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

Abstract

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 Dexon17, were generated. Gpr116 Dexon17 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 Gpr116Dexon17 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 Gpr116Dexon17 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.Keywords: pulmonary surfactant; G protein-coupled receptors; GPR116; surfactant metabolism; alveolar epithelium Pulmonary surfactant is synthesized by alveolar type II cells and is primarily composed of phospholipids, which constitute 80% of the total mass. The remaining components include neutral lipids, the lipid-associated surfactant proteins SFTPB and SFTPC, and the hydrophilic surfactant proteins SFTPA and SFTPD (1, 2). Saturated phosphatidylcholine (SatPC) is uniquely enriched in surfactant and, in concert with SFTPB and SFTPC, is fundamentally required to reduce surface tension at the air/liquid interface within the alveolus (3, 4). After synthesis, the lipid and lipidassociated proteins SFTPB and SFTPC are routed to and stored in membrane-enclosed secretory organelles called lamellar bodies. Through constitutive pathways or upon stimulation by secretagogues (including purines [5,6], b-agonists [7][8][9], and adenosine [10, 11]) or mechanical stretch (12), lamellar bodies fuse with the plasma membrane and are exocytosed from type II cells into the alveolar space.The quantity of surfactant in the mammalian lung, referred to as surfactant pool size, increases dramatically during late gestation to facilitate the transition to air breathing at birth. In the fetal lung, the majority of surfactant is stored within type I...

Targeted Disruption of Ig-Hepta/Gpr116 Causes Emphysema-like Symptoms That Are Associated with Alveolar Macrophage Activation

“…Ϫ/Ϫ Mice-It has been reported that IgHepta expression is induced in mouse lung at 18 dpc, whereas accumulation of pulmonary surfactant initiates after birth (27,28). We observed that expression levels of F8/40 (a macrophage marker), Mmp2, and Mmp9 are comparable in the lung between WT and Ig-Hepta Ϫ/Ϫ mice at 18.5 dpc (Fig.…”

“…Recent studies have shown that targeted disruption of IgHepta in mice results in abnormal lung structure with enlarged alveoli and in progressive accumulation of pulmonary surfactant and AMs (27)(28)(29). These abnormalities are similar to those seen in patients and animal models with emphysema (21,39,40), but the underlying mechanism of pathogenesis has not been elucidated.…”

“…These abnormalities are similar to those seen in patients and animal models with emphysema (21,39,40), but the underlying mechanism of pathogenesis has not been elucidated. Because AMs have been shown to play a pivotal role in the pathogenesis of emphysema, we determined and An extensive accumulation of the AMs occurs in the alveoli of Ig-Hepta Ϫ/Ϫ mice as early as 3 weeks of age and progressively increases thereafter (27,28). Most AMs are enlarged and become foamy with lipid-laden phagosomes due to extensive uptake of surfactant lipids (27)(28)(29).…”

“…1, C and D). The increased levels of several inflammatory cytokines/chemokines and the accumulation of foamy AMs (27)(28)(29) suggest that local inflammation occurs in the alveoli of Ig-Hepta Ϫ/Ϫ mice.…”

“…Recent studies have reported that Ig-Hepta is highly expressed in the alveolar type II cells and essential for homeostasis of pulmonary surfactant. Mice deficient in Ig-Hepta exhibit massive accumulation of pulmonary surfactant in the alveoli due to abnormal synthesis and catabolism of surfactant lipids and proteins in the alveolar type II cells (27)(28)(29). Ig-Hepta knock-out (IgHepta Ϫ/Ϫ ) mice also exhibit emphysema-like symptoms with enlarged alveoli, accumulation of foamy AMs, and increased expression of MMP-12 (27).…”