Regulation of ion transport by oxidants

Oxidative stress measured in vivo without an exogenous contrast agent using QUEST MRI

Decades of experimental studies have implicated excessive generation of reactive oxygen species (ROS) in the decline of tissue function during normal aging, and as a pathogenic factor in a vast array of fatal or debilitating morbidities. This massive body of work has important clinical implications since many antioxidants are FDA approved, readily cross blood-tissue barriers, and are effective at improving disease outcomes. Yet, the potential benefits of antioxidants have remained largely unrealized in patients because conventional methods cannot determine the dose, timing, and drug combinations to be used in clinical trials to localize and decrease oxidative stress. To address this major problem and improve translational success, new methods are urgently needed that non-invasively measure the same ROS biomarker both in animal models and patients with high spatial resolution. Here, we summarize a transformative solution based on a novel method: QUEnch-assiSTed MRI (QUEST MRI). The QUEST MRI index is a significant antioxidant-induced improvement in pathophysiology, or a reduction in 1/T1 (i.e., R1). The latter form of QUEST MRI provides a unique measure of uncontrolled production of endogenous, paramagnetic reactive oxygen species (ROS). QUEST MRI results to-date have been validated by gold standard oxidative stress assays. QUEST MRI has high translational potential because it does not use an exogenous contrast agent and requires only standard MRI equipment. Summarizing, QUEST MRI is a powerful non-invasive approach with unprecedented potential for (i) bridging antioxidant treatment in animal models and patients, (ii) identifying tissue subregions exhibiting oxidative stress, and (iii) coupling oxidative stress localization with behavioral dysfunction, disease pathology, and genetic vulnerabilities to serve as a marker of susceptibility.

RAGE-induced changes in the proteome of alveolar epithelial cells

The receptor for advanced glycation end-products (RAGE) is a pattern recognition receptor and member of the immunoglobulin superfamily. RAGE is constitutively expressed in the distal lung where it co-localizes with the alveolar epithelium; RAGE expression is otherwise minimal or absent, except with disease. This suggests RAGE plays a role in lung physiology and pathology. We used proteomics to identify and characterize the effects of RAGE on rat alveolar epithelial (R3/1) cells. LC-MS/MS identified 177 differentially expressed proteins and the PANTHER Classification System further segregated proteins. Proteins involved in gene transcription (RNA and mRNA splicing, mRNA processing) and transport (protein, intracellular protein) were overrepresented; genes involved in a response to stimulus were underrepresented. Immune system processes and response to stimuli were downregulated with RAGE knockdown. Western blot confirmed RAGE-dependent changes in protein expression for NFκB and NLRP3 that was functionally supported by a reduction in IL-1β and phosphorylated p65. We also assessed RAGE's effect on redox regulation and report that RAGE knockdown attenuated oxidant production, decreased protein oxidation, and increased reduced thiol pools. Collectively the data suggest that RAGE is a critical regulator of epithelial cell response and has implications for our understanding of lung disease, specifically acute lung injury.

SIGNIFICANCE STATEMENT

In the present study, we undertook the first proteomic evaluation of RAGE-dependent processes in alveolar epithelial cells. The alveolar epithelium is a primary target during acute lung injury, and our data support a role for RAGE in gene transcription, protein transport, and response to stimuli. More over our data suggest that RAGE is a critical driver of redox regulation in the alveolar epithelium. The conclusions of the present work assist to unravel the molecular events that underlie the function of RAGE in alveolar epithelial cells and have implications for our understanding of RAGE signaling during lung injury. Our study was the first proteomic comparison showing the effects of RAGE activation from alveolar epithelial cells that constitutively express RAGE and these results can affect a wide field of lung biology, pulmonary therapeutics, and proteomics.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

Knockout of mitochondrial voltage-dependent anion channel type 3 increases reactive oxygen species (ROS) levels and alters renal sodium transport

It has been suggested that voltage-dependent anion channels (VDACs) control the release of superoxide from mitochondria. We have previously shown that reactive oxygen species (ROS) such as superoxide (O₂^̇̄) and hydrogen peroxide (H₂O₂) stimulate epithelial sodium channels (ENaCs) in sodium-transporting epithelial tissue, including cortical collecting duct (CCD) principal cells. Therefore, we hypothesized that VDACs could regulate ENaC by modulating cytosolic ROS levels. Herein, we find that VDAC3-knockout(KO) mice can maintain normal salt and water balance on low-salt and high-salt diets. However, on a high-salt diet for 2 weeks, VDAC3-KO mice had significantly higher systolic blood pressure than wildtype mice. Consistent with this observation, after a high-salt diet for 2 weeks, ENaC activity in VDAC3-KO mice was significantly higher than wildtype mice. EM analysis disclosed a significant morphological change of mitochondria in the CCD cells of VDAC3-KO mice compared with wildtype mice, which may have been caused by mitochondrial superoxide overload. Of note, compared with wildtype animals, ROS levels in VDAC3-KO animals fed a normal or high-salt diet were consistently and significantly increased in renal tubules. Both the ROS scavenger 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (TEMPOL) and the mitochondrial ROS scavenger (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (mito-TEMPO) could reverse the effect of high-salt on ENaC activity and systolic blood pressure in the VDAC3-KO mice. Mito-TEMPO partially correct the morphological changes in VDAC3-KO mice. Our results suggest that knocking out mitochondrial VDAC3 increases ROS, alters renal sodium transport, and leads to hypertension.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Role of epithelial sodium channels in the regulation of lung fluid homeostasis

In utero, fetal lung epithelial cells actively secrete Cl(-) ions into the lung air spaces while Na(+) ions follow passively to maintain electroneutrality. This process, driven by an electrochemical gradient generated by the Na(+)-K(+)-ATPase, is responsible for the secretion of fetal fluid that is essential for normal lung development. Shortly before birth, a significant upregulation of amiloride-sensitive epithelial channels (ENaCs) on the apical side of the lung epithelial cells results in upregulation of active Na(+) transport. This process is critical for the reabsorption of fetal lung fluid and the establishment of optimum gas exchange. In the adult lung, active Na(+) reabsorption across distal lung epithelial cells limits the degree of alveolar edema in patients with acute lung injury and cardiogenic edema. Cl(-) ions are transported either paracellularly or transcellularly to preserve electroneutrality. An increase in Cl(-) secretion across the distal lung epithelium has been reported following an acute increase in left atrial pressure and may result in pulmonary edema. In contrast, airway epithelial cells secrete Cl(-) through apical cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels and absorb Na(+). Thus the coordinated action of Cl(-) secretion and Na(+) absorption is essential for maintenance of the volume of epithelial lining fluid that, in turn, maximizes mucociliary clearance and facilitates clearance of bacteria and debris from the lungs. Any factor that interferes with Na(+) or Cl(-) transport or dramatically upregulates ENaC activity in airway epithelial cells has been associated with lung diseases such as cystic fibrosis or chronic obstructive lung disease. In this review we focus on the role of the ENaC, the mechanisms involved in ENaC regulation, and how ENaC dysregulation can lead to lung pathology.

FXYD1 negatively regulates Na(+)/K(+)-ATPase activity in lung alveolar epithelial cells

Acute respiratory distress syndrome (ARDS) is clinical syndrome characterized by decreased lung fluid reabsorption, causing alveolar edema. Defective alveolar ion transport undertaken in part by the Na(+)/K(+)-ATPase underlies this compromised fluid balance, although the molecular mechanisms at play are not understood. We describe here increased expression of FXYD1, FXYD3 and FXYD5, three regulatory subunits of the Na(+)/K(+)-ATPase, in the lungs of ARDS patients. Transforming growth factor (TGF)-β, a pathogenic mediator of ARDS, drove increased FXYD1 expression in A549 human lung alveolar epithelial cells, suggesting that pathogenic TGF-β signaling altered Na(+)/K(+)-ATPase activity in affected lungs. Lentivirus-mediated delivery of FXYD1 and FXYD3 allowed for overexpression of both regulatory subunits in polarized H441 cell monolayers on an air/liquid interface. FXYD1 but not FXYD3 overexpression inhibited amphotericin B-sensitive equivalent short-circuit current in Ussing chamber studies. Thus, we speculate that FXYD1 overexpression in ARDS patient lungs may limit Na(+)/K(+)-ATPase activity, and contribute to edema persistence.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Receptor for advanced glycation end-products regulates lung fluid balance via protein kinase C-gp91(phox) signaling to epithelial sodium channels

The receptor for advanced glycation end-products (RAGE), a multiligand member of the Ig family, may play a crucial role in the regulation of lung fluid balance. We quantified soluble RAGE (sRAGE), a decoy isoform, and advanced glycation end-products (AGEs) from the bronchoalveolar lavage fluid of smokers and nonsmokers, and tested the hypothesis that AGEs regulate lung fluid balance through protein kinase C (PKC)-gp91(phox) signaling to the epithelial sodium channel (ENaC). Human bronchoalveolar lavage samples from smokers showed increased AGEs (9.02 ± 3.03 μg versus 2.48 ± 0.53 μg), lower sRAGE (1,205 ± 292 pg/ml versus 1,910 ± 263 pg/ml), and lower volume(s) of epithelial lining fluid (97 ± 14 ml versus 133 ± 17 ml). sRAGE levels did not predict ELF volumes in nonsmokers; however, in smokers, higher volumes of ELF were predicted with higher levels of sRAGE. Single-channel patch clamp analysis of rat alveolar epithelial type 1 cells showed that AGEs increased ENaC activity measured as the product of the number of channels (N) and the open probability (Po) (NPo) from 0.19 ± 0.08 to 0.83 ± 0.22 (P = 0.017) and the subsequent addition of 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine-N-oxyl decreased ENaC NPo to 0.15 ± 0.07 (P = 0.01). In type 2 cells, human AGEs increased ENaC NPo from 0.12 ± 0.05 to 0.53 ± 0.16 (P = 0.025) and the addition of 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine-N-oxyl decreased ENaC NPo to 0.10 ± 0.03 (P = 0.013). Using molecular and biochemical techniques, we observed that inhibition of RAGE and PKC activity attenuated AGE-induced activation of ENaC. AGEs induced phosphorylation of p47(phox) and increased gp91(phox)-dependent reactive oxygen species production, a response that was abrogated with RAGE or PKC inhibition. Finally, tracheal instillation of AGEs promoted clearance of lung fluid, whereas concomitant inhibition of RAGE, PKC, and gp91(phox) abrogated the response.

A critical review of the American Journal of Physiology-Lung Cellular and Molecular Physiology: 2012-2015

I have had the privilege of serving as Editor-in-Chief of the American Journal of Physiology: Lung Cellular and Molecular Physiology from 1/1/2012 to 1/1/2015 and have been reappointed for another 3-year term. When I took over as editor, I published an editorial in AJP-Lung in which I highlighted my vision and outlined the tasks to be accomplished to transform AJP-Lung into "The best place to publish basic, translational, and hypothesis-driven clinical lung research." Herein I review our accomplishments during the first term. As promised, we review each article submitted to this journal and our reviews always help the quality and impact of every paper. We recognized the contributions of junior authors by establishing a number of awards and increased the visibility of AJP-Lung by establishing Facebook and Blog electronic pages and sponsoring symposia in scientific meetings. Our impact factor increased from 3.523 in 2011 to 4.041 in 2012 and, thanks to our calls for papers, we are receiving large numbers of high-quality papers in all aspects of pulmonary cell biology and lung diseases. The best is yet to come.

CFTR and lung homeostasis

CFTR is a cAMP-activated chloride and bicarbonate channel that is critical for lung homeostasis. Decreases in CFTR expression have dire consequences in cystic fibrosis (CF) and have been suggested to be a component of the lung pathology in chronic obstructive pulmonary disease. Decreases or loss of channel function often lead to mucus stasis, chronic bacterial infections, and the accompanying chronic inflammatory responses that promote progressive lung destruction, and, eventually in CF, lung failure. Here we discuss CFTR's functional role airway surface liquid hydration and pH, in regulation of other channels such as the epithelial sodium channel, and in regulating inflammatory responses in the lung.

Interrelated role of cigarette smoking, oxidative stress, and immune response in COPD and corresponding treatments

Cigarette smoking (CS) can impact the immune system and induce pulmonary disorders such as chronic obstructive pulmonary disease (COPD), which is currently the fourth leading cause of chronic morbidity and mortality worldwide. Accordingly, the most significant risk factor associated with COPD is exposure to cigarette smoke. The purpose of the present study is to provide an updated overview of the literature regarding the effect of CS on the immune system and lungs, the mechanism of CS-induced COPD and oxidative stress, as well as the available and potential treatment options for CS-induced COPD. An extensive literature search was conducted on the PubMed/Medline databases to review current COPD treatment research, available in the English language, dating from 1976 to 2014. Studies have investigated the mechanism by which CS elicits detrimental effects on the immune system and pulmonary function through the use of human and animal subjects. A strong relationship among continued tobacco use, oxidative stress, and exacerbation of COPD symptoms is frequently observed in COPD subjects. In addition, therapeutic approaches emphasizing smoking cessation have been developed, incorporating counseling and nicotine replacement therapy. However, the inability to reverse COPD progression establishes the need for improved preventative and therapeutic strategies, such as a combination of intensive smoking cessation treatment and pharmaceutical therapy, focusing on immune homeostasis and redox balance. CS initiates a complex interplay between oxidative stress and the immune response in COPD. Therefore, multiple approaches such as smoking cessation, counseling, and pharmaceutical therapies targeting inflammation and oxidative stress are recommended for COPD treatment.

Preclinical anatomical, molecular, and functional imaging of the lung with multiple modalities

In vivo imaging is an important tool for preclinical studies of lung function and disease. The widespread availability of multimodal animal imaging systems and the rapid rate of diagnostic contrast agent development have empowered researchers to noninvasively study lung function and pulmonary disorders. Investigators can identify, track, and quantify biological processes over time. In this review, we highlight the fundamental principles of bioluminescence, fluorescence, planar X-ray, X-ray computed tomography, magnetic resonance imaging, and nuclear imaging modalities (such as positron emission tomography and single photon emission computed tomography) that have been successfully employed for the study of lung function and pulmonary disorders in a preclinical setting. The major principles, benefits, and applications of each imaging modality and technology are reviewed. Limitations and the future prospective of multimodal imaging in pulmonary physiology are also discussed. In vivo imaging bridges molecular biological studies, drug design and discovery, and the imaging field with modern medical practice, and, as such, will continue to be a mainstay in biomedical research.

Chronic ethanol exposure alters the lung proteome and leads to mitochondrial dysfunction in alveolar type 2 cells

The lungs can undergo irreversible damage from chronic alcohol consumption. Herein, we developed an animal model predisposed for edematous lung injury following chronic ingestion of alcohol to better understand the etiology of alcohol-related disorders. Using animal modeling, alongside high-throughput proteomic and microarray assays, we identified changes in lung protein and transcript in mice and rats, respectively, following chronic alcohol ingestion or a caloric control diet. Liquid chromatography-mass spectrometry identified several mitochondrial-related proteins in which the expression was upregulated following long-term alcohol ingestion in mice. Consistent with these observations, rat gene chip microarray analysis of alveolar cells obtained from animals maintained on a Lieber-DeCarli liquid alcohol diet confirmed significant changes in mitochondrial-related transcripts in the alcohol lung. Transmission electron microscopy revealed significant changes in the mitochondrial architecture in alcohol mice, particularly following lipopolysaccharide exposure. Chronic alcohol ingestion was also shown to worsen mitochondrial respiration, mitochondrial membrane polarization, and NAD(+)-to-NADH ratios in alveolar type 2 cells. In summary, our studies show causal connection between chronic alcohol ingestion and mitochondrial dysfunction, albeit the specific role of each of the mitochondrial-related proteins and transcripts identified in our study requires additional study.