APOPTOSIS IS NOT TRIGGERED IN NEURONAL CELLS BY PARTIAL RELEASE OF CYTOCHROME C BY LASER PULSES

K.W. Kinnally¹, C. Rieder², C.A. Mannella² and A. Khodjakov^2
1Division of Basic Sciences, New York University College of Dentistry, 345 East 24th Street, New York, NY, 10010
²Division of Molecular Medicine, Wadsworth Center, Albany, NY 12201

Release of proteins from mitochondria into the cytosol is an early event in apoptosis in many cell types and signals the commitment to apoptosis.  Several of these mitochondrial proteins, such as cytochrome c and AIF (apoptosis inducing factor), have specified roles in the apoptotic chain of degradative reactions.  An important mechanistic question is whether the release of protein from some mitochondria is a self-amplified signal, inducing apoptosis through a cell-wide cascade of mitochondrial protein release.  To test this hypothesis, a technique is needed that can selectively trigger the release of proteins from only a few mitochondria within a cell.  This was achieved in living neuronal cells using highly
focused laser pulses of green light.  Laser-induced rupture of the membranes of individual mitochondria leads to collapse of the membrane potential in, and loss of cytochrome c from, the targeted (but not neighboring) mitochondria.  Cells in which up to 15% of the mitochondria have been disrupted do not undergo apoptosis and instead divide normally.  Thus, protein release from mitochondria is not a self-amplifying signal and cannot, alone, trigger an apoptotic cascade. Microinjection experiments suggest all the cytochrome c must be released to induce apoptosis.  Together, these findings suggest the mitochondrial signal is all or none.

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