Targeted inactivation of the murine Abca3 gene leads to respiratory failure in newborns with defective lamellar bodies

Expression of ABCA3 transporter gene in Tegillarca granosa and its association with cadmium accumulation

Exposure to cadmium (Cd), a heavy metal, can cause strong and toxic side effects. Cd can enter the body of organisms in several ways, leading to various pathological reactions in the body. Tegillarca granosa is a kind of bivalve shellfish favored by people in the coastal areas of China. Bivalve shellfish can easily absorb heavy metal pollutants from water bodies while filter feeding. T. granosa is considered a hyper-accumulator of Cd, and the TgABCA3 gene is highly expressed in individuals with a high content of Cd-exposed blood clam. However, it is unclear whether TgABCA3 is involved in Cd ion transport in blood clam and the molecular mechanism for the mechanism of the Cd-induced responses for maintaining cell homeostasis. In this study, the complete cDNA of the TgABCA3 gene was analyzed to provide insights into the roles of TgABCA3 in resistance against Cd in blood clam. The complete sequence of TgABCA3 showed high identity to that of TgABCA3 from other bivalves and contained some classical motifs of ATP-binding cassette transport proteins. TgABCA3 expression in different tissues was measured using real-time quantitative polymerase chain reaction (qRT-PCR) and western blot analysis. The tissue-specific expression showed that TgABCA3 expression was highest in the gill tissue. The TgABCA3 expression in the gill tissue was silenced using the RNA interference technique. After TgABCA3 silencing, the TgABCA3 expression decreased, the Cd content increased, the oxygen consumption and ammonia excretion rates increased, and the ingestion rate decreased. These results showing that the extents of Cd accumulation and resulting toxic effects are related to expression levels and activity of TgABCA3 indicate that TgABCA3 has a protective function against Cd in the clam. This increase in Cd accumulation results in serious damage to the body, leading to the enhancement of its physiological metabolism. Therefore, the findings of the study demonstrated that TgABCA3 can participate in the transport of Cd ions in the blood clam through active transport and play a vital role in Cd detoxification.

Gene Therapeutics for Surfactant Dysfunction Disorders: Targeting the Alveolar Type 2 Epithelial Cell

Genetic disorders of surfactant dysfunction result in significant morbidity and mortality, among infants, children, and adults. Available medical interventions are limited, nonspecific, and generally ineffective. As such, the need for effective therapies remains. Pathogenic variants in the SFTPB, SFTPC, and ABCA3 genes, each of which encode proteins essential for proper pulmonary surfactant production and function, result in interstitial lung disease in infants, children, and adults, and lead to morbidity and early mortality. Expression of these genes is predominantly limited to the alveolar type 2 (AT2) epithelial cells present in the distal airspaces of the lungs, thus providing an unequivocal cellular origin of disease pathogenesis. While several treatment strategies are under development, a gene-based therapeutic holds great promise as a definitive therapy. Importantly for clinical translation, the genes associated with surfactant dysfunction are both well characterized and amenable to a gene-therapeutic-based strategy. This review focuses on the pathophysiology associated with these genetic disorders of surfactant dysfunction, and also provides an overview of the current state of gene-based therapeutics designed to target and transduce the AT2 cells.

Targeting the lung epithelium after intravenous delivery by directed evolution of underexplored sites on the AAV capsid

Advances in adeno-associated virus (AAV) engineering have provided exciting new tools for research and potential solutions for gene therapy. However, the lung has not received the same tailored engineering as other major targets of debilitating genetic disorders. To address this, here we engineered the surface-exposed residues AA452-458 of AAV9 capsid proteins at the three-fold axis of symmetry and employed a Cre-transgenic-based screening platform to identify AAV capsids targeted to the lung after intravenous delivery in mice. Using a custom image processing pipeline to quantify transgene expression across whole tissue images, we found that one engineered variant, AAV9.452sub.LUNG1, displays dramatically improved transgene expression in lung tissue after systemic delivery in mice. This improved transduction extends to alveolar epithelial type II cells, expanding the toolbox for gene therapy research for diseases specific to the lung.

Pulmonary surfactant: a unique biomaterial with life-saving therapeutic applications

Pulmonary surfactant is a complex lipoprotein mixture secreted into the alveolar lumen by type 2 pneumocytes, which is composed by tens of different lipids (approximately 90% of its entire mass) and surfactant proteins (approximately 10% of the mass). It is crucially involved in maintaining lung homeostasis by reducing the values of alveolar liquid surface tension close to zero at end-expiration, thereby avoiding the alveolar collapse, and assembling a chemical and physical barrier against inhaled pathogens. A deficient amount of surfactant or its functional inactivation is directly linked to a wide range of lung pathologies, including the neonatal respiratory distress syndrome. This paper reviews the main biophysical concepts of surfactant activity and its inactivation mechanisms, and describes the past, present and future roles of surfactant replacement therapy, focusing on the exogenous surfactant preparations marketed worldwide and new formulations under development. The closing section describes the pulmonary surfactant in the context of drug delivery. Thanks to its peculiar composition, biocompatibility, and alveolar spreading capability, the surfactant may work not only as a shuttle to the branched anatomy of the lung for other drugs but also as a modulator for their release, opening to innovative therapeutic avenues for the treatment of several respiratory diseases.

The Role of ABC Transporters in Lipid Metabolism and the Comorbid Course of Chronic Obstructive Pulmonary Disease and Atherosclerosis

Chronic obstructive pulmonary disease (COPD) ranks among the leading causes of morbidity and mortality worldwide. COPD rarely occurs in isolation and is often combined with various diseases. It is considered that systemic inflammation underlies the comorbid course of COPD. The data obtained in recent years have shown the importance of violations of the cross-links of lipid metabolism and the immune response, which are links in the pathogenesis of both COPD and atherosclerosis. The role of lipid metabolism disorders in the pathogenesis of the comorbid course of COPD and atherosclerosis and the participation of ATP-binding cassette (ABC) transporters in these processes is discussed in this article. It is known that about 20 representatives of a large family of ABC transporters provide lipid homeostasis of cells by moving lipids inside the cell and in its plasma membrane, as well as removing lipids from the cell. It was shown that some representatives of the ABC-transporter family are involved in various links of the pathogenesis of COPD and atherosclerosis, which can determine their comorbid course.

Neonatal respiratory failure due to novel compound heterozygous mutations in the ABCA3 lipid transporter

The ATP-binding cassette transporter member A3 (ABCA3) is a lipid transporter with a critical function in pulmonary surfactant biogenesis. Biallelic loss-of-function mutations in ABCA3 result in severe surfactant deficiency leading to neonatal respiratory failure with death in the first year of life. Herein, we describe a newborn with severe respiratory distress at birth progressing to respiratory failure requiring transplant. This patient was found to have a maternally inherited frameshift loss-of-function ABCA3 mutation and a paternally inherited synonymous variant in ABCA3 predicted to create a cryptic splice site. Additional studies showed reduced ABCA3 expression in hyperplastic alveolar epithelial type II cells and lamellar body alterations characteristic of ABCA3 deficiency, leading to a diagnosis of autosomal recessive ABCA3-related pulmonary surfactant dysfunction. This case highlights the need for an integrated, comprehensive approach for the diagnosis of inherited diseases when in silico modeling is utilized in the interpretation of key novel genetic mutations.

Alveolar lipids in pulmonary disease. A review

Lung lipid metabolism participates both in infant and adult pulmonary disease. The lung is composed by multiple cell types with specialized functions and coordinately acting to meet specific physiologic requirements. The alveoli are the niche of the most active lipid metabolic cell in the lung, the type 2 cell (T2C). T2C synthesize surfactant lipids that are an absolute requirement for respiration, including dipalmitoylphosphatidylcholine. After its synthesis and secretion into the alveoli, surfactant is recycled by the T2C or degraded by the alveolar macrophages (AM). Surfactant biosynthesis and recycling is tightly regulated, and dysregulation of this pathway occurs in many pulmonary disease processes. Alveolar lipids can participate in the development of pulmonary disease from their extracellular location in the lumen of the alveoli, and from their intracellular location in T2C or AM. External insults like smoke and pollution can disturb surfactant homeostasis and result in either surfactant insufficiency or accumulation. But disruption of surfactant homeostasis is also observed in many chronic adult diseases, including chronic obstructive pulmonary disease (COPD), and others. Sustained damage to the T2C is one of the postulated causes of idiopathic pulmonary fibrosis (IPF), and surfactant homeostasis is disrupted during fibrotic conditions. Similarly, surfactant homeostasis is impacted during acute respiratory distress syndrome (ARDS) and infections. Bioactive lipids like eicosanoids and sphingolipids also participate in chronic lung disease and in respiratory infections. We review the most recent knowledge on alveolar lipids and their essential metabolic and signaling functions during homeostasis and during some of the most commonly observed pulmonary diseases.

Lack of whey acidic protein (WAP) four-disulfide core domain protease inhibitor 2 (WFDC2) causes neonatal death from respiratory failure in mice

Respiratory failure is a life-threatening problem for pre-term and term infants, yet many causes remain unknown. Here, we present evidence that whey acidic protein (WAP) four-disulfide core domain protease inhibitor 2 (Wfdc2), a protease inhibitor previously unrecognized in respiratory disease, may be a causal factor in infant respiratory failure. Wfdc2 transcripts are detected in the embryonic lung and analysis of a Wfdc2-GFP knock-in mouse line shows that both basal and club cells, and type II alveolar epithelial cells (AECIIs), express Wfdc2 neonatally. Wfdc2-null-mutant mice display progressive atelectasis after birth with a lethal phenotype. Mutant lungs have multiple defects, including impaired cilia and the absence of mature club cells from the tracheo-bronchial airways, and malformed lamellar bodies in AECIIs. RNA sequencing shows significant activation of a pro-inflammatory pathway, but with low-quantity infiltration of mononuclear cells in the lung. These data demonstrate that Wfdc2 function is vitally important for lung aeration at birth and that gene deficiency likely causes failure of the lung mucosal barrier.

Pulmonary alveolar proteinosis

Pulmonary alveolar proteinosis (PAP) is a syndrome characterized by the accumulation of alveolar surfactant and dysfunction of alveolar macrophages. PAP results in progressive dyspnoea of insidious onset, hypoxaemic respiratory failure, secondary infections and pulmonary fibrosis. PAP can be classified into different types on the basis of the pathogenetic mechanism: primary PAP is characterized by the disruption of granulocyte-macrophage colony-stimulating factor (GM-CSF) signalling and can be autoimmune (caused by elevated levels of GM-CSF autoantibodies) or hereditary (due to mutations in CSF2RA or CSF2RB, encoding GM-CSF receptor subunits); secondary PAP results from various underlying conditions; and congenital PAP is caused by mutations in genes involved in surfactant production. In most patients, pathogenesis is driven by reduced GM-CSF-dependent cholesterol clearance in alveolar macrophages, which impairs alveolar surfactant clearance. PAP has a prevalence of at least 7 cases per million individuals in large population studies and affects men, women and children of all ages, ethnicities and geographical locations irrespective of socioeconomic status, although it is more-prevalent in smokers. Autoimmune PAP accounts for >90% of all cases. Management aims at improving symptoms and quality of life; whole-lung lavage effectively removes excessive surfactant. Novel pathogenesis-based therapies are in development, targeting GM-CSF signalling, immune modulation and cholesterol homeostasis.

Alveolar injury and regeneration following deletion of ABCA3

Adaptation to air breathing after birth is dependent upon the synthesis and secretion of pulmonary surfactant by alveolar type 2 (AT2) cells. Surfactant, a complex mixture of phospholipids and proteins, is secreted into the alveolus, where it reduces collapsing forces at the air-liquid interface to maintain lung volumes during the ventilatory cycle. ABCA3, an ATP-dependent Walker domain containing transport protein, is required for surfactant synthesis and lung function at birth. Mutations in ABCA3 cause severe surfactant deficiency and respiratory failure in newborn infants. We conditionally deleted the Abca3 gene in AT2 cells in the mature mouse lung. Loss of ABCA3 caused alveolar cell injury and respiratory failure. ABCA3-related lung dysfunction was associated with surfactant deficiency, inflammation, and alveolar-capillary leak. Extensive but incomplete deletion of ABCA3 caused alveolar injury and inflammation, and it initiated proliferation of progenitor cells, restoring ABCA3 expression, lung structure, and function. M2-like macrophages were recruited to sites of AT2 cell proliferation during the regenerative process and were present in lung tissue from patients with severe lung disease caused by mutations in ABCA3. The remarkable and selective regeneration of ABCA3-sufficient AT2 progenitor cells provides plausible approaches for future correction of ABCA3 and other genetic disorders associated with surfactant deficiency and acute interstitial lung disease.

The biology of the ABCA3 lipid transporter in lung health and disease

The lipid transporter, ATP-binding cassette class A3 (ABCA3), is a highly conserved multi-membrane-spanning protein that plays a critical role in the regulation of pulmonary surfactant homeostasis. Mutations in ABCA3 have been increasingly recognized as one of the causes of inherited pulmonary diseases. These monogenic disorders produce familial lung abnormalities with pathological presentations ranging from neonatal surfactant-deficiency-induced respiratory failure to childhood or adult diffuse parenchymal lung diseases for which specific treatment modalities remain limited. More than 200 ABCA3 mutations have been reported to date with approximately three quarters of patients presenting as compound heterozygotes. Recent advances in our understanding of the molecular basis underlying normal ABCA3 biosynthesis and processing and of the mechanisms of alveolar epithelial cell dysregulation caused by the expression of its mutant forms are beginning to emerge. These insights and the role of environmental factors and modifier genes are discussed in the context of the considerable variability in disease presentation observed in patients with identical ABCA3 gene mutations. Moreover, the opportunities afforded by an enhanced understanding of ABCA3 biology for targeted therapeutic strategies are addressed.

Neonatal Respiratory Failure with Retarded Perinatal Lung Maturation in Mice Caused by Reticulocalbin 3 Disruption

Reticulocalbin 3 (Rcn3) is an endoplasmic reticulum lumen protein localized to the secretory pathway. As a Ca2t-binding protein of 45 kDa (Cab45)/Rcn/ER Ca2t-binding protein of 55 kDa (ERC45)/calumenin (CREC) family member, Rcn3 is reported to function as a chaperone protein involved in protein synthesis and secretion; however, the biological role of Rcn3 is largely unknown. The results presented here, for the first time, depict an indispensable physiological role of Rcn3 in perinatal lung maturation by using an Rcn3 gene knockout mouse model. These mutant mice die immediately at birth owing to atelectasis-induced neonatal respiratory distress, although these embryos are produced with grossly normal development. This respiratory distress results from a failure of functional maturation of alveolar epithelial type II cells during alveogenesis. This immaturity of type II cells is associated with a dramatic reduction in surfactant protein A and D, a disruption in surfactant phospholipid homeostasis, and a disorder in lamellar body. In vitro studies further show that Rcn3 deficiency blunts the secretion of surfactant proteins and phospholipids from lung epithelial cells, suggesting a decrease in availability of surfactants for their surface activity. Collectively, these observations indicate an essential role of Rcn3 in perinatal lung maturation and neonatal respiratory adaptation as well as shed additional light on the mechanism of neonatal respiratory distress syndrome development.

ABCA3, a key player in neonatal respiratory transition and genetic disorders of the surfactant system

Genetic disorders of the surfactant system are rare diseases with a broad range of clinical manifestations, from fatal respiratory distress syndrome (RDS) in neonates to chronic interstitial lung disease (ILD) in children and adults. ABCA3 [ATP-binding cassette (ABC), subfamily A, member 3] is a lung-specific phospholipid transporter critical for intracellular surfactant synthesis and storage in lamellar bodies (LBs). Its expression is developmentally regulated, peaking prior to birth under the influence of steroids and transcription factors. Bi-allelic mutations of the ABCA3 gene represent the most frequent cause of congenital surfactant deficiency, indicating its critical role in lung function. Mutations affect surfactant lipid and protein processing and LBs’ morphology, leading to partial or total surfactant deficiency. Approximately 200 mutations have been reported, most of which are unique to individuals and families, which makes diagnosis and prognosis challenging. Various types of mutations, affecting different domains of the protein, account in part for phenotype diversity. Disease-causing mutations have been reported in most coding and some non-coding regions of the gene, but tend to cluster in the first extracellular loop and the second nucleotide-binding domain (NBD), leading to defective glycosylation and trafficking defects and interfering with ATP binding and hydrolysis respectively. Mono-allelic damaging and benign variants are often subclinical but may act as disease modifiers in lung diseases such as RDS of prematurity or associate with mutations in other surfactant-related genes. Diagnosis is complex but essential and should combine pathology and ultrastructure studies on lung biopsy with broad-spectrum genetic testing of surfactant-related genes, made possible by recent technology advances in the massive parallel sequencing technology.

Structural Features of the ATP-Binding Cassette (ABC) Transporter ABCA3

In this review we reported and discussed the structural features of the ATP-Binding Cassette (ABC) transporter ABCA3 and how the use of bioinformatics tools could help researchers to obtain a reliable structural model of this important transporter. In fact, a model of ABCA3 is still lacking and no crystallographic structures (of the transporter or of its orthologues) are available. With the advent of next generation sequencing, many disease-causing mutations have been discovered and many more will be found in the future. In the last few years, ABCA3 mutations have been reported to have important pediatric implications. Thus, clinicians need a reliable structure to locate relevant mutations of this transporter and make genotype/phenotype correlations of patients affected by ABCA3-related diseases. In conclusion, we strongly believe that the model preliminarily generated by these novel bioinformatics tools could be the starting point to obtain more refined models of the ABCA3 transporter.

Cellular mechanisms of alveolar pathology in childhood interstitial lung diseases: current insights from mouse genetics

PURPOSE OF REVIEW

Childhood interstitial lung diseases (ILDs) are a diverse class of disorders affecting the alveolar gas exchange region that lack specific treatments and are usually fatal. Here, we integrate recent insights into alveolar cell biology with histopathology from well characterized mutations of surfactant-associated genes. We take a reductionist approach by parsing discrete histological features and correlating each to perturbation of a particular function of the alveolar epithelial type II (AT2) cell, the central driver of disease, to generate a working model for the cellular mechanisms of disease pathogenesis.

RECENT FINDINGS

The application of genetically modified mice and single cell genomics has yielded new insights into lung biology, including the identification of a bipotent alveolar progenitor in development, mapping of adult AT2 stem cells in vivo, and demonstration that latent cooperative interactions with fibroblasts can be pathologically activated by targeted injury of the AT2 cell.

SUMMARY

As we learn more about individual and cooperative roles for alveolar cells in health, we can dissect how perturbations of specific cellular functions contribute to disease in childhood ILDs. We hope our updated model centered around the AT2 cell as the initiator of disease provides a cellular framework that researchers can build upon and revise as they identify the specific molecular signals within and between alveolar cells that mediate the diverse pathologic features, so that targeted pharmacologic and cell-based treatments for patients can ultimately be engineered.

Modeling human lung development and disease using pluripotent stem cells

Directed differentiation of human pluripotent stem cells (hPSCs) into mature cells, tissues and organs holds major promise for the development of novel approaches in regenerative medicine, and provides a unique tool for disease modeling and drug discovery. Sometimes underappreciated is the fact that directed differentiation of hPSCs also provides a unique model for human development, with a number of important advantages over model organisms. Here, I discuss the importance of using human stem cell models for understanding human lung development and disease.

Abca3 haploinsufficiency is a risk factor for lung injury induced by hyperoxia or mechanical ventilation in a murine model

BACKGROUND

Heterozygous ATP-binding-cassette subfamily A member 3 (ABCA3) mutations are associated with neonatal respiratory complications. In an adult murine model, we investigated whether Abca3 haploinsufficiency is a predisposing factor for lung injury induced by hyperoxia or mechanical ventilation.

METHODS

Abca3 haploinsufficient (Abca3(+/-)) and wild-type (WT) mice were prospectively randomized to 25 min of ventilation or 72 h of hyperoxia or left unchallenged in air.

RESULTS

As compared with WT mice, unchallenged Abca3(+/-) mice had significantly decreased lung phosphatidylcholine (PC) and phosphatidylglycerol (PG) levels (P < 0.02) and decreased lung compliance (P < 0.05). When ventilated for 25 min, Abca3(+/-) mice demonstrated a significantly greater increase in bronchoalveolar lavage (BAL) interleukins (P ≤ 0.01) and lung wet to dry ratio (P < 0.005). Hyperoxia resulted in increased compliance (P < 0.05) and total lung capacity (TLC) (P = 0.01) only in the Abca3(+/-) mice, consistent with enlarged alveolar spaces. The ratio of PC to PG in BAL-relevant for surfactant dysfunction-was significantly elevated by oxygen exposure, with the greatest increase in Abca3(+/-) mice.

CONCLUSION

In a murine model, Abca3 haploinsufficiency results in an altered biochemical and lung mechanical phenotype, as well as a greater lung injury induced by hyperoxia or mechanical ventilation. The inability to maintain a normal PC/PG ratio appears to play a key role.

ATP-binding cassette member A3 (E292V) gene mutation and pulmonary morbidity in very-low-birth-weight infants

AIM

ATP-binding cassette member A 3 (ABCA3) plays a critical role for the transport of surfactant phospholipids into the lamellar bodies of type II alveolar epithelial cells. Term infants carrying the E292V missense mutation of the gene encoding ABCA3 are likely to develop respiratory distress syndrome, and the mutation has also been linked to interstitial lung disease in paediatric patients. The aim of this study was to investigate the association of the E292V genotype with pulmonary morbidity in a large cohort of very-low-birth-weight (VLBW) infants.

METHODS

We performed a genetic association study with a prospective, population-based multi-centre cohort of 3177 VLBW infants born in 16 German study centres between 2003 and 2009 (German Neonatal Network). The ABCA3 genotype was determined by restriction fragment length polymorphism-PCR in genomic DNA samples derived from buccal swabs.

RESULTS

In a large cohort of 3177 VLBW infants, 11 individuals were found to be heterozygote for the E292V mutation (0.34%). After stratification according to ABCA3 genotype, no differences were noted for clinical characteristics, necessary treatments and neonatal pulmonary outcomes.

CONCLUSIONS

Within the size limits of our study cohort, the ABCA3 missense mutation E292V had no remarkable effect on pulmonary outcome in VLBW infants. Present results do not rule out the possibility that E292V phenotype is associated with minor difference in the morbidity.

A-Subclass ATP-Binding Cassette Proteins in Brain Lipid Homeostasis and Neurodegeneration

The A-subclass of ATP-binding cassette (ABC) transporters comprises 12 structurally related members of the evolutionarily highly conserved superfamily of ABC transporters. ABCA transporters represent a subgroup of "full-size" multispan transporters of which several members have been shown to mediate the transport of a variety of physiologic lipid compounds across membrane barriers. The importance of ABCA transporters in human disease is documented by the observations that so far four members of this protein family (ABCA1, ABCA3, ABCA4, ABCA12) have been causatively linked to monogenetic disorders including familial high-density lipoprotein deficiency, neonatal surfactant deficiency, degenerative retinopathies, and congenital keratinization disorders. Recent research also point to a significant contribution of several A-subfamily ABC transporters to neurodegenerative diseases, in particular Alzheimer's disease (AD). This review will give a summary of our current knowledge of the A-subclass of ABC transporters with a special focus on brain lipid homeostasis and their involvement in AD.

Conditional hypoxia inducible factor-1α induction in embryonic pulmonary epithelium impairs maturation and augments lymphangiogenesis

Hypoxia inducible factor (HIF) 1a, EPAS1 and NEPAS are expressed in the embryonic mouse lung and each isoform exhibits distinct spatiotemporal expression patterns throughout morphogenesis. To further assess the role of the HIF1a isoform in lung epithelial cell differentiation and homeostasis, we created transgenic mice that express a constitutively active isoform of human HIF-1a (HIF-1a three point mutant (TPM)), in a doxycycline-dependent manner. Expression of HIF1a TPM in the developing pulmonary epithelium resulted in lung hypoplasia characterized by defective branching morphogenesis, altered cellular energetics and impaired epithelial maturation, culminating in neonatal lethality at birth from severe respiratory distress. Histological and biochemical analyses revealed expanded glycogen pools in the pulmonary epithelial cells at E18.5, concomitant with decreased pulmonary surfactant, suggesting a delay or an arrest in maturation. Importantly, these defects occurred in the absence of apoptosis or necrosis. In addition, sub-pleural hemorrhaging was evident as early as E14.5 in HIF1a TPM lungs, despite normal patterning of the blood vasculature, consistent with defects in endothelial barrier function. Epithelial expression of HIF1a TPM also resulted in increased VEGFA and VEGFC production, an increase in the number of lymphatic vessels and indirect activation of the multiple Notch pathway components in endothelial precursor cells. Collectively, these data indicate that HIF-1a protein levels in the pulmonary epithelium must be tightly controlled for proper development of the epithelial and mesenchymal compartments.

Some ABCA3 mutations elevate ER stress and initiate apoptosis of lung epithelial cells

BACKGROUND

ABCA3 transporter (ATP-binding cassette transporter of the A subfamily) is localized to the limiting membrane of lamellar bodies, organelles for assembly and storage of pulmonary surfactant in alveolar epithelial type II cells (AECII). It transports surfactant phospholipids into lamellar bodies and absence of ABCA3 function disrupts lamellar body biogenesis. Mutations of the ABCA3 gene lead to fatal neonatal surfactant deficiency and chronic interstitial lung disease (ILD) of children. ABCA3 mutations can result in either functional defects of the correctly localized ABCA3 or trafficking/folding defects where mutated ABCA3 remains in the endoplasmic reticulum (ER).

METHODS

Human alveolar epithelial A549 cells were transfected with vectors expressing wild-type ABCA3 or one of the three ABCA3 mutant forms, R43L, R280C and L101P, C-terminally tagged with YFP or hemagglutinin-tag. Localization/trafficking properties were analyzed by immunofluorescence and ABCA3 deglycosylation. Uptake of fluorescent NBD-labeled lipids into lamellar bodies was used as a functional assay. ER stress and apoptotic signaling were examined through RT-PCR based analyses of XBP1 splicing, immunoblotting or FACS analyses of stress/apoptosis proteins, Annexin V surface staining and determination of the intracellular glutathion level.

RESULTS

We demonstrate that two ABCA3 mutations, which affect ABCA3 protein trafficking/folding and lead to partial (R280C) or complete (L101P) retention of ABCA3 in the ER compartment, can elevate ER stress and susceptibility to it and induce apoptotic markers in the cultured lung epithelial A549 cells. R43L mutation, resulting in a functional defect of the properly localized ABCA3, had no effect on intracellular stress and apoptotic signaling.

CONCLUSION

Our data suggest that expression of partially or completely ER localized ABCA3 mutant proteins can increase the apoptotic cell death of the affected cells, which are factors that might contribute to the pathogenesis of genetic ILD.

Pulmonary surfactant pathophysiology: current models and open questions

Pulmonary surfactant is an essential lipid-protein complex that stabilizes the respiratory units (alveoli) involved in gas exchange. Quantitative or qualitative derangements in surfactant are associated with severe respiratory pathologies. The integrated regulation of surfactant synthesis, secretion, and metabolism is critical for air breathing and, ultimately, survival. The goal of this review is to summarize our current understanding and highlight important knowledge gaps in surfactant homeostatic mechanisms.

Conditional deletion of Abca3 in alveolar type II cells alters surfactant homeostasis in newborn and adult mice

ATP-binding cassette A3 (ABCA3) is a lipid transport protein required for synthesis and storage of pulmonary surfactant in type II cells in the alveoli. Abca3 was conditionally deleted in respiratory epithelial cells (Abca3(Δ/Δ)) in vivo. The majority of mice in which Abca3 was deleted in alveolar type II cells died shortly after birth from respiratory distress related to surfactant deficiency. Approximately 30% of the Abca3(Δ/Δ) mice survived after birth. Surviving Abca3(Δ/Δ) mice developed emphysema in the absence of significant pulmonary inflammation. Staining of lung tissue and mRNA isolated from alveolar type II cells demonstrated that ∼50% of alveolar type II cells lacked ABCA3. Phospholipid content and composition were altered in lung tissue, lamellar bodies, and bronchoalveolar lavage fluid from adult Abca3(Δ/Δ) mice. In adult Abca3(Δ/Δ) mice, cells lacking ABCA3 had decreased expression of mRNAs associated with lipid synthesis and transport. FOXA2 and CCAAT enhancer-binding protein-α, transcription factors known to regulate genes regulating lung lipid metabolism, were markedly decreased in cells lacking ABCA3. Deletion of Abca3 disrupted surfactant lipid synthesis in a cell-autonomous manner. Compensatory surfactant synthesis was initiated in ABCA3-sufficient type II cells, indicating that surfactant homeostasis is a highly regulated process that includes sensing and coregulation among alveolar type II cells.

Genetic disorders of surfactant dysfunction

Mutations in the genes encoding the surfactant proteins B and C (SP-B and SP-C) and the phospholipid transporter, ABCA3, are associated with respiratory distress and interstitial lung disease in the pediatric population. Expression of these proteins is regulated developmentally, increasing with gestational age, and is critical for pulmonary surfactant function at birth. Pulmonary surfactant is a unique mixture of lipids and proteins that reduces surface tension at the air-liquid interface, preventing collapse of the lung at the end of expiration. SP-B and ABCA3 are required for the normal organization and packaging of surfactant phospholipids into specialized secretory organelles, known as lamellar bodies, while both SP-B and SP-C are important for adsorption of secreted surfactant phospholipids to the alveolar surface. In general, mutations in the SP-B gene SFTPB are associated with fatal respiratory distress in the neonatal period, and mutations in the SP-C gene SFTPC are more commonly associated with interstitial lung disease in older infants, children, and adults. Mutations in the ABCA3 gene are associated with both phenotypes. Despite this general classification, there is considerable overlap in the clinical and histologic characteristics of these genetic disorders. In this review, similarities and differences in the presentation of these disorders with an emphasis on their histochemical and ultrastructural features will be described, along with a brief discussion of surfactant metabolism. Mechanisms involved in the pathogenesis of lung disease caused by mutations in these genes will also be discussed.

ABCA12 maintains the epidermal lipid permeability barrier by facilitating formation of ceramide linoleic esters

Harlequin ichthyosis is a congenital scaling syndrome of the skin in which affected infants have epidermal hyperkeratosis and a defective permeability barrier. Mutations in the gene encoding a member of the ABCA transporter family, ABCA12, have been linked to harlequin ichthyosis, but the molecular function of the protein is unknown. To investigate the activity of ABCA12, we generated Abca12 null mice and analyzed the impact on skin function and lipid content. Abca12-/- mice are born with a thickened epidermis and die shortly after birth, as water rapidly evaporates from their skin. In vivo skin proliferation measurements suggest a lack of desquamation of the skin cells, rather than enhanced proliferation of basal layer keratinocytes, accounts for the 5-fold thickening of the Abca12-/- stratum corneum. Electron microscopy revealed a loss of the lamellar permeability barrier in Abca12-/- skin. This was associated with a profound reduction in skin linoleic esters of long-chain omega-hydroxyceramides and a corresponding increase in their glucosyl ceramide precursors. Because omega-hydroxyceramides are required for the barrier function of the skin, these results establish that ABCA12 activity is required for the generation of long-chain ceramide esters that are essential for the development of normal skin structure and function.

Harlequin ichthyosis model mouse reveals alveolar collapse and severe fetal skin barrier defects

Harlequin ichthyosis (HI), which is the most severe genodermatosis, is caused by loss-of-function mutations in ABCA12, a member of the ATP-binding cassette transporter family. To investigate the pathomechanism of HI and the function of the ABCA12 protein, we generated ABCA12-deficient mice (Abca12(-/-)) by targeting Abca12. Abca12(-/-) mice closely reproduce the human HI phenotype, showing marked hyperkeratosis with eclabium and skin fissure. Lamellar granule abnormalities and defective ceramide distribution were remarkable in the epidermis. Skin permeability assay of Abca12(-/-) fetuses revealed severe skin barrier dysfunction after the initiation of keratinization. Surprisingly, the Abca12(-/-) mice also demonstrated lung alveolar collapse immediately after birth. Lamellar bodies in alveolar type II cells of the Abca12(-/-) mice lacked normal lamellar structures. The level of surfactant protein B, an essential component of alveolar surfactant, was reduced in the Abca12(-/-) mice. Fetal therapeutic trials with systemic administration of retinoid or dexamethasone, which are effective for HI and respiratory distress, respectively, to the pregnant mother mice neither improved the skin phenotype nor extended the survival period. Our HI model mice reproduce the human HI skin phenotype soon after the initiation of fetal skin keratinization and provide evidence that ABCA12 plays pivotal roles in lung and skin barrier functions.