Role of the N-terminal seven residues of surfactant protein B (SP-B)

Surfactant protein B-derived peptides as endosomal escape enhancers for pulmonary delivery of siRNA

Respiratory diseases still cause significant mortality and morbidity worldwide, highlighting the need for new inhalable drugs. RNA therapeutics, which have the potential to modulate the expression of virtually any gene, could address this unmet medical need. Nevertheless, clinical translation requires the design of RNA formulations able to overcome the extra- and intracellular barriers in the lung. We previously discovered that the endogenous cationic amphiphilic surfactant protein B (SP-B) promotes cytosolic delivery of small interfering RNA (siRNA) in lung-related cell types via endosomal membrane fusion. However, to bypass drawbacks related to the use of animal-derived SP-B, there is a keen interest in developing synthetic SP-B analogues with comparable activity. Here, we show that native SP-B can successfully be replaced by smaller peptides, with the N-terminal heptapeptide and amphipathic helix being minimally required to promote siRNA-induced gene silencing. Peptidolipid-coated nanogels were designed and demonstrated equivalent siRNA delivery efficacy compared to state-of-the-art lipid nanoparticles (LNPs). Moreover, they exhibit enhanced resistance to vibrating mesh nebulization and reduced inflammatory activation of bronchial epithelial cells. Collectively, the discovery of SP-B peptides as RNA delivery enhancers holds promise for developing potent inhalable RNA formulations with favorable safety profiles, of value for the treatment of chronic inflammatory pathologies.

Bacterial expression, purification and folding of exceptionally hydrophobic and essential protein: Surfactant Protein-B (SP-B)

Lung Surfactant Protein B (SP-B) is essential for life. It is thus striking that, to this point, no method for making the full-length protein has been published and consequently we lack detailed understanding of SP-B's basic structure-function relationships, as well as an inability to make it for clinical use. The major challenge in producing SP-B lies with its exceptionally hydrophobic nature. In this work, we present a method to produce recombinant SP-B in bacteria that can be used to make the full-length protein as well as the product focused on here, which is a construct lacking the N-terminal 7 residues, rSP-B (Δ7NTC48S-SP-B-6His). The construct is produced as a fusion to Staphylococcus nuclease A (SN) in Escherichia coli C43 cells, a strain known to promote production of toxic and membrane recombinant proteins. After cleavage from SN, rSP-B is folded on column and then exchanged into the lipid or detergent system of choice. rSP-B prepared in this way exhibits the correct secondary structure and demonstrates surface activity. The yield obtained is 0.3 mg of purified rSP-B (Δ7NTC48S-SP-B-6His) per liter of initial bacterial culture. We expect this method for producing SP-B will be valuable in enabling basic research into SP-B's mechanisms, as well as possibly facilitating the inclusion of SP-B in lung surfactant formulations to treat common and frequently fatal lung conditions and in lung surfactant-based drug delivery.

Emulation of the structure of the Saposin protein fold by a lung surfactant peptide construct of surfactant Protein B

The three-dimensional structure of the synthetic lung Surfactant Protein B Peptide Super Mini-B was determined using an integrative experimental approach, including mass spectrometry and isotope enhanced Fourier-transform infrared (FTIR) spectroscopy. Mass spectral analysis of the peptide, oxidized by solvent assisted region-specific disulfide formation, confirmed that the correct folding and disulfide pairing could be facilitated using two different oxidative structure-promoting solvent systems. Residue specific analysis by isotope enhanced FTIR indicated that the N-terminal and C-terminal domains have well defined α-helical amino acid sequences. Using these experimentally derived measures of distance constraints and disulfide connectivity, the ensemble was further refined with molecular dynamics to provide a medium resolution, residue-specific structure for the peptide construct in a simulated synthetic lung surfactant lipid multilayer environment. The disulfide connectivity combined with the α-helical elements stabilize the peptide conformationally to form a helical hairpin structure that resembles critical elements of the Saposin protein fold of the predicted full-length Surfactant Protein B structure.

Pulmonary surfactant as a versatile biomaterial to fight COVID-19

The COVID-19 pandemic has wielded an enormous pressure on global health care systems, economics and politics. Ongoing vaccination campaigns effectively attenuate viral spreading, leading to a reduction of infected individuals, hospitalizations and mortality. Nevertheless, the development of safe and effective vaccines as well as their global deployment is time-consuming and challenging. In addition, such preventive measures have no effect on already infected individuals and can show reduced efficacy against SARS-CoV-2 variants that escape vaccine-induced host immune responses. Therefore, it is crucial to continue the development of specific COVID-19 targeting therapeutics, including small molecular drugs, antibodies and nucleic acids. However, despite clear advantages of local drug delivery to the lung, inhalation therapy of such antivirals remains difficult. This review aims to highlight the potential of pulmonary surfactant (PS) in the treatment of COVID-19. Since SARS-CoV-2 infection can progress to COVID-19-related acute respiratory distress syndrome (CARDS), which is associated with PS deficiency and inflammation, replacement therapy with exogenous surfactant can be considered to counter lung dysfunction. In addition, due to its surface-active properties and membrane-interacting potential, PS can be repurposed to enhance drug spreading along the respiratory epithelium and to promote intracellular drug delivery. By merging these beneficial features, PS can be regarded as a versatile biomaterial to combat respiratory infections, in particular COVID-19.

Treatment of Respiratory Distress Syndrome with Single Recombinant Polypeptides that Combine Features of SP-B and SP-C

Treatment of respiratory distress syndrome (RDS) with surfactant replacement therapy in prematurely born infants was introduced more than 30 years ago; however, the surfactant preparations currently in clinical use are extracts from animal lungs. A synthetic surfactant that matches the currently used nature-derived surfactant preparations and can be produced in a cost-efficient manner would enable worldwide treatment of neonatal RDS and could also be tested against lung diseases in adults. The major challenge in developing fully functional synthetic surfactant preparations is to recapitulate the properties of the hydrophobic lung surfactant proteins B (SP-B) and SP-C. Here, we have designed single polypeptides that combine properties of SP-B and SP-C and produced them recombinantly using a novel solubility tag based on spider silk production. These Combo peptides mixed with phospholipids are as efficient as nature-derived surfactant preparations against neonatal RDS in premature rabbit fetuses.

All-Atom Molecular Dynamics Simulations of Dimeric Lung Surfactant Protein B in Lipid Multilayers

Although lung surfactant protein B (SP-B) is an essential protein that plays a crucial role in breathing, the details of its structure and mechanism are not well understood. SP-B forms covalent homodimers, and in this work we use all-atom molecular dynamics simulations to study dimeric SP-B’s structure and its behavior in promoting lipid structural transitions. Four initial system configurations were constructed based on current knowledge of SP-B’s structure and mechanism, and the protein maintained a helicity consistent with experiment in all systems. Several SP-B-induced lipid reorganization behaviors were observed, and regions of the protein particularly important for these activities included SP-B’s “central loop” and “hinge” regions. SP-B dimers with one subunit initially positioned in each of two adjacent bilayers appeared to promote close contact between two bilayers. When both subunits were initially positioned in the same bilayer, SP-B induced the formation of a defect in the bilayer, with water penetrating into the centre of the bilayer. Similarly, dimeric SP-B showed a propensity to interact with preformed interpores in the bilayer. SP-B dimers also promoted bilayer thinning and creasing. This work fleshes out the atomistic details of the dimeric SP-B structures and SP-B/lipid interactions that underlie SP-B’s essential functions.

Synthetic surfactants with SP-B and SP-C analogues to enable worldwide treatment of neonatal respiratory distress syndrome and other lung diseases

Treatment of neonatal respiratory distress syndrome (RDS) using animal-derived lung surfactant preparations has reduced the mortality of handling premature infants with RDS to a 50th of that in the 1960s. The supply of animal-derived lung surfactants is limited and only a part of the preterm babies is treated. Thus, there is a need to develop well-defined synthetic replicas based on key components of natural surfactant. A synthetic product that equals natural-derived surfactants would enable cost-efficient production and could also facilitate the development of the treatments of other lung diseases than neonatal RDS. Recently the first synthetic surfactant that contains analogues of the two hydrophobic surfactant proteins B (SP-B) and SP-C entered clinical trials for the treatment of neonatal RDS. The development of functional synthetic analogues of SP-B and SP-C, however, is considerably more challenging than anticipated 30 years ago when the first structural information of the native proteins became available. For SP-B, a complex three-dimensional dimeric structure stabilized by several disulphides has necessitated the design of miniaturized analogues. The main challenge for SP-C has been the pronounced amyloid aggregation propensity of its transmembrane region. The development of a functional non-aggregating SP-C analogue that can be produced synthetically was achieved by designing the amyloidogenic native sequence so that it spontaneously forms a stable transmembrane α-helix.

Quenching of tryptophan fluorescence in a highly scattering solution: Insights on protein localization in a lung surfactant formulation

CHF5633 (Chiesi Farmaceutici, Italy) is a synthetic surfactant developed for respiratory distress syndrome replacement therapy in pre-term newborn infants. CHF5633 contains two phospholipids (dipalmitoylphosphatidylcholine and 1-palmitoyl-2oleoyl-sn-glycero-3-phosphoglycerol sodium salt), and peptide analogues of surfactant protein C (SP-C analogue) and surfactant protein B (SP-B analogue). Both proteins are fundamental for an optimal surfactant activity in vivo and SP-B genetic deficiency causes lethal respiratory failure after birth. Fluorescence emission of the only tryptophan residue present in SP-B analogue (SP-C analogue has none) could in principle be exploited to probe SP-B analogue conformation, localization and interaction with other components of the pharmaceutical formulation. However, the high light scattering activity of the multi-lamellar vesicles suspension characterizing the pharmaceutical surfactant formulation represents a challenge for such studies. We show here that quenching of tryptophan fluorescence and Singular Value Decomposition analysis can be used to accurately calculate and subtract background scattering. The results indicate, with respect to Trp microenvironment, a conformationally homogeneous population of SP-B. Trp is highly accessible to the water phase, suggesting a surficial localization on the membrane of phospholipid vesicles, similarly to what observed for full length SP-B in natural lung surfactant, and supporting an analogous role in protein anchoring to the lipid phase.

Model Lung Surfactant Films: Why Composition Matters

Lung surfactant replacement therapies, Survanta and Infasurf, and two lipid-only systems both containing saturated and unsaturated phospholipids and one containing additional palmitic acid were used to study the impact of buffered saline on the surface activity, morphology, rheology, and structure of Langmuir monolayer model membranes. Isotherms and Brewster angle microscopy show that buffered saline subphases induce a film expansion, except when the cationic protein, SP-B, is present in sufficient quantities to already screen electrostatic repulsion, thus limiting the effect of changing pH and adding counterions. Grazing incidence X-ray diffraction results indicate an expansion not only of the liquid expanded phase but also an expansion of the lattice of the condensed phase. The film expansion corresponded in all cases with a significant reduction in the viscosity and elasticity of the films. The viscoelastic parameters are dominated by liquid expanded phase properties and do not appear to be dependent on the structure of the condensed phase domains in a phase separated film. The results highlight that the choice of subphase and film composition is important for meaningful interpretations of measurements using model systems.

Micropipette Technique Study of Natural and Synthetic Lung Surfactants at the Air-Water Interface: Presence of a SP-B Analog Peptide Promotes Membrane Aggregation, Formation of Tightly Stacked Lamellae, and Growth of Myelin Figures

The present study is a microscopic interfacial characterization of a series of lung surfactant materials performed with the micropipette technique. The advantages of this technique include the measurement of equilibrium and dynamic surface tensions while acquiring structural and dynamic information at microscopic air-water interfaces in real time and upon compression. Here, we characterized a series of animal-derived and synthetic lung surfactant formulations, including native surfactant obtained from porcine lungs (NS); the commercial Curosurf, Infasurf, and Survanta; and a synthetic Super Mini-B (SMB)-containing formulation. It was observed that the presence of the natural hydrophobic proteins and, more strikingly, the peptide SMB, promoted vesicle condensation as thick membrane stacks beneath the interface. Only in the presence of SMB, these stacks underwent spontaneous structural transformations, consisting of the nucleation and growth of microtubes and in some cases their subsequent coiling into helices. The dimensions of these tubes (2-15 μm diameter) and their linear (2-3 μm/s) and volumetric growth rates (20-30 μm³/s) were quantified, and no specific effects were found on them for increasing SMB concentrations from 0.1 to 4%. Nevertheless, a direct correlation between the number of tubes and SMB contents was found, suggesting that SMB molecules are the promoters of tube nucleation in these membranes. A detailed analysis of the tube formation process was performed following previous models for the growth of myelin figures, proposing a combined mechanism between dehydration-rehydration of the lipid bilayers and induction of mechanical defects by SMB that would act as nucleation sites for the tubes. The formation of tubes was also observed in Infasurf, and in NS only after subsequent expansion and compression but neither in the other clinical surfactants nor in protein-free preparations. Finally, the connection between this data and the observations from the lung surfactant literature concerning the widely reported "near-zero surface tension" for lung surfactant films and intact alveolar surfaces is also discussed.

Stability of an amphipathic helix-hairpin surfactant peptide in liposomes

Surfactant protein B (SP-B; 79 residues) is a member of the saposin superfamily and plays a pivotal role in lung function. The N- and C-terminal regions of SP-B, cross-linked by two disulfides, were theoretically predicted to fold as charged amphipathic helices, suggesting participation in surfactant activities. Previous studies with oxidized Super Mini-B (SMB), a construct based on the N- and C-regions of SP-B (i.e., residues 1-25 and 63-78) joined with a designer turn (-PKGG-) and two disulfides, indicated that freshly prepared SMB in lipids folded as a surface active, α-helix-hairpin. Because other peptides modeled on α-helical SP domains lost helicity and surfactant activity on storage, experiments were here performed on oxidized SMB in surfactant liposomes stored at ~2-8°C for ≤5.5years. Captive bubble surfactometry confirmed low minimum surface tensions for fresh and stored SMB preparations. FTIR spectroscopy of fresh and stored SMB formulations showed secondary structures compatible with the peptide folding as α-helix-hairpin. A homology (I-TASSER) model of oxidized SMB demonstrated a globular protein, exhibiting a core of hydrophobic residues and a surface of polar residues. Since mass spectroscopy indicated that the disulfides were maintained on storage, the stability of SMB may be partly due to the disulfides bringing the N- and C-α-helices closer. Mass spectroscopy of stored SMB preparations showed some methionine oxidation, and also partial deacylation of surfactant phospholipids to form lyso-derivatives. However, the stable conformation and activity of stored SMB surfactant suggest that the active helix-hairpin resists these chemical changes which otherwise may lead to surfactant inhibition.