Anoctamin 1 induces calcium-activated chloride secretion and proliferation of renal cyst-forming epithelial cells

The ion channel Anoctamin 10/TMEM16K coordinates organ morphogenesis across scales in the urochordate notochord

During embryonic development, tissues and organs are gradually shaped into their functional morphologies through a series of spatiotemporally tightly orchestrated cell behaviors. A highly conserved organ shape across metazoans is the epithelial tube. Tube morphogenesis is a complex multistep process of carefully choreographed cell behaviors such as convergent extension, cell elongation, and lumen formation. The identity of the signaling molecules that coordinate these intricate morphogenetic steps remains elusive. The notochord is an essential tubular organ present in the embryonic midline region of all members of the chordate phylum. Here, using genome editing, pharmacology and quantitative imaging in the early chordate Ciona intestinalis we show that Ano10/Tmem16k, a member of the evolutionarily ancient family of transmembrane proteins called Anoctamin/TMEM16 is essential for convergent extension, lumen expansion, and connection during notochord morphogenesis. We find that Ano10/Tmem16k works in concert with the plasma membrane (PM) localized Na+/Ca2+ exchanger (NCX) and the endoplasmic reticulum (ER) residing SERCA, RyR, and IP3R proteins to establish developmental stage specific Ca2+ signaling molecular modules that regulate notochord morphogenesis and Ca2+ dynamics. In addition, we find that the highly conserved Ca2+ sensors calmodulin (CaM) and Ca2+/calmodulin-dependent protein kinase (CaMK) show an Ano10/Tmem16k-dependent subcellular localization. Their pharmacological inhibition leads to convergent extension, tubulogenesis defects, and deranged Ca2+ dynamics, suggesting that Ano10/Tmem16k is involved in both the "encoding" and "decoding" of developmental Ca2+ signals. Furthermore, Ano10/Tmem16k mediates cytoskeletal reorganization during notochord morphogenesis, likely by altering the localization of 2 important cytoskeletal regulators, the small GTPase Ras homolog family member A (RhoA) and the actin binding protein Cofilin. Finally, we use electrophysiological recordings and a scramblase assay in tissue culture to demonstrate that Ano10/Tmem16k likely acts as an ion channel but not as a phospholipid scramblase. Our results establish Ano10/Tmem16k as a novel player in the prevertebrate molecular toolkit that controls organ morphogenesis across scales.

Insights into the function and regulation of the calcium-activated chloride channel TMEM16A

The TMEM16A channel, a member of the TMEM16 protein family comprising chloride (Cl-) channels and lipid scramblases, is activated by the free intracellular Ca2+ increments produced by inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release after GqPCRs or Ca2+ entry through cationic channels. It is a ubiquitous transmembrane protein that participates in multiple physiological functions essential to mammals' lives. TMEM16A structure contains two identical 10-segment monomers joined at their transmembrane segment 10. Each monomer harbours one independent hourglass-shaped pore gated by Ca2+ ligation to an orthosteric site adjacent to the pore and controlled by two gates. The orthosteric site is created by assembling negatively charged glutamate side chains near the pore´s cytosolic end. When empty, this site generates an electrostatic barrier that controls channel rectification. In addition, an isoleucine-triad forms a hydrophobic gate at the boundary of the cytosolic vestibule and the inner side of the neck. When the cytosolic Ca2+ rises, one or two Ca2+ ions bind to the orthosteric site in a voltage (V)-dependent manner, thus neutralising the electrostatic barrier and triggering an allosteric gating mechanism propagating via transmembrane segment 6 to the hydrophobic gate. These coordinated events lead to pore opening, allowing the Cl- flux to ensure the physiological response. The Ca2+-dependent function of TMEM16A is highly regulated. Anions with higher permeability than Cl- facilitate V dependence by increasing the Ca2+ sensitivity, intracellular protons can replace Ca2+ and induce channel opening, and phosphatidylinositol 4,5-bisphosphate bound to four cytosolic sites likely maintains Ca2+ sensitivity. Additional regulation is afforded by cytosolic proteins, most likely by phosphorylation and protein-protein interaction mechanisms.

P2Y2R and Cyst Growth in Polycystic Kidney Disease

Key Points

Background

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by multiple bilateral kidney cysts that gradually enlarge, resulting in a decline in kidney function. Cyst growth is significantly driven by ATP-dependent chloride secretion mediated by the ion channel TMEM16A. This pathway is further augmented in advanced stages of the disease by hypoxia and activation of hypoxia-inducible factor (HIF)-1α. The mechanisms by which ATP leads to activation of TMEM16A and how HIF-1α contributes to cyst growth in vivo have remained elusive.

Methods

Mice with an inducible tubule-specific deletion of Pkd1 were compared with mice with an additional codeletion of the purinergic receptor P2y2r. Furthermore, animals were challenged by pharmacological activation of HIF-1α and Pkd1-deficient mice were treated with suramin, an antagonist of purinergic signaling. In addition, expression of P2Y2R, TMEM16A, and HIF-1α was analyzed in nephrectomy samples from 27 patients with ADPKD.

Results

Genetic deletion of P2y2r significantly inhibited cyst growth in vivo. In addition, aggravation of the polycystic phenotype mediated by pharmacological activation of HIF-1α was reduced by deletion of P2y2r. Application of suramin to pharmacologically inhibit purinergic signaling also suppressed cyst enlargement in vivo. Analysis of kidney samples from 27 patients with ADPKD revealed significant expression of P2Y2R at the luminal site of the cyst-lining epithelium.

Conclusions

P2Y2R was significantly expressed in human and mouse polycystic kidneys. Deletion and antagonism of P2Y2R reduced cyst enlargement in an ADPKD mouse model.

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.

The role of anoctamin 1 in liver disease

Liver diseases include all types of viral hepatitis, alcoholic liver disease (ALD), nonalcoholic fatty liver disease (NAFLD), cirrhosis, liver failure (LF) and hepatocellular carcinoma (HCC). Liver disease is now one of the leading causes of disease and death worldwide, which compels us to better understand the mechanisms involved in the development of liver diseases. Anoctamin 1 (ANO1), a calcium-activated chloride channel (CaCC), plays an important role in epithelial cell secretion, proliferation and migration. ANO1 plays a key role in transcriptional regulation as well as in many signalling pathways. It is involved in the genesis, development, progression and/or metastasis of several tumours and other diseases including liver diseases. This paper reviews the role and molecular mechanisms of ANO1 in the development of various liver diseases, aiming to provide a reference for further research on the role of ANO1 in liver diseases and to contribute to the improvement of therapeutic strategies for liver diseases by regulating ANO1.

Inhibition of TMEM16A improves cisplatin-induced acute kidney injury via preventing DRP1-mediated mitochondrial fission

Acute kidney injury (AKI) is associated with high morbidity and mortality. Our previous study has demonstrated that TMEM16A, a Ca2+-activated chloride channel, contributes to renal fibrosis progression in chronic kidney disease. However, whether TMEM16A is involved in AKI is still unknown. In this study, we established cisplatin AKI mice model and found that TMEM16A expression was upregulated in the injured kidney. In vivo knockdown of TMEM16A effectively prevented cisplatin-induced tubular cell apoptosis, inflammation and kidney function loss. Western blot and transmission electron microscopy (TEM) revealed that TMEM16A knockdown inhibited Drp1 translocation from the cytoplasm to mitochondria and prevented mitochondrial fission in tubular cells. Consistently, in cultured HK2 cells, knockdown or inhibition of TMEM16A by shRNA or its specific inhibitor suppressed cisplatin-induced mitochondrial fission and its associated energy dysfunction, ROS accumulation, and cell apoptosis via inhibiting Drp1 activation. Further investigation showed that genetic knockdown or pharmacological inhibition of TMEM16A inhibited cisplatin-induced Drp1 Ser-616 site phosphorylation through ERK1/2 signaling pathway, whereas overexpression of TMEM16A promoted this effect. Treatment with Drp1 or ERK1/2 inhibitor could efficiently prevent cisplatin-induced mitochondrial fission. Collectively, our data suggest that TMEM16A inhibition alleviated cisplatin-induced AKI by preventing tubular cell mitochondrial fission through the ERK1/2 / Drp1 pathway. Inhibition of TMEM16A may be a novel therapeutic strategy for AKI.

Pathogenic Relationships in Cystic Fibrosis and Renal Diseases: CFTR, SLC26A9 and Anoctamins

The Cl--transporting proteins CFTR, SLC26A9, and anoctamin (ANO1; ANO6) appear to have more in common than initially suspected, as they all participate in the pathogenic process and clinical outcomes of airway and renal diseases. In the present review, we will therefore concentrate on recent findings concerning electrolyte transport in the airways and kidneys, and the role of CFTR, SLC26A9, and the anoctamins ANO1 and ANO6. Special emphasis will be placed on cystic fibrosis and asthma, as well as renal alkalosis and polycystic kidney disease. In essence, we will summarize recent evidence indicating that CFTR is the only relevant secretory Cl- channel in airways under basal (nonstimulated) conditions and after stimulation by secretagogues. Information is provided on the expressions of ANO1 and ANO6, which are important for the correct expression and function of CFTR. In addition, there is evidence that the Cl- transporter SLC26A9 expressed in the airways may have a reabsorptive rather than a Cl--secretory function. In the renal collecting ducts, bicarbonate secretion occurs through a synergistic action of CFTR and the Cl-/HCO3- transporter SLC26A4 (pendrin), which is probably supported by ANO1. Finally, in autosomal dominant polycystic kidney disease (ADPKD), the secretory function of CFTR in renal cyst formation may have been overestimated, whereas ANO1 and ANO6 have now been shown to be crucial in ADPKD and therefore represent new pharmacological targets for the treatment of polycystic kidney disease.

KCNE1 does not shift TMEM16A from a Ca2+ dependent to a voltage dependent Cl- channel and is not expressed in renal proximal tubule

The TMEM16A (ANO1) Cl^- channel is activated by Ca²⁺ in a voltage-dependent manner. It is broadly expressed and was shown to be also present in renal proximal tubule (RPT). KCNQ1 is an entirely different K⁺ selective channel that forms the cardiac I_KS potassium channel together with its ß-subunit KCNE1. Surprisingly, KCNE1 has been claimed to interact with TMEM16A, and to be required for activation of TMEM16A in mouse RPT. Interaction with KCNE1 was reported to switch TMEM16A from a Ca2²⁺-dependent to a voltage-dependent ion channel. Here we demonstrate that KCNE1 is not expressed in mouse RPT. TMEM16A expressed in RPT is activated by angiotensin II and ATP in a KCNE1-independent manner. Coexpression of KCNE1 does not change TMEM16A to a voltage gated Cl^- channel and Ca²⁺-dependent regulation of TMEM16A is fully maintained in the presence of KCNE1. While overexpressed KCNE1 slightly affects Ca²⁺-dependent regulation of TMEM16A, the data provide no evidence for KCNE1 being an auxiliary functional subunit for TMEM16A.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Emerging therapies for autosomal dominant polycystic kidney disease with a focus on cAMP signaling

Autosomal dominant polycystic kidney disease (ADPKD), with an estimated genetic prevalence between 1:400 and 1:1,000 individuals, is the third most common cause of end stage kidney disease after diabetes mellitus and hypertension. Over the last 3 decades there has been great progress in understanding its pathogenesis. This allows the stratification of therapeutic targets into four levels, gene mutation and polycystin disruption, proximal mechanisms directly caused by disruption of polycystin function, downstream regulatory and signaling pathways, and non-specific pathophysiologic processes shared by many other diseases. Dysfunction of the polycystins, encoded by the PKD genes, is closely associated with disruption of calcium and upregulation of cyclic AMP and protein kinase A (PKA) signaling, affecting most downstream regulatory, signaling, and pathophysiologic pathways altered in this disease. Interventions acting on G protein coupled receptors to inhibit of 3',5'-cyclic adenosine monophosphate (cAMP) production have been effective in preclinical trials and have led to the first approved treatment for ADPKD. However, completely blocking cAMP mediated PKA activation is not feasible and PKA activation independently from cAMP can also occur in ADPKD. Therefore, targeting the cAMP/PKA/CREB pathway beyond cAMP production makes sense. Redundancy of mechanisms, numerous positive and negative feedback loops, and possibly counteracting effects may limit the effectiveness of targeting downstream pathways. Nevertheless, interventions targeting important regulatory, signaling and pathophysiologic pathways downstream from cAMP/PKA activation may provide additive or synergistic value and build on a strategy that has already had success. The purpose of this manuscript is to review the role of cAMP and PKA signaling and their multiple downstream pathways as potential targets for emergent therapies for ADPKD.

A 3D In Vivo Model for Studying Human Renal Cystic Tissue and Mouse Kidney Slices

(1) Background: Autosomal dominant polycystic kidney disease (ADPKD) is a frequent monogenic disorder that leads to progressive renal cyst growth and renal failure. Strategies to inhibit cyst growth in non-human cyst models have often failed in clinical trials. There is a significant need for models that enable studies of human cyst growth and drug trials. (2) Methods: Renal tissue from ADPKD patients who received a nephrectomy as well as adult mouse kidney slices were cultured on a chorioallantoic membrane (CAM) for one week. The cyst volume was monitored by microscopic and CT-based applications. The weight and angiogenesis were quantified. Morphometric and histological analyses were performed after the removal of the tissues from the CAM. (3) Results: The mouse and human renal tissue mostly remained vital for about one week on the CAM. The growth of cystic tissue was evaluated using microscopic and CT-based volume measurements, which correlated with weight and an increase in angiogenesis, and was accompanied by cyst cell proliferation. (4) Conclusions: The CAM model might bridge the gap between animal studies and clinical trials of human cyst growth, and provide a drug-testing platform for the inhibition of cyst enlargement. Real-time analyses of mouse kidney tissue may provide insights into renal physiology and reduce the need for animal experiments.

Loss of Polycystin-1 causes cAMP-dependent switch from tubule to cyst formation

Autosomal dominant polycystic kidney disease is the most common monogenic disease that causes end-stage renal failure. It primarily results from mutations in the PKD1 gene that encodes for Polycystin-1. How loss of Polycystin-1 translates into bilateral renal cyst development is mostly unknown. cAMP is significantly involved in cyst enlargement but its role in cyst initiation has remained elusive. Deletion of Polycystin-1 in collecting duct cells resulted in a switch from tubule to cyst formation and was accompanied by an increase in cAMP. Pharmacological elevation of cAMP in Polycystin-1-competent cells caused cyst formation, impaired plasticity, nondirectional migration, and mis-orientation, and thus strongly resembled the phenotype of Polycystin-1-deficient cells. Mis-orientation of developing tubule cells in metanephric kidneys upon loss of Polycystin-1 was phenocopied by pharmacological increase of cAMP in wildtype kidneys. In vitro, cAMP impaired tubule formation after capillary-induced injury which was further impaired by loss Polycystin-1.

•

Loss of Polycystin-1 switches renal cells from tubule to cyst formation

•

Deletion of Polycystin-1 leads to increase in cAMP

•

Elevation of cAMP in wildtype cells phenocopies Polycystin-1-deficient features

•

Features are: impaired plasticity, nondirectional migration, and mis-orientation

Blockade of TMEM16A protects against renal fibrosis by reducing intracellular Cl- concentration

BACKGROUND AND PURPOSE

Renal fibrosis is the final common outcome in most forms of CKD. However, the underlying causal mechanisms remain obscure. The present study examined whether TMEM16A, a Ca²⁺ -activated chloride channel, contributes to the progress of renal fibrosis.

EXPERIMENTAL APPROACH

Masson staining, western blot and immunohistochemistry were used to measure renal fibrosis and related proteins expression. MQAE was used to evaluate the intracellular Cl^- concentration.

KEY RESULTS

TMEM16A expression was significantly upregulated in fibrotic kidneys of unilateral ureteral obstruction (UUO) and high-fat diet murine models, and in renal samples of IgA nephropathy patients. In vivo knockdown of TMEM16A with adenovirus harboring TMEM16A-shRNA or inhibition of TMEM16A channel activity with its specific inhibitor CaCCinh-A01 or T16Ainh-A01 effectively prevented UUO-induced renal fibrosis and decreased protein expression of fibronectin, α-SMA and collagen in the obstructed kidneys. In cultured HK2 cells, knockdown or inhibition of TMEM16A suppressed TGF-β1-induced epithelial to mesenchymal transition, reduced snail1 expression and phosphorylation of Smad2/3 and ERK1/2, whereas overexpression of TMEM16A showed the opposite effects. TGF-β1 increased [Cl^- ]i in HK2 cells, which was inhibited by knockdown or inhibition of TMEM16A. Reducing [Cl^- ]i by low Cl^- culture medium significantly blunted TGF-β1-induced Smad2/3 phosphorylation and profibrotic factors expression. The profibrotic effects of TGF-β1 were also abrogated by the inhibitor of SGK1, a kinase whose activity was also suppressed by reducing [Cl^- ]i.

CONCLUSION AND IMPLICATIONS

Blockade of TMEM16A prevented the progression of kidney fibrosis, likely by suppressing [Cl^- ]i/SGK1/TGF-β1 signaling pathway. TMEM16A may be a potential new therapeutic target against renal fibrosis.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

The chloride channel CFTR is not required for cyst growth in an ADPKD mouse model

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the development of bilateral renal cysts which enlarge continuously, leading to compression of adjacent intact nephrons. The growing cysts lead to a progressive decline in renal function. Cyst growth is driven by enhanced cell proliferation and chloride secretion into the cyst lumen. Chloride secretion is believed to occur mainly by the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR), with some contribution by the calcium-activated chloride channel TMEM16A. However, our previous work suggested TMEM16A as a major factor for renal cyst formation. The contribution of CFTR to cyst formation has never been demonstrated in an adult ADPKD mouse model. We used mice with an inducible tubule-specific Pkd1 knockout, which consistently develop polycystic kidneys upon deletion of Pkd1. Cellular properties, ion currents, and cyst development in these mice were compared with that of mice carrying a co-deletion of Pkd1 and Cftr. Knockout of Cftr did not reveal any significant impact on cyst formation in the ADPKD mouse model. Furthermore, knockout of Cftr did not attenuate the largely augmented cell proliferation observed in Pkd1 knockout kidneys. Patch clamp analysis on primary renal epithelial cells lacking expression of Pkd1 indicated an only marginal contribution of CFTR to whole cell Cl^- currents, which were clearly dominated by calcium-activated TMEM16A currents. In conclusion, CFTR does not essentially contribute to renal cyst formation in mice caused by deletion of Pkd1. Enhanced cell proliferation and chloride secretion is caused primarily by upregulation of the calcium-activated chloride channel TMEM16A.

Therapeutic Potential for CFTR Correctors in Autosomal Recessive Polycystic Kidney Disease

BACKGROUND & AIMS

Autosomal recessive polycystic kidney disease (ARPKD) is caused by mutations in PKHD1, encoding fibrocystin/polyductin (FPC). Severe disease occurs in perinates. Those who survive the neonatal period face a myriad of comorbidities, including systemic and portal hypertension, liver fibrosis, and hepatosplenomegaly. The goal here was to uncover therapeutic strategies for ARPKD.

METHODS

We used wild-type and an FPC-mutant cholangiocyte cell line in 3-dimenional cysts and in confluent monolayers to evaluate protein expression using western blotting and protein trafficking using confocal microscopy.

RESULTS

We found that the protein level of the cystic fibrosis transmembrane conductance regulator (CFTR) was downregulated. The levels of heat shock proteins (HSPs) were altered in the FPC-mutant cholangiocytes, with HSP27 being downregulated and HSP90 and HSP70 upregulated. FPC-mutant cholangiocytes formed cysts, but normal cells did not. Cyst growth could be reduced by increasing HSP27 protein levels, by HSP90 and HSP70 inhibitor treatments, by silencing HSP90 through messenger RNA inhibition, or by the novel approach of treating the cysts with the CFTR corrector VX-809. In wild-type cholangiocytes, CFTR is present in both apical and basolateral membranes. FPC malfunction resulted in altered colocalization of CFTR with both apical and basolateral membranes. Whereas, treatment with VX-809, increasing HSP27 or inhibiting HSP70 or HSP90 restored CFTR localization toward normal values.

CONCLUSIONS

FPC malfunction induces the formation of cysts, which are fueled by alterations in HSPs and in CFTR protein levels and miss-localization. We suggest that CFTR correctors, already in clinical use to treat cystic fibrosis, could also be used as a treatment for ARPKD.

Development of a QPatch-Automated Electrophysiology Assay for Identifying TMEM16A Small-Molecule Inhibitors

The calcium-activated chloride channel, TMEM16A, is involved in airway hydration and bronchoconstriction and is a promising target for respiratory disease. Drug development efforts around channels require an electrophysiology-based assay for identifying inhibitors or activators. TMEM16A has proven to be a difficult channel to record on automated electrophysiology platforms due to its propensity for rundown. We developed an automated, whole-cell, electrophysiology assay on the QPatch-48 to evaluate small-molecule inhibitors of TMEM16A. In this assay, currents remained stable for a duration of roughly 11 min, allowing for the cumulative addition of five concentrations of compounds and resulted in reproducible IC₅₀s. The absence of rundown was likely due to a low internal free-calcium level of 250 nM, which was high enough to produce large currents, but also maintained the voltage dependence of the channel. Current amplitude averaged 6 nA using the single-hole QPlate and the channel maintained outward rectification throughout the recording. Known TMEM16A inhibitors were tested and their IC₅₀s aligned with those reported in the literature using manual patch-clamp. Once established, this assay was used to validate novel TMEM16A inhibitors that were identified in our high-throughput fluorescent-based assay, as well as to assist in structure–activity relationship efforts by the chemists. Overall, we demonstrate an easy to operate, reproducible, automated electrophysiology assay using the QPatch-48 for TMEM16A drug development efforts.

Gender-Dependent Phenotype in Polycystic Kidney Disease Is Determined by Differential Intracellular Ca2+ Signals

Autosomal dominant polycystic kidney disease (ADPKD) is caused by loss of function of PKD1 (polycystin 1) or PKD2 (polycystin 2). The Ca²⁺-activated Cl⁻ channel TMEM16A has a central role in ADPKD. Expression and function of TMEM16A is upregulated in ADPKD which causes enhanced intracellular Ca²⁺ signaling, cell proliferation, and ion secretion. We analyzed kidneys from Pkd1 knockout mice and found a more pronounced phenotype in males compared to females, despite similar levels of expression for renal tubular TMEM16A. Cell proliferation, which is known to be enhanced with loss of Pkd1^−/−, was larger in male when compared to female Pkd1^−/− cells. This was paralleled by higher basal intracellular Ca²⁺ concentrations in primary renal epithelial cells isolated from Pkd1^−/− males. The results suggest enhanced intracellular Ca²⁺ levels contributing to augmented cell proliferation and cyst development in male kidneys. Enhanced resting Ca²⁺ also caused larger basal chloride currents in male primary cells, as detected in patch clamp recordings. Incubation of mouse primary cells, mCCDcl1 collecting duct cells or M1 collecting duct cells with dihydrotestosterone (DHT) enhanced basal Ca²⁺ levels and increased basal and ATP-stimulated TMEM16A chloride currents. Taken together, the more severe cystic phenotype in males is likely to be caused by enhanced cell proliferation, possibly due to enhanced basal and ATP-induced intracellular Ca²⁺ levels, leading to enhanced TMEM16A currents. Augmented Ca²⁺ signaling is possibly due to enhanced expression of Ca²⁺ transporting/regulating proteins.

The Ca2+-activated chloride channel ANO1/TMEM16A: An emerging therapeutic target for epithelium-originated diseases?

Anoctamin 1 (ANO1) or TMEM16A gene encodes a member of Ca²⁺ activated Cl^– channels (CaCCs) that are critical for physiological functions, such as epithelial secretion, smooth muscle contraction and sensory signal transduction. The attraction and interest in ANO1/TMEM16A arise from a decade long investigations that abnormal expression or dysfunction of ANO1 is involved in many pathological phenotypes and diseases, including asthma, neuropathic pain, hypertension and cancer. However, the lack of specific modulators of ANO1 has impeded the efforts to validate ANO1 as a therapeutic target. This review focuses on the recent progress made in understanding of the pathophysiological functions of CaCC ANO1 and the current modulators used as pharmacological tools, hopefully illustrating a broad spectrum of ANO1 channelopathy and a path forward for this target validation.

Loss of PKD1 and PKD2 share common effects on intracellular Ca2+ signaling

In polycystic kidney disease (PKD) multiple bilateral renal cysts gradually enlarge causing a decline in renal function. Transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1) drive cyst enlargement. We demonstrated recently that a loss of PKD1 increases expression and function of TMEM16A in murine kidneys and in mouse M1 collecting duct cells. The data demonstrated that TMEM16A contributes essentially to cyst growth by upregulating intracellular Ca2+ signaling. Enhanced expression of TMEM16A and Ca2+ signaling increased both cell proliferation and fluid secretion, which suggested inhibition of TMEM16A as a novel therapy in ADPKD. About 15 % of all ADPKD cases are caused by mutations in PKD2. To analyze the effects of loss of function of PKD2 on Ca2+ signaling, we knocked-down Pkd2 in mouse primary renal epithelial cells in the present study, using viral transfection of shRNA. Unlike in Pkd1-/- cells, knockdown of PKD2 lowered basal Ca2+ and augmented store-operated Ca2+ entry, which was both independent of TMEM16A. However, disease causing purinergic Ca2+ store release was enhanced, similar to that observed in Pkd1-/- renal epithelial cells. The present data suggest pharmacological inhibition of TMEM16A as a treatment in ADPKD caused by mutations in both PKD1 and PKD2.

The Controversial Role of Fibrosis in Autosomal Dominant Polycystic Kidney Disease

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is characterized by the progressive growth of cysts but it is also accompanied by diffuse tissue scarring or fibrosis. A number of recent studies have been published in this area, yet the role of fibrosis in ADPKD remains controversial. Here, we will discuss the stages of fibrosis progression in ADPKD, and how these compare with other common kidney diseases. We will also provide a detailed overview of some key mechanistic pathways to fibrosis in the polycystic kidney. Specifically, the role of the 'chronic hypoxia hypothesis', persistent inflammation, Transforming Growth Factor beta (TGFβ), Janus Kinase/Signal Transducers and Activators of Transcription (JAK/STAT) and microRNAs will be examined. Evidence for and against a pathogenic role of extracellular matrix during ADPKD disease progression will be provided.

Macrophage migration inhibitory factor is regulated by HIF-1α and cAMP and promotes renal cyst cell proliferation in a macrophage-independent manner

Progressive cyst growth leads to decline of renal function in polycystic kidney disease. Macrophage migration inhibitory factor (MIF) was found to be upregulated in cyst-lining cells in a mouse model of polycystic kidney disease and to promote cyst growth. In addition, MIF can be secreted by tubular cells and may contribute to cyst growth in an autocrine manner. However, the underlying mechanisms leading to induction of MIF in cyst-lining cells remained elusive. Here, we demonstrate that hypoxia-inducible transcription factor (HIF) 1α upregulates MIF in cyst-lining cells in a tubule-specific PKD1 knockout mouse. Pharmacological stabilization of HIF-1α resulted in significant increase of MIF in cyst epithelial cells whereas tubule-specific knockout of HIF-1α prevented MIF upregulation. Identical regulation could be found for ABCA1, which has been shown to act as a transport protein for MIF. Furthermore, we show that MIF and ABCA1 are direct target genes of HIF-1α in human primary tubular cells. Next to HIF-1α and hypoxia, we found MIF being additionally regulated by cAMP which is a strong promotor of cyst growth. In line with these findings, HIF-1α- and cAMP-dependent in vitro cyst growth could be decreased by the MIF-inhibitor ISO-1 which resulted in reduced cyst cell proliferation. In conclusion, HIF-1α and cAMP regulate MIF in primary tubular cells and cyst-lining epithelial cells, and MIF promotes cyst growth in the absence of macrophages. In line with these findings, the MIF inhibitor ISO-1 attenuates HIF-1α- and cAMP-dependent in vitro cyst enlargement. KEY MESSAGES: • MIF is upregulated in cyst-lining cells in a polycystic kidney disease mouse model. • MIF upregulation is mediated by hypoxia-inducible transcription factor (HIF) 1α. • ABCA1, transport protein for MIF, is also regulated by HIF-1α in vitro and in vivo. • MIF is additionally regulated by cAMP, a strong promotor of cyst growth. • MIF-inhibitor ISO-1 reduces HIF-1α- and cAMP-dependent cyst growth.

Cyst growth in ADPKD is prevented by pharmacological and genetic inhibition of TMEM16A in vivo

In autosomal dominant polycystic kidney disease (ADPKD) multiple bilateral renal cysts gradually enlarge, leading to a decline in renal function. Transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1) are known to drive cyst enlargement. Here we demonstrate that loss of Pkd1 increased expression of TMEM16A and CFTR and Cl^- secretion in murine kidneys, with TMEM16A essentially contributing to cyst growth. Upregulated TMEM16A enhanced intracellular Ca²⁺ signaling and proliferation of Pkd1-deficient renal epithelial cells. In contrast, increase in Ca²⁺ signaling, cell proliferation and CFTR expression was not observed in Pkd1/Tmem16a double knockout mice. Knockout of Tmem16a or inhibition of TMEM16A in vivo by the FDA-approved drugs niclosamide and benzbromarone, as well as the TMEM16A-specific inhibitor Ani9 largely reduced cyst enlargement and abnormal cyst cell proliferation. The present data establish a therapeutic concept for the treatment of ADPKD.

Targeting chloride transport in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited kidney disease. Transepithelial fluid secretion is one of the key factors of cystogenesis in ADPKD. Multiple studies have suggested that fluid secretion across ADPKD cyst-lining cells is driven by the secretion of chloride, essentially mediated by the CFTR channel and stimulated by increased intracellular levels of 3',5'-cyclic adenosine monophosphate. This review focuses on the pathophysiology of fluid secretion in ADPKD based on the pioneering studies of Jared Grantham and colleagues, and on the follow-up investigations from the molecular level to the potential applications in ADPKD patients. Altogether, the studies of fluid and chloride transport in ADPKD paved the way for innovative therapeutic targets to prevent cyst volume expansion and thus, kidney disease progression.

Targeting of Intracellular TMEM16 Proteins to the Plasma Membrane and Activation by Purinergic Signaling

Anoctamins such as TMEM16A and TMEM16B are Ca²⁺-dependent Cl⁻ channels activated through purinergic receptor signaling. TMEM16A (ANO1), TMEM16B (ANO2) and TMEM16F (ANO6) are predominantly expressed at the plasma membrane and are therefore well accessible for functional studies. While TMEM16A and TMEM16B form halide-selective ion channels, TMEM16F and probably TMEM16E operate as phospholipid scramblases and nonselective ion channels. Other TMEM16 paralogs are expressed mainly in intracellular compartments and are therefore difficult to study at the functional level. Here, we report that TMEM16E (ANO5), -H (ANO8), -J (ANO9) and K (ANO10) are targeted to the plasma membrane when fused to a C-terminal CAAX (cysteine, two aliphatic amino acids plus methionin, serine, alanin, cystein or glutamin) motif. These paralogs produce Ca²⁺-dependent ion channels. Surprisingly, expression of the TMEM16 paralogs in the plasma membrane did not produce additional scramblase activity. In contrast, endogenous scrambling induced by stimulation of purinergic P2X7 receptors was attenuated, in parallel with reduced plasma membrane blebbing. This could suggest that intracellular TMEM16 paralogs operate differently when compared to plasma membrane-localized TMEM16F, and may even stabilize intracellular membranes. Alternatively, CAAX tagging, which leads to expression in non-raft compartments of the plasma membrane, may antagonize phosphatidylserine exposure by endogenous raft-located TMEM16F. CAAX-containing constructs may be useful to further investigate the molecular properties of intracellular TMEM16 proteins.

Regulation of TMEM16A by CK2 and Its Role in Cellular Proliferation

Casein kinase 2 (CK2) is a highly ubiquitous and conserved serine/threonine kinase that forms a tetramer consisting of a catalytic subunit (CK2α) and a regulatory subunit (CK2β). Despite being ubiquitous, CK2 is commonly found at higher expression levels in cancer cells, where it inhibits apoptosis, and supports cell migration and proliferation. The Ca²⁺-activated chloride channel TMEM16A shows similar effects in cancer cells: TMEM16A increases cell proliferation and migration and is highly expressed in squamous cell carcinoma of the head and neck (HNSCC) as well as other malignant tumors. A microscopy-based high-throughput screening was performed to identify proteins that regulate TMEM16A. Within this screen, CK2 was found to be required for proper membrane expression of TMEM16A. small interfering (si) RNA-knockdown of CK2 reduced plasma membrane expression of TMEM16A and inhibited TMEM16A whole cell currents in (cystic fibrosis bronchial epithelial) CFBE airway epithelial cells and in the head and neck cancer cell lines Cal33 and BHY. Inhibitors of CK2, such as TBB and the preclinical compound CX4549 (silmitasertib), also blocked membrane expression of TMEM16A and Ca²⁺-activated whole cell currents. siRNA-knockout of CK2 and its pharmacological inhibition, as well as knockdown or inhibition of TMEM16A by either niclosamide or Ani9, attenuated cell proliferation. Simultaneous inhibition of CK2 and TMEM16A strongly potentiated inhibition of cell proliferation. Although membrane expression of TMEM16A is reduced by inhibition of CK2, our data suggest that the antiproliferative effects by inhibition of CK2 are mostly independent of TMEM16A. Simultaneous inhibition of TMEM16A by niclosamide and inhibition of CK2 by silmitasertib was additive with respect to blocking cell proliferation, while cytotoxicity was reduced when compared to solely blockade of CK2. Therefore, parallel blockade TMEM16A by niclosamide may assist with anticancer therapy by silmitasertib.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

TMEM16A drives renal cyst growth by augmenting Ca2+ signaling in M1 cells

Abstract

Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca²⁺-activated TMEM16A Cl⁻ channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca²⁺ activated Cl⁻ channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl⁻ secretion. The present data demonstrate an increase in intracellular Ca²⁺ ([Ca²⁺]i) signals and Cl⁻ secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1−/− knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl⁻ upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca²⁺ levels and enhanced purinergic Ca²⁺ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca²⁺]i. Ca²⁺ signals, proliferation, and Cl⁻ secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in polycystic kidney disease.

Key messages

• ADPKD leads to continuous decline of renal function by growth of renal cysts.

• Knockdown of PKD1 or PKD2 increases TMEM16A expression.

• TMEM16A enhanced intracellular Ca²⁺ signals, Cl⁻ secretion, and proliferation.

• TMEM16A contributes to cyst growth in ADPKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01894-y) contains supplementary material, which is available to authorized users.

Global microRNA profiling in human urinary exosomes reveals novel disease biomarkers and cellular pathways for autosomal dominant polycystic kidney disease

MicroRNAs (miRNAs) play an important role in regulating gene expression in health and disease but their role in modifying disease expression in Autosomal Dominant Polycystic Kidney Disease (ADPKD) remains uncertain. Here, we profiled human urinary exosome miRNA by global small RNA-sequencing in an initial discovery cohort of seven patients with ADPKD with early disease (eGFR over 60ml/min/1.73m²), nine with late disease (eGFR under 60ml/min/1.73m²), and compared their differential expression with six age and sex matched healthy controls. Two kidney-enriched candidate miRNA families were identified (miR-192/miR-194-2 and miR-30) and selected for confirmatory testing in a 60 patient validation cohort by quantitative polymerase chain reaction. We confirmed that miR-192-5p, miR-194-5p, miR-30a-5p, miR-30d-5p and miR-30e-5p were significantly downregulated in patient urine exosomes, in murine Pkd1 cystic kidneys and in human PKD1 cystic kidney tissue. All five miRNAs showed significant correlations with baseline eGFR and ultrasound-determined mean kidney length and improved the diagnostic performance (area under the curve) of mean kidney length for the rate of disease progression. Finally, inverse correlations of these two miRNA families with increased expression in their predicted target genes in patient PKD1 cystic tissue identified dysregulated pathways and transcriptional networks including novel interactions between miR-194-5p and two potentially relevant candidate genes, PIK3R1 and ANO1. Thus, our results identify a subset of urinary exosomal miRNAs that could serve as novel biomarkers of disease progression and suggest new therapeutic targets in ADPKD.

Role of oxygen and the HIF-pathway in polycystic kidney disease

Kidney cyst growth in ADPKD is associated with regional hypoxia, presumably due to a mismatch between enlarged cysts and the peritubular capillary blood supply and compression of peritubular capillaries in cyst walls. Regional hypoxia leads to activation of hypoxia-inducible transcription factors, with the two main HIF isoforms, HIF-1 and HIF-2 expressed in cyst epithelia and pericystic interstitial cells, respectively. While HIF-2 activation is linked to EPO production, mitigating the anemia that normally accompanies chronic kidney disease, HIF-1 promotes cyst growth. HIF-dependent cyst growth is primarily due to an increase in chloride-dependent fluid secretion into the cyst lumen. However, given the broad spectrum of HIF-target genes, additional HIF-mediated pathways may also contribute to cyst progression. Furthermore, hypoxia can influence cyst growth through the generation of reactive oxygen species. Since cyst expansion aggravates regional hypoxia, a feedforward loop is established that accelerates cyst expansion and disease progression. Inhibiting the HIF pathway and/or HIF target genes that are of particular relevance for HIF-dependent cyst fluid secretion may therefore represent novel therapeutic approaches to retard the progression of APDKD.

Metanephric organ culture

Metanephric organ culture, or ex vivo embryonic kidney culture, was developed in the mid-twentieth century as a means to understand the development of the mammalian kidney and was used in early studies of polycystic kidney disease to explore mechanisms of renal cyst initiation by non-genetic factors. Following the identification of cystogenic genes, a resurgence of the use of metanephric organ culture occurred and has yielded insight into basic mechanisms of cystic dilation; facilitated identification of pathogenic pathways and potential therapeutic targets; and provided a system for evaluating therapeutic agents. This chapter provides detailed, step-by-step protocols with rationale and tips for the establishment, maintenance and treatment of metanephric organ cultures, and for performance of the most commonly employed secondary analyses of these cultures.

Contribution of Anoctamins to Cell Survival and Cell Death

Before anoctamins (TMEM16 proteins) were identified as a family of Ca2+-activated chloride channels and phospholipid scramblases, the founding member anoctamin 1 (ANO1, TMEM16A) was known as DOG1, a marker protein for gastrointestinal stromal tumors (GIST). Meanwhile, ANO1 has been examined in more detail, and the role of ANO1 in cell proliferation and the development of different types of malignomas is now well established. While ANO5, ANO7, and ANO9 may also be relevant for growth of cancers, evidence has been provided for a role of ANO6 (TMEM16F) in regulated cell death. The cellular mechanisms by which anoctamins control cell proliferation and cell death, respectively, are just emerging; however, the pronounced effects of anoctamins on intracellular Ca2+ levels are likely to play a significant role. Recent results suggest that some anoctamins control membrane exocytosis by setting Ca2+i levels near the plasma membrane, and/or by controlling the intracellular Cl- concentration. Exocytosis and increased membrane trafficking induced by ANO1 and ANO6 may enhance membrane expression of other chloride channels, such as CFTR and volume activated chloride channels (VRAC). Notably, ANO6-induced phospholipid scrambling with exposure of phosphatidylserine is pivotal for the sheddase function of disintegrin and metalloproteinase (ADAM). This may support cell death and tumorigenic activity of IL-6 by inducing IL-6 trans-signaling. The reported anticancer effects of the anthelminthic drug niclosamide are probably related to the potent inhibitory effect on ANO1, apart from inducing cell cycle arrest through the Let-7d/CDC34 axis. On the contrary, pronounced activation of ANO6 due to a large increase in intracellular calcium, activation of phospholipase A2 or lipid peroxidation, can lead to ferroptotic death of cancer cells. It therefore appears reasonable to search for both inhibitors and potent activators of TMEM16 in order to interfere with cancer growth and metastasis.

HIF-1α drives cyst growth in advanced stages of autosomal dominant polycystic kidney disease

Progressive cyst expansion in autosomal-dominant polycystic kidney disease has been shown to cause regional hypoxia in cystic and pericystic tissue. This results in up-regulation of hypoxia-inducible transcription factor (HIF) 1α in cyst epithelial cells. However, the functional impact of HIF-1α in cystic kidney disease was unknown. A new study has shown that HIF-1α promotes cyst progression in a mouse model of autosomal-dominant polycystic kidney disease.

Direct repression of anoctamin 1 (ANO1) gene transcription by Gli proteins

The Ca²⁺-activated Cl^- channel, anoctamin 1 (Ano1, also known as transmembrane protein 16A) contributes to intestinal pacemaking, fluid secretion, cellular excitability, and tissue development. The human ANO1 promoter contains binding sites for the glioma-associated oncogene (Gli) proteins. We investigated regulation of ANO1 transcription by Gli. ANO1 promoter activity was determined using a luciferase reporter system. Binding and functional effects of Glis on ANO1 transcription and expression were demonstrated by chromatin immunoprecipitation, small interfering RNA knockdown, PCR, immunolabeling, and recordings of Ca²⁺-activated Cl^- currents in human embryonic kidney 293 (HEK293) cells. Results from previous genome-wide association studies were used to test ANO1 promoter polymorphisms for association with disease. Gli1 and Gli2 bound to the promoter and repressed ANO1 transcription. Repression depended on Gli binding to a site close to the ANO1 transcriptional start site. Mutation of this site prevented Gli binding and transcriptional repression. Knockdown of Gli expression and inhibition of Gli activity increased expression of ANO1 RNA and Ca²⁺-activated Cl^- currents in HEK293 cells. A single-nucleotide polymorphism prevented Gli binding and showed association with irritable bowel syndrome. We conclude that Gli1 and Gli2 repress ANO1 by a novel mechanism that is independent of Gli cleavage and that has a role in gastrointestinal function.-Mazzone, A., Gibbons, S. J., Eisenman, S. T., Strege, P. R., Zheng, T., D'Amato, M., Ordog, T., Fernandez-Zapico, M. E., Farrugia, G. Direct repression of anoctamin 1 (ANO1) gene transcription by Gli proteins.

Drug Repurposing: The Anthelmintics Niclosamide and Nitazoxanide Are Potent TMEM16A Antagonists That Fully Bronchodilate Airways

There is an unmet need in severe asthma where approximately 40% of patients exhibit poor β-agonist responsiveness, suffer daily symptoms and show frequent exacerbations. Antagonists of the Ca2+-activated Cl- channel, TMEM16A, offers a new mechanism to bronchodilate airways and block the multiple contractiles operating in severe disease. To identify TMEM16A antagonists we screened a library of ∼580,000 compounds. The anthelmintics niclosamide, nitazoxanide, and related compounds were identified as potent TMEM16A antagonists that blocked airway smooth muscle depolarization and contraction. To evaluate whether TMEM16A antagonists resist use- and inflammatory-desensitization pathways limiting β-agonist action, we tested their efficacy under harsh conditions using maximally contracted airways or airways pretreated with a cytokine cocktail. Stunningly, TMEM16A antagonists fully bronchodilated airways, while the β-agonist isoproterenol showed only partial effects. Thus, antagonists of TMEM16A and repositioning of niclosamide and nitazoxanide represent an important additional treatment for patients with severe asthma and COPD that is poorly controlled with existing therapies. It is of note that drug repurposing has also attracted wide interest in niclosamide and nitazoxanide as a new treatment for cancer and infectious disease. For the first time we identify TMEM16A as a molecular target for these drugs and thus provide fresh insights into their mechanism for the treatment of these disorders in addition to respiratory disease.

Lipid Peroxidation Drives Renal Cyst Growth In Vitro through Activation of TMEM16A

BACKGROUND

Transepithelial chloride^- secretion, through the chloride channels cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1), drives cyst enlargement in polycystic kidney disease (PKD). Polycystic kidneys are hypoxic, and oxidative stress activates TMEM16A. However, mechanisms for channel activation in PKD remain obscure.

METHODS

Using tissue samples from patients with autosomal dominant PKD, embryonic kidney cultures, and an MDCK in vitro cyst model, we assessed peroxidation of plasma membrane phospholipids in human and mouse polycystic kidneys. We also used electrophysiologic Ussing chamber and patch clamp experiments to analyze activation of TMEM16A and growth of renal cysts.

RESULTS

Peroxidation of phospholipids in human and mouse kidneys as well as MDCK cysts in vitro is probably due to enhanced levels of reactive oxygen species. Lipid peroxidation correlated with increased cyst volume as shown in renal cultures and MDCK cysts in three-dimensional cultures. Reactive oxygen species and lipid peroxidation strongly activated TMEM16A, leading to depletion of calcium ion stores and store-operated calcium influx. Activation of TMEM16A- and CFTR-dependent chloride secretion strongly augmented cyst growth. Exposure to scavengers of reactive oxygen species, such as glutathione, coenzyme Q10, or idebenone (a synthetic coenzyme Q10 homolog), as well as inhibition of oxidative lipid damage by ferrostatin-1 largely reduced activation of TMEM16A. Inhibition of TMEM16A reduced proliferation and fluid secretion in vitro.

CONCLUSIONS

These findings indicate that activation of TMEM16A by lipid peroxidation drives growth of renal cysts. We propose direct inhibition of TMEM16A or inhibition of lipid peroxidation as potentially powerful therapeutic approaches to delay cyst development in PKD.

Transmembrane member 16A participates in hydrogen peroxide-induced apoptosis by facilitating mitochondria-dependent pathway in vascular smooth muscle cells

BACKGROUND AND PURPOSE

Transmembrane member 16A (TMEM16A), an intrinsic constituent of the Ca2+ -activated Cl- channel, is involved in vascular smooth muscle cell (VSMC) proliferation and hypertension-induced cerebrovascular remodelling. However, the functional significance of TMEM16A for apoptosis in basilar artery smooth muscle cells (BASMCs) remains elusive. Here, we investigated whether and how TMEM16A contributes to apoptosis in BASMCs.

EXPERIMENTAL APPROACH

Cell viability assay, flow cytometry, Western blot, mitochondrial membrane potential assay, immunogold labelling and co-immunoprecipitation (co-IP) were performed.

KEY RESULTS

Hydrogen peroxide (H2 O2 ) induced BASMC apoptosis through a mitochondria-dependent pathway, including by increasing the apoptosis rate, down-regulating the ratio of Bcl-2/Bax and potentiating the loss of the mitochondrial membrane potential and release of cytochrome c from the mitochondria to the cytoplasm. These effects were all reversed by the silencing of TMEM16A and were further potentiated by the overexpression of TMEM16A. Endogenous TMEM16A was detected in the mitochondrial fraction. Co-IP revealed an interaction between TMEM16A and cyclophilin D, a component of the mitochondrial permeability transition pore (mPTP). This interaction was up-regulated by H2 O2 but restricted by cyclosporin A, an inhibitor of cyclophilin D. TMEM16A increased mPTP opening, resulting in the activation of caspase-9 and caspase-3. The results obtained with cultured BASMCs from TMEM16A smooth muscle-specific knock-in mice were consistent with those from rat BASMCs.

CONCLUSIONS AND IMPLICATIONS

These results suggest that TMEM16A participates in H2 O2 -induced apoptosis via modulation of mitochondrial membrane permeability in VSMCs. This study establishes TMEM16A as a target for therapy of several remodelling-related diseases.

HIF-1α promotes cyst progression in a mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is mainly caused by mutations of the PKD1 gene and characterized by growth of bilateral renal cysts. Cyst growth is accompanied by regional hypoxia and induction of hypoxia-inducible factor (HIF)-1α in cyst-lining epithelial cells. To determine the relevance of HIF-1α for cyst growth in vivo we used an inducible kidney epithelium-specific knockout mouse to delete Pkd1 at postnatal day 20 or 35 to induce polycystic kidney disease of different severity and analyzed the effects of Hif-1α co-deletion and HIF-1α stabilization using a prolyl-hydroxylase inhibitor. HIF-1α expression was enhanced in kidneys with progressive cyst growth induced by early Pkd1 deletion, but unchanged in the milder phenotype induced by later Pkd1 deletion. Hif-1α co-deletion significantly attenuated cyst growth in the severe, but not in the mild, phenotype. Application of a prolyl-hydroxylase inhibitor resulted in severe aggravation of the mild phenotype with rapid loss of renal function. HIF-1α expression was associated with induction of genes that mediate calcium-activated chloride secretion. Thus, HIF-1α does not seem to play a role in early cyst formation, but accelerates cyst growth during progressive polycystic kidney disease. This novel mechanism of cyst growth may qualify as a therapeutic target.

Nephron-specific knockout of TMEM16A leads to reduced number of glomeruli and albuminuria

TMEM16A is a transmembrane protein from a conserved family of calcium-activated proteins that is highly expressed in the kidney. TMEM16A confers calcium-activated chloride channel activity, which is of importance for various cellular functions in secretory epithelia and involved in secretion-dependent renal cyst growth. However, its specific function in renal physiology has remained elusive so far. Therefore, we generated conditional nephron-specific TMEM16A-knockout mice and found that these animals suffered from albuminuria. Kidney histology demonstrated an intact corticomedullary differentiation and absence of cysts. Electron microscopy showed a normal slit diaphragm. However, the total number of glomeruli and total nephron count was decreased in TMEM16A-knockout animals. At the same time, glomerular diameter was increased, presumably as a result of the hyperfiltration in the remaining glomeruli. TUNEL and PCNA stainings showed increased cell death and increased proliferation. Proximal tubular cilia were intact in young animals, but the number of properly ciliated cells was decreased in older, albuminuric animals. Taken together, our data suggest that TMEM16A may be involved in ureteric bud branching and proper nephron endowment. Loss of TMEM16A resulted in reduced nephron number and, subsequently, albuminuria and tubular damage.

Comprehensive behavioral analyses of anoctamin1/TMEM16A-conditional knockout mice

AIMS

Anoctamin-1 (TMEM16A) is a calcium-activated chloride channel that is involved in numerous physiological conditions. Its role has been identified in electrophysiological and histological studies of genetic knockout animals. Recent cellular localization studies have shown that anoctamin-1 is co-expressed with presynaptic proteins, therefore its role in presynaptic terminals has been suggested. However, behavioral studies are lacking because conventional knockouts of anoctamin-1 are lethal after birth. In this study, we explored the role of anoctamin-1 in presynaptic terminals by analyzing the behavior of mice with conditional knockouts of anoctamin-1 in synapsin1-expressing cells.

MAIN METHODS

Using a synapsin1-Cre system, we selectively ablated anoctamin-1 in synapsin1 expressing cells. The mice were used in the behavioral experiments when they were between 6 and 9 months of age.

KEY FINDINGS

The mice with the conditional knockout of anoctamin-1 in synapsin1-expressing cells displayed impaired social behavior. In addition, the mice showed depressive-like behavior and decreased weight. However, these animals displayed normal locomotor activity, cognitive function, and motor coordination.

SIGNIFICANCE

These results suggested that anoctamin-1 is involved in psychiatric behavior because of its role in the regulation of synaptic transmission in presynaptic terminals.

Cell-specific mechanisms of TMEM16A Ca2+-activated chloride channel in cancer

TMEM16A (known as anoctamin 1) Ca2+-activated chloride channel is overexpressed in many tumors. TMEM16A overexpression can be caused by gene amplification in many tumors harboring 11q13 amplification. TMEM16A expression is also controlled in many cancer cells via transcriptional regulation, epigenetic regulation and microRNAs. In addition, TMEM16A activates different signaling pathways in different cancers, e.g. the EGFR and CAMKII signaling in breast cancer, the p38 and ERK1/2 signaling in hepatoma, the Ras-Raf-MEK-ERK1/2 signaling in head and neck squamous cell carcinoma and bladder cancer, and the NFκB signaling in glioma. Furthermore, TMEM16A overexpression has been reported to promote, inhibit, or produce no effects on cell proliferation and migration in different cancer cells. Since TMEM16A exerts different roles in different cancer cells via activation of distinct signaling pathways, we try to develop the idea that TMEM16A regulates cancer cell proliferation and migration in a cell-dependent mechanism. The cell-specific role of TMEM16A may depend on the cellular environment that is predetermined by TMEM16A overexpression mechanisms specific for a particular cancer type. TMEM16A may exert its cell-specific role via its associated protein networks, phosphorylation by different kinases, and involvement of different signaling pathways. In addition, we discuss the role of TMEM16A channel activity in cancer, and its clinical use as a prognostic and predictive marker in different cancers. This review highlights the cell-type specific mechanisms of TMEM16A in cancer, and envisions the promising use of TMEM16A inhibitors as a potential treatment for TMEM16A-overexpressing cancers.

Cell-specific regulation of proliferation by Ano1/TMEM16A in breast cancer with different ER, PR, and HER2 status

The calcium-activated chloride channel Ano1 (TMEM16A) is overexpressed in many tumors. However, conflicting data exist regarding the role of Ano1 in cell proliferation. Here, we performed immunohistochemistry to investigate the expression of Ano1 and Ki67 in 403 patients with breast cancer, and analyzed the association between the expression of Ano1 and Ki67 in breast cancer subtypes categorized according to estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). Ano1 expression was negatively correlated with Ki67 expression. Ano1 overexpression more frequently occurred in ER-positive or HER2-negative patients with the low expression of Ki67. Ano1 overexpression was associated with longer overall survival (OS) in breast cancer with the low expression of Ki67, especially in ER-positive, PR-positive, and HER2-negative breast cancer. Multivariate Cox regression analysis showed that Ano1 overexpression was a prognostic factor for longer overall survival in ER-positive, PR-positive, or HER2-negative patients with the low expression of Ki67. Furthermore, Ano1 promoted cell proliferation in ER-positive, PR-positive, and HER2-negative MCF7 cells, but inhibited cell proliferation in ER-negative, PR-negative, and HER2-negative MDA-MB-435S cells. Our findings suggest that Ano1 may differentially regulate cell proliferation in a subtype of breast cancer defined by ER, PR, and HER2. Combined expression of Ano1 and Ki67 may be used for predicting clinical outcomes of breast cancer patients with different subtypes of ER, PR, and HER2.

Mechanical Characterization of Microengineered Epithelial Cysts by Using Atomic Force Microscopy

Most organs contain interconnected tubular tissues that are one-cell-thick, polarized epithelial monolayers enclosing a fluid-filled lumen. Such tissue organization plays crucial roles in developmental and normal physiology, and the proper functioning of these tissues depends on their regulation by complex biochemical perturbations and equally important, but poorly understood, mechanical perturbations. In this study, by combining micropatterning techniques and atomic force microscopy, we developed a simple in vitro experimental platform for characterizing the mechanical properties of the MDCK II cyst, the simplest model of lumen-enclosing epithelial monolayers. By using this platform, we estimated the elasticity of the cyst monolayer and showed that the presence of a luminal space influences cyst mechanics substantially, which could be attributed to polarization and tissue-level coordination. More interestingly, the results from force-relaxation experiments showed that the cysts also displayed tissue-level poroelastic characteristics that differed slightly from those of single cells. Our study provides the first quantitative findings, to our knowledge, on the tissue-level mechanics of well-polarized epithelial cysts and offers new insights into the interplay between cyst mechanics and cyst physiology. Moreover, our simple platform is a potentially useful tool for enhancing the current understanding of cyst mechanics in health and disease.

Molecular, biophysical, and pharmacological properties of calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are a family of anionic transmembrane ion channels. They are mainly responsible for the movement of Cl^- and other anions across the biological membranes, and they are widely expressed in different tissues. Since the Cl^- flow into or out of the cell plays a crucial role in hyperpolarizing or depolarizing the cells, respectively, the impact of intracellular Ca²⁺ concentration on these channels is attracting a lot of attentions. After summarizing the molecular, biophysical, and pharmacological properties of CaCCs, the role of CaCCs in normal cellular functions will be discussed, and I will emphasize how dysregulation of CaCCs in pathological conditions can account for different diseases. A better understanding of CaCCs and a pivotal regulatory role of Ca²⁺ can shed more light on the therapeutic strategies for different neurological disorders that arise from chloride dysregulation, such as asthma, cystic fibrosis, and neuropathic pain.

MDCK cells are capable of water secretion and reabsorption in response to changes in the ionic environment

A prerequisite for tissue electrolyte homeostasis is highly regulated ion and water transport through kidney or intestinal epithelia. In the present work, we monitored changes in the cell and luminal volumes of type II Madin-Darby canine kidney (MDCK) cells grown in a 3D environment in response to drugs, or to changes in the composition of the basal extracellular fluid. Using fluorescent markers and high-resolution spinning disc confocal microscopy, we could show that lack of sodium and potassium ions in the basal fluid (tetramethylammonium chloride (TMACl) buffer) induces a rapid increase in the cell and luminal volumes. This transepithelial water flow could be regulated by inhibitors and agonists of chloride channels. Hence, the driving force for the transepithelial water flow is chloride secretion, stimulated by hyperpolarization. Chloride ion depletion of the basal fluid (using sodium gluconate buffer) induces a strong reduction in the lumen size, indicating reabsorption of water from the lumen to the basal side. Lumen size also decreased following depolarization of the cell interior by rendering the membrane permeable to potassium. Hence, MDCK cells are capable of both absorption and secretion of chloride ions and water; negative potential within the lumen supports secretion, while depolarizing conditions promote reabsorption.

P2Y2R is a direct target of HIF-1α and mediates secretion-dependent cyst growth of renal cyst-forming epithelial cells

Polycystic kidney diseases are characterized by numerous renal cysts that continuously enlarge resulting in compression of intact nephrons and tissue hypoxia. Recently, we have shown that hypoxia-inducible factor (HIF)-1α promotes secretion-dependent cyst expansion, presumably by transcriptional regulation of proteins that are involved in calcium-activated chloride secretion. Here, we report that HIF-1α directly activates expression of the purinergic receptor P2Y2R in human primary renal tubular cells. In addition, we found that P2Y2R is highly expressed in cyst-lining cells of human ADPKD kidneys as well as PKD1 orthologous mouse kidneys. Knockdown of P2Y2R in renal collecting duct cells inhibited calcium-dependent chloride secretion in Ussing chamber analyses. In line with these findings, knockdown of P2Y2R retarded cyst expansion in vitro and prevented ATP- and HIF-1α-dependent cyst growth. In conclusion, P2Y2R mediates ATP-dependent cyst growth and is transcriptionally regulated by HIF-1α. These findings provide further mechanistic evidence on how hypoxia promotes cyst growth.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.