Role of K(+) channel expression in polyamine-dependent intestinal epithelial cell migration

The metabolic impact of bacterial infection in the gut

Bacterial infections of the gut are one of the major causes of morbidity and mortality worldwide. The interplay between the pathogen and the host is finely balanced, with the bacteria evolving to proliferate and establish infection. In contrast, the host mounts a response to first restrict and then eliminate the infection. The intestine is a rapidly proliferating tissue, and metabolism is tuned to cater to the demands of proliferation and differentiation along the crypt-villus axis (CVA) in the gut. As bacterial pathogens encounter the intestinal epithelium, they elicit changes in the host cell, and core metabolic pathways such as the tricarboxylic acid (TCA) cycle, lipid metabolism and glycolysis are affected. This review highlights the mechanisms utilized by diverse gut bacterial pathogens to subvert host metabolism and describes host responses to the infection.

STIM1 promotes IPEC-J2 porcine epithelial cell restitution by TRPC1 signaling

Intestinal epithelial restitution is partly dependent on cell migration, which reseals superficial wounding after injury. Here, we tested the hypothesis that stromal interaction molecule 1(STIM1) regulates porcine intestinal epithelial cell migration by activating transient receptor potential canonical 1 (TRPC1) signaling. Results showed that the knockdown of STIM1 repressed cell migration after wounding, reduced the protein concentration of STIM1 and TRPC1, and decreased the inositol trisphosphate (IP3) content in IPEC-J2 cells (p < 0.05). However, overexpression of STIM1 obtained opposite results (p < 0.05). The inhibition of TRPC1 activity by treatment with SKF96365 in cells overexpressing wild-type and mutant STIM1 attenuated the STIM1 overexpression-induced increase of cell migration, STIM1, TRPC1 and IP3 (p < 0.05). In addition, polyamine depletion caused by α-difluoromethylornithine (DFMO) resulted in the decrease of above-mentioned parameters, and exogenous polyamine could attenuate the negative effects of DFMO on IPEC-J2 cells (p < 0.05). Moreover, the overexpression of STIM1 could rescue cell migration, the protein level of STIM1 and TRPC1, and IP3 content in polyamine-deficient IPEC-J2 cells (p < 0.05). These results indicated that STIM1 could enhance porcine intestinal epithelial cell migration via the TRPC1 signaling pathway. Inhibition of cell migration by polyamine depletion resulted from the reduction of STIM1 activity.

Effects of spermine on the proliferation and migration of porcine intestinal epithelial cells

Whether spermine promotes the repair of porcine intestinal epithelium damage through Ras-related C3 botulinum toxin substrate 1 (Rac1)/phospholipase C-γ1 (PLC-γ1) signaling remains unclear. The current study investigated the effects of spermine addition on the proliferation and migration of IPEC-J2 cells and the effects of Rac1/PLC-γ1 signaling on cell migration. We showed that the inhibitors of Rac1 (NSC-23766) and PLC-γ1 (U73122) reduced cell migration and decreased the protein levels of Rac1 and PLC-γ1 in the cells. Moreover, spermine promoted the proliferation and migration of the IPEC-J2 cells, that is, 1 µM spermine exhibited the best effect, and spermine treatment increased the protein levels of Rac1 and PLC-γ1. Further experiments showed that spermine treatment increased cell migration and enhanced Rac1 and PLC-γ1 protein levels, compared with NSC-23766 and U73122 treatments with spermine. In conclusion, spermine treatment promoted the repair of damaged porcine intestinal epithelium by accelerating cell proliferation and migration mediated by Rac1/PLC-γ1 signaling.

Metabolomic Profile of Weaned Pigs Challenged with E. coli and Supplemented with Carbadox or Bacillus subtilis

This study explored the metabolomic profiles in ileal mucosa and colon digesta in response to enterotoxigenic Escherichia coli F18 (ETEC) infection and dietary use of probiotics and low-dose antibiotics. Weaned pigs (n = 48, 6.17 ± 0.36 kg body weight) were randomly allotted to one of four treatments. Pigs in the negative control (NC) were fed a basal diet without ETEC challenge, whereas pigs in the positive control (PC), antibiotic, and probiotic groups were fed the basal diet, basal diet supplemented with 50 mg/kg of carbadox, or 500 mg/kg of Bacillus subtilis, respectively, and orally challenged with ETEC F18. All pigs were euthanized at day 21 post-inoculation to collect ileal mucosa and colon digesta for untargeted metabolomic profiling using gas chromatography coupled with time-of-flight mass spectrometry. Multivariate analysis highlighted a more distinct metabolomic profile of ileal mucosa metabolites in NC compared to the ETEC-challenged groups. The relative abundance of 19 metabolites from the ileal mucosa including polyamine, nucleotide, monosaccharides, fatty acids, and organic acids was significantly different between the NC and PC groups (q < 0.1). In colon digesta, differential metabolites including 2-monoolein, lactic acid, and maltose were reduced in the carbadox group compared with the probiotics group. In conclusion, several differential metabolites and metabolic pathways were identified in ileal mucosa, which may suggest an ongoing intestinal mucosal repair in the ileum of ETEC-challenged pigs on day 21 post-inoculation.

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

Regulatory role of l-proline in fetal pig growth and intestinal epithelial cell proliferation

l-proline (Pro) is a precursor of ornithine, which is converted into polyamines via ornithine decarboxylase (ODC). Polyamines plays a key role in the proliferation of intestinal epithelial cells. The study investigated the effect of Pro on polyamine metabolism and cell proliferation on porcine enterocytes in vivo and in vitro. Twenty-four Huanjiang mini-pigs were randomly assigned into 1 of 3 groups and fed a basal diet that contained 0.77% alanine (Ala, iso-nitrogenous control), 1% Pro or 1% Pro + 0.0167% α-difluoromethylornithine (DFMO) from d 15 to 70 of gestation. The fetal body weight and number of fetuses per litter were determined, and the small and large intestines were obtained on d 70 ± 1.78 of gestation. The in vitro study was performed in intestinal porcine epithelial (IPEC-J2) cells cultured in Dulbecco's modified Eagle medium-high glucose (DMEM-H) containing 0 μmol/L Pro, 400 μmol/L Pro, or 400 μmol/L Pro + 10 mmol/L DFMO for 4 d. The results showed that maternal dietary supplementation with 1% Pro increased fetal weight; the protein and DNA concentrations of the fetal small intestine; and mRNA levels for potassium voltage-gated channel, shaker-related subfamily, member 1 (Kv1.1) in the fetal small and large intestines (P < 0.05). Supplementing Pro to either gilts or IPEC-J2 cells increased ODC protein abundances and polyamine concentrations in the fetal intestines and IPEC-J2 cells (P < 0.05). In comparison with the Pro group, the combined administration of Pro and DFMO reduced the expression of ODC protein and spermine concentration in the fetal intestine, as well as the concentrations of putrescine, spermidine and spermine in IPEC-J2 cells (P < 0.05). Meanwhile, the percentage of cells in the S-phase and the mRNA levels of proto-oncogenes c-fos and c-myc were increased in response to Pro supplementation, whereas depletion of cellular polyamines with DFMO increased tumor protein p53 (p53) mRNA levels (P < 0.05). Taken together, dietary supplementation with Pro improved fetal pig growth and intestinal epithelial cell proliferation via enhancing polyamine synthesis.

Atractylenolide I stimulates intestinal epithelial repair through polyamine-mediated Ca2+ signaling pathway

BACKGROUND

An impairment of the integrity of the mucosal epithelial barrier can be observed in the course of various gastrointestinal diseases. The migration and proliferation of the intestinal epithelial (IEC-6) cells are essential repair modalities to the healing of mucosal ulcers and wounds. Atractylenolide I (AT-I), one of the major bioactive components in the rhizome of Atractylodes macrocephala Koidz. (AMR), possesses multiple pharmacological activities. This study was designed to investigate the therapeutic effects and the underlying molecular mechanisms of AT-I on gastrointestinal mucosal injury.

METHODS

Scratch method with a gel-loading microtip was used to detect IEC-6 cell migration. The real-time cell analyzer (RTCA) system was adopted to evaluate IEC-6 cell proliferation. Intracellular polyamines content was determined using high performance liquid chromatography (HPLC). Flow cytometry was used to measure cytosolic free Ca²⁺ concentration ([Ca²⁺]_c). mRNA and protein expression of TRPC1 and PLC-γ₁ were determined by real-time PCR and Western blotting assay respectively.

RESULTS

Treatment of IEC-6 cells with AT-I promoted cell migration and proliferation, increased polyamines content, raised cytosolic free Ca²⁺ concentration ([Ca²⁺]_c), and enhanced TRPC1 and PLC-γ₁ mRNA and protein expression. Depletion of cellular polyamines by DL-a-difluoromethylornithine (DFMO, an inhibitor of polyamine synthesis) suppressed cell migration and proliferation, decreased polyamines content, and reduced [Ca²⁺]_c, which was paralleled by a decrease in TRPC1 and PLC-γ₁ mRNA and protein expression in IEC-6 cells. AT-I reversed the effects of DFMO on polyamines content, [Ca²⁺]_c, TRPC1 and PLC-γ₁ mRNA and protein expression, and restored IEC-6 cell migration and proliferation to near normal levels.

CONCLUSION

Our data demonstrate that AT-I stimulates intestinal epithelial cell migration and proliferation via the polyamine-mediated Ca²⁺ signaling pathway. Therefore, AT-I may have the potential to be further developed as a promising therapeutic agent to treat diseases associated with gastrointestinal mucosal injury, such as inflammatory bowel disease and peptic ulcer.

c-Jun enhances intestinal epithelial restitution after wounding by increasing phospholipase C-γ1 transcription

c-Jun is an activating protein 1 (AP-1) transcription factor and implicated in many aspects of cellular functions, but its exact role in the regulation of early intestinal epithelial restitution after injury remains largely unknown. Phospholipase C-γ1 (PLCγ1) catalyzes hydrolysis of phosphatidylinositol 4,5 biphosphate into the second messenger diacylglycerol and inositol 1,4,5 triphosphate, coordinates Ca²⁺ store mobilization, and regulates cell migration and proliferation in response to stress. Here we reported that c-Jun upregulates PLCγ1 expression and enhances PLCγ1-induced Ca²⁺ signaling, thus promoting intestinal epithelial restitution after wounding. Ectopically expressed c-Jun increased PLCγ1 expression at the transcription level, and this stimulation is mediated by directly interacting with AP-1 and CCAAT-enhancer-binding protein (C/EBP) binding sites that are located at the proximal region of the rat PLCγ1 promoter. Increased levels of PLCγ1 by c-Jun elevated cytosolic free Ca²⁺ concentration and stimulated intestinal epithelial cell migration over the denuded area after wounding. The c-Jun-mediated PLCγ1/Ca²⁺ signal also plays an important role in polyamine-induced cell migration after wounding because increased c-Jun rescued Ca²⁺ influx and cell migration in polyamine-deficient cells. These findings indicate that c-Jun induces PLCγ1 expression transcriptionally and enhances rapid epithelial restitution after injury by activating Ca²⁺ signal.

Estrogen Receptors in Regulating Cell Proliferation of Esophageal Squamous Cell Carcinoma: Involvement of Intracellular Ca2+ Signaling

Esophageal cancer is a deadly disease in the esophagus with a poor prognosis. Over 90 % of esophageal cancer is esophageal squamous cell carcinoma (ESCC) and its pathogenic mechanisms remain unclear. Epidemiology study found a strong gender difference with a sex ratio of 8-9:1 in favor of males, but the molecular mechanisms for so striking gender difference are poorly understood so far. In the present study, we demonstrated the expression of estrogen receptors in human ESCC cells. 17β-E2 but not 17α-E2 was found to dose-dependently suppress the cell proliferation of human ESCC cells, which was attenuated by estrogen receptor antagonist ICI1 82,780. Furthermore, 17β -E2 but not 17α-E2 10 nM markedly induced both intracellular Ca²⁺ release and extracellular Ca²⁺ entry into ESCC cells, which was again attenuated by estrogen receptor antagonist ICI182,780. Taken together, our data clearly demonstrate that estrogen exerts anti-proliferative action on human ESCC cells likely through estrogen receptor-Ca²⁺ signaling pathway, which may provide a reasonable explanation on the striking male predominance of ESCC.

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters

The absorption of glucose is electrogenic in the small intestinal epithelium. The major route for the transport of dietary glucose from intestinal lumen into enterocytes is the Na⁺/glucose cotransporter (SGLT1), although glucose transporter type 2 (GLUT2) may also play a role. The membrane potential of small intestinal epithelial cells (IEC) is important to regulate the activity of SGLT1. The maintenance of membrane potential mainly depends on the activities of cation channels and transporters. While the importance of SGLT1 in glucose absorption has been systemically studied in detail, little is currently known about the regulation of SGLT1 activity by cation channels and transporters. A growing line of evidence suggests that cytosolic calcium ([Ca²⁺]_cyt) can regulate the absorption of glucose by adjusting GLUT2 and SGLT1. Moreover, the absorption of glucose and homeostasis of Ca²⁺ in IEC are regulated by cation channels and transporters, such as Ca²⁺ channels, K⁺ channels, Na⁺/Ca²⁺ exchangers, and Na⁺/H⁺ exchangers. In this review, we consider the involvement of these cation channels and transporters in the regulation of glucose uptake in the small intestine. Modulation of them may be a potential strategy for the management of obesity and diabetes.

Inhibition of Kv channel expression by NSAIDs depolarizes membrane potential and inhibits cell migration by disrupting calpain signaling

Clinical use of non-steroidal anti-inflammatory drugs (NSAIDs) is well known to cause gastrointestinal ulcer formation via several mechanisms that include inhibiting epithelial cell migration and mucosal restitution. The drug-affected signaling pathways that contribute to inhibition of migration by NSAIDs are poorly understood, though previous studies have shown that NSAIDs depolarize membrane potential and suppress expression of calpain proteases and voltage-gated potassium (Kv) channel subunits. Kv channels play significant roles in cell migration and are targets of NSAID activity in white blood cells, but the specific functional effects of NSAID-induced changes in Kv channel expression, particularly on cell migration, are unknown in intestinal epithelial cells. Accordingly, we investigated the effects of NSAIDs on expression of Kv1.3, 1.4, and 1.6 in vitro and/or in vivo and evaluated the functional significance of loss of Kv subunit expression. Indomethacin or NS-398 reduced total and plasma membrane protein expression of Kv1.3 in cultured intestinal epithelial cells (IEC-6). Additionally, depolarization of membrane potential with margatoxin (MgTx), 40mM K(+), or silencing of Kv channel expression with siRNA significantly reduced IEC-6 cell migration and disrupted calpain activity. Furthermore, in rat small intestinal epithelia, indomethacin and NS-398 had significant, yet distinct, effects on gene and protein expression of Kv1.3, 1.4, or 1.6, suggesting that these may be clinically relevant targets. Our results show that inhibition of epithelial cell migration by NSAIDs is associated with decreased expression of Kv channel subunits, and provide a mechanism through which NSAIDs inhibit cell migration and may contribute to NSAID-induced gastrointestinal (GI) toxicity.

RhoA enhances store-operated Ca2+ entry and intestinal epithelial restitution by interacting with TRPC1 after wounding

Early mucosal restitution occurs as a consequence of epithelial cell migration to resealing of superficial wounds after injury. Our previous studies show that canonical transient receptor potential-1 (TRPC1) functions as a store-operated Ca(2+) channel (SOC) in intestinal epithelial cells (IECs) and plays an important role in early epithelial restitution by increasing Ca(2+) influx. Here we further reported that RhoA, a small GTP-binding protein, interacts with and regulates TRPC1, thus enhancing SOC-mediated Ca(2+) entry (SOCE) and epithelial restitution after wounding. RhoA physically associated with TRPC1 and formed the RhoA/TRPC1 complexes, and this interaction increased in stable TRPC1-transfected IEC-6 cells (IEC-TRPC1). Inactivation of RhoA by treating IEC-TRPC1 cells with exoenzyme C3 transferase (C3) or ectopic expression of dominant negative RhoA (DNMRhoA) reduced RhoA/TRPC1 complexes and inhibited Ca(2+) influx after store depletion, which was paralleled by an inhibition of cell migration over the wounded area. In contrast, ectopic expression of wild-type (WT)-RhoA increased the levels of RhoA/TRPC1 complexes, induced Ca(2+) influx through activation of SOCE, and promoted cell migration after wounding. TRPC1 silencing by transfecting stable WT RhoA-transfected cells with siRNA targeting TRPC1 (siTRPC1) reduced SOCE and repressed epithelial restitution. Moreover, ectopic overexpression of WT-RhoA in polyamine-deficient cells rescued the inhibition of Ca(2+) influx and cell migration induced by polyamine depletion. These findings indicate that RhoA interacts with and activates TRPC1 and thus stimulates rapid epithelial restitution after injury by inducing Ca(2+) signaling.

Complementary roles of KCa3.1 channels and β1-integrin during alveolar epithelial repair

BACKGROUND

Extensive alveolar epithelial injury and remodelling is a common feature of acute lung injury and acute respiratory distress syndrome (ARDS) and it has been established that epithelial regeneration, and secondary lung oedema resorption, is crucial for ARDS resolution. Much evidence indicates that K(+) channels are regulating epithelial repair processes; however, involvement of the KCa3.1 channels in alveolar repair has never been investigated before.

RESULTS

Wound-healing assays demonstrated that the repair rates were increased in primary rat alveolar cell monolayers grown on a fibronectin matrix compared to non-coated supports, whereas an anti-β1-integrin antibody reduced it. KCa3.1 inhibition/silencing impaired the fibronectin-stimulated wound-healing rates, as well as cell migration and proliferation, but had no effect in the absence of coating. We then evaluated a putative relationship between KCa3.1 channel and the migratory machinery protein β1-integrin, which is activated by fibronectin. Co-immunoprecipitation and immunofluorescence experiments indicated a link between the two proteins and revealed their cellular co-distribution. In addition, we demonstrated that KCa3.1 channel and β1-integrin membrane expressions were increased on a fibronectin matrix. We also showed increased intracellular calcium concentrations as well as enhanced expression of TRPC4, a voltage-independent calcium channel belonging to the large TRP channel family, on a fibronectin matrix. Finally, wound-healing assays showed additive effects of KCa3.1 and TRPC4 inhibitors on alveolar epithelial repair.

CONCLUSION

Taken together, our data demonstrate for the first time complementary roles of KCa3.1 and TRPC4 channels with extracellular matrix and β1-integrin in the regulation of alveolar repair processes.

Caveolin-1 enhances rapid mucosal restitution by activating TRPC1-mediated Ca2+ signaling

Early rapid mucosal restitution occurs as a consequence of epithelial cell migration to reseal superficial wounds, a process independent of cell proliferation. Our previous studies revealed that the canonical transient receptor potential-1 (TRPC1) functions as a store-operated Ca(2+) channel (SOCs) in intestinal epithelial cells (IECs) and regulates epithelial restitution after wounding, but the exact mechanism underlying TRPC1 activation remains elusive. Caveolin-1 (Cav1) is a major component protein that is associated with caveolar lipid rafts in the plasma membrane and was recently identified as a regulator of store-operated Ca(2+) entry (SOCE). Here, we showed that Cav1 plays an important role in the regulation of mucosal restitution by activating TRPC1-mediated Ca(2+) signaling. Target deletion of Cav1 delayed gastric mucosal repair after exposure to hypertonic NaCl in mice, although it did not affect total levels of TRPC1 protein. In cultured IECs, Cav1 directly interacted with TRPC1 and formed Cav1/TRPC1 complex as measured by immunoprecipitation assays. Cav1 silencing in stable TRPC1-transfected cells by transfection with siCav1 reduced SOCE without effect on the level of resting [Ca(2+)]cyt. Inhibition of Cav1 expression by siCav1 and subsequent decrease in Ca(2+) influx repressed epithelial restitution, as indicated by a decrease in cell migration over the wounded area, whereas stable ectopic overexpression of Cav1 increased Cav1/TRPC1 complex, induced SOCE, and enhanced cell migration after wounding. These results indicate that Cav1 physically interacts with and activates TRPC1, thus stimulating TRPC1-mediated Ca(2+) signaling and rapid mucosal restitution after injury.

Atractylodes macrocephala Koidz promotes intestinal epithelial restitution via the polyamine--voltage-gated K+ channel pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Atractylodes macrocephala Koidz (AMK) has been used widely as a digestive and tonic in traditional Chinese medicine. AMK has shown noteworthy promoting effect on intestinal epithelial cell migration, which might represent a promising candidate for the treatment of intestinal mucosa injury. The aim of this study was to investigate the efficacy of AMK on intestinal mucosal restitution and the underlying mechanisms via IEC-6 cell migration model.

MATERIALS AND METHODS

A wounding model of IEC-6 cells was induced by a single-edge razor blade along the diameter of six-well polystyrene plates. The cells were grown in control cultures and in cultures containing spermidine (5 μmol/L, SPD, reference drug), alpha-difluoromethylornithine (2.5 mmol/L, DFMO, polyamine inhibitor), AMK (50, 100, and 200 μg/mL), DFMO plus SPD and DFMO plus AMK for 24h. The membrane potential (MP) and cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) were detected by flow cytometry, and polyamines content was determined via high-performance liquid chromatography (HPLC). The expression of Kv1.1 mRNA and protein levels were assessed by RT-qPCR and Western blot analysis, respectively. Cell migration assay was carried out using the Image-Pro Plus software. All of these indexes were used to evaluate the effectiveness of AMK.

RESULTS

(1) Treatment with AMK caused significant increases in cellular polyamines content, membrane hyperpolarization, an elevation of [Ca(2+)]cyt and an acceleration of cell migration in IEC-6 cells, as compared to control group. (2) AMK not only reversed the inhibitory effects of DFMO on the polyamines content, MP, and [Ca(2+)]cyt but also restored IEC-6 cell migration to control levels. (3) The Kv1.1 mRNA and protein expression were significantly increased by AMK treatment in control and polyamine-deficient IEC-6 cells.

CONCLUSIONS

The results of our current studies revealed that treatment with AMK significantly stimulates the migration of intestinal epithelial cells through polyamine-Kv1.1 channel signaling pathway, which could promote the healing of intestinal injury. These results suggest the potential usefulness of AMK to cure intestinal disorders characterized by injury and ineffective repair of the intestinal mucosa.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Evidence of K+ channel function in epithelial cell migration, proliferation, and repair

Efficient repair of epithelial tissue, which is frequently exposed to insults, is necessary to maintain its functional integrity. It is therefore necessary to better understand the biological and molecular determinants of tissue regeneration and to develop new strategies to promote epithelial repair. Interestingly, a growing body of evidence indicates that many members of the large and widely expressed family of K(+) channels are involved in regulation of cell migration and proliferation, key processes of epithelial repair. First, we briefly summarize the complex mechanisms, including cell migration, proliferation, and differentiation, engaged after epithelial injury. We then present evidence implicating K(+) channels in the regulation of these key repair processes. We also describe the mechanisms whereby K(+) channels may control epithelial repair processes. In particular, changes in membrane potential, K(+) concentration, cell volume, intracellular Ca(2+), and signaling pathways following modulation of K(+) channel activity, as well as physical interaction of K(+) channels with the cytoskeleton or integrins are presented. Finally, we discuss the challenges to efficient, specific, and safe targeting of K(+) channels for therapeutic applications to improve epithelial repair in vivo.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca²+ signaling by differentially modulating STIM1 and STIM2

Early epithelial restitution occurs as a consequence of intestinal epithelial cell (IEC) migration after wounding, and its defective regulation is implicated in various critical pathological conditions. Polyamines stimulate intestinal epithelial restitution, but their exact mechanism remains unclear. Canonical transient receptor potential-1 (TRPC1)-mediated Ca(2+) signaling is crucial for stimulation of IEC migration after wounding, and induced translocation of stromal interaction molecule 1 (STIM1) to the plasma membrane activates TRPC1-mediated Ca(2+) influx and thus enhanced restitution. Here, we show that polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca(2+) signaling by altering the ratio of STIM1 to STIM2. Increasing cellular polyamines by ectopic overexpression of the ornithine decarboxylase (ODC) gene stimulated STIM1 but inhibited STIM2 expression, whereas depletion of cellular polyamines by inhibiting ODC activity decreased STIM1 but increased STIM2 levels. Induced STIM1/TRPC1 association by increasing polyamines enhanced Ca(2+) influx and stimulated epithelial restitution, while decreased formation of the STIM1/TRPC1 complex by polyamine depletion decreased Ca(2+) influx and repressed cell migration. Induced STIM1/STIM2 heteromers by polyamine depletion or STIM2 overexpression suppressed STIM1 membrane translocation and inhibited Ca(2+) influx and epithelial restitution. These results indicate that polyamines differentially modulate cellular STIM1 and STIM2 levels in IECs, in turn controlling TRPC1-mediated Ca(2+) signaling and influencing cell migration after wounding.

Activation of Wnt3a signaling stimulates intestinal epithelial repair by promoting c-Myc-regulated gene expression

In response to mucosal injury, epithelial cells modify the patterns of expressed genes to repair damaged tissue rapidly. Our previous studies have demonstrated that the transcription factor c-Myc is necessary for stimulation of epithelial cell renewal during mucosal healing, but the up-stream signaling initiating c-Myc gene expression after injury remains unknown. Wnts are cysteine-rich glycoproteins that act as short-range ligands to locally activate receptor-mediated signaling pathways and correlate with the increased expression of the c-Myc gene. The current study tested the hypothesis that Wnt3a signaling is implicated in intestinal epithelial repair after wounding by stimulating c-Myc expression. Elevated Wnt3a signaling in intestinal epithelial cells (IEC-6 line) by coculturing with stable Wnt3a-transfected fibroblasts or ectopic overexpression of the Wnt3a gene enhanced intestinal epithelial repair after wounding. This stimulatory effect on epithelial repair was prevented by silencing the Wnt coreceptor LRP6 or by c-Myc silencing. Activation of the Wnt3a signaling pathway increased β-catenin nuclear translocation by decreasing its phosphorylation and stimulated c-Myc expression during epithelial repair after wounding. In stable Wnt3a-transfected IEC-6 cells, increased levels of c-Myc were associated with an increase in expression of c-Myc-regulated genes cyclcin D1 and cyclin E, whereas c-Myc silencing inhibited expression of cyclin D1 and cyclin E and delayed epithelial repair. These results indicate that elevated Wnt3a signaling in intestinal epithelial cells after wounding stimulates epithelial repair by promoting c-Myc-regulated gene expression.

Role of myosin light chain kinase in cardiotrophin-1-induced cardiac myofibroblast cell migration

Chemotactic movement of myofibroblasts is recognized as a common means for their sequestration to the site of tissue injury. Following myocardial infarction (MI), recruitment of cardiac myofibroblasts to the infarct scar is a critical step in wound healing. Contractile myofibroblasts express embryonic smooth muscle myosin, α-smooth muscle actin, as well as collagens I and III. We examined the effects of cardiotrophin-1 (CT-1) in the induction of primary rat ventricular myofibroblast motility. Changes in membrane potential (Em) and Ca2+entry were studied to reveal the mechanisms for induction of myofibroblast migration. CT-1-induced cardiac myofibroblast cell migration, which was attenuated through the inhibition of JAK2 (25 μM AG490), and myosin light chain kinase (20 μM ML-7). Inhibition of K+channels (1 mM tetraethylammonium or 100 μM 4-aminopyridine) and nonselective cation channels by 10 μM gadolinium (Gd3+) significantly reduced migration in the presence of CT-1. CT-1 treatment caused a significant increase in myosin light chain phosphorylation, which could be inhibited by incubation in Ca2+-free conditions or by application of AG490, ML-7, and W7 (100 μM; calmodulin inhibitor). Monitoring myofibroblast membrane potential with potentiometric fluorescent DiBAC4( 3 ) dye revealed a biphasic response to CT-1 consisting of an initial depolarization followed by hyperpolarization. Increased intracellular Ca2+, as assessed by fluo 3, occurred immediately after membrane depolarization and attenuated at the time of maximal hyperpolarization. CT-1 exerts chemotactic effects via multiple parallel signaling modalities in ventricular myofibroblasts, including changes in membrane potential, alterations in intracellular calcium, and activation of a number of intracellular signaling pathways. Further study is warranted to determine the precise role of K+currents in this process.

Hypoxic preconditioning enhances bone marrow mesenchymal stem cell migration via Kv2.1 channel and FAK activation

Transplantation using stem cells including bone marrow mesenchymal stem cells (BMSCs) is emerging as a potential regenerative therapy after ischemic attacks in the heart and brain. The migration capability of transplanted cells is a critical cellular function for tissue repair. Based on our recent observations that hypoxic preconditioning (HP) has multiple benefits in improving stem cell therapy and that the potassium Kv2.1 channel acts as a promoter for focal adhesion kinase (FAK) activation and cell motility, the present investigation tested the hypothesis that HP treatment can increase BMSC migration via the mechanism of increased Kv2.1 expression and FAK activities. BMSCs derived from green fluorescent protein-transgenic mice were treated under either normoxic (N-BMSC) or hypoxic (0.5% O(2)) (HP-BMSC) conditions for 24 h. Western blot analysis showed HP selectively upregulated Kv2.1 expression while leaving other K(+) channels, such as Kv1.5 and Kv1.4, unaffected. Compared with normoxic controls, significantly larger outward delayed rectifier K(+) currents were recorded in HP-BMSCs. HP enhanced BMSC migration/homing activities in vitro and after intravenous transplantation into rats subjected to permanent myocardial infarction (MI). The HP-promoted BMSC migration was inhibited by either blocking K(+) channels or knocking down Kv2.1. Supporting a relationship among HP, Kv2.1, and FAK activation, HP increased phosphorylation of FAK(397) and FAK(576/577), and this effect was antagonized by blocking K(+) channels. These findings provide novel evidence that HP enhances the ability of BMSCs to migrate and home to the injured region; this effect is mediated through a regulatory role of Kv2.1 on FAK phosphorylation/activation.

Sphingosine-1-phosphate regulates the expression of adherens junction protein E-cadherin and enhances intestinal epithelial cell barrier function

BACKGROUND

The regulation of intestinal barrier permeability is important in the maintenance of normal intestinal physiology. Sphingosine-1-phosphate (S1P) has been shown to play a pivotal role in enhancing barrier function in several non-intestinal tissues. The current study determined whether S1P regulated function of the intestinal epithelial barrier by altering expression of E-cadherin, an important protein in adherens junctions.

METHODS

Studies were performed upon cultured differentiated IECs (IEC-Cdx2L1 line) using standard techniques.

RESULTS

S1P treatment significantly increased levels of E-cadherin protein and mRNA in intestinal epithelial cells (IECs) and also led to E-cadherin localizing strongly to the cell-cell border. S1P also improved the barrier function as indicated by a decrease in 14C-mannitol paracellular permeability and an increase in transepithelial electrical resistance (TEER) in vitro.

CONCLUSIONS

These results indicate that S1P increases levels of E-cadherin, both in cellular amounts and at the cell-cell junctions, and leads to improved barrier integrity in cultured intestinal epithelial cells.

STIM1 translocation to the plasma membrane enhances intestinal epithelial restitution by inducing TRPC1-mediated Ca2+ signaling after wounding

Early epithelial restitution is an important repair modality in the gut mucosa and occurs as a consequence of epithelial cell migration. Canonical transient receptor potential-1 (TRPC1) functions as a store-operated Ca2+ channel (SOCs) in intestinal epithelial cells (IECs) and regulates intestinal restitution, but the exact upstream signals initiating TRPC1 activation after mucosal injury remain elusive. Stromal interaction molecule 1 (STIM1) is a single membrane-spanning protein and is recently identified as essential components of SOC activation. The current study was performed to determine whether STIM1 plays a role in the regulation of intestinal epithelial restitution by activating TRPC1 channels. STIM1 translocation to the plasma membrane increased after wounding, which was followed by an increase in IEC migration to reseal wounds. Increased STIM1 levels at the plasma membrane by overexpressing EF-hand mutant STIM1 enhanced Ca2+ influx through SOCs and stimulated IEC migration after wounding. STIM1 interacted with TRPC1 and formed STIM1/TRPC1 complex, whereas inactivation of STIM1 by STIM1 silencing decreased SOC-mediated Ca2+ influx and inhibited epithelial restitution. In cells overexpressing EF-hand mutant STIM1, TRPC1 silencing also decreased STIM1/TRPC1 complex, reduced SOC-mediated Ca2+ influx, and repressed cell migration after wounding. Our findings demonstrate that induced STIM1 translocation to the plasma membrane promotes IEC migration after wounding by enhancing TRPC1-mediated Ca2+ signaling and provide new insight into the mechanism of intestinal epithelial restitution.

A simplified method for quantifying cell migration/wound healing in 96-well plates

Cell migration plays a key role in both normal physiological and pathological conditions. The study of cell migration and its underlying mechanisms is of great significance in various fields of research, including basic biology and pharmaceutical development. The cell migration or scratch wounding assay is an easy and economical in vitro method that allows researchers to assess a large number of testing compounds. Even though this simple assay has been used for decades, researchers are still trying to modify such experimental protocols and wounding devices. In this study, an 8-channel mechanical "wounder" was designed for performing a cell migration assay, particularly in a 96-well culture plate format. With special designs of a guiding bar and adjustable pins for use with disposable pipette tips, this wounder confined the scratch area within the center of each well to ensure a perfect contact between the pins and the well surface. As a result, this mechanical wounder produces a uniform denudation of a cell monolayer in a 96-well plate with a wound size of around 600 microm. Using this improved wounding device, the effects of epidermal growth factor and DL-alpha-difluoromethylornithine on the reepithelialization of rat intestinal epithelial cells (IEC-6) and serum on the wound recovery of human umbilical vein endothelial cells were demonstrated. This wounder facilitates cell migration study and can be applicable for multiple sample analysis.

Rac1 promotes intestinal epithelial restitution by increasing Ca2+ influx through interaction with phospholipase C-(gamma)1 after wounding

Intestinal mucosal restitution occurs as a consequence of epithelial cell migration and reseals superficial wounds after injury. This rapid reepithelialization is mediated in part by a phospholipase C-gamma1 (PLC-gamma1)-induced Ca(2+) signaling, but the exact mechanism underlying such signaling and its regulation remains elusive. The small GTP-binding protein Rac1 functions as a pivotal regulator of several signaling networks and plays an important role in regulating cell motility. The current study tests the hypothesis that Rac1 modulates intestinal epithelial cell migration after wounding by altering PLC-gamma1-induced Ca(2+) signaling. Inhibition of Rac1 activity by treatment with its inhibitor NSC-23766 or Rac1 silencing with small interfering RNA decreased store depletion-induced Ca(2+) influx and suppressed cell migration during restitution, whereas ectopic overexpression of Rac1 increased Ca(2+) influx and promoted cell migration. Rac1 physically interacted with PLC-gamma1 and formed Rac1/PLC-gamma1 complex in intestinal epithelial cells. PLC-gamma1 silencing in cells overexpressing Rac1 prevented stimulation of store depletion-induced Ca(2+) influx and cell migration after wounding. Polyamine depletion inhibited expression of both Rac1 and PLC-gamma1, decreased Rac1/PLC-gamma1 complex levels, reduced Ca(2+) influx, and repressed cell migration. Overexpression of Rac1 alone failed to rescue Ca(2+) influx after store depletion and cell migration in polyamine-deficient cells, because it did not alter PLC-gamma1 levels. These results indicate that Rac1 promotes intestinal epithelial cell migration after wounding by increasing Ca(2+) influx as a result of its interaction with PLC-gamma1.

Formation of Kv2.1-FAK complex as a mechanism of FAK activation, cell polarization and enhanced motility

Focal adhesion kinase (FAK) plays key roles in cell adhesion and migration. We now report that the delayed rectifier Kv2.1 potassium channel, through its LD-like motif in N-terminus, may interact with FAK and enhance phosphorylation of FAK(397) and FAK(576/577). Overlapping distribution of Kv2.1 and FAK was observed on soma and proximal dendrites of cortical neurons. FAK expression promotes a polarized membrane distribution of the Kv2.1 channel. In Kv2.1-transfected CHO cells, formation of the Kv2.1-FAK complex was stimulated by fibronectin/integrin and inhibited by the K(+) channel blocker tetraethylammonium (TEA). FAK phosphorylation was minimized by shRNA knockdown of the Kv2.1 channel, point mutations of the N-terminus, and TEA, respectively. Cell migration morphology was altered by Kv2.1 knockdown or TEA, hindering cell migration activity. In wound healing tests in vitro and a traumatic injury animal model, Kv2.1 expression and co-localization of Kv2.1 and FAK significantly enhanced directional cell migration and wound closure. It is suggested that the Kv2.1 channel may function as a promoting signal for FAK activation and cell motility.

K+ channels and the cAMP-PKA pathway modulate TGF-beta1-induced migration of rat vascular myofibroblasts

Our previous studies have indicated that TGF-beta1 exerts its effect on the expression of A-type potassium channels (I(A)) in rat vascular myofibroblasts by activation of protein kinase C during the phenotypic transformation of vascular fibroblasts to myofibroblasts. In the present study, patch-clamp whole-cell recording and transwell-migration assays were used to examine the effects of TGF-beta1- and phorbol 12-myristate 13-acetate (PMA)-induced expression of I(A) channels on myofibroblast migration and its modulation by the protein kinase A (PKA) pathway. Our results reveal that incubation of fibroblasts with TGF-beta1 or PMA up-regulates the expression of I(A) channels and increases myofibroblast migration. Blocking I(A) channel expression by 4-aminopyridine (4-AP) significantly inhibits TGF-beta1- and PMA-induced myofibroblast migration. Incubation of fibroblasts with forskolin does not result in increased expression of I(A) channels but does cause a slight increase in fibroblast migration at higher concentrations. In addition, forskolin increases the TGF-beta1- and PMA-induced myofibroblast migration but inhibits TGF-beta1- and PMA-induced the expression of I(A) channels. Whole-cell current recordings showed that forskolin augments the delayed rectifier outward K(+) (I(K)) current amplitude of fibroblasts, but not the I(A) of myofibroblasts. Our results also indicate that TGF-beta1- and PMA-induced expression of I(A) channels might be related to increase TGF-beta1- or PMA-induced myofibroblast migration. Promoting fibroblast and myofibroblast migration via the PKA pathway does not seem to involve the expression of I(A) channels, but the modulation of I(K) and I(A) channels might be implicated.

Polyamine synthesis inhibition induces S phase cell cycle arrest in vascular smooth muscle cells

Polyamines are important for cell growth and proliferation and they are formed from arginine and ornithine via arginase and ornithine decarboxylase (ODC). Arginine may alternatively be metabolised to NO via NO synthase. Here we study if vascular smooth muscle cell proliferation can be reversed by polyamine synthesis inhibitors and investigate their mechanism of action. Cell proliferation was assessed in cultured vascular smooth muscle A7r5 cells and in endothelium-denuded rat arterial rings by measuring [3H]-thymidine incorporation and by cell counting. Cell cycle phase distribution was determined by flow cytometry and polyamines by HPLC. Protein expression was determined by Western blotting. The ODC inhibitor DFMO (1-10 mM) reduced polyamine concentration and attenuated proliferation in A7r5 cells and rat tail artery. DFMO accumulated cells in S phase of the cell cycle and reduced cyclin A expression. DFMO had no effect on cell viability and apoptosis as assessed by fluorescence microscopy. Polyamine concentration and cellular proliferation were not affected by the arginase inhibitor NOHA (100-200 microM) and the NO synthase inhibitor L-NAME (100 microM). Lack of effect of NOHA was reflected by absence of arginase expression. Polyamine synthesis inhibition attenuates vascular smooth muscle cell proliferation by reducing DNA synthesis and accumulation of cells in S phase, and may be a useful approach to prevent vascular smooth muscle cell proliferation in cardiovascular diseases.

Induced TRPC1 expression increases protein phosphatase 2A sensitizing intestinal epithelial cells to apoptosis through inhibition of NF-kappaB activation

Transient receptor potential canonical-1 (TRPC1) functions as a store-operated Ca2+ channel in intestinal epithelial cells (IECs), and induced TRPC1 expression sensitizes IECs to apoptosis by inhibiting NF-kappaB activation. However, the exact mechanism by which increased TRPC1 results in NF-kappaB inactivation remains elusive. Protein phosphatase 2A (PP2A) is a widely conserved protein serine/threonine phosphatase that is implicated in the regulation of a wide array of cellular functions including apoptosis. The present study tests the hypothesis that induced TRPC1 expression inhibits NF-kappaB activation by increasing PP2A activity through Ca2+ influx in IECs. The expression of TRPC1 induced by stable transfection with the wild-type TRPC1 gene increased PP2A activity as indicated by increases in levels of PP2A proteins and their phosphatase activity. Increased levels of PP2A activity in stable TRPC1-transfected IEC-6 cells (IEC-TRPC1) were associated with decreased nuclear levels of NF-kappaB proteins and a reduction in NF-kappaB-dependent transcriptional activity, although there were no changes in total NF-kappaB protein levels. Inhibition of PP2A activity by treatment with okadaic acid or PP2A silencing with small interfering RNA not only enhanced NF-kappaB transactivation but also prevented the increased susceptibility of IEC-TRPC1 cells to apoptosis induced by treatment with tumor necrosis factor-alpha (TNF-alpha)/cycloheximide (CHX). Decreasing Ca2+ influx by exposure to the Ca2+-free medium reduced PP2A mRNA levels, destabilized PP2A proteins, and induced NF-kappaB activation, thus blocking the increased sensitivity of IEC-TRPC1 cells to TNF-alpha/CHX-induced apoptosis. These results indicate that induced TRPC1 expression increases PP2A activity through Ca2+ influx and that increased PP2A sensitizes IECs to apoptosis as a result of NF-kappaB inactivation.

Bradykinin-induced microglial migration mediated by B1-bradykinin receptors depends on Ca2+ influx via reverse-mode activity of the Na+/Ca2+ exchanger

Bradykinin (BK) is produced and acts at the site of injury and inflammation. In the CNS, migration of microglia toward the lesion site plays an important role pathologically. In the present study, we investigated the effect of BK on microglial migration. Increased motility of cultured microglia was mimicked by B1 receptor agonists and markedly inhibited by a B1 antagonist but not by a B2 receptor antagonist. BK induced chemotaxis in microglia isolated from wild-type and B2-knock-out mice but not from B1-knock-out mice. BK-induced motility was not blocked by pertussis toxin but was blocked by chelating intracellular Ca2+ or by low extracellular Ca2+, implying that Ca2+ influx is prerequisite. Blocking the reverse mode of Na+/Ca2+ exchanger (NCX) completely inhibited BK-induced migration. The involvement of NCX was further confirmed by using NCX+/- mice; B1-agonist-induced motility and chemotaxis was decreased compared with that in NCX+/+ mice. Activation of NCX seemed to be dependent on protein kinase C and phosphoinositide 3-kinase, and resultant activation of intermediate-conductance (IK-type) Ca2+-dependent K+ currents (I(K(Ca))) was activated. Despite these effects, BK did not activate microglia, as judged from OX6 staining. Using in vivo lesion models and pharmacological injection to the brain, it was shown that microglial accumulation around the lesion was also dependent on B1 receptors and I(K(Ca)). These observations support the view that BK functions as a chemoattractant by using the distinct signal pathways in the brain and, thus, attracts microglia to the lesion site in vivo.

KCNA1 and TRPC6 ion channels and NHE1 exchanger operate the biological outcome of HGF/scatter factor in renal tubular cells

Hepatocyte growth factor (HGF) is a glycoprotein that induces in vitro epithelial tubular cell growth, motility, scattering and branching morphogenesis. The cell machineries that account for HGF biological effects are still unclear. In previous study, we found that HGF upregulated in epithelial tubular cell line (HK2) 3 genes: potassium channel KCNA1, calcium channel (transient receptor potential channel, subfamily C, member 6, TRPC6) and Na(+)/H(+) exchanger-1 (NHE1). In this study, we validated these results with reverse transcription PCR and WB analysis. To investigate whether KCNA1, TRPC6, NHE1 mediate the changes induced by HGF in HK2, we studied the effects of their inhibitors: 4-aminopyridine, charybdotoxin, dendrotoxin K inhibitors of KCNA1, lanthanum, N-(p-amylcinnamoyl) anthranilic acid inhibitors of TRPC6, 5-(N-ethyl-N-isopropyl)amiloride, cariporide inhibitors of NHE1. The inhibitors prevented HGF-induced growth, migration, cytoskeletal reorganization and tubulogenesis in HK2. These results indicate that KCNA1, TRPC6 and NHE1 are cell machineries that are exploited by HGF to effect its biological outcome in renal tubular cells.

Involvement of voltage-gated K+ and Na+ channels in gastric epithelial cell migration

Epithelial cell migration plays an important role in gastrointestinal mucosal repair. We previously reported that multiple functional ion channels, including a Ba(2+)-sensitive K(+) inward rectifier K(ir)1.2, 4-aminopyridine (4-AP)-sensitive voltage-gated K(+) channels K(v)1.1, K(v)1.6 and K(v)2.1, and a nifedipine-sensitive, tetrodotoxin (TTX)-insensitive voltage-gated Na(+) channel Na(v)1.5 were expressed in a non-transformed rat gastric epithelial cell line (RGM-1). In the present study, we further investigated whether these ion channels are involved in the modulation of gastric epithelial cell migration. Cell migration was determined by monolayer wound healing assay. Results showed that blockade of K(v) with 4-AP or Na(v)1.5 with nifedipine inhibited RGM-1 cell migration in the absence or presence of epidermal growth factor (EGF), which effectively stimulated RGM-1 cell migration. Moreover, high concentration of TTX mimicked the action of nifedipine, suggesting that the action of nifedipine was mediated through specific blockade of Na(v)1.5. In contrast, inhibition of K(ir)1.2 with Ba(2+), either in basal or EGF-stimulated condition, had no effect on RGM-1 cell migration. In conclusion, the present study demonstrates for the first time that voltage-gated K(+) and Na(+) channels are involved in the modulation of gastric epithelial cell migration.

Cell cycle-dependent expression of potassium channels and cell proliferation in rat mesenchymal stem cells from bone marrow

OBJECTIVE

Recently, our team has demonstrated that voltage-gated delayed rectifier K(+) current (IK(DR)) and Ca(2+)-activated K(+) current (I(KCa)) are present in rat bone marrow-derived mesenchymal stem cells; however, little is known of their physiological roles. The present study was designed to investigate whether functional expression of IK(DR) and I(KCa) would change with cell cycle progression, and whether they could regulate proliferation in undifferentiated rat mesenchymal stem cells (MSCs).

MATERIALS AND METHODS

Membrane potentials and ionic currents were recorded using whole-cell patch clamp technique, cell cycling was analysed by flow cytometry, cell proliferation was assayed with DNA incorporation method and the related genes were down-regulated by RNA interference (RNAi) and examined using RT-PCR.

RESULTS

It was found that membrane potential hyperpolarized, and cell size increased during the cell cycle. In addition, IK(DR) decreased, while I(KCa) increased during progress from G(1) to S phase. RT-PCR revealed that the mRNA levels of Kv1.2 and Kv2.1 (likely responsible for IK(DR)) reduced, whereas the mRNA level of KCa3.1 (responsible for intermediate-conductance I(KCa)) increased with the cell cycle progression. Down-regulation of Kv1.2, Kv2.1 or KCa3.1 with the specific RNAi, targeted to corresponding gene inhibited proliferation of rat MSCs.

CONCLUSION

These results demonstrate that membrane potential, IK(DR) and I(KCa) channels change with cell cycle progression and corresponding alteration of gene expression. IK(DR) and intermediate-conductance I(KCa) play an important role in maintaining membrane potential and they participate in modulation of proliferation in rat MSCs.

Polyamines and the intestinal tract

Owing to their high turnover, the intestinal mucosal cells have a particularly high requirement for polyamines. Therefore, they are an excellent charcol for the study of polyamine function in rapid physiological growth and differentiation. After a cursory introduction to the major aspects of polyamine metabolism, regulation, and mode of action, we discuss the contribution of the polyamines to the maintenance of normal gut function, the maturation of the intestinal mucosa, and its repair after injuries. Repletion of cellular polyamine pools with (D,L)-2-(difluoromethyl)ornithine has considerably improved our understanding of how the polyamines are involved in the regulation of normal and neoplastic growth. Unfortunately, the attempts to exploit polyamine metabolism as a cancer therapeutic target have not yet been successful. However, the selective inactivation of ornithine decarboxylase appears to be a promising chemopreventive method in familial adenomatous polyposis. Presumably, it relies on the fact that ornithine decarboxylase is a critical regulator of the proliferative response of the protooncogene c-myc, but not of its apoptotic response.

Delayed rectifier outward K+ current mediates the migration of rat cerebellar granule cells stimulated by melatonin

Melatonin (MT) may work as a neuromodulator through the associated MT receptors in the central nervous system. Previously, our studies have shown that MT increased the I(K) current via a G protein-related pathway. In the present study, patch-clamp whole-cell recording, transwell migration assays and organotypic cerebellar slice cultures were used to examine the effect of MT on granule cell migration. MT increased the I(K) current amplitude and migration of granule cells. Meanwhile, TEA, the I(K) channel blocker, decreased the I(K) current and slowed the migration of granule cells. Furthermore, the effects of MT on the I(K) current and cell migration were not abolished by pre-incubation with P7791, a specific antagonist of MT(3)R, but were eliminated by the application of the MT(2)R antagonists K185 and 4-P-PDOT. I(K) current and cell migration were decreased by the application of dibutyryl cyclic AMP (dbcAMP), which was in contrast to the MT effect on the I(K) current and cell migration. Incubation with dbcAMP essentially blocked the MT-induced increasing effect. Moreover, incubation of isolated cell cultures in the MT-containing medium also decreased the cAMP immunoreactivity in the granule cells. It is concluded, therefore, that I(K) current, downstream of a cAMP transduction pathway, mediates the migration of rat cerebellar granule cells stimulated by MT.

Depolarization and decreased surface expression of K+ channels contribute to NSAID-inhibition of intestinal restitution

Non-steroidal anti-inflammatory drugs (NSAIDs) contribute to gastrointestinal ulcer formation by inhibiting epithelial cell migration and mucosal restitution; however, the drug-affected signaling pathways are poorly defined. We investigated whether NSAID inhibition of intestinal epithelial migration is associated with depletion of intracellular polyamines, depolarization of membrane potential (E(m)) and altered surface expression of K(+) channels. Epithelial cell migration in response to the wounding of confluent IEC-6 and IEC-Cdx2 monolayers was reduced by indomethacin (100 microM), phenylbutazone (100 microM) and NS-398 (100 microM) but not by SC-560 (1 microM). NSAID-inhibition of intestinal cell migration was not associated with depletion of intracellular polyamines. Treatment of IEC-6 and IEC-Cdx2 cells with indomethacin, phenylbutazone and NS-398 induced significant depolarization of E(m), whereas treatment with SC-560 had no effect on E(m). The E(m) of IEC-Cdx2 cells was: -38.5+/-1.8 mV under control conditions; -35.9+/-1.6 mV after treatment with SC-560; -18.8+/-1.2 mV after treatment with indomethacin; and -23.7+/-1.4 mV after treatment with NS-398. Whereas SC-560 had no significant effects on the total cellular expression of K(v)1.4 channel protein, indomethacin and NS-398 decreased not only the total cellular expression of K(v)1.4, but also the cell surface expression of both K(v)1.4 and K(v)1.6 channel subunits in IEC-Cdx2. Both K(v)1.4 and K(v)1.6 channel proteins were immunoprecipitated by K(v)1.4 antibody from IEC-Cdx2 lysates, indicating that these subunits co-assemble to form heteromeric K(v) channels. These results suggest that NSAID inhibition of epithelial cell migration is independent of polyamine-depletion, and is associated with depolarization of E(m) and decreased surface expression of heteromeric K(v)1 channels.

Intestinal ribosomal p70(S6K) signaling is increased in piglet rotavirus enteritis

Recent identification of the mammalian target of rapamycin (mTOR) pathway as an amino acid-sensing mechanism that regulates protein synthesis led us to investigate its role in rotavirus diarrhea. We hypothesized that malnutrition would reduce the jejunal protein synthetic rate and mTOR signaling via its target, ribosomal p70 S6 kinase (p70(S6K)). Newborn piglets were artificially fed from birth and infected with porcine rotavirus on day 5 of life. Study groups included infected (fully fed and 50% protein calorie malnourished) and noninfected fully fed controls. Initially, in "worst-case scenario studies," malnourished infected piglets were killed on days 1, 3, 5, and 11 postinoculation, and jejunal samples were compared with controls to determine the time course of injury and p70(S6K) activation. Using a 2 x 2 factorial design, we subsequently determined if infection and/or malnutrition affected mTOR activation on day 3. Western blot analysis and immunohistochemistry were used to measure total and phosphorylated p70(S6K); [(3)H]phenylalanine incorporation was used to measure protein synthesis; and lactase specific activity and villus-crypt dimensions were used to quantify injury. At the peak of diarrhea, the in vitro jejunal protein synthetic rate increased twofold (compared with the rate in the uninfected pig jejunum), concomitant with increased jejunal p70(S6K) phosphorylation (4-fold) and an increased p70(S6K) level (3-fold, P < 0.05). Malnutrition did not alter the magnitude of p70(S6K) activation. Immunolocalization revealed that infection produced a major induction of cytoplasmic p70(S6K) and nuclear phospho-p70(S6K), mainly in the crypt. A downregulation of semitendinosus muscle p70(S6K) phosphorylation was seen at days 1-3 postinoculation. In conclusion, intestinal activation of p70(S6K) was not inhibited by malnutrition but was strongly activated during an active state of mucosal regeneration.

Polyamines are required for phospholipase C-gamma1 expression promoting intestinal epithelial restitution after wounding

Intestinal mucosal restitution occurs by epithelial cell migration, rather than by proliferation, to reseal superficial wounds after injury. Polyamines are essential for the stimulation of intestinal epithelial cell (IEC) migration during restitution in association with their ability to regulate Ca2+ homeostasis, but the exact mechanism by which polyamines induce cytosolic free Ca2+ concentration ([Ca2+]cyt) remains unclear. Phospholipase C (PLC)-gamma1 catalyzes the formation of inositol (1,4,5)-trisphosphate (IP3), which is implicated in the regulation of [Ca2+]cyt by modulating Ca2+ store mobilization and Ca2+ influx. The present study tested the hypothesis that polyamines are involved in PLC-gamma1 activity, regulating [Ca2+]cyt and cell migration after wounding. Depletion of cellular polyamines by alpha-difluoromethylornithine inhibited PLC-gamma1 expression in differentiated IECs (stable Cdx2-transfected IEC-6 cells), as indicated by substantial decreases in levels of PLC-gamma1 mRNA and protein and its enzyme product IP3. Polyamine-deficient cells also displayed decreased [Ca2+]cyt and inhibited cell migration. Decreased levels of PLC-gamma1 by treatment with U-73122 or transfection with short interfering RNA specifically targeting PLC-gamma1 also decreased IP3, reduced resting [Ca2+]cyt and Ca2+ influx after store depletion, and suppressed cell migration in control cells. In contrast, stimulation of PLC-gamma1 by 2,4,6-trimethyl-N-(meta-3-trifluoromethylphenyl)-benzenesulfonamide induced IP3, increased [Ca2+]cyt, and promoted cell migration in polyamine-deficient cells. These results indicate that polyamines are absolutely required for PLC-gamma1 expression in IECs and that polyamine-mediated PLC-gamma1 signaling stimulates cell migration during restitution as a result of increased [Ca2+]cyt.

The roles of calcium signaling and ERK1/2 phosphorylation in a Pax6+/- mouse model of epithelial wound-healing delay

Background

Congenital aniridia caused by heterozygousity at the PAX6 locus is associated with ocular surface disease including keratopathy. It is not clear whether the keratopathy is a direct result of reduced PAX6 gene dosage in the cornea itself, or due to recurrent corneal trauma secondary to defects such as dry eye caused by loss of PAX6 in other tissues. We investigated the hypothesis that reducing Pax6 gene dosage leads to corneal wound-healing defects. and assayed the immediate molecular responses to wounding in wild-type and mutant corneal epithelial cells.

Results

Pax6^+/- mouse corneal epithelia exhibited a 2-hour delay in their response to wounding, but subsequently the cells migrated normally to repair the wound. Both Pax6^+/+ and Pax6^+/- epithelia activated immediate wound-induced waves of intracellular calcium signaling. However, the intensity and speed of propagation of the calcium wave, mediated by release from intracellular stores, was reduced in Pax6^+/- cells. Initiation and propagation of the calcium wave could be largely decoupled, and both phases of the calcium wave responses were required for wound healing. Wounded cells phosphorylated the extracellular signal-related kinases 1/2 (phospho-ERK1/2). ERK1/2 activation was shown to be required for rapid initiation of wound healing, but had only a minor effect on the rate of cell migration in a healing epithelial sheet. Addition of exogenous epidermal growth factor (EGF) to wounded Pax6^+/- cells restored the calcium wave, increased ERK1/2 activation and restored the immediate healing response to wild-type levels.

Conclusion

The study links Pax6 deficiency to a previously overlooked wound-healing delay. It demonstrates that defective calcium signaling in Pax6^+/- cells underlies this delay, and shows that it can be pharmacologically corrected. ERK1/2 phosphorylation is required for the rapid initiation of wound healing. A model is presented whereby minor abrasions, which are quickly healed in normal corneas, transiently persist in aniridic patients, compromising the corneal stroma.

Induced TRPC1 expression sensitizes intestinal epithelial cells to apoptosis by inhibiting NF-kappaB activation through Ca2+ influx

Apoptosis occurs within crypts and at the intestinal luminal surface and plays a critical role in mucosal homoeostasis. NF-kappaB (nuclear factor-kappaB) is the central regulator of the transcription of genes involved in apoptosis, and its activity is highly regulated in the intestinal mucosa. We have recently demonstrated that TRPC1 (transient receptor potential canonical-1) is expressed in IECs (intestinal epithelial cells) and functions as a Ca2+ permeable channel activated by Ca2+ store depletion. The present study tests the hypothesis that TRPC1 channels are implicated in the regulation of apoptosis by inhibiting NF-kappaB through the induction of TRPC1-mediated Ca2+ influx in the IEC-6 line. The expression of TRPC1 induced by stable transfection of IEC-6 cells with the wild-type TRPC1 gene (IEC-TRPC1 cells) increased Ca2+ influx after Ca2+ store depletion and repressed NF-kappaB transactivation, which was associated with an increase in susceptibility to apoptosis induced by exposure to TNFalpha (tumour necrosis factor-alpha) plus CHX (cycloheximide) (TNF-alpha/CHX), or STS (staurosporine). By contrast, the induction of endogenous NF-kappaB activity, by the depletion of cellular polyamines, promoted resistance to apoptosis, which was prevented by the ectopic expression of the IkappaBalpha super-repressor. Furthermore, inhibition of TRPC1 expression by transfection with siRNA (small interfering RNA) targeting TRPC1 (siTRPC1) decreased Ca2+ influx, increased NF-kappaB transactivation, and prevented the increased susceptibility of IEC-TRPC1 cells to apoptosis. Decreasing Ca2+ influx by exposure to a Ca2+-free medium also induced NF-kappaB activity and blocked the increased susceptibility to apoptosis of stable IEC-TRPC1 cells. These results indicate that induced TRPC1 expression sensitizes IECs to apoptosis by inhibiting NF-kappaB activity as a result of the stimulation of Ca2+ influx.

Involvement of Kv1.1 and Nav1.5 in proliferation of gastric epithelial cells

In the present study, patch clamp experiments demonstrated the expression of multiple ionic currents, including a Ba2+-sensitive inward rectifier K+ current (IKir), a 4-aminopyridine- (4-AP) sensitive delayed rectifier K+ current (IKDR), and a nifedipine-sensitive, tetrodotoxin-resistant inward Na+ current (INa.TTXR) in the non-transformed rat gastric epithelial cell line RGM-1. RT-PCR revealed molecular identities of mRNAs for the functional ionic currents, including Kir1.2 for IKir, Kv1.1, Kv1.6, and Kv2.1 for IKDR, and Nav1.5 for INa.TTXR. Pharmacologic blockade of Kv and Nav, but not Kir, suppressed RGM-1 cell proliferation. To further elucidate which subtypes of the ion channels were involved in cell proliferation, RNA interference was employed to knockdown specific gene expression. Downregulation of Kv1.1 or Nav1.5 by RNA interference suppressed RGM-1 cell proliferation. To conclude, our study is the first to delineate the expression of ion channels and their functions as growth modulators in gastric epithelial cells.

TRPC1 functions as a store-operated Ca2+ channel in intestinal epithelial cells and regulates early mucosal restitution after wounding

An increase in cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)) results from Ca(2+) release from intracellular stores and extracellular Ca(2+) influx through Ca(2+)-permeable ion channels and is crucial for initiating intestinal epithelial restitution to reseal superficial wounds after mucosal injury. Capacitative Ca(2+) entry (CCE) induced by Ca(2+) store depletion represents a major Ca(2+) influx mechanism, but the exact molecular components constituting this process remain elusive. This study determined whether canonical transient receptor potential (TRPC)1 served as a candidate protein for Ca(2+)-permeable channels mediating CCE in intestinal epithelial cells and played an important role in early epithelial restitution. Normal intestinal epithelial cells (the IEC-6 cell line) expressed TRPC1 and TPRC5 and displayed typical records of whole cell store-operated Ca(2+) currents and CCE generated by Ca(2+) influx after depletion of intracellular stores. Induced TRPC1 expression by stable transfection with the TRPC1 gene increased CCE and enhanced cell migration during restitution. Differentiated IEC-Cdx2L1 cells induced by forced expression of the Cdx2 gene highly expressed endogenous TRPC1 and TRPC5 and exhibited increased CCE and cell migration. Inhibition of TRPC1 expression by small interfering RNA specially targeting TRPC1 not only reduced CCE but also inhibited cell migration after wounding. These findings strongly suggest that TRPC1 functions as store-operated Ca(2+) channels and plays a critical role in intestinal epithelial restitution by regulating CCE and intracellular [Ca(2+)](cyt).

Subcellular distribution of calcium-sensitive potassium channels (IK1) in migrating cells

Cell migration is crucial for wound healing, immune defense, or formation of tumor metastases. In addition to the cytoskeleton, Ca2+ sensitive K+ channels (IK1) are also part of the cellular "migration machinery." We showed that Ca2+ sensitive K+ channels support the retraction of the rear part of migrating MDCK-F cells by inducing a localized shrinkage at this cell pole. So far the molecular nature and in particular the subcellular distribution of these channels in MDCK-F cells is unknown. We compared the effect of IK1 channel blockers and activators on the current of a cloned IK1 channel from MDCK-F cells (cIK1) and the migratory behavior of these cells. Using IK1 channels labeled with a HA-tag or the enhanced green fluorescent protein we studied the subcellular distribution of the canine (cIK1) and the human (hIK1) channel protein in different migrating cells. The functional impact of cIK1 channel activity at the front or rear part of MDCK-F cells was assessed with a local superfusion technique and a detailed morphometric analysis. We show that it is cIK1 whose activity is required for migration of MDCK-F cells. IK1 channels are found in the entire plasma membrane, but they are concentrated at the cell front. This is in part due to membrane ruffling at this cell pole. However, there appears to be only little cIK1 channel activity at the front of MDCK-F cells. In our view this apparent discrepancy can be explained by differential regulation of IK1 channels at the front and rear part of migrating cells.

Polyamines regulate expression of E-cadherin and play an important role in control of intestinal epithelial barrier function

Epithelial cells line the gastrointestinal mucosa and form an important barrier that protects the subepithelial tissue against a wide array of noxious substances, allergens, viruses and luminal microbial pathogens. Restoration of mucosal integrity following injury and various environmental stresses requires epithelial cell decisions that regulate signaling networks controlling gene expression, survival, migration and proliferation. Recently, it has been shown that polyamines play an important role in the regulation of cell-cell interactions and are critical for maintenance of intestinal epithelial integrity. Both the function of polyamines in expression of adherens junction proteins and their possible mechanisms, especially in implication of intracellular Ca2+ and c-Myc transcription factor, are the subject of this review article.