Dynamic and differential regulation of NKCC1 by calcium and cAMP in the native human colonic epithelium

Aging-related impairment of neurogenic chloride secretion in human colon mucosa

Background

lderly individuals are more susceptible to chronic constipation, which may be linked to imbalanced mucosa secretion and absorption. Our research aims to explore the age-related alterations in epithelial chloride secretion within the human colon.

Methods

Colonic mucosal tissues were obtained from 9 young patients (aged 28-35 years), 10 middle-aged patients (aged 48-56 years), 10 elderly patients without constipation (aged 66-75 years), and 12 elderly patients with constipation (aged 65-78 years) who underwent surgery for colonic carcinoma. The epithelial chloride (Cl-) secretion was assessed using the short-circuit current (Isc) method. Comparative analysis was conducted on Cl- secretion induced by spontaneous activity, bethanechol, forskolin, veratridine, and electrical field stimulation (EFS) in the four groups. Additionally, investigations were carried out on changes in cholinergic and VIPergic Cl- secretion.

Results

The spontaneous Cl- secretion was not affected by aging. The increase in Isc induced by bethanechol and forskolin remained unaltered in aged colon. However, the veratridine-induced neurogenic Isc increment were significantly reduced with aging and constipation. The EFS-evoked Isc rising, which typically exhibiting a biphasic pattern, was inhibited by aging in a frequency-dependent manner. Administration of scopolamine and VIP6-28 to block cholinergic and vasoactive intestinal peptide (VIP) receptors led to smaller increases in the first and second phases of the EFS-evoked response in aged colons compared to young colons.

Conclusion

Significant impairments in neurogenic Cl- secretion occur in the aged colon, correlating with the degeneration of cholinergic and VIPergic nerves in the mucosa. This study could enhance our understanding of the pathophysiology of elderly constipation.

Association Between Genetically Proxied SLC12A2 Inhibition and Inflammatory Bowel Disease: A Mendelian Randomization Study

The global rise in hypertension prompts the use of medications to manage blood pressure. However, selecting first-line drugs remains challenging as their efficacy often stems from blood pressure reduction rather than specific pharmacological actions. Evaluating interactions between antihypertensive drugs and common diseases can aid tailored treatment. Here, we assess the potential link between antihypertensives and inflammatory bowel disease (IBD). Summary-level coronary heart disease (CHD) data (184,305 individuals), systolic BP (SBP) data (757,601 individuals), ulcerative ileocolitis data (361,188 individuals), ulcerative colitis data (364,454 individuals), other ulcerative colitis data (361,619 individuals), and ulcerative proctitis data (361,700 individuals) were all from genome-wide association studies (GWASs), FinnGen or eQTL studies publicly accessible. The DrugBank10 and ChEMBL11 databases function to identify genes encoding protein products targeted by active constituents of BP-lowering drugs. Summary-data-based MR (SMR) estimated the associations between expressions of drug target genes and symptoms of IBD. A multivariable MR study was further conducted to examine if the observed association was direct association. Subsequently, we collected blood samples from IBD patients in the Gastroenterology Department of Gastroenterology, the First Affiliated Hospital of Anhui Medical University and blood from healthy individuals at the physical examination center. Real-time quantitative PCR was employed to detect the expression changes of drug target genes in the peripheral blood of patients with IBD. Furthermore, we used Caco2 cells to construct an in vitro model of IBD, examined the expression of the target molecules, and verified the potential of Bumetanide to improve IBD. SMR analysis revealed that enhanced SLC12A2 gene expression in blood (equivalent to a one standard deviation increase) was a risk factor for ulcerative ileocolitis (beta = 0.5861, se = 0.2972, p = 0.0486) and enhanced gene expression of ACE was a protective factor. Additionally, SCNN1D and SLC16A1 played protective roles of IBD, while NR3C1 was identified as a risk factor. However, among these genes, only SLC12A2 was considered to influence the progress of inflammatory bowel disease through systolic blood pressure based on Mendelian randomization analysis results. Other genes may be associated with IBD depending on the expression of their own proteins, independent of changes in blood pressure. In the peripheral blood of IBD patients and in vitro experiments, SCL12A2 has been shown to be highly expressed in IBD. In vitro experiments have confirmed that Bumetanide can inhibit SCL12A2 to improve tight junctions, reduce inflammation levels, and ameliorate IBD symptoms. Therapeutic inhibition of SCL12A2 may benefit patients with IBD. In the future, this study may contribute to the selection of more personalized antihypertensive medications for different subgroups of hypertensive patients.

Aqueous extract of Descuraniae Semen attenuates lipopolysaccharide-induced inflammation by inhibiting ER stress and WNK4-SPAK-NKCC1 pathway

Sepsis causes systemic inflammatory responses and acute lung injury (ALI). Despite modern treatments, sepsis-related ALI mortality remains high. Aqueous extract of Descuraniae Semen (AEDS) exerts anti-endoplasmic reticulum (ER) stress, antioxidant and anti-inflammatory effects. AEDS alleviates inflammation and oedema in ALI. Sodium-potassium-chloride co-transporter isoform 1 (NKCC1) is essential for regulating alveolar fluid and is important in ALI. The NKCC1 activity is regulated by upstream with-no-lysine kinase-4 (WNK4) and STE20/SPS1-related proline/alanine-rich kinase (SPAK). This study aimed to investigate the effects of AEDS on lipopolysaccharide (LPS)-induced ALI model in A549 cells, considering the regulation of ER stress, WNK4-SPAK-NKCC1 cascades, inflammation and apoptosis. Cell viability was investigated by the CCK-8 assay. The expressions of the proteins were assessed by immunoblotting analysis assays. The levels of pro-inflammatory cytokines were determined by ELISA. The expression of cytoplasmic Ca2+ in A549 cells was determined using Fluo-4 AM. AEDS attenuates LPS-induced inflammation, which is associated with increased pro-inflammatory cytokine expression and activation of the WNK4-SPAK-NKCC1 pathway. AEDS inhibits the WNK4-SPAK-NKCC1 pathway by regulating of Bcl-2, IP3R and intracellular Ca2+. WNK4 expression levels are significantly higher in the WNK4-overexpressed transfected A549 cells and significantly decrease after AEDS treatment. AEDS attenuates LPS-induced inflammation by inhibiting the WNK4-SPAK-NKCC1 cascade. Therefore, AEDS is regarded as a potential therapeutic agent for ALI.

Tuft cell-derived acetylcholine promotes epithelial chloride secretion and intestinal helminth clearance

Epithelial cells secrete chloride to regulate water release at mucosal barriers, supporting both homeostatic hydration and the "weep" response that is critical for type 2 immune defense against parasitic worms (helminths). Epithelial tuft cells in the small intestine sense helminths and release cytokines and lipids to activate type 2 immune cells, but whether they regulate epithelial secretion is unknown. Here, we found that tuft cell activation rapidly induced epithelial chloride secretion in the small intestine. This response required tuft cell sensory functions and tuft cell-derived acetylcholine (ACh), which acted directly on neighboring epithelial cells to stimulate chloride secretion, independent of neurons. Maximal tuft cell-induced chloride secretion coincided with immune restriction of helminths, and clearance was delayed in mice lacking tuft cell-derived ACh, despite normal type 2 inflammation. Thus, we have uncovered an epithelium-intrinsic response unit that uses ACh to couple tuft cell sensing to the secretory defenses of neighboring epithelial cells.

The biogenesis of potassium transporters: implications of disease-associated mutations

The concentration of intracellular and extracellular potassium is tightly regulated due to the action of various ion transporters, channels, and pumps, which reside primarily in the kidney. Yet, potassium transporters and cotransporters play vital roles in all organs and cell types. Perhaps not surprisingly, defects in the biogenesis, function, and/or regulation of these proteins are linked to range of catastrophic human diseases, but to date, few drugs have been approved to treat these maladies. In this review, we discuss the structure, function, and activity of a group of potassium-chloride cotransporters, the KCCs, as well as the related sodium-potassium-chloride cotransporters, the NKCCs. Diseases associated with each of the four KCCs and two NKCCs are also discussed. Particular emphasis is placed on how these complex membrane proteins fold and mature in the endoplasmic reticulum, how non-native forms of the cotransporters are destroyed in the cell, and which cellular factors oversee their maturation and transport to the cell surface. When known, we also outline how the levels and activities of each cotransporter are regulated. Open questions in the field and avenues for future investigations are further outlined.

Involvement of sodium-glucose cotransporter-1 activities in maintaining oscillatory Cl- currents from mouse submandibular acinar cells

In salivary acinar cells, cholinergic stimulation induces elevations of cytosolic [Ca2+]i to activate the apical exit of Cl- through TMEM16A Cl- channels, which acts as a driving force for fluid secretion. To sustain the Cl- secretion, [Cl-]i must be maintained to levels that are greater than the electrochemical equilibrium mainly by Na+-K+-2Cl- cotransporter-mediated Cl- entry in basolateral membrane. Glucose transporters carry glucose into the cytoplasm, enabling the cells to produce ATP to maintain Cl- and fluid secretion. Sodium-glucose cotransporter-1 is a glucose transporter highly expressed in acinar cells. The salivary flow is suppressed by the sodium-glucose cotransporter-1 inhibitor phlorizin. However, it remains elusive how sodium-glucose cotransporter-1 contributes to maintaining salivary fluid secretion. To examine if sodium-glucose cotransporter-1 activity is required for sustaining Cl- secretion to drive fluid secretion, we analyzed the Cl- currents activated by the cholinergic agonist, carbachol, in submandibular acinar cells while comparing the effect of phlorizin on the currents between the whole-cell patch and the gramicidin-perforated patch configurations. Phlorizin suppressed carbachol-induced oscillatory Cl- currents by reducing the Cl- efflux dependent on the Na+-K+-2Cl- cotransporter-mediated Cl- entry in addition to affecting TMEM16A activity. Our results suggest that the sodium-glucose cotransporter-1 activity is necessary for maintaining the oscillatory Cl- secretion supported by the Na+-K+-2Cl- cotransporter activity in real time to drive fluid secretion. The concerted effort of sodium-glucose cotransporter-1, Na+-K+-2Cl- cotransporter, and apically located Cl- channels might underlie the efficient driving of Cl- secretion in different secretory epithelia from a variety of animal species.

Expression of the non-neuronal cholinergic system components in Malpighian tubules of Mythimna separata and evidence for non-neuronal acetylcholine synthesis

The non-neuronal cholinergic system, widely distributed in nature, is an ancient system that has not been well studied in insects. This study aims to investigate the key components of the cholinergic system and to identify the non-neuronal acetylcholine (ACh)-producing cells and the acting sites of ACh in the Malpighian tubules (MTs) of Mythimna separata. We found that non-neuronal ACh in MTs is synthesized by carnitine acetyltransferase (CarAT), rather than choline acetyltransferase (ChAT), as confirmed by using enzyme inhibitors and high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS/MS). Fluorescence in situ hybridization revealed the presence of CarAT mRNA within MTs, specifically localized in the principal cells. Immunohistochemistry showed strong staining for A-mAChR, a muscarinic acetylcholine receptor, in the principal cells. Pharmacological analysis further demonstrated that ACh acts through A-mAChR in the principal cells to increase the intracellular Ca2+ concentration. These findings provide compelling evidence for the existence of a non-neuronal cholinergic system in the MTs of M. separata, and the principal cells play a crucial role in ACh synthesis via CarAT.

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Extracellular Hydraulic Resistance Enhances Cell Migration

Cells migrating in vivo encounter microenvironments with varying physical properties. One such physical variable is the fluid viscosity surrounding the cell. Increased viscosity is expected to increase the hydraulic resistance experienced by the cell and decrease cell speed. The authors demonstrate that contrary to this expected result, cells migrate faster in high viscosity media on 2-dimensional substrates. Both actin dynamics and water dynamics driven by ion channel activity are examined. Results show that cells increase in area in high viscosity and actomyosin dynamics remain similar. Inhibiting ion channel fluxes in high viscosity media results in a large reduction in cell speed, suggesting that water flux contributes to the observed speed increase. Moreover, inhibiting actin-dependent vesicular trafficking that transports ion channels to the cell boundary changes ion channel spatial positioning and reduces cell speed in high viscosity media. Cells also display altered Ca2+ activity in high viscosity media, and when cytoplasmic Ca2+ is sequestered, cell speed reduction and altered ion channel positioning are observed. Taken together, it is found that the cytoplasmic actin-phase and water-phase are coupled to drive cell migration in high viscosity media, in agreement with physical modeling that also predicts the observed cell speedup in high viscosity environments.

Loss of NKCC1 function increases epithelial tight junction permeability by upregulating claudin-2 expression

Conditions that cause the loss of epithelial barrier integrity are often accompanied by dysregulation of tight junction protein expression and/or localization. Recently, we have reported that patients with mutations in SLC12A2, the gene encoding the basolateral Na+-K+-2Cl- cotransporter (NKCC1), suffer from severe gastrointestinal deficits, including chronic gastrointestinal inflammation, gastrointestinal hemorrhage, intestinal obstruction, and constipation. Although the intestinal inflammation observed in patients with loss of NKCC1 function may or may not be due to tight junction dysfunction, we investigated whether the loss of NKCC1 function affects paracellular ion transport and epithelial barrier function. Wild-type HT29-MTX-E12 and CRISPR/Cas9-mediated NKCC1 knockout (KO) HT29 clones were tested for tight junction protein expression and localization. Tightness of epithelial cell monolayer was assessed by measurement of transepithelial electrical resistance and permeability of molecular tracers in transwell filters. Tight junction protein localization was assessed by immunofluorescence. Loss of NKCC1 expression strongly increases the expression of claudin-2 and occludin in epithelial cell monolayers. Loss of NKCC1 significantly reduces the transepithelial electrical resistance (TER) indicating an increase in paracellular ions flux, consistent with upregulation of the cation-selective and channel-forming claudin-2. In addition, NKCC1-KO monolayers showed a significant increase in the paracellular flux of small molecules like fluorescein (0.33 kDa), whereas the permeability of higher molecular weight TRITC-Dextran (4 kDa and 70 kDa) remained unchanged. Thus, NKCC1 regulates tight junction protein expression and loss of NKCC1 function affects epithelial barrier integrity.

Colonic Fluid and Electrolyte Transport 2022: An Update

Colonic epithelial cells are responsible for maintaining a delicate balance between luminal secretion and the absorption of fluids and ions. This review aims to discuss and update the model of colonic electrolyte secretion and absorption via the cystic fibrosis transmembrane regulator (CFTR), epithelial sodium channel (ENaC), Na-K-Cl cotransporters (NKCC1 and 2), Na-H exchangers (NHE1–4), colonic H,KATPase, and several other key components involved in multi-level transepithelial ion transport. Developments in our understanding of the activity, regulation, localization, and relationships of these ion transporters and their interactions have helped forge a more robust understanding of colonic ion movement that accounts for the colonic epithelium’s role in mucosal pH modulation, the setting of osmotic gradients pivotal for fluid retention and secretion, and cell death regulation. Deviations from homeostatic ion transport cause diarrhea, constipation, and epithelial cell death and contribute to cystic fibrosis, irritable bowel syndrome (IBS), ulcerative colitis, and cancer pathologies. Signal transduction pathways that regulate electrolyte movement and the regulatory relationships between various sensors and transporters (CFTR as a target of CaSR regulation and as a regulator of ENaC and DRA, for example) are imperative aspects of a dynamic and comprehensive model of colonic ion homeostasis.

Intestinal secretory mechanisms and diarrhea

One of the primary functions of the intestinal epithelium is to transport fluid and electrolytes to and from the luminal contents. Under normal circumstances, absorptive and secretory processes are tightly regulated such that absorption predominates, thereby enabling conservation of the large volumes of water that pass through the intestine each day. However, in conditions of secretory diarrhea, this balance becomes dysregulated, so that fluid secretion, driven primarily by Cl- secretion, overwhelms absorptive capacity, leading to increased loss of water in the stool. Secretory diarrheas are common and include those induced by pathogenic bacteria and viruses, allergens, and disruptions to bile acid homeostasis, or as a side effect of many drugs. Here, we review the cellular and molecular mechanisms by which Cl- and fluid secretion in the intestine are regulated, how these mechanisms become dysregulated in conditions of secretory diarrhea, currently available and emerging therapeutic approaches, and how new strategies to exploit intestinal secretory mechanisms are successfully being used in the treatment of constipation.

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Comparative Effects of L. plantarum CGMCC 1258 and L. reuteri LR1 on Growth Performance, Antioxidant Function, and Intestinal Immunity in Weaned Pigs

Lactobacillus plantarum CGMCC 1258 and Lactobacillus reuteri LR1 are two important strains of probiotics. However, their different advantages in the probiotic effect of weaned pigs are still poorly understood. Therefore, the study was to investigate the comparative effects of dietary supplementation of L. plantarum CGMCC 1258 and L. reuteri LR1 on growth performance, antioxidant function, and intestinal immunity in weaned pigs. Ninety barrows [initial body weight (BW) = 6.10 ± 0.1 kg] 21 days old were randomly divided into 3 treatments with 5 replicates, each replicate containing 6 pigs. Pigs in control (CON) were fed a basal diet, and the basal diets supplemented with 5 × 10¹⁰ CFU/kg L. plantarum CGMCC 1258 (LP) or L. reuteri LR1 (LR) for 42 days, respectively. The results showed that LP increased (p < 0.05) serum superoxide dismutase (SOD), and decreased (p < 0.05) serum malondialdehyde (MDA) and the expression and secretion of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) in intestinal mucosa, but has no significant effect on growth performance and diarrheal incidence. However, LR increased (p < 0.05) final BW and average daily gain (ADG), reduced (p < 0.05) 29–42-day diarrheal incidence, decreased (p < 0.05) the expression and secretion of IL-1β, IL-6, TNF-α, and IFN-γ, and increased (p < 0.05) the expression of transforming growth factor-β (TGF-β) in intestinal mucosa. In addition, the serum glutathione peroxidase (GSH-PX), mRNA relative expression of Na+-K+-2Cl– co-transporter 1 (NKCC1) and cystic fibrosis transmembrane conductance regulator (CFTR) and the content of toll-like relative (TLR2) and TLR4 in the jejunum, and secretory immunoglobulin (sIgA) content of ileal mucosa were higher (p < 0.05) than LP. Collectively, dietary L. plantarum CGMCC 1258 improved intestinal morphology, intestinal permeability, intestinal immunity, and antioxidant function in weaned pigs. Dietary L. reuteri LR1 showed better growth performance, a lower incidence of diarrhea, better intestinal morphology, and a higher extent of immune activation in weaned pigs.

An Overview of Cell-Based Assay Platforms for the Solute Carrier Family of Transporters

The solute carrier (SLC) superfamily represents the biggest family of transporters with important roles in health and disease. Despite being attractive and druggable targets, the majority of SLCs remains understudied. One major hurdle in research on SLCs is the lack of tools, such as cell-based assays to investigate their biological role and for drug discovery. Another challenge is the disperse and anecdotal information on assay strategies that are suitable for SLCs. This review provides a comprehensive overview of state-of-the-art cellular assay technologies for SLC research and discusses relevant SLC characteristics enabling the choice of an optimal assay technology. The Innovative Medicines Initiative consortium RESOLUTE intends to accelerate research on SLCs by providing the scientific community with high-quality reagents, assay technologies and data sets, and to ultimately unlock SLCs for drug discovery.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Repeated anodal trans-spinal direct current stimulation results in long-term reduction of spasticity in mice with spinal cord injury

KEY POINTS

Spasticity is a disorder of muscle tone that is associated with lesions of the motor system. This condition involves an overactive spinal reflex loop that resists the passive lengthening of muscles. Previously, we established that application of anodal trans-spinal direct current stimulation (a-tsDCS) for short periods of time to anaesthetized mice sustaining a spinal cord injury leads to an instantaneous reduction of spasticity. However, the long-term effects of repeated a-tsDCS and its mechanism of action remained unknown. In the present study, a-tsDCS was performed for 7 days and this was found to cause long-term reduction in spasticity, increased rate-dependent depression in spinal reflexes, and improved ground and skill locomotion. Pharmacological, molecular and cellular evidence further suggest that a novel mechanism involving Na-K-Cl cotransporter isoform 1 mediates the observed long-term effects of repeated a-tsDCS.

ABSTRACT

Spasticity can cause pain, fatigue and sleep disturbances; restrict daily activities such as walking, sitting and bathing; and complicate rehabilitation efforts. Thus, spasticity negatively influences an individual's quality of life and novel therapeutic interventions are needed. We previously demonstrated in anaesthetized mice that a short period of trans-spinal subthreshold direct current stimulation (tsDCS) reduces spasticity. In the present study, the long-term effects of repeated tsDCS to attenuate abnormal muscle tone in awake female mice with spinal cord injuries were investigated. A motorized system was used to test velocity-dependent ankle resistance and associated electromyographical activity. Analysis of ground and skill locomotion was also performed, with electrophysiological, molecular and cellular studies being conducted to reveal a potential underlying mechanism of action. A 4 week reduction in spasticity was associated with an increase in rate-dependent depression of spinal reflexes, and ground and skill locomotion were improved following 7 days of anodal-tsDCS (a-tsDCS). Secondary molecular, cellular and pharmacological experiments further demonstrated that the expression of K-Cl co-transporter isoform 2 (KCC2) was not changed in animals with spasticity. However, Na-K-Cl cotransporter isoform 1 (NKCC1) was significantly up-regulated in mice that exhibited spasticity. When mice were treated with a-tsDCS, down regulation of NKCC1 was detected, and this level did not significantly differ from that in the non-injured control mice. Thus, long lasting reduction of spasticity by a-tsDCS via downregulation of NKCC1 may constitute a novel therapy for spasticity following spinal cord injury.

A dileucine motif in the COOH-terminal domain of NKCC1 targets the cotransporter to the plasma membrane

Na⁺-K⁺-2Cl^- cotransporter-1 (NKCC1) mediates the electroneutral transport of Na⁺, K⁺, and Cl^- and is normally localized to the basolateral membrane of polarized epithelial cells. We recently reported the first known solute carrier family 12 member 2 ( SLC12A2) mutation (we call NKCC1-DFX) that causes epithelial dysfunction in an undiagnosed disease program case. The heterozygous mutation leads to truncation of the COOH-terminal tail of the cotransporter, resulting in both mutant and wild-type cotransporters being mistrafficked to the apical membrane of polarized epithelial cells. Here we demonstrate by using consecutive truncations and site-directed mutagenesis of the COOH-terminal domain of NKCC1 that truncation of NKCC1 COOH domain uncouples the cotransporter from the lateral membrane. We identify a dileucine motif that, when mutated, leads to cotransporter accumulation in the cytoplasm and mistrafficking to the apical/subapical region of epithelial cells, thereby recapitulating the phenotype observed with the patient mutation. We show that truncation deletion and LL substitution mutants are trafficked out of the endoplasmic reticulum and trans-Golgi network but accumulate in early and late endosomes where they are degraded.

Failure of bumetanide to improve outcome after intracerebral hemorrhage in rat

After intracerebral hemorrhage (ICH), brain edema commonly occurs and can cause death. Along with edema, there are significant alterations in the concentrations of key ions such as sodium, potassium, and chloride, which are essential to brain function. NKCC1, a cation-chloride cotransporter, is upregulated after brain damage, such as traumatic injury and ischemic stroke. NKCC1 brings sodium and chloride into the cell, possibly worsening ion dyshomeostasis. Bumetanide, a specific NKCC1 antagonist, blocks the transport of chloride into cells, and thus should attenuate the increases in chloride, which should lessen brain edema and improve neuronal functioning post-ICH, as with other injuries. We used the collagenase model of ICH to test whether bumetanide treatment for three days (vs. vehicle) would improve outcome. We gave bumetanide beginning at two hours or seven days post-ICH and measured behavioural outcome, edema, and brain ion content after treatment. There was some evidence for a minor reduction in edema after early dosing, but this did not improve behaviour or lessen injury. Contrary to our hypothesis, bumetanide did not normalize ion concentrations after late dosing. Bumetanide did not improve behavioural outcome or affect lesion volume. After ICH, bumetanide is safe to use in rats but does not improve functional outcome in the majority of animals.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

The role of Gαq/Gα11 signaling in intestinal epithelial cells

Intestinal homeostasis and the coordinated actions of digestion, absorption and excretion are tightly regulated by a number of gastrointestinal hormones. Most of them exert their actions through G-protein-coupled receptors. Recently, we showed that the absence of Gαq/Gα11 signaling impaired the maturation of Paneth cells, induced their differentiation toward goblet cells, and affected the regeneration of the colonic mucosa in an experimental model of colitis. Although an immunohistochemical study showed that Gαq/Gα11 were highly expressed in enterocytes, it seemed that enterocytes were not affected in Int-Gq/G11 double knock-out intestine. Thus, we used an intestinal epithelial cell line to examine the role of signaling through Gαq/Gα11 in enterocytes and manipulated the expression level of Gαq and/or Gα11. The proliferation was inhibited in IEC-6 cells that overexpressed Gαq/Gα11 and enhanced in IEC-6 cells in which Gαq/Gα11 was downregulated. The expression of T-cell factor 1 was increased according to the overexpression of Gαq/Gα11. The expression of Notch1 intracellular cytoplasmic domain was decreased by the overexpression of Gαq/Gα11 and increased by the downregulation of Gαq/Gα11. The relative mRNA expression of Muc2, a goblet cell marker, was elevated in a Gαq/Gα11 knock-down experiment. Our findings suggest that Gαq/Gα11-mediated signaling inhibits proliferation and may support a physiological function, such as absorption or secretion, in terminally differentiated enterocytes.

Acetylcholine-related proteins in non-neoplastic appearing colonic mucosa from patients with colorectal neoplasia

The pathogenesis of colorectal neoplasia (CRN) has been associated with altered non-neuronal acetylcholine (ACh) metabolism. The aim of this study was to characterize expression, function, and cellular location of ACh-related proteins in biopsies obtained from endoscopic normal-appearing sigmoid colon in patients with and without CRN. Messenger-RNA (mRNA) levels of 17 ACh-related proteins were quantified by rt-qPCR. Functional responses to ACh, measured as electrogenic transepithelial short circuit current (SCC), were recorded using the Ussing chamber technique. Finally, cellular localization of choline transporter-like proteins (CTLs) and butyryl-cholinesterase enzyme (BChE) was determined by immunohistochemistry. mRNA expression of CTL1 and CTL4 was increased in patients with CRN (P = 0.002 and P = 0.04, respectively). In functional experiments, baseline SCC was increased in CRN patients. ACh induced rapid biphasic changes in SCC. An initial decreasing phase was observed in the minority of CRN patients versus the majority of controls (25% vs 69%, respectively, P = 0.031). For the second increasing phase of SCC, data indicated ACh-activation of two receptors. For both parts of the biphasic response, the half maximal effective concentration and maximal responses showed no difference between patient groups. Immunohistochemistry demonstrated CTL1, 3 and 4 and BChE to be localized to colonic crypt cells. We conclude that CRN is associated with increased expression of CTL1 and CTL4, augmented basal prostaglandin-dependent secretion, and altered functional channel response to ACh in human endoscopic normal-appearing colonic mucosa. The immunohistochemical findings support CTL1, CTL3, CTL4, and BChE to be involved in non-neuronal mucosal ACh metabolism.

The Zn2+-sensing receptor, ZnR/GPR39, upregulates colonocytic Cl- absorption, via basolateral KCC1, and reduces fluid loss

Administration of zinc, as a complement to oral rehydration solutions, effectively diminishes duration and severity of diarrhea, but it is not known whether it merely fulfills a nutritional deficiency, or if zinc has a direct role of regulating solute absorption. We show that Zn2+ acts via a specific receptor, ZnR/GPR39, to reduce fluid loss. Intestinal fluid secretion triggered by cholera toxin (CTx) was lower in WT mice compared to ZnR/GPR39 KO. In the absence of dietary Zn2+ we observed similar fluid accumulation in WT and ZnR/GPR39 KO mice, indicating that Zn2+ and ZnR/GPR39 are both required for a beneficial effect of Zn2+ in diarrhea. In primary colonocytes and in Caco-2 colonocytic cells, activation of ZnR/GPR39 enhanced Cl- transport, a critical factor in diarrhea, by upregulating K+/Cl- cotransporter (KCC1) activity. Importantly, we show basolateral expression of KCC1 in mouse and human colonocytes, thus identifying a novel Cl- absorption pathway. Finally, inhibition of KCC-dependent Cl- transport enhanced CTx-induced fluid loss. Altogether, our data indicate that Zn2+ acting via ZnR/GPR39 has a direct role in controlling Cl- absorption via upregulation of basolateral KCC1 in the colon. Moreover, colonocytic ZnR/GPR39 and KCC1 reduce water loss during diarrhea and may therefore serve as effective drug targets.

Characterization of Na+-K+-2Cl- Cotransporter Activity in Rabbit Lacrimal Gland Duct Cells

PURPOSE

We recently reported that isolated duct segments from rabbit lacrimal gland (LG) were able to secrete fluid in response to secretagogues, which were blocked completely by bumetanide. This suggests the functional involvement of Na+-K+-2Cl- cotransporter (NKCC1) in ductal fluid secretion. Therefore, the aim of this study was to investigate the activity profile of NKCC1 in isolated rabbit LG duct segments.

METHODS

Interlobular ducts were isolated from fresh rabbit LG tissue. Microfluorometry with the ammonium (NH4+)-pulse technique was used to elicit pH changes in duct cells, and the rate of bumetanide-sensitive cytosolic acidification after addition of NH4+ was used to quantify the activity of NKCC1.

RESULTS

While basal activity of NKCC1 was undetectable, low cytosolic chloride (Cl-) level and hyperosmotic challenge (390 mOsm) were able to increase the activity of NKCC1. Carbachol (100 μM) had no significant effect on NKCC1 activity. Elevation of cytosolic calcium (Ca2+) level with Ca2+-ionophore (A 23187, 1 μM) did not cause any alteration in the activity of the cotransporter while direct activation of protein kinase C (phorbol myristate acetate, 100 nM) increased its activity slightly but in a significant manner. Addition of either forskolin (10 μM), cell-permeable cAMP analogue (8-bromo cAMP, 100 μM) or vasoactive intestinal peptide (200 nM) resulted in a significant increase in the activity of NKCC1.

CONCLUSIONS

These results highlight the functional involvement of NKCC1 in LG duct secretion. These findings may facilitate our understanding of LG function and may contribute to the development of targeted pharmacologic interventions in case of dry eye disease.

Ae4 (Slc4a9) Anion Exchanger Drives Cl- Uptake-dependent Fluid Secretion by Mouse Submandibular Gland Acinar Cells

Transcellular Cl(-) movement across acinar cells is the rate-limiting step for salivary gland fluid secretion. Basolateral Nkcc1 Na(+)-K(+)-2Cl(-) cotransporters play a critical role in fluid secretion by promoting the intracellular accumulation of Cl(-) above its equilibrium potential. However, salivation is only partially abolished in the absence of Nkcc1 cotransporter activity, suggesting that another Cl(-) uptake pathway concentrates Cl(-) ions in acinar cells. To identify alternative molecular mechanisms, we studied mice lacking Ae2 and Ae4 Cl(-)/HCO3 (-) exchangers. We found that salivation stimulated by muscarinic and β-adrenergic receptor agonists was normal in the submandibular glands of Ae2(-/-) mice. In contrast, saliva secretion was reduced by 35% in Ae4(-/-) mice. The decrease in salivation was not related to loss of Na(+)-K(+)-2Cl(-) cotransporter or Na(+)/H(+) exchanger activity in Ae4(-/-) mice but correlated with reduced Cl(-) uptake during β-adrenergic receptor activation of cAMP signaling. Direct measurements of Cl(-)/HCO3 (-) exchanger activity revealed that HCO3 (-)-dependent Cl(-) uptake was reduced in the acinar cells of Ae2(-/-) and Ae4(-/-) mice. Moreover, Cl(-)/HCO3 (-) exchanger activity was nearly abolished in double Ae4/Ae2 knock-out mice, suggesting that most of the Cl(-)/HCO3 (-) exchanger activity in submandibular acinar cells depends on Ae2 and Ae4 expression. In conclusion, both Ae2 and Ae4 anion exchangers are functionally expressed in submandibular acinar cells; however, only Ae4 expression appears to be important for cAMP-dependent regulation of fluid secretion.

A human colonic crypt culture system to study regulation of stem cell-driven tissue renewal and physiological function

The intestinal epithelium is one of the most rapidly renewing tissues in the human body and fulfils vital physiological roles such as barrier function and transport of nutrients and fluid. Investigation of gut epithelial physiology in health and disease has been hampered by the lack of ex vivo models of the native human intestinal epithelium. Recently, remarkable progress has been made in defining intestinal stem cells and in generating intestinal organoid cultures. In parallel, we have developed a 3D culture system of the native human colonic epithelium that recapitulates the topological hierarchy of stem cell-driven tissue renewal and permits the physiological study of native polarized epithelial cells. Here we describe methods to establish 3D cultures of intact human colonic crypts and conduct real-time imaging of intestinal tissue renewal, cellular signalling, and physiological function, in conjunction with manipulation of gene expression by lentiviral or adenoviral transduction. Visualization of mRNA- and protein-expression patterns in cultured human colonic crypts, and cross-validation with crypts derived from fixed mucosal biopsies, is also described. Alongside studies using intestinal organoids, the near-native human colonic crypt culture model will help to bridge the gap that exists between investigation of colon cancer cell lines and/or animal (tissue) studies, and progression to clinical trials. To this end, the near native human colonic crypt model provides a platform to aid the development of novel strategies for the prevention of inflammatory bowel disease and cancer.

Ammonia exposure increases the expression of Na(+):K (+):2Cl (-) cotransporter 1a in the gills of the giant mudskipper, Periophthalmodon schlosseri

The giant mudskipper, Periophthalmodon schlosseri, is an obligate air-breathing teleost that can actively excrete ammonia against high concentrations of environmental ammonia. This study aimed to clone and sequence the Na (+) :K (+) :2Cl (-) cotransporter 1 (nkcc1) from the gills of P. schlosseri, and to determine the effects of ammonia exposure on its mRNA expression and protein abundance after pre-acclimation to slightly brackish water (salinity 3; SBW) for 2 weeks. The complete coding cDNA sequences of nkcc1a consisted of 3453 bp, coding for 1151 amino acid with an estimated molecular mass of 125.4 kDa. Exposure to 75 mmol l(-1) NH4Cl in SBW had no effect on the mRNA expression of nkcc1a. However, western blotting revealed a significant increase in the protein abundance of multiple T4-immunoreactive bands of molecular mass 170-250 kDa in the gills of P. schlosseri exposed to ammonia. Furthermore, immunofluorescence microscopy demonstrated the colocalization of the increased T4-immunoreactive protein with Na(+)/K(+)-ATPase (Nka) α-subunit to the basolateral membrane of certain ionocytes in the gills of the ammonia-exposed fish. As Nkcc1 is known to have a basolateral localization, it can be concluded that ammonia exposure led to an increase in the expression of glycosylated Nkcc1, the molecular masses of which were reduced upon enzymatic deglycosylation, in the gills of P. schlosseri. The dependency on post-transcriptional and post-translational regulation of branchial Nkcc1 in P. schlosseri would facilitate prompt responses to changes in environmental condition. As NH4 (+) can replace K(+), NH4 (+) could probably enter ionocytes through the basolateral Nkcc1a during active ammonia excretion, but increased influx of Na(+), NH4 (+) and 2Cl(-) would alter the transmembrane Na(+) gradient. Consequently, exposure of P. schlosseri to ammonia would also result in an increase in branchial activity of Nka with decreased NH4 (+) affinity so as to maintain intracellular Na(+) and K(+) homeostasis as reported elsewhere.

Increased spinal cord Na⁺-K⁺-2Cl⁻ cotransporter-1 (NKCC1) activity contributes to impairment of synaptic inhibition in paclitaxel-induced neuropathic pain

Microtubule-stabilizing agents, such as paclitaxel (Taxol), are effective chemotherapy drugs for treating many cancers, and painful neuropathy is a major dose-limiting adverse effect. Cation-chloride cotransporters, such as Na(+)-K(+)-2Cl(-) cotransporter-1 (NKCC1) and K(+)-Cl(-) cotransporter-2 (KCC2), critically influence spinal synaptic inhibition by regulating intracellular chloride concentrations. Here we show that paclitaxel treatment in rats significantly reduced GABA-induced membrane hyperpolarization and caused a depolarizing shift in GABA reversal potential of dorsal horn neurons. However, paclitaxel had no significant effect on AMPA or NMDA receptor-mediated glutamatergic input from primary afferents to dorsal horn neurons. Paclitaxel treatment significantly increased protein levels, but not mRNA levels, of NKCC1 in spinal cords. Inhibition of NKCC1 with bumetanide reversed the paclitaxel effect on GABA-mediated hyperpolarization and GABA reversal potentials. Also, intrathecal bumetanide significantly attenuated hyperalgesia and allodynia induced by paclitaxel. Co-immunoprecipitation revealed that NKCC1 interacted with β-tubulin and β-actin in spinal cords. Remarkably, paclitaxel increased NKCC1 protein levels at the plasma membrane and reduced NKCC1 levels in the cytosol of spinal cords. In contrast, treatment with an actin-stabilizing agent had no significant effect on NKCC1 protein levels in the plasma membrane or cytosolic fractions of spinal cords. In addition, inhibition of the motor protein dynein blocked paclitaxel-induced subcellular redistribution of NKCC1, whereas inhibition of kinesin-5 mimicked the paclitaxel effect. Our findings suggest that increased NKCC1 activity contributes to diminished spinal synaptic inhibition and neuropathic pain caused by paclitaxel. Paclitaxel disrupts intracellular NKCC1 trafficking by interfering with microtubule dynamics and associated motor proteins.

Luminal microbes promote monocyte-stem cell interactions across a healthy colonic epithelium

The intestinal epithelium forms a vital barrier between luminal microbes and the underlying mucosal immune system. Epithelial barrier function is maintained by continuous renewal of the epithelium and is pivotal for gut homeostasis. Breaching of the barrier causes mobilization of immune cells to promote epithelial restitution. However, it is not known whether microbes at the luminal surface of a healthy epithelial barrier influence immune cell mobilization to modulate tissue homeostasis. Using a mouse colonic mucosal explant model, we demonstrate that close proximity of luminal microbes to a healthy, intact epithelium results in rapid mucus secretion and movement of Ly6C⁺7/4⁺ monocytes closer to epithelial stem cells. These early events are driven by the epithelial MyD88-signaling pathway and result in increased crypt cell proliferation and intestinal stem cell number. Over time, stem cell number and monocyte–crypt stem cell juxtapositioning return to homeostatic levels observed in vivo. We also demonstrate that reduced numbers of tissue Ly6C⁺ monocytes can suppress Lgr5EGFP⁺ stem cell expression in vivo and abrogate the response to luminal microbes ex vivo. The functional link between monocyte recruitment and increased crypt cell proliferation was further confirmed using a crypt–monocyte coculture model. This work demonstrates that the healthy gut epithelium mediates communication between luminal bacteria and monocytes, and monocytes can modulate crypt stem cell number and promote crypt cell proliferation to help maintain gut homeostasis.

Canonical Wnt signals combined with suppressed TGFβ/BMP pathways promote renewal of the native human colonic epithelium

BACKGROUND

A defining characteristic of the human intestinal epithelium is that it is the most rapidly renewing tissue in the body. However, the processes underlying tissue renewal and the mechanisms that govern their coordination have proved difficult to study in the human gut.

OBJECTIVE

To investigate the regulation of stem cell-driven tissue renewal by canonical Wnt and TGFβ/bone morphogenetic protein (BMP) pathways in the native human colonic epithelium.

DESIGN

Intact human colonic crypts were isolated from mucosal tissue samples and placed into 3D culture conditions optimised for steady-state tissue renewal. High affinity mRNA in situ hybridisation and immunohistochemistry were complemented by functional genomic and bioimaging techniques. The effects of signalling pathway modulators on the status of intestinal stem cell biology, crypt cell proliferation, migration, differentiation and shedding were determined.

RESULTS

Native human colonic crypts exhibited distinct activation profiles for canonical Wnt, TGFβ and BMP pathways. A population of intestinal LGR5/OLFM4-positive stem/progenitor cells were interspersed between goblet-like cells within the crypt-base. Exogenous and crypt cell-autonomous canonical Wnt signals supported homeostatic intestinal stem/progenitor cell proliferation and were antagonised by TGFβ or BMP pathway activation. Reduced Wnt stimulation impeded crypt cell proliferation, but crypt cell migration and shedding from the crypt surface were unaffected and resulted in diminished crypts.

CONCLUSIONS

Steady-state tissue renewal in the native human colonic epithelium is dependent on canonical Wnt signals combined with suppressed TGFβ/BMP pathways. Stem/progenitor cell proliferation is uncoupled from crypt cell migration and shedding, and is required to constantly replenish the crypt cell population.

Sorbitol dehydrogenase overexpression and other aspects of dysregulated protein expression in human precancerous colorectal neoplasms: a quantitative proteomics study

Colorectal adenomas are cancer precursor lesions of the large bowel. A multitude of genomic and epigenomic changes have been documented in these preinvasive lesions, but their impact on the protein effectors of biological function has not been comprehensively explored. Using shotgun quantitative MS, we exhaustively investigated the proteome of 30 colorectal adenomas and paired samples of normal mucosa. Total protein extracts were prepared from these tissues (prospectively collected during colonoscopy) and from normal (HCEC) and cancerous (SW480, SW620, Caco2, HT29, CX1) colon epithelial cell lines. Peptides were labeled with isobaric tags (iTRAQ 8-plex), separated via OFFGEL electrophoresis, and analyzed by means of LC-MS/MS. Nonredundant protein families (4325 in tissues, 2017 in cell lines) were identified and quantified. Principal component analysis of the results clearly distinguished adenomas from normal mucosal samples and cancer cell lines from HCEC cells. Two hundred and twelve proteins displayed significant adenoma-related expression changes (q-value < 0.02, mean fold change versus normal mucosa ±1.4), which correlated (r = 0.74) with similar changes previously identified by our group at the transcriptome level. Fifty-one (∼25%) proteins displayed directionally similar expression changes in colorectal cancer cells (versus HCEC cells) and were therefore attributed to the epithelial component of adenomas. Although benign, adenomas already exhibited cancer-associated proteomic changes: 69 (91%) of the 76 protein up-regulations identified in these lesions have already been reported in cancers. One of the most striking changes involved sorbitol dehydrogenase, a key enzyme in the polyol pathway. Validation studies revealed dramatically increased sorbitol dehydrogenase concentrations and activity in adenomas and cancer cell lines, along with important changes in the expression of other enzymes in the same (AKR1B1) and related (KHK) pathways. Dysregulated polyol metabolism might represent a novel facet of metabolome remodeling associated with tumorigenesis.

Characterization of CFTR High Expresser cells in the intestine

The CFTR High Expresser (CHE) cells express eightfold higher levels of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel compared with neighboring enterocytes and were first identified by our laboratory (Ameen et al., Gastroenterology 108: 1016, 1995). We used double-label immunofluorescence microscopy to further study these enigmatic epithelial cells in rat intestine in vivo or ex vivo. CHE cells were found in duodenum, most frequent in proximal jejunum, and absent in ileum and colon. CFTR abundance increased in CHE cells along the crypt-villus axis. The basolateral Na(+)K(+)Cl(-) cotransporter NKCC1, a key transporter involved in Cl(-) secretion, was detected at similar levels in CHE cells and neighboring enterocytes at steady state. Microvilli appeared shorter in CHE cells, with low levels of Myosin 1a, a villus enterocyte-specific motor that retains sucrase/isomaltase in the brush-border membrane (BBM). CHE cells lacked alkaline phosphatase and absorptive villus enterocyte BBM proteins, including Na(+)H(+) exchanger NHE3, Cl(-)/HCO3(-) exchanger SLC26A6 (putative anion exchanger 1), and sucrase/isomaltase. High levels of the vacuolar-ATPase proton pump were observed in the apical domain of CHE cells. Levels of the NHE regulatory factor NHERF1, Na-K-ATPase, and Syntaxin 3 were similar to that of neighboring enterocytes. cAMP or acetylcholine stimulation robustly increased apical CFTR and basolateral NKCC1 disproportionately in CHE cells relative to neighboring enterocytes. These data strongly argue for a specialized role of CHE cells in Cl(-)-mediated "high-volume" fluid secretion on the villi of the proximal small intestine.

Regulatory mechanisms for glycogenolysis and K+ uptake in brain astrocytes

Recent advances in brain energy metabolism support the notion that glycogen in astrocytes is necessary for the clearance of neuronally-released K(+) from the extracellular space. However, how the multiple metabolic pathways involved in K(+)-induced increase in glycogen turnover are regulated is only partly understood. Here we summarize the current knowledge about the mechanisms that control glycogen metabolism during enhanced K(+) uptake. We also describe the action of the ubiquitous Na(+)/K(+) ATPase for both ion transport and intracellular signaling cascades, and emphasize its importance in understanding the complex relation between glycogenolysis and K(+) uptake.

Vasoactive intestinal peptide regulates sinonasal mucociliary clearance and synergizes with histamine in stimulating sinonasal fluid secretion

Mucociliary clearance (MCC) is the primary physical airway defense against inhaled pathogens and particulates. MCC depends on both proper fluid/mucus homeostasis and epithelial ciliary beating. Vasoactive intestinal peptide (VIP) is a neurotransmitter expressed in the sinonasal epithelium that is up-regulated in allergy. However, the effects of VIP on human sinonasal physiology are unknown, as are VIP's interactions with histamine, a major regulator of allergic disease. We imaged ciliary beat frequency, mucociliary transport, apical Cl(-) permeability, and airway surface liquid (ASL) height in primary human sinonasal air-liquid-interface cultures to investigate the effects of VIP and histamine. VIP stimulated an increase in ciliary beat frequency (EC50 0.5 μM; maximal increase ∼40% compared with control) and cystic fibrosis transmembrane conductance regulator (CFTR)-dependent and Na(+)K(+)2Cl(-) cotransporter-dependent fluid secretion, all requiring cAMP/PKA signaling. Histamine activated Ca(2+) signaling that increased ASL height but not ciliary beating. Low concentrations of VIP and histamine had synergistic effects on CFTR-dependent fluid secretion, revealed by increased ASL heights. An up-regulation of VIP in histamine-driven allergic rhinitis would likely enhance mucosal fluid secretion and contribute to allergic rhinorrhea. Conversely, a loss of VIP-activated secretion in patients with CF may impair mucociliary transport, contributing to increased incidences of sinonasal infections and rhinosinusitis.

COMMD1 interacts with the COOH terminus of NKCC1 in Calu-3 airway epithelial cells to modulate NKCC1 ubiquitination

Mice deficient in Na-K-2Cl cotransporter (NKCC1) have been generated by targeted disruption of the gene encoding NKCC1 involving the carboxy terminus (CT-NKCC1) but not the amino terminus. We hypothesize that the resulting physiological defects are due to loss of proteins interacting with CT-NKCC1. Using a yeast two-hybrid approach, adaptor protein COMMD1 was found to bind to CT-NKCC1 (aa 1,040-1,212). Binding was verified in a yeast-independent system using GST-COMMD1 and myc-CT-NKCC1. Truncated COMMD1 and CT-NKCC1 peptides were used in binding assays to identify the site of interaction. The results demonstrate concentration-dependent binding of COMMD1 (aa 1-47) to CT-NKCC1 (aa 1,040-1,134). Endogenous COMMD1 was detected in pull downs using recombinant FLAG-CT-NKCC1; this co-pull down was blocked by COMMD1 (aa 1-47). CT-NKCC1 (aa 1,040-1,137) decreased basolateral membrane expression of NKCC1, and COMMD1 (aa 1-47) increased NKCC1 membrane expression. Downregulation of COMMD1 using silencing (si)RNA led to a transient loss of endogenous COMMD1 but did not affect activation of NKCC1 by hyperosmotic sucrose. Hyperosmolarity caused a transient increase in NKCC1 membrane expression, indicating regulated trafficking of NKCC1; downregulation of COMMD1 using siRNA reduced baseline (unstimulated) NKCC1 expression and blunted a transient elevation in NKCC1 membrane expression caused by hyperosmolarity. Constitutive downregulation of COMMD1 in HT29 engineered cells exhibited loss of COMMD1 and decreased NKCC1 membrane expression with no effect on activation of NKCC1. Loss of COMMD1 in Calu-3 cells and in HT29 cells led to reduced ubiquitinated NKCC1. The results indicate a role for COMMD1 in the regulation of NKCC1 membrane expression and ubiquitination.

Capsaicin induces NKCC1 internalization and inhibits chloride secretion in colonic epithelial cells independently of TRPV1

Colonic chloride secretion is regulated via the neurohormonal and immune systems. Exogenous chemicals (e.g., butyrate, propionate) can affect chloride secretion. Capsaicin, the pungent ingredient of the chili peppers, exerts various effects on gastrointestinal function. Capsaicin is known to activate the transient receptor potential vanilloid type 1 (TRPV1), expressed in the mesenteric nervous system. Recent studies have also demonstrated its presence in epithelial cells but its role remains uncertain. Because capsaicin has been reported to inhibit colonic chloride secretion, we tested whether this effect of capsaicin could occur by direct action on epithelial cells. In mouse colon and model T84 human colonic epithelial cells, we found that capsaicin inhibited forskolin-dependent short-circuit current (FSK-I(sc)). Using PCR and Western blot, we demonstrated the presence of TRPV1 in colonic epithelial cells. In T84 cells, TRPV1 localized at the basolateral membrane and in vesicular compartments. In permeabilized monolayers, capsaicin activated apical chloride conductance, had no effect on basolateral potassium conductance, but induced NKCC1 internalization demonstrated by immunocytochemistry and basolateral surface biotinylation. AMG-9810, a potent inhibitor of TRPV1, did not prevent the inhibition of the FSK-I(sc) by capsaicin. Neither resiniferatoxin nor N-oleoyldopamine, two selective agonists of TRPV1, blocked the FSK-I(sc). Conversely capsaicin, resiniferatoxin, and N-oleoyldopamine raised intracellular calcium ([Ca(2+)](i)) in T84 cells and AMG-9810 blocked the rise in [Ca(2+)](i) induced by capsaicin and resiniferatoxin suggesting the presence of a functional TRPV1 channel. We conclude that capsaicin inhibits chloride secretion in part by causing NKCC1 internalization, but by a mechanism that appears to be independent of TRPV1.

Lubiprostone targets prostanoid signaling and promotes ion transporter trafficking, mucus exocytosis, and contractility

BACKGROUND AND AIM

Lubiprostone is a chloride channel activator in clinical use for the treatment of chronic constipation, but the mechanisms of action of the drug are poorly understood. The aim of this study was to determine whether lubiprostone exerts secretory effects in the intestine by membrane trafficking of ion transporters and associated machinery.

METHODS

Immunolabeling and quantitative fluorescence intensity were used to examine lubiprostone-induced trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR), sodium/potassium-coupled chloride co-transporter 1 (NKCC1), electrogenic sodium/bicarbonate co-transporter 1 (NBCe1), down-regulated in adenoma (DRA), putative anion transporter 1 (PAT1), sodium/proton exchanger 3 (NHE3), Ca(2+) activated chloride channel 2 (ClC-2) serotonin and its transporter SERT, E prostanoid receptors EP4 and EP1, sodium/potassium ATPase (Na-K-ATPase) and protein kinase A (PKA). The effects of lubiprostone on mucus exocytosis in rat intestine and human rectosigmoid explants were also examined.

RESULTS

Lubiprostone induced contraction of villi and proximal colonic plicae and membrane trafficking of transporters that was more pronounced in villus/surface cells compared to the crypt. Membrane trafficking was determined by: (1) increased membrane labeling for CFTR, PAT1, NKCC1, and NBCe1 and decreased membrane labeling for NHE3, DRA and ClC-2; (2) increased serotonin, SERT, EP4, EP1 and PKA labeling in enterochromaffin cells; (3) increased SERT, EP4, EP1, PKA and Na-K-ATPase in enterocytes; and (4) increased mucus exocytosis in goblet cells.

CONCLUSION

These data suggest that lubiprostone can target serotonergic, EP4/PKA and EP1 signaling in surface/villus regions; stimulate membrane trafficking of CFTR/NBCe1/NKCC1 in villus epithelia and PAT1/NBCe1/NKCC1 in colonic surface epithelia; suppress NHE3/DRA trafficking and fluid absorption; and enhance mucus-mobilization and mucosal contractility.

Functional Cftr in crypt epithelium of organotypic enteroid cultures from murine small intestine

Physiological studies of intact crypt epithelium have been limited by problems of accessibility in vivo and dedifferentiation in standard primary culture. Investigations of murine intestinal stem cells have recently yielded a primary intestinal culture in three-dimensional gel suspension that recapitulates crypt structure and epithelial differentiation (Sato T, Vries RG, Snippert HJ, van de Wetering M, Barker N, Stange DE, Van Es JH, Abo A, Kujala P, Peters PJ, Clevers H. Nature 459: 262-265, 2009). We investigated the utility of murine intestinal crypt cultures (termed "enteroids") for physiological studies of crypt epithelium by focusing on the transport activity of the cystic fibrosis transmembrane conductance regulator Cftr. Enteroids had multiple crypts with well-differentiated goblet and Paneth cells that degranulated on exposure to the muscarinic agonist carbachol. Modified growth medium provided a crypt proliferation rate, as measured by 5-ethynyl-2'-deoxyuridine labeling, which was similar to proliferation in vivo. Immunoblots demonstrated equivalent Cftr expression in comparisons of freshly isolated crypts with primary and passage 1 enteroids. Apparent enteroid differences in mRNA expression of other transporters were primarily associated with villous epithelial contamination of freshly isolated crypts. Microelectrode analysis revealed cAMP-stimulated membrane depolarization in enteroid epithelium from wild-type (WT) but not Cftr knockout (KO) mice. Morphological and microfluorimetric studies, respectively, demonstrated Cftr-dependent cell shrinkage and lower intracellular pH in WT enteroid epithelium in contrast to Cftr KO epithelium or WT epithelium treated with Cftr inhibitor 172. We conclude that crypt epithelium of murine enteroids exhibit Cftr expression and activity that recapitulates crypt epithelium in vivo. Enteroids provide a primary culture model that is suitable for physiological studies of regenerating crypt epithelium.

Chronic regulation of colonic epithelial secretory function by activation of G protein-coupled receptors

BACKGROUND

Enteric neurotransmitters that act at G protein-coupled receptors (GPCRs) are well known to acutely promote epithelial Cl(-) and fluid secretion. Here we examined if acute GPCR activation might have more long-term consequences for epithelial secretory function.

METHODS

Cl(-) secretion was measured as changes in short-circuit current across voltage-clamped T(84) colonic epithelial cells. Protein expression was measured by western blotting and intracellular Ca(2+) levels by Fura-2 fluorescence.

KEY RESULTS

While acute (15 min) treatment of T(84) cells with a cholinergic G(q) PCR agonist, carbachol (CCh), rapidly stimulated Cl(-) secretion, subsequent CCh-induced responses were attenuated in a biphasic manner. The first phase was transient and resolved within 6 h but this was followed by a chronic phase of attenuated responsiveness that was sustained up to 48 h. CCh-pretreatment did not chronically alter responses to another G(q)PCR agonist, histamine, or to thapsigargin or forskolin which elevate intracellular Ca(2+) and cAMP, respectively. This chronically acting antisecretory mechanism is not shared by neurotransmitters that activate G(s)PCRs. Conditioned medium from CCh-pretreated cells mimicked its chronic antisecretory actions, suggesting involvement of an epithelial-derived soluble factor but further experimentation ruled out the involvement of epidermal growth factor receptor ligands. Acute CCh exposure did not chronically alter surface expression of muscarinic M(3) receptors but inhibited intracellular Ca(2+) mobilization upon subsequent agonist challenge.

CONCLUSIONS & INFERENCES

These data reveal a novel, chronically acting, antisecretory mechanism that downregulates epithelial secretory capacity upon repeated G(q)PCR agonist exposure. This mechanism involves release of a soluble factor that uncouples receptor activation from downstream prosecretory signals.

Physiological relevance of cell-specific distribution patterns of CFTR, NKCC1, NBCe1, and NHE3 along the crypt-villus axis in the intestine

We examined the cell-specific subcellular expression patterns for sodium- and potassium-coupled chloride (NaK2Cl) cotransporter 1 (NKCC1), Na(+) bicarbonate cotransporter (NBCe1), cystic fibrosis transmembrane conductance regulator (CFTR), and Na(+)/H(+) exchanger 3 (NHE3) to understand the functional plasticity and synchronization of ion transport functions along the crypt-villus axis and its relevance to intestinal disease. In the unstimulated intestine, all small intestinal villus enterocytes coexpressed apical CFTR and NHE3, basolateral NBCe1, and mostly intracellular NKCC1. All (crypt and villus) goblet cells strongly expressed basolateral NKCC1 (at approximately three-fold higher levels than villus enterocytes), but no CFTR, NBCe1, or NHE3. Lower crypt cells coexpressed apical CFTR and basolateral NKCC1, but no NHE3 or NBCe1 (except NBCe1-expressing proximal colonic crypts). CFTR, NBCe1, and NKCC1 colocalized with markers of early and recycling endosomes, implicating endocytic recycling in cell-specific anion transport. Brunner's glands of the proximal duodenum coexpressed high levels of apical/subapical CFTR and basolateral NKCC1, but very low levels of NBCe1, consistent with secretion of Cl(-)-enriched fluid into the crypt. The cholinergic agonist carbachol rapidly (within 10 min) reduced cell volume along the entire crypt/villus axis and promoted NHE3 internalization into early endosomes. In contrast, carbachol induced membrane recruitment of NKCC1 and CFTR in all crypt and villus enterocytes, NKCC1 in all goblet cells, and NBCe1 in all villus enterocytes. These observations support regulated vesicle traffic in Cl(-) secretion by goblet cells and Cl(-) and HCO(3)(-) secretion by villus enterocytes during the transient phase of cholinergic stimulation. Overall, the carbachol-induced membrane trafficking profile of the four ion transporters supports functional plasticity of the small intestinal villus epithelium that enables it to conduct both absorptive and secretory functions.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

AE2 Cl-/HCO3- exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 Na(+)-K(+)-2Cl(-) cotransporter mediates basolateral Cl(-) uptake. However, additional evidence suggests that the AE2 Cl(-)/HCO(3)(-) exchanger, when coupled with the NHE1 Na(+)/H(+) exchanger or a Na(+)-HCO(3)(-) cotransporter (NBC), contributes to HCO(3)(-) and/or Cl(-) uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (I(sc)) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2(-/-)) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated I(sc) was the same in WT and AE2(-/-) colon. However, the I(sc) response in AE2(-/-) colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO(3)(-) resulted in robust cAMP-stimulated I(sc) in both AE2(-/-) and WT colon that was largely mediated by NKCC1, whereas removal of Cl(-) resulted in sharply decreased cAMP-stimulated I(sc) in AE2(-/-) colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated I(sc) in AE2(-/-) colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2(-/-) colon, Cl(-) secretion supported by basolateral NKCC1 is enhanced, whereas HCO(3)(-) secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.