(Patho-)Physiology of Na+/H+ Exchangers (NHEs) in the Digestive System

Intracellular pH dynamics regulates intestinal stem cell lineage specification

Intracellular pH dynamics is increasingly recognized to regulate myriad cell behaviors. We report a finding that intracellular pH dynamics also regulates adult stem cell lineage specification. We identify an intracellular pH gradient in mouse small intestinal crypts, lowest in crypt stem cells and increasing along the crypt column. Disrupting this gradient by inhibiting H+ efflux by Na+/H+ exchanger 1 abolishes crypt budding and blocks differentiation of Paneth cells, which are rescued with exogenous WNT. Using single-cell RNA sequencing and lineage tracing we demonstrate that intracellular pH dynamics acts downstream of ATOH1, with increased pH promoting differentiation toward the secretory lineage. Our findings indicate that an increase in pH is required for the lineage specification that contributes to crypt maintenance, establishing a role for intracellular pH dynamics in cell fate decisions within an adult stem cell lineage.

Whole Transcriptome Analysis of Differentially Expressed Genes in Cultured Nile Tilapia (O. niloticus) Subjected to Chronic Stress Reveals Signaling Pathways Associated with Depressed Growth

Chronic stress is a serious threat to aquaculture as it lowers fish growth performance and compromises fish welfare. The exact mechanism by which growth is retarded is, however, not clearly understood. This study sought to elucidate the gene expression profiles associated with chronic stress in cultured Nile tilapia (Oreochromis niloticus) reared for 70 days at different ammonia concentrations and stocking densities. Fish in the treatment groups showed negative growth, while the controls showed positive allometric growth. The specific condition factor (Kn) ranged from 1.17 for the controls to 0.93 for the ammonia and 0.91 for the stocking density treatments. RNA was extracted from muscle tissue using TRIzol followed by library construction and Illumina sequencing. Comparative transcriptome analysis revealed 209 differentially expressed genes (DEGs) (156 up- and 53 down-regulated) in the ammonia and 252 DEGs (175 up- and 77 down-regulated) in the stocking density treatment. In both treatments, 24 and 17 common DEGs were up- and down-regulated, respectively. DEGs were significantly enriched in six pathways associated with muscle activity, energy mobilization and immunity. The heightened muscular activity consumes energy which would otherwise have been utilized for growth. These results bring to fore the molecular mechanisms underlying chronic stress’ suppression of growth in cultured Nile tilapia.

Severe acute respiratory syndrome coronavirus 2 may cause liver injury via Na+/H+ exchanger

The liver has many significant functions, such as detoxification, the urea cycle, gluconeogenesis, and protein synthesis. Systemic diseases, hypoxia, infections, drugs, and toxins can easily affect the liver, which is extremely sensitive to injury. Systemic infection of severe acute respiratory syndrome coronavirus 2 can cause liver damage. The primary regulator of intracellular pH in the liver is the Na⁺/H⁺ exchanger (NHE). Physiologically, NHE protects hepatocytes from apoptosis by making the intracellular pH alkaline. Severe acute respiratory syndrome coronavirus 2 increases local angiotensin II levels by binding to angiotensin-converting enzyme 2. In severe cases of coronavirus disease 2019, high angi-otensin II levels may cause NHE overstimulation and lipid accumulation in the liver. NHE overstimulation can lead to hepatocyte death. NHE overstimulation may trigger a cytokine storm by increasing proinflammatory cytokines in the liver. Since the release of proinflammatory cytokines such as interleukin-6 increases with NHE activation, the virus may indirectly cause an increase in fibrinogen and D-dimer levels. NHE overstimulation may cause thrombotic events and systemic damage by increasing fibrinogen levels and cytokine release. Also, NHE overstimulation causes an increase in the urea cycle while inhibiting vitamin D synthesis and gluconeogenesis in the liver. Increasing NHE3 activity leads to Na⁺ loading, which impairs the containment and fluidity of bile acid. NHE overstimulation can change the gut microbiota composition by disrupting the structure and fluidity of bile acid, thus triggering systemic damage. Unlike other tissues, tumor necrosis factor-alpha and angiotensin II decrease NHE3 activity in the intestine. Thus, increased luminal Na⁺ leads to diarrhea and cytokine release. Severe acute respiratory syndrome coronavirus 2-induced local and systemic damage can be improved by preventing virus-induced NHE overstimulation in the liver.

Downregulation of SLC9A8 Promotes Epithelial-Mesenchymal Transition and Metastasis in Colorectal Cancer Cells via the IL6-JAK1/STAT3 Signaling Pathway

BACKGROUND

SLC9A8 has been shown to be involved in mucus layer formation, intestinal mucosal integrity, and hyperproliferation of colitis-associated tumor development. However, its effects on the epithelial-mesenchymal transition (EMT) and the metastasis of colorectal cancer (CRC) remain unknown.

AIMS

To explore whether SLC9A8 participates in EMT and the metastasis of CRC.

METHODS

Western blotting and immunohistochemistry were performed to evaluate the expression of SLC9A8 in CRC patients. At the cellular level, the effect of SLC9A8 on proliferation, migration, and invasion was measured using cell viability analysis, flow cytometry analysis, and Transwell assays. Mouse tumor xenograft and metastasis models were established to analyze whether knockdown of SLC9A8 increased tumor volume, tumor weight, and metastasis. Moreover, whether downregulated expression of SLC9A8 promotes EMT via activation of the IL6-JAK1-STAT3 signaling pathway was investigated.

RESULTS

SLC9A8 protein was downregulated in CRC tissues, and this downregulation was significantly associated with tumor size, lymph node status, pTNM stage, and poor prognosis. SLC9A8 overexpression markedly suppressed cell proliferation, migration, and invasion. Downregulation of SLC9A8 promoted CRC cell proliferation, migration, and invasion. Moreover, knockdown of SLC9A8 also increased tumor volume, tumor weight, and metastasis in vivo. Meanwhile, downregulation of SLC9A8 significantly promoted the in vitro migration of CRC cells via EMT by activating the IL6-JAK1/STAT3 signaling pathway.

CONCLUSIONS

Downregulation of SLC9A8 plays an important role in EMT and metastasis of CRC progression and may become a new potential therapeutic target for the treatment of CRC.

Landscape of sialylation patterns identify biomarkers for diagnosis and prediction of response to anti-TNF therapy in crohn's disease

Crohn's disease (CD), a subtype of inflammatory bowel disease (IBD), causes chronic gastrointestinal tract inflammation. Thirty percent of patients do not respond to anti-tumor necrosis factor (TNF) therapy. Sialylation is involved in the pathogenesis of IBD. We aimed to identify potential biomarkers for diagnosing CD and predicting anti-TNF medication outcomes in CD. Three potential biomarkers (SERPINB2, TFPI2, and SLC9B2) were screened using bioinformatics analysis and machine learning based on sialylation-related genes. Moreover, the combined model of SERPINB2, TFPI2, and SLC9B2 showed excellent diagnostic value in both the training and validation cohorts. Importantly, a Sial-score was constructed based on the expression of SERPINB2, TFPI2, and SLC9B2. The Sial-low group showed a lower level of immune infiltration than the Sial-high group. Anti-TNF therapy was effective for 94.4% of patients in the Sial-low group but only 15.8% in the Sial-high group. The Sial-score had an outstanding ability to predict and distinguish between responders and non-responders. Our comprehensive analysis indicates that SERPINB2, TFPI2, and SLC9B2 play essential roles in pathogenesis and anti-TNF therapy resistance in CD. Furthermore, it may provide novel concepts for customizing treatment for individual patients with CD.

The Physiological Function and Potential Role of the Ubiquitous Na+/H+ Exchanger Isoform 8 (NHE8): An Overview Data

The Na⁺/H⁺ exchanger transporters (NHE) play an important role in various biologic processes including Na⁺ absorption, intracellular pH homeostasis, cell volume regulation, proliferation, and apoptosis. The wide expression pattern and cellular localization of NHEs make these proteins pivotal players in virtually all human tissues and organs. In addition, recent studies suggest that NHEs may be one of the primeval transport protein forms in the history of life. Among the different isoforms, the most well-characterized NHEs are the Na⁺/H⁺ exchanger isoform 1 (NHE1) and Na⁺/H⁺ exchanger isoform 3 (NHE3). However, Na⁺/H⁺ exchanger isoform 8 (NHE8) has been receiving attention based on its recent discoveries in the gastrointestinal tract. In this review, we will discuss what is known about the physiological function and potential role of NHE8 in the main organ systems, including useful overviews that could inspire new studies on this multifaceted protein.

Levothyroxine treatment and gastric juice pH in humans: the proof of concept

PURPOSE

Despite the absorption of oral thyroxine (T4) occurs in the small bowel, several patients with gastric disorders show an increased need for T4. In vitro evidence suggested that medium pH variations interfere with T4 dissolution. This study was aimed at finding the proof of concept of a direct relationship between the minimal effective dose of T4 and the actual gastric juice pH.

PATIENTS AND METHODS

Among 311 consecutively thyroxine-treated patients, 61 bearing Hashimoto's thyroiditis (52 F/9 M; median age = 51 years) who complained persistent dyspepsia and/or upper abdominal symptoms following a noninvasive workup for gastrointestinal disorders, underwent EGDS with multiple biopsies and gastric juice pH measurement. All patients accepted to take thyroxine in fasting conditions, abstaining from eating or drinking for one hour.

RESULTS

Thyroxine requirement increased along with the rising gastric pH (ρ = 0.4229; p = 0.0007). A multivariate analysis revealed that gastric pH was, beside body mass index, the far more important independent variable in determining the effective dose of T4 (p = 0.001). The ROC curve revealed that the pH threshold for an increased thyroxine requirement was at 2.28, being the AUC by 78%. Subdividing patients by the histologic findings, it appeared a significant increase (p = 0.0025) along with the progressive damage of gastric mucosa.

CONCLUSION

The in vivo measurement of gastric pH highlighted its key role in determining the minimal effective dose of oral T4 and may explain the interference of food, of some drugs and gut disorders on levothyroxine treatment.

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Carbonic Anhydrases II, IX, and XII in Reflux Esophagitis

BACKGROUND

The pathogenesis of gastroesophageal reflux disease (GERD) has not been resolved in detail. Esophageal epithelial cells provide resistance to acidic reflux via several mechanisms, many of which involve buffering acid with bicarbonate and transporting protons. Carbonic anhydrases (CAs) are enzymes that control the acid-base balance by catalyzing the reversible hydration of carbon dioxide to produce bicarbonate and hydrogen ions.

AIMS

We aimed to determine the immunohistochemical expression patterns of CAII, CAIX, and CAXII in the normal esophageal squamous epithelium and in patients with GERD.

METHODS

We evaluated 82 biopsy samples, including 26 with a histologically normal esophagus, 26 with histologically mild esophagitis, and 30 with severe esophagitis. Expression patterns of CAII, CAIX, and CAXII in the esophageal squamous epithelium were determined by immunohistochemical staining.

RESULTS

Cytoplasmic CAII expression was predominantly detected in the upper luminal part of the squamous epithelium and was significantly (p < 0.01) increased in GERD. Expression of CAIX was essentially membranous. The isozyme was constantly present in the peripapillary cells. In the interpapillary areas, clustered expression was observed to emerge and increase significantly (p < 0.01) in esophagitis. CAXII expression was the most abundant of the isozymes and was mainly membranous. In the normal squamous epithelium, CAXII expression was confined to the basal layer; in severe esophagitis, CAXII expression increased significantly in both basal (p < 0.05) and superficial (p < 0.01) halves of the epithelium.

CONCLUSIONS

We demonstrate upregulated expression of CAII, CAIX, and CAXII in GERD. The increase in expression likely contributes to esophageal epithelial resistance to acidic reflux.

Prolonged NHE Activation may be both Cause and Outcome of Cytokine Release Syndrome in COVID-19

The release of cytokines and chemokines such as IL-1β, IL-2, IL-6, IL-7, IL-10, TNF-α, IFN-γ, CCL2, CCL3, and CXCL10 is increased in critically ill patients with COVID-19. Excessive cytokine release during COVID-19 is related to increased morbidity and mortality. Several mechanisms are put forward for cytokine release syndrome during COVID-19. Here we have mentioned novel pathways. SARS-CoV-2 increases angiotensin II levels by rendering ACE2 nonfunctional. Angiotensin II causes cytokine release via AT₁ and AT₂ receptors. Moreover, angiotensin II potently stimulates the Na+/H+ exchanger (NHE). It is a pump found in the membranes of many cells that pumps Na+ inward and H+ outward. NHE has nine isoforms. NHE1 is the most common isoform found in endothelial cells and many cells. NHE is involved in keeping the intracellular pH within physiological limits. When the intracellular pH is acidic, NHE is activated, bringing the intracellular pH to physiological levels, ending its activity. Sustained NHE activity is highly pathological and causes many problems. Prolonged NHE activation in COVID-19 may cause a decrease in intracellular pH through H+ ion accumulation in the extracellular area and subsequent redox reactions. The activation reduces the intracellular K+ concentration and leads to Na+ and Ca²⁺ overload. Increased ROS can cause intense cytokine release by stimulating NF-κB and NLRP3 inflammasomes. Cytokines also cause overstimulation of NHE. As the intracellular pH decreases, SARS-CoV-2 rapidly infects new cells, increasing the viral load. This vicious circle increases morbidity and mortality in patients with COVID-19. On the other hand, SARS-CoV-2 interaction with NHE3 in intestinal tissue is different from other tissues. SARS-CoV-2 can trigger CRS via NHE3 inhibition by disrupting the intestinal microbiota. This review aimed to help develop new treatment models against SARS-CoV-2- induced CRS by revealing the possible effects of SARS-CoV-2 on the NHE.

Functional characterization of the sodium/hydrogen exchanger 8 and its role in proliferation of colonic epithelial cells

Intestinal NaCl, HCO₃^-, and fluid absorption are strongly dependent on apical Na⁺/H⁺ exchange. The intestine expresses three presumably apical sodium-hydrogen exchanger (NHE) isoforms: NHE2, NHE3, and NHE8. We addressed the role of NHE8 [solute carrier 9A8 (SLC9A8)] and its interplay with NHE2 (SLC9A2) in luminal proton extrusion during acute and chronic enterocyte acidosis and studied the differential effects of NHE8 and NHE2 on enterocyte proliferation. In contrast to NHE3, which was upregulated in differentiated versus undifferentiated colonoids, the expression of NHE2 and NHE8 remained constant during differentiation of colonoids and Caco2Bbe cells. Heterogeneously expressed Flag-tagged rat (r)Nhe8 and human (h)NHE8 translocated to the apical membrane of Caco2Bbe cells. rNhe8 and hNHE8, when expressed in NHE-deficient PS120 fibroblasts showed higher sensitivity to HOE642 compared to NHE2. Lentiviral shRNA knockdown of endogenous NHE2 in Caco2Bbe cells (C2Bbe/shNHE2) resulted in a decreased steady-state intracellular pH (pH_i), an increased NHE8 mRNA expression, and augmented NHE8-mediated apical NHE activity. Lentiviral shRNA knockdown of endogenous NHE8 in Caco2Bbe cells (C2Bbe/shNHE8) resulted in a decreased steady-state pH_i as well, accompanied by decreased NHE2 mRNA expression and activity, which together contributed to reduced apical NHE activity in the NHE8-knockdown cells. Chronic acidosis increased NHE8 but not NHE2 mRNA expression. Alterations in NHE2 and NHE8 expression/activity affected proliferation, with C2Bbe/shNHE2 cells having lower and C2Bbe/shNHE8 having higher proliferative capacity, accompanied by amplified ERK1/2 signaling pathway and increased EGFR expression in the latter cell line. Thus, both Na⁺/H⁺ exchangers have distinct functions during cellular homeostasis by triggering different signaling pathways to regulate cellular proliferation and pH_i control.

Enteropathogenic Escherichia coli—A Summary of the Literature

Diarrheal disease is still a major public health concern, as it is still considered an important cause of death in children under five years of age. A few decades ago, the detection of enteropathogenic E. coli was made by detecting the O, H, and K antigens, mostly by agglutination. The recent protocols recommend the molecular methods for diagnosing EPEC, as they can distinguish between typical and atypical EPEC by identifying the presence/absence of specific virulence factors. EPEC are defined as diarrheagenic strains of E. coli that can produce attaching and effacing lesions on the intestinal epithelium while being incapable of producing Shiga toxins and heat-labile or heat-stable enterotoxins. The ability of these strains to produce attaching and effacing lesions enable them to cause localized lesions by attaching tightly to the surface of the intestinal epithelial cells, disrupting the surfaces of the cells, thus leading to the effacement of the microvilli. EPEC are classified on typical and atypical isolates, based on the presence or absence of E. coli adherence factor plasmids. All the EPEC strains are eae positive; typical EPEC strains are eae+, bfpA+, while atypical strains are eae+, bfpA−. No vaccines are currently available to prevent EPEC infections.

The therapeutic importance of acid-base balance

Baking soda and vinegar have been used as home remedies for generations and today we are only a mouse-click away from claims that baking soda, lemon juice, and apple cider vinegar are miracles cures for everything from cancer to COVID-19. Despite these specious claims, the therapeutic value of controlling acid-base balance is indisputable and is the basis of Food and Drug Administration-approved treatments for constipation, epilepsy, metabolic acidosis, and peptic ulcers. In this narrative review, we present evidence in support of the current and potential therapeutic value of countering local and systemic acid-base imbalances, several of which do in fact involve the administration of baking soda (sodium bicarbonate). Furthermore, we discuss the side effects of pharmaceuticals on acid-base balance as well as the influence of acid-base status on the pharmacokinetic properties of drugs. Our review considers all major organ systems as well as information relevant to several clinical specialties such as anesthesiology, infectious disease, oncology, dentistry, and surgery.

Towards an Integral Therapeutic Protocol for Breast Cancer Based upon the New H+-Centered Anticancer Paradigm of the Late Post-Warburg Era

A brand new approach to the understanding of breast cancer (BC) is urgently needed. In this contribution, the etiology, pathogenesis, and treatment of this disease is approached from the new pH-centric anticancer paradigm. Only this unitarian perspective, based upon the hydrogen ion (H+) dynamics of cancer, allows for the understanding and integration of the many dualisms, confusions, and paradoxes of the disease. The new H+-related, wide-ranging model can embrace, from a unique perspective, the many aspects of the disease and, at the same time, therapeutically interfere with most, if not all, of the hallmarks of cancer known to date. The pH-related armamentarium available for the treatment of BC reviewed here may be beneficial for all types and stages of the disease. In this vein, we have attempted a megasynthesis of traditional and new knowledge in the different areas of breast cancer research and treatment based upon the wide-ranging approach afforded by the hydrogen ion dynamics of cancer. The concerted utilization of the pH-related drugs that are available nowadays for the treatment of breast cancer is advanced.

Pathophysiology of hepatic Na+/H+ exchange (Review)

Na+/H+ exchangers (NHEs) are a family of membrane proteins that contribute to exchanging one intracellular proton for one extracellular sodium. The family of NHEs consists of nine known members, NHE1-9. Each isoform represents a different gene product that has unique tissue expression, membrane localization, physiological effects, pathological regulation and sensitivity to drug inhibitors. NHE1 was the first to be discovered and is often referred to as the 'housekeeping' isoform of the NHE family. NHEs are not only involved in a variety of physiological processes, including the control of transepithelial Na+ absorption, intracellular pH, cell volume, cell proliferation, migration and apoptosis, but also modulate complex pathological events. Currently, the vast majority of review articles have focused on the role of members of the NHE family in inflammatory bowel disease, intestinal infectious diarrhea and digestive system tumorigenesis, but only a few reviews have discussed the role of NHEs in liver disease. Therefore, the present review described the basic biology of NHEs and highlighted their physiological and pathological effects in the liver.