Lipid-Protein and Protein-Protein Interactions in the Pulmonary Surfactant System and Their Role in Lung Homeostasis

Abstract

Pulmonary surfactant is a lipid/protein complex synthesized by the alveolar epithelium and secreted into the airspaces, where it coats and protects the large respiratory air-liquid interface. Surfactant, assembled as a complex network of membranous structures, integrates elements in charge of reducing surface tension to a minimum along the breathing cycle, thus maintaining a large surface open to gas exchange and also protecting the lung and the body from the entrance of a myriad of potentially pathogenic entities. Different molecules in the surfactant establish a multivalent crosstalk with the epithelium, the immune system and the lung microbiota, constituting a crucial platform to sustain homeostasis, under health and disease. This review summarizes some of the most important molecules and interactions within lung surfactant and how multiple lipid-protein and protein-protein interactions contribute to the proper maintenance of an operative respiratory surface.

Keywords: antimicrobial activity; apoptosis; efferocytosis; inflammation; pulmonary surfactant film; respiratory air–liquid interface; surface tension; surfactant metabolism; tissue repair.

Figure 1

Protein/protein and protein/lipid interactions are essential for maintaining surfactant homeostasis at the alveolar spaces. Surfactant components are synthesized by AE2C and assembled in lamellar bodies (LB). Proper formation of these highly packed lipid organelles requires the presence of SP-B and ABCA3 [20,21]. Upon secretion to the alveolar fluid, surfactant is efficiently adsorbed into the air–liquid interface, in a process highly dependent on the presence of proteins SP-B and SP-C [22,23]. Multilayered membranous surfactant structures associate to the interfacial film forming a reservoir, which provides mechanical stability and a source of material upon respiratory dynamics [19]. Extracellular surfactant structures include a membrane network promoted by SP-A and SP-B (tubular myelin) [24] and membrane arrangements of different complexity [25]. After exposure to air, surfactant “used” membranes are converted into small vesicles [26] in a SP-D-mediated process [27] that can be at least partly re-uptaken by AE2C, a process promoted by SP-A [28]. SP-A also acts as a secretion inhibitory signal [29], in an opposing role to that of SP-B as secretion inducer [30]. Components captured by AE2C are routed to the recycling pathway or degraded [14]. Besides, alveolar macrophages account for a 20% of surfactant clearance [31], for which the presence of granulocyte-macrophage colony stimulating factor (GM-CSF) is required [32]. Alveolar homeostasis highly depends on the effective cross-talk between alveolar macrophages (AM) and AE2C and the maintenance of lipid homeostasis in both cells, involving a proper reverse cholesterol transport (RCT) mediated by ABCA1 and ABCG1 lipid transporters [33,34,35]. SP-A is represented as its most abundant octadecameric form; SP-B as double rings, each formed by six dimers; SP-C as monomers; SP-D as dodecamers.

Figure 2

Lipid–protein interactions in the mechanical stabilization of alveoli during breathing cycles. Surfactant proteins SP-B and SP-C facilitate surfactant dynamics by regulating the mechanical properties of surfactant membranes and films through direct interaction with fluid-like lipids [36], including PG. Binding of SP-B and SP-C to unsaturated and anionic lipids would produce protein partitioning into fluid domains that may contribute to highly curved membranes [37], promoting lipid polymorphism [38,39,40,41,42] and favoring a compression-driven enrichment of the interfacial film in the most surface active component of surfactant, DPPC. On the other hand, by binding to DPPC at solid/fluid phase interfaces, SP-A promotes demixing of surfactant lipids, facilitating the segregation of unsaturated phospholipids at the interface [43] and, thus, modeling the mechanical properties of the surfactant film [44]. In addition, surfactant proteins A, B and C stabilize multilayered interfacial structures and preclude the out-of-plane relaxation of the surfactant film at the end of expiration by promoting membrane-membrane contacts: SP-A (oligomer shown, octadecamer) binds simultaneously to different membranes through its carbohydrate recognition domains [10]; SP-B forms rings (shown as hexamers of dimers) and tubes that connect different bilayers [45,46], and SP-C maintains insertion of palmitoylated cysteins at its N-terminal segment into highly packed liquid-ordered regions of the interfacial film [47,48]. Surfactant membranes may be further stabilized during the breathing cycles by SP-A/SP-B and SP-B/SP-C interactions. Notice that surfactant proteins in the cartoon are not represented at equivalent scale.

Figure 3

Host response to lipopolysaccharide (LPS). LPS is composed of lipid A, the hydrophobic membrane-anchoring region, and a hydrophilic part formed by the core oligosaccharide region and the O-antigen. The core oligosaccharide region includes an outer core region, enriched in hexoses, and an inner core region proximal to lipid A that contains 3-deoxy-D-manno-octulosonic acid [184]. The O-antigen, also termed O-specific chain, contains up to 50 highly variable oligosaccharide units. Both the acyl chains and the negative charges of the phosphate groups bound to the diglucosamine backbone of lipid A are required for the recognition of LPS by cell receptors and the subsequent activation of the host immune system. LPS molecules associate forming non-lamellar supramolecular structures that are recognized by LBP, which transfers LPS monomers to CD14. LPS is subsequently transferred to the TLR4/MD2 complex, triggering complex dimerization and initiating the signaling cascade. Interaction of surfactant protein SP-A (functional form, octadecamer) with LPS modifies the aggregated LPS structure, inducing formation of lamellar phases, and therefore preventing the recognition of LPS by LBP.

Figure 4

Antimicrobial activity of surfactant lipids and proteins. Lung collectins contribute to microbial clearance by: (i) facilitating mucociliary clearance through microbial aggregation/agglutination; (ii) enhancing microbial phagocytosis via opsonization, upregulation of the expression of cell-surface receptors involved in microbial recognition by phagocytic cells, and regulation of the complement system; and (iii) favoring bacterial trapping by NETs through simultaneous binding to bacteria and NETs. In addition, SP-A (18-mer), SP-C and SP-D (here only 12-mers are represented) directly affect growth and viability of microorganisms by increasing the permeability of the microbial cell membrane. Surfactant lipids and proteins also inhibit microbial infectivity, preventing the adhesion and internalization of bacteria and viruses by alveolar epithelial cells.