Article pubs.acs.org/biochemistry
Environmental Pollutant Ozone Causes Damage to Lung Surfactant Protein B (SP-B) Joanna M. Hemming,† Brian R. Hughes,† Adrian R. Rennie,‡ Salvador Tomas,† Richard A. Campbell,§ Arwel V. Hughes,∥ Thomas Arnold,⊥ Stanley W. Botchway,# and Katherine C. Thompson*,† †
Department of Biological Sciences and Institute of Structural and Molecular Biology, Birkbeck College, University of London, Malet Street, London WC1E 7HX, U.K. ‡ Materials Physics, Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden § Institut Laue-Langevin, 71 Avenue des Martyrs, CS20156, 38042 Grenoble Cedex 09, France ∥ ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0QX, U.K. ⊥ Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, U.K. # STFC, Lasers for Science Facility, Central Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, U.K. S Supporting Information *
ABSTRACT: Lung surfactant protein B (SP-B) is an essential protein found in the surfactant fluid at the air−water interface of the lung. Exposure to the air pollutant ozone could potentially damage SP-B and lead to respiratory distress. We have studied two peptides, one consisting of the N-terminus of SP-B [SP-B(1−25)] and the other a construct of the N- and C-termini of SP-B [SP-B(1−25,63−78)], called SMB. Exposure to dilute levels of ozone (∼2 ppm) of monolayers of each peptide at the air−water interface leads to a rapid reaction, which is evident from an increase in the surface tension. Fluorescence experiments revealed that this increase in surface tension is accompanied by a loss of fluorescence from the tryptophan residue at the interface. Neutron and X-ray reflectivity experiments show that, in contrast to suggestions in the literature, the peptides are not solubilized upon oxidation but rather remain at the interface with little change in their hydration. Analysis of the product material reveals that no cleavage of the peptides occurs, but a more hydrophobic product is slowly formed together with an increased level of oligomerization. We attributed this to partial unfolding of the peptides. Experiments conducted in the presence of phospholipids reveal that the presence of the lipids does not prevent oxidation of the peptides. Our results strongly suggest that exposure to low levels of ozone gas will damage SP-B, leading to a change in its structure. The implication is that the oxidized protein will be impaired in its ability to interact at the air−water interface with negatively charged phosphoglycerol lipids, thus compromising what is thought to be its main biological function.
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unsaturated, phospholipids present such as 1-palmitoyl-2oleoyl-sn-glycero-3-phosphocholine (POPC) and its anionic analogue, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), are required to fluidize the otherwise fairly rigid DPPC monolayer and thus provide the required dynamical properties of the interfacial layer.2 The two proteins directly associated with the air−water interface, SP-B and SP-C, interact with the lipids and are implicated in the transfer of lipids to and retention of lipids at the interface.2 The presence of SP-B is vital for survival. Human infants born, at full term, without a working version of SP-B due to a genetic disorder do not survive.8−11 Mice with the SP-B gene deleted from both chromosomes die within days of birth.12 The presence of SP-C
he air−water interface of the lung requires a layer of surfactant material to prevent alveolar collapse.1 The exact composition of the surfactant material at the interface varies between species but contains mainly lipids, ∼90% by weight, and two hydrophobic proteins, surfactant protein B (SP-B) and surfactant protein C (SP-C), which make up the remaining 10%.2 A range of lipids are required for correct respiratory function, and various types of phospholipids constitute around 80% by weight of lung surfactant and neutral lipids, mostly cholesterol, ∼10%. Approximately half the total phospholipid present in humans is the saturated lipid 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC).3 However, the condition of premature infants, and animals, born with an insufficiency of lung surfactant does not dramatically improve if they are treated with a surfactant of pure DPPC, whereas treatment with lipid mixtures or lipid/protein mixtures has been shown to be very successful.4−7 The general consensus is that the other, mainly © XXXX American Chemical Society
Received: March 20, 2015 Revised: July 30, 2015
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DOI: 10.1021/acs.biochem.5b00308 Biochemistry XXXX, XXX, XXX−XXX
Article
Biochemistry
Figure 1. Primary structures of the full-length human protein SP-B (UniProt entry sp|P07988|201−279), N- and C-terminal construct SMB, and Nterminal peptide SP-B(1−25).
when in micelles and lipid bilayers.27,28 A recent review by Olmeda et al. discusses the likely structure of SP-B in lipid membranes based on sequence homology with other proteins in the saposin family, all of which interact with lipids.29 Structural studies have been performed on the peptide components of SP-B studied in this work, SP-B(1−25) and SMB. Gordon et al. studied the structure of SP-B(1−25) in POPG vesicles and found it to be predominantly α-helical; Kurutz and Lee reported that the shorter peptide SP-B(11−25) dissolved in methanol was α-helical, and Booth et al. studied the structure of C-terminal segment SP-B(63−78) in SDS micelles and found that it consisted of an amphipathic helix.30−32 Shanmukh et al. reported that the secondary structure of the monomeric form of SP-B(1−25) at the air−water interface depended upon the surrounding environment.33 The peptide had a predominantly α-helical structure when present as a pure film, a predominantly β-sheet secondary structure at the interface when present in 10 wt % mixtures with 4:1 DPPC/ DOPG lipid mixtures, where DOPG is the unsaturated lipid 1,2-dioleoyl-sn-glycero-3-phosphoglycerol, but was predominantly α-helical when present at 1 wt % in a 4:1 DPPC/DOPG lipid mixture. Waring et al. and Sarker et al. studied a short version of SMB, SP-B(8−25,63−78), and found the structure was consistent with two linked amphipathic α-helices and contained two intramolecular disulfide bonds.34,35 There have been a few neutron reflectometry studies of SP-B at the air−water interface36,37 and an X-ray reflectometry study of SP-B(1−25) at the air−water interface,38 but no one has used these methods to follow changes to SP-B upon its exposure to gas-phase ozone. In this work, we have used a variety of analytical methods to study the effects of exposure of SP-B(1−25) and SMB at the air− water interface to low levels of gas-phase ozone. We have investigated the behavior of monolayers containing just the peptides and also mixed monolayers containing both a peptide and a lipid component. The lipids studied were the most abundant lung lipid, DPPC, and the anionic lipid POPG. POPG was chosen as it is the most abundant anionic lipid in adult human lung surfactant,39 and it is believed that the interaction of SP-B with anionic lipids is important for lung function.2 Although anionic lipids in the lung are predominantly unsaturated,39 to selectively examine changes in the interaction of SMB to the anionic headgroup of lipids upon oxidation of SMB we also performed studies in which SMB and the saturated anionic lipid DPPG were exposed to ozone.
is less crucial, but a lack of SP-C is also linked to respiratory problems.13 Ozone is present as a secondary pollutant in ambient air. It has long been known that exposure to ozone, O3, leads to respiratory problems, increased hospital admissions, and death.14−16 What is not clear is the actual mechanism, or mechanisms, by which ozone exposure leads to respiratory problems. The surfactant at the air−water interface of the lung is one of the first lines of defense when humans are exposed to ozone present in ambient air. Several studies have attempted to investigate the effect of short- and longer-term ozone exposures in vivo on a pulmonary surfactant. Müller and co-workers17 and Putman and co-workers18 studied the surface tension of lung surfactant retrieved from rats after exposure to 0.8 ppm ozone for 2 and 12 h. The adsorption of the surfactant to the air− water interface after exposure to ozone was significantly slower, and the final surface tension reached was higher following exposure. It is known that unsaturated lipids present at the air−water interface will react readily with ozone.19−23 No reports of the exposure of proteins SP-B and SP-C to ozone have been published, but Kim et al. have reported a rapid reaction between a peptide composed of the first 25 amino acids of SPB, known as SP-B(1−25), at the air−water interface of an aqueous droplet and high levels of gas-phase ozone.24 Kim et al. studied the products of the reaction using field-induced droplet ionization mass spectrometry and reported that the oxidized product contained three more oxygen atoms than SP-B(1−25), which they deduced came from the oxidation of methionine to methionine sulfoxide and tryptophan to N-formylkynrenine. The signal-to-noise ratio for the product, SP-B(1−25) + 3O, was much lower than for the starting material, SP-B(1−25); hence, Kim et al. suggest that the oxidized form of SP-B(1−25) may be lost entirely from the interface, rationalized by the conversion of fairly hydrophobic residues into more hydrophilic species. In this work, we have studied the effect of ozone exposure at the air−water interface on two truncated versions of SP-B, SPB(1−25) and SP-B(1−25, 63−78), which is known as Super Mini B (SMB). SMB has been shown to better mimic the properties of full-length SP-B, compared to SP-B(1−25), although both SPB(1−25) and SMB improve lung function in surfactant deficient animal models.25,26 The sequences of the full-length human protein SP-B, SP-B(1−25), and SMB are shown in Figure 1. The full-length protein SP-B is a small, 79-amino acid residue, 8.7 kDa protein. Although hydrophobic, at physiological pH, it carries a positive charge of ∼7, which is thought to be important for selective interaction with negatively charged lipids such as phosphoglycerols.2 Peptides SMB and SP-B(1−25) carry positive charges of ∼7 and ∼4, respectively, at physiological pH. The seven cysteine residues in SP-B are all involved in disulfide bonds, three of which are intramolecular and the last of which is intermolecular to another SP-B monomer. A crystal or NMR structure of full-length protein SPB has not yet been reported, but circular dichroism and FTIR experiments reveal approximately 40−50% α-helical content
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MATERIALS AND METHODS The SP-B(1−25) and SMB peptides were supplied by Peptide Protein Research Ltd. The peptides were synthesized using standard Fmoc chemistry with a stated purity of >98%. The SMB peptide was subjected to air oxidation to produce the folded form with two internal disulfide bonds (Cys8−Cys49 and Cys11−CysC34), equivalent to the internal disulfides formed by full-length SP-B, as reported previously.25 All lipids used were supplied by Avanti Polar Lipids (Alabaster, AL). B
DOI: 10.1021/acs.biochem.5b00308 Biochemistry XXXX, XXX, XXX−XXX
Article
Biochemistry
treated with chloroform as described above. Extraction and analysis of SP-B(1−25) monolayers in this way were straightforward, and the results are presented. However, SMB transfers less readily into the aqueous SDS−PAGE buffer medium than SP-B(1−25), and we were unable to obtain consistent transfer of the peptide to the gel; thus, no results are presented. For the HPLC analysis, the material recovered after removal of the chloroform was dissolved in a small quantity of ethanol. The material extracted in this way was analyzed using HPLC with a C5 column (Jupiter 5 μm C5 300 Å, 250 × 4.6 mm, Phenomenex) and solvent of (i) ethanol and water [3:1 (w/w)] or (ii) propan-2-ol, both with 0.1% trifluoracetic acid, with a gradient of 0 to 50% solvent ii over 40 min with a flow rate of 0.5 mL min−1, and monitoring the absorption of the eluent at 254 nm. Fluorescence Microscopy Measurements. Monolayers of SMB were formed on an aqueous subphase contained in a plastic Petri dish with a glass bottom (MatTek). Fluorescent lifetime images, and maximal fluorescence intensities, were recorded using a setup described previously.41 Briefly, a custom-built two-photon microscope was constructed using scanning XY galvanometers (GSI Lumonics Ltd.). A diodepumped (Verdi V18) titanium sapphire (Mira F900) operating at 700−980 nm was used to pump an optical parametric oscillator (OPO, APE-Coherent, GmbH, Berlin, Germany) to generate a laser wavelength at 590 nm, with a pulse width of 180 fs and a repetition rate of 76 MHz. The laser beam was focused to a diffraction-limited spot using a water-immersion ultraviolet corrected objective (Nikon VC, 60×, NA 1.2) and specimens were illuminated on the microscope stage of a modified Nikon TE2000-U instrument with UV-transmitting optics. The tryptophan residue of SMB was excited using twophoton excitation. The intensity and lifetime of the emitted fluorescence light (∼380 nm) were recorded before and after the surface was exposed briefly to a 3 ppm source of ozone, flowing at a rate of 1 L min−1. Fluorescence emission was collected without descanning, bypassing the scanning system, and passed through a narrowband interference (UG11, Comar) filter used to isolate the UV light transmitted to the photomultiplier. Emission fluorescence was detected using an external fast microchannel plate photomultiplier tube (Hamamatsu R3809U-50) and recorded using a time-correlated single photon counting (TCSPC) PC module SPC830 (Becker and Hickl GmbH, Berlin, Germany). Fluorescence lifetime image microscopy was performed by synchronizing the XY galvanometer positions with the fluorescence decay. Neutron and X-ray Reflection Measurements. Reflection of neutrons and X-rays occurs where there is an interface between two materials with different refractive indices for the respective probe. The refractive index of a material to X-rays is related to the number of electrons in the component atoms, whereas for neutrons, the refractive index depends upon the nuclear properties of the particular isotopes present. The reflected beam can be used to determine properties such as the amount and thickness of the layer of material giving rise to the reflection, as explained later. Further details of the use of X-ray and neutron reflections to study monolayers at the air−water interface can be found in recent reviews.42−44 The reflectivity experiments described here were performed by adding a monolayer film of either pure peptide or a peptide/ lipid mixture to an aqueous subphase. The monolayer was compressed until the desired surface pressure was obtained;
Other reagents were obtained from commercial sources and of ≥98% purity. The chloroform was stabilized with 0.5−1% ethanol. The reaction between gas-phase ozone and monolayers of SP-B(1−25) and SMB either as pure peptide films or as mixtures with phospholipids at the air−water interface was studied under various conditions. In general, monolayers were prepared on an aqueous 50 mM, pH 7, sodium phosphate-buffered subphase contained in a PTFE-lined Langmuir trough (Nima Technology) housed in an environmental chamber to contain the ozone. The monolayers were prepared by slowly adding solutions of the peptide, or the peptide/lipid mixture, to the surface of the aqueous subphase using a Hamilton syringe. The peptide solutions were prepared as 0.2 mg mL−1 peptide dissolved in a 5:1 (v/v) chloroform/methanol solvent; lipid mixtures were prepared as 1 mg mL−1 lipid in chloroform. Binary mixtures of peptide and lipids were prepared by mixing the required amounts of each solution. After the addition of the peptide or peptide/lipid mixtures to the surface of the subphase, the solvent was allowed to evaporate under a flow of oxygen gas before the experiments were started. The experiments were performed by continuously flowing a dilute (varied between ∼0.1 and 3 ppm) mixture of ozone in oxygen (BOC, ≥99.5%) at a constant flow rate of 1 or 2 L min−1 through the chamber. The ozone was generated by passing dry oxygen through a commercial ozone generator (UVP) that generated ozone from the photolysis of molecular oxygen. Surface Pressure Measurements. The surface pressure, equal to the difference between the surface tension of the gas− water interface of pure water and that of the interface under study, was measured as the monolayer films of the peptides and peptide/lipid mixtures were held at a constant area and exposure to gas-phase ozone. The surface pressure was measured using a Wilhelmy plate made from Whatman chromatography grade filter paper. Experimental constraints meant that relatively low surface pressures (
