Amphipathic alpha-helical peptides based on surfactant apoprotein SP-A

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.

Structural changes of surfactant protein A induced by cations reorient the protein on lipid bilayers

Surfactant protein A (SP-A) is an octadecameric hydrophilic glycoprotein and is the major protein component of pulmonary surfactant. This protein complex plays several roles in the body, such as regulation of surfactant secretion, recycling and adsorption of surfactant lipids, and non-serum-induced immune response. Many of SP-A's activities are dependent upon the presence of cations, especially calcium. Here, we have studied in vitro the effect of cations on the interaction of purified bovine SP-A with phospholipid vesicles made of dipalmitoylphosphatidylcholine and unsaturated phosphatidylcholine. We have found that SP-A octadecamers exist in an "opened-bouquet" conformation in the absence of cations and interact with lipid membranes via one or two globular headgroups. Calcium-induced structural changes in SP-A lead to the formation of a clearly identifiable stem in a "closed-bouquet" conformation. This change, in turn, seemingly results in all of SP-A's globular headgroups interacting with the lipid membrane surface and with the stem pointing away from the membrane surface. These results represent direct evidence that the headgroups of SP-A (comprising carbohydrate recognition domains), and not the stem (comprising the amino-terminus and collagen-like region), interact with lipid bilayers. Our data support models of tubular myelin in which the headgroups, not the tails, interact with the lipid walls of the lattice.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

Protein composition of synthetic surfactant affects gas exchange in surfactant-deficient rats

Synthetic surfactant peptides offer an opportunity to standardize the protein composition of surfactant. We tested the effect of phospholipids (PL) with synthetic full-length SP-B1-78 (B), mutant B (Bser), KL4 peptide (UCLA-KL4), and palmitoylated SP-C1-35 (C) on oxygenation and lung function in a surfactant-deficient rat model. Sixty-four adult rats were ventilated with 100% oxygen, a tidal volume of 7.5 mL/kg, and a rate of 60/min. Their lungs were lavaged with saline until the arterial PO2 dropped below 80 torr, when 100 mg/kg surfactant was instilled. Surfactant preparations included: PL (PL surfactant), PL + 3% B (B surfactant), PL + 3% B and 1% C (BC surfactant), PL + 3% UCLA-KL4 (KL4 surfactant), PL + 3% Bser (Bser surfactant), and PL + 3% B and 1% UCLA-KL4 (BKL4 surfactant). Sixty minutes after surfactant instillation, positive end-expiratory pressure was applied for 5 min, and pressure-volume curves were determined in situ. The six surfactant preparations had a minimum surface tensions <10 mN/m on a Langmuir/Wilhelmy balance. Instillation of PL, Bser, and BKL4 surfactant increased mean arterial/alveolar PO2 (aADO2) ratios by 50-100% over postlavage values, whereas KL4 surfactant increased aADO2 ratios by 118%, B surfactant by 191%, and BC surfactant by 225%. Lung volumes at 30 cm H2O pressure were highest after treatment with BC surfactant, intermediate after B and KL4 surfactants, and lowest after BKL4, Bser, and PL surfactants. These data suggest that a surfactant preparation with a combination of synthetic B and C peptides surpasses synthetic B and KL4 surfactants in improving oxygenation and lung compliance in surfactant-deficient rats.

A synthetic segment of surfactant protein A: structure, in vitro surface activity, and in vivo efficacy

Surfactant protein A (SP-A) is a 248-residue, water-soluble, lipid-associating protein found in lung surfactant. Analysis of the amino acid sequence using the Eisenberg hydrophobic moment algorithm predicts that the SP-A segment spanning residues 114-144 has high hydrophobic moments, typical of lipid-associating amphipathic domains. The secondary structure, in vitro surface activity and in vivo lung activity of this SP-A sequence were studied with a 31-residue synthetic peptide analog (A114-144). Analysis of the secondary structure using circular dichroism and Fourier transform infrared spectroscopy indicated association with lipid dispersions and a dominant helical content. Surface activity measurements of A114-144 with surfactant lipid dispersions and the hydrophobic surfactant proteins B and C (SP-B/C) showed that A114-144 enhances surface activity under conditions of dynamic compression and respreading on a Langmuir/Wilhelmy surface balance. Synthetic surfactant dispersions containing A114-144 improved lung compliance in spontaneously breathing, 28-d premature rabbits to a greater degree than surfactant dispersions with synthetic SP-B/C and synthetic surfactant lipids alone. These observations indicate that inclusion of A114-144 may improve synthetic preparations currently used for surfactant replacement therapy.

Role of hydrophobic and hydrophilic forces in peptide-protein interaction: new advances

A considerable part of important biological processes is governed by the noncovalent association of peptides and proteins. Various types of intermolecular forces may be involved in the formation of these molecular assemblies. This review gives a brief account of the physicochemical bases of interactive forces, with special emphasis on their impact on various peptide-protein interactions; summarizes the newest biochemical and biophysical methods for the study of such interactions; and discusses the role of various hydrophilic and hydrophobic forces in peptide-protein interactions in various fields of life sciences, such as immunology, enzymology, receptor binding, and toxicology.

Biomimetic pulmonary surfactants

Considerable progress has been made in the development of defined mixtures of proteins or peptides with phospholipids which mimic the activity of natural pulmonary surfactants. Several of these biomimetic surfactants are active in animal models and clinical syndromes of surfactant deficiency. This review summarizes the structure and composition of natural surfactants and the development of defined mixtures of peptides and lipids that may be useful in the treatment of respiratory distress.