Control of Ion Transport by Tmem16a Expressed in Murine Intestine

COVID-19 Diarrhea Is Inflammatory, Caused by Direct Viral Effects Plus Major Role of Virus-induced Cytokines

BACKGROUND & AIMS

Diarrhea occurs in up to 50% of cases of COVID-19. Nonetheless, the pathophysiologic mechanism(s) have not been determined.

METHODS

This was examined using normal human enteroid monolayers exposed apically to live SARS-CoV-2 or non-replicating virus-like particles (VLPs) bearing the 4 SARS-CoV-2 structural proteins or irradiated virus, all of which bound and entered enterocytes.

RESULTS

Live virus and VLPs incrieased secretion of multiple cytokines and reduced mRNAs of ACE2, NHE3, and DRA. Interleukin (IL)-6 plus IL-8 alone reduced NHE3 mRNA and protein and DRA mRNA and protein. Neither VLPs nor IL-6 plus IL-8 alone altered Cl- secretion, but together they caused Cl- secretion, which was Ca2+-dependent, CFTR-independent, blocked partially by a specific TMEM16A inhibitor, and entirely by a general TMEM16 family inhibitor. VLPs and irradiated virus, but not IL-6 plus IL-8, produced Ca2+ waves that began within minutes of VLP exposure, lasted for at least 60 minutes, and were prevented by pretreatment with apyrase, a P2Y1 receptor antagonist, and general TMEM16 family inhibitor but not by the specific TMEM16A inhibitor.

CONCLUSIONS

The pathophysiology of COVID-19 diarrhea appears to be a unique example of a calcium-dependent inflammatory diarrhea that is caused by direct viral effects plus the virus-induced intestinal epithelial cytokine secretion.

The anoctamins: Structure and function

When activated by increase in intracellular Ca2+, anoctamins (TMEM16 proteins) operate as phospholipid scramblases and as ion channels. Anoctamin 1 (ANO1) is the Ca2+-activated epithelial anion-selective channel that is coexpressed together with the abundant scramblase ANO6 and additional intracellular anoctamins. In salivary and pancreatic glands, ANO1 is tightly packed in the apical membrane and secretes Cl-. Epithelia of airways and gut use cystic fibrosis transmembrane conductance regulator (CFTR) as an apical Cl- exit pathway while ANO1 supports Cl- secretion mainly by facilitating activation of luminal CFTR and basolateral K+ channels. Under healthy conditions ANO1 modulates intracellular Ca2+ signals by tethering the endoplasmic reticulum, and except of glands its direct secretory contribution as Cl- channel might be small, compared to CFTR. In the kidneys ANO1 supports proximal tubular acid secretion and protein reabsorption and probably helps to excrete HCO3-in the collecting duct epithelium. However, under pathological conditions as in polycystic kidney disease, ANO1 is strongly upregulated and may cause enhanced proliferation and cyst growth. Under pathological condition, ANO1 and ANO6 are upregulated and operate as secretory channel/phospholipid scramblases, partly by supporting Ca2+-dependent processes. Much less is known about the role of other epithelial anoctamins whose potential functions are discussed in this review.

Loureirin C extracted from Dracaena cochinchinensis S.C. Chen prevents rotaviral diarrhea in mice by inhibiting the intestinal Ca2+-activated Cl- channels

ETHNOPHARMACOLOGICAL RELEVANCE

Resina Draconis (RD) is the red resin of Dracaena cochinchinensis (Lour.) S.C. Chen and most used as a hemostatic drug in traditional Chinese medicine. Recent studies have reported that RD has a therapeutic effect on gastrointestinal diseases. Loureirin A, B, and C (LA, LB, and LC) are dihydrochalcone compounds isolated from RD.

AIM OF THE STUDY

Dehydration is the primary cause of death in rotaviral diarrhea. Inhibition of Ca2+-activated Cl- channels (CaCCs)-mediated Cl- secretion significantly reduced fluid secretion in rotaviral diarrhea. RD was used to treat digestive diseases such as diarrhea and abdominal pain; however, the pharmacological mechanism remains unclear. This study investigated the effects of RD and loureirin on intestinal Cl- channels and their therapeutic effects on rotavirus-induced diarrhea, aiming to reveal RD's molecular basis, targets, and mechanisms for treating rotaviral diarrhea.

MATERIALS AND METHODS

Cell-based fluorescence quenching assays were used to examine the effect of RD and loureirin on Cl- channels activity. Electrophysiological properties were tested using short-circuit current experiments in epithelial cells or freshly isolated mouse intestinal tissue. Fecal water content, intestinal peristalsis rate, and smooth muscle contraction were measured in neonatal mice infected with SA-11 rotavirus before and after LC treatment or adult mice.

RESULTS

RD, LA, LB, and LC inhibited CaCCs-mediated Cl- current in HT-29 cells and colonic epithelium. The inhibitory effect of LC on CaCCs was primarily on the apical side in epithelial cells, which may be partially produced by affecting cytoplasmic Ca2+ levels. LC significantly inhibited TMEM16A-mediated Cl- current. Characterization studies revealed that LC inhibited basolateral K+ channel activity without affecting Na+/K+-ATPase activity in the colonic epithelium. Although LC activated the cystic fibrosis transmembrane regulator in epithelial cells, its effect was not apparent in colonic epithelium. In vivo, LC significantly reduced the fecal water content, intestinal peristalsis rate, and smooth muscle contraction of mice infected with rotavirus.

CONCLUSION

RD and its active compound LC inhibit intestinal CaCCs activity, which might mediate the anti-rotaviral diarrheal effect of RD.

Management and Mechanisms of Diarrhea Induced by Tyrosine Kinase Inhibitors in Human Epidermal Growth Factor Receptor-2-Positive Breast Cancer

Breast cancer has the highest incidence among female malignancies, significantly impacting women's health. Recently, numerous HER2-targeted therapies have achieved excellent clinical outcomes. Currently, anti-HER2 drugs are divided into three main categories: monoclonal antibodies, small-molecule tyrosine kinase inhibitors, and antibody-coupled drugs (ADCs). The main toxic side effects of small molecule TKI-based therapy are diarrhea, hand-foot syndrome, rash, nausea, and vomiting. Diarrhea is a potential predictor of tumor response, affecting up to 95% of cancer patients treated with TKIs. Severe gastrointestinal toxicity can result in the need for dose reductions and treatment interruptions. This not only compromises the efficacy of TKIs but also deteriorates human nutrition and quality of life. The majority of individuals develop diarrhea within 7 days of starting treatment, with approximately 30% developing grade 3 or higher diarrhea within 2-3 days of starting treatment. The severity of diarrhea typically correlates with the dosage of most TKIs. Current prevention and management strategies are primarily empirical, focusing on symptom alleviation rather than addressing the toxicological mechanisms underlying TKI-induced diarrhea. Consequently, anti-diarrheal drugs are often less effective in managing this condition in cancer patients receiving TKIs. Moreover, our understanding of the toxicological mechanisms responsible for such diarrhea remains limited, underscoring the urgent need to identify these mechanisms in order to develop effective anti-diarrheal medications tailored to this specific context. This review aims to elucidate management approaches and mechanisms for diarrhea induced by TKIs during HER2-positive breast cance.

The Anion Channel TMEM16a/Ano1 Modulates CFTR Activity, but Does Not Function as an Apical Anion Channel in Colonic Epithelium from Cystic Fibrosis Patients and Healthy Individuals

Studies in human colonic cell lines and murine intestine suggest the presence of a Ca2+-activated anion channel, presumably TMEM16a. Is there a potential for fluid secretion in patients with severe cystic fibrosis transmembrane conductance regulator (CFTR) mutations by activating this alternative pathway? Two-dimensional nondifferentiated colonoid-myofibroblast cocultures resembling transit amplifying/progenitor (TA/PE) cells, as well as differentiated monolayer (DM) cultures resembling near-surface cells, were established from both healthy controls (HLs) and patients with severe functional defects in the CFTR gene (PwCF). F508del mutant and CFTR knockout (null) mice ileal and colonic mucosa was also studied. HL TA/PE monolayers displayed a robust short-circuit current response (ΔIeq) to UTP (100 µM), forskolin (Fsk, 10 µM) and carbachol (CCH, 100 µM), while ΔIeq was much smaller in differentiated monolayers. The selective TMEM16a inhibitor Ani9 (up to 30 µM) did not alter the response to luminal UTP, significantly decreased Fsk-induced ΔIeq, and significantly increased CCH-induced ΔIeq in HL TA/PE colonoid monolayers. The PwCF TA/PE and the PwCF differentiated monolayers displayed negligible agonist-induced ΔIeq, without a significant effect of Ani9. When TMEM16a was localized in intracellular structures, a staining in the apical membrane was not detected. TMEM16a is highly expressed in human colonoid monolayers resembling transit amplifying cells of the colonic cryptal neck zone, from both HL and PwCF. While it may play a role in modulating agonist-induced CFTR-mediated anion currents, it is not localized in the apical membrane, and it has no function as an apical anion channel in cystic fibrosis (CF) and healthy human colonic epithelium.

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.

The role of Transmembrane Protein 16A (TMEM16A) in pulmonary hypertension

Transmembrane protein 16A (TMEM16A), a member of the TMEM16 family, is the molecular basis of Ca2+-activated chloride channels (CaCCs) and is involved in a variety of physiological and pathological processes. Previous studies have focused more on respiratory-related diseases and tumors. However, recent studies have identified an important role for TMEM16A in cardiovascular diseases, especially in pulmonary hypertension. TMEM16A is expressed in both pulmonary artery smooth muscle cells and pulmonary artery endothelial cells and is involved in the development of pulmonary hypertension. This paper presents the structure and function of TMEM16A, the pathogenesis of pulmonary hypertension, and highlights the role and mechanism of TMEM16A in pulmonary hypertension, summarizing the controversies in this field and taking into account hypertension and portal hypertension, which have similar pathogenesis. It is hoped that the unique role of TMEM16A in pulmonary hypertension will be illustrated and provide ideas for research in this area.

Ion Channel Involvement in Tumor Drug Resistance

Over 90% of deaths in cancer patients are attributed to tumor drug resistance. Resistance to therapeutic agents can be due to an innate property of cancer cells or can be acquired during chemotherapy. In recent years, it has become increasingly clear that regulation of membrane ion channels is an important mechanism in the development of chemoresistance. Here, we review the contribution of ion channels in drug resistance of various types of cancers, evaluating their potential in clinical management. Several molecular mechanisms have been proposed, including evasion of apoptosis, cell cycle arrest, decreased drug accumulation in cancer cells, and activation of alternative escape pathways such as autophagy. Each of these mechanisms leads to a reduction of the therapeutic efficacy of administered drugs, causing more difficulty in cancer treatment. Thus, targeting ion channels might represent a good option for adjuvant therapies in order to counteract chemoresistance development.

Identification of evodiamine and rutecarpine as novel TMEM16A inhibitors and their inhibitory effects on peristalsis in isolated Guinea-pig ileum

The transmembrane member 16A (TMEM16A)-encoded Ca2+-activated Cl- channel (CaCC) is expressed in interstitial cells of Cajal (ICCs) and involved in the generation of the slow-wave currents of gastrointestinal (GI) smooth muscles. TMEM16A modulators have been shown to positively or negatively regulate the contraction of gastrointestinal smooth muscle. Therefore, targeting the pharmacological modulation of TMEM16A may represent a novel treatment approach for gastrointestinal dysfunctions such as constipation and diarrhoea. In this study, evodiamine and rutecarpine were extracted from the traditional Chinese medicine Evodia rutaecarpa and identified as novel TMEM16A inhibitors with comparable inhibitory effects. Their effects on intestinal peristalsis were examined. Whole-cell patch clamp results show that evodiamine and rutecarpine inhibited TMEM16A Cl- currents in CHO cells. The half-maximal inhibition values (IC50) of evodiamine and rutecarpine on TMEM16A Cl- currents were 11.8 ± 1.3 μΜ and 9.2 ± 0.4 μM, and the maximal effect values (Emax) were 95.8 ± 5.1% and 99.1 ± 1.6%, respectively. The Lys384, Thr385, and Met524 in TMEM16A are critical for evodiamine and rutecarpine's inhibitory effects. Further functional studies show that both evodiamine and rutecarpine can significantly suppress the peristalsis in isolated guinea-pig ileum. These findings demonstrate that evodiamine and rutecarpine are new TMEM16A inhibitors and support the regulation effect of TMEM16A modulators on gastrointestinal motility.

Action Mode of Gut Motility, Fluid and Electrolyte Transport in Chronic Constipation

Chronic constipation is a common gastrointestinal disorder, with a worldwide incidence of 14–30%. It negatively affects quality of life and is associated with a considerable economic burden. As a disease with multiple etiologies and risk factors, it is important to understand the pathophysiology of chronic constipation. The purpose of this review is to discuss latest findings on the roles of gut motility, fluid, and electrolyte transport that contribute to chronic constipation, and the main drugs available for treating patients. We conducted searches on PubMed and Google Scholar up to 9 February 2021. MeSH keywords “constipation”, “gastrointestinal motility”, “peristalsis”, “electrolytes”, “fluid”, “aquaporins”, and “medicine” were included. The reference lists of searched articles were reviewed to identify further eligible articles. Studies focusing on opioid-induced constipation, evaluation, and clinic management of constipation were excluded. The occurrence of constipation is inherently connected to disorders of gut motility as well as fluid and electrolyte transport, which involve the nervous system, endocrine signaling, the gastrointestinal microbiota, ion channels, and aquaporins. The mechanisms of action and application of the main drugs are summarized; a better understanding of ion channels and aquaporins may be helpful for new drug development. This review aims to provide a scientific basis that can guide future research on the etiology and treatment of constipation.

Calcium-activated chloride channel is involved in the onset of diarrhea triggered by EGFR tyrosine kinase inhibitor treatment in rats

EGFR tyrosine kinase inhibitors (TKIs) are mainly used to treat non-small cell lung cancer; however, adverse effects such as severe diarrhea represent a major obstacle towards the continuation of EGFR-TKIs therapy. Chloride channels, which control the fluid flow in the intestinal lumen, are proposed as an important target to remediate EGFR-TKIs-induced diarrhea, but the mechanism remains unclear. The aim of this study was to clarify the mechanism underlying EGFR-TKIs-induced diarrhea with a particular focus on the role of intestinal chloride channels. Here, we show that osimertinib-treated rats exhibit diarrhea and an increase in fecal water content without showing any severe histopathological changes. This diarrhea was attenuated by intraperitoneal treatment with the calcium-activated chloride channel (CaCC) inhibitor CaCCinh-A01. These findings were confirmed in afatinib-treated rats with diarrhea. Moreover, treatment with the Japanese traditional herbal medicine, hangeshashinto (HST), decreased fecal water content and improved fecal appearance in rats treated with EGFR-TKIs. HST inhibited the ionomycin-induced CaCC activation in HEK293 cells in patch-clamp current experiments and its active ingredients were identified. In conclusion, secretory diarrhea induced by treatment with EGFR-TKIs might be partially mediated by the activation of CaCC. Therefore, blocking the CaCC could be a potential new treatment for EGFR-TKI-induced diarrhea.

Transport properties in CFTR-/- knockout piglets suggest normal airway surface liquid pH and enhanced amiloride-sensitive Na+ absorption

Previous analysis of CFTR-knockout (CFTR-/-) in piglets has provided important insights into the pathology of cystic fibrosis. However, controversies exist as to the true contribution of CFTR to the pH balance in airways and intestine. We therefore compared ion transport properties in newborn wild-type (CFTR+/+) and CFTR-knockout (CFTR-/- piglets). Tracheas of CFTR-/- piglets demonstrated typical cartilage malformations and muscle cell bundles. CFTR-/- airway epithelial cells showed enhanced lipid peroxidation, suggesting inflammation early in life. CFTR was mainly expressed in airway submucosal glands and was absent in lungs of CFTR-/- piglets, while expression of TMEM16A was uncompromised. mRNA levels for TMEM16A, TMEM16F, and αβγENaC were unchanged in CFTR-/- airways, while mRNA for SLC26A9 appeared reduced. CFTR was undetectable in epithelial cells of CFTR-/- airways and intestine. Small intestinal epithelium of CFTR-/- piglets showed mucus accumulation. Secretion of both electrolytes and mucus was activated by stimulation with prostaglandin E2 and ATP in the intestine of CFTR+/+, but not of CFTR-/- animals. pH was measured inside small bronchi using a pH microelectrode and revealed no difference between CFTR+/+ and CFTR-/- piglets. Intracellular pH in porcine airway epithelial cells revealed only a small contribution of CFTR to bicarbonate secretion, which was absent in cells from CFTR-/- piglets. In contrast to earlier reports, our data suggest a minor impact of CFTR on ASL pH. In contrast, enhanced amiloride-sensitive Na⁺ absorption may contribute to lung pathology in CFTR-/- piglets, along with a compromised CFTR- and TMEM16A-dependent Cl^- transport.

TMEM16A deficiency: a potentially fatal neonatal disease resulting from impaired chloride currents

INTRODUCTION

TMEM16A is a calcium-activated chloride channel expressed in various secretory epithelia. Two siblings presented in early infancy with reduced intestinal peristalsis and recurrent episodes of haemorrhagic diarrhoea. In one of them, the episodes were characterised by hepatic pneumatosis with gas bubbles in the portal vein similar to necrotising enterocolitis of the newborn.

METHODS

Exome sequencing identified a homozygous truncating pathogenic variant in ANO1. Expression analysis was performed using reverse transcription PCR, western blot and immunohistochemistry. Electrophysiological and cell biological studies were employed to characterise the effects on ion transport both in patient respiratory epithelial cells and in transfected HEK293 cells.

RESULTS

The identified variant led to TMEM16A dysfunction, which resulted in abolished calcium-activated Cl^- currents. Secondarily, CFTR function is affected due to the close interplay between both channels without inducing cystic fibrosis (CF).

CONCLUSION

TMEM16A deficiency is a potentially fatal disorder caused by abolished calcium-activated Cl^- currents in secretory epithelia. Secondary impairment of CFTR function did not cause a CF phenotyp, which may have implications for CF treatment.

Dual role of Ca2+-activated Cl- channel transmembrane member 16A in lipopolysaccharide-induced intestinal epithelial barrier dysfunction in vitro

Dysfunction of intestinal epithelial Cl⁻ currents and channels have previously been reported in inflammatory intestinal diseases. However, the expression and function of the newly identified Ca²⁺-activated Cl⁻ channel transmembrane member 16A (TMEM16A) in the intestinal epithelium is unclear. In this study, we investigated the effects of TMEM16A on intestinal epithelial barrier function in vitro. Intestinal epithelial barrier dysfunction was modeled by lipopolysaccharide (LPS)-induced cell damage in intestinal epithelial IEC-6 cells and the effects of TMEM16A knockdown and overexpression on cell apoptosis and tight junctions were studied. Corresponding mRNA and protein expression levels were measured by quantitative real-time polymerase chain reaction, western blotting, and immunofluorescence analysis, respectively. TMEM16A expression was significantly increased by LPS, possibly via a process involving the transcription factor nuclear factor-κB and both Th1 and Th2 cytokines. Low- and high-dose LPS dysregulated tight junctions (high-myosin light-chain kinase expression) and cell apoptosis-dependent cell barrier dysfunction, respectively. TMEM16A aggravated cell barrier dysfunction in IEC-6 cells pretreated with low-dose LPS by activating ERK1/MLCK signaling pathways, but protected against cell barrier dysfunction by activating ERK/Bcl-2/Bax signaling pathways in IEC-6 cells pretreated with high-dose LPS. We concluded that TMEM16A played a dual role in LPS-induced epithelial dysfunction in vitro. The present results indicated the complex regulatory mechanisms and targeting of TMEM16A may provide potential treatment strategies for intestinal epithelial barrier damage, as well as forming the basis for future studies of the expression and function of TMEM16A in normal and inflammatory intestinal diseases in vivo.