alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

Sodium channels and the ionic microenvironment of breast tumours

Cancers of epithelial origin such as breast, prostate, cervical, gastric, colon and lung cancer account for a large proportion of deaths worldwide. Better treatment of metastasis, the main cause of cancer deaths, is therefore urgently required. Several of these tumours have been shown to have an abnormally high concentration of Na+ ([Na+ ]) and emerging evidence points to this accumulation being due to elevated intracellular [Na+ ]. This poses intriguing questions about the cellular mechanisms underlying Na+ dysregulation in cancer, and its pathophysiological significance. Elevated intracellular [Na+ ] may be due to alterations in activity of the Na+ /K+ -ATPase, and/or increased influx via Na+ channels and Na+ -linked transporters. Maintenance of the electrochemical Na+ gradient across the plasma membrane is vital to power many cellular processes that are highly active in cancer cells, including glucose and glutamine import. Na+ channels are also upregulated in cancer cells, which in turn promotes tumour growth and metastasis. For example, ENaC and ASICs are overexpressed in cancers, increasing invasion and proliferation. In addition, voltage-gated Na+ channels are also upregulated in a range of tumour types, where they promote metastatic cell behaviours via various mechanisms, including membrane potential depolarisation and altered pH regulation. Together, recent findings relating to elevated Na+ in the tumour microenvironment and how this may be regulated by several classes of Na+ channels provide a link between altered Na+ handling and poor clinical outcome. There are new opportunities to leverage this altered Na+ microenvironment for therapeutic benefit, as exemplified by several ongoing clinical trials.

Resveratrol downregulates ENaCs through the activation of AMPK in human colon cancer cells

Epithelial sodium channels (ENaCs) are critically engaged in a number of hallmarks of cancer progression, including proliferation, migration, invasion and apoptosis. Thus, the inhibition of ENaCs possesses therapeutic potential in cancer. Resveratrol, a natural polyphenol with anti-carcinogenic activity, is a potent activator of 5' AMP-activated protein kinase (AMPK) which reduces the abundance of ENaCs in the cell membrane by causing the internalization of the β subunit. However, the effect of resveratrol on ENaCs in cancer cells is unknown. Therefore, in this study, we aimed to investigate the effects of resveratrol on ENaCs in human colon cancer cells HCT116 and HT29. The influences of resveratrol either alone or together with AMPK inhibitor compound C (CC), and ENaC inhibitor amiloride on cell viability were examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The expression levels of phospho-AMPK Thr¹⁷² and βENaC in cells were determined by immunofluorescence staining, and the expression of apoptotic markers Caspase-3 and Caspase-9 were analyzed by qRT-PCR. Resveratrol was found to activate AMPK in a dose-dependent manner. Both AMPK activation by resveratrol and ENaC inhibition by amiloride decreased cell viability and increased apoptosis significantly. AMPK activation also reduced βENaC expression in cells. Our results suggest that ENaC inhibition through AMPK activation might be a potential mechanism underlying the anti-cancer effects of resveratrol.

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.

Cancer Salt Nostalgia

High-salt (sodium chloride) diets have been strongly associated with disease states and poor health outcomes. Traditionally, the impact of salt intake is primarily studied in cardiovascular diseases, hypertension and renal diseases; however, recently there has been increasing evidence demonstrating the role of salt in autoimmune diseases. Salt has been shown to modulate the inflammatory activation of immune cells leading to chronic inflammation-related ailments. To date, there is minimal evidence showing a direct correlation of salt with cancer incidence and/or cancer-related adverse clinical outcomes. In this review article, we will discuss the recent understanding of the molecular role of salt, and elucidate the apparent double-edged sword nature of the relationship between salt and cancer progression.

Differential expression of epithelial sodium channels in human RCC associated with the prognosis and tumor stage: Evidence from integrate analysis

Background: Epithelial sodium channels are disputed in renal cell carcinoma, but its functions and effects on clinical outcomes are not well understood. Materials and Methods: IHC and PT-PCR were used to detect ENaCα, β, γ, AVPR2, AQP2, and MR expression in the primary tumor and peritumoral tissues. GEPIA online tool was used to analyze the relationship between epithelial sodium channels and clinical-pathological characteristics. Tumor IMmune Estimation Resource online tool was used to investigate the immune profile relevant to epithelial sodium channels expression. Results: Quantitative RT-PCR analysis revealed that ENaCα, β, γ, AQP2, and AVPR2 mRNA were decreased in the RCC, but there was no difference in MR mRNA expression between kidney and RCC (p=0.238). The IHC analyses showed that the intensely positive staining of ENaCα, β, γ, AVPR2, and AQP in the renal tubular and the attenuated in the RCCs. MR displayed moderate staining in both RCC and normal tissue. With the promotion of staging, the expression of AQP2, AVPR2, and MR reduced gradually and predicted a better prognosis. Although ENaCα, β, and γ were unable to associate with staging, we still observed a high expression of ENaCβ and γ displayed a poorer prognosis of RCC. Conclusions: ENaCs shows an oncogene profile in RCC, drugs targeting epithelial sodium channel should be a possible therapeutic way to treat RCC. AVPR2 and MR exhibit an encouraging immunomodulatory function; patients with low expression of AVPR2 and MR may obtain more benefit from immunotherapy.

How Dysregulated Ion Channels and Transporters Take a Hand in Esophageal, Liver, and Colorectal Cancer

Over the last two decades, the understanding of how dysregulated ion channels and transporters are involved in carcinogenesis and tumor growth and progression, including invasiveness and metastasis, has been increasing exponentially. The present review specifies virtually all ion channels and transporters whose faulty expression or regulation contributes to esophageal, hepatocellular, and colorectal cancer. The variety reaches from Ca²⁺, K⁺, Na⁺, and Cl^- channels over divalent metal transporters, Na⁺ or Cl^- coupled Ca²⁺, HCO₃^- and H⁺ exchangers to monocarboxylate carriers and organic anion and cation transporters. In several cases, the underlying mechanisms by which these ion channels/transporters are interwoven with malignancies have been fully or at least partially unveiled. Ca²⁺, Akt/NF-κB, and Ca²⁺- or pH-dependent Wnt/β-catenin signaling emerge as cross points through which ion channels/transporters interfere with gene expression, modulate cell proliferation, trigger epithelial-to-mesenchymal transition, and promote cell motility and metastasis. Also miRs, lncRNAs, and DNA methylation represent potential links between the misexpression of genes encoding for ion channels/transporters, their malfunctioning, and cancer. The knowledge of all these molecular interactions has provided the basis for therapeutic strategies and approaches, some of which will be broached in this review.

Targeting Ion Channels for Cancer Treatment: Current Progress and Future Challenges

Ion channels are key regulators of cancer cell pathophysiology. They contribute to a variety of processes such as maintenance of cellular osmolarity and membrane potential, motility (via interactions with the cytoskeleton), invasion, signal transduction, transcriptional activity and cell cycle progression, leading to tumour progression and metastasis. Ion channels thus represent promising targets for cancer therapy. Ion channels are attractive targets because many of them are expressed at the plasma membrane and a broad range of existing inhibitors are already in clinical use for other indications. However, many of the ion channels identified in cancer cells are also active in healthy normal cells, so there is a risk that certain blockers may have off-target effects on normal physiological function. This review describes recent research advances into ion channel inhibitors as anticancer therapeutics. A growing body of evidence suggests that a range of existing and novel Na⁺, K⁺, Ca²⁺ and Cl^- channel inhibitors may be effective for suppressing cancer cell proliferation, migration and invasion, as well as enhancing apoptosis, leading to suppression of tumour growth and metastasis, either alone or in combination with standard-of-care therapies. The majority of evidence to date is based on preclinical in vitro and in vivo studies, although there are several examples of ion channel-targeting strategies now reaching early phase clinical trials. Given the strong links between ion channel function and regulation of tumour growth, metastasis and chemotherapy resistance, it is likely that further work in this area will facilitate the development of new therapeutic approaches which will reach the clinic in the future.

Intracellular delivery of liposome-encapsulated Finland trityl radicals for EPR oximetry

Tetrathiatriarylmethyl (TAM, trityl) radicals have found wide applications in electron paramagnetic resonance (EPR) oximetry. However, the biomedical applications of TAM radicals were exclusively limited to an extracellular region owing to their negatively charged nature. The intracellular delivery of TAM radicals still remains a challenge. In the present work, we report a liposome-based method to encapsulate the water-soluble Finland trityl radical CT-03 for its intracellular delivery. Using the thin lipid film hydration method, CT-03-loaded liposomes were prepared from DSPC/cholesterol/DOTAP with a mean size of 167.5 ± 2.4 nm and a zeta potential of 27.8 ± 0.8 mV. EPR results showed that CT-03 was entrapped into the liposomes and still exhibited good oxygen (O₂) sensitivity. Moreover, CT-03 was successfully delivered into HepG2 cells and HUVECs using the CT-03-loaded liposomes. Importantly, the combination of the liposome-encapsulated radical CT-03 and the other TAM radical CT02-H enabled simultaneous measurements of the intracellular and extracellular O₂ concentrations and O₂ consumption rates in HepG2 cells. Our present study provides a new approach for intracellular delivery of TAM radicals and could significantly expand their biomedical applications.

Sodium homeostasis in the tumour microenvironment

The concentration of sodium ions (Na+) is raised in solid tumours and can be measured at the cellular, tissue and patient levels. At the cellular level, the Na+ gradient across the membrane powers the transport of H+ ions and essential nutrients for normal activity. The maintenance of the Na+ gradient requires a large proportion of the cell's ATP. Na+ is a major contributor to the osmolarity of the tumour microenvironment, which affects cell volume and metabolism as well as immune function. Here, we review evidence indicating that Na+ handling is altered in tumours, explore our current understanding of the mechanisms that may underlie these alterations and consider the potential consequences for cancer progression. Dysregulated Na+ balance in tumours may open opportunities for new imaging biomarkers and re-purposing of drugs for treatment.

Potassium and Sodium Channels and the Warburg Effect: Biophysical Regulation of Cancer Metabolism

Ion channels are progressively emerging as a novel class of membrane proteins expressed in several types of human cancers and regulating the different aspects of cancer cell behavior. The metabolism of cancer cells, usually composed by a variable proportion of respiration, glycolysis, and glutaminolysis, leads to the excessive production of acidic metabolic products. The presence of these acidic metabolites inside the cells results in intracellular acidosis, and hinders survival and proliferation. For this reason, tumor cells activate mechanisms of pH control that produce a constitutive increase in intracellular pH (pHi) that is more acidic than the extracellular pH (pHe). This condition forms a perfect microenvironment for metastatic progression and may be permissive for some of the acquired characteristics of tumors. Recent analyses have revealed complex interconnections between oncogenic activation, ion channels, hypoxia signaling and metabolic pathways that are dysregulated in cancer. Here, we summarize the molecular mechanisms of the Warburg effect and hypoxia and their association. Moreover, we discuss the recent findings concerning the involvement of ion channels in various aspects of the Warburg effect and hypoxia, focusing on the role of Na+ and K+ channels in hypoxic and metabolic reprogramming in cancer.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

Hypertonicity-induced cation channels in HepG2 cells: architecture and role in proliferation vs. apoptosis

KEY POINTS

Na⁺ conducting hypertonicity-induced cation channels (HICCs) are key players in the volume restoration of osmotically shrunken cells and, under isotonic conditions, considered as mediators of proliferation - thereby opposing apoptosis. In an siRNA screen of ion channels and transporters in HepG2 cells, with the regulatory volume increase (RVI) as read-out, δENaC, TRPM2 and TRPM5 were identified as HICCs. Subsequently, all permutations of these channels were tested in RVI and patch-clamp recordings and, at first sight, HICCs were found to operate in an independent mode. However, there was synergy in the siRNA perturbations of HICC currents. Accordingly, proximity ligation assays showed that δENaC was located in proximity to TRPM2 and TRPM5 suggesting a physical interaction. Furthermore, δENaC, TRPM2 and TRPM5 were identified as mediators of HepG2 proliferation - their silencing enhanced apoptosis. Our study defines the architecture of HICCs in human hepatocytes as well as their molecular functions.

ABSTRACT

Hypertonicity-induced cation channels (HICCs) are a substantial element in the regulatory volume increase (RVI) of osmotically shrunken cells. Under isotonic conditions, they are key effectors in the volume gain preceding proliferation; HICC repression, in turn, significantly increases apoptosis rates. Despite these fundamental roles of HICCs in cell physiology, very little is known concerning the actual molecular architecture of these channels. Here, an siRNA screening of putative ion channels and transporters was performed, in HepG2 cells, with the velocity of RVI as the read-out; in this first run, δENaC, TRPM2 and TRPM5 could be identified as HICCs. In the second run, all permutations of these channels were tested in RVI and patch-clamp recordings, with special emphasis on the non-additivity and additivity of siRNAs - which would indicate molecular interactions or independent ways of channel functioning. At first sight, the HICCs in HepG2 cells appeared to operate rather independently. However, a proximity ligation assay revealed that δENaC was located in proximity to both TRPM2 and TRPM5. Furthermore, a clear synergy of HICC current knock-downs (KDs) was observed. δENaC, TRPM2 and TRPM5 were defined as mediators of HepG2 cell proliferation and their silencing increased the rates of apoptosis. This study provides a molecular characterization of the HICCs in human hepatocytes and of their role in RVI, cell proliferation and apoptosis.

Titanium dioxide nanoparticles induce mitochondria-associated apoptosis in HepG2 cells

Widespread applications of nanosized materials over the past decade have prompted investigations of desirable properties and potential hazards to humans and the environment.

MRP4 regulates ENaC-dependent CREB/COX-2/PGE2 signaling during embryo implantation

Multi-drug resistance protein 4 (MRP4), a potential chemotherapeutic target as well as a transporter for endogenous signaling molecules (e.g. prostaglandins), is known to be expressed in the endometrium, although its possible role(s) in the physiology of the endometrium remains unknown. Here, we show that MRP4 is upregulated at implantation window and localized to the basolateral membrane of the endometrial epithelium, the interface between the epithelium and stroma in mice. In human endometrial epithelial cells, MRP4 expression is upregulated by ENaC activation and the inhibition of MRP4 blocks ENaC-dependent PGE₂ release as well as phosphorylation of CREB. Intrauterine injection of MRP4 inhibitor in mice prior to implantation significantly downregulated implantation markers COX-2, Claudin4 and Lif, and reduced implantation rate. These results in together have revealed a previously undefined role of MRP4 in mediating ENaC-dependent CREB/COX-2/PGE₂ signaling essential to embryo implantation with implication in cancer progression as well.

Methods for cell volume measurement

Volume is an essential characteristic of a cell, and this review describes the main methods of its measurement that have been used in the past several decades. The discussed methods include various implementations of light scattering, estimates based on one or two cell dimensions, surface scanning, fluorescence confocal and transmission slice-by-slice imaging, intracellular volume markers, displacement of extracellular solution, quantitative phase imaging, radioactive methods, and some others. Suitability of these methods to some typical samples and applications is discussed. © 2017 International Society for Advancement of Cytometry.

Extraoral Taste Receptor Discovery: New Light on Ayurvedic Pharmacology

More and more research studies are revealing unexpectedly important roles of taste for health and pathogenesis of various diseases. Only recently it has been shown that taste receptors have many extraoral locations (e.g., stomach, intestines, liver, pancreas, respiratory system, heart, brain, kidney, urinary bladder, pancreas, adipose tissue, testis, and ovary), being part of a large diffuse chemosensory system. The functional implications of these taste receptors widely dispersed in various organs or tissues shed a new light on several concepts used in ayurvedic pharmacology (dravyaguna vijnana), such as taste (rasa), postdigestive effect (vipaka), qualities (guna), and energetic nature (virya). This review summarizes the significance of extraoral taste receptors and transient receptor potential (TRP) channels for ayurvedic pharmacology, as well as the biological activities of various types of phytochemical tastants from an ayurvedic perspective. The relative importance of taste (rasa), postdigestive effect (vipaka), and energetic nature (virya) as ethnopharmacological descriptors within Ayurveda boundaries will also be discussed.

Over-expression of ASIC1a promotes proliferation via activation of the β-catenin/LEF-TCF axis and is associated with disease outcome in liver cancer

Acid-sensing ion channels 1a (ASIC1a) has been reported to promote migration and invasion in liver cancer. However, the clinical significance and molecular mechanism of ASIC1a in liver cancer remain unknown. In the study, we found that ASIC1a is frequently up-regulated in liver cancer tissues. The over-expression of ASIC1a is associated with advanced clinical stage and poor prognosis. The pro-proliferative of ASIC1a is pH dependent. Knockout of ASIC1a by CRISPR/CAS9 inhibited liver cancer cell proliferation and tumorigenicity in vitro and in vivo through β-catenin degradation and LEF-TCF inactivation. Our results indicated a potential diagnostic marker and chemotherapeutic target for liver cancer.

Inflammatory role of high salt level in tumor microenvironment (Review)

Chronic inflammation is known to play a critical role in cancer development and progression. High salt is known to mediate several chronic inflammatory diseases including hypertension, myocardial infarction, neurological ischemic attack, autoimmune diseases and cancers. High salt level is shown to induce angiogenesis and immune-dysfunction, both of which play a direct role in cancer proliferation. Furthermore, salt has been suggested to enhance Warburg-like metabolic phenotype in cancer cells and at the same time also induce pro-tumor MΦ2-macrophage phenotype. Recent studies have identified several molecular targets such as tonicity specific transcript factor NFAT5/TonEBP, sodium ion channel γENaC, and vascular endothelial growth factor, VEGF, which are upregulated under high salt external environment. These molecular targets offer futuristic therapeutic application in precision medicine. In this review, we discuss the current understanding of the salt mediated metabolic and immune dysfunctions playing a potential role in cancerous changes.

ENaC/DEG in Tumor Development and Progression

The epithelial Na+ channel/degenerin (ENaC/DEG) superfamily, including the acid-sensing ion channels (ASICs), is characterized by a high degree of similarity in structure but highly diverse in physiological functions. These ion channels have been shown to be important in several physiological functions of normal epithelial cells, including salt homeostasis, fluid transportation and cell mobility. There is increasing evidence suggesting that ENaC/DEG channels are critically engaged in cancer cell biology, such as proliferation, migration, invasion and apoptosis, playing a role in tumor development and progression. In this review, we will discuss recent studies showing the role of ENaC and ASIC channels in epithelial cells and its relationship to the oncogenesis.

Potential Roles of Amiloride-Sensitive Sodium Channels in Cancer Development

The ENaC/degenerin ion channel superfamily includes the amiloride-sensitive epithelial sodium channel (ENaC) and acid sensitive ionic channel (ASIC). ENaC is a multimeric ion channel formed by heteromultimeric membrane glycoproteins, which participate in a multitude of biological processes by mediating the transport of sodium (Na⁺) across epithelial tissues such as the kidney, lungs, bladder, and gut. Aberrant ENaC functions contribute to several human disease states including pseudohypoaldosteronism, Liddle syndrome, cystic fibrosis, and salt-sensitive hypertension. Increasing evidence suggests that ion channels not only regulate ion homeostasis and electric signaling in excitable cells but also play important roles in cancer cell behaviors such as proliferation, apoptosis, invasion, and migration. Indeed, ENaCs/ASICs had been reported to be associated with cancer characteristics. Given their cell surface localization and pharmacology, pharmacological strategies to target ENaC/ASIC family members may be promising cancer therapeutics.

Mitogen-activated protein kinases as key players in osmotic stress signaling

BACKGROUND

Osmotic stress arises from the difference between intracellular and extracellular osmolality. It induces cell swelling or shrinkage as a consequence of water influx or efflux, which threatens cellular activities. Mitogen-activated protein kinases (MAPKs) play central roles in signaling pathways in osmotic stress responses, including the regulation of intracellular levels of inorganic ions and organic osmolytes.

SCOPE OF REVIEW

The present review summarizes the cellular osmotic stress response and the function and regulation of the vertebrate MAPK signaling pathways involved. We also describe recent findings regarding apoptosis signal-regulating kinase 3 (ASK3), a MAP3K member, to demonstrate its regulatory effects on signaling molecules beyond MAPKs.

MAJOR CONCLUSIONS

MAPKs are rapidly activated by osmotic stress and have diverse roles, such as cell volume regulation, gene expression, and cell survival/death. There is significant cell type specificity in the function and regulation of MAPKs. Based on its activity change during osmotic stress and its regulation of the WNK1-SPAK/OSR1 pathway, ASK3 is expected to play important roles in osmosensing mechanisms and cellular functions related to osmoregulation.

GENERAL SIGNIFICANCE

MAPKs are essential for various cellular responses to osmotic stress; thus, the identification of the upstream regulators of MAPK pathways will provide valuable clues regarding the cellular osmosensing mechanism, which remains elusive in mammals. The elucidation of in vivo MAPK functions is also important because osmotic stress in physiological and pathophysiological conditions often results from changes in the intracellular osmolality. These studies potentially contribute to the establishment of therapeutic strategies against diseases that accompany osmotic perturbation.

Aldosterone alters the chromatin structure of the murine endothelin-1 gene

Aldosterone increases sodium reabsorption in the renal collecting duct and systemic blood pressure. Paradoxically, aldosterone also induces transcription of the endothelin-1 (Edn1) gene to increase protein (ET-1) levels, which inhibits sodium reabsorption.

AIMS

Here we investigated changes in the chromatin structure of the Edn1 gene of collecting duct cell lines in response to aldosterone treatment. The Edn1 gene has a CpG island that encompasses the transcription start site and four sites in the 5' regulatory region previously linked to transcriptional regulation.

MATERIALS AND METHODS

The chromatin structure of the Edn1 gene was investigated using a quantitative PCR-based DNaseI hypersensitivity assay in murine hepatocyte (AML12), renal cortical collecting duct (mpkCCDC14), outer medullary collecting duct1 (OMCD1), and inner medullary collecting duct-3 (IMCD-3) cell lines.

KEY FINDINGS

The CpG island was uniformly accessible. One calcium-responsive NFAT element remained at low chromatin accessibility in all cell lines under all conditions tested. However, the second calcium responsive NFAT element located at -1563bp upstream became markedly more accessible in IMCD-3 cells exposed to aldosterone. Importantly, one established aldosterone hormone response element HRE at -671bp relative to the transcription start site was highly accessible, and another HRE (-551bp) became more accessible in aldosterone-treated IMCD-3 and OMCD1 cells.

SIGNIFICANCE

The evidence supports a model in which aldosterone activation of the mineralocorticoid receptor (MR) results in the MR-hormone complex binding at HRE at -671bp to open chromatin structure around other regulatory elements in the Edn1 gene.

Adaptive responses of cell hydration to a low temperature arrest

Slow cooling leads to a passive dehydration of cells, whereas rehydration during warming reflects the active regain of functionality. The ability to modulate such an energy demanding process could be instrumental in optimizing the cryo-arrest of living systems. In the present study, various levels of hypertonic stress were used to disturb the water content of cells and to define the energy profiles of aquaporins and (Na(+) conducting) cation channels during rehydration. Na(+) import was found to be the rate-limiting step in water restoration, whereas aquaporins merely played a permissive role. Indeed, regulated Na(+) import was increased 2-fold following cryo-arrests, thus facilitating the osmotic rehydration of cells. Freezing temperatures increased cell viscosity with a remarkable hysteresis and viscosity was a trigger of cation channels. The peptide hormone vasopressin was a further activator of channels, increasing the viability of post-cryo cells considerably. Hence, the hormone opens the path for a novel class of cryo-protectants with an intrinsic biological activity.

Genes Regulating Epithelial Polarity Are Critical Suppressors of Esophageal Oncogenesis

Esophageal cancer is an aggressive disease featured by early lymphatic and hematogenous dissemination, and is the sixth leading cause of cancer-related deaths worldwide. The proper formation of apicobasal polarity is essential for normal epithelium physiology and tissue homeostasis, while loss of polarity is a hallmark of cancer development including esophageal oncogenesis. In this review, we summarized the stages of esophageal cancer development associated with the loss or deregulation of epithelial cell apicobasal polarity. Loss of epithelial apicobasal polarity exerts an indispensable role in the initiation of esophageal oncogenesis, tumor progression, and the advancement of tumors from benign to malignant. In particular, we reviewed the involvement of several critical genes, including Lkb1, claudin-4, claudin-7, Par3, Lgl1, E-cadherin, and the Scnn1 gene family. Understanding the role of apicobasal regulators may lead to new paradigms for treatment of esophageal tumors, including improvement of prognostication, early diagnosis, and individually tailored therapeutic interventions in esophageal oncology.

Atomic force microscopy imaging reveals the formation of ASIC/ENaC cross-clade ion channels

ASIC and ENaC are co-expressed in various cell types, and there is evidence for a close association between them. Here, we used atomic force microscopy (AFM) to determine whether ASIC1a and ENaC subunits are able to form cross-clade hybrid ion channels. ASIC1a and ENaC could be co-isolated from detergent extracts of tsA 201 cells co-expressing the two subunits. Isolated proteins were incubated with antibodies against ENaC and Fab fragments against ASIC1a. AFM imaging revealed proteins that were decorated by both an antibody and a Fab fragment with an angle of ∼120° between them, indicating the formation of ASIC1a/ENaC heterotrimers.

Recombinant adeno-associated virus utilizes cell-specific infectious entry mechanisms

Understanding the entry and trafficking mechanism(s) of recombinant adeno-associated virus (rAAV) into host cells can lead to evolution in capsid and vector design and delivery methods, resulting in enhanced transduction and therapeutic gene expression. Variability of findings regarding the early entry pathway of rAAV supports the possibility that rAAV, like other viruses, can utilize more than one infectious entry pathway. We tested whether inhibition of macropinocytosis impacted rAAV transduction of HeLa cells compared to hepatocellular carcinoma cell lines. We found that macropinocytosis inhibitor cytochalasin D blocked rAAV transduction of HeLa cells (>2-fold) but enhanced (10-fold) transduction in HepG2 and Huh7 lines. Similar results were obtained with another macropinocytosis inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA). The augmented transduction was due to neither viral binding nor promoter activity, affected multiple rAAV serotypes (rAAV2, rAAV2-R585E, and rAAV8), and influenced single-stranded and self-complementary virions to comparable extents. Follow-up studies using CDC42 inhibitor ML141 and p21-activated kinase 1 (PAK1) siRNA knockdown also resulted in enhanced HepG2 transduction. Microscopy revealed that macropinocytosis inhibition correlated with expedited nuclear entry of the rAAV virions into HepG2 cells. Enhancement of hepatocellular rAAV transduction extended to the mouse liver in vivo (4-fold enhancement) but inversely blocked heart tissue transduction (13-fold). This evidence of host cell-specific rAAV entry pathways confers a potent means for controlling and enhancing vector delivery and could help unify the divergent accounts of rAAV cellular entry mechanisms.

IMPORTANCE

There is a recognized need for improved rAAV vector targeting strategies that result in delivery of fewer total particles, averting untoward toxicity and/or an immune response against the vector. A critical step in rAAV transduction is entry and early trafficking through the host cellular machinery, the mechanisms of which are under continued study. However, should the early entry and trafficking mechanisms of rAAV differ across virus serotype or be dependent on host cell environment, this could expand our ability to target particular cells and tissue for selective transduction. Thus, the observation that inhibiting macropinocytosis leads to cell-specific enhancement or inhibition of rAAV transduction that extends to the organismic level exposes a new means of modulating vector targeting.

Epoxyeicosatrienoic acids attenuating hypotonic-induced apoptosis of IMCD cells via γ-ENaC inhibition

Inner medulla collecting duct (IMCD) cells are the key part for urinary concentration. Hypotonic stress may trigger apoptosis of IMCD cells and induce renal injury. Epoxyeicosatrienoic acids (EETs) play an important role in anti-apoptosis, but their roles in hypotonic-induced apoptosis of IMCD cells are still unclear. Here we found increasing exogenous 11, 12-EET or endogenous EETs with Ad-CMV-CYP2C23-EGFP transfection decreased apoptosis of IMCD cells induced by hypotonic stress. Moreover, up-regulation of γ-ENaC induced by hypotonic stress was abolished by elevation of exogenous or endogenous EETs. Collectively, this study illustrated that EETs attenuated hypotonic-induced apoptosis of IMCD cells, and that regulation of γ-ENAC may be a possible mechanism contributing to the anti-apoptotic effect of EETs in response to hypotonic stress.

Up-regulation of Kv7.1 channels in thromboxane A2-induced colonic cancer cell proliferation

Thromboxane A2 (TXA2) is known to stimulate colonic cancer cell proliferation, although the mechanism has not been clarified. In this study, we compared the expression levels of Kv7.1 K(+) channels between human colorectal cancer tissue and the accompanying non-tumor mucosa. Kv7.1 proteins were found to be consistently up-regulated in the cancer tissues from different patients. Kv7.1 was also expressed in human colonic cancer cell lines. Treatment of colonic cancer cells with 9,11-epithio-11,12-methano-thromboxane A2 (STA2), a stable analogue of TXA2, significantly increased whole-cell K(+) currents sensitive to chromanol 293B, an inhibitor of Kv7.1 channels, in parallel with an increased expression of Kv7.1 proteins. In contrast, TXB2, an inactive metabolite of TXA2, had no effects on expression level and function of Kv7.1. A TXA2 receptor antagonist (SQ29548) and an inhibitor of cAMP-dependent protein kinase (Rp-8-Br-MB-cAMPS) inhibited STA2-induced increases in both Kv7.1 expression and chromanol 293B-sensitive K(+) currents. Interestingly, STA2-stimulated proliferation of colonic cancer cells was inhibited by chromanol 293B. These results suggest that Kv7.1 channels are involved in the TXA2-induced cancer cell proliferation and that they are up-regulated by the TXA2 receptor-mediated cAMP pathway.

Benzamil sensitive ion channels contribute to volume regulation in canine chondrocytes

BACKGROUND AND PURPOSE

Chondrocytes exist within cartilage and serve to maintain the extracellular matrix. It has been postulated that osteoarthritic (OA) chondrocytes lose the ability to regulate their volume, affecting extracellular matrix production. In previous studies, we identified expression of epithelial sodium channels (ENaC) in human chondrocytes, but their function remained unknown. Although ENaC typically has Na(+) transport roles, it is also involved in the cell volume regulation of rat hepatocytes. ENaC is a member of the degenerin (Deg) family, and ENaC/Deg-like channels have a low conductance and high sensitivity to benzamil. In this study, we investigated whether canine chondrocytes express functional ENaC/Deg-like ion channels and, if so, what their function may be.

EXPERIMENTAL APPROACH

Canine chondrocytes were harvested from dogs killed for unassociated welfare reasons. We used immunohistochemistry and patch-clamp electrophysiology to investigate ENaC expression and video microscopy to analyse the effects of pharmacological inhibition of ENaC/Deg on cell volume regulation.

KEY RESULTS

Immunofluorescence showed that canine chondrocytes expressed ENaC protein. Single-channel recordings demonstrated expression of a benzamil-sensitive Na(+) conductance (9 pS), and whole-cell experiments show this to be approximately 1.5 nS per cell with high selectivity for Na(+) . Benzamil hyperpolarized chondrocytes by approximately 8 mV with a pD2 8.4. Chondrocyte regulatory volume decrease (RVI) was inhibited by benzamil (pD2 7.5) but persisted when extracellular Na(+) ions were replaced by Li(+) .

CONCLUSION AND IMPLICATIONS

Our data suggest that benzamil inhibits RVI by reducing the influx of Na(+) ions through ENaC/Deg-like ion channels and present ENaC/Deg as a possible target for pharmacological modulation of chondrocyte volume.

The migratory capacity of human trophoblastic BeWo cells: effects of aldosterone and the epithelial sodium channel

Aldosterone is a key regulator of the epithelial sodium channel (ENaC) and stimulates protein methylation on the β-subunit of the ENaC. We found that aldosterone (100 nM) promotes cellular migration in a wound-healing model in trophoblastic BeWo cells. Here, we tested if the positive influence of aldosterone on wound healing is related to methylation reactions. Cell migration and proliferation were measured in BeWo cells at 6 h, when mitosis is still scarce. Cell migration covered 12.4, 25.3, 19.6 and 45.1 % of the wound when cultivated under control, aldosterone (12 h), 8Br-cAMP and aldosterone plus 8Br-cAMP, respectively. Amiloride blocked the effects of aldosterone alone or in the presence of 8Br-cAMP on wound healing. Wound healing decreased in aldosterone (plus 8Br-cAMP) coexposed with the methylation inhibitor 3-deaza-adenosine (3-DZA, 12.9 % reinvasion of the wound). There was an increase in wound healing in aldosterone-, 8Br-cAMP- and 3-DZA-treated cells in the presence of AdoMet, a methyl donor, compared to cells in the absence of AdoMet (27.3 and 12.9 % reinvasion of the wound, respectively). Cell proliferation assessed with the reagent MTT was not changed in any of these treatments, suggesting that cellular migration is the main factor for reinvasion of wound healing. Electrophysiological studies showed an increase in ENaC current in the presence of aldosterone. This effect was higher with 8Br-cAMP, and there was a decrease when 3-DZA was present. AdoMet treatment partially reversed this phenomenon. We suggest that aldosterone positively influences wound healing in BeWo cells, at least in part through methylation of the ENaC.

Flufenamic acid as an ion channel modulator

Flufenamic acid has been known since the 1960s to have anti-inflammatory properties attributable to the reduction of prostaglandin synthesis. Thirty years later, flufenamic acid appeared to be an ion channel modulator. Thus, while its use in medicine diminished, its use in ionic channel research expanded. Flufenamic acid commonly not only affects non-selective cation channels and chloride channels, but also modulates potassium, calcium and sodium channels with effective concentrations ranging from 10(-6)M in TRPM4 channel inhibition to 10(-3)M in two-pore outwardly rectifying potassium channel activation. Because flufenamic acid effects develop and reverse rapidly, it is a convenient and widely used tool. However, given the broad spectrum of its targets, experimental results have to be interpreted cautiously. Here we provide an overview of ion channels targeted by flufenamic acid to aid in interpreting its effects at the molecular, cellular, and system levels. If it is used with good practices, flufenamic acid remains a useful tool for ion channel research. Understanding the targets of FFA may help reevaluate its physiological impacts and revive interest in its therapeutic potential.

OSR1-sensitive small intestinal Na+ transport

The oxidative stress responsive kinase 1 (OSR1) contributes to WNK (with no K)-dependent regulation of renal tubular salt transport, renal salt excretion, and blood pressure. Little is known, however, about a role of OSR1 in the regulation of intestinal salt transport. The present study thus explored whether OSR1 is expressed in intestinal tissue and whether small intestinal Na(+)/H(+) exchanger (NHE), small intestinal Na(+)-glucose cotransport (SGLT1), and/or colonic epithelium Na(+) channel (ENaC) differ between knockin mice carrying one allele of WNK-resistant OSR1 (osr1(+/KI)) and wild-type mice (osr1(+/+)). OSR1 protein abundance was determined by Western blotting, cytosolic pH from BCECF fluorescence, NHE activity from Na(+)-dependent realkalinization following an ammonium pulse, SGLT1 activity from glucose-induced current, and colonic ENaC activity from amiloride-sensitive transepithelial current in Ussing chamber experiments. As a result, OSR1 protein was expressed in small intestine of both osr1(+/KI) mice and osr1(+/+) mice. Daily fecal Na(+), K(+), and H(2)O excretion and jejunal SGLT1 activity were lower, whereas small intestinal NHE activity and colonic ENaC activity were higher in osr1(+/KI) mice than in osr1(+/+) mice. NHE3 inhibitor S-3226 significantly reduced NHE activity in both genotypes but did not abrogate the difference between the genotypes. Plasma osmolarity, serum antidiuretic hormone, plasma aldosterone, and plasma corticosterone concentrations were similar in both genotypes. Small intestinal NHE3 and colonic α-ENaC protein abundance were not significantly different between genotypes, but colonic phospho-β-ENaC (ser633) was significantly higher in osr1(+/KI) mice. In conclusion, OSR1 is expressed in intestine and partial WNK insensitivity of OSR1 increases intestinal NHE activity and colonic ENaC activity.

Structural basis for the sheddase function of human meprin β metalloproteinase at the plasma membrane

Ectodomain shedding at the cell surface is a major mechanism to regulate the extracellular and circulatory concentration or the activities of signaling proteins at the plasma membrane. Human meprin β is a 145-kDa disulfide-linked homodimeric multidomain type-I membrane metallopeptidase that sheds membrane-bound cytokines and growth factors, thereby contributing to inflammatory diseases, angiogenesis, and tumor progression. In addition, it cleaves amyloid precursor protein (APP) at the β-secretase site, giving rise to amyloidogenic peptides. We have solved the X-ray crystal structure of a major fragment of the meprin β ectoprotein, the first of a multidomain oligomeric transmembrane sheddase, and of its zymogen. The meprin β dimer displays a compact shape, whose catalytic domain undergoes major rearrangement upon activation, and reveals an exosite and a sugar-rich channel, both of which possibly engage in substrate binding. A plausible structure-derived working mechanism suggests that substrates such as APP are shed close to the plasma membrane surface following an "N-like" chain trace.

The ΔC splice-variant of TRPM2 is the hypertonicity-induced cation channel in HeLa cells, and the ecto-enzyme CD38 mediates its activation

Hypertonicity-induced cation channels (HICCs) are key-players in proliferation and apoptosis but their molecular correlate remains obscure. Furthermore, the activation profile of HICCs is not well defined yet. We report here that, in HeLa cells, intracellular adenosine diphosphate ribose (ADPr) and cyclic ADPr (cADPr), as supposed activators of TRPM2, elicited cation currents that were virtually identical to the osmotic activation of HICCs. Silencing of the expression of TRPM2 and of the ecto-enzyme CD38 (as a likely source of ADPr and cADPr) inhibited HICC as well as nucleotide-induced currents and, in parallel, the hypertonic volume response of cells (the regulatory volume increase, RVI) was attenuated. Quantification of intracellular cADPr levels and the systematic application of extra- vs. intracellular nucleotides indicate that the outwardly directed gradient rather than the cellular activity of ADPr and cADPr triggers TRPM2 activation, probably along with a simultaneous biotransformation of nucleotides.Cloning of TRPM2 identified the ΔC-splice variant as the molecular correlate of the HICC, which could be strongly supported by a direct comparison of the respective Ca²⁺ selectivity. Finally, immunoprecipitation and high-resolution FRET/FLIM imaging revealed the interaction of TRPM2 and CD38 in the native as well as in a heterologous (HEK293T) expression system. We propose transport-related nucleotide export via CD38 as a novel mechanism of TRPM2/HICC activation. With the biotransformation of nucleotides running in parallel, continuous zero trans-conditions are achieved which will render the system infinitely sensitive.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.

Glioma-specific cation conductance regulates migration and cell cycle progression

In this study, we have investigated the role of a glioma-specific cation channel assembled from subunits of the Deg/epithelial sodium channel (ENaC) superfamily, in the regulation of migration and cell cycle progression in glioma cells. Channel inhibition by psalmotoxin-1 (PcTX-1) significantly inhibited migration and proliferation of D54-MG glioma cells. Both PcTX-1 and benzamil, an amiloride analog, caused cell cycle arrest of D54-MG cells in G(0)/G(1) phases (by 30 and 40%, respectively) and reduced cell accumulation in S and G(2)/M phases after 24 h of incubation. Both PcTX-1 and benzamil up-regulated expression of cyclin-dependent kinase inhibitor proteins p21(Cip1) and p27(Kip1). Similar results were obtained in U87MG and primary glioblastoma multiforme cells maintained in primary culture and following knockdown of one of the component subunits, ASIC1. In contrast, knocking down δENaC, which is not a component of the glioma cation channel complex, had no effect on cyclin-dependent kinase inhibitor expression. Phosphorylation of ERK1/2 was also inhibited by PcTX-1, benzamil, and knockdown of ASIC1 but not δENaC in D54MG cells. Our data suggest that a specific cation conductance composed of acid-sensing ion channels and ENaC subunits regulates migration and cell cycle progression in gliomas.

Metalloprotease meprin beta generates nontoxic N-terminal amyloid precursor protein fragments in vivo

Identification of physiologically relevant substrates is still the most challenging part in protease research for understanding the biological activity of these enzymes. The zinc-dependent metalloprotease meprin β is known to be expressed in many tissues with functions in health and disease. Here, we demonstrate unique interactions between meprin β and the amyloid precursor protein (APP). Although APP is intensively studied as a ubiquitously expressed cell surface protein, which is involved in Alzheimer disease, its precise physiological role and relevance remain elusive. Based on a novel proteomics technique termed terminal amine isotopic labeling of substrates (TAILS), APP was identified as a substrate for meprin β. Processing of APP by meprin β was subsequently validated using in vitro and in vivo approaches. N-terminal APP fragments of about 11 and 20 kDa were found in human and mouse brain lysates but not in meprin β(-/-) mouse brain lysates. Although these APP fragments were in the range of those responsible for caspase-induced neurodegeneration, we did not detect cytotoxicity to primary neurons treated by these fragments. Our data demonstrate that meprin β is a physiologically relevant enzyme in APP processing.

Amiloride docking to acid-sensing ion channel-1

Amiloride is a small molecule diuretic, which has been used to dissect sodium transport pathways in many different systems. This drug is known to interact with the epithelial sodium channel and acid-sensing ion channel proteins, as well as sodium/hydrogen antiporters and sodium/calcium exchangers. The exact structural basis for these interactions has not been elucidated as crystal structures of these proteins have been challenging to obtain, though some involved residues and domains have been mapped. This work examines the interaction of amiloride with acid-sensing ion channel-1, a protein whose structure is available using computational and experimental techniques. Using molecular docking software, amiloride and related molecules were docked to model structures of homomeric human ASIC-1 to generate potential interaction sites and predict which analogs would be more or less potent than amiloride. The predictions made were experimentally tested using whole-cell patch clamp. Drugs previously classified as NCX or NHE inhibitors are shown to also inhibit hASIC-1. Potential docking sites were re-examined against experimental data to remove spurious interaction sites. The voltage sensitivity of inhibitors was also examined. Using the aggregated data from these computational and experimental experiments, putative interaction sites for amiloride and hASIC-1 have been defined. Future work will experimentally verify these interaction sites, but at present this should allow for virtual screening of drug libraries at these putative interaction sites.