multiple extracellular agonists and intracellular second messengers. The regulation of NHE3 depends on its C-terminal cytoplasmic domain, which acts as a scaffold to bind multiple regulatory proteins and links NHE3 to the cytoskeleton. The cytoskeletal association occurs by both direct binding to ezrin and by indirect binding via ezrin binding to the C-terminus of the multi-PDZ domain containing proteins NHERF1 and NHERF2. This is a review of the domain structure of NHE3 and of the scaffolding function and role in the regulation of NHE3 of the NHE3 C-terminal domain.

Regulation of intestinal electroneutral sodium absorption and the brush border Na+/H+ exchanger by intracellular calcium

The intestinal electroneutral Na(+) absorptive processes account for most small intestinal Na(+) absorption in the period between meals and also for the great majority of the increase in ileal Na(+) absorption that occurs postprandially. In most diarrheal diseases, there is inhibition of neutral NaCl absorption. Elevated levels of intracellular calcium ([Ca(2+)](i)) are known to inhibit NaCl absorption and involve multiple components of the Ca(2+) signaling pathway. The BB Na(+)/H(+) exchanger NHE3 accounts for most of the recognized digestive changes in neutral NaCl absorption, as well as most of the changes in Na(+) absorption that occur in diarrheal diseases. Previous studies have examined several aspects of Ca(2+) regulation of NHE3 activity. These include phosphorylation, protein trafficking, and multiprotein complex formation. In addition, recent studies have demonstrated the role of the NHERF family of PDZ domain-containing proteins in Ca(2+) regulation of NHE3 activity, thereby adding a new level of complexity to understanding Ca(2+)-dependent inhibition of Na(+) absorption. In this article, we will review the current understanding of (1) Ca(2+) signaling events in intestinal epithelial cells; (2) Ca(2+) regulation of intestinal electroneutral sodium absorption, which includes NHE3; and (3) the role of the NHERF family of PDZ domain-containing proteins in Ca(2+) regulation of NHE3 activity. We will also present new data on using advanced imaging showing rapid BB NHE3 endocytosis in response to elevated [Ca(2+)](i).

Phospholipase C-gamma binds directly to the Na+/H+ exchanger 3 and is required for calcium regulation of exchange activity

Multiple studies suggest that phospholipase C-gamma (PLC-gamma) contributes to regulation of sodium/hydrogen exchanger 3 (NHE3) in the small intestine, although the mechanism(s) for this regulation remain unknown. We demonstrate here that PLC-gamma binds directly to the C terminus of NHE3 and exists in similar sized multiprotein complexes as NHE3. This binding is dynamic and decreases with elevated [Ca(2+)](i). The PLC-gamma-binding site in NHE3 was identified (amino acids 586-605) and shown to be a critical regulatory domain for protein complex formation, because when it is mutated, NHE3 binding to PLC-gamma as well as NHERF2 is lost. An inhibitory peptide, which binds to the Src homology 2 domains contained in PLC-gamma without interrupting binding of PLC-gamma to NHE3, was used to probe a non-lipase-dependent role of PLC-gamma. In the presence of this peptide, carbachol-stimulated calcium inhibition of NHE3 was lost. These results mirror previous studies with the transient receptor potential channel and suggest that PLC-gamma may play a common role in regulating the cell-surface expression of ion transporters.

Reaction-diffusion modeling ERK- and STAT-interaction dynamics

The modeling of the dynamics of interaction between ERK and STAT signaling pathways in the cell needs to establish the biochemical diagram of the corresponding proteins interactions as well as the corresponding reaction-diffusion scheme. Starting from the verbal description available in the literature of the cross talk between the two pathways, a simple diagram of interaction between ERK and STAT5a proteins is chosen to write corresponding kinetic equations. The dynamics of interaction is modeled in a form of two-dimensional nonlinear dynamical system for ERK-and STAT5a -protein concentrations. Then the spatial modeling of the interaction is accomplished by introducing an appropriate diffusion-reaction scheme. The obtained system of partial differential equations is analyzed and it is argued that the possibility of Turing bifurcation is presented by loss of stability of the homogeneous steady state and forms dissipative structures in the ERK and STAT interaction process. In these terms, a possible scaffolding effect in the protein interaction is related to the process of stabilization and destabilization of the dissipative structures (pattern formation) inherent to the model of ERK and STAT cross talk.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

K+ channel inhibition accelerates intestinal epithelial cell wound healing

Restitution is the process by which superficial interruptions in the gastrointestinal mucosa are repaired by the flattening and spreading of epithelial cells surrounding the damage. During this process, mucosal epithelial cells undergo extensive reshaping and cytoskeletal remodeling. K(+) channels, located primarily on the basolateral surface of gut epithelial cells, are central to both actin polymerization, via their control of membrane potential, and cell volume regulation. We questioned whether K(+) channels are involved in restitution using an in vitro model of intestinal epithelium, monolayers of the human colon carcinoma cell line T84. We report that pharmacologic K(+) channel inhibition accelerates wound healing in T84 cell monolayers. Both Ca(++)-dependent and constitutively active channels are involved, as indicated by the sensitivity to clotrimazole, charybdotoxin, and barium. The ability of clotrimazole to accelerate wound resealing was also observed in Caco-2 cell sheets. Pharmacologic stimulation of K(+) channel activity had no effect on the repair rate. Analysis of the resealing process by time lapse and confocal microscopy revealed that K(+) channel inhibitors abolished the initial wound retraction, briefly accelerated the repair rate, and altered the shape of the cell sheet abutting the injury during the early phase of resealing. We hypothesize that K(+) channel inactivation interrupts the coregulation of f-actin polymerization and volume control that is initiated by the healing process.

Interleukin-1beta increases urine flow rate and inhibits protein expression of Na(+)/K(+)-ATPase in the rat jejunum and kidney

The effect of interleukin-1beta (IL-1beta) on urine flow rates and Na(+)/K(+)-ATPase activity and expression was studied in rat intestinal and renal epithelia. The cytokine produced a significant diuretic effect and increased urine flow rate by around 10-fold compared with the control. This effect was considered to be secondary to the well-documented natriuretic effect of the cytokine described in the literature. On the other hand, we have shown previously that IL-1beta inhibits glucose absorption from the jejunum. As sodium excretion and glucose absorption are both dependent on Na(+)/K(+)-ATPase activity, the effect of the cytokine on the renal and intestinal pump was investigated. IL-1beta inhibited, in a dose-dependent manner, the activity of Na(+)/K(+)-ATPase in villus and crypt jejunal cells and in medullary and cortical kidney cells. Western blot analysis revealed a decrease in the protein expression of the enzyme, which was confirmed by the radiolabeled ouabain binding assay. The results suggest that the diuretic and natriuretic effect of IL-1beta and its inhibitory effect on glucose absorption are all due to downregulation of the Na(+)/K(+) pump in the kidney and jejunum.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.

Differences in Ca(2+) signaling underlie age-specific effects of secretagogues on colonic Cl(-) transport

Taurodeoxycholic acid (TDC) stimulates Cl(-) transport in adult (AD), but not weanling (WN) and newborn (NB), rabbit colonic epithelial cells (colonocytes). The present study demonstrates that stimuli like neurotensin (NT) are also age specific and identifies the age-dependent signaling step. Bile acid actions are segment and bile acid specific. Thus although TDC and taurochenodeoxycholate stimulate Cl(-) transport in AD distal but not proximal colon, taurocholate has no effect in either segment. TDC increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in AD, but not in WN and NB, colonocytes. In AD cells, TDC (5 min) action on Cl(-) transport needs intra- but not extracellular Ca(2+). NT, histamine, and bethanechol increase Cl(-) transport and [Ca(2+)](i) in AD, but not WN, distal colonocytes. However, A-23187 increased [Ca(2+)](i) and Cl(-) transport in all age groups, suggesting that Ca(2+)-sensitive Cl(-) transport is present from birth. Study of the proximal steps in Ca(2+) signaling revealed that NT, but not TDC, activates a GTP-binding protein, Galpha(q), in AD and WN cells. In addition, although WN and AD colonocytes had similar levels of phosphatidylinositol 4,5-bisphosphate, NT and TDC increased 1,4,5-inositol trisphosphate content only in AD cells. Nonresponsiveness of WN cells to Ca(2+)-dependent stimuli, therefore, is due to the absence of measurable phospholipase C activity. Thus delays in Ca(2+) signaling afford a crucial protective mechanism to meet the changing demands of the developing colon.

Effect of cinnamon, clove and some of their constituents on the Na(+)-K(+)-ATPase activity and alanine absorption in the rat jejunum

The effect of a water extract of some spices on the in vitro activity of the rat jejunal Na(+)-K(+)-ATPase was investigated. Extracts of nutmeg, cinnamon, clove, cumin, coriander, turmeric and caraway all inhibited the ATPase, while anise seed and white pepper exerted no significant effects. The extracts of clove and cinnamon had the most potent inhibitory effect on the intestinal ATPase as compared to extracts of other spices. They also inhibited the in vitro Na(+)-K(+)-ATPase activity in a crude kidney homogenate and the activity of an isolated dog kidney Na(+)-K(+)-ATPase. The alcoholic extract of cinnamon, compared to the aqueous extract, had a stronger inhibitory action on the jejunal enzyme and a lower IC(50) value, which was not significantly different from the one observed with cinnamaldehyde, the major volatile oil present cinnamon, suggesting that in alcoholic extracts cinnamaldehyde is the major inhibitory component. The IC(50) values of eugenol, aqueous clove extract and ethanolic clove extract all fell within the same range and were not significantly different from each other, suggesting that eugenol is the major inhibitory component in both alcoholic and aqueous extracts. Based on the IC(50) values, the order of sensitivity of the enzyme to the spices extracts is as follows: isolated dog kidney ATPase>rat kidney ATPase>rat intestine ATPase. The aqueous extracts of clove and cinnamon also significantly lowered the absorption of alanine from the rat intestine. It was concluded that the active principle(s) in clove and cinnamon can permeate the membrane of the enterocytes and inhibit the Na(+)-K(+)-ATPase that provides the driving force for many transport processes.

The epithelial Na(+)/H(+) exchanger, NHE3, is internalized through a clathrin-mediated pathway

Trafficking of the Na(+)/H(+) exchanger isoform 3 (NHE3) between sub-apical vesicles and apical membrane of epithelial cells is a suggested mechanism of regulation of NHE3 activity. When epitope-tagged NHE3 was stably expressed in NHE-deficient Chinese hamster ovary cells, a sizable fraction was found in recycling endosomes. This system was used to analyze the mechanism of endocytosis of NHE3. Immunofluorescence and radiolabeling experiments showed that inhibition of clathrin-mediated endocytosis using hypertonicity, acid treatment, or K(+) depletion inhibited internalization of NHE3. Moreover, transient transfection of an inhibitory mutant of dynamin (DynS45N) blocked the clathrin-mediated uptake of transferrin, as well as the endocytosis of NHE3. In ileal villus cells, endogenous NHE3 was also found to co-purify with isolated clathrin-coated vesicles, thereby confirming their association in native tissues. The role of COP-I subunits in the intracellular traffic of NHE3 was evaluated using ldlF cells, which bear a temperature-sensitive mutation in the epsilon-COP subunit. At the permissive temperature, NHE3 distributed normally, whereas at the restrictive temperature, which induces rapid degradation of epsilon-COP, NHE3 was still internalized, but its subcellular distribution was altered. These results indicate that endocytosis of NHE3 occurs primarily via clathrin-coated pits and vesicles and that normal intracellular trafficking of NHE3 involves an epsilon-COP-dependent step.

Development of an endogenous epithelial Na(+)/H(+) exchanger (NHE3) in three clones of caco-2 cells

Expression of endogenous Na(+)/H(+) exchangers (NHEs) NHE3 and NHE1 at the apical (AP) and basolateral (BL) membrane domains was investigated in three clones (ATCC, PF-11, and TC-7) derived from the human adenocarcinoma cell line Caco-2. In all three clones, NHE1 was the only isoform detected at the BL domain during 3 to 22 postconfluent days (PCD). In clone PF-11, the BL NHE1 activity increased up to 7 PCD and remained stable thereafter. Both NHE1 and NHE3 were found at the AP domain at 3 PCD and contributed 67 and 33% to the total AP Na(+)/H(+) exchange, respectively. The AP NHE3 activity increased significantly from 3 to 22 PCD, from 93 to 450 microM H(+)/s, whereas AP NHE1 activity decreased from 192 to 18 microM H(+)/s during that time. Similar results were obtained with the ATCC clone, whereas very little AP NHE3 activity was observed in clone TC-7. Surface biotinylation and indirect immunofluorescence confirmed these results and also suggested an increase in the number of cells expressing NHE3 being the major mechanism of the observed overall increase in NHE3 activity in PF-11 and ATCC clones. Phorbol 12-myristate 13-acetate (PMA, 1 microM) acutely inhibited NHE3 activity by 28% of control, whereas epidermal growth factor (EGF, 200 ng/ml) stimulated the activity by 18%. The effect of PMA was abolished by the protein kinase C (PKC) inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, suggesting involvement of PKC in the PMA-induced inhibition of NHE3. Similar magnitude of inhibition by PMA and stimulation by EGF was observed at 7 and 17 PCD, suggesting the development of regulatory mechanisms in the early postconfluent period. Taken together, these data suggest a close similarity of membrane targeting and regulation of endogenous NHE3 between Caco-2 cells and native small intestinal epithelial cells and support the usefulness of some Caco-2 cell clones as an in vitro model for studies on physiology of NHE3 in the intestinal epithelium.

NSP4 elicits age-dependent diarrhea and Ca(2+)mediated I(-) influx into intestinal crypts of CF mice

Homologous disruption of the murine gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) leads to the loss of cAMP-mediated ion transport. Mice carrying this gene defect exhibit meconium ileus at birth and gastrointestinal plugging during the neonatal period, both contributing to high rates of mortality. We investigated whether infectious mammalian rotavirus, the recently characterized rotaviral enterotoxin protein NSP4, or its active NSP4(114-135) peptide, can overcome these gastrointestinal complications in CF (CFTR(m3Bay) null mutation) mice. All three agents elicited diarrhea when administered to wild-type (CFTR(+/+)), heterozygous (CFTR(+/-)), or homozygous (CFTR(-/-)) 7- to 14-day-old mouse pups but were ineffective when given to older mice. The diarrheal response was accompanied by non-age-dependent intracellular Ca(2+) mobilization within both small and large intestinal crypt epithelia. Significantly, NSP4 elicited cellular I(-) influx into intestinal epithelial cells from all three genotypes, whereas both carbachol and the cAMP-mobilizing agonist forskolin failed to evoke influx in the CFTR(-/-) background. This unique plasma membrane halide permeability pathway was age dependent, being observed only in mouse pup crypts, and was abolished by either the removal of bath Ca(2+) or the transport inhibitor DIDS. These findings indicate that NSP4 or its active peptide may induce diarrhea in neonatal mice through the activation of an age- and Ca(2+)-dependent plasma membrane anion permeability distinct from CFTR. Furthermore, these results highlight the potential for developing synthetic analogs of NSP4(114-135) to counteract chronic constipation/obstructive bowel syndrome in CF patients.

G protein-coupled receptors in gastrointestinal physiology. III. Asymmetry in plasma membrane signal transduction: lessons from brush-border Na+/H+ exchangers

Signal transduction in epithelial cells adds another level of complexity to the signaling that occurs in symmetrical cells, in the form of the need to coordinate and keep separate signals at the apical and basolateral membranes. Regulation by protein kinases of ileal NaCl absorption and its component brush-border Na+/H+ exchanger are used as an example of how signaling in epithelial cells must deal with spatial localization of signals, protein-protein interactions, signaling molecules, and the involvement of the transport protein being regulated in collecting and focusing the signals generated at the receptor and beyond.

Ileal microvillar protein villin is tyrosine-phosphorylated and associates with PLC-gamma1. Role of cytoskeletal rearrangement in the carbachol-induced inhibition of ileal NaCl absorption

In ileal absorptive cells, carbachol inhibits NaCl absorption and its component brush border Na+/H+ exchanger, acting via basolateral membrane receptors. This carbachol effect involves (i) activation of brush border phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PLC) activity and brush border but not basolateral membrane translocation of PLC-gamma1 (Khurana, S., Kreydiyyeh, S., Aronzon, A., Hoogerwerf, W. A., Rhee, S. G., Donowitz, M., and Cohen, M. E. (1996) Biochem. J. 313, 509-518); and (ii) brush border tyrosine kinase(s) because mucosal but not serosal addition of the tyrosine kinase inhibitor genistein prevents the carbachol-induced inhibition of NaCl absorption and brush border Na+/H+ exchange. In the present work we identify a pool of villin (a brush border actin-binding protein) in the microvillus membrane fraction of rabbit ileum; this pool of villin is tyrosine-phosphorylated and associates with brush border membrane PLC-gamma1. Villin is present both in the Triton X-100-soluble and -insoluble fractions of the brush border. The Triton X-100-soluble pool is approximately 4-fold smaller than the brush border pool of villin that is present in the Triton X-100-insoluble fraction. Only the villin present in the Triton X-100-soluble fraction of ileal villus brush border associates with PLC-gamma1 and is tyrosine-phosphorylated. Carbachol increases the tyrosine phosphorylation of villin rapidly (as early as 30 s) and transiently. Carbachol also increases the amount of tyrosine-phosphorylated villin that associates with PLC-gamma1. These studies demonstrate that carbachol effects on NaCl absorption are accompanied by an increase in brush border PLC-gamma1 association with villin and an increase in tyrosine phosphorylation of villin. To study the role of cytoskeletal rearrangement in carbachol-induced inhibition of NaCl absorption, we used the F-actin stabilizing drug jasplakinolide. Jasplakinolide prevents the carbachol inhibition of ileal NaCl absorption. This suggests that F-actin severing is necessary for carbachol to inhibit ileal villus NaCl absorption. Since villin is known to sever actin, these studies suggest a role for villin in the signaling cascade that begins at the basolateral membrane with carbachol binding to its receptor and ends at the apical membrane in inhibition of NaCl absorption.

Bacteroides fragilis toxin exhibits polar activity on monolayers of human intestinal epithelial cells (T84 cells) in vitro

Strains of Bacteroides fragilis associated with diarrhea in children (termed enterotoxigenic B. fragilis, or ETBF) produce a heat-labile ca. 20-kDa protein toxin (BFT). The purpose of this study was to examine the activity of BFT on polarized monolayers of human intestinal epithelial cells (T84 cells). In Ussing chambers, BFT had two effects. First, BFT applied to either the apical or basolateral surfaces of T84 monolayers diminished monolayer resistance. However, the time course, magnitude, and concentration dependency differed when BFT was applied to the apical versus basolateral membranes. Second, only basolateral BFT stimulated a concentration-dependent and short-lived increase in short circuit current (Isc; indicative of C1- secretion). Time course experiments indicated that Isc returned to baseline as resistance continued to decrease, indicating that these two electrophysiologic responses to BFT are distinct. Light microscopic studies of BFT-treated monolayers revealed only localized cellular changes after apical BFT, whereas basolateral BFT rapidly altered the morphology of nearly every cell in the monolayer. Transmission and scanning electron microscopy after basolateral BFT confirmed a striking loss of cellular microvilli and complete dissolution of some tight junctions (zonula occludens) and zonula adherens without loss of desmosomes. The F-actin structure of BFT-treated monolayers (stained with rhodamine-phalloidin) revealed diminished and flocculated staining at the apical tight junctional ring and thickening of F-actin microfilaments in focal contacts at the basolateral monolayer surface compared to those in similarly stained control monolayers. BFT did not injure T84 monolayers, as assessed by lactic dehydrogenase release and protein synthesis assays. These studies indicate that BFT is a nonlethal toxin which acts in a polar manner on T84 monolayers to stimulate C1- secretion and to diminish monolayer resistance by altering the apical F-actin structure of these cells. BFT may contribute to diarrheal disease associated with ETBF infection by altering epithelial barrier function and stimulating C1- secretion.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Brush border phosphatidylinositol 3-kinase mediates epidermal growth factor stimulation of intestinal NaCl absorption and Na+/H+ exchange

In terminally differentiated ileal villus Na+-absorptive cells, epidermal growth factor (EGF) stimulates NaCl absorption and its component brush border Na+/H+ exchanger, acting via basolateral membrane receptors, and as we confirm here, a brush border tyrosine kinase. In the present study we show that brush border phosphatidylinositol 3-kinase (PI 3-kinase) is involved in EGF stimulation of NaCl absorption and brush border Na+/H+ exchange. In rabbit ileum studied with the Ussing chamber-voltage clamp technique, EGF stimulation of active NaCl absorption is inhibited by the selective PI 3-kinase inhibitor wortmannin. PI 3-kinase, a largely cytosolic enzyme, translocates specifically to the brush border of ileal absorptive cells following EGF treatment. This translocation occurs as early as 1 min after EGF treatment and remains increased at the brush border for at least 15 min. EGF also causes a rapid (1 min) and large (4-5-fold) increase in brush border PI 3-kinase activity. Involvement of PI 3-kinase activity in intestinal Na+ absorption is established further by studies done in the human colon cancer cell line, Caco-2, stably transfected with the intestinal brush border isoform of the Na+/H+ exchanger, NHE3 (Caco-2/NHE3 cells). Brush border Na+/H+ exchange activity was measured using the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)5-(6)-carboxyfluorescein. EGF added to the basolateral surface but not apical surface of Caco-2/NHE3 cells increased brush border Na+/H+ exchange activity. The EGF-induced increase in brush border Na+/H+ exchange activity was completely abolished in cells pretreated with wortmannin. EGF treatment caused increased tyrosine phosphorylation of PI 3-kinase in both ileal brush border membranes and Caco-2/NHE3 cells, suggesting that a tyrosine kinase upstream of the PI 3-kinase is involved in the EGF effects on Na+ absorption. In conclusion, the present study provides evidence in two separate intestinal models, the ileum and a human colon cancer cell line, that PI 3-kinase is an intermediate in EGF stimulation of intestinal Na+ absorption.