Surfactant Secretion Research Articles

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.

Human decidua-derived mesenchymal stem cells differentiate into functional alveolar type II-like cells that synthesize and secrete pulmonary surfactant complexes.

Lung alveolar type II (ATII) cells are specialized in the synthesis and secretion of pulmonary surfactant, a lipid-protein complex that reduces surface tension to minimize the work of breathing. Surfactant synthesis, assembly and secretion are closely regulated and its impairment is associated with severe respiratory disorders. At present, well-established ATII cell culture models are not available. In this work, Decidua-derived Mesenchymal Stem Cells (DMSCs) have been differentiated into Alveolar Type II- Like Cells (ATII-LCs), which display membranous cytoplasmic organelles resembling lamellar bodies, the organelles involved in surfactant storage and secretion by native ATII cells, and accumulate disaturated phospholipid species, a surfactant hallmark. Expression of characteristic ATII cells markers was demonstrated in ATII-LCs at gene and protein level. Mimicking the response of ATII cells to secretagogues, ATII-LCs were able to exocytose lipid-rich assemblies, which displayed highly surface active capabilities, including faster interfacial adsorption kinetics than standard native surfactant, even in the presence of inhibitory agents. ATII-LCs could constitute a highly useful ex vivo model for the study of surfactant biogenesis and the mechanisms involved in protein processing and lipid trafficking, as well as the packing and storage of surfactant complexes.

Evolution of Resistance to a Last-Resort Antibiotic in Staphylococcus aureus via Bacterial Competition

Antibiotic resistance is a key medical concern, with antibiotic use likely being an important cause. However, here we describe an alternative route to clinically relevant antibiotic resistance that occurs solely due to competitive interactions among bacterial cells. We consistently observe that isolates of Methicillin-resistant Staphylococcus aureus diversify spontaneously into two distinct, sequentially arising strains. The first evolved strain outgrows the parent strain via secretion of surfactants and a toxic bacteriocin. The second is resistant to the bacteriocin. Importantly, this second strain is also resistant to intermediate levels of vancomycin. This so-called VISA (vancomycin-intermediate S. aureus) phenotype is seen in many hard-to-treat clinical isolates. This strain diversification also occurs during in vivo infection in a mouse model, which is consistent with the fact that both coevolved phenotypes resemble strains commonly found in clinic. Our study shows how competition between coevolving bacterial strains can generate antibiotic resistance and recapitulate key clinical phenotypes.

ATP-gated P2X receptors in health and disease.

ATP-gated P2X receptors in health and disease.

Effect of P2X7 receptor knockout on AQP-5 expression of type I alveolar epithelial cells.

P2X7 receptors, ATP-gated cation channels, are specifically expressed in alveolar epithelial cells. The pathophysiological function of this lung cell type, except a recently reported putative involvement in surfactant secretion, is unknown. In addition, P2X7 receptor-deficient mice show reduced inflammation and lung fibrosis after exposure with bleomycin. To elucidate the role of the P2X7 receptor in alveolar epithelial type I cells we characterized the pulmonary phenotype of P2X7 receptor knockout mice by using immunohistochemistry, western blot analysis and real-time RT PCR. No pathomorphological signs of fibrosis were found. Results revealed, however, a remarkable loss of aquaporin-5 protein and mRNA in young knockout animals. Additional in vitro experiments with bleomycin treated precision cut lung slices showed a greater sensitivity of the P2X7 receptor knockout mice in terms of aquaporin-5 reduction as wild type animals. Finally, P2X7 receptor function was examined by using the alveolar epithelial cell lines E10 and MLE-12 for stimulation experiments with bleomycin. The in vitro activation of P2X7 receptor was connected with an increase of aquaporin-5, whereas the inhibition of the receptor with oxidized ATP resulted in down regulation of aquaporin-5. The early loss of aquaporin-5 which can be found in different pulmonary fibrosis models does not implicate a specific pathogenetic role during fibrogenesis.

The preterm pig as a model for acute lung disease after preterm birth (668.3)

Respiratory distresssyndrome and the more severe sequel bronchopulmonary dysplasia remain the leading causes of morbidity, mortality, and lifelong disability after preterm birth, with inadequate surfactant secretion a key contributor. We present the preterm pig as a translational model for lung disease after preterm birth. Preterm pigs were delivered by caesarian section at gestation days (GD) 98, 100, 102,and 104 (comparable to human fetuses delivered at 25, 28, 30, and 32 wks) and were provided intensive care following newborn neonatal intensive care procedures, including mechanical ventilation (MV) or supplemental oxygen and nutritional support. Expression of Surfactant Protein B (SP‐B),a surfactant‐associated protein absolutely required for postnatal lung function, was determined by western analysis using lung tissue harvested 24 h after delivery or earlier for pigs that became moribund. Pigs delivered at GD 98 and 100, had low SP‐B and MV is required for survival. SP‐B increases with gestational age. Moreover, provision of MV to preterm piglets delivered at GD 102 causes a dramatic increase in SP‐B expression and suggests MV can induce SP‐B secretion and reduce dependency on surfactant administration.Based on SP‐B expression, lung histology and pathology, and the responses to MV, the preterm pig is a translational large animal model for studying acute lung disease after preterm birth and for preclinical trials to evaluate interventions and improve MV protocols. The large litters of pigs reduce the numbers of pregnant animals needed, the attributes of the preterm pig allow investigators to refine experimental approaches, and the preterm pig replaces the baboon as a large animal model to study acute lung disease and injury after preterm birth.

Complementary roles for Rock1 and myosin light chain kinase in actin coat compression on fused secretory vesicles (538.4)

Alveolar type II epithelial cells secrete pulmonary surfactant via exocytosis of lamellar bodies (LBs). Surfactant is a poorly‐soluble lipo‐protein substance that does not readily diffuse out of fused LBs following opening of the fusion pore. LBs are coated with actin following fusion with the plasma membrane (PM) and actin coating and subsequent actin coat compression is essential to drive surfactant secretion. We have previously demonstrated that myosin II is involved in actin coat compression. However detailed mechanisms of myosin II activation and coat compression were still missing. Here we report that actin coat compression is a multiphasic process. High‐resolution life cell imaging and immunocytochemistry revealed that Rock1 and myosin light chain kinase 1 (MLCK) are recruited to LBs following fusion with the PM. Recruitment of Rock1 was preceded by transient recruitment and activation of Rho GTPases to the actin coat. Inhibition of Rock1 did not affect initial compression of the actin coat but significantly impaired full compression of fused vesicles. Inhibition of MLCK did not have a significant effect on actin coat compression at all. Combined inhibition of Rock1 and MLCK, however, completely blocks vesicle compression. Our data suggest that Rock1 and MLCK act via complementary pathways to regulate actin coat compression likely regulating activation of regulatory myosin II light chain.Grant Funding Source: Deutsche Forschungsgemeinschaft (DFG), BiU

Microfluidic shear stress-regulated surfactant secretion in alveolar epithelial type II cells in vitro

We investigated the role of flow-induced shear stress on the mechanisms regulating surfactant secretion in type II alveolar epithelial cells (ATII) using microfluidic models. Following flow stimulation spanning a range of wall shear stress (WSS) magnitudes, monolayers of ATII (MLE-12 and A549) cells were examined for surfactant secretion by evaluating essential steps of the process, including relative changes in the number of fusion events of lamellar bodies (LBs) with the plasma membrane (PM) and intracellular redistribution of LBs. F-actin cytoskeleton and calcium levels were analyzed in A549 cells subjected to WSS spanning 4-20 dyn/cm(2). Results reveal an enhancement in LB fusion events with the PM in MLE-12 cells upon flow stimulation, whereas A549 cells exhibit no foreseeable changes in the monitored number of fusion events for WSS levels ranging up to a threshold of ∼8 dyn/cm(2); above this threshold, we witness instead a decrease in LB fusion events in A549 cells. However, patterns of LB redistribution suggest that WSS can potentially serve as a stimulus for A549 cells to trigger the intracellular transport of LBs toward the cell periphery. This observation is accompanied by a fragmentation of F-actin, indicating that disorganization of the F-actin cytoskeleton might act as a limiting factor for LB fusion events. Moreover, we note a rise in cytosolic calcium ([Ca(2+)]c) levels following stimulation of A549 cells with WSS magnitudes ranging near or above the experimental threshold. Overall, WSS stimulation can influence key components of molecular machinery for regulated surfactant secretion in ATII cells in vitro.

Surfactant Secretion in LRRK2 Knock-Out Rats: Changes in Lamellar Body Morphology and Rate of Exocytosis

Leucine-rich repeat kinase 2 (LRRK2) is known to play a role in the pathogenesis of various diseases including Parkinson disease, morbus Crohn, leprosy and cancer. LRRK2 is suggested to be involved in a number of cell biological processes such as vesicular trafficking, transcription, autophagy and lysosomal pathways. Recent histological studies of lungs of LRRK2 knock-out (LRRK2 -/-) mice revealed significantly enlarged lamellar bodies (LBs) in alveolar type II (ATII) epithelial cells. LBs are large, lysosome-related storage organelles for pulmonary surfactant, which is released into the alveolar lumen upon LB exocytosis. In this study we used high-resolution, subcellular live-cell imaging assays to investigate whether similar morphological changes can be observed in primary ATII cells from LRRK2 -/- rats and whether such changes result in altered LB exocytosis. Similarly to the report in mice, ATII cells from LRRK2 -/- rats contained significantly enlarged LBs resulting in a >50% increase in LB volume. Stimulation of ATII cells with ATP elicited LB exocytosis in a significantly increased proportion of cells from LRRK2 -/- animals. LRRK2 -/- cells also displayed increased intracellular Ca2+ release upon ATP treatment and significant triggering of LB exocytosis. These findings are in line with the strong Ca2+-dependence of LB fusion activity and suggest that LRRK2 -/- affects exocytic response in ATII cells via modulating intracellular Ca2+ signaling. Post-fusion regulation of surfactant secretion was unaltered. Actin coating of fused vesicles and subsequent vesicle compression to promote surfactant expulsion were comparable in cells from LRRK2 -/- and wt animals. Surprisingly, surfactant (phospholipid) release from LRRK2 -/- cells was reduced following stimulation of LB exocytosis possibly due to impaired LB maturation and surfactant loading of LBs. In summary our results suggest that LRRK2 -/- affects LB size, modulates intracellular Ca2+ signaling and promotes LB exocytosis upon stimulation of ATII cells with ATP.

Does Epigenetic Regulation Plays a Critical Role in Acute Lung Injury?

Acute lung injury and the acute respiratory distress syndrome (ALI/ARDS) are severe respiratory conditions that remain as significant public health concerns, with ARDS mortality as high at 30-40% [1-4]. ALI/ARDS is primarily caused by pneumonia, it is also caused by a variety of other clinical disorders including pulmonary or non-pulmonary sepsis, major trauma, inhalation injury such as aspiration of gastric or oropharyngeal contents, drug over use, and blood products. ALI/ARDS features with widespread damage to the various types of cells in the lung and the structures of alveolar capillary membrane. The pathogenesis of ALI/ARDS involves three major overlapping phases, inflammatory phase, proliferative phase, and fibrotic phase. Pathological stimuli cause cytokine secretion and inflammatory cell infiltration to the injured site followed by the activation of pro-inflammatory mediator cascades. Many proinflammatory mediators including pro-inflammatory cytokines and chemokines are involved in the pathogenesis of ALI/ARDS. Increased amount of TNF-α, IL-1β, IL-6, and IL-8 are reported in BAL fluid and plasma of patients with ARDS, orchestrating with the low levels of inflammatory inhibitors such as IL-1α, IL-4, and IL-10. Inflammation in ALI/ARDS results in cell death, alveolar epithelial membrane leaking, and dysfunction of surfactant secretion. The followed injury repair mediates fibro proliferative response further impaired lung function.

The Expression of Surfactant Proteins A and D in the Intestines and Pancreas of Murine Fetuses

Purpose: Surfactant proteins exist in the digestive tract and may play an important role in the host defense. However, the expression of surfactant proteins in the premature digestive system remains unclear. The aim of this study was to investigate the expression of surfactant proteins in the intes-tines and pancreas of murine fetuses. Methods: Immunostaining for SP-A and SP-D was assessed in the small intestine and pancreas of ICR murine fetuses on days 15, 16, 17 and 18 of gestation (normal duration of pregnancy: 19 - 21 days). RT-PCR was performed to detect the expression of spa and spd mRNA in the small intestine and pancreas on day 16, 17 and 18 of gestation. Results: Immunoreactivity for SP-A and SP-D in the acinar cells of pancreas and intestinal mucosal surface were positive on day 16 of gestation onward. RT-PCR revealed that the expression of spa and spd mRNA was significant in the pancreas but weak in the small intestine. Conclusions: Our data revealed that surfactant proteins are present in the fetal intestines and pancreas and that a significant expression of spa and spd mRNA is detected in the fetal pancreas. Pancreas may be a possible organ involved in the synthesis and secretion of surfactant proteins into the intestinal lumen.

Influence of glucocorticoids, neuregulin-1β, and sex on surfactant phospholipid secretion from type II cells

Glucocorticoids induce lung fibroblasts to produce fibroblast-pneumocyte factor, a peptide that stimulates type II cells to synthesize pulmonary surfactant. This effect is known to be more apparent in cells derived from female fetuses, a characteristic that has been attributed to sex-linked differences in the fibroblasts. In the current study, it has been shown that dexamethasone enhances both β-adrenergic receptor (β-AR) activity (1.3- to 1.6-fold increase) and (-)-isoproterenol-induced secretion of surfactant (1.8- to 1.9-fold increase) in type II cells. However, fibroblast-conditioned media (FCM), prepared in the presence of dexamethasone, generates a much greater response to (-)-isoproterenol (3.1- to 3.8-fold increase). Furthermore, each of these effects is more pronounced if both cell types are female-derived. It is hypothesized that the enhanced response to glucocorticoids is the result of a synergistic effect between the steroid and a component of FCM. Neuregulin-1β (NRG1β), which is elevated in FCM generated in the presence of dexamethasone, influences not only the rate of surfactant secretion and the β-AR activity in type II cells, but also enhances in both sexes the cellular response to (-)-isoproterenol. These results suggest that NRG1β might be more effective than glucocorticoids in treating prematurely born male infants, which are known to respond poorly to glucocorticoids. Given that glucocorticoids are known to induce higher levels of β-AR mRNA, the effect of NRG1β, alone and in combination with dexamethasone, on β-AR gene expression was measured using qRT-PCR. Whereas NRG1β had no effect alone, in combination with dexamethasone it produced up to a 4.2-fold elevation in the level of β-AR mRNA.

Expansive Generation of Functional Airway Epithelium From Human Embryonic Stem Cells

Production of human embryonic stem cell (hESC)-derived lung progenitors has broad applicability for drug screening and cell therapy; however, this is complicated by limitations in demarcating phenotypic changes with functional validation of airway cell types. In this paper, we reveal the potential of hESCs to produce multipotent lung progenitors using a combined growth factor and physical culture approach, guided by the use of novel markers LIFRα and NRP1. Lung specification of hESCs was achieved by priming differentiation via matrix-specific support, followed by air-liquid interface to allow generation of lung progenitors capable of in vitro maturation into airway epithelial cell types, resulting in functional characteristics such as secretion of pulmonary surfactant, ciliation, polarization, and acquisition of innate immune activity. This approach provided a robust expansion of lung progenitors, allowing in vivo assessment, which demonstrated that only fully differentiated hESC-derived airway cells were retained in the distal airway, where they aided in physiological recovery in immunocompromised mice receiving airway injury. Our study provides a basis for translational applications of hESCs for lung diseases.

The emerging role of miR‐375 in cancer

MicroRNAs (miRNAs) are evolutionarily conserved, small noncoding RNAs that are believed to play fundamental roles in various biological processes through regulation of gene expression at the level of posttranscription. MiR-375 was first identified as a pancreatic islet-specific miRNA regulating insulin secretion. However, further study revealed that miR-375 is a multifunctional miRNA participating in pancreatic islet development, glucose homeostasis, mucosal immunity, lung surfactant secretion and more importantly, tumorigenesis. Recently, miR-375 has been found significantly downregulated in multiple types of cancer, and suppresses core hallmarks of cancer by targeting several important oncogenes like AEG-1, YAP1, IGF1R and PDK1. The alteration of miR-375 in cancer is caused by a variety of mechanisms, including the dysregulation of transcription factors, aberrant promoter methylation and so on. Reduced expression of miR-375 in tissue or circulation may indicate the presence of neoplasia as well as a poor prognosis of many malignant cancers. Moreover, miR-375 stands for a promising direction for developing targeted therapies due to its capacity to inhibit tumor cell growth in vitro and in vivo. Here, we summarize the present understanding of the tumor suppressive role of miR-375 in cancer progression; the mechanisms underlying the dysregulation of miR-375; the potential use of miR-375 in prognosis and diagnosis and the therapeutic prospects of miR-375 in cancer.

Prostaglandins before caesarean section for preventing neonatal respiratory distress

Respiratory distress (RD) can occur in both preterm and term neonates born through normal vaginal delivery or caesarean section (CS). It accounts for about 30% of neonatal deaths and can occur at any time following birth. Respiratory distress syndrome (RDS), transient tachypnoea (rapid breathing) of the newborn and persistent pulmonary hypertension (increased blood pressure of pulmonary vessels) of the newborn are the most frequent clinical presentations of neonatal RD. Prostaglandins are used in routine obstetric practice to ripen the uterine cervix and to trigger labour, with those of the E series being preferred over others due to the fact that they are more uteroselective. Administration of prostaglandins to an expectant mother before delivery causes reabsorption of lung fluid from the fetal lung and promotes surfactant secretion by inducing a catecholamine surge. As a result, significant reduction in neonatal respiratory morbidity following a CS could be obtained, leading to reduced long-term complications such as bronchopulmonary dysplasia (chronic lung disease with lung tissue modification) and asthma. The objective of this review was to determine if administration of prostaglandins before CS can improve respiratory outcomes of newborns. We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 September 2013). We also searched three clinical trial registries; ClinicalTrials.gov, the Australian New Zealand Clinical Trials Registry and the WHO Clinical Trials Registry Platform (ICTRP), for ongoing studies (24 June 2013). We included all published and unpublished randomised controlled trials comparing the use of prostaglandins with other treatments (including placebo) to reduce neonatal respiratory morbidity. Participants were pregnant women with an indication for a CS, and we compared administration of prostaglandins prior to CS with no treatment, placebo or another treatment. Two review authors independently assessed studies for inclusion and assessed trial quality, with the third review author referred to for settling any disagreements. Two review authors extracted data. Data were checked for accuracy. We used the Cochrane tool for assessing risk of bias in the included study and contacted the study authors to request additional information where appropriate. We found one randomised controlled trial with a low risk of bias which was carried out in a tertiary neonatal care centre in Australia. The study involved 36 women (18 received intravaginal prostaglandin E 2 gel and 18 received placebo).There was one case of neonatal respiratory distress in the control group, which the trialist reported as transient tachypnoea of the newborn (risk ratio (RR) 0.33, 95% confidence interval (CI) 0.01 to 7.68, one study, n = 36).None of the neonates required mechanical ventilation and the trial authors reported median Apgar scores at one and five minutes as being similar in both groups.There were no treatment-related side effects in either group. Noradrenaline concentrations (median values (range)) were reported as being significantly higher in the cord blood samples of the intervention group compared to the control group. Although the trial authors reported a significant increase in catecholamine levels in the intervention group, there was no significant difference in the respiratory outcomes between intervention and control groups. The quality of evidence was graded as low because the sample size was small and there were few events. No definite conclusions can thus be drawn on the effects of prostaglandins on neonatal respiratory outcomes from this review.

F-actin scaffold stabilizes lamellar bodies during surfactant secretion

Alveolar type 2 (AT2) cells secrete surfactant that forms a protective layer on the lung's alveolar epithelium. Vesicles called lamellar bodies (LBs) store surfactant. Failure of surfactant secretion, which causes severe lung disease, relates to the manner in which LBs undergo exocytosis during the secretion. However, the dynamics of LBs during the secretion process are not known in intact alveoli. Here, we addressed this question through real-time confocal microscopy of single AT2 cells in live alveoli of mouse lungs. Using a combination of phospholipid and aqueous fluorophores that localize to LBs, we induced surfactant secretion by transiently hyperinflating the lung, and we quantified the secretion in terms of loss of bulk LB fluorescence. In addition, we quantified inter-LB phospholipid flow through determinations of fluorescence recovery after photobleaching. Furthermore, we determined the role of F-actin in surfactant secretion through expression of the fluorescent F-actin probe Lifeact. Our findings indicate that, in AT2 cells in situ, LBs are held in an F-actin scaffold. Although F-actin transiently decreases during surfactant secretion, the LBs remain stationary, forming a chain of vesicles connected by intervesicular channels that convey surfactant to the secretion site on the plasma membrane. This is the first instance of a secretory process in which the secretory vesicles are immobile, but form a conduit for the secretory material.

Newborn Transition

The transition from intrauterine to extrauterine life is a complex adaptation. Although, in a sense, the entire time in utero is in preparation for this transition, there are many specific anatomic and physiologic changes that take place in the weeks and days leading up to labor that facilitate a healthy transition. Some, including increasing pulmonary vasculature and blood flow, are part of an ongoing process of maturation. Others, such as a reversal in the lung from secreting fluid to absorbing fluid and the secretion of pulmonary surfactant, are associated with the hormonal milieu that occurs when spontaneous labor is impending. Interventions such as elective cesarean birth or induction of labor may interfere with this preparation for birth. Postnatal interventions such as immediate clamping of the umbilical cord and oropharyngeal suction may also compromise the normal process of newborn transition. This article reviews the physiology of the fetal to newborn transition and explores interventions that may facilitate or hinder the optimal process.

Identification of Sec14‐like 3 as a novel lipid‐packing sensor in the lung

Pulmonary surfactant, a complex composed primarily of lipids and associated proteins, is synthesized in alveolar type II (ATII) cells and secreted into alveoli to prevent collapse during respiration. Although numerous studies have clarified the fundamental roles of pulmonary surfactant, the molecular mechanisms of transport and secretion of pulmonary surfactant remain totally unknown. Thus, we screened candidate genes by comparing genes with the expressed sequence tag (EST) libraries of embryonic and adult lungs by using the digital differential display method in the National Center for Biotechnology Information (NCBI) database. We identified Sec14-like 3 (Sec14L3) as a new class of lipid-associated proteins highly expressed in ATII cells. We found that Sec14L3 expression is >100-fold increased during the perinatal period in the lung. Furthermore, Sec14L3 bound to small-sized liposomes (30 nm in diameter), but not to large-sized liposomes (100 nm diameter), through its Sec14 domain. Because of the increased curvature, lipid-packing defects are more likely to occur in small-sized liposomes than in large-sized liposomes. Based on these results, we conclude that Sec14L3 is a new class of lipid-packing sensor. Sec14L3 may play important roles in the lung, such as intracellular lipid transport, surfactant maturation, and endo/exocytosis.

Orphan G Protein–Coupled Receptor GPR116 Regulates Pulmonary Surfactant Pool Size

Pulmonary surfactant levels within the alveoli are tightly regulated to maintain lung volumes and promote efficient gas exchange across the air/blood barrier. Quantitative and qualitative abnormalities in surfactant are associated with severe lung diseases in children and adults. Although the cellular and molecular mechanisms that control surfactant metabolism have been studied intensively, the critical molecular pathways that sense and regulate endogenous surfactant levels within the alveolus have not been identified and constitute a fundamental knowledge gap in the field. In this study, we demonstrate that expression of an orphan G protein-coupled receptor, GPR116, in the murine lung is developmentally regulated, reaching maximal levels 1 day after birth, and is highly expressed on the apical surface of alveolar type I and type II epithelial cells. To define the physiological role of GPR116 in vivo, mice with a targeted mutation of the Gpr116 locus, Gpr116(Δexon17), were generated. Gpr116(Δexon17) mice developed a profound accumulation of alveolar surfactant phospholipids at 4 weeks of age (12-fold) that was further increased at 20 weeks of age (30-fold). Surfactant accumulation in Gpr116(Δexon17) mice was associated with increased saturated phosphatidylcholine synthesis at 4 weeks and the presence of enlarged, lipid-laden macrophages, neutrophilia, and alveolar destruction at 20 weeks. mRNA microarray analyses indicated that P2RY2, a purinergic receptor known to mediate surfactant secretion, was induced in Gpr116(Δexon17) type II cells. Collectively, these data support the concept that GPR116 functions as a molecular sensor of alveolar surfactant lipid pool sizes by regulating surfactant secretion.

Lung Surfactant Levels are Regulated by Ig-Hepta/GPR116 by Monitoring Surfactant Protein D

Lung surfactant is a complex mixture of lipids and proteins, which is secreted from the alveolar type II epithelial cell and coats the surface of alveoli as a thin layer. It plays a crucial role in the prevention of alveolar collapse through its ability to reduce surface tension. Under normal conditions, surfactant homeostasis is maintained by balancing its release and the uptake by the type II cell for recycling and the internalization by alveolar macrophages for degradation. Little is known about how the surfactant pool is monitored and regulated. Here we show, by an analysis of gene-targeted mice exhibiting massive accumulation of surfactant, that Ig-Hepta/GPR116, an orphan receptor, is expressed on the type II cell and sensing the amount of surfactant by monitoring one of its protein components, surfactant protein D, and its deletion results in a pulmonary alveolar proteinosis and emphysema-like pathology. By a coexpression experiment with Sp-D and the extracellular region of Ig-Hepta/GPR116 followed by immunoprecipitation, we identified Sp-D as the ligand of Ig-Hepta/GPR116. Analyses of surfactant metabolism in Ig-Hepta+/+ and Ig-Hepta−/− mice by using radioactive tracers indicated that the Ig-Hepta/GPR116 signaling system exerts attenuating effects on (i) balanced synthesis of surfactant lipids and proteins and (ii) surfactant secretion, and (iii) a stimulating effect on recycling (uptake) in response to elevated levels of Sp-D in alveolar space.