Regulation of Phagosomal Acidification

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145

Diseases of gut inflammation such as neonatal necrotizing enterocolitis (NEC) result after an injury to the mucosal lining of the intestine, leading to translocation of bacteria and endotoxin (lipopolysaccharide). Intestinal mucosal defects are repaired by the process of intestinal restitution, during which enterocytes migrate from healthy areas to sites of injury. In an animal model of NEC, we determined that intestinal restitution was significantly impaired compared with control animals. We therefore sought to determine the mechanisms governing enterocyte migration under basal conditions and after an endotoxin challenge. Here we show that the cytoskeletal reorganization and stress fiber formation required for migration in IEC-6 enterocytes requires RhoA. Enterocytes were found to express the endotoxin receptor Toll-like receptor 4, which served to bind and internalize lipopolysaccharide. Strikingly, endotoxin treatment significantly inhibited intestinal restitution, as measured by impaired IEC-6 cell migration across a scraped wound. Lipopolysaccharide was found to increase RhoA activity in a phosphatidylinositol 3-kinasedependent manner, leading to an increase in phosphorylation of focal adhesion kinase and an enhanced number of focal adhesions. Importantly, endotoxin caused a progressive, RhoA-dependent increase in cell matrix tension/contractility, which correlated with the observed impairment in enterocyte migration. We therefore conclude that endotoxin inhibits enterocyte migration through a RhoA-dependent increase in focal adhesions and enhanced cell adhesiveness, which may participate in the impaired restitution observed in experimental NEC.Necrotizing enterocolitis (NEC) 1 is the leading cause of death from gastrointestinal disease in neonates and is second to respiratory disease as the overall leading cause of morbidity and mortality in this population (1-3). Clinical manifestations of NEC include abdominal distention, feeding intolerance, and systemic sepsis, which result from the destruction of the intestinal barrier (4, 5). Mucosal breakdown may occur as a result of a perinatal insult, such as hypoxia, which allows bacteria and bacterial by-products to breach the normally impermeable mucosal barrier and initiate a local and systemic inflammatory response (4,6,7,9). This causes the release of proinflammatory cytokines, including tumor necrosis factor, platelet-activating factor, nitric oxide, and endotoxin, which lead to further epithelial injury (10 -13). Following mucosal damage, healing occurs initially through the process of epithelial restitution, in which healthy enterocytes adjacent to the sites of injury migrate toward the denuded mucosa to bridge the defect (14 -18). We hypothesize that the intestinal barrier defect in NEC does not result from epithelial injury alone but also from impaired restitution. This paper focuses specifically on the process of enterocyte migration, which is the sine qua non of epithelial restitution and mucosal healing.Although a variety of cytokines may be importan...

Section: Methodsmentioning

confidence: 99%

Endotoxin Inhibits Intestinal Epithelial Restitution through Activation of Rho-GTPase and Increased Focal Adhesions

Çetin

Ford

Sysko

et al. 2004

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145

“…60 for details). The microscope and software set up used for ratio imaging have been described in detail elsewhere (60,61).…”

Section: Methodsmentioning

confidence: 99%

“…Thus, phagosomes containing live mycobacteria do not fuse with lysosomes nor do they acidify fully, yet acquire LAMP1 (15,16,61). To ensure that bona fide phagolysosomes are formed in the absence of ⑀-COP we measured phagosomal pH in FcR-ldl cells.…”

Section: Fusion Of Early Endosomes With Phagosomes In the Absencementioning

confidence: 99%

Role of COPI in Phagosome Maturation

Botelho¹,

Hackam²,

Schreiber³

et al. 2000

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Phagosomes mature by sequentially fusing with endosomes and lysosomes. Vesicle budding is presumed to occur concomitantly, mediating the retrieval of plasmalemmal components and the regulation of phagosomal size. We analyzed whether fission of vesicles from phagosomes requires COPI, a multimeric complex known to be involved in budding from the Golgi and endosomes. The role of COPI was studied using ldlF cells, that harbor a temperature-sensitive mutation in ⑀-COP, a subunit of the coatomer complex. These cells were made phagocytic toward IgG-opsonized particles by heterologous expression of human Fc␥RIIA receptors. Following incubation at the restrictive temperature, ⑀-COP was degraded in these cells and their Golgi complex dispersed. Nevertheless, phagocytosis persisted for hours in cells devoid of ⑀-COP. Retrieval of transferrin receptors from phagosomes became inefficient in the absence of ⑀-COP, while clearance of the Fc␥RIIA receptors was unaffected. This indicates that fission of vesicles from the phagosomal membrane involves at least two mechanisms, one of which requires intact COPI. Traffic of fluid-phase markers and aggregated IgG-receptor complexes along the endocytic pathway was abnormal in ⑀-COP-deficient cells. In contrast, phagosome fusion with endosomes and lysosomes was unimpaired. Moreover, the resulting phagolysosomes were highly acidic. Similar results were obtained in RAW264.7 macrophages treated with brefeldin A, which precludes COPI assembly by interfering with the activation of adenosine ribosylation factor. These data indicate that neither phagosome formation nor maturation are absolutely dependent on COPI. Our findings imply that phagosomal maturation differs from endosomal progression, which appears to be more dependent on COPI-mediated formation of carrier vesicles.

“…Acidification of phagosomes has been shown to occur by fusion with late endosomes and lysosomes in mammalian cells (52). We examined by using LysoTracker Red, a membranediffusible probe accumulated in acidic organelles (53), whether the PPV and phagosomes of the amoeba are acidified during maturation.…”

Section: Ehrab5 Is Not Associated With Endosomes or Lysosomes But Exhmentioning

confidence: 99%

Rab5-associated Vacuoles Play a Unique Role in Phagocytosis of the Enteric Protozoan Parasite Entamoeba histolytica

Saito‐Nakano

Yasuda

Nakada‐Tsukui

et al. 2004

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174

In mammals, Rab5 and Rab7 play a specific and coordinated role in a sequential process during phagosome maturation. Here, we report that Rab5 and Rab7 in the enteric protozoan parasite Entamoeba histolytica, EhRab5 and EhRab7A, are involved in steps that are distinct from those known for mammals. EhRab5 and EhRab7A were localized to independent small vesicular structures at steady state. Priming with red blood cells induced the formation of large vacuoles associated with both EhRab5 and EhRab7A ("prephagosomal vacuoles (PPV)") in the amoeba within an incubation period of 5-10 min. PPV emerged de novo physically and distinct from phagosomes. PPV were gradually acidified and matured by fusion with lysosomes containing a digestive hydrolase, cysteine proteinase, and a membrane-permeabilizing peptide amoebapore. After EhRab5 dissociated from PPV, 5-10 min later, the EhRab7A-PPV fused with phagosomes, and EhRab7A finally dissociated from the phagosomes. Immunoelectron and light micrographs showed that PPV contained small vesicle-like structures containing fluid-phase markers and amoebapores, which were not evenly distributed within PPV, suggesting that the mechanism was similar to multivesicular body formation in PPV generation. In contrast to Rab5 from other organisms, EhRab5 was involved exclusively in phagocytosis, but not in endocytosis. Overexpression of wild-type EhRab5 enhanced phagocytosis and the transport of amoebapore to phagosomes. Conversely, expression of an EhRab5Q67L GTP form mutant impaired the formation of PPV and phagocytosis. Altogether, we propose that the amoebic Rab5 plays an important role in the formation of unique vacuoles, which is essential for engulfment of erythrocytes and important for packaging of lysosomal hydrolases, prior to the targeting to phagosomes.