Subcellular fractionation by organelle electrophoresis: separation of phagosomes containing heat-killed yeast particles

Survival of pathogenic mycobacteria in macrophages is mediated through autophosphorylation of protein kinase G

Pathogenic mycobacteria survive within macrophages through the inhibition of phagosome-lysosome fusion. A crucial factor for avoiding lysosomal degradation is the mycobacterial serine/threonine protein kinase G (PknG). PknG is released into the macrophage cytosol upon mycobacterial infection, suggesting that PknG might exert its activity by interfering with host signaling cascades, but the mode of action of PknG remains unknown. Here, we show that PknG undergoes autophosphorylation on threonine residues located at the N terminus. In contrast to all other mycobacterial kinases investigated thus far, autophosphorylation of PknG was not involved in the regulation of its kinase activity. However, autophosphorylation was crucial for the capacity of PknG to promote mycobacterial survival within macrophages. These results will contribute to a better understanding of the virulence mechanisms of pathogenic mycobacteria and may help to design improved inhibitors of PknG to be developed as antimycobacterial compounds.

Analysis of mitochondria isolated from single cells

Bulk studies are not suitable to describe and study cell-to-cell variation, which is of high importance in biological processes such as embryogenesis, tissue differentiation, and disease. Previously, capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) was used to measure the properties of organelles isolated from millions of cells. As such, these bulk measurements reported average properties for the organelles of cell populations. Similar measurements for organelles released from single cells would be highly relevant to describe the subcellular variations among cells. Toward this goal, here we introduce an approach to analyze the mitochondria released from single mammalian cells. Osteosarcoma 143B cells are labeled with either the fluorescent mitochondrion-specific 10-N-nonyl acridine orange (NAO) or via expression of the fluorescent protein DsRed2. Subsequently, a single cell is introduced into the CE-LIF capillary where the organelles are released by a combined treatment of digitonin and trypsin. After this treatment, an electric field is applied and the released organelles electromigrate toward the LIF detector. From an electropherogram, the number of detected events per cell, their individual electrophoretic mobilities, and their individual fluorescence intensities are calculated. The results obtained from DsRed2 labeling, which is retained in intact mitochondria, and NAO labeling, which labels all mitochondria, are the basis for discussion of the strengths and limitations of this single-cell approach.

Protein kinase G from pathogenic mycobacteria promotes survival within macrophages

Pathogenic mycobacteria resist lysosomal delivery after uptake into macrophages, allowing them to survive intracellularly. We found that the eukaryotic-like serine/threonine protein kinase G from pathogenic mycobacteria was secreted within macrophage phagosomes, inhibiting phagosome-lysosome fusion and mediating intracellular survival of mycobacteria. Inactivation of protein kinase G by gene disruption or chemical inhibition resulted in lysosomal localization and mycobacterial cell death in infected macrophages. Besides identifying a target for the control of mycobacterial infections, these findings suggest that pathogenic mycobacteria have evolved eukaryotic-like signal transduction mechanisms capable of modulating host cell trafficking pathways.

Hijacking the host: survival of pathogenic mycobacteria inside macrophages

Mycobacteria are among the most persistent pathogens known today: one-third of the global population is latently infected with Mycobacterium tuberculosis, an organism that is responsible for approximately 2.5 million deaths each year. One key to the pathogenic potential of the mycobacteria lies in their capacity to resist destruction by macrophages. Although it has long been recognized that mycobacteria achieve such resistance by interfering with phagosome--lysosome fusion, recent work has shed some more light on the molecular basis of this highly efficient survival strategy.

Entry and survival of pathogenic mycobacteriain macrophages

Pathogenic mycobacteria, including Mycobacterium tuberculosis, are phagocytosed by macrophages but manage to survive within the mycobacterial phagosome. Recent work has shed some more light on the mechanisms of mycobacterial entry and survival inside macrophages. Two host cell components, the steroid cholesterol and a phagosomal coat protein termed TACO were found to play crucial roles in the establishment of an intracellular infection. This review describes how these findings may help to understand the circumvention of the normal trafficking routes inside host cells by mycobacteria.

Lectin-induced retardation of subcellular organelles during preparative density gradient electrophoresis: selective purification of plasma membranes

Plasma membranes (PM) are difficult to separate by conventional means from other cellular compartments. Using a density gradient electrophoresis (DGE) apparatus (7 cm, x 2.2 cm), mammalian subcellular organelles were separated from a total postnuclear supernatant. The sialic acid-binding lectin wheat germ agglutinin (WGA) permitted 1.5-fold electrophoretic retardation of plasma membranes lagging far behind endoplasmic reticulum, endosomes, Golgi and lysosomes (in order of increasing electrophoretic mobility). Mobilities of the latter organelles were not affected by wheat germ agglutinin. The retarded plasma membrane was monitored by surface iodination, the presence of Ca(++)- and Na+/K(+)-ATPases and by the presence of clathrin-coated pits using Western immunoblotting. In the presence of WGA two clathrin-containing compartments were detected; in the absence of WGA three clathrin populations were seen in the electropherogram: clathrin-coated vesicles, clathrin-coated pits (on the PM) and clathrin-coated structures on the trans-Golgi network (TGN). Both in the presence and absence of WGA, plasma membrane domains of different electrophoretic mobilities were detected.