Roles of the Rlim-Rex1 axis during X chromosome inactivation in mice

In female mice, the gene dosage from X chromosomes is adjusted by a process called X chromosome inactivation (XCI) that occurs in two steps. An imprinted form of XCI (iXCI) that silences the paternally inherited X chromosome (Xp) is initiated at the 2- to 4-cell stages. As extraembryonic cells including trophoblasts keep the Xp silenced, epiblast cells that give rise to the embryo proper reactivate the Xp and undergo a random form of XCI (rXCI) around implantation. Both iXCI and rXCI require the lncRNA Xist, which is expressed from the X to be inactivated. The X-linked E3 ubiquitin ligase Rlim (Rnf12) in conjunction with its target protein Rex1 (Zfp42), a critical repressor of Xist, have emerged as major regulators of iXCI. However, their roles in rXCI remain controversial. Investigating early mouse development, we show that the Rlim-Rex1 axis is active in pre-implantation embryos. Upon implantation Rex1 levels are downregulated independently of Rlim specifically in epiblast cells. These results provide a conceptual framework of how the functional dynamics between Rlim and Rex1 ensures regulation of iXCI but not rXCI in female mice.

UNDERSTANDING THE VITAL DETERMINANTS SHAPING LEARNERS' PHYSICAL ACTIVITYAND PSYCHOEMOTIONAL WELLBEING IN THE COVID-19 PERIOD

The Corona Virus (COV-19) epidemic significantly affected the educational environment, requiring a quick transition to distance and blended learning methods. This extraordinary disruption had an incredible impact on pupil's levels of physical activity (PA), psycho-emotional health (PEH) and engagement with academic material. The research aims to examine the vital determinants that influenced various areas of learners' lives during CoV-19. The purpose of this 600-person study was to collect data on the subjects' overall health and PA levels for the CoV-19 pandemic. The SPSS application was used to process the questionnaire's collected data. The information given reveals the respondents' degree of PA throughout the quarantine. According to the breakdown, 15% indicated low levels of PA, 39% reported medium levels and 46% reported high levels. The data show that, despite the respondents' different levels of PA, little PA predominated for most of them. The limitations of distance learning throughout quarantine and the prevalent recommendation of leaving residence for necessary reasons were blamed for this tendency. There were fewer prospects for higher-intensity PA due to these circumstances.

THE CORNEAL ENDOTHELIUM IN OCULAR SURFACE DISEASE AND GLAUCOMA: MECHANISMS OF DYSFUNCTION AND TREATMENT STRATEGIES

To determine risk factors and the overall incidence of ocular surface disorders in a cohort of long-term glaucoma patients. Utilizing simple clinical tools, cross-sectional observational research were constructed to evaluate ocular surface problems and indicators. Using a four-grade scale, ten queries regarding symptoms and indications on the cornea's surface were used to create an OSD severity score. The patients were divided into three groups: A, B, and C, depending on the result. The variables that increase the incidence of surface sickness were identified using a multinomial logistic regression. Five hundred and twenty patients made up the total population. According to the multivariate analysis, the patient's age, the number of daily eyedrops, any previous changes in topical treatment for ocular intolerance, intraocular pressure, and degree of glaucoma were all connected with the severity of ocular surface illness. Ocular surface disorders are frequently developed by patients getting treatment for primary open-angle glaucoma or ocular hypotension. which are less prevalent and serious in geriatric patients because their use greater drugs and have greater advanced glaucoma.

hnRNPL expression dynamics in the embryo and placenta

Heterogeneous nuclear ribonucleoprotein L (hnRNPL) is a conserved RNA binding protein (RBP) that plays an important role in the alternative splicing of gene transcripts, and thus in the generation of specific protein isoforms. Global deficiency in hnRNPL in mice results in preimplantation embryonic lethality at embryonic day (E) 3.5. To begin to understand the contribution of hnRNPL-regulated pathways in the normal development of the embryo and placenta, we determined hnRNPL expression profile and subcellular localization throughout development. Proteome and Western blot analyses were employed to determine hnRNPL abundance between E3.5 and E17.5. Histological analyses supported that the embryo and implantation site display distinct hnRNPL localization patterns. In the fully developed mouse placenta, nuclear hnRNPL was observed broadly in trophoblasts, whereas within the implantation site a discrete subset of cells showed hnRNPL outside the nucleus. In the first-trimester human placenta, hnRNPL was detected in the undifferentiated cytotrophoblasts, suggesting a role for this factor in trophoblast progenitors. Parallel in vitro studies utilizing Htr8 and Jeg3 cell lines confirmed expression of hnRNPL in cellular models of human trophoblasts. These studies coordinated regulation of hnRNPL during the normal developmental program in the mammalian embryo and placenta.

The permissive effects of glucose on receptor-operated potentiation of insulin secretion from mouse islets: a role for ERK1/2 activation and cytoskeletal remodelling

AIMS/HYPOTHESIS

Glucose plays two distinct roles in regulating insulin secretion from beta cells--an initiatory role, and a permissive role enabling receptor-operated secretagogues to potentiate glucose-induced insulin secretion. The molecular mechanisms underlying the permissive effects of glucose on receptor-operated insulin secretion remain uncertain. We have investigated the role of extracellular signal-regulated kinase 1/2 (ERK1/2) activation and consequent cytoskeletal remodelling in this process.

METHODS

Insulin release was measured from groups of isolated mouse islets using static incubation experiments and subsequent radioimmunoassay of samples. ERK1/2 activation was measured by western blotting of islet protein samples for both phosphorylated and total ERK1/2. Rhodamine-phalloidin staining was used to measure filamentous actin in dispersed primary beta cells.

RESULTS

Inhibition of ERK1/2 blocked potentiation of glucose-induced insulin release by the receptor-operated secretagogues kisspeptin, A568, exendin-4 and JWH015, although the agonists alone had minimal effects on ERK1/2 activation, suggesting a permissive rather than causal role for ERK1/2 activation in receptor-operated insulin release. Following pharmacological activation of ERK1/2 all agonists caused a significant increase in insulin release from islets incubated with sub-stimulatory levels of glucose. ERK1/2 inhibition significantly reduced the glucose-dependent decreases in filamentous actin observed in primary beta cells, while pharmacological dissociation of actin filaments enabled all receptor-operated secretagogues tested to significantly stimulate insulin release from islets at a sub-stimulatory glucose concentration.

CONCLUSIONS/INTERPRETATION

Glucose-induced ERK1/2 activation in beta cells mediates the permissive effects of stimulatory glucose concentrations on receptor-operated insulin secretagogues, at least in part through effects on actin depolymerisation and cytoskeletal remodelling.

“Hydrophobic” Surfactant Apoproteins and Augmentation of Phospholipid Recycling

A 10,000-11,000 molecular weight apoprotein was isolated from an ethanol-ether extract of rat lung surfactant and purified by silicic acid chromatography. The protein (Apo Et) significantly augmented the uptake of phospholipids in liposomal form by cultured rat granular pneumocytes by a time-dependent process that varied with protein concentration and liposome composition. The protein had no effect on cell viability and showed no phospholipase activity. The mechanism for this augmented phospholipid uptake is not known but could be due to an alteration of physical form of the phospholipids by the protein or to a receptor-mediated uptake of phospholipids. This protein may prove to be a physiologically important regulator of the recycling of lung surfactant phospholipids.

Mitochondrial localization of catalase provides optimal protection from H2O2-induced cell death in lung epithelial cells

Reactive oxygen species (ROS) can cause cell injury and death via mitochondrial-dependent pathways, and supplementation with antioxidants has been shown to ameliorate these processes. The c-Jun NH2-terminal kinase (JNK) pathway has been shown to play a critical role in ROS-induced cell death. To determine if targeting catalase (CAT) to the mitochondria provides better protection than cytosolic expression against H2O2-induced injury, the following two approaches were taken: 1) adenoviral-mediated transduction was performed using cytosolic (CCAT) or mitochondrial (MCAT) CAT cDNAs and 2) stable cell lines were generated overexpressing CAT in mitochondria ( n = 3). Cells were exposed to 250 μM H2O2, and cell survival, mitochondrial function, cytochrome c release, and JNK activity were analyzed. Although all viral transduced cells had a transient twofold increase in CAT activity, MCAT cells had significantly higher survival rates, the best mitochondrial function, and lowest JNK activity compared with CCAT and LacZ controls. The improved protection with MCAT was observed in primary type II lung epithelial cells and in transformed lung epithelial cells. In the three stable cell lines, cell survival directly correlated with extent of mitochondrial localization ( r = 0.60572, P < 0.05) and not overall CAT activity ( r = −0.45501, P < 0.05). Data indicate that targeting of antioxidants directly to the mitochondria is more effective in protecting lung epithelial cells against ROS-induced injury. This has important implications in antioxidant supplementation trials to prevent ROS-induced lung injury in critically ill patients.

Diminished Lung Compliance and Elevated Surfactant Lipids and Proteins in Nutritionally Obese Young Rats

Dietary-induced obesity is associated with increases in lung weight, lung volume, alveolar surface area, and number of lamellar bodies in alveolar epithelial type II cells. This suggests that alterations in lung compliance and surfactant content may also occur. The effects of dietary-induced obesity on lung function and surfactant composition were studied in newborn male rats raised in (1) small litters until weaning and then fed a high fat diet (Obese Group, n = 23) and (2) normal-sized litters until weaning and subsequently fed a normal rat diet (Control Group, n = 29). At age 8 weeks, lung function was measured in anesthetized, spontaneously breathing rats, and surfactant composition was analyzed in lung tissue and lavage fluid. The 8-week-old obese rats had a higher body weight (31%) and fat pad weight/body weight ratio (224%) than the Control Group. When compared with control animals, obese rats had an increased respiratory rate, reduced tidal volume, and decreased lung compliance (dynamic and specific). Disaturated phosphatidylcholine (DSPC) in lung tissue and surfactant pellets (large aggregates) and SP-A and SP-B levels in large aggregates were higher in obese than control rats. Phospholipid, DSPC, and triglyceride contents were also elevated in lung tissue in obese rats, suggesting intracellular lipid accumulation, but low relative to alveolar surface area. Thus, alterations in lung function and surfactant lipids and proteins occur in dietary-induced obesity in young rats. We speculate that intrapulmonary lipid deposition and possible surfactant deficiency relative to alveolar surface area may contribute to the reduction in lung compliance in obese rats.

Hyperoxia and nitric oxide reduce surfactant components (DSPC and surfactant proteins) and increase apoptosis in adult and fetal rat type II pneumocytes

Nitric oxide (NO) alone or in conjunction with hyperoxia can have protective or detrimental effects on the lung. Our hypothesis was that hyperoxia in conjunction with NO would result in increased cellular dysfunction and apoptotic cell death in adult and fetal Type II pneumocytes (TIIP) in a dose-dependent manner. The TIIP were obtained from adult and 19-day fetal rat lungs. The TIIP were then exposed to 100, 200 and 500 micro M of the NO-donor, Glyco-SNAP-2, alone or in conjunction with 95% oxygen for 24 h. While low-dose NO exposure alone did not increase cytotoxicity, in conjunction with hyperoxia, there was a significant dose-dependent increase in apoptotic cell death of adult TIIP as well as fetal TIIP. Choline incorporation into disaturated phosphatidylcholine was markedly decreased in adult TIIP while the fetal TIIP had similar values as controls. However, the mRNAs of surfactant proteins A, B and C as well as iNOS were significantly reduced in fetal TIIP. Exogenous peroxynitrite also increased nitrotyrosine formation in fetal TIIP as did hyperoxia and NO. The effect of hyperoxia and NO could be abrogated with catalase and superoxide dismutase. These findings may have significant clinical implications in the use of NO in premature infants.

Synexin and GTP increase surfactant secretion in permeabilized alveolar type II cells

We have previously suggested that synexin (annexin VII), a Ca(2+)-dependent phospholipid binding protein, may have a role in surfactant secretion, since it promotes membrane fusion between isolated lamellar bodies (the surfactant-containing organelles) and plasma membranes. In this study, we investigated whether exogenous synexin can augment surfactant phosphatidylcholine (PC) secretion in synexin-deficient lung epithelial type II cells. Isolated rat type II cells were cultured for 20-22 h with [(3)H]choline to label cellular PC. The cells were then treated with beta-escin, which forms pores in the cell membrane and releases cytoplasmic proteins including synexin. These cells, however, retained lamellar bodies. The permeabilized type II cells were evaluated for PC secretion during a 30-min incubation. Compared with PC secretion under basal conditions, the presence of Ca(2+) (up to 10 microM) did not increase PC secretion. In the presence of 1 microM Ca(2+), synexin increased PC secretion in a concentration-dependent manner, which reached a maximum at approximately 5 microg/ml synexin. The secretagogue effect of synexin was abolished when synexin was inactivated by heat treatment (30 min at 65 degrees C) or by treatment with synexin antibodies. GTP or its nonhydrolyzable analog beta:gamma-imidoguanosine-5'-triphosphate also increased PC secretion in permeabilized type II cells. The PC secretion was further increased in an additive manner when a maximally effective concentration of synexin was added in the presence of 1 mM GTP, suggesting that GTP acts by a synexin-independent mechanism to increase membrane fusion. Thus our results support a direct role for synexin in surfactant secretion. Our study also suggests that membrane fusion during surfactant secretion may be mediated by two independent mechanisms.

H+-and K+-dependence of Ca2+ uptake in lung lamellar bodies

Lung lamellar bodies maintain an acidic interior by an energy-dependent process. The acidic pH may affect the packaging of surfactant phospholipids, processing of surfactant proteins, or surfactant protein A-dependent lipid aggregation. The electron-probe microanalysis of lamellar body elemental composition has previously suggested that lamellar bodies contain high levels of calcium some of which may be in ionic form. In this study, we investigated the Ca2+ uptake characteristics in isolated lung lamellar bodies. The uptake of Ca2+ was measured by monitoring changes in the fluorescence of Fluo-3, a Ca2+ indicator dye. The uptake of Ca2+ in lamellar bodies was ATP-dependent and increased with increasing concentrations of Ca2+. At 100 nM Ca2+, the uptake was almost completely inhibited by bafilomycin A1, a selective inhibitor of vacuolar type H+-ATPase, or by NH4Cl, which raises the lamellar body pH, suggesting that the pH gradient regulates the uptake. The uptake of Ca2+ increased as the Ca2+ concentration was increased, but the relative contribution of bafilomycin A1-sensitive uptake decreased. At 700 nM, it comprised only 20% of the total uptake. These results suggest the presence of additional mechanism(s) for uptake at higher Ca2+ concentrations. At 700 nm Ca2+, the rate and extent of uptake were lower in the absence of K+ than in the presence of K+. The inhibitors of Ca2+-activated K+-channels, tetraethylammonium, Penitrem A, and 4-aminopyridine, also inhibited the K+-dependent Ca2+ uptake at 700 nM Ca2+. Thus the uptake of Ca2+ in isolated lung lamellar bodies appears to be regulated by two mechanisms, (i) the H+-gradient and (ii) the K+ transport across the lamellar body membrane. We speculate that lamellar bodies accumulate Ca2+ and contribute to regulation of cytosolic Ca2+ in type II cells under resting and stimulated conditions.

Coordinate packaging of newly synthesized phosphatidylcholine and phosphatidylglycerol in lamellar bodies in alveolar type II cells

Methylamine, a weak base, inhibits packaging of newly synthesized phosphatidylcholine (PC) in lamellar bodies in 20-22 h cultured alveolar type II cells, suggesting a role for acidic pH of lamellar bodies. In this study, we tested if (i) the packaging of PC is similarly regulated in freshly isolated type II cells and (ii) methylamine also inhibits the packaging of other surfactant phospholipids, particularly, phosphatidylglycerol (PG). The latter would suggest coordinated packaging so as to maintain the phospholipid composition of lung surfactant. During the short-term metabolic labeling experiments in freshly isolated type II cells, methylamine treatment decreased the incorporation of radioactive precursors into PC, disaturated PC (DSPC), and PG of lamellar bodies but not of the microsomes, when compared with controls. The calculated packaging (the percentage of microsomal lipid packaged in lamellar bodies) of each phospholipid was similarly decreased (approximately 50%) in methylamine-treated cells, suggesting coordinated packaging of surfactant phospholipids in lamellar bodies. Equilibrium-labeling studies with freshly isolated type II cells (as is routinely done for studies on surfactant secretion) +/- methylamine showed that in methylamine-treated cells, the secretion of PC and PG was decreased (possibly due to decreased packaging), but the phospholipid composition of released surfactant (measured by radioactivity distribution) was unchanged; and the PC content (measured by mass or radioactivity) of lamellar bodies was lower, but the PC composition (as percentage of total phospholipids) was unchanged when compared with control cells. We speculate that the newly synthesized surfactant phospholipids, PC, DSPC, and PG, are coordinately transported into lamellar bodies by a mechanism requiring the acidic pH, presumably, of lamellar bodies.

Antenatal triiodothyronine improves neonatal pulmonary function in preterm lambs

OBJECTIVE

To characterize 1) pulmonary gas exchange, 2) pulmonary function, 3) lung fluid and tissue phospholipid content, and 4) thyroid hormone in the premature lamb (0.85 of term) after intra-amniotic administration of 100 micrograms of triiodothyronine (T3) 2 weeks before delivery.

METHODS

Nine fetal lambs were given 100 micrograms of intra-amniotic T3 under ultrasound guidance at 112 +/- 1 days' gestation and delivered at 126 +/- 1 days (term = 149 days). Five saline-injected animals served as controls. Arterial blood gases, pulmonary mechanics, and lung volumes were compared between groups for 1 hour after delivery. At delivery, tracheal fluid and blood was taken for T3, and thyroxine (T4) levels. Tracheal fluid and lung tissues were assayed for total phosphorus and disaturated phosphatidylcholine.

RESULTS

Triiodothyronine-treated lambs had significantly higher mean arterial pH and lower PCO2 than controls (P < .05) with a trend toward higher mean PO2. The dynamic lung compliance was increased by 54% with a 40% proportional increase in tidal volume and minute ventilation in the T3-treated group (P < .05). Functional residual capacity increased 69% (P < .05) without a change in specific compliance. The tracheal fluid and pulmonary phospholipids and tracheal fluid and plasma T3 and T4 levels were not different between the two groups.

CONCLUSION

A single 100 micrograms dose of antenatal T3 significantly improves neonatal gas exchange and lung compliance. The improvement in lung function was not accompanied by an increase in pulmonary surfactant production. It is inferred that T3 improved lung function via accelerated structural development of the lung with an alternative possible effect on parenchymal connective tissue matrix.

Exudative lung injury is associated with decreased levels of surfactant proteins in a rat model of meconium aspiration

OBJECTIVE

Meconium aspiration syndrome remains a common cause of respiratory failure in neonates. The acute effects of meconium aspiration are inactivation of lung surfactant in vivo and in vitro. This study investigated the delayed effects of meconium on alveolar surfactant phospholipids and protein levels in spontaneously breathing animals.

METHODS

Twenty-two adult rats were given 4.3 mg of dry weight human meconium after endotracheal intubation. Rats were briefly mechanically ventilated in room air, extubated, then killed after 16 (n = 6), 24 (n = 6), 48 (n = 6), and 72 hours (n = 4). Control animals received the same volume of normal saline (n = 7) or no meconium (n = 7). Bronchoalveolar lavage and tissue specimens were evaluated for inflammatory cells, total proteins, surfactant phospholipids, and surfactant proteins.

RESULTS

Meconium caused exudative lung injury that was reflected in increased cell counts and proteins in alveolar lavage fluid. The peak injury occurred at 16 hours after instillation, whereas recovery occurred by 72 hours. Although total lavage fluid phospholipids did not change over time, phospholipid and dipalmitoyl phosphatidylcholine in large aggregates tended to decrease at 24 hours. Western blot analysis demonstrated time-dependent qualitative decreases in surfactant proteins A and B (SP-A, SP-B) in meconium-instilled animals compared with the controls. ELISA for SP-B confirmed the Western blot findings with total SP-B in large aggregate decreasing from 25 +/- 4 microg in controls to 6.6 +/- 0.8 microg at 24 hours of injury.

CONCLUSIONS

Our study suggests that the exudative lung injury with meconium instillation is associated with decreased levels of SP-A and SP-B in the large aggregate fraction of lung surfactant. We speculate that decreased secretion and/or increased degradation accounts for lower levels of SP-B in bronchoalveolar lavage fluid.

Ionic regulation of proton chemical (pH) and electrical gradients in lung lamellar bodies

This study investigated the pH (chemical) and electrical gradients in lamellar bodies, the acidic surfactant-secreting organelles of lung epithelial type II cells, by following the uptake of a weak fluorescent base, quinacrine, and a membrane potential-sensitive dye, bis-(3-phenyl-5-oxoisoxazol-4-yl)pentamethine oxonol (oxonol V). In isolated lung lamellar bodies, the ATP-dependent uptake of both agents could be inhibited by bafilomycin A1, a reportedly specific inhibitor of vacuolar-type H(+)-ATPase (V-ATPase) and could be dissipated by a protonophore, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, suggesting that the V-ATPase generates an electropositive interior. A closely linked uptake of Cl- neutralizes the positive electrical potential and increases the proton pump activity. The uptake of quinacrine, but not oxonol V, was decreased by Na+. This effect of Na+ could be prevented by dimethylamiloride, suggesting the presence of electroneutral Na+/H+ exchanger in lamellar body membranes. The initial rates of quinacrine and oxonol V uptake were increased by bumetanide, but only in the presence of Na+, K+, and Cl-, suggesting that the lamellar bodies also contain an outwardly directed electroneutral Na(+)-K(+)-2Cl- cotransporter. Thus three ion transporters, H(+)-translocating V-ATPase, Na+/H+ exchanger, and Na(+)-K(+)-2Cl- cotransporter, appear to determine the chemical and electrical gradients across the lamellar body membrane.

Activation of protein kinase C by 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-py rid ine carboxylic acid methyl ester (Bay K 8644), a calcium channel agonist, in alveolar type II cells

A role for calcium channels in the regulation of surfactant secretion is suggested by the observation that endothelin-1-stimulated surfactant secretion is inhibited by calcium channel blockers. 1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridi ne carboxylic acid methyl ester (Bay K 8644), a dihydropyridine derivative, stimulates voltage-dependent and non-voltage-dependent calcium channels in a number of cell types. This study demonstrates that Bay K 8644 increased phosphatidylcholine (PC) secretion in isolated lung epithelial type II cells in a time- and concentration-dependent manner with an EC50 of 100 +/- 8 nM (mean +/- SEM, N = 6). The secretagogue effect of Bay K 8644 was independently decreased in the absence of external calcium, or in the presence of nifedipine, a calcium channel antagonist, or inhibitors of protein kinase C (PKC). Bay K 8644 increased intracellular calcium from 130 +/- 8 to 230 +/- 14 nM (N = 6, P < 0.05), an effect that was blocked by nifedipine. Bay K 8644 also increased the membrane-associated PKC activity in a concentration-dependent manner. In the membranes from Bay K 8644-stimulated cells, the increase in calcium-dependent PKC was greater than that in the calcium-independent PKC, suggesting preferential translocation of calcium-dependent PKC to the membranes. We suggest that both elevated calcium and activation of PKC are required for calcium agonist Bay K 8644-induced surfactant secretion in type II cells.

Calcium-dependence of synexin binding may determine aggregation and fusion of lamellar bodies

Synexin (annexin VII) is a member of the annexin family of calcium and phospholipid binding proteins that promote calcium-dependent aggregation and fusion of lipid vesicles or secretory granules. We have previously suggested that synexin may be involved in membrane fusion processes during exocytosis of lung surfactant since it promotes fusion in vitro of lamellar bodies with plasma membranes. In this study, we characterized calcium-dependency of synexin binding to lamellar bodies and plasma membranes, since such binding is the initial, and, therefore, may be the rate-limiting step in membrane aggregation and fusion. The binding of biotinylated synexin to lamellar bodies and plasma membranes increased in a calcium-dependent manner reaching a maximum at approx. 200 microM Ca2+. Binding to lamellar bodies was completely inhibited by unlabelled synexin. Gel-overlay analysis showed that synexin bound to an approx. 76 kDa protein in the lamellar body and plasma membrane fractions. The calcium kinetics were noticeably similar for synexin binding to lamellar bodies and plasma membranes, aggregation of lamellar bodies, and fusion of lamellar bodies with lipid vesicles. At low calcium concentrations, aggregation of lamellar bodies could be increased with increasing synexin concentration, and arachidonic acid increased all three parameters (binding, aggregation, and fusion) in a similar manner. The effects of calcium and arachidonic acid on these three parameters suggest that synexin binding to lamellar bodies may be a rate-determining step for fusion during surfactant secretion. Furthermore, at near physiological calcium levels, the membrane fusion may be enhanced by elevated concentrations of synexin and polyunsaturated fatty acids.

Methylamine decreases trafficking and packaging of newly synthesized phosphatidylcholine in lamellar bodies in alveolar type II cells

Lung lamellar bodies, the storage organelles for lung surfactant phosphatidylcholine (PC), maintain an acidic pH that can be increased with weak bases. This study investigates the effect of a weak base, methylamine, on the pH in lamellar bodies and on the trafficking and packaging of newly synthesized PC in lamellar bodies. Methylamine increased the pH of isolated lung lamellar bodies and of lamellar bodies in intact cells. Metabolic labelling of isolated type II cells with [methyl-3H]choline showed that although methylamine (2.5-10 mM) did not alter the labelling of cellular or microsomal PC and disaturated PC, it decreased the labelling of the PC and disaturated PC in lamellar bodies. The packaging of PC in lamellar bodies (the specific activities ratio between the PC in lamellar bodies and the microsomal PC) also decreased in a time- and concentration-dependent manner. The cellular synthesis of PC or its packaging into lamellar bodies was unaltered by brefeldin A, suggesting that the Golgi was not involved in PC packaging. Although methylamine also increased surfactant secretion, the inhibition of PC packaging in lamellar bodies seems unrelated to the secretagogue effect, (1) on the basis of metabolic consequences of increased secretion and (2) because ATP, another secretagogue, did not inhibit PC packaging. Methylamine seems to inhibit PC packaging by inhibiting trafficking of PC to lipid-rich light subcellular fractions. Together our results suggest that the trafficking of surfactant PC into lamellar bodies might be sensitive to changes in the pH of lamellar bodies.

Receptor-mediated regulation of pulmonary surfactant secretion

Surfactant protein A (SP-A) regulates surfactant secretion via an SP-A specific type II cell membrane receptor (SPAR). We report here that two anti-SPAR monoclonal antibodies can modulate the secretory inhibition caused by SP-A. A2C and A2R are rat monoclonal antibodies raised independently and recognize a 32-kDa protein on rat alveolar type II cell membranes. Immunocytochemical studies show that these antibodies bind to isolated type II cells. Scatchard analysis confirms that SP-A binds alveolar type II cells through a single affinity receptor and shows that A2C and A2R recognize that same receptor. Both antibodies inhibit the binding of 125I-SP-A to isolated type II cells. The functional activity of this 32-kDa protein was studied by examining surfactant secretion in isolated type II cells. Surfactant phospholipid secretion was measured in cells that were exposed to various surfactant phospholipid secretagogues (ATP, dibutyryl cAMP, terbutaline, or ionomycin), +/-SP-A (100 ng/ml), +/-A2C or A2R. Both antibodies block the negative feedback loop by which SP-A inhibits surfactant secretion. This activity of A2C and A2R is dose-dependent and is independent of the secretagogue used. Thus, the 32-kDa type II cell membrane protein bound by A2C and A2R is the functional receptor on alveolar type II cell membranes and regulates type II cell surfactant secretion.

Protein kinase C in intracellular pH regulation in alveolar type II cells

The Na+/H+ exchanger and Na(+)-HCO3- cotransporter have been implicated in regulation of intracellular pH (pHi) in alveolar type II cells. This study demonstrates that activation of protein kinase C (PKC) stimulates both of these ion transporters in type II cells. Treatment of type II cells with 80 nM phorbol 12-myristate 13-acetate (PMA) increased the resting pHi in a time-dependent manner. Compared with control cells, the rates of recovery from an acid load increased with PMA treatment, reaching a maximum at 15 min, and returned to control levels by 3 h. The PMA-stimulated changes in recovery rate were sensitive to H-7, a PKC inhibitor. For PMA treatment up to 2 h, these recoveries were also sensitive to dimethylamiloride (DMA), an inhibitor of Na+/H+ exchanger activity, and to HCO3-, suggesting activation of both the Na+/H+ exchanger and the Na(+)-HCO3- cotransporter. After prolonged (3 h) treatment with PMA, however, the recovery was insensitive to DMA but was sensitive to HCO3-, suggesting that the Na+/H+ exchanger was no longer active and that most of the recovery was mediated by the Na(+)-HCO3- cotransporter. PMA treatment also altered the Na+ kinetics of the recovery from an acid load with respect to the Michaelis constant (Km) and maximal ion flux (Vmax), suggesting protein modifications of each transporter. We suggest that PKC activation in type II cells results in acute and long-term changes in pHi regulatory mechanisms mediated by the Na+/H+ exchanger and by the Na(+)-HCO3- cotransporter.

Meconium increases surfactant secretion in isolated rat alveolar type II cells

One of the underlying causes of pathophysiology of meconium aspiration syndrome is access of meconium to the alveolar space and inhibition of activity of lung surfactant. This study examines the effects of meconium on type II cell function by following surfactant secretion. Isolated rat alveolar type II cells were labeled with [methyl-3H]choline during the initial 21-22 h of incubation. During the subsequent 150 min of incubation, phosphatidylcholine (PC) secretion in the presence of 1% meconium was increased 250 +/- 11% (mean +/- SE, n = 23) over controls. The secretagogue effect was concentration-dependent and reached a maximum at 0.5% meconium. The meconium effect was not due to cellular toxicity as evaluated by vital dye exclusion, lactate dehydrogenase release, and PC synthesis. The secretagogue effect of meconium was associated with the particulate fraction pelleted by centrifugation of the suspension for 1 h at 100,000 x g. Heat treatment of meconium decreased the effect, suggesting the active component to be a protein. The effect of meconium was additive with that of 0.1 mM terbutaline, or 1 mM ATP, suggesting different pathways of action of each agent. The effect of meconium was reduced in the presence of 0.1 mM 4,4'-diisothiocyanato-2,2'-disulfonic acid, or 100 ng/mL surfactant protein A. These agents were previously shown to inhibit surfactant secretion in a stimulus-independent manner. Our results suggest that meconium at low concentrations is not toxic to type II cells, and a component of meconium, possibly a protein, increases PC secretion.

Stilbene disulfonic acids inhibit synexin-mediated membrane aggregation and fusion

Stilbene disulfonic acids inhibit surfactant secretion from lung epithelial type II cells by an undefined mechanism, and inhibit CD4 mediated cell-cell fusion. We have previously shown that lung synexin promotes in vitro fusion of lamellar bodies and plasma membranes, an obligatory process for surfactant secretion. This study investigates the effect of stilbene disulfonic acids, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), and 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (AMDS), on synexin-mediated liposome aggregation and fusion. Structurally, these three stilbene compounds differ in the number of isothiocyano groups present (DIDS = 2, SITS = 1, and AMDS = 0). At 10 micrograms synexin/ml, DIDS and SITS inhibited synexin-mediated liposome aggregation with an EC50 of 3.5 microM and 148 microM, respectively. In comparison, AMDS was least inhibitory (EC50 > 1 mM). Thus, the inhibitory potency (DIDS > SITS > AMDS) was partly dependent upon the number of isothiocyano groups. The EC50 was also dependent on synexin concentration. Stilbene disulfonic acids were also inhibitory for arachidonic acid-enhanced synexin-mediated liposome fusion. The EC50 for DIDS and SITS for fusion were similar to that for liposome aggregation. Ca(2+)-induced synexin polymerization, measured by 90 degrees light scattering, was increased by DIDS, suggesting binding of stilbene disulfonic acids to synexin. The binding of DIDS to synexin was dependent on the molar ratio of synexin to DIDS. These results indicate that stilbene disulfonic acids interact directly with synexin to inhibit membrane aggregation and fusion. Our results suggest that such inhibition of synexin activity may contribute towards inhibition of surfactant secretion by DIDS, and support a physiological role for synexin in lung surfactant secretion.

Protein kinase C in ATP regulation of lung surfactant secretion in type II cells

Previous studies provided indirect evidence for a role for protein kinase C (PKC) in ATP stimulation of surfactant secretion. The present study demonstrates that ATP increases PKC activity in the membrane fraction and decreases PKC activity in the cytosol fraction of alveolar type II cells, indicating translocation of PKC to the membranes. The kinetics of ATP concentration dependence of increases in phosphatidylcholine secretion and diacylglycerol content were similar, suggesting direct correlation between these two parameters. ATP also increased membrane PKC activity in a concentration-dependent manner. Almost one-half of the PKC activity in the cytosol and membrane fractions was Ca2+ independent. The ATP-induced increase was greater in membrane-associated Ca(2+)-dependent enzyme (233%) than in Ca(2+)-independent enzyme (121%). Desensitization of PKC by exposure of cells to phorbol esters decreased PKC activity in the membrane and cytosol fractions. In cells pretreated for 3 h with phorbol esters, PKC activity was near minimum, and ATP-stimulated secretion was lowest (< 40% of that observed in untreated cells). These results indicate that a major part of ATP-stimulated surfactant secretion in type II cells is mediated via activation of PKC.

Na(+)-dependent and Na(+)-independent systems of choline transport by plasma membrane vesicles of A549 cell line

Membrane vesicles of A549 lung cells accumulate choline by two pathways: the Na(+)-independent uphill uptake of choline [Michaelis-Menten constant (Km) approximately 44 microM; steady-state gradient approximately 45 at 5 microM external choline] is dependent on a transmembrane H+ gradient, is relatively insensitive to hemicholinium-3, is amiloride sensitive, and is abolished by valinomycin plus carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The Na(+)-dependent active choline uptake (Km approximately 4 microM, inhibitor constant for hemicholinium-3 approximately 0.1 microM), is specific for Na+, is amiloride and FCCP sensitive, and is electrogenic: the overshoot using K(+)-loaded vesicles and NaCl gradient was increased by valinomycin. The time of the overshoot peak, T was approximately 90 s in a NaSCN medium (or in presence of other lipid-soluble anions), a value close to that for alpha-aminoisobutyrate as substrate (T = approximately 1.5 min). T was lengthened in NaCl medium to approximately 10 min, and the overshoot was abolished by impermeant anions. External Cl- is not required for the choline uptake: valinomycin produced an overshoot in the presence of only impermeant anions, with T approximately 90 s. Most of the above properties are shared by the high-affinity Na(+)-dependent choline transport in synaptosomes. The characteristics of the Na(+)-dependent choline uptake by membrane vesicles of A549 cells are consistent with an electrogenic choline(+)-Na+ cotransport, with the rate-limiting anion (e.g., Cl-) influx balancing the positive charges transferred into the vesicles. The data are also consistent with an involvement of an amiloride-sensitive choline+/H+ antiport (or choline(+)-OH- symport) in the low- and high-affinity choline uptake pathways.

Inhibition of lung calcium-independent phospholipase A2 by surfactant protein A

The effect of lung surfactant protein A (SP-A) on lung phospholipase A2 (PLA2) activity was investigated. SP-A was purified from bovine surfactant obtained by lung lavage. PLA2 was assayed using radiolabeled 1,2-dipalmitoyl phosphatidylcholine (DPPC) in surfactant-like unilamellar liposomes with Ca(2+)-free acidic (pH 4) or 10 mM Ca2+, alkaline (pH 8.5) buffer. SP-A significantly inhibited Ca(2+)-independent acidic PLA2 of rat lung homogenate or isolated lamellar bodies but had no effect on the Ca(2+)-dependent alkaline enzyme. Lamellar body PLA2 was inhibited by 50% with 0.25 micrograms SP-A/microgram lamellar body protein. Similar inhibition by SP-A was observed when 1-palmitoyl,2-oleoyl PC (POPC) was the substrate. Binding assay showed binding of 125I-labeled SP-A to DPPC but not to POPC, indicating that removal of substrate was not the mechanism for inhibition of the enzyme by SP-A. Chemical reduction or alkylation of SP-A abolished its inhibitory effect on PLA2 activity. Inactivation of endogenous SP-A in isolated lamellar bodies or surfactant increased Ca(2+)-independent PLA2 activity in these fractions. The presence of SP-A in liposomes stimulated the uptake of DPPC by isolated granular pneumocytes in primary culture but significantly inhibited its degradation. These results indicate that the Ca(2+)-independent acidic PLA2 has a role in the metabolism of internalized surfactant phospholipid and that SP-A can modulate the activity of this enzyme.

Inhibition of Trimeresurus flavoviridis phospholipase A2 by lung surfactant protein A (SP-A)

A marked sequence homology has been noted between lung surfactant protein A (SP-A) and an inhibitor of phospholipase A2 (PLA2) isolated from the serum of Trimeresurus flavoviridis (Habu snake). This study evaluated the effect of SP-A on PLA2 activity from several sources. SP-A was isolated from bovine or rat lung surfactant by extraction with 1-butanol and octyl beta-D-glucopyranoside. The addition of SP-A produced a concentration-dependent inhibition of T. flavoviridis PLA2 that indicated non-competitive kinetics with Ki 5 micrograms/ml. Inhibition was reversed by heat inactivation, disulfide bond reduction or alkylation of SP-A, or by the presence of anti-SP-A antibody. Treatment of SP-A with endoglycosidase F or the presence of variation monosaccharides or lectins did not alter SP-A inhibition. Binding of PLA2 to SP-A was shown by ultrafiltration and was abolished by SP-A alkylation or the presence of SDS. The SP-A/PLA2 complex recovered from the ultrafilter had essentially no enzymatic activity, but activity was restored by treatment with mercaptoethanol. SP-A had no effect on activity of PLA2 from Naja naja, Crotalus atrox, or bovine pancreas. These results indicate that surfactant protein A selectively inhibits Trimeresurus phospholipase A2 activity and suggest that binding to the enzyme is the mechanism for inhibition.

Alkalosis- and ATP-induced increases in the diacyglycerol pool in alveolar type II cells are derived from phosphatidylcholine and phosphatidylinositol

Alkalosis and ATP increase surfactant secretion in alveolar type II cells, possibly via non-receptor- and receptor-mediated mechanisms respectively. We compared the effects of these two agonists on phosphatidylinositol (PI) and 1,2-diacylglycerol (DAG) pools and on phosphatidylcholine (PC) hydrolysis in alveolar type II cells. Alkalosis, caused by transfer of cells from 5% (control) to 0% CO2 in air, and ATP increased the secretion of surfactant compared with the controls. The stimulated secretion was inhibited by staurosporine, a protein kinase C inhibitor. DAG and PI contents of control cells were 50 +/- 1.1 (mean +/- S.E.M., n = 8) and 14 +/-0.8 nmol/mg phospholipid (n = 7) respectively. The DAG content increased by approximately 50 nmol (100%) within 5 s of treatment with both alkalosis and ATP, returned to control levels by 1 min, and increased again at 5 min by approximately 20 nmol. The PI content decreased maximally by approximately 6 nmol (40%) at 5 s and returned to control levels by 30 s with both alkalosis and ATP, but was unchanged thereafter. Mass-balance analysis of net changes in DAG and PI pools suggests that additional sources, possibly PC, must also contribute to the DAG increase. ATP or alkalosis also increased the hydrolysis of PC. The labelling of phosphocholine was increased (approximately 60%) at as early as 5 s and remained elevated at subsequent time points, whereas labelling of choline was higher only with ATP at 50 s and later, suggesting activation of phospholipase C by both agonists, and of phospholipase D by only ATP. Our studies demonstrate that ATP and alkalosis stimulate rapid hydrolysis of inositol and choline phospholipids to increase the DAG mass in type II cells, and that phospholipase C-stimulated PC hydrolysis is the major pathway for DAG formation.

Stimulation of lung surfactant secretion by endothelin-1 from rat alveolar type II cells

Endothelin-1 (ET-1), a potent vasoactive peptide released by endothelial cells, alters cytosolic Ca2+ and phosphoinositide metabolism in cells of myogenic and nonmyogenic origin. We evaluated the effect of ET-1 on surfactant phosphatidylcholine (PC) secretion from alveolar type II cells labeled with [methyl-3H]choline. ET-1 stimulated the secretion of PC in a time- and dose-dependent manner. Binding of 125I-labeled ET-1 to type II cell membranes was saturable at approximately 1 nM and suggested the presence of a single type of receptor. The secretagogue effect of ET-1 was independently inhibited by nifedipine and nitrendipine, both L-type calcium channel blockers, and removal of extracellular calcium. ET-1 also increased cellular diacylglycerol content by approximately 50% within 30 s, which could not be attenuated by pretreatment with nifedipine, suggesting an early activation of protein kinase C that was independent of Ca2+ influx. Further, ET-1-stimulated PC secretion was also blocked by inhibitors of protein kinase C. Collectively, these results indicate that the binding of ET-1 to type II cells is coupled to the activation of protein kinase C, which increases calcium influx through L-type calcium channels, and results in increased secretion of lung surfactant. Since ET-1 is released from pulmonary micro- and macrovasculature endothelial cells in close proximity to type II cells, our findings support the novel concept that endothelial cells interact with alveolar type II cells in a paracrine fashion to regulate surfactant secretion.

A rapid and gentle method for the isolation of genomic DNA from mycobacteria

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Inhibition of phosphatidylcholine secretion by stilbene disulfonates in alveolar type II cells

Various agents stimulate the secretion of lung surfactant from alveolar type II cells by increasing intracellular Ca2+, cyclic adenosine-3':5'-monophosphate (cAMP), or diacylglycerol. A few agents, including the purified surfactant protein A, are known to inhibit the secretion by an unknown mechanism. In the present study, we demonstrated that stilbene disulfonic acids, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), are potent but reversible inhibitors of lung surfactant secretion. The inhibition was concentration dependent, and the EC50 was 5 microM for DIDS and 50 microM for SITS. The inhibition was not specific to agonists for any one type of receptor, and was also observed for secretion stimulated by 8-bromo-cAMP, or tetradecanoyl phorbol acetate, suggesting that the site of inhibition was distal to the generation of intracellular second messengers. This was also supported by the failure of DIDS to block the stimulus-mediated increase in diacylglycerol content of type II cells. Further, DIDS and SITS were also inhibitory for basal secretion. Based on the reversibility of inhibition and the fact that inhibition was observed with both basal and stimulated secretion, we suggest that stilbene disulfonic acids affect a component of the exocytosis process that occurs at or near the plasma membrane.

Transport of choline by plasma membrane vesicles from lung-derived epithelial cells

A549 cells, a lung epithelium-derived cell line, were used as a model system to study choline transport by granular pneumocytes. Intact cells accumulated free choline against a concentration gradient by a low-affinity transport system with kinetic characteristics similar to that previously described for granular pneumocytes (Am. J. Respir. Cell Mol. Biol. 1: 455, 1989). Membrane vesicles prepared from these cells showed a 10-fold enrichment in plasma membrane marker enzymes with a vesicular H2O space of 5.7 +/- 0.05 (SE) microliters/mg protein. Vesicles showed a time- and concentration-dependent uptake of free [3H]choline in Na(+)-free medium. With 5 microM choline, choline uptake reached an apparent steady-state concentration gradient (inside/outside) of 50. 3H that was membrane associated ("bound" choline) represented approximately 5% of total uptake. In the presence of an initial gradient of NaCl, choline uptake showed an overshoot with a plateau value similar to Na(+)-free conditions; a similar effect was observed for plasma membrane vesicles from rat lung type 2 epithelial cells. The steady-state uptake of choline was inhibited at low pH (6.5) and by the presence of valinomycin or carbonyl cyanide p-tri-fluoromethoxyphenylhydrazone and was abolished when both were present. These results show that plasma membrane vesicles from A549 cells accumulate choline by binding to the membranes and by Na(+)-dependent and -independent transport mechanisms, the latter apparently reflecting a transmembrane proton gradient.

A competitive inhibitor of phospholipase A2 decreases surfactant phosphatidylcholine degradation by the rat lung

We have shown previously that radiolabelled phosphatidylcholine (PC) in liposomes or natural surfactant is removed from the alveolar space and metabolically recycled in a process that is stimulated by cyclic AMP (cAMP). In this study, we evaluated the effect of a transition-state phospholipid analogue (MJ33; 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol) that competitively inhibited acidic phospholipase A2 (PLA2) activity (pH 4.0) of lung homogenate by more than 97%, but had no effect on PLA2 activity at pH 8.5. MJ33 incorporated into unilamellar liposomes (dipalmitoyl PC/egg PC/cholesterol/phosphatidylglycerol, molar proportions 10:5:3:2) or co-sonicated with biosynthesized natural surfactant was instilled into the trachea of the anaesthetized rat; lungs were then removed for 2 h perfusion in the absence or presence of 0.1 mM-8-bromo cAMP. Total uptake for phospholipid was unchanged in the presence of the inhibitor MJ33. Degradation of labelled PC during 2 h perfusion in the absence of MJ33 was approx. 26% of that instilled for choline-labelled liposomal PC, 16% for liposomal PC labelled in the second fatty-acyl position, and 33% for choline-labelled natural surfactant. Degradation of PC was decreased by approx. 25-40% for each substrate in the presence of MJ33. Inhibition of lipid degradation depended on the mole fraction of MJ33 in the liposomes and was maximal at 1 mol%. These studies demonstrate a significant role for acidic Ca(2+)-independent PLA2 in the degradation of internalized alveolar PC, but further indicate that this enzyme accounts for a minor fraction of total lung PC metabolism.

Inhibitors of choline transport in alveolar type II epithelial cells

Isolated alveolar type II epithelial cells (granular pneumocytes) from rat lung accumulate free choline against a concentration gradient by an energy-dependent saturable transport process with apparent Km approximately 18 microM. In order to evaluate the structural requirements for choline transport by these cells, the inhibition of the initial rate of cellular uptake of [3H]choline (5 microM) by its analogue was measured. There was no significant inhibition of substrate uptake by analogues lacking an amino group while the presence of a quaternary nitrogen was most effective. N,N'-dimethylethanolamine (apparent Ki, 7 microM) and n-decylcholine (apparent Ki, 0.5 microM) were potent competitive inhibitors of choline transport. Substitution of the hydroxyl group in choline greatly diminished the inhibitory effect; fluorocholine, thiocholine, betaine, and betaine aldehyde showed little or no inhibition. This requirement for a hydroxyl group raises the possibility of hydrogen bonding of choline with the transport protein. The choline transport system in granular pneumocytes appears to differ from that in synaptosomes by the lower affinity of the carrier for substrate and for hemicholinium-3 and from that in erythrocytes by the role of the hydroxyl in the substrate molecule. The availability of inhibitory analogues for choline transport will facilitate isolation and study of the granular pneumocyte choline transport protein.

Dicyclohexylcarbodiimide and vanadate sensitive ATPase of lung lamellar bodies

Lung surfactant is synthesized in lung epithelial type II cells and stored in the lamellar bodies prior to its secretion onto the alveolar surface. The lamellar bodies, like other secretory organelles, maintain an ATP-dependent pH gradient that is sensitive to inhibitors of H(+)-ATPase. This report shows that the ATPase activity of lamellar bodies is enriched in a fraction prepared from lamellar bodies that were disrupted after isolation. The apparent Vmax for this enzyme was 150 nmol ATP hydrolyzed per min per mg protein and apparent Km for ATP was approximately 50 microM. The enzyme activity was sensitive to N-ethylmaleimide (NEM), dicyclohexylcarbodiimide (DCCD) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) (all inhibitors of vacuolar-type H(+)-ATPase) and vanadate (inhibitor of phosphoenzyme-type ATPase). Besides, the activity could also be inhibited with diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and Ca2+. Two proteins (of approximately 45 kDa and 17 kDa) of this fraction showed acid-stable phosphorylation with ATP. The labeling of proteins with ATP (-gamma-32P) could be chased with unlabelled ATP, suggesting that phosphorylation and dephosphorylation of these proteins is associated with the ATPase activity. Our results on inhibition characteristics of the enzyme activity suggest that besides a vacuolar type H(+)-ATPase, the lamellar bodies also contain a phosphoenzyme type ATPase that is sensitive to inhibitors of vacuolar type H(+)-ATPase.

In vitro fusion of lung lamellar bodies and plasma membrane is augmented by lung synexin

Lamellar bodies of lung epithelial type II cells undergo fusion with plasma membrane prior to exocytosis of surfactant into the alveolar lumen. Since synexin from adrenal glands promotes aggregation and fusion of chromaffin granules, we purified synexin-like proteins from bovine lung cytosolic fraction, and evaluated their effect on the fusion of isolated lamellar bodies and plasma membrane fractions. Synexin activity, which co-purified with an approx. 47 kDa protein (pI 6.8), was assessed by following calcium-dependent aggregation of liposomes prepared from a mixture of phosphatidylcholine:phosphatidylserine (PC:PS, 3:1, mol/mol). Lung synexin caused aggregation of liposomes approximating lung surfactant lipid-like composition, isolated lamellar bodies, or isolated plasma membrane fraction. Lung synexin promoted fusion only in the presence of calcium. It augmented fusion between lamellar bodies and plasma membranes, lamellar bodies and liposomes, or between two populations of liposomes. However, selectivity with regard to synexin-mediated fusion was observed as synexin did not promote fusion between plasma membrane and liposomes, or between liposomes of surfactant lipid-like composition and other liposomes. These observations support a role for lung synexin in membrane fusion between the plasma membrane and lamellar bodies during exocytosis of lung surfactant, and suggest that such fusion is dependent on composition of interacting membranes.

Alveolar uptake of lipid and protein components of surfactant

We investigated the clearance of radiolabeled natural surfactant from the alveolar space of the isolated perfused rat lung. 3H, 35S-natural surfactant was prepared from rat lungs that had been perfused with [methyl-3H]choline and [35S]methionine. The biosynthesized material contained greater than 95% of 3H in phosphatidylcholine (PC) and approximately 80% of 35S in surfactant protein A. Natural surfactant (1 mumol PC) was instilled into the trachea; lungs were analyzed 5 min later or after 2 h perfusion to determine surfactant uptake, defined as lung lavage-resistant 3H or 35S [% of instilled disintegrations per minute(dpm)]. Uptake at 5 min was 31.4 +/- 0.37% for 3H and 31.9 +/- 0.85% for 35S (mean +/- SE, n = 4). At 2 h, uptake was 46.6 +/- 0.96% for 3H and 45.8 +/- 1.1% for 35S (n = 7). In the presence of 0.1 mM 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), uptake at 2 h for both 3H and 35S was stimulated to approximately 57% of instilled dpm (n = 4). Microsomes and plasma membranes isolated from lung homogenates had a ratio of 3H to 35S that was similar to the original surfactant, whereas 3H/35S in isolated lamellar bodies was increased 2.1-fold. Degradation of lipid was indicated by finding 13.4 +/- 0.65% of homogenate 3H in the aqueous fraction of lung extract after 2 h perfusion; only 2.3 +/- 0.47% of 35S dpm were soluble in trichloroacetic acid, suggesting significantly less protein breakdown. Lipid degradation was increased more than twofold by 8-BrcAMP, whereas protein degradation was not changed significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP increases synthesis of surfactant-associated protein A by perfused rat lung

Synthesis and secretion of surfactant-associated protein were studied in isolated rat lungs perfused with [3H]phenylalanine or [35S]methionine in synthetic medium. Surfactant was isolated by lung lavage and density-gradient centrifugation followed by dialysis to remove unincorporated amino acid and extraction with ethanol-ether to yield a delipidated protein fraction. Incorporation of [3H]phenylalanine into the delipidated surfactant protein fraction showed a lag phase of approximately 3 h followed by progressive increase over the next 3 h at a rate of 1.6 nmol.mg protein-1.h-1. With 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP, 0.1 mM) added to the perfusate, the incorporation rate between 3 and 6 h was increased by 75%. 3H specific activity in a delipidated lamellar body-rich fraction isolated from lung homogenates was unchanged by 8-BrcAMP at 3 h but was increased by 45% at 6 h. The major peak of radioactivity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of surfactant and lamellar bodies corresponded to proteins of 27-36 kDa that were identified as surfactant protein A (SP-A) by immunoblot. In the presence of 8-BrcAMP during 6 h of perfusion, specific activity of 35S-labeled SP-A in immunoprecipitated protein was increased by 93% and the SP-A mRNA content of lung was increased 145%. These results show that isolated perfused lungs synthesize and secrete surfactant-associated proteins and that the presence of a permeable cAMP analogue in the lung perfusate leads to increased secretion followed by induction of synthesis for SP-A.

Regulation of lung surfactant secretion

Secretion of lung surfactant is the direct step in release of the lipoprotein-like product, synthesized in lung epithelial type II cells, onto the alveolar surface. Release of surfactant phosphatidylcholine (PC) proceeds via formation of surface pores during exocytosis of lamellar bodies. Surfactant secretion is regulated locally in the lung by changes in ventilation rate, possibly mediated by distension and altered intracellular pH. Secretion is also stimulated by various agents, including agonists for beta-adrenergic, purinoceptors, and vasopressin receptors and is associated with increased cytosolic Ca2+, cellular adenosine 3',5'-cyclic monophosphate, and activation of protein kinases. Limited studies suggest that secretion of surfactant protein A may be regulated by both cAMP-dependent and protein kinase C-dependent pathways. The integration of these various mechanisms for the in vivo regulation of surfactant secretion remains largely unexplored. Future research into the mechanisms involved in lamellar body fusion with the plasma membrane, role of protein phosphorylation, transient changes in cAMP and Ca2+, and coordination between the secretion of phospholipid and protein components of surfactant should enhance our understanding of secretion of surfactant "lipoprotein."

Choline transport by lung epithelium

The uptake of [3H]choline was investigated using isolated perfused rat lungs and primary cultures of granular pneumocytes isolated by tryptic digestion of rat lungs. Metabolic products were separated from free choline by chloroform:methanol extraction and column chromatography. Tissue-associated [3H]choline increased progressively in the perfused lung, and estimated mean intracellular concentration at 2 h was 12 times the extracellular concentration (5 microM). Choline uptake was inhibited by ventilation with CO and by perfusion with the choline analog, hemicholinium-3 (HC-3). Isolated granular pneumocytes also accumulated choline against a concentration gradient by an energy-dependent process. The concentration for half-maximal uptake, after correction for the diffusion component, was estimated at 18 +/- 4 microM (mean +/- SE; n = 3), and the estimated maximal rate of uptake was 213 +/- 44 pmol/min/microliter cell water. HC-3 inhibited uptake by approximately 50% at a concentration of 10(-4) M. There was no effect on uptake when Na+ in the medium was replaced by Li+ or N-methylglucamine+. These results indicate that granular pneumocytes possess a transport system that results in accumulation of choline against a concentration gradient. The characteristics of uptake indicate that this system is similar to the low affinity choline transport system of other organs.

The management of craniofacial pain in a pain relief unit

Craniofacial pain can be one of the more intractable problems that presents to the GDP. One management route can be referral to a specialist pain relief unit, where the facilities and expertise will be available for diagnosis, counselling, drug management and invasive therapy. In this article the authors describe the results achieved at one such unit, over a one-year period, for a group of 34 craniofacial pain sufferers. At the end of the year 30 patients reported total or partial relief from their pain.

Regulation of lung surfactant secretion by intracellular pH

We investigated secretion of lung surfactant phosphatidylcholine (PC) using isolated perfused rat lung preparation after labeling the lung lipids in vitro with [methyl-3H]choline. The perfusion medium was Krebs-Ringer bicarbonate buffer (pH 7.4) containing 10 mM glucose and 3% fatty acid-poor bovine serum albumin. After ventilation of lungs with air containing 5% CO2 (control) for 1 h, 0.91% +/- 0.04 (mean +/- SE, n = 6) of total lung lipid radioactivity (greater than 95% in PC) was recovered in the cell-free lavage fluid. The secretion of PC was increased with terbutaline (50 microM), 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP, 100 microM), phorbol L2-myristate 13-acetate (30 ng/ml), and ATP (1 mM), in each case by approximately 150%. Secretion of PC was also increased by 160% if the lungs were ventilated with air containing 0% CO2. The low CO2-mediated PC secretion was time and concentration dependent. The dose-response curve for 0-10% CO2 was S-shaped. The low CO2-induced increase in PC secretion could be largely reversed with diffusible weak acids (25 mM, acetate or butyrate) in the perfusion medium. An increase (70%) in secretion was also induced with 10 mM NH4Cl, suggesting a role for intracellular alkalosis. These observations suggest that intracellular alkalosis stimulates lung surfactant secretion. Alkalosis-stimulated secretion of PC was additive with that with terbutaline (5 X 10(-7) to 5 X 10(-4) M) or 10(-4) M 8-BrcAMP, suggesting that alkalosis effect was not mediated through the beta-adrenergic pathway of surfactant secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretagogues for lung surfactant increase lung uptake of alveolar phospholipids

This study evaluated the effect of known secretagogues for lung surfactant on the uptake of phospholipid from the aveolar space. Synthetic liposomes containing tracer [choline-methyl-3H]dipalmitoyl phosphatidylcholine (DPPC) were instilled endotracheally in anesthetized rats. Lungs were then isolated and perfused under control conditions or with addition of terbutaline (0.1 mM), ATP (1 mM), or a phorbol ester, tradecanoyl phorbol acetate (TPA, 30 ng/ml). Uptake of liposomes was defined as lavage-resistant accumulation of radioactivity in the lung. Uptake at 2 h was 18.2 +/- 0.93% of instilled radioactivity per minute (mean +/- SE, n = 4) under control conditions and was increased by 56-82% in the presence of each of the agonists (P less than 0.05). At 5 min after addition of terbutaline, uptake was unchanged but secretion of phosphatidylcholine (PC) into the alveolar space was significantly stimulated. Internalized diphosphatidylcholine was degraded to aqueous soluble metabolites and also converted to PC containing an unsaturated fatty acid, and this metabolism was significantly stimulated in the presence of each of the secretagogues. These results indicate that known secretagogues for surfactant increase uptake and metabolism of phospholipid, suggesting linkage of these processes in a physiologically regulated surfactant cycle.