Genetic Ablation of the ClC-2 Cl- Channel Disrupts Mouse Gastric Parietal Cell Acid Secretion

Metabolic regulator ERRγ governs gastric stem cell differentiation into acid-secreting parietal cells

Parietal cells (PCs) produce gastric acid to kill pathogens and aid digestion. Dysregulated PC census is common in disease, yet how PCs differentiate is unclear. Here, we identify the PC progenitors arising from isthmal stem cells, using mouse models and human gastric cells, and show that they preferentially express cell-metabolism regulator and orphan nuclear receptor Estrogen-related receptor gamma (Esrrg, encoding ERRγ). Esrrg expression facilitated the tracking of stepwise molecular, cellular, and ultrastructural stages of PC differentiation. EsrrgP2ACreERT2 lineage tracing revealed that Esrrg expression commits progenitors to differentiate into mature PCs. scRNA-seq indicated the earliest Esrrg+ PC progenitors preferentially express SMAD4 and SP1 transcriptional targets and the GTPases regulating acid-secretion signal transduction. As progenitors matured, ERRγ-dependent metabolic transcripts predominated. Organoid and mouse studies validated the requirement of ERRγ for PC differentiation. Our work chronicles stem cell differentiation along a single lineage in vivo and suggests ERRγ as a therapeutic target for PC-related disorders.

SLC26A9 in airways and intestine: secretion or absorption?

SLC26A9 is one out of 11 proteins that belong to the SLC26A family of anion transporters. Apart from expression in the gastrointestinal tract, SLC26A9 is also found in the respiratory system, in male tissues and in the skin. SLC26A9 has gained attention because of its modifier role in the gastrointestinal manifestation of cystic fibrosis (CF). SLC26A9 appears to have an impact on the extent of intestinal obstruction caused by meconium ileus. SLC26A9 supports duodenal bicarbonate secretion, but was assumed to provide a basal Cl- secretory pathway in airways. However, recent results show that basal airway Cl- secretion is due to cystic fibrosis conductance regulator (CFTR), while SLC26A9 may rather secrete HCO₃-, thereby maintaining proper airway surface liquid (ASL) pH. Moreover, SLC26A9 does not secrete but probably supports reabsorption of fluid particularly in the alveolar space, which explains early death by neonatal distress in Slc26a9-knockout animals. While the novel SLC26A9 inhibitor S9-A13 helped to unmask the role of SLC26A9 in the airways, it also provided evidence for an additional role in acid secretion by gastric parietal cells. Here we discuss recent data on the function of SLC26A9 in airways and gut, and how S9-A13 may be useful in unraveling the physiological role of SLC26A9.

Expression alteration and dysfunction of ion channels/transporters in the parietal cells induces gastric diffused mucosal injury

Gastric mucosal injuries include focal and diffused injuries, which do and do not change the cell differentiation pattern. Parietal cells loss is related to the occurrence of gastric mucosal diffused injury, with two phenotypes of spasmolytic polypeptide-expressing metaplasia and neuroendocrine cell hyperplasia, which is the basis of gastric cancer and gastric neuroendocrine tumor respectively. Multiple ion channels and transporters are located and expressed in the parietal cells, which is not only regulate the gastric acid-base homeostasis, but also regulate the growth and development of parietal cells. Therefore, alteration and dysregulation of ion channels and transporters in the parietal cells impairs the morphology and physiological functions of stomach, resulted in gastric diffused mucosal damage. In this review, multiple ion channels and transporters in parietal cells, including K⁺ channels, aquaporins, Cl^- channels, Na⁺/H⁺ transporters, and Cl^-/HCO₃^- transporters are described, and their roles in gastric diffused mucosal injury are discussed. We hope to drive researcher's attention to focus on the role of ion channels/transporters loss in the parietal cells induced gastric diffused mucosal injury.

Pathophysiological role of ion channels and transporters in gastrointestinal mucosal diseases

The incidence of gastrointestinal (GI) mucosal diseases, including various types of gastritis, ulcers, inflammatory bowel disease and GI cancer, is increasing. Therefore, it is necessary to identify new therapeutic targets. Ion channels/transporters are located on cell membranes, and tight junctions (TJs) affect acid–base balance, the mucus layer, permeability, the microbiota and mucosal blood flow, which are essential for maintaining GI mucosal integrity. As ion channel/transporter dysfunction results in various GI mucosal diseases, this review focuses on understanding the contribution of ion channels/transporters to protecting the GI mucosal barrier and the relationship between GI mucosal disease and ion channels/transporters, including Cl⁻/HCO₃⁻ exchangers, Cl⁻ channels, aquaporins, Na⁺/H⁺ exchangers, and K⁺ channels. Here, we provide novel prospects for the treatment of GI mucosal diseases.

Parallel intermediate conductance K+ and Cl- channel activity mediates electroneutral K+ exit across basolateral membranes in rat distal colon

Transepithelial K⁺ absorption requires apical K⁺ uptake and basolateral K⁺ exit. In the colon, apical H⁺-K⁺-ATPase mediates cellular K⁺ uptake, and it has been suggested that electroneutral basolateral K⁺ exit reflects K⁺-Cl^- cotransporter-1 (KCC1) operating in parallel with K⁺ and Cl^- channels. The present study was designed to identify basolateral transporter(s) responsible for K⁺ exit in rat distal colon. Active K⁺ absorption was determined by measuring ⁸⁶Rb⁺ (K⁺ surrogate) fluxes across colonic epithelia under voltage-clamp conditions. With zero Cl^- in the mucosal solution, net K⁺ absorption was reduced by 38%, indicating that K⁺ absorption was partially Cl^--dependent. Serosal addition of DIOA (KCC1 inhibitor) or Ba²⁺ (nonspecific K⁺ channel blocker) inhibited net K⁺ absorption by 21% or 61%, respectively, suggesting that both KCC1 and K⁺ channels contribute to basolateral K⁺ exit. Clotrimazole and TRAM34 (IK channel blockers) added serosally inhibited net K⁺ absorption, pointing to the involvement of IK channels in basolateral K⁺ exit. GaTx2 (CLC2 blocker) added serosally also inhibited net K⁺ absorption, suggesting that CLC2-mediated Cl^- exit accompanies IK channel-mediated K⁺ exit across the basolateral membrane. Net K⁺ absorption was not inhibited by serosal addition of either IbTX (BK channel blocker), apamin (SK channel blocker), chromanol 293B (K_V7 channel blocker), or CFTR_inh172 (CFTR blocker). Immunofluorescence studies confirmed basolateral membrane colocalization of CLC2-like proteins and Na⁺-K⁺-ATPase α-subunits. We conclude that active K⁺ absorption in rat distal colon involves electroneutral basolateral K⁺ exit, which may reflect IK and CLC2 channels operating in parallel.NEW & NOTEWORTHY This study demonstrates that during active electroneutral K⁺ absorption in rat distal colon, K⁺ exit across the basolateral membrane mainly reflects intermediate conductance K⁺ channels operating in conjunction with chloride channel 2, with a smaller, but significant, contribution from K⁺-Cl^- cotransporter-1 (KCC1) activity.

A Metformin-Responsive Metabolic Pathway Controls Distinct Steps in Gastric Progenitor Fate Decisions and Maturation

Cellular metabolism plays important functions in dictating stem cell behaviors, although its role in stomach epithelial homeostasis has not been evaluated in depth. Here, we show that the energy sensor AMP kinase (AMPK) governs gastric epithelial progenitor differentiation. Administering the AMPK activator metformin decreases epithelial progenitor proliferation and increases acid-secreting parietal cells (PCs) in mice and organoids. AMPK activation targets Krüppel-like factor 4 (KLF4), known to govern progenitor proliferation and PC fate choice, and PGC1α, which we show controls PC maturation after their specification. PC-specific deletion of AMPKα or PGC1α causes defective PC maturation, which could not be rescued by metformin. However, metformin treatment still increases KLF4 levels and suppresses progenitor proliferation. Thus, AMPK activates KLF4 in progenitors to reduce self-renewal and promote PC fate, whereas AMPK-PGC1α activation within the PC lineage promotes maturation, providing a potential suggestion for why metformin increases acid secretion and reduces gastric cancer risk in humans.

Physiological Significance of Ion Transporters and Channels in the Stomach and Pathophysiological Relevance in Gastric Cancer

Gastric cancer (GC) is a highly invasive and fatal malignant disease that accounts for 5.7% of new global cancer cases and is the third leading cause of cancer-related death. Acid/base homeostasis is critical for organisms because protein and enzyme function, cellular structure, and plasma membrane permeability change with pH. Various ion transporters are expressed in normal gastric mucosal epithelial cells and regulate gastric acid secretion, ion transport, and fluid absorption, thereby stabilizing the differentiation and homeostasis of gastric mucosal epithelial cells. Ion transporter dysfunction results in disordered ion transport, mucosa barrier dysfunction, and acid/base disturbances, causing gastric acid-related diseases such as chronic atrophic gastritis (CAG) and GC. This review summarizes the physiological functions of multiple ion transporters and channels in the stomach, including Cl⁻ channels, Cl⁻/HCO₃⁻ exchangers, sodium/hydrogen exchangers (NHEs), and potassium (K⁺) channels, and their pathophysiological relevance in GC.

The Integral Role of Tight Junction Proteins in the Repair of Injured Intestinal Epithelium

The intestinal epithelial monolayer forms a transcellular and paracellular barrier that separates luminal contents from the interstitium. The paracellular barrier consists of a highly organized complex of intercellular junctions that is primarily regulated by apical tight junction proteins and tight junction-associated proteins. This homeostatic barrier can be lost through a multitude of injurious events that cause the disruption of the tight junction complex. Acute repair after injury leading to the reestablishment of the tight junction barrier is crucial for the return of both barrier function as well as other cellular functions, including water regulation and nutrient absorption. This review provides an overview of the tight junction complex components and how they link to other plasmalemmal proteins, such as ion channels and transporters, to induce tight junction closure during repair of acute injury. Understanding the components of interepithelial tight junctions and the mechanisms of tight junction regulation after injury is crucial for developing future therapeutic targets for patients experiencing dysregulated intestinal permeability.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Chloride channel ClC- 2 enhances intestinal epithelial tight junction barrier function via regulation of caveolin-1 and caveolar trafficking of occludin

Previous studies have demonstrated that the chloride channel ClC-2 plays a critical role in intestinal epithelial tight junction (TJ) barrier function via intracellular trafficking of TJ protein occludin. To study the mechanism of ClC-2-mediated TJ barrier function and intracellular trafficking of occludin, we established ClC-2 over-expressing Caco-2 cell line (Caco-2^CLCN2) by full length ClC-2 ORF transfection. ClC-2 over-expression (Caco-2^CLCN2) significantly enhanced TJ barrier (increased TER by ≥2 times and reduced inulin flux by 50%) compared to control Caco-2^pEZ cells. ClC-2 over-expression (Caco-2^CLCN2) increased occludin protein level compared to control Caco-2^pEZ cells. Surface biotinylation assay revealed reduced steady state endocytosis of occludin in Caco-2^CLCN2 cells. Furthermore, ClC-2 over-expression led to reduction in caveolin-1 protein level and diminishment of caveolae assembly. Caveolae disruption increased TJ permeability in control but not ClC-2 over-expressing Caco-2^CLCN2 cells. Selective ClC-2 channel blocker GaTx2 caused an increase in caveolin-1 protein level and reduced occludin level. Delivery of cell permeable caveolin-1 scaffolding domain reduced the occludin protein level. Over all, these results suggest that ClC- 2 enhances TJ barrier function in intestinal epithelial cells via regulation of caveolin-1 and caveolae-mediated trafficking of occludin.

Transgenic and gene knockout mice in gastric cancer research

Mouse models are useful tool for carcinogenic study. They will greatly enrich the understanding of pathogenesis and molecular mechanisms for gastric cancer. However, only few of mice could develop gastric cancer spontaneously. With the development and improvement of gene transfer technology, investigators created a variety of transgenic and knockout/knockin mouse models of gastric cancer, such as INS-GAS mice and gastrin knockout mice. Combined with helicobacter infection and carcinogens treatment, these transgenic/knockout/knockin mice developed precancerous or cancerous lesions, which are proper for gene function study or experimental therapy. Here we review the progression of genetically engineered mouse models on gastric cancer research, and emphasize the effects of chemical carcinogens or infectious factors on carcinogenesis of genetically modified mouse. We also emphasize the histological examination on mouse stomach. We expect to provide researchers with some inspirations on this field.

Gastric acid secretion

PURPOSE OF REVIEW

The present review summarizes the past year's literature, both clinical and basic science, regarding neuroendocrine and intracellular regulation of gastric acid secretion and proper use of antisecretory medications.

RECENT FINDINGS

Gastric acid kills microorganisms, modulates the gut microbiome, assists in digestion of protein, and facilitates absorption of iron, calcium, and vitamin B12. The main stimulants of acid secretion are gastrin, released from antral G cells; histamine, released from oxyntic enterochromaffin-like cells; and acetylcholine, released from antral and oxyntic intramural neurons. Other stimulants include ghrelin, motilin, and hydrogen sulfide. The main inhibitor of acid secretion is somatostatin, released from oxyntic and antral D cells. Glucagon-like peptide-1 also inhibits acid secretion. Proton pump inhibitors (PPIs) reduce acid secretion and, as a result, decrease somatostatin and thus stimulate gastrin secretion. Although considered well tolerated drugs, concerns have been raised this past year regarding associations between PPI use and kidney disease, dementia, and myocardial infarction; the quality of evidence, however, is very low.

SUMMARY

Our understanding of the physiology of gastric secretion and proper use of PPIs continues to advance. Such knowledge is crucial for improved management of acid-peptic disorders.