Interactions of The Low Molecular Weight Group of Surfactant-associated Proteins (sp 5–18) With Pulmonary Surfactant Lipids

The interaction of the low molecular weight group of surfactant-associated proteins, SP 5–18, with the major phospholipids of pulmonary surfactant was studied by fluorescence measurements of liposomal permeability and fusion, morphological studies, and surface activity measurements. The ability of SP 5–18 to increase the permeability of large unilamellar lipid vesicles was enhanced by the presence of negatively charged phospholipid. The permeability of these vesicles increased as the protein concentration was raised and the pH was lowered. SP 5–18 also induced leakage from liposomes made both from a synthetic surfactant lipid mixture and from lipids separated from SP 5–18 during its purification from canine sources. When SP 5–18 was added to egg phosphatidylglycerol liposomes, the population of liposomes which became permeable leaked all encapsulated contents, while the remaining liposomes did not leak at all. The extent of leakage was higher in the presence of 3 mM calcium. SP 5–18 also induced lipid mixing between two populations of egg phosphatidylglycerol liposomes in the presence of 3 mM calcium, as monitored by resonance energy transfer between two different fluorescent lipid probes, N-(7-nitro-2,l,3-benzoxadiazol-4-yl)phosphatidylethanolamine and N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine. Negative-staining electron microscopy showed that the addition of SP 5–18 and 3 mM calcium produced vesicles twice the size of control egg phosphatidylglycerol liposomes. In addition, surface balance measurements revealed that the adsorption of liposomal lipids to an air/water interface was enhanced by the presence of SP 5–18, negatively charged phospholipids, and 3 mM calcium. These observations suggest a similar lipid dependence for the interactions observed in the fluorescence and adsorption experiments. © 1988, American Chemical Society. All rights reserved

Interaction of erythrocyte protein 4.1 with phospholipids. A monolayer and liposome study

We have studied the interaction of purified human erythrocyte protein 4.1 with phospholipid membranes by monitoring both the increase in surface pressure of monolayers at the air/water interface and the change in permeability in liposomes to fluorescent molecules, in the presence of protein 4.1. Protein 4.1 penetrated into monolayers of brain phosphatidylserine (PS) and egg phosphatidylcholine (PC), even above surface pressures of 30 mN/m. Protein 4.1 increased the permeability of negatively charged PS, but not PC, liposomes, measured as the increase in fluorescence when encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and p-xylenebispyridinium bromide (DPX) or carboxyfluorescein were released into the medium. The interaction of protein 4.1 with PS large unilamellar vesicles (LUV) was increased as the pH and the ionic strength were lowered, and decreased as the Ca2+ or Mg2+ concentrations and ionic strength were raised. In order to study the relevance of these measurements to the erythrocyte, we prepared LUV of synthetic lipid mixtures characteristic of both the inner and the outer membrane leaflets. Protein 4.1 increased the permeability of inner, but not outer, leaflet LUV at both pH 6.0 and 7.4. These observations suggest that negatively charged phospholipid domains around the protein 4.1 high-affinity protein-binding site(s) may contribute to the anchoring of protein 4.1 to the cytoplasmic surface of the red cell membrane. © 1988

Pulmonary Collectins Modulate Strain-Specific Influenza A Virus Infection and Host Responses

Collectins are secreted collagen-like lectins that bind, agglutinate, and neutralize influenza A virus (IAV) in vitro. Surfactant proteins A and D (SP-A and SP-D) are collectins expressed in the airway and alveolar epithelium and could have a role in the regulation of IAV infection in vivo. Previous studies have shown that binding of SP-D to IAV is dependent on the glycosylation of specific sites on the HA1 domain of hemagglutinin on the surface of IAV, while the binding of SP-A to the HA1 domain is dependent on the glycosylation of the carbohydrate recognition domain of SP-A. Here, using SP-A and SP-D gene-targeted mice on a common C57BL6 background, we report that viral replication and the host response as measured by weight loss, neutrophil influx into the lung, and local cytokine release are regulated by SP-D but not SP-A when the IAV is glycosylated at a specific site (N165) on the HA1 domain. SP-D does not protect against IAV infection with a strain lacking glycosylation at N165. With the exception of a small difference on day 2 after infection with X-79, we did not find any significant difference in viral load in SP-A(−/−) mice with either IAV strain, although small differences in the cytokine responses to IAV were detected in SP-A(−/−) mice. Mice deficient in both SP-A and SP-D responded to IAV similarly to mice deficient in SP-D alone. Since most strains of IAV currently circulating are glycosylated at N165, SP-D may play a role in protection from IAV infection