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

AMPK and glucose deprivation exert an isoform-specific effect on the expression of Na+,K+-ATPase subunits in cultured myotubes

In skeletal muscle, Na+,K+-ATPase (NKA), a heterodimeric (α/β) P-type ATPase, has an essential role in maintenance of Na+ and K+ homeostasis, excitability, and contractility. AMP-activated protein kinase (AMPK), an energy sensor, increases the membrane abundance and activity of NKA in L6 myotubes, but its potential role in regulation of NKA content in skeletal muscle, which determines maximum capacity for Na+ and K+ transport, has not been clearly delineated. We examined whether energy stress and/or AMPK affect expression of NKA subunits in rat L6 and primary human myotubes. Energy stress, induced by glucose deprivation, increased protein content of NKAα1 and NKAα2 in L6 myotubes, while decreasing the content of NKAα1 in human myotubes. Pharmacological AMPK activators (AICAR, A-769662, and diflunisal) modulated expression of NKA subunits, but their effects only partially mimicked those that occurred in response to glucose deprivation, indicating that AMPK does not mediate all effects of energy stress on NKA expression. Gene silencing of AMPKα1/α2 increased protein levels of NKAα1 in L6 myotubes and NKAα1 mRNA levels in human myotubes, while decreasing NKAα2 protein levels in L6 myotubes. Collectively, our results suggest a role for energy stress and AMPK in modulation of NKA expression in skeletal muscle. However, their modulatory effects were not conserved between L6 myotubes and primary human myotubes, which suggests that coupling between energy stress, AMPK, and regulation of NKA expression in vitro depends on skeletal muscle cell model.

Alveolar-capillary endocytosis and trafficking in acute lung injury and acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is associated with high morbidity and mortality but lacks specific therapeutic options. Diverse endocytic processes play a key role in all phases of acute lung injury (ALI), including the initial insult, development of respiratory failure due to alveolar flooding, as a consequence of altered alveolar-capillary barrier function, as well as in the resolution or deleterious remodeling after injury. In particular, clathrin-, caveolae-, endophilin- and glycosylphosphatidyl inositol-anchored protein-mediated endocytosis, as well as, macropinocytosis and phagocytosis have been implicated in the setting of acute lung damage. This manuscript reviews our current understanding of these endocytic pathways and subsequent intracellular trafficking in various phases of ALI, and also aims to identify potential therapeutic targets for patients with ARDS.

Signaling pathways and potential therapeutic targets in acute respiratory distress syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) is a common condition associated with critically ill patients, characterized by bilateral chest radiographical opacities with refractory hypoxemia due to noncardiogenic pulmonary edema. Despite significant advances, the mortality of ARDS remains unacceptably high, and there are still no effective targeted pharmacotherapeutic agents. With the outbreak of coronavirus disease 19 worldwide, the mortality of ARDS has increased correspondingly. Comprehending the pathophysiology and the underlying molecular mechanisms of ARDS may thus be essential to developing effective therapeutic strategies and reducing mortality. To facilitate further understanding of its pathogenesis and exploring novel therapeutics, this review provides comprehensive information of ARDS from pathophysiology to molecular mechanisms and presents targeted therapeutics. We first describe the pathogenesis and pathophysiology of ARDS that involve dysregulated inflammation, alveolar-capillary barrier dysfunction, impaired alveolar fluid clearance and oxidative stress. Next, we summarize the molecular mechanisms and signaling pathways related to the above four aspects of ARDS pathophysiology, along with the latest research progress. Finally, we discuss the emerging therapeutic strategies that show exciting promise in ARDS, including several pharmacologic therapies, microRNA-based therapies and mesenchymal stromal cell therapies, highlighting the pathophysiological basis and the influences on signal transduction pathways for their use.

Mild and moderate chronic hypercapnia elicit distinct transcriptomic responses of immune function in cardiorespiratory nuclei

Chronic hypercapnia (CH) is a hallmark of respiratory-related diseases, and the level of hypercapnia can acutely or progressively become more severe. Previously, we have shown time-dependent adaptations in steady-state physiology during mild (arterial Pco2 ∼55 mmHg) and moderate (∼60 mmHg) CH in adult goats, including transient (mild CH) or sustained (moderate CH) suppression of acute chemosensitivity suggesting limitations in adaptive respiratory control mechanisms as the level of CH increases. Changes in specific markers of glutamate receptor plasticity, interleukin-1ß, and serotonergic modulation within key nodes of cardiorespiratory control do not fully account for the physiological adaptations to CH. Here, we used an unbiased approach (bulk tissue RNA sequencing) to test the hypothesis that mild or moderate CH elicits distinct gene expression profiles in important brain stem regions of cardiorespiratory control, which may explain the contrasting responses to CH. Gene expression profiles from the brain regions validated the accuracy of tissue biopsy methodology. Differential gene expression analyses revealed greater effects of CH on brain stem sites compared with the medial prefrontal cortex. Mild CH elicited an upregulation of predominantly immune-related genes and predicted activation of immune-related pathways and functions. In contrast, moderate CH broadly led to downregulation of genes and predicted inactivation of cellular pathways related to the immune response and vascular function. These data suggest that mild CH leads to a steady-state activation of neuroinflammatory pathways within the brain stem, whereas moderate CH drives the opposite response. Transcriptional shifts in immune-related functions may underlie the cardiorespiratory network's capability to respond to acute, more severe hypercapnia when in a state of progressively increased CH.NEW & NOTEWORTHY Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Hypercapnia increases ACE2 expression and pseudo-SARS-CoV-2 entry in bronchial epithelial cells by augmenting cellular cholesterol

Patients with chronic lung disease, obesity, and other co-morbid conditions are at increased risk of severe illness and death when infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Hypercapnia, the elevation of CO2 in blood and tissue, commonly occurs in patients with severe acute and chronic lung disease, including those with pulmonary infections, and is also associated with high mortality risk. We previously reported that hypercapnia increases viral replication and mortality of influenza A virus infection in mice. We have also shown that culture in elevated CO2 upregulates expression of cholesterol synthesis genes in primary human bronchial epithelial cells. Interestingly, factors that increase the cholesterol content of lipid rafts and lipid droplets, platforms for viral entry and assembly, enhance SARS-CoV-2 infection. In the current study, we investigated the effects of hypercapnia on ACE2 expression and entry of SARS-CoV-2 pseudovirus (p-SARS-CoV-2) into airway epithelial cells. We found that hypercapnia increased ACE2 expression and p-SARS-CoV-2 uptake by airway epithelium in mice, and in cultured VERO and human bronchial epithelial cells. Hypercapnia also increased total cellular and lipid raft-associated cholesterol in epithelial cells. Moreover, reducing cholesterol synthesis with inhibitors of sterol regulatory element binding protein 2 (SREBP2) or statins, and depletion of cellular cholesterol, each blocked the hypercapnia-induced increases in ACE2 expression and p-SARS-CoV-2 entry into epithelial cells. Cigarette smoke extract (CSE) also increased ACE2 expression, p-SARS-CoV-2 entry and cholesterol accumulation in epithelial cells, an effect not additive to that of hypercapnia, but also inhibited by statins. These findings reveal a mechanism that may account, in part, for poor clinical outcomes of SARS-CoV-2 infection in patients with advanced lung disease and hypercapnia, and in those who smoke cigarettes. Further, our results suggest the possibility that cholesterol-lowering therapies may be of particular benefit in patients with hypercapnia when exposed to or infected with SARS-CoV-2.

Impact of Arterial CO2 Retention in Patients With Moderate or Severe ARDS

BACKGROUND

Lung-protective ventilation (reduced tidal volume and limited plateau pressure) may lead to CO2 retention. Data about the impact of hypercapnia in patients with ARDS are scarce and conflicting.

METHODS

We performed a non-interventional cohort study with subjects with ARDS admitted from 2006 to 2021 and with PaO2 /FIO2 ≤ 150 mm Hg. We examined the association between severe hypercapnia (PaCO2 ≥ 50 mm Hg) on the first 5 days after the diagnosis of ARDS and death in ICU for 930 subjects. All the subjects received lung-protective ventilation.

RESULTS

Severe hypercapnia was noted in 552 subjects (59%) on the first day of ARDS (day 1); 323/930 (34.7%) died in the ICU. Severe hypercapnia on day 1 was associated with mortality in the unadjusted (odds ratio 1.54, 95% CI 1.16-1.63; P = .003) and adjusted (odds ratio 1.47, 95% CI 1.08-2.43; P = .004) models. In the Bayesian analysis, the posterior probability that severe hypercapnia was associated with ICU death was > 90% in 4 different priors, including a septic prior for this association. Sustained severe hypercapnia on day 5, defined as severe hypercapnia present from day 1 to day 5, was noted in 93 subjects (12%). After propensity score matching, severe hypercapnia on day 5 remained associated with ICU mortality (odds ratio 1.73, 95% CI 1.02-2.97; P = .047).

CONCLUSIONS

Severe hypercapnia was associated with mortality in subjects with ARDS who received lung-protective ventilation. Our results deserve further evaluation of the strategies and treatments that aim to control CO2 retention.

Hypercapnia in COPD: Causes, Consequences, and Therapy

Chronic obstructive pulmonary disease (COPD) is a progressive respiratory disorder that may lead to gas exchange abnormalities, including hypercapnia. Chronic hypercapnia is an independent risk factor of mortality in COPD, leading to epithelial dysfunction and impaired lung immunity. Moreover, chronic hypercapnia affects the cardiovascular physiology, increases the risk of cardiovascular morbidity and mortality, and promotes muscle wasting and musculoskeletal abnormalities. Noninvasive ventilation is a widely used technique to remove carbon dioxide, and several studies have investigated its role in COPD. In the present review, we aim to summarize the causes and effects of chronic hypercapnia in COPD. Furthermore, we discuss the use of domiciliary noninvasive ventilation as a treatment option for hypercapnia while highlighting the controversies within the evidence. Finally, we provide some insightful clinical recommendations and draw attention to possible future research areas.

Mechanisms of Hypercapnia-Induced Endoplasmic Reticulum Dysfunction

Protein transcription, translation, and folding occur continuously in every living cell and are essential for physiological functions. About one-third of all proteins of the cellular proteome interacts with the endoplasmic reticulum (ER). The ER is a large, dynamic cellular organelle that orchestrates synthesis, folding, and structural maturation of proteins, regulation of lipid metabolism and additionally functions as a calcium store. Recent evidence suggests that both acute and chronic hypercapnia (elevated levels of CO₂) impair ER function by different mechanisms, leading to adaptive and maladaptive regulation of protein folding and maturation. In order to cope with ER stress, cells activate unfolded protein response (UPR) pathways. Initially, during the adaptive phase of ER stress, the UPR mainly functions to restore ER protein-folding homeostasis by decreasing protein synthesis and translation and by activation of ER-associated degradation (ERAD) and autophagy. However, if the initial UPR attempts for alleviating ER stress fail, a maladaptive response is triggered. In this review, we discuss the distinct mechanisms by which elevated CO₂ levels affect these molecular pathways in the setting of acute and chronic pulmonary diseases associated with hypercapnia.

Inhaled β2 Adrenergic Agonists and Other cAMP-Elevating Agents: Therapeutics for Alveolar Injury and Acute Respiratory Disease Syndrome?

Inhaled long-acting β-adrenergic agonists (LABAs) and short-acting β-adrenergic agonists are approved for the treatment of obstructive lung disease via actions mediated by β2 adrenergic receptors (β2-ARs) that increase cellular cAMP synthesis. This review discusses the potential of β2-AR agonists, in particular LABAs, for the treatment of acute respiratory distress syndrome (ARDS). We emphasize ARDS induced by pneumonia and focus on the pathobiology of ARDS and actions of LABAs and cAMP on pulmonary and immune cell types. β2-AR agonists/cAMP have beneficial actions that include protection of epithelial and endothelial cells from injury, restoration of alveolar fluid clearance, and reduction of fibrotic remodeling. β2-AR agonists/cAMP also exert anti-inflammatory effects on the immune system by actions on several types of immune cells. Early administration is likely critical for optimizing efficacy of LABAs or other cAMP-elevating agents, such as agonists of other Gs-coupled G protein-coupled receptors or cyclic nucleotide phosphodiesterase inhibitors. Clinical studies that target lung injury early, prior to development of ARDS, are thus needed to further assess the use of inhaled LABAs, perhaps combined with inhaled corticosteroids and/or long-acting muscarinic cholinergic antagonists. Such agents may provide a multipronged, repurposing, and efficacious therapeutic approach while minimizing systemic toxicity. SIGNIFICANCE STATEMENT: Acute respiratory distress syndrome (ARDS) after pulmonary alveolar injury (e.g., certain viral infections) is associated with ∼40% mortality and in need of new therapeutic approaches. This review summarizes the pathobiology of ARDS, focusing on contributions of pulmonary and immune cell types and potentially beneficial actions of β2 adrenergic receptors and cAMP. Early administration of inhaled β2 adrenergic agonists and perhaps other cAMP-elevating agents after alveolar injury may be a prophylactic approach to prevent development of ARDS.

TRAF2 Is a Novel Ubiquitin E3 Ligase for the Na,K-ATPase β-Subunit That Drives Alveolar Epithelial Dysfunction in Hypercapnia

Several acute and chronic lung diseases are associated with alveolar hypoventilation leading to accumulation of CO2 (hypercapnia). The β-subunit of the Na,K-ATPase plays a pivotal role in maintaining epithelial integrity by functioning as a cell adhesion molecule and regulating cell surface stability of the catalytic α-subunit of the transporter, thereby, maintaining optimal alveolar fluid balance. Here, we identified the E3 ubiquitin ligase for the Na,K-ATPase β-subunit, which promoted polyubiquitination, subsequent endocytosis and proteasomal degradation of the protein upon exposure of alveolar epithelial cells to elevated CO2 levels, thus impairing alveolar integrity. Ubiquitination of the Na,K-ATPase β-subunit required lysine 5 and 7 and mutating these residues (but not other lysines) prevented trafficking of Na,K-ATPase from the plasma membrane and stabilized the protein upon hypercapnia. Furthermore, ubiquitination of the Na,K-ATPase β-subunit was dependent on prior phosphorylation at serine 11 by protein kinase C (PKC)-ζ. Using a protein microarray, we identified the tumor necrosis factor receptor-associated factor 2 (TRAF2) as the E3 ligase driving ubiquitination of the Na,K-ATPase β-subunit upon hypercapnia. Of note, prevention of Na,K-ATPase β-subunit ubiquitination was necessary and sufficient to restore the formation of cell-cell junctions under hypercapnic conditions. These results suggest that a hypercapnic environment in the lung may lead to persistent epithelial dysfunction in affected patients. As such, the identification of the E3 ligase for the Na,K-ATPase may provide a novel therapeutic target, to be employed in patients with acute or chronic hypercapnic respiratory failure, aiming to restore alveolar epithelial integrity.

Maturation of the Na,K-ATPase in the Endoplasmic Reticulum in Health and Disease

Abstract

The Na,K-ATPase establishes the electrochemical gradient of cells by driving an active exchange of Na⁺ and K⁺ ions while consuming ATP. The minimal functional transporter consists of a catalytic α-subunit and a β-subunit with chaperon activity. The Na,K-ATPase also functions as a cell adhesion molecule and participates in various intracellular signaling pathways. The maturation and trafficking of the Na,K-ATPase include co- and post-translational processing of the enzyme in the endoplasmic reticulum (ER) and the Golgi apparatus and subsequent delivery to the plasma membrane (PM). The ER folding of the enzyme is considered as the rate-limiting step in the membrane delivery of the protein. It has been demonstrated that only assembled Na,K-ATPase α:β-complexes may exit the organelle, whereas unassembled, misfolded or unfolded subunits are retained in the ER and are subsequently degraded. Loss of function of the Na,K-ATPase has been associated with lung, heart, kidney and neurological disorders. Recently, it has been shown that ER dysfunction, in particular, alterations in the homeostasis of the organelle, as well as impaired ER-resident chaperone activity may impede folding of Na,K-ATPase subunits, thus decreasing the abundance and function of the enzyme at the PM. Here, we summarize our current understanding on maturation and subsequent processing of the Na,K-ATPase in the ER under physiological and pathophysiological conditions.

Graphic Abstract

Influence of Intermittent Hypoxia/Hypercapnia on Atherosclerosis, Gut Microbiome, and Metabolome

Obstructive sleep apnea (OSA), a common sleep disorder characterized by intermittent hypoxia and hypercapnia (IHC), increases atherosclerosis risk. However, the contribution of intermittent hypoxia (IH) or intermittent hypercapnia (IC) in promoting atherosclerosis remains unclear. Since gut microbiota and metabolites have been implicated in atherosclerosis, we examined whether IH or IC alters the microbiome and metabolome to induce a pro-atherosclerotic state. Apolipoprotein E deficient mice (ApoE-/- ), treated with IH or IC on a high-fat diet (HFD) for 10 weeks, were compared to Air controls. Atherosclerotic lesions were examined, gut microbiome was profiled using 16S rRNA gene amplicon sequencing and metabolome was assessed by untargeted mass spectrometry. In the aorta, IC-induced atherosclerosis was significantly greater than IH and Air controls (aorta, IC 11.1 ± 0.7% vs. IH 7.6 ± 0.4%, p < 0.05 vs. Air 8.1 ± 0.8%, p < 0.05). In the pulmonary artery (PA), however, IH, IC, and Air were significantly different from each other in atherosclerotic formation with the largest lesion observed under IH (PA, IH 40.9 ± 2.0% vs. IC 20.1 ± 2.6% vs. Air 12.2 ± 1.5%, p < 0.05). The most differentially abundant microbial families (p < 0.001) were Peptostreptococcaceae, Ruminococcaceae, and Erysipelotrichaceae. The most differentially abundant metabolites (p < 0.001) were tauro-β-muricholic acid, ursodeoxycholic acid, and lysophosphoethanolamine (18:0). We conclude that IH and IC (a) modulate atherosclerosis progression differently in distinct vascular beds with IC, unlike IH, facilitating atherosclerosis in both aorta and PA and (b) promote an atherosclerotic luminal gut environment that is more evident in IH than IC. We speculate that the resulting changes in the gut metabolome and microbiome interact differently with distinct vascular beds.

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.

Molecular mechanisms of Na,K-ATPase dysregulation driving alveolar epithelial barrier failure in severe COVID-19

A significant number of patients with coronavirus disease 2019 (COVID-19) develop acute respiratory distress syndrome (ARDS) that is associated with a poor outcome. The molecular mechanisms driving failure of the alveolar barrier upon severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection remain incompletely understood. The Na,K-ATPase is an adhesion molecule and a plasma membrane transporter that is critically required for proper alveolar epithelial function by both promoting barrier integrity and resolution of excess alveolar fluid, thus enabling appropriate gas exchange. However, numerous SARS-CoV-2-mediated and COVID-19-related signals directly or indirectly impair the function of the Na,K-ATPase, thereby potentially contributing to disease progression. In this Perspective, we highlight some of the putative mechanisms of SARS-CoV-2-driven dysfunction of the Na,K-ATPase, focusing on expression, maturation, and trafficking of the transporter. A therapeutic mean to selectively inhibit the maladaptive signals that impair the Na,K-ATPase upon SARS-CoV-2 infection might be effective in reestablishing the alveolar epithelial barrier and promoting alveolar fluid clearance and thus advantageous in patients with COVID-19-associated ARDS.

Elevated CO2 modulates airway contractility

Carbon dioxide (CO₂), a primary product of oxidative metabolism, can be sensed by eukaryotic cells eliciting unique responses via specific signalling pathways. Severe lung diseases such as chronic obstructive pulmonary disease are associated with hypoventilation that can lead to the elevation of CO₂ levels in lung tissues and the bloodstream (hypercapnia). However, the pathophysiological effects of hypercapnia on the lungs and specific lung cells are incompletely understood. We have recently reported using combined unbiased molecular approaches with studies in mice and cell culture systems on the mechanisms by which hypercapnia alters airway smooth muscle contractility. In this review, we provide a pathophysiological and mechanistic perspective on the effects of hypercapnia on the lung airways and discuss the recent understanding of high CO₂ modulation of the airway contractility.

Hypercapnia selectively modulates LPS-induced changes in innate immune and DNA replication-related gene transcription in the macrophage

Hypercapnia, the elevation of CO₂ in blood and tissues, commonly occurs in severe acute and chronic respiratory diseases and is associated with increased risk of death. Recent studies have shown that hypercapnia inhibits expression of select innate immune genes and suppresses host defence against bacterial and viral pneumonia in mice. In the current study, we evaluated the effect of culture under conditions of hypercapnia (20% CO₂) versus normocapnia (5% CO₂), both with normoxia, on global gene transcription in human THP-1 and mouse RAW 264.7 macrophages stimulated with lipopolysaccharide (LPS). We found that hypercapnia selectively downregulated transcription of LPS-induced genes associated with innate immunity, antiviral response, type I interferon signalling, cytokine signalling and other inflammatory pathways in both human and mouse macrophages. Simultaneously, hypercapnia increased expression of LPS-downregulated genes associated with mitosis, DNA replication and DNA repair. These CO₂-induced changes in macrophage gene expression help explain hypercapnic suppression of antibacterial and antiviral host defence in mice and reveal a mechanism that may underlie, at least in part, the high mortality of patients with severe lung disease and hypercapnia.

Elevated CO2 Levels Delay Skeletal Muscle Repair by Increasing Fatty Acid Oxidation

Muscle dysfunction often occurs in patients with chronic obstructive pulmonary diseases (COPD) and affects ventilatory and non-ventilatory skeletal muscles. We have previously reported that hypercapnia (elevated CO₂ levels) causes muscle atrophy through the activation of the AMPKα2-FoxO3a-MuRF1 pathway. In the present study, we investigated the effect of normoxic hypercapnia on skeletal muscle regeneration. We found that mouse C2C12 myoblasts exposed to elevated CO₂ levels had decreased fusion index compared to myoblasts exposed to normal CO₂. Metabolic analyses of C2C12 myoblasts exposed to high CO₂ showed increased oxidative phosphorylation due to increased fatty acid oxidation. We utilized the cardiotoxin-induced muscle injury model in mice exposed to normoxia and 10% CO₂ for 21 days and observed that muscle regeneration was delayed. High CO₂-delayed differentiation in both mouse C2C12 myoblasts and skeletal muscle after injury and was restored to control levels when cells or mice were treated with a carnitine palmitoyltransfearse-1 (CPT1) inhibitor. Taken together, our data suggest that hypercapnia leads to changes in the metabolic activity of skeletal muscle cells, which results in impaired muscle regeneration and recovery after injury.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

Hypercapnia Regulates Gene Expression and Tissue Function

Carbon dioxide (CO₂) is produced in eukaryotic cells primarily during aerobic respiration, resulting in higher CO₂ levels in mammalian tissues than those in the atmosphere. CO₂ like other gaseous molecules such as oxygen and nitric oxide, is sensed by cells and contributes to cellular and organismal physiology. In humans, elevation of CO₂ levels in tissues and the bloodstream (hypercapnia) occurs during impaired alveolar gas exchange in patients with severe acute and chronic lung diseases. Advances in understanding of the biology of high CO₂ effects reveal that the changes in CO₂ levels are sensed in cells resulting in specific tissue responses. There is accumulating evidence on the transcriptional response to elevated CO₂ levels that alters gene expression and activates signaling pathways with consequences for cellular and tissue functions. The nature of hypercapnia-responsive transcriptional regulation is an emerging area of research, as the responses to hypercapnia in different cell types, tissues, and species are not fully understood. Here, we review the current understanding of hypercapnia effects on gene transcription and consequent cellular and tissue functions.

Hypercapnia-Driven Skeletal Muscle Dysfunction in an Animal Model of Pulmonary Emphysema Suggests a Complex Phenotype

Patients with chronic pulmonary conditions such as chronic obstructive pulmonary disease (COPD) often develop skeletal muscle dysfunction, which is strongly and independently associated with poor outcomes including higher mortality. Some of these patients also develop chronic CO₂ retention, or hypercapnia, which is also associated with worse prognosis. While muscle dysfunction in these settings involve reduction of muscle mass and disrupted fibers’ metabolism leading to suboptimal muscle work, mechanistic research in the field has been limited by the lack of adequate animal models. Over the last years, we have established a rodent model of COPD-induced skeletal muscle dysfunction that allowed a disaggregated interrogation of the cellular and physiological effects driven by COPD from the ones unique to hypercapnia. We found that while COPD and hypercapnia synergistically contribute to muscle atrophy, they are antagonistic processes regarding fibers respiratory capacity. We propose that AMP-activated protein kinase (AMPK) is a crucial regulator of CO₂ signaling in hypercapnic muscles, which leads to both net protein catabolism and improved mitochondrial respiration to support a transition into a substrate-rich, fuel-efficient metabolic mode that allows muscle cells cope with the CO₂ toxicity.

High CO2 Levels Impair Lung Wound Healing

Delayed lung repair leads to alveolopleural fistulae, which are a major cause of morbidity after lung resections. We have reported that intrapleural hypercapnia is associated with delayed lung repair after lung resection. Here, we provide new evidence that hypercapnia delays wound closure of both large airway and alveolar epithelial cell monolayers because of inhibition of epithelial cell migration. Cell migration and airway epithelial wound closure were dependent on Rac1-GTPase activation, which was suppressed by hypercapnia directly through the upregulation of AMP kinase and indirectly through inhibition of injury-induced NF-κB-mediated CXCL12 (pleural CXC motif chemokine 12) release, respectively. Both these pathways were independently suppressed, because dominant negative AMP kinase rescued the effects of hypercapnia on Rac1-GTPase in uninjured resting cells, whereas proteasomal inhibition reversed the NF-κB-mediated CXCL12 release during injury. Constitutive overexpression of Rac1-GTPase rescued the effects of hypercapnia on both pathways as well as on wound healing. Similarly, exogenous recombinant CXCL12 reversed the effects of hypercapnia through Rac1-GTPase activation by its receptor, CXCR4. Moreover, CXCL12 transgenic murine recipients of orthotopic tracheal transplantation were protected from hypercapnia-induced inhibition of tracheal epithelial cell migration and wound repair. In patients undergoing lobectomy, we found inverse correlation between intrapleural carbon dioxide and pleural CXCL12 levels as well as between CXCL12 levels and alveolopleural leak. Accordingly, we provide first evidence that high carbon dioxide levels impair lung repair by inhibiting epithelial cell migration through two distinct pathways, which can be restored by recombinant CXCL12.

Hypercapnia: An Aggravating Factor in Asthma

Asthma is a common chronic respiratory disorder with relatively good outcomes in the majority of patients with appropriate maintenance therapy. However, in a small minority, patients can experience severe asthma with respiratory failure and hypercapnia, necessitating intensive care unit admission. Hypercapnia occurs due to alveolar hypoventilation and insufficient removal of carbon dioxide (CO₂) from the blood. Although mild hypercapnia is generally well tolerated in patients with asthma, there is accumulating evidence that elevated levels of CO₂ can act as a gaso-signaling molecule, triggering deleterious effects in various organs such as the lung, skeletal muscles and the innate immune system. Here, we review recent advances on pathophysiological response to hypercapnia and discuss potential detrimental effects of hypercapnia in patients with asthma.

Correlation of Arterial CO2 and Respiratory Impedance Values among Subjects with COPD

Chronic obstructive pulmonary disease (COPD) is a respiratory illness characterized by airflow limitation and chronic respiratory symptoms with a global prevalence estimated to be more than 10% in 2010 and still on the rise. Furthermore, hypercapnic subject COPD leads to an increased risk of mortality, morbidity, and poor QoL (quality of life) than normocapnic subjects. Series of studies showed the usefulness of the forced oscillation technique (FOT) to measure small airway closure. Traditional findings suggested that hypercapnia may not be the main treating targets, but recent findings suggested that blood stream CO₂ may lead to a worse outcome. This study aimed to seek the relationship between CO₂ and small airway closure by using FOT. Subjects with COPD (n = 124; hypercapnia 22 and normocapnia 102) were analyzed for all pulmonary function values, FOT values, and arterial blood gas analysis. Student’s t-test, Spearman rank correlation, and multi linear regression analysis were used to analyze the data. COPD subjects with hypercapnia showed a significant increase in R5, R20, Fres, and ALX values, and a greater decrease in X5 value than normocapnic patients. Also, multiple linear regression analysis showed R5 was associated with hypercapnia. Hypercapnia may account for airway closure among subjects with COPD and this result suggests treating hypercapnia may lead to better outcomes for such a subject group.

Endocytosis in the adaptation to cellular stress

Cellular life is challenged by a multitude of stress conditions, triggered for example by alterations in osmolarity, oxygen or nutrient supply. Hence, cells have developed sophisticated stress responses to cope with these challenges. Some of these stress programs such as the heat shock response are understood in great detail, while other aspects remain largely elusive including potential stress-dependent adaptations of the plasma membrane proteome. The plasma membrane is not only the first point of encounter for many types of environmental stress, but given the diversity of receptor proteins and their associated molecules also represents the site at which many cellular signal cascades originate. Since these signaling pathways affect virtually all aspects of cellular life, changes in the plasma membrane proteome appear ideally suited to contribute to the cellular adaptation to stress. The most rapid means to alter the cell surface proteome in response to stress is by alterations in endocytosis. Changes in the overall endocytic flux or in the endocytic regulation of select proteins conceivably can help to counteract adverse environmental conditions. In this review we summarize recent data regarding stress-induced changes in endocytosis and discuss how these changes might contribute to the cellular adaptation to stress in different systems. Future studies will be needed to uncover the underlying mechanisms in detail and to arrive at a coherent picture.

High CO2 Downregulates Skeletal Muscle Protein Anabolism via AMP-activated Protein Kinase α2-mediated Depressed Ribosomal Biogenesis

High CO₂ retention, or hypercapnia, is associated with worse outcomes in patients with chronic pulmonary diseases. Skeletal muscle wasting is also an independent predictor of poor outcomes in patients with acute and chronic pulmonary diseases. Although previous evidence indicates that high CO₂ accelerates skeletal muscle catabolism via AMPK (AMP-activated protein kinase)-FoxO3a-MuRF1 (E3-ubiquitin ligase muscle RING finger protein 1), little is known about the role of high CO₂ in regulating skeletal muscle anabolism. In the present study, we investigated the potential role of high CO₂ in attenuating skeletal muscle protein synthesis. We found that locomotor muscles from patients with chronic CO₂ retention demonstrated depressed ribosomal gene expression in comparison with locomotor muscles from non-CO₂-retaining individuals, and analysis of the muscle proteome of normo- and hypercapnic mice indicates reduction of important components of ribosomal structure and function. Indeed, mice chronically kept under a high-CO₂ environment show evidence of skeletal muscle downregulation of ribosomal biogenesis and decreased protein synthesis as measured by the incorporation of puromycin into skeletal muscle. Hypercapnia did not regulate the mTOR pathway, and rapamycin-induced deactivation of mTOR did not cause a decrease in ribosomal gene expression. Loss-of-function studies in cultured myotubes showed that AMPKα2 regulates CO₂-mediated reductions in ribosomal gene expression and protein synthesis. Although previous evidence has implicated TIF1A (transcription initiation factor-1α) and KDM2A (lysine-specific demethylase 2A) in AMPK-driven regulation of ribosomal gene expression, we found that these mediators were not required in the high CO₂-induced depressed protein anabolism. Our research supports future studies targeting ribosomal biogenesis and protein synthesis to alleviate the effects of high CO₂ on skeletal muscle turnover.

Helicobacter pylori infection impairs chaperone-assisted maturation of Na-K-ATPase in gastric epithelium

Helicobacter pylori infection always induces gastritis, which may progress to ulcer disease or cancer. The mechanisms underlying mucosal injury by the bacteria are incompletely understood. Here, we identify a novel pathway for H. pylori-induced gastric injury, the impairment of maturation of the essential transport enzyme and cell adhesion molecule, Na-K-ATPase. Na-K-ATPase comprises α- and β-subunits that assemble in the endoplasmic reticulum (ER) before trafficking to the plasma membrane. Attachment of H. pylori to gastric epithelial cells increased Na-K-ATPase ubiquitylation, decreased its surface and total levels, and impaired ion balance. H. pylori did not alter degradation of plasmalemma-resident Na-K-ATPase subunits or their mRNA levels. Infection decreased association of α- and β-subunits with ER chaperone BiP and impaired assembly of α/β-heterodimers, as was revealed by quantitative mass spectrometry and immunoblotting of immunoprecipitated complexes. The total level of BiP was not altered, and the decrease in interaction with BiP was not observed for other BiP client proteins. The H. pylori-induced decrease in Na-K-ATPase was prevented by BiP overexpression, stopping protein synthesis, or inhibiting proteasomal, but not lysosomal, protein degradation. The results indicate that H. pylori impairs chaperone-assisted maturation of newly made Na-K-ATPase subunits in the ER independently of a generalized ER stress and induces their ubiquitylation and proteasomal degradation. The decrease in Na-K-ATPase levels is also seen in vivo in the stomachs of gerbils and chronically infected children. Further understanding of H. pylori-induced Na-K-ATPase degradation will provide insights for protection against advanced disease.NEW & NOTEWORTHY This work provides evidence that Helicobacter pylori decreases levels of Na-K-ATPase, a vital transport enzyme, in gastric epithelia, both in acutely infected cultured cells and in chronically infected patients and animals. The bacteria interfere with BiP-assisted folding of newly-made Na-K-ATPase subunits in the endoplasmic reticulum, accelerating their ubiquitylation and proteasomal degradation and decreasing efficiency of the assembly of native enzyme. Decreased Na-K-ATPase expression contributes to H. pylori-induced gastric injury.

Sepsis therapies: learning from 30 years of failure of translational research to propose new leads

Sepsis has been identified by the World Health Organization (WHO) as a global health priority. There has been a tremendous effort to decipher underlying mechanisms responsible for organ failure and death, and to develop new treatments. Despite saving thousands of animals over the last three decades in multiple preclinical studies, no new effective drug has emerged that has clearly improved patient outcomes. In the present review, we analyze the reasons for this failure, focusing on the inclusion of inappropriate patients and the use of irrelevant animal models. We advocate against repeating the same mistakes and propose changes to the research paradigm. We discuss the long-term consequences of surviving sepsis and, finally, list some putative approaches-both old and new-that could help save lives and improve survivorship.

Activation of AMPK under Hypoxia: Many Roads Leading to Rome

AMP-activated protein kinase (AMPK) is known as a pivotal cellular energy sensor, mediating the adaptation to low energy levels by deactivating anabolic processes and activating catabolic processes in order to restore the cellular ATP supply when the cellular AMP/ATP ratio is increased. Besides this well-known role, it has also been shown to exert protective effects under hypoxia. While an insufficient supply with oxygen might easily deplete cellular energy levels, i.e., ATP concentration, manifold other mechanisms have been suggested and are heavily disputed regarding the activation of AMPK under hypoxia independently from cellular AMP concentrations. However, an activation of AMPK preceding energy depletion could induce a timely adaptation reaction preventing more serious damage. A connection between AMPK and the master regulator of hypoxic adaptation via gene transcription, hypoxia-inducible factor (HIF), has also been taken into account, orchestrating their concerted protective action. This review will summarize the current knowledge on mechanisms of AMPK activation under hypoxia and its interrelationship with HIF.

Hypercapnia Impairs Na,K-ATPase Function by Inducing Endoplasmic Reticulum Retention of the β-Subunit of the Enzyme in Alveolar Epithelial Cells

Alveolar edema, impaired alveolar fluid clearance, and elevated CO2 levels (hypercapnia) are hallmarks of the acute respiratory distress syndrome (ARDS). This study investigated how hypercapnia affects maturation of the Na,K-ATPase (NKA), a key membrane transporter, and a cell adhesion molecule involved in the resolution of alveolar edema in the endoplasmic reticulum (ER). Exposure of human alveolar epithelial cells to elevated CO2 concentrations caused a significant retention of NKA-β in the ER and, thus, decreased levels of the transporter in the Golgi apparatus. These effects were associated with a marked reduction of the plasma membrane (PM) abundance of the NKA-α/β complex as well as a decreased total and ouabain-sensitive ATPase activity. Furthermore, our study revealed that the ER-retained NKA-β subunits were only partially assembled with NKA α-subunits, which suggests that hypercapnia modifies the ER folding environment. Moreover, we observed that elevated CO2 levels decreased intracellular ATP production and increased ER protein and, particularly, NKA-β oxidation. Treatment with α-ketoglutaric acid (α-KG), which is a metabolite that has been shown to increase ATP levels and rescue mitochondrial function in hypercapnia-exposed cells, attenuated the deleterious effects of elevated CO2 concentrations and restored NKA PM abundance and function. Taken together, our findings provide new insights into the regulation of NKA in alveolar epithelial cells by elevated CO2 levels, which may lead to the development of new therapeutic approaches for patients with ARDS and hypercapnia.

AMP-Activated Protein Kinase (AMPK) at the Crossroads Between CO2 Retention and Skeletal Muscle Dysfunction in Chronic Obstructive Pulmonary Disease (COPD)

Skeletal muscle dysfunction is a major comorbidity in chronic obstructive pulmonary disease (COPD) and other pulmonary conditions. Chronic CO2 retention, or hypercapnia, also occur in some of these patients. Both muscle dysfunction and hypercapnia associate with higher mortality in these populations. Over the last years, we have established a mechanistic link between hypercapnia and skeletal muscle dysfunction, which is regulated by AMPK and causes depressed anabolism via reduced ribosomal biogenesis and accelerated catabolism via proteasomal degradation. In this review, we discuss the main findings linking AMPK with hypercapnic pulmonary disease both in the lungs and skeletal muscles, and also outline potential avenues for future research in the area based on knowledge gaps and opportunities to expand mechanistic research with translational implications.

Elevated CO2 regulates the Wnt signaling pathway in mammals, Drosophila melanogaster and Caenorhabditis elegans

Carbon dioxide (CO2) is sensed by cells and can trigger signals to modify gene expression in different tissues leading to changes in organismal functions. Despite accumulating evidence that several pathways in various organisms are responsive to CO2 elevation (hypercapnia), it has yet to be elucidated how hypercapnia activates genes and signaling pathways, or whether they interact, are integrated, or are conserved across species. Here, we performed a large-scale transcriptomic study to explore the interaction/integration/conservation of hypercapnia-induced genomic responses in mammals (mice and humans) as well as invertebrates (Caenorhabditis elegans and Drosophila melanogaster). We found that hypercapnia activated genes that regulate Wnt signaling in mouse lungs and skeletal muscles in vivo and in several cell lines of different tissue origin. Hypercapnia-responsive Wnt pathway homologues were similarly observed in secondary analysis of available transcriptomic datasets of hypercapnia in a human bronchial cell line, flies and nematodes. Our data suggest the evolutionarily conserved role of high CO2 in regulating Wnt pathway genes.

Stimulation of Na+/K+-ATPase with an Antibody against Its 4th Extracellular Region Attenuates Angiotensin II-Induced H9c2 Cardiomyocyte Hypertrophy via an AMPK/SIRT3/PPARγ Signaling Pathway

Activation of the renin-angiotensin system (RAS) contributes to the pathogenesis of cardiovascular diseases. Sodium potassium ATPase (NKA) expression and activity are often regulated by angiotensin II (Ang II). This study is aimed at investigating whether DR-Ab, an antibody against 4^th extracellular region of NKA, can protect Ang II-induced cardiomyocyte hypertrophy. Our results showed that Ang II treatment significantly reduced NKA activity and membrane expression. Pretreatment with DR-Ab preserved cell size in Ang II-induced cardiomyopathy by stabilizing the plasma membrane expression of NKA and restoring its activity. DR-Ab reduced intracellular ROS generation through inhibition of NADPH oxidase activity and protection of mitochondrial functions in Ang II-treated H9c2 cardiomyocytes. Pharmacological manipulation and Western blotting analysis demonstrated the cardioprotective effects were mediated by the activation of the AMPK/Sirt-3/PPARγ signaling pathway. Taken together, our results suggest that dysfunction of NKA is an important mechanism for Ang II-induced cardiomyopathy and DR-Ab may be a novel and promising therapeutic approach to treat cardiomyocyte hypertrophy.

AMPK alleviates high uric acid-induced Na+-K+-ATPase signaling impairment and cell injury in renal tubules

One of the mechanisms in hyperuricemia (HUA)-induced renal tubular injury is the impairment of Na⁺-K⁺-ATPase (NKA) signaling, which further triggers inflammation, autophagy, and mitochondrial dysfunction and leads to cell injury. Here, we used RNA sequencing to screen the most likely regulators of NKA signaling and found that the liver kinase B1(LKB1)/adenosine monophosphate (AMP)-activated protein kinase (AMPK)/ mammalian target of rapamycin (mTOR) pathway was the most abundantly enriched pathway in HUA. AMPK is a key regulator of cell energy metabolism; hence, we examined the effect of AMPK on HUA-induced dysregulation of NKA signaling and cell injury. We first detected AMPK activation in high uric acid (UA)-stimulated proximal tubular epithelial cells (PTECs). We further found that sustained treatment with the AMPK activator 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR), but not the AMPK inhibitor Compound C, significantly alleviated UA-induced reductions in NKA activity and NKA α1 subunit expression on the cell membrane by reducing NKA degradation in lysosomes; sustained AICAR treatment also significantly alleviated activation of the NKA downstream molecules Src and interleukin-1β (IL-1β) in PTECs. AICAR further alleviated high UA-induced apoptosis, autophagy, and mitochondrial dysfunction. Although AMPK activation by metformin did not reduce serum UA levels in hyperuricemic rats, it significantly alleviated HUA-induced renal tubular injury and NKA signaling impairment in vivo with effects similar to those of febuxostat. Our study suggests that AMPK activation may temporarily compensate for HUA-induced renal injury. Sustained AMPK activation could reduce lysosomal NKA degradation and maintain NKA function, thus alleviating NKA downstream inflammation and protecting tubular cells from high UA-induced renal tubular injury.

High serum levels of uric acid cause kidney tissue damage through cellular processes that have now been identified by researchers in China. Uric acid is a common component of urine, but causes damage if it is present in high levels in the blood (hyperuricemia). While investigating the mechanisms behind hyperuricemia, Zhibin Ye and co-workers at Fudan University in Shanghai recently showed that impairment of the Na⁺-K⁺-ATPase (NKA) signaling pathway, which regulates uric acid transportation through the kidneys, is a crucial feature of renal damage progression. The team have now shown that NKA is regulated by the AMP-activated protein kinase (AMPK) pathway, and that AMPK is enriched during the initial phases of hyperuricemia. Studies on rat models indicated that sustained AMPK activation restored NKA signaling, limiting damage from hyperuricemia.

Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair

Acute respiratory distress syndrome (ARDS) is a devastating disorder characterized by diffuse inflammation and edema formation. The main management strategy, low tidal volume ventilation, can be associated with the development of hypercapnic acidosis (HCA). Mesenchymal stem cells (MSCs) are a promising therapeutic candidate currently in early-phase clinical trials. The effects of HCA on the alveolar epithelium and capillary endothelium are not well established. The therapeutic efficacy of MSCs has never been reported in HCA. In the present study, we evaluated the effects of HCA on inflammatory response and reparative potential of the primary human small airway epithelial and lung microvasculature endothelial cells as well as on the capacity of bone marrow-derived MSCs to promote wound healing in vitro. We demonstrate that HCA attenuates the inflammatory response and reparative potential of primary human small airway epithelium and capillary endothelium and induces mitochondrial dysfunction. It was found that MSCs promote lung epithelial wound repair via the transfer of functional mitochondria; however, this proreparative effect of MSCs was lost in the setting of HCA. Therefore, HCA may adversely impact recovery from ARDS at the cellular level, whereas MSCs may not be therapeutically beneficial in patients with ARDS who develop HCA.-Fergie, N., Todd, N., McClements, L., McAuley, D., O'Kane, C., Krasnodembskaya, A. Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Hypercapnia Alters Expression of Immune Response, Nucleosome Assembly and Lipid Metabolism Genes in Differentiated Human Bronchial Epithelial Cells

Hypercapnia, the elevation of CO2 in blood and tissues, commonly occurs in severe acute and chronic respiratory diseases, and is associated with increased risk of mortality. Recent studies have shown that hypercapnia adversely affects innate immunity, host defense, lung edema clearance and cell proliferation. Airway epithelial dysfunction is a feature of advanced lung disease, but the effect of hypercapnia on airway epithelium is unknown. Thus, in the current study we examined the effect of normoxic hypercapnia (20% CO2 for 24 h) vs normocapnia (5% CO2), on global gene expression in differentiated normal human airway epithelial cells. Gene expression was assessed on Affymetrix microarrays, and subjected to gene ontology analysis for biological process and cluster-network representation. We found that hypercapnia downregulated the expression of 183 genes and upregulated 126. Among these, major gene clusters linked to immune responses and nucleosome assembly were largely downregulated, while lipid metabolism genes were largely upregulated. The overwhelming majority of these genes were not previously known to be regulated by CO2. These changes in gene expression indicate the potential for hypercapnia to impact bronchial epithelial cell function in ways that may contribute to poor clinical outcomes in patients with severe acute or advanced chronic lung diseases.

Hypercapnia increases airway smooth muscle contractility via caspase-7-mediated miR-133a-RhoA signaling

The elevation of carbon dioxide (CO2) in tissues and the bloodstream (hypercapnia) occurs in patients with severe lung diseases, including chronic obstructive pulmonary disease (COPD). Whereas hypercapnia has been recognized as a marker of COPD severity, a role for hypercapnia in disease pathogenesis remains unclear. We provide evidence that CO2 acts as a signaling molecule in mouse and human airway smooth muscle cells. High CO2 activated calcium-calpain signaling and consequent smooth muscle cell contraction in mouse airway smooth muscle cells. The signaling was mediated by caspase-7-induced down-regulation of the microRNA-133a (miR-133a) and consequent up-regulation of Ras homolog family member A and myosin light-chain phosphorylation. Exposure of wild-type, but not caspase-7-null, mice to hypercapnia increased airway contraction and resistance. Deletion of the Caspase-7 gene prevented hypercapnia-induced airway contractility, which was restored by lentiviral transfection of a miR-133a antagonist. In a cohort of patients with severe COPD, hypercapnic patients had higher airway resistance, which improved after correction of hypercapnia. Our data suggest a specific molecular mechanism by which the development of hypercapnia may drive COPD pathogenesis and progression.

AMP-Activated Protein Kinase as a Reprogramming Strategy for Hypertension and Kidney Disease of Developmental Origin

Suboptimal early-life conditions affect the developing kidney, resulting in long-term programming effects, namely renal programming. Adverse renal programming increases the risk for developing hypertension and kidney disease in adulthood. Conversely, reprogramming is a strategy aimed at reversing the programming processes in early life. AMP-activated protein kinase (AMPK) plays a key role in normal renal physiology and the pathogenesis of hypertension and kidney disease. This review discusses the regulation of AMPK in the kidney and provides hypothetical mechanisms linking AMPK to renal programming. This will be followed by studies targeting AMPK activators like metformin, resveratrol, thiazolidinediones, and polyphenols as reprogramming strategies to prevent hypertension and kidney disease. Further studies that broaden our understanding of AMPK isoform- and tissue-specific effects on renal programming are needed to ultimately develop reprogramming strategies. Despite the fact that animal models have provided interesting results with regard to reprogramming strategies targeting AMPK signaling to protect against hypertension and kidney disease with developmental origins, these results await further clinical translation.

Effects of hypercapnia in acute respiratory distress syndrome

In patients with acute respiratory distress syndrome (ARDS) hypercapnia is a marker of poor prognosis, however there is controversial information regarding the effect of hypercapnia on outcomes. Recently two studies in a large population of mechanical ventilation patients showed higher mortality associated independently to hypercapnia. Key roles responsible for the poor clinical outcomes observed in critically ill patients exposed to hypercapnia are not well known, two possible mechanisms involved are the effect of CO2 on the muscle and the alveolar epithelium. Hypercapnia frequently coexists with muscle atrophy and dysfunction, moreover patients surviving ARDS present reduced muscle strength and decreased physical quality of life. One of the possible mechanisms responsible for these abnormalities could be the effects of hypercapnia during the course of ARDS. More over controversy persists about the hypercapnia role in the alveolar space, in the last years there is abundant experimental information on its deleterious effects on essential functions of the alveolar epithelium.

Restoration of Megalin-Mediated Clearance of Alveolar Protein as a Novel Therapeutic Approach for Acute Lung Injury

Acute respiratory distress syndrome constitutes a significant disease burden with regard to both morbidity and mortality. Current therapies are mostly supportive and do not address the underlying pathophysiologic mechanisms. Removal of protein-rich alveolar edema-a clinical hallmark of acute respiratory distress syndrome-is critical for survival. Here, we describe a transforming growth factor (TGF)-β-triggered mechanism, in which megalin, the primary mediator of alveolar protein transport, is negatively regulated by glycogen synthase kinase (GSK) 3β, with protein phosphatase 1 and nuclear inhibitor of protein phosphatase 1 being involved in the signaling cascade. Inhibition of GSK3β rescued transepithelial protein clearance in primary alveolar epithelial cells after TGF-β treatment. Moreover, in a bleomycin-based model of acute lung injury, megalin^+/- animals (the megalin^-/- variant is lethal due to postnatal respiratory failure) showed a marked increase in intra-alveolar protein and more severe lung injury compared with wild-type littermates. In contrast, wild-type mice treated with the clinically relevant GSK3β inhibitors, tideglusib and valproate, exhibited significantly decreased alveolar protein concentrations, which was associated with improved lung function and histopathology. Together, we discovered that the TGF-β-GSK3β-megalin axis is centrally involved in disturbances of alveolar protein clearance in acute lung injury and provide preclinical evidence for therapeutic efficacy of GSK3β inhibition.

Intermittent Hypoxia and Hypercapnia Accelerate Atherosclerosis, Partially via Trimethylamine-Oxide

Obstructive sleep apnea (OSA) is a common disorder characterized by intermittent hypoxia and hypercapnia (IHC) during sleep. OSA has been shown to be a risk factor for atherosclerosis, but the relation of IHC to the induction or progression of atherosclerosis is not well understood. To dissect the mechanisms involved, we compared atherosclerotic lesion formation in two mouse models, i.e., apolipoprotein E (ApoE) and low density lipoprotein receptor (Ldlr)-deficient mice, with or without IHC exposure. Ten-week-old ApoE-/- or Ldlr-/- mice were fed a high-fat diet for 4 or 8 weeks while being exposed to IHC for 10 hours/day or room air (RA) for 24 hours/day. En face lesions of the aorta, aortic arch, and pulmonary artery (PA) were examined. Moreover, 3,3-dimethyl-1-butanol (DMB), an inhibitor of microbial trimethylamine (TMA) production, was used to determine the contribution of TMA-oxide (TMAO) to IHC-induced atherosclerosis. Eight weeks of IHC exposure expedited the formation of atherosclerosis in both the PA and aortic arch of ApoE-/- mice, but only in the PA of Ldlr-/- mice (ApoE-/- PA 8 wk, IHC 35.4 ± 1.9% versus RA 8.0 ± 2.8%, P < 0.01). The atherosclerotic lesions evolved faster and to a more severe extent in ApoE-/- mice as compared with Ldlr-/- mice (PA IHC 8 wk, ApoE-/- 35.4 ± 1.9% versus Ldlr-/- 8.2 ± 1.5%, P < 0.01). DMB significantly attenuated but did not totally eliminate IHC-induced PA atherosclerosis. Our findings suggest that IHC, a hallmark of OSA, accelerates the progression of atherosclerosis in the aorta and especially in the PA. This process is partly inhibited by DMB, demonstrating that microbial metabolites may serve as therapeutic targets for OSA-induced atherosclerosis.

Mechanisms of Organ Dysfunction in Sepsis

Sepsis-associated organ dysfunction involves multiple responses to inflammation, including endothelial and microvascular dysfunction, immune and autonomic dysregulation, and cellular metabolic reprogramming. The effect of targeting these mechanistic pathways on short- and long-term outcomes depends highly on the timing of therapeutic intervention. Furthermore, there is a need to understand the adaptive or maladaptive character of these mechanisms, to discover phase-specific biomarkers to guide therapy, and to conceptualize these mechanisms in terms of resistance and tolerance.

TGF-β inhibits alveolar protein transport by promoting shedding, regulated intramembrane proteolysis, and transcriptional downregulation of megalin

Disruption of the alveolar-capillary barrier is a hallmark of acute respiratory distress syndrome (ARDS) that leads to the accumulation of protein-rich edema in the alveolar space, often resulting in comparable protein concentrations in alveolar edema and plasma and causing deleterious remodeling. Patients who survive ARDS have approximately three times lower protein concentrations in the alveolar edema than nonsurvivors; thus the ability to remove excess protein from the alveolar space may be critical for a positive outcome. We have recently shown that clearance of albumin from the alveolar space is mediated by megalin, a 600-kDa transmembrane endocytic receptor and member of the low-density lipoprotein receptor superfamily. In the currents study, we investigate the molecular mechanisms by which transforming growth factor-β (TGF-β), a key molecule of ARDS pathogenesis, drives downregulation of megalin expression and function. TGF-β treatment led to shedding and regulated intramembrane proteolysis of megalin at the cell surface and to a subsequent increase in intracellular megalin COOH-terminal fragment abundance resulting in transcriptional downregulation of megalin. Activity of classical protein kinase C enzymes and γ-secretase was required for the TGF-β-induced megalin downregulation. Furthermore, TGF-β-induced shedding of megalin was mediated by matrix metalloproteinases (MMPs)-2, -9, and -14. Silencing of either of these MMPs stabilized megalin at the cell surface after TGF-β treatment and restored normal albumin transport. Moreover, a direct interaction of megalin with MMP-2 and -14 was demonstrated, suggesting that these MMPs may function as novel sheddases of megalin. Further understanding of these mechanisms may lead to novel therapeutic approaches for the treatment of ARDS.

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.

Hypercapnia Impairs ENaC Cell Surface Stability by Promoting Phosphorylation, Polyubiquitination and Endocytosis of β-ENaC in a Human Alveolar Epithelial Cell Line

Acute lung injury is associated with formation of pulmonary edema leading to impaired gas exchange. Patients with acute respiratory distress syndrome (ARDS) require mechanical ventilation to improve oxygenation; however, the use of relatively low tidal volumes (to minimize further injury of the lung) often leads to further accumulation of carbon dioxide (hypercapnia). Hypercapnia has been shown to impair alveolar fluid clearance (AFC), thereby causing retention of pulmonary edema, and may lead to worse outcomes; however, the underlying molecular mechanisms remain incompletely understood. AFC is critically dependent on the epithelial sodium channel (ENaC), which drives the vectorial transport of Na⁺ across the alveolar epithelium. Thus, in the current study, we investigated the mechanisms by which hypercapnia effects ENaC cell surface stability in alveolar epithelial cells (AECs). Elevated CO₂ levels led to polyubiquitination of β-ENaC and subsequent endocytosis of the α/β-ENaC complex in AECs, which were prevented by silencing the E3 ubiquitin ligase, Nedd4-2. Hypercapnia-induced ubiquitination and cell surface retrieval of ENaC were critically dependent on phosphorylation of the Thr615 residue of β-ENaC, which was mediated by the extracellular signal-regulated kinase (ERK)1/2. Furthermore, activation of ERK1/2 led to subsequent activation of AMP-activated protein kinase (AMPK) and c-Jun N-terminal kinase (JNK)1/2 that in turn phosphorylated Nedd4-2 at the Thr899 residue. Importantly, mutation of Thr899 to Ala markedly inhibited the CO₂-induced polyubiquitination of β-ENaC and restored cell surface stability of the ENaC complex, highlighting the critical role of Nedd4-2 phosphorylation status in targeting ENaC. Collectively, our data suggest that elevated CO₂ levels promote activation of the ERK/AMPK/JNK axis in a human AEC line, in which ERK1/2 phosphorylates β-ENaC whereas JNK mediates phosphorylation of Nedd4-2, thereby facilitating the channel-ligase interaction. The hypercapnia-induced ENaC dysfunction may contribute to impaired alveolar edema clearance and thus, interfering with these molecular mechanisms may improve alveolar fluid balance and lead to better outcomes in patients with ARDS.

Effects of hypercapnia on the lung

Gases are sensed by lung cells and can activate specific intracellular signalling pathways, and thus have physiological and pathophysiological effects. Carbon dioxide (CO₂ ), a primary product of oxidative metabolism, can be sensed by eukaryotic cells eliciting specific responses via recently identified signalling pathways. However, the physiological and pathophysiological effects of high CO₂ (hypercapnia) on the lungs and specific lung cells, which are the primary site of CO₂ elimination, are incompletely understood. In this review, we provide a physiological and mechanistic perspective on the effects of hypercapnia on the lungs and discuss the recent understanding of CO₂ modulation of the alveolar epithelial function (lung oedema clearance), epithelial cell repair, innate immunity and airway function.

Macrophage-epithelial interactions in pulmonary alveoli

Alveolar macrophages have been investigated for years by approaches involving macrophage extraction from the lung by bronchoalveolar lavage, or by cell removal from lung tissue. Since extracted macrophages are studied outside their natural milieu, there is little understanding of the extent to which alveolar macrophages interact with the epithelium, or with one another to generate the lung's innate immune response to pathogen challenge. Here, we review new evidence of macrophage-epithelial interactions in the lung, and we address the emerging understanding that the alveolar epithelium plays an important role in orchestrating the macrophage-driven immune response.