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Volume 2 (2002), Pages 1198-1208
Short Communication

Microscopic Photosensitization: A New Tool to Investigate the Role of Mitochondria in Cell Death

1Department of Biochemistry and Molecular Biology, PO Box 13D, Monash University, Victoria 3800, Australia
2Olympus Australia Pty. Ltd., 1/104 Ferntree Gully Rd., Oakleigh VIC 3166, Australia

Received 15 February 2002; Accepted 20 March 2002

Copyright © 2002 May-Ghee Lum et al.


Active involvement of mitochondria in cell death has been well-documented, but local apoptotic signaling between subsets of mitochondria has been poorly explored to date. Using mitochondrially localized CMXRos as a photosensitizer coupled to laser irradiation by confocal laser scanning microscopy, we demonstrate that partial irradiation of about half the mitochondria in human 143B TK cells induces rapid loss of mitochondrial membrane potential (ΔΨm) in nonirradiated mitochondria. Cells so partially irradiated show apoptotic indications, including mobilization of cytochrome c and binding of annexin V within 2 h following irradiation. The loss of ΔΨm in nonirradiated mitochondria did not occur in cells photoirradiated in the absence of CMXRos. Increasing the proportion of irradiated mitochondria in each cell (up to about 50%) generated a correspondingly greater percentage of cells in which nonirradiated mitochondria lost ΔΨm and which also showed apoptotic indications. Only at the highest level of irradiation (global for all mitochondria in one cell) were signs of necrosis evident (judged by uptake of propidium iodide). Because laser irradiation is specific to the subpopulation of mitochondria targeted, the data imply that a signal emanating from irradiated mitochondria is processed by their nonirradiated counterparts. We conclude that intermitochondrial signaling occurs in the subcellular response to induction of apoptosis.