INHIBITION OF CASPASE-LIKE ACTIVITIES PREVENTS THE APPEARANCE OF REACTIVE OXYGEN SPECIES AND DARK-INDUCED APOPTOSIS IN THE UNICELLULAR CHLOROPHYTE DUNALIELLA TERTIOLECTA(1).

When the chlorophyte alga Dunaliella tertiolecta Butcher is placed in darkness, a form of programmed cell death with many similarities to apoptosis is induced, including the induction of caspase-like proteases. Many uncertainties about the regulation and mediators that participate in the process remain. To examine the relationship between caspase-like activities and different apoptotic events (i.e., phosphatidylserine [PS] translocation), increases in membrane permeability and numbers of dead cells revealed by SYTOX-green staining, and the generation of reactive oxygen species (ROS), we used the broad-range caspase inhibitor Boc-D-FMK to block the activity of the whole class of caspase-like proteins simultaneously. In the presence of the inhibitor, ROS were not produced, and cells did not die. Loss of membrane asymmetry, indicated by external labeling of PS by annexin V, was apparent at midstages of light deprivation, although it did not conform to the typical pattern for PS exposure observed in metazoans or vascular plants, which occurs at early stages of the apoptotic event. Thus, we have evidence for a link between ROS and cell death involving caspase-like enzymes in an alga. The fact that caspase-like inhibitors prevent not only cell death, but also ROS and loss of cell membrane integrity and asymmetry, suggests that caspase-like proteases might have regulatory roles early in cell death, in addition to dismantling functions.