Vacuolar ATPase regulates surfactant secretion in rat alveolar type II cells by modulating lamellar body calcium

Overstretching alveolar epithelial type II cells decreases surfactant secretion via actin polymerization and intracellular trafficking alteration

Pulmonary surfactant is essential for maintaining proper lung function. Alveolar epithelial type II (AE2) cells secrete surfactants via lamellar bodies (LBs). In tidal loading during each breath, the physiological cyclic stretching of AE2 cells promotes surfactant secretion. Excessive stretching inhibits surfactant secretion, which is considered to contribute to the development of lung damage. However, its precise mechanism remains unknown. This study tested whether actin polymerization and intracellular transport are required for pulmonary surfactant secretion and the association of actin polymerization and transport in identical human AE2-derived A549 cells using live-cell imaging, not in the bulk cells population. We found that overstretching approximately doubled actin polymerization into filaments (F-actin) and suppressed LB secretion by half in the fluorescent area ratio, compared with physiological stretching (F-actin: 1.495 vs 0.643 (P < 0.01); LB: 0.739 vs 0.332 (P < 0.01)). An inhibitor of actin polymerization increased LB secretion. Intracellular tracking using fluorescent particles revealed that cyclic stretching shifted the particle motion perpendicularly to the direction of stretching according to the orientation of the F-actin (proportion of perpendicular axis motion prior particle: 0h 40.12 % vs 2h 63.13 % (P < 0.01)), and particle motion was restricted over time in the cells subjected to overstretching, indicating that overstretching regulates intracellular transport dynamics (proportion of stop motion particle: 0h 1.01 % vs 2h 11.04 % (P < 0.01)). These findings suggest that overstretching changes secretion through the cytoskeleton: overstretching AE2 cells inhibits pulmonary surfactant secretion, at least through accelerating actin polymerization and decreasing intracellular trafficking, and the change in actin orientation would modulate intracellular trafficking.

Novel opportunities from bioimaging to understand the trafficking and maturation of intracellular pulmonary surfactant and its role in lung diseases

Pulmonary surfactant (PS), a complex mixture of lipids and proteins, is essential for maintaining proper lung function. It reduces surface tension in the alveoli, preventing collapse during expiration and facilitating re-expansion during inspiration. Additionally, PS has crucial roles in the respiratory system's innate defense and immune regulation. Dysfunction of PS contributes to various respiratory diseases, including neonatal respiratory distress syndrome (NRDS), adult respiratory distress syndrome (ARDS), COVID-19-associated ARDS, and ventilator-induced lung injury (VILI), among others. Furthermore, PS alterations play a significant role in chronic lung diseases such as chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). The intracellular stage involves storing and releasing a specialized subcellular organelle known as lamellar bodies (LB). The maturation of these organelles requires coordinated signaling to organize their intracellular organization in time and space. LB's intracellular maturation involves the lipid composition and critical processing of surfactant proteins to achieve proper functionality. Over a decade ago, the supramolecular organization of lamellar bodies was studied using electron microscopy. In recent years, novel bioimaging tools combining spectroscopy and microscopy have been utilized to investigate the in cellulo intracellular organization of lamellar bodies temporally and spatially. This short review provides an up-to-date understanding of intracellular LBs. Hyperspectral imaging and phasor analysis have allowed identifying specific transitions in LB's hydration, providing insights into their membrane dynamics and structure. A discussion and overview of the latest approaches that have contributed to a new comprehension of the trafficking and structure of lamellar bodies is presented.

SARS-CoV-2 treatment effects induced by ACE2-expressing microparticles are explained by the oxidized cholesterol-increased endosomal pH of alveolar macrophages

Exploring the cross-talk between the immune system and advanced biomaterials to treat SARS-CoV-2 infection is a promising strategy. Here, we show that ACE2-overexpressing A549 cell-derived microparticles (AO-MPs) are a potential therapeutic agent against SARS-CoV-2 infection. Intranasally administered AO-MPs dexterously navigate the anatomical and biological features of the lungs to enter the alveoli and are taken up by alveolar macrophages (AMs). Then, AO-MPs increase the endosomal pH but decrease the lysosomal pH in AMs, thus escorting bound SARS-CoV-2 from phago-endosomes to lysosomes for degradation. This pH regulation is attributable to oxidized cholesterol, which is enriched in AO-MPs and translocated to endosomal membranes, thus interfering with proton pumps and impairing endosomal acidification. In addition to promoting viral degradation, AO-MPs also inhibit the proinflammatory phenotype of AMs, leading to increased treatment efficacy in a SARS-CoV-2-infected mouse model without side effects. These findings highlight the potential use of AO-MPs to treat SARS-CoV-2-infected patients and showcase the feasibility of MP therapies for combatting emerging respiratory viruses in the future.

Channels and Transporters of the Pulmonary Lamellar Body in Health and Disease

The lamellar body (LB) of the alveolar type II (ATII) cell is a lysosome-related organelle (LRO) that contains surfactant, a complex mix of mainly lipids and specific surfactant proteins. The major function of surfactant in the lung is the reduction of surface tension and stabilization of alveoli during respiration. Its lack or deficiency may cause various forms of respiratory distress syndrome (RDS). Surfactant is also part of the innate immune system in the lung, defending the organism against air-borne pathogens. The limiting (organelle) membrane that encloses the LB contains various transporters that are in part responsible for translocating lipids and other organic material into the LB. On the other hand, this membrane contains ion transporters and channels that maintain a specific internal ion composition including the acidic pH of about 5. Furthermore, P2X4 receptors, ligand gated ion channels of the danger signal ATP, are expressed in the limiting LB membrane. They play a role in boosting surfactant secretion and fluid clearance. In this review, we discuss the functions of these transporting pathways of the LB, including possible roles in disease and as therapeutic targets, including viral infections such as SARS-CoV-2.

Inter-laboratory variability of A549 epithelial cells grown under submerged and air-liquid interface conditions

In vitro cell models offer a unique opportunity for conducting toxicology research, and the human lung adenocarcinoma cell line A549 is commonly used for toxicology testing strategies. It is essential to determine whether the response of these cells grown in different laboratories is consistent. In this study, A549 cells were grown under both submerged and air-liquid interface (ALI) conditions following an identical cell seeding protocol in two independent laboratories. The cells were switched to the ALI after four days of submerged growth, and their behaviour was compared to submerged conditions. The membrane integrity, cell viability, morphology, and (pro-)inflammatory response upon positive control stimuli were assessed at days 3, 5, and 7 under submerged conditions and at days 5, 7, and 10 at the ALI. Due to the high variability of the results between the two laboratories, the experiment was subsequently repeated using identical reagents at one specific time point and condition (day 5 at the ALI). Despite some variability, the results were more comparable, proving that the original protocol necessitated improvements. In conclusion, the use of detailed protocols and consumables from the same providers, special training of personnel for cell handling, and endpoint analysis are critical to obtain reproducible results across independent laboratories.

Adhesion-GPCR Gpr116 (ADGRF5) expression inhibits renal acid secretion

The diversity and near universal expression of G protein-coupled receptors (GPCR) reflects their involvement in most physiological processes. The GPCR superfamily is the largest in the human genome, and GPCRs are common pharmaceutical targets. Therefore, uncovering the function of understudied GPCRs provides a wealth of untapped therapeutic potential. We previously identified an adhesion-class GPCR, Gpr116, as one of the most abundant GPCRs in the kidney. Here, we show that Gpr116 is highly expressed in specialized acid-secreting A-intercalated cells (A-ICs) in the kidney using both imaging and functional studies, and we demonstrate in situ receptor activation using a synthetic agonist peptide unique to Gpr116. Kidney-specific knockout (KO) of Gpr116 caused a significant reduction in urine pH (i.e., acidification) accompanied by an increase in blood pH and a decrease in pCO2 compared to WT littermates. Additionally, immunogold electron microscopy shows a greater accumulation of V-ATPase proton pumps at the apical surface of A-ICs in KO mice compared to controls. Furthermore, pretreatment of split-open collecting ducts with the synthetic agonist peptide significantly inhibits proton flux in ICs. These data suggest a tonic inhibitory role for Gpr116 in the regulation of V-ATPase trafficking and urinary acidification. Thus, the absence of Gpr116 results in a primary excretion of acid in KO mouse urine, leading to mild metabolic alkalosis ("renal tubular alkalosis"). In conclusion, we have uncovered a significant role for Gpr116 in kidney physiology, which may further inform studies in other organ systems that express this GPCR, such as the lung, testes, and small intestine.

Intonation of Nrf2 and Hif1-α pathway by curcumin prophylaxis: A potential strategy to augment survival signaling under hypoxia

BACKGROUND

Pulmonary surfactant oxidation leads to alveolar collapse- a condition often noticed in high altitude pulmonary edema (HAPE). The present study was aimed to determine the effect of curcumin prophylaxis in augmenting the phase II antioxidant enzymes and surfactant proteins expression in enabling the pulmonary surfactant homeostasis under hypoxia.

METHODS

A549 cells were exposed to 3% hypoxia for different time durations (1 h, 3 h, 6 h, 12 h and 24 h). The Cells were pretreated (1 h) with 10 μM curcumin and exposed to hypoxia. The in-vivo results were extrapolated into in-vivo system using male Sprague Dawley rats, exposed to a stimulated altitude of 7620 m for 6 h. The rats were supplemented with curcumin (50 mg/kgBW) 1 h prior to hypoxia exposure.

RESULTS

Results showed that, the expression of surfactant proteins (SPs) A and B decreased from 3 h of hypoxic exposure, whereas expression of SP-C and SP-D proteins were increased within 1 h of hypoxic exposure over control cells. Hypoxic exposure resulted into significant increase in protein and lipid peroxidation (p < 0.001), reduced levels of antioxidants (GSH, GPx and SOD) (p < 0.001) along with significant down regulation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and Heme oxygenase-1 (HO-1) in A549 cells over control. However, the curcumin supplementation both in-vitro and in-vivo resulted into increased expressions of HO-1 and Nrf2 significantly (p < 0.001), which enabled the cells in balanced expression of SPs with reduced levels of oxidants. Further curcumin significantly enhanced the levels of antioxidant enzymes in BALF along with stabilized expression of hypoxia inducible factor 1(HIF-1α) followed by reduced expression of vascular endothelial growth factor (VEGF) in lungs of rats. The immunohistochemistry observations provided substantial evidence of enhanced surfactant protein expressions in lungs of curcumin administered hypoxia exposed rats.

CONCLUSION

These results indicate that curcumin augment survival signaling by reinforcing the induction of phase II antioxidant enzymes thereby enabling the pulmonary surfactant homeostasis under hypoxia.

Oxidized Ferric and Ferryl Forms of Hemoglobin Trigger Mitochondrial Dysfunction and Injury in Alveolar Type I Cells

Lung alveoli are lined by alveolar type (AT) 1 cells and cuboidal AT2 cells. The AT1 cells are likely to be exposed to cell-free hemoglobin (Hb) in multiple lung diseases; however, the role of Hb redox (reduction-oxidation) reactions and their precise contributions to AT1 cell injury are not well understood. Using mouse lung epithelial cells (E10) as an AT1 cell model, we demonstrate here that higher Hb oxidation states, ferric Hb (HbFe(3+)) and ferryl Hb (HbFe(4+)) and subsequent heme loss play a central role in the genesis of injury. Exposures to HbFe(2+) and HbFe(3+) for 24 hours induced expression of heme oxygenase (HO)-1 protein in E10 cells and HO-1 translocation in the purified mitochondrial fractions. Both of these effects were intensified with increasing oxidation states of Hb. Next, we examined the effects of Hb oxidation and free heme on mitochondrial bioenergetic function by measuring changes in the mitochondrial transmembrane potential and oxygen consumption rate. In contrast to HbFe(2+), HbFe(3+) reduced basal oxygen consumption rate, indicating compromised mitochondrial activity. However, HbFe(4+) exposure not only induced early expression of HO-1 but also caused mitochondrial dysfunction within 12 hours when compared with HbFe(2+) and HbFe(3+). Exposure to HbFe(4+) for 24 hours also caused mitochondrial depolarization in E10 cells. The deleterious effects of HbFe(3+) and HbFe(4+) were reversed by the addition of scavenger proteins, haptoglobin and hemopexin. Collectively, these data establish, for the first time, a central role for cell-free Hb in lung epithelial injury, and that these effects are mediated through the redox transition of Hb to higher oxidation states.

H+-dependent inorganic phosphate uptake in Trypanosoma brucei is influenced by myo-inositol transporter

Trypanosoma brucei is an extracellular protozoan parasite that causes human African trypanosomiasis or "sleeping sickness". During the different phases of its life cycle, T. brucei depends on exogenous inorganic phosphate (P_i), but little is known about the transport of P_i in this organism. In the present study, we showed that the transport of ³²P_i across the plasma membrane follows Michaelis-Menten kinetics and is modulated by pH variation, with higher activity at acidic pH. Bloodstream forms presented lower P_i transport in comparison to procyclic forms, that displayed an apparent K_0.5 = 0.093 ± 0.008 mM. Additionally, FCCP (H⁺-ionophore), valinomycin (K⁺-ionophore) and SCH28080 (H⁺, K⁺-ATPase inhibitor) inhibited the P_i transport. Gene Tb11.02.3020, previously described to encode the parasite H⁺:myo-inositol transporter (TbHMIT), was hypothesized to be potentially involved in the H⁺:P_i cotransport because of its similarity with the Pho84 transporter described in S. cerevisiae and other trypanosomatids. Indeed, the RNAi mediated knockdown remarkably reduced TbHMIT gene expression, compromised cell growth and decreased P_i transport by half. In addition, P_i transport was inhibited when parasites were incubated in the presence of concentrations of myo-inositol that are above 300 μM. However, when expressed in Xenopus laevis oocytes, two-electrode voltage clamp experiments provided direct electrophysiological evidence that the protein encoded by TbHMIT is definitely a myo-inositol transporter that may be only marginally affected by the presence of P_i. These results confirmed the presence of a P_i carrier in T. brucei, similar to the H⁺-dependent inorganic phosphate system described in S. cerevisiae and other trypanosomatids. This transport system contributes to the acquisition of P_i and may be involved in the growth and survival of procyclic forms. In summary, this work presents the first description of a P_i transport system in T. brucei.

Sustained treatment of sickle cell mice with haptoglobin increases HO-1 and H-ferritin expression and decreases iron deposition in the kidney without improvement in kidney function

There is growing evidence that extracellular haemoglobin and haem mediate inflammatory and oxidative damage in sickle cell disease. Haptoglobin (Hp), the scavenger for free haemoglobin, is depleted in most patients with sickle cell disease due to chronic haemolysis. Although single infusions of Hp can ameliorate vaso-occlusion in mouse models of sickle cell disease, prior studies have not examined the therapeutic benefits of more chronic Hp dosing on sickle cell disease manifestations. In the present study, we explored the effect of Hp treatment over a 3-month period in sickle mice at two dosing regimens: the first at a moderate dose of 200 mg/kg thrice weekly and the second at a higher dose of 400 mg/kg thrice weekly. We found that only the higher dosing regimen resulted in increased haem-oxygenase-1 and heavy chain ferritin (H-ferritin) expression and decreased iron deposition in the kidney. Despite the decreased kidney iron deposition following Hp treatment, there was no significant improvement in kidney function. However, there was a nearly significant trend towards decreased liver infarction.

Adhesion GPCR Function in Pulmonary Development and Disease

Classic G-protein-coupled receptors (GPCRs) control multiple aspects of pulmonary physiology as demonstrated by loss-of-function experiments in mice and pharmacologic targeting of GPCRs for treatment of several pulmonary diseases. Emerging data demonstrate critical roles for members of the adhesion GPCR (aGPCR) family in pulmonary development, homeostasis, and disease. Although this field is still in its infancy, this chapter will review all available data regarding aGPCRs in pulmonary biology, with a particular focus on the aGPCR for which the most substantial data to date exist: Adgrf5.

CaMKII inhibition in type II pneumocytes protects from bleomycin-induced pulmonary fibrosis by preventing Ca2+-dependent apoptosis

The calcium and calmodulin-dependent kinase II (CaMKII) translates increases in intracellular Ca(2+) into downstream signaling events. Its function in pulmonary pathologies remains largely unknown. CaMKII is a well-known mediator of apoptosis and regulator of endoplasmic reticulum (ER) Ca(2+). ER stress and apoptosis of type II pneumocytes lead to aberrant tissue repair and progressive collagen deposition in pulmonary fibrosis. Thus we hypothesized that CaMKII inhibition alleviates fibrosis in response to bleomycin by attenuating apoptosis and ER stress of type II pneumocytes. We first established that CaMKII was strongly expressed in the distal respiratory epithelium, in particular in surfactant protein-C-positive type II pneumocytes, and activated after bleomycin instillation. We generated a novel transgenic model of inducible expression of the CaMKII inhibitor peptide AC3-I limited to type II pneumocytes (Tg SPC-AC3-I). Tg SPC-AC3-I mice were protected from development of pulmonary fibrosis after bleomycin exposure compared with wild-type mice. CaMKII inhibition also provided protection from apoptosis in type II pneumocytes in vitro and in vivo. Moreover, intracellular Ca(2+) levels and ER stress were increased by bleomycin and significantly blunted with CaMKII inhibition in vitro. These data demonstrate that CaMKII inhibition prevents type II pneumocyte apoptosis and development of pulmonary fibrosis in response to bleomycin. CaMKII inhibition may therefore be a promising approach to prevent or ameliorate the progression of pulmonary fibrosis.

A new role for an old drug: Ambroxol triggers lysosomal exocytosis via pH-dependent Ca²⁺ release from acidic Ca²⁺ stores

Ambroxol (Ax) is a frequently prescribed drug used to facilitate mucociliary clearance, but its mode of action is yet poorly understood. Here we show by X-ray spectroscopy that Ax accumulates in lamellar bodies (LBs), the surfactant storing, secretory lysosomes of type II pneumocytes. Using lyso- and acidotropic substances in combination with fluorescence imaging we confirm that these vesicles belong to the class of acidic Ca(2+) stores. Ax lead to a significant neutralization of LB pH, followed by intracellular Ca(2+) release, and to a dose-dependent surfactant exocytosis. Ax-induced Ca(2+) release was significantly reduced and slowed down by pretreatment of the cells with bafilomycin A1 (Baf A1), an inhibitor of the vesicular H(+) ATPase. These results could be nearly reproduced with NH3/NH4(+). The findings suggest that Ax accumulates within LBs and severely affects their H(+) and Ca(2+) homeostasis. This is further supported by an Ax-induced change of nanostructural assembly of surfactant layers. We conclude that Ax profoundly affects LBs presumably by disordering lipid bilayers and by acting as a weak base. The pH change triggers - at least in part - Ca(2+) release from stores and secretion of surfactant from type II cells. This novel mechanism of Ax as a lysosomal secretagogue may also play a role for its recently discussed use for lysosomal storage and other degenerative diseases.

Haptoglobin attenuates hemoglobin-induced heme oxygenase-1 in renal proximal tubule cells and kidneys of a mouse model of sickle cell disease

Sickle cell disease (SCD), a hereditary hemolytic disorder is characterized by chronic hemolysis, oxidative stress, vaso-occlusion and end-organ damage. Hemolysis releases toxic cell-free hemoglobin (Hb) into circulation. Under physiologic conditions, plasma Hb binds to haptoglobin (Hp) and forms Hb-Hp dimers. The dimers bind to CD163 receptors on macrophages for further internalization and degradation. However, in SCD patients plasma Hp is depleted and free Hb is cleared primarily by proximal tubules of kidneys. Excess free Hb in plasma predisposes patients to renal damage. We hypothesized that administration of exogenous Hp reduces Hb-mediated renal damage. To test this hypothesis, human renal proximal tubular cells (HK-2) were exposed to HbA (50μM heme) for 24h. HbA increased the expression of heme oxygenase-1 (HO-1), an enzyme which degrades heme, reduces heme-mediated oxidative toxicity, and confers cytoprotection. Similarly, infusion of HbA (32μM heme/kg) induced HO-1 expression in kidneys of SCD mice. Immunohistochemistry confirmed the increased HO-1 expression in the proximal tubules of the kidney. Exogenous Hp attenuated the HbA-induced HO-1 expression in vitro and in SCD mice. Our results suggest that Hb-mediated oxidative toxicity may contribute to renal damage in SCD and that Hp treatment reduces heme/iron toxicity in the kidneys following hemolysis.

MicroRNA-206 regulates surfactant secretion by targeting VAMP-2

Lung surfactant secretion is a highly regulated process. Our previous studies have shown that VAMP-2 is essential for surfactant secretion. In the present study we investigated the role of miR-206 in surfactant secretion through VAMP-2. VAMP-2 was confirmed to be a target of miR-206 by 3'-untranslational region (3'-UTR) luciferase assay. Mutations in the predicated miR-206 binding sites reduced the binding of miR-206 to the 3'-UTR of VAMP-2. miR-206 decreased the expression of VAMP-2 protein and decreased the lung surfactant secretion in alveolar type II cells. In conclusion, miR-206 regulates lung surfactant secretion by limiting the availability of VAMP-2 protein.

Characterization of V-ATPase inhibitor-induced secretion of cysteine-rich with EGF-like domains 2

We previously demonstrated that cysteine-rich with EGF-like domains 2 (CRELD2), a novel ER stress-inducible factor, is a secretory glycoprotein; however, the stimuli that induce CRELD2 secretion have not yet been characterized. In this study, we found that the perturbation of intravesicular acidification of cytoplasmic organelles in HEK293 cells stably expressing wild-type (wt) CRELD2 induced its secretion. In particular, Concanamycin A (CMA) and Bafilomycin A1 (Baf), inhibitors of vacuolar ATPase (V-ATPase), increased the secretion of CRELD2 without relying on its C-terminal structure. The levels of secretion of EGFP-fused CRELD2 (SP-EGFP-CRELD2), which consists of EGFP following the putative signal peptide (SP) sequence of CRELD2, from COS7 cells transiently transfected with this construct were also increased after each of the treatments, but their intracellular localization was barely affected by CMA treatment. Transient overexpression of 78-kDa glucose-regulated protein (GRP78) and protein disulfide isomerase (PDI) also increased the secretion of CRELD2 from HEK293 cells expressing wt CRELD2, whereas the perturbation of intravesicular acidification did not alter the expression of GRP78 and PDI in the HEK293 cells. We further studied the roles of intracellular calcium ions and the Golgi apparatus in the secretion of CRELD2 from HEK293 cells in which intravesicular acidification was perturbed. The treatment with calcium ionophore increased the secretion of wt CRELD2, while that with BAPTA-AM, an intracellular calcium chelator, did not reduce the CMA-induced CRELD2 secretion. By contrast, treatment with brefeldin A (BFA), which inhibits the transportation of proteins from the ER to the Golgi apparatus, almost completely abolished the secretion of wt CRELD2 from the HEK293 cells. In conclusion, we demonstrated that the intravesicular acidification by V-ATPase regulates the secretion of CRELD2 without relying on the balance of intracellular calcium ions and the expression of ER chaperones such as GRP78 and PDI. These findings concerning the role of V-ATPases in modulating the secretion of CRELD2, a novel ER stress-inducible secretory factor, may provide new insights into the prevention and treatment of certain ER stress-related diseases.

Long-chain acyl-CoA dehydrogenase deficiency as a cause of pulmonary surfactant dysfunction

Long-chain acyl-CoA dehydrogenase (LCAD) is a mitochondrial fatty acid oxidation enzyme whose expression in humans is low or absent in organs known to utilize fatty acids for energy such as heart, muscle, and liver. This study demonstrates localization of LCAD to human alveolar type II pneumocytes, which synthesize and secrete pulmonary surfactant. The physiological role of LCAD and the fatty acid oxidation pathway in lung was subsequently studied using LCAD knock-out mice. Lung fatty acid oxidation was reduced in LCAD(-/-) mice. LCAD(-/-) mice demonstrated reduced pulmonary compliance, but histological examination of lung tissue revealed no obvious signs of inflammation or pathology. The changes in lung mechanics were found to be due to pulmonary surfactant dysfunction. Large aggregate surfactant isolated from LCAD(-/-) mouse lavage fluid had significantly reduced phospholipid content as well as alterations in the acyl chain composition of phosphatidylcholine and phosphatidylglycerol. LCAD(-/-) surfactant demonstrated functional abnormalities when subjected to dynamic compression-expansion cycling on a constrained drop surfactometer. Serum albumin, which has been shown to degrade and inactivate pulmonary surfactant, was significantly increased in LCAD(-/-) lavage fluid, suggesting increased epithelial permeability. Finally, we identified two cases of sudden unexplained infant death where no lung LCAD antigen was detectable. Both infants were homozygous for an amino acid changing polymorphism (K333Q). These findings for the first time identify the fatty acid oxidation pathway and LCAD in particular as factors contributing to the pathophysiology of pulmonary disease.

Regulation of luminal acidification by the V-ATPase

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

Transport of inorganic phosphate in Leishmania infantum and compensatory regulation at low inorganic phosphate concentration

BACKGROUND

Proliferation of Leishmania infantum depends on exogenous inorganic phosphate (P_i) but little is known about energy metabolism and transport of P_i across the plasma membrane in Leishmania sp.

METHODS

We investigated the kinetics of ³²P_i transport, the influence of H⁺ and K⁺ ionophores and inhibitors, and expression of the genes for the Na⁺:P_i and H⁺:P_i cotransporters.

RESULTS

The proton ionophore FCCP, bafilomycin A₁ (vacuolar ATPase inhibitor), nigericin (K⁺ ionophore) and SCH28080 (an inhibitor of H⁺, K⁺-ATPase) all inhibited the transport of P_i. This transport showed Michaelis-Menten kinetics with K_0.5 and V_max values of 0.016±0.002mM and 564.9±18.06pmol×h^-1×10^-7cells, respectively. These values classify the P_i transporter of L. infantum among the high-affinity transporters, a group that includes Pho84 of Saccharomyces cerevisiae. Two sequences were identified in the L. infantum genome that code for phosphate transporters. However, transcription of the PHO84 transporter was 10-fold higher than the PHO89 transporter in this parasite. Accordingly, P_i transport and LiPho84 gene expression were modulated by environmental P_i variations.

CONCLUSIONS

These findings confirm the presence of a P_i transporter in L. infantum, similar to PHO84 in S. cerevisiae, that contributes to the acquisition of inorganic phosphate and could be involved in growth and survival of the promastigote forms of L. infantum.

GENERAL SIGNIFICANCE

This work provides the first description of a PHO84-like P_i transporter in a Trypanosomatide parasite of the genus Leishmania, responsible for many infections worldwide.

Surfactant phospholipid metabolism

Pulmonary surfactant is essential for life and is composed of a complex lipoprotein-like mixture that lines the inner surface of the lung to prevent alveolar collapse at the end of expiration. The molecular composition of surfactant depends on highly integrated and regulated processes involving its biosynthesis, remodeling, degradation, and intracellular trafficking. Despite its multicomponent composition, the study of surfactant phospholipid metabolism has focused on two predominant components, disaturated phosphatidylcholine that confers surface-tension lowering activities, and phosphatidylglycerol, recently implicated in innate immune defense. Future studies providing a better understanding of the molecular control and physiological relevance of minor surfactant lipid components are needed. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Combined atomic force microscopy-fluorescence microscopy: analyzing exocytosis in alveolar type II cells

Hybrid atomic force microscopy (AFM)-fluorescence microscopy (FM) investigation of exocytosis in lung epithelial cells (ATII cells) allows the detection of individual exocytic events by FM, which can be simultaneously correlated to structural changes in individual cells by AFM. Exocytosis of lamellar bodies (LBs) represents a slow form of exocytosis found in many non-neuronal cells. Exocytosis of LBs, following stimulation with adenosine-5'-triphosphate (ATP) and phorbol 12-myristate 13-acetate (PMA), results in a cation influx via P2X(4) receptors at the site of LB fusion with the plasma membrane (PM), which should induce a temporary increase in cell height/volume. AFM measurements were performed in single-line scans across the cell surface. Five minutes after stimulation, ATII cells revealed a cell height and volume increase of 13.7% ± 4.1% and 15.9 ± 4.8% (N = 9), respectively. These transient changes depend on exocytic LB-PM fusion. Nonstimulated cells and cells lacking LB fusions did not show a significant change in cell height/volume (N = 8). In addition, a cell height decrease was observed in ATII cells stimulated by uridine-5'-triphosphate (UTP) and PMA, agonists inducing LB fusion with the PM, but not activation of P2X(4) receptors. The cell height and volume decreased by -8.6 ± 3.6% and -11.2 ± 3.9% (N = 5), respectively. Additionally, low force contact and dynamic mode AFM imaging of cell areas around the nucleus after stimulation with ATP/PMA was performed. Fused LBs are more pronounced in AFM topography images compared to nonfused LBs, concluding that different "dynamic states" of LBs or locations from the PM are captured during imaging.

Spatio-temporal aspects, pathways and actions of Ca(2+) in surfactant secreting pulmonary alveolar type II pneumocytes

The type II cell of the pulmonary alveolus is a polarized epithelial cell that secretes surfactant into the alveolar space by regulated exocytosis of lamellar bodies (LBs). This process consists of multiple sequential steps and is correlated to elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) required for extended periods of secretory activity. Both chemical (purinergic) and mechanical (cell stretch or exposure to an air-liquid interface) stimuli give rise to complex Ca(2+) signals (such as Ca(2+) peaks, spikes and plateaus) that differ in shape, origin and spatio-temporal behavior. This review summarizes current knowledge about Ca(2+) channels, including vesicular P2X4 purinoceptors, in type II cells and associated signaling cascades within the alveolar microenvironment, and relates stimulus-dependent activation of these pathways with distinct stages of surfactant secretion, including pre- and postfusion stages of LB exocytosis.

Targeting reversible disassembly as a mechanism of controlling V-ATPase activity

Vacuolar proton-translocating ATPases (V-ATPases) are highly conserved proton pumps consisting of a peripheral membrane subcomplex called V1, which contains the sites of ATP hydrolysis, attached to an integral membrane subcomplex called Vo, which encompasses the proton pore. V-ATPase regulation by reversible dissociation, characterized by release of assembled V1 sectors into the cytosol and inhibition of both ATPase and proton transport activities, was first identified in tobacco hornworm and yeast. It has since become clear that modulation of V-ATPase assembly level is also a regulatory mechanism in mammalian cells. In this review, the implications of reversible disassembly for V-ATPase structure are discussed, along with insights into underlying subunit-subunit interactions provided by recent structural work. Although initial experiments focused on glucose deprivation as a trigger for disassembly, it is now clear that V-ATPase assembly can be regulated by other extracellular conditions. Consistent with a complex, integrated response to extracellular signals, a number of different regulatory proteins, including RAVE/rabconnectin, aldolase and other glycolytic enzymes, and protein kinase A have been suggested to control V-ATPase assembly and disassembly. It is likely that multiple signaling pathways dictate the ultimate level of assembly and activity. Tissue-specific V-ATPase inhibition is a potential therapy for osteoporosis and cancer; the possibility of exploiting reversible disassembly in design of novel V-ATPase inhibitors is discussed.

Fusion-activated Ca(2+) entry: an "active zone" of elevated Ca(2+) during the postfusion stage of lamellar body exocytosis in rat type II pneumocytes

Background

Ca²⁺ is essential for vesicle fusion with the plasma membrane in virtually all types of regulated exocytoses. However, in contrast to the well-known effects of a high cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c) in the prefusion phase, the occurrence and significance of Ca²⁺ signals in the postfusion phase have not been described before.

Methodology/Principal Findings

We studied isolated rat alveolar type II cells using previously developed imaging techniques. These cells release pulmonary surfactant, a complex of lipids and proteins, from secretory vesicles (lamellar bodies) in an exceptionally slow, Ca²⁺- and actin-dependent process. Measurements of fusion pore formation by darkfield scattered light intensity decrease or FM 1-43 fluorescence intensity increase were combined with analysis of [Ca²⁺]_c by ratiometric Fura-2 or Fluo-4 fluorescence measurements. We found that the majority of single lamellar body fusion events were followed by a transient (t_1/2 of decay = 3.2 s) rise of localized [Ca²⁺]_c originating at the site of lamellar body fusion. [Ca²⁺]_c increase followed with a delay of ∼0.2–0.5 s (method-dependent) and in the majority of cases this signal propagated throughout the cell (at ∼10 µm/s). Removal of Ca²⁺ from, or addition of Ni²⁺ to the extracellular solution, strongly inhibited these [Ca²⁺]_c transients, whereas Ca²⁺ store depletion with thapsigargin had no effect. Actin-GFP fluorescence around fused LBs increased several seconds after the rise of [Ca²⁺]_c. Both effects were reduced by the non-specific Ca²⁺ channel blocker SKF96365.

Conclusions/Significance

Fusion-activated Ca²⁺entry (FACE) is a new mechanism that leads to [Ca²⁺]_c transients at the site of vesicle fusion. Substantial evidence from this and previous studies indicates that fusion-activated Ca²⁺ entry enhances localized surfactant release from type II cells, but it may also play a role for compensatory endocytosis and other cellular functions.