Pulmonary Surfactant: The Key to the Evolution of Air Breathing Christopher B. Daniels and Sandra Orgeig Department of Environmental Biology, University of Adelaide, Adelaide, South Australia 5005, Australia Pulmonary surfactant controls the surface tension at the air-liquid interface within the lung. This sys- tem had a single evolutionary origin that predates the evolution of the vertebrates and lungs. The lipid composition of surfactant has been subjected to evolutionary selection pressures, partic- ularly temperature, throughout the evolution of the vertebrates. ungs have evolved independently on several occasions pendent units, do not necessarily stretch upon inflation but Lover the past 300 million years in association with the radi- unpleat or unfold in a complex manner. Moreover, the many ation and diversification of the vertebrates, such that all major fluid-filled corners and crevices in the alveoli open and close vertebrate groups have members with lungs. However, lungs as the lung inflates and deflates. differ considerably in structure, embryological origin, and Surfactant in nonmammals exhibits an antiadhesive func- function between vertebrate groups. The bronchoalveolar lung tion, lining the interface between apposed epithelial surfaces of mammals is a branching “tree” of tubes leading to millions within regions of a collapsed lung. As the two apposing sur- of tiny respiratory exchange units, termed alveoli. In humans faces peel apart, the lipids rise to the surface of the hypophase there are ~25 branches and 300 million alveoli. This structure fluid at the expanding gas-liquid interface and lower the sur- allows for the generation of an enormous respiratory surface face tension of this fluid, thereby decreasing the work required area (up to 70 m2 in adult humans). Generally, in nonmam- to separate the two surfaces. However, for surfactant to act as mals, lungs are baglike with either smooth walls or large, bel- an antiadhesive, the respiratory tissues must “fold” in on them- lows-shaped respiratory units (termed faveoli) extending from selves, possibly during exhalation, or when the ventilatory the outer wall of the lung into a central air space. Birds have period is punctuated by protracted nonventilatory periods at the most strikingly different lung structure, with a pair of small low lung volume. These conditions occur frequently in the parabronchial lungs connected to a series of air sacs. Air is ventilatory pattern of nonmammals. propelled via the airsacs, which act like bellows, in a unidi- rectional manner through the lung. The lungs consist of a The pulmonary surfactant system series of tubes (parabronchi) from which emanate the very- small-diameter, rigid air capillaries, which lie in close apposi- Pulmonary surfactant is a complex mixture of phospho- tion with blood capillaries and represent the site of gas lipids (PL), neutral lipids [particularly cholesterol (Chol)], and exchange. Some reptiles and amphibians have a complex and proteins. The PL are assembled in the endoplasmic reticulum dense arrangement of septate compartments. However, in and the Golgi apparatus of alveolar type II cells and are stored contrast to mammals, the lungs of fish, amphibians, and rep- in lamellar bodies until exocytosis (Fig. 1). The assembly of the tiles always lack a bronchial tree, a diaphragm, and a separate surfactant proteins into lamellar bodies is less clear and may pleuroperitoneal chamber, and they have respiratory units up proceed via the endoplasmic reticulum and Golgi apparatus to 100 times larger than alveoli of similar-sized mammals. to multivesicular bodies before combining with the lamellar But all lungs have one common characteristic. They are bodies (10). The source(s) of alveolar Chol remains unknown internal, fluid-lined, gas-holding structures that inflate and (16). The lamellar bodies consist of a dense proteinaceous deflate cyclically. As a result, all lungs face potential problems core with lipid bilayers arranged in concentric, stacked lamel- related to the surface tension of the fluid. Pulmonary surfac- lae surrounded by a limiting membrane. After the lamellar tant is produced in the lung to decrease surface tension of this bodies have been released into the alveolar space, they swell fluid lining (hypophase). Von Neergaard (quoted in Ref. 6) first and unravel into a characteristic cross-hatched structure demonstrated that the surface forces at the gas-liquid interface termed tubular myelin (Fig. 1). It is this structure that supplies of the lung contribute substantially to the retractive pressure, the lipids for the surface film, which regulates the surface ten- and hence static compliance, of the lung. However, surfactant sion of the liquid lining the lung (10). can also vary surface tension with the radius of curvature of The ability to lower and vary surface tension with changing each alveolus (or more accurately, regions within an alveolus) surface area is attributed to the interactions between the dis- so that the pressures within all alveoli are maintained at simi- aturated PLs (DSPs), particularly dipalmitoylphosphatidyl- lar values, permitting alveoli of different sizes to coexist. Sur- choline (DPPC), and the other lipids, such as the unsaturated factant also helps the narrowest airways to remain open, PLs (USPs) and Chol. Upon expiration, dynamic compression thereby reducing the resistance to air flow and controlling of the mixed surfactant film results in the “squeezing out” of fluid balance in the lung. However, the alveoli, being interde- USP and Chol, such that the surface film is enriched in DPPC 0886-1714/03 5.00 © 2003 Int. Union Physiol. Sci./Am. Physiol. Soc. News Physiol Sci 18: 151157, 2003; www.nips.org 10.1152/nips.01438.2003 151 FIGURE 1. A: schematic diagram of the life cycle of pulmonary surfactant (moving from bottom to top). Pulmonary surfactant components are synthesized in the endoplasmic reticulum (ER), transported to the Golgi aparatus, and packaged into lamellar bodies (LB). LB are secreted into the liquid lining the alveoli (hypophase) via exocytosis across the type II cell plasma membrane. Here the lamellar bodies swell and unravel, forming a cross-hatched structure termed tubu- lar myelin (TM), which consists of lipids and proteins. This structure supplies lipids to the surface film as well as the surface-associated phase (SAP). As the mixed molecular film is compressed, lipids are squeezed out of the film into the SAP to produce a dipalmitoylphosphatidylcholine (DPPC)-enriched film that is capa- ble of reducing surface tension (ST) to near 0 mN/m. It is possible that some lipids from the SAP reenter the surface film (dashed arrow). Lipids from the surface film and the SAP are eventually recycled and taken back up by the type II cell via endocytosis. The role of some of the surfactant proteins (SP-A, -B, and -C) in regulating these processes is indicated with n (stimulation) or p (inhibition). Reproduced with permission from Elsevier from Ref. 17. B: structure of the lung epithelium of the lizard Ctenophorus nuchalis, demonstrating the location of type II cells (t) in between the capillaries (c), which bulge into the airspace. The capillaries contain macrophages (m) and erythrocytes (r). Surfactant material (s) has been extruded into the airspace. C: electron micrograph demonstrating the surfactant structures: lamellar bodies (lb) and tubular myelin (tm) in the airspace of a lizard lung. B and C are reprinted with permission from Ref. 14. (Fig. 1A). The DPPC molecules can be compressed tightly into the surface film upon inspiration lowers the Tm of the lipid together by virtue of their two fully saturated fatty acid chains. mixture, enabling it to exist in the fluid state at the same body In so doing, they exclude water molecules from the air-liquid temperature. In this state, the lipids are able to disperse to coat interface, thereby eliminating surface tension (19). the surface of the expanding fluid layer. Lipids can exist in either a fluid liquid-crystalline state or in Temperature therefore has a profound influence on the a solid gel state. The transition between these two phases structure and function of surfactant lipids. Given that the occurs at the phase transition temperature (Tm) of that lipid. majority of animals have much lower body temperatures than For the surfactant lipids to spread over the alveolar surface homeothermic mammals, how can animals regulate the fluid- upon inspiration, the surfactant film must exist in the liquid- ity of their surfactant film at low and/or fluctuating body tem- crystalline state. Because DPPC has a Tm of 41°C, a pure peratures? Furthermore, the low metabolic rates of nonmam- DPPC film will exist in the gel form at mammalian body tem- mals also have profound implications for the rates of synthesis peratures and hence adsorb extremely slowly to the air-liquid of new components. Hence, for example, the need for addi- interface (19). The addition of other lipids, e.g., Chol or USP, tional quantities of USP in surfactant requires PL synthesis, 152 News Physiol Sci • Vol. 18 • August 2003 • www.nips.org which may take some time. Hence USPs do not appear to be the surfactant-secreting cells migrated with the air-filled out- appropriate for controlling fluidity of surfactant in the face of pouchings of the gut before the lung surfactant took on its cur- the very rapid or step changes in body temperature, which rent surface tension-controlling functions. Hence the surfac- commonly occur in most nonmammals and in torpid or hiber- tant system predated the evolution of lungs and was crucial for nating mammals. the evolution of air breathing. Chol is able to affect the fluidity of PLs directly. Chol is thought to increase the separation between PL molecules, thus Surfactant and the evolution of air breathing disrupting the intermolecular interactions between their head groups and allowing greater rotational movement. At 10% by Here we examine the evolution of the surfactant system in weight, or 20 mol%, Chol is the second most abundant lipid association with three of the major evolutionary steps for the component of pulmonary surfactant.