Regulation of SP-B and SP-C secretion in rat type II cells in primary culture

Rab38 Mutation and the Lung Phenotype

Rab38 is highly expressed in alveolar type II cells, melanocytes, and platelets. These cells are specifically-differentiated cells and contain characteristic intracellular organelles called lysosome-related organelles, i.e., lamellar bodies in alveolar type II cells, melanosomes in melanocytes, and dense granules in platelets. There are Rab38-mutant rodents, i.e., chocolate mice and Ruby rats. While chocolate mice only show oculocutaneous albinism, Ruby rats show oculocutaneous albinism and prolonged bleeding time and, hence, are a rat model of Hermansky-Pudlak syndrome (HPS). Most patients with HPS suffer from fatal interstitial pneumonia by middle age. The lungs of both chocolate mice and Ruby rats show remarkably increased amounts of lung surfactant and conspicuously enlarged lysosome-related organelles, i.e., lamellar bodies, which are also characteristic of the lungs in human HPS. There are 16 mutant HPS-mouse strains, of which ten mutant genes have been identified to be causative in patients with HPS thus far. The gene products of eight of the ten genes constitute one of the three protein complexes, i.e., biogenesis of lysosome-related organelle complex-1, -2, -3 (BLOC-1, -2, -3). Patients with HPS of the mutant BLOC-3 genotype develop interstitial pneumonia. Recently, BLOC-3 has been elucidated to be a guanine nucleotide exchange factor for Rab38. Growing evidence suggests that Rab38 is an additional candidate gene of human HPS that displays the lung phenotype.

When Is an Alveolar Type 2 Cell an Alveolar Type 2 Cell? A Conundrum for Lung Stem Cell Biology and Regenerative Medicine

Generating mature, differentiated, adult lung cells from pluripotent cells, such as induced pluripotent stem cells and embryonic stem cells, offers the hope of both generating disease-specific in vitro models and creating definitive and personalized therapies for a host of debilitating lung parenchymal and airway diseases. With the goal of advancing lung-regenerative medicine, several groups have developed and reported on protocols using defined media, coculture with mesenchymal components, or sequential treatments mimicking lung development, to obtain distal lung epithelial cells from stem cell precursors. However, there remains significant controversy about the degree of differentiation of these cells compared with their primary counterparts, coupled with a lack of consistency or uniformity in assessing the resultant phenotypes. Given the inevitable, exponential expansion of these approaches and the probable, but yet-to-emerge second and higher generation techniques to create such assets, we were prompted to pose the question, what makes a lung epithelial cell a lung epithelial cell? More specifically for this Perspective, we also posed the question, what are the minimum features that constitute an alveolar type (AT) 2 epithelial cell? In addressing this, we summarize a body of work spanning nearly five decades, amassed by a series of "lung epithelial cell biology pioneers," which carefully describes well characterized molecular, functional, and morphological features critical for discriminately assessing an AT2 phenotype. Armed with this, we propose a series of core criteria to assist the field in confirming that cells obtained following a differentiation protocol are indeed mature and functional AT2 epithelial cells.

Exogenous gene transfer of Rab38 small GTPase ameliorates aberrant lung surfactant homeostasis in Ruby rats

Background

Rab³8 small GTPase regulates intracellular transport in melanocytes and alveolar type II epithelial cells. Ruby rats carrying Rab³8 and other gene mutations exhibit oculocutaneous albinism, bleeding diathesis, and hence, are a rat model of human Hermansky-Pudlak syndrome (HPS). We previously showed that Long Evans Cinnamon (LEC) rats, one strain of the Ruby rats, developed aberrant lung surfactant homeostasis with remarkably enlarged lamellar bodies in alveolar type II cells.

Methods

A replication-deficient recombinant adenovirus expressing rat Rab³8 (Ad-Rab³8) was constructed. Alveolar type II cells were isolated from the LEC rats and tested for lung surfactant phosphatidylcholine secretion. The rats were also examined whether exogenous expression of Ad- Rab³8 could rescue the altered lung surfactant homeostasis in the lungs.

Results

Isolated type II cells infected with Ad-Rab³8 exhibited improved secretion patterns of [³H]phosphatidylcholine, i.e. increased basal hyposecretion and decreased agonist-induced hypersecretion. Endobronchial administration of Ad-Rab³8 improved the morphology of type II cells and lamellar bodies, reducing their sizes close to those of wild-type rats. The increased amounts of phosphatidylcholine and surfactant protein B in the lamellar body fractions were decreased in the Ad-Rab³8 infected lungs.

Conclusions

These results provide strong evidence that the aberrant lung surfactant homeostasis in the LEC rats is caused by Rab³8 deficit, and suggest that endobronchial delivery of the responsive transgene could be an effective method to ameliorate the abnormal lung phenotype in the animal model of HPS.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Mature Surfactant Protein-B Expression by Immunohistochemistry as a Marker for Surfactant System Development in the Fetal Sheep Lung

Evaluation of the number of type II alveolar epithelial cells (AECs) is an important measure of the lung's ability to produce surfactant. Immunohistochemical staining of these cells in lung tissue commonly uses antibodies directed against mature surfactant protein (SP)-C, which is regarded as a reliable SP marker of type II AECs in rodents. There has been no study demonstrating reliable markers for surfactant system maturation by immunohistochemistry in the fetal sheep lung despite being widely used as a model to study lung development. Here we examine staining of a panel of surfactant pro-proteins (pro-SP-B and pro-SP-C) and mature proteins (SP-B and SP-C) in the fetal sheep lung during late gestation in the saccular/alveolar phase of development (120, 130, and 140 days), with term being 150 ± 3 days, to identify the most reliable marker of surfactant producing cells in this species. Results from this study indicate that during late gestation, use of anti-SP-B antibodies in the sheep lung yields significantly higher cell counts in the alveolar epithelium than SP-C antibodies. Furthermore, this study highlights that mature SP-B antibodies are more reliable markers than SP-C antibodies to evaluate surfactant maturation in the fetal sheep lung by immunohistochemistry.

Altered lung surfactant system in a Rab38-deficient rat model of Hermansky-Pudlak syndrome

Several Long-Evans rat substrains carrying the phenotype of oculocutaneous albinism and bleeding diathesis are a rat model of Hermansky-Pudlak syndrome (HPS). The mutation responsible for the phenotype ( Ruby) was identified as a point mutation in the initiation codon of Rab38 small GTPase that regulates intracellular vesicle transport. As patients with HPS often develop life-limiting interstitial pneumonia accompanied by abnormal morphology of alveolar type II cells, we investigated lung surfactant system in Long-Evans Cinnamon rats, one strain of the Ruby rats. The lungs showed conspicuous morphology of type II cells containing markedly enlarged lamellar bodies. Surfactant phosphatidylcholine and surfactant protein B were increased in lung tissues and lamellar bodies but not in alveolar lumen. Expression levels of mRNA for surfactant proteins A, B, C, and D were not altered. Isolated type II cells showed aberrant secretory pattern of newly synthesized [3H]phosphatidylcholine, i.e., decreased basal secretion and remarkably amplified agonist-induced secretion. [3H]phosphatidylcholine synthesis and uptake by type II cells were not altered. Thus Rab38-deficient type II cells appear to carry abnormality in lung surfactant secretion but not in synthesis or uptake. These results suggest that aberrant lung surfactant secretion may be involved in the pathogenesis of interstitial pneumonia in HPS.

Adenosine A2B receptors are highly expressed on murine type II alveolar epithelial cells

The adenosine A(2B) receptor (A(2B)R) has a wide tissue distribution that includes fibroblasts and endothelial and epithelial cells. The recent generation of an A(2B)R(-/-) mouse constructed with a beta-galactosidase (beta-gal) reporter gene under control of the endogenous promoter has provided a valuable tool to quantify A(2B)R promoter activity (29). To determine the sites of expression of the A(2B) receptor in the mouse lung, histological and flow cytometric analysis of beta-gal reporter gene expression in various lung cell populations was performed. The major site of A(2B)R promoter activity was found to be the type II alveolar epithelial cells (AECs), identified by coexpression of prosurfactant protein C, with relatively less expression in alveolar macrophages, bronchial epithelial cells, and cells of the vasculature. Highly purified type II AECs were prepared by fluorescence-activated sorting of enhanced green fluorescent protein (eGFP)-positive cells from transgenic mice expressing eGFP under control of the surfactant protein C promoter (21). The type II cells expressed 89-fold higher A(2B)R mRNA than pulmonary leukocytes, and the A(2B)R was shown to be functional, as treatment of purified type II AECs with the nonspecific adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) induced an increase in intracellular cAMP greater that the beta-adrenergic agonist isoproterenol that was inhibited completely following treatment by ATL-802, a novel, highly potent (K(i) = 8.6 nM), and selective (>900 fold over other adenosine receptor subtypes) antagonist of the mouse A(2B)R.

A mutation in Rab38 small GTPase causes abnormal lung surfactant homeostasis and aberrant alveolar structure in mice

The chocolate mutation, which is associated with oculocutaneous albinism in mice, has been attributed to a G146T transversion in the conserved GTP/GDP-interacting domain of Rab38, a small GTPase that regulates intracellular vesicular trafficking. Rab38 displays a unique tissue-specific expression pattern with highest levels present in the lung. The purpose of this study was to characterize the effects of Rab38-G146T on lung phenotype and to investigate the molecular basis of the mutant gene product (Rab38(cht) protein). Chocolate lungs exhibited a uniform enlargement of the distal airspaces with mild alveolar destruction as well as a slight increase in lung compliance. Alveolar type II cells were engorged with lamellar bodies of increased size and number. Hydrophobic surfactant constituents (ie, phosphatidylcholine and surfactant protein B) were increased in lung tissues but decreased in alveolar spaces, consistent with a malfunction in lamellar body secretion and the subsequent cellular accumulation of these organelles. In contrast to wild-type Rab38, native Rab38(cht) proteins were found to be hydrophilic and not bound to intracellular membranes. Unexpectedly, recombinant Rab38(cht) proteins retained GTP-binding activity but failed to undergo prenyl modification that is required for membrane-binding activity. These results suggest that the genetic abnormality of Rab38 affects multiple lysosome-related organelles, resulting in lung disease in addition to oculocutaneous albinism.

Is there a role for beta-adrenoceptor agonists in the management of acute lung injury and the acute respiratory distress syndrome?

Despite improvements in general supportive care and ventilatory strategies designed to limit lung injury, no specific pharmacological therapy has yet proven to be efficacious in the management of acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS). Based on experimental studies, as well as studies of the ex-vivo human lung, pulmonary edema fluid clearance from the alveolar space can be augmented by both inhaled and systemic beta2-adrenoceptor agonists (beta2-agonists). Additionally, in the presence of lung injury, beta2-agonists may reduce lung vascular permeability. Treatment with beta2-agonists may also increase the secretion of surfactant and have anti-inflammatory effects. In view of these potentially beneficial effects, beta2-agonist therapy should be evaluated for the treatment of lung injury in humans, particularly because they are already in wide clinical use and do not seem to have serious adverse effects in critically ill patients.

Paracrine stimulation of surfactant secretion by extracellular ATP in response to mechanical deformation

We developed a heterologous system to study the effect of mechanical deformation on alveolar epithelial cells. First, isolated primary rat alveolar type II (ATII) cells were plated onto silastic substrata coated with fibronectin and maintained in culture under conditions where they become alveolar type I-like (ATI) cells. This was followed by a second set of ATII cells labeled with the nontransferable, vital fluorescent stain 5-chloromethylfluorescein diacetate to distinguish them from ATI cells. By morphometric analysis, equibiaxial deformation (stretch) of the silastic substratum induced comparable changes in cell surface area for both ATII and ATI cells. Surfactant lipid secretion was measured using cells metabolically labeled with [(3)H]choline. In response to 21% tonic stretch for 15 min, ATII cells seeded with ATI cells secreted nearly threefold more surfactant lipid compared with ATII cells seeded alone. ATI cells did not secrete lipid in response to stretch. The enhanced lipid secretion by ATII plus ATI cocultures was inhibited by treatment with apyrase and adenosine deaminase, suggesting that ATP release by ATI cells enhanced surfactant lipid secretion at 21% stretch. This was confirmed using a luciferase assay where, in response to 21% stretch, ATI cells released fourfold more ATP than ATII cells. Because ATI cells release significantly more ATP at a lower level of stretch than ATII cells, this supports the hypothesis that ATI cells are mechanosensors in the lung and that paracrine stimulation of ATII cells by extracellular ATP released from ATI cells plays a role in regulating surfactant secretion.

Defective surfactant secretion in a mouse model of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) in humans represents a family of disorders of lysosome-related organelle biogenesis associated with severe, progressive pulmonary disease. Human case reports and a mouse model of HPS, the pale ear/pearl mouse (ep/pe), exhibit giant lamellar bodies (GLB) in type II alveolar epithelial cells. We examined surfactant proteins and phospholipid from ep/pe mice to elucidate the process of GLB formation. The 2.8-fold enrichment of tissue phospholipids in ep/pe mice resulted from accumulation from birth through adulthood. Tissue surfactant protein (SP)-B and -C were increased in adult ep/pe mice compared with wild-type mice (WT), whereas SP-A and -D were not different. Large aggregate surfactant (LA) from adult ep/pe mice had decreased phospholipid, SP-B, and SP-C, with no differences in SP-A and -D compared with WT. Although LA from ep/pe animals exhibited an increased total protein-to-total phospholipid ratio compared with WT, surface tension was not compromised. Phospholipid secretion from isolated type II cells showed that basal and stimulated secretion from ep/pe cells were approximately 50% of WT cells. Together, our data indicate that GLB formation is not associated with abnormal trafficking or recycling of surfactant material. Instead, impaired secretion is an important component of GLB formation in ep/pe mice.

Surfactant protein C biosynthesis and its emerging role in conformational lung disease

Surfactant protein C (SP-C) is a hydrophobic 35-amino acid peptide that co-isolates with the phospholipid fraction of lung surfactant. SP-C represents a structurally and functionally challenging protein for the alveolar type 2 cell, which must synthesize, traffic, and process a 191-197-amino acid precursor protein through the regulated secretory pathway. The current understanding of SP-C biosynthesis considers the SP-C proprotein (proSP-C) as a hybrid molecule that incorporates structural and functional features of both bitopic integral membrane proteins and more classically recognized luminal propeptide hormones, which are subject to post-translational processing and regulated exocytosis. Adding to the importance of a detailed understanding of SP-C biosynthesis has been the recent association of mutations in the proSP-C sequence with chronic interstitial pneumonias in children and adults. Many of these mutations involve either missense or deletion mutations located in a region of the proSP-C molecule that has structural homology to the BRI family of proteins linked to inherited degenerative dementias. This review examines the current state of SP-C biosynthesis with a focus on recent developments related to molecular and cellular mechanisms implicated in the emerging role of SP-C mutations in the pathophysiology of diffuse parenchymal lung disease.

Surfactant protein B in type II pneumocytes and intra-alveolar surfactant forms of human lungs

Surfactant protein B (SP-B) is synthesized by type II pneumocytes as a proprotein (proSP-B) that is proteolytically processed to an 8-kD protein. In human type II pneumocytes, we identified not only proSP-B, processing intermediates of proSP-B, and mature SP-B, but also fragments of the N-terminal propeptide. By means of immunoelectron microscopy, proSP-B and processing intermediates were localized in the endoplasmic reticulum, Golgi vesicles, and few multivesicular bodies in type II pneumocytes in human lungs. A colocalization of fragments of the N-terminal propeptide and mature SP-B was found in multivesicular, composite, and some lamellar bodies. Mature SP-B was localized over the projection core of lamellar bodies and core-like structures in tubular myelin figures. In line with immunoelectron microscopy and Western blot analysis of human type II pneumocytes, a fragment of the N-terminal propeptide was also detected in isolated rat lamellar bodies. In conclusion, our data indicate that the processing of proSP-B occurs between the Golgi complex and multivesicular bodies and provide evidence that a fragment of the N-terminal propeptide and mature SP-B are transported together to the lamellar bodies. In human lungs, mature SP-B is involved in the structural organization of lamellar bodies and tubular myelin by the formation of core particles.

Bench-to-bedside review: beta2-Agonists and the acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a devastating constellation of clinical, radiological and pathological signs characterized by failure of gas exchange and refractory hypoxia. Despite nearly 30 years of research, no specific pharmacological therapy has yet proven to be efficacious in manipulating the pathophysiological processes that underlie this condition. Several in vitro and in vivo animal or human studies suggest a potential role for β₂-agonists in the treatment of ARDS. These agents have been shown to reduce pulmonary neutrophil sequestration and activation, accelerate alveolar fluid clearance, enhance surfactant secretion, and modulate the inflammatory and coagulation cascades. They are also used widely in clinical practice and are well tolerated in critically ill patients. The present review examines the evidence supporting a role for β₂-agonists as a specific pharmacological intervention in patients with ARDS.

Pulmonary surfactant secretion in briefly cultured mouse type II cells

There is little information on the regulation of surfactant secretion in mouse type II cells. We isolated type II cells from C57BL/6 and FVB mice, cultured them overnight, and then examined their response to known surfactant secretagogues. Secretion of phosphatidylcholine, surfactant protein (SP)-B and SP-C was stimulated by terbutaline, 5'-N-ethylcarboxyamidoadenosine (NECA), ATP, UTP, TPA, and ionomycin. Phosphatidylcholine secretion was increased approximately twofold by all agonists in both strains of mice. The response to terbutaline and NECA is the same as in rat type II cells, whereas the response to ATP, UTP, TPA, and ionomycin is considerably less. Secretion of SP-B and SP-C was increased sevenfold by terbutaline and threefold by ATP, effects similar to those in rat type II cells. The response to terbutaline was significantly decreased in type II cells from beta(2)-adrenergic receptor null mice. These data establish that briefly cultured type II cells provide a suitable model for investigation of surfactant secretion in normal and genetically altered mice.

Pulmonary type II cell hypertrophy and pulmonary lipoproteinosis are features of chronic IL-13 exposure

Interleukin (IL)-13, a key mediator of Th2-mediated immunity, contributes to the pathogenesis of asthma and other pulmonary diseases via its ability to generate fibrosis, mucus metaplasia, eosinophilic inflammation, and airway hyperresponsiveness. In these studies, we compared surfactant accumulation in wild-type mice and mice in which IL-13 was overexpressed in the lung. When compared with littermate controls, transgenic animals showed alveolar type II cell hypertrophy under light and electron microscopy. Over time, their alveoli also filled with surfactant in a pulmonary alveolar proteinosis pattern. At the same time, prominent interstitial fibrosis occurs. Bronchoalveolar lavage fluid from these mice had a three- to sixfold increase in surfactant phospholipids. Surfactant proteins (SP)-A, -B, and -C showed two- to threefold increases, whereas SP-D increased 70-fold. These results indicate that IL-13 is a potent stimulator of surfactant phospholipid and surfactant accumulation in the lung. IL-13 may therefore play a central role in the broad range of chronic pulmonary conditions in which fibrosis, type II cell hypertrophy, and surfactant accumulation occur.