Surfactant proteins SP-A and SP-D in human health and disease

Surfactant proteins A (SP-A) and D (SP-D) are lung surfactant-associated hydrophilic proteins that have been implicated in surfactant homeostasis and pulmonary innate immunity.They are collagen-containing C-type (calcium-dependent) lectins, called collectins, and are structurally similar to mannose-binding protein of the lectin pathway of the complement system. Being carbohydrate pattern-recognition molecules, they recognize a broad spectrum of pathogens and allergens via the lectin domain, with subsequent activation of immune cells via the collagen region, thus offering protection against infection and allergenic challenge. SP-A and SP-D have been shown to be involved in viral neutralization, clearance of bacteria, fungi, and apoptotic and necrotic cells, down-regulation of allergic reaction, and resolution of inflammation. Studies on single-nucleotide polymorphism, protein levels in broncho-alveolar lavage, and gene knock-out mice have clearly indicated an association between SP-A and SP-D and a range of pulmonary diseases. In addition, recent studies using murine models of allergy and infection have raised the possibility that the recombinant forms of SP-A and SP-D may have therapeutic potential in controlling pulmonary infection, inflammation, and allergies in humans.     

Surfactant proteins SP-A and SP-D as modulators of the allergic inflammation in asthma.

Surfactant proteins A and D (SP-A and SP-D) are members of the collectin family and bind to various motifs of microorganisms, particles and allergens. They play an important role in the first-line defense within the lung. Recent research has highlighted that these proteins not only augment innate immune responses to invading microorganisms but also act on adaptive immune functions like dendritic cell maturation and T cell proliferation. Both SP-A and SP-D downregulate the eosinophilic inflammation in murine asthma models and shift the cytokine profile towards a T helper cell type 1 response. In addition, they are effective at alleviating bronchial hyperresponsiveness. Although our knowledge about surfactant proteins as modulators of the allergic inflammatory reaction in asthma is still limited, the idea that surfactant proteins play a role in asthma has attracted increasing attention. In this review, the impact of the lung collectins SP-A and SP-D on asthmatic allergic inflammation and vice versa will be discussed.     

SP-A、SP-D与肺部天然免疫防御

肺表面活性物质脱辅基蛋白A、D(SP-A、SP-D)系C型凝集素超家族中胶凝素家族成员,是肺部重要的天然免疫防御分子,不仅能清除病原体,还参与免疫、炎症及过敏反应的调节.     

Lung surfactant proteins (SP-A and SP-D) in non-adaptive host responses to infection

The lung surfactant proteins A and D (SP-A and SP-D) are collectins composed of C-type lectin domains attached to collagen regions. SP-A and SP-D are mainly found in the surfactant covering the pulmonary epithelial cells, but are also produced by cells lining the gastrointestinal tract. The main role of SP-A and SP-D is to interact directly with carbohydrate on the surface of microbial pathogens, thereby initiating a variety of effector mechanisms. This review focuses on the non-adaptive host responses of SP-A and SP-D to infection. Interaction of SP-A and SP-D with phagocytes is discussed and the structure and function of the putative receptors for SP-A and SP-D is presented. SP-A and SP-D seem to be regulated in a way similar to acute-phase proteins in the course of inflammation and evidence for the involvement of SP-A and SP-D as immunomodulators as well as their role in clearing allergens and modulating effector mechanisms in allergic reactions is discussed.     

The lung collectins, SP-A and SP-D, modulate pulmonary innate immunity

Pulmonary surfactant, which covers the peripheral airway, is a mixture of lipids and proteins. The hydrophilic surfactant proteins A (SP-A) and D (SP-D) play important roles in host defense mechanisms of the lung. These proteins belong to a collectin subgroup in which lectin domains are associated with collagenous structures. Collectins involve mannose-binding lectin, and are considered to function in innate immune systems. SP-A and SP-D interact with various microorganisms and pathogen-derived components. They act as opsonins by binding and agglutinating pathogens. The lung collectins also possess direct inhibitory effects on bacterial growth. SP-A and SP-D associate with immune cells, and activate various cellular functions. The direct interactions of SP-A and SP-D with macrophages result in modulation of phagocytosis or the production of reactive oxygen species. Moreover, by associating with cell surface pattern-recognition receptors, SP-A and SP-D regulate inflammatory cellular responses such as the release of lipopolysaccharides-induced proinflammatory cytokines. Animal models of SP-A- or SP-D-deficiency reveal significant defect in host defense. Significant susceptibility to bacterial and viral infections, delayed microbial clearance, and overexpression of proinflammatory cytokines are observed in SP-A or SP-D knockout mice. A more complete understanding of the mechanisms is required, but the biological relevance of SP-A and SP-D against various respiratory infections has been increasingly recognized.     

Protective Roles of Pulmonary Surfactant Proteins, SP-A and SP-D, Against Lung Allergy and Infection Caused by Aspergillus fumigatus

Pulmonary surfactant proteins, SP-A and SP-D, are immune molecules which can directly interact with pathogens and allergens, stimulate immune cells and manipulate cytokine and chemokine profiles during host's immune response. Using an opportunistic fungal pathogen Aspergillus fumigatus (Afu), we have attempted to understand participation of SP-A and SP-D in the host immunity. Afu causes a systemic infection via lungs, called invasive aspergillosis (IPA) in immunocompromised subjects. In the immunocompetent subjects, it can cause an allergic disorder, called allergic bronchopulmonary aspergillosis (ABPA). Therapeutic administration of these proteins in a murine model of IPA can rescue mice from death. Treating mice, having ABPA, can suppress IgE levels, eosinophilia, pulmonary cellular infiltration and cause a marked shift from a pathogenic Th2 to a protective Th1 cytokine profile. These results highlight the potential of SP-A, SP-D and their recombinant forms, as novel therapeutics for lung allergy and infection.     

Protective roles of pulmonary surfactant proteins,SP-A and SP-D,against lung allergy and infection caused by Aspergillus fumigatus

Pulmonary surfactant proteins, SP-A and SP-D, are immune molecules which can directly interact with pathogens and allergens, stimulate immune cells and manipulate cytokine and chemokine profiles during host's immune response. Using an opportunistic fungal pathogen Aspergillus fumigatus (Afu), we have attempted to understand participation of SP-A and SP-D in the host immunity. Afu causes a systemic infection via lungs, called invasive aspergillosis (IPA) in immunocompromised subjects. In the immunocompetent subjects, it can cause an allergic disorder, called allergic bronchopulmonary aspergillosis (ABPA). Therapeutic administration of these proteins in a murine model of IPA can rescue mice from death. Treating mice, having ABPA, can suppress IgE levels, eosinophilia, pulmonary cellular infiltration and cause a marked shift from a pathogenic Th2 to a protective Th1 cytokine profile. These results highlight the potential of SP-A, SP-D and their recombinant forms, as novel therapeutics for lung allergy and infection.     

The immunoregulatory roles of lung surfactant collectins SP-A, and SP-D, in allergen-induced airway inflammation

It has become increasingly evident that pulmonary surfactant proteins, SP-A and SP-D, present in the alveolar and bronchial epithelial fluid linings, not only play significant functions in the innate defense mechanism against pathogens, but also are involved in immunomodulatory roles, which result in the protection against, and resolution of, allergen-induced airway inflammation. Studies on allergen-sensitized murine models, and asthmatic patients, show that SP-A and SP-D can: specifically bind to aero-allergens; inhibit mast cell degranulation and histamine release; and modulate the activation of alveolar macrophages and dendritic cells during the acute hypersensitive phase of allergic response. They also can alleviate chronic allergic inflammation by inhibiting T-lymphocyte proliferation as well as increasing phagocytosis of DNA fragments and clearance of apoptotic cell debris. Furthermore, it has emerged, from the studies on SP-D-deficient mice, that, when these mice are challenged with allergen, they develop increased eosinophil infiltration, and abnormal activation of lymphocytes, leading to the production of Th2 cytokines. Intranasal administration of SP-D significantly attenuated the asthmatic-like symptoms seen in allergen-sensitized wild-type, and SP-D-deficient, mice. These important findings provide a new insight of the role that surfactant proteins play in handling environmental stimuli and in their immunoregulation of airway inflammatory disease.     

Changes in lipid structure produced by surfactant proteins SP-A, SP-B, and SP-C

Pulmonary surfactant phospholipids may assume several different structures including tubular myelin, unilamellar and multilamellar vesicles, and others. These populations of materials appear to have similar phospholipid compositions but may differ in their association with surfactant proteins SP-A, SP-B, or SP-C. We have used electron microscopy to determine the changes in structure of simple lipid mixtures (phosphatidylglycerol, dipalmitoylphosphatidylcholine) produced by adding one or combinations of the three proteins. Adding SP-A to lipids generated multilamellar structures composed of membranes with fuzzy or particulate surfaces. In contrast, SP-B or SP-C generated discoidal particles and structures that appeared to be sheets of membrane formed by associated particles. Used together, SP-A and SP-B reorganized some of the lipid into tubular myelin, a structure that was not observed in SP-A, SP-C recombinants. These observations confirm the in vitro formation of tubular myelin reported by others and support the possibility that surfactant materials with defined structure can be produced in vitro for analyses of their molecular organizations.     

Polymorphisms of human SP-A, SP-B, and SP-D genes: association of SP-B Thr131Ile with ARDS

An allele association study of 19 polymorphisms in surfactant proteins SP-A1, SP-A2, SP-B, and SP-D genes in acute respiratory distress syndrome (ARDS) was carried out. Trend-test analysis revealed differences (p < 0.05) in the frequency of alleles for some of the microsatellite markers flanking SP-B, and for one polymorphism (C/T) at nucleotide 1580 [C/T (1580)], within codon 131 (Thr131Ile) of the SP-B gene. The latter determines the presence or absence of a potential N-linked glycosylation site. Multivariate analysis revealed significant differences only for the C/T (1580) polymorphism. When the ARDS population was divided into subgroups, idiopathic (i.e., pneumonia, etc.) or exogenic (i.e., trauma, etc.), significant differences were observed for the C/T (1580), for the idiopathic ARDS group, and the frequency of the C/C genotype was increased in this group. Based on the odds ratio, the C allele may be viewed as a susceptibility factor for ARDS. Although the expression of both C and T alleles occurs in heterozygous individuals, it is currently not known whether these alleles correspond to similar levels of SP-B protein. These data suggest that SP-B or a linked gene contributes to susceptibility to ARDS.     

Novel, non-radioactive, simple and multiplex PCR-cRFLP methods for genotyping human SP-A and SP-D marker alleles

We have previously identified an allele of the human SP-A2 gene that occurs with greater frequency in an RDS population [12]. Because of the importance of SP-A in normal lung function and its newly emerging role in innate host defense and regulation of inflammatory processes, we wish to better characterize genotypes of both SP-A1 and SP-A2 genes. It has been determined that SP-D shares similar roles in immune response. Therefore, in this report we 1) describe a novel, non radioactive PCR based-cRFLP method for genotyping both SP-A and SP-D; 2) describe two previously unpublished biallelic polymorphisms within the SP-D gene; 3) present the partial sequence of one new SP-A1 allele (6A14) and describe other new SP-A1 and SP-A2 alleles; and 4) describe additional methodologies for SP-A genotype assessment. The ability to more accurately and efficiently genotype samples from individuals with various pulmonary diseases will facilitate population and family based association studies. Genetic polymorphisms may be identified that partially explain individual disease susceptibility and/or treatment effectiveness.     

Immunohistochemical distribution of SP-D,compared with that of SP-A and KL-6, in interstitial pneumonias

The immunohistochemical distribution of SP-D was compared with that of SP-A and KL-6 using a monoclonal antibody in lung tissues of 15 cases of collagen vascular disease-associated interstitial pneumonia (CVD-IP), 4 cases of hypersensitivity pneumonitis (CHP), and 6 cases of other diseases to determine their differences in distribution. In this study, the main targets were alveolar epithelial cells, especially those in the regenerating stage, as well as lymph vessels and stroma. The cytoplasm of type II alveolar epithelial cells and Clara cells was positive for SP-D, with sharp margins; interestingly, however, during the process of regeneration large positive cells were intermingled with relatively small negative cells, even in the same row of cells. In sharp contrast, staining for SP-A and KL-6 was positive in the cytoplasm of all the regenerating alveolar epithelial cells, as well as Clara cells. Staining for KL-6 was usually positive in the surface of air spaces in linear fashion. Staining for SP-A was also positive in elastic fibers in vascular walls. In areas of destruction of pulmonary structures, loose stroma and the endothelial cells of lymph vessels as well as their contents were distinctly positive for SP-A and/or KL-6 but not SP-D. Judging from these results in pulmonary tissues of CVD-IP and HP, SP-D might be a marker for maturity of regenerating epithelial cells. Both SP-A and KL-6 were detected in intimate relationship to the stage of regeneration of alveolar epithelial cells and were expressed before SP-D. In addition, the lymph vessels play a very important role in transfer of KL-6 into the bloodstream.     

Neuronal nicotinic receptors: function,modulation and structure

In the present paper, we discuss the diversity of nicotinic receptors (nAChRs) expressed in hippocampal neurons and their functional properties. Three distinct types of whole-cell currents can be identified in hippocampal neurons by brief application of nicotinic agonists. These currents, which are referred to as types IA, II and III, were distinguished pharmacologically on the basis of their differential sensitivities to various nicotinic agonists and antagonists, and functionally on the basis of their rectification properties and rundown. Most of the hippocampal neurons show type IA current in response to nicotinic agonists, and the single channels that account for these currents in addition to having a very short open time and a high conductance, have a high Ca²⁺permeability and inactivate very fast. Based on the comparison of the properties of the nicotinic currents elicited in hippocampal neurons with those elicited by activation of nAChRs transiently expressed in oocytes, we have concluded that type IA currents may be subserved by α 7-bearing nAChRs, type II currents by α4β2 nAChRs, and type III currents by α3β4 nAChRs. Indeed,in-situhybridization shows that mRNAs coding for α7, α4 and β2 nAChR subunits are present in hippocampal neurons. These findings altogether have given new insights for the studies of neurophysiological processes and neuropathological conditions in which neuronal nAChRs may be implicated.     

Decidual expression and localization of human surfactant protein SP-A and SP-D,and complement protein C1q

Surfactant proteins SP-A and SP-D, and complement protein C1q are soluble innate immune pattern recognizing molecules. SP-A, SP-D and C1q have an overall similar structure composed of an N-terminal triple-helical collagen region that is followed by a trimeric globular domain. While SP-A and SP-D belong to the collectin family (collagen containing lectin), C1q is the first recognition subcomponent of the classical pathway of the complement system. Recently, SP-A, SP-D and C1q have been considered to play important roles in early and late pregnancy. However, their expression in early human decidua has not been examined. Here, we investigated whether SP-A, SP-D and C1q are expressed within first trimester decidua in humans and their expression is associated with trophoblasts and decidual stromal cells. Decidual samples from women undergoing elective vaginal termination of pregnancy during first trimester were obtained from 25 subjects. Immunohistochemical studies using anti-human SP-A, anti-human SP-D and anti-human C1q antibodies were performed on decidual tissue sections along with anti-vimentin and cytokeratin-7 antibodies to identify stromal cells and trophoblasts. The expression was also examined by immunostaining and PCR using decidual and stromal cells. C1q expression was significantly higher when compared to SP-A and SP-D in the first trimester human decidua. Double immunostaining revealed that all stromal cells and trophoblasts expressed SP-A, SP-D and C1q, while only few invasive trophoblasts expressed C1q. Thus, expression of SP-A, SP-D and C1q in human decidua during first trimester suggests potential role of SP-A, SP-D and C1q during the early stages of pregnancy including implantation, trophoblast invasion and placental development.     

Structural requirements for SP-D function in vitro and in vivo: therapeutic potential of recombinant SP-D

Surfactant protein D has multiple functions in innate immunity in the lung. The generation of SP-D knock-out mice has revealed a central role for this protein in the control of lung inflammation. Accumulating evidence in mouse models of infection and inflammation indicates that truncated recombinant forms of surfactant protein D are biologically active in vivo. This review addresses the structural requirements for recognised activities of SP-D in vitro and in vivo, with emphasis on evidence arising from studies with transgenic mice and mouse models of inflammatory lung disease. The potential of truncated recombinant forms of surfactant protein D as novel therapy for infectious and inflammatory disease is discussed.     

BAFF, APRIL and their receptors: structure, function and signaling

BAFF, APRIL and their receptors play important immunological roles, especially in the B cell arm of the immune system. A number of splice isoforms have been described for both ligands and receptors in this subfamily, some of which are conserved between mouse and human, while others are species-specific. Structural and mutational analyses have revealed key determinants of receptor-ligand specificity. BAFF-R has a strong selectivity for BAFF; BCMA has a higher affinity for APRIL than for BAFF, while TACI binds both ligands equally well. The molecular signaling events downstream of BAFF-R, BCMA and TACI are still incompletely characterized. Survival appears to be mediated by upregulation of Bcl-2 family members through NF-kappaB activation, degradation of the pro-apototic Bim protein, and control of subcellular localization of PCKdelta. Very little is known about other signaling events associated with receptor engagement by BAFF and APRIL that lead for example to B cell activation or to CD40L-independent Ig switch.     

Intracellular transport and kinesin superfamily proteins, KIFs: structure, function, and dynamics

Various molecular cell biology and molecular genetic approaches have indicated significant roles for kinesin superfamily proteins (KIFs) in intracellular transport and have shown that they are critical for cellular morphogenesis, functioning, and survival. KIFs not only transport various membrane organelles, protein complexes, and mRNAs for the maintenance of basic cellular activity, but also play significant roles for various mechanisms fundamental for life, such as brain wiring, higher brain functions such as memory and learning and activity-dependent neuronal survival during brain development, and for the determination of important developmental processes such as left-right asymmetry formation and suppression of tumorigenesis. Accumulating data have revealed a molecular mechanism of cargo recognition involving scaffolding or adaptor protein complexes. Intramolecular folding and phosphorylation also regulate the binding activity of motor proteins. New techniques using molecular biophysics, cryoelectron microscopy, and X-ray crystallography have detected structural changes in motor proteins, synchronized with ATP hydrolysis cycles, leading to the development of independent models of monomer and dimer motors for processive movement along microtubules.