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El sistema respiratorio es imprescindible para la obtención del oxígeno necesario para los procesos metabólicos. El intercambio gaseoso tiene lugar a través de la barrera alveolo-capilar, donde el oxígeno y el dióxido de carbono difunden entre el espacio aéreo en los alveolos y los eritrocitos en el torrente sanguíneo. El epitelio respiratorio representa la superficie más extensa del organismo expuesta a la atmósfera y se compone de dos tipos celulares, las células alveolares tipo I (ATI) y las células alveolares tipo II (ATII). Las células ATI constituyen la mayor superficie epitelial destinada a intercambio gaseoso en los alveolos, mientras que la principal función de las células ATII es la síntesis y el ensamblaje de los componentes del surfactante pulmonar en forma de ensamblados de membranas altamente empaquetadas en el interior de organelas ácidas, denominadas cuerpos lamelares. El principal estímulo fisiológico para la secreción de surfactante es el estiramiento del epitelio alveolar durante la inspiración, el cual provoca un incremento en la concentración de calcio citoplasmático que, a su vez, desencadena la fusión de los cuerpos lamelares con la membrana plasmática de las células ATII y finalmente, la liberación de surfactante al espacio alveolar. Una vez secretado, el surfactante pulmonar se adsorbe y se extiende a lo largo de la interfase aire-líquido alveolar, donde el surfactante estabiliza la superficie respiratoria durante los sucesivos ciclos de inspiración-espiración al reducir la tensión superficial. Además de esta función principal, el surfactante pulmonar también participa en la defensa pulmonar contra infecciones provocadas por virus, bacterias y hongos, ejerciendo actividades microbicidas directas u opsonizando patógenos y modulando las respuestas celulares inflamatorias..., The respiratory system is necessary to obtain the oxygen required for metabolic processes. Gas exchange takes place through the alveolar-capillary barrier, where oxygen and carbon dioxide diffuse between air space in alveoli and erythrocytes in the bloodstream. The respiratory epithelium represents the largest surface in the organism exposed to the atmosphere and comprises two cell types, alveolar type I (ATI) and alveolar type II (ATII) cells. ATI cells constitute the major gas exchange surface in the alveoli, whereas the main function of ATII cells is the synthesis and assembly of the components of pulmonary surfactant in the form of tightly packed membrane arrays inside acidic organelles, called lamellar bodies. The primary physiological stimulus for surfactant secretion is the mechanical stretching of alveolar epithelium during inspiration, which leads to a rise in the cytoplasmic calcium concentration that triggers the fusion of lamellar bodies with the plasma membrane of ATII cells and finally, surfactant release. Once secreted, pulmonary surfactant is adsorbed and spread along the alveolar air-liquid interface, where surfactant stabilizes the respiratory surface during successive inspiration-exhalation cycles by reducing the surface tension. Besides this main role, pulmonary surfactant also participates in lung defense against infection caused by virus, bacteria and fungi, exerting direct microbicidal activities or opsonization of pathogens and modulating the inflammatory cellular responses...

Surfactant protein B (SP-B), from the saposin-like family of proteins, is essential to facilitate the formation and proper performance of surface active films at the air-liquid interface of mammalian lungs, and lack of or deficiency in this protein is associated with lethal respiratory failure. Despite its importance, neither a structuralmodel nor amolecular mechanism of SP-B is available. The purpose of the present work was to purify and characterize native SP-B supramolecular assemblies to provide a model supporting structure-function features described for SP-B. Purification of porcine SP-B using detergentsolubilized surfactant reveals the presence of 10 nm ringshaped particles. These rings, observed by atomic force and electron microscopy, would be assembled by oligomerization of SP-B as a multimer of dimers forming a hydrophobically coated ring at the surface of phospholipid membranes or monolayers. Docking of rings from neighboring membranes would lead to formation of SP-B–based hydrophobic tubes, competent to facilitate the rapid flow of surface active lipids both between membranes and between surfactant membranes and the interface. A similar sequential assembly of dimers, supradimeric oligomers and phospholipid-loaded tubes could explain the activity of other saposins with colipase, cytolysin, or antibiotic activities, offering a common framework to understand the range of functions carried out by saposins. —Olmeda, B., García-Álvarez, B., Gómez, M. J., Martínez-Calle, M., Cruz, A., Perez-Gil, J. A model for the structure and mechanism of action of pulmonary surfactant protein B. FASEB J. 29, 4236–4247 (2015). www.fasebj.org, Ministerio de Economía y Competitividad (España), Comunidad de Madrid, Sección Deptal. de Bioquímica y Biología Molecular (Biológicas), Fac. de Ciencias Biológicas, TRUE, pub