Phagocytosis (and endocytosis) is an unusual cellular process which results in the formation of a novel subcellular organelle, the phagosome. This phagosome contains not only the internalised target of phagocytosis, but also the external medium, creating a new border between extracellular and intracellular environments. The boundary at the plasma membrane is, of course, tightly controlled and exploited in ionic cell signalling events. Although there has been much work on the control of phagocytosis by ions, notable Ca2+ ions influxing across the plasma membrane, increasing our understanding of the mechanism enormously, very little work has been done exploring the phagosome/cytosol boundary. In this paper, we have explored the changes in the intra-phagosomal Ca2+ ion content which occur during phagocytosis and phagosome formation in human neutrophils. Measuring Ca2+ ion concentration in the phagosome is potentially prone to artefacts as the intra-phagosomal environment experiences changes in pH and oxidation. However, by excluding such artefacts, we conclude that there are open Ca2+ channels on the phagosome, which allow Ca2+ ions to ‘drain’ into the surrounding cytosol. This conclusion was confirmed by monitoring the translocation of intracellularly expressed YFP-tagged C2 domain of PKC-γ. This approach marked regions of membrane at which Ca2+ influx occurred, the earliest being the phagocytic cup, and then the whole cell. This paper therefore presents data which has novel implications for understanding phagocytic Ca2+ signalling events, such as peri-phagosomal Ca2+ hotspots, and other phenomena.
Biology and Life Sciences, Cell and Developmental Biology
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