Protease Inhibitors Cause Necrotic Cell Death in Chlamydomonas reinhardtii by Inducing the Generation of Reactive Oxygen Species

The Cytotoxic Effects of Camptothecin and Mastoparan on the Unicellular Green Alga Chlamydomonas reinhardtii

We have recently reported that protease inhibitors affecting the activity of the proteasome cause necrotic cell death in Chlamydomonas reinhardtii instead of inducing apoptosis as shown for some mammalian cell lines. Therefore, we have studied other well-known inducers of apoptosis in mammalian cells for their effects on C. reinhardtii cells. Mastoparan caused rapid cell death without a prominent lag-phase under all growth conditions, whereas the cytotoxic effect of the topoisomerase I inhibitor camptothecin exclusively occurred during the cell-division phase. Essentially no differences between wall-deficient and wild-type cells were observed with respect to dose-response and time-course of camptothecin and mastoparan. In cultures of the wall-deficient strain, cell death was accompanied by swelling and subsequent disruption of the cells, established markers of necrosis. In case of the wild-type strain, camptothecin and mastoparan caused accumulation of apparently intact, but dead cells instead of cell debris due to the presence of the wall. Both in cultures of the wall-deficient and the wild-type strains, cell death was accompanied by an increase of the protein concentration in the culture medium indicating a lytic process like necrosis. Taking together, we have severe doubts on the existence of an apoptotic program in case of C. reinhardtii.

SpxB-mediated H2 O2 induces programmed cell death in Streptococcus sanguinis

Streptococcus sanguinis (S. sanguinis) is a commensal oral streptococci that produces hydrogen peroxide (H2 O2 ), and this production is dependent on pyruvate oxidase (SpxB) activity. In addition to its well-known role in intraspecies or interspecies competitions, recent studies have shown that H2 O2 produced by S. sanguinis under aerobic conditions not only upregulates biofilm formation and eDNA release but also regulates cell death without obvious cell lysis. Here, we report that S. sanguinis exhibits characteristic hallmarks of eukaryotic apoptosis when it encounters endogenous and exogenous H2 O2 . As the most common mode of programmed cell death (PCD), apoptosis is accompanied by a series of biochemical and morphological events, including DNA fragmentation, chromosome condensation, membrane potential depolarization, phosphatidylserine (PS) exposure, and caspase substrate binding protein activity changes. In addition, we also provide genetic evidence that there is decreased expression of the related DNA repair genes comEA, recA, dnaC, dinG, and pcrA in the wild-type compared to the isogenic spxB mutant in S. sanguinis. Our data suggest that endogenous H2 O2 is the most important agent in this development process in S. sanguinis.