Yeast Lacking theSRO7/SOP1-encoded Tumor Suppressor Homologue Show Increased Susceptibility to Apoptosis-like Cell Death on Exposure to NaCl Stress

Abstract

Yeast cells deleted for the SRO7/SOP1 encoded tumor suppressor homologue show increased sensitivity to NaCl stress. On exposure to growth-inhibiting NaCl concentrations, sro7⌬ mutants display a rapid loss in viability that is associated with markers of apoptosis: accumulation of reactive oxygen species, DNA breakage, and nuclear fragmentation. Additional deletion of the yeast metacaspase gene YCA1 prevents the primary fast drop in viability and diminishes nuclear fragmentation and DNA breakage. We also observed that NaCl induced loss in viability of wild-type cells is Yca1p dependent. However, a yeast strain deleted for both SRO7 and its homologue SRO77 exhibits NaCl-induced cell death that is independent on YCA1. Likewise, sro77⌬ single mutants do not survive better after additional deletion of the YCA1 gene, and both sro77⌬ and sro77⌬yca1⌬ mutants display apoptotic characteristics when exposed to growth-inhibiting salinity, suggesting that yeast possesses Yca1p-independent pathway(s) for apoptosis-like cell death. The activity of Yca1p increases with increasing NaCl stress and sro7⌬ mutants achieve levels that are higher than in wild-type cells. However, mutants lacking SRO77 do not enhance caspase activity when subject to NaCl stress, suggesting that Sro7p and Sro77p exert opposing effects on the cellular activity of Yca1p. INTRODUCTIONThe physiological cell death process of apoptosis is a morphologically distinct form of cellular suicide (Kerr, 2002), designed to remove potentially threatening or undesired cells (Vaux and Korsmeyer, 1999;Beers and McDowell, 2001). In animal cells, apoptosis occurs in an ordered series of event and is associated with activation of the programmed cell death machinery (Wyllie et al., 1980;Joza et al., 2002). Inappropriate regulation of apoptosis is linked to the pathogenesis of many human diseases, such as AIDS, cancer, autoimmune, and neurodegenerative disorders (Uren and Vaux, 1996). The process of programmed cell death can be triggered by a vast array of stimuli, and a complex network of regulators and effectors coordinates the process (Beers and McDowell, 2001;Joza et al., 2002). At the end of almost all apoptotic scenarios, a constellation of morphological characteristics emerge, including chromatin condensation, DNA fragmentation, flipping of phosphatidylserine to the outer leaflet of the plasma membrane, and breakage of the cells to small membrane-enclosed vesicles, so called apoptotic bodies (Wyllie et al., 1980). These structural features constitute the cytological hallmarks of apoptosis.The finding that cellular suicide occurs in most, if not all, multicellular organisms raises the question on how the mechanisms of apoptosis have evolved. Do these mechanisms have a common origin and have these processes first evolved in single-celled organisms? For unicellular organisms exposed to harsh conditions, cellular suicide may serve to propagate the genome to future generations if some of the members of a population are sacrificed to promote survival of others (Engelberg-Kulk...

Apoptosis in budding yeast caused by defects in initiation of DNA replication

“…A role for Yca1p in yeast apoptosis and the validity of the concept of apoptosis in yeasts was recently challenged by experiments that used extraordinarily lethal conditions allowing for a maximum of 0.001% survival (Wysocki and Kron, 2004). Unlike the Wysocki and Kron study, but similar to other studies that (like our study) used far less lethal conditions more appropriate for studying cell-death pathways (Bettiga et al, 2005;Fahrenkrog et al, 2004;Madeo et al, 2002b;Wadskog et al, 2004) MATa,112, (Fig. 2).…”

Metacaspases

“…17 Furthermore, metacaspase involvement in the yeast cell death models was investigated after in vivo staining for caspase activity by flow cytometry with FITC-labeled VAD-fmk (FITC-VAD-fmk). 17,[41][42][43][44] Likewise, in the fungus C. albicans, it was suggested that the antifungal compound Plagiochin E activated metacaspases based on the appearance of FITC-VAD-fmk labeled cells. 45 In the phytoplankton Chlamydomonas reinhardtii and other marine species, such as the diatom Thalassiosira pseudonana and the unicellular coccolithore Emiliania huxlei, increased caspase-like activity was correlated with increased accumulation of metacaspase mRNA and protein, thereby providing indirect evidence that metacaspases had caspase-like activity.…”

Bloodstream formTrypanosoma bruceidepend upon multiple metacaspases associated with RAB11-positive endosomes

“…They have been implicated in maintenance of the balance between death and proliferation required for plant embryogenesis Bozhkov et al, 2005). Evidence is also accumulating for a role in cell death of the Saccharomyces cerevisiae metacaspase YCA1 (Madeo et al, 2002;Herker et al, 2004;Bettiga et al, 2004;Wadskog et al, 2004;Madeo et al, 2004). Overexpression of YCA1 resulted in both fragmented DNA and chromatin condensation, and these features were abolished by co-incubation with the caspase inhibitor Z-VAD-fluoromethylketone (Madeo et al, Trypanosoma brucei possesses five metacaspase genes.…”