Metabolism of surfactant phosphatidylcholine molecular species in cftr(tm1HGU/tm1HGU) mice compared to MF-1 mice

Choline in cystic fibrosis: relations to pancreas insufficiency, enterohepatic cycle, PEMT and intestinal microbiota

BACKGROUND

Cystic Fibrosis (CF) is an autosomal recessive disorder with life-threatening organ manifestations. 87% of CF patients develop exocrine pancreas insufficiency, frequently starting in utero and requiring lifelong pancreatic enzyme substitution. 99% develop progressive lung disease, and 20-60% CF-related liver disease, from mild steatosis to cirrhosis. Characteristically, pancreas, liver and lung are linked by choline metabolism, a critical nutrient in CF. Choline is a tightly regulated tissue component in the form of phosphatidylcholine (Ptd'Cho) and sphingomyelin (SPH) in all membranes and many secretions, particularly of liver (bile, lipoproteins) and lung (surfactant, lipoproteins). Via its downstream metabolites, betaine, dimethylglycine and sarcosine, choline is the major one-carbon donor for methionine regeneration from homocysteine. Methionine is primarily used for essential methylation processes via S-adenosyl-methionine.

CLINICAL IMPACT

CF patients with exocrine pancreas insufficiency frequently develop choline deficiency, due to loss of bile Ptd'Cho via feces. ~ 50% (11-12 g) of hepatic Ptd'Cho is daily secreted into the duodenum. Its re-uptake requires cleavage to lyso-Ptd'Cho by pancreatic and small intestinal phospholipases requiring alkaline environment. Impaired CFTR-dependent bicarbonate secretion, however, results in low duodenal pH, impaired phospholipase activity, fecal Ptd'Cho loss and choline deficiency. Low plasma choline causes decreased availability for parenchymal Ptd'Cho metabolism, impacting on organ functions. Choline deficiency results in hepatic choline/Ptd'Cho accretion from lung tissue via high density lipoproteins, explaining the link between choline deficiency and lung function. Hepatic Ptd'Cho synthesis from phosphatidylethanolamine by phosphatidylethanolamine-N-methyltransferase (PEMT) partly compensates for choline deficiency, but frequent single nucleotide polymorphisms enhance choline requirement. Additionally, small intestinal bacterial overgrowth (SIBO) frequently causes intraluminal choline degradation in CF patients prior to its absorption. As adequate choline supplementation was clinically effective and adult as well as pediatric CF patients suffer from choline deficiency, choline supplementation in CF patients of all ages should be evaluated.

Phosphatidylcholine kinetics in neonatal rat lungs and the effects of rhuKGF and betamethasone

Surfactant, synthesized by type II pneumocytes (PN-II), mainly comprises phosphatidylcholine (PC) and is essential to prevent neonatal respiratory distress. Furthermore, PC is essential to lung tissue growth and maintenance as a membrane component. Recent findings suggest that the lung contributes to systemic lipid homeostasis via PC export through ABC-A1 transporter expression. Hence it is important to consider pharmacological interventions in neonatal lung PC metabolism with respect to such export. Five-day-old rats were treated with carrier (control), intraperitoneal betamethasone, subcutaneous recombinant human keratinocyte growth factor (rhuKGF), or their combination for 48 h. Animals were intraperitoneally injected with 50 mg/kg [D9-methyl]choline chloride 1.5, 3.0, and 6.0 h before death at day 7, and lung lavage fluid (LLF) and tissue were harvested. Endogenous PC, D9-labeled PC species, and their water-soluble precursors (D9-)choline and (D9-)phosphocholine were determined by tandem mass spectrometry. Treatment increased secreted and tissue PC pools but did not change equilibrium composition of PC species in LLF. However, all treatments increased specific surfactant components in tissue. In control rats, peak D9-PC in lavaged lung was reached after 3 h and was decreased at 6 h. Only 13% of this net loss in lavaged lung was found in LLF. Such decrease was not present in lungs treated with betamethasone and/or with rhuKGF. D9-PC loss at 3–6 h and PC synthesis calculated from D9 enrichment of phosphocholine indicated that daily synthesis rate is higher than total pool size. We conclude that lung tissue contributes to systemic PC homeostasis in neonatal rats, which is altered by glucocorticoid and rhuKGF treatment.

Effects of recombinant human keratinocyte growth factor on surfactant, plasma, and liver phospholipid homeostasis in hyperoxic neonatal rats

Respiratory distress and bronchopulmonary dysplasia (BPD) are major problems in preterm infants that are often addressed by glucocorticoid treatment and increased oxygen supply, causing catabolic and injurious side effects. Recombinant human keratinocyte growth factor (rhKGF) is noncatabolic and antiapoptotic and increases surfactant pools in immature lungs. Despite its usefulness in injured neonatal lungs, the mechanisms of improved surfactant homeostasis in vivo and systemic effects on lipid homeostasis are unknown. We therefore exposed newborn rats to 85% vs. 21% oxygen and treated them systemically with rhKGF for 48 h before death at 7 days. We determined type II pneumocyte (PN-II) proliferation, surfactant protein (SP) mRNA expression, and the pulmonary metabolism of individual phosphatidylcholine (PC) species using [D(9)-methyl]choline and tandem mass spectrometry. In addition, we assessed liver and plasma lipid metabolism, addressing PC synthesis de novo, the liver-specific phosphatidylethanolamine methyl transferase (PEMT) pathway, and triglyceride concentrations. rhKGF was found to maintain PN-II proliferation and increased SP-B/C expression and surfactant PC in both normoxic and hyperoxic lungs. We found increased total PC together with decreased [D(9)-methyl]choline enrichment, suggesting decreased turnover rather than increased secretion and synthesis as the underlying mechanism. In the liver, rhKGF increased PC synthesis, both de novo and via PEMT, underlining the organotypic differences of rhKGF actions on lipid metabolism. rhKGF increased the hepatic secretion of newly synthesized polyunsaturated PC, indicating improved systemic supply with choline and essential fatty acids. We suggest that rhKGF has potential as a therapeutic agent in neonates by improving pulmonary and systemic PC homeostasis.

Transient in utero disruption of cystic fibrosis transmembrane conductance regulator causes phenotypic changes in alveolar type II cells in adult rats

Background

Mechanicosensory mechanisms regulate cell differentiation during lung organogenesis. We have previously demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR) was integral to stretch-induced growth and development and that transient expression of antisense-CFTR (ASCFTR) had negative effects on lung structure and function. In this study, we examined adult alveolar type II (ATII) cell phenotype after transient knock down of CFTR by adenovirus-directed in utero expression of ASCFTR in the fetal lung.

Results

In comparison to (reporter gene-treated) Controls, ASCFTR-treated adult rat lungs showed elevated phosphatidylcholine (PC) levels in the large but not in the small aggregates of alveolar surfactant. The lung mRNA levels for SP-A and SP-B were lower in the ASCFTR rats. The basal PC secretion in ATII cells was similar in the two groups. However, compared to Control ATII cells, the cells in ASCFTR group showed higher PC secretion with ATP or phorbol myristate acetate. The cell PC pool was also larger in the ASCFTR group. Thus, the increased surfactant secretion in ATII cells could cause higher PC levels in large aggregates of surfactant. In freshly isolated ATII cells, the expression of surfactant proteins was unchanged, suggesting that the lungs of ASCFTR rats contained fewer ATII cells. Gene array analysis of RNA of freshly isolated ATII cells from these lungs showed altered expression of several genes including elevated expression of two calcium-related genes, Ca²⁺-ATPase and calcium-calmodulin kinase kinase1 (CaMkk1), which was confirmed by real-time PCR. Western blot analysis showed increased expression of calmodulin kinase I, which is activated following phosphorylation by CaMkk1. Although increased expression of calcium regulating genes would argue in favor of Ca²⁺-dependent mechanisms increasing surfactant secretion, we cannot exclude contribution of alternate mechanisms because of other phenotypic changes in ATII cells of the ASCFTR group.

Conclusion

Developmental changes due to transient disruption of CFTR in fetal lung reflect in altered ATII cell phenotype in the adult life.

Molecular and functional changes of pulmonary surfactant in response to hyperoxia

Surfactant comprises phosphatidylcholine (PC) together with anionic phospholipids, neutral lipids, and surfactant proteins SP-A to-D. Its composition is highly specific, with dipalmitoyl-PC, palmitoyl-myristoyl-PC, and palmitoyl-palmitoleoyl-PC as its predominant PC species, but with low polyunsaturated phospholipids. Changes in pulmonary metabolism and function in response to injuries depend on their duration and whether adaptation can occur. We examined in rats prolonged (7 days) versus acute (2 days) exposure to non-lethal oxygen concentrations (85%) with respect to the composition and metabolism of individual lung phospholipid molecular species. Progressive inflammation, structural alteration, and involvement of type II pneumocytes were confirmed by augmented bromodeoxyuridine incorporation, broadening of alveolar septa, and increased granulocyte, macrophage, SP-A, and SP-D concentrations. Surfactant function was impaired after 2 days, but normalized with duration of hyperoxia, which was attributable to inhibition but not to alteration in SP-B/C concentrations. Phospholipid pool sizes and PC synthesis by lung tissue, as assessed by [methyl-(3)H]-choline incorporation, were unchanged after 2 days, although after 7 days they were elevated 1.7-fold. By contrast, incorporation of labeled PC into tissue pools of surfactant and lung lavage fluid decreased progressively. Moreover, concentrations of arachidonic acid containing phospholipids were augmented at the expense of saturated palmitoyl-myristoyl-PC and dipalmitoyl-PC. We conclude a persisting impairment in the intracellular trafficking and secretion of newly synthesized PC, accompanied by a progressive increase in alveolar arachidonic acid containing phospholipids in spite of recovery of acutely impaired surfactant function and adaptive increase of overall PC synthesis.

Lipidomics of cellular and secreted phospholipids from differentiated human fetal type II alveolar epithelial cells

Maturation of fetal alveolar type II epithelial cells in utero is characterized by specific changes to lung surfactant phospholipids. Here, we quantified the effects of hormonal differentiation in vitro on the molecular specificity of cellular and secreted phospholipids from human fetal type II epithelial cells using electrospray ionization mass spectrometry. Differentiation, assessed by morphology and changes in gene expression, was accompanied by restricted and specific modifications to cell phospholipids, principally enrichments of shorter chain species of phosphatidylcholine (PC) and phosphatidylinositol, that were not observed in fetal lung fibroblasts. Treatment of differentiated epithelial cells with secretagogues stimulated the secretion of functional surfactant-containing surfactant proteins B and C (SP-B and SP-C). Secreted material was further enriched in this same set of phospholipid species but was characterized by increased contents of short-chain monounsaturated and disaturated species other than dipalmitoyl PC (PC16:0/16:0), principally palmitoylmyristoyl PC (PC16:0/14:0) and palmitoylpalmitoleoyl PC (PC16:0/16:1). Mixtures of these PC molecular species, phosphatidylglycerol, and SP-B and SP-C were functionally active and rapidly generated low surface tension on compression in a pulsating bubble surfactometer. These results suggest that hormonally differentiated human fetal type II cells do not select the molecular composition of surfactant phospholipid on the basis of saturation but, more likely, on the basis of acyl chain length.

Mass Spectrometric Analysis of Surfactant Metabolism in Human Volunteers Using Deuteriated Choline

Surfactant reduces surface tension at pulmonary air-liquid interfaces. Although its major component is dipalmitoyl-phosphatidylcholine (PC16:0/16:0), other PC species, principally palmitoylmyristoyl-PC, palmitoylpalmitoleoyl-PC, and palmitoyloleoyl-PC, are integral components of surfactant. The composition and metabolism of PC species depend on pulmonary development, respiratory rate, and pathologic alterations, which have largely been investigated in animals using radiolabeled precursors. Recent advances in mass spectrometry and availability of precursors carrying stable isotopes make metabolic experiments in human subjects ethically feasible. We introduce a technique to quantify surfactant PC synthesis in vivo using deuteriated choline coupled with electrospray ionization tandem mass spectrometry. Endogenous PC from induced sputa of healthy volunteers comprised 54.0 +/- 1.5% PC16:0/16:0, 9.7 +/- 0.7% palmitoylmyristoyl-PC, 10.0 +/- 1.0% palmitoylpalmitoleoyl-PC, and 13.1 +/- 0.3% palmitoyloleoyl-PC. Infusion of deuteriated choline chloride (3.6 mg/kg body weight) over 3 hours resulted in linear incorporation into PC over 30 hours. After a plateau of 0.61 +/- 0.04% labeled PC between 30 and 48 hours, incorporation decreased to 0.30 +/- 0.02% within 7 days. Compared with native PC, fractional label was initially lower for PC16:0/16:0 (31.9 +/- 8.3%) but was higher for palmitoyloleoyl-PC (21.0 +/- 1.2%), and equilibrium was achieved after only 48 hours. We conclude that infusion of deuteriated choline and electrospray ionization tandem mass spectrometry is useful to investigate surfactant metabolism in humans in vivo.

Residual cftr expression varies with age in cftr(tm1Hgu) cystic fibrosis mice: impact on morphology and physiology

Mouse models for cystic fibrosis (CF) mimic intestinal manifestations of the human disease, but the lung disease phenotypes are lacking in most strains. In this work, the issue was addressed whether aging of the respiratory tract leads to lung pathophysiology in the exon 10 insertional mutant cftr(tm1Hgu) mouse. Weight gain, body weight and life-span of cftr(tm1Hgu) mice were significantly reduced compared with control mice. cftr(tm1Hgu) mice expressed 20, 21 or 37% (median) of wild-type cystic fibrosis conductance transmembrane regulator (cftr) mRNA transcript in lungs, intestine and kidney. Wild-type cftr mRNA in renal and respiratory epithelia varied with age from levels similar to Ztm:MF1 controls at the age of 2 and 4 months to levels seen in patients with CFTR splice mutations beyond the age of 6 months. The morphology of the bronchi and more distal airways was apparently normal in cftr(tm1Hgu) mice during their first year of life. The alveolar surfactant phospholipid pool was increased in cftr(tm1Hgu) mice by 1.5- to 2-fold compared with Ztm:MF1 controls. Alveolar clearance of gamma-labelled scandium oxide - the first report of lung clearance measurement in living mice - was reduced in cftr(tm1Hgu) mice compared with littermate controls. Although no progressive lung pathology was seen in the cftr expression of cftr(tm1Hgu) mice, surfactant phospholipid homeostasis, and alveolar and mucociliary clearance were abnormal. Therefore, the described model is useful for studying the initial CF lung pathophysiology.

Molecular species compositions of lung and pancreas phospholipids in the cftr(tm1HGU/tm1HGU) cystic fibrosis mouse

Fatty acid analysis of phospholipid compositions of lung and pancreas cells from a cystic fibrosis transmembrane regulator (CFTR) negative mouse (cftr(-/-))suggested that a decreased concentration of docosahexaenoate (22:6(n-3)) and increased arachidonate (20:4(n-6)) may be related to the disease process in cystic fibrosis (CF). Consequently, we have determined compositions of the major phospholipids of lung, pancreas, liver, and plasma from a different mouse model of CF, the cftr(tm1HGU/tm1HGU) mouse, compared with ZTM:MF-1 control mice. Electrospray ionization mass spectrometry permitted the quantification of all of the individual molecular species of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylglycerol (PtdGly), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). There was no deficiency of 22:6(n-3) in any phospholipid class from lung, pancreas, or liver from mice with the cftr(tm1HGU/tm1HGU). Instead, the concentration of 20:4(n-6) was significantly decreased in plasma PtdCho species and in pancreas and lung species of PtdEtn, PtdSer, and PtdIns. These results demonstrate the variability of membrane phospholipid compositions in different mouse models of CF and suggest that in cftr(tm1HGU/tm1HGU) mice, the apparent deficiency was of 20:4n-6- rather than of 22:6n-3-containing phospholipid species. They highlight a need for detailed phospholipid molecular species analysis of cells expressing mutant CFTR from children with CF before the therapeutic effects of administering high doses of 22:6(n-3)-containing oils to children with CF can be fully evaluated.

Phosphatidylcholine molecular species in lung surfactant: composition in relation to respiratory rate and lung development

Surfactant reduces surface tension at the air-liquid interface of lung alveoli. While dipalmitoylphosphatidylcholine (PC16:0/ 16:0) is its main component, proteins and other phospholipids contribute to the dynamic properties and homeostasis of alveolar surfactant. Among these components are significant amounts of palmitoylmyristoylphosphatidylcholine (PC16:0/ 14:0) and palmitoylpalmitoleoylphosphatidylcholine (PC16:0/ 16:1), whereas in surfactant from the rigid tubular bird lung, PC16:0/14:0 is absent and PC16:0/16:1 strongly diminished. We therefore hypothesized that the concentrations of PC16:0/14:0 and PC16:0/16:1 in surfactants correlate with differences in the respiratory physiology of mammalian species. In surfactants from newborn and adult mice, rats, and pigs, molar fractions of PC16:0/14:0 and PC16:0/16:1 correlated with respiratory rate. Labeling experiments with [methyl-(3)H]choline in mice and perfused rat lungs demonstrated identical alveolar proportions of total and newly synthesized PC16:0/14:0, PC16:0/16:1, and PC16:0/16:0, which were much higher than those of other phosphatidylcholine species. In surfactant from human term and preterm neonates, fractional concentrations not only of PC16:0/16:0 but also of PC16:0/14:0 and PC16:0/ 16:1 increased with maturation. Our data emphasize that PC16:0/14:0 and PC16:0/16:1 may be important surfactant components in alveolar lungs, and that their concentrations are adapted to respiratory physiology.