MTOC reorientation occurs during FcgammaR-mediated phagocytosis in macrophages.

Cell polarization is essential for targeting signaling elements and organelles to active plasma membrane regions. In a few specialized cell types, cell polarity is enhanced by reorientation of the MTOC and associated organelles toward dynamic membrane sites. Phagocytosis is a highly polarized process whereby particles >0.5 microm are internalized at stimulated regions on the cell surface of macrophages. Here we provide detailed evidence that the MTOC reorients toward the site of particle internalization during phagocytosis. We visualized MTOC proximity to IgG-sRBCs in fixed RAW264.7 cells, during live cell imaging using fluorescent chimeras to label the MTOC and using frustrated phagocytosis assays. MTOC reorientation in macrophages is initiated by FcgammaR ligation and is complete within 1 h. Polarization of the MTOC toward the phagosome requires the MT cytoskeleton and dynein motor activity. cdc42, PI3K, and mPAR-6 are all important signaling molecules for MTOC reorientation during phagocytosis. MTOC reorientation was not essential for particle internalization or phagolysosome formation. However Golgi reorientation in concert with MTOC reorientation during phagocytosis implicates MTOC reorientation in antigen processing events in macrophages.

Chlamydia trachomatis vacuole maturation in infected macrophages.

Chlamydia trachomatis is an obligate intracellular bacterium responsible for one of the most common sexually transmitted diseases. In epithelial cells, C. trachomatis resides in a modified membrane-bound vacuole known as an inclusion, which is isolated from the endocytic pathway. However, the maturation process of C. trachomatis within immune cells, such as macrophages, has not been studied extensively. Here, we demonstrated that RAW macrophages effectively suppressed C. trachomatis growth and prevented Golgi stack disruption, a hallmark defect in epithelial cells after C. trachomatis infection. Next, we systematically examined association between C. trachomatis and various endocytic pathway markers. Spinning disk confocal time-lapse studies revealed significant and rapid association between C. trachomatis with Rab7 and LAMP1, markers of late endosomes and lysosomes. Moreover, pretreatment with an inhibitor of lysosome acidification led to significant increases in C. trachomatis growth in macrophages. At later stages of infection, C. trachomatis associated with the autophagy marker LC3. TEM analysis confirmed that a significant portion of C. trachomatis resided within double-membrane-bound compartments, characteristic of autophagosomes. Together, these results suggest that macrophages can suppress C. trachomatis growth by targeting it rapidly to lysosomes; moreover, autophagy is activated at later stages of infection and targets significant numbers of the invading bacteria, which may enhance subsequent chlamydial antigen presentation.

The role of alternative oxidase in modulating carbon use efficiency and growth during macronutrient stress in tobacco cells.

When wild-type (wt) tobacco (Nicotiana tabacum cv. Petit Havana SR1) cells are grown under macronutrient (P or N) limitation, they induce large amounts of alternative oxidase (AOX), which constitutes a non-energy-conserving branch of the respiratory electron transport chain. To investigate the significance of AOX induction, wt cells were compared with transgenic (AS8) cells lacking AOX. Under nutrient limitation, growth of wt cell cultures was dramatically reduced and carbon use efficiency (g cell dry weight gain g(-1) sugar consumed) decreased by 42-63%. However, the growth of AS8 was only moderately reduced by the nutrient deficiencies and carbon use efficiency values remained the same as under nutrient-sufficient conditions. As a result, the nutrient limitations more severely compromised the tissue nutrient status (P or N) of AS8 than wt cells. Northern analyses and a comparison of the mitochondrial protein profiles of wt and AS8 cells indicated that the lack of AOX in AS8 under P limitation was associated with increased levels of proteins commonly associated with oxidative stress and/or stress injury. Also, the level of electron transport chain components was consistently reduced in AS8 while tricarboxylic acid cycle enzymes did not show a universal trend in abundance in comparison to the wt. Alternatively, the lack of AOX in AS8 cells under N limitation resulted in enhanced carbohydrate accumulation. It is concluded that AOX respiration provides an important general mechanism by which plant cells can modulate their growth in response to nutrient availability and that AOX also has nutrient-specific roles in maintaining cellular redox and carbon balance.