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Pulmonary surfactant, a complex consisting of 90% lipids and 10% specific proteins, lines the alveoli of the lung and prevents alveolar collapse and transudation by lowering the surface tension at the air-liquid interface. Dipalmitoylphosphatidylcholine constitutes approximately 50% of the surfactant lipids and is primarily responsible for the surface tension-lowering property of the surfactant mixture. This phospholipid, together with the other surfactant phospholipids, is produced at the endoplasmic reticulum of the alveolar type II epithelial cells. The characteristic lamellar bodies in these cells serve as storage depot for the surfactant before this is secreted onto the alveolar surface. This article reviews the pathways via which the surfactant lipids are synthesized, our current knowledge of the regulation of these pathways, and what is known about intracellular traffic of phospholipids from their site of synthesis to the lamellar bodies.

Lipids were long believed to have a structural role in biomembranes and a role in energy storage utilizing cellular lipid droplets and plasma lipoproteins. Research over the last decades has identified an additional role of lipids in cellular signaling, membrane microdomain organization and dynamics, and membrane trafficking. These properties make lipids an attractive target for pathogens to modulate host cell processes in order to allow their survival and replication. In this review we will summarize the often ingenious strategies of pathogens to modify the lipid homeostasis of host cells, allowing them to divert cellular processes. To this end pathogens take full advantage of the complexity of the lipidome. The examples are categorized in generalized and emerging principles describing the involvement of lipids in host-pathogen interactions. Several pathogens are described that simultaneously induce multiple changes in the host cell signaling and trafficking mechanisms. Elucidation of these pathogen-induced changes may have important implications for drug development. The emergence of high-throughput lipidomic techniques will allow the description of changes of the host cell lipidome at the level of individual molecular lipid species and the identification of lipid biomarkers.

Surfactant protein C (SP-C) is synthesized by type II pneumocytes as a 21-kD propeptide (proSP-C) which is proteolytically processed to a 4.2-kD dipalmitoylated protein. To characterize the processing of proSP-C and the role of the cysteine protease cathepsin H, we studied the localization of proSP-C and cathepsin H in human as well as proSP-C in rat lungs, the enzymatic cathepsin H activity in isolated rat lamellar bodies, and the cleavage of human proSP-C by purified cathepsin H. Using antisera directed against the N-terminal E(11)-R(23) (NPROSP-C(11-23)), the C-terminal G(162)-G(174) domain (CPROSP-C(162-174)) of proSP-C, and against cathepsin H, immunogold labeling identified all three in electron-dense multivesicular bodies, but only NPROSP-C(11-23) and cathepsin H in composite as well as lamellar bodies of type II pneumocytes. Immuno double-labeling further distinguished electron-dense vesicles containing cathepsin H or electron light vesicles/multivesicular bodies containing proSP-C. Isolated lamellar bodies contained enzymatically active cathepsin H, a 6-kD proSP-C processing intermediate detected only by NPROSP-C(11-23), and mature SP-C. Using enzyme activities comparable to those in isolated lamellar bodies, purified cathepsin H generated a partially N-terminal processed proSP-C intermediate in vitro. In conclusion, our results indicate that after the fusion of electron-dense vesicles containing cathepsin H and electron-light vesicles or multivesicular bodies containing proSP-C, cathepsin H is involved in the first N-terminal processing step of proSP-C in electron-dense multivesicular bodies of type II pneumocytes.

Surfactant protein B (SP-B) is a 17-kDa dimeric protein produced by alveolar type II cells. Its main function is to lower the surface tension by inserting lipids into the air/liquid interface of the lung. SP-B's function can be mimicked by a 25-amino acid peptide, SP-B(1-25), which is based on the N-terminal sequence of SP-B. We synthesized a dimeric version of this peptide, dSP-B(1-25), and the two peptides were tested for their surface activity. Both SP-B(1-25) and dSP-B(1-25) showed good lipid mixing and adsorption activities. The dimeric peptide showed activity comparable to that of native SP-B in the pressure-driven captive bubble surfactometer. Spread surface films led to stable near-zero minimum surface tensions during cycling while protein free, and films containing SP-B(1-25) lost material from the interface during compression. We propose that dimerization of the peptide is required to create a lipid reservoir attached to the monolayer from which new material can enter the surface film upon expansion of the air/liquid interface. The dimeric state of SP-B can fulfill the same function in vivo.

Influenza A virus (IAV) infections are a major cause of respiratory disease of humans and animals. Pigs can serve as important intermediate hosts for transmission of avian IAV strains to humans, and for the generation of reassortant strains; this may result in the appearance of new pandemic IAV strains in humans. We have studied the role of the porcine lung collectins surfactant proteins D and A (pSP-D and pSP-A), two important components of the innate immune response against IAV. Hemagglutination inhibition assays revealed that both pSP-D and pSP-A display substantially greater inhibitory activity against IAV strains isolated from human, swine, and horse, than lung collectins from other animal species. The more potent activity of pSP-D results from interactions mediated by the asparagine-linked oligosaccharide located in the carbohydrate recognition domain of pSP-D, which is absent in SP-Ds from other species characterized to date. Presence of this sialylated oligosaccharide moiety enhances the anti-influenza activity of pSP-D, as demonstrated by assays of viral aggregation, inhibition of infectivity, and neutrophil response to IAV. The greater hemagglutination inhibitory activity of pSP-A is due to porcine-specific structural features of the conserved asparagine-linked oligosaccharide in the carbohydrate recognition domain of SP-A. A more efficient lung collectin-mediated immune response against IAV in pigs may play a role in providing conditions by which pigs can act as “mixing vessel” hosts that can lead to the production of reassortant, pandemic strains of IAV.

Summary1. Golgi complex, rough and smooth microsomes, plasma membranes, mitochondria and nuclei from rat liver were isolated and their purity assessed using specific marker enzymes.2. The various subcellular fractions were assayed for the following processes: biosynthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine, the conversion of phosphatidylserine into phosphatidylethanolamine, the formation of lecithin via N-methylation, and the activation of palmitic and octanoic acids.3. None of these processes were found to be present in Golgi complex. 4. The endoplasmic reticulum appears to be the principal site in the cell for the synthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine and the formation of lecithin. Interestingly, the biosynthesis of phosphatidylserine appears to be four times more active in rough than in smooth microsomes, which might suggest a ribosomal localization of this process.5. Except for CDPdiglyceride synthesis, mitochondria do not contain any of the synthesizing activities described in 4. Mitochondria are, however, the only site in the cell where phosphatidylserine is decarboxylated. This activity appears to be localized in the inner membrane.6. The activation of palmitate is localized predominantly in endoplasmic reticulum and mitochondria, though some activity was detected in plasma membranes as well. All other cell organelles, including Golgi and probably nuclei, did not contain significant palmitoyl-CoA synthetase activity. The sub-

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