Local photoactivation (green) shows that mitochondrial fusion is inhibited by apoptosis onset.

Mitochondrial networks fragment during apoptosis, and it is known that fission increases. Any fission increase is usually accompanied by a fusion increase, but on page 493 Karbowski et al. find that activating apoptosis blocks mitochondrial fusion.

The authors came up with a clever new technique to track individual organelles using a photoactivatable form of GFP with a mitochondrial targeting sequence. Aiming a laser at individual mitochondria in tagged cells activated the fluorescent tag, and the dilution of fluorescence provided a quantitative readout of organelle fusion and fission. Inducing apoptosis blocked mitochondrial fusion to a degree that could fully account for apoptotic mitochondrial fragmentation. The block in fusion occurs around the same time as Bax translocation to mitochondria and mitochondrial permeabilization, and before caspase activation.

The data suggest that a complete block in mitochondrial fusion is a normal part of apoptosis, and that this block is either fully or partially responsible for mitochondrial fragmentation. The authors are now adapting their assay to study mitochondrial fragmentation in more detail, and suggest that it could be used for high-resolution studies on the dynamics of other organelles. ▪