ABCA3+/- mice have significantly decreased amounts of phosphatidylcholine and phosphatidylglycerol in lung tissue

Introduction: ABCA3 (ATP-binding cassette A3 transporter) is found predominantly in the membrane of lamellar bodies in lung type II alveolar cells, where it mediates ATP-dependent import of surfactant phospholipids. Haplotype analysis of the ABCA3 gene has identified a polymorphism which is significantly associated with respiratory distress and bronchopulmonary dysplasia in premature infants. Heterozygosity of ABCA3 mutations is also known to affect the severity of lung disease associated with other surfactant protein mutations. ABCA3 deficiency in humans and mice results in decreased phospholipid content, absence of surfactant and irreversible respiratory distress leading to death. ABCA3^+/- (heterozygous for null allele) mice survive the neonatal period but have increased leukocyte count in their bronchoalveolar lavage fluid. As it is possible that loss of one functioning allele of ABCA3 translates into a change in lung phospholipid content, we investigated the phospholipid content of surfactant in ABCA3^+/- mice. Methods: The lung tissues of wildtype (WT) and ABCA3^+/- adult as well as neonatal mice were investigated for phopsholipid content (nmol/mg wet weight) by mass spectrometry. In each group, lung tissues from mice were dissected and total lipids extracted. These were then resuspended in chloroform, and an aliquot taken for mass spectrometry analysis.

Results: Total phospholipids were significantly lower in ABCA3^+/- adult mice as compared to WT mice (36.5 vs. 38.1, p=0.03). As the predominant phospholipids in the surfactant are phosphatidylcholine (PC), phosphatidylglycerol (PG) and phosphatidylinositol (PI), these were further investigated. Dipalmitoylated PC (DPPC) was significantly lower in the ABCA3^+/- mice as compared to WT in both newborns (2.1±0.4 vs. 2.8±0.3, p=0.04) and adults (4.2±0.4 vs. 4.5±0.5; p=0.01). In adults, this difference was seen only in the female mice (females 4.1 vs. 4.7, p=0.01; males 4.2 vs. 4.2). Similarly, levels of PC 32:1 and PC 34:2 were significantly lower. A corresponding increase in PCs with long acyl chains was observed. Total PG was significantly decreased in the ABCA3^+/- mice, both newborns and adults, with no difference observed between the sexes (p=0.01). Conclusion: ABCA3^+/- status results in a decrease in total phospholipids, PC and PG in the lungs. In addition, a significant reduction in DPPC and PC 32:1 is observed. As DPPC is the major surface-active component in surfactant, we speculate that a significant decrease of DPPC might translate into a greater susceptibility to lung injury either due to external stimuli or due to prematurity. However, this needs to be investigated in further studies.