Carbonic anhydrase II and alveolar fluid reabsorption during hypercapnia

Hypercapnia and its relationship with respiratory infections

INTRODUCTION

Hypercapnia is developed in patients with acute and/or chronic respiratory conditions. Clinical data concerning hypercapnia and respiratory infections interaction is limited.

AREAS COVERED

Currently, the relationship between hypercapnia and respiratory infections remains unclear. In this review, we summarize studies on the effects of hypercapnia on models of pulmonary infections to clarify the role of elevated CO2 in these pulmonary pathologies. Hypercapnia affects different cell types in the alveoli, leading to changes in the immune response. In vitro studies show that hypercapnia downregulates the NF-κβ pathway, reduces inflammation and impairs epithelial wound healing. While in vivo models show a dual role between short- and long-term effects of hypercapnia on lung infection. However, it is still controversial whether the effects observed under hypercapnia are pH dependent or not.

EXPERT OPINION

The role of hypercapnia is still a controversial debate. Hypercapnia could play a beneficial role in mechanically ventilated models, by lowering the inflammation produced by the stretch condition. But it could be detrimental in infectious scenarios, causing phagocyte dysfunction and lack of infection control. Further data concerning hypercapnia on respiratory infections is needed to elucidate this interaction.

Proteomics discovery of pulmonary hypertension biomarkers: Insulin-like growth factor binding proteins are associated with disease severity

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by sustained elevations of pulmonary artery pressure. To date, we lack circulating, diagnostic, and prognostic markers that correlate to clinical and functional parameters. In this study, we performed mass spectrometry-based proteomics analysis to identify circulating biomarkers of PAH. Plasma samples from patients with idiopathic pulmonary arterial hypertension (IPAH, N = 9) and matched normal controls (N = 9) were digested with trypsin and analyzed using data-dependent acquisition on an Orbitrap mass spectrometer. A total of 826 (false discovery rate [FDR] 0.047) and 461 (FDR 0.087) proteins were identified across all plasma samples obtained from IPAH and control subjects, respectively. Of these, 153 proteins showed >2 folds change (p < 0.05) between groups. Circulating levels of carbonic anhydrase 2 (CA2), plasma kallikrein (KLKB1), and the insulin-like growth factor binding proteins (IGFBP1-7) were quantified by immunoassay in an independent verification cohort (N = 36 PAH and N = 35 controls). CA2 and KLKB1 were significantly different in PAH versus control but were not associated with any functional or hemodynamic measurements. Whereas, IGFBP1 and 2 were associated with higher pulmonary vascular resistance, IGFBP2, 4, and 7 with decreased 6-min walk distance (6MWD), and IGFBP1, 2, 4, and 7 with worse survival. This plasma proteomic discovery analysis suggests the IGF axis may serve as important new biomarkers for PAH and play an important role in PAH pathogenesis.

The integrated stress response is tumorigenic and constitutes a therapeutic liability in KRAS-driven lung cancer

The integrated stress response (ISR) is an essential stress-support pathway increasingly recognized as a determinant of tumorigenesis. Here we demonstrate that ISR is pivotal in lung adenocarcinoma (LUAD) development, the most common histological type of lung cancer and a leading cause of cancer death worldwide. Increased phosphorylation of the translation initiation factor eIF2 (p-eIF2α), the focal point of ISR, is related to invasiveness, increased growth, and poor outcome in 928 LUAD patients. Dissection of ISR mechanisms in KRAS-driven lung tumorigenesis in mice demonstrated that p-eIF2α causes the translational repression of dual specificity phosphatase 6 (DUSP6), resulting in increased phosphorylation of the extracellular signal-regulated kinase (p-ERK). Treatments with ISR inhibitors, including a memory-enhancing drug with limited toxicity, provides a suitable therapeutic option for KRAS-driven lung cancer insofar as they substantially reduce tumor growth and prolong mouse survival. Our data provide a rationale for the implementation of ISR-based regimens in LUAD treatment.

Carbon dioxide-dependent signal transduction in mammalian systems

Carbon dioxide (CO2) is a fundamental physiological gas known to profoundly influence the behaviour and health of millions of species within the plant and animal kingdoms in particular. A recent Royal Society meeting on the topic of 'Carbon dioxide detection in biological systems' was extremely revealing in terms of the multitude of roles that different levels of CO2 play in influencing plants and animals alike. While outstanding research has been performed by leading researchers in the area of plant biology, neuronal sensing, cell signalling, gas transport, inflammation, lung function and clinical medicine, there is still much to be learned about CO2-dependent sensing and signalling. Notably, while several key signal transduction pathways and nodes of activity have been identified in plants and animals respectively, the precise wiring and sensitivity of these pathways to CO2 remains to be fully elucidated. In this article, we will give an overview of the literature relating to CO2-dependent signal transduction in mammalian systems. We will highlight the main signal transduction hubs through which CO2-dependent signalling is elicited with a view to better understanding the complex physiological response to CO2 in mammalian systems. The main topics of discussion in this article relate to how changes in CO2 influence cellular function through modulation of signal transduction networks influenced by pH, mitochondrial function, adenylate cyclase, calcium, transcriptional regulators, the adenosine monophosphate-activated protein kinase pathway and direct CO2-dependent protein modifications. While each of these topics will be discussed independently, there is evidence of significant cross-talk between these signal transduction pathways as they respond to changes in CO2. In considering these core hubs of CO2-dependent signal transduction, we hope to delineate common elements and identify areas in which future research could be best directed.

Regulation of AMPK activity by type 10 adenylyl cyclase: contribution to the mitochondrial biology, cellular redox and energy homeostasis

The downregulation of AMP-activated protein kinase (AMPK) activity contributes to numerous pathologies. Recent reports suggest that the elevation of cellular cAMP promotes AMPK activity. However, the source of the cAMP pool that controls AMPK activity remains unknown. Mammalian cells possess two cAMP sources: membrane-bound adenylyl cyclase (tmAC) and intracellularly localized, type 10 soluble adenylyl cyclase (sAC). Due to the localization of sAC and AMPK in similar intracellular compartments, we hypothesized that sAC may control AMPK activity. In this study, sAC expression and activity were manipulated in H9C2 cells, adult rat cardiomyocytes or endothelial cells. sAC knockdown depleted the cellular cAMP content and decreased AMPK activity in an EPAC-dependent manner. Functionally, sAC knockdown reduced cellular ATP content, increased mitochondrial ROS formation and led to mitochondrial depolarization. Furthermore, sAC downregulation led to EPAC-dependent mitophagy disturbance, indicated by an increased mitochondrial mass and unaffected mitochondrial biogenesis. Consistently, sAC overexpression or stimulation with bicarbonate significantly increased AMPK activity and cellular ATP content. In contrast, tmAC inhibition or stimulation produced no effect on AMPK activity. Therefore, the sAC-EPAC axis may regulate basal and induced AMPK activity and support mitophagy, cellular energy and redox homeostasis. The study argues for sAC as a potential target in treating pathologies associated with AMPK downregulation.

Role of Carbonic Anhydrases and Inhibitors in Acid-Base Physiology: Insights from Mathematical Modeling

Carbonic anhydrases (CAs) catalyze a reaction fundamental for life: the bidirectional conversion of carbon dioxide (CO2) and water (H2O) into bicarbonate (HCO3-) and protons (H+). These enzymes impact numerous physiological processes that occur within and across the many compartments in the body. Within compartments, CAs promote rapid H+ buffering and thus the stability of pH-sensitive processes. Between compartments, CAs promote movements of H+, CO2, HCO3-, and related species. This traffic is central to respiration, digestion, and whole-body/cellular pH regulation. Here, we focus on the role of mathematical modeling in understanding how CA enhances buffering as well as gradients that drive fluxes of CO2 and other solutes (facilitated diffusion). We also examine urinary acid secretion and the carriage of CO2 by the respiratory system. We propose that the broad physiological impact of CAs stem from three fundamental actions: promoting H+ buffering, enhancing H+ exchange between buffer systems, and facilitating diffusion. Mathematical modeling can be a powerful tool for: (1) clarifying the complex interdependencies among reaction, diffusion, and protein-mediated components of physiological processes; (2) formulating hypotheses and making predictions to be tested in wet-lab experiments; and (3) inferring data that are impossible to measure.

Gas Exchange Disturbances Regulate Alveolar Fluid Clearance during Acute Lung Injury

Disruption of the alveolar-capillary barrier and accumulation of pulmonary edema, if not resolved, result in poor alveolar gas exchange leading to hypoxia and hypercapnia, which are hallmarks of acute lung injury and the acute respiratory distress syndrome (ARDS). Alveolar fluid clearance (AFC) is a major function of the alveolar epithelium and is mediated by the concerted action of apically-located Na⁺ channels [epithelial Na⁺ channel (ENaC)] and the basolateral Na,K-ATPase driving vectorial Na⁺ transport. Importantly, those patients with ARDS who cannot clear alveolar edema efficiently have worse outcomes. While hypoxia can be improved in most cases by O₂ supplementation and mechanical ventilation, the use of lung protective ventilation settings can lead to further CO₂ retention. Whether the increase in CO₂ concentrations has deleterious or beneficial effects have been a topic of significant controversy. Of note, both low O₂ and elevated CO₂ levels are sensed by the alveolar epithelium and by distinct and specific molecular mechanisms impair the function of the Na,K-ATPase and ENaC thereby inhibiting AFC and leading to persistence of alveolar edema. This review discusses recent discoveries on the sensing and signaling events initiated by hypoxia and hypercapnia and the relevance of these results in identification of potential novel therapeutic targets in the treatment of ARDS.

Carbonic anhydrase inhibitor attenuates ischemia-reperfusion induced acute lung injury

Ischemia-reperfusion (IR)-induced acute lung injury (ALI) is implicated in several clinical conditions including lung transplantation, cardiopulmonary bypass surgery, re-expansion of collapsed lung from pneumothorax or pleural effusion and etc. IR-induced ALI remains a challenge in the current treatment. Carbonic anhydrase has important physiological function and influences on transport of CO₂. Some investigators suggest that CO₂ influences lung injury. Therefore, carbonic anhydrase should have the role in ALI. This study was undertaken to define the effect of a carbonic anhydrase inhibitor, acetazolamide (AZA), in IR-induced ALI, that was conducted in a rat model of isolated-perfused lung with 30 minutes of ischemia and 90 minutes of reperfusion. The animals were divided into six groups (n = 6 per group): sham, sham + AZA 200 mg/kg body weight (BW), IR, IR + AZA 100 mg/kg BW, IR + AZA 200 mg/kg BW and IR+ AZA 400 mg/kg BW. IR caused significant pulmonary micro-vascular hyper-permeability, pulmonary edema, pulmonary hypertension, neutrophilic sequestration, and an increase in the expression of pro-inflammatory cytokines. Increases in carbonic anhydrase expression and perfusate pCO₂ levels were noted, while decreased Na-K-ATPase expression was noted after IR. Administration of 200mg/kg BW and 400mg/kg BW AZA significantly suppressed the expression of pro-inflammatory cytokines (TNF-α, IL-1, IL-6 and IL-17) and attenuated IR-induced lung injury, represented by decreases in pulmonary hyper-permeability, pulmonary edema, pulmonary hypertension and neutrophilic sequestration. AZA attenuated IR-induced lung injury, associated with decreases in carbonic anhydrase expression and pCO₂ levels, as well as restoration of Na-K-ATPase expression.

Carbon dioxide-sensing in organisms and its implications for human disease

The capacity of organisms to sense changes in the levels of internal and external gases and to respond accordingly is central to a range of physiologic and pathophysiologic processes. Carbon dioxide, a primary product of oxidative metabolism is one such gas that can be sensed by both prokaryotic and eukaryotic cells and in response to altered levels, elicit the activation of multiple adaptive pathways. The outcomes of activating CO2-sensitive pathways in various species include increased virulence of fungal and bacterial pathogens, prey-seeking behavior in insects as well as taste perception, lung function, and the control of immunity in mammals. In this review, we discuss what is known about the mechanisms underpinning CO2 sensing across a range of species and consider the implications of this for physiology, disease progression, and the possibility of developing new therapeutics for inflammatory and infectious disease.

Evolutionary conserved role of c-Jun-N-terminal kinase in CO2-induced epithelial dysfunction

Elevated CO₂ levels (hypercapnia) occur in patients with respiratory diseases and impair alveolar epithelial integrity, in part, by inhibiting Na,K-ATPase function. Here, we examined the role of c-Jun N-terminal kinase (JNK) in CO₂ signaling in mammalian alveolar epithelial cells as well as in diptera, nematodes and rodent lungs. In alveolar epithelial cells, elevated CO₂ levels rapidly induced activation of JNK leading to downregulation of Na,K-ATPase and alveolar epithelial dysfunction. Hypercapnia-induced activation of JNK required AMP-activated protein kinase (AMPK) and protein kinase C-ζ leading to subsequent phosphorylation of JNK at Ser-129. Importantly, elevated CO₂ levels also caused a rapid and prominent activation of JNK in Drosophila S2 cells and in C. elegans. Paralleling the results with mammalian epithelial cells, RNAi against Drosophila JNK fully prevented CO₂-induced downregulation of Na,K-ATPase in Drosophila S2 cells. The importance and specificity of JNK CO₂ signaling was additionally demonstrated by the ability of mutations in the C. elegans JNK homologs, jnk-1 and kgb-2 to partially rescue the hypercapnia-induced fertility defects but not the pharyngeal pumping defects. Together, these data provide evidence that deleterious effects of hypercapnia are mediated by JNK which plays an evolutionary conserved, specific role in CO₂ signaling in mammals, diptera and nematodes.

Hypercapnia: a nonpermissive environment for the lung

Patients with severe acute and chronic lung diseases develop derangements in gas exchange that may result in increased levels of CO(2) (hypercapnia), the effects of which on human health are incompletely understood. It has been proposed that hypercapnia may have beneficial effects in patients with acute lung injury, and the concepts of "permissive" and even "therapeutic" hypercapnia have emerged. However, recent work suggests that CO(2) can act as a signaling molecule via pH-independent mechanisms, resulting in deleterious effects in the lung. Here we review recent research on how elevated CO(2) is sensed by cells in the lung and the potential harmful effects of hypercapnia on epithelial and endothelial barrier, lung edema clearance, innate immunity, and host defense. In view of these findings, we raise concerns about the potentially deleterious effects hypercapnia may have in patients with acute and chronic lung diseases.

Antigen retrieval to improve the immunocytochemistry detection of sigma-1 receptors and ER chaperones

Molecular chaperones localized at the endoplasmic reticulum (ER) lumen constitutively or cellular stress-dependently associate with a variety of proteins to promote their proper folding or to inhibit protein misfolding. ER chaperones preferentially form large complexes with co-chaperones and/or misfolded proteins in a highly crowded cellular environment that often hampers their detection by immunocytochemistry (ICC). This study establishes an antigen retrieval (AR) protocol to improve the ICC detection of ER chaperones in cultured cells using widely available antibodies against synthetic peptides. Among ten different antigen retrieval/fixation conditions, only the AR with Tris-HCl (pH 9.5) containing 6 M urea (80°C for 10 min) significantly improved the ICC detection of the novel ER chaperone sigma-1 receptor (Sig-1R) in Chinese hamster ovary cells. Extended fixation with 4% paraformaldehyde for 1 h effectively preserved the morphology of the ER under the AR condition. This method greatly enhanced the signal-to-noise ratio in Sig-1R ICC, thus allowing for semi-quantitative detection of protein upregulation under ER stress. The AR similarly improved the ICC detection of a series of other major ER chaperones, including BiP/GRP78, GRP94, calnexin, calreticulin, ERp57, protein disulfide isomerase, and cyclophilin B. The improved ICC methodology using the urea AR at 80°C may improve ICC of ER molecules as well as visualization of ER structure and substructures.

Extracellular signal-regulated kinase (ERK) participates in the hypercapnia-induced Na,K-ATPase downregulation

Hypercapnia has been shown to impair alveolar fluid reabsorption (AFR) by decreasing Na,K-ATPase activity. Extracellular signal-regulated kinase pathway (ERK) is activated under conditions of cellular stress and has been known to regulate the Na,K-ATPase. Here, we show that hypercapnia leads to ERK activation in a time-dependent manner in alveolar epithelial cells (AEC). Inhibition of ERK by U0126 or siRNA prevented both the hypercapnia-induced Na,K-ATPase endocytosis and impairment of AFR. Moreover, ERK inhibition prevented AMPK activation, a known modulator of hypercapnia-induced Na,K-ATPase endocytosis. Accordingly, these data suggest that hypercapnia-induced Na,K-ATPase endocytosis is dependent on ERK activation in AEC and that ERK plays an important role in hypercapnia-induced impairment of AFR in rat lungs.

Immunohistochemical localization and gene expression of carbonic anhydrase isoenzymes CA-II and CA-VI in canine lower airways and lung

The immunohistolocalization and gene expression of carbonic anhydrase (CA) isoenzymes CA-II and CA-VI in the canine lower airways and lung were examined using specific canine CA-II and CA-VI antisera and the RT-PCR method. Laryngeal, tracheal and bronchial epithelia, serous acinar and bronchiolar secretory cells and pulmonary great alveolar cells showed immunopositive reactions to anti-CA-II and anti-CA-VI antisera. However, all mucous cells showed immunonegative reactions. The physiological roles of CA-II and CA-VI in the lower airways and lung may involve the maintenance of pH balance and the protection of mucosal surfaces against the acidic milieu.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Sensing, physiological effects and molecular response to elevated CO2 levels in eukaryotes

Carbon dioxide (CO₂) is an important gaseous molecule that maintains biosphere homeostasis and is an important cellular signalling molecule in all organisms. The transport of CO₂ through membranes has fundamental roles in most basic aspects of life in both plants and animals. There is a growing interest in understanding how CO₂ is transported into cells, how it is sensed by neurons and other cell types and in understanding the physiological and molecular consequences of elevated CO₂ levels (hypercapnia) at the cell and organism levels. Human pulmonary diseases and model organisms such as fungi, C. elegans, Drosophila and mice have been proven to be important in understanding of the mechanisms of CO₂ sensing and response.

Effect of ventilation pressure on alveolar fluid clearance and beta-agonist responses in mice

High tidal volume ventilation is detrimental to alveolar fluid clearance (AFC), but effects of ventilation pressure (P) on AFC are unknown. In anesthetized BALB/c mice ventilated at constant tidal volume (8 ml/kg), mean AFC rate was 12.8% at 6 cmH(2)O P, but increased to 37.3% at 18 cmH(2)O P. AFC rate declined at 22 cmH(2)O P, which also induced lung damage. Increased AFC at 18 cmH(2)O P did not result from elevated plasma catecholamines, hypercapnia, or hypocapnia, but was due to augmented Na(+) and Cl(-) absorption. PKA agonists and beta-agonists stimulated AFC at 10 cmH(2)O P by upregulating amiloride-sensitive Na(+) transport. However, at 18 cmH(2)O P, PKA agonists and beta-agonists reduced AFC. At 15 cmH(2)O P, the AFC rate was intermediate (mean 26.6%), and forskolin and beta-agonists had no effect. Comparable P dependency of AFC and beta-agonist responsiveness was found in C57BL/6 mice. The effect on AFC of increasing P to 18 cmH(2)O was blocked by adenosine deaminase or an A(2b)-adenosine receptor antagonist, and could be mimicked by adenosine in mice ventilated at 10 cmH(2)O P. Modulation of adenosine signaling also resulted in altered responsiveness to beta-agonists. These findings indicate that, in the normal mouse lung, basal AFC rates and responses to beta-agonists are impacted by ventilation pressure in an adenosine-dependent manner.

Proteomic analysis of lamellar bodies isolated from rat lungs

Background

Lamellar bodies are lysosome-related secretory granules and store lung surfactant in alveolar type II cells. To better understand the mechanisms of surfactant secretion, we carried out proteomic analyses of lamellar bodies isolated from rat lungs.

Results

With peptide mass fingerprinting by Matrix Assisted Laser Desorption/Ionization – Time of Flight mass spectrometry, 44 proteins were identified with high confidence. These proteins fell into diverse functional categories: surfactant-related, membrane trafficking, calcium binding, signal transduction, cell structure, ion channels, protein processing and miscellaneous. Selected proteins were verified by Western blot and immunohistochemistry.

Conclusion

This proteomic profiling of lamellar bodies provides a basis for further investigations of functional roles of the identified proteins in lamellar body biogenesis and surfactant secretion.

AMP-activated protein kinase regulates CO2-induced alveolar epithelial dysfunction in rats and human cells by promoting Na,K-ATPase endocytosis

Hypercapnia (elevated CO(2) levels) occurs as a consequence of poor alveolar ventilation and impairs alveolar fluid reabsorption (AFR) by promoting Na,K-ATPase endocytosis. We studied the mechanisms regulating CO(2)-induced Na,K-ATPase endocytosis in alveolar epithelial cells (AECs) and alveolar epithelial dysfunction in rats. Elevated CO(2) levels caused a rapid activation of AMP-activated protein kinase (AMPK) in AECs, a key regulator of metabolic homeostasis. Activation of AMPK was mediated by a CO(2)-triggered increase in intracellular Ca(2+) concentration and Ca(2+)/calmodulin-dependent kinase kinase-beta (CaMKK-beta). Chelating intracellular Ca(2+) or abrogating CaMKK-beta function by gene silencing or chemical inhibition prevented the CO(2)-induced AMPK activation in AECs. Activation of AMPK or overexpression of constitutively active AMPK was sufficient to activate PKC-zeta and promote Na,K-ATPase endocytosis. Inhibition or downregulation of AMPK via adenoviral delivery of dominant-negative AMPK-alpha(1) prevented CO(2)-induced Na,K-ATPase endocytosis. The hypercapnia effects were independent of intracellular ROS. Exposure of rats to hypercapnia for up to 7 days caused a sustained decrease in AFR. Pretreatment with a beta-adrenergic agonist, isoproterenol, or a cAMP analog ameliorated the hypercapnia-induced impairment of AFR. Accordingly, we provide evidence that elevated CO(2) levels are sensed by AECs and that AMPK mediates CO(2)-induced Na,K-ATPase endocytosis and alveolar epithelial dysfunction, which can be prevented with beta-adrenergic agonists and cAMP.

High CO2 levels impair alveolar epithelial function independently of pH

Background

In patients with acute respiratory failure, gas exchange is impaired due to the accumulation of fluid in the lung airspaces. This life-threatening syndrome is treated with mechanical ventilation, which is adjusted to maintain gas exchange, but can be associated with the accumulation of carbon dioxide in the lung. Carbon dioxide (CO₂) is a by-product of cellular energy utilization and its elimination is affected via alveolar epithelial cells. Signaling pathways sensitive to changes in CO₂ levels were described in plants and neuronal mammalian cells. However, it has not been fully elucidated whether non-neuronal cells sense and respond to CO₂. The Na,K-ATPase consumes ∼40% of the cellular metabolism to maintain cell homeostasis. Our study examines the effects of increased pCO₂ on the epithelial Na,K-ATPase a major contributor to alveolar fluid reabsorption which is a marker of alveolar epithelial function.

Principal Findings

We found that short-term increases in pCO₂ impaired alveolar fluid reabsorption in rats. Also, we provide evidence that non-excitable, alveolar epithelial cells sense and respond to high levels of CO₂, independently of extracellular and intracellular pH, by inhibiting Na,K-ATPase function, via activation of PKCζ which phosphorylates the Na,K-ATPase, causing it to endocytose from the plasma membrane into intracellular pools.

Conclusions

Our data suggest that alveolar epithelial cells, through which CO₂ is eliminated in mammals, are highly sensitive to hypercapnia. Elevated CO₂ levels impair alveolar epithelial function, independently of pH, which is relevant in patients with lung diseases and altered alveolar gas exchange.