Endocytic pathway for surfactant protein A in human macrophages: binding, clathrin-mediated uptake, and trafficking through the endolysosomal pathway.
In the noninflamed lung, surfactant protein A (SP-A) acts as an anti-inflammatory molecule through its effects on macrophage (MPhi) function, modulating cytokine and reactive oxygen and nitrogen intermediate production. The receptors responsible for these effects of SP-A on human MPhi are not clear, although SP-A binding to several proteins has been described. In this study, we demonstrate high-affinity specific binding of SP-A to primary human MPhi. SP-A binding was inhibited by EGTA, indicating calcium dependence. However, mannan did not inhibit SP-A binding, suggesting that binding is mediated by a direct protein-protein interaction that does not involve carbohydrate recognition. Our laboratory has previously shown that SP-A is rapidly endocytosed by human MPhi into discrete vesicles. Although previous work indicates that SP-A is ultimately degraded by murine MPhi over time, the trafficking pathway of SP-A through MPhi after uptake has not been reported and is of potential biological importance. We examined trafficking of SP-A in human MPhi by electron and confocal microscopy and show for the first time that SP-A is endocytosed by primary human MPhi through clathrin-coated pits and colocalizes sequentially over time with the early endosome marker EEA1, late endosome marker lamp-1, and lysosome marker cathepsin D. We conclude that SP-A binds to receptor(s) on human MPhi, is endocytosed by a receptor-mediated, clathrin-dependent process, and trafficks through the endolysosomal pathway. These studies provide further insight into the interactions of SP-A with the MPhi cell surface and intracellular compartments that play important roles in SP-A modulation of lung MPhi biology.
Crowther
Joy E
JE
Dept. of Medicine, Ohio State Univ., 420 W. 12th Ave., 200 MRF, Columbus, OH 43210, USA.
Schlesinger
Larry S
LS
D002118:Calcium / Q000494:pharmacology; D002966:Clathrin / Q000502:physiology*; D022163:Clathrin-Coated Vesicles / Q000502:physiology; D004705:Endocytosis / Q000502:physiology*; D006801:Humans; D008081:Liposomes / Q000378:metabolism; D016676:Macrophages, Alveolar / Q000187:drug effects / Q000502:physiology*; D008351:Mannans / Q000494:pharmacology; D021381:Protein Transport / Q000502:physiology*; D037662:Pulmonary Surfactant-Associated Protein A / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism
D002966:Clathrin; D008081:Liposomes; D008351:Mannans; D037662:Pulmonary Surfactant-Associated Protein A; D011956:Receptors, Cell Surface; C067774:surfactant protein A receptor; D002118:Calcium